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Title: Discoveries and Inventions of the Nineteenth Century
Author: Routledge, Robert
Language: English
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[Illustration:
PLATE I.
THE GREAT WHEEL IN ACTION.
]
DISCOVERIES AND INVENTIONS
OF THE
NINETEENTH CENTURY
Who saw what ferns and palms were pressed
Under the tumbling mountain’s breast,
In the safe herbal of the coal?
But when the quarried means were piled,
All is waste and worthless, till
Arrives the wise selecting Will,
And, out of slime and chaos, Wit
Draws the threads of fair and fit.
Then temples rose, and towns, and marts,
The shop of toil, the hall of arts;
Then flew the sail across the seas
To feed the North from tropic trees;
The storm-wind wove, the torrent span,
Where they were bid the rivers ran;
New slaves fulfilled the poet’s dream,
Galvanic wire, strong-shouldered steam.
EMERSON.
DISCOVERIES AND INVENTIONS
OF THE
NINETEENTH CENTURY
BY
ROBERT ROUTLEDGE, B.Sc.,
SOMETIME ASSISTANT EXAMINER IN CHEMISTRY AND IN NATURAL PHILOSOPHY TO
THE UNIVERSITY OF LONDON
THIRTEENTH EDITION
REVISED AND PARTLY RE-WRITTEN, WITH ADDITIONS
CONTAINING FOUR HUNDRED AND FIFTY-SIX ILLUSTRATIONS
LONDON
GEORGE ROUTLEDGE AND SONS, LIMITED
BROADWAY, LUDGATE HILL
1900
PREFACE.
In the following pages an attempt has been made to present a popular
account of remarkable discoveries and inventions which distinguish the
XIXth century. They distinguish it not merely in comparison with any
previous century, but in comparison with all the centuries that have
preceded, in regard to far-reaching intellectual acquisitions, and to
material achievements, which together have profoundly affected our ways
of thinking and our habits of life. In the latter, the enormously
increased facilities of locomotion and international communication due
to railways and steam navigation have wrought the greatest changes.
These inventions depending primarily upon that of the steam engine, this
first claims our notice, although properly assignable to a period
preceding our era by a few years. Again, much of our material
advancement is connected with improvements in the manufacture of iron
and its applications in the form of steel, which have been especially
the work of the last half of the century. So great has been the progress
in this department, that for the present edition it has been found
necessary to re-write altogether the article devoted to it. Our social
conditions have also been greatly modified by the celerity of verbal
intercourse afforded by the telegraph and the telephone, and these
inventions have received appropriate notice in this work. In every
branch of science also we have reason to be proud of the discoveries our
era can claim, for they vastly excel in number and are not inferior in
range to those of all the ages taken together. From so large a field,
selection was of course necessary; and the instances selected have been
those which appeared to some extent typical, or those which seemed to
have the most direct bearing on the general advance of our time. The
topics comprise chiefly those great applications of mechanical
engineering and arts, and of physical and chemical science, in which
every intelligent person feels concerned; while some articles are
devoted to certain purely scientific discoveries that have excited
general interest.
The author has aimed at giving a concise but clear description of the
several subjects; and that without assuming on the part of the reader
any knowledge not usually possessed by young persons of either sex who
have received an ordinary education. The design has been to treat the
subjects as familiarly as might be consistent with a desire to impart
real information; while the popular character of the book has not been
considered a reason for regarding accuracy as unnecessary. On the
contrary, pains have been taken to consult the best authorities; and it
is only because the sources of information to which the author is under
obligation are so many, that he cannot acknowledge them in detail.
The present edition has been revised throughout, and such changes have
been made as were required to bring the matter into accordance with the
progress that has taken place since this book was first published in
1876. But details given in the former editions have at the same time
been retained where they served to indicate the successive stages of
improvement. It would, for example, be impossible in a section on steam
navigation, to omit some notice of the _Great Eastern_, and therefore
the drawings and the account of the construction of that remarkable ship
that appeared in the first edition, have been left with but slight
alterations in the present volume, although the vessel has since been
broken up. On the other hand, two sections are devoted to projects which
the XIXth century has not seen realised; but the XXth century will in
all probability shortly witness the completion of one or other of the
great canal schemes; and if the first submarine tunnel is destined not
to be one connecting England with the Continent, it will be one uniting
Great Britain with her sister isle.
1899.
* * * * *
For permission to make use of illustrations in this volume the author’s
and publishers’ thanks are due to the several proprietors of _The
Graphic_ (for Plates I., XI., and XII.)—of _The Engineer_ (for sketch
design of the Great Wheel, map and views of the Tower Bridge)—of _The
Scientific American_ (map of North Sea Canal); also to Mr. Walter B.
Basset (for Plate V.)—to “The Cassier Magazine Company” (for Edison’s
Kinetographic Theatre and the Hotchkiss Gun)—to “The Century Company”
(for portrait of M. Tesla, from a photograph by Sarony)—to “The
Incandescent Gas Light Company” (for cuts of burners, etc.)—to _The
Engineering Magazine_, and _The Engineering News_, both of New York—to
the Remington Company—to Mr. W. W. Greener, of Birmingham (for cuts of
rifles, etc., from his comprehensive book on “The Gun”)—to _The
Photogram_, Limited—to the Proprietors of _Nature_—to the Linotype
Company—and to Captains Hadcock and Lloyd (for illustrations of modern
artillery from their great work on the subject).
CONTENTS.
PAGE
INTRODUCTION 1
STEAM ENGINES 3
THE LOCOMOTIVE 14
PORTABLE ENGINES 24
THE STEAM HAMMER 25
IRON 29
IRON IN ARCHITECTURE 72
BIG WHEELS 81
TOOLS 85
THE BLANCHARD LATHE 96
SAWING MACHINES 98
RAILWAYS 101
THE METROPOLITAN RAILWAYS 114
THE PACIFIC RAILWAY 116
INCLINED RAILWAYS 125
STEAM NAVIGATION 129
RIVER AND LAKE STEAMBOATS OF AMERICA 144
SHIPS OF WAR 149
FIRE-ARMS 169
THE MILITARY RIFLE 178
RIFLED CANNON 190
MACHINE GUNS 218
TORPEDOES 227
SHIP CANALS 249
THE SUEZ CANAL 251
THE MANCHESTER SHIP CANAL 262
THE NORTH SEA CANAL 271
THE PANAMA AND NICARAGUA CANAL PROJECTS 272
IRON BRIDGES 276
GIRDER BRIDGES 280
SUSPENSION BRIDGES 284
CANTILEVER BRIDGES 291
THE TOWER BRIDGE, LONDON 297
THE GREAT BROOKLYN BRIDGE 303
PRINTING MACHINES 305
LETTERPRESS PRINTING 306
PATTERN PRINTING 321
HYDRAULIC POWER 324
PNEUMATIC DISPATCH 340
ROCK BORING 349
THE MONT CENIS TUNNEL 351
ROCK-DRILLING MACHINES 355
THE CHANNEL TUNNEL 364
THE ST. GOTHARD RAILWAY 371
LIGHT 380
SOME PHENOMENA OF LIGHT 382
VELOCITY OF LIGHT 384
REFLECTION OF LIGHT 388
REFRACTION 397
DOUBLE REFRACTION AND POLARISATION 399
CAUSE OF LIGHT AND COLOUR 408
THE SPECTROSCOPE 416
CELESTIAL CHEMISTRY AND PHYSICS 436
ROENTGEN’S X RAYS 445
SIGHT 452
THE EYE 454
VISUAL IMPRESSIONS 468
ELECTRICITY 481
ELEMENTARY PHENOMENA OF ELECTRICITY AND MAGNETISM 483
THEORY OF ELECTRICITY 487
ELECTRIC INDUCTION 488
DYNAMICAL ELECTRICITY 490
INDUCED CURRENTS 502
MAGNETO-ELECTRICITY 507
THE GRAMME MAGNETO-ELECTRIC MACHINE 511
ELECTRIC LIGHTING AND ELECTRIC POWER 519
THE NEW ELECTRICITY 538
THE ELECTRIC TELEGRAPH 547
TELEGRAPHIC INSTRUMENTS 553
TELEGRAPHIC LINES 572
THE TELEPHONE 581
LIGHTHOUSES 593
PHOTOGRAPHY 607
PHOTOGRAPHY IN COLOURS 630
PRINTING PROCESSES 632
STEREOTYPING 632
LITHOGRAPHY 636
OTHER PROCESSES 640
THE LINOTYPE MACHINE 645
RECORDING INSTRUMENTS 653
THE PHONOGRAPH 665
AQUARIA 675
THE CRYSTAL PALACE AQUARIUM 677
THE BRIGHTON AQUARIUM 682
GOLD AND DIAMONDS 687
GOLD 687
DIAMONDS 696
NEW METALS 714
INDIA-RUBBER AND GUTTA-PERCHA 724
INDIA-RUBBER 724
GUTTA-PERCHA 728
ANÆSTHETICS 731
EXPLOSIVES 740
MINERAL COMBUSTIBLES 751
COAL 751
PETROLEUM 757
PARAFFIN 761
COAL-GAS 764
COAL-TAR COLOURS 781
THE GREATEST DISCOVERY OF THE AGE 801
NOTES 811
INDEX 813
LIST OF ILLUSTRATIONS.
FIG. PAGE
Heading—Rain, Steam, and Speed (after Turner) 1
1. Portrait of James Watt 3
2. Newcomen’s Steam Engine 4
3. Watt’s Double-action Steam Engine 5
4. Governor and Throttle-Valve 6
4_a_. Watt’s Parallel Motion 8
5. Slide Valve 9
6. Section of Gifford’s Injector 11
7. Bourdon’s Pressure Gauge 12
8. Steam Generator 13
9. Section of Locomotive 15
10. Stephenson’s Link Motion 17
10_a_. G. N. R. Express Passenger Locomotive 19
10_b_. Joy’s Valve Gear 20
11. Locomotive after Explosion 22
12. Hancock’s Steam Omnibus 22
13. Nasmyth’s Steam Hammer 27
14. Merryweather’s Steam Fire-Engine 28
15. A Foundry 29
16. Aerolite in the British Museum 31
17. Blast Furnace 41
18. Section and Plan of Blast Furnace (obsolete type) 42
19. Section of a Reverberatory Furnace 45
20. Fibrous Fracture of Wrought Iron 47
21. Cup and Cone 49
22. Section of Blast Furnace 51
23. Experiments at Baxter House 58
24. Bessemer Converter 63
25. Model of Bessemer Steel Apparatus 65
26. Section of Regenerative Stoves and Open Hearth 68
26_a_. Rolling Mill 71
26_b_. The Eiffel Tower in course of construction 73
26_c_. The Eiffel Tower 75
26_d_. St. Paul Building, N. Y. 77
26_e_. Manhattan Insurance Co.’s Building in course of 79
erection
26_f_. Manhattan Insurance Co.’s Building nearly completed 80
26_g_. Original Design for the Great Wheel 82
27. Portrait of Sir Joseph Whitworth 85
28. Whitworth’s Screw Dies and Tap 86
29. Screw-cutting Lathe 87
30. Whitworth’s Measuring Machine 89
31. Whitworth’s Drilling Machine 91
32. Whitworth’s Planing Machine 93
33. Pair of Whitworth’s Planes or Surface Plates 94
34. Interior of Engineer’s Workshop 95
35. Blanchard Lathe 96
36. Vertical Saw 98
37. Circular Saw 99
38. Pit-Saw 100
39. Box Tunnel 101
40. Coal-pit, Salop 102
41. Sankey Viaduct 103
42. Rails and Cramp-gauge 104
43. Fish-plate 105
44. Section of Rails and Fish-plates 106
45. Conical Wheels 107
46. Centrifugal Force 107
47. Points 108
48. Signal Box on North London Railway 109
49. Post Office Railway Van 111
50. Gower Street Station, Metropolitan Railway 115
51. Map of the Route of Pacific Railway 117
52. Trestle Bridge 118
53. American Canyon 119
54. “Cape Horn” 121
55. Snow Plough 122
56. First Steam Railroad Train in America 123
57. Railway Embankment 124
57_a_. Train ascending the Rigi 126
57_b_. At the summit of the Rigi 127
58. The _Great Eastern_ at Anchor 129
59. Casting Cylinder of a Marine Steam Engine 131
60. Screw Propeller 132
61. Section of _Great Eastern_ Amidships 134
62. The _Great Eastern_ in course of construction 135
63. The _Great Eastern_ ready for launching 136
64. Comparative sizes of Steamships 137
65. The ss. _City of Rome_ 138
66. The _Castalia_ in Dover Harbour 140
67. The same—End View 141
68. Bessemer Steamer 142
68_a_. A Whaleback Steamer, No. 85, built at West 146
Superior, Wisconsin
69. H.M.S. _Devastation_ in Queenstown Harbour 149
70. Section of H.M.S. _Hercules_ 151
71. Section of H.M.S. _Inconstant_ 153
72. Section, Elevation and Plan of Turret of H.M.S. 154
_Captain_
73. H.M.S. _Captain_ 155
74. Diagram of H.M.S. _Captain_ 158
75. Ditto 159
76. H.M.S. _Glatton_ 162
77. H.M.S. _Thunderer_ 163
78. The _König Wilhelm_ 165
78_a_. The _Victoria_ leaving Newcastle-on-Tyne 166
78_b_. Firing at Floating Battery 168
79. Krupp’s Works at Essen, Prussia 169
80. Trajectory of a Projectile 174
81. Diagram for Trajectory of a Projectile 176
82. Muzzle-loading Musket and Rifles (obsolete 179
patterns)
83. The Minié Bullet 181
84. Greener’s Expanding Bullet 182
85. The Chassepot Rifle—Section of the Breech 183
86. Section of the Martini-Henry Lock 185
87. The Martini-Henry Rifle 186
88. The Mannlicher Magazine Rifle 188
89. The Magazine and Breech of the Mannlicher Rifle 189
90. 32–pounder, 1807 191
91. Whitworth Rifling and Projectile 193
92. 600–pounder Muzzle-loading Armstrong Gun 194
93. 35–ton Fraser Gun 195
94. Section of 9–in. Fraser Gun 196
95. Millwall Shield after being battered with Heavy 200
Shot—Front View
96. Rear View of the Millwall Shield 200
97. Comparative Sizes of 35 and 81–ton Guns 201
98. Diagram of Velocities and Pressures 205
99. Elswick 4·7–in. Q. F. Gun on Pivot Mounting 207
100. The Moncrieff Gun raised and ready for firing 209
101. Moncrieff Gun lowered for loading 209
102. 68–ton Gun on Elswick Hydro-Pneumatic Mounting 211
103. Mallet’s Mortar 213
104. 32–pounder Krupp Siege Gun, with Breech-piece open 214
105. The Citadel of Strasburg after the Prussian 215
Bombardment
105_a_. The Shrapnel and Segment Shells 217
105_b_. The Gatling Gun—Rear View 219
105_c_. The Gatling Gun—Front View 221
105_d_. The Montigny Mitrailleur 222
105_e_. A Hotchkiss Gun 224
106. Harvey’s Torpedo.—Working the Brakes 227
107. Submerged Torpedo 228
108. Mode of Firing Torpedo 230
109. Explosion of Whitehead’s Torpedo 231
110. Effect of the Explosion of Whitehead’s Torpedo 232
111. Experiment with a Torpedo charged with 10 lbs. Gun 233
Cotton
112. Explosion of Torpedo containing 67 lbs. Gun Cotton 234
113. Explosion of 432 lbs. Gun Cotton in 37 ft. Water 235
114. The same in 27 ft. Water 235
115. Section of Priming Case and Exploding Bolt 236
116. Harvey’s Torpedo 237
117. The same 238
118. The same 239
119. Official Trial of “Harvey’s Sea Torpedo” 239
120. Model of Submarine Guns 240
121. The Warner Experiment off Brighton 241
122. Portrait of M. Lesseps 249
123. The Sand-Glass 253
124. A Group of Egyptian Fellahs and their Wives 254
125. Dredges and Elevators at Work 255
126. Map of the Suez Canal 256
127. Port Saïd, the Mediterranean Entrance to the Suez 257
Canal
128. Bird’s-eye View of Port Saïd 258
129. One of the Breakwaters at Port Saïd 259
130. Lake Timsah and Ismaïlia 259
131. Railway Station at Ismaïlia 260
132. The Viceroy of Egypt cutting Embankment 261
133. Map of the Manchester Ship Canal, Western Portion 263
134. Map of the Manchester Ship Canal, Eastern Portion 263
135. A Cutting for the Manchester Ship Canal 265
136. Blasting Rocks for the Manchester Ship Canal 266
137. Manchester Ship Canal Works, Runcorn 267
137_a_. The French Steam Navvy 268
137_b_. The English Steam Navvy 269
137_c_. Sketch Map of the North Sea Canal 271
138. Britannia Bridge, Menai Straits 276
139. Diagram showing Strains 278
140. Ditto 279
141. Girder 279
142. Ditto 279
143. Ditto 280
144. Section of a Tube of the Britannia Bridge 281
145. Albert Bridge, Saltash 283
146. Clifton Suspension Bridge, near Bristol 285
147. Section of Shaft 286
147_a_. Clifton Suspension Bridge, Niagara 288
147_b_. Living Model of the Cantilever Principle 291
147_c_. Principal Dimensions of the Forth Bridge 294
147_d_. Map of the Tower Bridge and its Approaches 299
147_e_. The Tower Bridge 301
147_f_. Sketch 302
148. Newspaper Printing-Room 305
149. Inking Balls 306
150. Inking Roller 306
151. Diagram of Single Machine 308
152. Diagram of Perfecting Machine 309
153. Cowper’s Double Cylinder Machine 309
154. Tapes of Cowper’s Machine 310
155. Hopkinson and Cope’s Perfecting Machine 311
156. Section of Casting Apparatus 314
157. Diagram of the Walter Press 315
158. Hoe’s Type Revolving Cylinder Machine 317
159. Hoe’s “Railway” Machine 319
160. Napier’s Platen Machine 320
161. Roller for Printing Wall-Papers 322
162. Machine for Printing Paper-Hangings 323
163. Chain Testing Machine 324
164. Pascal’s Principle 325
165. Collar of Hydraulic Cylinder 326
166. Hydraulic Press 327
167. Section of Hydraulic Lift Graving Dock 331
168. Section of Column 332
169. Sir W. Armstrong’s Hydraulic Crane 335
170. Raising Tubes of Britannia Bridge 336
171. Press for Raising the Tubes 337
172. Head of Link-Bars 338
173. Apparatus to Prove Transmission of Pressure 339
174. Pneumatic Tubes and Carriages 340
175. Diagram of Tubes, &c. 342
176. Sending and Receiving Apparatus 343
177. Section of Receiving Apparatus 344
178. Sommeiller Boring Machines 349
179. Transit by Diligence over Mont Cenis 353
180. Burleigh Rock Drill on Tripod 356
181. The same on Movable Column 358
182. The same Mounted on Carriage 359
183. Diamond Drill Crown 360
184. Diamond Drill Machinery 363
185. Chart of the Channel Tunnel 367
186. Section of the Channel Tunnel 368
187. View of Dover 369
187_a_. Map of the St. Gothard Railway 372
187_b_. The Uppermost Bridge over the Maïenreuss 375
187_c_. The Bridges over the Maïenreuss, near Wasen 377
187_d_. Windings of the Line near Wasen 378
188. Contrasts of Light 380
189. Rays 382
190. Diagram 383
191. Telescopic Appearance of Jupiter and Satellites 384
192. Diagram 386
193, 194, 195. Diagrams 388
196. Diagram 389
197. Polemoscope 390
198. Apparatus for Ghost Illusion 391
198_a_. The Ghost Illusion 393
199. Illusion produced by Mirrors 394
200. A Stage Illusion 395
201. View of Venice—Reflections 396
202. Refraction 397
203. Diagram 398
204, 205. Diagrams of Crystals 400
206. Diagram 401
207. Diagram 403
208. Diagram 404
209. Polariscope 406
210. Section showing Polarisation 407
211. Iceland Spar, showing Double Refraction 407
212. Diagram 408
213. Diagram 410
214. Diagram 412
215. Portrait of Professor Kirchhoff 416
216. Diagram 417
217. Newton’s Experiment 418
218. Bunsen’s Burner on Stand 421
219. Spectroscope with one Prism 423
220. Miniature Spectroscope 426
221. The Gassiot Spectroscope 427
222. Browning’s Automatic Adjustment of Prisms 429
223. Apparatus for Spark Spectra 430
224. The Sorby-Browning Micro-Spectroscope 433
225. Section of Micro-Spectroscope, with Micrometer 434
226. Diagram 435
227. Section of Micro-Spectroscope 436
228. Solar Eclipse, 1869 439
229. The Planet Saturn 441
230. Solar Prominences, No. 1 442
231. Ditto, No. 2 443
232. Section of Amateur Star Spectroscope 444
232_a_. X. Ray Photo of Living Hand, Exposure 4 minutes 446
232_b_. Skiagraph of a Hand by Dr. Roentgen 448
232_c_. Metal objects photographed through Calico and sheet 450
of Aluminium
232_d_. Skiagraph of Layers of various substances 451
233. Portrait of Professor Helmholtz 452
234. Vertical Section of the Eye 454
235. Section of Retina 456
236. Diagram 457
237. Muscles of Eyes 459
238. Diagram 461
239. Diagram 464
240. Diagram 465
241. Ruete’s Ophthalmoscope 466
242. Diagram 467
243. Wheatstone’s Reflecting Stereoscope 469
244. Diagram 470
245. Diagram 471
246. The Telestereoscope 473
247. Lines 475
248, 249. Diagrams 476
250, 251. Diagrams 477
251_a_. Edison’s Kinetographic Theatre 479
252. Portrait of Sir W. Thomson 481
253. A simple Electroscope 485
254. The Gold-leaf Electroscope 489
255. The Leyden Jar 490
256. A Voltaic Element 491
257. Ampère’s Rule 492
258. Galvanometer 493
259. Daniell’s Cell and Battery 495
260. Grove’s Cell and Battery 495
261. Wire Ignited by Electricity 496
262. Duboscq’s Electric Lantern and Regulator 497
263. Decomposition of Water 498
264. Electro-plating 499
265. A Current producing a Magnet 500
266. An Electro-magnet 501
267. Ruhmkorff’s Coil 503
268. Discharge through Rarefied Air 504
268_a_. Large Induction Coil at the Old Polytechnic 505
Institution, London
269. Appearance of Spark on Looking-glass 507
270. Magneto-electric Spark 508
271. A Magnet producing a Current 509
272. Clarke’s Magneto-electric Machine 509
273. Magneto-electric Light 510
274. Diagram 511
275. Gramme Machine 512
276. Insulated Coils 513
277. Hand Gramme Machine 513
278. Gramme Machine, with eight Vertical Electro-Magnets 516
279. Gramme Machine, with Horizontal Electro-magnets 517
280. Gramme Machine 519
280_a_. The Alliance Machine 520
280_b_. Wilde’s Machine 521
280_c_. Siemens’ Dynamo 522
280_d_. The Brush Dynamo 523
280_e_. Siemens’ Regulator 524
280_f_. Jablochkoff Candle 525
280_g_. Electric Lamp 526
280_h_. Incandescent Lamp 529
280_i_. Poles with Single Arms for Suburban Roads.—The 533
Ontario Beach Railway, Rochester, N.Y.
280_j_. The Glynde Telepherage Line, on the system of the 534
late Fleeming Jenkin
280_k_. Diagrams 540
280_l_. The Tesla Oscillator 542
280_m_. M. Nikola Tesla 543
281. Portrait of Professor Morse 547
282. Double-Needle Instrument 554
283. Electro-magnetic Bells 555
284. Portable Single-Needle Instrument 556
285. Connections of Telegraph Line 558
286. Morse Recording Telegraph 559
287. Morse Transmitting Key 561
288. Morse Transmitting Plate 562
289. Step-by-step Movement 567
290. Froment’s Dials 567
291. Wheatstone’s Universal Dial Telegraph 568
292. Mirror Galvanometer 571
293. Telegraph Post and Insulators 573
294. Ditto 573
295. Wire Circuit 574
296. Wire and Earth Circuit 574
297. Submarine Cable 575
298. Making Wire for Atlantic Cable 577
299. Instrument Room at Valentia 578
300. Breaking of the Cable 579
301. Atlantic Telegraph Cable, 1866 580
302. Diagram 580
302_a_. Reiss’s Musical Telephone 584
302_b_. Bell’s Musical Telephone 585
302_c_. Superposition of Currents 587
302_d_. Bell’s Speaking Telephone 588
302_e_. Hughes’s Microphone 591
Lighthouse (heading) 593
303. Eddystone Lighthouse 594
304. Eddystone in a Storm 595
305. Revolving Light Apparatus 601
306. Stephenson’s Holophotal Light 604
307. Camera 607
308. Camera and Slide 615
309. Folding Camera 616
310. Lenses 617
311. Bath 619
311_a_. The Roll-Slide 622
312. Portrait of Aloysius Senefelder 632
313. Press for Stereotyping by Clay Process 633
313_a_. The Linotype Machine 645
313_b_. A Matrix 646
313_c_. Diagram of Movements 647
313_d_. A Line of Matrices 648
313_e_. A finished Line entering galley 649
313_f_. The Melting Pot and Mould Wheel 650
313_g_. The Finished Line 651
313_h_. Lines assembled into a “Form” 651
313_i_. Matrices dropping into Magazine 652
314. Recording Anemometer 653
315. Registration of Height of Barometer and Thermometer 655
316. Electric Chronograph 657
317. Negretti’s Deep-Sea Thermometer 661
318. Ditto, General Arrangement 662
319. Atmospheric Recording Instrument 663
319_a_. Traces of Vibrations of a Tuning-Fork 667
319_b_. Phonautographic Tracings of Different Vowel Sounds 667
319_c_. Diagram 668
319_d_. Phases of Sound Waves 668
319_e_. Edison’s Original Phonograph 670
319_f_. Diagrammatic Section of Phonograph 671
319_g_. The Graphophone 672
319_h_. Edison’s Perfected Phonograph 674
320. Domestic Aquarium 675
321. The Opelet 679
322. Viviparous Blenny 680
323. The Lancelet 681
324. Sea-Horses 683
325. Proteus anguinus 684
326. Mud-Fish 685
327. The Axolotl 686
328. Sorting, Washing, and Digging at the South African 687
Diamond Fields
329. Gold Miner’s Camp 689
330. Gold in Rocks 690
331. “Cradle” for Gold-washing 690
332. Pniel, from Jardine’s Hotel 702
333. Sifting at the “Dry Diggings” 703
334. Vaal River, from Spence Kopje 704
334_a_. Sketch Section of the Kimberley Diamond Mine 709
335. Portrait of Sir Humphrey Davy 714
336. Apparatus 717
337. Portrait of Mr. Thomas Hancock 724
338. Portrait of Sir James Young Simpson, M.D. 731
339. Railway Cutting 740
340. View on the Tyne 751
341. Fossil Trees in a Railway Cutting 752
342. Impression of Leaf in Coal Measures 753
343. Possible Aspect of the Forests of the Coal Age 754
344. The Fireside 756
345. View on Hyde and Egbert’s Farm, Oil Creek 761
346. View of City of London Gas-Works 764
347. Section of Gas-making Apparatus 765
348. The Retort 767
348_a_. Retort House of the Imperial Gas-Works 768
349. The Gas Governor 770
350. Bunsen’s Burner 772
351. Faraday’s Ventilating Gas-Burner 773
351_a_. Diagram 778
351_b_. Diagram 778
351_c_. Diagram 779
351_d_. Diagram 779
351_e_. Diagram 780
352. Apparatus for making Magenta 781
353. Iron Pots for making Nitro-Benzol 784
354. Section of Apparatus for making Nitro-Benzol 785
355. Apparatus for making Aniline 786
356. Section of Hollow Spindle 787
357. Portrait of J. Prescott Joule, F.R.S. 801
LIST OF PLATES.
PLATE I.
TO FACE
THE GREAT WHEEL IN ACTION _Title page_
PLATE II.
NORTH-EASTERN RAILWAY LOCOMOTIVE 18
PLATE III.
THE GREAT STEAM HAMMER, ROYAL GUN FACTORY, WOOLWICH 28
PLATE IV.
THE AMERICAN TRACT SOCIETY BUILDING 76
PLATE V.
GENERAL VIEW OF THE GREAT WHEEL AT EARL’S COURT 84
PLATE VI.
MOUNT WASHINGTON INCLINED TRACK 124
PLATE VII.
PIKE’S PEAK RAILROAD, ROCKY MOUNTAINS 128
PLATE VIII.
THE “CLERMONT” FROM A CONTEMPORARY DRAWING 130
PLATE IX.
THE “MARY POWELL” 144
PLATE X.
THE “NEW YORK” 148
PLATE XI.
H.M.S. “THE TERRIBLE” 168
PLATE XII.
THE 110–TON ARMSTRONG GUN 202
PLATE XIII.
THE FORTH BRIDGE 292
PLATE XIV.
THE TOWER BRIDGE IN COURSE OF CONSTRUCTION 298
PLATE XV.
THE BROOKLYN BRIDGE 304
PLATE XVI.
THE NORTH MOUTH OF THE GREAT TUNNEL, ST. GOTHARD RAILWAY 374
PLATE XVII.
SPECTRA (Coloured Plate) 422
[Illustration:
_Wind, Steam, and Speed_ (after TURNER).
]
INTRODUCTION.
Only by knowledge of Nature’s laws can man subjugate her powers and
appropriate her materials for his own purposes. The whole history of
arts and inventions is a continued comment on this text; and since the
knowledge can be obtained only by observation of Nature, it follows that
Science, which is the exact and orderly summing-up of the results of
such observation, must powerfully contribute to the well-being and
progress of mankind.
Some of the services which have been rendered by science in promoting
human welfare are thus enumerated by an eloquent writer: “It has
lengthened life; it has mitigated pain; it has extinguished diseases; it
has increased the fertility of the soil; it has given new securities to
the mariner; it has furnished new arms to the warrior; it has spanned
great rivers and estuaries with bridges of form unknown to our fathers;
it has guided the thunderbolt innocuously from heaven to earth; it has
lighted up the night with the splendour of the day; it has extended the
range of the human vision; it has multiplied the power of the human
muscles; it has accelerated motion; it has annihilated distance; it has
facilitated intercourse, correspondence, all friendly offices, all
dispatch of business; it has enabled man to descend to the depths of the
sea, to soar into the air, to penetrate securely into the noxious
recesses of the earth, to traverse the land in cars which whirl along
without horses, to cross the ocean in ships which run ten knots an hour
against the wind. These are but a part of its fruits, and of its
first-fruits; for it is a philosophy which never rests, which has never
attained, which is never perfect. Its law is progress. A point which
yesterday was invisible is its goal to-day, and will be its
starting-point tomorrow.”—(MACAULAY).
Thus every new invention, every triumph of engineering skill, is the
embodiment of some scientific idea; and experience has proved that
discoveries in science, however remote from the interests of every-day
life they may at first appear, ultimately confer unforeseen and
incalculable benefits on mankind. There is also a reciprocal action
between science and its application to the useful purposes of life; for
while no advance is ever made in any branch of science which does not
sooner or later give rise to a corresponding improvement in practical
art, so on the other hand every advance made in practical art furnishes
the best illustration of scientific principles.
The enormous material advantages which this age possesses, the cheapness
of production that has placed comforts, elegancies, and refinements
unknown to our fathers within the reach of the humblest, are traceable
in a high degree to the arrangement called the “division of labour,” by
which it is found more advantageous for each man to devote himself to
one kind of work only; to the steam engine and its numerous
applications; to increased knowledge of the properties of metals, and of
the methods of extracting them from their ores; to the use of powerful
and accurate tools; and to the modern plan of manufacturing articles by
processes of copying, instead of fashioning everything anew by manual
labour. Little more than a century ago everything was slowly and
imperfectly made by the tedious toil of the workman’s hand; but now
marvellously perfect results of ingenious manufacture are in every-day
use, scattered far and wide, so that their very commonness almost
prevents us from viewing them with the attention and admiration they
deserve. Machinery, actuated by the forces of nature, now performs with
ease and certainty work that was formerly the drudgery of thousands.
Every natural agent has been pressed into man’s service: the winds, the
waters, fire, gravity, electricity, light itself.
But so much have these things become in the present day matters of
course, that it is difficult for one who has not witnessed the
revolution produced by such applications of science to realize their
full importance. Let the young reader who wishes to understand why the
present epoch is worthy of admiration as a stage in the progress of
mankind, address himself to some intelligent person old enough to
remember the century in its teens; let him inquire what wonderful
changes in the aspect of things have been comprised within the
experience of a single lifetime, and let him ask what has brought about
these changes. He will be told of the railway, and the steam-ship, and
the telegraph, and the great guns, and the mighty ships of war—
“The armaments which thunderstrike the walls
Of rock-built cities, bidding nations quake,
And monarchs tremble in their capitals.”
He will be told of a machine more potent in shaping the destinies of our
race than warlike engines—the steam printing-press. He may hear of a
chemistry which effects endless and marvellous transformations; which
from dirt and dross extracts fragrant essences and dyes of resplendent
hue. He may hear something of a wonderful instrument which can make a
faint beam of light, reaching us after a journey of a thousand years,
unfold its tale and reveal the secrets of the stars. Of these and of
other inventions and discoveries which distinguish the present age it is
the purpose of this work to give some account.
[Illustration: JAMES WATT]
STEAM ENGINES.
To track the steps which led up to the invention of the Steam Engine,
and fully describe the improvements by which the genius of the
illustrious Watt perfected it at least in principle, are not subjects
falling within the province of this work, which deals only with the
discoveries and inventions of the present century. But as it does enter
into our province to describe some of the more recent developments of
Watt’s invention, it may be desirable to give the reader an idea of his
engine, of which all the more recent applications of steam are
modifications, with improvements of detail rather than of principle.
Watt took up the engine in the condition in which it was left by
Newcomen; and what that was may be seen in Fig. 2, which represents
Newcomen’s atmospheric engine—the first practically useful engine in
which a piston moving in a cylinder was employed. In the cut, the lower
part of the cylinder, _c_, is removed, or supposed to be broken off, in
order that the piston, _h_, and the openings of the pipes, _d_, _e_,
_f_, connected with the cylinder, may be exhibited. The steam was
admitted beneath the piston by the attendant turning the cock _k_, and
as the elastic force of the steam was only equal to the pressure of the
atmosphere, it was not employed to raise the piston, but merely filled
the cylinder, the ascent of the piston being caused by the weight
attached to the other side of the beam, which at the same time sent down
the pump-rod, _m_; and when this was at its lowest position, the piston
was nearly at the top of the cylinder, which was open. The attendant
then cut off the communication with the boiler by closing the cock, _k_,
at the same time opening another cock which allowed a jet of cold water
from the cistern, _g_, to flow through the opening, _d_, into the
cylinder. The steam which filled the cylinder was, by contact with the
cold fluid, instantly condensed into water; and as the liquefied steam
would take up little more than a two-thousandth part of the space it
occupied in the gaseous state, it followed that a vacuum was produced
within the cylinder; and the weight of the atmosphere acting on the top
of the piston, having no longer the elastic force of the steam to
counteract it, forced the piston down, and thus raised the pump-bucket
attached to the rod, _m_. The water which entered the cylinder from the
cistern, together with that produced by the condensation of the steam,
flowed out of the cylinder by the opening, _f_, the pipe from which was
conducted downwards, and terminated under water, the surface of which
was at least 34 ft. below the level of the cylinder; for the atmospheric
pressure would cause the cylinder to be filled with water had the height
been less. The improvements which Watt, reasoning from scientific
principles, was enabled to effect on the rude engine of Newcomen, are
well expressed by himself in the specification of his patent of 1769. It
will be observed that the machine was formerly called the “fire engine.”
[Illustration:
FIG. 2.—_Newcomen’s Steam Engine._
]
[Illustration:
FIG. 3.—_Watt’s Double-action Steam Engine._
]
“My method of lessening the consumption of steam, and consequently
fuel, in fire engines, consists of the following principles:—_First._
That vessel in which the powers of steam are to be employed to work
the engine (which is called the cylinder in common fire engines, and
which I call the steam-vessel), must, during the whole time the engine
is at work, be kept as hot as the steam that enters it; first, by
enclosing it in a case of wood, or any other materials that transmit
heat slowly; secondly, by surrounding it with steam or other heated
bodies; and thirdly, by suffering neither water nor any other
substance colder than the steam to enter or touch it during that
time.—_Secondly._ In engines that are to be worked either wholly or
partially by condensation of steam, the steam is to be condensed in
vessels distinct from the steam-vessels or cylinders, although
occasionally communicating with them,—these vessels I call condensers;
and whilst the engines are working, these condensers ought to be kept
at least as cold as the air in the neighbourhood of the engines by the
application of water or other cold bodies.—_Thirdly._ Whatever air or
other elastic vapour is not condensed by the cold of the condenser,
and may impede the working of the engine, is to be drawn out of the
steam-vessels or condensers by means of pumps, wrought by the engines
themselves or otherwise.—_Fourthly._ I intend in many cases to employ
the expansive force of steam to press on the pistons, or whatever may
be used instead of them, in the same manner in which the pressure of
the atmosphere is now employed in common fire engines. In cases where
cold water cannot be had in plenty, the engines may be wrought by this
force of steam only, by discharging the steam into the air after it
has done its office.—_Lastly._ Instead of using water to render the
pistons and other parts of the engines air- and steam-tight, I employ
oils, wax, resinous bodies, fat of animals, quicksilver, and other
metals in their fluid state.”
[Illustration:
FIG. 4.—_Governor and Throttle-Valve._
]
From the engraving we give of Watt’s double-action steam engine, Fig. 3,
and the following description, the reader will realize the high degree
of perfection to which the steam engine was brought by Watt. The steam
is conveyed to the cylinder through a pipe, B, the supply being
regulated by the throttle-valve, acted on by rods connected with the
governor, D, which has a rotary motion. This apparatus is designed to
regulate the admission of steam in such a manner that the speed of the
engine shall be nearly uniform; and the mode in which this is
accomplished may be seen in Fig. 4, where D D is a vertical axis
carrying the pulley, _d_, which receives a rotary motion from the
driving-shaft of the engine, by a band not shown in the figures. Near
the top of the axis, at _e_, two bent rods work on a pin, crossing each
other in the same manner as the blades of a pair of scissors. The two
heavy balls are attached to the lower arms of these levers, which move
in slits through the curved guides intended to keep them always in the
same vertical plane as the axis, D D. The upper arms are jointed at _f
f_ to rods hinged at _h h_ to a ring not attached to the axis, but
allowing it to revolve freely within it. To this ring at F is fastened
one end of the lever connected with the throttle-valve in a manner
sufficiently obvious from the cut. The position represented is that
assumed by the apparatus when the engine is in motion, the disc-valve,
_z_, being partly open. If from any cause the velocity of the engine
increases, the balls diverge from increased centrifugal force, and the
effect is to draw down the ring at F, and, through the system of levers,
to turn the disc in the direction of the arrows, and diminish the supply
of steam. If, on the other hand, the speed of the engine is checked, the
balls fall towards the axis, and the valve is opened wider, admitting
steam more freely, and so restoring its former speed to the engine. On
one side of the cylinder are two hollow boxes, E E, Fig. 3,
communicating with the cylinder by an opening near the middle of the
box. Each of these steam-chests is divided into three compartments by
conical valves attached to rods connected with the lever, H. These
valves are so arranged that when the upper part of the cylinder is in
communication with the boiler, the lower part is open to the condenser,
I, and _vice versâ_. The top of the cylinder is covered, and the
piston-rod passes through an air and steam-tight hole in it; freedom of
motion, with the necessary close fitting, being attained by making the
piston-rod pass through a _stuffing-box_, where it is closely surrounded
with greased tow. The piston is also packed, so that, while it can slide
freely up and down in the cylinder, it divides the latter into two
steam-tight chambers. In an engine of this kind, the elastic force of
the steam acts alternately on the upper and lower surfaces of the
piston; and the condenser, by removing the steam which has performed its
office, leaves a nearly empty space before the piston, in which it
advances with little or no resistance. On the rod which works the
air-pump, two pins are placed, so as to move the lever, H, up and down
through a certain space, when one pin is near its highest and the other
near its lowest position, and thus the valves are opened and closed when
the piston reaches the termination of its stroke. In the condenser, I, a
stream of cold water is constantly playing, the flow being regulated by
the handle, _f_. The steam, in condensing, heats the cold water, adding
to its bulk, and at the same time the air, which is always contained in
water, is disengaged, owing to the heat and the reduced pressure. Hence
it is necessary to pump out both the air and the water by the pump, J,
which is worked by the beam of the engine. In his engines Watt adopted
the heavy fly-wheel, which tends to equalize the movement, and render
insensible the effects of those variations in the driving power and in
the resistance which always occur. In the action of the engine itself
there are two positions of the piston, namely, where it is changing its
direction, in which there is no force whatever communicated to the
piston-rod by the steam. These positions are known as the “dead points,”
and in a rotatory engine occur twice in each revolution. The resistance
also is liable to great variations. Suppose, for example, that the
engine is employed to move the shears by which thick plates of iron are
cut. When a plate has been cut, the resistance is removed, and the speed
of the engine increases; but this increase, instead of taking place by a
sudden start, takes place gradually, the power of the engine being in
the meantime absorbed in imparting increased velocity to the fly-wheel.
When another plate is put between the shears, the power which the
fly-wheel has gathered up is given out in the slight diminution of its
speed occasioned by the increased resistance. But for the fly-wheel,
such changes of velocity would take place with great suddenness, and the
shocks and strains thereby caused would soon injure the machine. This
expedient, in conjunction with that admirable contrivance, the
“governor,” renders it possible to set the same engine at one moment to
forge an anchor, and at the next to shape a needle. One of the most
ingenious of Watt’s improvements is what is termed the “parallel
motion,” consisting of a system of jointed rods connecting the head of
the piston-rod, R, with the end of the oscillating beam. As, during the
motion of the engine, the former moves in a straight line, while the
latter describes a circle, it would be impossible to connect them
directly. Watt accomplished this by hinging rods together in form of a
parallelogram, in such a manner that, while three of the angles describe
circles, the fourth moves in nearly a straight line. Watt was himself
surprised at the regularity of the action. “When I saw it work for the
first time, I felt truly all the pleasure of novelty, _as if I was
examining the invention of another man_.”
[Illustration:
A B is half the beam, A being the main centre; B E, the main links,
connecting the piston-rod, F, with the end of the beam; G D, the
air-pump links, from the centre of which the air-pump-rod is
suspended; C D moves about the fixed centre, C, while D E is movable
about the centre D, itself moving in an arc, of which C is the
centre. The dotted lines show the position of the links and bars
when the beam is at its highest position.
FIG. 4_a_.—_Watt’s Parallel Motion._
]
Many improvements in the details and fittings of almost every part of
the steam engine have been effected since Watt’s time. For example, the
opening and closing of the passages for the steam to enter and leave the
cylinder is commonly effected by means of the slide-valve (Fig. 5). The
steam first enters a box, in which are three holes placed one above the
other in the face of the box opposite to the pipe by which the steam
enters. The uppermost hole is in communication with the upper part of
the cylinder, and the lowest with the lower part. The middle opening
leads to the condenser, or to the pipe by which the steam escapes into
the air. A piece of metal, which may be compared to a box without a lid,
slides over the three holes with its open side towards them, and its
size is such that it can put the middle opening in communication with
either the uppermost or the lowest opening, at the same time giving free
passage for the steam into the cylinder by leaving the third opening
uncovered. In A, Fig. 5, the valve is admitting steam below the piston,
which is moving upwards, the steam which had before propelled it
downwards now having free exit. When the piston has arrived at the top
of the cylinder, the slide is pushed down by the rod connecting it with
the eccentric into the position represented at B, and then the opposite
movement takes place. The slide-valve is not moved, like the old pot-lid
valves, against the pressure of the steam, and has other advantages,
amongst which may be named the readiness with which a slight
modification renders it available for using the steam “_expansively_.”
This expansive working was one of Watt’s inventions, but has been more
largely applied in recent times. In this plan, when the piston has
performed a part of its stroke, the steam is shut off, and the piston is
then urged on by the expansive force of the steam enclosed in the
cylinder. Of course as the steam expands its pressure decreases; but as
the same quantity of steam performs a much larger amount of work when
used expansively, this plan of cutting off the steam is attended with
great economy. It is usually effected by the modification of the
slide-valve, shown at C, Fig. 5, where the faces of the slides are made
of much greater width than the openings. This excess of width is called
the “_lap_,” and by properly adjusting it, the opening into the cylinder
may be kept closed during the interval required, so that the steam is
not allowed to enter the cylinder after a certain length of the stroke
has been performed. The slide-valve is moved by an arrangement termed
the eccentric. A circular disc of metal is carried on the shaft of the
engine, and revolves with it. The axis of the shaft does not, however,
run through the centre of the disc, but towards one side. The disc is
surrounded by a ring, to which it is not attached, but is capable of
turning round within it. The ring forms part of a triangular frame to
which is attached one arm of a lever that communicates the motion to the
rod bearing the slide. Expansive working is often employed in
conjunction with _superheated steam_, that is, steam heated out of
contact with water, after it has been formed, so as to raise its
temperature beyond that merely necessary to maintain it in the state of
steam, and to confer upon it the properties of a perfect gas. Experience
has proved that an increased efficiency is thus obtained.
[Illustration:
FIG. 5.—_Slide Valve._
]
The actual power of a steam engine is ascertained by an instrument
called the Indicator, which registers the amount of pressure exerted by
the steam on the face of the piston in every part of its motion. The
indicator consists simply of a very small cylinder, in which works a
piston, very accurately made, so as to move up and down with very little
friction. The piston is attached to a strong spiral spring, so that when
the steam is admitted into the cylinder of the indicator the spring is
compressed, and its elasticity resists the pressure of the steam, which
tends to force the piston up. When the pressure of steam below the
piston of the indicator is equal to that of the atmosphere, the spring
is neither compressed nor extended; but when the steam-pressure falls
below that of the atmosphere, as it does while the steam is being
condensed, then the atmospheric pressure forces down the piston of the
indicator until it is balanced by the tension of the now stretched
spring. The extension or compression of the spring thus measures the
difference between the pressure of the atmosphere and that of the steam
in the cylinder of the engine, with which the cylinder of the indicator
freely communicates.
From the piston-rod of the indicator a pencil projects horizontally, and
its point presses against a sheet of paper wound on a drum, which moves
about a vertical axis. This drum is made to move backwards and forwards
through a part of a revolution, so that its motion may exactly
correspond with that of the piston in the cylinder of the steam engine.
Thus, if the piston of the indicator were to remain stationary, a level
line would be traced on the paper by the movement of the drum; and if
the latter did not move, but the steam were admitted to the indicator,
the pencil would mark an upright straight line on the paper. The actual
result is that a figure bounded by curved lines is traced on the paper,
and the curve accurately represents the pressure of the steam at every
point of the piston’s motion. The position of the point of the pencil
which corresponds with each pound of pressure per square inch is found
by trial by the maker of the instrument, who attaches a scale to show
what pressures of steam are indicated.
If the pressure per square inch is known, it is plain that by
multiplying that pressure by the number of square inches in the area of
the piston of the engine, the total pressure on the piston can be found.
The pressure does not rise instantly when the steam is first admitted,
nor does it fall quite abruptly when the steam is cut off and
communication opened with the condenser. When the steam is worked
expansively, the pressure falls gradually from the time the steam is
shut off. Now, the amount of work done by any force is reckoned by the
pressure it exerts multiplied into the space through which that pressure
is exerted. Therefore the work done by the steam is known by multiplying
the pressure in pounds on the whole surface of the piston into the
length in feet of the piston’s motion through which that pressure is
exerted. The trace of the pencil on the paper—_i.e._, the _indicator
diagram_—shows the pressures, and also the length of the piston’s path
through which each pressure is exerted, and therefore it is not
difficult to calculate the actual work which is done by the steam at
every stroke of the engine. If this be multiplied by the number of
strokes per minute, and the product divided by 33,000, we obtain what is
termed the _indicated horse-power_ of the engine. The work done per
minute is divided by 33,000, because that number is taken to represent
the work that a horse can do in a minute: that is, the average work done
in one minute by a horse would be equal to the raising of the weight of
1,000 lbs. thirty-three feet high, or the raising of thirty-three pounds
1,000 feet high. The number, 33,000, as expressing the work that could
be done by a horse in one minute, was fixed on by Watt, but more recent
experiments have shown that he over-estimated the power of horses, and
that we should have to reduce this number by about one-third if we
desire to express the actual average working power of a horse. But the
power of engines having come to be expressed by stating the horse-power
on Watt’s standard, engineers have kept to the original number, which
is, however, to be considered as a merely artificial unit or term of
comparison between one engine and another; for the power of a horse to
perform work will vary with the mode in which its strength is exerted.
The source of the power which does the work in the steam engine is the
combustion of the coal in the furnace under the boiler. The amount of
work a steam engine will do depends not only on the quantity of steam
which is generated in a given time, but also upon the pressure, and
therefore the temperature at which the steam is formed.
[Illustration:
FIG. 6.—_Section of Giffard’s Injector._
]
The water constantly evaporating in the boiler of a steam engine is
usually renewed by forcing water into the boiler against the pressure of
the steam by means of a small pump worked by the engine. In the
engraving of Watt’s engine this pump is shown at M. But recently the
feed-pump has been to a great extent superseded by a singular apparatus
invented by M. Giffard, and known as Giffard’s Injector. In this a jet
of steam from the boiler itself supplies the means of propelling a
stream of water directly into the boiler. Fig. 6 is a section of this
interesting apparatus through its centre, and it clearly shows the
manner in which the current of steam is made to operate on the jet of
water. The steam from the boiler passes through the pipe A and into the
tube B through the holes. The nozzle of this tube is of a conical shape,
and its centre is occupied by a rod pointed to fit into the conical
nozzle, and provided with a screw at the other end, so that the opening
can be regulated by turning the handle, C. At D the jet of steam comes
in contact with the water which feeds the boiler, the arrangement being
such that the steam is driven into the centre of the stream of water
which enters by the pipe E, and is propelled by the steam jet through
another cone, F, issuing with such force from the orifice of the latter
that it is carried forward through the small opening at G into the
chamber H. Here the water presses on the valve K, which it raises
against the pressure of the steam and enters the boiler. The water
issuing from the cone, F, actually traverses an open space which is
exposed to the air, and where the fluid may be seen rushing into the
boiler as a clear jet, except a few beads of steam which may be carried
forward in the centre, the rest of the steam having been condensed by
the cold water. The steam, of course, rushes from the cone, B D, with
enormous velocity, which is partly communicated to the water. The pipe,
L, is for the water which overflows in starting the apparatus, until the
pressure in H becomes great enough to open the valve. The supplies of
water and of steam have to be adjusted according to the conditions of
pressure in the boiler, and according to the temperature of the
feed-water. It is found that when the feed-water is at a temperature
above 120° Fahrenheit, the injector will not work: the condensation of
the steam is therefore necessary to the result. For, as the steam is
continually condensed by the cold water, it rushes from D with the same
velocity as into a vacuum, and the water is urged on by a momentum due
to this velocity. We must observe, moreover, that the net result of the
operation is a lessening of the pressure in the boiler; for the entrance
of the feed-water produces a fall of temperature in the boiler, and the
bulk of steam expended is fourteen times the bulk of the water injected:
thus, although the apparatus before actual trial would not appear likely
to produce the required result, the effect is no more paradoxical than
in the case of the feed-pump. The injector has been greatly improved by
Mr. Gresham, who has contrived to make some of the adjustments
self-acting, and his form of the apparatus is now largely used in this
country. The injector is applicable to stationary, locomotive, or marine
engines.
Steam boilers are now always provided with one of _Bourdon’s_ gauges,
for indicating the pressure of the steam. The construction of the
instrument will easily be understood by an examination of Fig. 7. The
gauge is screwed into some part of the boiler, where it can always be
seen by the person in charge. The stop-cock A communicates with the
curved metallic tube C, which is the essential part of the contrivance.
This tube is of the flattened form shown at D, having its greatest
breadth perpendicular to the plane in which the tube is curved, and it
is closed at the end E, where it is attached to the rod F, so that any
movement of E causes the axle carrying the index-finger, F, to turn, and
the index then moves along the graduated arc. The connection is
sometimes made by wheelwork, instead of by the simple plan shown in the
figure. The front plate is represented as partly broken away, in order
to show the internal arrangement, which, of course, is not visible in
the real instrument, where only the index-finger and graduated scale are
seen, protected by a glass plate.
[Illustration:
FIG. 7.—_Bourdon’s Pressure Gauge._
]
When a curved tube of the shape here described is subjected to a greater
pressure on the inside than on the outside, it tends to become
straighter, and the end E moves outward; but when the pressure is
removed, the tube resumes its former shape. The graduations on the scale
are made by marking the position of the index when known pressures are
applied. The amounts of pressure, when the gauges are being graduated,
are known by the compression produced in air contained in another
apparatus. Gauges constructed on Bourdon’s principle are applied to
other purposes, and can be made strong enough to measure very great
pressures, such as several thousand pounds on the square inch; they may
also be made so delicate as to measure variations of pressure below that
of the atmosphere. The simplicity and small size of these gauges, and
the readiness with which they can be attached, render them most
convenient instruments wherever the pressure of a gas or liquid is
required to be known.
[Illustration:
FIG. 8.—_Steam Generator._
]
A point to which great attention has been directed of late years is the
construction of a boiler which shall secure the greatest possible
economy in fuel. Of the total heat which the fuel placed in the furnace
is capable of supplying by its combustion, part may be wasted by an
incomplete burning of the fuel, producing cinders or smoke or unburnt
gases, another part is always lost by radiation and conduction, and a
third portion is carried off by the hot gases that escape from the
boiler-flues. Many contrivances have been adopted to diminish as much as
possible this waste of heat, and so obtain the greatest possible
proportion of available steam power from a given weight of fuel. Boilers
wholly or partially formed of tubes have recently been much in favour.
An arrangement for quickly generating and superheating steam is shown in
Fig. 8, in connection with a high-pressure engine.
Steam engines are constructed in a great variety of forms, adapted to
the purposes for which they are intended. Distinctions are made
according as the engine is fitted with a condenser or not. When steam of
a low pressure is employed, the engine always has a condenser, and as in
this way a larger quantity of work is obtainable for a given weight of
fuel, all marine engines—and all stationary engines, where there is an
abundant supply of water and the size is not objectionable—are provided
with condensers. High-pressure steam may be used with condensing
engines, but is generally employed in non-condensing engines only, as in
locomotives and agricultural engines, the steam being allowed to escape
into the air when it has driven the piston to the end of the stroke. In
such engines the beam is commonly dispensed with, the head of the
piston-rod moving between guides and driving the crank directly by means
of a connecting-rod. The axis of the cylinder may be either vertical,
horizontal, or inclined. A plan often adopted in marine engines, by
which space is saved, consists in jointing the piston-rod directly to
the crank, and suspending the cylinder on trunnions near the middle of
its length. The trunnions are hollow, and are connected by steam-tight
joints, one with the steam-pipe from the boiler, and the other with the
eduction-pipe. Such engines have fewer parts than any others; they are
lighter for the same strength, and are easily repaired. The trunnion
joints are easily packed, so that no leakage takes place, and yet there
is so little friction that a man can with one hand move a very large
cylinder, whereas in another form of marine engine, known as the
side-lever engine, constructed with oscillating beams, the friction is
often very great.
_THE LOCOMOTIVE._
The first locomotive came into practical use in 1804. Twenty years
before, Watt had patented—but had not constructed—a locomotive engine,
the application of steam to drive carriages having first been suggested
by Robinson in 1759. The first locomotives were very imperfect, and
could draw loads only by means of toothed driving-wheels, which engaged
teeth in rack-work rails. The teeth were very liable to break off, and
the rails to be torn up by the pull of the engine. In 1813, the
important discovery was made that such aids are unnecessary, for it was
found that the “bite” of a smooth wheel upon a smooth rail was
sufficient for all ordinary purposes of traction. But for this
discovery, the locomotive might never have emerged from the humble duty
of slowly dragging coal-laden waggons along the tramways of obscure
collieries. The progress of the locomotive in the path of improvement
was, however, slow, until about 1825, when George Stephenson applied the
blast-pipe, and a few years later adopted the tubular boiler. These are
the capital improvements which, at the famous trial of locomotives, on
the 6th of October, 1829, enabled Stephenson’s “Rocket” to win the prize
offered by the directors of the Liverpool and Manchester Railway. The
“Rocket” weighed 4½ tons, and at the trial drew a load of tenders and
carriages weighing 12¾ tons. Its average speed was 14 miles an hour, and
its greatest, 29 miles an hour. This engine, the parent of the powerful
locomotives of the present day, may now be seen in the Patent Museum at
South Kensington. Since 1829, numberless variations and improvements
have been made in the details of the locomotive. In weight, dimensions,
tractive power and speed, the later locomotives vastly surpass the
earlier types.
[Illustration:
FIG. 9.—_Section of Locomotive_ (A.D. 1837).
]
Fig. 9 represents the section of a locomotive constructed _c._ 1837. The
boiler is cylindrical; and at one end is placed the fire-box, partly
enclosed in the cylindrical boiler, and surrounded on all sides by the
water, except where the furnace door is placed, and at the bottom, where
the fuel is heaped up on bars which permit the cinders to drop out. At
the other end of the boiler, a space beneath the chimney called the
smoke-box is connected with the fire-box by a great number of brass
pipes, open at both ends, firmly fixed in the end plates of the boiler.
These tubes are from 1¼ in. to 2 in. in diameter, and are very
numerous—usually about one hundred and eighty, but sometimes nearly
double that number. They therefore present a large heating surface to
the water, which stands at a level high enough to cover them all and the
top of the fire-box. The boiler of the locomotive is not exposed to the
air, which would, if allowed to come in contact with it, carry off a
large amount of heat. The outer surface is therefore protected from this
cooling effect by covering it with a substance which does not permit the
heat to readily pass through it. Nothing is found to answer better than
felt; and the boiler is accordingly covered with a thick layer of this
substance, over which is placed a layer of strips of wood ¾ in. thick,
and the whole is surrounded with thin sheet iron. It is this sheet iron
alone that is visible on the outside. The level of the water in the
boiler is indicated by a gauge, which is merely a very strong glass
tube; and the water carried in the tender is forced in as required, by a
pump (not shown in the Fig.). The steam leaves the boiler from the upper
part of the _steam-dome_, A, where it enters the pipe, B; the object
being to prevent water from passing over with the steam into the pipe.
The steam passes through the _regulator_, C, which can be closed or
opened to any extent required by the handle, D, and rushes along the
pipe, E, which is wholly within the boiler, but divides into two
branches when it reaches the smoke-box, in order to conduct the steam to
the cylinders. Of these there are two, one on each side, each having a
slide-valve, by means of which the steam is admitted before and behind
the pistons alternately, and escapes through the blast-pipe, F, up the
chimney, G, increasing the draught of the fire by drawing the flame
through the longitudinal tubes in proportion to the rush of steam; and
thus the rate of consumption of fuel adjusts itself to the work the
engine is performing, even when the loads and speeds are very different.
Though the plane of section passing through the centre of boiler would
not cut the cylinders, one of them is shown in section. H is the piston;
K the connecting-rod jointed to the crank, L, the latter being formed by
forging the axle with four rectangular angles, thus, __¦¯¯¦__; and the
crank bendings for the two cylinders are placed in planes at right
angles to each other, so that when one is at the “dead point,” the other
is in a position to receive the full power of the piston. There are two
safety valves, one at M, the other at N; the latter being shut up so
that it cannot be tampered with.
Locomotives are fitted with an ingenious apparatus for reversing the
engines, which was first adopted by the younger Stephenson, and is known
as the “link motion.” The same arrangement is employed in other engines
in which the direction of rotation has to be changed; and it serves
another important purpose, namely, to provide a means by which steam may
be employed expansively at pleasure. The link motion is represented in
Fig. 10, where A, B, are two eccentrics oppositely placed on the
driving-shaft, and their rods joined to the ends of the curved bar or
link, C D. A slit extends nearly the whole length of this bar, and in it
works the stud E, forming part of the lever, F, G, movable about the
fixed joint, G, and having its extremity, F, jointed to the rod H, that
moves the slide-valve. The weight of the link and the eccentric rods is
counterpoised with a weight, K, attached to the lever, I K, which turns
on the fixed centre, L. This lever forms one piece with another lever, L
M, with which it may be turned by pulling the handle of O P, connected
with it through the system of jointed rods. When the link is lowered, as
shown in the figure, the slide-valve rod will follow the movement of the
eccentric, B, while the backward and forward movement of the other
eccentric will only be communicated to the end of C, and will scarcely
affect the position of the stud E at all. By drawing the link up to its
highest position, the motion due to eccentric A only will be
communicated to the slide-valve rod, which will therefore be drawn back
at the part of the revolution where before it was pushed forward, and
_vice versâ_; hence the engine will be reversed. When the link is so
placed that the stud is exactly in the centre, the slide-valve will
receive no motion, and remain in its middle position, consequently the
engine is stopped. By keeping the link nearer or farther from its
central position, the throw of the slide-valve will be shorter or
longer, and the steam will be shut off from entering the cylinder when a
smaller or larger portion of the stroke has been performed.
[Illustration:
FIG. 10.—_Stephenson’s Link Motion._
]
Although Fig. 9 represents with sufficient clearness all the essential
parts of a locomotive, it should be observed that as actually
constructed for use on the different lines of railway the machine is
greatly modified in the arrangement and proportions of its parts. A
greater number of adjuncts and subsidiary appliances are also provided
for the more effective and convenient working of the engine, and for
giving control over the movement of the train, and these, in fact,
conduce much to the greater economy and safety with which trains are now
run. As the circumstances and conditions under which railways are worked
vary much in different parts of the world, the locomotive has to be
designed to meet the requirements of each case, and its general
appearance, details and dimensions are accordingly much diversified.
From among the many types of recent locomotives we select for
illustration and a short description the form of express passenger
engine that has lately been designed by Mr. T. W. Worsdell, the engineer
of the North Eastern Railway, and this will give the opportunity of
noticing some of the newest improvements, which are embodied in this
engine. See Plate II.
The plan of causing the steam to work expansively has already been
mentioned on pages 8 and 9, as used by cutting off the steam when part
of the stroke of the piston has been made. Another mode by which the
expansive principle has long been made use of in stationary and marine
engines is to allow the steam from the boiler to enter first a smaller
cylinder and from that, at the end of the stroke, to pass into a larger
one in which, as it expands, it exercises a diminished pressure. This
arrangement has been called the compound or double-cylinder engine, and
was known to possess certain advantages where high pressure steam was
made use of. Indeed, in marine engines the principle of “triple
expansion” is now quite commonly adopted—that is, the steam passes
successively into three cylinders of successively greater diameter. Mr.
Webb, the locomotive engineer of the London and North Western Railway,
appears to have been the first to make the “compounding” system a
practical success as applied to the locomotive. In Mr. Webb’s
arrangement there are three cylinders, two smaller ones for the
high-pressure steam from the boiler, and between these a single large
low-pressure cylinder which receives the steam that has done its work
from both the smaller cylinders. In Mr. Worsdell’s engine the original
and simpler locomotive construction of two cylinders has been adhered
to, and thus the general plan of the engine is unchanged except in the
larger size of the low-pressure cylinder. In the present engine the
stroke is 24 in.; the high-pressure cylinder has its internal diameter
20 in. and the low-pressure cylinder a diameter of 28 in. The
boiler-shell is made of steel, the fire-box is of copper, and there are
203 brass tubes, 1¾ in. diameter and 10 ft. 11 in. long, connecting the
fire-box with the smoke-box. The frame, and indeed most parts of the
engine, are also made of steel. The driving-wheels, which here are a
single pair, have a diameter of 7 ft. 7¼ in. The total “wheel-base” is
nearly 21 ft., and it will be observed that the forepart of the engine
is supported on a four-wheeled _bogie_. The _bogie_ is capable of a
certain amount of horizontal motion by turning round a swivel, but this
movement is controlled by springs, so that, notwithstanding the length
of the frame, the engine is enabled to take curves with great facility,
while its motion is perfectly steady even at the highest speeds. The
working pressure of the steam in the boiler is 170 lbs. on the square
inch. The steam which leaves the high-pressure cylinder is conveyed to
the low-pressure cylinder by a pipe that is led round the inside of the
smoke-box, and thus enters the larger cylinder after taking up heat that
would otherwise be wasted, so that its elastic force is fully
maintained. This circumstance, no doubt, contributes to the very marked
economy of fuel that has been effected by the compound engines. How
great the economy is found in the working will be seen by the following
results, which are taken from the actual records. The same train was
taken over the same rails in ordinary quick passenger traffic for
several journeys which, as performed in the same time by the compound
engine and by another otherwise similar non-compound engine, required
for the compound, 25,254 lbs. of coal; for the non-compound, 32,104
lbs.; or, the consumption of coal by the former was 28 lbs. per mile; by
the latter, 36 lbs. per mile. This represents a saving of about 21 per
cent. of the fuel. As the steam enters the high-pressure cylinder first,
it would not be possible to start the engine if it had stopped at one of
the “dead-points” on that side, without a special arrangement for
admitting the steam directly to the other cylinder in such cases. This,
of course, is required only for the first stroke, and Mr. Worsdell and
M. von Borries have contrived for this purpose an ingenious valve,
brought into operation when required by a touch from the engineer, and
then immediately adjusting itself automatically, so as to restore the
steam connections to their normal condition.
[Illustration:
PLATE II.
NORTH EASTERN RAILWAY LOCOMOTIVE.
]
[Illustration:
FIG. 10_a_.—_G.N.R. Express Passenger Locomotive._
]
Another type of the high-speed passenger engines used for express trains
on several of the great English railways is well represented by one of
the Great Northern Company’s locomotives, as depicted in Fig. 10_a_. In
this there are a single pair of driving wheels of very large diameter,
namely, 8 ft. 2 in., so that each complete movement of the pistons will
carry the engine forwards a length of nearly 26 ft. There are outside
cylinders, and therefore the driving axle is straight, and the leading
wheels are in two pairs, mounted on a _bogie_ which is capable of a
certain amount of independent horizontal rotation.
The Stephenson’s link motion, described on page 17, has lately been
often supplanted by another arrangement known as Joy’s valve gear, which
leaves the crank axle unencumbered with eccentrics, and, as taking up
less space, is generally now preferred for locomotives and also for
marine engines. Its principle is very simple, and will be readily
understood from the diagram in Fig. 10_b_, where _c_ is the spindle of
the slide-valves as in Fig. 5, but capable, we shall now suppose, of a
horizontal movement only. Jointed to it at D is a rod D E attached to a
block at E, which can move only within a slot in the strong bar E F in a
circular segment, the centre of which is at D. The bar we suppose for
the moment to be immovable, and disposed symmetrically to C D. Now let
an alternate up and down motion along the circular segment be given to
block E, and the effect will be to leave the centre, D, unchanged in
position, and, therefore, in that case the valve will not be moved at
all. Now this reciprocating movement is given to the block E by a system
of levers (not here shown), jointed to the connecting-rod (K, Fig. 9) in
such a manner that the rod D E is compelled to follow the movement of
the connecting-rod, but the end E must always travel in the circular
segment. We have hitherto supposed this segmental piece to be fixed, but
the engineer has the power of so turning it as to tilt either the upper
or lower part towards D. If, for instance, the guiding segment is fixed
as at II, the block in rising will push in the valve-spindle, and in
descending draw it out, as the length of the rod D E is invariable. But
if the guides be turned over so as to bring F nearer D than E, the same
movement of the block will give the reverse motions to the
valve-spindle.
[Illustration:
FIG. 10_b_.—_Joy’s Valve Gear._
]
From the great rapidity with which the machinery of the locomotive
moves, the different parts require to be carefully balanced in order to
prevent dangerous oscillations. For example, the centrifugal force of
the massive cranks, etc., is balanced by inserting between the spokes of
the driving wheels certain counterpoises, the weights and positions of
which are finally adjusted by trial. The engine is suspended by chains
and set in motion, and a pencil attached to one corner of the frame
marks on a horizontal card the form of the oscillation, usually by an
oval figure. When the diameter of this figure is reduced to about 1/16
inch, the adjustment is considered complete.
The power of a locomotive, of course, depends on the pressure of the
steam and the size of the cylinder, &c.; but a very much lower limit
than is imposed by these conditions is set to the power of the engine to
draw loads by the adhesion between the driving wheels and the rails. By
the term “adhesion,” which is commonly used in this case, nothing more
is really meant than the friction between surfaces of iron. When the
resistance of the load drawn is greater than this friction, the wheels
turn round and slip on the rails without advancing. The adhesion depends
upon the pressure between the surfaces, and upon their condition. It is
greater in proportion as the weight supported by the driving-wheels is
greater, and when the rails are clean and dry it is equal to from 15 to
20 per cent. of that part of the weight of the engine which rests on the
driving-wheels; but when the rails are moist, or, as it is called,
“greasy,” the tractive power may be only 5 per cent. of the weight;
about one-tenth may be taken as an average. Suppose that 30 tons of the
weight of a locomotive are supported by the driving-wheels, that
locomotive could not be employed to drag a train of which the resistance
would cause a greater pull upon the coupling-links of the tender than
they would be subject to if they were used to suspend a weight of 3
tons. The number of pairs of wheels in a locomotive varies from two to
five; most commonly there are three pairs; and one, two, or all, are
driven by the engine, the wheels being coupled accordingly; very often
two pairs are coupled.
The pressure at which the steam is used in the locomotive is sometimes
very considerable. A pressure equal to 180 lbs. on each square inch of
the surface of the boiler is quite usual. The greater economy obtained
by the employment of high-pressure steam acting expansively in the
cylinder, points to the probability of much higher pressures being
adopted. There is practically no limit but the power of the materials to
resist enormous strains, and there is no reason, in the nature of
things, why steam of even 500 lbs. per square inch should not be
employed, if it were found otherwise desirable. It need hardly be said
that locomotives are invariably constructed of the very best materials,
and with workmanship of the most perfect kind. The boilers are always
tested, by hydraulic pressure, to several times the amount of the
highest pressure the steam is required to have, and great care is
bestowed upon the construction of the safety-valves, so that the steam
may blow off when the due amount of pressure is exceeded. The explosion
of a locomotive is, considering the number of engines in constant use, a
very rare occurrence, and is probably in all cases owing to the sudden
generation of a large quantity of steam, and not to an excessive
pressure produced gradually. Among the causes capable of producing
explosive generation of steam may be mentioned the deposition of a hard
crust of stony matter, derived from the water; this crust allows the
boiler to be over-heated, and if water should then find its way into
contact with the heated metal, a large quantity of steam will be
abruptly generated. Or should the water in the boiler become too low,
parts of the boiler may become so heated that on the admission of fresh
water it would be suddenly converted into steam. When an explosion does
take place, the enormous force of the agent we are dealing with when we
bottle up steam in an iron vessel, is shown by the effects produced.
Fig. 11 is from a photograph taken from an exploded locomotive, where we
may see how the thick plates of iron have been torn like paper, and the
tubes, rods, and levers of the engine twisted in inextricable confusion.
[Illustration:
FIG. 11.—_Locomotive after Explosion._
]
[Illustration:
FIG. 12.—_Hancock’s Steam Omnibus._
]
Locomotive engines for propelling carriages on common roads were
invented many years ago, by Gurney, Anderson, Scott Russell, Hancock,
and others. One designed by Hancock is represented in Fig. 12. Such
engines do not appear to have found much favour, though the idea has
been successfully realized in the traction engines lately introduced.
Probably the application of steam power to the propulsion of vehicles
along common roads fell into neglect on account of the superior
advantages of railways, but the common road locomotive is at present
receiving some attention. In the tramways which are now laid along the
main roads in most large cities we see one-half of the problem solved.
It is not so much mechanical difficulties that stand in the way of this
economical system of locomotion, as the prejudices and interests which
have always to be overcome before the world can profit by new
inventions. The engines can be made noiseless, emitting no visible steam
or smoke, and they are under more perfect control than horses. But
vestries and parochial authorities offer such objections as that horses
would be frightened in the streets, if the engine made a noise; and if
it did not, people would be liable to be run over, and the horses be as
much startled as in the other case. But horses would soon become
accustomed to the sight of a carriage moving without equine aid, however
startling the matter might appear to them at first; and the objection
urged against the noiseless engines might be alleged against wooden
pavements, india-rubber tires, and many other improvements. It is highly
probable that in the course of a few years the general adoption of
steam-propelled vehicles will displace horses, at least upon tramways.
The slowness with which inventions of undeniable utility and of proved
advantage come into general use may be illustrated by the fact of some
great English towns and centres of engineering industry not having made
a single tramway until, in all the populous cities of the United States,
and in almost every European capital, tramways had been in successful
operation for many years. [1890.]
Some time has elapsed since the foregoing paragraph was written for an
earlier edition of this work, and during that period there has been an
advance in both practice and opinion; so that now it has become highly
probable that before the century ends a great change may be witnessed in
our modes of locomotion, even on ordinary roads. Already every town of
importance throughout the United Kingdom has been provided with
excellent tramways, along which, in not a few instances, horseless
vehicles roll smoothly, to the great convenience of the general public,
while not one of the difficulties and dangers to general street traffic
has been experienced that were so confidently predicted by those who
were unable to perceive that an innovation might be an improvement. The
now universally-popular bicycle has been continually receiving
improvements, of which there appears to be no end, and as the machine
and all the contrivances connected with it are so familiar to everyone,
there is no need here to do more than to refer to them, because they
have led the way to great improvements in ordinary carriages.
The steam-propelled vehicle for common roads has just been mentioned as
an invention belonging to the first half of the century, and the reasons
it did not find favour have been alluded to. There exists in the United
Kingdom a law concerning horseless carriages travelling on highways,
which was passed to apply to traction engines, and enacts that other
than horse vehicles are not to go along a road at a greater speed than
four miles an hour, and only two miles an hour through a town, and
moreover they are to be preceded by a man bearing a red flag, etc. But a
bill has been introduced (1895) into the legislature to amend this law,
and permit the British people to use on their common roads such light
self-propelled carriages as are becoming popular in France, as may be
seen from the following account:—
On Tuesday, 11th June, 1895, a race was started from Versailles to
Bordeaux and back, a distance of 727 miles or more for the double
journey. The first prize was the substantial sum of 40,000 francs
(£1,600), to which was attached the condition of the carriage seating
four persons, and other prizes were also to be awarded to various kinds
of automatic vehicles. No fewer than sixty-six vehicles were entered for
competition, and these were variously supplied with motive power from
steam, electricity, or petroleum spirit. The starting place was
Versailles at 12·9 p.m., and at 10·32 on Wednesday morning MM. Panhard &
Levassor’s petroleum carriage arrived at Bordeaux, whence, after a stop
of only four minutes, the return journey was begun, but shortly
afterward an accident caused a delay of one hour, but the carriage made
the whole distance at the average of 14·9 miles per hour. In this and
three other carriages belonging to the same firm, the propeller was the
Daimler motor. Though this carriage was the first to accomplish the trip
it received only the second prize, the condition of seating _four_
persons not having been complied with. The first prize fell to a
four-seated vehicle by Les Fils de Peugeot Frères, a firm who carried
off besides the third and fourth prizes. These carriages were also
driven by so-called petroleum motors. These motors are really gas
engines on the principle to be presently mentioned, but the gas is
produced by the vapourisation of a volatile constituent of petroleum
(benzoline). The Daimler motor is a compact combination of two cylinders
connected with a chamber containing the explosive mixture of gas and
air. The pistons perform their in and out strokes simultaneously, but
their working strokes alternately.
_PORTABLE ENGINES._
The application of steam power to agricultural operations has led to the
construction of engines specially adapted by their simplicity and
portability for the end in view. The movable agricultural engines have,
like the locomotives, a fire-box nearly surrounded by the water, and
horizontal tubes, and are set on wheels, so that they may be drawn by
horses from place to place. There is usually one cylinder placed
horizontally on the top of the boiler; and the piston-rod, working in
guides, is, as in the old locomotive, attached by a connecting-rod to
the crank of a shaft, which carries a fly-wheel, eccentrics, and pulleys
for belts to communicate the motion to the machines. Engines of this
kind are also much used by contractors, for hoisting stones, mixing
mortar, &c. These engines are made with endless diversities of details,
though in most such simplicity of arrangement is secured, that a
labourer of ordinary intelligence may, after a little instruction, be
trusted with the charge of the engine; while their economy of fuel,
efficiency, and cheapness are not exceeded in any other class of steam
engine.
Besides the steam engines already described or alluded to, there are
many interesting forms of the direct application of steam power. There
are, for example, the steam roller and the steam fire-engine. The former
is a kind of heavy locomotive, moving on ponderous rollers, which
support the greater part of the weight of the engine. When this machine
is made to pass slowly over roads newly laid with broken stones, a few
repetitions of the process suffice to crush down the stones and
consolidate the materials, so as at once to form a smooth road. Steam
power is applied to the fire engine, not to propel it through the
streets, but to work the pumps which force up the water. The boilers of
these engines are so arranged that in a few minutes a pressure of steam
can be obtained sufficient to throw an effective jet of water. The cut
at the end of this chapter represents a very efficient engine of this
kind, which will throw a jet 200 feet high, delivering 1,100 gallons of
water per minute. It has two steam cylinders and two pumps, each making
a stroke of two feet. These are placed horizontally, the pumps and the
air reservoir occupying the front part of the engine, while the vertical
boiler is placed behind. The steam cylinders, which are partly hid in
the cut by the iron frame of the engine, are not attached to the boiler,
which by this arrangement is saved from injurious strains produced by
the action of the moving parts of the mechanism. There are seats for
eight firemen, underneath which is a space where the hose is carried. A
first-class steam fire-engine of this kind, completely fitted, costs
upwards of £1,300.
A cheap and very convenient prime mover has lately come into use, which
has certain advantages over even the steam engine. Where a moderate or a
very small power is required, especially where it is used only at
intervals, the _gas engine_ is found to be more convenient. It is small
and compact, no boiler or furnace is required, and it can be started at
any moment. As now made, it works smoothly and without noise. The piston
is impelled, not by the expansive force of steam, but by that of heated
air, the heat being generated by the explosion of a mixture of common
coal gas and air within the cylinder itself. Thus a series of small
explosions has the same effect as the admissions of steam through a
valve. A due quantity of gas and air is introduced into the cylinder,
and is ignited by the momentary opening of a communication with a
lighted gas jet outside. But the machine is provided with a regulator or
governor, which so acts on the valve mechanism that this communication
is made at each stroke only when the speed of rotation falls below a
certain assigned limit, and thus the number of the explosions is less
than the number of strokes, unless its work absorbs the machine’s whole
energy, which, according to the size of the engine, may be from that of
a child up to 30–horse power.
_THE STEAM HAMMER._
Before the invention of the steam hammer, large forge hammers had been
in use actuated by steam, but in an indirect manner, the hammer having
been lifted by cams and other expedients, which rendered the apparatus
cumbersome, costly, and very wasteful of power, on account of the
indirect way in which the original source of the force, namely, the
pressure of the steam, had to reach its point of application by giving
the blow to the hammer. Not only did the necessary mechanism for
communicating the force in this roundabout manner interfere with the
space necessary for the proper handling of the article to be forged, but
the range of the fall of the hammer being only about 18 in., caused a
very rapid decrease in the energy of the blow when only a very
moderate-sized piece of iron was introduced. For example, a piece of
iron 9 in. in diameter reduced the fall of the mass forming the hammer
to one-half, and the work it could accomplish was diminished in like
proportion. Besides, as the hammer was attached to a lever working on a
centre, the striking face of the hammer was parallel to the anvil only
at one particular point of its fall; and again, as the hammer was always
necessarily raised to the same height at each stroke, there was
absolutely no means of controlling the force of the blow. When we
reflect on the fact that the rectilinear motion of the piston in the
cylinder of the engine had first to be converted into a rotary one, by
beams, connecting-rod, crank, &c., and then this rotary movement
transformed into a lifting one by the intervention of wheels, shafts,
cams, &c., while all that is required in the hammer is a straight
up-and-down movement, the wonder is that such an indirect and cumbersome
application of power should have for so many years been contentedly
used. But in November, 1839, Mr. Nasmyth, an eminent engineer of
Manchester, received a letter from a correspondent, informing him of the
difficulty he had found in carrying out an order received for the
forging of a shaft for the paddle-wheels of a steamer, which shaft was
required to be 3 ft. in diameter. There was in all England no forge
hammer capable of executing such a piece of work. This caused Mr.
Nasmyth to reflect on the construction of forge hammers, and in _a few
minutes_ he had formed the conception of the steam hammer. He
immediately sketched the design, and soon afterward the steam hammer was
a _fait accompli_, for Mr. Nasmyth had one at once executed and erected
at his works, where he invited all concerned to come and witness its
performances. Will it be believed that four years elapsed before this
admirable application of steam power found employment outside the walls
of Mr. Nasmyth’s workshops? After a time he succeeded in making those
best able to profit by such an invention aware of the new power—for such
it has practically proved itself, having done more to revolutionize the
manufacture of iron than any other inventions that can be named, except,
perhaps, those of Cort and Bessemer. The usual prejudice attending the
introduction of any new machine, however obvious its advantages are
afterward admitted to be, at length cleared away, and the steam hammer
is from henceforth an absolute necessity in every engineering workshop,
and scarcely less so for some of the early stages of the process of
manufacturing crude wrought iron. Whether blows of enormous energy or
gentle taps are required, or strokes of every gradation and in any
order, the steam hammer is ready to supply them.
[Illustration:
FIG. 13.—_Nasmyth’s Steam Hammer._
]
A steam hammer of the smaller kind is represented in Fig. 13. The
general mode of action will easily be understood. The steam is admitted
below the piston, which is thus raised to any required height within the
limits of the stroke. When the communication with the boiler is shut off
and the steam below the piston is allowed to escape, the piston, with
the mass of iron forming the hammer attached to the piston-rod, falls by
its own weight. This weight, in the large steam hammers, amounts to
several tons; and the force of the blow will depend jointly upon the
weight of the hammer, and upon the height from which it is allowed to
fall. The steam is admitted and allowed to escape by valves, moved by a
lever under the control of a workman. By allowing the hammer to be
raised to a greater or less height, and by regulating the escape of the
steam from beneath the piston, the operator has it in his power to vary
the force of the blow. Men who are accustomed to work the valves can do
this with great nicety. They sometimes exhibit their perfect control
over the machine by cracking a nut on the anvil of a huge hammer; or a
watch having been placed—face upwards—upon the anvil, and a moistened
wafer laid on the glass, a practised operator will bring down the
ponderous mass with such exactitude and delicacy that it will pick up
the wafer, and the watch-glass will not even be cracked. The steam
hammer has recently been improved in several ways, and its power has
been more than doubled, by causing the steam, during the descent, to
enter above the piston and add its pressure to the force of gravity.
Probably one of the most powerful steam hammers ever constructed is that
recently erected at the Royal Gun Factory at Woolwich, for the purpose
of forging great guns for the British Navy. It has been made by Nasmyth
& Co., and is in shape similar to their other steam hammers. Its height
is upwards of 50 ft., and it is surrounded with furnaces and powerful
cranes, carrying the huge iron tongs that are to grasp the glowing
masses. The hammer descends not merely with its own weight of 30 tons;
steam is injected behind the falling piston, which is thus driven down
with vastly enhanced rapidity and impulse. Of the lower portion of this
stupendous forge, nothing is visible but a flat table of iron—the
anvil—level with the floor of the foundry. But more wonderful, perhaps,
than anything seen aboveground, is the extraordinarily solid foundation
beneath. Huge tablets of foot-thick castings alternate with concrete and
enormous baulks of timber, and, lower down, beds of concrete, and piles
driven deep into the solid earth, form a support for the uppermost
plate, upon which the giant delivers his terrible stroke. Less than this
would render it unsafe to work the hammer to its full power. As the
monster works—soberly and obediently though he does it—the solid soil
trembles, and everything movable shivers, far and near, as, with a
scream of the steam, our ‘hammer of Thor’ came thundering down, mashing
the hot iron into shape as easily as if it were crimson dough, squirting
jets of scarlet and yellow yeast. The head of the hammer, which of
course works vertically, is detachable, so that if the monster breaks
his steel fist upon coil or anvil, another can be quickly supplied.
These huge heads alone are as big as a sugar-hogshead, and come down
upon the hot iron with an energy of more than a thousand foot-tons. By
the courteous permission of Major E. Maitland, Superintendent of the
Royal Gun Factories, we are enabled to present our readers with the view
of the monster hammer which forms the Plate III.
Mr. Condie, in his form of steam hammer, utilizes the mass of the
cylinder itself to serve as the hammer. The piston-rod is hollow, and
forms a pipe, through which the steam is admitted and discharged, and
the piston is stationary, the cylinder moving instead—between vertical
guides. A hammer face is attached to the bottom of the cylinder by a
kind of dovetail socket, so that if the striking surface becomes injured
in any way, another can easily be substituted. The massive framework
which supports the moving parts of Condie’s hammer has its supports
placed very far apart, so as to leave ample space for the handling of
large forgings.
[Illustration:
FIG. 14.—_Merryweather’s Steam Fire-Engine._
]
[Illustration:
PLATE III.
THE GREAT STEAM HAMMER, ROYAL GUN FACTORY, WOOLWICH.
]
[Illustration:
FIG. 15.—_A Foundry._
]
IRON.
“Iron and coal,” it has been well said, “are kings of the earth”; and
this is true to such an extent that there is scarcely an invention
claiming the reader’s attention in this book but what involves the
indispensable use of these materials. Again, in their production on the
large scale it will be seen that there is a mutual dependence, and that
this is made possible only by means of the invention we have begun with;
for without the steam engine the deep coal mines could not have the
water pumped out of them,—it was indeed for this very purpose that the
steam engine was originally contrived,—nor could the coal be efficiently
raised without steam power. Before the steam engine came into use iron
could not be produced or worked to anything like the extent attained
even in the middle of the nineteenth century, for only by steam power
could the blast be made effective and the rolling mill do its work. On
the other hand, the steam engine required iron for its own construction,
and this at once caused a notable increase in the demand for the metal.
Once more, the engine itself supplies no force; for without the fuel
which raises steam from the water in the boiler it is motionless and
powerless, and that fuel is practically _coal_. In consequence of thus
providing power, and also of supplying a requisite for the production of
iron, coal has acquired supreme industrial importance, so that all our
great trades and places of densest population are situated in or near
coal-fields. But what we have further to say about coal may be
conveniently deferred to a subsequent article, while we proceed to treat
of iron, and of the contrivances in which it plays an essential part.
Iron has also been called “the mainspring of civilization,” and the
significance of the phrase is obvious enough when we consider the
enormous number and infinite variety of the things that are made of it:
the sword and the ploughshare; all our weapons of war and all our
implements of peace; the slender needle and the girders that span wide
rivers; the delicate hair-spring of the tiny watch and the most
tenacious of cables; the common utensils of domestic life and the huge
battle-ships of our fleets; the smoothest roads, the loftiest towers,
the most spacious pleasure palaces. Such extensive applications of iron
for purposes so diverse have been rendered possible only by the greater
facility and cheapness of production, together with the better knowledge
of the properties of the substance and increased skill in its treatment,
that have particularly distinguished our century. Apart again from the
constructive uses of iron, it enters essentially into another class of
inventions of which the age is justly proud, namely, those which utilize
electricity in the production of light, mechanical power, and chemical
action; for it is on a quality possessed by iron, and by _iron alone_,
that the generation of current by the electric dynamo ultimately
depends. This peculiar property of iron, which was first announced by
Arago in 1820, and has since proved so fertile in practical
applications, is that a bar of the metal can, under suitable conditions,
be instantly converted into the most powerful of magnets, and as quickly
demagnetized. What these conditions are will be explained when we come
to treat of electricity.
[Illustration:
FIG. 16.—_Aerolite in the British Museum._
]
Besides the unique property of iron just referred to, and its
superlative utility in arts and industries, there are other
circumstances that give a peculiar interest to this metal. It is the
chief constituent of many minerals, and traces or small quantities are
found in most of the materials that make up the crust of the earth; it
is present also in the organic kingdoms, being especially notable in the
blood of vertebrate (_back-boned_) animals, of which it is an essential
component. Notwithstanding its wide diffusion, iron is not found
_native_, that is, as _metal_, but has to be extracted from its _ores_,
which are usually dull stony-looking substances, as unlike the metal as
can be conceived. In this respect it differs from gold, which is not met
in any other than the metallic state, in the form of _nuggets_, minute
crystals or branching filaments, and from metals such as silver, copper,
and a few others which also are occasionally found native. It is true
that rarely small quantities of metallic iron have been met with in the
form of minute grains disseminated in volcanic rocks; but in contrast
with the practical absence of metallic iron from terrestrial accessible
materials is the fact that masses of iron, sometimes of nearly pure
metal, occasionally descend upon the earth from interplanetary space.
These are _aerolites_, of which there are several varieties, some
consisting only of crystalline minerals without any metallic iron,
others of a mixture of minerals and metals, but the most common are of
iron, always alloyed with a small quantity of nickel, and usually
containing also traces more or less of a few other metals and known
chemical elements. The iron in some specimens has been found to amount
to 93 per cent. of the whole. These _aerolites_, or _meteorites_, as
they are also called, are of irregular shape and vary greatly in size,
which however is sometimes very considerable: one found in South America
was calculated to weigh 14 tons, another discovered in Mexico, 20 tons.
There is in the British Museum a good specimen of an iron meteorite,
which is represented in Fig. 16, where it will be observed that a
portion has been cut off to form a plane surface, which when polished
and etched by an acid, reveals a crystalline structure quite peculiar
and distinctive, so that such meteorites can be recognized with
certainty, even if they did not possess surface characters which are
easily observed and identified when once a specimen has been examined.
The fall of meteorites to the surface of the earth is comparatively
rare, but it has been witnessed by even scientific observers; as when
Gassendi, the French astronomer, saw in Provence the fall of a meteorite
weighing 59 lbs. In the _Transactions of the Royal Society_ for 1802 may
be found a detailed account of an instance in England of the fall which
took place in Yorkshire, on the 13th December 1795, of a stone 56 lbs.
in weight. Aerolites become ignited or incandescent by reason of the
great velocity with which they pass through the atmosphere, whereby the
air in front of them is condensed and heated, the heat often being
sufficient to liquefy or even vaporize the solid matter. The so-called
shooting stars are with good reason believed to be nothing but such
incandescent aerolites, and the aerolites themselves are regarded as
small asteroids, or scattered planetary dust, portions of which
occasionally coming within the sphere of the earth’s attraction are
drawn to its surface. Meteoric iron is too rare to be of any value as a
source of iron, but certain specimens have been found in which the metal
was malleable and of excellent quality. From such meteorites the natives
of India and other places have, it is said, sometimes forged weapons of
wonderful temper and keenness, and we may well imagine that when such
weapons have been made from iron that had actually been observed to fall
from the sky, they would be regarded as endowed with magical powers, so
that we may perhaps ascribe to such circumstances the origin of some of
the legends about enchanted swords, etc. It is significant also that in
some Egyptian inscriptions of the very highest antiquity, the word
indicating iron has for its literal meaning _stone of the sky_.
But as nature has hardly provided man with the _metal_ iron, he has been
obliged to find the art of extracting it from substances which are
utterly unlike the metal itself. In this case, as in many others, the
art has been discovered and practised ages before any scientific
knowledge of the nature of the processes employed had been acquired. The
idea prevails that there are such difficulties in extracting this metal;
that elaborate and complex appliances, not unlike those in use in modern
times, were requisite for the purpose; and therefore that the use of
iron is compatible only with a somewhat late period in man’s history,
and implies a comparatively advanced stage of civilization. Now there
undoubtedly are facts which tend to confirm this view; for instance, the
Spaniards who first colonized North America found the natives perfectly
familiar with the use of copper, but without any acquaintance with iron,
although the region abounded with the finest ferruginous minerals; and,
again, the archæologists who have examined the relics of ancient
civilizations and of pre-historic peoples about the shores of the
Mediterranean, find in the earliest of these relics weapons and
implements of rudely chipped stones, followed later by the use of
better-shaped and polished stones; hence the periods represented by
these, they have respectively designated by the terms _palæolithic_ and
_neolithic_—the old and the new stone ages. At some later time the stone
of these implements was gradually replaced by _bronze_, which is a
mixture of copper and tin, while as yet iron does not occur in any form
among the remains. In the latest layers, however, articles of iron are
found, and it is inferred that this metal came into use only after
bronze had been known for an indefinite period; hence these later
pre-historic periods have come to be respectively called _the bronze
age_ and _the iron age_. No doubt this succession really occurred in the
localities where the observations were made, but it would not be
justifiable to assume that the same was the case in every part of the
world, for much would depend on such circumstances as the presence or
absence of the essential minerals. We may also set against the supposed
difficulty of obtaining iron from the ores, the still greater complexity
of the methods required for the production of copper and of tin. Besides
this is the fact that the ores of tin are found but in very few places
in the world, and of these only the Cornwall mines, so well known in
ancient times, would be likely to furnish a supply to the places where
pre-historic bronzes are found; this implies that navigation and
commerce must have already made considerable progress. On the other
hand, iron has been produced and worked for untold ages by the negro
races all over Central Africa, and the method of treating the ore has no
doubt been that which is there still practised by certain scarcely
civilized tribes, and it is as simple as any metallurgical operation can
possibly be, requiring merely a hole dug in a clay bank, wherein the
fuel and minerals are piled up, and the mere wind supplies sufficient
blast to urge the fire to the needful temperature, or air is blown in
from rude bellows made of a pair of skins alternately raised and
compressed. These very primitive furnaces have in some places developed
into permanent clay structures, seven or eight feet in height. The
natives of Central Africa have therefore long known the method of
extracting iron, as well as of forging and casting it.
The nature and value of what has been done during the century in the
treatment of iron would not be intelligible without some description of
the ordinary processes of extracting the metal from the ores; and a
scientific understanding of these implies some acquaintance with
chemistry. Not because metallurgy has been developed from chemistry, for
the fact is rather the reverse; indeed, as we have seen, the art of
extracting iron from its ores was practised ages before chemistry as a
science was dreamt of. Although we may assume that many of our readers
have sufficient knowledge of chemistry to attach distinct ideas to such
few chemical terms as we shall have occasion to use, yet it may be of
advantage to others to have some preliminary notes of the character of
the chemical actions, and of some properties of the substances that will
have to be referred to. It is certainly the case that people in general,
and even people very well informed in other subjects, have but the
vaguest notions of the nature of chemical actions, and of the meaning of
the terms belonging to that science. For example, one of our most
popular and justly esteemed writers, treating of the very subject of
iron extraction, calls the ore a _matrix_, thereby implying that the
iron as metal is disseminated in detached fragments throughout the mass,
which is a conception inconsistent with the facts. The reader will be in
a more advantageous position for understanding the relation of the ores
of iron to the metal, if he will follow in imagination, or still better
in reality, a few observations and experiments like the following—of
which, however, he is recommended not to attempt the chemical part
unless he is himself practically familiar with the performance of
chemical operations, or can obtain the personal assistance of someone
who is. Taking, say, a few common iron nails, let him note some obvious
properties they possess: they have weight—are hard and tough so that
they cannot be crushed in a mortar—are opaque to light—if a smooth
surface be produced on any part, it will show that peculiar shiny
appearance which is called metallic lustre, in this case without any
decided colour—they are not dissolved by water as sugar or salt is—and
are attracted by a magnet. If several of the nails be heated to bright
redness they may be hammered on an anvil into one mass, and this may be
flattened out into a thin plate, or it may be shaped into a slender rod
and then drawn out into wire; or otherwise the nails may be converted
into the small fragments called iron filings. In these several forms the
nails, as nails, will have ceased to exist; but the material of which
they were formed will remain unchanged, and each and every part of it
however large or small will continue to exhibit all the properties noted
above as belonging to the substance of the nails, which in the cases
supposed has undergone merely _physical_ change of shape. Treating our
nails in yet another way, we may proceed to subject them to a _chemical_
change, by an experiment very simple in itself, but involving certain
precautions, by neglect of which the tyro in chemical operations would
incur some personal risks; these might however be obviated by using only
very small quantities of the materials (a mere pinch of iron filings and
a few drops of sulphuric acid), when the results would still be
sufficiently observable. A few of the iron nails having been placed in a
flask of thin glass, we pour upon them a mixture of oil of vitriol
(sulphuric acid) and water, which has previously been prepared by
gradually adding 1 measure of the acid to 5 measures of water. The
action that takes place is greatly accelerated by heat, and indeed the
contents should be heated to boiling by standing the flask on a layer of
fine sand spread on an iron plate and gently heated from below. The
nails will soon disappear, being completely dissolved by the acid
liquid, and the turbid solution should be filtered through filtering
paper as rapidly as possible and while still hot. This turbid and dirty
looking condition is due to foreign matters in the nails, for these
never consist of _pure_ iron. The filtered liquid is set aside to cool
in a closed vessel, in which after a time will be found a deposit of
crystals of a pale bluish-green colour. The liquor above these having
been poured off, the crystals are to be rinsed with a very small
quantity of _cold_ water, and then dried between folds of
blotting-paper, after which they are ready for examination. The quantity
of the diluted acid put into the flask should have a certain proportion
to the weight of the nails; about 5 _fluid ounces_ to 1 ounce of iron
will be found convenient, for if less is used the nails will not be
entirely dissolved, and an excess will tend to keep the crystals in
solution instead of depositing them when cold. The nails—as such—will
now have passed out of existence: can we say that the iron that formed
them exists in the crystals? Certainly not as the _metal_ iron, for
every property of the metal will have disappeared. The crystals are
brittle, can be crushed in a mortar—they are translucent—they show no
metallic lustre, but only glassy surfaces—they are readily dissolved by
water—they are not attracted by a magnet. The most powerful lens will
fail to show the least particle of iron in them; they have in their
properties no assignable relation to the metal of the nails, but are
matter of quite another sort; and be it noted that this entire
_otherness_ is the special and characteristic sign of _chemical change_.
So complete is the transformation in the case we have been considering
that it would never have been said that iron was contained in these
crystals, but rather that the metal had for ever passed out of
existence, but for one circumstance; and that is, that by subjecting the
crystals to certain processes of _chemical analysis_ we can again obtain
from them the iron in metallic state. Nay more, we should find the
weight of metal so obtained to be exactly equal to that of the pure iron
dissolved from the original nails, supposing of course that we operated
upon the whole of the crystalline matter so produced. The inference
therefore is that although every property of the iron appeared to be
absent from the crystals, the iron entering in them retained there its
original weight, and the correct statement of the change would be, that
in the crystals the iron had lost all its original properties SAVE ONE,
namely, its weight, or gravitating force, if we choose to call it so, a
property belonging to it in common with every material substance.
Chemical analysis can also separate from the crystals their other
constituents and weigh them apart—so much water and so much sulphuric
acid—and when to these weights that of the iron is added, the sum
exactly makes up the weight of the crystals.
A still simpler experiment, which may be performed by anyone with the
greatest ease, may serve as a further illustration of the profound
nature of the change in the properties of bodies brought about by
chemical combination, and it will also serve as the occasion of
directing attention to a remarkable circumstance that invariably
characterizes such changes, and one that should always be present in our
minds when we are considering them. A yard of flat _magnesium_ wire can
be bought for a few pence, and after its metallic character has been
observed in the silvery lustre disclosed by scraping the dull white
surface, a few inches is to be held vertically by a pair of tongs, or by
inserting one extremity in a cleft at the end of a stick, then the lower
part is brought into contact with a candle or gas flame. The metal will
instantly burn with a dazzlingly brilliant light, and some white smoke
(really fine white solid particles) will float into the air; but if a
plate be held under the burning metal, some of the smoke will settle
upon it, together with white fragments that have preserved some shape of
the metallic ribbon, but which a touch will reduce into a fine white
powder, identical with the well-known domestic medicine called “calcined
magnesia”—a substance totally different from the _metal magnesium_. The
reader will scarcely require to be told that in this burning the metal
is entering into combination with the oxygen of the air—by which that
invisible gas somehow becomes fixed in these solid white particles, so
entirely unlike itself. But this experiment might be so arranged that
the quantities of magnesium and oxygen entering into the magnesia could
be weighed. For this purpose special appliances would be required in
order to ensure complete combustion of the metal, for in the experiment
as just described some small particles are liable to be shielded from
the oxygen by a covering of magnesia, and the arrangement would have to
be such that the _whole_ of the white powder could be gathered up and
weighed. In the absence of such appliances, and of a delicate balance,
together with the skill requisite for their use, the reader must for the
time be contented to take our word for what would be the result. In
every experiment the magnesia would be found heavier than the metal
burned in the proportion of 5 to 3; in other words, magnesia always
contains (so the phrase runs) 3 parts of magnesium combined with 2 of
oxygen: never more nor less. A definite proportion between the weights
of the constituent substances characterizes every chemical combination,
and when this is once determined in a single sample of the compound, it
is determined for every portion of the same, wherever found or however
produced. But each compound has its own particular proportion, that is,
the quantitative relations are different for each. For example, the two
constituents of water, hydrogen and oxygen, are combined in the ratio of
1 to 8, etc.; and oxygen combines with metals in a ratio different in
each case. Then occasionally the same ratio of constituents occurs in
compounds of different composition. The elementary student is apt to
suppose that this is _because_ of the _law_ which he finds stated,
probably in almost the first page of his text-book: “Every compound
contains its elements in definite and invariable proportions”; and even
well-educated people entertain the idea of the fact being “governed by”
or “obeying” the law just quoted,—a misconception arising from the other
use of the word “law,” as signifying an enactment. The real case however
is the converse; namely, that a multitude of facts like that above
stated have governed the law, and caused it to be what it is—the general
statement of many observed facts.
We have assumed that the reader’s chemical knowledge had already made
him aware that in every case of ordinary combustion the oxygen of the
atmosphere is in the act of entering into combination with the burning
body: as with the magnesium, so with a coal fire, a gas flame, or a
burning candle; only in these last cases the products of the combustion
pass away invisibly. The candle by burning disappears from sight, but
its matter is not lost, and as in the case of magnesium, the compounds
it forms weigh more than the unburnt candle. The experiment is commonly
shown in courses of elementary lectures on chemistry, of so burning a
candle that the invisible products are retained in the apparatus,
instead of being dissipated in the atmosphere, and the increase of
weight of the burnt candle over the original one is demonstrated by the
balance. Important as is the part played by oxygen in all chemical
actions on the earth, the composition of the atmosphere was not
understood until the end of the eighteenth century, and it was well on
into the nineteenth before the quantities of its constituents were
accurately determined. Now everyone knows that air is mainly made up of
a _mixture_ of the two gases _oxygen_ and _nitrogen_. A _mixture_ of two
or more things is very different from a chemical _combination_ of them;
for in the former each ingredient retains its own properties. (See _Air_
in Index.) Nitrogen being an inert gas that takes no part in combustion,
or in the ordinary chemical actions of the air, acts therein simply as a
diluent of the oxygen. It is necessary in relation to our present
subject to bear this in mind, as well as the relative quantities of the
two gases in air. For our immediate purpose we may neglect the minor
constituents of air—such as watery vapour, carbonic acid, etc., of which
the total weight does not exceed one hundredth part of the whole—and
consider air as a mixture of 23 parts by weight of oxygen with 77 of
nitrogen, or calculated in volumes, 21 measures of oxygen with 79 of
nitrogen. Compounds of oxygen with nearly every one of the other seventy
or more chemical elements are known, and these compounds, which are
called _oxides_, are arranged by chemists under five or six classes,
forming as they do _basic radicles_, _acid radicles_, _saline oxides_,
etc. With some of these compounds belonging to different classes, we
must make acquaintance after noticing the elementary substance with
which the oxygen is united.
We begin with _carbon_, which forms the chief constituent of all our
combustibles. Some specimens of graphite, plumbago, or “blacklead”
consist of almost pure carbon (98 per cent.), and some varieties of wood
charcoal exceptionally contain 96 per cent.; but in ordinary charcoal
the percentage is much less. Coal, the most familiar of our solid fuels,
varies greatly in composition, carbon being the predominating
constituent, in amount from 57 to 93 per cent. Coke, another fuel much
used in metallurgical operations, is made by heating coal without access
of air, when a large quantity of gaseous substances is expelled. Coke
burns with an intense and steady heat without emitting any visible
smoke, but it does not ignite as readily as coal. Carbon forms two
different compounds with oxygen: both are invisible gases, but they
differ in the proportions of the constituents, and present different
properties. When carbon (coal, coke, or charcoal) is completely burnt,
that is, with an abundant supply of air, the product is _carbonic acid_
gas, in which 3 parts of carbon are combined with 8 of oxygen: when, on
the other hand, the carbon is burnt with a sufficiently restricted
access of air, the result is _carbonic oxide_ gas, in which 3 parts of
carbon are united with only 4 of oxygen. The reader will here observe
that the former contains just twice as much oxygen as the latter for the
same quantity of carbon. This fact and numberless others like it are
expressed or summed up by another _law_ of chemical combination which
states that when two elements combine in several different proportions
these are invariably such that the ratios in the several compounds will
be found to have exact and simple numerical relations; that is, such as
may, when reduced to their lowest terms, be expressed by the simple
integers 1, 2, 3, etc., as 1 : 2, 3 : 2; ... 8 : 9, etc. It comes to the
same thing if we compare together the weights A and A´ which are united
in each compound with any one identical weight of B, giving of course
the ratio A : B ÷ A´ : B. For instance, in the case just given, of
carbon and oxygen, 3 : 4 ÷ 3 : 8 = 2 : 1. This, which is simply stating
the facts, is called the _law of multiple proportions_. On a later page
will be found another illustration (see Index, _Nitrogen and Oxygen
Compounds_), and its expression in terms of the _atomic theory_, which
goes behind the facts (so to speak), but is extremely useful by
comprehending many other groups of facts in chemistry and in other
sciences. Carbonic acid gas is of course incombustible, but carbonic
oxide gas burns by uniting with the additional proportion of oxygen and
becoming carbonic acid. On the other hand, carbonic acid gas passing
over red-hot coals takes up from them the additional proportion of
carbon, and is, we may say, _unburnt_ into carbonic oxide. When we see a
pale blue flame flickering over the bright embers in a fire grate, it is
carbonic oxide burning back again by taking more oxygen from the air
above the coals. Carbonic oxide combines directly with two or three of
the metals, as, for instance, it forms a volatile compound with nickel,
at a certain temperature, and this is decomposed again at a higher
temperature. The like takes place with iron, although in very small
quantities, but the observation throws some light on the processes of
reduction. Carbonic oxide is neither acid nor basic, but carbonic acid
is an acid oxide, and as such unites with oxides of the basic class to
form another range of compounds. Thus, for example, the oxide of the
metal _calcium_ is quicklime, which is strongly basic, and this directly
combines with carbonic acid, forming a _neutral_ substance called in
systematic chemistry _calcium carbonate_, or more commonly but less
correctly, carbonate of lime, familiar to everyone in the compact state
as limestone, and marble, and in a more or less pulverent condition as
chalk. When any of these is heated to redness, carbonic acid is expelled
and quicklime remains. Like most oxides, quicklime forms a compound with
water, the combination being attended with the extrication of much heat,
the compact quicklime swelling and crumbling into _slaked lime_. The
chemist’s term for a compound of a _basic_ oxide with water is
_hydrate_, while that of an _acid_ oxide with water is for him properly
an _acid_, or in order to particularly distinguish this class, an
_oxy-acid_. It was however the older practice to give the name of acid
to the oxide alone, and this naming having found its way into popular
language is much more familiar to the non-scientific reader. The
systematic names of the two compounds of carbon and oxygen are _carbon
monoxide_ and _carbon dioxide_, but we shall use here the more familiar
terms carbonic oxide and carbonic acid.
We have now to call attention to a substance which contributes by far
the largest part to the solid crust of our globe. It is called _silica_,
from _silic-_, the Latin word for flint (without case suffix): it is
seen in flint, and very pure in rock crystal, quartz, agate, and
calcedony. It forms the essential part of every kind of sand and
sandstone, and is the principal ingredient of clay, granite, slate,
basalt, and many other minerals. Silica is the oxide of a quasi-metal
called _silicon_, which can be obtained from silica with difficulty, and
only by roundabout processes, presenting itself in different conditions
according to the process used. Silica is an acid oxide, and it readily
unites with most of the basic oxides when heated with them, forming a
class of compounds of different properties which are much modified in
admixtures containing two or more. Very few of these _silicates_ are
soluble in water, most of them are not: they are all fusible at various
temperatures, except silicate of alumina, of which fire-clay is chiefly
constituted. Alumina, it should be stated, is the oxide of the metal
aluminium. The silicates of lime and of magnesia fuse only with great
difficulty; but the silicates of iron and of manganese are easily fused,
and silicate of lead still more so. Glass is a mixture of silicates,
often of lime, soda, and alumina; sometimes of lead and potash mainly;
porcelain and pottery consist chiefly of silicate of alumina with
varying proportions of silicates of iron, of lime, etc.
It now remains only to mention two non-metallic elements that are nearly
always present in crude iron, but which the metallurgist strives to
eliminate, as they are in general very injurious to the quality of the
material even when their amount is very small. The first is _sulphur_,
well known as _brimstone_, also as _flowers of sulphur_, a yellow
coloured solid, which burns in the air. The product of the combustion is
an invisible gas of a readily recognized pungent odour: this is an
acid-forming oxide containing equal weights of sulphur and oxygen. There
is another oxide in which the weight of oxygen is one and a half times
that of the sulphur, and this is the radicle of the very active
_sulphuric acid_ or _oil of vitriol_. Sulphur, like oxygen, unites with
most of the other elements, forming compounds called _sulphides_. Of
these the iron compound called _pyrites_ is the best known, and its
occurrence in coal prevents the use of that material as fuel in contact
with iron or other metals. _Phosphorus_ is an element that occurs
naturally only in combination; in its separated state it is a very
inflammable solid. It combines directly with other substances and is
taken up by some fused metals in large quantities. In many cases a very
small proportion of it existing in a metal greatly modifies the
properties of that metal. Phosphorus forms several oxides, and these are
radicles of powerful acids, among which is _phosphoric acid_ that
combines with basic oxides to form _phosphates_.
We have now, in the few last paragraphs, set before the reader the
minimum of chemical knowledge that will enable him to follow the
_rationale_ of such processes of the modern treatment of iron and its
ores as we can here give an outline of. Although there are numberless
minerals from which some iron can be extracted, the name of _iron ore_
is confined to such as contain a sufficient amount to make the
extraction commercially profitable, and this requires that the mineral
should be capable of yielding at least one-fifth of its weight. The ores
are very abundant in many parts of the world, and they consist mainly of
_oxides_ and their _hydrates_, or of _carbonate_, or of carbonate mixed
with clay and silicates, sometimes also with coaly matters in addition.
The carbonate iron ores are often mixed with oxides. Each class of ore
is liable to be contaminated with phosphates and with sulphur. The
richest ore is the _magnetic iron ore_, which is found in enormous
masses in Sweden, Russia, and North America. It is an oxide containing
72·41 per cent. of iron. _Red hæmatite_ and _specular ore_ are varieties
of another oxide with 70 per cent. of iron: the former is a very pure
ore when compact. It is found in Lancashire, Cumberland, and South
Wales, and much has been imported from Spain, while America has abundant
supplies near Lake Superior. Specular iron ore forms brilliant
steel-like crystals which show the red colour of hæmatite only when
scratched or powdered. Elba was famous for this ore, which occurs also
in Russia and Sweden, and large deposits are met with in both North and
South America. Brown hæmatite is a hydrate of the former, containing 60
per cent. of iron; it abounds in France and Spain, where some kinds are
associated with a noteworthy quantity of _phosphate_ of iron. _Spathic_
or _sparry iron_ ore is, when pure, a collection of nearly colourless
transparent crystals, consisting of carbonate of iron; it contains about
48 per cent. of iron, and also some of the metal _manganese_, which last
circumstance makes it, as we shall see, particularly suitable for
producing certain kinds of steel—indeed it is sometimes called _steel
ore_. Large beds of it occur in Styria and Carinthia. _Clay iron stone_,
or _clay-band_, has been extensively mined in Britain. It is found
abundantly in Staffordshire, Yorkshire, Derbyshire, and South Wales. It
consists of carbonate of iron intimately mixed with clay. The quantity
of iron in some samples falls as low as 17 per cent., but it rises with
variations to as much as 50 per cent. Much of its importance arises from
the fact of its occurring in beds alternating with layers of coal,
limestone, and clay, so that the same pit is sometimes able to supply
firebricks for building the furnace, fuel for the smelting, and
limestone for the _flux_,—a combination of advantages that for long
enabled iron to be produced in England cheaper than elsewhere. The like
is true of the _blackband ore_, which, in addition to the same
ferruginous composition as the last, contains also so much combustible
or bituminous material that it can be _calcined_ (roasted) without
additional fuel. The deposits of _blackband_ in Lanarkshire and
Ayrshire, which were discovered only in 1801, have given great
industrial importance to the district. Yet another British ore must be
noticed, namely, the _Cleveland ironstone_ of the North Riding of
Yorkshire. This is a carbonate of a grey or bluish colour caused by the
presence of a little iron silicate. It contains also a considerable
amount of phosphorus.
How simple is the operation of obtaining iron from the ore has already
been stated—that it is necessary only to surround lumps of ore by fuel
in a fire urged by a natural or artificial blast, and then to hammer the
mass extracted from the furnace so as to weld together the scattered
particles of the metal, and at the same time squeeze out the associated
slag and cinders, in order to obtain a coherent malleable piece, which
can be reheated in a smith’s fire, and forged into any required form. It
is no wonder therefore that iron was so produced by the ancient Britons;
at any rate Cæsar found them well provided with iron implements and
weapons. No doubt the Romans brought their more advanced skill to the
working of the metal; but in the matter of treating the original ore,
the methods they pursued on an extensive scale in Britain were of the
rude kind already described. Indeed in localities where the Romans were
known to have carried on their operations, the remains of their workings
are almost always found on high ground, so that it may be inferred that
they relied upon the winds to fan their fires, and their operations were
incomplete and wasteful. The most extensive of them appear to have been
in Sussex and Monmouthshire, in which last county there are places where
the ground is in large areas covered by their cinders and refuse, and in
this about 30 or 40 per cent. of iron occurs, so that for some centuries
this material was found capable of being profitably reworked as a source
of the metal. Iron continued to be produced in England during the middle
ages with charcoal for fuel, but its export was forbidden, and whatever
steel was required had to be imported from abroad. Afterwards German
artisans were brought over for making steel, and soon afterwards the
importation of shears, knives, locks, and other articles was prohibited.
The native production of iron continued, and this consumed the forests
so rapidly for the supply of charcoal, that various Acts were passed to
restrain the iron-makers, in order to preserve the timber. In spite of
these, the arts of smelting and working iron advanced apace: bellows
were used for the blast, and then the works were brought down into the
valleys, where water power could be employed to work them. The scarcity
of charcoal fuel caused many attempts to supply its place with pit coal,
but these met with small success, partly on account of the coal
containing so much sulphur, and partly from the difficulty of obtaining
with it a sufficiently high temperature, especially as the blowing
apparatus was as yet very imperfect. At length, in the first half of the
seventeenth century, the problem was solved by Dud Dudley, whose process
was kept secret, but is believed to have consisted in supplying coal at
the top of a higher furnace, in such a manner that the coal was
converted into _coke_ by the heat of the escaping gases before it
reached the reducing zone of the furnace. This innovation was violently
opposed by the charcoal smelters, who persecuted the inventor in every
way, until their resistance was successful. But before the middle of the
next century coke was regularly used in iron smelting, the process
having been made successful by Darby at Coalbrookdale, and then many new
applications of cast iron came into vogue. Coke being a substance
burning less freely than charcoal, bellows were found inadequate to give
the necessary blast, and were displaced by _blowing cylinders_, actuated
at first by water wheels, but this uncertain and comparatively feeble
source of power was soon superseded by the steam engine, the “fire
engine,” for which, as we have seen, Watt obtained his patent in 1769.
The furnaces were not then all engaged in producing the fusible metal
now called _cast_ or _pig iron_ as are the huge blast furnaces we see at
the present time. Indeed it was much to the disgust of the old iron
smelter that occasionally his product turned out to be of the fusible
kind, unworkable by the hammer, which therefore he regarded as
worthless. At what date _cast iron_ was first used is uncertain; but
probably it was not long before the fourteenth century. The furnaces in
use up to that time were small square walled-in structures only 3 or 4
feet high, and their effect would not greatly exceed that of a smith’s
forge: but as improved blowing apparatus gave more power, they soon
became enlarged into oval or round brick towers from 10 to 15 feet high,
and they, like the small furnaces, could be made to yield either smith
iron or steel by modifying the charge and the manner of applying the
blast; while furnaces of dimensions exceeding a certain limit could no
longer be trusted to turn out malleable metal, but they produced instead
the cruder substance we call white pig iron, and this requires much
subsequent treatment before it is converted into malleable or “merchant
iron.” Nevertheless the demand for cast iron as such, and more
particularly the adoption of improved methods of deriving malleable iron
from it, caused further increase in the size and numbers of blast
furnaces, until in the early part of our century 30 feet was not an
unusual height, the highest one in England in 1830 attaining 40 feet.
The total make of pig iron in England was in that year nearly 700,000
tons, perhaps about fifty times as much as it was a century before, and
thirty years later (1860) it had risen to nearly 4,000,000 tons. These
figures show the extraordinary expansion of the British iron manufacture
in the earlier part of the century; and the still more extensive
applications of iron during the next twenty years had the effect of
almost doubling the produce in 1880, and of increasing also three-fold
the amount of foreign metal imported, raising it to 2,500,000 tons. The
reader will now, it is hoped, be prepared to follow with some interest a
brief account of the principal inventions which have brought about
results of such importance.
[Illustration:
FIG. 17.—_Blast Furnace (Obsolete Type)._
]
[Illustration:
FIG. 18.—_Section and Plan of Blast Furnace (Obsolete Type)._
]
Deferring for the moment any description of the latest blast furnaces,
we invite his attention to Fig. 17, which represents the furnace used in
the first half of our century, but which now is of an obsolete type,
Fig. 18 being the section and plan of the same. The lower part of Fig.
17 shows where the molten metal has been allowed to run out of the
furnace into channels made in dry sand; first a main stream, then
branches to right and left, each of these with smaller offsets on each
side of it. These smaller channels are the moulds for the _pigs_, so
called because of the fancied resemblance of their position with regard
to the branch that supplied them, to the litter of a sow. They are
easily broken off from the larger mass, and then form pieces about 3 ft.
long with a ᗜ-shaped section, 4 in. wide, the weight being from 60 to 80
lbs. This is iron of the crudest kind, and though it is often referred
to as “cast iron,” it is, as a matter of fact, not used in this state
for any castings, except those of the very roughest and largest kind: a
certain amount of purification is requisite in most cases. This is given
by fusing the metal—along with some form of oxide and often other
matters—in a _cupola furnace_, which is like a small blast furnace,
being from 8 ft. to 20 ft. high and uses coke for fuel with a cold
blast.
So far from being simply iron, pig contains a large and variable
proportion of other matters amounting often to 10 or 12 per cent.; and
these confer upon it its fusibility. The principal one is carbon, which
is found in the metal partly in the state of chemical combination with
it, and partly in the form of small crystals similar to those of
graphite or plumbago, disseminated through the mass. When there is a
comparatively small proportion of the carbon combined with the iron, the
substance is grey, and it can be filed or drilled or turned in a lathe.
In white cast iron the combined carbon predominates, or is sometimes
accompanied by scarcely any graphitic carbon; it is brittle and so very
hard that a file makes no impression. It fuses at a lower temperature
than the other varieties. A third kind is the _mottled_ cast iron, which
shows a large coarse grain when broken, and distinct points of separate
graphite particles; it is tougher than the others, and therefore when
cannon were made of cast iron this variety was preferred. The following
table giving the percentage composition of four samples of crude cast
iron will show their diversities.
┌─────────────────────────────────┬────────┬────────┬────────┬────────┐
│ │ White.│ White.│Mottled.│ Grey.│
├─────────────────────────────────┼────────┼────────┼────────┼────────┤
│Iron │ 88·81│ 89·304│ 93·29│ 90·376│
│Combined carbon │ 4·94│ 2·457│ 2·78│ 1·021│
│Graphite, or uncombined carbon │ ...│ 0·871│ 1·99│ 2·641│
│Silicon │ 0·75│ 1·124│ 0·71│ 3·061│
│Sulphur │ trace│ 2·516│ trace│ 1·139│
│Phosphorus │ 0·12│ 0·913│ 1·23│ 0·928│
│Manganese │ 5·38│ 2·815│ trace│ 0·834│
└─────────────────────────────────┴────────┴────────┴────────┴────────┘
The reader will observe that the last item in the table above is a
substance that he has not yet made the acquaintance of, namely,
_manganese_. This is a metal which in many of its chemical relations
much resembles iron, and ferruginous ores usually contain a greater or
less proportion of it. Manganese is of great importance in the
manufacture of steel, as we shall presently see; but as a separate metal
it has no application, and is obtainable in the metallic state with much
difficulty. One of its oxides has however very extensive applications in
the chemical arts, and others form acid radicles, which in combination
with potash or soda give rise to useful products. The well-known
“Condy’s fluid” is a solution of one of these.
We have seen how malleable iron or steely iron may be directly obtained
from the ores, but it has been found that on the large scale it is
necessary and more economical to operate on the pig iron produced by the
blast furnaces in such a manner as to remove the greater part of the
foreign substances.
[Illustration:
FIG. 19.—_Section of a Reverberatory Furnace._
]
The first step in the conversion of the pig iron usually taken has been,
and to a certain extent even is still, to remelt the metal in what is
termed a _finery furnace_, a kind of forge in which a charcoal fire is
urged by a cold blast, and so regulated that an excess of oxygen is
supplied, or rather more than would suffice to convert all the carbon of
the fuel into carbonic acid; although this is perhaps not absolutely
necessary, as carbonic acid would itself supply oxygen by suffering
reduction to carbonic oxide. At any rate the melted metal is exposed to
an oxidizing atmosphere and constantly stirred. Many different
arrangements of the furnace and details of the process have been used.
For instance, where the finest quality of malleable iron was not aimed
at, coke has been the fuel employed, and many shapes of furnaces, etc.,
have been contrived, and various additions of ores, oxides, etc., made
to the charge, according to local practice and the nature of the crude
iron. One marked effect of the operation is the final removal of nearly
all the _silicon_, which is burnt or oxidized into _silica_, and this at
once unites with oxide of iron, which is also formed, to produce a
readily fusible slag of silicate of iron, and in the production of this
silicate any sand attached to the pig will also take part. Much of the
carbon, amounting sometimes to more than half, is also eliminated as
carbonic oxide, and of what is left but little remains in the graphitic
state. The action on the phosphorus is usually less marked, but there is
always a notable reduction of the quantity. The sulphur is also lessened
in some degree, although when coke is used, the fuel has the
disadvantage of itself containing sulphur, phosphates, and other
deleterious matters. Sometimes a little lime is added to the charge to
take up the sulphur from the coke. The operation lasts some hours, the
fused metal being frequently stirred with an iron rod, until it assumes
a pasty granular condition, when the workman gradually collects it upon
the end of the rod into a ball of about three-quarters of a cwt. in
weight. These balls, or _blooms_ as they are called, are removed from
the furnace while still intensely hot, and at once submitted to powerful
pressure by means of some suitable mechanical arrangement, the effect
being to squeeze out the liquid slag and force the particles of metal
together by which the whole becomes partially welded into a more compact
mass. Then this mass is, while still hot, either hammered with gradually
increased force of the strokes, or in the more modern practice, passed
between iron rollers (these we shall presently describe), by which it is
shaped into a bar. The bars are afterwards cut into lengths, reheated
without contact of fuel, again hammered or re-rolled; and this process
is several times repeated when the best product is required. During the
first treatment of the blooms, and also in the subsequent hammering or
rolling, the oxygen of the atmosphere acts on the surface of the glowing
metal, so as to cover it with thin scales of oxide, and these, carried
into the interior of the mass, will give up their oxygen to any residual
silicon, carbon, etc., producing a little more slag, carbonic oxide,
phosphate of iron, etc., which by the pressure of the hammers or rolls
are ultimately forced out of the metal. It will be observed that in
producing the pig iron the chemical action is the separation of oxygen
from the metal, while conversely an oxidizing action is set up in the
finery and subsequent treatment, in order to burn off the foreign
ingredients. But this cannot be done without at the same time
re-oxidizing some of the iron itself, of which therefore there is always
a considerable loss, by its formation into slag (silicate), cinder,
foundry scale (oxide), etc. The quantity of iron lost depends of course
on many conditions, such as the care exercised in the operations, but it
occurs in all the processes that have been devised for the conversion in
question, even in the most modern: its amount may be taken to range
between 10 and 20 per cent. The reader is requested to bear in mind the
nature of the chemical actions that have just been described, for in
even the most recently invented processes the principle is the same in
nature and effect. So completely can the foreign elements be eliminated
by this, or some analogous process, such as we shall presently mention,
that the finest Swedish bar iron contains more than 99½ per cent. of the
metal, and in some cases only a very little carbon and a mere trace of
phosphorus remain, amounting together to less than 1 part in 2000. Such
metal is made from very pure ore, containing no sulphur and scarcely any
phosphorus, while charcoal is the fuel used in all the operations. As
already mentioned, the objection to the use of coke is the sulphur,
phosphates, and siliceous matters it contains. Toward the close of the
eighteenth century an invention came into use which obviated the
disadvantages of the cheaper fuel for converting crude iron. This was
the puddling furnace, brought into use after much experimenting by Henry
Cort in 1784. In it the pig iron is fused in a _reverberatory furnace_,
the form of which will be understood from Fig. 19, which is a diagram
showing such a furnace in section, where _f_ is the fire, _a_ an
aperture at which the fuel is introduced, _p_ the ash pit, _b_ is a low
wall of refractory material called the “bridge,” over which the flame
passes, and is by the low arched roof reflected or _reverberated_
downwards upon the charge, _c_, which is laid on a _hearth_, or iron
floor, having spaces below it where air circulates in order to prevent
it becoming too hot. In Cort’s original arrangement the bed of the
hearth was formed of sand, which gave rise to much inconvenience by
producing a quantity of the very fusible silicate of iron, that speedily
attacked the masonry of the furnace, and therefore a very important
improvement was devised some years later by S. B. Rogers, who made the
bed of his furnace of a layer of oxide of iron, spread on a cast iron
plate 1½ inches thick. In later times it has become usual to cover the
iron hearth with certain other refractory mixtures varied according to
circumstances, of oxide, ore, cinder, lime, etc. There is one of these
mixtures significantly designated “_bull-dog_” by the workmen. We may
mention here that it has, in more recent times, when very high
temperatures are obtainable, been found unnecessary to cause even the
flame to come into contact with the substances on the hearth, inasmuch
as the heat radiated from the flame and the intensely heated roof of the
furnace suffices, so that in consequence of this the roofs are now
constructed nearly flat. In the puddling furnace the melted metal is
constantly stirred, and no little skill is required to regulate the fire
by the damper on the chimney, and to admit the proper amount of air to
mix with the flame. The pig iron softens and melts gradually, until at
length it becomes perfectly liquid, at which stage it swells up and
appears to boil owing to the escape of carbonic oxide in numerous jets,
which burn with the characteristic pale blue flame. The puddler then
briskly stirs the mass to cause more complete oxidation of the carbon,
silicon, etc., by bringing the superficially formed oxide of iron into
the interior. As the iron loses its carbon, it assumes much the texture
of porridge, consisting of pasty lumps of malleable iron implexed with
the liquid slag (silicate of iron, etc.) which drips from the spongy
balls as the puddler collects them at the end of his stirring rod, as in
the finery operation. The next thing is to run the mass immediately
between powerful rolls (puddling rolls) by which the slag is squeezed
out, as before, and finally through the _finishing rolls_ that shape it
into bars or plates.
When a comparatively impure pig iron is used or when a better quality of
malleable metal is desired, the crude iron is submitted to a preliminary
treatment before puddling. This treatment, by a technical distinction,
called _refinery_, is practically identical with the _finery_ process
already described, except that instead of being collected into blooms,
the fluid metal is run out to form a layer 2 or 3 inches thick, and
this, before becoming quite solid, is suddenly cooled by having water
thrown over it, the result being a white, hard, brittle mass, which
broken into pieces is ready for the puddling furnace.
The operation that has been described is known as _hand puddling_, in
contradistinction to later methods in which it has been sought to
substitute some form of machine that will produce the same result
automatically, such as revolving furnaces, etc. It has been found
difficult to maintain these in good working order, and in England at
least mechanical puddling has never found much favour, but in the great
iron works of Creusot, in France, large revolving furnaces were in use
about 1880, which could turn out 20 tons of converted iron in 24 hours,
whereas the old hand puddling furnaces could in the same period produce
only 2½ or 3 tons, with two sets of men, the puddler and one assistant.
Of these mechanical furnaces it is unnecessary to give any account,
especially as the puddling process itself has nearly gone out of use,
having been superseded by more economical methods.
The use of rolls for treating the product of the puddling furnace, and
for making it into bars, was also an invention of Henry Cort’s, for
which he obtained a patent in 1783. This was in many respects an immense
improvement on the older system of hammering; it is still practised, and
by it shapes can be given to the metal scarcely possible on the older
system, while the tenacity of the metal is increased by the uniformity
given to the grain. The difference of chemical composition between cast
and wrought iron the reader has already been made acquainted with, and
there is quite as great a difference in their textures. The former, when
broken across, shows a distinctly crystalline structure, which we may
compare to that of loaf-sugar, while the latter exhibits grain, not
unlike that of a piece of wood. This fibrous structure depends upon the
mechanical treatment of the iron, and in rolled bars the fibres always
arrange themselves parallel to the length of the bar. Fig. 20 shows this
fibrous structure in a piece of iron where a portion has been wrenched
off. Like wood, wrought iron has much greater tenacity along the fibres
than across them; that is, a much less force is required to tear the
fibres asunder than to break them transversely. Consequently, to obtain
the greatest advantage from the strength of wrought iron, the metal must
be so applied that the chief force may act upon it in the direction of
the fibres. Near the beginning of our article on IRON BRIDGES (_q.v._)
the reader will find some illustrations of the very different resisting
powers of cast and wrought iron.
[Illustration:
FIG. 20.—_Fibrous Fracture of Wrought Iron._
]
Nothing in the way of inventions can be compared to those of Cort’s as
to the effect they have had in promoting the iron industry, until we
reach a period some years after the middle of our century; but we must
not neglect to recognize the scarcely inferior importance of Rogers’
improvement. Singularly enough, neither of these men reaped any benefit
from his inventions. Cort died in the last year of the eighteenth
century, quite a poor man, having been supported only by a niggardly
pension of some £160 from the Government, and leaving his family in
indigent circumstances. Yet a most eminent authority on iron questions
(Sir W. Fairbairn) estimated—some time about the middle of our era—that
the two inventions of Cort’s alone, the rolling-mill and the
reverberatory puddling furnace, had by that time added to the wealth of
Great Britain by an amount equivalent to six hundred million pounds
sterling. For many iron-masters had profited by these inventions,
amassing very great fortunes, in some instances also acquiring titles of
honour. Clearly to Cort and Rogers may be applied the _sic vos non
vobis_ saying.
We shall now turn to the improvements that have been effected in the
blast furnace, and of these none perhaps has been more marked than that
made by Neilson, when in 1828 he substituted heated air for the ordinary
cold air that had before always supplied the blast. It will be
remembered that the heat is due to the combination of only the oxygen of
the air with the carbon of the coke, but the greater part of the air—the
four-fifths of nitrogen—take no part in the action, beyond abstracting a
large proportion of the heat; but when the air is heated to a high
temperature before entering the furnace, the cooling effect of the
nitrogen is greatly obviated, and consequently a much higher temperature
is obtained at the place of combustion, and the requisite intensity of
heat is at once produced, which is most effective in completing the
fusion and separation from each other of the slags and iron, and also in
accomplishing the reduction of the oxide. But Neilson found that the net
result of burning some fuel to heat the air before entering the furnace
was a great economy of the total fuel required for smelting the ore. He
had to encounter many difficulties in carrying his invention into
practice; the iron ovens first used for heating the air were rapidly
oxidized; and when thick cast iron pipes were substituted, these were
liable to leak at the joints on account of the expansions and
contractions caused by changes of temperature. Then the new invention
had as usual to contend with established prejudices and misconceptions;
but it soon came into use in Scotland, where it effected a great saving;
inasmuch as it was found possible to use with the hot blast raw coal of
a certain kind, plentiful in Scotland, because the heat retained by the
ascending gases sufficed to convert the coal at the top of the charge
into coke.
It will be remembered that the active agent in the reduction of the ore
is the carbonic oxide gas formed by the incomplete combustion of the
carbon of the fuel; or what comes to the same thing, the absorption by
carbonic acid first produced of another proportion of carbon. The carbon
oxide robs the iron oxide of its oxygen to become itself changed into
carbonic acid. In reality however the action is more complex than this
in its chemical relations; for instance, metallic iron will under
certain circumstances act conversely on carbonic acid, and rob it of
half its oxygen. The net result of the reactions between carbon, iron,
iron oxide, and these gases depends mainly upon the temperature and
pressure and upon the relative quantities of each substance present. In
the gases escaping from the blast furnace there is always a large
quantity (nearly one-third) of carbonic oxide. At the blast furnaces in
work during the first half of our century the combustible gases were
allowed to burn to waste as they issued from the top of the furnace, in
the manner shown in Fig. 17, and at night the flames used to form a
weird and striking feature in the prospect of an iron-smelting region.
Instead of allowing the escaping gases to burn to waste, it became the
practice about 1860, and so continues, to draw them off and burn them
under steam boilers or use their flames for heating the blast. An
effective method of withdrawing the gases is shown in Fig. 21, which is
a section through the upper part of a smelting furnace, with the “cup
and cone” arrangement. The mouth of the furnace is covered by a shallow
iron cone _a_, open at the bottom, into which fits another cone _b_,
attached to a chain _c_, sustained by an arm of the lever _d_, which is
firmly held in position by the chain _e_, and is also provided with a
counterpoise _f_. When the mouth of the furnace is thus closed, the
gases find an exit by the opening _g_, seen behind the cones, and
leading into a downward passage, through which they are drawn by the
draught of a tall chimney to the place where they are burnt. The charge
for the furnace is filled into the hopper _a_, and at the proper time
the chain, _e_, is slackened when the weight of the material resting on
the suspended cone overcomes that of the counterpoise, and the charge
slides down over the surface of the cone _b_, which is immediately drawn
up again by the counterpoise, so that the opening into the air is at
once closed.
[Illustration:
FIG. 21.—_Cup and Cone._
]
The march of improvement in the blast furnace has been characterized
particularly in Britain and the United States by a great increase of
dimensions, which is found to promote economy in fuel, etc. In the
former country the furnace of the latter part of our century is commonly
from 70 to 80 feet high, and some have even been built with a height of
more than 100 feet, while in the States the tendency to build very high
furnaces is still more marked. A single large furnace may turn out as
much as 1,500 tons of pig iron in a week, and some in America, it is
said, actually produce as much as 2,500 tons. The more usual output of a
blast furnace is however much less than these amounts; but if we say
only one-half, or even one-third of these quantities, a state of things
is indicated very different from what obtained about 1837, when the best
Welsh furnaces produced only 200 tons a week. If we go back to the
beginning of the century, the difference is much more marked, for the
blast furnaces of that period could turn out only about 30 tons in a
week.
The proportions of fuel, ore, and limestone charged into the furnace
vary greatly according to the composition of the ore, the quality of
iron aimed at, and the practice of each manufacturer. It is usual
previously to calcine the carbonate ores and others also, in order to
expel the carbonic acid and the moisture, of which last all contain a
considerable amount: and sometimes the limestone is mixed with the ore
to undergo this preliminary process. The charge being conveyed from the
roasting kilns to the blast furnace while still hot effects an obvious
economy of fuel in the latter. In the case of hæmatite ore the
quantities of materials in one charge may be something like 54 cwt. of
ore, 9 cwt. of limestone, and 33 cwt. of coke. It is quite common to use
mixtures of different kinds of ore, so as to modify the quality of the
product according to particular requirements. The use of the limestone
is to take up silica, and the slag is found to consist mainly of
silicates of lime and alumina. The amount flowing from a blast furnace
of course varies much according to the conditions, and is larger than
would commonly be supposed; for the production of one ton of pig iron
involves the production of from ½ to 1½ tons of slag.
Fig. 22 represents in section the later type of blast furnace, which of
course is circular in plan. Its height may be taken as 80 feet, and the
diameter at the widest part of the interior as 22½ feet, narrowed to 20
feet near the top. The lowest portion, C, is called the _crucible_, the
bottom of which is the _hearth_, both formed of the most refractory
materials obtainable. The conical widening, B, above the crucible is the
_boshes_, and at the top is seen the “cup and cone” apparatus already
described, A, surmounted by the short cylindrical iron mouth, through
apertures in which the charges are tipped from the gallery, D, these
having been raised there in small trucks by hydraulic or other
elevators. The escaping gases leave the furnace by the exit, E, which
leads into the “down-come,” G, and they are conducted from it to the
“regenerative stoves” and dealt with as presently to be described. Our
section represents the masonry of the furnace as sustained by pillars,
P, at the outside of the lower part; these pillars support a strong ring
of iron plates upon which the wall rests. This arrangement has the
advantage of allowing the workmen the greatest freedom of access to
parts about the crucible, which require much attention. Here, at the
lowest part, is an aperture from which the liquid iron is allowed to run
out every five or six hours, it being plugged in the meantime by clay
and sand. The slag being much lighter than the iron, floats above it,
and runs off at a higher level over the _tympstone_. Opening into the
hearth are several orifices to admit the hot blast from the nozzles of
the _tuyères_, which of course do not project into the furnace itself;
but they are so near to the region of intensest heat that they would be
rapidly destroyed unless they were surrounded by a casing through which
a current of water is constantly running. The _tuyères_, of which there
may be 3 or 5, are supplied from the pipe seen at K. The earlier plans
of heating the air did not permit of a very high temperature being given
to the hot blast, about 600° F. being the limit; but the “regenerative”
stoves can supply a blast of more than 1,600° F., or not far below the
melting point of silver. Another great increase has been in the pressure
of the blast; 2 or 3 lbs. per square inch sufficed in the earlier
practice; but the lofty modern furnaces have to be supplied with the
blast at a pressure of 10 lbs. per square inch, and over. Even when
comparatively low pressures were the rule, a large ironworks required
much blowing power. The works formerly at Dowlais, in South Wales, for
instance, had an engine of 650 horse-power for the blowing engine, in
which a piston of 12 feet diameter moved in a cylinder 12 feet in
length. The quantity of air that passes into a blast furnace amounts to
thousands of tons per week, its weight being much greater than that of
all the ore, coke, and limestone put together.
[Illustration:
FIG. 22.—_Section of Blast Furnace._
]
It need scarcely be said that great care and expense are bestowed on the
construction of these furnaces. Only the best and most refractory
materials, such as firebricks, are used for the lining, and the exterior
is a casing of solid masonry, strengthened with iron bands. When a new
furnace is finished it takes a month or six weeks to put it into
operation; but when this is done it will remain in action night and day
continuously for a long period—perhaps for eight or ten years—before the
necessity for repairs requires a “blow out.” And the blow out and
restarting, without the cost of repairs, entail an outlay of several
hundred pounds.
The gases leaving the throat of the furnace consist mainly of nitrogen
and a little carbonic acid, together with about one-third of their
volume of the combustible gases, carbonic oxide, and some hydrogen; but
these last do not leave the furnace in an ignited state, because the
oxygen there has already been consumed. They are conducted by the
“down-come” pipe, G, Fig. 22, to a point at which, by means of a valve,
they can be directed to one or other of two circular towers entirely
filled with firebricks, arranged chequerwise, so as to form innumerable
passages between them. The furnace gases are admitted at the bottom of
the Cowper tower, or “regenerative stove,” into a flue to which a
regulated quantity of air has access, and there they are fired: the
flame ascending the flue to the upper part of the tower, thence
descends, communicating its heat to the firebricks, which soon acquire a
very high temperature, especially where the flame first enters, and the
burnt gases leave the tower for a tall chimney, leaving most of their
heat in the firebricks. When this action has continued for a sufficient
time, the connection of the regenerator with the throat of the furnace
is cut off, and the escaping gases are directed into the other
regenerator, and at the same time the blast from the blowing engine is
made to ascend among the firebricks of the first, where gaining
increasing temperature as it ascends—the stove being hottest at the
top—the air leaves the tower to be conducted to the _tuyères_ at such
high temperature as already mentioned. While the one regenerator is thus
heating the blast, the other is in its turn accumulating heat from the
flames of the escaping gases; and thus they are worked alternately, the
action being constantly reversed after suitable intervals.
When iron is combined with a much smaller proportion of carbon than in
cast iron, and contains little or no graphitic or uncombined carbon, we
have the very useful compound known as steel. In the earlier half of the
century it was customary to distinguish steel from malleable iron on the
one hand, and cast iron on the other. If the compound contained from 0·5
to 1·5 per cent. of carbon, it was called steel by some authorities,
while others extended these limits a little on either side. Later it was
found that the presence of elements other than carbon can confer steely
properties on iron, and indeed it is possible to have a metal containing
no carbon, but possessing the characteristic properties of steel. Sir
Joseph Whitworth proposed to classify a piece of metal according to its
tensile strength, without any regard to either its chemical composition
or its mode of manufacture: if it could not bear more than 30 tons per
square inch it should be considered iron, but if it had a higher tensile
strength, it should then be regarded as steel. To estimate the
engineering value a figure depending upon the elongation or stretching
of the specimen before breaking was to be added to the number of tons of
the breaking load. This stretching power of steel is in some cases of as
much importance as the tensile strength: the ordnance maker, for
instance, considers a steel with a breaking strength of 53 tons under an
elongation of 5 per cent. as _for his purposes_ to be rejected: while a
specimen showing a breaking strain of only 30 tons along with an
elongation of 35 per cent., on 2 inches of length, he will regard as
good. The tensile strength of steel depends in part on its composition,
in part on the mode of manufacture, and in part on the subsequent
treatment. The _average_ tensile strength of a wrought iron bar per
square inch of section is about 25 tons (30 is the maximum); while the
like average for steel is 43 tons, and some kinds of cast steel will
bear nearly 60 tons. Steel bars of a certain temper subjected by Sir
Joseph Whitworth to a process of hardening in oil showed a tensile
strength of even 90 tons per square inch. These figures will suffice to
show the great utility of steel in structures and machines. But steel
has besides a characteristic property which makes it extremely valuable
in a great variety of applications, namely, its capability of being
_tempered_. If a piece of steel is heated to dull redness and suddenly
cooled by plunging it into cold water, it becomes so extremely hard that
it cannot be acted on by a file; nay, its hardness may be made to rival
that of the diamond, which is the hardest substance known. Now by a
second operation this hardness can be reduced to any required degree:
this is done by re-heating the metal to a certain moderate degree
between 430° F. and 630° F. and again cooling it by immersion in some
cooling medium. In this “letting down” process, it is the highest
temperature that produces the greatest softening, and the properties of
the tempered steel will depend upon the precise degree to which the
metal has been reheated. For example, if the product be required for
making into sword blades, or watch-springs, and to possess much
elasticity, the proper temperature is between 550° F. and 570° F.; but
if the steel is to be suitable for saws the temperature must range
within a few degrees of 600° F., according to the fineness of the tool
intended; a lower temperature would give a metal too hard for them to be
sharpened with a file. On the other hand, sharp cutting instruments and
tools for working metals are obtained hard by tempering at lower degrees
than springs. In practice the index of the temperature is taken from the
colour of the film of oxide that gradually forms on a polished surface
of the metal as the heat is raised, and begins by a very pale yellow (at
430° F.), passing through deeper shades into brown, then through purple
into deep blue (at 570° F.), etc. The reader will now see why watch and
clock springs have their deep blue colour, and he can observe for
himself the whole series of colours by very gradually heating a piece of
polished steel over a small flame.
If we compare the chemical composition of wrought iron and of cast iron
with that of steel as regards the content of carbon, we see at once that
steel holds an intermediate position, so that if in the puddling furnace
we could arrest the decarbonization at a certain point we should obtain
steel; or if, on the other hand, we could put back into chemical
combination with the decarbonized wrought iron a due percentage of
carbon we should in that way also obtain steel. And it will be observed
that the oldest primitive furnaces could not have failed sometimes to
have produced steel as the net or final result of such actions. In fact,
steel always has been and still is produced on one or other of these two
principles, applied in divers ways, but severally and distinctly
directed to that end. Of the many more or less modified processes of
steel-making that have been in use, we need here but briefly mention a
few which were _the_ processes of the first sixty years of our century,
and are to a considerable extent still in operation, although eclipsed
in importance by two other processes that, since the date referred to,
have been supplying the metal in enormously increased quantities, and
which will have to be particularly described.
The most usual of the older processes of steel-making, still carried on
at Sheffield and elsewhere, is known as the _cementation process_: it
consists in heating bars of the best wrought iron in contact with
charcoal, at a high temperature, for three or four weeks. At Sheffield
the iron bars and charcoal are packed in alternate layers into troughs
14 ft. long by 3½ ft. deep and wide, constructed of slabs of siliceous
sandstone 6 in. thick. The last layer of charcoal at the top is covered
to a certain depth with a layer of refractory matter, and the flames
from a furnace beneath are made to envelop the stone troughs or _pots_,
as they are technically called, for a period of a week or more according
to the thickness of the bars operated upon. These are generally 3 in.
broad and from five- to six-eighths of an inch thick. When it is found
by withdrawing a test bar for examination that the operation is
complete, the fire is gradually diminished and the whole allowed to cool
slowly, which requires about a fortnight. Instead of only charcoal, a
mixture of powdered charcoal or soot with a little salt has been used by
some makers—which mixture, technically called _cement powder_, has given
its name to the process. In some works 16 tons or more of iron are
treated in one operation. The bars are found unchanged in form, but
increased in weight by perhaps 27 lbs. per ton, for carbon has combined
with the iron, being apparently transferred in the iron from one
particle to another. The surface of the bars becomes rough and uneven
from a multitude of blebs or blisters, and hence they are called
_blister bars_, and the steel of which they now consist is named
_blister steel_. In this conversion we may suppose that the iron at its
outer surface first enters into combination with carbon taken from the
carbonic oxide gas, which would be produced by combustion of the
charcoal with the limited quantity of air in its interstices, and the
oxygen thus set free would immediately seize again on the surrounding
charcoal, and by repeated changes of this kind in which the oxygen acts
as a carrier of carbon to the iron, in which it is transferred inwards
from particle to particle. The cause of the blisters has been much
discussed: probably the cause is the formation and escape of a volatile
compound of carbon and sulphur at the surface of the soft metal; for it
is known that nearly the whole of the little sulphur in the wrought iron
disappears in the cementation process. Blister steel is never
homogeneous, for near the surface it always contains more carbon than
within; the bars are therefore broken up into short lengths which are
carefully assorted, bound together with wire, heated, welded together
under a hammer or by rolling, and finally formed into a bar, which is
stamped with the outline of a pair of shears, and is then known as
_shear steel_, because this product was generally found the most
suitable for making the shears used in dressing cloth.
Another method of dealing with the blister steel is to charge crucibles
or pots having covers with 50 or 100 lbs. weight of the broken-up bars,
and subject the crucibles to a strong heat in a reverberatory furnace,
when the metal melts, and at the proper moment the contents of a great
number of pots are almost simultaneously poured into a mould to form an
_ingot_. The result is a very uniform steel of the finest texture, known
and highly esteemed as _cast steel_ or _crucible steel_. This steel is
much more fusible than iron, but less so than cast iron.
The production of steel by arresting at a certain stage the
decarbonizing of cast iron in the puddling furnace requires much
experience on the part of the workman, who has to learn when the desired
point has been reached by certain indications, such as the appearance of
the flame, or by the examination of a small sample of the fluid metal
withdrawn and rapidly cooled. Various additions to the charge in
definite proportions are generally made, such as scales of iron oxide,
or a quantity of an oxide ore (hæmatite, etc.) or other materials, the
most essential for a good product consisting of a little manganese in
some form. The result is _puddled steel_; and this, like blister steel,
can be converted into cast steel by fusion in crucibles, running into
ingot moulds, and subsequent treatment by hammering, pressing, rolling,
etc. In 1864 puddled steel was described as an article of great
commercial importance, but this it soon lost by the introduction of
simpler, cheaper, and more reliable processes. The methods and
improvements proposed for the production of steel have been exceedingly
numerous, as is shown by the records of the English Patent Office alone,
which contain up to the end of 1856 specifications of ninety-two patents
for different steel-manufacturing processes, while from 1857 to 1865,
the epoch-marking period of steel making, seventy-four more patents were
obtained for this purpose. It would be quite beyond our limits to make
special reference to these, and to the numerous patents which have since
been granted, but there is one of great importance in steel-making which
must be mentioned, and that is the patent for the employment in the
cementation process of carbide of manganese, taken out by J. M. Heath in
1839. This made England almost independent of the former large
importations of Swedish and Russian iron, and it caused an immediate
reduction of £40 in the price per ton of good steel, effecting a saving
which up to 1855 is calculated at not less than £2,000,000. Heath was
one of those who fail to benefit by their inventions, for his was boldly
appropriated by another person who took advantage of a verbal flaw in
the specification, and Heath did not obtain any redress from the law
courts until, after ten years’ litigation, a majority of Exchequer
judges reversed all the previous decisions against him (1853). In the
meantime the man had died, but as the patent was about to expire his
widow was on petition granted an extension of it for seven years. The
nature of the influence of manganese on steel-making has not been fully
explained, and there is some diversity of opinion on the subject, as it
is said—on the one hand, merely to remove or counteract the injurious
effects of sulphur or phosphorus; on the other, to impart to the steel
greater ductility, strength, and power of welding, tempering, etc.
The manufacture of _crucible_ or _cast steel_ has been carried on at
Essen in Prussia by the firm of A. Krupp & Co., on a scale surpassing
anything attempted elsewhere,—theirs being the largest steel-works in
the world, and remarkable for the variety and excellence of its
products. It began in so small a way that it is said only a single
workman was employed. To the Great Exhibition of 1851, at London,
Krupp’s firm sent a block of crucible cast steel weighing 2¼ tons, a
larger mass of the metal than had ever been shown before, and looked
upon with no little astonishment, for at that time steel was a precious
commodity, the price of refined steel ranging from £45 to £60 per ton.
At the next London Exhibition, in 1862, the Essen Works showed a block
of cast steel 20 tons in weight, and at the Vienna Exhibition of 1873,
one of 52 tons. This casting, which was first made of a cylindrical
shape, was forged into an octagonal form under an immense steam-hammer,
larger than the Woolwich hammer described on a previous page, for the
weight of the moving part is no less than 50 tons. This huge mass of
cast steel was of the finest quality; the forging into the prismatic
form was to show its malleability, for it was intended for the body of a
gun to have a bore of 14 inches. Since the period referred to, ingots of
more than 100 tons have been cast. That shown at Vienna was the product
of some 1,800 crucibles, each containing 65 lbs. of melted steel, which
had to be poured into the mould in a regular and continuous stream, so
that the metal might solidify into a perfectly uniform mass. Such work
can be done only by trained men, who act in regular ranks with military
precision, and in pairs emptying their crucibles into channels
previously assigned, then filing off to the other end of the rank to
receive another crucible, while the pair of men who were behind are
pouring out theirs, and so on in succession. The crucibles are emptied
into a number of channels formed of iron lined with fire-clay, and
leading down into the mould. Many precautions have to be taken to ensure
the regular progress of the operations, and all the time required to
fill the huge moulds may be counted by minutes.
The headpiece to our chapter on Fire-Arms gives but a very inadequate
idea of the magnitude of the Essen Works about 1870. A better notion
will be obtained from a few figures which we select from a list giving
some of the contents of the Essen Works in 1876. There were 1,109
furnaces of various kinds, of which 250 were for smelting; 77 steam
hammers, 294 steam engines, 18 rolling mills, 365 turning lathes, and
700 other machine tools; 24 miles of ordinary gauge railway for traffic
within the works; together with 10 miles of narrow gauge railway; 38
miles of telegraph lines, with 45 Morse apparatus, etc. (J. S. Jeans’
_Steel: its History, etc._, 1880). These figures belong, be it observed,
to the state of things in 1876; but we learn from a later authority that
in 1894 these works employed 15,000 men, and we must suppose that the
plant has been proportionately increased since the earlier period, when
10,000 men were employed.
In the year 1854 a regular system of records began to be kept of the
amounts of coal and ores raised in Great Britain, and also of the
quantities of the various metals produced. These show that in 1894 very
nearly three times as much coal was raised as in 1854, and that in the
same period the quantity of British pig iron smelted annually had
increased four-fold; these increases look small when compared with the
expansion of the steel production in Britain within the same period of
forty years, for this had enlarged _thirty-fold_. This extraordinary
development is attributable to the introduction of two processes by
either of which various steels of excellent quality, and adapted to a
great range of applications, can be produced cheaply and with certainty.
These processes are respectively known as the Bessemer and the Open
Hearth, and the reader should observe that with the main principles
involved in these he has already been made acquainted.
Henry Bessemer, who first saw the light in England in 1813, may be said
to have been born an inventor, for his father was one before him—a
Frenchman employed in the royal mint at Paris, afterwards appointed by
the Revolutionary authorities to superintend a public bakery; on an
accusation of giving short weight, thrown into prison, from which, and
probably from the guillotine, he escaped, and found employment in the
English mint. Subsequently he devised some notable improvements in the
art of producing letterpress type, and for many years carried on a
prosperous business as a typefounder. The son developed inventive
faculties at a very early age: in lathe engraving, dies, dating stamps,
etc. His name became familiar to everyone by his production of the
metallic powder long known as “Bessemer’s Gold Paint.” It became known
to Bessemer that the raw material of this substance, which was then sold
at £5, 10_s._ per lb., really cost only about one shilling per lb., and
he set himself to discover its composition and mode of manufacture. He
succeeded in this so well that he could produce the article at the
insignificant cost of four shillings a pound, and his first order for a
supply of it was at the rate of £4 per lb., and the business was
continued, realising profits of something like 1,000 per cent. at first.
For this article no patent was taken out, but Bessemer himself, assisted
by two trustworthy workmen, carried on the manufacture in secret, and he
some time afterwards rewarded the fidelity of his men by handing over
the business to them as a free gift. Then he took out patents for
improvements in the manufacture of oils, varnishes, sugar, plate glass,
etc. Several of his machines for these purposes were shown at the London
Exhibition of 1851. Bessemer is said to have obtained altogether some
150 patents, including those granted for inventions connected with our
subject. He may be regarded as the type of the very fortunate inventor,
since on the patents of the one process we are going to describe he
ultimately obtained royalties to the value of more than £1,057,000, and
this irrespective of profits derived from commercially working it
himself.
At the time of the Crimean War, Bessemer had some experiments made at
Vincennes with cylindrical projectiles he had devised for firing from
smooth-bore guns, yet so as to impart to the projectile at the same time
rotation about its axis. The experiments were successful, but it was
pointed out that the guns of cast iron then in use would not bear heavy
projectiles, and he was induced, at the suggestion of the Emperor
Napoleon III., to undertake some researches with the view of finding
metal more suitable for artillery. Bessemer, having then little
knowledge of the metallurgy of iron, applied himself on his return to
England to the study of the best books on the subject, visited the
principal iron-working districts, and began a series of experiments at a
small experimental installation he set up in London. There, after
repeated failures, he did at length succeed in producing a metal much
tougher than the cast iron then used, and a small model gun was
submitted to the Emperor, who encouraged Bessemer to persevere with his
experiments; which he did, though the expense was a great tax on his
capital, continued as the experiments were for two years and a half. But
by this time he had acquired a knowledge of many important facts, and
these gradually led him to the experimental realization of the idea he
had conceived, but only after many trials in which several thousand
pounds were expended. At length the agenda of the British Association
for the Cheltenham meeting of 1856 announced that a paper would be read
by H. Bessemer, entitled “The Manufacture of Iron and Steel without
Fuel.” It will be easily understood that a title in such terms would
give rise to much derisive incredulity; and we may imagine the
iron-masters on that occasion crowding into Section G, while asking each
other in the spirit of certain philosophers of old, “What will this
babbler say?” Some of what he did say may here be quoted, as at once
explanatory and historically memorable.
“I set out with the assumption that crude iron contains about 5 per
cent. of carbon; that carbon cannot exist at a white heat in the
presence of oxygen without uniting therewith and producing combustion;
that such combustion would proceed with a rapidity dependent on the
amount of surface of carbon exposed; and lastly, that the temperature
which the metal would acquire would be also dependent on the rapidity
with which the oxygen and carbon were made to combine; and consequently,
that it was only necessary to bring the oxygen and carbon together in
such a manner that a vast surface should be exposed to their mutual
action, in order to produce a temperature hitherto unattainable in our
largest furnaces.
[Illustration:
FIG. 23.—_Experiments at Baxter House._
]
“With a view of testing practically this theory, I constructed a
cylindrical vessel of 3 ft. in diameter and 5 ft. in height, somewhat
like an ordinary cupola furnace (see Fig. 23). The interior is lined
with firebricks, and at about 2 in. from the bottom of it I inserted
five _tuyère_ pipes, the nozzles of which are formed of well-burned
fire-clay, the orifice of each _tuyère_ being about three-eighths of an
inch in diameter; they are so put into the brick lining (from the outer
side) as to admit of their removal and renewal in a few minutes, when
they are worn out. At one side of the vessel, about half-way up from the
bottom, there is a hole made for running-in the crude metal, and on the
opposite side there is a tap-hole, stopped with loam, by means of which
the iron is run out at the end of the process. In practice this
converting vessel may be made of any convenient size, but I prefer that
it should not hold less than one nor more than five tons of fluid iron
at each charge; the vessel should be placed so near to the discharge
hole of the blast furnace as to allow the iron to flow along a gutter
into it. A small blast cylinder is required capable of compressing air
to about 8 lbs. or 10 lbs. to the square inch. A communication having
been made between it and the _tuyères_ before named, the converting
vessel will be in a condition to commence work; it will however on the
occasion of its first being used after re-lining with firebricks be
necessary to make a fire in the interior with a few baskets of coke, so
as to dry the brickwork and heat up the vessel for the first operation,
after which the fire is to be all carefully raked out at the
tapping-hole, which is again to be made good with loam: the vessel will
then be in readiness to commence work, and may be so continued without
any use of fuel until the brick lining, in the course of time, becomes
worn away, and a new lining is required. I have before mentioned that
the _tuyères_ are situated nearly close to the bottom of the vessel, the
fluid metal will therefore rise some 18 in. or 2 ft. above them; it is
therefore necessary, in order to prevent the metal from entering the
_tuyère_ holes, to turn on the blast before allowing the fluid crude
iron to run into the vessel from the blast furnace. This having been
done, and the metal run in, a rapid boiling up of the metal will be
heard going on within the vessel, the metal being tossed violently about
and dashed from side to side, shaking the vessel by the force with which
it moves; from the throat of the converting vessel flame will
immediately issue, accompanied by a few bright sparks such as are always
seen rising from the metal when running into the pig-beds. This state of
things will continue for about fifteen minutes, during which time the
oxygen in the atmospheric air combines with the carbon contained in the
iron, producing carbonic oxide, or carbonic acid gas, and at the same
time evolving a powerful heat. Now, as this heat is generated in the
interior of, and is diffused in innumerable fiery bubbles through, the
whole fluid mass, the metal absorbs the greater part of it, and its
temperature becomes immensely increased, and by the expiration of the
fifteen minutes before named that part of the carbon which appears
mechanically mixed and diffused throughout the crude iron has been
entirely consumed: the temperature however is so high that the
chemically combined carbon now begins to separate from the metal, as is
at once indicated by an immense increase in the volume of flame rushing
out of the throat of the vessel. The metal in the vessel now rises
several inches above its natural level, and a light frothy slag makes
its appearance and is thrown out in large foam-like masses. This violent
eruption of cinder generally lasts about five or six minutes, when all
further appearance of it ceases, a steady and powerful flame replacing
the shower of sparks and cinder which always accompanies the boil. The
rapid union of carbon and oxygen which thus takes place adds still
further to the temperature of the metal, while the diminished quantity
of carbon present allows a part of the oxygen to combine with the iron,
which undergoes combustion and is converted into an oxide. At the
excessive temperature that the metal has now acquired, the oxide as soon
as formed undergoes fusion, and forms a powerful solvent of those earthy
bases that are associated with the iron; the violent ebullition which is
going on mixes most intimately the scoria and metal, every part of which
is thus brought in contact with the fluid oxide, which will thus wash
and cleanse the metal most thoroughly from the silicon and other earthy
bases which are combined with the crude iron, while the sulphur and
other volatile matters which cling so tenaciously to iron at ordinary
temperatures are driven off, the sulphur combining with the oxygen and
forming sulphurous acid gas.
“The loss in weight of crude iron during its conversion into an ingot of
malleable iron was found, on a mean of four experiments, to be 12½ per
cent., to which will have to be added the loss of metal in the finishing
rolls. This will make the entire loss probably not less than 18 per
cent. instead of about 28 per cent., which is the loss on the present
system. A large portion of this metal is however recoverable by heating
with carbonaceous gases the rich oxides thrown out of the furnace during
the boil. These slags are found to contain innumerable small grains of
metallic iron, which are mechanically held in suspension in the slags
and may be easily recovered.
“I have before mentioned that after the boil has taken place a steady
and powerful flame succeeds, which continues without any change for
about ten or twelve minutes, when it rapidly falls off. As soon as this
diminution of flame is apparent the workman will know that the process
is completed, and that the crude iron has been converted into pure
malleable iron, which he will form into ingots of any suitable size and
shape by simply opening the tap-hole of the converting vessel and
allowing the fluid malleable iron to flow into the iron ingot moulds
placed there to receive it. The masses of iron thus formed will be free
from any admixture of cinder, oxide, or other extraneous matters, and
will be far more pure and in a forwarder state of manufacture than a
pile formed of ordinary puddle bars. And thus it will be seen that by a
single process, requiring no manipulation or particular skill, and with
only one workman, from three to five tons of crude iron pass into the
condition of several piles of malleable iron in from thirty to
thirty-five minutes, with the expenditure of about a third part the
blast now used in a finery furnace, with an equal charge of iron, and
with the consumption of no other fuel than is contained in the crude
iron.
“To those who are best acquainted with the nature of fluid iron, it may
be a matter of surprise that a blast of cold air forced into melted
crude iron is capable of raising its temperature to such a degree as to
retain it in a perfect state of fluidity after it has lost all its
carbon and is in the condition of malleable iron, which, in the highest
heat of our forges, only becomes softened into a pasty mass. But such is
the excessive temperature that I am enabled to arrive at with a properly
shaped converting vessel and a judicious distribution of the blast, that
I am enabled not only to retain the fluidity of the metal, but to create
so much surplus heat as to remelt all the crop-ends, ingot-runners, and
other scrap that is made throughout the process, and thus bring them,
without labour or fuel, into ingots of a quality equal to the rest of
the charge of new metal....
“To persons conversant with the manufacture of iron, it will be at once
apparent that the ingots of the malleable metal which I have described
will have no hard or steely parts, such as are found in puddled iron,
requiring a great amount of rolling to blend them with the general mass,
nor will such ingots require an excess of rolling to expel cinder from
the interior of the mass, since none can exist in the ingot, which is
pure and perfectly homogeneous throughout, and hence requires only as
much rolling as is necessary for the development of fibre; it therefore
follows that, instead of forming a merchant bar, or rail, by the union
of a number of separate pieces welded together, it will be far more
simple and less expensive to make several bars or rails from a single
ingot. Doubtless this would have been done long ago had not the whole
process been limited by the size of the ball which the puddler could
make.
“The facility which the new process affords of making large masses will
enable the manufacturer to produce bars that, in the old mode of
working, it was impossible to obtain; while at the same time it admits
of the use of more powerful machinery, whereby a great deal of labour
will be saved and the process be greatly expedited.... I wish to call
the attention of the meeting to some of the peculiarities which
distinguish cast steel from all other forms of iron, viz., the perfectly
homogeneous character of the metal, the entire absence of sand-cracks or
flaws, and its greater cohesive force and elasticity, as compared with
the blister steel from which it is made,—qualities which it derives
solely from its fusion and formation into ingots, all of which
properties malleable iron acquires in like manner by its fusion and
formation into ingots in the new process; nor must it be forgotten that
no amount of rolling will give the blister steel, although formed of
rolled bars, the same homogeneous character that cast steel acquires by
a mere extension of the ingot to some ten or twelve times its original
length....
“I beg to call your attention to an important fact connected with the
new process which affords peculiar facilities for the manufacture of
cast steel. At that stage of the process immediately following the boil
the whole of the crude iron has passed into the condition of cast steel
of ordinary quality. By the continuation of the process the steel so
produced gradually loses its small remaining portion of carbon, and
passes successively from hard to soft steel, and from soft steel to
steely iron, and eventually to very soft iron; hence, at a certain
period of the process, any quality of metal may be obtained. There is
one in particular which by way of distinction I call semi-steel, being
in hardness about midway between ordinary cast steel and soft malleable
iron. This metal possesses the advantage of much greater tensile
strength than soft iron; it is also more elastic, and does not readily
take a permanent set, while it is much harder and is not worn or
indented so easily as soft iron; at the same time it is not so brittle
or hard to work as ordinary cast steel. These qualities render it
eminently well adapted to purposes where lightness and strength are
specially required, or where there is much wear, as in the case of
railway bars, which from their softness and lamellar texture soon become
destroyed. The cost of semi-steel will be a fraction less than iron,
because the loss of metal that takes place by oxidation in the
converting vessel is about 2½ per cent. less than it is with iron; but
as it is a little more difficult to roll, its cost per ton may fairly be
considered to be the same as iron; but as its tensile strength is some
30 or 40 per cent. greater than bar iron, it follows that for most
purposes a much less weight of metal may be used than that so taken. The
semi-steel will form a much cheaper metal than any we are at present
acquainted with. These facts have not been elicited from mere laboratory
experiments, but have been the result of working on a scale nearly twice
as great as is pursued in our largest iron works, the experimental
apparatus doing 7 cwt. in thirty minutes, while the ordinary puddling
furnace makes only 4½ cwt. in two hours, which is made into six separate
balls, while the ingots or blooms are smooth, even prisms, 10 in. square
by 30 in. in length, weighing about equal to ten ordinary puddle balls.”
The startling novelty of the methods and results described in this paper
had the effect of paralyzing discussion at the time. But soon the voice
of detraction was heard; many iron-masters ridiculed the idea of
producing iron and steel without fuel, and indeed it may have been
observed, the title of the paper notwithstanding, that first the silicon
and carbon, and then the iron itself, really supplied the fuel. And we
must remember that malleable iron in a molten state was then deemed an
impossibility, for the hottest furnaces then known could not effect the
fusion, however prolonged their action might be, yet Bessemer was to
obtain five tons in this condition in the short space of half an hour
with no other aid than cold air. Then it was said that Bessemer’s
process of forcing air into melted cast iron had no claim of novelty,
for it had been tried before and found valueless. Some iron-masters on
trying experiments on a small scale and with imperfect appliances met
with failures, and discredited the process at once; but five large
establishments paid for licences sums amounting to £26,500 within three
weeks of the reading of the paper. At the works of the Dowlais Iron Co.,
in South Wales, who were the first licensees, the first converter was
set up under Bessemer’s personal superintendence, and at the first
operation five tons of iron were produced direct from the blast furnace
pig. This apparently satisfactory result proved quite otherwise when
this iron came to be practically tested; for it was found quite useless!
It was both “_cold-short_” and “red-short,” to use the technical
terms,—the former of which means that although the sample may be welded,
it is when cold brittle and rotten; the latter means that at a low red
heat it breaks and crumbles under the hammer. Further trials were made,
new experiments instituted, but the success that attended Bessemer’s
early experiments could not be repeated, and as yet no one knew the
reason why. Now it so happened that in the preliminary experiments an
exceptionally pure pig iron had been made use of containing little or no
phosphorus or sulphur, substances very deleterious in iron, and still
more so in steel. With the capital obtained by the sale of his licences
Bessemer quietly set to work to investigate the cause of his
non-success, making daily experiments with a ton or two of metal at a
time. These experiments extended over a period of two and a half years,
and upon them Bessemer and his partner spent about £16,000, besides the
£4,000 the preliminary researches had cost. But all difficulties were at
length overcome, and the process was now found capable of turning out
pure iron and steel when the pure pig iron of Sweden was used in the
converter. In the meantime the licensees had made no attempts
practically to carry out the process, which began to be denounced as
visionary: it was “a mare’s nest”; it was “a meteor that had passed
through the metallurgical world, but had gone out with all its sparks.”
When Bessemer again brought the subject before the public, he found that
no one believed in it; everyone said, “Oh, this is the thing that made
such a blaze two or three years ago, and which was a failure.” Neither
iron-makers nor steel-makers would now take it up. Bessemer and his
partner thereupon joined with three other gentlemen to establish at
Sheffield a steel-works of their own, where the invention should be
carried into full practice. In due time works were erected, and they
commenced to sell steel, receiving at first very paltry orders, for such
quantities as 28 lbs. or 56 lbs.; but the orders soon became larger, and
afterwards very much larger, for they were underselling the Sheffield
manufacturers by £20 a ton, and their steel was undistinguishable from
the higher priced article. Bessemer had now bought his licences back
again, and in the course of his second set of experiments had patented
each improvement as it occurred to him, finally bringing the mechanical
apparatus to the degree of efficiency requisite for practical working,
without which his primary idea would have been valueless. Before
directing the reader’s attention to the form the apparatus had assumed,
we may transcribe what Mr. Jeans, in the work above referred to, has
told about the commercial success of the Bessemer steel-making firm:—
“On the expiration of the fourteen years’ term of partnership of this
firm the works, which had been greatly increased from time to time out
of revenues, were sold by private contract for exactly twenty-four times
the amount of the whole subscribed capital, notwithstanding that the
firm had divided in profits during the partnership a sum equal to
fifty-seven times the gross capital, so that by the mere commercial
working of the process, apart from the patent, each of the five partners
retired after fourteen years from the Sheffield works with eighty-one
times the amount of his subscribed capital, or an average of nearly
cent. per cent., every two months,—a result probably unprecedented in
the annals of commerce.”
[Illustration:
FIG. 24.—_Bessemer Converter._
A, Front view, showing the mouth, _c_; B, Section.
]
The form of the Bessemer apparatus as it finally left the inventor’s
hands may now be considered: but in certain details and arrangements
some modifications, dictated by the experience and requirements of
individual establishments, have been made, leaving the principles of the
apparatus unchanged. Thus instead of making the converting vessel turn
on trunnions, it is sometimes constructed fixed, the fluid metal after
conversion being let out at a tap-hole; the number and size of the
_tuyères_ are varied; and so with the disposition of the air chamber or
_tuyère_ box, the pressure of the blast, the capacity of the converter
itself, etc. In capacity converters vary between 2½ tons and 10 tons;
one of medium size is shown in elevation and section in Fig. 24, and may
be described as an egg-shaped vessel about 15 ft. high and 6 ft.
diameter inside. It is strongly made of wrought iron in two parts bolted
together, and is lined inside with some thick infusible coating, of
which more is to be said presently. The converter swings on trunnions,
one of which is hollow, and admits the blast by the pipe _b_ to the base
of the vessel, whence it passes through the passages shown at _e_. The
thickness of the lining at _e_ may perhaps be 20 in., and passages for
the air are perforated in fire-clay _tuyères_, of which there may be
seven, each with seven perforations of half an inch diameter. To the
other trunnion is attached a toothed wheel which engages the teeth of a
rack receiving motion from hydraulic pressure. The iron for the
operation is melted in a furnace having its hearth above the level of
the converter; and to receive its charge the latter is turned so that
the molten cast iron may be poured in from a trough until its surface is
nearly on a level with the lowest of the _tuyères_. The blast having
been turned on, the hydraulic power is set to work and the converter is
slowly brought to an upright position. The pressure of the current of
air prevents any of the fluid metal from entering the blow-holes. The
blast of cold air is continued until all the silicon and carbon have
been removed by oxidation. If the production is to be steel, there is
then added to the contents of the converter, placed in position to
receive it, a certain weight of melted cast iron of a special
constitution, and the blow is resumed for a few minutes; or in more
recent practice this special metal is added to the fluid metal run out
of the converter into a spacious ladle in known quantity. On this
addition an intense action takes place, attended by an extremely
brilliant flame and a throwing out of cinder or slag. The metal thus
added to the decarbonized iron is a carbonized alloy of iron and
manganese obtained from an ore naturally containing the latter metal,
and scarcely any phosphorus or sulphur. The charcoal pig from this ore
is called _spiegeleisen_ (German = mirror-iron) from its brilliant
reflecting facets; it contains from 12 to 20 per cent. of manganese,
with about 5 per cent. of carbon, and a considerable proportion of
silicon. An exact chemical analysis of the particular spiegeleisen
having been previously made, it is known what proportion of it is to be
added to the decarbonized iron in order to convert this into a steel
with any required content of carbon. The manganese probably acts by
combining with oxide of iron diffused through the mass, and together
with the silicon forming the very easily separated slag which is
ejected.
The whole series of operations connected with the Bessemer process may
be easily followed by the help of Fig. 25, which is taken from a
beautiful model in the Museum of Practical Geology. This model, which
was presented to the museum by Mr. Bessemer himself, represents every
part of the machinery and appliances of the true relative sizes. C is
the trough, lined with infusible clay, by which the liquid pig iron is
conveyed to the converters, A. The hydraulic apparatus by which the
vessels are turned over is here below the pavement, but the rack which
turns the pinion on the axis of the converter is shown at B. The vessel
into which the molten steel is poured from the converter is marked E,
and this vessel is swung round on a crane, D, so as to bring it exactly
over the moulds, placed in a circle ready to receive the liquid steel,
which on cooling is turned out in the form of solid ingots. The valves
which control the blast, and those which regulate the movements of the
converter through the hydraulic apparatus, are worked by the handle seen
at H. The crane, or revolving table, D, is also under perfect control,
so that the crude pig iron is converted into steel, and the moulds are
filled with a rapidity and ease that are positively marvellous to a
spectator.
[Illustration:
FIG. 25.—_Model of Bessemer Steel Apparatus._
]
The development of the Bessemer process soon had the effect of so
reducing the price of steel that this material came into use for almost
every purpose for which iron had previously been employed, such as
railway bars, girders, etc., for bridges, boiler plates, etc., for all
which “steely iron” containing only 0·12 to 0·40 per cent. carbon proved
admirably adapted. The practical success of the Bessemer process had not
long been demonstrated commercially by the inventor and his partners at
Sheffield before other firms began the manufacture: so that in 1878
there were in Great Britain alone twenty-seven establishments making
Bessemer steel and using 111 converters. It may give an idea of the
magnitude the Bessemer steel manufacture had attained even at that time
if we quote the cost of erecting a complete plant for two 5–ton
converters: it was £44,400, as given in a detailed estimate. In all
these cases pig iron from ores free from phosphorus and sulphur had to
be used, for as we have seen the converter failed to eliminate these
vitiating elements. Imported pig ores had in general to be used, or pig
from the limited supply of British hæmatite ores in West Cumberland. The
Barrow Hæmatite Steel Company engaged in the production of Bessemer
steel on a very large scale, having by 1878 erected no fewer than
sixteen converters of the capacity of 6 tons each. In the meanwhile many
efforts were made to discover some method of eliminating phosphorus, so
that the ordinary qualities of British pig iron, and iron derived in any
part of the world from the coarse phosphorized ores, might be available
for the converter. Many of the methods then devised proved correct in
principle and feasible in practice; but as, for sundry reasons, none of
them came extensively into use, we need not here allude to them further.
The solution of the problem was announced in 1879. Some years before, G.
J. Snelus had come to the conclusion that with a siliceous lining it
would be impossible to eliminate phosphorus in the Bessemer converter,
and that some refractory substance of a basic character must be sought
for in order that the slag produced should be in a condition to absorb
the phosphoric acid as fast as it is produced. He patented in 1872 the
use of magnesian limestone as a material for the lining; as that
substance when intensely heated became very hard and stony, being in
that condition quite unaffected by water. Two young chemists, Messrs.
Thomas and Gilchrist, apparently without being aware of Mr. Snelus’s
conclusions, had also convinced themselves that the chief deficiency in
the Bessemer process was due to the excess of silica in the slag, and in
1874 they began to try the effect of basic linings, and also of basic
additions, such as lime, etc., to the charge in the converter, so that
the lining itself should not be worn out by entering into the slag.
Their results proved that phosphorus could be eliminated when the slag
contained excess of a strong base. An example of an operation at
Bolckow, Vaughan, & Company’s Eston works with the highly phosphorized
Cleveland pig iron may be quoted. The basic-lined converter received
first 9 cwt. of lime, then 6 tons of metal. When the blast at 25 lbs.
pressure was turned on, the silicon began at once to burn; for three
minutes the carbon was not affected, but for fourteen minutes longer it
regularly diminished, the silicon keeping pace with it. After the blow
had been continued for thirteen minutes from the commencement, the
converter was turned down to allow of the further introduction of 19½
cwt. of a mixture of two parts of lime with one of oxide of iron. So
long as 1·5 per cent. of carbon remained in the metal the phosphorus was
untouched, and at the end of the blow, _i.e._ when the flame dropped,
only one-third of it had been eliminated; it still formed 1 per cent. of
the metal. The blast continued for another two minutes brought it down
to ¼ per cent., and in one more minute only a trace was left. Most of
the sulphur was got rid of at the same time. From Cleveland pig, thus
de-phosphorized in the Bessemer converter, large quantities of steel
rails were rolled for the North Eastern Railway Company, and were found
entirely satisfactory, being as good as those made from the Cumberland
hæmatite steel. This de-phosphorized process has been brought into
operation wherever phosphoric ores are dealt with, and it has been
applied with equal success in the “open hearth” furnaces, of which we
have now to speak.
All discoveries and all inventions may be traced back to preceding
discoveries and inventions in an endless series, and it is only by its
precursors that each in its turn has been made possible. If we take one
of the greatest marvels brought into existence at nearly the close of
our epoch, namely, “wireless telegraphy,” we may follow up links of a
chain connecting it with the recorded observations of an ancient Greek
(Thales) who flourished seven centuries before our era, and even these
may not have been original discoveries of his. And it will have been
gathered from what has already been said that steel must have been
produced, however unwittingly, at the earliest period at which man began
to reduce iron from its ores. So the very latest, and for many purposes
the most extensively practised, process of modern steel-making, brought
indeed to working perfection mainly by the perseverance and scientific
insight of two individuals, is the result of the observation and the
accumulated experience of former generations. The observations and
experience here alluded to are chiefly those that follow two lines: one
concerning the properties of the metal itself, the other relating to the
means of commanding very high temperatures on a great scale. On this
occasion we are able almost to lay a finger on some proximate links of
the chain. Réaumur, the French naturalist, made steel in the early part
of the eighteenth century by melting cast iron in a crucible, and in
this liquid metal he dissolved wrought iron, the product being, as the
reader will now easily understand, the intermediate substance, steel;
and this was obtained of course at a temperature which was incapable of
fusing wrought iron by itself. He published in 1722 a treatise on “The
Art of converting Iron into Steel, and of softening Cast Iron.” For
this, and certain other metallurgical discoveries, Réaumur received a
life-pension equivalent to about £500 per annum,—a treatment very
different from that dealt out by the British to Henry Cort. The action
in Réaumur’s crucible is precisely that used on the large scale in
Siemens’ open hearth. But this last became possible only when Siemens
had worked out his “regenerative stove” or heat accumulator, the
development of an idea suggested by a Dundee clergyman in 1817.
A general notion of the Siemens’ regenerative stove will have been
already gained from the account given before of its application to the
modern type of blast furnace. Of the inventor himself, C. William
Siemens, it may be observed that he was one of a family of brothers, all
remarkable for their scientific attainments, and in many of his
researches and processes he was aided by his brothers Frederick and
Otto. In our article on “Electric Power and Lighting” there will be
found some notice of a few of Siemens’ inventions pertaining to those
subjects. A still more admirable invention of his is the electric
pyrometer, an instrument of the utmost utility for measuring, with an
accuracy previously unapproachable, the high temperature of furnaces,
etc. Indeed there are few departments of science, pure or applied, which
have not been enriched by the researches and contrivances of this
distinguished man, whose merits were acknowledged by the bestowal upon
him of the highest scientific and academical honours, and also of a
title, for he became Sir William Siemens.
[Illustration:
FIG. 26.—_Section of Regenerative Stoves and Open Hearth._
]
Siemens was much engaged from 1846 in conjunction with his brother
Frederick in experimental attempts, continued over a period of ten
years, at the construction of the regenerative gas furnace. At length,
in 1861, he proposed the application of his furnace to an “open hearth,”
and during the next few years some partial attempts to carry out his
process were made, and he himself had established experimental works at
Birmingham in order to mature his processes, while Messrs. Martin of
Sireuil, in France, having obtained licences under Siemens’ patents,
gave their attention to a modification of his process, by which they
succeeded in producing excellent steel. Siemens having in 1868 proved
the practicability of his plans by converting at his Birmingham works
some old phosphorized iron rails into serviceable steel, a company was
formed, and in 1869 the Landore Siemens’ Steel Works were established at
Landore in Glamorganshire, and a few years after, these had sixteen
Siemens open hearth melting furnaces at work, giving a total output of
1,200 tons of steel per week. The number of furnaces was subsequently
increased. Extensive works specially designed for carrying out the
Siemens and the Siemens-Martin process were shortly afterwards erected
at other places, as at Newtown, near Glasgow, Panteg in Wales, etc. In
Great Britain the open hearth process gradually gained upon the
Bessemer, until in 1893, when the total output of both kinds amounted to
nearly 3,000,000 tons, this was almost equally divided between them, and
since that period the steel made by the former has greatly surpassed in
amount that made by the latter.
How the regenerative stove, or heat accumulator, works, and how it is
applied in the open hearth process, the reader may learn by aid of the
diagram Fig. 26, in which however no representation of the disposition
of the parts in any actual furnace is given, nor any details of
construction beyond what is necessary to make the principle clear. On
the right and on the left of the diagram will be seen a pair of similar
chambers which are shown as partly below the level of the ground S S´,
such being a usual disposition. The outer walls of these chambers are
thick and the interior is entirely lined with the most refractory
fire-bricks, of which also is formed the partition in between each pair
of compartments, as well as the passages from the top of each opening on
the furnace H. Each chamber or compartment is filled with rows of
fire-bricks, laid chequerwise so as to leave a multitude of channels
between. At the bottom of the chamber on the left let us suppose
atmospheric air to be admitted by the channels A, A, A, and a
combustible gas which we may take to be a mixture of carbonic oxide with
some hydrogen is admitted in the same way to the second compartment on
the left through the passages G, G, G. Supposing the apparatus quite
cold in the first instance, the gas would ascend into the furnace H as
shown by the arrows, because it might be drawn by an up-draught in a
chimney connected with the six chambers shown at the bottom of the
right, and it would also tend to rise up into the space H by its lighter
specific gravity, and there it could be set on fire, when a volume of
flame would pass across to the right, a plentiful supply of air rushing
in through the air chamber from A, A, A, and the products of the
combustion, mainly hot carbonic acid gas and hot nitrogen gas, in
passing through the right-hand chambers, would make the bricks in both
compartments very hot after a time, for the current would divide itself
between the two passages, as indicated by the divided arrow. We have not
shown the valves by which the workman is able, by merely pulling a
lever, to shut off the air supply from A, A, A, and of gas from G, G, G,
and put these channels into direct communication with the up-draught
chimney, at the same time supplying gas at G´, G´, G´, and air at A´,
A´, A´. These rise up among the now heated bricks each in its own
compartment, but mix where they enter the furnace H, now hot enough to
set them on fire, and the gaseous products of combustion, hotter now
than before, descend among the fire-bricks of the left-hand
compartments, heating them in turn. After another period, say half an
hour, the valves are again reversed, and again gas and air both heated
burn in the space H, and their products supply still more heat to the
right-hand compartments. And so the action may be continued with a great
temperature each time produced by the combustion of the combining bodies
at increasingly higher temperatures. Thus, if cold gas and air by
combination give rise to 500° of heat, when the same combine, at say the
initial temperature of 400°, the result would be a temperature of 900°;
if burnt at this latter degree, then 900° + 500° would be reached, and
so on. It would seem as if there were no limit to the temperatures
obtainable in this way. But the nature of the materials of which the
furnace is constructed imposes a limit, for even the most refractory
matters yield at length, and the working would come to an end by the
fusing of the brickwork. The diagram is a section through the length of
the hearth (for it is usually oblong in plan), and the low arch above H
being exposed to the fiercest heat, is formed of the most refractory
“silica bricks,” that is, bricks made of coarsely ground silica held
together with a little lime; yet this extremely resisting material is
acted upon, and the arch has to be renewed every few months or sometimes
weeks. The hearth itself is supported by massive iron plates, shown in
the diagram by the thick lines, above which is laid a deep bed L, of
quartz sand or ganister, or where required a _basic_ lining, beaten hard
down, and forming a kind of basin with sides sloping down in all
directions to a point immediately below the centre of the fire-brick
door D, where is the aperture for tapping, stopped by a mixture of sand
and clay until the metal is ready for drawing off, when it runs outside
into an iron spout lined with sand and is received into the ingot
moulds. B in the figure represents the “bath,” as it is called, of
molten metal, which, in the larger furnaces, where 20 tons of metal is
operated on at once, may occupy an area of 150 square ft.
It need hardly be mentioned that there has to be a certain adjustment
between the volumes of air and of gas that pass into the regenerative
stoves, in order that the best effect may be obtained. Besides the limit
of temperature occasioned by the nature of the materials, there is a
chemical reason why the regenerative stoves cannot increase the
temperature indefinitely. It is noticed that when the temperature of the
furnace has become very high indeed, the flame over the hearth assumes a
peculiar appearance, being interrupted by dark spaces. These are
attributable to what is called in chemistry “dissociation,”—in this case
the dissociation of carbonic acid gas, which by the heat alone separates
into carbonic oxide and oxygen gases. In the same way these gases refuse
to combine if brought together heated beyond a certain temperature. This
phenomenon of dissociation is a general one, for it is found that for
any pair of substances there is a characteristic range of temperature
above or below which they refuse to combine. The gas used in these
stoves is either unpurified coal gas, or that produced by passing steam
over red-hot coal or coke.
We have spoken of the Siemens and the Siemens-Martin open hearth
processes. In the latter a charge of pig iron, say 1½ tons, is first
melted on the hearth, then about 2 tons of wrought iron is added in
successive portions, and in like manner nearly as much scrap steel
(_i.e._ turnings, etc.), the final addition being half a ton of
spiegeleisen containing 12 per cent. of manganese. A furnace of
corresponding dimensions will allow of three charges every twenty-four
hours. In the Siemens process it is not wrought iron or steel scrap that
is mainly used to decarbonize the pig, but a pure oxide ore. This is
thrown into the bath of molten metal in quantities of a few cwts. at a
time, when a violent ebullition occurs. When samples of the metal and of
the slag are found to be satisfactory, spiegeleisen or ferro-manganese
is added, and the charge is cast. This process takes a rather longer
time than the former, but gives steel of more uniform character. In both
processes, phosphorus is oxidized at the high temperature attained and
passes into the slag, which last floats of course on the molten metal
and is from time to time tapped off as the action proceeds.
[Illustration:
FIG. 26_a_.—_Rolling Mill._
]
Fig. 26_a_ shows a rolling mill with what is called a “two-high” train
for finishing bars by passing them between the grooves cut in the rolls
to give the required section. The rolls in the illustration turn in one
direction only, and therefore the bars after emerging from the larger
grooves have to be drawn back over the machine and set into a smaller
pair from the same side. This inconvenience is avoided in the
“three-high train,” on which three rolls revolve, and the bars can be
passed through them from one side to the other alternately. The celerity
with which a glowing steel ingot is without re-heating converted into a
straight steel rail 60 or 100 feet long, by passing a few times
backwards and forwards between the rolls, is very striking. These rolls
are made of solid steel, and in some cases have a diameter of 26 inches
or more.
_IRON IN ARCHITECTURE._
Everyone knows how much iron is used in those great engineering
structures that mark the present age, and of which a few examples will
be described in succeeding articles. One other feature of the nineteenth
century is the use of iron in architecture. Some have, indeed, protested
against the use of iron for this purpose, and would even deny the name
of architecture to any structure obviously or chiefly formed of that
material. Stone and wood, they say, are the only proper materials,
because each part must be wrought by hand, and cannot be cast or
moulded; and further, iron being liable to rust, suggests decay and want
of permanence, and these are characters incompatible with noble
building. All this can rest only on a relative degree of truth—as, for
instance, machinery is used to dress and shape both wood and stone, and
the permanence of even the latter is as much dependent on conditions as
that of iron. Iron used in architecture is hideous when applied in
shapes appropriate only to stone; but when it is disposed in the way
suggested by its own properties, and receives ornament suitable to its
own nature, the result is harmonious and graceful, and the structure may
display beauties that could be attained by no other materials. Be that
as it may, the great and lofty covered spaces that are required for our
railway stations and for other purposes could have been obtained only by
the free use of iron, and everyone can recall to mind instances of such
structure not devoid of elegance, in spite of the absence—the proper
absence—of the Classic “orders” or Gothic “styles.” The first notable
instance of the application of iron on a large scale was the erection of
the “Crystal Palace,” in Hyde Park, for the great Exhibition of 1851. It
was taken down and re-erected at Sydenham, and there it has become so
well known to everyone that any description of it is quite unnecessary
in this place.
As another conspicuous example of what may be done with iron, the Eiffel
Tower at Paris may be briefly described.
The idea of erecting a tower 1,000 feet high was not of itself new. It
had been entertained in England as early as 1833, in America in 1874,
and in Paris itself in 1881. It has been reserved for M. Gustave Eiffel,
a native of Dijon, who commenced to practise as an engineer in 1855, to
realize this ambitious project. He has long been occupied in the
construction of great railway bridges and viaducts, and in these he has
adopted a system peculiar to himself of braced wrought-iron piers
without masonry or cast-iron columns. He also was the first French
engineer to erect bridges of great span without scaffolding. In the
Garabit viaduct he planned an arch of 541 feet, crossing the Truyère at
a height of nearly 400 feet above it. One result of M. Eiffel’s studies
in connection with these lofty piers was his proposal to erect the tower
for the Paris Exhibition of 1889. This proposal met with great
opposition on the part of many influential people in Paris—authors,
painters, architects, and others protesting with great energy against
the modern Tower of Babel, which was, as they said, to disfigure and
profane the noble stone buildings of Paris by the monstrosities of a
machine maker, etc. etc. The Eiffel Tower is now constructed, and no one
has heard that it has dishonoured the monuments of Paris, for it has
been instead a triumph of French skill, the glory of its designer, and
the wonder of the Exhibition.
[Illustration:
FIG. 26_b_.—_The Eiffel Tower in course of construction._
]
The tower rests on four independent foundations, each at the angle of a
square of about 330 feet in the side, and it may be noted that the two
foundations near the Seine had to be differently treated from the other
two, where a bed of gravel 18 feet thick was found at 23 feet below the
surface, and where a bed of concrete, 7 feet thick, gave a good
foundation. The foundations next the river had to be sunk 50 feet below
the surface to obtain perfectly good foundations. Underlying the whole
is a deep stratum of clay; but this is separated from the foundations by
a layer of gravel of sufficient thickness. Above this are beds of
concrete, covering an area of 60 square metres, and on the concrete
rests a pile of masonry. Each of the four piles is bound together by two
great iron bars, 25 feet long and 4 inches diameter, uniting the masonry
by means of iron cramps, and anchoring the support of the structure,
although its stability is already secured by its mere weight. The tower
is of curved pyramidal form, so designed that it shall be capable of
resisting wind pressure, without requiring the four corner structures to
be connected by diagonal bracing. The four curved supports are, in fact,
connected with each other only by girders at the platforms on the
several stages, until at a considerable length they are sufficiently
near to each other to admit the use of the ordinary diagonals. The work
was begun at the end of January, 1887, and M. Eiffel notes how the
imagination of the workmen was impressed by the notion of the vast
height of the intended structure. Not steel, but iron is the material
used throughout, and the weight of it is about 7,300 tons, without
reckoning what is used in the foundations, and in the machinery
connected with the lifts, etc. It has long ago been found that stone
would be an unsuitable material for a structure of this kind, and it is
obvious that only iron could possibly have been used to build a tower of
so vast a height and within so short a space of time, for it was
completed in April, 1889. A comparison of heights with the loftiest
stone edifices may not be without interest. The highest building in
Paris is the dome of the Invalides, 344 feet; Strasburg Cathedral rises
to 466 feet; the Great Pyramid to 479 feet; the apex of the spire in the
recently completed Cathedral at Cologne to 522 feet. These are
overtopped by the lofty stone obelisk the Americans have erected at
Washington, which attains a height of more than 550 feet. Such spires
and towers have been erected only at the cost of immense labour. But
iron, which can be so readily joined by riveting, lends itself
invitingly to the skill of the constructor, more particularly by reason
of the wonderful tensile strength it possesses. It is scarcely possible
to convey any adequate idea of the great complicated network of bracings
by which in the Eiffel Tower each standard of the columns is united to
the rest to form one rigid pile. The horizontal girders unite the four
piers in forming the supports of the first storey some 170 feet above
the base. The arches which spring from the ground and rise nearly to the
level of these girders are not so much intended to add to the strength
of the structure as to increase its architectural effect. The first
storey stands about 180 feet above the ground, and is provided with
arcades, from which fine views of Paris may be obtained. Here there are
spacious restaurants of four different nationalities. And in the centre
of the second storey (380 feet high) is a station where passengers
change from the inclined lifts to enter other elevators that ascend
vertically to the higher stages of the tower. On the third storey, 900
feet above the ground, there is a saloon more than 50 feet square,
completely shut in by glass, whence a vast panorama may be contemplated.
Above this again are laboratories and scientific observatories, and,
crowning all, is the lighthouse, provided with a system of optical
apparatus for projecting the rays from a powerful electric light. This
light has been seen from the Cathedral at Orléans, a distance of about
70 miles.
[Illustration:
FIG. 26_c_.—_The Eiffel Tower._
]
The buildings of the Paris Exhibition of 1889 are themselves splendid
examples, not only of engineering skill, but of good taste and elegant
design in iron structures and their decorations. The vast _Salle des
Machines_ (machinery hall) exceeds in dimensions anything of the kind in
existence, for it is nearly a quarter of a mile long, and its roof
covers at one span its width of 380 feet, rising to a height of 150 feet
in the centre. This great hall is to remain permanently, as well as the
other principal galleries with their graceful domes.
The Eiffel Tower having proved one of the most striking features of the
great Paris Exhibition, and of itself a novelty sufficient to attract
visitors to the spot, and having, long before the Exhibition closed,
completely defrayed the expense of its construction, with a handsome
profit besides, its success has naturally provoked similar
enterprises,—as, for instance, at Blackpool, a seaside resort in
Lancashire, there has been erected an openwork metal tower, resembling
the Paris structure, but of far less altitude.
_Tall Buildings in American Cities._
In several of the great cities of the United States, the last few years
have witnessed a novel and characteristic development of the use of iron
in architecture. In many structures on the older continent, this
material has been frankly and effectively employed, forming the obvious
framework of the erection, even when the leading motive was quite other
than a display of engineering skill. The Crystal Palace at Sydenham and
other erections have been referred to, in which iron has taken its place
as the main component of structures designed more or less to fulfil
æsthetic requirements: the guiding principle in “tall office buildings”
in the cities of the Western continent is, on the contrary, avowedly
utilitarian. Iron has, of course, long been used in the form of pillars,
beams, etc., in ordinary buildings, and it is only the extraordinary
extension of this employment of it, after the lift or elevator had been
perfected, and the ground-space in great commercial centres was daily
becoming more valuable, that has led to the erection of structures of
the “sky-scraper” class in American cities. For a given plot at a stated
rent, a building of many stories, let throughout as offices, will
obviously bring to its owner a greater return than one of few stories.
The elevators now make a tenth story practically as accessible as a
third storey, and the tall building readily fills with tenants. No claim
for artistic beauty has been advanced for these structures, which aim
simply at being places of business, and if provision be made for
sufficient floor-space and daylight, and for artificial lighting,
heating, and ventilation, together with the ordinary conveniences of
modern life, and ready elevator service, nothing more is required by the
utilitarian spirit, that seeks only facilities for money-getting. These
tall buildings are usually erected on plots disproportionately small,
and the architectural effect is apt to be bizarre and incongruous,
especially when the structure shoots up skyward in some comparatively
narrow street amid more modest surroundings. They are really engineering
structures, but invested with features belonging to edifices of quite
another order of construction. If they are necessities of the place and
period, and are “come to stay,” it cannot be doubted but that decoration
of an appropriate and harmonious character will, in course of time, be
evolved along with them, when the conventionality that clings to
architecture shall be broken through, and a new style appear, as
consistent, and therefore as beautiful, in relation to the “tall office
building,” as were those of the Greek temple and the Gothic minster in
their free and natural adaptation.
[Illustration:
PLATE IV.
THE AMERICAN TRACT SOCIETY BUILDING.
]
[Illustration:
FIG. 26_d_.—_St. Paul Building, N. Y._
]
Here, apparently, is the opportunity for the advent of a new and
characteristic style. There is great ingenuity displayed in the
arrangement and internal finish of these buildings. But besides the
somewhat novel application of iron, the most notable circumstances
regarding them are the tendency to make them of greater and greater
height, and the wonderfully short time in which, upon occasion, they can
be run up. Chicago has recently been noted for its tall edifices, among
which may be named _The Reliance Building_, erected upon a site only 55
feet in breadth, but rising in fourteen stories to the height of 200
feet, and presenting the appearance of a tower. There are no cast iron
pillars, but the whole metal framework is of rolled steel, the columns
consisting of eight angle-sections, bolted together in two-story
lengths, adjoining columns breaking joint at each floor, and braced
together with plate girders, 24 inches deep, bolted to the face of the
columns, with which they form a rigid connection. Externally, the
edifice shows nothing but white enamelled terra-cotta and plate glass.
This building was originally a strongly-built structure of five stories,
the lower one being occupied as a bank. The foundations and the first
story were taken out, and prepared for the lofty edifice, the
superstructure being the while supported on screws. Then the three upper
stories were taken down, and the building was continued from the second
story, which was filled with tenants while the building was in course of
erection above.
[Illustration:
FIG. 26_e_.—_Manhattan Insurance Co.’s Building, in course of
erection._
]
Still more lofty edifices have been going skyward in other places.
Already in New York there are a great number of lofty piles due to the
introduction of the lifts or elevators, by which an office on the tenth
floor is made as convenient as one on the second. These buildings
usually receive the name of the owners of the structure, who occupy,
perhaps, only one floor. To mention only a few. There is the American
Tract Society building, with its twenty-three stories, 285 feet high,
which is one of the latest and handsomest of these tall piles in the
city. See Plate IV. Still loftier is the St. Paul building, fronting the
New York Post-Office at the junction of Park Row and Broadway. This
structure is splayed at the angle between Ann Street and Broadway, where
its width is 39½ feet, while its _loftiest_ part has frontages of about
30 feet along each of these thoroughfares. The height is no less than
313 feet above the pavement, and the number of stories is twenty-five.
This building is faced with light yellow limestone, and although it was
commenced only in the summer of 1895, it was expected to be ready for
occupation by the autumn of 1896. Even this great height is overtopped
by the Manhattan Life Insurance Company’s building, rising 330 feet, and
remarkable as perhaps beyond previous record of quickness in building a
gigantic structure. Obviously, the foundations of such a building must
be most seriously considered, prepared and tested, before the great bulk
of the building is begun, and in the _New York Engineering Magazine_ one
of the architects has given a full account, with complete illustrations,
of all the works, from the rock foundation to the completed edifice. A
description of the foundation work, though most interesting for the
professional engineer, would probably have little attraction for the
general reader; but its importance may be inferred from the fact of its
having taken nearly six months for its completion, while the huge
superstructure required only eight months. The eighteenth tier of beams
was reached in “three months from the time the foundations were ready on
which to set the first piece of steel, composing the bolsters that
support the cantilever system.... The substructure, which starts in
bed-rock and continues to the cellar-floor, consists of fifteen piers,
varying in size from 9 feet in diameter, to 21 feet 6 inches by 25 feet
square.... The number of bricks used in the piers amounted to 1,500,000.
From this it may be seen that a good-sized building was sunk out of
sight before any part of the superstructure could be begun.” An open
court within the main structure, special framing for the arrangements of
the company’s offices on the sixth floor, the great height and weight of
the tower, and the requisite provision for wind-bracing, delayed in some
degree a regular advance of the stories; but within three months no less
than 5,800 tons were placed in position. There were girders weighing 40
tons, many columns of 10 and 12 tons, and cantilevers of 80 tons weight
and 67 feet long. Strange to say, that in a building of this magnitude,
where such masses had to be raised 300 feet into the air, there was not
a single accident involving loss of life. When four stories of the steel
framework had been put up, the bricklayers were set to work, and they
followed the frame-setters throughout. After the masons came the
pipe-layers, with their ten miles of pipes, followed by electricians,
fixing their thirty-five miles of communicating wires. Thirty thousand
cubic feet of stone was cut and set on the Broadway front in eighty
days. Then craftsmen of the different trades followed each other, or
worked in harmony together, story after story upwards: the engineers for
boilers, heating, and elevators, the plumbers, the decorators, the
carpenters and cabinet-makers, the plasterers, the marble and tile
workers, the gasmen, etc. In fine, every story was completely finished
and ready for occupation in eight months after the start from the
foundations.
[Illustration:
FIG. 26_f_.—_Manhattan Insurance Co.’s Buildings nearly completed._
]
The shortness of the time in which these lofty buildings were run up is
not less remarkable than the completeness of their fittings, which
comprise everything requisite for communication within the premises and
in connection with the outer world. The elevators or lifts are the
perfection of mechanism in their way, and act with wonderful smoothness
and regularity; of these are usually two at least, as well as an ample
staircase. Notwithstanding all these appliances, some disastrous and
fatal conflagrations have occurred at buildings erected on the “tall”
principle; and as “business premises” of even 380 feet high are
projected, the authorities have been considering the desirability of
restricting the heights. It has been proposed that offices should not
exceed in height 200 feet; hotels, 150 feet; and private houses, 75
feet.
_BIG WHEELS._
The Paris example of an engineering feat upon an unprecedented scale
having proved sufficiently captivating for the general public to ensure
for itself a great commercial success, even amid the attractions of an
International Exhibition, was not lost upon the enterprising people of
the States when the “World’s Fair” at Chicago was in preparation in
1893. It was then that Mr. G. W. G. Ferris, the head of a firm of bridge
constructors at Pittsburg, conceived the idea of applying his
engineering skill to the erection of a huge wheel, revolving in a
vertical plane, with cars for persons to sit in, constituting, in fact,
an enormous “merry-go-round,” as the machine once so common at country
fairs was called. The novelty of the Chicago erection was, therefore,
not the general idea, but the magnitude of the scale, which, for that
reason, involved the application of the highest engineering skill, and
the solution of hitherto unattempted practical problems. Several
thousand pounds were, in fact, expended on merely preliminary plans and
designs. The great wheel at Chicago was 250 feet in diameter, and to its
periphery were hung thirty-six carriages, each seating forty persons. At
each revolution, therefore, 1,440 people would be raised in the air to
the height of 250 feet, and from that elevation afforded a splendid
prospect, besides an experience of the peculiar sensation like that of
being in a balloon, when the spectator has no perception of his own
motion, but the objects beneath appear to have the contrary movement,
that is to say, they seem to be sinking when he is rising, and _vice
versâ_. The axle of the Chicago wheel was a solid cylinder, 32 inches in
diameter and 45 feet long; on this were two hubs, 16 feet in diameter,
to which were attached spoke rods, 2½ inches in diameter, passing in
pairs to an inner crown, which was concentric with the outer rim, but 40
feet within it. The inner and outer crowns were connected together, and
the former joined to the crown of the twin wheel by an elaborate system
of trusses and ties, which, however, left an open space between the rims
of 20 feet from the outside. These last were formed of curved riveted
hollow beams, in section 25½ inches by 19 inches, and between them,
slung upon iron axles through the roofs, were suspended, at equal
intervals, the thirty-six carriages, each 27 feet long, and weighing 13
tons without its passengers, who added 3 tons more to the weight. The
wheel with its passengers was calculated to weigh about 1,200 tons, and
it rested on two pyramidal skeleton towers of ironwork 140 feet high,
having bases 50 feet by 60 feet. The wheel was moved by power applied at
the lowest point, the peripheries of both the rims having great cogs 6
inches deep and 18 inches apart, which engaged a pair of large
cog-wheels, carried on a shaft 12 inches in diameter.
[Illustration:
FIG. 26_g_.—_Original Design for the Great Wheel._
]
This curious structure was not begun until March, 1893, yet it was set
in motion three months afterwards, having cost about £62,500. The
Company had to hand over to the Exhibition one half of the receipts
after the big wheel had paid for its construction, but even then they
realised a handsome profit, and at the close of the World’s Fair, they
sold the machine for four-thirds of its cost, in order that it might be
re-erected at Coney Island.
No sooner had the great Ferris wheel at Chicago proved a financial
success than an American gentleman, Lieutenant Graydon, secured a patent
for a like machine in the United Kingdom; and as it has now become
almost a matter of course that some iron or steel structure, surpassing
everything before attempted, should form a part of each great
exhibition, a Company was at once formed in London, under the title of
“The Gigantic Wheel and Recreation Towers Co., Limited,” to construct
and work at the Earl’s Court Oriental Exhibition of 1895, a great wheel,
similar in general form to that of Chicago. But the design of the London
wheel had some new features, as will be seen from the sketches, Fig.
26_c_ (from _The Engineer_ of 20th April, 1894), and, moreover, having
been planned of larger dimensions than its American prototype, presented
additional engineering problems of no small complexity. After due
deliberation the scheme of the work was entrusted to Mr. Walter B.
Basset, a talented young engineer, connected with the firm of Messrs.
Maudslay, Sons, & Field, and already experienced in designing iron
structures. Under this gentleman, with the assistance of Mr. J. J.
Webster in carrying out some of the details, the work has been so
successfully accomplished that the “Great Wheel” of 1895 may be cited as
one of the crowning mechanical triumphs of the nineteenth century. The
original design has not been followed so far as regards the lower
platforms for refreshment rooms, &c. Plate V., for which we are indebted
to Mr. Basset, is a photographic representation of the actual structure.
The wheel at Earl’s Court exceeds the Ferris wheel in diameter by 50
feet, being 300 feet across. It is supported on two towers, 175 feet
high, each formed by four columns 4 feet square, built of steel plates
with internal diaphragms, and surmounted by balconies that may be
ascended in elevators raised by a weight of water, which, after having
been discharged into a reservoir under the ground level, is again pumped
up to the top of the towers. Between the balconies on each tower there
is also a communication _through the axle_ of the wheel, which, instead
of being solid as at Chicago, is a tube of 7 feet diameter, and 35 feet
long, made in sections, riveted together, of steel 1 inch thick, and
weighing no less than 58 tons. The raising and fixing in its high place
of such a mass of metal required specially ingenious devices, which have
been greatly appreciated by professional engineers. But for these
devices, the erection of scaffolding in the ordinary way of proceeding
would have entailed an outlay simply enormous. The axle is stiffened by
projecting rings, and, between pairs of these, the spoke rods are
attached by pins 3 inches in diameter. The axle was the production of
Messrs. Maudslay, Field & Co.; all the rest of the metal work was made
at the Arrol Works at Glasgow, and the carriages were constructed by
Brown, Marshall & Co., of Birmingham. The Earl’s Court wheel is turned
by a mechanism different from that of the Chicago wheel, for whereas the
latter was provided with cogs, the former has two chains, each 1,000
feet long and 8 tons weight, surrounding the periphery of the wheel on
either side. The chains go over drums in the engine-shed, from which
they pass underground to guide-pulleys, and as they unwind from the
Great Wheel, they again go over guide-pulleys to lead them back to the
drums. These chains are firmly held throughout in the jaws of V-shaped
grooves, and there are arrangements for taking up the slack. The drums
are actuated by wheel gearing, connected with two horizontal Robey steam
engines, each of 50 horse-power, one on either side, capable of being
worked singly or together. It is, however, found sufficient to use the
engine of one side only, and even then to work it at but 16 horse-power,
and the operation can be controlled by one man, who has also the command
of a brake. Both starting and stopping are accomplished with the
greatest smoothness and absence of strain or jar. There are forty
carriages, each 25 feet long, 9 feet wide, and 10 feet high. Each will
accommodate forty passengers, and these enter at the ends from eight
platforms at different heights from the ground, so arranged as to be on
the level of the eight lowest carriages while the wheel is stationary.
The passengers who have had their ride leave at the other end of the
carriages by eight similar platforms on the other side of the wheel.
After the change of passengers in one set of eight carriages, the wheel
is turned through exactly one-fifth of a revolution, which has the
effect of bringing the next eight carriages to the level of the
platforms, and it is again brought to a standstill whilst the change of
passengers is taking place; and so on, until the whole freight of say
1,600 persons has been changed during the five stoppages in one
revolution, for which about thirty-five minutes are required, and the
process of emptying and filling eight carriages at once is repeated.
There are first and second class carriages, the charge for the former
being two shillings, and for the latter one shilling; so that, reckoning
800 passengers of each class, one turn would bring to the treasury the
handsome sum of £120.
The sensations experienced in a journey on the Great Wheel are, as
already mentioned, comparable to those enjoyed by the aërial voyagers in
a balloon, where all perception of proper motion is lost, and it is the
world beneath that seems to recede and float away, presenting the while
a strangely changing panorama. Many people who have never made a balloon
ascent yet know the calm delight of floating in a boat without effort
down some placid stream, unconscious of any motion beyond that vaguely
inferred from the silent apparent gliding by of the banks. Very similar
are, in part, the feelings of the passenger who is almost imperceptibly
carried up into the air in a carriage of the Great Wheel, but the
vertical direction of the movement, and the gradual expansion of the
horizon as the vertex is approached, lend an unwonted novelty to the
situation. From the Earl’s Court Wheel the view is both interesting and
extensive, for on a clear day the prospect stretches as far as the Royal
Castle of Windsor.
The “Gigantic Wheel” at Earl’s Court was inaugurated on the 11th July,
1895, in the presence of an assemblage of 5,000 people, including many
distinguished personages, who were all treated to a ride. Plate I. shows
a portion of the wheel and carriages as in motion.
[Illustration:
PLATE V.
GENERAL VIEW OF THE GREAT WHEEL AT EARL’S COURT.
]
[Illustration:
FIG. 27.—_Sir Joseph Whitworth._
]
TOOLS.
Of the immense variety of tools and mechanical contrivances employed in
modern times, by far the greatest number are designed to impart to
certain materials some definite shape. The brickmaker’s mould, the
joiner’s plane, the stonemason’s chisel, the potter’s wheel, are
examples of simple tools. More elaborate are the coining press, the
machine for planing iron, the drilling machine, the turning lathe, the
rolling mill, the Jacquard loom. But all such tools and machines have
one principle in common—a principle which casual observers may easily
overlook, but one which is of the highest importance, as its application
constitutes the very essence of the modern process of _manufacture_ as
distinguished from the slow and laborious mode of making things by hand.
The principle will be easily understood by a single example. Let it be
required to draw straight lines across a sheet of paper. Few persons can
take a pen or pencil, and do this with even an approach to accuracy, and
at best they can do it but slowly and imperfectly. But with the aid of a
ruler any number of straight lines may be drawn rapidly and surely. The
former case is an instance of _making_ by hand, the latter represents
_manufacturing_, the ruler being the tool or machine. Let it be observed
that the ruler has in itself the kind of form required—that is to say,
straightness—and that in using it we copy or transfer this straightness
to the mark made on the paper. This is a simple example of the _copying
principle_, which is so widely applied in machines for manufacturing;
for, in all of these, materials are shaped or moulded by various
contrivances, so as to reproduce certain definite forms, which are in
some way contained within the machine itself. This will be distinctly
seen in the tools which are about to be described.
[Illustration:
FIG. 28.—_Whitworth’s Screw Dies and Tap._
]
Probably no one mechanical contrivance is so much and so variously
applied as the _Screw_. The common screw-nail, which is so often used by
carpenters for fastening pieces of metal on wood, or one piece of wood
to another, is a specimen of the screw with which everybody is familiar.
The projection which winds spirally round the nail is termed the
_thread_ of the screw, and the distance that the thread advances
parallel to the axis in one turn is called the _pitch_. It is obvious
that for each turn the screw makes it is advanced into the wood a depth
equal to the pitch, and that there is formed in the wood a hollow screw
with corresponding grooves and projections. Screws are formed on the
ends of the bolts, by which various parts are fastened together, and the
hollow screws which turn on the ends of the bolts are termed _nuts_. The
screws on bolts and nuts, and other parts of machines, were formerly
made with so many different pitches that, when a machine constructed by
one maker had to be repaired by another, great inconvenience was found,
on account of the want of uniformity in the shape and pitch of the
threads. A uniform system was many years ago proposed by Sir Joseph
Whitworth, and adopted by the majority of mechanical engineers, who
agreed to use only a certain defined series of pitches. The same
engineer also contrived a hand tool for cutting screws with greater
accuracy than had formerly been attained in that process. A mechanic
often finds it necessary to form a screw-thread on a bolt, and also to
produce in metal a hollow screw. The reader may have observed
gas-fitters and other workmen performing the first operation by an
instrument having the same general appearance as Fig. 28. This contains
hard steel _dies_, which are made to press on the bolt or pipe, so that
when the _guide-stock_ is turned by the handles, the required grooves
are cut out. The arrangement of these dies in Sir Joseph Whitworth’s
instrument is shown in Fig. 28, which represents the central part of the
guide-stock; A, B, C are the steel dies retained in their places, when
the instrument is in use, by a plate which can be removed when it is
necessary to replace one set of dies by another, according to the pitch
of thread required. The figure also shows the set of dies, A, B, C,
removed from the guide-stock. D is the work, pressed up against the
fixed die, A, by B and C, the pressure being applied to these last as
required by turning the nut, thus drawing up the key, E, so that the
inclined planes, _f_, _g_, press against similar surfaces forming the
ends of the dies. For producing the hollow screws, _taps_ are provided,
which are merely well-formed screws, made of hard steel and having the
threads cut into detached pieces by several longitudinal grooves, as
represented in the lower part of Fig. 28.
[Illustration:
FIG. 29.—_Screw-cutting Lathe._
]
The method of forming screws by dies and taps is, however, applicable
only to those of small dimensions, and even for these it is not employed
where great accuracy is required. Perfect screws can only be cut with a
lathe, such as that represented in Fig. 29. In this we must first call
the reader’s attention to the portion of the apparatus marked A, which
receives the name of the _slide-rest_. The invention of this contrivance
by Maudsley had the effect of almost revolutionizing mechanical art, for
by its aid it became possible to _produce true surfaces in the lathe_.
Before the slide-rest was introduced, the instrument which cut the wood
or metal was held in the workman’s hand, and whatever might be his skill
and strength, the steadiness and precision thus obtainable were far
inferior to those which could be reached by the grip of an iron hand,
guided by unswerving bars. The slide-rest was contrived by Maudsley in
the first instance for cutting screws, but its principle has been
applied for other purposes. This principle consists in attaching the
cutting tool to a slide which is incapable of any motion, except in the
one direction required. Thus the slide, A, represented in Fig. 29, moves
along the _bed_ of the lathe, B, carrying the cutter with perfect
steadiness in a straight line parallel to the axis of the lathe. There
are also two other slides for adjusting the position of the cutter; the
handle, _a_, turns a screw, which imparts a transverse motion to the
piece, _b_, and the tool receives another longitudinal movement from the
handle, _c_. The pieces are so arranged that these movements take place
in straight lines in precisely the required direction, and without
permitting the tool to be unsteady, or capable of any rocking motion. In
Whitworth’s lathe, between the two sides of the bed, and therefore not
visible in the figure, is a shaft placed perfectly parallel to the axis
of the lathe. One end of this shaft is seen carrying the wheel, C, which
is connected with a train of wheels, D, and is thus made to revolve at a
speed which can be made to bear any required proportion to that of the
mandril, E, of the lathe, by properly arranging the numbers of the teeth
in the wheels; and the machine is provided with several sets of wheels,
which can be substituted for each other. The greater part of the length
of this shaft is formed with great care into an exceedingly accurate
screw, which works in a nut forming part of the slide-rest. The effect,
therefore, of the rotation of the screw is to cause the slide-rest to
travel along the bed of the lathe, advancing with each revolution of the
screw through a space equal to its pitch distance. There is an
arrangement for releasing the nut from the guiding-screw, by moving a
lever, and then by turning the winch the slide-rest is moved along by a
wheel engaging the teeth of a rack at the back of the lathe. Now, if the
train of wheels, C D, be so arranged that the screw makes one revolution
for each turn of the mandril, it follows that the cutting tool will move
longitudinally a distance equal to the pitch of the guiding-screw while
the bar placed in the lathe makes one turn. Thus the point of the cutter
will form on the bar a screw having the same pitch as the guiding-screw
of the lathe.
Here we have a striking illustration of the copying principle, for the
lathe thus produces an exact copy of the screw which it contains. The
screw-thread is traced out on the cylindrical bar, which is operated
upon by the combination of the circular motion of the mandril with the
longitudinal movement of the slide-rest. By modifying the relative
amounts of these movements, screw-threads of any desired pitch can be
made, and it is for this purpose that the _change wheels_ are provided.
If the thread of the guiding-screw makes two turns in one inch, one
revolution of the wheel C will advance the cutter half an inch along the
length of the bar. If the numbers of teeth in the wheels be such that
the wheel D makes ten revolutions while C is making one, then in the
length of half an inch the thread of the screw produced by the cutter
will go round the core ten times, or, in technical language, the screw
will be of 1/20 inch pitch.
[Illustration:
FIG. 30.—_Whitworth’s Measuring Machine._
]
Since a screw turning in a nut advances only its pitch distance at each
revolution, a finely-cut screw furnishes an instrument well adapted to
impart a slow motion, or to measure minute spaces. Suppose a screw is
cut so as to have fifty threads in an inch, then each turn will advance
it 1/50 in.; half a turn 1/100 in.; a quarter of a turn, 1/200, and so
on. It is quite easy to attach a graduated circle to the head of the
screw, so that, by a fixed pointer at the circumference, any required
fraction of a revolution may be read off. Thus if the circle had two
hundred equal parts, we could, by turning the screw so that one division
passed the index, cause the screw to advance through 1/200 of 1/50 inch,
or 1/10000 part of an inch. This is the method adopted for moving the
cross-wires of the instruments for measuring very small spaces under the
microscope. Sir Joseph Whitworth, who has done so many great things in
mechanical art, was the first mechanician to perceive the importance of
extreme accuracy of workmanship, and he invented many beautiful
instruments and processes by which this accuracy might be attained. Fig.
30 represents one of his measuring machines, intended for practical use
in the workshop, to test the dimensions of pieces of metal where great
precision is required. The base of the machine is constructed of a rigid
cast iron bed bearing a fixed headstock, A, and a movable one, B, the
latter sliding along the bed, C, with a slow movement, when the handle,
D, is turned. This slow motion is produced by a screw on the axis, _a_,
working in the lower part of the headstock, just as the slide-rest is
moved along the bed of the lathe. The movable headstock, when it has
been moved into the position required, is firmly clamped by a
thumbscrew. The face of the bed is graduated into inches and their
subdivisions. Here it should be explained that the machine is not
intended to be used for ascertaining the absolute dimensions of objects,
but for showing by what fraction of an inch the size of the work
measured differs from a certain standard piece. Each headstock carries a
screw of 1/20 inch pitch, made with the greatest possible care and
accuracy. To the head of the screw in the movable headstock is attached
the wheel, _b_, having its circumference divided into 250 equal parts,
and a fixed index, _c_, from which its graduations may be counted. An
exactly similar arrangement is presented in connection with the screw
turning in the fixed headstock, but the wheel is much larger, and its
circumference is divided in 500 equal parts. It follows, therefore, that
if the large wheel be turned so that one division passes the index, the
bar moves in a straight line 1/500 of the 1/20 of an inch, that is,
1/10000 an inch. The ends of the bars, _d_ and _e_, are formed with
perfectly plane and parallel surfaces, and an ingenious method is
adopted of securing equality of pressure when comparisons are made. A
plate of steel, with perfectly parallel faces, called a _gravity-piece_,
or _feeler_, is placed between the flat end of the bar and the
standard-piece, and the pressure when the screw-reading is taken must be
just sufficient to prevent this piece of steel from slipping down, and
that is the case when the steel remains suspended and can nevertheless
be easily made to slide about by a touch of the finger. Thus any piece
which, with the same screw-readings, sustains the gravity-piece in the
same manner as the standard, will be of exactly the same length; or the
number of divisions through which the large wheel must be turned to
enable it to do so tells the difference of the dimensions in
ten-thousandth parts of an inch. By this instrument, therefore, gauges,
patterns, &c., can be verified with the greatest precision, and pieces
can be reproduced perfectly agreeing in their dimensions with a standard
piece. Thus, for example, the diameters of shafting can be brought with
the greatest precision to the exact size required to best fit their
bearings.
In another measuring machine on the same principle the delicacy of the
measurement has been carried still farther, by substituting for the
large divided wheel one having 200 teeth, which engage an endless screw
or worm. This will easily be understood by reference to Fig. 31, where a
similar arrangement is applied to another purpose. Imagine that a wheel
like P, Fig. 31, but with 200 teeth, has taken the place of E in Fig.
30, and that the wheel, T, on the axis of the endless screw is shaped
like E, Fig. 30. One turn of the axis carrying the endless screw,
therefore, turns the wheel through 1/200 of a revolution, and as this
axis bears a graduated head, having 250 divisions, the screw having 20
threads to the inch, is, when one division passes the index, advanced
through a space equal to 1/250 × 1/200 × 1/20, or 1/1000000 an inch;
that is, the one-millionth part of an inch. This is an interval so small
that ten times its length would hardly be appreciated with the highest
powers of the microscope, and the machine is far too delicate for any
practical requirements of the present day. It will indicate the
expansion caused by heat in an iron bar which has merely been touched
with the finger for an instant, and even the difference of length
produced by the heat radiated from the person using it. A movement of
1/1000000 of an inch is shown by the gravity-piece remaining suspended
instead of falling, and the piece falls again when the tangent-screw is
turned back through 1/250 of a revolution, a difference of reading
representing a possible movement of the measuring surface through only
2/1000000 an inch. This proves the marvellous perfection of the
workmanship, for it shows that the amount of play in the bearings of the
screws does not exceed one-millionth of an inch.
A good example of a machine-tool is the _Drilling Machine_, which is
used for drilling holes in metal. Such a machine is represented in Fig.
31, where A is the strong framing, which is cast in a single piece, in
order to render it as rigid as possible. The power is applied by means
of a strap round the speed pulley, B, by which a regulated speed is
communicated to the bevel wheel, C, which drives D, and thus causes the
rotation of the hollow shaft, E. In the lower part of the latter is the
spindle which carries the drilling tool, F, and upon this spindle is a
longitudinal groove, into which fits a projection on the inside of E.
The spindle is thus forced to rotate, and is at the same time capable of
moving up and down. The top of the spindle is attached to the lower end
of the rack, G, by a joint which allows the spindle to rotate freely
without being followed in its rotation by the rack, although the latter
communicates all its vertical movements to the spindle, as if the two
formed one piece. The teeth of the rack are engaged by a pinion, which
carries on its axis the wheel H, turned by an endless screw on the
shaft, I, which derives its motion by means of another wheel and endless
screw from the shaft, K. The latter is driven by a strap passing over
the _speed pulleys_, L and M, and thus the speed of the shaft K can be
modified as required by passing the strap from one pair of pulleys to
another. The result is that the rack is depressed by a slow movement,
which advances the drill in the work, or, as it is technically termed,
gives the _feed_ to the drill. By a simple piece of mechanism at N the
connection of the shafts K and I can be broken, and the handle O made to
communicate a more rapid movement to I, so as to raise up the drill in a
position to begin its work again, or to bring it quickly down to the
work, and then the arrangement for the self-acting feed is again brought
into play. By turning the wheel, P, the table, Q, on which the work is
fastened, is capable of being raised or lowered, by means of a rack
within the piece R, acted on by a pinion carried on the axle, P. The
table also admits of a horizontal motion by the slide S, and may besides
be swung round when required.
[Illustration:
FIG. 31.—_Whitworth’s Drilling Machine._
]
The visitor to an engineer’s workshop cannot fail to be struck with the
operation of the powerful _Lathes_ and _Planing Machines_, by which long
thick flakes or shavings of iron are removed from pieces of metal with
the same apparent ease as if the machine were paring cheese. The figure
on the opposite page represents one of the larger forms of the planing
machine, as constructed by Sir J. Whitworth. The piece of work to be
planed is firmly bolted down to the table, A, which moves upon the
Ꮩ-shaped surfaces, running its whole length, and accurately fitting into
corresponding grooves in a massive cast iron bed. The bevel wheel, of
which a portion is seen at B, is keyed on a screw, which extends
longitudinally from end to end of the bed. This screw works in nuts
forming part of the table, and as it turns in sockets at the ends of the
bed, it does not itself move forward, but imparts a progressive movement
to the table, and therefore to the piece of metal to be planed. As this
table must move backwards and forwards, there must be some contrivance
for reversing the direction of the screw’s rotation, and this is
accomplished in a beautifully simple manner by an arrangement which a
little consideration will enable any one to understand. It will be
observed that there are three drum-pulleys at C. Let the reader confine,
for the present, his attention to the nearest one, and picture to
himself that the shaft to which it is attached is placed in the same
horizontal plane as the axis of the screw and at right angles to it,
passing in front of bevel wheel B. A small bevel wheel turning with this
shaft, and engaging the teeth of the wheel B, may, it is plain,
communicate motion to the screw. Now let the reader consider what will
be the effect on the _direction_ of the rotation of B of applying the
bevelled pinion to the nearer or to the farther part of its
circumference, supposing the direction of the rotation of this pinion to
be always the same. He will perceive that the direction in one case will
be the reverse of that in the other. The shaft to which the nearest
pulley is attached carries a pinion engaging the wheel at its farther
edge, and therefore the rotation of this pulley in the same direction as
the hands of a watch causes the wheel B to rotate so that its upper part
moves towards the spectator. The farthest pulley, _a_, turns with a
hollow shaft, through which the shaft of the nearest pulley simply
passes, without any connection between them, and this hollow shaft
carries a pinion, which engages the teeth of B at the nearer edge, and,
in consequence, the rotation of the farther pulley, _a_, in the
direction of the hands of a watch, would cause the upper part of B to be
moving from the spectator. The middle pulley, _b_, runs loosely on the
shaft, and the driving-strap passes through the guide, _c_, and it is
only necessary to move this, so as to shift the strap from one drum to
another, in order to reverse the direction of the screw and the motion.
This shifting of the strap is done by a movement derived from the table
itself, on which are two adjustable stops, D and E, acting on an
arrangement at the base of the upright frame when they are brought up to
it by the movement of the table, so as not only to shift the strap, but
also to impart a certain amount of rotation to upright shaft, F, in each
direction alternately. The piece which carries the tools, G and H, is
placed horizontally, and can be moved vertically by turning the axis, I,
thus causing an equal rotation of two upright screws of equal pitch,
which are contained within the uprights and work in nuts, forming part
of the tool-box. The pieces carrying the tools are moved horizontally by
the screws which are seen to pass along the tool-box, and these screws
receive a certain regulated amount of motion at each reversal of the
movement of the table from the mechanism shown at K. The band-pulley, L,
receives a certain amount of rotation from the same shaft, and the
catgut band passing round the tops of the cylinders which carry the
cutters is drawn in alternate directions at the end of each stroke, the
effect being to turn the cutters half round, so as always to present
their cutting edges to the work. There are also contrivances for
maintaining the requisite steadiness in the tools and for adjusting the
depth of the cut. The cutting edge of the tools is usually of a Ꮩ-shape,
with the angle slightly rounded, and the result of the process is not
the production of a plane, but a grooved surface. But by diminishing the
amount of horizontal _feed_ given to the cutters, the grooves may be
made finer and finer, until at length they disappear, and the surface is
practically a plane. Planing machines are sometimes of a very large
size. Sir J. Whitworth has one the table of which is 50 ft. in length,
and the machine is capable of making a straight cut 40 ft. long in any
article not exceeding 10 ft. 6 in. high or 10 ft. wide.
[Illustration:
FIG. 32.—_Whitworth’s Planing Machine._
]
[Illustration:
FIG. 33.—_Pair of Whitworth’s Planes, or Surface Plates._
]
The copying principle is evident in this machine; for the plane surface
results from the combination of the straightness of the bed with the
straightness of the transverse slide along which the tools are moved. It
should, moreover, be observed that it is precisely this machine which
would be employed for preparing the straight sliding surfaces required
in the construction of planing and other machines, and thus one of these
engines becomes the parent, as it were, of many others having the same
family likeness, and so on _ad infinitum_. Thus, having once obtained
perfectly true surfaces, we can easily reproduce similar surfaces. But
the reader may wish to know how such forms have been obtained in the
first instance; how, for example, could a perfectly plane surface be
fashioned without any standard for comparison? This was first perfectly
done by Sir J. Whitworth, forty-five years ago. Three pieces of iron
have each a face wrought into comparatively plane surfaces; they are
compared together, and the parts which are prominent are reduced first
by filing, but afterwards, as the process approaches completion, by
scraping, until the three perfectly coincide. The parts where the plates
come in contact with each other are ascertained by smearing one of them
with a little oil coloured with red ochre: when another is pressed
against it, the surfaces of contact are shown by the transference of the
red colour. Three plates are required, for it is possible for the
prominences of No. 1 exactly to fit into the hollows of No. 2, but in
that case _both_ could not possibly exactly coincide with the surface of
No. 3; for if one of them did (say No. 1), then No. 3 must be exactly
similar to No. 2, and consequently when No. 2 was applied to No. 3,
hollow would be opposed to hollow and prominence to prominence. A little
reflection will show that only when the three surfaces are truly plane
will they exactly and entirely coincide with each other. The planes,
when thus carefully prepared, approach to the perfection of the ideal
mathematical form, and they are used in the workshop for testing the
correctness of surfaces, by observing the uniformity or otherwise of the
impression they give to the surface when brought into contact with it,
after being covered by a very thin layer of oil coloured by
finely-ground red ochre.
Fig. 33 represents a small pair of Whitworth’s planes. When one of these
is placed horizontally upon the other, it does not appear to actually
come in contact with it, for the surfaces are so true that the air does
not easily escape, but a thin film supports the upper plate, which
glides upon it with remarkable readiness (A). When, however, one plate
is made to slide over the other, so as to exclude the air, they may both
be lifted by raising the upper one (B). This effect has, by several
philosophers, been attributed to the mere pressure of the atmosphere;
but recent experiments of Professor Tyndall’s show that the plates
adhere even in a vacuum. The adhesion appears therefore to be due to
some force acting between the substances of the plates, and perhaps
identical in kind with that which binds together the particles of the
iron itself.
[Illustration:
FIG. 34.—_Interior of Engineer’s Workshop._
]
[Illustration:
FIG. 35.—_The Blanchard Lathe._
]
_THE BLANCHARD LATHE._
This machine affords a striking example of the application of the
copying principle which is the fundamental feature of modern
manufacturing processes. It would hardly be supposed possible, until the
method had been explained, that articles in shape so unlike geometrical
forms as gun-stocks, shoemakers’ lasts, &c., could be turned in a lathe.
The mode in which this is accomplished is, however, very simple in idea,
though in carrying that idea into practice much ingenious contrivance
was required. The illustration, Fig. 35, represents a Blanchard’s lathe,
very elegantly constructed by Messrs. Greenwood and Batley, of Leeds.
The first obvious difference between an ordinary lathe and Blanchard’s
invention is that in the former the work revolves rapidly and the
cutting-tool is stationary, or only slowly shifts its position in order
to act on fresh portions of the work, while in the latter the work is
slowly rotated and the cutting-tools are made to revolve with very great
velocity. Again, it will be observed that the headstock of the Blanchard
lathe, instead of one, bears _two_ mandrels, having their axes parallel
to each other. One of these carries the pattern, C, which in the figure
has the exact shape of a gun-stock that is to be cut in the piece of
wood mounted on the nearer spindle. One essential condition in the
arrangement of the apparatus is that the pattern and the work having
been fixed in similar and parallel positions, shall always continue so
at every point of their revolutions. This is easily accomplished by
placing exactly similar toothed wheels on the two axles, and causing
these to be turned by one and the same smaller toothed wheel or pinion.
The two axles must thus always turn round in the same direction and with
exactly the same speed, so that the work which is attached to one, and
the pattern which is fixed on the other, will always be in the same
phase of their revolutions. If, for example, the part of the wood which
is to form the upper part of the gun-stock is at the bottom, the
corresponding part of the pattern will also be at the bottom, as in the
figure, and both will turn round together, so that every part of each
will be at every instant in a precisely similar position. The wood to be
operated upon is, it must be understood, roughly shaped before it is put
into the lathe. The toothed wheels and the pinion which drives them are
in the figure hid from view by the casing, _h_, which covers them. The
pinion receives the power from a strap passing over _f_. The cutters are
shown at _e_; they are placed radially, like the spokes of a wheel, and
have all their cutting edges at precisely one certain distance from the
axis on which they revolve, so that they all travel through the same
circle. These cutting-edges, it may be observed, are very narrow, almost
pointed. The shaft carrying the cutters is driven at a very high speed,
by means of a strap passing over _k_ and _i_. The number of revolutions
made by the cutters in one second is usually more than thirty. The great
peculiarity of the lathe consists in the manner in which the position of
the cutters is made to vary. The axle which carries them rotates in a
kind of frame, which can move backwards and forwards, so that the
cutters may be readily put at any desired distance from the axis of the
work. Their position is, however, always dependent on the pattern, for,
fixed in a similar frame, _b_, which is connected with the former, is a
small disc wheel, _a_, having precisely the same radius as that of the
circle traced out by the cutters, and this disc is made by a strong
spring to press against the pattern. The cutters, being fixed in the
same rocking-frame which carries this guiding-wheel, must partake of all
its backward and forward motions, and as the cutting-wheel and the
guide-wheel are so arranged as to have always the same relative
positions to the axes of the two headstocks, it follows that the edges
of the cutters will trace out identically the same form as the
circumference of the guiding-disc. The latter is, of course, not driven
round, but simply turns slowly with the pattern by friction, for it is
pressed firmly against the pattern by a spring or weight acting on the
frame, in order that the cutters may be steadily maintained in their
true, but ever-varying, position. The rocking-frame receives a slow
longitudinal motion by means of the screw, _n_, so that the cutters are
carried along the work, and the guide along the pattern.
The whole arrangement is self-acting, so that when once the pattern and
the rough block of wood have been fixed in their positions, the machine
completes the work, and produces an exact repetition of the shape of the
pattern. It is plain that any kind of forms can be easily cut by this
lathe, the only condition being that the surface of the pattern must not
present any re-entering portions which the edge of the guide-wheel
cannot follow. The machine is largely used for the purposes named above,
and also for the manufacture of the spokes of carriage-wheels. The
limits of this article will not permit of a description of the beautiful
adjustments given to the mechanism in the example before us,
particularly in the arrangement for driving the cutters in a framework
combining lateral and longitudinal motions; but the intelligent reader
may gather some hints of these by a careful inspection of the figure.
The machine is sometimes made with the frame carrying the guide-wheel
and cutters, not rocking but sliding in a direction transverse to the
axes of the headstocks. It is extremely interesting to see the Blanchard
lathe at work, and observe how perfectly and rapidly the curves and form
of the patterns seem to grow, as it were, out of the rudely-shaped piece
of wood, which, of course, contains a large excess of material, or, in
the picturesque and expressive phrase of the workmen, _always gives the
machine something to eat_.
[Illustration:
FIG. 36.—_Vertical Saw._
]
_SAWING MACHINES._
With the exception of the last, all the machines hitherto described in
the present article are distinguished by this—they are tools which are
used to produce other machines of every kind. Without such implements it
would be impossible to fashion the machines which are made to serve so
many different ends. Another peculiarity of these tools has also been
referred to, namely, that they are especially serviceable, and indeed
essential, for the reproduction of others of the same class. Thus, the
accurate leading-screw of the lathe is the means used to cut other
accurate screws, which shall in their turn become the leading-screws of
other lathes, and a lathe which forms a truly circular figure is a
necessary implement for the construction of another lathe which shall
also produce truly circular figures. In these tools, therefore, we find
the copying principle, to which allusion has been already made, as the
great feature of all machines; but in order to bring this principle
still more clearly before the reader, we have described in the Blanchard
Lathe a machine of a somewhat different class, because it embodied a
very striking illustration of the principle in question. We are far from
having described all the implements of the mechanical engineer, or even
all the more interesting ones; for example, we have given no account of
the powerful lathes in which great masses of iron are turned, or of the
analogous machines, which, with so much accuracy, shape the internal
surfaces of the cylinders of steam engines, of cannons, &c. The history
of the steam engine tells us of the difficulties which Watt had to
contend with in the construction of his cylinders, for no machine at
that time existed capable of boring them with an approach to the
precision which is now obtained.
[Illustration:
FIG. 37.—_Circular Saw._
]
The kind of general interest which attaches to the tools we have already
described is not wanting in yet another class of machine-tools, namely,
those employed in converting timber into the forms required to adapt it
for the uses to which it is so extensively applied. And for popular
illustration, this class of tools presents the special advantage of
being readily understood as regards their purpose and mode of action,
while their simplicity in these respects does not prevent them from
showing the advantages of machine over hand labour. Everybody is
familiar with the up-and-down movement of a common saw, and in the
machine for sawing balks of timber into planks, represented in Fig. 36,
this reciprocating motion is retained, but there are a number of saws
fixed parallel to each other in a strong frame, at a distance
corresponding to the thickness of the planks. The saws are not placed
with their cutting edges quite upright, but these are a little more
forward at the top, so that as they descend they cut into the wood, but
move upwards without cutting, for the teeth then recede from the line of
the previous cut, while in the meantime the balk is pushed forward ready
for the next descent of the saw-frame. This pushing forward, or
_feeding_, of the timber is accomplished by means of ratchet-wheels,
which are made to revolve through a certain space after each descent of
the saw-frame, and, by turning certain pinions, move forward the
carriage on which the piece of timber is firmly fixed, so that when the
blades of the saws are beginning the next descent they are already in
contact with the edge of the former cut. To prevent the blades from
moving with injurious friction in the saw-cuts, these last are made of
somewhat greater width than the thickness of the blades, by the simple
plan of bending the teeth a little on one side and on the other
alternately. The rapidity with which the machine works, depends of
course on the kind of wood operated upon, but it is not unusual for such
a machine to make more than a hundred cuts in the minute. The figure
shows the machine as deriving its motion by means of a strap passing
over a drum, from shafting driven by a steam engine. This is the usual
plan, but sometimes the steam power is applied directly, by fixing the
piston-rod of a steam cylinder to the top of the saw-frame, and
equalizing the motion by a fly-wheel on a shaft, turned by a crank and
connecting-rod.
A very effective machine for cutting pieces of wood of moderate
dimensions is the _Circular Saw_, represented in Fig. 37. Here there is
a steel disc, having its rim formed into teeth; and the disc is made to
revolve with very great speed, in some cases making as many as five
hundred turns in a minute, or more than eight in a second. On the bench
is an adjustable straight guide, or fence, and when this has been fixed,
the workman has only to press the piece of wood against it, and push the
wood at the same time towards the saw, which cuts it at a very rapid
rate. Sometimes the circular saw is provided with apparatus by which the
machine itself pushes the wood forwards, and the only attention required
from the workman is the fixing of the wood upon the bench, and the
setting of the machine in gear with the driving-shaft. Similar saws are
used for squaring the ends of the iron rails for railways, two circular
saws being fixed upon one axle at a distance apart equal to the length
of the rails. The axle is driven at the rate of about 900 turns per
minute, and the iron rail is brought up parallel to the axle, being
mounted on a carriage, and still red hot, when the two ends are cut at
the same time by the circular saws, the lower parts of which dip into
troughs of water to keep them cool.
[Illustration:
FIG. 38.—_Pit-Saw._
]
[Illustration:
FIG. 39.—_Box Tunnel._
]
RAILWAYS.
[Illustration:
FIG. 40.—_Coal-pit, Salop._
]
Towards the end of last century, tramways formed by laying down narrow
plates of iron, were in use at mines and collieries in several parts of
England. These plates had usually a projection or flange on the inner
edge, thus—L, in order to keep the waggons on the track, for the wheels
themselves had no flange, but were of the kind used on ordinary roads.
These flat tramways were found liable to become covered up with dirt and
gravel, so that the benefit which ought to have been obtained from their
smoothness was in a great measure lost. _Edge rails_ were, therefore,
substituted, and the wheels were kept on the rails by having a _flange_
cast on the inner edge of the rim. The rails were then always made of
cast iron, for, although they were very liable to break, the great cost
of making them of wrought iron prevented that material from being used
until 1820, when the method of forming rails of malleable iron by
rolling came into use. The first time a tramway was used for the
conveyance of passengers was in 1825, when the Stockton and Darlington
Railway was opened—a length of thirty-seven miles. It appears that the
carriages were at first drawn by horses, although locomotives were used
on this and other colliery lines for dragging, at a slow rate, trains of
mineral waggons. At that time engineers were exercising their ingenuity
in overcoming a difficulty which never existed by devising plans for
giving tractive power to the locomotive through the instrumentality of
rack-work rails. It never occurred to them to first try whether the
adhesion of the smooth wheel to the smooth rail was not sufficient for
the purpose. During the first quarter of the present century the greater
part of the goods and much passenger traffic was monopolized by the
canals. It is quoted, as a proof of the careless manner in which this
service was performed, that the transport of bales of cotton from
Liverpool to Manchester sometimes occupied twice the length of time
required in their voyage across the Atlantic. When an Act of Parliament
authorizing the construction of a railway between Liverpool and
Manchester was applied for, the canal companies succeeded in retarding,
by their influence, the passing of that Act for two years. It was
passed, however, in 1828, and the construction of the line was proceeded
with. This line was at first intended only for the conveyance of goods,
especially of cotton and cotton manufactures, and the waggons were to be
drawn by horses. When the line was nearly finished the idea of employing
horses was, at the instigation of Mr. George Stephenson, abandoned in
favour of steam power. The directors were divided in opinion as to
whether the carriage should be dragged by ropes wound on large drums by
stationary engines, or whether locomotives should be employed. Finally,
the latter plan was adopted, and it was also suggested that passengers
might be carried. The directors offered a prize for the best locomotive,
and the result has been already mentioned. In the light of our
experience since that time, it is curious to read of the doubts then
entertained by skilful engineers about the success of the locomotive. In
a serious treatise on the subject, one eminent authority hoped “that he
might not be confounded with those hot-brained enthusiasts who
maintained the possibility of carriages being driven by a steam engine
on a railway at such a speed as twelve miles an hour.” When the “Rocket”
had accomplished the unprecedented velocity of twenty-nine miles an
hour, and the railway was opened for passengers as well as goods, the
thirty stage coaches daily plying between Liverpool and Manchester found
their occupation gone, and all ceased to run except one, which had to
depend on the roadside towns only, while the daily number of passengers
between the two cities rose at once from 500 to 1,600. In that
delightful book, Smiles’s “Life of George Stephenson,” may be found most
interesting details of the difficulties attending the introduction of
railways, especially with regard to the construction of this first
important line. Mr. Smiles relates how the promoters of the scheme
struggled against “vested interests;” how the canal proprietors,
confident at first of a secure and continuous enjoyment of their
monopoly, ridiculed the proposed railway, and continued their exorbitant
charges and tardy conveyance, pocketing in profits the prime cost of
their canal about every three years; how, roused into active opposition,
they did all in their power to thwart the new scheme; how the Lord Derby
and the Lord Sefton of that day, and other landowners, offered every
resistance to the surveyors; how the Duke of Bridgewater’s farmers would
not allow them to enter their fields, and the Duke’s gamekeepers had
orders to shoot them; how even a clergyman threatened them with personal
violence, and they had to do their work by stealth, while the reverend
gentleman was conducting the services in his church; how newspaper and
other writers declared that the locomotives would kill the birds,
prevent cows from grazing and hens from laying, burn houses, and cause
the extinction of the race of horses. All the civil engineers scouted
the idea of a locomotive railway, and Stephenson was held up to derision
as an ignoramus and a maniac by the “most eminent lawyers,” and the most
advanced and “respectable” professional C.E.s of the time. An article
appeared in the “Quarterly Review,” very favourable to the construction
of railways, but remarking in reference to a proposed line between
London and Woolwich: “What can be more palpably absurd and ridiculous
than the prospect held out of locomotives travelling _twice as fast as
stage coaches_! We should as soon expect the people of Woolwich to
suffer themselves to be fired off upon one of Congreve’s _ricochet_
rockets as trust themselves to the mercy of a machine going at such a
rate. We will back old Father Thames against the Woolwich Railway for
any sum. We trust that Parliament will, in all railways it may sanction,
limit the speed to _eight or nine miles an hour_, which we entirely
agree with Mr. Sylvester is as great as can be ventured on with safety.”
This passage, which reads so strangely now, may be seen in the
“Quarterly Review” for March, 1825. But still more curious appear now
the reports of the debates in Parliament, and of the evidence taken
before the Parliamentary Committee, in which we find the opinions and
fears of the best informed men of that period, and trace the frantic
efforts of the holders of the “vested interests” to retain them, however
obstructive of the public good.
[Illustration:
FIG. 41.—_Sankey Viaduct._
]
[Illustration:
FIG. 42.—_Rails and Cramp-gauge._
]
When it has been decided to construct a railway between two places, the
laying-out of the line is a subject requiring great consideration and
the highest engineering skill—for the matter is, on account of the great
cost, much more important than the setting-out of a common road. The
idea of a perfect railroad is that of a straight and level line from one
terminus to another; but there are many circumstances which prevent such
an idea from being ever carried into practice. First, it is desirable
that the line should pass through important towns situated near the
route; and then the cost of making the roadway straight and level, in
spite of natural obstacles, would be often so great, that to avoid it
detours and inclines must be submitted to—the inconvenience and the
increased length of road being balanced by the saving in the cost of
construction. It is the business of the engineer who lays out the line
to take all these circumstances into consideration, after he has made a
careful survey of the country through which the line is to pass. The
cost of making railways varies, of course, very much according to the
number and extent of the tunnels, cuttings, embankments, or other works
required. The average cost of each mile of railway in Great Britain may
be stated as about £35,000. The road itself when the rails are laid down
is called _the permanent way_, perhaps originally in distinction to the
temporary tramways laid down by the contractors during the progress of
the works. The permanent way is formed first of _ballast_, which is a
layer of gravel, stone, or other carefully chosen material, about 2 ft.
deep, spread over the roadway. Above the ballast and partly embedded in
it are placed the _sleepers_, which is the name given to the pieces of
timber on which the rails rest. These timbers are usually placed
transversely—that is, across the direction of the rails, in the manner
shown in Fig. 42. This figure also represents the form of rails most
commonly adopted, and exhibits the mode in which they are fastened down
to the sleepers by means of the iron _chairs_, _b_ _c_, the rail being
firmly held in its place by an oak wedge, _d_. These wedges are driven
in while the rails are maintained at precisely the required distance
apart by the implement, _e f_, called a _cramp gauge_, the chairs having
previously been securely attached to the sleepers by bolts or nails. The
double ⟙ form of rail has several important advantages, such as its
capability of being reversed when the upper surface is worn out, and the
readiness with which the ends of the rails can be joined by means of
_fish-plates_. These are shown in Fig. 43, where in a we are supposed to
be looking down on the rails, and in B to be looking at them sideways.
In Fig. 44 we have the rail and fish-plates in section. The holes in the
rails through which the bolts pass are not round but oval, so that a
certain amount of play is permitted to the ends of the rails.
[Illustration:
FIG. 43.—_Fish-plate._
]
It may easily be seen on looking at a line of rails that they are not
laid with the ends quite touching each other, or, at least, they are not
usually in contact. The reason of this is that space must be allowed for
the expansion which takes place when a rise in the temperature occurs.
If the rails are laid down when at the greatest temperature they are
likely to be subject to, they may then be placed in actual contact; but
in cold weather a space will be left by their contraction. For this
reason it is usual when rails are laid to allow a certain interval; thus
rails 20 ft. long laid when the temperature is 70°, are placed with
their ends 1/20th of an inch apart, at 30° 1/10th of an inch apart, and
so on. The neglect of this precaution has sometimes led to damage and
accidents. A certain railway was opened in June, and after an excursion
train had in the morning passed over it, the midday heat so expanded the
iron, that the rails became in some places elevated 2 ft. above the
level, and the sleepers were torn up; so that, in order to admit of the
return of the train, the rails had to be hastily relaid in a kind of
zigzag. In June, 1856, a train was thrown off the metals of the
North-Eastern Railway, in consequence of the rails rising up through
expansion.
[Illustration:
FIG. 44.—_Section of Rails and Fish-plates._
]
The distance between the rails in Great Britain is 4 ft. 8½ in., that
width having been adopted by George Stephenson in the construction of
the earlier lines. Brunel, the engineer of the Great Western, adopted,
however, in the construction of that railway, a gauge of 7 ft., with a
view of obtaining greater speed and power in the engines, steadiness in
the carriages, and increased size of carriages for bulky goods. The
proposal to adopt this gauge gave rise to a memorable dispute among
engineers, often called “The Battle of the Gauges.” It was stated that
any advantages of the broad gauge were more than compensated by its
disadvantages. The want of uniformity in the gauges was soon felt to be
an inconvenience to the public, and a Parliamentary Committee was
appointed to consider the subject. They reported that either gauge
supplied all public requirements, but that the broad gauge involved a
great additional outlay in its construction without any compensating
advantages of economy in working; and, as at that time 2,000 miles of
railway had been constructed on the narrow gauge, whereas only 270 miles
were in existence on the broad gauge, they recommended that future
railways should be made the prevailing width of 56½ in. The Great
Western line had engines, bridges, tunnels, viaducts, &c., on a larger
scale than any other railway in Britain. The difference of gauge was
after a time felt to involve so much inconvenience that lines which
adopted the 7–ft. gauge have since relaid the tracks at the more common
width. At the present day we find the Great Western Railway completely
reconstructed on the narrow gauge system, in order that trains may run
without interruption in connection with other lines.
[Illustration:
FIG. 45.—_Conical Wheels._
]
[Illustration:
FIG. 46.—_Centrifugal Force._
]
The wheels of railway carriages and engines differ from those of
ordinary carriages in being fastened in pairs upon the axles, with which
they revolve (see Fig. 45). The tire of the wheel is conical, the slope
being about 1 in 20; that is, in a wheel 5 in. broad the radius of the
outer edge is ¼ in. less than that of the inner; and the rails are
placed sloping a little inwards. The effect of this conical figure is to
counteract any tendency to roll off the rails; for if a pair of wheels
were shifted a little to one side, the parts of the tires rolling upon
the rails being then of unequal circumference, would cause the wheels to
roll towards the other side. The conical shape produces this kind of
adjustment so well that the flanges do not in general touch the rails.
They act, however, as safeguards in passing over curves and junctions.
In curves the outer line of rails is laid higher than the inner, so that
in passing over them the train leans slightly inwards, in order to
counteract what is called the centrifugal force, to which any body
moving in a curve is subject. This so-called force is merely the result
of that tendency which every moving body has to continue its motion in a
straight line. A very good example of the effect of this may be seen
when a circus horse is going rapidly round the ring. The inclination
inwards is still more perceptible when a rider is standing on the
horse’s back, as shown in Fig. 46. The earth’s attraction of gravity is
pulling the performer straight down, and the centrifugal force would of
itself throw her outwards horizontally. The resultant or combined effect
of both acts is seen in the exact direction in which she is leaning, and
it presses her feet on the horse’s back, the animal itself being under
similar conditions. It is obvious that the amount of centrifugal force,
and therefore of inward slope, will increase with the speed and
sharpness of the curve, and on the railways the rails are placed so that
the slope counteracts the centrifugal force when the train travels at
about the rate of twenty miles per hour.
[Illustration:
FIG. 47.—_Points._
]
A very important part of the mechanism of a railway is the mode of
passing trains from one line of rails to another. Engines and single
carriages are sometimes transferred by means of _turn-tables_, but the
more general plan is by _switches_, which are commonly constructed as
shown in Fig. 47. There are two rails, A and B, tapering to a point and
fixed at the other end, so that they have sufficient freedom to turn
horizontally. A train passing in the direction shown by the arrow would
continue on the main line, if the points are placed as represented; but
if they be moved so that the _long tongue_ is brought into contact with
the rail of the main line, then the train would run on to the side
rails. These _points_ are worked by means of a lever attached to the
rod, C, the lever being either placed near the rails, or in a
_signal-box_, where a man is stationed, whose sole duty it is to attend
to the points and to the signals. The interior of a signal-box near an
important junction or station is shown in Fig. 48, and we see here the
numerous levers for working the points and the signals, each of these
having a connection, by rods or wires, with the corresponding point or
signal-post. The electric telegraph is now an important agent in railway
signalling, and in a signal-box we may see the bells and instruments
which inform the pointsman whether a certain section of the line is
“blocked” or “clear.” The signals now generally used on British railways
are made by the semaphore, which is simply a post from which an arm can
be made to project. When the driver of the train sees the arm projecting
from the left-hand side of the post, it is an intimation to him that he
must stop his train; when the arm is dropped half-way, so as to project
45° from the post, it is meant that he must proceed cautiously; when the
arm is down the line is clear. These signals, of course, are not capable
of being seen at night, when their place is supplied with lamps,
provided with coloured glasses—red and green—and also with an uncoloured
glass. The lamp may have the different glasses on three different sides,
and be turned round so as to present the required colour; or it may be
made to do so without turning, if provided with a frame having red and
green glasses, which can be moved like spectacles in front of it. The
meanings of the various coloured lights and the corresponding semaphore
signals are these:
| White _All right_ Go on.
↿ Green _Caution_ Proceed slowly.
┓ Red _Danger_ Stop.
[Illustration:
FIG. 48.—_Signal-box on the North London Railway._
]
A very clear account of the mode of working railway signals on what is
now called the _block system_, together with a graphic description of a
signal-box, was given in a paper which appeared some years ago in “The
Popular Science Review,” from the pen of Mr. Charles V. Walker, F.R.S.,
the telegraph engineer to the South-Eastern Railway Company, who was the
first to organize an efficient system of electric signalling for
railways. We may remark that the signalling instruments on the
South-Eastern line, and indeed on all the lines at the present day,
address themselves both to the ear and to the eye, for they consist
of—first, bells, on which one, two, or more blows are struck, each
series of blows having its own particular meaning; and, second, of a
kind of miniature signal-post, with arms capable of being moved by
electric currents into positions similar to those of the arm of an
actual signal-post, so that the position of the arms is made always to
indicate the state of the line. One arm of the little signal-post—the
left—is red, and it has reference to _receding_ trains; the other—viz.,
the right—arm is white, and relates to _approaching_ trains. Mr. Walker
thus describes the signalling:
“The ordinary position of the arms of the electro-magnetic telegraph
semaphores will be down; that is to say, when the line is clear of all
trains, and business begins, say in early morning, all the arms will be
down, indicating that no train is moving. When the first train is ready
to start, say from Charing Cross, the signalman will give the proper
bell-signal to Belvidere—two, three, or four blows, according as the
train is for Greenwich, for North Kent, or Mid-Kent, or for the main
line; and the Belvidere man will acknowledge this by one blow on the
bell in reply, and without raising the Charing Cross red or left arm.
This is the signal that the train may go on; and when the train has
passed, so that the Charing Cross man can see the tail lights, he gives
the out signal a second time, which the Belvidere man acknowledges, at
the same time raising the red arm at Charing Cross, behind the train,
and so protecting it until it has passed him at Belvidere, when he
signals to that effect to Charing Cross, at the same time putting down
the red arm there, as an indication that the line is again clear. While
these operations are going on for down trains, others precisely similar,
but in the reverse direction, are going on for up trains.... One and the
same pressure on the key sends a bell signal and raises or depresses the
semaphore arm as the case may require, a single telegraph wire only
being required for the combined system, as for the more simple bell
system.” In one of the signal-boxes on the South-Eastern line, Mr.
Walker states, on a certain day 650 trains or engines were signalled and
all particulars accurately entered in a book, the entries requiring the
writing down of nearly 8,000 figures: an illustration of the amount of
work quietly carried on in a signal-box for the advantage and security
of the travelling public.
Mr. Walker also gives us a peep into the inside of one of the
signal-boxes, thus: “The interior of a large signal-box exhibits a very
animated scene, in which there are but two actors, a man and a boy, both
as busy as bees, but with no hurry or bustle. The ruling genius of the
place is the strong, active, intelligent signalman, standing at one end
of the apartment, the monarch for the time being of all he surveys.
Immediately before him in one long line, extending from side to side, is
a goodly array of levers, bright and clean from constant use and careful
tending, each one labelled for its respective duty. Before him to the
right and left are the various electro-magnetic semaphores, each one in
full view and adjusted in position to the pair of roads to which it is
appropriated, and all furnished with porcelain labels. Directly in front
of him is a screen, along which are arranged the various semaphore keys;
and on brackets, discreetly distributed, are the bells and gongs, the
twin companions each of its own semaphore. Before the screen are the
writing-desk and books, and here stands the youngster, the ministering
spirit, all on the alert to take or to send electric signals and to
record them, his time and attention being devoted alternately to his
semaphore keys and to his books, being immediately under the eye and
control of the signalman. This is no place for visitors, and the scenes
enacted here have little chance of meeting the public gaze; indeed, the
officers whose duties take them hither occasionally are only too glad to
look on, and say as little as may be, and not interrupt the active pair,
between whom there is evidently a good understanding in the discharge of
duties upon the accurate performance of which so much depends. Looking
on, the man will be seen in command of his rank and file: signals come,
are heard and seen by both man and boy; levers are drawn and withdrawn,
one, two, three, or more; the arms and the lamps on the gigantic masts
outside, of which there are three, well laden, are displayed as
required, distant signals are moved, points are shifted and roads made
ready; telegraph signals are acknowledged; and on looking out—for the
box is glazed throughout—trains are seen moving in accordance with the
signals made; and on the signal-posts at the boxes, right and left—for
here they are within easy reach of each other—arms are seen up and down
in sympathy with those on the spot, and with the telegraph signals that
have been interchanged. There is no cessation to this work, and there is
no confusion in it; one head and hand directs the whole, so that there
are no conflicting interests and no misunderstandings; all is done in
perfect tranquillity, and the great secret is that one thing is done at
a time. All this, which is so simple and so full of meaning to the
expert, is to the uninitiated intricate and vague; and though he cannot
at first even follow the description of the several processes, so
rapidly are they begun and ended, yet, as the cloud becomes thin, and
his ideas become clearer, he cannot fail to be gratified, and to be
filled with admiration at the great results that are brought about by
means so simple.”
[Illustration:
FIG. 49.—_Post Office Railway Van._
]
Most of the carriages used on railways are so familiar to everyone that
it is unnecessary to give any description of them. We give a figure of
one which, though of early type, has special features of interest, being
the well-designed Travelling Post Office, Fig. 49. In such vans as that
here represented letters are sorted during the journey, and for this
purpose the interior is provided with a counter and with pigeon-holes
from end to end. When the train stops bags may, of course, be removed
from or received into the van in the ordinary manner; but by a simple
mechanism bags may be delivered at a station and others taken up while
the train continues its journey at full speed. A bar can be made to
project from the side of the carriage, and on this the bag is hung by
hooks, which are so contrived that they release the bag when a rod,
projecting from the receiving apparatus, strikes a certain catch on the
van. The bag then drops into a netting, which is spread for its
reception; and in order to receive the bags taken up, a similar netting
is stretched on an iron frame attached to the van This frame is made to
fold up against the side of the carriage when not in use. When the train
is approaching the station where the bag is to be taken up, this frame
is let down, and a projecting portion detaches the bags, so that they
drop into the net, from which they are removed into the interior of the
vehicle. These travelling post offices are lighted with gas, and are
padded at the ends, so that the clerks may not be liable to injury by
concussions of the carriages.
England has had to borrow from the United States not a few hints for
such adaptations and appliances as tend to promote the comfort and
convenience of travellers by rail, especially on what we insularly call
long journeys. Some of these vehicles on the American railways are
luxurious hotels upon wheels; they contain accommodation for forty
persons, having a kitchen, hot and cold water, wine, china and linen
closets, and more than a hundred different articles of food, besides an
ample supply of tablecloths, table napkins, towels, sheets, pillowcases,
&c. Then there are other Pullman inventions, such as the “palace” and
the “sleeping” cars, in which the traveller who is performing a long
journey makes himself at home for days, or perhaps for a week, as, for
instance, while he is being carried across the American continent from
ocean to ocean at the easy rate of twenty miles an hour on the Pacific
and other connecting lines. Mr. C. Nordhoff, an American writer, giving
an account of his journey to the Western States, writes thus: “Having
unpacked your books and unstrapped your wraps in your Pullman or Central
Pacific palace car, you may pursue all the sedentary avocations and
amusements of a parlour at home; and as your housekeeping is done—and
admirably done—for you by alert and experienced servants; as you may lie
down at full length, or sit up, sleep, or wake at your choice; as your
dinner is sure to be abundant, very tolerably cooked, and not hurried;
as you are pretty certain to make acquaintances in the car; and as the
country through which you pass is strange and abounds in curious and
interesting sights, and the air is fresh and exhilarating—you soon fall
into the ways of the voyage; and if you are a tired business man or a
wearied housekeeper, your careless ease will be such a rest as certainly
most busy and overworked Americans know how to enjoy. You write
comfortably at a table in a little room called a ‘drawing-room,’
entirely closed off, if you wish it, from the remainder of the car,
which room contains two large and comfortable armchairs and a sofa, two
broad clean plate-glass windows on each side (which may be doubled if
the weather is cold), hooks in abundance for shawls, hats, &c., and
mirrors at every corner. Books and photographs lie on the table. Your
wife sits at the window sewing and looking out on long ranges of
snow-clad mountains or on boundless ocean-like plains. Children play on
the floor or watch at the windows for the comical prairie dogs sitting
near their holes, and turning laughable somersaults as the car sweeps
by. The porter calls you at any hour you appoint in the morning; he
gives half an hour’s notice of breakfast, dinner, or supper; and while
you are at breakfast, your beds are made up and your room or your
section aired. About eight o’clock in the evening—for, as at sea, you
keep good hours—the porter, in a clean grey uniform, comes in to make up
the beds. The two easy-chairs are turned into a berth; the sofa
undergoes a similar transformation; the table, having its legs pulled
together, disappears in a corner, and two shelves being let down furnish
two other berths. The freshest and whitest of linen and
brightly-coloured blankets complete the outfit; and you undress and go
to bed as you would at home.”
An important general truth may find a familiar illustration in the
subject now under notice. The truth in question may be expressed by
saying that, in all human affairs, as well as in the operations of
nature, the state of things at any one time is the result, by a sort of
growth, of a preceding state of things. And in this way it is certainly
true of inventions, that they never make their appearance suddenly in a
complete and finished state—like Minerva, who is fabled to have sprung
from the brain of Jupiter fully grown and completely armed; but rather
their history resembles the slow and progressive process by which
ordinary mortals attain to their full stature. We have already seen that
railways had their origin in the tramways of collieries; and, in like
manner, the railway carriage grew out of the colliery truck and the
stage coach; for when railway carriages to convey passengers were first
made, it did not occur to their designers that anything better could be
done than to place coach bodies on the frame of the truck; and
accordingly the early railway carriages were formed by mounting the body
of a stage coach, or two or three such bodies side by side, on the
timber framework which was supported by the flanged wheels. The cut,
Fig. 56, is from a painting in the possession of the Connecticut
Historical Society, and it represents the first railway train in America
on its trial trip (1831), in which sixteen persons took part, who were
then thought not a little courageous. Here we see that the carriages
were regular stage coaches, and the same was the case in England. But it
is very significant that, to this day, the stage coach bodies are
traceable in many of the carriages now running on English lines,
especially in the first-class carriages, where, in the curved lines of
the mouldings which are supposed to ornament the outside, one may easily
recognize the forms of the curved bodies of the stage coaches, although
there is nothing whatever, in the real framing of the timbers of the
railway carriage, which has the most distant relation to these curves.
Then again, almost universally on English lines, the old stage coach
door-handles are still retained on the first-class carriages, in the
awkward flat oval plates of brass which fold down with a hinge. Many
other points might be named which would show the persistence of the
stage coach type on the English railways. The cut, Fig. 56, proves that
the Americans set out with the same style of carriages; but North
America, as compared with the Old World, is _par excellence_ the country
of rapid developments, and there carriages, or cars, as our
Transatlantic cousins call them, have for a long time been made with
numerous improvements, and in forms more in harmony with the railway
system, than the conservatism of English ideas, still cleaving to the
stage coach type, permitted to be attempted in this country.
Railway travellers in the United States had long enjoyed the benefit of
comforts and convenience in the appointments of their carriages long
before any change had been effected in the general arrangements of the
vehicles provided by the railway companies in England. It is now indeed
a considerable number of years since this state of things has been
altered in the older country; as all the great lines, following the
example of the Midland Company, who first adopted the Pullman cars, have
constructed luxurious vehicles in which every elegance and comfort are
placed within the reach of the English traveller, and these improvements
are highly appreciated by all who have long journeys to make by day or
night.
The elegance and comfort of the arrangements are almost too obvious to
require description. We see the luxuriously padded chairs, which, by
turning on swivels, permit the traveller to adjust his position
according to his individual wishes, so that he can, with ease, place his
seat either to gaze directly on the passing landscape, or turn his face
towards his fellow-travellers opposite or on either side. The chairs are
also provided with an arrangement for placing the backs at any required
inclination, and the light and refined character of the decorations of
the carriage should not escape the reader’s notice. Pullman Cars of
another kind, providing sleeping accommodation for night journeys, are
also in use on the Midland line, and they are fitted up with the same
thoughtful regard to comfort as the Parlour Car.
The great engineering feats which have been accomplished in the
construction of railways are numerous enough to fill volumes. We give,
therefore, only a short notice of one or two recently constructed lines
which have features of special interest, concluding with a brief account
of such remarkable constructions as the railway by which the traveller
may now go up the Rigi, and the railways which ascend Vesuvius and Mt.
Pilatus.
_THE METROPOLITAN RAILWAYS._
When the traffic in the streets of London became so great that the
ordinary thoroughfares were unable to meet public requirements, the bold
project was conceived of making a railway under the streets. The
construction of a line of railway beneath the streets of a populous
city, amidst a labyrinth of gas-pipes, water-mains, sewers, &c., is
obviously an undertaking presenting features so remarkable that the
London Underground Railway cannot here be passed over without a short
notice. Its construction occupied about three years, and it was opened
for traffic in 1863. The line commencing at Paddington, and passing
beneath Edgware Road at right angles, reaches Marylebone Road, under the
centre of which it proceeds, and passing beneath the houses at one end
of Park Crescent, Portland Place, it follows the centre of Euston Road
to King’s Cross, where connection with the Great Northern and Midland
system is effected. Here the line bends sharply southwards, and proceeds
to Farringdon Street Station, the original terminus. A subsequent
extension takes an easterly direction and reaches Aldgate Station, the
nominal terminus. The crown of the arch which covers the line is in some
places only a few inches beneath the level of the streets; in other
places it is several feet below the surface, and, in fact, beneath the
foundations of the houses and other buildings. The steepest gradient on
the line is 1 in 100, and the sharpest curve has a radius of 200 yards.
The line is nearly all curved, there not being in all its length
three-quarters of a mile of straight rails. The difficulties besetting
an undertaking of this kind would be tedious to describe, but may
readily be imagined. The line traverses every kind of soil—clay, gravel,
sand, rubbish, all loosened by previous excavations for drains, pipes,
foundations, &c.; and the arrangements of these drains, water and
gas-pipes, had to be reconciled with the progress of the railway works,
without their uses being interfered with even for a time. Of the
stations the majority have roofs of the ordinary kind, open to the sky;
but two of them, namely, Baker Street and Gower Street, are completely
underground stations, and their roofs are formed by the arches of
brickwork immediately below the streets. The arrangements at these
stations show great boldness and inventiveness of design. The booking
offices for the up line are on one side of the road, and those for the
down line on the other. Fig. 50 represents the interior of the Gower
Street Station, and the other is very similar. In each the platforms are
325 ft. long and 10 ft. broad, and the stations are lighted by lateral
openings through the springing of the arch which forms the roof. This
arch is a portion of a circle of 32 ft. radius, with a span of 45 ft.
and a rise of 9 ft. at the crown. The lateral openings are arched at the
top and bottom, but the sides are flat. The width of each is 4 ft. 9
in., and the height outside 6 ft., increasing to 10 ft. at the ends
opening on the platform. The openings are entirely lined with white
glazed tiles, and the outward ends open into an area, the back of which
is inclined at an angle of 45°, and the whole also lined with white
glazed tiles, and covered with glass, except where some iron gratings
are provided for ventilation. The tiles reflect the daylight so
powerfully that but little gas is required for the illumination of the
station in the day-time. The arched roofs of these stations are
supported by piers of brickwork, 10 ft. apart, 5 ft. 6 in. deep, and 3
ft. 9 in. wide. In the spaces between the piers vertical arches, like
parts of the brick lining of a well, are wedged in, to resist the thrust
of the earth, and a straight wall is built inside of this between the
piers, to form the platform wall of the station. The tops of the piers
are connected by arches, and are thus made to bear the weight of the
arched roof, which has 2 ft. 3 in. thickness of brickwork at the crown,
and a much greater thickness towards the haunches.
[Illustration:
FIG. 50.—_Gower Street Station, Metropolitan Railway._
]
The benefit derived by the public from the completion of the
Metropolitan Railway was greatly increased by the subsequent
construction of another railway—“The Metropolitan District,” which,
joining the Metropolitan at Paddington, makes a circuit about the
west-end of Hyde Park, and passing close to the Victoria Terminus of the
London, Chatham, and Dover and the Brighton and South Coast Railways,
reaches Westminster Bridge, and then follows the Thames Embankment to
Blackfriars Bridge, where it leaves the bank of the river for the
Mansion House, Mark Lane and Aldgate stations. This line, taken in
conjunction with the Metropolitan, forms the “_inner circle_” of the
railway communication in London. The circuit was for a long time
incomplete at the east by the want of connection between the Mansion
House Station and that of Moorgate Street, although these stations are
but little more than half a mile apart. A line connecting these two
points has lately been constructed at great cost, and the public now
possess a complete circle of communication. The number of trains each
day entering and leaving some of the stations on the Metropolitan system
is very great. Moorgate Street Station—a terminus into which several
companies run—may have about 800 trains arriving or departing in the
course of a day.
_THE PACIFIC RAILWAY._
The remarkable development of railways which has taken place in the
United States has its most striking illustration in the great system of
lines by which the whole continent can be traversed from shore to shore.
The distance by rail from New York to San Francisco is 3,215 miles, and
the journey occupies about a week, the trains travelling night and day.
The traveller proceeding from the Eastern States to the far west has the
choice of many routes, but these all converge to Omaha. From this point
the Pacific Railroad will convey him towards the land of the setting
sun. The map, Fig. 51, shows the course of this railway, which is the
longest in the world. It traverses broader plains and crosses higher
mountains than any other. Engineering skill of the most admirable kind
has been displayed in the laying-out and in the construction of the
line, with its innumerable cuttings, bridges, tunnels, and snow-sheds.
[Illustration:
FIG. 51.—_Map of the Route of the Pacific Railway._
]
The road from Omaha to Ogden, near the Great Salt Lake—a distance of
1,032 miles—is owned by the Union Pacific Company, and the Central
Pacific joins the former at Ogden and completes the communication to San
Francisco, a further length of 889 miles—the whole distance from Omaha
to San Francisco being 1,911 miles. The Union Pacific was commenced in
November, 1865, and completed in May, 1869. There are at Omaha extensive
workshops provided with all the appliances for constructing and
repairing locomotives and carriages, and these works cover 30 acres of
ground, and give employment to several thousand men. The population of
Omaha rose during the making of the railway from under 3,000 in 1864 to
more than 16,000 in 1870, and it is now a flourishing town. A little
distance from Omaha the line approaches the Platte River, and the valley
of this river and one of its tributaries is ascended to Cheyenne, 516
miles from Omaha, the line being nowhere very far from the river’s
course. Cheyenne is 5,075 ft. higher above the sea than Omaha, the
elevation of which is 966 ft. The Platte River is a broad but very
shallow stream, with a channel continually shifting, owing to the vast
quantity of sand which its muddy waters carry down. This portion of the
line passing through a district where leagues upon leagues of fertile
land await the hand of the tiller, has opened up vast tracts of
land—hedgeless, gateless green fields, free to all, and capable of
receiving and supporting millions of human inhabitants. Cheyenne, a town
of 3,000 inhabitants, is entirely the creation of the railways, for
southward from Cheyenne a railway passes to Denver, a distance of 106
miles, through rich farming and grazing districts. Seven miles beyond
Cheyenne the line begins to ascend the Black Hills by steep gradients,
and at Granite Canyon, for example, the rise in five miles is 574 ft.,
or about 121 ft. per mile. Many lime-kilns have been erected in this
neighbourhood, where limestone is very abundant. A little beyond this
point the road is in many places protected by snow-sheds, fences of
timber, and rude stonework. At Sherman, 549 miles from Omaha, the line
attains the summit of its track over the Black Hills, and the highest
point on any railway in the world, being 8,242 ft. above the level of
the sea. Wild and desolate scenery characterizes the district round
Sherman, and the hills, in places covered with a dense growth of wood,
will furnish an immense supply of timber for years to come. The
timber-sheds erected over the line, and the fences beside it are not so
much on account of the depth of snow that falls, but to prevent it from
blocking the line by being drifted into the cuts by the high wind. A few
miles beyond Dale Creek at Sherman is the largest bridge on the line. It
is a trestle bridge, 650 ft. long and 126 ft. high, and has a very light
appearance—indeed, to an English eye unaccustomed to these _impromptu_
timber structures, it looks unpleasantly light. From Sherman the line
descends to Laramie, which is 7,123 ft. above the sea level and 24 miles
from Sherman, and here the railway has a workshop, for good coal is
found within a few miles. A fine tract of grazing land, 60 miles long
and 20 miles broad, stretches around this station, and it is said that
nowhere in the whole North American continent can cattle be reared and
fattened more cheaply. The line, now descending the Black Hills, crosses
for many miles a long stretch of rolling prairie, covered in great part
with sage-bush, and forming a tableland lying between the western base
of the Black Hills and the eastern base of the snowy range of the Rocky
Mountains, which latter reach an elevation of from 10,000 to 17,000 ft.
above the sea level and are perpetually covered with snow. Such
tablelands are termed in America “parks.” Before the line reaches the
summit of the pass by which it crosses the range of the snowy mountains,
it traverses some rough country among the spurs of the hills—through
deep cuts and under snow-sheds, across ravines and rivers, and through
tunnels. At Percy, 669 miles, is a station named after Colonel Percy,
who was killed here by the Indians when surveying for the line. He was
surprised by a party of the red men, and retreated to a cabin, where he
withstood the attack of his assailants for three days, killing several
of them; but at length they set fire to the cabin, and the unfortunate
Colonel rushing out, fell a victim to their ferocity. Near Creston, 737
miles from Omaha, the highest point of the chief range is reached,
though at an elevation lower by 1,212 ft. than the summit of the pass
where the line crosses the Black Hills, which are the advanced guard of
the Rocky Mountains. Here is the water-shed of the continent, for all
streams rising to the east of this flow ultimately into the
Atlantic,—while these, having their sources in the west, fall into the
Pacific. Before reaching Ogden the line passes through some grand
gorges, which open a way for the iron horse through the very hearts of
the mountains, as if Nature had foreseen railways and providently formed
gigantic cuttings—such as the Echo and Weber Canyons, which enable the
line to traverse the Wahsatch Mountains.
[Illustration:
FIG. 52.—_Trestle Bridge._
]
Echo Canyon is a ravine 7 miles long, about half a mile broad, flanked
by precipitous cliffs, from 300 to 800 ft. high, and presenting a
succession of wild and grand scenery. In Weber Canyon the river foams
and rushes along between the mountains, which rise in massive grandeur
on either side, plunging and eddying among the huge masses of rock
fallen from the cliffs above. Along a part of the chasm the railway is
cut in the side of the steep mountain, descending directly to the bed of
the stream. Where the road could not be carried round or over the spurs
of the mountains it passes through tunnels, often cut through solid
stone. A few miles farther the line reaches the city of Ogden, in the
state of Utah, the territory of the Mormons. This territory contains
upwards of 65,000 square miles, and though the land is not naturally
productive, it has, by irrigation, been brought into a high state of
cultivation, and it abounds in valuable minerals, so that it now
supports a population of 80,000 persons.
[Illustration:
FIG. 53.—_American Canyon._
]
We have now arrived at Ogden, where the western portion of the great
railway line connecting the two oceans unites to the Union Pacific we
have just described. This western portion is known as the Central
Pacific Railroad, and it stretches from Ogden to San Francisco, a
distance of 882 miles.
The portion of the line which unites Sacramento to Ogden, 743 miles, was
commenced in 1863 and finished in 1869, but nearly half of the entire
length was constructed in 1868, and about 50 miles west of Ogden, the
remarkable engineering feat of laying 10 miles of railway in one day was
performed. It was thus accomplished: when the waggon loaded with the
rails arrived at the end of the track, the two outer rails were seized,
hauled forward off the car, and laid upon the sleepers by four men, who
attended to this duty only. The waggon was pushed forwards over these
rails, and the process of putting down the rails was repeated, while
behind the waggon came a little army of men, who drove in the spikes and
screwed on the fish-plates, and, lastly, a large number of Chinese
workmen, with pickaxes and spades, who ballasted the line. The average
rate at which these operations proceeded was about 240 ft. of track in
77½ seconds, and in these 10 miles of railway there were 2,585,000
cross-ties, 3,520 iron rails, 55,000 spikes, 7,040 fish-plates, and
14,080 bolts with screws, the whole weighing 4,362,000 lbs.! Four
thousand men and hundreds of waggons were required, but in the 10 miles
all the rails were laid by the same eight men, each of whom is said to
have that day walked 10 miles and lifted 1,000 tons of iron rails.
Nothing but the practice acquired during the four previous years and the
most excellent arrangement and discipline could have made the
performance of such a feat possible as the laying of eight miles of the
track in six hours, which was the victory achieved by these stalwart
navvies before dinner.
The line crosses the great American desert, distinguished for its
desolate aspect and barren soil, and so thickly strewn with alkaline
dust that it appears almost like a snow-covered plain. The alkali is
caustic, and where it abounds no vegetation can exist, most of the
surface of this waste being fine, hard grey sand, mixed with the
fragments of marine shells and beds of alkali.
The third great mountain range of the North American continent is
crossed by this line, at an elevation of 7,043 ft. above the sea level.
The Sierra Nevada, as the name implies, is a range of rugged wild broken
mountain-tops, always covered with snow. The more exposed portions of
the road are covered with snow-sheds, solidly constructed of pine wood
posts, 16 in. or 20 in. across: the total length of snow-sheds on the
Sierra Nevada may equal 50 miles. These sheds sometimes take fire; but
the company have a locomotive at the Summit Station, ready to start at a
moment’s notice with cars carrying tanks of water. The snow falls there
sometimes to a depth of 20 ft. in one winter; and in spring, when it
falls into the valleys in avalanches, sweeping down the mountain-sides,
they pass harmlessly over the sloping roofs of the snow-sheds. Where the
line passes along the steep flank of a mountain, the roofs of these
snow-sheds abut against the mountain-side, so that the masses of snow,
gliding down from its heights, continue their slide without injury to
line, or sheds, or trains. Where, however, the line lies on level
ground, or in a ridge, the snow-sheds are built with a strong roof of
double slope, in order to support or throw off the snow. From Summit
(7,017 ft.) the line descends continuously to Sacramento, which is only
30 ft. above the sea level, and 104 miles from Summit. About 36 miles
from Summit, the great American Canyon, one of the wildest gorges in the
Sierra Nevada range, is passed. Here the American River is confined for
a length of two miles between precipitous walls of rock, 2,000 ft. in
height, and so steep that no human foot has ever yet followed the stream
through this tremendous gorge (Fig. 53). A few miles beyond this the
line is carried, by a daring feat of engineering, along the side of a
mountain, overhanging a stream 2,500 ft. below. This mountain is known
as “Cape Horn,” and is a place to try the nerves of timid people. When
this portion of the line was commenced, the workmen were lowered and
held by ropes, until they had hewn out a standing-place on the shelving
sides of the precipice, along whose dizzy height, where even the agile
Indian was unable to plant his foot, the science of the white man thus
made for his iron horse a secure and direct road. (Fig. 54.)
[Illustration:
FIG. 54.—“_Cape Horn._”
]
[Illustration:
FIG. 55.—_Snow Plough._
]
These lines of railway, connecting Omaha with Sacramento, are remarkable
evidences of the energy and spirit which characterize the Anglo-Saxon
race in America. The men who conceived the design of the Central Pacific
Railroad, and actually carried it into effect, were not persons
experienced in railway construction; but five middle-aged traders of
Sacramento, two of whom where drapers, one a wholesale grocer, and the
others ironmongers, believing that such a railway should be made, and
finding no one ready to undertake it, united together, projected the
railway, got it completed, and now manage it. These gentlemen were
associated with an engineer named Judah, who was a sanguine advocate of
the scheme, and made the preliminary surveys, if he did not plan the
line. The line is considered one of the best appointed and best managed
in the States; yet the project was at first ridiculed and pronounced
impracticable by engineers of high repute, opposed by capitalists, and
denounced by politicians. An eminent banker, who personally regarded the
scheme with hopefulness, would not venture, however, to take any stock,
lest the credit of his bank should be shaken, were he known to be
connected with so wild a scheme. And, indeed, the difficulties appeared
great. Except wood, all the materials required, the iron rails, the
pickaxes and spades, the waggons, the locomotives, and the machinery had
to be sent by sea from New York, round Cape Horn, a long and perilous
voyage of nine months duration, and transhipped at San Francisco for
another voyage of 120 miles before they could reach Sacramento. Add to
this that workmen were so scarce in California, and wages so high, that
to carry on the work it was necessary to obtain men from New York; and
during its progress 10,000 Chinamen were brought across the Pacific, to
work as labourers. Subscriptions came in very slowly, and before 30
miles of the line had been constructed, the price of iron rose in a very
short time to nearly three times its former amount. At this critical
juncture, the five merchants decided to defray, out of their own private
fortunes, the cost of keeping 800 men at work on the line for a whole
year. We cannot but admire the unswerving confidence in their enterprise
displayed by these five country merchants, unskilled in railway making,
unaided by public support, and even discouraged in their project by
their own friends. The financial and legal obstacles they successfully
surmounted were not the only difficulties to be overcome. They had the
engineering difficulties of carrying their line over the steep Sierra, a
work of four years; long tunnels had to be bored; one spring when snow
60 ft. in depth covered the track, it had to be removed by the shovel
for 7 miles along the road; saw-mills had to be erected in the
mountains, to prepare the sleepers and other timber work; wood and water
had to be carried 40 miles across alkali plains, and locomotives and
rails dragged over the mountains by teams of oxen. The chief engineer,
who organized the force of labourers, laid out the road, designed the
necessary structures, and successfully grappled with the novel problem
of running trains over such a line in all seasons, was Mr. S. S.
Montague. The requirements of the traffic necessitate not only solidly
constructed iron-covered snow-sheds, but massive snow-ploughs to throw
off the track the deep snow which could in no other way be prevented
from interrupting the working of the line. These snow-ploughs are
sometimes urged forward with the united power of eight heavy
locomotives. Fig. 55 represents one of these ploughs cleaning the line,
by throwing off the snow on to the sides of the track. The cutting
apparatus varies in its arrangements, some forms being designed to push
the snow off on one side, some on the other, and to fling it down the
precipices; and others, like the one represented, are intended merely to
throw it off the track.
[Illustration:
FIG. 56.—_The first Steam Railroad Train in America._
]
Sacramento is 1,775 miles from Omaha, and is connected with San
Francisco by a line 139 miles long. At San Francisco, or rather at
Oakland, 1,911 miles from Omaha and 3,212 miles from New York, is the
terminus of the great system of lines connecting the opposite shores of
the vast North American continent. San Francisco, situated on the
western shore of a bay, is connected with Oakland by a ferry; but the
railway company have recently constructed a pier, which carries the
trains out into the bay for 2¼ miles. This pier is strongly built, and
is provided with a double set of rails and a carriage-road, and with
slips at which ships land and embark passengers, so that ships trading
to China, Japan, and Australia can load and unload directly into the
trains, which may pass without change from the shores of the Pacific to
those of the Atlantic Ocean. San Francisco is a marvellous example of
rapid increase, for the population now numbers 170,000, yet a quarter of
a century ago 500 white settlers could not be found in as many miles
around its site. The first house was erected in 1846, and in 1847 not a
ship visited the bay, but now forty large steamships ply regularly,
carrying mails to China, Japan, Panama, South America, Australia, &c.,
and there are, of course, hundreds of other steamers and ships.
The descriptions we have given of only two lines of railway may suffice
to show that the modern engineer is deterred by no obstacles, but boldly
drives his lines through places apparently the most impracticable. He
shrinks from no operations however difficult, nor hesitates to undertake
works the mere magnitude of which would have made our forefathers stand
aghast. Not in England or America alone, but in almost every part of the
world, the railways have extended with wonderful rapidity; the continent
of Europe is embraced by a network of lines; the distant colonies of
Australia and New Zealand have thousands of miles of lines laid down,
and many more in progress; the map of India shows that peninsula
traversed in all directions by the iron roads; and in the far distant
East we hear of Japan having several lines in successful operation, and
the design of laying down more. In connection with such works, at home
and abroad, many constructions of great size and daring have been
designed and erected; many navigable rivers have been bridged, and not
seldom has an arm of the sea itself been spanned; hundreds of miles of
embankments and viaducts have been raised; hills have been pierced with
innumerable cuttings and tunnels, and all these great works have been
accomplished within the experience of a single generation of men, and
have sprung from one single successful achievement of Stephenson’s—the
Liverpool and Manchester Railway, completed and opened in 1830. We in
England should also have pride in remembering that the growth of the
railways here is due to the enterprise, industry, and energy of private
persons; for the State has furnished no funds, but individuals, by
combining their own resources, have executed the works, and manage the
lines for their common interest and the public good. It is said that the
amount of money which has been spent on railways in Great Britain is not
far short of 500 millions of pounds sterling. The greatest railway
company in the United Kingdom is the London and North-Western, which
draws in annual receipts about seven millions of pounds; and the total
receipts of all the railway companies would nearly equal half the
revenue of the State.
[Illustration:
FIG. 57.—_Railway Embankment near Bath._
]
[Illustration:
PLATE VI.
MOUNT WASHINGTON INCLINED TRACK.
]
_INCLINED RAILWAYS._
The construction of railways over lofty ranges of mountains will be
found illustrated by the brief notices in other pages of the Union
Pacific line in the United States, and of the St. Gothard railway over
the Alps. In such cases, the track has been to a great extent carried
over the spurs or along the sides of the mountains, so that such
inclines might be obtained as the ordinary locomotive was capable of
ascending. The expensive operation of tunnelling was resorted to only
where sinuous deviations from the more direct route involved a still
greater expenditure of initial cost, or a continual waste of time and
energy in the actual working of the line. Sometimes winding tracks,
almost returning by snake-like loops on their own route, as projected on
the map, were required in order that the ascent could be made with an
incline practicable for the ordinary locomotive. In the earlier
development of railways, there were to be met with cable inclines, where
the traction of the locomotive had to be superseded or supplemented by
that of a rope or chain wound round a drum actuated by a stationary
steam-engine. The more powerful locomotives of the present day are able
to mount grades of such inclination that the employment of cable
traction is no longer requisite, except in but a few cases. Railways had
carried passengers about in all parts of the world for many years before
the engineer addressed himself to the problem of easily and quickly
taking people up heights of steep and toilsome ascent, sought generally
for the sake of the prospect, etc. Such, at least, has been the object
of most of the inclined railways already constructed, but to this their
utility is by no means limited, and as their safety and stability has
been proved by many years of use, they may find wider applications than
the gratification of the tourist and pleasure-seeker.
The toothed rail or rack which was formerly supposed necessary to obtain
power of traction on rails has been already mentioned (p. 101), and as
early as 1812 such a contrivance appears to have been in use in England,
near Leeds, the invention of a Mr. Blenkinsop. This mode of traction
received no development or improvement worthy of notice until Mr. S.
Marsh constructed, in 1866, a railway ladder—for so it may be called—for
the ascent of Mount Washington in the United States. In this case there
was a centre rail formed of iron, angle iron laid between and parallel
to the metals on which ran the wheels of the carriages. In this centre
rail angle irons were connected by round bars of wrought iron, which the
teeth of a pinion of the locomotive engaged, so that a climbing action,
resembling somewhat that of a wheel entering on the successive rounds of
a ladder, was produced, and in this way an ascensive power was obtained
sufficient to overcome gravity, the gradient not much exceeding a rise
of one foot in three at any point (12 vertical to 32 horizontal). This
railway was completed in 1869, and for more than a quarter of a century
it has carried thousands of tourists to the summit of Mount Washington
without a single fatal accident. This system of ascending mountains was
soon adopted in Europe with certain improvements, for in 1870 an
inclined railway was constructed to the summit of the Rigi, in which a
system of involute gearing was substituted for the ladder-like rounds of
Mr. Marsh. A certain vibratory action, due to the successive engagements
of the teeth in the central rack, which was somewhat disagreeable for
passengers, was soon afterwards obviated in the Abt system, in which two
racks are used, with the teeth of one opposite the spaces of the other,
and a double pinion provided, so that greater uniformity in the acting
power is obtained. With certain modifications in detail, such as
horizontal instead of vertical pinions, this system has been largely
adopted wherever cables have been dispensed with. In the inclined
railway by which Mount Pilatus, near Lucerne, is now ascended,
horizontal teeth project from both sides of a centre rail, and these are
engaged by horizontal pinions. The incline here is very steep, being in
places nearly 30 degrees; teeth perpendicular to the plane of the
incline would have offered a less margin of safety than those on the
plan actually adopted. In some places, as among the Alps, and more
particularly in South America, there are railways in which the ordinary
mode of traction and that with the rack are combined; that is, where the
gradient exceeds the ordinary limit, a central rack-rail is laid down,
on approaching which the engineer slackens his speed, and allows
pinions, moved by the locomotive, to become engaged in the double rack,
by which he slowly climbs the steep ascent until a level tract is
reached which permits of the ordinary traction being resumed.
[Illustration:
FIG. 57_a_.—_Train Ascending the Rigi._
]
Instead of climbing the inclines by rack-work rails, there is another
system which offers great advantages for economy in working, and one
generally resorted to where the incline can be made in one vertical
plane. This is the balanced cable, in which the gravitation force of a
descending car or train is utilised to draw up, or assist to draw up,
the ascending car or train. These cars are attached to the ends of a
cable which passes round a drum at the top of the incline, and means are
provided, according to circumstances, so that the drum may be turned, or
its revolutions controlled by brakes. When there is a water supply at
the upper end of the incline, a simple and economical mode of working
the cable is available; for all that is necessary is to provide each car
with a water-tank capable of being rapidly filled and emptied. The upper
car is made the heavier when required, by filling its tank with water,
when it raises the lower car, and on itself arriving at the bottom, the
water is discharged before the load to be taken up is received.
[Illustration:
FIG. 57_b_.—_At the Summit of the Rigi._
]
Many inclined railways are now in operation in various parts of the
world, as at Mount Vesuvius, where two of the slopes have a combined
length of 10,500 feet; at Mount Supurga and at Mount San Salvatore there
are others. At Burgen-stock in Switzerland there is one having a slope
57 feet vertical to 100 feet horizontal. These are cable inclines; but a
rack is also used with a pinion regulated by a friction-brake to avoid
accident, in case of the cable parting. The largest inclined railway in
America is at the Catskill Mountains, where an ascent of 1,600 feet is
made in a horizontal distance of 6,780 feet. In this a novel plan has
been adopted for compensating the varying weight that has to be moved,
for it is obvious that at the commencement the load at the top of the
incline has to raise not only that at the bottom, but the whole weight
of the cable also, equal to 35,000 pounds of wire rope, and again after
the middle point has been passed, the descending power is constantly
increasing, while the load being raised is diminishing. Now, in order
that the engine may work with more uniform effect, the engineer has not
made the incline a straight line, but with the slope lightest at the
bottom and gradually increased towards the top, so that the line is
really a curve in the vertical plane, and has at every point just the
inclination required for balancing the weight of the wire cable, as this
shifts from the one track to the other. Instead of a rack pinion and
brake to control a too rapid descent from any accident, the cars are
provided with clutches, which are automatically thrown out on wooden
guard-rails, when a safe speed is exceeded. Inclined railways have also
been constructed to the summit of Snowdon, in North Wales, and to that
of the Jungfrau, in Switzerland.
[Illustration]
[Illustration:
PLATE VII.
PIKE’S PEAK RAILWAY, ROCKY MOUNTAINS.
]
[Illustration:
FIG. 58.—_The Great Eastern at Anchor._
]
STEAM NAVIGATION.
The first practically successful steamboat was constructed by Symington,
and used on the Forth and Clyde Canal in 1802. A few years afterwards
Fulton established steam navigation in American waters, where a number
of steamboats plied regularly for some years before the invention had
received a corresponding development in England, for it was not until
1814 that a steam-packet ran for hire in the Thames. From that time,
however, the principle was quickly and extensively applied, and steamers
made their appearance on the chief rivers of Great Britain, and soon
began also to make regular passages from one sea-port to another, until
at length, in 1819, a steamer made the voyage from New York to
Liverpool. It does not appear, however, that such ocean steam voyages
became at once common, for we read that in 1825 the captain of the first
steam-ship which made the voyage to India was rewarded by a large sum of
money. It was not until 1838 that regular steam communication with
America was commenced by the dispatch of the _Great Western_ from
Bristol. Other large steamers were soon built expressly for the passage
of the Atlantic, and a new era in steam navigation was reached when, in
1845, the _Great Britain_ made her first voyage to New York in fourteen
days. This ship was of immense size, compared with her predecessors, her
length being 320 ft., and she was moreover made of iron, while instead
of paddles, she was provided with a screw-propeller, both circumstances
at that time novelties in passenger ships. Fulton appears to have made
trial in America of various forms of mechanism for propelling ships
through the water. Among other plans he tried the screw, but finally
decided in favour of paddle-wheels, and for a long time these were
universally adopted. Many ships of war were built with paddle-wheels,
but the advantages of the screw-propeller were at length perceived. The
paddle-wheels could easily be disabled by an enemy’s shot, and the large
paddle-boxes encumbered the decks and obstructed the operations of naval
warfare. Another circumstance perhaps had a greater share in the general
adoption of the screw, which had long before been proposed as a means of
applying steam power to the propulsion of vessels. This was the
introduction of a new method of placing the screw, so that its powers
were used to greater advantage. Mr. J. P. Smith obtained a patent in
1836 for placing the propeller in that part of the vessel technically
called the _dead-wood_, which is above the keel and immediately in front
of the rudder. When the means of propulsion in a ship of war is so
placed, this vital part is secure from injury by hostile projectiles,
and the decks are clear for training guns and other operations. Thus
placed, the screw has been proved to possess many advantages over
paddle-wheels, so that at the present time it has largely superseded
paddle-wheels in vessels of every class, except perhaps in those
intended to ply on rivers and lakes. Many fine paddle-wheel vessels are
still afloat, but sea-going steamers are nearly always now built with
screw-propellers. In the application of the steam engine to navigation
the machine has received many modifications in the form and arrangement
of the parts, but in principle the marine engine is identical with the
condensing engine already described. The engines in steam-ships are
often remarkable for the great diameter given to the cylinders, which
may be 8 ft. or 9 ft. or more. Of course other parts of the machinery
are of corresponding dimensions. Such large cylinders require the
exercise of great skill in their construction, for they must be cast in
one piece and without flaws. The engraving, Fig. 59, depicts the scene
presented at the works of Messrs. Penn during the casting of one of
these large cylinders, the weight of which may amount to perhaps 30
tons. Only the top of the mould is visible, and the molten iron is being
poured in from huge ladles, moved by powerful cranes. In paddle vessels
the great wrought iron shaft which carries the paddle-wheels crosses the
vessel from side to side. This shaft has two cranks, placed at right
angles to each, and each one is turned by an engine, which is very
commonly of the kind known as the side-lever engine. In this engine,
instead of a beam being placed above the cylinder, two beams are used,
one being set on each side of the cylinder, as low down as possible. The
top of the piston-rod is attached to a crosshead, from each end of which
hangs a great rod, which is hinged to the end of the side-beam. The
other ends of the two beams are united by a cross-bar, to which is
attached the connecting-rod that gives motion to the crank. Another
favourite form of engine for steam-ships is that with oscillating
cylinders. The paddle-wheels are constructed with an iron framework, to
which flat boards, or floats, are attached, placed usually in a radial
direction. But when thus fixed, each float enters the water obliquely,
and in fact its surface is perpendicular to the direction of the
vessel’s course only at the instant the float is vertically under the
axis of the wheel. In order to avoid the loss of power consequent upon
this oblique movement of the floats, they are sometimes hung upon
centres, and are so moved by suitable mechanism that they are always in
a nearly vertical position when passing through the water. Paddle-wheels
constructed in this manner are termed _feathering_ wheels. They do not
appear, however, to possess any great advantage over those of the
ordinary construction, except when the paddles are deeply immersed in
the water, and this result may be better understood when we reflect that
the actual path of the floats through the water is not circular, as it
would be if the vessel itself did not move; for all points of the wheel
describe peculiar curves called _cycloids_, which result from the
combination of the circular with the onward movement.
[Illustration:
PLATE VIII.
THE “CLERMONT,” FROM A CONTEMPORARY DRAWING.
]
[Illustration:
FIG. 59.—_Casting Cylinder of a Marine Steam Engine._
]
The next figure, 60, exhibits a very common form of the screw propeller,
and shows the position which it occupies in the ship. The reader may not
at once understand how a comparatively small two-armed wheel revolving
in a plane perpendicular to the direction of the vessel’s motion is able
to propel the vessel forward. In order to understand the action of the
propeller, he should recall to mind the manner in which a screw-nail in
a piece of wood advances by a distance equal to its pitch at every turn.
If he will conceive a gigantic screw-nail to be attached to the vessel
extending along the keel,—and suppose for a moment that the water
surrounding this screw is not able to flow away from it, but that the
screw works through the water as the nail does in the wood,—he will have
no difficulty in understanding that, under such circumstances, if the
screw were made to revolve, it would advance and carry the vessel with
it. The reader may now form an accurate notion of the actual propeller
by supposing the imaginary screw-nail to have the thread so deeply cut
that but little solid core is left in the centre, and supposing also
that only a very short piece of the screw is used—say the length of one
revolution—and that this is placed in the dead-wood. Such was the
construction of the earlier screw-propellers, but now a still shorter
portion of the screw is used; for instead of a complete turn of the
thread, less than one-sixth is now the common construction. Such a strip
or segment of the screw-thread forms a _blade_, and two, three, four, or
more blades are attached radially to one common axis. The blades spring
when there are two from opposite points in the axis, and in other cases
from points on the same circle. The blades of the propeller are cut and
carved into every variety of shape according to the ideas of the
designer, but the fundamental principle is the same in all the forms. It
need hardly be said that the particles of the water are by no means
fixed like those of the wood in which a screw advances. But as the water
is not put in motion by the screw without offering some resistance by
reason of its inertia, this resistance reacting on the screw operates in
the same manner, but not to the same extent, as the wood in the other
case. When we know the pitch of the screw, we can calculate what
distance the screw would be moved forward in a given number of
revolutions if it were working through a solid. This distance is usually
greater than the actual distance the ship is propelled, but in some
cases the vessel is urged through the water with a greater velocity than
if the screw were working in a solid nut. The shaft which carries the
screw extends from the stem to the centre of the ship where the engines
are placed, and it passes outward through a bearing lined with wood, of
which _lignum vitæ_ is found to be the best kind, the lubricant for this
bearing being not oil but water. The screw would not have met with the
success it has attained but for this simple contrivance; for it was
found that with brass bearings a violent thumping action was soon
produced by the rapid rotation of the screw. The wearing action between
the wood and the iron is very slight, whereas brass bearings in this
position quickly wear and their adjustments become impaired. The
screw-shaft is very massive and is made in several lengths, which are
supported in appropriate bearings; there is also a special arrangement
for receiving the thrust of the shaft, for it is by this thrust received
from the screw that the vessel is propelled, and the strain must be
distributed to some strong part of the ship’s frame. There is usually
also an arrangement by which the screw-shaft can, when required, be
disconnected from the engine, in order to allow the screw to turn freely
by the action of the water when the vessel is under sail alone.
[Illustration:
FIG. 60.—_Screw Propeller._
]
A screw-propeller has one important advantage over paddle-wheels in the
following particular: whereas the paddle-wheels act with the best effect
when the wheel is immersed in the water to the depth of the lowest
float, the efficiency of the screw when properly placed is not
practically altered by the depth of immersion. As the coals with which a
steamer starts for a long voyage are consumed, the immersion is
decreased—hence the paddle-wheels of such a steamer can never be
immersed to the proper extent _throughout_ the voyage; they will be
acting at a disadvantage during the greater part of the voyage. Again,
even when the immersion of the vessel is such as to give the best
advantage to the paddle-wheels, that advantage is lost whenever a
side-wind inclines the ship to one side, or whenever by the action of
the waves the immersion of the paddles is changed by excess or defect.
From all such causes of inefficiency arising from the position of the
vessel the screw-propeller is free. The reader will now understand why
paddle-wheel steamers are at the present day constructed for inland
waters only.
A great impulse was given to steam navigation, by the substitution of
iron for wood in the construction of ships. The weight of an iron ship
is only two-thirds that of a wooden ship of the same size. It must be
remembered that, though iron is many times heavier than wood, bulk for
bulk, the required strength is obtained by a much less quantity of the
former. A young reader might, perhaps, think that a wooden ship must
float better than an iron one; but the law of floating bodies is, that
the part of the floating body which is below the level of the water,
takes up the space of exactly so much water as would have the same
weight as the floating body, or in fewer words, a floating body
displaces its own _weight_ of water. Thus we see that an iron ship,
being lighter than a wooden one, must have more buoyancy. The use of
iron in ship-building was strenuously advocated by the late Sir W.
Fairbairn, and his practical knowledge of the material gave great
authority to his opinion. He pointed out that the strains to which ships
are exposed are of such a nature, that vessels should be made on much
the same principles as the built-up iron beams or girders of railway
bridges. How successfully these principles have been applied will be
noticed in the case of the _Great Eastern_. This ship, by far the
largest vessel ever built, was designed by Mr. Brunel, and was intended
to carry mails and passengers to India by the long sea route. The
expectations of the promoters were disappointed in regard to the speed
of the vessel, which did not exceed 15 miles an hour; and no sooner had
she gone to sea than she met with a series of accidents, which appear,
for a time, to have destroyed public confidence in the vessel as a
sea-going passenger ship. Some damage and much consternation were
produced on board by the explosion of a steam jacket a few days after
the launch. Then the huge ship encountered a strong gale in Holyhead
Harbour, and afterwards was disabled by a hurricane in the Atlantic, in
which her rudder and paddles were so damaged, that she rolled about for
several days at the mercy of the waves. At New York she ran upon a rock,
and the outer iron plates were stripped off the bottom of the ship for a
length of 80 ft. She was repaired and came home safely; but the
companies which owned her found themselves in financial difficulties,
and the big ship, which had cost half a million sterling, was sold for
only £25,000, or only about one-third of her value as old materials.
The misfortunes of the _Great Eastern_, and its failure as a commercial
speculation in the hands of its first proprietors, have been quoted as
an illustration of the ill luck, if it might be so called, which seems
to have attended several of the great works designed by the Brunels—for
the Thames Tunnel was, commercially, a failure; the Great Western
Railway, with its magnificent embankments, cuttings, and tunnels, has
reverted to the narrow gauge, and therefore the extra expense of the
large scale has been financially thrown away; the Box Tunnel, a more
timid engineer would have avoided; and then there is the _Great
Eastern_. It is, however, equally remarkable that all these have been
glorious and successful achievements as engineering works, and the
scientific merit of their designers remains unimpaired by the merely
accidental circumstance of their not bringing large dividends to their
shareholders. Nor is their value to the world diminished by this
circumstance, for the Brunels showed mankind the way to accomplish
designs which, perhaps, less gifted engineers would never have had the
boldness to propose. The Box Tunnel led the way to other longer and
longer tunnels, culminating in that of Mont Cenis; but for the Thames
Tunnel—once ranked as the eighth wonder of the world—we should probably
not have heard of the English Channel Tunnel—a scheme which appears less
audacious now than the other did then; if no _Great Eastern_ had
existed, we should not now have had an Atlantic Telegraph. Possibly this
huge ship is but the precursor of others still larger, and it is
undoubtedly true that since its construction the ideas of naval
architects have been greatly enlarged, and the tendency is towards
increased size and speed in our steam-ships, whether for peace or war.
[Illustration:
FIG. 61.—_Section of Great Eastern Amidships._
]
[Illustration:
FIG. 62.—_The Great Eastern in course of Construction._
]
The accidents which had happened to the ship had not, however,
materially damaged either the hull or the machinery; and the _Great
Eastern_ was refitted, and afterwards employed in a service for which
she had not been designed, but which no other vessel could have
attempted. This was the work of carrying and laying the whole length of
the Atlantic Telegraph Cable of 1865, of which 2,600 miles were shipped
on board in enormous tanks, that with the contents weighed upwards of
5,000 tons. The ship has since been constantly engaged in similar
operations.[1] The _Great Eastern_ is six times the size of our largest
line-of-battle ships, and about seven times as large as the splendid
steamers of the Cunard line, which run between Liverpool and New York.
She has three times the steam power of the largest of these Atlantic
steamers, and could carry twenty times as many passengers, with coal for
forty days’ consumption instead of fifteen. Her length is 692 ft.;
width, 83 ft.; depth, 60 ft.; tonnage, 24,000 tons; draught of water
when unloaded, 20 ft.; when loaded, 30 ft.; and a promenade round her
decks would be a walk of more than a quarter of a mile. The vessel is
built on the cellular plan to 3 ft. above the water-line; that is, there
is an inner and an outer hull, each of iron plates ¾ in. thick, placed 2
ft. 10 in. apart, with ribs every 6 ft., and united by transverse
plates, so that in place of the ribs of wooden ships, the hull is, as it
were, built up of curved cellular beams of wrought iron. The ship is
divided longitudinally by two vertical partitions or bulkheads of
wrought iron, ½ in. thick. These are 350 ft. long and 60 ft. high, and
are crossed at intervals by transverse bulkheads, in such a manner that
the ship is divided into nineteen compartments, of which twelve are
completely water-tight, and the rest nearly so. The diagram (Fig. 61)
represents a transverse section, and shows the cellular construction
below the water-line. The strength and safety of the vessel are thus
amply provided for. The latter quality was proved in the accident to the
ship at New York; and the former was shown at the launch, for when the
vessel stuck, and for two months could not be moved, it was found that,
although one-quarter of the ship’s length was unsupported, it exhibited
no deflection, or rather the amount of deflection was imperceptible.
Fig. 62 is from a photograph taken during the building of the ship, and
Fig. 63 shows the hull when completed and nearly ready for launching,
while the vignette at the head of the chapter exhibits the big ship at
anchor when completely equipped. The paddle-wheels are 56 ft. in
diameter, and are turned by four steam engines, each having a cylinder 6
ft. 2 in. in diameter, and 14 ft. in length. The vessel is also provided
with a four-bladed screw-propeller of 24 ft. diameter, driven by another
engine having four cylinders, six boilers, and seventy-two furnaces. The
total actual power of the engines is more than that of 8,000 horses, and
the vessel could carry coals enough to take her round the world—a
capability which was the object of her enormous size. The vessel as
originally constructed contained accommodation for 800 first-class
passengers, 2,000 second class, and 1,200 third class—that is, for 4,000
passengers in all. The principal saloon was 100 ft. long, 36 ft. wide,
and 13 ft. high. Each of her ten boilers weighs 50 tons, and when all
are in action, 12 tons of coal are burnt every hour, and the total
displacement of the vessel laden with coal is 22,500 tons.
Footnote 1:
She was broken up for old iron, 1889.
[Illustration:
FIG. 63.—_The Great Eastern ready for Launching._
]
The use of steam power in navigation has increased at an amazing rate.
Between 1850 and 1860 the tonnage of the steam shipping entering the
port of London increased three-fold, and every reader knows that there
are many fleets of fine steamers plying to ports of the United Kingdom.
There are, for example, the splendid Atlantic steamers, some of which
almost daily enter or leave Liverpool, and the well-appointed ships
belonging to the Peninsular and Oriental Company. The steamers on the
Holyhead and Kingston line may be taken as good examples of first-class
passenger ships. These are paddle-wheel boats, and are constructed
entirely of iron, with the exception of the deck and cabin fittings.
Taking one of these as a type of the rest, we may note the following
particulars: the vessel is 334 ft. long, the diameter of the
paddle-wheels is 31 ft., and each has fourteen floats, which are 12 ft.
long and 4 ft. 4 in. wide. The cylinders of the engines are 8 ft. 2 in.
in diameter, and 6 ft. 6 in. long. The ship cost about £75,000. The
average passage between the two ports—a distance of 65½ miles—occupies 3
hours 52 minutes, and at the measured mile the vessel attained the speed
of 20·811 miles per hour. As an example of the magnificent vessels owned
by the Cunard Company, we shall give now a few figures relating to one
of their largest steam-ships, the _Persia_, launched in 1858, and built
by Mr. N. Napier, of Glasgow, for the company, to carry mails and
passengers between Liverpool and New York. Her length is 389 ft., and
her breadth 45 ft. She is a paddle-wheel steamer, with engines of 850
horse-power, having cylinders 100 in. in diameter with a stroke of 10
ft. The paddle-wheels are 38 ft. 6 in. in diameter, and each has
twenty-eight floats, 10 ft. 8 in. long and 2 ft. wide. The _Persia_
carries 1,200 tons of coal, and displaces about 5,400 tons of water.
[Illustration:
FIG. 64.—_Comparative Sizes of Steamships._
1838, _Great Western_; 1844, _Great Britain_; 1856, _Persia_; 1858,
_Great Eastern_.
A, Section amidships of _Great Eastern_; B, The same of _Great
Western_. Both on the same scale, but on a larger one than their
profiles.
]
A velocity of twenty-six miles per hour appears to be about the highest
yet attained by a steamer.[2] This is probably near the limit beyond
which the speed cannot be increased to any useful purpose. The
resistance offered by water to a vessel moving through it increases more
rapidly than the velocity. Thus, if a vessel were made to move through
the water by being pulled with a rope, there would be a certain strain
upon the rope when the vessel was dragged, say, at the rate of five
miles an hour. If we desired the vessel to move at double the speed, the
strain on the rope must be increased four-fold. To increase the velocity
to fifteen miles per hour, we should have to pull the vessel with nine
times the original force. This is expressed by saying that the
resistance varies as the square of the velocity. Hence, to double the
speed, the impelling force must be quadrupled, and as that force is
exerted through twice the distance in the same time, an engine would be
required of eight times the power—or, in other words, the power of the
engine must be increased in proportion to the _cube_ of the velocity; so
that to propel a boat at the rate of 15 miles an hour would require
engines twenty-seven times more powerful than those which would suffice
to propel it at the rate of five miles an hour.
Footnote 2:
This has now (1895) been far surpassed.—_Vide infra._
The actual speed attained by steam-ships with engines of a given power
and a given section amidships will depend greatly upon the shape of the
vessel. When the bow is sharp, the water displaced is more gradually and
slowly moved aside, and therefore does not offer nearly so much
resistance as in the opposite case; but the greater part of the power
required to urge the vessel forward is employed in overcoming a
resistance which in some degree resembles friction between the bottom of
the vessel and the water.
The wonderful progress which has, in a comparatively short time, taken
place in the power and size of steam-vessels, cannot be better brought
home to the reader than by a glance at Fig. 64, which gives the profiles
of four steamships, drawn on one and the same scale, thus showing the
relative lengths and depths of those vessels, each of which was the
largest ship afloat at the date which is marked below it, and the whole
period includes only the brief space of twenty years!—for this, surely,
is a brief space in the history of such an art as navigation. All these
ships have been named in the course of this article, but in the
following table a few particulars concerning each are brought together
for the sake of comparing the figures:
┌─────┬─────────────────┬─────────────────┬────────┬────────┐
│Date.│ Name. │ Propulsion. │Length. │Breadth.│
├─────┼─────────────────┼─────────────────┼────────┼────────┤
│1838 │_Great Western_ │Paddles │236 ft. │ 36 ft. │
│1844 │_Great Britain_ │Screw │322 ft. │ 51 ft. │
│1856 │_Persia_ │Paddles │390 ft. │ 45 ft. │
│1858 │_Great Eastern_ │Screw and paddles│690 ft. │ 83 ft. │
└─────┴─────────────────┴─────────────────┴────────┴────────┘
[Illustration:
FIG. 65.—_The s.s. City of Rome._
]
Several passenger ocean-going steamships have been built since the
_Persia_, of still greater dimensions, and of higher engine power. These
have generally been surpassed in late years by some splendid Atlantic
liners, such as the sister vessels owned by the International Navigation
Co., and now named respectively the _New York_ and the _Paris_. The
_City of Rome_, launched in 1881 by the Barrow Steamship Co., is little
inferior in length to the _Great Eastern_, although the tonnage is only
about one-third. The _City of Rome_ is 560 ft. long, 52 ft. wide, and 37
ft. deep. Her engines are capable of working up to 10,000 indicated
horse-power. Fig. 65 is a sketch of this ship, and shows that she
carries four masts and three funnels. The main shaft measures more than
2 ft. across, and the screw-propeller is 24 ft. in diameter. She has
accommodation for 1,500 passengers, and is fitted with all the
conveniences and luxuries of a well-appointed hotel. The International
Navigation Co.’s ship _Paris_, has made the passage across the Atlantic
in less than six days, and appears to be the fastest vessel in the
transatlantic service. In August, 1889, she made the run from shore to
shore in 5 days, 22 hours, 38 minutes.
The extraordinary increase in the speed of steamships that has been
effected within the last few years depends mainly upon the improvements
that have latterly been made in the marine engine—a machine of which we
have been unable to give an account, because its details are too
numerous and complicated to be followed out by the general reader.
Suffice it to say, that the use of higher steam pressures with compound
expansion (p. 18), condensers which admit of the same fresh water being
used in the boilers over and over again, and better furnace
arrangements, are among the more important of these improvements. But
not only have the limits of practicable speed been enlarged, but a
greater economy of fuel for the work done has been attained; the result
being that ocean carriage is now cheaper than ever. The outcome of this
will not cease with simply a greatly extended steam navigation, but
appears destined ultimately to produce effects on the world at large
comparable in range and magnitude with those that may be traced to the
use of the steam engine itself since its first invention.
Among the curiosities of steamboat construction may be mentioned a
remarkable ship which was built a few years ago for carrying passengers
across the English Channel without the unpleasant rolling experienced in
the ordinary steamboats. The vessel, which received the name of the
_Castalia_, was designed by Captain Dicey, who formerly held an official
position at the Port of Calcutta. His Indian experience furnished him
with the first suggestion of the new ship in the device which is adopted
there for steadying boats in the heavy surf. The plan is to attach a log
of timber to the ends of two outriggers, which project some distance
from the side of the vessel; or sometimes two canoes, a certain distance
apart, are connected together. Some of these Indian boats will ride
steadily in a swell that will cause large steamers to roll heavily.
Improving on this hint, Captain Dicey built a vessel with two hulls,
each of which acted as an outrigger to the other. Or, perhaps, the
_Castalia_ may be described as a flat-bottomed vessel with the middle
part of the bottom raised out of the water throughout the entire length,
so that the section amidships had a form like this—
[Illustration]
The two hulls were connected by what we may term “girders,” which
extended completely across their sections, forming transverse partitions
or bulkheads, and these girders were strongly framed together, so as to
form rigid triangles. These united the two hulls so completely, that
there was not any danger of the vessel being strained in a sea-way. The
decks were also formed of iron, although covered with wood, so that the
whole vessel really formed a box girder of enormous section.
[Illustration:
FIG. 66.—_The Castalia in Dover Harbour._
]
The reason why the steamers which until lately ran between Dover and
Calais, Folkestone and Boulogne, and other Channel ports, were so small,
was because the harbours on either side could not receive vessels with
such a draught as the fine steamers, for example, which run on the
Holyhead and Kingston line. Now, the _Castalia_ drew only 6 ft. of
water, or 1 ft. 6 in. less than the small Channel steamers, and would,
therefore, be able to enter the French ports at all states of the tide.
Yet the extent of the deck space was equalled in few passenger ships
afloat, except the _Great Eastern_ and some of the Atlantic steamers.
The vessel was 290 ft. in length, with an extreme breadth of 60 ft. The
four spacious and elegantly-fitted saloons—two of which were 60 ft. by
36 ft., and two 28 ft. by 26 ft.—and the roomy cabins, retiring rooms,
and lavatories, were the greatest possible contrast to the “cribbed,
cabined, and confined” accommodation of the ordinary Channel steamers.
There were also a kitchen and all requisites for supplying dinners,
luncheons, etc., on board. But besides the above-named saloons and
cabins, there was a grand saloon, which was 160 ft. long and 60 ft.
wide; and the roof of this formed a magnificent promenade 14 ft. above
the level of the sea. There was comfortable accommodation in the vessel
for more than 1,000 passengers.
The inner sides of the hulls were not curved like the outside, but were
straight, with a space between them of 35 ft. wide, and the hulls were
each 20 ft. in breadth, and somewhat more in depth. There were two
paddle-wheels, placed abreast of each other in the water-way between the
two hulls, and each of these contained boilers and powerful engines. The
designers of this vessel calculated that she would attain a speed of 14¾
knots per hour, but this result failed to be realized. Probably there
were no data for the effect of paddles working in a confined
water-space. The position of the paddles is otherwise an advantage, as
it leaves the sides of the vessel free and unobstructed. The ship had
the same form at each end, so it could move equally well in either
direction. There were rudders at both ends, and the steering qualities
of the ship were good. Although the speed of the _Castalia_ was below
that intended, the vessel was a success as regards steadiness, for the
rolling and pitching were very greatly reduced, and the miseries and
inconveniences of the Channel passage obviated.
[Illustration:
FIG. 67.—_The Castalia in Dover Harbour—End View._
]
The _Castalia_ is represented in Figs. 66 and 67. She was constructed by
the Thames Iron Shipbuilding Co., and launched in June, 1874, but after
she had been tried at sea, it was found necessary to fit her with
improved boilers, and this caused a delay in placing the vessel on her
station.
The _Castalia_ proved a failure in point of speed, and she was soon
replaced by another and more powerful vessel constructed on the same
general plan, and named the _Calais-Douvres_. But this twin-ship again
failed to answer expectations, and as the harbour on the French shore
was meanwhile deepened and improved, new and very fine paddle-wheel
boats, named the _Invicta_, _Victoria_, and _Empress_ have been placed
on the service. As the latter boat, at least, has steamed from Dover to
Calais, nearly twenty-six miles, under the hour, there is nothing more
to be desired in point of speed. A fourth vessel is to take the place of
the twin-ship, _Calais-Douvres_, and will receive the same name.
[Illustration:
FIG. 68.—_Bessemer Steamer._
]
Another very novel and curious invention connected with steam navigation
was the steamer which Mr. Bessemer built at Hull in 1874. This invention
also was to abolish all the unpleasant sensations which landsmen are apt
to experience in a sea voyage, by effectually removing the cause of the
distressing _mal de mer_. The ship was built for plying between the
shores of France and England, and the method by which he purposed to
carry passengers over the restless sea which separates us from our
Gallic neighbours was bold and ingenious. He designed a spacious saloon,
which, instead of partaking of the rolling and tossing of the ship, was
to be maintained in an absolutely level position. The saloon was
suspended on pivots, much in the same way as a mariner’s compass is
suspended; and by an application of hydraulic power it was intended to
counteract the motion of the ship and maintain the swinging saloon
perfectly horizontal. It was originally proposed that the movements
should be regulated by a man stationed for that purpose, where he could
work the levers for bringing the machinery into action, so as to
preserve the saloon in the required position. This plan was, however,
improved upon, and the adjustments made automatic. It may be well to
mention that it is a mistake to suppose that anything freely suspended,
like a pendulum, on board a ship rolling with the waves, will hang
vertically. If, however, we cause a heavy disc to spin very rapidly, say
in a horizontal plane, the disc cannot be moved out of the horizontal
plane without the application of some force. A very well-made disc may
be made to rotate for hours, and would, by preserving its original plane
of rotation, even show the effect of the earth’s diurnal motion. Mr.
Bessemer designed such a gyroscope to move the valves of his hydraulic
apparatus, and so to keep his swinging saloon as persistently horizontal
as the gyroscope itself. Mr. Bessemer’s ship was 350 ft. long, and each
end, for a distance of 48 ft., was only about 4 ft. from the line of
floating. Above the low ends a breastwork was raised, about 8 ft. high,
and 254 ft. long. In the centre, and occupying the space of 90 ft., was
the swinging saloon intended for first-class passengers. At either end
of this apartment were the engines and boilers. The engines were
oscillating and expansive, working up to 4,600 horse-power, which could
be increased to 5,000. There were two pairs of engines, one set at
either end of the ship, and each having two cylinders of 80 in. in
diameter, and a stroke of 5 ft., working with steam of 30 lbs. pressure
per square inch, supplied from four box-shaped boilers, each boiler
having four large furnaces. The paddle-wheels, of which there were a
pair on either side of the vessel, were 27 ft. 10 in. in diameter
outside the outer ring, and each wheel has twelve feathering floats. The
leading pair of wheels, when working at full speed, were to make
thirty-two revolutions per minute, and the following pair of wheels move
faster.
Entrance into the Bessemer saloon was gained by two broad staircases
leading to one landing, and a flexible passage from this point to the
saloon. The saloon rested on four steel gudgeons, one at each end, and
two close together near the middle. These were not only to support the
saloon, but also to convey the water to the hydraulic engines, by which
the saloon was to be kept steady. For this purpose the after one was
made hollow, and connected with the water mains from powerful engines,
and also with a supply-pipe leading to a central valve-box, by means of
which the two hydraulic cylinders on either side were supplied with
water. Between the two middle gudgeons, a gyroscope, worked by a small
turbine, filled with water from one of the gudgeons, enabled Mr.
Bessemer to dispense with the services of a man, and thus completed his
scheme of a steady saloon, by making the machinery completely automatic.
The saloon was 70 ft. long, 35 ft. wide, and 20 ft. high. The Bessemer
ship proved to be a total failure, and never went to sea as a passenger
boat.
On board of some modern war-ships where speed is essential, and where
the engines are driven at a very great number of revolutions per minute,
as in the case of torpedo-boat catchers, the vibration throughout the
whole of the vessel becomes extremely trying, not only for the nerves of
the crew, but for the security of the structure itself. The cause of
this vibration and consequent strain and loss of power is not far to
seek. The cylinders of marine engines are always of a large diameter, 6
feet, 8 feet, or even more sometimes, and the pistons and piston-rods
are necessarily of great strength and corresponding weight. Now, at
every half revolution of the engines, this heavy mass of piston and
piston-rod, though moving at an exceedingly high speed in the middle of
the stroke, has to be brought to a standstill, and an equal velocity in
the opposite direction imparted to it. A large portion of the power is
therefore uselessly expended in stopping a great moving mass, and
reversing its motion. All the force required to do this reacts on the
vessel’s frame. Many attempts have been made to construct rotatory
steam-engines, and some hundreds of patents taken out for such
inventions, which in general have a piston revolving about a shaft; but
the great friction, and consequent liability to wear out, have prevented
their practical use.
Lately, a method of using steam on the principle embodied in the water
turbine has been developed, and within the last six or seven years has
found successful application in propelling electro-dynamos at very high
speeds. In the steam turbine there are no pistons, piston-rods, or other
reciprocating parts, the effect depending on the same kind of reaction
that is taken advantage of in the water turbine (which has a high
efficiency in giving out a large proportion of energy), and the power is
applied with smoothness and an entire absence of the oscillations that
would shake to pieces any vessel that an ordinary steam-engine could
propel at the same rate.
The advantages of the steam turbine have been proved by the performances
of a small experimental vessel lately built at Newcastle, and
appropriately named the _Turbinia_. She is only 100 feet in length, and
9 feet in breadth, with a displacement of some 44 tons. Now the highest
record speed for any vessel of that size is 24 knots per hour; but the
_Turbinia_, in a heavy sea, showed, at a measured mile, the speed of 32¾
knots, which is believed to be greater than that of any craft now
afloat, being nearly 37¾ miles an hour, or equal to that of an ordinary
railway train. Besides that, it has been found by experiment, that an
arrangement of the blades of the screw propeller more suitable to high
velocities will enable a still greater speed to be obtained. The weight
of the turbine engines of this vessel is only 3 tons, 13 cwts., and the
whole weight of the machinery, including boilers and condensers, is only
22 tons, with an indicated H.P. of 1576, and a steam consumption of but
16 lbs. per hour. The weight of the turbine is only one-fifth of that of
marine engines of equal power; the space occupied is smaller; the
initial cost is less; not so much superintendence is required; the
charges of maintenance are diminished; reduced dimensions of propeller
and shaft suffice; vibration is eliminated; speed is increased; and
greater economy of fuel is secured.
_THE RIVER AND LAKE STEAM-BOATS OF AMERICA._
The chapter on “Steam Navigation,” in the foregoing pages, has dealt
mainly with the progress of the ocean-going steam-ship, from the
establishment of regular transatlantic services down to the building of
the splendid liners, the _New York_ and the _Paris_, and we have
recorded, in addition, the performances of the pair of hitherto
unsurpassed sister ships, the _Campania_ and the _Lucania_. The
importance and interest attaching to steam navigation is, however, by no
means confined to ocean-going vessels, and the chapter demands a
supplementary notice of the great developments of the steam-ship in
other parts of the world than Britain, more particularly where great
rivers, navigable for hundreds of miles, and lakes, spreading their
waters over vast areas, present conditions of traffic and opportunities
for adaptation to an extent that could not be required within the range
of Britain or British oceanic lines.
[Illustration:
PLATE IX.
THE “MARY POWELL.”
]
If the reader will cast his eye on the map of the United States, he will
see towards the northern boundary a great fresh-water system, comprising
five enormous lakes, the least of which is nearly two hundred, and the
largest nearly three hundred miles in length, in all presenting a total
area greater by far than that of England and Scotland together thrice
told. This lake system has a line of coast to be reckoned only by
thousands of miles, and for a long time an enormous traffic has been
carried across its waters by sailing vessels of all kinds, two- or
three-masted schooners, brigs, and other craft, carrying wood, stone,
lime, and other commodities. On the map, the position of the Detroit
River, which leads from the southern extremity of Lake Huron to Lake
Erie, will readily be recognised, and this strait, which is in the only
line of transport from the three great upper lakes, formerly presented
all the picturesqueness that crowds of boats of every build could
impart. Especially was this the case at Amherstburg, its southern
extremity, where sometimes a northern wind would make the passage
impracticable for several days in succession, and a fleet of a hundred
or two hundred sailing vessels would collect to await the opportunity of
a favouring breeze in order to carry them against the current to Port
Huron. Then, taking advantage of the right moment, they would set their
sails, and in a compact body move slowly up the strait. This was not
quick enough to meet the traffic, and, before long, larger vessels were
built, which were towed up and down the Detroit by steam-tugs. The next
step of replacing sailing ships by steam-vessels was not long in
following, and though there still exist fine specimens of sailing craft
on the lakes, their day may be said to be over. The navigation of these
lakes, before the extensive development of the railway systems near
their shores, comprised a large passenger traffic, which was carried on
by big paddle-wheel steamers, and at the time of the great westward set
of emigration to Michigan, Wisconsin, and Minnesota, these steamers were
crowded to their utmost capacity. The great improvement which in recent
years has become possible for passenger steamers in speed, cabin
accommodation, and other particulars, above all, the growth of great
cities on the shores, the progress of the territories adjoining the lake
system, and other circumstances, are now combining to renew the
passenger traffic on a larger scale than ever. “Fifteen millions of
people,” says Mr. H. A. Griffin, the Secretary of the Cleveland Board of
Control (_Engineering Magazine_, iv., 819), “now live upon the shore
lines of the lakes, or within six hours’ travel by rail, and nearly all
of that population is south of the United States boundary line. The
territory directly tributary to the lakes, north and south of the line,
is capable of easily maintaining a population of 100,000,000.... It does
not require a very lively imagination to foresee the Great Lakes
surrounded by the most prosperous and progressive people on earth, and
crossed and recrossed by scores of lines of passenger steam-ships, in
addition to a still greater number of freight lines.” The number of
first-class passenger steamers already launched or on the stocks is an
indication that the revival of passenger traffic will not lag or be
delayed.
The unique conditions and requirements of this lacustrine traffic were
bound to lead to types of vessels differing in many respects from the
steam-ships to be seen in the harbours of Great Britain. The
introduction of iron shipbuilding gave a great impetus to the
construction of the lake steamers, for vessels of more than 3,000 tons
could be built with a comparatively shallow draught of water (15½ feet),
which was one of the necessities of the situation. As far back as 1872,
iron shipbuilding had been fully established at Cleveland and Detroit,
and at the latter place scores of splendid steel steam-ships have been
turned out. The Cleveland builders have not been far behind, and
Buffalo, Milwaukee, Chicago, and other places, have followed suit. At
the beginning of 1893, there were on the lakes more than fifty vessels
of over 2,000 tons each, while the total number of steam vessels of all
kinds was considerably over 1,600, and sailing vessels with steam-tugs
counted over 2,000. The tonnage of the ships on the lakes has been
estimated at about 36 per cent. of the whole mercantile marine of the
United States, and it is said that 40,000 men are employed upon the
vessels. The total freight passing Detroit in 1892 was calculated to
exceed 34,000,000 tons, an amount greater than the whole foreign and
coasting trade of the port of London. There are more than thirty
shipbuilding concerns on the lakes, and some of them possess large dry
docks of their own; but there are also independent companies owning dry
docks of great size. Some of these shipbuilding establishments have
turned out steel ocean-going tugs, paddle and screw passenger steamers,
cargo-carrying boats, vessels for carrying railway trains across the
Detroit river, etc., etc.
[Illustration:
FIG. 68_a_.—_A Whaleback Steamer, No. 85, Built at West Superior,
Wisconsin._
]
The extent and importance which steam navigation has attained in a
definite region have been indicated in the preceding paragraphs; but an
attempt to show by illustration and description the several
characteristic forms the steam-ship has now assumed in these lacustrine
waters would carry us far beyond our allotted limits. The steam vessels
now on the lakes are almost exclusively actuated by screw-propellers,
whether they are passenger or freight boats. The boilers and engines are
near the stern, and the hulls are usually of great length; in fact, some
of these steamboats will compare in dimensions with the _Persia_, which
was the transatlantic marvel about the year 1857. (See p. 137.) Such is
the _Mariposa_, launched in 1892, which is 350 feet long and 45 feet
broad, carrying 3,800 net tons, with a draught of only 15½ feet. There
are others, 380 feet long, with engines of 7,000 horse-power, steaming
at 20 miles an hour, and providing ample accommodation for 600
passengers. The newest and most novel type of steam-ship on the lakes is
the “whaleback.” The celerity with which ships of this kind have been
constructed on occasion is perfectly marvellous. One of them, named the
_Christopher Columbus_, designed to carry passengers to and from the
World’s Fair at Chicago in 1893, was launched in fifty-six days after
the keel had been laid, yet it was a ship intended to carry 5,000
passengers, having a length over all of 362 feet, breadth 42 feet, depth
24 feet. The “whaleback” steamers are designed to give the greatest
carrying capacity with a given draught of water, and all the structures
usually fitted to the upper deck of a steamer are in them replaced by
the plain curved and closed deck, over which, when the vessel is in a
storm, waves may sweep harmlessly, thus avoiding the shocks received by
ships with high sides.
The river steam-boat was, as we have seen, nearly coeval with the
nineteenth century, and although its practicability was first
demonstrated in British waters, regular steam navigation was not
established until a few years afterwards, when, in 1807, Robert Fulton
placed on the River Hudson its first steam-boat. To this others were
soon added, so that in 1813 there were six steam-boats regularly plying
on the Hudson before a single one ran for hire on the Thames. An article
by Mr. Samuel Ward Stanton, in a recent number of _The Engineering
Magazine_, gives a very full account of the Hudson River steam-boats
from the beginning down to 1894, and to this article we are mainly
indebted for the details we are about to give.
The Hudson River washes the western shore of Manhattan Island, on which
stands by far the greater part of the city of New York, with its vast
population. The river is here straight, and has a nearly uniform width
of one mile; at New York it is commonly called the _North River_,
because of the direction of its course, for it descends from almost the
due north. It is not one of the great rivers of the United States as
regards length or extent of navigation; not, _e.g._, like the
Mississippi and the Missouri, which are ascended by steam-boats to
thousands of miles above their mouths; but it has one of the world’s
great capitals on its shores, and at the quays, which occupy both its
banks to the number of eighty or more, may be seen in multitudes some of
the finest ocean-going steamships, trading to every considerable port in
the world. The North River separates New York from what are practically
the populous suburbs of Jersey City and Hoboken, though these are
controlled by their own municipalities.
It was on the River Hudson that steam navigation was inaugurated by
Fulton with a vessel which was 133 feet long, 18 feet broad, and 7 feet
deep, and was named the _Clermont_. The speed attained was but five
miles an hour. The first trip was made on the 7th August, 1807, to
Albany, 150 miles up the river from New York, with twenty-four
passengers on board, and the new kind of locomotion was so well
patronised that during the following winter, when the Hudson navigation
had to be suspended on account of the ice, it was considered expedient
to enlarge the capacity of the boat by adding both to her length and
width; at the same time her name was changed to _The North River_, and
she plied regularly for several seasons afterwards. Her speed down the
river with the current was evidently greater than that of the first trip
up the river, for on 9th November, 1809, the New York _Evening Post_
announced that “The North River steam-boat arrived this afternoon in
twenty-seven and a half hours from Albany, with sixty passengers.”
The paddle-wheels were of a primitive form, and as they were unprovided
with paddle-boxes, the arrangement had the appearance of a great
undershot mill-wheel on each side of the boat, above the deck of which
was placed the steam-engine, a position it has retained in all these
river-boats, in which a huge, rhombus-shaped beam, oscillating high
above the deck, is a conspicuous feature. Another boat of much larger
dimensions was built the following year, having a tonnage of nearly 300,
and from that time there has been a more or less regular increase in the
sizes of the vessels, until in 1866 a tonnage of nearly 3,000 was
reached. In 1817 a vessel called the _Livingstone_ was launched, which
was able to go up to Albany in eighteen hours. In 1823 was launched the
_James Kent_, a novel feature in which vessel was the boiler made of
copper, and weighing upwards of 30 tons. It was so planned that if it
happened to burst, the hot water would be carried through the bottom of
the vessel by tubes or hollow pillars. From this it appears that
considerable apprehension existed as to the liability of the boilers
exploding. We are told that the cost of the copper boiler was in this
case nearly one-third of that of the whole vessel. The cabins are
described as having been very handsomely fitted up, and the speed was
such that fourteen hours sufficed for the trip up river to Albany. Many
fine boats were placed on the river during the twenty following years,
and these were marked by various improvements, as when, in 1840,
anthracite coal was for the first time substituted for wood as the fuel
for the furnaces, with the effect of reducing the cost of this item to
one-half. Then, in 1844, iron began to be used for constructing the
hulls, and a few years afterwards, steamers having a speed of twenty
miles an hour and over, became quite common. In 1865, and again in the
eighties, some four screw-propeller boats were built; but this type does
not appear to have found much favour on the Hudson, for the large
paddle-wheels and the single or double beam, working high above the
deck, have continued the almost universal form of construction. A very
popular and famous boat was placed on the Hudson in 1861. This was the
_Mary Powell_, called the “Queen of the Hudson,” which, although a boat
of moderate tonnage (983), was able on occasion to steam at the rate of
twenty-five miles an hour. This vessel was placed on the line between
New York and Rondont, and was still running in 1894.
One of the most modern and most elegant boats on the Hudson is the _New
York_, launched in 1887, and declared by Mr. Stanton to be one of the
finest river steam-boats in the world, well arranged, and beautifully
finished and furnished. She is built on fine lines, is 311 feet long, 40
feet broad, and with a tonnage of 1,552, draws only 12¼ feet of water.
She can steam at twenty miles an hour, and is placed on one of the New
York and Albany lines. Throughout the summer there are both day and
night boats for Albany, and the latter especially are of great size,
three stories high, and provided with saloons, state-rooms, and, in
fact, all the accommodation of a luxurious first-class hotel. The
vessels named in this notice include but a few of the splendid boats
that ply on the River Hudson, and, in respect of their numbers, speed,
and comfort, it may safely be asserted that they cannot be equalled on
any other river in the world.
[Illustration:
PLATE X.
THE “NEW YORK.”
]
[Illustration:
FIG. 69.—_H.M.S. Devastation in Queenstown Harbour._
]
SHIPS OF WAR.
“Take it all in all, a ship of the line is the most honourable thing
that man, as a gregarious animal, has ever produced. By himself,
unhelped, he can do better things than ships of the line; he can make
poems, and pictures, and other such concentrations of what is best in
him. But as a being living in flocks, and hammering out with alternate
strokes and mutual agreement, what is necessary for him in those flocks
to get or produce, the ship of the line is his first work. Into that he
has put as much of his human patience, common sense, forethought,
experimental philosophy, self-control, habits of order and obedience,
thoroughly wrought hand-work, defiance of brute elements, careless
courage, careful patriotism, and calm expectation of the judgment of
God, as can well be put into a space of 300 ft. long by 80 ft. broad.
And I am thankful to have lived in an age when I could see this thing so
done.” So wrote Mr. Ruskin about forty years ago, referring, of course,
to the old wooden line-of-battle ships. It may be doubted whether he
would have written thus enthusiastically about so unpicturesque an
object as the _Glatton_, just as it may be doubted whether the
armour-plated steamers will attain the same celebrity in romance and in
verse as the old frigates with their “wooden walls.” Certain it is that
the patience, forethought, experimental philosophy, thoroughly wrought
hand-work, careful patriotism, and other good qualities which Mr. Ruskin
saw in the wooden frigates, are not the less displayed in the new
ironclads.
Floating batteries, plated with iron, were employed in the Crimean War
at the instigation of the French Emperor. About the same time the
question of protecting ships of war by some kind of defensive armour was
forced upon the attention of maritime powers, by the great strides with
which the improvements in artillery were advancing; for the new guns
could hurl projectiles capable of penetrating, with the greatest ease,
any wooden ship afloat. The French Government took the initiative by
constructing _La Gloire_, a timber-framed ship, covered with an armour
of rolled iron plates, 4½ in. thick. The British Admiralty quickly
followed with the _Warrior_, a frigate similar in shape to the wooden
frigates, but built on an iron frame, with armour composed of plates 4½
in. thick, backed by 18 in. of solid teak-wood, and provided with an
inner skin of iron. The _Warrior_ was 380 ft. long, but only 213 ft. of
this length was armoured. The defensive armour carried by the _Warrior_,
and the ironclads constructed immediately afterwards, was quite capable
of resisting the impact of the 68 lb. shot, which was at that time the
heaviest projectile that could be thrown by naval guns. But to the
increasing power of the new artillery it soon became necessary to oppose
increased thickness of iron plates. The earlier ironclads carried a
considerable number of guns, which could, however, deliver only a
broadside fire, that is, the shots could, for the most part, be sent
only in a direction at right angles to the ship’s length, or nearly so.
But in the more recently built ironclads there are very few guns, which
are, however, six times the weight of the old sixty-eight pounders, and
are capable of hurling projectiles of enormous weight. The ships built
after the _Warrior_ were completely protected by iron plates, and the
thickness of the plates has been increased from time to time, with a
view of resisting the increased power which has been progressively given
to naval guns. A contest, not yet terminated, has been going on between
the artillerist and the ship-builder; the one endeavouring to make his
guns capable of penetrating with their shot the strongest defensive
armour of the ships, the other adding inch after inch to the thickness
of his plates, in order, if possible, to render his ship invulnerable.
[Illustration:
FIG. 70.—_Section of H.M.S. Hercules._
]
One of the finest of the large ironclads is the _Hercules_, of which a
section amidships is presented on the next page. This ship is 325 ft. in
length, and 59 ft. in breadth, and is fitted with very powerful engines
which will work up to 8,529 indicated horse-power. The tonnage is 5,226;
weight of hull, 4,022 tons; weight of the armour and its backing, 1,690
tons; weight of engines, boilers, and coals, 1,826 tons; total with
equipment and armament, 8,676 tons. Although the _Hercules_ carries this
enormous weight of armour and armament, her speed is very great,
excelling, in fact, that of any merchant steamer afloat, for she can
steam at the rate of nearly 17 miles an hour. She also possesses, in a
remarkable degree, the property which naval men call _handiness_; that
is, she can be quickly turned round in a comparatively small space. The
handiness of a steamer is tested by causing her to steam at full speed
with the helm hard over, when the vessel will describe a circle. The
smaller the diameter of that circle, and the shorter the time required
to complete it, the better will the vessel execute the movements
required in naval tactics. Comparing the performances of the _Warrior_
and the _Hercules_, we find that the smallest circle the former can
describe is 1,050 yards in diameter, and requires nine minutes for its
completion, whereas the latter can steam round a circle of only 560
yards diameter in four minutes. The section (Fig. 70) shows that, like
the _Great Eastern_, the _Hercules_ is constructed with a double hull,
so that she would be safe, even in the event of such an accident as
actually occurred to the _Great Eastern_, when a hole was made by the
stripping off of her bottom plates, 80 ft. long and 5 ft. wide. The
defensive armour of the _Hercules_ is, it will be observed, greatly
strengthened near the water-line, where damage to the ship’s side would
be most fatal. The outer iron plates are here 9 in. thick, while in
other parts the thickness is 8 in., and in the less important positions
6 in. The whole of the hull is, however, completely protected above the
water-line, and the iron plates are backed up by solid teak-wood for a
thickness of from 10 in. to 12 in. The teak is placed between girders,
which are attached to another iron plating 1½ in. thick, supported by
girders 2 ft. apart. The spaces between these girders are also filled
with teak, and the whole is lined with an inner skin of iron plating, ¾
in. thick. The belt along the water-line has thus altogether 11¼ in. of
iron, of which 9 in. are in one thickness, and this part is, moreover,
backed by additional layers of teak, as shown in the section; so that,
besides the 11¼ in. of iron, the ship’s side has here 3 ft. 8 in. total
thickness of solid teak-wood. The deck is also covered with iron plates,
to protect the vessel from vertical fire. The _Hercules_ carries eight
18–ton guns as her central battery, and two 12–ton guns in her bow and
stern: these guns are rifled, and each of the larger ones is capable of
throwing a shot weighing 400 lbs. The guns can be trained so as to fire
within 15° of the direction of the keel; for near the ends of the
central battery the ports are indented, and the guns are mounted on
Scott’s carriages, in such a manner that any gun-slide can be run on to
a small turn-table, and shunted to another port, just as a
railway-carriage is shunted from one line to another. Targets for
artillery practice were built so as to represent the construction of the
side of the _Hercules_, and it was found, as the result of many
experiments, that the vessel could not be penetrated by the 600 lb. shot
from an Armstrong gun, fired at a distance of 700 yds. The production of
such iron plates, and those of even greater thickness which have since
been used, forms a striking example of the skill with which iron is
worked. These plates are made by rolling, and it will be understood that
the machinery used in their formation must be of the most powerful kind,
when it is stated that plates from 9 in. to 15 in. thick are formed with
a length of 16 ft. and a breadth of 4 ft. The plates are bent, while red
hot, by enormous hydraulic pressure, applied to certain blocks, upon
which the plates are laid, the block having a height adjusted according
to the curve required. The operation requires great care, as it must be
accomplished without straining the parts in a manner injurious to the
strength of the plate.
[Illustration:
FIG. 71.—_Section of H.M.S. Inconstant._
]
Fig. 71 on the next page is the section of another ship of war, the
_Inconstant_, which has not, like the _Hercules_, been designed to
withstand the impact of heavy projectiles, but has been built mainly
with a view to speed. The _Inconstant_ has only a thin covering of iron
plating, except in that portion of the side which is above water, where
there is a certain thickness of iron diminishing from the water-line
upwards, but not enough to entitle the _Inconstant_ to be classed as an
armoured vessel. This ship, however, may be a truly formidable
antagonist, for she carries a considerable number of heavy guns, which
her speed would enable her to use with great effect against an adversary
incapable of manœuvring so rapidly. She could give chase, or could run
in and deliver her fire, escaping by her speed from hostile pursuit in
cases where the slower movements of a ponderous ironclad would be much
less effective. The _Inconstant_ carries ten 12–ton guns of 9 in.
calibre, and six 6–ton 7 in. guns, all rifled muzzle-loaders, mounted on
improved iron carriages, which give great facilities for handling them
The ship is a frigate 338 ft. long and 50 ft. broad, with a depth in the
hold of 17 ft. 6 in. She is divided by bulkheads into eleven water-tight
compartments. The engines are of 6,500 indicated horse-power, and the
vessel attains an average speed of more than 18½ miles per hour.
[Illustration:
FIG. 72.—_Section, Elevation, and Plan of Turret of H.M.S. Captain._
]
[Illustration:
FIG. 73.—_H.M.S. Captain._
]
A new system of mounting very heavy naval guns was proposed by Captain
Coles about 1861. This plan consists in carrying one or two very heavy
guns in a low circular tower or turret, which can be made to revolve
horizontally by proper machinery. The turret itself is heavily armoured,
so as to be proof against all shot, and is carried on the deck of the
ship, which is so arranged that the guns in the turret can be fired at
small angles with the keel. The British Admiralty having approved of
Captain Coles’ plans, two first-class vessels were ordered to be built
on the turret system. These were the _Monarch_ and the _Captain_—the
latter of which we select for description on account of the melancholy
interest which attaches to her. On page 155 a diagram is given
representing the profile of the _Captain_, in which some of the
peculiarities of the ship are indicated—the turrets with the muzzles of
two guns projecting from each being easily recognized. The _Captain_ was
320 ft. long and 53 ft. wide. She was covered with armour plates down to
5 ft. below the water-line, as represented by the dark shading in the
diagram. The outer plating was 8 in. thick opposite the turrets, and 7
in. thick in other parts. It was backed up by 12 in. of teak; there were
two inner skins of iron each ¾ in. thick, then a framework with
longitudinal girders 10 in. deep. The deck was plated in the spaces
opposite the turrets with iron 1½ in. thick. The _Captain_ was fitted
with twin screws—that is, instead of having a single screw, one was
placed on each side, their shafts being, of course, parallel with the
vessel’s length. The object of having two screws was not greater power,
for it is probable that a single screw would be more effectual in
propelling the ship; but this arrangement was adopted because it was
considered that, had only one screw been fixed, the ship might easily be
disabled by the breaking of a blade or shaft; whereas in the case of
such an accident to one of the twin screws, the other would still be
available. The twin screws could also be used for steering, and the
vessel could be controlled without the rudder, as the engines were quite
independent of each other, each screw having a separate pair. The
diameter of the screws was 17 ft. The erections which are shown on the
deck between the turrets afforded spacious quarters for the officers and
men. These structures were about half the width of the deck, and tapered
off to a point towards the turrets, so as leave an unimpeded space for
training the guns, which could be fired at so small an angle as 6° with
the length of the vessel. Above these erections, and quite over the
turrets, was another deck, 26 ft. wide, called the “hurricane deck.” The
ship was fully rigged and carried a large spread of canvas. But the
special features are the revolving turrets, and one of these is
represented in detail in Fig. 72, which gives a section, part elevation,
and plan. Of the construction of the turret, and of the mode in which it
was made to revolve, these drawings convey an idea sufficiently clear to
obviate the necessity of a minute description. Each turret had an
outside diameter of 27 ft., but inside the diameter was only 22 ft. 6
in., the walls being, therefore, 2 ft. 3 in. thick—nearly half this
thickness consisting of iron plating. Separate engines were provided for
turning the turrets, and they could also be turned by men working at the
handles shown in the figures. Each turret carried two 25–ton Armstrong
guns, capable of receiving a charge of 70 lbs. of gunpowder, and of
throwing a 600 lb. shot.
After some preliminary trials the _Captain_ was sent to sea, and behaved
so well, that Captain Coles and Messrs. Laird, her designer and
contractors, were perfectly satisfied with her qualities as a sea-going
ship. She was then sent in the autumn of 1870 on a cruise with the
fleet, and all went well until a little after midnight between the 6th
and 7th September, 1870, when she suddenly foundered at sea off Cape
Finisterre. The news of this disaster created a profound sensation
throughout Great Britain, for, with the exception of nineteen persons,
the whole crew of five hundred persons went down with the ship. Captain
Coles, the inventor of the turrets, was in the ill-fated vessel and
perished with the rest, as did also Captain Burgoyne, the gallant
commander, and the many other distinguished naval officers who had been
appointed to the ship; among the rest was a son of Mr. Childers, then
First Lord of the Admiralty. Although the night on which this
unfortunate ship went down was squally, with rain, and a heavy sea
running, the case was not that of an ordinary shipwreck in which a
vessel is overwhelmed by a raging storm. It might be said, indeed, of
the loss of the _Captain_ as of that of the _Royal George_:
“It was not in the battle;
No tempest gave the shock;
She sprang no fatal leak;
She ran upon no rock.”
[Illustration:
FIG. 74.
]
[Illustration:
FIG. 75.
]
One of the survivors, Mr. James May, a gunner, related that, shortly
after midnight he was roused from his sleep by a noise, and feeling the
ship uneasy, he dressed, took a light, and went into the after turret,
to see if the guns were all right. He found everything secure in the
turret, but that moment he felt the ship heel steadily over, and a heavy
sea having struck her on the weather side, the water flowed into the
turret, and he got out through the hole in the top of the turret by
which the guns were pointed, only to find himself in the water. He swam
to the steam-pinnace, which he saw floating bottom upwards, and there he
was joined by Captain Burgoyne and a few others. He saw the ship turn
bottom up, and sink stern first, the whole time from her turning over to
sinking not being more than a few minutes. Seeing the launch drifting
within a few yards, he called out, “Jump, men! it is your last chance.”
He jumped, and with three others reached a launch, in which were fifteen
persons, all belonging to the watch on deck, who had found means of
getting into this boat. One of these had got a footing on the hull of
the ship as she was turning over, and he actually walked over the bottom
of the vessel, but was washed off by a wave and rescued by those who in
the meantime had got into the launch. It appears that Captain Burgoyne
either remained on the pinnace or failed to reach the launch. Those who
were in that boat, finding the captain had not reached them, made an
effort to turn their boat back to pick him up, but the boat was nearly
swamped by the heavy seas, and they were obliged to let her drift. One
man was at this time washed out of the boat and lost, after having but
the moment before exclaimed, “Now, lads, I think we are all right.”
After twelve hours’ hard rowing, without food or water, the survivors,
numbering sixteen men and petty officers and three boys, reached Cape
Finisterre, where they received help and attention. On their arrival in
England, a court-martial was, according to the rules of the service,
formally held on the survivors, but in reality it was occupied in
investigating the cause of the catastrophe. The reader may probably be
able to understand what the cause was by giving his attention to some
general considerations, which apply to all ships whatever, and by a
careful examination of the diagrams, Figs. 74 and 75, which are copied
from diagrams that were placed in the hands of the members of the
court-martial. The letters b and g and the arrows are, however, added,
to serve in illustration of a part of the explanation. The vessel is
represented as heeled over in smooth water, and the gradations on the
semicircle in Fig. 74 will enable the reader to understand how the heel
is measured by angles. If the ship were upright, the centre line would
coincide with the upright line, marked o on the semicircle, and drawn
from its centre. Suppose a level line drawn through the centre of the
semicircle, and let the circumference between the point where the last
line cuts it and the point o be divided into ninety equal parts, and let
these parts be numbered, and straight lines drawn from the centre to
each point of division. In the figure the lines are drawn at every fifth
division, and the centre line of the ship coincides with that drawn
through the forty-fifth division. In this case the vessel is said to be
inclined, or heeled, at an angle of forty-five degrees, which is usually
written 45°. In a position half-way between this and the upright the
angle of heel would be 22½°, and so on. The reader no doubt perceives
that a ship, like any other body, must be supported, and he is probably
aware that the support is afforded by the upward pressure of the water.
He may also be familiar with the fact that the weight of every body acts
upon it as if the whole weight were concentrated at one certain point,
and that this point is called the centre of gravity of the body.
Whatever may be the position of the body itself, its centre of gravity
remains always at the same point with reference to the body. When the
centre of gravity happens to be within the solid substance of a body,
there is no difficulty in thinking of the force of gravitation acting as
a downward pull applied at the centre of gravity. But this point is by
no means always within the substance of bodies: as often as not it is in
the air outside of the body. Thus the centre of gravity of a uniform
ring or hoop is in the centre, where, of course, it has no material
connection with the hoop; but in whatever position the hoop may be
placed, the earth’s attraction pulls it _as if_ this central point were
rigidly connected with the hoop, and a string were attached to the point
and constantly pulled downwards. This explanation of the meaning of
centre of gravity may not be altogether superfluous, for, when the
causes of the loss of the _Captain_ were discussed in the newspapers, it
became evident that such terms as “centre of gravity” convey to the
minds of many but very vague notions. One writer in a newspaper enjoying
a large circulation seriously attributed the disaster to the
circumstance of the ship having lost her _centre of gravity_! The upward
pressure of water which supports a ship is the same upward pressure
which supported the water before the ship was there—that is, supported
the mass of water which the ship displaces, and which was in size and
shape the exact counterpart of the immersed part of the ship. Now, this
mass of water, considered as a whole, had itself a centre of gravity
through which its weight acted downwards, and through which it is
obvious that an equal upward pressure also acted. This centre of gravity
of the displaced water is usually termed the “centre of buoyancy,” and,
unlike the centre of gravity, it changes its position with regard to the
ship when the latter is inclined, because then the immersed part becomes
of a shape different for each inclination of the ship. Now, recalling
for an instant the fundamental law of floating bodies—namely, that the
weight of the water displaced is equal to the weight of the floating
body—we perceive that in the case of a ship there are two equal forces
acting vertically, viz., the weight of the ship or downward pull of
gravitation acting at G, Fig. 74, the centre of gravity of the ship, and
an equal upward push acting through B, the centre of buoyancy. It is
obvious that the action of these forces concur to turn a ship placed as
in Fig. 74 into the upright position. It is by no means necessary for
this effect that the centre of gravity should be below the centre of
buoyancy. All that is requisite for the stability of a ship is, that
when the ship is placed out of the upright position, these forces should
act to bring her back, which condition is secured so long as the centre
of buoyancy is nearer to the side towards which the vessel is inclined
than the centre of gravity is. When there is no other force acting on a
ship or other floating body, these two points are always in the same
vertical line. The two equal forces thus applied in parallel directions
constitute what is called in mechanics a “couple,” and the effect of
this in turning the ship back into the upright position is the same as
if a force equal to its weight were applied at the end of a lever equal
in length to the horizontal distance between the lines through B and G.
The righting force, then, increases in proportion to the horizontal
distance between the two points, and it is measured by multiplying the
weight of the ship in tons by the number of feet between the verticals
through G and B, the product being expressed in statical foot-tons, and
representing the weight in tons which would have to be applied to the
end of a lever 1 ft. long, in order to produce the same turning effect.
When a ship is kept steadily heeled over by a side wind, the pressure of
the wind and the resistance of the water through which the vessel moves
constitute another couple exactly balancing the righting couple. The
moment of the righting couple, or the righting force, or statical
stability as it is also called, is determined by calculation and
experiment from the design of the ship, and from her behaviour when a
known weight is placed in her at a known distance from the centre. Such
calculations and experiments were made in the case of the _Captain_, but
do not appear to have been conducted with sufficient care and
completeness to exhibit her deficiency in stability. After the loss of
the ship, however, elaborate computations on these points were made from
the plans and other data. The following table gives some of the results,
with the corresponding particulars concerning the _Monarch_ for the sake
of comparison:
┌───────────────────────────────────────────────┬──────────┬──────────┐
│ │_Monarch._│_Captain._│
├───────────────────────────────────────────────┼──────────┼──────────┤
│ I. Angle at which the edge of the deck is │ 28° │ 14° │
│ immersed │ │ │
│ II. Statical righting force in foot-tons at │ 12,542 │ 5,700 │
│ the angle at which the deck is immersed │ │ │
│III. Angle of greatest stability │ 40° │ 21° │
│ IV. Greatest righting force in foot-tons │ 15,615 │ 7,100 │
│ V. Angle at which the righting force ceases │ 59° │ 54° │
│ VI. Reserve of dynamical stability at an angle│ 6,500 │ 410 │
│ of 14° in _dynamical_ foot-tons │ │ │
└───────────────────────────────────────────────┴──────────┴──────────┘
From No. V. in the above table we learn that if the _Captain_ had been
heeled to 54°, the centre of gravity would have overtaken the centre of
buoyancy—that is, the two would have been in one vertical line. Any
further heeling would have brought the points into the position shown in
Fig. 75, where it is obvious that the action of the forces is now to
turn the vessel still more on its side, and the result is an upsetting
couple instead of a righting couple.
These figures and considerations refer to the case of the vessel
floating in smooth water, but the case of a vessel floating on a wave is
not different in principle. The reader may picture to himself the
diagrams inclined so that the water-line may represent a portion of the
wave’s surface; then he must remember that the very action which heaves
up the water in a sloping surface is so compounded with gravity, that
the forces acting through G and B retain nearly the same position
relatively to the surface as before.
No. VI. in the foregoing table requires some explanation. To heel a ship
over to a certain angle a certain amount of _work_ must be done, and in
the scientific sense _work_ is done only when something is moved through
a space against a resistance. When the weight of a ton is raised 1 ft.
high, one foot ton of work is said to be done; if 2 tons were raised 1
ft., or 1 ton were raised 2 ft., then two foot-tons of work would be
done, and so on. The same would be the case if a pressure equal to those
weights were applied so as to move a thing in any direction through the
same distances. It should be carefully noticed that the foot-ton is
quite a different unit in this case from what it is as the moment of a
couple. If we heel a ship over by applying a pressure on the masts, it
is plain that the pressure must act through a certain space, and the
same heel could be caused either by means of a smaller pressure or a
greater, according as we apply it higher up or lower down; but the space
through which it must act would vary, so that the product of the
pressure and space would, however, be always the same. No. VI. shows the
amount of work that would have to be done in order completely to upset
each of the vessels when already steadily heeled over to 14°. The
amounts in the two cases are so different that we can easily understand
how a squall which would not endanger the _Monarch_ might throw the
_Captain_ over. A squall suddenly springing up would do more than heel a
vessel over to the angle at which it is able to maintain it: it would
swing it beyond that position by reason of the work done on the sails as
they are moving over with the vessel, and the latter would come to a
steady angle of heel only after a series of oscillations. Squalls,
again, which, although suddenly springing up in this manner, could not
heel the ship over beyond the angle where the stability vanishes, might
yet do so if they were intermittent and should happen to coincide in
time with the oscillations of the ship—just as a series of very small
impulses, coinciding with the time of the vibrations of a heavy
pendulum, may accumulate so as to increase the range of vibration to any
extent. It is believed that in the case of the _Captain_ the pressure of
the wind on the underside of the hurricane assisted in upsetting the
vessel. This, however, could only have exerted a very small effect
compared to that produced by the sails. The instability of the _Captain_
does not appear to have been discovered by such calculations as were
made before the vessel went to sea. It was observed, however, that the
ship when afloat was 1 ft. 6 in. deeper in the water than she should
have been—in other words, the freeboard, or side of the ship out of the
water, instead of being 8 ft. high as intended, was only 6 ft. 6 in.,
and such a difference would have a great effect on the stability.
[Illustration:
FIG. 76.—_H.M.S. Glatton._
]
The turret system has been applied to other ships on quite a different
plan. Of these the _Glatton_ is one of the most remarkable. Her
appearance is very singular, and totally unlike that which we look for
in a ship, as may be seen by an inspection of Fig. 76, page 162. The
_Glatton_, which was launched in 1871, is of the _Monitor_ class, and
was designed by Mr. E. J. Reed, who has sought to give the ship the most
complete protection. With this view the hull is covered with iron plates
below the water-line, and the deck also is cased with 3 in. iron plates,
to resist shot or shell falling vertically. The base of the turret is
shielded by a massive breastwork, which is a peculiarity of this ship.
The large quantity of iron required for all these extra defences has, of
course, the effect of increasing the immersion of the vessel, and
therefore of diminishing her speed. The freeboard when the ship is in
ordinary trim is only 3 ft. high, and means are provided for admitting
water to the lowest compartment, so as to increase the immersion by 1
ft., thus reducing the freeboard to only 2 ft. when the vessel is in
fighting trim, leaving only that small portion of the hull above water
as a mark for the enemy. The water ballast can be pumped out when no
longer needed. The _Glatton_ is 245 ft. long and 54 ft. broad, and she
draws 19 ft. of water with the freeboard of 3 ft., displacing 4,865 tons
of water, while, with the 2 ft. freeboard, the displacement is 5,179
tons. This ship cost £210,000. Mr. Reed wished to construct a vessel of
much larger size on the same plan—a proposal to which, however, the
Admiralty did not then consent. The _Glatton_ is, nevertheless, one of
the most powerful ships of war ever built, and may be considered as an
impregnable floating fortress. Above the water-line the hull is covered
with armour plates 12 in. thick, supported by 20 in. of teak backing,
and an inner layer of iron 1 in. thick. Below the water-line the iron is
8 in. thick, and the teak 10 in. The revolving turret carries two 25–ton
guns, firing each a 600 lb. shot, and is covered by a massive plating of
iron 14 in. in thickness. Besides this the base of the turret is
protected by a breastwork rising 6 ft. above the hull. This breastwork
is formed of plates 12 in. thick, fastened on 18 in. of teak. The turret
rises 7 ft. above the breastwork, and therefore the latter in no way
impedes the working of the guns. The _Glatton_ has a great advantage
over all the other turret ships in having a perfectly unimpeded fore
range for her guns, for there is no mast or other object to prevent the
guns being fired directly over the bow. There are no sails, the mast
being intended only for flying signals and hoisting up boats, &c. The
hull is divided by vertical partitions into nine water-tight
compartments, and also into three horizontal flats—the lowest being
air-tight, and having arrangements for the admission and removal of
water, as already mentioned. The stem of the ship is protruded forwards
below the water for about 8 ft., thus forming a huge ram which would
itself render the _Glatton_ a truly formidable antagonist at close
quarters even if her guns were not used. The engines are capable of
being worked up to 3,000 horse-power, giving the ship a speed of 9½
knots per hour, and means are provided for turning the turret by steam
power. The turret can be rotated by manual labour, requiring about three
minutes for its complete revolution, but by steam power the operation
can be effected in half a minute. The commander communicates his orders
from the pilot-house on the hurricane deck to the engine-room,
steering-house, and turret, by means of speaking-tubes and electric
telegraphs. The _Glatton_ was not designed to be ocean-going, but is
intended for coast defence.
[Illustration:
FIG. 77.—_H.M.S. Thunderer._
]
The British navy contains two powerful turret-ships constructed on the
same general plan as the _Glatton_, but larger, and capable of steaming
at a greater speed, and of carrying coal for a long voyage. These sister
ships are named the _Devastation_, Fig. 69, and the _Thunderer_, Fig.
77. The _Thunderer_ has two turrets and a freeboard of 4 ft. 6 in. Space
is provided for a store of 1,800 tons of coal, of which the _Glatton_
can carry only 500 tons. The vessel is fitted with twin screws, turned
by two pairs of independent engines, capable of working up to 5,600
horse-power, and she can steam at the rate of 12 knots, or nearly 14
miles, an hour. With the large supply of coal she can carry, the
_Thunderer_ could make a voyage of 3,000 miles without re-coaling.
Though the freeboard of the heavily-plated hull is only 4 ft. 6 in., a
lighter iron superstructure, indicated in the figure by the light
shading, rises from the deck to the height of 7 ft., making the real
freeboard nearly 12 ft. This gives the ship much greater stability, and
prevents her from rolling heavily when at sea. The length is 285 ft. and
the width 58 ft., and the draught 26 ft. The hull is double, the
distance between the outer and inner skins of the bottom being 4 ft. 6
in. The framing is very strong and on the longitudinal principle, and
the keel is formed of Bessemer steel. Each turret is 24 ft. 3 in. in
internal diameter, and is built with five layers of teak and iron.
Beginning at the inside, there is a lining of 2⅝ in. iron plates; then 6
in. of teak in iron frames, arranged horizontally; 6 in. of armour
plates; 9 in. of teak, placed vertically; outside of all, 8 in. armour
plates. Each turret carries two Fraser 35–ton guns, rifled
muzzle-loaders. The turrets revolve by hand or by steam-power. There are
no sails, and thus a clear range for the guns is afforded fore and aft.
The bases of the turrets are protected by the armoured breastwork, of
which a portion is seen in the figure in advance of the fore turret.
Another very powerful ship of war, which possesses some special
features, is represented in the diagram on page 165, Fig. 78. This
vessel, named the _König Wilhelm_, was built at Blackwall for the
Prussian Government by the Thames Ironworks and Steam Shipbuilding
Company, from designs by Mr. Reed. Her length is 365 ft., width 60 ft.;
burthen, 6,000 tons; displacement, 8,500 tons. She is framed
longitudinally, that is, girders pass from end to end, about 7 ft.
apart, and the stem projects into a pointed ram. In this case also the
hull is double; there is, in fact, one hull within another, with a space
of 4½ ft. between them. The armour plates are 8 in. in thickness, with
10 in. of teak backing; but on the less important parts the thickness of
the iron is reduced to 6 in., and in some places to 4 in. This ship has
a broadside battery, and there are no turrets, but on the deck there
are, fore and aft, two semicircular shields, formed of iron plates and
teak, pierced with port-holes for cannon, and also with loop-holes for
muskets. From these a fore-and-aft fire may be kept up. The ship is
fully rigged, and has also steam engines of 7,000 horse-power, by
Maudslay and Co. Her armament consists of four three-hundred-pounders,
capable of delivering fore-and-aft as well as broadside fire, and
twenty-three other guns of the same size between decks. These guns are
all Krupp’s steel breech-loaders.
[Illustration:
FIG. 78.—_The König Wilhelm._
]
[Illustration:
FIG. 78_a_.—_The “Victoria” leaving Newcastle-on-Tyne._
]
The great contest of armour plates _versus_ guns has already been
alluded to, and to the remarks then made it may be added that, while on
the one hand, guns weighing 110 tons are mounted in turrets, ships are
already designed with 18 in. and even 20 in. of steel armour plates. It
would be very difficult to predict which side will sooner reach the
limit beyond which increase of size and power cannot go. The gradual
increase of thickness of plating, attended by increased weight of guns,
projectiles, and charges of powder, may be illustrated by stating in a
condensed form a few details of some ships, as regards the thickness of
armour, and its resisting power, which is nearly in proportion to the
square of its thickness; and also some particulars respecting the guns
originally carried by those ships.
┌──────────────┬──────────┬──────────┬──────────┬──────────┬──────────┐
│ │_Warrior._│ _Her- │_Glatton._│ _Thun- │ _Vic- │
│ │ │ cules._ │ │ derer._ │ toria._ │
├──────────────┼──────────┼──────────┼──────────┼──────────┼──────────┤
│Date when │ 1861 │ 1868 │ 1872 │ 1877 │ 1889 │
│ completed │ │ │ │ │ │
│Thickness of │ 4½ │ 9 │ 12 │ 14 │ 18 │
│ iron plating│ │ │ │ │ │
│ in inches │ │ │ │ │ │
│Relative │ 20 │ 81 │ 144 │ 196 │ 324 │
│ resisting │ │ │ │ │ │
│ power of │ │ │ │ │ │
│ plating │ │ │ │ │ │
│Guns carried │Cast iron,│ Wrought │ Wrought │ Wrought │ Steel, │
│ │ smooth │ iron, │ iron, │ iron, │ Rifled │
│ │ bore │ rifled │ rifled │ rifled │ │
│Weight of guns│ 4¾ │ 18 │ 25 │ 35 │ 111 │
│ in tons │ │ │ │ │ │
│Charge of │ 16 │ 60 │ 70 │ 120 │ 960 │
│ powder in │ │ │ │ │ │
│ lbs. │ │ │ │ │ │
│Weight of │ 68 │ 400 │ 600 │ 700 │ 1,800 │
│ projectiles │ │ │ │ │ │
│ in lbs. │ │ │ │ │ │
│Destructive │ 452 │ 3,863 │ 5,165 │ 8,404 │ 56,000 │
│ power of │ │ │ │ │ │
│ projectiles │ │ │ │ │ │
│ in foot-tons│ │ │ │ │ │
└──────────────┴──────────┴──────────┴──────────┴──────────┴──────────┘
One of the latest additions out of the thirty or forty armoured ships
that have been added to the British Navy since the preceding pages were
written is included in the above table for the sake of comparison. Our
ironclad fleet now includes vessels protected and armed in many
different ways. Some have the protective armour extended continuously
along the water-line, others have it for only a greater or less part of
their length. The armaments are also very diverse as to the size of the
guns and the way in which they are mounted. A few carry one or two of
the huge 110–ton gun mounted in massive revolving turrets; others have
their guns in central batteries, or in _barbettes_, and others again are
arranged as broadside ships; while these plans are also variously
combined so as to form a great number of different types. In the ships
built within the last 15 years, steel has been almost invariably used
instead of iron for the armour-plating. A great increase of speed has
been obtained in late years. The largest British armoured ships yet
launched have displacements between 10,000 and 12,000 tons, but another
class of first-rate line-of-battle ships of still greater size is in
process of construction, and of these it is estimated that four will be
completed in 1893. They are all of the same design and armament, and
will have a displacement of 14,150 tons, a length of 380 feet, and a
breadth of 75 feet. The armour plates at the sides will be 18 inches
thick. Each ship will carry four 67–ton breech loading rifled guns, ten
6–inch quick firing guns, and 18 other smaller guns, also quick firing.
These vessels are expected to realize a speed of about 20 miles per
hour; but this is somewhat less than a few of the heavy ironclads now
afloat have given by actual trial, a rate equal to 21⅓ miles an hour
having been attained by some. Several of our rapid unarmoured cruisers
are able to steam at 25 miles an hour.
Before the close of 1894, the British navy possessed no fewer than eight
of the largest armoured line of battle-ships mentioned in the foregoing
paragraph, each being of 14,150 tons displacement, and having engines of
13,000 horse-power. At the same period there were in course of
construction four ships surpassing even these in tonnage, though of
somewhat less engine-power. Two were building at Portsmouth, to be
called the _Majestic_ and the _Royal George_, whilst the _Jupiter_ was
in progress at Glasgow and the _Mars_ at Birkenhead. All these are very
heavily armoured vessels, each displacing 14,900 tons, provided with
engines of 12,000 horse-power, and a very effective armament of guns.
Among the powerful ships of the navy may now also be noted the _Blake_,
the _Blenheim_, which, although the displacement is only 500 tons
greater than that of _König Wilhelm_, have engines of nearly three times
the power, namely, of 20,000 horse-power. Of large armoured ships,
namely, those of _9,000 tons and upwards_, Great Britain now has afloat
at least fifty; and the advance that has taken place in the size and
power of war-ships during the last twenty years may be inferred by
reference to the foregoing paragraphs giving the dimensions, &c., of the
_Glatton_ and the _Thunderer_, which paragraphs are, for the sake of
comparison, allowed to appear as they did in the first edition (1876) of
this book. Besides these very large armoured vessels, of which the
smallest is nearly twice as big as the largest of twenty-five years ago,
the British navy comprises ships of every size and for every purpose,
and so many of them that their names and classifications would occupy
many pages.
Two recent additions representing new type of ships claim notice before
this article is concluded. These are first the _Terrible_, with a sister
ship the _Powerful_. The former, of which a representation[3] is given
in Plate V., is pronounced, for its size, armour, armament, and speed
taken together, to be the most powerful cruiser in the world. The length
is 538 ft., breadth 71 ft., depth 43 ft., and the displacement is 14,250
tons. A special object in the design of this vessel was high speed, and
she is provided with twin-screws and two engines, the combined effort of
which is equal to 25,000 horse-power. There are forty-eight boilers and
four funnels, the ship being capable of carrying 3,000 tons of coal. The
vessel is built on the lines of the great Atlantic steamers, and the
engines, guns, and magazines are protected by a thick curved armour
deck. The vessel has a speed of 22 knots, or 25⅓ miles per hour. Her
armament consists of two 22–ton guns, twelve 6–in. quick-firing, and
many other smaller machine guns, and she carries besides four submerged
torpedo tubes. A second ship to be noted is amongst those designed
mainly to exceed all other craft in speed, and ranging in tonnage from
3,800 to 4,500. The _Janus_, a torpedo-boat destroyer of this class, was
found, at a recent trial over a measured mile, to attain the then
unexampled speed of 28 knots, or 32¼ miles per hour. But even this has
been beaten by a new torpedo-boat destroyer, built by Messrs. Yarrow at
Poplar for the Russian Government, and launched in August, 1895. This
vessel, within a few hours after leaving the stocks, cut through the
water at the rate of 30·285 knots, or nearly 35 miles, per hour.
Footnote 3:
From _Graphic_, 1st June, 1895.
A sad fate befell the _Victoria_, which was one of the heaviest armed of
British ships (_vide_ page 129), when taking part in some naval
manœuvres off Tripoli, on the Syrian coast, where she was the flag-ship
of Admiral Tryon, commander-in-chief of the squadron. On the 22nd June,
1893, in consequence of an inconsiderate order given by the admiral
himself, the _Victoria_ was struck by the formidable ram of the
_Camperdown_ (10,600), and in fifteen minutes turned over and sank in
sight of the whole fleet, carrying down with her the admiral, 30
officers, and 320 men, out of a crew of 600. [1895.]
[Illustration:
FIG. 78_b_.—_Firing at a floating battery._
]
[Illustration:
PLATE XI. H.M.S. “TERRIBLE.”
]
[Illustration:
FIG. 79.—_Krupp’s Works, at Essen, Prussia_.
]
FIRE-ARMS.
The invention of gunpowder—or rather its use in war—appears at first
sight a device little calculated to promote the general progress of
mankind. But it has been pointed out by some historians that the
introduction of gunpowder into Europe brought about the downfall of the
feudal system with its attendant evils. In those days every man was
practically a soldier: the bow or the sword he inherited from his father
made him ready for the fray. But when cannons, muskets, and mines began
to be used, the art of war became more difficult. The simple possession
of arms did not render men soldiers, but a long special training was
required. The greater cost of the new arms also contributed to change
the arrangements of society. Standing armies were established, and war
became the calling of only a small part of the inhabitants of a country,
while the majority were left free to devote themselves to civil
employments. Then the useful arts of life received more attention,
inventions were multiplied, commerce began to be considered as
honourable an avocation as war, letters were cultivated, and other
foundations laid for modern science. If such have really been the
indirect results of the invention of gunpowder, we shall hardly share
the regret of the fine gentleman in “Henry IV.”:
“That it was great pity, so it was,
That villanous saltpetre should be digged
Out of the bowels of the harmless earth,
Which many a good tall fellow had destroyed
So cowardly.”
We often hear people regretting that so much attention and ingenuity as
are shown by the weapons of the present day should have been expended
upon implements of destruction. It would not perhaps be difficult to
show that if we must have wars, the more effective the implements of
destruction, the shorter and more decisive will be the struggles, and
the less the total loss of life, though occurring in a shorter time.
Then, again, the exasperated and savage feelings evoked by the
hand-to-hand fighting under the old system have less opportunity for
their exercise in modern warfare, which more resembles a game of skill.
But the wise and the good have in all ages looked forward to a time when
sword and spear shall be everywhere finally superseded by the
ploughshare and the reaping-hook, and the whole human race shall dwell
together in amity. Until that happy time arrives—
“Till the war-drum throbs no longer, and the battle flags are furl’d
In the Parliament of man, the Federation of the world—
When the common sense of most shall hold a fretful realm in awe,
And the kindly earth shall slumber, lapt in universal law,”—
we may consider that the more costly and ingenious and complicated the
implements of war become, the more certain will be the extension and the
permanence of civilization. The great cost of such appliances as those
we are about to describe, the ingenuity needed for their contrivance,
the elaborate machinery required for their production, and the skill
implied in their use, are such that these weapons can never be the arms
of other than wealthy and intelligent nations. We know that in ancient
times opulent and civilized communities could hardly defend themselves
against poor and barbarous races. But the world cannot again witness
such a spectacle as Rome presented when the savage hordes of Alaric
swarmed through her gates, and the mighty civilization of centuries fell
under the assaults of the northern barbarians. In our day it is the poor
and barbarous tribes who are everywhere at the mercy of the wealthy and
cultivated nations. The present age has been so remarkably fertile in
warlike inventions, that it may truthfully be said that the progress
made in fire-arms and war-ships within the second half of the nineteenth
century surpasses everything that had been previously accomplished from
the time gunpowder came into use. Englishmen have good reason to be
proud of the position taken by their country, and may feel assured that
her armaments will enable her to hold her own among the most advanced
nations of the world.
The subject of fire-arms embraces a very wide ground, as will appear if
we consider the many different forms in which these weapons are
constructed in order best to serve particular purposes. Pertaining to
this subject, attention must also be directed to the modern projectiles
and to the newer explosives that have largely taken the place of
ordinary gunpowder. The shot gun, fowling-piece, and sporting rifle
properly come under the head of fire-arms, and in the march of
improvement these forms have most commonly been in advance of military
muskets and rifles, the ingenuity bestowed on all their details being
worthy of admiration. Nevertheless it is to the implements of war that
general interest attaches; for on them depends so much the fate of
battles and the destiny of nations, that whenever any country is engaged
in war the question of arms becomes one of surpassing importance,
enlisting the patriotic instincts of every citizen. Hence in the
following pages our space will be devoted mainly to weapons of war, and
more particularly to those that have been adopted by our own country.
Everyone of course is aware that guns, cannon, and gunpowder are by no
means inventions of the nineteenth century; but there are fewer
acquainted with the fact that rifling, breech-loading, machine guns, and
revolvers were all invented and tried hundreds of years before. The
devices by which some of these ideas were sought to be realised in past
ages appear to us in some instances very primitive, not to say childish,
when compared with modern work: but it must be remembered that nearly
all the appliances required for producing such weapons had themselves to
wait for their invention until the nineteenth century; such, for
instance, as the steam-hammer, powerful and accurate tools, refined
measuring implements, material entirely reliable such as the new steel,
and also scientific investigations of all the conditions involved. The
military fire-arms are of so many different forms and patterns that we
can deal here with but a selection from the various services. If a rough
classification had to be made, the most obvious distinction would be
between the weapons the soldier carries in his hands (small-arms) and
those which are mounted on some kind of carriage and discharge
projectiles of much greater weight (ordnance). Ordnance again includes
guns mounted on forts, carried in ships, or taken with an army into the
field, in each case coming into action under different conditions.
Partaking somewhat of the nature of both field-guns and of small-arms
are the machine guns, of which the French mitrailleur was the first
example, afterwards developing into much more effective weapons in the
hands of Gatling, Gardner, Nordenfelt, Maxim, and Hotchkiss.
As much will have to be said about _rifling_ the bores of muskets and
cannon, we may here explain the nature and object of this device. The
projectiles used in all guns down to comparatively recent times were
almost invariably of spherical form, and could indeed scarcely be
otherwise with smooth-bore weapons. As the diameter of the shot would
necessarily be something less than that of the bore of the barrel, a
considerable loss of power would result from the escape of the powder
gases between the shot and the barrel, which escape is known as
_windage_. Another disadvantage of the spherical projectile is that for
the same weight of metal the air offers a greater resistance to its
passage, and consequently checks its speed more quickly than that of any
other circular form; for the air resistance is proportional to the
square of the diameter, and therefore if we take a ball of 1 in.
diameter and a cylinder of 1 in. in length, each having the same weight
of metal, the diameter of the cylindrical shot will be a little more
than four-fifths of an inch, and the air resistance to the ball will be
exactly half as much again as to the cylinder, that is, in the
proportion of 3 to 2. Again, the passage of the spherical shot within
the barrel of the gun will not be in a straight line, but in a series of
rebounds from side to side, and its direction on leaving the muzzle will
depend upon which part of the bore it just before impinges on, as from
that it will also take a rotatory “twist” that will in part determine
its path through the air.
Now if an elongated projectile were fired from a smooth-bore gun, its
course through the air would be erratic to a degree impossible to the
spherical shot, for it would turn end over end with deviations that
would make aiming impracticable. But if the elongated projectile is made
to spin rapidly enough about its longitudinal axis, it flies through the
air quite steadily, the axis of rotation remaining parallel to that of
the gun throughout the whole flight. The steadiness due to rapid
rotation has familiar examples in spinning tops, in gyroscopic tops, in
the way arrows are feathered so that the air may cause them to revolve
axially, and so on. The axial rotation of the projectile is effected by
ploughing out in the cylindrical barrel of the gun a number of spiral or
twisting grooves, which the projectile is compelled to follow as it
travels along the barrel, either by means of corresponding protuberances
formed upon its surface in the first instance, as in Jacob’s bullets, or
by studs let into it, as in the studded shots and shells for ordnance
which constituted at one time the regulation plan adopted by the British
Government; or otherwise by making the force of the explosion expand
some portion of the projectile in such a manner that this portion shall
completely fill up the grooves, thus preventing windage, and causing the
projectile to follow the twist of the grooves. This is the more general
method, especially since the adoption of breech-loading. The Lancaster
rifling, and that advocated by Whitworth, are the same in principle, but
differ in appearance, from the section of the barrel being made in the
one case oval, in the other hexagonal or polygonal, but with the twist
necessary to produce rotation.
Incident to the discharge of all fire-arms, great and small, is a
phenomenon of which we have to speak, because it is one which in the
mounting of heavy ordnance especially has to be taken into account. And
as it also illustrates in a very direct way one of the most general laws
of nature, while people often have very vague and erroneous ideas of its
cause and operation, it deserves the reader’s attention. In gunnery it
is called the _recoil_, and is familiar to anyone who has ever fired a
pistol, fowling-piece, or rifle, in the kick backwards felt at the
moment of the discharge. This law is in operation whenever the condition
of a body in respect to its rest or motion is changing. That is,
whenever a body at rest has motion given to, or if when already moving
it is made to go faster or slower, or to stop, or when the direction of
the motion is changed from that in a straight line. Now although these
changes or actions are frequently occurring before our eyes, the
operation in them of Newton’s third law of motion does not generally
present itself to common observation. This third law was stated by Sir
Isaac Newton thus:—“To every action there is always an opposite and
equal reaction.” Now the expanding gases due to the gunpowder explosion
press the bullet forwards and the barrel (with its attachments)
backwards, with the same pressure in both cases, but at the end of the
bullet’s passage along the bore the same velocity is not imparted to the
two bodies, because the same pressure acting for the same time on bodies
of unequal _mass_ always produces velocities that are inversely
proportional to the _masses_. The reader should try to acquire this
conception of _mass_, remarking that it is a something quite distinct
from that of _weight_. A given lump of metal, for instance, would have
exactly the same _mass_ in any part of the universe, whereas its weight
would depend upon its position; as, for instance, at the distance from
the earth of the moon’s orbit, it would _weigh_ only as 1/3600th part of
its weight at the earth’s surface, and if it could be carried to the
very centre of the earth it would there have no weight at all. Though
the lump of metal will have different weights at different parts of the
earth’s surface, it has been found (by experiment) that the weights of
bodies at any one place are proportional to their masses. Therefore the
same numbers that express the weights of bodies might also express their
masses; but for certain good reasons these quantities are referred to
different units. In England a piece of metal weighing 32 lbs. under
standard conditions is said to have mass = 1; and so on. As with the
_same pressure acting for the same time_, the velocities imparted are
inversely proportional to the masses, it follows that the number
expressing the velocity multiplied by that representing the mass in each
such case of action and reaction will give the same product, or in other
words the _amount of motion_ (momentum) will be the same. This is what
Newton meant by saying the reaction is _equal_ to the action. We may now
by way of illustration calculate the velocity of recoil of a rifle under
conditions similar to those that might occur in practice. Let us suppose
that the rifle, including the stock and all attachments, weighs 10 lbs.,
and that from it is fired a bullet weighing one-sixteenth of a pound,
with a velocity at the muzzle of 1,200 ft. per second. To obtain the
amount of motion or the momentum, we should here multiply the number
expressing the _mass_ of the bullet by 1,200, but for our present
purpose the weight numbers may be used for the sake of simplicity;
therefore 1/16 x 1,200 = 75 will represent (proportionately) the forward
momentum of the bullet, and according to Newton’s law the backward
momentum of the rifle will be, on the same scale, 75 also. We must
therefore find the number which multiplied by 10 will give 75, and this
obviously is 7·5. That is as much as to say that at the instant the
bullet is leaving the muzzle, the rifle itself, _if free to move_, would
be moving backward at the speed of 7½ ft. per second. Observe that this
result would be the same if the rifle were fired where weight is
non-existent; nor is the recoil due, as sometimes is erroneously
supposed, to the resistance of the air to the passage of the bullet
along the barrel, for even if the air were abolished, the recoil, so far
as due to the masses and velocities, would remain the same, as indeed
may be seen from the fact of our calculation taking no account of the
bore of the rifle or of the shape of the bullet, circumstances of the
utmost importance where atmospheric resistance is concerned.
The foregoing calculation however involves an assumption not in exact
conformity with actual conditions, by taking for granted that the
_centre of gravity_ of the rifle is in the line of the axis of the
barrel, while in fact this centre is almost always lower, and therefore
the kick of the recoil acts in part as a turning-over push, tending to
tilt up the muzzle of the gun, and for that reason the firer must hold
the weapon very firmly or he will miss his aim. When such a rifle as we
have supposed is fired, say from the shoulder, it would follow from the
above calculation that the backward kick of the recoil is equivalent to
a blow from a 10–lb. weight moving at the speed of 7½ ft. per second.
This would certainly be a very uncomfortable experience, but the
backward momentum must be met somehow. We have supposed that the gun is
free to move, but we know the firer presses it firmly against the
muscles of his shoulder, and the stock of the gun is spread out and
provided with a smooth hollow heel plate, so that any pressure from it
is felt as little as possible, especially as the muscle against which it
is applied acts as an elastic pad. With the rifle thus firmly held we
may regard the marksman and his rifle as forming only _one mass_, and
the centre of gravity of this being now much below the axis of the
barrel, the effect of the recoil tends to overthrow the man backwards;
but he learns to resist this by standing firmly, so that the elasticity
of his whole frame comes into play; and besides this, the mass factor of
the momentum being now so large, the velocity factor becomes
comparatively insignificant.
Although the momenta of gun and projectile are, according to Newton’s
law, _equal_ and opposite, the case is very different with regard to
their _energies_, or powers of doing work, for the measure of these is
jointly mass and _the square of the velocity_. The _energy_ (_vis viva_)
of a body of weight in pounds = W, moving with the velocity of v feet
per second is always Wv^2/64·4, that is, it will do this number of
foot-lbs. units of work before it comes to rest. It would require too
much space to demonstrate and fully explain here what this means, but
the reader may refer to our index under the entries “Energy” and “Work,”
or to any modern elementary treatise on dynamics. If the calculation be
made of the energies of the ball and of the rifle due to our calculated
velocities of recoil, it will be found that that of the ball is 160
times greater than that of the other, and the ball possesses this energy
in a much smaller compass.
[Illustration:
FIG. 80.—_Trajectory of a Projectile._
]
The course or track of a projectile through the air after it leaves the
gun is called the _trajectory_, and this has been studied both
experimentally and theoretically, with interesting results. Assuming
that the shot passed through empty space, or that the air offered no
resistance to its passage, it would be very easy to trace the path of a
projectile. Let us suppose that Fig. 80 represents a gun elevated at a
high angle. The moment the projectile leaves the muzzle, gravity begins
to act upon it, causing it to move vertically downwards with
ever-increasing velocity until it finally reaches the ground; the onward
uniform movement parallel to the axis of the piece being continued all
the time. We could find the position of the projectile at the end of
successive equal periods of time by drawing a straight line AC, a
prolongation of the axis of the piece, or a line of the same
inclination; on this we mark off equal distances representing by scale
the velocity of the projectile per second, the points B, C, D, E being
the positions the projectile would be in at the end of each successive
second if gravity did not act. In order to bring the diagram within
moderate compass, we suppose the projectile to have only the small
velocity of 115 ft. per second. At the end of the first second it would
be at B, but now suppose that gravity is allowed to act for one second,
it would at the end of that time have fallen 16 ft. vertically below B
and have arrived at _b_. Similarly we may set off by scale on verticals
through C, D, and E distances representing 64 ft., 144 ft., and 256 ft.
respectively. Because, for instance, the ball, without gravity acting,
would at the end of 3 seconds be at D, where we may suppose its course
arrested and gravity then allowed to act for 3 seconds to pull the ball
down from its position of rest at D; at the end of this period, gravity
alone acting, its position would be 144 ft. vertically below D, because
gravity pulls a body that distance in 3 seconds, and the actual position
3 seconds after the ball had left the muzzle would be at _d_, after it
had described the curved path A, _b_, _c_, _d_. Supposing _d_ to be the
highest point of the trajectory, another 3 seconds would bring the ball
along a downward curve, and at the end of 6 seconds from the discharge
it would be at a point on the same level as A. Now the complete curve
would be symmetrical on each side of a vertical line through its highest
point, and it would be in fact a regular _parabola_ with its vertex at
_d_.
The foregoing presupposes that the air offers no resistance to the
passage of the projectile through it. The fact however is quite
otherwise, for no sooner does the projectile begin its flight than its
velocity is constantly diminished by the air’s resistance. Now this
resistance is complex, depending upon a number of different conditions,
the effect of which can be taken into account only by extremely complex
calculations. Obviously it will vary according to the area of the
section presented by the projectile to the line of its flight, and again
by the shape of its front, for a pointed shot will cleave the air with
less resistance than one with a flat front. Then the density of the air
at the time will also enter into the calculation. The mass of the
projectile and also its velocity, upon which depend its _vis viva_,
energy, or power of overcoming resistance in doing work, will also have
to be considered. Most complex of all is the law, or rather laws (_i.e._
relations), which connect the air resistance with the velocity; for this
relation no definite expression has been found. It is a function of the
velocity (known only by experiment under defined conditions), and
varying with the velocity itself. Thus for velocities up to 790 ft. per
second, it is a function (determined experimentally) of the second power
or square of the velocity; between 790 ft. per second and 990 ft. per
second the law of resistance is changed and becomes a function of the
third power of the velocity; between 990 ft. and 1,120 ft. velocity the
law again changes and is related to the sixth power of the velocity;
between 1,120 ft. and 1,330 ft. the resistance is again related to the
third power of the velocity; and with higher speeds than that last named
it is again more nearly related to the square of the velocity. It will
be seen that to calculate the path of a projectile is really a very
difficult mathematical problem, and indeed one which can be solved only
approximately when all the known data are supplied.
The air resistance to the motion of a projectile is much greater than
before trial would be supposed. Let us take an experiment that has
actually been recorded, in which a bullet three-quarters of an inch in
diameter, weighing one-twelfth of a pound, was found to have a velocity
of 1,670 ft. per second at a distance of 25 ft. from the gun, and this
50 ft. farther was reduced to 1,550 ft. per second. Now if the reader
will calculate, according to the formula we have given above, the
_energy_ due to the bullet’s velocity at these points, he will find it
must have done 500 foot-lbs. units of work in traversing the 50 ft., and
as this could have been expended only in overcoming the resistance of
the air, we learn that this last must have been equivalent to a mean or
average pressure of 10 lbs. thrusting the bullet backwards.
It will be interesting to compare the difference in the trajectory of a
projectile under defined conditions, worked out with the air resistance
taken into account, compared with the trajectory when the air is
supposed to be non-existent. We find an example of the former problem
fully worked out by many elaborate mathematical formulæ in Messrs. Lloyd
and Hadcock’s treatise on Artillery. The problem is thus stated:—“An
11–in. breech-loading howitzer” (a howitzer is a piece of ordnance used
for firing at high angles) “fires a 600–lb. projectile with an initial
velocity of 1,120 foot-sec. at an elevation of 20°. Find the range, time
of flight, and angle of descent.” We shall calculate these points on the
suppositions adopted with regard to Fig. 80, and with no higher
mathematics than common multiplication and division.
It will have been observed that we supposed two motions that really take
place simultaneously to take place successively and independently: one
in the direction of the line of fire, due to the initial velocity; the
other vertically downwards, due to the action of gravity, the final
result being the same. This affords an excellent illustration of another
of Newton’s laws of motion, and should be considered by the reader in
this connection. The law itself admits of being stated in various ways,
as thus:—“Whenever a force acts on a body, it produces upon it exactly
the same change of motion in its own direction, whether the body be
originally at rest or in motion in any direction with any velocity
whatever—whether it be at the same time acted on by other forces or
not.” Or again: “When two forces act in any direction whatever on a body
free to move, they impress upon it a motion which is the _superposition_
(or compounding) of those that it would receive if each force acted
separately.” The law is given also in the following form (Thomson and
Tait):—“When any forces act on a body, then, whether the body be
originally at rest or moving with any velocity and in any direction,
each force produces in the body the exact change of motion which it
would have had had it acted singly on the body originally at rest.” In
all of these expressions the word “forces” is used, and a very
convenient word it is, but it may be noted in passing, nothing but a
word; for it stands for no real self-existing things, since, apart from
observed changes of motion in bodies, forces for us have no existence.
Nevertheless, it is useful for the sake of abbreviating statements about
changes of motion, to regard these actions as produced by imaginary
agents—imagined for the time and for this purpose, and therefore vainly
to be sought for in the realm of reality.
[Illustration:
FIG. 81.—_Diagram._
]
In dealing with the trajectory of the howitzer’s projectile through
airless space we have no concern with its diameter nor with its weight.
We use the little diagram, Fig. 81, to represent the motions,—_c_ being
a horizontal line, _a_, a vertical one, the angle at B is therefore a
right angle, and we assume that at A to be 20°. Now, the most elementary
geometry teaches us that every triangle having these angles will have
the lengths of its sides in the same invariable proportions one to
another whatever may be the size of the triangle itself, and it has been
found convenient to calculate these proportions once for all, not merely
for angle 20°, but for every angle up to 90°. Besides this, distinct
names have been given to the proportions of every side of the triangle
to each of the other two sides. Thus in the triangle before us, if we
take _a_, _b_, and _c_ to represent the numbers expressing the lengths
of the sides against which they are placed, _a_ divided by _b_, that is
_a_ ÷ _b_, or _a_/_b_, is called the _sine_ of angle 20°, while _c_/_b_
is named the _cosine_ of that angle, etc. These therefore are _numbers_
which are given in mathematical tables, and we find by these that _sine_
20° = 0·3420201, and _cosine_ 20° = 0·9396926, and these with the
initial velocity give us all the data we require. We may first find the
_time_ the projectile would take to reach the ground level, or strictly
that of the muzzle of the gun at B. Taking _t_ to stand for this time,
we know that AC = 1,120 × _t_, but CB will be the distance that a body
would fall from rest at C by the influence of gravity in that same time,
_t_, and it is known by experiment that this distance is 16·1 feet
multiplied by the _square_ of the time from rest in seconds. We have now
therefore the length of the line CB, and put _a_/_b_ = CB/AC = (16·1 ×
_t_^2)/(1,120 × _t_) = _sine_ 20° = ·3420201, and dividing numerator and
denominator by _t_ and multiplying the above 3rd and 5th expressions by
1,120, we have
16·1 × _t_ = 1,120 × ·3420201
1,120 × ·3420201
and therefore _t_ = ———————————————— = 23·7927 secs.
16·1
Having obtained the time, it will be easy to work out the lengths _b_
and _a_ as 26,648 ft. and 9114·1 ft. respectively; and as _c_/_b_ =
_cosine_ 20°, we have _c_ = 26,648 × ·9396926 = 25040·8 ft., which is
the _range_. The trajectory will be a curve (parabola) symmetrical on
each side of a vertical line half-way between A and B, and the length of
this line within the triangle will be equal to half of _a_, and in half
of 23·7927 seconds the projectile, supposed to move only along the line
AC, would reach the point where this vertical axis intersects AC. If
during this half-time it had been falling from rest at the same
intersection, it would have reached a point below by a space just one
quarter of CB (the spaces fallen through being as the _squares_ of the
times), and therefore at this its highest point its distance above AB
would also be one quarter the length of _a_ = 2278·525 ft., which
distance is called the _height_ of the trajectory; and the descending
curve being in every respect symmetrical to the ascending branch, the
angle at which this would be inclined to AB would be 20°, but in the
opposite direction to BAC, while the velocity would be the same as at A.
We may now compare these results with those calculated when the air
resistance is taken into account:—
┌─────────────────────────────────┬───────────┬───────────┬───────────┐
│ │Without air│With air │ │
│ │resistance.│resistance.│Difference.│
├─────────────────────────────────┼───────────┼───────────┼───────────┤
│Time of descent │23·7927 │22·61 sec. │–1·18 sec. │
│ │secs. │ │ │
│Angle of descent │20° │23° 49´ │+3° 49´ │
│Velocity of descent │1120 │868·8 │–251·2 │
│ │foot-secs. │foot-secs. │f.-s. │
│Range │25040·8 ft.│20,622 ft. │–4418·8 ft.│
│Height of trajectory │2278·5 ft. │1989 ft. │–288·5 ft. │
└─────────────────────────────────┴───────────┴───────────┴───────────┘
With the air resistance the trajectory will no longer be a symmetrical
curve: its highest point, instead of being on the vertical line midway
between A and B, will be on one 1,050 ft. nearer to B than to A, and the
descending branch will be steeper than the ascending. The total time, it
will be observed, is less, although the final, and therefore the mean,
velocity, is also less; but this shortening of the time is due to the
trajectory itself being much less in length. The range of the projectile
is decreased by 4,418 ft., or 1,473 yards, or more than four-fifths of a
mile. The loss of velocity at the descent is very notable, and the
reader will find it interesting to calculate the corresponding loss of
energy by the formula already given.
The reader should now easily understand that the projectile from a rifle
or gun discharged horizontally through airless space at the height of
16·1 ft. above a level plain would strike the ground in one second at a
range or distance from the gun exactly equal to the initial velocity, or
if the gun were on a tower and its axis 64·4 ft. above the plain, the
range would then be 2V. It will be seen therefore that, corresponding to
the range intended, there must be in general a certain inclination given
to the axis of the piece in aiming, and this is done by means of the
_sights_, one of which near the muzzle is usually fixed, while that next
the breech is adjustable by sliding along an upright bar, which is
graduated so that the proper elevation may be given for any required
range. These graduations are made from experiments, and of course have
reference only to some standard quantity and quality of ammunition and a
standard of weight, shape, and material in the projectile. Sometimes
large pieces of ordnance are laid by elevation in degrees, etc., marked
on their mounting, the angles being taken from a table prepared for that
particular gun and ammunition, from experiments at different ranges.
After these generalities about fire-arms we may enter upon certain
particulars about the construction of some varieties, beginning with
_THE MILITARY RIFLE._
[Illustration:
FIG. 82.—_Muzzle-loading Musket and Rifles_ (_obsolete patterns_).
A. Brown Bess and Bayonet; B. Brunswick Rifle; C. Enfield Rifle and
Bayonet.
]
In Fig. 82 are represented the muzzle-loading musket and muzzle-loading
rifles which formed the regulation weapons of the British infantry from
the beginning of the century up to the year 1864. Somewhat slow in its
earlier stages was the development of the modern military rifle from the
old smooth bore musket with its flint-lock, which was the ordinary
weapon of the British and other armies up to nearly the middle of this
century. Partly, perhaps, owing to the inherent conservatism of
government departments, and partly to the very serious outlay involved
in arming all the troops of a nation with a new weapon, it has happened
that many improvements in small arms were in use as applied to sporting
guns, long before they were adopted in the regulation weapons of armies.
The advance towards the modern arm of precision has been made along all
the several directions that converge in the latest product, and it may
be said that the most obvious of these are spiral rifling,
breech-loading, and improved ammunition. The improvements in any one of
these particulars would have been of little advantage unless the others
had been kept in line with it. How long antiquated systems may continue
in use may be illustrated by the case of the flint-lock, which was
retained in the British army from the time it superseded the old
match-lock, in the latter part of the seventeenth century, down to
almost the middle of this present nineteenth. It is quite possible that
not a few readers still in their fifties may never have seen a
flint-lock outside of a museum, yet this was the firing apparatus of the
weapon that used to be affectionately known to our soldiers as “Brown
Bess,” and that for a century and a half continued the regulation arm of
British troops helping Wellington to win his victories, and superseded
by the percussion musket only in 1842. The “Brown Bess” of the earlier
part of the century had a smooth-bore barrel of three-quarters of an
inch diameter (0·753 inch), and 39 inches long; this musket weighed,
with its bayonet, 11 lbs., 2 oz. The bullet was spherical, and made of
lead, in weight a little over one ounce. The diameter of the bullet was
slightly smaller than the bore of the barrel, because a closely fitting
ball could not be used, on account of the great force required to push
it home with a ram-rod. The bullet was therefore wrapped in loosely
fitting material, called a “patch,” and this made the gun easy to load,
even when the barrel was “fouled” by the solid residues that always
remain after the explosion of gunpowder. “Brown Bess” was credited with
a range of 200 yards, but its want of accuracy was such that the soldier
was directed not to fire until he could see the whites of the enemies’
eyes. But in 1800 one or two British regiments were armed with the
muzzle-loading rifle known as Baker’s, and again in 1835 these were
provided with the _Brunswick rifle_. These regiments afterwards became
known as the Rifle Brigade. The bullets in both cases were spherical,
and as the earlier pattern had a seven-grooved barrel, there was so much
difficulty in introducing the bullets into the muzzles that mallets had
to be used. The bullet of the Brunswick rifle was encircled by a
projecting band, which fitted into two rather deep grooves diametrically
opposite to each other in the barrel. This bullet, wrapped in some
slightly greased material, could be readily dropped into the muzzle, and
rammed home without difficulty. Moreover, whereas in Baker’s rifle the
grooves made only a quarter of a turn in the length of the barrel, the
grooves of the Brunswick rifle made more than one complete turn. This
was so much an improvement on “Brown Bess” that the effective range was
more than doubled. For the rank and file of the infantry regiments the
flint-lock smooth-bore musket was, however, the regulation weapon until
1842, when it was superseded by the percussion musket. The
percussion-cap is now comparatively little used, as, since the
introduction of cartridges containing their own means of ignition, it is
rapidly becoming a thing of the past. The copper percussion-cap, in the
form it still retains, was invented about 1816, and was universally
adopted for sporting-guns a long time before it was used for the
military weapon. In 1842 the percussion musket was definitely adopted as
the weapon of the British army, but up to that date the flint-locks
still continued to be made at Birmingham.
The barrel of the percussion musket then issued was shortly afterwards
rifled, when about the year 1852 the Minié system was adopted, and the
Government awarded to M. Minié, a Frenchman, the sum of £20,000 for the
bullet he had invented. What the meaning of this improvement was may now
be explained, and we must begin by mentioning the various forms of
grooving, or, at least, such forms as found some approval during the
present century, for grooved barrels had been tried long before. At
first the grooves appear to have been intended merely to receive the
fouling, and these were often made without any twist or spiral, but
parallel to the axis of the barrel. The grooves are hollow channels of
greater or less depth, and of various forms; square, triangular,
rounded, or of such a form that the inner line of a section of the
barrel would present the form of a ratchet wheel. The numbers of the
grooves made use of have varied between two and twelve, or more, and
different rates of twist, or numbers of turns of the spiral in the
length of the barrel have been resorted to, these ranging from half a
turn to twelve turns. The Brunswick rifle had been found wanting in
accuracy, when at length in 1846 General Jacobs proposed the adoption of
the conical bullet with projecting spiral ridges which fitted into
grooves cut in the rifle barrel. The difficulty in using muzzle-loading
rifles consisted in the force required to ram down the bullet, which had
to adapt itself to the grooves, and fill them up so that the gases due
to the explosion of the powder should not escape. If the bullet simply
dropped into the bore of the rifle easily, it did not effectually fill
the grooves, which then became channels of this _windage_, and if, on
the other hand, the leaden bullet was made to fill the grooves from the
muzzle, great force was required, and the time and effort expended in
ramming the missile home, detracted enormously from the efficiency of
the rifle as a military weapon. Mr. Lancaster produced rifles having a
slightly oval, instead of a circular, bore, making, of course, the
necessary twist within the barrel. A bullet of the corresponding
section, but nearly globular, much as if the projecting belt of the
Brunswick bullet had been laterally extended to its opposite poles,
could be easily dropped in at the muzzle, without force being required
to make it take grooves, the barrel being internally smooth throughout.
It was, however, soon found that this easy-fitting ball allowed a
considerable amount of _windage_, and the Minié system was definitely
adopted, in which advantage was taken of a fact observed some years
before by a French artillery officer, who found that an elongated leaden
bullet, if hollowed out at the base, was so expanded by the pressure of
the powder gases that the material was forced into the grooves of a
rifle. Minié made his bullet elongated, pointed in front, and hollowed
out part of its length by a conical space, the widest part of which was
at the base, and was covered by a small iron cup, that, when driven
inwards by the pressure of the gases, caused an expansion of the bullet
by which the lead was forced into the grooves of the rifling. But the
forces operating on the base of the bullet would at times cause the iron
cup to cut the bullet in two, and propel the anterior portion only,
leaving the base in the form of a ring clinging to the rifling. The
military authorities had many comparative trials carried out between the
smooth-bore percussion musket and the Minié rifle. The greater accuracy
of the latter may be inferred from the results of practice made by men
firing at a target 6 feet high and 20 feet broad; when at 100 yards
distance, 74 hits out of 100 shots were made from the musket, against 94
from the rifle; and the superiority of the latter, at longer ranges, was
increasingly marked. Thus at 260, 300, and 400 yards the respective
percentages of hits were for the musket 42, 16, 4½, but for the same
ranges the rifle gave 80, 55, 52.
[Illustration:
FIG. 83.—_The Minié Bullet._
]
Curiously enough, the principle of the expanding bullet had been brought
forward by the late Mr. W. Greener seventeen years before the government
prize was awarded to M. Minié. Mr. Greener’s bullets were of an oval
form, being half as long again as their diameter, with one end flattened
where the lead was excavated in a narrowing hollow nearly through the
bullet. In this opening was inserted the end of a tapering plug of hard
metal, and when the rifle was fired this plug was driven home, and the
lead thus expanded took the grooves, so preventing windage, and giving
range and accuracy; while allowing the piece to be loaded with as much
ease as the smooth-bore musket. The invention, though favourably
reported on by the military authorities at the time, did not receive the
attention it would seem to have deserved. However, in 1857, Mr.
Greener’s claim of priority for the first suggestion of the expanding
bullet was acknowledged by a government award of £1,000.
[Illustration:
FIG. 84.—_Greener’s Expanding Bullet._
]
Sir Joseph Whitworth, having been invited by the British military
authorities to institute experiments with a view to producing the best
type of rifle, with the help of the most perfect machinery, constructed
the barrels with a polygonal bore, a plan which he had before adopted
for large guns. The barrels were accurately bored out to a hexagonal
section, and experiments were made to find what number of turns in the
twist would give the projectile a sufficient rapidity of rotation to
maintain it during its flight parallel to its axis. It was found that
one turn in 20 inches was sufficient, and the projectile was made by
machinery to fit accurately but easily into the rifled bore, so that it
dropped into its place, and the loading could be expeditiously
performed. The bullet was long, compared with the bore, which was made
smaller than before, and it was found that the explosion caused it to
expand sufficiently to fill up the corners of the hexagon, so that there
was no loss from windage. The accuracy of aim of the Whitworth rifle was
superior to that of any weapon of the kind that had, up to that time,
been produced. When officially tried against the Enfield, its mean
deviation at 500 yards range was only 4½ inches, while that of the
Enfield at the same range was 27 inches. Mr. Whitworth had proved the
advantages of using a small bore, an elongated bullet, and a sharp twist
in the rifling; and it was acknowledged that as a military weapon his
rifle was superior to all other arms of similar calibre that had before
been produced. Some doubt appears to have been entertained, however, as
to whether the mechanical perfection of the trial rifles could be
maintained if they came to be manufactured on the large scale, and also
as whether an adequate supply of the polygonal ammunition would be
procurable when required. The Whitworth rifle was never adopted into the
government service, and soon after these trials in 1857, the adoption of
another type of weapon became imperative, as the results obtained by the
Germans with their needle-gun, demonstrated the enormous advantages of a
breech-loading rifle.
[Illustration:
FIG. 85.—_The Chassepot Rifle.—Section of the Breech._
]
The French then adopted the Chassepot rifle (so called after its
inventor), which embodied the same principle as the needle-gun, but with
improvements. This arm has a rifled barrel, with a breech mechanism of
great simplicity, which is represented in section in Fig. 85. The piece
marked B corresponds to what is called the “hammer” in the old lock used
with percussion-caps, and the first operation in charging the rifle
consists in drawing out B, as shown in the cut, until, by the spring, C,
connected with the trigger, A, falling into a notch, the hammer, if we
may so term it, is retained in that position. The effect of this
movement is to draw out also a small rod attached to the hammer, and
terminated in front by a needle, about ½ in. long, at the same time that
a spiral spring surrounding the rod is compressed, the spring being
fastened to the front end of the rod, and abutting against a screw-plug,
which closes the hinder end of F, and permits only the rod to pass
through it. The piece F, which is also movable, has projecting from its
front end a little hollow cylinder, through the centre of which the
needle passes, and this little cylinder has a collar, serving to retain
its position, an india-rubber ring surrounding a portion of the
cylinder, and forming a plug to effectually close the rear end of the
barrel. It will be noticed that the cylinder is continued by a smaller
projection, which forms a sheath for the point of the needle. The
movable breech-piece, F, is provided with a short lever, E, by which it
is worked. The second movement performed by the person who is charging
the piece is to turn this lever from a horizontal to a vertical
position, which thus causes the piece F to turn 90° about its axis, and
then by drawing the lever towards him he removes the piece F from the
end of the barrel, which, thus exposed, is ready to receive the
cartridge. The cartridge contains the powder and the bullet in one case,
the posterior portion containing also a charge of _fulminate_ in the
centre, and it is by the needle penetrating the case of the cartridge
and detonating this fulminate that the charge is exploded. When the
cartridge has been placed in the barrel, the piece F is pushed forward,
the metallic collar and india-rubber ring stop up the rear of the
barrel, and on turning the lever, E, into a horizontal position, the
breech is entirely closed. If now the trigger be drawn, the hammer is
released, and the spring carries it forward, at the same time impelling
the needle through the base of the cartridge-case, where it immediately
causes the explosion of the fulminate. The bullet is conical, and its
base having a slight enlargement, the latter moulds itself to the
grooves with which the barrel is rifled. When the piece has not to be
fired immediately, the lever is not placed horizontally, but in an
inclined position, in which the hammer cannot move forward, even if the
trigger be drawn. The Chassepot has an effective range of 1,093 yards,
and the projectile leaves the piece with a velocity of 1,345 ft. per
second, the trajectory being such that at 230 yards the bullet is only
18 in. above the straight line. The piece can be charged and fired by
the soldier in any position, and it was found that it could be
discharged from seven to ten times per minute, even when aim was taken
through the sights with which it is furnished, and fourteen or fifteen
times per minute without sighting. The ordinary rifled musket, which
this arm superseded, could only be fired twice in a minute, and could
only be loaded when the soldier was standing up.
Other nations followed either by adopting as their infantry arm some
form of breech-loader, or by converting their muzzle-loaders into
breech-loaders as a temporary expedient, pending the selection of some
more perfect type. When in 1864 a committee which had been appointed to
investigate the question of proper arms for our infantry, recommended
that that branch of the service should be supplied with breech-loaders,
our Government, considering that no form of breech-loader had up to that
time been invented which would unequivocally meet all the requirements
of the case, wisely determined that, pending the selection of a suitable
arm, the service muzzle-loaders should meanwhile be converted into
breech-loaders. The problem of how this was to be done was solved by the
gunmaker Snider, and in the “Converted Enfield” or “Snider” the British
army was provided for a time with an arm satisfying the requirements of
that period. This change of weapon was effected at a comparatively small
outlay, for the conversion cost less than twenty shillings an arm. The
breech action in the Snider consisted of a solid piece of metal which
closed the breech end of the barrel, and, being hinged on the right-hand
side parallel to the barrel, could be turned aside, making room for the
insertion into the conically widened bore of a metallic cartridge case,
invented by Colonel Boxer, which contained the projectile, the powder
charge, and the means of ignition in itself. A short backward movement
of the breech-lock caused a claw acting on the base of the spent
cartridge case to withdraw it from the barrel, and then the reaction of
a spring brought the breech-block back into position, after insertion of
a new cartridge. This cartridge proved very effective in increasing the
range and accuracy of the weapon. It should be mentioned that all the
breech-loading mechanisms are provided with arrangements by which the
metallic cases of the spent cartridges are automatically extracted from
the barrel. The authorities having, in 1866, offered gunmakers and
others handsome prizes for the production of rifles best fulfilling
certain conditions, nine weapons were selected out of 104 as worthy to
compete. No first prize was awarded, but the second was given to Mr.
Henry, while Mr. Martini was seventh on the list. In order to obtain a
weapon fulfilling all the requirements, a vast number of experiments
were made by the committee appointed for that purpose, as to best
construction of barrel, size of bore, system of rifling, kind of
cartridge, and other particulars, and assistance was rendered by several
eminent gunsmiths and engineers.
[Illustration:
FIG. 86.—_Section of Martini-Henry Lock._
]
After a severe competition it appeared that the best weapon would be
produced by combining Henry’s system of rifling with Martini’s mechanism
for breech-loading. The parts constituting the lock and the mechanism
for working the breech, shown in Fig. 86, are contained in a metal case,
to which is attached the woodwork of the stock, now constructed in two
parts. To this case is attached the butt of the rifle by a strong metal
bolt 6 in. in length, A, which is inserted through a hole in the
heel-plate. The part that closes the breech—termed the “block”—is marked
B. It turns loosely on a pin, C, passing through its rear end and fixed
into the case at a level somewhat higher than the axis of the barrel.
The end of the block is rounded off so as to form with the rear end of
the case, D, which is hollowed out to receive it in a perfect knuckle
joint. Let it be observed that this rounded surface, which is the width
of the block, receives the whole force of the recoil, no strain being
put on the pin, C, on which the block turns. In the experiments a leaden
pin was substituted, and the action of the mechanism was not in the
least impaired. This arrangement serves greatly to diminish the wear and
the possibility of damage from the recoil. As the pin on which the block
turns is slightly above the axis at the barrel, it follows that the
block, when not supported, immediately drops down below the barrel.
Behind the trigger-guard is a lever, E, working on a pin, F, fitted into
the lower part of the case. To this lever is attached a much shorter
piece called the “tumbler,” which projects into the case, G. It is this
tumbler which acts as a support for the block, and raises it into its
firing position or lowers it according as the lever, E, is drawn toward
a firer or pushed forward. How this is accomplished will be readily
understood by observing the form of the notch, H, in which the upper end
of the tumbler moves. It will be noticed that the piece being in the
position for firing, if the lever be pushed back, G slides away from the
shallower part of the notch into the deeper, and the block accordingly
falls into the position shown in the figure; and if again the lever is
drawn backward, G acting on H will raise the block to its former
position. The block or breech-piece is hollowed out on its upper
surface, I, so as to permit the cartridge to be readily inserted into
the exploding chamber, J. The centre of the block is bored out, and
contains within the vital mechanism for exploding the cartridge, namely,
a spiral spring, of which the little marks at K are the coils in
section. These coils pass round apiece of metal called the “striker,”
which is armed with a point, capable of passing through a hole in the
front face of the block exactly behind the percussion-cap of the
cartridge when the block is in the firing position. When the lever
handle is moved _forward_, it causes the tumbler, which works on the
same pin, to revolve, and one of its arms draws back the striker,
compressing the spring in so doing, so that as the block drops down the
point of the striker is drawn inwards. In this position the piece
receives the cartridge into the chamber. The lever, E, being now drawn
backward, the piece is forced into the notch, H, and the block is kept
firmly in its place; besides this, there is a further compression of the
spring by the tumbler, and in this position the spring is retained by
the rest-piece, L, which is pushed into a bend in the tumbler. By
pulling the trigger this piece is released, so that the tumbler can
revolve freely, and relieve the pent-up spring, whose elasticity impels
the striker forward, so that this enters the carriage directly. A very
important and ingenious part of this arrangement is the contrivance for
extracting the case of the exploded cartridge. The extractor turns on
the pin, M, and has two arms pointing upwards, N, which are pressed by
the rim of the cartridge pushed home into two grooves cut in the sides
of the barrel. It has another arm, O, bent only slightly upwards and
pointing towards the centre of the case, and forming an angle of about
80° with the above-mentioned upright arm; when, by pushing forward the
lever, its short arm drops into the recess, the block, no longer
supported, falls, and hits the point of the bent arm of extractor, so
causing the two upright arms to extract the cartridge-case a little way.
[Illustration:
FIG. 87.—_The Martini-Henry Rifle._
A, ready for loading; B, loaded and ready for firing.
]
The barrel is of steel; the calibre is 0·451 in. It is rifled on Mr.
Henry’s patent system. The section of the bore may be generally
described as a heptagon with re-entering angles at the junctions of the
planes, so that there are fourteen points of contact for the bullet,
viz., one in the middle of each plane, and one at each of the
re-entering angles. The twist of rifling is one turn in 22 in. The
charge consists of 85 grains of powder, and a bullet weighing 480
grains, of a form designed by Mr. Henry. The cartridge is of the same
general construction as the “Boxer” cartridge, used in the Snider rifle,
but it is bottle-shaped, the diameter being enlarged from a short
distance in rear of the bullet, in order to admit of its being made
shorter, and consequently stronger, than would be otherwise possible. A
wad of bees’-wax is placed between the bullet and powder, by which the
barrel is lubricated at each discharge. The sword-bayonet to be used
with this rifle is of a pattern proposed by Lord Elcho. It is a short
sword, broad towards the point, and furnished on a portion of the back
with a double row of teeth, so as to form a stout saw. It is so balanced
as to form a powerful chopping implement, so that, in addition to its
primary use as a bayonet, it will be useful for cutting and sawing
brushwood, small trees, &c.
The following are the principal particulars of weight, dimensions, &c.,
of the Martini-Henry rifle:
│Without bayonet 4 ft. 1 in.
Length of rifle │With bayonet fixed 5 ft. 8 in.
│Of barrel 2 ft. 9·22 in.
Calibre │ 0·451 in.
Rifling │Grooves 7
│Twist 1 turn in 22 in.
Weight │Without bayonet 8 lbs. 7 oz.
│With bayonet 10 lbs. 4 oz.
Bayonet │Length 2 ft. 1½ in.
│Weight without scabbard 1 lb. 8 oz.
Charge of powder │ 85 grains.
Weight of bullet │ 480 grains.
The rifle is sighted to 1,400 yards.
As an evidence of the accuracy of fire in this rifle, it may be stated
that of twenty shots fired at 1,200 yards, the mean absolute deflection
of the hits from the centre of the group was 2·28 ft. The highest point
in the trajectory at 500 yards is rather over 8 ft. so that the bullet
would not pass over a cavalry soldier’s head within that distance. The
trajectory of the Snider at the same range rises to nearly 12 ft. The
bullet will pass through from thirteen to seventeen ½ in. elm planks
placed 1 in. apart at 20 yards distance; the number pierced by the
Snider under similar circumstances being from seven to nine. As regards
rapidity of fire, twenty rounds have been fired in 53 seconds; and one
arm which had been exposed to rain and water artificially applied for
seven days and nights, and had during that time fired 400 rounds, was
then fired, without cleaning, twenty rounds in 1 minute 3 seconds.
Rifles of the Martini-Henry and Chassepot type were soon superseded, for
as early as 1876 Switzerland had armed her troops with a magazine rifle
of a smaller calibre than any then in use, and this weapon was found so
effective that in a few years after every European nation had followed
suit, as also had the United States and Japan, each country adopting
some particular pattern of a weapon with certain modifications. Of these
the Mannlicher and the Mauser are much used. A magazine rifle is one
that can be fired several times successively without reloading. Like
revolvers, the magazine arms repeat their fire, but instead of having
several distinct firing chambers, they have but one, from which the
empty cartridge cases are automatically extracted by the breech
mechanism, for the magazine rifle is necessarily a breech-loader. The
magazine rifle carries a supply of cartridges, which one after another
are brought into the firing chamber by the simple action of the breech
mechanism, so that the soldier is enabled to discharge several rounds in
any position without reloading. The several varieties of the magazine
rifle may be classed according to the position of the magazine. This may
be: First, in the stock; second, under the barrel; third, in a box under
the breech; or fourth, in a box above the breech. In the first and
second variety the cartridges are in line in a tube, out of which they
are moved on by a spiral spring, and this was the earlier form of the
weapon. The box above or below the breech is the later development, and
has the advantage of holding the cartridges lying side by side, and thus
in a position in which they are not so liable to injure each other as in
the tubular arrangements. Then, again, the movement of the cartridge in
the breechbox in arriving at the firing chamber is much less than in the
linear magazines, and the centre of gravity of the whole changes but
little when the supply is exhausted. With any of the varieties of
magazine a suitable modification of the mechanism may be adopted, so
that the weapon can at will be used as a single firing rifle, but
changeable in an instant to the magazine form. Again, the box magazine
may be made as a fixture on the rifle, or it may be detachable.
Commissions of military authorities had for several years been
deliberating upon the best models for their respective nations, while
Professor Hebler was working out his researches as to the best calibre
for military rifles. Hebler published a work showing the great
advantages of a bore one-third less in diameter than that commonly in
use, which was about 0·45 inch, as in our Martini-Henry. The
small-calibre rifle shoots straighter and hits harder than the large
bore one, and the recoil is less, and so is the weight of the weapon.
Lead is found to be too soft a material for the bullet of the small-bore
rifles, as it does not keep in the rifling, which has a sharper turn
than that in the older weapon; hence the bullets are now cased in steel
or nickel. These bullets have remarkable power of penetration. Some will
go through a steel plate 1¼ inch thick, making a clean hole in it, and
the Lebel bullet penetrates 15 inches of solid oak, at a distance of 220
yards. Such a missile would, therefore, be capable of going completely
through the bodies of several men or horses.
[Illustration:
FIG. 88.—_The Mannlicher Magazine Rifle._
]
The Germans, about 1888, adopted a magazine rifle known as the Mauser.
It had a fixed tubular magazine for eight rounds below the barrel, and a
breech mechanism of the Remington-Keene type. The French followed suit
with their famous Lebel gun, the construction of which was long kept
secret. It also has a fixed under barrel tubular magazine, and the
cartridges used with it contain smokeless powder. It is said that a new
gun of practically the same pattern has been adopted by Russia, but with
a detachable magazine to contain five rounds. The Russian gun will also
use smokeless powder. In England, a small-bore rifle of 0·303 inches
calibre is now issued to all troops. It has an under breechbox magazine,
modified from the Lee rifle. The box is detachable, so that the weapon
could normally be used as a single loader, and the magazine attached
only when required. But the British authorities have decided that the
magazine box is to be attached to the weapon by a chain. The first issue
of this pattern of rifle to British soldiers took place early in 1890.
The Austrians are adopting the Mannlicher pattern, in which the magazine
idea is embodied in a complete and practical form. This rifle has a
fixed box magazine below the breech. From this box, in which the
cartridges—five in number—lie side by side, they are fed up by springs
as they are disposed of by the movement of the breech mechanism. The
magazine is recharged by placing in it a tin case containing five
cartridges, and the case drops out when all the cartridges have been
fired. In this form there is of course no necessity for providing any
mechanism for holding the magazine in reserve while the rifle is used as
a single loader. As to calibre, the Austrian authorities follow other
countries in adopting a small bore, namely, 0·315 inch. Italy has
converted her single-fire Vetterli rifle into a magazine arm, with a box
something like the Mannlicher, and Belgium has adopted a gun of the same
type. The rate of fire from charged magazines of such guns as the “Lee,”
“Mannlicher” and “Vetterli,” worked with the right hand without bringing
the piece down from the shoulder is, for all of them, about one shot per
second; but the time that is required to recharge the magazines varies
much according to the contrivance used. The number of rounds the
magazine of a rifle is capable of containing when fully charged is from
5 to 12, or more, according to the difference of system. It is
considered that in the detachable Lee or the quick recharging Mannlicher
five rounds are ample for use at a critical moment.
[Illustration:
FIG. 89.—_The Magazine and Breech of the Mannlicher Rifle._
]
The calibre of the military rifle has been decreased with almost every
new pattern adopted. Thus, while the _old_ “Brown Bess” had a calibre of
0·75 inch, in the last issue of it the bore was reduced to 0·693 inch;
the Enfield (1852) had a bore of 0·577 inch; the Martini-Henry, 0·451
inch, which, in a newer pattern adopted in 1887, was reduced to 0·400
inch; and, finally, in the Lee-Metford, the calibre is only 0·303 inch.
A similar consecutive reduction of bores has taken place in the rifles
adopted by other countries, and one of the latest type, issued for the
use of the United States Navy, has a bore of only 0·236 inch, and it is
even expected that a still smaller one will become general. The
advantage of the narrow and lighter projectile is that while it has a
higher initial velocity with a given charge, its flight is less checked
by the resistance of the atmosphere, the section it presents being so
much less. Thus the bullet of 0·236 inch diameter has a section little
more than one-fourth that of the 0·45 inch bullet. The difference is
well shown in the comparative heights of the _trajectory_ (or path of
the bullet) of the Martini-Henry 0·450 inch bullet, and that of the
0·303 inch Lee-Metford (the latter with _cordite_ ammunition); for at a
range of 1,000 yards the former reaches to 48 feet above the line of
sight, while the latter rises to only 25 feet.
Some form of repeating or magazine rifle has now been adopted by all the
most important nations of the world. The number of shots contained in
the magazines varies from 5 to 12. In the British detachable box
magazine there are ten charges. The calibres of the barrels range in the
infantry patterns of different nations from 0·256 inch to 0·315 inch;
the explosive used in every case is some kind of smokeless powder, and
this, in the cartridge for the Lee-Metford, is _cordite_. The bullets
are not made simply of lead, but of lead coated with a harder metal or
alloy such as steel, cupro-nickel, nickel steel, or they consist
entirely of some of these alloys.
Although the magazine rifle is now the regulation weapon of the infantry
of all great armies, it is not improbable that at no distant future it
maybe superseded by one in which, as in certain machine guns, the force
of the recoil will be used for actuating the breech and lock movements.
Many patents have already been taken out for rifles on this principle,
and several patterns have actually been constructed, in which a merely
momentary contact of the breech-piece with the end of the barrel is
sufficient; the recoil of the barrel with the reaction of a spring
performs all the requisite movements with such rapidity that an amazing
speed of firing has been obtained. It is said that such an automatic gun
can send forth bullets at a perfectly amazing rate. Of course the
mechanism of such a gun is somewhat intricate, and it is impossible to
explain its construction and action without a great number of diagrams
and much description.
_RIFLED CANNON._
Having briefly sketched in the foregoing section the development of the
military rifle from such weapons as our own “Brown Bess,” down to the
repeating or magazine rifle, we now purpose to adopt a similar course
with regard to ordnance, giving also some particulars of the methods of
manufacture, etc., and following in general the order of history.
Naturally there is nothing that accelerates progress in warlike
inventions so much as the exigences of war itself. This is well
exemplified in circumstances attending the Crimean War, which was waged
in 1854 by England and France in alliance against Russia. The desire of
having ships that could run the gauntlet of the heavy guns mounted on
Russian forts led to the construction of _La Gloire_ and other
armour-plated vessels, as we have already seen, and a suggestion of the
French Emperor, as to improving metal for guns, made to Mr. Bessemer,
led incidentally but ultimately to the great revolution in the
manufacture of steel, although it is true that Krupp of Essen had begun
to produce small cast-steel ordnance as early as 1847. But what
determined the necessity for rifled ordnance was more particularly the
greater comparative effects obtained by the muzzle-loading rifles over
the field artillery then in use in the several engagements that took
place in the Crimea, especially in the battle of Inkerman (1854). The
rifles so much surpassed in accuracy at long ranges the smooth-bore
field-pieces firing spherical projectiles, that field artillery was on
the point of losing its relative importance, and even in the matter of
range the latter lost so much by windage that the men serving the
artillery could sometimes be leisurely picked off by the rifle
sharp-shooters. Inventors were soon at work on devising methods of
increasing the accuracy of ordnance fire with both light and heavy
pieces, and before the end of the war some cast-iron guns rifled on
Lancaster’s plan had been mounted on forts and in ships, without proving
very successful except in regard to increase of range when elongated
pointed projectiles were used with them.
[Illustration:
FIG. 90.—_32–pounder, 1807._
]
Now let us see of what kind was the ordnance used for some years after
the middle of the century, in order that we may be the better able to
appreciate the progress that has since been made. Ordnance is, as
already noticed, of several species, as guns mounted on fortresses,
naval guns, siege guns, field-guns, etc., and the size of the pieces
under each of those heads is distinguished sometimes in one, sometimes
in another of three different ways. We may name it by the weight of the
gun itself in tons or hundredweights, as “a 35–ton gun,” etc.; or by the
weight of its projectile, as “a 68–pounder,” etc.; or by its calibre,
that is the diameter of its bore, as “a 4–inch gun,” etc. We may take
the naval guns with which Nelson won his battles (Trafalgar, 1807) as
representative of all except field ordnance up to about 1856. They were
all made of cast iron, threw spherical projectiles, and were very rudely
mounted. The gun most commonly mounted on board our ships of war was the
32–pounder, weighing 32 cwt., shown with its carriage in Fig. 90. The
carriage was of wood, and consisted of two side pieces joined back and
front by two transverse pieces and carried by four low wooden wheels.
The trunnions of the gun fitted into bearings at the top of the
side-pieces, and were secured by iron plates that passed over them in a
semi-cylindrical form and were bolted down to the wood. The position of
the trunnions on the gun was always such that the breech end of the gun
preponderated, being supported on an adjustable wooden wedge; and when
the muzzle of the gun had to be lowered, this was done by raising the
breech end with handspikes and pushing in the wedge so far as to prevent
the breech from dropping down again. There was a vent or narrow passage
to contain a train of powder from the touch-hole at the upper part of
the breech to the rear of the charge. When the gun was fired, with its
muzzle protruding a little way out of the port-hole, the recoil would
trundle it inwards about its own length, when its course would be stayed
by a thick rope attached to the sides of the vessel; and by other tackle
it would be kept in position until loaded, when it would be allowed to
roll back, or would be drawn by ropes and pulleys out to the port-hole,
and by the same means such lateral inclination as might be required
could be given. This last adjustment was called _training_ the gun. A
32–pounder required the services altogether of a dozen or fourteen men,
but these by virtue of constant drill would learn to handle the clumsy
machine with a certain amount of expedition. If we except a notch on the
highest point of the muzzle, the pieces were devoid of anything of the
nature of sights, though sometimes marks were made on the adjustable
wedge under the breech to correspond with certain elevations. Nor were
sights required; for the mode of fighting then was to get quite close to
the adversary’s ship and pour in a _broadside_ by firing simultaneously
all guns on the enemy’s side when they had been _trained_ (by rough
methods), so as to concentrate their effect as much as possible on one
point of the antagonist. Nelson’s famous ship the _Victory_ carried a
few larger guns than the 32–pounders, namely, two 68–pounders, called
_carronades_ (from having first been cast at Carron in Scotland), and
some 42–pounders. The 32– and 42–pounders numbered together thirty, and
there were also as many 24–pounders, with forty 12–pounders. These were
all simply cast of the required dimensions, and were not made with the
one single improvement which after two centuries’ use of cast-iron guns
had been introduced into France about fifty years before, namely, the
_boring_ of the chase out of a solid casting.
On the outbreak of the Crimean War (1854) the minds of many inventors
were occupied by the problem of ordnance construction, and this also
engaged the attention among others of two of the most eminent British
mechanical engineers of the day. These were Sir W. Armstrong and Sir J.
Whitworth, who, with others, were invited by our War Department to
submit the best models of field and heavy guns their skill was severally
able to produce. Two years afterward, Sir W. Armstrong had, after many
experiments, completed a gun of 1·88 in. calibre. This had a forged
steel barrel 6 feet in length; but it was only after eight such forgings
had been bored and rejected on account of flaws revealed only by the
boring that a sound barrel was at length obtained. This barrel was
strengthened on the outside by _jackets_ made from coils of wrought iron
bars welded into a piece and shrunk on while hot (of which process we
shall have something more to say presently); the barrel was rifled with
many shallow grooves, and the pointed projectile, 3 calibres long, was
made of lead, for which afterwards iron coated with lead was
substituted. This gun was a breech-loader, the breech being closed by a
block let into a slot after loading, and then pressed against the barrel
by some turns of a screw which advanced parallel to the axis of the
piece, and was made hollow for loading through, before the closing block
was put in. In a trial of the various pieces ready in 1857, it was found
that the Armstrong gun made as just described had an accuracy and range
immensely greater than any weapon that had ever been tested, and the
Government authorities approved of the system of construction, except
that they preferred muzzle-loading pieces to breech-loading, as being
simpler in action, more easily kept in repair, and cheaper in original
cost and ammunition.
When Sir Joseph Whitworth’s gun was, in 1863, submitted to a competitive
trial against the Armstrong, as to their endurance and mode of ultimate
failure when fired with ever-increasing charges of powder and shot, at
the forty-second round the Armstrong breech-loader split, and at the
sixtieth the Armstrong muzzle-loader had one of its coils cracked; while
it was not until the ninety-second round that the Whitworth gun burst
violently into eleven pieces. These competing guns were 12–pounder
field-guns weighing 8 cwts., and from each 2,800 regulation rounds had
been fired before they were subjected to the bursting proofs. The result
of these trials being that the authorities considered that steel was not
then sufficiently reliable, and they decided to adopt the system of
building up rifled guns with iron jackets over an inner tube of steel.
Sir Joseph Whitworth made his guns entirely of steel, and they were
striking examples of beautiful and accurate workmanship. His system of
rifling consisted in forming the bore of the gun so that its section is
a regular hexagon, and the projectile is an elongated bolt with sides
exactly fitting the barrel of the gun: the projectile is, in fact, a
twisted hexagonal prism. Fig. 91 shows at the left-hand side the section
of the barrel, and on the right we see the form of the projectile on a
smaller scale, this last representing, in fact, the exact size and shape
of the bullet of the Whitworth _rifle_ mentioned on another page. Sir
Joseph’s guns were muzzle-loaders, and they were remarkable for their
long range and accuracy of fire. One of these guns, with a charge of 50
lbs. of gunpowder, threw a 250–lb. shot a distance of nearly six miles,
and on another occasion a 310–lb. shell was hurled through the air, and
first touched the ground at a distance of more than six and a quarter
miles from the gun. These distances are greater than any to which shot
or shell had previously been thrown.
[Illustration:
FIG. 91.—_Whitworth Rifling and Projectile._
]
As the material of these Whitworth guns was very costly, and very
perfect workmanship was required in the formation of the barrel and the
shots, the expense attending their manufacture and use was much greater
than that incurred in the case of the Armstrong guns. Sir W. Armstrong’s
estimate for a 35–ton gun was £3,500, and Sir J. Whitworth’s, £6,000.
The gun, as constructed at Woolwich on Mr. Fraser’s plan, was estimated
to cost £2,500. The first cost of a gun is a matter for consideration,
since each piece, even the strongest, is able only to fire a limited
number of rounds before it becomes unsafe or useless. It appears that no
cannon has yet been constructed capable of withstanding without
alteration the tremendous shocks given by the explosion of the
gunpowder, and these alterations, however small at any one discharge,
are summed up and ultimately bring to an end what may be termed the
“life of the piece.”
[Illustration:
FIG. 92.—_600–pounder Muzzle-loading Armstrong Gun._
]
About the year 1858 Sir William Armstrong (afterwards Lord Armstrong)
established at Elswick, Newcastle-on-Tyne, a manufactory of ordnance,
which has since developed into the great arsenal now so well known all
over the world. Here all the resources of science have been applied to
the problems of artillery, and experiments carried on with a prodigality
of cost and promptness of execution impossible at a government
establishment trammelled with official regulations. Here, and also at
Woolwich, our national ordnance factory, guns have since always been
constructed on the building-up plan advocated by Sir W. Armstrong, whose
principle consists in disposing of the fibre of the iron so as best to
resist the strains in the several parts of the gun. Wrought iron being
fibrous in its texture has, like wood, much more strength in the
direction of the grain than across it. The direction of the fibre in a
bar of wrought iron is parallel to its length, and in that direction the
iron is nearly twice as strong as it is transversely. A gun may give way
either by the bursting of the barrel or by the blowing out of the
breech. The force which tends to produce the first effect acts
transversely to the axis of the gun; hence the best way to resist it is
to wrap the iron round the barrel, so that the fibres of the metal
encircle it like the hoops of a cask. The force which tends to blow out
the breech is best resisted by disposing the fibres of the iron so as to
be parallel to the axis of the gun; hence Sir W. Armstrong makes the
breech-piece from a solid forging with the fibre in the required
direction. But the Elswick building-up principle involves much more than
the direction of the fibres of the iron, for each coil or jacket, after
having its spires welded together, was bored out on a lathe, and the
exterior of the part of the gun on which it was to be placed was also
turned with the utmost exactness, so that when the enveloping piece was
heated to a certain temperature and in this state brought into position,
it would in cooling compress the parts it encircled just to that degree
which careful calculations showed would best strengthen the gun without
unduly straining the metal at any part. The Elswick guns being built up
of several superimposed jackets of calculated lengths and thicknesses,
the means was afforded of distributing the tensions throughout the whole
mass of metal to the best advantage. In the simpler form, arranged by
Mr. Fraser, and for the sake of economy adopted by the authorities at
Woolwich in 1867, the greater part of the benefit derivable from
adjustment of tension was no doubt sacrificed to cheapness of
manufacture. These, and also the forms of Armstrong guns that have not
yet been described, ceased to be made after 1880, by which time steel
had replaced iron in every part of the construction and fittings of
guns, and muzzle-loading had been definitely abandoned in favour of
breech-loading.
[Illustration:
FIG. 93.—_35–ton Fraser Gun._
]
Now, in 1874, when the first edition of the present work was in
preparation, the Fraser-Woolwich guns were in full vogue, being spoken
of by the public press as the _ne plus ultra_ of artillery construction
in size, efficiency, and economy. When, accordingly, the author had been
privileged to visit the arsenal and witness the production of these guns
in every stage of their manufacture, he wrote a description of it which
is here retained as printed at the time, seeing that it may not be
without historical interest, particularly since great numbers of these
guns must still be extant, mounted on our forts in various parts of the
world, and seeing also that the description of the simpler formations
may render more easily to be understood future references to similar
operations in gun-making as have been retained in the later
developments. Of course, the following description was written in the
_present tense_, and therefore in perusing it the reader must constantly
bear in mind that the guns with which our ships of war have since been
equipped are in _every respect entirely different_ from
_The Fraser-Woolwich Guns, 1867–1880._
[Illustration:
FIG. 94.—_Section of 9 in. Fraser Gun._
]
Until the year 1867 the guns made at Woolwich were constructed according
to the original plan proposed by Sir W. Armstrong, and on this system
one of the large guns consisted of as many as thirteen separate pieces.
These guns, though unexceptionable as to strength and efficiency, were
necessarily so very costly that it became a question whether anything
could be done to lessen the expense by a simpler mode of construction or
by greater economy in the material. The problem was solved in the most
satisfactory manner by Mr. Fraser, of the Royal Gun Factory, who
proposed an important modification of the original plan, and the
adoption of a kind of iron cheaper than had been previously employed,
yet perfectly suited for the purpose. Mr. Fraser’s modification
consisted in building up the guns from only a few coils, instead of
several, the coils being longer than Sir W. Armstrong’s, and the iron
coiled upon itself two or even three times: a plan which enabled him to
supersede the breech-piece, formerly made in one large forging, by a
piece formed of coils. In order to perceive the increased simplicity of
construction introduced by Mr. Fraser, we need but glance at the section
of a 9 in. gun constructed according to his system, Fig. 94, and
remember that a piece of the same size made after the original plan had
ten distinct parts, whereas the Fraser is seen to have but four. Compare
also Figs. 92 and 93. We shall now describe the process of making the
Fraser 9 in. gun. The parts of the gun as shown in the section, Fig. 94,
are: 1, the steel barrel; 2, the B tube; 3, the breech-coil; 4, the
cascable screw. The inner steel barrel is made from a solid cylinder of
steel, which is supplied by Messrs. Firth, of Sheffield. This steel is
forged from a cast block, the casting being necessary in order to obtain
a uniform mass, while the subsequent forging imparts to it greater
solidity and elasticity. After the cylinder has been examined, and the
suitable character of the steel tested by trials with portions cut from
it, the block is roughly turned and bored, and is then ready for the
toughening process. This consists in heating the tube several hours to a
certain temperature in an upright furnace, and then suddenly plunging it
into oil, in which it is allowed to remain for a day. By this treatment
the tenacity of the metal is marvellously increased. A bar of the steel
1 in. square previous to this process, if subjected to a pull equal to
the weight of 13 tons, begins to stretch and will not again recover its
original form when the tension is removed, and when a force of 31 tons
is applied it breaks. But the forces required to affect the toughened
steel in a similar manner are 31 tons and 50 tons respectively. The
process, unfortunately, is not without some disadvantages, for the
barrel is liable to become slightly distorted and even superficially
cracked. Such cracks are removed by again turning and boring; the
hardness the steel acquires by the toughening process being shown by the
fact that in the first boring 8¼ in. diameter of _solid_ steel is cut
out in 56 hours, yet for this slight boring, in which merely a thin
layer is peeled off, 25 hours are required; and lest there should be any
fissures in the metal, which, though not visible to the eye, might make
the barrel unsound, it is filled with water, which is subjected to a
pressure of 8,000 lbs. per square inch. If under this enormous pressure
no water is forced outwards, the barrel is considered safe. It is now
ready to have the B tube shrunk on it.
The B tube, like certain other portions of these guns, is constructed
from coiled iron bars, and this constitutes one great peculiarity of Sir
W. Armstrong’s system. It has the immense advantage of disposing the
metal so that its fibres encircle the piece, thus applying the strength
of the iron in the most effective way. The bars from which the coils are
prepared are made from “scrap” iron, such as old nails, horse-shoes, &c.
A pile of such fragments, built up on a wooden framework, is placed in a
furnace and intensely heated. When withdrawn the scraps have by
semi-fusion become coherent, and under the steam hammer are soon welded
into a compact mass of wrought iron, roughly shaped as a square prism.
The glowing mass is now introduced into the rolling-mill, and in a few
minutes it is rolled out, as if it were so much dough, into a long bar
of iron. In order to form this into a coil it is placed in a very long
furnace, where it can be heated its entire length. When sufficiently
heated, one end of the bar is seized and attached to an iron core of the
required diameter, and the core being made to revolve by a steam engine,
the bar is drawn out of the furnace, winding round the core in a close
spiral, so that the turns are in contact. The coil is again intensely
heated, and in this condition a few strokes of the steam hammer in the
direction of its axis suffice to combine the spires of the coil into one
mass, thus forming a hollow cylinder.
The B tube for the 9 in. gun is formed of two double coils. When the two
portions have been completely welded together under the steam hammer,
the tube, after cooling, is roughly turned and bored. It is again fine
bored to the required diameter, and a register of the diameter every few
inches down the bore is made. These measurements are taken for the
purpose of adapting most accurately the dimensions of the steel barrel
to the bore of the B tube, as it is found that perfect exactness is more
easily obtained in turning than in boring. The steel barrel is therefore
again turned to a size slightly _larger_ than the bore of the B tube,
and is then placed muzzle end upwards, and so arranged that a stream of
water, to keep it cool, shall pass into it and out again at the muzzle,
by means of a syphon, while the B tube, which has been heated until it
is sufficiently expanded, is passed over it and gradually cooled.
If now the B tube were allowed to cool spontaneously, its ends would, by
cooling more rapidly than the central part, contract upon the steel
barrel and grip it firmly at points which the subsequent cooling would
tend to draw nearer together longitudinally, and thus the barrel would
be subjected to injurious strains. In order to prevent this, the B tube
is made to cool progressively from the breech end, by means of jets of
water made to fall upon it, and gradually raised towards the muzzle end,
which has in the meanwhile been prevented from shrinking by having
circles of gas-flames playing upon it.
The breech-coil, or jacket, is formed of three pieces welded together.
First, there is a triple coil made of bars 4 in. square, the middle one
being coiled in the reverse direction to the other two. After having
been intensely heated in a furnace for ten hours, a few blows on its end
from a powerful steam hammer welds its coils perpendicularly, and when a
solid core has been introduced, and the mass has been well hammered on
the sides, it becomes a compact cylinder of wrought iron, with the
fibres all running round it. When cold it is placed in the lathe, and
the muzzle end is turned down, leaving a shoulder to receive the
trunnion-ring. The C coil is double, welded in a similar manner to the B
coil, and it has a portion turned off, so that it may be enclosed by the
trunnion-ring.
The trunnion-ring is made by punching a hole in a slab of heated iron
first by a small conical mandrel, and then enlarging by repeating the
process with larger and larger mandrels. The iron is heated for each
operation, and the trunnions are at the same time hammered on and
roughly shaped—or, rather, only one has to be hammered on—for a portion
of the bar which serves to hold the mass forms the other. The
trunnion-ring is then bored out, and after having been heated to
redness, is dropped on to the triple breech-coil which is placed muzzle
end up, and the turned end of the C coil (of course, not heated) is then
immediately placed within the upper part of the trunnion-ring. The
latter in cooling contracts so forcibly as to bind the ends of the coils
together, and the whole can thus be placed in a furnace and heated to a
high temperature, so that when removed and put under the steam hammer,
its parts are readily wielded into one mass. The breech-coil in this
state weighs about 16 tons; but so much metal is removed by the
subsequent turnings and borings, that it is reduced to nearly half that
weight in the gun. It is then turned in a lathe of the most massive
construction, which weighs more than 100 tons. Fig. 34, page 95, is from
a drawing taken at Woolwich, and shows one of the large guns in the
lathe. No one who witnesses this operation can fail to be struck with
the apparent ease with which this powerful tool removes thick flakes of
metal as if it were so much cheese. The projections of the trunnions
prevent the part in which they are situated from being finished in this
lathe, and the gun has to be placed in another machine, where the
superfluous metal of the trunnion-ring is pared off by a tool moving
parallel to the axis of the piece. Another machine accomplishes the
turning of the trunnions, the “jacket” being made to revolve about their
axis. The jacket is then accurately bored out with an enlargement or
socket to receive the end of the B tube, and a hollow screw is cut at
the breech end for the cascable.
The portion of the gun, consisting of the steel barrel with the B tube
shrunk on it, having been placed upright with the muzzle downwards, the
breech-piece, strongly heated, is brought over it by a travelling crane,
and slips over the steel barrel, while the recess in it receives the end
of the B tube. Cold water is forced up into the inside of the barrel in
order to keep it cool. As the breech cools, which it is allowed to do
spontaneously, it contracts and grips the barrel and B tube with great
force. The cascable requires to be very carefully fitted. It is this
piece which plays so important a part in resisting the force tending to
blow out the end of the barrel. The cascable is a solid screw formed of
the very best iron, and its inner end is wrought by scraping and filing,
so that when screwed in there may be perfect contact between its face
and the end of the steel barrel. A small annular space is left at the
circumference of the inner end, communicating through a small opening
with the outside. The object of this is, that in case of rupture of the
steel barrel, the gases escaping through it may give timely warning of
the state of the piece.
Besides minor operations, there remain the important processes of
finishing the boring, and of rifling. The boring is effected in two
operations, and after that the interior is gauged in every part, and
“lapping” is resorted to where required, in order to obtain the perfect
form. Lapping consists in wearing down the steel by friction against
fine emery powder and oil, spread on a leaden surface. The piece is then
ready for rifling. The machinery by which the rifling is performed
cannot be surpassed for its admirable ingenuity and simplicity.
In this operation the gun is fixed horizontally, its axis coinciding
with that of the bar, which carries the grooving tools. This bar is
capable of two independent movements, one backwards and forwards in a
straight line in the direction of the length of the bar, and the other a
rotation round its axis. The former is communicated by a screw parallel
to the bar, and working in a nut attached to the end of it. For the
rotatory movement the bar carries a pinion, which is engaged by a rack
placed horizontally and perpendicularly to the bar, and partaking of its
backward and forward movement, but arranged so that its end must move
along another bar placed at an angle with the former. It is this angle
which determines the pitch of the rifling, and by substituting a curved
guide-bar for the straight one, an increasing twist may be obtained in
the grooves.
The projectile used with these guns is of a cylindrical form, but
pointed at the head, and the moulds in which these shots are cast are so
arranged that the head of the shot is moulded in iron, while the body is
surrounded with sand. The rapid cooling induced by the contact of the
cold metal causes the head of the shot to solidify very quickly, so that
the carbon in the iron is not separated as in ordinary casting. In
consequence of this treatment, the head of the shot possesses the
hardness of steel, and is therefore well adapted for penetrating iron
plates or other structures. The projectiles are turned in a lathe to the
exact size, and then shallow circular cavities are bored in them, and
into these cavities brass studs, which are simply short cylinders of a
diameter slightly larger than the cavities, are forced by pressure. The
projecting studs are then turned so as accurately to fit the spiral
grooves of the guns. Thus the projectile in traversing the bore of the
piece is forced to make a revolution, or part of a revolution, about its
axis, and the rapid rotation thus imparted has the effect of keeping the
axis of the missile always parallel to its original direction. Thus
vastly increased accuracy of firing is obtained.
[Illustration:
FIG. 95.—_Millwall Shield after being battered with Heavy Shot.—Front
View._
]
[Illustration:
FIG. 96.—_Rear View of the Millwall Shield._
]
Shells are also used with the Woolwich rifled guns. The shells are of
the same shape as the solid shots, from which they differ in being cast
hollow, and having their interior filled with gunpowder. Such shells
when used against iron structures require no fuse; they explode in
coming into collision with their object. In other cases, however, the
shells are provided with fuses, which cause the explosion when the shot
strikes. Fig. 93, page 195, represents one of the 35–ton guns, made on
the plan introduced by Mr. Fraser. This piece of ordnance is 16 ft.
long, 4 ft. 3 in. in diameter at the breech, and 1 ft. 9 in. at the
muzzle. The bore is about 1 ft. Each gun can throw a shot or bolt 700
lbs. in weight, with a charge of 120 lbs. of powder. It is stated that
the shot, if fired at a short range, would penetrate a plate of iron 14
in. thick, and that at a distance of 2,000 yards it would retain
sufficient energy to go through a plate 12 in. thick. The effect of
these ponderous missiles upon thick iron plates is very remarkable.
Targets or shields have been constructed with plates and timber backing,
girders, &c., put together in the strongest possible manner, in order to
test the resisting power of the armour plating and other constructions
of our ironclad ships. The above two cuts, Figs. 95 and 96, are
representations of the appearance of the front and back of a very strong
shield of this description, after having been struck with a few 600 lb.
shots fired from the 25–ton gun. The shots with chilled heads, already
referred to, sometimes were found to penetrate completely through the 8
in. front plate, and the 6 in. of solid teak, and the 6 in. of plating
at the back. The shield, though strongly constructed with massive plates
of iron, only served to prove the relative superiority of the artillery
of that day, which was at the time when our century had yet about thirty
years to run. Up to 1876 no confidence was placed in steel as a
resisting material, a circumstance perhaps not much to be wondered at,
as its capabilities had not then been developed by the newer processes
of manufacture, described in our article on Iron; nor had mechanicians
acquired the power of operating with large masses of the metal. Since
then it has come about that only steel is relied upon for efficiently
resisting the penetration of projectiles, iron being held of no account
except as a backing. There has always been a rivalry between the
artillerist and the naval constructor, and this contest between the
attacking and the defending agencies is well illustrated in the table on
page 166, where the parallel advance in the destructive power of guns
and in the resisting power of our war-ships is exhibited in a numerical
form.
[Illustration:
FIG. 97.—_Comparative Sizes of 35 and 81 ton Guns._
A, 35–ton; B, 81–ton.
]
The 35–ton Fraser guns were at the time of their production humorously
called in the newspapers “Woolwich infants”; but it was not long before
they might in another sense be called infants in comparison with a still
larger gun of 81 tons weight constructed at Woolwich shortly before
iron-coiling and muzzle-loading were set aside. Fig. 97 shows the
relative dimensions of the 35–ton and 81–ton guns: the latter was built
up in the same way as the 9–inch gun described above, but the coils were
necessarily longer and the chase was formed in three parts instead of
two. The total length of this gun was 27 feet, and the bore was about 24
feet long and 14 in. in diameter, and the weight of the shot about 1000
lbs., with sufficient energy to penetrate at a considerable distance an
iron plate 20 in. in thickness. It was for the manufacture of these very
large guns that the great steam hammer, represented in Plate III., was
erected at Woolwich.
* * * * *
The 81–ton gun was the largest muzzle-loader ever made in the national
gun factory at the time when such huge weapons were in request; but in
1876 its dimensions were surpassed by those of a few 100–ton guns built
at Elswick to the order of the Italian Government for mounting on their
most formidable ironclads. These guns have a calibre of 17·72 inches,
and are provided with a chamber of somewhat larger bore to receive the
charge of powder. They are built up on the Armstrong shrinkage
principle, and comprise as many as twenty different tubes, jackets,
hoops, screws, etc., and are undoubtedly the most powerful
muzzle-loading weapons ever constructed. It happened, just as these guns
were completed, that the British Government, apprehensive at the time of
a war with Russia, exercised its rights of purchasing two of them, one
to be mounted at Gibraltar, the other at Malta.
The Elswick establishment soon afterwards surpassed all its former
achievements in building great guns, by designing and constructing the
huge breech-loaders, one of which forms the subject of our Plate XII.
These are known as the Armstrong 110–ton guns; they are formed of solid
steel throughout, and their weight is accurately 247,795 lbs., or 110
tons 12 cwts. 51 lbs. The total length of the gun is 43 ft. 8 in., and
of this 40 ft. 7 in. is occupied by the bore, along which the rifling
extends 33 ft. 1 in. The calibre of the rifled part is 16¼ in., and the
diameter of the powder chamber is somewhat greater. The regulation
charge of powder weighs 960 lbs., although the guns are tested with
still greater charges. The weight of the projectile is 1,800 lbs., and
it leaves the muzzle with a velocity of 2,128 ft. per second, which is
equivalent to a dynamical energy of 56,520 foot-tons. What this means
will perhaps be better understood, not by describing experiments such as
those on the Millwall Shield, the results of which are depicted in Figs.
95 and 96, but by stating that if the shot from the 110–ton gun
encountered a solid wall of wrought iron a yard thick, it would pass
through it. The Elswick 110–ton gun is, in fact, the most powerful piece
of ordnance that has ever been constructed. There are no trunnions to
these great guns, but they are encircled by massive rings of metal,
between which pass strong steel bands that tie the gun to its carriage,
or, rather, to the heavy steel frame on which it is mounted, and which
slides on a couple of girders. The force of the recoil acts on a
hydraulic ram that passes through the lower part of the supporting
frame. The whole working of the gun is done by hydraulic power, and,
indeed, the same method has been applied by the Elswick firm to the
handling of all heavy guns. By hydraulic power, maintained automatically
by a pumping engine exercising a pressure of from 800 lbs. to 1,000 lbs.
per square inch, are operated the whole of the movements required for
bringing the cartridge and the projectile from the magazine; for
unscrewing the breech block, withdrawing it, and moving it aside; for
pushing home the shot and the cartridge to their places in the bore; for
closing the breech and screwing up the block; for rotating the turret
within which the gun is mounted, or in other cases for ramming the piece
in or out, and for elevating or depressing it. It is, indeed, obvious
that such ponderous masses of metal as form the barrels and projectiles
of these 110–ton and other guns of the larger sizes could not be handled
to advantage by any of the ordinary mechanical appliances. But by the
application of the hydraulic principle, a very few men are able to work
the largest guns with the greatest ease, for their personal labour is
thus reduced to the mere manipulation of levers. On board ship the power
required for working large guns has lately been sometimes supplied by a
system of shafting driven by a steam engine and provided with drums and
pulleys, exactly as in an engineer’s workshop. Great care has also been
bestowed upon the mounting of the smaller guns, which are so nicely
poised on their bearings and provided with such accurately fitted racks,
pinions, etc., that a steel gun of 10 ft. in length can easily be
pointed in any direction by the touch of a child’s hand. The mechanical
arrangements are now so admirably adapted for facility of working that,
unless in the rude shocks of actual warfare the nicely adjusted
machinery is found to be liable to be thrown out of gear, these
applications of the engineer’s skill may be considered as having done
all that was required to bring our modern weapons to perfection.
[Illustration:
PLATE XII.
THE 110–TON ARMSTRONG GUN.
]
With the construction of the 110–ton we arrive at a period when
commences a new era in guns—and especially in the armament of
war-ships—necessitated by various circumstances, amongst which may be
named the invention of torpedoes and the building of swiftly moving
torpedo-boats, and of still swifter “torpedo-boat catchers or
destroyers”; so that guns that could be worked only at comparatively
long intervals were at a great disadvantage. Again, about 1880, were
published the records of a most elaborate and important series of
researches conducted by Captain Noble and Sir F. Abel, the chemist of
our War Department. They had investigated all the conditions attending
the combustion of gunpowder in confined spaces, the nature and
quantities of the products, the temperature and pressures of the
confined gases, etc. The information thus afforded was extremely
valuable; but besides this, direct experiments made with actual guns
were carried out, more particularly at Elswick, in which the speed of
the projectiles at every few inches of their travel along the bore of
the piece was ascertained, and also the pressures of the powder gases at
any point. The way in which this is done we shall explain on another
page. (_See article on Recording Instruments._)
So long as muzzle-loading was in use, guns were necessarily made short,
for had they not to be run in from the port-holes and embrasures of
forts in order to be loaded? Now there was an obvious disadvantage in
this, for the projectile left the gun before the expansive force of the
gases had been spent that could have imparted additional velocity. When
however muzzle-loading was abandoned, and especially when strong and
trustworthy steel became available for the construction of the gun
throughout, there was no reason to waste in this way the power of the
charge, so that barrels were made lighter, much longer in proportion to
the calibre, and every part accurately adapted in strength to the strain
to be resisted. For instances of increasing length, take the 38–ton
12–inch guns built up at Woolwich (of only seven pieces) for H.M.S.
_Thunderer_ (see Fig. 93), on Mr. Fraser’s plan. These had a bore equal
to only 16 times their calibre, while in the Armstrong 100–ton guns the
bore is 21 calibres long; and in the 110–ton guns the total length of
the chase is 31 times the diameter of the rifled part. It has since been
the practice to make the bore of guns from 30 to 40 calibres in length.
The effect of a longer chase used with an appropriate charge is very
clearly and instructively shown by the diagram Fig. 98, which is by
permission copied from the very comprehensive work by Messrs. E. W.
Lloyd and A. G. Hadcock, entitled _Artillery: its Progress and Present
Position_. The reader should not pass over this diagram until he has
thoroughly understood it, for it is an excellent example of the graphic
method of presenting the results of scientific investigations. At the
lower part of the diagram there are drawn to scale half-sections of a
long and of a short gun. The horizontal line above is marked in equal
parts representing feet numbered from the base of the projectiles. The
upright line on the left numbered at every fourth division is the scale
for the pressures in tons per square inch on the base of the projectile,
and these are represented by the height of the plain curves above the
horizontal line at each point in the travel of the shot. The dotted
lines represent in the same way, but _not_ on the same scale as the
former, the velocity with which the base of the projectile passes every
point in the chase. The figures 2, 4, and 6 on the upright line at the
right-hand side refer only to pressures: the velocities scale is such
that the point where the dotted meets the right-hand one is 2,680 units
above the horizontal line, as the middle upright in the same way is
1,561 high, and the heights of the dotted lines represent each on the
same scale the velocities of the bases of the projectiles at the
corresponding parts of the chases. The shorter gun has the rifled part
of the chase 15·4 calibres long; the corresponding part of the longer is
nearly 33 calibres. The short 7–inch gun has a charge of 30 lbs. of
gunpowder, and its projectile weighed 115 lbs. The longer 6–inch gun was
not charged with gunpowder, but with the more powerful modern explosive
_cordite_ (see Index), of which there was 19·5 lbs., and its projectile
weighed 100 lbs. The charges were so adjusted that the shots had the
same initial maximum pressure of 20 tons per square inch applied to
them. Now the cordite, though much more powerful than gunpowder (that
is, a given weight will produce far more gas), is slower in its
ignition, continuing longer to supply gas. The maximum pressure, 20 tons
in both cases, is suddenly attained by the gunpowder gases, when the
shot has hardly moved 6 inches onward, and the pressure declines rapidly
as the moving shot leaves more space for the gas; while the cordite
gases produce their greatest pressure more gradually at a part where the
shot is already about 20 inches on its way, and not only do their
highest pressures continue for a greater distance,—but the decline is
far less rapid than in the other case. It will be observed by the
intersection of the dotted lines, that when the shots in each case have
moved about 2 ft. their velocities are equal. They finally leave the
muzzles with the velocities marked on the diagram, and if the reader
will apply the formula given on page 174 he will obtain their respective
energies in _foot-lbs._; but for large amounts like these it is more
usual to state the energy in _foot-tons_, which of course will be
arrived at by dividing the _foot-lbs._ numbers by 2,240, and these will
work out in the one case to 4978·9 ft.-tons, and in the other to 1942·5
ft.-tons. The shot from the long gun will therefore have more than 2½
times the destructive power of the other.
[Illustration:
FIG. 98.—_Diagram of Velocities and Pressure._
]
The operations required in constructing guns are multiform, and have to
be very carefully conducted so that the workmanship shall be of the best
quality. The finest ores are selected for reduction, and the steel is
obtained by the Siemens-Martin process already described. It must be
free from sulphur and phosphorus, and contain such proportions of
carbon, silicon, and manganese as experience has shown to be best, and
its composition is ascertained by careful chemical analysis before it is
used. The fluid steel is run into large ladles lined with fire-brick,
and provided with an opening in the bottom from which the metal can be
allowed to run out into the _ingot_ moulds, the size and proportions of
these being in accordance with the object required; some admitting of as
much as 80 tons at one operation. When a barrel or hoop is required of
not less than 6 inches internal diameter the ingot is cut to the
required length and roughly bored. The ingot is then heated, a long
cylindrical steel bar is put through the hole, and under a hydraulic
press the hot metal is squeezed into greater length and less diameter.
The hole first bored through the ingot is of somewhat greater, and the
steel bar (called a mandril) of less, diameter than required in the
finished piece. Portions are cut from each end of what is now called the
_forging_ and subjected to mechanical tests: if these are satisfactory,
the forging is rough bored and turned on the outside. It is then
annealed, by being heated and allowed to cool very slowly. The next
operation is to _harden_ the metal by raising it to a certain
temperature, at which it is immersed in rape oil until cold. Then the
piece is again annealed, and fine-turned and bored. All these operations
have to be performed not only on the barrel, but also on each hoop,
before the hoops are shrunk on, and the greatest nicety of measurement
is required in each piece. Then the gun has to be turned on the outside,
the screw for the breech piece cut, the bore rifled, etc. The object of
the annealing is to relieve the metal from internal strains. It will not
be wondered at that months are required for the construction of the
larger kind of guns. Thus at Elswick a 6 in. quick firing gun, upon
which men are employed night and day, cannot be completed in less than
five months, and sixteen months are required for making a 67–ton gun.
We may take as an illustration of the progress of modern artillery one
of the products of the Elswick factory which has just been referred to,
and for which the demand from all quarters has been unprecedentedly
great, namely, the 4·7 inch gun. This weapon is mounted in various
manners according to the position it has to occupy, whether for a land
defence, or on ship-board between decks, or on the upper deck. The
arrangement shown in Fig. 99, which is reproduced from Messrs. Lloyd &
Hadcock’s work, is known as the centre pivot mounting, and is suitable
for such a position as the upper deck of a ship. The reader should
compare the proportions and mounting of this weapon with those of the
old 32–pounder sketched in Fig. 90, observing the very much greater
comparative length of the modern weapon, and the mechanism for elevating
and training it (which, however, the scale of the drawing crowds into
too small a space to show as it deserves). C is a projection from the
breech, to which is attached the piston of the recoil press; at T is the
handle for training, which actuates a worm at V; the elevation is
regulated by the turning of the four-armed wheel. The long chase of the
gun projects in front; but the mounting and the breech machinery are
protected by shields of thick steel, of which the sections of two plates
are denoted by the dark upright parts in front. These are fixed; but a
movable plate above the gun can be raised or lowered into an inclined
position, for better taking sights. In the figure this is shown as open
and in a horizontal position. This gun is provided with sights by which
it can be aimed at night; that is, the sights can be illuminated by
small electric lamps suitably placed; the wires connecting these with
voltaic battery cells carried on the mounting are indicated. The figure
represents the gun as constructed about 1893, but the improvements that
are continually being made have brought about some modifications in the
details.
[Illustration:
FIG. 99.—_Elswick 4·7 inch Q. F. Gun on Pivot Mounting._
]
Very notable among the productions of the great Elswick factory are the
_quick firing_ guns. These were at first confined to guns of small
calibre, such as the 6–pounders. They are, of course, all
breech-loaders, and the powder and shot are both contained in a single
metallic cartridge case. A more formidable weapon of the same class is
the 45–pounder rapid firing gun, which, like the rest, is constructed
entirely of steel, with a total length of 16 ft. 2½ in., a calibre of
4·724 in., and a length of bore equal to 40 diameters. The weight of
this gun is 41 cwt., and it throws a shell of 45 lbs. weight with a
12–lb. charge of gunpowder. Quick firing guns having a calibre of 6 in.
are now also made in great numbers for arming our ironclads. The breech
block in the quick firing guns turns aside on a hinge, and after the
introduction of the cartridge it is closed and screwed up to its place
by a slight turn of a handle. The piece is then pointed and trained by
aid of mechanical gearing as in the case of the heavier guns. But Mr.
Hotchkiss has introduced a simpler method of elevating and training his
3–pounder and 6–pounder quick firing guns, by attaching to the rear, and
unaffected by the recoil, a shoulder piece against which the marksman
can lean, and move the weapon as he takes his aim. Though these guns
weigh respectively 4½ cwt. and 7 cwt., they can thus be pointed with the
greatest ease. The firing is done by pulling a trigger in what seems
like the stock of a pistol. The empty cartridge case is automatically
extracted from the firing chamber by the act of opening the breech, and
it drops to the ground. Ten or twelve rounds per minute can be fired
from these guns, and Lord Armstrong has advocated the use of a number of
them for naval armament in preference to that of a few ordinary
breech-loaders of more unwieldy dimensions. He has calculated that in a
given time a far greater weight of metal can be projected from a vessel
armed with quick firing guns than from one provided only with the
heavier class of cannons.
The breech pieces in the Elswick guns are closed on the “interrupted
screw” system—that is, a very large screw thread of V-shaped section is
cut in the barrel at the breech end, and a corresponding thread on the
principal part of the breech block, which is, of course, capable of
rotating about the axis. The screw threads, however, are not continuous,
segments parallel to the axis being cut away, the spaces in the outer
thread corresponding with the projecting parts in the inner, and _vice
versâ_, so that when the block is pushed home, one very small part of a
turn suffices to engage all the threads. The screw is also made conical,
and is so cut into steps, as it were, that great resisting power is
brought into play. The Elswick guns are provided with hydraulic buffers
for checking the recoil, and the principle is applied in various
modified forms. In some cases the pistons allow for the water a passage,
which towards the end gradually diminishes. This is the arrangement for
the 3–pounder rapid firing Hotchkiss gun, and the force of the recoil is
made at the same time to compress two springs, which serve to return the
gun to the firing position. This very handy gun is said to be able to
fire twenty rounds per minute. In Mr. Vavasseur’s plan of mounting, the
recoil is checked by ports, or openings, in the piston of a hydraulic
cylinder being gradually closed, which is easily arranged by making a
spiral groove within the cylinder, which gives a small axial motion to
part of the piston.
[Illustration:
FIG. 100.—_The Moncrieff Gun raised and ready for firing._
]
[Illustration:
FIG. 101.—_Moncrieff Gun lowered for loading._
]
An extremely effective plan for the defence of coasts and harbours was
originated by Colonel Moncrieff, when about 1863 he contrived a method
of mounting large guns on the disappearing system, by which almost
complete protection against hostile fire is given to both gun and
gunners. He utilizes the recoil as a means of bringing the gun down into
a protected position the moment it has been fired, and retains this
energy by a simple arrangement until the piece has been reloaded, when
it is allowed to expend itself by again raising the gun above the
parapet into the original firing position. The configuration and action
of Colonel Moncrieff’s gun-carriage will be understood by an inspection
of the annexed illustrations, where in Fig. 100 is shown the gun raised
above the parapet and ready for firing. When the discharge takes place,
the gun, if free, would move backwards with a certain speed, but the
disposition of the mounting is such that this initial velocity receives
no _sudden_ check, the force being expended in raising a heavy
counterpoise, and at the same time the gun is permitted to descend,
while maintaining a direction parallel to its firing position. At the
end of the descent, which, it must be understood, is caused by the force
of the recoil, and not by the counterpoise, for this more than balances
the weight of the gun, the latter is retained as shown in Fig. 101 until
it has been reloaded; and when it has again to be fired, it is released
so as to allow the descent of the counterpoise to raise it once more
into position. The great advantage of this invention is the protection
afforded to the artillerymen and gun while loading; and even the aiming
can be accomplished by mirrors, so that the men are exposed to no
danger, except from “vertical fire,” which involves but little risk.
Colonel Moncrieff took out a patent for his invention in 1864, but
committed the practical working out of his idea to the firm of Sir W. G.
Armstrong & Co., in whose hands the design was ultimately transformed
from the original somewhat cumbersome arrangement of the mounting into
the compact and manageable form shown in Fig. 102, which represents a
13·9 inch 68–ton breech-loading disappearing gun on the Elswick
hydro-pneumatic mounting. The principle of hydraulic power is fully
explained in our article on that subject, and an example of its
application to cranes as devised by Sir W. Armstrong is there described.
When guns began to be made very large, and projectiles weighing several
cwts. had to be dealt with, the application of power in some form became
essential for loading, running out, elevating, training, etc.; and
though steam-power naturally was first used, hydraulic power was adopted
at Elswick, and has been there applied to the mountings of large guns
with the greatest success by Mr. G. W. Rendel. To mention the various
arrangements in which this power is applied, or to attempt any
description of the elaborate machinery by which it is regulated, would
carry us far beyond our limits. But the powerful weapon depicted in Fig.
102 is designed to be worked only by the manual effort of a few men. In
this mounting the pressure of condensed air sustains the gun in the
firing position; that pressure, acting upon the water in the recoil
presses, having previously forced up their rams so as to turn into a
nearly vertical position the strong brackets or beams on which the
trunnions are supported. The recoil is checked in the usual way by the
forcing of the water through small ports or valves as the ram descends,
but these valves are so arranged that the water is in part forced back
into the air chamber, and there recompresses the air, to restore the
power for again raising the gun. The pressure in the air chamber when
the gun is down may be about 1,400 lbs. per square inch; when it is up
this will be reduced to perhaps one half by the expansion of the air in
doing work. We have here the reaction of compressed air taking the place
of the gravity of the counterpoise originally designed. There are in
this hydro-pneumatic mounting a number of adjusting appliances, such as
forcing pumps, brakes, etc., for regulating the pressures, or quantity
of liquid, as, for instance, when lowering the gun without any recoil
action in operation. Then again, with any change in the weight of
projectile or in the powder charge, there would be a corresponding
change in the power of the recoil, and therefore the necessity for
compensatory adjustments, which are made with great readiness. The
nicety with which the parts are adapted to each other in this mounting
must be obvious, when we observe the magnitude of the mass to be moved
with the least delay, and brought to rest, quite gently and exactly, in
a new position. Details cannot here be given even of the method by which
the valves in the recoil cylinders are automatically controlled for this
purpose. Means are also supplied for setting the gun, while still in its
protected position, to the required angle of elevation or depression.
The adjustment is made by the long rods attached near the breech and set
at their lower ends to the position giving the intended angle to the
raised gun. The varied and powerful strains to which the parts of this
mechanism are subject, and which have had to be calculated and provided
for, may be inferred from the enormous recoil energy of the gun, which
under ordinary conditions amounts to no less than 730 foot-tons. The gun
is provided with ordinary, and also with reflecting, sights, so that no
one need be exposed to the enemy’s fire. Protective armour above the gun
is not required, as the pit itself being usually on some elevation is
imperceptible to the enemy, and the gun is visible but for a few
seconds, forming a quite inconspicuous object. The pit in which the gun
is mounted is commonly lined with concrete. Italy, England, Norway,
Japan and other countries have appreciated the advantages of the
disappearing system in providing the most powerful coast defences yet
devised, and a great many guns have been mounted on this principle.
[Illustration:
FIG. 102.—_68–ton Gun on Elswick Hydro-Pneumatic Mounting._
]
An extraordinary piece of ordnance is represented in Fig. 103. It is one
of two huge mortars, the idea of which presented itself to Mr. Mallet
during the Crimean War, the intention being to throw into the Russian
lines spherical shells a yard in diameter, which would, in fact, have
constituted powerful mines, rendering it impossible for the
fortifications to continue tenable. Mallet’s original design was to
project these shells from mortars of no less than 40 tons weight. When
it was pointed out that the transport of so heavy a mass would be
impracticable, the design was changed to admit of the mortar being made
in pieces not exceeding eleven tons in weight, and built up where
required. During the most active period in the siege of Sebastopol this
plan was submitted to Lord Palmerston, who at once ordered two of these
apparently formidable pieces to be constructed, without waiting for
official examinations of the scheme, and the usual reports of
experts,—promptness in this case being considered of the utmost
importance. A contract was made with a private firm, who undertook to
deliver them in ten weeks. But the difficulties attending such
constructions not being understood at the time, delays arose, the
contractors failed, and two years elapsed before the mortars were
completed. In the meantime peace had been concluded, and the mortars
were never fired against any hostile works; but experiments were made
with one of them at Woolwich. The heaviest of the shells it was intended
to project weighed 2,940 lbs., and for this it was proposed to use a
charge of 80 lbs. of gunpowder. In the experiments the charges first
used were low, but gradually increased: when it was found that after
every few rounds repairs became necessary in consequence of the weak
points in the construction, and after the nineteenth round the mortar
was so much damaged that the trials were definitely discontinued. The
other mortar, though mounted, was never fired, but remains at Woolwich,
an object of some interest to artillerists, especially since there has
been some talk of reverting to this very old-fashioned form of ordnance
as a means of attacking ironclads in their most vulnerable direction by
the so-called vertical fire. In one of the rounds of the Mallet mortar
tried at Woolwich, a shell weighing 2,400 lbs. was thrown by a charge of
70 lbs. of gunpowder a distance of more than a mile and a half, and it
buried itself in the soil to a very great depth.
[Illustration:
FIG. 103.—_Mallet’s Mortar_
]
For high-elevation firing, howitzers will more probably be the form of
ordnance most in use. The range of the howitzer is determined by the
angle at which it is elevated, whereas with the mortar it is chiefly by
variation of the powder charge that the aim is adjusted. Many of the old
short 9 in. muzzle-loaders have already been converted into 11 in.
rifled howitzers, and these are likely to prove of great service in
defending our harbours and channels against war vessels.
Some account has been given in a preceding article of the great steel
works of Krupp & Co. at Essen, and the place has been noted as one of
the greatest gun factories in the world during the second half of our
century. The process there practised of casting crucible steel ingots,
and already described, is precisely that used in the first stage of
gun-making. The steel for guns put into the crucibles is a carefully
adjusted mixture of one quality of iron puddled into steel and subjected
to certain treatment; the other portion is made from a different quality
of iron from which all the carbon has been puddled out. The cast ingot
is forged under a great steam hammer, bored, turned, and steel hoops
shrunk upon it, in several layers, and other operations are performed
upon it like those which have already been mentioned. A 14 in. gun is
said to require sixteen months for its manufacture, and its cost to be
about £20,000.
[Illustration:
FIG. 104.—_32–pounder Krupp Siege Gun, with Breech-piece open._
]
Artillerists had long carried on a warm controversy as to the relative
merits of wrought iron and steel in gun construction, the latter
material being regarded with shyness on account of its want of
uniformity as formerly produced. Krupp however began as early as 1847 to
make guns of his excellent crucible steel, and through bad report and
good report confined himself to this material until, it is asserted, by
1878 he had supplied over 17,000 steel guns of all calibres. He began by
making a 3–pounder gun, but soon produced pieces of larger size, all of
which were bored and turned out of solid masses of metal. At a later
period the plan of shrinking on strengthening hoops of steel was
adopted. The Krupp guns have found extensive favour, and many very heavy
ones have been made, some indeed of greater weight than the 110–ton
Armstrong; but the excess of weight is due to the mass of metal which
the Krupp construction of the breech mechanism requires. Thus Krupp’s
120–ton gun has a muzzle energy of but 45,796 foot-tons, while that of
the Elswick piece is 55,105 foot-tons.
[Illustration:
FIG. 105.—_The Citadel of Strasburg after the Prussian Bombardment._
]
The breech arrangement in the Krupp guns consists of a lateral slot into
which slides a closing block after the charge has been inserted from the
rear. An obsolete form of this breech piece is seen in Fig. 104, which
represents a 32–pounder gun such as was used in sieges by the Prussians
in the Franco-German War. It will be observed here that the slot and
breech piece are of rectangular form; but this shape, causing the piece
to be weak where most strength was required, was afterwards altered into
a D-shaped section, the curved side being of course to the rear. That
difficulty which baffled the earliest attempts at breech-loading is the
same that has given much trouble to modern gunmakers. It consists in so
closing the breech that no escape of the powder gases can take place
there at the moment of discharge. When we remember that the momentary
pressure of the gases in the powder chamber may amount to more than 40
tons on the square inch, we can well understand the enormous velocity
with which they will rush forth from even the smallest interspace
between the base of the gun and the breech block, but we can hardly
realise without actual inspection the mechanical action they produce in
their passage: when once the escape occurs, a channel is cut in the
metal as if part had been removed by an instrument, and the piece in
that condition is disabled for further use. Several devices are in use
obtaining perfect closure of the breech, which is technically called
_obturation_ (Latin, _obturare_, to close up). One of these consists in
fitting closely into the circumference of the bore a ring of very
elastic steel, turned up at the edges towards the powder chamber. The
gas pressure forces the edge of this ring still more closely against the
interior of the powder chamber, much in the same way as the Bramah
collar acts in the hydraulic press (see Fig. 165). The shaded circle
shown on the breech piece in Fig. 104 is an additional device for
obtaining obturation. The Broadwell ring, as the above-mentioned
contrivance is called, is not used in English guns, but another plan of
obtaining a gas-joint has been much adopted, in which a squeezable pad
is by compression forced outwards to close up the bore.
A very long range was claimed for Krupp’s guns at the time of the
Franco-German War, for at the siege of Paris (1870) it was said they
could hurl projectiles to the distance of five miles, though probably
there was some exaggeration in this statement. There is no doubt however
that the Prussians had very effective and powerful artillery, as may be
gathered from Fig. 105, which is taken from a photograph of part of the
fortifications of Strasburg after the bombardment of that fortress. The
explosive shells used by the Prussians against masses of troops were not
precisely segment shells of the form already described, but the
principle and effect were the same, for the interior was built up of
circular rings, which broke into many pieces when the shell exploded.
Out of the very numerous forms in which modern ordnance is constructed,
we have been able to select but a few examples for illustration and
description. These will suffice, it is hoped, to give an idea of the
progress that the century has witnessed. It would be beyond our scope to
give details of the ingenious mechanical devices that have come to be
applied to guns: such as the breech-closing arrangements, the various
ways in which recoil is controlled and utilized, etc. A good
illustration, had space permitted, of the scientific skill applied to
ordnance would be found in the contrivances fitted to certain
projectiles in order to determine their explosion at the proper moment.
These are very different from the cap or time fuse that did duty in the
first half of the century. We have indeed said little of the projectiles
themselves beyond mention of the Palliser chilled shot and the obsolete
studded projectiles. We have not explained how bands of copper, or other
soft metal, are put round a certain part of the shot or shell, in order
that, being forced into the grooves, the axial rotation may be imparted,
or how windage is prevented by “gas checks” attached to the base of the
projectiles. We must now be contented to conclude this section by
showing the structure of two kinds of explosive shells which have been
much used.
Shrapnel shell takes its name from Lieutenant Shrapnel, who was its
inventor about the end of last century, but the projectile began to be
used only in 1808. Fig. 105_a_ is a section showing the shell as a case
containing a number of spherical bullets, of which in the larger shells
there are very many, the interspaces being filled with rosin, poured in
when melted; the bullets are thus prevented from moving about. The
figure shows the shell without the fuse or percussion apparatus, which
screws into the hollow at the front. The bursting charge of gunpowder is
behind the bullets, and when it explodes they travel forward with a
greater velocity than the shell, but with trajectories more or less
radiating, carrying with them wide-spreading destruction and death.
A shrapnel shell may be said to be a short cannon containing its charge
of powder in a thick chamber at the breech end; the sides of the fore
part of the shell are thinner than those of the chamber, and may be said
to form the barrel of the cannon. This cannon is loaded up to the muzzle
with round balls, which vary with the shell in size. An iron disc
between the powder and the bullets represents the wad used in ordinary
fowling-pieces. A false conical head is attached to the shell, so that
its outward appearance is very similar to that of an ordinary
cylindro-conoidal shell: that is to say, it looks like a very large long
Enfield bullet. The spinning motion which had been communicated to the
shell by the rifling of the gun from which it had been fired causes the
barrel filled with bullets to point in the direction of the object at
which the gun has been aimed. Consequently, when the shrapnel shell is
burst, or rather fired off, the bullets which it contained are streamed
forward with actually greater velocity than that at which the shell had
been moving; and the effect produced is similar to firing grape and
canister from a smooth-bore cannon at a short range.
[Illustration:
FIG. 105_a_.—_The Shrapnel and Segment Shells._
]
Segment shells were first brought into use by Lord Armstrong in 1858 in
connection with his breech-loading guns. The segment shell consists of a
thin casing like a huge conical-headed thimble, with a false bottom
attached to it. It is filled with small pieces of iron called
“segments,” cast into shapes which enable them to be built up inside the
outer casing into two or more concentric circular walls. The internal
surface of the inmost wall forms the cavity of the compound or segment
shell, and contains the bursting charge. The segment shell is fitted
with a percussion fuse, which causes it to explode when it strikes. In
the shrapnel shell, the powder charge is situated in rear of the
bullets, and consequently produces the chief effect in a forward
direction. In the segment shell, the powder is contained inside the
segments, and therefore produces the chief effect in a lateral
direction. When the shrapnel shell is burst at the right moment, its
effect is greatly superior to that of the segment shell; on the other
hand, the segment shell, when employed at unknown or varying distances,
is far more unlikely to explode at the proper time.
Shrapnel and segment shells can be used with field artillery, _i.e._,
9–pounders, 12–pounders, 16–pounders; and also with heavy rifled guns in
fortresses, viz., 40–pounders, 64–pounders, 7–in. and 9–in. guns. But
the conditions of their service are very different in each case. With
regard to field artillery, the distance of the enemy is rarely known,
and is constantly changing, and hence the men who have to adjust the
fuses would probably be exposed to the fire of the enemy’s artillery,
and, consequently, could not be expected to prepare the fuses with the
great care and nicety which are absolutely necessary to give due effect
to the shells. There are, however, some occasions when the above
objections would not hold good—as, for instance, when field artillery
occupy a position in which they wait the attack of an enemy advancing
over ground in which the distances are known.
Segment shells require no adjustment of their percussion fuse. They
enable the artillerymen to hit off the proper range very quickly, since
the smoke of the shell which bursts on striking tells them at once
whether they are aiming too high or too low.
With regard, however, to the service of heavy rifled guns in fortresses,
the conditions are quite different. In the first place, the distance of
all objects in sight would be well known beforehand; and in the second
place, the fuses of the shells would be carefully cut to the required
length in the bomb-proofs, where the men would be completely sheltered.
The 7–in. shrapnel contains 227 bullets, and a 9–in. shrapnel would
contain 500 bullets of the same size, and these shells could be burst
with extraordinary accuracy upon objects 5,000, 6,000, or 7,000 yards
off.
_MACHINE GUNS._
The name of machine guns has been applied to arms which may be regarded
as in some respects intermediate between cannons and rifles, since in
certain particulars they partake of the nature of both. Like the former,
they are fired from a stand or carriage, and in some of their forms
require more than one man for their working: in the calibre of their
barrels and the weight of their projectiles, they are assimilated to the
rifle, but they are capable of pouring forth their missiles in a very
rapid succession—so rapid indeed as practically to constitute volley
firing. The firing mechanism of the machine gun has always an automatic
character, but the rifle has acquired this feature, so that it cannot be
made a distinguishing mark: on the other hand, since machine guns have
been made to discharge projectiles of such weights as 1 lb. or 3 lb.
there is nothing to separate them from quick-firing ordnance unless it
be the automatic firing.
The idea of combining a number of musket-barrels into one weapon, so
that these barrels may be discharged simultaneously or in rapid
succession, is not new. Attempts were made two hundred years ago to
construct such weapons; but they failed, from the want of good
mechanical adjustments of their parts. Nor would the machine gun have
become the effective weapon it is, but for the timely invention of the
rigid metallic-cased cartridge. Several forms of machine guns have in
turn attracted much attention. There is the Mitrailleur (or
Mitrailleuse), of which so much was heard at the commencement of the
Franco-German War, and of whose deadly powers the French managed to
circulate terrible and mysterious reports, while the weapon itself was
kept concealed. Whether this arose from the great expectations really
entertained of the destructive effects of the mitrailleur, or whether
the reports were circulated merely to inspire the French troops with
confidence, would be difficult to determine. Our own policy in regard to
new implements of war is not to attempt to conceal their construction.
Experience has shown that no secret of the least value can long be
preserved within the walls of an arsenal, although the French certainly
apparently succeeded in surrounding their invention with mystery for a
while. The machine gun, or “battery,” invented by Mr. Gatling, an
American, is said by English artillerists to be free from many defects
of the French mitrailleur. In 1870 a committee of English military men
was appointed to examine the powers of several forms of mitrailleur,
with a view to reporting upon the advisability or otherwise of
introducing this arm into the British service. They recommended for
certain purposes the Gatling battery gun.
[Illustration:
FIG. 105_b_.—_The Gatling Gun.—Rear View._
]
In the Gatling the barrels, ten in number, are distinct and separate,
being screwed into a solid revolving piece towards the breech end, and
passing near their muzzles through a plate, by which they are kept
parallel to each other. The whole revolves with a shaft, turning in
bearings placed front and rear in an oblong fixed frame, and carrying
two other pieces, which rotate with it. These are the “carrier” and the
lock cylinder. Fig. 105_b_ gives a rear view, and Fig. 105_c_ a side
view, of the Gatling battery gun. The weapon is made of three sizes, the
largest one firing bullets 1 in. in diameter, weighing ½ lb., the
smallest discharging bullets of ·45 in. diameter. The small Gatling is
said to be effective at a range of more than a mile and a quarter, and
can discharge 400 bullets or more in one minute. Mr. Gatling thus
describes his invention:
“The gun consists of a series of barrels in combination with a grooved
carrier and lock cylinder. All these several parts are rigidly secured
upon a main shaft. There are as many grooves in the carrier, and as many
holes in the lock cylinder, as there are barrels. Each barrel is
furnished with one lock, so that a gun with ten barrels has ten locks.
The locks work in the holes formed in the lock cylinder on a line with
the axis of the barrels. The lock cylinder, which contains the lock, is
surrounded by a casing, which is fastened to a frame, to which trimmers
are attached. There is a partition in the casing, through which there is
an opening, and into which the main shaft, which carries the lock
cylinder, carrier, and barrels, is journaled. The main shaft is also at
its front end journaled in the front part of the frame. In front of the
partition in the casing is placed a cam, provided with spiral surfaces
or inclined planes.
“This cam is rigidly fastened to the casing, and is used to impart a
reciprocating motion to the locks when the gun is rotated. There is also
in the front part of the casing a cocking ring which surrounds the lock
cylinder, is attached to the casing, and has on its rear surface an
inclined plane with an abrupt shoulder. This ring and its projection are
used for cocking and firing the gun. This ring, the spiral cam, and the
locks make up the loading and firing mechanism.
“On the rear end of the main shaft, in rear of the partition in the
casing, is located a gear-wheel, which works to a pinion on the
crank-shaft. The rear of the casing is closed by the cascable plate.
There is hinged to the frame in front of the breech-casing a curved
plate, covering partially the grooved carrier, into which is formed a
hopper or opening, through which the cartridges are fed to the gun from
feed-cases. The frame which supports the gun is mounted upon the
carriage used for the transportation of the gun.
“The operation of the gun is very simple. One man places a feed-case
filled with cartridges into the hopper; another man turns the crank,
which, by the agency of the gearing, revolves the main shaft, carrying
with it the lock cylinder, carrier, barrels, and locks. As the gun is
rotated, the cartridges, one by one, drop into the grooves of the
carrier from the feed-cases, and instantly the lock, by its impingement
on the spiral cam surfaces, moves forward to load the cartridge, and
when the butt-end of the lock gets on the highest projection of the cam,
the charge is fired, through the agency of the cocking device, which at
this point liberates the lock, spring, and hammer, and explodes the
cartridge. As soon as the charge is fired, the lock, as the gun is
revolved, is drawn back by the agency of the spiral surface in the cam
acting on a lug of the lock, bringing with it the shell of the cartridge
after it has been fired, which is dropped on the ground. Thus, it will
be seen, when the gun is rotated, the locks in rapid succession move
forward to load and fire, and return to extract the cartridge-shells. In
other words, the whole operation of loading, closing the breech,
discharging, and expelling the empty cartridge-shells is conducted while
the barrels are kept in continuous revolving movement. It must be borne
in mind that while the locks revolve with the barrels, they have also,
in their line of travel, a spiral reciprocating movement; that is, each
lock revolves once and moves forward and back at each revolution of the
gun.
“The gun is so novel in its construction and operation that it is almost
impossible to describe it minutely without the aid of drawings. Its main
features may be summed up thus: 1st.—Each barrel in the gun is provided
with its own independent lock or firing mechanism. 2nd.—All the locks
revolve simultaneously with the barrels, carrier, and inner breech, when
the gun is in operation. The locks also have, as stated, a reciprocating
motion when the gun is rotated. The gun cannot be fired when either the
barrels or locks are at rest.”
There is a beautiful mechanical principle developed in the gun, viz.,
that while the gun itself is under uniform constant rotary motion, the
locks rotate with the barrels and breech, and at the same time have a
longitudinal reciprocating motion, performing the consecutive operations
of loading, cocking, and firing without any pause whatever in the
several and continuous operations.
The small Gatling is supplied with another improvement called the “drum
feed.” This case is divided into sixteen sections, each of which
contains twenty-five cartridges, and is placed on a vertical axis on the
top of the gun. As fast as one section is discharged, it rotates, and
brings another section over the feed aperture, until the whole 400
charges are expended.
[Illustration:
FIG. 105_c_.—_The Gatling Gun.—Front View._
]
After a careful comparison of the effects of field artillery firing
shrapnel, the committee concluded that the Gatling would be more
destructive in the open at distances up to 1,200 yards, but that it is
not comparable to artillery in effect at greater distances, or where the
ground is covered by trees, brushwood, earthworks, &c. The mitrailleur,
however, would soon be knocked over by artillery if exposed, and
therefore will probably only be employed in situations under shelter
from such fire. An English officer, who witnessed the effects of
mitrailleur fire at the battle of Beaugency, looks upon the mitrailleur
as representing a certain number of infantry, for whom there is not room
on the ground, suddenly placed forward at the proper moment at a
decisive point to bring a crushing fire upon the enemy. Many other
eye-witnesses have spoken of the fearfully deadly effect of the
mitrailleur in certain actions during the Franco-German War.
Mr. Gatling contends that, shot for shot, his machine is more accurate
than infantry, and certainly the absence of nerves will ensure
steadiness; while so few men (four) are necessary to work the gun that
the exposure of life is less. No re-sighting and re-laying are necessary
between each discharge. When the gun is once sighted its carriage does
not move, except at the will of the operator; and the gun can be moved
laterally when firing is going on, so as to sweep the section of a
circle of 12° or more without moving the trail or changing the wheels of
the carriage. The smoke of battle, therefore, does not interfere with
its precision.
Whatever may be the part this new weapon is destined to play in the wars
of the future, we know that every European Power has now provided itself
with some machine guns. The Germans have those they took from the
French, who adhere to their old pattern. The Russians have made numbers
of Gatlings, each of which can send out, it is said, 1,000 shots per
minute, and improvements have been effected, so as to obtain a lateral
sweep for the fire.
[Illustration:
FIG. 105_d_.—_The Montigny Mitrailleur._
]
A competitor to the Gatling presents itself in the Belgian mitrailleur,
the Montigny, Fig. 105_d_. This gun, like the Gatling, is made of
several different sizes, the smallest containing nineteen barrels and
the largest thirty-seven. The barrels are all fitted into a wrought iron
tube, which thus constitutes the compound barrel of the weapon. At the
breech end of this barrel is the movable portion and the mechanism by
which it is worked. The movable portion consists mainly of a short
metallic cylinder of about the same diameter as the compound barrel, and
this is pierced with a number of holes which correspond exactly with the
position of the gun-barrels, of which they would form so many
prolongations. In each of the holes or tubes a steel piston works
freely; and when its front end is made even with the front surface of
the short cylinder, a spiral spring, which is also contained in each of
the tubes, is compressed. The short cylinder moves as a whole backwards
and forwards in the direction of the axis of the piece, the movement
being given by a lever to the shorter arm of which the movable piece is
attached. When the gun is to be loaded this piece is drawn backwards by
raising the lever, when the spiral springs are relieved from
compression, and the heads of the pistons press lightly against a flat
steel plate in front of them. The withdrawal of the breech-block gives
space for a steel plate, bored with holes corresponding to the barrels,
to be slid down vertically; and this plate holds in each hole a
cartridge, the head of each cartridge being, when the plate has dropped
into its position, exactly opposite to the barrel, into which it is
thrust, when the movable breech-block is made to advance. The anterior
face of this breech-block is formed of a plate containing a number of
holes again corresponding to the barrels, and in each hole is a little
short rod of metal, which has in front a projecting point that can be
made to protrude through a _small_ aperture in the front of the plate,
the said small apertures exactly agreeing in position with the centres
of the barrels, and being the only perforations in the front of the
plate. The back of the plate has also openings through which the heads
of the pistons can pass, and by hitting the little pieces, or strikers,
cause their points to pass out through the apertures in front of the
plate, and enter the base of the cartridges, where _fulminate_ is
placed. The plate filled with cartridges has a bevelled edge, and the
points of the strikers are pushed back by it as it descends. The heads
of the pistons are separated until the moment of discharge from the
recesses containing the strikers by the flat steel plate or shutter
already mentioned. The effect, therefore, of pushing the breech-block
forward is to ram the cartridges into the barrels, and at the same time
the spiral springs are compressed, and the heads of the pistons press
against the steel shutter which separates them from the strikers, so
that the whole of the breech mechanism is thus closed up. When the piece
is to be fired a handle is turned, which draws down the steel shutter
and permits the pistons to leap forward one by one, and hit the
strikers, so that the points of the latter enter the cartridges and
inflame the fulminate. The shutter is cut at its upper edge into steps,
so that no two adjoining barrels are fired at once. The whole of the
thirty-seven barrels can be fired by one and a quarter turns of the
handle, which may, of course, be given almost instantly, or, by a slower
movement, the barrels can be discharged at any required rate.
The barrels of the machine guns we have described do not, as is
generally supposed, radiate; on the contrary, they are arranged in a
perfectly parallel direction. In consequence of this, the bullets are at
short ranges directed nearly to one spot. The Gatling gun was adopted as
a service weapon by the British navy, and in several minor actions it
had proved effective, but in its original form it was superseded by the
Gardner gun, in which the barrels are fixed horizontally side by side,
and are in number five or fewer; each barrel is able to fire 120 rounds
per minute. A new system of feed was afterwards applied to the Gatling
gun by Mr. Accles, by means of which this gun was greatly improved and
its rate of firing was increased to more than 1,000 rounds per minute;
indeed, 80 rounds have been fired from it within 2 seconds. The Gatlings
in this improved form have ten barrels, and are provided with feed
drums, each containing 104 cartridges, and capable, when empty, of being
almost instantly replaced by a full one. The contents of one drum can,
if necessary, be discharged in about 2¼ seconds, so that in this time
104 rifle bullets would be fired; or considerably more than the rate of
1,000 rounds per minute could easily be maintained. The weapon is so
mounted, that without moving its carriage it can be pointed at any angle
of elevation or depression, and through a considerable lateral range.
Mr. Nordenfelt has brought out a machine gun, which, on account of the
simplicity and strength of its firing mechanism, has proved the most
reliable weapon of its class, and it also has been adopted into the
British service, and indeed into that of nearly every nation in the
world. In this gun there are five barrels arranged as in the Gardner,
but the firing is operated by a lever working backwards and forwards at
the rate of 600 rounds per minute.
[Illustration:
FIG. 105_e_.—_A Hotchkiss Gun._
]
In the firing of all these weapons, by turning a crank, or moving a
lever at one side, any attempt at exact aiming must obviously be
difficult if not impossible, from the liability of the gun to get moved.
Several designs have been proposed for making the firing mechanism
entirely automatic so as to require no effort on the part of the firer,
whose attention can then be directed solely to pointing the piece. It
would not be easy to explain in detail the way in which this is
accomplished in these very ingenious guns; for while the principle of
their action is sufficiently clear, namely, that the force of the recoil
is made to extract the spent cartridge, open the breech, insert a fresh
cartridge, close the breech, and fire the charge, the mechanism of the
reacting springs, etc., by which this is effected could scarcely, even
by the aid of elaborate diagrams, be made intelligible to any other than
a gunsmith. The Maxim is one of those automatic guns: it has but one
barrel, and after the first discharge it will go on firing with
marvellous rapidity the cartridges supplied to it in a continuous chain,
and this without any deviation from such direction as may be given to it
by the operator, for he has neither crank to turn nor lever to move, but
merely sits behind an iron shield directing the weapon at will, which,
without interference, fires hundreds of shots per minute from one
barrel, so long as the long bands of cartridges are supplied to it.
Mr. Nordenfelt and Mr. Hotchkiss have also both contrived quick firing
guns for 1–lb., 3–lb., and 6–lb. projectiles, and these, it has been
thought, will be of great service in naval warfare as against torpedo
boats.
Though the automatic mechanism, whereby the breech operations are all
performed by the force of the recoil of the barrel, which is allowed to
slide backwards, and is then returned to its place by a spring, is too
complicated for illustration here, mention may be made of a quite recent
device by which the recoil action is dispensed with, and the mechanism
so far simplified that scarcely more than half the number of parts in
the lock mechanism are required. Imagine a closed tube beneath the
barrel, parallel to it, and communicating with it only by a small boring
near the muzzle; through this opening the expanding gases will pass, in
a degree depending on its size and position, and by their action on a
piston near the breech, impulses are supplied that will actuate the lock
mechanism so long as cartridges are supplied, as they may be in a
continuous band. A weapon of this construction has been already tried,
and its discharges are so rapid that the sound of them is described as
being quite deafening. This plan appears to be equally applicable to
small arms, and to machine or field guns. A very effective gun of the
kind, which fires ordinary rifle bullets, has been contrived by Mr.
Hotchkiss, and is represented in Fig. 105_e_. It is capable of sending
forth as many as 1,000 shots in one minute.
Modern ordnance has required certain modifications in the making of
gunpowder, so that the original name of _powder_ would now hardly be
applicable at all. The large charges now used, if introduced in the form
of fine powder, would certainly shatter the guns from the suddenness of
the exploding force. Hence the material is made up into larger or
smaller masses, generally rounded like small pebbles. The explosive used
for the huge 110–ton guns presents itself in the form of
chocolate-coloured hexagonal prisms, two or three inches long and about
an inch in diameter. These are obtained by compressing the specially
prepared material into moulds with a hydraulic press. The reason for
this process is that, in order to obtain precision and uniformity in the
effects, not only must the composition of the powder be always the same,
but the size, shape, weight, and number of the several portions that
make up the charge must be invariable. It has not been found possible to
fire one of these monster guns many times without such signs of
deterioration as would suggest a short “life” for each of them. But the
greatest necessity for modern fire-arms is a smokeless powder or other
explosive. It is obvious that the advantages of quick firing, whether of
large or of small fire-arms, are greatly reduced if the soldier or
gunner is prevented by smoke from taking aim. The invention of a
smokeless gunpowder has several times been announced, and great advances
have, indeed, been made towards its realization. Certain compositions,
which appeared to meet the requirement of being practically smokeless,
have, however, been found liable to chemical changes, or to corrode the
bore, or to possess other objectionable properties. In this country the
explosive coming into use as best adapted for quick firing guns, etc.,
presents itself in appearance like whitish or grey strings, and has
hence received the name of _cordite_. The composition and mode of
manufacture of these new substitutes for gunpowder are not readily
disclosed, each military authority jealously guarding its own secrets.
The problem of smokeless powder has, however, been almost completely
solved, for at a military review that took place on the Continent in
1889, the discharge of the rifles (loaded with blank cartridges, of
course) is said to have been attended with no more smoke than the puff
of a cigar. The new invention will cause some changes in military
tactics, for the manœuvres formerly executed under cover of the battle
smoke will no longer be possible. Some particulars as to the nature of
smokeless powders will be found in the article on “Explosives.”
[Illustration]
[Illustration:
FIG. 106.—_Harvey’s Torpedo. Working the Brakes._
]
TORPEDOES.
The notion of destroying ships or other structures by explosions of
gunpowder, contained in vessels made to float on the surface of the
water, or submerged beneath it, is not of very modern origin. Two
hundred and fifty years ago the English tried “floating petards” at the
siege of Rochelle. During the American War of Independence similar
contrivances were used against the British, and from time to time since
then “torpedoes,” as they were first termed by Fulton, have been
employed in warfare in various forms; but up to quite a recent period
the use of torpedoes does not appear to have been attended with any
decided success, and it is probable that but for the deplorable Civil
War in the United States we should have heard little of this invention.
During that bitter fratricidal struggle, however, when so much ingenuity
was displayed in the contrivance of subsidiary means of attack and
defence, the torpedo came prominently into notice, having been employed
by the Confederates with the most marked effects. It is said that
thirty-nine Federal ships were blown up by Confederate torpedoes, and
the official reports own to twenty-five having been so destroyed. This
caused the American Government to turn their attention to the torpedo,
and they became so convinced of the importance of this class of war
engine that they built boats expressly for torpedo warfare, and equipped
six _Monitors_ for the same purpose.
It has been well remarked that the torpedo plays the same part in naval
warfare as does the mine in operations by land. This exactly describes
the purpose of the torpedo where it is used defensively, but the
comparison fails to suggest its capabilities as a weapon of offence.
There are few occasions where a mine is made the means of attack, while
the torpedo readily admits of such an employment, and, used in this way,
it may become a conspicuous feature of future naval engagements. Many
forms of this war engine have been invented, but all may be classified,
in the first place, under two heads: viz., stationary torpedoes, and
mobile or offensive torpedoes; while independent distinctions may be
made according to the manner of firing the charge; or, again, according
to the mode of determining the instant of the explosion. The stationary
torpedo may be fixed to a pile or a raft, or attached to a weight; the
offensive torpedo may be either allowed to float or drift against the
hostile ships, or it may be propelled by machinery, or attached to a
spar of an ironclad or other vessel. The charge may be fired by a match,
by percussion, by friction, by electricity, or by some contrivance for
bringing chemicals into contact which act strongly upon each other, and
thus generate sufficient heat to ignite the charge. The instant of
explosion may be determined by the contact of the torpedo with the
hostile structure (in which case it is said to be “self-acting”), or by
clockwork, or at the will of persons directing the operations. In some
cases lines attached to triggers are employed; in others electric
currents are made use of.
[Illustration:
FIG. 107.—_Submerged Torpedo._
]
In the American Civil War the stationary torpedoes at first laid down
were self-acting, that is, they were so arranged as to explode when
touched by a passing vessel. Such arrangements present the great
disadvantages of being as dangerous to friendly as to hostile ships. The
operation of placing them is a perilous one, and when once sunk, they
can only be removed at great risk. Besides this, they cannot be relied
on for certain action in time of need, as the self-acting apparatus is
liable to get out of order. The superiority of the method of firing them
from the shore when the proper instant arrived, became so obvious that
the self-acting torpedo was soon to a great extent superseded by one so
arranged that an observer could fire it at will, by means of a
trigger-line or an electric current. Similar plans had often been
previously employed or suggested. For example, during the war between
Austria and Italy the Austrian engineers at Venice had very large
electric torpedoes sunk in the channels which form the approaches to the
city. They consisted of large wooden cases capable of containing 400
lbs. of gun-cotton, moored by chains to a wooden framework, to which
weights were lashed that sufficed to sink the whole apparatus, Fig. 107.
A cable containing insulated wires connected the torpedo with an
electrical arrangement on shore, and the explosion could take place only
by the operator sending a current through these wires. The torpedo was
wholly submerged, so that there was nothing visible to distinguish its
position. There was no need of a buoy or other mark, as in the case of
self-acting torpedoes, to warn friendly vessels off the dangerous spot,
and therefore nothing appeared to excite an enemy’s suspicions. But it
is, however, absolutely necessary that the defenders should know the
precise position of each of their submarine mines, so that they might
explode it at the moment the enemy’s ship came within the range of its
destructive action. This was accomplished at Venice in a highly
ingenious manner, by erecting a camera obscura in such a position that a
complete picture of the protected channels was projected on a fixed
white table. While the torpedoes were being placed in their positions an
observer was stationed at the table, who marked with a pencil the exact
spot at which each torpedo was sunk into the water. Further, those
engaged in placing the torpedoes caused a small boat to be rowed round
the spot where the torpedo had been placed, so as to describe a circle
the radius of which corresponded to the limit of the effective action of
the torpedo. The course of the boat was traced on the picture in the
camera, so that a very accurate representation of the positions of the
submarine mines in the channels was obtained. Each circle traced on the
table was marked by a number, and the wire in connection with the
corresponding torpedo was led into the camera, and marked with the same
number, so that the observer stationed in the camera could, when he saw
the image of an enemy’s ship enter one of the circles, close the
electric circuit of the corresponding wire, and thus instantly explode
the proper torpedo. The events of the war did not afford an opportunity
of testing practically the efficiency of these preparations.
Another mode of exploding torpedoes from the shore has been devised by
Abel and Maury. It has the advantage of being applicable by night as
well as by day. The principle will be easily understood with the
assistance of the diagram, Fig. 108, in which, for the sake of
simplicity, the positions of only three torpedoes, 1, 2, 3, are
represented.
[Illustration:
FIG. 108.—_Mode of Firing Torpedo._
]
In this arrangement two observers are required at different stations on
the shore. At each station—which should not, of course, be in any
conspicuous position—is a telescope, provided with a cross-wire, and
capable of turning horizontally about an upright axis. The telescope
carries round with it, over a circular table of non-conducting
substance, a metallic pointer which presses against narrow slips of
metal let into the circumference of the table. To each slip of metal a
wire passing to a torpedo is attached, and another wire is connected
with the axis of the pointer, so as to be put into electric contact with
each of the others when the pointer touches the corresponding piece of
metal on the rim of the table. The mode in which these wires are
connected with the torpedoes, the telescopes, and the electric apparatus
is shown by the lines in the diagram. At each station is a key, which
interrupts the electric circuit except when it is pressed down by the
operator. There are thus four different points at which contacts must be
simultaneously made before the circuit can be complete or a torpedo
explode. In the diagram three of these are represented as closed, and in
such a condition of affairs it only remains for the observer to depress
the handle of the key at station B to effect the explosion of torpedo
No. 2. The observer at station a is supposed to see the approaching
vessel in the line of torpedo No. 2, and recognizing this as an enemy’s
ship, he depresses the key at his station. The operator at B, by
following the course of the vessel with his telescope, will have brought
the pointer into contact with the wire leading to No. 2 torpedo, and he
then causes the explosion to take place by completing the circuit by
depressing his key. A modification of this plan is proposed by which the
position of the torpedoes is indicated by placing marks, such as
differently-coloured flags, or by night lamps with coloured glasses,
throwing their light only towards the telescopes. These marks are placed
in the line of direction of each torpedo from the telescope as at
_c_{1}_, _c_{2}_, _c_{3}_ and _b_{1}_, _b_{2 3}_; and if they can be put
at some distance, the position of the torpedo is determined with great
accuracy by the intersection of the lines of sight of the two
telescopes. Electric wires connect the stations and the torpedoes in the
same manner as we have before described. Such methods of firing
torpedoes are no doubt the most efficient, for the destructive charge
may be sunk so far below the surface that not a ripple or an eddy can
excite an enemy’s suspicion, or the channel appear otherwise than free
and unobstructed, while friendly ships may pass and repass without risk;
for the current which determines the explosion only passes when the two
sentinels complete the circuit by simultaneously depressing their keys.
Attempts have often been made to convert the torpedo into an offensive
weapon, by causing vessels containing explosive charges to drift by
currents, or otherwise, into contact with the enemy’s ships. The results
have been always unsatisfactory, as there is great uncertainty of the
machine coming into contact with its intended mark. Besides, it is easy
to defend vessels against such attacks by placing nets, &c., to
intercept the hostile visitors, especially if the attack is made by day,
and by night the chance that a torpedo drifting at random would strike
its object is very small indeed. One condition essential to the success
of such attacks is that the approach of the insidious antagonist may be
unobserved. Accordingly divers schemes have been projected for
propelling vessels wholly submerged beneath the surface of the water, so
that they may approach their object unperceived, and exert their
destructive effect precisely at that part of the vessel where damage is
most fatal, and where an ironclad vessel is most vulnerable, namely,
below the water-line. Vessels have been built, propelled by steam and so
contrived that their bodies are wholly submerged, only the funnel being
visible above the surface. These _quasi_ submarine ships carry small
crews, and are fitted with a long projecting spar in front, at the end
of which is carried the torpedo.
[Illustration:
FIG. 109.—_Explosion of Whitehead’s Torpedo._
]
[Illustration:
FIG. 110.—_Effect of the Explosion of Whitehead’s Torpedo._
]
The Federal navy sustained several disasters from torpedo-boats of this
kind. For example, the commander of the United States steamer
_Housatonic_ reported the loss of that vessel by a rebel torpedo off
Charleston on the evening of the 17th February, 1864, stating that about
8·45 p.m. the officer of the deck discovered something in the water
about 100 yards from, and moving towards, his ship. It had the
appearance of a plank moving in the water. It came directly towards the
ship, the time from when it was first seen till it was close alongside
being about two minutes; and hardly had it arrived close to the ship
before it exploded, and the ship began to sink. The torpedo-boat, with
its commander and crew, were lost, having, it is supposed, gone into the
hole made by the explosion, and sunk with the _Housatonic_. In general,
however, the performance of submarine boats has been unsatisfactory.
There is the difficulty of determining accurately the course of the
boat; there is great danger to the men manning it, as exemplified in the
case above; and there is again the problem of providing a means of
propulsion which shall enable such a boat to advance or retreat for,
say, a mile or more, without making its presence conspicuous by smoke or
otherwise. The latter condition would appear to exclude the use of steam
for such purposes, as the inevitable smoke and vapour would betray the
presence of the wily craft. Another power which has been proposed is air
strongly compressed, and recently a still more portable agent has been
suggested in solid carbonic acid, which is capable of exerting a
pressure of forty atmospheres by passing into the gaseous form. A
locomotive form of torpedo, invented by Mr. Whitehead, has the explosive
charge, which consists of about 18 lbs. of glyoxyline, placed in the
front part of a cigar-shaped vessel, the other part containing mechanism
for working a screw-propeller, by means of compressed air contained in a
suitable reservoir. This torpedo having been sunk a few feet below the
water, the motive power may be set in action by drawing a cord attached
to a detent, when the mechanical fish proceeds in a straight line under
the water. It is said that this torpedo is effective at 500 yards from
the ship attacked, and may even be made sufficiently powerful to travel
1,000 yards under the water. The great objection to such arrangements is
the uncertainty of the missile arriving at its destination, for even
supposing that the water were without currents, the least deviation from
the straight course would cause the torpedo to pass wide of the mark at
1,000 yards distant. It is said that at the experimental trials more
than one projector of such war engines has been startled by his machine,
after pursuing a circuitous submarine course, exploding in dangerous
proximity to the place whence it was sent off, the engineer narrowly
escaping being “hoist with his own petard.” The experiments which have
been made with Whitehead’s torpedo in smooth water appear, however, to
have been so far successful that we may probably hear of this invention
being put in practical operation in certain cases. Fig. 109 shows the
upthrow of water produced by the explosion of one of these torpedoes
against an old hulk. The large mass of water thus heaved up is a proof
of the mechanical energy of the explosion, and the effect on the hulk is
shown in Fig. 110, which exhibits the damage done to her timbers, from
the effects of which, it need hardly be said, she immediately sank. In
Fig. 112 we have the representation of the explosion of one of
Whitehead’s torpedoes containing 67 lbs. of gun-cotton, instead of the
glyoxyline. The accurate delineation of these pyramids of water could
not have been obtained but by the aid of instantaneous photography, and
it constitutes a good example of the great value of such an application
of that art, for the instantaneous photographs obtained in these
experiments enabled the engineers to calculate accurately the volume and
height of the column of water, which thus furnishes a measure of the
power of the explosion.
[Illustration:
FIG. 111.—_Experiment made by the Royal Engineers with a Torpedo
charged with 10 lbs. of Gun-Cotton._
]
The ordinary torpedo adopted by the British authorities for coast
defence consists of a cylinder of boiler plate, 4 ft. long and 3 ft. in
diameter. It is intended to contain 432 lbs. of loose gun-cotton,
equivalent in explosive energy to about a ton of gunpowder. The effect
of one of these torpedoes exploded 37 ft. beneath the surface of the
water is depicted in Fig. 113, and in Fig. 114 is shown the effect
produced when the same charge was exploded at the depth of 27 ft. below
the surface. Gun-cotton appears to be the most effective explosive for
torpedoes, if we may judge by the large volume of water heaved up, as
witness Fig. 111, which shows the result with a small torpedo,
containing only 10 lbs. of gun-cotton, exploded at a less depth than
those already mentioned. The ordinary torpedoes are moored by an anchor
attached to the torpedo, and floating above it is a buoy shaped like an
inverted cone. This cone contains a mechanical arrangement of such a
nature that when it is struck by a passing vessel, an electric circuit
is closed by bringing into contact two wires connecting the torpedo with
a voltaic battery on shore. While the apparatus may thus be at any
moment made fatal to a hostile vessel touching it, from the control it
is under by the engineer having the management of the battery contacts,
friendly vessels may pass over it with impunity.
[Illustration:
FIG. 112.—_Explosion of Whitehead’s Torpedo, containing 67 lbs. of
Gun-Cotton._
]
The employment of torpedoes develops, as a matter of course, a system of
defence against them. Nets spread across a channel will catch drifting
torpedoes, and stationary ones may be caused to explode harmlessly by
nets attached to spars pushed a great distance forward from the
advancing ship.
Before the final adoption of Whitehead’s torpedo, presently to be
described, the British Government had, after various official trials,
approved of a towing torpedo designed for offensive operations. It is
the invention of Commander Harvey, and is worthy of a detailed
description for the ingenuity of its construction.
[Illustration:
FIG. 113.—_Explosion of 432 lbs. of Gun-Cotton in 37 feet of Water._
]
[Illustration:
FIG. 114.—_Explosion of 432 lbs. of Gun-Cotton in 27 feet of Water._
]
The shape of Harvey’s torpedo, as may be noticed on reference to Fig.
118, is not symmetrical, but it has some remote resemblance to a boat,
though constructed with flat surfaces throughout. The outside case is
formed of wood well bound with iron, all the joints being made
thoroughly water-tight. The length is 5 ft. and the depth 1¾ ft., while
the breadth is only 6 in. Within this wooden case is another water-tight
case made of thick sheet copper, from the top of which two very short
wide tubes pass upwards to what we may term the deck of the wooden case.
These are the apertures through which the charge of gunpowder or other
explosive material is introduced; and when the tubes have been securely
stopped with corks, brass caps are screwed on. The centre of the
internal case is occupied by a copper tube, _g_, Fig. 115, which passes
the entire depth, and is soldered to the top and bottom of the copper
case, so that the interior of the tube has no communication with the
body of the torpedo, the principal charge merely surrounding it. Thus
the tube forms a small and quite independent chamber in the midst of the
large one, which latter is capable of containing 80 lbs. of gunpowder.
The copper tube or priming-case contains also a charge, _a_, which when
exploded bursts the tube, and thus fires the torpedo in its centre. The
priming charge is put in from the lower end of the tube, which is
afterwards closed by a cork and brass cap, _h_; for the centre of the
priming-case is occupied by a brass tube, _b_, closed at the bottom, but
having within a pointed steel pin projecting upwards. In this tube works
the exploding bolt _c d_, which requires a pressure of 30 or 40 lbs. to
force it down upon the steel pin. This pressure is communicated to the
bolt by the straight lever working in the slot at its head, _d_, and
itself acted on at its extremity by the curved lever to which it is
attached. Thus from the mechanical advantage at which the levers act a
moderate downward pressure suffices to force the exploding bolt to the
bottom of the brass tube. The lower end of this bolt has a cavity
containing an exploding composition sufficient in itself to fire the
torpedo, even independently of the priming charge contained in the
copper tube. This composition is safely retained in the end of the bolt
by a metallic capsule, _c_, which, when the bolt is forced down, is
pierced through by the steel pin at the bottom of the brass tube, and
then the explosion takes place. The bolts are not liable to explosion by
concussion or exposure to moderate heat, and they can be kept for an
indefinite period without deterioration.
[Illustration:
FIG. 115.—_Section of Priming-Case and Exploding Bolt._
]
The mode of producing the explosion is not stated: it consists probably
of an arrangement for bringing chemicals into contact. Besides the two
levers already mentioned, a shorter curved lever working horizontally
will be noticed. The object of this is to make a lateral pressure also
effective in forcing down the bolt—a result accomplished by attaching to
the short arm of the lever a greased cord, which, after passing
horizontally through a fairleader, runs through an eye (see Fig. 117) in
the straight lever, and has its extremity fastened so that a horizontal
movement of the short lever draws the other down. A very important part
of the apparatus is the safety key, _f_, Fig. 115, a wedge which passes
through a slot in the exploding bolt, and resting on the brass-work of
the priming-case, retains the muzzle 1 in. above the pin. Through the
eye of the safety key and round the bolts passes a piece of packthread,
_e_, which being knotted is strong enough to keep the key securely in
its place, but weak enough to yield when the strain is put on the line,
_d´_, used for withdrawing the safety key at the proper moment. This
line is attached to the eye of the key, and passes through one of the
handles forming the termination of the iron straps. As represented in
Fig. 117, it forms the centre one of the three coils of rope. The bottom
of the torpedo is ballasted with an iron plate, to which several
thicknesses of sheet lead can be screwed on as occasion requires. Fig.
117 shows the arrangement of the slings by which the torpedo is attached
to the tow-rope, and it will be seen that another rope passes backwards
through an eye in the stern to the spindle-shaped object behind the
torpedo. This is a buoy, of which two at least are always used, although
only one is represented in the figure. Each buoy, in length 4½ ft., is
made of solid layers of cork built up on an iron tube running through it
lengthways, so that the buoys admit of being strung upon the rope.
[Illustration:
FIG. 116.—_Harvey’s Torpedo._
]
Having thus described the construction of the torpedo, we proceed to
explain how it is used. It must be understood that if the torpedo and
its attached buoys are left stationary in the water, the tow-rope being
quite slack, the torpedo will, from its own weight, sink several feet
below the surface. But when they are _towed_, the strain upon the
tow-line brings the torpedo to the surface, to dip below it again as
often as the tow-line is slackened. There is another peculiarity in the
behaviour of the torpedo, and that is that, when towed, it does not
follow in the wake of the vessel, but diverges from the ship’s track to
the extent of 45°. Its shape and the mode in which it is attached to the
tow-line are designed so as to obtain this divergence. But, according as
the torpedo is required to diverge to the right or to the left, there
must be the corresponding shape and arrangement of tow-line and levers;
hence two forms of torpedo are required, the starboard and the port. The
figures represent the port torpedo, or that which is launched from the
left side of the torpedo-ship, and diverges to the left of its course.
The efficiency of the torpedo depends upon the readiness and certainty
with which it can be brought into contact with the hostile ship, and
this is accomplished by duly arranging the course of the torpedo vessel,
and by skilfully regulating the tow-line so as to obtain the requisite
amount of divergence, and to cause the torpedo to strike at the proper
depth. The tow-rope is wound on a reel, furnished with a powerful brake,
the action of which will be readily understood by inspection of Fig.
116, which represents also a similar smaller reel for the line attached
to the safety key. Leather straps, sprinkled with rosin to increase the
friction, encircle the drums of the reels, and can be made to embrace
them tightly by means of levers, so that the running out of the lines
can be checked as quickly as may be desired. Handles are attached to the
straps, so that they can be lifted off the drum when the line is being
drawn in by working the handles. When the torpedo is ready for action
and has been launched, a suitable length of tow-line, which is marked
with knots every ten fathoms, is allowed to run off its reel, while the
safety key-line is at the same time run off the small reel, care being
taken to avoid fouling or such strains on the line as would prematurely
withdraw the key. Fig. 106 will make clear the mode of controlling the
lines, but it is not intended to represent the actual disposition in
practice, where the men and the brakes would be placed under cover. On
the left of the figure a starboard torpedo is about to be launched; on
the right a port torpedo has been drawn under the ironclad and is in the
act of exploding, the safety key having been withdrawn by winding in its
line when the torpedo came into proximity to the attacked vessel.
[Illustration:
FIG. 117.—_Harvey’s Torpedo._
]
[Illustration:
FIG. 118.—_Harvey’s Torpedo._
]
[Illustration:
FIG. 119.—_Official Trial of “Harvey’s Sea Torpedo,” February, 1870._
]
When the torpedo has been launched over the vessel’s side, the latter
being in motion, the torpedo immediately diverges clear of the ship; and
when the buoys have also reached the water, the men working the reels
pay out the line steadily, occasionally checking the torpedo to keep it
near the surface, but avoiding a sudden strain upon the slacked
tow-rope, which would cause the torpedo to dive, and in shallow water
this might lead to the injury or loss of the torpedo. The torpedo can be
gradually veered out to the distance required, at the same time that the
safety-key is so managed that sufficient strain may be put upon it to
prevent it from forming a long bight astern of the torpedo, but avoiding
such a strain as would break the yarn holding the safety-key in its
place. The distance to which the tow-line may be paid will depend upon
the circumstances of the attack. More than 50 fathoms is, however, a
disadvantage, as the long bight of tow-lines makes the torpedo drag
astern. The torpedo can always be made to dive several feet below the
surface by suddenly letting out two or three fathoms of tow-line. The
torpedo vessel should, of course, be a steamer of considerable
speed—able to outstrip when necessary all her antagonists, and, as a
rule, it is found best to make the attack at night. Let us imagine two
ships of war at anchor, and parallel to each other at perhaps a distance
of 60 fathoms; and suppose that, under cover of darkness, a hostile
torpedo vessel boldly steams up between them, having launched both its
starboard and port torpedoes. In such a case neither ship could fire at
the torpedo vessel for fear of injuring the other, while the torpedo
vessel would in all probability succeed in bringing its floating mines
into contact with both its enemies.
[Illustration:
FIG. 120.—_Model of Submarine Guns._
]
Another device for submarine attacks upon vessels on which much
ingenuity has been expended is the submarine gun. It has been sought to
propel missiles beneath the surface of the water, these missiles being
usually provided with a charge which, on contact with the vessel’s side,
would explode, and by making a hole below the water-line, cause the
certain destruction of the ship. It is obvious that such a mode of
attack would reach the only vulnerable parts of a thickly-plated
ironclad, and therefore the project has been recently revived in several
forms. Fig. 120 is taken from the photograph of a model of an invention
of this kind. The guns which are to propel the submarine projectiles,
have port-holes formed by valves in such a manner that the gun when
loaded can be run out without allowing water to enter; it can then be
fired while the muzzle is below the surface, and again drawn in without
the port being at any time so opened that water can pour into the
vessel. All contrivances of this kind have hitherto been failures;
indeed, it does not appear possible that they could succeed, except at
very close quarters, for the resistance offered by water to a body
moving rapidly in it is extremely great, and, as we have already had
occasion to state, the resistance increases as the square of the
velocity, and probably in even a higher degree for very great
velocities. Any one who will remember the effort it requires to move
one’s hand quickly backwards and forwards through water will easily
understand that the resistance it presents would, in a comparatively
short space, check the speed of a projectile, however great that speed
might be at first. A good many years ago Mr. Warner produced a great
sensation by an invention which appears to have been essentially a
floating torpedo. The cut below, Fig. 121, represents the result of an
experiment publicly made by him off Brighton, in 1844, upon a barque,
which was towed out by a steamer to a distance of a mile and a half from
the shore. Mr. Warner was on board the steamer, and the barque was 300
yards astern. Five minutes after a signal had been made from the shore,
the torpedo was caused to explode, striking the barque amidships,
throwing up a large column of water and _débris_, shooting the mainmast
clean out of the vessel, the mizen going by the board, and dividing the
hull into two parts, so that she sank immediately. Yet this invention,
though apparently so successful, does not seem to have ever been put in
practice.
[Illustration:
FIG. 121.—_The Warner Experiment off Brighton._
]
The stationary torpedoes of the kind mostly used in the American Civil
War were, as already stated, _self-acting_; that is, they exploded when
touched by a passing vessel. They would now be more generally called
_self-acting mines_, and are to be distinguished from that form of the
weapon in which the explosion is determined by some manipulation on
shore, such as the closing of an electric circuit, when the hostile
vessel comes within the area of destructive action. This form receives
the name of _observation mines_. Stationary mines are essentially
instruments of defence, and as such are employed for the protection of
rivers and harbours. The self-acting varieties usually contain a charge
of 70 lbs. to 80 lbs. of gun-cotton, and are commonly arranged in lines.
Of course, when the occasion for which such mines have been laid down is
past, they must be removed, and the operation of picking them up is one
of great danger. The observation mines, on the other hand, do not
require immediate removal, and they can be taken up with little risk. In
the British service the observation mine contains about 500 lbs. of
gun-cotton, and a line of these is sometimes moored in a water-way, from
35 feet to 50 feet below the surface. The area of destructive action in
this case is a circle of about 30 feet radius, and therefore a line of
seven such mines laid across a channel at intervals of 120 feet apart
would ensure the almost certain destruction of any war vessel of
ordinary breadth that might attempt to pass up a river of 840 feet in
width. This is about the distance across the Thames near the Tower of
London, but the depth of the river there being only 12 feet at low-water
and 33 feet at high-water, would not suffice to give effect to the full
energy of so large a charge of gun-cotton; for it has been found that,
for a given charge, there is a certain depth under water at which its
explosion will produce the maximum effect, and this depth will be
greater with heavier charges than with light ones. The regulation
“observation mine” of the British service has a cylindrical case of
stout plate-iron, 32 inches in diameter and 34 inches high, with domed
ends. Within this gun-cotton is contained, in a wet condition, in a
number of copper envelopes, which have holes for access of water to wet
the charge from time to time as occasion requires, the wet condition
being the safest for the carriage of gun-cotton. The centre of the case
is a tin charged with some discs of dry gun-cotton, and the detonator
required to bring about the explosion of the whole charge when the
electrical contact is made, fires the fuze contained within the primer.
These cases are arranged to have a certain buoyancy, and are moored with
wire ropes to heavy iron sinkers, the mooring ropes being of such length
as to keep the explosive case at the proper depth below the surface, and
of sufficient strength to resist the force of the currents in the
waterway. There is also another type of submarine mine, operated by an
electric current, the circuit of which is closed by contact of a passing
vessel, if at the time a battery on shore is included in the circuit. In
this way the mines can be made harmless or dangerous for passing vessels
at the will of the operator on shore. The passing vessel is made to
complete the circuit by tilting over the cylindrical case so far that
some mercury contained in a small part of it is upset, and makes the
requisite metallic contact. This arrangement is known as the
_electro-contact_ mine.
In former pages of this article on torpedoes will be found
representations of the effects produced by _Whitehead’s torpedo_, which,
being automobile and travelling altogether under the surface of the
water, was capable of being made a very formidable weapon of offence.
When the earlier editions of this work were going through the press, it
was understood that the Whitehead torpedo left much to be desired as
regards speed, certainty of direction through the water, and perhaps in
other points, the inventor being constantly engaged in effecting
improvements. At that time particular pains were taken to keep secret
the nature of the most important parts of the internal mechanism. The
work of construction was carried on in a room with locked doors,
blocked-out windows, and a military guard outside. The earlier
experimental forms of this automobile torpedo were constructed in
complete secrecy by the inventor himself, with the help of only one
trusted, skilled mechanic and a boy, who was no other than Mr.
Whitehead’s own son. The history of the invention is very interesting,
and exemplifies the power of skill and perseverance to overcome a
multitude of difficulties, the result being a machine which is simply a
marvel of ingenuity and of delicate nicety of adaptation.
The first notion of the automobile torpedo appears to have occurred to
an Austrian naval officer; but it took rather the form of a small vessel
containing within itself some propelling power by which it could move
along the surface of the water, its course being directed by ropes or
guiding lines from the shore or from a ship. The fore part of the little
vessel was to hold an explosive, to be fired automatically by the
self-propelled torpedo coming into contact with the side of the hostile
vessel. The propelling power, as first suggested, was clockwork, if that
could be made efficient, or steam as an alternative. The Austrian
authorities, however, considered that it would be impracticable to
direct the course of the torpedo in the manner proposed, and that there
were also great objections to each of the methods of obtaining motive
power. The assistance of a thoroughly competent and skilful mechanician
was then sought, and Mr. Whitehead, at that time the director of an
engineering establishment at Fiume, devoted himself to solving the
problem of devising a torpedo which should be able to travel beneath the
surface of the water, and, when once started, should require no external
guidance to keep it on its proper course. After some years of
experimental labours, Mr. Whitehead produced the first form of the
weapon with which his name is associated, but to this he has since added
from time to time many ingenious improvements. A committee of experts
having been appointed by the Austrian Government to test the
capabilities of the new invention, it was made the subject of a long
series of trials, after which the committee recommended its immediate
adoption in the Austrian navy. The earlier form of the Whitehead torpedo
had, however, the defect already mentioned, of being sometimes very
erratic in its course; its speed was small (6 knots) compared with that
of the more recent patterns (30 knots), and its range of travel
proportionately less. The British Admiralty having invited Mr. Whitehead
to visit England with some specimens of his invention, a committee was
appointed to make complete trials of the capabilities of two weapons he
had brought with him. Although by this time great improvements had been
made on the original design, and in particular, Mr. Whitehead had almost
completely overcome the difficulty of keeping the torpedo at a uniform
depth during its course, by means of delicate adjustments in what we may
call the steering chamber (to be presently mentioned), much remained to
be accomplished before the weapon attained the perfection of the modern
patterns. Indeed, the inventor may be said to have from time to time
redesigned his contrivances, as when in 1876 the speed was increased to
18 knots, and again in 1884 more powerful engines brought up the speed
to 24 knots. Further improvements have been made by Mr. Whitehead, who
designed a new form of the weapon in 1889, and some of the more recent
patterns can now show a speed of 30 knots or more. The committee
appointed by the Admiralty to conduct experiments with the first pair of
torpedoes brought to England, after having tested them in various ways
for a period extending over six months, reported that they believed that
“any maritime nation failing to provide itself with submarine locomotive
torpedoes, would be neglecting a great source of power, both for offence
and defence.” Upon this recommendation the Admiralty immediately
purchased from Mr. Whitehead for £15,000 the secret of the internal
mechanism of his invention and the rights of manufacturing it. The
self-adjusting apparatus within the steering chamber, by means of which
the torpedo was kept at its due depth, was then a jealously-guarded
secret; but when the arrangement with Mr. Whitehead was effected, the
Government immediately set about the manufacture of these torpedoes on a
large scale. The artificers employed in making the Whitehead torpedoes
were now numerous, and the internal structure of these weapons could not
advantageously be altogether concealed from those who had to handle them
on board of the ships, so that it inevitably happened that some details
of their construction leaked out, and came into the possession of other
powers, whereupon all the maritime states followed the example of Great
Britain by providing their navies with Whitehead or some such form of
locomotive torpedo. It is no part of our plan to enter into the
mechanical _minutiæ_ of the Whitehead torpedo. We may, however, give the
reader such an idea of the external appearance and internal arrangement
of the Whitehead torpedo as will enable him to appreciate to some extent
the ingenuity and skill that have been brought to bear upon its
construction.
There are in existence many different patterns of the
weapon—twenty-four, it is said—and this is what might be expected from
the fact of its being produced at several different manufactories, each
striving to effect whatever improvements its resources will supply. Some
torpedoes have been made at Fiume, very many at Mr. Whitehead’s works at
Portland, as also at the Government establishment at Woolwich, while
private enterprise in this direction is encouraged by contracts with
some private firms, such as that of Messrs. Greenwood & Bately at Leeds.
The greatest diameter of the large torpedo is 18 inches, but in some it
is rather more, in others 14 inches or 16 inches; and the length may
vary between 14 feet and 19 feet. Many of our Whitehead torpedoes are
made of polished steel, but in the later patterns phosphor-bronze is
partly made use of, as being not liable to corrode. The interior of the
torpedo is divided by transverse partitions into five distinct
compartments. The foremost of these, called the “head,” contains the
explosive charge when the weapon is ready for use in actual warfare.
This section, which may occupy about one-sixth of the total length, is
an air-tight case made of phosphor-bronze, one-sixteenth of an inch
thick, and it is kept permanently charged with slabs of wet gun-cotton,
which may amount to 200 pounds weight in all, and is ready to be
attached by a screw and bayonet joint to the body of the torpedo; but
this is done only at the time immediately before it is required for its
destructive employment. Its place at other times, as when the torpedo is
used for drill practice, and to test its running powers, is occupied by
a dummy head of steel, of exactly the same shape and size, and packed
with wood in such a manner that its weight and centre of gravity are
like those of the explosive head when the latter is ready for action.
The wet gun-cotton requires a _detonative_ explosive of dry material
close to it, in order to determine its own detonation. The explosive
heads of the Whitehead are not fitted with the pistol and priming tube
until all is ready for the discharge of the weapon, as this would render
the handling of the torpedo highly dangerous. This priming apparatus is
merely a metallic tube that slips into a corresponding hollow in the
explosive head so far as to reach well within the wet gun-cotton charge,
although still separated from the latter by a metal casing. The
posterior extremity of the priming tube contains a few ounces of dry
gun-cotton, and just in front of this is a copper cap containing some
fulminate of mercury, which readily explodes when struck by the point of
a steel rod, occupying the centre of the tube and projecting a short
distance out at the “nose” of the torpedo, so as to be driven inwards by
the impact of the latter on a ship’s side. The explosion of the
fulminate causes the detonation of the dry gun-cotton at the bottom of
the priming tube, and this is taken up by the whole mass of the
explosive with destructive effect. The danger of premature or accidental
explosion by anything coming in contact with the projecting striker is
obviated by several checks which prevent any chance blow driving the rod
home against the fulminate charge. The anterior projecting end of the
rod has a screw thread worked upon it, and on this turns freely a nut
provided with wings like a small fan, revolving in such a manner that as
the torpedo is moved through the water, the nut is spun off, and the
striker is free to be driven back, except in so far as it is still
retained by a small copper pin, the breaking of which requires a
considerable blow. Again, the little fan above mentioned cannot begin to
spin off the rod until another pin or wedge has been withdrawn, which
operation is performed just before launching the weapon.
Immediately behind the exploding head of the torpedo is the air-chamber,
which occupies a considerable space in the length, _i.e._, about
one-third of the whole. This part is made of the toughest steel, nearly
⅓ of an inch thick, and contains the power actuating the motor, in the
form of air forced into it by powerful pumps on board the ship, until
the pressure reaches the enormous amount of 1,300 lbs. or more on the
square inch, or, at least, this is what is made use of in the newer
patterns when charged for action. In the largest size of the weapon the
weight of air injected may be more than 60 lbs., and, of course,
considerably detracts from the buoyancy of this part.
Behind the air-chamber comes another much shorter compartment we have
called the “steering chamber,” in which are contained the most ingenious
and delicate parts of the apparatus, namely, the mechanism by which this
extraordinary artificial fish adjusts itself, after the manner of a
living thing, to the required conditions. Among other contrivances, it
contains several valves controlling the action of the compressed air on
the engines, etc. The enormous pressure to which the air-chamber is
charged, if allowed to act unchecked, would give at first a power almost
sufficient to shatter the machinery, and, in order to prevent this, a
“reducing valve” is interposed so that only a moderate and uniform
pressure of air is allowed to act upon the engines. Then there is the
“starting valve” by which the air is admitted or cut off from the
engines, and still another valve which is contrived to delay the action
of the compressed air for the short interval during which the torpedo is
passing from the discharging tube until it enters the water. For during
this interval the propellers not having to act against the water, but
only against the resistance of the atmosphere, would be whirled round at
an enormous speed, and the machinery would sustain such shocks and
strains as might endanger the whole apparatus. It is to prevent this
that the “delay action valve” is provided.
The automatic apparatus by which the torpedo’s course is regulated is a
very remarkable part of the invention, and it admits of the nicest
adjustments. This was the crown of Mr. Whitehead’s ingenuity, but the
details were, by an arrangement between the government and the inventor,
not to be made public, though necessarily communicated to certain
officers in the service, and known to the chief artisans employed in
their fabrication. These persons are all, we believe, required to give
pledges not to divulge the arrangement of particular parts. But such
details could scarcely be made intelligible, even should they be
interesting, to the general reader. The principles upon which the
controlling apparatus are arranged may, however, be comprehended without
difficulty.
The tail of the torpedo is provided with two rudders, one in its central
vertical plane, and the other in its central horizontal plane. Their
action in directing the torpedo’s course is exactly that which the tail
supplies to a fish, or the rudder to a boat. Suppose that while the
torpedo is passing through the water the vertical rudder is by any means
turned towards one side, the course of the metallic fish will be
diverted towards that side; or again, a turning upwards of the
horizontal rudder would have the effect of directing the nose towards
the surface, and would make the torpedo rise, and so on. Now the
positions of the horizontal rudder are regulated from the “steering
chamber,” in which a heavy weight is suspended like a pendulum, so as to
be capable of swinging fore and aft. This pendulous weight actuates the
horizontal rudder through a system of rods and levers, so that when it
hangs vertically the horizontal rudder is level, but if from any cause
the nose of the torpedo were directed downwards, the pendulous weight
would come to a more forward position in the steering chamber, and would
raise the rudder, and thus turn the nose towards the surface until the
original horizontal position were regained. In the contrary case, of
course, the reverse action would take place. But the torpedo, while
preserving a horizontal position, might tend to sink to too great a
depth, or rise too near the surface, and this is prevented by another
adjustment, namely, a piston receiving the pressure of the water, which,
on the other side, is opposed by a spring. If the torpedo sinks a little
the pressure increases, the piston, which moves with perfect freedom
without allowing water to pass in, is forced inwards, and its movement
is communicated to the same levers that connect the pendulous weight
with the horizontal rudder, the latter is raised, and then the nose of
the torpedo is directed upwards, and it consequently approaches the
surface again. In the contrary case the spring, relieved from some of
the external pressure, operates the levers in the other direction.
The compartment immediately behind the “steering chamber” contains the
engines which are of the Brotherhood type, provided with _three_ single
acting cylinders. The three-fold throw prevents any possibility of the
engine getting on a “dead point.” Though this compartment is the
shortest in the torpedo, the engines in the larger sizes are capable of
indicating as much as thirty horse power. It has for simplicity been
stated above that the pendulous weight and the balanced piston act by
means of rods on the horizontal rudder; this was so in the early
patterns of the torpedo, but it was soon found that they did not do so
with sufficient steadiness and promptitude, and the force they could
apply was in the larger and swifter forms quite ineffective. Nowadays
the engine compartment always contains a little piece of apparatus which
is an arrangement of cylinder and piston, upon which the compressed air
acts in one or the other direction according to the way its slide-valve
is moved. It is this slide-valve that the rods from the “steering
chamber” move, and allow the force of the compressed air to turn the
rudder up or down. This auxiliary apparatus has the same relation to the
torpedo rudder that the steam-steering apparatus of a large vessel has
to its rudder. Although it is only about a few inches long, its power
and delicacy are such that the pressure of half an ounce on its slide
admits to its piston a force equal to 160 lbs., and its introduction has
given the torpedo the power of steadily steering itself.
Behind the engine compartment, but completely shut off from it, is
another almost empty division occupying a considerable part of the
length of the torpedo, and known as the “buoyancy chamber.” But it
contains, attached to the bottom of it, a certain amount of ballasting,
so adjusted to balance the weights of the other parts that the whole
floats horizontally, and at the same time preserving the tube in one
vertical position as regards its transverse diameter, _i.e._, so that
the horizontal rudder is always horizontal. The shaft from the engine
passes through this compartment, as also the rod from the small motor
that moves the horizontal rudder. These, of course, pass through
water-tight bearings.
At the tail of the torpedo, behind the rudders, are _two_ three-bladed
screw propellers, of which the anterior one is mounted on a tubular
shaft having a common axis with the other, but made to revolve in the
opposite direction by means of a bevel wheel mounted on each independent
shaft, with a third such wheel connecting them. The object of the double
screw is to obviate “slip,” that is, ineffective motion of the blades
through the water, and by this means the full power of the engines can
be developed; while any tendency to _deviation_ to right or left, due to
the rotation, is reduced to a minimum. We have spoken of one horizontal
and one vertical rudder, although externally there appear to be two of
each kind, right and left, above and below, on the tail of the torpedo.
These pairs, however, are so connected as to be always in the same
respective planes. The controlling mechanism acting in two different
ways on the horizontal rudder has been already indicated, but nothing
has yet been said about the vertical rudder. It is not moveable by
anything within the torpedo, but is commonly fixed by clamping screws in
or about the same vertical plane as the axis of the torpedo, and it
performs the same function as a kind of back fin, which, in the earlier
forms, extended nearly the whole length of the tube; and that is
obviating any tendency of the torpedo to roll about its axis. The
vertical rudder can also be fixed at a considerable inclination to the
axis should occasion require, and the effect of that would be to cause
the torpedo to pursue a circular course of greater or less radius,
according to the less or greater degree of inclination. Very rarely,
however, would this be required, and the vertical rudder may be
considered as fixed in the axial plane, or having such slight
inclination as may, on trial, have been found necessary to counteract
any tendency to lateral deviation.
There are several different methods for discharging the Whitehead
torpedoes from ships. They may be sent from a tube below the water-line,
but the arrangements for that purpose are complicated and difficult to
manage, while, on the other hand, the launch of the weapon is not
perceived by the enemy, and it is at the same time out of the reach of
any blow from a hostile missile while yet in its discharging tube. More
commonly the discharging tube is arranged above the water-level. On
regular torpedo boats, the tubes are sometimes mounted on pairs upon a
revolving table, provided with many nice adjustments, and even the
single above-water torpedo tube, as used between decks, is an apparatus
having somewhat complicated appliances. The torpedo is expelled from the
tube now preferably by a small charge of _cordite_. But in the Royal
Navy no fewer than some twenty different patterns of torpedo tubes have
been in use for the various sizes of torpedoes. In some of these,
compressed air, in others gunpowder or _cordite_, in others, again,
mechanical impulse propels the torpedo into its element. It would
obviously be impossible within our limits to enter into details of these
various constructions, or to attempt descriptions of _all_ the ingenious
contrivances applied to the torpedo itself, or to give an account of the
means of defence against mines and torpedoes, this last being a matter
belonging to naval tactics. The adoption of the torpedo as a naval
weapon has given rise to special types of boats adapted for its
employment, and these again have required other boats to destroy them
(“torpedo-boat destroyers” or “catchers”). Light draught and high speed
were desired in these last; but in many cases the intended speed was
inferior to that of the torpedo boats that were to be caught.
The following particulars about the British torpedo-boat destroyer
_Daring_ may be compared with those given of the cruiser _Majestic_. The
_Daring_ is 185 feet long, 7 broad, and she draws only 7 feet of water.
Her speed is about 28½ knots per hour, with a steam pressure in the
boilers of 200 lbs. per square inch, and an air pressure in the
stoke-holds equivalent to 3 inches of water (forced draught.)
The importance attached to the prospective use in war of the automobile
torpedo may be shown by the fact that at the end of 1890 the number of
torpedo boats built or laid down for England was 206, and for France
210, while other nations followed with numbers proportionate to their
means. Forty “torpedo-boat destroyers” were in building for the British
Navy towards the close of the year 1896, and now (March, 1897) it is
announced that the number of torpedo boats and torpedo-boat destroyers
in the French Navy is to be increased by 175.
[Illustration]
[Illustration:
FIG. 122.—_M. Ferdinand de Lesseps._
]
SHIP CANALS.
Artificial canals are amongst the oldest of inventions, for, centuries
ago, they have been constructed, even of very large dimensions, in
various parts of the world. There is in China, for instance, a great
canal, 900 miles in length and 200 feet broad, which is supposed to have
been made 800 years ago. The advantages of canals did not escape the
attention of the Egyptians, Greeks and Romans. We read of very early
attempts to cut through isthmuses, in order to form a water
communication between regions where other carriage would be long and
difficult. It appears to be admitted that canals connecting the Red Sea
with the Mediterranean existed some centuries before the Christian era,
and to cut the Isthmus of Corinth by a waterway was a cherished project
with several Roman Emperors, and now it appears that in this nineteenth
century this project will shortly be realized. But as the canal-lock is
but a comparatively modern invention, dating only from the fourteenth
century, and first used in Holland, all the canals anterior to that
period had to be designed as level cuts, a restriction which greatly
increased the difficulties of the problem. Canals were in use in various
parts of Europe, particularly in Holland and France, long before any
were constructed in England, as, for example, the Languedoc Canal,
which, by a cut of 150 miles, connects the Bay of Biscay with the
Mediterranean. It is 60 feet broad, and attains, at its highest level,
an elevation of 600 feet above the sea. The canal system in England was
first introduced in the middle of the eighteenth century, and soon
afterwards, the Duke of Bridgewater engaged the famous Brindley to
construct a canal, connecting his collieries at Worsley with Manchester,
about seven miles distant, and afterwards extended his scheme, so as to
open up a more direct water communication between Manchester and
Liverpool. Before the making of this canal, the cost of the carriage of
goods between these towns had been forty shillings per ton by land, and
twelve shillings by water. After that, they were conveyed with
regularity for six shillings per ton. The system was soon extended, so
as to connect the Trent with the Mersey, and the boldness of both the
projectors and their engineer in carrying out this scheme is memorable
in the history of such undertakings. Brindley was equal to the task of
coping with the difficulty of carrying his canal over what had hitherto
been supposed an insuperable obstacle, for he pierced Harecastle Hill
with a tunnel more than a mile and a half in length—a then unheard of
piece of engineering—to say nothing of several shorter tunnels, many
aqueducts, and scores of locks. The Duke of Bridgewater, who at one
period had been unable to raise £500 on his own bond for the prosecution
of his scheme, died in 1803, in receipt of a princely income from the
profits of his useful undertaking. For its creation, he had, however,
denied himself the present enjoyments of his patrimonial revenue, by
reducing his expenses at one period to the modest sum of £400 per annum.
Before his death, the Duke, for taxation purposes, estimated his income
at £110,000 per annum. Before the railway system was fully established a
network of canals had united the most populous places in England, the
total length of the waterways being not much less than two thousand
miles. With the rise of railways the importance of canals as channels
for the conveyance of merchandise declined. But, nevertheless, in
consequence of the continued increase of traffic and the great cheapness
with which goods can be carried by water, canals are often able to
compete with railways in the carriage of bulky or heavy goods when speed
of transit is not an object. The English canals have, therefore, never
been disused or abandoned, notwithstanding the ubiquitous ramifications
of the railway lines. Nay, the value of the Bridgewater Canal system,
about to be superseded so far as concerns the communication between
Liverpool and Manchester by the greater scheme we have presently to
describe, is such that £1,710,000 is now required for its purchase; and
that is the value in spite of four lines of railway connecting those
great towns, and all competing for the carriage of goods. In these
canals, designed for inland communication only, the navigation is
confined to boats or barges of very insignificant dimensions compared
with the sea-going ships that some great modern canals are constructed
to receive.
To the present century belongs the famous “Caledonian Canal,” as the
waterway is often called that extends in a straight line for more than
60 miles across Scotland, in north-east and south-west directions. The
canal work here was commenced in 1802, under the direction of Telford,
and though it was opened for traffic in 1822, the work as it now exists
was not completed until 1847. But the length of the actual canal
construction in this case did not much exceed 23 miles, for a natural
waterway, navigable for ships of any burden, is formed by the series of
narrow lakes that fill what is called the “Great Glen of the Highlands.”
This glen has many of the characteristics of a great artificial ditch:
its highest point is only 90 feet above the tide level in Loch Linnhe; a
circumstance not a little remarkable in so mountainous a country. What
is also remarkable is the great depth of these lakes, which in some
places exceeds 900 feet. The banks also are generally very steep, and
indeed at one time it was impracticable to pass along the shores of Loch
Ness, the longest of the lakes. But there are now good roads along both
banks. Although the ground traversed by the artificial channels of the
Caledonian Canal is chiefly alluvial, the cost of the undertaking proved
to be great, amounting, it is said, to about one and a quarter million
pounds sterling. Indeed, had it not been for the introduction of steam
navigation before the completion of the work, and the consequent
increase and facility of water conveyance, it is doubtful whether the
utility of this canal would have been commensurate with its cost, or its
receipts have made any profit for its promoters. By the Caledonian Canal
large steamers and other vessels may pass from sea to sea, and in the
summer time the steamers that traverse it are crowded with tourists
attracted by the magnificent scenery it presents throughout the greater
part of its length.
But whatever had previously been done in canal construction was
surpassed in enterprise and importance by Lesseps’ great work in Egypt.
_THE SUEZ CANAL._
As we have already seen, the idea of opening a waterway between the Red
Sea and the Mediterranean is by no means a product of the present
century. The ancient Egyptians do not appear to have cut directly
through the Isthmus, but Herodotus describes a canal made by Necho about
the year 600 B.C., from Suez through the Bitter Lakes to Lake Timsah and
then westward to Bubastis on the Nile. He mentions certain water gates,
and states that vessels took four days in sailing through. This canal
became silted up with sand ages ago, but it was cleared out again and
re-opened in the seventh century of our era by the Caliph Omar, and
traces of it are still visible. According to some recent discoveries in
the chief archives of Venice, as early as the end of the fifteenth
century, when Vasco da Gama had discovered the Cape of Good Hope, and
the Portuguese took that new route to India, hitherto the exclusive
property of the Venetian and Genoese merchants, a re-cutting of the
Isthmus of Suez was thought of. Plans were prepared and embassies sent
to Egypt for paving the way for the accomplishment of this great
enterprise, which, it is said, was only foiled by the persistent
opposition of some patricians, who were probably bribed by foreign gold
to prevent the execution of the plan. One of our Elizabethan poets,
Christopher Marlowe, appears, in the following lines, to have
anticipated M. de Lesseps:—
“Thence marched I into Egypt and Arabia,
And here, not far from Alexandria,
Whereat the Terrene and the Red Sea meet,
Being distant less than full a hundred leagues.
I meant to cut a channel to them both,
That men might quickly sail to India.”
For at that period travellers going to India in the famous sailing
ships, called “East Indiamen,” were obliged to sail round the Cape of
Good Hope and pass from the Southern to the Indian Ocean. The reader who
wishes to understand the importance of the Suez Canal should look at the
map of the Eastern Hemisphere, where he will have no difficulty in
finding the position of the vast continent of Africa, which is washed on
the north by the Mediterranean Sea, on the west by the Atlantic, on the
south by the Southern Ocean, and on the east and north-east by the
Indian Ocean and the Red Sea. If he now traces the waterway round
Africa, on coming to the head of the Red Sea he will find the only
interruption of the oceanic continuity in the narrow neck of land called
the Isthmus of Suez. But for this, ships might long ago have made
complete circuits round this vast, and, even as yet, but partially
explored continent. The circuit would, indeed, be a great one of some
15,000 miles; but the barrier that the Isthmus presented to
inter-oceanic communication between the eastern and the western worlds
was a piece of physical geography which has undoubtedly been a most
important factor in determining the course of history. It has been said
that had there existed at Suez a strait like that of Gibraltar or that
of Messina, instead of a sandy isthmus, the achievements of Diaz, Vasco
da Gama, and Columbus would have lost much of their significance; but
the advantages to the world’s commerce would have been incalculable, and
the progress of the race might have been more rapid.
The Emperor Napoleon I. had the idea of restoring the old canal; but it
was only when steam navigation had taken its place on the seas that the
scheme was looked upon as offering any chance of financial success. But
General Chesney, who made some surveys for the French Government in
1830, had come to the conclusion that there was a considerable
difference of level between the two seas—a difference, he calculated, of
about 30 feet. The existence of such a state of things would, of course,
have been very unfavourable for the undertaking; but the General’s
supposition was soon proved to have been erroneous.
The suggestion of carrying out the project of constructing a ship canal
through the Isthmus was seriously revived by Père Enfantin, the St.
Simonian, in the year 1833. He then induced M. Ferdinand Lesseps, the
French vice-consul, and Mehemet Ali, the Pasha of Egypt, to take some
practical measures towards its accomplishment. Surveys were made, but
owing to the breaking out of a plague, and to other causes, not much
more was heard of the scheme till 1845. In 1846 _La Société d’Etude du
Canal de Suez_ was formed, and among those who turned their attention to
the subject was Robert Stephenson. His report was wholly unfavourable to
the enterprise. He recommended the construction of a railway through
Egypt, and a line was accordingly made between Alexandria and Suez. But,
notwithstanding the opinion of Mr. Stephenson, M. Lesseps persevered
with wonderful energy, believing, on the report of other engineers, that
the scheme could be successfully carried out. It is right, however, to
state that Mr. Stephenson did not say it was impossible to complete the
Suez Canal—he merely gave it as his opinion that the cost of making the
canal, and keeping it in a proper state for navigation, would be so
great that the scheme would not pay. However, in 1854, the Viceroy of
Egypt signed the concession, and in 1860 the work was actually
commenced, but not on a plan that was advocated by the English engineers
of making the canal 25 feet above the sea level. There were also some
political and financial difficulties to be overcome. The Suez Canal
Company, it was said, had expended twelve millions of money in what was
considered to be chiefly shifting sands.
[Illustration:
FIG. 123.—_The Sand-Glass._
]
When the Suez Canal was projected, many prophesied evil to the
undertaking, from the sand of the Desert being drifted by the wind into
the canal, and others were apprehensive that where the canal was cut
through the sand, the bottom would be pushed up by the pressure of the
banks. They imagined that the sand would behave exactly like the ooze of
a soft peat-bog, through which, when a trench has been cut, the bottom
of the trench soon rises, for the soft matter has virtually the
properties of a liquid: it acts, in fact, exactly like very thick
treacle. Sand, however, is not possessed of liquid properties; it has a
definite angle of repose, which is not the case with thin bog. This
behaviour of sand is familiarly illustrated in the sand-glass, which the
diagram Fig. 123, will recall to mind. It may be observed that the sand
falling in a slender stream from the upper compartment is in the lower
one heaped up in a little mound, the sides of which preserve a nearly
constant inclination of about 30°. In this property it is distinctly
different from peat-bog or such-like material, which has no definite
angle of repose. It need hardly be said that all apprehensions as to the
safety of the canal from the causes here alluded to have proved
unfounded.
But if some English engineers appeared to oppose the project, another
eminent one, Mr. Hawkshaw, certainly helped it on at a moment when the
Viceroy of Egypt was losing confidence; and, had his opinion been
adverse to the project reported upon, the Viceroy would certainly not
have taken upon himself additional liability in connection with the
undertaking, and the money expended up to that date would have been
represented only by some huge mounds of sand and many shiploads of
artificial stone, thrown into the bottom of the sea to make the harbour
of Port Saïd. And that M. Lesseps appreciated the good offices of Mr.
Hawkshaw is shown from the fact that, when he introduced that engineer
to various distinguished persons, on the occasion of the opening of the
canal, he said, “This is the gentleman to whom I owe the canal.” It
cannot, therefore, be said of the English nation that they were jealous
of the peaceful work of their French neighbours, or opposed it in any
other sense but as a “non-paying” and apparently unprofitable scheme.
The Canal was opened in great state by Napoleon III.’s Empress Eugénie,
in November, 1869, when a fleet of fifty vessels passed through, and the
fact was thus officially announced in Paris:—“The canal has been
traversed from end to end without hindrance, and the Imperial yacht,
_Aigle_, after a splendid passage, now lies at her moorings in the Red
Sea.
“Thus are realized the hopes which were entertained of this great
undertaking—the joining of the two seas.
“The Government of the Emperor cannot but look with satisfaction upon
the success of an enterprise which it has never ceased to encourage. A
work like this, successfully accomplished in the face of so many
obstacles, does honour to the energetic initiative of the French mind,
and is a testimony to the progress of modern science.”
An Imperial decree was then issued, dated the 19th of November,
appointing M. de Lesseps to the rank of Grand Cross of the Legion of
Honour, in consideration of his services in piercing the Isthmus of
Suez.
The Suez Canal is 88 geographical, or about 100 statute miles long: its
average width is 25 yards, and the minimum depth, 26 feet. At intervals
of five or six miles, the canal is widened, for a short space, to 50
yards, forming thus sidings (_gares_) where only vessels can pass each
other. At these, therefore, a ship has often to wait until a file of
perhaps twenty steamers, coming the other way, has passed. Occasionally
a ship gets across, or “touches,” and then the canal is blocked for
hours. So much inconvenience has been found from the restricted
dimensions of the work, that in 1886 it was proposed to widen the canal,
or, alternatively, to construct a second canal, and use the two like the
lines of a railway, so that vessels would never have occasion to pass
each other. The amount of traffic is very large, and has been steadily
increasing. Thus, in 1874, the tonnage of the vessels passing through
was 5,794,400 tons; in 1880, the tonnage was 8,183,313, and the receipts
of the Company amounted to £2,309,218. In 1875, the British Government
purchased, from the Khedive, £4,000,000 worth of shares.
[Illustration:
FIG. 124.—_A Group of Egyptian Fellahs, and their Wives._
]
The Suez Canal is not so much a triumph of engineering as a monument of
successful enterprise and determination on the part of its great
promoter, M. Lesseps, in the face of great difficulties. According to
the original programme, the canal was to have been constructed by forced
labour, supplied by the Viceroy. The unhappy peasantry of the country,
called “fellahs,” were compelled to give their labour for a miserable
pittance of rice. No doubt, in ancient times, when forced labour was in
use, every peasant might cheerfully work, because it was for the general
benefit to bring sweet water from the Nile to other dry and thirsty
places in Egypt; but to be obliged to work at a waterway of salt, which
was only to be of use to foreigners who passed through the country,
could not be expected of human beings, and therefore the carrying out of
the work was not unaccompanied by cruelties of the nature attending
slave labour in other lands. This was one of the reasons why the late
Lord Palmerston opposed the canal scheme, for the kind hearted statesman
bore in mind the loss of health and life occasioned to poor Egyptians by
this mode of labour, and the more so because it had been originally
proposed that one of the conditions on which the French Company was to
take up the project should be the execution of the work by _free
labour_. In consequence, no doubt, of representations from free
countries, the Porte was induced to put a veto on the employment of
forced labour, and everyone thought that this would be the deathblow to
the completion of the canal: but M. Lesseps did not give way to despair,
and he since stated that if he had depended on the labours of the
fellahs only, the difficulties of the work never could have been
surmounted; and that, in fact, the successful prosecution of the work
was owing to his having turned his attention to the mechanical
contrivances used for dredging on the Thames and the Clyde, from which
he obtained better results in half the time and at half the cost.
[Illustration:
FIG. 125.—_Dredges and Elevators at Work._
]
[Illustration:
FIG. 126.—_Map of the Suez Canal._
]
The dredges used in the construction of the canal were of a new
description. They were wonderful mechanical contrivances, and but for
them the canal would not have been finished. They were not the
contrivance of M. Lesseps, but of one of the contractors, a
distinguished engineer, who received his technical education in France
but his practical experience in England. The use of the dredging
machines was prepared for by digging out a rough trough by spade work,
and as soon as it had been dug to the depth of from six feet to twelve
feet, the water was let in. After the water had been let in, the steam
dredges were floated down the stream, moored along the bank, and set to
work. The dredges were of two kinds. The great _couloirs_ consisted of a
long, broad, flat bottomed barge, on which stood a huge framework of
wood, supporting an endless chain of heavy iron buckets. The chain was
turned by steam, and the height of the axle was shifted from time to
time, so that the empty buckets, as they revolved round and round,
should always strike the bottom of the canal at a fixed angle. As they
were dragged over the soil they scooped up a quantity of mud and sand
and water, and as each bucket reached its highest point in the round, it
discharged its contents into a long iron pipe which ran out at right
angles to the barge. The further extremity of this pipe stretched for
some yards beyond the bank of the canal, and therefore, when the
dredging was going on, there was a constant stream of liquid mud pouring
from the pipe’s mouth upon the shore, and thus raising the height of the
embankment. When the hollow scooped out by the buckets had reached the
required depth, the dredge was moved to another place, and the same
process was repeated over and over again. These stationary dredges,
however, though very effective, required much time in moving, and the
lighter work of the canal was chiefly effected by movable dredges of a
smaller size. These machines were of the same construction as those
described; the only difference was that the mud raised by their agency
was not poured directly on shore by pipes attached to the dredges, but
was emptied in the first instance into large barges moored alongside the
dredge. These barges were divided into compartments, each of which
contained a railway truck, and when the barge was filled it steered away
to the bank, where an elevator was fixed. The trucks, filled with mud
were raised by a crane worked by steam power, and placed upon inclined
rails, attached to the elevator, which sloped upwards at an angle of 45
degrees towards the bank. They were then drawn up the rails by an
endless rope, and as each truck reached the end of the rails its side
fell open, the mud was shot out upon the bank, and the empty truck
returned by another set of rails to the platform on which the elevator
was placed, and was thence lowered into the barge to which it belonged.
As the elevator could unload and re-load a barge much faster than the
dredges could fill it with mud, each elevator was fed by half a dozen
dredges, and thus the mud raised from the canal by several dredges was
carted away without difficulty at one and the same time. As these
floating dredges were much easier to shift than those encumbered by the
long _couloir_ pipes, the work of excavating the bed went on much more
rapidly. But in places where there was any great mass of earth or sand
to be removed, the large _couloirs_ could scoop out a given volume in a
shorter time.
The traveller who wishes to see the canal should go to France, and,
embarking at the port of Marseilles, cross the Mediterranean Sea, and
steam to Port Saïd, which is about 150 miles east of the port of
Alexandria, where the isthmus is crossed by the railroad, and is used by
travellers to India, being known as the “overland route.” And this
railway conveys the mail to and from India, thus saving the great sea
voyage round Africa and the Cape of Good Hope. Nevertheless, it involves
two transhipments—from the steamer to the rail at Alexandria, and from
the railway to the steamer at Suez.
[Illustration:
FIG. 127.—_Port Saïd, the Mediterranean entrance to the Suez Canal._
]
Let us notice in order the places passed by the traveller in going from
Port Saïd to Suez and the Red Sea. The arrow (Fig. 126) points in the
direction of the compass, and shows that the canal runs very nearly from
north to south. Port Saïd is the little town at the northern or
Mediterranean entrance to the canal, situated on the flat sands at the
entrance of the canal, and is built chiefly of wood, with straight wide
streets and houses, and although it now contains several thousand
inhabitants, before the making of the canal was begun one hundred people
could hardly have been got together. The town contains nothing deserving
of notice, and has a striking resemblance to the newly settled cities of
America. But in it reside agents who represent numerous varied
interests—administrative, financial, mercantile and political. It is
provided with docks, basins, quays and warehouses, and has a harbour
stretching out a couple of miles or so into the sea, for to that
distance two piers, or rather breakwaters, run out.
Fig. 128 shows these two converging breakwaters, which have been built
out into the Mediterranean from the coast, the larger and more westerly
one being one mile and a half long, the shorter about a mile and a
quarter, and the distances between the two lighthouses erected on the
extremities of the breakwater being half a mile.
The piers are made of concrete which was cast in blocks weighing 10 tons
each. This composition has of late years been greatly approved by
engineers where stone cannot be procured. The sea-face of the great
canal in Holland is composed of a similar artificial stone, and it is
found to bear the wear and tear of the waves almost, if not quite, as
well as ordinary stone. It is stated that 25,000 blocks, each weighing
10 tons, were used. They were not laid with the regularity of ordinary
masonry, but had been dropped from large barges, so that they presented
a very rugged and uneven appearance (Fig. 129); but the object of
throwing out these great bulwarks is for the purpose of preventing the
sand brought down by the Nile silting in and closing up the canal. Along
the western pier there is, from this cause, a constant settlement of
sand, which was partially washed through the interstices left between
the blocks of artificial stone, and might have given some trouble by
forming sandbanks in the harbour; but this was prevented by the
introduction of smaller stones, which could readily be carried out in
boats at the low tide.
[Illustration:
FIG. 128.—_Bird’s-eye View of Port Saïd._
]
Beginning with the Mediterranean Sea and Port Saïd, there is a run of 28
miles to Kantara, through Lake Menzaleh. Although called a lake, it is,
in truth, nothing but a shallow lagoon or swamp, in which water-fowl of
all kinds are very abundant, the great flocks of white pelicans and pink
flamingoes being especially striking. The waters of this lagoon cover
lands that once were fertile, and the salt sea-sands doubtless conceal
the remains of many an ancient town.
[Illustration:
FIG. 129.—_One of the Breakwaters at Port Saïd._
]
Of all portions of the undertaking, this one, M. Lesseps states, was the
most arduous and difficult, though, at the time, it attracted the least
attention. A trough had to be dredged out of the bed of the shallow
lagoon, and on either side of this hollowed out space high sandbanks had
to be erected, and the difficulty of making a solid foundation for these
sand banks was found to be extreme. The difficulty, however, was
surmounted, and such is the excellence of the work, that the water
neither leaks out, nor does any of the brackish water of the lagoon
infiltrate and undermine the great embankments.
[Illustration:
FIG. 130.—_Lake Timsah and Ismaïlia._
]
At Kantara, the canal crosses the track of the highway between Cairo and
Syria—a floating bridge carries the caravans across; and near this spot
is stationed an Egyptian man-of-war, which supplies the police for the
proper watch and ward of the canal. From Kantara to El Fendane is a
distance of 15 miles—that is to say, to the southern extremity of Lake
Ballah, where the canal still passes through sand embankments, raised
within a mere. The lake is, however, almost dried up, and therefore the
difficulties which had to be surmounted at Lake Menzaleh were not felt
here.
The traveller may now be supposed to have arrived at Lake Timsah, where,
no doubt, in the days of the Pharaohs, a lake existed. When taken in
hand by M. Lesseps, it was a barren, sandy hollow, containing a few
shallow pools, through which a man could easily wade, but now it is
filled with the waters of the Mediterranean Sea. It is a pretty, inland,
salt water lake, about three miles in width. On the northern shore
stands the town, or, rather, small settlement of Ismaïlia, which is, in
fact, the “half way house” where most of the officials of the Suez Canal
Company resided, as they could get to either end of the canal with
greater facility, or to Cairo by the railroad, which comes to this
point, and continues, with the canal, to Suez.
[Illustration:
FIG. 131.—_Railway Station at Ismaïlia._
]
When the canal was opened, in November, 1869, Ismaïlia was the scene of
the most brilliant part of the opening ceremony, in which the French
Empress Eugénie, the Empress of Austria, the Crown Prince of Prussia,
and other distinguished personages took share. The Khedive built himself
a summer palace, and M. Lesseps erected a villa, and the town was most
artistically laid out, with every prospect of becoming a flourishing
place. But the drainage had been so entirely overlooked, that it is said
the sewage found its only outlet in the fresh water canal; and the
consequence was fever broke out and so infected the town, that it was
soon almost quite deserted. In 1882, Ismaïlia was once more the scene of
bustle and activity, for here was the base of Sir Garnet Wolseley’s
operations in his brilliant campaign against Arabi. The British Navy
entered the canal, and took possession of Ismaïlia, where the army and
the military stores were rapidly concentrated. From this place, Sir
Garnet advanced along the route of the railway and the Sweet Water
Canal, and, after storming the lines of Tel-el-Kebir, occupied Cairo,
without further resistance, after a campaign of only three weeks’
duration.
From Lake Timsah to the Bitter Lakes the canal again passes for eight
miles or so through the desert, where, by partial excavations by hand
labour and subsequent flooding to admit the dredges, it was considered
that a sufficiently deep channel could be made. The _couloirs_ were set
to work, when suddenly “a lion arose in their path” in the shape of a
great rock, about 80 feet in length, and lying 12 feet only below the
surface, and right in the middle of the main channel. If anything could
show the indomitable energy of M. Lesseps it was his courage in dealing
with this difficulty, and at a time when a few months only could elapse
before the advertised day of the opening. He attacked the sunken rock
with gunpowder. A large raft, or floor, supported on barges, was moored
over the sunken rock, and from this men, armed with long poles shod with
steel, drilled numerous holes, into which charges of gunpowder were
placed, and fired in the usual manner by the electric battery. This
temporary obstruction occurred opposite to the landing place at
Sérápeum.
[Illustration:
FIG. 132.—_The Viceroy of Egypt cutting the last embankment of the
Reservoir of the Plain of Suez, to unite the two seas—the
Mediterranean and the Red Sea._
]
Passing by Sérápeum, the traveller arrives at a vast expanse of water
called the “Bitter Lakes,” because the dry sandy hollow formerly
contained a marsh, or mere, of very brackish water. The possibility of
keeping this great area filled with sea water had been denied by the
opponents of the canal, who said the water would sink into the sand or
be evaporated by the intense heat of the sun; but none of these
prognostications have been verified, and it is now a great inland sea,
far surpassing Lake Timsah, being 25 miles long and from six to seven
miles wide. The only difficulty in filling this enormous natural basin
arose from the rapidity and force with which the waters flowed in. This
was done when the water at Suez was at low tide, and then subsequently
the Red Sea was allowed to flow in. Though the expanse of water in the
Bitter Lakes is great enough, the available channel is still narrow. But
the steamers can proceed at full speed, as here there are no banks to be
washed away.
Since the two seas have joined their waters, a strong current has set in
from south to north, but there is no eddy or fall at the place where the
waters meet. The tide runs up the canal with great force, and there is a
difference of six or seven feet between high and low water: but the tide
does not extend beyond the Bitter Lakes, where it is gradually diffused
and lost. The colour of the current of water from Suez is said to be
green, whilst that portion fed by the Mediterranean is blue. Since the
Bitter Lakes have been filled the mean temperature of the districts on
the banks has fallen 5° Centigrade. It is also stated that, although the
canal swarms with sea fish they keep to their respective ends of the
canal, as if the Mediterranean fish would not consort with those of the
Red Sea, or, rather, make themselves at home in strange waters. There is
also, perhaps, another cause, and that is the very bitter nature of the
water at the northern end of the Bitter Lakes, which acts as a natural
barrier, through which the fish may decline to pass.
The bed of the Bitter Lakes is the only portion of the canal’s course in
which it was not necessary to make a cutting. Buoys are laid down to
mark the best channel, but such is the width and depth of the water that
vessels need not exactly keep within them. Quitting the Bitter Lakes we
again enter the canal proper. In order to reach the vast docks which the
Suez Canal Company has constructed on the western coast of the Red Sea,
the canal is now quitted, and the vessel crosses the neck of the Red
Sea. The Cairo and Alexandria Railway has been extended two miles, and
is carried through the sea on an embankment, which lands the train close
to the docks and quays of the canal, so that passengers by the overland
route are able to embark from the train on board the steamer, and thus
escape the troublesome transhipment of themselves and luggage.
_THE MANCHESTER SHIP CANAL._
The project of constructing a ship canal to connect Manchester with the
sea appears to have been started just before the railway era, but it was
then abandoned, as the opening of the Liverpool and Manchester Canal
brought about an immediate reduction in the rates of carriage. Perhaps
it was the success of the Suez Canal which caused the revival of this
scheme, in 1880, combined with the depression of the cotton trade at
that period, when the Liverpool dock dues and the comparatively high
railway rates proved a heavier tax than usual on the great Lancashire
industry. The first definite steps were taken two years afterwards, when
two plans were submitted for the selection of a committee. One scheme
proposed to construct the canal without any locks; but, as Manchester is
60 feet above the sea level, there would, it was felt, be certain
inconveniences in loading or unloading ships in a deep depression. The
other plan was submitted by Mr. Leader Williams, a well known canal
engineer, who proposed to take the canal from Runcorn, a distance of 20
miles, and making use of locks. When Parliament was applied to for
powers authorizing the prosecution of the enterprise, there was, of
course, much opposition offered by the various interests involved, and
the inquires before the Committees of each House of Parliament were
unusually protracted, for they extended in all to 175 days, and the cost
to the promoters is said to have amounted to £150,000. Then, when the
Bill had passed, it was found that the capital (£8,000,000) could not be
raised owing to the financial depression, and partly also to some want
of confidence in the soundness of the undertaking on the part of the
Lancashire capitalists. But the promoters submitted the whole scheme to
a representative committee, who should consider any possible objections.
This committee reported (after sitting almost daily for five weeks) upon
every point, and were unanimous in pronouncing the undertaking to be
perfectly practicable and commercially sound. After this there was no
difficulty in raising the required capital, which was subscribed by
corporate bodies as well as private persons. The contract was let for
£5,750,000, and the work was commenced in November, 1887, the contractor
undertaking to have the canal completed and ready for traffic by January
1st, 1892.
[Illustration:
FIG. 133.—_Western Portion._
]
[Illustration:
FIG. 134.—_Eastern Portion._
FIGS. 133 AND 134.—_Map of the Manchester Ship Canal._
]
The Manchester Docks of this canal will cover an area of nearly 200
acres at the south-western suburb of that city, and from there the canal
traverses the Valley of the Irwell, following, indeed, the general
course of the river, but not its windings, so that the bed of the river
is, in the distance of eight miles, or down to its junction with the
Mersey, repeatedly crossed by the line of the canal. From the confluence
of the rivers, the canal traverses the Valley of the Mersey, for this is
the name retained by the combined streams. The course of the river, in
its progress towards the sea, now makes wider bends, but the canal
proceeds, by a slight and nearly uniform curve, to Latchford, near
Warrington, passing to the south of which last named place it follows a
straight line to Runcorn, which is at a distance of 23 miles from
Manchester. Here it reaches what is now the estuary of the Mersey, but
the embankments are continued along the southern shore to Eastham, where
the terminal locks are placed. In this part of the canal, the engineer
had difficulties to overcome of a different nature from those
encountered in the upper part, where it was chiefly a matter of cutting
across the ground intervening between the bends of the river, so as to
form for its waters a new and direct channel everywhere of the requisite
breadth and depth. But when Runcorn has been passed, and Weston Point
rounded, there is the mouth of the River Weaver to be crossed, and this
is marked by a great expanse of loose and shifting mud. Other affluents
of the Mersey are dealt with by means of sluices, and in one instance
the waters of a river are actually carried beneath the course of the
canal by conduits of 12 feet in diameter. The total length of the canal
from Manchester to the tidal locks at Eastham is 35 miles.
[Illustration:
FIG. 135.—_A Cutting for the Manchester Ship Canal._
]
[Illustration:
FIG. 136.—_Blasting Rocks for the Manchester Ship Canal._
]
The minimum width of the canal at the bottom is 120 feet, its depth 26
feet. But for several miles below Manchester this width will be
increased, so that ships may be moored along the sides, and yet
sufficient space left for the up and down lines of traffic in the
middle. In this way, works and manufactories on the banks will be able
to load and unload their cargoes at their own doors, and it may be
expected that the advantages so offered will cause the banks of the
canal to be much in request for the sites of works of all kinds. At the
several places where the locks are placed there will be a smaller and a
larger one, side by side, so that water shall not be needlessly used in
passing a moderate sized vessel through the greater locks. As these last
are 550 feet long and 60 feet wide, they are capable of receiving the
largest ships, whilst the smaller locks are 300 feet long and 40 feet
wide. Again, both the larger and the smaller are provided with gates in
the middle, so that only half their length may be used when that is
found sufficient. Coming down the canal from Manchester, the first set
of locks will be at Barton, about three miles distance, just below the
place where the Bridgewater Canal is carried across the Irwell, which is
now to become the ship canal, by means of the aqueduct of 1760, by which
Brindley became so famous. There is a story told about Brindley being
desirous of satisfying the duke about the practicability of his plan,
and requesting the confirmatory opinion of another engineer. When,
however, this gentleman was taken to the place where it was proposed to
construct the aqueduct, he shook his head, and said that he had often
heard of castles in the air, but had never before been shown where any
of them were to be erected. This aqueduct is about 600 feet long, and
the central one of its three arches spans the river at a height of
nearly 40 feet above the water. But the Manchester Ship Canal requires a
clear headway of 75 feet, and Mr. Williams is going to replace the fixed
stone structure by a swinging aqueduct, or trough of iron, which can be
turned round, so as to give a clear passage for ships in his canal. This
trough, or great iron box, will have gates at each end, and gates will
be provided in the aqueduct at each side, so that no water will be lost
when the water bridge is turned aside. But more than this; hydraulic
lifts have been designed, so that, in a few minutes, vessels can be
lowered from the Bridgewater Canal into the Manchester Canal, or raised
from the latter into the former while still floating in water. The
supply of water for the canal will be ample, as it has the rivers
Irwell, Mersey and Bollin, with their tributary streams, to draw from.
It should be mentioned that the terminal locks at Eastham will be of
somewhat larger dimensions than those already referred to, and will be
three in number. The largest, which is on the south or landward side,
will be 600 feet by 80 feet, the middle one 350 feet by 50 feet, and the
smallest one 150 feet by 30 feet. These three locks will be separated by
concrete piers 30 feet wide, on which will be placed the hydraulic
machinery for opening and closing the gates. Besides the ordinary gates,
there will be provided for each lock at Eastham an outer pair of
storm-gates that will be closed only in rough weather. These gates will
shut from the outside against the lock sills, and, by resisting the
force of wind and waves, will protect the ordinary tidal gates from
being forced open. The lock gates throughout will be made of a wood
obtained from British Guiana, and known as _greenheart_. This timber is
the product of a large tree (_Nectandra Rodiœi_) belonging to the laurel
family. It is a very heavy and close grained wood, the strength and
endurance of which have been proved many years ago by its use in
ship-building, etc., and some of the logs imported for the canal are
remarkably fine specimens, being 22 inches square and 60 feet long. A
pair of the largest gates weigh about 500 tons. The gates of the tidal
locks at Eastham will all be open for half the time of each tide, when
there will be a depth of water, above the sills, greater by 11 feet than
that of any dock in Liverpool or Birkenhead.
[Illustration:
FIG. 137.—_Manchester Ship Canal Works, Runcorn._
]
The way in which the difficulty is overcome of crossing the several busy
lines of railway that intersect the course of the new canal, so that
their traffic shall not be impeded, is one of special interest in this
bold scheme. The London and North Western Railway crosses the Mersey at
Runcorn by a bridge that leaves a clear headway of 75 feet at high
water, and it was determined that this headway should be maintained in
the bridges over the canal. The use of swing bridges on lines of railway
over which trains are constantly passing being out of the question, it
is necessary that the railways be carried over the canal at the required
height. It is accordingly laid down in the Act of Parliament that before
the Canal Company can cut the existing lines of railway it shall
construct permanent bridges, and carry over them lines rising by
gradients not exceeding 1 in 135, and not only so, but these deviation
lines must be previously given up to the several railway companies for
six months to be tried experimentally in that period for goods traffic.
The cost of constructing these deviation lines, which, in all, will not
be far short of 12 miles of new railway, will not be much less than
£500,000. The traffic of the canal will probably have great feeders at
certain points in the other canals and the railway lines that reach it.
For instance, the Bridgewater Canal, now incorporated with the greater
undertaking, will bring traffic from the Staffordshire potteries, the
river Weaver brings salt laden barges from Cheshire, and at other points
the railways will bring the produce of the excellent coal fields of
South Yorkshire and South Lancashire, which will be automatically
transferred from the waggons into ocean going steamships.
[Illustration:
FIG. 137_a._—_The French Steam Navvy._
]
[Illustration:
FIG. 137_b_.—_The English Steam Navvy._
]
Though the general notion of the construction of the canal as a deep,
wide trench, or cutting following the course shown on the map, is
sufficiently simple, the operation of carrying this into practice
involves the exercise of great skill and ingenuity in dealing with
mechanical obstacles. Man’s operations in the world consist but in
changing the position of masses of matter; and the properties of
matter—its inertia, cohesion, gravitation, etc., are the forces that
oppose his efforts. The quantity of matter to be shifted in excavating
this trench of thirty-five miles long across the country was no less
than sixty millions of tons. The number of “navvies” employed at one
time has been 15,000; but even this army of workmen would have made but
slow progress with a cutting of this magnitude, had not the “strong
shouldered steam” been also called into operation for scooping out the
soil. The illustrations (Figs. 137_a_ and #137_b_:fig137b) will show the
arrangement of two forms of “steam navvies” that were much used on the
works. One (Fig. 137_a_) is similar to the dredgers used for clearing
mud out of rivers and canals: it consists of a series of scoops, or
buckets, mounted on an endless chain, so as to scrape the material from
an inclined embankment and tip it into waggons for removal. The other
(Fig. 137_b_) may be compared to a gigantic ladle made to scrape against
the face of a cutting in rising, and filling each time its bucket with
nearly a ton of the material. It is most interesting to witness the
perfect control which the man at the levers exercises over this machine,
the movements of which he directs with as much precision as if he were
handling a spoon. One of these steam navvies is able to fill 600 waggons
or more—that is, to remove 3,000 tons of material—in one day; and as
many as eighty of them have been simultaneously used on the Canal works.
The value of the plant employed by the contractor is estimated at
£700,000, and the length of temporary railway lines (see Fig. 137), for
transport of the “spoil,” etc., is said to exceed 200 miles. There is a
main line running through from one end of the canal to the other, and
known to the workmen as the “Overland Route.” From this diverge numerous
branches, some to the bottom of the excavations in progress, others to
embankments down which is tipped out the “spoil,” as the dug out
material is called; while others connecting with brickfields and
quarries, or with existing canals and railway lines, serve to bring
supplies of the materials used in the constructions. Some 150
locomotives are constantly at work on these temporary lines, and the
coal consumed by them, and by the steam navvies, steam cranes, pumping
engines, etc., is equivalent to about two train loads every day.
Though the Manchester Ship Canal is to be nearly twice as wide as the
Suez Canal, its width for some miles below Manchester will be still
greater, for there the banks will form long continuous wharves for the
accommodation of the works and factories that are certain to be
attracted to the spot. Indeed, so obvious are the advantages of ocean
shipment, and so extensive the industries of South Lancashire, that it
is not improbable the whole course of the canal may, in process of time,
be lined with wharves, and the two great cities of Manchester and
Liverpool may be united by a continuous track of dense population. Be
that as it may, there seems every reason to believe that the undertaking
will be a financial success. Calculation has shown that if the cotton
alone that enters and leaves Manchester were carried by the canal at
half the rates charged by the railways, there would result not only an
annual saving of £456,000 to the cotton trade, but a clear profit to the
canal company sufficient to pay more than 3 per cent. interest on its
own capital. And, again, the railway and other local interests that have
hitherto been opposed to this great enterprise can hardly fail to be in
the long run benefited by the enlarged prosperity and increased general
trade and manufactures it will develop. So that it will presently be
found that there is room enough and work enough for both canal and
railways.
The Manchester Ship Canal, so far from having been ready for traffic on
the 1st January, 1892, was not completed until the end of 1893, and it
was only on the 16th December, 1893, that the directors and their
friends made the trial trip throughout its entire length, accomplishing
the distance of 35½ miles in 5½ hours. The total cost of the canal was
greatly in excess of the estimates, which placed it at eight million
pounds, as fifteen millions is the sum actually expended upon it. With
such a vast capital expenditure, it may be some time before the ordinary
shareholders can look for dividends, especially as there has not been
any sudden rush of traffic, such as many sanguine people expected. On
the other hand, traffic is continuously and steadily increasing, and
there is reason to believe that this great work will ultimately prove a
commercial, as it has an engineering, success.
[Illustration:
FIG. 137_c._—_Sketch Map of The North Sea Canal._
]
_THE NORTH SEA CANAL._
Like several other canals for sea going ships this last addition to the
achievements of modern engineering is but the realisation of a project
conceived at a long past period. The idea of a canal to connect the
Baltic and the North Sea dates back into the Middle Ages, and indeed a
short canal was constructed in 1389, which by uniting two secondary
streams of the peninsula really did provide a waterway between the two
seas. The inefficiency of this means of communication may be inferred
from the fact of there having been proposed since that period no fewer
than sixteen schemes of canalisation between these two seas, of which
the recently completed North Sea Canal is the sixteenth, and it need
hardly be said the greatest, so that in comparison with it the rest
vanish into insignificance. The canal was commenced in 1887, and on the
20th of June, 1895, it was opened by the reigning Emperor of Germany,
William II., with a very imposing naval pageant in which nearly a
hundred ships of war from the great navies of the world took part. A
glance at the accompanying sketch-map will show the great importance of
this canal as a highway of commerce. The entrance to the Baltic has
hitherto been round the peninsula of Denmark and through the narrow
“belts” and “sounds” that divide the Danish Islands, a course beset with
imminent perils to navigators, for the channels abound in rocks and
dangerous reefs, to say nothing about the frequent storms and the
impediments of ice floes. Yet as many as 35,000 vessels have lately had
to take that course annually, these representing a total tonnage of no
less than 20,000,000 tons. The figures speak for the magnitude of the
Baltic shipping intercourse with the rest of the world; while the losses
incurred in traversing these forbidding waters may be gathered from the
statement that since 1858, nearly 3000 ships have been wrecked in them,
and a greater number much damaged. Indeed, for large vessels, there is
hardly a more dangerous piece of navigation in all Europe. The
importance of this canal must not therefore be estimated solely by the
saving of length in ships’ course, though that is great, as the map
shows.
The North Sea Canal is 61 miles long, 200 ft. wide at the surface, 85
ft. wide at the bottom, and it will admit of vessels of 10,000 tons
register passing through, the average time of transit being about twelve
hours. The estimated cost of this undertaking was nearly eight and a
quarter million pounds sterling, and about one-third of this sum was
contributed by Germany, for whom the canal is of the greatest strategic
importance in case of war, for her fighting ships need not then traverse
foreign waters. The construction was therefore pushed forward with
unusual energy, as many as 8,600 men having been engaged on the works at
one time. An important naval station already exists at Kiel, the Baltic
end of the canal, where there is a splendid harbour. The engineer and
designer of this water-way is Herr Otto Baensch, who has devised much
ingenious machinery in connection with the immense tidal locks at the
extremities of the canal, and the swing bridges by which several lines
of railway are carried across it. In the construction of this canal
there were no vast engineering difficulties to be overcome, and hence
striking feats of mountain excavation or valley bridging are not to be
met with in its course, though in places there are some deep cuttings.
The methods of excavating and of steam dredging that were made use of
have already been illustrated in relation to the other works described
in this article. The country through which the canal passes does not
present any unusually picturesque features.
_THE PANAMA AND NICARAGUA CANAL PROJECTS._
The several undertakings described in our chapter on Ship Canals are now
all completed and in active operation, and but for financial
mis-management and dishonest speculations, the same might probably have
been said of another great project, the name of which was on everyone’s
lips a short time ago, but in which public interest has lately waned;
perhaps from a mistaken impression that the construction itself is
involved in a common ruin with the fortunes of so many of its promoters,
or that the scheme was frustrated by some unforeseen and insurmountable
engineering difficulties. These assumptions have so little justification
that it is quite probable that Lesseps’ last great project may yet be
completed under more favourable auspices, and the Panama Canal unite the
Atlantic and Pacific Oceans. The Panama Canal Company still exists, and
possesses not only a very large part of the work almost quite finished,
but all the extensive plant in perfect condition for resuming
operations. The original scheme provided for a tidal water-way between
the two oceans, without the intervention of a single lock. The canal was
to be nearly 47 miles in length, 100 feet wide at the surface of the
water, 72 feet wide at the bottom, and 29 feet deep. The entrances are
at Colon on the Atlantic side, and at Panama on the Pacific. The latter
is the eastern extremity, and the western one is on the Atlantic side,
owing to the configuration of the isthmus which curves round the Panama
Gulf that opens to the south. A railway crosses the isthmus between the
points already named, and the route of the canal is laid down almost
parallel with this railway, from which it is nowhere far distant. For
the first 20 miles from the Atlantic side the land is only at a very
moderate elevation above the sea-level, say 25 or 30 feet, but the next
11 miles is more hilly, the elevations reaching at some points 150 to
170 feet, but these are only for short distances. A few miles farther
on, they rise still higher, until at Culebra the highest point is met
with, about 323 feet above the sea-level, and a cut of this depth, 1,000
feet long, would be required. Through this highest part it has been
proposed to drive a tunnel, but the total extent of the deep cutting at
this part of the canal would be nearly 2 miles in length. This would no
doubt be a work of the most formidable magnitude, for it has been
calculated that no less than 24,000,000 cubic yards of material,
consisting for the most part of solid rock, would have to be removed. It
is not supposed, however, to offer any great difficulty in an
engineering point of view. Doubtless it would be costly, and would take
some time to accomplish. Another heavy piece of work would consist in
constructions for controlling a mountain torrent called the Rio Chagres,
through the valley of which the canal passes. This stream is very
variable in the quantity of water it discharges, rising in the rainy
season 45 feet above its ordinary level, and sending down forty times as
much water as it does in the dry season.
Mr. Saabye, an American engineer, who examined unofficially the works of
the Panama Canal in 1894, considers that about one half of the total
excavation has already been done, and one half of the total length of
the canal almost finished, and remaining in comparatively good
condition. At both ends, including 15 miles on the Atlantic side, there
is water 18 to 24 feet deep. “Besides the work already done, the Canal
Company has on hand, distributed at both terminals, and at convenient
points along the canal route, an immense stock of machinery, tools,
dredges, barges, steamers, tug-boats, and materials for continued
construction. At Panama, La Boca, and Colon, as well as along the canal,
are numerous buildings—large and small—for offices, workshops,
storehouses, and warehouses, and for lodging and boarding the men who
were employed on the work. The finished work, as well as all the
machinery, tools, materials, buildings, etc., are well taken care of and
looked after. The Canal Company employs one hundred uniformed policemen,
besides numerous watchmen, machinists, and others, whose sole duty
consists in watching the canal and looking after needed repairs of plant
and care of materials. In fact, the work and the whole plant is in such
a condition, so far as I could ascertain, that renewed construction
could be taken up and carried to a finish at any time it is desired to
do so, after the Company’s finances will permit.”
An enormous amount of money has already been expended on the Panama
Canal, and much of it lavishly and unnecessarily. A reorganised company
may probably be able to form such estimates of the probable cost of
completing the work under careful and efficient management, that
financial confidence in it maybe restored. The canal not only already
possesses the requisite plant, but the route has the special advantages
of assistance in transport from the railway everywhere at but a short
distance from it, and fine commodious harbours for its ocean mouths. If
it were finished as originally designed, vessels could pass through it
with one tide, say in about six hours. It is understood that before the
Panama enterprise is again proceeded with, the Company think that a sum
of about £25,000 should be expended in a complete survey and re-study of
all the conditions, and the results submitted to the most eminent
engineers.
A rival scheme for carrying a ship canal across the isthmus that divides
the Atlantic and Pacific Oceans is that known as the Nicaragua Canal, as
the proposed route is to cross Lake Nicaragua, an extensive sheet of
water situated some 400 or 500 miles north-west of the Panama Canal. The
lake is 110 miles long and 45 miles broad, and is on its western side
separated from the Pacific by a strip of land only 12 miles wide, having
at one point an elevation not exceeding 154 feet, which is probably the
lowest on the isthmus. The lake drains into the Caribbean Sea on the
east, by the San Juan river, a fine wide stream, 120 miles in length,
which is navigable for river boats from the Caribbean Sea up to the
lake, except near its upper part, where some rapids at certain times
prevent the passage of the boats. This canal project first took definite
form in 1850, when a survey was made and routes reported on. The scheme
attracted some attention in the United States, and in 1872, and again in
1885, further surveys and estimates were made at the instance of the
States Government. The earlier schemes provided for the rise and fall
between sea and lake-–108 feet, a considerable number of locks—eleven on
each side, making the total length from sea to sea 181 miles. The report
of the latter advocated the canalization of the San Juan by a very bold
measure, namely, the construction of an immense dam, by which the waters
were to be retained in the valley for many miles at the level of the
lake. A company was formed to promote the project, and again in 1890
there were more surveys and estimates made. This company actually
expended a considerable sum of money in attempting to improve the
harbour at Greytown, which would have formed the eastern terminus, but
had become silted up. But it was found afterwards that it would be
better to recommend the formation of an artificial harbour at another
point, by constructing two long piers running out into the sea, although
this change would involve the abandonment of a few hundred yards of
canal already excavated by the company near Greytown. The company has
also laid down about 12 miles of railway along the proposed route, with
wooden and iron sheds as workshops, offices, etc., and, moreover, had
dredges and other appliances at work. At this stage it was proposed that
the United States Government should guarantee the bonds of the Nicaragua
Canal Company to the extent of more than twenty million pounds sterling.
By an Act of Congress passed in March, 1895, a commission of engineers
was appointed for the purpose of ascertaining the feasibility,
permanence, and cost of construction and completion of the Nicaragua
Canal by the route contemplated. The report of this commission is an
elaborate and exhaustive review of the whole scheme based upon a
personal examination of the route, and on the plans, surveys, and
estimates made for the company, whose records, however, are stated in
the report to be deficient in the supply of many important data. The
Canal Company’s project provided for the improvement of Greytown
harbour, as already stated, and from that place the canal was to proceed
westward at the sea-level to the range of high ground on the eastern
side of the isthmus, which elevation was to be ascended by three locks
of unusual depth, and a deep cut more than 3 miles in length, through
rock to a maximum depth of 324 feet. After passing this enormous cut,
the route provides for a series of deep basins, in which the water is
confined by numerous dams or embankments, the canal excavations being
confined to short sections through higher ground separating these
basins. The total length of these embankments will be about 6 miles, and
their heights will vary from a few feet to more than seventy. About 31
miles from Greytown the canal reaches the San Juan river, which,
however, by means of an enormous dam across the valley at a place called
Ochoa, 69 miles below the point at which it receives the waters of Lake
Nicaragua, is there practically converted into an arm of the lake. This
dam, which would raise the water of the river 60 feet above its present
level, and would, of course, flood the valley back to the lake, is the
most notable feature of the project. Its maximum height would be about
105 feet, and the weirs on its crest, to discharge the surplus water,
would require a total length of nearly a quarter of a mile. Twenty-three
smaller embankments would also be needed for retaining the waters; the
river would have to be deepened in the upper part, and a channel dredged
out in the soft mud of the lake for 14 miles beyond the river. The big
Ochoa dam is said to have no precedent in engineering construction, on
account of its great height and the enormous volume of the waters it is
intended to retain. No doubt its construction and safe maintenance are
within the range of engineering skill, when a thoroughly exhaustive
survey of the site has been made, and the necessary funds are
forthcoming. From the western shore of the lake its level would also be
extended by another great dam crossing the valleys of the Tola and the
Rio Grande, with a length of 2,000 feet and a height of 90 feet. The
canal would then be carried to the sea-level by a series of locks. The
length of the canal from sea to sea would be 170 miles, but of this only
40 miles of channel would require to be excavated. The total cost of the
work, as estimated by the Nicaragua Canal Company, would be about
fifteen million pounds sterling, but the State Commission of Engineers
thinks about double that amount would be a safer calculation, and taking
into account the imperfection of the data, even this might be exceeded
in certain contingencies. The Government of the United States has been
urged to expend a few thousand pounds on another engineering commission,
to make complete surveys, and consider all the practical problems
involved, including the final selection of a route.
[Illustration:
FIG. 138.—_Britannia Bridge, Menai Straits._
]
IRON BRIDGES.
The credit of having invented the arch is almost universally assigned to
the ancient Romans, though the period of its introduction and the date
of its first application to bridge building are unknown. That some
centuries before the Christian era, the timber bridges of Rome had not
been superseded by those of more permanent construction is implied in
the legend of the defence of the gate by Horatius Cocles—a tale which
has stirred the heart of many a schoolboy, and is known to everybody by
Macaulay’s spirited verses, in which
“Still is the story told,
How well Horatius kept the bridge,
In the brave days of old.”
Some of the arched bridges built by the Romans remain in use to this day
to attest the skill of their architects. The Ponte Molo at Rome, for
example, was erected 100 B.C.; and at various places in Italy and Spain
many of the ancient arches still exist, as at Narni, where an arch of
150 ft. span yet remains entire. Until the close of the last century the
stone or brick arch was the only mode of constructing substantial and
permanent bridges. And in the present century many fine bridges have
been built with stone arches. The London and Waterloo Bridges across the
Thames are well-known instances, each having several arches of wide
span, attaining in the respective cases 152 ft. and 120 ft. The widest
arch in England, and one probably unsurpassed anywhere in its
magnificent stride of 200 ft., is the bridge across the Dee at Chester,
built by Harrisson in 1820. At the end of last century _cast_ iron began
to be used for the construction of bridges, a notable example being the
bridge over the Wear at Sunderland, of which the span is 240 ft. But
with the subsequent introduction of _wrought_ iron into bridge building
a new era commenced, and some of the great results obtained by the use
of this material will be described in the present article. In order that
the reader may understand how the properties of wrought iron have been
taken advantage of in the construction of bridges, a few words of
explanation will be necessary regarding the strains to which the
materials of such structures are exposed.
Such strains may be first mentioned as act most directly on the
materials of any structure or machine, and these are two in number,
namely, extension and compression. When a rope is used to suspend a
weight, the force exerted by the latter tends to stretch the rope, and
if the weight be made sufficiently great, the rope will break by being
pulled asunder. The weight which just suffices to do this is the measure
of the _tenacity_ of the rope. Again, when a brick supports a weight
laid upon it, the force tends to compress the parts of the brick or to
push them closer together, and if the force were great enough, the brick
would yield to it by being crushed. Now, a brick offers so great a
resistance to a crushing pressure, that a single ordinary red brick may
be capable of supporting a weight of 18 tons, or 40,320 lbs.—that is,
about 1,000 lbs. on each square inch of its surface. Thus the bricks at
the base of a tall factory chimney are in no danger of being crushed by
the superincumbent weight, although that is often very great. The
_tenacity_ of the brick, however, presents the greatest possible
contrast to its strength in resisting pressure, for it would give way to
a pull of only a few pounds. Cast iron resembles a brick to a certain
extent in opposing great resistance to being crushed compared to that
which it offers to being pulled asunder, while wrought iron far excels
the cast metal in tenacity, but is inferior to it in resistance to
compression.
The following table expresses the forces in tons which must be applied
for each square inch in the section of the metals, in order that they
may be torn apart or crushed:
┌───────────────────────┬──────────────────────┬──────────────────────┐
│ │ Tenacity per square │Crushing pressure per │
│ │ inch, in tons. │square inch, in tons. │
├───────────────────────┼──────────────────────┼──────────────────────┤
│Cast iron │ 8 │ 50 │
│Wrought iron │ 30 │ 17 │
│Iron wire │ 40 │ ... │
└───────────────────────┴──────────────────────┴──────────────────────┘
Besides the direct strains which tend to simply elongate or compress the
materials of a structure or of a machine, there are modes of applying
forces which give rise to transverse strains, tending to twist or wrench
the pieces or to bend them, or rupture them by causing one part of a
solid to slide away from the rest. Strains of this kind no doubt come
into play in certain subordinate parts of bridges of any kind; but if we
divide bridges according to the nature of the strains to which the
essential parts of the structure are subject, we may place in a class
where the materials are exposed to crushing forces only, all bridges
formed with stone and brick arches; and in a second class, where the
material is subjected to extension only, we can range all suspension
bridges; while the third class is made up of bridges in which the
material has to resist both compression and extension. This last
includes all the various forms of girder bridges, whether trussed,
lattice, or tubular. The only remark that need be here made on arched
bridges is, that when cast iron was applied to the construction of
bridges, the chief strength of the material lying in its resistance to
pressure, the principle of construction adopted was mainly the same as
that which governs the formation of the arch; but as cast iron has also
some tenacity, this permitted certain modifications in the adjustment of
the equilibrium, which are quite out of the question in structures of
brick and stone.
[Illustration:
FIG. 139.
]
[Illustration:
FIG. 140.
]
[Illustration:
FIG. 141.
]
[Illustration:
FIG. 142.
]
The general principle of the construction of girder bridges is easily
explained by considering a simple case, which is almost within
everybody’s experience. Let us suppose we have a plank supported as in
Fig. 139. The plank will by its own weight sink down in the centre,
becoming curved in the manner shown; or if the curvature be not
sufficiently obvious, it may always be increased by placing weights on
the centre, as at _g_. If the length of the plank had been accurately
measured when it was extended flat upon the ground, it would have been
found that the upper or concave surface, _a b_, had become shorter, and
the lower or convex surface, _c d_, longer when the plank is supported
only at the ends—a result sufficiently obvious from the figure it
assumes. It is plain, then, that the parts of the wood near the upper
surface are squeezed together, while near the lower surface the wood is
stretched out. Thus, the portions in the vicinity of the upper and lower
surfaces are in opposite conditions of strain; for in the one the
tenacity of the material comes into play, and in the other its power of
resisting compression. There is an intermediate layer of wood, however,
which, being neither extended or compressed, receives no strain. The
position of this is indicated by the line _e f_, called the _neutral
line_. If the plank, instead of being laid flat, is put upon its edge,
as in Fig. 140, the deflection caused by its weight will hardly be
perceptible, and it will in this position support a weight which in its
former one would have broken it down. There is in this case a neutral
line, _e f_, as before; but as the part which is most compressed or
extended is now situated at a greater distance from the neutral line,
the resistance of the material acts, as it were, at a greater leverage.
Again the portions near the neutral line are under no strain; they do
not, therefore, add to the strength, although they increase the weight
to be supported, and they may, for that reason, be removed with
advantage, leaving only sufficient wood to connect the upper and lower
portions rigidly together. The form of cast iron beams, Fig. 141, which
were used for many purposes, depends upon these principles. The
sectional area of the lower flange, which is subjected to tension, is
six times that of the upper one, which has to resist compression,
because the strength of cast iron to resist pressure is about six times
greater than its power of resisting a pull. If the upper flange were
made thicker, the girder would be weaker, because the increased weight
would simply add to the tension of the lower one, where, therefore, the
girder would be more ready to give way than before. If we suppose the
vertical web divided into separate vertical portions, and disposed as at
Fig. 142, the strength of the girder, and the principle on which that
strength depends, will be in no way changed, and we at once obtain the
box girder, which on a large scale, and arranged so that the roadway
passes through it, forms the tubular bridge. It is only necessary that
the upper part should have strength enough to resist the compressing
force, and the lower the extending force, to which the girder may be
subject; and wrought iron, properly arranged, is found to have the
requisite strength in both ways, without undue weight. The various forms
of trussed girders, the trellis and the lattice girders, now so much
used for railway bridges, all depend upon the same general principles,
as does also the Warren girder, in which the iron bars are joined so as
to form a series of triangles, as in Fig. 143.
[Illustration:
FIG. 143.
]
Girders have been made of wrought iron up to 500 ft. in length, but the
cost of such very long girders is so great, that for spans of this width
other modes of construction are usually adopted.
_GIRDER BRIDGES._
[Illustration:
FIG. 144.—_Section of a Tube of the Britannia Bridge._
]
The Britannia Bridge, which carries the Chester and Holyhead Railway
across the Menai Straits, is perhaps the most celebrated example of an
iron bridge on the girder principle. It was designed by Stephenson, but
the late Sir W. Fairbairn contributed largely by his knowledge of iron
to the success of the undertaking, if he did not, in fact, propose the
actual form of the tubes. Stephenson fixed upon a site about a mile
south of Telford’s great suspension bridge, because there occurred at
this point a rock in the centre of the stream, well adapted for the
foundation of a tower. This rock, which rises 10 ft. above the low-water
level, is covered at high water to about the same depth. On this is
built the central tower of the bridge, 460 ft. from the shore on either
side, where rises another tower, and at a distance from each of these of
230 ft. is a continuous embankment of stone, 176 ft. long. The towers
and abutments are built with slightly sloping sides, the base of the
central or Britannia tower being 62 ft. by 52 ft., the width at the
level where the tubes pass through it, a height of 102 ft., being
reduced by the tapering form to 55 ft. The total height of the central
tower is 230 ft. from its rock foundation. The parapet walls of the
abutments are terminated with pedestals, the summits of which are
decorated by huge lions, looking landwards. As each line of rails has a
separate tube, there are four tubes 460 ft. long for the central spans,
and four 230 ft. long for the shorter spans at each end of the bridge.
Each line of rails, in fact, traverses a continuous tube 1,513 ft. in
length, supported at intervals by the towers and abutments. The four
longer tubes were built up on the shore, and were floated on pontoons to
their positions between the towers, and raised to the required elevation
by powerful hydraulic machinery. The external height of each tube at the
central tower is 30 ft., but the bottom line forms a parabolic curve,
and the other extremities of the tubes are reduced to a height of 22¾
ft. The width outside is 14 ft. 8 in. Fig. 144 shows the construction of
the tube, and it will be observed that the top and bottom are cellular,
each of the top cells, or tubes, being 1 ft. 9 in. wide, and each of the
bottom ones 2 ft. 4 in. The vertical framing of the tube consists
essentially of bars of ⟙-iron, which are bent at the top and bottom, and
run along the top and bottom cells for about 2 ft. The covering of the
tubes is formed of plates of wrought iron, rivetted to ⟙- and ∟-shaped
ribs. The thickness of the plates is varied in different parts from ½
in. to ¾ in. The plates vary also in their length and width in the
different parts of the tubes, some being 6 ft. by 1¾ ft., and others 12
ft. by 2 ft. 4 in. The joints are not made by overlapping the plates,
but are all what are termed _butt_ joints, that is, the plates meet edge
to edge, and along the juncture a bar of ⟙-iron is rivetted on each
side, thus: ╬. The cells are also formed of iron plates, bolted together
by ∟-shaped iron bars at the angles. The rails rest on longitudinal
timber sleepers, which are well secured by angle-iron to the ⟙-ribs of
the framing forming the lower cells. More than two millions of rivets
were used in the work, and all the holes for them, of which there are
seven millions, were punched by special machinery. The rivets being
inserted while red hot, and hammered up, the contraction which took
place as they cooled drew all the plates and ribs very firmly together.
In the construction of the tubes no less than 83 miles of angle-iron
were employed, and the number of separate bars and plates is said to be
about 186,000. The expansion and contraction which take place in all
materials by change of temperature had also to be provided for in the
mode of supporting the tubes themselves. This was accomplished by
causing the tubes, where they pass through the towers, to rest upon a
series of rollers, 6 in. in diameter, and these were arranged in sets of
twenty-two, one set being required for each side of each tube, so that
in all thirty-two sets were needed. There are other ingenious
arrangements for the same purpose at the ends of the tubes resting on
the abutments, which are supported on balls of gun-metal, 6 in. in
diameter, so that they may be free to move in any manner which the
contractions and expansions of the huge tubes may require. Each of the
tubes, from end to end of the bridge, contains 5,250 tons of iron. The
mode in which these ponderous masses were raised into their elevated
position is described in the article on “Hydraulic Power,” as it
furnishes a very striking illustration of the utility and convenience of
that contrivance. The foundation-stone of the central tower was laid in
May, 1846, and the bridge was opened in October, 1850. The tubes have
some very curious acoustic properties: for example, the sound of a
pistol-shot is repeated about half a dozen times by the echoes, and the
tubular cells, which extend from one end of the bridge to the other,
were used by the workmen engaged in the erection as speaking-tubes. It
is said that a conversation may thus be carried on with a person at the
other end of the bridge, a distance of a quarter of a mile. The rigidity
of the great tubes is truly wonderful. A very heavy train, or the
strongest gale, produces deflections in the centre, vertical and
horizontal respectively, of less than one inch. But when ten or a dozen
men are placed so that they can press against the sides of the tube,
they are able, by timing their efforts so as to agree with the period of
oscillation proper to the tube, to cause it to swing through a distance
of 1¼ in.—an illustration of facts of great importance in mechanics,
showing that even the most strongly built iron structure has its own
proper period of oscillation as much as the most slender stretched wire,
and that comparatively small impulses can, by being isochronous with the
period of oscillation, accumulate, as it were, and produce powerful
effects. Bridges are often tried by causing soldiers to march over them,
and such regulated movements form the severest test of the freedom of
the structures from dangerous oscillation. The main tubes of the
Britannia Bridge make sixty-seven vibrations per minute. The expansion
and contraction occurring each day show a range of from ½ in. to 3 in.
The total cost of the structure was £601,865.
A stupendous tubular bridge has also been built over the St. Lawrence at
Montreal, and the special difficulties which attended its construction
render it perhaps unsurpassed as a specimen of engineering skill. The
magnitude of the undertaking may be judged of from the following
dimensions: Total length of the Victoria Bridge, Montreal, 9,144 ft., or
1¾ miles; length of tubes, 6,592 ft., or 1¼ miles: weight of iron in the
tubes, 9,044 tons; area of the surface of the ironwork, 32 acres; number
of piers, 24, with 25 spans between the piers, each from 242 ft. to 247
ft. wide.
[Illustration:
FIG. 145.—_Albert Bridge, Saltash._
]
Another singular modification of the girder principle occurs in the
bridge built by Brunel across a tidal river at Saltash, Fig. 145. Here
only a single line of rails is carried over the stream, which is,
however, 900 ft. wide, and is crossed by two spans of about 434 ft.
wide. A pier is erected in the very centre of the stream, in spite of
the obstacles presented by the depth of the water, here 70 ft., and by
the fact that below this lay a stratum of mud 20 ft. in depth before a
sound foundation could be reached. This work was accomplished by sinking
a huge wrought iron cylinder, 37 ft. in diameter and 100 ft. in height,
over the spot where the foundation was to be laid. The cylinder
descended by its own weight through the mud, and when the water had been
pumped out from its interior, the workmen proceeded to clear away the
mud and gravel, till the rock beneath was reached. On this was then
built, within the cylinder, a solid pillar of granite up to the
high-water level, and on it were placed four columns of iron 100 ft.
high, each weighing 150 tons. The two wide spans are crossed by girders
of the kind known as “bow-string” girders, each having a curved
elliptical tube, the ends of which are connected by a series of iron
rods, forming a catenary curve like that of a suspension bridge. To
these chains, and also to the curved tubes, the platform bearing the
rails is suspended by vertical suspension bars, and the whole is
connected by struts and ties so nicely adjusted as to distribute the
strains produced by the load with the most beautiful precision. When the
bridge was tested, a train formed wholly of locomotives, placed upon the
entire length of the span, produced a deflection in the centre of 7 in.
only. This bridge has sometimes been called a suspension bridge because
of the flexible chords which connect the ends of the bows; but this
circumstance does not in reality bring the bridge as a whole under the
suspension principle. The section of the bow-shaped tube is an ellipse,
of which the horizontal diameter is 16 ft. 10 in. and the vertical
diameter 12 ft., and the rise in the centre about 30 ft. Beside the two
fine spans which overleap the river, the bridge is prolonged on each
side by a number of piers, on which rest ordinary girders, making its
total length 2,240 ft., or nearly half a mile; 2,700 tons of iron were
used in the construction. As in the case of the Britannia Bridge, the
tubes were floated to the piers, and then raised by hydraulic pressure
to their position 150 ft. above the level of the water. The bridge was
opened by the late Prince Consort in 1860, and has received the name of
the Albert Bridge.
_SUSPENSION BRIDGES._
The general principle of the suspension bridge is exemplified in a chain
hanging between two fixed points on the same level. If two chains were
placed parallel to each other, a roadway for a bridge might be formed by
laying planks across the chains, but there would necessarily be a steep
descent to the centre and a steep ascent on the other side. And it would
be quite impossible by any amount of force to stretch the chains into a
straight line, for their weight would always produce a considerable
deflection. Indeed, even a short piece of thin cord cannot be stretched
horizontally into a perfectly straight line. It was, therefore, a happy
thought which occurred to some one, to hang a roadway from the chains,
so that it might be quite level, although they preserved the necessary
curve. In designing such bridges, the engineer considers the platform or
roadway as itself constituting part of the chain, and adjusts the loads
in such a manner that the whole shall be in equilibrium, so that if the
platform were cut into sections, the level of the road would not be
impaired.
Public attention was first strongly drawn to suspension bridges by the
engineer Telford, who, in 1818, undertook to throw such a bridge across
the Menai Straits, and the work was actually commenced in the following
year. The Menai Straits Suspension Bridge has been so often described,
that it will be unnecessary to enter here into a lengthy account of it,
especially as space must be reserved for some description of other
bridges of greater spans. The total length of this bridge is 1,710 ft.
The piers are built of grey Anglesea marble, and rise 153 ft. above the
high-water line. The distance between their centres is 579 ft. 10½ in.,
and the centres of the main chains which depend from them are 43 ft.
below the line joining the points of suspension. The roadway is 102 ft.
above the high-water level, and it has a breadth of 28 ft., divided into
two carriage-ways separated by a foot-track. The chains are formed of
flat wrought iron bars, 9 ft. long, 3¼ in. broad, and 1 in. thick. In
the main chains, of which there are sixteen, no fewer than eighty such
bars are found at any point of the cross section, for each link is
formed of five bars. These bars are joined by cross-bolts 3 in. in
diameter. The main chains are connected by eight transverse stays formed
of cast iron tubes, through which pass wrought iron bolts, and there are
also diagonal ties joining the ends of the transverse stays. The time
occupied in the construction was 6½ years, and the cost was £120,000.
This bridge has always been regarded with interest for being the first
example of a bridge on the suspension principle carried out on the large
scale, and also for its great utility to the public, who, instead of the
hazardous passage over an often stormy strait, have now the advantage of
a safe and level roadway.
[Illustration:
FIG. 146.—_Clifton Suspension Bridge, near Bristol._
]
The Clifton Suspension Bridge over the Avon, near Bristol, is noted for
having a wider span than any other bridge in Great Britain, and it is
remarkable also for the great height of its roadway. The distance
between the centres of the piers—that is, the distance of the points
between which the chains are suspended—is more than 702 ft. Part of the
ironwork for this bridge was supplied from the materials of a suspension
bridge which formerly crossed the Thames at London, and was removed to
make room for the structure which now carries the railway over the river
to the Charing Cross terminus. Five hundred additional tons of ironwork
were used in the construction of the Clifton Bridge, which is not only
much longer than the old Hungerford Bridge, but has its platform of more
than double the width, viz., 31 ft. wide, instead of 14 ft. A view of
this bridge is given in Fig. 146, where its platform is seen stretching
from one precipitous bank of the rocky Avon to the other, and the river
placidly flowing more than 200 ft. below the roadway. The picturesque
surroundings of this elegant structure greatly enhance its appearance,
and the view looking south from the centre of the bridge itself is
greatly admired, although the position may be at first a little trying
to a spectator with weak nerves. The work is also of great public
convenience, as it affords the inhabitants of the elevated grounds about
Clifton a direct communication between Gloucestershire and
Somersetshire, thus avoiding the circuitous route through Bristol, which
was required before the completion of the bridge.
[Illustration:
FIG. 147.
]
The use of iron wire instead of wrought bars has enabled engineers to
far exceed the spans of the bridges already described. The table on page
199 shows that iron wire has a tenacity nearly one-third greater than
that of iron bars, and this property has been taken advantage of in the
suspension bridge which M. Chaley has thrown over the valley at
Fribourg, in Switzerland. This bridge has a span of no less than 880
ft., and is constructed entirely of iron wires scarcely more than ⅒ in.
in diameter. The main suspension cables, of which there are two on each
side, are formed of 1,056 threads of wire, and have a circular section
of 5½ in. diameter. The length of each cable is 1,228 ft., and at
intervals of 2 ft. the wires are firmly bound together, so as to
preserve its circular form. But as the cable approaches the piers, the
wires are separated, and the two cables on each side unite by the
spreading out of the wires into one flat band of parallel wire, which
passes over the rollers at the top of the piers, and is again divided
into eight smaller cables, which are securely moored to the ground. Each
of the mooring cables is 4 in. in diameter, and is composed of 528
wires. In order to obtain a secure attachment for the mooring cables,
shafts were sunk in the solid rock 52 ft. deep, and the ingenious mode
in which, by means of inverted arches, an anchorage in the solid rock is
formed for the cables, will be understood by a reference to Fig. 147.
The cables pass downwards through an opening made in each of the middle
stones, and are secured at the bottom by stirrup-irons and keys. The
suspension piers are built of blocks of stone, very carefully shaped and
put together with cramps and ties, so as to constitute most substantial
structures. These piers are embellished with columns and entablatures,
forming Doric porticoes, enclosing the entrances to the bridge, which
are archways 43 ft. high and 19 ft. wide. The roadway is 21 ft. wide,
and is supported on transverse beams, 5 ft. apart, upon which is laid
longitudinal planking covered by transverse planking. The roadway beams
are suspended to the main cables by vertical wire cables, 1 in. in
diameter. The length of these suspension cables of course varies
according to their position, the shortest being ½ ft. and the longest 54
ft. in length. Each suspension cable is secured by the doubling back of
the wires over a kind of stirrup, through which passes a plate of iron,
supported by the two suspension cables, the latter being close together,
and, indeed, only separated by the thickness of the suspension cables,
which hang between them. The roadway has a slight rise towards the
centre, its middle point being from 20 to 40 in. above the level of the
ends, according to the temperature.
To test the stability of the bridge, fifteen heavy pieces of artillery,
accompanied by fifty horses and 300 people, were made to traverse it at
various speeds, and the results were entirely satisfactory. Indeed, a
few years afterwards the people of Fribourg had another wire bridge
thrown over the gorge of Gotteron, at about a mile from the former.
This, though not so long (640 ft.), spans the chasm at a great height,
and in this respect is probably not surpassed by any bridge in the
world—certainly not by any the length of which can compare with its own.
The height of the roadway above the valley is 317 ft., or about the same
as that of the golden gallery of St. Paul’s Cathedral above the street.
The structure is very light, and the sensation experienced when, looking
_vertically_ downwards through the spaces between the flooring boards,
you see the people below diminished to the apparent size of flies, and
actually feel yourself suspended in mid-air, is very peculiar, as the
writer can testify.
The Americans have, however, outspanned all the rest of the world in
their wire suspension bridges. They have thrown a suspension bridge of
800 ft. span over the Niagara at a height of 260 ft. above the water, to
carry not only a roadway for ordinary traffic, but a railway. Suspension
bridges are not well adapted for the latter purpose, but there seemed no
other solution of the problem possible under the circumstances. The
bridge, however, combines to a certain extent the girder with the
suspension principle. The girder which _hangs_ from the main cables (for
they are made of wire), carries the railway, and below this is the
suspended roadway for passengers and ordinary carriages. The engineer of
this work was Roebling, who also designed many other suspension bridges
in America.
The spans of any European bridges are far exceeded by that of the wire
suspension bridge which crosses the Ohio River at Cincinnati, with a
stride of more than 1,000 ft.; and this is, in its turn, surpassed by
another bridge which has been thrown over the Niagara. This bridge,
which must not be confounded with the one mentioned above, or with the
Clifton Bridge in England already described, merits a detailed
description from the audacity of its span, which is nearly a quarter of
a mile, and entitles it to the distinction of being the longest bridge
in the world of one span.
[Illustration:
FIG. 147_a_.—_Clifton Suspension Bridge, Niagara._
]
The new suspension bridge at the Niagara Falls, called the Clifton
Bridge, of which a view is given in Fig. 147_a_, is intended for the use
of passengers and carriages visiting the Falls, and it is also the means
of more direct communication between several small towns near the banks
of the river. The bridge is situated a short distance below the Falls,
crossing the river at right angles to its course at a point where the
rocks which form the banks are about 1,200 ft. apart. The distance
between the centres of the towers is 1,268 ft. 4 in., and the bridge has
by far the longest single span of any bridge in the world, the distance
between the points of suspension being more than twice that of the Menai
Bridge, and more than six times the span of the widest stone bridge in
England. This remarkable suspension bridge was constructed by Mr. Samuel
Keefer, and was opened for traffic on the 1st of January, 1869, the
actual time employed in the work having been only twelve months. The
cables and suspenders are made of wire, which was drawn in England at
Warrington and Manchester, and the wires for the main cables were made
of such a length, that each wire passed from end to end of the cable
without weld or splice. The length of each of the two main cables is
1,888 ft., and of this length 1,286 ft. usually hangs between the
suspending towers, the centre being about 90 ft. below the level of the
points of suspension. This last distance, however, varies considerably
with the temperature, for in winter the contraction produced by the cold
brings up the centre to 89 ft. below the level line, while in summer it
maybe 3 ft. lower. The centre of the bridge is about 190 ft. above the
water in summer, and 193 ft. in winter. The cables are each formed of
seven wire ropes, and each rope consists of seven strands, each strand
containing nineteen No. 9 Birmingham gauge wires of the diameter of
0·155 in. The cables of this bridge do not hang in vertical planes,
since in the centre they are only 12 ft. apart; while at the towers,
where they pass over the suspension rollers, they are 42 ft. apart. The
end of the platform which rests on the right bank is 5 ft. higher than
the other, and if a straight line were drawn from one end to the other,
the centre of the roadway would be in winter 7 ft. above it, and in
summer 4 ft. From each point of suspension twelve wire ropes, called
“stays,” pass directly to certain points of the platform. The stays are
not attached to the cables, but pass over rollers on the tops of the
towers, and are anchored in the rock, independently of the cables. The
longest stays are tangential to the curve formed by the main cables, and
they are fixed to the platform at a point about half-way to the centre.
Other stays proceed from the platform at intervals of 25 ft., between
the longest and the end of the bridge. The thickness of the stays is
varied according to the strain they have to bear, and they form not only
a great additional support to the platform, but they also serve to
stiffen the bridge and lessen the horizontal oscillations to which the
platform would be liable from the shifting loads it has to bear. There
are also stays which transversely connect the two cables. The wire ropes
by which the platform is suspended to the main cables are ⅝ths of an
inch in diameter, and have such a strength that the material would only
yield to a strain of 10 tons. These suspenders are placed 5 ft. apart
and are 480 in number, the lengths, of course, being different according
to the position. To each pair of suspenders is attached a transverse
beam, 13½ ft. long, 10 in. deep, and 2½ in. wide. Upon these beams—which
are, of course, 5 ft. apart from centre to centre—rests the flooring,
formed of two layers of pine planking 1½ in. thick; and the roadway thus
formed constitutes a single track 10 ft. in width. Along each side of
the platform is a truss the whole length of the bridge, formed of an
upper and a lower beam, 6½ ft. apart, united by ties and diagonal
pieces. The lower chord of the truss is 2 ft. below the road, and on it
rolled iron bars are bolted continuously from one end of the bridge to
the other. The last arrangement contributes greatly to stiffen the
platform, vertically and horizontally. In the central part of the bridge
the flooring-boards are bolted up to the cables, and there are studs
formed of 2 in. iron tubes, so that the platform cannot be lifted
vertically without raising the cables also; and as thus 81 tons of the
weight of the cables vertically rest upon the platform, great steadiness
is secured, inasmuch as the central part of the cables must partake of
any movement of the platform, and their weight greatly increases the
inertia to be overcome. In order still further to prevent oscillations
as much as possible, a number of “guys” are attached to the bridge.
These are wire ropes of the same thickness as the suspenders, and they
connect the platform with various points of the bank—some going
horizontally to the summit of the cliffs, others vertically, but the
majority obliquely. There are twenty-eight guys on the side of the
bridge next the falls, and twenty-six on the other side. The thickness
of the wire rope of which they are made being little more than ½ in.,
they are scarcely visible, or rather appear like spider lines. About 400
ft. of the length of the bridge in the centre is without either guys or
stays except two small steel ropes, which, tightly strained from cliff
to cliff, cross each other nearly at right angles at the centre of the
bridge. The suspension towers are pyramidal in form and are built of
white pine, the timbers being a foot square in section and very solidly
put together, so that they are capable of bearing forty times the load
which can ever be put upon them. The towers are surmounted by strong
frames of cast iron, to which are fixed the rollers carrying the cables
and stays to their anchorage. The weight of the bridge itself, together
with the greatest load it can be required to bear, amounts to 363 tons.
Its cost was £22,000, and it was constructed without a single accident
of any kind.
The foam of the great falls is carried by the stream beneath the bridge,
and in sunshine the spectator who places himself on the centre of its
platform sees in the spray driven by the wind, not a mere fragment of a
rainbow, or a semicircular arc, but the complete circle, half of which
appears beneath his feet. The gorge of the Niagara is very liable to
furious blasts of winds, for by its conformation it seems to gather the
aërial currents into a focus, so that a gentle breeze passing over the
surrounding country is here converted into a strong gale, sweeping down
with great force between the precipitous banks of the river. Indeed, one
would suppose that the cavern from which Æolus allows the winds to rush
out, must be situated near Niagara Falls. The bridge is not disturbed by
ordinary winds, although during its construction, before the stays and
guys were fixed, it was subject to considerable displacement from this
cause. The peculiar arrangement of the cables, by which they hang, not
vertically, but widening out from the centre of the bridge, giving what
has been termed the “cradle” form, has proved of the highest advantage,
so that, with the aid of the guys and stays, and the plan of attaching
the central part of the roadway to the cables, the bridge is believed to
be capable of withstanding without damage a gale having the force of 30
lbs. per square foot, although its total pressure on the structure might
then amount to more than 100 tons. The stability of the structure was
severely tested soon after its erection by a furious gale from the
south-west, by which the guys were severely strained; in fact, many of
them gave way. In one case an enormous block of stone, 32 tons in
weight, to which one of the guys was moored, was dragged up and moved 10
ft. nearer the bridge. This and some lateral distortion of the platform,
which was easily remedied, was all the damage sustained by the bridge.
By an increase of the strength of the guys, &c., and the addition of the
two diagonal steel wire ropes mentioned above, the bridge was soon made
stronger than before. Some years ago, when the Menai suspension bridge
was exposed to a storm of like severity, that structure suffered great
damage, the platform having been broken and some of it swept away. In
the great gale which swept down upon the Niagara bridge, although the
force of the wind was so great that passengers and carriages could not
make headway, the vertical oscillations of the bridge never exceeded 18
in., an amount which must be considered extremely satisfactory in a
bridge of the kind, having a span of nearly a quarter of a mile.[4]
Footnote 4:
Notwithstanding the skill displayed in its construction, this bridge
has, since the above account was written, been destroyed by a
tremendous hurricane.
[Illustration:
FIG. 147_b_.—_Living Model of the Cantilever Principle._
]
_CANTILEVER BRIDGES._
The great Forth Bridge, now (December, 1889) approaching completion, is
the first bridge on the cantilever and central girder principle that has
been erected in Great Britain, and it has also the distinction of being
by far the widest spanned bridge in all the world. We are told by the
engineers of the bridge that the cantilever and girder principle is by
no means new, for it has been adopted hundreds of years ago by
comparatively rude tribes in the construction of timber bridges, to
which it readily lends itself. Such bridges are described as having been
erected by the natives of Hindoostan, Canada, Thibet, etc., even at
remote periods. The principle of the cantilever and girder construction
was well illustrated by Mr. Baker, one of the engineers of the bridge,
at a lecture given by him at the Royal Institution, by means of what he
termed “a living model,” of which (Fig. 147_b_) shows the general
arrangement. Two men, seated on chairs, extend their arms and hold in
their hands sticks, of which the other ends butt against the chairs. The
central girder is represented by a shorter stick, suspended at _a_ and
_b_. We have here the representation of two double cantilevers, the
ropes at _c_ and _d_, connected with the weights, representing the
anchorages of the landward arms of the cantilevers. When a weight is
placed on _a b_, which was done in the “living model,” by a third man
seating himself thereon, a tensile strain comes into action in the ropes
and in the men’s arms, while the sticks abutting on the chairs have to
resist a compressing force, and the weight of the whole is borne by the
legs of the chairs, also under compression. Now let the reader imagine
the men’s heads to be 360 feet above the ground, and about a third of a
mile apart, while the distance between _a_ and _b_ is 350 feet, and he
will have a rough but sufficiently clear idea, not only of the principle
upon which the Forth Bridge is constructed, but also of the magnitude of
one of its spans. To complete the comparison, Mr. Baker further invited
his hearers to suppose that the pull upon each arm of the men is equal
to 10,000 tons, and that the legs of each chair press on the ground with
the weight of more than 100,000 tons.
The Forth Bridge spans the estuary at Queensferry nine miles north-west
from Edinburgh, and its purpose is to afford uninterrupted railway
communication along the eastern side of Scotland. It will, in effect,
shorten the railway journey between Edinburgh and Perth, or Aberdeen, by
nearly two hours. Queensferry had long been established as a usual place
for crossing the Forth, and readers of Scott’s “Antiquary” will remember
that the first chapter describes how Monkbarns and Lovel, by some
accidental delays to the coach, lost the tide, and had to wait, to sail
“with the tide of ebb and the evening breeze,” finding themselves, in
the meanwhile, pretty comfortable over a good dinner at the “Hawes Inn.”
This inn still stands, its situation being close to the southern end of
the great bridge. A design for the erection of a light suspension bridge
at the same spot was published at the beginning of the present century,
but although the spans were to be equal to those of the present bridge
(17,000 feet), the different scale of the projects may be inferred from
the total weight of iron to be used being estimated at 200 tons, while
50,000 tons will be required for the structure now approaching
completion.
In 1873, an Act of Parliament was obtained authorizing the construction
of a suspension bridge at Queensferry, to carry the railway over the
estuary. The design comprised practically two bridges, each carrying a
single line of rails, the bridges being braced together at intervals.
The central towers were to have been 600 feet high, or about 100 feet
loftier than any other erection then existing in the world. The designer
was the late Sir Thomas Bouch, and preparations were made for carrying
out the plans by the erection of workshops and the manufacture of bricks
for the piers. But the project was knocked on the head by the terrible
disaster at the Tay Bridge, in December, 1879, when several of the
central piers were overturned by the force of the wind, with swift
destruction to a passing train, which was precipitated into the water,
and every one of about ninety persons in the train perished. Sir Thomas
Bouch having been the designer of the Tay Bridge, public confidence in
his plan was shaken to such an extent, that the four railway companies
who were promoting the construction of the suspension bridge abandoned
the project in favour of a design on the cantilever and central girder
system, which was then brought forward by Mr. (now Sir John) Fowler and
Mr. Baker. When the Bessemer process had made steel attainable at a
cheap rate, these engineers recognized the advantages which cantilever
bridges, made of that material, presented for the wide spans required
for carrying railways across navigable rivers, and in 1865 they had
designed such a bridge, with 1,000 feet spans for a viaduct, across the
Severn, near the position of the present tunnel. It was not, however,
until 1881 that the designs for the Forth Bridge were published in
English and American engineering journals. These designs at once
attracted attention, and scarcely a year had elapsed before a railway
bridge was built for the Canadian and Pacific Railway, on the same
principle, and this has been followed by others since. It is, however,
absurd to allege that the engineers took their ideas from America,
merely because these smaller undertakings have been completed before the
great work that dwarfs them all was open for traffic. The construction
of the Forth Bridge on its present design was commenced in January,
1883. Its site at Queensferry is at a point where the estuary narrows,
and where, in the very middle of the channel, there is a small rocky
island, called Inchgarvie, that furnishes a solid foundation for the
great central pier. On each side of this island the channels are about
one-third of a mile wide, and more than 200 feet deep, and through them
the tide rushes with great velocity. The impossibility of building up
any intermediate piers, under such circumstances, is sufficiently
obvious—the currents must be crossed at one span, if a railway bridge
had to be made. The formation of the piers for such a work presented
many novel problems, and much of the work had to be commenced in deep
water; that is, the ground of rock or hard clay had to be prepared, in
some parts, as far as 90 feet below high water. Each pier stands on four
caissons, which are great tubes or drums of iron and steel, filled up
with concrete. Each weighed, when empty, about 400 tons, but when filled
up with concrete, the weight would be about 3,000 tons. The diameter of
each is 70 feet, and the deepest one is sunk 89 feet below the water,
and it was with no little labour that some of them were put in their
places. Each caisson has an outer and an inner tube, is 70 feet in
diameter at the base, and 60 feet at the top. Seven feet from the
bottom, an air-tight partition formed a chamber in the lower part of the
caisson, about 70 feet in diameter, by 7 feet high, and shafts
sufficiently large to admit the passage of men and tools led from the
top. Air was forced into this chamber, when the caisson had been sunk,
expelling the water, and then men descended through the shafts and
locks, in which a high pressure of air was also maintained, and
excavated the material at the bottom, until the caisson had, by its own
weight, sunk to the depth required. The work in this air chamber was
carried on by means of electric lights, and ten or twelve weeks were
occupied in sinking each caisson. The pressure of the air in the working
chamber was sometimes as high as 35 pounds per square inch, or
sufficient to maintain the mercurial column in a barometer 72 inches
high, instead of the ordinary 29 or 30 inches. It was found that the
labour in the compressed air chamber could not be done by our home
workmen, as they were quite unaccustomed to the high air pressures
required to keep out the water; but arrangements were made for the
assistance of a staff of French workmen, inured to the conditions by
long working under water in the construction of the docks at Antwerp.
[Illustration:
PLATE XIII.
THE FORTH BRIDGE.
]
The stores, offices and workshops, situated on a slight eminence near
the south end of the bridge, are very extensive, occupying, it is said,
an area of 50 acres. Here are great furnaces, cranes and machinery for
shaping and fitting the steel plates and bars ready for taking their
appointed places in the vast structure. An hydraulic crane may, for
instance, be seen lifting a ton weight flat steel plate that has been
heated to redness in a regenerative gas furnace, and transferring it to
an hydraulic press, where it is quickly and quietly bent to the required
shape. The plate is then cooled, and, when the edges have been planed,
it is placed in position with the adjoining plates, and the rivet holes
are drilled by an ingenious machine, specially designed by Mr. Arrol,
the contractor, for that purpose. It works upon 8–feet lengths of the
tubes, and simultaneously cuts ten rivet holes at different points in
the circumference. All the different parts of the structure are
temporarily fitted together to ascertain that every piece is properly
adjusted. They are then marked according to the position they are to
take, and are laid aside until they are wanted. Thus the work at the
bridge has proceeded without any awkward hitches arising from ill
adjusted sections being brought together. At times, 1,800 tons of
finished steel-work has been turned out of these shops in a month, and
this material, which was supplied by the Steel Company of Scotland, has
been found thoroughly trustworthy in every respect. Its strength is
one-half greater than that of the best wrought-iron, and the plates have
thrice the ductility of iron plates. The steel plates for the great
tubes are supplied in lengths of 16 feet, and of different thicknesses,
between ⅜ths of an inch and 1¼ inch.
[Illustration:
FIG. 147_c_.—_Principal Dimensions of the Forth Bridge._
]
The sketch, Fig. 147 _c_, shows the general dimensions of the bridge
proper, or that part of the viaduct which will actually span the
estuary. Of the three great piers that support the cantilevers, it will
be observed that the central one, which rests on Inchgarvie, is wider
than the other two. Each consists mainly of four tubes, 12 feet in
diameter, made of plates of steel 1¼ inch in thickness, and these rise
to the highest part of the bridge, which is 361 feet above the water, so
that the structure is as lofty as St. Paul’s Cathedral. These great
tubes are not placed vertically, but incline inwards towards the top, so
that while the “straddle legs” of each pair are 120 feet apart at the
base, they are only 33 feet apart at the top. These lofty columns are
also braced together diagonally by other steel tubes—that is, a tube
passes from the foot of every column to each of the other three. At the
base of each column, the lowest spanning member springs also (which
appears like an arch, but is not so), as a tube of 12 feet diameter.
Thus abutting or resting on enormously thick plates of steel that cap
the masonry of each pier, are five tubular steel limbs, three of which
are 12 feet in diameter, and two are 8 feet, and, besides these five,
girder members diverge from nearly the same centre. One of the large
tubular members is the first strut that rises obliquely to support the
upper structure. From the point where this strut meets the upper member,
a stay passes downwards with an opposite inclination to the lower
member, from its point of junction with which another strut rises, and
so on. All the struts, as being subject to compressing force, are made
of steel tubes; the straight upper members and the stays are lattice
braced girders of rectangular section. The apparent curve of the lower
member—for it is really made up of sections of straight tubes—may
suggest the notion of an arch; but the reader must remember that the
principle of this bridge has no relation to that of the arch. The
cantilevers do not unite the long arms they stretch, but each is an
independent structure with its own perfect stability, and it will not be
clutched on or locked up to its neighbours by the central girders. The
weight of one of these 1,700 feet spans is about 16,000 tons, and the
heaviest train loads might be two coal trains, weighing together, say
800 tons, or only one-twentieth of the dead weight of the structure.
But, what would not generally be supposed, the pressure of the wind is
an element of much more importance in considering the stability of the
bridge than the weight of the rolling load. It is to resist the wind
pressure that the lofty columns that are only 33 feet apart at the top
across the bridge, plant their bases 120 feet asunder. The estimated
lateral pressure of the wind on one of the cantilevers, assuming it as
equal to 56 lbs. per square foot, would amount to 2,000 tons. These
strains are so fully provided for that the engineers are confident that
a hurricane of such a force as would desolate the country would leave
the Forth Bridge intact, even if the wind blew in opposite directions on
the two arms of the cantilever. To rend asunder the top ties, a pull
equivalent to the weight of 45,000 tons would be required, whilst the
utmost strain that passing trains could possibly bring upon these ties
would be less than 2,000 tons. A striking illustration of the strength
of these huge brackets was lately given by Mr. Baker himself, when in a
public lecture he assured his audience that half a dozen of our
ponderous modern ironclads might be hung from the cantilevers. Everyone
knows that a bracket requires to be strongest nearest the base, and the
lower steel arms that stretch out 680 feet each diminish in diameter
until at the end it has decreased to five feet, and the pairs approach
each until, from being 120 feet apart at the base, they are only 33 feet
apart at the ends. The central girders will each weigh about 1,000 tons,
and only one end of each will be attached to a cantilever, the other
ends will simply rest on what are called “rocking columns,” so that
there may be freedom of motion to allow play for the changes of position
that will be induced by changes of temperature expanding or contracting
the huge masses of metal.
The reader can hardly have failed to observe that the chief element in
the stability of the structure depends upon balancing a great mass of
metal on the one side of a pier by an equal mass on the other side. But
while each end of the central cantilever bears half the weight of a
central girder, the two shoreward cantilevers have this load at their
inner ends only. How is their balance maintained? In this way: the
shoreward arms are made about 10 feet longer than those that stretch
over the water and their extremities are also loaded with about 1,000
tons of iron, built up within the shore piers.
The lofty columns of the piers were erected without any external
staging, from a temporary platform surrounding the piers and supporting
the necessary machinery. The weight of this platform with the machinery
on it was about 400 tons, and as the work proceeded it was raised as
required by hydraulic machines placed within the vertical columns. As
the height of these increased, the men and materials had to be conveyed
to the platform by cages moving between guide ropes and worked by steam
engines. From this platform were constructed not only the main columns,
but the great diagonal tubes, the bracing girders, and the viaduct
girder. The cantilevers were also put together without scaffolding. When
the first few feet of the lower member had been built out from the base,
a movable platform was hung round it, and on this platform were the
cranes for putting the plates into position, the furnace for heating the
rivets, and the hydraulic riveter of specially designed construction,
without noise or hammering, the riveting being completed by the
application of a pressure equal to 3 tons per square inch. The building
up of the cantilever arms on either side of each pier always proceeded
at the same rate, so that the balance was constantly maintained. This
building out from each side of the pier, without the necessity of
relying upon any temporary scaffolding from below, is one great
advantage of the cantilever system, as it is both easier and safer than
a system which relies upon the temporary scaffolding raised from below.
The Forth is for the time the longest spanned bridge in the world; but
it may not retain that honour long, for the legislature of the United
States has already authorized the construction of a cantilever bridge,
the spans of which are to be 2,480 feet. Still more gigantic is the
project lately put forward by some competent French engineers of
bridging the English Channel from Folkestone to Cape Grisnez in 70 spans
on the cantilever system. The designs have been completed and the
calculations made, and no one doubts of the engineering practicability
of the scheme. But the cost is estimated at about 34 million pounds
sterling, or nearly six times as much as that required for constructing
the proposed Channel Tunnel; so that the scale could be turned in favour
of the bridge only if the political reasons that were opposed to the
tunnel were held not to be applicable to the bridge. But it is difficult
to conceive that the existing traffic could ever be developed to such an
extent as to make an undertaking of this magnitude a commercial success.
Since the above account was written, the Forth Bridge was formally
opened on the 4th March, 1890, by the Prince of Wales, in the presence
of a great gathering of railway directors, eminent engineers, and other
distinguished persons from all parts. A very strong gale was blowing at
the time, and at this very hour the bridge was therefore subjected to
another severe but undesigned test of its stability. The perfect
steadiness and security of the structure impressed all who were present
on that occasion, and the train crossed the bridge, exposed to a wind
pressure, registered by the gauge, of 25 lbs. per square foot. At the
luncheon following the opening ceremony, the Prince announced that
baronetcies had been conferred upon Mr M. W. Thompson (the chairman of
the Bridge Company) and upon Sir John Fowler, and that Mr. Baker and Mr.
Arrol, the contractor for the works, were to be knighted. Sir John
Fowler, the engineer-in-chief, was born in 1817, and has been engaged in
many other important works of railway construction in Yorkshire, in that
of the London and Brighton Railway, in the Sheffield Waterworks, &c. The
Metropolitan Railway in London, which also was carried out by Sir John
Fowler, would alone suffice to make him famous as an engineer. Sir
Benjamin Baker is a much younger man, who has had a large and varied
practice in railway engineering in various parts of the world. He is in
much request on the American continent, and is now engaged in carrying
out a ship railway in Canada and a tunnel under the Hudson at New York.
Sir William Arrol began life at nine years of age as a “piecer” in a
cotton mill, but was afterwards apprenticed as an engineer. Subsequently
he was employed as a foreman by engineering firms in Glasgow. In 1866,
he began business on his own account at Dalmarnock, and obtained
contracts at first for smaller then for larger works connected with
bridge and viaduct building. He is distinguished for the energy and
inventive resources he displays in carrying out his undertakings.
_THE TOWER BRIDGE, LONDON._
A little more than four years after the opening of the Forth Bridge, in
June 1894, another great enterprise which had been commenced eight years
before, was inaugurated by the Prince and Princess of Wales as
representatives of Her Majesty the Queen. This was the Tower Bridge,
which not only is one of the most important public works of the century,
but one that presents features of interest and novelty that have never
before been combined in any single structure. The want of an adequate
communication between the shores of the Thames eastward of London Bridge
had long been felt, and was for years a subject of serious consideration
for the Metropolitan authorities. The congested state of the traffic
across London Bridge has often furnished a spectacle for the sight-seer,
and figures are not wanting to show that the number of foot-passengers
alone who daily traverse that bridge, which altogether is only 54 feet
wide, would be equal to the whole population of many considerable
cities: for in 1882 a count showed the daily average of pedestrians to
be 110,525, while the number of vehicles was 22,242. There was much
difference of opinion as to the best method of providing the required
means of communication; but there was an almost universal agreement as
to its position being selected just eastward of the Tower of London. The
map of the districts connected by the Tower Bridge which is given in
Fig. 147_d_, will show a reader who has any acquaintance with London the
suitability of the site. The problem of traversing the river at this
point involved complex conditions as affecting the vehicular traffic and
the navigation, and many different schemes were proposed and examined,
comprised under the three heads of bridges, tunnels and ferries. But a
ferry is always an imperfect means of communication, liable to accidents
and interruptions from fogs, and in severe weather from ice, rendering
the transit impossible for sometimes many days together. A tunnel
beneath the river would, of course, leave the navigation without
impediment, but among its special disadvantages are the great expense of
construction and maintenance, for it has been found that tunnels beneath
waterways are very costly in both respects. Besides, there would have to
be long inclined approaches at each end, and the cost would be
enormously increased by the amount of valuable land these would occupy.
It was indeed proposed that the tunnel should be provided instead with
hydraulic lifts at each end, like those often found in connection with
the sub-ways at railway stations; but such would have to be of
Brobdignagian dimensions, and would daily entail heavy expense. Then, as
regards the bridges, schemes of various kinds were proposed, some even
bridging the whole 850 feet width of the river at a single span, but all
distinguishable by these important characteristics: they either provided
a high level roadway which requires long inclines to reach it, but
permitted lofty-masted ships to pass under it; or, on the other hand,
the roadway was to be made at a low level with a clear headway above the
water of moderate height. While avoiding the inclined approaches, this
plan would either prevent fully rigged vessels passing to the wharves
above the bridge, or some part of the structure would have to open or
swing aside, that the ships might pass through the opening, thus
completely interrupting the pedestrian and vehicular traffic for the
time, with an amount of inconvenience that may be imagined when, as
often happens, twenty large ships or more might pass in the course of a
day, each causing a stoppage of five minutes in the road traffic. Nor
would it be without risks that large vessels could pass through a
comparatively narrow opening in a strong tide-way. Plans for sub-ways,
for high level roadways and for low level roadways, were examined by
Parliamentary Committees when powers to construct the works were
successively applied for by the Metropolitan authorities, and much
valuable evidence having been given, such objectionable features of each
scheme as have been already referred to were duly noted. At length in
1878, Mr. Horace Jones, the late architect to the City of London, in a
report on the various projects, suggested the general plan on which the
present bridge is built, and this having been approved of by the Common
Council, steps were taken to obtain Parliamentary powers to raise the
necessary capital and to proceed with the works; but, for various
reasons, it was not until 1885 that the Act authorising the undertaking
was passed. In the meantime Mr. John Wolfe Barry was appointed engineer
of the structure, while Mr. Jones was to superintend the architectural
details; but after having received the honour of knighthood in 1885, he
died in the same year; and Mr. Barry, reconsidering the joint design,
introduced some new features and somewhat modified the architectural
expression of the structure. One striking point of originality about the
Tower Bridge is that while it is essentially an iron and steel
construction as much as the Forth Bridge, the heavy stiff metal-work is
encased in masonry of elegant and appropriate architectural design, by
which the general desire that the bridge should harmonize so far as
might be, with the ancient historical fortress it adjoins, has been
happily realised. Then again, by the ingenious engineering, the public
have the advantage of a low level roadway, while the largest vessels may
pass freely through a wide space without risk. These apparently
incompatible advantages have been obtained by the adoption of what is
the _bascule_ principle on a hitherto unattempted scale. _Bascule_ is a
French engineering term, which is probably less familiar to most of our
readers than the thing itself. It is applied to the platform of a
draw-bridge which turns as the lid of a box does on its hinges, to
afford a passage over the stream or moat when it is horizontal, and when
drawn up vertically denies such passage. Smaller _bascule_ bridges on
exactly the same plan as in the Tower Bridge may often be seen in places
having docks or canals, such as Hull, &c. In these a flap or platform is
let down from each side from the vertical position, in which the
water-way is open until the free edges meet together to form the
roadway. These platforms turn on horizontal pivots, and are
counterpoised by loads of stone or metal, so that they are without
difficulty raised and lowered by a winch or handle that turns a cogged
pinion engaging the teeth of a large quadrant.
[Illustration:
PLATE XIV.
THE TOWER BRIDGE IN COURSE OF CONSTRUCTION.
]
[Illustration:
“THE ENGINEER” SWAIN ENG.
FIG. 147_d_.—_Map of the Tower Bridge and its Approaches._
]
The following general description of the Tower Bridge is mainly
abstracted from a very full and excellent account of it drawn up in 1894
by Mr. J. E. Tuit, engineer to Sir W. Arrol & Co., the contractors, in
which are embraced the whole of the technical details of the structure.
The map, Fig. 147_d_, shows the site of the bridge and its approaches,
of which the northern one begins close to the mint and passes along the
east side of the Tower of London to the northern abutment. This approach
is formed of a series of brick arches, and is nearly 1,000 feet long and
35 feet wide in the roadway, with a footpath 12½ feet wide on either
side of it. The incline is only a rise of 1 in 60, but the southern
approach is slightly steeper, namely, 1 in 40 leaving the street level
at Tooley Street. At each abutment there are also stairs connecting the
banks of the river with the roadway of the bridge. The width of the
river between the two abutments is 880 feet, and this is divided, as
shown in Fig. 147_e_, into two side spans, each 270 feet wide, and one
central span of 200 feet clear, making together 740 feet, the river
piers, each of which is 70 feet wide, completing the total span. The
clear headway above high water, when the _bascules_ or leaves are down,
is, in the middle span, 29½ feet in the centre, but only 15 feet at the
ends; but when the leaves are raised for ships to pass, it is about 143
feet. The headway at the shore sides of the piers is 27 feet, but this
is lessened to 23 feet and 20 feet at the north and south abutments
respectively. The roadway and footpaths are continued along the side
spans of the same width as on the approaches, but over the central span
the road is 32 feet, and each footway 8½ feet wide. The river piers are
said to be the largest in the world of the same kind, and their great
area was necessitated by the nature of the London clay on which they
rest, which was found incapable of bearing a load much exceeding four
tons per square foot without some risk of undue settlement.
The part of the piers below the bed of the river is formed of concrete,
while the upper part is brickwork, set in cement and faced with Cornish
granite. Upon each of the river piers rest four octagonal columns, built
up of flat steel plates, connected together at their edges by splayed
angle-bars. The columns are 120 feet high, and 5½ feet in diameter;
those on each pier are securely braced together, at certain stages also
by plate girders, 6 feet deep, to form a floor or landing, and the tops
of the columns are similarly joined together. At the height of 143 feet
above high water there are two footways, each 12 feet wide and 230 feet
long, carried on girders over the central span, and supported by the
columns on each pier. It must be noted that all the roadway, and, in
fact, all the practical and useful structure of the bridge, depend upon
the steel-work alone, which is supported mainly by the eight octagonal
columns just mentioned. The architectural features, which so
appropriately clothe all the steel columns, are added for æsthetic
considerations, and their masonry takes no part in bearing the weights
and strains of the structure. Indeed, the stone-work of the towers is
carefully separated from the columns, which were covered with canvas
while the masonry was built round them, and spaces were left at every
point where compression of the steel-work would bring weight upon the
stone-work. This investment of the metal-work by beautiful architecture
is, as already mentioned, one of the most original features of the Tower
Bridge. The view of the work in progress, as given in Plate VIII., which
is one of the many beautiful illustrations in Mr. Tuit’s book, will give
the reader an opportunity of judging how much the structure gains in
sightliness by the addition of the architectural features. Two hydraulic
lifts are placed in each tower to convey pedestrians to and from the
higher level footways, when the moving parts of the bridge are open, and
stairs also are provided for the same purpose for those who prefer them
to using the lifts.
[Illustration:
FIG. 147_e._—_The Tower Bridge._
]
Length of Bridge with its approaches 2680 feet.
Length of Northern approach 1000 feet.
Length of Southern approach 800 feet.
Width between N. and S. abutments 830 feet.
Width of central span 200 feet.
Width of side spans, each 270 feet.
Depth of River at high water under central span 33½ feet.
Depth of River at lowest tides under central span 12 feet.
Clear headway at high water when the leaves are down (varies 20 to 29½
from one part of the bridge to another) feet.
Clear headway in centre span at high water with the leaves 143 feet.
raised
The side spans are really suspension bridges, but the chains have only
two links, connected at the lowest point by a pin 2½ feet in diameter,
while their higher ends are supported on the columns of the piers, and
on similar but shorter columns on the abutments. The horizontal pulls of
the chains on the piers are made to balance each other by connecting the
chains to tie bars stretching across the central span, and the landward
ends of the chains, after passing over the lower columns of the
abutments, are securely anchored in enormous masses of concrete.
Each of the opening parts, or _bascules_, or leaves, as they may be
called, consists of four girders 18½ feet apart, rigidly braced
together, and connected at the pier end with a great shaft, 48 feet long
and 1 foot 9 inches in diameter, which turns in massive bearings,
resting upon four fixed girders. The leaf is counterbalanced on the
shore side of the pivot shaft by 350 tons of lead and iron; the short
leverage of the centre-weight and small space available for it required
the greater part of this weight to be of lead, rather than of the less
expensive metal. The pivot shaft passes through the centre of gravity of
the whole, so that, although the total weight is nearly 1,200 tons, no
very great power is required to set it in motion, as the pivot shaft
rests on rollers to diminish the friction. The power for moving the leaf
is applied to toothed quadrants of 42 feet radius, of which two are
fixed to the outside girders of each leaf, and are geared into cogs
moved by eight large hydraulic engines, with six accumulators, into
which water is pumped by two engines, each of 360 horse-power.
The total length of the bridge, including the approaches, is just half a
mile, and the height of the towers from the foundations is 293 feet, so
that if one of them were placed beside St. Paul’s Cathedral, it would
compare with it in height as shown in the sketch, Fig. 147_f_.
[Illustration:
FIG. 147_f_.—_Sketch._
]
_THE GREAT BROOKLYN BRIDGE._
The Clifton Bridge at Niagara Falls, which for a time had the
distinction of being the longest in span of any suspension bridge in the
world, has been fully described in previous pages; but more recently
this bridge has been surpassed in span, and in all other respects, by a
structure that immediately connects two of the most populous localities
in the United States of America. The Island of Manhattan, which is
occupied by the city of New York proper, has a population of nearly two
millions, and a strait on its eastern side, connecting Long Island Sound
with New York Harbour, alone divides it from the other great seats of
population, called respectively Long Island City and Brooklyn. This
channel is about ten miles long, and of a varying width, which may
average three-quarters of a mile. There are many ferries between the
opposite shores, and the waters are busy with steamers, sailing-boats,
tugs, and craft of all kinds, engaged either in traffic with ports near
at hand, or in trade with distant lands. At the southern end of this
strait, near the point of its junction with New York Bay, is the
narrowest part of its course, and it is here that it is crossed by the
magnificent suspension bridge, known indifferently as the East River
Bridge, or Brooklyn Bridge, which provides land communication between
New York, with its population of two millions, and Brooklyn, the fourth
city of the States in point of size, with inhabitants numbering about
one million. Brooklyn is largely a residential place for persons whose
daily business is in New York. It has wide, well-planned streets, many
shaded by the luxuriant foliage of double rows of trees, and possesses
parks, public buildings, institutes, churches, etc., on a scale
commensurate with its importance.
The central span of Brooklyn Bridge, from tower to tower, is 1,595 feet,
and each shore part, extending from the tower to the anchorage of the
cables, is 930 feet span, while the two approaches beyond the anchorage
together add 2,534 feet to the total length, which is 5,989 feet, or
considerably over a mile. The centre span, it will be observed, is much
greater than that of the Niagara Falls Clifton Bridge, which was less
than one quarter of a mile, whereas the Brooklyn Bridge span extends to
something approaching one-third of a mile, or, more exactly, a few yards
longer than three-tenths. The width of the Brooklyn is another one of
its remarkable features, for this is no less than 85 feet, and includes
two roadways for ordinary vehicles, and two tramway tracks, on which the
carriages are moved by an endless cable, worked by a stationary engine
on the Brooklyn side. There is also a footpath, 13 feet wide, for
pedestrians. In this structure, as in many other suspension bridges,
advantage has been taken of the great tenacity of steel wire as compared
with iron bars. But here the wires are not twisted in strands like
ropes, but are laid straight together, and bound into a cylindrical
form, each wire being 3,572 feet long, and extending from end to end of
the cables, which are four in number, each calculated to bear a strain
of 12,200 tons. The number of wires in each cable is very great, for
instead of about the thousand of which the stranded wire cables usually
consist, there are 5,296 steel wires wrapped closely round, and forming
a cylinder 15¾ inches in diameter. Each wire is galvanised, that is,
coated with zinc, and then coated with oil. The towers over which the
cables pass are of masonry, and rise to 272 feet above high-water; their
dimensions at the water level are 140 feet by 50 feet, which offsets
diminish until at the top they are 120 feet by 40 feet. At the anchor
structures, the cables enter the masonry at nearly 80 feet above
high-water, and pass 28 feet into the stonework for connection with the
anchor chains. The anchorages are masses of masonry, measuring at the
base 129 feet by 119 feet, and at the top 117 feet by 104 feet, with a
height of 89 feet in front and 85 feet in the rear. The weight of each
anchor-plate is 23 tons. The roadway of the bridge is suspended from the
cables above the buildings and streets between the towers and the
anchorages. The approaches, on the Brooklyn side 971 feet, on the New
York side 1,563 feet, are carried on stonework arches, which are
utilised as warehouses, but where these approaches cross streets, iron
bridges are thrown over. The clear headway between the centre of the
roadway over the river at high-water is 135 feet, so that there is no
obstruction to navigation, and the headway at the towers is 119 feet, so
that the roadway rises towards the centre about 3 feet 3 inches in 100
feet. The two towers comprise more than 85,000 cubic yards of masonry,
and for various purposes 13,670 tons of concrete were used. The work was
commenced in January, 1870, and the first wire was carried across on
29th May, 1877. The bridge was opened to the public on the 24th of May,
1883, and the tramway four months later. The bridge was made free for
pedestrians in 1891, and in 1894 the tram-car fares were reduced to five
cents (2½_d._) for two journeys. In that year, 41,927,122 passengers
were carried on the cars. The average number of persons daily crossing
the bridge is estimated at about 115,000, although on one day (11th
Feb., 1895) as many as 225,645 passengers have been carried on the cars.
The cost of the work connected with this great bridge was $15,000,000
(£3,125,000).
In relation to the subject of wide-spanning bridges, the erection has
been contemplated of structures which would surpass in magnitude and
boldness any of those yet named. Thus, in 1894, the New York Chamber of
Commerce proposed to throw across the River Hudson, which washes the
western side of New York, a bridge with a clear span of 3,200 feet
(six-tenths of a mile), and 500 feet clear height; and the project was
declared by an eminent and experienced engineer to be quite feasible.
[Illustration:
PLATE XV.
THE BROOKLYN BRIDGE.
]
[Illustration:
FIG. 148.—_Newspaper Printing-Room, with Walter Machines._
]
PRINTING MACHINES.
A volume might be filled with descriptions of the machines which in
every department of industry have taken the place of slow and laborious
manual labour. But if even we selected only such machines as from the
beautiful mechanical principles involved in their action, or from their
effects in cheapening for everybody the necessaries and comforts of
life, might be considered of universal interest, the limits of the space
we can afford for this class of inventions would be far exceeded. The
machines for spinning, for weaving fabrics, for preparing articles of
food, are in themselves worthy of attention; then there is a little
machine which in almost every household has superseded one of the most
primitive kinds of hand-work, and that is the sewing machine. But all
these we must pass over, and confine our descriptions of special
machines to a class in which the interest is of a still more general and
higher character, since their effect in promoting the intellectual
progress of mankind is universally acknowledged. We need hardly say that
we allude to Printing Presses, and if we add a few lines on printing
machines other than those which have given us cheap literature, it is
because these other machines also have contributed to the general
culture by giving us cheap decorative art, and in their general
principles they are so much akin to the former that but little
additional description is necessary.
_LETTERPRESS PRINTING._
The manner in which the youthful assistants of printers came to receive
their technical appellation of “devils” has been the subject of many
ingenious explanations. One of these is to the effect that the earlier
productions of the press, having imitated the manuscript characters, the
uninitiated supposed the impressions were produced by hand-copying, and
in consequence of their rapid production and exact conformity with each
other, it was thought that some diabolical agency must have been
invoked. Another story relates that one of Caxton’s first assistants was
a negro boy, who of course soon became identified in the popular mind
with an imp from the nether world. A very innocent explanation is put
forward in another tale, relating that one of the first English printers
had in his employment a boy of the name of De Ville, or Deville, which
name was soon corrupted into the now familiar title, and became the
inheritance of this youth’s successors in the craft. Perhaps a more
probable and natural explanation might be found in the personal
appearance which the apprentices must have presented, with hands, and no
doubt faces also, smeared over with the black ink which it was their
duty to manipulate. For the ink was formerly always laid upon large
round pads or balls of leather, stuffed with wool. When these balls,
Fig. 149, which were, perhaps, about 12 in. in diameter, had received a
charge of ink, the apprentice dabbed the one against the other, working
them with a twisting motion, and after having obtained a uniform
distribution of the ink on their surfaces with many dexterous
flourishes, he applied them to the face of the types with both hands,
until all the letters were completely and evenly charged. The operation
was very troublesome, and much practice was required before the
necessary skill was obtained, while it was always a most difficult
matter to keep the balls in good working condition.
[Illustration:
FIG. 149.—_Inking Balls._
]
[Illustration:
FIG. 150.—_Inking Roller._
]
The first important step towards the possibility of a printing machine
was made, when for these inking balls was substituted a cylindrical
roller, mounted on handles, Fig. 150. The body of the roller is of wood,
but it is thickly coated with a composition which unites the qualities
of elasticity, softness, and readiness to take up the ink and distribute
it evenly over the types. The materials used for this composition are
chiefly glue and treacle, and sometimes also tar, isinglass, or other
substances. Glycerine and various other materials have also been
proposed as suitable ingredients for these composition rollers, but it
is doubtful whether the original compound is not as efficacious as any
yet tried. The composition is not unlike india-rubber in its appearance
and some of its properties. Fig. 150 represents equally the mode in
which the roller is applied to the type in hand presses, and that in
which it is charged with ink, by being moved backwards and forwards over
a smooth table upon which the ink has been spread.
From the time of the first appearance of printing presses in Europe down
to almost the beginning of the present century, a period of 350 years,
no improvement in the construction appears to have been attempted. They
were simply wooden presses with screws, on exactly the same plan as the
cheese-presses of the period. Earl Stanhope first, in 1798, made a press
entirely of iron, and he provided it with an excellent combination of
levers, so that the “platen,” or flat plate which overlies the paper and
receives the pressure, is forced down with great power just when the
paper comes in contact with the types. Such presses are capable of
turning out about 250 impressions per hour, and it should be noted that
the very finest book printing is still done by presses upon this
principle. One reason is that in such cases, where it is desired to
print with the greatest clearness and depth of colour, the ink employed
is much thicker, or stiffer, and requires more thorough distribution and
application to the type than a machine can effect. Stanhope’s press was
not of a kind to meet the desire for rapid production, to which the
increasing importance of newspapers gave rise. The first practical
success in this direction was achieved by König, who, in 1814, set up
for Mr. Walter, the proprietor of the “Times,” two machines, by which
that newspaper was printed at the rate of 1,100 impressions per hour,
the machinery being driven by steam power.
The “Times” of the 28th November, 1814, in the following words made its
readers acquainted with the fact that they had in their hands for the
first time a newspaper printed by steam power:
“Our journal of this day presents to the public the practical result of
the greatest improvement connected with printing since the discovery of
the art itself. The reader of this paragraph now holds in his hand one
of many thousand impressions of ‘The Times’ newspaper, which were taken
off by a mechanical apparatus. A system of machinery almost organic has
been devised and arranged, which, while it relieves the human frame of
its most laborious efforts in printing, far exceeds all human powers in
rapidity and dispatch. That the magnitude of the invention may be justly
appreciated by its effects, we shall inform the public that after the
letters are placed by the compositors, and enclosed in what is called
the ‘form,’ little more remains for man to do than to attend upon and
watch this unconscious agent in its operations. The machine is then
merely supplied with paper, itself places the form, inks it, adjusts the
paper to the form newly inked, stamps the sheet, and gives it forth to
the hands of the attendant, at the same time withdrawing the form for a
fresh coat of ink, which itself again distributes, to meet the ensuing
sheet now advancing for impression, and the whole of these complicated
acts is performed with such a velocity and simultaneousness of movement
that no less than 1,100 sheets are impressed in one hour. That the
completion of an invention of this kind, not the effect of chance, but
the result of mechanical combinations, methodically arranged in the mind
of the artist, should be attended with many obstructions and much delay
may be readily admitted. Our share in this event has, indeed, only been
the application of the discovery, under an agreement with the patentees,
to our own particular business; yet few can conceive, even with this
limited interest, the various disappointments and deep anxiety to which
we have for a long course of time been subjected. Of the person who made
the discovery we have little to add. Sir Christopher Wren’s noblest
monument is to be found in the building which he erected: so is the best
tribute of praise which we are capable of offering to the inventor of
the printing machine comprised in the preceding description, which we
have feebly sketched, of the powers and utility of his invention. It
must suffice to say further, that he is a Saxon by birth, that his name
is König, and that the invention has been executed under the direction
of his friend and countryman, Bauer.”
[Illustration:
FIG. 151.—_Diagram of Cowper and Applegath’s Single Machine._
]
Each of the machines erected by König for the “Times” printed only one
side of the sheet, so that when they had been half printed by one
machine, they had then to be passed through the other, in order to be
“perfected,” as it is technically termed. These machines were greatly
improved by Messrs. Applegath and Cowper, who contrived also a
modification by which the sheets could be perfected in one and the same
machine. As the principle of these machines has been followed, with more
or less diversity of detail, in most of the printing machines at present
in use, it is very desirable to lay that principle clearly before the
reader. The diagram, Fig. 151, will make the action of Applegath and
Cowper’s single-printing machine easily understood. The type is set up
on a flat form, A B, which occupies part of the horizontal table, C D,
the rest of which, A C, is the inking table. E is a large cylinder,
covered with woollen cloth, which forms the “blanket.” The paper passes
round this cylinder, and it is pressed against the form. The small black
circles, _f_, _g_, _h_, _k_, _l_, _m_, _n_, represent the rollers for
distributing the ink. _f_ is called the _ductor_ roller. This roller,
which revolves slowly, is made of metal, and parallel to it is a plate
of metal, having a perfectly straight edge, nearly, but not quite,
touching the cylinder, and at the other side, as well as at the
extremities, bent upwards, so as to form a kind of trough, to contain
the ink, as a reservoir. The slow rotation of the ductor conveys the ink
to the next roller, which is covered with composition, and being made to
move backwards and forwards between the ductor roller and the table at
certain intervals, it is termed the _vibrating_ roller. The ink having
thus reached the inking-table, is spread evenly thereon by the
_distributing rollers_, _h_, _k_, and it is taken up from the inking
table, as the latter passes under, by the _inking_ rollers, _l_, _m_,
_n_. The table, C D, as a whole is constantly moving right and left in a
horizontal direction, so that the form passes alternately under the
impression cylinder, E, and the inking rollers, _l_, _m_, _n_. The axles
of the inking and distributing rollers are made long and slender, and
instead of turning in fixed bearings, they rest in slots or notches, in
order that, as the form passes below them, they may be raised, so that
they rest on the inking slab, and on the types, only by their own
weight. They are placed not quite at right angles to the direction of
the table, but a little diagonally. The sliding motion caused by this,
helps very much in the uniform spreading of the ink. By these
arrangements the form is evenly smeared with ink, since each inking
roller passes over it _twice_ before it returns to meet the paper under
E.
[Illustration:
FIG. 152.—_Diagram of Applegath and Cowper’s Perfecting Machine._
]
[Illustration:
FIG. 153.—_Cowper’s Double Cylinder Machine._
]
Fig. 152 is a similar diagram, to show the action of the double or
perfecting printing machine, in which the sheets are printed on both
sides. It will be observed that the general arrangement of impression
cylinder, rollers, &c., is represented in duplicate, but reversed in
direction. There are also two cylinders, B B, the purpose of which, as
may be gathered from an inspection of the diagram, is to reverse the
sheets of paper, so that after one side has been printed under the
cylinder, E´, the blank surface may be turned downward, ready to receive
the impression from the form, A B. Fig. 153 gives a view of the Cowper
and Applegath double machine, as actually constructed. The man standing
up is called the _feeder_ or _layer-on_. He pushes the sheets forward,
one by one, towards the tapes, which carry them down the farther side of
the more distant cylinder, under which they pass, receiving the
impression; and so on in the manner already indicated in the diagram,
Fig. 152, until finally they reach a point where, released by the
separation of the two sets of tapes, they are received by the
_taker-off_ (the boy who is represented seated on the stool), and are
placed by him on a table. The bed or table which carries the form moves
alternately right and left, impelled by a pinion acting in a rack
beneath it, in such a manner that the direction of the table’s motion is
changed at the proper moment, while the driving pulley continues to
revolve always in the same direction. The movements of the table and of
the cylinders are performed in exact harmony with each other, for these
pieces are so connected by trains of wheels and rack-work that the
sheets of paper may always receive the impression in the proper position
as regards the margins, and therefore, when the sheets are printed on
both sides, the impressions will be exactly opposite to each other. This
gives what is technically called “true register,” and as this cannot be
secured unless the paper travels over both cylinders at precisely the
same rate, these are finished with great care by turning their surfaces
in a lathe to exactly the same diameter. The action of the machine will
not be fully understood without a glance at the arrangement of the
endless tapes which carry the paper on its journey. The course of these
may be followed in Fig. 154, and a simple inspection of the diagram will
render a tedious description unnecessary.
[Illustration:
FIG. 154.—_Tapes of Cowper’s Machine._
]
In Fig. 155 we have a representation of a steam-power printing machine,
such as is now very largely used for the ordinary printing of books,
newspapers of moderate circulation, hand-bills, &c., and in all the
ordinary work of the printing press. In this the table on which the form
is placed has a reciprocating motion, but the large cylinder moves
continuously always in the same direction. The feeder, or layer-on,
places the sheet of paper against certain stops, and at the right moment
the sheet is nipped by small steel fingers, and carried forwards to the
cylinder, which brings it into contact with the inked type. This is done
with much accuracy of register, for the impression cylinders gear in
such a manner with the rest of the parts that their revolutions are
synchronous. This is a perfecting machine, for the paper, after having
received the impression on one side, is carried by tapes round the other
cylinder, where it receives the impression on the other side, “set-off
sheets” being passed through the press at the same time. The axles of
the impression cylinders are mounted at the ends of short rocking beams,
by small oscillations of which the cylinders are alternately brought
down upon, or lifted off, the form passing below them. A machine of this
kind can print 900 impressions per hour, even of good bookwork, and for
newspaper or other printing, where less accuracy and finish are
required, it may be driven at such a rate as to produce 1,400 perfected
impressions per hour.
[Illustration:
FIG. 155.—_Messrs. Hopkinson and Cope’s Perfecting Machine._
]
The machines used for lithographic printing by steam power are almost
identical in their general arrangement with that just described, which
may be taken as a representative specimen of the modern printing
machine.
To such machines as those already described the world is indebted for
cheap books, cheap newspapers, and cheap literature in general. But
when, with railways and telegraphs, came the desire for the very latest
intelligence, the necessities of the newspaper press, as regards
rapidity of printing, soon required a greater speed than could possibly
be attained by any of the flat form presses; for in these the table,
with the forms placed upon it, is unavoidably of a considerable weight,
and this heavy mass has to be set in motion, stopped, moved in the
opposite direction, and again stopped during the printing of each sheet.
The shocks and strains which the machine receives in these alternate
reversals of the direction of the movement impose a limit beyond which
the speed cannot be advantageously increased. When Mr. Applegath was
again applied to by the proprietors of the “Times” to produce a machine
capable of working off a still larger number of impressions, he decided
upon abandoning the plan of reciprocating movement, and substituting a
continuous rotary movement of the type form. And he successfully
overcame the difficulties of attaching ordinary type to a cylindrical
surface. The idea of placing the type on a rotating cylinder is due to
Nicholson, who long ago proposed to give the types a wedge shape, so
that the pieces of metal would, like the stones of an arch, exactly fit
round the cylindrical surface. The wedge-shaped types were, however, so
liable to be thrown from their places by the centrifugal force, that
Nicholson proposed also certain mechanical methods of locking the types
together after they had been placed on the circumference of the drum.
The plan he suggested for this purpose involved, however, such an
expenditure of time and trouble that his idea was never carried into
practice. Mr. Applegath used type of the ordinary kind, which was set up
on flat surfaces, forming the sides of a prism corresponding to the
circumference of his revolving type cylinder, which was very large and
placed vertically. The flat surfaces which received the type were the
width of the columns of the newspaper, and the type forms were firmly
locked up by screwing down wedge-shaped rules between the columns at the
angles of the polygon. These form the “column rules,” which make the
upright lines between the columns of the page, and by their shape they
served to securely fix the type in its place. The diameter of the
cylinder to which the form was thus attached was 5 ft. 6 in., but the
type occupied only a portion of its circumference, the remainder serving
as an inking table. Round the great cylinder eight impression rollers
were placed, and to each impression roller was a set of inking rollers.
At each turn, therefore, of the great cylinder eight sheets received the
impression. These cylinders were, as already stated, placed vertically,
and, as it was necessary to supply the sheets from horizontal tables, an
ingenious arrangement of tapes and rollers was contrived, by which each
sheet was first carried down from the table into a vertical position,
with its plane directed towards the impression roller, in which position
it was stopped for an instant, then moved horizontally forwards round
the impression cylinder, and was finally brought out, suspended
vertically, ready for a taker-off to place on his pile. This machine
gave excellent results as to speed and regularity. From 10,000 to 12,000
impressions could be worked off in an hour, and the advantage was
claimed for it of keeping the type much cleaner, by reason of its
vertical position. The power of this machine may be judged of from one
actual instance. It is stated that of copies of the “Times” in which the
death of the Duke of Wellington was announced, 14th November, 1852, no
less than 70,000 were printed in one day, and the machines were not once
stopped, either to wash the rollers or to brush the forms. It may be
mentioned, in order to give a better idea of the magnitude of the
operation of printing this one newspaper, that one average day’s copies
weigh about ten tons, and that the paper for the week’s consumption
fills a train of twenty waggons.
At the “Times” office and elsewhere, the vertical machine has some years
ago been superseded by others with horizontal cylinders. The fastest,
perhaps, of all these printing machines is that which is now known as
the “Walter Press,” so called either because its principle was suggested
by the proprietor of the “Times,” or merely out of compliment to him.
The improvements which are embodied in the Walter Press have been the
subject of several patents taken out in the names of Messrs. MacDonald
and Calverley, and it is to these improvements that we must now direct
the attention of the reader. But we must premise that such machines as
the Walter Press became possible only by the discovery of the means of
rapidly producing what is called a stereotype plate from a form of type.
A full account of the methods of effecting this is reserved for a
subsequent article, but here it may suffice to say, that when a thick
layer of moist cardboard, or rather a number of sheets of thin unsized
paper pasted together and still quite moist, is forced down upon the
form by powerful pressure, a sharp even mould of the type is obtained,
every projection in the latter producing a corresponding depression in
the _papier maché_ mould. When the paper mould is dry, it may be used
for forming a _cast_ by pouring over it some fusible metallic alloy,
having the properties of becoming liquid at a temperature which will not
injure the mould, of taking the impressions sharply, and of being
sufficiently hard to bear printing from. One of the improvements in
connection with the Walter Press is in the mode of forming cylindrical
stereotype casts from the paper mould. For this purpose the mould is
placed on the _internal_ surface of an iron semi-cylinder, with the face
which has received the impression of the type inwards. The central part
of the semi-cylinder is occupied by a cylindrical iron core, which is
adjusted so as to leave a uniform space between its convex surface and
the concave face of the mould. Into this space is poured the melted
metal, and its pressure forces the mould closely against the concave
cylindrical surface to which it is applied, so that the thickness
becomes quite uniform. The iron core has a number of grooves cut round
it, and these produce in the cast so many ribs, or projections, which
encircle the inner surface, and serve both to strengthen the cast and
afford a ready means of obtaining an exact adjustment. Not the complete
cylinder, but only half its circumference, is cast at once, the axis of
the casting apparatus being placed horizontally, and the liquid metal
poured in one unbroken stream between the core and the mould from a
vessel as long as the cylinders. Fig. 156 is a section of the casting
apparatus, in which _a_ is the core, _b_ the _papier maché_ mould, _c_
the iron semi-cylinder containing it, _d_ the metal which has been
poured in at the widened space, _e_. When the metal has solidified, the
core is simply lifted off, and the cast is then taken out, in the form
of a semi-cylinder, the internal surface of which has exactly the
diameter of the external surface of the roller of the machine on which
it is to be placed, in company with another semi-cylindrical plate, so
that the two together encircle half the length of the roller, and when
another pair of semi-cylinders have been fixed on the other part of the
roller, the whole matter of one side of the newspaper sheet, usually
four pages, is ready for printing. One great advantage of working from
stereotype casts made in this way is that the form-bearing cylinder of
the machine has no greater circumference than suffices to afford space
for the matter on one side of the paper. The casts are securely fixed on
the revolving cylinder by elbows, which can be firmly screwed down. The
casts are usually made to contain one page each, so that four
semi-cylinders, each half the length of the revolving cylinder, are
fixed on the circumference of the latter. The process of casting in no
way injures the paper mould, which is in fact generally employed to
produce several plates.
[Illustration:
FIG. 156.
]
[Illustration:
FIG. 157.—_Diagram of the Walter Press._
]
The Walter Machine is not fed with separate sheets of paper, but takes
its supply from a huge roll, and itself cuts the paper into sheets after
it has impressed it on both sides. This is done by a very simple but
effective plan, which consists in passing the paper between two
equal-sized rollers, the circumference of which is precisely the length
of the sheets to be cut. These rollers grip the paper, but only on the
marginal spaces; and on the circumference of one of them, and parallel
to its axis, is a slightly projecting steel blade, which fits into a
corresponding recess, or groove, in the circumference of the other, and
at this time the whole width of the sheet is firmly held by a projecting
piece acted on by a spring. Although the Walter Machine, as actually
constructed, presents to the uninitiated spectator an apparently endless
and intricate series of parallel cylinders and rollers, yet it is in
reality exceedingly simple in principle, as may be seen by the diagram
given in Fig. 157. In this we may first direct the reader’s attention to
the two cylinders, F_{1}, F_{2}, which bear the stereotype casts—one of
the matter belonging to one side of the sheet, the other of the matter
belonging to the other side, for the Walter Press is a perfecting
machine—and the web of paper having been printed by F_{1}, against which
it is pressed by the roller, P_{1}, passes straight, as shown by the
dotted line, to the second pair of cylinders, in order to be printed on
the other side; and here, of course, the form cylinder, F_{2}, is below,
and the impression cylinder, P_{2}, above, and an endless cleaning
blanket is supplied to the latter to receive the _set-off_. The web of
paper then passes between the cutting rollers, C, C_{1}, by which it is
cut in sheets. But the knife has a narrow notch in the centre, and one
at each end, so that the paper is not severed at those parts, narrow
strips or tags being left, which maintain for a while a slight
connection. But the tapes, _t_{1}_, _t_{2}_, between which the paper is
now carried, are driven at a rather quicker rate than the web issues
from C, C_{1}; and the result is, that the tags are torn, and the sheet
becomes separated from the portion next following it. Thus, as a
separate sheet, it arrives at the horizontal tapes, _h_, and is brought
to another set of tapes mounted on the frame, _r_, rocking about the
centre, _c_, by which it is brought finally to the tapes, _f_{1}_,
_f_{2}_, which by the movement of _r_ receive the sheets alternately. A
sheet-flyer, _s_, oscillates between the tapes, _f_{1}_, _f_{2}_; and as
fast as the sheets arrive, lays them down right and left alternately,
and it only remains for the piles, _p_{1}_, _p_{2}_ so formed, to be
removed. The inking apparatus of each form-cylinder is indicated by the
series of rollers marked I_{1}, I_{2}; and in this part of the machine
there are also some improvements over former presses, for the
distributing rollers are not made of composition, but of iron, turned
with great exactness to a true surface, and arranged so as not quite to
touch each other. At D is an apparatus for damping the paper, in which
there are hollow perforated cylinders, covered by blanket, and filled
with some porous material, which is kept constantly wet. These cylinders
being made to rotate rapidly, the centrifugal force causes the water to
find its way uniformly to the outside. Here the paper also passes
between rollers intended to flatten and to stretch it. At R is the great
roll of paper, from which the machine takes its supply. These rolls
contain, perhaps, five miles length of paper, and at first it was a
matter of some difficulty to fix them firmly on their wooden axles, so
that they might be steadily unwound; but the contrivers of the Walter
Press make these spindles as tight as may be required by forming them in
wedge-shaped pieces, which can be made to increase the thickness of the
spindle by drawing one upon another by screws.
The great speed of the Walter Machine is secured by the paper being
drawn by the machine itself from a continuous web, instead of being laid
on in a separate sheet, so that the machine is not dependent on the
dexterity of the layers-on, who are besides necessarily highly-skilled
workmen, and therefore a great economy of wages results from using a
machine which does not require their services; and as the Walter Press
also itself lays down the perfected sheets, the necessary attendants are
as few as possible. The waste of paper and loss of time by stoppages are
said to be extremely small with this machine.
Fig. 148 will give some idea of the appearance of the printing-room
where one of the leading London daily papers is being printed by Walter
Presses.
Another fast printing machine is the type revolving cylinder machine
invented by Colonel Richard M. Hoe, and manufactured by the well-known
firm of Hoe and Company, New York, with whose name the history of fast
printing machines must ever be associated. In these machines the type is
placed on the circumference of a cylinder which rotates about a
horizontal axis, and the difficulties of securely locking up the type
are successfully overcome. The machines are made with two, four, six,
eight, or ten impression cylinders, and at each revolution of the great
cylinder the corresponding number of impressions are produced. The
engraving on the opposite page, Fig. 158, represents the two-cylinder
machine, and an examination of the figure will render its general action
intelligible. The form of type occupies about one-fourth of the
circumference of the great cylinder, the remainder being used as an
ink-distributing surface. Round this main cylinder, and parallel to it,
are placed smaller impression cylinders, from two to ten in number,
according to the size of the machine. When the press is in operation,
the rotation of the main cylinder carries the type form to each
impression cylinder in succession, and it there impresses the paper,
which is made to arrive at the right time to secure true register. One
person is required for each impression cylinder, to supply the sheets of
paper, which have merely to be laid in a certain position, when, at the
proper moment, they are seized by the “grippers,” or fingers of the
machine, and after having been printed, are carried out by tapes, and
laid in heaps by self-acting sheet-flyers, by which the hands which are
required to receive and pile the sheets in other machines are dispensed
with. The ink is contained in a fountain placed beneath the main
cylinder, and is conveyed by means of rollers to the distributing
surface of the main cylinder. This surface, being lower than that of the
type forms, passes by the impression cylinders without touching them.
For each impression cylinder there are two inking rollers, receiving
their supply of ink from the distributing surface of the main cylinder.
These inking rollers, the bearings of which are, by springs, drawn
towards the axis of the main cylinder, rise as the form passes under
them, and having inked it, they again drop on to the distributing
surface. Each page of the matter is locked up on a detachable segment of
the large cylinder, which segment constitutes its bed and chase. The
column-rules are parallel with the shaft of the cylinder, and are
consequently straight, while the head, advertising, and dark rules have
the form of segments of a circle. The column-rules are in the shape of a
wedge, with the thin end directed towards the axis of the cylinder, so
as to bind the types securely. These wedge-shaped column-rules are held
in their place by tongues projecting at intervals along their length,
and sliding in grooves cut crosswise in the face of the bed. The spaces
in the grooves between the column-rules are accurately fitted with
sliding blocks of metal level with the surface of the bed, the ends of
the blocks being cut away underneath, to receive a projection on the
sides of the tongues of the column-rules. The locking up is effected by
means of screws at the foot of each page, by which the type is held as
securely as in the ordinary manner upon a flat bed. The main cylinder of
the machine represented in Fig. 158 has a diameter of 3 ft. 9 in., and
its length is, according to the size of the sheets to be printed, from 4
ft. 5 in. to 7 ft. 4 in. The whole is about 20 ft. long, 10 ft. wide,
including the platforms, and a height of 9 ft. in the room in which it
is placed suffices for its convenient working. The steam power required
is from one to two horse-power, according to the length of the main
cylinder. The speed of these machines is limited only by the ability of
the feeders to supply the sheets fast enough. The ten-cylinder machine
has, of course, ten impression cylinders, instead of two, and there are
ten feeding-tables, arranged one above the other, five on each side. The
main cylinder has a diameter of 4 ft. 9 in., and is 6 ft. 8 in. long.
The machine occupies altogether a space of 31 ft. by 16 ft., and its
height is 18 ft. A steam engine of eight horse-power is sufficient to
drive the ten-cylinder machine, which is then capable of producing
25,000 impressions per hour. The mechanism of the larger machines is
precisely similar to that of the two-cylinder machine, except such
additional devices as are necessary to carry the paper to and from the
main cylinder at four, six, eight, or ten points of its circumference.
Much admirable contrivance is displayed in the manner of disposing
feeders as closely as possible round the central cylinder.
[Illustration:
FIG. 158.—_Messrs. Hoe’s Type Revolving Cylinder Machine._
]
In some machines, such as Messrs. Hoe’s, Fig. 158, the sheet-flyers are
interesting features, for they form an efficient contrivance for laying
down and piling up, with the greatest regularity, sheet after sheet as
it issues from the press. The sheet-flyer is in fact an automatic
taker-off, and therefore it supersedes the services of the boy who would
otherwise be required. It is simply a light wooden framework of parallel
bars, turning on one of its sides as a centre; and the tapes carrying
the sheet, passing down between the bars, bring the paper down upon the
frame, where its progress is then stopped, the frame makes a rapid turn
on its centre, lays down the sheet, and quickly rises to receive another
from the tapes. One can hardly see a printing machine in action without
being struck with the deftness with which the sheet-flyer does its duty;
for the precision with which it receives a sheet, lays it down, and then
quickly returns, to be ready for the next, suggest to the mind of the
spectator rather the movements of a conscious agent than the motions of
an unintelligent piece of mechanism. The sheet-flyer is seen at the
left-hand side of Fig. 158, where it is in the act of laying down a
sheet on the pile it has already formed.
[Illustration:
FIG. 159.—_Messrs. Hoe’s “Railway” Machine._
]
[Illustration:
FIG. 160.—_Napier’s Platen Machine._
]
The modern improvements in printing presses are well illustrated by the
machine represented on the opposite page, Fig. 159, which has been
designed by the Messrs. Hoe to work exclusively by hand. It is intended
for the newspaper and job work of a country office, and it works easily,
without noise or jar, by turning the handle always in the same
direction, producing 800 impressions in an hour. The bed moves backwards
and forwards on wheels running on rails, the reciprocating movement
being derived from the circular one by means of a crank. From the mode
in which the table is carried backwards and forwards, the manufacturers
call this the “Railway Printing Machine.” The paper is fed to the
underside of the cylinder, which, after an impression has been given,
remains stationary while the bed is returning, and while the layer-on is
adjusting his sheet of paper. The axle of the impression cylinder
carries a toothed wheel working in a rack on the bed or table, the wheel
having at two parts of its circumference the teeth planed off so as to
permit of the return of the table without moving the impression
cylinder, which is again thrown into gear with the rack by a catch, so
that the same tooth of the rack always enters the same space on the
toothed wheel, and thus a good register is secured. The impression
cylinder remains unaltered, whatever may be the size of the type form,
it being only necessary to place the forward edge of the form always on
the same line of the bed. Machines of a very similar construction, but
driven by steam power, are used in lithographic printing; and in some of
these machines advantage is elegantly taken of the fact that, when a
wheel rolls along, the uppermost point of its circumference is always
moving forward at exactly twice the velocity of its centre. Hence, if
the table of a printing machine rests on the _circumference_ of wheels,
a backward and forward movement of the centres of these wheels, produced
by the throw of a crank through a space of 2 ft., would produce a
rectilineal reciprocating movement through a distance of 4 ft. of a
table resting on the circumference of the wheels. Any reader who is
interested in geometry or mechanics would do well to convince himself
that the lowest point of the wheel of a railway carriage, for example,
is stationary (considered while it is the lowest point), that the
_centre_ of the wheel is moving forwards with the velocity of the train,
and that the highest point of the wheel is moving forwards with just
twice the speed of the train. There is no difficulty about the rate of
rectilineal motion of the centre, but the reader cannot possibly
perceive the truth of the statement regarding the lowest and highest
points unless he reflects on the subject, or puts it to the test of
experiment. Another form of press which is used for good book printing
is represented in the engraving, Fig. 160, which shows Napier’s platen
machine. There the action is similar to that of the ordinary hand
presses as regards the mode in which the paper is pressed against the
face of the type; but the movements are all performed by steam power,
applied through the driving belt, shown in the figure.
The various kinds of printing machines adapted to each description of
work are too numerous to admit of even a passing mention here; but those
which have been described may fairly be considered as representing the
leading principles of modern improvements. This article relates only to
the mechanism by which an impression is transferred from a form to the
surface of paper: the interesting and novel _processes_ by which the
form itself may be produced—processes which have amazingly abridged the
printers’ labour and extended the resources of the art—deserve a
separate chapter, and will furnish matter for an article on Printing
Processes, which will be the better understood by being placed after
chapters wherein the scientific bases of some of these processes are
discussed.
_PATTERN PRINTING._
The machines used for printing patterns are, in principle, very similar
to those for letterpress printing; but the circumstance of several
different colours having frequently to go to the production of one
pattern leads to the multiplication, in the present class of machines,
of the apparatus for distributing the colours and impressing the
materials. Pattern printing machines are most extensively used for
impressing fabrics, such as calicoes, muslins, &c., and for producing
the wall-papers for decorating apartments. The machines employed for
calicoes and for papers are so much alike, that to describe the one is
almost to describe the other.
The papers intended for paper-hangings are, in the first instance,
covered with a uniform layer of the colour which is to form the ground,
and this is done even in the case of papers which are to have a white
ground. The colours thus laid on, and those which are applied by the
machine, are composed of finely-ground colouring matters mixed with thin
size or glue to a suitable consistence, and the ground-tint is given by
bringing the upper surface of the paper, as it is mechanically unwound
from a great roll, into contact with an endless band of cloth emerging
from a trough containing a supply of the fluid colour. The paper then
passes over a horizontal table, where the layer of colour is uniformly
distributed over its surface by brushes moved by machinery, and the
paper, after having been thoroughly dried, is ready to receive the
impressions. The impressions may be given by flat blocks of wood on
which the pattern is carved in relief, or from revolving cylinders on
which the pattern is similarly carved. The former is the process of hand
labour called “block printing,” and it requires much skill and care on
the part of the operator; but with these, excellent results are
obtained, as a correct adjustment of the positions of the parts of the
pattern can always be secured. The latter is the mode of printing
mechanically on rollers, corresponding with the type-bearing cylinders
of the machines already described; but for pattern printing on paper
they are made of fine-grained wood, mounted on an iron axle, and they
are carved so that the design to be printed stands out in relief on
their surface. One of these rollers is represented in Fig. 161, and it
should be clearly understood that each colour in the pattern on a
wall-paper requires a separate roller, the design cut on which
corresponds only with the forms the particular colour contributes to the
pattern. Such rollers being necessarily somewhat expensive, as the
pattern is usually repeated many times over the cylindrical surface, the
plan has been adopted of fastening a mass of hard composition in an iron
axle, and when this has been turned to a truly cylindrical surface, it
is made to receive plates of metal, formed of a fusible alloy of lead,
tin, and nickel. These plates are simply casts from a single carved
wooden mould of the pattern, which has thus only once to be formed by
hand. The plates are readily bent when warmed, and are thus applied to
the cylindrical surface, to which they are then securely attached. It is
found advantageous to cover the prominent parts of the rollers which
produce the impressions with a thin layer of felt, as this substance
takes up the colours much more readily than wood or metal, and leaves a
cleaner impression.
[Illustration:
FIG. 161.—_Roller for Printing Wall-Papers._
]
The machine by which wall-papers are printed is represented in Fig. 162,
where it will be observed that the impression cylinder has a very large
diameter, and that a portion of its circumference forms a toothed wheel,
which engages a number of equal-sized pinions placed at intervals about
its periphery. Each pinion being fixed on the axle of a pattern-bearing
roller, these are all made to revolve at the same rate. There is,
however, some adjustment necessary before that exact correspondence of
the impressions with each other is secured, which is shown on the
printed pattern by each colour being precisely in its appointed place.
The rollers are constantly supplied with colour by endless cloths, which
receive it from the troughs that are shown in the figure, one trough
being appropriated to each roller. Some of these machines can print as
many as eighteen or twenty different colours at once, by having that
number of rollers; and it is easy to see how, by dividing each trough
into several vertical compartments, in each of which a different colour
is placed, it would be possible to triple or even quadruple the number
of colours printed by one machine.
The machinery by which calicoes are printed is almost identical in
construction with that just described, and presents the same general
appearance. There is, however, an important difference in the rollers,
which in calico printing are of copper or bronze, and have the design
engraved upon their polished cylindrical surface, not in relief, but in
hollows. After the whole surface of the roller becomes charged with
colour, there is in the machine a straight-edge, which removes the
colour from the smooth surface, leaving only what has entered into the
hollow spaces of the design, which, as the roller comes round to the
cloth, yield it up to the surface of the latter. Thus, by a self-acting
arrangement, the rollers are charged with colour, cleaned, and made to
give up their impressions to the stuff by parting with the colour in the
hollows. Rollers having patterns in relief are also used in calico
printing, the mechanism being then almost identical with that of the
former machine. It need hardly be said that great pains are taken in the
construction of such machines to have each part very accurately
adjusted, so that the impression may fall precisely upon the proper
place, without any blurring or confusion of the colours, and the fact
that an intricate design, having perhaps eighteen or twenty tints, can
be thus mechanically reproduced millions of times speaks volumes for the
accuracy and finish of the workmanship which are bestowed on such
printing machines.
[Illustration:
FIG. 162.—_Machine for Printing Paper-Hangings._
]
[Illustration:
FIG. 163.—_Chain-Testing Machine at Messrs. Brown and Lenox’s Works,
Millwall._
]
HYDRAULIC POWER.
If a hollow sphere, _a_, Fig. 173, be pierced with a number of small
holes at various points, and a cylinder, _b_, provided with a piston,
_c_, fitted into it, when the apparatus is filled with water, and the
piston is pushed inwards, the water will spout out of all the orifices
equally, and not exclusively from that which is opposite to the piston
and in the direction of its pressure. The jets of water so produced
would not, as a matter of fact, all pursue straight paths radiating from
the centre of the sphere, because gravity would act upon them; and all,
except those which issued vertically, would take curved forms. But when
proper allowance is made for this circumstance, each jet is seen to be
projected with equal force in the direction of a radius of the sphere.
This experiment proves that when pressure is applied to any part of a
liquid, that pressure is transmitted _in all directions equally_. Thus
the pressure of the piston—which, in the apparatus represented in the
figure, is applied in the direction of the axis of the cylinder only—is
carried throughout the whole mass of the liquid, and shows itself by its
effect in urging the water out of the orifices in the sphere in all
directions; and since the force with which the water rushes out is the
same at every jet, it is plain that the water must press equally against
each unit of area of the inside surface of the hollow sphere, without
regard to the position of the unit.
If we suppose the piston to have an area of one square inch, and to be
pushed inwards with a force of 10 lbs., it cannot be doubted that the
square inch of the inner surface of sphere immediately opposite the
cylinder will receive also the pressure of 10 lbs.; and since the
pressures throughout the interior of the hollow globe are equal, every
square inch of its area will also be pressed outwards with a force equal
to 10 lbs. Hence, if the total area of the interior be 100 square
inches, the whole pressure produced will amount to a hundred times 10
lbs.
[Illustration:
FIG. 164.—_Pascal’s Principle._
]
[Illustration:
FIG. 165.—_Collar of Hydraulic Cylinder._
]
That water or any other liquid would behave in the manner just described
might be deduced from a property of liquids which is sufficiently
obvious, namely, the freedom with which their particles move or slide
upon each other. The equal transmission of pressure in all directions
through liquids was first clearly expressed by the celebrated Pascal,
and it is therefore known as “Pascal’s principle.” He said that “if a
closed vessel filled with water has two openings, one of which is a
hundred times as large as the other; and if each opening be provided
with an exactly-fitting piston, a man pushing in the small piston could
balance the efforts of a hundred men pushing in the other, and he could
overcome the force of ninety-nine.” Pascal’s principle—which is that of
the hydraulic press—may be illustrated by Fig. 164, in which two tubes
of unequal areas, _a_ and _b_, communicate with each, and are supposed
to be filled with a liquid—water, for example, which will, of course,
stand at the same level in both branches. Let us now imagine that
pistons exactly fitting the tubes, and yet quite free to move, are
placed upon the columns of liquid—the larger of which, _b_, we shall
suppose to have five times the diameter, and therefore twenty-five times
the sectional area, of the smaller one. A pressure of 1 lb. applied to
the smaller piston would, in such a case, produce an upward pressure on
the larger piston of 25 lbs.; and in order to keep the piston at rest,
we should have to place a weight of 25 lbs. upon it. Here then a certain
force appears to produce a much larger one, and the extent to which the
latter may be increased is limited only by the means of increasing the
area of the piston. Practically, however, we should not by any such
arrangement be able to prove that there is exactly the same proportion
between the total pressures as between the areas, for the pistons could
not be made to fit with sufficient closeness without at the same time
giving rise to so much friction as to render exact comparisons
impossible. We may, however, still imagine a theoretical perfection in
our apparatus, and see what further consequences may be deduced,
remembering always that the actual results obtained in practice would
differ from these only by reason of interfering causes, which can be
taken into account when required. We have supposed hitherto that the
pressures of the pistons exactly balance each other. Now, so long as the
system thus remains in equilibrium no _work_ is done; but if the
smallest additional weight were placed upon either piston, that one
would descend and the other would be pushed up. As we have supposed the
apparatus to act without friction, so we shall also neglect the effects
due to difference in the levels of the columns of liquid when the
pistons are moved; and further, in order to fix our ideas, let us
imagine the smaller tube to have a section of 1 square inch in area, and
the larger one of 25 square inches. Now, if the weight of the piston,
_a_, be increased by the smallest fraction of a grain, it will descend.
When it has descended a distance of 25 in., then 25 cubic inches of
water must have passed into _b_, and, to make room for this quantity of
liquid, the piston with the weight of 25 lbs. upon it must have risen
accordingly. But since the area of the larger tube is 25 in., a rise of
1 in. will exactly suffice for this; so that a weight of 1 lb.
descending through a space of 25 in., raises a weight of 25 lbs. through
a space of 1 in. This is an illustration of a principle holding good in
all machines, which is sometimes vaguely expressed by saying that _what
is gained in power is lost in time_. In this case we have the piston,
_b_, moving through the space of 1 in. in the same time that the piston
a moves through 25 in.; and therefore the _velocity_ of the latter is
twenty-five times greater than that of the former, but the time is the
same. It would be more precise to say, that what is gained in force is
lost in space; or, that no machine, whatever may be its nature or
construction, is of itself capable of doing _work_. The “mechanical
powers,” as they are called, can do but the work done upon them, and
their use is only to change the relative amounts of the two factors, the
product of which measures the work, namely, space and force. Pascal
himself, in connection with the passage quoted above, clearly points out
that in the new mechanical power suggested by him in the hydraulic
press, “the same rule is met with as in the old ones—such as the lever,
wheel and axle, screw, &c.—which is, that the distance is increased in
proportion to the force; for it is evident that as one of the openings
is a hundred times larger than the other, if the man who pushes the
small piston drives it forward 1 in., he will drive backward the large
piston one-hundredth part of that length only.” Though the hydraulic
press was thus distinctly proposed as a machine by Pascal, a certain
difficulty prevented the suggestion from becoming of any practical
utility. It was found impossible, by any ordinary plan of packing, to
make the piston fit without allowing the water to escape when the
pressure became considerable. This difficulty was overcome by Bramah,
who, about the end of last century, contrived a simple and elegant plan
of packing the piston, and first made the hydraulic press an efficient
and useful machine. Fig. 166 is a view of an ordinary hydraulic press,
in which a is a very strong iron cylinder, represented in the figure
with a part broken off, in order to show that inside of it is an iron
piston or ram, _b_, which works up and down through a water-tight
collar; and in this part is the invention by which Bramah overcame the
difficulties that had previously been met with in making the hydraulic
press of practical use. Bramah’s contrivance is shown by the section of
the cylinder, Fig. 165, where the interior of the neck is seen to have a
groove surrounding it, into which fits a ring of leather bent into a
shape resembling an inverted U. The ring is cut out of a flat piece of
stout leather, well oiled and bent into the required shape. The effect
of the pressure of the water is to force the leather more tightly
against the ram, and as the pressure becomes greater, the tighter is the
fit of the collar, so that no water escapes even with very great
pressures. To the ram, _b_, Fig. 166, a strong iron table, _c_, is
attached, and on this are placed the articles to be compressed. Four
wrought iron columns, _d d d d_, support another strong plate, _e_, and
maintain it in a position to resist the upward pressure of the goods
when the ram rises, and they are squeezed between the two tables. The
interior of the large cylinder communicates by means of the pipe, _f f_,
with the suction and force-pump, _g_, in which a small plunger, _o_,
works water-tight. Suppose that the cylinders and tubes are quite filled
with water, and that the ram and piston are in the positions represented
in the figure. When the piston of the pump, _g_, is raised, the space
below it is instantly filled with water, which enters from the
reservoir, _h_, through the valve, _i_, the valve _k_ being closed by
the pressure above it, so that no water can find its way back from the
pipe, _f_, into the small cylinder. When the piston has completed its
ascent, the interior of the small cylinder is therefore completely
filled with water from the reservoir; and when the piston is pushed
down, the valve, _i_, instantly closes, and all egress of the liquid in
that direction being prevented, the greater pressure in _g_ forces open
the valve, _k_, and the water flows along the tube, _f_, into the large
cylinder. The pressure exerted by the plunger in the small cylinder,
being transmitted according to the principles already explained,
produces on each portion of the area of the large plunger equal to that
of the smaller an exactly equal pressure. In the smaller hydraulic
presses the plunger of the forcing-pump is worked by a lever, as
represented in the figure at _n_; so that with a given amount of force
applied by the hand to the end of the lever, the pressure exerted by the
press will depend upon the proportion of the sectional area of _b_ to
that of _o_, and also upon the proportion of the length _m n_, to the
length _m l_. To fix our ideas, let us suppose that the distance from
_m_ of the point _n_ where the hand is applied is ten times the distance
_m l_, and that the sectional area of _b_ is a hundred times that of
_o_. If a force of 60 lbs. be applied at _n_, this will produce a
downward pressure at _m_ equal to 60 × 10, and then the pressure
transmitted to the ram of the great cylinder will be 60 × 10 × 100 =
60,000 lbs. The apparatus is provided with a safety-valve at _p_, which
is loaded with a weight; so that when the pressure exceeds a desired
amount, the valve opens and the water escapes. There is also an
arrangement at _q_ for allowing the water to flow out when it is desired
to relieve the pressure, and the water is then forced out by the large
plunger, which slowly descends to occupy its place. The body of the
cylinder is placed beneath the floor in such presses as that represented
in Fig. 166, in order to afford ready access to the table on which the
articles to be compressed are placed.
[Illustration:
FIG. 166.—_Hydraulic Press._
]
The force which may, by a machine of this kind, be brought to bear upon
substances submitted to its action, is limited only by the power of the
materials of the press to resist the strains put upon them. If water be
continually forced into the cylinder of such a machine, then, whatever
may be the resistance offered to the ascent of the plunger, it must
yield, or otherwise some part of the machine itself must yield, either
by rupture of the hydraulic cylinder, or by the bursting of the
connecting-pipe or the forcing-pump. This result is certain, for the
water refuses to be compressed, at least to any noticeable degree, and
therefore, by making the area of the plunger of the force-pump
sufficiently small, there is no limit to the pressure per square inch
which can be produced in the hydraulic cylinder; or, to speak more
correctly, the limit is reached only when the pressure in the hydraulic
cylinder is equal to the cohesive strength of the material (cast or
wrought iron) of which it is formed. It has been found that when the
internal pressure per square inch exceeds the cohesive or tensile
strength of a rod of the metal 1 in. square (see page 207), no increase
in the thickness of the metal will enable the cylinder to resist the
pressure. Professor Rankine has given the following formula for
calculating the external radius, R, of a hollow cylinder of which the
internal radius is _r_, the pressure per square inch which it is desired
should be applied before the cylinder would yield being indicated by
_p_, while _f_ represents the tensile strength of the materials:
R = _r_√((_f_ + _p_)/(_f_ – _p_))
We may see in this formula that as the value of _p_ becomes more and
more nearly equal to _f_, the less does the divisor (_f_ – _p_) become,
and therefore the greater is the corresponding value of R; and when _f_
= _p_, or _f_ – _p_ = 0, the interpretation would be that no value of R
would be sufficiently great to satisfy the equation. Thus a cylinder,
made of cast iron, of which the breaking strain is 8 tons per square
inch, would have its inner surface ruptured by that amount of internal
pressure, and the water passing into the fissures would exert its
pressure with ever-increasing destructive effect.
With certain modifications in the proportions and arrangement of its
parts, the hydraulic press is used for squeezing the juices from
vegetable substances, such as beetroots, &c., for pressing oils from
seeds, and, in fact, all purposes where a powerful, steady, and easily
regulated pressure is needed. Cannons and steam boilers are tested by
hydraulic pressure, by forcing water into them by means of a force-pump,
just as it is forced into the cylinder of the hydraulic press described
above. This mode of testing the strength has several great advantages;
for not only can the pressure be regulated and its amount accurately
known; but in case the cannon or steam boiler should give way, there is
no danger, for it does not explode—the metal is simply ruptured, and the
moment this takes place, the water flows out and the strain at once
ceases.
The strength of bars, chains, cables, and anchors is also tested by
hydraulic power, and the engraving at the head of this article, Fig.
163, represents the hydraulic testing machine at the works of Messrs.
Brown and Lenox, the eminent chain and anchor manufacturers, of
Millwall. Immediately in front of the spectator are the force-pumps, and
the steam engine by which they are driven. It will be observed that four
plungers are attached to an oscillating beam in such a manner that the
water is continuously forced into the hydraulic cylinder. The outer pair
of plungers are of much larger diameter than the inner pair, in order
that the supply of water may be cut off from the former when the
pressure is approaching the desired limit, and the smaller pair alone
then go on pumping in the water, the pressure being thus more gradually
increased. Behind the engine and forcing pump is the massive iron
cylinder, where the pressure is made to act on a piston, which is forced
towards that end of the cylinder seen in the drawing. The piston is
attached to a very thick piston-rod, moving through a water-tight collar
at the other end of the cylinder. The effect of the hydraulic pressure
is, therefore, to draw the piston-rod into the cylinder, and not, as in
the apparatus represented in Fig. 166, to force a plunger out. The head
of the piston-rod is provided with a strong shackle, to which the chains
to be tested can be attached. In a line with the axis of the cylinder is
a trough, some 90 ft. long, to hold the chain, and at the farther end of
the trough is another very strong shackle, to which the other end of the
chain is made fast. A peculiarity of Messrs. Brown and Lenox’s machine
is the mode in which the tension is measured. In many cases it is deemed
sufficient to ascertain by some kind of gauge the pressure of the water
in the hydraulic cylinder, and from that to deduce the pull upon the
chain; but the Messrs. Brown have found that every form of gauge is
liable to give fallacious indications, from variations of temperature
and other circumstances, and they prefer to measure the strain directly.
This is accomplished by attaching the shackle at the farther extremity
of the trough to the short arm of a lever, turning upon hard steel
bearings, the long arm of this lever acting upon the short arm of
another, and so on until the weight of 1 lb. at the end of the last
lever will balance a pull on the chain of 2,240 lbs., or 1 ton. The
tension is thus directly measured by a system of levers, exactly
resembling those used in a common weighing machine, and this is done so
accurately that even when a chain is being subjected to a strain of many
tons, an additional pull, such as one can give to the shackle-link with
one hand, at once shows itself in the weighing-room. The person who has
charge of this part of the machine places on the end of the lever a
weight of as many pounds as the number of tons strain to which the chain
to be tested has to be submitted. The engineer sets the pump in action,
the water is rapidly forced into the cylinder, the piston is thrust
inwards, and the strain upon the chain begins; the engineer then cuts
off the water supply from the larger force-pumps, and the smaller pair
go on until the strain becomes sufficient to raise the weight, and then
the person in the weighing-room, by pulling a wire, opens a valve in
connection with the hydraulic cylinder, which allows the water to
escape, and the strain is at once taken off. This testing machine, which
is capable of testing cables up to 200 tons or more, was originally
designed by Sir T. Brown, the late head of the firm, and not only was
the first constructed in the country, but remains unsurpassed in the
precision of its indications.
The testing of cables, which we have just described, is a matter of the
highest importance, for the failure of cables and anchors places ships
and men’s life in great danger, since vessels have frequently to ride
out a storm at anchor, and should the cables give way, a ship would then
be almost entirely at the mercy of the winds and waves. Hence the
Government have, with regard to cables and anchors, very properly made
certain stringent regulations, which apply not only to the navy but to
merchant shipping. The chain-cable is itself a comparatively modern
application of iron, for sixty years ago our line-of-battle ships
carried only huge hempen cables of some 8 in. or 9 in. diameter.
Chain-cables have now almost entirely superseded ropes, though some
ships carry a hempen cable, for use under peculiar circumstances. The
largest chain-cables have links in which the iron has a diameter of
nearly 3 in., and these cables are considered good and sound when they
can bear a strain of 136 tons. Such are the cables used in the British
navy for the largest ships. Of course, there are many smaller-sized
cables also in use, and the strains to which these are subjected when
they are tested in the Government dockyards vary according to the
thickness of the iron; but it is found that nearly one out of every four
cables supplied to the Admiralty proves defective in some part, which
has to be replaced by a sounder piece. The chain-cables made by Messrs.
Brown and Lenox for the _Great Eastern_ are, as might have been
expected, of the very stoutest construction; the best workmanship and
the finest quality of iron having been employed in their manufacture.
These cables were tested up to 148 tons, a greater strain than had ever
before been applied as a test to any chain, and it was found that a pull
represented by at least 172 tons was required to break them. It is
difficult to believe that a teacup-full of cold water shoved down a
narrow pipe is able to rend asunder the massive links which more than
suffice to hold the huge ship securely to her anchors, but such is
nevertheless the sober fact. The regulations of the Board of Trade
require that every cable or anchor sold for use in merchant ships is to
be previously tested by an authorized and licensed tester, who, if he
finds it bears the proper strain, stamps upon it a certain mark.
The means which is afforded by hydraulic power of applying enormous
pressures has been taken advantage of in a great many of the arts, of
which, indeed, there are few that have not, directly or indirectly,
benefited by this mode of modifying force. An illustration, taken at
random, may be found in the machinery employed at Woolwich for making
elongated rifle-bullets. The bullets are formed by forcing into dies,
which give the required shape, little cylinders of solid lead, cut off
by the machine itself from a continuous cylindrical rod of the metal.
The rod, or rather filament, of lead is wound like a rope on large
reels, from which it is fed to the machine. It is in the production of
this solid leaden rope or filament that hydraulic pressure is used.
About 4 cwt. of melted lead is poured into a very massive iron cylinder,
the inside of which has a diameter of 7½ in., while the external
diameter is no less than 2 ft. 6 in., so that the sides of the cylinder
are actually 11¼ in. thick. When the lead has cooled so far as that it
has passed into a half solid state, a ram or plunger, accurately fitting
the bore of the cylinder, is forced down by hydraulic pressure upon the
semi-fluid metal. This plunger is provided with a round hole throughout
its entire length, and as it is urged against the half solidified metal
with enormous pressure, the lead yields, and is forced out through the
hole in the plunger, making its appearance at the top as a continuous
cylindrical filament, quite solid, but still hot. This is wound upon the
large iron reels as fast as it emerges from the opening in the plunger,
and these reels are then taken to the bullet-shaping machine, which
snips off length after length of the leaden cord, and fashions it into
bullets for the Martini-Henry rifle. The leaden pipes which are so much
used for conveying water and gas in houses are made in a similar manner,
metal being forced out of an annular opening, which is formed by putting
an iron rod, having its diameter of the required bore of the pipe, in
the middle of the circular opening. The lead in escaping between the rod
and the sides of the opening takes the form of a pipe, and is wound upon
large iron reels, as in the former case.
[Illustration:
FIG. 167.—_Section of Hydraulic Lift Graving Dock._
]
[Illustration:
FIG. 168.—_Section of Column._
]
Another interesting application of hydraulic power is to the raising of
ships vertically out of the water, in order to examine the bottoms of
their hulls, and effect any necessary repairs. The hydraulic lift
graving dock, in which this is done, is the invention of Mr. E. Clark,
who, under the direction of Mr. Robert Stephenson, designed the
machinery and superintended the raising of the tubes of the Britannia
Bridge, where a weight of 1,800 tons was lifted by only three presses.
The suitability of the hydraulic press for such work as slowly raising a
vessel was doubtless suggested to him in connection with this
circumstance, and the durability, economy, and small loss of power which
occurs in the action of the press, pointed it out as particularly
adapted for this purpose. The ordinary dry dock is simply an excavation,
lined with timber or masonry, from which the tide is excluded by a gate,
which, after the vessel has entered the dock at high water, is closed;
and when the tide has ebbed, and left the vessel dry, the sluice through
which the water has escaped is also closed. In a tideless harbour the
water has to be pumped out of the dock, and this last method is also
adopted even in tidal waters, so that the docks may be independent of
the state of the tides. The lift of Clark’s graving dock is a direct
application of the power of the hydraulic press, and we select for
description the graving dock constructed at the Victoria Docks for the
Thames Graving Dock Company, whose works occupy 26 acres. Fig. 167 is a
transverse section of this hydraulic lift graving dock, in which there
are two rows of cast iron columns, 5 ft. in diameter at the base, where
they are sunk 12 ft. in the ground, and 4 ft. in diameter above the
ground. The clear distance between the two rows is 60 ft., and the
columns are placed 20 ft. apart from centre to centre, sixteen columns
in each row, thus giving a length of 310 ft. to the platform, but
vessels of 350 ft. in length may practically be lifted. The bases of the
columns, one of which is represented in section in Fig. 168, are filled
with concrete, on which the feet of the hydraulic cylinders rest. The
outer columns support no weight, but act merely as guides for the
crossheads attached to the plungers. The height of the columns is 68½
ft., and a wrought iron framed platform connects the columns at the top.
In order that any inequalities in the height of the rams may be
detected, a scale is painted on each column, to mark the positions of
the crossheads. The hydraulic cylinders, which are within these columns,
have solid rams of 10 in. diameter, with a stroke of 25 ft., and on the
tops of these are fastened the crossheads, 7½ ft. long, made of wrought
iron, and supporting at the ends bars of iron, to the other ends of
which the girders of the platform are suspended. The girders are,
therefore, sixteen in number, and together form a gridiron platform,
which can be raised or lowered with the vessel upon it. The thirty-two
hydraulic cylinders were tested at a pressure of more than 3 tons per
square inch. The water is admitted immediately beneath the collars at
the top (this being the most accessible position) by pipes of only ½ in.
diameter, leading from the force-pumps, of which there are twelve, of 1⅞
in. diameter, directly worked by a fifty horse-power steam engine. The
presses are worked in three groups—one of sixteen, and two of eight
presses,—so arranged that their centres of action form a sort of tripod
support, and the presses of each group are so connected that perfect
uniformity of pressure is maintained. The raising of a vessel is
accomplished in about twenty-five minutes, by placing below the vessel a
pontoon, filled in the first instance with water, and then raising the
pontoon with the vessel on it, while the water is allowed to escape from
the pontoon through certain valves; then when the girders are again
lowered, the pontoon, with the vessel on it, remains afloat. Thus in
thirty minutes a ship drawing, say, 18 ft. of water is lifted on a
shallow pontoon, drawing, perhaps, only 5 ft., and the whole is floated
to a shallow dock, where, surrounded with workshops, the vessel, now
high and dry, is ready to receive the necessary repairs. The number of
vessels which can thus be docked is limited only by the number of
pontoons, and thus the same lift serves to raise and lower any number of
ships, which are floated on and off its platform by the pontoons. With a
pressure in the hydraulic cylinders of about 2 tons upon each square
inch, the combined action of these thirty-two presses would raise a ship
weighing 5,000 tons.
Hydraulic power has been used not only for graving docks, as shown in
the above figures, but also for dragging ships out of the water up an
inclined plane. The machinery for this purpose was invented by Mr.
Miller for hauling ships up the inclined plane of “Martin’s slip,” at
the upper end of which the press cylinder is placed, at the same slope
as the inclined plane, and the ship is attached, by means of chains, to
a crosshead fixed on the plunger. Hydraulic power has also been used for
launching ships, and the launch of the _Great Eastern_ is a memorable
instance; for the great ship stuck fast, and it was only by the
application of an immense pressure, exerted by hydraulic apparatus, that
she could be induced to take to the water. Water pressure is also
applied to hoists for raising and lowering heavy bodies, and in such
cases the pressure which is obtained by simply taking the water supply
from an elevated source, or from the water-main of a town, is sometimes
made use of, instead of that obtained by a forcing pump. The lift at the
Albert Hall, South Kensington, by which persons may pass to and from the
gallery without making use of the stairs, is worked by hydraulic
pressure in the manner just mentioned. In such lifts or hoists there is
a vertical cylinder, in which works a leather-packed piston, having a
piston-rod passing upwards through a stuffing-box in the top of the
cylinder. The upper end of the piston-rod has a pulley of 30 in. or 36
in. diameter, attached to it, and round this pulley is passed a chain,
one end of which is fixed, and the other fastened to the movable cage or
frame. So that the cage moves with twice the speed of the piston, and
the length of the stroke of the latter is one-half of the range of the
cage.
Sir William Armstrong has applied hydraulic power to cranes and other
machines in combination with chains and pulleys. His hydraulic crane is
represented by the diagram, Fig. 169, intended to show only the general
disposition of the principal parts of this machine, which is so
admirably arranged that one man can raise, lower, or swing round the
heaviest load with a readiness and apparent ease marvellous to behold.
Here it is proper to mention once for all, that the pressure for the
hydraulic machines is obtained not only by natural heads of water, or by
forcing-pumps worked by hand, but very frequently by forcing-pumps
worked by steam power. It is usual to have a set of three pumps with
their plungers connected respectively with three cranks on one shaft,
making angles of 120° with each other. A special feature of Sir W.
Armstrong’s hydraulic crane is the arrangement by which the engines are
made to be always storing up power by forcing water into the vessel,
_a_, called the “accumulator.” The accumulator—which in the diagram is
not shown in its true position—may be placed in any convenient place
near the crane, and consists of a large cast iron cylinder, _b_, fitted
with a plunger, _c_, moving water-tight through the neck of the
cylinder. To the head of the plunger is attached by iron cross-bars, _d
d_, a strong iron case filled with heavy materials, so as to load the
plunger, _c_, with a weight that will produce a pressure of about 600
lbs. upon each square inch of the inner surface of the cylinder. The
water is pumped into the cylinder by the pumping engines through the
pipe, _f_, and then the piston rises, carrying with it the loaded case,
guided by the timber framework, _g_, until it reaches the top of its
range, when it moves a lever that cuts off the supply of steam from the
pumping engine. When the crane is working the water passes out of the
cylinder, _a_, by the pipe, _h_, and exerts its pressures on the
plungers of the smaller cylinders; and the plunger of the accumulator,
in beginning its descent again, moves the lever in connection with the
throttle-valve of the engine, and thus again starts the pumps, which
therefore at once begin to supply more water to the accumulator. The
latter is, however, large enough to keep all the several smaller
cylinders of the machine at work even when they are all in operation at
once. Fig. 169 shows a sketch elevation and a ground plan of the crane
as constructed to carry loads of 1 ton, but the size of the cylinders is
somewhat exaggerated, and all details, such as pipes, guides, valves,
rods, &c., are omitted. The hydraulic apparatus is entirely below the
flooring—only the levers by which the valves are opened and closed
appearing above the surface. The crane-post, _i_, is made of wrought
iron: it is hollow and stationary; the jib, _k_, is connected with the
ties, _l_, by side-pieces, _n_, which are joined by a cross-piece at
_m_, turning on a swivel and bearing the pulley, _u_. The jib and the
side-pieces are attached at _o_ to a piece turning round the crane-post,
and provided with a friction roller, _p_, which receives the thrust of
the jib against the crane-post; the same piece is carried below the
flooring and is surrounded with a groove, which the links of the chain,
_q_, fit. This chain serves to swing the crane round, and for this
purpose the hydraulic cylinders, _r_, _r´_, come into operation. The
plungers of these have each a pulley, over which passes the chain _q_,
having its ends fastened to the cylinders, so that when, by the pressure
of the water, one plunger is forced out, the other is pushed in, and the
chain passing round the groove at _s_ swings the jib round. The
cylinders are supplied with water by pipes—omitted in the sketch, as are
also those by which the water leaves the cylinders. These pipes are
connected with valves—also omitted on account of the scale of the
diagram being too small to show their details—so that the movement of a
lever, _t_, in one or the other direction at the same time connects one
cylinder with the supply and the other with the exit-pipe. When the
crane is swinging round, the sudden closing of the valves would produce
an injurious shock, and to prevent this relief-valves are provided on
both the supply and exit-pipes communicating with each cylinder. When,
therefore, the valves are closed, the impetus of the jib and its load
acting on the chain, and through that on the plungers, continues to move
the latter, the motion is permitted to take place by the relief-valves
opening, and allowing water to enter or leave the cylinders against the
pressure of the water. There is also a self-acting arrangement by which,
when these plungers have moved to the extent of their range in either
direction, the valves are closed. The chain of the crane rests on guide
pulleys, and passing over the pulley _u_, goes down the centre of the
crane-post to the pulley _v_, and thence passes backwards and forwards
over a series of three pulleys at _w_ and two at _x_, and is fastened at
its end to the cylinder, _y_. As there are thus six lines of chain, when
the plunger of the lifting cylinder comes 1 ft. out, 6 ft. of chain pass
over the guide pulley, _u_. The plunger, when near the end of its stroke
in either direction, is made to move a bar—not shown—which closes the
valve. When the crane is loaded, the load is lowered by simply opening
the exhaust-valve, when the lift-plunger will be forced back into its
cylinder by the pull on the chain. But as the chain may require to be
lowered when there is no load upon it, although a bob is provided at _z_
to draw the chain down, it would be disadvantageous to increase the
weight of this to the extent required for forcing back the lifting
plunger. A _return_ cylinder is therefore made use of, the plunger of
which has but a small diameter, and is connected with the head of the
lift-plunger, so that it forces the latter back when the lift-cylinder
is put in communication with the exhaust-pipe. The water is admitted to
the lifting cylinder from the accumulator by a valve worked by a lever,
which, when moved the other way, closes the communication and opens the
exhaust-pipe, and then the pressure in the return cylinder, which is
constant, drives in the plunger of the lifting cylinder. The principle
of the accumulator may plainly be used with great advantage even when
manual labour is employed, for a less number of men will be required for
working the pumps to produce the effect than if their efforts had to be
applied to the machine only at the time it is in actual operation, for
in the intervals they would, in the last case, be standing idle.
Apparatus on the same plan has been used with advantage for opening and
shutting dock gates, moving swing bridges, turn-tables, and for other
purposes where a considerable power has to be occasionally applied.
[Illustration:
FIG. 169.—_Sir W. Armstrong’s Hydraulic Crane._
]
[Illustration:
FIG. 170.—_Raising Tubes of the Britannia Bridge._
]
[Illustration:
FIG. 171.—_Press for Raising the Tubes._
]
[Illustration:
FIG. 172.—_Head of Link-Bars._
]
A famous example of the application of hydraulic power was the raising
of the great tubes of the Britannia Bridge. As already stated, the tubes
were built on the shore, and were floated to the towers. This was done
by introducing beneath the tubes a number of pontoons, provided with
valves in the bottom, so as to admit the water to regulate the height of
the tube according to the tide. The great tubes were so skilfully guided
into their position that they appeared to spectators to be handled with
as much ease as small boats. The mode in which they were raised by the
hydraulic presses wall be understood from Fig. 170, where A is one of
the presses and C the tube, supported by the chains, B. The tubes were
suspended in this manner at each end, and as the great tubes weighed
1,800 tons, each press had, therefore, to lift half this weight, or 900
tons. The ram or plunger of the pump was 1 ft. 8 in. in diameter, and
the cylinder in which it worked was 11 in. thick. Two steam engines,
each 40 horse-power, were used to force the water into the cylinders.
These cylinders were themselves remarkable castings, for each contained
no less than 22 tons of iron. Notwithstanding the great thickness of the
metal, an unfortunate accident occurred while the plungers were making
their fourth ascent, for the bottom of one of the cylinders gave way—a
piece of iron weighing nearly a ton and a half having been forced out,
which, after killing a man who was ascending a rope ladder to the press,
fell on the top of the tube 80 ft. below, and made in it a deep
indentation. The accident occasioned a considerable delay in the
progress of the work, for a new cylinder had to be cast and fitted. Such
an accident would assuredly have caused the destruction of the tube
itself but for the foresight and prudence of the engineer in placing
beneath the ends of the vast tube as it ascended slabs of wood 1 in.
thick, so that it was impossible for the tube to fall more than 1 in. It
must be stated that as the tube was lifted each step, the masonry was
built up from below, and then as the next lift proceeded inch by inch, a
slab of wood was placed under the ends. Although by the giving way of
the cylinder of the hydraulic press the end of the tube fell through no
greater space than 1 in., the momentum was such that beams calculated to
bear enormous weights were broken. At the time of the accident the
pressure in the cylinder did not exceed that which it was calculated to
bear or that which is frequently applied in hydraulic presses for other
purposes. Some scientific observers attributed the failure of the
cylinder to the oscillating of the tube. It had been found when the
similar tubes of the bridge over the Conway were being raised, that when
the engines at each end made their strokes simultaneously, a dangerous
undulation was set up in the tube, and it was therefore necessary to
cause the strokes of the engines to take place alternately. The chains
by which the tubes were suspended were made of flat bars 7 in. wide and
about 1 in. thick, being rolled in one piece, with expanded portions
about the “eye,” through which the connecting-bolts pass. The links of
the chain consisted of nine and eight of these bars alternately—the bars
of the eight-fold links being made a little thicker than those of the
nine-fold, so as to have the same aggregate strength. The mode in which
the hydraulic presses were made to raise the tubes is very clearly
described by Sir William Fairbairn in his interesting work on the Conway
and Britannia Bridges, and his account of the mode of raising the tubes
is here given in his own words, but with letters referring to Fig. 171:
“Another great difficulty was to be overcome, and it was one which
presented itself to my mind with great force, viz., in what manner the
enormous weight of the tube was to be kept suspended when lifted to the
height of 6 ft., the proposed travel of the pump, whilst the ram was
lowered and again attached for the purpose of making another lift. Much
time was occupied in scheming means for accomplishing this object, and
after examining several projects, more or less satisfactory, it at last
occurred to me that, by a particular formation of the links (of the
chain by which the tubes were to be suspended) we might make the chains
themselves support the tube. I proposed that the lower part of the top
of each link, immediately below the eye, should be formed with square
shoulders cut at right angles to the body of the link (Fig. 172). When
the several links forming the chain E were put together, these shoulders
formed a bearing surface, or “hold,” for the crosshead B attached to the
top of the ram A of the hydraulic pump. But the upper part of this
crosshead, C C, was movable, or formed of clips, which fitted the
shoulders of the chain, and were worked by means of right- and
left-handed screws, and could be made either to clip the chain
immediately under the shoulders when the ram of the pump was down and a
lift about to be made, or be withdrawn at pleasure. Attached to the
large girders F were a corresponding set of clips, D D, which were so
placed and adjusted as to height that when the ram of the pump was at
the top there was distance between the two sets of clips equal to twice
the length of the travel of the pump, or the length of the two sets of
the links of the chain. To render the action of the apparatus more
clear, suppose the tube resting on the shelf of masonry in the position
that it was left in after the operation of floating was completed, and
the chains attached, and everything ready for the first lift, the ram of
the pump being necessarily down. The upper set of clips attached to the
crosshead are forced under the shoulders of the links, and the lower set
of clips attached to the frames resting upon the girders are drawn back,
so as to be quite clear of the chain; the pumps are put into action
simultaneously at both ends of the tube, and the whole mass is slowly
raised until it has reached a height of 6 ft. from its original
resting-place. The clips attached to the crosshead, B, have so far been
sustaining the weight, but it will be observed that by the time the pump
has ascended to its full travel, the square shoulders of another set of
links have come opposite to the lower clips on the girders, D, and these
clips are advanced under the shoulders of the links, and the rams being
allowed to descend a little, they in their turn sustain the load and
relieve the pumps. The upper clips being withdrawn, the rams are allowed
to descend, and after another attachment, a further lift of 6 ft. is
accomplished; and thus, by a series of lifts, any height may be
attained. The fitness of this apparatus for its work was admirable, and
the action of the presses was, as Mr. Stephenson termed it, delightful.”
[Illustration:
FIG. 173.—_Apparatus to prove Transmission of Pressure in all
directions._
]
[Illustration:
FIG. 174.—_Pneumatic Tubes and Carriages._
]
PNEUMATIC DISPATCH.
When the use of the electric telegraph became general, it was found
necessary to establish in all large towns branch stations, from which
messages were conveyed to the central station, or to which they were
sent, either by messengers who carried the written despatch, or by
telegraphing between the central and branch stations. The latter had the
disadvantages of rendering the original message liable to an additional
chance of incorrect transmission, and when an unusually great number of
despatches had to be sent to or from a particular branch station, there
was necessarily great delay in the forwarding of them. The plan of
sending the written messages between the central stations by bearers was
unsatisfactory on account of the time occupied. These inconveniences led
to the invention of a system for propelling, by the pressure of air, the
papers upon which the messages were written through tubes connecting the
stations. This was first carried into practice by the Electric and
International Telegraph Company, who, in this way, connected their
central station in London with their City branch stations. The apparatus
was designed and erected by Mr. L. Clark and Mr. Varley in 1854. The
first tube laid down was from Lothbury and the Stock Exchange—a distance
of 220 yards. This tube had an inside diameter of only 1½ in.; but a
larger tube, having a diameter of 2¼ in. was, some years afterwards,
laid between Telegraph Street and Mincing Lane—a distance of 1,340
yards—and was used successfully. In these tubes the carriers were pushed
forward by the pressure of the atmosphere, a vacuum having been produced
in front by pumping out the air. The plan of propelling the carrier by
compressing the air behind it was also tried with good results, and, in
fact, with a gain of speed; for, while a carrier occupied 60 or 70
seconds in passing from Telegraph Street to Mincing Lane when drawn by a
vacuum, it accomplished its journey in 50 or 55 seconds when it was shot
forwards by compressed air, the difference in pressure before and behind
it being the same in each case. A great deal of trouble was occasioned
when the vacuum system was used, by water being drawn in at the joints
of the pipes. This water sometimes accumulated to such a degree,
especially after wet weather, that it completely overcame the power of
the vacuum to draw the air through it, by lodging in the vertical
portions of the tube, where they passed to the upper floors of the
central station. This was remedied by improving the construction of the
joints, and by arranging a syphon for drawing off any water which might
be present. The best construction of the carrier was another matter
which required some experience to discover. It was found that
gutta-percha, or papier maché covered with felt, was the most efficient
material. The tubes found by Mr. Varley to give the best results were
formed of lead covered externally with iron pipes. The joints were made
perfectly smooth in the inside by means of a heated steel mandrel, on
which they were formed, so that the tube was of one perfectly uniform
bore throughout. An ingenious arrangement was also adopted by which the
air itself was made to do the work of opening and closing the valves,
and even that of removing the carrier from the tube: when, by a
telegraphic bell, rung from the distant station, it was announced that a
carrier was dispatched, the attendant at the receiving station had only
to touch for a second a knob marked “receive,” which put the tube in
communication with the vacuum, in which condition it remained until the
arrival of the carrier, which, by striking against a pad of
india-rubber, released the detent, and thus cut off the vacuum. The
carrier then fell out of the receiver and dropped into a box placed to
catch it. When a carrier was sent, it was placed in the tube, and a
button marked “send” was touched, by which a communication was opened
with a vessel of compressed air and the end of the tube behind the
carrier was immediately closed by a slide, the movements being all
performed by the air itself. On the arrival of the carrier, the boy at
the receiving station rang an electric bell to signal its reception; and
the sender then touched another knob marked “cut off,” which caused the
supply of compressed air to be cut off, and the slide to be withdrawn
from the end of the tube, which was then ready either to receive or send
carriers. By this arrangement there was no waste of power, for the
reservoirs of compressed air or of vacuum were only drawn upon when the
work was actually required to be done.
The tubes laid down by the Telegraph Company are still in active
operation; but at the new Central Telegraph Station the automatic valves
of Messrs. Clark and Varley appear to be dispensed with, and the
attendants perform the work of closing the tube, shutting off the
compressed air, &c., by a few simple movements.
In December, 1869, Messrs. Siemens were commissioned by the
Postmaster-General to lay tubes on their system from the General Post
Office to the Central Telegraph Station; and the work having been
accomplished in February, 1870, and proving perfectly satisfactory after
six weeks’ trial, it was decided to connect in the same manner Fleet
Street and the West Strand office at Charing Cross with the Central
Station. The system proposed by the Messrs. Siemens consisted in forming
a circuit of tubes, through which the carriers might be continually
passing in one direction. The diagram, Fig. 175, will give an idea of
the manner in which it was designed to arrange the tubes between the
Central Telegraph Station and Charing Cross. The arrows indicate the
direction in which the air rushes through the tubes; A is the piston in
the cylinder, and valves are so arranged as to pump air out of the
chamber V, and compress it into the chamber P. This plan has been
departed from, so far as regards the Charing Cross Station, for want of
space there prevented the tube being curved with a radius large enough
to convey the carriers without their being liable to stick, and
consequently, these are not carried round in the tube. The passage of
carriers being stopped here, there are, in point of fact, two tubes: an
“up” tube and a “down” tube. But these are connected by a sharp bend, so
that though the tube is continuous as regards the air current, it is
interrupted as regards the circulation of the carriers. The tubes are of
iron, 3 in. internal diameter, made in lengths of about 19 ft.; and for
the turns and bends, pieces are curved with a radius of 12 ft. Both
lines are laid side by side in a trench at about a foot depth below the
streets. The ends of the adjacent lengths form butt joints, so that the
internal surface is interrupted as little as possible, and there is a
double collar to fasten the lengths together. Arrangements are also made
for removing from the inside of the tubes water or dirt, or matter which
may in any manner have got in.
[Illustration:
FIG. 175.—_Diagram of Tubes, &c._
]
[Illustration:
FIG. 176.—_Sending and Receiving Apparatus.—Transverse Section._
]
One special feature of Messrs. Siemens’ invention is the plan by which
the carriers are introduced into and removed from the tube at any
required station without the circulation of the air being interfered
with. The simple yet ingenious mechanism by which this is effected will
be understood from the sections shown in Figs. 176 and 177. The figures
represent the position of the apparatus when placed to receive a
carrier; A´ is the receptacle into which the carrier is shot by the air
rushing from A towards A´´. This receptacle is ᗜ-shaped, the curve of
the Ⅾ corresponding with that of the tube, and the upper flat part
admitting of a piece of plate glass being inserted, through which the
attendant may perceive when a carrier arrives. The progress of the
carrier is arrested by a perforated plate, B, which allows the air to
pass. The ends of this receptacle are fixed in two parallel plates, F
F´, which also receive the ends of the plain cylinder, having precisely
the same diameter as the tube, A. These plates are connected also by
cross-pieces, D E, the whole forming a sort of frame, which turns upon E
as a centre; and according as it is put in the position shown by the
plain line in Fig. 176, or in that indicated by the dotted lines, causes
the receiving tube or the hollow cylinder to form part of the main tube,
the cross-piece, D, serving as a handle for moving the apparatus. It
should be remarked that the plates are made to fit the space cut out of
the main tube with great nicety, otherwise much loss of power would
result from leakage. When the hollow cylinder is in a line with the main
tube, it is plain that the carrier will not be stopped, as the tube is
then continuous and uninterrupted. In this hollow cylinder also the
carrier to be sent is deposited after the rocking frame has been placed
on it, Fig. 177; then, on drawing the handle, the hollow cylinder is
brought into the circuit, and the carrier at once shoots off. To stop a
carrier, the receiving-tube is put in by another movement of the handle,
and when the carrier arrives, it is removed by bringing the open
cylinder, or _through tube_, into the circuit, and thus making the
receiver ready for having the carrier pushed out of it by a rod which is
made to slide out by moving a handle. In order to avoid the obstruction
to the movement of the air which would be caused by the carrier while in
the receiving-tube, a pipe, G, is provided, through which the air
chiefly passes when the perforations of the plate, B, are closed by the
presence of a carrier. In this pipe at H is a throttle-valve, which is
opened by tappets, K, on the rocking frames when the receiver is in
circuit, and again closed when the open tube is substituted. The current
thus suffers no interruption by the action of the apparatus.
[Illustration:
FIG. 177.—_Receiving Apparatus.—Longitudinal Section._
]
The carriers are small cylinders of gutta-percha, or papier maché,
closed at one end, and provided with a lid at the other. They are
covered with felt or leather, and at the front they are furnished with a
thick disc of drugget or leather, like the leathers of a common
water-pump, but fitting quite loosely in the tube. Such a carrier, being
placed in the tube at the Central Station, Fig. 175, will be carried by
the current in the direction of the arrows to the Charing Cross Station,
where its progress will be interrupted; but according to the original
plan it would continue its journey until it again reached the Central
Station, where it would be intercepted by the diaphragm, Fig. 175. But
the carrier is stopped, if at any station the receiving-tube is placed
in circuit, and this is done when an electric signal indicates to the
station that a carrier intended for it has been dispatched. The tubes
are worked on the “block system,” that is, each section is known to be
clear before a carrier is allowed to enter it, and a bell is provided,
which is struck by a little lever, moved by each carrier in its passage
through, so that the attendant at each station knows when a carrier has
shot along the “through tube” of the station. This mode of working the
tubes renders the liability to accidents much less, but their carrying
power might be increased by dispatching carriers at regular and very
short intervals of time, when the limit would be only in the ability of
the attendants to receive a carrier and open the circuit in sufficient
time to allow the next following one to proceed without stoppage. The
length of the lines of tube laid down on this system, with the times
required for the carriers to traverse them, are stated below, the
pressure and the vacuum being respectively equal to the absolute
pressures of 22 lbs. and 5½ lbs. on each square inch of the reservoirs
during the experiments:
┌────────────────────────────────────────┬──────┬──────┐
│ │Yards.│M. S. │
│Telegraph Station to General Post Office│ 852│ 1 54│
│General Post Office to Temple Bar │ 1,206│ 2 28│
│Temple Bar to General Post Office │ 1,206│ 2 10│
│General Post Office to Telegraph Station│ 852│ 1 13│
│ │ —————│ —————│
│ │ 4,116│ 7 45│
└────────────────────────────────────────┴──────┴──────┘
When the air was not compressed, but the vacuum only was used, the air
being allowed to enter the other end of the tube at the ordinary
atmospheric pressure, the time required for the carrier to traverse the
circuit was 10 minutes 23 seconds. In this case the vacuum was
maintained, so that the air was constantly in movement; but when the
experiment was tried by allowing the air in the tube to become
stationary, placing a carrier at one end, and then opening communication
with the vacuum reservoir at the other, the carrier required 13½ minutes
to complete the journey. This is explained by the fact of the greater
part of the air having to be exhausted from the tube before the carrier
could be set in motion.
The utility and advantage of the pneumatic system is well seen when its
powers are compared with the wires. Thus, a single carrier, which may
contain, say, twenty-seven messages, can be sent every eight minutes;
and since not more than one message per minute could be transmitted by
telegraph wire, even by the smartest clerks, the real average being
about two minutes for each message, it follows that only four messages
could be sent in the time required for a single carrier to traverse the
up tube, and to do the work which could be done by the tube seven wires
and fourteen clerks would be required.
Mr. R. S. Culley, the official telegraph engineer, states as his
experience of the relative wear and tear of the carriers in these iron
tubes and in the smooth lead tubes, that it had been found necessary to
renew the felt covering of eighty-two dozen of the carriers used for
three months in the iron tubes, while in the same period only
thirty-eight dozen of those used in the lead tubes required to be
recovered. The numbers of carriers sent and received by the pneumatic
tubes on the 21st of November, 1871, between 11 a.m. and 4 p.m., were:
Iron tubes │ 135
2¼ in. lead tubes 1,170│
│1,697
1½ in. lead tubes 527│
The mileage of the carriers sent was much greater in the lead than in
the iron pipes, although the total lengths of each kind were
respectively 5,974 yards and 6,826 yards. The result is remarkable, as
showing the effect of apparently slight differences when their operation
is summed up by numerous repetitions.
The circuit at Charing Cross having been divided on account of the
difficulty mentioned above, the tubes act as separate pipes—one for “up”
traffic (_i.e._, to Central Telegraph Station), the other for “down”
(_i.e._, from the Central Station). The air, however, still accomplishes
a circuit, being exhausted at one end and compressed at the other. A
very noticeable and curious difference is found between the times
required by the carriers to perform the “up” and the “down” journeys:
An “up” carrier requires 6·5 minutes
A “down” carrier requires 12·5 minutes
————
Together 19·0 minutes
When two pipes were separated at Charing Cross so that the air no longer
circulated from one to the other, but both were left open to the
atmosphere, while the “up” pipe was worked by a vacuum only and the
“down” pipe by pressure only, the times were for
An “up” carrier 8·5 minutes
A “down” carrier 11·3 minutes
————
Together 19·8 minutes
The time, therefore, for the whole circuit was practically the
same—whether the tubes were worked by a continuous current of air or
separated, and one worked by the vacuum and the other by pressure. It
was also seen that when the tubes were connected so that the air current
was continuous, and the pump producing a vacuum at one end and a
compression at the other, the neutral point where the pressure was equal
to that of the atmosphere was not found midway between the two
extremities—that is, at Charing Cross Station—but much nearer the vacuum
end. When the tubes were disconnected, it appeared, as already shown by
the figures given above, that there was a gain of speed on the down
journey, and a loss of speed on the up journey; and as the requirements
of the traffic happened to require greater dispatch for the down
journeys, the tubes have been worked in this manner.
It has been proposed to convey letters by pneumatic dispatch between the
General and Suburban Post Offices, and the Post Office authorities have
even consulted engineers on the practicability of sending the Irish
mails from London to Holyhead by this system. It was calculated,
however, that although the scheme could be carried out, the proportion
of expense for great speeds and long distances would be enormously
increased. A speed of 130 miles per hour was considered attainable, but
the wear and tear of the carriers would be extremely great at this high
velocity, and it was considered doubtful whether this circumstance might
not operate seriously against the practical carrying out of the plan.
The prime cost would be very great, for the steam power alone which
would be requisite would amount to 390 horse-power for every four miles.
We thus see that very high velocities would introduce a new order of
difficulties in the practical working. The case as regards the velocity
with which electric signals can be sent round the world is very
different.
An amusing hoax appears to have been perpetrated by some waggish
telegraph clerk on an American gentleman at Glasgow, with regard to the
pneumatic system of sending messages; for the gentleman sent to the
“Boston Transcript” a letter, in which he relates that having sent a
telegraphic message from Glasgow to London, he received in a few minutes
a reply which indicated a mistake somewhere, and then he went to the
Glasgow telegraph office, and asked to see his message.
“The clerk said, ‘We can’t show it to you, as we have sent it to
London.’ ‘But,’ I replied, ‘you must have my original paper here. I wish
to see that.’ He again said, ‘No, we have not got it: it is in the post
office at London.’ ‘What do you mean?’ I asked. ‘Pray, let me see the
paper I left here half an hour ago.’ ‘Well,’ said he, ‘if you must see
it, we will get it back in a few minutes, but it is now in London.’ He
rang a bell, and in five minutes or so produced my message, rolled up in
pasteboard.... I inquired if I might see a message sent. ‘Oh, yes; come
round here.’ He slipped a number of messages into the pasteboard scroll,
popped it into the tube, and made a signal. I put my ear to the tube and
heard a slight rumbling noise for seventeen seconds, when a bell rang
beside me, indicating that the scroll had arrived at the General Post
Office, 400 miles off. It almost took my breath away to think of it.”
In the journal called “Engineering,” into which this curious letter was
copied, it is pointed out that to travel from London to Glasgow, a
distance of 405 miles, in seventeen seconds, the carrier must have moved
at the rate of 24 miles per second, or 5 miles a second faster than the
earth moves in its orbit, and the carrier would have in such a case
become red hot by its friction against the tube before it had travelled
a single second.
A plan of conveying, not telegraph messages, but parcels, was proposed
and carried into effect some time ago, and more recently has been
applied to lines of tubes in connection with the General Post Office.
These tubes pass from Euston Station down Drummond Street, Hampstead
Road, Tottenham Court Road, to Broad Street, St. Giles’s, whence, with a
sharp bend, they proceed to the Engine Station at Holborn, and then to
the Post Office. The tube is formed chiefly of cast iron pipes of a
⌓-shaped section, 4 ft. 6 in. wide and 4 ft. high, in 9 ft. lengths.
There are curves with radii of 70 ft. and upwards, and at these parts
the tube is made of brickwork and not of iron. The carriages run on four
wheels, and are so constructed that the ends fit the tubes nearly, and
the interval left is partly closed by a projecting sheet of india-rubber
all round. The carriages are usually sent through the tube in trains of
two or three, and the trains are drawn forward by an exhausting
apparatus formed by a fan, 22 ft. in diameter, worked by two horizontal
steam engines having cylinders 24 in. in diameter and a stroke of 20 in.
The air rushes by centrifugal force from the circumference of the fan,
and is drawn in at the centre, where the exhaust effect is produced. The
tubes which convey the air from the main tube open into the latter at
some distance from its extremities, which are closed by doors, so that
after the carriage passes the entrance of the suction tube, its momentum
is checked by the air included between it and the doors, which air is,
of course, compressed by the forward movement of the carriage. At the
proper moment the doors are opened by a self-acting arrangement, and the
carriage emerges from the tube. There are two lines of tube—an “up” and
a “down” line—and means are provided for rapidly transferring the
carriages from one to the other at the termini. The time occupied in the
transit is about 12 minutes. Some of the inclines have as much slope as
1 in 14, yet loads of 10 or 12 tons weight are drawn up these gradients
without difficulty. The mails are sent between Euston Station and the
Post Office by means of these tubes. Passengers have also made the
journey as an experiment by lying down in the carriages. Fig. 174 shows
one of the carriages and the entrance to the tubes.
Great expectations have been formed by some persons of the applications
of pneumatic force. Some have suggested its use for moving the trains in
the proposed tunnel between England and France. But calculations show
that for long distances and large areas such modes of imparting motion
are enormously wasteful of power. Thus, in the tunnel alluded to it must
be remembered that not only the train, but the whole mass of air in the
tunnel would have to be drawn or pushed forward. The drawing of a train
through by exhausting the air would be very similar to drawing it
through by a rope; in fact, the mass of air may be regarded as a very
elastic rope, but by no means a very light one, or one that could be
drawn through without some opposing force which has a certain
resemblance to friction coming into operation. Indeed, it has been
calculated that in the case named, only five per cent. of the total
power exerted by the engines in exhausting the air could possibly
produce a useful effect in moving the train.
Air has also been made the medium for conveying intelligence in another
manner than by shooting written messages through tubes, for its property
of transmitting pressure has been applied to produce at a distance
signals like those made use of in the electric telegraph system. A few
years ago, an apparatus for this object was contrived by Signor
Guattari, whose invention is known as the “Guattari Atmospheric
Telegraph.” In this there is a vessel charged with compressed air by a
compression-pump, and the pressure is maintained by the same means,
while the reservoir is being drawn upon. A valve is so arranged that the
manipulator can readily admit the compressed air to a tube extending to
the station where the signals are received, at which the pressure is
made to move a piston as often as the sender opens the valve. This
movement is made to convey intelligence when a duly regulated succession
of impulses is sent into the tube—the receiving apparatus being arranged
either to give visible or audible signals, or to print them on slips of
paper, according to any of the methods in use with the electric
telegraph. Certain advantages over the electric system are claimed for
this pneumatic telegraph—as, for example, greater simplicity and less
liability to derangement. The tubes, which are merely leaden piping of
small bore, are also exempt from the inconvenient interruptions which
electric communication sometimes suffers from electrical disturbances in
the atmosphere. The pneumatic system is easily arranged, and from its
great simplicity any person can in a few hours learn to use the whole
apparatus, while it is calculated that the expense of construction and
working would not be above half of that incurred for the electric
system. For telegraphs in houses, ships, warehouses, and short lines,
this invention will doubtless prove very serviceable; but for long lines
a much greater force of compression would be required, and the time
needed for the production of an impulse at the distant ends of the tubes
would be considerably increased. [1875].
[Illustration]
[Illustration:
FIG. 178.—_The Sommeiller Boring Machines._
]
ROCK BORING.
Allusion has already been made to one great characteristic of our age,
namely, the replacement, in every department of industry, of manual
labour by machines. A brief notice of even the main features of the
various contrivances which have been made to take the place of men’s
hands would more than occupy this volume. Accordingly, we must omit all
reference to many branches of manufacture, although the products may be
of very great utility, and the processes of very high interest; and in
taking one example here and another there, we must be guided mainly by
the extent and depth of the influence which the new invention appears
destined to exert. This consideration has, with scarcely an exception,
decided the selection of the topics already discussed, and it has also
determined the introduction of the present article, which relates to
machines of no less general importance than the rest, although at first
sight it might seem to enter upon the details of merely a special branch
of industry. But so general are the interests connected with the subject
we are about to lay before our readers, that we are not sure it would
not have been more logical to have placed the present article before all
the rest. For whence comes the iron of which our steam engines, tools,
rails, ships, cannon, bridges, and printing presses are made?—whence
comes the fuel which supplies force to the engines?—whence come, in
fine, the substances which form the _matériel_ of every art? Plainly
from the earth—the nurse and the mother of all, and in most cases from
the bowels of the earth, for her treasures are hidden far below the
surface—the coal, and the ores of iron and other metals, are not ready
to our hand, exposed to the light of day. The railways also, and the
canals, can be made only on condition that we cut roads through the
solid rocks, and pierce with tunnels the towering mountains. Hence the
tools which enable us to penetrate into the substance of the earth
present the highest general interest from a practical point of view, and
this interest is enhanced by the knowledge of the structure and past
history of our planet acquired in such operations.
The operations by which solid rocks are penetrated in the sinking of
shafts for mines, or in the driving of tunnels, drifts, headings,
galleries, or cuttings for railways, mines, or other works, are easily
understood. In the first place a number of holes—perhaps 3 ft. or 4 ft.
deep and 2 in. or 3 in. in diameter—are formed in the rock. The holes
are then charged with gunpowder or other explosive materials, a
slow-burning match is adjusted, the miners retire to a safe distance,
the explosion takes place—detaching, shattering, and loosening masses of
the rock more or less considerable; and then gangs of workmen clear away
the stones and _débris_ which have been detached by the explosion, and
the same series of operations is renewed. The holes for the blasting
charges are formed by giving repeated blows on the rock with a kind of
chisel called a _jumper_—the end of which is formed of very hard steel,
so that the rock is in reality chipped away. The _débris_ resulting from
this operation is cleared away from time to time by a kind of auger or
some similar contrivance. But for many purposes it is necessary to drill
holes in rocks to great depths, hundreds of feet perhaps, as for
example, in order to ascertain the nature of underlying strata, or to
verify the presence of coal or other minerals before the expense of
sinking a shaft is incurred. These bore-holes were commonly formed in
exactly the same manner as the blast-holes already mentioned, by
repeated blows of a chisel or jumper, which was attached to the end of a
rod; and as the hole deepened, additional lengths of rod were joined on,
and the rods were withdrawn from time to time to admit of the removal of
the _débris_ by augers, or by cylinders having a valve at the bottom.
The reciprocating movement is given to the chisels and rods either by
hand or by steam or water power. When the length of the rods becomes
considerable, of course the difficulty of giving the requisite blows in
rapid succession is greatly increased, for the whole length of rods has
to be lifted each time, and if allowed to fall with too much violence,
the breaking of the chisel or the rods is the inevitable result. The
time requisite for drawing out the rods, removing the fragments chipped
out, and again attaching the rods and lowering, also increases very much
as the bore gets deeper. Messrs. Mather and Platt, the Manchester
engineers, have, in order to obviate these difficulties, constructed
machines in which the chipping or cutting is done by the fall of a tool
suspended from a rope, the great advantage resulting from the
arrangement being the facility and rapidity with which the tools used
for the cutting and for the removal of the _débris_ are lowered to their
work and drawn up. It is necessary in using the jumper, whether in
cutting blast-holes or bore-holes, to give the tool a slight turn after
each blow, in order that the rock may be chipped off all round, and the
action of the tool equalized. Many attempts have been made to drill
rocks after the fashion in which iron is drilled—that is, by drilling
properly so called, in which the tool has a rapid rotary motion. But
even in comparatively soft rock, it is found that no steel can
sufficiently withstand the abrading action of the rock, for the tool
becomes quickly worn, and makes extremely slow progress. We shall have
presently to return to the subject of bore-holes; but now let us turn
our attention to an example which will illustrate the nature and
advantages of the machinery which has in recent times been applied to
work the jumpers by which the holes for blasting are formed.
_THE MONT CENIS TUNNEL._
The successful construction, by the direction of Napoleon, of a broad
and easy highway from Switzerland into Italy, crossing the lofty Alps
amid the snows and glaciers of the Simplon, has justly been considered a
feat of skill redounding to the glory of its designers. But we have
recently witnessed a greater feat of engineering skill, for we have seen
the Alps conquered by the stupendous work known as the Mont Cenis
Tunnel. This tunnel is 7½ English miles in length; but it is not the
mere length which has made the undertaking remarkable. The mountain
which is pierced by the tunnel is formed entirely of hard rock, and what
added still more to the apparently impracticable character of the
proposal when first announced was the circumstance that it was quite
impossible to sink vertical shafts, so that the work could not, as in
the usual process, be carried on at several points simultaneously, but
must necessarily be continued from the two extremities only, a
restriction which would occasion a vast loss of time and much expense,
to say nothing of the difficulties of ventilating galleries of more than
three miles in length. The reader must bear in mind that the importance
of this question of ventilation depends not simply on the renewing of
the air contaminated by the respiration of the workmen, but on the quick
removal of the noxious gases produced in the explosions of the blasting
charges. A work surrounded by such difficulties would probably have
never been attempted had not Messrs. Sommeiller and Co. invited the
attention of engineers to an engine of their invention, worked by
compressed air, and capable of automatically working “jumpers” which
could penetrate the hardest rock. These rock-boring machines, having
been examined by competent authorities in the year 1857, were pronounced
so efficient that the execution of the long-spoken-of Alpine tunnel was
at once resolved upon, and before the close of that year the work had
actually been commenced, after a skilful and accurate survey of the
proposed locality had been made, and the direction of the tunnel set
out. The tunnel does not pass through Mont Cenis, although the post road
from St. Michel to Susa passes over part of Mont Cenis, which gives its
name to the pass. The mountain really pierced by the tunnel is known as
the Grand Vallon, and the tunnel passes almost exactly below its summit,
but at a depth the perpendicular distance of which is as nearly as
possible one mile. The northern end of the tunnel is near a village
named Fourneaux.
Pending the construction of the Sommeiller machines, and other machinery
which was to supply the motive force, the work of excavation was
commenced at both ends, in 1857, in the ordinary manner, that is, by
hand labour, and in 1858 surveys of the greatest possible accuracy were
meanwhile made, in order that the two tunnels might be directed so that
they would meet each other in the heart of the mountain. The reader will
at once perceive that the smallest error in fixing on the direction of
the two straight lines which ought to meet each other would entail very
serious consequences. The difficulties of doing this may be conceived
when we remember that the stations were nearly 8 miles apart, separated
by rugged mountains, in a region of snows, mists, clouds, and winds,
over which the levels had to be taken, and a very precise triangulation
effected. So successfully were these difficulties overcome, and so
accurately were the measurements and calculations made, that the
junction of the centre lines of the completed tunnel failed by only a
_few inches_, a length utterly insignificant under the conditions.
The work was carried on by manual labour only, until the beginning of
1861, for it was found, on practically testing the machinery, that many
important modifications had to be made before it could be successfully
employed in the great work for which it was designed. After the
machinery had been set to work, at the Bardonnêche end, breakages and
imperfections of various parts of the apparatus, or the contrivances for
driving it, caused delay and trouble, so that during the whole of 1861
the machines were in actual operation for only 209 days, and the
progress made averaged only 18 in. per day, an advance much less than
could have been effected by manual labour. The engineers, not
disheartened or deterred by these difficulties and disappointments,
encountered them by making improvement after improvement in the
machinery as experience accumulated, so that a wonderful difference in
the rate of progress showed itself in 1862, when the working days
numbered 325, and the average rate of advance was _three feet nine
inches per day_.
At the Fourneaux extremity more time was required for the preparation of
the air-compressing machinery, and the machines had been at work in the
other extremity, with more or less interruption, for nearly two years
before the preparations at Fourneaux were completed.
The illustration at the head of this article, Fig. 178, represents the
Sommeiller machines at work, the motive power being compressed air,
conveyed by tubes from receivers, into which it is forced until the
pressure becomes equal to that of six atmospheres, or 90 lbs. per square
inch. The compression was effected by taking advantage of the natural
heads of water, which were made to act directly in compressing the air;
the pressure due to a column of water 160 ft. high being made to act
upwards, to compress air, and force it through valves into the
receivers; then the supply of water was cut off, and that which had
risen up into the vessel previously containing air was allowed to flow
out, drawing in after it through another valve a fresh supply of air;
and then the operations were repeated by the water being again permitted
to compress the air, and so on, the whole of the movements being
performed by the machinery itself. The compressed air, after doing its
work in the cylinders of the boring tools, escaped into the atmosphere,
and in its outrush became greatly cooled, a circumstance of the greatest
possible advantage to the workmen, for otherwise, from the internal
warmth of the earth, and that produced by the burning of lights,
explosions of gunpowder, and respiration, the heat would have been
intolerable. At the same time, the escaping air afforded a perfect
ventilation of the workings while the machines were in action. At other
times, as after the explosion of the charges, it was found desirable to
allow a jet of air to stream out, in order that the smoke and carbonic
acid gas should be quickly cleared away. Even had the work been done by
manual labour alone, a plentiful supply of compressed air would have
been required merely for ventilation, so that there was manifest
advantage in utilizing it as the motive power of the machines.
[Illustration:
FIG. 179.—_Transit by Diligence over Mont Cenis._
]
The experience gained in the progress of the work suggested from time to
time many improvements in the machinery and appliances, which finally
proved so effectual that the progress was accelerated beyond
expectation. At the end of 1864, when the machines had been in work
about four years, it was calculated that the opening of the tunnel might
be looked for in the course of the year 1875. But in point of fact it
happened that on the 25th December, 1870, perforator No. 45 bored a hole
from Italy into France, by piercing the wall of rock, about 4 yards
thick, which then separated the workings from each other. The centre
lines of the two workings, as set out from the different sides of the
mountain, failed to coincide by only a foot, that set out on the
Fourneaux side being this much higher than the other, but their
horizontal directions exactly agreeing. The actual length of the tunnel
was found to be some 15 yards longer than the calculated length, the
calculation having given 7·5932 miles for the length, whereas by actual
measurement it was found to be 7·6017 miles. The heights above the
sea-level of the principal points are these:
Feet.
Fourneaux, or northern entrance 3,801
Bardonnêche, or southern entrance 4,236
Summit of tunnel 4,246
Highest point of mountain vertically over the tunnel 9,527
The tunnel is lined with excellent brick and stone arching, and it is
connected with the railways on either side by inclined lines, which are
in part tunnelled out of the mountain, so that the extremities of the
tunnel referred to above are not really entered by the trains at all;
but these lateral tunnels join the other and increase the total distance
traversed underground to very nearly 8 miles, or more accurately, 7·9806
miles. The time required by a train to pass from one side to the other
is about 25 minutes. What a contrast is this to the old transit over the
Mont Cenis pass by “diligence”! We have the scene depicted in Fig. 179,
where we perceive, sliding down or toiling up the steep zigzag ascents,
a series of curious vehicles drawn by horses with perpetually jingling
bells.
The cost of the Mont Cenis Tunnel was about £3,000,000 sterling, or
upwards of £200 per yard; but as a result of the experience gained in
this gigantic work, engineers consider that a similar undertaking could
now be carried out for half this cost. It is supposed that the profit to
the contractors for the Mont Cenis Tunnel was not much less than £100
per yard. The greatest number of men directly employed on the tunnel at
one time was 4,000, and the total horse-power of the machinery amounted
to 860. From 1857 to 1860, by hand labour alone, 1,646 metres were
excavated; from 1861 to 1870 the remaining 10,587 metres were completed
by the machines. The most rapid progress made was in May, 1865, in which
month the tunnel was driven forward at one end the length of 400 feet.
When the workings were being carried through quartz, a very hard rock,
the speed was greatly reduced—as, for example, during the month of
April, 1866, when the machines could not accomplish more than 35 ft.
The perforators used in the Mont Cenis Tunnel were worked by compressed
air, conveyed to a small cylinder, in which it works a piston, to the
rod of which the jumper is directly attached. The air, being admitted
behind the piston, impels the jumper against the rock, and the tool is
then immediately brought back by the opening of a valve, which admits
compressed air in front of the piston, at the same time that the air
which has driven it forward is allowed to escape, communication with the
reservoir of compressed air having previously been closed behind it. The
whole of these movements are automatic, and they are effected in the
most rapid manner, four or five blows being struck in every second, or
between two and three hundred in one minute. Water was constantly forced
into the holes, so as to remove the _débris_ as quickly as it was
formed. A number of these machines were mounted on one frame, supported
on wheels, running on the tramway which was laid along the gallery. The
perforators had no connection with each other, for each one had its own
tube for the conveyance of compressed air, and its own tube to carry the
water used for clearing out the hole, and the cylinders were so fixed on
the frames that the jumpers could be directed in any desired manner
against any selected portion of the rock. They were driven to an average
depth of about 2½ ft., and the process occupied from forty to fifty
minutes. When a set of holes had thus been formed, the cylinders were
shifted and another series commenced, until about eighty holes had been
bored, the formation of the whole number occupying about six or seven
hours, and the holes being so arranged that the next operation would
detach the rock to the required extent. The flexible tubes, which
conveyed the air and water to the machines from the entrances, were then
removed from the machines and stowed away, the frame bearing the
perforators was drawn back along the tramway, workmen advanced whose
duty it was to wipe out the holes, charge them with powder, and fix the
fuses ready for the explosion. When the slow-burning match was ignited,
all retired behind strong wooden barricades, at a safe distance, until
the explosion had taken place; and after the compressed air had been
allowed to stream into the working, so as to clear away all the smoke
and gas generated by the explosion, the workmen ran up on a special
tramway the waggons which were to carry away all the detached stones;
and when this had been done, the floor was levelled, the tramways were
lengthened, and the frame bearing the drilling machines was brought up
to begin a fresh series of operations, which were usually repeated about
twice in the course of every twenty-four hours. A great part of the rock
consists of very hard calcareous schist, interspersed with veins of
quartz, one of the hardest of all rocks, which severely tries the temper
of the steel tools, for a few blows on quartz will not unfrequently
cause the point of a jumper to snap off.
_ROCK-DRILLING MACHINES._
Several forms of rock-drills, or perforators, have been constructed on
the same principle as that used in the Mont Cenis Tunnel, and a
description of one of them will give a good notion of the general
principle of all. We select a form devised by Mr. C. Burleigh, and much
used in America, where it has been very successfully employed in driving
the Hoosac Tunnel, effecting a saving in the cost of the drilling
amounting to one-third of the expense of that operation, and effecting
also a still greater saving of time, for the tunnel, which is 5 miles in
length, is to be completed in four years, instead of twelve, as the
machines make an advance of 150 ft. per month, whereas the rate by hand
labour was only 49 ft. per month. These machines are known as the
“Burleigh Rock Drills,” and have been patented in England for certain
improvements by Mr. T. Brown, who has kindly supplied us with the
following particulars:
[Illustration:
FIG. 180.—_Burleigh Rock Drill on Tripod._
]
The Burleigh perforator acts by repeated blows, like Bartlett and
Sommeiller’s, but its construction is more simple, and the machine is
lighter and not half the size, while its action is even superior in
rapidity and force. The Burleigh machines are composed of a single
cylinder, the compressed air or steam acting directly on the piston,
without the necessity of flywheel, gearing, or shafting. The regular
rotation of the drills is obtained by means of a remarkably simple
mechanical contrivance. This consists of two grooves, one rectilinear,
the other in the form of a spiral cut into the piston-rod. In each of
these channels, or grooves, is a pin, which works freely in their
interior: these pins are respectively fixed to a concentric ring on the
piston-rod. A ratchet wheel holds the ring, and the pin slides into the
curve, causing it to turn always in the same direction, without being
able to go back. By this eminently simple piece of mechanism, the
regular rotation of the drill-holder is secured. The slide-valve is put
into motion by the action of a projection, or ball-headed piston-rod, on
a double curved momentum-piece, or trigger, which is attached to the
slide-rod or spindle by a fork, thus opening and shutting the valve in
the ascent and descent of the piston. Fig. 180 represents one of the
machines attached in this instance by a clamp to the frame of a tripod.
The principal parts of the machine are the cylinder, with its piston,
and the cradle with guide-ways, in which the cylinder travels. The
action of the piston is similar to that of the ordinary steam hammer,
with this difference, that, in addition to the reciprocating, it has
also a rotary, motion. The drill-point is held in a slip-socket, or
clamp, at the end of the piston-rod, by means of bolts and nuts. The
drill-point rotates regularly at each stroke of the piston, making a
complete revolution in every eighteen strokes. For hard rocks it is
generally made with four cutting edges, in the form of a St. Andrew’s
cross, thus striking the rock in seventy-two places in one revolution,
each cutting edge chipping off a little of the stone at each stroke in
advance of the one preceding. The jumper makes, on an average, 300 blows
per minute, and such is the construction of the machine, that the blows
are of an elastic, and not of a rigid, nature, thus preventing the
drill-point from being soon blunted. It has been found in practice, that
a drill-point used in the Burleigh machine can bore on an average 20 ft.
of Aberdeen granite without re-sharpening. As the drill pierces the
rock, the machine is fed down the guide-ways of the cradle by means of
the feed-screw (see Fig. 180), according to the nature of the rock and
the progress made. When the cylinder has been fed down the entire length
of the feed-screw, and if a greater depth of hole is required, the
cylinder is run back, and a longer drill is inserted in the socket at
the end of the piston-rod. The universal clamp may be attached to any
form of tripod, carriage, or frame, according to the requirements of the
work to be done; it enables the machines to work vertically,
horizontally, or at any angle.
The following advantages are claimed for this machine: Any labourer can
work it; it combines strength, lightness, and compactness in a
remarkable degree, is easily handled, and is not liable to get out of
order. No part of the mechanism is exposed; it is all enclosed within
the cylinder, so there is no risk of its being broken. It is applicable
to every form of rockwork, such as tunnelling, mining, quarrying, open
cutting, shaft-sinking, or submarine drilling; and in hard rock, like
granite, gneiss, ironstone, or quartz, the machine will, according to
size, progress at the incredible rate of _four inches_ to _twelve inches
per minute_, and bore holes from ¾ in. up to 5 in. diameter. It will, on
an average, go through 120 ft. of rock per day, making forty holes, each
from 2 ft. to 3 ft. deep, and it can be used at any angle and in any
direction, and will drill and clear itself to any depth up to 20 ft.
The following extract from the “Times,” September 24th, 1873, gives an
account of some experiments with the machine, made at the meeting of the
British Association in that year, before the members of the Section of
Mechanical Science:
“Yesterday, considerable interest was taken in this section, as it had
been announced that a ‘Burleigh Rock Drilling Machine’ would be working
during the reading of a paper by Mr. John Plant. The machine was not,
however, in the room, but was placed in the grounds outside, where it
was closely examined by the members after the adjournment, and seen in
full operation, boring into an enormous block of granite. The aspect of
the machine cannot be called formidable in any respect, for it looks
like a big garden syringe, supported upon a splendid tripod; but when at
work, under about 80 lbs. pressure of compressed air, it would be deemed
a very revolutionary agent indeed, against whose future power the
advocates for manual labour in the open quarry, the tunnel, and even the
deep mine, may well look aghast. Placed upon a block of granite a yard
deep, the machine was handled and its parts moved by the fair hands of
many of the lady associates of scientific proclivities; but once the
source of power was turned on, the drill began its poundings, eating
holes 2 in. in diameter in the block of granite, and making a honeycomb
of it as easily as a schoolboy would demolish a sponge cake. It pounds
away at the rate of 300 strokes, and progresses forward about 12 in., in
the minute, making a complete revolution of the drill in eighteen
strokes, and keeping the hole free of the pounded rock. The machine was
fixed to work at any angle, almost as readily as a fireman can work his
hose; and its adaptation to a wide range of stone-getting, by drilling
for blasting, and cutting large blocks for building and engineering,
with a saving of capital and labour, was admitted by many members of the
section. The tool is called the ‘Burleigh Rock Drill,’ invented by Mr.
Charles Burleigh, a gentleman hailing from Massachusetts, United States.
The patent is the property of Messrs. T. Brown and Co., of London. The
principal feature of this new machine is, that it imitates in every way
the action of the quarryman in boring a hole in the rock.”
[Illustration:
FIG. 181.—_Burleigh Rock Drill on Movable Column._
]
Many forms of carriages and supports have, from time to time, been made
to suit the work for which the ‘Burleigh’ machines have been required.
The machine is attached to these carriages, or supports, by means of the
universal clamp, by which it can be worked in any direction and at any
angle. Of these carriages we select for notice only two forms, one of
which is shown in Fig. 181. This carriage can be used to great advantage
in adits and drifts. It consists of an upright column, with a screw
clamp-nut for holding and raising or lowering the machine, the whole
being mounted on a platform which can slide right across the carriage,
and thus the machine can be brought to work on any point of a heading.
It is secured in position by means of a jack-screw in the top of the
column; and as the carriage is mounted on wheels, it is easily moved to
permit of blasting. Fig. 182 represents a carriage which is the result
of many years’ experience with mining machinery, and it is considered a
very perfect appliance. It is constructed of wood and iron, and it runs
on wheels. The supports for the machines, four of which may be mounted
at once, are two horizontal bars, the lower of which can be raised or
lowered, as may be necessary. The two parallel sides of the carriage are
joined only at the upper side, and there is nothing to prevent it from
being run into the heading, though the way between the rails may be
heaped up with broken rock, if only the rails are clear. Drilling, and
the removal of the broken rock, may then proceed simultaneously; for, by
means of a narrow gauge inside the carriage rails, small cars may be
taken right up to the _débris_. It is made in different sizes, to suit
the dimensions of the tunnel required. To give the carriage steadiness
in working, it is raised from the wheels by jack-screws, and held in
position by screws in a similar manner to the carriage represented in
Fig. 181.
[Illustration:
FIG. 182.—_Burleigh Rock Drills mounted on a Carriage._
]
[Illustration:
FIG. 183.
]
An extremely interesting system of drilling rocks—totally different from
that on which the machines we have just described are constructed—has,
within the last few years, been introduced by Messrs. Beaumont and
Appleby. What does the reader think of boring holes in rocks with
diamonds? It has long been a matter of common knowledge that the diamond
is the hardest of all substances, and that it will scratch and wear down
any other substances, while it cannot itself be scratched or worn by
anything but diamond. In respect to wearing down or abrading hard
stones, the diamond, according to experiments recently made by Major
Beaumont, occupies a position over all other gems and minerals to a
degree far beyond that which has been generally attributed to it; for in
these experiments it was found that on applying a diamond, or rather a
piece of the “carbonate” about to be described, fixed in a suitable
holder, to a grindstone in rapid rotation, the grindstone was quickly
worn down; but on repeating a similar experiment with sapphires and with
corundum, it was these which were worn down by the grindstone. Without,
on the present occasion, entering into the natural history of the
diamond, we may say that there are, besides the pure colourless
transparent crystals so highly prized as gems, several varieties of
diamond, and that those which are tinged with pink, blue, or yellow, are
far from having the same value for the jeweller. Then there is another
impure variety called _boort_, which appears to be employed only to
furnish a powder by which the brilliants are ground and polished. In the
diamond gravels of Brazil, from which we derive our regular supply of
these gems, there was discovered in 1842 a curious variety of
dark-coloured diamond, in which the crystalline cleavage, or tendency to
split in certain directions (which belongs to the ordinary stones),
appears to be almost absent; and the substance might be regarded as a
transition form between the diamond and graphite but for its hardness.
This substance was until lately used for the same purposes as _boort_,
which is a nearer relative of the pure crystal, and like it, splits
along certain planes. It received from the miners the name of
“_carbonado_,” and with regard to the application we are considering, it
has turned out to be a sort of Cinderella among diamonds; for its
unostentatious appearance is more than compensated for by its surpassing
all its more brilliant sisters in the useful property to which reference
has been made. This Brazilian term is doubtless the origin of the
English name by which the substance in question is known among the
English diamond merchants, who call it “carbonate”—an unfortunate word,
for it is used in chemistry with an entirely different signification.
“Carbonate” it is, however, which supplies the requirements of the
rock-drill, and the selected stones are set in a crown, or short tube,
of steel, represented by _c_ in Fig. 183. In this they are secured as
follows: holes are drilled in the rim of the tube, and each hole is then
cut so that a piece of the diamond exactly fits it, and when this piece
has been inserted, the metal is drawn round by punches, so as almost to
cover the stone, leaving only a point projecting, _b b_. The portions of
the crown between the stones are somewhat hollowed out, as at _a_, for a
purpose which will presently be mentioned. The crown thus set with the
boring gems is attached to the end of a steel tube, by which it is made
to rotate with a speed of about 250 revolutions per minute while pressed
against the rock to be bored. Water is forced through the steel tube,
and passing out between the rock and the crown, especially under the
hollows, _c c_, makes its escape between the outside of the boring-tube
and the rock, thus washing away all the _débris_ and keeping the drill
cool. The pressure with which the crown is forced forward depends, of
course, on the nature of the rock to be cut, and varies from 400 lbs. to
800 lbs. In this way the hardest rocks are quickly penetrated—sometimes,
for example, at the rate of 4 in. per minute, compact limestone at 3
in., emery at 2 in., and quartz at the rate of 1 in. per minute. It is
found that, even after boring through hundreds of feet of such
materials, the diamonds are not in the least worn, but as fit for work
as before: they are damaged only when by accident one of the stones gets
knocked out of its setting; and this machine surpasses all in the
rapidity with which it eats its way through the firmest rocks. This, it
must be observed, is the special privilege of the diamond drill—that,
since the begemmed steel crown and the boring-rods are alike tubular,
the rock is worn away in an annular space only, and a solid cylinder of
stone is detached from the mass, which cylinder passes up with the
hollow rods, where, by means of certain sliding wedges, it is held fast,
and is drawn away with the rods.
When the diamond drill is used merely for driving the holes for
blasting, this cylinder of rock is not an important matter; but there is
an application of the drill where this cylinder is of the greatest
value, furnishing as it does a perfect, complete, and easily preserved
section of the whole series of strata through which the drill may pass
when a bore-hole is sunk in the operation of searching for minerals
(which is so significantly called in the United States “prospecting,” a
phrase which seems to be making its way in England in mining
connections); for the core is uniformly cylindrical, the surface is
quite smooth, and any fossils which may be present come up uninjured, so
far as they are contained in the solid core, and thus the strata are
readily recognized. Contrast this with the old method, where the
bore-hole in prospecting is made by the reciprocating action imparted to
a steel tool, and merely the _pounded_ material is obtained, usually in
very small fragments, by augers or sludge-pumps: the fossils, which
might afford the most valuable indications, crushed and perhaps
incapable of being recognized; and instead of the beautifully definite
and continuous cylinder, a mere mass of _débris_ is brought up. In the
prospecting-bores the diameter of the hole is from 2 in. to 7 in. The
size adopted depends on the nature of the strata to be penetrated, and
on the depth to which it is proposed to carry the boring. When the
strata are soft, the operation is commenced with a bore of 7 in., and
when this has been carried to an expedient depth, the danger of the
sides of the hole falling in is avoided by putting down tubes, and then
the diamond drill, fixed to tubes of a somewhat smaller diameter, will
be again inserted, and the boring recommenced; or the hole can be
widened, so as to receive the lining-tubes. Of course, in boring through
hard rocks, such as compact limestones, sandstone, &c., no lining-tubes
are necessary.
In a very interesting paper, read before the members of the Midland
Institute of Mining Engineers, by Mr. J. K. Gulland, the engineer of the
Diamond Rock-Boring Company, who have the exclusive right of working the
patents for this remarkable invention, that gentleman concludes by
remarking that “the leading feature of the diamond drill is that it
works without percussion, thus enabling the holing of rocks to be
effected by a far simpler class of machinery than any which has to
strike blows. Every mechanical engineer knows, often enough to his cost,
that he enters upon a new class of difficulties when he has to recognize
it as a normal state of things with any machinery he is designing that
portions of it are brought violently to rest. These difficulties
increase very much when the power, as in the case of deep bore-holes,
has to be conveyed for a considerable distance. Where steel is used a
percussive action is necessitated, as, if a scraping action is used, the
drill wears quicker than the rock. The extraordinary hardness of the
diamond places a new tool in our hands, as its hardness, compared with
ordinary rock, say granite, is practically beyond comparison. Putting
breakages on one side, a piece of “carbonate” would wear away thousands
of times its own bulk of granite. Irrespective of the private and
commercial success which this invention has attained, it is a boon to a
country such as ours, where minerals constitute in a great measure our
national wealth and greatness.”
The advantages of the diamond drill may be illustrated by the case of
what is termed the Sub-Wealden Exploration. From certain geological
considerations, which need not be entered upon here, several eminent
British and continental geologists have arrived at the conclusion that
it is probable that coal underlies the Wealden strata of Kent and
Sussex, and that it may be perhaps met with at a workable depth. If such
should really prove to be the case, the industrial advantages to the
south of England would be very great, for the existence of coal so
comparatively near to the metropolis would prove not only highly
lucrative to the owners of the coal, but confer a direct benefit upon
thousands by cheapening the cost of fuel. A number of property owners
and scientific men, having resolved that the matter should be tested by
a bore, raised funds for the purpose, and a 9 in. bore had been carried
down to a depth of 313 ft. in the ordinary manner, when a contract was
entered into with the Diamond Rock-Boring Company for a 3 in. bore
extracting a cylinder of rock 2 in. in diameter. The company, as a
precautionary measure, lined the old hole with a 5 in. steel tube; and
in spite of some delay caused by accidents, they increased the depth of
the hole to 1,000 ft. in the interval from 2nd February, 1874, to 18th
June, 1874–-the progress of the work being regarded with the greatest
interest by the scientific world. Unfortunately, the further progress of
the work has been prevented by an untoward event, namely, the breaking
of the boring-rod, or rather tube; and, although the company is prepared
with suitable tackle for extracting the tubes in case of accidents of
this kind, and generally succeeds in lifting them by a taper tap, which,
entering the hollow of the tube, lays hold of it by a few turns—yet, in
this instance, where there have been special difficulties, the
extraction of so great a length of tubes is, as the reader may imagine,
by no means an easy task. Six attempts have been made to remove the
boring-rods which have dropped down; but so difficult has this operation
proved, that, all these efforts having failed, it has been decided to
abandon the old work and commence a new boring on an adjacent spot. A
contract has been entered into with the Diamond Boring Company, who have
undertaken to complete the first 1,000 ft. for £600, which is only £200
more than it would have cost to completely line the old bore-holes with
iron tubes—an operation which was contemplated by the committee in
charge of the exploration. The terms agreed to by the company are very
favourable to the promoters of the Sub-Wealden Exploration, although the
cost of the second 1,000 ft. will be £3,000 more; and the committee are
relying upon the public for contributions to enable them to carry on
their enterprise. It is most probable that funds will be forthcoming,
and should the boring result in the finding of coal measures beneath the
Wealden strata, all the nation will be the richer and participate in the
advantages resulting from an undertaking carried on by private persons.
Already a totally unexpected source of wealth has been met with by the
old bore showing the existence of considerable beds of gypsum in these
strata, and the deposits of gypsum are about to be worked. Whether coal
be found or not found, there is no doubt that a bore-hole going down
2,000 ft. will greatly increase our geological knowledge, and may reveal
facts of which we have at present no conception.
[Illustration:
FIG. 184.—_The Diamond Drill Machinery for deep Bores._
]
The boring-tubes, it maybe remarked, are made in 6 ft. lengths, and are
so contrived that the joints are nearly flush—that is, there is no
projection at the junctions of the tubes. Fig. 184 is engraved from a
photograph of the machinery used for working the diamond drill when
boring a hole for “prospecting.” This looks at first sight a very
complicated machine, but in reality each part is quite simple in its
action, and is easily understood when its special purpose has been
pointed out. We cannot, however, do more than indicate briefly the
general nature of the mechanism. The reader will on reflection perceive
that, although the idea of causing a rod to rotate in a vertical hole
may be simple, yet in practically carrying it out a number of different
movements and actions have to be provided for in the machinery. The
weight of the rods cannot be thrown on the cutters, nor borne by the
moving parts of the machine—hence the movable disc-shaped weights
attached to the chains are to balance the weight of the boring-rods as
the length of the latter is increased. There must also be a certain
amount of _feed_ given to the cutters, regulated and adjusting itself to
avoid injurious excess: hence a nut which feeds the drill is encircled
by a friction-strap in which it merely slips round without advancing the
cutter when the proper pressure is exceeded. There must be means of
throwing this into or out of gear, or advancing the tool in the work and
of withdrawing it—hence the handles seen attached to the brake-straps.
Water must be drawn from some convenient source, and caused to pass down
the drill-tube—hence the force-pump seen in the lowest part of the
figure. The rods must be raised by steam power and lowered by mechanism
under perfect control—hence suitable gearing is provided for that
purpose.
The reader may be interested in learning what is the cost of
“prospecting” with this unique machinery. The company usually undertake
to bore the first 100 ft. for £40, but the next 100 ft. cost £80–-that
is, for 200 ft. £120 would be charged; the third 100 ft. would cost
£120–-that is to say, the first 300 ft. would cost £240, and so on—each
lower 100 ft. costing £40 more than the 100 ft. above it. Some of the
holes bored have been of very great depth, and have been executed in a
marvellously short space of time. Thus, in 54 days, a depth of 902 ft.
was reached at Girrick in a boring for ironstone; another for coal at
Beeston reached 1,008 ft.; and at Walluff in Sweden 304½ ft. were put
down in one week!
These machines are peculiarly suitable for submarine boring, for they
work as well under water as in the air; and they will no doubt be put
into requisition in the preliminary experiments about to be made for
that great project which bids fair to become a sober fact—the Channel
Tunnel between England and France; and as, by the time these pages will
be before the public, the work of the greatest and boldest rock-boring
yet attempted will have commenced, and the scheme itself will be the
theme of every tongue, the Author feels that the present article would
be incomplete without some particulars of the great enterprise. [1875.]
_THE CHANNEL TUNNEL._
The notion of connecting England and France by a submarine line of
railways is not of the latest novelty, but has been from time to time
mooted by the engineers of both countries. The most carefully prepared
scheme, however, is embodied in the joint propositions of Sir J.
Hawkshaw and Messrs. Brunlees and Low among English engineers; and those
of M. Gamond on the French side, which these gentlemen have prepared at
the invitation of the promoters of the scheme, give the clearest and
most authentic account of the considerations on which this gigantic
enterprise will be based, and from this document we draw the following
passages:
The undersigned engineers, some of whom have been engaged for a
series of years in investigating the subject of a tunnel between
France and England, having attentively considered those
investigations and the facts which they have developed, beg to
report thereon jointly for the information of the committee.
These investigations supported the theory that the Straits of
Dover were not opened by a sudden disruption of the earth at that
point, but had been produced naturally and slowly by the gradual
washing away of the upper chalk; that the geological formations
beneath the Straits remained in the original order of their
deposit, and were identical with the formations of the two shores,
and were, in fact, the continuation of those formations.
Mr. Low proposed to dispense entirely with shafts in the sea, and
to commence the work by sinking pits on each shore, driving
thence, in the first place, two small parallel driftways or
galleries from each country, connected at intervals by transverse
driftways. By this means the air could be made to circulate as in
ordinary coal-mines, and the ventilation be kept perfect at the
face of the workings.
Mr. Low laid his plans before the Emperor of the French in April,
1867, and in accordance with the desire of his Majesty, a
committee of French and English gentlemen was formed in
furtherance of the project.
For some years past Mr. Hawkshaw’s attention has been directed to
this subject, and ultimately he was led to test the question, and
to ascertain by elaborate investigations whether a submarine
tunnel to unite the railways of Great Britain with those of France
and the Continent of Europe was practicable.
Accordingly, at the beginning of the year 1866, a boring was
commenced at St. Margaret’s Bay, near the South Foreland; and in
March, 1866, another boring was commenced on the French coast, at
a point about three miles westward of Calais; and simultaneously
with these borings an examination was carried on of that portion
of the bottom of the Channel lying between the chalk cliffs on
each shore.
The principal practical and useful results that the borings have
determined are that on the proposed line of the tunnel the depth
of the chalk on the English coast is 470 ft. below high water,
consisting of 175 ft. of upper or white chalk and 295 ft. of lower
or grey chalk; and that on the French coast the depth of the chalk
is 750 ft. below high water, consisting of 270 ft. of upper or
white chalk and 480 ft. of lower or grey chalk; and that the
position of the chalk on the bed of the Channel, ascertained from
the examination, nearly corresponds with that which the geological
inquiry elicited.
In respect to the execution of the work itself, we consider it
proper to drive preliminary driftways or headings under the
Channel, the ventilation of which would be accomplished by some of
the usual modes adopted in the best coal-mines.
As respects the work itself, the tunnel might be of the ordinary
form, and sufficiently large for two lines of railway, and to
admit of being worked by locomotive engines, and artificial
ventilation could be applied; or it might be deemed advisable, on
subsequent consideration, to adopt two single lines of tunnel. The
desirability of adopting other modes of traction may be left for
future consideration.
Such are the essential passages of the report which, in 1868, was
submitted to the Government of the Emperor Louis Napoleon, and was made
the subject of a special commission appointed by the Emperor to inquire
into the subject in all its bearings. The commission presented its
report in 1869, and these are the chief conclusions contained in it:
I. The commission, after having considered the documents relative
to the geology of the Straits, which agree in establishing the
continuity, homogeneity, and regularity of level of the _grey
chalk_ between the two shores of the Channel,
Are of opinion that driving a submarine tunnel in the lower part
of this chalk is an undertaking which presents reasonable chances
of success.
Nevertheless they would not hide from themselves the fact that its
execution is subject to contingencies which may render success
impossible.
II. These contingencies maybe included under two heads: either in
meeting with ground particularly treacherous—a circumstance which
the known character of the grey chalk renders improbable; or in an
influx of water in a quantity too great to be mastered, and which
might find its way in either by infiltration along the plane of
the beds, or through cracks crossing the body of the chalk.
Apart from these contingencies, the work of excavation in a soft
rock like grey chalk appears to be relatively easy and rapid; and
the execution of a tunnel, under the conditions of the project, is
but a matter of time and money.
III. In the actual state of things, and the preparatory
investigations being too incomplete to serve as a basis of
calculation, the commission will not fix on any figure of expense
or the probable time which the execution of the permanent works
would require.
The chart, Fig. 185, and the section, Fig. 186, will give an idea of the
course of the proposed tunnel, which will connect the two countries
almost at the nearest points. The depth of the water in the Channel
along the proposed line nowhere exceeds 180 ft.—little more than half
the height of St. Paul’s Cathedral, which building would, therefore, if
sunk in the midst of the Channel, still form a conspicuous object rising
far above the waves. But the tunnel will pass through strata at least
200 ft. below the bottom of the Channel, rising towards each end with a
moderate gradient; and from the lower points of these inclines the
tunnel will rise slightly with a slope of 1 in 2,640 to the centre, or
just sufficient for the purposes of drainage. On the completion of the
tunnel a double line of rails will be laid down in it, and trains will
run direct from Dover to Calais. Companies have already been formed in
England under the presidency of Lord Richard Grosvenor, and in France
under that of M. Michel Chevalier, and the legislation of each country
has sanctioned the enterprise. Verily the real magician of our times is
the engineer, who, by virtually abolishing space, time, and tide, is
able to transport us hither and thither, not merely one or two—almost
like the magicians we read of in the “Arabian Nights,” with their
enchanted horses or wonderful carpets—but by hundreds and by tens of
hundreds.
[Illustration:
FIG. 185.—_Chart of the Channel Tunnel._
]
The “Daily News” of January 22nd, 1875, in presenting its readers with a
chart of the proposed tunnel, offered also the following sensible and
interesting comment on the subject:
“This long-debated project has at length emerged from the region of
speculation, and is entering the stage of practical experiment. On this
side the Channel a company has been formed to carry out the work, and on
the other side the French Minister of Public Works has presented to the
Assembly a Bill authorizing a French company to co-operate with the
English engineers. The enterprise is one worthy of the nations which
have in the present generation joined the two shores of the Atlantic by
an electric cable, and cut a ship canal through the Isthmus of Suez, and
of the age which has obliterated the old barrier of the Alps. All these
gigantic undertakings seemed almost as bold in conception and as
difficult of execution as the great work now about to commence. Those
twenty miles of sea have long been crossed by telegraph lines; they will
soon be bridged, as it were, by splendid steamers; but even our own
generation, accustomed as it is to gigantic engineering works, has
scarcely regarded the construction of a railway underneath the waves as
within the reach of possibility. M. Thomé de Gamond, who first made the
suggestion five and thirty years ago, was long regarded as an
over-sanguine person, who did not recognize the inevitable limits of
human skill and power. A tunnel under twenty miles of stormy sea seemed
very much like an engineer’s dream, and it is only within the last few
years that it has been regarded as a feasible project. Of its
possibility, however, there seems now to be no manner of doubt. It is
merely a stream of sea-water, and not a fissure in the earth, which
divides us from the Continent. Prince Metternich was right in speaking
of it as a ditch. The depth is nowhere greater than one hundred and
eighty feet; and so far as careful soundings can ascertain the condition
of the soil underneath the water, it consists of a smooth unbroken bed
of chalk. The success of the experiment depends on this bed of chalk
being continuous and whole. Should any very deep fissure exist, which is
extremely improbable, the tunnel may probably not be driven through it.
But given, what every indication shows to exist, a homogeneous chalk bed
some hundreds of feet in thickness, the driving of a huge bore for
twenty miles through it is a mere question of time, money, and
organization, and as the engineers have these resources at their
command, they are sanguine, and we may even say confident, of success.
[Illustration:
FIG. 186.—_Section of the Channel Tunnel._
]
[Illustration:
FIG. 187.—_View of Dover._
]
“The method by which it is proposed that the excavation shall be made is
in some respects similar to that which was successfully employed in
tunnelling the Alps. Mont Cenis was pierced by machinery adapted to the
cutting of hard rock; the chalk strata under the Channel are to be bored
by an engine, invented by Mr. Dickenson Brunton, which works in the
comparatively soft strata like a carpenter’s auger. A beginning will be
made simultaneously on both sides of the Channel, and the effort will at
first be limited to what we may describe as making a clear hole through
from end to end. This small bore, or driftway as it is called, will be
some seven or nine feet in diameter. If such a communication can be
successfully made, the enlargement will be comparatively easy. Mr.
Brunton’s machine is said to cut through the chalk at the rate of a yard
an hour. We believe that those which were used in the Mont Cenis Tunnel
cut less than a yard a day of the hard rock of the mountain. Two years,
therefore, ought to be sufficient to allow the workers from one end to
shake hands with those from the other side. The enlargement of the
driftway into the completed tunnel would take four years’ more labour
and as many millions of money. The millions, however, will easily be
raised if the driftway is made, since the victory will be won as soon as
the two headways meet under the sea. One of the great difficulties of
the work is shared with the Mont Cenis Tunnel, the other is peculiar to
the present undertaking. The Alps above the one, and the sea above the
other, necessarily prevent the use of shafts. The work must be carried
on from each end; and all the _débris_ excavated must be brought back
the whole length of the boring, and all the air to be breathed by the
workmen must be forced in. The provision of a fit atmosphere is a mere
matter of detail. In the great Italian tunnel the machines were moved by
compressed air, which, being liberated when it had done its work,
supplied the lungs of the workers with fresh oxygen. The Alpine
engineers, however, started from the level of the earth: the main
difficulty of the Submarine Tunnel seems to be that it must have as its
starting-point at each end the bottom of a huge well more than a hundred
yards in depth. The Thames Tunnel, it will be remembered, was
approached, in the days when it was a show place, by a similar shaft,
though of comparatively insignificant depth. This enterprise may indeed
be said to bear something like the relation to the engineering and
mechanical skill of the present day which Brunel’s great undertaking
bore to the powers of an age which looked on the Thames Tunnel as the
eighth wonder of the world. Probably the danger which will be incurred
in realizing the larger scheme is less than that which Brunel’s workmen
faced.
“It is, of course, impossible for any estimate to be formed of the risks
of this enormous work. They have been reduced to a minimum by the
mechanical appliances now at our disposal, but they are necessarily
considerable. The tunnel is to run, as we understand, in the lower
chalk, and there will be, as M. de Lesseps told the French Academy, some
fifty yards of soil—a solid bed of chalk, it is hoped—between the
sea-water and the crown of the arch. Moreover, an experimental half-mile
is to be undertaken on each side before the work is finally begun; the
engineers, in fact, will not start on the journey till they have made a
fair trial of the way. Altogether the beginning seems to us to be about
to be made with a combination of caution and boldness which deserves
success, even though it should be unable to command it. Unforeseen
difficulties may arise to thwart the plans, but the enterprise, so far,
is full of promise. The opening of such a communication between this
country and the Continent will be a pure gain to the commercial and
social interests on both sides. It obliterates the Channel so far as it
hinders direct communication, yet keeps it intact for all those
advantages of severance from the political complications of the
Continent, which no generation has more thoroughly appreciated than our
own. The commercial advantages of the communication must necessarily be
beyond all calculation. A link between the two chief capitals of Western
Europe, which should annex our railway system to the whole of the
railways of the Continent, would practically widen the world to pleasure
and travel and every kind of enterprise. The 300,000 travellers who
cross the Channel every year would probably become three millions if the
sea were practically taken out of the way by a safe and quick
communication under it. The journey to Paris would be very little more
than that from London to Liverpool. It is, however, quite needless to
enlarge on these advantages. The Channel Tunnel is the crowning
enterprise of an age of vast engineering works. Its accomplishment is to
be desired from every point of view, and, should it be successful, it
will be as beneficent in its results as the other great triumphs of the
science of our time.”
The Channel Tunnel is not yet a _fait accompli_, although the
preliminary trial works have been made at both ends. Drift-ways of some
ten feet diameter have been cut beneath the waters of the strait, and
instead of the experimental half mile mentioned in the foregoing
paragraph, the works have been pushed forward on the English side for
about a mile and a quarter with complete success. As was anticipated, no
physical difficulties were met with, for the machines did their work
with the greatest ease, and the drift has now remained for some years
practically free from any infiltration of water. These results indicate
that the scheme might be completed with speed and safety. Parliament,
however, has refused to allow the undertaking to proceed, being moved to
this course by the opinions of military authorities, who see dangers to
England in the completion of this enterprise, or at least such a
disturbance of the British complacency at the notion that our island
might be reached otherwise than “by the inviolate sea,” that the whole
land would be liable to terrors and alarms from invasion by stratagem.
It is represented that huge fortresses and a special army for that
purpose would become necessary to guard the mouth of the tunnel were it
made. This is, perhaps, the kind of objection which such an enterprise
could not fail to raise. But it can hardly be expected that all the
commercial and international advantages which the realization of the
scheme would undoubtedly secure are for ever to stand in abeyance for
such opinions as have, for the present, caused the operations to be
suspended. It has been pointed out that there are many ways of instantly
rendering such a tunnel impracticable in case of a sudden alarm. But the
necessity could only arise after a supposed paralysis or destruction of
such army and navy as Britain could bring together to defend her land.
Perhaps military skill will presently devise less costly methods of
defence than those authorities now suppose the tunnel would require; or,
even if such armaments were really necessary for our sense of insular
security, the expense might be no unprofitable outlay for the advantages
to be gained. It is satisfactory to know that the promoters of the
scheme are sanguine of the subsidence of the military and political
prejudices, which are now the only obstacles to its accomplishment. A
somewhat unexpected result from the operations in connection with the
experimental driftways has been the discovery, on the Kentish coast, of
seams of coal underlying the chalk at a workable depth.
_THE ST. GOTHARD RAILWAY._
Since the completion of the Mont Cenis Tunnel, a still greater piece of
rock boring has been begun and finished in the great tunnel of the St.
Gothard Railway. The construction of a railway to connect Italy with
Switzerland, was a project conceived as far back as 1838, when the first
railway company in the latter country was constructed. The route of the
proposed line was a matter of much debate, not alone on account of
difference of engineering opinions, but also by reason of the various
competing interests that would have to be reconciled and induced to
co-operate in the work. The St. Gothard route was only one of the
several schemes that were advocated, and the first decisive step appears
to have been taken at Lucerne, where, in 1853, a meeting was called by
the authorities of the canton to consider the merits of the project; the
result being that the Lucerne Government addressed to the Federal
Council a representation of the advantages this route would afford. More
discussion ensued, and it was only when Switzerland appeared likely to
have no share in the traffic between the Milan district and the more
northern parts of Europe that, in 1861, the partizans of the St. Gothard
route appointed a provisional committee to take action in the matter.
This committee had plans prepared, and sent a deputation to obtain the
assent of the Italian Government. The canton of Tessin, through which
the projected line, or its then surviving rival, was designed to pass,
became a lively scene in the game of speculation, for promoters rushed
in to secure, if possible, concessions which they might sell at a very
advanced price to the winning party. For this purpose came to that poor
Swiss canton Jews and Christians from every land. The St. Gothard route
gained the day, and a Union was, in 1863, formed by the concurrence of
the two principal Swiss railways and fifteen of the cantons most
interested in the scheme. Difficulties and delays were, however,
encountered before the necessary compacts could be concluded with the
neighbouring states—and then there came the war of 1867. So that it was
not until the latter part of 1872 that the construction of the line was
actually entered upon. Before the great work of piercing the St. Gothard
had been completed, the undertaking was embarrassed by financial
difficulties arising from the fact of the lines on the Italian side
costing more than double the estimated amount. The Swiss Government,
however, voted a special subsidy, and the work, which had been suspended
for a while, was proceeded with; much attention being paid to its
economical prosecution. In 1881, when the line was opened, the mails
were carried between Zurich and Bellinzona in seven hours, instead of in
thirty hours as previously required for transit by the excellently
appointed mail carriages under the Federal Administration.
[Illustration:
FIG. 187_a_.—_Map of the St. Gothard Railway._
]
Besides the great tunnel, the St. Gothard line has some unique devices
in railway construction which cannot fail to interest the reader.
Several of the passes over the Alps have been made use of from time
immemorial. We know that Hannibal led his Carthaginian hosts over one of
them, and that they have been traversed by Roman legions, as well as by
Germanic hordes. But, although the St. Gothard is the most direct of all
the routes, it never afforded a passage to armies or migratory tribes.
The road through this pass was not formed by the use of any elaborate
appliances for overcoming the natural obstacles: it was rather the work
of simple peasants and mountain shepherds, with such rough constructions
in wood as might give a sufficiently secure passage across the torrents
and gorges. The old road keeps beside the Reuss from the head of the
lake of Lucerne until it reaches the highest level of the pass, where
the water-shed occurs. It then descends steeply, with many twists and
windings, to the banks of the Ticino, and it follows the course of this
river to its embouchure at Tresa, on Lago Maggiore. The railway follows
the same course, except that it cuts off the higher part of the pass by
the great tunnel piercing the mountain. The scenery throughout could,
perhaps, be nowhere equalled for the variety of its wild grandeur.
The great tunnel of the St. Gothard passes from Gœschenen, on the Reuss,
beneath the col of the pass, and emerges close to the village of Airolo,
on the banks of the Ticino. The length of this tunnel is rather more
than nine and a quarter miles, so that it is about one and a half miles
longer than the Mont Cenis Tunnel. Its northern end is 3,638 feet above
the sea level; its southern end is higher, namely, 3,756 feet; but there
is an intermediate point in the tunnel higher than either-–3,786
feet—and from this there is a uniform incline in each direction. The
tunnel is 300 yards beneath the lowest part of the valley of Andermatt,
and the summits of the mountains it traverses are at least a mile above
it. The motive power by which the rock-drilling machines used in driving
the tunnel were actuated was, as in the case of the Mont Cenis Tunnel,
compressed air; and the power used for compressing the air was, in this
case also, a head of water,—but this was not applied in the same way.
The waters of the Reuss at the northern side, and those of the Tremola
and of the Ticino at the southern side, were taken at a considerable
height in very large cast-iron pipes, and were made to act upon powerful
turbines that gave motion to the compressing machines. These were
capable of compressing the air so that its volume was reduced to
one-twentieth, and the pressure it then exercised would, of course, be
equal to that of twenty atmospheres, or about 300 lbs. on the square
inch,—or more than three times as much as was made use of in the Mont
Cenis Tunnel. The compressed air, carried through pipes to the head of
the workings in the rock, was there allowed to exert its force on the
pistons of the perforators in the manner already described. There was,
in fact, a continual repetition of exactly the same cycle of operations
of boring, charging, firing, etc., that are mentioned on page 355. A
large quantity of the compressed air was always allowed to rush into the
work immediately after each blasting, in order that the smoke and other
products might be driven out and the atmosphere rendered fit for
respiration. In attacking the mountain simultaneously from each side it
was, of course, essential that the tunnels should be driven in precisely
the same direction, and therefore the positions of the points of
departure had to be determined by very careful surveys. At Gœschenen,
the gorge of the Reuss did not naturally admit of a sufficient distance
of vision to fix the direction with the required accuracy, and it became
necessary to pierce a thick mass of rock with a special tunnel for the
purpose of taking a sight sufficiently far back. At Airolo, again, the
tunnel had to enter the valley by curving towards the village; and here
a provisional gallery had to be driven in the straight line.
[Illustration:
PLATE XVI.
THE NORTH MOUTH OF THE GREAT TUNNEL, ST. GOTHARD RAILWAY.
]
Several contractors competed for the work of constructing this great
tunnel, and it was at first supposed that an Italian company, which was
managed by some of the principal engineers engaged on the Mont Cenis,
would be almost certain to obtain the contract. The promoters, however,
intrusted the work to a private individual, M. Louis Favre, of Geneva.
This gentleman undertook to complete the tunnel in eight years, at the
price of 2,800 francs per mètre for the work of excavation merely,
exclusive of masonry, etc. This cost would be not far from £101 per
English yard. The contract was signed on August 7th, 1872, and on
September 12th of the same year M. Favre commenced operations at the
southern end, and the work at the northern end was begun on October 9th
following. The operations were carried on with great energy, and even
during the period of the Company’s financial difficulties there was no
stoppage of the works between Gœschenen and Airolo. It has been
suggested that it was largely due to the regular and successful progress
of this great piece of rock boring that the Company were enabled to
re-establish themselves on a basis that ensured the completion of the
whole undertaking. The contractor, on his part, did not fail to
encounter many physical difficulties. At the southern end much trouble
was caused by torrents of water gushing from the soil, many of these
being of great volume and force; in fact, the work was here carried on
for nearly a whole year in the midst of water—for the ground for the
first mile consisted of glacial and other deposits, which were
intersected by subterranean water-courses. Reaching the solid rock was
here a relief. But at Gœschenen little of loose formation was met with;
but the rock encountered was of extreme hardness—consisting, indeed, of
almost pure quartz, which had the effect of quickly blunting the points
of even the best tempered tools. But another kind of difficulty had to
be overcome when the workings got beneath the vale of Urseren. Here, at
several places, layers of argillaceous matter were found between the
masses of hard rock. These layers were easy enough to pierce through,
but on account of the pressure of the rocks in which they were
interspersed, they were squeezed out and gradually protruded within the
tunnel, which would soon have become entirely obstructed. At first a
very massive lining of timber was tried, but it was soon found that this
must be replaced by a solid vaulting of stone. The first vault failed to
sustain the pressure, and so did the second, although the thickness of
the material was more than a yard. In some places these operations had
to be several times repeated, and from this cause the cost of parts of
the tunnel has been nearly £1,000 per yard.
[Illustration:
FIG. 187_b._—_The Uppermost Bridge over the Maïenreuss._
]
The instances above mentioned may be taken as mere specimens of the
physical difficulties attending a work of this kind. There are often
others arising from the unusual circumstances under which the workmen
are placed, and others again from accidental causes alone. M. Favre
experienced some of these, as, for example, when one year a fire
destroyed the greater part of the village of Airolo; another year there
was a strike on the part of the workmen. The high temperature in the
workings was, especially towards the end, a source of great trouble. The
cause of the heat is no doubt the same as that which is held to support
the theory of the earth’s central heat. Numberless observations have
established the fact that the temperature of the earth’s crust increases
as we go deeper. The increase appears not to be uniform in different
places—at least there is much discrepancy in the estimates that have
been made. But as a sufficient approximation to a general statement, it
may be taken as proved that for every seventy feet or so that you go
below the surface of the ground, there is an increase of the temperature
of the strata equal to 1° Fahrenheit. Now, the workmen in the two
sections of the tunnel had, at last, to carry on their labour in a
temperature of more than 100° Fahrenheit. This, perhaps, might have been
one cause of some unprecedented kinds of malady that appeared amongst
the tunnel labourers. M. Favre himself was not destined to witness the
completion of his great undertaking, for, on July 19th, 1879, as he was
returning from an inspection of the tunnel, he fell into the arms of his
companions, struck down by a fatal attack of apoplexy. On February 29th,
1880, the last fuse required to blast down the rock separating the two
tunnels was fired by one of the few workmen who had been engaged in the
operations during the whole period from their commencement. It was found
that the two tunnels met exactly and coincided in direction.
[Illustration:
FIG. 187_c_.—_The Bridges over the Maïenreuss near Wasen._
]
The construction of such a line of railway as the St. Gothard tries the
skill of the engineer, and taxes all the resources of his art. The
problems presented by the nature of the route, and the requirements of
the iron road, have in this case been successfully solved by bold
expedients—by new and ingenious devices. The reader will readily
understand that the ordinary cart road may wander about, so to speak, of
its own will; it is not confined to the limited gradient of the line; or
obliged to make its turns and curves of at least a certain radius. Now,
there are portions of the valley where the general slope is too steep
for the railway to follow, and where it was necessary to form it in
zig-zags, so that certain sections of the gorge or valley may be several
times traversed by the line returning upon itself. Fig. 187_d_ is a view
showing an incident of this kind, and one of the most interesting spots
on the route. The dark line on the spectator’s right is the track of the
railway; the white trace, which in the lower part of the view is seen on
the other side of the Reuss, is the ordinary road. If this last be
followed up the valley, it will be seen to cross first the Reuss, and
then a tributary stream (the _Maïenreuss_) descending through a gorge on
the right, after which it zig-zags up a hill to the village of _Wasen_
(the church of which village is seen crowning the eminence in the centre
of our view), and then it continues its course up the valley, passing
through a small village, and disappearing over the shoulder of a hill on
the right bank of the river. Let us now carefully follow the railway
from where the train at the bottom of the picture is seen ascending the
gradient. The line presently passes under a bridge, and then enters the
tunnel, near to the entrance of which a small building will be noticed.
The course of the tunnel is shown by the _curve_ marked in dots, for
this tunnel makes a round within the rock, and the railway emerges to
day again at a point lower down in the course of the valley than at the
entrance to the tunnel, but at a higher level. It is seen in the figure
appearing from behind the rocks in the right-hand lower corner, passing
under a short tunnel and continuing along the mountain side. The curved
tunnel resembles in direction part of the turn of a corkscrew; it is one
of a series of _helicoidal_ tunnels of which there are several examples
on the line. The entrance to this tunnel is 2,539 feet, the exit 2,654
feet, above the sea level. It is known as _Pfaffensprung_ (Monk’s Leap)
Tunnel. The line again enters a short tunnel, and immediately crosses
the deep gorge of the Maïenreuss, to plunge again into another tunnel at
the base of this hill on which Wasen stands. Higher up it crosses the
Reuss and enters the helicoidal tunnel of _Wattingen_ (dotted line). On
emerging from this, the line re-crosses the Reuss, and may now be traced
_down_ the valley, but higher up on the mountain side, coming in the
reverse direction, and after passing _Wasen_ on the other side,
re-crossing the Maïenreuss gorge by a second bridge. Then turning back
again through another helicoidal tunnel (_Leggistein_) the line crosses
the Maïenreuss for the third time, and continues its course up the
valley. Fig. 187_c_ gives us a near view of _Wasen_, and a glimpse up
the gorge of the Maïenreuss from its junction with the Reuss. The bridge
with the large single arch is that which carries the ordinary road, and
higher up we see the three iron bridges that carry the railway backwards
and forwards in its doublings. We can well imagine the perplexity of
anyone ascending the valley in the train for the first time, and
ignorant of the peculiarities of this extraordinary railway. In crossing
the first, or lowest bridge, over the Maïenreuss, he would catch a
glimpse of the church of Wasen, perched on its hill, high above him, and
on his right. After being carried through more tunnels, and over more
bridges, he would some minutes afterwards be disposed to think that his
eyes were deceiving him, for there, still on his right, he would see the
same church, but now on about the same level as the train. Again, after
more tunnels and bridges, the church would once more appear, transferred
to the left of the line, and sunk very far down. These several
apparitions of the same building in different positions, after the train
has seemed to have been pursuing its onward course the while,—which
course would not be judged by any impressions the traveller would
usually receive to be other than rectilinear,—are indeed a regular
bewilderment to the inexperienced traveller. He is then obliged finally
to resign himself passively to be carried he knows not whither or how,
for his sense of direction is completely at fault;—the train comes out
of tunnels which seem turned the wrong way; the river, which he expected
to find on the left hand, he sees on the right; and the Reuss appears to
have reversed the direction of its flow.
[Illustration:
FIG. 187_d_.—_Windings of the Line near Wasen._
]
It is understood that the St. Gothard line has been a great commercial
success, for the number of passengers entering and leaving Italy by that
route has been enormous, and still shows a large annual increase.
Indeed, the prosperity of the line has been so great that the project
has been revived of carrying another railway over the Alps to connect
Italy and Switzerland by way of the Simplon. If this scheme should be
carried out, the mountains will be pierced by a tunnel of a length
double that of the St. Gothard.
[Illustration:
FIG. 188.
]
LIGHT.
The foregoing pages have been devoted to the description of inventions
or operations in which mechanical actions are the most obvious features.
Some of the contrivances described have for their end and object the
communication of motion to certain bodies, others the arrangement of
materials in some definite form, and all are essentially associated with
the idea of what is called _matter_. But we are now about to enter on
another region—a region of marvels where all is enchanted ground—a
region in which we seem to leave far behind us our grosser conceptions
of matter, and to attain to a sphere of more refined and subtile
existence. For we are about to show some results of those beautiful
investigations in which modern science has penetrated the secrets of
Nature by unfolding the laws of light—
“Light
Ethereal, first of things, quintessence pure.”
The diversity and magnificence of the spectacles which, by day as well
as by night, are revealed to us by the agency of light, have been the
theme of the poet in every age and in every country. It cannot fail to
arrest the attention to find Science declaring that all the loveliness
of the landscape, the fresh green tints of early summer and the golden
glow of autumn, the brilliant dyes of flowers, of insects, of birds, the
soft blue of the cloudless sky, the rosy hues of sunset and of dawn, the
chromatic splendour of rubies, emeralds, and other gems, the beauties of
the million-coloured rainbow,—are all due to light—to light alone, and
are not qualities of the bodies themselves, which merely _seem_ to
possess the colours. The following quaint stanzas, in which a poet of
the seventeenth century addresses “Light” have a literal correspondence
with scientific truth:
“All the world’s bravery, that delights our eyes,
Is but thy several liveries;
Thou the rich dye on them bestowest,
Thy nimble pencil paints this landscape as thou goest.
“A crimson garment in the rose thou wearest:;
A crown of studded gold thou bearest;
The virgin lilies, in their white,
Are clad but with the lawn of almost naked light.
“The violet, Spring’s little infant, stands
Girt in thy purple swaddling-bands;
On the fair tulip thou dost dote;
Thou clothest it in a gay and parti-coloured coat.”
All these beauties are indeed derived from the imponderable and
_invisible_ agent, light; and the variety and changefulness of the
effects we may constantly observe show that light possesses the power of
impressing our visual organs in a thousand different ways, modified by
the surrounding circumstances, as witness that ever-shifting
transformation scene—the sky. In the skies of such a climate as that of
England there are ceaseless changes and ever-beautiful effects,
producing everywhere more perfect and diversified pictures than the
richest galleries can show. In the night how changed is the spectacle,
when the sun’s more powerful rays are succeeded by the soft light of the
moon, sailing through the azure star-bestudded vault! What limitless
scope for the artist is afforded by these innumerable modifications of a
single subtile agent, in light and shade, brightness and obscurity, in
the contrasts and harmonies of colours, and in the countless hues
resulting from their mixtures and blendings!
It will be necessary, before attempting to explain the discoveries and
inventions which prove how successfully science, aided by the powerful
mathematical analysis of modern times, has acquired a knowledge of the
ways of light, to discuss such of the ordinary phenomena as have a
direct bearing upon the subjects to be considered.
[Illustration:
FIG. 189.—_Rays._
]
_SOME PHENOMENA OF LIGHT._
It may be considered as a matter of common experience that light is able
to pass through certain bodies, such as air and gases, pure water,
glass, and a number of other liquids and solids, which, by virtue of
this passage of light, we term _transparent_, in opposition to another
class of bodies, called _opaque_, through which light does not pass.
That light traverses a vacuum may be held as proved by the light of the
sun and stars reaching us across the interplanetary spaces; but it may
also be made the subject of direct experiment by an apparatus described
below, Fig. 190. Another fact, very obvious from common observation, is
that light usually travels in straight lines. Some familiar experiences
may be appealed to for establishing this fact. For example, every one
has observed that the beams of sunlight which penetrate an apartment
through any small opening pursue their course in perfectly straight
lines across the atmosphere, in which their path is rendered visible by
the floating particles of dust. It is by reason of the straightness with
which rays of light pursue their course that the joiner, by looking
along the edge of a plank, can judge of its truth, and that the engineer
or surveyor is able by his theodolite and staff to set out the work for
rectilinear roads or railways. On a grander scale than in the sunbeam
traversing a room, we witness the same fact in the effect represented in
Fig. 189, where the sun, concealed from direct observation, is seen to
send through openings in the clouds, beams that reveal their paths by
lighting up the particles of haze or mist contained in the atmosphere.
It is not the air itself which is rendered visible; but whenever a beam
of sunlight, or of any other brilliant light, is allowed to pass through
an apartment which is otherwise kept dark, the track of the beam is
always distinctly visible, and, especially if the light be concentrated
by a lens or concave mirror, the fact is revealed that the air, which
under ordinary circumstances appears so pure and transparent, is in
reality loaded with floating particles, requiring only to be properly
lighted up to show themselves.
[Illustration:
FIG. 190.
]
Professor Tyndall, in the course of some remarkable researches on the
decomposition of vapours by light, wished to have such a glass tube as
that represented in Fig. 190, filled with air perfectly free from these
floating particles. When the beam of the electric lamp passed through
the exhausted tube, no trace of the existence of anything within the
tube was revealed, for it appeared merely like a black gap cut out of
the visible rays that traversed the air; thus proving that light,
although the agent which makes all things become visible, _is itself
invisible_—that, in fact, we see not light, but only illuminated
substances. When, however, air was admitted to the tube, even after
passing through sulphuric acid, the beam of the light became clearly
revealed within the tube, and it was only by allowing the air to stream
very slowly into the exhausted glass tube through platinum pipes, packed
with platinum gauze and intensely heated, that Professor Tyndall
succeeded in obtaining air “optically empty,” that is, air in which no
floating particles revealed the track of the beams. The destruction of
the floating matter by the incandescent metal proves the particles to be
organic; but a more convenient method of obtaining air free from all
suspended matter was found by Professor Tyndall to be the passing of the
air through a _filter of cotton wool_. It must not be supposed that it
is only occasionally, or in dusty rooms, laboratories, or lecture-halls,
that the air is charged with organic and other particles—
“As thick as motes in the sunbeams.”
“The air of our London rooms,” says Tyndall, “is loaded with this
organic dust, nor is the country air free from its pollution. However
ordinary daylight may permit it to disguise itself, a sufficiently
powerful beam causes the air in which the dust is suspended to appear as
a semi-solid, rather than as a gas. Nobody could, in the first instance,
without repugnance, place the mouth at the illuminated focus of the
electric beam and inhale the dust revealed there. Nor is this disgust
abolished by the reflection that, although we do not see the nastiness,
we are drawing it in our lungs every hour and minute of our lives. There
is no respite to this contact with dirt; and the wonder is, not that we
should from time to time suffer from its presence, but that so small a
portion of it would appear to be deadly to man.” The Professor then goes
on to develop a very remarkable theory, which attributes such diseases
as cholera, scarlet fever, small pox, and the like, to the inhalation of
organic _germs_ which may form part of the floating particles. But we
must return to our immediate subject by a few words on the
_VELOCITY OF LIGHT._
[Illustration:
FIG. 191.—_Telescopic appearance of Jupiter and Satellites._
]
It may be stated at once, that this velocity has the amazing magnitude
of 185,000 miles in one second of time, and that the fact of light
requiring time to travel was first discovered, and the speed with which
it does travel was first estimated, about 200 years ago, by a Danish
astronomer, named Roemer, by observations on the eclipses of the
satellites of Jupiter. The satellites of Jupiter are four in number, and
as they revolve nearly in plane of the planet’s orbit, they are subject
to frequent eclipses by entering the shadow cast by the planet; in fact,
the three inner satellites at every revolution. Fig. 191 represents the
telescopic appearance of the planet, from a drawing by Mr. De La Rue,
and in this we see the well-known “belts,” and two of the satellites,
one of which is passing across the face of the planet, on which its
shadow falls, and is distinctly seen as a round black spot, while the
other may be noticed at the lower right-hand corner of the cut. The
satellite next the planet (Io) revolves round its primary in about 42½
hours, and consequently it is eclipsed by plunging into the shadow of
Jupiter at intervals of 42½ hours, an occurrence which must take place
with the greatest regularity as regards the duration of the intervals,
and which can be calculated by known laws when the distance of the
satellite from the planet has been determined. Nevertheless, Roemer
observed that the actual intervals between the successive immersions of
Io in the shadow of Jupiter did not agree with the calculated period of
rotation when the distance between Jupiter and the earth was changing,
in consequence chiefly of the movement of the latter (for Jupiter
requires nearly twelve years to complete his revolution, and may,
therefore, be regarded as stationary as compared for a short time with
the earth). Roemer saw also, that when this distance _was increasing_,
the observed intervals between the successive eclipses were a little
greater, and that when the distance _was decreasing_ they were a little
less, than the calculated period. And he found that, supposing the
earth, being at the point of its orbit nearest to Jupiter, to recede
from that planet, the _sum of all the retardations_ of the eclipses
which occur while the earth is travelling to the farthest point of its
orbit, amounts to 16½ minutes, as does also the _sum of the
deficiencies_ in the period when the earth, approaching Jupiter, is
passing from the farthest to the nearest point of her orbit. While,
however, the earth is near the points in her orbit farthest from, or
nearest to Jupiter, the distance between the two planets is not
materially _changing_ between successive eclipses, and _then_ the
observed intervals of the eclipses coincide, with the period of the
satellite’s rotation. The reader will, after a little reflection, have
no difficulty in perceiving that the 16½ minutes represent the time
which is required by the light to traverse the diameter of the earth’s
orbit; or, if he should have any difficulty, it may be removed by
comparing the case with the following.
Let us suppose that from a railway terminus trains are dispatched every
quarter of an hour, and that the trains proceed with a common and
uniform velocity of, say, one mile per minute. Now, a person who remains
stationary, at any point on the railway, observes the trains passing at
regular intervals of fifteen minutes, no matter at what part of the line
he may be placed. But now, let us imagine that a train having that very
instant passed him, he begins to walk along the line towards the place
from which the trains are dispatched: it is plain that he will meet the
next train before fifteen minutes—he would, in fact, meet it a mile
higher up the line than the point from which he began his walk fourteen
minutes before; but the train, taking a minute to pass over this mile,
would pass his point of departure just fifteen minutes after its
predecessor. And our imaginary pedestrian, supposing him to continue his
journey at the same rate, would meet train after train at intervals of
fourteen minutes. Similarly, if he walked away from the approaching
trains, they would overtake him at intervals of sixteen minutes. And
again, it would be easy for him to calculate the speed of the trains,
knowing that they passed over each point of the line every fifteen
minutes. Thus, suppose him to pass _down_ the line a distance known to
be, say, a quarter of a mile; suppose he leaves his station at noon, the
moment a train has passed, and that he takes, say an hour, to arrive at
his new station a quarter of a mile lower; here, observing a train to
pass at fifteen seconds after one o’clock, and knowing that it passed
his original station at one, he has a direct measure of the speed of the
trains. Here we have been explaining a discovery two centuries old; but
our purpose is to prepare the reader for an account of how the velocity
of light has been recently measured in a direct manner, and it certainly
appears a marvellous achievement that means have been found to measure a
velocity so astounding, not in the spaces of the solar system, or along
the diameter of the earth’s orbit, but within the narrow limits of an
ordinary room! The reliance with which the results of these direct
measures will be received, will be greatly increased by the knowledge of
the astronomical facts with which they show an entire concordance. In
taking leave of Roemer, we may mention that his discovery, like many
others, and like some inventions which have been described in this book,
did not for some time find favour with even the scientific world, nor
was the truth generally accepted, until Bradley’s discovery of the
aberration of light completely confirmed it.
[Illustration:
FIG. 192.
]
To two gifted and ingenious Frenchmen we are indebted for independent
measurements of the velocity of light by two different methods. The
general arrangement of M. Fizeau’s method is represented in Fig. 192, in
which the rays from a lamp, L, after passing through a system of lenses,
fall upon a small mirror, M N, formed of unsilvered plate-glass inclined
at an angle of 45° to the direction of the rays; from this they are
reflected along the axis of a telescope, T, by the lens of which being
rendered parallel, they become a cylindrical beam, B, which passes in a
straight line to a station, D, at a distance of some miles (in the
actual experiment the lamp was at Suresnes and the other station at
Montmartre, 5½ miles distant) whence the beam is reflected along the
same path, and returns to the little plate of glass at M N, passing
through which it reaches the eye of the observer at E. At W is a toothed
wheel, the teeth of which pass through the point F, where the rays from
the lamp come to a focus; and as each tooth passes, the light is stopped
from issuing to the distant station. This wheel is capable of receiving
a regular and very rapid rotation from clockwork in the case, C,
provided with a register for recording the number of its revolutions. If
the wheel turns with such a speed that the light permitted to pass
through one of the spaces travels to the mirror and back in exactly the
same time that the wheel moves and brings the next space into the tube,
or the second space, or the third, or any _space_, the reflected light
will reach the spectator’s eye just as if the wheel were stationary; but
if the speed be such that a _tooth_ is in the centre of the tube when
the light returns from the mirror, then it will be prevented from
reaching the spectator’s eye at all, so long as this particular speed is
maintained, but either a decrease or an increase of velocity would cause
the luminous image to reappear. Speeds between those by which the light
is seen, and those by which it entirely disappears, cause it to appear
with merely diminished brilliancy. It is only necessary to observe the
speed of the wheel when the light is at its brightest, and when it
suffers complete eclipse, for then the time is known which is required
for space and tooth respectively to take the place of another space—and
hence the time required for the light to pass to the mirror and back is
found.
M. Foucault’s method is similar in principle to that used by Wheatstone
in the measurement of the velocity of electricity. He used a mirror
which was made to revolve at the rate of 700 or 800 turns _per second_,
and the arrangement of the apparatus was such as to admit of the
measurement of the time taken by light to pass over the short space of
about four yards! More recently, however, he has modified and improved
his apparatus by adopting a most ingenious plan of maintaining the speed
of the mirror at a determined rate, which he now prefers should be 400
turns per second, while the light is reflected backwards and forwards
several times, so that it traverses a path of above 20 yards in length.
The time taken by the light to travel this short distance is, of course,
extremely small, but it is accurately measured by the clockwork
mechanism, and found to be about the 1/150000000th of a second! The
results of these experiments of Foucault’s make the velocity of light
several thousand miles per second less than that deduced from the
astronomical observation of Roemer and Bradley, in which the distance of
the earth from the sun formed the basis of the calculations; and hence
arose a surmise that this distance had been over-estimated. That such
had, indeed, been the case was confirmed almost immediately afterwards
by a discussion among the astronomers as to the correctness of the
accepted distance, the result of which has been that the mean distance,
which was formerly estimated at 95 millions of miles, has, by careful
astronomical observations and strict deductions, been now estimated at
between 91 and 92 millions of miles. The famous transit of Venus
December 9th, 1873–-to observe which the Governments of all the chief
nations of the world sent out expeditions—derived its astronomical and
scientific importance from its furnishing the means of calculating, with
greater correctness than had yet been attained, the distance of the
earth from the sun.
[Illustration:
FIG. 193.
]
[Illustration:
FIG. 194.
]
[Illustration:
FIG. 195.
]
_REFLECTION OF LIGHT._
Long before plate glass backed by brilliant quicksilver ever reflected
the luxurious appointments of a drawing-room; long before looking-glass
ever formed the mediæval image of “ladye fair”; long before the haughty
dames of imperial Rome were aided in their toilettes by _specula_; long
before the dark-browed beauties of Egypt peered into their brazen
mirrors; long, in fact, before men knew how to make glass or to polish
metals, their attention and admiration must have often been riveted by
those perfect and inverted pictures of the landscape, with its rocks,
trees, and skies, which every quiet lake and every silent pool presents.
Enjoyment of the spectacle probably prompted its imitation by the
formation artificially of smooth flat reflecting surfaces; and no doubt
great skill in the production of these, and their application to
purposes of utility, coquetry, and luxury, preceded by many ages any
attempt to discover the laws by which light is reflected. The most
fundamental of these laws are very simple, and for the purpose we have
in view, it is necessary that they should be borne in mind. Let A B,
Fig. 193, be a _plane reflecting surface_, such as the surface of pure
quicksilver or still water, or a polished surface of glass or metal, and
let a ray of light fall upon it in the direction, I O, meeting the
surface at O, it will be reflected along a line, O R,—such that if at
the point O we draw a line, O P, perpendicular to the surface, the
incident ray, I O, and the reflected ray, O R, will form equal angles
with the perpendicular—in other words, the angle of incidence will be
equal to the angle of reflection, and the perpendicular, the incident
ray, and the reflected ray, will all be in one plane perpendicular to
the reflecting plane. It would be quite easy to prove from this law that
the luminous rays from any object falling on a plane reflecting surface
are thrown back just _as if_ they came from an object placed behind the
reflecting surface symmetrically to the real object. The diagrams in
Figs. 194 and 195 will render this clear. In the second diagram, Fig.
195, it will be noticed that only the portion of the mirror between Q
and P takes any part in the action, and therefore it is not necessary,
in order to see objects in a plane mirror, that the mirror should be
exactly opposite to them; thus the portion O Q might be removed without
the eye losing any part of the image of the object A B.
[Illustration:
FIG. 196.
]
There are many very interesting and important scientific instruments in
which the laws of reflection from plane surfaces are made use of—such,
for example, as the _sextant_ and the _goniometer_; but passing over all
these, we may say a word about the formation of several images from one
object by using two mirrors. It has already been explained that the
action of a plane mirror is equivalent to the placing of objects behind
it symmetrically disposed to the real object. The reflections, or
_virtual images_ in the mirror, behave optically exactly _as if_ they
were themselves real objects, and are reflected by other mirrors in
precisely the same manner. From this it follows that two planes inclined
to each other at an angle of 90° give three images of an object placed
between them, the images and the object apparently placed at the four
angles of a rectangle. When the mirrors are inclined to each other at an
angle of 60°, five images are produced, which, with the original object,
show an hexagonal arrangement. The formation of these by the principle
of symmetry is indicated in Fig. 196. It was these symmetrically
disposed images which suggested to Sir David Brewster the construction
of the instrument so well known as the _kaleidoscope_, in which two—or,
still better, three—mirrors of black glass, or of glass blackened on one
side, are placed in a pasteboard tube inclined to each other at 60°: one
end of the tube is closed by two parallel plates of glass; the outer one
ground, but the inner transparent, leaving between them an interval, in
which are placed fragments of variously-coloured glass, which every
movement of the instrument arranges in new combinations. At the other
end of the tube is a small opening—on applying the eye to which one sees
directly the fragments of glass, with their images so reflected that
beautifully symmetrical patterns are produced; and this with endless
variety. When this instrument was first made in the cheap form in which
it is now so familiarly known, it obtained a popularity which has
perhaps never been equalled by any scientific toy, for it is said that
no fewer than 200,000 kaleidoscopes were sold in London and Paris in one
month.
[Illustration:
FIG. 197.—_Polemoscope._
]
By way of contrast to the mirrors of the kaleidoscope harmlessly
producing beautiful designs, by symmetrical images of fragments of
coloured glass, we show the reader, in Fig. 197, mirrors which are
reflecting quite other scenes, for here is seen the manner in which even
the plane mirror has been pressed into the service of the stern art of
war. The mirrors are employed, not like those of Archimedes, to send
back the sunbeams from every side, and by their concentration at one
spot to set on fire the enemy’s works, but to enable the artillerymen in
a battery to observe the effect of their shot, and the movement of their
adversaries, without exposing themselves to fire by looking over the
parapet of their works. The contrivance has received the appropriate
name of _Polemoscope_ (πολεμος, _war_, and σκοπεω, _to view_), and it
consists simply, as shown in the figure, of two plane mirrors so
inclined and directed, that in the lower one is seen by reflection the
localities which it is desired to observe.
[Illustration:
FIG. 198.—_Apparatus for Ghost Illusion._
]
We return once more to the arts of peace, in noticing the advantage
which has been lately taken of plane mirrors for the production of
spectral and other illusions, in exhibitions and theatrical
entertainments, the improvement in the manufacture of plate-glass having
permitted the production of enormous sheets of that substance. Among the
most popular exhibitions of this class was that known as “Pepper’s
Ghost,” the arrangement of the mirrors having been the subject of a
patent taken out by Mr. Pepper and Mr. Dircks jointly. The principle on
which the production of the illusion depends, may be explained by the
familiar experience of everybody who has noticed that, in the twilight,
the glass of a window presents to a person inside of a room the images
of the light or bright objects in the apartment, while the objects
outside are also visible through the glass. As, by night coming on, the
reflections increase in brilliancy, the darkness outside is almost
equivalent to a coat of black paint on the exterior surface of the
glass; but, on the contrary, in the daylight no reflection of the
interior of the room is visible to the spectator inside, on looking
towards the window. The reflections are present, nevertheless, in the
day-time as well as at night, only they are overpowered and lost when
the rays which reach the eye _through_ the glass are relatively much
more powerful. Even in the day-time the image of a lighted candle is
usually visible, in the absence of direct sunshine, against a dark
portion of the exterior objects as a back-ground. The visibility, or
otherwise, of the internal objects by reflection, and of the external
objects seen through the glass, depends entirely on the relative
intensities of the illumination, for the more illuminated side
overpowers and conceals the other, just as the rising sun causes the
stars “to pale their ineffectual fires.” Hence, on looking through the
window on a dark night, we cannot see objects out of doors unless we
screen off the reflection of the illuminated objects in the room. If the
rays transmitted through the glass, and those which are reflected, have
intensities not very different, we see then the reflected images mixed
up in the most curious manner with the real objects. It is exactly in
this way that the ghosts are made to appear in the illusion of which we
are speaking. The real actors are seen through a large plate of
colourless and transparent glass, and from the front surface of this
glass rays are reflected which apparently proceed from a phantom taking
a part in the scene among the real actors. The arrangement is shown in
Fig. 198, where E G is the stage, separated from the auditorium, h, by a
large plate of transparent glass, E F, placed in an inclined position,
and not visible to the spectators, for the lights in front are turned
down, and the stage is also kept comparatively dark. Parallel to the
large plate of glass is a silvered mirror, C D, placed out of the
spectators’ sight, and receiving the rays from a person at A, also out
of sight of the spectators, and strongly illuminated by an oxy-hydrogen
lime-light at B. The manner in which the rays are reflected from the
silvered mirror to the plate-glass, and hence reflected so as to reach
the spectators and give them the impression of a figure standing on the
stage at G, is sufficiently indicated by the lines drawn in the diagram.
The apparitional and unsubstantial character of the image is derived
from its seeming transparency, and from the manner in which it may be
made to melt away, by diminishing the brightness of the light which
falls on the real person. The introduction of the second mirror was a
great improvement, for by this the phantom is made to appear erect,
while its original stands in a natural attitude. Whereas, with only the
plate-glass, E F, the _ghost_ could not be made to appear upright,
unless, indeed, as was sometimes done, the plate was inclined at an
angle of 45°, and the actor of the ghost lay horizontally beneath it. A
scene of the kind produced by the improved apparatus, is represented in
Fig. 198_a_.
[Illustration:
FIG. 198_a_.—_The Ghost Illusion._
]
Another illusion is produced by the help of a large silvered mirror,
placed at an inclination of 45°, sloping backwards from the floor, and,
in consequence, presenting to the spectators the image of the ceiling,
which appears to them the back of the scene. The mirror is perforated
near the centre by an opening, through which a person passes his head,
and, all his body being concealed by the mirror, the effect produced is
that of a head floating in the air. Means are provided of withdrawing
the mirror, when necessary, while the curtain is down, and then the real
back of the scene appears, which, of course, is exactly similar to the
false one painted on the ceiling. Fig. 199 represents a scene produced
at the Polytechnic by a somewhat similar arrangement of mirrors, under
the management of Mr. Pepper. Plane mirrors were employed in another
piece of natural magic which this gentleman exhibited to the public, who
were shown a kind of large box, or cabinet, raised from the floor, and
placed in the middle of the stage, so that the spectators might see
under it and all round it. Inside of the box were two silvered mirrors
the full height of it, and these were hinged to the farther angles, so
that each one being folded with its face against a side of the box,
their backs formed the apparent sides, and were painted exactly the same
as the real interior of the box. When the performer enters the box, the
door is closed for an instant, while he, stepping to the back, turns the
mirrors on their hinges until their front edges meet, where an upright
post in the middle of the box conceals their line of junction. The
performer thus places himself behind the mirrors in the triangular space
between them and the back of the box, while the mirrors, now inclined at
angles of 45° to the sides, reflect images of these to the spectators
when the door is opened, and the spectators see then the box apparently
empty, for the reflection of the sides appears to them as the back of
the cabinet. The entertainment was sometimes varied by a skeleton
appearing, on the door being opened, in the place of the person who
entered the cabinet. It is hardly necessary to say that the skeleton was
previously placed in the angle between the mirrors where the performer
conceals himself.
[Illustration:
FIG. 199.—_Illusion produced by Mirrors._
]
[Illustration:
FIG. 200.—_A Stage Illusion._
]
To the same inventive gentleman, whose ingenious use of plane mirrors
has thus largely increased the resources of the public entertainer, is
due another stage illusion, the effect of which is represented in Fig.
200; and, although it does not depend on reflection, it may be
introduced here as showing how the perfection of the manufacture of
plate-glass, which makes it available for the ghost exhibition, can be
applied in another way in dramatic spectacles. The female form, here
supposed to be seen in a dream by the sleeper, is not a reflection,
although she appears floating in mid-air, strangely detached from all
supports, but the real actress. This is accomplished by making use of
the transparency of plate-glass, a material strong enough to afford the
necessary support, and yet invisible under the circumstances of the
exhibition.
But it is not behind the turned-down footlights, or in the exhibitions
of the showman, that we find the most beautiful illustrations of the
laws of reflection. In the quiet mountain mere, amid the sweet freshness
of nature, we may often see tree, and crag, and cliff, so faithfully
reproduced, that it needs an effort of the understanding to determine
where substance leaves off and shadow begins, a condition of the liquid
surface indicated in two lines by Wordsworth:
“The swan, on still St. Mary’s Lake,
Floats double, swan and shadow.”
The landscape painter is always gratified if he can introduce into his
picture some piece of water, and it can hardly be doubted that much of
the charm of lakes and rivers is due to their power of reflecting. Look
on Fig. 201, a view of some buildings at Venice; and, in order to see
how much of its beauty is owing to the quivering reflections, imagine
the impression it would produce were the place of the water occupied by
asphalte pavement, or a grass lawn. The condition of the reflections
here represented is perhaps even more pleasing than that produced by
perfect repose: they are in movement, and yet not broken and confused:
“In bright uncertainty they lie,
Like future joys to Fancy’s eye.”
[Illustration:
FIG. 201.—_View of Venice—Reflections._
]
_REFRACTION._
That light moves in straight lines is a statement which is true only
when the media through which it passes are uniform; for it is easily
proved that when light passes from one medium to another, a change of
direction takes place at the common surface of the media in all rays
that meet this surface otherwise than perpendicularly. As a consequence
of this, it really is possible to see round a corner, as the reader may
convince himself by performing the following easy experiment. Having
procured a cup or basin, Fig. 202, let him, by means of a little
bees’-wax or tallow, attach to the bottom of the vessel, at R, a small
coin. If he now places the cup so that its edge just conceals the coin
from view, and maintains his eye steadily in the same position as at I,
he will, when water is poured into the cup, perceive the coin apparently
above the edge of the vessel in the direction I R´, that is, the bottom
of the cup will appear to have risen higher. Since it is known that in
each medium the rays pass in straight lines, the bending which renders
the coin visible can therefore only take place at the common junction of
the media, or, in other words, the ray, R O, passing from the object in
a straight line through the water, is bent abruptly aside as it passes
out at the surface of the water, A B, and enters the air, in which it
again pursues a straight course, reaching the eye at I, where it gives
the spectator an impression of an object at R´. This experiment is also
an illustration of the cause of the well-known tendency we have to
under-estimate the depth of water when we can see the bottom. The broken
appearance presented by an oar plunged into clear water is due to
precisely the same cause. The curious exaggerated sizes and distorted
shapes of the gold-fish seen in a transparent globe have their origin in
the same bending aside of the rays. This deviation which light undergoes
in passing obliquely from one medium into another is known by the name
of _refraction_, and it is essential for the understanding of the sequel
that the reader should be acquainted with some of the laws of this
phenomenon, although their discovery by Snell dates two centuries and a
half anterior to the present time. Let T O, Fig. 203, be a ray of light
which falls obliquely upon a plane surface, A B, common to two different
media, one of which is represented by the shaded portion of the figure,
A B C D, of which C D represents another plane surface, parallel to the
former. If the ray, T O, suffered no refraction, it would pursue its
course in a straight line to _R´_; but as a matter of fact it is found
that such a ray is always bent aside at O, if the medium A B C D is more
or less dense than the other. If, for example, A B C D is water, and the
medium above it glass, then the ray entering at O will take the course O
_R_; but if A B C D is a plate of glass with water above and below it,
the ray will take the course T O, O R, R B, suffering refraction on
entering the glass, and again on leaving it, so that R B will emerge
from the glass parallel to its original direction at T O. If through the
point of incidence, O, we suppose a line, O P, to be drawn perpendicular
to the surface, A B, then we may say that the ray in passing from the
rarer medium (water, air, &c.) into the denser medium (glass, &c.) is
bent towards the perpendicular, or normal, as at O; but that on leaving
the denser to enter the rarer medium, as at R, it is bent away from the
perpendicular. In other words, the angle _b_ O _a_ is less than the
angle _m_ O T, and O R forms a less angle with R P´ than R B´ does. It
is also a law of _ordinary_ refraction that the normal, O P, at the
point of incidence, the incident ray, T O, and the refracted ray, O R,
are all in the same plane. Besides, there is the important and
interesting law discovered by Snell and by Descartes, which may thus be
explained with reference to Fig. 203. On the incident and refracted
rays, T O and O R, let us suppose that any equal distances, O _d_ and O
_b_, are measured off from O, and that from each of the points _a_ and
_b_, perpendiculars, _a m_ and _b n_, are drawn to the normal, P P,
which passes through O; then it is found that, whatever may be the angle
of incidence, T O P, or however it is made to vary, the length of the
line _a m_ bears always the same proportion to the line _b n_ for the
same two media. Thus, if A B C D be water, and T O enters it out of the
air, the length of the line _a m_ divided by the length of the line _a
b_ will always (whatever slope T O may have) give the quotient 1·33.
This number is, therefore, a constant quantity for air and water, and is
called the index of refraction for air into water. The law just
explained is expressed by the language of mathematics thus: For two
given media the ratio of the sines of the angles of incidence and of
refraction is constant.
[Illustration:
FIG. 202.
]
[Illustration:
FIG. 203.
]
It is an axiom in optical science that a ray of light when sent in the
opposite direction will pursue the same path. Thus in Fig. 203 the
direction of the light is represented as from T towards B´; but if we
suppose B´ R to be an incident ray, it would pursue the path B´ R, R O,
O T, and in passing out of the denser medium, A B C D at O, its
direction is farther from the normal, P P, or O T, as the law of sines,
_a m_ will be always longer than _n b_, and will bear a constant ratio
to it. Suppose the angle R O P to increase, then P O B will become a
right angle; that is, the emergent ray, O T, will just graze the
surface, A B, when the angle R O P has some definite value. If this last
angle be further increased, _no light at all will pass out of the
medium_ A B C D, but the ray R O will be totally reflected at O back
into the medium, A B C D, according to the laws of reflection. The angle
which R O forms with O P when O T just skims the surface, A B, is termed
the _limiting angle_, or the _critical angle_, and its value varies with
the media. The reader may easily see the total reflection in an
aquarium, or even in a tumbler of water, when he looks up through the
glass at the surface of the water, which has then all the properties of
a perfect mirror.
The power of lenses to form images of objects is entirely due to these
laws of refraction. The ordinary double-convex lens, for example, having
its surfaces formed of portions of spheres, refracts the rays so that
_all_ the rays which from _one luminous point_ fall upon the lens, meet
together again at a point on the other side, the said point being termed
their _focus_. It is thus that _images_ of luminous bodies are formed by
lenses. An explanation of the construction and theory of lenses cannot,
however, be entered into in this place.
One important remark remains to be made—namely, that in the above
statement of the laws of reflection and refraction, certain limitations
and conditions under which they are true and perfectly general have not
been expressed; for the mention of a number of particulars, which the
reader would probably not be in a condition to understand, would only
tend to confuse, and the explanation of them would lead us beyond our
limits. Some of these conditions belong to the phenomena we have to
describe, and are named in connection with them, and others, which are
not in immediate relation to our subject, we leave the reader to find
for himself in any good treatise on optics.
_DOUBLE REFRACTION AND POLARIZATION._
About two hundred years ago, a traveller, returning from Iceland,
brought to Copenhagen some crystals, which he had obtained from the Bay
of Roërford, in that island. These crystals, which are remarkable for
their size and transparency, were sent by the traveller to his friend,
Erasmus Bartholinus, a medical man of great learning, who examined them
with great interest, and was much surprised by finding that all objects
viewed through them appeared double. He published an account of this
singular circumstance in 1669, and by the discovery of this property of
Iceland spar, it became evident that the theory of refraction, the laws
of which had been studied by Snell and by Huyghens a few years before,
required some modification, for these laws required only one refracted
ray, and Iceland spar gave two. Huyghens studied the subject afresh, and
was able, by a geometrical conception, to bring the new phenomena within
the general theory of light. Iceland spar is chemically carbonate of
lime (calcium carbonate), and hence is also called calc spar, and, from
the shape of the crystals, it has also been termed rhombohedral spar.
The form in which the crystals actually present themselves is seen in
Fig. 204, which also represents the phenomenon of double refraction.
Iceland spar splits up very readily, but only along certain definite
directions, and from such a piece as that represented in Fig. 204 a
perfect rhombohedron, such as that shown in Fig. 206, is readily
obtained by cleavage; and then we have a solid having six lozenge-shaped
sides, each lozenge or side having two obtuse angles of 101° 55´, and
two acute angles of 78° 5´. Of the eight solid corners, such as A B C,
&c., six are produced by the meeting of one obtuse and two acute angles,
and _the remaining two solid corners are formed by the meeting of three
obtuse angles_. Let us imagine that a line is drawn from one of these
angles to the other: the diagonal so drawn forms the _optic axis_ of the
crystal, and a plane passing through the optic axis, A B, Fig. 205, and
through the bisectors of the angles, E A D and F B G, marks a certain
definite direction in the crystal, to which also belong all planes
parallel to that just indicated. Any one of such planes forms what is
termed a “principal section,” to which we shall presently refer.
[Illustration:
FIG. 204.
]
[Illustration:
FIG. 205.
]
[Illustration:
FIG. 206.
]
It will be observed that in Fig. 204 the white circle on a black ground
seen through the crystal is doubled; but that, instead of being white as
the circle really is, the images appear grey, except where they overlap,
and there the full whiteness is seen. If we place the crystal upon a dot
made on a sheet of paper, or having made a small hole with a pin in a
piece of cardboard, hold this up to the light, and place the crystal
against it, we see apparently two dots or two holes. The two images
will, if the dot or hole be sufficiently small, appear entirely detached
from each other. Now, if, keeping the face of the crystal against the
cardboard or paper, the observer turn the crystal round, he will see one
of the images revolve in a circle round the other, which remains
stationary. The latter is called the _ordinary_ image, and the former
the _extraordinary_ image. Let us place the crystal upon a straight
black line ruled on a horizontal sheet of paper, Fig. 205, and let us
suppose, in order to better define the appearance, that we place it so
that the _optic axis_, A B, is in a plane perpendicular to the paper, A
being one of the two corners where the three obtuse angles meet, and B
the other, and the face, A B C D, parallel to E G H B, which touches the
paper. Then, according to the laws of ordinary refraction, if we look
_straight_ down upon the crystal, we should see through it the line I K,
unchanged in position—that is, the ray would pass perpendicularly
through the crystal as shown by L M—and, in fact, a part of the ray does
this, and gives us the _ordinary_ image, O O´; but another part of the
ray departs from the laws of Snell and Descartes, and, following the
course L N Y´, enters the eye in the direction N Y´, producing the
impression of another line at L´, which is the _extraordinary_ ray, E
E´. If the crystal be turned round on the paper, E E´ will gradually
approach O O´, and the two images will coincide when the _principal
section_ is parallel to the line I K; but the coincidence is only
apparent, and results from the superposition of the two images—for a
mark placed on the line drawn on the paper will show two images, one of
which will follow the rotation of the crystal, and show itself to the
right or left of the _ordinary_ image, according as C is to the right or
left of A. So that there are really in every portion of the crystal two
images on the line, one of which turns round the other, and the
coalescence of the two images twice in each revolution is only apparent,
for the different parts of the lengths of the images do not coincide. On
continuing the revolution of the crystal after they apparently coincide,
the images are again seen to separate, the _extraordinary_ one being now
displaced on the other side, or always towards the point, C. Thus, then,
the ray, on entering the crystal, bifurcates, one branch passing through
the crystal and out of it in the same straight line, just as it would in
passing through a piece of glass, while the other is refracted at its
entrance into the crystal, although falling perpendicularly upon its
face, and again at its exit. And again, when a beam of light, R _r_,
Fig. 206, falls obliquely on a crystal of Iceland spar, it divides at
the face of the crystal into two rays, _R_ O, and _r_ E; the former,
which is the ordinary ray, follows the laws of ordinary refraction—it
lies in the plane of incidence, and obeys the law of sines, just as if
it passed through a piece of plate-glass. The _extraordinary_ ray, on
the other hand, departs from the plane of incidence, except when the
latter is parallel to the _principal section_, and the ratio of the
sines of the angles of incidence and refraction varies with the
incidence. The reader who is desirous of studying these curious
phenomena of _double_ refraction, and those of polarization, is strongly
recommended to procure some fragments of Iceland spar, which he can very
easily cleave into rhombohedra, and with these, which need not exceed
half an inch square, or cost more than a few pence, he can demonstrate
for himself the phenomena, and become familiar with their laws. He will
find very convenient the simple plan recommended by the Rev. Baden
Powell, of fixing one of the crystals to the inside of the lid of a
pill-box, through which a small hole has been made, and through the hole
and the crystal view a pin-hole in the bottom of the box, turning the
lid, and the crystal with it, to observe the rotation of the image. The
same arrangement will serve, by merely attaching another rhomb of spar
within the box, to study the very interesting facts of the polarization
to which we are about to claim the reader’s attention.
The curious phenomena which have just been described, although in
themselves by no means recent discoveries, have led to some of the most
interesting and beautiful results in the whole range of physical
science. The examination and discussion of them by such able
investigators as Huyghens, Descartes, Newton, Fresnel, Malus, and
Hamilton, have largely conduced to the establishment of the undulatory
hypothesis—that comprehensive theory of light, which brings the whole
subject within the reach of a few simple mechanical conceptions.
It was at first supposed that it was only one of the rays which are
produced in double refraction that departed from the ordinary laws, and
Iceland spar was almost the only crystal known to have the property in
question. At the present day, however, the substances which are known to
produce double refraction are far more numerous than those which do not
possess this property, for, by a more refined mode of examination than
the production of double images, Arago has been able to infer the
existence of a similar effect on light in a vast number of bodies.
Crystals have also been found which split up a ray of light entering
them into two rays, neither of which obeys the laws of Descartes. It
may, in fact, be said that, with the exception of water, and most other
liquids, of gelatine and other colloidal substances, and of
well-annealed glass, there are few bodies which do not exercise similar
power on light.
[Illustration:
FIG. 207.
]
On examining the two rays which emerge from a rhomb of Iceland spar, on
which only one ray of ordinary light has been allowed to fall, we find
that these emergent rays have acquired new and striking properties, of
which the incident ray afforded no trace; for, if we allow the two rays
emerging from a rhomb of the spar to fall upon a second rhomb, we shall
find, on viewing the images produced, that their intensity varies with
the position into which its second crystal is turned. Thus, if we place
a rhomb of the spar upon a dot made on a sheet of white paper, we shall
have, as already pointed out, two images of equal darkness. But, in
placing a second rhomb of the spar upon the first, in such a manner that
their _principal sections_ coincide, and the faces of one rhomb are also
parallel to the faces of the other, we shall still see _two_ equally
intense images of the dot, only the images will be more widely separated
than before, and no difference will be produced by separating the
crystals if the parallelism of the planes of their respective principal
sections be preserved. Here, then, is at once a notable difference
between a ray of ordinary light and one that emerges from a rhomb of
Iceland spar; for, in the case of rays of ordinary light, we have seen
that the second rhomb would divide each ray into two, whereas it is
incapable (in the position of crystals under consideration) of dividing
either the ordinary or the extraordinary ray which emerges from the
first rhomb. If, still keeping the second rhomb above the other, we make
the former rotate in a horizontal plane, we may observe that, as we turn
the upper crystal so that the planes of the _principal sections_ form a
small angle with each, each image will be doubled, and, as the upper
crystal is turned, each pair of images exhibits a varying difference of
intensity. The ordinary ray in entering the second crystal is divided by
it into a second ordinary ray and a second extraordinary ray, the
intensities of which vary according to the angle between the principal
sections. When the two principal sections are parallel to one plane,
that is, when the angle between them is either 0° or 180°, the
extraordinary image disappears, and only the ordinary one is seen, and
with its greatest intensity. When the two _principal sections_ are
perpendicular to each other, that is, when the second crystal has been
turned through either 90° or 270°, the extraordinary has, on the
contrary, its greatest intensity, and the ordinary one disappears. When
the principal section of the second crystal has been turned into any
intermediate position, such as through 45° and 135°, or any odd multiple
of 45°, both images are visible and have equal intensities. This
experiment shows that the two rays which emerge from the first crystal
have acquired new properties, that each is affected differently by the
second crystal, according as the crystal is presented to it in different
directions round the ray as an axis. The ray of light is no longer
uniform in its properties all round, but appears to have acquired
different sides, as it were, in passing through the rhomb of Iceland
spar. This condition is indicated by saying that the ray is _polarized_,
and the first rhomb of spar is termed the _polarizer_, while the second
rhomb, by which we recognize the fact that both the _ordinary_ and the
_extraordinary_ rays emerge having different sides, has received the
name of _analyser_. But, in order to study conveniently all the
phenomena in Iceland spar, we should have crystals of a considerable
size, otherwise the two rays do not become sufficiently separated so as
to make it an easy matter to intercept one of them while we examine the
other. A very ingenious mode of getting rid of one of the rays was
devised by Nicol, and as his apparatus is much used for experiments on
polarized light, we shall state the mode of constructing _Nicol’s
Prism_. It is made from a rhomb of Iceland spar, Fig. 207, in which _a_
and _b_ are the corners where the three obtuse angles meet, all equal.
If we draw through _a_ and _b_ lines bisecting the angles _d a c_ and _f
h g_, and join _a b_, these lines will all be in one plane, which is a
principal section of the crystal, and contains the axis, _a b_. Now
suppose another plane, passing through _a b_, to be turned so that it is
at right angles to the plane containing _a b_ and the bisectors: this
plane would cut the sides of the crystal in the lines _a i_, _i h_, _b
k_, _k a_; and in making the Nicol prism, the crystal is cut into two
along this plane, and the two pieces are then cemented together by
_Canada balsam_. A ray of light, R, entering the prism, undergoes double
refraction; but the ordinary ray, meeting the surface of the Canada
balsam at a certain angle greater than the limiting angle, is totally
reflected, and passes out of the crystal at O; while the extraordinary
ray, meeting the layer of balsam at a less angle than _its_ limiting
angle, does not undergo total reflection, but passes through the balsam,
and emerges in the direction of E, completely polarized, so that the ray
is unable to penetrate another Nicol’s prism of which the principal
section is placed at right angles to that of the first.
[Illustration:
FIG. 208.
]
Among other crystals which possess the property of doubly refracting,
and therefore of polarizing, is the mineral called _tourmaline_, which
is a semi-transparent substance, different specimens having different
tints. In Fig. 208, A, B, represent the prismatic crystals of
tourmaline, and C shows a crystal which has been cut, by means of a
lapidary’s wheel, into four pieces, the planes of division being
parallel to the axis of the prism. The two inner portions form slices,
having a uniform thickness of about 1/20 in., and when the faces of
these have been polished, the plates form a convenient polarizer and
analyser. Let us imagine one of the plates placed perpendicularly
between the eye and a lighted candle. The light will be seen distinctly
through it, partaking, however, of the colour of the tourmaline; and if
the plate be turned round so that the direction of the axis of the
crystal takes all possible positions with regard to the horizon, while
the plane of the plate is always perpendicular to the line between the
eye and the candle, _no change whatever will be seen in the appearance
of the flame_. But if we fix the plate of crystal in a given position,
let us say with the axial direction vertical, and place between it and
the eye the second plate of tourmaline, the appearances become very
curious indeed, and _the candle is visible or invisible according to the
position of this second plate_. When the axis of the second is, like
that of the first, vertical, the candle is distinctly seen; but when the
axis of the second plate is horizontal, no rays from the candle can
reach the eye. If the second plate be slowly turned in its own plane,
the candle becomes visible or invisible at each quarter of a revolution,
the image passing through all degrees of brightness. Thus the luminous
rays which pass through the first plate are polarized like those which
emerge from a crystal of Iceland spar. It is not necessary that the
plates used should be cut from the same crystal of tourmaline, for any
two plates will answer equally well which have been cut parallel to the
axes of the crystals which furnished them. In the case of tourmaline the
extraordinary ray possesses the power of penetrating the substance of
the crystal much more freely than the ordinary ray, which a small
thickness suffices to absorb altogether. It may be noted that in the
simple experiment we have just described, the plate of tourmaline next
the candle forms the _polarizer_, and that next the eye the _analyser_;
and that until the latter was employed, the eye was quite incapable of
detecting the change which the light had undergone in passing through
the first plate, for the unassisted eye had no means of recognizing that
the rays emerged with sides. The usual manner of examining light, to
find whether it is polarized, is to look through a plate of tourmaline
or a Nicol’s prism, and observe whether any change in brightness takes
place as the prism or plate is rotated. Now, it so happened that in 1808
a very eminent French man of science, named Malus, was looking through a
crystal of Iceland spar, and seeing in the glass panes of the windows of
the Luxembourg Palace, which was opposite his house, the image of the
setting sun, he turned the crystal towards the windows, and instead of
the two bright images he expected to see, he perceived only one; and on
turning the crystal a quarter of a revolution, this one vanished as the
other image appeared. It was, indeed, by a careful analysis of this
phenomenon that Malus founded a new branch of science, namely, that
which treats of polarized light; and his views soon led to other
discoveries, which, with their theoretical investigations, constitute
one of the most interesting departments of optical science, as
remarkable for the grasp it gives of the theory of light as for the
number of practical applications to which it has led.
The accidental observation of Malus led to the discovery that when a ray
of ordinary light falls obliquely on a mirror—not of metal, but of any
other polished surface, such as glass, wood, ivory, marble, or
leather—it acquires by reflection at the surface the same properties
that it would acquire by passing through a Nicol’s prism or a plate of
tourmaline: in a word, it is polarized. Thus, if a ray of light is
allowed to fall upon a mirror of black glass at an angle of incidence of
54° 35´, the reflected ray will be found to be polarized in the plane of
reflection—that is, it will pass freely through a Nicol’s prism when the
principal section is parallel to the plane of reflection; but when it is
at right angles to the latter, the reflected ray will be completely
extinguished by the prism—that is, it is completely polarized. If the
angle of the incident ray is different from 54° 35´, then the reflected
ray is not completely intercepted by the prism—it is not completely but
only partially polarized. The angle at which maximum polarization takes
place varies with the reflecting substance; thus, for water it is 53°,
for diamond 68°, for air 45°. A simple law was discovered by Sir David
Brewster by which the polarizing angle of every substance is connected
with its refractive index, so that when one is known, the other may be
deduced. It may be expressed by saying that the polarizing angle is that
angle of incidence which makes the reflected and the refracted rays
perpendicular to each other. The refracted rays are also found to be
polarized in a plane perpendicular to that of reflection.
[Illustration:
FIG. 209.—_Polariscope._
]
[Illustration:
FIG. 210.
]
[Illustration:
FIG. 211.—_Iceland Spar showing Double Refraction._
]
Instruments of various forms have been devised for examining the
phenomena of polarized light. They all consist essentially of a
polarizer and an analyser, which may be two mirrors of black glass
placed at the polarizing angle, or two bundles of thin glass plates, or
two Nicol’s prisms, or two plates of tourmaline, or any pair formed by
two of these. Fig. 209 represents a polariscope, this instrument being
designed to permit any desired combination of polarizer and analyser,
and having graduations for measuring the angles, and a stage upon which
may be placed various substances in order to observe the effects of
polarized light when transmitted through them. It is found that thin
slices of crystals placed between the polarizer and analyser exhibit
varied and beautiful effects of colour, and by such effects the doubly
refracting power of substances can be recognized, where the observation
of the production of double images would, on account of their small
separation, be impossible. And the polariscope is of great service in
revealing structures in bodies which with ordinary light appear entirely
devoid of it—such, for example, as quill, horn, whalebone, &c. Except
liquids, well-annealed glass, and gelatinous substances, there are, in
fact, few bodies in which polarized light does not show us the existence
of some kind of structure. A very interesting experiment can be made by
placing in the apparatus, shown in Fig. 210, a square bar of
well-annealed glass; on examining it by polarized light, it will be
found that before any pressure from the screw C is applied to the glass,
it allows the light to pass equally through every part of it; but when
by turning the screw the particles have been thrown into a state of
strain, as shown in the figure, distinct bands will make their
appearance, arranged somewhat in the manner represented; but the shapes
of the figures thus produced vary with every change in the strain and in
the mode of applying the pressure.
[Illustration:
FIG. 212.
]
_CAUSE OF LIGHT AND COLOUR._
We have hitherto limited ourselves to a description of some of the
phenomena of light, without entering into any explanation of their
presumed causes, or without making any statements concerning the nature
of the agent which produces the phenomena. Whatever this cause or agent
may be, we know already that light requires time for its propagation,
and two principal theories have been proposed to explain and connect the
facts. The first supposes light to consist of very subtile matter shot
off from luminous bodies with the observed velocity of light; and the
second theory, which has received its great development during the
present century, regards luminous effects as being due to movements of
the particles of a subtile fluid to which the name of “ether” has been
given. Of the existence of this ether there is no proof: it is imagined;
and properties are assigned to it for no other reason than that if it
did exist and possess these properties, most of the phenomena of light
could be easily explained. This theory requires us to suppose that a
subtile imponderable fluid pervades all space, and even interpenetrates
bodies—gaseous, liquid, and solid; that this fluid is enormously
elastic, for that it resists compression with a force almost beyond
calculation. The particles of luminous bodies, themselves in rapid
vibratory motion, are supposed to communicate movement to the particles
of the ether, which are displaced from a position of equilibrium, to
which they return, executing backwards and forwards movements, like the
stalks of corn in a field over which a gust of wind passes. While an
ethereal particle is performing a complete oscillation, a series of
others, to which it has communicated its motion, are also performing
oscillations in various phases—the adjacent particle being a little
behind the first, the next a little behind the second, and so on, until,
in the file of particles, we come to one which is in the same phase of
its oscillation as the first one. The distance of this from the first is
called the “length of the luminous wave.” But the ether particles do
not, like the ears of corn, sway backwards and forwards merely in the
direction in which the wave itself advances: they perform their
movements in a direction perpendicular to that in which the wave moves.
This kind of movement may be exemplified by the undulation into which a
long cord laid on the ground may be thrown when one end is violently
jerked up and down, when a wave will be seen to travel along the cord,
but each part of the latter only moves perpendicularly to the length.
The same kind of undulation is produced on the surface of water when a
stone is thrown into a quiet pool. In each of these cases the parts of
the rope or of the water do not travel along with the wave, but each
particle oscillates up and down. Now, it may sometimes be observed, when
the waves are spreading out on the surface of a pool from the point
where a stone has been dropped in, that another set of waves of equal
height originating at another point may so meet the first set, that the
crests of one set correspond with the hollows of the other, and thus
strips of nearly smooth water are produced by the superposition of the
two sets of waves. Let Fig. 212 represent two systems of such waves
propagated from the two points A A, the lines representing the crests of
the waves. Along the lines, _b b_, the crests of one set of waves are
just over the hollows of the other set; so that along these lines the
surface would be smooth, while along C C the crests would have double
the height. Now, if light be due to undulation, it should be possible to
obtain a similar effect—that is, to make two sets of luminous
undulations destroy each other’s effects and produce darkness: in other
words, we should be able, _by adding light to light, to produce
darkness!_ Now, this is precisely what is done in a celebrated
experiment devised by Fresnel, which not only proves that darkness may
be produced by the meeting of rays of light, but actually enables us to
measure the lengths of the undulations which produce the rays.
[Illustration:
FIG. 213.
]
In Fig. 213 is a diagram representing the experiment of the two mirrors,
devised by Fresnel. We are supposed to be looking down upon the
arrangement: the two plane mirrors, which are placed vertically, being
seen edgeways, in the lines, M O, O N, and it will be observed that the
mirrors are placed _nearly_ in the same upright plane, or, in other
words, they form an angle with each other, which is nearly 180°. At L is
a very narrow upright slit, formed by metallic straight-edges, placed
very close together, and allowing a direct beam of sunlight to pass into
the apartment, this being the only light which is permitted to enter.
From what has been already said on reflection from plane mirrors, it
will readily be understood that these mirrors will reflect the beams
from the slit in such a manner as to produce the same effect, in every
way, as if there were a real slit placed behind each mirror in the
symmetrical positions, A and B. Each virtual image of the slit may,
therefore, be regarded as a real source of light at A and at B; thus,
for example, it will be observed that the actual lengths of the paths
traversed by the beams which enter at L, and are reflected from the
mirrors, are precisely the same as if they came from A and B
respectively. The virtual images may be made to approach as near to each
other as may be required, by increasing the angle between the two
mirrors, for, when this becomes 180°, that is, when the two mirrors are
in one plane, the two images will coincide. If, now, a screen be placed
as at F G, a very remarkable effect will be seen; for, instead of simply
the images of the two slits, there will be visible a number of vertical
coloured bands, like portions of very narrow rainbows, and these
coloured bands are due to the two sources of light, A and B; for, if we
cover or remove one of the mirrors, the bands will disappear and the
simple image of the slit will be seen. If, however, we place in front of
L a piece of coloured glass, say red, we shall no longer see
rainbow-like bands on the screen, but in their place we shall find a
number of strips of red light and dark spaces alternately, and, as
before, these are found to depend upon the _two_ luminous sources, A and
B. We must, therefore, come to the conclusion that the two rays exercise
a mutual effect, and that, by their superposition, they produce darkness
at some points and light at others. These alternate dark and light bands
are formed on the screen at all distances, and the spaces between them
are greater as the two images, A and B, are nearer together. Further,
with the same disposition of the apparatus, it is found that when yellow
light is used instead of red, the bands are closer together; when green
glass is substituted for yellow, blue for green, and violet for blue,
that the bands become closer and closer with each colour successively.
Hence, the effect of coloured bands, which is produced when pure
sunlight is allowed to enter at L, is due to the superposition of the
various coloured rays from the white light. Let us return to the case of
the red glass, and suppose that the distance apart of the two images, A
and B, has been measured, by observing the angle which they subtend at
C, and by measuring the distance, C O D, or rather, the distance C O L.
Now, the distances of A and B from the centre of each dark band, and of
each light band, can easily be calculated, and it is found that the
_difference between the two distances_ is always the same for the same
band, however the screen or the mirrors may be changed. On comparing the
_differences_ of the distances of A and B in case of bright bands, with
those in the case of dark ones, it was found that the former could be
expressed by the even multiples of a very small distance, which we will
call _d_, thus:
0, 2_d_, 4_d_, 6_d_, 8_d_, ...
while the differences for the dark bands followed the odd multiples of
the same quantity, _d_, thus:
_d_, 3_d_, 5_d_, 7_d_, 9_d_, ....
These results are perfectly explained on the supposition that light is a
kind of wave motion, and that the distance, _d_, corresponds to _half
the length of a wave_. We have the waves entering L, and pursuing
different lengths of path to reach the screen at F G, and, if they
arrive in opposite phases of undulation, the superposition of two will
produce darkness. The undulations will plainly be in opposite phases
when the lengths of paths differ by an _odd_ number of _half-wave_
lengths, but in the same phase when they differ by an _even_ number.
Hence, the length of the wave may be deduced from the measurement of the
distances of A and B from each dark and light band, and it is found to
differ with the colour of the light. It is also plain that, as we know
the velocity of light, and also the length of the waves, we have only to
divide the length that light passes over in one second, by the lengths
of the waves, in order to find how many undulations must take place in
one second. The following table gives the wave-lengths, and the number
of undulations for each colour:
┌───────┬──────┬───────────────────┐
│Colour.│Number│ Number of │
│ │ of │Oscillations in one│
│ │Waves │ second. │
│ │in one│ │
│ │inch. │ │
├───────┼──────┼───────────────────┤
│Red │40,960│514,000,000,000,000│
│Orange │43,560│557,000,000,000,000│
│Yellow │46,090│578,000,000,000,000│
│Green │49,600│621,000,000,000,000│
│Blue │53,470│670,000,000,000,000│
│Indigo │56,560│709,000,000,000,000│
│Violet │60,040│750,000,000,000,000│
└───────┴──────┴───────────────────┘
These are the results, then, of such experiments as that of Fresnel’s,
and although such numbers as those given in the table above are apt to
be considered as representing rather the exercise of scientific
imagination than as real magnitudes actually measured, yet the reader
need only go carefully over the account of the experiment, and over that
of the measurement of the velocity of light, to become convinced that by
these experiments _something_ concerned in the phenomena of light has
really been measured, and has the dimensions assigned to it, even if it
be not actually the distance from crest to crest of ether waves—even,
indeed, if the ether and its waves have no existence. But by picturing
to ourselves light as produced by the swaying backwards and forwards of
particles of ether, we are better able to think upon the subject, and we
can represent to ourselves the whole of the phenomena by a few simple
and comparatively familiar conceptions.
As an example of the facility with which the ether theory lends itself
to aiding our notions of the phenomena of light, take the explanation of
polarization. Let us suppose that we are looking at a ray of light along
its direction, and that we can see the particles of ether. We should, in
such a case, see them vibrating in planes having every direction, and
their paths, as so seen, would be represented by an indefinite number of
the diameters of a circle. Now, suppose we make the ray first pass
through a rhomb of Iceland spar: we should, if we could see the
vibrating particles in the emergent ordinary and extraordinary rays,
perceive them swaying backwards and forwards across the direction of the
rays in two planes only, as represented by the lines, B D and A C, in
the two circles, O _o_ and E _e_, Fig. 214–-that is, half the particles
would be vibrating in the direction B D, and the other half in the
direction A C; and further, the two directions would be at right angles
to each other—the vibrations forming the extraordinary ray being
performed in a plane at right angles to that in which the vibrations
producing the ordinary ray take place. If—these planes being in the
position indicated in 1, Fig. 214–-we turn the crystal round through
90°, they would rotate with it, and would come severally into the
position shown in 2, Fig. 214.
[Illustration:
FIG. 214.
]
It was at one time objected to the theory which represents light as due
to wave-like movements that, just as the vibrations which constitute
sound spread in all directions, and go round intercepting bodies,
enabling us, for example, to hear the sound of a bell even when a
building intervenes, so if vibrations really produce light, these would
extend within the shadows, and we ought to perceive light within the
shadows, bending, as it were, round the edge of the shadow-casting body.
This objection, which at one time presented a great difficulty for the
wave theory, was triumphantly removed by the discovery that the luminous
vibrations do extend into the shadow, and that this is in reality never
completely dark. It is true that, although we can hear round a corner,
we are in general unable to see round it; but it should be noticed that
in the case of hearing, the sound is much weakened by intervening
objects, and that there are what may be termed _sound shadows_. A ray of
light produces sensible effects only in the direction of its
propagation; but it can be shown that the successive portions of the
waves advancing along it are centres of lateral disturbances producing
new or secondary waves in all directions, which, however, interfere with
and destroy each other. When an opaque screen intercepts a portion of
the principal wave, it also stops a number of oblique or secondary
waves, which would interfere more or less with the rest. Under ordinary
circumstances, the remaining oblique or secondary rays are quite
insensible in the presence of the direct light. But, with an apparatus
which will cost but the two or three minutes’ time required to construct
it, the reader may see for himself that light is able to _pass round an
obstacle_, and he may witness directly phenomena of the same order as
those presented in the experiment of Fresnel’s mirrors, which require
costly apparatus for their production. He has only to take two fragments
of common window-glass, and having made a piece of tinfoil adhere to one
surface of each piece of glass, cut, with a sharp penknife, the finest
possible slit in each piece of tinfoil, making the slit from ½ in. to 1
in. in length. If he will then hold one piece of glass about 2 ft. from
his eye, so that it may be in the line between his eye and the sun (or
other luminous body), and hold the other piece close to his eye with its
slit parallel to that in the first piece, he will see the latter not
simply as a line of light, but parallel to it a number of
brilliantly-coloured rainbow-like bands will be seen on either side. If,
instead of receiving the light from the sun, or from a candle-flame, the
light given off by a spirit-lamp, with a piece of salt on its wick, be
used, bright yellow stripes will be seen with dark spaces between them.
Or, if the piece of glass next the sun be red-coloured, instead of plain
glass, no rainbow-like bands will be visible, but a number of bright red
stripes alternating with dark bands will be seen. The reader will have
probably now little difficulty in perceiving that these can be easily
explained as the results of interferences of a kind quite analogous to
those of the waves of water represented in the diagram, Fig. 212. The
rainbow-like stripes are due to the different wave-lengths of the
different colours, as a consequence of which the bright and dark bands
would be formed at different positions. Our limits do not admit of a
full explanation of these beautiful effects, but the reader requiring
further information would peruse with the greatest advantage portions of
Sir John Herschel’s “Familiar Lectures on Scientific Subjects.”
The undulatory theory gives also an easy explanation of colours; they
being, according to the theory, only the effects, as already stated, of
the different rates of vibrations of the ether. If the ether particles
perform 514,000,000,000,000 oscillations in a second, we receive the
impression we call red colour; if they execute 750,000,000,000,000
vibrations, the impression produced on our organ of sight is
different—we call it violet; and so on. Thus science teaches us that
visual impressions so different as red, green, blue, violet, and other
distinct colours, are, in reality, all due to movements of one and the
same——something; and that the different sensations of colour we
experience, arise merely from different rates of recurrence in these
movements. In the subsequent article we shall have occasion to show that
ordinary light, such as that of the sun, or of a candle, contains rays
of every imaginable colour, mixed together in such proportions, that
when this light falls upon a piece of paper, or upon snow, we have, in
looking at these objects, the sensation of _whiteness_. But, if the
light falls upon any substance which is able, in some way, to absorb or
destroy some of the vibrations, the admixture of which makes up “_white
light_,” as it is called, then that object sending back to our eyes the
rays formed of the remaining group of vibrations, gives us the sensation
of _colour_. Suppose, for example, a substance to be so constituted that
it is capable of absorbing, or quenching in some way, all the vibrations
of the ether which occur at a quicker rate than 520,000,000,000,000 in a
second: such a substance would send back to our eyes only the vibrations
which constitute red light (see table, page 411), and we should say the
substance in question had a _red_ colour. Similarly, if the substance
gave back only the vibrations which have the quickest rates, we should
call the substance of a _violet_ character. The agent which produces in
our visual organs the impression of colour is, therefore, not in the
objects, but in the light which falls upon them. The rose is red, not
because it has redness in itself, but because the light which falls upon
it contains some rays in which there are movements that occur just the
number of times per second that gives us the impression we call redness;
in short, the colour comes not from the flower but from the light.
“But,” the reader might say, “the rose is always red by whatever light I
see it, and therefore the colour must be in the flower. Whether I view
it by sunlight, or moonlight, or candlelight, or gaslight, I invariably
see that _it is red_.” Now, it is precisely this circumstance—the
seemingly invariable association of the object with a certain
impression—in this case, redness—that leads our judgment astray, and
makes us believe that the colour is in the object. Most people live out
their lives without anything occurring to them which would give them the
least idea that the colours of the objects they see around them are not
in these objects themselves, but are derived from the light that falls
upon the objects. And it required the comparison of many observations
and experiments, and some clear reasoning, to establish a truth so
unlike the most settled convictions of ordinary minds.
The point in question is fortunately one extremely easy of experiment,
since we have simple means of producing light in which the vibrations
corresponding to only one colour are present. The reader is strongly
recommended to try the following experiment for himself. Let him procure
a spirit-lamp, and place on the wick a piece of common salt about as
large as a pea. Let the lamp be lighted in a room from which all other
light is completely excluded, and bring near the flame a red rose or a
scarlet geranium. The flower will be seen with _all its redness gone_—it
will appear of an ashy grey or leaden colour. A ball of bright scarlet
wool, such as ladies use to work brilliant patterns for cushions, &c.,
held near this flame, is apparently transformed into a ball of the
homely grey worsted with which, about a century ago, old ladies might be
seen industriously darning stockings. The experiment is, perhaps, even
more striking when, a little distance from the spirit-lamp, is placed a
feeble light of the ordinary kind, a rushlight for example. The ball of
wool, held near the latter, shows vivid scarlet, but, brought near the
spirit-lamp with the salted wick, is pale, ashy grey. Moving thus the
ball of worsted, first to one light then to the other, gives a most
convincing and striking proof of the entire illusion we are under as to
colour being an inherent quality of substances. Similar experiments may
be multiplied indefinitely. A bouquet, viewed by the rushlight, shows
the so-called _natural_ colours of the flowers; viewed by the salted
flame, roses, verbenas, violets, larkspurs, and leaves, all appear of
the uniform ashy grey, and only _yellow_ flowers come out in their
_natural_ colours. A picture, say a chromo-lithograph after one of the
most gorgeous landscapes that Turner ever painted, appears a work in
monochrome, and gives exactly the effect of a sepia or indian-ink
drawing. The most blooming complexion vanishes, and the countenance
assumes a cadaverous aspect very startling to persons of weak nerves;
the lips especially, which might have rivalled pink coral by ordinary
light, take a repulsive livid hue. All these effects may be seen to
greater advantage by using the gas-flame of a Bunsen’s burner, having a
lump of salt placed in the flame; or by means of a piece of _fine_ wire
gauze, about six inches square, supported about two or three inches
above an ordinary gas-burner, from which the gas is allowed to issue
without being lighted, but when to the top of the wire gauze, which is
strewed with small fragments of salt, a light is applied, the gas will
ignite only above the gauze, without the flame passing down to the
burner below.
A fuller explanation of these strange appearances may be gathered from
the subsequent article; but it may suffice now to state that spirit, or
gas burned in the way we have indicated, gives off little or no light of
any kind. If, however, common salt be introduced into the flame, then
light—but light of only one particular colour—is given off, and that
colour is yellow. There are no red, or green, or blue, or violet
vibrations given off; and as the objects on which the light falls cannot
supply these, it follows that with this light no impression
corresponding to these colours can be produced on the eye, whatever may
be the objects upon which it falls. Such experiments, not simply read
about but actually performed, cannot fail to convince an intelligent
person that the colours come from the light and not from the object. Of
course, it is not denied that there is in each substance something that
determines which are the rays absorbed, and which are the rays reflected
to the eye—something that can destroy certain waves, but is powerless
over others that rebound from the substance, and reaching the eye, there
produce their characteristic impressions. And it is but this power of
sending back only certain rays among the multitude which a sunbeam
furnishes, that can be attributed to objects when we say that they have
such or such a colour. In this sense, then, we may properly say that
_the rose is red_, but it is also at the same time undeniably true that
_the redness is not in the rose_.
Let it not be supposed that such scientific conclusions as those we have
arrived at tend in any way to rob Nature of her beauty, or that our
sense of the loveliness of colour is in any danger of being blunted by
thus tracing out, as far as may be, the causes and sources of our
sensations. The poets have occasionally said harsh things of
science—indeed, one goes so far as to stigmatize the man of science as
one who would “untwist the rainbow” and “botanize upon his mother’s
grave;” and another thus laments dispelled illusions:
“When Science from Creation’s face
Enchantment’s veil withdraws,
What lovely visions yield their place
To cold material laws!”
Now, in the case we have been considering, the scientific view is surely
as beautiful as the ordinary one. We can, it is true, no longer regard
the objects as having in themselves the colours which common observation
attributes to them, but we look upon the material world as being, so to
speak, the neutral canvas upon which Light, the great painter, spreads
his varied tints, although, unlike the real canvas of an artist, which
is not only neutral, but receives indifferently whatever hues are laid
upon it, the objects around us exercise a selective effect—as if the
picture of Nature were produced by each part of the canvas refusing all
the tints save one, but itself supplying none. The tendency of the study
of science to increase our interest in the great spectacle of Nature,
and to enhance our appreciation of her charms, has been more justly
indicated by another poet—thus:
“Nor ever yet
The melting rainbow’s vernal tinctured hues
To me have shone so pleasing, as when first
The hand of Science pointed out the path
In which the sunbeams gleaming from the west
Fall on the watery cloud, whose darksome veil
Involves the orient.”
[Illustration:
FIG. 215.—_Portrait of Professor Kirchhoff._
]
THE SPECTROSCOPE.
Many of the modern discoveries and inventions already described in these
pages have been instances of practical applications of science to the
every-day wants of mankind; but the chief interest of the subject we now
enter upon flows mainly from other sources than direct applications of
its principles in useful arts, although these applications are already
neither few nor unimportant. But that which, in the highest degree,
claims our attention and excites our admiration in the revelations of
the spectroscope is the wonderful and wholly unexpected extent to which
this instrument has enlarged our knowledge of the universe, and the
apparently inadequate means by which this has been accomplished. A
little triangular piece of glass gives us power to rob the stars of
their secrets, and tells more about those distant orbs than the wildest
imagination could have deemed attainable to human knowledge. One of the
most acute philosophers of the present century, a profound thinker who
devoted his mind to the consideration of the mutual relations of the
sciences, declared emphatically, not very many years ago, that all we
could know of the heavenly bodies must ever be confined to an
acquaintance with their motions, and to such a limited acquaintance with
their features as the telescope reveals in the less distant ones. A
knowledge of their composition, he expressly asserted, could never be
attained, for we could have no means of chemically examining the matter
of which they are constituted. Such was the deliberate utterance of a
man by no means disposed to underrate the power of the human mind in the
pursuit of truth. And such might still have been the opinion of the
learned and of the unlearned, but for the remarkable train of
discoveries which has led us to the construction of instruments
revealing to us the nature of the substances entering into the
constitution of the heavenly bodies. We have now, for example, the same
certainty about the existence of iron in our sun, that we have about its
existence in the poker and tongs on the hearth. The last few years have
seen the dawn of a new science; and two branches of knowledge which
formerly seemed far as the poles asunder—namely, astronomy and
chemistry—have their interests united in this new science of celestial
chemistry. The progress which has been made in this department of
spectroscopic research is so rapid, and the field is so promising, that
the well-instructed juvenile of the future, instead of idly repeating
the simple lay of _our_ childhood:
“Twinkle, twinkle, little star,
How I _wonder_ what you are!”
will probably only have to direct his sidereal spectroscope to the
object of his admiration in order to obtain exact information as to what
the star is, chemically and physically.
[Illustration:
FIG. 216.
]
The results which have already been obtained in celestial chemistry, and
other branches of spectroscopic science, are so surprising, and
apparently so remote from the range of ordinary experience, that the
reader can only appreciate these wonderful discoveries by tracing the
steps by which they have been reached. A few fundamental phenomena of
light have already been spoken of in the foregoing article; and an
acquaintance with these will have prepared the reader’s mind for a
consideration of the new facts we are about to describe. In discussing,
in the foregoing pages, the subject of refraction, we have, in order
that the reader’s attention might not be distracted, omitted all mention
of a circumstance attending it, when a beam of ordinary light falls upon
a refracting surface, such as that represented in Fig. 203. The laws
there explained apply, in fact, to elementary rays, and not to ordinary
white light, which is a mixture of a vast multitude of elementary rays,
red, yellow, green, &c. When such a beam falls obliquely upon a piece of
glass, the ray is, at its entrance, broken up into its elements, for
these, being refracted in different degrees by the glass, each pursues a
different path in that medium, as represented by Fig. 216. Each
elementary ray obeys the laws which have been explained, and therefore
each emerges from the second surface of the plate parallel to the
incident ray, and, in consequence of this, the separation is not
perceptible under ordinary circumstances with plates of glass having
parallel surfaces. But, if the second surface be inclined so as to form
such an angle with the first that the rays are rendered still more
divergent in their exit, then the separation of the light into its
elementary coloured rays becomes quite obvious. Such is the arrangement
of the surfaces in a prism, and in the triangular pieces of glass which
are used in lustres.
For the fundamental experimental fact of our subject, we must go back
two centuries, when we shall find Sir Isaac Newton making his celebrated
analysis of light by means of the glass prism. We shall describe
Newton’s experiment, for, although it was performed so long ago, and is
generally well known, it will render our view of the present subject
more complete; and it will also serve to impress on the reader an
additional instance of the world’s indebtedness to that great mind, when
we thus trace the grand results of modern discovery from their source.
“It is well,” is the remark of a clear thinker and eloquent writer, “to
turn aside from the fretful din of the present, and to dwell with
gratitude and respect upon the services of ‘those mighty men of old, who
have gone down to the grave with their weapons of war,’ but who, while
they lived, won splendid victories over ignorance.”
[Illustration:
FIG. 217.—_Newton’s Experiment._
]
The experiment of Sir Isaac Newton will be readily understood from Fig.
217, where C is the prism, and A C represents the path of a beam of
sunlight allowed to enter into a dark apartment through a small _round_
hole in a shutter, all other light being excluded from the apartment. In
this position of the prism, the rays into which the sunbeam is broken at
its entrance into the glass were bent upwards, and at their emergence
from the glass they were again bent upwards, still more separated, so
that when a white screen was placed in their path, instead of a white
circular image of the sun appearing, as would have been the case had the
light been merely refracted and not split up, Newton saw on the screen
the variously-coloured band, D D, which he termed the _spectrum_. The
letters in the figure indicate the relative positions of the various
colours, red, orange, yellow, green, blue, &c., by their initial
letters. The spectrum, or prolonged coloured image of the sun, is red at
the end, R, where the rays are least refracted, and violet at the other
extremity, where the refraction is greatest, while, in the intermediate
spaces, yellow, green, and blue pass by insensible gradations into each
other. Newton varied his experiment in many ways, as, for example, by
trying the effect of refraction through a second prism on the
differently coloured rays. He found that the second prism did not divide
the yellow rays, for instance, into any other colour, but merely bent
them out of the straight course, to form on the second screen a somewhat
broader band of yellow, and similarly with regard to the others. From
these, and a number of other experiments described in his “Opticks,” (A.
D. 1675), Newton concludes, “that if the sun’s light consisted of but
one sort of rays, there would be but one colour in the whole world, nor
would it be possible to produce any new colour by reflections and
refractions, and, by consequence, the variety of colours depends upon
the composition of light.” ... “And if, at any time, I speak of light
and rays, or coloured, or endued with colours, I would be understood to
speak not philosophically and properly, but grossly, and accordingly to
such conceptions as vulgar people in seeing all these experiments would
be apt to frame. For the rays, to speak properly, are not coloured. In
them there is nothing else than a certain power and disposition to stir
up a sensation of this or that colour. For, as sound in a bell, a
musical string, or other sounding body, is nothing but a trembling
motion, and in the air nothing but that motion propagated from the
object, and in the sensorium ‘tis a sense of that motion under the form
of a sound; so colours in the object are nothing but a disposition to
reflect this or that sort of rays more copiously than the rest: in the
rays they are nothing but their dispositions to propagate this or that
motion into the sensorium, and in the sensorium they are sensations of
these motions under the form of colours.”
These memorable investigations of Newton’s have been the admiration of
succeeding philosophers, and even poets have caught inspiration from
this theme:
“Nor could the darting beam of speed immense
Escape his swift pursuit and measuring eye.
E’en Light itself, which everything displays.
Shone undiscovered, till his brighter mind
Untwisted all the shining robe of day;
And, from the whitening undistinguished blaze,
Collecting every ray into his kind,
To the charmed eye educed the gorgeous train
Of parent colours. First the flaming red
Sprung vivid forth; the tawny orange next;
And next delicious yellow—by whose side
Fell the kind beams of all-refreshing green;
Then the pure blue, that swells autumnal skies,
Ethereal played; and then, of sadder hue
Emerged the deepened indigo, as when
The heavy-skirted evening droops with frost,
While the last gleamings of refracted light
Died in the fainting violet away.
These, when the clouds distil the rosy show,
Shine out distinct adown the watery bow;
While o’er our heads the dewy vision bends
Delightful—melting on the fields beneath.
Myriads of mingling dyes from these result,
And myriads still remain.—Infinite source
Of beauty! ever blushing—ever new!
Did ever poet image aught so fair,
Dreaming in whispering groves, by the hoarse brook,
Or prophet, to whose rapture Heaven descends?”
The spectra which Newton obtained by admitting the solar beams through a
circular aperture, were, however, not simple spectra. The circular beam
may be considered as built up of flat and very thin bands of light,
parallel to the edges of the prism, and a simple ray would be formed by
one of these flat bands; as the round opening would allow an indefinite
number of such rays to enter, each would produce its own spectrum on the
screen, and the actual image would be formed of a number of spectra
overlapping each other. When the aperture by which the light is admitted
consists merely of a narrow slit, or line, parallel to the edges of the
prism, we obtain what is termed a _pure spectrum_. When the prism is
properly placed, an eye, viewing the fine slit through it, sees a
spectrum formed, as it were, of a succession of virtual images of the
slit in all the elementary coloured rays.
The person who first examined the solar spectrum in this manner was the
English chemist Wollaston, who, in 1802, found that the spectrum thus
observed was not continuous, but that it was crossed at intervals by
dark lines. Wollaston saw them by placing his eye directly behind the
prism. Twelve years later, namely, in 1814, the German optician
Fraunhofer devised a much better mode of viewing the spectrum; for,
instead of looking through the prism with the naked eye, he used a
telescope, placing the prism and the telescope at a distance of 24 ft.
from the slit, the virtual image of which was thus considerably
magnified. The prism was so placed that the incident and refracted rays
formed nearly equal angles with its faces, in which circumstance the ray
is least deflected from its direction, and the position is therefore
spoken of as being that of _minimum deviation_. It can be shown that
this position is the only one in which the refracted rays can produce
clear and sharp virtual images of the slit, and therefore it is
necessary in all instruments to have the prism so adjusted. Fraunhofer
then saw that the dark lines were very numerous, and he found that they
always kept the same relative positions with regard to the coloured
spaces they crossed; that these positions did not change when the
material of which the prism was made was changed; and that a variation
in the refracting angle of the prism did not affect them. He then made a
very careful map, laying down upon it the position of 354 of the lines
out of about 600 which he counted, and indicated their relative
intensities, for some are finer and less dark than others. The most
conspicuous lines he distinguished by letters of the alphabet, and these
are still so indicated; and the dark lines in the solar spectrum are
called “Fraunhofer’s Lines.” These lines, as will appear in the sequel,
are of great importance in our subject. A few of the more obvious ones
are shown in No. 1, Plate XVII. Fraunhofer found that these lines were
always produced by sunlight, whether direct, or diffused, or reflected
from the moon and planets; but that the light from the fixed stars
formed spectra having different lines from those in the sun—although he
recognized in some of the spectra a few of the same lines he found in
the solar spectrum. The fact of these differences in the spectra of the
sun and fixed stars proved that the cause of the dark lines, whatever it
might be, must exist in the light of these self-luminous bodies, and not
in our atmosphere. It was, however, some years afterwards ascertained
that the passage of the sun’s light through the atmosphere does give
rise to some dark bands in the spectrum; for it was found that certain
lines make their appearance only when the sun is near the horizon, and
its rays consequently pass through a much greater thickness of air.
Sir D. Brewster first noticed in 1832 that certain coloured gases have
the power of absorbing some of the sun’s rays, so that the spectrum,
when the rays are made to pass through such a gas before falling on the
prism, is crossed by a series of dark lines—altogether different from
Fraunhofer’s lines, though these are also present. The gas in which this
property was first noticed is that called “nitric peroxide”—a
brownish-red gas, of which even a thin stratum produces a well-marked
series of dark lines. The same property was soon discovered in the
vapours of bromine, iodine, and a certain compound of chlorine and
oxygen. Each substance furnishes a system of lines peculiar to itself:
thus the vapour of bromine, although it has almost exactly the same
colour as nitric peroxide, gives a totally different set of lines.
These, therefore, do not depend on the mere colour of the gas or vapour,
and this is conclusively proved by the fact of many coloured vapours
producing no dark lines whatever: the vapour of tungsten chloride, for
example, although in colour so exactly like bromine vapour that the two
cannot be distinguished by the eye, yields no lines whatever.
[Illustration:
FIG. 218.—_Bunsen’s Burner on a stand._
]
In Fig. 218 is represented a lamp for burning coal-gas, which is
constantly used by chemists as a source of heat. It is known as
“Bunsen’s burner,” from its inventor the celebrated German chemist. It
consists of a metal tube, 3 in. or 4 in. long, and ⅓ in. in diameter, at
the bottom of which the gas is admitted by a small jet communicating
with the elastic tube which brings the gas to the apparatus. A little
below the level of the jet there are two lateral openings which admit
air to the tube. The gas, therefore, becomes mixed with air within the
tube, and this inflammable mixture streams from the top of the tube and
readily ignites on the approach of a flame, the mixture burning with a
pale bluish flame of a very high temperature. This little apparatus is
not only the most useful pieces of chemical apparatus ever devised, but
it furnishes highly instructive illustrations of several points in
chemical and physical science; and to some of these we invite the
reader’s attention, as they have an immediate bearing on our present
subject. Coal-gas is a mixture of various compounds of the two
elementary bodies, hydrogen and carbon; and when the gas burns, these
substances are respectively uniting with the oxygen of the air,
producing water and carbonic acid gas. Now, when coal-gas is burnt in
the ordinary manner as a source of light, the supply of oxygen is too
small to admit of the complete combustion of all its constituents; and
as the oxygen more eagerly seizes upon the hydrogen than upon the
carbon, a large proportion of the latter thus set free from its hydrogen
compound is deposited in the flame in the solid form, and is there
intensely heated. The presence of solid carbon in an ordinary gas flame
is easily proved by holding in it a cold fragment of porcelain, or a
piece of metal, which will become covered with soot. In the flame of the
Bunsen burner there is no soot, because the increased supply of oxygen,
afforded by previously mixing the gas with air, enables the whole of the
constituents of the gas to be completely burnt; and this is of the
greatest advantage to the chemist, who always desires to have the
vessels he heats free from soot, in order that he may observe what is
taking place within them. The flame of Bunsen’s lamp becomes that of an
ordinary sooty gas flame, when the two orifices which admit the air at
the bottom of the tube are closed up, and then, of course, the
temperature cannot be so high as when the whole constituents of the gas
are completely burnt, but the flame becomes highly luminous; whereas
when the orifices are open it gives so little light, that in a dark room
one cannot see a finger held 20 in. from the lamp. Plainly the cause of
this difference is connected with the presence or absence of the heated
particles of solid carbon. The non-luminous flame contains no solid
particles; the bright part of the other flame is full of them. To these
heated particles of solid carbon we are, then, indebted for the light
which burning coal-gas supplies. And, since we are able by such
artificial illumination to distinguish colours, the white-hot carbon
must give off rays of all degrees of refrangibility, and we should
expect to find in the spectrum produced by such a flame, the red,
yellow, green, and other coloured rays. And such is indeed the spectrum
which these incandescent carbon particles produce: it resembles the
solar spectrum, but _there is an entire absence of dark lines_, so that
the appearance is that represented in No. 1, Plate XVII., if we suppose
the Fraunhofer lines removed. If the pale blue flame of the Bunsen’s
burner be similarly examined, the spectrum, No. 14, Plate XVII., shows
that only a few rays of certain refrangibilities are emitted, forming
bright lines here and there, but of little intensity, while the whole of
the other rays are absent. This shows that while the highly heated solid
gives off all rays from red to violet without interruption, the still
more highly heated gases give off only a few selected rays.
It has long been known that some substances impart certain colours to
flames, and such substances have been long employed to produce coloured
effects in fireworks, &c. But coloured flames do not appear to have been
examined by the prism until 1822, when Sir John Herschel described the
spectra of strontium, copper, and of some other substances, remarking
that “The colours thus communicated by the different bases to flame
afford in many cases a ready and neat way of detecting extremely minute
quantities of them.” A few years later, Fox Talbot described the method
of obtaining a monochromatic flame, by using in a spirit-lamp diluted
alcohol in which a little salt has been dissolved. The paper in which he
describes this and other observations concludes thus: “If this opinion
should be correct and applicable to the other definite rays, a glance at
the prismatic spectrum of flame may show it to contain substances which
it would otherwise require a laborious chemical analysis to detect.”
Here we have the first hint of that spectrum analysis which has provided
the chemist with a method of surpassing delicacy for the detection of
metallic elements. The spectra of coloured flames were also subsequently
examined and described by Professor W. A. Miller, but the most complete
investigation into the subject was made by Professors Kirchhoff and
Bunsen, who also contrived a convenient instrument, or _spectroscope_,
for the examination and comparison of different spectra. The instrument
has received many improvements and modifications, but the essential
parts are one or more prisms; a slit, through which the light to be
examined is allowed to enter; a tube, having at the other end a lens to
render parallel the rays from the slit; a telescope, through which the
spectrum is viewed; and usually some apparatus by which the positions of
the different lines may be identified.
[Illustration:
PLATE XVII. SPECTRA.
]
[Illustration:
FIG. 219.—_Spectroscope with one Prism._
]
A very elegant instrument, made by Mr. John Browning, of the Strand, is
represented in Fig. 219. It has a single prism, made of glass, of great
power in dispersing the rays. The prism is supported on a little stage,
placed in the middle of a horizontal circular brass table about 6 in. in
diameter. On the left is seen a tube, about 15 in. long, at the outer
extremity of which is the slit, formed of pieces of metal very
accurately shaped. One of these pieces slides in a direction at right
angles to the slit, and, by means of a spring and a fine screw, can be
very nicely adjusted, so that an opening of any degree of fineness can
be readily obtained. In front of the slit is a small glass prism, with
its edges parallel to the slit, but only half its height. The bases of
this prism are formed of two sides of a square and its diagonal, and, as
shown in the figure, one side is parallel to the face of the slit, and
the other to the axis of the tube. Rays of light coming from a source on
the left of the slit (as seen in the figure) will, therefore, enter this
little prism, and be totally reflected (see page 399) by the diagonal
surface, down the axis of the tube through the lower half only of the
slit. This is the only office of this prism, which has nothing to do
with the dispersion of the rays: the use to which it is put will be seen
presently. It is fixed in such a manner that, when required, it can be
turned aside with the touch of a finger, and the _whole_ length of the
slit exposed. A peculiarity in these instruments of Mr. Browning’s is
the admirable arrangement for determining the position of any line in a
spectrum. For this purpose, the eye-piece of the telescope is provided
with a pair of cross-wires, and the telescope itself, which is about 18
in. in length, moves in a horizontal plane round the axis of the
circular brass table, from which an arm projects, carrying a ring into
which the telescope screws. This arm carries a _vernier_ along the limb
of the circular table, which is very accurately divided into thirds of
degrees, so that with the aid of the vernier the angular position of the
telescope can be read off to a minute, that is, to 1/60th of a degree.
The arm carrying the telescope is provided with a screw for clamping it
in any desired position while the readings are taken. On placing in
front of the slit the flame of a Bunsen’s burner, the spectrum produced
by any substance in this flame will, when the instrument is in proper
adjustment, be seen on looking through the telescope, and the
cross-wires being also in view, the point of their intersection may be
brought into coincidence with any line of the spectrum, and the
telescope being clamped in this position, the angular reading thus taken
determines the position of the line. Thus, for example, the angular
positions in which the principal Fraunhofer’s lines are seen having been
observed and recorded, the angular position of any line in another
spectrum will at once determine its position among the Fraunhofer lines;
or the spectrum may be mapped by laying down the angular readings of the
lines by means of a scale of equal parts. And, again, in the little
prism in front of the slit we have the means of bringing two spectra in
view at once, one being from a light directly in front, and the other
from a light at the side. The two spectra are seen one above the other,
and the coincidence or difference of their lines may be directly
observed. When the instrument is in use, the prism and the ends of the
tube are covered with a black cloth, loosely thrown over them, by which
all stray light is shut out. The author has had in use for several years
one of these instruments, and he cannot forbear expressing his perfect
satisfaction with its powers, which he finds amply sufficient for all
ordinary chemical purposes, while the accuracy of the workmanship is
really wonderful, considering the very moderate price of the instrument.
The substances the spectra of which are most conveniently examined are
the metals of the alkalies and alkaline earths. Small quantities of the
salts of these metals, placed in a loop of fine platinum wire, impart
characteristic colours to the flame of a Bunsen burner or to that of a
spirit-lamp. For the examination of the spectra the former is to be
preferred, as the lines come out much more vividly. Indeed, at
temperatures higher than that of the Bunsen’s burner, such as in the
flame of pure hydrogen, or in the voltaic arc, some substances give out
additional lines. In Plate XVII., Nos. 2 to 9, is shown the appearance
of the spectra produced by the Bunsen’s burner when salts of the metals
are held in the flame in the manner already mentioned, and the spectra
are examined with the instrument just described. One of the simplest of
these spectra is that produced by sodium compounds, such as common salt.
The smallest particle of this substance imparts an intense yellow colour
to the flame, and the spectrum is found to take the form of a single
bright yellow line—No. 3. It has been estimated that the presence of the
(1/100000000)_th part of a grain_ of sodium can be detected by the
production of this line. Indeed, the very delicacy of this sodium
reaction renders it almost impossible to get rid of this line, for
sodium is found to be present in almost everything,—a fact the earlier
observers of spectra were not aware of, for they attributed this yellow
line to water, which was the only substance they knew to be so generally
diffused. If a platinum wire be heated in the flame of the Bunsen burner
until all the sodium indications have disappeared, it suffices to remove
the wire, and, without allowing it to come into contact with anything,
to leave it exposed to the air for a few minutes, to cause it again to
give the characteristic yellow colour when again plunged into the flame.
This is due to the fact that the element is contained in all the
floating particles which pervade the atmosphere. The spectroscope is not
required to show the presence of the sodium on the platinum which has
been exposed to the air, the colour imparted to the flame being plainly
visible to the eye, and it needs only the Bunsen burner and 2 in. of
platinum wire to prove the fact, and also to show that mere contact with
the fingers is enough to highly charge the wire with sodium compounds.
Any volatile compound of potassium gives the spectrum represented by No.
2, the principal lines being a red line and one in the extreme violet,
the latter being somewhat difficult to observe. There is also a third
rather ill-defined red line, and a portion of a faint continuous
spectrum. Salts of strontium impart a bright red colour to the flame,
and the spectrum they produce is shown by No. 6, in which are seen
several bright red lines and a fainter blue one. Calcium, which also
gives a reddish colour to flame, furnishes an entirely different set of
lines (No. 5), and barium salt another, containing numerous lines,
especially some very vivid green ones.
In all the cases we have named, and whenever bright-lined spectra are
furnished by substances placed in the flame of a lamp, or in burning
hydrogen gas, or in the intensely hot voltaic arc, there is evidence
that the substances are converted into vapour or gas. We have already
seen how hot solid carbon gives a continuous spectrum, while carbon in
the state of gaseous combination gives most of the bright lines seen in
the spectrum of coal-gas (No. 14). It is observed also that the more
readily volatized are the salts, the more vivid are the bright lines
they produce when heated in a flame. It must be understood that each
element gives it own characteristic lines, that these are always in
precisely the same position in the spectrum, that no substance produces
a line in exactly the same position as another, however near two lines
due to different substances may, in some cases, appear; and also, that
however the salts of the different metals are mixed together, each
produces its own lines, and each ingredient may be recognized. And this
is done in an instant by an experienced observer—a mere glance at the
superposed spectra of, perhaps, half a dozen metals, suffices to inform
him which are present. There is also a peculiarity in this optical mode
of recognizing the presence of bodies which gives the subject the
highest interest, namely, the circumstance that the spectrum is produced
and the bodies recognized, however far from the observer the luminous
gas may be placed, the only condition required being that the rays reach
the instrument.
Until Kirchhoff and Bunsen’s spectroscopic investigations, lithium was
supposed to be a rare metal, occurring only in a few minerals. It
happens that this substance yields a remarkable spectrum (No. 4), for it
gives an extremely vivid line of a splendid red colour, accompanied by
only one other, a feeble yellow line; and the reaction is of very great
delicacy, for 1/6000000 of a grain can easily be detected, and an eye
which has once seen the red line readily recognizes it again. A single
drop of a mineral water containing lithium has been found to distinctly
produce the red line, in cases where the quantity contained in a quart
of the water would have escaped ordinary chemical analysis. The
spectroscope has shown that lithium, so far from occurring in only four
or five minerals, is a substance very widely diffused in nature. In the
waters of the ocean, in mineral and river waters, in most plants, in
wines, tea, coffee, milk, blood, and muscle, this metal has been found.
Dr. Roscoe states that the ash of a cigar, when moistened with
hydrochloric acid, and held in a platinum wire in the flame of the
Bunsen’s burner, at once shows the principal lines of sodium, potassium,
calcium, and lithium. Salts of lithium and of strontium both impart a
rich crimson tint to flames, and it is hardly possible to detect any
difference in these colours with the naked eye; but, as the reader may
see on comparing spectra No. 4 and No. 6, the prism makes a wide
distinction.
Matter for a very interesting chapter in the history of prismatic
analysis has been furnished by the discovery of four new elements by
means of the spectroscope. In 1860 Bunsen observed that the residue,
after evaporation, of a certain mineral water, yielded spectra with
bright lines which he had not seen before. He concluded that they were
due to some unknown elements, and, in order to separate these, he
evaporated many tons of the water, and was rewarded by the discovery of
two alkaline metals, _cæsium_ and _rubidium_. The delicacy of the
spectrum reaction may be inferred from the fact of a ton weight of the
water containing only three grains of the salts of each of these
substances. Rubidium gives a splendid spectrum, containing red, yellow,
and green lines, and also two characteristic violet lines; while cæsium
has orange, yellow, and green lines, and two very beautiful blue lines,
by which it is easily recognized.
About the same time, Mr. W. Crookes discovered, in a mineral from the
Hartz, another elementary body, the existence of which was first
indicated to him by the characteristic spectrum it produces, namely, a
single splendid green line (No. 8 spectrum). In 1864 two German chemists
discovered, also in the Hartz, a fourth new element, which was detected
by two well-defined lines in the more refrangible end of the
spectrum—(see spectrum No. 9, in the plate). This metal was named
Indium, in reference to the colour of its lines, and the names of the
other three—cæsium, rubidium, and thallium, are also derived from the
colours of their characteristic lines.
[Illustration:
FIG. 220.—_Miniature Spectroscope._
]
Although the reader may, from such representations of the spectra as
those given in Plate XVII., form some idea of their appearance, he would
find his knowledge of the subject much clearer if he had the opportunity
of examining for himself the actual phenomena. We have already
recommended the performance of certain easy experiments involving no
outlay, but, in the matter of spectroscopes, carefully finished optical
and mechanical work is absolutely necessary in the appliances. It
fortunately happens that one eminent optician, at least, has made it his
study to produce good spectroscopic apparatus at the lowest possible
cost, and if the reader be interested in this subject, and desirous of
trying experiments himself, he can, for a very moderate sum, be equipped
with all the appliances for examining the phenomena we have described.
He has only to procure, in the first place, a small direct-vision
spectroscope, such as that represented of its actual size in Fig. 220,
which is sold by Mr. Browning for twenty-two shillings; secondly, a
Bunsen’s burner, a few feet of india-rubber tubing, two inches of
platinum wire, and a few grains of the salts of lithium, strontium,
thallium, &c. The whole expense will probably be covered by adding four
shillings to the cost of the spectroscope, and the reader will then be
in a position to see for himself the principal Fraunhofer lines, the
spectra of the metals already referred to, and the absorption bands of
the gases which have been mentioned, as well as the absorption bands in
liquids which will be spoken of in the sequel.
[Illustration:
FIG. 221.—_The Gassiot Spectroscope._
]
The splitting up of a beam of light into its elements—which it is the
office of the prism to produce—is accomplished by a single prism to a
certain degree only. It separates the red from the green, for example;
but the colours pass into each by insensible gradations through orange,
yellow, and greenish yellow. If we allow the rays to fall upon a second
prism after emerging from the first, the separation is carried further;
the red, for instance, is spread out into different kinds of red, and so
on with the rest. And the greater the number of prisms, the greater is
the extension which is given to the spectrum. Now, just as by increasing
the power of the telescope, new stars become visible, whose light was
before too faint, and nebulæ, or stars which before seemed single, are
resolved into clusters of individual stars—so, by increasing the power
of the spectroscope by employing two, four, or more prisms, lines which
appear single by the less powerful instruments are, in some instances,
resolved into groups of lines, and new lines come into view, which
before were too faint to show themselves. For example, if we view the
Fraunhofer lines through a spectroscope like that in Fig. 220, but
having two prisms instead of one, we shall see that the D line is not
really a single line, but is formed of two lines close together. If we
use greater dispersive power by employing a greater number of prisms, we
shall observe with solar light that when these two D lines are
sufficiently separated, several other lines make their appearance
between them. In this way the number of dark lines in sunlight, which
have been carefully mapped by Kirchhoff and others, amount to upwards of
2,000; and no doubt there are many more lines waiting a still more
powerful instrument. Fig. 221 is copied from a large spectroscope made
by Mr. Browning for Mr. Gassiot. It has nine or more highly dispersive
glass prisms; the telescope and the tube bearing the slit have focal
lengths of 18 in., the lenses having a diameter of 1½ in.; the telescope
is provided with a slow motion for taking the angular position; and
there is a third tube provided with a micrometer, by which the position
of the lines can be measured to 1/10000th of an inch.
The instruments we have mentioned, except the miniature spectroscope,
show only a portion of the spectra at once, a movement of the telescope
being requisite to bring each part into view. It has been already stated
that the only position of the prism which will make the lines clear and
well defined is that in which the _deviation_ is the least. In using
trains of prisms it is therefore necessary to adjust each prism for the
part of the spectrum which may be under observation. This is a tedious
process, and it has been obviated by a useful invention of Mr.
Browning’s, by which the adjustment is rendered automatic—that is, the
movements of the telescope are communicated to the prisms in such a
manner that they place themselves into the proper position for producing
clear images of the slit, whatever may be the refrangibility of the rays
under examination: Fig. 222 shows the arrangement as it appears when
viewed from above. The train of six prisms can be so arranged that the
ray after passing through six of them shall be totally reflected by a
surface of the last prism, and pursue again its path through the six
prisms in the reverse direction, becoming more and more dispersed by
each prism until it emerges parallel to the axis of the telescope. The
power of the instrument is, therefore, equivalent to that of one with
twelve prisms; but it can be used at pleasure with any dispersive power,
from two to twelve prisms.
[Illustration:
FIG. 222.—_Browning’s Automatic Adjustment of Prisms._
]
By making use of one of the Bunsen burners, the lines which are
characteristic of some ten or twelve metals are readily seen when one of
their more volatile salts is converted into vapour. For this purpose
their chlorides are usually employed, but the reactions are common to
all their salts. It is necessary that the metal should exist in the
flame in the state of highly heated vapour or gas, in order that its
characteristic rays should be given off. We usually introduce compounds
of these metals into the flame; but there is reason to believe that
these are decomposed in the flame, and the disassociated metal takes the
form of glowing gas, a small quantity of which suffices for the
production of the bright lines. No doubt the other constituent of the
compound, the chlorine for example, is also set free in the gaseous
form; but since the spectrum of the metal only is visible, we may infer
that at the temperature of the flame, the non-metallic elements are not
sufficiently luminous to produce a spectrum. When we repeat the
experiments with salts of the less volatile metals, we obtain no
spectra—the temperature of the flame not being sufficiently high to
convert these into vapour. Other methods have, therefore, to be resorted
to, and advantage is taken of the fact discovered by Faraday, that an
electric spark is nothing but highly heated matter. The spectroscope
gives us reason to believe that this matter, which is formed of the
substances between which the spark passes, is in the gaseous state; for
it is found, on examining sparks passing between two pieces of each
metal, that characteristic bright lines are produced. If one of the
metals already named is submitted to this examination, the same lines
are found which are seen in the spectra produced by the salts of the
metal volatized in the flame, but in some cases additional bright lines
appear in the spark spectrum. With the heavier metals the spark, or the
electric arc, is, however, the only means of igniting their vapours. The
usual mode of doing this is to make the discharges of a large induction
coil pass between the two fine wires of the metals, placed about a
quarter of an inch apart. A Leyden jar is commonly employed to condense
the discharge, and thus produce a still higher temperature. Mr. Browning
has contrived the neat little apparatus shown in Fig. 223, in which the
jar is superseded by a more compact and convenient condenser inside of
the box, so that it is only necessary to attach one terminal of the coil
to the binding-screw, seen outside of the end of the box, and place the
other wire from the coil in the binding-screw of one or the other of the
pieces of apparatus supported by the upright rod. Of these it is the one
on the right which at present engages our attention. Within a small
glass cylinder are two sliding rods, terminated by screw-clips, which
hold finely-pointed pieces of the metal under examination. The slit of
the spectroscope is placed close to the glass cylinder, and when a very
rapid succession of sparks is passing, the bright lines are seen
continuously. The spectra of metals examined in this way are found to
yield a very large number of lines. Thus the spectrum of calcium has 75
lines, and that of iron no fewer than 450 lines. Our limits will not
permit of an account of many interesting particulars relating to these
spectra, which include those of all the 50 metallic elements. It should,
perhaps, be stated that a modified mode of producing spectra by sparks
is sometimes found useful. This consists in causing sparks to pass
between a solution of some salt of the metal and a piece of platinum
wire. The apparatus for this purpose is that shown on the left side of
the upright in Fig. 223.
[Illustration:
FIG. 223.—_Apparatus for Spark Spectra._
]
It remains to describe the method of producing spectra of the gaseous
non-metallic elements, such as oxygen, nitrogen, hydrogen, &c. For this
purpose electricity is again made use of. It has been found that while
an electric discharge cannot take place across a perfect vacuum, and air
or gas, at ordinary densities, offers much resistance to the passage of
electricity, on the other hand, a highly rarefied gas permits the
discharge to take place through it with great facility. This is seen in
Geissler’s tubes, where a succession of discharges from a Ruhmkorff’s
coil causes the tubes to appear filled with light—due to the heating to
incandescence of a very minute quantity of the gas. The eye readily
recognizes difference of colour in the light given off by the different
gases, and when this light is examined by the spectroscope, bright
lines, characteristic of each gas, are observed. Nos. 12 and 13, in
Plate XVII., are the spectra of hydrogen and of nitrogen respectively,
which appear when the gases are examined in the manner just described.
In this manner the spectra of chlorine, bromine, iodine, oxygen,
sulphur, phosphorus, &c., may be studied. Silicon and some other solid
non-metallic elements present great difficulties to the spectroscopist,
for these elements cannot be volatized at any temperature we can
command, and the spectra of their elements can only be inferred from
those of their compounds. But unfortunately the spectra are found to
vary with the nature of the compound, and thus it happens that in the
case of carbon, for example, no definite spectrum can be assigned to the
element. The flame of coal-gas, burning in the air, as in the Bunsen
burner, gives the spectrum No. 14; but if this is compared with the
spectrum of the flame of burning _cyanogen_ (a compound of carbon and
nitrogen), the two are found to differ greatly. The cyanogen spectrum
has the two pale broad bands of violet-blue, the four blue lines, the
two green lines, and the brightest of the greenish yellow which are seen
in the coal-gas spectrum. But it has in addition a characteristic series
of violet lines, a series of bright blue, two or three crimson and red
lines, and bands in the orange, and several green lines, none of which
occur in the coal-gas spectrum. These additional lines are not due to
nitrogen, for, with perhaps the exception of some red lines, they do not
coincide in position with any of the nitrogen lines. The spectrum of
hydrogen, No. 12, should be noticed, as its three lines are very
distinct, and it will be observed that they exactly coincide in their
position with the three Fraunhofer lines, C, F, and G, in No. 1.
There is another branch of this extensive subject to which we have now
to invite the reader’s attention. The power of certain gases to absorb
or stop certain rays of an otherwise continuous spectrum has already
been mentioned; but this property is by no means confined to gases, for
certain liquids and solids do this in a high degree. There is a
remarkable metallic element, named _didymium_. It is a rare substance,
and its presence cannot with certainty be detected by any ordinary
tests. Its salts, however, form solutions _without colour_, or nearly
so, which have the power of strongly absorbing certain rays. If we hold
before the slit of the spectrum a small tube containing a solution of
any one of the salts, and allow the rays from the sun, or from a
luminous gas or candle-flame, to pass through it, we see the spectrum
crossed by certain well-defined very dark bands. A spectrum of this kind
is called an _absorption spectrum_, and the position, number, width,
&c., of dark bands are found to be as peculiar to each substance as are
the bright lines in the spectra of the elements. The method of observing
them when produced by solutions is very simple. The liquid is contained
in a small test-tube, which is placed in front of the slit; or, more
conveniently, the liquid is put into a _wedge-shaped_ vessel, and thus
the thickness of the stratum of liquid through which the rays pass can
easily be varied, so that the best results may be obtained. The
absorption spectra are produced by many compound substances. A striking
absorption spectrum is seen when a solution in alcohol of the green
colouring matter of leaves (_chlorophyll_) is examined; for several
distinct bands are seen, one in the red being especially well marked.
Many other coloured bodies exhibit characteristic absorption bands, as,
for example, permanganate of potash, uranic salts, madder, port wine,
and magenta. The bands are so peculiar for each substance, that if
so-called port wine, for example, owe its colour to colouring matter
other than that of the grape, such as logwood, &c., the adulteration can
be instantly detected by a glance at the absorption spectrum. As,
however, the absorption bands are not, like the bright lines of metals,
definite images of the slit, but rather broad portions of the spectra,
it is very desirable in examining such spectra to compare them directly
with those of known substances, by throwing two spectra into one field,
by means of a side reflecting prism, as already described.
Perhaps one of the most interesting examples of absorption spectra is
that of blood. A single drop of blood in a tea-cupful of water will show
its characteristic spectrum when it is properly examined. If the blood
is arterial or oxidized blood, two well-marked dark bands are visible;
but if venous or deoxidized blood be used, we see, instead of the two
dark bands, a single one in an intermediate position. These differences
have been proved to be due to oxidization and deoxidization of a
constituent of the blood, called _hæmoglobin_, and by using appropriate
chemical reagents, the same specimen of blood may be made to exhibit any
number of alternations of the two spectra, according as oxidants or
reducing reagents are employed. It would be possible by an examination
of the absorption spectrum of a drop of arterial blood to pronounce that
a person had died of suffocation from the fumes of burning charcoal. In
such case, the supply of oxygen being cut off, the hæmoglobin of the
whole of the blood in the system becomes deoxidized.
The beautiful delicacy of these spectrum reactions has permitted the
spectroscope to be applied to the microscope with signal success by Mr.
Browning, working in conjunction with Mr. Sorby, who has devoted great
attention to this subject. The Sorby-Browning instrument is a
direct-vision spectroscope, with a slit, lens, &c., placed above the
eye-piece of the microscope. By receiving the light through a single
drop of an absorptive liquid placed under the object-glass of the
microscope, the characteristic bands are made visible. The
micro-spectroscope is also a valuable instrument for examining the
absorption bands which are found in the light reflected from solid
bodies, for the smallest fragment suffices to fill the field of the
microscope. Mr. Sorby is able to obtain most unmistakably the dark bands
peculiar to blood from a particle of the matter of a blood-stain
weighing less than 1/1000th part of a grain. It is plain from this that
the spectroscope must sometimes prove of great service in giving
evidence of crime from traces which would escape all ordinary
observation.
The micro-spectroscope, in its most complete form, is represented in
Fig. 224. As may be seen from the figure, the apparatus consists of
several parts. The prism is contained in a small tube, which can be
removed at pleasure; below the prism is an achromatic eye-piece, having
an adjustable slit between the two lenses; the upper lens being
furnished with a screw motion to focus the slit. A side slit, capable of
adjustment, admits, when required, a second beam of light from any
object whose spectrum it is desired to compare with that of the object
placed on the stage of the microscope. This second beam of light strikes
against a very small prism suitably placed inside the apparatus, and is
reflected up through the compound prism, forming a spectrum in the same
field with that obtained from the object on the stage. A is a brass tube
carrying the compound direct-vision prism, and has a sliding arrangement
for roughly focussing.
[Illustration:
FIG. 224.—_The Sorby-Browning Micro-Spectroscope._
]
B, a milled head, with screw motion to finely adjust the focus of the
achromatic eye-lens.
C, milled head, with screw motion to open or shut the slit _vertically_.
Another screw, H, at right angles to C, regulates the slit horizontally.
This screw has a larger head, and when once recognized cannot be
mistaken for the other.
D D, an apparatus for holding a small tube, that the spectrum given by
its contents may be compared with that from any other object on the
stage.
E, a screw, opening and shutting a slit to admit the quantity of light
required to form the second spectrum. Light entering the aperture near E
strikes against the right-angled prism which we have mentioned as being
placed inside the apparatus, and is reflected up through the slit
belonging to the compound prism. If any incandescent object is placed in
a suitable position with reference to the aperture, its spectrum will be
obtained, and will be seen on looking through it.
F shows the position of the field lens of the eye-piece.
G is a tube made to fit the microscope to which the instrument is
applied. To use this instrument, insert G like an eye-piece in the
microscope tube. Screw on to the microscope the object-glass required,
and place the object whose spectrum is to be viewed on the stage.
Illuminate with stage mirror if transparent, with mirror and lieberkühn
and dark well if opaque, or by side reflector, bull’s-eye, &c. Remove A,
and open the slit by means of the milled head, H, at right angles to D
D. When the slit is sufficiently open the rest of the apparatus acts
like an ordinary eye-piece, and any object can be focussed in the usual
way. Having focussed the object, replace A, and gradually close the slit
till a good spectrum is obtained. The spectrum will be much improved by
throwing the object a little out of focus.
Every part of the spectrum differs a little from adjacent parts in
refrangibility, and delicate bands or lines can only be brought out by
accurately focussing their own parts of the spectrum. This can be done
by the milled head, B. Disappointment will occur in any attempt at
delicate investigation if this direction is not _carefully attended to_.
When the spectra of very small objects are to be viewed, powers of from
½ in. to 1/20th, or higher, may be employed. Blood, madder, aniline
dyes, permanganate of potash solution, are convenient substances to
begin experiments with. Solutions that are too strong are apt to give
dark clouds instead of delicate absorption bands. Small cells or tubes
should be used to hold fluids for examination.
Mr. Browning has still further improved the micro-spectroscope by the
ingenious arrangement for measuring the positions of the lines, which is
represented in Fig. 225, and the construction and the use of which he
thus described in a paper read before the Microscopical Society:
[Illustration:
FIG. 225.—_Section of Micro-Spectroscope with Micrometer._
]
Attached to the side is a small tube, A A. At the outer part of this
tube is a blackened glass plate, with a fine clear white pointer in the
centre of the tube. The lens, C, which is focussed by sliding the milled
ring, M, produces an image of the bright pointer in the field of view by
reflection from the surface of the prism nearest the eye. On turning the
micrometer, M, the slide which holds the glass plate is made to travel
in grooves, and the fine pointer is made to traverse the whole length of
the spectrum.
It might at first sight appear as if any ordinary spider’s web or
parallel wire micrometer might be used instead of this contrivance. But
on closer attention it will be seen that as the spectrum will not permit
of magnification by the use of lenses, the line of such an ordinary
micrometer could not be brought to focus and rendered visible. The
bright pointer of the new arrangement possesses this great
advantage—that it does not illuminate the whole field of view.
If a dark wire were used, the bright diffused light would almost obscure
the faint light of the spectra, and entirely prevent the possibility of
seeing, let alone measuring, the position of lines or bands in the most
refrangible part of the spectrum.
To produce good effects with this apparatus the upper surface of the
compound prism, P, must make an angle of exactly 45° with the sides of
the tube. Under these circumstances the limits of correction for the
path of the rays in their passage through the dispersing prisms are very
limited and must be strictly observed. The usual method of correcting by
the outer surface is inadmissible. For the sake of simplicity, some of
the work of the lower part of the micro-spectroscope is omitted in the
engraving. As to the method of using this contrivance: With the
apparatus just described, measure the position of the principal
Fraunhofer’s lines in the solar spectrum. Let this be done _carefully_,
in _bright_ daylight. A little time given to this measurement will not
be thrown away, as it will not require to be done again. Note down the
numbers corresponding to the position of the lines, and draw a spectrum
from a scale of equal parts. About 3 in. will be found long enough for
this spectrum; but it may be made as much longer as is thought
desirable, as the measurements will not depend in any way on the
distance of these lines apart, but only on the micrometric numbers
attached to them. Let this scale be done on cardboard and preserved for
reference. Now measure the position of the dark bands in any absorption
spectra, taking care for this purpose to use lamplight, as daylight will
give, of course, the Fraunhofer lines, which will tend to confuse your
spectrum. If the few lines occurring in most absorption spectra be now
drawn to the same scale as the solar spectrum, on placing the scales
side by side, a glance will show the exact position of the bands in the
spectrum relatively to the Fraunhofer lines, which thus treated form a
natural and unchangeable scale (see diagram, Fig. 226). But for purposes
of comparison it will be found sufficient to compare the two lists of
numbers representing the micrometric measures, simply exchanging copies
of the scale of Fraunhofer lines, or the numbers representing them will
enable observers at a distance from each other to compare their results,
or even to work simultaneously on the same subject.
[Illustration:
FIG. 226.
]
A simpler form of the micro-spectroscope is also made by Mr. Browning at
a very modest price, and if the reader possesses a microscope, and
desires to examine these interesting subjects for himself, he will do
well to procure this instrument, instead of that represented in Fig.
220, as it will also answer better for other purposes. A section of the
instrument is shown in Fig. 227. When used with the microscope it is
slipped into the place of the eye-piece. There is an adjustable slit, a
reflecting prism, by which two different spectra may be examined at
once, and a train of five prisms for dispersing the rays. It can be used
equally well for seeing the bright lines of metals and the Fraunhofer
lines, and for viewing any two spectra simultaneously. These
direct-vision spectroscopes are better adapted for general use by those
who have not several different instruments, than such forms as that
shown in Fig. 229, for in the direct-vision instruments the whole extent
of the spectrum is visible at one view, which is by no means the case
with the larger instruments.
[Illustration:
FIG. 227.—_Section of Micro-Spectroscope._
]
_CELESTIAL CHEMISTRY AND PHYSICS._
We now approach that portion of our subject in which its interest
culminates, for however remarkable may be some of the above-named
results of this searching optical analysis, they are surpassed by those
which have been obtained in the field upon which we are about to enter.
The cause of the dark lines which Fraunhofer observed in the light of
the sun and of certain stars remained unexplained, he only establishing
the fact that they must be due to some absorptive power existing in the
sun and stars themselves, and not to anything in our atmosphere. It was
reserved for Professor Kirchhoff, of the University of Heidelberg, to
show the full significance of the dark lines. Fraunhofer had, on his
first observation of the lines, noticed that the D lines were coincident
with the bright lines in the spectrum of sodium. This interesting fact
may be readily observed with any spectroscope which permits of the two
spectra being simultaneously viewed. The bright line (or lines if the
spectroscope be powerful) of the metal is seen as a prolongation of the
dark D solar line. Even with an instrument like that shown in Fig. 220
the coincidence may be noticed. Let the observer receive into the
instrument the rays in diffused daylight only, when he will still see
the principal Fraunhofer lines distinctly, and let him note the exact
position of the D line, while he brings in front of the slit the flame
of a spirit-lamp charged with a little salt. He will then see the bright
yellow line replacing the dark D line, and by alternately removing and
putting back the lamp he will be soon convinced of the perfectly
identical position of the lines.
This fact remained without explanation from 1814 to 1859, when Kirchhoff
accidentally found, to his surprise, that the dark D line could be
produced artificially. He says: “In order to test in the most direct
manner possible the frequently asserted fact of the coincidence of the
sodium lines with the D lines, I obtained a tolerably bright solar
spectrum, and brought a flame coloured by sodium vapour in front of the
slit. I then saw the dark lines D, change into bright ones. The flame of
a Bunsen’s lamp threw the bright sodium lines upon the solar spectrum
with unexpected brilliancy. In order to find out the extent to which the
intensity of the solar spectrum could be increased without impairing the
distinctness of the sodium lines, I allowed the full sunlight to shine
through the sodium flame, and, to my astonishment, I saw that the _dark
lines_, D, _appeared with an extraordinary_ degree of clearness. I then
exchanged the sunlight for the Drummond’s or oxy-hydrogen lime-light,
which, like that of all incandescent solid or liquid bodies, gives a
spectrum containing no dark lines. When this light was allowed to fall
through a suitable flame, coloured by common salt, _dark_ lines were
seen in the spectrum in the position of the sodium lines. The same
phenomenon was observed if, instead of the incandescent lime, a platinum
wire was used, which, being heated in the flame, was brought to a
temperature near its melting point, by passing an electric current
through it. The phenomenon in question is easily explained, upon the
supposition that the sodium flame absorbs rays of the same degree of
refrangibility as those it emits, whilst it is perfectly transparent for
all other rays.” (Quoted in Roscoe’s Lectures on “Spectrum Analysis.”)
When the light of ignited lime was similarly made to pass through flames
containing the incandescent vapours of potassium, barium, strontium,
&c., the bright lines which these substances would have produced had the
lime-light not been present were found to be in every case changed into
dark lines, occupying the very same positions in the spectrum. In such
experiments the flames containing the metals in the vapourized state do
all the time really give off those rays which are peculiar to each
substance; but when a more intense illumination—such as the lime-light,
the electric arc, or direct sunlight—passes through them, the rays of
the spectrum produced by the intense light overpower those given off by
the relatively feebly coloured flames, and hence the portions of the
spectrum which are occupied by these, appear black. But as the intense
light would give a perfectly continuous spectrum if the incandescent
metallic vapour allowed the rays corresponding to its lines to pass
through it, the inference is obvious that each vapour absorbs those
particular rays which it has itself the power of emitting, but allows
all others to pass freely through it. Besides the experimental proofs of
this fact which have been already adduced, many others might be named.
The flame of a spirit-lamp with a salted wick appears opaque and smoky
when we look through it at a large flame of burning hydrogen, also
coloured by sodium; for the rays emitted by the latter do not penetrate
the former, which, in consequence of its feebler light, appears dark by
comparison. Again, if an exhausted tube containing metallic sodium be
heated so as to convert the sodium into vapour, the tube viewed by the
light of a sodium flame appear to contain a black smoke, and the light
from the flame will no more pass through it than through a solid object;
yet the tube appears perfectly transparent when viewed by ordinary
light, and the light from a lithium or other coloured flame would also
pass freely. Kirchhoff was led by purely theoretical reasoning to
conclude that all luminous bodies have precisely the same power of
absorbing certain rays of light as they have of emitting them at the
same temperature, and he thus brought luminous rays under the same
general law which had previously been established for radiant heat by
Prevost, Dessains, Balfour Stewart, and others. Here, then, a law was
arrived at, and, abundantly confirmed by direct experiment as regards
the more volatile metals, it was ready to supply the most satisfactory
explanation of the coincidences which were everywhere discovered to
exist between the Fraunhofer lines and those which belong to terrestrial
substances. For Kirchhoff also found, when mapping the very numerous
lines seen in the spark spectrum of iron, that for each of the 90 bright
lines of iron which he then observed, there was a dark line in the solar
spectrum exactly corresponding in position. The number of observed
bright lines in the iron spectrum has been since extended to 460, and
yet each is found to have its exact counterpart in a solar dark line.
So many coincidences as these made it certain that these dark lines and
the bright lines of iron must have a common cause, for the chances
against the supposition that the agreement was merely accidental are
enormous. Kirchhoff actually calculated, by the theory of probabilities,
the odds against the supposition. He found it represented by
1,000,000,000,000,000,000 to 1. The result arrived at in the case of
sodium at once suggested the explanation that these lines were produced
by an absorptive effect of the vapour of iron. Now, the existence of
such a vapour in our atmosphere could not be admitted, while the
temperature of the sun was known to be exceedingly high, far higher,
indeed, than any temperature we can produce by electricity, or any other
means. Hence, Kirchhoff concluded that his observations proved the
presence of the vapour of iron in the sun’s atmosphere with as much
certainty as if the iron had been actually submitted to chemical tests.
By the same reasoning, Kirchhoff also demonstrated the existence in the
solar atmosphere of calcium, chromium, magnesium, nickel, barium,
copper, and zinc. To these, other observers have added strontium,
cadmium, cobalt, manganese, lead, potassium, aluminium, titanium,
uranium, and hydrogen. It has also been demonstrated that a considerable
number of the Fraunhofer lines are due to absorption in our atmosphere
by its gases and aqueous vapour. This demonstration of the existence of
iron and nickel in the sun is an interesting pendent to the known
composition of many meteorites which reach us from interplanetary space.
Kirchhoff was led to believe that the central part of the sun is formed
of an incandescent solid or liquid, giving out rays of all
refrangibility, just as white-hot carbon does; that round this there is
an immense atmosphere, in which sodium, iron, aluminium, &c., exist in
the state of gas, where they have the power of absorbing certain rays;
that the solar atmosphere extends far beyond the sun, and forms the
corona; and that the dark sunspots, which astronomers have supposed to
be cavities, are a kind of cloud, floating in the vaporous atmosphere.
During total eclipses of the sun, certain red-coloured prominences have
been noticed projecting from the sun’s limb, and visible only when the
glare of its disc is entirely intercepted by the moon. Fig. 228
represents a total eclipse, and will give a rude notion of the
appearance of the red prominences seen against the fainter light of the
_corona_, which extends to a considerable distance beyond the sun’s
disc. Now, two distinguished men of science simultaneously and
independently made the discovery of a mode of seeing these red
prominences, even when the sun was unobscured. M. Janssen was observing
a total eclipse of the sun in India, and the examination by the
spectroscope of the light emitted from the red prominences showed him
that they were due to immense columns of incandescent hydrogen, for he
recognised the red line and blue lines which belong to the spectrum of
this gas (see No. 12, Plate XVII.). Mr. Norman Lockyer at the same time
also succeeded in viewing the solar prominences in London without an
eclipse. He found a red line perfectly coinciding in position with
Fraunhofer’s C line and that of hydrogen, another nearly coinciding with
F, and a third yellow line near D. Soon after this, Dr. Huggins
discovered a mode of observing the shape of the red prominences at any
time, by using a powerful train of prisms and a wide slit, so that the
changes in the forms of the red flames can be followed. Now, since the
red prominences give off only a few rays of particular refrangibility,
it is not difficult to understand that the light of the sun might be, as
it were, so diluted by stretching out the spectrum, by means of a train
of many prisms, that almost only the red rays, C, should enter the
telescope, and occupy the field with sufficient intensity to overpower
all others, and produce an image of the object from which they
originated. The nature of this action may be illustrated thus: If we
hold vertically a prism, and look through it at a candle-flame, we may
perceive a lengthened-out image of the flame, showing the succession of
prismatic colours, and formed, as it were, of a red image of the flame
close to a yellow one, and so on, but presenting no defined form. If,
still viewing this spectrum, we introduce into the flame on a platinum
wire a piece of common salt, we shall perceive a well-defined yellow
image of the candle start out, because the rays which are emitted by the
incandescent sodium, being all of one refrangibility, the prism simply
refracts without dispersing them. The dispersion which weakens the light
of the continuous spectrum by lengthening it out, does not sensibly
detract from the brilliancy of the bright lines, as their breadth is
scarcely increased—they are refracted but not dispersed. Hence, when a
sufficient number of prisms is employed, the bright lines of the solar
_chromosphere_ may be seen in full sunshine, in spite of the greater
intensity of the light emanating from the _photosphere_, which produces
the continuous spectrum. The bright C line is, of course, a virtual
image of the slit produced by rays of that particular refrangibility;
but by using a very high dispersive power, the slit may be opened so
wide that the C rays form in the telescope a red image of the prominence
from which they issue, since their light will predominate over that of
any rays belonging to the continuous spectrum.
[Illustration:
FIG. 228.—_Solar Eclipse, 1869._
]
In the hands of Mr. Norman Lockyer the science of the physical and
chemical constitution of the sun has made rapid progress, and new facts
are continually being observed, which serve to furnish more and more
definite views. Mr. Lockyer considers that, extending to a great
distance around the sun is an atmosphere of comparatively cooler
hydrogen, or perhaps of some still lighter substance which is unknown to
us. It is this which forms what is termed the _corona_, or circle of
light which is seen surrounding the sun in a total eclipse. Immersed in
this, and extending to a much smaller distance from the nucleus of the
sun, is another envelope, termed the _chromosphere_, consisting of
incandescent hydrogen and some glowing vapours of magnesium and calcium.
The brightest part of this envelope, which lies nearest the sun, is that
which gives off the red rays by which the prominences may be observed
without an eclipse. These prominences have been shown to be tremendous
outbursts of glowing hydrogen, belched up with sometimes an enormous
velocity from below, since they have been observed to spring up 90,000
miles in a few minutes. Beneath the chromosphere, and nearer to the body
of the sun, are enormous quantities of the vapours of the different
elements—sodium, iron, &c.—to which the dark lines of the solar spectrum
are due. This stratum Mr. Lockyer calls the _reversing layer_, because
it reverses (turns to dark) the lines which would otherwise have
appeared bright, just as Kirchhoff’s sodium vapour did in the experiment
described on page 437. Beneath the reversing layer is the _photosphere_,
from which emanates the light that is absorbed in part by the reversing
layer, and which there is good reason to believe is either intensely
heated solid or liquid matter.
In 1861 Dr. Huggins devoted himself, with an ardour which has since
known no remission, to the extension of prismatic analysis to the other
heavenly bodies. The difficulties of the investigations were great.
There was first the small quantity of light which a star sends to the
spectator; this was obviated by the use of a telescope of large
aperture, which admitted and brought to a focus many more rays from the
star, and therefore the brightness of the image was proportionately
increased. Not so the size of the image: the case of the fixed stars for
this always remains a mere point. It was, of course, necessary to drive
the telescope by clockwork, so that the light of the star might be
stationary on the field of the spectroscope. As the spectrum of the
image of the star formed by the object-glass would be a mere line,
without sufficient breadth for an observation of the dark or light lines
by which it might be crossed, it is necessary to spread out the image so
that the whole of the light may be drawn out into a very narrow line,
having a length no greater than will produce a spectrum broad enough for
the eye to distinguish the lines in it. This is accomplished by means of
a cylindrical lens placed in the focus of the object-glass, and
immediately in front of the slit. Covering one-half of the slit is a
right-angled prism by which the light to be compared with that of the
star is reflected into the slit. The light is usually that produced by
taking electric sparks between wires of the metal in the manner already
described. The dispersive power of the spectroscope was furnished by two
prisms of very dense glass, and the spectrum was viewed through a
telescope of short focal length. Dr. Huggins’s observations lead him to
the conclusion that the planets Mars, Jupiter, and Saturn possess
atmospheres, as does also the beautiful ring by which Saturn is
surrounded; for he noticed in the spectrum of each different dark lines
not belonging to the solar spectrum.
[Illustration:
FIG. 229.—_The Planet Saturn._
]
Passing to the results obtained in the case of the fixed stars, we may
remind the reader of the enormous distance of the bodies which are
submitted to the new method of analysis. Sir John Herschel gives the
following illustration of the remoteness of Sirius—supposed to be one of
the nearest of the fixed stars: Take a globe, 2 ft. in diameter, to
represent the sun, and at a distance of 215 ft. place a pea, to give the
proportionate size and distance of the earth. If you wish to represent
the distance of Sirius _on the same scale_, you must suppose something
placed _forty thousand miles_ away from the little models of sun and
earth. But not only do we know with certainty some of the substances
contained in Sirius, but the star spectroscope has taught us a great
deal about orbs so remote, that their distance is absolutely
unmeasurable. About Aldebaran we know that there are hydrogen gas and
vapours of magnesium, iron, calcium, sodium, and some four or five other
elements. Generally the lines indicate the presence of hydrogen in these
distant suns; but there is, at least, one remarkable exception in α
_Orionis_, the spectrum of which yields no trace of the hydrogen lines,
although it is evident that magnesium, sodium, calcium, &c., are
present. The spectra of celestial bodies are of several kinds. Many of
the stars have, like our sun, a continuous spectrum crossed by dark
lines. Such is that of Sirius, No. 10, Plate XVII. Others have, however,
both dark and bright lines, and some are marked by only three bright
spaces. Of the spectra of the nebulæ some have three bright lines (see
No. 11, Plate XVII.), and the bodies producing them are, therefore, to
be considered as masses of incandescent gas, while some give continuous
spectra. One of the bright lines in the spectra of the nebulæ coincides
with one of the hydrogen lines, and another—the brightest of the
three—with one of the brightest nitrogen lines; but the third does not
agree with any with which it has as yet been compared. The inference
from these appearances is that the nebulæ contain hydrogen and nitrogen,
but the absence of the other lines of these substances has not been
fully explained; although the observation of Dr. Huggins, that when the
light of incandescent nitrogen and hydrogen is gradually obscured by
interposing layers of neutral tinted glass, the lines corresponding with
those in the nebular spectra are the last to disappear, seems to suggest
a probable solution of the difficulty.
[Illustration:
FIG. 230.—_Solar Prominences, No. 1._
]
There is another very interesting line of spectroscopic research in the
power the prism gives us of estimating the velocity with which the
distances of the stars from our system are increasing or diminishing. On
closely examining the hydrogen lines of Sirius, and comparing them with
the bright lines of hydrogen rendered incandescent by electric
discharges in a Geissler tube, the spectrum of which his instrument
enabled him to place side by side with that of the star, Mr. Huggins was
surprised to find that the lines in the latter did not exactly coincide
in position with those of the former, but appeared slightly nearer the
red end of the spectrum. This indicated a longer wave-length, or
increased period of vibration, according to the theory of light, which
would be accounted for by a receding motion between Sirius and the
earth, just as the crest of successive waves of the sea would overtake a
boat going in the same direction at longer intervals of time than those
at which they would pass a fixed point, while, if the boat were meeting
the waves, these intervals would, on the other hand, be shorter. Hence
if the position of the lines in the spectrum depends on the periods of
vibration, that position will be shifted towards the red end when the
luminous body is receding from the earth with a velocity comparable to
that of light, and towards the violet end when the motion is one of
approach. The change in refrangibility observed by Mr. Huggins
corresponded with a receding velocity of 41·4 miles per second, and when
from this was subtracted the known speed with which the earth’s motion
round the sun was carrying us from the star at the time, the remainder
expressed a motion of recession amounting to about twenty miles a
second, which motion, there is reason to believe, is chiefly due to a
proper movement of Sirius. These deductions from prismatic observations
are of the highest value astronomically, since they will eventually
enable the real motions of the stars to be determined, for ordinary
observation could only show us that component of the motion which is at
right angles to the visual ray, while this gives the component along the
visual ray. In the same manner, it is inferred that Arcturus, a bright
star in the constellation _Boötes_, is approaching us with a velocity of
fifty-five miles per second.
[Illustration:
FIG. 231.—_Solar Prominences, No. 2._
]
When the solar spots are examined with the spectroscope, the dark image
of the slit produced by the hydrogen line, F, is observed to show a
strange crookedness when it is formed by rays from different parts of
the spot. This distortion is due to the same cause as the displacement
of the stellar lines, namely, motions of approach or recession of the
masses of glowing hydrogen. Mr. Norman Lockyer, to whom we are indebted
for the most elaborate investigations of the solar surface, has
calculated, from the position of the lines, the velocities with which
masses of heated hydrogen are seen bursting upwards, and those which
belong to the down-rushes of cooler gas. Velocities as great as 100
miles per second were, in this way, inferred to occur in some of the
storms which agitate the solar surface. Two drawings of a solar storm,
given by Mr. Lockyer, are shown in Figs. 230 and 231. These are
representations of one of the so-called red prominences, the first
giving its appearance at five minutes past eleven on the morning of
March 14th, 1869, and the last showing the same _ten minutes
afterwards_. The enormous velocity which these rapid changes imply will
be understood when it is stated that this prominence was 27,000 miles
high. “This will give you some idea,” says Mr. Lockyer, “of the
indications which the spectroscope reveals to us, of the enormous forces
at work in the sun, merely as representing the stars, for everything we
have to say about the sun the prism tells us—and it was the first to
tell us—we must assume to be said about the stars. I have little doubt
that, as time rolls on, the spectroscope will become, in fact, almost
the pocket companion of every one amongst us; and it is utterly
impossible to foresee what depths of space will not in time be gauged
and completely investigated by this new method of research.”
The light of comets has also been examined by the spectroscope, and many
interesting results arrived at. Our limits do not, however, permit us to
enter into a discussion of these interesting subjects.
Fig. 232 is a section of another of Mr. Browning’s popular instruments,
which is named by him the “Amateur’s Star Spectroscope.” It exhibits
very distinctly the different spectra of the various stars, nebulæ,
comets, &c.
[Illustration:
FIG. 232.—_Section of Amateur’s Star Spectroscope._
]
The reader who is desirous of learning more of this fascinating subject
is referred to Dr. Roscoe’s elegant volume, entitled, “Lectures on
Spectrum Analysis.” This work, which is embellished with handsome
engravings and illustrated by coloured maps and spectra, gives a clear
and full account of every department of the subject, and in the form of
appendices, abstracts of the more important original papers are
supplied, while a complete list is given of all the memoirs and
publications relating to the spectroscope which have been published.
This brief account of the spectroscope and its revelations, which is all
that our space permits us to give, will not fail to awaken new thoughts
in the mind of a reader who has obtained even a glimpse of the nature of
the subject, especially in relation to that branch of which we have last
treated, for in every age and in every region the stars have attracted
the gaze and excited the imagination of men. The belief in their
influence over human affairs was profound, universal, and enduring; for
it survived the dawn of rising science, being among the last shades of
the long night of superstition which melted away in the morning of true
knowledge. Even Francis Bacon, the father of the inductive philosophy,
and old Sir Thomas Browne, the exposer of “Vulgar Errors,” believed in
the influences of the stars; for while recognizing the impostures
practised by its professors, they still regarded astrology as a science
not altogether vain. It was reserved for the mighty genius of Newton to
prove that in very truth there are invisible ties connecting our earth
with those remote and brilliant bodies—ties more potent than ever
astrology divined; for he showed that even the most distant orb is bound
to its companions and to our planet by the same power that draws the
projected stone to the ground. And now the spectroscope is revealing
other lines of connection, and showing that not gravitation alone is the
sympathetic bond which unites our globe to the celestial orbs, but that
there exists the closer tie of a common constitution, for they are all
made of the same matter, obeying the same physical and chemical laws
which belong to it on the earth. We learn that hydrogen, and magnesium,
and iron, and other familiar substances, exist in these inconceivably
distant suns, and there exhibit the identical properties which
characterize them here. We confirm, by the spectroscope, the fact
partially revealed by other lines of research, that the stars which
appear so fixed, are, in reality, careering through space, each with its
proper motion. We learn also that the stars are the theatres of vast
chemical and physical changes and transformations, the rapidity and
extent of which we can hardly conceive. There is, for example, the case
of that wonderful star in the constellation of the Crown, which, in
1866, suddenly blazed out, from a scarcely discernible telescopic star,
to become one of the most conspicuous in the heavens, and the bright
lines its beams produced in the spectroscope revealed the fact that this
abrupt splendour was due to masses—who can imagine how vast?—of
incandescent hydrogen. This brightness soon waned, and τ _Coronæ
Borealis_ reverted once more to all but telescopic invisibility. The
seeming fixity of the stars is an illusion of the same nature as that
which prevents a casual observer from recognizing their apparent diurnal
motion, and now we have also ample evidence that permanence of physical
condition, even in the stars, is impossible. Everywhere in the universe
there is motion and change; there is no pause, no rest, but a continual
unfolding, an endless progression.
“Know the stars yonder,
The stars everlasting,
Are fugitive also,
And emulate, vaulted,
The lambent heat-lightning
And fire-fly’s flight.”
ROENTGEN’S X RAYS.
[Illustration:
FIG. 232_a_.—_Living Hand. Exposure, 4 Minutes._
]
On page 507 reference will be made to certain remarkable effects
observed by Mr. Crookes when the electric discharges from an induction
coil are passed through very highly exhausted tubes. These
phosphorescent and mechanical effects Mr. Crookes attributed to streams
of “radiant matter” shot off from the _negative_ pole with immense
velocities—the matter not being that of the electrode itself, but
particles of the extremely rarefied residual gas, which, being
comparatively few, could mostly traverse the tube in straight lines
without coming into collision with their fellows, and thus a class of
phenomena, different from the striated discharges in the ordinary and
less highly exhausted Geissler tubes, comes into view. The emanations
from the negative pole, or _cathode_, in highly rarefied gases became
known as the “cathode rays,” and they began to be further examined by
other observers, and more particularly in 1894 by Hittorf, and by M.
Lenard, a Hungarian physicist, who found that they pass through thin
plates of metal, and through wood and other substances not transparent
to ordinary light. It was also observed by Lenard, and also previously
by Hertz, that there are several kinds of cathode rays, which differ
from each other as regards their powers of exciting phosphorescence,
capability of being deflected by a magnet, and the degrees in which they
are absorbed by various media. But universal attention was drawn to this
subject by the announcement, at the end of 1895, of certain discoveries
made by Dr. W. K. Roentgen, a professor of physics at Wurzburg. He
covered a highly-exhausted Crookes’ tube with black cardboard, and found
that when the discharge of a large induction coil was passed through the
tube in a dark room, a piece of paper coated on one side with
platino-cyanide of barium, and held near the covered tube, glowed with a
brilliant fluorescence, no matter which side of the paper was turned
towards the tube; and even at a distance of two yards some fluorescence
was still visible. On experimenting with various bodies interposed
between the covered tube and the fluorescent screen, it was found that
the emanations passed through nearly every substance with more or less
facility. The screen lit up when placed behind a book of a thousand
pages, also behind two packs of cards. A single layer of tin-foil
scarcely threw a shadow, and several thicknesses were required to
produce a distinct effect. Deal boards, an inch thick, offered little
resistance. A very thick plate of aluminium (6/10 inch) reduced the
fluorescence, but still allowed some rays to pass. The hand held before
the fluorescent screen showed a dark shadow of the bones only, with but
a faint outline of their fleshy investment. Copper, silver, gold,
platinum, and lead, in comparatively small thicknesses, intercept these
rays. Thus a plate of lead only five hundredths of an inch thick almost
stops them. Increase of thickness increases the resistance to their
passage in all cases; but the comparative transparency of a body cannot
be deduced from its thickness and density. Many other bodies besides
platino-cyanide of barium become fluorescent under the influence of
these rays, such as certain kinds of glass, Iceland spar, rock-salt,
etc. Dr. Roentgen is convinced that these rays are not the cathode rays
or any part of them; but as the theoretical nature of the new rays has
not yet been explained, he has preferred to provisionally call them the
X rays, a denomination doubtless suggested by the use of the symbol _x_
in algebra to represent unknown quantities.
The source of the X rays, Roentgen states, is at the place where the
cathode rays strike the walls of the exhausted tube, and produce the
most brilliant phosphorescence; but they cannot be cathode rays which
have merely passed through the glass, for, contrary to what has been
observed with respect to the latter, they cannot be deflected by a
magnet. Nor is glass the only substance in which they can be generated,
for they were obtained from an apparatus in which the cathode rays were
made to impinge upon a plate of aluminium nearly one-tenth of an inch
thick. Photographic dry plates are also sensitive to the X rays, and
their power to pass through wood, ebonite, etc., makes the experiments
of testing the opacity, or otherwise, of various objects for them quite
easy. It is necessary merely to place the object on the closed cover of
the dark slide, and place the whole under the vacuum tube; all the
exposure, which is somewhat prolonged in most cases, may be made in
ordinary light. But the light-tight boxes, in which photographic plates
are packed, cannot, of course, be brought near the apparatus, as they
are completely permeable to the X rays, and their whole contents may be
rendered useless. The impression obtained on the photographic plate is
not so much a photograph as a _shadow_ of the interposed object—a shadow
more or less dense in the positive print according to the permeability
of the object, and the length of the exposure. These photographic
results have sometimes been called “shadowgrams,” “radiograms,”
“radiographs,” “skiagraphs,” etc. The word _skiagraph_ appears the most
appropriate designation. That the emanations from the phosphorescing
substance on which the cathode rays impinge are entitled to be also
called rays, appears from the regularity of the shadows thrown on the
fluorescent screen or photographic plate; and the fact of their
propagation in straight lines was proved by Dr. Roentgen obtaining a
_pin-hole_ photograph of the phosphorescing part of the vacuum tube,
when the latter was enveloped in black paper. Why a _pin-hole_ and not a
lens was used for taking this photograph will presently appear.
[Illustration:
FIG. 232_b_.—_Skiagraph of a Hand, by Dr. Roentgen. The Third Finger
has a Ring on it._
]
One of Dr. Roentgen’s experiments excited the attention and interest of
the general public, as well as of the scientific world, in the most
extraordinary degree, and though its announcement was received in some
quarters with incredulity, experimenters in all parts of the world
immediately set themselves at work to test the truth of the alleged
discovery. Electrical apparatus of different kinds, with various
adjustments, were employed, with results that were in some cases
failures, in others confirmations of the German professor’s statements,
and not unfrequently the variations in the conditions gave rise to
increased knowledge of the phenomena generally. The experiment just
alluded was one in which a dry photographic plate contained in one of
the camera dark slides, now so familiar to every one, was placed (with
the slide still closed by its wooden cover of nearly one quarter of an
inch thick) a few inches below the Crookes’ tube, and the hand of a
living-person being extended on the outside of the cover, a shadow of
the bones of the hand, as if seen through the surrounding tissues, was
obtained. Much popular misconception as to the powers of the “new
photography” arose from want of knowledge of the process by which these
strange pictures were obtained, the common notion being that these
photographs were produced by some method of using a camera, and that
outlines of people’s bodies and skeletons could be taken
instantaneously, not only through their clothes, but through doors and
walls. Much nearer the mark was the allusion of a scientific writer as
to the possibility of the new process realising Dickens’ description of
Marley’s ghost: “His body was transparent, so that Scrooge, observing
him, and looking through his waistcoat, could see the two buttons on his
coat behind.” The value of the new discovery for medical and surgical
purposes was immediately recognised, and very soon its application was
successfully practised.
Dr. Roentgen found that the X rays are incapable of refraction, in this
respect differing from ordinary light (see pages 397 and following), and
among the experiments which most impressed and astonished his auditors
when he was lecturing at Potsdam on his new discovery before the
Imperial Court of Germany, was one in which he showed the X rays passing
in a straight line through water without undergoing refraction. The rays
pass without interruption equally through the substances, whether these
be coherent or in a layer of fine powder of the same thickness, and this
again shows that there can be no regular reflection or refraction.
Prisms and lenses, whether of glass, ebonite, or aluminium, fail to
afford evidence of refractive action, hence the X rays cannot be focused
like those of ordinary light, and that is why the photograph of the
vacuum tube had to be taken by a pin-hole. Again, glass lenses could not
be used, because this substance, so transparent to light, is
particularly opaque to the X rays, and would in a great degree intercept
them, while lenses of ebonite and of aluminium, which were tried, were
inoperative on account of the irrefrangibility of the rays.
As to the nature of the rays themselves, Dr. Roentgen rejects the notion
of their being “ultra-violet” rays, which was suggested by some. The
meaning of this term is seen when it is understood that a great distance
beyond the violet end of the visible spectrum there are radiations,
revealed by their photographic impressions, so that the whole spectrum
is really some eight times as long as the visible part. In consequence
of these ultra-violet rays acting on the photographic plate, it is
possible, as has long been known, to take a photograph in the dark. The
eye is quite insensible to the X rays also, and although these, as we
have seen, readily pass through the bodily tissues, it may be placed
quite near the discharge tube, the latter being enveloped in black
paper, without causing any sensation. That the new rays are in some way
allied to light is the opinion held by Dr. Roentgen, and he is inclined
to consider them as due to _longitudinal_ vibrations in the ether; that
is, instead of the transverse waves to which light is attributed, these
resemble the waves of sound, in so far that they move in the direction
of propagation. This would account for the absence of any distinct
refraction, or polarisation, which seems to characterise the X rays.
Their connection with certain electric Maxwell-Hertz waves (see p. 541)
is more problematical, as the mathematical formulæ for these admit only
transverse oscillation. But on the assumption of certain conditions, due
to the action of electricity, etc., on highly rarefied air, the possible
existence of longitudinal vibrations has been deduced by admitting a
certain variation in some of the factors of the Maxwell formulæ.
Other suggestions have been advanced in order to make the observed facts
concerning the X rays fit into established theories, but so far these
attempts have been unsuccessful. It would seem as if our present
conceptions of light, electricity, the ether and matter, will have to be
profoundly modified and enlarged in order to bring these and other
recently discovered phenomena within their scope. Since the publication
of Dr. Roentgen’s paper, his results have received confirmation in every
quarter, and many new observations have been added, some of which seem
to tend not so much to elucidate the phenomena, as to prove them even
more complicated than was at first supposed. Such was the announcement
in June, 1896, of the discovery of several varieties of X rays.
[Illustration:
FIG. 232_c_.—_Metal Objects taken through Calico and Sheet of
Aluminium._
]
In the meantime, various modifications have been made in the forms of
the tubes and electrodes, and divers arrangements have been used for the
exciting electrical apparatus. Thus it has been found that the X rays
are given off from platinum more copiously than from glass, aluminium,
or any other substance, and by using a tube closed by a “platinum
window,” on which the cathode rays impinge, Mr. Gifford has been able to
reduce the time of exposure for obtaining a skiagraph of the bones on
the hand to half a minute, whereas twenty times that period was formerly
required. Another form of tube is advertised by Brady & Martin of
Newcastle, with which, in conjunction with a new screen, a coil giving a
5–inch spark will, it is stated, yield a good skiagraph of the hand in
_two seconds_, which appears to be the shortest time yet attained.
Another firm of tube-makers, Newton & Co., London, state that their
special form of tube, excited by a coil giving a 6–inch spark, and used
with their fluorescent screen, “will work right through the human body,
showing the heart, liver, spine, ribs, the movements of the heart and of
the diaphragm, etc.” It has recently been observed that the best results
are obtained when there is a certain, but as yet undefined, relation
between the degree of rarefaction of the residual gas in the tube, and
the intensity or frequency of the electric discharges, and that these
should be accommodated to the work required. Thus, for example, if a
skiagraph of the hand be attempted with an apparatus in which these
factors are carried to too high a degree, the resulting X rays will pass
through the bones almost as freely as through the surrounding tissues,
and their shadows will therefore not appear. If, on the other hand, the
contrary conditions hold, an incomplete or maybe no result will be
found. This seems to explain the failures that have sometimes occurred
when tubes of apparently identical construction have been used in the
hands of the same, or of different, observers. Perhaps more depends also
on the time of exposure. For instance, if a short exposure be given in
the case of the hand, the photograph will be merely a silhouette of that
member; with a little longer exposure, this will show the nails; with
still longer time, the shadow of the fleshy parts begins to grow faint
and the skeleton to appear. With yet more prolonged exposure only the
bones will show, in their various degrees of opacity, and the shadows of
these will gradually disappear as the time of exposure is increased,
until at length the image will be entirely effaced. The considerable
differences as to distinctness of the various tissues, which are
exhibited by the published prints of hand shadows, are thus explicable.
[Illustration:
FIG. 232_d_.—_A Skiagraph of Layers of Various Substances._
]
[Illustration:
FIG. 233.—_Portrait of Professor Helmholtz._
]
SIGHT.
The investigations of modern science have borne rich fruit, not only by
vastly extending our knowledge of the universe of things around us, but
also making us acquainted with the mode in which certain agents act upon
our bodily organs, and by revealing, up to a certain point, what may be
termed the mechanism of that most wonderful thing—the human mind—or, at
least, that part which is immediately concerned in the perceptions of an
external world. Of all the physical influences which affect the human
mind, those due to light are the most powerful and the most agreeable.
One of the most ancient of philosophers says, in the simple words which
are appropriate to the expression of an undeniable truth, “Truly the
light is sweet, and a pleasant thing it is for the eyes to behold the
sun.” The impression produced by light alone is a source of pleasure—a
cheering influence of the highest order; and there is a special
character in the pleasing effects of light, from the circumstance that
they do not exhaust the sense so quickly as do even pleasurable
impressions on other organs—such as sweet tastes, fragrant odours, or
agreeable sounds. Sight is not liable to that satiety which soon
overtakes the enjoyment of sensations arising from the other senses; it
possesses, therefore, a refinement of quality of which the rest are
devoid. Sight converses with its objects at a greater distance than does
any other sense, and it furnishes our minds with a greater variety of
ideas. Indeed, our mental imagery is most largely made up of
reminiscences of visual impressions; for when the idea of anything is
brought up in our minds by a word, for example, there arises, in most
cases, a more or less vivid presentation of some visible appearance. Our
visual impressions are also longer retained in memory or idea than any
other class of sensations.
The nature of the impressions we receive through the eye is extremely
varied; for we thus perceive not only the difference between light and
darkness, but in the sensations of colour we have quite another class of
effects, while the lustre and sparkle of polished and brilliant objects
add new elements of beauty and variety. We find examples of the latter
qualities in the verdant sheen of the smooth leaf, in the splendid
reflections of burnished gold, in the bright radiance of glittering
gems, and “in gloss of satin and glimmer of pearls.” The eye is also the
organ which conveys to our minds the impressions of visible motion, with
all those pleasures of exciting spectacle which enter so largely into
our enjoyment of life. It likewise discriminates the forms, sizes, and
distances of objects; but by a process long misunderstood, and dependent
upon a set of perceptions which, although precisely those whence we
derive our most fundamental notions of the objects around us, have been
completely overlooked in that time-honoured enumeration of the senses
which recognizes only five.
If such be the extent to which our minds are dependent upon the
wonderful apparatus of the eye, it may easily be imagined what must be
the comparative narrowness of mental development in those who have never
enjoyed this precious sense, and the feeling of deprivation in those,
who, having enjoyed it, have unfortunately lost it. Well may our sublime
poet despairingly ask—
“Since light so necessary is to life,
And almost life itself—if it be true
That light is in the soul—
The all in every part: why was the sight
To such a tender ball as the eye confined,
So obvious and so easy to be quenched?”
—for he himself, in his own person, experienced this deprivation, and he
thus touchingly, in his great work, laments his loss:
“Thus with the year
Seasons return; but not to me returns
Day, or the sweet approach of even or morn,
Or sight of vernal bloom, or summer’s rose,
Or flocks, or herds, or human face divine;
But cloud instead, and ever-during dark
Surround me; from the cheerful ways of men
Cut off; and for the book of knowledge fair
Presented with a universal blank
Of Nature’s works—to me expunged and rased,
And wisdom at one entrance quite shut out.”
An organ which is the instrument of so many nice discriminations as is
the eye must, of course, present the most delicate adjustment in its
parts. So much has in recent times been learnt of the nature of its
mechanism; of the relation between the impressions made upon it and the
judgments formed by the mind therefrom; of the illusions which its very
structure produces; of the defects to which it is liable; and of its
wonderfully refined physiological elements—that a branch of science
sufficiently extensive to require a large part of a studious lifetime
for its complete mastery has grown up under the hands of modern
physiologists, physicists, and psychologists. To some of the results of
their labour we would invite the reader’s attention; and in order to
render the account of them intelligible, we must, to a certain extent,
describe “things new and old.”
_THE EYE._
[Illustration:
FIG. 234.—_Vertical Section of the Eye._
]
The form of the human eye and the general arrangement of its parts may
be understood by referring to Fig. 234, which is a section of the
eyeball. It has a form nearly globular, and is covered on the outside by
a tough firm case, A, named the _sclerotic coat_, which is, for the most
part, white and opaque. This covering it is which forms what is commonly
termed the “white of the eye;” but in the front part of the eyeball it
loses its opacity, and merges into a transparent substance, termed the
_cornea_, B. The cornea has a greater convexity than the rest of the
exterior of the eyeball, so that it causes the front part of the eye to
have a somewhat greater projection than would result from its general
globular form. This sclerotic coat—with its continuation, the
cornea—serves to support and protect the more delicate parts within, and
is itself kept in shape by the _humours_, which fill the whole of the
interior. The greater space is occupied by the _vitreous humour_, C; but
the space immediately behind the transparent cornea is filled with the
_aqueous humour_, D. The latter is little else than pure water, and the
former is like thin transparent jelly. The cavities containing these two
humours are separated by the transparent double convex lens, E, called
the _crystalline lens_, which, in consistence, resembles very thick
jelly or soft gristle. The outward surface of this lens has a flatter
curvature than the inner surface. Immediately in front of the
crystalline lens is found the _iris_, F, which may be described as a
curtain having in the middle a round hole. The iris is the part which
varies in colour from one individual to another—being blue, brown, grey,
&c.; and the aperture in its centre is the dark circular spot termed the
_pupil_.
The general disposition of the parts of the eye with regard to light
will be most easily understood by comparing it with an optical
instrument, to which it bears no little resemblance, namely, the _camera
obscura_, so well known in connection with photography. We may picture
to ourselves a still more complete resemblance, by imagining that the
lens of the camera is single, that we have fixed in front of it a
watch-glass, with the convex side outwards, and that we have filled with
water the whole of the interior of the camera, including the space
between the watch-glass and the lens. The _focussing-screen_ of the
camera corresponds with the inner surface of the back of the eyeball,
about which we shall presently have more to say. Now, even if the camera
had no lens, but were simply a box filled with water, and having in
front the watch-glass, fixed in the manner just mentioned, we could
obtain the images of objects on the screen, as a consequence of the
curvature of the watch-glass. It would, however, in this case, be
necessary to have the camera much longer, or, in other words, the rays
would be brought to a focus at a greater distance than if we put in the
glass lens, which would, thus placed in the water, cause the rays to
converge to a focus at a much shorter distance, although its effect when
surrounded by water would be less powerful than in the air. There we see
the effect of the crystalline lens of the eye in bringing the rays to a
focus within a much shorter distance than that which would be required
had there been present only the curved cornea, and the aqueous and
vitreous humours of the eye, which are but little different from pure
water in their optical properties.
If we _focus_ the camera by adjusting the distance between the lens and
the screen so as to get a distinct image of a near object, we should
find, on directing the instrument to a distant one, that the image would
be blurred and indistinct, and the lens would have to be moved nearer to
the screen; or we could get the image of the distant object distinct by
replacing the lens by another lens in the same position, but having some
flatter curvature. It is plain that the same object would be gained if
our lens could be made of some elastic material, which, on being pulled
out radially at its edges, could be made to assume the required degree
of flatness without losing its lenticular form. Now, it is precisely
with an automatic adjustment of this kind that the _crystalline lens_ of
the eye is provided, for the lens is suspended by an elastic ligament,
G, by the tension of which its surfaces are more flattened than they
would otherwise be; but when the tension of this ligament is relaxed, by
the action of certain delicate muscles which draw it down, the
elasticity of the lens causes it to assume a more convex form.
[Illustration:
FIG. 235. _Section of Retina._
]
These optical adjustments give, on the inner surface of the coats of the
eye, a more or less perfect real image of the objects to which the eye
is directed, and it is on the back part of this inner surface that the
network of nerves, called the _retina_, H, is spread out. The sclerotic
coat, already spoken of, is lined internally with another, named the
_choroid_, which is composed of delicate blood-vessels, intermingled
with a tissue of cells filled with a substance of an intensely black
colour. It is upon this last layer that the delicate membrane of the
retina is spread out between the choroid and the vitreous humour.
The retina is, in part, an expansion of the fibres of the optic nerve
over the back part of the eyeball. If we suppose the globe of this cut
vertically into two portions, and so divide the front from the back part
of the eye, the retina would be seen spread out on the concave surface
of the back part, and in the middle of this part, opposite the
crystalline lens, would be seen a spot in which the retina assumes a
yellowish colour, and in the centre of this, a little round pit or
depression. The spot is called the _macula lutea_, or _yellow spot_, and
the little central pit, which is of the highest importance in vision, is
termed the _fovea centralis_. A little way from the yellow spot, and
nearer the nose, is a point from which a number of fibres are seen to
radiate, and this is, in fact, the part at which the optic nerve enters
the eyeball, and from which it sends out its ramifications over the
retina. This part, for a reason which will shortly appear, is called the
_blind spot_.
When the minute structure of the retina is examined by the microscope,
its physiological elements are found to undergo very remarkable
modifications at the yellow spot. In the retina, although the total
thickness does not exceed the 1/80th part of an inch, no fewer than
eight or ten different essential or nervous layers have been
distinguished. Fig. 235 rudely represents a section. The lowest stratum,
A, which is next the choroid, and forms about a quarter of the total
thickness, is formed of a multitude of little rod-shaped bodies, _a_,
ranged side by side, and among these are the conical or bottle-shaped
bodies, _b_. This lowest stratum of the retina is called the _layer of
rods and cones_. At their front extremities the rods and cones pass into
very delicate fibres, which, going through an extremely fine layer of
fibres, B, are connected with a series of small rounded bodies, which
form the layer of _nuclei_, C, separated by a layer of nervous fibres,
D, from a granular layer, E, in front of which is a stratum of still
finer granules, F, underlying a layer of ganglionic nerve-cells, G, of a
larger size than any of the other elements, and these ganglionic cells
send out numerous branching nerve-fibres, forming the layer H. Finally,
on the front surface of the retina there is a thin stratum formed of
fibres, which issue from the optic nerve, K, Fig. 234, and in fact
constitute the expansion of this nerve on the inner surface of the
eyeball. The terminations of some, at least, of these nerve-fibres have
been traced, and have been found to form junctions with those branching
from the ganglionic cells.
[Illustration:
FIG. 236.
]
Of the part played by each of these delicate structures in exciting
visual impressions little is yet known. How light, or the pulsations of
ether, if such there be, is ultimately converted into sensation will
probably for ever remain a mystery, although it is quite likely that the
kind of visual impression which is conveyed by each part of the
elaborate structure of the retina may ultimately be distinguished. One
curious result of modern investigation is that _light falling directly
upon fibres of the optic nerve is quite incapable of exciting any
sensation whatever_. Light has no more effect on this nerve and its
fibres than it would have on any other nerve of the body if exposed to
its action. The apparatus of rods, cones, and other structures are
absolutely essential to enable light to give that stimulus to the optic
nerve which, conveyed to the brain, is converted into visual sensations.
So if this apparatus were absent in our organs of vision, in vain would
the optic nerve proper be spread out over the interior of the eyeball:
we should be no more able to _see_ with such eyes than we are able to
see with our hands.
We now invite the reader’s careful consideration to the diagram, Fig.
236, which is a section of the retina through the yellow spot. The upper
part of the figure is the front, and the deep depression is the little
pit already spoken of—the _fovea centralis_. The lowest dark line
represents the basement membrane of the retina, and immediately above is
seen the layer of rods and cones, and the various strata already spoken
of are represented in their due order in the marginal parts of the
diagram. Now observe the remarkable modifications of the nervous
structures in the neighbourhood of the _fovea centralis_, some of which
are visible in the diagram. In the first place, the cones are there much
longer, more slender, and more closely set, so that there is a far
greater number of them on a given surface; but the rods are
comparatively few, and are, in fact, not found at all under the floor of
the little pit. The layer of _nuclei_, into which the cones extend, is
thinner, and is found almost immediately below the anterior surface, for
all the other layers thin out in the fovea in a very curious manner. It
is, however on the margin of the fovea that the stratum of ganglionic
cells, G, Fig. 235, attains its greatest thickness, for there it is
formed by the superposition of eight or ten cells, being here thicker
than any other layer, while it is so thinned off towards the margin of
the retina that it no longer forms even a continuous stratum. This
layer, however, becomes much thinner _in the fovea_, which contains, in
fact, but few superposed cells. The tint of the yellow spot is said to
be derived from a colouring matter, which affects all the layers except
that of the cones. The centre of the yellow spot, where the _fovea
centralis_ is situated, is extremely transparent, and is so delicate
that it is very easily ruptured, and has frequently been taken for an
aperture.
We should not have risked wearying the reader with these details
concerning the little pit in the centre of the retina had it not
possessed an extreme importance in the mechanism of the eye, a fact
which he will at once appreciate when we say that _of the whole surface
of the retina, the only spot where the image of an object can produce
distinct vision is the fovea centralis_. Since this is undoubtedly true,
it follows that the physiological elements which we there find are
precisely those which are most essential for producing this effect. The
case may be exemplified by recurring to the comparison of the eye with a
photographer’s camera, by supposing his screen to be of such a nature
that only on one _very small spot_ near its centre could a distinct
image be possibly obtained of just one point of an object. Such a defect
in his camera would render the photographer’s art impossible, and this
defect (if it may be so called) in the eye would render it almost
equally useless, had not an adjustment, which more than compensates for
it, been afforded in the extreme _mobility_ of our organs of vision.
This adjustment is so perfect that people in general do not even suspect
that the image of _each point_ of an object which they distinctly see
must be formed on one particular spot on the retina—a spot about
one-tenth of the diameter of an ordinary pin-head! We may venture,
without any disrespect to the reader, to assume that the chances are
that it is new to him to learn how each letter in the lines beneath his
eye must successively, but momentarily, form its image in the very
little pit in the centre of his retina; and the chances are at least a
hundred to one that, even if aware of this, he has passively received
the statement, and that he has not made the least attempt to _realize
the truth for himself_. Yet nothing is easier. Let him request a friend
to slowly peruse some printed page, while he meanwhile intently watches
his friend’s eyes. He will then perceive that before a single word can
be read there is a _movement_ of the eyeballs, which are, quite
unconsciously to the person reading, so directed that the image of each
letter (for the area of distinct vision is incapable of receiving more
than this at once) shall fall upon the only parts of the retinæ from
which a distinct impression can be conveyed along the optic nerve. Thus
it is that the eye, without any conscious effort of the observer, is
directed in succession to the various points of an object, and it is
only by an effort of will in fixing the eyes upon one spot that one
becomes aware of the blurred and confused forms of all the rest of the
visual picture. Yet so readily do the eyeballs turn to any part of the
indistinct picture on which the attention is fixed, that it is not
improbable a person unversed in such experiments, wishing to verify our
conclusions by looking, say, at one spot on the opposite wall, will be
very apt, in thinking of the features of the rest of the picture, to
direct his eyes there, and then declare that he, at least, sees no such
vague forms. If such be his experience, the correction is easy. He has
only to ask some one to watch closely his eyes while he repeats the
experiment, and after a few trials he will succeed in maintaining the
requisite immobility of the eyeballs—a condition upon which the success
of many such experiments depends.
[Illustration:
FIG. 237.—_Muscles of Eyes._
The muscles of the eyeballs viewed from above:—B, the internal rectus;
E, the external rectus; S, the superior rectus; T, the superior
oblique, passing through a loop of ligament at U, and turning
outwards and downwards to its insertion at C. The inferior rectus
and the inferior oblique are not visible in the figure: the superior
rectus is removed from the right eyeball in order to show the optic
nerve N.
]
This extreme mobility of the eyeballs more than compensates for the loss
of the clear and well-defined picture, for it calls into action one of
the most sensitive of all the impressions of which we are capable, and
one which possesses in so high a degree the power of uniting with our
other sensations, that this sixth sense has been, as already stated,
utterly overlooked, except by the more modern students of the nature of
our sensations. It is usually termed the _muscular sense_, and to it are
due some of the nicest distinctions of impressions of which we are
capable. The muscles of every part of our frame take their part in
producing impressions in our minds, and those of the eyeballs have a
very large share in furnishing us with ideas of forms and motions. Fig.
237 is a diagram showing the general arrangement of these muscles; and
their anatomical designations, which need not much concern us at
present, are given beneath the figure. The wonder is, that the
sensations arising from the relative conditions of parts so few, should
afford us the immense variety of notions referrible for their origin to
these muscles only. We take one example in illustration. Suppose we
watch the flight of a bird, at such an elevation that no part of the
landscape comes into the field of view at all; and that, again, we
follow with the eye, under similar circumstances, the path of a rocket.
We can unhesitatingly pronounce the motions unlike, and yet in each case
there was no visual impression present but that of the object focussed
upon the yellow spot. But the movement of _the muscles in one case_ was
different from that in the other. Nay more, we can form such a judgment
of the motion as to pronounce that the object followed such and such a
curve—we may recognize the parabola in one path, and the circle,
perhaps, in the other. And this kind of discrimination arises from the
fact, that when we have, maybe times without number, previously looked
at parabolas and circles, in diagrams perhaps, the muscles of the
eyeballs have performed just the same series of movements, as point
after point of the line was made to form its image on the yellow spot.
This is not the only class of impressions that these muscles are capable
of affording; there is, for example, little doubt that they aid us in
estimating distance. But space will not permit further discussion of
this subject.
Although the blurred and indefinite retinal picture may be compensated,
and perhaps more than compensated, by the readiness with which the eyes
move, it is, of course, possible that greater precision and delicacy of
visual impression over the whole surface of the retina might be
consistent with a still greater increase of our powers of perception.
There are instances in which the absence of finish, as it may be termed,
in all but one little spot in the picture, proves a real inconvenience
and a sensible deprivation. Perhaps a friend calls our attention to the
fact that a balloon is sailing through the air, or some fine morning,
hearing in the fields the blithe song of the sky-lark, we look up and
vainly try to bring the small image upon the place of distinct vision.
Now, if an image which falls upon any other part of the retina is
perceived, even indistinctly, an instant suffices to direct the eyes
into the exact position requisite for clear vision—an example of the
marvellous precision with which impressions are put in relation to each
other by the unconscious action of the brain. But while an image on the
fovea, only 1/6000th of an inch diameter, produces a distinct sensation,
it is found that if the image falls on the retina at a point some
distance from the yellow spot, the image must be 150 times larger in
order to produce any impression; and it is in consequence of the image
of balloon or bird not having the requisite size to give any impression
to the less sensitive portion of the retina, that we grope blindly, as
it were, until by chance the image falls near the yellow spot, when the
tentative motion of the eyeballs is instantly arrested, and the image
fixed. On the other hand, the field of indistinct vision which the eye
takes in is extremely wide, for bright objects are thus perceived, even
when their direction forms an angle laterally of nearly 90° with the
axis of the eye; and, if the object be not only bright, but in motion,
its presence is noticed under such circumstances with still greater
ease. Thus, an observer scanning the heavens would have a perception of
a shooting star anywhere within nearly half the hemisphere. The range
is, however, less than 90° in a vertical direction.
We have said that the fibres of the optic nerve, entering the back part
of the eyeball, at K, Fig. 234, ramify over the anterior surface of the
retina in fibres which form a layer of considerable relative thickness.
The light, therefore, first encounters these nerves, and only after
traversing their transparent substance does it reach the deeper seated
layer of rods and cones, where it excites some action that is capable of
stimulating the optic nerve. These rods and cones might naturally be
supposed to be merely accessory to the fibres of the optic nerve, had we
not the following conclusive evidence that the cones play a necessary
part in the action, and that it is only through them that light acts
upon the optic nerve:
[Illustration:
FIG. 238.
]
1. The cones are more developed and more numerous in the spot where
vision is most distinct.
2. The “blind spot” is full of fibres of the optic nerve, but is
absolutely insensible to light, and is without rods or cones.
3. We can distinguish an image on the fovea, having only 1/6000th of an
inch diameter; but on the other parts of the retina the images must have
larger dimensions. It is found that the size of the smallest
distinguishable images agrees nearly with the diameters of the cones at
the respective parts.
To some readers the fact will doubtless be new, that a considerable
portion of the eye is quite insensible to light, namely, that portion
already designated as the “blind spot.” A simple experiment, made by
help of Fig. 238, will prove this. Place the book so that the length of
the figure may be parallel to the line joining the eyes, and let the
right eye be exactly opposite the white cross, and at a distance from it
of about 11 in. If the left eye be now closed, while with the right the
cross is steadily viewed so that it is _always_ clear and distinct, the
white circle will completely disappear, and the ground will appear of a
uniform black colour. In order to insure success, the observer must be
careful not to _look at_ the white circle, but at the cross, and some
persons find this more difficult than others. The position of the blind
spot in the eye has been already mentioned, and its significance in
showing the insensibility to light of the fibres of the optic nerve has
been pointed out. In the table of the dimensions of some parts of the
eye, which, for convenience of reference, is given together below, it
will be seen that the diameter of the blind spot is considerable
compared with the size of the retina, its greatest diameter being about
8/100 in. The length on the retina of the image of a man at a distance
of 6 ft. or 7 ft. is not greater than this, so that in a certain
position with regard to the eye a person would, like the white circle,
be quite invisible. In like manner, by looking steadily in a certain
direction with one eye, the image of the full moon may be made to fall
upon the blind spot, and the luminary then becomes invisible, and would
be so even if its apparent diameter were eleven times greater; so that
if we suppose eleven full moons ranged in a line, the whole would be
quite invisible to a person looking towards a certain point of the sky
at no great angular distance from them.
The following are the dimensions in English inches of some parts of the
eye:
In.
Diameter of the entrance of the optic nerve 0·08
Distance of centre of optic nerve from centre of yellow spot 0·138
Diameter of _fovea centralis_ 0·008
Diameter of the nerve-cells of the retina 0·0005
Diameter of the _nuclei_ 0·00003
Diameter of the rods 0·00004
Diameter of the cones in yellow spot 0·00018
Length of rods 0·0016
Length of cones in yellow spot 0·0008
Thickness of retina at the back of the eye 0·0058
By means of an instrument to be presently described, the ophthalmoscope,
it is possible to view directly the whole surface of the retina, and to
observe the inverted images of the objects there depicted. It is thus
observed that it is only on the parts near the yellow spot that the
images are formed with clear and sharp definition. Away from this the
definition is less perfect; and besides the diminished sensitiveness of
the retina, this circumstance contributes to the vagueness of the visual
picture, although the falling off in clearness of vision at a very
little distance from the yellow spot is far more marked than the loss of
definition in the image there formed.
Until within the last few years it has been most confidently asserted by
many authors that the eye, considered as an _optical instrument_, is
absolutely perfect, and entirely free from certain defects to which
artificial instruments are liable. Thus Dr. W. B. Carpenter states, in
his “Animal Physiology” (1859): “The eye is much more remarkable for its
perfection as an optical instrument than we might be led to suppose from
the cursory view we have hitherto taken of its functions; for, by the
peculiarities of its construction, certain faults and defects are
avoided, to which all ordinary optical instruments are liable.” Among
the imperfections which are completely corrected in the eye, he names
“spherical aberration” and “chromatic aberration”—both of which give
rise to certain defects in optical instruments. But by recent careful
investigations it has been conclusively shown that the eye is not free
from chromatic aberration; that it has defects analogous to spherical
aberration; and that there are, besides, certain optical imperfections
in its structure, which are avoided in the artificial instruments.
Professor Helmholtz, one of the most distinguished of German
mathematicians, physicists, and physiologists, whose great work on
“Physiological Optics” is the most complete treatise on the subject
which has ever appeared, is so far from considering the eye as possessed
of all optical perfections that he remarks that, should an optician send
him an instrument having like _optical_ defects, he would feel justified
in sending it back. The defects which may be traced in the eye,
_considered as an optical instrument_, do not, however, he admits,
detract from the excellence of the eye _considered as the organ of
vision_.
When we find that Sir Isaac Newton pointed out the chromatic aberration
of the eye two centuries ago—when we find that D’Alembert, in 1767,
proved that the lenses of the eye might have as great a dispersive power
as glass without the want of achromatism necessarily becoming
noticeable—when we find that the celebrated optician Dolland, the
inventor of the achromatic lens, showed that the refractions which take
place in the eye all tend to bring the violet rays towards the axis more
than the red—when we find that Maskelyne the astronomer, Wollaston the
physicist, Fraunhofer the optician, and other scarcely less
distinguished men of science, have made actual measurements of the
distances of the _foci_ in the human eye for the different rays of the
spectrum—when we find how these defects have so long ago been observed,
examined, and measured as to their amount—the persistence with which
writer after writer has asserted the achromatism of the human eye
appears so extraordinary, that it can only be accounted for by the
prevalence of the preconceived notion that the eye is absolutely
perfect—a notion not without its reason and grounds, in the fact of the
exquisite adaptation of the organ of sight to the needs of humanity.
Although the want of achromatism in the eye thus escapes ordinary
notice, it is, on the other hand, easy to render it evident by simple
experiments. If, for example, we view from a certain distance the solar
spectrum projected on a white screen, it will be found that, when we see
the red end quite distinctly, the violet end will, at the same time,
appear vague and confused, and _vice versâ_. The author believes that
the following very simple experiment will at once convince any person
that the fact is as stated. Procure a small piece of blue or _violet_
stained glass, and another piece of _red_ glass, and, having cut out of
an opaque screen a rectangular opening, say ½ in. long and ¼ in. wide,
place the glasses close to it, so that one-half the opening is covered
by the red glass and the other half by the violet glass, the two being
placed so that, on looking through the screen, a violet square and a red
square are visible. The opaque screen may be made of black paper,
cardboard, or tinfoil, and the edges of the opening must be cut
perfectly even. On looking through this arrangement, held at a distance
of about two feet from the eye, both squares may be seen distinctly by a
person of ordinary vision; but, at a distance of five inches from the
eye, he will find it impossible to see the squares otherwise than with
vague and ill-defined edges. This is because the crystalline lens cannot
adapt its curvature so as to bring the rays from the object to a focus
on the retina. Now, by trial, the nearest distance at which each of the
coloured squares becomes visible may be found, and it will be observed,
that the violet square is first sharply defined at a less distance than
the red, whereas, if the eye brought the red and violet rays to a focus
at the same point, the smallest distance of distinct vision would
coincide in both cases.
The reader may observe the same fact for himself, in even a still
simpler manner, by turning to Fig. 238, page 461. When the white circle
is viewed by one eye, at a distance of about a foot, and an opaque
screen, such as a coin, is held close to the eye, so that the pupil is
half covered by it, the one side of the white circle will appear
bordered by a narrow fringe of blue, and the other side by a narrow
fringe of orange. If the opaque screen be shifted from one side of the
pupil to the other, the colours will change places, the orange appearing
always on the same side of the white circle as the screen is held before
the eye. The same appearances are presented in a still more marked
degree when the full moon is made the subject of the experiment.
The diagram, Fig. 239, shows the course of the red and violet rays from
a luminous point, A, the refraction being supposed to take place at
B_{1} B_{2}. The violet rays after refraction form the cone, B_{1}, E,
B_{2}, and E is their focus; the red rays form the cone, B_{1}, F,
B_{2}, and have a focus at F. The position of the retina would be
intermediate between E and F, and is indicated by C_{1}, C_{2}. It will
be noticed that the violet rays cross, and are received on the retina in
the same circle, G G, so that the colours, then blended, would be
separately imperceptible; but the point would produce a diffused
circular image of the blended colours.
In viewing an object—the moon, for example—the accommodation of the eye
is like that indicated in the diagram. The distinct image due to the red
rays would be formed behind the retina, and that due to the violet rays
would be in front of it. In the image on the retina the most intense
rays—such as the orange, yellow, and green—are those which are blended
by the adjustment of the eye, and the red and violet form images more
out of focus (to use a common expression), and a very little larger than
the more intense image. We might expect that a white disc would
therefore appear with a fringe of colour, resulting from a mixture of
red and violet; but the fringe is too narrow, and the colour itself too
feeble, to become perceptible. When, however, the pupil of the eye is
half covered, the red and violet images are displaced in different
directions, the position of the retina being too far forward for the
one, and too far back for the other. The coincidence therefore ceasing,
the colours show themselves at the margins of the image.
[Illustration:
FIG. 239.
]
The non-perception under ordinary circumstances of the chromatic
aberration of the eye is largely due to the greater intensity of the
colours which differ least in their refrangibilities. The clearness of
our vision does not, therefore, practically suffer from this defect of
the eye. Professor Helmholtz constructed lenses which rendered his eyes
really achromatic, and looking through these when the pupil was half
covered, no coloured fringes were seen at the edges of dark or light
objects, or when the objects were looked at with an imperfect
accommodation of the eye. He was, however, unable to detect any increase
of clearness or distinctness of vision by the correction.
The eye is also subject to other aberrations and irregular refractions,
which are special to itself; for example, with moderately illuminated
objects the crystalline lens produces images apparently well defined,
and nothing is visible to suggest the absence of uniformity in its
structure. But when the light is intense, and concentrated in a small
object surrounded by a dark field, the irregular structure of the
crystalline lens shows itself in the most marked manner. Every one must
have noticed the appearance presented by the distant street-lamps on a
dark night, and by the stars. The latter we know to be for us mere
points of light, and their images produced by perfect lenses would also
be mere points; instead of which we see what seem to be rays issuing
from the star, an appearance which has given rise to the ordinary
representation of a star as a figure having several rays. That no such
rays actually do emanate from the real star may be easily proved: first,
by concealing the luminous point from view, by means of a small object
held up as a screen. If the rays had any existence outside of the eye,
they would still be seen; instead of which, the whole of them disappear
when the luminous point, or, in the case of the street-lamp, when the
flame, is covered by the screen. A second proof that the origin of the
phenomenon is in the eye, and not in the object, is afforded by the fact
that if, while attentively observing the rays, we incline the head, the
rays turn with the eyes, so that when the head is resting on the
shoulder the ray which appeared vertical becomes horizontal. The cause
of these divergences from the regular image lies in the fact of the
crystalline lens being built up of fibres which have refractive powers
somewhat different from that of the intermediate substance. These fibres
are arranged in layers parallel to the surfaces of the crystalline lens,
and the direction of the fibres in each layer is generally from the
centre to the circumference; but towards the axis they form, by bending,
a kind of six-rayed figure, as shown in Fig. 240, which represents the
arrangement of the fibres of the external layers of the lens. In the
outermost layers the branches of the star-shaped figure are subdivided
into secondary branches, which give rise to more complicated figures.
When we view by night a very brilliant but small light, even these
subdivisions may be traced in the radiating figure.
[Illustration:
FIG. 240.
]
The light which enters the eye is partly absorbed by the black pigment
of the choroid, and partly sent back by diffused reflection from the
retina through the crystalline lens and pupil. The image of a luminous
body as depicted on the retina of another person cannot be seen by us
under ordinary circumstances, because, by the principle of reversibility
already mentioned as of universal application in optics, the rays which
issue from the retinal images are refracted on leaving the eye, and
follow the same paths by which they entered it, so that they are sent
back to the object. An observer cannot see the retinal image of a candle
in another person’s eye, unless he allows the rays to enter his own, and
this cannot be done directly, because the head of the observer would be
interposed between the candle and the eye observed, and the light would
then be intercepted. By holding a piece of unsilvered plate glass
vertically, we may reflect the light of a candle into the eye of another
person, and then the light thrown out from the retinal image of the
candle will, on again meeting the surface of the glass, be in part
reflected to its source, and in part pass through the glass, on the
other side of which it may be received into the eye of an observer. The
positions of the observed and observing eye may be described as exactly
opposite to and near each other, while the candle is placed to one side
in the plane separating the two eyes, and the glass is held so that it
forms an angle of 45° with the line joining the pupils. Under these
circumstances the observer may see the light at the back of the eye, but
he will not be able to distinguish anything clearly, because his own eye
cannot accommodate itself so as to bring to a focus the rays coming from
the retina of the other, since these rays are refracted by the media
through which they emerge. But, by means of suitable lenses interposed
between the two eyes, the retina and all its details may be distinctly
seen and examined. Such an arrangement of lenses and a reflecting
surface constitute the instrument called the _ophthalmoscope_ (οφθαλμος,
_the eye_) of which there are many forms, but all constructed on the
principle just indicated. This principle was first pointed out by
Helmholtz, who described the first ophthalmoscope in 1851.
[Illustration:
FIG. 241.—_Ruete’s Ophthalmoscope._
]
Ruete’s ophthalmoscope is represented in Fig. 241. The parts of the
instrument are supported on a stand, C, and about the vertical axis of
this the column, D, and the arms, H and K, can turn freely and
independently; E is a concave metallic mirror, about 3 in. in diameter,
and having an aperture in its centre through which the observer, B,
looks. The arm, H, merely carries a black opaque screen, which serves to
shield the eye of B from the light of the lamp, and to reduce, if
required, the amount of light passing through the aperture in the
mirror. The arm, K, which is about a foot in length, carries two
uprights which slide along it, and in each of these slides a rod bearing
a lens, which can thus be adjusted into any required position. The
instrument is used in an apartment where all light but that of the lamp
can be excluded. In the instrument just described an inverted image is
obtained, which is sufficient for ordinary medical purposes, but this
construction does not allow of the examination of retinal images, which
is best performed with an instrument having a plane mirror.
The appearance presented by the back of the eye when viewed in the
ophthalmoscope is represented in Fig. 242. The retina appears red,
except at the place where the optic nerve enters, which is white. On the
reddish ground the retinal blood-vessels can be distinguished; A, A, A,
branches of the retinal artery, have a brighter red colour, and more
strongly reflect the light than the branches, B, B, B, of the retinal
vein. Among these, and especially towards the margin, are seen, more or
less distinctly, the broader vessels of the choroid. Above the optic
nerve and a little to the right may be observed the _fovea centralis_.
[Illustration:
FIG. 242.
]
During the last twenty years the ophthalmoscope has been the chief means
of extending the knowledge of oculists regarding the diseased and
healthy conditions of the eye. In this way the substance of the lens and
the state of the humours can be directly seen, the causes of impaired
vision can be discovered, and the nature of many maladies made out with
certainty. This modern invention, by which the interesting spectacle of
the interior of the living eye can be observed, has therefore been far
from proving a barren triumph of science. Many insidious maladies can
thus be detected, and may be successfully treated before the organ has
become hopelessly diseased. In some cases the ophthalmoscope gives the
most certain evidence of the existence of obscure and unsuspected
diseases of other parts of the body.
_VISUAL IMPRESSIONS_.
Everybody knows that, however well the flat picture of an object may
imitate the colours and forms of nature, we are never deceived into
supposing that we have the real object before us. There must, therefore,
be something different in the conditions under which we see real objects
from those under which we view their pictures. The most favourable
circumstances for receiving an illusive impression of solidity from a
flat picture, is when we view it from a fixed position and with one eye.
This is because one means by which we unconsciously estimate distances
depends upon the changes in the perspective appearances of objects
caused by changes in our point of view. In many cases these changes in
the perspective are the only means we have of judging of the relative
distances of objects. But there is another circumstance which is still
more intimately connected with our perception of solidity. Each eye
receives a slightly different image of the objects before us (unless
these be extremely remote), inasmuch as they are viewed from a different
point. When the objects are very near, the two retinal images may differ
considerably, as the reader may convince himself by viewing with each
eye, alternately, objects immediately before him, while the other eye is
closed, and the head all the while motionless. The nearer objects will
plainly appear to shift their positions as seen against the back-ground
of the more distant objects; and a somewhat more careful observation
will reveal changes of perspective, or apparent form, in every one of
these objects. An extreme case is presented in that of a playing card,
or thin book, held in the plane which divides the eyes. The back or the
face, the one side or the other, will be seen, according as the right or
the left eye is opened. If we close the left eye, the displacement and
change of apparent form produced by a slight movement of the head are
sufficiently obvious; a movement of the head 2½ in. to the left causes a
decided change in the relative positions of adjacent objects. It is
plain, however, that it is precisely from a point 2½ in. to the left
that the left eye views these objects, and hence the perspective
appearance seen by the left eye must have the difference due to this
shifting of the point of view.
On the other hand, if one looks at a picture, or flat surface, placed
immediately in front, no change in the relative positions of its parts
is discernible by viewing it with either eye alternately. Not but that
there is a difference in the retinal images in the two cases, but there
is an absence of any point of comparison by which the change may be
judged. If we take a photograph of a statue, it will, when viewed by one
or the other eye, present the difference of the retinal images which is
due to a flat surface; the parts of the photographic image will be of
slightly different proportions as seen by each eye. If, instead of the
photograph we have before our eyes a statuette, each eye will see a
quite different view: the right eye will see a portion which is
invisible to the left eye, and _vice versâ_, and, in fact, we shall see
more than half round the object. Here, then, we have certain differences
of the retinal pictures when solid objects are viewed, and these
differences by innumerable repetitions have, unconsciously to ourselves,
become associated with notions of solidity, of something having length,
breadth, and depth, or thickness. The marvellous delicacy of these
perceptions will be alluded to hereafter.
Let us suppose that the lenses of two cameras are fixed in the positions
occupied by the two eyes, and that a photograph is taken in each camera,
the subject being, for example, a statuette. It is obvious that the
differences of the two photographs would correspond with the differences
of the two retinal images, and that, if a person could view with the
right eye only the photograph taken in the right-hand camera, and with
the left eye the left-hand photograph only, there would be formed on the
retinæ of his eyes images very nearly corresponding with those which the
actual object would produce, and the result would be, if these retinal
pictures occupied the proper position on the eyes, that the impression
of solidity would be produced, which is called the _stereoscopic
effect_.
This may be done without the aid of any instrument, as almost any person
may discover after some trials with nothing but a _stereoscopic slide_,
if he can succeed in maintaining the optic axis of his eyes quite
parallel. In such a case he will observe the stereoscopic effect by the
fusing together, as it were, into one sensation, of the impression
received by the right eye from the right photograph, with that received
by the left eye from the left photograph. But as each eye will, at the
same time, have the photograph intended for the other in the field of
view, the observer will be conscious of a non-stereoscopic image on each
side of the central stereoscopic one. These outside images are, however,
very distracting, for the moment the attention is in the least directed
to them, the optic axes converge to the one side or the other, losing
their parallelism, and the stereoscopic effect vanishes, because the
images no longer fall in the usual positions on the retinæ. It is, in
consequence, only after some practice that one succeeds in readily
viewing stereoscopic slides in this manner, but the acquirement is a
convenient one when a person has rapidly to inspect a number of such
slides, for he can see them stereoscopically without putting them in the
instrument. Many persons, however, find great difficulty in acquiring
this power. In such cases it is well to begin by separating the two
photographs by means of a piece of cardboard, covered with black paper
on both sides. When this is held in the plane between the eyes, each eye
sees only its own photograph, and the observer is not troubled with the
two exterior images. After a little practice in this way, the cardboard
may usually be dispensed with, and the observer will insensibly have
acquired the habit of viewing the slides stereoscopically, without any
aid whatever.
[Illustration:
FIG. 243.—_Wheatstone’s Reflecting Stereoscope._
]
Instruments have, however, been contrived which enable one to obtain the
desired result without effort; and one form of these is now tolerably
well known to everybody. The first stereoscope was the invention of
Wheatstone. The reflecting stereoscope is represented in Fig. 243, and
consists essentially of two plane metallic mirrors inclined to the front
of the instrument at angles of 45°, so that in each of them the observer
sees only the design which belongs to it. The rays reach the eyes as if
they came from images placed in front of the observer; and the two
images having the proper differences, produce together the impression of
solid objects.
[Illustration:
FIG. 244.
]
Brewster’s stereoscope—which is far more widely known than
Wheatstone’s—has two acute prisms, or, more usually, two portions of a
convex lens are cut out, and placed with their margins or thin parts
inwards, and they thus produce the same effect as would be obtained by
combinations of a prism with a convex lens. Another very common form of
the stereoscope has merely two convex lenses. The effect of the convex
lenses is to increase the apparent size of the images by diminishing the
divergence of the rays emitted by each point, producing the appearance
of larger designs seen at a greater distance. The effect of the prism is
to give the rays the direction which they would have if they proceeded
from an object placed in a position immediately between the two designs,
and an additional element by which we estimate distance, namely, the
convergence of the optic axes, is made to aid in the illusion, when the
rays proceeding from the two different pictures have approximately the
inclination that they would have if they emanated from real objects at
the place where the image is apparently formed. The box or case in which
the lenses or lenticular prisms are placed takes various forms. One of
the most common is represented in Fig. 244, but the stand on which it is
mounted is not a necessary part of the instrument, although it is
sometimes convenient. A handsome form is met with as a square case,
enclosing a number of photographic stereoscopic views mounted on an
endless chain in such a manner that they are brought successively into
view by turning a knob on the outside. When an instrument of this kind
is fitted up with a series of the beautiful landscape transparencies,
which are produced by certain continental photographers, a more perfect
reproduction of the impressions derived from nature, exclusive of
colour, cannot be conceived. We seem to be present on the very spots
which are so truthfully depicted by the subtile pencil of the sunbeam;
we feel that we have but to advance a foot in order to mix with the
passengers in the streets of Paris or of Rome, and that a single step
will bring us on the mountain-side, or place us on the slippery glacier;
at our own fireside we can feel the forty centuries looking down upon us
from the heights of those grand Egyptian pyramids, and find ourselves
bodily confronted with the mysterious Sphinx, still asking the solution
of her enigma. The truth and force with which these stereoscopic
photographs reproduce the relief of buildings are such, that when one
sees for the first time the real edifice of which he has once examined
the stereoscopic images, it no longer strikes him as new or unknown; for
he derives from the actual scene no impression of form that he has not
already received from the image.
[Illustration:
FIG. 245.
]
But of all subjects of stereoscopic photography the glaciers are,
perhaps, those which best show the power of the instrument as far
surpassing all other resources of graphic presentation. The most careful
painting fails to convey a notion of the strange glimmer of light which
fills the clefts of the ice, seen through the transparent substance
itself. The simple photograph commonly presents nothing but a confused
mass of grey patches; but combine in the stereoscope two such
photographs, each formed of nothing but slightly different grey patches,
and a surprising effect is at once produced: the masses of ice assume a
palpable form, and the beautiful effects of light transmitted or
reflected by the translucent solid reveal themselves. Another very
beautiful class of subjects for stereoscopic slides is found in those
marvellous instantaneous photographs, which seize and fix the images of
the waves as they dash upon the shore. Here a scene which has tasked the
power of the greatest painter is brought home to us with such force and
vividness that we all but hear the wild uproar of the breakers.
But for the art of photography the stereoscope would not thus be ready
to minister to our enjoyment, for no pictures wrought by man’s handiwork
could approach the requisite accuracy which the two stereoscopic
pictures must possess. All attempts to produce such pictures by
engraving or lithography have failed, except only in the case of linear
geometrical designs, such as representations of crystals. A very useful
and suggestive application of the stereoscope has been made to the
illustration of a treatise on _solid geometry_, where the lines
representing the planes, being drawn in proper perspective, the reader
by placing a simple stereoscope over the plates sees the planes stand
out in relief before him, and the multitude of lines, angles, &c., which
in a simple drawing might be distracting even for a practised
geometrician, assume a clear and definite form. The difference between
the two retinal pictures of objects is so slight, that when the objects
are at a little distance, ordinary observation fails to discover it
without the aid of special instruments; and an inspection of the pair of
photographs in a stereoscopic slide will convince any one that, even in
these, close and careful observation is required to perceive the
difference.
Some of the principles of stereoscopic drawings may be seen exemplified
by the pair we give in Fig. 245. With this figure the reader may attempt
the experiment of seeing the stereoscopic effect without the
stereoscope. When he has succeeded in doing this, or when he fuses the
images together by placing a simple stereoscope over the page, he will
find the result very singular; for he will receive the impression of a
solid crystal of some dark polished substance—black lead, for
instance—placed on a surface of the same material. The edges of the
solid will appear to have a certain lustre, such as one sees on the
edges of a real crystal. The reason of this impression being produced by
two drawings, one of which is formed by black lines on a white ground,
while the other has white lines on a black ground, is probably due to
the circumstance that we very often see in nature the _lustrous_ edges
of an object with one eye only. That is, one eye is in the path of the
rays which are regularly reflected from the object, while the other is
not,—a fact which may be verified in an instant by looking first with
one eye and then with the other, at a polished pencil, or similar
object, when placed in a certain position.
There is a kind of modification of the reflecting stereoscope, known
under the name of the _pseudoscope_, which is highly instructive, as
showing how much our notions of the solidity of objects are due to the
differences of the retinal images. In the pseudoscope the rays reach the
eyes after passing through rectangular prisms in such a manner that
objects on the right appear on the left, and objects on the left appear
on the right; but the images agree by reason of the symmetry of the
reflection, although the image of the objects that without the
instrument would be formed in the right eye is, by the action of the
prisms, formed in the left eye, and _vice versâ_. The impressions
produced are very curious: convex bodies appear concave—a coin, for
example, seems to have the image hollowed out, a pencil appears a
cylindrical cavity, a globe seems a concave hemisphere, and objects near
at hand appear distant, and so on. These illusions are, however, easily
dispelled by any circumstance which brings before the mind our knowledge
of the actual forms, and by a mental effort it is possible to perceive
the actual forms even with the pseudoscope, and indeed to revert
alternately, with the same object, from convexity to concavity. This
last effect is very curious, for the object appears to abruptly change
its form, becoming alternately hollow and projecting, according as the
mind dwells upon the one notion or the other; but the experiment is
attended with a feeling of effort, which is very fatiguing to the eyes.
Professor Helmholtz has contrived another very curious instrument,
depending on the same principles as the stereoscope. He terms it the
_telestereoscope_, and while the effect of the pseudoscope is to reverse
the relief of objects, the telestereoscope merely exaggerates this
relief; hence this instrument is well adapted for making those objects
which from their distance present no stereoscopic effect, stand out in
relief. The distance between our eyes is not sufficiently great to give
us sensibly different views of very distant objects, and what the
telestereoscope does is virtually to separate our eyes to a greater
distance. Fig. 246 is a horizontal section of the instrument. L and R
represent the position of the eyes of the spectator; _a, b_, are two
plane mirrors at 45° to his line of sight; A, B, are two larger plane
mirrors, respectively nearly parallel to the former. _c d a_ L and _f g
b_ R show the paths of rays from distant objects, and it is obvious that
the right eye will obtain a view of the objects identical with that
which would be presented to an eye at R´, while the left eye has
similarly the picture of the objects as seen from the point L´. The four
mirrors are mounted in a box, and means are provided for adjusting the
positions of the larger mirrors, as may be required. With this
instrument the distant objects in a landscape—a range of mountains, for
example—which present to the naked eye little or no appearance of
relief, have their projections and hollows revealed in the most curious
manner.
[Illustration:
FIG. 246.—_The Telestereoscope._
]
It is upon a similar principle that stereoscopic views of some of the
celestial bodies have been obtained. Admirable stereoscopic slides of
the moon have been produced by photographing her at different times,
when the illumination of the surface is the same, but when, in
consequence of her _libration_, somewhat different views of our
satellite are presented to us. Two such photographs, properly combined
in the stereoscope, give not only the spherical form in full relief, but
all the details of the surface: the mountains, craters, valleys, and
plains are seen in their true relative projection.
The telestereoscope may be inverted, so to speak, and its effect
reversed; for an arrangement of mirrors similarly disposed, but on such
a scale as will permit the eyes to be respectively in the lines _c d_
and _f g_, would reflect from objects in the direction L R rays which
would have but little of the difference of direction to which the
stereoscopic effect is due. Hence solid objects viewed with such an
instrument appear exactly like flat pictures, the effect being far more
marked than in simply viewing them with one eye.
An ingenious method of exhibiting a stereoscopic effect to an audience
has been contrived by Rollmann. He draws on a black ground two linear
stereoscopic designs—that for the left eye with red lines, that for the
right eye with blue. Each individual in the audience is provided with a
piece of blue glass and a piece of red: he places the red glass before
the left eye, the blue glass before the right: each eye thus receives
only the picture intended for it, for the blue lines cannot be seen
through the red glass, or the red lines through the blue glass. The
diagrams may, of course, be projected on a screen by a magic lantern, in
which case the circumstances are even more favourable. Duboscq has
arranged a kind of opera-glass, so that a person may view appropriate
designs on the large scale, and arrangements have been also contrived by
which the stereoscopic effect may be seen in moving figures.
Every student of this interesting subject should examine a few
stereoscopic images produced by simple lines representing geometrical
figures, or the photographs of the model of a crystal, as these exhibit
in the most striking manner the conditions requisite for the production
of stereoscopic effects. A person having a little skill in perspective
and geometry might construct the two stereoscopic images of a body
defined by straight lines, but the drawings must be executed with
extreme exactitude, for the least deviation would produce the most
marked effect in the stereoscopic appearance. The production of
stereoscopic photographs now forms a considerable branch of industrial
art. At first, these photographs were made by taking the two different
views with the same camera at two operations. But there were
difficulties in obtaining uniformity of depth in the impressions, and
the change in the shadows produced by the earth’s rotation showed
itself—although the interval between the two exposures might not exceed
three or four minutes. The increased shadows in such cases show
themselves in the stereoscope, like dark screens suspended in the air.
It was Sir David Brewster who, in 1849, first proposed the plan now
universally adopted, of producing the views simultaneously by twin
cameras forming their images on different parts of the same sensitive
plate, the centres of the lenses being placed at the same distance apart
as a man’s eyes, that is, from 2½ to 3 in. This is, of course, the only
manner in which instantaneous views can be secured. Helmholtz, however,
advocates the photographs of remote objects being taken at a much
greater distance apart, for they otherwise present little appearance of
relief. By selecting from an assortment of slides, two views of the
Wetterhorn, taken from different points in the Grindelwald valley, and
combining these in the stereoscope, he found that a far more distinct
idea of the modelling of the mountain could be thus obtained than even a
spectator of the actual scene would receive by viewing the mountain from
any one point. Such a mode of combining the photographs would produce in
the stereoscope the same effect as the telestereoscope would in the
landscape, but the effect would be caused to a proportionately far
higher degree.
The date of Wheatstone’s first publication regarding the stereoscope was
1833; but a complete description and theory of the instrument was not
published until five years afterwards. Brewster first made public, in
1843, his invention of the stereoscope with lenses, which is now so
familiar to us, and few scientific instruments have become so quickly
and extensively popular; certainly no other simple and inexpensive
instrument has contributed so largely to the amusement and instruction
of our domestic circles. And, to the philosopher who studies the nature
of our perceptions, the stereoscope has been even more instructive, for,
instead of vague surmises, it provided him with the solid ground of
experiment on which to found his theories. The literature of this one
subject—stereoscopic effect—is extensive enough to occupy a tolerably
long book-shelf. It dates from 300 B.C., when Euclid touched upon the
subject in his Optics; and after a lapse of more than eighteen centuries
it was taken up by Baptista Porta, in 1583; but the whole development of
this subject belongs almost entirely to the last half-century.
[Illustration:
FIG. 247.
]
The part which the muscles of the eyes take in our perceptions of form
has been already alluded to, and it may be interesting to illustrate
this point by a curious example or two of illusions arising from their
movements. If our reader will glance at Fig. 247, he will see that the
lines, _a b_ and _c d_, appear to be farther apart towards the centre
than at the ends, while _f g_ and _h i_, on the other hand, appear
nearest together in the middle. He will hardly be convinced that in each
case the lines are quite parallel until he has actually measured the
distances. A still more striking example of the same kind of illusion is
shown by Fig. 248, due to Zöllner. This appears a sort of pattern, in
which the broad bands are not upright, but sloping alternately to the
right and left, and with the spaces between the lines wider at one end
than the other. The lines in the figure are, however, strictly parallel.
The illusion by which they appear divergent and convergent is still more
strongly felt when the book is held so that the wider bands are inclined
at an angle of 45° to the horizon. There is another illusion here with
reference to the short lines, which will appear to be opposite to the
white spaces on the other side of the long lines to which they are
attached. That these illusions are really due to movements of the eyes
may be proved by viewing the designs in any manner which entirely
prevents the movement, as by fixing the gaze on one spot in the case of
Fig. 247, when the illusion will vanish; but this plan is not so easily
applied to Fig. 248. A convincing proof, however, will be found in the
appearance of these figures when they are viewed by the instantaneous
light of the electric spark, as when a Leyden jar is discharged in a
dark room. The reader viewing the figures, held near the place where the
spark appears, will see them distinctly without the illusions as to the
non-parallelism of the lines. In the absence of an electrical machine,
or coil and jar, the reader may have an opportunity of seeing the
figures by flashes of lightning at night, when the result will be the
same.
[Illustration:
FIG. 248.
]
There is a property of the eye which has led to the production of many
amusing and curious illusions. This property in itself is no new
discovery, for its presence and effects must have been noticed ages ago.
The property in question is illustrated when we twirl round a stick or
cord, burning with a red glow at the end. We seem to trace a _circle_ of
fire; but as the glowing spark cannot be in more than one point of the
circle at once, it is plain that the impression produced on the eye must
remain until the spark has completed its journey round the circle, and
reaching each point successively renews the luminous impression. Like
other subjects relating to vision, this phenomenon has been carefully
examined in recent times, and its laws accurately determined.
The fact which is obvious from such an experiment, may be thus stated:
Visual impressions repeated with sufficient rapidity produce the effect
of objects continually present. This persistence of the visual
impressions is easily made the subject of experiment by means of rapidly
rotating discs; and in the common toy called a “colour top” we have a
ready means of verifying some of the conclusions of science on this
subject. Some very interesting results may be obtained by an apparatus
as simple as this, regarding the laws of the phenomenon we are
considering, and the effects of various mixtures of tints and colours.
The well-known toy, the thaumatrope, depends on the same principle. In
this a piece of cardboard is painted on one side, with a bird, for
example, and on the other side with a cage: when the cardboard is
twirled round very rapidly by means of a cord fixed at opposite points
of its length, both bird and cage become visible at once, and the bird
appears in the cage.
[Illustration:
FIG. 249.
]
[Illustration:
FIG. 250.
]
[Illustration:
FIG. 251.
]
A still more ingenious application of this principle we owe to Plateau,
who described it in 1833, under the title of the _phenakistiscope_; and
also to Stampfer, who independently devised the same arrangement about
the same time, and named it the _stroboscopic disc_. The reader may, at
almost any toy-shop, purchase one of them, provided with a number of
amusing figures; or he may easily construct for himself one which will
exemplify the principle. He requires no other materials than a piece of
cardboard, and his only tools may be a sharp penknife, a pair of
compasses, and a flat ruler. Let him draw on his cardboard a circle of 8
in. diameter, and divide its circumference by eight equidistant points.
From these radii should be drawn with the point of the compasses, and
equal distances from the centre marked off upon them, to fix the centres
of the small circles, which must all have exactly the same size (say, 1
in. in diameter) and be marked by a distinct line. In these are to be
marked the hand of a clock-face in the positions shown in Fig. 249; and
finally, in the direction of the radii, narrow slips are to be cut out
of the cardboard as shown. If a pin be put through the centre of the
disc, attaching it thus to the flat end of a cork, so that it can freely
rotate in its own plane, and the disc be turned rapidly round, as in
Fig. 250, in front of a looking-glass, while the spectator looks through
the slits, he will see the hand on the little dial apparently turning
round, with rather a jerky movement it is true, somewhat like the
dead-beat seconds-hand that is sometimes seen on clocks. The illusion is
best when the slits are so narrow that only one of the several images is
visible by reflection, namely, that which is adjacent to the slit. Thus,
as the disc rotates, each little circle is visible for an instant as the
slit passes in front of the spectator’s eye; and if the rotation be
sufficiently rapid, the impression of the disc is permanent, as it is
constantly being renewed by the successive circles, while, on the
contrary, the hands, having different positions, produce images in
different positions, giving the appearance of a jerky rotation. The
instruments sold in the shops have sometimes a thin metallic disc with
the slits in it, and a series of designs printed in smaller paper discs.
The paper discs may be screwed on the other disc as required, and a
button on a pulley with an endless band is provided for producing the
rotation more conveniently. Fig. 251 shows one of the pictures for a
disc with twelve slits, and the effect produced by it is that of a
dancing figure.
Another arrangement for showing the same illusion has lately become a
very popular toy, and quite deservedly so, for it has the advantages of
requiring no looking-glass, and of making the effect visible to a number
of persons at the same time. This apparatus, which has been termed the
Zoetrope, consists simply of a cylindrical box, like a drum with the
upper end cut off. It is mounted on a pivot, which permits its revolving
rapidly about its vertical axis when touched by the finger. The cylinder
has a number of equidistant vertical slits round the upper part of its
circumference. The figures which produce the illusion are printed on a
slip of paper, which is placed in the lower part of the drum, and when
this is in rapid rotation, and the figures are viewed through the slits,
the illusion is produced in exactly the same manner as in the revolving
disc.
[Illustration:
FIG. 251_a_.—_Edison’s Kinetographic Theatre._
]
At the end of the article on the phonograph in a subsequent page, the
reader will find a remark as to the effect that might be produced by a
combination of that instrument with instantaneous and simultaneous
photographs of some famous speaker. This combination has now been
accomplished by the great inventive genius to whom we are indebted for
the phonograph. Mr. Edison has done this so effectively that he may be
said to have given life to the _zoetrope_ by the perfection in which the
ocular illusion is produced together with the audible manifestations
that keep time with it. The amount of thought and ingenuity expended on
this new contrivance, which Edison has called the _kinetoscope_, will
scarcely be appreciated by anyone who has not given some consideration
to the many practical difficulties that have been overcome. No wonder
that the announcement made at the beginning of 1892 should have been
received with incredulity, for it was to the effect that Edison had
contrived some happy combination of photography and electricity by which
a man (presumably one who could afford to pay for luxuries) might sit in
his own room and see the moving forms of the actors in an opera
projected on a screen before his eyes, while at the same time he would
hear their voices singing. Every movement, every change of expression,
every glance of the eye, and, in fact, all that was visible to the
spectator in front of the stage would appear on the screen, while not a
note of vocalist, or chord of orchestra, would fail to reach the ear.
And all this was to be evoked at will, and repeated as often as desired,
not, therefore, of course, as a presentation of what was taking place at
the time, but as a reproduction of some previous performance. This
wonderful result has virtually been attained by the application of
delicate and ingenious machinery designed to make the phonograph and the
camera work synchronously. The first part of the problem was the
production of a succession of so-called instantaneous photographs at an
extremely rapid rate. In the actual apparatus forty-six photographs are
taken every second, a feat which would beforehand be thought
impracticable. This is accomplished by making use of a band of sensitive
celluloid film, which alone admits of being moved and stopped with the
desired rapidity. The movement is imparted by an electric motor, and the
arrangement is such that for each exposure the film is held stationary
for 9/10ths of 1/46th of a second, during which the lens is uncovered,
then for the remaining ⅒th it is covered, while at the same time the
film is jerked forward so as to expose a fresh surface to receive a new
impression. Obviously the mass moved and stopped with this rapidity
(which without the stoppages is at the rate of 26 miles an hour) must be
small, and it is found that photographs about 1 in. in diameter cannot
be much exceeded in view of this condition. The lens has to be entirely
stopped or screened during the tenth of the short interval (1/460th of a
second) in which the onward movement of the film is taking place, and it
has to be practically open during the remaining 9/10ths of the interval
(9/460ths of one second) in which the film is held stationary in order
to receive the photographic image. These alternations of movement and
stoppage must take place with the utmost regularity, and Edison has used
a beautifully regulated electro-motor as the active power, which also
simultaneously moves a phonograph so that sights and sounds shall
proceed in step, for it is thus they have to be reproduced. This is done
by developing the band of film, and from it printing photographic
_positives_ on a similar band, whose images are successively projected
on a screen by means of a lantern with a step by step movement, exactly
the same as that by which the original photographs are taken, while the
phonographic cylinder is so timed as to give off to a loud-speaking
instrument the sounds that accompanied the photographs. A description of
the ingenious mechanism by which all this is accomplished is not
suitable for these pages, for it is the result, rather than the details
of apparatuses, that interest the general reader. In a simpler form of
kinetoscope the positive images on the band of film are viewed directly
by single observers, each looking through magnifying glasses; in this a
disc with 46 slits revolves, and in its passage, as each slit
momentarily permits a view of the image, an electric flash
simultaneously lights it up. The same principle is, of course, used in
the screen projections. From the very great number of impressions made
on the eye in one second, there is none of that jerky movement that is
observable in the older appliances. Mr. Edison has found it necessary to
provide a special stage, or rather small theatre, in which the actors of
the little dramas may be photographed with every advantage in the way of
lighting, &c. Fig. 251_a_ shows this kinetographic theatre with the
electric camera in action. The subjects reproducible in the kinetoscope
include the most rapid movements, such as quick dances, blacksmiths
hammering on an anvil, &c., or incidents of ordinary life involving much
gesture and change of facial expression, and nothing can be more amusing
than to see all these shown to the life by the images on the screen, or
by the pictures viewed through the lens, especially if at the same time
the phonograph is made to emit the corresponding sounds.
[Illustration:
FIG. 252.—_Portrait of Sir W. Thomson._[5]
]
Footnote 5:
Now Lord Kelvin.
ELECTRICITY.
About sixty years ago a popular book was published having for its theme
the advantages which would flow from the general diffusion of scientific
knowledge. Great prominence was, of course, given to the utility of
science in its direct application to useful arts, and many scientific
inventions conducing to the general well-being of society were duly
enumerated. Under the head of electricity, however, the writer of that
book mentioned but few cases in which this mysterious agent aided in the
accomplishment of any useful end. The meagre list he gives of the
instances in which he says “_even_ electricity and galvanism might be
rendered subservient to the operations of art,” comprises only orreries
and models of cornmills and pumps turned by electricity, the designed
splitting of a stone by lightning, and the suggestion of Davy that the
upper sheathing of ships should be fastened with copper instead of iron
nails, with a hint that the same principle might be extended in its
application. At the present day the applications of electricity are so
numerous and important, that even a brief account of them would more
than fill the present volume. Electricity is the moving power of the
most remarkable and distinguishing invention of the age—the telegraph;
it is the energy employed for ingeniously measuring small intervals of
time in chronoscopes, for controlling time-pieces, and for firing mines
and torpedoes; it is the handmaid of art in electro-plating and in the
reproduction of engraved plates, blocks, letterpress, and metal work; it
is the familiar spirit invoked by the chemist to effect marvellous
transformations, combinations, and decompositions; it is a therapeutic
agent of the greatest value in the hands of the skilful physician. Such
an extension of the practical applications of electricity as we have
indicated implies a corresponding development of the science itself;
and, indeed, the history of electricity during the present century is a
continuous record of brilliant discoveries made by men of rare and
commanding genius—such as Davy, Ampère, and Faraday. To give a complete
account of these discoveries would be to write a treatise on the
science; and although the subject is extremely attractive, we must pass
over many discoveries which have a high scientific interest, and present
to the reader so much of this recently developed science as will enable
him to comprehend the principles of a few of its more striking
applications.
The science of electricity presents some features which mark it with
special characters as distinguished from other branches of knowledge. In
mechanics and pneumatics and acoustics we have little difficulty in
picturing in our minds the nature of the actions which are concerned in
the phenomena. We can also extend ideas derived from ordinary experience
to embrace the more recondite operations to which heat and light may be
due, and, by conceptions of vibrating particles and undulatory ether,
obtain a mental grasp of these subtile agents. But with regard to
electricity no such conceptions have yet been framed—no hypothesis has
yet been advanced which satisfactorily explains the inner nature of
electrical action, or gives us a mental picture of any pulsations,
rotations, or other motions of particles, material or ethereal, that may
represent all the phenomena. Incapable as we are of framing a distinct
conception of the real nature of electricity, there are few natural
agents with whose ways we are so well acquainted as electricity. The
_laws_ of its action are as well known as those of gravitation, and they
are far better known than those which govern chemical phenomena or the
still more complex processes of organic life.
Definite as are the laws of electricity, there is no branch of natural
or physical science on which the ideas of people in general are so
vague. Spectators of the effects of this wonderful energy—as seen
violently and destructively in the thunderstorm, and silently and
harmlessly in the Aurora—knowing vaguely something of its powers in
traversing the densest materials, in giving convulsive shocks, and in
affecting substances of all kinds—the multitude regard electricity with
a certain awe, and are always ready to attribute to its agency any
effect which appears mysterious or inexplicable. The popular ignorance
on this subject is largely taken advantage of by impostors and
charlatans of every kind. Electric and magnetic nostrums of every form,
electric elixirs, galvanic hair-washes, magnetized flannels, polarized
tooth-brushes, and voltaic nightcaps appear to find a ready sale, which
speaks unmistakably of the less than half-knowledge which is possessed
by the public concerning even the elements of electrical science.
Electricity has also a special position with regard to its intimate
connection with almost every other form of natural energy. Evolved by
mechanical actions, by heat, by movements of magnets, and by chemical
actions, it is capable in its turn of reproducing any of these. It plays
an important, but as yet an undefined, part in the physiological actions
constantly going on in the organized body, and is, in fact,
all-pervading in its influence over all matter, organic and inorganic—a
secret power strangely but universally concerned in all the operations
of nature. We are compelled to regard electricity not as a kind of force
acting upon otherwise inert matter, but rather as an affection or
condition of which every kind of matter is capable, although we are
still unable to form a conjecture of the precise nature of the action.
We have now to address ourselves to the task of unfolding so much of the
science as will enable the reader to understand the leading principles
of such important applications as electro-plating, illumination, and the
telegraph; and this will necessarily include an account of the grand
discovery of the identity, or at least intimate connection, of magnetism
and electricity.
_ELEMENTARY PHENOMENA OF MAGNETISM AND ELECTRICITY._
The distinctive property of a magnet is, as everybody knows, to attract
pieces of iron, and this property having been observed by the ancients
in a certain ore of iron which was found near the city of Magnesia, in
Asia Minor, the property itself came to be called Magnetism. A bar of
steel, if rubbed with the natural magnet or loadstone, acquires the same
property, and if the bar be suspended horizontally or poised on a pivot,
it will settle only in one definite direction, which in this country is
nearly north and south. If a narrow magnetized bar be plunged into iron
filings, it will be found that these are attracted chiefly by the ends
of the bar, and not at all by the centre. It appears as if the magnetic
power were concentrated in the extremities of the bar, and these are
termed its poles, the pole at the end of the bar which points to the
north is called the _north pole_ of the magnet, and the other is named
the _south pole_. If a north pole of one magnet be presented to the
north pole of another, they will repel each other, and the same
repulsion will take place between the south poles, whereas the north
pole of one magnet attracts the south pole of another. In other words,
poles of the same name repel each other, but poles of opposite names
attract each other, or still more concisely, _like poles repel, unlike
poles attract each other_.
Magnetism acts through intervening non-magnetic matter with undiminished
energy. Thus, the attractions and repulsions of magnetic poles manifest
themselves just as strongly when the poles are separated by a stratum of
wood or stone as when merely air intervenes, and the attraction of small
pieces of iron by a magnet takes place through the interposed palm of
one’s hand without diminution. A delicately suspended needle in even a
remote apartment of a large building moves whenever a cart passes in the
street. It is almost too well known to require mention here, that iron
and steel are the only common substances which are capable of plainly
exhibiting magnetic forces, and, indeed, there are no known substances
capable of so powerful a magnetization as these. But the difference in
the magnetic behaviour of iron and steel is not so well understood, and
it is a point of importance for our subject, and connected with a
fundamental law which governs all magnetic manifestations. A piece of
pure iron is very readily cut with a file, whereas a piece of steel may
be so hard that the file makes no impression upon it whatever; and hence
a piece of pure iron, or rather iron holding no carbon in combination,
and possessed of no steely quality, is often spoken of as _soft iron_.
When a piece of soft iron is placed near the pole of a magnet, the iron
becomes, for the time, a magnet. If iron filings be sprinkled over it,
they will arrange themselves about the parts of the iron respectively
nearest and farthest from the magnet, thus showing that the piece of
soft iron has acquired magnetic poles. It will be found on examining
these poles that the one nearest the magnet is of the contrary name to
the pole of the magnet, and the farthest is of the same name. The
conversion of the soft iron into a magnet by the influence of a magnetic
pole is termed _induction_. It need hardly be said that the inductive
effect is more powerful in proportion to the shortness of the distance
separating the piece of soft iron from the magnetic pole, and, of
course, the effect is at its maximum when there is actual contact.
Induction thus explains, by aid of the law of the poles, the attraction
which a magnet exercises over pieces of iron, for it is plain that the
inductive influence is accompanied by attraction between the two
contiguous oppositely-named poles of the magnet, and of the piece of
iron. But attraction is not the only force, for the pole developed at
the farthest portion of the piece of iron being of the same name as the
inducing pole, these will be mutually repulsive. The attractive force
will, however, be more powerful on account of the shorter distance at
which it is exerted, and will predominate over the repulsive force,
particularly at short distances, because then the difference will be
relatively greater. At distances from the inducing pole relatively great
to the distance between the two poles of the piece of iron, the
difference may be so small that its effect in attracting the piece of
soft iron will be imperceptible, and then the piece of iron acted on by
two (nearly) equal parallel forces, will be subject to what is termed in
mechanics a _couple_, the only effect of which is to turn the body into
such a position that the opposing forces act along the same line. The
definite direction assumed by a freely suspended needle may be explained
by supposing that the earth itself is a magnet having a _south_ pole in
the _northern_ hemisphere, and a _north_ pole in the _southern_
hemisphere, the line joining these poles being shorter than the axis of
the earth, and not quite coinciding with it in position; and the fact of
the needle being turned round but not bodily attracted is then easily
accounted for, the attractive and repulsive forces being reduced to a
_couple_ in the manner just explained.
If the attempt be made to turn a piece of steel into a magnet, by the
induction of a magnetic pole, the same results will be obtained as in
the case of soft iron, but in a much feebler degree, and with this
difference: the piece of steel does not lose its magnetism when the
inducing magnet is withdrawn, whereas in the case of the soft iron every
trace of magnetism vanishes the instant the inducing pole is removed.
And if the pole of the magnet be not only put in contact with one end of
the piece of steel, but rubbed on it, the piece will acquire permanent
and powerful magnetism. Hence it will be noticed that a piece of soft
iron can by the mere approximation of a magnetic pole be converted in an
instant into a magnet, and by the removal of the magnet can as instantly
be deprived of its magnetism, and made to revert into its ordinary
condition; while steel is not so readily magnetized, but retains its
magnetism permanently.
[Illustration:
FIG. 253.—_A simple Electroscope._
]
The elementary phenomena of electricity are extremely simple and easy of
demonstration, and as the whole science rests upon inferences derived
from these, the reader would do well to perform the following simple
experiments for himself. Apparatus is represented in Fig. 253, but the
only essential portion is a straw, B, suspended from any convenient
support by a very fine filament of _white silk_. To one or both ends of
the straw a little disc of gilt paper, or a small ball of elder-pith or
of cork, should be attached, so that the straw may be balanced
horizontally. Now rub on a piece of woollen cloth a bit of sealing-wax,
or a stick of sulphur, or a piece of amber, or a penholder, paper-knife,
or comb made of ebonite, and immediately present the substance to the
ball at the end of the straw. It will be first attracted to the rubbed
surface, but after coming into contact with it, repulsion will be
manifested and the ball will separate, and may be chased round the
circle by following it with the excited body. The attraction of light
bodies by amber after it has been rubbed appears to be the one solitary
electrical observation recorded by the ancients, but it has given its
name to the science, ελεκτρον being the Greek name for amber. The cause,
then, of this property is named _electricity_, and bodies which exhibit
it are said to be _electrified_. The reader will remark that these words
_explain_ nothing: they are used merely to _express_ a certain state of
matter and the entirely unknown cause of that state. Let the pith or
cork ball at the end of the straw be again charged with electricity, by
bringing it into contact with a piece of sealing-wax or ebonite which
has just been electrified by friction. In this condition it will, as we
have just seen, be repelled by the substance which charged it, and on
trial it will be found to be repelled also by all the substances we have
named, after they have been excited by friction. But if, while still
charged with the electricity communicated to it by contact with
sealing-wax, sulphur, ebonite, or amber, we present to it a warm and dry
glass tube which has just been rubbed with dry silk, we shall find that
the ball will be strongly attracted. After contact with the glass,
repulsion will take place, and the ball will refuse again to come into
contact with the excited glass. In this condition, however, it will be
immediately attracted by rubbed sealing-wax or ebonite, and so on
alternately: the ball when repelled by the wax is attracted by the
glass, and when repelled by the glass is attracted by the wax.
These simple experiments prove that, whatever electricity may be, there
are two kinds of it, or, at least, it manifests two opposite sets of
forces. The electricity evolved by the friction of glass with silk was
formerly called _vitreous_ electricity, and that shown by excited resin,
sealing-wax, amber, &c., was named _resinous_ electricity. These names
have now been respectively replaced by the terms _positive_ and
_negative_. It must be understood that these terms imply no actual
excess or defect, but are purely distinguishing terms, just as we speak
of the _up_ and _down_ line of a railway, without implying an
inclination in one direction or the other. A fact of great importance in
electrical theory is discovered when the substances in which electricity
is developed are carefully examined: it is found that one kind is never
produced without the other simultaneously appearing. Thus, the silk
which has been used for rubbing the glass in the above experiments will
be found to exhibit the same electricity as sealing-wax or ebonite. And,
further, the _quantities_ of positive and negative electricity evolved
are always found to be equal, or equivalent to each other; that is, if
they are put together they completely neutralize or destroy each other’s
effects. We have used the word “quantity,” implying that electricity can
be measured. No doubt, whatever electricity may be, there may be more or
less of it; but can we measure an imponderable, invisible, impalpable
thing, incapable of isolation? What we really measure when we say that
we measure electricity is the attractive or repulsive force: we balance
this against some other force (that of gravitation, for example), and we
say, so much weight lifted represents so much electricity.
If we try to electrify a piece of metal by holding it in the hand and
rubbing it against woollen cloth, silk, or other substance, we shall
fail in the attempt: no signs of electricity will thus be shown by the
metal. Hence bodies were formerly divided into two classes—those which
could be electrified by friction, and those which could not. It was
afterwards found, however, that there was no real ground for this
division, but that, on the contrary, _no two bodies can be rubbed
together, even if they are made of the same substances, without positive
electricity appearing in one, and an equivalent quantity of negative
electricity in the other_. The real difference between bodies which
prevents the manifestation of electricity in many cases depends upon the
fact that electricity is able to traverse some substances with great
facility, while others prevent its passage. Thus, if we suspend
horizontally a hempen cord by white silk attached to the ceiling, so
that the hempen cord comes in contact with nothing but the silk, we
shall find, on presenting a piece of excited ebonite to one end of the
cord, that electric attraction of light bodies will be manifested at the
other. If a silk cord be substituted for the hempen one, no such effect
will be observed. The hemp is, therefore, said to be a _conductor_, and
the silk a _non-conductor_. Again, if we substitute for one of the silk
threads suspending the cord a piece of twine, or a wire, we shall fail
to obtain any electric manifestations at the remote end, because the
electricity will be carried off into the earth by the conducting powers
of these substances. On the other hand, filaments of glass or ebonite
may be used, instead of the silk, with the same effect: they do not
allow the electricity to run through them to the ground, and are
therefore termed, like the silk, _insulators_ of electricity. The
distinction of bodies into conductors on the one hand, and into
non-conductors or insulators on the other, is of paramount importance in
the science and in all its applications. This distinction, however, is
not an absolute one: there is no substance so perfect an insulator that
it will not permit any electricity to pass, and there is no conductor so
perfect that it does not offer resistance to the passage. Substances may
be arranged in a list which presents a gradation from the best conductor
to the best insulator. The metals are by far the best conductors, but
there is great relative diversity in their conductive power. Silver,
copper, and gold are much the best conductors among the metals, iron
offering eight times, and quicksilver fifty times, the resistance of
silver. Coke, charcoal, aqueous solutions, water, vegetables, animals,
and steam are all more or less conductors, while among the substances
called insulators may be named, in order of increasing insulating power,
india-rubber, porcelain, leather, paper, wool, silk, mica, glass, wax,
sulphur, resins, amber, gum-lac, gutta-percha, and ebonite. It will now
be obvious why the electricity developed by the friction of a piece of
metal fails to manifest itself under ordinary circumstances, as, for
instance, when held in the hand: the metal and the body being both
conductors, the electricity escapes. But if the piece of metal be held
by an insulating handle of glass or ebonite, the electrified condition
may easily be observed.
_THEORY OF ELECTRICITY._
The few elementary facts which have been pointed out are absolutely
necessary for the foundation of what is sometimes termed the theory of
electricity, but which is properly no theory,—at least, not a theory in
the same sense as gravitation is a theory explaining the motions of the
planets, or even in the sense in which the hypothesis of the ether and
its movements explains the phenomena of light. It is absolutely
necessary to have a conception of some kind which may serve to connect
in our minds the various phenomena of electricity, if it were only to
enable us the more easily to talk about them. In default of any
supposition which will shadow forth what actually occurs in these
phenomena, we have recourse to what has been aptly termed _a
representative fiction_: we picture to ourselves the actions as due to
_imaginary fluids_—fluids which we know _do not exist_, but are as much
creations of the mind as Macbeth’s air-drawn dagger; not, however, like
his “false creation,” proceeding from “the heat-oppressed brain,” but
intellectual fictions, consciously and designedly adopted for the
purpose of enabling us the better to think of the facts, to readily
co-ordinate them, and to express them in simple and convenient language.
Non-scientific persons hearing this language usually mistake its
purport, and imagine that the actual existence of an “electric fluid” is
acknowledged. The accounts which appear in the newspapers of the damage
done by thunderstorms are often amusing from the objectivity which the
reporter attributes to the “electric fluid.” It is described, perhaps,
as “entering the building,” “passing down the chimney,” then “proceeding
across the floor,” “rushing down the gas-pipes,” “forcing its way
through a crevice, and then streaming down the wall,” &c., in terms
which imply the utmost confidence of belief in the existence of the
“fluid.” With this intimation that the hypothesis of electric fluids is
merely, then, a “_façon de parler_,” the reader will not be misled by
the following brief explanation of the elementary facts in the language
of the theory.
In the natural state all bodies contain an indefinite quantity of an
imponderable subtile matter, which may be called “neutral electric
fluid.” This fluid is formed by a combination of two different kinds of
particles, positive and negative, which are present in equal quantities
in bodies not electrified; but when there is in any body an excess of
one kind of particles, that body is charged accordingly with positive or
negative electricity. Both fluids traverse with the greatest rapidity
certain substances termed _conductors_; but they are retained amongst
the molecules of _insulating_ substances, which prevent their movement
from point to point. When one body is rubbed against another, the
neutral electric fluid is decomposed—the positive particles go to one
body, the negative with which these positive particles were before
united pass to the other body. The particles of the same name repel each
other, but particles of opposite names attract each other; and it is
this attraction which is overcome when the electricities are separated
by friction or in any other manner.
It will be observed that the above is nothing but the statement of the
elementary facts in the language of the hypothesis. This system of the
two fluids readily lends itself to the explanation of nearly all the
phenomena presented in what is termed _static electricity_—that is, in
those phenomena where the actions are conceivably due to a more or less
permanent separation of the fluids. The grand discoveries in electricity
turn, however, upon quite another condition, namely, one in which the
two hypothetical fluids must be imagined as constantly combining, and
here the utility of the hypothesis is less marked. Inasmuch, however, as
there can be no doubt regarding the identity of the agent operating in
the two sets of circumstances, the facts of _dynamical electricity_ must
still be expressed in the same language, with the aid of any additional
conceptions which may give us more grasp of the subject.
_ELECTRIC INDUCTION._
In all electrical phenomena an inductive action occurs, which resembles
that which we have already indicated with regard to magnetism. Thus, if
we take an insulated metallic conductor in the uncharged state, and
bring it near an electrified body, we shall find that the conductor,
while still at a considerable distance, will give signs of an electrical
charge. Suppose we have a cylindrical conductor, and that we present one
end of it to the electrified body, but at such a distance that no spark
shall pass, we shall find, if the charge on the electrified body be
strong and the conductor be brought sufficiently near, that on bringing
the finger near the insulated cylinder, a spark passes. While the
cylinder continues in the same position with regard to the electrified
body, no further sparks can be drawn from it; but if the distance
between the two bodies be increased, the insulated cylinder will be
found to have another charge of electricity, which will again produce a
spark. And by repeating these movements we may obtain as many sparks as
we desire by these mechanical actions, without in the least drawing upon
the charge on the original electrified body. The electrophorus is a
device for obtaining electricity by this plan, and several rotatory
electrical machines have lately been invented which yield large supplies
of electricity by a similar inductive action.
[Illustration:
FIG. 254.
_The Gold-leaf Electroscope._
]
It is found that in such a case as that we have above supposed, if the
electrified body is charged with positive electricity, the uncharged
conductor brought near it has its electricities separated—the negative
attracted and held by the attraction of the positive charge in the parts
of the cylinder nearest the inducing body; while the corresponding
quantity of positive electricity is driven towards the most remote parts
of the insulated conductor. It is this last which gives the spark in the
first case, and if it be not thus withdrawn from the conductor, it
re-combines with the negative electricity when the conductor is
withdrawn from the neighbourhood of the electrified body, and the
conductor then reverts to the natural or unelectrified state. But the
contact of a conducting body with the conductor while it is under the
influence of the electrified body withdraws only positive electricity,
the negative—being held, as it were, by the attraction of the positive
electricity of the charged body—is not thus removed, and in this
condition it is sometimes called _disguised_ or _dissimulated_
electricity—a term the propriety of which is doubtful. The excess of
negative “fluid” which the conductor thus acquires shows itself,
however, only when the inducing body has been withdrawn. Precisely
similar effects will take place, _mutatis mutandis_, if the electrified
body has a negative charge. A demonstration of inductive effects is
readily afforded in the action of the gold-leaf _electroscope_, Fig.
254, in which two strips of gold-leaf are suspended within a glass case
from wire passing through the top, and terminated in a metal plate. This
instrument is often used for showing the existence of very small
electric charges. Let a stick of sealing-wax be rubbed and held, say, a
foot or more from the plate of the electroscope, the leaves will diverge
with negative electricity. The sealing-wax being retained in the same
position, touch the plate for an instant with the finger. This will
remove the negative charge, but the _positive_ electricity will be
retained on the plate by the attraction of the negative of the
sealing-wax. Now remove the sealing-wax, when the dissimulated charge
will spread itself over the whole insulated metallic portion of the
electroscope, and the leaves will diverge with a strong charge of
_positive_ electricity. If an excited glass tube is brought near the
electroscope, the leaves will now diverge still more; if the sealing-wax
is replaced in its former position, the leaves will collapse. In all
these cases the electrified body parts with none of its own electricity
by developing electrical effects in the neighbouring bodies.
The inductive actions we have described take place through the air,
which is a non-conductor, and such actions may be made to take place
through any other non-conductor. With solid non-conductors, such as
glass, gutta-percha, &c., the inducing body may be brought very near to
the conductor on which it is to act; for the intervening solid
substance, or _dielectric_, as it has been appropriately called, opposes
a resistance to the combination of the opposite electricities, and the
inductive effects are greatly intensified by the approximation. Faraday
discovered that the amount of inductive action with a given charge is
also dependent upon the nature of the dielectric, and that the electric
forces act upon the particles of the dielectric, circumstances which are
of the greatest importance, as we shall presently find, in practical
telegraphy. The most familiar instance of induction is probably well
known to the reader in the Leyden jar, Fig. 255, which is simply a
wide-mouthed bottle of thin glass, covered internally and externally
with tin-foil to within a few inches of the neck. The inner coating
communicates by means of a rod and chain with a brass knob. Such a jar
admits of the accumulation of a larger quantity of electricity than the
conductor of a machine will retain. A very few turns of the machine will
suffice usually to charge the conductor to the fullest extent; but if it
be put in communication with the knob of a jar, a great many more turns
will be required to attain the same charge in the conductor, and the
excess of electricity represented by these additional turns will have
accumulated within the jar—an effect due to the “dissimulated”
electricity of its exterior.
[Illustration:
FIG. 255.—_The Leyden Jar._
]
Everybody knows the result when a metallic communication is established
between the exterior and the interior of a charged Leyden jar. There is
a very bright spark, a snap, and the jar is “discharged.” Everybody
knows, also, the sensation experienced when his body takes the place of
the metallic communication, or forms part of the circuit through which
the communication takes place. Everybody knows that the shock then felt
may also be experienced at the same moment by any number of persons who
join hands, under such conditions that they also form a part of the line
of communication. Such facts irresistibly suggest the notion of
something passing through the whole chain, and this notion is in perfect
harmony with the hypothesis of the “fluids,” for we have only to suppose
that it is one or both of these which rush through the circuit the
instant the line of communication is complete. It was one of Franklin’s
discoveries that the electrical charges of the Leyden jar do not reside
in the metallic coatings; for he made a jar with removable inside and
outside coatings, which, properly taken from the glass, showed no signs
of electrification, yet when replaced the jar was found to be again
highly charged. This would seem to show that the charge clings to, or
penetrates within, the glass.
_DYNAMICAL ELECTRICITY._
Let us take a vessel containing water, to which some sulphuric acid has
been added, Fig. 256, and in the liquid plunge a plate of copper, C, and
a plate of _pure_ zinc, Z, keeping the plates apart from each other. As
it is not easy to obtain zinc perfectly free from admixture of other
metals, an artifice is commonly resorted to for obtaining a surface of
pure metal, by rubbing a plate of the ordinary metal with quicksilver,
which readily dissolves pure zinc, but is without action on the iron and
other metals with which the zinc is contaminated, while the quicksilver
is not acted upon by the diluted acid, but is merely the vehicle by
which the pure zinc is presented to the liquid. Under the conditions we
have described, no action will be perceived, no gas will be given off,
nor will the zinc dissolve in the acid. If the electrical condition of
the portion of the copper-plate which is out of the liquid be examined
by means of a _delicate_ electroscope, it will be found to possess a
very weak charge of _positive_ electricity, and a similar examination of
the zinc plate will show the existence on it of a feeble charge of
_negative_ electricity. If the two plates be made to touch each other,
or if a wire be attached to each plate, as shown in the figure, and the
wires be brought into contact outside of the vessel, an action in the
liquid is immediately perceptible at the surface of the _copper_ plate,
when a multitude of small bubbles of hydrogen gas will at once make
their appearance, and the gas will be given off continuously from the
copper plate so long as there is metallic contact through the wires, or
otherwise, between the two plates, or until the acid is saturated with
zinc—for in this action the zinc is dissolving, and, in consequence,
liberating hydrogen, which strangely makes its appearance, not at the
place where the chemical action really occurs, namely, at the surface of
the zinc which is in contact with the acid, but at the surface of the
copper which is not acted upon by the acid.
[Illustration:
FIG. 256.—_A Voltaic Element._
]
It is known that when we establish a metallic communication between two
bodies charged with equivalent quantities of positive and negative
electricities respectively, these combine and neutralize each other, and
all signs of electricity vanish. It is obvious that the contact of the
two wires has this effect, as the signs of electric charge which were
before discoverable in each of the plates are no longer found while the
wires are in contact. But the charges reappear the instant the contact
is broken, the chemical action ceasing at the same time. If the wire
connecting the two plates outside of the vessel be carefully examined,
it will be found, so long as the chemical action is going on, to be
endowed with new and very remarkable properties. If this wire be
stretched horizontally over a freely suspended magnetic needle, and
parallel to it, the needle will be deflected from its position, and, if
the wire be placed very near it, will point nearly east and west,
instead of north and south. Now, this effect is produced by any part
whatever of the wire, and it instantly ceases if the wire be cut at any
point. These facts at once suggest the idea of its being due to
something flowing through the wire, so long as metallic continuity is
preserved. This idea is much strengthened when we find that the action
of the connecting wire upon the magnetic needle is quite definite—or, in
other words, there are indications which correspond with the notion of
direction. For when the wire, which we shall still suppose to be
stretched horizontally _above_ the needle and parallel to its direction,
is so connected with the plates immersed in the acid that the portion
which approaches the south-pointing pole of the needle proceeds from the
copper plate, while the portion above the north pole is in connection
with the zinc plate, then the north end of the needle will always be
deflected towards the west—whereas, if the connections be made in the
contrary manner, the deflection will be in the opposite direction; and
if the wire be below the needle, the contrary deflections will be
observed with the same connections. The discovery of the action of such
a wire on the magnetic needle was made by Œrsted in 1819, and it is a
discovery remarkable for the wonderful extent of the field which it
opened out, both in the region of pure science and in that of practical
utility.
[Illustration:
FIG. 257.—_Ampère’s Rule._
]
Since by such experiments as those just mentioned the notion of a
_current_ is arrived at, the mind recurs to the fiction of the “fluids,”
and pictures the “positive fluid” as rushing in one direction, and the
“negative fluid” in the other, to seek a re-combination into “neutral
fluid.” But we must never lose sight of the fact that these ideas are
consciously adopted as representative fictions to help our thoughts—just
as John Doe and Richard Roe, imaginary parties to an imaginary lawsuit,
used to be named in legal documents, in order to explain the nature of
the proceedings. Failing, then, to find anything really flowing along
our wire, it is still absolutely necessary, seeing there is something
definite in its action, to assign a direction to the supposed current;
and it has been agreed that we shall represent the current as flowing
from the positively charged body to the negatively charged body—that is,
in the case we have been considering, from the copper to the zinc
through the wire. When this conventional representation has been
adopted, the action on a magnetic needle can easily be defined and
remembered by an artifice proposed by Ampère. In Fig. 257, let N S
represent the magnetized needle, N being the pole which points towards
the north, and S the south pole. Let C be the end of the wire connected
with the copper plate, and Z that connected with the zinc. The current
is therefore supposed to flow in the direction indicated by the arrows
in a wire above the needle and in the wire placed below. Now, suppose
that a man is swimming in the current in the same direction it is
flowing, _and with his face towards the needle, then the north pole of
the needle will always be deflected towards his left_. With the
direction of current represented in the figure, the pole, N, will be
thrown forward from the plane of the paper, or towards the spectator.
The reader who desires to study the mutual action of currents and
magnets will find it necessary to fix this idea in his mind. He will now
be able to see that if the wire be coiled round the needle, as shown by
the lines and arrows, Fig. 257, so that the same current may circulate
in reverse directions above and below the magnet, its effects in
deviating the needle will everywhere concur—that is, the action of each
part will be to turn the north pole towards the left. It is, therefore,
plain that if the wire conveying the current be passed several times
round the magnetic needle, the deflecting force will be increased; and a
current, which would, by merely passing above or below the magnet,
produce no marked deflection, might be made to produce a considerable
effect if carried many times round it. The arrangement for this purpose
is shown in Fig. 258, where it will be perceived that the needle is
surrounded by a coil of wire, so that the current circulates many times
about it, and the effects of each part of the circuit concur in
deflecting the needle. Such an arrangement of the wire and needle
constitutes what is called the _galvanometer_, an instrument used to
discover the existence and direction of electric currents.
[Illustration:
FIG. 258.—_Galvanometer._
]
The arrangement of metals and acid which we have described is termed a
_voltaic couple_, _element_, or _cell_; and a great controversy has long
been carried on among men of science as to the place at which the
development of electricity has its origin. Three-quarters of a century
ago, the effect was attributed by Volta to the mere contact of the two
dissimilar metals. In the experiment we have described this contact,
supposing the wires to be of copper, would occur at the junction of the
wire and the zinc plate. Now, by joining the copper plate of such a cell
to the zinc plate of another cell, the copper of that to the zinc of a
third, and so on, it is evident that the number of dissimilar contacts
might be indefinitely increased, and the electric power should be
proportionately augmented. It is found that this is really the case, but
Volta’s explanation has been opposed by another which regards the
chemical action in the cells as the real origin of the electric
manifestations. This last explanation, supported by many apparently
conclusive experiments of Faraday and others, has been generally
accepted. Galvanic batteries—as a series of cells joined together in a
certain manner are termed—have been constructed, in which there is no
contact of dissimilar metals; and no electric _current_ can be obtained
from an apparatus in which no chemical action takes place. The contact
theory in a modified form has recently been revived by Sir W. Thomson
and others. In this it is now maintained that some _separation_ of
electricities really does take place by contact of dissimilar
_substances_, but that a _current_ can be produced only when this
separation is continually renewed by chemical actions. Be the true
explanation what it may, the fact is undoubted that by joining cell to
cell, we can really obtain vastly more powerful effects. If we take a
single cell, such as that represented in Fig. 256, and connect the
plates with a long and thin wire, we shall find that the current flowing
through each part of the circuit is much weaker than when we connect the
plates with a short and thick wire. In other words, the action in the
latter case, when the wire is stretched over a magnetic needle, will be
more powerful than in the former. By using a long and thin wire the
current may be so weakened that it becomes necessary to surround the
needle with many coils of the wire to produce a marked deflection.
Again, much depends upon the material; thus a copper wire conveys a much
more powerful current than a German silver one of the same dimensions.
There thus appears to be a certain analogy between the flow of
electricity along conductors to that of water through pipes. The longer
and narrower are the pipes, the less is the quantity of water forced
through them by a given head; and similarly, the resistance to the
passage of a current increases with the length and narrowness of the
conducting wire. When all other circumstances are the same, the
_electrical resistance_ of a conductor varies directly as its length and
inversely as its sectional area. Hence the current flowing in the
apparatus represented in Fig. 256 would be increased by making the wire
thicker, and by making it shorter by bringing Z and C nearer together,
and by making the area they expose to the liquid larger; for in the
liquid also the current flows as indicated by the arrow, a fact which
may be proved by the deflection of a magnetized needle suspended above
the vessel. The magnitude of the current depends, then, upon two
opposing forces, namely, that which continuously separates the
electricities, or drives them apart to re-combine through the circuit,
and that which opposes their passage. The former, which is termed the
_electromotive force_, originates, according to some, from the mere
contact of dissimilar materials, according to others from the chemical
action. Now, we may increase the strength of the current in a given
arrangement, either by increasing the electromotive force, or by
diminishing the resistance. The increase of the strength of the current,
produced by merely pouring more acid into the vessel, Fig. 256, is due,
according to the chemical theory, to the former cause; according to the
contact theory, to the latter. By multiplying the cells we increase the
electromotive forces: the current receives, so to speak, an onward shove
in each cell, but with each cell we introduce an additional resistance.
Hence, it follows, that when the resistance of the circuit outside of
the cells is extremely small, the current produced by a single cell is
as powerful as that produced by a thousand. But when the external
resistance is great, as when long thin wires are used, the united
electromotive forces of a number of cells are needed to drive the
current through the circuit. The strength of a current, C, is therefore
expressible by the following simple formula, in which _r_ stands for the
internal resistance, and _e_ for the electromotive force in each cell;
_n_ represents the number of cells in the battery, these being supposed
exactly similar in every respect; R is the sum of the resistances in the
circuit outside of the battery.
_ne_
C = ————————
_nr_ + R
It is easily seen that the smaller R is made, the more nearly does the
strength of the current become independent of the number of cells.
[Illustration:
FIG. 259.—_Daniell’s Cell and Battery._
]
[Illustration:
FIG. 260.—_Grove’s Cell and Battery._
]
But many modifications have been made in the materials and form of the
cells, by which greater power and duration of action have been attained.
Our space permits a description of only two forms, and these must be
described without a discussion of the principles upon which their
increased efficiency depends. Daniell’s constant cell is represented in
Fig. 259, where D is a battery of ten such cells, A is a cylindrical
vessel of copper, C is a tube of porous earthenware, closed at the
bottom, and within it is suspended the solid rod of amalgamated zinc, B.
The copper vessel and the zinc rod are provided with screws by which
wires may be attached. In the copper vessel is placed a saturated
solution of sulphate of copper, and some crystals of the same substance
are placed on the perforated shelf within the vessel. The porous tube is
filled with diluted sulphuric acid. When the battery is in action the
zinc is dissolved by the sulphuric acid, and metallic copper is
continually deposited upon the internal surface of the copper vessel.
Daniell’s battery, in some form or other, is much used for telegraphs
and for electrotyping. Grove’s cell is shown in section in Fig. 260. The
external vessel is made of a rectangular form in glazed earthenware or
glass. It contains a thick plate of amalgamated zinc, A, A, bent
upwards, and between the two portions a flat porous cell, C, C, is
placed, filled with strong nitric acid, in which is immersed a thin
sheet of platinum. The outside vessel is charged with water, mixed with
about ⅛th of sulphuric acid. D represents a battery of four such cells,
in which the mode of connecting the platinum of one to the zinc of the
next may be noticed. The terminal platinum and zinc form the _poles_ of
the battery, and to them the wires are attached which convey the
current. The substitution of plates of coke for the platinum gives the
form of battery known as Bunsen’s, which is also sometimes made with
circular cells. Gover’s and Bunsen’s are much more powerful arrangements
than Daniell’s, but the latter has the advantage as regards the duration
and uniformity of its action.
[Illustration:
FIG. 261.—_Wire ignited by electricity._
]
When the current produced by a battery of a dozen or more such cells is
conveyed by a wire, it is observed that this wire becomes sensibly hot,
and, if the wire be thin enough, the heat may be sufficiently great to
heat the wire to redness. By stretching a piece of platinum wire between
two separate rods which convey the current, as represented in Fig. 261,
the length of wire through which the current passes may be adjusted so
as to give any required amount of light, and the wire may even be heated
to the fusing-point of platinum. This property of electricity has some
interesting applications, as, for example, in firing mines and other
explosive charges, and in some surgical operations. A still more
interesting exhibition of heating and luminous effects is observed when
the terminals of a battery of many cells are connected with two rods of
coke, or gas-retort carbon. When the pointed ends of the rods are
brought into contact, the current passes, and the points begin to glow
with an intensely bright light, and if they are then separated from each
other by an interval of ⅒th of an inch or more, according to the power
of the battery, a luminous arc extends between them, emitting so intense
a light that the unprotected eye can hardly support it. This luminous
arc is called the _voltaic arc_, and it excels all other artificial
lights in brilliancy, a fact due to the extremely high temperature to
which the carbon particles are heated, the temperature being, perhaps,
the highest we can attain. It must not be supposed that in this
brilliant light we see electricity: the light is due to the same cause
as the light of a candle or gas flame, namely, incandescent particles of
solid carbon. These particles are carried from one carbon point to the
other, and it is found that the positive pole rapidly loses its
substance, which is partly deposited on the negative pole. But in order
to obtain a steady light, it is requisite to keep the pieces of carbon
at one invariable distance; and therefore the transference of the
material from one pole to the other, and the loss by combustion, must be
compensated by a slow movement of the carbons towards each other.
Several kinds of apparatus are used for this purpose, but they all
depend upon the principle of regulating the motions by the action of an
electro-magnet, formed by the current itself, which becomes weaker as
the carbons are farther apart. The movement is communicated to the
apparatus by clockwork. Duboscq’s electric lantern is shown in Fig. 262,
with enlarged images of the carbon points projected on a screen. The
mechanism of the regulator is contained within the cylindrical box
immediately below the lantern. The supports of both carbons are moved;
that which bears the positive carbon pole being advanced twice as fast
as the other, and thus the light is maintained at the same level, for
the positive carbon wears away twice as fast as the other. The light is
more brilliant when charcoal is used instead of coke, but then it is
necessary to operate in a vacuum, to avoid the combustion of the
charcoal. The voltaic arc has recently been applied to illuminate
lighthouses, and for other purposes, and will probably soon be more
widely employed, for a cheap and convenient mode of producing a uniform
current of electricity has recently been discovered and will be
presently described.
[Illustration:
FIG. 262.—_Duboscq’s Electric Lantern and Regulator._
]
[Illustration:
FIG. 263.—_Decomposition of Water._
]
The current which is maintained by the chemical action taking place in
the cells of the battery can also be made to do chemical work outside of
the battery. When the poles of the battery are terminated by wires or
plates of platinum, and these are plunged into water acidulated with
sulphuric acid, bubbles of gas are seen to rise rapidly from each wire,
or _electrode_, as it is termed. Fig. 263 shows an arrangement by which
these gases may be collected separately, and examined, by simply placing
over each electrode an inverted glass tube, filled also with the
acidulated water. The gases collect at the tops of the tubes, displacing
the water, and it is found that from the wire connected with the zinc
end of the battery, or negative electrode, hydrogen gas is given off,
while at the positive electrode oxygen gas is liberated, in volume
precisely equal to half that of the hydrogen. This being the proportion
in which these two substances combine to produce water, it appears that
in the passage of the current a certain quantity of water is decomposed;
and the quantity thus decomposed is in reality a measure of the current,
all the other effects of which are found to be proportional to this.
When the electricity in a current is said to be measured, it is simply
the power of the current to deflect a magnet, or the quantity of gas it
can liberate, or some other such effect, which is in fact measured. The
discharge of a Leyden jar through such an apparatus as that represented
in Fig. 263 would present no perceptible decomposition of the water; yet
such a discharge passed through the arms and body produces, as everybody
knows, a painful shock, and is accompanied by a bright spark and a
noise, while the simultaneous contact of the fingers with the positive
and negative poles of the galvanic battery occasions neither shock nor
spark. Thousands of discharges from large jars must be passed through
acidulated water to liberate the amount of gas which a battery current
of a second’s duration will produce. The electricity of the jar is often
spoken about as having a higher _tension_ than that of the battery, but
the latter sets an immensely greater quantity of electricity in motion.
The idea may be illustrated thus: Suppose we have a small cistern of
water placed at a great height, and that this water could fall to the
ground in one mass. The fall of the small quantity from a great height
would be capable of producing very marked instantaneous effects, such as
smashing, as with a blow, any structure upon which it might fall. This
would correspond with the small quantity of electricity which passes in
the discharge of a Leyden jar. Contrast this with the case in which we
allow a very large quantity of water to descend from a very small
height—as when the water of a reservoir is flowing down a gently
inclined channel. It is plain that a different kind of effect might be
produced in this case; the current might be made, for instance, to turn
a water-wheel, which the more forcible impact of the small quantity of
water in the case first supposed would have broken into pieces.
[Illustration:
FIG. 264.—_Electro-plating._
]
It is probable that the apparent decomposition of water by the electric
current is in reality a secondary effect, and that it is the sulphuric
acid which is decomposed. When, instead of acidulated water, we place in
the apparatus a solution of sulphate of copper, it is found that
metallic copper is deposited on the negative electrode, and sulphuric
acid collects at the positive electrode. The metal is deposited in a
firm and coherent state, and the useful applications of this deposition
of metals are of great interest and importance. For, in a similar
manner, gold, silver, lead, zinc, and other metals may be made to form
thin uniform layers over any properly prepared surface. The immense
advantages which the arts have derived from electro-plating illustrate
in a convincing manner the benefits which physical science can confer on
society at large.
The process of electro-plating may be practised by the aid of apparatus
of very simple character. Fig. 264 shows all that is necessary for
obtaining perfect casts in copper of seals, small medals, &c. A A is a
section of a common tumbler; B B is a tube, made by rolling some brown
paper round a ruler, uniting the edge with sealing-wax, and closing the
bottom by a plug of cork, round which the paper may be tied by a string,
or in any other convenient manner. The tumbler contains a solution of
sulphate of copper, and the tube is filled with water, to which about
one-twentieth of its bulk of sulphuric acid has been added. A strip of
_amalgamated_ zinc, or a piece of thick amalgamated zinc wire, is placed
in the tube, and a piece of copper bell-wire is twisted round the top of
it, and has attached to its other extremity, and immersed in the copper
solution, the article which is to be covered with copper. We may suppose
that this is to be a cast in white wax or in plaster of one side of a
medal. The cast is carefully covered with black lead by means of a soft
brush, and the copper wire is inserted in such a manner as to be in
contact with the black lead at some part. When the apparatus has been
left for some hours in the position represented, a deposit of copper
will be found over the blackleaded surface, and it will be a perfect
impression of the wax cast.
Such a copper cast, or any article in copper having a perfectly clean
surface, can be readily covered by a film of silver by means of a
similar arrangement, where a solution of cyanide of potassium, in which
some chloride of silver has been dissolved, is made to take the place of
the sulphate of copper. Electro-plating with the precious metals has
become a commercial industry of great importance; and this process has
completely superseded the old plan of covering the metallic article to
be plated with an amalgam of silver or of gold, and then exposing it to
heat, which volatized the mercury, leaving a thin film of gold or of
silver adhering to the baser metal. On the large scale a battery of
several cells is used for electro-plating, and the articles are immersed
in the metallic solutions as the negative poles of the battery; any
required thickness of deposit being given according to the length of the
time they remain. At the works of Messrs. Elkington, of Birmingham,
these operations are conducted on a grand scale. The liquid there
employed for silvering is a solution of cyanide of silver in cyanide of
potassium, and the positive pole is formed of a plate of silver, which
dissolves in proportion as the metal is deposited on the negative pole.
As the charging of batteries is a troublesome operation, and their
action is liable to variations which affect the strength of the
currents, the more uniform, more convenient, and more economical mode of
producing currents by magneto-electricity, which will presently be
described, has been to a great extent substituted for the voltaic
battery.
[Illustration:
FIG. 265.—_A Current producing a Magnet._
]
The wire conveying a current not only affects a magnetic needle in the
manner already described, but itself possesses magnetic properties, of
which, indeed, its action on the needle is the result and the
indication. If such a wire be plunged into iron filings, it will be
found that the filings are attracted by it: they cling in a layer of
uniform thickness round its whole circumference and along its whole
length, and the moment the connection with the battery is broken they
drop off. This experiment shows that every part of the wire conveying a
current is magnetic, and it may be proved that the action is not
intercepted by the interposition of any non-magnetic material. Thus the
action of the wire upon the magnetic needle takes place equally well
through glass, copper, lead, or wood. Consequently, if we cover the wire
with a layer of gutta-percha, or over-spin it with silk or cotton, we
shall obtain like results on our filings, and if we coil the covered
wire round a bar of iron, while the non-conducting covering of the wire
will compel the current to circulate through all the turns of the coil,
it will not interfere with the magnetic action on each particle of the
bar. Whenever this is done it is found that the iron is converted into a
powerful magnet so long as the current passes. Fig. 265 represents in a
striking manner the result when the current is made to circulate through
numerous convolutions of the wire; and as each turn adds its effect to
that of the rest, magnets of enormous strength may be formed by
sufficiently increasing the number of the turns. The end of the iron bar
is shown projecting from the axis of the coil, and below it is placed a
shallow wooden bowl, containing a number of small iron nails. The
instant the battery connection is completed these nails leap up to the
magnetic pole, and group themselves round it in the manner shown in the
cut; and again, when the current is interrupted, the iron reverts to its
ordinary condition, the magnetism vanishes, and the nails drop down in
an instant. These effects may be produced again and again, as often as
the current flows and is broken. A magnet so produced is called an
_electro-magnet_, to distinguish it from the ordinary permanent steel
magnets. By coiling the conducting wire round a bar of iron which has
been bent into the form of a horse-shoe, very powerful magnets may be
produced, and enormous weights may be supported by the force of the
magnetic attraction so evoked. Fig. 266 represents the apparatus for
experiments of this kind, in which weights exceeding a ton can be
sustained.
[Illustration:
FIG. 266.—_An Electro-magnet._
]
Here, then, we have a striking instance of the subtile agent
electricity, evoked by the contact of a few pieces of zinc with dilute
acid, showing itself capable of exerting an enormous mechanical force.
Engines have been constructed in which this force is turned to account
to produce rotatory motion as a source of power. Such engines have
certain advantages for special purposes; but the money cost for
expenditure of material for power so obtained is, at least, sixty times
greater than in the case of the steam engine. It is, however, in
producing mechanical effects at a distance that the electric current
finds the most interesting practical application of its magnetic
properties. These are the actions which are so extensively utilized in
the construction of telegraphic instruments, of clocks regulated by
electric communication with a standard time-keeper, and of many
ingenious self-registering instruments. The telegraph will be described
in the next article, and we shall also have occasion in subsequent
articles to describe some of the other applications of electro-magnetic
and electro-chemical force.
_INDUCED CURRENTS._
These very remarkable phenomena were discovered by the illustrious
Faraday, in 1830, and this discovery, and that of magneto-electricity,
may be ranked among the most memorable of his many brilliant
contributions to electric science. Let two wires be stretched parallel
and very near to each other, but not in contact. Let the extremities of
one wire, which we shall term A, be connected with a galvanometer (page
415), so that the existence of any current through the wire may be
instantly indicated. Let the two extremities of the other wire, B, be
put into connection with the poles of a battery. The moment the
connection is complete, and the battery current _begins_ to rush through
B, a deflection of the galvanometer needle will be observed, indicating
a current of very short duration through A in the opposite direction to
the battery current through B. This induced current, which is called the
_secondary_ current, does not continue to flow through A: it occurs
merely at the time the _primary_ or battery current is established; and
though the latter continues to flow through the wire, B, no further
effect is produced in the other wire. When, however, the battery
connection is broken, and the primary current ceases to flow, at that
instant there is set up in the wire, A, another momentary secondary
current, but this one is in the _same_ direction as the battery current.
This is termed the _direct secondary_ current, in opposition to the
former, which is called the _inverse_ current.
These effects are much more powerful when, instead of lengths of
straight wire, or single circles of wires, we use two coils of wire, one
of which, namely, that which conveys the primary currents, is placed in
the axis of the other. It must be distinctly understood that the
secondary currents are of momentary duration only; they are not produced
at all while the battery _is flowing_, but only at the time of its
commencement and cessation. If, however, we make the primary coil so
that it can be slid in and out of the axis of the other, then while the
primary current is continuously flowing, we can produce secondary
currents in the other coil, by causing the coils to approach or recede
from each other. As we bring the coils near each other, and slide the
primary into the secondary, the current in the latter is _inverse_; when
the one coil is receding from the other, it is _direct_. These
mechanical actions are not produced without expenditure of force, for
the approaching coils repel each other and the receding coils attract
each other. The setting up of the battery current in the primary coil
when placed within the other is equivalent to bringing it, with the
current flowing, from an immense distance in an extremely small time.
Similarly, when the battery current is broken, it is equivalent to an
instantaneous recession. The effects, therefore, are proportionately
powerful. It is found, also, and this we shall presently refer to more
fully, that when, instead of the primary coil, a magnet is similarly
moved into, or removed from, the axis of the secondary coil, currents in
opposite directions are set up in the latter without any battery being
used at all. The direction of these currents is the same as would be
produced by a primary current that would form, in a piece of iron placed
in the axis of the coil, an electro-magnet with poles similarly situated
to those of the magnet so introduced or withdrawn. Hence, by placing a
bar of soft iron in the axis of the primary coil, the secondary currents
will be produced with increased force. When a long secondary coil,
having the turns of its wire well insulated from each other, surrounds a
primary coil provided with a core of soft iron, or still better, with a
bundle of annealed iron wires, a series of powerful discharges, like
those of a Leyden jar, may be obtained between the terminals of the
secondary coil, when the battery contact is made and broken in rapid
succession.
[Illustration:
FIG. 267.—_Ruhmkorff’s Coil._
]
Such induction coils have been very carefully and skilfully constructed
by Ruhmkorff, and are therefore often called “Ruhmkorff’s Coils.” One of
these is represented in Fig. 267. A B is the coil, and the apparatus is
provided with what is termed a _condenser_, which consists of layers of
tin-foil placed between sheets of thick paper, and alternately connected
so that one set communicates with one extremity of the primary coil, and
the other with the other. This condenser is conveniently contained in
the wooden base of the instrument. Its introduction has greatly
increased the intensity of the secondary current, and sparks of 18 in.
or 20 in. in length have been obtained in the place of very short ones.
It should be stated that of the two secondary currents, only one has
sufficient intensity to traverse the secondary circuit when there is any
break in its continuity. This is the _direct secondary current_, or that
which is produced on breaking the primary circuit. The reason is that
the commencing current in the primary circuit induces in the spires of
its own coil an inverse current, and the battery current therefore
attains its full strength gradually, but still in a very short time;
while, on the cessation of the battery current, the same induction sends
a wave of electricity through the primary coil in the same direction,
and then the current ceases abruptly. Consequently, in the latter case,
the induced electricity of the secondary coil is set in motion in much
less time, and therefore possesses much greater intensity.
The magnetism of the iron core is usually made use of to break and make
the current, by the attraction of a piece of iron attached to a spring,
which, by moving towards the end of the core, separates from a point in
connection with the battery, and, the current no longer flowing, the
magnetism ceases, and the spring again brings back the iron and renews
the contact.
[Illustration:
FIG. 268.—_Discharge through Rarefied Air._
]
By means of such coils many surprising effects have been produced.
Perhaps one of the most beautiful experiments in the whole range of
physical science is made by causing the discharges of the secondary coil
to take place through an exhausted vessel in the manner represented in
Fig. 268. A beautiful light fills the interior of the vessel, and the
terminals appear to glow with a strange radiance—one being surrounded
with a kind of blue halo and another with a red. On reversing the
direction of the currents, which is done by the little apparatus at the
right-hand end of the coil in Fig. 267, the blue and the red radiance
change places. Beautiful flashes of light may also be made to appear in
the vessel, having the most marked resemblance to the streamers of the
Aurora Borealis. When, instead of vessels almost free from common air,
we repeat the experiment with tubes containing an extremely small
residue of some other gas, such as hydrogen, carbonic acid, &c., the
colour of the light and other appearances change Geissler’s tubes have
already been spoken of in connection with the spectroscope; but,
independently of that, the various beautiful appearances which such
tubes have been made to present, by the introduction of fluorescent
substances and other devices, render the induction coil an instrument of
the highest interest to the scientific amateur. Then there are striking
physiological and other effects which the coil is capable of producing.
For instance, we are able by its instrumentality to produce from
atmospheric air unlimited quantities of that singular modification of
oxygen which is called _ozone_. The electricity of the coil has been
used for firing mines, torpedoes and cannons, and for lighting the
gas-burners of large buildings.
[Illustration:
FIG. 268_a_.—_Large Induction Coil at the old Polytechnic Institution,
London._
]
The late Mr. Apps, who was well known as a skilful constructor of
scientific apparatus, devoted much attention to improving the induction
coil, and he made a very large one for the Polytechnic Institution in
Regent Street, London, which Institution was at that time the home of
popular science, under the direction of Mr. Pepper. This coil is
represented in Fig. 268_a_, surrounded by the somewhat scenic
accessories which were then supposed to be required for making science
attractive to the multitude. Externally, the coil appeared as a
cylinder, nearly 5 feet long and 20 inches in diameter. From each end
projected smaller cylinders. All these and also the two upright pillars
upon which the apparatus was supported were covered with ebonite. The
large cylinder contained the primary coil, which was made of copper wire
one-tenth of an inch in diameter and 3,770 yards long, covered with
cotton thread, and making about 6,000 turns round the central core. This
primary coil was inclosed in an ebonite tube ½-inch thick, and outside
of the tube, occupying 4 feet 2 inches of its length, was the secondary
coil, containing 150 miles of silk covered wire, ·015 inch diameter, and
very carefully arranged for insulation, so as to resist the tension of
the electricity when the coil was in action. The condenser contained 750
square feet of tin-foil, and 40 Bunsen cells supplied the current for
the primary coil. The power of this instrument was very great, for it
would give a spark through the air of more than two feet in length, and
the discharge could perforate a certain thickness of glass. It would
charge a battery of Leyden jars having 40 square feet of tin-foil by
only three breaks of contact in the primary circuit, so that the
discharge would deflagrate considerable lengths of wire. The appearance
of the spark, with this, as with other large induction coils, may be
described as a thick line of light, surrounded by a reddish halo of less
brilliancy, and this halo, unlike the line of the spark, had a sensible
duration. The reddish glow might be blown aside by a current of air when
a series of discharges was taking place, and partly separated from the
denser looking line of light. The latter is no doubt formed by intensely
heated particles of the metals between which the discharge takes place,
while the former is probably due to the incandescence of the oxygen and
nitrogen gases in the air. The disc shown in our illustration behind the
coil was for carrying six Geissler tubes, to display the pretty
experiment of the various colours of the luminous discharge in different
attenuated gases. When the coil was first mounted it was provided with
an ordinary contact-breaker, but as the strong sparks were found to very
soon destroy the contact points, a contact-breaker was substituted on
Foucault’s plan. In this, the contacts are made by a platinum tipped
wire dipping into mercury, that occupies the bottom of a strong glass
vessel and forms part of the circuit. The vessel is filled with alcohol,
which is a non-conductor, and it is therefore in the midst of this
liquid that the contacts are made and broken. This apparatus is shown in
the illustration, on the table at the left. A favourite experiment at
the Polytechnic was to connect one of the discharging wires of the coil
with the back of a large looking-glass, and bring the other wire to the
front. In this case the sparks assumed a peculiar appearance, for they
became thin and wiry-looking, and divided into many branches. They were
very bright, and the noise of the discharges, was crackling and quite
different from that produced by the blow of the flaming sparks taken
through the air. Their appearance is represented in Fig. 269. The
effects in this experiment were probably due to the spark taking a path
on the surface of the glass determined by points of moisture or other
inequalities.
[Illustration:
FIG. 269.—_Spark on the Looking-glass._
]
Ruhmkorff’s coil has been of great advantage to the electrician, for it
supplies a stream of _high tension_ electricity like that of the common
machine, but more readily and conveniently. M. Ruhmkorff was the first
person to obtain the great prize of £2,000, which the late Emperor of
the French (Napoleon III.) directed, in 1852, should be awarded every
five years for the most useful application of the voltaic battery. But
no award had been made until 1864, when the inventor of the induction
coil was properly considered worthy of it. This invention was the means
of bringing into notice a new range of interesting phenomena, especially
those attending the discharge passed through highly exhausted vessels.
Investigations into the circumstances which modify the appearances, and
especially into the nature of the _stratified discharge_ in which the
vessels are filled with bands or flakes of light separated by dark
intervals, have long engaged the attention of some of our ablest
physicists. Remarkable results were obtained by Mr. Crookes with very
highly exhausted vessels. These showed not only beautiful fluorescent
luminous effects, but in them the discharge could produce mechanical
actions, and Mr. Crookes was led to regard it as a stream of radiant
matter.
_MAGNETO-ELECTRICITY._
When it had been shown that an electric current was capable of evoking
magnetism, it seemed reasonable to expect that the reverse operation of
obtaining electric currents by means of magnets should be possible.
Faraday succeeded in solving this interesting problem in November, 1831,
and one of his earliest, simplest, and most convincing experiments for
the demonstration of the production of electricity by a magnet is
represented in Fig. 270. A B is a strong horse-shoe magnet, C is a
cylinder of soft iron, round which a few feet of silk-covered copper
wire are wound; one end of the wire terminates in a little copper disc,
and the other end is bent, as shown at D, so that it is in contact with
the disc, but pressing so lightly against it that any abrupt movement of
the bar causes the point of the wire and the disc to separate. When the
bar is allowed to fall upon the poles of the magnet, the separation
occurs, and again when it is suddenly pulled off; and on each occasion a
very small but brilliant spark is observed where the contact of the wire
and disc is broken. It was in allusion to this experiment that a
contributor to “Blackwood’s Magazine” wrote:
Around the magnet, Faraday
Is sure that Volta’s lightnings play;
But how to draw them from the wire?
He took a lesson from the heart;
‘Tis when we meet, ‘tis when we part,
Breaks forth the electric fire.
[Illustration:
FIG. 270.—_Magneto-electric Spark._
]
[Illustration:
FIG. 271.—_A Magnet producing a Current._
]
[Illustration:
FIG. 272.—_Clarke’s Magneto-electric Machine._
]
If a coil of fine insulated wire be passed many times round a hollow
cylinder, open at the ends, and the extremities of the wire connected
with a galvanometer at some distance, then if into the axis of the coil,
A B, Fig. 271, a steel magnet be suddenly introduced, an immediate
deflection of the needle takes place; but after a few oscillations it
returns to its former position. When the magnet is quickly withdrawn,
the needle receives a momentary impulse in the opposite direction. The
magnetization and demagnetization of the iron core in the induction coil
would, therefore, of itself cause the induced currents already
described, for these actions are equivalent to sudden insertion and
withdrawal of a magnet. If we suppose C, in Fig. 271, to represent, not
a magnet, but a piece of soft iron—the reader will remember that this
soft iron can be, as often as required, magnetized and demagnetized by
simply bringing near one end of it the pole of a permanent magnet (see
page 484). Upon this principle many ingenious machines have been
constructed for producing electric currents by the relative motions of
magnets and of soft iron cores surrounded by wires. Clarke’s machine is
shown in Fig. 272. A is a powerful steel magnet fixed to the upright. A
brass spindle passing between the poles can be made to rotate very
rapidly by the multiplying-wheel, E, on which a handle is fixed. There
are two short cylinders of soft iron parallel to the spindle, united
together by the transverse piece of iron, D, which turns with the
spindle. Each bar is surrounded by a great length of insulated copper
wire, and the ends of the wires are so connected with springs which
press against a portion of the spindle, which is here partly formed of a
non-conducting material, that the currents generated in the coils,
although in different directions as they approach a pole and recede from
it, are nevertheless made to flow in one direction in the external
circuit. R R in the figure represent two brass handles, which are
grasped by a person wishing to experience the shocks the machine can
give when the wheel is turned. When the terminals of the coil are
provided with insulating handles and connected with pointed pencils of
charcoal, the electric light can readily be produced by expenditure of
mechanical effort in turning the handle. The arrangement of the points
for this purpose is shown in Fig. 273, and we shall presently see what
advantage has been drawn from this experiment on a great scale as a
source of light.
[Illustration:
FIG. 273.—_Magneto-electric Light._
]
It will be observed that during the revolution of the armatures, as the
wire-covered iron cores are termed, there are two maximum and two
minimum points at which the currents are strongest and weakest. These
variations may be lessened by increasing the number of armatures and of
magnets, and Mr. Holmes arranged a machine with eighty-eight coils and
sixty-six magnets, and the connections were so contrived that the
currents always flowed in the same direction in the external circuit.
This machine required 1¼ horse-power to drive it when the currents were
flowing, but much less when the circuit was interrupted, and it was
designed for, and successfully applied to, the production of the
electric light for lighthouse illumination. Instead of steel magnets
which gradually lose their strength, it is obvious that electro-magnets
might be employed, but this source of electricity is costly,
troublesome, and inconstant. Mr. Wilde hit upon the idea of using a
small magneto-electric machine with permanent steel magnets, to generate
the current for exciting a larger electro-magnet, and the current from
this produced a still more powerful electro-magnet, from which a
magneto-electric current could be collected and applied. The same idea
was subsequently applied in other forms, as by shunting off a portion of
the current produced from the mere residual magnetism of an
electro-magnet, to pass through its own coils and evoke a stronger
magnetism, which again reacts by producing a more powerful current, and
so on continually; the limit being dependent only on the mechanical
force employed, and on the power of the wires to convey the electricity,
for they become very hot, and, unless artificially cooled, the
insulating material would be destroyed. The armatures used in Wilde’s,
Ladd’s, and other machines of this kind, are quite different in
arrangement from those of Clarke’s machine, and are far superior. They
are formed of a long bar of soft iron, of a section like this, Ꮋ, and
the wire is wound longitudinally between the flanges from end to end of
the bar, up one side and down the other. This armature rotates about its
longitudinal axis between the pairs of the poles of a file of horse-shoe
magnets, either permanent, or electro-magnets excited by the
magneto-electric currents. In this case opposite poles are induced along
the edges of the bar, and these poles are reversed at each half-turn.
The intensity of the induced currents increases with the velocity with
which the armature is made to revolve up to a certain point; but because
the magnetization of the soft iron requires a sensible time to be
effected, and the poles are reversed at every half-turn, it is found
that a speed increasing beyond the limit is attended by decrease of the
intensity of the current. The intensity in such machines has, therefore,
a definite limit. But in a modification of the magneto-electric machine,
which has quite recently been invented by M. Gramme, the limit is vastly
extended by the ingenious disposition of the iron core and armatures,
and his machines appear to solve the problem of the cheap production of
steady and powerful electric currents, so that electricity will soon be
applied in processes of manufacture where the cost of electrical power
has hitherto placed it out of the question. We shall now endeavour to
explain the principle on which the Gramme machine depends, and describe
some forms in which it is constructed.
_THE GRAMME MAGNETO-ELECTRIC MACHINE._
[Illustration:
FIG. 274.
]
[Illustration:
FIG. 275.—_Gramme Machine for the Laboratory or Lecture Table._
]
Let X, Fig. 274, be a coil of covered wire; then while a bar magnet, B
A, is advancing towards it and passing through it, as at M, a current
will flow through the coil and along a wire connecting its ends, _s s_.
The current will change its direction as the centre of the magnet is
leaving the coil to advance in the direction, B A. If A A´ be a bar of
soft iron, with the coil fixed upon it, we can still excite currents in
the coil by magnetizing the bar inductively. If the pole of a permanent
magnet be carried along from A´ to M in a direction parallel to the bar,
but not touching it, the part of the bar immediately opposite will be a
pole of opposite name, and the advance of this induced pole towards M
will be attended with a current in the coil, and its recession by an
opposite current. It need hardly be mentioned that the same result is
attained if the magnetic pole is stationary, and the bar with the coil
upon it moved in proximity to it. Now imagine that the bar is bent into
a ring, the ends, A A´, being united. If the ring be made to turn round
its centre in its own plane, and near a magnetic pole, it is plain that
when the coil is approaching this pole a current will be produced in it,
and when it is receding, an opposite current. Let the number of coils be
increased, and each coil in turn will be the seat of a current, or of
the electrical state which tends to produce a current. In Fig. 275 the
reader may see how this disposition is realized. The figure shows a form
of the Gramme Machine adapted for the lecture-table or laboratory. A M´
B M is the soft iron ring, covered with a series of separate coils
placed radially, O is a compound horse-shoe steel magnet, S its south
pole, N its north pole, each pole being armed with a block of soft iron
hollowed into the segment of a circle and almost completely embracing
the circle of coils. The magnetism of each pole is strongly developed in
the interior faces of these armatures. The inductive action tends to
produce two equal and opposite currents, which, like the currents of two
similar voltaic batteries joined by their like poles, neutralize each
other in the connected coils, but flow together through an external
circuit. Fig. 276 will make clear the manner in which the coils, B B,
are placed on the ring, A. The length of wire in each coil is the same,
and the extremities are attached to strips of copper, R R, which are
fixed on the spindle of the machine. The two ends of each wire are
connected with two consecutive strips, while the coils are insulated
from each other, and thus each coil, like the element of a battery,
contributes to the aggregate current. The currents are drawn off, as it
were, from these axial conductors at two opposite points of the ring, by
springs very lightly touching them on each side of the spindle, as may
be seen in Fig. 275. In Fig. 277 is another arrangement of the table
apparatus with the magnet vertical, and formed according to the new plan
suggested by M. Jamin, who finds the best magnets are made by tying
together thin strips of steel.
[Illustration:
FIG. 276.—_Insulated Coils surrounding an Annulus of Iron Wires._
]
[Illustration:
FIG. 277.—_Hand Gramme Machine, with Jamin’s Magnet._
]
But the importance of this invention consists in the facility which it
affords for cheaply producing electricity on a scale adapted for
industrial operations, for the deposition of metals, for artificial
light, and for chemical purposes. The great importance of a cheap
electric light for lighthouses prompted the British Government to permit
the inventor to exhibit the light thus produced from the Clock Tower of
the Houses of Parliament; for the signal light during the sittings of
the House had previously been produced by a gas-light. This electric
light was produced by a powerful Gramme machine, such as that shown in
Fig. 278, driven by a small steam engine in the vaults of the Houses of
Parliament, and the ordinary carbon points, reflectors, &c., were used
in the Clock Tower, where the light was exhibited; copper wire ½ inch
diameter being used to convey the current from the machine to the
carbons. The result of these experiments may be gathered from the
following extract from an official report made by the engineers of the
Trinity House:
“Pursuant to the instructions received from the Deputy Master to furnish
you with my opinion on the relative merits of the electric and gas
lights under trial at the Clock Tower, Westminster, I beg to submit the
following report:—On the evening of the 1st ultimo I was accompanied by
Sir F. Arrow (who kindly undertook to check my observations by his
experience) to the Westminster Palace, where we met Captain Galton,
R.E., Dr. Percy, and some gentlemen connected with the electric and gas
apparatus under trial. I was informed that the stipulations under which
the lights were arranged were, that they be fixed white to illuminate a
sector of the town surface of 180°, having a radius of three miles. I
first examined the Gramme magneto-electric machine, in use for producing
the currents of electricity. This machine we found attached by a leather
driving-belt to the steam engine belonging to the establishment. We then
proceeded to the Clock Tower, where we found the electric lamp, at an
elevation of 250 ft. The Wigham gas apparatus was placed at the same
elevation, within a semi-lantern of twelve sides, about 8½ ft. in
diameter, and 10 ft. 3 in. high in the glazing. Near the centre of the
lantern were three large Wigham burners, each composed of 108 jets.
After the examination of the apparatus, we proceeded to Primrose Hill,
for the purpose of comparing the electric and gas lights at a distance
of three miles. The evening, which was wet and rather misty, was
admirably suited to our purpose, ordinary gas-lights being barely
visible at a distance of one mile.”
The results of a photometric comparison of the electric and gas lights
were as under, the machine making 389 revolutions per minute, and
absorbing 2·66 horse-power; the illuminating power of the gas used being
25 candles, and the quantity consumed 300 cubic ft. per hour.
┌───────────────────────────────────────────────┬──────────┬──────────┐
│ │ Electric │Wigham Gas│
│ │ Light. │ Burner. │
│ │ │108 jets. │
├───────────────────────────────────────────────┼──────────┼──────────┤
│Relative intensity of lights │ 945·56│ 370·56│
│ Or as │ 100 │ 39·19│
│Illuminating power in standard sperm candles as│ 3,066 │ 1,199 │
│ units │ │ │
└───────────────────────────────────────────────┴──────────┴──────────┘
“_Electric Light._—Total cost per session £174 5_s._ 0_d._, being equal
to 5_s._ 7_d._ per hour of exhibition of the light. Details shown in the
full report. Gas Light.—Total cost per session of one burner of 108
jets, £159 15_s._ 3_d._, equal to 5_s._ 1·4_d._ per hour of exhibition
of light, and £296 3_s._ 4_d._, equal to 9_s._ 5·9_d._ per hour of
exhibition of the light, when using three burners of 108 jets each.
Details shown in the full report. It will be observed from the
photometric measurements, before referred to, of the electric light and
108–jet gas burner, that in the case of the electric light we have at
our disposal for distribution over the required area an illuminant
radiating freely in space equal to 3,066 candles; with the gas light we
have an illuminant radiating freely in space equal to 1,199 candles. It
is to be remembered that in dealing with the small electric spark as the
focus of a dioptric apparatus for distribution over the required area,
the light can be more perfectly utilized than with the large gas flame
of the Wigham burner, owing to its very small dimensions as compared
with the latter. The relative cost and efficiency of the three modes of
illumination may be summed up as follows:
┌──────────────────────────────────────────┬────────┬─────────────────┐
│ │ELECTRIC│ GAS. │
│ │ LIGHT. │ │
├──────────────────────────────────────────┼────────┼────────┬────────┤
│ │ │ One │ Three │
│ │ │108–jet │108–jet │
│ │ │Burner. │Burners.│
├──────────────────────────────────────────┼────────┼────────┼────────┤
│Cost of light per hour, in pence │ 67 │ 61·4 │ 113·9 │
│ Or as │ 100 │ 91·6 │ 170 │
│Cost of light per candle per hour in pence│ ·0219 │ ·0512 │ ·0317 │
│ Or as │ 100 │ 233·8 │ 144·7 │
│Cost of light from a dioptric apparatus │ ·00118 │ ·00310 │ ·00275 │
│ for fixed light per standard candle per │ │ │ │
│ hour expressed in pence │ │ │ │
│ Or as │ 100 │ 262·7 │ 233·1 │
└──────────────────────────────────────────┴────────┴────────┴────────┘
“Thus by adopting the electric light as a standard of intensity and
cost, there is shown a superiority over the gas in intensity of 65·2 per
cent. when using one 108–jet burner, and 27·1 per cent. when using three
108–jet burners. There is also shown a saving in cost per candle or unit
of light per hour of 162·7 per cent. when using one 108–jet burner, and
133·1 per cent. when using three of these burners, forming a triform
gas-light. It is further to be remembered that the triform gas-light
actually represents the maximum power obtainable at present by gas; but
no reference has been made to the power of increase capable in the
electric light by the adoption of two magneto-electric machines. By
having the machine and lamp in duplicate, as estimated, and which I
consider a necessity to insure perfect confidence in the regular
exhibition of the electric light, this light can be doubled in intensity
during such evenings as the atmosphere is found to be so thick as to
impair its efficiency. This double power would be obtained at the
trifling additional cost of coals and carbons consumed during the time
this increased power may be found to be necessary; this additional cost
I estimate at 4_d._ per hour. With the arrangement proposed for the
electric light, I consider this powerful illuminant, if manipulated by
careful attendants, perfectly reliable: in proof of this I may state
that the electric light at the Souter Point Lighthouse, on the coast of
Durham, has now been exhibited two years and a half, and the light has
never been known to fail for one minute.”
[Illustration:
FIG. 278.—_Gramme Machine, with Eight Vertical Electro-magnets._
]
[Illustration:
FIG. 279.—_Gramme Machine, with Horizontal Electro-magnets._
]
Fig. 278 represents one of the light-producing machines. The
electro-magnets are excited by a portion of the currents they themselves
produce, they retaining sufficient residual magnetism to develop the
currents. There is a pair of current-collectors on each side. This
machine weighs 1,540 lbs., its height is 3 ft., and width 2 ft. It will
produce a light having the intensity of 500 Carcel lamps, which may be
doubled by increasing the speed. Fig. 279 is another form which is also
adapted for illuminating purposes, and, when made with fewer coils, for
electrotyping purposes also. There are in this also two sets of
current-collectors, and by means of a connecting cylinder (seen at the
base of the machine) the currents can be combined for quantity and for
tension as may be required. This machine is only about 2 ft. square, and
it produces a light equal to 200 burners; but this may be increased, as
the following table shows:
┌─────────────────────┬─────────────────────┬─────────────────────────┐
│Number of revolutions│Intensity of light in│ Remarks. │
│ per minute. │ Carcel Lamps. │ │
├─────────────────────┼─────────────────────┼─────────────────────────┤
│ 650│ 77│No heating and no sparks.│
│ 850│ 125│No heating and no sparks.│
│ 880│ 150│No heating and no sparks.│
│ 900│ 200│No heating and no sparks.│
│ 935│ 250│A little heat, no sparks.│
│ 1,025│ 290│Heat and sparks. │
└─────────────────────┴─────────────────────┴─────────────────────────┘
The value of M. Gramme’s invention for electro-plating is proved by the
fact of its adoption by Messrs. Christofle of Paris, whose
electro-plating establishment is one of the largest in the world. This
firm has no fewer than fourteen of these machines at work, and each is
capable of depositing 74 ozs. of silver per hour. There is little doubt
that the electric current will now soon be employed for reducing metals.
Thus fine copper, which is worth 3_s._ or 4_s._ per lb., may perhaps be
obtained at about the cost of ordinary copper; potassium, sodium, and
aluminium at less than half their present price; and magnesium, calcium,
and other rare metals at prices which will bring them into commercial
use. The machine shown in Fig. 280 is intended for electro-plating and
for general purposes: it supplies the means of readily and cheaply
plating with copper, or with any other metal, such articles as steam
pipes, boiler tubes, ship plates, guns, bolts, nails, marine engines,
machinery, culinary vessels, cisterns, &c. The advantage of protecting
iron or other material from corroding agents is obvious; and as iron
coated with copper is available not only for useful, but also for
artistic, purposes, as a cheap substitute for bronze, this invention
will doubtless lead to a greatly extended application of bronzed iron in
buildings and ornamental structures.
The machine well illustrates how mechanical work may be changed into
electricity, and electricity caused to do work. The power required to
drive the machine at a given speed is much less when no current is being
drawn from it, than when the current is flowing. If the current from one
machine is sent through the armature of another, the latter revolves,
and may be made to do work. Thus _power_ may be conveyed to a distance
by electricity, with only the loss caused by the resistance of the
conducting wires. If, when two machines are thus connected, the
direction of rotation in the first one be suddenly reversed, the
armature of the second will almost immediately stop, and then resume its
motion in the opposite direction. A very interesting experiment can be
performed when the circuit connecting the two machines is made to
include a certain length of platinum wire. When both machines are in
motion, the platinum exhibits no heating effects; but if the second
machine be stopped by an assistant while the rotation of the first is
continued, the wire is raised to a red heat. In this way it is shown
that motion, electricity, and heat are related to each other, and are
mutually convertible; for on the stopping of the second machine, the
electricity being no longer used up, so to speak, in producing motion,
has its power transformed into heat.
The Gramme machine has also been ingeniously employed for railway brakes
on some of the Belgian lines; and it is applicable to telegraphy, where
the cost of zinc, acids, batteries, &c., is a considerable item. It is
impossible to predict the many applications for manufacturing purposes
which will be made of electricity, now a cheap, reliable, and convenient
mode has been discovered of producing currents of any required strength.
Though by no means the first or only machine by which mechanical force
can be converted into dynamical electricity, it shows an immense advance
on any former one in the regularity of the action, and in the capability
of being driven at a very high rate of speed without the inconvenient
accompaniments of the heating of the conductors and destructive sparks
at the movable contacts. There can be no doubt of the importance of this
machine for use in lighthouses, and for metallurgical and chemical
purposes, and the inventor believes the time will come when all large
ocean-going vessels will carry an electric light at the masthead. The
light would be sufficiently powerful to show rocks or land five or six
miles ahead, and an additional safeguard of incalculable value would be
thus provided for those “that go down to the sea in ships, that do
business in great waters.”
[Illustration:
FIG. 280.
]
_ELECTRIC LIGHTING AND ELECTRIC POWER._
[Illustration:
FIG. 280_a_.—_The Alliance Machine._
]
It was mentioned in the last section that the introduction of so
convenient and reliable a means of producing electrical currents as the
Gramme machine, would cause electricity to be largely applied for
illuminating and other purposes. The Gramme machine was first made in
1870, and it attracted much attention, as the principle of combining the
currents was quite different from that used in previous magneto-electric
machines. In fact, the Gramme machine yielded quite unexpected results,
and the principle employed in it opened a new field. The development
that has taken place in the applications of electricity within the
twenty years since 1870 has been truly marvellous. The electric light
appears to have been first used in lighthouses about 1862, and the
machines by which the current was produced were, in principle,
combinations of a great number of Clarke’s machines (see page 509). One
such machine was invented by Mr. Holmes, and was used for the
illumination of the South Foreland Lighthouse in 1862. Another similar
form of still earlier invention had been set up in Paris as early as
1855,—not, indeed, for the purposes of illumination, but for a project
which failed. Its arrangement had been originally suggested by a Belgian
physicist in 1849; and the machine of 1855, having received certain
improvements, afterwards became very well known by the name of the
_Alliance Company’s_ machine, or simply the _Alliance_ machine. It is
represented in its improved form in Fig. 280_a_. Here ranges of steel
horse-shoe magnets will be observed, each magnet weighing about 40 lbs.
and made of six plates of tempered steel, held together with screws.
Each of the eight rows of magnets contains seven, and thus sixteen poles
are presented at uniform distances, arranged in circles. Carried on the
central axle are six discs, which revolve between the circles of sixteen
poles, and on the circumference of each disc are sixteen equidistant
bobbins or coils of insulated wire, so that the whole of the sixteen
coils are opposite to the sixteen poles at the same moment. The
extremities of the wires at the coils are connected with proper
adjustments for gathering up the currents, and by means of these the
coils may be arranged either for tension or for quantity, like the
elements of a battery (page 494).
[Illustration:
FIG. 280_b_.—_Wilde’s Machine._
]
Wilde’s machine, which has been mentioned in page 511, is shown in fig.
280_b_. It will be observed that this consists of a small machine, M,
with permanent steel magnets, and the current from these circulates
through the coils of the electro magnets, A B. The arrangement of the
armatures, bobbin, commutators, etc., is the same in both cases. But as
a speed of 2,500 revolutions per minute was needed, it was necessary to
keep the bearings, T T, from heating by causing cold water to circulate
through them. Mr. Ladd arranged a machine on the same principle as
Wilde’s, by suppressing the permanent magnets, but availing himself of
the _residual_ magnetism of the iron core to bring about the induction.
A machine of this kind was shown at the Paris Exhibition of 1867, and
people were quite astonished to see electrical power capable of
producing a brilliant light developed by a small machine 2 ft. long, 1
ft. wide, and 9 in. high. But the great velocity of rotation, and the
consequent heating of the bearings, left much to be desired before a
really practical machine could be produced.
[Illustration:
FIG. 280_c_.—_Siemens’ Dynamo._
]
In the newest Siemens’ machine, represented in fig. 280_c_, the Gramme
principle is made use of, as the revolving coil is of large diameter,
and it consists of a copper cylinder, on which are wound a number of
juxtaposed coils like those of a galvanometer. The revolving cylinder is
surrounded by the poles of a system of electro-magnets excited by the
whole of the induced current being passed through their coils. In a
paper describing this machine, Siemens first made use of the term
“dynamo-electric machine,” and this expression, contracted to the single
word DYNAMO, has since been universally employed to designate machines
of this kind. The modifications in the forms and arrangements of the
different dynamos that have been invented in late years are endless, and
every week patents are granted for further improvements and fresh
combinations of the parts. It would be quite beyond the scope of this
work to enumerate all the forms of the dynamo that have been favourably
spoken of; but we shall content ourselves by adding a drawing of the
Brush dynamo (Fig. 280_d_), which has been so largely used for electric
lighting in the United States. In this dynamo we have a Gramme ring, but
the number of coils on it is reduced to eight, the intervals being
filled up with pieces of iron, and the ring revolves in a vertical plane
between the poles of two double oblong electro-magnets, which are
arranged with poles of the same name opposite to each other. The
commutators shown in the nearer part convert the alternately reversed
currents generated in the coils into a direct continuous one. They are
formed with bundles of wires, as in the Gramme machine.
[Illustration:
FIG. 280_d._—_The Brush Dynamo._
]
[Illustration:
FIG. 280_e._—_Siemens’ Regulator._
]
But the providing of a cheap and efficient source of current
electricity, although an absolutely necessary step, would not have been
capable of bringing about the present development of electric lighting,
unless the appliances by which the current is made to manifest itself as
light had not also been brought nearly to perfection. The conditions
required to maintain a steady light from a current of electricity
passing between carbon points have been already explained on page 497,
and a representation of Dubosc’s electric lantern and regulator is
shown. The regulator systems that have been invented since it became
obvious that the light of the electric arc admitted of practical
application on the large scale are very numerous. The earlier forms of
regulator, which were used only for scientific purposes—such as lantern
projections on screens, experiments on light, etc.—were complicated in
their arrangements and uncertain in their action, for great variations
in the light sometimes took place, and occasionally it would, indeed, be
extinguished, and then again shine out as brightly as before. Nearly all
the regulators that have come into use depend upon movements controlled
by electro-magnetic actions produced automatically as the distance
between the carbon changes. It would, however, lead us too far into the
technicalities of the subject to explain minutely the mechanism of any
particular form of the mechanical regulators, and the results depend so
often upon the minute details, that it would be difficult to trace the
action without a set of large and complete drawings. Perhaps the
regulators that have been most used are those of Serrin, Siemens, Brush,
Thomson, Houston and Edison. But nearly every inventor has produced
different forms of his apparatus; Siemens, for instance, has patented
eight or ten regulators. Fig. 280_e_ shows the mechanism of one of the
last named inventor’s regulators, in which the two actions required for
the separation and approach of the carbons are determined respectively
by the vibrations of the rocking lever, M Y L, actuated by the
electro-magnet, E, and the simple weight of the upper carbon-holder, A
A. When the lamp is not in circuit, the lever, L, is thrown back by a
spring, the tension of which is regulated by the screw, R, so that the
catch, Q, is disengaged from the wheel, I. The train of wheels is then
free to revolve by action of the rack, A, supporting the weight of the
upper carbon, until the motion stops by the carbons touching each other.
Now let the lamp be connected up, and the current will pass from C,
through the electro-magnet, the mass of the apparatus, and return by the
wire connecting the lower carbon-holder with Z. The carbon points will
glow, but the magnet then attracting M moves the lever, L, the piece, Q,
engages the wheel I, pushing it one tooth forward. But this movement of
the lever establishes a contact at X, so that the current abandons the
electro-magnet, to pass the shorter way, and M being no longer
attracted, the lever is pushed back by the spring, the contact at X is
broken, and the magnet being again excited the lever turns as before,
and Q pushes I round the space of another tooth. These alternating
actions succeed each other with great rapidity, and effect the
separation of the carbons through the train of wheels acting on the
racks. These movements continue until, in a second or two, the
separation of the carbons has become so great, that the current passing
through the electro-magnet is no longer able to operate against the
weight of the upper carbon-holder, and this happens when an arc of
proper size is produced, this required result being brought about by
proper adjustment of the parts of the apparatus, marked by the letters
R, K and X. But as the carbons are consumed, the increase of the length
of this arc further weakens the current, until the spring attached to
the lever, L, prevails over the attractive force of the electro-magnet
on M, and thus withdraws the catch, Q, altogether, when the wheels being
free to turn, the weight operates to bring the carbons nearer together,
until, with the lessened resistance, the energy of the current is
restored, and Q again comes into play to arrest the approximating
movement. It may be seen, from the above explanation, that this lamp is
automatic; in other words, when it has once been properly adjusted, it
is lighted by merely completing the circuit. For fixing the carbons
properly in their holders there are, of course, other regulating screws.
How very nearly perfection the automatic regulation of the arc electric
lamp has been brought by such contrivances as these, will be obvious to
all who have noticed the steadiness that has been attained in all the
modern installations.
[Illustration:
FIG. 280_f_.—_Jablochkoff Candle._
]
An ingenious plan was devised by Jablochkoff for dispensing with all
mechanism for regulating the distance of the carbons. This invention is
known as the _electric candle_, and is of great interest from the fact
that it was with this arrangement that the electric light was, for the
first time, practically employed for street and theatre illumination.
This was in 1878, when visitors to Paris, during the Exhibition, were
astonished by the splendid displays in the Avenue de l’Opéra, at the
shops of the Louvre, and at some of the theatres. Then it was shown, for
the first time, that electric lighting was not merely a scientific
curiosity, but a new and formidable rival to gas. The Jablochkoff
candles were also subsequently used in the electric lamps on the Thames
Embankment. The principle of the contrivance will be understood from
fig. 280_f_. Two carbons, C and D, are placed parallel at a little
distance apart, and the space between them is filled up with plaster of
Paris, kaolin, or some similar material, through which the current will
not pass, but which burns, fuses, volatilises, or crumbles away by the
heat produced by the passage of the current between the two carbons.
These carbons are, of course, fixed in insulated holders, and to start
the candle a small tip of carbon paste is made to connect the carbons at
the top. The Jablochkoff candles must be used with currents rapidly
alternating in direction. The reason for this is, that otherwise one of
the carbons (the positive one) would be consumed quicker than the other,
and that would cause the distance between them to increase, until it
became so great that the current would cease to pass, and the light
would go out. In order to obtain such alternating currents with the
Gramme machine, a special apparatus had to be devised to change its
direct into alternately reversed currents; but, dynamos intended to
supply electric lights are now made without commutators, and they supply
rapidly succeeding currents in opposite directions. In certain types of
dynamos, again, the armature coils are stationary, and it is the field
magnets that are made to revolve, and in these cases, not even a sliding
contact is required, but the end of the armature coils are directly and
permanently connected with the main circuit. But as these dynamos are
self-exciting, the electricity induced in a few of the armature coils is
collected apart from the main circuit, and passed through the
electro-magnets of the machine itself, after the alternate currents
have, by means of a commutator, been converted into one direct
continuous current.
[Illustration:
FIG. 280_g_.—_Electric Lamp._
]
The arc electric light, as used for the illumination of streets and
public places, is too intense and concentrated to be pleasant to the
eye, and therefore it has been found necessary to surround it by globes
of enamelled glass, or of porcelain, or of ground glass, or of frosted
glass. By these expedients for diffusing and softening the light, it is
rendered much more acceptable, but this advantage is gained at the cost
of a considerable loss of the whole illuminating power, a loss which is,
probably, never less than 10 per cent., but is usually much greater. The
globes used in Paris, with the Jablochkoff candles, were of enamelled
glass, and the apparatus was arranged, as shown in Fig. 280_g_, where it
is partly represented in section, and with a part of the globe broken
off, in order to show one of the candles placed in the holder which
connects it with the circuit. In each lamp several candles were mounted,
in some cases four; but the lamps in the Place de l’Opéra held twelve.
At first there were mechanical arrangements, automatic and otherwise, by
which, when the candle was burned down the current could be turned on to
another. But M. Jablochkoff afterwards discovered that there was really
no need for such a mechanism. For when the whole of the candles are
simultaneously and equally connected with the circuit conductors, it is
found that one of them will more easily transmit the current than any
one of the rest, and when that particular one has once been lighted by
the heat developed, the current will pass almost entirely through the
arc, any loss through the connecting strip of carbon, at the tops of the
other candles, being quite insignificant. When the first of the candles
has burnt down completely, until the insulating porcelain holder
separates the carbons, the current will at once re-establish itself at
the top of one of the remaining carbons, and so on, while one is left.
The arc electric light has not been brought to its present position
without the expenditure of much care and ingenuity in the preparation of
the carbons used for its production. When Davy first produced the
voltaic arc, the electrodes he used were simply sticks of charcoal.
These were very quickly consumed, and a more durable form of carbon was
sought for. This was found by Foucault, who made use of rods sawn out of
the carbonaceous residue left in the retorts in the process of making
coal-gas. This substance was, however, by no means uniform or
sufficiently pure, and the light obtained was consequently unsteady.
Many experiments were made in preparing special carbons. Pounded coke,
coke and charcoal, were mixed with syrup or tar into a paste, which was
moulded and compressed, and then the sticks were kept in covered vessels
at a high temperature for many hours. Acids were used for purification,
and also alkalis, to remove silica. At the present time there are
several manufacturers of electric light carbons who carry on extensive
operations by processes which probably are very similar one to another,
and which may well be represented by M. Carré’s, whose carbons have the
highest reputation. M. Carré prefers a mixture of powdered coke,
calcined lampblack, and a syrup made of sugar and gum. The whole is well
mixed and incorporated, water being added from time to time to make up
for loss by evaporation, and to give the paste the proper degree of
consistence. The paste is then subjected to compression, by which it is
forced through draw-holes, and the carbons, having been piled up in
covered crucibles, are exposed for a certain time to a high temperature.
As a practical illuminant for lighthouses, the arc electric light came
into use many years ago (1862) as we have already seen. This was when
the generator of the current was the magneto-electro machine; but, now,
when this generator has developed into the modern dynamo, the cost of
the electric supply has been enormously reduced, so that, power for
power, electric lights may be worked at half the former cost, and with
greater convenience and certainty. Light for light, electrical
illumination is said to be far cheaper than gas. Again, the arc electric
light has properties which have caused it to be employed, not only in
every important lighthouse in England, France, Russia, America, and
elsewhere, but most ships of war are provided with means of projecting a
beam of electric light in any direction, in order that the presence of
torpedo boats, etc., may be discovered at night, or harbours entered and
signals made under circumstances when such operations would be otherwise
impossible. It was by the use of the electric light that, in 1886, one
of the Peninsular and Oriental Company’s steamers passed safely through
the Suez Canal, at night, and the experiment was so satisfactory, that
the canal authorities placed beacons and light-buoys to guide such
vessels, as, being provided with electric apparatus, were enabled to
hold their proper course between its banks. The use of projected beams
for watching the movements of enemies, and for signalling to great
distances in time of war, has been recognized by all the great military
powers. The advantage of the electrical light in some mines, in
subterranean and submarine operations and generally, in work that has to
be carried on at night by large bodies of men, is constantly finding
illustration. Few readers are unacquainted with the brilliant effect of
the arc lamps in exhibitions, parks, &c.; at out of door fêtes, or
applied to the illumination of fountains, such as those at the Paris
Exhibition of 1889.
The arc lamps are used in series; that is, where there are a certain
number of lamps to be supplied, the same electrical current circulates
through the whole of them, and this, of course, must have force enough
to overcome the resistance of the whole circuit. Thus, at each lamp, the
intensity of the illumination must necessarily be very great. A solution
was long sought to the problem of so dividing the current energy, that
it might be made to produce lights, of moderate intensity, at a greater
number of points. When Mr. Edison, shortly after having invented the
phonograph, announced that he had solved the problem of the electric
light division, there was a great panic amongst the holders of shares in
gas companies, and a heavy fall in this kind of stock immediately
occurred. As it turned out, the alarm was unnecessary, for gas was not
to be superseded, immediately and definitely, by electricity.
Nevertheless, it is by virtue of the principle that was contained in
Edison’s invention, that electric lighting has assumed the wide-spread
importance it has at the present day, and that it is now actually
ousting gas as an illuminant in the business and domestic premises of
our large towns, and in theatres, libraries, and other places of resort.
The principle which has brought about this great development of electric
illumination is that shown in a simple form in Fig. 261. It appears,
however, that as early as 1841, a platinum wire, made incandescent with
a battery current, was proposed as a source of light, and in 1845,
carbon was used in the form of slender rods, by King, and also by J. W.
Starr, in the United States. Both inventors inclosed their carbons in
glass tubes, from which the air was exhausted, so that the carbon might
not burn away. In the following year, Greener and Staite turned their
attention to lamps of this kind, and, again, in 1849, Petrie worked on
the same subject. After that, the problem ceased to engage attention,
until, in 1873, a Russian man of science, named Lodyguine, took the
matter up and patented a carbon incandescent lamp, which did not,
however, prove a practical success, and although the idea was worked out
in various ways by Konn, Reynier, Trouvé, and others, the apparatus they
designed was, in every case, lacking in simplicity, and certainty of
action. The Edison incandescent lamp, the announcement of the discovery
of which so fluttered the gas companies, about 1878, was a reversion to
the plan of an incandescent metallic wire. This wire was made of an
alloy of platinum and iridium, which was adopted by Edison on account of
the very high temperature required for its fusion. And in order to
prevent the temperature from quite reaching that point, the wire was
arranged in a spiral within which was a rod of metal that, by its
dilatation with a certain temperature, caused a contact to be made which
diverted part of the current through a shorter circuit, and thus lowered
the temperature of the spiral to within the assigned limits. But the
advantages presented by carbon over metallic conductors led Edison to
attempt the formation of filaments by charring first slips of paper,
afterwards slips of bamboo. About the same time Mr. J. W. Swan, of
Newcastle-on-Tyne, was experimenting in the same direction, and, in the
latter part of the year 1880, he exhibited the first incandescent lamps
shown in England. Swan’s carbon filaments were prepared from cotton
threads which had previously been steeped in dilute sulphuric acid,
washed, and passed through draw holes to give them an uniform section.
They are thus made perfectly homogeneous throughout, and, after having
been wound on pieces of earthenware to the required shape, they are
carbonized by packing in powdered charcoal and heating. These filaments
are very thin, but solid and elastic. The arrangement of the lamp (see
Fig. 280_h_) is extremely simple: the filament of carbon bent into a
horse-shoe form, or turned so as to form a loop, is inclosed in a glass
bulb of a globular or egg shape, about two inches in diameter. The
extremities of the filament are connected in an ingenious manner to two
platinum wires that pass outward through the glass into which they are
fused, and terminate either in binding screws or in two small loops. The
bulb is exhausted first by an ordinary air-pump, and then by a Sprengel
mercurial pump, the current of electricity being sent through the
filament during the last stages of the process, and finally the bulb is
hermetically sealed. The light yielded by these lamps is mild and
steady, and its intensity depends on the electric current sent through
them; but this may, it is said, be carried as high as to make the light
equal to that of twenty candles. Each horse power of force expended on
the dynamo suffices to maintain ten of these lamps. At the Exhibition of
Electrical Apparatus at Paris in 1881, the Swan lamp received the gold
medal as being the best system in its class. The Swan and the Edison
patents are now worked together by one Company, and the productions of
this Company are very largely used, although there are several more or
less modified systems of glow lamps prepared by other manufacturers.
[Illustration:
FIG. 280_h_.
]
The great advantages offered by electric glow lamps over gas-lights
caused them to be speedily adopted by the most enterprising managers of
theatres and places of amusement. Mr. D’Oyly Carte had the Savoy
Theatre, in London, completely fitted up with these lamps in 1881. The
light was soft and agreeable, it did away with the risks of fire both
for the audience and the performers: for the footlights and scene-lights
were also electric glow lamps, and the coolness of the house and greater
purity of the air were at once appreciated. Several other London
theatres have since adopted the incandescent electric lamps, and it is
obvious that the system will become universal. In all ocean-going
passenger steamers, electric lighting of the saloons and cabins is now
the rule. No mode of illumination so readily adapts itself to the
production of artistic and decorative effects as the glow lamps: for the
covering glasses may be tinted of any required shade, and the lights may
be placed in any position. Small glow lamps are occasionally used as
personal adornments, when placed, for instance, as part of a lady’s
head-dress amidst diamonds, a novel effect of great brilliancy is
produced. It need hardly be said that in this application the wearer is
not required to carry a dynamo about with her, for the electricity is
supplied in a manner much more convenient for this purpose by a device
presently to be described. For several years electric incandescent
lamps, supplied by the like means, have been in action every night in
the carriages of the trains running between London and Brighton, and
more recently the Company have had electric reading lamps of five
candle-power fitted up in the carriages of the main line trains. They
are placed at the backs of the seats just above the passengers’ head.
When anyone wishes to make use of one of these lamps, he places a penny
in a slot, and then, on pressing a knob, the light appears, and at the
end of half an hour it is automatically extinguished; but, of course, it
can again be made to appear by another penny dropped in the slot, and so
on every half-hour as long as may be required.
To maintain the electric light (whether arc or incandescent) quite
steady, the greatest uniformity in the speed of the dynamo is essential;
and if the prime mover by which it is worked, whether steam-engine,
gas-engine, water-wheel, or turbine, is not perfectly regular in its
action, the lights will fluctuate in brightness, and thus produce an
effect which is very unpleasant. This is entirely obviated by the
adjunct we have now to describe, which not only is most efficient as a
regulator, but is, moreover, of still more importance by also providing
the means of storing up the electrical energy in a portable form. The
reader will have understood that in a voltaic cell the production of an
electric current is the concomitant of a chemical union of substances
within the cell (p. 493). Now, in the experiment shown in Fig. 263 (p.
498), it is the reverse of combination—namely, the decomposition of the
water that is supposed to be effected under the influence of the current
from a galvanic battery, and the poles are so connected that the
direction of the current in the liquid while the decomposition is
proceeding is from the wire in the O tube to that in the H tube. If the
experiment be interrupted by removing the battery, and then putting a
galvanometer (Fig. 258) in its place, the galvanometer will immediately
indicate a current passing through the apparatus in a direction the
_reverse_ of the former one—that is, in the liquid it goes from H to O,
and the volumes of the gases will slowly diminish while water is
reproduced by imperceptible and gradual re-combination. Batteries can be
made by joining up a series of arrangements like Fig. 263, consisting of
nothing but strips of platinum surrounded by hydrogen and oxygen gases
and the intervening acidified water. Analogous results are obtainable by
cells containing other compounds with suitable metallic poles, for when
decomposition has been effected through a series of such cells by a
sufficiently powerful current from a _primary_ battery, the series of
cells will constitute, on removal of the primary battery, a _secondary_
battery, for when the terminals of this are joined, the current will
flow in the reversed direction while the separated parts of the original
compounds are re-combining within the cells. These _secondary_ batteries
are called also _polarisation_ batteries. A form of secondary battery
was contrived some years ago (1859) by M. Gaston Planté, in which the
current of the primary battery was made to act on plates of lead
immersed in dilute sulphuric acid. The effect was to coat one of the
lead plates of each pair with lead oxide; and in the action of the
secondary battery this was reversed, and the plates gradually returned
to their original condition, when, of course, the current ceased. Some
improvements were made in the Planté battery by Faure, who coated one of
a pair of very thin lead plates at once with a film of red oxide of
lead, and used a layer of felt to separate it from the other plate. Such
arrangements have been called “accumulators”; another term applied to
them is “storage batteries”; but it is not to be supposed that in them
electricity is stored or, so to speak, bottled up. They consist merely
of such an arrangement of materials as that when a current (_direct_,
not alternating) from a dynamo is passing, certain substances are placed
in a position of chemical separation in such a manner that in
re-combining an equable current of electricity is produced in the
conductor externally uniting them. We need not notice some slight
modifications of the Faure cells that have been lately introduced, as no
new principle is involved. The light of incandescent lamps worked by the
Faure accumulator is perfectly free from the fluctuations which may
usually be noticed when the lamps are directly connected with the dynamo
only. Even if the engine should stop altogether, the light may be
maintained for hours. The accumulator has also the advantage of giving
out the electric energy that may have been imparted to it days before;
so that when a house is fitted up with an independent electric light
installation, there is no necessity for running the dynamo all the time
the lamps are in use, as two or three days weekly may suffice to charge
all the accumulators. Then there is the portability of the accumulator,
which permits electrical energy to be made use of in situations where
dynamos and prime movers would be impossible. It is said that a large
Faure cell weighing about 140 lbs. can receive and give out energy equal
to one horse power for one hour. In the arrangement for the reading
lamps in railway carriages referred to above, accumulators are placed
under the seats; and it need hardly be said that when the electric light
has been seen in a _coiffure_, a small Faure cell concealed about the
wearer’s person has supplied the current. A very interesting and useful
application of the accumulator is the portable electric light lamp for
miners made by the Edison-Swan Company. It is simply an incandescent
lamp protected by a strong glass cover attached to the side of a
cylindrical case containing a four-celled accumulator. This lamp is
provided with an ingenious contrivance by which the circuit would be
interrupted, if by accident the outer glass cover of the lamp were
broken. Let us now see what another new development of the applications
of electricity gains by the use of accumulators by turning our attention
to the _electro-motor_.
At the Vienna Exhibition of 1873, the Gramme Company showed two of their
machines, and it is said that when one of these machines was at rest, a
workman connected the ends of two covered copper wires with the other
machine, thinking that these were placed to carry the current from that
machine when in movement. Everybody was surprised when, without any
power from the machinery, the ring was soon in rapid rotation. These
wires were in fact joined up to the other Gramme machine which was
already in action, and it was the current from this that set the former
in motion. There is no reason why this story should not be perfectly
true, although there are good reasons for believing that the
_electro-motor_ was the result of no such accidental circumstance. The
attractions and repulsions between the poles of electro-magnets was soon
seen to supply an available source of motive power, and the subject has
been already mentioned on page 518. Professor Jacobi, of St. Petersburg,
seems to have been the first who constructed an electro-magnetic engine,
the exciting power being the current supplied by a voltaic battery. This
was in 1834, and in a few years afterwards the Professor applied his
engine to a small paddle-wheel boat, 28 feet long, which was
electrically propelled for several days, but at a slow speed. The engine
in this case was virtually a magneto-electric machine worked backwards,
that is, instead of applying power to turn the machine and so produce a
current of electricity, the current was supplied by the battery and
produced power. In 1850, an electro-motor of five horse power was shown
by an American, Mr. Page, the principle of which may be illustrated by
supposing a reversal of the action represented in Fig. 271, thus: if,
instead of producing currents by moving the magnet, C, in and out of the
coil, A B, we substitute a battery for D, we can, by alternating the
direction of the current through the coil, cause a reciprocating motion
of the magnet, C, and this again may be described as a magneto-electric
machine worked backward. It was soon recognized that no practical
electro-motor was adequate to the production of such high powers as the
steam engine supplies, and that the cost must necessarily many times
exceed that of steam power. But certain advantages, nevertheless,
pertained to the electro-motor in certain positions, as instance in
safety, and where a small force only was occasionally required. Now,
when the Gramme machine was invented to supply currents of electricity
under conditions much more favourable than the magneto-electric machines
it superseded, and at a cost vastly less than that of any voltaic
battery, it is highly improbable that the relation of the new current
generator to the production of electro-motive power would long be
overlooked.
The electro-motor may, therefore, be considered simply as a dynamo
worked backward, and almost any form of dynamo may in this way be used
as an electro-motor, that is, a current being supplied either from a
battery or from a dynamo, the motor converts the electrical energy into
mechanical energy. Any dynamo that supplies a direct and continuous
current can thus be used; but there are certain conditions which make it
desirable to somewhat modify the proportions and arrangement of the
several parts when the machine is for motor purposes.
In general, any source of current may be used, but in the applications
of the electro-motor there are chiefly two methods in practice of
supplying the current. The one takes the current from a dynamo in
motion, the other from an accumulator which has previously been
“charged” by a dynamo.
Both of these methods are used in the familiar and interesting
application of the electro-motor to the propulsion of carriages on
tramways and railways. For the latter, indeed, an attempt was made
half-a-century ago on the Edinburgh and Glasgow railway, to employ the
force of an electro-magnetic machine actuated by a battery. This was in
1842, and although this electric locomotive was fitted up completely, it
did not attain a speed of more than four miles an hour. The weight with
the batteries, carriage, etc., exceeded five tons. But in the recent
inventions which have been in practical operation in many places, it is
found quite easy to dispense with any current producer on the electric
locomotive itself, for the electricity is supplied by a fixed dynamo and
the current is transmitted along the line by a conductor from which a
sliding contact conveys it to the electro-motor, which is attached to
the framework of the carriage and acts on the driving axles of the
wheels directly or by toothed gear. In such cases the return current is
carried either by another conductor or by the rails themselves. In
another arrangement one rail conveys the current to the locomotive and
the other returns it. When the rails are so used they have, of course,
to be insulated from the ground and laid with special electrical contact
pieces joining their consecutive lengths, and all the carriage wheels
have to be insulated, so that the currents shall flow only through the
coils of the electro-motor. A railway on this system has been worked at
Berlin for some time, and a short tramway on the same plan has lately
been opened at Brighton. The Bessborough and Newry Electric Railway
(Ireland) uses a single separate conductor three miles long, and the
power is supplied at a very small cost from a dynamo station near the
middle of the line, where water power is taken advantage of to drive a
large turbine. Quite recently electric propulsion has been adopted on
some of the short tunnel lines in London, and it is quite probable that
ultimately the system will be adopted throughout the whole course of the
underground railways, with the view of obtaining a purer and more
agreeable atmosphere.
[Illustration:
FIG. 280_i_.—_Poles with Single Arms for Suburban, Roads.—The Ontario
Beach Railway, Rochester, N. Y._
]
A very light electric railway has been designed, in which the cars run
along rails attached to posts at such a height above the ground as may
be required to make the line level, or with only slight gradients. The
rails also serve as conductors. This is known as the telepherage system,
and it is found to be well adapted for light loads in an undulating
country.
[Illustration:
FIG. 280_j_.—_The Glynde Telepherage Line, on the system of the late
Fleeming Jenkin._
]
The other plan which makes use of accumulators commends itself for
application to ordinary tramway carriages, because no conductors are
required along the line, and each car can move independently. The chief
objection is the great weight of the accumulators and the space they
occupy, although they are usually placed under the seats without much
inconvenience. There are at present (January, 1890) six electric
tramcars running in London, and the accumulator system would no doubt
have been applied largely as the motive power for the ordinary street
omnibus, but for the difficulty of controlling them under the momentum
of the great mass of the accumulators, etc. The same objection lies
against the use of the accumulators and motors for propelling tricycles,
although such machines have really been used. But accidents such as
occasionally happen to such vehicles would be attended with additional
risks of injury from the acids of the secondary battery, etc. But there
is one mode of using electric propulsion, that is free from every
objection and, indeed, offers great advantages. Only two years ago the
first electric boat on the Thames was tried experimentally between
Richmond and Henley, and the result was entirely in favour of the
electric over the steam launch. The Faure battery, or so-called “storage
cells,” are arranged beneath the floor of the boat for most of its
length in the smaller boats, and the electro-motor is directly coupled
with the screw shaft. The electric launch has these advantages: perfect
safety, freedom from dirt and smoke, no thumping or vibrating, no noise
of steam discharge, or smell of hot oil, no engineer or stoker is
required, and much larger space available for passengers. One of these
electric launches, not going full speed, is able to travel sixty miles
without having the accumulators recharged. A considerable number of
these launches are already in use, and many more are in course of
construction. They are made of all sizes, from the smallest to those
that will carry quite a large company, and may be used for excursion
parties on the river. The description of one of these last states that
she is 65 feet in length, and 10 feet across the beam. She can carry
sixty passengers, and twenty can dine in the saloon at one time. There
are lavatories, pantries, dressing rooms, etc., and a brass railed upper
deck, with an awning. At night this boat is lighted up with electric
glow-lamps, the current for these also being supplied by the
accumulators. The Electric Launch Company has stations with Gramme
machines at work to charge cells ready to replace exhausted ones at
several places, namely Hampton, Staines, Maidenhead, Boulter’s Lock,
Henley, Reading and Oxford. There is every prospect of a general
extension of the electric propulsion of boats, and visitors to the
Electrical Exhibition at Edinburgh, in 1890, will find electric launches
taking holiday makers as far as Linlithgow. The boats will be like those
on the Thames, fitted with the Immisch motor. Some electricians are now
sanguine enough to believe that even for large vessels electricity will
yet be able to compete with steam in special cases.
The modes of using electric propulsion that we have just noticed furnish
a very interesting chain of conversions of one form of force into
another, with a reversal of the order of transformation at a certain
point. Let us begin with the carbonic acid gas that existed in the
atmosphere of the carboniferous geological period. The solar emanations
were absorbed, and used by the leaves of the plants to separate the two
elements of the gas,—the plant retaining the one in its substance and
returning the other to the air. The plant becomes coal; and ages
afterwards the particles of the two separated elements are ready to
re-unite and give out in the form of heat all the energy that was
absorbed by their separation. This heat is in the steam-engine converted
into the energy of mechanical power. This mechanical power is in the
dynamo expended in moving copper wires through a magnetic field. Every
schoolboy who has played with a common steel magnet—and what boy has
not?—knows that the space immediately round the magnet is the seat of
strange attractive and repulsive force, for he has felt their pulls and
pushes on pieces of iron or steel. This mysterious space is the magnetic
field, and although a person would not be able to perceive that
mechanical force is expended when he moves a single copper ring across
such a field, he will readily become conscious of the fact when he moves
a number at once that form a closed circuit; and he should not omit the
opportunity of feeling this for himself if he is allowed to turn the
handle of such a machine as that represented in Figs. 275 or 277. The
mechanical power is absorbed in the dynamo because the movement induces
an electric current that would of itself produce motion in the machine
in the opposite direction. However, the electricity induced by magnetism
and motion is made to pass through the Faure cell or accumulator, when
it does chemical work by separating oxide of lead from sulphuric acid,
leaving these substances in a position to unite together again, when
this action produces a reverse current of electricity through an
external metal circuit. The coils of the electro-motor form this
circuit; the electricity induces magnetism, and the magnetism gives rise
to visible motion and mechanical power.
From what has been already said, it will be obvious that a pair of
covered copper wires connecting a dynamo with an electro-motor becomes a
very convenient means of _carrying power_ from one place to another.
There are situations in which shafts, belts, or any other mechanical
expedients are troublesome or impossible to use for this purpose. For
instance, a dynamo working at the mouth of a tunnel or coal-pit may be
made to drive any machinery within with nothing between but the
motionless wires. Or a single dynamo will supply moderate power to a
number of small workshops, provided each has an electro-motor, with no
other connection than a pair of copper wires. This arrangement is found
very advantageous for light work and where power is required
occasionally, as in watch-making, the manufacture of philosophical
instruments, etc. Such moderate power is occasionally in demand also in
private houses, to drive sewing machines, lathes, etc.; and it is
obtainable from the same source as the current for lighting. Private
installations for lighting purposes usually have a dynamo driven by a
gas engine, and working into a set of accumulators. It seems not a
little remarkable that if the gas were burnt in the ordinary way instead
of being used in the gas engine, it would give only a fraction of the
amount of the light it causes to be given out by the electric light
lamps. But at the present time, houses and business premises are
supplied with electricity by companies who carry electric mains through
the streets. In England these electric mains, which are thick insulated
copper wires, are inclosed in iron pipes and laid beneath the pavement,
like the gas mains. In the United States, where electric illumination is
much used, the conductors have been usually carried overhead like
telegraph wires, but not a few fatal accidents have occurred from these
conductors falling into the streets. There is no reason to doubt but
that in a short time it will be as common for households to draw upon
such electric mains for their supply of light and power as it now is to
draw gas and water from common mains. The electric supply companies have
central stations in suitable positions, where very large and powerful
dynamos are regularly driven by steam power. These stations are provided
with appliances for measuring the currents and for duly controlling the
energy sent out. What will appear very extraordinary when we remember
that electricity is in itself unknown, is that the quantity supplied to
each house or establishment can be actually measured, and is paid for by
meter as in the case of gas. As already said (page 498) electricity can
only be measured by its effects, and it is the chemical effect which it
is found convenient to use for the purpose we are speaking of. The plan
is simply this: two plates of zinc dip into a solution of sulphate of
zinc, and from the one to the other there is sent through the solution
one-thousandth part of the current to be measured. While the current
passes, zinc is deposited on the plate towards which the current goes in
the solution, and if this plate is periodically weighed this furnishes
the measure of the total current. But how is just one-thousandth of the
whole current taken off from the rest and made to circulate through the
measuring apparatus? This is very easily done by taking advantage of the
law of derived circuits, which for our present purpose may be stated
thus: when a current of electricity finds two different circuits along
which it can pass, it will divide and circulate through both of them,
but the greater part will pass through the circuit of less resistance
(if there be any inequality), and by adjusting the resistances of the
circuits we can divide the current between the two partial or derived
circuits in any required proportions. Electric resistances, it may be
mentioned, depend upon the length, section, and nature of the conductor,
and are very easily measured and adjusted.
While the method just explained serves very well to measure the quantity
of electricity that has passed through a conductor in a given period,
provided that the current has always been in the same direction, it will
be sufficiently obvious that it would fail altogether in the case of
alternating currents. And, in fact, even in the case supposed this mode
of measurement does not take account of the real energy set in motion. A
reference to page 498, where the differences of electric currents are
mentioned that are commonly spoken of—_tension_ and _quantity_—will show
that electric effects depend upon more than the _quantity_ of
electricity passing. Forms of apparatus have been devised for recording
the total energy supplied; but their construction and principles are too
complex to be here explained. In some cases high tension currents are
required, in others it is quantity and not tension that is sought for;
and there are ways of transforming the qualities of currents so that the
same source shall supply electricity of either class. An example of this
may have been noticed in the action of the Ruhmkorff coil, where the
mere interruption of the primary or battery circuit, which possesses so
little tension that of itself it could not give rise to a spark,
nevertheless produces a wave of electricity in the secondary circuit of
a tension so high that sparks several feet long may be produced by it.
A somewhat recent application of the electric current of the dynamo may
be just mentioned here. It is what is known as electrical welding, and
depends upon the heat developed by currents being proportioned to the
electrical resistance for each part of the circuit. The heat thus
generated, where the current passes between two surfaces of metal, even
of considerable dimensions, is sufficient to bring them to a semi-fluid
condition, so that when simply pressed together they coalesce into one
mass. In this way pieces of iron work can be welded together in
situations where it would be either inconvenient or impossible to heat
them by furnaces.
The reader who has followed the last article will probably be prepared
to admit that “the magnetic field” is one of the most wonderful things
in the whole realm of inorganic nature, as all the powerful effects we
have been describing are the results of merely moving wires through it.
A wire conveying an electrical current so modifies the space surrounding
it, or so acts upon the unknown pervading medium, that conductors moved
in it, have other currents generated in them. An intermittent current,
like that in the primary circuit of the induction coil, is equivalent to
a movement of the magnetic field in regard to the secondary coil, so
that the general principle in the coil and the dynamo is fundamentally
the same. Quite recently, Professor Elihu Thomson has shown some very
novel mechanical effects of repulsions and rotations of conductors
placed near the poles of a coil through which rapidly alternating
currents are passing. [1890.]
We already hear of natural forces which have hitherto in a manner run to
waste being now utilised in man’s service by the advantage taken of the
capability of a slender wire to convey power. A notable instance is in
the case of the famous Falls of Niagara. Here the head of water is used
to drive turbines; our readers must not run away with any notion of huge
water-wheels being placed below the falls. But from the high level of
the water above the falls a tunnel has been cut which brings the water
into pipes 7½ feet in diameter, and these deliver it into three
turbines, in passing through which it develops a force of 5,000 horse
power, and this force is communicated to a steel shaft 2½ feet in
diameter, connected with the revolving parts of the dynamo. Mr. G.
Forbes, the engineer, states that the company who have undertaken this
enterprise are supplying, with a handsome profit to themselves,
electrical current or power at ⅛th of a penny per unit, for which
English companies charge sixpence. That is, Niagara supplies power at
1/48th of the price it can be obtained from coal.
The fact that mechanical power can be brought from a distance to
everyone’s door by a slender wire, and at small cost, suggests the
possibility of great social and industrial changes being effected in the
future by that one condition. Think of the abolition of factory chimneys
and smoke, nay, even of the abolition of the factory system itself, for
cheap power transmission seems to promise much in that direction, and
there is a shadowing forth of still more in
_THE NEW ELECTRICITY._
The Leyden jar and a few of its most obvious and common effects have
been touched upon already, (page 490); but the phenomena which are
revealed by a careful study of its charge and discharge show that these
are by no means of the simple kind that has generally been supposed.
Thus, for instance, if the magnetising effects of what is called current
electricity be borne in mind, especially the _definiteness_ of this
action as regards the _direction_ of the current (_cf._ Fig. 257), it
would follow that if instead of the iron bar in Fig. 265 we place within
the coil some unmagnetised steel needles we should find after passing a
current or discharge that these have become converted into permanent
magnets, and that their north poles are always towards the left of the
supposed current. Years ago experiments were made to ascertain whether
the discharges of a Leyden jar repeatedly passed through a coil would
magnetise needles in the same way, because it had been assumed that the
discharge is simply a current of extremely short duration and of quite
definite direction. As far back as 1824 it had, however, been observed
that the needles were magnetised sometimes in the wrong direction, yet
no attempt was made to explain this—it was sometimes merely mentioned in
the books as “anomalous magnetisation.” Dr. Henry of Washington, U.S.A.,
experimented on the subject, and in 1842 referred this action to a
condition of the discharge which had never before been suggested. He
says “we must admit _the existence of a principal discharge in one
direction, and then several reflex actions backward and forward, each
more feeble than the preceding, until the equilibrium is obtained_.”
Some five years afterwards Helmholtz had independently arrived at the
same conclusion, and from the fact that when a _succession_ of Leyden
jar discharges are sent through the voltameter (Fig. 263) the water is
indeed decomposed, but _both_ oxygen and hydrogen are evolved at _each_
electrode. Sir William Thomson (now Lord Kelvin) examined the question
from a theoretical point of view, and in a masterly mathematical paper
published by him in 1853 not only showed that the discharge must be of
an oscillating character, but gave the form of equation by which the
rate of oscillation is determined.
Faraday proved, as has already been stated, that the matter of the
dielectric takes part in such condensing actions as that of the Leyden
jar. The electrical charge enters into the glass, the particles of which
are thrown into a certain state of strain or tension (which Faraday
called polarisation), and the discharge of the jar is their release from
that tension. So that it appears that whatever electricity may be, it
can in some way become bound up with the particles of ordinary matter
like glass and other dielectrics, and exert force upon them, which force
acts always in two opposite directions. It is the opposition of the form
or direction in which the electrical effect is manifested that gave rise
to the conception of the two “fluids”—the “positive” and the “negative.”
If these “fluids” really existed it would surely have been possible to
give to an insulated body an absolute charge of either of them. But this
can never be done; if, for instance, you have in the middle of a room a
metallic sphere charged with positive electricity, the necessary
condition is that on the walls of the apartment or on surrounding
objects there is an exactly equivalent quantity or negative electricity.
The number of oscillations or alternate momentary currents in a single
discharge of a Leyden jar is enormous. Theory shows that under ordinary
circumstances they must be enumerated by hundreds of thousands, if not
by millions; that is, the apparently instantaneous spark is really made
up of say a million surgings to and fro of the electric influence. But
theory also shows that the frequency of these oscillations can be
controlled or adjusted through an indefinite range. A general notion of
the requisite conditions may be obtained by the analogy of sound, and
for this we may take the familiar case of the strings of a musical
instrument, say the violin, or the harp. Everybody knows that when a
stretched string or wire is pulled a little aside it is in a state of
lateral strain, striving by its elastic force to return to its position
of rest, and if it is suddenly let go it not only rapidly regains that
position, but by the inertia of its motion is carried beyond it against
its elastic force, which, however, again brings it back, and the
movement is continued nearly up to the point at which it was originally
released, this swinging movement persisting for an indefinite period,
during which the vibrations, which have an ascertainable and perfectly
regular frequency, are communicated to the sounding-board of the
instrument and from that to the air, by which they are conveyed to the
ear and affect the auditor as a musical note, which note is higher as
the number of vibrations per second is greater. Everybody will have
observed that in the violin the note yielded by each open string is
higher as the tension becomes greater by turning the peg to tighten it;
that the same string will, without any change in its tension, yield
higher notes as shorter lengths of it are employed. Another circumstance
upon which the pitch of the note depends may also be illustrated in the
violin, in which it will be noted that the G string, which gives the
lowest notes, is loaded with wire wound spirally round it. Here, then,
are three circumstances that collectively determine the pitch or number
of vibrations of a string—tension, length, weight; and if you give the
measures of these to a mathematician he can tell you the note the string
will emit, for the number of vibrations is given (when the measures are
expressed in the proper units) by the formula
_√t_
_n_ = ——————
_2l√w_
This shows that we have only to adjust suitably the tension, length, and
weight of a string in order to make it vibrate at any rate we please.
Now in the oscillation of currents in the Leyden jar discharge there are
conditions which correspond, by analogy at least, with those that
determine the vibrations of a stretched string. These conditions are of
course electrical, and they are definable in terms of electric units,
which need not be discussed here. As we are leading the reader to the
modern view of electricity, which sets aside the fluid theories and
regards electricity as having no separate existence, but as being merely
the manifestation of some condition of a universally pervading medium,
the same, in fact, as the luminiferous ether, it is curious to remark
that these electrical oscillations would seem to attribute to the
incompressible and imponderable ether something very much like the
characteristic property of matter we call inertia, by virtue of which
the released cord flies past its position of equilibrium to the other
side. Or may this quality be dependent on the matter of the dielectric
in which the ether is, as it were, entangled?
The oscillatory character of the Leyden jar discharge was elegantly
demonstrated before a large audience in a lecture given by Professor O.
Lodge at the Royal Institution a few years ago. Clearly it is impossible
to render perceptible to the senses the millions of periodic discharges
that take place in the marvellously short space of time taken up by a
spark, but by doing what is analogous to slackening the tension of the
stretched string or increasing its length, that is by increasing the
_static capacity_, which means using a large number of jars combined
into a battery, and at the same time causing the discharge to pass
through coils (the effect of these is to increase the _self-induction_
of the circuit—called also _impedance_), an arrangement corresponding
with loading the string, Dr. Lodge was able to bring down the rate of
oscillation to 5,000 per second, when, instead of the crack of the
ordinary discharge, a very shrill continuous sound was heard. The
addition of another coil gave another load, and when the rate was thus
reduced to about 500, the note emitted was that of the C above the
middle A of the piano. With the rate of oscillation thus reduced, it
became easy to render the discontinuity of the discharge visible by
means of revolving mirrors, as in the well-known acoustical
demonstrations.
[Illustration:
FIG. 280_k_.
]
Professor Lodge has devised an experiment which again shows the analogy
of electrical oscillations with those by which sound is produced. It is
well known that a vibrating tuning-fork will set another fork of the
same pitch to vibrate also by mere approximation. A and B (Fig. 280_k_)
are two exactly similar Leyden jars, the inner and outer coatings of
each being connected by a wire enclosing a considerable area in its
circuit, which in the case of A contains an air gap across which sparks
pass when the coatings are connected with the poles of an electrical
machine. The circuit of B is provided with an adjustable sliding piece
C, and the coatings are almost connected with each other by a strip of
tinfoil hanging over the rim but not quite reaching to the outer
coating. When the jars are placed so that their wire circuits are
parallel, and sparks are passing across the air interval of A’s circuit,
a position of the slider on the other can be found when sparks also pass
between the tin-foil and the outer coating. But if the slider be moved
from this position, the two circuits will no longer be in unison, and
the sparks in B will cease. This response of the oscillations in one jar
to those set up in another of the same vibratory period is called
_electrical resonance_.
Dr. Hertz, a professor in the University of Bonn, has opened out new
paths to investigators by a brilliant series of researches which have
shown that in the dielectric surrounding an electrical system executing
very rapid oscillations there are waves of electro-motive and magnetic
force. These researches are not capable of any condensed description
here, and the reasoning is of a kind that appears mainly to the expert
physicist. One of his modes of investigation required oscillations of
extreme rapidity, and he obtained them by attaching to each pole of an
induction coil a metal plate, and between these plates, which were in
the same vertical plane, passed a stout wire interrupted by an air gap
in its centre provided with small brass balls. The rate of oscillation
of this arrangement was calculated as the hundred-millionth part of 1·4
second. In conjunction with this system Hertz made use of a very simple
apparatus he called a resonator, which consisted merely of a piece of
copper wire bent into a circle of about 28 inches diameter. The ends of
the wire did not, however, meet, but were fitted with two balls, or with
a ball and a point, and an arrangement by which the air gap between them
could be very finely adjusted and measured. This resonator was, of
course, prepared as to be in electrical tune with the original vibrator,
and with it Hertz was able to examine the condition of the surrounding
space. When held in the hand near the vibrator he found that sparks
crossed the air space in the resonator, and that the length of the air
space across which the sparks would pass varied with the position of the
resonator. When the plane of the resonator was parallel with the metal
planes of the vibrator and its axis in the horizontal line drawn
perpendicularly through the vibrator’s air space, the sparks passed
readily when the air space of the resonator was at the same time
vertically above or below its centre, but they ceased entirely when it
was level with the centre. He obtained these sparks when the resonator
was held—in free space, be it understood—in the above-mentioned position
even at a distance from the vibrator of 13 yds., the length of the
apartment. By examining the results with other positions of his
resonator and by other and varied experiments, Hertz was able to prove
the existence of definite waves of electro-magnetic and electro-motive
forces, to measure their lengths, and to show that they are capable of
reflection, refraction, and even polarization by the same laws that hold
with the extremely short but enormously rapid vibrations constituting
light. It may here be mentioned that the existence of currents in the
resonator can be shown by a Geissler tube being made to take the place
of the air space, which tube is thus lighted up without any metallic or
visible connection with any electrical apparatus whatever, the only
requisite conditions being that its circuit be tuned to the vibrator,
and in a certain position in relation to the axis of the spark space of
the latter. Hertz has also shown that electro-magnetic disturbances
(transversal waves) are propagated in space with a determinate velocity
akin to that of light, and in short the outcome of his investigations,
as well as of those undertaken by others, has been a vindication of
Clerk Maxwell’s splendid theory by which light is regarded as an
electro-magnetic action. Professor Righi of Bologna, having succeeded in
obtaining shorter electrical waves than anyone before—namely, 4/10ths of
an inch instead of about 20 inches—was able with them to repeat all the
phenomena of optics such as reflection, refraction, circular
polarization, interference, &c. It appears then almost certain that
light and electro-magnetic waves or radiations are but one and the same
affection of a pervading medium we call the ether.
By following up in certain directions lines of research suggested by the
investigations of Maxwell, Lodge, Hertz and others, and by an unreserved
acceptance of the ether theory of light, electricity and magnetism, some
wonderful practical results have recently been obtained by M. Nikola
Tesla, an electrical engineer now resident in New York. The experiments
shown by Tesla in his public lectures have excited great interest in
scientific circles, and have by many persons been witnessed with
something like astonishment.
[Illustration:
FIG. 280_l_.—_The Tesla Oscillator._
]
[Illustration:
FIG. 280_m_.—_M. Nikola Tesla._
]
One of the first objects of M. Tesla was to obtain alternating currents
of high tension and great frequency. It may be seen from Fig. 272 that
the movement of coils of wire in a magnetic field generates currents,
and it has been stated that these currents are in alternately opposite
directions as the coils approach or recede from the magnetic poles. In
the machine represented in Fig. 280_a_, each revolution would produce 16
reversals of current. Tesla constructed a rotatory machine which gave
20,000 alternations of current in one second, because it had 400 poles
and could be rotated at a very high speed. But of course the number of
poles and the speed of the machine could not be increased beyond certain
practical limits. By a happy application of the known principle of
harmonic oscillations, in which all the rotatory movements of
fly-wheels, coils and poles could be dispensed with, Tesla simplified
the alternate current generator, reducing the moving parts to the
minimum at the same time that he obtained a greater number of
alternations and almost perfect regularity in their periodicity. The way
in which this has been accomplished may be gathered from a careful
inspection of Fig. 280_l_ compared with the following explanation. This
illustration, it should be understood, is merely a diagram in which
details of mechanism are altogether omitted, and only so much shown as
will serve to explain the principle. We shall take the mechanical part
first, and direct the reader’s attention to the means by which an iron
rod is made to perform very rapid to-and-fro movements in the direction
of its length, and to do that with perfectly isochronous periods, which
may be made longer or shorter at will, and which are quite independent
of very considerable variations in the motive power. The diagram
represents the apparatus in section, and the central part of it marked
by letters P and P´ is a piston through which passes what may be called
a piston-rod A, which projects some distance out of the cylinder at both
ends. The piston is shown in the diagram in its central position, where
the impelling power has no action to move it as will presently be seen.
This moving power we may assume to be the compressed air applied through
the ports I I´. Just to the right of the upper one of these on the
diagram will be observed in the piston a slot S opening into a hollow T,
which communicates directly with the space on the left of the piston.
The same arrangement, with directions reversed, is seen on the other
side of the piston. If now the piston were pushed a little to the left
of the position shown in the diagram, the compressed air rushing from I
through the slot into the opening S T would impel the piston towards the
right, and it would be carried onward by its inertia beyond the position
shown in the figure towards the right, but in doing this the access of
the compressed air on the left would be cut off, and the slot
communicating with the space on the right hand would allow the
compressed air to act in the space P, checking the further advance of
the piston to the right, acting like a spring or elastic cushion, and
again driving the piston to the left, during which movement the air that
has done its work is allowed to escape at the outlet O O. The same cycle
of operations will be rapidly repeated, but the rate of oscillation
admits of control, for the larger the air chamber in which the air is
compressed by the momentum of the piston and rod, the less will it be
compressed and the less powerfully it will resist, while with a smaller
capacity of air-chamber the more powerful will be the back spring of the
imprisoned air. On the other hand, the mass that is moved may be
increased; that is the weight of the rod, &c., may be increased. In any
case the oscillations will be perfectly regular, because the force which
tends to bring the piston to its position of equilibrium will be always
proportionate to its distance from that point. So that we have here a
rod shooting in and out shuttle-wise with the utmost regularity and with
almost any desired rapidity, controllable under precisely the like
conditions as the stretched string already mentioned, for as the tension
of the string is the measure of the force with which it strives to
regain its position of equilibrium, so the compression of air in the
chamber behind the piston; and as the loaded string vibrates slower, so
will the loaded piston. So much for the mechanical part of this machine,
for we may omit all details of valves, &c. The electrical arrangement is
very simple and of the greatest efficiency. On each projecting end of
the piston are wound coils of insulated copper wire, which being shot in
and out across a powerful magnetic field between the jaws of very large
electro-magnets M M´ cut the “lines of force” to the best advantage, and
from these coils alternating currents of high tension and frequency are
gathered up. The vibrating rod is steadied by working in bearings (not
shown). The electro-magnets are actuated as usual by coils of insulated
wire surrounding their iron cores. In the motion of the moving coils
there are electrical forces called into play which in mechanical effect
control the movement in the same way as the air-springs, and as these
electrical forces admit of certain adjustments and have calculable
effects, the _mechanical period_ of the machine and the _electrical_ one
can be made to accord, and thus to, as it were, sustain each other, and
assure a perfectly isochronous periodicity, even with considerable
variations of the impelling force. Though we have supposed compressed
air as the actuating agent, steam has been applied in some slightly
modified forms of the machine, and sometimes at the high pressure of 350
lbs. per square inch. Such is Tesla’s alternating current producer, or
the _Tesla Oscillator_, as it has been called. This, of course, is a
very different thing from the vibrator of disruptive discharge already
mentioned in connection with the experiments of Professor Hertz. Tesla
also uses the disruptive discharge, and what with the high frequency and
the great tension of his currents, he obtains electric oscillations of
hitherto unequalled rapidity, calculable at thousands of millions per
second. He claims, indeed, to be able to agitate the ether at rates of
undulation comparable with those of light itself (500 billions per
second). Some of the experiments he has shown certainly lend support to
such an explanation. The lighting of electric lamps with but one
metallic connection, and that held in a person’s hand, and causing
Geissler tubes to light up without any metallic connections whatever,
and making gas at ordinary pressures luminous, a lump of charcoal
contained in a closed glass vessel to become red-hot while the vessel is
merely held in the hand, are certainly phenomena that cannot be
explained on the old lines. The space between two large surfaces of
metal 15 feet apart, and forming the poles of an oscillatory system, is
shown to be full of light-forming influences, as when phosphorescent
substances contained in closed glass vessels glow intensely, the glass
being apparently no obstacle. According to Tesla, you make space and
matter equally permeable to ethereal undulations when these are tuned,
so to speak, to the proper frequency.
Many of the strange effects Tesla has shown are referable to the
principle of electric resonance; such are the powers of a coil with no
metallic connections with any other apparatus and removed, by a distance
of many feet, from any current-conveying wires. Tesla’s workshop was an
apartment 40 feet long and 20 wide, and the wires connecting the poles
of his oscillator were carried round the walls, while in the centre of
the workshop stood a very large but entirely insulated coil, between the
terminals of which an ordinary incandescent lamp was placed. This lamp
was brilliantly illuminated when the oscillator was in action. The
electric qualities of this coil were so adjusted that its currents came
into tune with the ethereal vibrations propagated from the conductor
round the room. But further, a single hoop of copper wire of the proper
diameter and thickness could be brought into unison with the coil, and
when held in the hand over the latter, even at a considerable distance,
incandescent lamps attached to it were lighted up by the induced
currents. Many other novel experiments have been shown by M. Tesla, but
they need not here be described, as they have yet to be connected with
the logical study of the entire class of phenomena. M. Tesla speaks
somewhat sanguinely of being ultimately able to convey signals, and even
power, to a distance, not merely with one wire but with no wires at all!
Another thing he looks forward to is to set the electricity, or rather
the ether that interpenetrates the matter of the whole earth, into a
state of agitation. This seems what is commercially termed “a large
order;” but we have seen that every Leyden jar, every coil, and in fact
every electrical system, has its own period, and if by any possibility
we could discover, or by chance hit upon the earth’s electric vibration
period, it is not antecedently impossible that even the comparatively
small efforts of such oscillatory vibrations as we could produce, would
by their accumulation agitate the earth’s ether. It is well known that
very small impulses, so tuned as to correspond with the natural period
of a considerable mass, will produce striking mechanical effects. Thus,
a troop of soldiers passing over a bridge have often been known to break
down a structure that would have supported their mere weight many times
over, because they were all marching together and with a step
corresponding in time with the oscillatory period of the bridge. It is
now always enjoined in the military orders that troops in crossing a
bridge must “break step.” Another familiar illustration of the
accumulation of small synchronous impulses is the experiment of singing
into a glass goblet the note corresponding with its vibration period.
The singer merely by sustaining this note for a short time often
succeeds in shivering the glass into fragments. M. Tesla believes that
he has already succeeded in agitating the earth’s ether to some extent;
he does at least obtain flaming purple streamers passing into the air
from one end of a coil, while the other is connected with the earth.
These discoveries and theories appear likely to lead to many unforeseen
results, valuable for both science and its applications, and such as may
far surpass the expectations of those who take less enthusiastic views
of the matter than M. Tesla and his friends do. The theoretical
properties of the ether and the conditions of it, which are held capable
of making it the scene and the medium of all the hitherto so-called
ponderable and imponderable forces, have not been completely worked out.
The experiments that have been already made show that disturbances of
very different kinds may be propagated in the ether by undulations of
any length from less than 1/60000th part of an inch, as in the case of
violet light already spoken of, to the 1,200 miles attributed to certain
electrical conditions.
The foregoing sentences, describing the discoveries of Hertz and others,
had not long been penned before it had become possible to announce that
they had borne fruit in as extraordinary an invention as could have
distinguished the close of an extraordinary century. It is the
realization of what the most accomplished electrician would not long
before have pronounced a dream—namely, _wireless telegraphy_. The
general principle of it should not be obscure after the account of the
“Hertzian waves”; but our space does not permit a description of details
of its working out in a practical form by a young Italian electrician,
Signor Marconi. We have already seen that a Geissler tube, when its
circuit is properly attuned, can be lighted up by the magneto-electric
disturbance propagated without material contacts, and this itself would
constitute a method of signalling to a distance. On the same principle,
a discharge may be determined by the “wave” between conductors in
certain adjustable conditions of electric tension, and in this way local
circuits may be brought into play, and ordinary telegraphic effects
produced, as described in the following article. The actual apparatus to
receive the ethereal impulses is extremely simple—merely a little fine
metallic dust (nickel and silver) in a glass tube included in the
resonator circuit by a wire at each end, touching the dust. This gathers
together, or coheres (hence the apparatus is called the _coherer_),
under the magneto-electric influence, a local battery discharge then
passes, completing a circuit, and the dust has to be shaken loose again
by a mechanical agitation. Marconi has been able to signal over a
distance of forty-three miles.
[Illustration:
FIG. 281.—_Portrait of Professor Morse._
]
THE ELECTRIC TELEGRAPH.
More than two centuries ago a learned Italian Jesuit, named Strada, gave
a fanciful account of a method by which he supposed two persons might
communicate with each other, however far they might be separated. He
conceived two needles magnetized by a loadstone of such virtue, that the
needles balanced on separate pivots ever afterwards pointed in parallel
directions; and if one were turned to any point, the other also
sympathetically moved in complete accordance with it. The happy
possessors of these sympathetic needles, each having his needle mounted
on a dial marked with the same letters and words similarly inscribed,
would be able to communicate their thoughts to each other at
preconcerted hours, by movements and pauses of the wonderful needles.
The poet Akenside, when describing, in his “Pleasures of the
Imagination,” the effect of association in bringing ideas before our
minds, illustrates his point by a happy allusion to Strada’s conceit.
Here is the passage:
“For when the different images of things,
By chance combined, have struck the attentive soul
With deeper impulse, or, connected long,
Have drawn her frequent eye; howe’er distinct
The external scenes, yet oft the ideas gain
From that conjunction an eternal tie
And sympathy unbroken. Let the mind
Recall one partner of the various league—
Immediate, lo! the firm confederates rise.
‘Twas thus, if ancient fame the truth unfold,
Two faithful needles, from the informing touch
Of the same parent stone, together drew
Its mystic virtue, and at first conspired
With fatal impulse quivering to the pole.
Then—though disjoined by kingdoms, though the main
Rolled its broad surge betwixt, and different stars
Beheld their wakeful motions—yet preserved
The former friendship, and remembered still
The alliance of their birth. Whate’er the line
Which one possessed, nor pause nor quiet knew
The sure associate, ere, with trembling speed,
He found its path, and fixed unerring there.”
In our own day this fancy of Strada’s has been literally and completely
realized in all save the convenient portability of the sympathetic
dials; but this and the other forms of apparatus which are now so
familiar in electric telegraphy were produced by no sudden inspiration
occurring to a single individual. Great inventions are ever the outcome
not of the labours of one but of a hundred minds, and the progress of
the electric telegraph might be traced, step by step, from the first
suggestions, made more than a century ago, of employing, for the
communication of intelligence at a distance, the imperfect electric
means then known. The men who then attempted to utilize the mysterious
agency of electricity failed to produce a practical telegraph, because
the conditions of electrical excitation known at that time gave no scope
for the realization of their project. Not the less do they deserve our
grateful remembrance for the faith and energy with which they strove to
overcome the difficulties of their task. Voltaic electricity was first
proposed as the means of conveying signals to a distance in 1808,
immediately after the discovery of the power of the pile to decompose
water; and the method of communicating the signals was based upon this
property. Sömmering proposed to arrange thirty-five pairs of electrodes,
formed by gold pins passed through the bottom of a glass vessel
containing acidulated water. Each pair of pins was marked by a letter of
the alphabet or a numeral, and attached to distinct wires, which could
be put into connection with a pile at the sending station. The signals
were made by the gas evolved from these electrodes indicating the letter
intended. The number of wires required and the slowness of working were
great objections, and this system never came into practical use,
although it was afterwards proposed to diminish the number of the wires
from thirty-five to two—by so varying the amounts of gas given off and
the periods of time as to form an intelligible system of signals. Ten or
twelve years after, Mr. Ronalds, of Hammersmith, invented an ingenious
system by which letters on a dial could be pointed out at a distance by
frictional electricity. Two dials, on which the letters, &c., were
marked, were each placed behind a screen having an aperture, which
permitted only one letter to be seen at once; and the dial was mounted
on the seconds arbor of a clock with a dead-beat escapement. A pair of
pith balls hung in front, insulated and connected by means of an
insulated wire with the similar pair at the other end of the line, where
the other clock and dial were placed. The clocks were regulated to go as
nearly as possible at the same rate, so that at each end of the line the
same letters were simultaneously displayed. It was easy, however, at any
time to start the clocks together at the same letter by a signal
previously agreed upon, and all that was really required was a
synchronous motion of the discs during the time the signals were being
sent. The insulated wire received from a small electrical machine a
charge, which caused the pith balls at both ends to diverge; and the
moment the wire was discharged, the balls collapsed suddenly and
simultaneously, and this discharge was effected by the sender of the
message at the instant that the letter he wished to indicate appeared at
the opening in front of his dial. Since the same letter was at the same
instant visible at the other end also, it was indicated to the receiver
of the message by the collapse of the pith balls. Ronalds worked this
telegraph experimentally with a wire 525 ft. long, but it was never
adopted practically. On communicating to the Admiralty the power of his
invention, he was informed that “_telegraphs of any kind were wholly
unnecessary, and no other than the one in use would be adopted_.”
The memorable discovery of electro-magnetism by Œrsted in 1819 was soon
followed by attempts to apply it to the production of signals at a
distance. Ampère first pointed out the possibility of making an electric
telegraph with needles surrounded by wires; but he proposed to have a
separate needle and wire for each signal to be transmitted. If Ampère
had but thought of producing signals by different combinations of two
movements, as Schweigger had before suggested for Sömmering’s telegraph,
thus making two wires and two needles suffice, the practical
introduction of the electric telegraph would have dated some twenty
years earlier than it actually did. In 1835 Baron Schilling exhibited an
electric telegraph with five magnetic needles, and he afterwards
improved upon it so far as to reduce the number of needles and
conductors to one—for to him the happy thought seems first to have
occurred that one needle could be made to produce many signals by
different combinations of its movements—sometimes to the right,
sometimes to the left. Thus two movements to the left might stand for A,
three for B, four for C, one to left followed by one to left for D, and
so on. Schilling’s apparatus does not appear to have had the requisite
qualities for practical working on the large scale. From this time,
however, telegraphic inventions succeeded each other rapidly, and we
meet with the names of Gauss, Weber, Steinheil, and others, as inventors
and discoverers in the region of practical science which was now fairly
opened, The first two used the magneto-electric machine to give motion
to the needle; and the thought of using the metals of the railway line
as conductors having occurred to Gauss, he found, on making the attempt,
that the insulation was imperfect, but he perceived that the great
apparent conductibility of the earth would allow of its being
substituted for one of the metallic communicators.
But the first who succeeded, after long and persevering effort, in
giving a practical character to the electric telegraph, was undoubtedly
Professor Wheatstone. He had for some years been engaged in electrical
researches before, in 1837–-a memorable year for telegraphic
inventions—he took out a patent in conjunction with Mr. W. Fothergill
Cooke. In their telegraph there were five magnetic needles, arranged in
a horizontal row, each needle being in a vertical position, and the
various letters of the alphabet were indicated by the convergence of the
needles towards the point at which the letter was marked on the dial.
The first electric telegraph constructed in England was made on this
system on the London and Blackwall Railway. In 1838, Messrs. Wheatstone
and Cooke had reduced the number of needles to two, and many other
improvements were effected in the apparatus for signalling, it being
made possible for any number of intermediate stations to receive the
messages. Several great railway companies erected lines with five lines
of wire, but the expense of so many conductors was found to be
considerable, and Messrs. Cooke and Wheatstone, after reducing the
number of needles and conductors to two, ultimately (1845) patented an
instrument with a single needle. It was about this time that an incident
occurred which strongly drew the attention of the general public to the
electric telegraph, which had, up to that time, been considered as the
more immediate concern of the railway companies. A foul crime had been
committed at Salthill, by the murder of a woman named Hart; and Tawell,
the suspected murderer, was traced to Slough station, and there it was
found he had taken the train to London; a description of his person was
telegraphed, with instructions to the police to watch his movements on
his arrival at Paddington. He was accordingly followed, apprehended,
tried, convicted, and executed. This incident has been graphically and
circumstantially described by Sir Francis B. Head, in connection with an
anecdote recording a curiously expressed recognition of the value of the
telegraph in furthering the ends of justice. We give the passage in
full:
“Whatever may have been his fears, his hopes, his fancies, or his
thoughts, there suddenly flashed along the wires of the electric
telegraph, which were stretched close beside him, the following words:
‘A murder has just been committed at Salthill, and the suspected
murderer was seen to take a first-class ticket for London by the train
which left Slough at 7·42 p.m. He is in the garb of a Quaker, with a
brown great-coat on, which reaches nearly down to his feet. He is in the
last compartment of the second first-class carriage.’ And yet, fast as
these words flew like lightning past him, the information they
contained, with all its details, as well as every secret thought that
had preceded them, had already consecutively flown millions of times
faster; indeed, at the very instant that, within the walls of the little
cottage at Slough, there had been uttered that dreadful scream, it had
simultaneously reached the judgment-seat of Heaven! On arriving at the
Paddington Station, after mingling for some moments with the crowd, he
got into an omnibus, and as it rumbled along he probably felt that his
identity was every minute becoming confounded and confused by the
exchange of fellow-passengers for strangers, that was constantly taking
place. But all the time he was thinking, the cad of the omnibus—a
policeman in disguise—knew that he held his victim like a rat in a cage.
Without, however, apparently taking the slightest notice of him, he took
one sixpence, gave change for a shilling, handed out this lady, stuffed
in that one, until, arriving at the Bank, the guilty man, stooping as he
walked towards the carriage door, descended the steps, paid his fare,
crossed over to the Duke of Wellington’s statue, where, pausing for a
few moments, anxiously to gaze around him, he proceeded to the Jerusalem
Coffee-house, thence over London Bridge to the Leopard Coffee-house in
the Borough, and, finally, to a lodging-house in Scott’s Yard, Cannon
Street. He probably fancied that, by making so many turns and doubles,
he had not only effectually puzzled all pursuit, but that his appearance
at so many coffee-houses would assist him, if necessary, in proving an
_alibi_; but, whatever may have been his motives or his thoughts, he had
scarcely entered the lodging when the policeman—who, like a wolf, had
followed him every step of the way—opening his door, very calmly said to
him—the words, no doubt, were infinitely more appalling to him even than
the scream that had been haunting him—‘Haven’t you just come from
Slough?’ The monosyllable, ‘No,’ confusedly uttered in reply,
substantiated his guilt. The policeman made him his prisoner; he was
thrown into jail, tried, found guilty of wilful murder, and hanged. A
few months afterwards, we happened to be travelling by rail from
Paddington to Slough, in a carriage filled with people all strangers to
one another. Like English travellers, they were mute. For nearly fifteen
miles no one had uttered a single word, until a short-bodied,
short-necked, short-nosed, exceedingly respectable-looking man in the
corner, fixing his eyes on the apparently fleeting posts and rails of
the electric telegraph, significantly nodded to us as he muttered aloud,
‘Them’s the cords that hung John Tawell!’”
So far we have followed Wheatstone and Cooke, because these gentlemen
were the first who in any country made the electric telegraph a success
on the great scale. Elsewhere than in England, laboratories and
observatories had been connected by experimental lines, and models had
been exhibited to Emperors, but these two Englishmen were the first to
construct a telegraph for practical use. It must not, however, be
supposed that they are entitled to be considered the exclusive inventors
of the electric telegraph, for we have already named other distinguished
investigators who contributed their share to this remarkable invention.
And some years before Wheatstone and Cooke had patented their first
needle telegraph, the first ideas of a system which has largely
superseded the needles for ordinary telegraphic purposes, had presented
themselves to a mind capable of developing them into the most efficient
form of telegraphic apparatus which we possess. In October, 1832, among
the passengers on board the steamship _Sully_, bound from France to the
United States, was a talented American artist who had gained some
reputation in his profession. A casual conversation with his
fellow-passengers on electricity, and the plan by which Franklin drew it
from the clouds along a slender wire, suggested to the artist the
possibility of thus communicating intelligence by signals at a distance.
He named his notion to a fellow-passenger, Dr. Jackson, an American
professor, who had devoted some attention to electrical science, and
this gentleman suggested several possible (and impossible) methods in
which the thing might, as he thought, be accomplished. None of these
suggestions, however, indicated the direction in which the idea
afterwards took practical form in Morse’s hands. Jackson had among his
baggage in the hold, and therefore inaccessible on the voyage, a
galvanic battery and an electro-magnet, and these he described to the
painter by the aid of rough sketches. When, some years afterwards, Morse
had realized his ideas of electric communication, and success was
bringing him the favour of fortune, Jackson advanced a claim to a share
in the invention, and a famous lawsuit, Jackson _v._ Morse, was ended by
a verdict in favour of Morse, which public and scientific opinion has
unanimously endorsed. In reference to this matter, Mr. R. Sabine, the
author of an excellent little treatise on “The History and Progress of
the Electric Telegraph,” has thus placed the subject in its true light:
“Two men came together. A seed-word, sown, perhaps, by some purposeless
remark, took root in fertile soil. The one, profiting by that which he
had seen and read of, made suggestions, and gave explanations of
phenomena and constructions only imperfectly understood by himself, and
entirely new to the other. The theme interested both, and became a
subject of daily conversation. When they parted, the one forgot or was
indifferent to the matter, whilst the other, more in earnest, followed
it up with diligence, toiling and scheming ways and means to realize
what had only been a dream common to both. His labours brought him to
the adoption of a method not discussed between them, and Morse became
the acknowledged inventor of a great system. Fame and fortune smiling
upon the inventor, it was natural enough that Jackson, awakening from
his unfortunate indolence, should remember his share in their earlier
interchange of ideas, that had, perhaps, first directed Morse’s
attention to the subject of telegraphy. And, although we are compelled
to pronounce dishonest those attempts which Jackson made to claim the
later and proper invention of Morse—that of the _electro-magnetic
recorder_—and strong as is our confidence in the spotless integrity of
our friend, we cannot entirely ignore Jackson—little as he has done—nor
deny him an inferior place amongst those men whose names are associated
with the history and progress of the electric telegraph in America.”
From the time of this chance conversation with Dr. Jackson, Morse
devoted his mind entirely to the subject of telegraphic communication,
and although then more than forty years of age, he abandoned the
profession in which he had already gained some distinction, and with the
energy and elastic power of adaptability which characterize the American
mind, he gave himself up to this new pursuit to such good purpose, that
a few years afterwards saw his telegraph system completely established
in the United States, where the lines now exceed 20,000 miles in length.
At the instigation of the late Emperor of the French, the Governments of
France, Belgium, Holland, Austria, Sweden, Russia, Turkey, and the Papal
States, combined to award to Professor Morse, in recognition of his
services to practical science, the sum of £16,000. It was in 1836 that
Morse had first brought his notions into a practical form, but his
apparatus has since received many improvements at his own hands, or by
the useful modifications of it which have been proposed by others. The
transmitting key invented by Morse has proved a valuable piece of
apparatus, and its simplicity has contributed much to the success of his
invention. Telegraphs on this system were erected in America in 1837,
and the Morse apparatus is now more extensively used than any other in
every country.
In 1840 Professor Wheatstone had succeeded in most ingeniously applying
electro-magnetism in such a manner as actually to realize Strada’s
sympathetic needles, by having the letters of the alphabet arranged
round the circumference of a circle, and pointed at by a revolving hand.
Such a dial is provided at each end of the line, and the sender of the
message has only to make the index of his own dial pause for an instant
at any letter; the hand of his correspondent’s dial will also pause at
the same letter. These dial telegraphs are particularly convenient for
many purposes, as they do not require a trained telegraphist to read or
send the messages. Wheatstone’s plan has been greatly simplified by
Breguet, of Paris, and others, and it is much used in mercantile and
public establishments. From the foregoing discursive historical
indication of the progress of the electric telegraph we shall now
proceed to describe the systems most commonly employed in practical
telegraphy, with a brief reference to some other interesting forms; and
in following these descriptions, the reader will find the advantage of
an acquaintance with the electrical facts discussed in the last article,
with which facts we shall presume he has become to a certain extent
familiar.
In every telegraphic system there are three distinct portions of the
apparatus, which may be separately considered, as they may be variously
combined. We have—
1º. The apparatus for producing the electricity, such as batteries,
magneto-electric machines, &c.
2º. The conductors, or wires, which convey the electricity.
3º. The apparatus for sending and for receiving the messages.
Of the first we shall have little to add to what has been said in the
last article; and before entering upon the description of the second, it
will be better to discuss the third division.
_TELEGRAPHIC INSTRUMENTS._
Telegraphs may conveniently be classed according to the mode in which
the actions of the sender produce their effect at the point where the
message is received. A first class may include those in which the
current is made to deflect magnetized needles; a second may comprise
those in which the current, by magnetizing soft iron, causes an index to
travel along a dial and point to the letter intended; a third may
embrace those in which the same action on soft iron is made to print the
despatches, either in ordinary type or in conventional signs; while in a
fourth class we may put the instruments which give their indications by
sounds only. It is obvious that in some of these systems signs only are
used, and a special training and acquaintance with the symbols is
necessary, while in the rest the ordinary alphabetic letters are shown
or recorded. In the former case the apparatus is simpler, and therefore
for the general business of public telegraphy it is almost exclusively
employed; while for private purposes, where it is often required that
the messages should be dispatched and received by persons not acquainted
with the symbolic language, the dial telegraph, or that which prints the
message in ordinary characters, will continue to be employed, in spite
of the greater complexity and greater liability to derangement of the
apparatus.
In the needle telegraphs the essential part of the apparatus is a
multiplier (page 493), having its needle mounted vertically on a
horizontal axis, to which is also attached an indicator, visible on the
face of the instrument, and formed either of a light strip of wood, or
of another magnetized needle, having its poles placed in the reverse
position to those of the needle within the coil. When the current is
sent through the latter, the index is deflected to the right or left,
according to the direction in which the current passes. Fig. 282
represents the exterior of one of Wheatstone and Cooke’s double-needle
instruments, now almost entirely superseded, where needles are used at
all, by the single-needle instrument. The face of the instrument is
marked with letters and signs, which were supposed to aid the memory of
the telegraphist, and the movements of the needles were chosen rather
with that view than any other. We need not here give the code of
signals, as the double instrument is now obsolete, and the code for the
single-needle instrument, which was devised by Wheatstone and Cooke, has
been in most cases superseded by one corresponding with the Morse code,
a deflection to the right representing a dot, and a deflection to the
left a dash.
[Illustration:
FIG. 282.—_The Double-Needle Instrument._
]
The smaller case surmounting the instrument, Fig. 282, contains a bell
or alarum, which serves to call the attention of the clerk at the
receiving station. The first electric bell-alarum was invented by
Wheatstone and Cooke. It was simply a clock alarum, put in motion by a
wound-up spring. The spring was released at the proper moment by a
detent, which was removed by the attraction of a soft iron armature to
the core of a small electro-magnet, formed by the line wire itself; but
when the current, on account of the length of the line, was too weak to
produce a sufficiently strong electro-magnet, Wheatstone caused it to
close the circuit of a local battery. The electric alarum has been
modified in a thousand ways, and as electric alarums or bells are now
coming into common use in hotels, and even private houses, we give in
Fig. 283 a representation of one of the simplest forms, in which the
bell is rung continuously by the electric current so long as the circuit
is closed. The action is very simple: a soft iron armature, A, is
attached to the steel spring, B, and prolonged into a hammer, C, which
strikes the bell, D, every time the armature is attracted to the
electro-magnet. The armature and the spring, E, form part of the
circuit, which is continued by connectors to F, and through the coils to
G. The spring, E, does not follow the armature in its motion towards the
electro-magnet, and consequently the circuit is broken before the
armature touches the magnet; but the hammer strikes the bell, and the
elasticity of the spring, B, brings the armature back into contact with
E, the circuit is closed, and the motions are repeated, so that the bell
is struck a rapid succession of blows. This _make-and-break_ movement is
precisely similar to that with which Ruhmkorff’s coils are usually
provided.
[Illustration:
FIG. 283.—_Electro-Magnetic Bells._
]
Below the dial of the instrument, in Fig. 282, may be seen two handles.
Each of these is connected with an arrangement constituting the
transmitting apparatus, by which the metallic contacts are varied
according to the position of the handles. When the handle is vertical,
all communication with the battery in connection with the instrument is
cut off, but the coils are ready to receive any current from the
line-wires. When the handle is turned to the right or left, the contacts
are such that the battery current flows into the line, and deflects to
the right or left the needles of both receiving and transmitting
instruments. The single-needle instrument as now made is of a very
simple and inexpensive construction, and it is the form principally used
in connection with the working of lines of railway. One may see at every
station in the United Kingdom the little vertical needle, mounted in the
centre of a small perfectly plain green dial-plate; for the letters and
signs with which it was formerly the practice to cover the dial have
been found to distract the eye more than they aid the memory. A boy will
after a few weeks’ practice learn to read the signals and to transmit
messages with considerable rapidity.
[Illustration:
FIG. 284.—_Portable Single-Needle Instrument._
]
The field telegraph lines, which are used in actual warfare to enable
the commander of an army to communicate with every part of his forces,
require as the essential condition for their construction rapidity of
erection and removal, and the greatest possible simplicity and
portability in the sending and receiving instruments. The wires are
fastened to trees, or other fixed supports, where such are available,
but artificial supports are provided in light poles which admit of being
readily planted in the ground and removed. In cases where it is
inexpedient or impossible to use these, the conductor may be laid along
the ground, but must then be well insulated with some non-conducting
material, which is capable of withstanding the action of the weather. A
kind of cable is usually employed, in which is the conductor, made of
copper, protected and strengthened by hemp fibres and covered with some
non-conducting material. No form of needle telegraph instrument could be
simpler than that represented in Fig. 284, which has been designed for
military purposes. The communicator, or transmitting apparatus, here
shows an arrangement very compact, and not easily deranged. The springs,
A B, press against the piece of metal marked C, with which good contact
is insured by providing the springs with several projecting steel
points. D, E are finger-keys of ebonite or ivory; underneath are two
points of a metallic conductor on which the springs can be pressed down
by a touch of the finger. This conductor is in communication with the
binding-screw, F, from which a wire proceeds to the negative or zinc end
of the battery, while the piece, C, is in metallic connection with G, to
which a wire proceeding to the positive or copper end of the battery is
attached. From B a wire, H, communicates through the hinge with one end
of the coil, the upper end of which is connected through the upper hinge
with a binding-screw not visible in the figure, and to this the end of
the line conductor is attached. From A a wire K passes to another
binding-screw, by which the earth connection is made. A current arriving
by the line traverses the coils and passes through H and B into C, hence
by A into the earth through K. When D is depressed the current from the
battery passing from G through C, A, and K, into the earth, and thus to
the distant station, returns through the coils of the instrument there
and along the line wire, through the coils, L L, and by H, B, D and F,
to the negative pole of the battery. The reader will have little
difficulty in tracing the course of the reverse currents, whether sent
or received, which deflect the needles in the opposite direction.
The field telegraph instrument selected by the War Department of the
United States Government is also extremely simple, communicating its
signals, not by the deflections of a needle, but by the blows on an
electro-magnet of its armature. The letters are indicated by various
combinations of two signals—one, a single stroke of the armature; and
the other, two blows in very rapid succession. The alphabet used is the
“General Service Flag Code” of the American army and navy, and the
signal numerals of this code are indicated by contacts of the
transmitting key—one contact producing a single blow of the armature,
implying the numeral 1, and two rapidly succeeding contacts causing two
blows, which stand for the numeral 2. The signals are read merely by the
sound made by the stroke of the armature. In the table below the code is
given, dots being used to represent the contacts of the key in the
“sending” instrument, and the blows of the armature in the “receiving”
instrument—the single dots standing for one contact or sound, and the
double dots for the double blows:
┌────────┬──────────┬────────────────────┐
│Letters.│Flag Code.│ Telegraph Signals. │
├────────┼──────────┼────────────────────┤
│ A │2 2 │·· ·· │
│ B │2 1 1 2 │·· · · ·· │
│ C │1 2 1 │· ·· · │
│ D │2 2 2 │·· ·· ·· │
│ E │1 2 │· ·· │
│ F │2 2 2 1 │·· ·· ·· · │
│ G │2 2 1 1 │·· ·· · · │
│ H │1 2 2 │· ·· ·· │
│ I │1 │· │
│ J │1 1 2 2 │· · ·· ·· │
│ K │2 1 2 1 │·· · ·· · │
│ L │2 2 1 │·· ·· · │
│ M │1 2 2 1 │· ·· ·· · │
│ N │1 1 │· · │
│ O │2 1 │·· · │
│ P │1 2 1 2 │· ·· · ·· │
│ Q │1 2 1 1 │· ·· · · │
│ R │2 1 1 │·· · · │
│ S │2 1 2 │·· · ·· │
│ T │2 │·· │
│ U │1 1 2 │· · ·· │
│ V │1 2 2 2 │· ·· ·· ·· │
│ W │1 1 2 1 │· · ·· · │
│ X │2 1 2 2 │·· · ·· ·· │
│ Y │1 1 1 │· · · │
│ Z │2 2 2 2 │·· ·· ·· ·· │
└────────┴──────────┴────────────────────┘
There are similar signals for the numerals and for a few often-recurring
syllables.
The telegraphs we have hitherto described leave no record of the
despatches sent, and hence the messages cannot be read at leisure, and
errors which may occur in the transmission cannot be traced to their
source. A system which registers the messages as actually received has
plainly many advantages over those which merely give a visible or
audible signal without leaving any trace. Hence many contrivances have
been proposed for making the receiving apparatus print the message in
ordinary characters. Such instruments are necessarily very much more
complicated in their construction than those we have already mentioned,
and by no means so simple as the system we are about to describe,
namely, the Morse Telegraph, which is now so largely used, being
universally adopted in America and on the continent of Europe; and,
since the telegraphic communication in Great Britain came into the hands
of the Post-office authorities, here, also, the Morse is the system most
approved.
[Illustration:
FIG. 285.—_Connections of a Telegraphic Line, with Morse Instruments._
]
The general arrangement of the transmitters, batteries, receiving
instruments, &c., should be first studied in its simplest form, as
represented by the diagram, Fig. 285. M represents the vertical coils of
an electro-magnet upon which we are supposed to be looking down; the
armature, A, is attached to a lever, F, which, by the attraction of the
electro-magnet is therefore drawn down. In the position of the
connections, as represented, no current is passing, but if K be pressed
down so as to make connection at 1, at the same time it is broken at 2,
a current will pass in from the positive pole of battery, B, into the
line by 1, 3, L, L´, and through 3´, 2´ through the coils of the
electro-magnet at M´ into the earth, and so back to the negative pole,
Z. The armature, A´, will be attracted so long as the current continues.
Similarly, contact made at 1´ and broken at 2´, will affect the
electro-magnet, M, from the battery at B´. It should be noticed here
that it is not a question of the reversal of currents sent from the same
battery; the key merely enables the operator to send a current in one
direction, so as to affect the distant electro-magnet whenever or so
long as he depresses the key. We shall now examine the construction of
the Morse receiving apparatus, one of the most complete forms of which
is depicted in Fig. 286. In the present description we wish the reader
to consider only the portion of the apparatus towards the left, and to
suppose the absence of the electro-magnet at the right-hand side, with
all the appliances immediately connected with it. He must regard the
electro-magnet, A, as corresponding with M´ in Fig. 285, and remember
that it is in the power of the distant operator at K to throw the
current of his battery through the coils of A, by simply depressing his
key. When the current passes the armature, B, it is attracted, and the
lever, C, to which it is attached, turns on its bearings at D, and the
end, E, of its longer arm is pressed upwards. At this end of the lever,
in the earlier form of the instrument, was a blunt steel point which,
while the armature was attracted to the electro-magnet, was pressed into
a shallow groove in a metallic roller. Between the roller and the steel
point a paper ribbon, half an inch wide, K, was unwound from the drum,
L, by the two rollers, M and N, which grip the paper between them as
they are turned by clockwork within the case, F.
[Illustration:
FIG. 286.—_Morse Recording Telegraph._
]
An important improvement was effected when, instead of steel points for
embossing the message, the Morse instrument was provided with an
arrangement for printing the signals in ink; since the pressure required
for embossing the paper is considerably greater than that needed merely
to bring it into contact with the edge of a little inked disc. In the
inking arrangement the strip of paper travels just below the margin of a
vertical disc, turned by the clockwork, and having its plane parallel to
the length of the paper strip. The narrow edge of this disc is kept
charged with printer’s ink, which it receives from a roller. The end of
the lever connected with the armature of the electro-magnet is formed of
a light strip of metal carrying a narrow projection at the end, over
which the paper passes, just beneath, but not touching, the inking disc.
When the current passes, the little projection is lifted up, and raises
the paper into contact with the ink, printing either a dot or a dash
according to the duration of the current. The amount of force required
to raise an inch or two of the length of the paper ribbon through a
space not greater than the twentieth of an inch is but small, and much
less than would be required to emboss the paper; so that in a great many
cases the part of the apparatus which is represented in Fig. 286, on the
right, may be dispensed with. In other cases it is, however, necessary;
as when, from the length of the line, the currents are too feeble to
give clear indications with the printing lever; and we shall, therefore,
presently describe its arrangement and purpose.
The clockwork is actuated by a spring, wound by the handle G, but its
action is suspended by a detent, which is released by touching the lever
H. When the clockwork is in action and the current constantly
circulating in the coils, a continuous line, parallel to the length of
the ribbon, would be printed upon it, in consequence of the contact with
the inking-disc, P, being maintained; but when a momentary current only
rushes through the coils, the armature attracted but for an instant,
gives rise to merely a dot on the passing paper, while a current of a
little duration will cause the paper to be marked with a short line or
dash.
The dot and the dash are the elementary signs of the Morse code of
signals, and these are producible according to the time the contact key
is held down at the distant station. By employing various combinations
of these two signs, the letters of the alphabet, numerals, &c., are
indicated. In selecting the combinations Professor Morse had regard to
the frequency with which the different letters recur in the English
language. Thus, for the letter E, which is more frequently used than any
other, the symbol chosen was a single dot; and for T, which is the next
most frequently employed, the dash was plainly the most appropriate;
then the four only possible combinations of the signs in pairs fell to
the next most frequent letters, and so on. The following table gives the
complete Morse code. The eye of the reader will doubtless detect a kind
of symmetry in the arrangement of the signs for the first five and last
five numerals:
ALPHABET.
┌──────────────────────────┬─────────┐
│ Letter. │ Sign. │
├──────────────────────────┼─────────┤
│ A │·- │
│ Ä │·-·- │
│ B │-··· │
│ C │-·-· │
│ D │-·· │
│ E │· │
│ É │··-·· │
│ F │··-· │
│ G │--· │
│ H │···· │
│ I │·· │
│ J │·--- │
│ K │-·- │
│ L │·-·· │
│ M │-- │
│ N │-· │
│ O │--- │
│ Ö │---· │
│ P │·--· │
│ Q │--·- │
│ R │·-· │
│ S │··· │
│ T │- │
│ U │··- │
│ Ü │··-- │
│ V │···- │
│ W │·-- │
│ X │-··- │
│ Y │-·-- │
│ Z │--·· │
│ Ch │---- │
└──────────────────────────┴─────────┘
NUMERALS.
┌──────────────────────────┬─────────┐
│ Numeral. │ Sign. │
├──────────────────────────┼─────────┤
│ 1 │·---- │
│ 2 │··--- │
│ 3 │···-- │
│ 4 │····- │
│ 5 │····· │
│ 6 │-···· │
│ 7 │--··· │
│ 8 │---·· │
│ 9 │----· │
│ 0 │----- │
└──────────────────────────┴─────────┘
PUNCTUATION, &c.
┌──────────────────────────┬─────────┐
│ │ Sign. │
├──────────────────────────┼─────────┤
│Full stop │······ │
│Colon │---··· │
│Semicolon │-·-·-· │
│Comma │·-·-·- │
│Interrogation │··--·· │
│Exclamation │--··-- │
│Hyphen │-····- │
│Apostrophe │·----· │
│[6]Fraction-line │------ │
│[7]Inverted commas │·-··-· │
│[7]Parenthesis │-·--·- │
│Italics or underlined │··--·- │
│New line │·-·-·· │
└──────────────────────────┴─────────┘
Footnote 6:
To be placed between the numerator and denominator of a vulgar
fraction.
Footnote 7:
To be placed before and after the words to which they refer.
OFFICIAL SIGNALS.
┌──────────────────────────┬─────────┐
│ │ Sign. │
├──────────────────────────┼─────────┤
│Public message │··· │
│Official Telegraph message│·- │
│Private message │·--· │
│Call │-·-·-·- │
│Correction, or rub out │···-· │
│Interruption │·········│
│Conclusion │·-·-·-· │
│Wait │·-··· │
│Receipt │·-··-··-·│
└──────────────────────────┴─────────┘
The length of a dot being taken as a unit, the length of a = 3 dots.
dash
The space between the signs composing a letter = 1 dot.
The space between two letters of a word = 3 dots.
The space between two following words = 6 dots.
[Illustration:
FIG. 287.—_Morse Transmitting Key._
]
[Illustration:
FIG. 288.—_Morse Transmitting Plate._
]
Fig. 287 is a view of the Morse transmitting key. A B is a brass lever,
moving in bearings at C, and provided at the end of its longer arm with
a large knob or button of some insulating material. Steel pins are
screwed in at B and D, and they are so adjusted that while that at B is
pressed against the projection, E, by the action of the spring, F, when
the knob, K, is pressed, contact is broken at B, and established at D. D
and E are each provided with a binding-screw, so that wires may be
attached in the manner indicated in Fig. 285. When the key is in the
position shown, a current arriving by the line-wire passes from the
fulcrum, C, of the lever through the contacts into the apparatus. When
the knob is pressed down the battery current enters the lever by the
contact at D, and passes into the line from the fulcrum, C. The clerks
who are called upon to transmit messages usually soon learn to time the
contacts very accurately in accordance with the code of signals, so as
to produce the dashes and lines with accuracy. However, with certain
persons some difficulty was found in acquiring the requisite uniformity,
and to obviate any objection on this score, Morse invented an
arrangement for facilitating the signalling, which is represented in
Fig. 288. This is a smooth tablet of a non-conducting substance, such as
ivory, except the shaded portions, which are plates of metal having
their surfaces even with that of the ivory, and all soldered to a plate
of metal beneath the ivory, which places them all in communication with
each other and with the binding-screw, C. The lengths of the strips of
metal and those of the spaces between them correspond with the dots and
dashes of the Morse alphabet as marked on the tablet. The battery wire
is connected with the binding-screw, C, and the line-wire terminates in
an elastic and flexible coil of insulated wire, which is attached to a
short rod having an insulated handle and terminated by a blunt platinum
point. This the transmitter takes in his hand and draws uniformly along
the line of metal strips belonging to the letter which he wishes to
telegraph. The circuit is closed while the point of the style is passing
across the metallic strips. This arrangement appears to be but little
used, but it is nevertheless admirable for its simplicity, and is
described here as a good illustration of the mode in which the varied
duration of the contacts is able to produce the signals of the Morse
alphabet. With the ordinary transmitting key a clerk is able to
telegraph, on the average, twenty or twenty-five words in a minute, but
the receiving apparatus is capable of recording three times as many.
Morse also invented a system of transmitting the messages automatically,
by setting up the message in a kind of type, just as ordinary letters
are arranged for printing. The type, if it may be so called, had simple
projections like the slips of metal, corresponding with each letter in
Fig. 288. The lines of the message were drawn under a contact-lever,
which closed the circuit when lifted up by the projections. Thus the
speed of transmission could be very greatly increased, and a single wire
and apparatus had its capacity of conveying a great number of messages
in a given time proportionately enlarged.
We have now to ask the reader’s attention to the details of the
apparatus in Fig. 286, the use of which has not already been pointed
out. The electro-magnet, O O´, and the parts immediately connected with
it, form what is called a _relay_. The object of this may be illustrated
by supposing that the instrument is at one end of a long line, such as
that between Edinburgh and London. Let us suppose it is at Edinburgh:
the currents sent from London by a battery of convenient size might not
be powerful enough to magnetize the soft iron of A with sufficient
intensity to give clearness to the signals. They are, therefore, made to
circulate in the electro-magnet, O, where they act by attracting the
armature, W, which has the form of a split tube of soft iron, attached
to a very light lever, Q, adjusted with great delicacy, and so that it
moves by little magnetic force. The end of the lever works between two
adjustable screws, R and S, which are electrically insulated, except
that R is in communication with one extremity of the coils of the
electro-magnet, A. Q is in metallic communication through the pillar, T,
and the binding-screw, U, with the zinc end of a battery at Edinburgh,
which is called the local battery, the other pole of which communicates
with the other ends of the coils, A, through the screw, U´. When no
current from London is passing through O, Q is held down by the spring,
W´, and the circuit of the local battery is broken; but the instant the
line-current passes, the armature, W, is attracted, and Q makes contact
with R, the current from the local battery rushes through the coils, A,
and the appropriate movements of the printing lever are effected by its
action. X is a spring for drawing down the lever, and it is provided
with a screw for adjusting its tension, and Y, Z, are screws for
limiting the extent of motion of the lever; under P is the little
projection by which the band of paper is pressed against the
inking-disc; _l_ and _e_ are respectively the screws for the line and
earth connections.
An extremely ingenious system of signalling, by which the speed could be
greatly increased, has been devised by Sir Charles Wheatstone, and is
largely adopted by the British postal authorities. In this system the
message is first translated into telegraphic language by a machine,
which punches certain holes in a strip of stiff paper. The apparatus
originally designed for this purpose by the inventor is thus described
by him in the Juror’s Report, International Exhibition of 1862:
“Long strips of paper are perforated by a machine constructed for the
purpose, with apertures grouped to represent the letters of the alphabet
and other signs. A strip thus prepared is placed in an instrument
associated with a source of electric power, which, on being set in
motion, moves it along, and causes it to act on two pins in such a
manner that when one of them is elevated the current is transmitted to
the telegraphic circuit in one direction, when the other is elevated it
is transmitted in the reverse direction. The elevations and depressions
of these pins are governed by the apertures and intervening intervals.
These currents, following each other indifferently in these two opposite
directions, act upon a writing instrument at a distant station in such a
manner as to produce corresponding marks on a slip of paper, moved by
appropriate mechanism.
“The first apparatus is a _perforator_, an instrument for piercing the
slips of paper with the apertures in the order required to form the
message. The slip of paper passes through a guiding groove, at the
bottom of which an opening is made sufficiently large to admit of the
to-and-fro motion of the upper end of a frame containing three punches,
the extremities of which are in the same transverse line. Each of these
punches, the middle one of which is smaller than the two external ones,
may be separately elevated by the pressure of a finger-key.
“By the pressure of either finger-key, simultaneously with the elevation
of its corresponding punch, in order to perforate the paper, two
different movements are successively produced: first, the raising of a
clip which holds the paper firmly in its position; and secondly, the
advancing motion of the frame containing the three punches, by which the
punch which is raised carries the slip of paper forward the proper
distance. During the reaction of the key consequent on the removal of
the pressure, the clip first fastens the paper, and then the frame falls
back to its normal position. The two external keys and punches are
employed to make the holes, which, grouped together, represent letters
and other characters, and the middle punch to make holes which mark the
intervals between the letters.
“The second apparatus is the _transmitter_, the object of which is to
receive the slips of paper prepared by the perforator, and to transmit
the currents in the order and direction corresponding to the holes
perforated in the slip. This it effects by mechanism somewhat similar to
that by which the perforator performs its functions. An eccentric
produces and regulates the occurrence of three distinct movements: 1.
The to-and-fro motion of a small frame which contains a groove fitted to
receive the slip of paper, and to carry it forward by its advancing
motion. 2. The elevation and depression of a spring-clip, which holds
the slip of paper firmly during the receding motion, but allows it to
move freely during the advancing motion. 3. The simultaneous elevation
of three wires placed parallel to each other, resting at one of their
ends over the axis of the eccentric, and their free ends entering
corresponding holes in the grooved frame. These three wires are not
fixed to the axis of the eccentric, but each end of them rests against
it by the upward pressure of a spring; so that when a light pressure is
exerted on the free end of either of them, it is capable of being
separately depressed. When the slip of paper is not inserted the
eccentric is in action; a pin attached to each of the external wires
touches during the advancing and receding motions of the frame a
different spring; and an arrangement is adopted, by means of insulation
and contacts properly applied, by which, while one of the wires is
elevated, the other remains depressed; the current passes to the
telegraphic circuit in one direction, and passes in the other direction
when the wire before elevated is depressed, and _vice versâ_; but while
both wires are simultaneously elevated or depressed the passing of the
current is interrupted. When the prepared slip of paper is inserted in
the groove, and moved forward whenever the end of one of the wires
enters an aperture in its corresponding row, the current passes in one
direction, and when the end of the other wire enters an aperture of the
other row, it passes in the other direction. By this means the currents
are made to succeed each other _automatically_ in their proper order and
direction to give the requisite variety of signals. The middle wire only
acts as a guide during the operation of the current.
“The wheel which drives the eccentric may be moved by the hand, or by
the application of any motive power. Where the movement of the
transmitter is effected by machinery, any number may be attended to by
one or two assistants. This transmitter requires only a single
telegraphic wire.
“The third apparatus is the _recording_ or _printing apparatus_, which
prints or impresses legible marks on a strip of paper, corresponding in
their arrangement with the apertures in the perforated paper. The pens
or styles are elevated or depressed by their connection with the moving
parts of the electro-magnets. The pens are entirely independent of each
other in their action, and are so arranged that when the current passes
through the coils of the electro-magnet in one direction, one of the
pens is depressed, and when it passes in the contrary direction the
other is depressed; when the currents cease, light springs restore the
pens to their elevated points. The mode of supplying the pens with ink
is the following: A reservoir about an eighth of an inch deep, and of
any convenient length and breath, is made in a piece of metal, the
interior of which may be gilt in order to avoid the corrosive action of
the ink; at the bottom of this reservoir are two holes, sufficiently
small to prevent by capillary attraction the ink from flowing through
them; the ends of the pens are placed immediately above these small
apertures, which they enter when the electro-magnets act upon them,
carrying with them a sufficient charge of ink to make a legible mark on
a ribbon of paper passing beneath them. The motion of the paper ribbon
is produced and regulated by apparatus similar to those employed in
other register and printing telegraphs.”
The mode by which Wheatstone proposed to indicate the letters was novel,
consisting in dots only, the numbers and positions of which in two lines
along the paper ribbon distinguished the letters—the system of combining
the symbols being still identical with the Morse code, only the dash was
replaced by a dot in the lower lines:
WHEATSTONE’S DOT SIGNALS.
˙⠄ ⠄˙˙˙ ⠄˙⠄˙ ⠄˙˙ ˙ ˙˙⠄˙ ⠄⠄˙ ˙˙˙˙ ˙˙ ˙⠄⠄⠄
A B C D E F G H I J
MORSE’S DOT AND DASH.
·- -··· -·-· -·· · ··-· -· ···· ·· ·-
A B C D E F G H I J
A single dot in the upper line stood for E, in the lower line for T; a
dot in the upper line, followed by one in the lower line a little to the
right, represented A; one in the lower line, followed by another in the
upper line, indicated N; and so on. By the dot printing it is said that
Wheatstone would signal 700 letters per minute. There were, however,
objections to the new code of signals: all the world had agreed to use
the Morse alphabet, and it was perhaps less liable to incorrect reading;
and for other reasons this more rapid signalling was unsuitable for
submarine lines. The apparatus has therefore been modified to suit the
dot and dash system of signals, and great improvements have been
effected by Sir Charles on the original instruments, with a view of
increasing the rapidity of transmission as much as possible. The paper
as punched for the Morse signals shows a row of equidistant holes in the
middle, by which the paper is guided uniformly forward, and in the outer
rows are holes arranged in pairs, either exactly opposite to each other
or obliquely—the former produce dots at the receiving station, the
latter dashes. From 60 to 100 words can thus be sent and printed in one
minute, and the automatic transmitting system can be applied to the
needle, or any other form of telegraph.
After a clerk has for some time been habituated to working with the
Morse instrument, he is able to read the message from the different
sounds made by the armature, as dashes or dots are respectively marked,
and he usually _listens_ to the message, and transcribes it at once into
ordinary language by the ear alone. This observation soon led to the
adoption of sound alone as the means of signalling, and an instrument on
this plan has already been referred to.
Among the more remarkable forms of recording telegraphs, that of Hughes
may be mentioned, in which the message is printed at the receiving
station in distinct Roman characters; and as only a single instantaneous
current is required to be sent for each letter, the speed with which a
message can be dispatched is about three times as great as with the
Morse instrument. These advantages are, however, obtained only at the
cost of great delicacy and complexity in the apparatus, so that it is
unfit for ordinary use, although it is much employed on important lines,
where competent operators and skilled mechanics and electricians are at
hand to keep it duly regulated. This machine is too complicated for a
full description in these pages, although it is the best form of
type-printing telegraph, and possesses a special feature in the fact
that the printing is done whilst the wheel carrying the types is in
rapid rotation. The reader will find full and untechnical descriptions
of this and of all the more important forms of telegraphic apparatus in
Mr. R. Sabine’s useful “History and Progress of the Electric Telegraph,”
or in Lardner’s work as edited by Sir Charles Bright.
[Illustration:
FIG. 289.—_The Step-by-step Movement._
]
[Illustration:
FIG. 290.—_Froment’s Dials._
]
From the numerous forms of dial telegraphs we select two for
description. All these instruments are characterized by what is called
the “step-by-step” movement, and differ in their mechanical details, and
in the nature of the apparatus for producing the currents, some being
driven by electro-magnets and others by galvanic batteries. Their
principle may be easily explained. Suppose that a ratchet-wheel, having
twenty-six teeth, is mounted on an axis carrying a hand over a dial
having the letters of the alphabet inscribed upon it. A simple
arrangement in connection with an electro-magnet, somewhat like the
escapement of a clock, will serve to advance the wheel by one tooth each
time a current passes. The diagram, Fig. 289, will at once make this
principle clear. E is the electro-magnet, F the armature, separated by
the spring, S, from the magnet, except when the current passes, when the
catch, C, draws down the tooth in which it is engaged, so that a tooth
passes under the point at D; and when the current ceases, the spring, S,
brings up the catch to engage the succeeding tooth, and thus the hand
moves step by step in the direction of the arrow, advancing each time
the electric circuit is closed by one twenty-sixth of a revolution. In
Fig. 290 is represented lecture-table models of a step-by-step
indicating and transmitting instrument, as constructed by M. Froment, of
Paris. These instruments are supposed to be at the extremities of a long
line of wire. The left-hand figure is the manipulator, or sending
instrument, in which the operator has merely to quickly turn round the
index in the direction of the hands of a watch, by means of the knob, P,
until it points to the desired letter, pause at the letter for an
instant, and then quickly continue the movement until his index points
to the cross at the top of the dial, where he pauses if the word is
spelt out, and, if not, continues the rotation until he arrives at the
next letter, and so on. All these movements and pauses the hand on the
indicator will accurately repeat, and the reason of this may be seen by
observing that the battery contacts are made by the projections on the
metallic wheel, R, which turn with the index. The spring, N, is always
in contact with the wheel, but the spring, M, has such a shape that
contact is alternately made and broken as the projections and spaces
pass it. It is obvious that the needle of the indicator will therefore
advance over the same letters as the index of the communicator.
[Illustration:
FIG. 291.—_Wheatstone’s Universal Dial Telegraph._
]
A very elegant dial instrument has been invented by Sir Charles
Wheatstone, in which magneto-electric currents are made use of. In Fig.
291 communicator and indicator are represented mounted in one case, or
small box. The larger dial is the communicator, and its circumference is
divided into thirty equal spaces, in which are the twenty-six letters of
the alphabet, three punctuation marks, and a +. In an inner circle are
two series of numerals and other signs. About the circumference of the
dial are thirty small buttons or projecting keys, conveniently arranged,
so as to be readily depressed by the touch of a finger. Inside of the
box a strong permanent horse-shoe magnet is fixed, and near its poles a
pair of armatures of soft iron cores with insulated wire coils revolve
when the handle, A, is turned, as in the machines described in the last
article. In this manner a series of waves or short currents of
electricity are produced in the conductors when the circuit is complete,
and the currents are alternately in opposite directions, so that fifteen
revolutions of the coils will produce fifteen currents in one direction
and fifteen in the other. A pinion on the same spindle as the coils
works with a wheel on the axis carrying the pointer on the dial, so that
the pointer makes a complete revolution as often as the handle, A, makes
fifteen turns. Each of the thirty currents will pass through the
indicator, I, and through the line to the distant station, where they
will, by a step-by-step movement, advance the needle of the indicator.
So that the hand of the dial and the needle of the indicator at the
sending station, and that of the indicator at the distant station, will
all simultaneously be pointing to the same letter on their respective
dials; and they would continue to move round these, ever pointing to the
same letter, so long as the handle, A, is turned. How, then, is the
sender to cause the needle of his correspondent’s instrument to pause at
any desired letter? Not by stopping the revolution of the handle, A, for
that could not be done so as to send just the right number of currents,
inasmuch as the rotating armatures could not be instantly stopped. The
mode of causing the indicators to pause at any required letter is as
simple as it is ingenious. It has been already mentioned that the
step-by-step movement takes place at every current which passes through
the line, including the two indicators, and that thirty such currents
pass at each revolution of the pointer of the communicator. But when
these currents no longer flow, the indicators, of course, stop; and the
stoppage of the movements is reconciled with the continuous production
of the currents by having a series of little levers, each connected with
one of the buttons, and so arranged that when one of these has been
pushed down, the lever stops the revolution when it has come round of an
arm on the same central axis as the pointer, and riding loosely on a
hollow spindle, which bears the toothed wheel, driven by the pinion
already spoken of. The projecting arm is provided with a spring, which
falls between the teeth of the wheel, so that the arm is with certainty
carried round with the wheel. But where a button has been pushed down,
its lever catches the arm, lifting its spring away from the teeth of the
wheel. So long as the key remains down, the arrested arm makes a short
metallic circuit by its contact, and no currents pass into the line, for
they take the shortest path. The key is raised only when another is
depressed, and then the arm and the pointer immediately resume their
revolution until they again become stationary at the letter
corresponding with the key which has been pushed down. Suppose the key
of +, the zero of the dial, to be down, which is the proper condition of
the apparatus when a message has to be dispatched. The operator having
rung a bell at the distant end, to call the attention of the person who
receives his message, begins to turn the handle, A, at the rate of about
two revolutions per second. In this state of affairs no current is
passing into the line, and the fingers of both his communicator and
indicator remain stationary, as does also that of the indicator at the
distant end of the line. Now, suppose he has to spell the word “FOX.” He
turns the handle A continuously with his right hand the whole time he is
sending the message; and, manipulating the keys with his left, he
depresses that opposite to the letter F. By this action the key opposite
+ is raised, for the levers are pressed into notches against a
watch-chain, which has just enough _slack_ to allow one lever to enter a
notch, and therefore the pressure of another lever always raises the key
last depressed. When the operator presses down the F key, the + rises,
the radial arm is instantly released, and with the index is carried on
to F, where it stops; and the contacts will have, during that movement,
sent six currents into the line, so that the fingers of both indicators
will also point to F. When the pointer of the communicator has made just
a visible pause at F, he pushes down the key of O, and all the three
pointers recommence their journeys towards that letter. The operator
must, of course, wait until they have reached it and paused an instant,
when he depresses the button opposite X; and when the index has pointed
at that, he pushes down the + key, whereby the fingers all arrest their
movements at that point, indicating that the word is completed. In the
case supposed the word is completed by a single revolution of the
pointers; but this is, of course, not usually the case; thus, in
indicating the syllable “PON,” nearly three complete revolutions would
be required.
This admirable little instrument was designed for the use of private
persons, and is largely used in London and elsewhere. Its great
compactness and simplicity of operation render it highly suitable for
this purpose. There is no battery required, and all the inconvenient
attention demanded by a battery is therefore dispensed with. On the
other hand, the magnets gradually lose their power, and after a time
must be re-magnetized; and the electro-motive force developed in these
instruments is insufficient for lengths of line much exceeding 100
miles. For shorter lines, and for the purposes for which they are
designed, these instruments are perfection.
Very interesting forms of telegraph are those in which a despatch is not
merely written or printed, but actually transcribed as a _facsimile_ of
the writing in the original; and in this way it is possible for a design
to be drawn telegraphically at the distance of hundreds of miles. Like
the Hughes’ printing telegraph, the instruments which produce these
apparently marvellous results require synchronous movements at the two
stations. But although they are scientifically successful, there appears
to be no public demand for these copying telegraphs. One of the best
known is Bonelli’s, which dispatches its messages automatically when
they have been set up in raised metal types precisely similar to the
Roman capitals in the type of the ordinary printer. In Bonelli’s and
most other copying telegraphs the impressions are produced by chemical
decompositions—effected at the receiving station on the paper prepared
to receive the message. By Bonelli’s instrument it is said that when the
type has been set up, messages can be sent at the extraordinary rate of
1,200 words in one minute of time! The action of this system is such
that it is proved to be possible to reproduce in a few seconds—at York,
say—the very characters of a page of type the moment before set up in
London. The limits of our space will not admit of details of this
invention; but we here place before the reader a _facsimile_ of the
letters printed by it at the receiving stations.
[Illustration: BONELLI’S CHEMICAL TELEGRAPH]
We have to describe two other forms of instruments for receiving
telegraphic signals, both contrived with consummate skill by Sir William
Thomson, and, though exhibiting no new principle in any of their parts,
both fine examples of beautiful adjustment of materials for a desired
end. In these forms of apparatus, the delicacy of the mechanical
construction, and the accurate relations of one part to another, have
produced results of the greatest practical importance. Fig. 292
represents the _mirror galvanometer_, an instrument which has not only
proved of the highest value in scientific researches, but is of the
first importance in submarine telegraphy. It is in principle nothing
more than the single-needle telegraph, and it is exceedingly simple in
construction. A very small and light magnet, such as might be formed by
a fragment of the mainspring of a watch, ⅜ths of an inch long, say, is
attached to the back of a little circular mirror, made of extremely thin
silvered glass, also about ⅜ths of an inch in diameter. The mirror and
magnet are suspended by a single cocoon-fibre, so fine as to be almost
invisible, in the centre of a coil, A, of fine silk-covered copper wire.
In front of the suspended mirror, in the axis of the coil, is placed a
lens of about four feet focal distance, and opposite to this is a screen
having a slit, B, in the centre, behind which is placed a paraffin lamp,
D. The screen is provided with a paper scale, C, divided into equal
parts, and is placed at the distance of about two feet from the little
mirror. It follows, from this arrangement, that when the light passing
through the slit falls upon the mirror, it is reflected again through
the lens, and an image of the slit is seen on the scale. This image is
immediately above the slit when the beam falls perpendicularly upon the
mirror, and this condition may be brought about by properly placing the
apparatus with regard to the magnetic meridian. The directive power of
the earth over the little suspended magnet is, however, _almost_
annulled by properly fixing the steel magnet, E, which slides upon the
upright rod, so that the suspended magnet is thus free to obey the least
force impressed upon it by a current passing through the coil. And when
the mirror is deflected through a certain angle, the image on the scale
will be deflected to twice that angle, and thus the smallest movements
of the suspended magnet are readily recognized; not only by reason of
the length of the beam of light, which forms a weightless index, but
because they are doubled by this increased angular deflection.
[Illustration:
FIG. 292.—_The Mirror Galvanometer._
]
When the signals are being rapidly transmitted through a long submarine
line, the currents at the receiving station are much enfeebled and
retarded, and the result is that the movements of a suspended needle
have by no means the decided character which is seen in the instruments
connected with land lines. The signals through a submarine cable could
not therefore be received by any apparatus which required a certain
strength of current; but the mirror galvanometer indicates every change
in the currents, and the apparently irregular motions of the spot of
light can be interpreted by a skilled clerk, who, by long experience,
recognizes, in quite dissimilar effects, the same signal sent by the
clerk at the other end in precisely the same way. Thus a first contact,
corresponding with a dot of the Morse alphabet, may cause the light to
move some distance on the scale, a second contact immediately succeeding
moves it but a little way farther, and a third may occasion a movement
hardly perceptible.
The messages sent by the mirror galvanometer must be read as they are
received; and, as a telegraphic instrument, it is wanting in the
manifest advantages attending a recording instrument. Sir W. Thomson
has, however, devised another receiving instrument of great delicacy,
which is termed the _syphon recorder_. We cannot here describe its
admirable mechanical and electrical details, but the chief feature is
that the attractions and repulsions of the currents are made to produce
oscillations in a syphon formed of an extremely fine glass tube, the
shorter branch of which dips in a trough of ink, and the longer branch
terminates opposite to, but not touching, a band of paper, which is
continuously and regularly drawn along by clockwork while the message is
being received. The tube is a mere hair-like hollow filament of glass,
and the ink, which would not itself flow from a tube of so fine a bore,
is squirted out by electrical repulsion when the insulated reservoir in
which it is contained is electrified at the receiving station by an
ordinary machine. The message as written by this instrument appears
thus:
[Illustration: _the syphon recorder_]
The reader, on comparing these signals with the Morse code on page 560,
will have no difficulty in discovering their relation to it.
_TELEGRAPHIC LINES._
It now remains to give some account of the _line_, that is, the
conductor by which the sending and receiving instruments are united, and
along which the currents flow. Overhead lines are nearly always
constructed with iron wires, which are usually ⅙ in. in diameter, and
are coated with some substance to protect them from oxidation. Zinc is
often used for this purpose, the wire being drawn through melted zinc,
by which it becomes covered with a film of this metal—a process known as
“galvanizing” iron. Another mode is to cover the wires with tar, or to
varnish them from time to time with boiled linseed oil, and this _must_
be done in populous places, where the gases in the air are liable to act
upon the zinc. Sometimes _underground_ wires are used, and these are
often made of copper, covered with gutta-percha, and are laid in wooden
troughs, or in iron pipes. They are protected by having tape or other
material, saturated with tar or bitumen, wound round them. The poles
employed to suspend the overhead wires are generally made of larch or
fir, of such a length that when securely fixed in the ground they rise
12 ft. to 25 ft. above it, and at the top have a diameter of about 5 in.
About thirty poles are required for each mile, and every tenth pole
forms a “stretching-post,” being made stronger than the others and
provided with some appliance by which the wires can be tightened when
required. The wires are attached to the posts by insulating supports;
but at every pole there is always some “leakage,” the amount of which
depends on the form, material, and condition of the insulators. Glass is
quite unsuitable, because its surface strongly attracts moisture, which
thus forms a conducting film. All things considered, porcelain is found
to be the best insulating material for this purpose, since moisture is
not readily deposited on its surface, and even rain runs off without
wetting it; and it is durable, strong, and clean. Fig. 293 shows a
telegraph post, with brown salt-glazed stoneware insulators, shaped like
hour-glasses, with a perspective view and section of one of them.
Another form of insulator, shown in Fig. 294, has a stalk or hook of
porcelain, with a notch, into which the wire is simply lifted, and is
protected above by a porcelain bell. This form, or some modification of
it, is that most generally used.
[Illustration:
FIG. 293.—_Telegraph Post and Insulators._
]
[Illustration:
FIG. 294.
]
It need hardly be remarked that only a single wire is required with most
of the modern instruments for communication between any two places. Each
of the many wires often seen attached to the telegraph posts along a
road or railway represents a distinct line of communication—that is, one
wire may connect the two termini, another may join an intermediate
station and a terminus, a third may belong to two intermediate stations,
and so on. We have already alluded to the discovery by Steinheil of the
apparent conducting power of the earth; and if we must continue to think
of complete circuits, we must regard the earth as replacing for
telegraphic purposes the second or return wire, which was at first
supposed essential. For instance, when a battery current had to be sent
from Station A, Fig. 295, which we may suppose to be London, to Station
B, which we may call Slough, it was at first thought requisite to
provide a wire for the return of the current after it had traversed the
coils at the receiving station. But now the connections are made as
shown in Fig. 296, where the return wire is dispensed with, except a
small portion at each end, which is connected with a large plate of
copper buried in the earth; the arrows show the direction of the
current, according to the commonly received notion. By this plan the
current is increased in intensity, for the “earth circuit” appears to
offer less resistance than the copper wire. The view, however, which
regards the earth not as a conductor in the same sense as the wire, but
as the great _reservoir or storehouse of electricity_, accords better
with known facts.
[Illustration:
FIG. 295.—_Wire Circuit._
]
[Illustration:
FIG. 296.—_Wire and Earth Circuit._
]
The spread of telegraph lines, and the extent to which this mode of
communication is used by the public, may be illustrated by a few
particulars regarding the Central Telegraphic Office in London. The
management of all the public telegraph lines in Great Britain is now in
the hands of the Post Office authorities, and the arrangements at the
central office in London are an admirable specimen of administrative
organization. The Central Telegraph Office occupies a very large and
handsome building opposite the General Post Office, St.
Martin’s-le-Grand. In one vast apartment in this building, containing
ranges of tables, in all three-quarters of a mile long, may be seen
upwards of six hundred telegraph instruments, besides a number of
stations for the receipt and transmission of bundles of messages by
pneumatic dispatch. The number of clerks employed in working the
instruments is 1,200, and about three-fourths of these are females. The
wires from each instrument are conducted below the floor of the
apartment to a board where they terminate in binding-screws, marked with
the number of the instrument. The same board has binding-screws, with
battery connections, and others which form the terminals of the
telegraph lines, and thus the requisite connections are readily made.
The batteries are placed in a lower room, which contains about 23,000
cells of Daniell’s construction, formed into nearly 1,000 distinct
batteries, in each of which the number of cells varies according to the
length of the line through which the current has to pass. Thus, the
battery which supplies the currents that are sent through the coils of
the instrument at Edinburgh consists of 60 cells, but one-sixth of that
number suffices for some of the short lines. The instrument almost
exclusively used is the Morse recorder, and Wheatstone’s automatic
punching machine and transmitters are in constant employment. There are
also some examples of other instruments to be seen in operation, such as
the Hughes type printing telegraph, the American sounder, a few A, B, C,
dial instruments, and a solitary specimen of a double-needle instrument.
Upwards of 30,000 messages pass through this office each day.
[Illustration:
FIG. 297.—_Submarine Cable between Dover and Calais._
]
But the most striking achievements in connection with telegraphy are the
great submarine lines which unite the Old and New Worlds. Morse and
Wheatstone about the same time (1843) independently experimented with
sub-aqueous insulated wires, and their success gave rise to numerous
projects for submarine lines. How far any of these might have been
practical need not here be discussed, but it fortunately happened that
some years after this, the electrical properties of gutta-percha were
recognized, and this material, so admirably adapted for forming the
insulating covering of wires, was taken advantage of by Brett and Co.,
who obtained the right of establishing an electric telegraph between
France and England, and they succeeded in laying down the first
submarine cable. This cable extended from Dover to Cape Grisnez near
Calais, and the experiment proved successful; but, unfortunately, the
cable was severed within a week by the sharp rocks on which it rested
near the French coast. It proved, however, the excellent insulating
property of the new material, and demonstrated the possibility of
submarine telegraphic communication. Another cable was manufactured, in
which the gutta-percha core was protected by a covering of iron wires
laid specially on the exterior, and thus combining greater security with
a far larger amount of tenacity. A view and section of this—the first
practically successful submarine cable—are given in Fig. 297 of the real
size. It has four separate copper wires, each insulated with a covering
of gutta-percha, and the whole was spun with tarred hemp into the form
of a rope, and protected with an outer covering of ten of the thickest
iron wires wound spirally upon it. The cable when complete was 27 miles
in length, and each mile weighed 7 tons. This cable was laid in 1851,
and from that time it has been in constant use, with the exception of a
few interruptions from accidental ruptures. Its success immediately led
to the construction of other cables connecting England with Ireland,
Belgium, Holland, &c. In 1855 the practicability of an Atlantic cable
was no longer doubted, and £350,000 were soon subscribed by the public
for the project. A cable was manufactured weighing 10 tons to the mile,
and in August, 1857, 338 miles of it had been successfully paid out by
the ships when the cable parted. Better paying-out apparatus was now
devised—self-releasing brakes were constructed, so that the cable should
not be exposed to too great a strain; and in 1858 another cable,
requiring a strain of 3 tons to break it, was manufactured, and the
laying of it commenced in mid-ocean—the _Mægera_ and _Agamemnon_ going
in opposite directions, and paying out as they proceeded. Twice the
cable was severed, twice the ships met and repaired the injury; but the
third time, when they were 200 miles apart, the cable again broke. But
again the attempt was repeated, and this time success crowned the
effort; for on the 5th of August the two continents were telegraphically
connected. Unfortunately the electric continuity failed after the cable
had been a month in use.
Seven years elapsed before another endeavour was made; but the
experience gained in the unsuccessful attempt was not lost; and in 1865
another cable had been constructed, and the _Great Eastern_ was employed
in laying it. In this the conductor was composed of seven copper wires
twisted into one strand, covered with several layers of insulating
material, and covered externally with eleven stout iron wires, each of
which was itself protected by a covering of hemp and tar. This cable was
2,600 miles long, and contained 25,000 miles of copper wire, 35,000
miles of iron wire, and 400,000 miles of hempen strands, or more than
sufficient to go twenty-four times round the world. It was carefully
made, mile by mile, formed into lengths of 800 miles, and shipped on
board the _Great Eastern_ in enormous iron tanks, which weighed, with
their contents, more than 5,800 tons. This cable was manufactured by
Messrs. Glass and Elliot, at Greenwich, to whom the iron wire for the
outer covering was furnished by Messrs. Webster and Horsfall, of
Birmingham. Fig. 298 represents the workshops with the iron wire in
process of making. The great ship sailed from Valentia on the 23rd of
July, 1865, and the paying out commenced. Constant communication was
kept up with the shore, and signals exchanged with the instrument-room
at Valentia, which is represented in Fig. 299, where, among various
instruments invented by Sir W. Thompson, may be seen his mirror
galvanometer. After several mishaps, which required the cable to be
raised for repairs after it had been laid in deep water, the _Great
Eastern_ had paid out about 1,186 miles of cable, and was 1,062 miles
from Valentia, when a loss of insulation in the cable was discovered by
the electricians on board. This indicated some defect in the portion
paid out, and the usual work of raising up again had to be once more
resorted to. During this process the cable parted, and Fig. 300 shows
the scene on board the _Great Eastern_ produced by this occurrence, as
represented by an artist of the “Illustrated London News” who
accompanied the expedition. The broken cable was caught several times by
grapnels, and raised a mile or more from the bottom, but the tackle
proved unable to resist the strain, and four times it broke; and after
the spot had been marked by buoys, the _Great Eastern_ steamed home to
announce the failure of the great enterprise. For this 5,500 miles of
cable had altogether been made, and 4,000 miles of it lay uselessly at
the bottom of the ocean, after a million and a quarter sterling had been
swallowed up in these attempts.
[Illustration:
FIG. 298.—_Making Wires for Atlantic Telegraph Cable._
]
[Illustration:
FIG. 299.—_The Instrument-Room at Valentia._
]
But these disasters did not crush the hopes of the promoters of the
great enterprise, and in the following year the _Great Eastern_ again
sailed with a new cable, the construction of which is shown of the
actual size, in Fig. 301. In this there is a strand of seven twisted
copper wires, as before, forming the electric conductor; round this are
four coatings of gutta-percha; and surrounding these is a layer of jute,
which is protected by ten iron wires (No 10, B.W.G) of Webster and
Horsfall’s homogeneous metal, twisted spirally about the cable; and each
wire is enveloped in spiral strands of Manilla hemp. The _Great Eastern_
sailed on the 13th of July, and on the 28th the American end of the
cable was spliced to the shore section in Newfoundland, and the two
continents were again electrically connected. They have since been even
more so, for the cable of 1865 was eventually fished up, and its
electrical condition was found to be improved rather than injured by its
sojourn at the bottom of the Atlantic. It was spliced to a new length of
cable, which was successfully laid by the _Great Eastern_, and was soon
joined to a Newfoundland shore cable. There were now two cables
connecting England and America, and one connecting America and France
has since been laid. At the present time upwards of 20,000 miles of
submerged wires are in constant use in various parts of the world.
[Illustration:
FIG. 300.—_The Breaking of the Cable._
]
Certain interesting phenomena have been observed in connection with
submarine cables, and some of the notions which were formerly
entertained as to the speed of electricity have been abandoned, for it
has been ascertained that electricity cannot properly be said to have a
velocity, since the same quantity of electricity can be made to traverse
the same distance with extremely different speeds. No effect can be
perceived in the most delicate instruments in Newfoundland for one-fifth
of a second after contact has been made at Valentia; after the lapse of
another fifth of a second the received current has attained about seven
per cent. of its greatest permanent strength, and in three seconds will
have reached it. During the whole of this time the current is flowing
into the cable at Valentia with its maximum intensity. Fig. 302
expresses these facts by a mode of representation which is extremely
convenient. Along the line O X the regular intervals of time in tenths
of seconds are marked, commencing from O, and the intensity of the
current at each instant is expressed by the length of the upright line
which can be drawn between O X and the curve. The curve therefore
exhibits to the eye the state of the current throughout the whole time.
If after nearly a second’s contact with the battery the cable be
connected with the earth at the distant end, the rising intensity of the
current will be checked and then immediately begin to decline somewhat
more gradually than it rose, as indicated by the descending branch of
the curve in Fig. 302. A little reflection will show the unsuitability
for such currents of instruments which require a fixed strength to work
them. We may remark that, supposing a receiving instrument were in
connection with the Atlantic Cable which required the maximum strength
of the received current to work it, the sending clerk would have to
maintain contact for three seconds before this intensity would be
reached, and then, after putting the cable to earth, he would have to
wait some seconds before the current had flowed out. Several seconds
would, therefore, be taken up in the transmission of one signal, whereas
by means of the mirror galvanometer about one-fourteenth of this time
suffices, and the syphon recorder will write the messages twelve times
as fast as the Morse instrument. The cause of the gradual rise of the
current at the distant end of a submarine cable must be sought for in
the fact that the coated wire plays the part of a Leyden jar, and the
electricity which pours into it is partly held by an inductive action in
the surrounding water. The importance of Sir W. Thomson’s inventions as
regards rapidity of signalling, upon which the commercial success of the
Atlantic Cable greatly depends, will now be understood.
[Illustration:
FIG. 301.—_Atlantic Telegraph Cable, 1866._
]
[Illustration:
FIG. 302.
]
By furnishing the means of almost instantaneous communication between
distant places, the electric telegraph has enabled feats to be performed
which appear strangely paradoxical when expressed in ordinary language.
When it is mentioned as a sober fact that intelligence of an event may
actually reach a place before the time of its occurrence, a very
extraordinary and startling statement appears to be made, on account of
the ambiguous sense of the word _time_. Thus it appears very marvellous
that details of events which may happen in England in 1876 can be known
in America in 1875, but it is certainly true; for, on account of the
difference of longitude between London and New York, the hour of the day
at the latter place is about six hours behind the time at the former. It
might, therefore, well happen that an event occurring in London on the
morning of the 1st of January, 1876, might be discussed in New York on
the night of the 31st of December, 1875. There are on record many
wonderful instances of the celerity with which, thanks to electricity,
important speeches delivered at a distant place are placed before the
public by the newspapers. And there are stories in circulation
concerning incidents of a more romantic character in connection with the
telegraph. The American journals not long ago reported that a wealthy
Boston merchant, having urged his daughter to marry an unwelcome suitor,
the young lady resolved upon at once uniting herself to the man of her
choice, who was then in New York, _en route_ for England. The electric
wires were put in requisition; she took her place in the telegraph
office in Boston, and he in the office in New York, each accompanied by
a magistrate; consent was exchanged by electric currents, and the pair
were married by telegraph! It is said that the merchant threatened to
dispute the validity of the marriage, but he did not carry this threat
into execution. The following _jeu d’esprit_ appeared a short time ago
in “Nature,” and, we strongly suspect, has been penned by the same hand
as the lines quoted from “Blackwood,” on page 508.
ELECTRIC VALENTINE.
(_Telegraph Clerk_ ♂ _to Telegraph Clerk_ ♀.)
“‘The tendrils of my soul are twined
With thine, though many a mile apart;
And thine in close-coiled circuits wind
Around the magnet of my heart.
“‘Constant as Daniell, strong as Grove;
Seething through all its depths like Smee;
My heart pours forth its tide of love,
And all its circuits close in thee.
“‘Oh tell me, when along the line
From my full heart the message flows,
What currents are induced in thine?
One click from thee will end my woes!’
“Through many an Ohm the Weber flew,
And clicked this answer back to me—
‘I am thy Farad, staunch and true,
Charged to a Volt with love for thee.’”
[NOTE BY THE EDITOR.—_Ohm_, standard of electric resistance; _Weber_,
electric current; _Volt_, electro-motive force; _Farad_, capacity (of a
condenser).]
_THE TELEPHONE._
Of more recent invention than any of the classes of instruments already
mentioned for electrical communication at a distance is the telephone,
which differs widely from the rest in many notable particulars. Though
the telephone completely realized what had for years before been the
dream of physicists, the first announcement of its capabilities was
received, even by the scientific world, with some pause of incredulity;
but when its powers were demonstrated, it created no small sensation. It
has now, within a few years afterwards, become so familiar as an
appliance of ordinary life and business, that people in general are less
impressed by the wonder of it than were their fathers half a century ago
by the electric telegraphs of Wheatstone and of Morse. Like all other
inventions, it was led up to by preceding discoveries and tentative
efforts. It will be unnecessary here to trace those successive steps
with minuteness, or to attempt to adjust the claims of merit or priority
that have been put forward for different inventors, but a notice of some
of the stages in the evolution of this wonderful contrivance may be of
interest. If the reader has no previous knowledge of the physical nature
of sounds in relation to music, and especially to articulate speech, he
should now refer to the brief explanation given in a subsequent chapter,
at the commencement of the section on the Phonograph. He should,
however, bear in mind that in that explanation are included some
acoustical discoveries of a later date than some of the inventions we
are here to speak of, or, at least, the real causes of which give other
qualities than pitch to sound, had not been fully demonstrated when the
notion of the electric telephone was conceived.
When the electric telegraph came into use and it was found possible to
use it for communication of intelligence to great distances, it is not
surprising that the further problem of transmitting by electricity, not
signals merely, but audible speech, should be suggested. Perhaps the
first scientific person who avowed a belief in the possibility of doing
this, and even indicated the direction in which the solution of the
problem was to be sought, was a Frenchman of science, M. Charles
Bourseul. In 1854, he pointed out that sounds are caused by vibrations,
and reach the ear by like vibrations of the intervening medium, and,
although he could not say what took place in the modifications of the
organs of speech by which syllables are produced, he inferred that these
syllables could reach the ear only by vibrations of the medium, and that
if these vibrations could be reproduced the syllables would be
reproduced. He suggests that a man might speak near a flexible disc,
which the vibrations of his voice would throw into oscillatory movements
that could be caused to make and break a battery circuit, and that, at a
distance, the currents might be arranged to produce the like vibrations
in another disc. The weak point of this scheme was the want of any
suggestion as to the mode in which this last effect was to be produced.
Even when this part of the problem was solved in a few years afterwards,
as we shall presently see, it was musical—and not articulate—sound that
could be transmitted by an arrangement, using make and break contacts.
The reader, who has understood what has been said of electrical
currents, and also the account of the compounded vibrations in
articulate sounds introduced into our section on the phonograph, should
have little difficulty in seeing this must necessarily be the case, for
the contacts could only give the succession of the vibrations by
currents of equal intensity, and could not, like the yielding wax of the
phonograph cylinder, correspond with their relative intensities. M.
Bourseul pointed out advantages which would arise from the transmission
of speech by electricity, such as simplicity of apparatus and facility
in use—for, unlike the telegraph, no skilled operators would be
needed—to signal messages, or time spent in spelling out the words
letter by letter. He says that he had made some experiments, which
promised a favourable result, but demanded time and patience, and that
he is certain that, in a more or less distant future, speech will be
transmitted by electricity, so that what is spoken in Vienna may be
heard in Paris. One cannot help thinking that if M. Bourseul had but
pursued his experiments a little longer, he would not improbably have
achieved the invention of the speaking telephone, for which the world
had to wait twenty years longer. As it is, we cannot but admire his
scientific foresight and his confidence in the ultimate realization of
his idea.
But before this came to pass, an intermediate stage was reached in the
apparatus contrived by M. Reiss, a schoolmaster of Friedrichsdorf, who,
in 1860, solved the problem of electrically transmitting musical tones.
So far as concerned the reproduction of the sounds, this telephone was
founded upon a discovery, made in 1837, by an American physicist, named
Page, which was this: At the moment a bar of iron is magnetized, by
sending a current through a coil surrounding it, as shown in Fig. 265, a
slight but sharp click is heard. The transmitting apparatus was, in
principle, Mr. Scott’s phono-autograph (described in the section on the
phonograph), which had been invented in 1855. The tracing style of this
was replaced in Reiss’ apparatus by a small disc of platinum, connected
by a very light spring of the same metal with a binding-screw for the
battery connection. Nearly in contact with the little disc was a
platinum point, so arranged that the slightest oscillation of the
membrane would bring them into actual contact and thus close the
circuit. Worthy of remark is the very primitive nature of the materials
with which Reiss made his first experimental apparatus. The receptacle
for the voice was simply a large bung hollowed out into a conical
cavity, and the membrane was supplied by the skin of a German sausage,
while the clicking bar of the receiver was a stout knitting needle,
surrounded by a coil of covered copper wire and stuck into the bridge of
a violin, which, by acting as a sounding board, made the clicks produced
in the needle distinctly audible. M. Reiss finally produced his
telephone in the form shown in Fig. 302_a_, where _I_ is the receiver;
_B_, the voltaic battery; _I I_, the receiver; _c c_ is a coil of
insulated wire, surrounding a slender iron rod, mounted on the supports,
_f f_, which rest on the sounding board, _g g_. The transmitter consists
of the hollow box, _A_, provided with a trumpet-mouthed opening in one
side and having at the top a circular piece cut out, across which is
stretched a membrane with the little disc of platinum, _n_, fixed in its
centre. When a person applying his mouth to A sings into the box, the
membrane is thrown into vibrations corresponding with the notes, and at
each vibration a contact is made and a click is emitted from the distant
sounding box. The tones are concentrated by covering this box with the
perforated lid. It was afterwards found that a trumpet mouth fitted into
the receiver was still more effective. Reiss tried to use his
arrangement for transmitting speech, but without success, although
occasionally a syllable could be very indistinctly heard. An instrument,
with springs so nicely adjusted that slight vibrations did not separate
the platinum from actual contact, but merely caused change of pressure,
has indeed been made to convey articulate sounds, although the
arrangement was not essentially different from that of M. Reiss. This
mode of action is, however, a different thing, and we shall presently
see that very effective speech transmitters have been constructed by
applying it in a more refined way. This musical telephone could give the
pitch of the sounds in the song but not their quality (_timbre_), and
the receiver added to the main system of vibration other sets that
belonged to itself, the result being a shrill and by no means pleasing
tone, recalling that of a penny trumpet. Messrs. C. and L. Wray
afterwards effected some considerable improvements in M. Reiss’s
telephone, with the object of intensifying the effects and producing
better tones.
[Illustration:
FIG. 302_a_.—_Reiss’ Musical Telephone._
]
[Illustration:
FIG. 302_b_.—_Bell’s Musical Telephone._
]
A further step towards the speaking telephone may be illustrated by an
earlier invention of Mr. Graham Bell, a native of Scotland, who had
settled in the United States. Mr. Bell’s inventions, it may be
mentioned, were by no means the results of fortunate accidents or of
unsought and spontaneous flashes of conception, but rather the outcome
of long, patient and systematic studies. His father, Mr. Alexander
Melville Bell, of Edinburgh, had assiduously cultivated acoustic
science, and had in conjunction with his son, undertaken special
researches into the mechanism of the organs of speech, the elements of
articulate speech in different languages, and the musical components of
vocal sounds. When Graham afterwards pursued these studies in the light
of the fuller investigation carried out by Helmholtz, he was naturally
led to the application of electricity to acoustic transmission. After
some experiments in the production of vowel sounds by combinations of
electric tuning forks, he invented a telephone for reproducing musical
sounds at a distance, which was a great improvement on that of Reiss,
and involved another principle, which indeed is the same as that
utilized in his more mature invention of the speaking telephone. As a
like explanation of the action would apply in both cases, the reader
will find his advantage in following the observations we have to make on
the earlier instrument. This consisted of what was virtually two sets of
electric tuning forks, each set being acted upon by one electro-magnet.
Fig. 302_b_ will suffice to show the general form of the arrangement. A
plate of steel is bent twice at right angles longitudinally, and is
magnetized so that any transverse slice of it would constitute an
ordinary horse shoe magnet. This is seen endways in Fig. 302_b_ at M,
and N. and S. will indicate the north and south poles respectively. To
each limb of this broad magnet is attached a plate of steel, T, cut into
teeth, just in the same way as the steel plate in a common musical box
or mechanical piano, except that the teeth are not pointed. These are
tuned to give severally in pairs the notes of the musical scale when
thrown into vibration. Between the prongs of the series of tuning forks
thus formed is an electro-magnet, L, made of a bar of soft iron, I,
wound longitudinally by a coil, one end of which makes an earth
connection at E and the other is connected by the wire, W W´, to
complete the circuit through the coil of the distant apparatus. It will
be observed that the receiving and transmitting instruments are exactly
alike. Now, suppose one of these teeth is struck or otherwise thrown
into vibration, the result will be, since the free ends of the teeth are
magnetic poles, that alternating electric currents will be generated in
the coil of the electro-magnet (see page 509), and these will flow
through the entire circuit, including the coil of the distant
instrument, where the magnetism generated will alternately attract and
repel the polar extremities of the teeth in the steel plate. It will be
understood, of course, that the fellow prong of the fork will vibrate
also, and will simultaneously approach to and recede from the soft iron
core, so that being of opposite polarity, the effect on the
electro-magnet will be doubled. The action on the distant electro-magnet
will be a rapid series of reversals of the polarities of the core, and
hundreds of times in every second the ends of the steel teeth will be
alternately attracted and repelled. But not all of these will thereby be
thrown into vibration—only the one pair which were tuned into unison
with the former can and will respond to that particular series of
impulses, and the consequence will be that the same note will be emitted
by the receiving instrument. If two or more notes of the transmitter be
simultaneously thrown into vibration, the same notes will be heard from
the receiver, for each series of currents will flow along the wire
independently, just as if the other did not exist, and each will produce
its particular effect on the transmitter. In this way an air played on
the one instrument is heard also from the other, with all its accents
and combinations. But more than this, if a tune be played on a musical
instrument near the sender, or if a song be sung, the air will be
reproduced by the distant receiver. The reason of this is that the steel
tongues take up, or are thrown into movement by, the vibrations that
have the same periodicity. The manner in which a vibratory body responds
to impulses of its own periodicity may be easily shown by exposing the
wires of a piano and raising the dampers, when, if a note be sung near
the instrument, it will be found that a number of the wires respond,
namely, those that are capable of vibrating synchronously with the
constituent vibrations of the voice, for neither a voice nor a sounding
wire gives forth one simple system of vibrations, the audible effect
being due to the superposition or composition of several diverse
elementary systems. With the same arrangement another experiment may be
made, as an illustration of a matter important for our subject. Let the
different vowels be sung to the piano-wire on the same note or pitch,
and in the responses to each a difference of the quality of the sound
will be noticed, although the piano will not distinctly give back the
vowel itself. It would, however, do so if a number of its wires were
strung with certain definite relations in pitch to that of the
fundamental note and in unison with the voice components of the vowel
sound.
[Illustration:
FIG. 302_c_.—_Superposition of Currents._
]
It has been said above that two systems of electrical currents of
different periodicity would flow along one wire independently of each
other, but it should be explained that this takes place by a composition
of the currents, for it is evident that at any given instant the wire
can only be in one of three conditions, viz.: (1) with no current
flowing; (2) with a current in the positive direction; (3) with a
current in the negative direction. Such must always be the case, and,
therefore, it should be clearly understood how this is consistent with
the superposition of currents of different periodicities, a matter which
the diagram, Fig. 302_c_, is intended to illustrate. Suppose the _flow
of time_ to be represented by the dotted lines from _a_ to _b_, the
whole length of which we may call 1/100th of a second, and that the
current passing through the wire is represented in intensity and
direction by the plain lines; the intensity by distance above or below
the dotted line; the direction being positive where the plain line is
above, and negative when it is below the dotted straight line, and of
course no current at all occurs at the instant when the change of
direction takes place. The line A will thus represent alternating
currents, rising and sinking in intensity, and changing from one
direction to the other, going through 600 regularly recurring phases in
one second of time. Similarly, B may represent another series of
currents, having here a periodicity of 500 in one second of time. These
are here supposed to have greater intensity than the former. If the two
currents are sent through one wire their effects are superposed, so that
the actual electrical state of the wire would be represented by the
curve C, which is compounded from the two others, and where it will be
observed the rise and fall of the current, its maxima and minima, no
longer recur at regular intervals within the space of the 1/100th of the
second, the whole of that period being taken up by a less regular series
of changes, the cycle being repeated only 100 times in the second. The
same diagram might serve to illustrate the motions of, say, a particle
of air or the drum of the ear in acoustic vibration, the distances above
and below the straight line being taken to represent the displacements
from the position of rest on one side and the other. If the sounds of an
organ or piano consisted of only these primary vibrations, B would
roughly[8] represent the movements of the wires, the air and the drum of
the ear, when the note _si_{3}_ was sounded alone; A when the note
_re_{4}_ was more faintly sounded alone, and then C, if these notes were
sounded together, would correspond with the movements of the drum of the
ear. The movements it actually makes when we hear speech, or even a
single musical note, are, however, a thousand-fold more complex, for no
musical instrument gives out a note with a single set of vibrations, the
fundamental one being always accompanied by other sets diversely related
to it, according to the class of instrument. In some cases, fifteen or
sixteen sets of vibrations have been distinguished along with the
fundamental note, without exhausting the possible number. Of a like
order of complexity will be the currents which the wire of a speaking
telephone must convey, and the difference between the undulatory nature
of the currents in Bell’s musical telephone and any produced by mere
make and break contacts, as in Reiss’ arrangement, will be obvious, and
recognized as an important step towards the solution of the problem of
transmitting speech. When Mr. Bell invented his instrument, he was
seeking for a method of simultaneously transmitting by one wire several
messages by audible _signs_ merely; and by the method used in his
musical telephone this is practicable, for all that would be required
would be pairs of transmitters and receivers, each adjusted to one
single particular note. Another point that should be noted is that in
the Bell musical telephone no battery is used, for the currents are
those generated by magneto-electric induction, and the circuit through
the wires and coils are completed by earth connections.
Footnote 8:
The lines A and B in the diagram have not harmonic ordinates.
[Music]
[Illustration:
FIG. 302_d_.—_Bell’s Speaking Telephone._
]
In passing from the invention of the musical to that of the speaking
telephone, Mr. Bell passed from the more complex to the more simple
instrument, for of all apparatus by which communication can be carried
on at a distance, the Bell speaking telephone is one of the simplest. He
had only to make its vibrating disc of Scott’s phono-autograph into a
magnetized body, capable of producing currents in an electro-magnet coil
in the same way as did the vibrating plates in his musical telephone.
The Bell speaking telephone was publicly exhibited for the first time at
Philadelphia, in 1876, and was shown the same year to the British
Association by Sir William Thomson, who pronounced it the wonder of
wonders. For the first time in England, the instrument in a still
simpler form was exhibited by Mr. Preece, at the Plymouth meeting of the
British Association in 1877, and of nearly the same construction as is
still often used, although, as we shall presently see, for battery
telephones the transmitting apparatus is now made of larger dimensions,
of a different shape and on a different principle. We shall describe the
simple form in which transmitter and receiver are identical, each
consisting externally of a small cylindrical wooden or ebonite box, and
with a handle three or four inches in length of the same material. Fig.
302_d_ is a section of the instrument where N S is a cylindrical steel
magnet, on one end of which is wound the small coil B, made of fine silk
covered copper wire, the extremities of which pass through the handle M
at _f f_, and are connected by the binding screws _I I´_ with the line
wire C C´. Close to the coil covered end of the magnet is a very thin
diaphragm of iron, L L´, and when this is thrown into vibration by the
voice speaking into the trumpet-mouth opening, R R´, its movements
produce currents in the coil according to the principles that have
already been explained, for it will be observed that the iron disc is
magnetized by the inductive action of the permanent magnet N S. These
currents passing through the coil of the receiving instrument raise or
lower the intensity of the magnetic force in it, so that the distant
disc reproduces the vibrations of the transmitter. Such is at least an
obvious explanation of the action of this very simple arrangement; but
from a number of experiments and observations that have been made with
modifications of the instruments, it would appear that other and much
more complex phenomena concur in producing the effects. It has indeed
been suggested—and the idea is supported by numerous experiments—that,
in these telephonic transmissions of speech, vibrations are concerned
which are not at all of the mechanical kind we have been dealing with in
these explanations, but are _molecular_.
The Bell telephone is used by speaking distinctly before the mouth-piece
of the transmitter, while the listener at the other end of the line
applies the mouth-piece of his instrument to his ear, and one wire is
sufficient with good earth connections, although sometimes a second wire
is employed to complete the circuit. It is also found advantageous to
have two instruments in the circuit at each end, so that one may be held
to the ear while the operator is speaking through the other. In this
way, a rapid conversation can be carried on with the greatest ease, or
again, an instrument may be held at each ear, by which arrangement the
words are more distinctly heard. It is not necessary to shout, as this
has no effect, but to speak with a clear intonation, and some voices are
found to suit better than others. The vowel sounds are best transmitted,
except that of the English _e_, which, with the letters _g_, _j_, _k_,
and _q_, are always somewhat imperfectly transmitted. A song is very
distinctly heard, both in the words and the air, and the voice of the
person singing is readily recognized. Several instruments may be
included in one circuit at different stations, so that half a dozen
persons may take part in a conversation, and questions and answers may
be understood even when crossing each other. If two distinct telephone
circuits have their wires laid for a certain distance (two miles) near
each other, say a foot or more apart, and without any connection
whatever, listeners at the end of the one line will hear the
conversation exchanged through the other line. Other forms of the
instruments have been arranged, by which a large audience may hear
sounds produced at a distance, as, for instance, when a cornet-à-piston
was played in London, it was heard by thousands of people assembled in
the Corn Exchange at Basingstoke.
It would be impossible within our limits to even briefly describe the
great number of improvements and modifications of Bell’s system that
were devised by various persons soon after the invention was brought
out, and many additional complications were introduced into some of the
arrangements. Advantage was also taken to a greater or less extent of
another principle affecting the strength of electric currents, to which
we have now to call the reader’s attention, and to exemplify by one of
the simplest instruments, leaving detailed accounts of the various forms
in which it has been applied to be found in special treatises. The
reader should first turn back to page 400, where he will see an
expression of the strength of a battery current. It will be observed
that the current may be increased or diminished by diminishing or
increasing R, the external resistance, without changing the other terms.
Now M. Du Moncel discovered, as far back as 1856, that an increase of
pressure between two conductors in contact, and conveying a current,
caused a diminution of the electrical resistance, and this discovery was
utilized for telephonic purposes by Mr. Edison in his invention of the
carbon transmitter (1876). In this there is no magnet, and a stretched
membrane may take the place of the metallic plate, although a circle of
photographers’ ferro-type plate gives better results. A pad of
india-rubber, cork, or other material is fixed on the plate, and rests
upon a carbon disc, which again is in contact with a metallic conductor.
Between the latter and the carbon the current from a constant battery
passes. When the plate is thrown into vibration by speaking into the
mouth-piece, the variations of pressure conveyed to the carbon cause
variations in the resistance of its electrical contact, and thus a
series of undulations are produced in the current, and these affect the
electro-magnet of a Bell receiving instrument in the circuit as before,
so that the sounds are reproduced. It is now time to say a word about
the share in the invention of the speaking telephone which has been
claimed by Mr. Elisha Gray, also of the United States, who, at the time
Mr. Bell applied for the patent for his instruments, produced drawings
and descriptions of a plan he had devised for transmitting speech by
undulating electrical currents, and it has been admitted that the plan
he had conceived was perfect in principle. He proposed to use a battery
current, and his receiving instrument was nearly the same as Bell’s. The
undulations of the current were also determined, as in Edison’s
telephone, by changes in the external resistance, but this was effected
in a different, though equally simple manner. To a membrane stretched
across the lower end of a short wide tube that formed the mouth-piece of
the transmitter, and was placed vertically, was attached a piece of
platinum wire, conveying the current and dipping into a liquid of
moderate conductivity, but not quite touching another platinum electrode
fixed at the bottom of the vessel containing the liquid. The space of
liquid traversed by the current being thus varied by the oscillations of
the membrane, the resulting variations of the resistance produced the
requisite undulations in the intensity of the current. Both Mr. Bell and
Mr. Gray applied for patents on the 14th February, 1876, but the
American Patent Office recognized the claim of the former as prior.
[Illustration:
FIG. 302_e_.—_Mr. Hughes’ Microphone._
(B _and_ R _are merely diagrammatic_.)
]
Du Moncel’s observation was applied by Mr. Hughes in the construction of
an instrument, which he named the _microphone_. This was in the same
year that Edison had brought out his carbon telephone, and a certain
similarity, resulting from the identity of the principle employed, led
to an acrimonious controversy on what were supposed to be rival claims.
But the microphone differs so much in arrangement and performance from
the other instrument as to constitute a distinct invention. The
instrument, if it may be so called, is simplicity itself, in the form
represented in Fig. 302_e_, which is one of the most sensitive. There, C
and C´ are two small blocks of carbon, fixed on a small upright piece of
wood. Two cup shaped cavities are hollowed out in the carbon blocks, and
these serve to hold loosely, in a nearly vertical position, a small rod
of gas retort carbon pointed at the ends. This rod is only about one
inch in length, and the lower end merely rests on the bottom of the cup
in C´, while the other is capable of moving about in the upper cavity,
the vertical position being nearly maintained in a state of unstable
equilibrium. The carbons are in the circuit of a voltaic cell or small
battery, B, in the line through a Bell receiving instrument, which may
be at a distance. When the microphone is to be used, it is placed on a
table with a cushion or several folds of wadding beneath its base. If
the receiver be applied to the ear of a listener, he will distinctly
hear every word pronounced by one speaking near the microphone, even in
a low tone; but a loud voice may be heard when the speaker is 20 or 30
feet from the instrument. The minutest vibrations conveyed to the stand
are perceived at the receiver as loud noises. The tread of a fly walking
over the board, S, is heard like the tramp of a horse, and the ticks of
a watch are audible in the receiver when the ear is several inches away
from it. The slight touch of a feather on the stand is distinctly
audible, and a current of air impinging upon it is reproduced as the
noise of a stream of water. The microphone is, in fact, the most
sensitive detector of vibrations that is known, and its employment as a
transmitter has brought the telephone to its present perfection. It has
been constructed in an endless variety of forms, according to the
purposes for which it is intended, and its simplicity is as wonderful as
its extreme sensitiveness. We will further illustrate these qualities by
an experiment of Mr. Willoughby Smith’s on the same principle. Instead
of the two carbon blocks, he laid on the table, in parallel positions,
two small rat tail files, and completed the circuit by a third file,
laid across the others at right angles. This arrangement constituted so
sensitive a transmitter that the listener at the distant Bell receiver
could hear even the faint sound of the speaker’s breathing. Even three
common pins, similarly crossed, make an effective transmitter. The
feebleness of the variations in the current requisite to make the Bell
receiver produce sounds is extraordinary, and a very weak battery
current is sufficient, even under the circumstances of ordinary
practical use. Still more remarkable is the fact that in favourable
conditions the microphone is capable of transmitting sound without any
battery at all, but merely with connections to earth, when the ticking
of a watch placed upon the stand has been distinctly heard at the
distance of nearly one-third of a mile, and speech, also, has been
transmitted with unusual distinctness with the battery left out and
merely a few drops of water placed at the carbon contacts; indeed, it is
said that, even without the water, the voice may be heard. This effect
has been attributed to the carbons and water forming a battery
themselves, and in the latter to the moisture of the speaker’s breath
supplying the fluid element. But, again, the microphone will not only
transmit speech, but, under certain arrangements, it will reproduce it
(when one of the carbon electrodes is attached to a membrane), although
the result is less distinct than with the Bell receiver. It is, however,
not so easy to explain how mere variations of current intensity can
produce the effect where there can be no magnetic attractions and
repulsions. We must, no doubt, look for the cause in some other property
of electric currents. The transmitters used in various lines of
telephonic communication, erected by the Post Office or by companies in
Great Britain, are generally applications of the principle of the
microphone, and not of that of either Mr. Bell’s or Mr. Edison’s
original instrument. But more recently, Mr. Edison has most ingeniously
adapted variations of sliding friction, as modified by the action of the
undulatory current on a liquid electrolyte between the sliding surfaces
to the production of a loud speaking telephonic receiver—that is, one by
which the sounds are made audible to a large assembly. From this
instrument, the notes of a cornet-à-piston, played in Brighton, have
been distinctly heard throughout a large hall in London.
Another curious transmitter is formed of a fine jet of water traversed
by an electric current. Acoustic vibrations are easily set up in the
jet, and these modify its conductivity so as to produce corresponding
undulations of current intensity.
It would take long to point out the many scientific applications of so
sensitive an instrument as the microphone with its Bell receiver. As a
medium for conveying speech to a distance, whether for purposes of peace
or war, its use is sufficiently obvious. Some curiosities of musical
transmission have been noticed, and such experiments are repeated from
time to time with increasing success. It has been applied to many
purposes in surgery and medicine. In many cases of deafness it has made
conversation easy. Even the passage of the molecules of gases, when
diffusing through porous partitions, Mr. Chandler Roberts has by its
means made audible. The distances to which speech can now be transmitted
are considerable, as conversations have been carried on by persons
nearly 300 miles apart.
[Illustration]
LIGHTHOUSES.
Who does not regard with interest the lighthouses which at night throw
their friendly beams across the sea, to guide the mariner in his course,
and warn him of perils from sunken rock or treacherous shoal? The modern
lighthouse, with its beautiful appliances, is entirely the result of the
applied science of our age; and it affords a fine example of the manner
in which experiments, carried on to determine natural laws apparently of
an abstract character and without any obvious direct utility, give rise
to inventions of the highest importance and most extended usefulness.
The lofty structures which were erected near certain ancient harbours,
and of which the Pharos of Alexandria is the most memorable example,
burned on their summits open fires of wood; and whatever beacons existed
from that time down to the end of last century were merely blazing fires
of wood or coal. The lighthouses of the South Foreland, which were
established in 1634, displayed coal fires until 1790, and the
lighthouses in the Isle of Man were first illuminated with oil only in
1816. Down to the beginning of the present century, therefore, the
modern lighthouses showed no improvement on the ancient plan. Even the
Tour de Cordouan, at the mouth of the Garonne river, which was completed
in 1610, and is one of the most famous of modern lighthouses, from its
great height (200 ft.), and the care which has always been given to
render it efficient, showed down to 1780 merely a fire of billets of
wood, the upward loss of the light being diminished by a rude reflector
in the form of an inverted cone. In the improved means of obtaining
artificial light, and in the admirable optical apparatus by which that
light is utilized, we find the vast superiority of modern lighthouses.
But these are sometimes erected on isolated, and almost submerged,
rocks, exposed to the fury of the waves. The difficulties which have to
be overcome in their construction cause some lighthouse towers to rank
among the best specimens of engineering skill. We may, therefore,
consider under the present head—the towers; the sources of light; the
optical apparatus and its accessories.
[Illustration:
FIG. 303.—_The Eddystone Lighthouse._
]
One of the best-known lighthouses on the English coast is that on the
Eddystone Rock, about 14 miles S.S.W. from Plymouth. The structure which
now[9] stands upon this rock was the work of Smeaton, and was completed
in 1759. The stones forming the lower courses of this tower, which is
represented in Fig. 303, half in section and half in elevation, are
dovetailed into the rock itself and into each other. The masonry is
carried up in a solid mass for about 12 ft., the stone used being
granite, which also constitutes the whole of the exterior masonry. The
four upper apartments are formed with arched roofs, the side-thrust of
which is counteracted by iron chains surrounding the tower. These
chains, which are bedded in lead, were placed in their positions while
hot, and by their contraction bound the structure together with great
force. The masonry of the tower is 68 ft. high, and this is surmounted
by the light-room, the total height from the lowest course of stonework
to the gilt ball at the top being 94 ft., or nearly half that of the
London Monument. The diameter at the base is 26 ft., and that at the top
15 ft. The light-room is of an octagonal shape, and is made of iron
framework, glazed with thick plate glass. Below this are two
store-rooms, a kitchen, and a bed-room. The Eddystone has now breasted
the storms of more than a hundred years, and it remains as firm as the
rock it is built on. Fig. 304 is a picture of this noble lighthouse,
with the British fleet passing close to it, during a furious gale on the
22nd of October, 1859, or exactly a century after the completion of the
structure. The incident of the man in the water, which occupies the
foreground, is not an imaginary one, for it is recorded that the
_Trafalgar_ stopped in the midst of the storm to pick up a man who had
fallen overboard. For eighteen hours the ships encountered the fury of
the tempest, keeping out at sea in open order throughout the night. They
wore in at dawn, came up the Channel in line of battle, steamed into
Portland, and took up their anchorage without the loss of a sail, a
spar, or a rope-yarn.
Footnote 9:
Smeaton’s tower proving unsafe, has since been taken down and
replaced, in 1882, by one from Mr. Douglass’ design.
[Illustration:
FIG. 304.—_The Eddystone in a Storm._
]
The lighthouse tower on the Bell Rock is 100 ft. high, 42 ft. in
diameter at the base, and 15 ft. at the top. The Inchcape Rock, on which
it is placed, is the scene of Southey’s ballad of “Ralph the Rover,” and
the lighthouse here is one of the most serviceable on the Scottish
coast, for the dangerous spot on which it is placed lies in the direct
track of all vessels entering the Firth of Tay from the German Ocean.
The rock is submerged at spring tides to the depth of 12 ft. The tower
bears a close resemblance in shape to that of the Eddystone: it is
circular and faced with massive blocks of granite. The lower part, to
the height of 30 ft., is solid, and the door is reached by a bronze
ladder. The building contains five apartments, and a cistern for storing
fresh water for the use of the keepers, who have sometimes to remain in
their solitary situation for six or eight weeks together, the weather
preventing the possibility of any communication with the shore.
Still loftier than the tower on the Bell Rock is that which rises in the
midst of the Skerryvore Reef, 12 miles from Tyree, a small island off
the coast of Argyleshire. This building may be taken as a typical
specimen of a detached lighthouse, and the difficulties overcome in its
construction attest the skill of the engineer, Mr. Alan Stevenson, who
has written a highly interesting account of the work. The rocks here are
of _gneiss_, an extremely hard formation, and their surfaces are worn as
smooth as glass by the action of the water. On one of a numerous series
of these small islets, where only a narrow strip of rock, a few feet
wide, remains above the surface at high water, and this divided by
rugged lumps into narrow gullies, through which the sea constantly
rushes, the lighthouse is built. The work was commenced in 1838 by the
erection of a temporary wooden barrack on piles at a little distance
from the site chosen for the foundation. In a gale during the winter the
whole of this structure was swept away in one night. Another, more
strongly secured, was built the following summer, and in this Mr.
Stevenson and his men remained sometimes for fourteen days together, the
weather preventing any passage to or from the shore: here the men were
sometimes awakened from their hard-earned repose by the water pouring
over the roof, and by its rushing through the crevices, while the
erection swayed and reeled on its supports. Mr. Stevenson relates that
one night the men became so alarmed for the stability of their shelter
that some descended, and sought in cold and darkness a firmer footing on
the rocks. Two summers were occupied in cutting the foundations, and the
blasting of the rock in so narrow a space was an operation attended with
no little danger. A small harbour had to be formed at the rocks for the
vessels bringing the ready-prepared stones of the building from the
quarries, where also piers were built expressly for the shipment of the
materials. In designing his tower, the engineer preferred to oppose the
force of the waves by the weight of his structure, rather than to rely
on dovetailed or joggled-jointed stones. Measurements were made of the
force of the waves, which at Skerryvore was sometimes equivalent to a
pressure of 4,335 lbs. on the square foot; and calculations based on
these measurements showed that the mere weight of the superstructure
would amply suffice to keep the stones immovable. Nearly 59,000 cubic
feet of stone were used, or about five times the quantity contained in
the Eddystone Lighthouse, and the total cost of the building was
£87,000.
The use of iron, as a building material advantageously replacing stone,
has extended to lighthouses, and many have been constructed entirely of
cast and wrought iron, or partly of iron and partly of gun-metal, which
is not readily acted on by the sea spray. Such lighthouses are cheap,
easily and quickly erected, strong enough to bear shocks and vibrations,
and proof against fire, lightning, and earthquakes. The lighthouse on
Morant Point, Jamaica, is made of iron, cast in England; and it was
erected in a few months at a cost of one-third of that of a stone tower
of the same altitude. Its height is 105 ft., and the shaft is formed of
iron plates in segments of 10 ft. high, which are bolted together at
their flanges. At Gibbs Hill, Bermuda, is a lighthouse 130 ft. high,
constructed in the same manner.
So inefficient, inconvenient, and uncertain were the lamps or other
means of artificial illumination known up to nearly the beginning of the
present century, that nothing better could be found for the Eddystone
Lighthouse for forty years after its erection than tallow candles stuck
in a hoop—a means of illumination which would scarcely now be tolerated
even in a booth at a village fair. To M. Argand, a Frenchman, we are
indebted for the first great improvement in lamps. The admirable
invention which bears his name is, as everybody knows, an oil lamp with
a tubular wick, which occupies the annular space between two metallic
tubes, in such a manner that a current of air rises through the inner
tube, and thus reaches the interior of the flame. This current, and the
current which supplies the exterior, are increased by surrounding the
flame with a tall glass chimney; and a contraction of the chimney, just
above the flame, aids greatly in distributing the air, so as to insure
the complete combustion of the oil. In the original lamp the supply of
oil to the flame depended on the capillary attraction in the meshes of
the wick. M. Carcel applied clockwork to continuously pump up the oil
into the burner, so that, by overflowing, it was maintained at an
invariable level. This arrangement added greatly to the intensity and
steadiness of the light; and, on account of the uniformity of its flame,
the Carcel lamp has been selected as a standard to which, in France,
photometric determinations are referred.
The power of the Argand lamp, as employed in lighthouses, is greatly
increased by the plan of employing several concentric wicks instead of
one. Between these wicks there are, of course, open spaces, through
which the air obtains access to the flame, and the current of air is
made more rapid by the use of a very tall chimney. The large amount of
heat produced by the combustion of so much oil in a small space is
partly carried off by the excess of oil which is made to overflow the
burner—about four times the quantity consumed being constantly pumped up
into the burner for this purpose. Lighthouse lamps are made with two,
three, and four wicks; and the oil is forced up in the burners either by
clockwork or by the pressure of a piston loaded with a weight. The
following table gives the sizes of the burners and the illuminating
powers of the lamps:
┌──────┬──────┬────────┬─────────┐
│Order │Number│Diameter│Intensity│
│ of │ of │ in │of Light │
│Light.│Wicks.│inches. │in Carcel│
│ │ │ │ Lamps. │
├──────┼──────┼────────┼─────────┤
│ 1 │ 4 │3½ │ 23│
│ 2 │ 3 │2–15/16 │ 15│
│ 3 │ 2 │1¾ │ 5│
└──────┴──────┴────────┴─────────┘
The quantity of oil consumed in these lamps is less than that
proportional to the increase of the light:—for example, although the
four-wick lamp gives twenty-three times the light of the simple Argand,
it only consumes nineteen times the quantity of oil. The oil used in
these lamps is colza; but experiments have been made with a view of
introducing petroleum, which has the advantages, not only of being
cheaper and uncongealable by cold, but of giving a whiter and more
brilliant light. Hitherto, however, this substance has answered only
with lamps of one wick.
Coal-gas has been applied to the illumination of lighthouses, and as it
gives a light of great brilliancy and steadiness, when consumed in
proper burners, it has certain advantages over oil lamps, which have
caused it to be employed in situations where a supply can be readily
obtained. The light produced by lime, ignited by the combustion of
coal-gas or hydrogen mixed with oxygen, has also been suggested; but
this plan is not without risk of interruptions and of dangerous
accidents, and it has been considered inadvisable to entrust the
apparatus to the persons who commonly take charge of lighthouses.
The electric light has been very successfully applied in certain
lighthouses, since the mode of producing steady currents by
magneto-electric[10] machines has come into use. The lighthouses at the
South Foreland have been thus illuminated by a machine constructed by
Mr. Holmes in 1862. A very powerful electric light is exhibited from the
lighthouse on Cape Grisnez; and the adoption of this source of light has
been extending, as it is far more intense than any other artificial
light, and can be sent in more concentrated beams across the sea, on
account of its being emitted from a space which is practically a point.
These circumstances cause the beams of the electric light to possess
greater power of penetrating the atmosphere than those from any other
source.
Footnote 10:
Now superseded by the dynamo-electric machines.
But it is perhaps in the optical apparatus of lighthouses that the
greatest improvements and most admirable inventions are to be found.
When only the blaze of an open fire furnished the guide to the mariner,
the means resorted to in order to throw across the sea the light which
issued from the flames upwards or landwards, appear to have been of the
rudest kind, even where such attempts were made at all. The inverted
cone on the Tour de Cordouan has been already mentioned, and we read of
cases in which screens of sheet brass were placed on the landward side,
to throw back the light seaward.
Here it may be proper to examine the conditions which determine how the
light can be made most available for the guidance of the mariner.
Everybody knows that the light from a luminous body spreads out from it
in all directions equally. Thus, if we simply place an electric light on
a tower such as that on the Bell Rock, but few of the luminous rays can
benefit the mariner: namely, those which fall upon the sea or are
directed to the horizon. A much larger portion of the light will stream
upwards and be lost in space; another part will descend towards the base
of the tower, and be equally wasted. Again, if the situation of our
lighthouse were on the shore of the mainland, all the light which passes
landwards, whether horizontally or not, would be entirely lost for our
purpose. Even if, in the case of an isolated lighthouse, we can send out
all the light in a nearly level zone over the sea to the horizon, the
intensity of the illumination will diminish, on account of the widening
space, as the distance increases. The question, therefore, arises
whether it is possible to send the whole of the light in one unbroken
beam, not liable to this kind of enfeeblement, so that the only loss it
can experience may be absorption by the imperfectly transparent
atmosphere.
There are two means of gathering up all the otherwise useless beams, and
sending them in such a direction as to reach the eye of the distant
mariner. The one is by reflection from mirrors, and the other by
refraction through lenses. The apparatus employed in the first process
is termed _catoptric_, and in the latter _dioptric_.
When a luminous point is placed at the focus of a parabolic mirror, all
the rays which fall upon the mirror are reflected by it in a direction
parallel to its axis, so that they form a cylindrical beam. This is the
method which was adopted in the first improvements effected in
lighthouses. The parabolic reflector was first used at the Tour de
Cordouan in 1780, and soon afterwards metallic reflectors became the
ordinary appliances of lighthouses, and they are still largely used.
Such reflectors are made of sheet copper, thickly plated with silver,
about 6 oz. of this metal being applied to 16 oz. of copper. They are
formed by carefully beating a circular sheet of the plated copper into a
concave shape, which is finally brought to the exact curve by the aid of
gauges, and is then turned and polished. The largest of these reflectors
have a diameter of 2 ft. at the _mouth_, as it is termed, for the
reflector comes forward in advance of the lamp, the chimney and burner
passing through openings in the metal. The flame of the lamp occupies
such a position that its brightest part is in the focus of the mirror;
but since the focus is a point merely, whereas the flame has a certain
magnitude, it follows that the want of coincidence of the other luminous
points with the focus produces a certain divergence in the reflected
rays, so that the beam is not accurately cylindrical. This, however, is
far from being a disadvantage practically, for it has the effect of
widening a little the strip of sea illuminated by the beams. But all
that portion of the light which escapes from the mouth of the mirror
without being reflected is radiated in the ordinary manner, and is
practically lost. We shall presently see how even this light may be
gathered up and brought into the main beam.
Let us suppose a number of such reflectors, each with its own lamp,
placed in a horizontal circle, so as to throw their beams towards
different points of the compass. If eight lamps were so placed, eight
beams of light would stream out across the water, like eight spokes of a
wheel; eight sectors would, however, be left unilluminated, and for
ships in these spaces the lighthouse would be virtually non-existent:
its rays could only reach vessels within the eight narrow strips
traversed by the beams. If we double the number of reflectors in the
circle, or if we arrange another series of eight in a circle above or
below the others, so that a lamp in the second circle coincides
vertically with an interval in the first, the effect will be that we
shall have sixteen beams, and sixteen dark sectors, instead of eight;
that is, only a very small part of the expanse of water will receive the
benefit of the light. It must be remembered that the breadth of the
cylindrical beam would not be greater than the diameter of the mirrors,
and that the space illuminated by it has the same breadth at all
distances; or rather, that this is nearly the case, for the light does
not all issue precisely from the focus of the mirror. Thus, even if we
use a very great number of mirrors, we shall succeed in illuminating but
an extremely small proportion of the sea horizon. This evil is met by
giving a horizontal rotatory motion to the reflectors, causing the beams
to sweep over the whole expanse of the waters; and thus from every ship
the light will be visible for an instant. The rotation is produced by
clockwork, duly regulated, so that an uniform motion is obtained. The
regular appearances and eclipses of the light prevent the mariner from
mistaking for a lighthouse a bright star near the horizon or an
accidental fire on the coast; and, further, it being necessary that the
lighthouses along any particular coast should be readily distinguishable
from each other, it becomes easy, by assigning to each lighthouse a
different period of revolution, to individualize them, so that the
mariner shall be in no danger of confounding one with another.
But when the lighthouses on a certain extent of coast are numerous, this
mode of distinguishing them becomes inconvenient, as mistakes might
easily be made in small differences of time; and it would be inexpedient
to keep long intervals of darkness. Hence other methods have been
resorted to in addition—such as red lights, or lights alternately red
and white. The following are the distinctions made use of among the
Scottish lighthouses, including the double lighthouses, which give a
leading line to the navigator:
1. Fixed lights.
2. Revolving lights.
3. Revolving, with red and white beams alternately.
4. Revolving, with alternately two white beams and one red.
5. Revolving, with alternately two red beams and one white.
6. Flashing, in which the light increases and decreases at regular
intervals.
7. Intermittent, in which, by means of a revolving screen, the light
is abruptly cut off and exhibited.
8. Double fixed lights.
9. Double revolving lights, which appear and disappear at the same
instant.
The efficiency of reflectors depends on the state of polish of the
surface, and even with the most brilliant polish there is a very large
loss of light: in the ordinary condition of lighthouse reflectors, it is
found that one-half of the light is lost at the surface of the mirrors.
An attempt was made in England, about the beginning of the present
century, to substitute glass lenses for mirrors. But it was found that,
in spite of the loss occurring in reflection, the mirrors produced a
more intense beam. No doubt the person who made the attempt did not
observe the true conditions of the problem. It was Fresnel, the
illustrious Frenchman, whose name has already been mentioned in these
pages, who successfully solved the problem. He saw that it would be
necessary to give the lenses a short focal length, and at the same time
to have their diameters very great. The dimensions required by these
conditions far exceeded any that could be given to lenses formed in the
ordinary manner; and even if they could be so formed, the great
thickness of glass which would be necessary would diminish the
transparency, and unduly increase the weight of the apparatus to the
detriment of the revolving apparatus. An idea now occurred to Fresnel’s
mind, which, although similar to previous projects, he conceived
independently, and was undoubtedly the first to carry out. This was the
idea of the _lentille à échelons_, or “lens in steps.” The construction
of this will be understood from Fig. 305, where _a b_ is a section of a
lens in steps, and the dotted line, _c_, shows the thickness an ordinary
lens of the diameter _a b_ would have. Fresnel kept only the marginal
part of such a lens; and inside of the ring formed by this, he fitted
the margin of a second large lens having the same focal distance; inside
of this another ring, and so on; and in the centre a large lens of
moderate thickness. He also placed above and below the lens the
concentric prisms, _e e´_ and _f f´_, which, by refraction and total
reflections (see page 399), send the rays parallel to the axis of the
lens. Fresnel also contrived methods of economically grinding such
lenses and prisms with precision.
[Illustration:
FIG. 305.—_Revolving Light Apparatus._
]
Fresnel saw that it would be useless to apply lenses in lighthouse
illumination unless the intensity of the light given out by the
single-wick Argand lamps then in use could be considerably increased,
without much enlarging the flame. Accordingly he devoted himself, in
conjunction with his friend Arago, to this preliminary consideration.
Their studies and experiments led them to the construction of the lamp
with several concentric wicks—by which a brilliancy of light is
obtainable twenty-five times greater than that of the single-wick
Argand. The light which the improved lamp, when combined with Fresnel’s
lenses, could send to the horizon, was equivalent to that which would be
given by the united beams of 4,000 Argand lamps without optical
apparatus; and it was eight times greater than any which could be
produced by the reflectors then in use. The first apparatus constructed
on Fresnel’s plan was placed on the Tour de Cordouan in July, 1823.
France led the van in the erection of the most perfect lighthouses in
the world, and it was not until 1835 that, by the strenuous advocacy of
Mr. Alan Stevenson, a dioptric apparatus was employed in a British
lighthouse; but at the present time Fresnel’s principle has been adopted
in the majority of British lighthouses. Fig. 305 is a part elevation,
with the section, of a catadioptric apparatus of the first class. In
plan it is a regular octagon, and it sends out eight beams, which are
directed to the horizon, and made to sweep over the sea by its regular
rotation, produced by clockwork contained in the case, A. The whole
frame is very accurately balanced, and turns on its bearings, and the
rollers, _h_, _h_, with great smoothness and steadiness. The moving
power is given by the descent of a weight attached to a chain or cord,
which is wound round a barrel. One train of wheels is connected with
apparatus for regulating the speed, and to this an indicator is attached
which registers the number of revolutions made in an hour. There is also
a contrivance of some kind for maintaining the motion while the weight
is being wound up. The reader will observe that all the light of the
lamp, L, is utilized, except that which is directed towards the base and
top of the apparatus—a quantity less than one-fifth of the whole. About
45 per cent. of the light emitted by the lamp falls on the refracting
lenses; 22½ on the upper reflecting prisms; and 13½ on the lower
reflecting prisms. The brightest part of the flame is placed so that the
beams from it are directed towards the sea horizon, and the space
between the horizon and the neighbourhood of the lighthouse receives
ample light from the other parts of the flame. Thus a ship, or any part
of the sea within the range of the lighthouse, will see the light
appearing at regular intervals, as one after another of the eight beams
passes across it, the intervals being one eighth of the time in which
the apparatus completes its revolution. The zones of totally reflecting
prisms, shown at _e e´_, _f f´_, Fig. 305, were not adopted in British
lighthouses until 1844, when the Skerryvore light was exhibited with the
complete apparatus represented in the drawing.
The optical apparatus for lighthouses is constructed of certain sizes,
adapted to the different situations in which it is to be used. The
apparatus we have just described is made in six forms, according to the
_order_ of light required. The first three orders are for sea lights,
the rest for harbour lights; and the following are the dimensions of the
apparatus for each order of revolving or fixed lights:
┌─────────────┬─────────────┬─────────────┬───────────────────────────┐
│ Order. │ Height in │ Internal │ Number of Reflecting │
│ │ Inches. │ Diameter in │ Prisms. │
│ │ │ Inches. │ │
├─────────────┼─────────────┼─────────────┼─────────────┬─────────────┤
│ │ │ │ In Upper │ In Lower │
│ │ │ │ Zone. │ Zone. │
├─────────────┼─────────────┼─────────────┼─────────────┼─────────────┤
│ 1 │ 106½│ 72½│ 18 │ 8 │
│ 2 │ 83½│ 55 │ 16 │ 4 │
│ 3 │ 61½│ 39½│ 11 │ 4 │
│ 4 │ 29 │ 19¾│ 5 │ 4 │
│ 5 │ 21¾│ 14¾│ 5 │ 4 │
│ 6 │ 17½│ 12 │ 5 │ 4 │
└─────────────┴─────────────┴─────────────┴─────────────┴─────────────┘
When a revolving apparatus of the above description is erected on shore,
a reflector of suitable shape and dimensions is placed on the landward
side of the lamp, so as to throw its rays back upon itself and towards
the lenses which are directed seaward.
Fresnel also constructed glass apparatus for fixed lights. If we require
to send the light equally towards the horizon in all directions at once,
the problem is capable of solution, either by a proper form of glass
apparatus or by a proper form of mirrors. Suppose the section, _e c f_,
Fig. 305, to revolve about a vertical axis passing through the lamp, it
would sweep out a form which, when executed in glass, would spread out
all the light falling upon it into one horizontal sheet. Fresnel was
obliged to content himself with an approximation to this shape, formed
by a prismatic frame of many sides, containing straight horizontal bars
of glass, having the section _e c f_. The light is not quite uniformly
distributed by such apparatus, but the difficulty and expense attending
the formation of prismatic rings were very great when Fresnel
constructed this apparatus. Such rings can now be produced economically
and accurately, and therefore the fixed-light apparatus is now
constructed of circular glass rings, mounted in sections in such a
manner that a vertical section through the axis of the apparatus would
cut them in the form represented at _e c f_. Instead of forming the
metal framework in which the glass is mounted with vertical ribs, it is
made with the ribs placed somewhat diagonally, in order that the dark
sectors which would be produced by the shadows of upright ribs may be
avoided. It should be understood that the forms of the glass in each
side of the octagonal apparatus represented in the figure are produced
by the revolution of the same section, _e c f_, about the horizontal
axis, _d g_.
An ingenious promoter of the catoptric system has contrived to solve the
same problem by mirrors. The form of these may be understood by the aid
of Fig. 306, which, however, relates to another contrivance. Suppose
that the lines A B, A´ B´, are turned about C D as an axis, all three
preserving their relative positions, A B and A´ B´ would sweep out two
parabolic cones, which would have the property of reflecting in a
horizontal direction all rays falling upon them from a lamp placed at L.
But glass, as a material for lighthouse apparatus, has so many
advantages over metal that it is probable that metallic reflectors will
soon be entirely obsolete. The polish of the metal is very readily
destroyed, and as it is constantly liable to be tarnished, the frequent
cleaning required is apt to produce a scratched state of the surface,
even when great care is used. Far greater accuracy of form can be
imparted to glass than to metal reflectors. And then there is the great
loss of light occurring at even the most highly polished surfaces of
metal: a loss which is far greater than that occasioned by the
refraction and reflections of the glass apparatus. There are cases,
however, in which it is desirable to throw the whole of the light into
one beam, and this cannot be done without reflecting the light from one
side. Mr. Alan Stevenson contrived an excellent apparatus for this
purpose, and the diagram, Fig. 306, will explain its nature. L is a
point representing the source of light, A B, B´ A´, a parabolic metallic
mirror. All the rays between L A and L B, and all between L A´ and L
B´—that is, all those which fall upon the mirror—will be reflected
parallel to L G; but those between L B and L B´ would escape from the
mouth of the mirror, B B´, as a diverging cone. This is prevented by
placing the lens, H I, the focus of which is at L, so as to convert the
diverging cone, I L H, into the cylindrical beam, E H I F; and thus half
the light emitted from the luminous point is sent in one direction. A
hemispherical reflector, C K D, of which L is the centre, receives the
other half, which is thus thrown back through L, and then follows the
same course as the direct rays. For the metallic reflector, C K D, Mr.
Stevenson afterwards substituted a system of glass zones; of which O P Q
represents the sections. These had the same effect as the metallic
reflectors, without the loss of light occasioned by the latter. The
inner surface of the glass, C K D, is hemispherical, and the prismatic
zones are such as would be produced by turning the section about L K (or
C D) as an axis. The dotted lines show the course of a ray of light, L
_m_, which, meeting the hemispherical surface perpendicularly, passes
straight through it, and is totally reflected at _m_ by the inclined
surface, and again at _n_, so that it returns to L by the path _n_ L.
Reflecting glass prisms were also substituted for the metallic mirror, A
B, B´ A´, and thus the use of metal has been entirely dispensed with in
this apparatus. This light has been termed by Mr. Stevenson the
_holophotal_ (ὁλο, _entire_, φως, _light_). Such an apparatus will form
the intensest beam that a given source of illumination can yield. On the
other hand, when a fixed light is distributed to the whole horizon
simultaneously, the illuminating power of the source is taxed to the
utmost. These two cases may be considered the extreme modes of disposing
of the light, while the parcelling of it into several beams, as effected
by the apparatus represented in Fig. 305, is an intermediate mode.
[Illustration:
FIG. 306.—_Stevenson’s Holophotal Light._
]
It may be interesting to mention that the holophotal light at Baccalieu,
in Newfoundland, is visible in clear weather from another point 40 miles
distant. So long a range as this is seldom possible at sea, on account
of the rounded form of the earth rendering it necessary to raise the
light nearly 1,000 ft. above the water, if it is required to be visible
at 40 miles’ distance. A shorter distance generally suffices for the
requirements of the navigator; and therefore lighthouse towers rising
from the water are seldom carried to a greater height than something
between 100 ft. and 150 ft. A light elevated 100 ft. above the water
would be seen from the deck of a vessel 14 miles distant, and from the
masthead a much greater distance.
The optical apparatus of a lighthouse is protected by an outer metal
framework glazed with thick plate glass. This framework is made of iron,
or of gun-metal—the latter being preferred on account of the frequent
painting which iron needs in order to preserve it from corrosion. The
glass is carefully fitted into the framework, so as to avoid exposure to
strains from the shocks and vibrations to which a lighthouse is exposed.
The keepers are always provided with a store of panes of glass, ready
for fitting into their places in case of accidents. Sometimes the glass
is broken by large sea-birds dashing against it, and by pebbles which
are thrown up by the waves, or driven by the wind against the panes. It
is the interior of this lantern which forms the light-room already
spoken of. Great pains have been bestowed on the proper ventilation of
these light-rooms, as not only must the air have access to the lamp to
supply the flame, but the carbonic acid which escapes from the chimney
of the lamp must be promptly removed. Another serious inconvenience of
an ill-ventilated light-room would be the condensation, in the inner
surface of the plate glass, of the aqueous vapour, which is also a
product of the combustion.
The lenses and circular prisms for lighthouses are usually made of crown
glass, and are ground by fixing them on a large revolving iron table, on
which they are bedded in plaster of Paris and cemented by pitch—great
care being taken to place them in the exact position required, for only
about one-eighth of an inch is allowed for grinding down to shape the
glass as it comes from the moulds. Sand, emery, and finally rouge, are
used with water for the grinding and polishing processes. The cost of
the optical apparatus alone of a light of the first order, like that
shown in Fig. 305, amounts to upwards of £1,500. The lenses and prisms
are very carefully adjusted in their framework after this has been
fixed, and no plan of testing the adjustment has been found more
efficient than that of viewing the sea horizon through them from the
position which the flame will occupy.
The men to whom the charge of a lighthouse is confided undertake a duty
involving the gravest responsibilities, and demanding unremitting care.
In those lighthouses where a number of reflectors are hung upon a
revolving frame, the extinction of one lamp may not be a matter of much
consequence; but where only one lamp is used, life and death depend upon
its burning. To isolated lighthouses—such as those of Skerryvore and the
Bell Rock—four keepers are appointed, and one of these is always on
shore on leave, so that the men may be relieved at intervals; for it has
been found that a residence in these lonely towers cannot be continued
long together without bad effects. The duties of the lighthouse-keepers
must be performed with the greatest regularity. The glasses of the
light-room and the optical apparatus are carefully cleaned every
morning; the lamps are supplied with oil, the wicks trimmed or renewed,
the machinery oiled and adjusted, and everything prepared in readiness
for the evening. At sunset the lamps are lighted, and one keeper takes
his watch until midnight, when he is relieved by another, who maintains
the vigil till sunrise, when the lamps are extinguished.
The expediency of the regulation appointing three men to be always at
the lighthouse may be illustrated by an incident which occurred about
the beginning of the present century at the lighthouse on the “Smalls,”
a rock in the Bristol Channel. Two keepers held watch over the light on
that rock, which for months together is sometimes cut off from all
communication with the shore. At the time alluded to, after the weather
had for two weeks prevented access to the lighthouse, it was rumoured
among the seafaring men of the neighbouring ports that something was
wrong at the “Smalls,” for a signal of distress had been observed; but
the boats could not go within speaking distance, although many attempts
were made to reach the rock. The relatives of the men became anxious,
and night after night watched for the light. But the light never failed
to appear at the proper hour. After four months came calmer weather, and
then a boat brought to shore one lightkeeper alive, the other dead. What
the former felt when he found his comrade to be dying in their dreadful
isolation, or what his emotions were when he found himself there alone
with the lifeless body, is not recorded. But the thought occurred to him
that he must not commit the body to the waves, lest any suspicion of
foul play might fall upon himself. He therefore contrived a sort of
coffin for the dead man, and dragging it up to the gallery of the
lighthouse, tied it there. Punctually and faithfully for four long
months did he perform all the duties of his position, keeping watch from
twilight till dawn in that lonely light-room, while his ghastly charge
remained there within sight. But he came on shore strangely altered—a
sad, silent, gloomy, worn man—so that even his intimate friends hardly
knew him.
Here we close this brief account of the modern lighthouse, and of its
beautiful appliances, by which Science “has given new securities to the
mariner,” in addition to those with which she furnished him when she
showed him the use of the compass, supplied him with the chronometer,
and placed the sextant in his hands. How anxiously must the seaman who
has been prevented by unfavourable skies from ascertaining his exact
position, and has been trusting to the log and the compass to work his
reckoning, scan the horizon for the first glimpse of the hospitable
light beacon, which seems to say that the country he is approaching has
been watching for his coming, and welcomes him to its shores.
[Illustration]
[Illustration:
FIG. 307.
]
PHOTOGRAPHY.
No other of our nineteenth century inventions is at once so beautiful,
so precious, so popular, so appreciated as photography. It is exercising
a beneficial influence over the social sentiments, the arts, the
sciences of the whole world—an influence not the less real because it is
wide-spread and unobtrusive. The new art cherishes domestic and friendly
feelings by its ever-present transcripts of the familiar faces, keeping
fresh the memory of the distant and the dead; it keeps alive our
admiration of the great and the good by presenting us with the
lineaments of the heroes, the saints, the sages of all lands. It
gratifies, by faithful portrayals of scenes of grandeur and beauty, the
eyes of him who has neither wealth nor leisure for travel. It has
improved pictorial art by sending the painter to the truths of nature;
it has reproduced his works with marvellous fidelity; it has set before
the multitude the finest works of the sculptor. It is lending invaluable
aid to almost every science. The astronomer now derives his mathematical
data from the photograph; by its aid the architect superintends the
erection of distant buildings, the engineer watches over the progress of
his designs in remote lands, the medical man amasses records of morbid
anatomy, the geologist studies the anatomy of the earth, the ethnologist
obtains faithful transcripts of the features of every race. To the mind
of an intelligent reader numberless instances will present themselves,
not only of the utility of photography in the narrower sense of the
term, but of its higher utility in ministering to our love of the
beautiful in art and in nature.
Effects produced by chemical changes to which the rays of the sun give
rise are matters of common observation. The fading of the colour in the
portions of a fabric which are exposed to the light is a familiar
instance; and the bleaching of linen under the influence of sunshine in
the presence of moisture is a well-known operation. Decompositions
produced by light in certain compounds of silver soon attracted the
attention of chemists, and the remarkable activity of the solar rays in
causing the combination of hydrogen and chlorine gases has been even
made the means of measuring the intensity of light. When equal volumes
of these two gases are mixed together in the dark, they may be kept for
an indefinite period without change, provided only that the mixture be
preserved from access of light. But the instant it is exposed to the
direct rays of the sun, or to an intense light, such as that of burning
magnesium, the two gases suddenly unite with a loud explosion, in which
the glass vessel containing them is shattered into atoms. The product is
an intensely acid invisible gas, called hydrochloric acid; and if the
mixture is exposed to the diffused light of day, instead of the direct
rays of the sun, then the production of hydrochloric acid will take
place gradually, and with a rapidity depending on the intensity of the
light.
Of vastly more importance than the small operations of the laboratory
and the bleach-field are the changes which the sun’s rays silently and
unobtrusively effect in the vegetable world. The chemical effect of
light here appears to reside in its power of separating oxygen from
substances with which it is combined. The green parts of plants absorb
from the atmosphere the carbonic acid gas, which is constantly produced
by the respiration of men and animals, and by combustion, and other
processes. Under the influence of sunshine, this carbonic acid is
decomposed within the tissues of the plant; the oxygen is restored to
the atmosphere; the carbon with which it was united is retained to build
up the structure of the plant. In a similar manner light separates the
oxygen from the hydrogen of water, and the former gas is given off by
the leaves, while the hydrogen enters into the composition of the plant.
The carbon, which forms so large an element in the food of plants, is
chiefly obtained in this way; and the abundance of the supply of oxygen
thus thrown into the atmosphere may be inferred from the fact that a
single leaf of the water-lily will in the course of one summer give off
nearly eleven cubic feet of oxygen. But for this continual restoration
of oxygen to the atmosphere, animal life would soon disappear from the
face of the earth. It is the office of the vegetable world not only to
furnish a supply of organic matter as food for animals, but when the
materials of that food have been converted into oxidized products in the
animal system, and returned to the atmosphere as carbonic acid and
aqueous vapour, the sunshine, acting on the vegetable structure (chiefly
on the delicate tissue of the leaf), tears apart the oxygen and the
other substance. These are, therefore, once more capable of combination,
by which they may again supply the animal with heat and the other
energies of life.
Those actions of light which have been last referred to are called by
the chemist _reducing actions_, a term which he applies to the cases in
which a compound is made to part with its oxygen or other similar
element: when the remaining ingredient is a metal, the operation by
which the other has been removed is always called _reduction_. On the
other hand, the inverse operations by which oxygen, chlorine, &c., are
fixed upon other bodies, are distinguished as processes of _oxidation_.
Light is the means of determining each of these kinds of changes,
according to the conditions and the nature of the substances exposed to
its action. Thus moist chloride of silver will retain its white colour
if preserved in the dark; but if exposed to sunlight, it quickly
acquires a violet tint, which deepens in intensity until it has become
black. The dark matter was formerly admitted to be silver; for it was
known that the finely divided metal has this appearance, that during the
process the compound gives off chlorine, and that when nitric acid is
poured upon the darkened matter, reddish fumes are given off, exactly as
when the acid acts upon pure silver. The use of silver nitrate as a
marking-ink for linen depends upon a similar alteration of the salt
within the fibres; and the same reduction takes place when to a solution
of the nitrate in water organic matter is added. If a piece of white
silk be dipped into a solution of chloride of gold, and exposed to the
sun’s rays while still wet, the silk becomes first green, then purple,
and finally a film of metallic gold will be found overspreading its
surface. Many other chlorides and analogous compounds are similarly
affected by sunlight. On the other hand, chlorides, as we have already
seen, and oxygen, fix on hydrogen and on organic substances with greater
energy under the influence of light. A large series of chemical
compounds are obtained by means of the augmented affinity of chlorine
for hydrogen induced by the rays of the sun.
It was in availing himself of an action of the latter class that, in
1813, Joseph Nicéphore Niepce[11] established photography; for he was
the first to obtain a permanent sun-picture. Twelve years before this,
Wedgwood and Davy had copied paintings made on glass, and the profiles
of objects, the shadows of which were projected upon a piece of white
paper, or white leather, saturated with a solution of nitrate of silver.
The images so obtained could not be fixed, as no means was then known of
removing the silver salts which had not been acted upon during the
exposure; and the pictures soon blackened in every part when exposed to
the light. The application of the _camera obscura_, and the fixing of
the image so obtained, define the commencement of the art of
photography. The process of Niepce, which was termed _heliography_, was
conducted by smearing a highly polished metallic plate with a certain
resinous substance known as “bitumen of Judæa,” and this was exposed to
the image formed in the camera for some hours. The action of the light
was such, that the resin, which before exposure was soluble in oil of
lavender, became insoluble in that substance. Hence, on treating the
plate after exposure with that solvent, only the deep shadows dissolved
away, the lights being represented by the undissolved resin. The
brightly polished parts of the plate, which were uncovered by the
removal of the resin, appeared dark when made to reflect dark objects,
while the resin remaining unchanged on the plate appeared light in
comparison.
Footnote 11:
Born at Chalon-sur-Saône, died 1833.
In 1826 a French artist, named Daguerre,[12] who had already made some
reputation as a painter of dioramas, entered into a sort of partnership
with Niepce, into whose process he introduced some improvements; but,
dissatisfied with the slowness of this proceeding, he invented a process
of his own, by which pictures of great beauty could be produced with all
the shadows, lights, and half-tints faithfully rendered; while the time
of exposure in the camera was reduced to twenty minutes. In this process
the burnished surface of silver formed the shadows. A plate of copper,
coated with pure silver, had the silvered surface polished to the
highest degree, and it was then exposed to the vapour of iodine until a
thin yellow film had been produced uniformly over the silver. It was
then placed in the camera; and, although when withdrawn no image was
perceptible, a latent image was nevertheless present; for when the plate
was exposed to the vapour of mercury, that substance attached itself to
the parts of the plate in proportion as they had been acted upon by the
light. Means were adopted by Daguerre for fixing the picture; and after
his processes had been made public in 1839, several important
improvements were proposed by other persons. By using bromine as well as
iodine the sensitiveness of the plates was so much increased that the
time required for exposure was reduced to two minutes, so that about the
year 1841 portraits began to be taken by this process.
Footnote 12:
L. J. M. Daguerre, born 1787, died 1851.
The world at large, which profits most by great inventions, has little
idea at what cost of intense application, concentrated thought, and
heroic perseverance, such discoveries are made. What his discovery must
have cost Daguerre may be inferred from an anecdote related by J.
Baptiste Dumas, the distinguished French chemist and statesman. At the
close of one of his popular lectures in 1825–-_fourteen years before
Daguerre had perfected his process_—a lady came up to him and said,
“Monsieur Dumas, I have to ask you a question of vital importance to
myself. I am the wife of Daguerre, the painter. He has for some time let
the idea possess his mind that he can fix the images of the camera. Do
you, as a man of science, think it can ever be done, or is my husband
mad?” “In the present state of our knowledge we are unable to do it,”
replied Dumas; “but I cannot say it will always remain impossible, or
set down as mad the man who seeks to do it.” The French Government, with
an honourable recognition of the merits of Daguerre, and of Niepce who
had passed away poor and almost unknown, awarded to the former a pension
of 6,000 francs (£240), and to Isidore Niepce, the son of the latter, a
pension of 4,000 francs, one-half to be continued to their widows.
But Daguerre’s process had no sooner been brought to perfection than it
began to be supplanted by a rival method, devised by an Englishman, Mr.
Fox Talbot, who had published his process six months before that of
Daguerre was given to the world, and who, therefore, was unacquainted
with the details of the latter. The first of Mr. Talbot’s publications
contained only an improved mode of preparing a sensitive paper for
copying prints, by applying them to it and causing the light to pass
through the paper of the print, so that the parts of the sensitive paper
protected by the opaque black lines were not acted upon by the light.
The paper was first dipped in a solution of chloride of sodium, and then
in one of nitrate of silver, the result being the formation in the pores
of the paper of chloride of silver, a substance much more quickly
affected by light than the nitrate of silver used by Davy and Wedgwood.
The impression so obtained was a _negative_, that is, the lights and
shades of the original were reversed; but when this negative was again
copied by the same process, it produced a perfect copy of the original
print, for the lights and shades were of course reversed from those in
the negative proof. Thus from one negative any number of positive or
natural copies could be produced; and this point in Mr. Talbot’s
invention is one great feature of photography as now practised. In 1841,
Mr. Talbot obtained a patent for a process he called the _Calotype_, but
which, in his honour, has since been known as the Talbotype. A sheet of
paper is soaked, first in a solution of nitrate of silver, and then in
one of iodide of potassium, by which it becomes covered with iodide of
silver; it may then be dried. It is prepared for the camera by brushing
it over with a solution of gallic acid containing a little nitrate of
silver. By this last process its sensitiveness is greatly increased, and
an exposure in the camera for a few seconds, or minutes, according to
the power of the light, suffices to impress the paper with a latent or
invisible image, which reveals itself when the paper is treated with a
fresh portion of the gallic acid mixture. The Talbotype is the
foundation of the methods of photography now in general use; but, before
we describe these, it may be proper to mention some other substances
which have been found sensitive to light, and to discuss the nature of
the invisible images which are first produced in these processes.
The _art_ of photography has outstripped the _science_—in other words,
the nature and laws of the chemical actions by which its beautiful
effects are produced are not yet clearly understood, and some quite
recent discoveries seem to show that we have yet much to learn before a
complete theory of the chemical action of light can be proposed. Some
results which have been established may be mentioned, as they show those
curious effects of light to be more general than would be supposed from
a description of photographic processes dependent on silver salts only.
It has been found that certain acids, certain salts, and certain
compounds containing only two elements—of which one is a metal—have a
tendency to split up, or resolve themselves into their several
constituents, when exposed to the action of light. On the other hand,
chlorine, bromine, and iodine exhibit, under the same conditions, an
exalted affinity for the hydrogen of organic matters. These tendencies
concur when the compounds above referred to are associated with organic
materials, as in photography. Solution of nitrate of silver is blackened
when it is exposed to light on a piece of paper which has been dipped
into the solution; but a piece of white unglazed porcelain similarly
treated shows no change. A solution of nitrate of uranium in pure water
is not changed by light; but a solution of the same salt in alcohol
becomes green, and deposits oxide of uranium. The reducing action of the
light is insufficient of itself to accomplish the decomposition of the
salt in the first case; but the presence of the organic matter
determines this decomposition in the second case. Bichromate of
potassium is by itself not easily decomposed by light; but when it is
mixed with sugar, starch, gum, or gelatine, the sunbeams readily reduce
it. It is remarkable that the gelatine, gum, or starch becomes insoluble
by thus taking up oxygen, and the gelatine loses its property of
swelling up in water. We shall presently see the advantages which have
been drawn from these circumstances.
It is not necessary that the light should act upon both the organic
substance and the oxidizing substance at the same time. If paper
impregnated with iodide of silver and gallic acid be placed in the
camera, the image soon appears; but if, as in the Talbotype, the iodide
of silver only be acted upon by the light, no image is perceptible on
withdrawing the paper from the camera. The action of the light has
nevertheless imparted to the silver salt a tendency to reduction; for
when the paper is afterwards dipped into a solution of gallic acid, the
image immediately appears. In order to distinguish these two actions,
the substance which receives and preserves the latent impression from
the light is called the _sensitive_ substance, and that which reveals
the latent image is termed the _developing_ substance. A considerable
number of substances having this relation to each other have been
observed, and the following table of instances—cited by Niepce de
Saint-Victor, the nephew of the original inventor—will give some idea of
their variety:
┌───────────────────┬────────────────────────┬────────────────────────┐
│ Sensitive │ Developing Substance. │ Results. │
│ Substances in the │ │ │
│ paper exposed to │ │ │
│ the action of the │ │ │
│ Light. │ │ │
├───────────────────┼────────────────────────┼────────────────────────┤
│None, _i.e._, plain│A salt of silver │Black image. │
│ paper. │ │ │
│ │ │ │
│Nitrate of silver, │Gallic acid, or sulphate│Black image. │
│ or iodide of │ of iron. │ │
│ silver. │ │ │
│ │ │ │
│ │{ Water │By prolonged action of │
│ │ │ light, a grey image of│
│ │ │ protoxide of uranium; │
│ │ │ the image disappears │
│ │ │ when paper is kept in │
│ │ │ the dark, but shows │
│ │ │ itself again in the │
│ │ │ light. │
│Nitrate of uranium.│{ │ │
│ │{ Red prussiate of │Intensely red positive │
│ │ potash │ image; becomes blue by│
│ │ │ sulphate of iron. │
│ │ │ │
│Nitrate of uranium │Nitrate of silver or │Unchangeable │
│ and tartaric │ chloride of gold. │ images—resembling │
│ acid. │ │ those of ordinary │
│ │ │ photographs. │
│ │ │ │
│Chloride of gold. │Nitrate of uranium, │. . . . . . │
│ │ sulphate of iron, │ │
│ │ sulphate of copper, │ │
│ │ bichloride of mercury,│ │
│ │ salt of tin. │ │
│ │ │ │
│ │{ Sulphate of iron │Blue-black image. │
│Gallic acid. │{ │ │
│ │{ Red prussiate of │Blue image. │
│ │ potash │ │
│ │ │ │
│Red prussiate of │Water, bichloride of │Blue image, hastened by │
│ potash. │ mercury, gallic acid, │ acids and by heat. │
│ │ salt of silver, salt │ │
│ │ of cobalt. │ │
│ │ │ │
│Bichloride of │Protochloride of tin, │. . . . . . │
│ mercury. │ soda, potash, sulphide│ │
│ │ of sodium. │ │
│ │ │ │
│Chromic acid, or │Salts of silver │Purple-red positive │
│ bichromate of │ │ image. │
│ potash. │ │ │
│ │ │ │
│ │{ Blue litmus │Red image. │
│Starch. │{ Iodide of potassium │Reddish brown image. │
│ │{ White indigo │Blue positive image. │
│ │{ Campeachy wood │Red positive image. │
└───────────────────┴────────────────────────┴────────────────────────┘
These are only a few of the instances in which actions of this kind have
been observed. It is remarkable that the order of the first two columns
in this table may be inverted without changing the result. Thus, instead
of exposing iodide of silver to the light and developing the image with
gallic acid, one may expose a paper saturated with gallic acid solution,
and develop with iodide of potassium and nitrate of silver. The first
reaction noted in the table deserves some remark: it is not peculiar to
paper, but is common to most organic materials, such as albumen,
collodion starch, fabrics, and indeed to organic matters in general,
provided they are not of a black colour. Tartaric acid, sulphate of
quinine, and nitrate of uranium increase this sensibility. The paper
which has been impressed preserves its undeveloped image for a prolonged
period if kept in darkness; and it has been found that one piece of
paper can impart the image to another by simple contact in the dark.
What is still more remarkable, the invisible impressions on a piece of
paper may be transferred to another not in contact by merely placing it
opposite the first, and separated by an interval of a quarter of an
inch. No satisfactory explanation of these phenomena has been advanced,
but many conjectures have been made. One of these supposes that some
unknown intermediate products are formed, which are, in the case of the
latent image on paper, very oxidizable; but in the case of silver salts,
&c., very reducible, so that the addition of a silver salt in the first
case, and of organic matter in the second, only completes the phenomena
by ordinary chemical action. Niepce de Saint-Victor, however, found that
a surface of freshly broken porcelain alone will receive a latent
impression from light, and will reduce in those places sensitive salts
of silver. He believes that the light in these latent images is simply
stored up, and that its energy remains fixed to the surfaces until the
occasion of its producing a chemical action.
When a pure solar spectrum is made to fall upon paper rendered sensitive
by silver salts, the effect is observed to be greatest near the
Fraunhofer line H (No. 1, Plate XVII.), and it is prolonged with
decreasing intensity beyond the violet end of the spectrum, while
towards the other end it terminates about the line F. When other
sensitive substances are used, the range of photographic power in the
spectrum is modified. It has been found that when a daguerrotype plate
which has been impressed by the light in the camera is afterwards
exposed to the red or yellow rays of the spectrum, it loses its property
of condensing the mercurial vapours. This destruction of photographic
impression by red or yellow light has a practical application of great
importance, for it permits the processes of preparing paper and plates
to be carried on in a laboratory lighted by windows having yellow or
red, instead of the ordinary colourless, glass. Thus we see that it is
by no means the whole of the solar rays which are concerned in producing
photographic images; nay, there are some which even tend to destroy the
impressions produced by others. The fact that it is not the light, but
only certain rays in the sunbeam, may be proved very conclusively by an
experiment with a glass bulb filled with a mixture of equal volumes of
hydrogen and chlorine gases. When such a bulb is exposed to the light of
the sun or of burning magnesium, which is made to reach it by passing
through a piece of _red_ glass, no explosion takes place; but if the
bulb be covered only with a piece of _blue_ or _violet_ glass, the
explosion is produced just as quickly as if it were exposed to the
unaltered rays.
The visible spectrum obtained in the experiment described on page 318 is
far from constituting the only radiations which reach us from the sun.
For invisible beams of heat, less refrangible than the red rays, are
found beyond the red end of the spectrum; and another invisible spectrum
stretches far beyond the violet end, formed of rays recognized only by
their chemical activity. It is these which effect photographic actions,
and though they are in part more highly refrangible than any of the rays
producing the visible spectrum, a large portion are refracted within its
limits, so that the maximum of photographic action in a spectrum is
usually near the violet end. When we wish to examine the spectrum of the
heat rays, it is necessary to replace the glass prism by one made of
rock salt, for glass absorbs these heat rays. It also intercepts a great
part of the most refrangible rays; for when a prism of _quartz_ is
substituted for the glass one, the spectrum becomes greatly extended at
the violet end. The dark Fraunhofer lines which cross the visible
spectrum are represented also in great numbers in the invisible
spectrum: in photographs of the _ultra-violet_ rays more than 700 dark
lines have been counted. It has been proposed to employ quartz lenses in
the photographic camera; but there is reason to believe that the
increased transparency of such lenses for the chemical rays would be
counterbalanced by certain disadvantages attending the use of quartz.
The beauty of the images which are formed in the camera obscura long ago
gave rise to the desire of fixing them permanently. We know how
perfectly photography has already satisfied that desire, so far as the
_forms_ are concerned. The very perfection of the results obtained in
this direction increases our regret at our inability to fix also the
colours, and secure the picture, not in grey or brown tones of reduced
silver, but with all the glowing hues of nature. An observation made by
Herschel, Davy, and others, seemed at one time to hold out hopes of a
possible realization of chromatic photographs. It was noticed that the
images developed upon chloride of silver, of the different parts of the
solar spectrum, partook somewhat of the colours of the rays which
produced them. Edmond Becquerel made a plate of polished silver, placed
in dilute hydrochloric acid, form the positive pole of a battery. The
plate thus became coated with an extremely thin layer of chloride of
silver, which, as its thickness augmented, exhibited the series of
colours due to the action of light on thin films. The operation was
stopped when the plate had become of a violet colour for the second
time; it was then washed, dried, polished with the finest tripoli, and
heated to 212° F., the whole of these operations having been carried on
in the dark. When this plate was exposed for about two hours to the
solar spectrum, fixed by proper appliances which counteracted the
apparent motion of the sun, the luminous rays were found to have
impressed the plate with their respective colours. The yellow was
somewhat pale, but the red, green, and violet were exhibited in their
true tints. A theoretical explanation has been advanced, which supposes
that yellow light, for example, renders the surface of the plate on
which it falls peculiarly capable of receiving and transmitting
vibrations corresponding to those of yellow light. Just as a stretched
cord responds to its own musical note, the modified plate gives back,
out of all the vibrations which fall upon it in ordinary light, only
those of which it has itself acquired the periodicity. But since the
plate has not lost its sensitiveness to take on other rates of
vibrations, it receives other impressions, which first weaken and then
overcome the former, and, therefore, the colour necessarily vanishes.
This kind of difficulty seems to be a necessary concomitant of every
attempt in this direction; and all the hopes founded on results yet
obtained have been disappointed by the rapid fading of the images.
The comparative cheapness and convenience of Talbot’s process, and
especially the facilities which it afforded for the multiplication of
proofs, gave an immense impulse to photographic art. But the irregular
and fibrous structure of paper prevented the attainment of the beautiful
sharpness of outline and clear definition of detail which the plates of
Daguerre presented. Sir John Herschel suggested the use of glass plates
coated with sensitive photographic films, and Niepce de Saint-Victor
succeeded in fixing upon glass layers of albumen (white of egg)
containing the silver salts, a method which is still used to some
extent. The art received, however, its greatest stimulus from the
improvements which ensued on the application of _collodion_ to this
purpose. Collodion (κολλα, glue; in allusion to its adhesiveness) is the
name which has been given to a solution in ether of gun-cotton, or of a
substance nearly allied to it. Its employment was suggested by Le Grey
of Paris, but the late Mr. Archer was the first to carry the idea into
practice, and the process which he described in “The Chemist,” in 1851,
is virtually that which is now almost universally adopted. This process
has now been tested, for nearly a quarter of a century, by the united
experience of photographers all over the world, and it is agreed that it
is surpassed by no other, for it secures every quality which a
photograph can possess.[13] The minor details of the method can be, and
are, infinitely varied; scarcely two experienced photographers will be
found working the process in identically the same manner throughout.
Before giving an outline of the collodion process, it may be well to say
something respecting the chief instrument of photography—the camera.
Footnote 13:
(1875) But see below, page 541.
[Illustration:
FIG. 308.
]
The ordinary photographic camera is almost too well known to require
description. In its simplest form, Fig. 308, it is merely a rectangular
box, in front of which is placed the lens, which slides in a tube, that
its position may be adjusted so as to bring the rays to a focus on the
surface of a piece of ground glass at the opposite end. This glass is
fitted into a light frame, which slides in grooves, so that it can be
raised vertically out of its position, and replaced by another frame, B,
which contains a recess for the reception for the sensitive plate, and a
sliding screen which protects it from light until the right moment. When
this frame is placed in the camera, the sensitive surface occupies the
same position as that of the ground glass, and the sliding screen is
drawn up the moment before the operator removes from the front of the
lens a cap which he places there after adjusting the focus. The sliding
screen is usually made with a narrow strip at the lower part, joined to
the rest by a hinge, so that when it has been drawn up it may be
retained in its position, and placed out of the way, by being folded
down horizontally. There is commonly provision for two plates in one
frame, the slides, &c., being doubled, and the plates placed back to
back, as shown at B, Fig. 308. The camera is usually made in two parts,
as shown in the figure, that at the back sliding within the other, so
that a wider range for adjustment is obtained, and the same camera may
even be used with lenses of different focal lengths. Many improvements
have been made in the camera, by which it has been rendered more
portable, and capable of more adjustments to suit varying circumstances.
Fig. 309 represents a “bellows” or folding camera, which appears to
supply every requirement for the studio. It is copied from Messrs.
Negretti and Zambra’s catalogue, as are also the other figures of
photographic apparatus here given. Fig. 307 represents a camera for
taking stereoscopic views, fitted with two lenses, so that the two views
are taken simultaneously on one plate.
[Illustration:
FIG. 309.
]
No piece of apparatus used by the photographer is of so much importance
as the lens; for good pictures cannot be obtained without well-defined,
sharp images on the sensitive plate, and these images must have
sufficient intensity to produce the required amount of chemical action
in a short space of time. The formation of an image by means of a lens
which is thickest at the centre is tolerably familiar to everybody; for
most persons must have noticed that the lens of a pair of spectacles, or
of an eye-glass, will produce an inverted image of the window-frame on a
sheet of white paper, held a certain distance behind the lens. But the
diagrams by which the paths of the rays are usually represented seem to
convey a false impression to an ordinary reader, who usually goes away
with the idea that somehow three rays are sent off by the object, and
that one goes through the middle of the lens, and the other two meet it
and produce an image. Let us suppose that, by means of a circular
eye-glass, the image of a window is projected on a piece of white paper:
a straight line passing through the centre of the glass perpendicular to
its plane will meet the window and image each at a certain point. The
point in which it meets the image is the _focus_ of _innumerable_ rays,
which issue from the point in the window; that is, of the whole light
sent out in every direction by the point a certain portion falls upon
the lens, and by the refraction it undergoes in passing through it, the
rays are again brought together at the point in the image. Thus the
original point in the object is the apex of a solid cone of rays (if we
may say so), of which the lens is the base, and the point in the image
is the apex of another cone, having also the lens as its base. These
cones would be termed _right_ cones, because their bases are
perpendicular to their _axes_, or central lines. But they represent the
rays from only _one point_ of the object. Let us now consider how the
image of another point is formed, say one in the highest part of the
object which forms an image on the screen. Those rays which are sent out
by this point, and fall upon the lens, form now an _oblique_ cone, of
which the lens is the base, and the central ray will pass through the
middle of the lens and continue its journey on the other side with
little or no change of direction, forming also the axis of another
oblique cone, constituted of the refracted rays, all of which will meet
together at the lowest part of the image. Similar cones of incident and
refracted rays, all having the lens as base, and all of them cones more
or less oblique, will be formed by the light from each point of the
object. Thus, the rays which issue from each point are brought together
again in a series of points which have the same position with regard to
each other, and collectively form an inverted image.
On carefully looking at the image, say of a window-frame, formed by a
simple lens, the reader will observe two defects. The first is that the
image cannot be made equally clear and well defined at the centre and at
the edges: the adjustment which gives clear definition of one part
leaves the other with blurred outlines. The second defect, which is best
seen with large lenses, consists in coloured fringes surrounding the
outlines of the objects. This depends upon the unequal refrangibility of
the various rays, but it is obviated in _achromatic_ lenses, which are
formed of two or more different kinds of glass, so adapted that the
refracting power of the compound lens is retained, and the most powerful
rays of the spectrum are brought to a common focus. Such are the lenses
always used in the photographic camera, and the skill of the optician is
taxed to so combine them as to obtain, not only the union of the
principal rays in one focus, but the greatest possible flatness of field
in the image, the largest amount of light, the widest angle without
distortion of the picture, and other qualities.
[Illustration:
FIG. 310.
]
Photographers have even been so fastidious in the matter of lenses as to
require all the perfection of finish which is given to the
object-glasses of astronomical telescopes. Mr. Dallmeyer has made
photographic lenses which cost upwards of £250; but it is doubtful
whether the pictures formed by these would show any marked superiority
over those produced by lenses costing only one-fifth of that amount.
Fig. 310 shows the construction of the combination usually employed for
taking photographic portraits. A is a section showing the forms and
positions of the different lenses; B is an external view of the brass
mounting of the lens. It is provided with a flange, C, which is attached
by screws to the woodwork of the camera; and within the short tube, of
which this is a part, slides the tube carrying the lenses, being
furnished with a rack and pinion moved by the milled head, E. D is a cap
for covering up the front of the sliding tube. A slit in the tube admits
of plates of metal, perforated with circular openings, being inserted.
The openings are of various sizes; and these “stops” or diaphragms
enable the operator to regulate the amount of light; and to cut off when
required the rays passing through the marginal parts of the lens.
It now remains to describe in a few words a method of photography which
was, and still is, much practised, namely, the _collodion process_. The
collodion solution is prepared by dissolving one part of pyroxylin
(gun-cotton) in ninety parts of ether and sixty of alcohol. The
pyroxylin for this purpose may be obtained by steeping cotton-wool for a
few minutes in a mixture of nitre and sulphuric acid, with certain
precautions which need not here be mentioned. To the solution of
collodion is added a certain quantity of iodide of potassium, or of
iodide of ammonium; and sometimes other substances also are mixed with
the solution with a view of increasing the sensitiveness of the plate
when ready for exposure. Some of the collodion solution is poured on a
well-cleaned plate of glass, which is placed horizontally; it spreads
over the plate, and the excess having been poured back into the bottle,
the evaporation of the liquids leaves the glass covered with a thin
uniform transparent film, which firmly adheres. The next operation is to
render the plate sensitive by means of the “silver bath.” This is a
neutral solution of nitrate of silver, one part to fifteen of pure
water, which is placed in a trough of glass or porcelain, Fig. 311. By
the aid of a proper support the plate is introduced quickly and steadily
into the solution, immediately after the collodion film has been formed
on its surface. In two or three minutes the layer of collodion becomes
impregnated with iodide of silver, and when taken out of the bath, the
plate exhibits a creamy-looking surface. The operation of sensitizing
the plate by the silver bath must be performed in a room to which no
light has access, except that which has passed through _red_ or _yellow
glass_, or a semi-transparent yellow screen.
The plate is now ready for immediate exposure in the camera. It is
placed in the dark slide, in which it is conveyed to the camera; and
there the image of the object is allowed to fall upon it for a time,
which varies, according to the intensity of the light and the nature of
the object, from 3 seconds to 45 seconds. The slide is withdrawn from
the camera, and taken again to the “dark” room, _i.e._, where only
_yellow_ or _red_ light can reach it. If the plate be now examined, it
will be found to present no trace of an image. A latent one, however,
exists; and it is developed by pouring over the plate a solution of
pyrogallic acid—one part to 480 of water, with commonly a little alcohol
and acetic acid added. When it is desired to intensify the image still
more, a few drops of the nitrate of silver solution is added to the
_developing solution_ immediately before pouring it on the plate. When
the picture has become sufficiently distinct, it is washed with pure
water, and then immersed in a strong solution of hyposulphite of soda.
The last operation is termed by photographers “fixing” the picture, and
the substance employed in it is invaluable to the art. It acts as a
ready solvent of all the salts of silver which remain on the plate; and
the discovery of this property of the hyposulphites by Sir J. Herschel,
in 1839, marked an era in photography. The picture is then thoroughly
washed in cold water, in order that the hyposulphite of soda may be
entirely dissolved out. It is then dried, warmed before a fire, and
finally the film is covered with a coat of transparent varnish, by which
it is protected from mechanical injury. The image here is
_negative_—that is, the strongest lights of the object appear as the
darkest tints in the picture, and _vice versâ_. From it any number of
_positive_ pictures may be obtained by means of the sensitive paper
prepared with chloride of silver as in Fox Talbot’s plan.
As it is a tedious, and perhaps, in some cases, an impossible operation
to completely remove all traces of silver salts and hyposulphites from
photographs, they have frequently been found to fade; but this is rarely
the case with well-prepared specimens. Processes have, however, been
devised by which absolute permanence is secured for the photograph. One
of the best of these is known as the Carbon Printing Process, and, as
improved by Mr. Swan, it is thus practised:
A solution of gelatine is coloured by the addition of Indian ink, or any
other pigment which will give the desired tone. This solution is spread
over sheets of paper which are then dried. In this condition the paper
may be preserved for any length of time without any special precautions.
When it is required for use, it is floated, with the gelatine-covered
side downwards, in a solution of bichromate of potash, and then dried;
but these operations must be carried on in the dark. The paper is
exposed under a negative photograph, with which its prepared side is in
contact. The effect of the light is to render insoluble the gelatine on
all those parts on which it has fallen, and this action extends to a
depth in the layer proportionate to the intensity of the illumination.
The object is, therefore, to wash away all the _soluble_ gelatine and
the colour with which it is mixed; but this soluble gelatine is mainly
on the side of the film which is in contact with the paper. The gelatine
surface is therefore made to adhere to another piece of paper by means
of some substance insoluble in water; and when this has been done, the
whole is immersed in warm water. Then the soluble gelatine is soon
dissolved; the first paper floats off, and the insoluble gelatine,
holding the Indian ink or other colouring matter in its substance,
remains attached by the cement. As the thickness of the layer rendered
insoluble is in proportion to the intensity of the light passing through
each part of the negative, the picture will be presented in all the
proper gradations of light and shade.
[Illustration:
FIG. 311.
]
The “wet collodion” process, that has been described on the preceding
page, maintained an almost undisputed hold for more than twenty years in
the practice of photography in all branches, and it was not until after
the publication of the first edition of the present work that a new era
in the art was commenced by the introduction of what is known as the
_dry plate gelatino-bromide process_, to which the present enormous
popularity of photography as a recreative art is due. The difficulties
of manipulation, the necessity for extensive experience, and for special
and cumbersome appliances were obstacles it at once removed. And not
only so, but the whole scope of the art was extended; for work that was
before supposed impracticable, even to the most expert professional
photographer, became the amusement of the amateur. Here, we may remark
in passing, that photography is greatly indebted for this, and many
other improvements, to the enthusiasm of the amateur, which has
accelerated the development of the art to a remarkable extent. The
collodion process itself admitted of being modified as a dry plate
method, by coating the film with a preservative solution of tannin, gum,
albumen, or other substance, and then drying the plates, of course in a
dark place. This plan made it possible to practise out-door photography
with ease, and such plates were, at one time, much used for landscape
photography, but they have now been almost superseded by the gelatine
plates. It was Mr. Bennet, who, in 1874, first introduced the use of
sensitive emulsions of gelatine, and the advantages offered by their
use, caused them to be soon adopted by landscape and amateur
photographers. In 1878, Mr. Bennet showed, that these plates could be
made wonderfully rapid in their action, so that portraits, etc., could
be taken by them in an unprecedentedly short time. The preparation of
the dry gelatine plates was then commenced on a large scale, and these
were found so convenient, and reliable in use, that they were adopted by
the professional photographers, who had hitherto adhered to the wet
collodion and silver bath, from long habit and established associations.
The collodion processes are, however, still much used, and are preferred
by many to the gelatine plates; indeed, it is admitted, that only by the
former can certain desirable qualities of negatives be obtained, which
are of great importance in some applications of the art.
There are, it need hardly be said, many modifications of the processes
recommended for preparing gelatino-bromide dry plates, and each
manufacturer of the various kinds offered for sale has, no doubt, his
own special plan and formula. In all, a very fine and carefully selected
quality of gelatine is the medium in which the sensitive salts are
embedded. An “emulsion” is prepared by adding to warm gelatine solution
exactly determined quantities of solutions of certain compounds, of
which a bromide (usually bromide of potassium) and silver nitrate are
the essential ones, together with a small proportion of iodide of
potassium. Minute quantities of iodine, hydrochloric acid, etc., are
also often prescribed as additions. The mixture has to be heated, at the
boiling temperature, for three quarters of an hour, then cooled, and
mixed with more gelatine solution, or, instead of using acid and iodine
and boiling, a little ammonia is added. When cold and set, the gelatine
is washed with cold water, while squeezed through canvas, or after it
has been cut into thin strips. It is then drained, dissolved at a gentle
heat, and filtered warm. The clean glass plates are coated over with it,
at the temperature of 120° F., and are set aside in a perfectly
horizontal position until the gelatine has set, when they are placed for
twenty-four hours in a drying cupboard, maintained at 80° F. It will be
understood that these operations are conducted in a room where no light
enters, except through a frame of ruby-coloured glass, and the plates,
when dry, are carefully packed and stored in light-tight boxes. They are
marvellously sensitive, and receive the photographic impression in about
one-sixtieth (1/60th) of the time required for wet collodion plates.
Half a second exposure in the camera may be sufficient to impress the
image of a well lighted landscape, even when a very small stop is used,
and it is not unusual to employ for extra sensitive plates, a so-called
“instantaneous shutter,” when the exposure may be no more than 1/80th to
1/100th of a second, and yet obtain a perfectly strong image. Dry plates
are manufactured in vast numbers in many large establishments, and the
operations are carried on to a great extent by the aid of machinery, by
which the plates are uniformly coated and automatically carried into
drying chambers, etc.
If photography were popular before the introduction of the dry
gelatino-bromide plates, it has since become a hundred-fold more so.
Indeed, the camera is now seen everywhere, and few are the family
circles in which at least one amateur practitioner of the art is not to
be found; indeed, the technical terms of the art have become “Familiar
in their mouths as household words.” The daguerrotype, notwithstanding
its cost, had no sooner become a practicable process for taking
likenesses, than it began to supersede miniature painting, and how
rapidly it rose into general favour may be inferred from the fact that,
in 1850, ten years after its introduction, it was estimated that in the
United States of America, at least ten thousand persons had made it
their profession, and, probably half as many more were occupied in
making and selling chemicals, plates, cameras, lenses, mounting cases,
and other apparatus connected with its practice. Such being the demand
for photographic portraits, at the period when the sitter had, as we
have already seen, to remain motionless for two whole minutes in
sunlight, we can hardly be surprised at the increased popularity the art
has acquired in the last decade, when a picture can be produced with
one-hundredth the length of sitting, and at about the same reduction of
cost. It may here be mentioned, that Daguerre’s process is still
occasionally used for special purposes; it was, for instance, the method
selected for obtaining the photographic records in the expedition sent
out by the French Government, in 1874, to observe the transit of Venus.
[Illustration:
FIG. 311_a_.—_The Roll-Slide._
]
The dry plate processes have given an immense impulse to landscape
photography, and travellers have been able to bring back authentic
representations of the scenery and inhabitants from every part of the
globe. This advantage arises from the fact that having the camera, and
its appurtenances, the tourist or traveller is not obliged to carry
anything about with him except his plates, and when these have once been
exposed in the camera, and stowed away in light-tight boxes, the latent
images may be developed months, or even years, afterwards. But glass
plates are heavy, and are liable to accidental breakage. Inventive
ingenuity has been actively at work for the past few years, to find a
means of obviating these remaining inconveniences. The first method
adopted was to employ paper instead of glass, as a support for the
sensitive gelatine film. The paper, having been cut to the proper size,
is placed on a _film-carrier_, which is usually a thin plate of ebonite,
by which the paper is kept flat. These carriers take the place of the
glass plates in the ordinary dark slide, and after exposure in the usual
way, the papers are removed in the dark room and made up into
light-tight packages, where, of course, a large number will occupy but a
small space, and the weight of them be wholly negligible. Many persons
make use of this arrangement, which has the advantages of simplicity and
of requiring no special apparatus. But an improvement was soon brought
out, which consists in substituting for the carriers and pieces of
sensitive paper a continuous roll of the material. For this purpose a
special piece of apparatus, called the roll-holder, is made to take the
place of the dark slide at the back of the camera. The arrangement will
be readily understood from Fig. 311_a_. The figure shows the apparatus
in section, but only the disposition of the principal parts, most of the
mechanical details being omitted. R R´ are two metallic or wooden
rollers, which admit of being readily put in their places and taken out.
Upon one of these, R, the full length of the material is previously
wound, and the free end is passed over another roller, _r´_, and across
the opening at E O, where the exposure is made. There is in front of
this a dark slide (not here shown) to be drawn up when everything is
ready for uncovering the lens. Immediately behind the paper is a flat
plate of ebonite, E, or a smooth black board, the object of which is to
keep the material quite flat as it passes over the opening to the
roller, _r´_, which guides it to the roll, R´, on which it is wound as
required. S S´ are two small rollers always pressed by springs against
the rolls to prevent the turns working loose. There is a registering
apparatus outside in connection with one of the rollers, _r_, or _r´_,
to show when the proper length of material has been wound across the
opening for a new exposure; and at the same time a mark is automatically
made on the paper to indicate where the negatives are to be separated
for development by cutting the paper. Some forms of the apparatus also
call the operator’s attention to the sufficient winding of the roll by
an audible signal, a stroke on a little bell tells that everything is
ready for a new exposure. In some cases the number of exposures already
made is registered by figures that appear on the outside. The paper in
these processes is used only as a temporary support; for after the
negative has been developed in the ordinary way, the sensitive gelatine
film is removed from it and made to adhere firmly on a plate of clear
glass, from which prints are taken as usual. The operations required for
the transferring require considerable dexterity of manipulation, and to
both the paper and the glass special preparations have to be applied,
before and after the transference of the film. This plan, therefore, of
“stripping films” involves so great a number of delicate and somewhat
troublesome operations that very many photographers have preferred to
encounter the labour and risks of carrying about with them the more
easily manageable glass plates. But what if some grainless, transparent
substance could replace the paper in these rolls so that the negatives
might be ready for printing from when merely developed and fixed? Many
trials have been made to find this desideratum. A material sufficiently
translucent, even, and of tenacity enough to bear the stretching strain
between the rollers has, it is believed, been discovered in a very
singular substance previously used for other purposes. The reader is no
doubt familiar with it as the substitute for ivory in combs, knife
handles, and other small articles. It is called _celluloid_, and is a
composition the principal ingredients of which would never be guessed
from its appearance—namely gun-cotton and camphor! This material is
prepared in a plastic condition that enables it to be shaped into any
required form. It can be drawn into threads or rolled out into very thin
films. Thin plates of it have been used in photography as a substitute
for glass, for the sake of lightness, before its employment as a
transparent film in the roll-holders. We have now at length the
equipment of the travelling photographer reduced to the utmost
conceivable limits of lightness and compactness. Thus the complete
apparatus required for taking hundreds of pictures of a good size need
not be more than a few pounds in weight, and can easily be carried in
the hand. But even quite small negatives can now be very readily printed
in a few seconds on paper, with an enlargement of many times the
original dimensions. The resources of the photographic art appear indeed
to be endless; but a mere statement of even the more interesting of
these would lead us beyond our limits, and descriptions of the details
of manipulation are out of our province altogether. But a few of the
more recent applications and developments of the art scarcely or not at
all alluded to in the foregoing pages should receive some attention.
The extraordinary sensitiveness of the gelatine-bromide film which makes
it possible to impress on it a photographic image in the merest fraction
of a second of time, enables us to take pictures of objects in rapid
motion. Express trains at their highest speed have been successfully
photographed, and so has almost every moving object in nature. The
photographs that have been taken of men, of birds, horses, and other
animals in every phase of their most rapid actions, have solved many
disputed and perplexing problems as to the nature of their movements,
and sometimes the solutions have been of a very unexpected kind. Taking
a photographic “shot” at a bird has become almost more than a figure of
speech; for there are contrivances by which a bird on the wing may be
aimed at with the lens, and hit off on the sensitive plate with a
certainty surpassing that of the fowling-piece. There are also
photographic repeaters by which six or more successive photographs of
the bird, etc., can be taken in a single second. Mr. Muybridge has
published a number of such photographs of the horse, and by projection
of the different images on a screen from a magic lantern, in rapid
succession, he has been able to reproduce the visual appearance of
horses trotting, leaping, galloping, etc., on the principle of the
zoetrope (page 399). Photography has afforded wonderfully delicate
observations in many departments of science, by recording phenomena too
rapid for the eye to seize, or too recondite for direct perception. A
few examples may be mentioned. First, the advantage of photographing the
lines of spectra, such as those described in our article on the
spectroscope, will at once suggest themselves, and accordingly this
method of recording spectra has been largely used, and in the hands of
Mr. Lockyer, Dr. Draper, and others has been successfully applied to the
study of the solar and stellar spectra. But more than this, it is the
sensitive photographic plate that has enabled us to explore the region
of the solar spectrum lying far beyond its visible limits in the red and
in the violet rays. The ultra-violet portion of the spectrum is shown
photographically to be occupied by multitudes of the thin insensitive
spaces—breaks in the continuity of the active rays—which are impressed
on the photographic print as black lines, similar in every respect to
the lines mapped out in the visible spectrum by Fraunhofer. It is known
by these that the ultra-violet spectrum, produced by glass prisms,
extends to a distance beyond the last visible rays of nearly double the
space occupied by the colour spectrum. The principal lines, or rather
the greater groups of lines in the invisible spectrum, are distinguished
by the capital letters of the alphabet, in continuation of Fraunhofer’s
method, beginning from H and nearly exhausting the letters of the
alphabet to designate them. These are photographed _in the dark_; for
all the solar beams that are allowed to enter the stereoscope are first
passed through blue glass of such a depth that every kind of emanation
capable of affecting the human eye is intercepted.
Another extremely interesting example of the application of the art to
scientific research is celestial photography. An image of the sun may be
impressed on a sensitive plate in an ordinary camera, in an amazingly
short space of time, but such image is much too small to show any of the
markings on the disc of our luminary, even when the image is magnified,
for its diameter is only about ⅒th of an inch for each 12 inches of the
focal length of the lens. In order to obtain an image of 4 inches
diameter, a lens of 40 feet focal length must therefore be used. The
first attempts in solar photography appear to have been made in France,
in 1845, and the solar prominences were daguerrotyped in 1851; but it
was not until 1860, that Mr. De La Rue succeeded in obtaining some
beautiful negatives of the phenomena presented in an eclipse of the sun,
and was thus enabled to determine a great astronomical problem, by
showing that the red flames, or prominences, really belonged to the sun
itself. At the present time, photographs of parts of the sun’s disc are
regularly taken at Kew, and other observatories, without the very long
and heavy telescopes, which introduced many mechanical difficulties into
the operation; for, by means of Foucault’s siderostat, the great lens
and the photographic apparatus can be used in one fixed position. The
siderostat is an instrument on which a flat mirror, made of glass worked
to a perfect plane and silvered externally, is caused by clockwork to
follow the motion of the sun, so that the reflected beams can be
projected in any required direction unchangeably, and, therefore the
image of the sun (or other heavenly bodies) viewed in the mirror, is
absolutely stationary. The lens, carried in a short tube, has its axis
directed to this image, just as it would be pointed at the luminary
itself. In solar photography, the exposure is made through a very narrow
slit in an opaque screen, which is caused to move rapidly in front of
the image. Very fair photographic images of the sun, of several inches
diameter, can, however, be obtained with an ordinary telescope of five
feet or so focal length, by substituting a small photographic lens and
camera in the eye-piece, and by enlarging the image in printing.
As early as 1840, Dr. Draper succeeded in daguerrotyping the moon, but
it was not until 1851, that lunar photographs, obtained by Professor
Bond, another American astronomer, were first exhibited in England. Many
other distinguished experimenters have since successfully turned their
attention to this subject, such as Dancer, of Manchester, Secchi,
Crookes, Huggins, Phillips, and De La Rue. The latter, and also Mr. Fry,
by photographing the moon, at different periods of her libration, have
obtained very beautiful and interesting _stereoscopic_ prints of our
satellite, in which she presents to the eye the roundness and solidity
of a cannon ball. Mr. Rutherford, in America, had an object glass of 11¼
inches diameter, made expressly with correction for the chemical rays,
and with this instrument he has produced some of the finest photographs
of the moon that have yet been taken. Reflecting telescopes, which have
the advantage of uniting all the rays in one focus, have been used with
excellent results, and it is said that some taken with the great
reflector at Melbourne, where also the atmospheric conditions are very
favourable, are almost perfect.
Excellent photographs of the planets have also been taken by Mr.
Common and others; but they are of course small, and have contributed
so far, much less to our astronomical knowledge than those already
mentioned. Very different are the results obtained in what, a short
time ago, appeared a less promising field. The image of a so-called
fixed star, in even the most powerful telescopes, presents itself as a
mere luminous point, and this is the case whether the star is one of
the brightest or one of the least conspicuous. The telescopic
appearance is simply a more or less brilliant point. The various
degrees of brightness which distinguish one star from another (_stella
enim a stellâ differt in claritate_), and which the unassisted eye
attributes to difference of size, led, long before the invention of
telescopes, to a classification of them accordingly. The brightest
stars are said to be of the 1st “magnitude,” those of the next
inferior degree of brilliancy, of the 2nd “magnitude,” and so on, down
to the 6th, which includes the faintest star discernible by an acute
eye under favourable circumstances. But stars too faint to be thus
seen came into view in the field of the telescope, and therefore those
of the 7th magnitude, and beyond, are termed _telescopic_ stars, and
each additional power given to the instrument brings others in view
that previously were invisible. The classification has been carried
down to the 18th or 20th magnitude, which expresses the limit of
visibility with the most powerful telescopes yet constructed. In the
methods hitherto employed for this classification, there is
necessarily much that is arbitrary and vague, and it is quite common
to find a different magnitude assigned to the same star by different
authorities. Now the photographic plate enables the astronomer to
determine the relative brightness of stars quite definitely. Everyone
knows that the time required to impress an image on the sensitive
plate is longer, as that image is less luminous. Hence, by finding the
time required for the images of different stars to be impressed, we
have a measure of their relative luminosities. Suppose the image of a
group of stars is allowed to act on a plate for, say, 5 seconds, we
should find only the brightest stars represented. If a second plate
have double the exposure given, it would be impressed by the images of
not only the brightest stars of the group, but also by those of the
next degree of brilliancy; and a third plate exposed for 20 seconds
would show more stars than the two former exposures. So that plate
after plate might be exposed under the same group for successively
longer and longer intervals indefinitely. Exposures extending over
hours have been made, notably by Mr. Common in England, and by Mr.
Gill at the Cape of Good Hope, showing not only how magnitude may be
determined to any extent, and the heavens most accurately mapped out,
but with this very remarkable result:—_thousands of stars, invisible
even in the most powerful telescopes, are portrayed in the
photographs_. Let us consider for a moment the significance of this
fact with regard to the new space-exploring powers it has placed in
the hands of science. The number of stars visible to the unassisted
eye in the whole expanse of the heavens has been variously estimated,
but the figures usually given lie between 3,000 and 4,000, and the
highest estimate for the most acute eyesight, under the most
favourable atmospheric conditions, places the limit at 5,000. The
brightest star in the heavens is Sirius, and Sir. J. Herschel
ascertained that its light is about 324, that of an average star of
the 6th magnitude. Taking the average luminosities of stars of the
first six magnitudes, Sir W. Herschel, from his own observations,
represents their relative brightness by the following figures: 100;
25; 12; 6; 2; 1. The different degrees of brightness seen is,
probably, due to the following three causes, combined in various
proportions: (1) the different sizes of these luminaries themselves;
(2) differences in their intrinsic luminosity; and, (3) differences in
their distances from us. And it is also extremely probable that the
last is generally by far the largest factor of the three. It has been
found by photometrical experiments, that the light we receive from the
sun is 20,000,000,000 (twenty thousand million) times more than that
of Sirius. If we suppose Sirius to be in reality only as large and as
bright as our sun, it follows that its distance from us must be no
less than 13,433,000,000,000 miles. The distance of stars of the 16th
magnitude has been estimated to be such that their light—travelling at
the rate of 185,000 miles per second—takes between five and six
thousand years to reach us. For a long time no sensible parallax could
be discovered in any of the fixed stars; that is, no change in their
positions was discernible when viewed from points 183,000,000 miles
apart, namely from the extremities of a diameter of the earth’s orbit.
In other words, if we suppose the line of the length just mentioned to
form the base of a triangle, having a star at its vertex, the angle
formed by the sides is so small that the most refined instruments
failed to measure it. In recent times, however, the parallax of a few
stars—about a dozen or so—has been detected and approximately
measured. The greatest observed parallax belongs to in α the
constellation of the Centaur, a star of the first magnitude, 30° from
the south pole of the heavens, and of this the parallax amounts to but
a little more than nine-tenths of a second of angular measurement,
corresponding with a distance of nearly 20,000,000,000,000 miles, a
space which takes light 3½ years to pass over. This star is,
therefore, believed to be the nearest of any to our system. The
smallest parallax that has been measured in any of these few stars is
a fraction of a second of angle corresponding with a distance twenty
times greater than the other, and requiring seventy years for light to
traverse it. Now, as the photographic plate shows us stars of
magnitudes indefinitely smaller even than the telescopic sixteenth, we
cannot but marvel at the manner in which the light travelling from
these suns in the immeasurable depths of space, and taking untold
thousands (nay, millions, it may be) of years in its journey is yet
able so to agitate the atoms of our silver compounds that images of
things that will themselves, probably, never be seen by mortal eyes
are presented to our view. A circumstance requiring explanation will
occur to the reader’s mind in connection with stellar photography; and
that is, how does it happen that, if the image of a star is a mere
point, it nevertheless impresses the plate as a visible dot? It is
probably because the point is a centre whence the photographic
influence radiates laterally on the plate to a small but yet sensible
distance.
Among the cosmic objects presented to our observation there are none
more fully charged with interest and instruction than the _Nebulæ_.
These are faintly luminous patches, in some few cases visible to the
naked eye, but for the most part telescopic. The milky way, which
extends round the celestial sphere, is a very conspicuous phenomenon of
the same kind. A few other hazy, cloudlike patches are seen in various
parts of the heavens, visible on a clear moonless night when the eye is
directed towards the proper quarter. The well known group of the
Pleiades sometimes presents this appearance, but most persons are able
by the unassisted vision to discern in it a group of six stars at least,
and an opera-glass or ordinary hand telescope easily resolves the object
into a cluster of 20 or 30 distinct stars. Telescopes of higher powers
bring more stars into view, and as many as 118 have been counted in the
group. There are several other groups of this kind perceptible to the
naked eyes merely as diffused patches of light, but resolvable by the
telescope into thickly clustered groups of minute stars; but in many of
the resolvable nebulæ the separate stars appear spread on a back-ground
of diffused luminosity. Again, there are other nebulæ which telescopes
of the highest powers we possess fail to resolve at all. Not only has
the photographic method shown stellar components of some of these last,
but it has depicted the form of nebulæ never seen at all, and whose
existence was previously unknown and unsuspected. For example, the
photograph has revealed the existence of a back-ground of nebulous
patches to the stars of the Pleiades—a thing that had never before been
suspected, although the group has been repeatedly observed by the most
powerful telescopes. Those who are at all acquainted with astronomy,
will understand the significance of this discovery for the science. The
results already obtained afford a marvellous support to the famous
speculation known as the nebular hypothesis. And as the forms of these
objects are accurately shown for us by their own light, changes in their
appearance may thus be detected as time goes on which may serve to lift
the above named theory into the region of demonstrated truth. The nebulæ
which neither telescope nor camera can resolve are such as the
spectroscope proves to be masses of glowing gas or vapour.
It has been already mentioned that the light from these immeasurably
distant stars and nebulæ is so faint that the most sensitive
photographic plates have to be exposed for hours. This would be a matter
of no difficulty if the clockwork mechanism by which the apparatus is
made to follow the apparent motion of the heavens could be constructed
with absolute perfection. But as this is not obtainable, even with the
most careful workmanship, and the smallest jar or irregularity would
distort and confuse the images, this source of disturbance is eliminated
in the following manner: attached to the photographing apparatus and
driven with it is a telescope, provided with cross wires, and through
this an observer views some star during the whole period of the
exposure, his business being to keep the image of the star accurately on
the cross wire, which he is enabled to do by having the means of
slightly modifying the movement of the clockwork. In the Paris
Exhibition of 1889 were shown many very fine large photographic prints
of nebulæ (notably of great nebula in Orion), which have recently been
obtained in this manner, and those nebulæ that had been photographically
resolved had the stellar components marked with wonderful distinctness.
Comets and meteorites have been photographed, and even the _aurora
borealis_ and the lightning’s path have been brought within the camera’s
ken.
Space would fail us to describe the many applications now found for
photography in microscopy, in medicine and surgery, in anthropology, in
commerce, and in the arts. It is obvious also from the improvements that
are continually made, that many of these applications have not yet
received their full developments. Photography has been enlisted into the
service of the army and navy, and regular courses of instruction in the
art are given in their training schools. A well equipped photographic
waggon now accompanies every army corps, and in almost every ship of
war, some proficient operator is to be found. By an ingenious
combination of photography, aerostatics and electricity, it is possible
to obtain with perfect safety accurate information of the disposition of
an enemy’s forces and fortifications. A small captive balloon is sent
up, to which is attached a camera. At a height of a few hundred yards,
the balloon is practically safe from any projectiles, and in its cable
are interwoven two electric wires by which currents are conveyed to
electro-magnets, which produce all the movements required for any number
of exposures. Jurisprudence has found its account in recognizing the
art, for the photograph is received in evidence for proving identity,
etc. The administration of the criminal law takes advantage of the art
to secure the likeness of prisoners for future identification, and the
modern instantaneous process renders unnecessary the subjects’
concurrence with the operation. Again, if the “hue and cry” has to be
raised for an individual “wanted” for any offence, and a photographic
likeness of him is procurable, thousands of copies can be made of it in
a few hours, by night as easily as by day, and distributed to every
police station in the whole country.
Modern processes now enable us to obtain prints from negatives in as
many seconds as a few years ago hours were required, and this by
artificial light. A process of printing lately introduced and yielding
artistic results which deserve to find more general favour, is that
called the _platinotype_. Instead of the ordinary print produced on
lightly glazed paper by the reduction of silver compounds, and of
questionable permanency, the image is formed in the paper by metallic
platinum, the most changeless of all possible substances under ordinary
influences. The pictures are of a rich velvety black, with soft
gradations, and the surface is without glaze or glare. The print has, in
fact, the appearance and all the best qualities of the most highly
finished mezzotint engraving, combined with the minute fidelity
characteristic of the photograph. The problem of producing a photograph
in colours, permanently showing nature’s tints in all their gradations,
has still a great fascination for some experimenters, and startling
announcements are made from time to time of some discovery in this
direction. It does not appear, however, that any success has really been
arrived at, beyond the results long ago obtained by Becquerel as
described on page 614; and, indeed, as our knowledge of the science of
the subject increases, the less likely does the possibility of
photographing colours appear. It is, however, never safe to lay down the
limits of discovery in science.[14] Note that precisely in the matter of
rendering colour even in its due gradation of tone or luminous
intensity, the photograph is quite untruthful. Everybody has noticed how
unnaturally dark and heavy the foliage of trees appears in the prints;
if we suppose a lady in a blue dress, with yellow trimmings, to sit for
her portrait, the photograph will show her in a white dress with black
trimmings; a sitter with light yellow or auburn hair will appear of
quite a dark complexion; if you photograph a lemon and a plum together,
the latter will probably come out lighter than the former; or if a
daffodil be the subject, the flower will be drawn in tones much darker
than the leaves. This incorrectness of tone relations can, however, be
greatly lessened by the device of reducing the quantity of the blue
rays, by interposing a piece of optically plane yellow-tinted glass, by
using the sensitive plates tinted with certain coal-tar dyes, which are
now prepared and sold under the name of “ortho-chromatic plates,” or by
both methods combined.
Footnote 14:
See page 630.
If any illustration were needed of the great popularity now attained by
the practice of photography, reference might be made to the large number
of periodicals devoted to the subject, and appearing weekly,
fortnightly, quarterly or annually, in every civilised country, and also
to the multitudes of societies that have been formed for the promotion
of the art. In Great Britain alone there are now at least 150 such
societies in active operation, and they are correspondingly numerous
elsewhere. If, when we consider all that has been accomplished up to the
present time, with the jubilee year of photography scarcely passed, and
observe the increasing numbers of its cultivators guided by the
explanations of its phenomena that science is beginning to furnish, we
can expect a corresponding progress in the next fifty years, then the
centenary may be reached with a roll of achievements that could we know
them now we should think marvellous.
As already remarked elsewhere, the practical side of photography has
outstripped the theoretical one, for so far its progress has been much
less indebted for processes and technic to the direct guidance of
science than almost any other of our Nineteenth Century acquisitions,
such as telegraphy, electric lighting, etc. The materials employed, and
the mode of manipulation, have certainly _not_ been deduced from
previous knowledge of the nature of light or from the laws of chemistry,
although when, by repeated trials and happy guesses, the right direction
had been found, the field into which it led could be more easily
explored under the direction of chemistry and physics. But even yet the
fundamental principle, or the precise nature of the action of light on
certain compounds, has not been definitely made out, and although some
theories on the subject have been proposed, no one has been generally
accepted as an adequate explanation of the known facts, and still less
have any quantitative relations been established for these actions. The
photographer cannot compose a formula for the composition of his
emulsions and developers from assured data like those that enable the
chemist to weigh out with accuracy the constituents that go to produce a
required compound.
The attainment of permanency in its products, which, by several
processes, photography can now boast of, is one of its triumphs, and
will tend greatly to enlarge the sphere of its utility. For example, we
have a public institution, known as the National Portrait Gallery, in
which it is sought to gather together and preserve the likenesses of the
most eminent Englishmen, and presentments of such of far less fidelity
than photographic portraits are eagerly sought after. It has been
suggested that something like a National Gallery of _permanent
photographic portraits_ of the chief men of their time would be a
fitting and acceptable legacy to the public of the future. This idea has
much to recommend it, particularly as authentic likenesses would thus be
secured for the nation beyond the chance of loss.
Photography has been applied in preparing blocks in relief for printing
along with letterpress in the same way as woodcut blocks. The process
has the great advantage of producing in a wonderfully short time a
perfect facsimile of the artist’s drawing without the intervention of
any engraver. A plate of zinc, brass, or copper, coated with a dried
film of bichromated albumen, is exposed to light under the transparent
negative of a drawing in pure line, that is, one having in it only lines
of uniform colour throughout. The parts of the film reached by the
light, which correspond with the lines of the original design, are
rendered insoluble, while the rest can readily be removed by water.
These unprotected parts have then to be removed by the action of acids,
but these are used alternately with the application to the plate of
certain compositions, the purpose of which is to prevent lateral erosion
of the lines in relief before the requisite depth of the metal has been
removed. Fig. 147_f_ is the reproduction of a pen-and-ink sketch by this
or some similar process. But nature and the ordinary photograph show us
graduated tones which ordinary printers’ ink cannot really reproduce,
inasmuch as it is incapable of gradation, and can give the _effect_ of
gradation only by such devices as are mentioned on page 642 (last
sentence). Now, the photograph cannot yield a printing-block until its
continuous tones are broken up into lines or dots. Not a few methods of
doing this have been contrived, but that which is by far the most
commonly used, and is most successfully practised on the commercial
scale, is simple in principle, although in actual working it calls for
much experience and skill. The negative is taken upon a wet collodion
plate, in front of which, within the camera, and at a very short
distance (say 1/30th inch) from the film, is a transparent _screen_,
bearing two sets of parallel opaque lines at right angles to each other.
These lines are mechanically ruled with the utmost regularity, and are
separated by only very small intervals. There may be from 80 to 200 of
them in the space of one inch, according to the class of work required.
The effect of this is that the light reaches the photographic film
through a series of minute transparent squares, the sides of which may
be only from the 1/140th to the 1/400th of an inch in length. Now it is
found that the brighter lights from the original positive, after passing
these small apertures, spread so as to more or less cover the opposite
parts of the negative, while the feebler lights, from the shades of the
original, impress the plate to a less degree, the developed image in
these showing, perhaps, merely a small dot or, in the very darkest
parts, a blank. In this way, then, may the photographic negative be
obtained with a granulated texture following in graduation the tones of
the original. After this, the rest is easy, for the process of exposing
a metal plate, coated with a sensitive film under the negative, and of
etching it with acids, etc., is essentially the same as in the
foregoing. Such is the _half-tone process_, which is now so largely
superseding wood and other engraving. It is unnecessary to describe
technical details here, such as the employment of _bitumen of Judæa_ as
the coating for the metal plate, or how the image must be reflected into
the lens from a mirror to avoid a reversal in the final print, etc.
There are endless modifications of the processes briefly mentioned
above, and some of these are guarded as valuable trade secrets. Several
of the illustrations in this work are prepared by the half-tone process,
of which plates I., IV., V., etc., are examples, and they should be
examined with a strong lens, in order that the different rendering of
the light and the dark parts may be compared.
_PHOTOGRAPHY IN COLOURS._
It is the statement as to the futility of assigning limits to scientific
discovery that has been justified by facts. The preceding edition of
this work was not long in the hands of its readers before the solution
of the problem of photography in colours was announced from Paris,
where, at the close of 1890, the physicist M. Lippmann had succeeded in
photographing the solar spectrum in its natural colours, and at the
beginning of 1891, he was able to exhibit at the Academy of Science
untouched photographs of a stained glass window in three colours, of a
dish of oranges and red flowers, and of a gorgeously coloured parrot,
all in their natural tints. The method employed had no apparent relation
to that of Becquerel, but was of the simplest, and, moreover, one which
any reader who has followed the first few pages of our section on the
“Causes of Light and Colours” will have little difficulty in completely
understanding, if he has devoted a little attention to Fresnel’s
interference experiment. M. Lippmann took a photographic plate, coated
to a greater depth than usual with a gelatine film containing the
sensitive salts of silver, and in the camera this plate was exposed with
the glass towards the lens, while at the other side of the film was a
metallic reflecting surface, namely, quicksilver. Supposing a ray of red
light to enter the glass and traverse the film, it would be reflected
from the metallic surface, and would meet the direct ray within the
substance of the film, with a difference of length of path that would
produce the interferences already described, and so give rise to
alternate lines or bands of darkness and brightness. It would, of
course, be in the lines of maximum brightness that the silver would be
first deposited by the photographic action, and these microscopically
fine lines or striæ of silver would give back, from ordinary light, a
colour corresponding to the waves of red light that produced them.
Similarly with the other colours. Anyone may observe the production of
colour from ordinary white light in the iridescent tints of
mother-of-pearl, where the effects are due to the varying distances of
fine edges of the layers of the substance. If an impression is taken
from a piece of mother-of-pearl by solid paraffin, or by white wax, or
even by common red sealing-wax, the colours will seem to be adhering to
the impression, but the operation may be repeated times without number.
It is the distance apart of the lines or striæ that determinates the
colour, and this is always some definite multiple of the wave lengths,
given on p. 411, for the various colours. M. Lippmann’s products are
true colour photographs, and they form a new and elegant experimental
demonstration of the doctrine of luminiferous undulations.
The colour effects of nature have also been reproduced by taking
photographs of the same scene through coloured glass. Thus a screen of
yellow glass will intercept the blue and the red rays, and the sensitive
film will be impressed with images of objects containing yellow rays
only, and that in proportion to the quantity of these rays that enter
into any given tint. Similarly with images taken through red and blue
glasses. The positives from these partial images being projected by
three optical lanterns on the same space on a screen, and each being
coloured by passing through tinted glasses like the original, the
superposed images thus combined give a very lively impression of the
natural colours in all their gradations.
Among the many processes for reproducing photographs by non-photographic
processes, some have been more or less successfully combined with colour
printing. Some of these productions are very effective, and are more
attractive to many persons than the monochromatic tints of ordinary
photographs.
[Illustration:
FIG. 312.—_Portrait of Aloysius Senefelder._
]
PRINTING PROCESSES.
As it is beyond contradiction that printing is one of those inventions
which have most influenced the progress of mankind, so it will be
admitted that certain modern processes, by greatly facilitating the
operations, and vastly extending the resources, of the art, possess an
interest and importance surpassed by few of the subjects we have
discussed. In a former article the reader has been made acquainted with
the steam printing-press and other applications of machinery by which
the impressions of a form of type, or of a pattern, can be rapidly
multiplied. Here we have to describe some ingenious methods of preparing
the forms or originals for letterpress and other printing, and certain
beautiful processes for multiplying drawings, engravings, and pictures.
_STEREOTYPING._
This term is applied to the process of obtaining the impression of a
form of movable types, or of a woodcut, on a plate of metal which can be
printed from. These plates, after the required number of copies have
been printed, can be stored away; and they are ready for use whenever
another issue of the work is required. When the pages that are to be
stereotyped have been set up in ordinary type, there are several methods
by which the stereotype plates may be obtained from them; or rather,
there are several materials used to form the matrix or mould in which
the metal is cast. When plaster of Paris is used, the form is first
slightly oiled, to prevent adhesion of the plaster; a thin mixture of
plaster and water is then poured upon the form, which is surrounded by a
raised rim, to retain the plaster. The thin plaster is carefully led
into all the recesses of the type, and then some thicker material is
poured on. The plaster soon sets, and is lifted off the type, and, after
drying, is ready to receive the molten metal of which the stereotype
plate is formed. This metal is an easily fusible alloy of lead,
antimony, and other metals, which takes the form of the mould with great
accuracy, and is, when solid, sufficiently hard to print from.
[Illustration:
FIG. 313.—_Press for Stereotyping by Clay Process._
]
Another plan is to make use of prepared clay, spread upon an iron plate,
for the formation of the mould. The face of the type is brushed with
benzine, the plate with the clay is laid upon it, and pressure is
applied. The whole is then dried in a slow oven, and the clay, when
detached from the type, is ready to form the mould. The advantages of
the clay process are that the type does not require to be afterwards
cleaned from oil, and that the material does not fill up the deeper
spaces of the form, so that a thinner stratum of metal suffices to form
the stereotype plate.
A third mode of obtaining the mould has been already mentioned in
connection with the Walter Printing Press (page 313), in the working of
which the _papier maché_ process is ingeniously made to supply the
curved stereotype plates for the cylinders. This process is also largely
used for other newspaper presses, and sometimes for bookwork, as it
forms an invaluable means of expeditiously obtaining a number of
stereotype plates from the movable types. This production of a number of
similar forms makes it possible to strike off a very large number of
copies in a short time, for many presses can be employed simultaneously.
For the paper process a number of sheets of tissue-paper are pasted
together, and the moist paper is laid upon the form; then the operator,
by light strokes of a brush, beats down the paper into the hollows of
the type, beginning at the centre of the page, and going towards the
margins. A sheet of stout unsized paper, called “plate paper,”
constitutes the upper layer; and when the whole has been well beaten
down upon the type, pressure is applied by means of a screw acting upon
a plate of iron covering the whole. In this condition a gentle heat,
produced by steam, is made to completely dry and harden the paper
matrix, which is very soon fit to be used for casting the metal. The
apparatus for this purpose consists of a hollow iron table, within which
steam is made to circulate. On this the form is placed, and the platen
is pressed down upon it by means of a screw. In many cases the platen
also is heated by steam, to accelerate still further the drying of the
matrix, which is effected in about four or five minutes. One paper
matrix, by careful use, will serve for the production of a series of
casts without receiving any damage from the molten metal, as this is
fusible at a low temperature.
The mould for casting flat stereotype plates from the paper matrix is
made of iron, and has parallel surfaces, which admit of being so
adjusted that the thickness required in the plates may be obtained very
nearly. The paper matrix is laid on the horizontal iron bed of the
mould; gauge-bars are adjusted, which retain it in its position; and
then the second plate is folded down—the distance between that and the
paper being determined by the gauge-bars. The cover is secured by
clamping-screws, and then the mould is turned upright to receive the
metal, which is removed, when solid, after the mould has been turned
back into its horizontal position.
However the stereotype plates have been produced, it is necessary
accurately to adjust their thickness by planing off some of the material
from the back. The edges have also to be cut and trimmed to the exact
dimensions required by the press. Various machines have been devised for
effecting all these operations with accuracy and dispatch. The plates
are afterwards mounted on wooden or metal blocks to bring them to the
height of ordinary type.
A fourth method of producing plates for the same purpose as the
stereotype plates already described is by _electrotyping_. This method
appears to have been introduced as early as 1840, but the first results
were not without imperfections. Now, however, this plan is almost
universally applied to bookwork and woodcut illustrations. Many of our
popular illustrated periodicals have so large a circulation that the
wooden blocks would necessarily be spoiled by being used in steam
presses long before they had yielded the required number of impressions;
and the method has also the great advantage of securing the original
engraving from the chance of accidental damage, by which a block is
sometimes irretrievably injured. Hence woodcut illustrations are now
always printed from electrotype copies of the engraved blocks, whether
the work itself be printed from movable type or not. But the electrotype
or stereotype process is always resorted to in the case of a work,
whether illustrated or not, when it is foreseen that a re-issue will be
demanded. These processes are also of great advantage to the practical
printer, because when the pages set up in type have received their final
corrections, he can take the casts, and then the type may be
distributed—that is, returned to the cases ready for the compositors to
use for other work.
The electrotype process is almost as simple as those for producing
stereotype plates by casting, and its productions excel these by their
great durability and extreme exactness of reproduction. We may take it
for granted that the reader is familiar with the fact that ordinary
letterpress characters and woodcuts are printed from forms, in which the
black portions are in _relief_. For woodcuts the artist makes the
drawing, in reversed position, on a block of finely-grained boxwood, in
which the fibres of the wood are perpendicular to the surface. The
engraver hollows out all the parts which in the impression remain white,
while all the parts which are to receive the ink and produce the black
parts of the impression must be left at the original level. The wooden
blocks thus engraved would serve to produce a certain number of
impressions, which could be taken off by careful hand-printing without
perceptible damage to the block. But the pressure necessary for printing
inevitably crushes the projecting parts of the block; and the
impressions, after a certain number, lose their sharpness. This is
especially the case in machine printing; but not only does the
electrotype cast present a surface capable of bearing hard usage much
better than those of the hardest wood, but even if the number of
impressions required should wear out the metal plate, it can easily be
replaced by another cast from the original block.
The mould which serves to give the electrotype cast may be made either
of gutta-percha softened by a gentle heat and applied to the wood, or of
wax. In either case a powerful pressure is applied, in order to force
the yielding substance to take the forms of the engraved block or of the
metal type. Wax is now generally preferred; the yellow wax used for this
purpose is melted, and poured into a shallow pan; when it has become
solid, it is sprinkled over with finely-powdered pure blacklead, which
is brushed over the surface, and then the excess is removed by blowing
with bellows made for the purpose. Thus prepared, the wax is placed over
the type-form or wooden block in a powerful press, sometimes worked by
hydraulic power; but more frequently a _toggle_ press is employed, in
which the pressure is given by a screw and crank-wheel acting on two
_elbow joints_, or _toggles_. For the information of non-mechanical
readers it may be stated that a “toggle” consists of two bars jointed
together, and placed _nearly_ in a straight line: when a pressure is
applied to the joint, tending to bring the rods still more nearly into a
straight line, their extremities are thrust apart with a great force,
which increases indefinitely as the rectilinear position is approached.
In the electrotyper’s press there are two toggles constructed of very
broad bars, or rather thick plates, for they have nearly the width of
the bed of the press. With this machine a very powerful and regular
pressure is applied; and the wax in a few minutes takes a sharp
impression, embracing all the most delicate details of the work, and
becomes at the same time very hard. The impression, of course, has
hollows corresponding to the projections of the wooden block or
type-form, and _vice versâ_. The face of the wax mould is now very
carefully and completely blackleaded, a soft brush being used in the
process. It is then placed in the solution of sulphate of copper, and
the blacklead receives a deposit of copper, in the manner explained in a
former page (498). In about forty or fifty hours a firm, compact
deposit, about as thick as the finger-nail, covers the blackleaded
surface, forming a perfect reproduction of even the most minute details
of the engraved block or letterpress form.
The next operation has for its object the removal of the thin shell of
copper from the wax. This is effected by exposing the mould to a gentle
heat by immersing it in hot water, or by placing it on a hollow iron
table which is heated by steam. The wax is run off into a proper
receptacle for future use, and any portion adhering to the copper is
removed by the action of naphtha or of a solution of potash. The thin
copper shell is then tinned on the back, and an alloy of lead with some
tin and antimony, forming the _backing metal_, is poured on it, to the
depth of about one-eighth of an inch. When this has become solid the
backing is planed, so that the compound plate may have a certain regular
thickness, and that the back surface may be parallel to the face. The
edges are cut by a circular saw and trimmed by machine-tools, and the
plate is rendered perfectly even, and adjusted with the greatest
possible exactness to the required thickness. It is prepared for the
press by being screwed down upon a block of wood of a certain thickness,
so that the face of the plate may have the same height as common type,
the screws passing through the margin or other hollow parts of the face
of the cast. No more enduring surface than the copper of these
electrotype casts, backed up by the hard alloy, has yet been discovered.
_LITHOGRAPHY._
To Aloysius Senefelder, a musician attached to one of the theatres in
Munich, whose portrait appears at the head of this article, is due the
invention of the art of lithography. It is said he used to arrange his
musical compositions on a kind of slates, formed of flakes of the
limestone which is found in the neighbourhood of Munich. One day a
memorandum which he had made in this manner happened to fall into a
slop-bucket full of greasy water; on withdrawing the piece of stone, he
noticed with surprise that the grease had attached itself to the
characters, while the rest of the stone remained quite clean. Such an
incident might have happened to each one of a thousand men, and its
significance might not be perceived; but it suggested great
possibilities to Senefelder, who, applying himself for some years with
ingenuity and perseverance to experiments with the Munich limestone,
became, in the year 1800, the inventor of a new art. Though he was no
chemist, and was unskilled in mechanics and in drawing, yet within four
years from his first observation he had succeeded in finding the proper
materials for his crayons and the appropriate acids for acting on the
stone, in contriving a suitable press for taking the impressions, and in
producing samples of lithographic work in various styles of art. He
endeavoured to keep his processes secret, and having obtained the
exclusive right of exercising his invention in his own country, he
attempted to carry on all the operations himself. Little by little,
however, the general nature of the process became known, and although
the details were jealously concealed, ingenious persons in France and
elsewhere, by force of experiment, succeeded in re-inventing the art for
themselves, and Senefelder never profited by his invention as he should
have done.
The first lithographic press in London was established by Mr. Hullmandel
in 1810. The value of lithography as a means of multiplying works of art
was soon afterwards proved by the publication of a magnificent series of
picturesque delineations of the quaint architecture of the old towns of
Flanders and Germany, drawn on the stone by Samuel Prout. The late Mr.
J. D. Harding largely contributed to the popularity of lithography by
the landscapes which he drew on the stone, and thus placed in the hands
of every one, prints in which all the freedom and force of the artist’s
work were secured. The French designers excel in fine-art lithography,
and many beautiful productions of their crayons have been published in
every department of pictorial illustration.
The best lithographic stones come from Germany; but for some kinds of
work stones from other localities are used, on account of their less
cost. Thus, in England, a stone yielded by the white lias formation near
Bath has been found to possess the requisite qualities. The stones for
lithography are prepared in much the same way as slabs of marble are
polished; that is to say, by rubbing one slab against another with sand
and water. When the stones have thus been brought to a plane surface,
they are finished according to the purpose for which they are intended.
If they are intended to receive written characters, they are polished to
a very smooth surface by means of pumice-stone. But if they are to take
drawings, then a certain uniform grain is given by means of
finely-sifted sand, the operation being performed in a similar manner to
that in which the stones are dressed, only pressure is not applied to
the upper stone. The stones, after being washed and dried, are carefully
covered on their prepared surfaces with thin paper, and are sent out for
use.
When the stone is employed to reproduce written characters, or drawings
imitating those done with a pen, _lithographic ink_ is made use of with
an ordinary pen, a ruling-pen, a fine brush, or a pen which the
lithographer makes for the occasion out of thin metallic plates. The
composition of the ink varies much: the usual ingredients are wax,
gum-mastic, gum-lac, soap, and lampblack. This composition forms a
solid, which is rubbed down with water to a thick liquid when required
for use. The characters have, of course, to be written on the stone in a
reversed position, and the lithographer acquires the habit of doing this
with neatness and dexterity. He is provided with a looking-glass for
viewing his work, in order to see the effect which will be given by the
impression, for the looking-glass shows the characters in their usual
position, just as the image of ordinary writing seen in it is reversed,
showing, in fact, the very appearance the characters present on the
stone. For a drawing, a _lithographic crayon_ is used, made of wax,
soap, grease, lampblack, and other ingredients. With this the drawing is
made on the stone exactly as on paper, save the necessary reversals.
When the design has been placed on the stone, a liquid containing nitric
acid and gum is poured over it. This liquid acts on all the parts of the
stone not protected by the ink or crayon: they are thus rendered
incapable of receiving printing-ink, while the protected parts have the
impression more strongly fixed; for when the stone has been well washed
with water, and turpentine has afterwards been applied, so that all the
matter used in marking the design is dissolved away, the seemingly
obliterated characters reappear when—after the stone has been lightly
wiped with a damp sponge—the roller charged with printer’s ink is
applied. The ink is taken up by the stone only at those places which
have not been acted on by the acid. The impression is obtained by laying
a sheet of damp paper on the inked stone and applying pressure by means
of a roller, under which the stone passes. The stone is moistened with
water after each impression before the inking-roller is again applied.
The lithographic stone, like other originals used in printing, is liable
to deteriorate when large numbers of impressions are taken from it. This
would be a serious drawback in lithography, but for a method of renewing
the impression, which renders it unnecessary for the artist to retouch
his work. This is the process of _transferring_, which is practised by
the aid of a certain kind of paper specially prepared by a coating of
paste. On this a proof is taken from the original drawing on the stone,
and the still moist sheet is then applied to another stone, with the
face downwards, and passed under the press. The effect of the pressure
is to cause the adherence of the layer of paste to the stone; and when
the paper has been thoroughly wetted at the back, it may be removed,
leaving the paste still adhering to the stone, with the impression
beneath it. When water is applied, the paste is washed off, while the
ink of the impression remains attached to the stone, there reproducing
the design drawn on the first stone. The transferred design is treated
in exactly the same manner as the original drawing, acid being poured
over the stone, &c., and the impressions obtained by the same method of
successively sponging, inking, and pressing. The transferred drawing may
be made to yield another transfer, and so on indefinitely; but when a
large number of impressions from one design are required, it is usual to
make at once from the original as many transfers to separate stones as
will yield the required number of impressions without deterioration. In
this way as many as 70,000 copies have been taken from a single drawing
without their showing any marked difference in the character of the
impressions.
The transfer process is also applied to place on the stone characters
which have been written with a pen in the ordinary manner on prepared
paper. In this way a person’s handwriting is so accurately reproduced in
the impressions that it is often very difficult to detect the
interposition of the lithographic stone, and the impression often passes
as the immediate production of the writer’s pen. It is obvious that
drawings etched with the pen on transfer-paper can be printed from in
the same manner. And line engravings, which have been originally
produced by cutting hollow lines on polished plates of copper, can be
printed lithographically by transferring an impression to the stone. By
transfer also the impressions of raised types or of woodcuts can be
printed from the stone when desirable.
A beautiful and important application of lithography to the reproduction
of pictures in colours has been so successfully carried out that a new
branch of the art, termed _chromo-lithography_, now gives facsimiles of
water-colour drawings and of paintings in oil. The copies of
water-colour drawings especially are remarkable for their artistic
qualities, and it is undeniable that these cheap reproductions of good
paintings have done much to extend the knowledge of art. It is not
contended that a chromo-lithograph, for example, after one of old
William Hunt’s rustic figures, or birds’ nests with banks of primroses,
can possess the wonderful refinement of the original; but it will
nevertheless convey much of the artist’s sentiment. Such transcripts of
the works of our best artists adorn the homes of thousands who have
never perhaps had the opportunity of even seeing the painter’s original
handiwork. In many a remote settlement in distant colonies, as in many
an English home, the chromo-lithograph is the brightest of the household
art treasures.
The principle of chromo-lithography consists in printing on the same
paper with inks of various colours from different stones successively,
so as to produce, by the juxtaposition and superposition of the various
tints, the effect of a coloured drawing or painting. The artistic
effects of the best chromo-lithographs require a great number of
printings for their production, in some cases as many as twenty
different stones being employed. The stones and colours for such
productions require true artists to prepare them, persons who can
thoroughly understand and enter into the spirit of the original work.
The first operation consists in the preparation of a faithful but
spirited outline of the original, etched on transfer-paper, from which
the outline is placed on a lithographic stone. This sketch we have
called an outline, but it is in reality something more; for it should
suggest all the markings and limits of tints which belong to the
original. This first sketch has some points marked on the margin by dots
or crosses, which serve to secure true register in the subsequent
processes; that is, the impressions of the successive tints are so
placed on the press that these points coincide in each impression.
From the first stone as many impressions of the sketch are transferred
in light ink to other stones as there are colours required in the
reproduction. To each colour a special stone is assigned, on which the
lithographer, guided by the slight impression of the sketch, draws with
the ordinary black crayon the form which that colour is to produce on
the paper. Much artistic skill and judgment are required to do this in
such a manner as to obtain a clear and harmonious final result. The
gradations of the colours, and their blendings by superposition, must be
carefully regarded. When the form and limits of each colour have been
skilfully laid down upon its own stone, the surface is acted on by the
acid, it is washed, the ink is dissolved off by turpentine, the stone is
sponged, and the roller charged with ink of the appropriate tint is
passed over it. The ink, as before, adheres only to the parts over which
the crayon has passed, and an impression may be drawn off. Each of the
other stones is similarly treated, and when the whole are ready, a proof
is taken by giving the same sheet of paper the whole series of
impressions in their proper order and colours, with the greatest
possible accuracy of register. If any alterations appear desirable, they
are made accordingly, by aid of certain devices which need not be here
described, and when a satisfactory result has been obtained, the
printing of the whole series of impressions is proceeded with. When the
number of these is very large, transfers of each stone are taken as in
ordinary lithography, only with certain extra precautions for obtaining
precision in the register.
The brilliant effects produced by using gold and silver in lithography
are obtained by using a kind of varnish, instead of coloured ink, for
printing those parts where the metal is to appear. When this varnish has
acquired a certain stickiness by partial drying, powdered gold or silver
is applied, and this attaches itself only to the varnish; when the sheet
is dry it is passed under a burnished steel roller, the pressure of
which imparts a brilliant lustre to the metal.
A method of colour-printing, in some respects resembling that of
chromo-lithography, is practised by printing in variously coloured inks
from a series of wooden blocks. This admits of far greater expedition in
working off the impressions than the process with stones. The gradations
of the coloured inks and powdered tints are produced in the same manner
as those of ordinary woodcuts in black and white; and when the colours
are well chosen, and care is taken to secure the accurate superposition
of the impressions, very pleasing effects can be produced by this means.
The coloured prints which are from time to time issued as supplements to
the “Illustrated London News” are produced by this process, and are no
doubt well known to the reader. Our plate of spectra, No. XVII., is an
example of another method of printing in colours.
_OTHER PROCESSES._
In recent times a great number of printing processes have been devised,
but only a few have found their way into practical use, and some of
these have scarcely been so extensively applied as their merits appear
to deserve: either because the public demand has been insufficient to
bring these inventions into common use, or the cost of working them has
been too great. There is no doubt of their scientific success, whatever
may be their commercial value as competing with cheaper and readier
methods. We shall first describe the plan which has been termed _Nature
Printing_.
This process is applicable only to certain objects which possess, or may
be made to assume, a flat form. It has been most successfully applied to
botanical specimens, the impressions of the leaves, flowers, and other
parts of plants being given with an accuracy and minuteness of detail
which the finest work of an engraver could never attain. In fact, the
prints may be examined with a microscope, and they then reveal the
minute structure of the object with wonderful clearness and delicacy.
The notion of nature printing originated with M. Auer, the
Superintendent of the Imperial Printing Office at Vienna; but the
process was introduced into England, with certain improvements, by Mr.
H. Bradbury. Supposing the object to be printed is a plant or the frond
of a fern, it is first thoroughly dried by being pressed between folds
of blotting-paper by means of a screw-press. The paper is changed
several times, and, when necessary, the drying is accelerated by a
gentle heat. When the specimen is perfectly dry, it requires very
careful handling, for it is then generally extremely brittle. It is laid
upon a sheet of pure soft lead, the face of which has been formed into a
perfectly even surface, smooth and bright as a mirror. Mr. Bradbury
encountered some difficulties in attempting to produce a surface of this
kind, for small irregularities of the lead surface showed themselves;
but Mr. James Wood succeeded in preparing for him a machine by which the
lead is planed and polished in one operation. The object having been
carefully laid upon the bright and smooth surface of the lead, a
powerful pressure is applied by passing the plate between a pair of
polished steel rollers. The effect of this is to embed the plant in the
soft metal, which thus receives even the most delicate markings of the
object. The next operation is the careful and patient removal of the
object from the plate; and as this is very brittle, it will be easily
understood that it does not in general come away entirely, but portions
will be left embedded in the metal. The skill of the operator is shown
by destroying these by means of a blowpipe-flame, without in the least
fusing the lead, which would of course ruin the impression.
When the whole has been removed, the leaden plate will have been
engraved, as it were, by the object itself; and in this state the plate
will yield impressions with ink in the same manner as an engraved copper
plate. But in the soft metal the image would soon be obliterated, and
therefore a facsimile of its impression is obtained in copper by the
electrotype process. For this end the lead is covered with a varnish,
except on the face, and thus the deposit of copper takes place only
where it is required, and the current of electricity is continued until
a proper thickness of deposit has been obtained. This electrotype has
all the hollow forms of the lead plate in relief, and it is used only
for the preparation of another electrotype. For this purpose its face is
brushed over with fine, pure blacklead, in order to prevent the deposit
from becoming incorporated with it, while the rest of the plate is
varnished. When it is placed in the electrotyping solution the copper is
deposited on the blackleaded face, and the action is continued until the
layer of metal has acquired the thickness of one-eighth of an inch. It
is then removed from the matrix, and is ready for the printer, who deals
with it in the ordinary manner of copper-plate printing, except that he
uses a softer paper, and this is forced by the pressure into the
depressions in the plate, so that the impression is really embossed on
the paper. Coloured inks are also used instead of black; for instance,
to the leaves green-coloured ink is applied, and to the stems, &c.,
brown ink.
Several works on certain branches of natural history have been very
appropriately illustrated in this way; among these, perhaps, no more
beautiful example is to be found than in “The Ferns of Great Britain and
Ireland,” with text by Lindley and Moore. The merits of the
nature-printing process appear to be the accuracy of outline in the flat
form, and the delicacy of detail in parts projecting from the surface.
The impressions cannot present artistic or natural shading in the
objects; for the depth of colour will be in proportion to the projection
of the part, whereas in nature the darkest shades are seen in the
deepest recesses.
A copper plate, cut in the ordinary manner—as a line engraving, for
example—soon deteriorates, as the pressure applied for each impression
taken from it tends to close up the lines. It has therefore been
necessary, where a plate has to yield a large number of impressions, to
make use of steel instead of copper. But the electrotype has given the
means of multiplying indefinitely facsimiles of engraved copper plates,
so that in many cases a number of these are prepared, and used so long
as they continue to yield clear impressions, the original plates
engraved by the artist only furnishing the matrix. The mode of
reproducing the plates by electrotyping from the original engraved
plates is identical with that just described for obtaining the plates
for nature printing from the leaden plates.
Another process of wider interest, and producing very beautiful results,
is known as the Woodbury printing process, from the name of its
inventor. It is a mode of photographically forming a picture in relief,
from which printing blocks are obtained in much the same manner as in
the nature-printing process. But the subject which is thus printed is a
photograph; and it is only because in the actual production of the
impression on paper the agency of light is not called into play that it
is not described under the head of photography, for it is an ingenious
mode of causing the photograph to engrave its own image on a metal
plate. It is founded on a fact which has already been noticed, namely,
the insolubility of gelatine which has been mixed with a bichromate and
exposed to the action of light. Mr. Woodbury has obtained the best
results with a solution of Nelson’s opaque gelatine, 1 oz. of which is
dissolved in 5 oz. of water, and to each ounce of the solution 15 grains
of ammonium bichromate are added. When a layer of this mixture, which is
of course prepared in the dark, is exposed to the action of light under
a negative photograph, the gelatine is rendered insoluble under those
parts of the negative through which the light passes, that is, in the
parts corresponding with the dark shades in the original object, and the
depth of the layer thus rendered insoluble in each part will depend on
the relative thickness of the silver deposit in the negative photograph.
Thus, in the half-tints the insoluble layer will not be so deep as under
the parts of the negative through which the light passes without
interruption. But the differences of depth will appear when the soluble
gelatine has been dissolved away on the side of the layer which is
farther from the negative. Hence, Mr. Woodbury spreads his layer of
bichromated gelatine on a sheet of plate-glass, previously coated with
collodion, and when the gelatine has become dry, the double film is
detached from the glass and exposed under a negative, the collodion side
being uppermost and in contact with the photograph. After exposure the
film is temporarily attached to another piece of glass, by means of a
solution of India-rubber, and is then immersed in warm water, which
quickly dissolves the soluble parts of the gelatine. Thus a counterpart
in relief of the photograph is obtained. This is allowed to dry, and the
next operation consists in obtaining an impression from it in metal:
this Mr. Woodbury at first obtained by electric deposition, but he has
discovered a much more expeditious process, which one would hardly have
supposed possible before actual trial. The dry hard gelatine is placed
upon a flat, truly-surfaced steel plate, with the collodion surface
downward, a plate of soft metal is placed upon the gelatine, and the
whole is subjected to a pressure of about four tons per square inch in a
hydraulic press. In one minute a perfect impression of the gelatine
relief, down to the smallest detail, is formed in the soft metal; and,
strangely enough, the delicate sculpture which the light has executed on
the gelatine is not in the least injured, but will stamp its image on an
indefinite number of metal plates in the same manner.
The reader will understand that the impressed plate of metal now bears a
hollow sculpture representing the image of the original object from
which the negative photograph was taken, the darkest shades of the
object being represented by the deepest depressions in the plate, while
the highest lights are represented by portions of the metal at the
level, or nearly so, of the surface of the plate. From this plate the
prints on paper are obtained as follows: The plate is placed
horizontally, with its impressed face upwards, and a quantity of a
certain kind of ink is placed upon it. The composition of this ink, if
ink it may be termed, is one of the ingenious parts of this elegant
process. It is made of gelatine, coloured with some suitable transparent
or semi-transparent pigments, and it is poured on the plate in a warm
and fluid state, and in quantity more than sufficient to fill all the
hollows. A sheet of paper is placed over the plate, and a moderate
pressure is applied, when the excess of ink is squeezed out and escapes.
That which remains in the hollows of the plate, becoming set by cooling,
adheres to and is removed with the paper, giving in each part a force of
tint proportional to its quantity, that is, according to the depth of
the hollow in the plate. The paper is laid aside to dry, and although
the picture has at first a certain relief, yet the gelatine ink dries
down, the picture becoming so flat that no difference of the surface is
perceptible. It will be observed that this mode of printing rests upon a
distinctly new principle—namely, the production of shades and gradations
of tints by the varying quantity of the ink laid upon the different
parts of the paper. The method is in this respect identical with that by
which the water-colour painter produces his gradations; for the colour
is applied in transparent layers, and the depth of the tint produced
depends upon the mass of the pigment laid on, and is greater or less
according as the white of the paper is more or less visible through the
film of colouring matter. The gradations of tint in wood and steel
engraving and in lithographs are dependent upon quite another
principle—namely, the varying distribution of spots, patches, or lines
in black ink of uniform intensity. The Woodbury print has all the detail
and clearness of the photograph, together with a certain softness,
produced by the transparency of the colouring matter, not found in the
ordinary photographic print. The method admits of any desired tint being
given to the prints, and these are perfectly unchangeable by light. Thus
the result is a print which secures every good quality of a photograph
without any of the unpleasant ones, such as hardness, harsh tints,
opacity, fugaciousness. The prints may be taken on plates of glass, and
they then form beautiful transparencies. Such prints constitute most
admirable slides for the magic lantern, since the semi-transparent
colouring matter, and the soft gradations, produce charming effects.
Another ingenious invention of Mr. Woodbury’s provides a means of making
the sunbeam engrave a mezzotint copper plate from a photograph. The
action of light on bichromated gelatine is here again taken advantage
of. A film is prepared similar to that used in the above-described
Woodbury process proper, but the gelatine is mixed with some powdered or
granular material, so that it may give rise to a granulated texture in
the resulting plate. This film is treated exactly in the same way as
before with regard to exposing, washing with warm water, drying, &c. The
product is a very thin sheet, having a mezzotint-like surface, with more
or less grain according to the action of the light. The white parts are
perfectly freed from the granular matter by the solution of the
gelatine, while in the darkest parts there is the greatest accumulation.
The dry film in this condition is pressed into soft metal, and by a
double process of electrotyping and subsequent facing with steel, a
plate is obtained fit for printing at the copper-plate press. The firm
of Messrs. Goupil and Co., of Paris, extensively employ this process for
the preparation of the illustrations in that elegant publication, “The
Portfolio.” Another method of photographic engraving lately projected by
Mr. Woodbury is the following: a plate of steel is covered with a layer
of gelatine, mixed with a certain proportion of gum and glucose, and
dried in a dark room. This is exposed to the action of light under a
transparent photograph on glass. When afterwards this gelatine layer is
breathed upon, the moisture attaches itself to the portions which have
not been acted on by the light, and these become more or less sticky.
Sand or emery sifted to three different degrees of fineness is then
sprinkled over the plate, beginning with the coarsest, which attaches
itself to the most sticky parts. The less sticky parts are incapable of
retaining these larger particles; while the finest sand, which is
sprinkled on last, is held by parts of the plate that are even very
slightly sticky; but the places where the light has been intense are
dry, and none of the sand adheres. The gelatine layer is then completely
dried, and the plate, being covered with another of soft metal, is
placed in a press, by which a granular impression is produced on the
soft metal, and this may then be copied in copper by the electrotype
process. The larger particles of sand produce deeper depressions in the
plate, and thus a gradation of tint is obtained.
Amongst other applications of the gelatine relief devised by Mr.
Woodbury is that of producing a watermark in paper. A very delicate
relief is firmly attached to a plate of steel or zinc, and when paper is
rolled in contact with these plates, it receives an impression of the
design, all the delicate half-tints being represented in the slight
opacity of the paper. Mr. Woodbury is at present engaged in perfecting a
method for wedding his own process to that of chromo-lithography, by
first printing the different tints on the paper, and then transferring
the Woodbury prints to the top of the colours. The transparency of the
gelatine and ink is such that the most brilliant effects are attainable
in this way.
Bichromated gelatine is also the agent employed in _photolithography_,
the image of a negative photograph being thus rendered insoluble in a
layer of gelatine spread on the stone, which is acted on by acids, &c.,
in the usual way, after the soluble portions have been removed by water.
As there are also methods of using the lithographic process with plates
of zinc instead of stones, so there are processes of impressing the
image photographically upon the zinc. Of the general nature of the
processes of _zincography_, _photolithography_, and _photozincography_
the reader will now probably be able to form some idea, but the details
need not here be described. The last two, and some other processes for
printing photographic effects mechanically, all labour under the defect
of imperfectly rendering the _half-tints_ of a picture. This remark does
not apply to the Woodbury process. The photo-lithographic process gives
marvellous results in cases where no gradations are required. Thus a
whole page of the _Times_ newspaper may be lithographed in a space not
exceeding half of this page, and although the characters may be
indistinguishable to the naked eye, a lens will show them perfectly.
Similarly, we may obtain within the compass of an octavo page a
photo-lithograph of one of Hogarth’s large engravings, which will show
every touch of the original artist’s _burin_.
There is reason to hope that the time is not far distant when all our
tedious mechanical methods of reproducing drawings by wood or steel
engravings will be superseded by processes which will give us absolute
facsimiles of every touch of the artist’s pencil; and when some process,
giving all the delicacy and truthfulness of Mr. Woodbury’s prints, will
supply us with faithful transcripts of nature for book illustration at a
cost not exceeding that of the ordinary methods. So far as relates to
one style of drawing, these requirements appear to be nearly realized in
the process termed the _graphotype_, which reproduces mechanically, in
the form of a metal plate with all the lines in relief, a design which
the artist has etched on a flat surface. This is effected in the
following manner: Chalk is powdered very finely, and sifted through wire
gauze having very narrow meshes. A quantity of this is spread upon a
smooth plate of metal, and subjected to an intense pressure by means of
an hydraulic press. The particles of the chalk cohere into a mass,
having sufficient firmness to admit of its surface being drawn upon in
the same manner as a block of boxwood. The drawing is effected with an
ink composed of lampblack and glue, a finely-pointed camel’s-hair brush
being employed; but the shades must be produced by lines and strokes as
in wood engraving. When the ink is quite dry, the surface is rubbed with
a fitch brush or with velvet; and by this brushing the particles of
chalk not protected by the inked strokes are loosened and carried off.
In a short time the chalk between the strokes becomes quite hollowed
out; and when a depth of about one-eighth of an inch has been attained,
every line remains standing in relief exactly as in an engraved wood
block. A strong solution of silicate of potash is then poured upon the
chalk, which its chemical action converts into a kind of stone without
in any way altering the forms. Although this artificial stone is quite
hard, so that impressions may at once be taken from it, yet it is
incapable of enduring the wear and tear of the printing-press.
Accordingly a mould is taken from it, and this is made, by some of the
processes of casting or electrotyping already described, to furnish a
metal stereotype plate.
_THE LINOTYPE MACHINE._
[Illustration:
FIG. 313_a_.—_The Linotype Machine._
]
Among recent inventions in connection with printing, the _linotype
machine_ calls for special mention. In this machine a great number of
actions are combined and co-ordinated with the utmost ingenuity, but
such mechanism does not lend itself to popular description, and we must
confine ourselves to a statement of what it effects, recommending the
reader to avail himself of some opportunity of seeing the apparatus at
work. It will not then be needful to give details of every one of the
very numerous parts, which present in the _ensemble_ a great appearance
of complication, the more so that much ingenuity has been exerted to
make the machine compact, which is a practical point of great
importance. The disposition of parts is not, therefore, that which is
calculated to show each movement clearly to the spectator, but that by
which the least space is occupied. The machine is driven by belting from
a main shaft, turned by a steam-engine, gas-engine, electro-motor, or
other regular source of power, and rotated at such a rate that the main
pulley of the machine itself (14½ inches in diameter) shall make about
60 revolutions per minute. Fig. 313_a_ shows the general aspect of the
machine and seat for the one operator required, but as we are not
undertaking a detailed and complete description of the whole mechanism,
no letters of reference are given; but the reader will be able, from the
following diagrams, to identify the more important parts, and form a
general idea of their action and purpose. In this machine great use is
made of the contrivances called _cams_, several of which may be observed
in the sketch towards the side of the machine on the left, being fixed
on and turning with its main shaft. They consist of plates, or open rims
of various forms, which move levers, etc., in any required way, and at
any required period of the revolution.
[Illustration:
FIG. 313_b_.—_A Matrix._
]
[Illustration:
FIG. 313_c_.—_Diagram of Movements._
]
[Illustration:
FIG. 313_d_.—_A Line of Matrices._
]
The linotype is not a type-setting and type-distributing machine, but
one in which the form is stereotyped line by line; hence its name of
_linotype_. The mould, or matrix, is made up of a number of brass
matrices, each of which consists of a flat plate having on its edge a
letter incised. One of these is represented on Fig. 313_b_ wherein _a_
is the hollow letter. At the upper end the plate is cut into a number of
notches like the teeth of a saw, only that some of the teeth have their
points cut off, leaving steps, as it were, with faces parallel to the
longer edges of the matrix. There may be seen one of these at _b_, and
on the opposite side of the V-shape, three may be observed. The number
and arrangement of the cut-away notches is different for the matrix of
each letter (or sign), and special to it. The meaning of this will be
seen presently. The diagram Fig. 313_c_ will help us to see how these
matrices are assembled by touches of the finger on the required letters
as marked on the keyboard at D. The matrices are assorted and stored in
separate channels in the “matrix” magazine, A, a portion of its cover
being here represented as broken off in order to show the channels. It
will readily be understood that, by a system of levers connected with
each key, the corresponding matrix is released by means of an escapement
(B´), and falls down one of the channels E on to the travelling belt F,
which conveys it to composing stick G, in which the matrices
successively assemble in the order to constitute a line (Fig. 313_c_),
in which observe that the several words are separated by spaces formed
by long wedges of steel, the thick ends of which hang down considerably
below the line of matrices. These are dropped one by one from a store at
I (Fig. 313_c_), when required, by a touch on the key-bar J; two of them
are shown in position in the assembling stick G. In Fig. 313_a_ a bell
is seen in front of the keyboard, and this is automatically rung by a
mechanical device when the line of matrices is approaching in length to
that allotted to the work. At this point the operator has to consider
whether he can complete the line with another, or with how many
syllables of a word, and he touches the keys of the required letters.
The assembling stick then contains all the matrices comparatively
loosely packed side by side, for the words are as yet separated by only
the thin edges of the space wedges. A touch of the operator on a lever
brings into play another part of the mechanism by which the composed
line is bodily lifted a short way, then moved horizontally, and conveyed
to the “mould wheel,” in which there is a slot, adjustable in length and
width, and the line is here firmly pressed against the face of the wheel
in such a way that the slot coincides with the line of hollow letters on
the edges of the matrices, as shown in Fig. 313_b_. This moulding
arrangement is not the least ingenious device in this machine, and well
deserves attention. Before the moulding takes place, but while the line
is in its place, the wedge spaces are pushed up through the matrices by
another portion of the mechanism, and thus the line is immediately
“justified,” as the printers term it; that is, the wedges rise up,
separating the words, more or less, until the line has exactly its
assigned length, and the words are, at the same time, separated by equal
spaces. A melting-pot behind the mould-wheel contains a quantity of
fusible metal, resembling stereotype metal, which is maintained at just
the temperature of fluidity by a regulated gas burner. At the right
moment a plunger is forced into the fluid mass, causing it to rise
through a kind of spout to the level of the slot in the wheel, and be
forced through that into the line of letters. The metal instantly
solidifies in the mould, the line of matrices is removed on a bar to a
new position at R, Fig. 313_c_, and the wheel then makes a quarter of a
turn, bringing the mould from the horizontal into a vertical position
(Fig. 313_e_). The linotype is subjected to the operation of certain
knives (not shown), by which it is pared smoothly to the exact thickness
and height required, and finally ejected, as shown in Fig. 313_e_,
dropping in its proper order into a receiving galley. The line, as
completed, has the shape represented in Fig. 313_g_, and a number of
these lines assembled constitute a “form,” answering all the purposes of
the ordinary forms consisting of separate type. These last, after having
served their purpose, must be “distributed,” that is, each single letter
must be returned to the case from which it was taken by the compositor;
but the linotype form, after use, is simply returned to the melting-pot
for its metal to be recast into new forms. The forms can, of course,
remain standing for any length of time at the mere expense of keeping
the metal unemployed. One advantage of the linotype is that the printing
is all done from new clean-faced forms, instead of the old and
dull-faced characters of ordinary type that have been much used, but
have to be resorted to under ordinary circumstances.
[Illustration:
FIG. 313_e_.—_A Finished Line entering Galley._
]
[Illustration:
FIG. 313_f_.—_The Melting Pot and Mould Wheel._
]
It may occur to the reader that errors in linotype would be much more
difficult of correction than those occurring with the ordinary type
composed by hand. If by chance a wrong matrix appears in the line, this
can be changed by hand at once; but supposing that the operator
overlooks some error in reading the assembled line, which, observe, he
reads with the characters arranged as they will appear in the
impression, or that he has misread his manuscript, and the line is cast,
assembled into a “form” with the rest, and then in the printed proof the
error is discovered, how is it to be rectified? Simply by removing the
faulty linotype from the form, and casting a new one. This is so quickly
and easily done that it has been found by actual test between linotype
and ordinary type matter containing the same defects, that the former
could be corrected in less than one-third of the time required for the
latter.
We left the line of matrices at R (Fig. 313_c_), and we must now
indicate the method by which each is automatically returned to its own
magazine, an operation for which much ingenious mechanism has been
contrived, of which the details cannot be well described in this place.
The line having reached R, the space wedges are disengaged from it and
removed to their receptacle at I, while the matrices become engaged by
their teeth in the grooves of a horizontal bar, and then the bar is
grasped by a lever which lifts it up to the distributing arrangement at
the top of the machine, where the teeth of the matrices come to the
exact level of the grooves of the distributor bar T. The line is then
pushed laterally, the sides of the matrices become engaged in the
hollows of two parallel screws U, by which, while suspended only by such
of their inclined teeth as the corresponding groove of the distributor
can support, they are made to slowly travel along from left to right
until each reaches a certain point, namely, that at which its sustaining
V grooves on the bar are interrupted by cuts which permit it to drop
into its own special magazine. A little consideration will show how, by
various combinations of the notches on the matrix, and corresponding
cuts at the right places in the grooves of the bar, each matrix may be
made to move along until it reaches a determinate place, and there
dropped. Compare Fig. 313_b_ and Fig. 313_i_. Each matrix thus again
deposited in its proper magazine has completed the circuit of the
machine, or, at least, has passed from the bottom of its magazine to the
assembling stick, hence to the mould, and, by the distributor, finds its
way back to the top of its magazine, whence, in its turn, it will
descend to perform again the same duty.
[Illustration:
FIG. 313_g_.—_The Finished Line._
]
[Illustration:
FIG. 313_h_.—_Lines assembled into a “Form.”_
]
It must be understood that, beyond the operator’s touches on the
key-board, and that required to send off the assembled line to the
moulding apparatus, all the actions are done automatically without the
interference of the operator, who, while one line is getting moulded,
raised up, and distributed, calmly proceeds with the composition of the
following one.
[Illustration:
FIG. 313_i_.—_Matrices dropping into Magazine._
]
The rate at which the work is produced is very great. One good operator
with one machine can, it is said, turn out, hour by hour, matter that
would be equivalent to two and a half pages of this book, arranged solid
or without break. There are, of course, record performances of
exceptional operators who have completed more than twice as much as this
in a single hour.
[Illustration:
FIG. 314.—_Recording Anemometer._
]
RECORDING INSTRUMENTS.
Sir John Herschel, in enumerating at the close of his inestimable
“Discourse on the Study of Natural Philosophy” the causes of the rapid
development of the physical sciences in modern times, assigns a
prominent place to the improvement of scientific apparatus, especially
of those instruments by which exact measurements or observations are
made. The accurate and elaborate instruments which serve for the
delicate and precise determinations and observations of modern science
require for their production a very advanced state of mechanical art,
such as is indicated by the perfection of the tools we described in a
former article; and these tools are themselves, on the other hand, the
outcome of accurate knowledge, and another proof of the interaction
between science and practical art. Since precise observations and
accurate measurements form the essential bases of every science, its
progress will be accelerated by every improvement in its instruments
which increases their delicacy and exactness. Indeed, hardly any branch
of knowledge becomes entitled to be called a science until it rests upon
quantitative data of some kind. Chemistry was nothing but a confused
collection of vague notions until the exact determinations of the
balance were employed, and the proportions of the substances combining
or separating in chemical actions were found to be related by certain
simple and very definite laws. In all branches of inquiry there is the
same necessity of quantitative comparisons: lengths, angles, surfaces,
volumes, masses, durations must be compared with standards of their own
kind; motions, forces, pressures, temperatures, lights must be measured.
The case of chemistry shows the line along which other sciences are
advancing. Physiology has made great strides since instruments of
precision have been used in its investigations, and as some of these are
of the kind we here propose to treat of, they will be described in the
sequel. To recording instruments, meteorology is also largely indebted
for the remarkable progress which it is making, and which will soon
place this branch of knowledge in a condition to supply the most
striking illustration of the difference between a science founded on
accurate measurements and a mass of vague observations.
The obvious advantage of a recording instrument (say, for example, such
a one as that represented in Fig. 314, which registers the force and
direction of the wind) is that the results are obtained without the
immediate attention of an observer, and they can be continuously
recorded at every instant, day and night; but there is another and yet
greater advantage in certain kinds of instruments which write their own
records, in the fact that they can be made to register results which
would altogether escape direct observation. It is said that a practised
astronomical observer will correctly record the time of a phenomenon to
nearly the tenth of a second; but there are cases in which we may desire
to estimate time to the thousandth part of a second or less. An
investigation of M. Foucault has already been named in which a far less
interval of time was concerned (page 387); but the recording instruments
we have to mention here are of use for enabling us to make certain
instantaneous actions mark the time of their occurrence with the
greatest precision, and also for enabling us to note the variations in
actions which are too rapid to be directly observed in their various
phases.
Fig. 315 is a diagram which will serve to explain the method in which
the height of the barometer and the thermometer are registered in the
ingenious _metereograph_, invented by Professor Hough, of Dudley
Observatory. The contrivance has the advantage of performing the
operation for both instruments, with a single piece of mechanism and on
the same sheet of paper. The diagram is not intended to indicate the
actual arrangement of the parts of the apparatus, but merely to explain
the principle of its action. Let A represent a cylinder about 6 in. in
diameter and 7 in. high, covered with a sheet of paper, ruled with
certain lines, some parallel to the axis, and others perpendicular to
those. This drum revolves by clockwork, controlled by a pendulum, at a
certain regular rate of, say, one turn in seven days. B is a metallic
bar or lever, about 2 ft. in length, mounted on an axis or fulcrum at C.
At D is a pencil or style projecting from the extremity of the bar
opposite the centre of the drum, but not in actual contact with the
paper. E and F are platinum wires attached to the lever at about 3 in.
distance from the fulcrum, C; E passes into the open tube of a mercurial
thermometer, G, and F into the shorter branch of a syphon barometer, H.
The clockwork has other offices to perform besides turning the drum, A,
on its axis; and one of these is to alternately elevate and depress the
lever, B, every half-hour. If the end, F, be depressed, it is plain that
the wire will come into contact with the metallic float, which is
supported by the mercury and follows its movements. If, therefore, wires
from a battery, K, including an electro-magnet, I, in their circuit, be
connected with the bar at C, and with the mercury at H, when the wire at
F touches the float, the current will pass and the armature of the
electro-magnet will be attracted. The movement of the armature is so
arranged that it causes a blow to be given to the end of the bar D, so
that the pen there marks a dot on the drum, thus indicating its height
at the time, and therefore that of the mercury in H. When the lever is
depressed at the other end, the wire, E, similarly completes the circuit
through the mercury in the thermometer, and the height of the latter can
be known from the dot which is similarly impressed on the lower part of
the paper. These movements may be made with almost any degree of
precision required. The clockwork is also made to raise the hammers
which strike the pen against the drum, at the instant the electric
current passes.
[Illustration:
FIG. 315.—_Registration of Height of Barometer and Thermometer._
]
The instrument, as actually constructed, registers also the height of a
wet-bulb thermometer, by another wire requiring a lower depression of
the lever to bring it into contact with the mercury in a wet-bulb
thermometer. A complete double motion of the lever requires one hour,
and in that interval the heights of the barometer and both thermometers
are each recorded once. The wet and dry-bulb thermometers are registered
within a minute of each other, and half an hour elapses between the
barometer and thermometer records.
Another invention of Professor Hough’s is a barometer which marks a
continuous pencil-line on a revolving cylinder, by which the variations
of the mercury are shown for every instant of the day. Another part of
the arrangement is a machine for automatically printing on paper in
ordinary characters the height of the mercury to the thousandth part of
an inch.
A very simple and trustworthy record of thermometric and barometric
heights is obtained by photography at Kew and elsewhere. A sheet of
sensitive paper passes horizontally at a uniform speed behind the tube
of the instrument, so that the only light it can receive must pass
through the glass. A lamp is placed in front, and a portion of the paper
is protected from its rays by the mercury, while those which pass
through the tube above the mercury make their impression on the paper,
and thus record the indications of the instrument.
Fig. 314 represents part of another ingenious meteorological instrument
invented by Mr. J. E. H. Gordon, and made by Mr. Apps. It is an
electrical anemometer, for indicating and registering the direction and
force of the wind. The apparatus consists of an external portion, which
is of course fixed on some high and exposed part of the building; and
the indicating and registering instrument, which communicates with the
former only by insulated wires connected with a galvanic battery, and
which may be placed on any convenient table within the house. The
registering apparatus in this instrument is very neat and compact, and
the reader will no doubt be able to form a sufficiently good idea of its
nature from the portion which is visible in the cut, and from the
knowledge of similar apparatus he may have derived from the descriptions
already given in the article on the electric telegraph.
Modes of making phenomena record the time and duration of their own
occurrence are now much used in all scientific investigations; and in
connection with the electric _chronograph_ or _chronoscope_ which we are
about to describe, few more efficient or elegant methods of
“interrogating nature”—to use Bacon’s phrase—have yet been devised. The
reader who has never seen an instrument of this kind will be the better
able to understand its principle by a simple illustration, which may
very easily be made a practical one by himself if he has a tuning-fork
at hand. Let him fix the tuning-fork firmly into a board in an upright
position, by inserting the part usually held in the hand into a hole in
the board; and then attach to the fork, by means of a little bees’-wax,
a short bristle, which is to project from the extremity of one prong in
a direction perpendicular to the plane in which the prongs vibrate. He
has now only to provide himself with a piece of glass a few inches
square in order to obtain a record of the vibrations of the fork when
sounding. By the help of another piece of board it will be easy to
arrange a guide by which the piece of glass can be made to fall down by
its own weight in a plane parallel to the prongs, and in such a manner
that the free end of the bristle shall just touch its surface during the
whole time of its descent. Now let the surface of the glass be blackened
in the flame of a candle. If the glass be allowed to slide down when the
fork is not vibrating, the end of the bristle, by removing the lampblack
from the surface as the glass falls, would trace out a vertical line.
If, on the other hand, the blackened surface were itself not moved, but
simply brought into contact with the end of the bristle, while the fork
was sounding, there would be marked only a very short horizontal line,
corresponding with the extent of the vibratory movements of the prong.
When the glass is allowed to fall while the prong is in motion, the
combination of the horizontal movements of the bristle, and the vertical
one of the glass, will produce a waved line, which will exhibit
perfectly regular curves if the glass has been moved with uniform
velocity. It is plain that if the time taken by the glass to pass in
front of the bristle were accurately known, the number of movements per
second executed by the prong of the fork could be found. On the other
hand, if the rate of vibration of the fork be known, the time occupied
in the passage of the glass may accurately be read off. If this simple
experiment be understood, the principle of the electric chronograph will
be clear. Substitute for the sliding glass a cylinder covered with white
glazed paper which has been coated with lampblack in the same manner;
suppose the cylinder to revolve at a uniform rate, while a tuning-fork
is similarly writing its vibrations on the surface of the paper, and let
the same mechanism which turns the cylinder, slowly draw the
sounding-fork along a straight slide parallel to the axis of the
cylinder. The waved line will not form a complete circle on the surface
of the cylinder, but will be traced out in a spiral, owing to the
combined motions of the fork and the cylinder. As the number of
movements per second of a vibrating body emitting a given note are
accurately determined and perfectly regular, the waved line on the
cylinder thus furnishes an exact measure of small intervals of time, the
utility of which will presently be seen.
[Illustration:
FIG. 316.—_The Electric Chronograph._
]
Fig. 316 represents the apparatus as actually constructed. A is the
cylinder covered with the blackened paper, and driven by clockwork
contained in the case, B, the rate of movement being regulated by the
conical pendulum at C, so as to be approximately uniform. D is a lever
for starting and stopping the movement. The clockwork also causes the
carriage, E, to slide along the bars, F. This carriage bears three
electro-magnets, and to the armature of each a fine pointed strip of
metal is attached so as just to touch the surface of the cylinder. The
movement of the armatures takes place in a direction parallel to the
axis of the cylinder, and they are acted on by independent currents,
each electro-magnet having its own binding-screws for the attachment of
wires. The armature of one of these is a steel bar, which vibrates in
the manner of the prong of a tuning-fork, at a rate known by the note it
gives out. Sometimes, for delicate experiments, the movements of this
armature are checked and controlled by introducing into its electric
circuit a contact-breaker, formed of a real tuning-fork, the vibrations
of which are maintained by electro-magnets. Another electro-magnet on
the carriage, E, can be connected with the pendulum of a standard clock,
so that seconds may be also marked on the cylinder, as larger units in
the division of the time. In an electric chronograph, which was
exhibited at the International Exhibition of 1862, an ingenious and
excellent mode of making and breaking the electric contacts from the
standard pendulum was adopted. Two short vertical glass tubes, placed
side by side, had each near its lower end a small horizontal branch;
these branches were placed in a line with each other, with their ends in
very close proximity. The larger tubes were filled with mercury which
flowed into the horizontal branches, and the two streams joined in the
narrow space between the ends of the tubes. Although the mercury was
here unsupported by the glass, and surrounded by air only, it did not
run down, for the space was so small that the cohesion of the mercury
itself sufficed to keep the drop hanging between the two open ends of
the tubes. A very thin sheet of mica was carried by an arm of the
pendulum, so placed that at each complete oscillation the mica entered
the space between the two tubes and divided the mercury, and thus all
electrical communication between the two reservoirs was cut off. The
mica remained during one beat of the pendulum in this position; and at
the commencement of the return beat was withdrawn, allowing the divided
columns of mercury to flow together again, and complete the electric
circuit. This admirable make-and-break arrangement acted with the
greatest regularity: the mercury was not spilt, as might have been
expected, there was no friction, and any oxide formed by the spark which
passed when the current was interrupted was removed by the mica. The
pendulum provided with such an arrangement allows the current to pass,
and interrupts it, during alternate seconds, and the result is that the
cylinder is marked with a regularly divided broken line, thus,
¯|_|¯|_|¯|_, the establishment and interruption of the current at the
end of each second being marked with great sharpness and precision.
The third electro-magnet of the apparatus represented in Fig. 316 is
acted on by currents through the wires, G, H. The point attached to its
armature traces a plain spiral line on the revolving cylinder, except at
the instant when the current is established or interrupted. And the
phenomenon to be timed is in some way made to accomplish the making and
breaking of this circuit. This may be perhaps better understood by an
example. It has sometimes happened that the boxes employed in the
pneumatic dispatch stick fast in the tubes, and resist all efforts to
dislodge them by manœuvres with the compressed or rarefied air, or other
means. In such a case it becomes necessary to ascertain with tolerable
accuracy the position of the obstruction, so that the tube may be cut at
the right place and the obstacle removed. The known velocity of sound
has been ingeniously used for this purpose; the electric chronograph
being made the means of ascertaining, to a small fraction of a second,
the time required for the report of a pistol to be propagated through
the air of the tube and reflected back from the obstruction. An elastic
membrane is spread over the open extremity of the pneumatic tube; this
membrane has in the centre a little disc of platinum, in electric
connection with one of the wires, G, H. The other wire passes to a
galvanic cell or battery, and the return wire from the battery is
connected with a platinum point, the distance of which from the disc can
be adjusted with nicety by means of a screw, so that the circuit may be
complete only when the platinum point and the platinum disc are in
contact. The screw is adjusted so as to bring the point as near as
possible to the disc without actual contact. The chronograph cylinder
having been set in motion, a small pistol is fired into the pneumatic
tube through a side opening. Sound-waves of alternate compression and
rarefaction pass along the tube, and are reflected backwards and
forwards many times in succession between the obstacle and the membrane.
By these the membrane is alternately forced out and drawn in, making and
breaking the electric contact accordingly, and thus causing the point of
the electro-magnet to describe in the cylinder an indented line, the
intervals of which indicate the time the sound requires to traverse the
tube to the obstruction; and thus the position of the latter may be
known with sufficient accuracy.
A very interesting application of the electric chronograph is to the
measurement of the velocities of projectiles. The science of gunnery has
acquired an exactness unknown before electricity was made to carry
messages from the cannon-ball in its swiftest flight, and to write the
record of its own course. Instruments for thus measuring the velocities
of projectiles have been contrived by several electricians, among whom
Wheatstone appears to have been the first. The principle in most of
these chronographs is precisely the same as that on which the apparatus
represented in Fig. 316 is constructed. The action of the projectile
which is electrically indicated is the severing of a slender wire, which
is stretched from side to side of a wooden frame, so that it passes
continuously backwards and forwards in parallel lines. Thus a kind of
screen is formed, through which the missile must pass, and in its
passage must rupture the wire. If the wire conveys a current of
electricity, this current is therefore interrupted at the moment the
ball passes. Sometimes the immediate effect of the breaking of the wire
is mechanical, as in Wheatstone’s arrangement, where the wire is
stretched by a weight over a series of pulleys, and attached to a
contact-maker, which completes the circuit when it is set free by the
rupture of the wire. A similar arrangement in the screens has been
proposed by Mr. Siemens for the establishment of circuits in connection
with charged Leyden jars, the sparks of the discharges being made to
take place at the surface of a revolving cylinder of polished steel,
where the place is shown by the spot they leave on the metal. M.
Pouillet’s chronoscope dispenses with the revolving cylinder, and
measures the _duration_ of the current established by the projectile at
one part of its course and cut off at another, by the arc through which
the needle of a galvanometer is impelled.
The instrument which has been most employed in this country by
artillerists is that invented by Professor Bashforth. Its indications
are extremely accurate, for readings may be taken to the two-thousandth
part of a second. From ten to fifteen screens are placed in the path of
the projectile at distances asunder which may vary from 15 ft. to 150
ft., but which, of course, are carefully measured. Each screen is formed
of a wire carrying an independent current, which also circulates in the
coils of an electro-magnet. The ten electro-magnets have styles attached
to their armatures in such a manner that when all the currents are
passing, ten parallel spiral lines are traced on the surface of a
vertical revolving cylinder, about 12 in. long and 4 in. diameter. The
cylinder is covered with a sheet of highly-glazed paper coated with
lampblack; and when the sheet is removed, the lampblack may be fixed on
the paper if desired, and the record made permanent. The equal intervals
of time are marked on the same cylinder by another electro-magnet
connected with a pendulum beating half-seconds. The axis of the
revolving cylinder, &c., carries a heavy flywheel, which is set in
motion by the hand at the rate of about three revolutions in two
seconds, and the rest of the mechanism is thrown into gear just before
the signal to fire is given. The arrangement of the electric connections
at the screens is such that at the moment of the rupture of the wire,
the circuit is broken for an instant only. At this moment, the iron of
the corresponding electro-magnet ceasing to attract its armature, the
latter is drawn away by a spring; but the re-establishment of the
current immediately brings it back, and the style continues to trace the
same spiral line as before. The passage of the ball through the screen
is therefore marked by a little notch in the spiral line, thus, ——ᐱ——,
and the point where the deviation begins indicates the time at which the
ball passed. In order to read off this time, a straight-edge is applied
to the cylinder parallel to its axis; and by means of a scale sliding
upon the straight-edge the distances between the notches on the several
spirals are compared with those between the pendulum marks.
To record the instant at which a projectile passes determined points in
a cannon’s bore, lateral plugs are screwed in, each having, just
projecting into the bore, a small steel ball, which, pressed outwards by
the passing projectile, causes a cutter to divide the primary wire of a
Ruhmkorff coil, whereupon the spark that passes in the secondary circuit
leaves its record on a uniformly moving disc. Each plug has its own
battery, coil, and disc.
A special feature of recording instruments may be exemplified by certain
applications of the principle to the investigation of physiological
actions. A skilled physician is often able to detect in the pulse of his
patient certain characteristics besides the mere rate, which are highly
significant as regards the condition of the circulatory system. The
range of these indications has been greatly extended by an instrument
invented by MM. Chauveau and Marey, by which the pulse is made to write
down a graphic representation of its action. The patient’s arm having
been placed on a suitable support, a little stud covered with soft
leather is lightly pressed against the artery by a spring. The stud is
in contact with the shorter end of a very light lever, the other
extremity of which is furnished with a point, which registers its
movements on a cylinder of blackened metal, made to rotate and advance
longitudinally by clockwork; or the record is taken on strips of flat
smoked glass. As the motion is much magnified by the lever, every
variation in the pressure of the blood in the artery during the beat of
the pulse is distinctly and faithfully indicated. From the line so
traced the physician may obtain infallible data for judging of the
condition of the heart, the action of its valves, &c. It is marvellous
to observe the manner in which the curves of the _sphygmograph_, as the
instrument is termed, change their form when certain drugs are
administered: the change in some cases occurs immediately, so that the
eye can detect by the inspection of the sphygmographic curve almost the
instant at which the drug was introduced into the system, and the nature
of its action on the heart.
[Illustration:
FIG. 317.—_Negretti’s Deep-Sea Thermometer._
]
Another instrument which is doing good service in the hands of medical
investigators is the _spirograph_, in which the rise and fall of the
chest in breathing are similarly traced by the motions of a lever. In
this instrument a small pad, which presses on the chest, communicates
its movements to an elastic membrane, which, like the skin of a
drumhead, covers one end of a cylindrical box maintained in a fixed
position relatively to the person of the patient. The air in this box is
in communication, by means of a flexible tube, with the interior of
another similarly closed box; the elastic membrane of the latter acts
against the short end of a lever, which is made to register its
movements as in the sphygmograph, for the compression of the air caused
by the rise of the chest is conveyed to the second box through the
flexible tube. The curves furnished by this instrument also give
valuable indications, and exhibit marked changes under any influence in
the least degree affecting the respiratory system.
The value of a self-registering instrument for solving problems, the
intricacy of which is increased by the multiplicity and rapidity of the
actions to be observed, cannot be better illustrated than by the success
with which Professor Marey has thus studied some complicated actions of
locomotion, as related in his extremely interesting work entitled, “La
Machine Animale,” a translation of which has appeared in “The
International Series.” The action of the horse in the various paces,
walking, trotting, galloping, &c., has been an endless subject of
discussion, with no other data than the shoe-marks left in soft ground,
and the general appearance of the animal’s movements to an observer. But
M. Marey—by means of elastic bags containing air, communicating the
pressure through flexible tubes, so as to move little levers, which
write their traces on a revolving cylinder impelled by clockwork, and
carried by the rider,—has completely and finally settled all the points
in dispute. It is now definitely known how the horse’s feet are placed
on the ground in each of his paces, and the actual and relative time
that each foot remains down. The instruments are also made to register
the vertical movements of the animal, so that a complete record of its
motion can be obtained.
[Illustration:
FIG. 318.—_Negretti’s Deep-Sea Thermometer, general arrangement._
]
It was long a difficulty to obtain data as to the temperature of the sea
at great depths below the surface. It is obvious that the ordinary
maximum and minimum registering thermometers would not give the
temperature at any particular depth to which they might be submerged,
but merely the temperature of the warmest or coldest stratum of water
through which they passed in their descent or ascent. No plan which has
been devised to obviate these difficulties appears to have been attended
with success, until a quite recent invention of Messrs. Negretti and
Zambra supplied the desideratum, and furnished a convenient instrument
for trustworthy determination of the temperature of the ocean at any
required depth. The same firm long ago constructed thermometers for
deep-sea soundings, with bulbs protected from the pressure of the water
by an outer covering of thick glass surrounding the delicate bulb of the
thermometer, between which and the outer casing a space was left,
partially filled with mercury, so that heat might readily pass to or
from the inner bulb without the latter being exposed to the
superincumbent pressure. The new recording deep-sea thermometer differs,
however, from all other registering thermometers by containing mercury
only, without alcohol, or springs, or other removable indices, and,
consequently, it is free from liability to derangement. The following is
the description of the instrument:
In the first place it must be observed that the bulb of the thermometer
is protected so as to resist the pressure of the ocean, which varies
according to depth, that of 3,000 fathoms being something like 3 tons
pressure on the square inch. The new instrument is in shape like a
syphon with parallel legs, all in one piece, and having a continuous
communication, as shown in Fig. 317. The scale of the thermometer is
pivoted on a centre, and being attached in a vertical position to a
simple apparatus (which will be presently described), is lowered to any
depth that may be desired. In its descent the thermometer acts as an
ordinary instrument, the mercury rising or falling according to the
temperature of the stratum through which it passes; but so soon as the
descent ceases, and a reverse motion is given to the line, so as to pull
the thermometer towards the surface, the instrument turns once on its
centre, first bulb uppermost, and afterwards bulb downwards. This causes
the mercury, which was in the left-hand column, first to pass into the
dilated syphon-bend at the top, and thence into the right-hand tube,
where it remains, indicating on a graduated scale the exact temperature
at the time the thermometer was turned over. The cut shows the position
of the mercury _after_ the instrument has been turned on its centre. A
is the bulb; B the outer coating or protecting cylinder; C is the space
of rarefied air, which is reduced if the outer casing be compressed; D
is a small glass plug, as in Negretti and Zambra’s maximum thermometer,
which at the moment of turning cuts off the mercury in the tube from
that in the bulb, thereby ensuring that none but the former can be
transferred into the indicating column; E is an enlargement made in the
bend, so as to enable the mercury to pass quickly from one tube to
another in revolving; and F is the indicating tube, or thermometer
proper. When the thermometer is put in motion, as soon as the tube has
acquired a slightly oblique position, the mercury breaks off at the
point D, runs into the curved and enlarged portion, E, and eventually
falls into the tube, F, as the instrument resumes its original vertical
position.
[Illustration:
FIG. 319.—_The Atmospheric Recording Instrument._
]
The contrivance for turning the thermometer over at the bottom of the
sea may be described as a vertical propeller, to which the instrument is
pivoted. So long as the instrument is descending the propeller is lifted
out of gear and revolves free; but as soon as the ascent commences, the
action is reversed, the propeller falls into gear with a pinion
connected with the thermometer, and by these means the thermometer is
turned over, and after one turn it remains locked and immovable. The
engraving, Fig. 318, shows the general arrangement, T being the
thermometer, S a metal screw connected with the frame of the thermometer
by a wheel-and-pinion movement at W; S^† is the stop for arresting the
movement of the thermometer when it has made one complete turn.
The atmospheric recording thermometer (Fig. 319) differs from the
deep-sea thermometer by not having the double or protected bulb, as it
is not required to resist pressures. In this form of the instrument, the
thermometer is turned over by a simple clock movement, which can be set
to any hour that may be desired. It is fixed on the clock, and when the
hand arrives at the hour determined upon, and to which the clock has
been set as an alarum clock is set, a spring is released, and the
thermometer turns over as before described. A wet and dry-bulb
hygrometer is also arranged on the same plan. For observatories, or
where it is important to obtain hourly or half-hourly records of the
temperature, twelve or more thermometers are placed on a frame, and
these are turned over by clockwork one after the other at every hour or
half-hour as required.
The reader can hardly fail to perceive that powerful aid to the
investigation of the laws of nature must be afforded by such instruments
as we have described. And we have but taken an example here and there of
the scientific uses of the recording principle, selecting those that are
most readily understood, or that are connected with matters coming home
to the business and bosom of every one. The science of meteorology does
not deal with subjects which furnish merely amusing speculation for the
hour. Forecasts of storms and cyclones would often save many lives and
much valuable property; and our dependence upon meteorological
conditions cannot be more forcibly illustrated than by reference to the
disastrous floods which this year (1875) desolated some districts of
France. Meteorology has received a great impulse from the introduction
of recording instruments; and the vast number of results which are now
hourly recorded must lead to the certain development of the science, and
its reduction to exact laws. For even the winds obey laws—laws as
definite as those which control the motions of the planets; and could we
but take into account _the whole_ of the circumstances upon which the
movements and other conditions of the atmosphere depend, we should be
able to forecast the weather with the same certainty as—thanks to the
great and simple law of gravitation—we predict eclipses or other
astronomical phenomena. Already, by aid of the telegraph, it is often
possible to send a day’s warning of approaching storms to localities
lying in their probable track. The Signal Service, which is a Department
of the United States War Office, has a corps of meteorological observers
spread over the length and breadth of the States, who send every eight
hours, to a Central Office in Washington, a report of the force and
direction of the wind, height of the barometer, &c. The officer at
Washington sends back by telegraph to the public press a synopsis of
each day’s weather, and points out what weather will probably follow;
but if any city or port be threatened with a storm, special telegrams
are sent. Thus, a warning of the approach of a great storm, which
entered the American continent at San Francisco on the 22nd Feb., 1871,
was sent to Cheyenne, Omaha, and Chicago, twenty-four hours before the
storm reached these cities, which it was foreseen lay in its track.
Although the hurricane did much damage at some of these places, it would
probably have been far more destructive had not the inhabitants been
prepared for its approach.
An elegant form of _barograph_ or recording barometer has been brought
out, which is small, but sufficiently accurate for all ordinary
purposes. It is founded on the _aneroid_, which, as everybody knows, is
an instrument for indicating atmospheric pressure by the changes of form
it produces in a thin circular metallic box, partially exhausted of air.
The ordinary form of the aneroid is very sensitive and portable
(sometimes it is made only the size of a small watch), and bears an
index needle moving over a graduated disc; which arrangement is in the
barograph aneroid, replaced by a long lever, carrying an ink tracing
point in contact with the face of a cylinder that is caused by clockwork
to make one revolution per week. On the cylinder is spread a printed
paper diagram, divided by lines for each day of the week and each hour
of the day, and on this the tracing point marks a continuous curve,
showing all the fluctuations of the barometric pressure. The diagram is
removed at the end of the week, and a fresh form adjusted to the
cylinder. The impressed papers thus form a permanent and continuous
record, from which the height of the barometer, at any given moment, may
be read off.
_THE PHONOGRAPH._
Everything yet contrived in the way of recording instruments is eclipsed
in wonder and interest by one which is among the latest marvels of the
age. It is a recording instrument, and more than a recording instrument,
for it can reproduce to the senses the very phenomena it records; and
these same phenomena are the most familiar in their effects, and, at the
same time, so subtle and delicate, that the impressions they convey are
not generally thought of otherwise than in connection with our finest
intellectual and emotional perceptions. We are alluding to the
_phonograph_, which can register for us music and song, and articulate
human speech in all their tones and modulations, and, like an aërial
spirit, address them to the ear again, as often as we wish, and thus
Inform the cell of hearing, dark and blind;
Intricate labyrinth, more dread for thought
To enter than oracular cave,
Strict passage through which sighs are brought,
And whispers for the heart, their slave;
And shrieks, that revel in abuse
Of shivering flesh; and warbled air,
Whose piercing sweetness can unloose
The chains of frenzy, or entice a smile
Into the ambush of despair;
Hosannas pealing down the long-drawn aisle,
And requiems answered by the pulse that beats
Devoutly, in life’s last retreats!
In order that the reader may understand the action of the phonograph, it
is necessary that he should know something of the science of sound. Then
we must remember that this word is commonly used to express sometimes
those sensations of which the ear is the organ, and at other times the
external cause of those sensations. It is with the former meaning that
we use such expressions as “a sweet sound”; with the latter, such
phrases as “sound travels.” It will not be necessary to speak of the
physiology of the organ of hearing; but attention should be directed to
the different kinds of audible perceptions we can distinguish, let us
suppose, when listening to a song: First, there is the pitch, or the
notes in the musical scale, which, by their particular sequence,
constitute the air or melody. Second, there is the degree of loudness or
lowness of the notes. Third, the enunciation, or those differences by
which we distinguish, for example, the vowels _a_, _e_, _i_, _o_, _u_,
one from another. Fourth, the quality of the voice by which we can
distinguish between two vocalists singing the same vowel on the same
note with equal loudness. Observe that these four kinds of sound
perceptions are independent one of another. The last kind of difference
may also be well illustrated by the instance of musical instruments of
different kinds sounding the same note, in which case the difference of
the quality or _timbre_ is readily recognized. We have now to show the
nature of the mechanical movements outside of us which act on the ear
and reach our perception as sounds, giving the distinguishable
impressions that have been enumerated; and for the present we shall
consider the case of such musical sounds as those just referred to.
That the source of a sustained sound is an elastic body in a state of
vibration is a fact of which, in most cases, we are easily made aware by
the evidence of sight and touch; as a bell, a violoncello string, a
pianoforte wire, or a tuning-fork. On p. 656 is described a simple
method by which a tuning-fork may be made to write down its own
vibrations, and the more exact plan of recording them on the surface of
blackened paper on a revolving and advancing cylinder has also been
referred to. By the intervention of appropriate apparatus, a similar
record may be obtained from all sounding bodies. From observations of
this kind, and others in which totally different methods are used for
counting the number of vibrations per vibrations made in a given time,
it is known that the _pitch_ of the sound or note depends on the
rapidity of the vibrations—the pitch rises with the number of them per
second, and the relationship between the notes of a musical scale
depends entirely on these numbers. Thus, when the vibrations for the
eight notes of an octave are counted, the numbers always have this
_proportion_, beginning from the lowest note-–24, 27, 30, 32, 36, 40,
45, 48. Thus of the two notes—
[Music]
as produced on musical instruments tuned to the concert pitch of the
present day, the lower corresponds with 264 complete vibrations per
second, the higher with 528. It will be observed, too, that all the
harmonies are determined by some simple ratio in the rates of vibration:
the interval of the _fourth_ is 3, 4; that of the _fifth_ is 2, 3, etc.
Another easily discoverable fact is that the loudness of the sound
depends upon the amplitude of the vibrations. This is sufficiently
obvious by a few experiments with a tuning fork; and by close
examination of such tracings as have been mentioned, we shall soon
become aware of another circumstance—namely, that the vibrations not
simple, but that the larger or general movement has one or more sets of
small vibrations within it. In Fig. 319_a_, A is the curve that would be
traced by the tuning-fork in a state of simple vibration; B and C are
tracing such as are given by a fork in two of its modes of vibration.
The fork gives out its proper or fundamental note in both cases; but the
ear recognizes a difference in the quality of the sound due to the
smaller and more numerous vibrations. Differences of the same kind are
recognized in the notes produced by different musical instruments; but
these are usually more complicated, and their forms are characteristic
of the particular quality of the tone, which is thus shown to be due to
the superposing of several related systems of vibration upon the
fundamental one. Thus three out of the four qualities of sound
recognized by the ear have had their physical causes assigned. As for
the fourth—namely, the distinction we perceive among the different
vowels sung on the same note—it has a physical origin identical with the
last. For since parts of the vocal organs assume different positions in
enunciating the different vowels, they constitute for the time being so
many varied musical instruments, and the graphical traces of the sounds
(for they can be obtained) show a corresponding modification. Here, in
Fig. 319_b_, for example, are represented the tracings of the vibrations
given to the air in various vowel sounds. It is also through the
vibrations conveyed by the air to the little membrane called the drum of
the ear that the sensations of sound are received, and of the nature of
these vibrations a few words must be said presently. In considering the
different qualities of sound, we have so far confined ourselves to
sustained musical notes, as, for instance, the vowel sounds in singing.
This has been done to show the relations of rapidity of vibration to
pitch and for simplicity of illustration of the superposition of
vibrations, etc. Two other remarks must be added—viz., that the
vibrations of musical tones are _isochronous_—that is, whether the note
be loud or soft, the same time is taken up in each vibration
corresponding with the same fundamental note. Other vocal sounds than
sustained vowel notes are found to be due to still more complicated
combinations of vibrations of shorter duration, and _noises_, as
distinguished from musical sound, are formed also by the superposition
of a greater or less number of systems of vibrations, the rapidities of
which are wanting in harmonic relations such as we have pointed out
belong to the musical scale. Even in noises, however, there is often one
or more predominant systems of vibrations which a musical ear can
detect. If the reader will hold a pencil or penholder at one end and tap
with it on the edge of the table, passing in quick succession to parts
progressively nearer where the pencil is held, he will hardly fail to
recognize a rising pitch in the little noises.
[Illustration:
FIG. 319_a_.—_Traces of Vibrations of a Tuning-Fork._
]
[Illustration:
FIG. 319_b_.—_Phonautographic Tracings of Different Vowel Sounds._
]
[Illustration:
FIG. 319_c_.—_Diagram._
]
[Illustration:
FIG. 319_d_.—_Phases of Sound Waves._
]
A word or two must be said as to the way in which sound is transmitted
through the air. This progression is commonly spoken of as a wave
motion, but it must not be thought of as taking place in the form
familiar to us as waves on water; still less must the reader confound it
with the sinuous lines shown in the graphical representations of
vibrations given in the figures. It is rather a series of rapid
pulsations of the particles of the air taking place in the direction in
which the sound is propagated, and resembling waves on water only by
presenting periodical phases in uniform succession. The difference may
be illustrated from what may be seen in a field of wheat when the wind
is blowing over it. The stalks bend down, and rise again when the breeze
has passed, and thus the general appearance of the waves of the sea is
produced. If we confine our attention, however, to the motion of the
several ears of the wheat in a file of stalks, we shall obtain a clearer
notion of what takes place in the so-called waves of sound. The
positions of the stalks at some one instant of time may be represented
by the diagram, Fig. 319_c_. Each stalk is swinging backwards and
forwards like an inverted pendulum, and the successive phases of these
vibrations bring the adjacent ears nearest to each other about _i_, and
farthest apart at _a_ and _a´_. The places of these, and of all the
intermediate degrees of approximation and retreat, pass along the file.
Instead of the ears of wheat swinging on their elastic stalks, suppose
particles of air approaching and receding by virtue of the elasticity by
which they resist compression and recover from it, and you will obtain
an elementary idea of what takes place in the transmission of sound.
Fig. 319_d_ is a picture of a column of air acted upon by a tuning-fork.
The swiftly advancing prong is compressing the air in front of it, and
in swinging back it will tend to leave a vacuum behind it by which the
air is partially rarefied; and these alternate condensations and
rarefactions will travel along through the air by virtue of its
elasticity, and the mechanical action by which they are able to agitate
a stretched membrane (or other elastic body), so that its vibrations
will correspond with them in period and magnitude, may be easily
understood. The vibration produced is a simple one, but any number of
other systems may pass at the same time, and each one will be propagated
as if the rest did not exist, just as we may see different systems of
undulations moving on the surface of water. It should be observed that
the velocity of propagation is the same whatever may be the period or
the magnitude of the vibrations. The high and the low, the loud and the
soft notes of a piece of music played at a distance, all take the same
time in reaching the ear. Light as are the particles of air, the
mechanical actions which a number of them carrying strong vibratory
impulses will produce, may be illustrated by the rattling of window
panes by a loud peal of thunder, and may be bodily felt by a person
standing close to a very large bell while the hour is striking.
We have referred to instruments for registering sound, and even vocal
sounds, before anything has been said of the construction of the
phonograph, and it is, in fact, many years since the problem was solved
of recording the vibrations produced by speech. Mr. Leo Scott, in 1856,
invented an instrument, called the _Phonautograph_, which did this. It
consisted of a cone of sheet zinc like a large ear-trumpet, across the
smaller end of which was stretched a membrane, having attached to it a
very light style, which left a record of the vibrations of the membrane
on a blackened cylinder properly disposed to receive the tracing. When
any sound was produced near the open end of the cone, the impulses
reflected from its internal surface were concentrated on the membrane,
throwing it into corresponding vibrations. Now, this process could be
reversed if the tracing could be made to give back again to the style
its original movements, these transferred to the membrane would throw
the air within the cone into corresponding vibrations, and the sounds
that gave rise to the tracing would be reproduced. Yet Mr. Scott seems
to have suggested no such possibilities for his instrument; but a few
years after the invention of the phonautograph, M. Cros deposited at the
Academy of Sciences in Paris a sealed paper, which was opened after Mr.
Edison had patented the phonograph (1877), and found to contain
suggestions of how this might be done, but describing no experiments in
which any approach had been made towards realizing the conditions laid
down. To Mr. Edison belongs the honour of solving the problem by the
invention of the phonograph, which was patented by him in 1877. The
device which he happily hit upon for converting the phonautograph into a
phonograph was very simple in principle, and consisting merely in
substituting a sheet of tinfoil for the blackened paper in Scott’s
apparatus, the mechanism required for reproducing articulate human
speech was thus found, contrary to all expectations that had previously
been entertained, to be essentially of a remarkably simple character,
for the arrangement of the parts was even more direct than in the
phonautograph itself. This is not derogatory to the merit of the
inventor, for every invention depends upon something previously
attained, and the discovery of suitable materials for the various parts
of the machine, and the many delicate adjustments of their forms and
disposition to secure the required object, demanded the application of
very remarkable experimental skill. The phonograph differs from the
phonautograph by giving up what it has registered in the original form
and material, and thus it is a speaking machine. It is a speaking
machine which _reproduces_ articulate speech, not produces it. Much
ingenuity has been devoted to the construction of speaking machines
which should be capable of _producing_ the sounds of the human voice. By
throwing into vibration the air contained in cavities of certain shapes,
it was long ago found possible to produce sounds closely resembling
those of a voice singing particular vowels, and a real speaking machine
that could articulate words was exhibited in America before the
phonograph had been brought out. It was constructed by Mr. Faber, and
formed a very complicated arrangement, in which all the organs of human
speech were imitated. There were bellows acting like the lungs; a larynx
with various diaphragms, a mouth with movable tongue and lips, and a
tube to resemble the cavity of the nose. The positions and connections
of these parts were determined by levers acted from a key-board, like
that of a piano, and by certain pedals. By moving these in proper order,
the machine pronounced words distinctly enough, but in a strange
drawling tone. So like, however, were the sounds to those of the human
voice, that some accused the exhibitors of imposition, and unjustly
credited ventriloquism instead of mechanism with the results. It will be
observed that it is the function of the phonograph to reproduce, not
produce, human speech, and the mechanical arrangements of the instrument
are simplicity itself compared with Faber’s speaking machine.
[Illustration:
FIG. 319_e_.—_Edison’s Original Phonograph._
]
[Illustration:
FIG. 319_f_.—_Diagrammatic Section of Phonograph._
]
[Illustration:
FIG. 319_g_.—_The Graphophone._
]
Fig. 319_e_ shows the form of the phonograph as designed by Mr. Edison
in 1877. It had a brass cylinder (A) upon which a narrow helical groove
was cut, and was mounted upon an axle (B), having a narrow screw-thread
corresponding with the groove on the cylinder, and working in the
upright (C), so that when the handle was turned the cylinder revolved,
and at the same time advanced in the direction of its axis. A heavy
fly-wheel (D) was attached, in order that the rate of motion might be
nearly uniform. A sheet of tinfoil, or of very thin copper, was wrapped
round the brass cylinder, and on this metallic foil rested the steel
point attached to the vibrating diaphragm, which was mounted in the ring
(F). This point was always adjusted so as to be over the helical groove
in the cylinder, and made to touch the tinfoil with a regulated
pressure. E shows the manner of firmly supporting the diaphragm in such
a manner that it could be readily removed from the cylinder when the
latter had to be covered afresh with tinfoil, or the cylinder adjusted
for reproducing the sounds. The relation of the diaphragm and point to
the tinfoil is shown in Fig. 319_f_, which represents the apparatus in
section. The tracing point (_t_) is not attached directly to the
vibrating diaphragm, but to an adjustable spring (_s_), and interposed
between the spring and the thin metallic diaphragm is a little pad,
formed of a ring of small india-rubber tubing. When the mouthpiece (M)
is spoken into, the sound vibrations reach the diaphragm (_g g_) through
the opening (_o_), and the movements thus communicated to the point
(_t_) which indents the tinfoil to various depths, and with varying
frequency, as the handle is turned, bringing the whole length of the
groove in succession to be operated upon. When the instrument is
required to reproduce the speech so easily recorded, all that is
necessary is to allow the indentations to re-act on the point that made
them. The cylinder is re-adjusted to the tracing point at the end at
which it began, the cylinder is set in motion, and the traces made on
the tinfoil move the point up and down, the vibrating disc (_g_)
following its movements, and thus communicating to the air a system of
impulses which are the counterparts in period, force and succession of
those that originally entered at _o_. It was usual to attach a conical
mouthpiece to the ring (F) in order to concentrate the reproduced
sounds, which might then be heard in all parts of a large room. When, in
reproducing the sounds, the cylinder was turned with the same velocity
as when the words were spoken, the pitch of the voice issuing from the
instrument was the same. If it were turned quicker, the pitch was
raised; if slower, lowered. The words registered on the tinfoil could be
reproduced two or three times, but with decreasing distinctness, as the
tracings gradually become obliterated. However, the sheet could be
removed from the cylinder, and the speech reproduced at any place at any
time afterwards by means of a similar instrument, and a method of
stereotyping was proposed for preserving the records. The original
phonograph was greatly improved when well regulated clockwork was used
for imparting motion instead of the winch. Mr. Edison contrived a
modification of the machine, which made it much easier of manipulation,
by substituting for the cylinder a flat plate on which a spiral groove
was cut. The plate was turned by clockwork, while the vibrating point
was made to follow the groove from the centre to the circumference. The
phonograph in its original form reproduced speech with peculiarities of
its own. The quality was metallic, and reminded one of the intonation of
the street Punch. It will easily be understood that the disc itself must
necessarily have its own systems of vibration, and these will be further
modified by the action of all the other parts. Mr. Edison’s expectations
of the capabilities of the instrument not being realized, he turned his
attention, after several unsuccessful attempts at its improvement, to
the electric light and other subjects, at the same time declaring his
conviction that the perfection of the instrument would be but a matter
of time; in fact, within a very few years afterwards such improvements
were made on Mr. Edison’s instrument as went far to justify this
prophecy. These were the work of Dr. Chichester Bell and Mr. Tainter,
who, after long continued experiments, found in paraffin wax, with a
small admixture of some other substances, a better material for
receiving the impressions. A cutting style made to act upon this cuts
out a fine groove, the bottom of which is not a series of indentations,
but a continuous wavy curve, representing every degree of inflection the
vibrating diaphragm. In the new form of the instrument, FIG. 319_g_,
which was called the _graphophone_, to distinguish it from Edison’s, the
cylinder does not move forward: it is the diaphragm that advances
parallel to the revolving axle. The cylinder is driven by a treadle,
like a sewing-machine, and there is an ingenious arrangement by which
the speed is controlled so that it can be maintained quite uniform. The
movement of cylinder and style can be instantly arrested by touching a
button, and as readily re-started. Quite recently Mr. Edison has
returned to improving the phonograph by using rather thick, solid
cylinders of wax, which are previously prepared for use by the
instrument itself paring them down to a truly cylindrical and perfectly
uniform surface, the result being a great increase of clearness in the
speech and tones of music. Mr. Edison’s new instruments are driven by
small electro-motors, and the speed is regulated by a centrifugal
governor. It is said that these wax cylinders are capable of giving out
the same record for a thousand times without perceptible sign of
deterioration; and when the cylinders are required to receive a fresh
impression, a former one can easily be pared off. The machines can be
arranged so as to sound loud enough to be heard by a large assembly; but
the quality of the tones of speech or music is most perfect when
conveyed from the receiving chamber in front of the diaphragm to one or
both of the auditor’s ears by means of a short elastic tube. Half a
dozen persons can thus hear the record on the cylinder with such
marvellous distinctness as to be able to recognize the tones of a known
voice. The very latest form of the instrument, as it has just left Mr.
Edison’s hands, is represented in Fig. 319_h_. In this one single very
small diaphragm serves both for recording and reproducing the sounds.
This is made of extremely thin glass, to which is attached a small
projection made of celluloid, which acts on a bar that carries the
recording point. The configuration of this point is most ingenious and
peculiar, for it is, in fact, double, one part being shaped like a
gouge, which cuts into the walls of the minute depression traced on the
wax cylinder, while a style-shaped part impresses the wax with punctured
indentations. The shaping of its forms is a difficult and delicate
operation, for they are very small and are cut in sapphire. The
reproduced speech given out by this instrument is said to possess the
properties of sharpness and clearness in a remarkable degree. The
machine is provided also with a sapphire cutting edge, by which an old
record may be pared off by the very motion of the cylinder in receiving
a new one. This phonograph is put in movement by ingenious mechanical
devices, for giving uniform rotation from such motive power as may be
supplied by the foot or by water or by clockwork. This improved
instrument lays claim to practical utility, and its manufacture will, it
is stated, be shortly commenced on a large scale.
Quite recently there has been established in America a big manufactory
of phonographs in the form of a toy, which is sure to become very
popular everywhere. Here they turn out daily several hundred _real
speaking dolls_, which contain clockwork actuating a phonographic
cylinder impressed with the words of some childish story or simple
rhyme, such as “Jack and Jill,” “Mary Had a Little Lamb,” etc. Each doll
of course repeats its little tale as often as the clockwork is set
going. These toys are adapted to all nationalities; for, besides many
English, there are a number of French, German, Italian, etc., girls
employed doing nothing all day long but addressing appropriate words to
each little automaton’s waxen cylinder.
[Illustration:
FIG. 319_h_.—_Edison’s Perfected Phonograph._
]
The capabilities of the phonograph suggest some curious applications
that may be made of it. For example, the songs of a fine singer may
thus, in all their modulations, reach people in distant lands, or be
made audible to future generations. Thousands of people in England have
heard with their ears, through Mr. Edison’s instruments lately brought
over by Col. Gournaud, songs and speeches, and pieces of concerted
music, sung, said, or played in America months before. Music can be
bottled up, so to speak, without the consent of the originators; and,
indeed, it is said that an eminent _prima donna_ has applied for an
injunction to restrain certain _phonographers_ from reproducing her
vocal triumphs with their instruments. A speech of Mr. Gladstone’s,
delivered in England, has been phonographically heard in New York with
great applause. There is no reason but what, with a loud speaking
phonograph uttering an orator’s very words and tones, while
instantaneous photographs of his successive gestures and attitudes are
projected on a screen, a true and lively impression of his eloquence
might be conveyed centuries after his decease. One is almost led to
speculate as to the consequences if these nineteenth century inventions
had been antedated by a few thousand years: what stores of knowledge we
might now possess! and how pleasant it would be thus
To hear each voice we feared to hear no more!
Behold each mighty shade revealed to sight,
The Bactrian, Samian sage, and all who taught the right!
[Illustration:
FIG. 320.—_The Domestic Aquarium._
]
AQUARIA.
Under the date of May 28th, 1665, the curious gossiping diary of Samuel
Pepys contains this entry: “Thence to see my Lady Pen, where my wife and
I were shown a fine rarity; of fishes kept in a glass of water, that
will live so for ever—and finely marked they are, being foreign.” This
doubtless refers to the now well-known gold fishes, which about the time
alluded to were introduced into Europe from China, where they had
probably been for ages reared and kept in captivity, chiefly for the
sake of ornament. Perhaps the reader may be disposed to think that,
therefore, the aquarium cannot be distinctively a nineteenth century
invention, nor at all a modern invention, in principle at least; but
merely the “glass of water,” or the globe of gold fish on a larger
scale. Such a notion would be quite incorrect, for the principles which
are embodied in the modern aquarium were not recognized and applied
until quite recently. Aquatic animals kept for a period in vessels in
which the water is changed from time to time cannot be considered as
properly forming an aquarium. The beauty and value of a well-regulated
aquarium depend not merely on the opportunities it affords of studying
the habits of the animals; the spectacle it presents has a far wider
interest, as illustrating and confirming the conclusions of science
regarding certain great principles which govern the whole animal and
vegetable life of this terraqueous globe. Perhaps in the whole range of
nature nothing is more wonderful than the direct interdependence of
animal and vegetable life, and the exact balance between them, which
preserves the composition of the atmosphere unchanged. The constituents
of the atmosphere have an immediate relation to both forms of life. No
animal can live without a supply of oxygen gas, which it absorbs and
replaces by carbonic acid gas. The latter, on the other hand, is
absorbed by plants, for these, under the influence of light, decompose
the carbonic acid, returning the oxygen to the atmosphere, thus
purifying the air by again fitting it for the respiration of animals.
It might be supposed that animals which live entirely beneath the
surface of water are removed from the influence of atmospheric oxygen,
and that they form exceptions to this law. But such is not the case, for
water absorbs and holds in solution a certain quantity of air, the
oxygen of which is taken up by aquatic animals. In the lower forms of
animals inhabiting water, the absorption of this vital element takes
place at the general surface of the body; but in the more highly
organized creatures there are special organs appropriated to this
purpose, of which the gills of a fish may be cited as a typical example.
The giving out of carbonic acid is an action as universal in the animal
world as the absorption of oxygen, and all aquatic animals tend to
charge the water in which they live with this gas. Fish, or any other
water animals, will soon die if they are placed in water from which all
the air has previously been expelled by boiling, or by placing under the
receiver of an air-pump. In this case the creature dies from want of
oxygen; but it would also die, even if supplied with oxygen, were the
poisonous carbonic acid emitted by itself allowed to accumulate in the
liquid. In nature, this carbonic acid forms the food of aquatic plants
and sea-weeds, and these restore oxygen to the water. If a bunch of
watercresses be placed in a bottle filled with water, and exposed to
strong sunshine, the leaves may soon be seen covered with small bubbles
of gas. This gas may be collected and examined by a suitable arrangement
of the bottle, and it will be found to be pure oxygen.
The merit of having first imitated the plan of nature for the
preservation of aquatic animals appears to belong to Mr. Ward, the
inventor of the “Wardian cases” for ferns and other plants. He, in 1841,
formed in London a fresh-water aquarium, in which, for the first time,
the animals were kept in a healthy condition by the compensating action
of plants. Mr. Gosse, Dr. Price, and others, made experiments with
marine animals and plants, about 1850. Mr. Mitchell, who was then
secretary to the Zoological Society of London, saw about this time a
small aquarium on the balancing principle at Dr. Bowerbank’s, and this
suggested the erection of the fish-house in the Zoological Gardens,
Regent’s Park. This was opened in 1853, being the first public aquarium
ever constructed. The tanks remain at the present time in nearly their
original condition, and this aquarium has been remarkable, not only as
predecessor of the many public aquaria which have since been erected,
but for having given rise to a movement in favour of aquaria as domestic
establishments. The setting-up of household aquaria became almost the
rage of the day, and so many books and magazine articles devoted to the
subject appeared during the ten years following the establishment of the
Regent’s Park aquarium, that the literature of the subject is quite
considerable. Mr. Gosse showed how water for marine aquaria could be
produced by adding to fresh water the solid constituents of sea-water;
and, in the marine aquaria of some inland towns far distant from the
sea, this artificial sea-water is the only kind used. After the
establishment of the Regent’s Park aquarium, public aquaria were opened
successively in Dublin, Galway, Edinburgh, Scarborough, Weymouth, the
Crystal Palace, Brighton, Manchester, and Southport; and on the
continent at Paris, Hamburg, Hanover, Boulogne, Havre, Brussels,
Cologne, Vienna, and Naples; also in North America at San Francisco, and
in other places. The general interest in public aquaria, and especially
marine aquaria on the large scale, seemed to increase as the comparative
failure of the domestic tanks lessened the taste for them. The causes of
the failure so often attending the attempt to maintain aquaria on the
small scale arise partly from the amateur naturalist’s want of exact
knowledge, and the great amount of attention and care required, and
partly from the inherent difficulties of the subject. An aquarium, even
on the largest scale, and with every appliance that science can suggest,
only represents, after all, _a few_ of the conditions which actually
exist in nature; but in small vessels, with a limited quantity of water,
without the continual motion of the liquid, which belongs naturally to
seas and streams, and with circumstances of light and temperature widely
different from those which are obtained in nature, it is not surprising
that the success of domestic aquaria should be but very partial, and
that the taste for them should have declined accordingly.
Many public aquaria proved commercial failures; but we select for
special description two which have been thoroughly efficient, and are
remarkable for size, reputation, and successful management. The
arrangements at these two institutions as regards the aëration and
renewal of the water are, however, quite different. Some plan by which
the same sea-water might be supplied with oxygen, and kept in a clear
and pure condition, was necessary for the very existence of the inland
marine aquarium at the Crystal Palace, whereas the position of Brighton
made the natural sea-water more available. The success of the former
method at the Crystal Palace Aquarium, under the judicious system
adopted by Mr. W. A. Lloyd, the superintendent, perhaps renders this
aquarium one of the most interesting, in a scientific point of view, of
any yet in operation. The water here is never changed by the addition of
sea-water; but fresh water is added as required, simply to supply the
loss by evaporation; and any solid constituents which the animals may
abstract from the water as material for their shells is replaced, so
that the ordinary composition of sea-water is maintained. This is merely
imitating Nature, for the evaporation from the surface of the sea is
compensated by the fall of rain and the influx of rivers, the latter
constantly bringing in the various salts held in solution. The following
particulars regarding the Crystal Palace Aquarium are derived from Mr.
Lloyd’s excellent handbook, which contains not only clear descriptions
of the inhabitants of the tanks, but interesting historical notices and
a well-written disquisition on the principles which should regulate the
construction and management of aquaria.
_THE CRYSTAL PALACE AQUARIUM._
The building was commenced in July, 1870, and was opened in August,
1871. It was designed by Mr. Driver, of Victoria Street, and presents an
admirable simplicity, which entirely accords with the purpose for which
it was erected. The whole available space has been occupied, and nothing
has been wasted on unmeaning or fantastic embellishments. Even the
decorative shams, in which ordinary painters delight, have been
excluded. No part of the walls or of the woodwork is painted to look
like marble, or even to imitate oak. The building, which is about 400
ft. long and 70 ft. broad, is situated at the north end of the Palace,
partially occupying the site of the portion which was so unfortunately
burnt down in 1866. It is but one storey high, and besides a large
reservoir beneath the floor, holding 100,000 gallons of sea-water, there
is a series of sixty tanks, with thick plate-glass fronts, which
collectively contain 20,000 gallons of water. This water, weighing over
1,000,000 lbs., was brought from the coast and conveyed to the Palace by
the Brighton Railway Company at a very moderate rate. For many weeks
after the water was placed in the reservoir and tanks it was very
turbid, from taking up the lime used in their construction and in that
of the rockwork. In this condition it was very alkaline; but the lime
was slowly precipitated by the carbonic acid of the air, the water
became clear, and vegetation appeared in the tanks. The great capacity
of the reservoir facilitates the cleansing of the water; for, supposing
that the water in one of the tanks, holding, say, 6,000 gallons, became
turbid from any cause, the water from this tank could be run off into
the reservoir, where its mixture with the much larger quantity would not
sensibly affect the purity of the mass, from which within half an hour
the tank could again be filled.
All the tanks are constantly receiving water from the clear and cool
reservoir below, in which there are no animals, so that the motion of
the water in the tanks, like that of the ocean, is incessant. The water
issues from the pump at a rate (indicated by a counter) of from 5,000 to
7,000 gallons per hour. The pump is worked by a steam engine of three
horse-power, and the machinery requires the unremitting attention of
three engineers, who succeed each other by turns, each working for eight
hours. Two sets of the machinery—pumps, steam engines, and boilers—are
provided, one being always kept in reserve, ready for use in case of any
accident. Even in winter, when, from the lower temperature, the water
contains the largest amount of oxygen, it is found that the stopping of
the circulation of the water for only a few hours occasions manifest
discomfort to some of the animals. The water is poured into the two
centre tanks in an equally divided stream, and by a simple fall of a few
inches from tank to tank it flows by two routes to the lowest tank, from
which it passes into the reservoir below. This incessant circulation of
the water constantly exposes fresh surfaces to the action of the air, by
which oxygen is absorbed. But besides this, other small streams of water
are made to forcibly enter the tanks from jets, by which a large
quantity of air is carried down in very small bubbles. The removal of
carbonic acid is accomplished by the vegetation which spontaneously
makes its appearance in sea-water under suitable circumstances. It has
been found quite unnecessary to introduce purposely any kind of
sea-weeds, for the spores of low forms of vegetation are always present
in the water, and they develop rapidly under the stimulus of light.
Indeed, one of the difficulties of aquarium management is to avoid this
excessive vegetation by limiting the light as much as possible, and yet
leave sufficient illumination for the observation of the animals. The
amount of light falling upon each tank is very carefully attended to at
the Crystal Palace, and where it cannot be diminished sufficiently to
check the overgrowth of vegetation, without at the same time interfering
with a proper view of the animals, certain molluscs and fishes which
live upon _algæ_ are put into the tanks to consume them. This
spontaneous vegetation is so vigorous that a comparatively small
quantity suffices to remove from the water all the carbonic acid which
it may derive from the animals and decomposing matters.
It should be mentioned that at this aquarium the water is never
filtered, but its clearness is obtained merely by the perfect system of
circulation. The unused food and excrementitious matters are oxygenated
by the air which the water abundantly holds in solution—thanks to the
surface exposed in its constant circulation, the injection of the jets
of water carrying minute bubbles of air into the mass of water, and the
gas given off by the vegetation. The whole process of purification is
therefore chemical, and the success and excellent adaptation of the
system may be judged from the fact that the water seen in masses 9 ft.
deep appears perfectly clear and bright. The building is very cool in
summer: even in extremely hot weather the temperature of the air within
it is never higher than 68° F., and that of the water in the tanks never
exceeds 63°. In winter the temperature of the air is maintained by
hot-water pipes at from 60° to 65°, and the temperature of the water at
about 55°. On winter evenings the aquarium is illuminated with gas, and
the habits of many nocturnal animals can then be conveniently studied.
[Illustration:
FIG. 321.—_The Opelet_ (_Anthea cereus_).
]
“All the animals in this aquarium,” says Mr. Lloyd, “have to be fed
constantly; and as for the sea-anemones—of which there are in the
aquarium over 5,000 individuals—every one of them has a morsel of food
proportioned to its size, and according to the condition of the water,
given it at frequent intervals with a pair of wooden forceps by an
attendant who makes this his sole occupation—as these flower-like
creatures, being so non-locomotive as to be almost absolutely fixed,
cannot pursue their food, or in an aquarium obtain it in any other
manner. They are here deprived of the action of the waves, which in the
actual ocean brings them nutriment, which is arrested by their outspread
and waving tentacles. The food consumed by a few of the animals now
present in the aquarium is vegetable, consisting of green weeds (_Ulva_,
_Porphyra_, _Enteromorpha_, _&c._), but by far the greater number have
animal food given them. This consists of shrimps, alive or dead, crabs,
mussels, oysters, and fish, but they are never fed on butcher’s meat.”
The creatures known as “sea-anemones” are well represented in the
Crystal Palace Aquarium. The observer cannot fail to be struck by their
resemblance to flowers, from the radiated arrangement of their
tentacles, and the beautiful colours they often exhibit. The opelet
(_Anthea cereus_), Fig. 321, is perhaps the most beautiful among British
species, and is a conspicuous denizen of the Aquarium, where its long
green tentacles, tipped with lilac, are commonly seen expanded or
twisting about like so many snakes. These tentacles are stretched out in
search of food, and when by chance an unlucky shrimp or other suitable
prey merely touches a tentacle, it is seized and held with remarkable
pertinacity, the rest of the tentacles closing round it. The mouth of
the creature, placed in the centre of the disc, then expands to an
extraordinary size, and the prey is quickly lodged in the capacious
digestive sac of the _actinia_, where the soft parts are soon dissolved,
and the hard indigestible residue is ejected by the mouth. The tentacles
of _Anthea_, and of other species belonging to the same subdivision of
the animal kingdom, are furnished with an immense multitude of curious
organs, which consist of cells or minute bags, containing coiled up
within them a slender highly elastic filament. When these cells are
compressed, the filament shoots out of its capsule to a surprising
length; and it has been supposed that the adhesive power of the
tentacles depends upon these filiferous capsules; while it is not
improbable that some virulent fluid is also emitted from the cells, for
the victims appear as if paralysed almost as soon as they are seized.
Our knowledge of these animals has been largely extended by the
opportunities of observing their habits which are afforded by marine
aquaria.
[Illustration:
FIG. 322.—_The Viviparous Blenny_ (_Zoarces viviparus_).
]
To obtain the variety of animals requisite for stocking a public
aquarium is by no means an easy matter; for the animals must be good
specimens, in a healthy condition, uninjured by their capture or
transport from the sea. The Crystal Palace Aquarium Company have at
Plymouth a large pond, which communicates with the sea at every tide;
and this, under the superintendence of the company’s resident agent,
serves as a store for animals. Similar arrangements exist at Southend,
Weymouth, Tenby, and other places. The specimens are brought to Sydenham
by fast trains—special facilities being afforded by the railway
companies for this purpose. The mode of carrying the animals depends
upon their nature, and is sometimes a matter of no little difficulty.
All fishes, except perhaps eels and blennies, must be carried in a
sufficient bulk of water; and then the due oxygenation of the water and
the removal of the carbonic acid can be but very imperfectly
accomplished. A considerable mass of water is absolutely necessary in
such cases, and the difficulties and cost of the transit are much
increased by its weight. In warm weather the quantity of oxygen retained
in the water is materially diminished, and under such circumstances the
creatures would soon perish. On the other hand, in very cold weather the
temperature may be so far reduced below that suited to their habits that
death may also result from this cause. Crabs, lobsters, sea-anemones,
sea-urchins, and similar animals can in general be carried without being
immersed in a mass of water. These animals are placed in layers of wet
sea-weed contained in baskets, so that the air has access to the
moisture which covers the bodies of the animals, which is prevented from
drying up by the humidity. As in this case the small quantity of water
exposes a very large surface to the air, oxygen is plentifully supplied.
Mr. Lloyd points out that it is owing to the readiness with which mere
films of water are aërated, that it has been found possible to convey to
Australia the eggs of salmon and trout, and hatch them there. They could
not have been carried in water, but they were successfully conveyed when
surrounded by a cool and very moist atmosphere. This mode of
transmission is much more economical and convenient than the plan of
carrying the creatures in water, and it is therefore resorted to
whenever the organization of the animal permits.
[Illustration:
FIG. 323.—_The Lancelet_ (_Amphioxus lanceolatus_).
]
Specimens of a very remarkable creature are, or lately were, exhibited
at this aquarium in the lancelet (_Amphioxus lanceolatus_), Fig. 323,
which animal itself is a comparatively recent discovery. It is about 2½
in. long, and although it is fish-like in form, it presents so many
points of structure common to lower animals, that it is looked upon by
naturalists as a link between the molluscs and the fishes—being the
lowest of the latter in organization. The creature can hardly be said to
possess a skeleton, the tissues representing that structure are so soft.
It has no definite brain, but it possesses olfactory and optic organs of
a rudimentary kind.
_THE BRIGHTON AQUARIUM._
The Brighton Aquarium, already so well known as a place of popular
resort, is a structure of considerable architectural pretensions, and is
the largest establishment of the kind in existence. The idea of this
undertaking appears to have originated with Mr. E. Birch, the engineer
of the actual structure, who, having, in 1866, visited the aquarium at
Boulogne, perceived that the construction at Brighton of a marine
aquarium on a very extended scale offered every promise of commercial
success. The promoters, in 1868, obtained from Parliament an act
authorizing them to acquire a certain site for the aquarium, but
imposing such limits as to the height of the structure that it was
necessary to place the greater part of the building below the level of
the ground, a matter involving considerable engineering difficulties.
The aquarium is situated close to the Chain Pier and immediately below
the cliff, the building being protected from the waves by a strong
sea-wall of concrete and Portland stone. The building was definitely
opened in August, 1873, while the meeting of the British Association was
being held in the town. Its length is no less than 715 ft., and its
average width 100 ft. The predominant element in the architectural style
of the building is Italian. The following particulars as to the
arrangement and dimensions of the various parts of the building are
derived from the official guide-book:
Entering the gates at the western end, the visitor finds himself at the
top of a flight of granite steps leading to the entrance court, 60 ft.
by 40 ft. The front elevation of the building is 18 ft. high, and
consists of five arches, with terra-cotta columns and enrichments. On
the frieze running round the sides are the appropriate words, “And God
said, Let the waters bring forth abundantly the moving creature that
hath life.” On the northern side of the entrance court is the
restaurant; and on the southern side a series of niches ornamented with
vases. From this outer court, the entrance hall, which is 80 ft. by 45
ft., is approached through three doors. This is furnished with
reading-tables and supplied regularly with periodicals, journals, and
telegrams; while between the pillars supporting the roof are handsome
pedestals, surmounted with large glass vases containing the smaller
interesting marine and fresh-water animals, which would be lost to view
in the larger tanks. In one of the recesses facing the entrance are four
microscopes, in which specimens illustrative of subjects in natural
history connected with the aquarium are constantly exhibited. To the
north of the hall lie the general manager’s offices, the retiring-rooms,
kitchen, &c.; and eastwards, in a direct line with the restaurant, is
the entrance to the western corridor of the aquarium proper. This
corridor, which contains a great many tanks, is the longest of any: it
extends 220 ft., and is broken by a central vestibule, 55 ft. by 45 ft.
The roof, which is groined, is constructed of variegated bricks, and
rests upon columns of Bath stone, polished serpentine marble, and
Aberdeen granite, the carved capitals of the columns having appropriate
marine subjects. On each side are placed the first two series of tanks,
twenty-one in number. These increase in dimensions from 11 ft. by 10 ft.
upwards, the largest measuring over 100 ft. in length by 40 ft. in
width, and holding 110,000 gallons of sea-water. This colossal tank is
the largest in the building, and is devoted to the exhibition of
porpoises, turtles, and other animals of large size. The next largest
tank, 50 ft. by 30 ft., is immediately opposite.
[Illustration:
FIG. 324.—_Sea-Horses_ (_Hippocampus brevirostris_).
]
The eastern end of the western corridor opens upon the conservatory,
which serves as an approach to the rockwork, fernery, and picturesque
cascade, and also to the eastern corridor. Some artificial rockwork,
skirting the north side of the conservatory, is traversed by a stream of
water, broken up at intervals so as to form numerous little bays and
ponds, and utilized for the reception of seals and the larger reptilia.
In the side-space between the conservatory and the second or eastern
corridor are six octagonal table-tanks, of elegant design, for the
exhibition of some of the smaller and more rare marine animals, and, at
the eastern extremity, apparatus which serves to illustrate the hatching
and development of trout and salmon. The entire length of this second
corridor is about 160 ft., one side of the eastern portion, which is 90
ft. by 23 ft., being devoted to the exhibition of fresh-water animals.
At the end of the corridor are situated the curator’s offices and the
naturalists’ room, fitted with open tanks and all necessary appliances;
and the engines, pumps, &c., for supplying the water, and keeping it
constantly aërated.
The system adopted for aërating the water at the Brighton Aquarium is
quite different from that used at the Crystal Palace. In the former the
water is pumped directly from the sea into reservoirs formed under the
floors, and capable of holding 500,000 gallons, which can be filled in
ten hours. From these the water is pumped up into the tanks as required;
but there is no general circulation through the system of tanks and
reservoirs. Each tank is treated independently, and its water is aërated
and kept moving by the injection of air at the lower part, effected by
steam power.
The popularity of the Brighton Aquarium may be judged of from the fact
that the average daily number of visitors is about 9,000, and that on
some occasions nearly twice that number pass the turnstiles. Among the
specialities of the establishment are herring and mackerel, which it has
hitherto been considered impossible to preserve in confinement for any
length of time. They are now thriving well in the Aquarium, although
these fishes are both extremely impatient of confinement. The herring
feed readily upon small shrimps, in catching which they display a
wonderful activity. Fig. 324 shows the curious fish called the
“sea-horse” (_Hippocampus_), from the singular resemblance of the front
part of the body to a horse’s head, or, at least, to that form which
conventionally represents the “knight” among a set of chessmen. The tail
of the creature is prehensile, and enables it to cling to sea-weeds and
other bodies. The sea-horses have thriven well in the Brighton Aquarium,
and also in that at the Crystal Palace. The latest novelties are the
_Proteus_ from the dark caves of Adelsburg, axolotls from Mexico, the
mud-fish (_Lepidosiren annectans_) from the Gambia, and the
telescope-fish from Shanghai. Some of these creatures are of great
interest from the circumstance of their forming the connecting-links
between fishes and reptiles.
[Illustration:
FIG. 325.—_Proteus anguinus._
]
There are, therefore, now on view at the Brighton Aquarium specimens of
three species of animals possessing a high interest for naturalists and
others—not so much because their existence has been discovered in recent
times, as because they are illustrations of the great law of gradation
which exists in nature. Their position in the scale of organization is
so intermediate between reptiles and fishes, that naturalists have not
entirely agreed as to the kingdom to which these ought to be assigned.
Fig. 326 represents _Lepidosiren annectans_, which has gills covered by
flaps, and not exposed as they are in ordinary amphibious animals; and
is provided with four fins, or rudimentary legs, according as the reader
may choose to call them. The creature’s nostrils do not communicate with
the mouth, but are merely two blind sacs, as in fishes. The _Proteus
anguinus_, shown in Fig. 325, is an eel-like creature, only met with in
the subterranean waters of the Grotto of the Maddalena at Adelsburg. It
has four imperfectly developed legs, and gills reduced to mere fringe,
while there are lungs extending nearly the whole length of the abdomen.
The optical organs are entirely undeveloped, being represented merely by
two specks. The axolotl, Fig. 327, inhabits certain Mexican lakes, and
is remarkable for preserving, through the whole period of its life, the
gills for aquatic respiration, which other _amphibia_ possess in the
tadpole stage only.
[Illustration:
FIG. 326.—_The Mud-fish (Lepidosiren annectans)._
]
The mania for domestic aquaria which was at its height some years ago,
and the great popularity of public marine aquaria wherever they are
properly managed, express the real interest which is felt in the varied
forms of animal life, of which the aquarium affords the opportunity of
observing new and unknown phases. The progress of the science which
treats of the organization of the animal kingdom has made rapid strides
during the present century. Among the remarkable truths which have been
acquired is the fact of the unity of the plan which pervades the animal
kingdom. Each kind of animal has much in common with the kind above it,
and with the kind below it: a certain community of organization pervades
the whole, which is knit into one by the gradational forms which may be
observed connecting, like links of a chain which cannot be broken, the
more defined modifications from each other. It is their position in the
scale of organization which, in the eyes of the philosophic naturalist,
gives so much interest to some of the forms of life which have been
figured above.
[Illustration:
FIG. 327.—_The Axolotl._
]
[Illustration:
FIG. 328.—_Sorting, Washing, and Digging at the South African Alluvial
Diamond-Fields._
]
GOLD AND DIAMONDS.
It need hardly be said that gold and diamonds are named under nineteenth
century discoveries in relation to the newly-found fields which have
yielded these highly-prized substances in remarkable abundance.
_GOLD._
This precious metal is met with in nearly all parts of the world, and
its splendid colour, high lustre, the ease with which it may be wrought,
and its property of ever remaining untarnished, have caused it to be
greatly esteemed for ornamental purposes from the earliest historical
ages. No doubt the store set upon gold is derived from its suitability
for decorative uses; and its comparative scarcity enhances the regard in
which it is held. Its use, as a standard of value, is justified by the
general estimation in which it is held, and by the fact that the amount
of labour required to obtain the metal is on the whole tolerably
uniform. It is one of the few metals which are found in nature in the
uncombined state, but its separation from the materials with which it is
associated requires the performance of a certain amount of work, in
whatever form the metal may occur. Its general distribution is another
advantage attending its selection as the standard of value. It occurs in
England and Wales; in Spain, in France, in Hungary, in Piedmont, and in
other parts of Europe; in various localities in Asia; in both divisions
of the New World; in the remaining quarter of the globe, where it was
obtained even in very ancient times, for South-East Africa was probably
the locality to which a naval expedition was despatched by King
Solomon—“they came to Ophir, and fetched from thence gold.” Australia
also has, in the last half of our century, yielded much gold.
Gold is never met with in regular veins, but in primitive or igneous
rocks, or in deposits formed by the disintegration of these. In
Australia the metal is associated with quartz, in slate rocks
geologically equivalent to the Cambrian formations of England and Wales;
and in California it is also chiefly found in material which has been
formed by the wearing down of quartz and granite rocks. Before the
discoveries in California and Australia most of the gold in circulation
was obtained from auriferous iron pyrites. The first finding of gold in
California occurred in September, 1847, when a Mr. Marshall, the
proprietor of a saw-mill on the Sacramento River, observed some
glistening grains among the sand in his mill-race. The news soon spread,
and the inhabitants of the town of San Francisco, then numbering about
two hundred persons, were greatly excited thereby. When it became known
that gold was really to be found, multitudes flocked to California, the
population of San Francisco rapidly increased, and at the present day
the city contains nearly a thousand times as many inhabitants as it did
at the time gold was first discovered. The annual value of the metal
found in California averaged about £23,000,000 for ten years after 1851;
but this subsequently declined to less than half in 1872.
Sir Roderick Murchison, the distinguished geologist, pointed out the
great probability of the existence of gold in Australia many years
before the precious metal was actually found. It has, however, been
stated that gold was met with in Australia so long ago as 1788.
Considering the mode in which the metal occurs, it seems strange that
the emigrants who occupied the auriferous districts as agriculturists
did not long ago discover the riches which Nature had scattered over the
surface of the soil. No doubt, their attention was too much devoted to
their sheep and cattle to notice the glittering particles which might be
seen in the water-courses, and it would probably never enter their minds
that the eagerly desired metal could lie exposed to view on the surface
of the land. But the announcement of the discoveries in California
induced men to look at the soil more attentively, and in April, 1851,
Mr. Hargreaves appears to have found at Bathurst the first gold met with
in Australia. Four months afterwards the metal was also picked up at
Ballarat, Victoria, and the gold-fields so discovered proved even richer
than those of Sydney.
The effect of this discovery on the colony of Victoria proved
marvellous. The population, which in 1851 was 77,000, had in 1867 become
660,000; in the same period the land under cultivation expanded from
57,000 acres to 631,000, and the value of property rose enormously when
the grazier’s estimate of its worth was replaced by that of the miner.
The authorities of the colony from the first regulated the mining
operations by enactments defining the rights of the miners to the
“claims,” as the allotments of land for working upon are termed; and
thus disorder and lawlessness were almost unknown. Fig. 329 will give
the reader a notion of the appearance of a miners’ settlement in the
Australian gold-fields in the earlier period.
[Illustration:
FIG. 329.—_Gold Miners’ Camp._
]
The fundamental rocks in the colony of Victoria belong to the oldest
series of strata. They answer to the Silurian formation which exists in
Cumberland, Wales, and Scotland. Although the strata of the rocks are
much bent, and they have been worn down by the action of water, they are
as a whole but little altered, consisting chiefly of sandstones and
shales. These strata are interpenetrated by innumerable veins of quartz,
which vary in thickness from 1/16 in. to 150 ft. It is in these quartz
veins that the gold is seen in its original matrix. The metal is
sometimes in the form of grains or flakes, or in moss-like threads,
embedded in the quartz; sometimes in the form of well-defined crystals,
sometimes in rough lumps or _nuggets_. Fig. 330 shows three of the
various modes in which the gold is found disseminated through quartz.
Overlying the more ancient rocks with their auriferous quartz veins are
various rocks of different ages; and as these have been in part formed
by the wearing down of the older rocks, they also are in general
auriferous, and contain the gold in detached pieces, varying in size
from particles of fine dust to the huge nugget, containing 2,280 oz., or
nearly £10,000 worth of pure gold, which was found at Dunolly.
The soil, which has been formed by the disintegration of masses of
auriferous quartz, is full of gold, so that a patch of such soil 12 ft.
square has been known to yield 30 oz. of gold by a very rough kind of
washing to the depth of 1 ft. Soil of this kind has been carried down by
rivers and streams ages ago; and the lighter particles having been
carried off by the water, while the gold, from its greater specific
gravity, remained at the bottom of the stream, the sands and gravel of
these river-beds are very rich in gold. In many instances the ancient
water-courses have been entirely covered by igneous rocks, such as
basalt, which have flowed over the land in a molten state. The
gold-miner often finds his reward in burrowing beneath these basalts and
lavas, following the bed of the ancient river, and recovering its
long-buried treasures.
[Illustration:
FIG. 330.—_Gold in Rocks._
]
[Illustration:
FIG. 331.—_“Cradle” for Gold-washing._
]
The methods of carrying on the gold-seeking operations vary according to
the nature of the deposit which is worked and the resources of the
miner. The simplest, which was that most practised in the early days of
the gold-fields, consists in throwing into a tub several shovelsful of
the surface soil, and in pouring in water while the contents of the tub
are stirred about with a spade. The lighter matters are washed away, but
the gold by its great specific gravity remains behind. An improvement in
this, but still a very rude process, is practised by aid of the
_cradle_, Fig. 331, which is merely the trunk of a tree, hollowed out,
and provided with transverse partitions and ribs. The auriferous earth
is thrown into the upper compartment, which is then filled with water.
The cradle is rocked, so that the water may wash away all but the gold
and the heavy stones. Any particles of the former which may be carried
out of the head of the cradle will be stopped by the ribs which cross
the lower part. Machines for puddling by horse-power are now in use, and
other contrivances have superseded the tub and the cradle in
_surface-washing_. The auriferous earth obtained by excavating the soil
from pits is washed in a similar manner, as is also the material reached
by penetrating the deeper tertiary deposits, and by driving adits or
tunnels along the ancient river-beds beneath the layers of basalt.
A mode of washing accumulations of auriferous earths by streams of water
is employed where circumstances are favourable. A long inclined channel
is constructed, and lined with boards; or, when the natural inclination
of the soil requires it, a long trough is constructed and supported on
trestles. The trough is made of sawn boards, 1½ in. thick, in sections
12 ft. long, and it has a width of about 1 ft., the sides being from 8
in. to 2 ft. high. The inclination of the troughs is from 8 in. to 24
in. in 12 ft., and depends upon the abundance of the water: the more
water, the steeper is the slope. The bottoms of the troughs are crossed
by a number of transverse bars, which arrest the auriferous particles in
their descent. The _sluice_, or series of troughs, may be from 50 ft. to
several hundreds or even thousands of feet in length, and the cost from
£100 to £8,000. The earth is thrown in at the upper part of the trough,
and it is gradually washed down, the water being allowed to flow in some
cases by night as well as by day, but commonly in the day-time only, as
the troughs must be watched, to see that they do not become choked up,
and the soil washed out by the overflowing water. The _run_ goes on for
six or ten days, and then the current is stopped for a _cleaning-up_,
which occupies from half a day to a day. For this operation the stream
of water is stopped, and quicksilver is used to dissolve the grains of
gold from the sand, &c., collected by the _riffle-bars_. The quicksilver
is afterwards expelled from the amalgam by heat, and the gold remains as
a porous mass.
Sometimes, instead of shovelling the earth into the troughs, it can be
washed out of its position into suitable channels by means of a powerful
jet of water. This mode of working, which is termed _hydraulic jet
sluicing_, offers great advantages where the natural conditions admit of
its adoption. In this plan, instead of bringing the auriferous earth to
the water, the water is brought to the earth by a flexible pipe, like
the hose of a fire-engine, from a reservoir about 200 ft. higher, and
the stream is directed upon the material by a nozzle. This powerful jet
of water is used to separate and carry away the earth to the head of a
system of channels and troughs, like those already described. The hose
has a diameter of 8 in., but the orifice of the nozzle from which the
water issues is contracted, in order to increase the force of the jet.
The hydraulic jet sluicing requires from three to six men to work it,
and the material of a hill can be carried into the sluices in less time
than a hundred persons could do it by spades. Immense quantities of
earth are removed in this way, and fatal accidents are not infrequent
from the falling masses burying the men who carry the pipe. The force of
the jet of water itself is another source of danger, for broken limbs
and even fatal injuries have often been caused by it. The number of
accidental deaths occurring in hydraulic jet sluicing operations in the
colony of Victoria is reported to average about 60 in a year. Material
which has been worked before often yields a considerable amount of gold
when the operations are repeated; and in localities favourable to the
hydraulic jet system, the work can be carried on with little labour. In
this way three men have been known to extract in one week from dirt
washed for the third time, gold of the value of £330.
The gold which is embedded in quartz and other minerals, as shown in
Fig. 330, is obtained by crushing the material in stamping machines,
which are usually constructed with logs of wood shod with iron. In
another form of crushing-mill two large cast-iron rollers are used
instead of stampers. From the crushed material the particles of gold are
extracted by amalgamation with mercury, which is afterwards removed by
distillation.
The richness of the Victoria gold-fields may be inferred from the fact
that, up to the year 1868, 36,835,692 oz. had been obtained, the value
of which is no less than £147,342,767. The total value of the gold then
annually obtained throughout the whole world is estimated at about 20
millions of pounds sterling. When gold was found so plentifully in
California and Australia, it was supposed by some that its value as a
monetary standard would be affected. This has not happened, although the
prices obtained for the metal by its producers were considerably lower
in the last decade of the century than about 1867. The total annual
output of gold throughout the world is of course variable, and no doubt
there are also variations in the demand; but, so far, the fluctuations
have been relatively small, and there has been no such depreciation by
excess of production as in the case of silver. Yet the increased
production of gold after the discoveries made about the middle of the
century was beyond precedent. It has been estimated that between 1850
and 1875 the total value amounted to £600,000,000, showing an annual
average twelve times greater than that for the period between 1700 and
1850. Between 1875 and 1890 there was a falling off in the supply, the
annual average becoming only £20,000,000 in value. But since the
last-named date there has been a rise year by year, and at the close of
the century the value of the gold produced throughout the world in one
year may not be less than £40,000,000.
As already remarked, the distribution of gold is world-wide; and it has
happened in recent times, that just as one source of supply has shown
signs of failure, other fields have been discovered and have attracted
thousands of eager seekers to new regions. So, when the Californian
supply was falling off, there came the rush to Australia, where easily
worked alluvial deposits or rich veins continued for years to reward the
toil of the gold-finder, though in an ever-lessening degree, until in
1886 or 1887 the centre of attraction was shifted. But at a later period
fresh discoveries in Australia again raised the productiveness of that
quarter; and still more recently, the announcement of the existence of
much auriferous deposit in the valley of the Yukon River (Klondyke), and
in various localities of British Columbia, drew thousands to desolate
and undeveloped districts, in spite of the extremities of hardship and
destitution that might be endured.
The discovery of 1886 takes us to South Africa, a region with which also
our next section is mainly concerned, and the scene of an activity
unprecedented in the annals of gold-mining. From circumstances
immediately connected with our present subject, the close of our century
finds public attention intensely occupied with affairs at the austral
extremity of the “dark continent.” The history of South Africa, from the
time when, in 1486, the tempest-driven Portuguese mariner, Bartholomew
Diaz, first struck its shores at the promontory he named the “Cape of
Storms” (_Cabo Tormentoso_), and when, eleven years afterwards, the
celebrated Vasco de Gama sailed round it on his memorable voyage to
India, is one which, in many respects, presents features of peculiar
interest. It is not our province to enter into details of these, but it
may be stated that the “Cape of Good Hope”—the more auspicious
designation which the King of Portugal substituted for that of Diaz—was,
towards the end of the seventeenth century, colonized by Dutch and some
French settlers, and afterwards Table Bay became a regular port of call
for Dutch, English, and other ships trading to India. The Cape was taken
possession of by the British in 1795, but restored to the Dutch in 1803,
only to be three years after (1806) resumed by England, under whose rule
“Cape Colony” has since remained. The abolition of slavery in all
British dominions, enacted in 1833, was the occasion of great
dissatisfaction to the descendants of the Dutch settlers, who inhabited
isolated farmsteads, their possessions consisting chiefly of great herds
of cattle, tended by slaves. These people, or at least the majority of
them, resolved to quit the confines of British territory, and seek fresh
fields and pastures new in unoccupied regions north of the Orange River,
so that from 1835 to 1838 there was a continued “exodus of the emigrant
farmers.” The story of the following years, with its exciting events and
the vacillating policy of successive British Governments, must be
perused elsewhere: suffice it to say here, that the settlements of the
“emigrant farmers” had by 1854 established themselves into two separate
States, nominally recognising Great Britain as the “paramount power,”
but practically independent of it; for, at the last-named date, the
autonomy of “The Orange River Free State” was acknowledged, and two
years before that another section of the Boers, _i.e._ of the “emigrant
farmers,” who had settled beyond the Vaal River, was absolved from
British allegiance, and, restricted only by a claim of certain suzerain
powers, was constituted into “The South African Republic,” of which the
precise boundaries were at length determined by the “Convention of
London” in 1884. This territory is usually called for shortness the
_Transvaal_, and here in 1854 the existence of gold was first announced;
but the Boer authorities at once prohibited further prospecting,
fearing, and perhaps with reason, that the winning of the precious metal
within their bounds might disturb their pastoral quietude. The Boer
character has been the unique product of a race withdrawn for two
centuries from contact with European and civilized culture, living in
widely separated dwellings with scarcely other associations than cattle
and enslaved blacks. The Boer is described as of a type which draws away
from the enterprising man of modern times towards the primitive
patriarch centred in his flocks and herds: he hates innovations, and
greatly distrusts strangers; he would rather keep, in a box under his
bed, any money he may possess than employ it, or his own energies, in
developing the immense mineral resources of his territory, in which are
included not only gold, but copper, silver, lead, iron, and abundance of
coal.
After some years the prohibition against the exploitation of gold in the
Transvaal was withdrawn, and several localities in the Republic
subsequently became small capitals of gold-mining industry. The most
notable were Leydenburg and Barberton, at which latter place as many as
10,000 gold-seekers were congregated when the discovery of 1886 drew
most of them away. These communities were formed almost entirely by the
influx of people from beyond the Boer boundaries, mainly, of course,
English-speaking people from the Cape Colony, Australia, America, etc.
Their operations were hampered by the Transvaal legislation, and impeded
by the absence of adequate means of communication, which was a
characteristic feature of the Boers’ unprogressiveness; nevertheless,
gold-mining has been pursued in some of these localities ever since,
though with varying fortune. What drew nearly all the gold-seekers of
the Transvaal and of adjoining regions at once to the north side of the
Vaal River was the discovery there of _real gold mines_. This was at a
district within the Transvaal territory, named Witwatersrandt (=
White-waters-ridge), the designation which has been reduced by
abbreviation or affection to “The Randt”—or, _anglice_, the Rand.
It is singular that although the Randt district had been explored by
expert prospectors between 1877 and 1891, the outcrops of the auriferous
reefs entirely escaped their notice. But when the first hint of the
existence of these deposits was bruited abroad, it was the Kimberley men
who were foremost in surveying the spot. By the Kimberley men we mean
those who so soon had by lucky chance lighted in 1870 upon the rich and
apparently inexhaustible diamond mines, as related in our next section.
By the time of the announcement of gold-finds on the north side of the
Vaal River many of these men had become rich—very rich indeed. If they
had been so disposed they might then have returned to their native
countries with enviable fortunes, but it was just as the affairs of
their diamond companies had been settled by consolidation on a
satisfactory basis, and the spirit of discovery and adventure was still
strong upon them, that they resolved personally to explore the new El
Dorado. To a wild desolate region they proceeded, enduring there and on
the track thither the like discomforts they had experienced in their
earlier quest. But they took with them experts provided with all
appliances for ascertaining the prospective value of the alleged
discovery, and, when convinced of its reality, they purchased from the
Boer possessors their land at the price demanded, and it was not long
before they had chemists and engineers at work, having, at the cost of
making their own roads, had brought to the spot the necessary machinery
and appliances. The usual influx of workers, builders, speculators,
etc., followed, and in a wonderfully short space of time a town sprang
up where in 1886 there had been only a single poor farm. The town grew
rapidly to the dimensions of a city inhabited by 150,000 people. Its
name is Johannesburg.
In his book on South Africa, the late Lord Randolph Churchill,
describing Johannesburg in 1891, says that it has much of the appearance
of an English manufacturing town, but without noise, smoke, or dirt.
“The streets are crowded with a busy, bustling, active, keen,
intelligent-looking throng. There are gathered together human beings
from every quarter of the globe, the English possessing an immense
predominance. The buildings and general architecture of the town attain
an excellent standard, style having been consulted and sought after,
stone and bricks the materials, corrugated iron being confined to the
roofs, solidity, permanence, and progress being the general
characteristics.”
The Randt mines having drawn into the South African Republic great
numbers of enterprising workers, who have acquired wealth and built
cities, it would have been expected that they would have been permitted
to acquire the ordinary rights of citizenship. The Boers’ character,
however, has been manifested by their refusal of such rights, and by
their exacting grievous imposts from these _Uitlanders_ (Out-landers, or
strangers), who, being for the most part of English race, are finding
the injustice too hard to be borne, and greatly strained relations
between the Transvaal and Great Britain have again supervened. Indeed,
the situation has become so serious that it is feared actual war may
result, and that is why, in almost the last year of our century, people
are looking anxiously at the position of affairs in South Africa.
The geological conditions of the Randt are these: the upper series of
beds in the _Karoo formation_, which extends over the greater part of
South Africa, consist of quartzose strata, and in the district in
question these are much broken, faulted, and variously inclined. They
are interstratified with beds of sandstone and with the layers of
gold-bearing conglomerate, of which last there are several parallel to
one another and not far separated, ranging in their several thicknesses
from 6 inches to 6 feet, the thickest being known as the main reef.
These reefs form an oval basin,—that is, they dip with varying angle
towards a centre, and crop out at their up-turned edges. Johannesburg is
situated nearly 6,000 feet above the sea-level, on an elevated ridge,
along which for 30 miles east, and nearly the same westwards, the
northern outcrop extends, curving towards the south, while the southern
edge of the basin appears in the Orange Free State, where it has been
traced for a distance of 130 miles. There a shaft, sunk to the great
depth of 2,400 feet, found the main reef with undiminished richness. The
outcrop of the reefs stretches east and west for 130 miles, and the
distance between north and south is 30 miles. From such data it has been
inferred that the reefs contain altogether not less than 450 million
pounds worth of gold. The conglomerate of these reefs consists of
rounded quartz pebbles (which contain no gold), and pieces of sandstone
and of argillaceous material, the whole cemented together into a very
hard mass by iron pyrites. This last is the matrix in which the gold
exists, in the form, for the most part, of minute scarcely visible
crystals. To a depth of from 50 to 150 feet, air and moisture have acted
on the pyritic matter, and the material of the reef becoming in
consequence easily disintegrated, has yielded by mere mechanical
treatment most of its gold, whereas by the same operations on the
underlying hard, tough conglomerate, only about half its gold could be
obtained. Hence, after breaking up the ore, the pyritic matter is sorted
out and transported to the stamp battery, reduced to powder, from which
about five-eighths of the contained gold is removed by quicksilver. The
residue is concentrated by washing in a special machine called the “Frue
vanner,” and the _concentrates_, after roasting in order to oxidize base
metals, are subjected to the action of chlorine gas, by which the gold
is converted into a soluble chloride, from the solution of which it is
precipitated by ferrous sulphate. The _tailings_, _slimes_, and other
residues are further acted on by a solution of potassium cyanide, which
dissolves the minute remaining particles of gold, and from the solution
the metal is obtained by electrolysis. By these supplementary chemical
processes the total of the gold recovered from the ore is raised to 90
per cent. or more of all that chemical analysis shows to exist.
When it is said that the reefs are arranged in a basin-like form, it
must be understood that this applies to their general disposition, for
the regularity of geometrical shape does not belong to geological
basins. There are considerable variations in the inclinations of the
reefs: at some places they are nearly vertical, but generally they dip
towards the centre at various angles, a slope between 25° and 45° being
quite usual; and the inclination becomes less and less the deeper they
go, so that it is presumed that the beds are level towards the centre of
the basin. In the Randt the vertical shaft is rather the exception, the
entrance to the mine usually following the inclination of the reefs, and
the trucks of ore are drawn up sloping rails. From the inclined adits
horizontal galleries are excavated right and left at various depths by
which the main reef is worked, and there are cross cuts by which the
reefs to the north and south may be reached. The most active district of
the Randt is that which extends eastward of Johannesburg, where a long
succession of tall chimneys and winding headgears together with the
other appurtenances are visible. But there is nothing of a picturesque
character about a gold mine, more than is presented by the aspect of an
ordinary colliery.
The importance of the Randt gold-fields does not consist in the actual
richness of the crude material, which indeed in places here and there
cannot be profitably worked,—in mining parlance, it is not “pay ore.” It
is rather the great ascertained extent of these gold-bearing beds and
the general persistence of their character throughout that give to the
Randt its unique character amongst metalliferous workings. This
contrasts with the comparative uncertainty attending the exploitation of
auriferous quartz veins, which occur in detached unconnected patches,
that often end suddenly where least expected. There are in the Randt
nearly one hundred companies working mines, and of these there are many
that pay very handsome dividends on their original capital. A few pay
100 per cent., while a considerable number distribute 25 per cent. and
upwards; so that some of these Gold Companies are amongst the richest
and most influential financial houses in the world. The Randt is second
only to the United States in the quantity it adds annually to the
world’s production.
_DIAMONDS._
In ancient times, and down to a comparatively late period, the only
region from which were derived all the diamonds that found their way to
Europe, was India, where Golconda was long celebrated for the productive
mines in its neighbourhood, and for the high estimation in which fine
specimens of their yield were held. In the seventeenth century these
mines employed 60,000 persons, it is said; and in other districts of
India diamond-seeking has also been carried on from time immemorial. A
gradual decrease in the finds of Indian diamonds has long been observed,
and the supremacy the East had so long enjoyed as the purveyor of gems
was in the earlier part of the eighteenth century transferred to another
hemisphere. In 1727 the diamond was first discovered in Brazil; or
rather, we might say, was then first discerned there. For the
gold-seekers in washing the sands of certain Brazilian rivers had found
numberless specimens which they either threw aside as worthless, or,
seeing them prettier pebbles than the rest, used them as counters in
their card games; their true nature was not recognized, because the
rough diamond has by no means the attractive appearance of the cut and
polished brilliant flashing with refractive radiance. It must have been
these last, and not diamonds in their natural state, that presented
themselves to the imagination of the poet when he penned the line—
Or deep with diamonds in the _flaming_ mine.
The announcement of some diamonds having been found in America had no
effect on the prices in the Indian market, but the exports that soon
after came from Brazil in great abundance quite changed the conditions
of the trade, for in the first fifty years their value was estimated at
no less than £12,000,000 sterling. As already stated, the presence of
diamonds in Brazil was not recognized until 1727, and then by the
accident of one Lobo, an inhabitant of the gold district of Minas
Geräes, who had been in India and had seen rough diamonds there,
observing the resemblance; he took some of the Brazilian stones to
Lisbon, where their identity with the products of the Indian mines was
established. But the European dealers, alarmed lest this discovery
should depreciate the value of their stocks of Indian gems, spread a
report that the so-called diamonds from Brazil were but the refuse of
the Indian mines that had been sent to Brazil. This had the effect of
stopping for a time the sale of the Brazilian diamonds; but the traders
in these were not above taking a hint from their rivals—_fas est et ab
hoste doceri_—for they carried their diamonds to Bengal, and there sold
them as Indian stones at Indian prices. For nearly one hundred and forty
years after this Brazil was by far the most productive diamond region in
the whole world, and especially after 1754, when diamond-seekers
congregated by thousands in the very rich fields of Bahia, a district of
Brazil. Nor have the places above mentioned been by any means the only
localities in Brazil where diamond-finders have been at work; but the
production has decreased and has lost its relative importance by the
South African discoveries that about 1870 caused an entire change in the
diamond industry, and the high prices of the Brazilian gems no longer
capable of being maintained, the fall in value has rendered the workings
less remunerative than formerly. We may now pass over with mere mention,
discoveries of diamondiferous districts in North America, Australia, and
elsewhere.
While rejecting as entirely inapplicable and inexcusable by any stretch
of poetic licence the epithet _flaming_ for the diamond mine, we must
question whether the word _mine_, that as the customary word we have
continued to use, does not convey an equally false notion of the nature
of the workings to which hitherto reference has been made. For these in
most cases are nothing more than holes, very much like gravel pits in
the side of a hill. The diamonds which have so far been in question are
usually found among alluvial sands or gravels, the water-worn fragments
of disintegrated rocks. These are in many cases carried down by rivers,
and the diamonds under such circumstances are very frequently
accompanied by gold; indeed, it is the search for gold that has in many
cases led to their discovery. In the dry season of the year, which
extends from April to October, the lessened currents of certain of the
Brazilian streams are diverted from their course into canals, so as to
leave dry the bed of the stream, and here the mud is dug out to the
depth of six or eight feet or more, and transported near the washing
huts, these operations being continued throughout the dry season. When
this is over the digging is necessarily interrupted by great volumes of
water that fill the rivers and streams, and the diamond-seekers devote
their attention to washing the mud that has been collected. About one
cwt. of this is placed in a long trough, and water is made to flow in,
while the negro labourer stirs up the mass with his hands, until the
water runs off clear, all the particles of mud having been washed away.
The residual gravel is then very carefully examined, stone by stone, and
any diamonds found are handed to the overseer, who watches all
proceedings from an elevated seat. These Brazilian diamonds are mostly
of a small size: occasionally, but very rarely, stones of quite
exceptional value are found, but perhaps not one in 10,000. Formerly
when in the Brazilian fields a negro slave found one of 18 carats, or
more (18 carats = 72 grains), he not only obtained his freedom, but was
rewarded with gifts, and for the finding of smaller stones commensurate
rewards were given. The value of a diamond of the larger sizes depends
upon so many adventitious circumstances that it would not be easy for
any one to state the money’s worth of an 18–carat stone, but,
considering too that the price increases in a more rapid ratio than the
weight, we may to some extent draw an inference from the published
values in 1867 of smaller Brazilian brilliants, perfectly white, pure,
and flawless, when one of 5 carats (20 grains) in weight was priced at
£350. As the rough diamond gives a brilliant of only half its weight, we
may from the above assume an 18–carat stone to be worth in its finished
state at least £1,000.
It may well be asked what are the qualities possessed by the diamond
which have caused it to be so highly valued as an adornment all the
world over; and here it will be proper to invite the reader’s attention
to the chemical as well as to the physical character of the diamond. The
most obvious and attractive quality of the cut brilliant is its
unsurpassable lustre, which is due to its high refractive power. In a
section of our article on light the subject of refraction has been dealt
with, and an explanation given of the _index of refraction_. That of the
diamond is the highest known, being 2·50 to 2·75; other precious stones
have indices ranging from 1·58 to 1·78; those of glass and of quartz are
between 1·50 and 1·57. It follows from the known laws of refraction that
the _limiting_ or critical angle is less for diamond than for other
substances, as, for example, glass, for the posterior surface of a
diamond will totally reflect all the light that falls upon it at any
angle with the normal greater than 24°; glass will totally reflect only
when the incidence is greater than about 42°: hence the diamond reflects
from its farther surface about 64 per cent. of rays that glass similarly
situated would allow to pass outwards without reflection.
Another property in which the diamond excels all other substances is
hardness. It is the hardest substance in nature; for a diamond will
_scratch_ every other, but by none can it be _scratched_, except by
another diamond. Not but that by the application of a file the edges of
a diamond or brilliant may be notched and broken; but this would be
through sheer mechanical force tearing the substance, and would be a
test of _brittleness_, not of _hardness_. These two properties have not
unfrequently been confounded, as when it was foolishly prescribed as a
test for the genuineness of a diamond, that it should be placed on an
anvil and struck with a hammer. No doubt many good and valuable stones
have been sacrificed by this ignorant treatment. The hardness of the
diamond does not prevent its being reducible to powder when so required.
Again, diamonds are sometimes in such a condition of internal strain
that very slight shocks are sufficient to cause them to separate into
fragments. We read of diamonds that are suspected to be in this
condition being packed for transmission within raw potatoes. The extreme
hardness of the diamond secures it from all those accidental abrasions
and injuries to which softer materials are liable, so that it does not
deteriorate by age or use. It is unaffected also by any chemical
substances.
In chemical composition the diamond is pure carbon, one of the most
commonly diffused of the elementary bodies, as it enters into the
constitution of the atmosphere, of all organic bodies, and of a vast
number of mineral substances. Carbon in a less pure form also occurs
naturally as _graphite_, _plumbago_ or _black lead_, and in other
conditions comes into ordinary use as already explained in our article
on Iron. It was only towards the end of the eighteenth century that the
composition of the diamond was demonstrated by the celebrated French
chemist, Lavoisier, who actually burnt a diamond in oxygen gas, and
found the resulting product to be carbonic acid gas, identical with that
obtained by similarly burning a piece of charcoal. Soon afterwards
another French chemist, Clouet, confirmed Lavoisier’s conclusion by
producing _steel_ from pure iron and diamond heated together, an
experiment of much significance when considered in the light of the
remarkable relation between these substances, which is one of the latest
discoveries of our century. It should be observed that Clouet’s result
implies a fusion of the diamond as well as of the iron in the act of
entering into chemical combination.
Like nearly every solid substance of definite chemical composition, this
pure carbon takes the crystalline form. The phenomena of crystallization
are of the highest interest and beauty, for in them we see shapeless
matter fashioning itself into definite and often perfect geometrical
solids, as if it had been wrought by the hand of some mathematical
artist. Every substance forms crystals of some one shape when the
conditions are identical, and one essential condition for any
crystallization is that the particles should be capable of free movement
in arranging themselves, and this condition can occur only when the
substance is in the state of liquid or of gas. Crystals are commonly
deposited from solutions when the solvent evaporates or is cooled down;
or they are formed when a fused substance solidifies. In either case the
crystals are the larger and more perfect as they are allowed the greater
time to form. Now, carbon in any of its conditions has been found to be
absolutely infusible and insoluble, and therefore the origin of the
diamond has long been a puzzle to scientific men, very diverse surmises
having been propounded on this subject. Some have thought it was
separated from carbonic acid by the action of heat, or of electricity;
others, that the carbon had been gasified by subterranean heat; others,
among whom were Newton and the German chemist, Liebig, believed that
heat had nothing to do with it, but that the crystals slowly separated
from vegetable matters (hydro-carbons) in the process of decomposition
under some unknown conditions; others, that the diamond crystallized out
from liquid carbonic acid, holding under pressure some unknown form of
carbon in solution; others, that carbon was ejected by volcanic action
in a fused state; and so on. We hope to show that the problem has at
length been solved, and how.
The shapes of the natural crystals of the diamond must not be confounded
with those of the cut brilliants. The most frequently met with of the
former is the octahedron, or eight-sided figure, such as would result
from two square pyramids joined base to base, the triangles forming the
sides of the pyramids being of such a height that the three pairs of
opposite points are equidistant one from another, so that the octahedron
enclosed in a cube would have an apex in the middle of each surface of
the cube. There are other shapes of diamond crystals, but they are all
related to the cube, that is, they are all obtainable from the cube by
successively slicing off edges and angles. The natural diamond sometimes
has as many as 48 faces formed by such a process. This will easily be
understood by the reader if he will take a cube of common soap and
perform on it these operations _gradually_ with a sharp knife, taking
care always to make the new faces he produces equally inclined to the
adjoining ones. He may begin by cutting off a tiny piece from one corner
of the cube, forming a small equilateral triangle; then let him do the
same at two opposite corners, and again at all the eight corners. Then
he should make the cuts larger and larger, always producing equal sized
equilateral triangles so long as these can be formed. In every case he
will have shaped out such forms as belong to diamond crystals. Instead
of this, he may pare off one or more edges of the cube, or he may in
various ways combine the two operations, and he will probably be
surprised at the variety of forms producible in this manner, all derived
from the original cube and all representing possible forms of natural
diamonds, and indeed those of any substance that crystallizes in the
_cubical system_. A model of the diamond octahedron can be readily made
from the description already given, and the whole series of operations
will constitute an elementary but very instructive lesson in the science
of crystallography.
Diamonds are liable to occur with every imaginable distortion, so as to
be scarcely recognizable by their external form. A very pure smooth
uncut diamond, belonging to the Rajah of Mattam in Borneo, is shaped
exactly like a pear, two inches in length. By the way, battles have been
fought for the possession of this gem, and it is said that,£200,000 was
vainly offered for it. The diamond, notwithstanding its hardness, splits
with comparative ease in certain planes, and by such cleavage (a
property common to all crystals) the octahedral form commonly emerges.
It was not until the middle of the fifteenth century that the art of
cutting the diamond into regular facets was practised, and this can be
done only by the aid of diamond powder, prepared by crushing fragments
and faulty stones in a hard steel mortar. The first operation is to
split the stone by its natural cleavage, and the rough facets so
produced of two diamonds are ground together until they are quite
smooth. The grinding of other facets and the polishing are effected on
horizontal discs of steel making 2,000 revolutions per minute, and
overspread with diamond powder mixed with olive oil.
The external surface of the diamond in its natural state is often very
rough, the stone being always coated with a more or less opaque crust,
so that its translucent interior is concealed or veiled; but when the
reflection from its inner surfaces pierces this veil it glows as if
lighted from within, giving that peculiar appearance which is called its
“fire.” The surfaces of the diamond crystals are very often curved
instead of being flat, and the dodecahedral shape, when this is the
case, takes on an almost globular appearance. Diamonds of all colours
are found, as well as the highly esteemed colourless stones. Yellow ones
of various tints are frequent,—orange, brown, and pink are not very
rare; but red, green, blue, and black are almost unique, at least in a
condition to form large and perfect gems, and are accordingly much
prized. The black diamonds found in Borneo are so hard that ordinary
diamond powder has no effect whatever upon them; they have to be
manipulated with their own dust. The nature of the substances that
impart these colours to the diamond has never been made out; they must
be excessively small in quantity. When a diamond is burnt in air or in
oxygen gas by aid of a large burning glass or otherwise, an extremely
minute quantity of ash remains, and this often retains the shape of the
stone, in the form of a most delicate network; and of the composition of
the ash, this much has been made out: it contains silica and _iron_. We
shall find that the presence of the last named element, although but in
the merest trace, is not without significance.
The purely utilitarian uses of the diamond are few, but of importance.
The most familiar is in the glazier’s tool for cutting glass, and in
connection with this we may mention a fact not generally known, namely,
that though any point of a diamond will _scratch_ glass, it is only by a
natural point of the crystal, and that point of a certain shape, that
glass can be _cut_. Another kind of diamond, valueless as a gem, has
been turned to good account in Major Beaumont’s invention, described in
our section on Rock Drilling Machines, to which the reader is referred.
Minute diamonds are employed for writing on glass, for very fine
engraving, etc.
Having now said sufficient about diamonds in general to give the reader
an interest in the subject, and yet but little more than was needed to
impart the information necessary for following the further development
of the theme, we approach the discoveries in this connection which have
specially distinguished our century. We must transfer the reader’s
attention to South Africa, and if he can refer to any recent map of that
region, particularly to one showing its physical features, it will be of
advantage.
In 1867 some children, playing near the banks of the Orange River, found
what they thought to be merely a pebble prettier than the rest. A
neighbour seeing the stone in the children’s possession, obtained it
from their mother for a trifle. It passed through several hands, and was
bought at last by the Governor of the colony for £500. The discovery
shortly afterwards of other diamonds in the same locality attracted
numbers of persons to the district, and especially to the banks of the
Vaal River, which speedily became the scene of a great search for
diamonds. Though this search was confined to merely the surface of the
soil, it was attended with considerable success, and many fine diamonds
rewarded the diligence of the eager seekers. One of the most remarkable
stones for its great size, which equalled that of a walnut, was
discovered by a Kaffir. When this gem had finally reached the hands of
Messrs. Hunt and Roskell, of London, its value was estimated at no less
than £25,000. News of these discoveries having spread, a rush set in for
the diamond-fields of the Vaal River, and the banks of this stream soon
presented an animated spectacle. Europeans flocked to the spot, London
jewellers sent agents, and the inevitable Jews appeared on the scene to
purchase the precious gems from the lucky finders. It turned out that
many of the larger stones had a slightly yellow tinge, varying in
different specimens from the palest straw to a decided amber colour,
and, as this detracted greatly from their value, no little
disappointment and loss were sometimes experienced when the gems came to
be sold in London and Paris.
One of the first settlements which sprang up on the banks of the Vaal
River was a place called Pniel, of which the reader may form some idea
from Fig. 332, which is copied from a sketch actually taken from the
windows of Jardine’s Hotel. It was then only a little straggling
village, chiefly of wooden sheds or corrugated iron erections, with but
two or three more substantial structures. The diamonds which were found
in this neighbourhood were obtained from gravel which lay on the slopes
of the hills rising from the river. The mode of conducting the search
for diamonds in these gravels was simple enough. The first operation was
the washing of the material, in order to remove sand and dirt, and this
process was usually performed at the margin of the river, where the
gravel was brought down in carts and deposited in a suitable place, at
which a cradle was erected. The cradle was simply a strong wooden
framing sustaining sieves of wirework or perforated metal, placed one
above the other, those at the top having the largest meshes, so that the
lowest would only permit sand or very small pebbles to pass through. The
cradle was capable of receiving a rocking movement, and while the gravel
was thus sorted, water was freely poured on the uppermost layer, so that
the stuff was in a short time thoroughly cleansed and sorted. When this
had been accomplished, the gravel was thrown in successive lots on a
table, at which the digger sat and rapidly examined it for diamonds by
help of a flat piece of wood or iron (see Fig. 328). The larger gems
were readily detected, and indeed could be picked out from among the
pebbles on the sieve before the stuff was thrown on the sorting-table.
Crystals of quartz, which sometimes glisten among the mass, often
excited groundless delight in the bosom of the inexperienced worker.
[Illustration:
FIG. 332.—_Pniel, from Jardine’s Hotel_ (_c._ 1870).
]
On the payment of certain fees, the digger obtained a “claim,”—that is,
he acquired the right of working an assigned portion of the soil. But if
the claim had been left unworked for a week, it might be, in mining
parlance, “jumped”—that is, any person might take possession of it, or
jump into it, on procuring a proper licence.
Since the first rush of diamond-seekers to the river-banks, the stones
were abundantly found elsewhere, namely, at the “dry diggings,” where
the soil, dug out with a pick or shovel, was sifted first through rough
sieves, afterwards through sieves having fine wire meshes The sieve, in
such cases, was often suspended by thongs of hide between two upright
poles, in the manner represented in Fig. 333. The miner was thus enabled
to swing the sieve rapidly about, until the sand and dirt were
separated, when the remaining gravel was emptied on the sorting-table in
the manner before described. As the idea was formerly entertained that
diamonds lie only on or near the surface of the soil, the early miners
seldom penetrated more than a foot or two beneath the surface. But it
was discovered that, so far from it being true that diamonds are present
in superficial deposits only, the finest stones are met with at
considerable depths to which no defined limit can be assigned; thus in
sinking a well large diamonds have been found at 100 ft. below the
surface. When these facts became known, many of the abandoned claims
were worked over again down to a depth of 30 ft. or 40 ft.
[Illustration:
FIG. 333.—_Sifting at the “Dry Diggings”_ (_c._ 1870).
]
The rapid rise of localities under such conditions may be illustrated by
the case of Du Toit’s Pan, which is the centre of a dry-digging
district, and grew in a wonderfully short space of time from nothing to
be a town hiving several large hotels, two churches, several public
billiard-rooms, a hospital, and a theatre. In 1871 the _claims_ at this
place, each 30 ft. square, sold at prices varying from £1 to £50–-the
person who worked a claim paying also a small monthly sum for the
licence. But those who were lucky enough to have obtained the first
possession of the claims at another famous dry-digging locality, named
Colesberg Kopje, at the cost of only the licence at 10_s._ per month,
must have been still more fortunate, and have realized an enormous
percentage on their investments; for, four months afterwards the ruling
prices at the last-mentioned place were £2,000 and £4,000 per claim.
This great increase in value cannot be wondered at, if the accounts
related of the value of the diamonds found here are true. For instance,
it is stated that one individual, who just before the great rush had
bought a claim for £50, found in it diamonds worth £20,000. Colesberg
has become a populous town, with good buildings and regularly laid-out
streets, while a great camp of tents and other temporary structures
still surround it on all sides.
At all the towns above-mentioned newspapers were published, relating
chiefly to matters interesting to the miners—giving, for example, lists
of “finds,” with the names of the lucky finders. It is curious that the
term “diamondiferous” has, in these localities, come to be used as a
general term denoting excellence of any kind. Thus, when it is desired
to apply an epithet of superlative praise to a pickaxe or to a piece of
furniture, this significant adjective is made use of; and a salesman in
the diamond-fields will not hesitate to speak of _diamondiferous_ coats
and trousers!
[Illustration:
FIG. 334.—_The Vaal River, from Spence Kopje_ (_c._ 1870).
]
It will be seen that the early diamond-seekers at the Cape followed very
primitive methods, by simply washing in sieves the gravel and sand
shovelled out of the river banks; and indeed, it was only when, about
1871, they began to dig deeper that their working seems entitled to be
called mining. The “dry-digging” operations began at the since famous Du
Toit’s Pan, by the circumstance of a Boer farmer finding to his great
surprise diamonds sticking in the walls of his house, which had been
built of mud. When the locality of this mud was examined by digging,
more diamonds were found; and when the excavation was continued
downwards, still more. At this place and at four others, all within a
circuit of less than four miles diameter, have been developed the
richest diamond _mines_ in the world, throwing into the shade the
produce of all the river gravel washings; and what is still more
remarkable, showing no signs of exhaustion after nearly thirty years of
working, but rather the contrary. The locality soon presented a scene of
the most active industry, and it was not long before the town sprang up
which has since become celebrated all the world over—Kimberley, the
diamond capital. Kimberley is situated at the northern part of the
British territory known as Cape Colony, not far within its boundary, and
about 14 miles from the Vaal River, in Lat. 28° 43´ S., Long. 24° 46´ E.
It lies in a north-easterly direction from Cape Town, at a distance of
about 550 miles. When the existence of diamonds at the Cape became
known, a great influx of strangers seeking fortune set in to a land that
had failed to offer the attractions to colonists that America and
Australia did. Before the establishment of the overland route opened a
more direct way to India, China, and Australia, Cape Colony owed
whatever importance it had to its position as a provisioning and coaling
station for ships and steamers. As a British settlement it was little
regarded, and its somewhat somnolent condition would have been deepened
by the opening of the Suez Canal in 1869, had not the diamond discovery
in that very same year brought about a great change. But the early
diamond-seekers found their land of promise a wilderness without roads
and without habitations, for the development of civilization did not
then extend far from the coast. It is true, that here and there, at
great distances apart, a few primitive missionary stations might be
found, like that of Pniel shown in one of our cuts, which also represent
the inhospitable aspect of the country. One cannot but admire the pluck
of the adventurers, who, though unversed in their quest, encountered in
its prosecution prolonged toils and many hardships. But they were young
men, and their perseverance gained its reward. They came from all parts:
from Britain, from America, from Australia, from Germany, even from
Russia.
The finding at Du Toit’s Pan, and at contiguous places, of diamonds at
some depth below the surface of the soil, led to geological examinations
of the district, which ultimately resulted in discoveries of the highest
interest and importance, as will now be explained, with first a few
words about the external features of the country.
A traveller directing his steps northward from the sea-shore at almost
any part of the southern coast of Cape Colony will be faced by several
successive ranges of mountains, or what will appear to be such, running
more or less parallel to the coast, and of no great elevation. When he
has reached the summits of these heights he will not find corresponding
declivities on the northern side, but nearly level plains, bounded
northwards by other similar ranges. Supposing him to set out at a point,
say, 150 miles east of Cape Agulhas (the most southern point of Africa),
he will, about 50 miles from the shore, have reached the top of the
third of the great escarpments which rise up like the stages of a
gigantic terrace, and having thus gained the ridge of the Black
Mountains, he will see one of these almost level plains stretching
before him a breadth of 80 miles, for the most part arid and
inhospitable, with a much greater length east and west, and bounded on
the north by a portion of the range of elevations that in an almost
unbroken line runs through Cape Colony to Delagoa Bay nearly parallel
with the coast, at a distance from it between 100 and 150 miles. This
extensive plain is known as the _Groot Karoo_ (Great Karoo),—_karoo_
being the generic name for such plains in South Africa. After crossing
the Great Karoo, our traveller, on mounting the last far-reaching step
of the Brobdignagian staircase, may find himself on the summit of the
Nieuveldt Mountains, at an altitude of nearly 10,000 feet above the
sea-level, attained in several widely separated stages within a distance
of 140 miles. From the summit of these elevations there is no descent by
terraces northwards, but the high tableland or plateau stretches away
for hundreds of miles, descending by only a gentle slope towards the
Orange River, but maintaining an average altitude of nearly 6,000 feet,
and extending far beyond the Orange River towards the Equator. Kimberley
is situated about 50 miles north of the Orange River, and 4,042 feet
above the sea-level.
It was soon observed that the Karoos had common geological characters,
consisting in a certain series of shales, coal, limestones, etc., and
this series naturally came to be called the “Karoo formation,” just as
we in England speak of the Wealden formation, etc.; and it was found
that it extended over a greater part of Central South Africa, covering
an area of at least 200,000 square miles, with an estimated thickness of
5,000 feet. The reader need not imagine that a boring nearly a mile in
depth had to be made for the ascertainment of this last dimension, if he
will remember what has been said in the last paragraph about escarpments
of the rocks looking everywhere towards the coast. There is reason to
believe that these beds were originally the sedimentary deposits of a
vast fresh-water lake, or inland sea, far back in geological times. But
here we need only concern ourselves with the development of the Karoo
beds about Kimberley. There the ground is covered by a sandy soil of a
red colour, for it contains much iron. Below this there is a layer of
decomposed basalt, also containing much iron, its thickness varying from
20 to 90 feet. This lies upon a bed of very combustible shale, with
carbon and iron pyrites, 250 feet thick, which from its great
development here is known as Kimberley shale; then, after a conglomerate
stratum 10 feet thick, is found a very hard compact rock, resembling
hornblende, extending 400 feet downwards, and resting on another hard
rock of quartz, also 400 feet in depth. These beds are _nearly_
horizontal, but dipping a little towards the north. In speaking of them
collectively we may use the local term of the miners and call them “the
reef.”
Now, there are a few certain spots near Kimberley, and two or three
elsewhere, in which the strata forming “the reef” are not found, but
something quite different. These may be compared to large dry wells,
extending vertically downwards to unknown depths, which have been filled
up with matters from below. They are called _pipes_, but they are
uncommonly large ones; for though of a somewhat irregular circular or
oval shape, their diameters range from 200 to 500 feet. Nor must it be
supposed that the enclosing reef presents itself as a smooth wall, as
the name “pipe” might suggest. These _pipes_ are true diamond mines.
They are believed to have been formed by an eruptive action originating
from below at a great depth, and this was not by the escape of red-hot
lava or other molten rocks, but by that of steam or other gases. It is
known that the eruptive forces acted from below, for the edges of some
of the strata are seen in places in the walls of the reef that surround
the pipes to be turned a little _upwards_. It is known that the erupted
matter was not molten lava or rock, for the shale and other strata show
none of those changes of character near the pipes which would have
resulted from igneous action, and for the same reason the gas or steam
that escaped by these pipes could not have been highly heated. It must
therefore have forced its way through the strata by enormous tension or
pressure, and this either at one terrific outburst or possibly by the
gradual enlargement of smaller volcanic chimneys. These blow-holes are
filled with a mixture of subterranean débris, as if mud had been forced
up from below, carrying with it an extraordinary variety of rock
fragments and crystallized minerals. These are embedded in a mass of a
bluish-green colour much resembling indurated clay (but nearly as hard
as ordinary sandstone), and this on long exposure to the weather
crumbles down to a yellow friable substance. More than eighty different
kinds of minerals of the volcanic class have been found in this
_breccia_, as it is termed by geologists, and it is remarked that these
fragments could not have been exposed to any great heat, for their edges
show no signs of fusion. There are also embedded in the agglutinating
substance large masses of the surrounding strata, sometimes having an
area of several thousand square feet, and these are called in miners’
parlance “floating reef.” The cementing material is named “blue ground,”
and the same when crumbled down by exposure is known as “yellow ground.”
These colours are due to oxides of iron, which in the unaltered ground
give the blue-green tint, being lower oxides; but are converted by
absorption of oxygen into yellow and higher oxides. The upper part of
the pipes is filled to a depth of about 70 feet with “yellow ground,”
produced by the penetration of atmospheric influences. Blue ground and
yellow ground alike contain diamonds, and the yield of these is pretty
regular at all depths in the same mine (some have been explored down to
nearly 2,000 feet), although it varies considerably from one mine to
another, and in some the east side is often richer than the west. Thus
in one load (1,600 lbs.) of _ground_ from Du Toit’s Pan, in 1890, the
quantity of diamonds found averaged less than 2 grains (0·5 carat),
while Kimberley yielded 1·25 to 1·5 carats (5 to 6 grains). It is
singular that the stones from mines quite close together are so
distinctly different in character, that the Kimberley merchants can tell
at once the source of any particular parcel. This would indicate that
the blue mud was not forced up the several pipes at one and the same
time, carrying with it diamonds from one birthplace.
The existence of the diamondiferous pipes is pointed out by no
indication on the surface, which is covered nearly uniformly with the
red sandy soil already spoken of; although indeed the site of the
Kimberley mine was marked by a slight elevation, and that of Du Toit’s
Pan by one of the depressions there called _pans_, which, at least in
the wet season, are receptacles for surface water. The Wesselton mine,
which was found only in the last decade of the century, about a mile
from Du Toit’s Pan, also showed a surface depression, and that had been
utilised as a depositing place for dry rubbish. At a later period the
“Leicester mine” was accidentally discovered 40 miles away. At
Jagersfontein, in the Orange River Free State, 60 miles from the
Kimberley mines, is another pipe which yields the finest diamonds of
any, commanding prices nearly the double of those paid for the De Beers
and Kimberley gems, being in fact their nearest commercial rival. The
proprietorship of the Kimberley group having in 1889 become united in
the hands of one company, known as the “De Beers Consolidated Mines,”
this company is able practically to control the diamond market, as it
has sometimes turned out in a year as much as 3 million carats of
diamonds, which sell for about £3,500,000. Up to the end of 1892, 10
tons of diamonds had been derived from these mines, representing a value
of £60,000,000 sterling. In 1895, the De Beers Company sold diamonds to
the amount of £3,105,958, the total expense of working for that year
being £1,704,813,—the net profit was £1,401,145. The effect of
consolidating all the Kimberley diamond interest into the De Beers
Company has been to give an almost complete monopoly to this last, which
has however found it advantageous to restrict its production to an
annual output of about £3,000,000 in value, as the putting of a larger
quantity of diamonds on the market would cause lowering of their price,
and a diminution of the profits all round. The reason is, that though
the world at large annually spends between 4 and 4½ million pounds
sterling in the purchase of diamonds, yet it would not by a reduction in
their price be induced to spend proportionately more. The company are
sufficiently supplied by only two of their mines, the Kimberley and the
De Beers, the expenses of working these being also relatively smaller
than is the case with the others. It may be of interest to compare the
quantities of diamonds that have so far been produced from the world’s
greatest fields, leaving out Borneo, the Ural Mountains, Australia,
etc., as comparatively insignificant. Estimated produce of India, from
the remotest period, 10 million carats; of Brazil (since 1728), 12
million carats; of South Africa, in only 19 years, 57 million carats.
At the time of the discovery of the Kimberley mine (July 1871) it was
divided into about 500 claims, each 31 feet square, and between these
were roadways across; but when the claims were excavated to a depth of
100 feet or more the roadways became unsafe, and, the “blue ground”
underneath them being too tempting always to be left for their support,
they began to fall in, and the mine was often threatened with ruin from
this cause. The state of things became still worse when the unsupported
walls of the “pipe” itself began to collapse, so that by 1878 a quarter
of the claims were buried in the ruins of the reef. These falls
continued, and although very large sums were year after year expended in
removing the fallen reef, the cost amounting in 1882 to 2 million pounds
sterling, it was found at last that very few of the claims could be
regularly worked, and when in 1883 a tremendous fall of 250,000 cubic
yards of reef took place, covering half the area of the mine, it became
necessary to adopt another mode of working, namely, a regular system of
underground mining. Vertical shafts were sunk at a considerable distance
from the pipe itself, and tunnels from these carried through at
different levels, with a system of galleries so arranged that all the
“blue ground” is removable without danger to the miners. The whole mine
is illuminated by electric lights, and the different kinds of labour are
carried on by distinct sets of workmen, some of whom drill holes for the
reception of dynamite cartridges, others shovel the material into
trucks, others again wheel the trucks along tram lines, which converge
to a space where their contents are discharged into skips holding four
truck loads, in which they are hoisted to the surface at about the rate
of 400 loads per hour. This goes on day and night, the miners working in
three shifts of eight hours each. About 8,000 persons are employed,
6,500 of whom are blacks.
Fig. 334_a_ is a sketch section of the Kimberley diamond mine,
approximately to scale, and a glance at this will elucidate the
foregoing description. The thick vertical and horizontal lines show the
positions of the shafts and galleries that have at various times been
excavated, the lowest gallery being connected with a shaft a
considerable distance from the pipe, towards the right, but out of the
range of the sketch. The fringed lines at the top, with dates, give some
idea of the forms of the excavations until the final fall of reef that
determined the resort to subterranean working.
[Illustration:
FIG. 334_a_.—_Sketch Section of the Kimberley Diamond Mine._
]
When the “blue ground” has come to the surface, how are the diamonds to
be extracted from the hard mass? how can a stone of a few grains weight
be found amongst 1,600 lbs. of miscellaneous matter—a thing perhaps not
larger than a peppercorn in four cubic feet of compact material? The
“blue ground” is spread out on levelled and carefully prepared areas
called “depositing floors,” and there, after a few months’ exposure, all
but the very hardest pieces crumble down, the atmospheric action being
accelerated by turning the material over with harrows, and by occasional
waterings. The “blue ground” from the De Beers mine requires at least
six months of this treatment, and it contains a certain proportion of
refractory lumps that would not disintegrate in perhaps less than two
years. These lumps are coarsely crushed between rollers, and the
fragments are spread over slowly moving tables, from which any larger
diamonds are picked off; the fragments left go through smaller crushers,
and are subjected to still greater concentration. The depositing floors
of the De Beers mine are laid out as rectangles, 600 yards long by 200
yards wide, each holding about 50,000 loads. They occupy several square
miles, and as the “blue ground” spread upon them is always one of the
most valuable assets of the company, the quantity of it forms an
important item in the balance-sheets, and the amount that can be
realized from it can be estimated with sufficient closeness, on account
of the nearly uniform distribution of the diamonds. Thus in June 1895,
the 3,360,256 loads then on the floors were put down as equivalent to
nearly 1 million pounds sterling. When the “ground,” thoroughly
weathered, has become yellow and friable, it is transferred to the
washing machinery, by which about 99 per cent, of the original
non-diamondiferous material is removed, and, thus concentrated, the
gravel is together with the mechanically crushed material submitted to
the action of a machine called the _pulsator_, where the gravel is first
assorted into sizes by being turned about within an inclined iron
cylinder perforated with several stages of round holes of diameters
successively of 2, 3, 4 and 6 sixteenths of an inch. The pieces that are
too coarse to pass through the largest holes are taken to the _sorting
house_ direct; but the stones that have passed through the cylinder drop
according to their sizes into four separate sieves called at Kimberley
_jigs_, from the well-known mining term _jigger_, applied to a man who
washes ores in a sieve. The several jigs into which passes the now
assorted gravel have screens with meshes corresponding to the holes in
the cylinder; and by a very ingenious arrangement the concentration is
carried to the point at which the diamonds can be individually picked
out. The “jigs” themselves do not move, but all over the meshes of the
screen is spread a layer of leaden bullets, which prevent a too rapid
passage through the screens, while the material is kept moving in water,
by that liquid _pulsating_ or emerging in quickly succeeding gushes from
below the meshes, and thus carrying off the lighter matters, while those
of greater specific gravity, including the diamonds, work their way
downwards between the bullets and through the meshes, and are received
in boxes which are periodically carried to the sorting house.
When the now much concentrated diamondiferous gravel reaches the sorting
house, the remaining operation consists merely in picking the diamonds
out. But simple as this operation is, it has to be conducted
systematically. In the sorting house are long tables covered with plates
of iron, and placed in a good light. Upon these is thrown the wet
gravel, but not promiscuously; the different sizes being set apart, the
sorter spreads out the heap before him with a flat piece of zinc, picks
out the diamonds and drops them into a small box. Only white men in whom
confidence can be placed are allowed to deal with largest sized
material, for this offers the strongest temptation to purloiners, as in
this of course the most valuable stones are met with. This material,
after the first search, is submitted to the scrutiny of another person,
to see that no diamond has been overlooked; but the smaller assortments
are examined by blacks, who are closely supervised by white men. The
value of the diamonds occasionally sorted out in a single day may reach
£10,000.
At the diamond mines little trust is reposed in the honesty of the
blacks. Below ground and above ground they work under the constant
surveillance of white men, and they live in “compounds” which are
spacious areas—perhaps of 20 acres in extent—enclosed by lofty iron
fences, and containing long rows of corrugated iron erections divided
into rooms, each appropriated to a score of natives. Food, etc., is
supplied from a store at less than ordinary prices, and the company find
fuel and water gratis, and provide a well equipped hospital and medical
attendance. There are swimming baths, and ample recreation grounds for
dancing, etc. The natives of each of the many tribes keep by themselves
apart, and follow their own fancies. They receive good wages, and some
of them save money. They are not allowed out of the “compound” or the
mine, except to work on the depositing floor, which they do under guard.
They accept their restrictions voluntarily, making agreements for a
certain term, three months being the least. Those who leave, as many do
to spend their earnings, often “not wisely but too well,” usually
return. The depositing floors are surrounded by fences 7 feet high,
unscalably and impenetrably armed with barbed wire; and as here robbery
would have the readiest chance, where the largest stones might be met
with, extraordinary precautions are taken, watch and ward being
maintained by day and by night. Not more vigilantly did Cerberus keep
the entrance of Pluto’s domain, nor the wakeful dragon guard the golden
apples of the Hesperides, than the patrols observe the depositing
floors. At night powerful electric searchlights are made to play across
the enclosures, so that unauthorized movements can scarcely escape
detection. Besides these provisions against theft, the laws of the
Colony prohibit any attempt at illicit dealing in diamonds, under a
penalty of two years’ penal servitude.
The maximum penalties for contravention of the Diamond Laws are,
however, much more severe, and that to an extraordinary degree. Thus any
unlicensed dealer is liable to a fine of £1,000, or fifteen years’
imprisonment, or both. And the authorized dealers are required to keep a
most minute record of all their transactions, to send a copy of it every
month to the head of the police, and to produce it when required. It is
needless to say that extraordinary precautions are taken to prevent the
native workmen from secreting diamonds. And any person even finding a
diamond, and neglecting to report the circumstance to the proper quarter
at once, is liable to the pains and penalties above mentioned.
The “blue ground” was at first supposed to be the original home of the
diamond, within which it had somehow taken its shape. But no
satisfactory explanation was forthcoming as to the state of the carbon
before its solidification into the crystalline form. The more general
opinion has been in favour of a volcanic origin due to very high
temperature; and although the “blue ground” itself is clearly not the
ordinary erupted matter of volcanoes due to igneous fusion, the geology
points to the district having been the scene of very active and
extensive volcanic energies at more than one remote period, for the bed
of the Karoo inland sea has been several times covered by level sheets
of molten matter extruded somewhere from below; but not through the
“pipes,” which were blown out ages afterwards. The strata of basalt and
of hornblendic mineral, which extend horizontally over great areas in
the Karoo formation, are of igneous origin, as are also some nearly
vertical dykes of trap rock, about 7 feet wide, that are found
traversing the “blue ground” in certain directions. These intrusive
dykes are of course more recent than the formation of the blue ground,
and that is itself later than the production of the pipes. The fact of
many fragments of crystals being found in the “blue ground” does not
comport with the theory that supposes it to be the matrix; and besides
this, many of the diamonds show scratches, and as these are producible
only by other diamonds, it would appear that they must all have
travelled in company, some part of their journey at least.
Carbon in any form is quite infusible at the highest temperature we have
hitherto been able to produce, although an incipient softening under the
influence of the electric arc has been suspected. Professor Dewar, an
English chemist, basing his data on analogies with other substances, and
on purely theoretical grounds, has calculated that the melting
temperature of carbon is near 3,600° C. (6,512° F.), and that it cannot
remain in a liquid state at a temperature exceeding 5,527° C., when its
vapour would have a tension equivalent to a pressure of 15 tons on the
square inch. So far as these deductions are correct, both the melting
point of carbon and the boiling point of its liquid must lie within the
range of temperature expressed by 3,600° C. and 5,527° C. The most
intense heat we can produce is that developed in the electric arc
discharge, and an eminent French chemist and metallurgist, M. Moissan,
by employing special arrangements and very powerful currents, has thus
been able to obtain in his “electric furnace” a temperature estimated at
3,500° C., which nearly approaches the lower of the above-mentioned
limits, and he has thereby produced many new and unexpected chemical
combinations of refractory elements. Among the most striking of his
results is the formation artificially of real crystalline diamonds. He
found that carbon is freely dissolved by several of the metals in fusion
at the temperature of the electric furnace. When the carbon separated
from the metals, as they cooled and became solid, it was always in the
condition of graphite. The carbons of the electric poles were readily
attacked by molten iron, and it was from the solution of carbon in iron
that Moissan prepared his diamonds. The fact of carbon thus combining
with iron was of course no discovery, as the reader already knows; and
the resulting combination was found, on allowing the metal to cool, to
be simply cast iron, the greater part of the carbon separating out in
the graphitic form. But M. Moissan, having studied the conditions of the
Kimberley mines, and recognizing the probability of the diamonds having
taken their origin at very great depths, where the pressure due to the
weight of superincumbent strata would be immense, was struck with the
idea of _pressure_ being in some way a factor in their formation; and it
occurred to him that the carbon might separate from its liquid condition
in the iron in the crystalline, and not in the graphitic form, if the
solidification could be effected under great pressure. The apparently
insurmountable difficulty of applying an enormous pressure to a small
quantity of molten iron (half a pound) yielded to the experimenter’s
ingenuity. He took advantage of the circumstance that cast iron at the
moment of solidification expands, a property upon which depends its use
for many purposes. If then the fused mass were suddenly cooled on the
outside, we should have a shell of solid iron enclosing a nucleus of
still fluid metal, which, on cooling in its turn, would tend to expand,
and by so doing would exert a great pressure within the shell by which
it was confined. At first Moissan plunged his glowing crucible into cold
water, but a method of more rapidly cooling it was to immerse it in
melted lead. It seems a strange proceeding to cool the crucible by
surrounding it with hot metal, yet the _difference_ of the temperatures
was sufficient to produce the desired effect, the cooling contact of
water not really operating on the intensely heated body, which becomes
separated from the liquid by a coating of _steam_. When the mass of iron
was dissolved off, diamonds of all kinds were found in the residue, and,
though extremely small, some crystals were perfect in shape and colour;
every variety that occurred in the mines being found reproduced in tiny
size. There was also some graphite in the residue. Many more crystals of
“pure water” were obtained by the lead-cooling than by the
water-cooling, as the former process gave some flawless cubes and
octahedra. The largest of the set was only 1/50 inch across, and
although of perfect form when first extracted, within the course of
three months it had spontaneously split up into fragments.
There was evidently no danger of M. Moissan’s manufacture of diamonds
from coke causing consternation at Kimberley; though it would not be
without interest to speculate upon the consequences had the French
_savant_ achieved the greater triumph of turning out carbon crystals in
every respect equal to the productions of nature’s own laboratories.
What a drop there would have been in the shares of the De Beers Mines
Consolidated! What heaviness of heart would have fallen upon those great
ladies who exult in the exclusive possession of priceless tiaras and
precious necklaces flashing with the resplendent gems! From a scientific
point of view, M. Moissan’s fabrication of even those minute crystals,
which so soon spontaneously crumbled into fragments, is a distinct and
valuable success; for, notwithstanding their diminutive size and
instability, they show us that art has so far succeeded in imitating the
processes of nature, that some of her secrets have been revealed. Though
we know the exact chemical composition of all kinds of crystallized
minerals, very very few of these have we been able to imitate
artificially. Nor is this to be wondered at; for nature’s resources are
immense compared with ours: she can command temperatures unlimited by
which to form her solutions or liquefactions; prodigious pressures to
keep them close; and time immeasurable—geological time—in which to let
them cool, and their particles freely coalesce into geometric forms.
Human agency, being obviously unable to reproduce, even on the smallest
scale, such conditions as attended the deposition and slow cooling of
the earth’s crust, may not hope to rival the products of the planet’s
prime. So the fair owners of the earth-born gems may possess their souls
in peace, free from any fear of the chemists’ crucibles; and the
Kimberley Diamond Companies are not likely to suffer panics from the
results of scientific researches, and probably will continue to pay
their handsome dividends for time indefinite.
But curiously enough, a discovery of the latest years of our century has
revealed the existence of diamonds in a region not mapped by the most
advanced of geographers—a region which indeed cannot be defined by
degrees of latitude and longitude. In the recesses of an unquestionable
meteorite—one of those celestial lumps of iron of which mention has been
made in the earlier pages of this volume—real diamonds have been found.
These quite resembled the products of M. Moissan’s experiments, being
extremely small, but including clear and perfectly shaped crystals,
associated with black ones, and also with much graphite in more or less
definite forms. So very limited, however, could be the quantity of
diamonds obtainable from this hitherto unsuspected source, that even if
they rivalled in quality the finest stones from the South African mines,
it might be difficult to form a “Company” for their _exploitation_.
Still, there is the possibility of some one falling in with a little
meteorite containing some mature full-sized carbon crystals, and such a
one might be considered equally fortunate with the finder of the famous
Australian nugget “Welcome” (£25,000). The association of diamonds with
the ferruginous matter of the “blue ground” in the Kimberley pipes,
their crystallization out of iron in M. Moissan’s experiments, and their
presence in iron meteorites, would seem to point to special relations
between the two elements, iron and carbon. Some of these relations are
exemplified in another way by the profound modification effected in the
physical properties of iron, by its combination with a very small
quantity of carbon, as in some kinds of steel; or again, by the
differences between white cast iron and grey cast iron, as determined by
the condition of the carbon in each.
[Illustration:
FIG. 335.—_Portrait of Sir Humphrey Davy._
]
NEW METALS.
The chemistry of the nineteenth century can boast of a series of
discoveries more brilliant and more numerous than ever belonged to any
other science within a like period. And the advantage to the world must
have been great, for chemistry more directly than any other branch of
knowledge ministers to the useful arts and promotes the comfort and
well-being of society. The science itself, as it now exists, is almost
the creation of the present age. But its recent developments cannot be
here discussed; nor, of the immense number of new products with which it
has enriched the world, can more than a very few be brought under the
reader’s notice in the remaining pages of the present work. Among the
most striking of the remarkable series of discoveries by which Sir
Humphrey Davy penetrated the mysteries of matter was the isolation of
the alkali metals—a circumstance which marks an important era in the
history of chemistry. That the alkalies were oxides of unknown metals
had indeed been previously surmised by chemists, from the fact of their
behaving like metallic oxides in neutralizing and combining with acids
to form the class of compounds called _salts_. All attempts to decompose
these alkalies had proved fruitless until Davy separated the metal
_potassium_ from potash, in 1807. When, however, this alkali had once
been proved a compound, more correct ideas were introduced into chemical
science; the nature of other alkalies and earths was explained in like
manner, and new and powerful re-agents were placed in the hands of the
chemist.
Davy first obtained potassium by exposing to the action of the voltaic
current a fragment of potash which had become moist on the surface by
exposure to the air. The battery was formed of the then unprecedented
combination of two hundred pairs of 6–inch plates on Wollaston’s plan,
which was constructed for the Royal Institution of London. The heat
produced by the passage of the current fused the potash, and globules of
metallic potassium were separated at the negative wire. This method
yielded the metal in very small quantities only, and at a great cost.
Gay Lussac and Thenard soon afterwards found that potassium could be
obtained more cheaply and in greater abundance when fused potash was
made to flow over iron-turnings heated to whiteness in a gun-barrel, and
the hydrogen and potassium vapour were passed into a cooled receiver, in
which the latter body was condensed. The metal is now obtained by
heating potassium carbonate with charcoal. For this purpose it suffices
to heat crude tartar in a covered vessel from which air is excluded. The
tartar is first calcined in a crucible until all combustible vapour has
been driven off. The charred mass, which now consists of potassium
carbonate mixed with finely-divided carbon, is then broken into lumps
and quickly introduced into a wrought-iron retort, which is heated in a
furnace to nearly a white heat. A receiver in the form of a flat iron
box, 12 in. long, 5 in. wide, and ¼ in. deep, is adapted to the neck of
the retort, and is kept cooled by the application of a wet cloth on the
outside. The potassium thus obtained is not pure, and it must be
distilled in an iron retort, as otherwise a powerfully detonating
compound is apt to be formed by a portion of the metal combining with
carbonic oxide.
Immediately after his discovery of potassium Davy obtained sodium in the
same manner, and Gay Lussac and Thenard also procured it by the same
process they used for the sister metal. Sodium is now extracted on the
manufacturing scale for use as an agent in the reduction of two other
metals, of which we shall have to speak. A mixture of dried sodium
carbonate, powdered charcoal, and chalk is heated in wrought-iron
cylinders, about 4 ft. long, 5 in. internal diameter, and ½ in. thick.
The chalk takes no part in the chemical action, but is added in order to
give the sodium carbonate when it fuses a pasty consistence, and thus
prevent the separation of the charcoal. A number of these iron cylinders
are set in a reverberatory furnace; but they are coated with fire-clay
and enclosed in earthenware tubes, to prevent their destruction by the
intense heat. To one end of each cylinder a receiver is adapted, of the
form and dimensions already described for potassium. The other extremity
is closed by an iron plug, luted with fire-clay. When the charge in a
cylinder is exhausted, a fresh one is introduced by removing the plug,
taking out the residue, and inserting a new supply of the mixture made
up in a canvas bag. The operation is therefore continuous, and the metal
obtained is nearly pure, as sodium does not exhibit the same tendency as
potassium to form compounds with carbonic oxide.
Potassium and sodium are extremely soft metals; they are lighter than
water, upon which they float, at the same time rapidly decomposing that
compound by displacing half the hydrogen, which is set on fire by the
heat. The instant a piece of potassium touches the surface of water, a
violet flame bursts forth; but with sodium no flame appears unless the
metal is dropped on warm water, or prevented from swimming about. Since
these metals are thus capable of displacing hydrogen from its
combination with oxygen at ordinary temperatures, it follows that they
must have a powerful affinity for oxygen; and, indeed, they can only be
preserved in rock oil, for they rapidly combine with the oxygen of the
air. The great attraction of these metals for oxygen, and for chlorine
and other similar bodies, induces the chemist to employ them for
separating such bodies from their combination with other metals. Sodium
is generally employed for this purpose, as being far cheaper than
potassium.
Among the sixty-nine elementary or undecomposable substances which,
variously combined, constitute the whole material of our planet, so far
as we are acquainted with it, no fewer than fifty-six are metals. Of
these fifty-six metals very few are found in a free or uncombined state,
like the gold described in the last article. On the contrary, the whole
of the metallic elements of the globe, with insignificant exceptions,
exist in nature in a state of combination with one or more of the other
thirteen non-metallic substances. In this condition they form the stony
masses which are termed the ores of the more common metals, and they
constitute also the earths, the metallic bases of which were, until
recent times, unsuspected and unknown. Davy followed up his discovery of
the metals of potash and soda by experimental demonstrations that the
earths _alumina_, _magnesia_, and others, were really oxides of metals;
and when the nature of these substances had once been established,
chemists soon devised means for readily obtaining their metallic bases
in an isolated form. The new metals which have been thus isolated all
deserve the attention of the chemist; and the general reader will
probably also regard with interest the processes by which two of these
new metals, for which practical applications have been found, are
extracted, and the properties which have caused them to be produced on
the commercial scale. These are _aluminium_, the metallic base of common
clay; and _magnesium_, the metallic base of common magnesia, and Epsom
salts, and a constituent of dolomite, or magnesian limestone.
Aluminium was first isolated by Œrsted, in 1827, by decomposing its
chloride by means of potassium. The chlorine leaves the aluminium to
combine with the potassium, and thus the former is set free. Wöhler
effected some improvements in Œrsted’s process, and he first obtained
the metal in malleable globules. It is, however, to Deville that we are
indebted for the invention, in 1854, of a process which admitted of
application on a manufacturing scale. He obtains chloride of aluminium
by mixing alumina (the oxide of the metal) with powdered charcoal made
into a paste with oil, and heating the mixture in a tubular earthenware
retort, like those sometimes used in the manufacture of coal-gas, while
a current of dry chlorine is made to pass through the vessel. The
charcoal combines with the oxygen, forming carbonic oxide, a permanent
invisible gas; and the aluminium unites with the chlorine, giving rise
to aluminium chloride, which, being volatile, sublimes into a chamber
lined with glazed tiles, in which it condenses as a yellow translucent
mass. The metal is reduced from the chloride in the following manner: A
tube of hard glass, about an inch and a half in diameter, is placed over
a furnace, or chaffing-dish, as shown in Fig. 336, where D C is the
tube, and G G an iron pan for containing the red-hot charcoal. Into the
part of this tube marked E, about half a pound of dry aluminium chloride
has previously been introduced, and is kept in its place by plugs of
asbestos. A current of dry hydrogen gas, perfectly free from air, is
passed through the tube; the gas being generated in the vessel, A, and
in B passed over some substance which removes from it all moisture. The
aluminium chloride is then gently heated by placing red-hot charcoal
beneath it, so that any hydrochloric acid it may contain may be
expelled. A long narrow porcelain tray, or “boat,” containing pieces of
sodium, F, is then introduced into the tube; and, the current of
hydrogen being still maintained, heat is applied to the part of the tube
containing the sodium, and the aluminium chloride is made to distil over
by a regulated heat. As it passes over the sodium, it is reduced with a
vivid glow. The aluminium is set free, and collects in the tray with the
double chloride of sodium and aluminium which is produced by the
reaction. The tray is removed and more strongly heated in a porcelain
tube through which a current of hydrogen is passing, and the metal is
thus obtained in globules.
[Illustration:
FIG. 336.
]
Messrs. Bell, of Newcastle, undertook the manufacture of aluminium by a
system founded on this process. The first step is the preparation of
pure alumina, which may be obtained by igniting ammonia alum, or by
precipitating from a solution of alum free from iron, or from sodium
aluminate made from the mineral called _bauxite_. The precipitate of
hydrated alumina, mixed with charcoal and common salt, is made into
balls and dried. These balls, which are about as large as an orange, are
placed in upright earthenware retorts, which are heated to redness,
while a current of dry chlorine is passed through them. The volatile
double chloride of aluminium and sodium distils over, and is condensed
in chambers lined with earthenware. This substance is mixed with
powdered fluor-spar, or with _cryolite_ (itself a compound of
aluminium), which serves as a flux; and small pieces of sodium are
interspersed throughout the mixture. The proportions are ten parts of
the double chloride, five of fluor-spar, and two of sodium. This mixture
is thrown upon the hearth of a reverberatory furnace, and the doors are
shut to exclude air. A very intense action occurs: the chlorine,
quitting the aluminium, seizes on the sodium, and their combination is
attended by an enormous increase of temperature. The fused aluminium is
run off from the furnace together with the slags which are produced by
the operation. In this way, with a furnace having a hearth 16 ft.
square, about 16 lbs. of aluminium can be obtained in one operation.
Rose, the eminent German metallurgist, prefers to obtain aluminium from
cryolite, which is a compound of sodium, aluminium, and fluorine, found
in large quantities in Greenland. It is powdered and mixed with common
salt, and with the mixture a certain quantity of sodium cut into small
pieces is uniformly mingled. The whole is thrown into a heated crucible,
previously lined with a fused mixture of cryolite and salt, and more of
the same mixture is poured upon the contents of the crucible, which is
then covered and exposed to a red heat for two hours. The aluminium
generally collects into buttons, which may be easily melted together by
heating them in a crucible with common salt.
It will be obvious, from the preceding account of the processes of
extracting aluminium, that the cost of the metal must depend upon that
of sodium; and the same remark will apply to the case of magnesium. It
is interesting to observe how the price of the alkaline metals has
decreased as improved processes have been devised, and as the scale of
production has increased with the commercial demand for the article.
Prepared by Gay Lussac and Thenard’s process, these metals were produced
in but small quantities, and were sold at £5 per oz. When the mode of
reducing them by charcoal came into operation, the price fell to 30_s._
per oz.; and the researches of Deville so far improved the processes,
that in 1854 sodium could be procured for 5_s._ per oz. Mr. Gerhard, of
Battersea, subsequently manufactured sodium, so that it can now be
retailed at less than 1_s._ per oz. The price of aluminium before
Deville’s investigations was about 24_s._ per oz., but now the metal can
be purchased at about one-eighth of that cost. [1875.]
Aluminium is a white malleable metal, in colour and hardness not unlike
zinc. Its colour is not so white as that of silver, as it has a marked
bluish tint. It can be rolled into very thin sheets, and by rolling it
becomes harder and more elastic. It can also be drawn into fine wire. It
is remarkably sonorous, and a suspended bar gives out a clear musical
note when struck. Perhaps no property of aluminium more strikes a
person, who examines the metal for the first time, than its lightness.
It is, in fact, only two and a half times as heavy as water, while zinc
is seven times, silver ten and a half times, and gold more than nineteen
times as heavy as water. It retains its lustre in dry or in moist air
for any length of time, and at all ordinary temperatures. It is not
acted upon by nitric or sulphuric acids, but is attacked by hydrochloric
acids and by alkaline solutions with great energy. It has great rigidity
and tenacity, and can be turned, chased, and filed with the greatest
ease, and without clogging the tools. In the Paris Exhibition,[15] M.
Christofle showed spoons and forks and a cup made from it; and it may be
mentioned, as showing the hardness and strength of the metal, that the
cup could be allowed to fall on a stone pavement without being indented.
The metal gives a good impression by casting; and by striking under a
die, some admirable medals have been produced in it. Aluminium has
hitherto been chiefly used for ornamental articles, and for purposes
where lightness and rigidity are desirable, such as in the tubes of
telescopes, opera glasses, beams of balances, &c. Its unalterability and
admirable working qualities have also caused it to be used for cheap
trinkets and ornaments—such as watch-cases, bracelets, combs, seals,
penholders, candlesticks, &c. It is, however, incapable of receiving the
lustrous polish of silver, as it has a decidedly blue tint, so that it
will probably never replace silver for ornamental plate; but it would be
a good material for egg and mustard-spoons, as it is quite unaffected by
the sulphur compounds which so readily tarnish silver. It has been
suggested that if aluminium could be procured cheaply enough, “its
hardness, lightness, and incapability of rusting would render it
admirably adapted for the helmets and cuirasses of the cavalry; it would
make splendid field-guns, as strong as the present ones, and not
one-third of their weight; and, in sheets, it might serve as an
incorrodible roofing, far lighter and more durable than zinc. It would
admirably replace copper, if not silver, for the purpose of coinage. A
crown-piece in aluminium would hardly weigh more than a shilling in
silver, or a piece the size of a penny about as much as a copper
farthing. The same qualities of lightness, hardness, and incorrodibility
also excellently fit it for the beams of delicate balances, and for the
small weights used by the analytical chemist. It would make admirable
utensils for the more delicate operations of cooking—replacing the
copper ones, which render pickles and soups so poisonous. It is
extremely sonorous, and would make capital bells.”
Footnote 15:
Of 1867.
Some difficulty in working the metal has occurred from the want of any
suitable solder. This difficulty has been overcome by electrolytically
coating the metal with copper at the place where it has to be united
with others, and then soldering the copper in the ordinary manner.
Aluminium readily forms alloys with copper, silver, and iron. The alloys
with copper vary in colour from white to golden yellow, according to the
proportion of the metals. Some of these alloys are very hard and possess
excellent working qualities. The alloy of copper with 10 per cent. of
aluminium, which is called _aluminium bronze_, has been manufactured by
Messrs. Bell in considerable quantities. It is made by melting a
quantity of very pure copper in a plumbago crucible, and when the
crucible has been removed from the furnace, the solid aluminium is
dropped in. An extraordinary increase of temperature then occurs: the
whole mass becomes white hot, and unless the crucible be made of a
highly refractory material, it is fused by the heat developed in the
combination of the two metals, although it may have stood the heat
necessary for the fusion of copper.
The qualities of aluminium bronze have been investigated by Lieut.-Col.
Strange, who finds that the alloy possesses a very high degree of
tensile strength, and also great power of resisting compression, its
rigidity, or power of resisting cross strains, is also very great; in
other words, a bar of the alloy, fixed at one end and acted on at the
other by a transverse force tending to bend it, offers great
resistance,—namely, three times as much as gun-metal. An advantage
attending the use of the alloy for many delicate purposes is found in
its small expansibility by heat; it is therefore well adapted for all
finely-graduated instruments. It is very malleable, has excellent
sounding properties, and resists the action of the atmosphere. It works
admirably with cutting tools, turns well in the lathe, and does not clog
the files or other tools. It is readily made into tubes, or wires, or
other desired forms. The elasticity it possesses is very remarkable; for
wires made of it are found to answer better for Foucault’s pendulum
experiment than even those of steel. These admirable qualities would
seem to recommend the alloy for many applications in which it might be
expected to excel other metals. It appears, however, that the demand for
it has not met the expectations of the manufacturers, and the production
has been somewhat diminished of late, although it is used to some extent
for chains, pencil-cases, toothpicks, and other trinkets. When more than
10 per cent. of aluminium is added to the copper, the alloy produced is
weaker; and if the proportion is increased beyond a certain extent, the
bronze becomes so brittle that it may be pulverized in a mortar.
The metal _magnesium_ was first prepared, in 1830, by the French chemist
Bussy, by a process similar to that by which Deville obtained aluminium.
Bussy heated anhydrous magnesium chloride with potassium in a porcelain
crucible; and when the vessel had cooled, and the soluble residue had
been dissolved out by water, the metal was found as a grey powder, which
could be melted into globules. The recognition of the metal as the base
of magnesia is, however, due to Davy. About a quarter of a century after
Bussy’s discovery Deville having shown that sodium could be substituted
for potassium in such reductions, the metal became more cheaply
producible, and soon afterwards Bunsen and Roscoe pointed out its value
as a source of light. Mr. Sonstadt devoted himself to the elaboration of
a method of working Deville’s process on the large scale, and he
succeeded in establishing a company in Manchester for the manufacture.
The process as carried on at the company’s works in Salford is thus
described in the “Mechanics’ Magazine,” 30th August, 1867:
“Lumps of rock magnesia (magnesium carbonate) are placed in large jars,
into which hydrochloric acid in aqueous solution is poured. Chemical
action at once ensues: the chlorine and the magnesium embrace, and the
oxygen and carbon pass off in the form of carbonic acid. The result is
magnesium in combination with chlorine, and the problem now is how to
dissolve this new alliance—to get rid of the chlorine and so obtain the
magnesium. First, the water must be evaporated, which would be easy
enough if not attended with a peculiar danger. To get the magnesium
chloride perfectly dry it is necessary to bring it to a red heat; but
this would result in the metal dropping its novel acquaintance with
chlorine and resuming its ancient union with oxygen. To avert this
re-combination, the magnesium chloride whilst yet in solution is mixed
with sodium chloride (_i.e._, common salt), and thus fortified, the
aggressions of oxygen whilst drying are kept off. The mixture is exposed
in broad open pans over stoves, and when sufficiently dry, the double
salt is scraped together and placed in an iron crucible, in which it is
heated until melted, whereby the last traces of water are driven off. It
is then stowed away until required in air-tight vessels, to prevent
deliquescence. Here comes in that curious metal, sodium, also discovered
by Davy. Five parts of the mixed magnesium and sodium chlorides, mingled
with one part of sodium, are placed in a strong iron crucible with a
closely-fitting lid, which is then screwed down. The crucible is heated
to redness in a furnace, and its contents being fused, the sodium takes
the chlorine from the magnesium. When the crucible has been lifted from
the fire and allowed to cool, the lid is removed and a solid mass is
discovered, which, when tumbled out and broken up, reveals magnesium in
nuggets of various sizes and shapes, bright as silver.”
The crude metal also presents itself in the crucible as small grains,
and even as a black powder. The whole is carefully separated from the
refuse; it is purified by distillation in a current of hydrogen gas; and
it is afterwards melted and cast into ingots. Magnesium is a very light
metal, its specific gravity being only 1·743; that is, it is only one
and three-quarter times heavier than water. When heated in the air it
takes fire, and is rapidly converted into the oxide, magnesia. In the
form of wire or of narrow ribbon, it burns easily in the air, producing
a light of dazzling brilliancy, which among artificial modes of
illumination is rivalled only by the electric light. This is the chief
use at present made of the metal. Lamps have been contrived for burning
the wire in such a manner as to obtain a steady light, the wire being
pushed forward at a regulated rate by clockwork. The magnesium light is
rich in the rays which act upon sensitive photographic plates, and it
has been successfully employed in obtaining photographs of dark
interiors, such as vaults or caverns, and for the exploration of mines
and other dark places. The brilliancy of the firework displays which can
be produced by magnesium far surpasses that obtainable by any other
material used by the pyrotechnist. In such exhibitions balloons are sent
up having burning magnesium attached to them; and the metal in the state
of filings is also mixed with other materials. But magnesium is still a
very costly metal, and while the firework-makers find it too expensive
for common use, they complain that its brilliancy in occasional displays
dulls by contrast the effect of the ordinary fireworks, with which the
spectators are no longer satisfied.
Magnesium wire is not produced by drawing, as the metal is not ductile.
The wire is formed by a method identical with that used in the
fabrication of the leaden rope for making bullets (p. 330); that is to
say, the metal is forced in a heated and softened state through a small
opening in an iron cylinder. The intensity of the magnesium light has
been measured by Bunsen and Roscoe. They say that 72 grains of
magnesium, when properly burnt, evolve as much light as 74 stearine
candles burning for ten hours, and consuming 20 lbs. of stearine. Lamps
in which magnesium may be steadily burnt are made by Mr. F. W. Hart, of
London. In the more elaborate forms of these lamps, there are springs
and wheels for pushing forward the magnesium ribbon, or a strand of
magnesium wire, into the flame of a spirit-lamp; while at the same time
the magnesium wire is made to revolve on its axis, in order to overcome
its tendency to bend down, which would be a great disadvantage when the
light is used for optical apparatus. But for ordinary purposes a much
simpler arrangement suffices: the magnesium ribbon or wire is coiled on
a drum, from which it is drawn off by passing between two little
rollers, which are turned by hand. The wire or ribbon is drawn off the
drum by the rollers, and pushed forward through a guiding tube, which
brings it into the apex of the flame of a spirit-lamp. In this simpler
form of lamp the rate is, of course, directly dependent on the person
who turns the winch of the feeding-rollers; but in the automatic lamp
there are appliances for adjusting the rate; the suitable speed must be
first found by trial, and then the apparatus is to be regulated
accordingly. By means of these lamps photographs can be taken as quickly
as with sunlight, on account of the abundance of chemically-active rays
given out by the burning magnesium. It has been found that an equivalent
of magnesium, in combining with oxygen, liberates a larger amount of
heat than the equivalent quantity of any other metal, not excluding even
potassium. Magnesium forms alloys with several other metals, such as
lead, tin, mercury, gold, silver, platinum. All these alloys are
brittle, and have a granular or crystalline fracture. They are too
readily acted on by air and moisture to be of any service in the arts.
The alloy of 85 parts of tin with 15 of magnesium is hard and brittle;
its colour is lavender, although both constituents are white, or nearly
so; and it decomposes water at ordinary temperatures. Both metallic
magnesium and aluminium furnish useful re-agents to the scientific
chemist. The latter metal, when fused, dissolves boron, silicon, and
titanium, and on cooling deposits these elements in the crystalline
form, this being the only known process for artificially preparing them
in the crystalline state.
Since the above paragraphs were written, the price of sodium has been
further greatly reduced, and it can now (1890) be purchased in bulk at
about 4_s._ per lb. This cheapness has brought the substance into use
for the reduction of other metals and one consequence has been a great
fall in the price of aluminium. At Salindres, in France, the process of
obtaining this metal that has been described on page 587, has been in
use for many years, during which considerable quantities of aluminium
have been produced, the output for 1882 being stated as 5,280 lbs.
Aluminium has lately been prepared by a company at Wallsend-on-Tyne from
_cryolite_, a mineral which is found only in Greenland, but occurs there
in great abundance. Cryolite is a double fluoride of aluminium and
sodium, and the processes for its reduction consist in fusing it with
common salt in a reverberatory furnace, drawing off the mixture into an
iron vessel, and stirring into the fused mass a certain quantity of
sodium. This produces a violent action, on the cessation of which the
slag is poured off, and the metallic aluminium is found as a “button” at
the bottom of the converter. For obtaining a purer metal, the fusion is
made in crucibles, and the sodium is added in two operations without
removing the crucible. The yield of aluminium is about 8 per cent. of
the weight of cryolite, and three parts of sodium are required to
furnish one part of aluminium. Another large manufactory of aluminium is
in operation at Oldbury, near Birmingham. There is a special difficulty
in the metallurgy of aluminium, arising from the fact of the qualities
of the metal being much deteriorated by the presence of a very small
amount of foreign matters such as iron, silicon, &c., at the same time
that no process has been found for purifying the product from these
substances. If the aluminium is to be pure it must be so prepared at the
first. Electrolysis has been proposed as a means of reducing the
compounds, and obtaining the metal free from admixtures. Experiments
seem to show that the dynamo-electric machine may be applied to this
purpose, as well as to the reduction of sodium compounds, when certain
practical difficulties arising from the chemical energies of the
liberated substances have been overcome. What is called the “electric”
furnace has been successfully used in the production of aluminium
bronze. It is a rectangular iron box, 5 feet long, 1 foot deep, and 15
inches wide, with electrodes formed of rods of carbon 30 inches long and
3 inches in diameter. It is charged with a mixture of 25 parts of
corundum (native crystallized oxide of aluminium), 12 parts of carbon,
and 50 parts of granulated copper. This is covered at the top by lumps
of charcoal, and a lid is fastened over the whole. The current from a
powerful dynamo is sent through the carbons, and in about ten minutes
the copper is melted, when the electrodes, at first only a few inches
apart, are moved to an increased distance, and the strength of current
increased. The corundum is reduced, the aluminium alloying itself with
copper, and the oxygen combining with the carbon to form carbon
monoxide, which is driven off. The resulting alloy is cast into ingots,
its percentage of aluminium ascertained, and then it is melted with
enough copper to produce aluminium bronze (page 719). The price of
aluminium, which was as already stated about 3_s._ per ounce in 1875,
has been so much reduced that the metal may now (1890) be purchased for
11_s._ or 12_s._ per lb. We may therefore expect to see wider
applications of its excellent qualities. Though the price per lb. is
still much higher than that of copper-–22 or more times as much—the
metal is so much lighter that a lb. of aluminium occupies nearly 3⅓
times the space of a lb. of copper, so that, taking bulk for bulk,
aluminium is only about seven times as dear as copper. [1890.]
When first introduced by Deville, in 1854, aluminium cost £20 per lb.;
but its prospective value for application in the arts was recognised,
and in two or three years afterwards it was put on the market at 40_s._
per lb. It was then, as already remarked, applied to many purposes where
lightness is desirable, such as for the tubes of telescopes,
opera-glasses, the mounting of photographic lenses, &c. And in 1888,
when the production of sodium had been cheapened and applied to the
separation of aluminium, the price of the latter metal fell to 18_s._
per lb. In the meantime, the cheap electricity of the dynamo caused
attention to be again directed to the original electrolytic method; but
many difficulties in detail had to be overcome in applying this process
on the commercial scale. At length the sodium process was superseded;
and by the beginning of 1890, a Swiss company was producing aluminium at
11_s._ per lb. In the course of the following year they succeeded in
bringing the price down to 2_s._ per lb.; and again three years later,
namely at the beginning of 1894, they could offer the metal at 1_s._
7_d._ per lb. The conditions required for effecting this great reduction
were found in driving the dynamo machinery by water-power, and in an
abundant supply of cryolite at moderate cost. This cheapness of
production at once placed the Swiss company in the position of being the
largest and most successful aluminium manufacturers in the world, so
that in 1892 they had realised a net profit of £21,563, paying their
shareholders 8 per cent., and, further, in 1893, the net profit was half
as much again, and the dividend was increased to 10 per cent. A British
aluminium company has recently been formed in London for acquiring the
rights of working all the processes of the successful Swiss company,
purchasing outstanding English patents, amalgamating with certain
existing companies, and for working the _bauxite_ deposits in Ireland,
&c., &c. There is every reason to believe that an important result of
this enterprise will be a still further reduction in the price of this
metal, and consequently a great extension of its applications. And now
(September, 1895) we have already heard of a further reduction in the
price of this metal, which, at the present time, can be purchased in
bulk for about 1_s._ 6_d._ per lb.
[Illustration:
FIG. 337.—_Portrait of Mr. Thomas Hancock._
]
INDIAN-RUBBER AND GUTTA-PERCHA.
_INDIAN-RUBBER._
Researches into the history of the human race in remote ages have
revealed the fact, that before man knew how to extract metals from their
ores, his only implements were formed of stone; and before he became
acquainted with iron, there was an intermediate period in which the more
easily obtained metal, copper, had to serve as the material for all
tools and weapons. Hence archæologists speak of the stone age, the
bronze age, and the age of iron. If we were obliged to name the
nineteenth century after the material which distinctively serves in it
for the most extensive and varied uses, surely we should call it the Age
of Indian-rubber!
The industrial application of Indian-rubber is entirely modern. The
substance itself appears, however, to have been known to the natives of
Peru from time immemorial, and to have been used for the preparation of
some kind of garments. Although the first specimens were sent to Europe
so long ago as 1736, and the substance was from that time submitted to
many investigations, no other use was found for it up to the year 1820
than to efface from paper the marks made by pencils. From this it
derives the name by which it is commonly known. It has also been called
“gum elastic,” and _caoutchouc_ from the Indian name. Crude caoutchouc
is the product obtained by the spontaneous solidifying of the milky
juice of certain tropical plants—such as the _Hævea elastica_, _Jatropha
elastica_, and the _Siphonia cautshu_. The first grows chiefly in South
America, and in the basin of the Amazon forms immense forests. At a
certain season each year bands of persons, called “_seringarios_,” armed
with hatchets, visit these forests for the purpose of extracting the
caoutchouc. They make incisions into the trunk, and the milky juice
immediately runs out, and drops into a vessel placed to receive it, and
attached to the tree by means of a lump of clay. In about three hours
the juice ceases to flow, and the _seringario_ collects the products of
the incisions in one large vessel. By dipping a board into this vessel,
it becomes covered with the juice; and when this is allowed to dry, the
caoutchouc remains as a thin brownish yellow layer. The caoutchouc is
not dissolved in the juice, but is merely suspended in it; and to hasten
the drying and coagulation of the liquid, the board is warmed over a
smoky fire made with green wood. When alternate immersions and drying
have covered the board with a sufficient thickness of caoutchouc, the
layer is slit open with a knife, and the board is withdrawn. This is the
best kind of crude caoutchouc, because it is free from all admixture of
foreign bodies except the carbon derived from the smoky flame. The
_bottle_ Indian-rubber is moulded on pear-shaped lumps of clay, which
are covered with successive layers of the milky juice; when a sufficient
thickness has been attained, the clay is removed by soaking in water.
Up to 1820, as already mentioned, Indian-rubber was used only for
effacing pencil-marks, and about that time a piece half an inch square
sold for two shillings and sixpence. But the extreme elasticity and
extensibility of this singular substance was attracting the attention of
practical men in England, Scotland, and France. One of the earliest
patents obtained in this country for applications of caoutchouc was
taken out by Mr. Thomas Hancock, of Newington, in 1820. This gentleman
has written an account of the Indian-rubber manufacture from the
commencement, and the book is extremely interesting from the clear and
simple manner in which the inventor describes how he effected one
improvement after another in his processes and machinery. Mr. Hancock
had, previous to his turning his attention to Indian-rubber, no
acquaintance with chemistry; but he was skilled in mechanical
engineering and the use of tools, and this knowledge proved to be
precisely the kind most valuable for dealing with the first stages of
caoutchouc manufacture. His first patent was for the use of
Indian-rubber for the wrists of gloves, for braces, for garters, for
boots and shoes instead of laces, and for other similar purposes. The
rings for the wrists of gloves, &c., were simply cut from the bottle
Indian-rubber by machinery the patentee himself contrived for that
purpose. Mr. Hancock next arranged an apparatus for flattening the raw
Indian-rubber by warmth and pressure, so as to make it available for the
soles of boots, &c. He relates the practical difficulties he had to
encounter in his operations, and the manner in which he overcame them.
He soon noticed and utilized the fact that two clean freshly-cut
surfaces of caoutchouc, when pressed together, cohere and unite
perfectly. This further led him to devising a machine by which all the
waste cuttings and parings might be worked up. This machine consisted of
a cylinder revolving within a cover, both being provided with steel
teeth, by which the pieces of caoutchouc placed between them were torn
into shreds, and then kneaded into a solid coherent mass of homogeneous
texture. The first machine of this kind made by Mr. Hancock would work
up about 1 lb. of Indian-rubber; but now machines on the same principle
are in use operating on more than 200 lbs. of material at once, and
turning it out on a roll 6 ft. long, and 10 in. or 12 in. in diameter.
While Hancock was thus successful in mechanically working Indian-rubber,
Macintosh, of Glasgow, found means of effecting its solution by
coal-naphtha, and he obtained, in 1823, a patent for the application of
his discovery to the fabrication of waterproof garments. Waterproof
cloth, or “Macintosh,” is prepared by varnishing one side of a suitable
fabric with a solution of caoutchouc, or by covering one side of a cloth
with a thin film, and then bringing it into contact with a second piece
similarly prepared—the two caoutchouc layers becoming incorporated when
the double cloth is passed between rollers. Other solvents for
Indian-rubber have been discovered in ether, chloroform, sulphide of
carbon, and rectified turpentine. By treatment with these liquids it
swells up, and eventually dissolves, producing a viscid ropy mass,
which, by evaporation of the solvent, leaves the caoutchouc with all its
original elasticity. By the use of these last-named solvents the
persistent and disagreeable odour occasioned by coal-oil is avoided. Mr.
Hancock relates that when the manufacturers had overcome all obstacles,
and had succeeded in producing thin, light, pliable, and perfectly
waterproof fabrics, they had to encounter another quite unexpected
difficulty—the tailors set their faces against the new material, and
could not be induced to make it up! The manufacturers were, therefore,
obliged themselves to fashion waterproof garments, and retail them to
the public. This, however, turned out to be a benefit, for the seams
were made waterproof, so as to exclude even the little water which would
otherwise pass in by capillary attraction at the stitches.
It will now be observed that there are two distinct modes of working
caoutchouc: by dealing, viz., with the solid material, or with the
solutions. Thus, from a solid disc of caoutchouc long ribbons of the
material may be cut, and these ribbons, by being passed between a set of
circular knives, may be divided into a number of square threads. These
threads may then be drawn out to six or ten times their length; and, if
wound and maintained in this state of tension for forty-eight hours in a
warm place, they will lose their condition of tension, and their
elongated form will become their natural or unstrained one. In this
manner are the Indian-rubber threads prepared, which, covered with silk
or other material, form elastic fabrics such as those used in the sides
of boots. The circumstance of caoutchouc, when heated for some hours at
a temperature a little above the boiling-point of water, retaining
whatever form it has during the heating, is the basis of methods of
obtaining thin sheets and other forms of the material. Tubes are made by
forcing the heated caoutchouc through an annular opening by application
of great pressure; it sets in cooling, preserving a section
corresponding with the orifice through which it issues. In another mode
of forming tubes, a paste composed of caoutchouc, oxide of zinc, and
lime, is formed into sheets, which are cut into strips. The strips are
folded longitudinally, and the edges are cut together at an angle of 45°
with the surface, so that the cut surfaces may meet each other when the
strip is rolled on a mandril to give it a cylindrical form. A slight
pressure suffices to solder together the cut surfaces, and the tube is
then “vulcanized” by a process to be presently described.
The dissolved caoutchouc serves to prepare waterproof garments, round
threads, sheets of Indian-rubber, &c. Fabrics are coated with the
solution by pouring it on the material as it is passing horizontally
from a roller. A straight-edge, under which the charged cloth passes,
distributes the caoutchouc in a uniform layer, the thickness of which is
regulated by the space between the knife-edge and the fabric. When
sulphide of carbon is the solvent used, its evaporation is complete in
about ten minutes, but with other solvents two or three hours are
required. The caoutchouc is usually mixed with lampblack before being
spread on the cloth, and the article is finished by giving the
Indian-rubber layer a coat of gum-lac varnish. Sheets of Indian-rubber
are obtained by spreading fifteen or twenty layers over a cloth, which
is afterwards detached by wetting it with a solvent.
Threads of circular section are manufactured from a paste of caoutchouc,
made by dissolving that substance in sulphide of carbon mixed with 8 per
cent, of alcohol. This paste is placed in a cylinder, out of which it is
forced by a piston through a number of circular holes, whence it issues
in the form of filaments. These are received upon a stretched cloth,
which moves along, carrying the parallel threads, until the sulphide of
carbon has evaporated.
A modification of caoutchouc, possessing very valuable qualities for
many purposes, was discovered by Mr. Charles Goodyear, and largely
applied by him in the United States to the fabrication of waterproof
boots. In 1842 these boots were imported into Europe, and it was seen
that this form of the material had the advantages of not sticking to
other bodies at any ordinary temperatures, and of preserving its
elasticity even in the coldest weather, whereas ordinary Indian-rubber
becomes rigid by cold. The cut edges of this variety of caoutchouc do
not cohere by pressure. Mr. Goodyear attempted to keep his process a
secret; but Mr. Hancock, having soon detected the presence of sulphur in
the American preparations, set to work to discover how that substance
was made to combine with the caoutchouc. He succeeded, and he obtained a
patent for sulphurizing Indian-rubber before the original inventor had
applied for one. Mr. Hancock found that a sheet of caoutchouc immersed
in melted sulphur at 250° F., gradually absorbed from 12 to 15 per cent,
of its weight of sulphur; and, further, that this does not in any way
alter its properties. When, however, the sulphurated substance was for a
few minutes exposed to a temperature of 300°, it acquired new qualities,
which were precisely those of the modification employed by Mr. Goodyear
for his impervious boots. This transformation effected by sulphur Mr.
Hancock called _vulcanization_; and _vulcanized Indian-rubber_ is now
employed in nearly all the innumerable applications of caoutchouc,
provided the presence of sulphur is not absolutely objectionable.
Goodyear’s process consists in mixing the sulphur with the caoutchouc,
the suitable proportion (7 to 10 per cent.) having been determined
beforehand, and the sulphur ground up with the Indian-rubber in the
masticating machine, or disseminated through the viscid liquid if a
solution is used, or dissolved in the solvent employed. This gives
better results than Hancock’s process, because the sulphurization is
more uniform, and this method is therefore more largely employed. When
the various articles have been fabricated in the ordinary manner from
the mixture of caoutchouc and sulphur, they are enclosed in vessels,
where they are submitted for two or three hours to the action of steam
under a pressure of nearly 4 atmospheres, so that the steam may have a
temperature of about 280° F. A still easier method, due to Mr. Parkes,
consists in steeping the articles (which in this case should be thin) in
a solution of one part of chloride of sulphur in sixty of bisulphide of
carbon. The object becomes vulcanized by simple exposure to the air,
without the aid of heat. But this process is said to be liable to cause
the article afterwards to become brittle. The addition of oxide of zinc,
carbonate of lead, and other substances, is found to yield a product
better adapted for certain purposes than one in which only sulphur is
used.
The list of applications of vulcanized Indian-rubber would be a very
long one; but as a great number of these applications must be known to
everybody, it will be unnecessary to specify them. It has lately been
used for carriage-springs, for the tires of wheels, and for the rollers
of mangles. Its employment in the construction of portable boats,
pontoons, life-buoys, dresses for divers and for the preservation of
life at sea, air-tight bags and cushions, air and water beds, cushions
of billiard-tables, are a few of the thousand instances of its utility
which might be quoted.
When the proportion of sulphur mixed with the caoutchouc is increased to
25 or 35 per cent., another product having qualities entirely different
from those of vulcanized Indian-rubber is obtained when the mixture is
heated. This is the jet-black substance termed _ebonite_ or _vulcanite_,
which is made into such articles as combs, paper-knives, buttons, canes,
portions of ornamental furniture, and plates of electrical machines. It
is in many cases an excellent substitute for horn and for whalebone,
while for insulating supports, &c., in electric apparatus, it is
unrivalled. It has a full black colour and takes a bright polish; and it
may be cut, or filed, or moulded. It is very tough, hard, and durable.
In the transformation of Indian-rubber into vulcanite, the temperature
must be somewhat higher than that required for the production of the
vulcanized Indian-rubber. The caoutchouc used is very carefully purified
before it is incorporated with the sulphur; and the yellow paste formed
by the mixture is subjected to the contact of steam at a temperature of
about 310°.
_GUTTA-PERCHA._
Gutta-percha is a substance very like Indian-rubber in its chemical
properties, having the same composition, although in outward appearance
very different. It was first sent to Europe in 1822, but did not become
an article of commerce until 1844. It is the solidified juice of a tree
(_Isonandra percha_) which abounds in Borneo and Malacca. The trunk of
the tree grows to a diameter of 6 ft., but as timber it is valueless.
When an incision is made through the bark and into the wood, a milky
juice flows out, which speedily solidifies. Gutta-percha is a very tough
substance, but is without the elasticity of Indian-rubber. It differs
from the latter, too, in becoming softened by a gentle heat, and it will
then readily take and retain any impressions with great sharpness and
fidelity. Thus beautiful mouldings and other ornamental objects are
easily made. It also has the valuable quality of welding when softened
by heat. It is a non-conductor of electricity, and it is largely used
for covering telegraph-wires, and especially for forming an insulating
coating in submarine cables. It seems to have become known precisely at
the time it was required for this purpose, and the success of ocean
telegraphy is largely owing to its valuable properties. It is employed
as a substitute for leather in soling shoes and boots, and in forming
straps and bands for driving machinery; also in the preparation of tubes
used for conveying liquids, and for speaking-tubes. Dilute mineral acids
have no action upon it, and hence it is especially valuable for making
bottles to contain hydrofluoric acid, which attacks glass. A drawback to
the use of gutta-percha is its tendency to become oxidized when exposed
to light and air, by which it entirely loses its power of becoming
plastic by heat, and is converted into a brittle substance. But in the
dark, or under water, it may, however, be preserved for an indefinite
period without change.
Mr. Charles Hancock, in 1847, patented a machine for cutting the
gutta-percha into slices. In this machine there is a circular iron
plate, with three radial slots, in which knives are fixed somewhat in
the manner of the cutting tool of a spokeshave. The lumps of
gutta-percha drop against these knives as the plate is driven round, and
the material is cut into slices, which have a thickness determined by
the projection which has been given to the blades. Sometimes an upright
chopper is used, with straight or curved blades. These slices are
immersed in hot water, until they are softened, and they are then
subjected to the action of rollers armed with toothed blades, called
“breakers,” and also to the action of the mincing cylinder, which is
furnished with radiating blades, and revolves partly immersed in the
water. The material is carried out of the hot water to these machines by
endless webs mounted on rollers. The breakers and mincing cylinders make
about 800 revolutions per minute. The gutta-percha, thus reduced to
fragments, is carried forward again by endless webs into cold water,
where it is thoroughly washed and separated from the impurities, which
fall to the bottom, while the lighter gutta-percha floats on the surface
of the water.
Gutta-percha, like caoutchouc, can be combined with sulphur. The best
product is obtained when a small proportion of sulphur is used along
with some metallic sulphide. Mr. C. Hancock uses 48 parts of
gutta-percha, 1 of sulphur, and 6 of antimony sulphide. These
ingredients are thoroughly mixed and put into a boiler, where they are
heated under pressure for an hour or two. Another method of treating
gutta-percha was also devised by Mr. C. Hancock, who found that when
this strange substance was exposed to nitric oxide gas (which is given
off when nitric acid acts on copper) it became quite smooth, and
acquired an almost metallic lustre, losing also all its stickiness.
Another modification is formed by treating gutta-percha with chloride of
zinc; and yet another by the action of a solvent, such as turpentine, a
sulphide, sulphur, and carbonate of ammonia, employed simultaneously.
Mr. Hancock mixes all these materials together in a “masticator,” and
then applies heat to them while confined in a vessel under pressure. The
product of these operations is a very singular modification of
gutta-percha, in which the material assumes a spongy, elastic condition,
and in this form it is used to form the stuffing of sofas, easy chairs,
&c. Among the purposes to which gutta-percha has been applied besides
the general one of waterproof tissues and fabrics, may be named the
formation of straps, belts, bandages, cups, and other vessels, rollers
for cotton-spinning machinery, hammers of pianofortes, cards for
wool-carding, hammercloths, life-preservers, and trusses.
Gutta-percha is made into strips, bands, cords, or threads of any
required section, by passing sheets of suitable thickness between
rollers provided with grooves and cutting edges. For strips and bands
the sheets are passed through the machine cold, and divided by the
cutting-edges. But for round cords or threads sheets are supplied to the
rollers from a receptacle in which they acquire a temperature of about
200° F. The material is forced to take the form of the grooves, the
operation in this case being analogous to that of rolling iron bars. The
gutta-percha cords are received as they issue from the rollers in a tank
of cold water, from which they pass on, to be wound on reels or drums.
It is obvious that cords of any section may be formed by making use of
grooves of suitable shape. Tubes of gutta-percha are made by forcing the
softened substance out of an annular orifice: it is received into
vessels filled with cold water. Telegraph-wires are covered by a similar
process—the copper wire being made to pass through the centre of a
circular opening with the gutta-percha surrounding it. Picture-frames,
&c., are made by forcibly pressing warm gutta-percha into the warmed
moulds. Gutta-percha tubing is largely used everywhere for the
speaking-tubes by which persons in remote apartments of even the largest
building can converse. This is one of the labour-saving inventions of
our day. It must have struck every one who has seen these speaking-tubes
in operation in a large establishment, what a vast amount of running to
and fro they save, and how much they expedite business by the convenient
means they afford of giving orders and directions to persons in distant
apartments. This tubing is also used for the conveyance of liquids, and
it has been proposed to employ it instead of the ordinary leaden piping
used for carrying water. It may seem to the reader that gutta-percha is
too fragile a material to resist the pressure to which water-pipes are
exposed. But, judging from some experiments made by the engineer of the
Birmingham Waterworks, the power of gutta-percha tubing to resist
pressure is quite extraordinary, and far beyond what would be supposed.
The tubes experimented on had diameters of ¾ and ⅞th of an inch
respectively. The water from the main, where the pressure was that
caused by a head of 200ft., was in communication with these pipes for
several weeks, and they were found unaltered in any way. In order to
test the strength of the tubes, and find the greatest pressure they
would bear, the engineer then had them connected with a hydraulic
proving-pump; and here, when exposed to the highest pressure at which
the ordinary water-pipes were tested, namely, to 250 lbs. on the square
inch, they also remained intact. The pressure was afterwards increased
to 337 lbs., but without any damage to the tubes.
The increasing importance of gutta-percha may be inferred from the
continually augmented importation of the crude substance into this
country. In 1850 only 11,000 cwt. were imported, but the quantity has
increased year by year; and in 1872 we received nearly 46,000 cwt. The
demand is still increasing; but there is reason to apprehend that under
the stimulus of a rising market, the producers have collected the
gutta-percha wastefully and with great destruction of the trees, so that
it is not improbable that if the demand still increases, there may be a
gutta-percha famine. The concreted juices of certain other trees have
been proposed as substitutes for gutta-percha. None of these have as yet
come into practical use. The increase in a few years of the quantity of
Indian-rubber imported into the United Kingdom is perhaps more
extraordinary. From the tables given in Mr. Hancock’s book, it appears
that our imports of caoutchouc were 853,000 lbs. in 1850, but by 1855
they had amounted to 5,000,000 lbs.
[Illustration:
FIG. 338.—_Portrait of Sir James Young Simpson, M.D._
]
ANÆSTHETICS.
The discovery which is indicated by the somewhat unfamiliar word[16]
which heads this article is perhaps the greatest which has ever been
made in connection with the science of medicine. At least, there is no
other discovery of modern times which has so largely and directly
contributed to the assuagement of human suffering. Nay, in this respect
there is perhaps in the whole annals of the healing art no other which
can rival it, if we except that famous one of Jenner’s which has
arrested the ravages of small-pox. During the last thirty years, all the
more formidable operations of the surgeon have been, in almost every
case, performed with a happy unconsciousness on the part of the patient.
In unconsciousness, induced by the same means, has relief also been
found for severe suffering arising from other causes. The substances
which are denoted by the word “anæsthetics” differ from the drugs which
the older surgeons sometimes administered before an operation, in order
to lull the patient’s sense of pain. They differ in their nature and in
the mode of their administration; by the certainty and completeness of
their action; by the entirely transient effects they produce, which pass
off without leaving a trace.
Footnote 16:
From α (αν), privative, and αισθητικος, capable of perceiving or
feeling.
To the great chemist whose name has already been mentioned as the
discoverer of the metals of the alkalies and alkaline earths we are
indebted for the first of the remarkable class of bodies we are about to
discuss. The first work that Davy published had for its title
“Researches, Chemical and Philosophical, chiefly concerning Nitrous
Oxide and its Respiration.” This was in the year 1800, when the
philosopher had hardly completed his twenty-first year. The work caused
no little sensation in the scientific world, and it was in consequence
of the reputation he acquired by these researches that Davy was
appointed to the chemical professorship at the Royal Institution. Davy
was not the original discoverer of nitrous oxide, but he first entered
upon a full investigation of its properties, and announced the singular
effect produced by its inhalation. The kind of transient intoxication
and propensity to laughter which it excites have obtained for this
compound the familiar name of _laughing gas_. Davy had by experiment on
his own person proved the anæsthetic properties of this gas, for he had
a tooth painlessly extracted when under its influence, and he says in
the work above named that “as nitrous acid gas seems capable of
destroying pain, it could probably be used with advantage in surgical
operations where there is no effusion of blood.” Davy’s observations and
suggestions were destined to lie barren for nearly half a century, but
they nevertheless formed the basis of the great results which have since
been attained.
Before proceeding farther, it will perhaps be well to make the
unscientific reader acquainted with the chemistry of nitrous oxide. We
may presume that he knows that atmospheric air is a _mixture_ of the two
invisible gases, nitrogen and oxygen (the small quantity of carbonic
acid also present need not now be considered). When a known quantity of
air is passed over red-hot copper turnings, contained in a tube, the
whole of the oxygen is seized upon by the copper, and only the nitrogen
issues from the tube, and may be collected. Some of the copper is thus
converted into oxide, and the increase of the weight of the tube’s
contents shows the weight of oxygen contained in the air, while the
weight of nitrogen may be known from the volume collected. In this way
the chemist analyses atmospheric air, and determines that 100 parts by
weight of dry air contain about 79 of nitrogen and 21 of oxygen; or, by
measure, about four times as much of the former as of the latter. Now,
chemists are acquainted with no fewer than five _different_ substances
which contain nothing but nitrogen and oxygen. These substances are
either gases, or can be changed into the gaseous form by heat, and they
can all be analysed in the same manner as air. The results of such
analyses show in 100 parts by weight of each substance the following
proportions of its constituents:
┌────────┬──────┬──────┬──────┬──────┬──────┐
│ │No. 1.│No. 2.│No. 3.│No. 4.│No. 5.│
├────────┼──────┼──────┼──────┼──────┼──────┤
│Nitrogen│63·64 │46·67 │36·84 │30·44 │25·93 │
│Oxygen │36·36 │53·33 │63·16 │69·56 │74·07 │
└────────┴──────┴──────┴──────┴──────┴──────┘
In casting the eye over this table, no relation will probably be
detected between the five cases. But if we write down, not the
quantities of nitrogen and oxygen contained in 100 parts of each
compound, but the quantity of oxygen which in each compound is united to
some fixed quantity of nitrogen, we shall at once detect a remarkable
law: thus, taking 28 as the fixed weight of nitrogen, for reasons which
need not be here explained:
┌────────┬──────┬──────┬──────┬──────┬──────┐
│ │No. 1.│No. 2.│No. 3.│No. 4.│No. 5.│
├────────┼──────┼──────┼──────┼──────┼──────┤
│Nitrogen│ 28 │ 28 │ 28 │ 28 │ 28 │
│{Oxygen │ 16 │ 32 │ 48 │ 64 │ 80 │
│{ or │16 × 1│16 × 2│16 × 3│16 × 4│16 × 5│
└────────┴──────┴──────┴──────┴──────┴──────┘
Chemists have a sort of shorthand method of expressing the composition
of substances, which may be conveniently illustrated by the case before
us. Let it be agreed that the letter N shall not only represent
nitrogen, but always _fourteen_ parts by weight—grains, ounces, &c.,
&c.,—of nitrogen; and that, similarly, O shall stand for _sixteen_ parts
by weight of oxygen. It is plain that the composition of the compound
No. 2 may be represented by simply writing down “NO;” and that of No. 4,
in which there is just double the proportion of oxygen, by “NOO.” But to
avoid an unnecessary repetition of the same symbol, when it has to be
taken more than once, a small figure is written after and a little below
it. Thus, for OO, “O_{2}” is written. The proportional composition of
each of the five compounds will now be obvious from the following
symbols:
┌────────────┬────────────┬────────────┬────────────┬────────────┐
│ No. 1. │ No. 2. │ No. 3. │ No. 4. │ No. 5. │
├────────────┼────────────┼────────────┼────────────┼────────────┤
│ N_{2}O │ NO │ N_{2}O_{3} │ NO_{2} │ N_{2}O_{5} │
└────────────┴────────────┴────────────┴────────────┴────────────┘
These symbols may be regarded as merely a compendious expression of the
composition of each substance—as a shorthand statement of the _facts_ of
analysis. But to the majority of chemists the symbols have a deeper
significance; for they are taken as representing the _atoms_ of each
element which enter into each smallest possible particle of a compound;
they express a certain theory of the ultimate constitution of matter.
Thus, if we suppose that there exist indivisible particles of nitrogen
and of oxygen, and that each smallest particle, or _molecule_, of the
compounds under consideration is constituted of a certain definite and
invariable number of each kind of atoms; and, further, if we suppose
that an _atom_ of oxygen is heavier than one of nitrogen in the
proportion of 16 to 14, or 8 to 7, we shall have a simple theoretical
_explanation_ of the relations in the proportions already pointed out.
In fact, these would result from the simplest combinations of the two
kinds of _atoms_; and we can picture each one of the smallest particles
of the several bodies as thus constituted:
┌────────────┬────────────┬────────────┬────────────┬────────────┐
│ No. 1. │ No. 2. │ No. 3. │ No. 4. │ No. 5. │
├────────────┼────────────┼────────────┼────────────┼────────────┤
│ ● ● │ │ ● ● │ ● │ ● ● │
│ ○ │ ● ○ │ ○ ○ ○ │ ○ ○ │ ○ ○ ○ │
│ │ │ │ │ ○ ○ │
├────────────┼────────────┼────────────┼────────────┼────────────┤
│ N_{2}O │ NO │ N_{2}O_{3} │ NO_{2} │ N_{2}O_{5} │
└────────────┴────────────┴────────────┴────────────┴────────────┘
The black circles represent nitrogen atoms, and the open ones oxygen
atoms; the symbols are placed below in order that their relation to the
supposed atomic constitution may be obvious at a glance. While the
symbol of a compound must always accord with its percentage composition,
the latter does of itself determine the symbol or formula. A number of
other circumstances, which cannot here be discussed, are taken into
account as evidence of the constitution of the molecule.
This digression on chemical formulæ will, it is hoped, enable the
general reader, who may not previously have been acquainted with them,
to perceive their significance, instead of passing them over as
unintelligible cabalistic letters when they occur in the following
pages. With this object, it may be added that the elements, hydrogen,
carbon, and chlorine, are respectively represented by H, C, and Cl; and
that the proportional quantities, which are also implied in the symbols,
and are those by which H, C, and Cl combine with other bodies, are 1,
12, and 35·5 respectively. Another point which should be understood is
that the properties and behaviour of a chemical compound are different,
and usually extremely different, from those of any of its constituents.
This is well illustrated in the subject we are considering. Atmospheric
air is a _mixture_ (not a compound) of nitrogen and oxygen gases, and
all its properties are intermediate between those of its ingredients
taken separately. Nitrous oxide, N_{2}O, has properties not possessed by
either constituent separately. For example, it is very soluble in water,
whereas oxygen is very slightly so, and nitrogen still less. The other
compounds we have referred to differ widely from nitrous oxide and from
each other in their properties.
Nitrous oxide is an invisible gas, having a slightly sweetish taste and
smell. It is dissolved by water, which, at ordinary temperatures, takes
up about three-fourths of its volume of the gas. By cold and great
pressure the gas may be condensed into a colourless liquid. The gas is
obtained in a pure state by gently heating the salt called ammonium
nitrate, which is formed by neutralizing pure nitric acid with carbonate
of ammonia. The action which occurs may be explained thus: the hydrogen
of the ammonium unites with a portion of the oxygen of the nitric acid,
forming water, whilst the remainder of the oxygen combines with the
nitrogen. As chemical actions are regarded as either separations or
unions of atoms, they can be expressed by what is called a _chemical
equation_, the left-hand side of which shows the arrangement of the
atoms before the action, and the right-hand side the arrangement after
it, the sign of equality being read as “produce” or “produces.” But the
validity of the equations, like that of the symbolic formulæ, is quite
independent of the existence of atoms; for the equation always rests on
certain facts, namely, the relations between the quantities of the
substances which enter into, and those which are produced by, a chemical
action. Thus, in the present case the action may be symbolically
expressed as follows:
H_{4}N NO_{3} = 2H_{2}O + N_{2}O
Ammonium nitrate. Water. Nitrous oxide.
The equation expresses the fact that every 80 parts by weight of
ammonium nitrate, which are used in this reaction, split up into 36 of
water and 44 of nitrous oxide.
No attempt seems to have been made to turn Davy’s suggestion to
practical account; but in courses of chemical lectures at the hospitals
and elsewhere the peculiar physiological properties of nitrous oxide
have, since Davy’s announcement, always been demonstrated by some person
inhaling the gas. In the medical schools the students often operated on
a comrade who was under the influence of nitrous oxide to the extent of
bestowing sundry pinches and cuffs, which fully proved the anæsthetic
qualities of the nitrous oxide. In 1818 Faraday pointed out the
similarity between the effects of _ether_ and of nitrous oxide, and from
that time Professor Turner regularly included among the experiments of
his course of chemistry the inhalation of the vapour of ether by one of
the students. This was done by simply pouring a little ether into a
bladder of air, and by means of a tube drawing the mixed air and vapour
into the mouth. Until 1844 the effects of nitrous oxide and of ether
vapour remained without application, although thus continually
demonstrated in lectures. At the close of that year, Mr. Horace Wells, a
dentist, of Hartford, Connecticut, U.S.A., witnessed the usual
experiments with nitrous oxide at a public lecture. At his request the
lecturer attended at Mr. Wells’s residence the following day, to
administer to him the nitrous oxide, in order that he might try its
efficacy in annulling pain, for he was himself to have a tooth extracted
by a brother dentist. His exclamation on finding the operation
painlessly over was, “A new era in tooth-pulling!” Mr. Wells continued
his experiments on the use of nitrous oxide in dental operations, but he
did not apparently obtain uniform results, for he pronounced its effects
uncertain, and he gave it up. On the occasion when Mr. Wells’s tooth was
extracted, Dr. W. T. G. Morton was present, and he soon afterwards found
that under the influence of ether vapour, teeth might be painlessly
extracted and surgical operations performed. Dr. Morton attempted to
conceal the substance he used under the name of “letheon,” for which he
obtained a patent. But the well-known and characteristic odour of ether
declared the nature of the “letheon;” and Dr. Bigelow having in
consequence tried ether, found it to produce all the effects of
“letheon.” So the matter was no longer a secret. Dr. Morton was,
therefore, the person who first applied ether vapour, and the extraction
of a tooth was the occasion of its first application. This was in 1846.
It was used for the first time in England on the 19th of December, 1846,
also for the extraction of a tooth; and two days afterwards Mr. Liston,
the eminent surgeon, performed the operation of amputating the thigh
while his patient was under the influence of ether. The employment of
ether in surgical operations quickly spread, and its administration in
hospitals became general throughout Europe and America.
The chemical constitution of ether, and its relation to alcohol, may be
indicated by the following formulæ:
HOH HO(C_{2}H_{5}) (C_{2}H_{5})O(C_{2}H_{5})
Water. Alcohol. Ether.
If we suppose one of the hydrogen atoms in the molecule of water to be
removed and replaced by the group (C_{2}H_{5}), the result is alcohol.
If, now, (C_{2}H_{5}) be substituted in the alcohol for the remaining
atom of hydrogen, we get a particle of _ether_. Ether was discovered in
1540, and described as sweet oil of vitriol, but its real nature was
first pointed out by Liebig. It is prepared by distilling a mixture of
sulphuric acid and alcohol. It is a colourless transparent liquid,
extremely volatile, and possessing a peculiar and powerful odour. It
evaporates so rapidly that a drop allowed to fall from a bottle on a
warm day may be converted into vapour before it reaches the ground. When
its vapour is inhaled in sufficient proportion mixed with air, it soon
produces a complete insensibility to pain. In the case of a full-grown
man who inhales air containing 45 per cent. of the vapour, about 2 drams
per minute of the liquid are consumed. The air is allowed to stream over
the surface of the liquid in a proper apparatus, where it takes up the
vapour, and the two pass through a flexible tube to a piece fitting over
the mouth and nostrils of the patient. The effects produced are
progressive, and may be thus described:
For about two minutes after the beginning of the inhalation, the patient
retains his mental faculties, and has some power of controlling his
movements, but in a confused and disordered manner. At the end of the
third minute he is unconscious; there are no voluntary movements, but
muscular contractions may agitate the frame. At the end of the fourth
minute, the only perceptible movements are the motions of the chest in
respiration. If the inhalation be discontinued at the end of the fourth
minute, when 1 oz. of ether will have evaporated, similar stages are
passed through in reverse order during recovery. The condition reached
at the end of the fourth minute continues about two minutes; the
intermediate state lasts three or four minutes; the condition of
confused intellect and will about five minutes. This is succeeded by a
feeling of intoxication and exhilaration, which continues for ten or
fifteen minutes. It was probably this excitement of the system produced
by ether which has caused it to be superseded—in Britain, at least—in
about twelve months after its adoption, by _chloroform_.
Chloroform appears to have been independently discovered in 1831, by
Soubeiran, and by an American chemist, Guthrie. It is usually procured
by distilling a mixture of bleaching powder, spirits of wine, and water.
Chloroform is a colourless volatile liquid, of an odour much more
agreeable than that of ether. Its composition is represented by
CHCl_{3}. The merit of having first applied the singular properties of
this substance to the alleviation of human suffering belongs to the late
Sir J. Y. Simpson, of Edinburgh. Its use as an anæsthetic was apparently
suggested to this eminent professor by Mr. Waldie, of Liverpool. It was
first applied at Edinburgh on the 15th November, 1847; and when its
efficacy had been proved, it was soon extensively used, and in Europe,
at least, almost entirely superseded ether, as being more rapid and
certain in its action, not producing injurious excitement, and being
pleasanter to inhale. A notion prevailed that chloroform was not only
more powerful in its operation than ether, but also more safe. In
January, 1848, its administration, however, proved fatal to a patient;
and since then a certain number of casualties of this kind have occurred
with chloroform, ether, and other anæsthetics.
The patient is often made to inhale the vapour of chloroform by merely
holding before his mouth and nostrils a sponge or handkerchief, on which
a small quantity of the liquid has been poured. Dr. Snow contrived an
apparatus for administering the vapour with more regularity. A metal box
adapted to the shape of the face is made to cover the mouth and
nostrils. This piece has two valves, one of which admits the air and
vapour from an elastic tube connected with the apparatus containing the
chloroform, and prevents its return; the other valve is a flap opening
outwards, which allows the expired air to escape. There is also an
adjustment for admitting directly into the mouthpiece more or less
atmospheric air.
The sensations first experienced when chloroform is inhaled are said to
be agreeable. Many persons have described the feeling as resembling
rapid travelling in a railway carriage; there is a singing in the ears,
and when the power of vision ceases, and the person is no longer
conscious of light, the sensation is that of entering a tunnel. After
this there is a lessened sensibility to pain; and in the next stage the
unconsciousness to outward impressions is deeper, but the mental
faculties, though impaired, are not wholly suspended, for the patient
may speak, and usually dreams something which he afterwards remembers.
When the person is still more under the influence of the chloroform, no
voluntary motions take place, although there may be some inarticulate
muttering. Dr. Snow describes several conditions which may be observed
in patients undergoing operations under the influence of chloroform.
First, the patient may preserve the most perfect quietude without a sign
of consciousness or sensation; this is the most usual condition. Second,
he may moan, or cry, or flinch under the operation, without, however,
having the least memory of any pain when he recovers. Third, the patient
may talk, laugh, or sing during the operation; but what he says is
altogether devoid of reference to what is done. Fourth, he may be
conscious of what is taking place, and may look on while some minor
operation is proceeding, without feeling it, or without feeling it
painfully. This is often the condition of the patient as the effect is
passing off, while some smaller operation is still proceeding. Fifth,
the patient may complain he is being hurt; but afterwards, when the
effect of the chloroform has passed off, he will assert that he felt no
pain whatever. When the chloroform has been inhaled for but a short
time, the patient becomes conscious in about five minutes after its
discontinuance; but with a longer inhalation the period of
unconsciousness may last for perhaps ten minutes. The return of
consciousness takes place with tranquillity: not unfrequently the
patient’s first speech, even after a serious operation, often being an
assertion that the chloroform has not taken effect.
In the strongest degree of ether and chloroform effects, all the muscles
of the body are relaxed; the limbs hang down, or rest in any position in
which they are placed; the eyelids droop over the eyes, or remain as
they are placed by the finger; the breathing is deep, regular, and
automatic; there is often snoring, and this is, indeed, characteristic
of the deepest degree of unconsciousness; the relaxation of the muscles
renders the face devoid of expression, and with a placid appearance, as
if the person were in a sound natural sleep. He is perfectly passive
under every kind of operation. The breathing and the action of the heart
proceed all the while with unimpaired regularity. It is, however, known
by experiments on animals that if the inhalation be prolonged beyond the
period necessary to produce these effects, the respiratory functions are
interfered with by the insensibility extending to the nerves on which
they depend. The breathing of an animal thus treated becomes irregular,
feeble, or laborious, and death ensues. However nearly dead from
inhalation of ether vapour the animal may be, provided respiration has
not actually ceased, it always recovers when allowed to breathe fresh
air. Of course, the etherization is never carried to this stage with
human beings.
Air containing 2 grs. of chloroform in 100 cubic inches suffices to
induce insensibility; but 5 grs. in 100 cubic inches is found a more
suitable proportion. Dr. Snow, who strongly disapproved of the uncertain
and irregular mode of administering chloroform on a handkerchief or
sponge, contrived the inhaling apparatus already described. The air
before reaching the mouth and nostrils of the patient passes through a
vessel containing bibulous paper moistened with chloroform. This vessel
he surrounds with water at the ordinary temperature of the air, in order
to supply the heat absorbed by the conversion of the liquid into vapour,
so that the formation of the latter may go on regularly. The same
thoughtful arrangement formed part of the ether-inhaler he had
previously contrived.
The extraordinary effects of ether and chloroform have introduced new
and important facts into psychological science, and have illustrated and
extended some of the most interesting results of physiological research.
Let us trace the action of these substances, and explain it as far as
may be. Nitrous oxide, ether vapour, and chloroform vapour are all
soluble in watery fluids. The lungs present a vast surface bathed by
watery fluids, and therefore these gases are largely absorbed; and by a
well-known process, they pass directly into the blood, through the
delicate walls of the capillary vessels. The odour of ether can be
detected in any blood drawn from persons under its influence. Ether, or
chloroform, thus brought into the general current of the circulation, is
quickly carried to all parts of the body, and thus reaches the
nerve-centres. On these it produces characteristic effects by suspending
or paralysing nervous action: why or how this effect takes place is
unknown. The nervous centres are not all acted upon in an equal
degree—some require a larger quantity of the drug to affect them at all.
The parts of the nervous system first affected are the cerebral lobes,
which are known to be the seat of the intellectual powers. The
_cerebellum_—the function of which there is reason to believe is the
regulation and coordination of movements—is the next to yield to the
influence. Then follow the spinal nerves, which are the seat of
sensibility and motive power. This is as far as the action can safely be
carried: the nervous centre called the _medulla oblongata_, which is
placed at the junction of the brain and the spinal cord, still performs
its functions—one of the most important of which is to produce the
muscular contractions that keep the respiratory organs in action. We
have seen, by the effects of further etherization in animals, that when
this part of the system is affected, the animal dies from a stoppage of
the respiration.
But, unfortunately, there have been instances in which death has been
caused by the administration of ether and chloroform even under the most
skilful management. But these occurrences were not the result of the
inhalation having been carried so far as to stop respiration: in some
cases the patient has died before the first stage of insensibility.
These fatal cases have all been marked by a sudden paralysis of the
heart—that organ has abruptly ceased to act. Why in these, certainly a
very small percentage of patients, the action of the drug should at once
take effect on the heart has not yet been explained. The rhythmic action
of the heart depends upon nervous centres enclosed within its own
substance, so that this organ is to a certain extent independent; but it
is connected with the other nervous centres by the branches of a
remarkable nerve which proceeds from the _medulla oblongata_, and also
by another set of nerves which come from the chain of ganglia called the
_sympathetic nerve_. The nerve connecting the heart with the _medulla_
is a branch of that called the _pneumo-gastric_, and it is a
well-established fact that the action of the heart may be arrested by
irritation of this nerve. The comparatively few fatalities which have
attended the use of anæsthetics may, therefore, be due either to an
immediate action on the nerve-centres of the heart, or possibly to a
mediate action through the _medulla_ and the pneumo-gastric nerve.
Soon after the introduction of ether the use of nitrous oxide was
discontinued by the dentists, on account of the apparent uncertainty of
its action. Within the last few years, however, its employment in the
extraction of teeth has been revived by Dr. Evans, of Paris, who found
that to insure certainty in its action, the great point is the
inhalation of the gas in a pure state and without admixture of air.
Nitrous oxide seems now to be extensively used by dentists, and thus
Davy’s experiment of 1800 is repeated and verified daily in thousands of
cases, and to the great relief of hundreds who probably never heard his
name.
Other bodies, such as amylene (C_{5}H_{10}), carbon tetrachloride
(CCl_{4}), &c., have been tried as substitutes for ether and chloroform;
but having been found less efficacious or more dangerous, their use has
been abandoned. It might be instructive to reflect how much unnecessary
pain would have been spared to mankind had ether and chloroform been
known and applied at an earlier age. We know not what other beneficent
gifts chemistry may yet have in store for the alleviation of suffering,
but it is unlikely that even ether and chloroform are her _derniers
mots_. It should be remembered that the chemists who discovered and
examined these bodies were attracted to the work by nothing but the love
of their science. They had no idea how invaluable these substances would
afterwards prove. The chemist of the present day, whose labour is often
its own reward, may be cheered and stimulated in his toil by the thought
that while no discovery is ever lost, but goes to fill its appropriate
place in the great edifice of science, even the most apparently
insignificant truth may directly lead to invaluable results for humanity
at large.
What strange things the ancient thaumaturgists might have done had they
been possessed of the secret of chloroform or of nitrous oxide! What
miracles they would have wrought—what dogmas they would have sanctioned
by its aid! But the remarkable effects produced by the inhalation of
certain gases or vapours were not altogether unknown to the
ancients—although these effects were then attributed to anything but
their real cause. It is related that a number of goats feeding on Mount
Parnassus came near a place where there was a deep fissure in the earth,
and thereupon began to caper and frisk about in the most extraordinary
manner. The goatherd observing this, was tempted to look down into the
hole, to see what could have caused so extraordinary an effect. He was
himself immediately seized with a fit of delirium, and uttered wild and
extravagant words, which were supposed to be prophecies. The knowledge
of the presumed divine inspiration spread abroad, and at length a temple
in honour of Apollo was erected on the spot. Such was the origin of the
famous Oracle of Delphi, where the Pythoness, the priestess of Apollo,
seated on a tripod placed over the mysterious opening, delivered the
response of the god to such as came to consult the oracle. It is stated
by the ancient writers, that when she had inhaled the vapour, her eyes
sparkled, convulsive shudders ran through her frame, and then she
uttered with loud cries the words of the oracle, while the priests who
attended took down her incoherent expressions, and _set them in order_.
These possessions by the spirit of divination were sometimes violent.
Plutarch mentions a priestess whose frenzy was so furious, that the
priests and the inquirers alike fled terrified from the temple; and the
fit was so protracted that the unfortunate priestess herself died a few
days afterwards.
[Illustration:
FIG. 339.—_A Railway Cutting._
]
EXPLOSIVES.
The illustration above will serve to remind the reader of the great
importance of explosive agents in the operations of civil industry. By
reason of the more impressive and exciting spectacles which attend the
use of such agents in warfare, we are rather apt to lose sight of their
far more extensive utility as the giant forces whose aid man invokes
when he wishes to rend the rock in order to make a road for his steam
horse, or in order to penetrate into the bowels of the earth in search
of the precious ore. A little reflection will show that if such work had
to be done with only the pickaxe, the chisel, and the crowbar, the
progress would be painfully slow; and railway cuttings through masses of
compact limestone, like that represented in Fig. 339, for example, would
be well-nigh impossible. The formation of cuttings and tunnels, and the
removal of rocks in mining operations, are not the only service which
explosive agents render to the industrial arts; there is, besides other
uses which might be enumerated, the preparation of foundations for
buildings, bridges, harbours, and lighthouses. The use of gunpowder in
all such operations as those which have been referred to is too well
known to require description. But of late years gunpowder has been to a
great extent superseded for such purposes by two remarkable products of
modern chemistry, called _gun-cotton_ and _nitro-glycerine_. Military
art has also benefited by at least one of these products; and the use of
charges of gun-cotton for torpedoes has already been described and
illustrated in these pages.
It is not a little curious that the two most terribly powerful
explosives known to science should be prepared from two most harmless
and familiar substances. The nice, soft, clean, gentle cotton-wool, in
which ladies wrap their most delicate trinkets, becomes, by a simple
chemical transformation, a tremendously powerful explosive; and the
clear, sweet, bland liquid, glycerine, which they value as a cosmetic
for its emollient properties, becomes, by a like transformation, a still
more terrifically powerful explosive than the former. It is, perhaps,
even more curious that having undergone the transformation which confers
upon it these formidable qualities, neither cotton-wool nor glycerine is
changed in appearance. The former remains white and fleecy; the latter
is still a colourless syrupy-looking liquid.
The fibres which form cotton, linen, paper, and wood, are composed
almost entirely of a substance which is known to the chemist as
_cellulose_ or _cellulin_. That this substance, as it exists in the
fibres of linen and in sawdust, could be converted into an explosive
body by the action of nitric acid, appears to have been first observed
by the French chemist, Pelouze, in 1838. The action with cellulose in
the form of cotton-wool was more fully examined by Professor Schönbein,
of Basle, who, in 1846, first described the method of preparing
_gun-cotton_, and suggested some uses for it. He directs that one part
of finely-carded cotton-wool should be immersed in fifteen parts of a
mixture of equal measures of strong sulphuric and nitric acids; that
after the cotton has remained in the mixture for a few minutes, it
should be removed, plunged in cold water, and washed until every trace
of acid has been removed, and then carefully dried at a temperature not
exceeding the boiling-point of water.
After Professor Schönbein had demonstrated the power of the new agent in
blasting, and its projectile force in fire-arms, its manufacture on a
large scale was undertaken at several places. Messrs. Hall commenced to
make it at their gunpowder works at Faversham, and a manufactory was
also established near Paris. In July, 1847, a fearful explosion of
gun-cotton occurred at the Faversham works, which was believed to have
been caused by the spontaneous detonation of that substance. This
induced Messrs. Hall to discontinue the manufacture as too dangerous;
and they even destroyed a large quantity of the product which they had
in hand by burying it in the ground. The making of gun-cotton was soon
afterwards discontinued also by the French, who did not find the
substance to possess all the qualities fitting it for military use. The
Prussian Government also began to make gun-cotton; but the experiments
were put a stop to by the explosion of their factory. An eminent
artillery officer in the Austrian service, General von Lenk, undertook a
thorough examination of the manufacture and properties of gun-cotton for
military purposes. He introduced several improvements into the processes
of the manufacture; and the Austrian Government established works at
Hïrtenberg, with a view to the adoption of gun-cotton as a substitute
for gunpowder in fire-arms. It has some undoubted advantages over
powder, for it neither heats the gun nor fouls it, and it produces no
smoke. Notwithstanding this the Austrians have not abandoned the use of
gunpowder in favour of gun-cotton.
Gun-cotton, as a military agent, has a strenuous advocate in Professor
Abel, who presides over the Chemical Department of the British War
Office. To this gentleman we are indebted for great improvements in the
manufacture of gun-cotton, and for a more complete investigation of its
properties. Professor Abel’s processes were put in practice at a
manufactory which the Government established at Waltham Abbey; and
Messrs. Prentice also set up works at Stowmarket.
Some details of the mode in which the manufacture of gun-cotton was
carried on at Stowmarket may be of interest. The cotton was first
thoroughly cleansed and carefully dried; and these operations are of
great importance, for unless they are well performed, the product is
liable to explode spontaneously. The cotton was then weighed out in
charges of 1 lb., and each charge was completely immersed in a separate
vessel, containing a cold mixture of sulphuric and nitric acids. After a
short immersion the cotton was removed from the liquid, and with about
ten times its own weight of acids adhering to it, each charge was placed
in a separate jar, where it was allowed to remain for forty-eight hours.
The vessels were kept cool during the whole period by being placed in a
trough through which cold water was flowing. On removal from the jars,
the cotton was freed from adhering acid by being placed in a centrifugal
drying machine. It was then drenched with a large quantity of cold
water, and dried, washed again in a stream of cold water for forty-eight
hours, and the operations of alternately washing for forty-eight hours
and drying were repeated eight times. The drying was effected by placing
the material in cylinders of wire-gauze, which were whirled round by a
steam engine at the rate of 800 revolutions per minute, so that the
water was expelled by centrifugal force. The cotton was next reduced to
a pulp by a process similar to that which is employed in paper-making,
and the moist pulp was rammed into metallic cylinders by hydraulic
pressure, in order that it might be brought into forms suitable for use
in blasting, &c. The pulp was put into these moulds while wet, but the
water was nearly all expelled by the compression. The cylinders of
gun-cotton thus obtained were then covered with paper-parchment, and
finally dried at a steam temperature, with many precautions. The
compression of the cotton pulp, by bringing a large quantity of the
material into a smaller bulk, causes a greater concentration of the
explosive energy, and this is a matter of great importance in blasting.
We may now consider what chemistry has to teach concerning the nature of
the action by which cotton-wool is converted into gun-cotton. Cotton
itself is nearly pure cellulose. The chemical composition of cellulose
may be represented most simply by the formula C_{6}H_{10}O_{5}. Nitric
acid is a powerful oxidizing agent, and is constantly used in chemistry
to fix oxygen in various substances; but another kind of action exerted
by nitric acid in certain cases consists in the substitution of a
portion of its atoms for hydrogen, by which the residue of the particle
of nitric acid is converted into water. The formula for nitric acid may
be written HO NO_{2}, and it will be seen that by changing NO_{2} for H,
water, HOH, would be produced. This is precisely the kind of action
which occurs when cellulose is converted into _nitro-cellulose_. Two or
three, or more, atoms of hydrogen may be taken out of cellulose, and
replaced by two or three, or more, groups NO_{2}, and the result will be
a different kind of _nitro-cellulose_, according to the number of atoms
in the molecule replaced by NO_{2}. Several varieties of gun-cotton are
known, these being doubtless the result of the differences here alluded
to. The action producing di-nitro-cellulose is represented by this
equation:
C_{6}H_{10}O_{5} + 2HNO_{3} = C_{6}H_{8}(NO_{2})_{2}O_{5} + 2H_{2}O.
Cellulose. Nitric acid. Di-nitro-cellulose. Water.
The equation shows that water is produced by the reaction, and the
sulphuric acid which is used in the preparation performs no further part
than to take up this water, which would otherwise go to dilute the rest
of the nitric acid. The union of sulphuric acid and water is attended
with great heat, hence the necessity of cooling the vessels in making
the gun-cotton. Quite other products would be formed if the mixture
became heated.
The action of nitric acid on glycerine is of the same kind as that on
cellulose. When glycerine is allowed to drop into a cooled mixture of
nitric acid and sulphuric acid, the eye can detect little or no
difference between the appearance of the liquid which collects in the
bottom of the vessel and the glycerine dropped in. The product of the
action is, however, the terrible _nitro-glycerine_, a heavy,
oily-looking liquid, which explodes with fearful violence. Even a
single drop placed on a piece of paper, and struck on an anvil,
detonates violently and with a deafening report. The chemical change
which is effected in the glycerine (C_{3}H_{8}O_{3}), is the
substitution of three NO_{2} groups for three of hydrogen, producing
C_{3}H_{5}(NO_{2})_{3}O_{3}, or tri-nitro-glycerine. The general
reader may perhaps marvel that the chemist should be able not only to
count the number of atoms which go to make up the particles of a
compound body, but to say that they are arranged so and so: that the
atoms do not form an indiscriminate heap, but that they are connected
in an assignable manner. The reader is no doubt aware that these
compound particles are extremely small, and he may reasonably wonder
how science can pronounce upon the structure of things so small. He
may be more perplexed to learn that a calculation made by Sir W.
Thompson shows that the particles of water, for instance, cannot
possibly be more than the 1/250000000th of an inch in diameter, and
may be only 1/20th of that size. The truth is that the very existence
of atoms and molecules is an assumption. Like the undulatory ether, it
is an hypothesis which is adopted to simplify and connect our ideas,
and not a demonstrated reality. But the atomic hypothesis has so wide
a scope that some philosophers hold the existence of atoms and
molecules as almost a known fact. Be that as it may, the chemist in
assigning to a body a certain _molecular formula_ really does nothing
but express the results of certain experiments he has made upon it.
With one re-agent it is decomposed in this manner, with another in
that. By certain treatment it yields an acid, a salt; so much carbonic
acid, such a weight of water, is acted on or remains unaltered; gives
a precipitate or refuses to do so. Such are the _facts_ which the
chemist conceives are co-ordinated and expressed by the formula he
gives to a substance. The best formula is that which accords with the
greatest number of the properties of the body—which includes as many
of the facts as possible. It follows, therefore, that a formula which
aims at expressing more than the mere percentage composition of the
body—which, in the language of the atom hypothesis, seeks to represent
the mode in which the atoms are grouped in the molecule, but which in
reality represents only reactions, is written according as the chemist
considers this or that group of reactions more important. These
remarks might be illustrated by filling this page with the different
formulæ (a score or more) which have been proposed as representing the
constitution (_reactions?_) of one of the best-known of organic
compounds, namely, acetic acid.
Whether atoms really exist, and their arrangement in the particles of
bodies can be deduced from the phenomena, or not, the fact is undeniable
that these ideas have given chemists a wonderful grasp of the facts of
their science. The consistency and completeness of the explanation
afforded by these theories are ever being extended by modifications
which enable them to embrace more and more facts. Some of the properties
of the substance we are now considering confirm in a remarkable manner
the theoretical views which are expressed in its constitutional formula.
We may first consider the nature of gunpowder, and by comparing it with
nitro-glycerine, endeavour to explain the greater power of the latter
substance. Gunpowder is a mixture of charcoal, sulphur, and nitre, the
latter constituting three-fourths of its weight. Nitre supplies oxygen
for the combustion of the charcoal, which is thus converted into
carbonic acid, and the sulphur, which is added to increase the rapidity
of the combustion, is also oxidized. The products of the action are,
however, numerous and complicated, but the important result is the
sudden generation of a quantity of carbonic acid, nitrogen, carbonic
oxide, hydrogen, and other gases, which at the oxidizing temperature and
pressure of the air would occupy a space 300 times greater than the
powder from which they are set free; but the intense heat attending the
chemical action dilates the gases, so that at the moment of explosion
they would occupy a space at least 1,500 times greater than the
gunpowder. The materials of which gunpowder is composed are finely
powdered, in order that each portion shall be in immediate contact with
others, which shall act upon it. Plainly, the more thorough the
incorporation of the materials—that is, the more finely ground and
intimately mixed they are—the more rapid will be the inflammation of the
powder.
Looking now at the crude formula of nitro-glycerine,
C_{3}H_{5}N_{3}O_{9}, the reader will remark that the molecule contains
more than sufficient oxygen to form carbonic acid with all the carbon
atoms, and water with all the hydrogen atoms; for the C_{6} in _two_
molecules of nitro-glycerine would take only O_{12} to form 6CO_{2}; and
the H_{10}, to be converted into 5H_{2}O, would only need O_{5}; thus
there would be an excess of oxygen. Now, it may occur to the reflective
reader that in every molecule of nitro-glycerine the carbon and hydrogen
are already associated with as much oxygen as they can take up: that
they are, in fact, already burnt, and that no further union is possible.
But from chemical considerations it has been deduced that in the
nitro-glycerine molecule the oxygen atoms, except only three, which are
partially and _imperfectly_ joined to carbon, are united to nitrogen
atoms only. The constitution of the molecule may be represented by
arranging, as below, the letters which stand for the atoms, and by
joining them with lines, which shall stand for the bonds by which the
atoms are united.
O H H H O
| | | | |
N—O—C—C—C—O—N
| | | | |
O H O H O
|
O=N=O
We see here that the hydrogen atoms are completely, and the carbon atoms
partially, detached from the oxygen atoms; and therefore these atoms are
in the condition of the separated carbon and oxygen atoms in gunpowder.
Only the pieces of matter which lie side by side in gunpowder are in
size to the molecules of nitro-glycerine as mountains to grains of sand.
The mixture of the materials is then so much more intimate in
nitro-glycerine, since atoms which can rush together are actually within
the limits of the molecules; and these molecules have such a degree of
minuteness, that 25 millions, at least, could be placed in a row within
the length of an inch. We know that the finer the grains and the more
intimate the mixture, the quicker will gunpowder inflame; but here we
have a mixture far surpassing in minute subdivision anything we can
imagine as existing in gunpowder. Hence the combination in the case of
nitro-glycerine must be instantaneous, whereas that in gunpowder, quick
though it be, must still require a certain interval. If it take a
thousandth of a second for the gases to be completely liberated from a
mass of gunpowder, and only one-millionth of a second for a vast
quantity of carbonic acid, nitrogen, and steam to be set free from
nitro-glycerine, the destructive effect will be much greater in the
latter case. Again, the volume of the gases liberated from
nitro-glycerine in its detonation have at least 5,000 times the bulk of
the substance. We have entered into these chemical considerations, at
some risk of wearying the reader, with the desire of affording him a
clue to the singular properties of nitro-glycerine and gun-cotton, which
we are about to describe.
The nature of the chemical changes which may be set up in an explosive
substance, and the rapidity with which these changes proceed throughout
a mass of the material, are greatly modified by the conditions under
which the action takes place. If a red-hot wire be applied to a small
loose tuft of gun-cotton, it goes off with a bright flash without
leaving any smoke or any other residue. Thus, when the substance is
quite unconfined, no explosion occurs. If the cotton-wool be made into a
thread, and laid along the ground, it will burn at the rate of about 6
in. per second; if it be twisted into a yarn, the combustion will run
along at the rate of 6 ft. per second; but if the yarn be enclosed in an
Indian-rubber tube, the ignition proceeds at the rate of 30 ft. in a
second. If to a limited surface of gun-cotton, such as one end of a
length of gun-cotton yarn, a source of heat is applied—the temperature
of which is high enough to set up a chemical change, but not high enough
to inflame the resulting gases (carbonic oxide, hydrogen, &c.)—the
cotton burns comparatively slowly, rather smouldering than inflaming.
If, however, a _flame_ be applied, the gun-cotton flashes off with great
rapidity, because the heat applied sets fire to the gaseous products of
the chemical action. But if the gun-cotton be confined so that the gases
cannot escape, the combustion becomes rapid however set up. The reason
is that if the gases escape into the air, they carry off so much of the
heat produced by the smouldering gun-cotton, that the temperature does
not rise to the extent required to produce the flaming ignition, in
which the products are completely oxidized. If a mass of gun-cotton be
enclosed in a capacious vessel from which the air has been removed, and
the gun-cotton be ignited by means of a wire made hot by electricity,
the cotton will at first only burn in the slow way without flame; but as
the gases accumulate and exert a pressure which retards the abstraction
of heat accompanying their formation, the temperature will rise and
attain the degree necessary for the complete and rapid chemical changes
involved in the flaming combustion. Thus, the more resistance is offered
to the escape of the gases, the more rapid and perfect is the combustion
and explosive force produced by the ignition. Now, the explosion of
gun-cotton has been found to be too rapid when it is packed into the
powder-chamber of a gun, for its tendency is to burst the gun before the
ball has been fairly started. Hence a material like gunpowder, in which
the combustion is more gradual, is better suited for artillery. The
ignition of gunpowder, though rapid, is not instantaneous, and therefore
we can speak of it as more or less gradual. Indeed, in even the most
violent explosives, some time is doubtless required for the propagation
of the action from particle to particle. This extreme rapidity of
combustion, and consequent rending power, which is so objectionable in a
gun-chamber, makes gun-cotton a most powerful bursting charge for
shells, and, when it is enclosed in strong receptacles, for torpedoes
also.
But by the researches of Nobel, Professor Abel, and others, it has been
discovered—and this is, perhaps, the most remarkable discovery in
connection with explosives—that gun-cotton, nitro-glycerine, and other
explosive bodies, are capable of producing explosions in a manner quite
different from that which attends their ignition by heat. The violence
of this kind of explosion is far greater than that due to ordinary
ignition, for the action takes place with far greater rapidity
throughout the mass, and is, indeed, practically instantaneous. It
appears to be produced by the mere mechanical agitation or vibrations
which are communicated to the particles of the substance. Turning back
to the representation of the molecule of nitro-glycerine on page 744, it
will not be difficult to imagine that this may be an unstable kind of
structure; that the atoms of oxygen are prevented from rushing into
union with those of hydrogen and carbon only by the interposition of the
nitrogen; and that an agitation of the structure might shake all the
atoms loose, and leave them free to re-combine according to their
strongest affinities. Nitro-glycerine is by no means so ready to
_inflame_ as is gun-cotton: it is said that the flame of a match may be
safely extinguished by plunging it into the liquid; and when a
sufficient heat is applied to a quantity of the liquid in the open air,
it will burn quietly and without explosion. Even when nitro-glycerine is
confined, the application of heat cannot always be made to produce its
explosion; or, at least, the circumstances under which it can do so are
not accurately known, and the operation is difficult and uncertain. On
the other hand, nitro-glycerine explodes violently even when freely
exposed to the air if there be exploded in contact with it a _confined_
charge of gunpowder, or a detonating compound such as fulminating
powder. Gun-cotton possesses the same property of exploding by
concussion, which appears indeed to be a general one belonging to all
explosive bodies. According to recent researches, even gunpowder is
capable of a detonative explosion. A mass of gunpowder confined with a
certain proportion of gun-cotton, which is itself set off by fulminate
of mercury, is said to exert an explosive force four times greater than
that developed by the ignition of the gunpowder in the ordinary manner.
It has also been found that _wet_ gun-cotton can be exploded by
concussion, and the force of the explosion is unimpaired even when the
material is saturated with water. This makes it possible to use
gun-cotton with greater safety, as it may be transported and handled in
the wet condition without risk, and it preserves its properties for an
indefinite period without being deteriorated by the water. Some
experiments illustrating the extraordinary force of the detonative
explosions of gun-cotton and nitro-glycerine will give the reader an
idea of their power.
A palisade, constructed by sinking 4 ft. into the ground trunks of trees
18 in. in diameter, was completely destroyed in some experiments at
Stowmarket by the explosion of only 15 lbs. of gun-cotton. Huge logs
were sent bounding across the field to great distances, and some of the
trees were literally reduced to match-wood. The gun-cotton, be it
observed, was simply laid on the ground exposed to the air. The
destructive powers of nitro-glycerine are even greater. A tin canister,
containing only a few ounces of nitro-glycerine, is placed, without
being in any way confined, on the top of a smooth boulder stone of
several tons weight; it is exploded by a fuse containing fulminating
powder, which is fired from a distance by electricity. There is a
report, and the stone is found in a thousand fragments. The last
experiment shows one of the advantages of nitro-glycerine over gunpowder
as a blasting material, beyond its far greater power, which is about ten
times that of gunpowder. A charge of gunpowder inserted in a vertical
hole tends to force out a conical mass, the apex of which is at the
space occupied by the charge. With nitro-glycerine, and also with
gun-cotton, which last has almost six times the force of gunpowder, a
powerful rending action is exerted _below_ as well as above the charge.
Again, in blasting with gunpowder the charge must be confined, and the
hole is filled in above the charge with tightly rammed materials,
forming what is termed the _tamping_. But nitro-glycerine requires no
tamping: a small, thin metallic core containing the charge is simply
placed in the drill-hole, or the liquid itself is poured in, and a
little water placed above it. The effect of the explosion of
nitro-glycerine in “striking down,” when apparently no resistance is
offered, will seem very strange to the reader who is oblivious of
certain fundamental principles of mechanics. The force of the explosion
is due entirely to the sudden production of an enormous volume of gas,
which at the ordinary pressure would occupy several thousand times the
bulk of the material from which it is produced. This gas, by the law of
the equality of action and reaction, presses down upon the stone with
the same force that it exerts to raise the superincumbent atmosphere.
The pressure of the gas at the moment of its liberation is enormous; but
the atmosphere cannot instantaneously yield to this, for time is
required to set the mass of air in motion, and the wave of compression
advances slowly compared with the rapidity of the explosion. Hence the
air acts, practically, like a mass of solid matter, against which the
gases press, and which yields less readily than the rock, so that the
blow which is struck takes visible effect on the latter. Now, with
gunpowder, the evolution of gas is less rapid, the atmosphere has time
to yield, and the reaction has not the same violence. The rapidity of
the evolution of gas from the exploding nitro-glycerine is so great,
that the gases, though apparently unconfined, are not so in reality; for
the atmosphere acts as a real and very efficient tamping.
When nitro-glycerine first came into use for blasting purposes, it was
used in the liquid form under the name of “blasting oil;” but the
dangers attending the handling of the substance in this state are so
great, that it is now usual to mix the liquid with some powdered
substance which is itself without action, and merely serves as a vehicle
for containing the nitro-glycerine. To mixtures of this kind the names
“_dynamite_,” “_dualine_,” “_lithofracteur_” &c., have been given.
It is now hardly necessary to point out that the discovery of these new
explosives has largely extended our power over the rocks, enabling works
to be executed which would have been considered impracticable with less
powerful agents. It is true that the most fearful disasters have been
accidentally produced by the new explosives; but such occasional
devastation is the price exacted for the possession of powerful agents.
And just as in other cases—steam, for example—where great forces are
dealt with, so these new powers must be managed with unceasing care, and
placed in the hands of only skilful and intelligent men.
The products of the combustion of gunpowder are not all gaseous, but
include solid compounds, such as carbonate and sulphate of potassium. It
is these that give rise to the smoke seen when a gun is discharged, and
which, in rapid firing, soon obscures the sight of the objects aimed at.
They are also the causes of the fouling of the bore. Gun-cotton is quite
unexceptionable in these respects, and that prompted the attempts made
soon after its introduction to use it instead of gunpowder in fire-arms.
But the explosion of gun-cotton was found too sudden and violent for
guns and rifles, so that many serious accidents in consequence occurred.
The next thing done was to lessen the rapidity of the explosion by using
gun-cotton mixed with ordinary cotton, or twisted in threads round some
inert substance—in fact, to mitigate the violence of the shock by some
mechanical disposition of the material. The introduction of rapid firing
guns and repeating rifles forced on the problem of a smokeless powder;
and as the plan of replacing nitrate of potassium, in ordinary
gunpowder, by nitrate of ammonium was found to be attended with loss of
the keeping quality of the powder, other materials, such as picric acid,
which forms also the basis of the explosive called _mélinite_, have been
proposed. The composition of _mélinite_ was long a mystery, and that of
the smokeless powder adopted by the French was so carefully concealed
that many experiments had to be made by other nations to discover some
similar preparation, which was found possible by combining certain
substances with gun-cotton so as to modify the violence of its
explosion, and produce a manageable material having the required
properties. The British Government, after many experiments and much
careful testing, decided to adopt _cordite_, made of nitro-glycerine, in
which, by the aid of volatile solvent, di-nitro-cellulose is dissolved,
together with a little mineral oil. The semi-fluid composition, forced
through a round hole or die, comes out like a thread or cord, which the
evaporation of the volatile solvent leaves with very much the appearance
of common brown window-cord. This material has the several advantages of
keeping well, of being _uniform_ in its propulsive powers, of being
capable of imparting as high a velocity as a much larger charge of the
ordinary black gunpowder, while at the same time exercising a less
pressure on the chase of the gun.
It will have become obvious from the preceding paragraphs that,
according to the conditions under which an explosive is to be used,
selection must be made of the most suitable. For example, the substances
employed for propelling projectiles from guns must not have the violent
rending power of certain others, which, by this very property, are most
useful for blasting operations; and, again, although explosives of this
last kind are inadmissible as projectile agents, they are of the kind
best adapted for use in shells where it is the disruptive action that is
required. Also in blasting operations, the explosive has to be adapted
to the nature of the work, and it has been found that a substance which
has worked well in driving a heading for a tunnel through one kind of
rock may prove both slow in progress, and more costly in expenditure,
when some different kind of rock is reached. Besides this, regard must
be had in blasting operations to the nature of the effect required,
which is in some instances a shattering of the rock into fragments, in
others a detachment of it in masses. Thus, in the working of a slate
quarry, the explosive used must not be of a nature to shiver the rock
into useless splinters, but must operate in such a manner that compact
masses may be separated from the mountain side in a condition suitable
for cleaving, by appropriate tools, into numberless broad laminæ, which,
trimmed into rectangular shape, constitute our well-known roofing
slates. The blasting used on a coal seam must be so conducted as to
yield the material as much as possible in big lumps or cobbles rather
than in slack. When granite is blasted for the purpose of obtaining
building stones, the explosive must be one that, by its comparatively
slow action, divides the compact rock into the largest possible blocks.
On the other hand, when granite is blasted merely with the object of
removing it, as when a tunnel has to be driven through a mass of it, the
most disintegrating agent is then the best. The common popular
expressions by which the two classes of explosives just referred to are
distinguished are “high explosives” and “low explosives.” Dynamite may
be taken as a type of the former, and gunpowder a type of the latter. As
will be gathered from what is to follow, no definite separation between
these classes can be fixed, but in a general way it may be said that,
where a destructive, rather than a propelling or pressure effect is
required, the explosive used is one brought into operation by a
concussive or detonating priming, and acting mostly by detonation within
itself, such as dynamite, &c.
Whereas, up to nearly the middle of the nineteenth century, gunpowder
was practically the only explosive in use for either civil or military
purposes, the close of the century can show a list of several hundred
preparations that have been proposed or actually used in its stead. The
names by which these are put forward are expressive sometimes of an
ingredient in their composition, such as “ammonia dynamite,”
“cellulosa,” “mica powder,” “dynamite au carbon,” “dynamite de boghead,”
&c.; and sometimes the inventor’s name, as “So-and-so’s powder or
explosive”; sometimes of the strength of the mixture under various
fanciful names, such as “dynamite,” “heraklin,” “vigorite,” &c., &c.;
sometimes the names relate to the appearance of the compound, as “white
gunpowder,” “blasting gelatine,” &c., &c.; and sometimes to other
circumstances, such as “pudrolithe,” “saxifragine,” “safety powder,”
&c., &c. A very long list might be given of the substances severally
used in these various compositions. It will be sufficient to indicate
the general nature of the several classes into which the new explosives
may be divided. By turning back to p. 746, the reader will be reminded
of the composition of gunpowder, and of the part played therein by the
nitre (nitrate of potassium). Now a considerable number of the recently
patented explosives are simply modified gunpowders, which all contain
some nitrate, replacing wholly, or in part, the nitrate of potassium,
while sulphur is an ingredient of nearly all, and in many, the charcoal
of gunpowder is partly or wholly replaced by other carbonaceous
materials, such as sawdust, coal-dust, tan, starch, paraffin,
lycopodium, graphite, peat, flour, bran, &c. Certain mineral salts enter
into the composition of some, such as sulphate of iron, carbonate of
copper, sulphide of antimony, &c., &c.
In another class of the newer explosives chlorate of potassium takes the
place of the nitrate as the oxygen supplying material, with similar
variations as to the carbonaceous matter as are referred to above.
Yellow prussiate of potash and sugar sometimes replace both the charcoal
and sulphur of gunpowder in this class. Explosives of this chlorate
class are usually dangerous to manufacture, and are often very
sensitive, and also liable to changes by keeping, which render them
still more dangerous.
The next class of preparations brings us to the “high explosives,”
properly so called, and it is among these that most notable preparations
are met with. Of all the explosive nitro-compounds, gun-cotton was the
first practically employed (_vide_ p. 741); but very soon after
nitro-glycerine was discovered by Sobrero when working in Pelouze’s
laboratory. This afterwards became known as “blasting oil,” but it was
many years before nitro-glycerine came into use as an explosive, namely,
when, about 1860, Nobel, a Swedish engineer, had established factories
for its production as an agent for blasting. At first there were
difficulties and dangers attending its use, and it was only when Nobel
had discovered the detonation method of setting free its tremendous
energy that the new era of “high explosives” really commenced. Between
1860 and 1870 such a number of appalling catastrophes occurred in the
handling of the new “blasting oil” that in several European countries
its use was entirely prohibited. And, in England at least, this
prohibition remains, for “in a liquid state this explosive cannot be
sold in, or imported into this country. It is manufactured under the
strict provision that it is forthwith made up into dynamite or some
kindred licensed explosive.” ... “The only source, practically speaking,
of nitro-glycerine on a commercial scale in this country is the factory
of Nobel’s Explosive Co. (Ltd.) at Ardeer, in the county of Ayr.”[17]
Nitro-glycerine being so extremely dangerous to handle in the liquid
form led Nobel to propose its use in an altered condition, by causing it
to be absorbed by some inert porous material, the most suitable being a
siliceous earth found in Germany, and there known as _kieselguhr_, of
which one part will absorb three times its weight of liquid
nitro-glycerine. Here we have the original dynamite, but now other
substances are used for absorbing the liquid, and there are, indeed,
dynamites of two different classes:
1. Dynamites with inert absorbents.
2. Dynamites with absorbents which are themselves combustible, or
explosive.
Footnote 17:
Major Cundill, H.M.’s Inspector of Explosives.
Of the latter class there are endless varieties. One that has latterly
been much used is called “blasting gelatine,” and is practically a
combination of nitro-glycerine and nitro-cotton, this last ingredient
being a less nitrated cellulose than gun-cotton. Blasting gelatine
contains a very large percentage of nitro-glycerine (93–95 per cent.),
and has the appearance of stiff jelly of a pale yellow colour. It may be
of interest to remark that this second class of dynamites admits of
well-defined sub-divisions according to the nature of the absorbent, as:
(_a_) Charcoal, or other simple carbonaceous material.
(_b_) Gunpowder, or other nitrate or chlorate mixtures.
(_c_) Gun-cotton, or other nitro-compounds.
[Illustration:
FIG. 340.—_View on the Tyne._
]
MINERAL COMBUSTIBLES.
Certain mineral combustibles may fairly claim attention in a work
treating of the discoveries of the nineteenth century, not because these
bodies have been known and used only in recent times, but for other
reasons. The true nature of coal—that most important of all
combustibles—its relation to the past history of the earth, and to the
present and future interests of mankind; the work it will do; the extent
of the supply still existing in the bowels of the earth; the innumerable
chemical products which it yields—are subjects on which the knowledge
gained during the present century forms a body of discovery of the most
interesting and important kind. Another substance we have to mention,
though not a modern discovery, has lately been found in far greater
abundance, and is now so largely used for various purposes, that it has
become an important article of commerce.
_COAL._
[Illustration:
FIG. 341.—_Fossil Trees in a Railway Cutting._
]
Most persons know, or at least have been told, that coal is fossil
vegetable matter,—the long-buried remains of ancient forests. But
probably many receive the statement, not perhaps with incredulity, but
with a certain vague notion that it is, after all, merely a daring
surmise. And, indeed, nothing is at first sight more unlike stems, or
leaves, or roots of plants than a lump of coal. Then everybody knows
that coal is found thousands of feet beneath the surface of the earth,
whereas plants can grow only in the light of the sun. One begins to
understand the matter only when the teachings of geology have shown him
that, so far from the crust of the earth being, as he is apt to suppose,
fixed and unchangeable, it is in a state of constant fluctuation.
Changes in the levels of the ground are always going on: in one place it
is rising, in another sinking; here a tract of land is emerging from the
ocean, there a continent is subsiding beneath the water. The extreme
slowness with which these changes proceed causes them to escape all
ordinary observation. The case may be compared to the hour-hand on the
dial, which a casual spectator might pronounce quite stationary, since
the observation of a few seconds fails to detect its movement. As the
whole period comprehended in human annals counts but as a second of
geological time, it cannot be wondered at that it required a vast
accumulation of facts, and much careful and patient deduction from them,
before a conclusion was reached so apparently contradictory of
experience. It is, indeed, startling to learn that “the sure and
firm-set earth” is in a state of flow and change. Even the “everlasting
hills” give evidence that their materials were collected at the bottom
of the sea, and we know that the water which runs down their sides is
slowly but surely carrying them back particle by particle. Of the
magnitude of the changes which the surface of the earth has undergone in
times past, and which are still imperceptibly but constantly proceeding,
the ordinary experience of mankind can of itself give no example. But
such changes have sufficed to entomb a vast quantity of relics of the
innumerable forms of vegetation which flourished and waved their
branches in the sun, ages upon ages before the advent of man.
[Illustration:
FIG. 342.—_Impression of Leaf found in Coal Measures (Pecopteris)._
]
It may be thought impossible that vegetable matter should have so
changed as to become a dense, black, glistening, brittle mass, showing
no obvious forms of leaves or texture of wood. But no one who has seen
how a quantity of damp hay closely pressed together will, after a time,
become heated and change in colour to black, can have any difficulty in
comprehending how chemical and mechanical actions may completely alter
the aspect of vegetable matter. We have, however, the most direct
evidence of the vegetable origin of coal in the numberless
unquestionable forms of trees and plants met with in all coal strata.
Sometimes the trunks of the trees fossilized into stony matter are found
upright in the very situation in which they grew. Thus in Fig. 341 is
represented the appearance exhibited by the trunks and roots of some
fossil trees, which were exposed to view in the formation of a railway
cutting between Manchester and Bolton. In every coal-field also
beautiful impressions of the stems and leaves of plants are met with—one
common form of which is shown in Fig. 342. Most of the plants so found
belong to the flowerless division of the vegetable kingdom. Some are
closely allied to the ferns of the present day—to the common
“mare’s-tail” (_Equisetum_), to the club-moss, and to other well-known
plants. The firs and pines of the coal age are scarcely distinguishable
from existing species. If a fragment of ordinary coal be ground to a
very thin slice—so thin as to be transparent—and placed under the
microscope, it will show a number of minute rounded bodies, which are,
there is good reason to believe, nothing else than the spores or seeds
of plants, closely resembling the existing club-mosses. The spores of
the club-moss (_Lycopodium_) are so full of resinous matter, that they
are used for making fireworks and the flashes of lightning at theatres.
It is, therefore, extremely probable that the bitumen of coal is due to
the resin of similar spores, altered by the effects of subterranean
heat. The immense abundance of these little spores in the coal is a
proof that they accumulated in the ancient forests as the mosses grew,
and therefore the matter of coal was not accumulated under water or
washed down into the sea; for these little spores are extremely light,
and they cannot be wetted by water, and therefore they would have
floated on the surface, and would not have been found so diffused
throughout the coal. Fig. 343 is a picture of the possible aspect of the
ancient forests of the coal age. In the humid atmosphere which probably
prevailed at that period, the large tree-ferns and gigantic club-mosses,
which are conspicuous in the picture, must have flourished luxuriantly.
[Illustration:
FIG. 343.—_Possible Aspect of the Forests of the Coal Age._
]
The immense importance of coal for domestic purposes will be obvious
from the fact that it is estimated that in the United Kingdom alone no
less than 30,000,000 tons are annually consumed in house fires. Another
great use of coal is in the smelting, puddling, and working of iron, and
this probably consumes as much as our domestic fireplaces. Then there is
the vast consumption by steam engines, by locomotives, and by
steamboats. Another purpose for which coal is largely used is the making
of illuminating gas; and to the foregoing must also be added the
quantity which goes to feed the furnaces necessary in so many of the
arts—such as in the manufacturing of glass, porcelain, salt, chemicals,
&c. The quantity of coal raised in Great Britain was not accurately
known until 1854, when it was ordered that a register should be kept,
and an annual return made. The following figures, in round numbers, are
the returns published up to 1873. The table is continued in Note A.
Year. Coal raised, in Tons.
1854 64,661,000
1855 64,453,000
1856 66,645,000
1857 65,395,000
1858 65,008,000
1859 71,979,000
1860 83,208,000
1861 85,635,000
1862 83,638,000
1863 88,292,000
1864 92,787,000
1865 98,150,000
1866 101,630,000
1867 104,500,000
1868 103,141,000
1869 107,427,000
1870 110,289,000
1871 117,352,000
1872 123,497,000
1873 127,017,000
The first return showed our annual produce to be 64,661,000 tons. The
amount did not greatly vary until 1859, when there was an increased
production of nearly seven millions of tons; in 1860 a further increase
of eleven millions of tons more. Since then the quantity annually raised
has been increasing. Comparing the quantity which has been raised in any
year after 1863 with that raised ten years before, we see that the
increase in ten years is nearly half as much again; or, that at the
present rate of increase the amount annually raised doubles itself at
least every twenty years. Now, the question arises, How long can this go
on? However great may be the store of coal, it must sooner or later come
to an end. Is it possible to calculate how long our coals will last? and
what are the results of such calculations? These calculations have been
made; but there are great discrepancies in the results, for the
estimates of the amount of available coal still remaining vary greatly,
and different views are held regarding the rate of consumption in the
future. A very liberal estimate, by an excellent authority, of the
quantity of coal remaining under British soil, makes it 147,000 millions
of tons. With a consumption stationary at the present rate, this will
last 1,200 years; with an increase of consumption of 3,000,000 tons a
year, 276 years; but if the consumption continues to increase in the
same geometrical ratio it has hitherto followed, the supply will
scarcely last 100 years. It cannot be conceived, however, that this last
will be the real case, for the increasing depth to which it will be
necessary to go will soon cause a great increase in the cost, and thus
effectually check the consumption. Great Britain will, however, be
compelled to retire from the coal trade altogether, by the cheaper
supplies which other countries will yield, long before the absolute
exhaustion of her own coal-fields. It is calculated that the coal-fields
of North America contain thirteen times as much as those of all Europe
put together. Coal is also found abundantly in India, China, Borneo,
Eastern Australia, and South Africa; and it is believed that these
stores will supply the world for many thousand years.
[Illustration:
FIG. 344.—_The Fireside._
]
Seeing, then, that our supply of coal has a limit, and that at the
present increasing rate of consumption, the chief source of the wealth
of Great Britain must necessarily be exhausted in a few more centuries,
it behoves us to turn our mineral treasures to the best account, and to
adopt every possible means of obtaining from our coal its whole
available heat and force. The amount of avoidable waste of which we are
guilty in the consumption of coal is enormous. This is especially the
case in its domestic use, where probably nineteen-twentieths of the heat
produced is absolutely thrown away—sent off from the earth to warm the
stars. In England people look upon the wide open fireplace as the image
and type of home comfort. No doubt there are, from long use and habit,
many pleasing associations which cluster round the domestic hearth; but
we, to whom it is given to “look before and after,” must think what it
takes to feed that wide-throated chimney. All but a very small fraction
of the heat thus escapes, and is lost to man and the world for ever; and
surely we shall deserve the curses of our descendants if we continue
recklessly to throw away a treasure which, unlike the oil in the widow’s
cruse, is never renewed—for there is no contemporaneous formation of
coal. Thanks to the enhanced price of coal during the last few years,
some attention has been directed to contrivances for the economical
consumption of coal in its domestic, as well as in its manufacturing,
applications.
A time, however, will sooner or later come, when the whole available
coal shall have been consumed. What will then be the fuel of the
engines, and steamboats, and locomotives of the future? The reader may
think that then it will only be necessary to burn wood. But wood is
already being consumed from the face of the earth much more rapidly than
it is produced. How, then, can it be available when coal fails? The
truth is, we are now consuming not merely the wood which the sun-rays
are building up in our own time, but in hewing down the forests we are
using the sun-work of a century, while in coal we have the forests of
untold ages at our disposal—the accumulated combustible capital stored
up during an immense period of the earth’s existence. Upon this
stored-up capital we are now living, our current receipts of sun-force
being wholly inadequate to meet our expenditure. The coal is the
sun-force of former ages; and it is from this we are now deriving the
energy which performs most of our work. George Stephenson long ago
declared that his locomotives were driven by sunshine—by the sunshine of
former ages bottled up in the coal. And he was right. The mechanical
energy of our steam engines, and the chemical energy of our blast
furnaces, are derived from the combustion of vegetable matter, in which
the heat and light of the sun—our present sun or that of the coal
ages—are in some way stored up. The burning of wood or coal is,
chemically, the reverse action to that performed by the sunlight: by the
former carbon and oxygen are united, by the latter they are separated.
We foresee, then, a future period—however distant may be that future—in
which the world’s capital shall have been exhausted, and the energies
which are now employed in doing the world’s work will no longer be
available. But the reader will perhaps think that by improvements in the
steam engine, and in other ways, means will be found of getting more and
more work out of coal. It is true that we obtain from coal only a
_fraction_ of its available energy; but the whole work which could, by
any possible process, be done by the combustion of coal is _definite and
limited_, although its amount is large. A pound of coal burnt in one
minute sets free an amount of energy which would, if it could all be
made available, do as much as 300 horses working in the same time. But,
again, the reader may think, even if at some distant future the supplies
of fuel for the steam engines of our remote posterity should fail, that
before that time some other form of force than steam or heat engines
will have been discovered—some application of electricity, for example.
Now, it will appear, from principles which will be discussed in a
subsequent article, that not only is there no probability of such a
discovery, but that now, when the relations of the whole available
energies of the globe have been traced and defined, Science can find no
ground for admitting such a possibility under the present condition of
the universe.
_PETROLEUM._
When coal is heated in closed vessels, there are given off, as we shall
presently see, a number of gaseous and volatile products—many being
compounds of carbon and hydrogen—which condense to liquids or solids at
ordinary temperatures. Carbon is by far the largest constituent of coal,
which commonly contains only about 10 per cent. of other substances,
although the proportions vary very widely. Another important constituent
of coal is its hydrogen, and the value of coal as a source of heat
depends almost entirely upon the carbon and hydrogen it contains. Carbon
is one of the most remarkable of all the elements of the globe for its
power of entering into an enormous number of compounds. Thus, for
example, the compounds of carbon with only hydrogen are innumerable; but
they are all definite, and their composition is expressible by the
admirable system of chemical symbols, of which the reader has now, it is
hoped, some definite notion. Perhaps these hydro-carbons are among the
best evidences which could be adduced that modern science has obtained a
grasp of certain conceptions which have a real correspondence with the
actual facts of nature, even as regards the intimate constitution of
matter. This is not the place to enter into a complete exposition of
this subject. We may, however, invite the reader’s attention to a few
simple facts. A very large number of compounds of carbon and hydrogen
are known. If the percentage compositions of these be compared together,
it is only the eye of a most expert arithmetician which can detect any
relation between the proportions of the constituents in the various
compounds. The chemist, however, by associating such of these compounds
as resemble each other in their general properties, finds that they can
be arranged in series, in which the composition is accurately expressed
by multiples of the proportions: C = 12, H = 1. And not only so, the
different series themselves form a series of series, having a simple
relation to each other. Thus, confining ourselves to some of the known
hydro-carbons, we have the following:
┌──────────────────────┬──────────────────────┬──────────────────────┐
│ A │ B │ C │
├──────────────────────┼──────────────────────┼──────────────────────┤
│ C H_{4} │ C H_{2} │ │
│ C_{2}H_{6} │ C_{2}H_{4} │ C_{2}H_{2} │
│ C_{3}H_{8} │ C_{3}H_{6} │ C_{3}H_{4} │
│ C_{4}H_{10} │ C_{4}H_{8} │ C_{4}H_{6} │
│ C_{5}H_{12} │ C_{5}H_{10} │ C_{5}H_{8} │
│ C_{6}H_{14} │ C_{6}H_{12} │ C_{6}H_{10} │
│ &c. │ &c. │ &c. │
│ C_{_n_}H_{2_n_ + 2} │ C_{_n_}H_{2_n_} │ C_{_n_}H_{2_n_ – 2} │
└──────────────────────┴──────────────────────┴──────────────────────┘
┌──────────────────────┬──────────────────────┬──────────────────────┐
│ D │ E │ F │
├──────────────────────┼──────────────────────┼──────────────────────┤
│ │ │ │
│ │ │ │
│ C_{3}H_{2} │ │ │
│ C_{4}H_{4} │ C_{4}H_{2} │ │
│ C_{5}H_{6} │ C_{5}H_{4} │ C_{5}H_{2} │
│ C_{6}H_{8} │ C_{6}H_{6} │ C_{6}H_{4} │
│ &c. │ &c. │ &c. │
│ C_{_n_}H_{2_n_ – 4} │ C_{_n_}H_{2_n_ – 6} │ C_{_n_}H_{2_n_ – 8} │
└──────────────────────┴──────────────────────┴──────────────────────┘
This table might be indefinitely extended, but enough is given to enable
the intelligent reader to discover the laws connecting these formulæ.
The series headed B, it will be observed, have all the same percentage
composition. Why, then, one formula rather than another? The answer to
this question is the statement of a theoretical law upon which the whole
science of modern chemistry is based; for it has the same relation to
that science as gravitation has to astronomy. It is a matter of fact
that all gases, whatever their chemical nature, expand alike with the
same application of heat, and all obey the same law, which connects
volumes and pressures. These are very remarkable uniformities, for gases
in this respect exhibit the most decided contrast to liquids and solids.
The volume of each solid and of each liquid has its own special
relations to temperature and pressure: here there is endless diversity.
The volumes of all gases have one and the same relation to temperature
and pressure: here there is absolute uniformity. As an explanation of
these and other facts relating to gases, Amedeo Avogadro, in 1811, put
forward this hypothesis—_Equal volumes of all gases, under like
circumstances of temperature and pressure, contain the same number of
molecules_. This hypothesis was revived by Ampère a few years later, and
sometimes is called his. A necessary consequence of this law is that the
weights of the molecules of gases are proportional to their densities or
specific gravities. Hence when the percentage composition of a
hydro-carbon has been determined, by burning or oxidizing it in such a
manner as to obtain and weigh the products, carbonic acid and water, the
next thing the chemist does is to obtain the weight of a volume of the
gas. The number of times this exceeds the weight of hydrogen gas, under
the same conditions, expresses how many times the molecule is heavier
than the hydrogen molecule. Now, the chemist’s unit of weight in these
inquiries is the weight of a single _atom_ of hydrogen; and, as there
are grounds for believing that the _molecule_ of hydrogen consists of
two atoms of that substance, its weight = 2. Now, if the molecule of
marsh gas, the first hydro-carbon in the above list, has the composition
assigned, it will be 12 + 4 = 16 times heavier than the _atom_ of
hydrogen, and 16/2 = 8 times heavier than the _molecule_ of hydrogen.
Hence, if Avogadro’s law be correct, marsh gas should be just eight
times heavier than hydrogen gas; which is really the fact. The formula
expressing the composition of the molecule of a hydro-carbon, or of any
chemical compound whatever, is always so fixed that the same relations
may hold; and almost the first thing a chemist does in examining a new
body is to endeavour to obtain it in the state of gas.
The first four members of the series headed A are gases at ordinary
temperatures, the fifth is a gas at temperatures above the
freezing-point, and a liquid at lower temperatures; the next following
members are liquids which boil (that is, are converted into gases) at
temperatures rising with each additional carbon atom about 20° F. The
specific gravities and boiling-points of these liquids augment as we
pass from one hydro-carbon to the next, and the lower members of the
series are solids, fusing at temperatures higher and higher as the
number of carbon atoms is greater. Similar gradations of properties are
exhibited by the other series of hydro-carbons. Petroleum or rock-oil is
the name given to liquid hydro-carbons found in nature, and consisting
chiefly of compounds belonging to the series marked A in the above list.
Some varieties of petroleum hold in solution other hydro-carbons, and in
some cases paraffin is extracted from the oils by exposing the liquid to
cold, when the solid crystallizes out. Paraffin is a solid belonging to
the B series, and it is for the most part obtained by heating certain
minerals.
Deposits of liquid hydro-carbons, perhaps formed by a kind of natural
subterranean distillation from coal or other fossil organic matter,
exist in various localities. These deposits have long been known and
utilized at Rangoon, in Burmah, and on the shores of the Caspian Sea. At
Rangoon the mineral oil is obtained by sinking wells about 60 ft. deep
in a kind of clay soil, which is saturated with it. The oily clay rests
upon a bed of slate also containing oil, and underneath this is coal. It
may be supposed that subterranean heat, acting upon the coal, has
distilled off the petroleum, which has condensed in the upper strata.
This petroleum, when distilled in a current of steam, leaves about 4 per
cent of residue, and the volatile portion contains about one-tenth of
its weight of a substance (paraffin) which is solid at ordinary
temperatures. After an agitation with oil of vitriol, and another
distillation, _rock oil_ or _naphtha_ is obtained, which, however, is
still a mixture of several distinct chemical compounds. Mineral oils
have also been found in China, Japan, Hindostan, Persia, the West India
Islands, France, Italy, Bavaria, and England. In one of the Ionian
Islands there are oil-springs which have flowed, it is said, over 2,000
years.
But it is the recently discovered and extremely copious springs and
wells in Pennsylvania and Canada which have given a vastly extended
importance to the trade in mineral oil. Rock oil is now used in enormous
quantities as the cheapest illuminating oil, and that which furnishes
the most intense light. Its consumption as a lubricating oil for
machines has also been very large. Mineral oil was occasionally found at
various places in the United States, and sometimes used by the
inhabitants of the locality before the recent discoveries; but it was
not until August, 1859, that it was met with in large quantities. About
this time a boring which was made at Oil Creek, Pennsylvania, reached an
abundant source, for 1,000 gallons a day were drawn from it for many
weeks. The news of the discovery of this copious oil-spring spread
rapidly: thousands of persons flocked to the neighbourhood in hopes of
easily making a fortune by “striking oil.” Before the end of 1860 more
than a thousand wells had been bored, and some of these had yielded
largely. The regions of North America in which petroleum has been found
cover a large part of the States, and comprise Pennsylvania, New York,
Ohio, Michigan, Kentucky, Tennessee, Kansas, Illinois, Texas, and
California. In the vicinity of Oil Creek the bore-holes are usually
about 3 in. or 4 in. in diameter, and are often 500 ft. deep, and even
800 ft. is not uncommon. To make a bore-hole 900 ft. deep, and procure
all the requisites—steam engines, barrels, &c., for pumping the
oil—costs about $5,000. In 1869 many of these wells still yielded
regularly 300 barrels a day, but the supply has not continued with the
same abundance. One of the luckiest wells flowed at its first opening at
the rate of about 25,000 barrels a day. The apparatus used for working
the oil-wells is very simple—a rude derrick, a small steam engine, a
pump, and some barrels and tubs being all that is necessary. Fig. 345
will give the reader an idea of the scene presented by a cluster of
oil-wells in the Oil Creek region. Oil Creek received its name before
the petroleum trade was established, from the oil found floating on the
surface of the water. It is on the Alleghany River, about 150 miles
above Pittsburg, and here at its mouth is situated Oil City. There is a
wharf in Pittsburg for the oil traffic, and the barrels are brought down
the river in flats, or the oil is poured into very large flat boxes,
divided into compartments, which are then closed, and the boxes floated
down in groups of twenty or more. The refining process consists in
placing the crude oil into a large iron retort, connected with a
condenser formed of a coil of iron pipes, surrounded by cold water. Heat
is applied, and the lighter hydro-carbons (naphtha) come over first.
After the naphtha, the oils which are used for illuminating purposes
distil off. A current of steam is then forced into the retort, and this
brings over the heavy oils which are used for greasing machinery. A
black tarry oil yet remains; and, finally, after the separation of this,
a quantity of coke. The products are subjected to certain processes of
purification, which need not here be described. The magnitude of the
American oil trade may be inferred from the fact that in the second year
of its existence, from January to June, 1862, more than 4,500,000
gallons were exported from four seaports. This can hardly be wondered
at, considering the extremely low price at which this excellent
illuminating and lubricating agent can be produced. Refined petroleum
can be bought at Pittsburg for 16 cents. per gallon. It is believed by
some that the supplies of petroleum which exist in various localities
are so abundant that they will furnish illuminating oils to the whole
world for centuries.
[Illustration:
FIG. 345.—_View on Hyde and Egbert’s Farm, Oil Creek._
]
_PARAFFIN._
In the course of some researches on the substances contained in the tar,
which is obtained by heating wood in close vessels, Reichenbach found a
white translucent substance, to which he gave the above name, because it
was not acted upon by any of the ordinary chemical reagents, such as
sulphuric acid, nitric acid, &c. This substance, which is composed of
carbon and hydrogen only, is not unlike spermaceti; it is colourless,
translucent, and without smell or taste. But when slightly warmed, it
becomes very plastic, and may then be moulded with the greatest ease—and
in this respect it differs from spermaceti. Paraffin melts at from 88°
to 150° C., to a colourless liquid, which is so fluid that it may be
filtered through paper like water, and at a higher temperature it can be
distilled unchanged. Paraffin does not dissolve in water, and is but
slightly soluble in alcohol. In ether, naphtha, turpentine, benzol, and
sulphide of carbon, it dissolves very readily. When heated with sulphur,
it is decomposed: the sulphur seizes upon its hydrogen, sulphuretted
hydrogen is given off, and the carbon is separated; and this action has
been proposed as a ready means of obtaining pure sulphuretted hydrogen
for laboratory use. It is probable that paraffin is a mixture of various
hydro-carbons, having a composition expressible by the formula,
C_{_n_}H_{2_n_}; for different specimens fuse at different temperatures,
according as the paraffin has been obtained from one or the other
source.
In the year 1847, Dr. Lyon Playfair directed the attention of Mr. James
Young, then of Manchester, to a dense petroleum which issued from the
crevices of the coal in a Derbyshire mine. It was soon found that this
substance yielded a distillation—a pale yellow oil—which, on cooling,
deposited solid paraffin. Mr. Young, recognizing the importance of this
discovery, had an establishment at once erected on the spot, and the
work of extracting paraffin was carried on until the supply of the
petroleum had become nearly exhausted. Forced to seek for other sources
of paraffin, Mr. Young was fortunate enough, after many trials, to
discover that a species of bituminous coal, which occurs at Boghead,
near Bathgate, in the county of Linlithgow, yielded by distillation
annually large quantities of paraffin. In 1850 he procured a patent for
“treating bituminous coals to obtain paraffin, and oil containing
paraffin, therefrom.” This method consisted in distilling the coal in an
iron retort, gradually heated up to low redness, and kept at that
temperature until the volatile products ceased to come off. Under this
patent, Mr. Young developed the manufacture of paraffin into a new and
important branch of industry. The oil which first comes over in the
distillation of the Boghead mineral is largely used for illuminating
purposes under a variety of names besides that of _paraffin oil_, which
term is, we believe, chiefly applied to a less volatile portion,
extensively used for lubricating machinery, and consisting of liquid
hydro-carbons of the same percentage composition as solid paraffin,
which substance it also holds in solution. Mr. Young’s process consisted
in placing the mineral in a retort encased in brickwork—an arrangement
which caused the temperature of the retort to be more uniform than if
the heat of the furnace had been applied to it directly. The retorts
were placed vertically, and they were fed with the mineral by a hopper
at the top. The products of the distillation passed through a worm tube
surrounded by cold water into a cooled receiver. The result of the first
distillation was a crude oily matter, differing from tar in being
lighter than water, and in not drying-up when exposed to the air. This
crude oil was then several times alternately treated with sulphuric acid
and caustic potash, and distilled; and when about two-thirds of the oil
had been separated from the rest, as an oil for burning and lubricating
purposes, the residue yielded paraffin, or “paraffin wax,” as it is
sometimes called. It is estimated that in Scotland no less than 800,000
tons of shale are annually distilled for mineral hydro-carbons, with a
consumption of 500,000 tons of fuel. It is believed that about
25,000,000 gallons of crude oil are thus obtained, and from this 350,000
gallons of illuminating oil, 10,000 tons of lubricating oil, and 5,800
tons of solid paraffin are produced. Among the products exhibited in the
International Exhibition of 1862, was a block of beautifully translucent
paraffin, of nearly half a ton weight.
Paraffin is also obtained on the continent by distilling a variety of
coal termed _lignite_. The tar which comes over is distilled, until
nothing but coke remains. The condensed products are then treated with
caustic soda, in order to remove carbolic acid and other substances.
After washing with water, the oils are treated with sulphuric acid, in
order to remove basic substances. The oil is again washed, and is then
rectified by another distillation. The products which successively come
over are, if necessary, separated by being collected in different
vessels; but sometimes they are mixed together, and sent into the market
as illuminating oils under various names, such as “photogen,” “solar
oil,” &c. Oils having a specific gravity about 0·9 are collected apart,
and are placed in tanks in a very cool place. In the course of a few
weeks the solid paraffin, which is dissolved in the other hydro-carbons,
crystallizes out. The liquid oils are drawn off, and the crude paraffin,
which is of a dark colour, is freed from adhering oil by a centrifugal
machine, and afterwards by pressure applied by hydraulic power. It then
undergoes several other processes of purification before it is obtained
as a colourless translucent solid.
Several thousand tons of paraffin are annually consumed for making
candles, which is the most important application of the material. The
variation in the fusing-points of different specimens is doubtless due
to admixtures in greater or less proportion of other more easily fusible
hydro-carbons. It was on account of the imperfect separation of these
that the candles first made from paraffin were so liable to soften and
bend, and felt greasy to the touch. Paraffin for candle-making is
sometimes mixed with a certain proportion of other substances, such as
palmitic acid, &c. Among the patented applications of paraffin are the
lining of beer-barrels, and the preserving of fruits, jams, and meat.
Some kinds of paraffin are also used in the manufacture of matches.
Liebig once expressed a wish that coal-gas might be obtained in a solid
form: “It would certainly be esteemed one of the greatest discoveries of
the age if any one could succeed in condensing coal-gas into a white,
dry, odourless substance, portable and capable of being placed in a
candlestick or burned in a lamp.” Now, it is curious that paraffin has
nearly the same composition as good coal-gas: it burns with a bright and
smokeless flame, and beautiful candles are formed of it, which burn like
those made of the finest wax. When the fused paraffin first assumes the
solid form, it is transparent like glass; and if it could be retained in
that condition, we might have the pleasing novelty of transparent
candles. But the particles seek to arrange themselves in crystalline
forms, and the substance soon takes on its white semi-opaque appearance.
The great richness of the Boghead mineral in paraffin, which appears to
exist in it ready formed, prevented for many years any successful
competition by the working of other sources of supply. But paraffin is
an abundant constituent of Rangoon petroleum, and considerable
quantities may be obtained by distilling peat, and other fossil
substances. All petroleums and paraffins are, in fact, mixtures of a
number of hydro-carbons, which in many cases cannot be entirely
separated from each other. The accidents which have from time to time
occurred with some of these combustibles, and have caused legislative
enactments with regard to them, are due to the imperfect removal by
distillation of the more volatile bodies, which rise in vapour at
ordinary temperatures. Explosions of the hydro-carbons can occur only
under the same circumstances as with coal-gas; that is to say, the
application of a flame to a mixture of the vapour with atmospheric air.
[Illustration:
FIG. 346.—_View of the City of London Gas-works._
]
COAL-GAS.
When coal is burning in a common fire, we may see jets of smoky gas
issuing from the pieces of coal before they become red hot. This vapour,
coming in contact with flame in another part of the fire, may often be
observed to ignite, thus supplying an instance of gas-lighting in its
most elementary form. In the ordinary fire the air has free access, and
the inflammable gases and vapours continue to burn with flames more or
less bright, and when these have ceased the carbonaceous portion
continues afterward to glow until nearly the whole has been consumed,
except the solid residue which we call the ashes. These ashes in general
contain a portion of unconsumed carbon, mixed with what is chemically
_the ash_, namely, certain incombustible salts, constituting the white
part of the ashes. If, however, we heat the coal in a vessel which
prevents access of air, and allows the gases to escape, the coal is
decomposed much in the same way as when it is burnt in the open fire;
but the products formed are no longer burnt, the supply of oxygen being
cut off. Every one knows the familiar experiment of filling the bowl of
a common clay tobacco-pipe with powdered coal, then covering it with a
dab of clay, and placing it in a fire. The gas which soon comes from the
stem of the pipe does not take fire unless a light be applied, when it
may be seen to burn with a bright flame, and after the flow of gas has
ceased, nearly the whole of the carbon of the coal will be found
unconsumed in the bowl of the pipe. This simple experiment illustrates
perfectly the first step in the manufacture of coal-gas, namely, the
process of heating coal to redness in closed vessels, by which operation
the substances originally contained in coal are destroyed, and their
elements enter into new combinations.
These elements are few in number; for, except the very small portion
which remains as incombustible white ash, coal is constituted of carbon,
hydrogen, oxygen, nitrogen, and a little sulphur. All the varied and
interesting products obtained by the destructive distillation of coal
are combinations of two or more of these four or five elements.
Illuminating gas is far from being the only product when coal is heated
without access of air; for of the numerous substances volatized at the
red heat of the gas-retort a great number are not only incapable of
affording light, but liable to generate noxious compounds when burnt.
Besides this there are numerous bodies which, though leaving the retort
in the gaseous form, immediately assume the liquid or solid state at
ordinary temperatures. All such substances must be separated before
permanent gases are obtained fit for illuminating purposes and capable
of being carried through pipes to distant places. Thus an important part
of the apparatus for gas manufacture consists in arrangements for
separating the condensable bodies, and for removing useless or injurious
gases from the remainder.
[Illustration:
FIG. 347.—_Section of Gas-making Apparatus._
]
The products resulting from the destructive distillation of the coal
may, therefore, be classified as—_a_, solids left behind in the retort;
_b_, solids and liquids condensed by cooling the vapours which issue
from the retort; _c_, coal-gas—a mixture of gases from which certain
useless and noxious constituents must be removed. Fig. 347 is intended
to give a diagrammatic view of the apparatus employed in the generation,
purification, and storage of gas, the various parts being shown in
section. A is the furnace containing several retorts, of which B is one.
From each retort a tube, _d_, rises vertically, and curving downward
like an inverted U, it enters a long horizontal cylinder, _f_; half
filled with water, beneath the surface of which the open end of the
recurved tube dips. The cylinder containing water passes horizontally
along the whole range of furnaces in the gas-works, and is known as the
_hydraulic main_. It is here, then, the tar and the moisture first
condense, and the pipe is always kept half full of these liquids, so
that the ends of the pipes, _d_, from the retorts, dipping beneath its
surface, form traps or water-valves, which allow any retort to be opened
without permitting the gas to escape. As the tar accumulates in the
hydraulic main, it flows over through a pipe, _g_, leading downwards
into the _tar-well_, H. The gases take the same course; but while the
tar flows down the vertical tube, R, the gases pass on through _j_ into
the condensers or refrigerators. Gas cannot escape from the open end of
the tube, for it is always closed by the liquids—tar and ammoniacal
liquor—which accumulate and flow over the top of the open inner vessel
into the cistern, S, from which they are drawn off from time to time by
the stop-cock, I. Although when the gas has arrived at this cistern much
of its tar and ammoniacal vapours have been condensed, a portion is
still retained by reason of the high temperature of the gas; and this
has to be removed before it is permitted to enter the purifier. This is
the object of passing the gas through the series of pipes, _j j_,
forming the _condenser_. These are kept cool by the large surface they
expose to the air, and, when necessary, cold water from the cistern, K,
may be made to flow over them. The tar and other liquids condense in the
iron chest, T, which is so divided by partitions as to compel the gas to
pass through the whole series of tubes; and as the liquid accumulates,
it also overflows into the tar-well. The cooled gas then enters the
purifier, L L, in which are layers of slaked lime placed on a number of
shelves. By contact with the extensive surface of slaked lime the gas
has its sulphuretted hydrogen, carbonic acid, and some other impurities,
removed; and it then, through the tube _n_, enters the gasholder, in
which it is stored up for use.
Hydrated oxide of iron is now much used for purifying coal-gas. The
oxide is mixed with sawdust, and placed in layers 10 in. thick. Sulphide
of iron and water is formed; and when the mixture has ceased to absorb
any more, it is removed and exposed to a current of air; the hydrated
oxide is thus reproduced and sulphur set free. The process may be
repeated many times in succession, until the absorbent power is impaired
by the accumulation of sulphur.
The gasholder—or “gasometer,” as it is often improperly named—is an
immense cylindrical bell, made of wrought iron plates, and inverted in a
tank of water, in which it rises or falls. It is counterpoised by
weights attached to chains passing over pulleys, so as to press the gas
with a small force in order to drive it along the main, which
communicates with the pipes supplying it to the various consumers. The
pressure impelling the gas through the mains does not in general exceed
that of a column of water two or three inches high.
It will be necessary, after this slight outline describing the essential
parts of the apparatus, to enter more fully into the details of the
several parts.
The retorts are constructed of wrought iron, cast iron, or earthenware,
and in shape are cylindrical, with a diameter of 12 in. to 18 in., or
more, and a length of 6 ft. to 10 ft. Though sometimes circular in
section, other forms are commonly used—such as the elliptical, and
especially the ⌓-shaped. The retorts are closed except at the mouth-end,
Fig. 348, from the top of which rises the stand-pipe, A, which has
usually a diameter of about 5 in. When the charge has been introduced,
the mouth is closed by a plate of iron, B, closely and securely applied
by means of a screw, C, as shown in the figure—a perfectly tight joint
being obtained by a luting of lime mortar spread on the part of the lid
which comes into contact with the mouth of the retort. The retorts are
always set horizontally in the furnace—each furnace usually including a
set of five retorts. The charge of coals is introduced on a tray of
sheet iron adapted to the size of the retort, which, when properly
pushed in, is inverted so as turn out the contents, and then withdrawn.
The time required to completely expel the volatile constituents from the
charge in a gas retort varies very much, because there are great
diversities in the composition of the different kinds of coal employed.
Some varieties of coal, such as cannel, are easily decomposed, and the
operation may be complete in about three hours; while other kinds may
require double that time. The quantity of gas procurable from a given
weight of coal also varies according to the kind of coal made use of.
Thus, while a hundredweight of cannel may give 430 cubic feet of gas,
the same weight of Newcastle coals will yield but 370 cubic feet. The
nature of the gases given off from a retort will be different at the
different stages of the operation.
The scene presented by the retort-house of a large gas manufactory, when
viewed at night, is a singular spectacle. The strange lurid gleams which
shoot out amid the general darkness as the retorts are opened to
withdraw the coke, and the black forms of the workmen partially
illuminated by the glare, or flitting like dark shadows across it, form
a picture which might engage the pencil of a Rembrandt. In Fig. 348_a_
is depicted the retort-house at the Imperial Gas Works, King’s Cross.
Here the retorts are arranged in several tiers—the coal being brought,
and the coke withdrawn, by the aid of an iron carriage running on rails
parallel to the line of furnaces.
[Illustration:
FIG. 348.—_The Retort._
]
In the process of heating, a proper regulation of the temperature is of
the highest importance. It is found that when the retorts are heated to
bright cherry-red, the best results are obtained. At a lower temperature
a larger quantity of condensable vapours are given off, which collect in
the gasholders and distributing pipes as solid or liquid, and occasion
much inconvenience, while the quantity of gas obtained is decreased. On
the other hand, if the temperature be too high, some of the gases are
decomposed, and the quantity of carbon contained in the product is so
much diminished as seriously to impair the illuminating power. Again,
every second the gases after their production remain in the red-hot
retort diminishes their light-giving value; for those hydro-carbons on
which the luminiferous power of the gas depends, are then liable to
partial decomposition; a portion of their carbon is deposited on the
walls of the retort in a dense layer, gradually choking it up, while the
liberated hydrogen does not add to the illuminating but to the heating
constituents of the gas. A plan has been patented by Mr. White, of
Manchester, for rapidly removing the illuminating gases from the retort
by sweeping them out by means of a current of what has been termed
“water gas.” This water gas is produced by causing steam to pass over
heated coke, and is a mixture of carbonic acid, carbonic oxide, and
hydrogen. Though only two of these are combustible gases—and even they
do not yield light by their combustion, and, by adding to the bulk of
the gas, serve rather to dilute it—yet it has been found that in some
cases twice the amount of light is obtainable by White’s process than
the same weight of coal supplies when treated in the ordinary manner.
[Illustration:
FIG. 348_a_.—_Retort House of the Imperial Gas-Works, King’s Cross,
London._
]
The hydraulic main, as already mentioned, being kept half full of tar
into which the lower ends of the dip-pipes descend, prevents the gas
from escaping through the stand-pipes when the lid of a retort is
removed for the introduction of a fresh charge. The hydraulic main is
from 12 to 18 in. diameter, and the dip pipes pass into it by gas-tight
joints. Various forms of purifiers are in use besides the simple one
already mentioned. Some of these have arrangements for agitating the gas
with a purifying liquid by mechanical means, the motion being supplied
by a steam engine.
The gasholder, as it sinks in the water of the cistern, presses with
less force on the contained gas, and unless this inequality of pressure
were counteracted there would be very unequal velocities in the flow of
gas from the burner. The equality of pressure is obtained by making the
weight of the chains by which the gasholder is suspended equal to half
the weight the gasholder loses in the same length of its motion.
Gasholders are also constructed without chains or counterpoises, as
these are found to be unnecessary where the height of the gasholder does
not exceed half its width. In such cases, especially when the vessel is
very large, the difference of pressure at the highest and lowest
position is quite inconsiderable, and nothing more is necessary than
that upright guides or pillars be placed to preserve the vertical motion
of the vessel. Another improvement, which enables a lofty gasholder to
be used without increasing the depth of the tank, consists in forming
the gasholder of several cylinders, which slide in and out of one
another like the draw-tubes of a telescope. Each cylinder has a groove
formed by turning up the iron inside the rim, and at the top of the next
cylinder the edge is turned outwards so as to drop in the groove or
channel, which thus forms a gas-tight joint, for it is of course filled
with water as it rises. The pressure is, however, more accurately
regulated by an apparatus called the _governor_, through which the gas
passes in before it enters the mains. The construction and action of the
regulator will be understood from Fig. 349, where A represents a kind of
miniature gasholder, inverted in the cistern, B. From the centre of the
interior of the bell hangs a cone, C, within the contracted orifice of
the inlet-pipe. If this cone be drawn up, the size of the orifice, D, is
reduced, and, on the other hand, by its descent it enlarges the opening
through which the gas passes outward. By properly adjusting the weights
of the counterpoise, E, such a position of the cone may be found that
the gas passes into the mains at an assigned pressure. Suppose, now,
that from any cause the pressure of gas in F increases, that pressure
acting upon the inverted bell, A, causes it to rise and carry with it
the cone, which, by narrowing the orifice of the outlet, checks the flow
of gas. Similarly, a decrease of pressure in the mains would be followed
by the descent of the cone, and consequently freer egress of gas. In
hilly towns it is necessary to fix regulators of this kind at certain
heights in order to equalize the pressure. It is found that a difference
of 30 ft. in level affects the pressure of gas in the same main to about
the same amount as would a column of water one-fifth of an inch high,
the pressure being least at the lowest point.
[Illustration:
FIG. 349.—_The Gas Governor._
]
Coal-gas is a mixture of several gases, and these may be classified as,
first, the light-giving gases, or those which burn with a luminous
flame; secondly, gases which burn with a non-luminous flame, and which
therefore contribute to the _heat_, and not to the light, of a
gas-flame, and have the effect of diluting the gas; third, gases and
vapours which are properly termed impurities, as they are either
incombustible or by their combustion give rise to injurious products. Of
the first kind the principal is olefiant gas, a gas which burns with a
brilliant white flame without smoke. It is a compound of hydrogen and
carbon, six parts by weight of carbon being combined with one part by
weight of hydrogen. Besides olefiant gas other gaseous hydro-carbons are
found in smaller quantities. These contain a larger proportion of carbon
than olefiant gas. The second class contains hydrogen, light carburetted
hydrogen, and carbonic oxide. Hydrogen is one element of water, of which
it forms one-ninth of the weight. It burns with a flame giving
singularly little light, but having intensely heating power; in fact,
one of the brightest lights we can produce is obtained by allowing the
flame of burning hydrogen to heat a piece of lime. Light carburetted
hydrogen, like olefiant gas, is a compound of hydrogen and carbon, but
the proportion of carbon to hydrogen is only half what it is in olefiant
gas, namely, three parts to one. This gas enters largely into the
composition of coal-gas, and occurs naturally in the coal seams, being,
in fact, the dreaded _fire-damp_ of the miner. It is much lighter than
olefiant gas, for while that gas is of nearly the same specific gravity
as atmospheric air, light carburetted hydrogen is only a little more
than half that specific gravity. It is this ingredient of coal-gas which
renders it so light as to be available for inflating balloons. It burns
with either a bluish or a slightly yellow flame, yielding hardly any
light. Olefiant gas and the other luminiferous hydro-carbons, when
exposed to a bright red heat, split up for the most part into this gas
and carbon. This explains the importance of rapidly removing the gas
from the retort in which it is generated, a point which has been
referred to above. Carbonic oxide is a gas which one may often see
burning with a pale blue flame above the glowing embers of a common
fire, the flame giving, however, little light. It is a compound of
carbon and oxygen, containing only one-half the quantity of oxygen which
its carbon is capable of uniting with, and therefore ready to unite with
another proportion, which it does in burning, carbonic acid being the
product.
The third class of constituents of coal-gas—the impurities—are those
which the manufacturer strives to remove by passing the gas over lime,
milk of lime, oxide of iron, &c. Sulphuretted hydrogen, a compound of
sulphur and hydrogen, has an extremely nauseous odour resembling that of
rotten eggs. It is always formed in the distillation of coal, and if not
removed from the gas in the process of purification, it has a very
objectionable effect; for one product of its combustion is sulphurous
acid, and in a room where such gas is burnt much damage may be done by
the acid vapours; for example, the bindings of books, &c., soon become
deteriorated from this cause. The detection of sulphuretted hydrogen in
coal-gas is quite easy, for it is only necessary to hold in a current of
the gas a piece of paper dipped in a solution of the acetate of lead. If
in a few minutes the paper becomes discoloured the presence of
sulphuretted hydrogen is indicated.
But the _bête noire_ of the gas-maker is a substance called “sulphide of
carbon,” which is formed whenever sulphur and carbonaceous matters are
brought together at an elevated temperature. Sulphide of carbon is, in
the pure state, a colourless liquid, of an intensely offensive odour,
resembling the disagreeable effluvia of putrefying cabbages. The liquid
is extremely volatile, and coal-gas usually contains some of its vapour.
When too high a temperature is used in the generation of the gas, it
contains a large quantity of this deleterious ingredient, especially if
the amount of sulphur contained in the coal is at all considerable. This
sulphide of carbon vapour is very inflammable, and one product of its
combustion is a large quantity of sulphurous acid. This substance cannot
be removed from coal-gas by any process sufficiently cheap to admit of
its application on the large scale. It is said, however, that by passing
the gas over a solution of potash in methylated spirit, the sulphide of
carbon vapour can be completely got rid of. The price of these materials
renders the process available in special cases only, where the damage
done by the sulphurous acid would be serious, as in libraries, &c.
Besides the impurities we have already enumerated, many others are
present in greater or less quantity. Carbonic acid—the gas resulting
from the complete combustion of carbon—should be entirely removed by the
lime purifiers, as the presence of even a small percentage detracts
materially from the illuminating power. This gas is not inflammable and
cannot support combustion. It has decided acid properties, and readily
unites with alkaline bases forming carbonates: it is upon this behaviour
that its removal by lime depends. The illuminating power of coal-gas
containing only 1 per cent. of carbonic acid is reduced thereby by about
one-fifteenth of its whole amount.
The proper mode of burning the gas so as to obtain the maximum amount of
light it is capable of yielding requires a compliance with certain
physical and chemical conditions. The artificial production of light
depends upon the fact that by sufficiently heating any substance, it
becomes luminous, and the higher the temperature the greater the
luminosity. The light emitted by solid bodies moderately heated is at
first red in colour; as the temperature rises it becomes yellow, which
gradually changes to white when the heat becomes very intense. The
widest difference exists, however, in the temperature required to render
solids or liquids luminous, and that needed to cause gases to give off
light. In all luminous flames the light is emitted by _solid_ particles
highly heated. Every luminous gas-flame contains solid particles of
carbon, as may be easily shown by the soot deposited on any cold
body—such as a piece of metal—introduced into the flame. On the other
hand, the flame of burning hydrogen, which produces only aqueous vapour,
furnishes no light, but a heat so intense, that a piece of lime
introduced into the jet becomes luminous to a degree hardly supportable
by the eye. The conditions requisite, therefore, for burning
illuminating gas are, first, just such a supply of air as will prevent
particles of carbon from escaping unconsumed in the form of smoke, and
yet not enough to burn up the carbon before it has separated from the
hydrogen, and passed through the flame in the _solid_ state; second, the
attainment of the highest possible temperature in the flame, compatible
with the former condition. When the supply of oxygen is not in excess,
the hydrogen of the gaseous hydro-carbon appears to burn first; the
carbon is set free, and its solid particles immersed in the flame of the
burning hydrogen are there intensely heated; but ultimately reaching the
outer part of the flame, they enter into combination with the oxygen of
the air, producing carbonic acid; or if present in excessive quantity,
they are thrown off as smoke. If the purpose of burning the gas is to
obtain heating effects only, this is accomplished by supplying air in
such quantities, that the carbon enters into combination with oxygen in
the body of the flame, without a previous separation from the hydrogen
with which it is combined. In this case a higher temperature is
attained, and the flame is wholly free from smoke; so that vessels of
any kind placed over it remain perfectly clean and free from the least
deposit of soot. The last result is of great advantage in chemical
processes, especially where glass vessels require to be heated, for the
chemist retains an uninterrupted view of the actions taking place in his
flasks and retorts.
[Illustration:
FIG. 350.—_Bunsen’s Burner._
]
No better illustration of the nature of the combustion in a gas-flame
can be found than is furnished by Bunsen’s burner, Fig. 350, now
universally employed as a source of heat in chemical laboratories. In
this burner the gas issues from a small orifice at the level of _a_,
near the bottom of the tube, _b_, which is open at the top, and is in
free communication at the bottom with openings through which air enters
and mixes with the gas, as they rise together in the tube and are
ignited at the top. If the pressure of the gas be properly regulated,
the flame does not descend in the tube, but the mixture burns at the top
of the tube, producing a pale blue flame incapable of emitting light,
but much hotter than an ordinary flame, for the combustion is much
quicker. If the openings at _a_ be stopped, the supply of air to the
interior of the tube is cut off, and then the gas burns at the top of
the tube, _b_, in the ordinary manner, giving a luminous flame. Ordinary
gas-jets burning in the streets, at open stalls or shops, may be seen on
a windy night to have their light almost extinguished by the increased
supply of oxygen, carried mechanically into the body of the flame, the
white light instantly changing to pale blue. The disappearance of the
light in such cases is due, as in Bunsen’s burner, to the supply of
oxygen in sufficient quantity to combine at once with the carbon as well
as the hydrogen of the hydro-carbons.
[Illustration:
FIG. 351.—_Faraday’s Ventilating Gas-burner._
]
The burners now chiefly used for the consumption of coal-gas for
illuminating purposes are the bat’s-wing, the fish-tail, and various
forms of Argand. The bat’s-wing burner is simply a fine slit cut in an
iron nipple, and it produces a flat fan-like flame. The fish-tail is
formed by boring two holes so that two jets of gas inclined at an angle
of about 60° infringe on each other and produce a flat sheet of flame.
The Argand, in its simplest form, consists of a tubular ring perforated
with a number of small holes from which the gas issues. Many
modifications of this kind of burner have been devised, in all of which
a glass chimney is requisite to obtain a current of air sufficient to
consume the gas without smoke, and it is important that the height of
the chimney should be adapted to the amount of light required if the gas
is to be used economically. Argand burners are specially advantageous
where a concentrated light is required. Fig. 351 represents a
ventilating gas-burner, contrived by Faraday, the object being to remove
from the apartment the whole of the products of the combustion of the
gas. A is the pipe conveying the gas to the Argand burner, B, the flame
of which is enclosed in the usual cylindrical glass chimney, C C, open
at the top. This is enclosed in a wider glass cylinder closed at the top
by a double disc of talc, D D, and opening at its base into the
ventilating tube, E E. The direction of the currents produced by the
heat of the flame is shown by the arrows. The whole is entirely enclosed
by a globe of ground glass. Means are provided for regulating the
draught in the pipe, E E, which, when heated, creates of itself a strong
current of air through the apparatus.
The illuminating power of coal-gas may be measured directly by comparing
the intensity of the light emitted by a gas-flame consuming a known
quantity of gas per hour with the light yielded by some standard source.
The standard usually employed is a spermaceti candle burning at the rate
of 120 grains of sperm per hour. It is not necessary that the candle
actually used should consume exactly this amount, but the consumption of
sperm by the candle during the course of each experiment is ascertained
by the loss of weight, and the results obtained are easily reduced to
the standard of 120 grains per hour. An instrument is used for
determining the relative intensities of the illumination, called
Bunsen’s photometer. It consists of a graduated rule, or bar of wood or
metal, about 10 ft. long. At one end of this bar is placed the standard
candle, at the other is the gas-flame. A stand slides along the rule
supporting a circular paper screen at the level of the two flames, and
at right angles to the line joining them. This paper screen is made of
thin writing-paper, which has been brushed over with a solution of
spermaceti, except a spot in the centre, or, more simply, a grease-spot
is made in the middle of a piece of paper. In consequence the paper
surrounding the spot is much more transparent; yet when it is placed so
that both sides are equally illuminated, a spectator will not perceive
the spot in the centre when viewing the screen on either side. When the
screen has been placed by trial in such a position between the two
sources of light, it is only necessary to measure its distance from each
flame in order to compute the number of times the illuminating power of
the gas-flame exceeds that of the candle. This computation is based on
the fact that the intensity of the light from any source diminishes as
the square of the distance from the source. Thus, if a sheet of paper be
illuminated by a candle at 2 ft. distance, it will receive only
one-fourth of the light that would fall upon it were its distance but 1
ft., and if removed to 3 ft. distance it has only one-ninth of the
light. In the instrument used for measuring the illuminating power of
gas the rule is graduated in accordance with this law, so that the
relative intensities may be read off at once. The gas passes through a
meter for measuring accurately the quantity per minute which is consumed
by the burner, and there is also a gauge for ascertaining the pressure.
Another mode of estimating the illuminating power of coal-gas is by
determining the quantity of carbon contained in a given volume. For, in
general, the richness of the gas in carbon is a fair index of the
quantity of its luminiferous constituents. This may be readily effected
by exploding the gas with oxygen, and measuring the amount of carbonic
acid produced. Still more accurate determinations of the illuminating
value of gas may be obtained by a detailed chemical analysis.
The illuminating power of any gas is so calculated that it represents
the number of times that the light emitted by a jet of the gas, burning
at the rate of 5 cubic feet per hour, exceeds the light given off by the
standard sperm candle burning 120 grains of sperm per hour. For example,
when it is said that the illuminating power of London gas is 13, it is
meant that when the gas is burnt in an ordinary burner at the rate of 5
cubic feet per hour, the light is equal to that given by thirteen sperm
candles burning together 13 × 120 grains per hour. The quality of gas
varies very much, as it depends upon the kind of coal employed, and upon
the mode in which the manufacture is conducted. The following are the
results of experiments made to determine the illuminating power of the
gas supplied to several large towns:
Candles.
London 12·1
Paris 12·3
Birmingham 15·0
Berlin 15·5
Carlisle 16·0
Liverpool 22·0
Manchester 22·0
Glasgow 28·0
The relative quantities of tar, ammonia water, and coke yielded in
various gas manufactories also vary very considerably for the same
reasons.
In the early days of gas illumination the consumers were charged
according to the number of burners; but this arrangement proved so
unsatisfactory that the _gas-meter_ became a necessity, and already in
1817 meters had been devised, which were not essentially different from
those now in use. Although gas is used in so many houses, there are few
persons who have any notion of the mechanism of the gas-meter. Our space
will not allow full details of the construction, but the following
particulars may be mentioned. In the ordinary “wet” meter there is a
drum divided into four compartments by radiating partitions; this drum
revolves on a horizontal axis, and the lower half of the drum, or rather
more, is beneath the surface of water contained in the case, the water
being at the same level inside and outside the drum. The gas enters one
of the closed chambers formed between the surface of the water and a
partition of the drum. Its pressure tends to increase the size of the
chamber, hence the drum revolves. The preceding division of the drum
being filled with gas, this is driven into the exit pipe by the motion
of the drum, as it is included in a space comprised between the water
and a partition. Each division in turn comes into communication with the
gas-main, and as it is filled passes on towards the position in which a
passage is opened for it to the exit-pipe. Each turn of the drum,
therefore, carries forward a definite quantity of gas, and the only
thing necessary is a train of wheels, to register the number of
revolutions made by the drum. The “wet” meter is much inferior in almost
every respect to the “dry” meter, in which no water is used. The
principle of the “dry” meter is very simple. The gas pours into an
expanding chamber, partly constructed of a flexible material, and which
may be compared to the bellows of a circular accordion. The expansion is
made to compress another similar chamber, already filled with gas, which
is thus forced through the exit-pipe. When the first chamber has
expanded to a definite volume, it moves a lever, and this reverses the
communications. The expanded chamber is now opened to the exit-pipe, and
the other to the entrance-pipe, and so on alternately. A train of wheels
registers the number of movements on a set of dials.
Recent years have brought no essential changes in the methods of gas
making, although of course improvements in many minor details of the
processes and of the apparatus have been effected. These demand no
description at our hands, as they are of interest only to those
concerned with the actual technology of gas-making, nor need some of the
later forms of burners for using the gas be noticed, as these are
sufficiently familiar. They really do effect a considerable economy in
the consumption of gas, especially in cases where a more powerful light
is required. But the reader will have already learnt from a foregoing
section on Electric Lighting that the importance of gas as an illuminant
is already on the wane. Indeed, it will not be too much to say that,
before the close of the present century, every town will have its
streets, and still more certainly, all its places of public assembly,
such as theatres, concert halls, churches, libraries, &c., fitted with
installations for electric illumination, and even in shops and private
houses, it is probable that before long, gas will be superseded by the
electric light. Some of the disadvantages of burning gas have already
been referred to, and the danger attending its accidental escape into
apartments is illustrated by the yearly tale of victims to suffocation
and violent explosions. The inherent disadvantage of gas used as an
illuminant, is the enormous quantity of heat produced by its combustion,
compared with the amount of light evolved. The absolute quantity of heat
required to render a body highly luminous is really very small, for
masses of matter almost inappreciable become very luminous, provided
only that their _temperature_ be sufficiently raised. Thus, for example,
the few residual particles of gas in a Geissler’s tube (p. 431) become
incandescent by electrical discharges, while the number of them is too
small to sensibly heat the glass vessel, and the very attenuated carbon
filament in an electric glow lamp suffices by the mere contraction and
concentration of the current within it raising its temperature high
enough, to diffuse a brighter light than a large gas-flame. This
explains the fact alluded to elsewhere, that if instead of burning the
gas we use it in a gas engine, driving a dynamo connected with an
electric light installation, we shall obtain a much greater luminous
effect. As there is no combustion, the surrounding air is neither heated
nor deteriorated with gaseous products and smoke.
Without any rivalry from the electric-light, gas, as a domestic
luminant, has now met with a competitor on the ground of cheapness in
the mineral oils mentioned in the preceding article. If these could be
deprived of their unpleasant odour, and a perfectly safe lamp contrived
for burning them, it would be only under very favourable conditions that
gas could compete with them on the score of economy. But of late years
two applications of gas to other purposes than to illumination will have
been observed. First to heating, for warming, cooking, and other
domestic purposes, and also in various processes in the arts. In all the
appliances so used, the principle of Bunsen’s burner (p. 722) is
adopted, and stoves, fireplaces, and kitchen-ranges, heated by gas have
obvious advantages in their greater cleanliness and readiness. The other
new application of gas is as a motive power in the gas engine, by which
a very convenient supply of mechanical energy is afforded. There can be
little doubt that in the future, gas will be greatly used for these
purposes, and perhaps be for them consumed as largely as at present. A
singular thing in the history of gas-manufacture is the great value that
the bye-products have attained, that is to say, the ammoniacal liquor,
the coke, and especially the tar. So many valuable substances are now
derived from this last, that even if coal should cease to be
destructively distilled for gas, the operation would still be largely
carried on if only for the tar.
A jet of hydrogen gas burning in a dark room is all but invisible, yet
no gas can give so intense a heat. The lime-light, which no doubt is
perfectly familiar to everyone as an illuminant in magic lantern
projections, is simply a jet of mixed hydrogen and oxygen gases directed
on a piece of lime, which is rendered incandescent by the heat. The
flame of the Bunsen burner (p. 772) is distinguishable only by a very
pale blue colour, and it is impossible to discern objects, or to read by
its light in an otherwise dark room. But if a piece of thin platinum
wire formed into a coil, as by twisting round a pencil, be introduced
into the flame, the wire will glow with great brilliancy, and its
thickness will seem much increased. It will, in fact, emit so much light
that reading by its glow becomes easy. This shows that, as already
stated, a solid will give off light at a temperature which scarcely
suffices to make a gas visible. Thus a Bunsen burner flame can be made
to give light simply by putting into it some incombustible solid, which
itself incapable or suffering any chemical change under the conditions,
nevertheless becomes luminous by merely acquiring the temperature of the
almost invisible heated gas. The cause of the luminosity of the ordinary
gas burner, as compared with the almost invisible Bunsen burner flame,
has, indeed, been already explained on a previous page, but the
phenomenon is again, by the experiment just referred to, brought clearly
before our attention; and it becomes obvious that substances other than
the carbon of the hydro-carbon constituents of the coal gas will emit
rays of light. Chemical analysis shows that by far the larger proportion
of the constituents of ordinary coal-gas consist of gases which do not
themselves produce luminous flames, and that, taking 16 candle-gas,
about 10 candles of the illuminating power is due to compounds of which
the gas does not contain more than 4 per cent. Nearly half the bulk of
purified coal-gas is hydrogen, which itself gives no light whatever when
burnt; marsh-gas, which burns with only a slight luminosity, forms 35
per cent. of ordinary coal-gas; and there is usually present about 7 per
cent. of carbonic oxide, which in burning gives only a pale blue flame.
This shows that by far the greatest product of the combustion of
coal-gas is not light but heat. The flame of hydrogen is much the
hottest known, and as that gas enters so largely into the composition of
coal-gas, and the complete combustion of all the other constituents
takes place when the gas is previously mixed with air, as in the Bunsen
burner, we are provided with an economical means of obtaining high
temperatures. But coal-gas was in the first instance intended to provide
us with a cheap illuminant, and although for some time the gas itself
was very impure, and it was long before the crude appliances for burning
it were superseded by contrivances giving steadier and more brilliant
lights, such as the Argand and the regenerative burners. It is only
quite recently that the full illuminating possibilities of coal-gas have
been developed by the happy notion of converting the heating power of
its flame into light-giving power, by the simple plan of suspending a
suitable solid over the hot but non-luminous Bunsen burner.
The manner in which an effective method of doing this was discovered is
not a little curious. The construction of the ordinary incandescent
electric lamp, Fig. 280_h_, involves the necessity of enclosing the
carbon filament in an exhausted glass bulb; and it was when Auer von
Welsbach was engaged in attempting to find some substance that could be
brought into incandescence by the electric current, and yet be
incombustible even in the open air, that his investigations led to the
invention we have now to describe—an invention apparently destined to
give a new lease of life to coal-gas illumination.
[Illustration:
FIG. 351_a_.
]
[Illustration:
FIG. 351_b_.
]
It is singular also that Welsbach, in seeking for the most suitable
materials for heating to incandescence in the Bunsen burner flame,
should find them in certain very rare minerals, containing a group of
elements formerly of interest only to the scientific chemist, and up to
that time devoid of any practical applications. The names of these
elements, the oxides of which are called “earths,” will, of course, be
strange to non-chemical readers, but we give their names, with the
remark that the nearest familiar substance they at all resemble is
aluminium, of which the oxide, or “earth,” is alumina. These rare
metals, the oxides of which are the materials of the Welsbach “mantle,”
are all discoveries of the present century, or nearly so, and they are
called lanthanum, zirconium, thorium, cerium, didymium, yttrium, erbium,
&c. They occur as silicates or phosphates very sparingly, and in a few
localities in Norway; but some of them have now been found more
abundantly in America. The minerals, from which for the most part the
oxides are obtained, are called _monazite_, _orthite_, and _thorite_. It
was found after many trials that a blend of these earths in certain
proportions gives a mantle that yields a pure white light, while any
preponderance of one or another would impart some tint to the light.
This proper blending of the constituents forms a great improvement on
the first mantles, which generally shed a greenish light.
[Illustration:
FIG. 351_c_.
]
[Illustration:
FIG. 351_d_.
]
The _mantles_ are made by an ingenious process, in which a network of
cotton thread is knitted into the form of a tube; this is cut up into
suitable lengths, and a piece attached to form the top. The network is
then saturated with a solution of the nitrates of the rare earths
above-mentioned, and dried on glass rods. After this a loop of asbestos
thread is passed through the top, by which the _mantle_ may be attached
to its support. The mantle is now shaped to the required form, and the
cotton thread burnt off, when a thin skeleton of the oxides is left
reproducing the form of the original network. The mantle is again
strongly heated, and after cooling is dipped into a solution of
collodion, dried, and carefully laid in a box. The collodion serves to
strengthen the mantle sufficiently for transit, for it is very frail,
and would otherwise be liable to fall in pieces by slight shocks. Fig.
351_d_ is a full-sized representation of the completed mantle, and Fig.
351_e_ shows it mounted on the burner, where a rather small flame is
allowed for the first time to play upon it, by which the collodion is
quickly burnt off, and then the chimney-glass is placed over it, as in
Fig. 351_a_. In the earlier forms of lamp the lighting of the gas was a
matter requiring some delicacy of manipulation, for a rude shock, or an
awkward touch might cause the mantle to crumble into ruin, but now the
makers fit their lamps with a by-pass by which a very small flame is
maintained within the lamp ready to light up the gas when that is fully
turned on. (Fig. 351_c_.) The makers have also now made the lamps
available for street lighting, to which the fragility of the mantle was
formerly an obstacle, as it was liable to collapse by the tremor of the
traffic. This risk has been obviated by providing a spring to support
the mantle at the base. (Fig. 351_b_.)
[Illustration:
FIG. 351_e_.
]
The qualities of the Welsbach lamp have been examined by competent
persons, and from the statements they supply, we extract the following
particulars. The light is, for the same gas consumption, seven times
that of an ordinary gas burner; more than four times that of an Argand
burner; more than twice that of a “regenerative” lamp. It follows, of
course, that, light for light, the products of combustion, such as
carbonic acid, heat, &c., amount to only something like ⅙th of those
produced by ordinary burners, and the consumption of the gas is perfect,
there being absolutely no smoke. Though the mantles have to be renewed
about three times a year, when the burners are in constant use, the
total cost, light for light, is only ¼th of that of ordinary burners.
The light of the Welsbach burner is whiter than ordinary gaslight. It is
rich in the blue rays, and, therefore, more like daylight, permitting
well the comparison of shades of colour, and it is excellently suited
for workers with the microscope, &c. This new gas-lighting must also be
a great boon to photographers using artificial illumination, for the
actinic power is, with the same visual illumination, nearly twice that
of the ordinary gas flame.
[Illustration:
FIG. 352.—_Apparatus for making Magenta._
]
COAL-TAR COLOURS.
Coal-tar is an exceedingly complex material, being a mixture of a great
number of different substances. The following table shows the chemical
name of many of the substances obtainable from the coal-tar. It must not
be supposed that these substances exist ready formed in the coal, and
that they are merely expelled by the heat. We can understand better how
heat, acting upon an apparently simple substance like coal, and one
containing so few elements, is able to produce so large a variety of
different bodies, if we remember that heat is the agent most often
employed to effect chemical changes, and that from even two elements,
variously combined, bodies differing entirely from each other are
producible.
SUBSTANCES FOUND IN COAL-TAR.
_a._ COMPOUNDS OF CARBON AND HYDROGEN.
Hydrides of amyl, hexyl, heptyl, nonyl, and decyl.
Amylene, hexylene, heptylene, octylene, nonylene, decylene; (paraffin).
Benzol, toluol, xylol, cumol, cymol.
Naphthalene.
Anthracene.
Pyrene.
Chrysene.
_b._ COMPOUNDS OF CARBON, HYDROGEN, AND OXYGEN.
Phenol, cresol, phlorol.
Rosolic acid, brunolic acid.
_c._ COMPOUNDS OF CARBON, HYDROGEN, AND NITROGEN.
Aniline, toluidine.
Pyridine, picoline, lutidine, collidine, parvoline, coridine, rubidine,
viridine.
Leucoline, lepidine, cryptidine.
Cespitine, pyrrol.
This list contains only the names of substances which have actually been
found in the coal-tar, and it is certain that a number of products must
have escaped notice. It is obvious, too, that by using coal of different
kinds, and by varying the temperature and pressure at which the
operation of distilling the coal is effected, we shall probably be able
to increase the number of possible constituents of coal-tar almost
indefinitely. The list above presents to the non-chemical reader a
string of quite unfamiliar names; but, though the system of nomenclature
in chemistry is far from perfect, yet each of these names has a meaning
for the chemist beyond the mere designation of a substance. The chemical
name aims at showing, or at least suggesting, the composition of a body
and the general class to which it belongs. This may be illustrated by
the names of hydro-carbons in the above list. The five compounds headed
by benzol have many properties in common, and each one is entirely
different in its chemical behaviour to those which follow amylene. The
Greek numerals enter into the names of the latter, in order to express,
in this case, the number of atoms of carbon which are supposed to be
contained in each ultimate particle of the body. We write down in
parallel columns the names of these two classes of bodies, together with
the _symbols_ which represent their composition, reminding the reader
that the letter C represents carbon; the letter alone indicating _one_
atom of that element, but, when followed by a small figure, it implies
that number of carbon atoms; in like manner H, N, and O represent atoms
of hydrogen, nitrogen, and oxygen respectively.
Hexylene C_{6}H_{12}
Heptylene C_{7}H_{14}
Octylene C_{8}H_{16}
Nonylene C_{9}H_{18}
Decylene C_{10}H_{20}
Benzol C_{6}H_{6}
Toluol C_{7}H_{8}
Xylol C_{8}H_{10}
Cumol C_{9}H_{12}
Cymol C_{10}H_{14}
If these lists be carefully examined, it will be observed that there is
a regular progression in the constituent atoms, so that each set of
substances forms a series, the differences being always the same. The
various bodies contained in the coal-tar are separated from each other
by taking advantage of the fact that each substance has its own
boiling-point; that is, there is a certain temperature, different for
each body, at which it will rise into vapour quickly and continuously.
Benzol, for example, boils at 82° C., toluol at 114° C., and phenol at
188° C.; so that, if we apply heat to a mixture of these three
substances, the benzol will boil when the temperature reaches 82°, and
will pass away in vapour, carrying off heat, so that the temperature
will not rise until all the benzol has been driven off; then, when the
temperature reaches 114°, the toluol will begin to come off, but not
until that has all passed over into the receiver will the temperature
rise above 114°; and the phenol remaining will distil only at 188°.
Another mode of separating bodies when mixed together is by treating
them with a liquid which acts on, or dissolves out, some of the
constituents, but not the rest. The coal-tar, as it is received from the
gas-works, is placed in large stills, capable, perhaps, of holding
several thousand gallons, and usually made of wrought iron. Stills
sufficiently good for the purpose are commonly constructed from the
worn-out boilers of steam engines. The application of heat, of course,
causes the more volatile substances to come over first. These are
condensed and collected apart until products begin to come off which are
heavier than water. The first portion of the distillate, containing the
lighter liquids, is termed “coal naphtha.” The process is continued, and
heavier liquids come over, forming what is called in the trade the “dead
oil.” Pitch remains behind in the retort, from which it is usually run
out while hot, but sometimes the distillation is carried a step further.
The chief colour-producing substances contained in coal-tar are benzol,
toluol, phenol, naphthalene, and anthracene. The aniline which is
present in the tar is very small in amount, and if this ready-formed
aniline were our only supply, it would be impossible to make colours
from it on an industrial scale. The first of the above-named substances,
benzol, was discovered by Faraday, in 1825, in liquid produced by
strongly compressing gas obtained from oil. He called it bicarburet of
hydrogen; but afterwards another chemist, having procured the same body
by distilling benzoic acid with lime, termed it _benzine_. It readily
dissolves fats and oils; and is used domestically for removing
grease-spots, cleaning gloves, &c., and in the arts as a solvent of
india-rubber and gutta-percha. It is a very limpid, colourless liquid,
very volatile, and, when pure, is of a peculiar but not disagreeable
odour. It boils at 82° C., and, cooled to the freezing-point of water,
it solidifies into beautiful transparent crystals, a property which is
sometimes taken advantage of to separate it in a state of purity from
other liquids which do not so solidify.
Benzol is very inflammable, and its vapour produces an explosive mixture
with air. The vapour, which is invisible, will run out of any leak in
the apparatus, like water, and flow along the ground. Accidents have
occurred from this cause, and a case is on record in which the vapour
having crept along the floor of the works, was set on fire by a furnace
forty feet away from the apparatus, the flame, of course, running back
to the spot from which the vapour was issuing. Benzol is a dreadful
substance for spreading fire should it become ignited, for, being
lighter than water, it floats upon its surface, and therefore the flames
cannot be extinguished in the ordinary way. The discovery of the
presence of benzol in coal-tar was made by Hofman in 1845. It is
obtained from the light oil of coal-tar by first purifying this liquid
by alternately distilling it with steam and treating with sulphuric acid
several times. The product so obtained is a colourless liquid, sold as
“rectified coal naphtha,” which, however, has again to be several times
re-distilled with a careful regulation of the temperature, so that the
benzol may be distilled off from other substances, boiling at a somewhat
higher temperature, with which it is mixed. Even then the resulting
liquid (commercial benzol) contains notable quantities of toluol. If
benzol be added in small quantities at a time to very strong and warm
nitric acid, a brisk action takes place, and when after some time water
is added, a yellow oily-looking liquid falls to the bottom of the
vessel. The benzol will have disappeared, for nitric acid under such
circumstances acts upon it by taking out of each particle an atom of
hydrogen, which it replaces by a _group_ of atoms of nitrogen and
oxygen, and, instead of benzol, we have the yellow oil, _nitro-benzol_.
Chemists are accustomed to represent actions of this kind by what is
called a _chemical equation_, the left-hand side showing the symbols
representing the constitution of the bodies which are placed together,
and the right hand the symbols of the bodies which result from the
chemical action. Here is the equation representing the action we have
described:
C_{6}H_{6} + NO_{2}OH = C_{6}H_{5}(NO_{2}) + HOH
Benzol. Nitric acid. Nitro-benzol. Water.
[Illustration:
FIG. 353.—_Iron Pots for making Nitro-Benzol._
]
[Illustration:
FIG. 354.—_Section of Apparatus for making Nitro-Benzol._
]
Nitro-benzol has a sweet taste and a fragrant odour. It is known in
commerce under the names of artificial oil of bitter almonds and essence
of mirbane, and it has been used for perfuming soap. The chemical action
between benzol and concentrated nitric acid is so violent that, when
nitro-benzol first had to be manufactured on the large scale, great
difficulty was experienced on account of the serious explosions which
occurred. The apparatus now used in making nitro-benzol on the large
scale is represented in Fig. 353, which shows some of the cast-iron
pots, of which there is usually a long row. These pots are about 4½ ft.
in diameter, and the same in depth. Each is provided with a stirrer,
which is made to revolve by a bevil-wheel, _c_, on its spindle, working
with a pinion on a shaft, _b_, driven by a steam engine. A layer of
water is kept on the tops of the lids, the water being constantly passed
in and drawn off through the pipes, _d_, in order to keep it cool. For
the chemical action is, as usual, attended with heat, which vaporizes
some of the benzol, but the cold lid re-condenses the vapour, which
would otherwise escape with the nitrous fumes that pass off by the pipe,
_a_. There is at _e_ an opening, through which the material may be
introduced, and in the bottom of the vessel is an aperture through which
the products may be drawn off. Fig. 354 shows a section of one of the
cast-iron vessels, and exhibits the mode in which the spindle of the
stirrer passes through the lid. In the cup, _a_, filled with a liquid, a
kind of inverted cup, which is attached to the spindle, turns round
freely. It would not do to choose water for the liquid in this cup, for
water would, by absorbing the nitrous fumes, form an acid capable of
attacking and destroying the spindle. Nothing has been found to answer
better for this purpose than nitro-benzol itself. The charge introduced
into these vessels is a mixture of nitric and sulphuric acids together
with the benzol. During the action, which may last twelve or fourteen
days, no heat is applied, for the mixture becomes hot spontaneously, and
in fact care must be taken that it does not become too hot. The
nitro-benzol thus obtained is purified by washing with water and
solution of soda.
[Illustration:
FIG. 355.—_Apparatus for making Aniline._
]
If nitro-benzol were brought into contact with ordinary hydrogen gas, no
action whatever would take place. But it is well known to chemists that
gases which are just being liberated from a compound have at the
_instant of their generation_ much more powerful chemical properties
than they possess afterwards. Gases in this condition are said to be in
the _nascent state_. If we submit nitro-benzol to the action of nascent
hydrogen we find a remarkable change is produced. This change consists,
first, in the hydrogen robbing the nitro-benzol of all its oxygen atoms;
second, in the addition of hydrogen to the remainder; third, in some
re-arrangement of the atoms, by which a new body is formed. Not that
these changes are successive, or that we actually know the movement of
atoms, but we are thus able to form ideas which correspond with the
final result. The new substance is named _aniline_. It is regarded by
chemists as a base; that is, a substance capable of neutralizing and
combining with an acid to form a _salt_. Its composition is represented
by the symbols C_{6}H_{5} H_{2}N. Aniline was found in coal-tar in 1834,
and even its colour-producing power was noticed, for its discoverer
named it _kyanol_, in allusion to the blue colour it produced with
chloride of lime. Later it was obtained by distilling indigo with
potash, and hence received its present name from _anil_, the Portuguese
for indigo. The quantity of aniline contained in the tar is quite
insignificant.
Aniline is prepared from nitro-benzol on the large scale by heating it
with acetic acid and iron filings or iron borings, a process which
rapidly changes the nitro-benzol into aniline. The equation representing
the change is—
C_{6}H_{5}NO_{2} + H_{6} = C_{6}H_{5}H_{2}N + 2H_{2}O.
Nitro-benzol. Hydrogen. Aniline. Water.
The operation is effected in the apparatus represented in Fig. 355. It
consists of a large iron cylinder, within which works a paddle on a
vertical revolving spindle, which, being hollow, is also a pipe to
convey high pressure steam within the apparatus. Fig. 356 is a section
of the hollow spindle, in which _f_ is the pivot at the bottom of the
cylinder on which it turns; _d_ is the stirring paddle; _e_ is an
aperture admitting the steam from the pipe, _c_, forming the shaft of
the paddle, which is made to revolve by the bevil-wheel. The steams
enters by the elbow-pipe, which has a nozzle ground to fit the head of
the vertical revolving pipe, upon which it is pressed down by the screw.
When the materials have been introduced into the cylinder, the stirrer
is set in motion, and superheated steam is sent down the pipe; the
aniline is volatilized and passes with the steam through the pipe, which
is connected with a worm surrounded by cold water. The aniline is
purified by another distillation over lime or soda. When pure, aniline
is a colourless, somewhat oily-looking liquid, of a feeble aromatic
odour. Under the influence of light and air it becomes of a brownish
tint, in which condition it usually presents itself in commerce. It
scarcely dissolves in water, but is readily soluble in alcohol, ether,
&c.
[Illustration:
FIG. 356.—_Section of Hollow Spindle,—Aniline Apparatus._
]
It was Mr. Perkin who, in 1856, first obtained from aniline a substance
practically available for dyeing. Let it be noticed that when Mr. Perkin
discovered _aniline purple_, he was not engaged in searching for
dye-stuffs, but was carrying on a purely scientific investigation as to
the possibility of artificially preparing quinine. With this view,
having selected a substance into the composition of which nitrogen,
hydrogen, and carbon enter in exactly the same proportions as they occur
in quinine, but differing from it by containing no oxygen, he thought it
not improbable that by oxidizing this body he might obtain quinine. In
this he was disappointed, for the result was a dirty reddish-brown
powder. Being desirous, however, of understanding more fully the nature
of this reddish powder, he proceeded to try the effects of oxidation on
other similarly constituted but more simple bodies. For this purpose he
fortunately selected _aniline_, which, when treated with sulphuric acid
and bichromate of potash, he found to yield a perfectly black product.
Persevering in his experiments by examining this black substance, he
obtained, by digesting it with spirits of wine, the now well-known
“aniline purple.” Mr. Perkin, having determined to make the aniline
purple on the large scale, patented his process, and succeeded in
overcoming the many obstacles incident to the establishment of a new
manufacture requiring as its raw material products not at that time met
with as commercial articles. The process is now carried on the large
scale by mixing sulphuric acid and aniline in the proportions in which
they combine to form the sulphate of aniline, and dissolving by boiling
with water in a large vat. Bichromate of potash is dissolved in water in
another large vat. When both solutions are cold, they are mixed together
in a still larger vessel and allowed to stand a day or two. A fine black
powder settles on the bottom of the vessel in large quantities; this is
collected in filters, washed with water, and dried. This powder is not
aniline purple alone, but a mixture of this with other products,
presenting a very unpromising appearance; but when it has been digested
for some time with diluted methylated spirit, all the colouring matter
is dissolved out, and is obtained from the solution by placing the
latter in a still, where the spirit is distilled off and collected for
future use, while all the colouring matter remains behind, held in
solution by the water. From this aqueous solution the mauve is thrown
down by adding caustic soda. It is collected, washed, and drained until
of a pasty consistence, in which condition it is sent into the market.
It can be obtained in crystals, but the commercial article is seldom
required in this form, as the additional expense is not compensated by
any superiority in the practical applications of the colour. Mauve is
readily soluble in spirits of wine, but not very soluble in water. Its
tinctorial power is so great that _one-tenth of a grain_ suffices to
impart quite a deep colour to a gallon of water. Silk and woollen
fabrics have an extraordinary attraction for this colouring matter,
which attaches itself very firmly to their fibres. If some white wool is
dipped into even a very dilute solution, the colour is quickly absorbed.
Mauve is more permanent than any other coal-tar colour, being little
affected by the prolonged action of light.
Mauve is chemically a salt of a base which has been termed “mauveine.”
Mauveine itself is a nearly black crystalline powder, which forms
solutions of a dull blue-violet tint, but when an acid is added to such
a solution the tint is at once changed to purple. Mauveine is a powerful
base, displacing ammonia from its compounds. The commercial crystallized
mauve is the acetate of mauveine.
The process by which Mr. Perkin originally obtained mauve from aniline
evidently depends upon the well-known oxidizing property of bichromate
of potash, and experiments were accordingly made with other, oxidizing
bodies and aniline; in fact, patents were taken out for the use of
nearly every known oxidizing chemical. Three years after Mr. Perkin’s
discovery of mauve, M. Verguin, of Lyons, obtained, by treating _crude_
aniline with chloride of tin, the bright red colouring matter now known
as magenta. It was found also that crude aniline, when treated with
other metallic chlorides, nitrates, or other salts, which are oxidizing
agents less powerful than bichromate of potash, yields this bright red
colouring matter. A process patented by Medlock, in 1860, in which
arsenic acid is the oxidizing agent, has almost entirely superseded, in
England at least, all the others yet proposed for the manufacture of
magenta. It is not a little remarkable that magenta would not have been
discovered had M. Verguin and others operated on _pure_ aniline instead
of on the ordinary commercial article. For it was found subsequently by
Dr. Hofman that pure aniline cannot be made to yield magenta: the
presence of another body is necessary. A reference to the table of
coal-tar constituents will show that there is a hydro-carbon named
“toluol.” This substance is of a similar nature to benzol, and has a
boiling-point so little above that of benzol, that in the rough methods
of separation usually employed, a notable quantity of toluol is carried
over with the benzol, and is always present in the commercial article.
In the processes which benzol undergoes for conversion into aniline, the
toluol accompanies it in a series of parallel transformations, resulting
in the production of a base termed “toluidine”—similar to aniline—being,
however, in its pure state a solid at ordinary temperatures. We write
down the symbols representing the composition of the bodies formed in
the two cases in order to clearly show this:
Benzol C_{6}H_{6}
Nitro-benzol C_{6}H_{5}(NO_{2})
Aniline C_{6}H_{5}NH_{2}
Toluol C_{7}H_{8}
Nitro-toluol C_{7}H_{7}(NO_{2})
Toluidine C_{7}H_{7}NH_{2}
This aniline prepared from commercial benzol always contains some
toluidine; and it is essential for the production of magenta that this
substance should be operated on along with the aniline. Whether the
presence of some toluidine is also necessary for the production of mauve
and other colours is not yet known, but they are always prepared from
commercial benzol. It is certain that pure aniline yields no magenta,
neither does pure toluidine; but a mixture supplies it in abundance. For
the preparation of magenta the best proportions for this mixture would
be about three parts of aniline to one of toluidine; but, in practice,
it is not necessary to obtain the two substances separately, as benzol,
mixed with a sufficient quantity of toluol, may be obtained by
regulating the distillation. The apparatus used in the production of
magenta is shown in Fig. 352. It consists of a large iron pot set over a
furnace in brickwork, and having a lid with a stuffing-box, through
which passes a spindle carrying a stirrer. A bent tube rises from the
lid, and is connected with a worm surrounded by cold water, for the
purpose of condensing the aniline which is vapourized in the process.
The aniline, containing a due amount of toluidine, is mixed in this
apparatus with about one and a half times its weight of a saturated
solution of arsenic acid (H_{3}AsO_{4}). The fire is lighted and kept up
for several hours: water first, and lastly aniline, distil over. When
the operation is ended, steam is blown through the apparatus, thus
carrying off an additional portion of aniline. The crude product is then
boiled with water, the solution filtered, and common salt added, which
precipitates an impure magenta. This is afterwards dissolved and
recrystallized several times. The crystals of this magenta—like those of
many of the coal-colour products—have a peculiar greenish metallic
lustre; they dissolve in warm water, forming a deep purplish-red
solution. The chemical composition of magenta has been investigated by
Dr. Hofman, who found it to be a salt of an organic base, to which he
gave the name of “rosaniline.” This rosaniline is easily obtained from
magenta by addition to its solution of an alkali. While all its salts
are intensely coloured, rosaniline itself is a perfectly colourless
substance, becoming reddened by exposure to the air, as it absorbs
carbonic acid, thus passing to the condition of a salt. Rosaniline,
then, displays its chromatic powers only when it is combined with an
acid. This property is sometimes shown at lectures in a striking manner
by dipping a piece of paper into a colourless solution of rosaniline,
and exposing it to the air, when, as the rosaniline absorbs carbonic
acid, the paper changes from white to red. A more elegant form of the
same experiment is to dip a white rose into a solution of rosaniline
containing a little ammonia. As the ammonia escapes, or is expelled by a
current of warm air, the same kind of action occurs, and the white rose
changes to red—as if by magic, the emblem of the House of York is
transformed into the badge of Lancaster! The chemical nature of
rosaniline is regarded as analogous to that of ammonia—it is, in fact,
looked upon by chemists as a sort of ammonia, in each particle of which
some atoms of hydrogen have been replaced by certain _groups_ of carbon
and hydrogen atoms—some of these groups being derived from the aniline
and others from the toluidine. The particular salt of rosaniline which
constitutes the crude product of the action on the aniline and
toluidine, depends on the substance employed to effect the oxidation. If
a chloride, the resulting product is chloride of rosaniline; if a
nitrate, it is the nitrate; and so on. The magenta which is formed in
the first instance by the process we have described is an arseniate of
rosaniline; but in the subsequent processes, it is converted into the
chloride—the salt usually sold as magenta. Other salts of rosaniline are
made on the large scale—especially the acetate, the beautiful crystals
of which have the advantage of being very soluble.
Magenta attaches itself strongly to animal fibres, but the colour is
somewhat fugacious under the action of sunlight. It is used not only as
a dye, but more largely as the raw material from which a number of other
beautiful colours are obtained. For this reason it is manufactured on an
enormous scale, thousands of tons being produced annually, and the money
value of the colour produced from it must be reckoned by thousands of
pounds. Yet aniline was a few years ago merely a curiosity never met
with out of the laboratory of the scientific chemist. It is stated that
a single firm now makes more than twelve tons of aniline weekly, and on
its premises may be seen tanks, in each of which 30,000 gallons of
magenta solution is depositing its crystals. If a salt of rosaniline be
heated with aniline, the colour changes gradually through purple to
blue, while ammonia is at the same time given off. This is the colour
known as aniline blue, “bleu du Lyons,” &c. In its preparation it has
been found that the best results are obtained by employing the salt of
some weak acid—acetate of rosaniline, for example—and pure aniline, that
is, aniline free from toluidine. The operation is conducted in iron pots
very similar to those used in making magenta, but smaller. These pots
are not set over a fire, but a number of them are placed in a large
vessel containing oil, by which they can be maintained at a regulated
temperature when the oil is heated. The crude product undergoes several
purifications, and the aniline blue is supplied in commerce in powder,
or dissolved in spirits of wine. It is insoluble in water, and this has
been an obstacle to its employment; but recently a similar substance has
been obtained in a soluble form, and is extensively used for dyeing
wool, under the name of “Nicholson’s blue.” Other blues have been
similarly prepared, and from the same two substances, magenta and
aniline, a colour known as “violet imperial” was formerly made in very
large quantities, but it has been superseded by the colours about to be
described. It may be well to mention that these blues and violets have
been found to contain bases formed of rosaniline, in which one, two, or
three atoms of hydrogen are replaced by the group C_{6}H_{5}. This group
of atoms will be noticed to belong to aniline, and chemists have named
it phenyl, and, therefore, bases of these coloured salts are
respectively named phenyl-rosaniline, di-phenyl-rosaniline,
tri-phenyl-rosaniline. But Dr. Hofman found that other groups of atoms
besides C_{6}H_{5} may be made to take the place of H in rosaniline. By
acting on rosaniline or its salts with iodides of ethyl, C_{2}H_{5}I, or
iodide of methyl, CH_{3}I, he obtained a beautiful series of violets, of
which many shades could be produced, varying from red-purple to blue.
These are the colours so well known as Hofman’s violets, and are
prepared on the large scale by heating a solution of magenta (chloride
of rosaniline) in alcohol or wood spirit, with the iodide of ethyl or
the iodide of methyl. The nature and proportions of the ingredients are
regulated according to the tint required. The vessels are hermetically
closed during the heating, which is accomplished by steam admitted into
a steam-jacket surrounding the vessel. The crude product has to be
separated from the substances with which it is mixed, and the colouring
matter is finally obtained, presenting in the solid state the peculiar
semi-metallic lustre so characteristic of these products. Like the other
colours, Hofman’s violets are salts of _colourless_ bases, which, as
indicated above, are substitution products of rosaniline. The tints they
produce incline to red, violet, or blue, according as one, two, or three
hydrogen atoms are replaced by the ethyl or methyl groups. Colours have
also been obtained from mauve and iodide of ethyl—for example, the dye
known in commerce as “dahlia.” Other colours are procured from magenta
by treating it with various compounds: one such is the “Britannia
violet,” discovered also by Mr. Perkin, who procures it from magenta and
a hydrocarbon-bromide derived from the action of bromine or common
turpentine. This is a very useful colour, and is largely used in dyeing
and printing violets, of which any shades may be obtained.
Another derivative of rosaniline is the aniline green. It is obtained by
dissolving the rosaniline salt in dilute sulphuric acid, adding crude
_aldehyde_ (a substance obtained by acting with oxidizing agents on
alcohol). The mixture is heated until a sample dissolves in acidulated
water with a blue tint; it is poured out into boiling water containing
in solution hyposulphite of sodium, boiled, the liquid filtered; and the
green dye, if required in the solid state, is precipitated by carbonate
of sodium. Aniline green dyes wool and silk, the latter especially, of a
magnificent green; perhaps as beautiful a colour as any of the coal-tar
series, and one which has the singular advantage among greens of looking
as beautiful in artificial light as in daylight. The manner in which
this dye was discovered is somewhat curious. It is related by Mr. Perkin
of a dyer, named Chirpin, that he was trying to render permanent a
_blue_ colouring matter, which had been found could be produced from
rosaniline by the action of aldehyde and sulphuric acid. After a number
of fruitless attempts at fixing it, he confided his perplexities to a
photographic friend, who evidently thought that if it was possible to
fix a photograph, anything else might be fixed in like manner, for he
recommended his confidant to try hyposulphite of sodium. On making the
experiment, however, the dyer did not succeed in fixing his blue, but
converted it into the splendid aldehyde green. Like other colouring
matters we have described, this is a salt of a colourless base
containing sulphur. Like rosaniline, the colourless base takes on the
characteristic colour of its salts by merely absorbing carbonic acid
from the air.
Again, by a modification of the process for producing the Hofman
violets, another green of an entirely different constitution may be
obtained. It is bluer in tint than the former, and is much used for
cotton and silks, under the name of “iodine green.”
In the manufacture of magenta there is formed a residuum or bye-product,
consisting of a resinous, feebly basic substance, from which Nicholson
obtained a dye, imparting to silk and wool a gorgeous golden yellow
colour. This dye cannot be obtained directly, but is always produced in
greater or less quantity when magenta is made on the large scale, and is
separated during the purification. By first dyeing the silk or wool with
magenta, and then with this dye, which is commercially known as
“phosphine,” brilliant scarlet tints are obtained. The yellow colours
have been found to be salts of a base termed chrysaniline, a sort of
chemical relative of rosaniline, as may be seen in comparing the formulæ
which represent their constitution, with which we place also the symbol
for another substance obtained by submitting rosaniline to the influence
of nascent hydrogen. This body, _leucaniline_, again yields rosaniline
very readily when the hydrogen is removed by oxidizing agents. It will
be noticed that the three bodies form a series the members of which
differ only by H_{2}, thus indicating their close relationship.
C_{20}H_{17}N_{3} Chrysaniline.
C_{20}H_{19}N_{3} Rosaniline.
C_{20}H_{21}N_{3} Leucaniline.
Some idea will have been obtained from the foregoing particulars of the
great colour-supplying capabilities of aniline; but we have not yet
exhausted the utility of this interesting substance. It is probable that
the letters on the page now under the reader’s eye owe their blackness
to an aniline product. For after all the salts furnishing the lovely
tints we have mentioned have been extracted, there is in their
manufacture a final residuum, and from this an intense black is
obtained, which is largely used in the manufacture of printing-ink.
We have mentioned _phenol_ as a substance yielding colours. Phenol is
the body now so well known as a disinfectant under the name of “carbolic
acid,” a name given to it by its discoverer, Runge, who prepared it from
coal-tar, in 1834. Phenol forms colourless crystals, which dissolve to
some extent in water, and very readily in alcohol. It is a powerful
antiseptic, that is, it arrests the process of putrefaction in animal or
vegetable bodies, and it is also highly poisonous. The constitution of
phenol is given by the formula C_{6}H_{5} OH, in which the reader will
recognize the same group of atoms already indicated as entering into the
aniline derivatives. From some of these phenol may in fact be obtained,
and although it cannot be formed _directly_ from benzol, phenol can be
made to furnish benzol. When crude phenol is treated with a sulphuric
acid and oxalic acid, a substance is obtained which presents itself as a
brittle resinous mass of a brown colour, with greenish metallic lustre.
This substance is called _rosolic acid_ by chemists, but in commerce it
is known as _aurine_, and is used for dyeing silk of an orange colour,
which, however, is not very permanent. But by heating rosolic acid with
liquid ammonia, a permanent red dye is procured which has been termed
_peonine_, and has been much used for woollen goods. But it lately had
the reputation of exerting a poisonous action, producing blistering and
sores when stockings or other articles dyed with it were worn in contact
with the skin. It is now, therefore, less extensively employed.
_Coralline_, another body identical with or very similar to the former,
is similarly prepared from rosolic acid by heating it with ammonia under
pressure.
Again, by heating coralline with aniline, a blue dye, known as
“azurine,” or “azuline,” was formerly made in large quantities; but it
has been supplanted by the aniline blues already described.
When phenol is acted upon by nitric acid new compounds are produced,
standing in the same relation to phenol as nitro-benzol does to benzol.
The final result of the action of nitric acid on phenol is _picric
acid_, called also “carbazotic acid,” and, more systematically,
“tri-nitro-phenol;” for it is regarded as phenol in which three of the
hydrogen atoms have been replaced by the group NO_{2} thus,
C_{6}H_{2}(NO_{2})_{3} OH. It forms bright yellow-coloured crystals, and
its solution readily imparts a bright pure yellow colour to wool, silk,
&c. It received the name of picric acid (πικρος, _bitter_) from the
exceedingly bitter taste of even an extremely diluted solution. It is
said that picric acid is employed as an adulterant in bitter ale instead
of hops. Now, the colouring power of picric acid is so great, that even
the minute quantity which could be used to impart bitterness to beer is
recognizable by dipping a piece of white wool into the beer, when, if
picric acid be present, the wool acquires a clear yellow tint. Besides
its employment as a yellow, it is useful for procuring green tints by
combination with the blues. Picric acid again furnishes, by treatment
with _cyanide of potassium_, a deep red colour, consisting of an acid
which, when combined with ammonia, furnishes a magnificent colouring
material—which is, in fact, _murexide_, a dye identical with the famous
Tyrian purple of the ancients, and formerly obtainable only from certain
kinds of shell-fish.
Naphthaline—another of the colour-yielding substances of coal-tar—is,
like benzol, a hydro-carbon, but one belonging to quite another chemical
series. Its formula is C_{10}H_{8}, and it has an interest to chemists
altogether apart from its industrial uses, from having been the subject
of the classic researches of the French chemist, Laurent—researches
which resulted in the introduction of new and fertile ideas into
chemical science, contributing largely to its rapid progress.
Naphthaline forms colourless crystals, which, like camphor, slowly
volatilize at ordinary temperatures, and are readily distilled in a
current of steam. It is thus sufficiently volatile to escape complete
deposition in the condensers of the gas-works, and to be partly carried
over into the mains, where its collection occasions some trouble. Nitric
acid acts upon naphthaline in a manner analogous to that in which it
acts on benzol, forming nitro-naphthaline, which, in its turn, submitted
to the action of iron filings and acetic acid, is transformed into a
base called “naphthylamine.” The salts of naphthylamine are coloured
products which, in some cases, have been found available as dyes. There
is a crimson colour, and a yellow largely used under the name of
“Manchester yellow,” for imparting to silk and wool a gorgeous golden
yellow colour. Another coloured derivative of naphthaline, called
“carminaphtha,” was discovered by Laurent in the course of his
researches.
It would be easy to fill this volume with descriptions of the
properties, and modes of preparing the numerous colouring matters that
have been obtained from coal-tar products. In order to give the reader
an idea of the extent to which the tar products have been made to
minister to our sense of the beautiful, a list is here given of the
principal colouring matters from these sources that have been employed
in the arts. The various names under which a product has been
commercially known are in most cases given. It must be understood that
the same name is frequently applied to products chemically distinct, and
some of the names which appear as synonyms may also in reality indicate
different substances.
LIST OF COAL-TAR COLOURS.
I. COLOURS DERIVED FROM ANILINE AND TOLUIDINE.
_Blues and Violets._
Mauve, aniline purple, Perkin’s violet, violine, mauve, rosaniline,
anodising, &c.
Aniline blue, rosaniline blue, Hofman’s blue, bleu de Paris, bleu de
Lyons, bleu de Mulhouse, bleu de Mexique, bleu de nuit, bleu lumière,
night blue.
Hofman’s blue.
Nicholson’s blue, soluble blue.
Hofman’s violet, rosaniline violet.
A long series of red and blue violets, bearing Hofman’s name and
distinguished in commerce by adding R or B, according to the redness or
the blueness of the tint, ranging from RRRR to BBBB.
Dahlia.
Toluidine blue.
Violet de Paris.
Mauvaniline.
Violaniline.
Regina blue, opal blue, bleu de Fayolle, violet de Mulhouse.
Britannia violet.
Violet imperial.
And many others.
_Reds._
Aniline red, new red, magenta, solferino, aniline, rougé, roseine,
azaline.
Rubine, rubine imperial.
Chrysaniline red.
(_The above are all salts of rosaniline_)
Xylidine, tar red, soluble red.
_Yellows._
Chrysaniline, phosphine, aniline yellow, yellow fuschine.
Chrysotoluidine.
Dinaline.
Field’s orange.
_Greens._
Aldehyde green, aniline green, viridine, emeraldine.
Iodine green, iodide of methyl green, iodide of ethyl green.
Perkin’s green.
_Browns._
Havanna brown.
Bismarck brown, aniline brown, Napoleon brown, aniline maroon.
_Greys and Blacks._
Aniline grey, argentine.
Argentine black.
II.—COLOURS DERIVED FROM PHENOL.
_Blues and Violets._
Isopurpuric acid, Grénat.
Azuline, azurine.
_Reds._
Picramic acid.
Coralline, peonine.
Red coralline.
_Yellows._
Picric acid, carbazotic acid.
Aurine, rosolic acid.
_Green._
Chloropicrin.
_Browns._
Picrate of ammonia.
Isopurpurate of potash.
Phenyl brown, phenicine.
III.—COLOURS DERIVED FROM NAPHTHALENE.
_Reds._
Pseudoalizarine, naphthalic red.
Roseonaphthaline, carminaphtha.
_Yellows._
Binitronaphthaline, naphthaline yellow, golden yellow, Manchester
yellow.
And others.
The introduction of aniline colours into dyeing and calico-printing has
caused quite a revolution in these arts, the processes having become
much more simple, and the facilities for obtaining every variety of tint
largely increased. The arts of lithography, type-printing,
paper-staining, &c., have also profited by the coal-tar colours. For
such purposes the colour is prepared by fixing it on alumina, a process
in which much difficulty was at first experienced, for the colours are
themselves almost all of a basic nature. The desired result is now
attained by fixing them on the alumina with tannic or benzoic acid.
These lakes produce brilliant printing-inks, which are extensively used.
The aniline colours are also employed for coloured writing-inks, tinted
soaps, imitations of bronzed surfaces, and for a variety of other
purposes.
Not many years ago coal-tar was a valueless substance: it was actually
given away by gas-makers to any one who chose to fetch it from the
works. It was then “matter in the wrong place;” but Mr. Perkin’s
discovery led to its being put in the right place, and it has become the
raw material of a manufacture creating an absolutely new industry, which
has developed with amazing rapidity. This industry dates from only 1856,
and in 1862 the annual value of its products was more than £400,000. Dr.
Hofman, in reporting on the coal-tar colours shown at the Paris
Exhibition of 1867, computed the value at that time at about £1,250,000,
although the products were much cheaper than before. Large manufactories
have been established in Great Britain, in France, Germany, Switzerland,
America, and other countries. The possibility of such an industry is an
interesting illustration of the manner in which the progress made in any
one branch of practical science may lead to unexpected developments in
other quarters. The quantity of aniline obtained from coal-tar is very
small compared to the amount of coal used, as may be seen from the
following table, in which the respective weights of the various products
required in the manufacture of _mauve_ are arranged as given by Mr.
Perkin for the produce of 100 lbs. of coal.
lbs. oz.
Coal 100 0
Coal-tar 10 12
Coal-tar naphtha 0 8½
Benzol 0 2¾
Nitro-benzol 0 4¼
Aniline 0 2¼
Mauve 0 0¼
From this we may perceive that had not the manufacture of gas been
greatly extended, so as to yield a large aggregate produce of tar, the
requisite supply for the manufacture of aniline would not have been
attainable; and the industrial application of the previously worthless
bye-product reacts upon gas manufacture by cheapening the price of that
commodity, thus tending still more to extend its use.
Although anthracene has already been named as one of the
colour-producing substances found in coal-tar, we have not in the list
of coal-tar colours included the colouring matter which anthracene is
capable of yielding. The reason is that this case stands apart in some
respects from the rest. The colours derived from aniline and the other
substances already enumerated are instances of the production of bodies
not found in nature—mauve, magenta, &c., do not, so far as we know,
exist in nature. Their artificial formation was a production of
substances absolutely new. The colour of which we have now to treat is,
on the other hand, found in nature, and from its occurrence in the
_rubia tinctoria_, the roots of that plant have for ages been employed
as a source of colour, and are well known in this country as “madder.”
The plant is grown largely in Holland, in France, in the Levant, and in
the south of Russia.[18] Madder is used in enormous quantities for
dyeing reds and purples: the well-known “Turkey red” is due to the
colouring matter of this root. The total annual value of the madder
grown is calculated to reach nearly 2½ million pounds sterling. More
than forty years ago it was discovered that the madder-root yielded a
colouring substance, to which the name of “alizarine” was bestowed, from
_alizari_, the commercial designation of madder in the Levant. The
alizarine does not exist in the fresh root, but is produced in the
ordinary processes of preparing the root and dyeing with it, in
consequence of a peculiar decomposition or fermentation. Alizarine may
be procured from dried madder by simply submitting it to sublimation,
when beautiful orange needle-shaped crystals of alizarine may be
obtained. It is nearly insoluble in water, but readily dissolves in hot
spirits of wine. Acids do not dissolve it, but potash dissolves it
freely, striking a beautiful colour; with lime, barytes, and oxide of
iron, it forms purple lake, and with alumina a beautiful red lake.
According to Dr. Schunck, of Manchester, to whose investigations we are
indebted for much of our knowledge of madder, the root contains a bitter
uncrystallizable substance called “rubian,” which, under the action of
certain ferments, and of acids and alkalies, is decomposed into a kind
of sugar, and into alizarine and other colouring matters. The ferment,
which in the process of extracting the colouring matter from the roots
causes the formation of alizarine, is contained in the root itself.
Footnote 18:
The natural Order to which the madder plant belongs is interesting
from the number of its members which supply us with useful products.
That valuable medicine, quinine, is obtained from plants belonging to
this family, as is also ipecacuanha, and other articles of the
_materia medica_. _Coffea arabica_, which furnishes the coffee-berry,
is another member.
We have already seen how an investigation relating to a question of pure
chemical science accidentally led Mr. Perkin to the discovery of
mauve—the precursor of the long range of beautiful colours already
described. The mode of artificially preparing alizarine, so far from
being an accidental discovery, was sought for and found in 1869 by two
German chemists, Graebe and Liebermann. The researches of these chemists
were conducted in a highly scientific spirit. Instead of making attempts
to produce alizarine by trying various processes on first one body, then
another, to see if they could hit upon some tar product, or other
substance, which would yield the desired product, they began by
operating analytically on alizarine itself. Just as a mechanic ignorant
of horology, required to make a watch, would be more likely quickly to
succeed in his task by taking a watch to pieces to see how it is put
together, than if he had tried all manner of arranging springs and
wheels until he hit upon the right way; so these chemists set themselves
to take alizarine to pieces, in order to see from what materials they
might be able to put it together. They decomposed alizarine, and among
the products found a hydro-carbon identical in all its properties with
_anthracene_.
Anthracene was discovered in coal-tar by Laurent in 1832, and its
properties were investigated by Anderson in 1862. It may be remarked
that such investigations were not conducted with a view to any
industrial uses of anthracene, but merely for the sake of chemistry as a
science. Certainly no one could have supposed at that time that the
slightest relation existed between anthracene and madder. Anthracene is
a white solid hydro-carbon, which comes over only in the last stages of
the distillation of coal-tar, accompanied by naphthaline, from which it
is easily separated by means of spirits of wine, by which the
naphthaline is readily dissolved, but the anthracene scarcely. Anderson,
in 1861, discovered, among other results, that anthracene, C_{14}H_{10},
by treatment with nitric acid became changed into oxy-anthracene,
C_{14}H_{8}O_{2}; and this reaction we shall see is a step in the
process of procuring alizarine from anthracene. Phenol, as already
mentioned, can be made to yield benzol, by a process of deoxidization.
With a view to similarly obtaining a hydro-carbon from alizarine, Graebe
and Liebermann passed its vapours over heated zinc filings, and thus
produced anthracene from alizarine. It now remained to find a means of
reversing this process, that is, so to act on anthracene as to produce
alizarine, and this was effected by treating anthracene with bromine,
forming a substance which, on fusing with caustic potash, yielded
_alizarate of potash_, from which pure alizarine resulted by treatment
with hydrochloric acid. A much cheaper method was, however, necessary
for manufacturing purposes, and it was found in a process by which
oxy-anthracene, C_{14}O_{8}H_{2}, is treated at a high temperature with
strong sulphuric acid, and the product so formed heated with a strong
solution of potash, yielding alizarate of potassium as before. Many
other interesting substances appear to be formed in the reactions, but
the nature of these bodies has as yet been imperfectly investigated. No
doubt whatever can be entertained of the identity of natural with
artificial alizarine; and the production of this substance, the first
instance of a natural colouring matter made artificially, may be
regarded as a great triumph of chemical science. It was not long ago
supposed that the chemical bodies found in plants or animals, or
produced by vital actions, could not possibly be formed by any
artificial process from their elements. The laws which presided at their
formation were, it was conceived, wholly different from those which
governed the chemicals of the laboratory, for they were held to act
exclusively under the influence of a mysterious agent, namely, “vital
force.” It was supposed, for example, that from pure carbon, oxygen, and
hydrogen, no chemist would ever be able to produce such a compound as
acetic acid. Accordingly the domain of chemical science, previous to the
end of the first quarter of the present century, was divided by an
impassable barrier into the two regions of organic and inorganic
chemistry. Now, however, the chemist is able to build up in his
laboratory from their very elements a great number of the so-called
_organic_ bodies. And it is quite possible to do this in the case of
alizarine; that is, a chemist having in his laboratory the elements,
hydrogen, carbon, oxygen, &c., could actually build up the substance
which gives its value to madder.
The quantity of anthracene procurable from coal-tar is, unfortunately,
comparatively small, for it is found that from the distillation of 2,000
tons of coal only one ton of anthracene can be obtained. The use of
artificial alizarine would doubtless entirely supplant the employment of
madder-root if anthracene could be obtained in larger quantities; and
the change would be highly advantageous to this country, for as no
madder is grown in Great Britain, and we consume nearly half the whole
annual growth, it follows that every year a million pounds sterling go
out of the country for this commodity. When anthracene is produced from
coal in sufficient abundance, this sum will be available for the support
of our own population. In the meantime, the manufacture of artificial
alizarine is restricted only by the supply of its raw material.
The foregoing paragraphs of the present article, which were written for
the first edition of this work, not long after the introduction of
artificial alizarine, require some supplementary reference to the
subsequent progress of discovery and to the increased importance of the
manufacture of the coal-tar colours on the large scale. Since the first
introduction of alizarine as a commercial product, the substance has
received much attention from chemists. The constitution of the body
called above _oxy-anthracene_ is now better understood, and its chemical
relationship is more clearly indicated by the systematic name of
_anthraquinone_, which it now bears. The process of the manufacture of
alizarine has received some advantageous modifications, and the
artificial product may now be said to have entirely displaced the
madder-root in dyeing. But, more than this, chemists have found means of
preparing a number of “derivatives” of alizarine, many of which are
either colouring matters or are easily converted into such. Nearly
thirty of these substances have been described, and several of them have
found extensive industrial applications. We may mention _alizarine
blue_, C_{17} H_{9} NO_{4}, and another substance, produced by combining
that with _sodium bi-sulphite_, and having the formula C_{17} H_{9}
NO_{4} 2Na H SO_{3}. This last, manufactured largely, and sold under the
name of “_alizarine blue S._,” is remarkable for being one of the most
permanent of all colouring matters. It is said to be a faster colour
than even indigo blue, which, indeed, it is rapidly replacing in dyeing,
where it is applicable to cotton with a chromium mordant and to silk
with one of alumina. Two other colouring matters have also been derived
from anthracene, and are much used in dyeing; one is commercially named
_anthracene purple_, the other is _anthracene green_, which supplies the
calico printer with very fast shades of olive-green.
Several of the substances enumerated in the list of coal-tar colours, in
pages 689 and 690, are now but little used, or altogether abandoned in
dyeing and calico printing, because either their beautiful hues prove
too fugitive, or other bodies of the same class can be produced at a
much cheaper rate. The range of choice is now of the amplest, for
chemical discovery has been wonderfully active, but in many cases the
real nature and relationship of the artificial colouring matters
enumerated above have only quite recently been made out. Mauve (now
called _rosaline_), for example, the oldest of all the colour-tar
colours, and one which, as we have seen, was manufactured on an
extensive scale many years ago, is now scarcely made at all, because
much cheaper violets have taken its place. The science of the tinctorial
substances has lately taken a much more distinct form, and this
knowledge has borne fruit for industrial purposes. It would be out of
place here to review what has been done in this way, but a few facts
will show the richness of the field. It was only in 1886 that the true
chemical constitution of a class of coal-tar derivatives, called
_azines_, was first made out. They present themselves as pale yellow or
orange coloured crystallized solids, which melt at a comparatively high
temperature and may be distilled without decomposition. Although highly
coloured substances themselves, before they are converted into fast dyes
they require further treatment, which introduces into their molecules
another group of atoms. An almost indefinite number of such compounds
are theoretically possible, but from only a very few of them many useful
dye stuffs are now prepared on the large scale. Amongst the most
important of these are “neutral red,” “neutral violet,” and two other
violet colouring matters, “red dyestuff,” “fuchsia,” “giroflé,” “Magdala
red,” “indazine” and “Basle blue.”
Among the colouring matters before enumerated are “aniline yellow” and
“Bismarck brown.” Their real nature was not understood until a few years
ago; and though the use of the aniline yellow itself has been abandoned
on account of its fugacity, the substance has been found a most prolific
parent, which has supplied dye stuffs of the most diverse and brilliant
hues. These form what chemists term the _azo colours_, and they have
been manufactured in great variety and on a very large scale. In 1876,
the class of them called _chrysoidines_ was introduced, and again, in
1878, _tropœolines_. Great numbers of different azo colours have been
sent into commerce under various names, such as “butter yellow,”
“_crocein scarlet_,” “_Biebrich scarlet_,” “_Congo red_,” “_Bordeaux
G._,” “fast red,” &c., &c. About 140 of these azo dyes have been
described, and the commercial importance of this one class of compounds
alone may be inferred from the fact of no fewer than 200 patents having
been taken out for processes relating to their manufacture in the eleven
years from 1878 to 1888.
It would not be difficult to fill this book with instances of the way in
which the resources of modern life have been increased by chemistry
alone, a science almost entirely the creation of the present century.
Many of the processes of manufacture in which chemistry is applied to
the production of articles of every-day use have been so often
described, that they may be assumed to be already so well known as to
offer few elements of novelty to the general reader, whose interest
would also be likely to flag if he were carried over a long range of
even the brilliant discoveries that are so delightful and instructive
for the special students of this science. There is no parallel to the
rapidity of the progress made by the younger branch of the science which
concerns itself with the chemistry of one element—namely, carbon and its
various combinations, and it is from these carbon compounds that our
examples have been drawn. In the explosives, we have some of these
compounds supplying resistless forces for rending rocks, and furnishing
in warfare the most dreadful powers of destruction. In anæsthetics, we
see beneficent applications of others in alleviating suffering and
annulling pain; and again we have just shown how richly another set of
them can minister to our sense of beauty. The discussion of these topics
has afforded an opportunity for bringing before the reader some of the
laws or summarized statements of experimental facts, and also some of
those symbolical conceptions of the constitution of compounds, which
together furnish the clues that guide the chemist through the vast
labyrinth of the endless transformations of matter. The results attained
show that the notions expressed by such words as _atom_, _molecule_,
_compound radical_, _structural formula_, etc., have a true
representative correspondence with something in the actual constitution
of bodies.
[Illustration:
FIG. 357.—_James Prescott Joule, F.R.S._
]
THE GREATEST DISCOVERY OF THE AGE.
The indulgent reader who may have followed the course of the foregoing
pages, will perhaps peruse the title of this article with some little
bewilderment. His attention has been drawn to one after another of a
series of remarkable and important discoveries, and he will naturally
wonder what can be the discovery which is greater than any of these.
Now, a discovery is great in proportion to the extent and importance of
the results that flow from it. These results may be immediate and
practical, as in the case of vaccination; or they may be scientific and
intellectual, as in Newton’s discovery of the identity of the force
which draws a stone to the ground with that which holds the planets in
their orbits. Such discoveries as most enlarge our knowledge of the
world in which we live, by embracing in simple laws a vast field of
phenomena, are precisely those which are most prolific in useful
applications. If we admit, as we must, the truth of Bacon’s aphorism,
which declares that “Man, as the minister and interpreter of nature, is
limited in act and understanding by his observation of the order of
nature; neither his understanding nor his power extends farther,”[19]
then it would be easy to show that the discovery of which we have to
treat, more than any other, must be of immense practical service to
mankind in every one of the ways in which a knowledge of the order of
nature can be of use, viz.:—“First, In showing in how to avoid
attempting impossibilities. Second, In securing us from important
mistakes in attempting what is, in itself, possible, by means either
inadequate or actually opposed to the end in view. Third, In enabling us
to accomplish our ends in the easiest, shortest, most economical, and
most effectual manner. Fourth, In inducing us to attempt, and enabling
us to accomplish, objects which, but for such knowledge, we should never
have thought of undertaking.”[20]
Footnote 19:
“Homo naturæ minister et interpres, tantum facit et intelligit quantum
de naturæ ordine re vel mente observaverit: nec amplius scit aut
potest.”—_Novum Organum, Aphor._ I.
Footnote 20:
Sir J. Herschel.
A great principle, like that which we are about to explain to the
reader, is too vast in its bearings for its discovery and elaboration to
have been the work of an individual. This truth, and indeed the whole of
our knowledge, is but the result of the development and growth of
pre-existing knowledge. In fact, every discovery, however
brilliant—every invention, however ingenious, is but the expansion or
improvement of an antecedent discovery or invention. In strictness,
therefore, it is impossible to say where the first germ of even our
newest notions may be found. Our latest philosophy can be shown to be
the result of progressive modifications of ideas of remote ages. Hence
every great truth, every grand invention, has in reality been the
offspring of many minds; but we record as _the_ discoverers and
inventors those men who have made the longest strides in the path of
progress, and whose genius and labours have overcome obstacles defying
ordinary efforts.
The extent of the field which is covered by the principle we have in
view is so vast—embracing, as it does, the whole phenomena of the
universe—that it will not be possible to do more within our limits than
give the reader a general notion of the principle itself. It may be
useful to instance a truth which has a similar generality and
significance, and which has also acquired the force of an axiom, because
it is verified every hour. It is that greatest generalization of
chemistry, affirming that in all its transformations _matter is
indestructible_, and can no more be destroyed than it can be called into
being at will. This truth is so well established, that some philosophers
have asserted that an opposite state of things is _inconceivable_. But
it was not always known; and there are at the present day untutored
minds which not only believe that a substance destroyed by fire is
utterly annihilated, but what they find _inconceivable_ is the continued
existence of the substance in an invisible form. The candle burns away,
its matter vanishes from our view; but if we collect the invisible
products of the combustion, we find in them the whole substance of the
candle in union with the atmospheric oxygen. We may, in imagination,
follow the indestructible atoms of carbon in their migrations, from the
atmosphere to the plant, which is eaten by the animal and goes to form
its fat, and from the tallow, by combustion, back into the atmosphere
again. The notion of the real identity of matter under changing forms
has been expressed by our great dramatist in a well-known passage, which
is remarkable for its philosophic insight, when we consider the age in
which it was written:
HAMLET. To what base uses we may return, Horatio! Why may not
imagination trace the noble dust of Alexander, till he find it
stopping a bung-hole?
HORATIO. ‘Twere to consider too curiously to consider so.
HAMLET. No, faith, not a jot; but to follow him thither with
modesty enough, and likelihood to lead it. As thus: Alexander
died, Alexander was buried, Alexander returneth to dust; the dust
is earth; of earth we make loam; and why of that loam, whereto he
was converted, might they not stop a beer-barrel?
Imperial Cæsar, dead, and turned to clay,
Might stop a hole to keep the wind away;
O, that the earth, which kept the world in awe,
Should patch a wall to expel the winter’s flaw!
Now the greatest discovery of our age is that force, like matter, is
indestructible, and that it can no more be created than can matter. The
reader may perhaps think the statement that we cannot create force is in
contradiction to experience. He will be disposed to ask, What is the
steam engine for but to create force? Do we not gain force by the
pulley, the lever, the hydraulic press? And are not tremendous forces
produced when we explode gunpowder or nitro-glycerine? When the
principle with which we are here concerned has been developed and stated
in accurate terms, it is hoped the reader will see the real nature of
these contrivances. We are, however, aware that it is quite impossible
within the limits of a short article to do much more than indicate a
region of discovery abounding with results which may be yet unfamiliar
to some. Into this, if so minded, they should seek for further guidance,
which they will pleasantly find in the pages of Dr. Tyndall’s “Heat
considered as a Mode of Motion,” and in a little work by Professor
Balfour Stewart, entitled “The Conservation of Energy,” and quite
fascinating from the clearness and simplicity of its style. We may
continue our humble task of merely illustrating the general nature of
this, in reality the most important, subject which we have had occasion
to bring under the reader’s notice.
Perhaps the first step should be to point out the fact of the various
forces of nature—mechanical action, heat, light, electricity, magnetism,
chemical action—being so related that any one can be made to produce all
the rest directly or indirectly. Some examples of the conversion of one
form of force into another occur in the foregoing pages. Thus, on page
485 an experiment is described in which electricity produces a
mechanical action; electricity is also shown, on page 496, to produce
heat; on page 491 chemical action; on page 501 magnetism. Then, as
instances of the inverse actions, there is on page 488, in the first
paragraph on “Electric Induction,” an account of the mode in which
mechanical movements may give rise to electricity; and in the
experiments in pages 508, 509, and particularly in the account of the
Gramme machine, page 511, it is shown how mechanical movements can,
through magnetism, produce electricity. The voltaic element, page 491,
and the galvanic batteries, are instances of chemical action supplying
electricity. On page 518 a striking instance is mentioned of changes in
the forms of force. Every lighted candle is a case of chemical action
giving rise to light; and interesting examples of the inverse relation
are referred to on page 608. On page 168 is represented the conversion
of arrested motion into heat and light. We have, indeed, sufficient
examples to arrange a series of these conversions of forces in a circle.
Thus, chemical action (oxidation in the animal system) supplies muscular
power, this sets in motion a Gramme machine, the motion is converted
into electricity, the electricity produces the electric light, and light
causes chemical action, and with this the cycle is complete. In the
steam engine heat is converted into mechanical force, and many cases
will present themselves to the reader’s mind in which mechanical actions
give rise to heat. The doctrine of a mutual dependence and
convertibility among all the forms of force was first definitively
taught in England by Mr. (now Justice) Grove, in 1842; and almost
simultaneously Dr. Meyer promulgated similar views in Germany. Mr. Grove
subsequently embodied his doctrine in a treatise, called “The
Correlation of the Physical Forces,” which has seen several editions.
But this teaching included much more than a mere connection between the
various forces, for it extended to quantitative relations. It declared
that a given amount of one force always produced a definite amount of
another, that a certain quantity of heat, for example, would give rise
to a certain amount of mechanical action, and that this amount of
mechanical action was the _equivalent of the heat_ which produced it,
and would in its turn reproduce all that heat. These last doctrines,
however, rested on a speculative basis, until Mr. James Prescott Joule,
of Manchester, carried out a most patient, laborious, and elaborate
experimental investigation of the subject. His labours placed the truth
of the numerical equivalence of forces on a foundation which cannot be
shaken; and he accomplished for the principle of the indestructibility
of force what Lavoisier did for that of the indestructibility of
matter—he established it on the incontrovertible basis of accurate and
conclusive experiment. His determination of the value of the _mechanical
equivalent of heat_ especially is a model of experimental research; and
subsequent investigators have, by diversified methods, confirmed the
accuracy of his results. A great part of his work consisted in finding
what quantity of heat would be produced by a given quantity of _work_.
Before we proceed to give an indication of one of Dr. Joule’s methods of
making this determination, we may point out that if a weight be raised a
certain height, the work which is done in raising it will be given out
by the weight in its descent. If you carry a 1 lb. weight to the top of
the London Monument, which is 200 ft. high, you perform 200 units of
work. When the weight is at the top, the work is not lost; for let the
weight be attached to a cord passing over a pulley, and it will, as it
descends, draw up to the top another 1 lb. weight.[21] If you drop the
weight so that it falls freely, it descends with a continually
increasing velocity, strikes the pavement, and comes to rest. Still your
work is not lost. The collision of the weight and the pavement develops
_heat_, just as in the case of the experiment depicted on page 168, but
to a less degree—the increase of temperature might not be sensible to
the touch, but could be recognized by delicate instruments. Your work,
then, has now changed into the form of heat—the weight and the pavement
are hotter than before. This heat is carried off by contiguous
substances. But still your work is not lost, for it has made the earth
warmer. The heat, however, soon flows away by radiation from the earth,
and is diffused into space. The final result of your work is, then, that
a certain _measurable_ quantity of heat has been sent off into space. Is
your work now finally lost? Not so: in reality, it is only diffused
throughout the universe in the form of radiant heat of low intensity.
Yet it is lost for ever for useful purposes; for from this final form of
diffused heat there is no known or conceivable process by which heat can
be gathered up again.
Footnote 21:
See Note B, at the end.
Dr. Joule arranged paddles of brass or iron, so that they could turn
freely in a circular box containing water or quicksilver. From the sides
of the box partitions projected inwards, which contained openings that
permitted the divided arms of the paddle to pass, and preventing the
liquid from moving _en masse_, thus caused a churning action when the
paddle was turned. Now, every one who has worked a rotatory churn knows
that a considerable resistance is offered to this action; but every one
does not know that under these circumstances the liquid becomes warmer.
It was Dr. Joule’s object to discover how much the temperature of his
liquid was raised by a measured quantity of work. He used very delicate
thermometers, and had to take a number of precautions which need not
here be described; and he obtained the definite quantity of work by the
descent of a known weight through a known distance, a cord attached to
the weight being wound on a drum, which communicated motion to the
paddle. The experiments were conducted with varying circumstances, to
avoid chances of error, and were repeated very many times until uniform
and consistent indications were always obtained. The result of the
experiments showed that 772 units of work (foot-pounds) furnished heat
which would raise the temperature of 1 lb. of water from 32° to 33° F.,
which is the unit of heat. This number, 772, is a constant of the
greatest importance in scientific and practical calculations, and is
called “_the mechanical equivalent of heat_.” The amount of _work_ it
represents is sometimes called a “Joule,” and is always represented in
algebraical formulæ by “J.” Mr. Joule’s first paper appeared in 1843,
and soon afterwards various branches of the subject of “The Equivalence
and Persistence of Forces” were taken up by a number of able men, who
have advanced its principles along various lines of inquiry. Among the
most noted contributors to this question we find the names of Sir
William Thomson, Helmholtz, James Thomson, Rankin, Clausius, Tait,
Andrews, and Maxwell.
In the steam engine the case is the inverse of that presented by the
above-named experiment of Dr. Joule’s. Here we have heat producing work.
Now, the quantity of steam which enters the cylinder of a steam engine
may be found, and the temperature of the steam can be determined, and
from these the amount of _heat_ which passes into the cylinder per
minute, say, can be calculated. A large portion of this heat is, in an
ordinary engine, yielded up to the condensing water, and another part is
lost by conduction and radiation from the cylinder, condenser, pipes,
&c. But both these quantities can be estimated. When the amount is
compared with that entering the cylinder in the steam, a difference is
always found, which leaves a quantity of heat unaccounted for. When this
quantity is compared with the _work_ done by the engine in the same
interval (which work can be measured as described on page 10), it is
always found that for every 772 units of work a unit of heat has
disappeared from the cylinder. The numerical relation between work and
heat which is established in these two cases has been tested in many
quite different ways; and, within the limits of experimental errors,
always with the same numerical result. But equally definite quantitative
relations are known to exist among all the other forms of force; and the
manner in which these are convertible into each other has already been
indicated, although want of space prevents full illustration of this
part of the subject. It may, however, be seen that each form of force
can be mediately or immediately converted into mechanical effect, hence
each is expressible in terms of work. That is to say, we can assign to a
unit of electricity, for example, a number expressing the work which it
would do if entirely converted into work; and the same number also
expresses the work which would be required to produce the unit of
electricity. An ounce of hydrogen in combining with 8 oz. of oxygen
produces a certain measurable quantity of heat. If that heat, say = H,
were all converted into work, we now know that the work would = HJ.
Hence we can express a definite chemical action in terms of _work_. The
same is generally true of all physical forces, though in some cases,
such as light, vital action, &c., the quantitative relations have not
yet been definitely determined.
Since, then, all the forces with which we are acquainted are expressible
(though the exact relations of some have yet to be discovered) in terms
of work, it is found of great advantage to consider the power of doing
work as the common measure of doing all these. Thus, if we define
_energy_ as that which does, or that which is capable of doing, work, we
have a term extremely convenient in the description of some aspects of
our subject. Thus we can now speak of the _energies of nature_, instead
of the _forces_. And all forces, active or passive, may be summed up in
one word—_energy_. And, further, the great discovery of the conservation
of forces under definite equivalents, may be summed up very briefly in
this statement—THE AMOUNT OF ENERGY IN THE UNIVERSE IS CONSTANT. To make
this statement clear requires that a distinction between two forms of
every kind of energy be pointed out. To recur to the example before
imagined: if you carry the 1 lb. weight up the Monument, and deposit it
on the ledge at the top, it might lie there for a thousand years before
it was made to give back the work you had performed upon it. That work
has been, in a manner, _stored up_ by the _position_ you have given to
your weight. Now, in taking up the weight, you expended energy—you
really performed work: that is an instance of energy in operation, and
may be termed “actual energy.” In what form does the energy exist during
the thousand years we may suppose your weight to lie at the top of the
Monument? It is ready to yield up your work again at any moment it is
permitted to descend, and it possesses therefore during the whole period
a _potential energy_ equal in amount to the _actual energy_ you bestowed
upon it. A similar distinction between actual and potential energy
exists with regard to every form of force. If by any means you separate
an atom of carbon from an atom of oxygen, you exert actual energy. The
process is analogous to carrying up the weight. The atoms when separated
possess _potential_ energy,—they can rush together again, like the
weight to the earth, and in doing so will give out the work which was
expended on their separation. A parallel illustration might be drawn
from electrical force.
A typical example of the storing up of energy is furnished by a
crossbow. The moment a man begins to bend the bow he is doing work,
because he pulls the string in opposition to the bow’s resistance to a
change in its form; and it is plain that the amount of energy thus
expended is measurable. Suppose, now, the bow has been bent and the
string caught in the notch, from which it is released by drawing the
trigger when the discharge of the bow is desired. The bow may be
retained for an indefinite period in the bent condition, and in this
state it possesses, in the form of _potential energy_, all the work
which has been expended in bending it, and which it will, in fact, give
out, in some way or other, whenever the trigger is drawn. To fix our
ideas, let us suppose that to draw the string over the notch required a
pull of 50 lbs. over a space of 6 in.; that is equivalent to 50 × ½ = 25
units of work. Now let the bow be used to shoot an arrow weighing ¼ lb.
vertically upwards. The height in feet to which the arrow will rise
multiplied into its weight in pounds will be the work done upon it by
the bow. Now, we say that experiment proves that in the case supposed
the arrow would rise just 100 ft., so that the work done _by_ the bow (¼
× 100 = 25) would be precisely that done _upon it_. For the sake of
simplicity, we keep this illustration free from the mention of
interfering causes, which have to be considered and allowed for when the
matter is put to the real test of quantitative experiment. The instance
of the crossbow brings into notice a highly instructive circumstance,
which is this: the bow, which it may have taken the strength of a
Hercules to bend, will shoot its bolt by the mere touch of a child on
the trigger. In the same way, when a man fires a gun, he merely permits
the _potential_ energy contained in the charge to convert itself into
_actual_, or _kinetic_, energy. The real source of the energy, in the
case of the child discharging the crossbow, is the muscular power of the
man who drew it; the real source of the energy in exploding gunpowder is
the separation of carbon atoms from oxygen atoms, and that has been done
by the sun’s rays, as truly as the string was pulled away from the bow
by muscular power. If we turn our attention to nitro-glycerine or to
nitro-cellulose, we can, by following the chemical actions giving rise
to these substances, in like manner trace their energies to our great
luminary. The unstable union by which oxygen and nitrogen atoms are
locked up in the solid and liquid forms of nitro-cellulose and
nitro-glycerine is also the work of the sun; for nitrogen acids, or
rather nitrates, are produced naturally under certain electrical and
other conditions of the atmosphere, which are due, directly or
indirectly, to the sun’s action; and they cannot be formed artificially,
except by imitating the natural conditions, as by passing electric
sparks through air, &c.
It will now be understood, as regards the wonderful relations between
animal and vegetable life, which have already been alluded to more than
once, how the sun, by expending actual energy, separates atoms of carbon
from atoms of oxygen in the leaves of plants, and confers upon these a
position of advantage, _i.e._, potential energy; and how animals,
absorbing the separated carbon in the form of food, and inhaling the
separated oxygen in the air they breathe, cause the conversion of the
potential into actual energy, which appears in the heat, movements, and
vital functions of the animal body. In coal we have the energy which
plants absorbed from the sun ages ago, stored up in a potential form.
The carbon atoms are ready to rush into union with oxygen atoms, and
convert their energy of position into the energies developed by chemical
action, viz., heat, light, &c. Energy is thus constantly shifting its
form from actual to potential, and _vice versâ_, and exhibiting itself
under the various transformations of force, as when sun-force changes to
chemical action, chemical action to heat, heat to electricity, &c.
Energy is, indeed, the real modern PROTEUS—constantly assuming different
shapes, difficult to grasp if not held in fetters; now taking on the
form of a lion, now of a flame of fire, a whirlwind, a rushing stream.
As sober, literal matter of fact we catch glimpses of energy under these
very forms.
The greatest discovery of the age has, as already indicated, immediate
and important practical bearings. The amount of thought which, even in
the present day, is devoted by unscientific mechanics to the old problem
of perpetual motion is far greater than is generally supposed. The
principle of the conservation of energy shows that this is an
impossibility; that the inventor who seeks to create force might just as
well try to create matter; that the production of a perpetually moving
self-sustaining machine is as far removed from human power as the
bringing into existence of a new planet. In force, as in matter, the law
is inexorable—_ex nihilo nihil fit_. Again, knowing the definite amount
of energy obtainable from the combustion of a pound of coal, we can
compare the amount we actually procure from it in our steam engines with
this theoretical quantity as the limit towards which our improvements
should bring us continually nearer, but which we can never exceed, or,
indeed, even reach. The schemers of perpetual motion are not the only
class of speculators who pursue objects which are incompatible with our
principle. There are many who seek to accomplish desirable ends by
inadequate means: who, for example, are aiming perhaps to accomplish the
reduction of ores by a quantity of fuel less than that mechanically
equivalent to the work, or who conceive that by adding to coal some
substance which itself is unchanged, an indefinitely greater amount of
heat may be liberated by the combustion.
Enough has been said to show that the energies of animal life can be
traced to the sun as their source. The sun builds up the plant,
separating oxygen from carbon. The animal—directly or mediately by
devouring other animals—takes the carbonaceous matter of the plant, and
reunites it with oxygen. In the plant the sun winds up the spring which
gives life to the animal mechanism; for the winding-up of a spring and
the separation of the atoms having chemical affinities are alike
instances of supplying potential energy. In the animal there is a
running-down of the potential into actual energy. It is plain also that
of the total energy radiated from the sun in every direction, the earth
receives but a very small part (1/2300000000). By far the larger part is
diffused into space, where, for all such purposes as those with which we
are concerned, it is lost. The heat which the sun sends out in a year is
calculated to be equal to that which would be produced by the combustion
of a layer of coal 17 miles thick over the whole surface of the
luminary. Is the sun, then, a flaming fire? By no means. Combustion is
not possible at its temperature; and as we know the substances which
enter into its composition are the same as those we find in the earth,
we know that the chemical energies of such substances could not supply
the sun’s expenditure. Passing over as unsatisfactory an explanation
which might occur to some minds—namely, that the sun was created hot at
the beginning, and has so continued—there are two theories which attempt
to account for the sun’s heat. One is that of Meyer, who supposes the
heat is due to the continual impact of meteorites drawn to the sun by
its gravity; and the other is that of Helmholtz, who attributes the heat
to the continual condensation of the substance of the sun. Helmholtz
calculates that a shrinking of the sun’s diameter by only 1/10000th of
its present amount, would supply heat to last for two thousand years;
while the condensation of the substance of the sun to the density of the
earth would cover the sun’s expenditure for 17,000,000 of years. There
is great probability that both theories may be correct, and that the
cause of the sun’s heat may be considered as due in general terms to
aggregation of matter, by which the original potential energy of
position is converted into the actual energy of heat and light. Now,
however immense may be our planetary system, the sun being continually
throwing off this energy into space, there must come a time when the
supplies of meteorites will fail, and when the great globe of the sun
will have shrunk to its smallest dimensions. We see, then, that heat and
light are produced by the aggregation of matter; the heat and light are
radiated into space; the small fraction intercepted by our globe is the
source of almost every movement—the original stuff, so to speak, out of
which all terrestrial forces are made. The sun produces the winds, the
thunderstorms, the electric currents of the Aurora, the phenomena of
terrestrial magnetism, and is the source of vegetable and animal life.
The waves, the rains, the mountain torrents, the flowing rivers, are the
work of the sun’s emanations.
In the illustration of the energy expended on raising a weight
afterwards dropped, we traced that energy into the final form of heat of
a low temperature radiated into space. It would be easy to show that all
energy ultimately takes the same form. Now, although it is easy to
convert work into heat, there is no conceivable process by which
uniformly-diffused heat can again be made to do any kind of work. The
case may be compared to water, which in moving down from a higher to a
lower level may be made to perform any variety of work. But when all the
water has passed down from the higher level to the lower, it can no
longer do any work. Whenever work is done by the agency of heat, there
is always a passing from a higher temperature to a lower—a transference
of heat from a hotter body to a colder. If the condenser of the steam
engine had the same temperature as the steam, the machine would not
work. Not only do all the energies in operation on the face of the earth
continually run down into the form of radiant heat sent off by the earth
into space; but our sun’s energy, and that of the suns of other systems,
are also continually passing off into space; and the final effect must
be a uniform diffusion of heat in a universe in which none of the varied
forms of energy we now behold in operation will be possible, because all
will have run down to the same dead level of uniformly-diffused heat.
This startling corollary from the principle of the conservation of
energy has been worked out by Sir W. Thomson under the title of “The
Dissipation of Energy.” It leads us to contemplate a state of things in
which all light and life will have passed away from the universe—a
condition which the poet’s terrible dream of darkness, “which was not
all a dream,” seems to shadow forth—
“The bright sun was extinguished, and the stars
Did wander darkling in the eternal space,
Rayless and pathless; and the icy earth
Swung blind and blackening in the moonless air.
* * * * *
The world was void,
The populous and the powerful was a lump,
Seasonless, herbless, treeless, manless, lifeless—
A lump of death—a chaos of hard clay.
The rivers, lakes, and ocean all stood still,
And nothing stirred within their silent depths.
* * * * *
The waves were dead; the tides were in their grave,
The Moon, their mistress, had expired before;
The winds were withered in the stagnant air,
And the clouds perished; Darkness had no need
Of aid from them—She was the Universe.”
The doctrine of this persistence and dissipation of energy completely
harmonizes with the grand speculation termed the “nebular hypothesis,”
which regards the universe as having originally consisted of uniformly
diffused matter, which, being endowed with the power of gravitation,
aggregated round certain centres. This process is still going on; and,
according to modern speculations, light and life and motion are but
manifestations of this primæval potential energy being converted into
actual energy, and degrading ultimately into the form of
universally-diffused heat. To quote the closing sentences of the
eloquent passage in which Professor Tyndall concludes the work mentioned
above, “To nature nothing can be added, from nature nothing can be taken
away; the sum of her energies is constant, and the utmost man can do in
the pursuit of physical truth, or in the applications of physical
knowledge, is to shift the constituents of the never-varying total. The
law of conservation rigidly excludes both creation and annihilation.
Waves may change to ripples, and ripples to waves; magnitude may be
substituted for number, and number for magnitude; asteroids may
aggregate to suns, suns may resolve themselves into floræ and faunæ, and
floræ and faunæ melt in air: the flux of power is eternally the same. It
rolls in music through the ages, and all terrestrial energy—the
manifestations of life as well as the display of phenomena—are but the
modulations of its rhythm.”
The discoveries to which we have here endeavoured to attract the
reader’s attention thus give rise to conceptions of the utmost grandeur
and interest. We see that the sum of Nature’s energies is constant; that
all the manifestations of force are but the transference of power from
one position to another. And we have recognized the material source of
all our terrestrial energies in the sun. Two theories have already been
mentioned by which it is sought to account for the sun’s heat—the
meteoric theory of Meyer and Thomson, and the shrinkage theory of
Helmholtz. These both assume gravitation as the primal force from which
the supply of heat and other energies must be drawn, and they assume
also that the laws of radiation and of the degradation of temperature in
the transformation of heat into other forces, as we find them operating
at the earth’s surface, are equally in action in every region of space.
Hence is deduced that conception of the final state of the universe as
one of merely equally diffused temperature admitting of no further
transformation. This speculation presents the _universe_ in the aspect
of a clock, now indeed going, but when once run down, incapable of ever
being again wound up. There seems in this view a want of symmetry, so to
speak; we miss the feeling of harmonious _rhythm_ to which Tyndall
refers. There is, however, another cosmic theory, well supported by
accumulating facts, which assigns to gravitation a less important part
in the production of solar heat and in the evolution of worlds, and it
is one which supplies also a basis for the explanation of such phenomena
as aerolites, comets, variable stars, the inclination of planets’ axes
to their orbits, the proper motion of our sun, and that of the so called
fixed stars, of all of which the nebular hypothesis fails to give any
account; while, on the other hand, the _impact theory_, as it has been
named, includes the other, and goes beyond it. The reader who desires to
pursue this subject may be referred to Croll’s book on Stellar
Evolution.
In the last few paragraphs we have been dealing with speculations as
much as with discoveries. But indeed the former are the offspring of the
latter, as certainly as one invention becomes the parent of others. The
human mind never rests contented with the knowledge and mastery of
nature actually gained, but ever seeks to pass beyond and attain still
greater power. The volume we are now bringing to a close has given but
brief and imperfect indications of specimens, taken here and there, of
what has been done during the short period of one century. We may draw
an augury for the future of man’s dominion from the powers his
Promethean spirit has already grasped:
“The lightning is his slave; heaven’s utmost deep
Gives up her stars, and like a flock of sheep
They pass before his eye, are numbered, and roll on!
The tempest is his steed, he strides the air;
And the abyss shouts from her depth laid bare,
“Heaven, hast thou secrets? Man unveils me; I have none.”
NOTES A AND B.
_Note A_—_Continuation of Table on page 755, showing the quantity of
Coals raised annually in Great Britain._
Year. Coal raised in Tons.
1874 126,590,108
1875 133,306,458
1876 134,125,166
1877 134,179,968
1878 132,612,063
1879 133,720,393
1880 146,969,409
1881 154,184,300
1882 156,499,977
1883 163,737,327
1884 160,757,779
1885 159,351,418
1886 157,518,482
1887 162,121,576
1888 169,935,219
1889 176,916,724
1890 181,614,280
1891 185,479,126
1892 181,786,871
1893 164,325,795
1894 188,277,525
_Note B_—_CONSERVATION OF ENERGY._—_Page 804._
The statement here should have been more explicit, as it has reference
to a state of things not to be realised in practice. Like the well-known
“first law of motion,” it can neither be demonstrated _à priori_, nor
proved by any direct and simple experiment. The first law of motion
asserts that a body in motion, not acted on by any external force, will
continue to move in a straight line, and with a uniform velocity. Now we
cannot place a body in such a position that it will not be acted upon by
some external forces; but the more we lessen the effect of external
forces, the more nearly is the motion straight and uniform. Similarly in
the case supposed, the intention is to show that the weight carried up
is in a position to do just as much work as was done upon it. We must
suppose several impracticable but conceivable conditions in order to
eliminate considerations which do not concern the theoretical question;
we must suppose the cord to be weightless and absolutely devoid of
rigidity; the pulley to have no mass or inertia, that is to require no
force to set it in motion, and to move without any friction; the air to
offer no resistance; and the force of gravity to be uniform throughout
the space. Some approximation to these conditions is practicable, as,
for example, the pulley might be the lightest possible, and turn on
friction wheels, the cord might be the finest silk thread, and so on.
But it is not the influence of these external forces we are considering,
but only the energy due to the position of the raised weight. Assuming,
therefore, the disturbing conditions absolutely eliminated, it is not
difficult to see that no downward force or pressure, however small,
could be applied for ever so short a time, to the upper weight without
setting the system in motion. The motion would be an accelerated one so
long as the force was applied, it would become uniform when the force
ceased to act; it would have a velocity proportionate to the force. In
any case, after a time the descending weight would reach the ground, and
for our point of view it is quite immaterial whether the time occupied
by the movement were 5 minutes or 5,000 years, for be it observed, time
does not enter into the definition of _work_ as it does into that of
“horse-power.” Then by pushing the conceived conditions to their limits,
we may see that without considering any question of conversion of motion
into heat, the raised weight can, in theory at least, give back again
the energy spent upon it.
INDEX
A.
Abel, Professor, 746.
Accumulators, 530.
Adhesion of locomotive, 21.
Advantages of present age, 2.
Aerolites, 30.
Air, 734.
Albert Bridge, Saltash, 283.
Alizarine, 797.
“_Alliance_” magneto-electric machine, 520.
Aluminium, 717.
bronze, 719.
reduced cost of, 723.
American Tract Society building, 78.
Ampère’s hypothesis, 750.
rule, 492, 549.
_Amphioxus_, 679.
ANÆSTHETICS, 731.
Anemometer, 656.
Angle, limiting, or critical, 399.
Aniline, 787.
black, 793.
blue, 790.
green, 791.
purple, 788.
Anomalous magnetisation, 538.
_Anthea Cereus_, 678.
Anthracene, 796, 797.
Applegath printing machine, 312.
and Cowper, ditto, 308.
Apps’s anemometer, 656.
induction coil, 506.
AQUARIA, 675.
Arago, 599.
Architecture, use of iron in, 72.
Argand gas burners, 773.
lamps, 595.
Armours, ships’, strengths of, 166.
Armstrong 110–ton gun, 202.
Armstrong’s guns, 192.
hydraulic crane, 333.
Atoms, 733, 743.
Aurora, 504.
Australian gold, 688.
Austrian torpedoes, 229.
“_Automobile_” competition, 23.
Axolotl, 686.
B.
Bacon, Francis, 799.
Balloon, photographic, 628.
Basic process (Gilchrist’s), 66.
Battery, galvanic, 493.
Battery, secondary, 530.
Baxter House experiments, 58.
Bell Rock Lighthouse, 593.
Bells, electro-magnetic, 554.
Benzol, 783.
Bessemer, 26, 56.
Channel steamer, 142.
converter, 63.
iron, 62.
process, 64.
steel, 56–67.
BIG WHEELS, 81.
Bitter Lakes, 261.
BLANCHARD LATHE, THE, 86.
Blast furnace, 40.
Blind spot in eye, 460.
Blister steel, 54.
Blood spectra, 431.
Boilers of steam engines, 13.
Boring for coals, 361.
Bourdon’s pressure gauge, 12.
Bourseul, M., 582.
Box girders, 280.
Breakwater, 258.
Breakwaters for Suez Canal, 258.
BREECH-LOADING RIFLES, 182.
Brewster, Sir D., 405, 420, 470, 474.
Bridge, projected Channel, 296.
Bridgewater Canal, 250, 266.
BRIGHTON AQUARIUM, 682.
Britannia Bridge, 280.
raising tubes, 336.
British Aluminium Co., 723.
British navy in 1894, 167.
BROOKLYN BRIDGE, 303.
“Brown Bess,” 178.
Browning’s micro-spectroscopes, 434.
spectroscope, 422, 432.
Brunel, 283.
Brunswick rifle, 180.
Brush dynamo, 522.
Bullet, Greener’s expanding, 182.
Lebel, 188.
Minié, 180.
Bullets, machinery for making, 330.
Bunsen and Kirchhoff, 422.
and Roscoe, 720.
Bunsen’s battery, 496.
burner, 421.
Bye-products in gas making, 772.
C.
Cable railways, 126.
Cæsium, 426.
Caissons for Forth Bridge, 293.
_Calais-Douvres_, steamship, 141.
Caledonian Canal, 250.
Calico printing machines, 321.
California, discovery of gold in, 688.
Camera, 613.
Canal, Caledonian, 250.
Manchester Ship, 262.
Nicaragua, 274.
North Sea, 271.
Panama, 272.
Suez, 251.
CANTILEVER BRIDGES, 291.
“Cape Horn,” 120.
_Captain_, H.M.S., 141.
Carbon transmitter, 590.
Carbonic oxide, 44, 48.
Carbons for arc lights, 527.
Carcel lamp, 597.
Carpenter, Dr. W. B., 462.
Carriages, railway, 111.
for rock drills, 358.
Carriers in pneumatic tubes, 344.
Cars, Pullman, 112.
_Castalia_, steamship, 139.
Cast-iron, composition of, 43.
Cast steel, 54.
Catoptric lighthouse apparatus, 599.
CAUSE OF LIGHT AND COLOUR, 408.
CELESTIAL CHEMISTRY AND PHYSICS, 436.
Celluloid, 622.
Central Telegraph Office, London, 574.
Centres of gravity and buoyancy, 149.
Centrifugal force, 107.
Chains, 330.
Chain-testing machine, 329.
Channel Bridge (projected), 296.
steamers, 142.
CHANNEL TUNNEL, 364.
Chassepot rifle, 182.
Chemical action of light, 608.
equations, 734.
nomenclature, 782.
symbols, 733.
work of electricity, 497.
Chloroform, 608.
Chromatic aberration of eye, 462.
Chromo-lithography, 638.
Chronograph}
Chronoscope} electric, 656.
Cincinnati Suspension Bridge, 287.
_City of Rome_, steamship, 139.
Clark’s hydraulic lift graving dock, 331.
Clarke’s magneto-electric machine, 509.
Clay process, stereotyping by, 633.
Clerk Maxwell’s theory of light, 541.
“Clermont,” the, 147.
Clifton Suspension Bridge, near Bristol, 285.
Niagara, 287.
COAL, 751.
Coal in Kent, 371.
COAL-GAS, 764.
COAL-TAR COLOURS, 781.
Code, telegraphic, of American War Department, 528.
Morse’s, 560.
Wheatstone’s dot, 565.
Cold-short iron, 62.
Colesberg, 703.
Collodion process, 618.
Colour printing, 639.
Colours not in the objects, 413.
photography of, 628.
Comets, spectra of, 444.
Composition rollers, 406.
Condie’s steam-hammer, 28.
Copying principle, 86.
Cordite, 748.
Corona, 438.
Cort’s puddling furnace, 45.
Couple, mechanical, 149.
Cramp gauge, 129.
Croll, on Stellar Evolution, 810.
Crookes, 507.
Crystal Palace, an example of use of iron in architecture, 72.
Crystal Palace, 72.
AQUARIUM, 677.
Crystalline lens, 455.
Cup and cone, 49.
Current, electric, 492.
induced, 502.
measurement of, 536.
Currents in submarine cables, 579.
D.
Daguerre, 609.
Daguerreotype, 610.
Daimler motor, 24.
Dallmeyer, 617.
Daniell’s battery, 495.
De Beers Mines, 707.
Delphi, oracle at, 739.
Dial telegraphs, 566.
Diamond cutting, 700.
Diamond, qualities of the, 698.
rock drill, 359.
Diamondiferous, 703.
Diamonds, 696.
use of, 701.
Dioptric lighthouse apparatus, 600.
Discoveries, progressive, 802.
Dissipation of energy, 808.
Distinct vision, 458.
D lines of sodium spectrum, 425, 441.
Dolls, talking, 674.
Domestic consumption of coal, 755, 756.
DOUBLE REFRACTION AND POLARIZATION, 399.
Dredges, Suez Canal, 255.
Drilling machine, 90.
Dry digging, 702.
Duboscq’s electric lamp, 497.
Du Moncel, 590.
DYNAMICAL ELECTRICITY, 490.
Dynamo, Siemens’, 522.
Brush, 522.
E.
Earl’s Court, Great Wheel at, 83.
Earth’s circuit, 574.
Ebonite, 728.
Eccentric, 9.
Eclipse of sun, 438.
Eddystone lighthouse, 594.
Edison, 669, 670, 674.
Edison’s kinetoscope, 478.
Eiffel Tower, the, 72.
ELECTRIC LIGHTING AND ELECTRIC POWER, 519.
Electric current, 790.
furnace, 722.
ELECTRIC INDUCTION, 488.
launch, 534.
light, cost of, 43.
Electric light in lighthouses, 515.
telegraph, 598.
torpedo, 547.
tramway, 532.
welding, 537.
ELECTRICITY, 481.
ELECTRICITY, THE NEW, 538.
Electrode, 497.
Electro-magnet, 500.
Electromotive force, 494.
Electro-plating, 499, 518.
Electrotyping, 634.
Elementary bodies, 716.
ELEMENTARY PHENOMENA OF MAGNETISM AND ELECTRICITY, 483.
Elswick 4·7–in. gun, 206.
guns, 194.
Energy, 806.
Ether, 735.
the luminiferous, 408.
Exhaustion of coal, 755, 756, 757.
Expansive working of steam, 8, 17.
Explosion by concussion, 745.
of locomotive, 21.
of torpedoes, 229.
EXPLOSIVES, 225, 740.
different effects of, 748.
names and classes of, 750.
EYE, THE, 451.
dimensions of some parts of, 462.
Eye not optically perfect, 462.
Eyeballs, muscles of, 461.
F.
Fairbairn, Sir W., 280.
Faraday, 506, 508, 735.
ventilating gas-burner, 773.
Faure’s accumulator, 530.
Fellahs, 255.
Ferris wheel, Chicago, 81.
Field telegraphs, 555.
FIRE-ARMS, 169.
Fish-plates, 105.
Fizeau, 386.
Floating matter in air, 383.
Fluids, electric, 487.
Fly-wheels, 7.
Force, conservation of, 804.
electromotive, 494.
FORTH BRIDGE, THE, 311.
Foucault, 387.
Fovea centralis, 456, 457.
Fraser-Woolwich guns, 195.
Fraunhofer’s lines, 420, 436.
Fresnel’s mirrors, 409.
measurement of velocity of light, 600.
Fribourg Suspension Bridge, 286.
Froment’s dial telegraph, 567.
Furnace, electric, 322.
G.
Galvanic batteries, 493, 494.
Galvanometer, 493.
mirror, 570.
Gas engine, 25.
governor, 769.
holder, 766.
making apparatus, 765.
meters, 775.
pressure, 769.
retorts, 766.
Gases of blast furnace, 49.
Gatling battery gun, or mitrailleur, 219.
Gauge, broad and narrow, 106.
Bourdon’s pressure, 12.
Geissler’s tubes, 505.
Ghost, Pepper’s, 392.
Giffard’s injector, 11.
GIRDER BRIDGES, 280.
Glass, strains in, 407.
_Glatton_, H. M. S., 161.
Glynde, electric railway, 534.
GOLD, 686.
GOLD AND DIAMONDS, 687.
Gold-mining operations, 690.
Goodyear, Mr., 727.
Governor of steam engines, 6.
Gower Street Station, 114.
Gramme magneto-electric machine, the, 511.
Graphophone, 672.
Graphotype, 644.
Gray, 590.
GREAT BROOKLYN BRIDGE, 303.
_Great Eastern_, 133, 152, 330, 465, 578.
GREATEST DISCOVERY OF THE AGE, 801.
Greener’s expanding bullet, 182.
Grove, Sir W. R., 804.
Grove’s battery, 495.
Gun, 32–pounder, 191.
68–pounder, 192.
35–ton, 201.
81–ton, 201.
100–ton, 201.
110–ton, 202.
Elswick 4·7–in., 206.
Maxim, 225.
Moncrieff, 208.
Nordenfelt, 223.
Gun-cotton, 747.
torpedoes, 233.
Gunpowder, 734.
Guns, Armstrong’s, 192.
Elswick, 194.
Fraser-Woolwich, 195.
Krupp’s, 214.
quick-firing, 206.
submarine, 240.
GUTTA-PERCHA, 728.
H.
Half-tone process, 629.
Hancock, Mr. Charles, 729.
Mr. Thomas, 725.
Harvey’s torpedoes, 234.
Heat produced by electric current, 502.
Heat spectrum, 613.
Heating by gas, 776.
Helmholtz, 462, 464, 472, 474.
Henry, on Leyden jar discharge, 538.
_Hercules_, H.M.S., 150.
Hertz, Professor, 541.
Hippocampus, 664.
Hoe’s printing machines, 316, 318.
Holmes’ magneto-electric machine, 520.
Holophotal light, 604.
Holyhead and Kingston steamers, 136.
Horseless carriages (_Automobiles_), 23.
Horse-power, 10.
Hot-blast, 48.
Hotchkiss quick-firing guns, 208.
Hough’s metereograph, 654.
Howitzers, 213.
Hudson River steam navigation, 147.
Hughes’ printing telegraph, 560.
microphone, 590.
HYDRAULIC POWER, 324.
I.
Iceland spar, 399.
Illuminating power of gas, 774.
Illusion by movement of eye, 475.
by persistence of vision, 476.
stage, 290.
Images formed by lenses, 399, 616.
Impact theory of Stellar Evolution, 718.
Incandescent electric light, 528.
Incandescent gas-burners, 777.
INCLINED RAILWAYS, 125.
_Inconstant_, H.M.S., 152.
INDIA-RUBBER, 724.
INDIA-RUBBER AND GUTTA-PERCHA, 724.
Indicator, 9.
INDUCED CURRENTS, 502.
Induction coils, 503.
Injector, Giffard’s, 11.
Instantaneous photography, 623.
INTRODUCTION, 1.
IRON, 29.
IRON BRIDGES, 276.
IRON IN ARCHITECTURE, 72.
Iron, cast, 40.
chemical changes of, 34.
lighthouses, 596.
meteoric, 32.
ores, 39.
pig, 40.
ships, 133.
smelting, 39.
utility of, 30.
wrought, 47.
Ismaïlia, 260.
J.
Jablochkoff’s electric candle, 525.
Jackson, 551.
Jacobi, 531.
Jamin’s magnet, 513.
Johannesburg, 694.
Joule, 804, 805.
Joy’s valve gear, 20.
Jupiter, 384.
K.
Kaleidoscope, 389.
Karoos, 705.
Kimberley, 705.
Kinetographic theatre, 479.
Kinetoscope, 478.
Kirchhoff, 422.
Klondyke, 692.
_König_ or _Kaiser Wilhelm_, ironclad, 164.
König’s printing machine, 308.
Krupp’s guns, 214.
steel, 55.
works, 56.
L.
Lake Timsah, 260.
Lathe at Woolwich, 198.
Lathe, Blanchard, 96.
screw-cutting, 87.
Lap of slide-valve, 9.
Lebel rifle, 188.
Lens, formation of image by, 616.
in steps, 600.
photographic, 616.
Lepidosiren, 685.
LETTERPRESS PRINTING, 306.
Leyden jar, 490.
discharge, 538.
LIGHT, 380.
electric, 497.
invisible, 383.
LIGHTHOUSES, 593.
Limiting angle, 399.
Link motion, 16.
Linotype, the, 645.
Lithium, 425.
LITHOGRAPHY, 636.
Liverpool and Manchester Railway, 14.
Lock-gates, 267.
Locks in Manchester Ship Canal, 264, 266.
LOCOMOTIVE, THE, 14.
balancing of, 20.
compound, 18.
Lodge, Professor O., 540.
M.
MACHINE-GUNS, 218.
Madder, 796.
Magazine rifles, 187.
Magnesium, 720.
Magnetic field, 537.
Magnetism produced by current, 500.
Magneto-electric machines, 496, 507, 508.
MAGNETO-ELECTRICITY, 506.
Mallet’s Mortars, 212.
Malus, 405.
MANCHESTER SHIP CANAL, 262.
Manganese, 43.
Manhattan Life Insurance buildings, 78.
Mannlicher rifle, 187, 189.
Manufacturing _v._ making, 85.
Map, Channel Tunnel, 364.
Manchester Ship Canal, 263.
North Sea Canal, 271.
Pacific Railway, 117.
St. Gothard Railway, 372.
Suez Canal, 256.
Tower Bridge, etc., 299.
Marconi, 546.
Martini-Henry rifle, 184.
_Mary Powell_, the, 148.
Matter indestructible, 576.
Mauser rifle, 187, 188.
Maxim gun, 225.
Measuring machines, 88.
Mélinite, 748.
Menai Straits bridges, 280, 284.
Meteoric iron, 32.
Meteorites, 30.
Meteorology, importance of, 664.
Metereographs, 654.
Meters, gas, 775.
METROPOLITAN RAILWAY, THE, 114.
Microphone, 481.
Minié bullet, 180.
Mineral combustibles, 751.
Mines, submarine, 241.
Mirror, galvanometer, 570.
Mirrors, plane, 388.
Mirrors, illusions, 391–395.
Mitrailleur, 218.
Molecules, 733, 743.
_Monarch_, H.M.S., 156.
Moncrieff’s gun carriages, 208.
MONT CENIS TUNNEL, 351.
Montigny mitrailleur, 222.
Morse’s code, 560.
instruments, 558.
plate, 562.
telegraphic line, 556.
transmitting key, 561.
Mortar, Mallet’s, 212.
Mount Washington Inclined Railway, 125.
Musical sound, 666.
N.
Naphthaline, 669.
Napier’s platen machine, 321.
Nasmyth’s steam hammer, 26.
Nature knowledge, 1.
printing, 640.
Nebulæ, 626.
Needle telegraphs, 553.
Negretti and Zambra’s recording thermometer, 659.
NEW METALS, 714.
_New York_, the, 148.
Newton’s prism experiment, 418.
Niagara Suspension Bridge, 287.
Falls, 537.
NICARAGUA CANAL, 274.
Nichol’s prism, 403.
Niepce, J. N., 609.
de Saint-Victor, 611, 615.
Nitro-benzol, 784.
Nitrogen and oxygen compounds, 732.
Nitro-glycerine, 734.
Nordenfelt gun, 223.
NORTH SEA CANAL, 271.
Note A—Production of coal, 812.
Note B—Conservation of energy, 812.
O.
Œrsted’s experiments, 548.
Oil springs, 760.
Oldbury, manufacture of aluminium, 722.
Ophthalmoscope, 423.
Optical apparatus of lighthouses, 598.
Orders of lighthouse apparatus, 602.
Organic bodies, 798.
Oscillating engines, 14
P.
PACIFIC RAILWAY, THE, 116.
Paddle-wheels, 130.
Page, Mr., 532.
PANAMA CANAL, 272.
Papier-maché stereotype process, 633.
PARAFFIN, 761.
oils, 762, 763.
Parallel motion, 8.
Paris Exhibition, buildings of, 76.
Pascal’s principle, 325.
PATTERN PRINTING, 321.
Pepper, J. H., 392, 393, 505.
Percussion cap, 180.
PETROLEUM, 757.
Phenakistiscope, 476.
Phenomena of light, some, 382.
Phonautograph, 666.
PHONOGRAPH, 665.
Photographic camera, 615.
PHOTOGRAPHY, 607.
Photography, celestial, 636.
in colours, 614, 630.
in the dark, 613.
X-Ray, 447.
Photolithography, 644.
Photozincography, 644.
Pig iron, 40.
Planes, Whitworth’s, 94.
Planets, photographs of, 636.
Planing machines, 92.
Plants in coal measures, 753.
Plaster of Paris, stereotype process, 633.
PNEUMATIC DISPATCH, 340.
force, 333.
Pniel, 701.
Points, railway, 108.
Polariscope, 405.
Polarizer, 403.
Polytechnic institution, Regent Street, 505.
PORTABLE ENGINES, 24.
Portable telegraphic instruments, 556.
Portrait, Davy, 714.
Helmholtz, 452.
Joule, 789.
Kirchhoff, 416.
Lesseps, 249.
Morse, 547.
Senefelder, 632.
Simpson, 731.
Tesla, 572.
Thomson, 481.
Watt, 3.
Whitworth, 85.
Port Saïd, 257.
Post-office railway van, 111.
Potassium, 715.
Powder, smokeless, 748.
Power, horse, 10.
hydraulic, 324.
of steam engine, 9.
of locomotive, 21.
Powers, mechanical, 32.
Pressure gauge, 12.
transmitted in fluids, 324.
Principle, the copying, 86.
of the cantilever, 291.
PRINTING MACHINES, 305.
processes, 632.
telegraphs, 570.
Process blocks, 629.
Progress of mankind, 2.
Projectiles, 166.
air’s resistance to, 175.
deviation, 199.
long range of Whitworth, 193.
speed of, measured, 659.
trajectory of, 174.
Propagation of sound, 668.
Prospecting, 361.
_Proteus anguinus_, 684.
the modern, 807.
Pseudoscope, 472.
Puddling furnace, 45.
Q.
Queensferry, 292.
Quick-firing guns, 206.
Hotchkiss, 208.
R.
RAILWAYS, 101.
Great Western, 106.
Metropolitan, 114.
Midland, 112.
London and Manchester, 102, 124.
London and Woolwich, 103.
Pacific, 116.
St. Gothard, 371.
Stockton and Darlington, 101.
Randt, the, 694.
Rangoon petroleum, 759.
Rays polarized, 401.
Réaumur’s steel, 67.
Recoil, 172.
RECORDING INSTRUMENTS, 653.
Red-short iron, 62.
REFLECTION OF LIGHT, 388.
Reflection in water, 396.
total, 399.
REFRACTION, 397.
double, 399.
Regulators for electric lamps, 523.
Reiss, 583.
Reliance building, the, 78.
Resistance, electrical, 494.
Resonance, electric, 540, 541.
Retina, 456.
Reverberatory furnace, 45.
Rifle, Brunswick, 180.
Chassepot, 182.
Lebel, 188.
Lee, 188.
Mannlicher, 187, 189.
Martini-Henry, 184.
magazine, 187.
Mauser, 187, 188.
military, 178.
Minié, 181.
Snider Enfield, 184.
Vetterli, 189.
Whitworth, 182.
RIFLED CANNON, 190.
Rifles, breech-loading, 182.
reduction of bores of, 189.
Rifling, 171.
guns, 199.
Righi, Professor, 541.
Rigi, railways ascending the, 126.
RIVER STEAMBOATS, 144.
ROCK BORING, 349.
“Rocket,” 14.
ROCK-DRILLING MACHINES, 355.
ROENTGEN’S X-RAYS, 445.
Rolling iron, 71.
Ronald’s telegraph, 548.
Roscoe, 425, 437, 444.
and Bunsen, 664.
Royal Gun Factory, Woolwich, 27.
Ruete’s ophthalmoscope, 466.
Ruhmkorff’s coil, 730.
S.
Saint Paul building, the, 78.
Saltash Bridge, 283.
San Francisco, 123.
Sand, properties of, 253.
Saw, circular, 100.
SAWING MACHINES, 98.
Saturn, 440.
Schilling’s telegraph, 549.
Science, benefits of, 1.
Science and useful arts, 2.
Scott’s phonautograph, 669.
Screw, 86.
cutting lathe, 87.
dies and taps, 86.
propeller, 116, 117.
Sea anemones, 679.
horses, 684.
Secondary batteries, 530.
Segment shells, 217.
Senefelder, 556, 636.
Shear steel, 54.
SHIP CANALS, 249.
SHIPS OF WAR, 149.
Shrapnel shells, 216.
Siemens, 67.
Siemens’ dynamo, 522.
pneumatic tubes, 341.
regenerator, 68.
regulator, 523.
Siemens-Martin steel, 70.
Sight, 452.
Signals, railway, 108.
Silver plating by electricity, 500.
Sirius, 440.
Skerryvore, 596.
Skiagraphs, 447.
Slide rest, 87.
Smokeless powder, 226.
Snider rifle, 184.
Snow-plough, 123.
Sodium, 715.
Some phenomena of light, 382.
Sommeiller perforators, 351.
Sömmering’s telegraph, 548.
Sound, 665.
waves, 640.
Sounding telegraph, 566.
South African diamond fields, 701.
Speaking machine, 670.
tubes, 730.
Spectra, absorption, 431.
bright lined, 425.
of permanent gases, 430.
of salts, 424.
of stars, 440.
spark, 420.
SPECTROSCOPE, 416.
Spectrum, continuous, 422.
lithium, 425, 436.
of sodium, 424.
pure, 420.
solar, 418, 613.
Sphygmograph, 660.
Spiegeleisen, 64.
Stage illusions, 395.
Stars, distance of, 440.
motion of, 442.
spectra of, 440.
Steamboats, river, 144.
Steam engine, agricultural, 23.
domestic, 25.
forms of, 14.
Newcomen’s, 3.
portable, 23.
Watt’s double-acting, 6.
Watt’s improvements, 4.
STEAM ENGINES, 3.
Steam carriages, 21.
expansive working of, 8, 17.
STEAM HAMMER, THE, 25.
STEAM NAVIGATION, 129.
Steam fire engine, 23.
“navvies,” 269.
rollers, 23.
superheated, 9.
Steamships, comparative sizes of, 138.
recent improvements in, 139.
speed of, 137.
Steel, 52, 68.
Bessemer, 58.
blister, 54.
cast, 54.
Krupp’s, 55.
puddled, 55.
Réaumur’s, 67.
shear, 54.
Siemens, 68.
tempering, 53.
tensile strength of, 53.
Stellar evolution, 810.
Stephenson, George, 14, 102.
Robert, 280.
ST. GOTHARD RAILWAY, 371.
Stereoscope, Brewster’s refracting, 470.
Wheatstone’s reflecting, 469.
Stereoscopic effect, 469.
lustre, 472.
views, 470, 474.
STEREOTYPING, 642.
Stevenson, Alan, 602, 603.
Storage battery, 530.
Strada, 547.
Stratified discharge, 517.
Stroboscopic disc, 476.
Submarine cables, 575.
Sub-Wealden, exploration, 362.
SUEZ CANAL, THE, 251.
Sun, elements in, 438.
constitution, 438.
Superheated steam, 9.
Surface plates, Whitworth’s, 94.
SUSPENSION BRIDGES, 284.
Sympathetic needles, 547.
Syphon recorder, 571.
Swan’s carbon process, 619, 481.
T.
Table, air resistance to projectile, 177.
benzol and toluol compounds, 789.
coal raised in Great Britain, 727.
coal-tar, colours, 794.
composition of cast iron, 43.
dimensions of parts of eye, 462.
dimensions of steamships, 138.
electric light _v._ gas, 514, 515.
floatation of _Monarch_ and _Captain_, 160.
formulæ of hydro-carbon, 758.
Gramme machine, 518.
hydro-carbon in coal-tar, 782.
illuminating power of gases, 774.
lighthouse apparatus, 602.
lighthouse lamps, 597.
Martini-Henry rifle, 187.
nitrogen and oxygen compounds, 731, 732.
photographic actions, 612.
products from 100 lbs. of coal, 796.
ships of war, 166.
telegraph code, Morse’s, 560.
telegraph, war department, 557.
tenacities of iron, 277.
wave-lengths of colours, 411.
Wheatstone’s dot signals, 565.
Talbot, 610.
Talbotype, 611.
TALL BUILDINGS, 76.
Tawell, arrest of, 550.
TELEGRAPHIC INSTRUMENTS, 553.
TELEGRAPHIC LINES, 572.
Telegraph poles, 572.
Telegraphs in Great Britain, 574.
Telegraphy, wireless, 546.
Tel-el-Kebir, 260.
Telepherage, 549.
TELEPHONE, THE, 581.
Telestereoscope, 472.
Tenacities of iron, 277.
Tension of electricity, 498.
Tesla, Nikola, 542.
oscillator, 542.
Tesla’s experiments, 545.
Thallium, 426.
_The Terrible_, H.M.S., 167.
Thomson, Sir W., 483, 570, 571, 805, 808.
Throttle valve, 6.
_Thunderer_, H.M.S., 164.
“Times” newspaper, 387, 312.
TOOLS, 85.
Torpedo-boats, 231.
TORPEDOES, 227.
Tour de Cordouan, 593.
Tourmaline, 404.
TOWER BRIDGE, THE, 297.
Trajectory of projectile, 174.
Tramways, 22.
Transfer process, 638.
Transvaal, 693.
Tunnel, Mont Cenis, 351.
St. Gothard, 373.
Tunnels, helicoidal, 379.
Turbine, 144.
_Turbinia, the_, 144.
Turret ships, 154.
Tyndall, 383, 803, 809.
U.
Undulation of the ether, 409.
V.
Vaal River, 701.
Valentine, electric, 581.
Valve, throttle, 6.
slide, 8.
Vegetable nature of coal, 753.
VELOCITY OF LIGHT, 384.
Venice, view in, 396.
Vetterli rifle, 189.
Victoria, Australia, gold-field of, 688.
Victoria Bridge, Montreal, 282.
_Victoria_, H.M.S., 166, 168.
Vision, persistence of, 476.
VISUAL IMPRESSIONS, 468.
Voltaic element, 490.
Vulcanite, 728.
Vulcanized india-rubber, 727.
W.
Wall papers, machines for printing, 322.
Walter press, 323.
Warner’s torpedo experiment, 241.
Warren girder, 280.
_Warrior_, H.M.S., 150.
Warships, new types of, 167.
Water decomposed by electricity, 498.
Waterproof cloths, 726.
Watt, 3, 4, 8, 14.
Welding, electric, 537.
Wells, Horace, 735.
Welsbach incandescent gas lights, 777.
Whaleback, 147.
Wheatstone, 387, 470, 474, 552.
and Cooke’s telegraph, 548, 554.
Wheatstone’s automatic telegraph, 564.
dial telegraph, 568.
dot signals, 565.
Wheels of railway carriages, 107.
Whitehead’s torpedoes, 232, 242.
Whitworth’s guns, 193.
rifle, 182.
Wire-drawing for Atlantic cables, 576.
Wireless telegraphy, 546.
Woodbury printing process, 641.
process for engraving photographs, 643.
Woolwich, 27.
“infants,” 201.
Work, 11, 12, 160, 325, 804.
X.
X-RAYS, 445.
Y.
Young’s paraffin oil, 762.
Z.
Zinc, amalgamated, 490.
Zincography, 644.
Zoetrope, 478.
Zöllner, 475.
EDINBURGH: PRINTED BY MORRISON AND GIBB LIMITED
------------------------------------------------------------------------
TRANSCRIBER’S NOTES
1. Added “near Bristol” after “146. Clifton Suspension Bridge,” on p.
xii.
2. Added “PLATE IX.” to caption of illustration facing p. 144.
3. Added “PLATE XI.” to caption of illustration facing p. 168.
4. Switched the captions for “Fig. 99.” facing p. 210 with the caption
for “Fig. 102” facing p. 206. The captions were out of order and
did not agree with the content of the illustration.
5. Changed “a calibre of 4,724 in.” to “a calibre of 4·724 in.” on p.
#208. Otherwise the muzzle opening would be larger than its
length. Confirmed online.
6. Changed “the two rows in” to “the two rows is” on p. 332.
7. Added “O T” after “P P, or” on p. 398.
8. Added “PLATE XVII.” to caption of illustration facing p. 422.
9. Changed “screw motion to finally” to “screw motion to finely” on p.
433.
10. Changed “which strangely makes it” to “which strangely makes its” on
p. 491.
11. Changed “Duke of Wellington’s statute” to “Duke of Wellington’s
statue” on p. 550.
12. Changed “-” to “·-” for the Morse Code for A in the table on p. 565
to agree with the text that follows.
13. Changed “vegetation which spontaneously makes it” to “vegetation
which spontaneously makes its” on p. 678.
14. Added “INDEX” header on p. 813.
15. Silently corrected typographical errors.
16. Retained anachronistic and non-standard spellings as printed.
17. Enclosed italics font in _underscores_.
18. Superscripts are denoted by a carat before a single superscript
character or a series of superscripted characters enclosed in
curly braces, e.g. M^r. or M^{ister}.
19. Subscripts are denoted by an underscore before a series of
subscripted characters enclosed in curly braces, e.g. H_{2}O.
*** End of this LibraryBlog Digital Book "Discoveries and Inventions of the Nineteenth Century" ***
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