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Title: The Steam Engine Explained and Illustrated (Seventh Edition) - With an Account of its Invention and Progressive Improvement, and its Application to Navigation and Railways; Including also a Memoir of Watt
Author: Lardner, Dionysius
Language: English
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Transcriber's note:
Text enclosed by underscores is in italics (_italics_).
The notation "^{n}" means that n is superscript. The notation
"_{n}" means that n is subscript.
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[Illustration: WATT.
Engraved by H. Adlard, from a Drawing by H. Corbould, taken with
the permission of James Watt, Esq.
FROM THE STATUE BY CHANTREY.
London: Taylor & Walton, Upper Gower Street.]
THE STEAM ENGINE EXPLAINED AND ILLUSTRATED;
With an Account of Its Invention and Progressive Improvement,
and Its Application to Navigation and Railways;
Including also a Memoir of Watt.
by
DIONYSIUS LARDNER, D.C.L. F.R.S.
&c. &c.
SEVENTH EDITION,
Illustrated by Engravings on Wood.
London:
Printed for Taylor and Walton,
28. Upper Gower Street.
MDCCCXL.
London:
Printed by A. Spottiswoode,
New-Street-Square.
TO
THE RIGHT HONOURABLE
HENRY LORD BROUGHAM AND VAUX,
FELLOW OF THE ROYAL SOCIETY,
AND
MEMBER OF THE NATIONAL INSTITUTE OF FRANCE,
AS A MARK OF PUBLIC RESPECT
AND
A TESTIMONY OF PRIVATE REGARD,
THIS WORK
INSCRIBED, BY HIS ATTACHED FRIEND,
THE AUTHOR.
ADVERTISEMENT.
The Drawings for several of the Cuts in this Volume have been
taken, by the permission of Mr. Weale, from the admirable Plates
annexed to the last edition of Tredgold on the Steam Engine and on
Steam Navigation. This acknowledgment is especially due for the
Illustrations which abound in this Volume.
_London, June, 1840._
[Illustration: LONDON ENTRANCE TO THE BIRMINGHAM RAIL-ROAD.]
CONTENTS.
CHAPTER I.
PRELIMINARY MATTER.
Page
The Steam Engine, a Subject of popular Interest 4
Effects of Steam 5
Great Power of Steam 7
Object of this Work 9
Disputes respecting the Invention 11
Hero of Alexandria's Machine 13
Blasco De Garay's Proposition to propel Vessels
by a Machine 16
Solomon De Caus 17
Giovanni Branca proposes to work Mills by Steam 22
Marquis of Worcester 23
Mechanical Properties of Fluids 25
Elastic and Inelastic Fluids 25
Elasticity of Gases 28
Effects of Heat 29
Application of these Principles to the Engines
of Hero, De Caus, and Lord Worcester 30
Sir Samuel Morland 34
Denis Papin 36
Atmospheric Pressure 38
Weight of Air 39
Pressure of Air 41
Barometer 41
Elastic Force of Air and Gases 42
Force obtained by a Vacuum 43
Common Pump 43
Rarefaction by Heat 44
Process of filling Thermometers 44
Papin's Method of producing a Vacuum 44
His Discovery of the Condensation of Steam 45
Thomas Savery 47
CHAP. II.
ENGINES OF SAVERY AND NEWCOMEN.
Savery's Engine 49
Boilers and their Appendages 50
Working Apparatus described 51
Mode of Operation 52
Defects of Savery's Engine 58
Newcomen's Engine described by Papin 62
Newcomen and Cawley obtain a Patent for
Atmospheric Engine 65
Accidental Discovery of Condensation by Injection 69
Potter's Discovery of the Method of working
the Valves 71
His Contrivance improved by the Substitution
of a Plug Frame 72
Advantages of the Atmospheric Engine over that
of Savery 72
The Power of Savery's Engine restricted 73
It contained no new Principle 73
Its practical Superiority 73
CHAP. III.
EARLY CAREER AND DISCOVERIES OF JAMES WATT.
Atmospheric Engine improved by Beighton 75
Smeaton's Improvements in the Atmospheric Engine 76
Brindley obtains a Patent for Improvement in 76
Invents the Self-regulating Feeder 76
Infancy of James Watt 77
His Descent and Parentage 77
Anecdotes of his Boyhood 78
His early Acquirements 79
Goes to London 80
Returns to Glasgow 80
Appointed Instrument-maker to the University 81
Opens a Shop in Glasgow 81
His Friends and Patrons 81
Professor Robison's Remarks on Watt's personal
Character 82
His industrious and studious Habits 82
His Attention first directed to Steam 83
Experiments on High-pressure Engine 83
Repairs an Atmospheric Model 84
Experimental Inquiry consequent on this 84
Its Results 84
Observes great Defects in the Atmospheric Engine 85
His first Attempt to improve it 85
His early Experiments on Steam 87
Discovery by Experiment of the Expansion which
Water undergoes in Evaporation 90
Discovers the latent Heat of Steam 91
Informed by Dr. Black of the Theory of latent Heat 93
His Improvement not due to Black 93
CHAP. IV.
EXPOSITION OF PHYSICAL PRINCIPLES.
Construction of Thermometer 98
Method of graduating it 99
Freezing and boiling Points 99
Latent Heat of Water 101
Quantity of Heat necessary to convert Ice into
Water, first noticed by Dr. Black 101
Examination of the analogous Effects produced
by the continued Application of Heat to Water
in the liquid State 102
Process of Boiling 104
Reconversion of Steam into Water 104
Conversion of Water into Steam 105
Latent Heat of Steam 107
Boiling Point varies 108
Different in different Places 109
Inquiry whether a Diminution of Pressure will
produce a corresponding Effect on the boiling
Point 112
Table showing the Temperature at which Water
will boil under different Pressures of the
Atmosphere 113
Mechanical Force of Steam 115
Facts to be observed in 117
CHAP. V.
FURTHER DISCOVERIES OF WATT.
Watt finds that Condensation in the Cylinder
is incompatible with a due Economy of Fuel 119
Conceives the Notion of condensing out of the
Cylinder 120
Discovers separate Condensation 121
Invents the Air Pump 123
Substitutes Steam Pressure for Atmospheric
Pressure 123
Invents the Steam Case, or Jacket 124
His first Experiments to realise these
Inventions 125
His experimental Apparatus 125
His experimental Models fitted up at Delft
House, in Glasgow 128
Difficulties of bringing the improved Engines
into Use 129
Watt first employed by Roebuck as a Civil
Engineer 130
His Partnership with Roebuck 130
His first Patent 130
Experimental Engine at Kinneal 131
Abstract of the Act of Parliament for the
Extension of his Patent 132
Description of his single-acting Steam Engine 133
CHAP. VI.
WATT'S ENGINES.
Correspondence of Watt with Smeaton 145
Failure of Condensation by Surface 146
Improvements in Construction of Piston 147
Method of Packing 148
Improvements in boring the Cylinder 149
Disadvantages of the new compared with the old
Engines 150
Greatly increased Economy of Fuel 150
Economy of the Engine 151
Expedients to force the new Engines into Use 151
Correspondence of Boulton 153
Correspondence with Smeaton 155
Efficiency of Fuel in the new Engines 156
Discovery of the expansive Action of Steam 157
Watt states it in a Letter to Dr. Small 157
Its Principle explained 158
Mechanical Effects resulting from it 162
Computed Effect of cutting off Steam at
different Portions of the Stroke 163
Produces a variable Power 163
Expedients for equalising the Power 164
Expansive Principle in Watt's Engines limited 165
Its more extensive Application in the Cornish
Engines 165
CHAP. VII.
DOUBLE-ACTING ENGINE.
Common Steam 168
Superheated Steam 170
Laws of Dalton and Gay Lussac 171
Law of Mariotte 171
Relation between Temperature and Pressure of
common Steam 171
Effects of the Expansion of common Steam 173
Mechanical Effects of Steam 173
Method of equalising the expansive Force 174
Hornblower's Engine 175
Woolf's Engine 176
Watt's Attempts to extend the Steam Engine to
Manufactures 178
Papin's projected Applications of the Steam
Engine 178
Savery's Application of the Engine to move
Machinery 180
Jonathan Hull's Application to Water Wheels 180
Champion of Bristol applies the Atmospheric
Engine to raise Water 181
Stewart's Application of the Engine to
Mill-work 182
Wasbrough's Application of the Fly-wheel and
Crank 183
Reasons why Watt's single-acting Steam Engine
was not adapted to produce continuous uniform
Motion of Rotation 184
Watt's Second Patent 186
Sun-and-Planet Wheels 187
Valves of double-acting Engine 189
CHAP. VIII.
DOUBLE-ACTING ENGINE.
Methods of connecting the Piston-rod and Beam
in the double-acting Engine 193
Rack and Sector 194
Parallel Motion 195
Connection of Piston-rod and Beam 195
Connecting Rod and Crank 203
Fly-wheel 205
Throttle-valve 207
Governor 209
Construction and Operation of the
double-acting Engine 216
Eccentric 225
Cocks and Valves 227
Single-clack Valves 227
Double-clack Valves 228
Conical Valves 228
Slide Valves 229
Murray's Slides 229
D Valves 230
Seaward's Slides 235
Single Cock 238
Two-way Cock 239
Four-way Cock 239
Pistons 242
Common hemp-packed Piston 242
Woolf's Piston 243
Metallic Pistons 244
Cartwright's Engine 245
Cartwright's Piston 247
Barton's Piston 248
CHAP. IX.
BOILERS AND FURNACES.
Analysis of Coal 252
Process of Combustion 253
Heat evolved in it 254
Form and Structure of Boiler 255
Waggon Boiler 255
Furnace 256
Method of feeding it 257
Combustion of Gas in Flues 260
Williams's Patent for Method of consuming
unburned Gases 260
Construction of Grate and Ash-pit 261
Magnitude of heating Surface of Boiler 262
Steam-space and Water-space in Boiler 263
Position of Flues 264
Method of feeding Boiler 265
Method of indicating the Level of Water in
Boiler 266
Level Gauges 266
Self-regulating Feeders 267
Steam Gauge 270
Barometer Gauge 272
Watt's Invention of the Indicator 274
Counter 278
Safety-valve 279
Fusible Plugs 280
Self-regulating Damper 281
Brunton's Self-regulating Furnace 283
Gross and useful Effect of an Engine 285
Power and Duty of Engines 287
Horse-power of Steam Engines 289
Evaporation proportional to Horse-power 290
Sources of Loss of Power 292
Absence of good practical Rules for Power 292
Common Rules followed by Engine-makers 292
Duty distinguished from Power 294
Duty of Boilers 294
Proportion of Stroke to Diameter of Cylinder 295
Duty of Engines 296
Cornish System of Inspection 297
Table showing the Improvement of Cornish Engines 298
Beneficial Effects of Cornish Inspection 299
Successive Improvements on which the increased
Duty of Engines depends, traced by John Taylor
in his "Records of Mining" 299
CHAP. X.
LIFE OF WATT.
Watt's Friends and Associates at Birmingham 302
His Invention of the Copying Press 302
Heating Apartments and Buildings by Steam 303
Watt's Machine for drying Linen 303
His Share in the Discovery of the Composition
of Water 303
The Merit of this Discovery shared between
Cavendish, Lavoisier, and Watt 305
Anecdote of Watt's Activity of Mind 309
His Introduction of the Use of Chlorine in
Bleaching 310
One of the Founders of the Pneumatic
Institution at Clifton 310
His first Marriage 311
Death of his first Wife 311
His second Marriage 311
Death of his younger Son 311
Extracts from his Letters 312
Character of Watt by Lord Brougham 313
Extract from Sir Walter Scott's "Monastery" on
the Character of Watt 314
Sketch of the Life of Watt from the "Scotsman"
by Lord Jeffrey 315
Occupation of his old Age 318
His Invention of Machine for copying Sculpture 318
His last Days 318
Monuments to his Memory 319
Inscription by Lord Brougham on the Pedestal
of the Monument in Westminster Abbey 320
CHAP. XI.
LOCOMOTIVE ENGINES ON RAILWAYS.
Page
High-pressure Engines 322
One of the earliest Forms of the Steam Engine 322
Description of Leupold's Engine 323
Non-condensing Engine of Messrs. Trevethick
and Vivian 324
Construction of a Machine for moving Carriages
on Railroads 328
Effects of Railway Transport 329
Moral and political Consequences of 334
History of the Locomotive Engine 337
Construction of Locomotive Engine by Blinkensop 337
Messrs. Chapman's Contrivance 337
Walking Engine 337
Mr. Stephenson's Engines at Killingworth 339
Liverpool and Manchester Railway 342
The Directors offer a Prize for the best
Locomotive Engine 344
Experimental Trial 344
The "Rocket," "Sanspareil," and "Novelty" 344
Admirable Arrangement in the Rocket 345
Description of the "Sanspareil" 347
Description of the "Novelty" 349
The Superiority of the "Rocket" 350
Method of subdividing the Flue into Tubes 353
Progressive Improvement of Locomotive Engines 354
Dr. Lardner's Experiments in 1832 357
Adoption of Brass Tubes 360
Great Expense of Locomotive Power 361
Mr. Booth's Report 362
Detailed Description of the most improved
Locomotive Engines 365
Substitution of Brass for Copper Tubes
ascribed to Mr. Dixon 370
Power of Locomotive Engines 379
Position of the Eccentrics 379
Pressure of Steam in the Boiler 401
Dr. Lardner's Experiments in 1838 406
Resistance to Railway Trains 407
Dr. Lardner's Experiments on the Great Western
Railway 408
Experiments on Resistance 409
Restrictions on Gradients 410
Compensating Effect of Gradients 412
Experiment with the "Hecla" 412
Disposition of Gradients should be uniform 415
Methods of surmounting steep Inclinations 415
CHAP. XII.
LOCOMOTIVE ENGINES ON TURNPIKE ROADS.
Railways and Stone Roads compared 420
Gurney's Steam Carriage 423
The Boiler of Gurney's Engine 423
His Method of cleansing Boilers 428
Convenience and Safety of Steam Carriages 432
Two Methods of applying Locomotive Engines
upon common Roads 434
Horse Carriages compared with Steam 435
Extract from Mr. Farey's Evidence before the
House of Commons 435
Hancock's Steam Carriage 436
How it differs from that of Mr. Gurney 437
Ogle's Locomotive Carriage 438
Dr. Church's Steam Engine 439
CHAP. XIII.
STEAM NAVIGATION.
Form and Arrangement of Marine Engines 441
Arrangement of the Engine-room 446
Marine Boilers 448
Effects of Sea Water in Boilers 450
Remedies for them 451
Blow-off Cocks 452
Indicators of Saltness 453
Seaward's Indicator 454
His Method of blowing out 454
Field's Brine Pumps 456
Tubular Condensers applied by Mr. Watt 457
Hall's Condensers 458
Substitution of Copper for Iron Boilers 460
Process of Stoking 462
Watt's Expedient of attaching Felt to the
Boiler Surface 463
Means of economising Fuel 463
Number and Arrangement of Furnaces and Flues 463
Howard's Marine Engine 464
Application of the expansive Principle in
Marine Engines 466
Recent Improvements of Messrs. Maudslay and
Field 467
Humphrey's Marine Engine 470
Common Paddle-wheel 472
Feathering Paddles 474
Galloway's Patent for a Paddle-wheel with
movable Paddles 476
Split Paddle 478
Proportion of Power to Tonnage 480
Improved Efficiency of Marine Engines 482
Iron Steam Vessels 483
Steam Navigation to India 484
CHAP. XIV.
AMERICAN STEAM NAVIGATION.
Steam Navigation first established in America 487
Circumstances which led to it 488
Attempts of Fitch and Rumsey to apply the
single-acting Engine to the Propulsion of
Vessels 489
Stevens of Hoboken commences Experiments in
Steam Navigation 489
Experiments of Livingstone and Fulton 489
Fulton's first Boat 490
The Hudson navigated by Steam 491
Extension and Improvement of River Navigation 492
American Steamers 494
Difference between them and European Steamers 494
Steamers on the Hudson 494
American Paddle-wheels 495
Sea-going American Steamers 496
Speed attained by American Steamers 497
Lake Steamers 499
The Mississippi and its Tributaries 499
Steam-boats navigating it 500
Their Structure and Machinery 500
New Orleans Harbour 503
Steam Tugs 503
APPENDIX.
_On the Relation between the Temperature, Pressure, and_
_Density of Common Steam._
Empirical Formula of Biot, showing the
Relation between the Pressure and Temperature 505
Empirical formula of Southern 506
Tredgold 506
Mellet 506
De Pambour 506
MM. Dulong and Arago 506
Law of the Expansion of elastic Fluids,
discovered by Dalton and Gay Lussac 506
Formula for the Relation between the Volumes
and Temperatures 507
Law of Mariotte 507
Table of Pressures, Temperatures, Volumes, and
Mechanical Effects of Steam 509
Empirical Formulæ for the Relation between the
Volume of Water and that of the Steam produced
by its Evaporation under given Pressures 511
Formula of Navier 511
Modified by De Pambour 511
_On the Expansive Action of Steam._
Mechanical Effect produced during a given
Extent of Expansion 511
Mechanical Effect produced during Evaporation
and subsequent Expansion 512
Application to double-acting Engines 513
Formula for Pressure of Steam in Cylinders 514
Formula for total Mechanical Effect per Minute
of Steam when cut off at any proposed Part of
the Stroke 514
Formulæ exhibiting the Relation between the
Resistance of the Load, the Resistances of the
Engine, the Evaporation, the Speed of the
Piston, and the Magnitude of the Cylinder 515
Formulæ showing the Relation between the Power
of the Engine, the Evaporation, and the useful
Load 516
Formulæ for the _useful Effect_ and the _Duty_ 517
Estimates of the several Sources of
Resistances 518
Tables to facilitate the Computation of the
Effects of Expansive Engines 519
Table of the Areas of Pistons 520
EXAMPLES of the Application of these Formulæ 521
INDEX. 523
[Illustration: VIADUCT, NEAR WATFORD, BIRMINGHAM RAIL-ROAD.]
[Pg001]
THE STEAM ENGINE.
[Pg003]
[Illustration: HERO OF ALEXANDRIA.]
CHAPTER I.
THE STEAM ENGINE, A SUBJECT OF POPULAR INTEREST. — THE OBJECT
OF THIS WORK. — DISPUTES RESPECTING THE INVENTION. — HERO. — DE
GARAY. — DE CAUS. — BRANCA. — MARQUIS OF WORCESTER. — PHYSICAL
PRINCIPLES. — ELASTIC AND INELASTIC FLUIDS. — THEIR
PROPERTIES. — APPLICATION OF THESE PRINCIPLES TO THE ENGINES
OF HERO, DE CAUS, AND LORD WORCESTER. — SIR SAMUEL MORLAND. —
PAPIN. — ATMOSPHERIC PRESSURE. — THE WEIGHT OF AIR. — LESS AT
GREATER HEIGHTS. — BAROMETER. — PRESSURE OF AIR. — ELASTIC
FORCE OF AIR AND GASES. — FORCE PRODUCED BY A VACUUM. — COMMON
PUMP. — RAREFACTION BY HEAT. — PAPIN'S METHODS OF PRODUCING A
VACUUM. — HIS DISCOVERY OF THE CONDENSATION OF STEAM. —
SAVERY.
(1.) That the history of the invention of a piece of mechanism,
and the description of its structure, operation, and [Pg004]
uses, should be capable of being rendered the subject matter of a
volume, destined not alone for the instruction of engineers or
machinists, but for the information and amusement of the public in
general, is a statement which at no very remote period would have
been deemed extravagant and incredible.
Advanced as we are in the art of rendering knowledge popular, and
cultivated as the public taste is in the appreciation of the
expedients by which science ministers to the uses of life, there
is still perhaps but one machine of which such a proposition can
be truly predicated: it is needless to say that that machine is
the STEAM ENGINE. There are many circumstances attending this
extraordinary piece of mechanism which impart to it an interest so
universally felt. Whether we regard the details of its structure
and operation, the physical principles which it calls into play,
and the beautiful contrivances by which these physical principles
are rendered available;—or, passing over these _means_, we direct
our attention to the _ends_ which they attain, we are equally
filled with astonishment and admiration. The history of the steam
engine offers to our notice a series of contrivances which, for
exquisite and refined ingenuity, stand without any parallel in the
annals of mechanical science. These admirable inventions, unlike
other results of scientific inquiry, have also this peculiarity,
that, to understand their excellence and to perceive their beauty,
no previous or subsidiary knowledge is necessary, save what may be
imparted with facility and clearness in the progress of the
explanation and development of the machine itself. A simple and
clear exposition, divested of needless technicalities and aided by
well-selected diagrams, is all that is necessary to render the
construction and operation of the steam engine, in all its forms,
intelligible to persons of plain understanding and moderate
information.
But if the contrivances by which this vast power is brought to bear
on the arts and manufactures, be rendered attractive by their great
mechanical beauty, how much more imposing will the subject become
when the effects which the steam engine has produced upon the
well-being of the human race are considered. It has penetrated the
crust of the earth, and drawn from beneath it boundless treasures
[Pg005] of mineral wealth, which, without its aid, would have been
rendered inaccessible; it has drawn up, in measureless quantity,
the fuel on which its own life and activity depend; it has relieved
men from their most slavish toils, and reduced labour in a great
degree to light and easy superintendence. To enumerate its present
effects, would be to count almost every comfort and every luxury of
life. It has increased the sum of human happiness, not only by
calling new pleasures into existence, but by so cheapening former
enjoyments as to render them attainable by those who before could
never have hoped to share them: the surface of the land, and the
face of the waters, are traversed with equal facility by its power;
and by thus stimulating and facilitating the intercourse of nation
with nation, and the commerce of people with people, it has knit
together remote countries by bonds of amity not likely to be
broken. Streams of knowledge and information are kept flowing
between distant centres of population, those more advanced
diffusing civilisation and improvement among those that are more
backward. The press itself, to which mankind owes in so large a
degree the rapidity of their improvement in modern times, has had
its power and influence increased in a manifold ratio by its union
with the steam engine. It is thus that literature is cheapened,
and, by being cheapened, diffused; it is thus that Reason has taken
the place of Force, and the pen has superseded the sword; it is
thus that war has almost ceased upon the earth, and that the
differences which inevitably arise between people and people are
for the most part adjusted by peaceful negotiation.
Deep as the interest must be with which the steam engine will be
regarded in every civilised country, it presents peculiar claims
upon the attention of the people of Great Britain. Its invention
and progressive improvement are the work of our own time and our
own country; it has been produced and matured almost within the
last century, and is the exclusive offspring of British genius,
fostered and sustained by British enterprise and British capital.
The steam engine is a mechanical contrivance, by which coal, wood,
or other fuel is rendered capable of executing any [Pg006] kind
of labour. COALS are by it made to spin, weave, dye, print and
dress silks, cottons, woollens, and other cloths; to make paper,
and print books upon it when made; to convert corn into flour; to
express oil from the olive, and wine from the grape; to draw up
metal from the bowels of the earth; to pound and smelt it, to melt
and mould it; to forge it; to roll it, and to fashion it into
every desirable form; to transport these manifold products of its
own labour to the doors of those for whose convenience they are
produced; to carry persons and goods over the waters of rivers,
lakes, seas, and oceans, in opposition alike to the natural
difficulties of wind and water; to carry the wind-bound ship out
of port; to place her on the open deep ready to commence her
voyage; to throw its arms around the ship of war, and place her
side by side with the enemy; to transport over the surface of the
deep persons and information, from town to town, and from country
to country, with a speed as much exceeding that of the ordinary
wind, as the ordinary wind exceeds that of a common pedestrian.
Such are the virtues, such the powers, which the steam engine has
conferred upon COALS. The means of calling these powers into
activity are supplied by a substance which nature has happily
provided in unbounded quantity in every part of the earth; and
though it has no price, it has inestimable value: this substance
is WATER.
A pint of water may be evaporated by two ounces of coals. In its
evaporation it swells into two hundred and sixteen gallons of
steam, with a mechanical force sufficient to raise a weight of
thirty-seven tons a foot high. The steam thus produced has a
pressure equal to that of common atmospheric air; and by allowing
it to expand, by virtue of its elasticity, a further mechanical
force may be obtained, at least equal in amount to the former. A
pint of water, therefore, and two ounces of common coal, are thus
rendered capable of doing as much work as is equivalent to
seventy-four tons raised a foot high.
The circumstances under which the steam engine is worked on a
railway are not favourable to the economy of fuel. Nevertheless
a pound of coke burned in a locomotive engine [Pg007] will
evaporate about five pints of water. In their evaporation they
will exert a mechanical force sufficient to draw two tons weight
on the railway a distance of one mile in two minutes. Four horses
working in a stage-coach on a common road are necessary to draw
the same weight the same distance in six minutes.
A train of coaches weighing about eighty tons, and transporting
two hundred and forty passengers with their luggage, has been
taken from Liverpool to Birmingham, and back from Birmingham to
Liverpool, the trip each way taking about four hours and a
quarter, stoppages included. The distance between these places by
the railway is ninety-five miles. This double journey of one
hundred and ninety miles is effected by the mechanical force
produced in the combustion of four tons of coke, the value of
which is about five pounds. To carry the same number of passengers
daily between the same places by stage-coaches on a common road,
would require twenty coaches and an establishment of three
thousand eight hundred horses, with which the journey in each
direction would be performed in about twelve hours, stoppages
included.
The circumference of the earth measures twenty-five thousand
miles; and if it were begirt with an iron railway, such a train as
above described, carrying two hundred and forty passengers, would
be drawn round it by the combustion of about thirty tons of coke,
and the circuit would be accomplished in five weeks.
In the drainage of the Cornish mines the economy of fuel is much
attended to, and coals are there made to do more work than
elsewhere. A bushel of coals usually raises forty thousand tons of
water a foot high; but it has on some occasions raised sixty
thousand tons the same height. Let us take its labour at fifty
thousand tons raised one foot high. A horse worked in a fast
stage-coach pulls against an average resistance of about a quarter
of a hundred weight. Against this he is able to work at the usual
speed through about eight miles daily: his work is therefore
equivalent to one thousand tons raised one foot. A bushel of coals
consequently, as used in Cornwall, performs as much labour as a
day's work of one hundred such horses. [Pg008]
The great pyramid of Egypt stands upon a base measuring seven
hundred feet each way, and is five hundred feet high, its weight
being twelve thousand seven hundred and sixty millions of pounds.
Herodotus states, that in constructing it one hundred thousand men
were constantly employed for twenty years. The materials of this
pyramid would be raised from the ground to their present position
by the combustion of about four hundred and eighty tons of coals.
The Menai Bridge consists of about two thousand tons of iron, and
its height above the level of the water is one hundred and twenty
feet. Its mass might be lifted from the level of the water to its
present position by the combustion of four bushels of coal.
The enormous consumption of coals produced by the application of
the steam engine in the arts and manufactures, as well as to
railways and navigation, has of late years excited the fears of
many as to the possibility of the exhaustion of our coal-mines.
Such apprehensions are, however, altogether groundless. If the
present consumption of coal be estimated at sixteen millions of
tons annually, it is demonstrable that the coal-fields of this
country would not be exhausted for many centuries.
But in speculations like these, the probable, if not certain
progress of improvement and discovery ought not be overlooked; and
we may safely pronounce that, long before such a period of time
shall have rolled away, other and more powerful mechanical agents
will supersede the use of coal. Philosophy already directs her
finger at sources of inexhaustible power in the phenomena of
electricity and magnetism. The alternate decomposition and
recomposition of water, by magnetism and electricity, has too
close an analogy to the alternate processes of vaporisation and
condensation, not to occur at once to every mind: the development
of the gases from solid matter by the operation of the chemical
affinities, and their subsequent condensation into the liquid
form, has already been essayed as a source of power. In a word,
the general state of physical science at the present moment, the
vigour, activity, and sagacity with which researches in it are
prosecuted in every civilised [Pg009] country, the increasing
consideration in which scientific men are held, and the personal
honours and rewards which begin to be conferred upon them, all
justify the expectation that we are on the eve of mechanical
discoveries still greater than any which have yet appeared; and
that the steam engine itself, with the gigantic powers conferred
upon it by the immortal Watt, will dwindle into insignificance in
comparison with the energies of nature which are still to be
revealed; and that the day will come when that machine, which is
now extending the blessings of civilisation to the most remote
skirts of the globe, will cease to have existence except in the
page of history.
(2.) The object of the present volume will be to deliver, in an
easy and familiar style, an historical view of the invention of the
steam engine, and an exposition of its structure and operation in
the various forms in which it is now used, and of its most
important applications in the arts of life, especially in transport
by land and water. It is hoped that the details of these subjects
may be rendered easily intelligible to all persons of ordinary
information, whether urged by that natural and laudable spirit of
inquiry awakened by contemplating effects on the material and
social condition of our species, so rapid and so memorable as those
which have followed the invention of the steam engine, and by the
pleasure which results from the perception of the numerous
instances of successful contrivances and beautiful applications of
science to art which it unfolds,—or impelled by the exigencies of
trade or profession to acquire an acquaintance with a machine on
which, more than any other, the prosperity of our commercial and
manufacturing interests depends. It will be our aim to afford to
the former class all the information which they can require; and,
if this work be not as comprehensive in its scope, and as minute in
its details, as some of the latter may wish, it will at least serve
as an easy and convenient introduction to other works more
voluminous, costly, and detailed, but less elementary in their
matter, and less familiar in their style.
In explaining the different forms of steam engine which have been
proposed in the course of the progressive improvement [Pg010] of
that machine from its early rude and imperfect state to its
present comparatively perfect form, it will be necessary to advert
to various physical phenomena and mechanical principles, which,
however obvious to those who are conversant with matters of
science, must necessarily be at least imperfectly known by the
great majority of our readers. To refer for information on such
topics to other works on Mechanics and general Physics, would be
with most readers ineffectual, and with all unsatisfactory. In
former editions of the present work, we consigned these necessary
general principles of physics and mechanics to a preliminary
chapter; but it appears, on the whole, more convenient not to
remove the exposition of the principle from the place where its
application is required. We shall therefore pause as we proceed,
where these difficulties occur, to give such explanation and
illustration as may seem best suited to render them intelligible
and interesting to the unscientific reader.
The history of the arts and manufactures affords no example of any
invention the credit for which has been claimed by so many
different nations and individuals as that of the steam engine. The
advocates of the competitors for this honour have urged their
pretensions, and pressed their claims, with a zeal which has
occasionally outstripped the bounds of discretion, and the contest
has not unfrequently been tinged with prejudices, national and
personal, and characterised by a degree of asperity altogether
unworthy of so noble a cause, and beneath the dignity of science.
"When a question is clearly proposed, it is already half resolved."
Let us see whether a careful attention to this maxim will aid us
in the investigation of the origin of the steam engine. The source
of the power of that machine is found in the following natural
phenomena.
_First._ When fire is applied to water, the liquid swells into
vapour, and in undergoing this change exerts, as has been already
stated, a considerable amount of mechanical force. This force may,
by proper means, be rendered applicable to any purpose for which
labour or power is needful.
_Second._ The vapour so produced is endowed with the property of
elasticity, in virtue of which it is capable of [Pg011] swelling
or expanding into increased dimensions, exerting, as it expands, a
force, the energy of which is gradually diminished as the
dimensions of the vapour are increased. This mechanical force is
likewise capable of being applied to any useful purpose for which
labour or power is necessary.
_Third._ This vapour is capable, by proper means, of being
reconverted into water; and when so reconverted, it shrinks into
its original dimensions, deserting the large space which it
occupied as vapour, and leaving that space a vacuum. It is known
in physics, that when a vacuum is produced, surrounding bodies
have a tendency to rush into it with a definite amount of force.
Consequently any agent which produces a vacuum, becomes a source
of a considerable amount of mechanical power. By its reconversion
into water, therefore, steam again becomes a mechanical agent.
Such are the natural phenomena in which are found the original
sources of all steam power. In some forms of steam engine one of
these is used, and in some another, and in some the application of
all of them is combined; but in no existing form of steam engine
whatever is there any other source of mechanical power.
Neither these nor any other natural forces can be applied
_immediately_ to any useful purpose. The interposition of
mechanism is indispensable; on the invention and contrivance of
that mechanism depends altogether the useful application of these
natural forces.
The world owes the steam engine then partly to _discovery_, and
partly to _invention_.
He that discovered the fact, that mechanical force was produced in
the conversion of water into steam, must be justly held to be a
sharer in the merit of the steam engine, even though he should
never have practically applied his discovery. The like may be said
of him who first discovered the source of the mechanical power
arising from the expansion of steam.
The discoverer of the fact, that steam being reconverted into
water greatly contracted its dimensions, and thereby produced a
vacuum, is likewise entitled to a share of the credit. [Pg012]
The mechanism by which these natural forces have been rendered so
universally available as a moving power, is very various and
complicated, and cannot be traced to one inventor. "If a
watchmaker," says M. Arago, "well instructed in the history of his
art, were required to give a categorical answer to the question,
Who has invented watches? he would remain mute; but the question
would be divested of much of its difficulty if he were required
separately to declare who discovered the use of the main spring,
the different forms of escapement, or the balance wheel." So it is
with the steam engine. It is a combination of a great variety of
contrivances, distinct from each other, which are the production
of several inventors. If, however, one name more than the rest be
entitled to special notice; if _he_ is entitled to the chief
credit of the invention who by the powers of his mechanical genius
has imparted to the steam engine that form, and conferred upon it
those qualities, on which mainly depends its present extensive
utility, and by which it has become an agent of transcendant
power, spreading its beneficial effects throughout every part of
the civilised globe, then the universal voice will, as it were by
acclamation, award the honour to one individual, whose pre-eminent
genius places him far above all other competitors, and from the
application of whose mental energies to this machine may be dated
those grand effects which render it a topic of interest to all for
whom the progress of civilisation has any attractions. Before the
era rendered memorable by the discoveries of JAMES WATT, the steam
engine, which has since become an object of such universal
interest, was a machine of extremely limited power, inferior in
importance and usefulness to most other mechanical agents used as
prime movers; but, from that epoch, it is scarcely necessary here
to state, that it became a subject not of British interest only,
but one having an important connection with the progress of the
human race.
HERO OF ALEXANDRIA, 120 B. C.
[Illustration: _Fig._ 1.]
(3.) The discovery of the fact, that a mechanical force is
produced when water is evaporated by the application of heat,
[Pg013] must be considered as the first capital step in the
invention of the steam engine. It is recorded in a work entitled
_Spiritalia seu Pneumatica_, that Hero of Alexandria contrived a
machine, 120 years before the Christian era, which was moved by
the mechanical force of the vapour of water. The principle of this
machine admits of easy explanation: When a fluid issues from any
vessel in which it is confined, that vessel suffers a force equal
to that with which the fluid escapes from it, and in the opposite
direction. If water issues from an orifice, a pressure is produced
behind the orifice corresponding to the force with which the water
escapes. If a man discharge a gun, the gases produced by the
explosion of the powder issue with a certain force from the
muzzle, and his shoulder is driven backwards by the recoil with a
corresponding force. If the muzzle, instead of being presented
forwards, were turned at right angles to the length of the gun,
then, as the gases of explosion would escape sideways, the recoil
would likewise take place sideways, and the shooter, instead of
being driven backward, would be made to spin round as a dancer
pirouettes. This was the principle of Hero's steam engine. A small
globe or ball was placed on pivots at A and B (_fig._ 1.), on
which it was capable of revolving: steam was supplied through one
of these pivots from one of the tubes D C E F, which communicated
with the boiler. This steam filled the globe A I B K, and also the
arms I H and K G. A lateral orifice, represented at G, near the
end of these arms, allowed the steam to escape in a jet, and the
reaction, producing a recoil, had a tendency to drive the arm
round. A small orifice at H, on the other side of the tube,
produced a like effect. In the same manner, any convenient number
of arms might be provided, surrounding the globe and communicating
with its interior like the spokes of a wheel. Thus these arms,
having lateral orifices for the escape of the steam, all placed so
that the recoil may [Pg014] tend to turn the globe in the same
direction, a rotatory motion might be communicated to any
machinery which it was desired to move.
After having been allowed to slumber for nearly two thousand
years, this machine has recently been revived, and engines
constructed similar to it are now working in these countries. In
the proper place we shall describe Avery's Rotatory Engine, which
it will be seen is, not only in its principle, but almost in its
details, the machine of HERO OF ALEXANDRIA.
Although the elastic force of steam was not reduced to numerical
measure by the ancients, nor brought under control, nor applied to
any useful purpose, yet it appears to have been recognised in
vague and general terms. Aristotle, Seneca, and other ancient
writers, accounted for earthquakes by the sudden conversion of
water into steam within the earth. This change, according to them,
was effected by subterranean heat. Such tremendous effects being
ascribed to steam, it can scarcely be doubted that the Greeks and
Romans were acquainted with the fact, that water in passing into
vapour exercises considerable mechanical power. They were aware
that the earthquakes, which they ascribed to this cause, exerted
forces sufficiently powerful to extend the natural limits of the
ocean; to overturn from their foundations the most massive
monuments of human labour; to raise islands in the midst of seas;
and to heave up the surface of the land of level continents so as
to form lofty mountains.
Such notions, however, resulted not as consequences of any exact
or scientific principles, but from vague analogies derived from
effects which could not fail to have been manifested in the arts,
such as those which commonly occurred in the process of casting in
metal the splendid statues which adorned the temples, gardens, and
public places of Rome and Athens. The artisan was liable to the
same accidents to which modern founders are exposed, produced by
the casual presence of a little water in the mould into which the
molten metal is poured. Under such circumstances, the sudden
formation of steam of an extreme pressure produces, as is well
known, explosions attended with destructive effects. The Grecian
[Pg015] and Roman artisans were subject to such accidents; and the
philosopher, generalising such a fact, would arrive at a solution
of the grander class of phenomena of earthquakes and volcanoes.
Before natural phenomena are rendered subservient to purposes of
utility, they are often made to minister to the objects of
superstition. The power of steam is not an exception to this rule.
It is recorded in the Chronicles, that upon the banks of the Weser
the ancient Teutonic gods sometimes marked their displeasure by a
sort of thunderbolt, which was immediately succeeded by a cloud
that filled the temple. An image of the god _Busterich_, which was
found in some excavations, clearly explains the manner in which
this prodigy was accomplished by the priests. The head of the
metal god was hollow, and contained within it a pot of water: the
mouth, and another hole, above the forehead, were stopped by
wooden plugs; a small stove, adroitly placed in a cavity of the
head under the pot, contained charcoal, which, being lighted,
gradually heated the liquid contained in the head. The vapour
produced from the water, having acquired sufficient pressure,
forced out the wooden plugs with a loud report, and they were
immediately followed by two jets of steam, which formed a dense
cloud round the god, and concealed him from his astonished
worshippers.[1]
Among other amusing anecdotes showing the knowledge which the
ancients had of the mechanical force of steam, it is related that
Anthemius, the architect of Saint Sophia, occupied a house next
door to that of Zeno, between whom and Anthemius there existed a
feud. To annoy his neighbour, Anthemius placed on the ground floor
of his own house several close digesters, or boilers, containing
water. A flexible tube proceeded from the top of each of these,
which was conducted through a hole made in the wall between the
houses, and which communicated with the space under the floors of
the rooms in the house of Zeno. When Anthemius desired to annoy
his neighbour, he lighted fires under his boilers, and the steam
produced by them rushed in such quantity and with [Pg016] such
force under Zeno's floors, that they were made to heave with all
the usual symptoms of an earthquake.[2]
BLASCO DE GARAY, A. D. 1543.
(4.) In the year 1826, M. de Navarrete published, in Zach's
Astronomical Correspondence, a communication from Thomas Gonzales,
Director of the royal archives of Simancas, giving an account of
an experiment reported to have been made in the year 1543, in
which a vessel was propelled by a machine having the appearance of
a steam engine.
Blasco de Garay, a sea captain, proposed in that year to the Emperor
Charles V. to propel vessels by a machine which he had invented,
even in time of calm, without oars or sails. Notwithstanding the
apparent improbability attending this project, the Emperor ordered
the experiment to be made in the port of Barcelona, and the 17th of
June, 1543, was the day appointed for its trial. The commissioners
appointed by Charles V. to attend and witness the experiment were
Don Henry of Toledo, Don Pedro of Cardona, the treasurer Ravago, the
vice chancellor and intendant of Catalonia, and others. The vessel
on which the experiment was made was the Trinity, 200 tons burthen,
which had just discharged a cargo of corn at Barcelona. Garay
concealed the nature of his machinery, even from the commissioners.
All that could be discovered during the trial was, that it consisted
of a large boiler containing water, and that wheels were attached to
each side of the vessel, by the revolution of which it was
propelled. The commissioners having witnessed the experiment, made a
report to the king, approving generally of the invention,
particularly on account of the ease and promptitude with which the
vessel could be put about by it.
The treasurer Ravago, who was himself hostile to the project,
reported that the machine was capable of propelling a vessel at the
rate of two leagues in three hours; but the other commissioners
stated that it made a league an hour at the least, and that it put
the vessel about as speedily as would be accomplished with a galley
worked according to the common [Pg017] method. Ravago reported that
the machinery was too complicated and expensive, and that it was
subject to the danger of the boiler bursting.
After the experiment was made, Garay took away all the machinery,
leaving nothing but the framing of wood in the arsenals of
Barcelona.
Notwithstanding the opposition of Ravago, the invention was
approved, and the inventor was promoted and received a pecuniary
reward, besides having all his expenses paid.
From the circumstance of the nature of the machinery having been
concealed, it is impossible to say in what this machine consisted;
but as a boiler was used, it is probable, though not certain, that
steam was the agent. There have been various machines proposed, of
which a furnace and boiler form a part, and in which the agency of
steam is not used. The machine of Amontons furnishes an example of
this. It is most probable that the contrivance of Garay was
identical with that of Hero. The low state of the arts in Spain in
the sixteenth century would be incompatible with the construction
of any machine requiring great precision of execution. But the
simplicity of Hero's contrivance would have rendered its
construction and operation quite practicable. As to the claims to
the invention of the steam engine advanced by the advocates of De
Garay, founded on the above document, a refutation is supplied by
the admission, that though he was rewarded and promoted by the
government of the day, in consequence of the experiment, and
although the great usefulness of the contrivance in towing ships
out of port, &c., was admitted, yet it does not appear that a
second experiment was ever tried, much less that the machine was
ever brought into practical use.
SOLOMON DE CAUS, 1615.
(5.) Solomon De Caus was engineer and architect to Louis XIII.,
king of France, before the year 1612. In that year he entered the
service of the Elector Palatine, who married the daughter of King
James I., with whom he came to England. He was there employed by
the Prince of Wales in ornamenting the gardens of his house at
Richmond. [Pg018] During his sojourn in England he composed and
published at London, in the same year, a Treatise on Perspective.
This person was the author of a work entitled, "_Les Raisons des
Forces Mouvantes, avec diverses Machines tant utiles que
plaisantes_," which was apparently composed at Heidelburg, but
published at Franckfort, in 1615. The same work was subsequently
republished in Paris in 1623.
The treatise commences with definitions of what were then
considered the four elements: earth, air, fire, and water. Air is
defined to be a cold, dry, and light element, capable of
compression, by which it may be rendered very violent. He says,
"The violence will be great when water exhales in air by means of
fire, and that the said air is enclosed: as, for example, take a
ball of copper of one or two feet diameter, and one inch thick,
which being filled with water by a small hole, which shall be
strongly stopped with a peg, so that neither air nor water can
escape, it is certain that if we put the said ball upon a great
fire, so that it will become very hot, that it will cause a
compression so violent, that the ball will burst in pieces, with a
noise like a petard."
The effect which is here described is due to the combined pressure
of the heated air contained in the ball and the high pressure
steam raised from the water, but much more to the latter than to
the former. It is evident, however, from the language of De Caus,
that he ascribes the force entirely to the air, and seems to
consider that the force of the air proceeded from the water which
exhaled in it.
The first theorem is, "that the parts of the elements mix together
for a time, and then each returns to its place" (the elements here
referred to being apparently air and water). Upon this subject the
following is an example: "Take a round vessel of copper, soldered
close on every side, and with a tube, whereof one end approaches
nearly to the bottom of the vessel, and the other end, which
projects on the outside of the vessel, has a stop-cock; there is
also a hole in the top of the vessel, with a plug to stop it. If
this vessel will contain three pots of water, then pour in one pot
of water, and place the vessel on the fire about three or four
minutes, leaving the hole open; then take the vessel off the
[Pg019] fire, and a little after pour out the water at the hole,
and it will be found that a part of the said water has been
evaporated by the heat of the fire. Then pour in one pot of water
as before, and stop up the hole and the cock, and put the vessel
on the fire for the same time as before; then take it off, and let
it cool of itself, without opening the plug, and after it is quite
cold pour out the water, and it will be found exactly the same
quantity as was put in. Thus we see that the water which was
evaporated (the first time that the vessel was put on the fire) is
returned into water the second time when that vapour has been shut
up in the vessel, and cooled of itself."
In the description of these experiments, the processes of
evaporation and condensation are obscurely indicated; but there is
no intimation that the author possessed any knowledge of the
elastic force of steam. His theorem is, that the parts of the
element water mix for a time with the parts of the element air;
that fire causes this mixture, and that on removing the fire, and
dissipating the heat, then the parts of the water mixed with air
return to their proper place, forming again part of the water.
There is no indication of a _change of property_ of the water in
passing into vapour. It is difficult to conceive, if De Caus had
been aware that the vapour of water possessed the same violent
force which he distinctly and in terms ascribes to air, or if he
had been aware that in effect the vapour of the water produced by
the fire was a fluid, possessing exactly the same mechanical
qualities, and producing the same mechanical effects as air, that
he would not have expressed himself clearly on the subject.
He proceeds to give another demonstration that heat will cause the
particles of water to mix with those of air.
"After having put the measure of water into the vessel, and shut
the vent-hole, and opened the cock, put the vessel on the fire,
and put the pot under the cock; then the water of the vessel,
raising itself by the heat of the fire, will run out through the
cock; but about one sixth or one eighth part of the water will not
run out, because the violence of the vapour which causes the water
to rise proceeds from the [Pg020] said water; which vapour goes
out through the cock after the water with great violence. There is
also another example in quicksilver, or mercury, which is a fluid
mineral, but being heated by fire, exhales in vapour, and mixes
with the air for a time; but after the said vapour is cooled, it
returns to its first nature of quicksilver. The vapour of water is
much lighter, and therefore it rises higher," &c. &c.
In this second demonstration there appears to be some obscure
indication of the force of steam in the words "because of the
violence of the vapour which causes the water to rise," &c.
The fifth theorem is the following:—
"_Water will mount by the help of fire higher than its level_,"
which is explained and proved in the following terms:—
[Illustration: _Fig._ 2.]
"The third method of raising water is by the aid of fire. On this
principle may be constructed various machines: I shall here
describe one. Let a ball of copper marked A; well soldered in
every part, to which is attached a tube and stop-cock marked D, by
which water may be introduced; and also another tube marked B C,
which will be soldered into the top of the ball, and the lower end
C of which shall descend nearly to the bottom of the ball without
touching it. Let the said ball be filled with water through the
tube D, then shutting the stop-cock D, and opening the stop-cock
in the vertical tube B C, let the ball be placed upon a fire the
heat acting upon the said ball will cause the water to rise in the
tube B C."
In the apparatus as here described, the space enclosed in the
boiler above the surface of the water is filled with air. By the
action of the fire, two effects are produced: first, the air
enclosed above the water, being heated, acquires increased
elasticity, and presses with a corresponding force on the surface
of the water. By this means a column of water will be driven up
the tube A B at such a height as will balance the elasticity of
the heated air confined in the boiler; but besides [Pg021] this
the water contained in the boiler being heated, will produce
steam, which being mixed with air contained in the boiler, will
likewise press with its proper elasticity on the surface of the
water, and will combine with the air in raising a column of water
in the tube A B. In the above description of the machine, the
force which raises the water in the tube A B is ascribed to the
fire, no mention being made of the water, or of the vapour or
steam produced from it having any agency in raising the water in
the tube A B.
Antecedently to the date of this invention, the effect of heat in
increasing the elastic force of air was known, and so far as the
above description goes, the whole operation might be ascribed to
the air by a person having no knowledge whatever of the elasticity
of steam. M. Arago, however, who, on the grounds of this passage
in the work of De Caus, claims for him a share of the honour of
the invention of the steam engine, contends that the agency of
steam in this apparatus was perfectly known to De Caus, although
no mention is made of steam in the above description, because in
the second demonstration above quoted he uses the words, "the
violence of the vapour which causes the water to rise proceeds
from the said water; which vapour goes out from the cock after the
water with great violence." By these words M. Arago considers that
De Caus expresses the quality of elasticity proper to the vapour,
and that the context justifies the inference, that to this
elasticity he ascribed the elevation of the water in the tube C B.
There appears to be some uncertainty attending the birthplace of
De Caus. In the _Biographie Universelle_ he is said to have been
born and to have died in Normandy. M. Arago assigns Dieppe, or its
neighbourhood, as his birthplace.
There was another engineer and architect, Isaac De Caus, a native
of Dieppe, who published a work in folio, entitled "_Nouvelle
Invention de Lever l'Eau plus haut que sa Source, avec quelque
Machines mouvantes, par le Moyen de l'Eau, et un Discours de la
Conduite d'Icelle_." This volume is without a date, but from the
nature of its contents it would appear to have been published
before the work of Solomon De Caus already cited. The drawings and
machines described in both [Pg022] are exactly the same; but the
definitions and theorems quoted above on raising water by fire are
not given in the work of Isaac. It seems, therefore, that Solomon
De Caus re-published, with additions, the work of Isaac De Caus.
From the same birthplace being assigned to both these authors, as
well as from the similarity of their pursuits, it is likely they
were members of the same family, and from their christian names
they were probably Jews.
The work cited above, was dedicated to Louis XIII., and in the
dedication Solomon De Caus calls himself the subject of that
monarch; and in the privilege prefixed to the work he is
designated, "Our well-beloved Solomon De Caus, master engineer,
being at present in the service of our dear and well-beloved
cousin, the Prince Elector Palatine, has made known to us,"
&c.—"we, desiring to gratify the said De Caus, he being our
subject," &c.
It is therefore certain, whatever may have been the birthplace of De
Caus, that he was at least a subject of France. The circumstance of
his work being written in French, though published beyond the Rhine,
is also an argument in favour of his being a native of that country.
GIOVANNI BRANCA, 1629.
(6.) Giovanni Branca of Loretto in Italy, an engineer and
architect, proposed to work mills of different kinds by steam
issuing from a large æolopile, and blowing against the vanes of a
wheel. Branca was the author of many ingenious mechanical
inventions, a collection of which he dedicated to M. Cenci, the
governor of Loretto. These were published in a work printed at
Rome in 1629. It is a thin quarto, entitled "_Le Machine volume
nuovo, et di molto artificio da fare effetti maravigliosi tanto
Spiritali quanto di Animale Operatione, arichito di bellissime
figure. Del Sig. Giovanni Branca, Cittadino Romano. In Roma,
1629._" The work contains sixty-three engravings, accompanied by
descriptions in Italian and Latin. Branca's steam engine,
represented in the twenty-fifth plate, consists of a wheel
furnished with flat vanes upon its rim, like the boards of a
paddle wheel. The steam is produced in a close vessel, and made to
issue with violence from the extremity [Pg023] of a pipe directed
against the vanes, and causes the wheel to revolve. This motion
being imparted by the usual mechanical contrivances, any machinery
may be impelled by it. Different useful applications of this power
are contained in the work, viz. pestles and mortars for pounding
materials to make gunpowder, and rolling stones for grinding the
same; machines for raising water by buckets, for sawing timbers,
for driving piles, &c. &c.
This method of applying the force of steam has no analogy to any
application of steam in modern engines.
EDWARD SOMERSET, MARQUIS OF WORCESTER, 1663.
(7.) Of all the names which figure in the early annals of steam,
by far the most remarkable is that of the Marquis of Worcester,
who has left a description of a machine in a work, entitled "The
Scantling of One Hundred Inventions," which has been generally in
this country considered as giving him a right to the honour of
having been the inventor of the steam engine.
Lord Worcester having been engaged on the side of the Royalists in
the civil wars of the revolution, lost his fortune, and went to
Ireland, where he was imprisoned. He escaped from thence, and
reached France; from that country he ventured to London, as a
secret agent of Charles II., but was detected, and imprisoned in
the Tower, where he remained until the restoration, when he was
set at liberty. Tradition has connected the invention of the steam
engine with the following anecdote:—One day, during his
imprisonment, Lord Worcester observed the lid of the pot in which
his dinner was being cooked, suddenly forced upwards by the vapour
of the water which was boiling in it. Reflecting on this, it
occurred to him that the same force which raised the cover of the
pot might be rendered, when properly applied, a useful and
convenient moving power. After he recovered his liberty, he
accordingly proceeded to carry into effect this conception. The
contrivance to which he was ultimately led is described in the
following terms in the sixty-eighth invention, in the work above
named:—
"I have invented an admirable and forcible way to drive [Pg024]
up water by fire; not by drawing or sucking it upwards, for that
must be, as the philosopher terms it, _infra sphœrum activitatis_,
which is but at such a distance. But this way hath no bounder if
the vessels be strong enough. For I have taken a piece of whole
cannon whereof the end was burst, and filled it three quarters
full of water, stopping and screwing up the broken end, as also
the touch-hole, and making a constant fire under it; within
twenty-four hours, it burst and made a great crack. So that,
having a way to make my vessels so that they are strengthened by
the force within them, and the one to fill after the other, I have
seen the water run like a constant fountain stream forty feet
high. One vessel of water rarefied by fire driveth up forty of
cold water, and a man that tends the work has but to turn two
cocks; that one vessel of water being consumed, another begins to
force and refill with cold water, and so successively; the fire
being tended and kept constant, which the self-same person may
likewise abundantly perform in the interim between the necessity
of turning the said cocks."
Since the date of the publication of the "Century of Inventions"
was the year 1663, the experiments here mentioned must have been
made before that year. The description of the machine here given,
as well as others in the same work, was intended by the author,
not to convey a knowledge of the nature of the mechanism which he
used, but only to express the effects produced, and to indicate
the physical principle on which they depended. It should also be
observed, that an air of mystery was thrown by Worcester over the
accounts of all the machines which he described; and therefore any
obscurity in the above description ought not to be regarded as an
evidence against his claim to the discovery of the mechanical
agency of steam, so far as that agency is indicated by the effects
said by him to be produced. The above account is, however,
sufficiently distinct and explicit to enable any one possessing a
knowledge of the mechanical qualities of steam to perceive the
general nature of the machine described. To render this machine,
and that of De Caus, previously described, intelligible to those
who are not familiar with physical science, we must here explain
some general principles on which their agency depends. [Pg025]
(8.) Fluid bodies are of two kinds, inelastic fluids, or liquids,
and elastic fluids, or gases. Of the former of these classes,
water is the most familiar example, and of the latter, air.
These two species of fluids are each distinguished by peculiar
mechanical properties.
[Illustration: _Fig._ 3.]
(9.) The constituent particles of a liquid are distinguished from
those of solids by having little or no coherence; so that unless
the mass be confined by the sides of the vessel which contains it,
the particles will fall asunder by their gravity. A mass of
liquid, therefore, unlike a solid, can never retain any particular
form, but will accommodate itself to the form of the vessel in
which it is placed. It will press against the bottom of the vessel
which contains it with the whole force of its weight, and it will
press against the sides with a force proportional to the depth of
the particles in contact with the sides measured from the surface
of the liquid above. This lateral pressure also distinguishes
liquids from solids. Let us take for illustration the case of a
square or a cubical vessel, A B C D, _fig._ 3. If a solid body,
such as a piece of lead, be cut to the shape of this vessel, so as
to fit in it without pressing with any force against its sides,
the mechanical effect which would be produced by it when placed in
the vessel, would be merely a pressure upon the bottom, B C, the
amount of which would be equal to the weight of the metallic mass.
No pressure would be exerted against the sides; for the coherence
of the particles of the solid maintaining them in their position,
the removal of the sides would not subject the solid body
contained in the vessel to any change.
Now let us suppose this solid mass of lead to be rendered liquid
by being melted. The constituent particles will then be deprived
of that cohesion by which they were held together; they will
accordingly have a tendency to separate, and fall asunder by their
gravity, and will only be prevented from actually doing so by the
support afforded to them by the sides, [Pg026] A B, D C, of the
vessel. They will therefore produce a pressure against the sides,
which was not produced by the lead in its solid state. This
pressure will vary at different depths: thus a part of the side of
the vessel at P will receive a pressure proportional to the depth
of the point P below the surface of the lead. If, for example, we
take a square inch of the inner surface of the side of the vessel
at P, it will sustain an outward pressure equal to the weight of a
column of lead having a square inch for its base, and a height
equal to P A. And, in like manner, every square inch of the sides
of the vessel will sustain an outward pressure equal to the weight
of a column of lead having a square inch for its base, and a
height equal to the depth of the point below the surface of the
lead.
(10.) We have here proceeded upon the supposition that no force
acts on the upper surface A D of the lead. If any force presses A
D downwards, that force would be transferred to the bottom by the
lead, and would produce a pressure on the bottom B C equal to its
own amount in addition to the weight of the lead; and if the lead
were solid, this would be the only additional mechanical effect
which such a force acting on the surface A D of the lead would
produce. But if, on the other hand, the lead were liquified, then
the force now adverted to, acting on the surface A D, would not
only produce a pressure on the bottom B C, equal to its own amount
in addition to the weight of the lead, but it would also produce a
pressure against every part of the sides of the vessel, equal to
that which it would produce upon an equal magnitude of the surface
A D.
Thus if we suppose any mechanical cause producing a pressure on
the surface A D amounting to ten pounds on each square inch, the
effect which would be produced, if the lead were solid, would be
an additional pressure on the base B C amounting to ten pounds per
square inch. But if the lead were liquid, besides this pressure on
each square inch of the base B C, there would likewise be a
pressure of ten pounds on every square inch of the sides of the
vessel.
All that has been here stated with respect to a square or a
cubical vessel will be equally applicable to a vessel of any other
form. [Pg027]
(11.) The second class of fluids are distinguished from liquids by
the particles not merely being destitute of cohesion, but having a
tendency directly the reverse, to repel each other, and fly
asunder with more or less force. Thus if a vessel, such as that
represented in _fig._ 3., were filled with a fluid of this kind,
being open at the top, and not being restrained by any pressure
incumbent upon it, the particles of the fluid would not rest in
the vessel by their gravity, as those of the liquid would do; but
they would, by their mutual repulsion, fly asunder, and rise out
of the vessel, as smoke is seen to rise from a chimney, or steam
from the spout of a kettle. Let us suppose, then, that the vessel
in which an elastic fluid is contained is closed on every side by
solid surfaces. In fact, let us imagine that the square or cubical
vessel represented in _fig._ 3. is closed by a square lid at the
top A D, having contained in it an elastic fluid, such as
atmospheric air.
If such a cover, or lid, had been placed upon a liquid, the cover
would sustain no pressure from the fluid, nor would any mechanical
effect be produced, save those already described in the case of
the open vessel; but when the fluid contained in the vessel is
elastic, as is the case with air, then the elasticity (by which
name is expressed the tendency of the particles of the fluid to
fly asunder) will produce peculiar mechanical effects, which have
no existence whatever in the case of a liquid.
It is true that, supposing the fluid to be air or any other gas or
vapour, a pressure will be produced upon the bottom B C of the
vessel equivalent to the weight of such fluid, and lateral
pressures will be produced on the different points of the sides by
the weight of that part of the fluid which is above these points;
but gases and vapours are bodies of such extreme levity, that
these effects due to their weight are neglected in practice.
Putting, then, the weight of the air contained in the vessel out
of the question, let us consider the effect of its elasticity. If
the vessel, as already described, be supposed to contain
atmospheric air in its ordinary state, the tendency of the
constituent particles to fly asunder will be such as to produce on
every square inch of the inner surface of the vessel [Pg028] a
pressure amounting to fifteen pounds; this pressure being, as
already stated, quite independent of the weight of the air. In
fact, this pressure would continue to exist if the air contained
in the vessel actually ceased to have weight by being removed from
the neighbourhood of the earth, which is the cause of its gravity.
(12.) Different gases are endowed with different degrees of
elasticity, and the same gas may have its elasticity increased or
diminished, either by varying the space within which it is
confined, or by altering the temperature to which it is exposed.
If the space within which an elastic fluid is enclosed be
enlarged, its elasticity is found to diminish in the same
proportion. Thus if the air contained in the vessel A B C D
(_fig._ 3.) be allowed to pass into a vessel of twice the
magnitude, the elasticity of the particles will cause them to
repel each other, so that the same quantity of air shall diffuse
itself throughout the larger vessel, assuming double its former
bulk. Under such circumstances, the pressure which it would exert
upon the sides of the larger vessel would be only half that which
it had exerted on the sides of the smaller vessel. If, on the
other hand, it were forced into a vessel of half the magnitude of
A B C D, as it might be, then its elasticity would be double, and
it would press on the inner surface of that vessel with twice the
force with which it pressed on that of the vessel A B C D.
This power of swelling and contracting its dimensions according to
the dimensions of the vessel in which it is confined, or to the
force compressing it, is a quality which results immediately from
elasticity, and is consequently one which is peculiar to the gases
or elastic fluids, and does not at all appertain to liquids. If
the liquid contained in the vessel A B C D were transferred to a
vessel of twice the magnitude, it would only occupy half the
capacity of that vessel, and it could not by any means be
transferred, as we have supposed the air or gas to be, to a vessel
of half the dimensions, since it is inelastic and incompressible.
(13.) The elasticity of gases is likewise varied by varying the
temperature to which they are exposed; thus, in general, [Pg029]
if air or any other gas be augmented in temperature, it will
likewise be increased in elasticity; and if, on the other hand, it
be diminished in temperature, it will be likewise diminished in
its elastic force. The more heated, therefore, any air or gas
confined in a vessel becomes, the greater will be the force with
which it will press on the inner surface of that vessel, and tend
to burst it.
(14.) The same body may, by the agency of heat, be made to pass
successively through the different states of solid, liquid, and
gas, or vapour. The most familiar and obvious example of these
successive transitions is presented by water. Exposed to a certain
temperature, water can only exist as a solid; as the temperature
is increased, the ice, or solid water, is liquefied; and by the
continued application of heat, this water again undergoes a
change, and assumes the form, and acquires the mechanical
qualities, of air or gas: in such a state it is called STEAM.
This is a common property of all liquids. If they be exposed for a
sufficient length of time to a sufficient degree of heat, they
will always be converted into elastic fluids. These are usually
distinguished from air and other permanent gases, which never are
known to exist in the liquid form, by the term _vapour_, by which,
therefore, must be understood an elastic fluid which at common
temperatures exists in the liquid or solid state; by _steam_ is
expressed the vapour of water; and by _gases_, those elastic
fluids which like air are never known—at least, under ordinary
circumstances—to exist in any other but the elastic form.
(15.) When a liquid is caused, by the application of heat, to take
the form of an elastic fluid, or is evaporated, besides acquiring
the property of elasticity, it always undergoes a considerable
change of bulk. The amount of this change is different with
different liquids, and even with the same liquid it varies with
the circumstances under which the change is produced.
(16.) When water is evaporated under ordinary circumstances,—that
is, when exposed to no other external pressure than that of the
atmosphere,—it increases its volume about seventeen-hundred-fold.
Thus a cubic inch of liquid [Pg030] water would form about
seventeen hundred cubic inches of common steam. If, however, the
water be confined by a greater pressure than that produced by the
common atmosphere, then the increase of volume which takes place
in its evaporation would be less in proportion.
These important physical circumstances are now only indicated in a
general way. As we proceed with our account of the invention and
improvement of the steam engine, they will be developed more fully
and accurately.
(17.) After duly considering what has been just explained, no
difficulty will be found in comprehending the principles on which
the first rude attempts at the mechanical application of steam
already stated depend. In the apparatus ascribed to _Hero_ of
Alexandria, the elasticity of the vapour contained in the arms of
the revolving ball causes that vapour to issue from the lateral
orifices in the arms, such as that of G, _fig._ 1. As these
orifices, however, are exposed to the common atmosphere pressing
inwards with a force, the mean amount of which has been stated to
be about fifteen pounds per square inch, it follows that the steam
cannot escape from these orifices until its pressure or elasticity
exceeds this amount, and that when it does, the force with which
it will so escape will be the excess of its elasticity above that
of the atmosphere; and it is the reaction produced by this
difference of pressure, causing the arms to recoil, which will
give motion to the machine.
In the case of the apparatus of _De Caus_ (5.), the heat of the
fire acting on the vessel D C (_fig._ 2.) will raise the
temperature of the water contained in it, and also of the air
confined within it above the surface of that water. This air, as
it is increased in temperature, will also increase in elasticity;
it will therefore press on the surface of the water with increased
force, and will gradually force the water upwards in the tube; and
this effect would continue until all the water in the vessel would
be forced up the tube.
But at the same time that the heat acting on the vessel increases
the temperature of the air above the water, it also produces a
partial evaporation of the water, so that more or less steam is
mixed with the air in the vessel above the surface [Pg031] of the
water; and this steam possessing elasticity, unites with the air
in pressing on the surface of the water, and in raising it in the
tube.
[Illustration: _Figs._ 4, 5, and 6.]
Let us now revert to the brief account of the engine of the
Marquis of Worcester, described in "The Century of Inventions." We
collect from that description that the vessel in which the water
was evaporated was separate from those which contained the water
to be elevated; also that there were two vessels of the like
description, the contents of which were alternately elevated by
the pressure of the "water rarefied by the fire;" in other words
by steam; and that the water was raised in an uninterrupted
stream, by the management of two cocks communicating with these
vessels and with the boiler. The following is such an apparatus as
would answer this description. Let E (_fig._ 4.) be the vessel
containing the water to be evaporated, placed over a proper
furnace A; let S be a pipe to allow the steam produced from the
boiling water in E to pass into the vessels where its mechanical
action is required. Let R represent a cock or regulator, having in
it a curved passage, leading from S to the tube T, when the lever
or handle L is in the position represented by the cut; but leading
to the tube T′, when the lever L is turned one quarter of a
revolution to the right, as represented in _fig._ 5. By the
shifting of this lever, therefore, the steam pipe S may be made to
communicate alternately with the tubes T and T′. The tubes T and
T′ are carried respectively to two vessels V and V′, which are
filled with the water required to be raised. In these [Pg032]
vessels tubes enter at C and C′, descending nearly to the bottom:
these tubes have valves at B and B′, opening upwards, by which
water will be allowed to pass into the vertical tube F, but which
will not allow it to return downwards, the valves B and B′ being
then closed by the weight of the water above them.
Let G G′ be a pipe entering the sides of the vessels V and V′, for
the purpose of filling them with the water to be raised: let K be
a cock having a curved passage similar to the cock R, and leading
to a tube by which water is supplied from the reservoir or other
source from which the water to be raised is drawn. When the cock K
is placed as represented in _fig._ 4., the water from the
reservoir will flow through the curved passage in the cock K into
the tube G′, and thence into the vessel V′; but when this cock is
turned one quarter round, by shifting the lever to the left, it
will take the position represented in _fig._ 6., and the water
will flow through the curved passage into the tube G, and thence
into the vessel V. Let us now suppose the vessel V already filled
with water to be elevated, and the vessel V′ to have discharged
its contents. The cock R is turned, so as to allow the steam
generated in the boiler E to pass into the tube T, and thence into
the upper part of the vessel V, while the cock K is turned so as
to allow the water from the reservoir to pass into the tube G′,
and thence into the vessel V′. The steam collecting in the upper
part of the vessel V′ presses with its elastic force on the
surface of the water therein, and forces the water upwards in the
tube C; it passes through the valve B, which it opens by the
upward pressure received from the action of the steam, and thence
into the tube F, its descent into the tube C′ being prevented by
the valve V′, which can only be opened upwards. As the steam is
gradually supplied from the boiler E, the water in the vessel V is
forced up the tube C, through the valve B, and into the tube F,
until all the contents of the vessel V above the lower end of the
tube C have been raised. In the meanwhile, the vessel V′ has been
filled with water, through the cock K: when this has been
accomplished, the man who attends the machine shifts the cocks R
and K, so as to give them the position represented in _fig._ 5.
and _fig._ 6. [Pg033] In this position, the steam from the
boiler, being excluded from the tube T, will be conducted to the
tube T′, and thence to the vessel V′, while the water from the
reservoir will be excluded from the tube G′, and conducted through
the tube G to the vessel V. The vessel V will thus be replenished
and, by a process similar to that already described, the contents
of the vessel V′ will be forced up the tube C′, through the valve
B′, and into the tube F; its descent into the tube C being
prevented by the valve B, which will then be closed. After the
contents of the vessel V′ have thus been raised, and the vessel V
replenished, the two cocks R and K are once more shifted, and the
contents of V raised while V′ is replenished, and so on.
[Illustration: _Fig._ 4, 5, and 6.]
If, having comprehended the apparatus here described, the reader
refers to the description of the Marquis of Worcester's machine,
he will find that all the conditions therein laid down are
fulfilled by it. One vessel (E) of "water rarefied by fire" may by
such means "drive up forty (or more) of cold water; and the man
that tends the work has but to turn two cocks, that one vessel (V)
of water being consumed, another (V′) begins to force and refill
with cold water, and so on successively, the fire being tended and
kept constant; which the self-same person may likewise abundantly
perform, in the interim between the necessity of turning the said
cocks."
On comparing this with the contrivance previously suggested by De
Caus, it will be observed, that even if De Caus [Pg034] knew the
physical agent by which the water was driven upwards in the
apparatus described by him, still it was only a method of causing
a vessel of boiling water to empty itself; and before a repetition
of the process could be made, the vessel should be refilled, and
again boiled. In the contrivance of Lord Worcester, on the other
hand, the agency of the steam was employed in the same manner as
it is in the steam engines of the present day, being generated in
one vessel, and used for mechanical purposes in another. Nor must
this distinction be regarded as trifling or insignificant, because
on it depends the whole practicability of using steam as a
mechanical agent. Had its action been confined to the vessel in
which it was produced, it never could have been employed for any
useful purpose.
Although many of the projects contained in Lord Worcester's work
were in the highest degree extravagant and absurd, yet the engine
above described is far from being the only practicable and useful
invention proposed in it. On the contrary, many of his inventions
have been reproduced, and some brought into general use since his
time. Among these may be mentioned, stenography, telegraphs,
floating baths, speaking statues, carriages from which horses can
be disengaged if unruly, combination locks, secret escutcheons for
locks, candle moulds, the rasping mill, the gravel engine, &c.
SIR SAMUEL MORLAND, 1683.
(18.) Sir Samuel Morland was the son of a baronet of the same name,
who had received his title at the restoration for some services to
the royalist party, performed by him during the wars of the
Commonwealth. He appears to have devoted much attention to
mechanics, in which he attained some celebrity. He was the reputed
inventor of several ingenious contrivances, such as the drum capstan
for ships, the plunger pump, &c. He also investigated various
questions in acoustics, and among others, the determination of the
best form for the speaking-trumpet.
In 1680, Sir Samuel Morland was appointed Master [Pg035] of the
Works to Charles II., and in the following year was sent to
France, to execute some waterworks for Louis XIV. In 1683, while
in France, he wrote in the French language, a work entitled
"_Elevation des Eaux par toute sorte de Machines, reduite à la
Mesure, au Poids et à la Balance. Presentée à sa Majesté très
Chrestienne, par le Chevalier Morland, Gentilhomme Ordinaire de la
Chambre Privée, et Maistre des Méchaniques du Roi de la Grande
Brétagne, 1683._" This book is preserved in manuscript in the
Harleian Collection in the British Museum. It is written on
vellum, and consists of only thirty-eight pages. It contains
tables of measures and weights, theorems for the calculation of
the volumes of cylinders, the weights of columns of water, the
thickness of lead for pipes, and is concluded by a chapter on
steam, consisting of four pages, of which the following is a
translation:—
"_The principles of the new force of fire invented by Chevalier
Morland in 1682, and presented to His Most Christian Majesty in
1683_:—
"'Water being converted into vapour by the force of fire, these
vapours shortly require a greater space (about 2000 times) than the
water before occupied, and sooner than be constantly confined would
split a piece of cannon. But being duly regulated according to the
rules of statics, and by science reduced to measure, weight, and
balance, then they bear their load peaceably (like good horses), and
thus become of great use to mankind, particularly for raising water,
according to the following table, which shows the number of pounds
that may be raised 1800 times per hour to a height of six inches by
cylinders half filled with water, as well as the different diameters
and depths of the said cylinders.'"
There is nothing in the description here given which can indicate
the form of the machine by which Morland proposed to render the
force of steam a useful mover. It is, however, remarkable, that at
this early period, before experiments had been made on the
expansion which water undergoes in evaporation, he should have
given so near an approximation to [Pg036] the actual amount of
that expansion. It is scarcely supposable that such an estimate
could be obtained by him otherwise than by experiment.
The work containing the above description was not printed; but a
work bearing nearly the same title, containing, however, no
mention of the force of steam, was published by him in Paris in
the year 1685. In this he describes various experiments made by
him at St. Germains on the weight of the water of the Seine, and
gives weights of the columns of water, the contents of cylinders,
&c.
Soon after the publication of this work, Morland returned to
England, and resided near the court till his death. The celebrated
John Evelyn mentioned having paid a visit to him at his house at
Hammersmith, in 1695, when he had become aged and blind, but was
still remarkable for his mechanical ingenuity. "On the 25th of
October," says Evelyn, "the Archbishop and myself went to
Hammersmith to visit Sir Samuel Morland, who was entirely blind; a
very mortifying sight. He showed us his invention of writing
(short-hand), which was very ingenious; also his wooden kalendar,
which instructed him all by feeling; and other pretty and useful
inventions of mills, pumps, &c.; and the pump he had erected, that
serves water to his garden and to passengers, with an inscription,
and brings from a filthy part of the Thames near it a most perfect
and pure water."[3]
He died at Hammersmith, in January 1696; and before his death, as
a penance for his past life, was guilty of the eccentricity of
burying in the ground six feet deep a great collection of music
which he possessed.[4]
DENIS PAPIN, 1688.
(19.) Denis Papin, a native of Blois in France, and professor of
mathematics at Marbourg, is the name which stands next recorded in
the progressive invention of the steam engine. To this philosopher
is due the discovery of one of the qualities of steam, to the
proper management of which is owing much of the efficacy of the
modern steam engine. [Pg037]
Papin was born at Blois in France. He devoted his youth to the
study of medicine, in which he took a degree at Paris. The
revocation of the Edict of Nantes having driven him into exile, he
went to England, where the celebrated Boyle associated him in
several of his experiments with the air-pump, and caused him to be
elected a fellow of the Royal Society in 1681. Having been invited
to Germany by the Landgrave of Hesse, he discharged during several
years the duties of professor of mathematics at the university of
Marbourg, where he died in 1710. Notwithstanding his discoveries
respecting the agency of steam, he never received any mark of
distinction in his own country. The truth is, the importance and
value of these investigations were not apparent until long
afterwards.
This philosopher conceived the idea of producing a moving power by
means of a piston working in a cylinder, in the manner which we
shall now briefly explain.
[Illustration: _Fig._ 7.]
Let A B (_fig._ 7.) be a cylinder open at the top, and let a
piston P be fitted into it, so as to move in it air tight. At the
bottom of the cylinder suppose an opening provided, which can be
closed at pleasure, by a stop-cock, or otherwise, so that the
communication may be opened and closed at will between the
interior of the cylinder and the external air. This stop-cock
being opened, let the piston be drawn upwards till it reach the
top of the cylinder. Let the stop-cock at the bottom be then
removed, and imagine that some means can be supplied by which the
air within the cylinder can be suddenly annihilated. The piston,
now at the top, will have above it the pressure of the atmosphere;
and having no air below, it will be resisted in its descent by no
force save that arising from its friction with the cylinder. If,
then, the force of the air above the piston be greater than the
resistance arising from this friction, the piston will descend
with the excess of this force, and will continue so to descend
until it reach the bottom of the cylinder. Having attained that
position, let us [Pg038] suppose the stop-cock in the bottom
opened, so as to allow the external air to pass freely below the
piston. The piston may now be drawn to the top of the cylinder
again, offering no resistance save that of its weight, and its
friction with the cylinder. Having reached the top of the cylinder
once more, let the stop-cock be closed, and the air included
within the cylinder once more annihilated. A second descent of the
piston will take place, with the same force as before, and in like
manner the process may be continued indefinitely.
Now, if it should appear that means could be provided suddenly and
repeatedly to annihilate the air within the cylinder, and that the
pressure of the atmosphere above the piston should exert a force
compared with which the weight of the piston and its friction are
trifling, it is evident that a moving power would be obtained
which would be capable, by proper mechanism, of being applied to
any useful purpose, but which would more especially be applicable
to the working of pumps, the motion of which corresponds with that
which has been just ascribed to the piston in the cylinder. Such
were the first ideas of Papin. But in order to enable those who
are not conversant with physical science fully to appreciate their
importance, it will be necessary here to explain some of the
mechanical properties of atmospheric air.
(20.) The atmosphere is the thin, transparent, colourless, and
therefore invisible, fluid in which we live and move, which by
respiration sustains animal life, and is otherwise connected with
various important functions of organised matter. This fluid is so
light and attenuated, that it might at first be doubted whether it
be really a body at all; and, indeed, the name expressing
incorporeal beings, _spirit_, is a word in its origin signifying
_air_.[5] The air, however, is light only as compared with other
material substances, which exist in a more condensed state: it
possesses the quality of weight as absolutely as the most solid
and massive bodies in nature, and to render this quality manifest,
it is only necessary to submit a sufficient quantity of air to any
of the usual tests of gravitation. [Pg039]
A direct demonstration of this may be given by the following
experiment:—On the mouth of a flask let a stop-cock be fastened so
as to be air-tight. The interior of the flask may then be put into
free communication with the external air, or that communication may
be cut off at pleasure, by opening or closing the stop-cock. If a
syringe be applied to the mouth of the flask, the stop-cock being
open a part of the air contained in it may be drawn out. After this,
the stop-cock being closed, and the syringe detached, let the
flask be placed in the dish of a good balance, and accurately
counterpoised by weights in the other dish. This counterpoise will
then represent the weight of the flask, and of the air which has
remained in it. If the stop-cock be now opened, air will immediately
rush in, and replace that which the syringe had withdrawn from the
flask; and immediately the dish of the balance containing the flask
will sink by the effect of the weight of the air thus admitted into
the flask.
If the weight of quantity of air so small as to be capable of
being withdrawn by a syringe from an ordinary flask be thus of
sensible amount, it may be easily imagined that the vast mass of
atmosphere extending from the surface of the earth upwards, to a
height not ascertained with precision, but certainly not being
less than thirty miles, must be very considerable. Such a force,
pressing as it must constantly do, upon the surfaces of all
bodies, whether solid or fluid, and resisting and modifying their
movements, would play an important part in all mechanical
phenomena; and it is, therefore, not sufficient merely to have
recognised its existence, but it is most needful to measure its
amount with that degree of certainty and precision, which will
enable us to estimate its effects on those phenomena which we
shall have to investigate.
(21.) The amount of the pressure of the atmosphere on each square
inch of horizontal surface on which it rests, is obviously the
weight of the column of air extending from that square inch of
surface upwards to the top of the atmosphere. This force is
measured by the following means:—
[Illustration: _Fig._ 8.]
[Illustration: _Fig._ 9.]
Take a glass tube, A B (_fig._ 8.), above 32 inches long, open at
one end A, and closed at the other end B, and let it [Pg040] be
filled with mercury (quicksilver). Let a glass vessel or cistern
C, containing a quantity of mercury, be also provided. Applying
the finger at A, so as to prevent the mercury in the tube from
falling out, let the tube be inverted, and the end, stopped by the
finger, plunged into the mercury in C. When the end of the tube is
below the surface of the mercury in C (_fig._ 9.), let the finger
be removed. It will be found that the mercury in the tube will
not, as might be expected, fall to the level of the mercury in the
cistern C, which it would do were the end B open, so as to admit
the air into the upper part of the tube. On the other hand, the
level D of the mercury in the tube will be nearly 30 inches above
the level C of the mercury in the cistern.
The cause of this effect is, that the weight of the atmosphere
rests on the surface C of the mercury in the cistern, and tends
thereby to press it up, or rather to resist its fall in the tube;
and as the fall is not assisted by the weight of the atmosphere on
the surface D (since B is closed), it follows, that as much
mercury remains suspended in the tube above the level C, as the
weight of the atmosphere is able to support.
If the section of the tube were equal to the magnitude of a square
inch, the weight of the column of mercury in the tube above the
level C would be exactly equal to the weight of the atmosphere on
each square inch of the surface C.
(22.) If such an apparatus be observed from time to time, it will
be found that the column of mercury sustained in the tube will be
subject to variation between certain limits, never falling below
twenty-eight inches, and never rising above thirty-one inches.
This variation of the mercurial column is produced by a
corresponding variation in the weight of the atmosphere.
If the apparatus be transported to any height above its ordinary
position, it will have a less quantity of atmosphere above it, and
therefore the surface of the mercury in the cistern will be
pressed by a less weight, and consequently the [Pg041] column of
mercury will fall proportionally. In virtue of this effect, such
an instrument has been rendered a means of measuring heights, such
as the heights of mountains, the ascents of balloons, &c. &c.
(23.) If a proper scale be attached to the tube containing the
mercurial column, showing the absolute height of the column
sustained at any time, and indicating its changes of height, the
instrument becomes a BAROMETER.
Two cubic inches of mercury weigh very nearly one pound
avoirdupois.[6] Hence, when the barometric column measures thirty
inches, the weight of the atmosphere resting on each square inch
of surface is about fifteen pounds.
(24.) It is an established property of fluids, that they press
equally in all directions; and air, like every other fluid,
participates in this quality. Hence, it follows, that when the
downward pressure or weight of the atmosphere is fifteen pounds on
the square inch, the lateral, upward, and oblique pressures are of
the same amount. But, independently of the general principle, it
may be satisfactory to give experimental proof of this.
[Illustration: _Fig._ 10.]
Let four glass tubes, A, B, C, D (_fig._ 10.), be constructed of
sufficient length, closed at one end, A, B, C, D, and open at the
other. Let the open ends of three of them be bent, as represented
in the tubes B, C, D. Being previously filled with mercury, let
them all be gently inverted, so as to have their closed ends up,
as here represented. It will be found that the mercury will be
sustained in all, and that the difference of the levels in all
will be the same.[7] Thus, the mercury is sustained in A by the
upward pressure of the atmosphere; in B, by its horizontal or
lateral pressure; in C, by its downward pressure; [Pg042] and in
D, by its oblique pressure: and, as the difference of the levels
is the same in all, these pressures are exactly equal.
(25.) The same arrangement by which the pressure of the atmosphere
is measured by a mercurial column of equivalent weight, also
supplies the means of measuring the pressure or elasticity of
atmospheric air, or any other gas or vapour, whether in a more or
less compressed or rarefied state; and as instruments constructed
on this principle are of considerable use in steam engines, we
shall take this occasion to explain their principle and
application.
In the experiments described in (21), the space D B in the top of
the barometer-tube, from which the mercury descended, is a vacuum.
If, however, it were occupied by a quantity of air in a rarefied
state, or any other gas or vapour, such gas or vapour would press
on the surface of the mercury at D, with a force determined by its
elasticity. In that case, the atmospheric pressure acting on the
surface of the mercury C in the cistern, would be balanced by the
combined forces of the weight of the mercurial column sustained in
the tube, and the elasticity of the gas or vapour in the upper
part of it. Now if we know the actual amount of the atmospheric
pressure,—that is to say, the height of the column of mercury
which it would be capable of sustaining,—we should then be able
to determine the pressure of the rarefied air in the space C D.
For example, let us suppose that the barometric column, when B D
(_fig._ 9.) is a vacuum, measures thirty inches: the atmospheric
pressure, therefore, would be equal to the weight of a column of
mercury of that height. Let us suppose that the elasticity of the
gas or vapour occupying the upper part of the tube D B causes the
column to fall to the height of twenty-six inches: it is evident,
then, that the pressure of the air in the top of the tube would be
equal to the weight of a column of mercury of four inches. In
fine, to determine the pressure of the rarefied gas or vapour in
the top of the tube, it is only necessary to observe the
difference between the height of the column of mercury actually
sustained in the tube, and the column sustained at the same time
and [Pg043] place in a common barometer: the difference of the
two will be the column of mercury whose weight will represent the
pressure of the vapour or gas in the top of the tube.
(26.) Whenever the air contained in any vessel or other enclosed
space has by any means had its pressure reduced so as to be
rendered less than that of the external air, the external air will
have a tendency to rush into such vessel or enclosed space with a
force proportionate to the excess of the pressure of such external
air over that of the air within; and if any communication be
opened between the interior of such vessel or enclosed space, and
the external air, the latter will rush in until an equilibrium be
established between the pressures within and without. It is
evident that the force thus obtained by diminishing the pressure
of air within a vessel may be applied to any mechanical purpose.
It is by such means that water is raised in an ordinary pump. A
portion of the air contained between the piston of the pump and
the surface of the water below, is withdrawn by the action of the
piston, and the pressure of the air remaining under the piston is
thereby diminished. The superior pressure of the atmosphere upon
the external surface of the water in the well then forces up a
column of water in the pump-barrel, and this is continued as the
air is more and more rarefied by the action of the piston. By
whatever means, therefore, the air can be wholly or partially
withdrawn from any space, a mechanical power will be thereby
developed, proportional in its amount and efficacy to the quantity
of air so withdrawn. If, however, such air be withdrawn by any
mechanical process, such as by a syringe, by a common pump, or by
an air-pump, the quantity of force expended in withdrawing it is
always equivalent to the amount of mechanical power obtained by
the vacuum or partial vacuum so produced. Indeed the power
expended is greater than the power so obtained, inasmuch as the
friction, leakage, &c. of the exhausting apparatus must be allowed
for.
(27.) There are, however, various other means by which air may be
partially expelled from a vessel besides the direct application of
mechanical force. Thus if heat be applied to [Pg044] the vessel,
the air, as has been already explained, will acquire increased
elasticity, and will rush from the vessel with a force proportionate
to the excess of its elasticity above that of the external air, and
this process may be continued by increasing the heat to which the
vessel is exposed, until a very considerable portion of the air has
been expelled. If the orifice by which the air has escaped be then
closed, and the vessel be allowed to cool, the air within, by having
its temperature reduced to that of the external air, will lose all
the elasticity which it had gained from the heat, and will be in the
same condition as if an equivalent quantity of air had been
withdrawn by any mechanical agent. The external air, therefore, will
have a tendency to rush in with a force corresponding to the
difference of pressures.
The process of filling thermometers with mercury shows one use of
producing a high degree of rarefaction by heat. To construct the
instrument it is necessary to fill the bulb and a part of the tube
with mercury; but the bore of the tube is so small that the
mercury cannot be introduced by any ordinary means. It is
therefore held over flame until heated to a high temperature. The
air within it gradually increasing in pressure as its temperature
is raised, is forced through the small bore of the tube, until the
pressure of the air within becomes no more than equal to the
pressure of the external atmosphere; this air being so rarefied
that quantity in the bulb bears a very small proportion to its
contents at common temperatures. The mouth of the tube is then
plunged into mercury, and as the bulb cools, the air within it
loses its elasticity, and the superior pressure upon the external
surface forces the mercury into the tube. This continues until the
air remaining within the bulb has been so contracted, that its
pressure combined with the weight of the mercury, shall balance
the atmospheric pressure. The tube is then reversed, and the air
which remained rises in a bubble to the surface, and escapes.
(28.) Let us now return to the proceedings of Papin. How great a
power would result from such a machine as he conceived, will be
apparent, if it be considered that the unresisted atmosphere
exercises a pressure of about fifteen pounds on [Pg045] each
square inch of surface exposed to it, and that if the piston in
the cylinder imagined by Papin, had a diameter of only one foot,
its superficial magnitude would be about 114 square inches. The
pressure of the atmosphere upon it, therefore, would be 114 times
fifteen pounds, or 1710 pounds. Papin first proposed to produce
the vacuum under the piston by means of common air pumps, worked
by a water-wheel; and by such means he conceived that the power of
a river, stream, or waterfall might be conveyed by pipes to a
distance. While he was in England, in 1687, he laid his
contrivance before the Royal Society of London, but was met by
objections and difficulties, the nature of which he does not
explain.
It is, however, apparent, from what has been already explained,
that such a method of proceeding would amount to a mere transfer
of power, and would not, properly speaking, be itself a moving
force: the moving power would, in reality, be the force of the
water by which the water-wheel would be driven; and the air-pumps,
tubes, together with the piston and cylinder, would be merely
means of conveying the power of the water-wheel to the objects to
be moved, or the machinery to be driven. Papin states, that, long
before this, he had attempted to expel the air from his cylinder
by means of gunpowder; but, notwithstanding all the precautions
which he could take, there always remained a considerable
quantity; so much, indeed, as to deprive the vacuum of more than
half its proper force. At length he adopted an expedient for the
production of a vacuum which forms a most important step in the
progressive invention of the steam engine, and which gives to
Papin's name a high place in the history of that machine. This
method is explained in the following paragraph of a work published
by Papin in 1695, at Cassel, entitled "_Recueil de diverses Pièces
touchant quelques nouvelles Machines_", p. 53.
"I have endeavoured," says he, "to attain this end (viz. the
production of a vacuum in the cylinder) in another way. As water
has the property of elasticity, when converted into steam by heat,
and afterwards of being so completely recondensed by cold, that
there does not remain the least [Pg046] appearance of this
elasticity, I have thought that it would not be difficult to work
machines in which, by means of a moderate heat and at a small
cost, water might produce that perfect vacuum which has vainly
been sought by means of gunpowder."
This remarkable passage is given in the work just cited, as an
extract from the "Leipsic Acts," of August, 1690.
Let us pause here to explain more fully this important discovery.
(29.) We have explained that, in its conversion into vapour, by
the application of heat, water, besides acquiring the property of
elasticity, undergoes a vast enlargement of bulk, filling, under
ordinary circumstances, about 1700 times more space than it
occupied in the liquid form. This fact was known generally, though
not with numerical accuracy, by Papin, having been the foundation
of the machines previously invented and published by De Caus and
Lord Worcester; the happy idea of reversing the process occurred
to him. If water in its conversion into steam swelled into many
hundred times its original bulk, it would necessarily follow, that
steam, being reconverted into water, would shrink into its
primitive dimensions. Papin therefore saw, that if he could by any
means expel the air from his cylinder under the piston, and
replace it by the pure vapour of water, he could cause that vapour
to be reconverted into a comparatively minute quantity of water by
depriving it of the heat which sustained it in the state of steam,
and that by accomplishing this, the space in the cylinder under
the piston would become a vacuum; that by such means, the pressure
of the atmosphere above the piston would take full effect, and
would urge the piston down; that by introducing more steam under
the piston, it might be again raised by the elastic force of the
steam, the destruction of which by cold water would again produce
the descent of the piston with the same mechanical force; and that
in this way the alternate ascent and descent of the piston might
be continued indefinitely.
In accordance with these ideas, Papin constructed a model
consisting of a small cylinder, in which was placed a solid
piston; [Pg047] and in the bottom of the cylinder under the
piston was contained in a small quantity of water. The piston
being in immediate contact with this water, so as to exclude the
atmospheric air, on applying fire to the bottom of the cylinder,
steam was produced, the elastic force of which raised the piston
to the top of the cylinder; the fire being then removed, and the
cylinder being cooled by the surrounding air, the steam was
condensed and reconverted into water, leaving a vacuum in the
cylinder into which the piston was pressed by the force of the
atmosphere. The fire being applied and subsequently removed,
another ascent and descent were accomplished; and in the same
manner the alternate motion of the piston might be continued.
Papin described no other form of machine by which this property
could be rendered available in practice; but he states generally,
that the same end may be attained by various forms of machines
easy to be imagined.[8]
THOMAS SAVERY, 1698.
(30.) The discovery of the method of making a vacuum by the
condensation of steam was reproduced, before 1698, by Captain
Thomas Savery, to whom a patent was granted in that year for a
steam engine to be applied to the raising of water, &c. Savery
proposed to combine the machine described by the Marquis of
Worcester with an apparatus for raising water by suction into a
vacuum produced by the condensation of steam.
Savery appears to have been ignorant of the publication of Papin,
and stated that his discovery of the condensing principle arose
from the following circumstance:—
Having drunk a flask of Florence at a tavern, and flung the empty
flask on the fire, he called for a basin of water to wash his
hands. A small quantity which remained in the flask began to boil,
and steam issued from its mouth. It occurred to him to try what
effect would be produced by inverting the flask and plunging its
mouth in the cold water. Putting on a thick glove to defend his
hand from the heat, he seized the [Pg048] flask, and the moment
he plunged its mouth in the water the liquid immediately rushed up
into the flask and filled it.
Savery stated that this circumstance immediately suggested to him
the possibility of giving effect to the atmospheric pressure by
creating a vacuum in this manner. He thought that if, instead of
exhausting the barrel of a pump by the usual laborious method of a
piston and sucker, it was exhausted by first filling it with
steam, and then condensing the same steam, the atmospheric
pressure would force the water from the well into the pump-barrel,
and into any vessel connected with it, provided that vessel were
not more than about thirty-four feet above the elevation of the
water in the well. He perceived also, that, having lifted the
water to this height, he might use the elastic force of steam in
the manner described by the Marquis of Worcester to raise the same
water to a still greater elevation, and that the same steam which
accomplished this mechanical effect would serve, by its subsequent
condensation, to reproduce the vacuum, and draw up more water. It
was on this principle that Savery constructed the first engine in
which steam was ever brought into practical operation.
[Illustration: BRANCA'S ENGINE.]
FOOTNOTES:
[1] Arago, Eloge historique de James Watt; p. 22.
[2] Ibid., p. 21. note.
[3] Farey, Treatise on the Steam Engine, p. 93.
[4] Arago, sur les Machines à Vapeur, Annuaire, 1829, p. 165
[5] SPIRITUS, _breath_ or _air_.
[6] Exactly 15·68 oz. = 0·98 lb.
[7] This experiment with the tube A requires to be very
carefully executed, and the tube should be one of small bore.
[8] Recueil de diverses Pièces touchant quelques nouvelles
Machines, p. 38.
[Pg049]
[Illustration: SAVERY'S ENGINE.]
CHAP. II.
ENGINES OF SAVERY AND NEWCOMEN.
SAVERY'S ENGINE. — BOILERS AND THEIR APPENDAGES. — WORKING
APPARATUS. — MODE OF OPERATION. — DEFECTS OF THE ENGINE. —
NEWCOMEN AND CAWLEY. — ATMOSPHERIC ENGINE. — ACCIDENTAL
DISCOVERY OF CONDENSATION BY INJECTION. — HUMPHREY POTTER
MAKES THE ENGINE WORK ITSELF. — ADVANTAGES OF THE ATMOSPHERIC
ENGINE OVER THAT OF SAVERY. — IT CONTAINED NO NEW PRINCIPLE. —
ITS PRACTICAL SUPERIORITY.
(31.) The steam engine contrived by Savery, like every other which
has since been constructed, consists of two parts, essentially
distinct. The first is that which is employed to [Pg050] generate
the steam, which is called the boiler; and the second, that in
which the steam is applied as a moving power.
[Illustration: _Fig._ 11.]
The former apparatus in Savery's engine consists of two strong
boilers, sections of which are represented at D and E in _fig._
11.; D the greater boiler, and E the less. The tubes T and T′
communicate with the working apparatus, which we shall presently
describe. A thin plate of metal R, is applied closely to the top
of the great boiler D, turning on a centre C, so that by moving a
lever applied to the axis C on the outside of the top, the sliding
plate R can be brought from the mouth of the one tube to the mouth
of the other alternately. This sliding valve is called the
_regulator_, since it is by it that the communications between the
boiler and two steam vessels (hereafter described) are alternately
opened and closed, the lever which effects this being moved at
intervals by the hand of the attendant.
Two _gauge cocks_ are represented at G, G′, the use of which is to
determine the depth of water in the boiler. One, G, has its lower
aperture a little above the proper depth; and the other, G′, a
little below it. Cocks are attached to the upper ends G, G′, which
can be opened or closed at pleasure. The steam collected in the
top of the boiler pressing on the surface of the water, forces it
up in the tubes G, G′, if their lower ends be immersed. Upon
opening the cocks G, G′, if water be forced from both, there is
too much water in the boiler, since the mouth of G is _below_ its
level. If steam issue from both, there is too little water in the
boiler, since the mouth of G′ is _above_ its level. But if steam
issue from G, and water from G′, the water in the boiler is at its
proper level. This ingenious contrivance for determining the level
of the water in the boiler is the invention of Savery, and is used
in many instances at the present day.
The mouth of the pipe G should be at a level of a little less
[Pg051] than one third of the whole depth, and the mouth of G′ at
a level little lower than one third; for it is requisite that
about two thirds of the boiler should be kept filled with water.
The tube I forms a communication between the greater boiler D and
the lesser or feeding boiler E, descending nearly to the bottom of
it. This communication can be opened and closed at pleasure by the
cock K. A gauge pipe is inserted similar to G, G′, but extending
nearly to the bottom. From this boiler a tube F extends, which is
continued to a cistern C (_fig._ 12.), and a cock is placed at M,
which, when opened, allows the water from the cistern to flow into
the feeding boiler E, and which is closed when that boiler is
filled. The manner in which this cistern is supplied will be
described hereafter.
Let us now suppose that the principal boiler is filled to the
level between the gauge pipes, and that the subsidiary boiler is
nearly full of water, the cock K and the gauge cocks G G′ being
all closed. The fire being lighted beneath D, and the water
boiled, steam is produced, and is transmitted through one or other
of the tubes T, T′, to the working apparatus. When evaporation has
reduced the water in D below the level of G′, it will be necessary
to replenish the boiler D. This is effected thus:—A fire being
lighted beneath the feeding boiler E, steam is produced in it
above the surface of the water, which, having no escape, presses
on the surface so as to force it up in the pipe I. The cock K
being then opened, the boiling water is forced into the principal
boiler D, into which it is allowed to flow until water issues from
the gauge cock G′. When this takes place, the cock K is closed,
and the fire removed from E until the great boiler again wants
replenishing. When the feeding boiler E has been exhausted, it is
replenished from the cistern C (_fig._ 12.), through the pipe F,
by opening the cock M.
(32.) We shall now describe the working apparatus in which the
steam is used as a moving power.
Let V V′ (_fig._ 12.) be two steam vessels communicating by the
tubes T T′ (marked by the same letters in _fig._ 11.) with the
greater boiler D.
[Illustration: _Fig._ 12.]
Let S be a pipe, called the _suction pipe_, descending into
[Pg052] the well or reservoir from which the water is to be
raised, and communicating with each of the steam vessels through
tubes D D′, by valves A A′, which open upwards. Let F be a pipe
continued from the level of the engine to whatever higher level it
is intended to elevate the water. The steam vessels V V′
communicate with the _force-pipe_ F by valves B B′, which open
upwards, through the tubes E E′. Over the steam vessels and on the
force-pipe is placed a small cistern C, already mentioned, which
is kept filled with cold water from the force-pipe, and from the
bottom of which proceeds a pipe terminated with a cock G. This is
called the _condensing pipe_, and can be brought alternately over
each steam vessel. From this cistern another pipe communicates
with the feeding boiler (_fig._ 11.), by the cock M.[9]
The communication of the pipes T T′ with the boiler can be opened
and closed alternately, by the regulator R (_fig._ 11.), already
described.
Now suppose the steam vessels and tubes to be all filled with
common atmospheric air, and that the regulator be placed so that
the communication between the tube T and the boiler be opened, the
communication between the other tube T′ and the boiler being
closed, steam will flow into V through T. At first, while the
vessel V is cold, the steam will be condensed, and will fall in
drops of water on the bottom and sides of the vessel. The
continued supply of steam from the boiler will at length impart
such a degree of heat to the vessel V, that it will cease to
condense it. Mixed with the heated air [Pg053] contained in the
vessel V, it will have an elastic force greater than the
atmospheric pressure, and will therefore force open the valve B,
through which a mixture of air and steam will be driven until all
the air in the vessel V will have passed out, and it will contain
nothing but the pure vapour of water.
When this has taken place, suppose the regulator be moved so as to
close the communication between the tube T and the boiler, and to
stop the further supply of steam to the vessel V; and at the same
time let the condensing pipe G be brought over the vessel V, and
the cock opened so as to let a stream of cold water flow upon it.
This will cool the vessel V, and the steam with which it is filled
will be condensed and fall in a few drops of water, leaving the
interior of the vessel a vacuum. The valve B will be kept closed
by the atmospheric pressure. But the elastic force of the air
between the valve A and the surface of the water in the well, or
reservoir, will open A, so that a part of this air will rush in,
and occupy the vessel V. The air in the suction pipe S, being thus
allowed an increased space, will be proportionally diminished in
its elastic force, and its pressure will no longer balance that of
the atmosphere acting on the external surface of the water in the
reservoir. This pressure will, therefore, force water up in the
tube S until its weight, together with the elastic force of the
air above it, balances the atmospheric pressure. When this has
taken place, the water will cease to ascend.
Let us now suppose that, by shifting the regulator, the
communication is opened between T and the boiler, so that steam
flows again into V. The condensing cock G being removed, the
vessel will be again heated as before, the air expelled, and its
place filled by the steam. The condensing pipe being again allowed
to play upon the vessel V, and the further supply of steam being
stopped, a vacuum will be produced in V, and the atmospheric
pressure will force the water through the valve A into the vessel
V, which it will nearly fill, a small quantity of air, however,
remaining above it.
Thus far the mechanical agency employed in elevating the water is
the atmospheric pressure; and the power of steam is no further
employed than in the production of a vacuum. [Pg054] But, in
order to continue the elevation of the water through the force
pipe F, above the level of the steam vessel, it will be necessary
to use the elastic pressure of the steam. The vessel V is now
nearly filled by the water which has been forced into it by the
atmosphere. Let us suppose that, the regulator being shifted
again, the communication between the tube T and the boiler is
opened, the condensing cock removed, and that steam flows into V.
At first, coming in contact with the cold surface of the water and
that of the vessel, it is condensed; but the vessel is soon
heated, and the water formed by the condensed steam collects in a
sheet or film upon the surface of the water in V, so as to form a
surface as hot as boiling water.[10] The steam then being no
longer condensed, presses on the surface of the water with its
elastic force; and when that pressure becomes greater than the
atmospheric pressure, the valve B is forced open, and the water
issuing through it, passes through E into the force-pipe F; and
this is continued until the steam has forced all the water from V,
and occupies its place.
The further admission of steam through T is once more stopped by
moving the regulator; and the condensing pipe being again allowed
to play on V, so as to condense the steam which fills it, produces
a vacuum. Into this vacuum, as before, the atmospheric pressure
will force the water, and fill the vessel V. The condensing pipe
being then closed, and steam admitted through T, the water in V
will be forced by its pressure through the valve B and tube E into
F, and so the process is continued.
We have not yet noticed the other steam vessel V′, which, as far
as we have described, would have remained filled with common
atmospheric air, the pressure of which on the valve A′ would have
prevented the water raised in the suction pipe S from passing
through it. However, this is not the case; for, during the entire
process which has been described in V, similar effects have been
produced in V′, which we have only omitted to notice to avoid the
confusion which the two processes might produce. It will be
remembered, that after the steam, in the first instance, having
flowed from the boiler [Pg055] through T, has blown the air out
of V through B, the communication between T and the boiler is
closed. Now the same motion of the regulator which closes this,
opens the communication between T′ and the boiler; for the sliding
plate R (_fig._ 11.) is moved from the one tube to the other, and
at the same time, as we have already stated, the condensing pipe
is brought to play on V. While, therefore, a vacuum is being
formed in V by condensation, the steam, flowing through T′, blows
out the air through B′, as already described in the other vessel
V; and while the air in S is rushing up through A into V, followed
by the water raised in S by the atmospheric pressure, the vessel
V′ is being filled with steam, and the air is completely expelled
from it.
The communication between T and the boiler is now again opened,
and the communication between T′ and the boiler closed by moving
the regulator R (_fig._ 11.) from the tube T to T′; at the same
time the condensing pipe is removed from over V, and brought to
play upon V′. While the steam once more expels the air from V
through B, a vacuum is formed by condensation in V′, into which
the water in S rushes through the valve A′. In the mean time V is
again filled with steam. The communication between T and the
boiler is now closed, and that between T′ and the boiler is
opened, and the condensing pipe removed from V′, and brought to
play on V. While the steam from the boiler forces the water in V′
through B′ into the force-pipe F, a vacuum is being produced in V,
into which water is raised by the atmospheric pressure.
Thus each of the vessels V V′ is alternately filled from S, and
the water thence forced into F. The same steam which forces the
water from the vessels into F, having done its duty, is condensed,
and brings up the water from S, by giving effect to the
atmospheric pressure.
During this process, two alternate motions or adjustments must be
constantly made; the communication between T and the boiler must
be opened, and that between T′ and the boiler closed, which is
done by one motion of the regulator. The condensing pipe at the
same time must be brought from V to play on V′, which is done by
the lever placed upon it. Again [Pg056] the communication between
T′ and the boiler is to be opened, and that between T and the
boiler closed; this is done by moving back the regulator. The
condensing pipe is brought from V′ to V by moving back the other
lever, and so on alternately.
For the clearness and convenience of description, some slight and
otherwise unimportant changes have been made in the position of
the parts. A perspective view of this engine is represented at the
head of this chapter. The different parts already described will
easily be recognised.
The engine of Savery was very clearly described in a small work
published in London in 1702, entitled, _The Miner's Friend, or an
Engine to raise Water by Fire described, and the Manner of Fixing
it in Mines; with an Account of the several Uses it is applicable
unto, and an Answer to the Objection made against it; by Thomas
Savery, Gentleman_. This volume was dedicated to William III. (to
whom the engine had been exhibited at Hampton Court palace), to
the Royal Society, and to the mining adventurers of England. The
following are the uses to which Savery proposed the engine should
be applied: _First_, to raise water for turning all sorts of
mills; _second_, supplying palaces and houses with water, and
supplying means of extinguishing fire therein by the water so
raised; _third_, the supplying cities and towns with water;
_fourth_, draining fens or marshes; _fifth_, for ships; _sixth_,
the drainage of mines.
Dr. Harris, in his _Lexicon Technicum, or Dictionary of Arts and
Sciences_, mentions a machine of Savery's for propelling a vessel
in a calm, by paddle-wheels placed at the side; but it does not
appear that Savery contemplated the application of a steam engine
to work these wheels.
It is only from scattered passages in publications of the day that
it can be ascertained to what extent the engines of Savery were
practically applied. In his address to the Royal Society, he
speaks of the "difficulties and expense which he encountered in
instructing artisans to make engines according to his wish; but
that after much experience the workmen had become such masters of
the thing, that they bound themselves to deliver the engines
'exactly tight and fit for [Pg057] service, and such as he
(Savery) dare warrant them to every one that has occasion for
them.'"
In his address to the miners of England he also says, "that the
frequent disorders and cumbersomeness of water engines then in use
encouraged him to invent engines to work by this new force; that
though they were obliged to encounter the oddest and almost
insuperable difficulties, yet he spared neither time, pains, nor
money, till he had conquered them."
In Bradley's _Improvements of Planting and Gardening_, 1718, the
author thus speaks of an engine erected by Savery:—
"Supposing the situation of a house or garden to be a considerable
height above any pond, river, or spring, and that it has at
present no other conveniency of water than what is brought
continually by men or horses to it. In this case, the wonderful
invention of the late Mr. Savery, F.R.S., for raising water by
fire, will not only supply the defect, by flinging up as much
water as may be desired, but may be maintained with very little
trouble and very small expense.
"It is now about six years since Mr. Savery set up one of them for
that curious gentleman Mr. Balle, at Cambden House, Kensington,
near London, which has succeeded so well that there has not been
any want of water since it has been built; and, with the
improvements since made to it, I am apt to believe will be less
subject to be out of order than any engine whatever."
It is remarkable that, notwithstanding the high pressure steam
necessary for the operation of Savery's engine, he does not appear
to have adopted the obvious expedient of a safety valve. The
safety valve had been previously known, having been invented about
the year 1681, by Papin, for his digester, which was a close
boiler, contrived by him for stewing meat and digesting bones, by
submitting them to a higher temperature than that of water boiling
in an open vessel.
The safety valve which has ever since been used for steam boilers
of every kind is a valve which opens outwards, and is fitted to an
aperture in the boiler, so as to be steam tight. It is pressed
down by a weight, the amount of which is regulated by the maximum
pressure to which it is intended the steam [Pg058] shall be
limited. Thus, if the magnitude of the valve be a square inch, and
the pressure of the steam be limited to 10 lbs. per square inch
above the pressure of the atmosphere, then the valve would be
loaded with a weight of 10 lbs.; but as it was found necessary to
vary from time to time the limiting pressure of the steam, or the
load of the safety valve, these valves were usually constructed so
as to be held down by the pressure of a lever having a sliding
weight upon it. By moving the weight on the arm of the lever, the
pressure on the valve could be increased or diminished at the
discretion of the engineer. This contrivance was first applied to
Savery's engines, by Desaguliers, about the year 1717, before
which year Savery died.
It is justly observed by Mr. Farey, in his treatise on the steam
engine, that, "when a comparison is made between Captain[11]
Savery's engine and those of his predecessors, the result will be
in every respect favourable to his character as an inventor, and
as a practical engineer; all the details of his invention are made
out in a masterly style, and accidents and contingencies are
provided for, so as to render it a real working engine; whereas De
Caus, the Marquis of Worcester, Sir Samuel Morland, and Papin,
though ingenious philosophers, only produced mere outlines, which
required great labour and skill of subsequent inventors to fill
up, and make them sufficiently complete to be put in execution."
About the year 1718 further improvements were made in the
construction of Savery's engine, by Dr. Desaguliers; but it is
probable that some of these were suggested by the proceedings of
the inventors of the atmospheric engine, which shall presently
describe.
(33.) In order duly to appreciate the value of improvements, it is
necessary first to perceive the defects which these improvements
are designed to remove. Savery's steam engine, considering how
little was known of the value and properties of steam, and how low
the general standard of mechanical knowledge was in his day, is
certainly highly [Pg059] creditable to his genius. Nevertheless
it had very considerable defects, and was finally found to be
inefficient for the most important purposes to which he proposed
applying it.
At the time of this invention, the mines in England had greatly
increased in depth, and the process of draining them had become
both expensive and difficult; so much so, that it was found in
many instances that their produce did not cover the cost of
working them. The drainage of these mines was the most important
purpose to which Savery proposed to apply his steam engine.
It has been already stated that the pressure of the atmosphere
amounts to about fifteen pounds on every square inch. Now, a
column of water, whose base is one square inch, and whose height
is thirty-four feet, weighs about fifteen pounds. If we suppose
that a perfect vacuum were produced in the steam vessels V V′
(_fig._ 12.) by condensation, the atmospheric pressure would fail
to force up the water, if the height of the top of these vessels
above the water to be raised exceeded thirty-four feet. It is
plain, therefore, that the engine cannot be more than thirty-four
feet above the water which it is intended to elevate. But in fact
it cannot be so much; for the vacuum produced in the steam vessels
V V′ is never perfect. Water, when not submitted to the pressure
of the atmosphere, will vaporise at a very low temperature, as we
shall hereafter explain; and it was found that a vapour possessing
a considerable elasticity would, notwithstanding the condensation,
remain in the vessels V V′ and the pipe S, and would oppose the
ascent of the water. In consequence of this, the engine could
never be placed with practical advantage at a greater height than
twenty-six feet above the level of the water to be raised.
(34.) When the water is elevated to the engine, and the steam
vessels filled, if steam be introduced above the water in V, it
must first balance the atmospheric pressure, before it can force
the water through the valve B. Here, then, is a mechanical
pressure of fifteen pounds per square inch expended, without any
water being raised by it. If steam of twice that elastic force be
used, it will elevate a column in F of thirty-four feet in height;
and if steam of triple the force be used, it will raise a column
of sixty-eight feet high, [Pg060] which, added to twenty-six feet
raised by the atmosphere, gives a total lift of ninety-four feet.
In effecting this, steam of a pressure equal to three times that
of the atmosphere acts on the inner surface of the vessels V V′.
One third of this bursting pressure is balanced by the pressure of
the atmosphere on the external surface of the vessels; but an
effective pressure of thirty pounds per square inch still remains,
tending to burst the vessels. It was found that the apparatus
could not be constructed to bear more than this with safety; and,
therefore, in practice, the lift of such an engine was limited to
about ninety perpendicular feet. In order to raise the water from
the bottom of the mine by these engines, therefore, it was
necessary to place one at every ninety feet of the depth; so that
the water raised by one through the first ninety feet should be
received in a reservoir, from which it was to be elevated the next
ninety feet by another, and so on.
Besides this, it was found that sufficient strength could not be
given to those engines, if constructed upon a large scale.
They were, therefore, necessarily very limited in their
dimensions, and were incapable of raising the water with
sufficient speed. Hence arose a necessity for several engines at
each level, which greatly increased the expense.
(35.) These, however, were not the only defects of Savery's
engines. The consumption of fuel was enormous, the proportion of
heat wasted being much more than what was used in either forcing
up the water, or producing a _vacuum_. This will be very easily
understood by attending to the process of working the engine
already described.
When the steam is first introduced from the boiler into the steam
vessels V V′, preparatory to the formation of a vacuum, it is
necessary that it should heat these vessels up to the temperature
of the steam itself; for until then the steam will be condensed
the moment it enters the vessel by the cold surface. All this
heat, therefore, spent in raising the temperature of the steam
vessels is wasted. Again, when the water has ascended and filled
the vessels V V′, and steam is introduced to force this water
through B B′ into F, it is immediately condensed by the cold
surface in V V′, and does not [Pg061] begin to act until a
quantity of hot water, formed by condensed steam, is collected on
the surface of the cold water which fills these vessels. Hence
another source of the waste of heat arises.
When the steam begins to act upon the surface of the water in V
V′, and to force it down, the cold surface of the vessels is
gradually exposed to the steam, and must be heated while the steam
continues its action; and when the water has been forced out of
the vessel, the vessel itself has been heated to the temperature
of the steam which fills it, all which heat is dissipated by the
subsequent process of condensation. It must thus be evident that
the steam used in forcing up the the water in F, and in producing
a vacuum, bears a very small proportion indeed to what is consumed
in heating the apparatus after condensation.
(36.) There is also another circumstance which increases the
consumption of fuel. The water must be forced through B, not only
against the atmospheric pressure, but also against a column of
sixty-eight feet of water. Steam is therefore required of a
pressure of forty-five pounds on the square inch. Consequently the
water in the boiler must be boiled under this pressure. That this
should take place, it is necessary that the water should be raised
to a temperature considerably above 212°, even so high as 275°;
and thus an increased heat must be given to the boiler.
Independently of the other defects, this intense heat weakened and
gradually destroyed the apparatus.
Savery was the first who suggested the method of expressing the
power of an engine with reference to that of horses. In this
comparison, however, he supposed each horse to work but eight
hours a day, while the engine works for twenty-four hours. This
method of expressing the power of steam engines will be explained
hereafter.
(37.) The failure of the engines proposed by Captain Savery in the
work of drainage, from the causes which have been just mentioned,
and the increasing necessity for effecting this object, arising
from the large property in mines which became every year
unproductive by being flooded, stimulated the ingenuity [Pg062]
of mechanics to contrive some means of rendering those powers of
steam exhibited in Savery's engine available.
Thomas Newcomen, the reputed inventor of the atmospheric engine, was
an ironmonger, or, according to some, a blacksmith, in the town of
Dartmouth in Devonshire. From his personal acquaintance and
intercourse with Dr. Hooke, the celebrated natural philosopher, it
is probable that he was a person of some education, and therefore
likely to be above the position of a blacksmith. Being in the habit
of visiting the tin mines in Cornwall, Newcomen became acquainted
with the engine invented by Savery, and with the causes which led to
its inefficiency for the purposes of drainage.
It has been stated that Papin, about the year 1690, proposed the
construction of an engine working by the atmospheric pressure
acting on one side of a piston against a vacuum produced by the
condensation of steam on the other side. Papin was not conscious
of the importance of this principle; for, so far from ever having
attempted to apply it to practical purposes, he probably never
constructed, even on a small scale, any machine illustrating it.
On the contrary, he abandoned the project the moment he was
informed of the principle and structure of the steam engine of
Savery; and he then proposed an engine for raising water, acting
by the expansive force of steam similar to Savery's, but
abandoning the method of working by a vacuum.
This engine is described by Papin in a work published in 1707.
[Illustration: _Fig._ 13.]
A (_fig._ 13.) is an oval boiler, having a safety-valve B, which
limits the pressure of the steam. It is connected with a cylinder
C, by a curved pipe having a stop-cock at D. A pipe with a
stop-cock G opens from the top of the cylinder into the
atmosphere, and a safety-valve F is placed upon the cylinder. A
hollow copper piston H moves freely in the cylinder, and floats
upon the water. O is a funnel with a valve L in the bottom,
opening downwards, through which the cylinder C may be filled with
water to the level of the top of the funnel. A close air-vessel
communicates with the cylinder C by the curved tube, and has a
valve K opening upwards. The force-pipe through which the water is
raised communicates [Pg063] with the air-vessel I. If the cock D
be shut, and the cock G opened, water poured into the funnel O
will rise into the cylinder C, the air which fills the cylinder
escaping through the open pipe G. When the cylinder is thus filled
with water, let the cock G be closed, and the cock D opened. The
steam from the boiler, after heating the metal of the cylinder,
will force the piston downwards, and drive the water through the
curved tube into the vessel I, from which its return is prevented
by the valve K, which is closed by its weight. The air which
filled the vessel I will then be compressed, and by its elasticity
will drive a column of water up the pipe N. After the contents of
the cylinder have been thus discharged it may be refilled in the
same manner, and the process repeated.
It will be perceived that this project is nothing more than a
reproduction of the engine of the Marquis of Worcester. In the
preface to the work containing this description, Papin gives an
extract from a letter addressed by him to Leibnitz in 1698, from
which it appears that he had abandoned his idea of working the
piston by the atmospheric pressure acting against a vacuum,
considering it to be a contrivance inferior [Pg064] to the engine
now described. "We now raise water," he says, "by the force of
fire, _in a more advantageous manner than that which I had
published some years before_; for besides the suction, we now also
use the pressure which the water exerts upon other bodies in
dilating itself by heat; instead of which I before employed the
suction only, the effects of which are more limited."
From documents which have been preserved in the Royal Society, it
appears that Newcomen was acquainted with Papin's writings, and
therefore probably first derived from them the suggestion which he
subsequently realised in the atmospheric engine. Among some papers
of Dr. Hooke's have been found notes for the use of Newcomen, on
Papin's method of transmitting the force of a stream or fall of
water to a distance by pipes. Hooke dissuaded Newcomen from
attempting any machine on this principle, which, as first proposed
by Papin, was impracticable. He exposed the fallacy of Papin's
first project in several discourses before the Royal Society, and
considered his improved edition of it, though free from fallacy,
as impracticable.
Papin's project for producing a vacuum under a piston by
condensing the steam having been published in the _Actæ
Eruditorum_, in Latin, in 1690, and in French, at Cassel, in 1695,
and subsequently, in the _Philosophical Transactions_, in England
in 1697, cannot be supposed to be unknown to Dr. Hooke; and if
known to him, would probably have been communicated to Newcomen.
Dr. Hooke died in 1703, some years before the date of Newcomen's
invention.
John Cawley, who was the associate of Newcomen in his experiments
and inquiries, was a plumber and glazier of the same town.
Newcomen and Cawley obtained a patent for the atmospheric engine
in 1705, in which Savery was associated, he having previously
obtained a patent for the method of producing a vacuum by the
condensation of steam, which was essential to Newcomen's
contrivance. It was not, however, until about the year 1711 that
any engine had been constructed under this patent.
In the latter end of that year, according to Desaguliers, the
patentees "made proposals to drain a colliery at Griff, in
[Pg065] Warwickshire, in which work five hundred horses were
constantly employed. This proposal not being accepted, they
contracted, in the following March, to drain water for Mr. Back of
Wolverhampton, where, after many laborious attempts, they
succeeded in making their engine work; but not being either
philosophers to understand the reason, or mathematicians enough to
calculate the power and proportions of the parts, they very
luckily, by accident, found what they sought for."
[Illustration: _Fig._ 14.]
Newcomen resumed the old method of raising the water from the
mines by ordinary pumps, but conceived the idea of working these
pumps by some moving power less expensive than that of horses. The
means whereby he proposed effecting this, was by connecting the
end of the pump-rod D (_fig._ 14.) by a chain with the arch head A
of a [Pg066] working-beam A B, playing on an axis C. The other
arch head B of this beam was connected by a chain with the rod E
of a solid piston P, which moved air-tight in a cylinder F. If a
vacuum be created beneath the piston P, the atmospheric pressure
acting upon it will press it down with a force of fifteen pounds
per square inch; and the end A of the beam being thus raised, the
pump-rod D will be drawn up. If a pressure equivalent to the
atmosphere be then introduced below the piston, so as to
neutralise the downward pressure, the piston will be in a state of
indifference as to the rising or falling; and if in this case the
rod D be made heavier than the piston and its rod, so as to
overcome the friction, it will descend, and elevate the piston
again to the top of the cylinder. The vacuum being again produced,
another descent of the piston, and consequent elevation of the
pump-rod, will take place; and so the process may be continued.
Such was Newcomen's first conception of the _atmospheric engine_;
and the contrivance had much, even at the first view, to recommend
it. The power of such a machine would depend entirely on the
magnitude of the piston; and being independent of highly elastic
steam, would not expose the materials to the destructive heat
which was necessary for working Savery's engine. Supposing a
perfect vacuum to be produced under the piston in the cylinder, an
effective downward pressure would be obtained, amounting to
fifteen times as many pounds as there are square inches in the
section of the piston.[12] Thus, if the base of the piston were
100 square inches, a pressure equal to 1500 pounds would be
obtained.
(38.) In order to accomplish this, two things were necessary: 1.
To make a speedy and effectual vacuum below the [Pg067] piston in
the descent; and, 2. To contrive a counterpoise for the atmosphere
in the ascent.
The condensation of steam immediately presented itself as the most
effectual means of accomplishing the former; and the elastic force
of the same steam previous to condensation an obvious method of
effecting the latter. Nothing now remained to carry the design
into execution, but the contrivance of means for the alternate
introduction and condensation of the steam; and Newcomen and
Cawley were accordingly granted a patent in 1707, in which Savery
was united, in consequence of the principle of condensation for
which he had previously received a patent being necessary to the
projected machine. We shall now describe the _atmospheric engine_,
as first constructed by Newcomen:—
The boiler K (_fig._ 14.) is placed over a furnace I, the flue of
which winds round it, so as to communicate heat to every part of
the bottom of it. In the top, which is hemispherical, two
gauge-cocks G G′ are placed, as in Savery's engine, and a _puppet
valve_ V, which opens upward, and is loaded at one pound per
square inch; so that when the steam produced in the boiler exceeds
the pressure of the atmosphere by more than one pound on the
square inch, the valve V is lifted, and the steam escapes through
it, and continues to escape until its pressure is sufficiently
diminished, when the valve V again falls into its seat. This valve
performs the office of the safety-valve in modern engines.
The great steam-tube is represented at S, which conducts steam
from the boiler to the cylinder; and a feeding pipe T, furnished
with a cock, which is opened and closed at pleasure, proceeds from
a cistern L to the boiler. By this pipe the boiler may be
replenished from the cistern, when the gauge cock G′ indicates
that the level has fallen below it. The cistern L is supplied with
hot water, by means which we shall presently explain.
(39.) To understand the mechanism necessary to work the piston,
let us consider how the supply and condensation of steam must be
regulated. When the piston has been forced to the bottom of the
cylinder by the atmospheric pressure acting against a vacuum, in
order to balance that pressure, [Pg068] and enable it to be drawn
up by the weight of the pump-rod, it is necessary to introduce
steam from the boiler. This is accomplished by opening the cock R
in the steam pipe S. The steam being thus introduced from the
boiler, its pressure balances the action of the atmosphere upon
the piston, which is immediately drawn to the top of the cylinder
by the weight of the pump-rod D. It then becomes necessary to
condense this steam, in order to produce a vacuum. To accomplish
this, the further supply of steam must be cut off, which is done
by closing the cock R. The supply of steam from the boiler being
thus suspended, the application of cold water on the external
surface of the cylinder becomes necessary to condense the steam
within it. This was done by enclosing the cylinder within another,
leaving a space between them.[13] Into this space cold water was
allowed to flow from a cock M placed over it, supplied by a pipe
from the cistern N. This cistern is supplied with water by a pump
O, which is worked by the engine.
The cold water supplied from M, having filled the space between
the two cylinders, abstracts the heat from the inner one; and
condensing the steam, produces a vacuum, into which the piston is
forced by the atmospheric pressure. Preparatory to the next
descent, the water which thus fills the space between the
cylinders, and which is warmed by the heat abstracted from the
steam, must be discharged, in order to give room for a fresh
supply of cold water from M. An aperture, furnished with a cock,
is accordingly provided in the bottom of the cylinder, through
which the water is discharged into the cistern L; and being warm,
is adapted for the supply of the boiler through T, as already
mentioned.
The cock R being now again opened, steam is admitted below the
piston, which, as before, ascends, and the descent is again
accomplished by closing the cock R, and opening the cock M,
admitting cold water between the cylinders, and thereby condensing
the steam below the piston.
The condensed steam, thus reduced to water, will collect [Pg069]
in the bottom of the cylinder, and resist the descent of the
piston. It is therefore necessary to provide an exit for it, which
is done by a valve opening _outwards_ into a tube which leads to
the feeding cistern L, into which the condensed steam is driven.
That the piston should continue to be air-tight, it was necessary
to keep a constant supply of water over it; this was done by a
cock similar to M, which allowed water to flow from the pipe M on
the piston.
(40.) Soon after the first construction of these engines, an
accidental circumstance suggested to Newcomen a much better method
of condensation than the application of cold water on the external
surface of the cylinder. An engine was observed to work several
strokes with unusual rapidity, and without the regular supply of
the condensing water. Upon examining the piston, a hole was found
in it, through which the water, which was poured on to keep it
air-tight, flowed, and instantly condensed the steam under it.
On this suggestion Newcomen abandoned the external cylinder, and
introduced a pipe H, furnished with a cock Q, into the bottom of
the cylinder, so that, on turning the cock, the pressure of the
water in the pipe H, from the level of the water in the cistern N,
would force the water to rise as a jet into the cylinder, and
would instantly condense the steam. This method of condensing by
injection formed a very important improvement in the engine, and
is still used.
(41.) Having taken a general view of the parts of the atmospheric
engine, let us now consider more particularly its operation.
When the engine is not working, the weight of the pump-rod D
(_fig._ 14.) draws down the beam A, and draws the piston to the
top of the cylinder, where it rests. Let us suppose all the cocks
and valves closed, and the boiler filled to the proper depth. The
fire being lighted beneath it, the water is boiled until the steam
acquires sufficient force to lift the valve V. When this takes
place, the engine may be started. For this purpose the regulating
valve R is opened. The steam rushes in, and is first condensed by
the cold cylinder. After a short time the cylinder acquires the
temperature of the steam, which then [Pg070] ceases to be
condensed, and mixes with the air which filled the cylinder. The
steam and heated air, having a greater force than the atmospheric
pressure, will open a valve placed at the end X of a small tube in
the bottom of the cylinder, and which opens outwards. From this
(which is called the _blowing valve_[14]) the steam and air rush
in a constant stream, until all the air has been expelled, and the
cylinder is filled with the pure vapour of water. This process is
called _blowing_ the engine preparatory to starting it.
When it is about to be started, the engine-man closes the
regulator R, and thereby suspends the supply of steam from the
boiler. At the same time he opens the _condensing valve_ H[15];
and thereby throws up a jet of cold water into the cylinder. This
immediately condenses the steam contained in the cylinder, and
produces the vacuum. (The atmosphere cannot enter the _blowing_
valve, because it opens _outwards_, so that no air can enter to
vitiate the vacuum.) The atmospheric pressure above the piston now
takes effect, and forces it down in the cylinder. The descent
being completed, the engine-man closes the condensing valve H, and
opens the regulator, R. By this means he stops the play of the jet
within the cylinder, and admits the steam from the boiler. The
first effect of the steam is to expel the condensing water and
condensed steam which are collected in the bottom of the cylinder,
through the tube Y, containing a valve which opens _outwards_
(called the _eduction valve_), which leads to the hot cistern L,
into which this water is therefore discharged.
When the steam admitted through R ceases to be condensed, it
balances the atmospheric pressure above the piston, and thus
permits it to be drawn to the top of the cylinder by the weight of
the rod D. This ascent of the piston is also assisted by the
circumstance of the steam being somewhat stronger than the
atmosphere.
When the piston has reached the top, the regulating valve R is
closed, and the condensing valve H opened, and another descent
produced, as before, and so the process is continued. [Pg071]
The manipulation necessary in working this engine was, therefore,
the alternate opening and closing of two valves; the regulating
and condensing valves. When the piston reached the top of the
cylinder, the former was to be closed, and the latter opened; and,
on reaching the bottom, the former was to be opened, and the
latter closed.
(42.) The duty of working the engine requiring no great amount of
labour, or skill, was usually entrusted to boys, called, _cock
boys_. It happened that one of the most important improvements
which has ever been made in the working of steam engines was due
to the ingenuity of one of these boys. It is said that a lad,
named _Humphrey Potter_, was employed to work the cocks of an
atmospheric engine, and being tempted to escape from the
monotonous drudgery to which his duty confined him, his ingenuity
was sharpened so as to prompt him to devise some means by which he
might indulge his disposition to play without exposing himself to
the consequences of suspending the performance of the engine. On
observing the alternate ascending and descending motion of the
beam above him, and considering it in reference to the labour of
his own hands, in alternately raising and lowering the levers
which governed the cocks, he perceived a relation which served as
a clue to a simple contrivance, by which the steam engine, for the
first time, became an automaton. When the beam arrived at the top
of its play, it was necessary to open the steam valve by raising a
lever, and to close the injection valve by raising another. This
he saw could be accomplished by attaching strings of proper length
to these levers, and tying them to some part of the beam. These
levers required to be moved in the opposite direction when the
beam attained the lowest point of its play. This he saw could be
accomplished by strings, either connected with the outer arm of
the beam, or conducted over rods or pulleys. In short, he
contrived means of so connecting the levers which governed the two
cocks by strings with the beam, that the beam opened and closed
these cocks with the most perfect regularity and certainty as it
moved upwards and downwards.
Besides rendering the machine independent of manual [Pg072]
superintendence, this process conferred upon it much greater
regularity of performance than any manual superintendence could
ensure.
This contrivance of Potter was very soon improved by the
substitution of a bar, called a _plug frame_, which was suspended
from the arm of the beam, and which carried upon it pins, by which
the arms of the levers governing the cocks were struck as the
plug-frame ascended and descended, so as to be opened and closed
at the proper times.
The engine thus improved required no other attendance except to
feed the boiler occasionally by the cock T, and to attend the
furnace.
(43.) However the merit of the discovery of the physical
principles on which the mechanical application of steam depends
may be awarded, it must be admitted that the engine contrived by
Newcomen and his associates, considered as a practical machine,
was immeasurably superior to that which preceded it; superior,
indeed, to such a degree, that while the one was incapable of any
permanently useful application, the other soon became a machine of
extensive utility in the drainage of mines; and, even at the
present time, the atmospheric engine is not unfrequently used in
preference to the modern steam engine, in districts where fuel is
abundant and cheap; the expense of constructing and maintaining it
being considerably less than that of an improved steam engine. The
low pressure of the steam used in working it, rendered it
perfectly safe. While Savery's engine, to work with effect,
required that the steam confined in the vessels should have a
bursting pressure amounting to about thirty pounds per square
inch, the pressure of steam in the boiler and cylinder of the
atmospheric engine required only a pressure about one pound per
square inch. The high pressure also of the steam used in Savery's
engine, was necessarily accompanied, as we shall presently
explain, by a greatly increased temperature. The effect of this
was, to weaken and gradually destroy the vessels, especially those
which, like the steam vessels V and V′ (_fig._ 12.), were
alternately heated and cooled.
Besides these defects, the power of Savery's engines was [Pg073]
also very restricted, both as to the quantity of water raised and as
to the height to which it was elevated. On the other hand, the
atmospheric engine was limited in its power only by the dimensions
of its piston. Another considerable advantage which the atmospheric
engine possessed over that of Savery, was the facility with which it
was capable of driving machinery by means of the working-beam. The
merit, however, of Newcomen's engine, regarded as an invention, and
apart from merely practical considerations, must be ascribed
principally to its mechanism and combinations. We find in it no new
principle, and scarcely even a novel application of a principle. The
agency of the atmospheric pressure acting against a vacuum, or
partial vacuum, had been long known: the method of producing a
vacuum by the condensation of steam had been suggested by Papin, and
carried into practical effect by Savery. The mechanical power
obtained from the direct pressure of the elastic force of steam,
used in the atmospheric engine to balance the atmosphere during the
ascent of the piston, was suggested by De Caus and Lord Worcester.
The boiler, gauge pipes, and the regulator, were all borrowed from
the engine of Savery. The idea of using the atmospheric pressure
against a vacuum or partial vacuum, to work a piston in a cylinder,
had been suggested by Otto Guericke, an ingenious German
philosopher, who invented the air-pump; and this, combined with the
production of a vacuum by the condensation of steam, was
subsequently suggested by Papin. The use of a working-beam could not
have been unknown. Nevertheless, the judicious combination of these
scattered principles must be acknowledged to deserve considerable
credit. In fact, the mechanism contrived by Newcomen rendered a
machine which was before altogether inefficient, highly efficient:
and, as observed by Tredgold, such a result, considered in a
practical sense, should be more highly valued than the fortuitous
discovery of a physical principle. The method of condensing the
steam by the sudden injection of water, and of expelling the air and
water from the cylinder by the injection of steam, are two
contrivances not before in use, which are quite essential to the
[Pg074] effective operation of the engine. These processes, which
are still necessary to the operation of the improved steam engine,
appear to be wholly due to the inventors of the atmospheric engine.
[Illustration: ATMOSPHERIC ENGINE.]
FOOTNOTES:
[9] This pipe is represented as proceeding from the force-pipe
above the cistern C, in the perspective view of Savery's
engine at the head of this chapter.
[10] Hot water being lighter than cold, floats on the surface.
[11] "Captain" is a title given in Cornwall to the
superintendent of the works connected with a mine.
[12] As the calculation of the power of an engine depends on
the number of square inches in the section of the piston, it
may be useful to give a rule for computing the number of
square inches in a circle. The following rule will always give
the dimensions with sufficient accuracy:—_Multiply the number_
_of inches in the diameter by itself; divide the product by 14,_
_and multiply the quotient thus obtained by 11, and the result_
_will be the number of square inches in the circle_. Thus, if
there be 12 inches in the diameter, this multiplied by itself
gives 144, which divided by 14 gives 10-4/44, which multiplied
by 11 gives 115, neglecting fractions. There are, therefore,
115 square inches in a circle whose diameter is 12 inches.
[13] The external cylinder is not represented in the diagram.
[14] Also called the _snifting_ valve, from the peculiar noise
made by the air and steam escaping from it.
[15] Also called the _injection valve_.
[Pg075]
[Illustration: GREENOCK, IN 1824.]
CHAP. III.
PROGRESS OF THE ATMOSPHERIC ENGINE. — SMEATON'S IMPROVEMENTS.
— BRINDLEY, ENGINEER OF THE BRIDGEWATER CANAL. — INVENTS THE
SELF-REGULATING FEEDER. — JAMES WATT. — HIS DESCENT AND
PARENTAGE. — ANECDOTES OF HIS BOYHOOD. — HIS EARLY AQUIREMENTS.
— GOES TO LONDON. — RETURNS TO GLASGOW. — IS APPOINTED
INSTRUMENT-MAKER TO THE UNIVERSITY. — OPENS A SHOP IN GLASGOW.
— HIS FRIENDS AND PATRONS. — ADAM SMITH. — DR. BLACK. — ROBERT
SIMSON. — PROFESSOR ROBISON. — WATT'S PERSONAL CHARACTER. —
INDUSTRIOUS AND STUDIOUS HABITS. — HIS ATTENTION FIRST
DIRECTED TO STEAM. — EXPERIMENTS ON HIGH-PRESSURE STEAM. —
REPAIRS AN ATMOSPHERIC MODEL. — EXPERIMENTAL INQUIRY
CONSEQUENT ON THIS. — ITS RESULTS. — DISCOVERS THE GREAT
DEFECTS OF THE ATMOSPHERIC ENGINE. — DISCOVERY BY EXPERIMENT
OF THE EXPANSION WHICH WATER UNDERGOES IN EVAPORATION. —
DISCOVERS THE LATENT HEAT OF STEAM. — IS INFORMED BY DR. BLACK
OF THE THEORY OF LATENT HEAT.
(44.) The atmospheric engine was brought to a state of
considerable efficiency and improvement by Mr. Beighton, in 1718.
From that time it continued in use without any change in its
[Pg076] principle, and with little improvement in its structure,
for half a century. Although engines of this kind continued to be
extensively constructed, they were usually executed by ordinary
mechanics, incapable of applying to them the just principles of
practical science; and, consequently, little attention was paid to
their proportions. It was not until about the year 1772, that Mr.
John Smeaton, the celebrated engineer, applied the powers of his
mind to the investigation of this machine, as he had previously
done with such success to wind and water mills. Although he did
not introduce any new principle into the atmospheric engine, yet
it derived greatly augmented power from the proportions which he
established for engines of different magnitudes.
In 1759, Mr. James Brindley, whose name is so celebrated as the
engineer of the Duke of Bridgewater's canal, obtained a patent for
some improvements in the atmospheric engine. He proposed that the
boiler should be made of wood and stone, with a stove or
fire-place of cast iron within it, so that the fire should be
surrounded on every side by water. The chimney was to be an iron
pipe or tube, conducted through the boiler; so that the heated
air, in passing from the fire, should impart a portion of its heat
to the water. He also proposed a method of feeding the boiler,
which, by self-acting machinery, would keep the water in the
boiler at a fixed level, independently of any attention on the
part of the engine-man. This was to be accomplished by a buoy or
float upon the surface of the water in the boiler, which should
communicate with a valve in the feed-pipe, so that when the level
of the water in the boiler fell, the float or buoy, falling with
it, would open the valve and supply the feed. It is stated, in the
_Biographia Britannica_, that Mr. Brindley, in 1756, undertook to
erect an engine at Newcastle-under-Lyne; but he is said to have
been discouraged by the obstacles which were thrown in his way,
and to have abandoned the steam engine.
The interval between the invention of the atmospheric engine, and
the amelioration it received at the hands of Smeaton, has been
rendered memorable by the advent of one who was destined to work a
mighty change in the condition [Pg077] of the human race by the
application of his vast genius to the adaptation of steam power to
the uses of life.
(45.) JAMES WATT was born at Greenock, in Scotland, on the
nineteenth day of January, in the year 1736.[16]
The great-grandfather of Watt, a farmer in Aberdeenshire, was killed
in one of the battles of Montrose. The victorious party, not
thinking death a sufficient expiation for the political opinions in
support of which he had fought and bled, punished him in the person
of his son, by confiscating his little property. Thomas Watt, the
son, thus deprived of support, was received by distant relations,
and, for a time, applied himself to study, by which he was enabled,
after the restoration of tranquillity, to establish himself at
Greenock as a teacher of practical mathematics and navigation. He
resided in the burgh or barony of Crawford's Dyke, and attained a
position of sufficient respectability to be elected to the office of
baron-baillie, or chief magistrate, and died in 1734, at the
advanced age of ninety-two years.
Thomas Watt had two sons. The elder, John, adopted the profession
of his father, and was a teacher of mathematics and navigation at
Glasgow: he died in 1737, at the age of fifty years. The second
son, James, the father of the celebrated engineer, was, during a
quarter of a century, treasurer of the town council of Greenock,
and a local magistrate. He was remarked for the ardent zeal and
enlightened spirit with which he discharged his public duties. His
business was that of a ship-chandler, builder, and general
merchant; but, unhappily, notwithstanding his active industry, he
lost, in the decline of his life, by unsuccessful commercial
speculations, a part of the property which he had so honourably
acquired. He died in 1782, at the age of eighty-four years.
JAMES WATT, to whom the world is so largely indebted for the
extension and improvement of steam power, had from his birth an
extremely delicate constitution. From his mother, [Pg078] whose
family name was Muirhead, he received his first lessons in
reading, and he learned from his father writing and arithmetic.
Although he was entered as a pupil in the grammar school of
Greenock, yet such was his delicate state of health, that his
attendance there was so interrupted by constant indisposition that
he could derive but little benefit from the opportunities of
instruction which it afforded. For a great period of the year he
was confined to his room, where he devoted himself to study
without the aid of instruction. It was in the retirement of the
sick chamber that the high intellectual faculties of Watt, which
were destined to produce such precious fruits, began to unfold
themselves. He was too sickly to be subjected to the restraints
which the business of education usually imposes on children. His
parents, therefore, found it necessary to leave him at liberty to
choose his occupations and amusements. The following anecdotes
will show the use he made of this freedom.
A friend of his father found the boy one day stretched upon the
hearth tracing with chalk various lines and angles. "Why do you
permit this child," said he, "to waste his time so; why not send
him to school?" Mr. Watt replied, "You judge him hastily; before
you condemn us, ascertain how he is employed." On examining the
boy, then six years of age, it was found that he was engaged in
the solution of a problem of Euclid!
Having observed the tendency of his son's mind, Mr. Watt placed at
his disposal a collection of tools. These he soon learned to use
with the greatest skill. He took to pieces and put together, again
and again, all the children's toys which he could procure; and he
was constantly employed in making new ones. Subsequently he used
his tools in constructing a little electrical machine, the sparks
proceeding from which became a great subject of amusement to all
the playfellows of the poor invalid.
Though endowed with great retentive powers, Watt would probably
never have figured among the prodigies of a common school: he would
have been slow to commit his lessons to memory, from the repugnance
which he would feel to repeat like a parrot anything which he did
not perfectly [Pg079] understand. The natural tendency of his mind
to meditate on whatever came before it, would give him, to
superficial observers, the appearance of dullness. Happily, however,
he had a parent who was sufficiently clear-sighted, and who
entertained high hopes of the growing faculties of his son. More
distant and less sagacious relations were not so sanguine. One day
Mrs. Muirhead, the aunt of the boy, reproaching him for what she
conceived to be listless idleness, desired him to take a book and
occupy himself usefully. "More than an hour has now passed away,"
said she, "and you have not uttered a single word. Do you know what
you have been doing all this time? You have taken off, and put on,
repeatedly, the lid of the tea-pot; you have been holding the
saucers and the spoons over the steam, and you have been
endeavouring to catch the drops of water formed on them by the
vapour. Is it not a shame for you to waste your time so?"
Mrs. Muirhead was little aware that this was the first experiment
in the splendid career of discovery which was subsequently to
immortalise her little nephew. She did not see, as we now can, in
the little boy playing with the tea-pot, the great engineer
preluding to those discoveries which were destined to confer on
mankind benefits so inestimable.
One of the social qualities of mind which was remarkable
throughout his life, was the singular felicity and grace with
which he related anecdotes. This power was manifested even in his
earliest childhood. The following is an extract from a letter
written by Mrs. Marion Campbell, his cousin, and the playfellow of
his childhood:—
"He was not fourteen when his mother brought him to Glasgow to
visit a friend of hers; his brother John accompanied him. On Mrs.
Watt's return to Glasgow, some weeks after, her friend said, 'You
must take your son James home; I cannot stand the degree of
excitement he keeps me in; I am worn out for want of sleep. Every
evening before ten o'clock, our usual hour of retiring to rest, he
contrives to engage me in conversation, then begins some striking
tale, and, whether humorous or pathetic, the interest is so
overpowering that the family all listen to him with breathless
attention, and hour after hour strikes unheeded.'" [Pg080]
Watt had a younger brother, John, who was subsequently lost by
shipwreck, in a voyage from Scotland to the United States. This lad,
having determined on following the business of his father, left James
more completely at liberty to choose his own occupation. But such a
choice was difficult for a student who commanded equal success in
every thing to which he directed his attention.
The excursions which he was in the habit of making on the Scottish
mountains surrounding Loch Lomond, naturally directed his
attention to botany and mineralogy, in each of which he attained
considerable knowledge. His love of anecdote and romance was
likewise gratified by the scenery which he enjoyed in these walks;
and the traditions and popular songs with which they made him
acquainted. When from ill-health, as constantly happened, he was
confined to the house, he devoted himself to chemistry, natural
philosophy, and even to medicine and surgery. In chemistry he
acquired some experimental skill, and studied with eager zeal the
elements of natural philosophy by S'. Gravesande. His own unhappy
maladies prompted him to read works on surgery and medicine; and
to such an extent did the activity of his mind impel him on these
subjects, that he was found one day dissecting, in his room, the
head of a child, who had died of some unknown disease, with a view
to ascertain the cause of its death.
In 1775, at the age of nineteen, at the recommendation of Dr.
Dick, professor of natural philosophy in the university of
Glasgow, he went to London, where he employed himself in the house
of Mr. John Morgan, a mathematical instrument maker, in Finch
Lane, Cornhill, to whom he apprenticed himself for three years. He
remained, however, only a year, at the expiration of which
(probably owing to his delicate state of health) he was released
from his apprenticeship, and returned to Glasgow, with the
intention of establishing himself in business as an optician and
mathematical instrument maker. In the fulfilment of this
intention, however, he was obstructed by the interposition of the
Corporation of Trades in that town, who regarded him as an
intruder, not qualified by the necessary apprenticeship to carry
on business. All means of conciliation being [Pg081] exhausted,
the Professors of the University interfered, and gave him the use
of three apartments within the college, for carrying on his
business, and likewise appointed him mathematical instrument maker
to the University. Soon afterwards the opposition of the local
trades seems to have given way, and he opened a shop in Glasgow
for the sale of mathematical instruments.
After the celebrity at which he has arrived, it will be easily
believed that every trace of his earlier connection with Glasgow
college is carefully cherished. There are accordingly preserved at
that place little instruments and pieces of apparatus of exquisite
workmanship, which were executed entirely by the hand of Watt, at
a time when he was not in a condition to command the aid of
workmen under him.
At the time of obtaining this appointment in the University, Watt
was in his twenty-first year. His natural talents and winning
manners were speedily the means of gaining for him the esteem and
friendship of all those eminent persons connected at the time with
that university whose regard was most valued. Among these the
earliest of his friends and patrons were—ADAM SMITH, the author of
"The Wealth of Nations;" BLACK, afterwards celebrated for his
chemical discoveries, and more especially for his theory of latent
heat; and ROBERT SIMSON, rendered illustrious by his works on
ancient geometry. In releasing Watt from the persecution of the
Glasgow corporation, these distinguished persons first imagined that
they were conferring a benefit merely on an industrious and clever
artisan, whose engaging manners won their regard; but a short
acquaintance with him was sufficient to convince them how superior
his mind was to his position, and they conceived towards him the
most lively friendship. His shop became the common rendezvous, the
afternoon lounge, of all who were most distinguished for literary
and scientific attainments among the professors and students. There
they met to discuss the topics of the day in art, science, and
literature. Among these students, the name which afterwards attained
the highest distinctions, and among these distinctions, not the
least, the lasting personal friendship and esteem of Watt himself,
was ROBISON, [Pg082] the author of a well known work on Mechanics,
and one of the contributors to the _Encyclopœdia Britannica_.
The following extract from an unpublished manuscript by Robison
himself will show at once the estimation in which Watt was held,
and will illustrate one of the most interesting traits of his
personal character:—
"I had always, from my earliest youth, a great relish for the
natural sciences, and particularly for mathematical and mechanical
philosophy, when I was introduced by Drs. Simson, Dick, and Moor,
gentlemen eminent for their mathematical abilities, to Mr. Watt. I
saw a workman, and expected no more; but was surprised to find a
philosopher as young as myself, and always ready to instruct me. I
had the vanity to think myself a pretty good proficient in my
favourite study, and was rather mortified at finding Mr. Watt so
much my superior. . . . Whenever any puzzle came in the way of any
of the young students, we went to Mr. Watt. He needed only to be
prompted, for every thing became to him the beginning of a new and
serious study, and we knew that he would not quit it till he had
either discovered its insignificancy, or had made something of it.
He learnt the German language in order to peruse Leupold's
'Theatrum Machinarum;' so did I, to know what he was about.
Similar reasons made us both learn the Italian language. * * *
When to his superiority of knowledge is added the _naïve_
simplicity and candour of Mr. Watt's character, it is no wonder
that the attachment of his acquaintances was strong. I have seen
something of the world, and am obliged to say I never saw such
another instance of general and cordial attachment to a person
whom all acknowledged to be their superior. But that superiority
was concealed under the most amiable candour, and a liberal
allowance of merit to every man. Mr. Watt was the first to ascribe
to the ingenuity of a friend things which were nothing but his own
surmises, followed out and embodied by another. I am the more
entitled to say this, as I have often experienced it in my own
case."
Watt never permitted the inquiries which arose out of these
reunions to interfere with the discharge of the duties of his
workshop. There he passed the day, devoting the [Pg083] night to
study. Every inquiry appeared to him to be attractive in
proportion to its difficulty, and to have charms in proportion as
it was removed from the common routine of his business. As an
example of this may be mentioned the fact, that, being himself so
insensible to the charms of music that he could not distinguish
one note from another, he was actually induced to undertake the
construction of an organ, in which he was nevertheless completely
successful. The instrument he constructed, as might have been
expected, contained many improvements in its mechanism; but what
is much more remarkable, its tone and its musical qualities
commanded the admiration of all the professional musicians who
heard it. In the construction of this instrument Watt showed that
vigorous spirit of investigation which characterised all the
subsequent labours of his life. He made out the scale of
temperament by the aid of the phenomena of beats, of which he
could only obtain a knowledge by a profound but obscure work
published by Dr. Robert Smith of Cambridge.
The earliest occasion on which the attention of Watt is said to
have been called to the agency of steam, was in the year 1759,
when his friend Robison entertained some speculations for applying
that agent as a means of propelling wheel carriages; and he
consulted Watt on the subject. No record, however, has been
preserved of any experiments which were tried on this occasion;
nor does it appear that the inquiry was carried farther than a
verbal discussion, such as habitually took place on other subjects
of science between Watt and his friends.
(46.) In 1762, Watt tried some experiments on the force of steam
at a high pressure, confined in a close digester; and he then
constructed a small model to show how motion could be obtained
from that power. The practicability of what has since been called
the _High Pressure Engine_, was demonstrated by him on this
occasion; but he did not pursue the inquiry, on account of the
supposed danger of working with such compressed steam as was
required.
It is usual to provide, in the cabinets of experimental apparatus
for the instruction of the students of universities, [Pg084]
small working models of the most useful machines. In the
collection for the illustration of the lectures delivered to the
Natural Philosophy class in the University of Glasgow was a
working model of Newcomen's atmospheric engine, applied to a pump
for raising water; which, however, had never been found to work
satisfactorily. The Professor of Experimental Philosophy of that
day, Dr. John Anderson (the founder of the celebrated Andersonian
Institution), sent this model in 1763 to Watt's workshop, to be
repaired. Its defects soon disappeared, and it was made to work to
the satisfaction of the professor and students.
This simple discharge of his duty, however, did not satisfy the
artisan; and his wonted activity of mind rendered this model a
subject of profound meditation, and led him into a course of
practical inquiry respecting it, which formed the commencement of a
most brilliant career of mechanical discovery. The improvement—we
might almost say the creation—of the steam engine, by this great
man, must not therefore be regarded, as so often happens with
mechanical discoveries, as the result of fortuitous observation, or
even of a felicitous momentary inspiration. Watt, on the other hand,
conducted his investigation by a course of deep thought, and of
experiments marked by the last refinement of delicacy and address.
If he had received a more extended and liberal education, one would
have thought that he had adopted for his guide the celebrated maxim
of Bacon:—
"To write, speak, meditate, or act, when we are not provided with
_facts_ to direct our thoughts, is to navigate a coast full of
dangers without a pilot, and to launch into the immensity of the
ocean without either rudder or compass."
The model which he had repaired, had a cylinder of only two inches
diameter, and six inches stroke. After he had put it in complete
order, he found, that although the boiler was much larger in
proportion to the cylinder than those of real engines, yet, that
it was incapable of supplying the cylinder with steam in
sufficient quantity to keep it at work. To enable it to continue
to move, he found it necessary to lessen the quantity of water
raised by its pump, so as to [Pg085] reduce the load on its
piston very much below the proper standard according to the common
rules for large engines.
He ascribed the great inferiority in the performance of the model,
compared with the performance of the large engines, to the small
size of the cylinder, and to its material. The cylinder of the
model was brass, while those of large engines were of cast iron;
and brass being a better conductor of heat than iron, he concluded
that more heat in proportion was lost from this cause in the
model, than in the larger engines. He observed that the small
cylinder was so heated when the steam was admitted into it, that
it could not be touched by the hand; but, nevertheless, that this
heat contributed nothing to its performance, inasmuch as before
the piston descended, the cylinder required to be cooled.
(47.) His first attempt to improve the engine, was by using a
wooden cylinder instead of an iron one. He accordingly made a
model with a cylinder of wood, soaked in linseed oil, and baked to
dryness. With this he made numerous experiments, and found that it
required a less quantity of water to be thrown into the cylinder
to condense the steam, and that it was worked with a less supply
of steam from the boiler than was necessary with the metallic
cylinder.
Still he found that the force with which the piston descended was
considerably less than that which the atmospheric pressure ought
to supply, supposing a tolerably perfect vacuum to be produced
under the piston. This led him to suspect that the water injected
into the cylinder was not perfectly effectual in condensing the
steam. The experiments which he had previously made on the
increased temperature at which water boils under pressures greater
than that of the atmosphere, led him by analogy to the conclusion
that it would boil at lower temperatures if it were submitted to a
pressure less than the atmosphere, and he was aware that Dr.
Cullen and others had then recently discovered that in vacuo,
water would boil at so low a temperature as 100°. These notions
suggested the probability that the water injected into the
cylinder being heated by the condensed steam, might produce vapour
of a low temperature [Pg086] and reduced pressure under the
piston, which would account for the deficiency he observed in the
power of the engine.
No means occurred to him by which he could ascertain, by direct
experiment, the temperatures at which water would boil under
pressures less than that of the atmosphere. He sought, however, to
determine it by the following method. Having ascertained, by
repeating and multiplying the experiments which he had tried in
1762, on high-pressure steam, he obtained a table of the
temperatures at which water boils at various pressures greater
than that of the atmosphere. These results he laid down in a
series forming a curve, of which the abscissa represented the
temperatures, and the ordinates the pressures. He then continued
this curve, backwards as it were, and obtained, by analogy, an
approximation to the boiling temperatures, corresponding to
pressures less than that of the atmosphere. In other words, having
obtained by his experiments a notion, however imperfect, of the
law or rule observed by the temperatures at which water boils at
different pressures _greater_ than that of the atmosphere, he
calculated by the same law or rule what the pressures would be at
different pressures _less_ than that of the atmosphere.
Applying these results to the interior of the cylinder of the
atmospheric engine, he obtained an approximation to the pressure
of the vapour which would be produced from the warm water formed
by the cold water injected into the cylinder, and the steam
condensed by it; and he accordingly found that vapour, having a
pressure seriously injurious to the power of the engine would be
produced in the cylinder, unless considerably more water of
injection was thrown in than was customary.
It was apparent that the actual quantity of steam usefully
employed in the cylinder at each stroke, was only the quantity
which filled the cylinder; and therefore, in order to ascertain
the quantity of steam lost by the imperfections of the machine, it
was necessary to compare the actual quantity of steam transmitted
by the boiler to the cylinder at each stroke, with the quantity
which would just fill the cylinder. The difference would of course
be wasted. But to determine [Pg087] the actual quantity of steam
supplied by the boiler to the cylinder, there was no other means
than by observing the quantity of water evaporated in the boiler.
That being observed, it was necessary to know the quantity of
steam which that water formed; and it was therefore necessary to
determine the quantity or volume of steam which a given volume of
water produced.
(48.) On considering more attentively the operation of the
machine, the following circumstances gradually unfolded themselves
to him.
Let us suppose the piston at the top of the cylinder, and the
space in the cylinder below it, filled with steam so as to balance
the pressure of the atmosphere above the piston. Under such
circumstances the steam, as will presently be explained, must have
the temperature of boiling water. But that the steam should have,
and should maintain, this temperature, it was evidently necessary
that the inner surface of the cylinder in contact with it should
have the same temperature: for if it had a lower temperature, it
would take heat from the steam, and reduce the temperature of the
latter. Now the cylinder being a mass of metal, has a quality in
virtue of which heat passes freely through its dimensions, so that
its inner surface could not be maintained at a temperature more
elevated than that of its dimensions extending from the inner
surface to the outer surface. Therefore, to maintain the steam
contained in the cylinder at the proper temperature, it was
essential that the whole of the solid metal composing the cylinder
should be itself at that temperature.
Things being in this state, it was required that a vacuum should
be produced under the piston to give effect to the atmospheric
pressure above it, by relieving it from the pressure below. This,
indeed, would appear to have been attained by introducing as much
cold water within the cylinder as would be sufficient to reconvert
the steam contained in it into water; but Watt found, in his
experiments on the atmospheric model, that the piston would not
descend with the proper force, unless a vastly greater quantity of
water were introduced into the cylinder than the quantity which he
had ascertained to be [Pg088] necessary for the reconversion of
the steam into water. The cause of this he perceived and fully
explained.
If we suppose as much, and no more, cold water introduced into the
cylinder as would reconvert the steam contained in it into water,
then we should have in the bottom of the cylinder a quantity of
warm water with a vacuum above it: but the entire mass of metal
composing the cylinder itself, which was previously at the
temperature of boiling water, would still be at the same
temperature. The warm water, resting in contact with this metal in
the bottom of the cylinder, would be immediately heated by it, and
would rise in its temperature, while the metal of the cylinder
itself would be somewhat lowered in temperature by the heat which
it would thus impart to the warm water contained in it. Under
these circumstances, as we shall presently explain, steam would be
produced from the water, which would fill the cylinder; and
although such steam would not have a mechanical pressure equal in
amount to the atmosphere, and therefore would not altogether
prevent the piston from descending if it had no load to move, yet
it would deprive the engine of so great a portion of its
legitimate power as to render it altogether inefficient. But this
defect would be removed by throwing into the cylinder a sufficient
quantity of cold water, not only to destroy the steam contained in
it, but also to cool the entire mass of metal composing the
cylinder itself, until it would be reduced to such a temperature
that the vapour proceeding from the water contained in it would
have so small a pressure that it would not seriously or
injuriously obstruct the descent of the piston.
The piston being made to descend with such force as to render the
machine practically efficient, it would then be necessary again to
make it ascend; and to accomplish this, Watt found that the boiler
should supply a quantity of steam many times greater than was
necessary to fill the cylinder. Mature reflection on the
circumstances which have been just explained, enabled him to
discover how this undue quantity of steam was rendered necessary.
Let it be recollected, that when the piston has reached the bottom
of the cylinder, the whole mass of the cylinder, and [Pg089] the
piston itself, are reduced to so low a temperature that the vapour
of water, having the same temperature, has no pressure sufficiently
great to obstruct the action of the machine. When, in order to make
the piston ascend, steam is introduced from the boiler into the
cylinder under the piston, this steam encounters, in the first
instance, the cold surfaces of the metal forming the bottom of the
cylinder and the bottom of the piston. The first effect of this is
to convert the steam which comes from the boiler into water, an
effect which is produced by that steam imparting its heat to the
metal with which it comes into contact. This destruction of steam
continues until the metal exposed to contact with it has been heated
up to the temperature of boiling water. Then, and not till then, the
steam below the piston will have a pressure equal to that of the
atmosphere above it, and the piston will begin to ascend. As it
ascends, however, the sides of the cylinder which it exposes to the
contact of the steam are cold, and partially destroy the steam.
Steam, therefore, must be supplied from the boiler to replace the
steam thus destroyed; nor can the piston reach the top of the
cylinder until such a quantity of steam shall have flowed from the
boiler into the cylinder, as shall be sufficient not only to fill
the cylinder under the piston, but likewise, by its condensation, to
raise the whole mass of the cylinder and piston to the temperature
of boiling water.
Such were the circumstances which forced themselves upon the
attention of Watt, in the course of repairing, and subsequently
trying, the model of the atmospheric engine, at Glasgow. Being
informed generally of the uses of the engine in the drainage of
mines, and of the vast expense attending its operation, by reason
of the quantity of fuel which it consumed, he saw how important
any improvement would be by which the extensive sources of waste
which had thus presented themselves could be removed. He saw also,
that all that portion of steam which was expended, not in filling
the cylinder under the piston, but in heating the great mass of
metal composing the cylinder and piston, from a low temperature to
that of boiling water, upon each stroke of the piston, was so much
heat lost, and that the proportion of the fuel expended in
evaporating the steam thus wasted would be saved, if by any
[Pg090] expedient _he could make the piston descend without
cooling the cylinder_. But in order to estimate the full amount of
this waste, and to discover the most effectual means of preventing
it, it was necessary to investigate the quantity of heat necessary
for the evaporation of a given quantity of water; also, the
quantity of steam which a given quantity of water would produce,
as well as other circumstances connected with the temperature and
pressure of steam. He, therefore, applied himself to make
experiments with a view to elucidate these questions; and
succeeded in obtaining results which led to the discovery of some
of the most important of those physical phenomena, on the due
application of which, the efficacy of the steam engine, which he
afterwards invented, depended, and which also form striking facts
in the general physics of heat.
(49.) The first question to which he directed his experiments, was
the determination of the extent to which water enlarged its
volume, or magnitude, when it passed into steam. To ascertain
this, he filled a thin Florence flask with steam, of a pressure
equal to the atmosphere, and weighed it accurately. The same flask
was then filled with water, and weighed again. Finally, the weight
of the flask itself was ascertained. It is evident, that by such
means, the exact weight of the steam which filled the flask, and
of the same bulk of water, would be obtained. He found that the
water weighed about eighteen hundred times more than the steam;
from whence he inferred that the steam which filled the flask
contained about eighteen hundred times less water than the flask
would contain.[17]
[Pg091] Having once ascertained this point, he was able, by
observing the quantity of water evaporated in the boiler of the
atmospheric model, to compute the volume of steam which was
supplied to the cylinder. It was evident, that for every cubic
inch of water evaporated in the boiler, eighteen hundred cubic
inches of steam were supplied to the cylinder. Having accurately
observed the evaporation of the boiler for a short time, and the
number of strokes made by the piston in the same time, he found
that the quantity of water evaporated in the boiler would supply
about four times as much steam as the cylinder would require. He
consequently inferred, that about three-fourths of the steam
produced was wasted.
The next question to which he directed his experiments, was to
ascertain the quantity of cold water necessary to be injected into
the cylinder, in order to condense the steam contained in it. To
ascertain this, he attached a pipe to a boiler, by which he was
enabled to conduct the steam from the boiler into a glass jar
containing cold water at fifty-two degrees of temperature. The
steam, as it passed from the boiler through the pipe, was
condensed by the cold water, and continued to be so condensed,
until, by the heat which it imparted to the water, the latter
began to boil, and would then condense no more steam. On comparing
the water in the glass jar, when boiling, with the water
originally contained in it at fifty-two [Pg092] degrees, the
quantity was found to be increased in the proportion of six to
seven, very nearly; from which he inferred, that to reduce one
ounce of steam to water, it was necessary to mix about six ounces
of cold water with it.
He was further led to the conclusion, that steam contains a vast
quantity of heat, by the following experiment. He heated, in a
close digester, a quantity of water several degrees above the
common boiling point. When thus heated, by opening a stop-cock, he
allowed the compressed steam to escape into a cold vessel; in
three or four seconds, he found that the heat of the water in the
digester was reduced from a very high temperature to the common
boiling point; yet, that all the steam which escaped from it, and
which carried off with it the superabundant heat, formed only a
few drops of water when condensed; from which he inferred, that
this small quantity of water, in the form of steam, contained as
much heat as was sufficient to raise all the water in the digester
from the boiling point to the temperature at which it was before
the steam was allowed to escape.
Having thus ascertained the exact quantity of cold water which
ought to be injected into the cylinder in order to condense the
steam which filled the cylinder, he found, on comparing the
quantity necessary to be injected in order to enable the piston to
descend, that this quantity was about four times as great as that
which was necessary to condense the steam. This led him to the
conclusion, that about four times as much heat was destroyed in
the cylinder as needed to be destroyed, if the object were the
mere condensation of the steam. This result fully corroborated the
other conclusion, deduced, from the proportion which he found
between the quantity of steam supplied by the boiler and the
actual contents of the cylinder.
(50.) Watt was forcibly struck with these circumstances, not only
on account of their importance in an economical point of view,
when their relation to steam power was considered, but still more
so, as indicating phenomena in the physics of heat altogether
novel to him.
He, therefore, eagerly sought his friend Dr. Black, to whom he
communicated these results. Then, for the first time, he [Pg093]
was informed, by Black, of the theory of LATENT HEAT, which had
recently been discovered by him, and of which these very phenomena
formed the basis.
Some passages in the works of Dr. Robison produced an erroneous
impression, that a large share of the merit of the discoveries of
Watt which have been just explained was due to Dr. Black, to whose
instructions on the subject of latent heat Watt was represented to
have owed the knowledge of those facts which led to his principal
inventions and improvements. We shall here give, in the words of
Watt himself, his explanation of the circumstances which led to
this error. This explanation is given in a letter addressed by
Watt to Dr. Brewster, in May 1814, and prefixed to the third
volume of Brewster's edition of Robison's Mechanical Philosophy:—
"The representations of friends whose opinions I highly value
induce me to avail myself of this opportunity of noticing an error
into which not only Dr. Robison, but apparently also Dr. Black,
has fallen, in relation to the _origin_ of my improvements upon
the steam engine, and which not having been publicly controverted
by me, has, I am informed, been adopted by almost every subsequent
writer upon the subject of latent heat.
"Dr. Robison, in the article Steam Engine, after passing an
encomium upon me, dictated by the partiality of friendship,
qualifies me as the '_pupil_ and intimate friend of Dr. Black,'—a
description which not being there accompanied with any inference,
did not particularly strike me at the time of its first perusal.
He afterwards, in the dedication to me of his edition of Dr.
Black's lectures upon chemistry, goes the length of supposing me
to have professed to owe my improvements upon the steam engine to
the instructions and information I had received from that
gentleman, which certainly was a misapprehension; as, though I
have always felt and acknowledged my obligations to him for the
information I had received from his conversation, and particularly
for the knowledge of the doctrine of latent heat, I never did nor
_could_ consider my improvements as originating in those
communications. He is also mistaken in his assertion (p. 8. of the
preface to the above work), that 'I had attended two courses
[Pg094] of the doctor's lectures;' for, unfortunately for me, the
necessary avocations of my business prevented me from attending
his or any other lectures at college; and as Dr. Robison was
himself absent from Scotland for four years at the period referred
to, he must have been misled by erroneous information. In p. 184.
of the lectures, Dr. Black says, 'I have the pleasure of thinking
that the knowledge we have acquired concerning the nature of
elastic vapours, in consequence of my fortunate observation of
what happens in its formation and condensation, has contributed in
no inconsiderable degree to the public good by _suggesting_ to my
friend Mr. Watt of Birmingham, then of Glasgow, his improvement on
this useful engine' (meaning the steam engine of which he is then
speaking). There can be no doubt from what follows in his
description of the engine, and from the very honourable mention
which he has made of me in various parts of his lectures, that he
did not mean to lessen any merit that might attach to me as an
inventor; but, on the contrary, he was always disposed to give me
fully as much praise as I deserved.
"And were that otherwise doubtful, it would, I think, be evident
from the following quotation from a letter of his to me, dated
13th February 1783, where, speaking of an intended publication by
a friend of mine, on subjects connected with the history of steam,
he says, 'I think it is very proper for you to give him a short
account of your discoveries and speculations; _and particularly_
_to assert clearly and fully your sole right to the honour of_
_the improvements of the steam engine_.' And in a written
testimonial which he very kindly gave me, on the occasion of a
trial at law against a piracy of my invention in 1796-7, after
giving a short account of the invention, he adds, '_Mr. Watt was_
_the sole inventor of the capital improvement and contrivance_
_above mentioned._'
"Under this conviction of his candour and friendship, it is very
painful to me to controvert any assertion or opinion of my revered
friend; yet, in the present case I find it necessary to say, that
he appears to me to have fallen into an error; and I hope, in
addition to my assertion, to make that appear by the short history
I have given of my invention, in my [Pg095] notes upon Dr.
Robison's essay, as well as by the following account of the state
of my knowledge previous to my receiving any explanation of the
doctrine of latent heat; and also from that of the facts which
principally guided me in the invention.
"It was known very long before my time, that steam was condensed
by coming into contact with cold bodies, and that it communicated
heat to them; witness the common still, &c. &c.
"It was known, by some experiments of Dr. Cullen and others, that
water and other liquids boiled in vacuo at very low heats; water
below 100°.
"It was known to some philosophers that the capacity or
equilibrium of heat, as we then called it, was much smaller in
mercury and tin than in water.
"It was also known that evaporation caused the cooling of the
evaporating liquid, and bodies in contact with it.
"I had myself made experiments to determine the following facts:—
"First, the capacities of heat for iron, copper, and some sorts of
wood, comparatively with water.
"Second, the bulk of steam compared with that of water.
"Third, the quantity of water evaporated in a certain boiler by a
pound of coals.
"Fourth, the elasticities of steam at various temperatures greater
than that of boiling water, and an approximation to the law which
it followed at other temperatures.
"Fifth, how much water in the form of steam was required every
stroke by a small Newcomen's engine, with a wooden cylinder six
inches diameter, and twelve inches stroke.
"Sixth, the quantity of cold water required in every stroke to
condense the steam in that cylinder, so as to give it a working
power of about 7 lb. on the inch.
"Here I was at a loss to understand how so much cold water could
be heated so much by so small a quantity of water in the form of
steam; and I accordingly applied to Dr. Black, and then first
understood what was called latent heat.
"But this theory, though useful in determining the quantity of
injection necessary where the quantity of water [Pg096]
evaporated by the boiler, and used by the cylinder, was known, and
in determining, by the quantity and heat of the hot water emitted
by Newcomen's engines, the quantity of steam required to work them
did not lead to the improvements I afterwards made in the engine.
These improvements proceeded upon the old established fact, that
steam was condensed by the contact of cold bodies; and the later
known one, that water boiled in vacuo at heats below 100°, and
consequently that a vacuum could not be obtained unless the
cylinder and its contents were cooled every stroke to below that
heat."
[Illustration: LOCH LOMOND.]
FOOTNOTES:
[16] We are indebted for many of the anecdotes of the life of
Watt to the _Eloge Historique_, recently published by M. Arago,
who was furnished with all the documents and circumstances
relating to this celebrated person which were considered proper
for publication, by his son, the present James Watt, Esq., of
Aston Hall, near Birmingham, and to the notes added to this
memoir by Mr. Muirhead, a relative of Mr. Watt.
[17] The following is the account of these experiments given
in Watt's own words:—
"It being evident that there was a great error in Dr.
Desagulier's calculations of Mr. Beighton's experiments on the
bulk of steam, a Florence flask, capable of containing about a
pound of water, had about one ounce of distilled water put
into it; a glass tube was fitted into its mouth, and the
joining made tight by lapping that part of the tube with
packthread covered with glazier's putty. When the flask was
set upright, the tube reached down near to the surface of the
water, and in that position the whole was placed in a tin
reflecting oven before a fire until the water was wholly
evaporated, which happened in about an hour, and might have
been done sooner, had I not wished the heat not much to exceed
that of boiling water. As the air in the flask was heavier
than the steam, the latter ascended to the top, and expelled
the air through the tube. When the water was all evaporated,
the oven and flask were removed from the fire, and a blast of
cold air was directed against one side of the flask, to
collect the condensed steam in one place. When all was cold,
the tube was removed, the flask and its contents were weighed
with care; and the flask being made hot, it was dried by
blowing into it by bellows, and when weighed again was found
to have lost rather more than four grains, estimated at 4-1/3
grains. When the flask was filled with water, it was found to
contain about 17-1/8 ounces avoirdupois of that fluid which
gave about 1800 for the expansion of water converted into
steam of the heat of boiling water.
"This experiment was repeated with nearly the same result, and
in order to ascertain whether the flask had been wholly filled
with steam, a similar quantity of water was for the third time
evaporated; and, while the flask was still cold, it was placed
inverted with its mouth (contracted by the tube) immersed in a
vessel of water, which it sucked in as it cooled, until in the
temperature of the atmosphere it was filled to within half an
ounce measure of water.
"In repetitions of this experiment at a later date, I
simplified the apparatus by omitting the tube, and laying the
flask upon its side in the oven, partly closing its mouth by a
cork, having a notch on one side, and otherwise proceeding as
has been mentioned."
[Pg097]
[Illustration: GLASGOW.]
CHAP. IV.
EXPOSITION OF PHYSICAL PRINCIPLES. — THERMOMETER. — METHOD OF
GRADUATING IT. — FREEZING AND BOILING POINTS. — LATENT HEAT OF
WATER. — QUANTITY OF HEAT NECESSARY TO CONVERT ICE INTO WATER.
— QUANTITY OF HEAT GIVEN OUT BY WATER IN BEING CONVERTED INTO
ICE. — PROCESS OF BOILING. — OF RECONVERSION OF STEAM INTO
WATER. — QUANTITY OF HEAT NECESSARY TO CONVERT WATER INTO
STEAM. — BOILING POINT OF WATER. — DIFFERENT IN DIFFERENT
PLACES. — DEPENDS ON THE BAROMETER. — VARIES WITH THE
PRESSURE. — EXPERIMENTAL PROOF OF THIS. — BOILS AT LOWER
TEMPERATURES THAN 212° UNDER PRESSURES LESS THAN THE
ATMOSPHERE. — SUM OF LATENT AND SENSIBLE HEAT OF STEAM ALWAYS
THE SAME. — THE FUEL NECESSARY TO EVAPORATE WATER THE SAME,
WHATEVER BE THE TEMPERATURE OR PRESSURE AT WHICH IT IS
EVAPORATED. — MECHANICAL FORCE OBTAINED BY EVAPORATION. — THIS
FORCE NEARLY THE SAME UNDER ALL CIRCUMSTANCES.
(51.) We shall pause here to put the reader in possession of the
physical and mechanical principles connected with the evaporation
of water and other liquids, which are necessary to enable him to
understand the full extent of the value and the merit of the
discoveries of Watt, and to comprehend the [Pg098] structure and
operation of the steam engine in its improved form, as it has
passed to us from his hands.
As we shall frequently have occasion to refer to the indications
of a thermometer, we shall first explain the principle of that
instrument as it is commonly used in this country.
The thermometer is an instrument used for the purpose of measuring
and indicating the temperature or sensible heat of material
substances.
Heat, like all other physical agents, can only be measured by its
effects. One of these effects best suited for this purpose, is the
change of dimension which all bodies undergo in consequence of
their change of temperature. In general, when heat is applied to a
material substance, that substance undergoes an enlargement of
bulk; and if heat be abstracted from it, it suffers a diminution
of bulk. This variation of magnitude is not always in the same
proportion as the increase or diminution of temperature; but it is
so when applied to certain substances and between certain limits.
One of the substances whose expansion and contraction through an
extensive range of temperature has been found to be nearly
uniform, and which is attended with other convenient qualities for
a thermometer, is the liquid called _mercury_ or _quicksilver_. A
mercurial thermometer is constructed in the following way:—
A glass tube is made with a small and uniform bore: upon the end
of this tube, a bulb is blown, having a magnitude very great
compared with the bore of the tube. Let us suppose this bulb and a
part of the tube to be filled with mercury. If the mercury
contained in the bulb be heated, it will expand, and being more
susceptible of expansion than the glass which contains it, the
bulb will be too small for its augmented volume: the mercury in
the bulb can only, therefore, obtain room for its increased bulk
by pressing the mercury in the tube upwards, which it will
accordingly do. The increase of volume which the mercury in the
bulb therefore undergoes, will be exhibited by the increased
length of the column in the tube. Since the bore of the tube is
made so exceedingly minute compared with the magnitude of the
bulb, a very small quantity of mercury forced [Pg099] from the
bulb into the tube, will cause a considerable increase of the
length of the column. Small degrees of expansion will therefore be
rendered very apparent, and may be accurately measured. The
following is the method by which the thermometer called
_Fahrenheit's thermometer_ is graduated.
The tube and bulb being prepared and supplied with mercury, as
already explained, let the instrument be plunged in a vessel of
melting ice. It will be found that the mercury will stand in the
tube at a certain point, from which it will not vary so long as
any ice remains not completely melted in the vessel. Let a mark be
made on the tube, or on a scale attached to the tube, at the point
corresponding to the top of the column: the point thus marked is
called the _freezing point_.
Now let the instrument be immersed in a vessel of boiling water,
the barometer at the time having the height of thirty inches. It
will be found that so long as the water is kept boiling, the
column of mercury in the tube will remain stationary. Let the
point corresponding with the top of the column be marked on the
tube, or on the scale attached to it. This is called the _boiling
point_. Let the space on the scale between the freezing and
boiling points be now divided into 180 equal parts: each of these
parts is called a _degree_. Let the same divisions be continued
upon the scale below the freezing point, until thirty-two
divisions be taken; let the lowest division be then marked 0, and
let the successive divisions upwards from that be numbered 1, 2,
3, &c. In like manner, let the same divisions be continued above
the boiling point, as far as the tube will admit.
It is evident that, under these circumstances, the freezing point
will be marked by 32, and the boiling point by 212. It is usual to
express the degrees of a thermometer in the same manner as the
degrees of a circle, by placing a small ° above the number. Thus
the freezing point is expressed by 32°, and the boiling point by
212°.
The reason the degrees were commenced at 32° below the freezing
point was, because, when the thermometer was invented, that
temperature was supposed to be the lowest degree of cold possible,
being that of a certain mixture of [Pg100] snow and salt. This,
however, has since been found to be an error, very much lower
temperatures being obtained by various physical expedients.
The temperature of a body is, then, that elevation to which the
thermometer would rise when immersed in that body. Thus, if in
plunging the thermometer in water we found the mercury to rise or
fall to the division marked 100, we should then say, the
temperature of the water was 100°.
Let us suppose a spirit lamp, or other regular source of heat,
applied to a bath of mercury, so as to maintain the mercury at a
fixed temperature of 200°, and let another vessel, containing a
quantity of ice at a temperature of 20° be immersed in the
mercury. Let a thermometer be placed in the mercury, and another
in the ice. The following effects will then ensue. The thermometer
immersed in the ice will be observed gradually to rise from 20°
upwards, until it indicates the temperature of 32°. It will then
become stationary, and the ice which had hitherto remained in a
solid state will begin to melt and be converted into water. This
process of liquefaction will continue for a considerable time,
during which the thermometer immersed in the ice will constantly
be maintained at 32°. At the moment, however, when the last
portion of ice is liquefied, the thermometer will begin again to
rise. The coincidence of this ascent of the thermometer with the
completion of the liquefaction of the ice, may be very easily
observed, because the ice being lighter, bulk for bulk, than
water, will float on the surface, and so long as a particle of it
remains unmelted it will be distinctly seen.
Now it cannot be doubted that, during the whole of this process,
the mercury, supposed to be maintained at 200°, constantly imparts
heat to the ice; yet, from the moment the liquefaction begins,
until it is completed, no increased temperature is exhibited by
the thermometer immersed in the melting ice. If during this part
of the process no heat were received by the ice from the mercury,
the consequence would be, that the application of the lamp would
cause the temperature of the mercury to rise above 200°, which may
be easily demonstrated by withdrawing the vessel of ice from the
mercurial bath during the process of liquefaction. The moment
[Pg101] it is withdrawn, the thermometer immersed in the mercury,
instead of remaining fixed at 200°, will begin to rise, although
the action of the lamp remains the same as before; from which it
is evident that the heat which now causes the mercury to rise
above 200° was before received by the melting ice.
The heat which thus enters ice in the process of liquefaction, and
which is not indicated by the thermometer, is for this reason
called _latent heat_. It will be perceived that this phrase is the
name of a fact, and not of an hypothesis. That heat really enters
the water, and is contained in it, has been established by the
experiments; and to declare that it is present there, is to
declare an established fact. To call it by the name _latent_ heat,
is to declare another established fact, viz., that it is not
sensible to the thermometer.
These facts show us that heat is capable of existing in bodies in
two distinct states, in one of which it is sensible to the
thermometer, and in the other not. Heat which is sensible to the
thermometer is called, for distinction, _sensible_ or _free heat_.
It may be here observed, that heat which is sensible to the
thermometer is also perceptible by the senses, and heat not
sensible to the thermometer is not perceptible by the senses.
Thus, ice at 32° and water at 32° _feel_ equally cold, and yet we
have seen that the latter contains considerably more heat than the
former.
Dr. Black, who first noticed the remarkable fact to which we have
now alluded, inferred that ice is converted into water by
communicating to it a certain quantity or dose of heat, which
enters into combination with it in a manner analogous to that
which takes place when bodies combine chemically. The heat, thus
combined with the solid ice, loses its property of affecting the
senses or the thermometer, and the effects therefore bear a
resemblance to those cases of chemical combination in which the
constituent elements change their sensible properties when they
form the compound.
The fact that the thermometer immersed in the ice remains stationary
only as long as the process of liquefaction is going on, shows that
this absorption of heat is necessarily connected with that process,
and that, were it not for the conversion of [Pg102] the solid ice
into liquid water, the heat which is so received would be sensible,
and would cause the thermometer immersed in the ice to rise. Before
the time of Black it was supposed that the slightest addition of
heat would cause solid ice to be converted into water, and that the
thermometer would immediately pass from the freezing temperature to
higher degrees. The experiments above described, however, show the
falsehood of such a supposition. If, while the mercurial bath, in
which the ice is immersed, is maintained at the temperature of 200°,
the length of time necessary to complete the liquefaction of the ice
be observed, it would be found that that time is about twenty-eight
times the length of time which it would take to raise the liquid
water from 32° to 37°; and if it be assumed that the same quantity
of heat is imparted to the ice, during the process of liquefaction,
during each minute, as is imparted to the water, during each minute,
in rising from 32° to 37°, it will follow, that to liquefy the ice
requires twenty-eight times as much heat as is necessary to raise
the water from 32° to 37°. It appears, therefore, that, instead of a
small quantity of heat being necessary to melt the ice, a very
considerable portion is absorbed in that process.
Having ascertained the remarkable fact, that heat is absorbed in a
large quantity in the conversion of ice into water, without
rendering the body so absorbing it warmer, let us now inquire what
the exact quantity of heat so absorbed is. We have already stated
that, if the quantity communicated in equal times be the same, the
heat necessary to liquefy a given weight of ice would be
twenty-eight times as much as would be necessary to raise the same
weight of water from 32° to 37°; or, if the heat necessary to
raise water through every 5° be the same, that quantity of heat
would be sufficient to raise water from 32° to 172°: and hence we
infer, that as much heat is absorbed in the liquefaction of a
given quantity of ice as would raise the same quantity of water
through 140 degrees of the thermometric scale.
(52.) Let us now examine the analogous effects produced by the
continued application of heat to water in the liquid state.
Let a small quantity of water be placed in a glass flask of
considerable size, and then closed so as to prevent the escape
[Pg103] of any vapour. Let this vessel be now placed over the
flame of a spirit lamp, so as to cause the water it contains to
boil. For a considerable time the water will be observed to boil,
and apparently to diminish in quantity, until at length all the
water disappears, and the vessel is apparently empty. If the
vessel be now removed from the lamp, and suspended in a cool
atmosphere, the whole of the interior of its surface will
presently appear to be covered with a dewy moisture; and at length
a quantity of water will collect in the bottom of it, equal to
that which had been in it at the commencement of the process. That
no water has at any period of the experiment escaped from it, may
be easily determined, by performing the experiment with the glass
flask suspended from the arm of a balance, counterpoised by a
sufficient weight suspended from the other arm. The equilibrium
will be preserved throughout, and the vessel will be found to have
the same weight, when to all appearance it is empty, as when it
contains the liquid water. It is evident, therefore, that the
water exists in the vessel in every stage of the process, but that
it becomes invisible when the process of boiling has continued for
a certain length of time; and it may be shown that it will
continue to be invisible, provided the flask be exposed to a
temperature considerably elevated. Thus, for example, if it be
suspended in a vessel of boiling water, the water which it
contains will continue to be invisible; but the moment it is
withdrawn from the boiling water, and exposed to the cold air, the
water will again become visible, as above mentioned, forming a dew
on the inner surface, and finally collecting in the bottom, as in
the commencement of the experiment.
In fact, the liquid has, by the process of boiling, been converted
into _vapour_, or _steam_, which is a body similar in its leading
properties to common air, and, like it, is invisible. It will
hereafter appear that it likewise possesses the property of
elasticity, and other mechanical qualities enjoyed by gases in
general.
(53.) Again, let an open vessel be filled with water at 60°, and
placed in a mercurial bath, which is maintained, by a fire or lamp
applied to it, at the temperature of 230°. Place a thermometer in
the water, and it will be observed gradually to [Pg104] rise as
the temperature of the water is increased by the heat which it
receives from the mercury in which it is immersed. The water will
steadily rise in this manner until it attains the temperature of
212°; but here the thermometer immersed in it will become
stationary. At the same time the water contained in the vessel
will become agitated, and its surface will present the same
appearance as if bubbles of air were rising from the bottom, and
issuing at the top. A cloudy vapour will be given off in large
quantities from its surface. This process is called _ebullition_
or _boiling_. If it be continued for any considerable time, the
quantity of water in the vessel will be sensibly diminished; and
at length every particle of it will disappear, and the vessel will
remain empty. During the whole of this process, the thermometer
immersed in the water will remain stationary at 212°.
Now, it will be asked, what has become of the water? It cannot be
imagined that it has been annihilated. We shall be able to answer
this by adopting means to prevent the escape of any particle of
matter from the vessel containing the water, into the atmosphere
or elsewhere. Let us suppose that the top of the vessel containing
the water is closed, with the exception of a neck communicating
with a tube, and let that tube be carried into another close
vessel removed from the cistern of heated mercury, and plunged in
another cistern of cold water. Such an apparatus is represented in
_fig._ 15.
[Illustration: _Fig._ 15.]
A is a cistern of heated mercury, in which the glass vessel B,
containing water, is immersed. From the top of the vessel B
proceeds a glass tube C, inclining downwards, and entering a glass
vessel D, which is immersed in a cistern E of cold water. If the
process already described be continued until the water by constant
ebullition has disappeared, as already mentioned, [Pg105] from
the vessel B, it will be found that a quantity of water will be
collected in the vessel D; and if this water be weighed, it will
be found to have exactly the same weight as the water had which
was originally placed in the vessel B. It is, therefore, quite
apparent that the water has passed by the process of boiling from
the one vessel to the other; but, in its passage, it was not
perceptible by the sight. The tube C and the upper part of the
vessel B, had the same appearance, exactly, as if they had been
filled with atmospheric air. That they are not merely filled with
atmospheric air may, however, be easily proved. When the process
of boiling first commences, it will be found that the tube C is
cold, and the inner surface dry. When the process of ebullition
has continued a short time, the tube C will become gradually
heated, and the inner surface of it covered with moisture. After a
time, however, this moisture disappears, and the tube attains the
temperature 212°. In this state it continues until the whole of
the water is discharged from the vessel B to the vessel D.
(54.) These effects are easily explained. The water in the vessel
B is incapable of receiving any higher temperature than 212°,
consistently with its retaining the liquid form. Small portions,
therefore, are constantly converted into steam by the heat
received from the surrounding mercury, and bubbles of steam are
formed on the bottom and sides of the vessel B. These bubbles,
being very much lighter, bulk for bulk, than water, rise rapidly
through the water, just in the same manner as bubbles of air
would, and produce that peculiar agitation at its surface which
has been taken as the external indication of boiling. They escape
from the surface, and collect in the upper part of the vessel. The
steam thus collected, when it first enters the tube C, is cooled
below the temperature of 212° by the surface of the tube; and
consequently, being incapable of remaining in the state of vapour
at any lower temperature than 212°, it is reconverted into water,
and forms the dewy moisture which is observed in the commencement
of the process on the interior of the tube C. At length, however,
the whole of the tube C is heated to the temperature of 212°, and
the moisture which was previously collected upon its inner
[Pg106] surface is again converted into steam. As the quantity of
steam evolved from the water in B increases, it drives before it
the steam previously collected in the tube C, and forces it into
the vessel B. Here it encounters the inner surface of this vessel,
which is kept constantly cold by being surrounded with the cold
water in which it is immersed; and the vapour, being thus
immediately reduced below the temperature of 212°, is reconverted
into water. At first it collects in a dew on the surface of the
vessel D; but as this accumulates, it drops into the bottom of the
vessel, and forms a more considerable quantity. As the quantity of
water is observed to be gradually diminished in the vessel B, the
quantity will be found to be gradually increased in the vessel D;
and if the operation be suspended at any stage of the process, and
the water in the two vessels weighed, it will be found that the
weight of the water in D is exactly equal to the weight which the
water in B has lost.
(55.) The demonstration is, therefore, perfect, that the gradual
diminution of the boiling water in the vessel B is produced by the
conversion of that water into steam by the heat. In the process
first described, when the top of the vessel B was supposed to be
open, this steam made its escape into the air, where it was first
dispersed, and subsequently cooled in separate particles, and was
deposited in minute globules of moisture on the ground and on
surrounding objects.
(56.) In reviewing this process, we are struck by the fact, that
the continued application of heat to the vessel B is incapable of
raising the temperature of the water contained in it above 212°.
This presents an obvious analogy to the process of liquefaction,
and leads to inquiries of a similar nature, which are attended
with a like result. We must either infer, that the water, having
arrived at 212°, received no more heat from the mercury; or that
such heat, if received, is incapable of affecting the thermometer;
or, finally, that the steam which passes off carries this heat
with it. That the water receive heat from the mercury, will be
proved by the fact, that, if the vessel B be removed from the
mercury, other things remaining as before, the temperature of the
mercury will rapidly rise, and if the fire be continued, it will
even boil; but so long as the [Pg107] vessel B remains immersed,
it prevents the mercury from increasing in temperature. It
therefore receives that heat which would otherwise raise the
temperature of the quicksilver.
[Illustration: _Fig._ 16.]
If a thermometer be immersed in the steam which collects in the
upper part of the vessel B, it will show the same temperature (of
212°) as the water from which it is raised. The heat, therefore,
received from the mercury, is clearly not imparted in a sensible
form to the steam, which has the same temperature in the form of
steam as it had in the form of water. What has been already
explained respecting liquefaction would lead us, by analogy, to
suspect that the heat imparted by the mercury to the water has
become latent in the steam, and is instrumental to the conversion
of water into steam, in the same manner as heat has been shown to
be instrumental to the conversion of ice into water. As the fact
was in that case detected by mixing ice with water, so we shall,
in the present instance, try it by a like test, viz. by mixing
water with steam. Let about five ounces and a half of water, at
the temperature of 32°, be placed in a vessel A (_fig._ 16.), and
let another vessel B, in which water is kept constantly boiling at
the temperature of 212°, communicate with A by a pipe C proceeding
from the top, so that the steam may be conducted from B, and
escape from the mouth of the pipe at some depth below the surface
of the water in A. As the steam issues from the pipe, it will be
immediately reconverted into water by the cold water which it
encounters; and, by continuing this process, the water in A will
be gradually heated by the steam combined with it and received
through the pipe C. If this process be continued until the water
in A is raised to the temperature of 212°, it will boil. Let it
then be weighed, and it will be found to weigh six ounces and a
half: from whence we infer, that one ounce of water has been
received from the vessel B in the form of steam, and has been
reconverted into water by the inferior temperature of the water in
A. Now, this ounce of water received in the form of steam into the
vessel A had, when in that form, the temperature of 212°. It is
now [Pg108] converted into the liquid form, and still retains the
same temperature of 212°; but it has caused the five ounces and a
half of water with which it has been mixed, to rise from the
temperature of 32° to the temperature of 212°,—and this, _without
losing any temperature itself_. It follows, therefore, that, in
returning to the liquid state, it has parted with as much heat as
is capable of raising five times and a half its own weight of
water from 32° to 212°. This heat was combined with the steam,
though not sensible to the thermometer; and was, therefore,
_latent_. Had it been sensible in the water in B, it would have
caused the water to have risen through a number of thermometric
degrees, amounting to five times and a half the excess of 212°
above 32°; that is, through five times and a half 180°; for it has
caused five times and a half its own weight of water to receive an
equal increase of temperature. But five times and a half 180° is
990°, or, to use round numbers (for minute accuracy is not here
our object), 1000°. It follows, therefore, that an ounce of water,
in passing from the liquid state at 212° to the state of steam at
212°, receives as much heat as would be sufficient to raise it
through 1000 thermometric degrees, if that heat, instead of
becoming latent, had been sensible.
(57.) In order to derive all the knowledge from these experiments
which they are capable of imparting, it will be necessary to
examine very carefully how water comports itself under a variety
of different circumstances.
If water be boiled in an open vessel, with a thermometer immersed,
on different days, it will be observed that the fixed temperature
which it assumes in boiling will be subject to a variation within
certain small limits. Thus, at one time, it will be found to boil
at the temperature of 210°; while, at others, the thermometer
immersed in it will rise to 213°; and, on different occasions, it
will fix itself at different points within these limits. It will
also be found, if the same experiment be performed at the same
time in distant places, that the boiling points will be subject to
a like variation. Now, it is natural to inquire what cause
produces this variation; and we shall be led to the discovery of
the cause, by examining what other physical effects undergo a
simultaneous change. [Pg109]
If we observe the height of the barometer at the time of making
each experiment, we shall find a very remarkable correspondence
between it and the boiling temperature. Invariably, whenever the
barometer stands at the same height, the boiling temperature will
be the same. Thus, if the barometer stands at 30 inches, the
boiling temperature will be 212°. If the barometer fall to 29-1/2
inches, the thermometer stands at a small fraction above 211°. If
the barometer rise to 30-1/2 inches, the boiling temperature rises
to nearly 213°. The variation in the boiling temperature is, then,
accompanied by a variation in the pressure of the atmosphere
indicated by the barometer; and it is constantly found that the
boiling point will remain unchanged, so long as the atmospheric
pressure remains unchanged, and that every increase in the one
causes a corresponding increase in the other.
(58.) From these facts it must be inferred, that the pressure
excited on the surface of the water has a tendency to resist its
ebullition, and to make it necessary, before it can boil, that it
should receive a higher temperature; and, on the contrary, that
every diminution of pressure on the surface of the water will give
an increased facility to the process of ebullition, or will cause
that process to take place at a lower temperature. As these facts
are of the utmost importance in the theory of heat, it may be
useful to verify them by direct experiment.
If the variable pressure excited on the surface of the water by
the atmosphere be the cause of the change in the boiling
temperature, it must happen, that any change of pressure produced
by artificial means on the surface of the water must likewise
change the boiling point, according to the same law. Thus, if a
pressure considerably greater than the atmospheric pressure be
excited on a liquid, the boiling point may be expected to rise
considerably above 212°; and, on the other hand, if the surface of
the water be relieved from the pressure of the atmosphere, and be
submitted to a considerably diminished pressure, the water would
boil below 212°.
[Illustration: _Fig._ 17.]
Let B (_fig._ 17.) be a strong spherical vessel of brass, supported
on a stand S, under which is placed a large spirit lamp L, or other
means of heating it. In the top of this vessel are three apertures,
in two of which are screwed a [Pg110] thermometer T, the bulb of
which enters the hollow brass sphere, and a stop-cock C, which may
be closed or opened at pleasure, to confine the steam, or allow it
to escape. In the third aperture at the top, is screwed a long
barometer tube, open at both ends. The lower end of this tube
extends nearly to the bottom of the spherical vessel B. In the
bottom of this vessel is placed a quantity of mercury, the surface
of which rises to some height above the lower end of the tube A.
Over the mercury is poured a quantity of water, so as to half fill
the vessel B. Matters being thus arranged, the screws are made
tight, so as to confine the water, and the lamp is allowed to act on
the vessel; the temperature of the water is raised, and steam is
produced, which, being confined within the vessel, exerts its
pressure on the surface of the water, and resists its ebullition.
The pressure of the steam acting on the surface of the water is
communicated to the surface of the mercury, and it forces a portion
of the mercury into the tube A, which presently rises above the
point where the tube is screwed into the top of the vessel B. As the
action of the lamp continues, the thermometer T exhibits a gradually
increasing temperature; while the column of mercury in A shows the
force with which the steam presses on the surface of the water in
B,—this column being balanced by the pressure of the steam. Thus,
the temperature and pressure of the steam at the same moment may
always be observed by inspecting the thermometer T and the tube A.
When the column in the tube A has risen to the height of 30 inches
above the level of the mercury in the vessel B, then the pressure of
the steam will be equivalent to double the pressure of the
atmosphere, because, the tube A being open at the top, the
atmosphere presses on the [Pg111] surface of the mercury in it. The
thermometer T will be observed gradually to rise until it attains
the temperature of 212°; but it will not stop there, as it would do
if immersed in water boiled in an open vessel. It will, on the other
hand, continue to rise; and when the column of mercury in A has
attained the height of 30 inches, the thermometer T will have risen
to 251°,—being 39° above the ordinary boiling point.
During the whole of this process, the surface of the water being
submitted to a constantly increasing pressure, its ebullition is
prevented, and it continues to receive heat without boiling. That
it is the increased pressure which resists its ebullition, and
causes it to receive a temperature above 212°, may be easily
shown. Let the stop-cock C be opened; immediately the steam in B,
having a pressure considerably greater than that of the
atmosphere, will rush out, and will continue to issue from C,
until its pressure is balanced by the atmosphere. At the same time
the column of mercury in A will be observed rapidly to fall, and
to sink below the orifice by which it is inserted in the vessel B.
The thermometer T will also fall until it attains the temperature
of 212°. At that point, however, it will remain stationary; and
the water will now be distinctly heard to be in a state of rapid
ebullition. If the stop-cock C be once more closed, the
thermometer will begin to rise, and the column of mercury
ascending in A will be again visible.
If, instead of a stop-cock being at C, the aperture were made to
communicate with a valve, like the safety-valve of a steam engine,
loaded with a certain weight,—say at the rate of 15 lbs. on the
square inch,—then the thermometer T, and the mercury in the tube
A, would not rise indefinitely as before. The thermometer would
continue to rise till it attained the temperature of 251°; and the
mercury in the tube A would rise to the height of 30 inches. At
this limit the resistance of the valve would be balanced by the
pressure of the steam; and as fast as the water would have a
tendency to produce steam of a higher pressure, the valve would be
raised and the steam suffered to escape; the thermometer T and the
column of mercury in A remaining stationary during this process.
If the valve were loaded more heavily, the phenomena would be
[Pg112] the same, only that the mercury in T and A would become
stationary at certain heights. But, on the other hand, if the
valve were loaded at a less pressure than 15 lbs. on the square
inch, then the mercury in the two tubes would become stationary at
lower points.
(59.) These experiments show that every increase of pressure above
the ordinary pressure of the atmosphere causes an increase in the
temperature at which water boils. We shall now inquire whether a
diminution of pressure will produce a corresponding effect on the
boiling point.
This may be easily accomplished by the aid of an air pump. Let
water at the temperature of 200° be placed in a glass vessel under
the receiver of an air pump, and let the air be gradually
withdrawn. After a few strokes of the pump, the water will boil;
and if the mercurial gauge of the pump be observed, it will be
found that its altitude will be about 23-1/2 inches. Thus the
pressure to which the water is submitted has been reduced from the
ordinary pressure of the atmosphere expressed by the column of 30
inches of mercury, to a diminished pressure expressed by 23-1/2
inches; and we find that the temperature at which the water boils
has been lowered from 212° to 200°. Let the same experiment be
repeated with water at the temperature of 180°, and it will be
found that a further rarefaction of the air is necessary, but the
water will at length boil. If the gauge of the pump be now
observed, it will be found to stand at about fifteen inches,
showing, that at the temperature of 180° water will boil under
half the ordinary pressure of the atmosphere. These experiments
may be varied and repeated; and it will be always found, that, as
the pressure is diminished or increased, the temperature at which
the water will boil will be also diminished or increased.
(60.) The same effects may be exhibited in a striking manner
without an air pump, by producing a vacuum by the condensation of
steam. Let a small quantity of water be placed in a thin glass
flask, and let it be boiled by holding it over a spirit lamp. When
the steam is observed to issue abundantly from the mouth of the
flask, let it be quickly corked and removed from the lamp. The
process of boiling will then cease, and the water will become
quiescent; but if the flask be plunged [Pg113] in a vessel of
cold water, the water it contains will again pass into a state of
violent ebullition, thus exhibiting the singular fact of water
being boiled by cooling it. This effect is produced by the cold
medium in which the flask is immersed, causing the steam above the
surface of the water in it to be condensed, and therefore
relieving the water from its pressure. The water, under these
circumstances, boils at a lower temperature than when submitted to
the pressure of the uncondensed vapour.
(61.) There is no limit to the temperature to which water may be
raised, if it be submitted to a sufficient pressure to resist its
tendency to take the vaporous form. If a strong metallic vessel be
nearly filled with water, so as to prevent the liquid from
escaping by any force which it can exert, the water thus inclosed
may be heated to any temperature whatever without boiling; in
fact, it may be made red-hot; and the temperature to which it may
be raised will have no limit, except the strength of the vessel
containing it, or the point at which the metal of which it is
formed may begin to soften or to be fused.
(62.) The following table will show the temperature at which water
will boil under different pressures of the atmosphere corresponding
to the altitudes of the barometer between 26 and 31 inches.
Barometer. Boiling Point.
26 inches 204°·91
26·5 205°·79
27 206°·67
27·5 207°·55
28 208°·43
28·5 209°·31
29 210°·19
29·5 211°·07
30 212°
30·5 212°·88
31 213°·76
From this table it appears, that, for every tenth of an inch which
the barometric column varies between these limits, the boiling
temperature changes by the fraction of a degree expressed by the
decimal ·176, or nearly by the vulgar fraction 1/6.
(63.) In the experiment already described, by which the latent
[Pg114] heat of steam was determined, the water was supposed to be
boiled under the ordinary pressure of the atmosphere. Having seen,
however, that water may boil at different temperatures, under
different pressures, the inquiry presents itself, whether the heat
absorbed in vaporisation at different temperatures, and under
different pressures, is subject to any variation? Experiments of
the same nature as those already described, instituted upon water
in a state of ebullition at different temperatures, as well below
as above 212°, have led to the discovery of a very remarkable fact
in the theory of vapour. It has been found that the heat absorbed
by vaporisation is always less, the higher the temperature at
which the ebullition takes place; and less, by the same amount as
the temperature of ebullition is increased. Thus, if water boil at
312°, the heat absorbed in ebullition will be less by 100° than if
it boiled at 212°; and again, if water be boiled under a
diminished pressure, at 112°, the heat absorbed in vaporisation
will be 100° more than the heat absorbed by water boiled at 212°.
It follows, therefore, that the actual consumption of heat in the
process of vaporisation must be the same, whatever be the
temperature at which the vaporisation takes place; for whatever
heat is saved in the sensible form, is consumed in the latent
form, and _vice versâ_.
Let us suppose a given weight of water at the temperature of 32°
to be exposed to any regular source by which heat may be supplied
to it. If it be under the ordinary atmospheric pressure, the first
180° of heat which it receives will raise it to the boiling point,
and the next 1000° will convert it into steam. Thus, in addition
to the heat which it contains at 32°, the steam at 212° contains
1180° of heat. But if the same water be submitted to a pressure
equal to half the atmospheric pressure, then the first 148° of
heat which it receives will cause it to boil, and the next 1032°
will convert it into vapour. Thus, steam at the temperature of
180° contains a quantity of heat more than the same quantity of
water at 32°, by 1032° added to 148°, which gives a sum of 1180°.
Steam, therefore, raised under the ordinary pressure of the
atmosphere at 212°, and steam raised under half that pressure at
180°, contain the same quantity of heat,—with this difference
[Pg115] only—that the one has more latent heat, and less sensible
heat, than the other.
From this fact, that the sum of the latent and sensible heats of
the vapour of water is constant, it follows that the same quantity
of heat is necessary to convert a given weight of water into
steam, at whatever temperature, or under whatever pressure, the
water may be boiled. It follows, also, that, in the steam engine,
equal weights of high-pressure and low-pressure steam are produced
by the same consumption of fuel; and that, in general, the
consumption of fuel is proportional to the quantity of water
vaporised, whatever the pressure of the steam may be.[18]
(64.) Having explained the conditions under which, by supplying
heat to water, it is converted into steam, and, by abstracting
heat from steam, it may be reconverted into water, let us now
consider the mechanical force which is developed in these
phenomena.
[Illustration: _Fig._ 18.]
Let A B (_fig._ 18.) be a tube, or cylinder, the base of which is
equal to a square inch, and let a piston P move in it so as to be
steam-tight. Let it be supposed, that under this piston there is,
in the bottom of the cylinder, a cubic inch of water between the
bottom of the piston and the bottom of the tube; let the piston be
counterbalanced by a weight W acting over a pulley, which will be
just sufficient to counterpoise the weight of the piston, so as
leave no force tending to keep the piston down, except the force
of the atmosphere acting above it. Under the circumstances here
supposed, the piston being in contact with the water, and all air
being excluded, it will be pressed down by the weight of the
atmosphere, which we will suppose to be fifteen pounds, the
magnitude of the piston being a square inch. [Pg116]
Now let the flame of a lamp be applied at the bottom of the tube;
the water under the piston having its temperature thereby
gradually raised, and being submitted to no pressure save that of
the atmosphere above the piston, it will begin to be converted
into steam when it has attained the temperature of 212°. According
as it is converted into steam, it will cause the piston to ascend
in the tube until all the water has been evaporated. If the tube
were constructed of sufficient length, the piston then would be
found to have risen to the height of about seventeen hundred
inches, or one hundred and forty-two feet; since, as has been
already explained, water passing into steam under the ordinary
pressure of the atmosphere undergoes an increase of bulk in the
proportion of about seventeen hundred to one.
Now in this process, the air above the piston, which presses on it
with a force equal to fifteen pounds, has been raised one hundred
and forty-two feet. It appears, therefore, that, by the
evaporation of a cubic inch of water under a pressure equal to
fifteen pounds per square inch, a mechanical force of this amount
is developed.
It is evident that fifteen pounds raised one hundred and forty-two
feet successively, is equivalent to one hundred and forty-two
times fifteen pounds raised one foot. Now, one hundred and
forty-two times fifteen is two thousand one hundred and thirty,
and therefore the force thus obtained is equal to two thousand one
hundred and thirty pounds raised one foot high. This being within
about 110 pounds of a ton, it may be stated, in round numbers,
that, by the evaporation of a cubic inch of water under these
circumstances, a force is obtained equal to that which would raise
a ton weight a foot high.
The augmentation of volume which water undergoes in passing into
steam under the pressure here supposed, may be easily retained in
the memory, from the accidental circumstance that a cubic inch of
water is converted into a cubic foot of steam, very nearly. A
cubic foot contains one thousand seven hundred and twenty-eight
cubic inches,—which is little different from the proportion which
steam bears to water, when raised under the atmospheric pressure.
[Pg117]
(65.) It will, therefore, be an advantage to retain in memory the
following general facts:—
1. _A cubic inch of water evaporated under the ordinary
atmospheric pressure, is converted into a cubic foot of steam._
2. _A cubic inch of water evaporated under the atmospheric
pressure, gives a mechanical force equal to what would raise about
a ton weight a foot high._
(66.) Let us, again, suppose the piston P (_fig._ 23.) to be
restored to its original position, with the liquid water beneath
it; and, in addition to the weight of the atmosphere which before
pressed it down, let us suppose another weight of fifteen pounds
laid upon it, so that the water below shall be pressed by double
the weight of the atmosphere. If the lamp were now applied, and at
the same time a thermometer were immersed in the water, it would
be found that the water would not begin to be converted into steam
until it attained the temperature of about 250°. The piston would
then begin, as before, to ascend, and the water to be gradually
converted into vapour. The water being completely evaporated, it
would be found that the piston would be raised to a height little
more than half its former height, or 72 feet. The mechanical
effect, therefore, thus obtained, will be equivalent to double the
former weight raised half the former height.
In like manner, if the piston were loaded with thirty pounds in
addition to the atmosphere, the whole pressure on the water being
then three times the pressure first supposed, the piston would be
raised to somewhat more than one third of its first height by the
evaporation of the water. This would give a mechanical force
equivalent to three times the original weight raised a little more
than one third of the original height.
In general, as the pressure on the piston is increased, the height
to which the piston would be raised by the evaporation of the
water will be diminished in a proportion somewhat less than the
proportion in which the pressure on the piston is increased. If
the temperature at which the water is converted into steam under
these different pressures were the same, then the height to which
the piston would be raised by the evaporation of the water would
be diminished in precisely [Pg118] the same proportion as the
pressure on the piston is increased; and, in that case, the whole
mechanical force developed by the evaporation of the water would
remain exactly the same under whatever pressure the water might be
boiled. We shall explain hereafter the extent to which the
variation of temperature in the water and steam corresponding to
the variation of pressure modifies this law; but, as the effect of
the difference of temperatures is not considerable, it will be
convenient to register in the memory the following important
practical conclusion:—
(67.) _A cubic inch of water converted into steam will supply a
mechanical force very nearly equal to a ton weight raised a foot
high; and this force will not be subject to considerable
variation, whatever be the temperature or pressure at which the
water may be evaporated._
[Illustration: GLASGOW.]
FOOTNOTES:
[18] The preceding paragraphs, and some other parts of the
present volume on the general properties of Heat, are taken
from my Treatise on Heat, in the _Cabinet Cyclopœdia_, to
which those who desire more detailed explanation and more
copious illustration should refer.
[Pg119]
[Illustration: GLASGOW COLLEGE.]
CHAP. V.
WATT FINDS THAT CONDENSATION IN THE CYLINDER IS INCOMPATIBLE
WITH A DUE ECONOMY OF FUEL. — CONCEIVES THE NOTION OF
CONDENSING OUT OF THE CYLINDER. — DISCOVERS SEPARATE
CONDENSATION. — INVENTS THE AIR-PUMP. — SUBSTITUTES STEAM
PRESSURE FOR ATMOSPHERIC PRESSURE. — INVENTS THE STEAM CASE,
OR JACKET. — HIS FIRST EXPERIMENTS TO REALISE THESE
INVENTIONS. — HIS EXPERIMENTAL APPARATUS. — DIFFICULTIES OF
BRINGING THE IMPROVED ENGINES INTO USE. — WATT PRACTISES AS A
CIVIL ENGINEER. — HIS PARTNERSHIP WITH ROEBUCK. — HIS FIRST
PATENT. — DESCRIPTION OF HIS SINGLE-ACTING STEAM ENGINE.
(68.) At the period to which we have now brought the history of
the invention of the steam engine, Watt had obtained, chiefly by
his own experiments, a sufficient knowledge of the phenomena which
have been just explained, to enable him to arrive at the
conclusion that a very small proportion of the whole mechanical
effect attending the evaporation was really rendered available by
the atmospheric engine; and that, [Pg120] therefore, extensive
and injurious sources of waste existed in its machinery.
He perceived that the principal source of this wasteful
expenditure of power consisted in the quantity of steam which was
condensed at each stroke of the piston, in heating the cylinder
previous to the ascent of the piston. Yet, as it was evident that
that ascent could not be accomplished in a cold cylinder, it was
apparent that this waste of power must be inevitable, unless some
expedient could be devised, by which _a vacuum could be maintained
in the cylinder, without cooling it_. But, to produce such a
vacuum, the steam must be condensed; and, to condense the steam,
its temperature must be lowered to such a point that the vapour
proceeding from it shall have no injurious pressure; yet, if
condensed steam be contained in a cylinder at a high temperature,
it will return to the temperature of the cylinder, recover its
elasticity, and resist the descent of the piston.
Having reflected on these circumstances, it became apparent to
Watt, that a vice was inherent in the structure of the atmospheric
engine, which rendered a large waste of power inevitable; this
vice arising from the fact, that the condensation of the steam was
incompatible with the condition of maintaining the elevated
temperature of the cylinder in which that condensation took place.
It followed, therefore, either that the steam must be imperfectly
condensed, or that the condensation could not take place in the
cylinder. It was in 1765, that, pondering on these circumstances,
the happy idea occurred to him, that the production of a vacuum
could be equally effected, though _the place_ where the
condensation of the steam took place were not the cylinder itself.
He saw, that if a vessel in which a vacuum was produced were put
into communication with another containing an elastic fluid, the
elastic fluid would rush into the vacuum, and diffuse itself
through the two vessels; but if, on rushing into such vacuum, this
elastic fluid, being vapour, were there condensed, or restored to
the liquid form, that then the space within the two vessels would
be equally rendered a vacuum;—that, under such circumstances, one
of the vessels might be maintained at any temperature, however
high, while [Pg121] the other might be kept at any temperature,
however low. This felicitous conception formed the first step in
that splendid career of invention and discovery which has
conferred immortality on the name of Watt. He used to say, that
the moment the idea of separate condensation occurred to
him,—that is, of condensing, in one vessel kept cold, the steam
coming from another vessel kept hot,—all the details of his
improved engine rushed into his mind in such rapid succession,
that, in the course of a day, his invention was so complete that
he proceeded to submit it to experiment.
[Illustration: _Fig._ 19.]
To explain the first conception of this memorable invention; let a
tube or pipe, S (_fig._ 19.), be imagined to proceed from the
bottom of the cylinder A B to a vessel, C, having a stop-cock, D,
by which the communication between the cylinder and the vessel C
may be opened or closed at pleasure. If we suppose the piston P at
the top of the cylinder, and the space below it filled with steam,
the cylinder and steam being at the usual temperature, while the
vessel C is a vacuum, and maintained at a low temperature. Then,
on opening the cock D, the steam will rush from the cylinder A B
through the tube S, and, passing into the cold vessel C, will be
condensed by contact with its cold sides. This process of
condensation will be rendered instantaneous if a jet of cold water
is allowed to play in the vessel C. When the steam thus rushing
into C, has been destroyed, and the space in the cylinder A B
becomes a vacuum, then the pressure of the atmosphere being
unobstructed, the piston will descend with the force due to the
excess of the pressure of the atmosphere above the friction. When
it has descended, suppose the stop-cock D closed, and steam
admitted from [Pg122] the boiler through a proper cock or valve
below the piston, the cylinder and piston being still at the same
temperature as before. The steam on entering the cylinder, not
being exposed to contact with any surface below its own
temperature, will not be condensed, and therefore will immediately
cause the piston to rise, and the piston will have attained the
top of the cylinder when as much steam shall have been supplied by
the boiler as will fill the cylinder. When this has taken place,
suppose the communication with the boiler cut off, and the cock D
once more opened: the steam will again rush through the pipe S
into the vessel C, where encountering the cold surface and the jet
of cold water, it will be condensed, and the vacuum, as before,
will be produced in the cylinder A B; that cylinder still
maintaining its temperature, the piston will again descend, and so
the process may be continued.
(69.) Having carried the invention to this point, Watt saw that
the vessel C would gradually become heated by the steam which
would be continually condensed in it. To prevent this, as well as
to supply a constant jet of cold water, he proposed to keep the
vessel C submerged in a cistern of cold water, from which a pipe
should conduct a jet to play within the vessel, so as to condense
the steam as it would pass from the cylinder.
But here a difficulty presented itself, against which it was
necessary to provide. The cold water admitted through the jet to
condense the steam, mixed with the condensed steam itself, would
gradually collect in the vessel C, and at length choke it. To
prevent this, Watt proposed to put the vessel C in communication
with a pump F, which might be wrought by the engine itself, and by
which the water, which would collect in the bottom of the vessel
C, would be constantly drawn off. This pump would be evidently
rendered the more necessary, since more or less atmospheric air,
always combined with water in its common state, would enter the
vessel C by the condensing jet. This air would be disengaged in
the vessel C by the heat of the steam condensed therein; and it
would rise through the tube S, and vitiate the vacuum in the
cylinder;—an effect which would be rendered the more injurious,
[Pg123] inasmuch as, unlike steam, this elastic fluid would be
incapable of being condensed by cold. The pump F, therefore, by
which Watt proposed to draw off the water from the vessel C, might
also be made to draw off the air, or the principal part of it.
The vessel C was subsequently called a _condenser_; and, from the
circumstances just adverted to, the pump F has been called the
_air-pump_.
These—namely, the cylinder, the condenser, and the air-pump—were
the three principal parts in the invention, as it first presented
itself to the mind of Watt—and even before it was reduced to a
model, or submitted to experiment. But, in addition to these,
other two improvements offered themselves in the very first stage
of its progress.
In the atmospheric engine, the piston was maintained steam-tight
in the cylinder by supplying a stream of cold water above it, by
which the small interstices between the piston and cylinder would
be stopped. It is evident that the effect of this water as the
piston descended would be to cool the cylinder, besides which any
portion of it which might pass between the piston and cylinder and
which would pass below the piston, would boil the moment it would
fall into the cylinder, which itself would be maintained at the
boiling temperature. This water, therefore, would produce steam,
the pressure of which would resist the descent of the piston.
Watt perceived, that even though this inconvenience were removed
by the use of oil or tallow upon the piston, still, that as the
piston would descend in the cylinder, the cold atmosphere would
follow it; and would, to a certain extent, lower the temperature
of the cylinder. On the next ascent of the piston, this
temperature would have to be again raised to 212° by the steam
coming from the boiler, and would entail upon the machine a
proportionate waste of power.
If the atmosphere of the engine-house could be kept heated to the
temperature of boiling water, this inconvenience would be removed.
The piston would then be pressed down by air as hot as the steam
to be subsequently introduced into it. On further consideration,
however, it occurred to Watt that it would be still more
advantageous if the cylinder itself could be [Pg124] worked in an
atmosphere of steam, having only the same pressure as the
atmosphere. Such steam would press the piston down as effectually
as the air would; and it would have the further advantage over
air, that if any portion of it leaked through between the piston
and cylinder, it would be condensed, which could not be the case
with atmospheric air. He therefore determined on surrounding the
cylinder by an external casing, the space between which and the
cylinder he proposed to be filled with steam supplied from the
boiler. The cylinder would thus be enclosed in an atmosphere of
its own, independent of the external air, and the vessel so
enclosing it would only require to be a little larger than the
cylinder, and to have a close cover at the top, the centre of
which might be perforated with a hole to admit the rod of the
piston to pass through, the rod being made smooth, and so fitted
to the perforation that no steam should escape between them. This
method would be attended also with the advantage of keeping the
cylinder and piston always heated, not only inside but outside;
and Watt saw that it would be further advantageous to employ the
pressure of steam to drive the piston in its descent instead of
the atmosphere, as its intensity or force would be much more
manageable; for, by increasing or diminishing the heat of the
steam in which the cylinder was enclosed, its pressure might be
regulated at pleasure, and it might be made to urge the piston
with any force that might be required. The power of the engine
would therefore be completely under control, and independent of
all variations in the pressure of the atmosphere.
(70.) This was a step which totally changed the character of the
machine, and which rendered it a STEAM ENGINE instead of an
ATMOSPHERIC ENGINE. Not only was the vacuum below the piston now
produced by the property of steam, in virtue of which it is
reconverted into water by cold; but the pressure which urged the
piston into this vacuum was due to the elasticity of steam.
The external cylinder, within which the working cylinder was
enclosed, was called THE JACKET, and is still very generally used.
[Illustration: _Fig._ 20.]
(71.) The first experiment in which Watt attempted to [Pg125]
realise, on a small scale, his conceptions, was made in the
following manner. The cylinder of the engine was represented by a
brass syringe A B (_fig._ 20.) an inch and a third in diameter, and
ten inches in length, to which a top and a bottom of tin plate was
fitted. Steam was conveyed by a pipe, S, from a small boiler into
the lower end of this syringe, a communication being made with the
upper end of the syringe by a branch pipe D. For the greater
convenience of the experiment, it was found desirable to invert the
position of the cylinder, so that the steam should press the piston
P upwards instead of downwards. The piston-rod R therefore was
presented downwards. An eduction pipe E was also inserted in the top
of the cylinder, which was carried to the condenser. The piston-rod
was made hollow, or rather a hole was drilled longitudinally through
it, and a valve was fitted at its lower end, to carry off the water
produced by the steam, which [Pg126] would be condensed in the
cylinder in the commencement of the process. The condenser used in
this experiment operated without injection, the steam being
condensed by the contact of cold surfaces. It consisted of two thin
pipes F, G of tin, ten or twelve inches in length, and the sixth of
an inch in diameter, standing beside each other perpendicularly, and
communicating at the top with the eduction pipe, which was provided
with a valve opening upwards. At the bottom these two pipes
communicated with another tube I of about an inch in diameter, by a
horizontal pipe, having in it a valve, M, opening towards I, fitted
with a piston K, which served the office of the air-pump, being
worked by the hand. This piston, K, had valves in it opening
upwards. These condensing pipes and air-pump were immersed in a
small cistern, filled with cold water. The steam was conveyed by the
steam-pipe S to the bottom of the cylinder, a communication between
the top and bottom of the cylinder being occasionally opened by a
cock, C, placed in the branch pipe. The eduction pipe leading to the
condenser also had a cock, L, by which the communication between the
top of the cylinder and the condenser might be opened and closed at
pleasure. In the commencement of the operation, the cock N admitting
steam from the boiler, and the cock L opening a communication
between the cylinder and the condenser, and the cock C opening a
communication between the top and bottom of the cylinder, being all
open, steam rushed from the boiler, passing through all the pipes,
and filling the cylinder. A current of mixed air and steam was thus
produced through the eduction pipe E, through the condensing pipes F
and G, and through the air-pump I, which issued from the valve H in
the eduction pipe, and from the valve in the air-pump piston, all of
which opened upwards. The steam also in the cylinder passed through
the hole drilled in the piston-rod, and escaped, mixed with air,
through the valve in the lower end of that rod. This process was
continued until all the air in the cylinder, pipes, and condenser,
was blown out, and all these spaces filled with pure steam. The
cocks L, C, and N, were then closed, and the atmospheric pressure
closed the valve H and the valves in the air-pump piston. The cold
surfaces condensing the steam in [Pg127] the pipes F and G, and in
the lower part of the air-pump, a vacuum was produced in these
spaces. The cock C being now closed, and the cocks L and N being
open, the steam in the upper part of the cylinder rushed through the
pipe E into the condenser, where it was reduced to water, so that a
vacuum was left in the upper part of the cylinder. The steam from
the boiler passing below the piston, pressed it upwards with such
force, that it lifted a weight of eighteen pounds hung from the end
of the piston-rod. When the piston reached the top of the cylinder,
the cocks L and N were closed, and the cock C opened. All
communication between the cylinder and the boiler, as well as
between the cylinder and the condenser, were now cut off, and the
steam in the cylinder circulated freely above and below the piston,
by means of the open tube D. The piston, being subject to equal
forces upwards and downwards, would therefore descend by its own
weight, and would reach the bottom of the cylinder. The air-pump
piston meanwhile being drawn up, the air and the condensed steam in
the tubes F and G were drawn into the air-pump I, through the open
horizontal tube at the bottom. Its return was stopped by the valve
M. By another stroke of the air-pump, this water and air were drawn
out through valves in the piston, which opened upwards. The cock C
was now closed, and the cocks L and N opened, preparatory to another
stroke of the piston. The steam in the upper part of the cylinder
rushed, as before, into the tubes F and G, and was condensed by
their cold surfaces, while steam from the boiler coming through the
pipe S, pressed the piston upwards. The piston again ascended with
the same force as before, and in the same manner the process was
continually repeated.
(72.) The quantity of steam expended in this experimental model in
the production of a given number of strokes of the piston was
inferred from the quantity of water evaporated in the boiler; and
on comparing this with the magnitude of the cylinder and the
weight raised by the pressure of the steam, the contrivance was
proved to affect the economy of steam, as far as the imperfect
conditions of such a model could have permitted. A larger model
was next constructed, having an outer cylinder, or steam case,
surrounding the working cylinder, and [Pg128] the experiments
made with it fully realised Watt's expectations, and left no doubt
of the great advantages which would attend his invention. The
weights raised by the piston proved that the vacuum in the
cylinder produced by the condensation was almost perfect; and he
found that when he used water in the boiler which by long boiling
had been well cleared of air, the weight raised was not much less
than the whole amount of the pressure of the steam upon the
piston. In this larger model, the cylinder was placed in the usual
position, with a working lever and other apparatus similar to that
employed in the Atmospheric Engine.
(73.) It was in the beginning of the year 1765, Watt being then in
the twenty-ninth year of his age, that he arrived at these great
discoveries. The experimental models just described, by which his
invention was first reduced to a rude practical test, were fitted
up at a place called Delft House, in Glasgow. It will doubtless,
at the first view, be a matter of surprise that improvements of
such obvious importance in the economy of steam power, and capable
of being verified by tests so simple, were not immediately adopted
wherever atmospheric engines were used. At the time, however,
referred to, Watt was an obscure artisan, in a provincial town,
not then arrived at the celebrity to which it has since attained,
and the facilities by which inventions and improvements became
public were much less than they have since become. It should also
be considered that all great and sudden advances in the useful
arts are necessarily opposed by the existing interests with which
their effects are in conflict. From these causes of opposition,
accompanied with the usual influence of prejudice and envy, Watt
was not exempt, and was not therefore likely suddenly to
revolutionise the arts and manufactures of the country by
displacing the moving powers employed in them, and substituting an
engine, the efficacy and power of which depended mainly on
physical principles, then altogether new and but imperfectly
understood.
Not having the command of capital, and finding it impracticable to
inspire those who had, with the same confidence in the advantages
of his invention which he himself felt, he was [Pg129] unable to
take any step towards the construction of engines on a large
scale. Soon after this, he gave up his shop in Glasgow, and
devoted himself to the business of a Civil Engineer. In this
capacity he was engaged to make a survey of the river Clyde, and
furnished an elaborate and valuable Report upon its projected
improvements. He was also engaged in making a plan of the canal,
by which the produce of the Monkland Colliery was intended to be
carried to Glasgow, and in superintending the execution of that
work. Besides these, several other engineering enterprises
occupied his attention, among which may be mentioned, the
navigable canal across the isthmus of Crinan, afterwards completed
by Rennie; improvements proposed in the ports of Ayr, Glasgow, and
Greenock; the construction of the bridges at Hamilton, and at
Rutherglen; and the survey of the country through which the
celebrated Caledonian canal was intended to be carried.
"If, forgetful of my duties as the organ of this academy," says M.
Arago (whose eloquent observations on the delays of this great
invention, addressed to the assembled members of the National
Institute of France, we cannot forbear to quote), "I could think
of making you smile, rather than expressing useful truths, I would
find here matter for a ludicrous contrast. I would call to your
recollection the authors, who at our weekly sittings demand with
all their might and main (_à cor et à cris_) an opportunity to
communicate some little remark—some small reflection—some
trifling note, conceived and written the night before; I would
represent them to you cursing their fate, when according to your
rules, the reading of their communication is postponed to the next
meeting, although during this cruel week, they are assured that
their important communication is deposited in our archives in a
sealed packet. On the other hand, I would point out to you the
creator of a machine, destined to form an epoch in the annals of
the world, undergoing patiently and without murmur, the stupid
contempt of capitalists,—conscious of his exalted genius, yet
stooping for eight years to the common labour of laying down
plans, taking levels, and all the tedious calculations connected
with the routine of common engineering. While in this conduct you
cannot fail to recognise the serenity, [Pg130] the moderation,
and the true modesty of his character, yet such indifference,
however noble may have been its causes, has something in it not
altogether blameless. It is not without reason that society visits
with severe reprobation those who withdraw gold from circulation
and hoard it in their coffers. Is he less culpable who deprives
his country, his fellow citizens, his age, of treasures a thousand
times more precious than the produce of the mine; who keeps to
himself his immortal inventions, sources of the most noble and
purest enjoyment of the mind, who abstains from conferring upon
labour those powers, by which would be multiplied in an infinite
proportion the products of industry, and by which, with advantage
to civilisation and human nature, he would smooth away the
inequalities of the conditions of man."[19]
(74.) Although Watt was thus attracted by pursuits foreign to his
recent investigations respecting the improvement of steam power,
he never lost sight of that object. It was not until the year
1768, three years after his great discoveries, that any step was
taken to enable him to carry them into effect on a large scale. At
that time his friends brought him into communication with Dr.
Roebuck, the proprietor of the Carron Iron Works, who rented
extensive coal works at Kinneal from the Duchess of Hamilton. Watt
was first employed by Roebuck as a civil engineer; but when he
made known to him the improvements he had projected in the steam
engine, Roebuck proposed to take out a patent for an engine on the
principle of the model which had been fitted up at Delft House,
and to join Watt in a partnership, for the construction of such
engines. Sensible of the advantages to be derived from the
influence of Roebuck, and from his command of capital, Watt agreed
to cede to him two thirds of the advantages to be derived from the
invention. A patent was accordingly taken out on the fifth of
January, 1769, nearly four years after the invention had been
completed; and an experimental engine on a large scale was
constructed by him, and fitted up at Kinneal House. In the first
trial this machine more than fulfilled Watt's anticipations. Its
[Pg131] success was complete. In the practical details of its
construction, however, some difficulties were still encountered,
the greatest of which consisted in packing the piston, so as to be
steam-tight. The principle of the new engine did not admit of
water being kept upon the piston, to prevent leakage, as in the
old engines; he was therefore obliged to have his cylinders much
more accurately bored, and more truly cylindrical, and to try a
great variety of soft substances for packing the piston, which
would make it steam-tight without great friction, and maintain it
so in a situation perfectly dry, and at the temperature of boiling
water.
While Watt was endeavouring to overcome these and other
difficulties, in the construction of the machine, his partner, Dr.
Roebuck, became embarrassed, by the failure of his undertaking in
the Borrowstowness coal and salt works; and he was unable to
supply the means of prosecuting with the necessary vigour the
projected manufacture of the new engines.
The important results of Watt's labours having happily at this
time become more publicly known, Mr. Matthew Boulton, whose
establishment at Soho, near Birmingham, was at that time the most
complete manufactory for metal-work in England, and conducted with
unexampled enterprise and spirit, proposed to purchase Dr.
Roebuck's interest in the patent. This arrangement was effected in
the year 1773, and in the following year Mr. Watt removed to Soho,
where a portion of the establishment was allotted to him, for the
erection of a foundery, and other works necessary to realise his
inventions on a grand scale.
The patent which had been granted in 1769 was limited to a period
of fourteen years, and would consequently expire about the year
1783. From the small progress which had hitherto been made in the
construction of engines upon the new principle, and from the many
difficulties still to be encountered, and the large expenditure of
capital which must obviously be incurred before any return could
be obtained, it was apparent that unless an extension of the
patent right could be obtained, Boulton and Watt could never
expect any advantage adequate to the risk of their great [Pg132]
enterprise. In the year 1774 an application was accordingly made
to parliament for an extension of the patent, which was supported
by the testimony of Dr. Roebuck, Mr. Boulton, and others, as to
the merits and probable utility of the invention. An Act was
accordingly passed, in 1775, extending the term of the patent
until the year 1800.
(75.) The following abstract of this Act may not be uninteresting
at this time, when the anticipations expressed in it have been so
successfully and extensively realised:—
"An Act for vesting in James Watt, engineer, his executors,
administrators, and assigns, the sole use and property of certain
steam engines, commonly called fire engines, of his invention,
throughout his majesty's dominions, for a limited time:
"And whereas the said James Watt hath employed many years, and a
considerable part of his fortune, in making experiments upon steam
engines, commonly called fire engines, with a view to improve
those very useful machines, by which several very considerable
advantages over the common steam engines are acquired; but upon
account of the many difficulties which always arise in the
execution of such large and complex machines, and of the long time
requisite to make the necessary trials, he could not complete his
intention before the end of the year 1774, when he finished some
large engines as specimens of his construction, which have
succeeded, so as to demonstrate the utility of the said invention:
"And whereas, in order to manufacture these engines with the
necessary accuracy, and so that they may be sold at moderate
prices, a considerable sum of money must be previously expended in
erecting mills and other apparatus; and as several years and
repeated proofs will be required before any considerable part of
the public can be fully convinced of the utility of the invention,
and of their interest to adopt the same, the whole term granted by
the said letters patent may probably elapse before the said James
Watt can receive an advantage adequate to his labour and
invention:
"And whereas, by furnishing mechanical power at much less expense,
and in more convenient forms, than has hitherto been done, his
engines may be of great utility, in facilitating [Pg133] the
operations in many great works and manufactures of this kingdom;
yet it will not be in the power of the said James Watt to carry
his invention into that complete execution which he wishes, and so
as to render the same of the highest utility to the public of
which it is capable, unless the term granted by the said letters
patent be prolonged, and his property in the said invention
secured for such time as may enable him to obtain an adequate
recompense for his labour, time, and expense:
"To the end, therefore, that the said James Watt may be enabled
and encouraged to prosecute and complete his said invention, so
that the public may reap all the advantages to be derived
therefrom in their fullest extent: it is enacted,
"That from and after the passing of this Act, the sole privilege
and advantage of making, constructing, and selling the said
engines hereinbefore particularly described, within the kingdom of
Great Britain, and his majesty's colonies and plantations abroad,
shall be, and are hereby declared to be, vested in the said James
Watt, his executors, administrators, and assigns, for and during
the term of twenty-five years," &c. &c.
(76.) Thus protected and supported, Watt now directed the whole
vigour of his mind to perfect the practical details of his
invention, and the result was, the construction on a large scale
of the engine which has since been called his SINGLE ACTING STEAM
ENGINE.
It is necessary to recollect, that notwithstanding the extensive and
various application of steam power in the arts and manufactures, at
the time to which our narrative has now reached, the steam engine
had never been employed for any other purpose save that of raising
water by working pumps. The motion, therefore, which was required
was merely an upward force, such as was necessary to elevate the
piston of a pump, loaded with the column of water which it raised.
The following then is a description of the improved engine of Watt,
by which such work was proposed to be performed:—
[Illustration: _Fig._ 21.]
In the cylinder represented at C (_fig._ 21.), the piston P moves
steam-tight. It is closed at the top, and the piston [Pg134] rod,
being accurately turned, runs in a steam-tight collar, B,
furnished with a stuffing-box, and is constantly lubricated with
melted tallow. A funnel is screwed into the top of the cylinder,
through which, by opening a stop-cock, melted [Pg135] tallow is
permitted from time to time to fall upon the piston within the
cylinder, so as to lubricate it, and keep it steam-tight. Two
boxes, A A, called the upper and lower steam boxes, contain valves
by which steam from the boiler may be admitted and withdrawn.
These steam boxes are connected by a tube of communication T, and
they communicate with the cylinder at the top and bottom by short
tubes represented in the figure. The upper steam box A contains
one valve, by which a communication with the boiler may be opened
or closed at pleasure. The lower valve box contains two valves.
The lower valve I communicates with the tube T′, leading to the
condenser D, which being opened or closed, a communication is made
or cut off at pleasure, between the cylinder C and the condenser
D. A second valve, or upper valve H, which is represented closed
in the figure, may be opened so as to make a free communication
between the cylinder C and the tube T, and by that means between
the cylinder C, below the piston and the space above the piston.
The condenser D is submerged in a cistern of cold water. At the
side there enters it a tube, E, governed by a cock, which being
opened or closed to any required extent, a jet of cold water may
be allowed to play in the condenser, and may be regulated or
stopped, at pleasure. This jet, when playing, throws the water
upwards in the condenser towards the mouth of the tube T′, as
water issues from the rose of a watering pot. The tube S proceeds
from the boiler, and terminates in the steam box A, so that the
steam supplied from the boiler constantly fills that box. The
valve G is governed by levers, whose pivots are attached to the
framing of the engine, and is opened or closed at pleasure, by
raising or lowering the lever G′. The valve G, when open, will
therefore allow steam to pass from the boiler through the short
tube to the top of the piston, and this steam will also fill the
tube T. If the lower valve H be closed, its circulation beyond
that point will be stopped; but if the valve H be open, the valve
I being closed, then the steam will circulate equally in the
cylinder, above and below the piston. If the valve I be open, then
steam will rush through the tube T′ into the condenser; but this
escape of the steam will be [Pg136] stopped, if the valve I be
closed. The valve H is worked by the lever H′, and the valve I by
the lever I′.
The valve G is called the upper steam valve, H the lower steam
valve, I the exhausting valve, and E the condensing valve.
From the bottom of the condenser D proceeds a tube leading to the
air-pump, which is also submerged in the cistern of cold water. In
this tube is a valve M, which opens outwards from the condenser
towards the air-pump. In the piston of the air-pump N is a valve
which opens upwards. The piston-rod Q of the air-pump is attached
to a beam of wood called a plug frame, which is connected with the
working beam by a flexible chain playing on the small arch-head
immediately over the air-pump. From the top of the air-pump barrel
above the piston proceeds a pipe or passage leading to a small
cistern, B, called the hot well. The pipe which leads to this
well, is supplied with a valve, K, which opens outwards from the
air pump barrel towards the well. From the nature of its
construction, the valve M admits the flow of water from the
condenser towards the air-pump, but prevents its return; and, in
like manner, the valve K admits the flow of water from the upper
part of the air-pump barrel into the hot well B, but obstructs its
return.
Let us now consider how these valves should be worked in order to
move the piston upwards and downwards with the necessary force. It
is in the first place necessary that all the air which fills the
cylinder, the tubes and the condenser shall be expelled. To
accomplish this it is only necessary to open at once the three
valves G, H, and I. The steam then rushing from the boiler through
the steam-pipe S, and the open valve G will pass into the cylinder
above the piston, will fill the tube T, pass through the lower
steam valve H, will fill the cylinder C below the piston, and will
pass through the open valve I into the condenser. If the valve E
be closed so that no jet shall play in the condenser, the steam
rushing into it will be partially condensed by the cold surfaces
to which it will be exposed; but if the boiler supply it through
the pipe S in sufficient abundance, it will rush with violence
through the cylinder and all the passages, and its pressure in
the [Pg137] condenser D, combined with that of the heated air
with which it is mixed, will open the valve M, and it will rush
through mixed with the air into the air-pump barrel N. It will
press the valves in the air-pump piston upwards, and, opening
them, will rush through, and will collect in the air-pump barrel
above the piston. It will then, by its pressure, open the valve K,
and will escape into the cistern B.
Throughout this process the steam, which mixed with the air fills
the cylinder, condenser and air-pumps will be only partially
condensed in the last two, and it will escape mixed with air
through the valve K, and this process will continue until all the
atmospheric air which at first filled the cylinder, tubes,
condenser and air-pump barrel shall be expelled through the valve
K, and these various spaces shall be filled with pure steam. When
that has happened let us suppose all the valves closed. In closing
the valve I the flow of steam to the condenser will be stopped,
and the steam contained in it will speedily be condensed by the
cold surface of the condenser, so that a vacuum will be produced
in the condenser, the condensed steam falling in the form of water
to the bottom. In like manner, and for like reasons, a vacuum will
be produced in the air-pump. The valve M, and the valves in the
air-pump piston will be closed by their own weight.
By this process, which is called _blowing through_, the
atmospheric air, and other permanent gases, which filled the
cylinder, tubes, condenser and air-pump are expelled, and these
spaces will be a vacuum. The engine is then prepared to be
started, which is effected in the following manner:—The upper
steam valve G is opened, and steam allowed to flow from the boiler
through the passage leading to the top of the cylinder. This steam
cannot pass to the bottom of the cylinder, since the lower steam
valve H is closed. The space in the cylinder below the piston
being therefore a vacuum, and the steam pressing above it the
piston will be pressed downwards with a corresponding force. When
it has arrived at the bottom of the cylinder the steam valve G
must be closed, and at the same time the valve H opened. The valve
I leading to the condenser being also closed, the steam [Pg138]
which fills the cylinder above the piston is now admitted to
circulate through the open valve H below the piston, so that the
piston is pressed equally upwards and downwards by steam, and
there is no force to resist its movement save its friction with
the cylinder. The weight of the pump rods on the opposite end of
the beam being more than equivalent to overcome this the piston is
drawn to the top of the cylinder, and pushes before it the steam
which is drawn through the tube T, and the open valve H, and
passes into the cylinder C below the piston.
[Illustration: _Fig._ 22.]
When the piston has thus arrived once more at the top of the
cylinder, let the valve H be closed, and at the same time the
valves G and I opened, and the condensing cock E also opened, so
as to admit the jet to play in the condenser. The steam which
fills the cylinder C below the piston, will now rush through the
open valve I into the condenser which has been hitherto a vacuum,
and there encountering the jet, will be instantly converted into
water, and a mixture of condensed steam and injected water will
collect in the bottom of the condenser. At the same time, the
steam proceeding from the boiler by the steam pipe S to the upper
steam box A, will pass through the open steam valve G to the top
of the piston, but cannot pass below it because of the lower steam
valve H being closed. The piston, thus acted upon above by the
pressure of the steam, and the space in the cylinder below it
being a vacuum, its downward motion is resisted by no force but
the friction, and it is therefore driven to the bottom of the
cylinder. During its descent the valves G, I, and E remained open.
At the moment it arrives at the bottom of the cylinder, all these
three valves are closed, and the valve H opened. The steam which
fills the cylinder above the piston is now permitted to circulate
below it, by the open valve H, and the piston being consequently
pressed equally upwards and downwards will be drawn upwards as
before by the preponderance of the pump rods at the opposite end
of the beam. The weight of these rods must also be sufficiently
great to draw the air-pump piston N upwards. As this piston rises
in the air-pump, it leaves a vacuum below it into which the water
and air collected in the condenser will be drawn through the valve
M, which opens outwards. When the [Pg139] air-pump piston has
arrived at the top of the barrel, which it will do at the same
time that the steam piston arrives at the top of the cylinder, the
water and the chief part of the air or other fluids which may have
been in the condenser will be drawn into the barrel of the
air-pump, and the valve M being closed by its own weight, assisted
by the pressure of these fluids they cannot return into the
condenser. At the moment the steam piston arrives at the top of
the cylinder, the valve H is closed, and the three valves G, I,
and E are opened. The effect of this change is the same as was
already described in the former case, and the piston will in the
same manner and from the same causes be driven downwards. The
air-pump piston will at the same time descend by the force of its
own weight, aided by the weight of the plug-frame attached to its
rod. As it descends, the air below it will be gradually compressed
above the surface of the water in the bottom of the barrel, until
its pressure becomes sufficiently great to open the valves in the
air-pump piston. When this happens, the valves in the air-pump
piston, as represented on a large scale in _fig._ 22., will be
opened, and the air will pass through them above the piston. When
the piston comes in contact with the water in the bottom of the
barrel, this water will likewise pass through the open valves.
When the piston has arrived at the bottom of the air-pump barrel,
the valves in it will be closed by the pressure of the fluids
above them. The next ascent of the steam piston will draw up the
air-pump piston, and with it the fluids in the pump barrel above
it. As the air-pump [Pg140] piston approaches the top of its
barrel, the air and water above it will be drawn through the valve
K into the hot cistern B. The air will escape in bubbles through
the water in that cistern, and the warm water will be deposited in
it.
The magnitude of the opening in the condensing valve E, must be
regulated by the quantity of steam admitted to the cylinder. As
much water ought to be supplied through the injection valve as
will be sufficient to condense the steam contained in the
cylinder, and also to reduce the temperature of the water itself,
when mixed with the steam, to a sufficiently low degree to prevent
it from producing vapour of a pressure which would injuriously
affect the working of the piston. It has been shown, that five and
a half cubic inches of ice-cold water mixed with one cubic inch of
water in the state of steam would produce six and a half cubic
inches of water at the boiling temperature. If then the cylinder
contained one cubic inch of water in the state of steam, and only
five and a half cubic inches of water were admitted through the
condensing jet, supposing this water, when admitted, to be at the
temperature of 32°, then the consequence would be that six and a
half cubic inches of water at the boiling temperature would be
produced in the condenser. Steam would immediately arise from
this, and at the same time the temperature of the remaining water
would be lowered by the amount of the latent heat taken up by the
steam so produced. This vapour would rise through the open
exhausting valve I, would fill the cylinder below the piston, and
would impair the efficiency of the steam above pressing it down.
The result of the inquiries of Watt respecting the pressure of
steam at different temperatures, showed, that to give efficiency
to the steam acting upon the piston it would always be necessary
to reduce the temperature of the water in the condenser to 100°.
Let us then see what quantity of water at the common temperature
would be necessary to produce these effects.
If the latent heat of steam be taken at 1000°, a cubic inch of
water in the state of steam may be considered for the purposes of
this computation, as equivalent to one cubic inch of water at
1212°. Now the question is, how many cubic inches of water at 60°
must be mixed with this, in order that the [Pg141] mixture may
have the temperature of 100°? This will be easily computed. As the
cubic inch of water at 1212° is to be reduced to 100°, it must be
deprived of 1112° of its temperature. On the other hand, as many
inches of water at 60° as are to be added, must be raised in the
same mixture to the temperature of 100°, and therefore each of
these must receive 40° of temperature. The number of cubic inches
of water necessary to be added will therefore be determined by
finding how often 40° are contained in 1112°. If 1112 be divided
by 40, the quotient will be 27·8. Hence it appears, that to reduce
the water in the condenser to the temperature of 100°, supposing
the temperature of the water injected to be 60°, it will be
necessary to supply by the injection cock very nearly twenty-eight
times as much water as passes through the cylinder in the state of
steam; and therefore if it be supposed that all the water
evaporated in the boiler passes through the cylinder, it follows
that about twenty-eight times as much water must be thrown into
the condenser as is evaporated in the boiler.
From these circumstances it will be evident that the cold cistern
in which the condenser and air-pump are submerged, must be
supplied with a considerable quantity of water. Independently of
the quantity drawn from it by the injection valve, as just
explained, the water in the cistern itself must be kept down to a
temperature of about 60°. The interior of the condenser and
air-pump being maintained by the steam condensed in them at a
temperature not less than 100°; the outer surfaces of these
vessels consequently impart heat to the water in the cold cistern,
and have therefore a tendency to raise the temperature of that
water. To prevent this, a pump called the _cold pump_, represented
at L in _fig._ 21., is provided. By this pump water is raised from
any convenient reservoir, and driven through proper tubes into the
cold cistern. This cold pump is wrought by the engine, the rod
being attached to the beam. Water being, bulk for bulk, heavier
the lower its temperature, it follows that the water supplied by
the cold pump to the cistern will have a tendency to sink to the
bottom, pressing upwards the warmer water contained in it. A
waste-pipe is provided, by which this [Pg142] water is drained
off, and the cistern therefore maintained at the necessary
temperature.
From what has been stated, it is also evident that the hot well B,
into which the warm water is thrown by the air-pump, will receive
considerably more water than is necessary to feed the boiler. A
waste-pipe, to carry off this, is also provided; and the quantity
necessary to feed the boiler is pumped up by a small pump, O, the
rod of which is attached to the beam, as represented in _fig._
21., and which is worked by the engine. The water raised by this
pump is conducted to a reservoir from which the boiler is fed, by
means which will be hereafter explained.
We shall now explain the manner in which the machine is made to
open and close the valves at the proper times. By referring to the
explanation already given, it will be perceived that at the moment
the piston reaches the top of the cylinder, the upper steam valve
G must be open, to admit the steam to press it down; while the
exhausting valve I must be opened, to allow the steam to pass to
the condenser; and the condensing valve E must be opened, to let
in the water necessary for the condensation of the steam; and at
the same time the lower steam valve H must be closed, to prevent
the passage of the steam which has been admitted through G. The
valves G, I, and E must be kept open, and the valve H kept closed,
until the piston arrives at the bottom of the cylinder, when it
will be necessary to close all the three valves, G, I, and E, and
to open the valve H, and the same effects must be produced each
time the piston arrives at the top and bottom of the cylinder. All
this is accomplished by a system of levers, which are exhibited in
_fig._ 21. The pivots on which these levers play are represented
on the framing of the engine, and the arms of the levers G′, H′,
and I′, communicating with the corresponding valves G, H, and I,
are represented opposite a bar attached to the rod of the
air-pump, called the _plug frame_. This bar carries certain pegs
and detents, which act upon the arms of the several levers in such
a manner that, on the arrival of the beam at the extremities of
its play upwards and downwards, the levers are so struck that the
valves are opened and closed at the proper [Pg143] times. It is
needless to explain all the details of this arrangement. Let it be
sufficient, as an example of all, to explain the method of working
the upper steam valve G. When the piston reaches the top of the
cylinder, a pin strikes the arm of the lever G′, and throws it
upwards: this, by means of the system of levers, pulls the arm of
the valve G downwards, by which the upper steam valve is raised
out of its seat, and a passage is opened from the steam pipe to
the cylinder. The valve is maintained in this state until the
piston reaches the bottom of the cylinder, when the arm G′ is
pressed downwards, by which the arm G is pressed upwards, and the
valve restored to its seat. By similar methods the levers
governing the other three valves, H, I, and E, are worked.
[Illustration: _Fig._ 23.]
[Illustration: _Fig._ 24.]
The valves used in these engines were of the kind called _spindle
valves_. They consisted of a flat circular plate of bell metal, A
B, _fig._ 23., with a round spindle passing perpendicularly
through its centre, and projecting above and below it. This valve,
having a conical form, was fitted very exactly, by grinding into a
corresponding circular conical seat, A B C D, _fig._ 24., which
forms the passage which it is the office of the valve to open and
close. When the valve falls into its seat, it fits the aperture
like a plug, so as entirely to stop it. The spindle plays in
sockets or holes, one above and the other below the aperture which
the valve stops; these holes keep the valve in its proper
position, so as to cause it to drop exactly into its place.
In the experimental engine made by Mr. Watt at Kinneal, he used
cocks, and sometimes sliding covers, like the regulator described
in the old engines; but these he found very soon to become leaky.
He was, therefore, obliged to change them for the spindle valves
just described, which, being truly [Pg144] ground, and accurately
fitted in the first instance, were not so liable to go out of
order. These valves are also called _puppet clacks_, or _button
valves_.
In the earlier engines constructed by Watt, the condensation was
produced by the contact of cold surfaces, without injection. The
reason of rejecting the method of condensing by injection was,
doubtless, to avoid the injurious effects of the air, which would
always enter the condenser, in combination with the water of
condensation, and vitiate the vacuum. It was soon found, however,
that a condenser acting by cold surfaces without injection, being
necessarily composed of narrow pipes or passages, was liable to
incrustation from bad water, by which the conducting power of the
material of the condenser was diminished; so that, while its outer
surface was kept cold by the water of the cold cistern, the inner
surface might, nevertheless, be so warm that a very imperfect
condensation would be produced.
[Illustration: SOHO, BIRMINGHAM.]
FOOTNOTES:
[19] Eloge, p. 308.
[Pg145]
[Illustration: BIRMINGHAM.]
CHAP. VI.
CORRESPONDENCE OF WATT WITH SMEATON. — FAILURE OF CONDENSATION
BY SURFACE. — IMPROVEMENTS IN CONSTRUCTION OF PISTON. — METHOD
OF PACKING. — IMPROVEMENTS IN BORING THE CYLINDERS. —
DISADVANTAGES OF THE NEW COMPARED WITH THE OLD ENGINES. —
GREATLY INCREASED ECONOMY OF FUEL. — EXPEDIENTS TO FORCE THE
NEW ENGINES INTO USE. — CORRESPONDENCE WITH SMEATON. —
EFFICIENCY OF FUEL IN THE NEW ENGINES. — DISCOVERY OF THE
EXPANSIVE ACTION OF STEAM. — WATT STATES IT IN A LETTER TO DR.
SMALL. — ITS PRINCIPLE EXPLAINED. — MECHANICAL EFFECT
RESULTING FROM IT. — COMPUTED EFFECT OF CUTTING OFF STEAM AT
DIFFERENT PORTIONS OF THE STROKE. — PRODUCES A VARIABLE POWER.
— EXPEDIENTS FOR EQUALISING THE POWER. — LIMITATION OF THE
EXPANSIVE PRINCIPLE IN WATT'S ENGINES. — ITS MORE EXTENSIVE
APPLICATION IN THE CORNISH ENGINES.
(77.) In a letter addressed by Watt to Smeaton, dated April, 1766,
Watt refers to some of these practical difficulties which he had
to encounter. "I have been," says he, "tormented with exceedingly
bad health, resulting from the operation of an anxious mind, the
natural consequence of staking everything [Pg146] upon the cast
of a die; for in that light I look upon every project which has
not received the sanction of repeated success.
"I have made considerable alterations in our engine lately,
particularly in the condenser. That which I used at first was
liable to be impaired, from incrustations from bad water;
therefore we have substituted one which works by an injection. In
pursuing this idea I have tried several kinds, and have at last
come to one, which I am not inclined to alter. It consists of a
jack-head pump, shut at bottom, with a common clack bucket, and a
valve in the cover of the pump, to discharge the air and water.
The eduction steam pipe, which comes from the cylinder,
communicates with this pump both above and below the bucket, and
has valves to prevent anything from going back from the pump to
the eduction pipe. The bucket descends by its own weight, and is
raised by the engine when the great piston descends, being hung to
the outer end of the great lever: the injection is made both into
the upper part of this pump and into the eduction pipe, and
operates beyond my ideas in point of quickness and perfection."
Besides the difficulty arising from incrustation, Watt found the
tubulated condensers, and indeed all other expedients for
condensing by cold surfaces, subject to a fatal objection. They
did not condense instantaneously, and although they were capable
of ultimately effecting the condensation, yet that process was not
completed until a great part of the stroke of the piston was made.
Thus during more or less of the stroke the uncondensed steam
resisted the piston, and robbed the moving power of a part of its
effect. This objection has ever attended condensation by surface.
[Illustration: _Fig._ 25.]
[Illustration: _Fig._ 26.]
(78.) Another source of difficulty arose from the necessity of
constructing the piston and cylinder with greater precision than
had been usual in the old engines. To fit the cover to the
cylinder so as to be steam-tight; to construct the piston rod so
as to move through it without allowing the escape of steam, and
yet at the same time without injurious friction; to connect the
piston rod with the piston, so as to drive the [Pg147] latter
through the cylinder with a perfectly straight and parallel
motion; to make such connection perfectly centrical and firm, and
yet to allow the piston in its ascent to come nearly into contact
with the cover of the cylinder—were all difficulties peculiar to
the new engine. In the atmospheric engine the shank of the piston
rod was rough and square, and the rod was secured to the piston by
two or four branches or stays, as represented in _fig._ 25. It is
evident that such a construction would be inadmissible in an
engine in which the piston in its ascent must be brought nearly
into contact with the close cover of the cylinder. Besides this
the piston rod of an atmospheric engine might throughout its whole
length have any form which was most convenient, and required no
other property than the strength necessary to work the beam. In
the new engine, on the contrary, it was necessary that it should
be accurately turned and finely polished, so as to pass through
the hole in the top of the cylinder, and be maintained in it
steam-tight. This was effected by a contrivance called a
_stuffing-box_ B, represented in _fig._ 26. A hole is made in the
cover of the cylinder very little greater in magnitude than the
diameter of the piston rod. Above this hole is a cup in which,
around the piston, is placed a stuffing of hemp or tow, which is
saturated with oil or melted tallow. This collar of hemp is
pressed down by another piece, also perforated with a hole through
which the piston rod plays, and which is screwed down on the said
collar of hemp.
(79.) Although the imperfect manner in which the interior of the
cylinders was then formed impaired the efficiency of the [Pg148]
new engines, yet such imperfections were not so injurious as in
the old atmospheric engines. Any imperfection of form of the inner
surface of the cylinder would necessarily cause more or less steam
or air to escape between the piston and cylinder. In the improved
engine this steam passing into the vacuum below the piston would
rush into the condenser, and be there condensed, so that its
effect in resisting the motion of the piston would necessarily be
trifling. But on the other hand, any escape of air between the
piston and cylinder of an atmospheric engine would introduce an
elastic fluid under the piston, which would injuriously affect the
action of the machine.
[Illustration: _Fig._ 27.]
To make the pistons move sufficiently steam-tight in these early
imperfect cylinders, Watt contrived a packing formed of a collar
of hemp, or tow, as represented in _fig._ 27. The bottom of the
piston was formed of a circular plate of a diameter nearly, but
not altogether equal to the interior diameter of the cylinder. The
part of the piston above this was considerably less in diameter,
so that the piston was surrounded by a circular groove or channel
two inches wide, into which hemp or soft rope, called _gasket_,
was run, so as to form the packing. The top of the piston was
placed over this, having a rim or projecting part, which entered
the circular groove and pressed upon the packing, the cover being
pressed downwards by screws passing through the piston. The lower
part of the groove round the piston was rounded with a curve, so
that the pressure on the packing might force the latter against
the inner surface of the cylinder. This packing was kept supplied
with melted tallow, as already described, from the funnel, screwed
into the top of the cylinder. The metallic edges of the piston
were by this means prevented from coming into contact with the
surface of the cylinder, which was only pressed upon by the
stuffing or packing projecting beyond these.
(80.) Improved methods of boring soon, however, relieved [Pg149]
the engine from a part of these imperfections, and Watt writes to
Mr. Smeaton in the letter above quoted as follows:—
"Mr. Wilkinson has improved the art of boring cylinders; so that I
promise, upon a 72 inch cylinder, being not further distant from
absolute truth than the thickness of a thin sixpence in the worst
part. I am labouring to improve the regulators; my scheme is to
make them acute conical valves, shut by a weight, and opened by
the force of the steam. They bid fair for success, and will be
tried in a few days."
The person here alluded to was Mr. John Wilkinson, of Bersham near
Chester, who, about the year 1775, contrived a new machine for
accurately boring the insides of cylinders. The cylinder being
first obtained from the foundery with a surface as accurate as the
process of casting would admit, had its inner surface reduced to
still greater accuracy by this machine, which consists of a
straight central bar extended along the axis of the cylinder,
which was made to revolve slowly round it. During the operation of
boring, the borer or cutter was fitted to slide along this bar,
which being perfectly straight, served as a sort of ruler to guide
the borer or cutter in its progress through the cylinder. In this
manner the interior surface of the cylinder was rendered not only
true and straight in its longitudinal direction, but also
perfectly circular in its cross section.
The grease found to be most eligible for lubrication was the
tallow of beef or mutton; but in the earlier cylinders this was
soon consumed by reason of the imperfection of the boring, and the
piston being left dry ceased to be steam-tight. To prevent this,
Watt sought for some substance, which while it would thicken the
tallow, and detain it around the piston, would not be subject to
decomposition by heat. Black lead dust was used for this purpose,
but was soon found to wear the cylinder. In the mean while,
however, the improved method of boring supplied cylinders which
rendered this expedient unnecessary.
When the inner surface of the cylinder is perfectly true and
smooth, the packing of the piston is soon rendered solid and hard,
being moulded to the cylinder by working, so as to fit it
perfectly. When by wear it became loose, it was [Pg150] only
necessary to tighten the screws by which the top and bottom of the
piston were held together. The packing being compressed by those
means, was forced outwards towards the surface of the cylinder, so
as to be rendered steam-tight.
(81.) It was not until about the year 1778, nine years after the
date of the patent, and thirteen after the invention of separate
condensation, that any impression was produced on the mining
interests by the advantages which were presented to them by these
vast improvements. This long interval, however, had not elapsed
without considerable advantage; for although all the great leading
principles of the contrivance were invented so early as the year
1765, yet the details of construction had been in a state of
progressive and continued improvement from the time Watt joined
Dr. Roebuck, in 1769, to the period now adverted to.
The advantages which the engine offered in the form in which it
has been just described, were numerous and important, as compared
even with the most improved form of the atmospheric engine; and it
should be remembered, that that machine had also gone on
progressively improving, and was probably indebted for some of its
ameliorations to hints derived from the labours of Watt, and to
the adoption of such of his expedients as were applicable to this
imperfect machine, and could be adopted without an infraction of
his patent.
In the most improved forms to which the atmospheric engine had
then attained, the quantity of steam wasted at each stroke of the
piston was equal to the contents of the cylinder. Such engines,
therefore, consumed twice the fuel which would be requisite, if
all sources of waste could have been removed. In Watt's engines,
the steam consumed at each stroke of the piston amounted only to
1-1/4 times the contents of the cylinder. The waste steam,
therefore, per stroke, was only a quarter of what was usefully
employed. The absolute waste, therefore, of the best atmospheric
engines was four times that of the improved engine, and
consequently the saving of fuel in the improved engines amounted
to about three eighths of all the fuel consumed in atmospheric
engines of the same power. [Pg151]
(82.) But independently of this saving of steam, which would
otherwise be wasted, the power of Watt's engine, as compared with
the atmospheric engine, was so much augmented that the former
would work against a resistance of ten pounds on the square inch
under the same circumstances in which the latter would not move
against more than seven pounds. The cause of this augmentation of
power is easily explained. In the atmospheric engine the
temperature of the condensed steam could not be reduced below 152°
without incurring a greater loss than would be compensated by the
advantage to be obtained from any higher degree of condensation.
Now steam raised from water at 152° has a pressure of nearly four
pounds per square inch. This pressure, therefore, acted below the
piston resisting the atmospheric pressure above. In Watt's engine,
however, the condenser was kept at a temperature of about 100°, at
which temperature steam has a pressure of less than one pound per
square inch. A resisting force upon the piston of three pounds per
square inch was therefore saved in Watt's engine as compared with
the atmospheric engine.
(83.) Besides these direct sources of economy, there were other
advantages incidental to Watt's engine. An atmospheric engine
possessed very limited power of adaptation to a varying load. The
moving power being the atmospheric pressure, was not under
control, and, on the other hand, was subject to variations from
day to day and from hour to hour, according to the changes of the
barometer. In the first construction of such an engine, therefore,
its power being necessarily adapted to the greatest load which it
would have to move, whenever the load upon its pumps was
diminished, the motion of the piston in descending would be
rapidly accelerated in consequence of the moving power exceeding
the resistance. By this the machinery would be subject to sudden
shocks, which were productive of rapid wear, and exposed the
machinery to the danger of fracture. To remedy this inconvenience,
the following expedient was provided in the atmospheric engine:
whenever the load on the engine was materially diminished, the
quantity of water admitted through the injection valve to condense
the steam was proportionally [Pg152] diminished. An imperfect
condensation being therefore produced, vapour remained in the
cylinder under the piston, the pressure of which resisted the
atmosphere, and mitigated the force of the machine. Besides this,
a cock was provided in the bottom of the cylinder, called an _air
cock_, by which atmospheric air could be admitted to resist the
piston whenever the motion was too rapid.
These expedients, however, were all attended with a waste of fuel
in relation to the work done by the engine; for it is evident that
the consumption of steam was necessarily the same, whether the
engine was working against its full load or against a reduced
resistance.
On the other hand, in the improved engine of Watt, when the load, to
work against which the engine exerted its full power, was
diminished, a cock or valve was provided in the steam pipe leading
from the boiler, which was called a _throttle valve_, by adjusting
which the passage in that pipe could be more or less contracted. By
regulating this cock the supply of steam from the boiler was
checked, and the quantity transmitted to the cylinder diminished, so
that its effect upon the piston might be rendered equal to the
amount of the diminished resistance. By this means the quantity of
steam transmitted to the cylinder was rendered exactly proportional
to the work which the engine had to perform. If, under such
circumstances, the boiler was worked to its full power, so as to
produce steam as fast as it would when the engine was working at
full power, then no saving of fuel would be effected, since the
surplus steam produced in the boiler would necessarily escape at the
safety valve. But in such case the fireman was directed to limit the
fuel of the furnace until the discharge at the safety valve ceased.
By these expedients, the actual consumption of fuel in one of
these improved engines was always in the exact proportion of the
work which it performed, whether it worked at full power or at any
degree under its regular power.
(84.) Notwithstanding these and other advantages attending the new
engines, Boulton and Watt experienced difficulties all but
insurmountable in getting them into use. No manufactory existed in
the country possessing machinery capable of [Pg153] executing
with the necessary precision the valves and other parts which
required exact execution, and the patentees were compelled to
construct machinery at Soho for this purpose; and even after they
succeeded in getting the cylinders properly bored, the piston rods
exactly turned and polished, the spindle valves constructed so as
to be steam-tight, and every other arrangement completed which was
necessary for the efficiency of the machine, the novelty of the
engine, and the difficulty which was supposed to attend its
maintenance in good working order, formed strong objections to its
adoption.
To remove such objections, great sacrifices were necessary on the
part of Boulton and Watt; and they accordingly resolved to
undertake the construction of the new engines without any profit,
giving them to the parties requiring their use at first cost, on
the condition of being remunerated by a small share of what they
would save in fuel.
"We have no objection," writes Mr. Boulton, "to contract with the
Carron Company to direct the making of an engine to return the
water for their mills. * * * * We do not aim at profits in engine
building, but shall take our profits out of the saving of fuel; so
that if we save nothing, we shall take nothing. Our terms are as
follows: we will make all the necessary plans, sections, and
elevations for the building, and for the engine with its
appurtenances, specifying all cast and forged iron work, and every
other particular relative to the engine. We will give all
necessary directions to your workmen, which they must implicitly
obey. We will execute, for a stipulated price, the valves, and all
other parts which may·require exact execution, at Soho; we will
see that all the parts are put together, and set to work,
properly; we will keep our own work in repair for one year, and we
have no other objection to seven years than the inconvenience of
the distance. We will guarantee that the engine so constructed
shall raise at least 20,000 cubic feet of water twenty-four feet
high with each hundred weight of coals burnt.
"When all this is done, a fair and candid comparison shall be made
between it, and your own engine, or any other engine in Scotland,
from which comparison the amount of savings in fuel shall be
estimated, and that amount being [Pg154] divided into three
parts, we shall be entitled to one of those parts, in recompense
for our patent licence, our drawings, &c. &c. Our own share of
savings shall be estimated in money, according to the value of
your coals delivered under the boiler, and you shall annually pay
us that sum, during twenty-five years from the day you begin to
work; provided you continue the use of the engine so long. And in
case you sell the engine, or remove it to any other place, you
must previously give us notice, for we shall then be entitled to
our third of the savings of fuel, according to the value of coals
at such new place. This is a necessary condition, otherwise the
engine which we make for you at an expense of two thousand pounds
may be sold in Cornwall for ten thousand pounds.
"Such parts of the engine as we execute at Soho we will be paid
for at a fair price; I conclude, from all the observations I have
had an opportunity of making, that our engines are four times
better than the common engines. In boilers, which are a very
expensive article, the savings will be in proportion to the
savings of coal. If you compare our engine with the common engine
(not in size, but in power), you will find the original expense of
erecting one to be nearly the same.
"Mr. Wilkinson has bored us several cylinders, almost without
error; that of fifty inches diameter, which we put up at Tipton,
does not err the thickness of an old shilling in any part; so that
you must either improve your method of boring, or we must furnish
the cylinder to you."
The reluctance of mining companies to relinquish the old engines,
even on these terms, led them to propose to Mr. Watt to grant them a
licence for the use of his condenser, to be applied to the
atmospheric engine, without the introduction of other improvements.
Such a proposition was made to him by Mr. Smeaton, in the year 1778,
to which he returned the following answer:—
"I have several times considered the propriety of the application
of my condensers to common engines, and have made experiments with
that view upon our engine at Soho, but have never found such
results as would induce me to try [Pg155] it any where else; and,
in consequence, we refused to make that application to Wheal
Virgin engines in Cornwall, and to some others; our reasons were,
that though it might have enabled them to have gone deeper with
their present engines, yet, the savings of fuel would not have
been great, in comparison to the complete machine. By adding
condensers to engines that were not in good order, our engine
would have been introduced into that country (which we look upon
as our richest mine) in an unfavourable point of view, and without
such profits as would have been satisfactory either to us or to
the adventurers; and if we had granted the use of condensers to
one, we must have done so to all, and thereby have curtailed our
profits, and perhaps injured our reputation. Besides, where a new
engine is to be erected, and to be equally well executed in point
of workmanship and materials, an engine of the same power cannot
be constructed materially cheaper on the old plan than on ours;
for our boiler and cylinder are much smaller, and the building,
the lever, the chains, together with all the pump and pit work,
are only the same. * * * *
"We charge our profits in proportion to the saving made in fuel by
our engine, when compared with a common one which burns the same
kind of coals; we ask one third of these savings to be paid us
annually, or half yearly; the payment being redeemable in the
option of our employer, at ten years' purchase; and when the coals
are low priced, we should also make some charge as engineers. In
all these comparisons our own interest has made us except your
(Mr. Smeaton) improved engines, unless we were allowed a greater
proportion of the savings."
Their exertions to improve the manufacture of engines at Soho is
shown by the following letter from Mr. Boulton, in the same
correspondence to Mr. Smeaton:—
"We are systematising the business of engine making, as we have
done before in the button manufactory; we are training up workmen,
and making tools and machines to form the different parts of Mr.
Watt's engines with more accuracy, and at a cheaper rate than can
possibly be done by the ordinary methods of working. Our workshop
and apparatus will be of [Pg156] sufficient extent to execute all
the engines which are likely to be soon wanted in this country;
and it will not be worth the expense for any other engineers to
erect similar works, for that would be like building a mill to
grind a bushel of corn.
"I can assure you from experience, that our small engine at Soho
is capable of raising 500,000 cubic feet of water 1 foot high with
every 112 lbs. of coals, and we are in hopes of doing much more.
Mr. Watt's engine has a very great advantage in mines, which are
continually working deeper: suppose, for instance, that a mine is
50 fathoms deep, you may have an engine which will be equal to
draining the water when the mine is worked, to 100 fathoms deep,
and yet you can constantly adapt the engine to its load, whether
it be 50 or 100 fathoms, or any intermediate depth; and the
consumption of coals will be less in proportion when working at
the lesser than at the greater depths; supposing it works, as our
engines generally do, at 11 lbs. per square inch, when the mine
becomes 100 fathoms deep."
(85.) The great improvement which has been introduced within the
last half century, in the details of Watt's steam engine, will be
rendered manifest by comparing the effects of a given weight of
fuel here supplied by Mr. Boulton with the effects which the same
weight of fuel is now known to produce in the best pumping engines
worked in Cornwall. One of these engines, in good working order,
has been known to raise 125,000,000 lbs. 1 foot high, by the
combustion of a bushel of coals. But the average performance of
even the best engines is below this amount. If we take it at
90,000,000, this will be equivalent to the weight of about 1-1/2
million cubic feet of water, a bushel of coals being 3/4 cwt. It
will therefore follow that, with the present engines, one hundred
weight of coals is capable of raising about two million cubic feet
of water one foot high, being a duty four times that assigned to
the early engines by Mr. Boulton.
(86.) At the time that Watt, in conjunction with Dr. Roebuck,
obtained the patent for his improved engine, the idea occurred to
him, that the steam which had impelled the piston in its descent
rushed from the cylinder with a mechanical force much more than
sufficient to overcome any resistance [Pg157] which it had to
encounter in its passage to the condenser; and that such force
might be rendered available as a moving power, in addition to that
already obtained from the steam during the stroke of the piston.
This notion involved the whole principle of the expansive action
of steam, which subsequently proved to be of such importance in
the performance of steam engines. Watt was, however, so much
engrossed at that time, and subsequently, by the difficulties he
had to encounter in the construction of his engines, that he did
not attempt to bring this principle into operation. It was not
until after he had organised that part of the establishment at
Soho which was appropriated to the manufacture of steam engines,
that he proceeded to apply the expansive principle. Since the date
of the patent which he took out for this (1782), was subsequent to
the application of the same principle by another engineer, named
Hornblower, it is right to state, that the claim of Mr. Watt to
this important step in the improvement of the steam engine, is
established by a letter addressed by him to Dr. Small, of
Birmingham, dated Glasgow, May, 1769:—
"I mentioned to you a method of still doubling the effect of the
steam, and that tolerably easy, by using the power of steam
rushing into a vacuum, at present lost. This would do little more
than double the effect, but it would too much enlarge the vessels
to use it all: it is peculiarly applicable to wheel engines, and
may supply the want of a condenser, where the force of steam only
is used; for open one of the steam valves, and admit steam until
one fourth of the distance between it and the next valve is filled
with steam, then shut the valve, and the steam will continue to
expand, and to press round the wheel, with a diminishing power,
ending in one fourth of its first exertion. The sum of the series
you will find greater than one half, though only one fourth of
steam was used. The power will indeed be unequal, but this can be
remedied by a fly, or by several other means."
In 1776 the engine, which had been then recently erected at Soho,
was adapted to act upon the principle of expansion. When the
piston had been pressed down in the cylinder for a certain portion
of the stroke, the further supply of steam [Pg158] from the
boiler was cut off, by closing the upper steam valve, and the
remainder of the stroke was accomplished by the expansive power of
the steam which had already been introduced into the cylinder.
(87.) To make this method of applying the force of steam
intelligible, some previous explanation of mechanical principles
will be necessary.
If a body which offers a certain resistance be urged by a certain
moving force, the motion which it will receive will depend on the
relation between the energy of the moving force and the amount of
the resistance opposed to it. If the moving force be precisely
equal to the resistance, the motion which the body will receive
will be perfectly uniform.
If the energy of the moving force be greater than the resistance,
then its surplus or excess above the amount of resistance will be
expended in imparting momentum to the mass of the body moved, and
the latter will, consequently, continually acquire augmented
speed. The motion of the body will, therefore, be in this case
accelerated.
If the energy of the moving force be less in amount than the
resistance, then all that portion of the resistance which exceeds
the amount of the moving force will be expended in depriving the
mass of the body of momentum, and the body will therefore be moved
with continually diminished speed until it be brought to rest.
(88.) Whenever, therefore, a uniform motion is produced in a body,
it may be taken as an indication of the equality of the moving
force to the resistance; and, on the other hand, according as the
speed of the body is augmented or diminished, it may be inferred
that the energy of the moving force has been greater or less than
the resistance.
It is an error to suppose that rest is the only condition possible
for a body to assume when under the operation of two or more
mechanical forces which are in equilibrium. By the laws of motion
the state of a body which is not under the operation of any
external force must be either in a state of rest or of uniform
motion. Whichever be its state, it will suffer no change if the
body be brought under the operation of two or more forces which
are in equilibrium; for to suppose [Pg159] such forces to produce
any change in the state of the body, whether from rest to motion,
or _vice versâ_, or in the velocity of the motion which the body
may have previously had, would be equivalent to a supposition that
the forces applied to the body being in equilibrium were capable
of producing a dynamical effect, which would be a contradiction in
terms. This, though not always clearly understood by mere
practical men, or by persons superficially informed, is, in fact,
among the fundamental principles of mechanical science.
(89.) When the piston is at the top of the cylinder, and about to
commence its motion downwards, the steam acting upon it will have
not only to overcome the resistance arising from the friction of
the various parts of the engine, but will also have to put in
motion the whole mass of matter of the piston pump rods, pump
pistons, and the column of water in the pump barrels. Besides
imparting to this mass the momentum corresponding to the velocity
with which it will be moved, it will also have to encounter the
resistance due to the preponderance of the weight of the water and
pump rods over that of the steam piston. The pressure of steam,
therefore, upon the piston at the commencement of the stroke must,
in accordance with the mechanical principles just explained, have
a greater force than is equal to all the resistances which it
would have to overcome, supposing the mass to be moving at a
uniform velocity. The moving force, therefore, being greater than
the resistance, the mass, when put in motion, will necessarily
move with a gradually augmented speed, and the piston of the
engine which has been described in the last chapter would
necessarily move from the top to the bottom of the cylinder with
an accelerated motion, having at the moment of its arrival at the
bottom a greater velocity than at any other part of the stroke. As
the piston and all the matter which it has put in motion must at
this point come to rest, the momentum of the moving mass must
necessarily expend itself on some part of the machinery, and would
be so much mechanical force lost. It is evident, therefore,
independently of any consideration of the expansive principle, to
which we shall presently refer, that the action of the [Pg160]
moving power in the descent of the piston ought to be suspended
before the arrival of the piston at the bottom of the cylinder, in
order to allow the momentum of the mass which is in motion to
expend itself, and to allow the piston to come gradually to rest
at the termination of the stroke.
Thus, if we were to suppose that after the piston had descended
through three fourths of the whole length of the cylinder, and had
acquired a certain velocity, the steam above it were suddenly
condensed, so as to leave a vacuum both above and below it, the
piston, being then subject to no impelling force, would still move
downwards, in virtue of the momentum it had acquired, until the
resistance would deprive it of that momentum, and bring it to
rest; and if the remaining fourth part of the cylinder were
necessary for the accomplishment of this, then it is evident that
that part of the stroke would be accomplished without further
expenditure of the moving power.
In fact, this part of the stroke would be made by the expenditure
of that excess of moving power, which, at the commencement of the
stroke, had been employed in putting the machinery and its load in
motion, and in subsequently accelerating that motion.
Although under such circumstances the resistance, during the
operation of the moving power, shall not have been at any time
equal to the moving power, since while the motion was accelerated
it was less, and while retarded greater than that power, yet as
the whole moving power has been expended upon the resistance, the
mechanical effect which the moving power has produced under such
circumstances will be equal to the actual amount of that power. If
in an engine of this kind the steam was not cut off till the
conclusion of the stroke, a part of the moving power would be lost
upon those fixed points in the machinery which would sustain the
shock produced by the instantaneous cessation of motion at the end
of the stroke.
Independently, therefore, of any consideration of the expansive
principle, it appears that, in an engine of this kind, the steam
ought to be cut off before the completion of the stroke. [Pg161]
[Illustration: _Fig._ 28.]
(90.) To render the expansive action of steam intelligible, let A
B (_fig._ 28.) represent a cylinder whose area we will suppose,
for the sake of illustration, to be a square foot, and whose
length, A B, shall also be a foot. If steam of a pressure equal to
the atmosphere be supplied to this cylinder, it will exert a
pressure of about one ton on the piston; and if such steam be
uniformly supplied from the boiler, the piston will be moved from
A to B with the force of one ton, and that motion will be uniform
if the piston be opposed throughout the same space by a resistance
equal to a ton. When the piston has arrived at B, let us suppose
that the further supply of steam from the boiler is stopped by
closing the upper steam valve, and let us also suppose the
cylinder to be continued downwards so that B C shall be equal to A
B, and suppose that B C has been previously in communication with
the condenser, and is therefore a vacuum. The piston at B will
then be urged with a force of one ton downwards, and as it
descends the steam above it will be diffused through an increased
volume, and will consequently acquire a diminished pressure. We
shall, for the present, assume that this diminution of pressure
follows the law of elastic fluids in general; that it will be
decreased in the same proportion as the volume of the steam is
augmented. While the piston, therefore, moves from B downwards it
will be urged by a continually decreasing force. Let us suppose,
that by some expedient, it is also subject to a continually
decreasing resistance, and that this resistance decreases in the
same proportion as the force which urges the piston. In that case
the motion of the piston would continue uniform. When the piston
would arrive at P′, the middle of the second cylinder, then the
space occupied by the steam being increased in the proportion of 2
to 3, the pressure on the piston would be diminished in the
proportion of 3 to 2, and the pressure at B being one ton, it
would be two-thirds of a ton at P′. In like manner when the piston
would arrive at C, the space occupied by the steam being double
that which [Pg162] it occupied when the piston was at B, the
pressure of the steam would be half its pressure at B, and
therefore at the termination of the stroke, the pressure on the
piston would be half a ton.
If the space from B to C, through which the steam is here supposed
to act expansively, be divided into ten equal parts, the pressure
on the piston at the moment of passing each of those divisions
would be calculated upon the same principle as in the cases now
mentioned. After moving through the first division, the volume of
the steam would be increased in the proportion of 10 to 11, and
therefore its pressure would be diminished in the proportion of 11
to 10. The pressure, therefore, driving the piston at the end of
the first of these ten divisions would be 10/11ths of a ton. In
like manner, its pressure at the second of the divisions would be
10/12ths of a ton, and the third 10/13ths of a ton; and so on, as
indicated in the figure.
Now if the pressure of the steam through each of these divisions
were to continue uniform, and, instead of gradually diminishing,
to suffer a sudden change in passing from one division to another,
then the mechanical effect produced from B to C would be obtained
by taking a mean or average of the several pressures throughout
each of the ten divisions. In the present case it has been
supposed that the force on the piston at B was 2240 pounds. To
obtain the pressure in pounds corresponding to each of the
successive divisions, it will therefore only be necessary to
multiply 2240 by 10, and to divide it successively by 11, 12, 13,
&c. The pressures, therefore, in pounds, at each of the ten
divisions, will be as follows:—
1st 2036·3
2d 1866·6
3d 1723·1
4th 1600·0
5th 1493·3
6th 1400·0
7th 1317·6
8th 1244·4
9th 1179·0
10th 1120·0
If the mean of these be taken by adding them together [Pg163] and
dividing by 10, it will be found to be 1498 pounds. It appears,
therefore, that the pressures through each of the ten divisions
being supposed to be uniform (which however, strictly, they are
not,) the mechanical effect of the steam from B to C would be the
same as if it acted uniformly throughout that space upon the
piston with a force of about 1500 pounds, being rather less than
three-fourths of its whole effect from A to B.
But it is evident that this principle will be equally applicable
if the second cylinder had any other proportion to the first. Thus
it might be twice the length of the first; and in that case, a
further mechanical effect would be obtained from the expansion of
the steam.
The more accurate method of calculating the effect of the
expansion from B to C, would involve more advanced mathematical
principles than could properly be introduced here; but the result
of such a computation would be that the actual average effect of
the steam from B to C would be equal to a uniform pressure through
that space, amounting to one thousand five hundred and forty-five
pounds, being greater than the result of the above computation,
the difference being due to the expansive action through each of
the ten divisions, which was omitted in the above computation.
(91.) It is evident that the expansive principle, as here explained,
involves the condition of a variation in the intensity of the moving
power. Thus, if the steam act with a uniform energy on the piston so
long as its supply from the boiler continues, the moment that supply
is stopped, by closing the steam valve, the steam contained in the
cylinder will fill a gradually increasing volume by the motion of
the piston, and therefore will act above the piston with a gradually
decreasing energy. If the resistance to the moving power produced by
the load, friction, &c. be not subject to a variation corresponding
precisely to such variation in the moving power, then the
consequence must be that the motion imparted to the load will cease
to be uniform. If the energy of the moving power at any part of the
stroke be greater than the resistance, the motion produced will be
accelerated; if it be less, the motion will be retarded; and if it
be at one time greater, and another [Pg164] time less, as will
probably happen, then the motion will be alternately accelerated and
retarded. This variation in the speed of the body moved will not,
however, affect the mechanical effect produced by the power,
provided that the momentum imparted to the moving mass be allowed to
expend itself at the end of the stroke, so that the piston may be
brought to rest as nearly as possible by the resistance of the load,
and not by any shock on any fixed points in the machine. This is an
object which, consequently, should be aimed at with a view to the
economy of power, independently of other considerations connected
with the wear and tear of the machinery. So long as the engine is
only applied to the operation of pumping water, great regularity of
motion is not essential, and, therefore, the variation of speed
which appears to be an almost inevitable consequence of any
extensive application of the expansive principle, is of little
importance. In the patent which Watt took out for the application of
the expansive principle, he specified several methods of producing a
uniform effect upon a uniform resistance, notwithstanding the
variation of the energy of the power which necessarily attended the
expansion of the steam. This he proposed to accomplish by various
mechanical means, some of which had been previously applied to the
equalisation of a varying power. One consisted in causing the piston
to act on a lever, which should have an arm of variable length, the
length increasing in the same proportion as the energy of the moving
power diminished. This was an expedient which had been already
applied in mechanics for the purpose of equalising a varying power.
A well-known example of it is presented in the main-spring and fuzee
of a watch. According as the watch goes down, the main-spring
becomes relaxed, and its force is diminished; but, at the same time,
the chain by which it drives the fuzee acts upon a wheel or circle,
having a diameter increased in the same proportion as the energy of
the spring is diminished.
Another expedient consisted in causing the moving power, when
acting with greatest energy, to lift a weight which should be
allowed to descend again, assisting the piston when the energy of
the moving force was diminished. [Pg165]
Another method consisted in causing the moving force, when acting
with greatest energy, to impart momentum to a mass of inert
matter, which should be made to restore the same force when the
moving power was more enfeebled. We shall not more than allude
here to these contrivances proposed by Watt, since their
application has never been found advantageous in cases where the
expansive principle is used.
(92.) The application of the expansive principle in the engines
constructed by Boulton and Watt, was always very limited, by
reason of their confining themselves to the use of steam having a
pressure not much exceeding that of the atmosphere. If the
principle of expansion, as above explained, be attentively
considered, it will be evident that the extent of its application
will mainly depend on the density and pressure of the steam
admitted from the boiler. If the density and pressure be not
considerable when the steam is cut off, the extent of its
subsequent expansion will be proportionally limited. It was in
consequence of this, that this principle from which considerable
economy of power has been derived, was applied with much less
advantage by Mr. Watt than it has since been by others, who have
adopted the use of steam of much higher pressure. In the engines
of Boulton and Watt, where the expansive principle was applied,
the steam was cut off after the piston had performed from one half
to two thirds of the stroke, according to the circumstances under
which the engine was worked. The decreasing pressure produced by
expansion was, in this case, especially with the larger class of
engines, little more than would be necessary to allow the momentum
of the mass moved to spend itself, before the arrival of the
piston at the end of the stroke.
Subsequently, however, boilers producing steam of much higher
pressure were applied, and the steam was cut off when the piston
had performed a much smaller part of the whole stroke. The great
theatre of these experiments and improvements has been the mining
districts in Cornwall, where, instead of working with steam of a
pressure not much exceeding that of the atmosphere, it has been
found advantageous to use steam whose pressure is at least four
times as great as [Pg166] that of the atmosphere; and instead of
limiting its expansion to the last half or fourth of the stroke,
it is cut off after the piston has performed one fourth part of
the stroke or less, all the remainder of the stroke being
accomplished by the expansive power of the steam, and by momentum.
[Illustration: BRIDGE OVER THE CLYDE AT HAMILTON, DESIGNED BY
WATT.]
[Pg167]
[Illustration: DOUBLE-ACTING ENGINE, ZINC WORKS, CITY ROAD,
LONDON.]
CHAP. VII.
PROPERTIES OF STEAM. — COMMON STEAM. — SUPERHEATED STEAM. —
LAW OF DALTON AND GAY LUSSAC. — LAW OF MARIOTTE. — RELATION
BETWEEN TEMPERATURE AND PRESSURE OF COMMON STEAM. — EFFECTS OF
THE EXPANSION OF COMMON STEAM. — MECHANICAL EFFECTS OF STEAM.
— METHOD OF EQUALISING THE EXPANSIVE FORCE. — HORNBLOWER'S
ENGINE. — WOOLF'S ENGINE. — WATT'S ATTEMPTS TO EXTEND THE
STEAM ENGINE TO MANUFACTURES. — PAPIN'S PROJECTED APPLICATIONS
OF THE STEAM ENGINE. — SAVERY'S APPLICATIONS OF THE ENGINE TO
MOVE MACHINERY. — JONATHAN HULL'S APPLICATION TO WATER WHEELS.
— STEWART'S APPLICATION OF THE ENGINE TO MILL WORK. —
WASHBOROUGH'S APPLICATION OF THE FLY WHEEL AND CRANK. — WATT'S
SECOND PATENT. — DOUBLE-ACTION VALVES.
(93.) Since the application of the expansive action of steam
involves the consideration of its properties when it ceases to be
in contact with the water from which it was produced, and likewise
the variation of its pressure in different states of [Pg168]
density and at different temperatures, it is necessary here to
explain some of the most important of these properties of vapour.
Steam may exist in two states, distinguished from each other by
the following circumstances:—
1st. It may be such that the abstraction from it of any portion of
heat, however small, will cause its partial condensation.
2d. It may be such as to admit of the abstraction of heat from it
without undergoing any other change than that which air would
undergo under like circumstances, viz. a diminution of temperature
and pressure.
(94.) We shall call, for distinction, the former _Common Steam_,
and the latter _Superheated Steam_.
[Illustration: _Fig._ 29.]
To explain the circumstances out of which these properties arise,
let B (_fig._ 29.) be imagined to be a vessel filled with water,
communicating by a pipe and stopcock with another vessel A, which
in the commencement of the process may be conceived to be filled
with air. Let D be a pipe and stopcock at the top of this vessel.
If the vessel B be heated, and the two cocks be opened, the steam
proceeding from the water in B will blow the air out of the vessel
A through the open stopcock D, in the same manner as air is blown
from a steam engine. When the vessel A by these means has been
filled with pure steam, let both stopcocks be closed. If the steam
in A, under these circumstances, have a pressure of 15 lbs. per
square inch, its temperature will be found to be 213°. Now, if any
heat be abstracted from this steam, its temperature will fall, and
a portion of it will be reconverted into water.
Again, suppose the vessel A to be filled with pure steam which has
been produced from the heated water in B, the stopcock C being
open. Let the stopcock C be then closed, and the water in B be
heated to a higher temperature, the temperature and pressure of
the steam in A being observed. If the stopcock C be now opened,
the steam in A will be immediately observed to rise to the more
elevated temperature which has been imparted to the water in B,
and at the same time it will acquire an increased pressure.
[Pg169]
The increase of temperature which it has received would of itself
produce an increased pressure; but that this is not the sole cause
of the augmented pressure in the present case might be proved by
weighing the vessel A. It would be found to have increased weight,
which could only arise from its having received from the water in
B an additional quantity of vapour. The increased pressure
therefore, which the steam in A has acquired, is due conjointly to
its increased density and its increased temperature. In general,
if the water in the vessel B be raised or lowered in temperature,
the steam in the vessel A will rise and fall in temperature in a
corresponding manner, always having the same temperature as the
water in B. If the weight of the vessel A were observed, it would
be found to increase with every increase of temperature, and to
diminish with every diminution of temperature, proving that the
augmented temperature of the water in B produces an augmented
density of the steam in A. The same pressure would be found always
to correspond to the same temperature and density, so that if the
numerical amount of any one of the three quantities, the
temperature, the pressure, or the density, were known, the other
two must necessarily be determined, the same temperature always
corresponding to the same pressure, and _vice versâ_. And in like
manner, steam produced under these circumstances of the same
density cannot have different pressures. It must be observed that
the steam here produced receives all the heat which it possesses
from the water from which it is raised. Now it is easily
demonstrable, that this is the least quantity of heat which is
compatible with the steam maintaining the vaporous form; for if
the stopcock C be closed so as to separate the steam in A from the
water in B, and that any portion of heat, however small, be then
abstracted from the steam in A, some portion of the steam will be
reconverted into water.
This then, according to the definition already given, is _Common
Steam_.
(95.) Let us now suppose that the vessel A, being in communication
with the vessel B by the open stopcock, has been filled with pure
steam of any given temperature. The steam which it thus contains
will be common steam, and, as has been [Pg170] shown (94.), it
cannot lose any portion of heat, however small, without being
partially condensed; but let the stopcock C be closed, and let the
steam in A be then exposed to any source of heat by which its
temperature may be raised any required number of degrees. From the
steam thus obtained heat may be abstracted without producing any
condensation; and such abstraction of heat may be continued
without producing condensation, until the steam is cooled down to
that temperature at which it was raised from the water in B, when
the stopcock C was opened. Any further reduction of temperature
would be attended with condensation.
If after increasing the temperature of the steam in A, the
stopcock C being shut so as to render it superheated steam, its
pressure be observed, the pressure will be found to be increased,
but not to that amount which it would have been increased had the
steam in A been raised to the same temperature by heating the
water in B to that temperature, and keeping the stopcock open. In
fact, its present augmented pressure will be due only to its
increased temperature, since its density remains unchanged. But if
in these circumstances the stopcock C be suddenly opened, the
pressure of the steam in A will as suddenly rise to that pressure
which in common steam corresponds to its temperature; and if the
vessel A were weighed, it would be found to have increased in
weight, proving that the steam contained in it has received
increased density by an increased quantity of vapour proceeding
from the water in A. In fact, by opening the stopcock the steam
which was before superheated steam, has become common steam. It
has the greatest density which steam of that temperature can have;
and consequently, if any heat be abstracted from it, a partial
condensation will ensue.
To render these general principles more intelligible, let us
suppose that the water in B is raised to the temperature of 213°,
the stopcock C being open; the vessel A will then be filled with
steam of the same temperature, and having a pressure of 15 lbs.
per square inch. This will be common steam. If the stopcock be now
closed, and the whole apparatus be exposed to the temperature of
243°; the steam in A will preserve the same density, but its
pressure will be [Pg171] increased from 15 lbs. to a little more
than 16 lbs. per square inch. Let the stopcock C be then opened
and while the temperature of the steam in A shall continue to be
243°, the pressure will suddenly rise from 16 lbs. to about 26
lbs. per square inch. The weight of the steam in A will be at the
same time increased in the same proportion of 16 to 26 as its
pressure. The steam thus produced in A will then be common steam,
and any abstraction of heat from it would be attended with partial
condensation.
(96.) The law, according to which the pressure of elastic fluids
in general, whether gases or vapours, increases with their
temperature, was simultaneously discovered by Dalton and Gay
Lussac. If the pressure which the gas or vapour would have at the
temperature of melting ice, were expressed by 10,000, then the
increase of pressure which it would receive for every degree of
temperature by which it would be raised, its volume being supposed
to be preserved, would be expressed by 208-1/3. Thus, if the
pressure of gas, or vapour, on a surface of a certain magnitude at
the temperature of 32° were 10,000 ounces, then the same gas or
vapour would acquire an additional pressure of 208-1/3 ounces for
every degree of temperature which would be imparted to it above
32°. This law is common to all gases and vapours.
It may be objected that water cannot exist in the state of vapour
under the usual pressures at so low a temperature as melting ice.
This, however, does not hinder the application of the above law,
for that law will equally hold good by computing the pressure
which the vapour would have if it were a permanent gas, and if it
could therefore exist in the elastic form at that low temperature.
(97.) Another law, common to all elastic fluids, and of equal
importance with the former, was discovered by Mariotte. By this
law it appears that every gas or vapour, so long as its
temperature is unchanged, will have a pressure directly
proportional to its density. If therefore, while we compress steam
into half its volume, we could preserve its temperature unaltered,
we should increase its pressure in a two-fold proportion; but if
the process of compression should cause its temperature to
increase, [Pg172] then its increase of pressure will be greater
than its increase of density, since it will be due conjointly to
the increase of density and to the increase of temperature. In
this case the increased pressure may be deduced from the combined
application of the two laws just explained; that of Mariotte will
determine that increase of pressure which is due to the increase
of density, and that of Dalton and Gay Lussac will determine the
further increase of pressure which will be due to the increase of
temperature. The full investigation of these effects, and the
formulæ expressing them, will be found in the Appendix to this
volume.
(98.) The fixed relations which exist between the temperatures of
common steam and its pressure and density, have never been
discovered from any general physical principles. The pressures and
the densities however, which correspond to a great variety of
temperatures throughout the thermometric scale, have been
ascertained by extensive series of experiments instituted by
philosophers of this and other countries. From a comparison of the
temperatures and pressures thus found by experiment, empirical
formulæ have been constructed, which exhibit, with an approximation
sufficiently close for practice, this relation; and these formulæ
may accordingly be used for the computation of tables exhibiting the
pressures, temperatures, and densities of common steam; and such
tables will have sufficient numerical accuracy for all practical
purposes. These formulæ, and the tables resulting from them, will be
found in the Appendix to this volume.
(99.) It has been explained, that to effect the conversion of
water into steam, it is only necessary to impart to it as much
heat as, added to the temperature which it has, would, if it
continued in the liquid form, raise it to the temperature of
1212°. This condition is necessary, and sufficient to effect the
transition of water into vapour. If, for example, as much heat
were imparted to the water evaporated, as would maintain it in the
liquid state to 1300°, then the steam so produced would be
superheated steam, having 80° of heat more than is necessary to
maintain it in the vaporous form. From such steam, therefore, 80°
of heat may be abstracted without producing any condensation.
[Pg173]
(100.) Common steam being raised from water at any pressure and
temperature, and being afterwards separated from the water, if the
same steam be compressed into a small volume, or allowed to expand
into a greater volume, it will still maintain its quality of
common steam, and will have the same pressure and temperature,
whatever volume it may assume, as it would have if immediately
raised from water at that pressure. Thus if steam be raised from
water under a pressure of 30 lbs. per square inch, and, being
separated from the water, be allowed to dilate, until its pressure
is reduced to 15 lbs. per square inch, its temperature will then
be reduced to 213°, which is that temperature which it would have
if immediately raised from water under a pressure of 15 lbs. per
square inch; and if any heat be abstracted from such steam,
whether under its original pressure, or under the diminished
pressure of 15 lbs. per square inch, a condensation will be
produced, the amount of which will be the same, if the same
quantity of heat be abstracted from the steam. These are
consequences which immediately flow from the fact, that the sum of
the latent and sensible heats of steam is always the same.[20]
It appears, therefore, that supposing the steam used in an engine
to receive no additional heat after it leaves the boiler, however
it may be changed in its density by subsequent expansion, it will
still retain its character of common steam, and cannot lose any
portion of heat, however small, without suffering partial
condensation. The mechanical force also exerted by such steam,
after expansion, must be computed in the same manner as if it were
raised immediately.
(101.) If the law of Mariotte were strictly applicable to steam,
its mechanical effect would be the same as has been already
explained in all states of density; but since its temperature will
rise and fall as its density is increased or diminished, a
corresponding change will be produced in its [Pg174] mechanical
efficacy. It is therefore necessary in the calculation of the
mechanical effect of steam, whether it be used at a uniform
pressure without the principle of expansion, or with the
application of that principle to any given extent, to take into
account the combined operation of the laws of Mariotte and Dalton.
Formulæ exhibiting the relation between the temperatures,
pressures, volumes, and densities of steam, and the mechanical
effect produced by the evaporation of water, whether acting with
or without expansion, together with the tables necessary for the
practical application of these, will be found in the Appendix.
(102.) One of the methods of equalising the varying force of
expanding steam, would be to work it at the same time in two
cylinders connected with the same beam; so that while its force in
one would be augmented, its force in the other would be
diminished, the combination of the two producing a uniform effect.
Soon after the expansive principle was promulged by Mr. Watt, this
expedient was accordingly resorted to by an engineer named
Hornblower.
[Illustration: _Fig._ 30.]
In the year 1781, Hornblower conceived the notion of working an
engine with two cylinders of different sizes, by allowing the
steam to flow freely from the boiler until it fills the smaller
cylinder, and then permitting it to expand into the greater one,
employing it thus to press down two pistons in the following
manner.
Let C, _fig._ 30., be the centre of the great working-beam,
carrying two arch heads, on which the chains of the piston rods
play. The distances of these arch heads from the centre C must be
in the same proportion as the length of the cylinders, in order
that the same play of the beam may correspond to [Pg175] the
plays of both pistons. Let F be the steam-pipe from the boiler,
and G a valve to admit the steam above the lesser piston. H is a
tube by which a communication may be opened by the valve I,
between the top and bottom of the lesser cylinder B. K is a tube
communicating by the valve L, between the bottom of the lesser
cylinder B and the top of the greater cylinder A. M is a tube
communicating, by the valve N, between the top and bottom of the
greater cylinder A; and P a tube leading to the condenser by the
exhausting valve O.
At the commencement of the operation, suppose all the valves
opened, and steam allowed to flow through the engine until the air
be completely expelled, and then let all the valves be closed. To
start the engine, let the exhausting valve O and the steam valves
G and L be opened, as in _fig._ 30. The steam will flow freely
from the boiler, and press upon the lesser piston, and at the same
time the steam below the greater piston will flow into the
condenser, leaving a vacuum in the greater cylinder. The valve L
being opened, the steam which is under the piston in the lesser
cylinder will flow through K, and press on the greater piston,
which, having a vacuum beneath it, will consequently descend. At
the commencement of the motion, the lesser piston is as much
resisted by the steam below it, as it is urged by the steam above
it; but after a part of the descent has been effected, the steam
below the piston, in the lesser cylinder, passing into the
greater, expands into an increased space, and therefore loses part
of its elastic force. The steam above the lesser piston retaining
its full force by having a free communication with the boiler by
the valve G, the lesser piston will be urged by a force equal to
the excess of the pressure of this steam above the diminished
pressure of the expanded steam below it. As the pistons descend,
the steam which is between them is continually increasing in its
bulk, and therefore decreasing in its pressure, from whence it
follows, that the force which resists the lesser piston is
continually decreasing, while that which presses it down remains
the same, and therefore the effective force which impels it must
be continually increasing. [Pg176]
On the other hand, the force which urges the greater piston is
continually decreasing, since there is a vacuum below it, and the
steam which presses it is continually expanding into an increased
bulk.
[Illustration: _Fig._ 31.]
Impelled in this way, let us suppose the pistons to have arrived
at the bottoms of the cylinders, and let the valves G, L, and O,
be closed, and the valves I and N opened. No steam is allowed to
flow from the boiler, G being closed, nor any allowed to pass into
the condenser, since O is closed, and all communication between
the cylinders is stopped by closing L. By opening the valve I, a
free communication is made between the top and bottom of the
lesser piston through the tube H, so that the steam which presses
above the lesser piston will exert the same pressure below it, and
the piston is in a state of indifference. In the same manner the
valve N being open, a free communication is made between the top
and bottom of the greater piston, and the steam circulates above
and below the piston, and leaves it free to rise. A counterpoise
attached to the pump-rods, in this case, draws up the piston, as
in Watt's single engine; and when they arrive at the top, the
valves I and N are closed, and G, L, and O, opened, and the next
descent of the pistons is produced in the manner already
described, and so the process is continued.
The valves are worked by the engine itself, by means similar to
some of those already described. By computation, we find the power
of this engine to be nearly the same as a similar engine on Watt's
expansive principle. It does not, however, appear, that any
adequate advantage was gained by this modification of the
principle, since no engines of this construction are now made.
(103.) The use of two cylinders was revived by Arthur Woolf in 1804,
who, in this and the succeeding year, obtained patents for the
application of steam raised under a high pressure to double-cylinder
engines. The specification of his patent states, that he has proved
by experiment that steam raised [Pg177] under a safety-valve loaded
with any given number of pounds upon the square inch will, if
allowed to expand into as many times its bulk as there are pounds of
pressure on the square inch, have a pressure equal to that of the
atmosphere. Thus, if the safety-valve be loaded with four pounds on
the square inch, the steam, after expanding into four times its
bulk, will have the atmospheric pressure; if it be loaded with 5, 6,
or 10 lbs. on the square inch, it will have the atmospheric pressure
when it has expanded into 5, 6, or 10 times its bulk, and so on. It
was, however, understood in this case, that the vessel into which it
was allowed to expand should have the same temperature as the steam
before it expands.
It is very unaccountable how a person of Mr. Woolf's experience in
the practical application of steam could be led into errors so
gross as those involved in the averments of this patent; and it is
still more unaccountable how the experiments could have been
conducted which led him to conclusions not only incompatible with
all the established properties of elastic fluids, but even
involving in themselves palpable contradiction and absurdity. If
it were admitted that every additional pound avoirdupois which
should be placed upon the safety-valve would enable steam, by its
expansion into a proportionally enlarged space, to attain a
pressure equal to the atmosphere, the obvious consequence would
be, that a physical relation would subsist between the atmospheric
pressure and the pound avoirdupois! It is wonderful that it did
not occur to Mr. Woolf, that, granting his principle to be true at
any given place, it would necessarily be false at another place,
where the barometer would stand at a different height! Thus, if
the principle were true at the foot of a mountain, it would be
false at the top of it; and if it were true in fair weather, it
would be false in foul weather, since these circumstances would be
attended by a change in the atmospheric pressure, without making
any change in the pound avoirdupois.[21]
[Pg178]
(104.) For several years after the extension of Watt's first
patent had been obtained from parliament, he was altogether
engrossed by the labour of bringing to perfection the application
of the steam-engine to the drainage of mines, and in surmounting
the numerous difficulties which presented themselves to its
general adoption, even after its manifold advantages were
established and admitted. When, however, these obstacles had been
overcome, and the works for the manufacture of engines for pumping
water, at Soho, had been organised and brought into active
operation, he was relieved from the pressure of these anxieties,
and was enabled to turn his attention to the far more extensive
and important uses of which he had long been impressed with the
conviction that the engine was capable. His sagacious mind enabled
him to perceive that the machine he had created was an infant
force, which by the fostering influence of his own genius would
one day extend its vast power over the arts and manufactures, the
commerce and the civilisation of the world. Filled with such
aspirations, he addressed his attention about the year 1779, to
the adaptation of the steam-engine to move machinery, and thereby
to supersede animal power, and the natural agents, wind and water.
The idea that steam was capable of being applied extensively as a
prime mover, had prevailed from a very early period; and now that
we have seen its powers so extensively brought to bear, it will
not be uninteresting to revert to the faint traces by which its
agency was sketched in the crude speculations of the early
mechanical inventors.
(105.) Papin, to whom the credit of discovering the method of
producing a vacuum by the condensation of steam is due, was the
earliest and most remarkable of those projectors. With very limited
powers of practical application, he was, nevertheless, peculiarly
happy in his mechanical conceptions; and had his experience and
opportunities been proportionate to the clearsighted character of
his mind, he would doubtless have anticipated some of the most
memorable of his successors in the progressive improvement of the
steam engine.
In his work already cited, after describing his method of
imparting an alternate motion to a piston by the atmospheric
[Pg179] pressure acting against a vacuum produced by the
condensation of steam, he stated that his invention, besides being
applicable to pumping water, could be available for rowing vessels
against wind and tide, which he proposed to accomplish in the
following manner.
Paddle-wheels, such as have since been brought into general use,
were to be placed at the sides, and attached to a shaft extending
across the vessel. Within the vessel, and under this shaft, he
proposed to place several cylinders supplied with pistons, to be
worked by the atmospheric pressure. On the piston-rods were to be
constructed racks furnished with teeth: these teeth were to work in
the teeth of wheels or pinions, placed on the shaft of the
paddle-wheels. These pinions were not to be fixed on the shaft, but
to be connected with it by a ratchet; so that when they turned in
one direction, they would revolve without causing the shaft to
revolve; but when driven in the other direction, the catch of the
ratchet-wheel would act upon the shaft so as to compel the shaft and
paddle-wheels to revolve with the motion of the pinion or wheel upon
it. By this arrangement, whenever the piston of any cylinder was
forced down by the atmospheric pressure, the rack descending would
cause the corresponding pinion of the paddle-shaft to revolve; and
the catch of the ratchet wheel, being thus in operation, would cause
the paddle-shaft and paddle-wheels also to revolve; but whenever the
piston would rise, the rack driving the pinion in the opposite
direction, the catch of the ratchet wheel would merely fall from
tooth to tooth, without driving the paddle-shaft.
It is evident that by such an arrangement a single cylinder and
piston would give an intermitting motion to the paddle-shaft, the
motion of the wheel being continued only during the descent of the
piston; but if several cylinders were provided, then their motion
might be so managed, that when one would be performing its
ascending stroke, and therefore giving no motion to the
paddle-shaft, another should be performing its descending stroke,
and therefore driving the paddle-shaft. As the interval between
the arrival of the piston at the bottom of the cylinder and the
commencement [Pg180] of its next descent would have been, in the
imperfect machine conceived by Papin, much longer than the time of
the descent, it was evident that more than two cylinders would be
necessary to insure a constantly acting force on the paddle-shaft,
and, accordingly, Papin proposed to use several cylinders.
In addition to this, Papin proposed to construct a boiler having a
fireplace surrounded on every side by water, so that the heat
might be imparted to the water with such increased rapidity as to
enable the piston to make four strokes per minute. These projects
were promulged in 1690, but it does not appear that they were ever
reduced to experiment.
(106.) Savery proposed, in his original patent, in 1698, to apply
his steam engine as a general prime mover for all sorts of
machinery, by causing it to raise water to make an artificial
fall, by which overshot water-wheels might be driven. This
proposal was not acted on during the lifetime of Savery, but it
was at a subsequent period partially carried into effect. Mr.
Joshua Rigley erected several steam engines on this principle at
Manchester, and other parts of Lancashire, to impel the machinery
of some of the earliest manufactories and cotton mills in that
district. The engines usually raised the water from sixteen to
twenty feet high, from whence it was conveyed to an overshot
wheel, to which it gave motion. The same water was repeatedly
elevated by the engine, so that no other supply was necessary,
save what was sufficient to make good the waste. These engines
continued in use for some years, until superseded by improved
machines.[22]
(107.) In 1736, Jonathan Hulls obtained a patent for a method of
towing ships into or out of harbour against wind and tide. This
method was little more than a revival of that proposed by Papin in
1690. The motion, however, was to be communicated to the
paddle-shaft by a rope passing over a pulley fixed on an axis, and
was to be maintained during the returning stroke of the piston by
the descent of a weight which was elevated during the descending
stroke. There is no record, however, of this plan, any more than
that of Papin, ever having been reduced to experiment.
(108.) During the early part of the last century the [Pg181]
manufactures of this country had not attained to such an extent as
to render the moving power supplied by water insufficient or
uncertain to any inconvenient degree; and accordingly mills, and
other works in which machinery required to be driven by a moving
power, were usually built along the streams of rivers. About the
year 1750 the general extension of manufactures, and their
establishment in localities where water power was not accessible,
called the steam engine into more extensive operation. In the year
1752, Mr. Champion, of Bristol, applied the atmospheric engine to
raise water, by which a number of overshot wheels were driven.
These were applied to move extensive brass-works in that
neighbourhood, and this application was continued for about twenty
years, but ultimately given up on account of the expense of fuel
and the improved applications of the steam engine.
About this time Smeaton applied himself with great activity and
success to the improvement of wind and water mills, and succeeded
in augmenting their useful effect in a twofold proportion with the
same supply of water. From the year 1750 until the year 1780 he
was engaged in the construction of his improved water mills, which
he erected in various parts of the country, and which were
imitated so extensively that the improvement of such mills became
general. In cases where a summer drought suspended the supply of
water, horse machinery was provided, either to work the mill or to
throw back the water. These improvements necessarily obstructed
for a time the extension of steam power to mill work; but the
increase of manufactures soon created a demand for power greatly
exceeding what could be supplied by such limited means.
In the manufacture of iron, it is of great importance to keep the
furnaces continually blown, so that the heat may never be abated
by day or night. In the extensive ironworks at Colebrook Dale,
several water-wheels were used in the different operations of the
manufacture of iron, especially in driving the blowers of the iron
furnaces. These wheels were usually driven by the water of a
river, but in the summer months the supply became so short that it
was insufficient to work them all. Steam engines were accordingly
erected to [Pg182] return the water for driving these wheels.
This application of the engine as an occasional power for the
supply of water-wheels having been found so effectual, returning
engines were soon adopted as the permanent and regular means of
supplying water-wheels. The first attempt of this kind is recorded
to have been made by Mr. Oxley, in 1762, who constructed a machine
to draw coals out of a pit at Hartley colliery, in Northumberland.
It was originally intended to turn the machine by a continuous
circular motion received from the beam of the engine; but that
method not being successful, the engine was applied to raise water
for a wheel by which the machine was worked. This engine was
continued in use for several years, and though it was at length
abandoned, on account of its defective construction, it
nevertheless established the practicability of using steam power
as a means of driving water wheels.[23]
(109.) In the year 1777, Mr. John Stewart read a paper before the
Royal Society, describing a method for obtaining a continued
circular motion for turning all kinds of mills from the
reciprocating motion of a steam engine. He proposed to accomplish
this by means of two endless chains passing over pulleys, which
should be moved upwards and downwards by the motion of the engine,
in the manner of a window sash. The joint pins of the links of the
two chains worked in teeth at the opposite sides of a cog wheel,
to which they imparted a circular motion, first by one chain, and
then by the other, acting alternately on opposite sides of the
wheel. One chain impelled it during the descent of the piston, and
the other during the ascent; but one of these chains always passed
over its pulleys so as to produce no effect on one side of the cog
wheel, whilst the other chain worked on the opposite side to turn
it round. For this purpose each chain was provided with a catch,
to prevent its circulating over its pulleys in one direction, but
to allow it free motion in the other. The cog wheel thus kept in
revolution might be applied to the axis of any mill which the
engine was required to work. Thus, if it were applied to a
flour-mill, the millstone itself would perform the office of a
fly-wheel to regulate the intermission of [Pg183] the power, and
in other mills a fly-wheel might be added for this purpose.
The hints obtained by Mr. Stewart from Papin's contrivance, before
mentioned, will not fail to be perceived. In Mr. Stewart's paper
he notices indirectly the method of obtaining a continued circular
motion from a reciprocating motion by means of a crank or winch,
which, he says, occurs naturally in theory, but in practice would
be impossible, from the nature of the motion of the engine, which
depends on the force of the steam, and cannot be ascertained in
its length. Therefore, on the first variation, the machine would
be either broken in pieces or turned back. Such an opinion,
pronounced by a man of considerable mechanical knowledge and
ingenuity, against a contrivance which, as will presently appear,
proved in practice, not less than in theory, to be the most
effectual means of accomplishing the end here pronounced to be
impossible, is sufficiently remarkable. It might cast some doubt
on the extent of Mr. Stewart's practical knowledge, if it did not
happen to be in accordance with a judgment so generally
unimpeachable as that of Mr. Smeaton. This paper of Mr. Stewart's
was referred by the council of the Royal Society to Mr. Smeaton,
who remarked upon the difficulty arising from the absolute
stopping of the whole mass of moving power, whenever the direction
of the motion is changed; and observed, that although a fly-wheel
might be applied to regulate the motion, it must be such a large
one as would not be readily controlled by the engine itself; and
he considered that the use of such a fly-wheel would be a greater
incumbrance to a mill than a water-wheel to be supplied by water
pumped up by the engine. This engineer, illustrious as he was, not
only fell into the error of Mr. Stewart in respect of the crank,
but committed the further blunder of condemning the very expedient
which has since rendered the crank effectual. It will presently
appear that the combination of the crank and fly-wheel have been
the chief means of establishing the dominion of the steam engine
over manufactures.
(110.) In 1779, Mr. Matthew Wasbrough, an engineer at Bristol,
took out a patent for the application of a steam engine [Pg184]
to produce a continuous circular motion by means of ratchet
wheels, similar to those previously used by Mr. Oxley, at Hartley
colliery; to which, however, Mr. Wasbrough added a fly-wheel to
maintain and regulate the motion. Several machines were
constructed under this patent; and among others, one was erected
at Mr. Taylor's saw-mills and block manufactory at Southampton. In
1780, one was erected at Birmingham, where the ratchet work was
found to be subject to such objections, that one of the persons
about the works substituted for it the simple crank, which has
since been invariably used. A patent was taken out for this
application of the crank in the same year, by Mr. James Pickard,
of Birmingham. It will presently appear, however, that the
suggestion of this application of the crank was derived from the
proceedings of Watt, who was at the same time engaged in similar
experiments.
(111.) The single-acting steam engine, as constructed by Watt, was
not adapted to produce continuous uniform motion of rotation, for
the following reasons:—
_First._ The effect required was that of an uniformly acting
force. The steam engine, on the other hand, supplied an
intermitting force. Its operation was continued during the
descending motion of the piston, but it was suspended during the
ascent of the piston. To produce the continued effect now
required, either its principle of operation should be altered, or
some expedient should be devised for maintaining the motion of the
revolving shaft during the ascent of the piston, and the
consequent suspension of the moving power.
_Secondly._ The action of the steam engine was rectilinear. It was
a power which acted in a straight line, viz., in the direction of
the cylinder. The motion, however, required to be produced, was a
circular motion—a motion of rotation around the axis or shaft of
the mill.
The steps by which Watt proceeded to accomplish these objects have
been recorded by himself as follows, in his notes upon Dr.
Robison's article on the steam engine:—
"I had very early turned my mind to the producing of continued
motion round an axis; and it will be seen, by reference to my
first specification in 1769, that I there described [Pg185] a
steam wheel, moved by the force of steam, acting in a circular
channel against a valve on one side, and against a column of
mercury, or some other fluid metal, on the other side. This was
executed upon a scale of about six feet diameter at Soho, and
worked repeatedly, but was given up, as several practical
objections were found to operate against it; similar objections
lay against other rotative engines, which had been contrived by
myself and others, as well as to the engines producing rotatory
motions by means of ratchet wheels.
"Having made my single reciprocating engines very regular in their
movements, I considered how to produce rotative motions from them
in the best manner; and amongst various schemes which were
subjected to trial, or which passed through my mind, none appeared
so likely to answer the purpose as the application of the crank,
in the manner of the common turning lathe; but as the rotative
motion is produced in that machine by impulse given to the crank
in the descent of the foot only, it requires to be continued in
its ascent by the energy of the wheel, which acts as a fly; being
unwilling to load my engine with a fly-wheel heavy enough to
continue the motion during the ascent of the piston (or with a
fly-wheel heavy enough to equalise the motion, even if a
counterweight were employed to act during that ascent), I proposed
to employ two engines, acting upon two cranks fixed on the same
axis, at an angle of 120° to one another, and a weight placed upon
the circumference of the fly-wheel at the same angle to each of
the cranks, by which means the motion might be rendered nearly
equal, and only a very light fly-wheel would be requisite.
"This had occurred to me very early; but my attention being fully
employed in making and erecting engines for raising water, it
remained _in petto_ until about the year 1778 or 1789, when Mr.
Wasbrough erected one of his ratchet-wheel engines at Birmingham,
the frequent breakages and irregularities of which recalled the
subject to my mind, and I proceeded to make a model of my method,
which answered my expectations; but having neglected to take out a
patent, the invention was communicated by a workman employed
to [Pg186] make the model, to some of the people about Mr.
Wasbrough's engine, and a patent was taken out by them for the
application of the crank to steam engines. This fact the said
workman confessed, and the engineer who directed the works
acknowledged it; but said, nevertheless, that the same idea had
occurred to him prior to his hearing of mine, and that he had even
made a model of it before that time; which might be a fact, as the
application to a single crank was sufficiently obvious.
"In these circumstances, I thought it better to endeavour to
accomplish the same end by other means, than to enter into
litigation; and if successful, by demolishing the patent, to lay
the matter open to every body. Accordingly, in 1781, I invented
and took out a patent for several methods of producing rotative
motions from reciprocating ones; amongst which was the method of
the sun-and-planet wheels. This contrivance was applied to many
engines, and possesses the great advantage of giving a double
velocity to the fly-wheel; but is perhaps more subject to wear,
and to be broken under great strains, than a simple crank, which
is now more commonly used, although it requires a fly-wheel of
four times the weight, if fixed upon the first axis; my
application of the double engine to these rotative machines
rendered the counterweight unnecessary, and produced a more
regular motion."
(112.) Watt's second patent here referred to, was dated 25th
October, 1781, and was entitled "A patent for certain new methods
of applying the vibrating or reciprocating motions of steam or
fire engines to produce a continued rotative or circular motion
round an axis or centre, and thereby to give motion to the wheels
of mills and other machines."
All the methods specified in this patent were intended to be
worked by the single-acting engine, already described, a
counterweight being applied to impel the machinery during the
returning stroke of the engine, which weight would be elevated
during the descent of the piston. There were five different
expedients proposed in the specification for producing a rotatory
motion; but, of these five, two only were ever applied in
practice. [Pg187]
(113.) Suppose a rod or bar attached by a pin or joint at the
upper extremity to the working end of the beam of the engine, and
by a similar pin or joint at the lower extremity to an iron wheel
fixed on the extremity of the axis of the fly-wheel. One half of
this wheel is formed of a solid semicircle of cast iron, while the
other half is constructed of open spokes, so as to be as light as
is consistent with strength. The position of the wheel on the axis
is such that during the returning stroke of the piston, when the
operation of the steam is suspended, the heavy semicircle of the
wheel will be descending, and by its weight will draw down the
connecting bar, and thereby draw down the working end of the beam,
and draw up the piston in the cylinder. When the piston descends
and is driven by the power of the steam, the heavy semicircle of
the above-mentioned wheel will be drawn upwards, and in the same
way the motion will be continued.
[Illustration: _Fig._ 32.]
(114.) The second method of producing a rotatory motion, which was
subsequently continued for many years in practical operation, was
that which was called the _Sun-and-planet Wheels_. A toothed wheel
A (_fig._ 32.), called the sun wheel, was fixed on the axle of the
fly-wheel, to which rotation was to be imparted. The wheel B,
called the planet wheel, having an equal diameter, was fastened on
the end I of the connecting rod H I, so as to be incapable of
revolving. During the descent of the piston, the working end of
the beam was drawn upwards, and the end I of the connecting rod
travelled from C to D, through the dotted semicircle C I D. The
wheel B not being capable of revolving on the centre I, would,
during this motion, drive the sun wheel A. During the ascent of
the steam piston, the working end of the beam would descend, and
the centre I of [Pg188] the planet wheel B would be driven
downwards from D to C, through the other dotted semicircle, and
would consequently continue to drive the sun wheel round in the
same direction.
This contrivance, although in the main inferior to the more simple
one of the crank, is not without some advantages; among others, it
gives to the sun wheel double the velocity which would be
communicated by the crank; for in the crank one revolution only on
the axle is produced by one revolution of the crank, but in the
sun-and-planet wheel, two revolutions of the sun wheel are
produced by one of the planet wheel; thus a double velocity is
obtained from the same motion of the beam. This will be evident
from considering that when the planet wheel is in its highest
position, its lowest tooth is engaged with the highest tooth of
the sun wheel; as the planet wheel passes from the highest
position, its teeth drive those of the sun wheel before them, and
when it comes into the lowest position, the highest tooth of the
planet wheel is engaged with the lowest of the sun wheel: but then
half of the sun wheel has _rolled off_ the planet wheel, and,
therefore, the tooth which was engaged with it in its highest
position, must now be distant from it by half the circumference of
the wheel, and must, therefore, be again in the highest position;
so that while the planet wheel has been carried from the top to
the bottom, the sun wheel has made a complete revolution.
This advantage of giving an increased velocity may be obtained
also by the crank, by placing toothed wheels on its axle.
Independently of the greater expense attending the construction of
the sun-and-planet wheel, its liability to go out of order, and
the rapid wear of the teeth, and other objections, rendered it
inferior to the crank, which has entirely superseded it.
(115.) Although by these contrivances Watt succeeded in obtaining
a continuous circular motion from the reciprocating motion of
the steam engine, the machine was still one of intermitting,
instead of continuous action. The expedient of a counterweight,
elevated during the descending stroke, and giving back the power
expended on it in the interval of the returning stroke, did not
satisfy the [Pg189] fastidious mechanical taste of Watt. He
soon perceived that all which he proposed to accomplish by the
application of two cylinders and pistons working alternately,
could be attained with greater simplicity and effect by a single
cylinder, if he could devise means by which the piston might be
impelled by steam upwards as well as downwards. To accomplish
this, it was only necessary to throw the lower end of the cylinder
into alternate communication with the boiler, while the upper end
would be put into communication with the condenser. If, for
example, during the descent of the piston, the upper end of the
cylinder communicated with the boiler, and the lower end with the
condenser; and, on the other hand, during the ascent of the
piston, the lower end communicated with the boiler, and the upper
end with the condenser; then the piston would be driven
continually, whether upwards or downwards, by the power of steam
acting against a vacuum. Watt obtained his third patent for this
contrivance, on the 12th of March, 1782.
This change in the principle of the machine involved several other
changes in the details of its mechanism.
[Illustration: _Fig._ 33.]
(116.) It was necessary, in the first place, to provide means for
admitting and withdrawing the steam at either end of the cylinder.
For this purpose let B and B′ (_fig._ 33.) be two steam-boxes, B
the upper, and B′ the lower, communicating respectively with the
top and bottom of the cylinder by proper passages D D′. Let two
valves be placed in B, one, S, above the passage D, and the other,
C, below it; and in like manner two other valves in the lower
valve-box, B′, one, S′, above the passage D′, and the other, C′,
below it. Above the valve S in the upper steam-box is an opening
at which the steam-pipe from the boiler enters, and below the
valve C is another opening, at which enters the exhausting-pipe
leading to the condenser. In like manner, above the valve S′ in
the lower steam-box enters a steam-pipe leading from the boiler,
and below the valve C′ enters an exhausting-pipe leading to
[Pg190] the condenser. It is evident, therefore, that steam can
always be admitted above the piston by opening the valve S, and
below it by opening the valve S′; and, in like manner, steam can
be withdrawn from the cylinder above the piston, and allowed to
pass to the condenser, by opening the valve C, and from below it
by opening the valve C′.
[Illustration: _Fig._ 34.]
Supposing the piston P to be at the top of the cylinder, and the
cylinder below the piston to be filled with pure steam, let the
valves S and C′ be opened, the valves C and S′ being closed as
represented in _fig._ 34. Steam from the boiler will, therefore,
flow in through the open valve S, and will press the piston
downwards, while the steam that has filled the cylinder below the
piston will pass through the open valve C′ into the exhausting-pipe
leading to the condenser, and being condensed will leave the
cylinder below the piston a vacuum. The piston will, therefore, be
pressed downwards by the action of the steam above it, as in the
single-acting engine. Having arrived at the bottom of the cylinder,
let the valves S and C′ be both closed, and the valves S′ and C be
opened, as represented in _fig._ 34. Steam will now be admitted
through the open valve S′ and through the passage D′ below the
piston, while the steam which has just driven the piston downwards,
filling the cylinder above the piston, will be drawn off through the
open valve C, and the exhausting-pipe, into the condenser, leaving
the cylinder above the piston a vacuum. The piston will, therefore,
be pressed upwards by the action of the steam below it, against the
vacuum above it, and will ascend with the same force as that with
which it had descended.
This alternate action of the piston upwards and downwards may
evidently be continued by opening and closing the valves alternately
in pairs. Whenever the piston is at the top of the cylinder, as
represented in _fig._ 33., the valves S and C′, that is, the upper
steam-valve and the lower exhausting-valve, are opened, and the
valves C and S′, that is, the upper exhausting-valve and the lower
steam-valve, are closed; and [Pg191] when the piston has arrived at
the bottom of the cylinder, as represented in _fig._ 34., the valves
C and S′, that is, the upper exhausting-valve and the lower
steam-valve, are opened, and the valves S and C′, that is, the upper
steam-valve and the lower exhausting-valve, are closed.
If these valves, as has been here supposed, be opened and closed
at the moments at which the piston reaches the top and bottom of
the cylinder, it is evident that they may be all worked by a
single lever connected with them by proper mechanism. When the
piston arrives at the top of the cylinder, this lever would be
made to open the valves S and C′, and at the same time to close
the valves S′ and C; and when it arrives at the bottom of the
cylinder, it would be made to close the valves S and C′, and to
open the valves S′ and C.
If, however, it be desired to cut off the steam before the arrival
of the piston at the termination of its stroke, whether upwards or
downwards, then the steam-valves must be closed before the arrival
of the piston at the end of its stroke; and as the exhausting-valve
ought to be left open until the stroke is completed, these valves
ought to be moved at different times. In that case separate levers
should be provided for the different valves. We shall, however,
return again to the subject of the valves which regulate the
admission of steam to the cylinder and its escape to the condenser.
(117.) It will be remembered that in the single-acting engine the
process of condensation was suspended while the piston ascended in
the cylinder, and therefore the play of the jet of cold water in the
condenser was stopped during this interval. In the double-acting
engine, however, the flow of steam from the cylinder to the
condenser is continued, whether the piston ascends or descends, and
therefore a constant condensation of steam must be produced. The
condensing jet, therefore, does not in this case, as in the former,
play with intervals of intermission. A constant jet of cold water
must be maintained in the condenser.
It will presently appear that in the double-acting engine applied
to manufactures, the motion of the piston was subject to more or
less variation of speed, and the quantity of steam [Pg192]
admitted to the cylinder was subject to a corresponding change.
The quantity of steam, therefore, drawn into the condenser was
subject to variation, and required a considerable change in the
quantity of cold water admitted through the jet to condense it. To
regulate this, the valve or cock by which the water was admitted
into the condenser was worked in the double-acting engine by a
lever furnished with an index, by which the quantity of condensing
water admitted into the condenser could be regulated. This index
played upon a graduated arch, by which the engine-man was enabled
to regulate the supply.
[Illustration: HEATHFIELD HOUSE, NEAR BIRMINGHAM, THE RESIDENCE OF
WATT.]
FOOTNOTES:
[20] These effects are explained in my Treatise on Heat; and
they have lately been verified by experiments made with
locomotive engines by M. de Pambour, who found that the steam
raised from the boiler of a locomotive engine, under a
pressure of above 50 lbs. per square inch, was in the state of
common steam as it issued from the chimney at a very diminished
pressure and at a lower pressure.
[21] It is strange that this absurdity has been repeatedly
given as unquestionable fact in various encyclopædias, as well
as in by far the greater number of treatises expressly on the
subject.
[22] Farey, Treatise on the Steam Engine, p. 122.
[23] Farey on the Steam Engine, p. 297.
[Pg193]
[Illustration: DOUBLE-ACTING ENGINE.—CITY SAW-MILLS.]
CHAP. VIII.
METHODS OF CONNECTING THE PISTON-ROD AND BEAM IN THE
DOUBLE-ACTING ENGINE. — RACK AND SECTOR. — PARALLEL MOTION. —
CONNECTING ROD AND CRANK. — FLY-WHEEL. — THROTTLE-VALVE. —
GOVERNOR. — CONSTRUCTION AND OPERATION OF THE DOUBLE-ACTING
ENGINE. — ECCENTRIC. — COCKS AND VALVES. — SINGLE-CLACK VALVE.
— DOUBLE-CLACK VALVE. — CONICAL VALVES. — SLIDE VALVES. —
MURRAY'S SLIDES. — THE D VALVE. — SEAWARD'S SLIDES. — SINGLE
COCK. — FOUR-WAY COCK. — PISTONS. — COMMON HEMP-PACKED PISTON.
— WOOLFE'S PISTON. — METALLIC PISTONS. — CARTWRIGHT'S ENGINE.
— CARTWRIGHT'S PISTON. — BARTON'S PISTON.
(118.) In the single-acting engine, the force of the piston acted
on the beam only during its descent; and this force was
transmitted from the piston to the beam, as we have seen, by a
flexible chain, extended from the end of the piston-rod, [Pg194]
and playing upon the arch head of the beam. In the double-acting
engine, however, the force of the steam pressing the piston
upwards must likewise be transmitted to the beam, so as to drive
the latter upwards while the piston ascends. This action could not
be accomplished by a chain connecting the piston with the arch
head of the beam.
Where the mechanical action to be transmitted is a _pull_, and not
a _push_, a flexible chain, cord, or strap, is sufficient; but if
a _push_ or _thrust_ is required to be transmitted, then the
flexibility of the medium of mechanical communication afforded by
a chain renders it inapplicable. In the double-acting engine,
during the descent, the piston-rod still pulls the beam down; and
so far a chain connecting the piston-rod with the beam would be
sufficient to transmit the action of the one to the other; but in
the ascent, the beam no longer pulls up the piston-rod, but is
pushed up by it. A chain from the piston-rod to the arch head, as
described in the single-acting engine, would fail to transmit this
force. If such a chain were used with the double engine, where
there is no counterweight on the opposite end of the beam, the
consequence would be, that in the ascent of the piston the chain
would slacken, and the beam would still remain depressed. It is
therefore necessary that some other mechanical connection be
contrived between the piston-rod and the beam, of such a nature
that in the _descent_ the piston-rod may _pull_ the beam down, and
may _push_ it up in the _ascent_.
[Illustration: _Fig._ 35.]
Watt first proposed to effect this by attaching to the end of the
piston-rod a straight rack, faced with teeth, which should work in
corresponding teeth raised on the arch head of the beam, as
represented in _fig._ 35. If his improved steam engines required no
further precision of operation and construction than the atmospheric
engines, this might have been sufficient; but in these engines it
was indispensably necessary that the piston-rod should be guided
with a smooth and even motion through the stuffing-box in the top of
the cylinder, otherwise any shake or irregularity would cause it to
work loose in the stuffing-box, and either to admit the air, or to
let the steam escape. Under these circumstances, the motion of
[Pg195] the rack and toothed arch head were inadmissible, since it
was impossible by such means to impart to the piston-rod that smooth
and equable motion which was requisite. Another contrivance which
occurred to Watt was, to attach to the top of the piston-rod a bar,
which should extend above the beam, and to use two chains or straps,
one extending from the top of the bar to the lower end of the arch
head, and the other from the bottom of the bar to the upper end of
the arch head. By such means the latter strap would pull the beam
down when the piston would descend, and the former would pull the
beam up when the piston would ascend. These contrivances, however,
were superseded by the celebrated mechanism since called the
_Parallel Motion_, one of the most ingenious mechanical combinations
connected with the history of the steam engine.
(119.) It will be observed that the object was to connect by some
inflexible means the end of the piston-rod with the extremity of
the beam, and so to contrive the mechanism, that while the end of
the beam would move alternately up and down in part of a circle,
the end of the piston-rod connected with the beam should move up
and down in a straight line. If the end of the piston-rod were
fastened upon the end of the beam by a pivot without any other
connection, it is evident that, being moved up and down in the
arch of a circle, it would be drawn to the left and the right
alternately, and would consequently either be broken or bent, or
would work loose in the stuffing-box. Instead of connecting the
end of the rod immediately with the end of the beam by a pivot,
Watt proposed to connect them by certain moveable rods, so
arranged that, as the end of the beam would move up and down in
the circular arch, the rods would so accommodate themselves to
that motion, that the end connected with the piston-rod should not
be disturbed from its rectilinear course.
To explain the principle of the mechanism called the parallel
motion, let us suppose that O P (_fig._ 36.) is a rod or lever
moveable on a centre O, and that the end P of this rod shall move
through a circular arch P P′ P″ P‴ a vertical plane, and let its
play be limited by two stops S, which shall prevent its ascent
above the point P, and its descent below [Pg196] the point P‴.
Let the position of the rod and the limitation of its play be such
that the straight line A B drawn through P and P‴, the extreme
positions of the lever O P, shall be a vertical line.
[Illustration: _Fig._ 36.]
Let _o_ be a point on the other side of the vertical line A B, and
let the distance of O to the right of A B be the same as the
distance of _o_ to the left of A B. Let _o p_ be a rod equal in
length to O P, moving like O P on the centre _o_, so that its
[Pg197] extremity _p_ shall play upwards and downwards through the
arch _p p′ p″ p‴_, its play being limited in like manner by stops
_s_.
Now, let us suppose that the ends P _p_ of these two rods are joined
by a link P _p_, the connection being made by a pivot, so that the
angles formed by the link and the rods shall be capable of changing
their magnitude. This link will make the motion of one rod depend on
that of the other, since it will preserve their extremities P _p_
always at the same distance from each other. If, therefore, we
suppose the rod O P to be moved to the position O P‴, its extremity
P tracing the arch P P′ P″ P‴, the link connecting the rods will at
the same time drive the extremity _p_ of the rod _o p_ through the
arch _p p′ p″ p‴_ so that when the extremity of the one rod arrives
at P‴, the extremity of the other rod will arrive at _p‴_. By this
arrangement, in the simultaneous motion of the rods, whether upwards
or downwards, through the circular arches to which their play is
limited, the extremities of the link joining them will deviate from
the vertical line A B in opposite directions. At the limits of their
play, the extremities of the link will always be in the line A B;
but in all intermediate positions, the lower extremity of the link
will be to the right of A B, and its upper extremity to the left of
A B. So far as the derangement of the lower extremity of the link is
concerned, the matter composing the link would be transferred to the
right of A B, and so far as the upper extremity of the link is
concerned, the matter composing it would be transferred to the left
of A B.
By the combined effects of these contrary derangements of the
extremities of the link from the vertical line, it might be
expected that a point would exist, in the middle of the link,
where the two contrary derangements would neutralise each other,
and which point would therefore be expected to be disturbed
neither to the right nor to the left, but to be moved upwards and
downwards in the vertical line A B. Such is the principle of the
parallel motion; and in fact the middle point of the link will
move for all practical purposes accurately in the vertical line A
B, provided that the angular play of the levers O P and _o p_ does
not exceed a certain [Pg198] limit, within which, in practice,
their motion may always be restrained.
To trace the motion of the middle point of the link more minutely,
let P P′ P″ P‴ be four positions of the lever O P, and let _p_ _p′_
_p″_ _p‴_ be the four corresponding positions of the lever _o p_. In
the positions O P _o p_, the link will take the position P _p_, in
which the entire link will be vertical, and its middle point _x_
will therefore be in the vertical line A B.
When the one rod takes the position O P′, the other rod will have
the position _o p′_; and the link will have the position P′ _p′_.
The middle point of the link will be at _x′_, which will be found
to be on the vertical line A B. Thus one half of the link P′ _x′_
will be to the left of the vertical line A B; while the other
half, _p′ x′_, will be to the right of the vertical line; the
derangement from the vertical line affecting each half of the link
in contrary directions.
Again, taking the one rod in the position O P″, the corresponding
position of the other rod will be _o p″_, and the position of the
link will be P″ _p″_. If the middle point of the link in this
position be taken, it will be found to be at _x″_, on the vertical
line A B; and, as before, one half of the link P″ _x″_ will be
thrown to the left of the vertical line, while the other half _p″
x″_, will be thrown to the right of the vertical line.
Finally, let the one rod be in its lowest position, O P‴, while
the other rod shall take the corresponding position, _o p‴_. The
direction of the link P‴ _p‴_ will now coincide with the vertical
line; and its middle point _x‴_ will therefore be upon that line.
The previous derangement of the extremities of the rod, to the
right and to the left, are now redressed, and all the parts of the
rod have assumed the vertical position.
It is plain, therefore, that by such means the alternate motion of
a point such as P or _p_, upwards and downwards in a circular
arch, may be made to produce the alternate motions of another
point _x_, upwards and downwards in a straight line.
(120.) Although the guidance of the air-pump rod in a true
vertical line is not so necessary as that of the steam piston,
[Pg199] and as the air-pump piston is always brought down by its
own weight and that of its rod, the connection of the air-pump
piston-rod with the beam, by any contrivance of the kind now
described, was not so necessary. Nevertheless, by a slight
addition to the mechanical contrivance which has been just
described, Watt obtained the means of at once preserving the true
rectilinear motion of both piston-rods.
[Illustration: _Fig._ 37.]
Let the lever represented by O P in _fig._ 36. be conceived to be
prolonged to twice its length, as represented in _fig._ 37., so
that O P′ shall be twice O P. Let the points P _p_ be connected by
a link as before. Let a link P′ _x′_, equal in length to the link
P _p_ be attached to the point P′, and let the extremity _x′_ of
this link be connected with the point _p_ by another link, equal
in length to P P′, by pivots at _x′_ and _p_, so that the figure P
P′ _x′ p_ shall be a jointed parallelogram, the angles of which
will be capable of altering their magnitude with every change of
position of the rods _o p_ and O P. Thus, when the rod O P
descends, the angles of the parallelogram at P and _x′_ will be
diminished in magnitude, while the angles at P′ and _p_ will be
increased in magnitude. Now, let a line be conceived to be drawn
from O to _x′_. It is evident that that line will pass through the
middle point of the link _p_ P, for the triangle O P _x_ is in all
respects similar to the greater triangle O P′ _x′_ only on half
the scale, so that every side of the one is [Pg200] half the
corresponding side of the other. Therefore P _x_ is half the
length of P′ _x′_; but P′ _x′_ was made equal to P _p_, and
therefore _p x_ is half of P _p_, that is to say, _x_ is the
middle point of P _p_.
It has been already shown, that in the alternate motion of the
rods _o p_, O P in ascending and descending, the point _x_ is
moved upwards and downwards in a true vertical line. Now since the
triangle O P _x_ is in all respects similar to O P′ _x′_, and
subject to a similar motion during the ascent and descent of the
rods, it is apparent that the point _x′_ must be subject to a
motion in all respects similar to that which affects the points
_x_, except that the point _x′_ will move through double the
space. In fact, the principle of the mechanism is precisely
similar to that of the common pantograph, where two rods are so
connected as that the motion of the one governs the motion of the
other, so that whatever line or figure may be described by one, a
similar line or figure must be described by the other. Since,
then, the point _x_ is moved upwards and downwards in a vertical
straight line, the point _x′_ will also be moved in a vertical
straight line of double the length.
If such an arrangement of mechanism as has been here described can
be connected with the beam of the steam engine, so that while the
point _x′_ is attached to the top of the steam piston, and the
space through which it ascends and descends shall be equal to the
length of the stroke of that piston, the point _x_ shall be
attached to the rod of the air-pump piston, the stroke of the
latter being half that of the steam piston, then the points _x′_
and _x_ will guide the motion of the two pistons so as to preserve
them in true vertical straight lines.
The manner in which these ideas are reduced to practice admits of
easy explanation: let the point O be the centre of the great
working beam, and let O P′ be the arm of the beam on the side of
the steam cylinder. Let P be a pivot upon the beam, at the middle
point between its centre O and its extremity P′; and let the links
P p, P′ _x′_, and P _p_ be jointed together, as already described.
Let the point or pivot _o_ be attached to some part of the fixed
framing of the engine or engine house, and let the rod _o p_,
equal to half the arm of the beam, be attached by a pivot to the
corner of the parallelogram at [Pg201] _p_. Let the end of the
steam piston-rod be attached to the corner of the parallelogram
_x′_, and let the end of the air-pump be attached to the middle
point _x_ of the link P _p_; by which arrangement it is evident
that the rectilinear motion of the two piston-rods will be
rendered compatible with the alternate circular motions of the
points P′ and P on the beam.
Among the many mechanical inventions produced by the fertile
genius of Watt, there is none which has excited such universal,
such unqualified, and such merited admiration as that of the
parallel motion. It is indeed impossible, even for an eye
unaccustomed to view mechanical combinations, to behold the beam
of a steam engine moving the pistons, through the instrumentality
of the parallel motion, without an instinctive feeling of pleasure
at the unexpected fulfilment of an end by means having so little
apparent connection with it. When this feeling was expressed to
Watt himself, by those who first beheld the performance of this
exquisite mechanism, he exclaimed with his usual vivacity, that he
himself, when he first beheld his own contrivance in action, was
affected by the same sense of pleasure and surprise at its
regularity and precision. He said, that he received from it the
same species of enjoyment that usually accompanies the first view
of the successful invention of another person.
"Among the parts composing the steam engine, you have doubtless,"
says M. Arago, "observed a certain articulated parallelogram. At
each ascent and descent of the piston, its angles open and close
with the sweetness—I had almost said with the grace—which charms
you in the gestures of a consummate actor. Follow with your eye
alternately the progress of its successive changes, and you will
find them subject to the most curious geometrical conditions. You
will see, that of the four angles of the jointed parallelogram,
three describe circular arches, but the fourth which holds the
piston-rod is moved nearly in a straight line. The immense utility
of this result strikes mechanicians with even less force than the
simplicity of the means by which Watt has attained it."
The parallel motion, of which there are several other varieties,
depending, however, generally upon the same [Pg202] principle,
formed part of a patent which Mr. Watt obtained in the year 1784,
another part of which patent was for a locomotive engine, by which
a carriage was to be propelled on a road. In a letter to Mr.
Smeaton dated 22d October, in the same year, Watt says,—
"I have lately contrived several methods of getting entirely rid
of all the chains and circular arches about the great levers of
steam engines, and nevertheless making the piston-rods ascend and
descend perpendicularly, without any sliding motions or
right-lined guides, merely by combinations of motions about
centres; and with this further advantage, that they answer equally
well to push upwards as to pull downwards, so that this method is
applicable to our double engines which act both in the ascent and
descent of their pistons.
"A rotative engine of this species with the new motion which is
now at work in our manufactory (but must be sent away very soon)
answers admirably. It has cost much brain work to contrive proper
working gear for these double engines, but I have at last done it
tolerably well, by means of the circular valves, placed in an
inverted position, so as to be opened by the force of the steam;
and they are kept shut by the working gear. We have erected an
engine at Messrs. Goodwyne and Co.'s brewery, East Smithfield,
London."
[Illustration: _Fig._ 38.]
(121.) By the contrivance which has been explained above, the
force of the piston in ascending and descending would be conveyed
to the working end of the beam; and the next problem which Watt
had to solve was, to produce by the force exerted by the working
end of the beam in ascending and descending a continuous motion of
rotation. In the first instance he proposed to accomplish this by
a crank placed upon the axle to which rotation was to be imparted,
and driven by a rod connecting it with the working end of the
beam. Let K (_fig._ 38.) be the centre, to which motion is to be
imparted by the working end H of the beam. On the axle K suppose a
short lever K I to be fixed so that when K I is turned round the
centre K, the axle must turn with it. Let an iron rod, the weight
of which shall balance the piston and piston-rod at the other end
of the beam, be connected by joints with the working end H of the
beam, and the extremity I of the [Pg203] lever K I. As the end H
of the beam is moved upwards and downwards, the lever K I will be
turned round the centre K, taking successively the positions
represented by faint lines in the figure; and thus a motion of
continued rotation will be imparted to the axle K.
This simple and effectual expedient of producing a continued
rotatory motion by a crank was abandoned by Watt, as already
explained, by reason of a patent having been obtained upon
information of his experiments surreptitiously procured. To avoid
litigation, he therefore substituted for the crank the sun and
planet wheel already described; but at the expiration of the
patent, which restricted the use of the crank, the sun and planet
wheel was discontinued in Watt's engine, and the crank restored.
(122.) Whether the crank or the sun and planet wheel be used,
there is still a difficulty in the maintenance of a regular motion
of rotation. In the various positions which the crank and
connecting rod assume throughout a complete revolution, there are
two in which the moving power loses all influence in impelling the
crank. These positions are those which the crank assumes when the
piston is at the top and bottom of the [Pg204] cylinder, and is
just about to change the direction of its motion. When the piston
is at the bottom of the cylinder, the pivot I (_fig._ 38.), by
which the connecting rod H I is attached to the end of the crank,
is immediately over the axle K of the crank, and under the pivot
H, which joins the upper end of the connecting rod with the beam.
In fact, in this position the connecting rod and crank are in the
same straight line, extending from the end of the beam to the axle
of the crank. The steam, on entering the cylinder below the
piston, and pressing it upwards, would produce a corresponding
downward force on the connecting rod at H, which would be
continued along the connecting rod and crank to the axle K. It is
evident that such a force could have no tendency to turn the crank
round, but would expend its whole energy in pressing the axle K
downwards.
The other position in which the power loses its effect upon the
crank is when the piston is at the top of the cylinder. In this
case, the working end of the beam will be at the lowest point of
its play, and the crank-pin I will be immediately below the axle
K; so that K will be placed immediately between H and I. When the
steam presses on the top of the piston, it will expend its force
in drawing the end H of the connecting rod upwards, by which the
crank-pin I will likewise be drawn upwards. It is evident that
this force can have no effect in turning the crank round, but will
expend its whole energy in producing an upward strain on the axle
K.
If the crank were absolutely at rest in either of the positions
above described, it is apparent that the engine could not be put
in motion by the steam; but if the engine has been previously in
motion, then the mass of matter forming the crank, and the axle on
which the crank is formed, having already had a motion of
rotation, will have a tendency to preserve the momentum it has
received, and this tendency will be sufficient to throw the crank
K I out of either of those critical positions which have been
described. Having once escaped these dead points, then the
connecting rod forming an angle, however obtuse or acute, with the
crank, the pressure or pull upon the former will have a tendency
to produce rotation in the latter. As the crank revolves, however,
the influence [Pg205] of the connecting rod upon it will vary
according to the angle formed by the connecting rod and crank.
When that angle is a right angle, then the effect of the
connecting rod on the crank is greatest, since the force upon it
has the advantage of the whole leverage of the crank; but
according as the angle formed by the crank and connecting rod
becomes more or less acute or obtuse in the successive attitudes
which they assume in the revolution of the crank, the influence of
the connecting rod over the crank varies, changing from nothing at
the two dead points already described, to the full effect produced
in the two positions where they are at right angles. In
consequence of this varying leverage, by which the force with
which the connecting rod is driven by the steam is transmitted to
the axle on which the crank revolves, a corresponding variation of
speed would necessarily be produced in the motion imparted to the
crank. The speed at the dead points would be least, being due
altogether to the momentum already imparted to the revolving mass
of the crank and axle; and it would gradually increase and be
greatest at the points where the effect of the crank on the
connecting rod is greatest. Although this change of speed would
not affect the actual mechanical efficacy of the machine, and
although the same quantity of steam would perform the same work at
the varying velocity as it would do if the velocity were
regulated, yet this variation of speed would be incompatible with
the purposes to which it was now proposed that the steam engine
should be applied in manufactures. In these a regular uniform
motion should be imparted to the main axle.
(123.) One of the expedients which Watt proposed for the
attainment of this end was, by placing two cranks on the same
axle, in different positions, to be worked by different cylinders,
so that while one crank should be at its dead points, the other
should be in the attitude most favourable for its action. This
expedient has since, as we shall see, been carried into effect in
steam vessels; but one more simple and efficient presented itself
in the use of a _fly-wheel_.
On the main axle driven by the crank Watt placed a large wheel of
metal, as represented in _fig._ 43., called a _fly-wheel_. This
wheel being well constructed, and nicely balanced on its [Pg206]
axle, was subject to very little resistance from friction; any
moving force which it would receive it would therefore retain, and
would be ready to impart such moving force to the main axle
whenever that axle ceased to be driven by the power. When the
crank, therefore, is in those positions in which the action of the
power upon it is most efficient, a portion of the energy of the
power is expended in increasing the velocity of the mass of matter
composing the fly-wheel. As the crank approaches the dead points,
the effect of the moving power upon the axle and upon the crank is
gradually enfeebled, and at these points vanishes altogether. The
momentum which has been imparted to the fly-wheel then comes into
play, and carries forward the axle and crank out of the dead
points with a velocity very little less than that which it had
when the crank was in the most favourable position for receiving
the action of the moving power.
By this expedient, the motion of revolution received by the axle
from the steam piston is subject to no other variation than just
the amount of change of momentum in the great mass of the
fly-wheel, which is sufficient to extricate the crank twice in
every revolution from the mechanical dilemma to which its peculiar
form exposes it; and this change of velocity may be reduced to as
small an amount as can be requisite by giving the necessary weight
and magnitude to the fly-wheel.
(124.) By such arrangements the motion imparted to the main axle K
would be uniform, provided that the moving power of the engine be
always proportionate to the load which it drives. But in the
general application of the steam engine to manufactures it was
evident that the amount of the resistance to which any given
machine would be subject must be liable to variation. If, for
example, the engine drive a cotton-mill, it will have to impart
motion to all the spinning frames in that mill. The operation of
one or more of these may from time to time be suspended, and the
moving power would be relieved from a corresponding amount of
resistance. If, under such circumstances, the energy of the moving
power remained the same, the velocity with which the machines
would be driven would be subject to variation, being increased
whenever the operation of any portion of the machines usually
[Pg207] driven by it is suspended; and, on the other hand,
diminished when any increased number of machines are brought into
operation. In fine, the speed would vary nearly in the inverse
proportion of the load driven, increasing as the load is
diminished, and _vice versâ_.
On the other hand, supposing that no change took place in the
amount of the load driven by the engine, and that the same number
of machines of whatever kind would have to be continually driven,
the motion imparted to the main axle would still be subject to
variation by the changes inevitable to the moving power. The
piston of the engine being subject to an unvaried resistance, a
uniform motion could only be imparted to it, by maintaining a
corresponding uniformity in the impelling power. This would
require a uniform supply of steam from the boiler, which would
further imply a uniform rate of evaporation in the boiler, unless
means were provided in the admission of steam from the boiler to
the cylinder to prevent any excess of steam which might be
produced in the boiler from reaching the cylinder.
[Illustration: _Fig._ 39.]
[Illustration: _Fig._ 40.]
This end was attained by a contrivance afterwards called the
_throttle-valve_. An axis A B (_figs._ 39, 40.) was placed across
the steam pipe in a ring of cast-iron D E, of proper thickness. On
this axis was fastened a thin circular plate T, of nearly the same
diameter as the steam pipe. On the outer end B of this axle was
placed a short lever or handle B C, by which it could be turned.
When the circular plate T was turned into such a position as to be
at right angles to the length of the tube, it stopped the passage
within the tube altogether, so that no steam could pass from the
boiler to the engine. On the other hand, when the handle was
turned through a fourth of a revolution from this position, then
the circular plate T had its plane in the direction of the length
of the tube, so that its edge would be presented towards the
current of steam flowing from the boiler to the cylinder. In that
position the passage within the tube [Pg208] would be necessarily
unobstructed by the throttle-valve. In intermediate positions of
the valve, as that represented in _figs._ 39, 40., the passage
might be left more or less opened, so that steam from the boiler
might be admitted to the cylinder in any regulated quantity
according to the position given to the lever B C.
A view of the throttle-valve taken by a section across the steam
pipe is exhibited in _fig._ 40., and a section of it through the
axis of the steam pipe is represented in _fig._ 39. The form of
the valve is such, that, if accurately constructed, the steam in
passing from the boiler would have no effect by its pressure to
alter any position which might be given to the valve; and any
slight inaccuracy of form which might give a tendency to the steam
to alter the position would be easily counteracted by the friction
of the valve upon its axle. The latter might be regulated at
pleasure.
By this expedient, however the evaporation of water in the boiler
might vary within practical limits, the supply of steam to the
cylinder would be rendered regular and uniform. If the boiler became
too active, and produced more steam than was necessary to move the
engine with its load at the requisite speed, then the throttle-valve
was shifted so as to contract the passage and limit the supply of
steam. If, on the other hand, the process of evaporation in the
boiler was relaxed, then the throttle-valve was placed with its edge
more directed towards the steam. Independently of the boiler, if the
load on the engine was lightened, then the same supply of steam to
the cylinder would unduly accelerate the motion. In this case,
likewise, the partial closing of the throttle-valve would limit the
supply of steam and regulate the motion; and if, on the other hand,
the increase of load upon the engine rendered necessary an increased
supply of steam, then the opening of the throttle-valve would
accomplish the purpose. By these means, therefore, a uniform motion
might be maintained, provided the vigilance of the engine-man
was sufficient for the due management of the lever B C, and
provided that the furnace under the boiler was kept in sufficient
activity to supply the greatest amount of steam which would be
necessary [Pg209] for the maintenance of a uniform motion with the
throttle-valve fully opened.
(125.) Watt, however, soon perceived that the proper manipulation
of the lever B C would be impracticable with any degree of
vigilance and skill which could be obtained from the persons
employed to attend the engine. He, therefore, adapted to this
purpose a beautiful application of a piece of mechanism, which had
been previously used in the regulation of mill-work, and which has
since been well known by the name of the _Governor_, and has
always been deservedly a subject of much admiration.
The governor is an apparatus by which the axle of the fly-wheel is
made to regulate the throttle-valve, so that the moment that the
axle begins to increase its velocity, it shifts the position of
the throttle-valve, so as to limit the supply of steam from the
boiler, and thereby to check the increase of speed. And on the
other hand, whenever the velocity of the axle is diminished, the
lever B C is moved in the contrary direction, so as to open more
fully the passage for the steam, and accelerate the motion of the
engine.
A small grooved wheel A B (_fig._ 41.) is attached to a vertical
spindle supported in pivots or sockets C and D, in which it is
capable of revolving. An endless cord works in the groove A B, and
is carried over proper pulleys to the axle of the fly-wheel, where
it likewise works in a groove. When this cord is properly
tightened the motion of the fly-wheel will give motion to the
wheel A B, so that the velocity of the one will be subject to all
the changes incidental to the velocity of the other. By this means
the speed of the grooved wheel A B may be considered as
representing the speed of the fly-wheel, and of the machinery
which the axle of the fly-wheel drives.
[Illustration: _Fig._ 41.]
It is evident that the same end might be attained by substituting
for the grooved wheel A B a toothed wheel, which might be
connected by other toothed wheels, and proper shafts, and axles
with the axle of the fly-wheel.
A ring or collar E is placed on the upright spindle, so as to be
capable of moving freely upwards and downwards. To this ring are
attached by pivots two short levers, E F, the [Pg210] pivots or
joints at E allowing these levers to play upon them. At F these
levers are joined by pivots to other levers F G, which cross each
other at H, where an axle or pin passes through them, and attaches
them to the upright spindle C D. These intersecting levers are
capable, however, of playing on this axle or pin H. To the ends G
of these levers are attached two heavy balls of metal I. The
levers F G pass through slits in a metallic arch attached to the
upright spindle, so as to be capable of revolving upon it. If the
balls I are drawn outwards from the vertical axis, it is evident
that the ends F of the levers will be drawn down, and therefore
the pivots E likewise drawn down. In fact, the angles E F H will
become more acute, and the angle F E F more obtuse. By these means
the sliding ring E will be drawn down. To this sliding ring E, and
immediately above it, is attached a grooved collar, which slides
on the vertical spindle upwards and downwards with the ring E. In
the grooved collar are inserted the prongs of a fork K, formed at
the end of the lever K L, the fulcrum or pivot of the lever being
at L. By this arrangement, when the divergence of the balls I
causes the collar E to be drawn down, the fork K, whose prongs are
inserted in the groove of that collar, is likewise drawn down;
and, on the other hand, when, by reason of the balls I falling
towards the [Pg211] vertical spindle, the collar E is raised, the
fork K is likewise raised.
The ascent and descent of the fork K necessarily produce a
contrary motion in the other end N of the lever. This end is
connected by a rod, or system of rods, with the end M of the short
lever which works the throttle-valve T. By such means the motion
of the balls I, towards or from the vertical spindle, produces in
the throttle-valve a corresponding motion; and they are so
connected that the divergence of the balls I will cause the
throttle-valve to close, while their descent towards the vertical
spindle will cause it to open.
These arrangements being comprehended, let us suppose that, either
by reason of a diminished load upon the engine or an increased
activity of the boiler, the speed has a tendency to increase. This
would impart increased velocity to the grooved wheel A B, which
would cause the balls I to revolve with an accelerated speed. The
centrifugal force which attends their motion would therefore give
them a tendency to move from the axle, or to diverge. This would
cause, by the means already explained, the throttle-valve T to be
partially closed, by which the supply of steam from the boiler to
the cylinder would be diminished, and the energy of the moving
power, therefore, mitigated. The undue increase of speed would
thereby be prevented.
If, on the other hand, either by an increase of the load, or a
diminished activity in the boiler, the speed of the machine was
lessened, a corresponding diminution of velocity would take place
in the grooved wheel A B. This would cause the balls I to revolve
with less speed, and the centrifugal force produced by their
circular motion would be diminished. This force being thus no
longer able fully to counteract their gravity, they would fall
towards the spindle, which would cause, as already explained, the
throttle-valve to be more fully opened. This would produce a more
ample supply of steam to the cylinder, by which the velocity of
the machine would be restored to its proper amount.
[Illustration: _Fig._ 42.]
(126.) The principle which renders the governor so perfect a
regulator of the velocity of the machine is difficult to be
[Pg212] explained without having recourse to the aid of the
technical language of mathematical physics. As, however, this
instrument is of such great practical importance, and has
attracted such general admiration, it may be worth while here to
attempt to render intelligible the mechanical principles which
govern its operation. Let S (_fig._ 42.) be the point of
suspension of a common pendulum S P, and let P O P′ be the arch of
its vibration, so that the ball P shall swing or vibrate
alternately to the east and to the west of the lowest point O,
through the arches O P′ and O P. It is a property of such an
instrument that, provided the arch in which it vibrates be not
considerable in magnitude, the time of its vibration will be the
same whether the arch be long or short. Thus, for example, if the
pendulum, instead of vibrating in the arch P P′, vibrated in the
arch _p p′_, the time which it would take to perform its
vibrations would be the same. If, however, the magnitude of the
arch of vibration be increased, then a variation will take place
in the time of vibration; but unless the arch of vibration be
considerably increased, this variation will not be great.
Now let it be supposed that while the pendulum P P′ continues to
vibrate east and west through the arch P P′, it shall receive such
an impulse from north and south as would, if it were not in a
state of previous vibration, cause it to vibrate between north and
south, in an arch similar to the arch P P′. This second vibration
between north and south [Pg213] would not prevent the continuance
of the other vibration between east and west; but the ball P would
be at the same time affected by both vibrations. While, in virtue
of the vibration from east to west, the ball would swing from P to
P′, it would, in virtue of the other vibration, extend its motion
towards the north to a distance from the line W E equal to half a
vibration, and will return from that distance again to the
position P′. While returning from P′ to P, its second vibration
will carry it towards the south to an equal distance on the
southern side of W E, and it will return again to the position P.
If the combination of these two motions or vibrations be
attentively considered, it will be perceived that the effect on
the ball will be a circular motion, precisely similar to the
circular motion of the balls of the governor already described.
Now the time of vibration of the pendulum S P between east and
west will not in any way be affected by the second vibration,
which it is supposed to receive between north and south, and
therefore the time the pendulum takes in moving from P to P′ and
back again from P′ to P will be the same whether it shall have
simultaneously or not the other vibration between north and south.
Hence it follows that the time of revolution of the circular
pendulum will be equal to the time of similar vibrations of the
same pendulum, if, instead of having a circular motion, it were
allowed to vibrate in the manner of a common pendulum.
If this point be understood, and if it also be remembered that the
time of vibration of a common pendulum is necessarily the same
whether the arch of vibration be small or great, it will be easily
perceived that the revolving pendulum or governor will have nearly
the same time of revolution whether it revolve in a large circle
or a small one: in other words, whether the balls revolve at a
greater or a less distance from the central spindle or axis. This,
however, is to be understood only approximately. When the angle of
divergence of the balls is as considerable as it usually is in
governors, the time of revolution at different distances from the
axis will therefore be subject to some variation, but to a very
small one. [Pg214]
The centrifugal force (which is the name given in mechanics to
that influence which makes a body revolving in a circle fly from
the centre) depends conjointly on the velocity of revolution, and
on the distance of the revolving body from the centre of the
circle. If the velocity of revolution be the same, then the
centrifugal force will increase in the same proportion as the
distance of the revolving body from the centre. If, on the other
hand, the distance of the revolving body from the centre remain
the same, the centrifugal force will increase in the same
proportion as the square of the time of vibration diminishes, or,
in other words, it will increase in the same proportion as the
square of the number of revolutions per minute. It follows from
this, therefore, that the greater is the divergence of the balls
of the governor, and the more rapidly they revolve, the greater
will be their centrifugal force. Now this centrifugal force, if it
were not counterbalanced, would give the balls a constant tendency
to recede from the centre; but from the construction of the
apparatus, the further they are removed from the centre the
greater will be the effect of their gravitation in resisting the
centrifugal force.
It is evident that the ball at P will have a greater tendency to
fall by gravitation towards O than it would have at _p_, because
the acclivity of the arch descending towards O at P is greater
than its acclivity at _p_. The gravitation, therefore, or tendency
of the ball to fall towards the central axis being greater at P
than at _p_ it will be able to resist a greater centrifugal force.
This increased centrifugal force, which the ball would have
revolving at the distance P above what it would have at the
distance _p_, is produced partly by the greater distance of the
ball from the central axis, and partly by the greater velocity of
its motion. But it will be evident that the time of its revolution
may nevertheless be the same, or nearly the same, at both
distances. If it should appear that the actual velocity of its
motion of revolution at P be greater than its velocity at _p_, in
the same proportion as the circles in which they revolve, then it
is evident that the time of revolution would be as much increased
by the greater space which P will have to travel over, as it will
have to be [Pg215] diminished by the greater speed with which
that space is traversed. The time of revolution, therefore, may be
the same, or nearly the same, in both cases.
If this explanation be comprehended, it will not be difficult to
apply it to the actual case of the governor. If a sudden increase
of the energy of the moving power, or a diminution of the load,
should give the machine an increased velocity, then the increased
speed of the balls of the governor will give them an increased
centrifugal force, which for the moment will be greater than the
tendency of their gravitation to make them fall towards the
vertical axis. This centrifugal force, therefore, prevailing, the
balls will recede from the axis; but as they recede, their
gravitation towards the vertical axis will, as has been already
explained, be increased, and will become equal to the centrifugal
force produced by the increased velocity, provided that velocity
do not exceed a certain limit. When the balls, by diverging, get
such increased gravitation as to balance the centrifugal force,
then they will continue to revolve at a fixed distance from the
vertical axis. When this happens, the time of the revolution must
be nearly the same as it was before their increased divergence; in
other words, the proportion of the moving power to the load will
be so restored by the action of the levers of the governor on the
throttle-valve that the machine will move at its former velocity,
or nearly so.
The principle on which the governor acts, as just explained,
necessarily supposes temporary disarrangements of the speed. In
fact, the governor, strictly speaking, does not maintain a uniform
velocity, but restores it after it has been disturbed. When a
sudden change of motion of the engine takes place, the governor
being immediately affected will cause a corresponding alteration
in the throttle-valve; and this will not merely correct the change
of motion, but it will, as it were, overdo it, and will cause a
derangement of speed of the opposite kind. Thus if the speed be
suddenly increased to an undue amount, then the governor being
affected will first close the throttle-valve too much, so as to
reduce the speed below the proper limit. This second error will
again affect the governor in the contrary way, and the speed
[Pg216] will again be increased rather too much. In this way a
succession of alterations of effect will ensue until the governor
settles down into that position in which it will maintain the
engine at the proper speed.
To prevent the inconvenience which would attend any excess of such
variations, the governor is made to act with great delicacy on the
throttle-valve, so that even a considerable change in the
divergence of the balls shall not produce too much alteration in
the opening of that valve: the steam in the boiler should have at
least 2 lbs. per square inch pressure more than is generally
required in the cylinder. This excess is necessary to afford scope
for that extent of variation of the power which it is the duty of
the throttle-valve to regulate.
The governor is usually so adjusted as to make thirty-six
revolutions per minute, when in uniform motion; but if the motion
is increased to the rate of thirty-nine revolutions, the balls
will fly to the utmost extent allowed them, being the limitation
of the grooves in which their rods move; and if, on the other
hand, the speed be diminished to thirty-four revolutions per
minute, they will collapse to the lowest extent of their play. The
duty of the governor, therefore, is to correct smaller casual
derangements of the velocity; but if any permanent change to a
considerable extent be made either in the load driven by the
machine or in the moving power supplied to it from the boiler,
then a permanent change is necessary to be made in the connection
between the governor and the throttle-valve, so as to render the
governor capable of regulating those smaller changes to which the
speed of the machine is liable.
(127.) Having thus explained the principal mechanical contrivances
provided by Watt for the maintenance and regulation of the
rotatory motion to be produced by his double-acting steam engine,
let us now consider the machine as a whole, and investigate the
process of its operation. A section of this engine is represented
in _fig._ 43.
[Illustration: _Fig._ 43.]
Steam is supplied from the boiler to the cylinder by the steam
pipe S. The throttle-valve T in that pipe, near the cylinder, is
regulated by a system of levers connected with [Pg217] the
governor. The piston P is accurately fitted in the steam cylinder
C by packing, as already described in the single-acting engine.
This piston, as it moves, divides the cylinder into two
compartments, between which there is no communication by which
steam or any other elastic fluid can pass. The upper steam box B
is divided into three compartments by the two valves. Above the
upper steam valve V is a compartment communicating with the steam
pipe; below the upper exhausting valve E is another compartment
communicating with the eduction pipe which leads to the condenser.
By the valves V and E a communication may be opened or closed
between the boiler on the one hand, or the condenser on the other,
and the top of the cylinder. The continuation S′ of the steam pipe
leads to the lower box B′, which, like the upper, is divided into
three compartments by two valves V′ and E′. The upper compartment
communicates with the steam pipe, and thereby with the boiler; and
the lower compartment communicates with the eduction pipe, and
thereby with the condenser. By means of the two valves V′ and E′,
a communication may be opened or closed between the steam pipe on
the one hand, or the exhausting pipe on the other, and the lower
part of the cylinder. The four valves V, E, V′, and E′ are
connected by a system of levers with a handle or spanner _m_,
which, being driven downwards or upwards, is capable of opening or
closing the valves in pairs, in the manner already described
(116.). The condensers, the air-pump, and the hot-water pump, are
in all respects similar to those already described in the
single-acting engine, except that the condensing jet is governed
by a lever I, by which it is allowed to play continually in the
condenser, and by which the quantity of water admitted through it
is regulated. The cold-water pump N is worked by the engine as
already described in the single-acting engine, and supplies the
cistern in which the air-pump and condenser are submerged, so as
to keep down its temperature to the proper limit. On the air-pump
rod R are two pins properly placed, so as to strike the spanner
_m_, upwards and downwards, at the proper times, when the piston
approaches the termination of the stroke at the top or bottom of
the cylinder. The pump L [Pg218] conducts the warm water drawn by
the air-pump from the condenser to a proper reservoir for feeding
the boiler. The vertical motion of the piston-rod in a straight
line is rendered compatible with the circular motion of the end of
the beam by the parallel motion already described. The point _b_,
on the beam, moves upwards and downwards in a circular arch, of
which the axis of the beam is the centre. In like manner the point
_d_ of the rod _d c_ moves upwards and downwards, in a similar
arch of which the fixed pivot _c_ is the centre. The joint or bar
_d b_, which joins these two pivots, will be moved so that its
middle point _e_ will ascend and descend nearly in a straight
line, as has been already explained (120.); [Pg219] opposite this
point _e_ is attached the piston-rod of the air-pump, which is
accordingly guided upwards and downwards by this means. The
jointed parallelogram _b d g f_ is attached to the beam by pivots;
and, as has been explained (120.), the point _g_ will be moved
upwards and downwards in a straight line, through twice the space
through which the point _e_ is moved. To the point _g_ the rod of
the steam piston is attached. Thus, the rods of the steam piston
and air-pump are moved by the same system of jointed bars, and
moved through spaces which are in the proportion of two to one.
Although this system of jointed rods forming the parallel motion,
appears in the figure to consist only of one parallelogram _b d g
f_, and one rod _c d_, called the _radius rod_, it is, in fact,
double, a similar parallelogram and radius rod being attached to
corresponding points, and in the same manner on the other side of
the beam; but from the view given in the cut, the one set of rods
hides the other. The two systems of rods thus attached to opposite
sides of the beam at several inches asunder, are connected by
cross rods, the ends of which form the pivots or joints, and
extend between the parallelograms. The ends of these rods are only
visible in the figure. It is to the middle of one of these rods,
the end of which is represented at _e_, that the air-pump
piston-rod is attached; and it is to the middle of another, the
end of which is represented at _g_, that the steam piston-rod is
attached. These two piston-rods, therefore, are driven, not
immediately by either of the parallelograms forming the parallel
motion, but by the bars extending between them.
To the working end of the beam H is attached a rod of cast-iron O,
called the _connecting rod_, the lower end of which is attached to
the crank by a pivot. The weight of the connecting rod is so made,
that it shall balance the weight of the piston-rods of the air-pump
and cylinder on the other side of the beam; and the weight of the
piston-rod of the cold-water pump N nearly balances the weight of
the piston-rod of the hot-water pump L. Thus, so far as the weights
of the machinery are concerned, the engine is in equilibrium, and
the piston would rest in any position indifferently in the cylinder.
The axis of the fly-wheel on which the crank is formed is [Pg220]
square in the middle part, where the fly-wheel is attached to it,
but has cylindrical necks at each end, which rest in sockets or
bearings supported by the framing of the machine, in which sockets
the axis revolves freely. On the axle of the crank is placed the
fly-wheel, and connected with its axle is the governor Q, which
regulates the throttle-valve T in the manner already described.
Let us now suppose the engine to be in full operation. The piston
being at the top of the cylinder, the spanner _m_ will be raised
by the lower pin on the air-pump rod, and the upper steam valve V,
and the lower exhausting valve E′, will be opened, while the upper
exhausting valve E and the lower steam valve V′ are closed. Steam
will, therefore, be admitted above the piston, and the steam which
filled the cylinder below it will be drawn off to the condenser,
where it will be converted into water. The piston will, therefore,
be urged by the pressure of the steam above it to the bottom of
the cylinder. As it approaches that limit, the spanner _m_ will be
struck downwards by the upper pin on the air-pump rod, and the
valves V and E′ will be closed, and at the same time the lower
steam valve V′ and the upper exhausting valve E will be opened.
Steam will, therefore, be admitted below the piston, while the
steam above it will be drawn off into the condenser, and converted
into water. The pressure of the steam, therefore, below the piston
will urge it upwards, and in the same manner the motion will be
continued.
While this process is going on in the cylinder and the condenser,
the water formed in the condenser will be gradually drawn off by
the operation of the air-pump piston, in the same manner as
explained in the single-acting engine; and at the same time the
hot water thrown into the hot well by the air-pump piston will be
carried off by the hot-water pump L.
Such are the chief circumstances attending the continuance of the
operation of the double-acting engine. It is only necessary here to
recall what has been already explained respecting the operation of
the fly-wheel. The commencement of the motion of the piston from the
top and bottom of the cylinder is produced, not by the pressure of
the steam upon it upwards or downwards, which must, for the reasons
[Pg221] already explained, be entirely inefficient; but by the
momentum of the fly-wheel, which extricates the crank from those
positions in which the moving power cannot affect it.
The manner in which the motion of the crank affects the connecting
rod at the dead points produces an effect of great importance in
the operation of the engine. When the crank-pin is approaching the
lowest point of its play, and therefore the piston approaching the
top of the cylinder, the motion of the crank-pin becomes nearly
horizontal, and consequently its effect in drawing the connecting
rod and the working end of the beam downwards and the piston
upwards, is extremely small. The consequence of this is, that as
the piston approaches the top of the cylinder, its motion becomes
very rapidly retarded; and as the motion of the crank-pin at its
lowest point is actually horizontal, the piston is brought to a
state of rest by this gradually retarded motion at the top of the
cylinder. In like manner, when the crank-pin moves from its dead
point upwards, its motion at first is very nearly horizontal, and
consequently its effect in driving the working end of the beam
upwards, and the piston downwards, is at first very small, but
gradually accelerated. The effect of this upon the piston is, that
it arrives at and departs from the top of the stroke with a very
slow motion, being absolutely brought to rest at that point.
The same effect is produced when the piston arrives at the bottom
of the cylinder. This retardation and suspension of the motion of
the piston at the termination of the stroke affords time for the
process of condensation to be effected, so that when the moving
power of the steam upon the piston can come into action, the
condensation shall be sufficiently complete. As the piston
approaches the top of the cylinder, and its motion becomes slow,
the working gear is made to open the lower exhausting valve; the
steam enclosed in the cylinder below the piston, and which has
just driven the piston upwards, presses with an elastic force of
17 lbs. per square inch on every part of the interior of the
cylinder, while the uncondensed vapour in the condenser presses
with a force of about 2 lbs. per square inch. The steam,
therefore, will have a tendency to rush from the cylinder to the
[Pg222] condenser through the open exhausting valve, with an
excess of pressure amounting to 15 lbs. per square inch, while the
piston pauses at the top of the cylinder. This process goes on,
and when the piston has descended by the motion of the fly-wheel,
a sufficient distance from the top of the cylinder to call the
moving force of the steam into action, the exhaustion will be
complete, and the pressure of the uncondensed vapour in the
cylinder will become the same as in the condenser.
The pressure of steam in the cylinder, and of uncondensed vapour
in the condenser, varies, within certain limits, in different
engines, and therefore the amount here assigned to them must be
taken merely as an example.
The size of the valves by which the steam is allowed to pass from
the cylinder to the condenser should be such as to cause the
condensation to take place in a sufficiently short time, to be
completed when the steam impelling the piston is called into
action.
Watt, in the construction of his engines, made the
exhaustion-valves with a diameter which was one fifth of the
diameter of the cylinder, and therefore the actual magnitude of
the aperture for the escape of the steam was one twenty-fifth of
the magnitude of the cylinder; but the spindle of the valve
diminished this so that the available space for the escape of
steam did not exceed one twenty-seventh of the magnitude of the
cylinder. This was found to produce a sufficiently rapid
condensation.
It was usual to make the steam valves of the same magnitude as the
exhausting valves, but the flow of steam through the former was
resisted by the throttle-valve, while no obstruction was opposed
to its passage through the latter.
The rapidity with which the cylinder must be exhausted by the
condenser will, however, depend upon the velocity with which the
piston is moved in it. The magnitude, therefore, of the exhausting
valves which would be sufficient for an engine which acts with a
slow motion would be too small where a rapid motion is required.
In the single-acting steam engine, where the moving force always
acted downwards on the piston, the pressure upon [Pg223] all the
joints of the machinery by which the force of the piston was
conveyed to the working parts, always took place in the same
direction, and consequently whatever might be the mechanical
connection by which the several joints were formed, the pins by
which they were connected, must always come to a bearing in their
respective sockets, however loosely they may have been fitted. For
the same reason, however, that the arch head and chain were
abandoned as a means of connecting the steam piston with the beam,
and the parallel motion substituted, it was also necessary in the
double-acting engine, where all joints whatever were driven
alternately in opposite directions, to fit the connecting pins
with the greatest accuracy in their sockets, and to abandon all
connection of the parts by chains. If any sensible looseness was
left in the joints, a violent jerk would be produced every time
the motion of the piston was reversed. Any looseness either in the
pivots or joints of the parallel motion of the working beam, the
connecting rod, or crank, would, at every change of stroke, be so
accumulated as to produce upon the machinery the effects of
percussion, and would consequently be attended with the danger of
straining and breaking the moveable parts of the mechanism.
To secure, therefore, the necessary accuracy of the joints, Watt
contrived that every joint in the engine should admit of the size of
the socket being exactly adapted to the size of the pin, so as
always to make a good fitting by closing the socket upon the pin,
when any looseness would be produced by wear. With this view, all
the joints were fitted with sockets made of brass or gun-metal,
capable of adjustment. Each socket was composed of two pieces,
accurately fitted into a cell or groove, in which one of the brasses
can be moved towards the other by means of a wedge or screw. Each
brass has in it a semi-cylindrical cavity, and the two cavities
being opposed to each other, form a socket for the joint-pin. One of
the two brasses can always be tightened round that pin, so as to
enclose it tight between the two semi-cylindrical cavities, and to
prevent any looseness taking place. The brasses, and other parts of
such a joint, are represented [Pg224] in _fig._ 44. These joints
still continue to be used in the engines as now constructed.
[Illustration: _Fig._ 44.]
The motion of the working beam, and the pump-rods which it drives,
and of the connecting rod, ought, if the whole were constructed
with perfect precision, to take place in the same or parallel
vertical planes; but this supposes a perfection of execution which
could hardly have been expected in the early manufacture of such
engines, whatever may have been attained by improvements which
have been since made. In the details of construction, Watt saw
that there would be a liability to lateral strain, owing to the
planes of the different motions not being truly vertical and truly
parallel, and that if a provision were not made for such lateral
motion, the machinery would be subject to constant strain in its
joints and rapid wear. He provided against this by constructing
the main joints by which the great working lever was connected
with the pistons and connecting rod, so as to form universal
joints, giving freedom of motion laterally as well as vertically.
The great lever, or working beam, was so called from being
originally made from a beam of oak. It is now, however,
universally constructed of cast-iron. The connecting rod is also
made of cast-iron, and attached to the beam and to the crank by
axles or pivots.
The mechanism by which the four valves are opened and closed, is
subject to considerable variation in different engines. They have
been described above as being opened and closed simultaneously by
a single lever. Sometimes, however, they are opened alternately in
pairs by two distinct levers driven by two pins attached to the
air-pump rod. One pin strikes the lever, which opens and closes
the upper steam valve, and lower exhausting valve; the other
strikes that which opens and closes the lower steam valve and
upper exhausting valve.
Since the date of the earlier double-acting engines, constructed
by Boulton and Watt, a great variety of mechanical expedients have
been practised for working the valves, by which the steam is
admitted to and withdrawn from the [Pg225] cylinder. We shall
here describe a few of these methods:—
(128.) The method of working the valves by pins on the air-pump
rod driving levers connected with the valves has been, in almost
all modern double-acting machines, superseded by an apparatus
called an _eccentric_, by which the motion of the axle of the
fly-wheel is made to open and close the valves at the proper
times.
[Illustration: _Fig._ 45.]
An eccentric is a metallic circle attached to a revolving axle, so
that the centre of the circle shall not coincide with the centre
round which the axle revolves. Let us suppose that G (_fig._ 45.),
is a square revolving shaft. Let a circular plate of metal B D,
having its centre at C, have a square hole cut in it, corresponding
to the shaft G, and let the shaft G pass through this square
aperture, so that the circular plate B D shall be fastened upon the
shaft, and capable of revolving with it as the shaft revolves. The
centre C of the circular plate B D will be carried round the centre
G of the revolving shaft, and will describe round it a circle, the
radius of which will be the distance of the centre C of the circular
plate from the centre of the shaft. Such circular plate so placed
upon a shaft, and revolving with it, is _an eccentric_.
Let E F be a metallic ring, formed of two semicircles of metal
screwed together at H, so as to be capable, by the adjustment of
the screws, of having the circular aperture formed by the ring
enlarged and diminished within certain [Pg226] small limits. Let
this circular aperture be supposed to be equal to the magnitude of
the eccentric B D. To the circular ring E F let an arm L M be
attached. If the ring E F be placed around the eccentric B D, and
that the screws H be so adjusted as to allow the eccentric B D to
revolve within the ring E F, then while the eccentric revolves,
the ring not partaking of its revolution, the arm L M will be
alternately driven to the right and to the left, by the motion of
the centre C of the eccentric as it revolves round the centre G of
the axle. When the centre C of the eccentric is in the same
horizontal line with the centre G, and to the left of it, then the
position of L M will be that which is represented in _fig._ 45.;
but when, after half a revolution of the main axle, the centre C
of the eccentric is thrown on the other side of the centre G, then
the point M will be transferred to the right, to a distance equal
to twice the distance C G. Thus as the eccentric B D revolves
within the ring E F, that ring, together with the arm L M, will be
alternately driven, right and left, through a space equal to twice
the distance between the centre of the eccentric and the centre of
the revolving shaft.
If we suppose a notch formed at the extremity of the arm L M, which
is capable of embracing a lever N M, moveable on a pivot at N, the
motion of the eccentric would give to such a lever an alternate
motion from right to left, and _vice versâ_. If we suppose another
lever N O connected with N M, and at right angles to it, forming
what is called a bell-crank, then the alternate motion received by
M, from right to left, would give a corresponding motion to the
extremity O of the lever N O, upwards and downwards. If this last
point O were attached to a vertical arm or shaft, it would impart to
such arm or shaft an alternate motion upwards and downwards, the
extent of which would be regulated by the length of the levers
respectively.
By such a contrivance the revolution of the fly-wheel shaft is
made to give an alternate vertical motion of any required extent
to a vertical shaft placed near the cylinder, which may be so
connected with the valves as to open and close them. Since the
upward and downward motion of this vertical shaft is governed by
the alternate motion of the centre [Pg227] C to the right and to
the left of the centre G, it is evident that by the adjustment of
the eccentric upon the fly-wheel shaft, the valves may be opened
and closed at any required position of the fly-wheel and crank,
and therefore at any required position of the piston in the
cylinder.
Such is the contrivance by which the valves, whatever form may be
given to them, are now almost universally worked in double-acting
steam engines.
* * * * *
Having described the general structure and operation of the steam
engine as improved by Watt, we shall now explain, in a more
detailed manner, some parts of its machinery which have been
variously constructed, and in which more or less improvements have
been made.
OF THE COCKS AND VALVES.
(129.) In the steam engine, as well as in every other machine in
which fluids act, it is necessary to open or close, occasionally,
the tubes or passages through which these fluids move. The
instruments by which this is accomplished are called cocks or
valves.
Cocks or valves may be classified by the manner in which they are
opened: 1st, they may be opened by a motion similar to the lid of
a box upon its hinges; 2d, they may be opened by being raised
directly upwards, in the same manner as the lid of a pot or
kettle; 3d, they may be opened by a sliding motion, like that of
the sash of a window or the lid of a box which slides in grooves;
4th, they may be opened by a motion of revolution, in the same
manner as the cock of a beer-barrel is opened or closed. The term
_valve_ is more properly applied to the first and second of these
classes; the third class are usually called _slides_, and the
fourth _cocks_.
(130.) The single clack valve is the most simple example of the
first class. It is usually constructed by attaching to a plate of
metal larger than the aperture which the valve is intended to
stop, a piece of leather, and to the under side of this leather
another piece of metal smaller than the aperture. The leather
[Pg228] extending on one side beyond the larger metallic plate,
and being flexible, forms the hinge on which the valve plays. Such
a valve is usually closed by its own weight, and opened by the
pressure of the fluid which passes through it. It is also held
closed more firmly by the pressure of the fluid whose return it is
intended to obstruct. An example of this valve occurs in the steam
engine, in the passage between the condenser and the air-pump. The
aperture which it stops is there a seat inclined at an angle whose
inclination is such as to render the weight of the valve
sufficient to close it. In cases where the valve is exposed to
heat, as in the example just mentioned, where it is continually in
contact with the hot water flowing from the condenser to the
air-pump, the use of leather is inadmissible, and in that case the
metallic surface of the valve is ground smooth to fit its seat.
The extent to which such a valve should be capable of opening,
ought to be such that the aperture produced by it shall be equal
to the aperture which it stops. This will be effected if the angle
through which it rises be about 30°.
[Illustration: _Fig._ 46.]
The valve by which the air and water collected in the bottom of
the air-pump are admitted to pass through the air-pump piston is a
double clack, consisting of two semicircular plates, having the
hinges on the diameters of these semicircles, as represented in
_fig._ 46.
(131.) Of the valves which are opened by a motion perpendicular to
their seat, the most simple is a flat metallic plate, made larger
than the orifice which it is intended to stop, and ground so as to
rest in steam-tight contact with the surface surrounding the
aperture. Such a valve is usually guided in its perpendicular
motion by a spindle passing through its centre, and sliding in
holes made in cross bars extending above and below the seat of the
valve.
The conical steam-valves, which have been already described
(116.), usually called spindle-valves, are the most common of this
class. The best angle to be given to the conical seat is found in
practice to be 45°. With a less inclination the valve has a
tendency to be fastened in its seat, and a greater inclination
would cause the top of the valve to occupy [Pg229] unnecessary
space in the valve-box. The area, or transverse section of the
valve-box, should be rather more than double the magnitude of the
upper surface of the valve, in order to allow a sufficiently free
passage for the steam, and the play of the valve should be such as
to allow it to rise from its seat to a height not less than one
fourth of the diameter of its upper surface.
The valves coming under this class are sometimes formed as spheres
or hemispheres resting in a conical seat, and in such cases they
are generally closed by their own weight, and opened by the
pressure of the fluid which passes through them.
(132.) One of the advantages attending the use of slides, compared
with the other form of valves, is the simplicity with which the
same slide may be made to govern several passages, so that a
single motion with a slide may perform the office of two or more
motions imparted to independent valves.
In most modern engines the passage of the steam to and from the
cylinder is governed by slides of various forms, some of which we
shall now explain.
[Illustration: _Fig._ 47.]
(133.) In _figs._ 47. and 48. is represented a slide-valve
contrived by Mr. Murray of Leeds. A B is a steam-tight case
attached to the side of the cylinder; E F is a rod, which receives
an alternate motion, upwards and downwards, from the eccentric, or
from whatever other part of the engine is intended to move the
slide. This rod, passing through a stuffing-box, moves the slide G
upwards and downwards. S is the mouth of the steam pipe coming
from the boiler; T is the mouth of a tube or pipe leading to the
condenser; H is a passage leading to the top, and I to the bottom,
of the cylinder. In the position of the slide represented in
_fig._ 47., the steam coming from the boiler through S passes
through the space H to the top of the cylinder, while the steam
from the bottom of the cylinder passes through the space I into
the tube T, and goes to the condenser. When the rod [Pg230] E F
is raised to the position represented in _fig._ 48., then the
passage H is thrown into communication with the tube T, while the
passage I is made to communicate with the tube S. Steam,
therefore, passes from the boiler through I below the piston,
while the steam which was above the piston, passing through H into
T, goes to the condenser. Thus the single slide G performs the
office of the four valves described in (116.).
[Illustration: _Fig._ 48.]
(134.) The slide G has always steam of a full pressure behind it,
while the steam in front of it escaping to the condenser, exerts
but little pressure upon it. It is therefore always forcibly
pressed against the surfaces in contact with which it moves, and
is thereby maintained steam-tight. Indeed this pressure would
rapidly wear the rubbing surfaces, unless they were made
sufficiently extensive, and hardened so as to resist the effects
of the friction. Where fresh water is used, as in land boilers,
the slide may be made of hardened steel; and in the case of marine
boilers, it may be constructed of gun-metal. In this and all other
contrivances in which the apertures by which the steam is admitted
to and withdrawn from the piston are removed to any considerable
distance from the top and bottom of the cylinder, there is a waste
of steam, for the steam consumed at each stroke of the piston is
not only that which would fill the capacity of the cylinder, but
also the steam which fills the passage between the slide G and the
top or bottom of the cylinder. Any arrangement which would throw
the passages H and I on the other side of the slide G, that is,
between S and G, instead of being, as they are, between G and the
top and bottom of the cylinder, would remove this defect. This is
accomplished by a slide, which is usually called the D valve,
because, being semi-cylindrical in its form, and hollow, its cross
section resembles the letter D. This slide, which is that which at
present is in most general use, is represented in _figs._ 49, 50.;
E is the rod by which the slide is moved, passing [Pg231] through
a stuffing-box F; G G is the slide represented by a vertical
section, _a a_ being a passage in it extending from the top to the
bottom; S is the mouth of the great steam pipe coming from the
boiler; P is the pipe leading to the condenser; T H is a hollow
space formed in the slide always in communication with the steam
pipe S, and consequently always filled with steam from the boiler.
A transverse section of the slide and cylinder is represented in
_fig._ 51., where _a_ represents the top of the passage marked _a_
in _fig._ 49. In the position of the slide represented in _fig._
49., the steam filling the space T H has access to the top of the
cylinder, but is excluded from the bottom. The steam which was
below the piston, passing up the passage _a_, escapes through the
tube P to the condenser. When the piston has descended, the rod E
moves the slide downwards, so as to give it the position
represented in FIG. 50. The steam in T H has now access to the
bottom of the cylinder, while the steam above the piston passing
through P escapes to the condenser. In this way the operation of
the piston is continued and the steam consumed at each stroke only
exceeds the capacity of the cylinder by what is necessary to fill
the passages between the slide and the cylinder.
[Illustration: _Fig._ 49.]
[Illustration: _Fig._ 50.]
[Illustration: _Fig._ 51.]
In a slide constructed in this manner, the steam filling the space
T H has a tendency to press the slide back, so as to break the
contact of the rubbing surfaces, and thereby to cause the steam to
leak from the space T H to the back of the slide. This is
counteracted by the packing _x_, at the back of the slide.
In engines of very long stroke, the extent of the rubbing surfaces
of slides of this kind renders it difficult to keep [Pg232] them
in steam-tight contact and to insure their uniform wear. In such
cases, therefore, separate slides, upon the same principle, are
provided at the top and bottom of the cylinder, moved, however, by
a single rod of communication.
(135.) In slides, as we have here described them, the same motion
which admits steam to either end of the cylinder, withdraws it
from the other end. Such an arrangement is only compatible with
the operation of a cylinder which works without expansion; for in
such a cylinder the full flow of steam to the piston is only
interrupted for a moment during the change of position of the
slide. But if the steam act expansively, it would be necessary to
move the slide, so as to stop its flow to one end of the cylinder,
without at the same time obstructing the escape of steam from the
other end to the condenser. It would therefore be necessary that
the slide should close the passage leading to the cylinder at one
end, without at the same time obstructing the communication
between the passage from the cylinder to the condenser at the
other end. On the arrival of the piston, however, at the bottom of
the cylinder, it would be necessary immediately to put the lower
passage to the cylinder in communication with the steam pipe, and
the upper passage in communication with the condenser. This would
necessarily suppose two motions of the slide as well as some
modifications in its length. Let the length of the slide be such
that when the passage to the top of the cylinder is stopped, the
lower part of the slide shall not reach the passage to the lower
part of the cylinder; and let such a provision be made in the
mechanism by which the rod E governing the slide is driven that it
shall receive two motions during the descent of the piston, the
first to be imparted to it at the moment the steam is to be cut
off, and the second just before the termination of the stroke. Let
the position of the slide, at the commencement of the stroke, be
represented in _fig._ 52., and let it be required that the steam
shall be cut off at one half of the stroke. When the piston has
made half the stroke, the rod governing the slide is moved
downwards, so as to throw the slide into the position represented
in _fig._ 53. The passage between the steam pipe and the cylinder
is [Pg233] now stopped at both ends; but the passage from the
bottom of the cylinder to the condenser remains open. During the
remainder of the stroke, therefore, the steam in the cylinder
works expansively. As the piston approaches the bottom of the
cylinder, another motion is imparted to the rod governing the
slide, by which the latter is thrown into the position represented
in _fig._ 54. Steam now flows below the piston while the steam
above it passes to the condenser. In a similar manner, by two
motions successively imparted to the slide during the ascent of
the piston, the steam may be cut off at half stroke; and it is
evident that by regulating the time at which these motions are
given to the slide, the steam may be worked expansively, to any
required extent.
[Illustration: _Fig._ 52.]
[Illustration: _Fig._ 53.]
[Illustration: _Fig._ 54.]
It is easy to conceive various mechanical means by which, in the
same engine, the point at which the steam is cut off may be
regulated at pleasure.
In cases where the motion of the piston is very rapid, as in
locomotive engines, it is desirable that the passages to and from
the cylinder should be opened very suddenly. This is difficult to
be accomplished with any form of slide consisting of a single
aperture; but if, instead of admitting the steam to the cylinder
by a single aperture, the same magnitude of opening were divided
among several apertures, then a proportionally less extent of
motion in the slide would clear the passage for the steam, and
consequently greater suddenness of opening would be effected.
[Pg234]
The great advantages in the economy of fuel resulting from the
application of the expansive principle have, of late years forced
themselves on the attention of engineers, and considerable
improvements have been made in its application, especially in the
case of marine engines used for long voyages, in which the economy
of fuel has become an object of the last importance. The mechanism
by which expansive slides are moved, is made capable of adjustment,
so that the part of the stroke at which the steam is cut off, can be
altered at pleasure. The working power of the engine, therefore,
instead of being controlled by the throttle-valve, is regulated by
the greater or less extent to which the expansive principle is
applied. Steam of the same pressure is admitted to the cylinder in
all cases; but it is cut off at a greater or less portion of the
stroke, according to the power which the engine is required to
exert.
The last degree of perfection has been conferred on this principle
by connecting the governor with the mechanism by which the slide
is moved, so that the governor instead of acting on the
throttle-valve, is made to act upon the slide. By this means when,
by reason of any diminution of the resistance, the motion of the
engine is accelerated, the balls of the governor diverging shift
the cam or lever which governs the slide, so that the steam is cut
off after a shorter portion of the stroke, the expansive principle
is brought into greater play, and the quantity of steam admitted
to the cylinder at each stroke is diminished. If, on the other
hand, the resistance to the machine be increased, so as to
diminish the velocity of the engine, then the balls collapsing the
levers of the governor shift the cam which moves the slides, so as
to increase the portion of the stroke made by the piston before
the steam is cut off, and thereby to increase the amount of
mechanical power developed in the cylinder at each stroke. The
extent to which the expansive principle is capable of being
applied, more especially in marine engines, has been hitherto
limited by the necessity of using steam of very high pressure,
whenever the steam is cut off after the piston has performed only
a small part of the stroke. A method, however, is now (March,
1840) under experimental trial, by [Pg235] Messrs. Maudsley and
Field, by which the expansive principle may be applied to any
required extent without raising the steam in the boiler above the
usual pressure of from three to five pounds per square inch. This
method consists in the use of a piston of great magnitude. The
force urging the piston is thus obtained not by an excessive
pressure on a limited surface, but by a moderate pressure diffused
over a large surface. The entire moving force acting on the piston
before the steam is cut off, is considerably greater than the
resistance; but during the remainder of the stroke this force is
gradually enfeebled until the piston is brought to the extremity
of its play.
[Illustration: _Fig._ 55.]
(136.) Mr. Samuel Seaward, of the firm of Messrs. Seawards,
engineers, has contrived an improved system of slides, for which
he has obtained a patent. A section of Seaward's slides is
represented in _fig._ 55. The steam pipe proceeding from the
boiler to the cylinder is represented at A A, and it communicates
with passages S and S′ leading to the top and bottom of the
cylinder. These passages are formed in nozzles of iron or other
hard metal cast upon the side of the cylinder. These nozzles
present a smooth face outwards, upon which the slides B B′, also
formed with smooth faces, play. The slides B B′ are attached by
knuckle-joints to rods E E′, which move through stuffing-boxes,
and the [Pg236] connection of these rods with the slides is such
that the slides have play so as to detach their surfaces easily
from the smooth surfaces of the nozzles when not pressed against
these surfaces. The steam in the steam pipe A A will press against
the backs of the slides B B′, and keep their faces in steam-tight
contact with the smooth surfaces of the nozzles. These slides may
be opened or closed by proper mechanism at any point of the
stroke. When steam is to be admitted to the top of the cylinder,
the upper slide is raised and the passage S opened; and when it is
to be admitted to the bottom of the cylinder, the lower slide is
raised and the passage S′ opened; and its communication to the top
or bottom of the cylinder is stopped by the lowering of these
slides respectively. On the other side of the cylinder are
provided two passages C C′ leading to a pipe G, which is continued
to the condenser. On this pipe are cast nozzles of iron or other
metal presenting smooth faces towards the cylinder, and having
passages D D′ communicating between the top and bottom of the
cylinder respectively and the pipe G G leading to the condenser.
Two slides _b b'_, having smooth faces turned from the cylinder,
and pressing upon the faces of the nozzles D D′, are governed by
rods playing through stuffing-boxes, in the same manner as already
described. The faces of these slides being turned from the
cylinder, the steam in the cylinder having free communication with
them, has a tendency to keep them by its pressure in steam-tight
contact with the surfaces in which the apertures leading to the
condenser are formed. These two slides may be opened or closed
whenever it is necessary.
When the piston commences its descent, the upper steam slide is
raised, so as to open the passage S, and admit steam above the
piston; and the lower exhausting slide _b′_ is also raised, so as
to allow the steam below the piston to escape through G to the
condenser, the other two passages S′ and C being closed by their
respective slides. The slide which governs S is lowered at that
part of the stroke at which the steam is intended to be cut off,
the other slides remaining unchanged; and when the piston has
reached the bottom of the cylinder, the lower steam slide opens
the passage S′, and [Pg237] the upper exhausting slide opens the
passage C; and at the same time the lower exhausting slide closes
the passage C′. Steam being admitted below the piston through S′,
and at the same time the steam above it being drawn away to the
condenser through the open passage C and the tube G, the piston
ascends. When it has reached that point at which the steam is
intended to be cut off, the slide which governs S′ is lowered, the
other slides remaining unaltered, and the upward stroke is
completed in the same manner as the downward.
These four slides may be governed by a single lever, or they may
be moved by separate means. From the small spaces between the
several slides and the body of the cylinder, it will be evident
that the waste of steam by this contrivance will be very small.
In the slide valves commonly used, the packing of hemp at the back
of the slide, by which the pressure necessary to keep the slide in
steam-tight contact is obtained, requires constant attention from
the engine-man while the engine is at work. Any neglect of this
will produce a corresponding loss in the power of the engine; and
accordingly it is found that in many cases where engines work
inefficiently, the defect is owing either to ignorance or want of
attention on the part of the engine-man in the packing of the
slides. In Seaward's slides no hemp packing is used, nor is any
attention on the part of the engine-man required after the slides
are first adjusted. The slides receive the pressure necessary to
keep them in steam-tight contact with the surfaces of the nozzles
from the steam itself, which acts behind them.
The eduction and steam slides being independent of each other,
they may be adjusted so that the engine shall work expansively in
any required degree; and this may be accomplished either by
working the slides by separate mechanism, or by a single
eccentric.
One of the advantages claimed by the patentees for these slides
is, that the engines are secured from the accidents which arise
from the accumulation of water within the steam cylinder. If such
a circumstance should occur, the action of the piston will press
the water against the faces of the steam [Pg238] slides, and the
play allowed to them by their connection with the rods which move
them permits their faces to be raised from the surfaces of the
nozzles, so that the water collected in the cylinder shall be
driven into the steam pipe, and sent back from thence to the
boiler.
[Illustration: _Fig._ 56.]
[Illustration: _Fig._ 57.]
[Illustration: _Fig._ 58.]
(137.) Of the cocks or valves which are opened and closed by the
motion of an axis passing through their centre, the throttle-valve,
whether worked by hand or by the governor, is an example. But the
most common form for cocks is that of a cylindrical or slightly
conical plug (_fig._ 56.), inserted in an aperture of corresponding
magnitude passing across the pipe or passage which the cock is
intended to open or close. One or more holes are pierced
transversely in the cock, and when the cock is turned so that these
holes run in the direction of the tube, the passage through the tube
is opened; but when the passage through the cock is placed at right
angles to the tube, then the sides of the tube stop the ends of the
passage in the cock, and the passage through the tube is obstructed.
The simple cock is designed to open or close the passage through a
single tube. When the cock is turned, as in _fig._ 57., so that the
passage through the cock shall be at right angles to the length of
the tube, then the passage through the tube is stopped; but when the
cock is turned from that position through a quarter of a revolution,
as in _fig._ 58., then the passage through the cock takes the
direction of the passage through the tube, and the cock is opened,
and the passage through the tube unobstructed. In such a cock the
passage may be more or less _throttled_ by [Pg239] adjusting the
position of the cock, so that a part of the opening in it shall be
covered by the side of the tube.
(138.) It is sometimes required to put one tube or passage
alternately in communication with two others. This is accomplished
by a _two-way cock_. In this cock the passage is curved, opening
usually at points on the surface of the cock, at right angles to
each other. Such a cock has already been described, and its use
illustrated in the description of the Marquis of Worcester's
engine (17.); the two-way cock, as represented at K and R (_fig._
4.), being the means by which steam and water are alternately
supplied to the two forcing vessels.
[Illustration: _Fig._ 59.]
[Illustration: _Fig._ 60.]
(139.) When it is required to put four passages alternately in
communication by pairs, a _four-way cock_ is used. Such a cock has
two curved passages (_fig. 59._), each similar to the curved
passage in the two-way cock. Let S C B T be the four tubes which
it is required to throw alternately into communication by pairs.
When the cock is in the position _fig._ 59., the tube S
communicates with T, and the tube C with B. By turning the cock
through a quarter of a revolution, as in _fig._ 60., the tube S is
made to communicate with B, and the tube C with T; and if the cock
continue to be turned at intervals through a quarter of a
revolution, these changes of communication will continue to be
alternately made. It is evident that this may be accomplished by
turning the cock continually in the same direction.
The four-way cock is sometimes used as a substitute for the valves
or slides in a double-acting steam engine to conduct the steam to
and from the cylinder. If S represent a pipe conducting steam from
the boiler, C that which leads to the condenser, T the tube which
leads to the top of the cylinder, and B that which leads to the
bottom, then when the cock is in the position (_fig._ 59.), steam
would flow from the boiler to [Pg240] the top of the piston,
while the steam below it would be drawn off to the condenser; and
in the position (_fig._ 60.), steam would flow from the boiler to
the bottom of the piston, while the steam above it would be drawn
off to the condenser. Thus by turning the cock through a quarter
of a revolution towards the termination of each stroke, the
operation of the machine would be continued.
One of the disadvantages which is inseparable from the use of a
four-way cock for this purpose is the loss of the steam at each
stroke, which fills the tubes between the cock and the ends of the
cylinder. This disadvantage could only be avoided by the
substitution of two two-way cocks (138.) instead of a four-way
cock. A two-way cock at the top of the cylinder would open an
alternate communication between the cylinder and steam pipe, and
the cylinder and condenser, while a similar office would be
performed by another two-way cock at the other end.
The friction on cocks of this description is more than on other
valves; but this is in some degree compensated by the great
simplicity of the instrument. When the cock is truly ground into
its seat, being slightly conical in its form, the pressure of the
steam has a tendency to keep the surfaces in contact; but this
pressure also increases the friction, and has a tendency to wear
the seat of the cock into an elliptical shape. Consequently, such
cocks require to be occasionally ground and refitted.
(140.) The four-way cock, as above described, admits the steam to
one end of the piston at the same moment that it stops it at the
other end. It would therefore be inapplicable where steam is
worked expansively. A slight modification, however, analogous to
that already described in the slides, will adapt it to expansive
action. This will be accomplished by giving to one of the passages
through the cock one aperture larger than the other, and working
the cock so that this passage shall always be used to conduct
steam to the cylinder; also by enlarging both apertures of the
other passage, and using it always to conduct steam from the
cylinder. The effect of such an arrangement will be readily
understood.
[Illustration: _Fig._ 61.]
[Illustration: _Fig._ 62.]
[Illustration: _Fig._ 63.]
[Illustration: _Fig._ 64.]
Let the position of the cock at the commencement of the [Pg241]
descending stroke be represented in _fig._ 61. Steam flows from S
through T to the top of the cylinder, while it escapes from B
through C from the bottom of the cylinder. When the piston has
arrived at that point at which the steam is to be cut off, let the
cock be shifted to the position represented in _fig._ 62. The
passage of steam from the boiler is now stopped, but the escape of
steam from the bottom of the cylinder through C continues, and the
cock is maintained in this position until the piston approaches
the bottom of the cylinder, when it is further shifted to the
position represented in _fig._ 63. Steam now flows from S through
B to the bottom of the cylinder, while the steam from the top of
the cylinder escapes through C to the condenser. When the piston
has arrived at that point where the steam is to be cut off, the
cock is shifted to the position represented in _fig._ 64. The
communication between the steam and the bottom of the piston is
now stopped, while the communication between the top of the
cylinder and the condenser is still open. During the next double
stroke of the piston the position of the cock is similarly
changed, but in the contrary direction, and in the same way the
motion is continued. Under these circumstances the cock, instead
[Pg242] of being moved constantly in the same direction, as in the
case of the common four-way cock, will require to be moved
alternately in opposite directions.
PISTONS.
(141.) The office of a piston being to divide a cylinder into two
compartments by a movable partition which shall obstruct the passage
of any fluid from one compartment to the other, it is evident that
the two conditions which such an instrument ought to fulfil are,
_first_, that the contact of its sides with the surface of the
cylinder shall be so close and tight throughout its entire play that
no steam or other fluid can pass between them; _secondly_, that it
shall be so free from friction, notwithstanding this necessary
tightness, that it shall not absorb any injurious quantity of the
moving power.
Since, however accurately the surfaces of the piston and cylinder
may be constructed, there will always be in practice more or less
imperfection of form, it is evident that the contact of the
surface of the piston with the cylinder throughout the stroke can
only be maintained by giving to the circumference of the piston
sufficient elasticity to accommodate itself to such inequalities
of form. The substance, whatever it may be, used for this purpose,
and by which the piston is surrounded, is called _packing_.
In steam pistons the material used for packing must be such as is
capable of resisting the united effects of heat and moisture.
Hence leather and other animal substances are inapplicable.
The packing used for steam pistons is therefore of two kinds,
_vegetable packing_, usually hemp, or _metallic packing_.
The common hemp-packed piston has been already in part described
(79.). The bottom of the piston is a circular plate just so much
less in diameter than the cylinder as is sufficient to allow its
free motion in ascending and descending. A little above its lowest
point this plate begins gradually to diminish in thickness, until
its diameter is reduced to from one to two inches less than that
of the cylinder, leaving therefore around [Pg243] it a hollow
space, as represented in _fig._ 65. The cover of the piston is a
plate similarly formed, being in like manner gradually reduced in
thickness downwards, so as to correspond with the lower plate. In
the hollow space which thus surrounds the piston a packing of
unspun hemp or soft rope, called _gasket_, is introduced by
winding it round the piston so as to render it an even and compact
mass. When the space is thus filled up, the top of the piston is
attached to the bottom by screws. The curved form of the space
within which the hempen packing is confined is such that when the
screws are tightened, that part of the packing which is nearest to
the top and bottom of the piston is forced against the cylinder,
so as to produce upon the two parallel rings as much pressure as
is necessary to render it steam-tight. When by use the packing is
worn down so as to produce leakage, the cover of the cylinder must
be removed, and the screws connecting the top and bottom of the
piston tightened: this will force out the packing and render the
piston steam-tight. This packing is lubricated by melted tallow
let down upon the piston from the funnel inserted in the top of
the cylinder, furnished with a stop-cock to prevent the escape of
steam. The lower end of the piston-rod is formed slightly conical,
the thickest part of the cone being downward. It is passed up
through the piston, and a nut or wedge between the top and bottom
is inserted so as to secure the piston in its position upon the
rod.
[Illustration: _Fig._ 65.]
The process of removing the top of the cylinder for the purpose of
tightening the screws in the piston is one of so laborious a
nature, that the men entrusted with the superintendence of these
machines are tempted to allow the engine to work notwithstanding
injurious leakage at the piston, rather than incur the labour of
tightening the screws as often as it is necessary to do so.
To avoid this inconvenience, the following method of [Pg244]
tightening the packing of the piston without removing the lid of
the cylinder, was contrived by Woolf. The head of each of the
screws was formed into a toothed pinion, and as these screws were
placed at equal distances from the centre of the piston, these
several pinions were driven by a large toothed wheel, revolving on
the piston-rod as an axis. By such an arrangement it is evident
that if any one of the screws be turned, a like motion will be
imparted to all the others through the medium of the large central
wheel. Woolf accordingly formed, on the head of one of the screws,
a square end. When the piston was brought to the top of the
cylinder, this square end entered an aperture made in the under
side of the cover of the cylinder. This aperture was covered by a
small circular piece screwed into the top of the cylinder, which
was capable of being removed so as to render the square head of
the screw accessible. When this was done, a proper key being
applied to the square head of the screw, it was turned; and by
being turned, all the other screws were in like manner moved. In
this way, instead of having to remove the cover of the cylinder,
which in large cylinders was attended with great labour and loss
of time, the packing was tightened by merely unscrewing a piece in
the top of the cylinder not much greater in magnitude than the
head of one of the screws.
This method was further simplified by causing the great circular
wheel already described to move upon the piston-rod, not as an
axis, but as a screw, the thread being cut upon a part of the
piston-rod which worked in a corresponding female screw cut upon
the central plate. By such means, the screw whose head was let
into the cover of the cylinder which turned, would cause this
circular plate to be pressed downwards by the force of the screw
constructed on the piston-rod. This circular plate thus pressed
downwards, acted upon pins or plugs which pressed together the top
and bottom of the cylinder in the same manner as they were pressed
together by the screws connecting them as already described.
METALLIC PISTONS.
(142.) The notion of constructing a piston so as to move
steam-tight in the cylinder without the use of packing of
vegetable [Pg245] matter was first suggested by the Rev. Mr.
Cartwright, a gentleman well known for other mechanical
inventions. A patent was granted in 1797 for a new form of steam
engine, in which he proposed to use the vapour of alcohol to work
the piston instead of the steam of water: and since the principle
of the engine excluded the use of lubrication by oil or tallow, he
substituted a piston formed of metallic rings pressed against the
surface of the cylinder by springs, so as to be maintained in
steam-tight contact with it, independently either of packing or
lubrication. Although the engine for which this form of piston was
intended never came into practical use, yet it is so simple and
elegant in its structure, and forms a link so interesting in the
history of the steam engine, that some explanation of it ought not
to be omitted in this work.
The steam-pipe from the boiler is represented cut off at B (_fig._
66.); T is a spindle-valve, for admitting steam above the piston,
and R is a spindle-valve in the piston; D is a curved pipe forming
a communication between the cylinder and the condenser, which is
of very peculiar construction. Cartwright proposed effecting a
condensation without a jet, by exposing the steam to contact with
a very large quantity of cold surface. For this purpose, he formed
his condenser by placing two cylinders nearly equal in size, one
within the other, allowing the water of the cold cistern in which
they were placed to flow through the inner cylinder, and to
surround the outer one. Thus, the thin space between the two
cylinders formed the condenser.
[Illustration: _Fig._ 66.]
The air-pump is placed immediately under the cylinder, and the
continuation of the piston-rod works its piston, which is solid
and without a valve. F is the pipe from the condenser to the
air-pump, through which the condensed steam is drawn off through
the valve G on the ascent of the piston, and on the descent this
is forced through a tube into a hot well H, for the purpose of
feeding the boiler through the feed-pipe I. In the top of the hot
well H is a valve which opens inwards, and is kept closed by a
ball floating on the surface of the liquid. The pressure of the
condensed air above the surface of the liquid in H forces it
through I into the boiler. When the air accumulates in too great a
degree [Pg246] in H, the surface of the liquid is pressed so low
that the ball falls and opens the valve, and allows it to escape.
The air in H is that which is pumped from the condenser with the
liquid, and from which it was disengaged.
Let us suppose the piston at the top of the cylinder: it strikes
the tail of the valve T, and raises it, while the stem of the
piston-valve R strikes the top of the cylinder, and is pressed
into its seat. A free communication is at the same time open
between the cylinder, below the piston and the condenser, through
the tube D. The pressure of the steam [Pg247] thus admitted above
the piston acting against the vacuum below it, will cause its
descent. On arriving at the bottom of the cylinder, the tail of
the piston-valve R will strike the bottom, and it will be lifted
from its seat, so that a communication will be opened through it
with the condenser. At the same moment, a projecting spring K,
attached to the piston-rod, strikes the stem of the steam-valve T,
and presses it into its seat. Thus while the further admission of
steam is cut off, the steam above the piston flows into the
condenser, and the piston being relieved from all pressure, is
drawn up by the momentum of the fly-wheel, which continues the
motion it received from the descending force. On the arrival of
the piston again at the top of the cylinder, the valve T is opened
and R closed, and the piston descends as before, and so the
process is continued.
The mechanism by which motion is communicated from the piston to
the fly-wheel is peculiarly elegant. On the axis of the fly-wheel
is a small wheel with teeth, which work in the teeth of another
larger wheel L. This wheel is turned by a crank, which is worked
by a cross-piece attached to the end of the piston-rod. Another
equal-toothed wheel M is turned by a crank, which is worked by the
other end of the cross-arm attached to the piston-rod.
One of the peculiarities of this engine is, that the liquid which
is used for the production of steam in the boiler circulates
through the machine without either diminution or admixture with
any other fluid, so that the boiler never wants more feeding than
what can be supplied from the hot well H. This circumstance forms
an important feature in the machine, as it allows of ardent
spirits being used in the boiler instead of water, which, since
they boil at low heats, promised a saving of fuel. The inventor
proposed that the engine should be used as a still, as well as a
mechanical power, in which case the whole of the fuel would be
saved.
[Illustration: _Fig._ 67.]
[Illustration: _Fig._ 68.]
(143.) That part of Cartwright's piston which in the common piston
is occupied by the packing of gasket, already explained (141.),
was filled by a number of rings, one placed within and above
another, and divided into three or four [Pg248] segments. Two
rings of brass were made of the full size of the cylinder, and so
ground as to fit the cylinder nearly steam-tight. These were cut
into several segments A A A (_fig._ 67.), and were placed one
above the other, so as to fill the space between the top and
bottom plates of the piston. The divisions of the segments of the
one ring were made to fit between the divisions of the other.
Within these another series of rings, B B B, were placed,
similarly constructed, so as to fit within the first series in the
same manner as the first series were made to fit within the
cylinder. The joints of the upper series of each set of rings are
exhibited in the plan (_fig._ 67.); the places of the joints of
the lower series are shown by dotted lines; the position of the
rings of each series one above the other is shown in the section
(_fig._ 68.). The joints of the inner series of rings are so
placed as to lie between those of the outer series, to prevent the
escape of steam which would take place by one continued joint from
top to bottom of the packing. The segments into which the rings
are divided are pressed outwards by steel springs in the form of
the letter V, the springs which act upon the outer series of
segments abutting upon the inner series, and those which act on
the inner series abutting upon the solid centre of the piston:
these springs are represented in _fig._ 67.
[Illustration: _Fig._ 69.]
[Illustration: _Fig._ 70.]
(144.) An improved form was given to the metallic piston by
Barton. Barton's piston consists of a solid cylinder of cast iron,
represented at A in section in _fig._ 69., and in plan in [Pg249]
_fig._ 70. In the centre of this is a conical hole, increasing in
magnitude downwards, to receive the piston-rod, in which the
latter is secured by a cross-pin B. A deep groove, square in its
section, is formed around the piston, so that while the top and
bottom of the piston form circles equal in magnitude to the
section of the cylinder, the intermediate part of the body of the
piston forms a circle less than the former by the depth of the
groove. Let a ring of brass, cast iron, or cast steel, be made to
correspond in magnitude and form with this groove, and let it be
divided as represented in _fig._ 70., into four segments C C C C,
and four corresponding angular pieces D D D D. Let the groove
which surrounds the piston be filled by the four segments with the
four wedge-like angular pieces within them, and let the latter be
urged against the former by eight spiral springs, as represented
in _fig._ 69. and _fig._ 70. These springs will abut against the
solid centre by the piston, and will urge the segments C against
the cylinder. The spiral springs which urge the wedges are
confined in their action by steel pins which pass through their
centre, and by being [Pg250] confined in cylindrical cavities
worked into the wedges and into corresponding parts of the solid
centre of the piston, as the segments C wear, the springs urge the
wedges outwards, and the points of the latter protruding, are
gradually worn down so as to fill up the spaces left between the
segments, and thus to complete the outer surface of the piston.
Various other forms of metallic pistons have been proposed, but as
they do not differ materially in principle from those we have just
described, it will not be necessary here to describe them.
[Illustration: ENGINE AT THE CITY SAW MILLS.]
[Pg251]
[Illustration: FURNACE AT THE CITY SAW MILLS.]
CHAP. IX.
CONSTITUENTS OF COAL. — PROCESS OF COMBUSTION. — HEAT EVOLVED
IN IT. — FORM AND STRUCTURE OF BOILER. — WAGGON BOILER. —
FURNACE. — METHOD OF FEEDING IT. — COMBUSTION OF GAS IN FLUES.
— CONSTRUCTION OF GRATE AND ASH-PIT. — MAGNITUDE OF HEATING
SURFACE OF BOILER. — STEAM-SPACE AND WATER-SPACE IN BOILER. —
POSITION OF FLUES. — METHOD OF FEEDING BOILER. — LEVEL GAUGES.
— SELF-REGULATING FEEDERS. — STEAM-GAUGE. — BAROMETER-GAUGE. —
INDICATOR. — COUNTER. — SAFETY-VALVE. — FUSIBLE PLUGS. —
SELF-REGULATING DAMPER. — SELF-REGULATING FURNACE. — POWER AND
DUTY OF ENGINES. — HORSE-POWER OF STEAM ENGINES. — EVAPORATION
PROPORTIONAL TO HORSE-POWER. — SOURCES OF LOSS OF POWER. —
ABSENCE OF GOOD PRACTICAL RULES FOR POWER. — COMMON RULES
FOLLOWED BY ENGINE MAKERS. — DUTY DISTINGUISHED FROM POWER. —
DUTY OF BOILERS. — PROPORTION OF STROKE TO DIAMETER OF
CYLINDER. — DUTY OF ENGINES. — CORNISH SYSTEM OF INSPECTION. —
ITS GOOD EFFECTS. — HISTORICAL DETAIL OF THE DUTY OF CORNISH
ENGINES.
(145.) The machinery which has been explained in the preceding
chapters, consisting of the cylinder with its passages and valves,
the piston-rod, parallel motion, beam, connecting-rod and crank,
together with the condenser, air-pump, and other appendages,
having no source of moving power in themselves, must be regarded
as mere instruments by which the mechanical effect developed by
the furnace and the boiler is transmitted to the working point and
so [Pg252] modified as to be adapted to the uses to which the
machine is applied. The boiler is at once a magazine in which the
moving power is stored in sufficient quantity to supply the
demands of the engine and an apparatus in which that power is
fabricated. The mechanical effect evolved in the conversion of
water into steam by heat, is the process by which the power of the
steam-engine is produced, and space is provided in the boiler,
capacious enough to contain as much steam as is necessary for the
engine, besides a sufficient quantity of water to continue that
supply undiminished, notwithstanding the constant drafts made upon
it by the cylinder: even the water itself, from the evaporation of
which the mechanical power is produced, ought to be regarded as an
instrument by which the effect of the heat of the combustible is
rendered mechanically efficient, inasmuch as the same heat,
applied not only to other liquids but even to solids, would
likewise be productive of mechanical effects. The boiler and its
furnace are therefore parts of the steam-engine, the construction
and operation of which are entitled to especial attention.
(146.) COAL, the combustible almost universally used in
steam-engines, is a substance, the principal constituents of which
are _carbon_ and _hydrogen_, occasionally mixed with sulphur in a
small proportion, and earthy incombustible matter. In different
sorts of coal the proportions of these constituents vary, but in
coal of good quality about three quarters of the whole weight of
the combustible is carbon.
When carbon is heated to a temperature of about 700° in an
atmosphere of pure oxygen, it will combine chemically with that
gas, and the product will be the gas called _carbonic acid_. The
volume of carbonic acid produced by this combination, will be
exactly equal to that of the oxygen combined with the carbon, and
therefore the weight of a given volume of the gas will be
increased by the weight of carbon which enters the combination. It
is found that two parts by weight of oxygen combined with three of
carbon, form carbonic acid. The weight of the carbonic acid,
therefore, produced in the combustion, will be greater than the
weight of the oxygen, bulk for bulk, in the proportion of five to
two, the volume being the same and the gases being [Pg253]
compared at the same temperatures and under equal pressures. In
this combination heat is evolved in very large quantities. This
effect arises from the heat previously latent in the carbon and
oxygen being rendered sensible in the process of combustion. The
carbonic acid proceeding from the combustion is by such means
raised to a very high temperature, and the carbon during the
process acquires a heat so intense as to become luminous; no
flame, however, is produced.
Hydrogen, heated to a temperature of about 1000°, in contact with
oxygen will combine with the latter, and a great evolution of heat
will attend the process; the gases will be rendered luminous, and
flame will be produced. The product of this process will be water,
which being exposed to the intense heat of combustion, will be
immediately converted into steam. Hydrogen combines with eight
times its own weight of oxygen, producing nine times its own
weight of water.
Hydrogen gas is, however, not usually disengaged from coal in a
simple form, but combined chemically with a certain portion of
carbon, the combination being called carburetted hydrogen. Pure
hydrogen burns with a very faintly luminous blue flame, but
carburetted hydrogen gives that bright flame occasionally having
an orange or reddish tinge, which is seen to issue from burning
coals: this is the gas used for illumination, being expelled from
the coal by the process of coking, and conducted to the various
burners through proper pipes.
The sulphur, which in a very small proportion is contained in
coals, is also combustible, and combines in the process of
combustion with oxygen, forming sulphurous acid: it is also
sometimes evolved in combination with hydrogen, forming
sulphuretted hydrogen.
Atmospheric air consists of two gases, azote and oxygen, mixed
together in the proportion of four to one; five cubic feet of
atmospheric air consisting of four cubic feet of azote and one of
oxygen. Any combustible will combine with the oxygen contained in
atmospheric air, if raised to a temperature somewhat higher than
that which is necessary to cause its combustion in an atmosphere
of pure oxygen.
If coals, therefore, or other fuel exposed to atmospheric [Pg254]
air, be raised to a sufficiently high temperature, their
combustible constituents will combine with the oxygen of the
atmospheric air, and all the phenomena of combustion will ensue.
In order, however, that the combustion should be continued, and
should be carried on with quickness and activity, it is necessary
that the carbonic acid, and other products, should be removed from
the combustible as they are produced, and fresh portions of
atmospheric air brought into contact with it; otherwise the
combustible would soon be surrounded by an atmosphere composed
chiefly of carbonic acid to the exclusion of atmospheric air, and
therefore of uncombined oxygen, and consequently the combustion
would cease, and the fuel be extinguished. To maintain the
combustion, therefore, a current of atmospheric air must be
constantly carried through the fuel: the quantity and force of
this current must depend on the quantity and quality of the fuel
to be consumed. It must be such that it shall supply sufficient
oxygen to the fuel to maintain the combustion, and not more than
sufficient, since any excess would be attended with the effect of
absorbing the heat of combustion, without contributing to the
maintenance of that effect.
Heat is communicated from body to body in two ways, by radiation
and by contact.
Rays of heat issue from a heated body, and are dispersed through
the surrounding space in a manner, and according to laws, similar
to those which govern the radiation of light. The heat thus
radiated meeting other bodies is imparted to them, and penetrates
them with more or less facility according to their physical
qualities.
A heated body also brought into contact with another body of lower
temperature, communicates heat to that other body, and will
continue to do so until the temperature of the two bodies in
contact shall be equalised. Heat proceeds from fuel in a state of
combustion in both these ways: the heated fuel radiates heat in
all directions around it, and the heat thus radiated will be
imparted to all parts of the furnace which are exposed to the
fuel.
The gases, which are the products of the combustion, escape from
the fuel at a very high temperature, and consequently, in
acquiring that temperature they absorb a considerable [Pg255]
quantity of the heat of combustion. But besides the gases actually
formed in the process of combustion, the azote forming four fifths
of the air carried through the fuel to support the combustion,
absorbs heat from the combustible, and rises into the upper part
of the furnace at a high temperature. These various gases, if
conducted directly to the chimney, would carry off with them a
considerable quantity of the heat. Provision should therefore be
made to keep them in contact with the boiler such a length of time
as will enable them to impart such a portion of the heat which
they have absorbed from the fuel, as will still leave them at a
temperature sufficient, and not more than sufficient, to produce
the necessary draft in the chimney.
(147.) The forms of boiler which have been proposed as the most
convenient for the attainment of all these requisite purposes have
been very various. If strength alone were considered, the
spherical form would be the best; and the early boilers were very
nearly hemispheres, placed on a slightly concave base. The form
adopted by Watt, called the waggon boiler, consists of a
semi-cylindrical top, flat perpendicular sides, flat ends, and a
slightly concave bottom. The steam intended to be used in boilers
of this description did not exceed the pressure of the external
atmosphere by more than from 3 to 5 lbs. per square inch; and the
flat sides and ends, though unfavourable to strength, could be
constructed sufficiently strong for this purpose. In a boiler of
this sort, the air and smoke passing through the flues that are
carried round it, are in contact at one side only with the boiler.
The brickwork, or other materials forming the flue, must therefore
be non-conductors of heat, that they may not absorb any
considerable portion of heat from the air passing in contact with
them. A boiler of this form is represented in _fig._ 71.
The grate and a part of the flues are rendered visible by the
removal of a portion of the surrounding masonry in which the
boiler is set. The interior of the boiler is also shown by cutting
off one half of the semi-cylindrical roof. A longitudinal vertical
section is shown in _fig._ 72., and a cross section in _fig._ 73.
A horizontal section taken above the level of the grate, and below
the level of the water in the boiler, showing [Pg256] the course
of the flues, is given in _fig._ 74. The corresponding parts in
all the figures are marked by the same letters.
[Illustration: _Fig._ 71.]
The door by which fuel is introduced upon the grate is represented
at A, and the door leading to the ash-pit at B. The fire bars at C
slope downwards from the front at an angle of about 25°, giving a
tendency to the fuel to move from the front towards the back of
the grate. The ash-pit D is constructed of such a magnitude, form,
and depth, as to admit a current of atmospheric air to the
grate-bars, sufficient to sustain the combustion. The form of the
ash-pit is usually wide below, contracting towards the top.
[Illustration: _Fig._ 72.]
The fuel when introduced at the fire-door A, should be laid on that
part of the grate nearest to the fire-door, called the dead plates:
there it is submitted to the process of coking, by which the gases
and volatile matter which it contains are expelled, and being
carried by a current of air, admitted [Pg257] through small
apertures in the fire-door over the burning fuel in the hinder part
of the grate, they are burnt. When the fuel in front of the grate
has been thus _coked_, it is pushed back, and a fresh feed
introduced in front. The coal thus pushed back soon becomes vividly
ignited, and by continuing this process, the fuel spread over the
grate is maintained in the most active state of combustion at the
hinder part of the grate. By such an arrangement, the smoke produced
by the combustion of the fuel may be burnt before it enters the
flues. The flame and heated air proceeding from the burning fuel
arising from the grate, and rushing towards the back of the furnace,
passes over the _fire-bridge_ E, and is carried through the flue F
which passes under the boiler. This flue (the cross section of which
is shown in _fig._ 73., by the dark shade put under the boiler) is
very nearly equal in width to the bottom of the boiler, the space at
the bottom of the boiler, near the corners, being only what is
sufficient to give the weight of the boiler support on the masonry
forming the [Pg258] sides of the flue. The bottom of the boiler
being concave, the flame and heated air as they pass along the flue
rise to the upper part by the effects of their high temperature, and
_lick_ the bottom of the boiler from the fire-bridge at E to the
further end G.
[Illustration: _Fig._ 73.]
[Illustration: _Fig._ 74.]
At G the flue rises to H, and turning to the side of the boiler at
I I, conducts the flame in contact with the side from the back to
the front; it then passes through the flue K across the front, and
returns to the back by the other side [Pg259] flue L. The side
flue is represented, stripped of the masonry, in _fig._ 71., and
also appears in the plan in _fig._ 74., and in the cross section
in _fig._ 73. The course of the air is represented in _fig._ 74.
by the arrows. From the flue L the air is conducted into the
chimney at M.
By such an arrangement, the flame and heated air proceeding from
the grate are made to circulate round the boiler, and the length
and magnitude of the flues through which it is conducted should be
such, that when it shall arrive at the chimney its temperature
shall be reduced, as nearly as is consistent with the maintenance
of draught in the chimney, to the temperature of the water with
which it is in contact.
The method of feeding the furnace, which has been described above,
is one which, if conducted with skill and care, would produce a
much more perfect combustion of the fuel than would attend the
common method of filling the grate from the back to the front with
fresh fuel, whenever the furnace is fed. This method, however, is
rarely observed in the management of the furnace. It requires the
constant attention of the stokers (such is the name given to those
who feed the furnaces). The fuel must be supplied, not in large
quantities, and at distant intervals, but in small quantities and
more frequently. On the other hand, the more common practice is to
allow the fuel on the grate to be in a great degree burned away,
and then to heap on a large quantity of fresh fuel, covering over
with it the burning fuel from the back to the front of the grate.
When this is done, the heat of the ignited coal acting upon the
fresh fuel introduced, expels the gases combined with it and,
mixed with these, a quantity of carbon, in a state of minute
division, forming an opaque black smoke. This is carried through
the flues and drawn up the chimney. The consequence is, that not
only a quantity of solid fuel is sent out of the chimney
unconsumed, but the hydrogen and other gases also escape unburned,
and a proportional waste of the combustible is produced; besides
which, the nuisance of an atmosphere filled with smoke ensues.
Such effects are visible to all who observe the chimneys of
steam-vessels, while the engine is in operation. When the furnaces
are thus filled with fresh fuel, a large volume of [Pg260] dense
black smoke is observed to issue from the chimney. This gradually
subsides as the fuel on the grate is ignited, and does not
reappear until a fresh feed is introduced.
This method of feeding, by which the furnace would be made to
consume its own smoke, and the combustion of the fuel be rendered
complete, is not however free from counteracting effects. In
ordinary furnaces the feed can only be introduced by opening the
fire-doors, and during the time the fire-doors are opened a volume
of cold air rushes in, which passing through the furnace is
carried through the flues to the chimney. Such is the effect of
this in lowering the temperature of the flues, that in many cases
the loss of heat occasioned is greater than any economy of fuel
obtained by the complete consumption of smoke. Various methods,
however, may be adopted by which fuel may be supplied to the grate
without opening the fire-doors, and without disturbing the supply
of air to the fire. A hopper built into the front of the furnace,
with a moveable bottom or valve, by which coals may be allowed to
drop in from time to time upon the front of the grate, would
accomplish this.
(148.) In order to secure the combustion of the gases evolved from
the coals placed in the front of the grate, it is necessary that a
supply of atmospheric air should be admitted with them over the
burning fuel. This is effected by small apertures or regulators,
provided in the fire-doors, governed by sliding-plates, by which
they may be opened or closed to any required extent.
A patent has recently been granted to Mr. Williams, one of the
directors of the City of Dublin Steam Navigation Company, for a
method of consuming the unburned gases which escape from the
grate, and are carried through the flues. This method consists in
introducing into the flue tubes placed in a vertical position, the
lower ends of which being inserted in the bottom of the flue are
made to communicate with the ash-pit, and the upper ends of which
are closed. The sides and tops of these tubes are pierced with
small holes, through which atmospheric air drawn from the ash-pit
issues in jets. The oxygen supplied by this air immediately
combines with the carburetted hydrogen, which [Pg261] having
escaped from the furnace unburned is carried through the flues at
a sufficient temperature to enter into combination with the oxygen
admitted through holes in the tubes. A number of jets of flame
thus proceed from these holes, having an appearance similar to the
flame of a gas-lamp.
It is evident that such tubes must be inefficient unless they are
placed in the flues so near the furnace, that the temperature of
the unburned gases shall be sufficiently high to produce their
combustion.
(149.) The magnitude of the grate and ash-pit must be determined by
the rate at which the evaporation is required to be conducted in the
boiler and the quality of the fuel. It must be a matter of regret,
that the proportions of the various parts of steam-engines, with
their boilers and furnaces, have not been determined by any exact or
satisfactory experiments; and those who project and manufacture the
engines themselves, are not less in ignorance on those points than
others. With coals of the common quality a certain average
proportion must exist between the necessary magnitude of the
grate-surface and the quantity of water to be evaporated in a given
time in the boiler. But what that proportion is for any given
quality of fuel, is at present unascertained. Each engine-maker
follows his own rule, and the rule thus followed is in most cases a
matter of bare conjecture, unsupported by any experimental evidence.
Some engine-makers will allow a square foot of grate-surface for
every cubic foot of water per hour, which is expected to be
evaporated in the boiler; others allow only half a square foot: and
practice varies between these limits. Bituminous coals which melt
and cake, and which burn with much flame and smoke, must be spread
more thinly on the grate than other descriptions of fuel, otherwise
a considerable quantity of combustible gases would be dismissed into
the flues unburnt. Such coals therefore, other circumstances being
the same, require a larger portion of grate-surface; and the same
may be said of coals which produce clinkers in their combustion, and
form lumps of vitrified matter on the grate, by which the spaces
between the grate-bars are speedily closed up. When such fuel is
used, the grate-bars require to be frequently raked [Pg262] out,
otherwise the spaces between them being obstructed, the draught
would become insufficient for the due combustion of the fuel.
To facilitate the raking out of the grate, the bars are placed
with their ends towards the fire-door: they are usually made of
cast-iron, from two to two inches and a half wide on the upper
surface, with intervals of nearly half an inch between them. The
bars taper downwards, their under surfaces being much narrower
than their upper, the spaces between them thus widening, to
facilitate the fall of the ashes between them. The grate-bars
slope downwards from the front to the back. The height of the
centre of the bottom of the boiler, above the front of the grate,
is usually about two feet, and about three feet above the back of
it. The concave bottom of the boiler, however, brings its surfaces
at the slide closer to the grate.
(150.) Between the evaporating power of the boiler, and the
magnitude of surface it exposes to the action of the furnace,
there is a relation which, like that of the grate surface, has
never been ascertained by any certain or satisfactory experimental
investigation; much less have the different degrees of efficiency
attending different parts of the boiler-surface been determined.
That part of the surface of the boiler immediately over and around
the grate, is exposed to the immediate radiation of the burning
fuel, and is therefore probably the most efficient in the
production of steam. The tendency of flame and heated air to rise,
would naturally bring them in the flues into closer contact with
those parts of the boiler-surface which are horizontal in their
position, and which form the tops of the flues, than with those
which are lateral or vertical in their position, and which form
the sides of the flues. In a boiler constructed like that already
described, the flue-surface therefore, which would be most
efficient, would be the concave bottom of the boiler extending
from the fire-bridge to its remote end. In some boilers,
especially those in which steam of high pressure is produced, the
form is cylindrical, the middle flue being formed into an
elliptical tube the greater axis of which is horizontal from end
to end of the boiler. It seems doubtful, however, [Pg263] whether
in such a boiler the heat produces any useful effect on the water
below the flue, the water above being always at a higher
temperature, and therefore lighter than that below, and
consequently no currents being established between the upper and
lower strata of the water.
It was considered by Mr. Watt, but we are not aware on what
experimental grounds, that from eight to ten square feet of
heating surface were sufficient to produce the evaporation of one
cubic foot of water per hour. The practice of engine-makers since
that time has been to increase the allowance of heating surface
for the same rate of evaporation. Engine-builders have varied very
much in this respect, some allowing twelve, fifteen, and even
eighteen square feet of surface for the same rate of evaporation.
It must, however, still be borne in mind, that whether this
increased allowance did or did not produce the actual evaporation
imputed to it, has not been, as far as we are informed, ever
accurately ascertained. The production of a given rate of
evaporation by a moderate heat diffused over a larger surface,
rather than by a fiercer temperature confined to a smaller
surface, is attended with many practical advantages. The plates of
the boiler acted upon by the fire are less exposed to oxydisation,
and the boiler will be proportionally more durable.
(151.) Besides presenting to the action of the fire a sufficient
surface to produce steam at the required rate, the capacity of the
boiler must be proportioned to the quantity of water to be
evaporated. The space within the boiler is appropriated to a
twofold purpose: 1st, To contain the water to be evaporated; 2dly,
To contain a quantity of ready made steam for the supply of the
cylinder. If the space appropriated to the steam did not bear a
considerable proportion to the magnitude of the cylinder, the
momentary expansion of the steam passing to the cylinder from the
boiler at each stroke would reduce the pressure of the steam in a
great proportion, and unless the pressure in the boiler were
considerably greater than that which the steam is intended to have
in the cylinder, the pressure in the latter would be reduced below
the proper amount. The proportion of the [Pg264] steam space in
the boiler to the magnitude of the cylinder has been very
variously estimated, nor can it be said that any practical rule of
a general kind has been adopted. It is held by some that the
steam-space will be sufficient if it contain five times the
quantity of steam consumed at each stroke, while others maintain
that it should contain at least ten times that quantity, and
opinions vary between these limits.
(152.) The proportion of water-space in the boiler to its
evaporating power should also be regulated, so that the
introduction of the feed at a comparatively low temperature may
not unduly chill the water in the boiler. Supposing the feed to be
introduced in a low pressure boiler at the temperature of 100°,
and that the necessary temperature within the boiler be 225°, the
quantity of water it contains should be about five times the
quantity evaporated, and therefore also five times the quantity
introduced through the feed per hour. For every cubic foot of
water per hour therefore, intended to be evaporated by the boiler,
water-space for five cubic feet should be provided. It is,
however, right to repeat that this (like almost every other so
called rule) is the result not of any exact general calculation,
but one deduced from the custom which has obtained among the
manufacturers of steam-engines.
(153.) The surface of the water in the boiler should always be
above the range of the flues. When the heated air in the flues
acts upon a part of the boiler within which water is contained,
the water within receiving an increased temperature becomes, bulk
for bulk, lighter than the strata of water above it, and ascends.
It is replaced by the descending strata, which, in their turn
receiving increased temperature, rise to the surface; or if the
action of the heat convert the water into steam, the bubbles of
steam rise to the surface, fresh portions of water continually
coming into contact with the boiler-plates on which the heated air
or flame acts. By this process the boiler-plates are continually
cooled, either by being successively washed by water at a lower
temperature, or by the heat taken from them becoming latent in the
steam bubbles formed in contact with them. But if the heat act
[Pg265] upon a part of the boiler containing steam within it,
which steam being a slow recipient of heat, and no currents being
established, nor any phenomenon produced in which heat is rendered
latent, the heat of the fire communicated to the boiler-plates
accumulates in them, and raises their temperature to an injurious
degree. The plates may by this means be softened, so as to cause
the boiler to burst, or the difference between the expansion of
the highly heated plates thus exposed to fire in contact with
steam and that of the plates which are cooled by contact with
water, may cause the joinings of the boiler-plates to open, and
the boiler to leak. By whatever means, therefore, the boiler be
fed, care should be taken that the evaporation should not be
allowed to reduce the level of the water in it below the highest
flue.
(154.) As the water by which the boiler is fed must always have a
much lower temperature than that at which the boiler is
maintained, the supply of the feed will have a constant tendency
to lower the temperature of the water, and this tendency will be
determined by the proportion between the magnitude of the feed and
the quantity of water in the boiler.
Since it is requisite that the level of the water in the boiler
shall not suffer any considerable change, it is evident that the
magnitude of the feed must be equal to the quantity of water
evaporated. If it were less, the level of the water would
continually fall by reason of the excess of the evaporation over
the feed; and if it were greater, the level would rise by the
accumulation of water in the boiler. If therefore the quantity of
water-space allowed in the boiler be five times the volume of
water evaporated per hour, the quantity introduced by the feed per
hour, whether continuously or at intervals, must be of the same
amount. Since the process of evaporation is continuous, the
variation of level of water in the boiler will be entirely
dependent on the intervals between the successive feeds. If the
feed be continuous, and always equal to the evaporation, then the
level of the water in the boiler will undergo no change; but if
while the evaporation is continuous the feed be made at intervals,
then the change of level of water in the boiler as [Pg266] well
as its change of temperature, will be subject to a variation
proportional to the intervals between the successive feeds. It is
manifest, therefore, that the feed should either be uninterrupted
or be supplied at short intervals, so that the change of level and
temperature of the water in the boiler should not be considerable.
(155.) Different methods have been, from time to time, suggested
for indicating the level of the water in the boiler. We have
already mentioned the two gauge-pipes used in the earlier
steam-engines (31.), and which are still generally continued.
There are, however, some other methods which merit our attention.
[Illustration: _Fig._ 75.]
A weight F (_fig._ 75.), half immersed in the water in the boiler,
is supported by a wire, which, passing steam-tight through a small
hole in the top, is connected by a flexible string, or chain,
passing over a wheel W, with a counterpoise A, which is just
sufficient to balance F when half immersed. If F be raised above the
water, _A_ being lighter will no longer balance it, and F will
descend pulling up A, and turning the wheel W. If, on the other
hand, F be plunged deeper in the water, A will more than balance it,
and will pull it up, so that the only position in which F and A will
balance each other is, when F is half immersed. The wheel W is so
adjusted, that when two pins placed on its rim are in the horizontal
position, the water is at its proper level. Consequently it follows,
that if the water rise above this level, the weight F is lifted and
A falls, so that the pins come into another position. If, on the
other hand, the level of the water fall, F falls and A rises, so
that the pins assume a different position. Thus, in general, the
position of the pins becomes an indication of the quantity of water
in the boiler.
(156.) Another method is to place a glass tube (_fig._ 76.), with
one end T entering the boiler above the proper level, and the
other end T′ entering it below the proper level. It must [Pg267]
be evident that the water in the tube will always stand at the
same level as the water in the boiler, since the lower part has a
free communication with that water, while the surface is submitted
to the pressure of the same steam as the water in the boiler. This
and the last-mentioned gauge have the advantage of addressing the
eye of the engineer at once, without any adjustment; whereas the
gauge-cocks must be both opened, whenever the depth is to be
ascertained.
[Illustration: _Fig._ 76.]
These gauges, however, require the frequent attention of the
engine-man; and it becomes desirable either to find some more
effectual means of awakening that attention, or to render the
supply of the boiler independent of any attention. In order to
enforce the attention of the engine-man to replenish the boiler
when partially exhausted by evaporation, a tube was sometimes
inserted at the lowest level to which it was intended that the
water should be permitted to fall. This tube was conducted from
the boiler into the engine-house, where it terminated in a
mouth-piece or whistle, so that whenever the water fell below the
level at which this tube was inserted in the boiler, the steam
would rush through it, and issuing with great velocity at the
mouth-piece, would summon the engineer to his duty with a call
that would rouse him even from sleep.
[Illustration: _Fig._ 77.]
(157.) In the most effectual of these methods, the task of
replenishing the boiler should still be executed by the engineer;
and the utmost that the boiler itself was made to do, was to give
due notice of the necessity for the supply of water. The
consequence was, among other inconveniences, that the level of the
water was subject to constant variation.
To remedy this a method has been invented, by which [Pg268] the
engine is made to feed its own boiler. The pipe G (_fig._ 77.),
which leads from the hot water pump, terminates in a small cistern
C in which the water is received. In the bottom of this cistern, a
valve V is placed, which opens upwards, and communicates with a
feed-pipe, which descends into the boiler below the level of the
water in it. The stem of the valve V is connected with a lever
turning on the centre D, and loaded with a weight F dipped in the
water in the boiler in a manner similar to that described in
_fig._ 75., and balanced by a counterpoise A in exactly the same
way. When the level of the water in the boiler falls, the float F
falls with it, and pulling down the arm of the lever raises the
valve V, and lets the water descend into the boiler from the
cistern C. When the boiler has thus been replenished, and the
level raised to its former place, F will again be raised, and the
valve V closed by the weight A. In practice, however, the valve V
adjusts itself by means of the effect of the water on the weight
F, so as to permit the water from the feeding-cistern C to flow in
a continued stream, just sufficient in quantity to supply the
consumption from evaporation, and to maintain the level of the
water in the boiler constantly the same.
By this arrangement the boiler is made to replenish itself, or,
more properly speaking, it is made to receive such a supply, as
that it never wants replenishing, an effect which no effort of
attention on the part of an engine-man could produce. But this is
not the only good effect produced by this contrivance. A part of
the steam which originally left the boiler, and having discharged
its duty in moving the piston, was condensed and reconverted into
water, and lodged by the air-pump in the hot well (_fig._ 77.), is
here again restored to the source from which it came, bringing
back all the unconsumed portion of its heat preparatory to being
once more put in circulation through the machine.
The entire quantity of hot water pumped into the cistern C, is not
always necessary for the boiler. A waste-pipe may be provided for
carrying off the surplus, which may be turned to any purpose for
which it may be required; or it may be discharged into a cistern
to cool, preparatory to [Pg269] being restored to the cold
cistern, in case water for the supply of that cistern be not
sufficiently abundant.
[Illustration: _Fig._ 78.]
(158.) Another method of arranging a self-regulating feeder is
shown in _fig._ 78. A is a hollow ball of metal attached to the
end of a lever, whose fulcrum is at B. The other arm of the lever
C is connected with the stem of a spindle-valve, communicating
with a tube which receives water from the feeding-cistern. Thus,
when the level of the water in the boiler subsides, the ball A
preponderating over the weight of the opposite arm, the lever
falls, the arm C rises and opens the valve, and admits the feeding
water. This apparatus will evidently act in the same manner and on
the same principles as that already described. [Pg270]
The mouth of the tube by which the feed is introduced should be
placed at that part of the boiler which is nearest the end of the
flues which issue into the chimney. By such means the temperature
of the water in contact with those flues will be lowest at the
place where the temperature of the heated air intended to act upon
it is also lowest. The difference of the temperatures will
therefore be greater than it would be if the point of the boiler
containing water of a higher temperature was left in contact with
this part of the flue.
[Illustration: _Fig._ 79.]
(159.) It is necessary to have a ready method of ascertaining at
all times the pressure of the steam which is used in working the
engine. For this purpose a bent tube containing mercury is
inserted into some part of the apparatus, which has free
communication with the steam. Let A B C (_fig._ 79.) be such a
tube. The pressure of the steam forces the mercury down in the leg
A B, and up in the leg B C. If the mercury in both legs be at
exactly the same level, the pressure of the steam must be exactly
equal to that of the atmosphere; because the steam pressure on the
mercury in A B balances the atmospheric pressure on the mercury in
B C. If, however, the level of the mercury in B C be above the
level of the mercury in B A, the pressure of the steam will exceed
that of the atmosphere. The excess of its pressure above that of
the atmosphere may be found by observing the difference of the
level of the mercury in the tubes B C and B A, allowing a pressure
of one pound on each square inch for every two inches in the
difference of the levels.
If, on the contrary, the level of the mercury in B C should fall
below its level in A B, the atmospheric pressure will [Pg271]
exceed that of the steam, and the quantity of the excess may be
ascertained exactly in the same way.
If the tube be glass, the difference of levels of the mercury
would be visible; but it is most commonly made of iron; and in
order to ascertain the level, a thin wooden rod with a float is
inserted in the open end of B C, so that the portion of the stick
within the tube indicates the distance of the level of the mercury
from its mouth. A bulb or cistern of mercury might be substituted
for the leg A B, as in the common barometer. This instrument is
called the _steam-gauge_.
If the steam-gauge be used as a measure of the strength of the
steam which presses on the piston, it ought to be on the same side
of the throttle-valve (which is regulated by the governor) as the
cylinder; for if it were on the same side of the throttle-valve
with the boiler, it would not be affected by the changes which the
steam may undergo in passing through the throttle-valve, when
partially closed by the agency of the governor.
For boilers in which steam of very high pressure is used, as in
those of locomotive engines, a steam-gauge, constructed on the above
principle, would have inconvenient or impracticable length. In such
boilers the pressure of the steam is equal to four or five times
that of the atmosphere, to indicate which the column of mercury in
the steam-gauge would be four or five feet in height. In such cases
a thermometer-gauge may be used with advantage. The principle of
this gauge is founded on the fact, that between the pressure and
temperature of steam produced in contact with water there is a fixed
relation, the same temperature always corresponding to the same
pressure. If, therefore, a thermometer be immersed in the boiler
which shall show the temperature of the steam, a scale may be
attached to it, on which shall be engraved the corresponding
pressures. Such gauges are now very generally used on locomotive
engines.
[Illustration: _Fig._ 80.]
(160.) The force with which the piston is pressed depends on two
things, 1st, the actual strength of the steam which presses on it;
and, 2dly, on the actual strength of the vapour which resists it.
For although the vacuum produced by the method of separate
condensation be much more perfect than [Pg272] what had been
produced in the atmospheric engines, yet still some vapour of a
small degree of elasticity is found to be raised from the hot
water in the bottom of the condenser before it can be extracted by
the air-pump. One of these pressures is indicated by the
steam-gauge already described; but still, before we can estimate
the force with which the piston descends, it is necessary to
ascertain the force of the vapour which remains uncondensed, and
resists the motion of the piston. Another gauge, called the
barometer-gauge, is provided for this purpose. A glass tube A B
(_fig._ 80.), more than thirty inches long and open at both ends,
is placed in an upright or vertical position, having the lower end
B immersed in a cistern of mercury C. To the upper end is attached
a metal tube, which communicates with the condenser, in which a
constant vacuum, or rather high degree of rarefaction, is
sustained. The same vacuum must therefore exist in the tube A B,
above the level of the mercury, and the atmospheric pressure on
the surface of the mercury in the cistern C will force the mercury
up in the tube A B, until the column which is suspended in it is
equal to the difference between the atmospheric pressure and the
pressure of the uncondensed steam. The difference between the
column of mercury sustained in this instrument and in the common
barometer, will determine the strength of the uncondensed steam,
allowing a force proportional to one pound per square inch for
every two inches of mercury in the difference of the two columns.
In a well-constructed engine which is in good order, there is very
little difference between the altitude in the barometer-gauge and
the common barometer.
To compute the force with which the piston descends, thus becomes a
very simple arithmetical process. First, ascertain the difference of
the levels of the mercury in the steam-gauge; this gives the excess
of the steam pressure above the atmospheric pressure. Then find the
height of the mercury in the barometer-gauge; this gives the excess
of the atmospheric pressure above the uncondensed steam. Hence, if
these two heights be added together, we shall obtain the [Pg273]
excess of the impelling force of the steam from the boiler, on the
one side of the piston, above the resistance of the uncondensed
steam on the other side: this will give the effective impelling
force. Now, if one pound be allowed for every two inches of mercury
in the two columns just mentioned, we shall have the number of
pounds of impelling pressure on every square inch of the piston.
Then, if the number of square inches in the section of the piston be
found, and multiplied by the number of pounds on each square inch,
the force with which it moves will be obtained.
From what we have stated it appears that, in order to estimate the
force with which the piston is urged, it is necessary to refer to
both the barometer and the steam-gauge. This double computation
may be obviated by making one gauge serve both purposes. If the
end C of the steam-gauge (_fig._ 79.), instead of communicating
with the atmosphere were continued to the condenser, we should
have the pressure of the steam acting upon the mercury in the tube
B A, and the pressure of the uncondensed vapour which resists the
piston acting on the mercury in the tube B C. Hence the difference
of the levels of the mercury in the tubes would at once indicate
the difference between the force of the steam and that of the
uncondensed vapour, which is the effective force with which the
piston is urged.
(161.) But these methods of determining the effective force by
which the piston is urged, can only be regarded as approximations,
and not very perfect ones. If the condensation of steam on one
side of the piston were instantaneously effected, or the
uncondensed vapour were of the same tension during the whole
stroke; and if, besides this, the pressure of steam on the piston
were of uniform intensity from the beginning to the end of the
stroke, then the steam and barometer gauges taken together would
become an accurate index of the effective force of steam on the
piston: but such is not the case. When the steam is first admitted
through the steam-valve it acts on the piston with a pressure
which is first slightly diminished, and afterwards a little
increased, until it arrives at that part of the stroke at which
the steam-valve is closed, after which the pressure is diminished.
The [Pg274] pressure, therefore, urging the piston is subject to
variation; but the pressure of the uncondensed vapour on the other
side of the piston is subject to still greater change. At the
moment the exhausting-valve is opened, the piston is relieved from
the pressure upon it by the commencement of the condensation; but
this process during the descent of the piston is gradual, and the
vacuum is rendered more and more perfect, until the piston has
nearly attained the limit of its play. These variations, both as
well of the force urging the piston as of the force resisting it,
are such as not to be capable of being accurately measured by a
mercurial column, since they would produce oscillations in such a
column, which would render any observations of its mean height
impracticable.
To measure the mean efficient force of the piston, taking into
account these circumstances, Mr. Watt invented an instrument,
which, like all his mechanical inventions, has answered its
purpose perfectly, and is still in general use. This instrument,
called an _indicator_, consists of a cylinder of about 1-3/4 inch
in diameter, and 8 inches in length. It is bored with great
accuracy, and fitted with a solid piston moving steam-tight in it
with very little friction. The rod of this piston is guided in the
direction of the axis of the cylinder through a collar in the top,
so as not to be subject to friction in any part of its play. At
the bottom of the cylinder is a pipe governed by a stop-cock and
turned in a screw, by which the instrument may be screwed on the
top of the steam-cylinder of the engine. In this position, if the
stop-cock of the indicator be opened, a free communication will be
made between the cylinder of the indicator and that of the engine.
The piston-rod of the indicator is attached to a spiral spring,
which is capable of extension and compression, and which by its
elasticity is capable of measuring the force which extends or
compresses it in the same manner as a spring steel-yard or
balance. If a scale be attached to the instrument at any point on
the piston-rod to which an index might be attached, then the
position of that index upon the scale would be governed by the
position of the indicator-piston in its cylinder. If any force
pressed the indicator-piston upwards, so as to compress the
spring, [Pg275] the index would rise upon the scale; and if, on
the other hand, a force pressed the indicator-piston downwards,
then the spiral spring would be extended, and the index on the
piston-rod descend upon the scale. In each case the force of the
spring, whether compressed or extended, would be equal to the
force urging the indicator-piston, and the scale might be so
divided as to show the amount of this force.
Now, let the instrument be supposed to be screwed upon the top of
the cylinder of a steam-engine, and the stop-cock opened so as to
leave a free communication between the cylinder of the indicator
below its piston and the cylinder of the steam-engine above the
steam-piston. At the moment the upper steam-valve is opened, the
steam rushing in upon the steam-piston will also pass into the
indicator, and press the indicator-piston upwards: the index upon
its piston-rod will point upon the scale to the amount of pressure
thus exerted. As the steam-piston descends, the indicator-piston
will vary its position with the varying pressure of the steam in
the cylinder, and the index on the piston-rod will play upon the
scale, so as to show the pressure of the steam at each point
during the descent of the piston.
If it were possible to observe and record the varying position of
the index on the piston-rod of the indicator, and to refer each of
these varying positions to the corresponding point of the descending
stroke, we should then be able to declare the actual pressure of the
steam at every point of the stroke. But it is evident that such an
observation would not be practicable. A method, however, was
contrived by Mr. Southern, an assistant of Messrs. Boulton and Watt,
by which this is perfectly effected. A square piece of paper, or
card, is stretched upon a board, which slides in grooves formed in a
frame. This frame is placed in a vertical position near the
indicator, so that the paper may be moved in a horizontal direction
backwards and forwards, through a space of fourteen or fifteen
inches. Instead of an index a pencil is attached to the indicator of
the piston-rod: this pencil is lightly pressed by a spring against
the paper above mentioned, and as the paper is moved in a horizontal
direction [Pg276] under the pencil, would trace upon the paper a
line. If the pencil were stationary this line would be straight and
horizontal, but if the pencil were subject to a vertical motion, the
line traced on the paper moved under the pencil horizontally would
be a curve, the form of which would depend on the vertical motion of
the pencil. The board thus supporting the paper is put into
connexion by a light cord carried over pulleys with some part of the
parallel motion, by which it is alternately moved to the right and
to the left. As the piston ascends or descends, the whole play of
the board in the horizontal direction will therefore represent the
length of the stroke, and every fractional part of that play will
correspond to a proportional part of the stroke of the steam-piston.
[Illustration: _Fig._ 81.]
The apparatus being thus arranged, let us suppose the steam-piston
at the top of the cylinder commencing its descent. As it descends,
the pencil attached to the indicator piston-rod varies its height
according to the varying pressure of the steam in the cylinder. At
the same time the paper is moved uniformly under the pencil, and a
curved line is traced upon it from right to left. When the piston
has reached the bottom of the cylinder, the upper exhausting-valve
is opened, and the steam drawn off to the condenser. The
indicator-piston being immediately relieved from a part of the
pressure acting upon it descends, and with it the pencil also
descends; but at the same time the steam-piston has begun to ascend,
and the paper to return from left to right under the pencil. While
the steam-piston continues to ascend, the condensation becomes more
and more perfect, and the vacuum in the cylinder, and therefore also
in the indicator, being gradually increased in power, the
atmospheric pressure above the indicator-piston presses it downwards
and stretches the spring. The pencil meanwhile, with the paper
moving under it from right to left, traces a second curve. As the
former curve showed the actual pressure of the steam impelling the
piston in its descent, this latter will show the pressure of the
uncondensed steam raising the piston in its ascent, and a comparison
of the two will exhibit the effective force on the piston. _Fig._
81. represents such a diagram as would be [Pg277] produced by this
instrument. A B C is the curve traced by the pencil during the
descent of the piston, and C D E that during its ascent. A is the
position of the pencil at the moment the piston commences its
descent, B is its position at the middle of the stroke, and C at the
termination of the stroke. On closing the upper steam-valve and
closing the exhausting-valve, the indicator-piston being gradually
relieved from the pressure of the steam the pencil descends, and at
the same time the paper moving from left to right, the pencil traces
the curve C D E, the gradual descent of this curve showing the
progressive increase of the vacuum. As the atmospheric pressure
constantly acts above the piston of the indicator, its position will
be determined by the difference between the atmospheric pressure and
the pressure of the steam below it; and therefore the difference
between the heights of the pencil at corresponding points in the
ascending and descending stroke, will express the difference between
the pressure of the steam impelling the piston in the ascent and
resisting it in the descent at these points. Thus at the middle of
the stroke, the line B D will express the extent to which the spring
governing the indicator-piston would be stretched by the difference
between the force of steam impelling the piston at the middle of the
descending stroke, and the force of steam resisting it at the middle
of the ascending stroke. The force therefore measured by the line B
D will be the effective force on the piston at that point; and the
same may be said of every part of the diagram produced by the
indicator.
The whole mechanical effect produced by the stroke of the piston
being composed of the aggregate of all its varying effects
throughout the stroke, the determination of its amount [Pg278] is
a matter of easy calculation by the measurement of the diagram
supplied by the indicator. Let the horizontal play of the pencil
from A to C be divided into any proposed number of equal parts,
say ten: at the middle of the stroke, B D expresses the effective
force on the piston, and if this be considered to be uniform
through the tenth part of the stroke, as from _f_ to _g_, then the
number of pounds expressed by B D multiplied by the tenth part of
the stroke expressed in parts of a foot, will be the mechanical
effect through that part of the stroke expressed in pounds' weight
raised one foot. In like manner _m n_ will express the effective
force on the piston after three fourths of the stroke have been
performed, and if this be multiplied by a tenth part of the stroke
as before, the mechanical effect similarly expressed will be
obtained; and the same process being applied to any successive
tenth part of the stroke, and the numerical results thus obtained
being added together, the whole effect of the stroke will be
obtained, expressed in pounds' weight raised one foot.
(162.) By means of the indicator, the actual mechanical effect
produced by each stroke of the engine can be obtained, and if the
actual number of strokes made in any given time be known, the
whole effect of the moving power would be determined. An
instrument called a _counter_ was also contrived by Watt, to be
attached either to the working beam or to any other reciprocating
part of the engine. This instrument consisted of a train of
wheel-work with governing hands or indices moved upon divided
dials, like the hand of a clock. A record of the strokes was
preserved by means precisely similar to those by which the hands
of a clock or time-piece indicated and recorded the number of
vibrations of the pendulum or balance-wheel.
(163.) To secure the boiler from accidents arising from the steam
contained in it acquiring an undue pressure, a safety-valve is
used, similar in principle to those adopted in the early engines.
This valve is represented in _fig._ 71. at N. It is a conical
valve, kept down by a weight sliding on a rod upon it. When the
pressure of the steam overcomes the force of this weight, it
raises the valve and escapes, being carried off through the tube.
[Pg279]
With a view to the economy of heat, this waste steam tube is
sometimes conducted into the feeding cistern, where the steam
carried off by it is condensed, and heats the feeding water.
The magnitude of the safety-valve should be such that, when open,
steam should be capable of passing through it as rapidly as it is
generated in the boiler. The superficial magnitude, therefore, of
such valves must be proportional to the evaporating power of the
boiler. In low pressure boilers the steam is generally limited to
five or six pounds' pressure per square inch, and consequently the
load over the safety-valve in pounds would be found by multiplying
the superficial magnitude of its smallest part by these numbers.
In boilers in which the steam is maintained at a higher pressure,
it would be inconvenient to place upon the safety-valve the
necessary weight. In such cases a lever is used, the shorter arm
of which presses down the valve, and the longer arm is held down
by a weight capable of adjustment, so that the pressure on the
valve may be regulated at discretion. Two safety-valves should be
provided on all boilers, one of which should be locked up, so that
the persons in care of the engine should have no power to increase
the load upon it. In such case, however, it is necessary that a
handle connected with the valve should project outside the box
containing it, so that it may always be possible for the engineer
to ascertain that the valve is not locked in its seat, a
circumstance which is liable to happen.
Sometimes also two safety-valves are provided, one loaded a little
heavier than the other. The escape of steam from the lighter valve
in this case gives notice to the engine-man of the growing
increase of pressure, and warns him to check the production of
steam. The lever by which the safety-valve is held down is
sometimes acted on by a spiral spring, capable of being so
adjusted as to produce any required pressure on the valve. This
arrangement is adopted in locomotive engines, where steam of very
high pressure is used; and in such cases also there are always
provided two such valves, one of which cannot be increased in its
pressure.
The pipe by which the boiler is fed with water will [Pg280]
necessarily act as a safety-valve, for when the pressure of the
steam increases in an undue degree, it will press the water in the
boiler up through the feed-pipe, so as to discharge it into the
feed-cistern, a circumstance which would immediately give notice
of the internal state of the boiler. The steam-gauge, already
described (_fig._ 79.), would also act as a safety-valve; for if
the pressure of steam in the boiler should be so augmented as to
blow the mercury out of the steam-gauge, the steam would then
issue through the gauge, and the pressure of the boiler be
reduced, provided that the magnitude of the tube forming the
steam-gauge were sufficient for this purpose.
(164.) In high pressure boilers which are exposed to extreme
temperatures and pressures, and which are therefore subject to
danger of explosion, a plug of metal is sometimes inserted, which is
capable of being fused at a temperature above which the boiler
should not be permitted to be raised. If the pressure of steam
increase beyond the proper limit, the temperature of the water and
steam will undergo a corresponding increase; and if the metal of the
plug be capable of being fused at such a temperature, the plug will
fall out of the boiler, and the steam and water will issue from it.
Various alloys of metal are fusible at temperatures sufficiently low
for this purpose. An alloy composed of one part of lead, three of
tin, and five of bismuth, will fuse at the common temperature of
boiling water; and alloys of the same metals, in various
proportions, will fuse at different temperatures from 200° to 400°.
Although fusible plugs may be used, in addition to other means of
insuring safety, they ought not to be exclusively relied on at the
ordinary working pressure of the boiler. The fusible plug ought to
be capable of more than resisting the pressure; but if it be so,
its point of fusion would be one at which the steam would have a
pressure of at least two atmospheres above its working pressure.
The plug would therefore be capable of being fused only as soon as
the steam would acquire a pressure of 30 lbs. per inch above its
regular working pressure.
When a boiler ceases to be worked, and the furnace has been
extinguished, the space within it appropriated to steam [Pg281]
will be left a vacuum by the condensation of the steam with which
it was previously filled. The external pressure of the atmosphere
acting on the boiler would, under such circumstances, have a
tendency to crush it inwards. To prevent this, a safety-valve is
provided, opening inwards, and balanced by a weight sufficient to
keep it closed until it be relieved from the pressure of the steam
below.
A large aperture closed by a flange secured with screws,
represented at O in _fig._ 71., called the _man-hole_, is provided
to admit persons into the boiler for the purpose of cleaning or
repairing its interior.
(165.) The manner in which the governor regulates the supply of
steam from the boiler to the cylinder, proportioning the quantity
to the work to be done, and thereby sustaining a uniform motion,
has been already explained (p. 125.). Since then the _consumption_
of steam in the engine is subject to variation, owing to the
various quantities of work it may have to perform, it is evident
that the _production_ of steam in the boiler should be subject to
a proportional variation. For otherwise, one of two effects would
ensue: the boiler would either fail to supply the engine with
steam, or steam would accumulate in the boiler from being produced
in too great abundance, and would escape at the safety-valve, and
thus be wasted.
In order to vary the production of steam in proportion to the
demands of the engine, it is necessary to stimulate or mitigate
the furnace, as the evaporation is to be augmented or diminished.
The activity of the furnace must depend on the current of air
which is drawn through the grate-bars, and this will depend on the
magnitude of the space afforded for the passage of that current
through the flues. A plate called a _damper_ is accordingly placed
with its plane at right angles to the flue, so that by raising and
lowering it in the same manner as the sash of a window is raised
or lowered, the space allowed for the passage of air through the
flue may be regulated. This plate might be regulated by the hand,
so that by raising or lowering it the draught might be increased
or diminished, and a corresponding effect produced on the [Pg282]
evaporation in the boiler: but the force of the fire is rendered
uniformly proportional to the rate of evaporation by the following
arrangement, without the intervention of the engineer. The column
of water sustained in the feed-pipe (_figs._ 71, 72.) represents
by its weight the difference between the pressure of steam within
the boiler and that of the atmosphere. If the engine consumes
steam faster than the boiler produces it, the steam contained in
the boiler acquires a diminished pressure, and consequently the
column of water in the feed-pipe will fall. If, on the other hand,
the boiler produce steam faster than the engine consumes it, the
accumulation of steam in the boiler will cause an increased
pressure on the water it contains, and thereby increase the height
of the column of water sustained in the feed-pipe. This column
therefore necessarily rises and falls with every variation in the
rate of evaporation in the boiler. A hollow float P is placed upon
the surface of the water of this column; a chain connected with
this float is carried upwards, and passed over two pulleys, after
which it is carried downwards through an aperture leading to the
flue which passes beside the boiler: to this chain is attached the
damper. By such an arrangement it is evident that the damper will
rise when the float P falls, and will fall when the float P rises,
since the weight of the damper is so adjusted, that it will only
balance the float P when the latter rests on the surface of the
water.
Whenever the evaporation of the boiler is insufficient, it is
evident from what has been stated, that the float P will fall and
the damper will rise, and will afford a greater passage for air
through the flue. This will stimulate the furnace, will augment
its heating power, and will therefore increase the rate of
evaporation in the boiler. If, on the other hand, the production
of steam in the boiler be more than is requisite for the supply of
the engine, the float will be raised and the damper let down, so
as to contract the flue, to diminish the draught, to mitigate the
fire, and therefore to check the evaporation. In this way the
excess, or defect, of evaporation in the boiler is made to act
upon the fire, so as to render the heat proceeding from the
combustion as nearly as possible proportional to the wants of the
engine. [Pg283]
(166.) The method of feeding the furnace by hand through the
fire-door being subject to the double objection of admitting more
cold air over the fuel than is necessary for its combustion, and
the impracticability of insuring that regular attendance on the
part of the stokers, directed the attention of engineers to the
construction of self-regulating furnaces. The most effectual of
these, and that which has come into most general use, was invented
by Mr. William Brunton of Birmingham.
The advantages proposed to be attained by him were those expressed
in his patent:—
"First, I put the coal upon the grate by small quantities, and at
very short intervals, say every two or three seconds. 2dly, I so
dispose of the coals upon the grate, that the smoke evolved must
pass over that part of the grate upon which the coal is in full
combustion, and is thereby consumed. 3dly, As the introduction of
coal is uniform in short spaces of time, the introduction of air
is also uniform, and requires no attention from the fireman.
"As it respects economy: 1st, The coal is put upon the fire by an
apparatus driven by the engine, and so contrived that the quantity
of coal is proportioned to the quantity of work which the engine
is performing; and the quantity of air admitted to consume the
smoke is regulated in the same manner. 2dly, The fire-door is
never opened, excepting to clean the fire; the boiler, of course,
is not exposed to that continual irregularity of temperature which
is unavoidable in the common furnace, and which is found
exceedingly injurious to boilers. 3dly, The only attention
required is to fill the coal-receiver every two or three hours,
and clean the fire when necessary. 4thly, The coal is more
completely consumed than by the common furnace, as all the effect
of what is termed stirring up the fire (by which no inconsiderable
quantity of coal is passed into the ash-pit), is attained without
moving the coal upon the grate."
A circular grate is placed on a vertical revolving shaft; on the
lower part of this shaft, under the ash-pit, is placed a toothed
wheel driven by a pinion. This pinion is placed on another vertical
shaft, which ascends above the boiler; and [Pg284] on the other end
of this is placed a bevelled wheel driven by a pinion. This pinion
is attached to a shaft, which takes its motion from the axis of the
fly-wheel, or any other revolving shaft connected with the engine. A
constant motion of revolution is therefore imparted to the circular
grate, and its velocity being proportional to that of the engine,
will necessarily be also proportional to the quantity of fuel which
ought to be consumed. Through that part of the boiler which is over
the fire-grate a vertical tube or opening is made directly over that
part of the furnace which is most distant from the flues. Over this
opening a hopper is placed, which contains the fuel by which the
boiler is to be fed; and in the bottom of this hopper is a sliding
valve, capable of being opened or closed, so as to regulate the
quantity of fuel supplied to the fire-grate. The fuel dropping in in
small quantities through this open valve falls on the grate, and is
carried round by it, so as to leave a fresh portion of the grate to
receive succeeding feeds. The coals admitted through the hopper are
previously broken to a proper size; and in some forms of this
apparatus there are two rollers, at a regulated distance asunder,
the surfaces of which are formed into blunt angular points, and
which are kept in slow revolution by the engine. Between these
rollers the coals must pass before they reach the valve through
which the furnace is fed, and they are thus broken and reduced to a
regulated size. The valve which regulates the opening through which
the feed is admitted, is connected by chains and pulleys with the
self-regulating damper already described, so that in proportion as
the damper is raised, the valve governing the feed may be opened.
Thus, while the quantity of air admitted by the damper is increased
according to the demands of the engine, the quantity of fuel
admitted for the feed is increased by opening the valve in the
bottom of the hopper in the same proportion. Apertures are also
provided in the front of the grate, governed by regulators, by which
the quantity of air necessary and sufficient to produce the
combustion of the gas evolved from the fuel is admitted, these
openings being also connected with the self-regulating damper.
A considerable portion of the heat imparted to the water [Pg285]
in the boiler escapes by radiation from the surface of the boiler,
steam-pipes, and other parts of the machinery in contact with the
steam and hot water. The effects of this are rendered very
apparent in marine engines, where a large quantity of water is
found to be condensed in the great steam-pipes leading from the
boiler to the cylinder. In stationary land boilers this loss of
heat is usually diminished, and in some cases in a great degree
removed, by surrounding the boiler with non-conducting substances.
In some cases the boiler is built round in brick work. In
Cornwall, where the economy is regarded perhaps to a greater
extent than elsewhere, the boiler and steam-pipes are surrounded
with a packing of sawdust, which being almost a non-conductor of
heat, is impervious to the heat proceeding from the surfaces with
which it is in contact, and consequently confines all the heat
within the boiler. In marine boilers it has been the practice
recently to clothe the boiler and steam-pipes with a coating of
felt, which is attended with a similar effect. When these remedies
are properly applied, the loss of heat proceeding from the
radiation of the boiler is reduced to an extremely small amount.
The engine-houses of some of the Cornish engines, where the boiler
generates steam at a very high temperature, are nevertheless
frequently maintained at a lower temperature than the external
air, and on entering them they have in a great degree the effect
of a cave.
(167.) All mechanical action is measured by the amount of force
exercised, or resistance overcome, and the space through which
that force has acted, or through which the resistance has been
moved.
The gross amount of mechanical action developed by the moving
power of an engine, is expended partly on moving the engine
itself, and partly on overcoming the resistance on which the
engine is intended to act. That part of the mechanical energy of
the moving power which is expended on the resistance or load which
the engine moves exclusively, and of the power expended on moving
the engine itself, is called _the useful effect_ of the machine.
The _gross effect_, therefore, exceeds the _useful effect_ by the
[Pg286] amount of power spent in moving the engine, or which may
be wasted or destroyed in any way by the engine.
It is usual to express and estimate all mechanical effect whatever
by nature of the resistance overcome, by an equivalent weight
raised a certain height. Thus, if an engine exerts a certain power
in driving a mill, in drawing a carriage on a road, or in
propelling a vessel on water, the resistance against which it has
to act must be equal to a definite amount of weight. If a carriage
be drawn, the traces are stretched by the tractive power, by the
same tension that would be given to them if a certain weight were
appended to them. If the paddle-wheels of a boat are made to
revolve, the water opposes to them a resistance equal to that
which would be produced, if instead of moving the water the wheel
had to raise some certain weight. In any case, therefore, weight
becomes the exponent of the energy of the resistance against which
the moving power acts.
But the amount of mechanical effect depends conjointly on the
amount of resistance, and the space through which that resistance
is moved. The quantity of this effect, therefore, will be
increased in the same proportion, whether the quantity of
resistance or the space through which that resistance is moved be
augmented. Thus, a resistance of one hundred pounds, moved through
two feet, is mechanically equivalent to a resistance of two
hundred pounds moved through one foot, or of four hundred pounds
moved through six inches. To simplify, therefore, the expression
of mechanical effect, it is usual to reduce it invariably to a
certain weight raised one foot. If the resistance under
consideration be equivalent to a certain weight raised through ten
feet, it is always expressed by ten times the amount of that
weight raised through one foot.
It has also been usual in the expression of mechanical effect, to
take the pound weight as the unit of weight, and the foot as the
unit of length, so that all mechanical effect whatsoever is
expressed by a certain number of pounds raised one foot.
(168.) The gross effect of the moving power in a steam-engine, is
the whole mechanical force developed by the evaporation [Pg287]
of water in the boiler. A part of this effect is lost by the
partial condensation of the steam before it acts upon the piston,
and by the imperfect condensation of it subsequently: another
portion is expended on overcoming the friction of the different
moving parts, and in acting against the resistance which the air
opposes to the machine. If the motion be subject to sudden shocks,
a portion of the power is then lost by the destruction of momentum
which such shocks produce. But if those parts of the machine which
have a reciprocating motion be, as they ought to be, brought
gradually to rest at each change of direction, then no power is
absorbed in this way.
(169.) The useful effect of an engine is variously denominated
according to the relation under which it is considered. If it be
referred to the time during which it is produced, it is called
POWER.
(170.) If it be referred to the fuel, by the combustion of which
the evaporation has been effected, it is called DUTY.
(171.) When steam-engines were first brought into use, they were
commonly applied to work pumps for mills which had been previously
worked or driven by horses. In forming their contracts, the first
steam-engine builders found themselves called upon to supply
engines capable of executing the same work as was previously
executed by some certain number of horses. It was therefore
convenient, and indeed necessary, to be able to express the
performance of these machines by comparison with the animal power
to which manufacturers, miners, and others, had been so long
accustomed. When an engine, therefore, was capable of performing
the same work in a given time as any given number of horses of
average strength usually performed, it was said to be an engine of
so many horses' power. Steam-engines had been in use for a
considerable time before this term had acquired any settled or
uniform meaning, and the nominal power of engines was accordingly
very arbitrary. At length, however, the use of steam-engines
became more extended, and the confusion and inconvenience arising
out of all questions respecting the performance of engines,
rendered it necessary that some fixed [Pg288] and definite
meaning should be assigned to the terms by which the powers of
this machine were expressed. To have abandoned the term
_horse-power_, which had been so long in use, would have been
obviously inconvenient; nor could there be any objection to its
continuance, provided all engine-makers, and all those who used
engines, could be brought to agree upon some standard by which the
unit of horse-power might be defined. The performance of a horse
of average strength working for eight hours a day was therefore
selected as a standard, or unit, of steam-engine power. Smeaton
estimated that such an animal, so working, was capable of
performing a quantity of work equal in its mechanical effect to
22,916 lbs. raised one foot per minute, while Desaguliers
estimated the same power at 27,500 lbs. raised through the same
height in the same time. The discrepancy between these estimates
probably arose from their being made from the performances of
different classes of horses. Messrs. Boulton and Watt caused
experiments to be made with the strong horses used in the
breweries in London, and from the result of these trials they
assigned 33,000 lbs. raised one foot per minute, as the value of a
horse's power. This is the unit of engine-power now universally
adopted; and when an engine is said to be of so many horses'
power, what is meant is, that that engine, in good working order
and properly managed, is capable of moving a resistance equal to
33,000 lbs. through one foot per minute. Thus an engine of ten
horse-power is one that would raise 330,000 lbs. weight one foot
per minute.
Whether this estimate of an average horse's power be correct or
not, in reference to the actual work which the animal is capable
of executing, is a matter of no present importance in its
application to steam-power. The steam-engine is no longer used to
replace the power of horses, and therefore no contracts are based
upon such a comparison. The term horse-power, therefore, as
applied to steam-engines, must be understood to have no reference
whatever to the actual animal power, but must be taken as a term
having no other meaning than the expression of the ability of the
[Pg289] machine to move the amount of resistance above mentioned
through one foot per minute.
(172.) It has been already explained (67.) that the conversion of
a given volume of water into steam is productive of a certain
definite amount of mechanical force, this amount depending on the
pressure under which the water is evaporated, and the extent to
which the expansive principle is used in working the steam. It is
evident that this amount of mechanical effect is a major limit,
which cannot be exceeded by the power of the engine.
If the steam be not worked expansively, then the whole power of
the water, transmitted in the form of steam from the boiler to the
working machinery, will be a matter of easy calculation, when the
pressure at which the steam is worked is known. A table,
exhibiting the mechanical power of a cubic foot of water converted
into steam at various pressures, expressed in an equivalent number
of pounds' weight raised one foot high, is given in the Appendix
to this volume. Where much accuracy is sought for, the pressure at
which the steam is used must be taken into account; but by
reference to the table it will be seen, that when steam is worked
without expansion, its mechanical effect varies very little with
the pressure. It may therefore be assumed, as has been already
stated, that for every cubic inch of water transmitted in the form
of steam to the cylinders, a force is produced, represented by a
ton weight raised a foot high. Now, as 33,000 lbs. is very nearly
15 tons, it follows that 15 cubic inches of water converted into
steam per minute, or 900 cubic inches per hour, will produce a
mechanical force equal to one horse. If, therefore, to 900 cubic
inches be added the quantity of water per hour necessary to move
the engine itself, independently of its load, we shall obtain the
quantity of water per hour which must be supplied by the boiler to
the engine for each horse-power, and this will be the same
whatever may be the magnitude or proportions of the cylinder.
(173.) The quantity of power expended in working the engine
itself, independently of that required to move its load, will be
less in proportion to the degree of perfection which [Pg290] may
be attained in the construction of the engine, and to the order in
which it is kept while working. Engines vary one from another so
much in these respects, that it is scarcely possible to lay down
any general rules for the quantity of power to be allowed over and
above what is necessary to move the load. The means whereby
mechanical power is expended in working the engine may be
enumerated as follows:—
_First._ Steam in passing from the boiler to the cylinder is
liable to lose its temperature by the radiation of the steam-pipes
and other passages through which it is conducted. Since the steam
produced in the boiler is in contact with water, it will be common
steam (94.), and consequently the least loss of heat will cause a
partial condensation. To whatever extent this condensation may be
carried, a proportional loss of power, in reference to the heat
obtained from the fuel, will be entailed upon the engine.
It has been said that the force necessary to move the steam from
the boiler to the cylinder through passages more or less
contracted, subject to the friction of the pipes and tubes through
which it moves, should be taken into account in estimating the
power, and a corresponding deduction made. This, however, is not
the case: the steam having passed into the cylinder remains common
steam, its pressure being diminished by reason of the force
expended in thus moving it from the boiler to the cylinder. But
its mechanical efficacy at the reduced pressure is not sensibly
different from the efficacy which it had in the boiler. If at the
reduced pressure its volume were the same, then a loss of effect
would be sustained equivalent to the difference of the pressures;
but its volume being augmented in very nearly the same proportion
as its pressure is diminished, the mechanical efficacy of a given
weight of steam in the cylinder will be sensibly the same as in
the boiler.
_Second._ The radiation of heat from the cylinder and its
appendages, will cause a partial condensation of steam, and
thereby produce a diminished mechanical effect.
_Third._ The steam, which at each stroke of the piston fills the
passages between the steam-valves and the piston, at the [Pg291]
moment the latter commences the stroke will be inefficient. If it
were possible for the piston to come into steam-tight contact with
each end of the cylinder, and that the steam-valve should be in
immediate contact with the side or top of the piston, then the
whole of the steam which would pass through the steam-valve would
be efficient; but as some space, however small, must remain
between the piston and the ends of the cylinder, and between the
side of the cylinder and the steam-valve, there will always be a
volume of steam bearing a sensible proportion to the magnitude of
the cylinder, which at each stroke of the piston will be
inefficient. This volume of steam is called the _clearance_.
_Fourth._ Since the piston must move in steam-tight contact with
the cylinder, it must have a definite amount of friction with the
sides of the cylinder by whatever means it may be packed. This
friction will produce a corresponding resistance to the moving
power.
_Fifth._ The various joints of the machinery where steam is
contained are subject to leakage, and whatever amount of steam
shall thus escape must be placed to the account of power lost.
_Sixth._ When the eduction-valve is opened to admit the steam to
the condenser, a certain force is required to expel the steam from
the cylinder. This force reacts upon the piston, and counteracts
to a proportional extent the moving power of the steam on the
other side. Besides this the water in the condenser cannot be
conveniently reduced below the temperature of about 100°, and at
this temperature steam has a pressure of about 1 lb. per square
inch. This vapour will continue to fill the cylinder, and will
resist the moving power which impels the piston.
_Seventh._ Power must be provided for opening and closing the
valves or slides, for working the air-pump, hot-water pump, and
cold-water pump, and finally to overcome the friction on the
journals and centres of the parts of the parallel motion, the main
axle of the beam, the connecting rod, crank, and fly-wheel axle.
It will be apparent how very much these sources of resistances
must vary in different engines, and how rough [Pg292] an
approximation any general estimate must be of their gross amount.
(174.) There are many circumstances which obstruct the practical
application of any standard of engine-power: the magnitude of
furnace, and the extent of heating surface necessary to produce
any required rate of evaporation in the boiler, are unascertained;
each engine-maker has his own rule in these matters, and all the
rules are equally unsupported by any experimental test entitled to
respect. Thus the circumstances that govern the rate of
evaporation in the boiler may be regarded as almost wholly
unknown. But supposing the rate of evaporation to be ascertained,
the amount of power absorbed by the condensation of steam on its
passage to the cylinder, the imperfect condensation of the same
steam after it has worked the piston, the friction of the various
moving parts of the machinery, and, above all, the difference of
effect of these losses of power in engines constructed on
different scales of magnitude, are absolutely unknown. We are,
therefore, not placed in a condition to assign any thing more than
a general account of what has been the practice of engine-makers
in constructing engines which are nominally of a certain power.
In common low-pressure engines of the larger kind, to which class
alone we at present refer, it has been usual, with the same fuel
and under like circumstances, to allow from 10 to 18 square feet
of heating surface in the boiler for every nominal horse-power of
the engine. Within these wide limits the practice of engine-makers
has varied. It is not, however, to be supposed, that the boiler
with 18 square feet of surface per horse-power has the same
evaporating power as that which has but 10. This difference,
therefore, amounts to nothing more than different manufacturers of
steam-engines putting into circulation boilers having powers
_really_ different while they are _nominally_ the same. The
magnitude of the cylinder is regulated by the nominal power of the
engine, and it is usual so to regulate the evaporating power of
the boiler, that the piston shall move at the average rate of 200
feet per minute. This being assumed, it is customary to allow
about 22 square inches of piston [Pg293] surface for every
nominal horse-power of the engine. If this power were in
conformity to the standard already defined, this amount of surface
moved at 200 feet per minute would be impelled by a pressure
amounting to 7-1/2 lbs. per square inch. The safety-valve of the
boiler of such engines is usually loaded at from 4 to 5 lbs. per
square inch, and consequently the steam in the boiler will have a
pressure of from 19 to 20 lbs. per square inch. If, therefore, the
effective pressure on the piston be really only 7-1/2 lbs. per
square inch, the pressure expended in overcoming the friction of
the engine, and the loss consequent on the partial condensation of
steam on one side and its imperfect condensation on the other,
would amount to from 12 to 13 lbs. per square inch, or nearly
double the assumed useful effect of the engine.
Messrs. Maudslay and Field are accustomed to allow an evaporation
of ten gallons, or 1·6 cubic feet of water per hour, for each
nominal horse-power of the engine. They also allow about 22 square
inches of piston surface per nominal horse-power, the piston being
supposed to move at the rate of 200 feet per second.[24]
The quantity of grate surface necessary in proportion to the power
of the engine, has been equally unascertained, and engine-makers
vary in their practice from half a square foot to one square foot
per nominal horse-power.
The proportion which the magnitude of the heating surface of the
boiler, and the fire surface of the grate bears to the evaporating
power of the boiler, has not been determined by experiment, nor,
so far as we are informed, by any well-ascertained practical
results.
The estimates or rather conjectures of engine-makers, of the
evaporation necessary to produce one horse-power, vary from one to
two cubic feet of water per hour. It has been [Pg294] already
shown that the evaporation of 900 cubic inches, or little more
than half a cubic foot per hour, evolves a gross mechanical effect
representing one horse-power; from which it appears, that if the
evaporation of the boilers of steam engines were what engineers
suppose them to be, the gross mechanical power produced in them
for every nominal horse-power of the engine varies in actual
amount from the power of two to that of four horses.
The above estimates must be understood as referring to
double-acting steam engines above thirty-horse power. The
circumstances attending the performance of single-acting engines
applied to the drainage of mines, have been ascertained with much
greater precision. This has been mainly owing to a spirited system
of general inspection, which has been established in Cornwall, to
which we shall hereafter more particularly advert.
(175.) In expressing the duty of engines, it would have been
desirable that the duty of the boiler should have been separated
from that of the engine.
The duty of a boiler is estimated by the volume of water
evaporated by a given quantity of fuel, independently of the time
which such evaporation may take. The duty, therefore, will be
expressed by the number of cubic feet of water evaporated, divided
by the number of bushels of coal necessary for that evaporation,
supposing the bushel of coal to be the unit of fuel. It will be
observed that the _duty_ of an engine or boiler is entirely
distinct from, and independent of, its _power_. One boiler may be
greater than another in power to any extent, while it may be equal
to or less than it in duty. A bushel of coals may evaporate the
same number of cubic feet of water under two boilers, but may take
twice as great a time to produce such evaporation under one than
under the other. In such a case the power of one boiler will be
double that of the other, while their duty will be the same.
In like manner, a bushel of coals consumed in working two engines
may produce the same useful effect, but it may produce that useful
effect in the one in half the time it takes to produce it in the
other. In that case the _duty_ of the engines will be the same,
but the _power_ of the one will be double that of the other.
[Pg295]
In fine, _power_ has reference to _time_,—_duty_, to _fuel_. The
more rapidly the engine produces its mechanical effect, the
greater its power will be, whatever may be the fuel consumed in
working it. And, on the other hand, the greater the useful effect
produced by a given weight of fuel, the greater will be the duty,
however long the time may be which the fuel may take to produce
the useful effect.
(176.) The proportion of the stroke to the diameter of the
cylinder must be determined by the velocity intended to be given
to the piston. With the same capacity of cylinder, and the same
evaporation in the boiler, the velocity of the piston will augment
as the magnitude of its diameter is diminished.
The proportion of the diameter to the stroke of the cylinder is
very various. In engines used for steam-vessels the length of the
cylinder very little exceeds its diameter. In land engines,
however, the proportion of the length to the diameter is greater.
It is maintained by some that the proportion of the diameter and
length of the cylinder should be such as to render its surface
exposed to the cooling of the external air, the smallest possible.
Tredgold has maintained that since, during the stroke, the steam
is gradually exposed to contact with the surface of the cylinder
from the top to the bottom, the mean surface exposed in contact
with steam being half that of the entire cylinder, the proportion
of the diameter to the stroke should be such that the surface of
half the length of the cylinder, added to the magnitude of the top
and bottom, shall be a minimum. If this principle be admitted,
then the best proportion of the diameter to the stroke would be
that of one to two, the length of the stroke being twice the
diameter of the cylinder; but since the whole surface of the
cylinder is constantly exposed to the cooling effects of the air,
and since in the intervals of the stroke there is no sensible
change of the temperature of the surface, the loss of heat by
cooling will in effect be the same, especially in double-acting
engines, as if the cylinder were constantly filled with steam. If
this be admitted, then the object should be to give the cylinder
such a proportion, that its entire surface, including the top and
bottom, shall be a minimum. [Pg296] The proportion given by this
condition would be very nearly that which is observed in the
cylinders of marine engines, viz. that the length of the cylinder
should be equal to its diameter.
If in a low-pressure engine the pressure of steam in the cylinder
be taken at 17 lbs. per square inch, then the volume of steam will
be about fifteen hundred times that of the water which produces
it. For every cubic foot of water, therefore, in the effective
evaporation of the boiler, 1500 cubic feet of steam will be passed
through the cylinder. If it be intended that the motion of the
piston shall be at the rate of 25 strokes per minute, or 1500
strokes per hour, then the capacity of that portion of the
cylinder between the steam-valve and the piston at the end of the
stroke, must consist of half as many cubic feet as there are cubic
feet per hour evaporated in the boiler. If the steam, therefore,
be cut off at half stroke, the number of cubic feet of space in
the cylinder will be equal to the number of cubic feet of water
effectively evaporated by the boiler; and if a cubic foot of water
effectively evaporated be taken as the measure of a horse-power,
then there would be as many cubic feet in the capacity of the
cylinder as is equal to the nominal power of the engine.
(177.) The duty of engines varies according to their form and
magnitude, the circumstances under which they are worked, and the
purposes to which they are applied. In double-acting engines
working without expansion, the coal consumed per nominal
horse-power per hour varies from 7 to 12 lbs. An examination of
the steam-logs of several government steamers made by me a few
years since, gave, as the average of consumption of fuel at that
time of the best class of marine engines, about 8 lbs. per nominal
horse-power per hour. Since, however, no account could be obtained
of the actual evaporation of water in the boiler, nor, with the
necessary degree of precision, of the quantity and pressure of the
steam which passed through the cylinders, this estimate must be
regarded as an approximation subject to several causes of error.
The question of the duty of boilers and engines applied to the
[Pg297] general purposes of manufactures and navigation, is one
which has not yet been satisfactorily investigated; and it were
much to be desired that the proprietors of such engines should
combine to establish a strict analysis of their performance in
reference to their consumption of fuel, their evaporation of
water, and their useful effects. The results of such an
investigation, if properly conducted, would perhaps tend more to
the improvement of the steam engine than any discoveries in
science, or inventions in mechanical detail likely to be made in
the present stage of the progress of that machine.
(178.) A strict investigation of this kind has been for many years
carried on respecting the performance of the steam engines used
for the drainage of the mines in Cornwall; and it has been
attended with effects the most beneficial to the interests of
those concerned in them. The engines to which this important
inquiry has been applied being used for the purpose of pumping,
are generally single-acting engines, in which steam is used
expansively to a great extent. The steam is produced under a very
high pressure in the boiler, and being admitted to the cylinder is
cut off after a small portion of the entire stroke has been made,
the remainder of the stroke being produced by the expansion of the
steam.
About the year 1811, a number of the proprietors of the principal
Cornish mines agreed to establish this system of inspection, under
the management and direction of Captain Joel Lean, and to publish
monthly reports. In these reports were stated the following
particulars:—1. The load per square inch on the piston; 2. The
consumption of coal in bushels; 3. The number of strokes made by
the engine; 4. The length of the strokes in the pumps; 5. The load
in pounds; 6. The duty of the engine, expressed by the number of
pounds raised one foot high by the consumption of a bushel of
coals; 7. The number of strokes per minute; 8. The diameter and
stroke of the cylinder, and a general description of the engine.
When these reports were commenced, the number of engines brought
under inspection was twenty-one. In the year 1813 it increased
to twenty-nine; in 1814 to thirty-two; in 1820 the number
reported upon increased [Pg298] to forty; in 1828 the number was
fifty-seven; and in 1836 it was sixty-one. This gradual increase
in the number of engines brought under this system of inspection,
was produced by the good effects which attended it. These
beneficial consequences were manifested, not only in the improved
performance of the same engines, but in the gradually improved
efficiency of those which were afterwards constructed.
The following table taken from the statement of the duty of
Cornish engines by Thomas Lean and brother, lately published by
the British Association, will show in a striking manner the
improvement of the Cornish engines, from the commencement of this
system of inspection to the present time. The duty is expressed by
the number of pounds raised one foot high by the consumption of a
bushel of coals.
———————————————————————————————————————————————————————————————
| No. of | Average Duty of the | Average Duty of the best
Years.|Engines.| Whole. | Engine.
——————+————————+————————————————————-+———————————————————————-—
1812 | 21 | 19,300,000 |
1813 | 29 | 19,500,000 | 26,400,000
1814 | 32 | 20,600,000 | 32,000,000
1815 | 35 | 20,500,000 | 28,700,000
1816 | 35 | 23,000,000 | 32,400,000
1817 | 35 | 26,500,000 | 41,600,000
1818 | 36 | 25,400,000 | 39,300,000
1819 | 40 | 26,300,000 | 40,000,000
1820 | 46 | 28,700,000 | 41,300,000
1821 | 45 | 28,200,000 | 42,800,000
1822 | 52 | 28,900,000 | 42,500,000
1823 | 52 | 28,200,000 | 42,100,000
1824 | 49 | 28,300,000 | 43,500,000
1825 | 56 | 32,000,000 | 45,400,000
1826 | 51 | 30,500,000 | 45,200,000
1827 | 51 | 32,100,000 | 59,700,000
1828 | 57 | 37,100,000 | 76,800,000
1829 | 53 | 41,700,000 | 77,000,000
1830 | 56 | 43,300,000 | 78,000,000
1831 | 58 | 43,400,000 | 71,100,000
1832 | 59 | 45,000,000 | 85,000,000
1833 | 56 | 46,600,000 | 84,300,000
1834 | 52 | 47,800,000 | 90,900,000
1835 | 51 | 47,800,000 | 91,700,000
1836 | 61 | 46,600,000 | 85,400,000
1837 | 58 | 47,000,000 | 87,200,000
1838 | 61 | 48,700,000 | 84,200,000
——————+————————+————————————————————-+———————————————————————-—
[Pg299] As an example of the beneficial effects produced upon the
efficiency of an individual engine by the first application of
this system of inspection, the case of the Stray Park engine may
be mentioned. This engine, constructed by Boulton and Watt, had a
sixty inch cylinder, and when first reported in 1811, its duty
amounted to 16,000,000 pounds. After having been reported on for
three years, its duty was found to have increased to 32,000,000;
this estimate being taken from the average result of twelve
months' performance. Its duty was doubled in less than three
years.
It will appear, by inspection of the duties registered in the
preceding table, that the augmentation of the efficiency of the
engines has not been the effect of any great or sudden improvement,
but has rather resulted from the combination of a great number of
small improvements in the details of the operation of these
machines. In these improvements more is due to the successful
application of practical experience than to any new principles
developed by scientific research. Mr. John Taylor, in his "Records
of Mining," has traced the successive improvements on which the
increased duty of engines depends, and has connected these
improvements with their causes in the order of their dates. The
following results, abridged from his estimates, may not be
uninteresting:—
In 1769, soon after the date of the earliest discoveries of Mr.
Watt, but before they had come into practical application, Smeaton
computed that the average duty of fifteen atmospheric engines,
working at Newcastle-on-Tyne, was 5,590,000. The duty of the best
of these engines was 7,440,000, and that of the worst 3,220,000.
In 1772, Smeaton commenced his improvements on the atmospheric
engine, and raised the duty to 9,450,000.
In 1776, Watt obtained a duty of 21,600,000.
At this time Smeaton acknowledged that Watt's engines gave a duty
amounting to double that of his own.
In 1778-79, Watt reported a duty of 23,400,000.
From 1779 to 1788, Watt introduced the application of expansion,
and raised the duty to 26,600,000. [Pg300]
In 1798, an engine by Boulton and Watt, erected at Herland, was
reported as giving a duty of 27,000,000.
This engine, which was probably the best which at that time had
ever been erected, attracted the particular attention of Mr. Watt,
who, on visiting Cornwall, went to see it, and had many
experiments tried with it. It was under the care of Mr. Murdock,
the agent of Messrs. Boulton and Watt in Cornwall. When Mr. Watt
inspected it he pronounced it perfect, and that further
improvement could not be expected. How singular an instance this
of the impossibility, even of the most sagacious, to foresee the
results of mechanical improvement! In twenty years afterwards the
average duty of the best engine was nearly 40,000,000, and in
forty years it was above 84,000,000.
[Illustration: BOILER MANUFACTORY.]
FOOTNOTES:
[24] If 22 square inches of piston surface be allowed to
represent a horse-power, the power of an engine may always be
computed by dividing the square of the diameter of the piston
expressed in inches by 28. And, on the other hand, to find the
diameter of piston which would correspond to any given power,
multiply the number of horses' power by 28, and take the
square root of the product. These rules, however, cannot be
applied if the piston be supposed to move with any other
velocity; since, in that case, the same amount of piston
surface would cease to represent a horse-power, unless the
effective pressure on the piston were at the same time
changed.
[Pg301]
[Illustration: WATT'S CHAPEL IN HANDSWORTH CHURCH.]
CHAP. X.
NOTICE OF THE LIFE OF MR. WATT. — HIS FRIENDS AND ASSOCIATES
AT BIRMINGHAM. — INVENTION OF THE COPYING PRESS. — HEATING BY
STEAM. — DRYING LINEN BY STEAM. — THEORY OF THE COMPOSITION OF
WATER. — FIRST MARRIAGE OF WATT. — DEATH OF HIS FIRST WIFE. —
HIS SECOND MARRIAGE. — DEATH OF HIS YOUNGER SON. — EXTRACTS
FROM HIS LETTERS. — CHARACTER OF WATT BY LORD BROUGHAM. — BY
SIR WALTER SCOTT. — BY LORD JEFFREY. — OCCUPATION OF HIS OLD
AGE. — INVENTION OF MACHINE FOR COPYING SCULPTURE. — HIS LAST
DAYS. — MONUMENTS.
(179.) Having brought this historical analysis of the invention
and application of the steam engine to the date of the decease of
the illustrious man, to the powers of whose mind the world stands
indebted for the benefits conferred upon [Pg302] mankind by that
machine, it will perhaps not be deemed an improper digression in
this work, to devote some pages to a notice of the principal
labours of the same mind in other departments of art and science,
and to circumstances connected with his personal history and the
close of his life, which cannot fail to possess general interest.
At the period when Watt, having connected himself in partnership
with Boulton, went to reside at Soho, near Birmingham, a number of
persons, some of whom have since attained great celebrity by their
discoveries and their works, and all of whom were devoted to
inquiries connected with the arts and sciences, resided in that
neighbourhood. Among these may be mentioned PRIESTLEY, whose
discoveries in physical science have rendered his name immortal;
DARWIN, the philosopher and poet; WITHERING, a distinguished
physician and botanist; KEIR, a chemist, who published a translation
of Macquer, with annotations; GALTON, the ornithologist; and
EDGEWORTH, whose investigations respecting wheeled carriages and
other subjects, have rendered him well known. A society was formed
by these and other individuals, of which Boulton and Watt were
leading members, the meetings of which were held monthly on the
evening of full moon, and which was thence called the _Lunar
Society_. At the meetings of this society, subjects connected with
the arts and sciences were discussed, and out of those discussions
occasionally arose suggestions not unattended with important and
advantageous consequences. At one of these meetings, Darwin stated
that he had discovered a pen formed with two quills, by means of
which, at a single operation, an original and a copy of a letter
might be produced. Watt almost instantly observed that he thought he
could find a better expedient, and that he would turn it in his mind
that night. By the next morning the COPYING PRESS was invented, for
which he afterwards obtained a patent.
This machine, which is now so generally used in counting-houses,
consists of a rolling-press, by which a leaf of thin paper,
previously damped, is pressed upon the letter to be copied. The
writing, of which the ink is not yet quite dry, leaves its
impression upon the thin paper thus pressed upon [Pg303] it, and
the copy taken in this manner is read through the semi-transparent
paper. If a letter be written with ink suitable for this purpose,
a copy may be taken at any time within several hours after the
letter is written.
The method of heating apartments and buildings by steam, which has
since been improved and brought into extensive use, was likewise
brought forward by Watt. Although this contrivance had been
previously pointed out by Sir Hugh Platt about the middle of the
seventeenth century, and by Colonel Cooke in 1745, yet these
suggestions remained barren. Mr. Watt gave detailed methods of
heating buildings by steam[25]; and also invented a machine for
drying linen by steam, a description of which he communicated to
Dr. Brewster, which was read in December, 1824, before the Society
for promoting Useful Arts in Scotland.[26]
But the circumstance, exclusive of those connected with the
invention of the steam engine, which is by far the most memorable
in the career of Watt, is the share which he had in the discovery
of the composition of water. As this circumstance has recently
excited much interest, and led to some controversy, we shall here
state, as distinctly as possible, the leading facts connected with
it.
Water, which was so long held to be a simple element, has, in
modern times, been proved to be a substance consisting of two
aeriform bodies or gases chemically combined. These two gases are
those called in chemistry _oxygen_ and _hydrogen_. If eight grains
weight of oxygen be mixed with one grain weight of hydrogen, and
the mixture be submitted to such effects as would cause the
chemical combination of these two airs, it would be converted into
nine grains weight of pure water.
If, on the other hand, nine grains weight of pure water be
submitted to any conditions which would separate its constituent
parts, the result would be eight grains weight of oxygen gas, and
one grain weight of hydrogen gas. There are a variety of methods
in physics by which these effects would be [Pg304] produced. It
will be sufficient here to state one method of producing each of
the above changes.
If eight grains weight of oxygen be inclosed in a strong vessel
with one grain weight of hydrogen, all other substances being
excluded, and the mixture be inflamed, an explosion will take
place, the gases will disappear, and a small quantity of water
will be the only substance remaining in the vessel. If this water
be weighed, it will be found to weigh exactly nine grains.
It is known that the metals have a strong attraction for oxygen
gas, and this attraction is promoted by elevating their
temperature. If a glass tube be filled with iron wire heated to
redness, and to one end of this tube a small vessel of boiling
water be attached, the steam evolved from the water will force its
way through the spaces between the red-hot wires in the tube, and
would be expected to issue from the remote end; but if the
substance issuing from the remote end of the tube be examined, it
will be found to be not steam, but hydrogen gas. If the quantity
of this gas be ascertained by weight, and also the quantity of
weight lost by the vessel of water at the other end of the tube,
it will be found that the loss of weight of the water by
evaporation will be nine times the weight of the hydrogen which
has issued from the remote end of the tube. If the weight of the
tube with the wire contained in it be next ascertained, it will be
found to be increased by eight times the weight of the hydrogen
which has issued from its remote end. From this it follows that
the weight of the hydrogen which has escaped from the tube, added
to the increase of weight which has been given to the wire in the
tube, makes up the whole weight of the water evaporated. If the
wire in the tube be next examined, it will be found that it has
suffered oxydation, or, in other words, that a new substance has
been formed in it called the oxyde of iron,—such substance being
a chemical compound formed of oxygen gas and iron.
It follows, therefore, that in this process the vapour of the
water, in passing through the tube, has been decomposed, and that,
having given up to the iron its oxygen, the hydrogen [Pg305]
alone escaped from the other end; and for every nine grains weight
of steam which passed through the tube, eight grains of oxygen
have been combined with the iron, and one grain of hydrogen has
escaped from the end of the tube.
Such are the class of effects on which the modern discovery of the
composition of water has been based. The merit of that discovery
has been shared between the celebrated English chemist, CAVENDISH,
and the not less celebrated French chemist, LAVOISIER, the chief
merit, however, being ascribed to the former.
We shall now briefly state the facts which led to this discovery,
with their dates, which will necessarily show the share which Watt
had in it.
When pure hydrogen gas is burned in an atmosphere of common air,
the process which takes place is now known to be nothing more than
the chemical combination of the hydrogen with eight times its own
weight of oxygen taken from the atmosphere, and the product of the
combustion is a quantity of water nine times the weight of the
hydrogen consumed. In the year 1776, Macquer, a well-known chemist
of that day, having held a saucer of white porcelain over a flame
of hydrogen which was burning at the mouth of a bottle, observed
that no smoke was produced and no soot deposited on the saucer. On
the other hand, he found that after the lapse of some time drops
of a clear pellucid liquid were perceptible on the saucer: this
liquid he submitted to analysis, and found it to be pure water.
Macquer mentioned this fact without comment or inference. It did
not occur to him that the water thus produced upon the saucer was
a substance which contained the hydrogen, which disappeared upon
combustion from the bottle.
On the 18th of April, 1781, Mr. Warltire addressed a letter to Dr.
Priestley, dated Birmingham, which letter is published in Dr.
Priestley's _Experiments on Air_, printed at Birmingham in 1781,
in which Warltire informs Priestley that he had fired a mixture of
hydrogen and common air in close glass vessels, and that, although
previously to firing the mixture the vessels were clean and dry, a
dewy deposit was [Pg306] observed afterwards on their sides. In
fact, water was present which was not present before.
The mixture was in this case fired by passing an electric spark
through the vessel; and it is now known that the effect produced
was the combination of the hydrogen, which formed part of the
mixture of airs in the vessel with the oxygen, which also formed
part of the same mixture.
It appears, from expressions in Warltire's letter, that the same
experiment had been previously made by Priestley, and the same
result observed by him.
The inference deduced from this by Warltire, and apparently
acquiesced in by Priestley, was, that whenever hydrogen was fired
in atmospheric air, the moisture, which is always more or less
sustained in the latter, was deposited; but neither of these
chemists perceived the real cause of the production of the water.
In the beginning of 1783, and not later than the 21st of April,
this experiment of Warltire and Priestley was repeated by
Cavendish, with this difference, that, instead of exploding the
mixture of hydrogen and common air, Cavendish exploded a mixture
of hydrogen and oxygen. He observed that water was present after
the explosion, but _inferred nothing_.
In a published paper dated April, 1783, Priestley announced a
further and most important result of his experiments. This was,
that in examining the weight of water produced by the explosion of
a mixture of oxygen and hydrogen, _that weight was found to be
precisely equal to the sum of the weights of the two gases_, which
disappeared in the process.
Immediately on observing this, Priestley, being then, as has been
already stated, Watt's near neighbour, communicated to the latter
what he had observed; upon which Watt immediately, viz. by a letter
dated the 26th of the same month, declared that the inevitable
consequence which followed from Priestley's observations was, that
water was a substance compounded of oxygen and hydrogen deprived of
[Pg307] a quantity of heat which was previously latent in them.[27]
The letter containing this inference was communicated immediately by
Priestley to Sir Joseph Banks, then President of the Royal Society,
to be laid before that body; and it is accordingly printed with its
proper date in the 74th volume of the _Philosophical Transactions_.
About two months after the date of Mr. Watt's letter just quoted,
Lavoisier made experiments on the combustion of oxygen and
hydrogen, and read a memoir before the Academy of Sciences in
Paris, in which his views of the formation of water by the
combination of these gases were developed. This paper, by
Lavoisier, was afterwards printed in the Memoirs of the Academy in
the year 1784. The experiments are there stated to have been made
in the month of June, 1783; and it is stated that Sir Charles
Blagden, who was present at the experiments, told Lavoisier that
Mr. Cavendish had already burned the same gases in close vessels,
and obtained a very sensible quantity of water.
On the 15th of January, 1784, the celebrated paper by Cavendish,
entitled "Experiments on Air," was read before the Royal Society,
and in this paper the composition of water by the union of oxygen
and hydrogen is explained.
In a controversy which afterwards ensued on the respective
[Pg308] claims of Cavendish and Lavoisier to credit for the
discovery of the composition of water, Sir Charles Blagden stated
that he had told Lavoisier, in June, 1783, more than Lavoisier
acknowledged, that he had not only told him that water was
produced by the combustion of the gases, but that his information
embraced the whole theory of the composition of water. This
declaration of Blagden was subsequent in date to January, 1784,
and there is no evidence of any explanation of this theory, verbal
or otherwise, having been given by Cavendish, or any other person,
antecedent to April, 1783.
From this brief statement of the facts and dates it will appear
that the merit of the discovery of the FACT, that the weight of
water resulting from the combustion of oxygen and hydrogen, is
equal to the sum of the weights of the oxygen and hydrogen which
disappear in the combustion, is due to Priestley; and that the
merit of the INFERENCE from that fact, that water is a compound
body, whose constituents are oxygen and hydrogen, is due to
Watt.[28] Whether those who subsequently deduced the same
inference, and promulgated the same theory, were or were not
informed of Mr. Watt's solution of the phenomenon, or what credit
may be due to any person, however eminent, who at any time
posterior to Mr. Watt's letter to Priestley, asserted that they
had, at a time antecedently to that, made the same inference
without having published it, or communicated it in such a manner
as to establish their claim upon rational and credible evidence,
are questions which we shall not here discuss, being contented
with establishing the right of Mr. Watt to the merit of the
discovery of the THEORY which explained the FACT discovered by
Priestley.
Even in his declining years, after he had withdrawn from the
active pursuits of his business, the least excitement was
sufficient to call into play the slumbering powers of his
inventive genius. No object could present itself to his notice
[Pg309] without receiving from that genius adaptation in form and
construction to useful purposes. As an example of this restless
activity of mind the following anecdote may be mentioned:—
A company at Glasgow had erected on the right bank of the Clyde
extensive buildings and powerful engines for supplying water to
the town. After this expense it was found that a source of water,
of very superior quality, existed on the left bank of the river.
To change the site of the establishment, after the expense which
had been incurred in its erection could not be contemplated, and
they therefore proposed to carry across the bottom of the river a
flexible suction pipe, the mouth of which should terminate in the
source from which the pure water was to be derived. This pipe was
to be supported by a flooring constructed upon the bed of the
river; but it was soon apparent that the construction of such a
flooring on a shifting and muddy bottom, full of inequalities, and
under several feet depth of water would require a greater
expenditure of capital than could with propriety be afforded. In
this difficulty the aged mechanician, for whom Glasgow itself had
been the earliest stage of professional labour, was applied to,
and instantly solved the problem. His attention is said to have
been attracted by a lobster which had been served at table: he set
himself about to contrive how, by mechanism, he could make an
apparatus of iron with joints which should have all the
flexibility of the tail of the lobster. He therefore proposed that
an articulated suction-pipe, capable of accommodating itself to
all the inequalities and to the possible changes of the bed of the
river, should be carried across it; that this flexible pipe should
be two feet in diameter, and one thousand feet in length. This
project the company accordingly caused to be executed after the
plans and drawings of Watt with the most complete success.[29]
[Pg310] Among the less prominent, though not less useful services
rendered by Watt to his country, may be mentioned the introduction
of the use of chlorine in bleaching. That invention of Berthollet
was introduced into England by Watt after his visit to Paris at
the close of the year 1786. He constructed all the necessary
apparatus for it, directed its erection, and superintended its
first performances. He then left it to his wife's father, Mr.
Macgregor, to carry on the processes.
When the properties of the gases began to occupy the attention of
chemists, attempts were made to apply them as a means of curing
diseases of the lungs. Dr. Beddoes pursued this inquiry with great
activity, and established, through the means of private
subscription, at Clifton, an institution in which this method of
cure was carefully investigated. The Pneumatic Institution (for so
it was called) has been rendered celebrated for having at its head
Humphry Davy, just then commencing his scientific career. Among
its founders was also numbered James Watt. Not content, however,
with affording the institution the sanction of his name, he
designed and caused to be constructed, at Soho, the apparatus used
for making the gases and administering them to the patients.
As the exalted powers of the mind of Watt, unfolded in his
numerous mechanical and philosophical inventions and discoveries,
have commanded the admiration and respect of his species, the
affection and love of his fellow men would not have been less
conciliated, had the qualities of his heart, as developed in his
private and personal relations, been as well known as the products
of his genius.
In the year 1764, Watt being then in the twenty-ninth year of his
age, married his cousin, Miss Miller. At this time he had fallen
into a state of despondency from his disappointments, which
produced a serious attack of nervous illness. The accomplishments
and superior understanding, the mildness of temper and goodness of
disposition of his wife, soon restored him to health. Of this
marriage four children, two sons and two daughters, were the
issue. Two of these children died in infancy; another, a daughter,
was married to Mr. Miller of Glasgow; and the fourth is the
[Pg311] present Mr. James Watt. In September, 1773, while her
husband was engaged in the design of the Caledonian canal in the
North of Scotland, Mrs. Watt died in child-bed of a fifth child,
who was still-born: "Would that I might here transcribe," says M.
Arago, "in all their simple beauty, some lines of the journal in
which he daily recorded his inmost thoughts, his fears, his hopes!
Would that you could see him, after this heavy affliction, pausing
on the threshold of that home, where 'HIS KIND WELCOMER' awaited
him no more; unable to summon courage to enter those rooms where
he was never more to meet 'THE COMFORT OF HIS LIFE!' Possibly, so
faithful a picture of a very deep sorrow might at last put to
silence those obstinate theorists, who, without being struck by
the thousands of instances to the contrary, do yet refuse
qualities of the heart to every man whose intellect has been
fostered by the fertile, sublime, and imperishable truths of the
exact sciences!"
After the lapse of some years Watt married Miss Macgregor, a
person who is represented to have possessed qualities of mind
which rendered her a companion every way suitable to her husband.
This lady survived Watt, and died in 1832 at an advanced age. Two
children were the issue of this second marriage.
In the year 1800 the extended patent right, which had been granted
to Boulton and Watt for their improved engine, expired, and at
this time Mr. Watt retired altogether from business. He was
succeeded by his two sons, the present Mr. James Watt, and
Gregory, one of the children of his second marriage. The works at
Soho continued to be conducted by the present Mr. Boulton, the son
of the partner of Mr. Watt, and the two Messrs. Watt. In 1804
Gregory Watt died at the age of twenty-seven, of a disease of the
chest. This afflicting event was deeply felt by Mr. Watt; but he
did not sink under it into that state of despondency in which he
has been represented to have fallen by M. Arago. On the contrary,
he continued to show the same activity of mind which had
characterised his whole [Pg312] life; nor did he lose that
interest which he always took in the pursuit of literature and in
society. The state of his feelings under this affliction is shown
by the following extracts from letters written by him at that
time, which have been published by Mr. Muirhead.
"Heathfield, January 26th, 1805.
* * "I, perhaps, have said too much to you and Mrs. Campbell
on the state of my mind. I, therefore, think it necessary to
say that _I am not low spirited_; and were you here, you would
find me as cheerful in the company of my friends as usual; my
feelings for the loss of poor Gregory are not passion, but a
deep regret that such was his and my lot.
"I know that all men must die, and I submit to the decrees
of nature, I hope with due reverence to the Disposer of
Events. Yet one stimulus to exertion is taken away, and,
somehow or other, I have lost my relish for my usual avocations.
Perhaps time may remedy that in some measure; meanwhile,
I do not neglect the means of amusement which are in my power."
"Heathfield, April 8th, 1805.
* * "It is rather mortifying to see how easily the want of
even the best of us is dispensed with in the world; but it is
very well it should be so. We here, however, cannot help
feeling a terrible blank in our family. When I look at my
son's books, his writings and drawings, I always say to
myself, where are the mind that conceived these things, and
the hands that executed them? In the course of nature, he
should have said so of mine; but it was otherwise ordered, and
our sorrow is unavailing. As Catullus says:—
—— 'Nunc it, per iter tenebricosum,
Illuc, unde negant redire quemquam.
At vobis male sit, malæ tenebræ
Orci, quæ omnia bella devoratis!'
"But Catullus was a heathen; let us hope that he (G.) is now
rejoicing in another and a better world, free from our cares,
griefs, and infirmities. Some one has said, I shall not wholly
die; and Gregory's name, his merits and virtues, will live at
least as long as those do who knew him. You are not, from
this, to conceive that we give way to grief; on the contrary,
you will find us as cheerful as we ought to be, and as much
disposed to enjoy the friends we have left as ever; but we
should approach to brutes if we had no regrets."
Mr. Watt, at the date of these letters, had entered on his
seventieth year, a period after which great mental exertions are
rarely made. [Pg313]
In the summer of 1819, symptoms of indisposition manifested
themselves which soon rendered Watt aware of his approaching
dissolution. "I am very sensible," said he to his afflicted
friends, "of the attachment you show me, and I hasten to thank you
for it, as I am now come to my last illness." He died on the 25th
of August, 1819. His remains were deposited in the church of
Handsworth, near his estate of Heathfield. His son has raised over
his grave a Gothic chapel, in the centre of which is placed a
statue by Chantrey.
The personal character of Watt could not fail to excite the
admiration and the love of those distinguished persons, whose
pride and happiness it was to be admitted to a share in the
friendship of the great engineer. Among these were reckoned some
of the men who will leave upon the present age the deepest and
most lasting impressions of their genius, and such persons have
bequeathed to posterity the sentiments with which he inspired
them. We cannot here do more justice to the personal character of
the subject of this notice than by repeating the portraiture of it
which has been given by three of the most distinguished of his
friends, and of the most illustrious men of the present age.
At a meeting convened in 1824, for erecting a monument to Watt,
Lord Brougham pronounced a speech, from which we extract the
following observations:—
"I had the happiness of knowing Mr. Watt, for many years, in the
intercourse of private life; and I will take upon me to bear a
testimony in which all who had that gratification I am sure will
join, that they who only knew his public merit, prodigious as
that was, knew but half his worth. Those who were admitted to
his society will readily allow that anything more pure, more
candid, more simple, more scrupulously loving of justice, than
the whole habits of his life and conversation, proved him to be,
was never known in society. One of the most astonishing
circumstances in this truly great man, was the versatility of
his talents. His accomplishments were so various, the powers of
his mind were so vast, and yet of such universal application,
that it was hard to say whether we should most admire the
extraordinary grasp of his understanding, or the accuracy of
nice research with which he could bring it to bear upon the most
minute objects of investigation. I forget of whom it was said,
that his mind resembled the trunk of an elephant, which can pick
up [Pg314] straws, and tear up trees by the roots. Mr. Watt, in
some sort, resembled the greatest and most celebrated of his own
inventions, of which we are at a loss whether most to wonder at
the power of grappling with the mightiest objects, or of
handling the most minute; so that, while nothing seems too large
for its grasp, nothing seems too small for the delicacy of its
touch, which can cleave rocks, and pour forth rivers from the
bowels of the earth, and, with perfect exactness, though not
with greater ease, fashion the head of a pin, or strike the
impress of some curious die. Now, those who knew Mr. Watt, had
to contemplate a man whose genius could create such an engine,
and indulge in the most abstruse speculations of philosophy, and
could at once pass from the most sublime researches of geology
and physical astronomy, the formation of our globe, and the
structure of the universe, to the manufacture of a needle or a
nail; who could discuss, in the same conversation, and with
equal accuracy, if not with the same consummate skill, the most
forbidding details of art and the elegances of classical
literature, the most abstruse branches of science and the
niceties of verbal criticism.
"There was one quality in Mr. Watt which most honourably
distinguished him from too many inventors, and was worthy of
all imitation—he was not only entirely free from jealousy,
but he exercised a careful and scrupulous self-denial, and was
anxious not to appear, even by accident, as appropriating to
himself that which he thought belonged to others. I have heard
him refuse the honour universally ascribed to him, of being
the inventor of the steam engine, and call himself simply its
improver; though, in my mind, to doubt his right to that
honour, would be as inaccurate as to question Sir Isaac
Newton's claim to his greatest discoveries, because Descartes
in mathematics, and Galileo in astronomy and mechanics, had
preceded him; or to deny the merits of his illustrious
successor, because galvanism was not his discovery, though,
before his time, it had remained as useless to science as the
instrument called a steam engine was to the arts before Mr.
Watt. The only jealousy I have known him to betray, was with
respect to others, in the nice adjustment he was fond of
giving to the claims of inventors. Justly prizing scientific
discovery above all other possessions, he deemed the title to
it so sacred, that you might hear him arguing by the hour to
settle disputed rights; and if you ever perceived his temper
ruffled, it was when one man's invention was claimed by, or
given to another; or when a clumsy adulation pressed upon
himself that which he knew to be not his own."
In the preface to the _Monastery_ Sir Walter Scott speaks of Watt
in the following terms:—
"There were assembled about half a score of our northern
lights. * * Amidst this company stood Mr. Watt, the man whose
genius discovered the means of multiplying our national
resources to a degree, perhaps, even beyond his own stupendous
powers of calculation and combination; bringing the treasures
of the abyss to the summit of the earth—giving the feeble arm
of man the momentum of an Afrite—commanding manufactures to
arise as the rod of the prophet produced water in the
desert—affording the means of dispensing with that time and
tide which wait for no man—and of sailing without that wind
which defied the command and threats of Xerxes himself. This
potent commander of the elements—this abridger of time and
space—this magician, whose cloudy [Pg315] machinery has
produced a change on the world, the effects of which,
extraordinary as they are, are, perhaps, only now beginning to
be felt—was not only the most profound man of science—the
most successful combiner of powers, and calculator of numbers,
as adapted to practical purposes—was not only one of the most
generally well informed, but one of the best and kindest of
human beings.
"There he stood, surrounded by the little band I have
mentioned of northern literati, men not less tenacious,
generally speaking, of their own fame and their own opinions,
than the national regiments are supposed to be jealous of the
high character which they have won upon service. Methinks I
yet see and hear what I shall never see or hear again. In his
eighty-second year, the alert, kind, benevolent old man, had
his attention alive to every one's question, his information
at every one's command.
"His talents and fancy overflowed on every subject. One gentleman
was a deep philologist—he talked with him on the origin of the
alphabet, as if he had been coeval with Cadmus; another a
celebrated critic—you would have said the old man had studied
political economy and belles lettres all his life. Of science it is
unnecessary to speak—it was his own distinguished walk. And yet,
Captain Clutterbuck, when he spoke with your countryman, Jedediah
Cleishbotham, you would have sworn he had been coeval with Claverse
and Burley, with the persecutors and persecuted, and could number
every shot the dragoons had fired at the fugitive Covenanters. In
fact, we discovered that no novel of the least celebrity escaped
his perusal, and that the gifted man of science was as much
addicted to the productions of your native country, in other words,
as shameless and obstinate a peruser of novels, as if he had been a
very milliner's apprentice of eighteen."
In the Edinburgh newspaper, called the _Scotsman_, of the 4th
September, 1819, immediately after the decease of Watt, the
following sketch was published from the pen of Lord Jeffrey:—
"This name fortunately needs no commemoration of ours; for he
that bore it survived to see it crowned with undisputed and
unenvied honours; and many generations will probably pass away
before it shall have gathered 'all its fame.' We have said
that Mr. Watt was the great _improver_ of the steam engine;
but, in truth, as to all that is admirable in its structure,
or vast in its utility, he should rather be described as its
_inventor_. It was by his inventions, that its action was so
regulated as to make it capable of being applied to the finest
and most delicate manufactures, and its power so increased, as
to set weight and solidity at defiance. By his admirable
contrivance, it has become a thing stupendous alike for its
force and its flexibility—for the prodigious power which it
can exert, and the ease, and precision, and ductility with
which it can be varied, distributed, and applied. The trunk of
an elephant, that can pick up a pin or rend an oak, is as
nothing to it. It can engrave a seal, and crush masses of
obdurate metal before it—draw out, without breaking, a thread
as fine as gossamer, and lift a ship of war like a bauble in
the air. It can embroider muslin, and forge anchors—cut steel
into ribands, and impel loaded vessels against the fury of the
winds and waves. [Pg316]
"It would be difficult to estimate the value of the benefits
which these inventions have conferred upon this country. There
is no branch of industry that has not been indebted to them;
and, in all the most material, they have not only widened most
magnificently the field of its exertions, but multiplied a
thousand fold the amount of its productions. It is our
improved steam engine that has fought the battles of Europe,
and exalted and sustained, through the late tremendous
contest, the political greatness of our land. It is the same
great power which now enables us to pay the interest of our
debt, and to maintain the arduous struggle in which we are
still engaged (1819), with the skill and capital of countries
less oppressed with taxation. But these are poor and narrow
views of its importance. It has increased indefinitely the
mass of human comforts and enjoyments, and rendered cheap and
accessible all over the world the materials of wealth and
prosperity. It has armed the feeble hand of man, in short,
with a power to which no limits can be assigned; completed the
dominion of mind over the most refractory qualities of matter;
and laid a sure foundation for all those future miracles of
mechanic power which are to aid and reward the labours of
after generations. It is to the genius of one man, too, that
all this is mainly owing; and certainly no man ever bestowed
such a gift on his kind. The blessing is not only universal,
but unbounded; and the fabled inventors of the plough and the
loom, who were deified by the erring gratitude of their rude
contemporaries, conferred less important benefits on mankind
than the inventor of our present steam engine.
"This will be the fame of Watt with future generations; and it is
sufficient for his race and his country. But to those to whom he
more immediately belonged, who lived in his society and enjoyed his
conversation, it is not, perhaps, the character in which he will be
most frequently recalled—most deeply lamented—or even most highly
admired. Independently of his great attainments in mechanics, Mr.
Watt was an extraordinary, and in many respects a wonderful man.
Perhaps no individual in his age possessed so much and such varied
and exact information—had read so much, or remembered what he had
read so accurately and well. He had infinite quickness of
apprehension, a prodigious memory, and a certain rectifying and
methodising power of understanding, which extracted something
precious out of all that was presented to it. His stores of
miscellaneous knowledge were immense; and yet less astonishing than
the command he had at all times over them. It seemed as if every
subject that was casually started in conversation with him, had
been that which he had been last occupied in studying and
exhausting;—such was the copiousness, the precision, and the
admirable clearness of the information which he poured out upon it
without effort or hesitation. Nor was this promptitude and compass
of knowledge confined in any degree to the studies connected with
his ordinary pursuits. That he should have been minutely and
extensively skilled in chemistry and the arts, and in most of the
branches of physical science, might perhaps have been conjectured;
but it could not have been inferred from his usual occupations, and
probably is not generally known, that he was curiously learned in
many branches of antiquity, metaphysics, medicine, and etymology;
and perfectly at home in all the details of architecture, music,
and law. He was well acquainted, too, with most of the modern
languages, and familiar with their most recent literature. Nor was
it at all extraordinary to hear the great mechanician and engineer
detailing and expounding, for hours together, [Pg317] the
metaphysical theories of the German logicians, or criticising the
measures or the matter of the German poetry.
"His astonishing memory was aided, no doubt, in a great measure,
by a still higher and rarer faculty—by his power of digesting,
and arranging in its proper place, all the information he
received; and of casting aside and rejecting, as it were
instinctively, whatever was worthless or immaterial. Every
conception that was suggested to his mind seemed instantly to
take its place among its other rich furniture, and to be
condensed into the smallest and most convenient form. He never
appeared, therefore, to be at all incumbered or perplexed with
the _verbiage_ of the dull books he perused, or to the idle talk
to which he listened; but to have at once extracted, by a kind
of intellectual alchemy, all that was worthy of attention, and
to have reduced it, for his own use, to its true value and to
its simplest form. And thus it often happened, that a great deal
more was learned from his brief and vigorous account of the
theories and arguments of tedious writers, than an ordinary
student could ever have derived from the most painful study of
the originals; and that errors and absurdities became manifest
from the mere clearness and plainness of his statement of them,
which might have deluded and perplexed most of his hearers
without that invaluable assistance.
"It is needless to say that, with those vast resources, his
conversation was at all times rich and instructive in no ordinary
degree: but it was, if possible, still more pleasing than wise; and
had all the charms of familiarity with all the substantial
treasures of knowledge. No man could be more social in his spirit,
less assuming or fastidious in his manners, or more kind and
indulgent toward all who approached him. He rather liked to
talk,—at least in his latter years; but though he took a
considerable share of the conversation, he rarely suggested the
topics on which it was to turn, but readily and quietly took up
whatever was presented by those around him, and astonished the idle
and barren propounders of an ordinary theme by the treasures which
he drew from the mine they had unconsciously opened. He generally
seemed, indeed, to have no choice or predilection for one subject
of discourse rather than another; but allowed his mind, like a
great cyclopædia, to be opened at any letter his associates might
choose to turn up, and only endeavoured to select from his
inexhaustible stores, what might be best adapted to the taste of
his present hearers. As to their capacity he gave himself no
trouble; and indeed such was his singular talent for making all
things plain, clear, and intelligible, that scarcely any one could
be aware of such a deficiency in his presence. His talk, too,
though overflowing with information, had no resemblance to
lecturing or solemn discoursing, but, on the contrary, was full of
colloquial spirit and pleasantry. He had a certain quiet and grave
humour which ran through most of his conversation; and a vein of
temperate jocularity, which gave infinite zest and effect to the
condensed and inexhaustible information which formed its main
staple and characteristic. There was a little air of affected
testiness, and a tone of pretended rebuke and contradiction, with
which he used to address his younger friends, that was always felt
by them as an endearing mark of his kindness and familiarity; and
prized, accordingly, far beyond all the solemn compliments that
ever proceeded from the lips of authority. His voice was deep and
powerful, though he commonly spoke in a low and somewhat monotonous
tone, which harmonised admirably with the weight and brevity of his
observations, and set off to the greatest advantage the pleasant
[Pg318] anecdotes, which he delivered with the same grave brow, and
the same calm smile playing soberly on his lips. There was nothing
of effort, indeed, or impatience, any more than of pride or levity,
in his demeanour; and there was a finer expression of reposing
strength, and mild self-possession in his manner, than we ever
recollect to have met with in any other person. He had in his
character the utmost abhorrence for all sorts of forwardness,
parade, and pretensions; and, indeed, never failed to put all such
impostures out of countenance, by the manly plainness and honest
intrepidity of his language and deportment.
"In his temper and dispositions, he was not only kind and
affectionate, but generous, and considerate of the feelings of
all around him; and gave the most liberal assistance and
encouragement to all young persons who showed any indications of
talent, or applied to him for patronage or advice. His health,
which was delicate from his youth upwards, seemed to become
firmer as he advanced in years; and he preserved, up almost to
the last moment of his existence, not only the full command of
his extraordinary intellect, but all the alacrity of spirit and
the social gaiety which had illumined his happiest days. His
friends in this part of the country never saw him more full of
intellectual vigour and colloquial animation—never more
delightful or more instructive—than in his last visit to
Scotland in autumn 1817. Indeed, it was after that time that he
applied himself, with all the ardour of early life, to the
invention of a machine for mechanically copying all sorts of
sculpture and statuary; and distributed among his friends some
of its earliest performances, as the productions of a young
artist just entering on his eighty-third year.
"This happy and useful life came, at last, to a gentle close.
He had suffered some inconvenience through the summer; but was
not seriously indisposed till within a few weeks of his death.
He then became perfectly aware of the event which was
approaching; and with his usual tranquillity and benevolence
of nature, seemed only anxious to point out to the friends
around him, the many sources of consolation which were
afforded by the circumstances under which it was about to take
place. He expressed his sincere gratitude to Providence for
the length of days with which he had been blessed, and his
exemption from most of the infirmities of age; as well as for
the calm and cheerful evening of life that he had been
permitted to enjoy, after the honourable labours of the day
had been concluded. And thus, full of years and honours, in
all calmness and tranquillity, he yielded up his soul without
pang or struggle; and passed from the bosom of his family to
that of his God."
The English nation has ever shown itself insensible to the claims
of genius and high intellectual endowments, except where the
results have been brought directly to bear in statesmanship or
war. Of this inability to appreciate the highest order of
intellectual excellence Watt affords a striking example. When it
was suggested to the British government by those better capable
than that government was of appreciating the genius of this great
man, that the nation would do itself honour by erecting a splendid
monument at his own [Pg319] cost to him to whom it was so deeply
indebted for the extension of its resources and the augmentation
of its power, the reply was that such a measure could not be
adopted as it might be drawn into a precedent in like cases
thereafter! A precedent in like cases!! When will the time arrive
when the world will produce a like case? The monument which has
been erected in Westminster Abbey was in fact raised by private
subscription, the nation having thus stigmatised itself through
the act of its government with the everlasting disgrace of
refusing the honour proposed to it. The other statues and
monuments which have been erected to this great man, have been for
the most part raised by the filial piety and the never-dying
affection and veneration of the present Mr. James Watt. A statue
has been presented by him to the University of Glasgow, and placed
in one of the halls of that college. The inhabitants of Greenock
have also erected a marble statue of Watt, for which, and for a
library, a building has been erected at the expense of about
3,500_l._ which has been defrayed by Mr. James Watt. A colossal
bronze statue has been erected on a handsome granite pedestal,
standing at one of the corners of George Square, Glasgow. The
monument in Westminster Abbey, erected by the subscription raised
at the public meeting already alluded to, is a colossal statue of
Carrara marble, by Chantrey.
Watt was elected a fellow of the Royal Society of Edinburgh in
1784; of the Royal Society of London in 1785; a member of the
Batavian Society in 1787; and a corresponding member of the
Institut of France in 1808. The degree of Doctor of Laws was
conferred upon him by the University of Glasgow, in 1806; and in
1814, the highest scientific honour which can be attained by a
philosopher, was conferred on him by the Academy of Sciences of
the Institut of France, who nominated him one of its eight foreign
associates.
On the pedestal of the monument in Westminster Abbey is engraved
the following inscription from the pen of Lord Brougham:—
[Pg320]
NOT TO PERPETUATE A NAME WHICH MUST ENDURE WHILE THE PEACEFUL
ARTS FLOURISH, BUT TO SHOW THAT MANKIND HAVE LEARNED TO HONOUR
THOSE WHO BEST DESERVE THEIR GRATITUDE, THE KING HIS MINISTERS,
AND MANY OF THE NOBLES AND COMMONERS OF THE REALM RAISED THIS
MONUMENT TO
JAMES WATT,
WHO DIRECTING THE FORCE OF AN ORIGINAL GENIUS, EARLY EXERCISED
IN PHILOSOPHIC RESEARCH TO THE IMPROVEMENT OF THE STEAM
ENGINE, ENLARGED THE RESOURCES OF HIS COUNTRY, INCREASED THE
POWER OF MAN, AND ROSE TO AN EMINENT PLACE AMONG THE MOST
ILLUSTRIOUS FOLLOWERS OF SCIENCE AND THE REAL BENEFACTORS OF
THE WORLD. BORN AT GREENOCK MDCCXXXVI. DIED AT HEATHFIELD IN
STAFFORDSHIRE MDCCCXIX.
[Illustration: WATT'S CHAPEL IN HANDSWORTH CHURCH.]
FOOTNOTES:
[25] See Buchanan on the Economy of Fuel and Management of
Heat, especially as it relates to heating and drying by means
of Steam.
[26] See Brewster's Edinburgh Encyclopædia, article
STEAM-DRYING MACHINE.
[27] The following are the words in which Watt makes this
remarkable announcement to Priestley:—
"Let us now consider what obviously happens in the deflagration
of the inflammable (hydrogen) and dephlogisticated air (oxygen).
These two kinds of air unite with violence; they become red hot,
and upon cooling, totally disappear. When the vessel is cooled,
a quantity of water is found in it equal to the weight of the
air employed. This water is then the only remaining product of
the process; and water, light, and heat are all the products.
"Are we not then authorised to conclude, that water is composed
of dephlogisticated air (oxygen) and phlogiston (hydrogen),
deprived of part of their latent or elementary heat; that
dephlogisticated or pure air (oxygen) is composed of water
deprived of its phlogiston (hydrogen), and united to elementary
heat and light; and that the latter are contained in it in a
latent state, so as not to be sensible to the thermometer or to
the eye; and if light be only a modification of heat, or a
circumstance attending it, or a component part of the
inflammable air (hydrogen), then pure or dephlogisticated air
(oxygen) is composed of water deprived of its phlogiston
(hydrogen), and united to elementary heat."
[28] Those who desire to investigate this controversy more in
detail will find very full information on the subject in the
Translation of Arago's Eloge, with notes and appendix by J. P.
Muirhead, Esq. Murray, London, 1839.
[29] An account of this remarkable apparatus, accompanied by an
engraving made from a drawing supplied by Watt, was communicated
by Sir John Robison to the _Edinburgh Philosophical Journal_ in
1820. _See_ vol. iii, p. 60.
[Pg321]
[Illustration]
CHAP. XI.
LOCOMOTIVE ENGINES ON RAILWAYS.
NON-CONDENSING ENGINES. — LEUPOLD'S ENGINE.-TREVETHICK AND
VIVIAN. — EFFECTS OF RAILWAY TRANSPORT. — HISTORY OF THE
LOCOMOTIVE ENGINE. — BLENKINSOP. — MESSRS. CHAPMAN. — WALKING
ENGINE. — MR. STEPHENSON'S ENGINES AT KILLINGWORTH. —
LIVERPOOL AND MANCHESTER RAILWAY. — EXPERIMENTAL TRIAL. — THE
ROCKET. — THE SANSPAREIL. — THE NOVELTY. — SUBSEQUENT
IMPROVEMENTS IN THE LOCOMOTIVE ENGINE. — LARDNER'S EXPERIMENTS
IN 1832. — ADOPTION OF BRASS TUBES. — MR. BOOTH'S REPORT. —
DETAILED DESCRIPTION OF THE MOST IMPROVED LOCOMOTIVE ENGINES.
— POWER OF LOCOMOTIVE ENGINES. — EVAPORATION OF BOILERS. —
LARDNER'S EXPERIMENTS IN 1838. — RESISTANCE TO RAILWAY TRAINS.
— RESTRICTIONS ON GRADIENTS. — COMPENSATING EFFECT OF
GRADIENTS. — EXPERIMENT WITH THE HECLA. — METHODS OF
SURMOUNTING STEEP INCLINATIONS.
(180.) In the various modifications of the steam engine which we
have hitherto considered, the pressure introduced on one side of the
piston derives its efficacy either wholly or partially from the
vacuum produced by condensation on the other side. This always
requires a condensing apparatus, and a constant and abundant supply
of cold water. An engine of this kind must therefore necessarily
have considerable dimensions and weight, and is inapplicable to uses
in which a small and light machine only is admissible. If the
condensing apparatus be dispensed with, the piston will always be
resisted by a force equal to the atmospheric [Pg322] pressure, and
the only part of the steam pressure which will be available as a
moving power, is that part by which it exceeds the pressure of the
atmosphere. Hence, in engines which do not work by condensation,
steam of a much higher pressure than that of the atmosphere is
indispensably necessary, and such engines are therefore called
_high-pressure engines_.
We are not, however, to understand that every engine, in which
steam is used of a pressure exceeding that of the atmosphere, is
what is meant by an _high-pressure engine_; for in the ordinary
engines in common use, constructed on Watt's principle, the
safety-valve is loaded with from 3 to 5 lbs. on the square inch;
and in Woolf's engines, the steam is produced under a pressure of
40 lbs. on the square inch. These would therefore be more properly
called _condensing engines_ than _low-pressure engines_; a term
quite inapplicable to those of Woolf. In fact, by _high-pressure
engines_ is meant engines in which no vacuum is produced, and,
therefore, in which the piston works against a pressure equal to
that of the atmosphere.
In these engines the whole of the condensing apparatus, viz. the
cold-water cistern, condenser, air-pump, cold-water pump, &c., are
dispensed with, and nothing is retained except the boiler,
cylinder, piston, and valves. Consequently, such an engine is
small, light, and cheap. It is portable also, and may be moved, if
necessary, along with its load, and is therefore well adapted to
locomotive purposes.
(181.) High-pressure engines were one of the earliest forms of the
steam engine. The contrivance, which is obscurely described in the
article already quoted (7.), from the Century of Inventions, is a
high-pressure engine; for the power there alluded to is the
elastic force of steam working against the atmospheric pressure.
Newcomen, in 1705, applied the working-beam, cylinder, and piston
to the atmospheric engine; and Leupold, about 1720, combined the
working-beam and cylinder with the high-pressure principle, and
produced the earliest high-pressure engine worked by a cylinder
and piston. The following is a description of Leupold's engine:—
[Pg323]
[Illustration: _Fig._ 82.]
A (_fig._ 82.) is the boiler, with the furnace beneath it; C C are
two cylinders with solid pistons P P′, connected with the
working-beams B B′, to which are attached the pump-rods R R′, of
two forcing pumps F F′, which communicate with a great force-pipe
S; G is a _four-way cock_ (66.) already described. In the position
in which it stands in the figure, the steam issues from below the
piston P into the atmosphere, and the piston is descending by its
own weight; steam from the boiler is at the same time pressing up
the piston P′, with a force equal to the difference between the
pressure of the steam and that of the atmosphere. Thus the piston
R of the forcing-pump is being drawn up, and the piston P′ is
forcing the piston R′ down, and thereby driving water into the
force-pipe [Pg324] S. On the arrival of the piston P at the
bottom of the cylinder C, and P′ at the top of the cylinder C′,
the position of the cock is changed as represented in _fig._ 83.
The steam, which has just pressed up the piston P′, is allowed to
escape into the atmosphere, while the steam, passing from the
boiler below the piston P, presses it up, and thus P ascends by
the steam pressure, and P′ descends by its own weight. By these
means the piston R is forced down, driving before it the water in
the pump-cylinder into the force-pipe S, and the piston R′ is
drawn up to allow the other pump-cylinder to be re-filled; and so
the process is continued.
[Illustration: _Fig._ 83.]
A valve is placed in the bottom of the force-pipes, to prevent the
water which has been driven into it from returning. This valve
opens upwards; and, consequently, the weight of the water pressing
upon it only keeps it more effectually closed. On each descent of
the piston, the pressure transmitted to the valve acting upwards
being greater than the weight of the water resting upon it, forces
it open, and an increased quantity of water is introduced.
(182.) From the date of the improvement of Watt until the
commencement of the present century, non-condensing engines were
altogether neglected in these countries. In the year 1802, Messrs.
Trevethick and Vivian constructed the first non-condensing engine
of this kind which was ever brought into extensive practical use
in this kingdom. A section of this machine, made by a vertical
plane, is represented in _fig._ 84.
The boiler A B is a cylinder with flat circular ends. The
fire-place is constructed in the following manner:—A tube enters
the cylindrical boiler at one end; and, proceeding onwards near
the other extremity, is turned and recurved, so as to be carried
back parallel to the direction in which it entered. It is thus
conducted out of the boiler, at another part of the same end at
which it entered. One of the ends of this tube communicates with
the chimney E, which is carried upwards as represented in the
figure. The other mouth is furnished [Pg325] with a door; and in
it is placed the grate, which is formed of horizontal bars,
dividing the tube into two parts; the upper part forming the
fire-place, and the lower the ash-pit. The fuel is maintained in a
state of combustion, on the bars, in that part of the tube
represented at C D; and the flame is carried by the draught of the
chimney round the curved flue, and issues at E into the chimney.
The flame is thus conducted through the water, so as to expose the
latter to as much heat as possible.
[Illustration: _Fig._ 84.]
A section of the cylinder is represented at F, immersed in the
boiler, except a few inches of the upper end, where the four-way
cock G is placed for regulating the admission of the steam. A tube
is represented at H, which leads from this four-way cock into the
chimney; so that the waste steam, after working the piston, is
carried off through this tube, and passes into the chimney. The
upper end of the piston-rod is furnished with a cross-bar, which
is placed in a direction at right angles to the length of the
boiler, and also to the [Pg326] piston-rod. This bar is guided in
its motion by sliding on two iron perpendicular rods fixed to the
sides of the boiler, and parallel to each other. To the ends of
this cross-bar are joined two connecting rods, the lower ends of
which work two cranks fixed on an axis extending across and
beneath the boiler, and immediately under the centre of the
cylinder. This axis is sustained in bearings formed in the legs
which support the boiler, and upon its extremity is fixed the
fly-wheel as represented at B. A large-toothed wheel is placed on
this axis; which, being turned with the cranked axle, communicates
motion to other wheels; and through them, to any machinery which
the engine may be applied to move.
[Illustration: _Fig._ 85.]
As the four-way cock is represented in the figure, the steam
passes from the boiler through the curved passage G above the
piston, while the steam below the piston is carried off through a
tube which does not appear in the figure, by which it is conducted
to the tube H, and thence to the chimney. The steam, therefore,
which passes above the piston presses it downwards; while the
pressure upwards does not exceed that of the atmosphere. The
piston will therefore descend with a force depending on the excess
of the pressure of the steam produced in the boiler above the
atmospheric pressure. When the piston has arrived at the bottom of
the cylinder, the cock is made to assume the position represented
in _fig._ 85. This effect is produced by the motion of the
piston-rod. The steam now passes from above the piston, through
the tube H, into the chimney, while the steam from the boiler is
conducted through another tube below the piston. The pressure
above the piston, in this case, does not exceed that of the
atmosphere; while the pressure below it will be that of the steam
in the boiler. The piston will therefore ascend with the
difference of these pressures. On the arrival of the piston at the
top of the cylinder, the four-way cock is again turned to the
position represented in _fig._ 85., and the piston again descends;
and in the same manner the process is continued. A safety-valve is
placed on the boiler at V, loaded with a weight W, proportionate
[Pg327] to the strength of the steam with which it is proposed to
work.
In the engines now described, this valve was frequently loaded at
the rate of from 60 to 80 lbs. on the square inch. As the boilers of
high-pressure engines were considered more liable to accidents from
bursting than those in which steam of a lower pressure was used,
greater precautions were taken against such effects. A second
safety-valve was provided, which was not left in the power of the
engine-man. By this means he had a power to diminish the pressure of
the steam, but could not increase it beyond the limit determined by
the valve which was removed from his interference. The greatest
cause of danger, however, arose from the water in the boiler being
consumed by evaporation faster than it was supplied; and therefore
falling below the level of the tube containing the furnace. To guard
against accidents arising from this circumstance, a hole was bored
in the boiler, at a certain depth, below which the water should not
be allowed to fall; and in this hole a plug of metal was soldered
with lead, or with some other metal, which would fuse at that
temperature which would expose the boiler to danger. Thus, in the
event of the water being exhausted, so that its level would fall
below the plug, the heat of the furnace would immediately melt the
solder, and the plug would fall out, affording a vent for the steam,
without allowing the boiler to burst. The mercurial steam-gauge,
already described, was also used as an additional security. When the
force of the steam exceeded the length of the column of mercury
which the tube would contain, the mercury would be blown out, and
the tube would give vent to the steam. The water by which the boiler
was replenished was forced into it by a pump worked by the engine.
In order to economise the heat, this water was contained in a tube
T, which surrounded the pipe H. As the waste steam, after working
the piston, passed off through H, it imparted a portion of its heat
to the water contained in the tube T, which was thus warmed to a
certain temperature before it was forced into the boiler by the
pump. Thus a part of the heat, which was originally [Pg328] carried
from the boiler in the form of steam, was returned again to the
boiler with the water with which it was fed.
It is evident that engines constructed in this manner may be
applied to all the purposes to which the condensing engines are
applicable.
(183.) Two years after the date of the patent of this engine, its
inventor constructed a machine of the same kind for the purpose of
moving carriages on railroads; and applied it successfully, in the
year 1804, on the railroad at Merthyr Tydvil, in South Wales. It
was in principle the same as that already described. The cylinder
however was in a horizontal position, the piston-rod working in
the direction of the line of road: the extremity of the
piston-rod, by means of a connecting rod, worked cranks placed on
the axletree, on which were fixed two cogged wheels: these worked
in others, by which their motion was communicated finally to
cogged wheels fixed on the axle of the hind wheels of the
carriage, by which this axle was kept in a state of revolution.
The hind wheels being fixed on the axletree, and turning with it,
were caused likewise to revolve; and so long as the weight of the
carriage did not exceed that which the friction of the road was
capable of propelling, the carriage would thus be moved forwards.
On this axle was placed a fly-wheel to continue the rotatory
motion at the termination of each stroke. The fore wheels are
described as being capable of turning like the fore wheels of a
carriage, so as to guide the vehicle. The projectors appear to
have contemplated, in the first instance, the use of this carriage
on common roads; but that notion seems to have been abandoned, and
its use was only adopted on the railroad before mentioned. On the
occasion of its first trial, it drew after it as many carriages as
contained ten tons of iron a distance of nine miles; which stage
it performed without any fresh supply of water, and travelled at
the rate of five miles an hour.
(184.) Capital and skill have of late years been directed with
extraordinary energy to the improvement of inland transport; and
this important instrument of national wealth and civilisation has
received a proportionate impulse. Effects are now witnessed,
which, had they been narrated a few years [Pg329] since, could
only have been admitted into the pages of fiction or volumes of
romance. Who could have credited the possibility of a ponderous
engine of iron, loaded with some hundred passengers, in a train of
carriages of corresponding magnitude, and a large quantity of
water and coal, taking flight from Manchester and arriving at
Liverpool, a distance of above thirty miles, in little more than
an hour? And yet this is a matter of daily and almost hourly
occurrence. The rapidity of transport thus attained is not less
wonderful than the weights transported. Its capabilities in this
respect far transcend the exigencies even of the two greatest
commercial marts in Great Britain. Loads, varying from fifty to
one hundred and fifty tons, are transported at the average rate of
fifteen miles an hour; and in one instance we have seen a load—we
should rather say a _cargo_—of waggons, conveying merchandise to
the amount of two hundred and thirty tons gross, transported from
Liverpool to Manchester at the average rate of twelve miles an
hour.
The astonishment with which such performances must be viewed,
might be qualified, if the art of transport by steam on railways
had been matured, and had attained that full state of perfection
which such an art is always capable of receiving from long
experience, aided by great scientific knowledge, and the unbounded
application of capital. But such is not the present case. The art
of constructing locomotive engines, so far from having attained a
state of maturity, has not even emerged from its infancy. So
complete was the ignorance of its powers which prevailed, even
among engineers, previous to the opening of the Liverpool railway,
that the transport of heavy goods was regarded as the chief object
of the undertaking, and its principal source of revenue. The
incredible speed of transport, effected even in the very first
experiments in 1830, burst upon the public, and on the scientific
world, with all the effect of a new and unlooked-for phenomenon.
On the unfortunate occasion which deprived this country of Mr.
Huskisson, the wounded body of that statesman was transported a
distance of about fifteen miles in twenty-five minutes, being at
the rate of thirty-six miles an hour. The revenue of the road
arising from passengers since its opening, [Pg330] has, contrary
to all that was foreseen, been nearly double that which has been
derived from merchandise. So great was the want of experience in
the construction of engines, that the company was at first
ignorant whether they should adopt large steam engines fixed at
different stations on the line, to pull the carriages from station
to station, or travelling engines to drag the loads the entire
distance. Having decided on the latter, they have, even to the
present moment, laboured under the disadvantage of the want of
that knowledge which experience alone can give. The engines have
been constantly varied in their weight and proportions, in their
magnitude and form, as the experience of each successive month has
indicated. As defects became manifest they were remedied;
improvements suggested were adopted; and each year produced
engines of such increased power and efficiency, that their
predecessors were abandoned, not because they were worn out, but
because they had been outstripped in the rapid march of
improvement. Add to this, that only one species of travelling
engine has been effectively tried; the capabilities of others
remain still to be developed; and even that form of engine which
has received the advantage of a course of experiments on so grand
a scale to carry it towards perfection, is far short of this
point, and still has defects, many of which, it is obvious, time
and experience will remove.
If, then, the locomotive engine, subject thus to all the
imperfections inseparable from a novel contrivance—with the
restrictions on the free application of skill and capital, arising
from the nature of the monopolies granted to railway companies—with
the disadvantage of very limited experience, the great parent of
practical improvement, having been submitted to experiments hitherto
only on a limited scale, and confined almost to one form of
machine;—if, under such disadvantages, such effects have been
produced as are now daily witnessed by the public, what may not be
looked for from this extraordinary power when the enterprise of the
country shall be more unfettered—when greater fields of experience
are opened—when time, ingenuity, and capital have removed or
diminished existing imperfections, and have brought to light new and
more powerful principles? This is not mere speculation [Pg331] on
abstract possibilities, but refers to what is in actual progress.
The points of greatest wealth and population—the centres of largest
capital and most active industry throughout the country—will soon
be connected by lines of railway; and various experiments are
proposed, with more or less prospect of success, for the application
of steam engines on stone roads where the intercourse is not
sufficient to render railways profitable.
The important commercial and political effects attending such
increased facility and speed in the transport of persons and goods,
are too obvious to require any very extended notice here. A part of
the price (and in many cases a considerable part) of every article
of necessity or luxury, consists of the cost of transporting it from
the producer to the consumer; and consequently every abatement or
saving in this cost must produce a corresponding reduction in the
price of every article transported; that is to say, of every thing
which is necessary for the subsistence of the poor, or for the
enjoyment of the rich—of every comfort, and of every luxury of
life. The benefit of this will extend, not to the consumer only, but
to the producer: by lowering the expense of transport of the
produce, whether of the soil or of the loom, a less quantity of that
produce will be spent in bringing the remainder to market, and
consequently a greater surplus will reward the labour of the
producer. The benefit of this will be felt even more by the
agriculturist than by the manufacturer; because the proportional
cost of transport of the produce of the soil is greater than that of
manufactures. If two hundred quarters of corn be necessary to raise
four hundred, and one hundred more be required to bring the four
hundred to market, then the net surplus will be one hundred. But if
by the use of steam carriages the same quantity can be brought to
market with an expenditure of fifty quarters, then the net surplus
will be increased from one hundred to one hundred and fifty
quarters; and either the profit of the farmer, or the rent of the
landlord, must be increased by the same amount.
But the agriculturist would not merely be benefited by an
increased return from the soil already under cultivation. Any
[Pg332] reduction in the cost of transporting the produce to
market would call into cultivation tracts of inferior fertility,
the returns from which would not at present repay the cost of
cultivation and transport. Thus land would become productive which
is now waste, and an effect would be produced equivalent to adding
so much fertile soil to the present extent of the country. It is
well known, that land of a given degree of fertility will yield
increased produce by the increased application of capital and
labour. By a reduction in the cost of transport, a saving will be
made which may enable the agriculturist to apply to tracts already
under cultivation the capital thus saved, and thereby increase
their actual production. Not only, therefore, would such an effect
be attended with an increased extent of cultivated land, but also
with an increased degree of cultivation in that which is already
productive.
It has been said, that in Great Britain there are above a million
of horses engaged in various ways in the transport of passengers
and goods, and that to transport each horse requires as much land
as would, upon an average, support eight men. If this quantity of
animal power were displaced by steam engines, and the means of
transport drawn from the bowels of the earth, instead of being
raised upon its surface, then, supposing the above calculation
correct, as much land would become available for the support of
human beings as would suffice for an additional population of
eight millions; or, what amounts to the same, would increase the
means of support of the present population by about one third of
the present available means. The land which now supports horses
for transport would then support men, or produce corn for food.
The objection that a quantity of land exists in the country
capable of supporting horses alone, and that such land would be
thrown out of cultivation, scarcely deserves notice here. The
existence of any considerable quantity of such land is extremely
doubtful. What is the soil which will feed a horse and not feed
oxen or sheep, or produce food for man? But even if it be admitted
that there exists in the country a small portion of such land,
that portion cannot exceed, nor indeed equal, what would be
sufficient for the number of horses [Pg333] which must after all
continue to be employed for the purposes of pleasure, and in a
variety of cases where steam must necessarily be inapplicable. It
is to be remembered, also, that the displacing of horses in one
extensive occupation, by diminishing their price must necessarily
increase the demand for them in others.
The reduction in the cost of transport of manufactured articles,
by lowering their price in the market, will stimulate their
consumption. This observation applies of course not only to home
but to foreign markets. In the latter we already in many branches
of manufactures command a monopoly. The reduced price which we
shall attain by cheapness and facility of transport will still
further extend and increase our advantages. The necessary
consequence will be, an increased demand for manufacturing
population; and this increased population again reacting on the
agricultural interests, will form an increased market for that
species of produce. So interwoven and complicated are the fibres
which form the texture of the highly civilised and artificial
community in which we live, that an effect produced on any one
point is instantly transmitted to the most remote and apparently
unconnected parts of the system.
The two advantages of increased cheapness and speed, besides
extending the amount of existing traffic, call into existence new
objects of commercial intercourse. For the same reason that the
reduced cost of transport, as we have shown, calls new soils into
cultivation, it also calls into existence new markets for
manufactured and agricultural produce. The great speed of transit
which has been proved to be practicable, must open a commerce
between distant points in various articles, the nature of which
does not permit them to be preserved so as to be fit for use
beyond a certain time. Such are, for example, many species of
vegetable and animal food, which at present are confined to
markets at a very limited distance from the grower or feeder. The
truth of this observation is manifested by the effects which have
followed the intercourse by steam on the Irish Channel. The
western towns of England have become markets for a prodigious
quantity of Irish produce, which it had been previously [Pg334]
impossible to export. If animal food be transported alive from the
grower to the consumer, the distance of the market is limited by
the power of the animal to travel, and the cost of its support on
the road. It is only particular species of cattle which bear to be
carried to market on common roads and by horse carriages. But the
peculiar nature of a railway, the magnitude and weight of the
loads which may be transported on it, and the prodigious speed
which may be attained, render the transport of cattle, of every
species, to almost any distance, both easy and cheap. In process
of time, when the railway system becomes extended, the metropolis
and populous towns will therefore become markets, not as at
present to districts within limited distances of them, but to the
whole country.
The moral and political consequences of so great a change in the
powers of transition of persons and intelligence from place to place
are not easily calculated. The concentration of mind and exertion
which a great metropolis always exhibits, will be extended in a
considerable degree to the whole realm. The same effect will be
produced as if all distances were lessened in the proportion in
which the speed and cheapness of transit are increased. Towns at
present removed some stages from the metropolis, will become its
suburbs; others, now at a day's journey, will be removed to its
immediate vicinity; business will be carried on with as much ease
between them and the metropolis, as it is now between distant points
of the metropolis itself. Let those who discard speculations like
these as wild and improbable, recur to the state of public opinion,
at no very remote period, on the subject of steam navigation. Within
the memory of persons who have not yet passed the meridian of life,
the possibility of traversing by the steam engine the channels and
seas that surround and intersect these islands, was regarded as the
dream of enthusiasts. Nautical men and men of science rejected such
speculations with equal incredulity, and with little less than scorn
for the understanding of those who could for a moment entertain
them. Yet we have witnessed steam engines traversing not these
channels and seas alone, but sweeping the face of the waters round
every coast in Europe. The [Pg335] seas which interpose between our
Asiatic dominions and Egypt, and those which separate our own shores
from our West Indian possessions, have offered an equally
ineffectual barrier to its powers, and the establishment of a
regular steam communication between the capitals of the Old and New
World has ceased to be a question of practicability, having become
merely one of commercial profit. If steam be not used as the only
means of connecting the most distant points of our planet, it is not
because it is inadequate to the accomplishment of that end, but
because the supply of the material, from which at the present moment
it derives its powers, is restricted by local and accidental
circumstances.[30]
We propose in the present chapter to lay before our readers some
account of the means whereby the effects above referred to have
been produced; of the manner and degree in which the public have
availed themselves of these means; and of the improvements of
which they seem to us to be susceptible.
(185.) It is a singular fact, that in the history of this
invention considerable time and great ingenuity were vainly
expended in attempting to overcome a difficulty, which in the end
turned out to be purely imaginary. To comprehend distinctly the
manner in which a wheel carriage is propelled by steam, suppose
that a pin or handle is attached to the spoke of the wheel at some
distance from its centre, and that a force is applied to this pin
in such a manner as to make the wheel revolve. If the tire of the
wheel and the surface of the road were absolutely smooth and free
from friction, so that the face of the tire would slide without
resistance upon the road, then the effect of the force thus
applied would be merely to cause the wheel to turn round, the
carriage being stationary, the surface of the tire slipping or
sliding upon the road as the wheel is made to revolve. But if, on
the other hand, the pressure of the face of the tire upon the road
is such as to produce between them such a degree of adhesion as
will render it impossible for the wheel to slide or slip upon the
road by [Pg336] the force which is applied to it, the consequence
will be, that the wheel can only turn round in obedience to the
force which moves it by causing the carriage to advance, so that
the wheel will roll upon the road, and the carriage will be moved
forward, through a distance equal to the circumference of the
wheel, each time it performs a complete revolution.
It is obvious that both of these effects may be partially produced;
the adhesion of the wheel to the road may be insufficient to prevent
slipping altogether, and yet it may be sufficient to prevent the
wheel from slipping as fast as it revolves. Under such circumstances
the carriage would advance and the wheel would slip. The progressive
motion of the carriage during one complete revolution of the wheel
would be equal to the difference between the complete circumference
of the wheel and the portion through which in one revolution it has
slipped.
When the construction of travelling steam engines first engaged
the attention of engineers, and for a considerable period
afterwards, a notion was impressed upon their minds that the
adhesion between the face of the wheel and the surface of the road
must necessarily be of very small amount, and that in every
practical case the wheels thus driven would either slip
altogether, and produce no advance of the carriage, or that a
considerable portion of the impelling power would be lost by the
partial slipping or sliding of the wheels. It is singular that it
should never have occurred to the many ingenious persons who for
several years were engaged in such experiments and speculations,
to ascertain by experiment the actual amount of adhesion in any
particular case between the wheels and the road. Had they done so,
we should probably now have found locomotive engines in a more
advanced state than that to which they have attained.
To remedy this imaginary difficulty, Messrs. Trevethick and Vivian
proposed to make the external rims of the wheels rough and uneven,
by surrounding them with projecting heads of nails or bolts, or by
cutting transverse grooves on them. They proposed, in cases where
considerable elevations were to be ascended, to cause claws or
nails to project from the surface during the ascent, so as to take
hold of the road. [Pg337]
In seven years after the construction of the first locomotive
engine by these engineers, another locomotive engine was
constructed by Mr. Blinkensop, of Middleton Colliery, near Leeds.
He obtained a patent, in 1811, for the application of a rack-rail.
The railroad thus, instead of being composed of smooth bars of
iron, presented a line of projecting teeth, like those of a
cog-wheel, which stretched along the entire distance to be
travelled. The wheels on which the engine rolled were furnished
with corresponding teeth, which worked in the teeth of the
railroad, and, in this way, produced a progressive motion in the
carriage.
The next contrivance for overcoming this fictitious difficulty,
was that of Messrs. Chapman, who, in the year 1812, obtained a
patent for working a locomotive engine by a chain extending along
the middle of the line of railroad, from the one end to the other.
This chain was passed once round a grooved wheel under the centre
of the carriage; so that, when this grooved wheel was turned by
the engine, the chain being incapable of slipping upon it, the
carriage was consequently advanced on the road. In order to
prevent the strain from acting on the whole length of the chain,
its links were made to fall upon upright forks placed at certain
intervals, which between those intervals sustained the tension of
the chain produced by the engine. Friction-rollers were used to
press the chain into the groove of the wheel, so as to prevent it
from slipping. This contrivance was soon abandoned, for the very
obvious reason that a prodigious loss of force was incurred by the
friction of the chain.
The following year, 1813, produced a contrivance of singular
ingenuity, for overcoming the supposed difficulty arising from the
want of adhesion between the wheels and the road. This was no
other than a pair of mechanical legs and feet, which were made to
walk and propel in a manner somewhat resembling the feet of an
animal.
[Illustration: _Fig._ 86.]
A sketch of these propellers is given in _fig._ 86. A is the
carriage moving on the railroad, L and L′ are the legs, F and F′
the feet. The foot F has a joint at O, which corresponds to the
ankle; another joint is placed at K, which corresponds to the
knee; and a third is placed at L, which corresponds to [Pg338]
the hip. Similar joints are placed at the corresponding letters in
the other leg. The knee-joint K is attached to the end of the
piston of the cylinder. When the piston, which is horizontal, is
pressed outwards, the leg L presses the foot F against the ground,
and the resistance forces the carriage A onwards. As the carriage
proceeds, the angle K at the knee becomes larger, so that the leg
and thigh take a straighter position; and this continues until the
piston has reached the end of its stroke. At the hip L there is a
short lever L M, the extremity of which is connected by a cord or
chain with a point S, placed near the shin of the leg. When the
piston is pressed into the cylinder, the knee K is drawn towards
the engine, and the cord M S is made to lift the foot F from the
ground; to which it does not return until the piston has arrived
at the extremity of the cylinder. On the piston being again driven
out of the cylinder, the foot F, being placed on the road, is
pressed backwards by the force of the piston-rod at K; but the
friction of the ground preventing its backward motion, the
re-action causes the engine to advance: and in the same manner
this process is continued.
Attached to the thigh at N, above the knee, by a joint, is a
horizontal rod N R, which works a rack R. This rack has beneath it
a cog-wheel. This cog-wheel acts in another rack below it. By
these means, when the knee K is driven _from_ the engine, the rack
R is moved _backwards_; but the cog-wheel acting on the other rack
beneath it, will move the latter _in the contrary direction_. The
rack R being then moved _in the_ [Pg339] _same direction with the
knee_ K, it follows that the other rack will always be moved _in a
contrary direction_. The lower rack is connected by another
horizontal rod with the thigh of the leg L F′, immediately above
the knee at N′. When the piston is forced _inwards_, the knee K′
will thus be forced _backwards_; and when the piston is forced
_outwards_, the knee K′ will be drawn _forwards_. It therefore
follows, that the two knees K and K′ are pressed _alternately
backwards_ and _forwards_. The foot F′, when the knee K′ is drawn
forward, is lifted by the means already described for the foot F.
It will be apparent, from this description, that the piece of
mechanism here exhibited is a contrivance derived from the motion
of the legs of an animal, and resembling in all respects the fore
legs of a horse. It is however to be regarded rather as a specimen
of great ingenuity than as a contrivance of practical utility.
(186.) It was about this period that the important fact was first
ascertained that the adhesion or friction of the wheels with the
rails on which they moved was amply sufficient to propel the
engine, even when dragging after it a load of great weight; and
that in such case, the progressive motion would be effected
without any slipping of the wheels. The consequence of this fact
rendered totally useless all the contrivances for giving wheels a
purchase on the road, such as racks, chains, feet, &c. The
experiment by which this was determined appears to have been first
tried on the Wylam railroad; where it was proved, that when the
road was level, and the rails clean, the adhesion of the wheels
was sufficient, in all kinds of weather, to propel considerable
loads. By manual labour it was first ascertained how much weight
the wheels of a common carriage would overcome without slipping
round on the rail, and having found the proportion which that bore
to the weight, they then ascertained that the weight of the engine
would produce sufficient adhesion to drag after it on the railroad
the requisite number of waggons.[31]
In 1814, an engine was constructed at Killingworth, by Mr.
Stephenson, having two cylinders with a cylindrical [Pg340]
boiler, and working two pair of wheels, by cranks placed at right
angles; so that when the one was in full operation, the other was
at its dead points. By these means the propelling power was always
in action. The cranks were maintained in this position by an
endless chain, which passed round two cogged wheels placed under
the engine, and which were fixed on the same axles on which the
wheels were placed. The wheels in this case were fixed on the
axles, and turned with them.
[Illustration: _Fig._ 87.]
This engine is represented in _fig._ 87., the sides being open, to
render the interior mechanism visible. A B is the cylindrical
boiler; C C are the working cylinders; D E are the cogged wheels
fixed on the axle of the wheels of the engine, and surrounded by
the endless chain. These wheels being equal in magnitude, perform
their revolutions in the same time; so that, when the crank F
descends to the lowest point, the crank G rises from the lowest
point to the horizontal position D; and, again, when the crank F
rises from the lowest point to the horizontal position E, the
other crank rises to the highest point; and so on. A very
beautiful contrivance was adopted in this engine, by which it was
suspended on springs of steam. Small cylinders, represented at H,
are screwed by flanges to one side of the boiler, and project
within it a few inches; they have free communication at the top
with the water or steam of the boiler. Solid pistons are
represented at I, which move steam-tight in these [Pg341]
cylinders; the cylinders are open at the bottom, and the
piston-rods are screwed on the carriage of the engine, over the
axle of each pair of wheels, the pistons being presented upwards.
As the engine is represented in the figure, it is supported on
four pistons, two at each side. The pistons are pressed upon by
the water or steam which occupies the upper chamber of the
cylinder; and the latter being elastic in a high degree, the
engine has all the advantage of spring suspension. The defect of
this method of supporting the engine is, that when the steam loses
that amount of elasticity necessary for the support of the
machine, the pistons are forced into the cylinders, and the
bottoms of the cylinders bear upon them. All spring suspension is
then lost. This mode of suspension has consequently since been
laid aside.
In an engine subsequently constructed by Mr. Stephenson, for the
Killingworth railroad, the mode adopted of connecting the wheels
by an endless chain and cog-wheels was abandoned; and the same
effect was produced by connecting the two cranks by a straight
rod. All such contrivances, however, have this great defect, that,
if the fore and hind wheels be not constructed with dimensions
accurately equal, there must necessarily be a slipping or dragging
on the road. The nature of the machinery requires that each wheel
should perform its revolution exactly in the same time; and
consequently, in doing so, must pass over exactly equal lengths of
the road. If, therefore, the circumference of the wheels be not
accurately equal, that wheel which has the lesser circumference
must be dragged along so much of the road as that by which it
falls short of the circumference of the greater wheel; or, on the
other hand, the greater wheel must be dragged in the opposite
direction, to compensate for the same difference. As no mechanism
can accomplish a perfect equality in four, much less in six,
wheels, it may be assumed that a great portion of that dragging
effect is a necessary consequence of the principle of this
machine; and even were the wheels, in the first instance,
accurately constructed, it is not possible that their wear could
be so exactly uniform as to continue equal.
(187.) The next stimulus which the progress of this [Pg342]
invention received, proceeded from the great national work
undertaken at Liverpool, by which that town and the extensive
commercial mart of Manchester were connected by a double line of
railway. When this project was undertaken, it was not decided what
moving power it might be most expedient to adopt as a means of
transport on the proposed road: the choice lay between horse
power, fixed steam engines, and locomotive engines; but the first,
for many obvious reasons, was at once rejected in favour of one or
other of the last two.
The steam engine may be applied, by two distinct methods, to move
waggons either on a turnpike road or on a railway. By the one
method the steam engine is fixed, and draws the carriage or train
of carriages towards it by a chain extending the whole length of
road on which the engine works. By this method the line of road
over which the transport is conducted is divided into a number of
short intervals, at the extremity of each of which an engine is
placed. The waggons or carriages, when drawn by any engine to its
own station, are detached, and connected with the extremity of the
chain worked by the next stationary engine; and thus the journey
is performed, from station to station, by separate engines. By the
other method the same engine draws the load the whole journey,
travelling with it.
The Directors of the Liverpool and Manchester railroad, when that
work was advanced towards its completion, employed, in the spring
of the year 1829, Messrs. Stephenson and Lock, and Messrs. Walker
and Rastrick, experienced engineers, to visit the different
railways, where practical information respecting the comparative
effects of stationary and locomotive engines was likely to be
obtained; and from these gentlemen they received reports on the
relative merits, according to their judgment of the two methods.
The particulars of their calculations are given at large in the
valuable work of Mr. Nicholas Wood on railways; to which we refer
the reader, not only on this, but on many other subjects connected
with the locomotive steam engine, into which it would be foreign
to our object to enter. The result of the comparison of the two
systems was, that the capital [Pg343] necessary to be advanced to
establish a line of stationary engines was considerably greater
than that which was necessary to establish an equivalent power in
locomotive engines; that the annual expense by the stationary
engines was likewise greater; and that, consequently, the expense
of transport by the latter was greater, in a like proportion. The
subjoined table exhibits the results numerically:—
—————————————————————————————————————————————————————————————————
| | | Expense of
| | | taking a
| | Annual | Ton of Goods
| Capital. | Expense. | a Mile.
——————————————————-+——————————————-+——————————————-+—————————————
| £ s. d.| £ s. d.|
Locomotive engines | 58,000 0 0 | 25,517 8 2 | 0·164 penny
Stationary engines | 121,496 7 0 | 42,031 16 5 | 0·269
|——————————————-+——————————————-+—————————————
Locomotive system | | |
less | 63,496 7 0 | 16,514 8 3 | 0·105
——————————————————-+——————————————-+——————————————-+—————————————
On the score of economy, therefore, the system of locomotive
engines was entitled to a preference; but there were other
considerations which conspired with this to decide the choice of
the Directors in its favour. An accident occurring in any part of
a road worked by stationary engines must necessarily produce a
total suspension of work along the entire line. The most vigilant
and active attention on the part of every workman, however
employed, in every part of the line, would therefore be necessary;
but, independently of this, accidents arising from the fracture or
derangement of any of the chains, or from the suspension of the
working of any of the fixed engines, would be equally injurious,
and would effectually stop the intercourse along the line. On the
other hand, in locomotive engines an accident could only affect
the particular train of carriages drawn by the engine to which the
accident might occur; and even then the difficulty could be
remedied by having a supply of spare engines at convenient
stations along the line. It is true that the _probability_ of
accident is, perhaps, less in the stationary than in the
locomotive system; but the _injurious consequences_, when accident
_does_ happen, are prodigiously greater in the former. "The one
system," says Mr. Walker, "is like a chain extending from
Liverpool to Manchester, the failure [Pg344] of a single link of
which would destroy the whole; while the other is like a number of
short and unconnected chains," the destruction of any one of which
does not interfere with the effect of the others, and the loss of
which may be supplied with facility.
The decision of the Directors was, therefore, in favour of
locomotive engines; and their next measure was to devise some
means by which the inventive genius of the country might be
stimulated to supply them with the best possible form of engines
for this purpose. With this view, it was proposed and carried into
effect to offer a prize for the best locomotive engine which might
be produced under certain proposed conditions, and to appoint a
time for a public trial of the claims of the candidates. A premium
of five hundred pounds was accordingly offered for the best
locomotive engine to run on the Liverpool and Manchester railway;
under the condition that it should produce no smoke; that the
pressure of the steam should be limited to fifty pounds on the
inch; and that it should draw at least three times its own weight,
at the rate of not less than ten miles an hour; that the engine
should be supported on springs, and should not exceed fifteen feet
in height. Precautions were also proposed against the consequences
of the boiler bursting; and other matters not necessary to mention
more particularly here. This proposal was announced in the spring
of 1829, and the time of trial was appointed in the following
October. The engines which underwent the trial were, the Rocket,
constructed by Mr. Stephenson; the Sanspareil, by Hackworth; and
the Novelty, by Messrs. Braithwaite and Ericson. Of these, the
Rocket obtained the premium. A line of railway was selected for
the trial, on a level piece of road about two miles in length,
near a place called Rainhill, between Liverpool and Manchester;
the distance between the two stations was a mile and a half, and
the engine had to travel this distance backwards and forwards ten
times, which made altogether a journey of thirty miles. The Rocket
performed this journey twice: the first time in 2 hours 14 minutes
and 8 seconds; and the second time in 2 hours 6 minutes and 49
seconds. Its speed at different parts of the journey varied: its
greatest rate of motion was [Pg345] rather above 29 miles an
hour; and its least, about 11-1/2 miles an hour. The average rate
of the one journey was 13-4/10 miles an hour; and of the other,
14-2/20 miles. This was the only engine which performed the
complete journey proposed, the others having been stopped from
accidents which occurred to them in the experiment. The Sanspareil
performed the distance between the stations eight times,
travelling 22-1/2 miles in 1 hour 37 minutes and 16 seconds. The
greatest velocity to which this engine attained was something less
than 23 miles per hour. The Novelty had only passed twice between
the stations when the joints of the boiler gave way, and put an
end to the experiment.
(188.) The great object to be attained in the construction of
these engines was, to combine with sufficient lightness the
greatest possible heating power. The fire necessarily acts on the
water in two ways: first, by its radiant heat; and second, by the
current of heated air which is carried by the draught through the
flues, and finally passes into the chimney. To accomplish this
object, therefore, it is necessary to expose to both these sources
of heat the greatest possible quantity of surface in contact with
the water. These ends were attained by the following admirable
arrangement in the Rocket:—
[Illustration: _Fig._ 88.]
[Illustration: _Fig._ 89.]
This engine is represented in _fig._ 88. It is supported on four
wheels; the principal part of the weight being thrown on one pair,
which are worked by the engine. The boiler consists of a cylinder
six feet in length, with flat ends; the chimney issues from one
end, and to the other end is attached a square box B, the bottom
of which is furnished with the grate on which the fuel is placed.
This box is composed of two casings of iron, one contained within
the other, having between them a space about three inches in
breadth; the magnitude of the box being three feet in length, two
feet in width, and three feet in depth. The casing which surrounds
the box communicates with the lower part of the boiler by a pipe
marked C; and the same casing at the top of the box communicates
with the upper part of the boiler by another pipe marked D. When
water is admitted into the boiler, therefore, it flows freely,
through the pipe C, into the casing which [Pg346] surrounds the
furnace or fire-box, and fills this casing to the same level as
that which it has in the boiler. When the engine is at work, the
boiler is kept about half filled with water; and, consequently,
the casing surrounding the furnace is completely filled. The steam
which is generated in the water contained in the casing finds its
exit through the pipe D, and escapes into the upper part of the
boiler. A section of the engine, taken at right angles to its
length, is represented at _fig._ 89. Through the lower part of the
boiler pass a number of copper tubes of small size, which
communicate at one end with the fire-box, and at the other with
the chimney, and form a passage for the heated air from the
furnace to the chimney. The ignited fuel spread on the grate at
the bottom of the fire-box disperses its heat by radiation, and
acts in this manner on the whole surface of the casing surrounding
the fire-box; and thus raises the temperature of the thin shell of
water contained in that casing. The chief [Pg347] part of the
water in the casing, being lower in its position than the water in
the boiler, acquires a tendency to ascend when heated, and passes
into the boiler; so that a constant circulation of the heated
water is maintained, and the water in the boiler must necessarily
be kept at nearly the same temperature as the water in the casing.
The air which passes through the burning fuel, and which fills the
fire-box, is carried by the draught through the tubes which extend
through the lower part of the boiler; and as these tubes are
surrounded on every side with the water contained in the boiler,
this air transmits its heat through these tubes to the water. It
finally issues into the chimney, and rises by the draught. The
power of this furnace must necessarily depend on the power of
draught in the chimney; and to increase this, and at the same time
to dispose of the waste steam after it has worked the piston, this
steam is carried off by a pipe L, which passes from the cylinder
to the chimney, and escapes there in a jet which is turned
upwards. By the velocity with which it issues from this jet, and
by its great comparative levity, it produces a strong current
upwards in the chimney, and thus gives force to the draught of the
furnace. In _fig._ 89. the grate-bars are represented at the
bottom of the fire-box at F. There are two cylinders, one of which
works each wheel; one only appearing in the drawing _fig._ 88.,
the other being concealed by the engine. The spokes which these
cylinders work are placed at right angles on the wheels; the
wheels being fixed on a common axle, with which they turn.
In this engine, the surface of water surrounding the fire-box,
exposed to the action of radiant heat, amounted to twenty square
feet, which received heat from the surface of six square feet of
burning fuel on the bars. The surface exposed to the action of the
heated air amounted to 118 square feet. The engine drew after it
another carriage, containing fuel and water; the fuel used was
coke, for the purpose of avoiding the production of smoke.
(189.) The Sanspareil of Mr. Hackworth is represented in _fig._
90.; the horizontal section being exhibited in _fig._ 91.
[Illustration: _Fig._ 90.]
[Illustration: _Fig._ 91.]
The draught of the furnace is produced in the same manner as in
the Rocket, by ejecting the waste steam coming from [Pg348] the
cylinder into the chimney; the boiler, however, differs
considerably from that of the Rocket. A recurved tube passes
through the boiler, somewhat similar to that already described in
the early engine of Messrs. Trevethick and Vivian. In the
horizontal section (_fig._ 91.), D expresses the opening of the
furnace at the end of the boiler, beside the chimney. The
grate-bars appear at A, supporting the burning fuel; and a curved
tube passing through the boiler, and terminating in the chimney,
is expressed at B, the direction [Pg349] of the draught being
indicated by the arrow; C is a section of the chimney. The
cylinders are placed, as in the Rocket, on each side of the
boiler; each working a separate wheel, but acting on spokes placed
at right angles to each other. The tube in which the grate and
flue are placed diminishes in diameter as it approaches the
chimney. At the mouth where the grate was placed, its diameter was
two feet; and it was gradually reduced, so that, at the chimney,
its diameter was only fifteen inches. The grate-bars extended five
feet into the tube. The surface of water exposed to the radiant
heat of the fire was sixteen square feet; and that exposed to the
action of the heated air and flame was about seventy-five square
feet. The magnitude of the grate, or sheet of burning fuel which
radiated heat, was ten square feet.
(190.) The Novelty, of Messrs. Braithwaite and Ericson, is
represented in _fig._ 92.; and a section of the generator and
boiler is exhibited in _fig._ 93.; the corresponding parts in the
two figures are marked by the same letters.
[Illustration: _Fig._ 92.]
A is the generator or receiver containing the steam which works
the engine; this communicates with a lower generator B, which
extends in a horizontal direction the entire length of the
carriage. Within the generator A is contained the furnace F, which
communicates in a tube C, carried up through the generator, and
terminated at the top by sliding shutters, which exclude the air,
and which are only opened to supply fuel to the grate F. Below the
grate the furnace is not open, as usual, to the atmosphere, but
communicates, [Pg350] by a tube E, with a bellows D; which is
worked by the engine, and which forces a constant stream of air,
by the tube E, through the fuel on F, so as to keep that fuel in
vivid combustion. The heated air contained in the furnace F is
driven on, by the same force, through a small curved tube marked
_e_, which circulates like a worm (as represented in _fig._ 93.)
through the horizontal generator or receiver; and, tapering
gradually, until reduced to very small dimensions, it finally
issues into the chimney G. The air in passing along this tube,
imparts its heat to the water by which the tube is surrounded, and
is brought to a considerably reduced temperature when discharged
into the chimney. The cylinder, which is represented at K, works
one pair of wheels, by means of a bell-crank, the other pair, when
necessary, being connected with them.
[Illustration: _Fig._ 93.]
In this engine, the magnitude of the surface of burning fuel on
the grate-bars is less than two square feet; the surface exposed
to radiant heat is nine and a half square feet; and the surface of
water exposed to heated air is about thirty-three square feet.
The superiority of the Rocket may be attributed chiefly to the
greater quantity of surface of the water which is exposed to the
action of the fire. With a less extent of grate-bars than the
Sanspareil, in the proportion of three to five, it exposes a
greater surface of water to radiant heat, in the proportion of
four to three; and a greater surface of water to heated air, in
the proportion of more than three to two. It was found that
the Rocket, compared with the Sanspareil, consumed fuel, in
the evaporation of a given quantity of water, [Pg351] in the
proportion of eleven to twenty-eight. The suggestion of using the
tubes to conduct through the water the heated air to the chimney
is due to Mr. Booth, treasurer of the Liverpool and Manchester
Railway Company.
(191.) The object to be effected in the boilers of these engines
is, to keep a small quantity of water at an excessive temperature,
by means of a small quantity of fuel kept in the most active state
of combustion. To accomplish this, it is necessary, first, so to
shape the boiler, furnace, and flues, that the water shall be in
contact with as extensive a surface as possible, every part of
which is acted on, either immediately, by the heat radiating from
the fire, or mediately, by the air which has passed through the
fire, and which finally rushes into the chimney: and, secondly,
that such a forcible draught should be maintained in the furnace,
that a quantity of heat shall be extricated from the fuel, by
combustion, sufficient to maintain the water at the necessary
temperature, and to produce the steam with sufficient rapidity. To
accomplish these objects, therefore, the chamber containing the
grate should be completely surrounded by water, and should be
below the level of the water in the boiler. The magnitude of the
surface exposed to radiation should be as great as is consistent
with the whole magnitude of the machine. The comparative advantage
which the Rocket possessed in these respects over the other
engines will be evident on inspection. In the next place, it is
necessary that the heat, which is absorbed by the air passing
through the fuel, and keeping it in a state of combustion, should
be transferred to the water before the air escapes into the
chimney. Air being a bad conductor of heat, to accomplish this it
is necessary that the air in the flues should be exposed to as
great an extent of surface in contact with the water as possible.
No contrivance can be less adapted for the attainment of this end
than one or two large tubes traversing the boiler, as in the
earliest locomotive engines: the body of air which passed through
the centre of these tubes had no contact with their surface, and,
consequently, passed into the chimney at nearly the same
temperature as that which it had when it quitted the fire. The
only portion of air which imparted its heat to the water [Pg352]
was that portion which passed next to the surface of the tube.
Several methods suggest themselves to increase the surface of
water in contact with a given quantity of air passing through it.
This would be accomplished by causing the air to pass between
plates placed near each other, so as to divide the current into
thin strata, having between them strata of water, or it might be
made to pass between tubes differing slightly in diameter, the
water passing through an inner tube, and being also in contact
with the external surface of the outer tube. Such a method would
be similar in principle to the steam-jacket used in Watt's steam
engines, or to the condenser of Cartwright's engine already
described. But, considering the facility of constructing small
tubes, and of placing them in the boiler, that method, perhaps,
is, on the whole, the best in practice; although the shape of a
tube, geometrically considered, is most unfavourable for the
exposure of a fluid contained in it to its surface. The air which
passes from the fire-chamber, being subdivided as it passes
through the boiler by a great number of very small tubes, may be
made to impart all its excess of heat to the water before it
issues into the chimney. This is all which the most refined
contrivance can effect. The Rocket engine was traversed by
twenty-five tubes, each three inches in diameter; and the
principle has since been carried to a much greater extent.
The abstraction of a great quantity of heat from the air before it
reaches the chimney is attended with one consequence, which, at
first view, would present a difficulty apparently insurmountable;
the chimney would, in fact, lose its power of draught. This
difficulty, however, was removed by using the waste steam, which
had passed from the cylinder after working the engine, for the
purpose of producing a draught. This steam was urged through a jet
presented upwards in the chimney, and driven out with such force
in that direction as to create a sufficient draught to work the
furnace.
It will be observed that the principle of draught in the Novelty
is totally distinct from this: in that engine the draught is
produced by a bellows worked by the engine. The question, as far
as relates to these two methods, is, whether more power [Pg353]
is lost in supplying the steam through the jet, as in the Rocket,
or in working the bellows, as in the Novelty. The force requisite
to impel the steam through the jet must be exerted by the
returning stroke of the piston, and, consequently, must rob the
working effect to an equivalent amount. On the other hand, the
power requisite to work the bellows in the Novelty must be
subducted from the available power of the engine. The former
method has been hitherto found to be the more effectual and
economical.
The importance of these details will be understood, when it is
considered that the only limit to the attainment of speed by
locomotive engines is the power to produce, in a given time, a
certain quantity of steam. Each stroke of the piston causes one
revolution of the wheels, and consumes four cylinders full of
steam: consequently, a cylinder of steam corresponds to a certain
number of feet of road travelled over: hence it is that the
production of a rapid and abundant supply of heat, and the
imparting of that heat quickly and effectually to the water, is
the key to the solution of the problem to construct an engine
capable of rapid motion.
The method of subdividing the flue into tubes was carried much
further by Mr. Stephenson after the construction of the Rocket;
and, indeed, the principle was so obvious, it is only surprising
that, in the first instance, tubes of smaller diameter than three
inches were not used. In engines since constructed, the number of
tubes vary from ninety to one hundred and twenty, the diameter
being reduced to two inches or less; and in some instances tubes
have been introduced, even to the number of one hundred and fifty,
of one and a half inch diameter. In the Meteor, twenty square feet
are exposed to radiation, and one hundred and thirty-nine to the
contact of heated air; in the Arrow, twenty square feet to
radiation, and one hundred and forty-five to the contact of heated
air. The superior economy of fuel gained by this means will be
apparent by inspecting the following table, which exhibits the
consumption of fuel which was requisite to convey a ton weight a
mile in each of four engines, expressing also the rate of the
motion:— [Pg354]
————————————————————————————————————————————————————————————
| Average Rate of | Consumption of Coke
Engines. | Speed in Miles per | in Pounds per Ton
| Hour. | per Mile.
————————————————————————————————————————————————————————————
No. 1. Rocket | 14 | 2·41
2. Sanspareil | 15 | 2·47
3. Phœnix | 12 | 1·42
4. Arrow | 12 | 1·25
————————————————————————————————————————————————————————————
(192.) Since the period at which this railway was opened for the
actual purposes of transport, the locomotive engines have been in
a state of progressive improvement. Scarcely a month has passed
without suggesting some change in the details, by which fuel might
be economised, the production of steam rendered more rapid, the
wear of the engine rendered slower, the proportionate strength of
the different parts improved, or some other desirable end
obtained.
Engines constructed in the form of the Rocket, were subject to two
principal defects. The cylinders, being placed outside the engine,
were exposed to the cold of the atmosphere, which produced a waste
of heat more or less considerable by condensation. The points at
which the power of the steam to turn the wheels was applied, being
at the extremities of the axle and on the exterior of the wheel, a
considerable strain was produced, owing to the distance of the
point of application of the power from the centre of resistance.
If it were possible that the impelling power could act in drawing
the train at all times with equal energy on both sides of the
engine, then no injurious strain would be produced; but from the
relative position of the points on the opposite wheels to which it
was necessary to attach the connecting rods, it was inevitable
that, at the moment when one of the pistons exerts its full power
in driving the wheel, the other piston must be altogether
inactive. The impelling power, therefore, at alternate moments of
time, acted on opposite wheels, and on each of them at the
greatest possible distance from the centre of the axle.
[Illustration: _Fig._ 94.]
(193.) The next step in the improvement of the machine was made
with a view to remove these two defects. The cylinders were
transferred from the exterior of the engine to the [Pg355]
interior of the casing called the smoke-box, B, _fig._ 94., which
supports the chimney, and which receives the heated air issuing
from the tubes which traverse the boiler. Thus placed, the
cylinders are always maintained as hot as the air which issues
from the flues, and all condensation of steam by their exposure is
prevented. The piston-rods are likewise brought closer together,
and nearer the centre of the engine: the connecting rods, no
longer attached to the wheels, are made to act upon two cranks
constructed upon the axle of the wheels, and placed at right
angles to each other. From the position of these cranks, one would
always be at its dead point when the other is in full action. The
action of the steam upon them would, therefore, be generally
unequal; but this would not produce the same strain as when the
connecting rods are attached to points upon the exterior of the
wheels, owing to the cranks being constructed on the axle at
points so much nearer its centre. By this means it was found that
the working of the machine was more even, and productive of much
less strain, than in the arrangement adopted in the Rocket, and
the earlier engines. On the other hand, a serious disadvantage was
incurred by a double-cranked axle. The weakness necessarily
arising from such a form of axle could only be removed by great
thickness [Pg356] and weight of metal; and even this precaution,
at first, did not prevent their occasional fracture. The forging
of them was, however, subsequently much improved: the cranks,
instead of being formed by bending the metal when softened by
heat, were made by cutting the square of the crank out of the
solid metal; and now it rarely happens that one of these axles
fails.
The adoption of smaller tubes, and a greater number of them, with
a view more perfectly to extract the heat from the air in passing
to the chimney, rendered a more forcible draft necessary. This was
accomplished by the adoption of a more contracted blast-pipe
leading from the eduction-pipes of the cylinders and presented up
the chimney. A representation of such a blast-pipe, with the two
tubes leading from the cylinders and uniting together near the
point, which is presented up the chimney, is given at _p p_ in
_fig._ 104. The engine thus improved is represented in _fig._ 94.
A represents the cylindrical boiler, the lower half of which is
traversed by tubes, as described in the Rocket. They are usually
from eighty to one hundred in number, and about an inch and a half
in diameter; the boiler is about seven feet in length; the
fire-chamber is attached to one end of it, at F, as in the Rocket,
and similar in construction: the cylinders are inserted in a
chamber at the other end, immediately under the chimney. The
piston-rods are supported in the horizontal position by guides;
and connecting rods extend from them, under the engine, to the two
cranks placed on the axle of the large wheels. The effects of an
inequality in the road are counteracted by springs, on which the
engine rests; the springs being below the axle of the great
wheels, and above that of the less. The steam is supplied to the
cylinders, and withdrawn, by means of the common sliding valves,
which are worked by an eccentric wheel placed on the axle of the
large wheels of the carriage. The motion is communicated from this
eccentric wheel to the valve by sliding rods. The stand is placed
for the attendant at the end of the engine, next the fire-place F;
and two levers L project from the end which communicate with the
valves by means of rods, by which the engine is governed so as to
reverse the motion. [Pg357]
The wheels of these engines have been commonly constructed of wood
with strong iron ties, furnished with flanges adapted to the
rails. But Mr. Stephenson afterward substituted, in some
instances, wheels of iron with hollow spokes. The engine draws
after it a tender carriage containing the fuel and water; and,
when carrying a light load, is capable of performing the whole
journey from Liverpool to Manchester without a fresh supply of
water. When a heavy load of merchandise is drawn, it is usual to
take in water at the middle of the trip.
(194.) In reviewing all that has been stated, it will be perceived
that the efficiency of the locomotive engines used on this railway
is mainly owing to three circumstances: 1st, The unlimited power
of draft in the furnace, by projecting the waste steam into the
chimney; 2d, The almost unlimited abstraction of heat from the air
passing from the furnace, by arrangement of tubes traversing the
boiler; and, 3d, Keeping the cylinders warm, by immersing them in
the chamber under the chimney. There are many minor details which
might be noticed with approbation, but these constitute the main
features of the improvements.
The successive introduction of improvements in the engines, some
of which we have mentioned, was accompanied by corresponding
accessions to their practical power, and to the economy of fuel.
In the spring of the year 1832, I made several experiments on the
Manchester Railway, with a view to determine, in the actual state
of the locomotive engines at that time, their powers with respect
to the amount of load and the economy of fuel, from which I select
the following as examples:—
I.
On Saturday, the 5th of May, the engine called the "Victory"
took 20 waggons of merchandise, weighing gross 92 tons 19 cwt.
1 qr., together with the tender containing fuel and water, of
the weight of which I have no account, from Liverpool to
Manchester (30 miles), in 1 h. 34 min. 45 sec. The train
stopped to take in water half-way, for 10 minutes, [Pg358]
not included in the above-mentioned time. On the inclined
plane rising 1 in 96, and extending 1-1/2 mile, the engine was
assisted by another engine called the "Samson," and the ascent
was performed in 9 minutes. At starting, the fire-place was
well filled with coke, and the coke supplied to the tender
accurately weighed. On arriving at Manchester, the fire-place
was again filled, and the coke remaining in the tender
weighed. The consumption was found to amount to 929 pounds net
weight, being at the rate of one third of a pound per ton per
mile.
Speed on the level was 18 miles an hour; on a fall of 4 feet
in a mile, 21-1/2 miles an hour; fall of 6 feet in a mile,
25-1/2 miles an hour; on the rise over Chatmoss, 8 feet in a
mile, 17-5/8 miles an hour; on level ground sheltered from the
wind, 20 miles an hour. The wind was moderate, but direct
ahead. The working wheels slipped three times on Chatmoss, and
the train was retarded from 2 to 3 minutes.
The engine, on this occasion, was not examined before or after
the journey, but was presumed to be in good working order.
II.
On Tuesday, the 8th of May, the same engine performed the same
journey, with 20 waggons, weighing gross 90 tons 7 cwt. 2
qrs., exclusive of the unascertained weight of the tender. The
time of the journey was 1 h. 41 min. The consumption of coke
1040 lbs. net weight, estimated as before. Rate of speed:—
Level 17-5/8 miles per hour.
Fall of 4 feet in a mile 22
—— 6 22-1/2
Rise of 8 15
On this occasion there was a high wind ahead on the quarter,
and the connecting rod worked hot, owing to having been keyed
too tight. On arriving at Manchester, I caused the cylinders
to be opened, and found that the pistons were [Pg359] so
loose, that the steam blew through the cylinders with great
violence. By this cause, therefore, the machine was robbed of
a part of its power during the journey; and this circumstance
may explain the slight decrease in speed, and increase in the
consumption of fuel, with a lighter load, in this journey,
compared with that performed on the 5th of May.
The Victory weighs 8 tons 2 cwt., of which 5 tons 4 cwt. rest
on the drawing wheels. The cylinders are 11 inches diameter,
and 16 inches stroke, and the diameter of the drawing wheels
is 5 feet.
III.
On the 29th of May, the engine called the "Samson" (weighing
10 tons 2 cwt., with 14-inch cylinders, and 16-inch stroke;
wheels 4 feet 6 inches diameter, both pairs being worked by
the engine; steam 50 lbs. pressure, 130 tubes) was attached to
50 waggons, laden with merchandise; net weight about 150 tons;
gross weight, including waggons, 223 tons 6 cwt. The tender
weighed 7 tons, making a gross load (including the engine) of
240 tons 8 cwt. The engine with this load travelled from
Liverpool to Manchester (30 miles) in 2 hours and 40 min.,
exclusive of delays upon the road for watering, &c.; being at
the rate of nearly 12 miles an hour. The speed varied
according to the inclinations of the road. Upon a level, it
was 12 miles an hour; upon a descent of 6 feet in a mile, it
was 16 miles an hour; upon a rise of 8 feet in a mile, it was
about 9 miles an hour. The weather was calm, the rails very
wet; but the wheels did not slip, even in the slowest speed,
except at starting, the rails being at that place soiled and
greasy with the slime and dirt to which they are always
exposed at the stations. The coke consumed in this journey,
exclusive of what was raised in getting up the steam, was 1762
lbs., being at the rate of a quarter of a pound per ton per
mile.
(195.) The great original cost, and the heavy expense of keeping
the engines used on the railway in repair, have pressed severely
on the resources of the undertaking. One of the best [Pg360]
constructed of the later engines costs originally 1500_l._ and
sometimes more. The original cost, however, is far from being the
principal source of expense: the wear and tear of these machines,
and the occasional fracture of those parts on which the greatest
strain has been laid, have greatly exceeded what the directors had
anticipated. Although this source of expense must be in part
attributed to the engines not having yet attained that state of
perfection, in the proportion and adjustment of their parts, of
which they are susceptible, and to which experience alone can
lead, yet there are some obvious defects which demand attention.
The heads of the boilers are flat, and formed of iron, similar to
the material of the boilers themselves. The tubes which traverse
the boiler were, until recently, copper, and so inserted into the
flat head or end as to be water-tight. When the boiler was heated,
the tubes were found to expand in a greater degree than the other
parts of the boiler; which frequently caused them either to be
loosened at the extremities, so as to cause leakage, or to bend
from want of room for expansion. The necessity of removing and
refastening the tubes caused, therefore, a constant expense.
It will be recollected that the fire-place is situated at one end
of the boiler, immediately below the mouths of the tubes: a
powerful draft of air, passing through the fire, carries with it
ashes and cinders, which are driven violently through the tubes,
and especially the lower ones, situated near the fuel. These tubes
are, by this means, subject to rapid wear, the cinders continually
acting upon their interior surface. After a short time it becomes
necessary to replace single tubes, according as they are found to
be worn, by new ones; and it not unfrequently happens, when this
is neglected, that tubes burst. After a certain length of time the
engines require new tubing. This wear of the tubes might possibly
be avoided by constructing the fire-place in a lower position, so
as to be more removed from their mouths; or, still more
effectually, by interposing a casing of metal, which might be
filled with water, between the fire-place and those tubes which
are the most exposed to the cinders and ashes. The unequal
expansion of the tubes [Pg361] and boilers appears to be an
incurable defect, if the present form of the engine be retained.
If the fire-place and chimney could be placed at the same end of
the boiler, so that the tubes might be recurved, the unequal
expansion would then produce no injurious effect; but it would be
difficult to clean the tubes, if they were exposed, as they are at
present, to the cinders. The next source of expense arises from
the wear of the boiler-heads, which are exposed to the action of
the fire.
A considerable improvement was subsequently introduced into the
method of tubing, by substituting brass for copper tubes. I am not
aware that the cause of this improvement has been discovered; but
it is certain, whatever be the cause, that brass tubes are subject
to considerably slower wear than copper ones.
(196.) The expense of locomotive power having so far exceeded what
was anticipated at the commencement of the undertaking, it was
thought advisable, about the beginning of the year 1834, to
institute an inquiry into the causes which produced the
discrepancy between the estimated and actual expenses, with a view
to the discovery of some practical means by which they could be
reduced. The directors of the company, for this purpose, appointed
a sub-committee of their own body, assisted by Mr. Booth, their
treasurer, to inquire and report respecting the causes of the
amount of this item of their expenditure, and to ascertain whether
any and what measures could be devised for the attainment of
greater economy. A very able and satisfactory report was made by
this committee, or, to speak more correctly, by Mr. Booth.
It appears that, previous to the establishment of the railway,
Messrs. Walker and Rastrick, engineers, were employed by the
company to visit various places where steam power was applied on
railways, for the purpose of forming an estimate of the probable
comparative expense of working the railway by locomotive and by
fixed power. These engineers recommended the adoption of
locomotive power; and their estimate was, that the transport might
be effected at the rate of ·278 of a penny, or very little more
than a farthing per ton per mile. In the year [Pg362] 1833, five
years after this investigation took place, it was found that the
actual cost was ·625 of a penny, or something more than a
halfpenny, per ton per mile, being considerably above double the
estimated rate. Mr. Booth very properly directed his inquiries to
ascertain the cause of this discrepancy, by comparing the various
circumstances assumed by Messrs. Walker and Rastrick, in making
their estimate, with those under which the transport was actually
effected. The first point of difference which he observed was the
_speed_ of transport: the estimate was founded on an assumed speed
of ten miles an hour, and it was stated that a four-fold speed
would require an addition of 50 per cent. to the power, without
taking into account wear and tear. Now, the actual speed of
transport being double the speed assumed in the statement, Mr.
Booth holds it to be necessary to add 25 per cent. on that score.
The next point of difference is in the amount of the loads: the
estimate is founded upon the assumption, that every engine shall
start with its full complement of load, and that with this it
shall go the whole distance. "The facts, however, are," says Mr.
Booth, "that, instead of a _full load_ of profitable carriage
_from_ Manchester, about half the waggons _come back empty_; and,
instead of the tonnage being conveyed the whole way, many thousand
tons are conveyed only half the way; also, instead of the daily
work being uniform, it is extremely fluctuating." It is further
remarked, that in order to accomplish the transport of goods from
the branches and from intermediate places, engines are despatched
several times a-day, from both ends of the line, _to clear the
road_; the object of this arrangement being rather to lay the
foundation of a beneficial intercourse in future, than with a view
to any immediate profit. Mr. Booth makes a rough estimate of the
disadvantages arising from these circumstances, by stating them at
33 per cent. in addition to the original estimate.
The next point of difference is the fuel. In the original
estimate, _coal_ is assumed as the fuel, and it is taken at the
price of five shillings and ten-pence per ton: now the act of
parliament forbids the use of coal which would produce smoke; the
company have, therefore, been obliged to use _coke_, at [Pg363]
seventeen shillings and sixpence a ton.[32] Taking coke, then, to
be equivalent to coal, ton for ton, this would add ·162 to the
original estimate.
These several discrepancies being allowed for, and a proportional
amount being added to the original estimate, the amount would be
raised to ·601 of a penny per ton per mile, which is within one
fortieth of a penny of the actual cost. This difference is
considered to be sufficiently accounted for by the wear and tear
produced by the very rapid motion, more especially when it is
considered that many of the engines were constructed before the
engineer was aware of the great speed that would be required.
"What, then," says Mr. Booth, in the Report already alluded to, "is
the result of these opposite and mutually counteracting
circumstances? and what is the present position of the company in
respect of their moving power? Simply, that they are still in a
course of experiment, to ascertain practically the best
construction, and the most durable materials, for engines required
to transport greater weights, and at greater velocities, than had,
till very recently, been considered possible; and which, a few years
ago, it had not entered into the imagination of the most daring and
sanguine inventor to conceive: and farther, that these experiments
have necessarily been made, not with the calm deliberation and quiet
pace which a salutary caution recommends,—making good each step in
the progress of discovery before advancing another stage,—but
amidst the bustle and responsibilities of a large and increasing
traffic; the directors being altogether ignorant of the time each
engine would last before it would be laid up as inefficient, but
compelled to have engines, whether good or bad; being aware of
various defects and imperfections, which it was impossible at the
time to remedy, yet obliged to keep the machines in motion, under
all the disadvantages of heavy repairs, constantly going on during
the night, in order that the requisite number of engines might be
ready for the morning's work. Neither is this great experiment yet
complete; it is still going forward. But the most prominent
difficulties have been in a great measure surmounted, [Pg364] and
your committee conceive that they are warranted in expecting, that
the expenditure in this department will, ere long, be materially
reduced,—more especially when they consider the relative
performances of the engines at the _present time_, compared with
what it was two years ago."
In the half year ending 31st December, 1831, the six best engines
performed as follows:—
Miles.
Planet 9,986
Mercury 11,040
Jupiter 11,618
Saturn 11,786
Venus 12,850
Etna 8,764
——————
Making in all 66,044
——————
In the half year ending 31st December, 1833, the six best engines
performed as follows:—
Miles.
Jupiter 16,572
Saturn 18,678
Sun 14,552
Etna 17,763
Ajax 11,678
Firefly 15,608
——————
Making in all 95,851
——————
(197.) Since the date to which the preceding observations refer,
the locomotive engine has undergone several improvements in detail
of considerable importance; among which, the addition of a third
pair of wheels deserves to be particularly mentioned. An engine
supported on three pair of wheels has great security in the event
of the fracture of any one of the axles,—the remaining axles and
wheels being sufficient for the support of the machine. Connected
with this change is another, recommended by Mr. Robert Stephenson,
by which the flanges are removed from the driving wheels, those
upon the remaining pairs of wheels being sufficient to keep the
engine in its position upon the rails. We shall now describe a
locomotive engine similar in construction to those almost [Pg365]
universally used at present on railroads, as well in this kingdom
as in other countries.[33]
The external appearance of the engine and tender is shown in the
engraving at the head of this chapter. In _fig._ 97. is exhibited
a vertical section of the engine made by a plane carried through
its length; and in _fig._ 98. is exhibited a corresponding section
of its tender,—the tender being supposed to be joined on to the
engine at the part where the connecting points appear to be broken
in the drawing. In _fig._ 99. is exhibited the plan of the working
machinery, including the cylinders, pistons, eccentrics, &c. which
are under the boiler, by the operation of which the engine is
driven. _Fig._ 100. represents the tender, also taken in plan.
In _fig._ 101. is represented an elevation of the hinder end of
the engine next the fire-box; and in _fig._ 102. is represented a
cross vertical section through the fire-box, and at right angles
to the length of the engine, showing the interior of the boiler
above and beside the fire-box, the rivets and bolts connecting the
internal and external fire-boxes, the regulator, steam funnel, and
steam dome.
In _fig._ 103. is represented an elevation of the front of the
engine next the smoke-box, showing the cylinder covers W, buffers
T, &c.; and in _fig._ 104. is represented a section of the
interior of the smoke-box, made by a vertical plane at right
angles to the engine, showing the tube plate forming the foremost
end of the boiler, the branches S of the steam-pipe leading to the
cylinders, the blast-pipe _p_, the cylinders H, and the chimney G.
The same letters of reference are placed at corresponding parts in
the different figures.
The boiler, as has been explained in the engines already
described, is a cylinder placed upon its side, the section of
which is exhibited at A, _fig._ 97. The fire-box consists of two
casings of metal, one within the other. The fire-grate is
represented at D. The tubes by which the products of combustion
are [Pg366] drawn from the fire-box to the smoke-box F are
represented at E. Upon the smoke-box is erected the chimney G. In
the engine from which this drawing has been taken, and which was
used on the London and Birmingham Railway, the boiler is a
cylinder 7-1/2 feet long, and 3-1/2 feet in diameter. It is formed
of wrought-iron plates 5/16 of an inch in thickness, overlapping
each other, and bound together by iron rivets 7/8 of an inch in
diameter and 1-3/4 inch apart. One of these rivets, as it joins
two plates, is represented in _fig._ 95. The boiler is clothed
with a boarding of wood _a_, an inch in thickness, and bound round
by iron hoops screwed together at the bottom. Wood being a slow
conductor of heat, this covering has the effect of keeping the
boiler warm, and checking the condensation of steam which would
otherwise be produced by the rapid motion of the engine through
the cold air.
[Illustration: _Fig._ 95.]
[Illustration: _Fig._ 96.]
The external fire-box, B B, is a casing nearly square in its plan,
being four feet wide outside, and three feet seven and a half
inches long, measured in the direction of the boiler. It is
constructed of wrought-iron plates, similar to those of the
boiler. This box descends about two feet below the boiler, the top
being semi-cylindrical, as seen in _fig._ 102., of a somewhat
greater diameter than the boiler, and concentrical with it. The
front of the fire-box next the end of the boiler has a circular
opening equal in size to the end of the boiler. To the edge of
this opening the boiler is fastened by angle irons, and rivets in
the manner represented in _fig._ 96. These rivets are seen in
section in _fig._ 97.
The internal fire-box C, _fig._ 97., is similar in shape to the
external, only it is flat at the top, and close every where except
at the bottom. Between it and the external fire-box an open space
of three inches and a half is left all round, and on the side next
the boiler this space is increased to four inches. This internal
fire-box is made of copper plates, 7/16 [Pg367] of an inch in
thickness, every where except next the boiler, where the thickness
is 7/8.
As the sides and front of the external fire-box, and all the
surfaces bounding the internal fire-box, are flat, their form is
unfavourable for the resistance of pressure. Adequate means are,
therefore, provided for strengthening them. The plates forming the
internal fire-box are bent outwards near the bottom, until they
are brought into contact with those of the external fire-box, to
which they are attached by copper rivets, as represented at _f_ in
_fig._ 97. The plates forming the bounding surfaces of the two
fire-boxes are fastened together by stays represented at _k_ in
_figs._ 97. and 102. These stays, which are of copper, have a
screw cut upon them through their whole length, and holes are made
through the plates of both fire-boxes tapped with corresponding
threads. The copper screws are then passed through them, and
rivets formed on their heads within and without, as seen in _fig._
102. These screw rivets connect all parts of the plating of the
two fire-boxes which are opposed to each other: they are placed at
about four inches apart over the sides and back of the internal
fire-place and that part of the front which is below the boiler.
[Illustration: _Fig._ 97.
LONGITUDINAL VERTICAL SECTION OF A LOCOMOTIVE ENGINE.]
[Illustration: _Fig._ 98.
LONGITUDINAL VERTICAL SECTION OF THE TENDER.]
[Illustration: _Fig._ 99.
PLAN OF THE WORKING MACHINERY OF A LOCOMOTIVE ENGINE.]
[Illustration: _Fig._ 100.
PLAN OF THE TENDER.]
[Illustration: _Fig._ 101.
ELEVATION OF THE HINDER END OF A LOCOMOTIVE ENGINE.]
[Illustration: _Fig._ 102.
CROSS VERTICAL SECTION OF THE ENGINE THROUGH THE FIRE-BOX.]
[Illustration: _Fig._ 103.
ELEVATION OF THE FOREMOST END OF THE ENGINE.]
[Illustration: _Fig._ 104.
CROSS VERTICAL SECTION OF ENGINE THROUGH THE SMOKE-BOX.]
As the top of the internal fire-box cannot be strengthened by
stays of this kind, ribs of wrought-iron, which are seen in their
length at _l_, in _fig._ 97., and of which an end view is seen in
_fig._ 102., are attached by bolts to it. These ribs are hollowed
out, as seen in _fig._ 97., between bolt and bolt, in order to
break their contact with the roof of the fire-box, and allow a
more free passage to the heat through it. If they were in
continuous contact with the fire-box, the metal composing them
would become more highly heated, and would soon wear out, besides
intercepting heat from the water. This part of the fire-box is
subject to rapid wear, unless care be taken that the level of the
water be preserved at its proper height in the boiler. Even when
the boiler is properly filled, the depth of water above the roof
of the fire-box is not considerable, and on the least neglect the
roof may be exposed to the contact of steam, in which case it will
soon be destroyed.
To prevent accidents arising from this cause, a leaden plug,
[Pg368] represented at _m_, _figs._ 97. and 102., is inserted in
the roof of the internal fire-box. If the water be allowed to
subside, this plug will melt out before the copper is very
injuriously heated, and the steam rushing out at the aperture will
cause the fire to be extinguished.
Copper fire-boxes are almost universally used; but sometimes, from
the consideration of cheapness, the internal fire-box is
constructed of iron.
In the plating which forms the back of the external fire-box, an
oval aperture is formed, as represented in the back view of the
engine, _fig._ 101., for the fire-door _g_. The plating of the
internal fire-box around this aperture is bent at right angles to
meet that of the external fire-box, to which it is fastened by a
row of copper rivets. The fire-door is formed of two plates of
wrought-iron, riveted together with a space of nine inches and a
half between them. The air between these plates being an imperfect
conductor of heat, keeps the outer plate of the fire-door at a
moderate temperature.
In that part of the surface of the internal fire-box which forms
the end of the boiler, holes are made to receive the extremities
of the tubes, by which the air proceeding from the fire is drawn
to the smoke-box at the remote end of the boiler. These tubes are
represented in longitudinal section at E, _fig._ 97., and their
ends are seen in the surface of the internal fire-box in _fig._
102., and in the remote end of the boiler where they terminate in
the smoke-box in _fig._ 104. These tubes are formed of the best
rolled brass, and their thickness in the engine, to which we now
refer, is 1/13 of an inch. After the brass plating is bent into
the form of a tube, and being overlapped, is properly soldered
together, and the edges smoothed off, the tubes are made perfectly
cylindrical by being drawn through a circular steel die.
[Illustration: _Fig._ 105.]
The tube-plates (as those parts of the boiler ends in which the
tubes are inserted are called) are bored with holes in corresponding
positions, truly cylindrical, and corresponding in magnitude to the
tubes, so that the tubes, when passed into them, will be just in
contact with them. The length of the tubes is so regulated, that
when extending from end to end of the boiler, and passing through
the holes, they shall [Pg369] project at each end a little beyond
the holes. The manner of fastening them so as to be water-tight is
as follows:—A steel hoop or ferrule, made slightly conical, a
section of which is exhibited at C. _fig._ 105., the smaller end of
which is a little less than the internal diameter of the tube, but
which increases towards the outer end, is driven in as represented
in the figure. It acts as a wedge, and forces the tube into close
contact with the edges of the hole in the tube-plate.
When particular tubes in a boiler are worn out, and require to be
replaced, their removal is easily effected. It is only necessary
to cut the steel ferrule on the inside, and to bend it off from
contact with the tube, by which means it can be loosened and
withdrawn, and the tube removed.
In the engine to which this description refers there were one
hundred and twenty-four tubes, the external diameter of which was
1-5/8 inch. The distance between tube and tube was 3/4 of an inch.
The number of tubes vary in different engines, some having so many
as one hundred and fifty, while the number in some is less than
ninety. The evaporating power of an engine greatly depends on the
proper number and magnitude of its tubes; and the experience which
engineers have had on railways have led them gradually to increase
the number of tubes, and diminish their magnitude. In the Rocket,
already mentioned as having gained the prize on the opening of the
Liverpool and Manchester Railway, the number of tubes was
twenty-four, and their diameter three inches; but in all the
engines subsequently made their number was augmented, and their
diameter diminished. The practical inconvenience which limits the
size of the tubes is their liability to become choked by cinders
and ashes, which get wedged in them when they are too small, and
thereby obstruct the draft, and diminish the evaporating power of
the boiler. The tubes now in use, of about an inch and a [Pg370]
half internal diameter, not only require to be cleared of the
ashes and cinders, which get fastened in them after each journey,
but it is necessary throughout a journey of any length that the
tubes should be picked and cleaned by opening the fire door at
convenient intervals.
The substitution of brass for copper tubes, which has been already
mentioned as so great an improvement in the construction of
locomotive engines, is ascribed to Mr. Dixon, who suggested them
in 1833, being then the resident engineer of the Liverpool and
Manchester Railway. They are said to last six or eight times as
long as copper tubes of the same dimensions.
When tubes fail, they are usually destroyed by the pressure of the
water crushing them inwards: the water enters through the rent made
in the tube, and flowing upon the fire extinguishes it. When a
single tube thus fails upon a journey, the engine, notwithstanding
the accident, may generally be made to work to the end of its
journey by plugging the ends of the broken tube with hard wood; the
water in contact with which will prevent the fire from burning it
away.
Tubes of the dimensions here referred to weigh about sixteen
pounds, and lose from six to seven pounds before they are worn
out. Their cost is about one pound each.
The tubes act as stays, connecting the ends of the boiler to
strengthen them. Besides these, there are rods of wrought iron
extended from end to end of the boiler above the roof of the
internal fire-place. These rods are represented at _o_ in their
length in _fig._ 97., and an end view of them is seen in _fig._
102. The smoke-box F, _fig._ 97. 104., containing the cylinders,
steam-pipe, and blast-pipe, is four feet wide, and two feet long.
It is formed of wrought iron plates, half an inch thick on the
side next the boiler, and a quarter of an inch elsewhere. The
plates are riveted in the same manner as those of the fire-box
already described. From the top of the smoke-box, which, like the
fire-box, is semi-cylindrical, as seen in elevation in _fig._
103., and in section _fig._ 104., rises the chimney G, fifteen
inches diameter, and formed of 1/8 inch iron plates, riveted and
bound round by hoops. It is flanged to the top of the [Pg371]
smoke-box, as represented in _fig._ 104. Near the bottom of the
smoke-box the working cylinders are placed, side by side, in a
horizontal position, with the slide valves upwards. In the top of
the external fire-box a circular aperture is formed fifteen inches
in diameter, and upon this aperture is placed the steam-dome T
(_figs._ 97. 101, 102.) two feet high, and attached around the
circular aperture by a flange and screw secured by nuts. This
steam dome is made of brass 3/8 inch thick. In stationary boilers,
where magnitude is not limited, it has been already explained,
that the space allowed for steam is sufficiently large to secure
the complete separation of the vapour from the spray which is
mixed with it when it issues immediately from the water. In
locomotive boilers sufficient space cannot be allowed for this,
and the separation of the water from the steam is effected by the
arrangement here represented. A funnel-shaped tube _d′_ (_figs._
97. 102.), with its wide end upwards, rises into the steam-dome,
and reaches nearly to the top of it. This funnel bends towards the
back of the fire-box, and is attached by a flange and screws to
the great steam-pipe S, which traverses the whole length of the
boiler. The steam rising from the boiler fills the steam-dome T,
and descends in the funnel-shaped tube _d′_. The space it has thus
to traverse enables the steam to disengage itself almost
completely from the priming. The wider part of the great
steam-pipe _a_ is flanged and screwed at the hinder end to a
corresponding aperture in the back plate of the fire-box. This
opening is covered by a circular plate, secured by screws, having
a stuffing-box in its centre, of the same kind as is used for the
piston-rods of steam-cylinders. Through this stuffing-box the
spindle _a″_ of the regulator passes, and to its end is attached a
winch _h′_, by which the spindle _a″_ is capable of being turned.
This winch is limited in its play to a quarter of a revolution.
The other end of the spindle _a″_ is attached to a plate _e′_ seen
edgeways in _fig._ 97., and the face of which is seen in _fig._
102.: this circular plate _e_ is perforated with two apertures
somewhat less than quadrants. That part of the plate, therefore,
which remains not pierced forms two solid pieces somewhat greater
than quadrants. This plate is ground so as to move in steam-tight
[Pg372] contact with a fixed plate under it, which terminates at
the wide end of the conical mouth of the steam-pipe S. This fixed
circular plate is likewise pierced with two nearly quadrantal
apertures, corresponding with those in the movable plate _e′_.
When the movable plate _e′_ is turned round by the winch _h′_, the
apertures in it may be made to correspond with those of the fixed
circular plate on which it moves, in which position the steam-pipe
S communicates with the funnel _d′_ by the two quadrantal
apertures thus open. If, on the other hand, the winch _h′_ be
moved from this position through a quarter revolution, then the
quadrantal openings in the movable plate will be brought over the
solid parts of the fixed plate on which it moves, and these solid
parts being a little more than quadrants, while the openings are a
little less, all communication between the steam-pipe S and the
funnel _d′_ will be stopped, for in this case the quadrantal
openings in the fixed and movable plates respectively will be
stopped by the solid parts of these plates. It will be evident
that as the winch _h′_ of the regulator is moved from the former
position to the latter, in every intermediate position the
aperture communicating between the funnel _d′_ and the steam-pipe
S will be less in magnitude than the complete quadrant. It will in
fact be composed of two openings having the form of _sectors_ of a
circle less than a quadrant, and these sectors may be made of any
magnitude, however small, until the opening is altogether closed.
By such means the admission of steam from the boiler to the
steam-pipe S may be regulated by the winch _h′_.
The steam being admitted to the steam-pipe passes through it to
the front end of the boiler, and the pipe being enclosed within
the boiler the temperature of the steam is maintained. The
steam-pipe passing through the tube-plate at the front end of the
boiler is carried to a small distance from the tube-plate in the
same direction, where it is flanged on to a cross horizontal pipe
proceeding to the right and to the left as represented in _fig._
104. This cross pipe is itself flanged to two curved steam-pipes S
(_fig._ 104.), by which the steam is conducted to the valve-boxes
V V. The lower ends of these curved arms are flanged on to the
valve-boxes of the two cylinders [Pg373] at the ends nearest to
the boiler. The opening of one of these is exhibited in the right
hand cylinder in _fig._ 99. By these pipes the steam is conducted
into the valve-boxes or steam-chests, from which it is admitted by
slide-valves to the cylinders to work the pistons in the same
manner as has been already described in the large stationary
engines.
On the upper sides of the cylinders are formed the steam-chests or
valve-boxes, which are exhibited at U (_figs._ 97. 99. 104.).
These are made of cast-iron half an inch thick, and are bolted to
the upper side of each cylinder. At the front end they are also
secured by bolts to the smoke-box, and at the hinder end are
attached to the tube-plate. These valve-boxes communicate with the
passages _m_ and _n_ _fig._ 99. leading to the top and bottom of
the cylinder: these are called the steam-ports. They also
communicate with a passage _o_ leading to the mouth of a curved
horizontal pipe _p′_ connecting the front ends of the two
cylinders, as seen in _figs._ 99. 104. These curved pipes unite in
a single vertical pipe _p_, called the _blast-pipe_, seen in
_figs._ 97. 104.: this vertical pipe becomes gradually small
towards the top, and terminates a little above the base of the
funnel or chimney G. In the valve-box is placed the slide-valve
_v_ to which is attached the spindle _l′_. This spindle moves
through a stuffing-box _k′_, and is worked by gearing, which will
be described hereafter. According to the position given to the
slide, a communication may be opened between the steam-chest, or
the waste-port, and either end of the cylinders. Thus when the
slide is in the position represented in _fig._ 97. the steam-chest
communicates with the front end of the cylinder, while the
waste-port communicates with the hinder end. If, on the other
hand, the spindle _l′_ being pressed forward, move the slide to
its extreme opposite position, the steam-port _n_ would
communicate with the waste-port _o_, while the steam-chest would
communicate with the steam-port _m_, steam would, therefore, be
admitted to the hinder end of the cylinder, while the foremost end
would communicate with the waste-port. It will be perceived that
this arrangement is precisely similar to that of the slide-valves
already described (133.). The slide-valve is represented on a
larger scale in _fig._ 106., where A is the hinder steam-port,
[Pg374] B the foremost steam-port, and C the waste-port. The
surfaces D, separating the steam-ports from the waste-ports, are
called the bars: they are planed perfectly smooth, so that the
surfaces F and G of the slide-valve, also planed perfectly smooth,
may move in steam-tight contact with them. These surfaces are kept
in contact by the pressure of the steam in the steam-chest, by
which the slide-valve is always pressed down. In its middle
position, as represented by the dotted lines in the figure, both
the steam-ports are stopped by the slide-valve, so that at that
moment no steam is admitted to either end of the cylinder. On
either side of this intermediate position the slide has an inch
and a half play, which is sufficient to open successively the two
steam-ports.
[Illustration: _Fig._ 106.]
The cylinders are inserted at one end in the plate of the
smoke-box, and at the other in the tube-plate of the boiler. They
are closed at either end by cast iron covers, nearly an inch
thick, flanged on by bolts and screws. In the cover of the
cylinder attached to the tube-plate is a stuffing-box, in which
the piston rod plays. The metallic pistons used in locomotive
engines do not differ materially from those already described, and
therefore need not be here particularly noticed. From their
horizontal position they have a tendency to wear unequally in the
cylinders, their weight pressing them on one side only; but from
their small magnitude this effect is found to be imperceptible in
practice. In the engine here described the stroke of the piston is
eighteen inches, and this is the most usual length of stroke in
locomotive engines. The piston, in its play, comes at either end
within about half an inch of the inner surface of the covers of
the cylinders, this space being allowed to prevent collision. In
the foremost cover of the cylinder is inserted a cock _q′_ (_fig._
97. 99.), by which any water which may collect in the cylinder by
condensation or priming may be discharged. A cock _r′_ (_fig._
97.), communicating with a small tube proceeding from the branches
of the waste pipe _p′_ (_fig._ 104.), is likewise provided to
discharge from that pipe any water which may be [Pg375] collected
in it. After the steam has been admitted to work the piston
through the slide-valve, and has been discharged through the
waste-port by shifting that valve, it passes through the pipe _p′_
into the blast-pipe _p_, from the mouth of which it issues, with
great force, up the funnel G. When the motion of the engine is
rapid, the steam from the two cylinders proceeds in an almost
uninterrupted current from the blast-pipe, and causes a strong
draft up the chimney. The heated air which passes from the mouths
of the tubes into the smoke-box is drawn up by this current, and a
corresponding draft is produced in the fire-box.
[Illustration: _Fig._ 107.]
The piston-rods Y terminate in a fork, by which they are attached
to cross heads Z, the ends of which are confined by guide-bars A′,
in which they are allowed to play backwards and forwards through a
space equal to the stroke of the piston. To these cross heads Z,
between the prongs of the fork in which the piston terminates, are
attached the foremost ends of the connecting rods B′. These rods
are, therefore, driven backwards and forwards by the motion
imparted to the cross head Z by the piston-rods Y. The connecting
rods B′ are attached at the hinder ends to two cranks formed upon
the axles C′ of the driving wheels D′. These two cranks are formed
upon the axles precisely at right angles to each other. The
left-hand crank is represented in its horizontal position, in
_fig._ 99., and the right-hand crank is seen in its vertical
position. A cranked axle is represented on a larger scale in
_fig._ 107., and the two cranks are seen in a position oblique to
the plane of the figure. As this axle is the instrument by which
the impelling force is conveyed to the load, and as it has to
support a great portion of the weight of the engine, it is
constructed with great strength and precision. It is made all in
one [Pg376] piece, and of the best wrought iron called Back
Barrow, or scrap iron. In the engine here described its extreme
length is six feet and a half, and its diameter is five inches. At
the centre part A it is cylindrical, and is increased to five
inches and a quarter at C, where the cranks are formed. The sides
D of the cranks are four inches thick, and the crank pins B, which
are truly cylindrical, are five inches diameter, and three inches
in length, the brasses at the extremities of the connecting rods
which play upon them having a corresponding magnitude. The
distance from the centre of the crank-pins B to the centre of the
axle A must be exactly equal to half the stroke of the piston, and
is, therefore, in this case precisely nine inches. Upon the parts
F, which are seven inches and a half long, the great driving
wheels are firmly fastened, so as to be prevented from turning or
shaking upon the axle. The axle projects beyond the wheels at G,
where it is reduced to three inches and an eighth diameter. These
projecting parts G are five inches long, having collars at the
outer ends. Brasses are fixed at the outside frame of the engine
which rest upon these projections G of the axle, and upon these
brasses the weight of the engine is supported. The entire axle is
accurately turned in a lathe, and each of the crank-pins B is
likewise turned by suspending the axle on centres corresponding
with the centres of the crank-pins, and made on strong cast iron
arms, which are firmly fixed on the ends of the axle, and project
beyond the cranks so as to balance the axle, and enable it to turn
round on the centre of the crank-pin. The axle is by such means
made perfectly true, and the cranks are made of exactly the proper
length, and precisely at right angles to each other. The corners
of the cranks are champered off, as shown in the figure, and the
ends of the cylindrical parts well rounded out.
The strength and accuracy of construction indispensable in these
cranked axles, in order to make them execute their work, render
them very expensive. Those which are here described cost about
50_l._ each. When properly constructed, however, they are seldom
broken, but are sometimes bent when the engine escapes from the
rails.
The proper motion to admit and withdraw the steam from [Pg377]
either end of the cylinder is imparted to the slide-valves by
eccentrics, in a manner and on a principle so similar to that
already described in large stationary engines, that it will not be
necessary here to enter into any detailed explanation of the
apparatus for communicating this motion, which is exhibited in
plan and section in _figs._ 97. 99. The eccentrics are attached to
the cranked axles at E′ E″. The eccentric E′ imparts motion by a
rod _e″_ to a lever _h″_, formed on an axle extending across the
frame of the engine. This conveys motion to another lever _l″_,
projecting from the same axle. This lever _l″_ is jointed to
horizontal links _m″_, which at the foremost ends are attached to
the spindle _l′_, by which the slide is driven. By these means the
motion received by the eccentric from the great working axle
conveys to the spindle _l′_ an alternate movement backwards and
forwards, and the points at which it is reversed will be regulated
by the position given to the eccentric upon the great axle. The
eccentric is formed in two separate semicircles, and is keyed on
to the great axle, and consequently any position may be given to
it which may be required. The position to be given to the
eccentrics should be such that they shall be at right angles to
their respective cranks, and they should be fixed a quarter of a
revolution behind the cranks so as to move the slides to that
extent in advance of the pistons, since by the position of the
levers _h″_ and _l″_, the motion of the eccentric becomes reversed
before it reaches the valve spindle.
The performance of the engine is materially affected by the
position of the eccentrics on the working axle. The slide should
begin to uncover the steam-port a little before the commencement
of the stroke of the piston, in order that the steam impelling the
piston should be shut off, and the steam about to impel it in the
contrary direction admitted before the termination of the stroke.
Through this small space the steam, therefore, must act in
opposition to the motion of the piston. This is called the _lead_
of the slide, and the extent generally given to it is about a
quarter of an inch. This is accomplished by fixing the eccentrics
not precisely at right angles to the respective cranks, but a
little in advance of that position. The introduction of the steam
to [Pg378] the piston before the termination of the stroke has
the effect of bringing it gradually to rest at the end of the
stroke, and thereby diminishing the jerk or shock produced by the
rapid change of motion. In stationary engines, where the
reciprocations of the engine are slow, the necessity for this
provision does not arise; but in locomotive engines in which the
motion of the piston is changed from four to six times in a
second, it becomes necessary. The steam admitted to the piston
before the termination of the stroke acts as a spring-cushion to
assist in changing its motion, and if it were not applied, the
piston could not be kept tight upon the piston-rod. Another
advantage which is produced by allowing some lead to the slide is
that the waste steam which has just impelled the piston begins to
make its escape through the waste-port before the commencement of
the next stroke, so that when the impelling steam begins to
produce the returning stroke, there is less waste steam on the
other side of the piston to resist it.
When the motion of the engine is very rapid, the resistance of the
waste steam, as it escapes from the blast-pipe to the piston, has
been generally supposed to be very considerable, though we are not
aware of any direct experiments by which its amount has been
ascertained. In the account of the locomotive engine which has
been here described, supplied by Mr. Stephenson for the last
edition of Tredgold on the Steam Engine, he states, that the
average resisting pressure of the waste steam throughout the
stroke is 6 lbs. per square inch, when running at the usual rate
of from 25 to 28 miles an hour, and that at greater velocities
this negative pressure has been found to increase to more than
double that amount. No experiments are, however, cited from which
this inference has been drawn.
It has been also thought that the pressure of steam upon the
piston in the cylinder, at high velocities, is considerably below
the pressure of steam in the boiler; but this has not been, so far
as we are informed, ascertained by any satisfactory experimental
test. Mr. Stephenson likewise states, that this loss of pressure,
causes the negative pressure or resistance of the waste steam to
amount to [Pg379] from 30 to 40 per cent. of the positive
pressure upon the piston when the engine is running very fast, and
that therefore the power of the engine is diminished nearly one
half.
But it will be perceived that besides the uncertainty which
attends the estimate of the actual amount of pressure on the
piston compared with the pressure of steam in the boiler, the
inference here drawn does not appear to be compatible with what
has been already proved respecting the mechanical effect of steam.
No change of pressure which may take place between the boiler and
the cylinder can affect the practical efficacy of the steam. As
the steam passes through the engine, whatever change of pressure
it may be subject to, it still remains common steam; and though
its pressure may be diminished, its volume being increased in a
nearly equal proportion, its mechanical effect will remain the
same. The power of the engine, therefore, estimated as it ought to
be, by the whole mechanical effect produced, will not be altered
otherwise than by the effect of the increased resistance produced
by the blast-pipe. What that resistance is, we repeat, has not, so
far as we know, been ascertained by direct experiment, and there
are circumstances attending it which render it probable that, even
at high velocities, it is less in amount than Mr. Stephenson's
estimate.
The position of the eccentrics which is necessary to make the
pistons drive the engine forward must be directly the reverse of
that which would cause them to drive the engine backwards. To be
able, therefore, to reverse the motion of the engine, it would
only be necessary to be able to reverse the position of the
eccentrics, which may be accomplished by either of two expedients.
_First_, The eccentrics may be capable of revolving on the great
working axle, and also of sliding upon it through a small space.
Their revolution on the axle may be checked by letting a pin
attached to a collar fastened on the axle fall into a hole on the
side of the eccentric. Such a pin will drive the eccentric round
with the axle, and the position of this pin and the hole will
determine the position of the eccentric with reference to the
crank. At a short distance [Pg380] on the other side of the
eccentric may be a corresponding collar with a pin in the opposite
position. By moving the eccentric longitudinally on the axle, the
former pin may be withdrawn from the hole, and the latter allowed
to fall into the hole on the other side. Proper mechanism may be
provided by which the position of the eccentric may thus be
reversed in reference to the crank, and by such means the motion
of the engine may be reversed.
_Secondly_, Supposing the eccentrics which drive the engine
forward to be immovably fixed upon the axle, two other eccentrics
may be provided attached to other parts of the same axle, and
having a position exactly the reverse with reference to the
cranks. Proper mechanism may be provided, by which either or both
pairs of eccentrics may be thrown in or out of gear. Such are the
means adopted in the engine which has been already described. The
eccentrics for driving the engine backwards are placed outside the
cranks at F′ F″. A hand lever _w″_ _fig._ 101. is provided, by
which the engine man may throw either pair of eccentrics into or
out of gear, so as to make the engine work either backwards or
forwards.
[Illustration: 108.]
[Illustration: 109.]
[Illustration: 110.]
As all the moving parts of the engine require to be constantly
lubricated with oil to diminish the friction, and keep them cool,
oil-cups for this purpose are fixed upon them. In some engines
these oil-cups are attached separately to all the moving parts: in
others they are placed near each other in a row on the boiler, and
communicate by small tubes with the several parts required to be
lubricated. One of these is requisite for each end of the
connecting rods, for each of the guides of the piston-rods, for
the piston-rod itself, the spindle of the slide-valve, and other
parts. An elevation of one of these oil-cups is shown in _fig._
108., a vertical section in _fig._ 109., and horizontal plan in
_fig._ 110. The cup A is made of brass with a cover B. This cover
has a piece projecting from it turning upon a pin in a socket C at
the side of the cup A, and square at the end, resting upon a small
spring at the bottom of the socket to hold it either open or shut.
In the bottom of the [Pg381] cup is inserted an iron tube D
extending nearly to the top. This tube projects from the bottom of
the cup, where it is tapped for the purpose of fixing the cup on
the part of the engine which it is intended to lubricate. The hole
into which the cup is screwed communicates with the rubbing
surface, and some cotton thread is passed through the tube dipping
into the oil in the cup at the one end and touching the moving
part at the other. This thread acts as a siphon, and constantly
drops oil on the rubbing surface.
[Illustration: _Fig._ 111.]
The tender is a carriage attached behind the engine and close to it,
carrying coke for the supply of the furnace, and water for the
boiler. The coke is contained in the space R″, (_fig._ 98. 100.)
surrounded by a tank I″ containing water to feed the boiler. The
feed for the boiler is conducted from the tank through a pipe
descending downwards and in a curved direction, P″ Q″, _fig._ 98.,
and connected with a horizontal pipe K, _fig._ 97. A cock is
provided at P″, by which the supply of water to this pipe may be cut
off at pleasure. Another cock is provided at _t′_, _fig._ 97., where
the curved pipe joins the horizontal pipe by which the quantity of
water supplied to K may be regulated by opening the cock more or
less fully. The handle of this cock rises through the floor of the
engine, so that the engineer may regulate it at discretion. The pipe
K being conducted under the engine, as represented in _fig._ 97.,
terminates in a vertical pipe, of greater diameter, containing two
valves, both of which open upwards, and between these valves to this
vertical pipe is attached a force-pump, by which the water is drawn
from the horizontal pipe K into the vertical pipe K′, and from the
latter is driven into a delivery-pipe by which it is forced into the
boiler. The details of the interior of this feed-pump are
represented on a larger scale in _fig._ 111. The extremity of the
horizontal pipe K′ is represented in section at H, where it is
joined on by a screw to the bottom of the vertical pipe which is
represented in _fig._ 97. at K, and which is here represented in
section. The vertical pipe, represented in _fig._ 97. consists of
several parts screwed together by nuts and bolts passing through
flanges. The lowest piece I is attached by a flange to the piece L:
within these is contained the valve Q resting in a seat made
conical, so that the ball [Pg382] which forms the valve shall rest
in water-tight contact with it. The ball is turned and ground to an
accurate sphere, and whatever position it assumes upon its seat its
contact will be perfect. It is guided in its upward and downward
motion by several vertical bars which confine it, and which are
united at the top, so as to limit the upward motion of the ball. A
screw V′ is inserted in the bottom of the piece I, by removing which
access can be obtained to the valve. The piece L is secured to the
short pipe G by nuts and bolts passed through a flange. The pipe G
is cast upon the end of the feed-pump A. On the foremost end of this
feed-pump is constructed a stuffing-box C of the usual form, having
a gland D forced against packing by nuts and screws E. The plunger B
is turned so as to be truly cylindrical, and moves in water-tight
contact through the gland D. The plunger not being in contact with
the inner surface of the pump-barrel A, the latter need not be
ground. The horizontal rod by which the plunger B is driven is
attached at its foremost extremity to an arm which projects from the
rod of the steam-piston, and consequently this plunger is moved
through a space equal to the stroke of the steam-piston. In this
case that space is eighteen inches. The [Pg383] upper end of the
vertical tube G is attached by screws and a flange to a piece P
containing a valve R similar in all respects to the lower valve Q,
and like it opening upwards. A screw V is introduced at the top by
which access may be obtained to this valve. This screw also presses
on the crown of the guides of the valve, so as to hold it down by
regulated pressure. At the side of this upper piece P is inserted a
horizontal tube M connected with the end of the delivery-pipe N.
This latter is continued to the boiler with which it communicates at
the fire-box. When the plunger B is drawn out of the pump-barrel A,
the spherical valve Q being relieved from its downward pressure is
raised, and water passes from the pipe H through the valve Q into
the vertical pipe G; the lower valve Q then closes and stops the
return of the water. The plunger B returning into the pump-barrel A
then forces the water against the upper valve R and drives it
through the delivery-tube N, from which its return is prevented by
the valve R. When the delivery-tube N is filled with water
throughout its whole length, every stroke of the plunger will
evidently drive into the boiler a volume of water equal to the
magnitude of a part of the plunger eighteen inches in length.
Until within the last few years, locomotive engines were supported
on only four wheels; they are, however, now almost universally
supported on six, the driving wheels being in the middle. To give
greater security to the position of the engine between the rails
it is usual to construct flanges on the tires of all the six
wheels. Mr. Stephenson, however, has been in the practice of
constructing the driving wheels without flanges, and with tires
truly cylindrical, depending on the flanges of the two pairs of
smaller wheels to maintain the engine between the rails. The
wheels of the engine here described are constructed in this
manner. The driving wheels D′ are fixed on the cranked axle C′,
and are five feet in diameter. The other wheels L′ M′, the one
being placed immediately behind the smoke-box, and the other
immediately behind the fire-box, are each three feet six inches in
diameter, and have a flange upon their tires, which running on
the [Pg384] inside of each rail keeps the engine between the
rails. Each pair of these small wheels, like the driving-wheels,
is fixed upon their axle. The axles are 3-5/8 inches diameter, and
project beyond the wheels, the projecting part supporting the
frame of the engine and turning in brasses. Upon these brasses
rest springs, which bear the whole weight of the engine. These
springs having nothing between them and the road but the wheels
and axles intercept and equalise the sudden shocks produced by the
rapid motion upon the road.
When an engine is required for the transport of very heavy loads,
such as those of merchandise, the adhesion of one pair of working
wheels is found to be insufficient, and, in such cases, one of the
two pairs of wheels L′ M′ is made of the same diameter as the
wheels which are placed upon the working axle, and a bar is
attached to points on the outside of the wheels at equal distances
from their centre, connecting them in such a manner that any force
applied to make one pair of wheels revolve must necessarily impart
the same motion to the other pair. By such means the force of the
steam is made to drive both pairs of wheels, and consequently a
proportionally increased adhesion is obtained.
The velocity which an engine is capable of imparting to the load
which it draws depends upon the rate at which the pistons are
capable of being moved in the cylinders. By every motion of each
piston backwards and forwards one revolution of the driving wheels
is produced, and by each revolution of the driving wheels,
supposing them not to slip upon the rails, the load is driven
through a distance upon the road equal to their circumference. As
the two cylinders work together, it follows, that a quantity of
steam sufficient to fill four cylinders supplied by the boiler to
the engine will move the train through a distance equal to the
circumference of the driving wheels; and in accomplishing this,
each piston must move twice from end to end of the cylinder; each
cylinder must be twice filled with steam from the boiler; and that
steam must be twice discharged from the cylinder through the
blast-pipe into the chimney.
[Pg401] If the driving wheels be five feet in diameter their
circumference will be fifteen feet seven inches. To drive a train
with a velocity of thirty miles an hour, it will be necessary that
the engine should be propelled through a space of forty-five feet
per second. To accomplish this with five-feet wheels they must be
therefore made to revolve at the rate of very nearly three
revolutions per second; and as each revolution requires two
motions of the piston in the cylinder, it follows that each piston
must move three times forwards and three times backwards in the
cylinder in a second; that steam must be admitted six times per
second from the steam-chest to each cylinder, and discharged six
times per second from each cylinder into the blast-pipe. The
motion, therefore, of each piston, supposing it to be uniform,
must divide a second into six equal parts, and the puffs of the
blast-pipe in the chimney must divide a second into twelve equal
parts. The motion of the slides and other reciprocating parts of
the machinery must consequently correspond.
This motion of the reciprocating parts of the machinery being
found to be injurious to it, and to produce very rapid wear,
attempts have been made to remedy the defect, and to obtain
greater speed with an equal or diminished rate of motion of the
piston, by the adoption of driving wheels of greater diameter, and
on several of the great lines of railway the magnitude of the
wheels for the passenger-engines have been increased to five feet
and a half and six feet diameter; but such engines have not been
sufficiently long in use to afford grounds for forming a practical
estimate of their effects. Experiments of a much bolder
description have, however, been tried on one of the great lines of
railway by the adoption of driving wheels of much greater
diameter. In some cases their magnitude has been increased even to
ten feet; but from various experiments to which these engines have
been submitted by myself and others, as well as from the
experience which appears to be obtained from the results of their
ordinary work, it does not appear that any advantages have
attended them, and they have been accordingly for the most part
abandoned.
The pressure of steam in the boiler is limited by two
safety-valves, [Pg402] represented in _fig._ 97. at N and O. The
valve at N is under the control of the engineer, but the valve at
O is inaccessible to him. The structure of the safety-vale
represented at N is exhibited on a larger scale in _fig._ 112.,
which represents its section, and _fig._ 113., which shows a plan
of the valve-seat with the valve removed. The valve A, which is
made of brass, is mitred round the edge at an angle of 45°, and
has a spindle, or stalk B, cast upon it, projecting downwards from
the middle of it. The valve-seat C is also made of brass, and cast
with a flange at the bottom to attach it to the boiler. The mitred
surface of the valve is ground into the valve-seat, so as to rest
in steam-tight contact with it. Across the valve-seat, which is
two and a half inches in diameter, is cast a thin piece D, seen in
plan in _fig._ 113. and in section in _fig._ 112. which extends
from the top to the bottom, and has a longitudinal hole through
it, in which the spindle B of the valve works: by this hole it is
guided when it rises from its seat. A projection E is cast upon
the seat of the valve, in which a standard F is inserted. This
standard is forked at the top, and receives the end of a lever G,
which turns in it upon a centre. A rod H is jointed to this lever
by another pin at three inches from the former, and the lower end
of this rod, ground to a point, presses upon the centre of the
valve A. At the other end of the lever, which is broken off in
_fig._ 112., at a distance of three feet from the centre pin,
inserted in the fork of the pillar F, the rod of a common
spring-balance _w_, _fig._ 101., is attached by a finger-nut _n_.
The bottom of this spring-balance is secured on to the fire-box.
This balance is screwed up by the finger-nut on the valve-lever
until the required pressure on the lever is produced through the
medium of the rod H, this pressure being generally fifty pounds
per square inch above the atmosphere. When the pressure of the
steam in the boiler exceeds this, the valve A is raised from its
seat, and the steam escapes.
[Illustration: _Fig._ 112.]
[Illustration: _Fig._ 113.]
It is evident that the sliding weight by which the pressure
[Pg403] of the safety-valve is sometimes regulated in stationary
engines would not be admissible in a locomotive engine, since the
motion of the engine would constantly jolt it up and down, and
cause the steam to escape. One of the disadvantages attending the
use of the spring-valve is that it cannot be opened to let the
steam escape without increasing its force, so that the steam, when
escaping, must really have a greater pressure than that to which
the valve has been previously adjusted. The longer the lever is,
the greater will be this difference of pressure, inasmuch as a
given elevation of the pin governing the rod H would cause a
proportionally greater motion in that end of the lever attached to
the spring.
The second safety-valve O is enclosed in a case, so that it is
inaccessible, and its purpose is to limit the power of the
engineer to increase the pressure of steam in the boiler. This
valve is similar in construction to the former, but instead of
being held down by a lever, is pressed upon by several small
elliptical springs placed one above another over the valve, and
held down by a screw which turns in a frame Y, fixed into the
valve-seat. By this screw the pressure on the valve can be
adjusted to any required degree; and if the open safety-valve be
screwed down to a greater pressure, the steam will begin to escape
from this second valve.
Also in the case where the boiler produces surplus steam faster
than its escape can be effected at the valve N, the pressure will
sometimes be increased until the valve O is opened, and its escape
will take place from both valves.
The whole weight of the engine bears upon those parts of the six
axles R′, _fig._ 99., which project beyond the wheels. Boxes are
formed in which these parts of the axles turn, and through the
medium of which the weight of the engine rests upon them. Over
these boxes are constructed oil or grease cups, by means of which
the axles are constantly lubricated. It is usual to lubricate the
axles of the engine itself with oil: the axles of the tender, and
other coaches and waggons, are lubricated with a mixture of oil
and tallow. In the middle of the box in which the axle turns, and
between the two oil-cups, is cast a socket, in which the end of
the spindle on [Pg404] which the spring presses rests. The
springs are composed of a number of steel-plates, laid, in the
usual manner, one above the other, increasing in length upwards.
In the engine here described, the plates forming the springs of
the driving wheels are thirteen in number, each of which is four
inches in width, and 5/16ths of an inch in thickness. The springs
upon the other wheels are three inches in width. The springs of
the driving wheels are below the axle, while those of the smaller
wheels are above it.
Buffers D″ are placed behind the tender, which act upon a spring C
(_fig._ 100.), to break the collision, when the waggons or
carriages strike upon the tender, and similar buffers are attached
to all passenger-coaches. Some of these buffers are constructed
with a system of springs similar to C, but more elastic, and
combined in greater number under the framing of the carriage, so
that a considerable play is allowed to them. In some cases the
rods of the buffers are made to act upon strong spiral springs
inserted in the sides of the framing of the carriage. This
arrangement gives greater play to the buffers; and as every coach
in a train has several buffers, the combined effect of these is
such, that a considerable shock given to either end of the train
may be rendered harmless by being spent upon the elasticity of
these several systems of springs.
In order to give notice of the approach of a train, a
steam-whistle Z′, _fig._ 97. 101., is placed immediately above the
fire-box at the back of the engine. This is an apparatus composed
of two small hemispheres of brass, separated one from the other by
a small space. Steam is made to pass through a hollow space
constructed in the lower hemisphere, and escapes from a very
narrow circular opening round the edge of that hemisphere, rushing
up with a force proportionate to its pressure. The edge of the
upper hemisphere presented downwards encounters this steam, and an
effect is produced similar to the action of air in organ pipes. A
shrill whistle is produced, which can be heard at a very
considerable distance, and, differing from all ordinary sounds, it
never fails to give timely notice of the approach of a train.
The water tank I″, _fig._ 98. 100., which is constructed on the
tender, is formed of wrought-iron plates 1/8 of an inch thick,
[Pg405] riveted at the corners by angle iron already described.
This tank is 9 feet long, 6-3/4 feet wide, and 2-1/4 feet deep.
The top is covered with a board K″, and a raised platform N″ is
constructed behind, divided into three parts, covered with leads,
which open on hinges. The middle lid covers an opening to the tank
by which water is let in: the lids at either side cover boxes in
which are contained the tools necessary to be carried with the
engine. The curved pipe P″, _fig._ 98., leading from the bottom of
the tank to the pipe Q″, is of copper. The pipe Q″, connecting the
latter with the feed-pipe K′, _fig._ 99., is sometimes formed of
leather or India-rubber cloth, having a spiral spring on the
inside to prevent it from collapsing. It is necessary that this
pipe Q″ should have a power of yielding to a sufficient degree to
accommodate itself to the inequalities of motion between the
engine and tender. A metal pipe is sometimes used, supplied with a
double ball and socket, and a telescopic joint, having sufficient
play to allow for the lateral and longitudinal inequalities of
motion of the engine and tender. The weight of an engine, such as
that here described, supplied with its proper quantity of water
and fuel, is about 12 tons: the tender, when empty, weighs about
3-1/4 tons; and when filled with water and fuel its weight is 7
tons. The tank contains 700 gallons of water, and the tender is
capable of carrying about 800 weight of coke. This supply is
sufficient for a trip of from thirty to forty miles with an
ordinary load.
(198.) It is not usual to express the power of locomotive engines
in the same manner as that of other engines by the term
horse-power. Indeed, until the actual amount of resistance opposed
to these machines, under the various circumstances in which they
are worked, shall be ascertained with some degree of precision, it
is impossible that their power or efficiency can be estimated with
any tolerable degree of approximation. The quantity of water
evaporated, and passed in steam through the cylinders, supplies a
major limit to the power exerted; but even this necessary element
for the calculation of the efficacy of these machines has not been
ascertained by a sufficiently extensive course of observation and
experiment. Mr. Stephenson states, that the engine which [Pg406]
has been here described is capable of evaporating 77 cubic feet of
water per hour, while the early locomotives could only evaporate
16 cubic feet per hour. This evaporation, however, is inferior to
that which I have ascertained myself to be produced by engines in
regular operation on some of the northern railways. In an
experiment made in July, 1839, with the Hecla engine, I found that
the evaporation in a trip of ninety-five miles, from Liverpool to
Birmingham, was at the rate of 93·2 cubic feet per hour, and in
returning the same distance it was at the rate of 85·7 cubic feet
per hour, giving a mean of 89 cubic feet per hour nearly. The
Hecla weighed 12 tons; and its dimensions and proportions
corresponded very nearly with those of the engine above described.
In a course of experiments which I made upon the engines then in
use on the Grand Junction Railway in the autumn of 1838 I found
that the ordinary evaporating power of these engines varied from
eighty to eighty-five cubic feet per hour.
Engines of much greater dimensions, and consequently of greater
evaporating power, are used on the Great Western Railway. In the
autumn of 1838 experiments were made upon these engines by Mr.
Nicholas Wood and myself, when we found that the most powerful
engine on that line, the North Star, drawing a load of 110-1/2
tons gross, engine and tender inclusive, at 30-1/2 miles an hour,
evaporated 200 cubic feet of water per hour. The same engine
drawing a load of 194-1/2 tons at 18-1/2 miles an hour evaporated
141 cubic feet per hour, and when drawing 45 tons at 38-1/2 miles
an hour evaporated 198 cubic feet of water per hour.
It has been already shown that a cubic foot of water evaporated
per hour produces a gross amount of mechanical force very little
less than two-horse power, and consequently the gross amount of
mechanical power evolved in these cases by the evaporation of the
locomotive boilers will be very nearly twice as many horse-power
as there are cubic feet of water evaporated per hour. Thus the
evaporation of the Hecla, in the experiments made in July, 1839,
gave a gross power of about one hundred and eighty horses, while
the evaporation of the North Star gave a power of about four
hundred horses. In stationary engines about half the gross
[Pg407] power evolved in the evaporation is allowed for waste,
friction, and other sources of resistance not connected with the
load. What quantity should be allowed for this in locomotive
engines is not yet ascertained, and therefore it is impossible to
state what proportion of the whole evaporation is to be taken as
representing the useful horse-power.
(199.) The great uniformity of resistance produced by the traction
of carriages upon a railway is such as to render the application
of steam power to that purpose extremely advantageous. So far as
this resistance depends on mechanical defects, it is probably
rendered as uniform as is practicable, and in proportion to the
quantity of load carried is reduced to as small an amount as it is
likely to attain under any practicable circumstances. Until a
recent period this resistance was ascribed altogether, or nearly
so, to mechanical causes. The inequalities of the road-surface,
the friction of the axles of the wheels in their bearings, and the
various sources of resistance due to the machinery of the engine,
being the principal of these resistances, were for the most part
independent of the speed with which the train was moved; and it
was accordingly assumed in all calculations respecting the power
of locomotive engines that the resistance would be practically the
same whatever might be the speed of the train. It had been well
understood that so far as the atmosphere might offer resistance to
the moving power this would be dependent on the speed, and would
increase in a very high ratio with the speed; but it was
considered that the part of the resistance due to this cause
formed a fraction of the whole amount so insignificant that it
might be fairly disregarded in practice, or considered as a part
of the actual computed resistance taken at an average speed.
It has been, until a late period, accordingly assumed that the
total amount of resistance to railway trains which the locomotive
engines have had to overcome was about the two hundred and
fiftieth part of the gross weight of the load drawn: some
engineers estimated it at a two hundred and twentieth; others at a
two hundred and fiftieth; others at a three hundred and thirtieth
part of the load; and the two hundred and fiftieth part of the
gross load drawn may perhaps be [Pg408] considered as a mean
between these much varying estimates. What the experiments were,
if any, on which these rough estimates were based, has never
appeared. Each engineer formed his own valuation of this effect,
but none produced the experimental grounds of their opinion. It
has been said that the trains run down the engine, or that the
drawing chains connecting the engine slacken in descending an
inclination of sixteen feet in a mile, or 1/330. Numerous
experiments, however, made by myself, as well as the constant
experience now daily obtained on railways, show that this is a
fallacious opinion, except at velocities so low as are never
practised on railways.
(200.) In the autumn of 1838 a course of experiments was commenced
at the suggestion of some of the proprietors of the Great Western
Railway Company, with a view to determine various points connected
with the structure and the working of railways. A part of these
experiments were intended to determine the mean amount of the
resisting force opposed to the moving power, and this part was
conducted by me. After having tried various expedients for
determining the mean amount of resistance to the moving power, I
found that no method gave satisfactory results except one founded on
observing the motion of trains by gravity down steep inclined
planes. When a train of waggons or coaches is placed upon an
inclined plane so steep that it shall descend by its gravity without
any moving power, its motion, when it proceeds from a state of rest,
will be gradually accelerated, and if the resistance to that motion
was, as it has been commonly supposed to be, uniform and independent
of the speed, the descent would be uniformly accelerated: in other
words, the increase of speed would be proportional to the time of
the motion. Whatever velocity the train would gain in the first
minute, it would acquire twice that velocity at the end of the
second minute, three times that velocity at the end of the third
minute, and so on; and this increase of velocity would continue to
follow the same law, however extended the plane might be. That such
would be the law which the descending motion of a train would follow
had always been supposed, up to the time of the experiments now
referred to; and it was even maintained by some that [Pg409] such a
law was in strict conformity with experiments made upon railways and
duly reported. The first experiments instituted by me at the time
just referred to afforded a complete refutation of this doctrine. It
was found that the acceleration was not uniform, but that with every
increase of speed the acceleration was lessened. Thus if a certain
speed were gained by a train in one second when moving at five miles
an hour, a much less speed was gained in one second when moving ten
miles an hour, and a comparatively small speed was gained in the
same time when moving at fifteen miles an hour, and so on. In fact,
the augmentation of the rate of acceleration appeared to diminish in
a very rapid proportion as the speed increased: this suggested to me
the probability that a sufficiently great increase of speed would
destroy all acceleration, and that the train would at length move at
a uniform velocity. In effect, since the moving power which impels a
train down an inclined plane of uniform inclination is that fraction
of the gross weight of the train which acts in the direction of the
plane, this moving power must be necessarily invariable; and as any
acceleration which is produced must arise from the excess of this
moving power over the resistance opposed to the motion of the train,
from whatever causes that resistance may arise, whenever
acceleration ceases, the moving force must necessarily be equal to
the resistance; and therefore, when a train descends an inclined
plane with a uniform velocity, the gross resistance to the motion of
the train must be equal to the gross weight of the train resolved in
the direction of the plane; or, in other words, it must be equal to
that fraction of the whole weight of the train which is expressed by
the inclination of the plane. Thus if it be supposed that the plane
falls at the rate of one foot in one hundred, then the force
impelling the train downwards will be equal to the hundredth part of
the weight of the train. So long as the resistance to the motion of
the train continues to be less than the hundredth part of its
weight, so long will the motion of the train be accelerated; and the
more the hundredth part of the weight exceeds the resistance, the
more rapid will the acceleration be; and the less the hundredth part
of the weight [Pg410] exceeds the resistance, the less rapid will
the acceleration be. If it be true that the amount of resistance
increases with the increase of speed, then a speed may at length be
attained so great that the amount of resistance to the motion of the
train will be equal to the hundredth part of the weight. When that
happens, the moving power of a hundredth part of the weight of the
train being exactly equal to the resistance to the motion, there is
no excess of power to produce acceleration, and therefore the motion
of the train will be uniform.
Founded on these principles, a vast number of experiments were
made on planes of different inclinations, and with loads of
various magnitudes; and it was found, in general, that when a
train descended an inclined plane, the rate of acceleration
gradually diminished, and at length became uniform; that the
uniform speed thus attained depended on the weight, form, and
magnitude of the train and the inclination of the plane; that the
same train on different inclined planes attained different uniform
speeds—on the steeper planes a greater speed being attained. From
such experiments it followed, contrary to all that had been
previously supposed, that the amount of resistance to railway
trains had a dependence on the speed; that this dependence was of
great practical importance, the resistance being subject to very
considerable variation at different speeds, and that this source
of resistance arises from the atmosphere which the train
encounters. This was rendered obvious by the different amount of
resistance to the motion of a train of coaches and to that of a
train of low waggons of equal weight.
The former editions of this work having been published before the
discovery which has resulted from these experiments, the average
amount of resistance to railway trains, there stated, and the
conclusions deduced therefrom, were in conformity with what was
then known. It was stated that the resistance to the moving power
was practically independent of the speed, and on level rails was
at the average rate of about seven pounds and a half per ton. This
amount would be equivalent to the gravitation of a load down an
inclined plane falling 1/300, and consequently in ascending such a
plane the moving power would have to encounter twice [Pg411] the
resistance opposed to it on a level. As it was generally assumed
that a locomotive-engine could not advantageously vary its
tractive power beyond this limit, it was therefore inferred that
gradients (as inclinations are called) ought not to be constructed
of greater steepness than 1/300. It was supposed that in
descending gradients more steep than this the train would be
accelerated and would require the use of the brake to check its
motion, while in ascending such planes the engine would be
required to exert more than twice the ordinary tractive power
required on level rails. As the resistance produced by the air was
not taken into consideration, no distinction was made between
heavy trains of goods presenting a frontage and magnitude bearing
a small proportion to their gross weight and lighter trains of
passenger-coaches presenting great frontage and great magnitude in
proportion to their weight. The result of the experiments above
explained leads to inferences altogether at variance with those
which have been given in former editions of the present work, and
which were then universally admitted by railway engineers. The
tendency of the results of these experiments show that low
gradients on railways are not attended with the advantageous
effects which have been hitherto ascribed to them; that, on the
contrary, the resistance produced by steeper gradients can be
compensated by slackening the speed, so that the power shall be
relieved from as much atmospheric resistance by the diminution of
velocity as is equal to the increased resistance produced by the
gravity of the plane which is ascended. And, on the other hand, in
descending the plane the speed may be increased until the
resistance produced by the atmosphere is increased to the same
amount as that by which the train is relieved of resistance by the
declivity down which it moves. Thus, on gradients, the inclination
of which is confined within practical limits, the resistance to
the moving-power may be preserved uniform, or nearly so, by
varying the velocity.
(201.) The series of experiments which have established these
general conclusions have not yet been sufficiently extended
and varied to supply a correct practical estimate of the limit
which it would be most advantageous to impose upon the [Pg412]
gradients of railways; but it is certain that railways may be laid
down, without practical disadvantage, with gradients considerably
steeper than those to which it has been hitherto the practice to
recommend as a limit.
The principle of compensation by varied speed being admitted, it
will follow that the time of transit between terminus and terminus
of a line of railway laid down with gradients, varying from twenty
to thirty feet a mile, will be practically the same as it would be
on a line of the same length constructed upon a dead level; and
not only will the time of transport be equal, but the quantity of
moving power expended will not be materially different. The
difference between the circumstances of the transport in the two
cases will be merely that, on the undulating line, a varying
velocity will be imparted to the train and a varying resistance
opposed to the moving power; while on the level line the train
would be moved at a uniform speed, and the engine worked against a
uniform resistance. These conclusions have been abundantly
confirmed by the experiments made in last July with the Hecla
engine above referred to. The line of railway between Liverpool
and Birmingham on which the experiment was made extended over a
distance of ninety-five miles, and the gradients on which the
effects were observed varied from a level to thirty feet per mile,
a great portion of the line being a dead level. The following
table shows the uniform speed with which the train ascended and
descended the several gradients, and also the mean of the ascent
and descent in each case, as well as the speed upon the level
parts of the line:—
——————————————————————————————————————————————————
| Speed. |
|———————————————-————————————————————————
Gradient.| Ascending. | Descending. | Mean.
——————————————————————————————————————————————————
One in |Miles per hour.|Miles per hour.|
177 | 22·25 | 41·32 | 31·78
265 | 24·87 | 39·13 | 32·00
330 | 25·26 | 37·07 | 31·16
400 | 26·87 | 36·75 | 31·81
532 | 27·35 | 34·30 | 30·82
590 | 27·37 | 33·16 | 30·21
650 | 29·03 | 32·58 | 30·80
| | |————————
Level | | | 30·93
——————————————————————————————————————————————————
[Pg413] From this table it is apparent that the gradients do
possess the compensating power with respect to speed already
mentioned. The discrepancies existing among the mean values of the
speed are only what may be fairly ascribed to casual variations in
the moving power. The experiment was made under favourable
circumstances: little disturbance was produced from the
atmosphere; the day was quite calm. In the same experiment it was
found that the water evaporated varied very nearly in proportion
to the varying resistance, and the amount of that evaporation may
be taken as affording an approximation to the mean amount of
resistance. Taking the trip to and from Birmingham over the
distance of 190 miles, the mean evaporation per mile was 3·36
cubic feet of water. The volume of steam produced by this quantity
of water will be determined approximately by calculating the
number of revolutions of the driving wheels necessary to move the
engine one mile. The driving wheels being 5 feet in diameter,
their circumference was 15·7 feet, and consequently in passing
over a mile they would have revolved 336·3 times. Since each
revolution consumes four cylinders full of steam, the quantity of
steam supplied by the boiler to the cylinders per mile will be
found by multiplying the contents of the cylinder by four times
336·3, or 1345·2.
The cylinders of the Hecla were 12-1/2 inches diameter, and 18
inches in length, and consequently their contents were 1·28 cubic
feet for each cylinder: this being multiplied by 1345·2 gives
1721·86 or 1722 cubic feet of steam per mile. It appears,
therefore, that supposing the priming either nothing or
insignificant, which was considered to be the case in these
experiments, 3·36 cubic feet of water produced 1722 cubic feet of
steam, of the density worked in the cylinders. The ratio,
therefore, of the volume of this steam to that of the water
producing it, was 1722 to 3·36, or 512·5 to 1. The pressure of
steam of this density would be 54·5 pounds per square inch.[34]
Such, therefore, was the limit of the average total pressure of
the steam in the cylinders. In this experiment the safety-valve of
the boiler was screwed down to 60 pounds per square [Pg414] inch
above the atmospheric pressure, which was therefore the major
limit of the pressure of steam in the boiler; but as the actual
pressure in the boiler must have been less than this amount, the
difference between the pressure in the cylinder and boiler could
not be ascertained. This difference, however, would produce no
effect on the moving power of the steam, since the pressure of
steam in the cylinders obtained by the above calculation is quite
independent of the pressure in the boiler, or of any source of
error except what might arise from priming. The pressure of 54·5
pounds per square inch, calculated above, being the total pressure
of the steam on the pistons, let 14·5 pounds be deducted from it,
to represent the atmospheric pressure against which the piston
must act, and the remaining 40 pounds per square inch will
represent the whole available force drawing the train and
overcoming all the resistances arising from the machinery of the
engine, including that of the blast-pipe. The magnitude of a
12-1/2 inch piston being 122·7 square inches, the total area of
the two pistons would be 245·2 square inches, and the pressure
upon each of 40 pounds per inch would give a total force of 9816
on the two pistons. Since this force must act through a space of
three feet, while the train is impelled through a space of 15·7
feet, it must be reduced in the proportion of 3 to 15·7, to obtain
its effect at the point of contact of the wheels upon the rails:
this will give 1875 pounds as the total force exerted in the
direction of the motion of the train. The gross weight of the
train being 80 tons, including the engine and tender, this would
give a gross moving force along the road of about 23·4 pounds per
ton of the gross load, this force being understood to include all
the resistances due to the engine. This resistance corresponds to
the gravitation of a plane rising at the rate of 1/95, and
therefore it appears that such would be the inclination of the
plane by the gravitation of which the gross resistance would be
doubled, instead of such inclination being about 1/300, as has
been hitherto supposed.
Since the remarkable and unexpected results of this series of
experiments became known various circumstances were brought to
light, which were before unnoticed, and which [Pg415] abundantly
confirm them. Among these may be mentioned the fact, that in
descending the Madeley plane, on the Grand Junction Railway, which
falls for above three miles at the rate of twenty-nine feet a
mile, the steam can never be entirely cut off. But, on the other
hand, to maintain the necessary speed in descending, the power of
the engine is always necessary. As this plane greatly exceeds that
which would be sufficient to cause the free motion of the train
down it, the power of the engine expended in descending it,
besides all that part of the gravitating power of the plane which
exceeds the resistance due to friction and other mechanical causes
must be worked against the atmosphere.
This estimate of the resistance is also in conformity with the
results of a variety of experiments made by me with trains of
different magnitudes down inclined planes of various inclinations.
(202.) In laying out a line of railway the disposition of the
gradients should be such as to preserve among them as uniform a
character as is practicable, for the weight and power of the
engine must necessarily be regulated by the general steepness of
the gradients. Thus if upon a railway which is generally level,
like that between Liverpool and Manchester, one or two inclined
planes of a very steep character occur, as happens upon that line,
then the engine which is constructed to work upon the general
gradients of the road is unfit to draw the same load up those
inclinations which form an exception to the general character of
the gradients. In such cases some extraordinary means must
generally be provided for surmounting those exceptionable
inclinations. Several expedients have been proposed for this
purpose, among which the following may be mentioned:—
1. Upon arriving at the foot of the plane the load is divided, and
the engine carries it up in several successive trips, descending
the plane unloaded after each trip. The objection to this method
is the delay which it occasions—a circumstance which is
incompatible with a large transport of passengers. From what has
been stated, it would be necessary, when the engine is fully
loaded on a level, to divide its load into two or more parts, to
be successively [Pg416] carried up when the incline rises 52 feet
per mile. This method has been practised in the transport of
merchandise occasionally, when heavy loads were carried on the
Liverpool and Manchester line, upon the Rainhill incline.
2. A subsidiary or assistant locomotive engine may be kept in
constant readiness at the foot of each incline, for the purpose of
aiding the different trains, as they arrive, in ascending. The
objection to this method is the cost of keeping such an engine
with its boiler continually prepared, and its steam up. It is
necessary to keep its fire continually lighted, whether employed
or not; otherwise, when the train would arrive at the foot of the
incline, it should wait until the subsidiary engine was prepared
for work. In cases where trains would start and arrive at stated
times, this objection, however, would have less force. This method
is at present generally adopted on the Liverpool and Manchester
line.
3. A fixed steam-engine may be erected on the crest of the
incline, so as to communicate by ropes with the train at the foot.
Such an engine would be capable of drawing up one or two trains
together, with their locomotives, according as they would arrive,
and no delay need be occasioned. This method requires that the
fixed engine should be kept constantly prepared for work, and the
steam continually up in the boiler.
4. In working on the level, the communication between the boiler
and the cylinder in the locomotives may be so restrained by
partially closing the throttle-valve, as to cause the pressure
upon the piston to be less in a considerable degree than the
pressure of steam in the boiler. If under such circumstances a
sufficient pressure upon the piston can be obtained to draw the
load on the level, the throttle-valve may be opened on approaching
the inclined plane, so as to throw on the piston a pressure
increased in the same proportion as the previous pressure in the
boiler was greater than that upon the piston. If the fire be
sufficiently active to keep up the supply of steam in this manner
during the ascent, and if the rise be not greater in proportion
than the power thus obtained, the locomotive will draw the load up
the incline without further assistance. It is, however, to be
observed, that in this case [Pg417] the load upon the engine must
be less than the amount which the adhesion of its working wheels
with the railroad is capable of drawing; for this adhesion must be
adequate to the traction of the same load up the incline,
otherwise, whatever increase of power might be obtained by opening
the throttle-valve, the drawing wheels would revolve without
causing the load to advance. This method has been generally
practised upon the Liverpool and Manchester line in the transport
of passengers; and, indeed, it is the only method yet discovered
which is consistent with the expedition necessary for that species
of traffic.
In the practice of this method considerable aid may be derived
also by suspending the supply of feeding water to the boiler
during the ascent. It will be recollected that a reservoir of cold
water is placed in the tender which follows the engine, and that
the water is driven from this reservoir into the boiler by a
forcing pump, which is worked by the engine itself. This pump is
so constructed that it will supply as much cold water as is equal
to the evaporation, so as to maintain constantly the same quantity
of water in the boiler. But it is evident, on the other hand, that
the supply of this water has a tendency to check the rate of
evaporation, since in being raised to the temperature of the water
with which it mixes it must absorb a considerable portion of the
heat supplied by the fire. With a view to accelerate the
production of steam, therefore, in ascending the inclines, the
engine man may suspend the action of the forcing pump, and thereby
stop the supply of cold water to the boiler; the evaporation will
go on with increased rapidity, and the exhaustion of water
produced by it will be repaid by the forcing pump on the next
level, or still more effectually on the next descending incline.
Indeed the feeding pump may be made to act in descending an
incline, if necessary, when the action of the engine itself is
suspended, and when the train descends by its own gravity, in
which case it will perform the part of a brake upon the descending
train.
5. The mechanical connexion between the piston of the cylinder and
the points of contact of the working wheels with the road may be
so altered, upon arriving at the incline, as to [Pg418] give the
piston a greater power over the working wheels. This may be done
in an infinite variety of ways, but hitherto no method has been
suggested sufficiently simple to be applicable in practice; and
even were any means suggested which would accomplish this, unless
the intensity of the impelling power were at the same time
increased, it would necessarily follow that the speed of the
motion would be diminished in exactly the same proportion as the
power of the piston over the working wheels would be increased.
Thus, on the inclined plane, which rises fifty-five feet per mile,
upon the Liverpool line, the speed would be diminished to nearly
one fourth of its amount upon the level.
[Illustration]
FOOTNOTES:
[30] Some of the preceding observations on inland transport,
as well as other parts of the present chapter, appeared in
articles written by me in the _Edinburgh Review_ for October,
1832, and October, 1834.
[31] Wood on Railroads, 2d edit.
[32] The cost of coke has risen considerably since the date of
this report.
[33] I am indebted to the enlarged edition of Tredgold on the
Steam Engine, published by Mr. Weale, for the drawings of this
engine. The details of the machine are very fully given in
that work, the description of them being supplied by Mr.
Stephenson himself.
[34] See Table of Pressures, Temperatures, and Volumes, in
appendix.
[Pg419]
[Illustration]
CHAP. XII.
LOCOMOTIVE ENGINES ON TURNPIKE ROADS.
RAILWAYS AND STONE ROADS COMPARED. — MR. GURNEY'S STEAM
ENGINE. — CONVENIENCE AND SAFETY OF STEAM CARRIAGES. —
HANCOCK'S STEAM ENGINE. — OGLE'S STEAM ENGINE. — TREVETHICK'S
INVENTION. — DR. CHURCH'S STEAM ENGINE.
(203.) We have hitherto confined our observations on steam-power,
as a means of transport by land, to its application on railways.
But modern speculation has not stopped there; various attempts
have been made, and attended with more or less success, to work
steam-carriages on common roads. The mere practicability of this
project had long been regarded as very questionable; but enough
has been done to show that the only doubt which can attend it, is
as to whether it can be profitably resorted to, as a means of
transport, and this question [Pg420] has been materially affected
by the recent extension of railways. In comparing the effect of a
stone road with an iron railway, there are two circumstances which
give great superiority and advantage to the latter: first, the
resistance opposed by a railway to the moving power, no matter
what that moving power may be, is considerably less in proportion
to the load than on a stone road. The average resistance on a good
level stone road, to the motion of carriages drawn at the speed
usually attained by the application of horse-power, may be taken
at about a thirty-sixth part of the load, while the resistance to
a load drawn upon a railway _at the same speed_ probably does not
amount to a tenth part of this resistance. Thus the moving power,
whatever it may be, would produce on a railway ten times the
useful effect which it would produce on a stone road; secondly,
the resistance which is opposed to the moving power on a level
railway is much more uniform than on a stone road, and,
consequently, the moving power is less subjected to jerks and
inequalities. This renders the application of inanimate power more
easy on the railway. Those inequalities of surface which increase
the amount of resistance on stone roads as compared with railways
also produce a jolting motion in the carriage, to counteract
which, the use of springs become necessary. These springs render
the motion of that part of the carriage which rests upon them
different from that part of the carriage which supports them; and
in the application of steam-machinery it becomes necessary so to
connect the moving power with the wheels that the machinery may
have one motion, and the wheels which are put in mechanical
connexion with that machinery, and driven by it, shall have
another motion. This, it is true, is the case with locomotive
engines on railways; but owing to the greater smoothness and
equality of the railway surface the difference between the motion
of the carriage body suspended on springs and that of the wheels
is much less than it would be on a stone road.
But besides the greater smoothness of railways compared with stone
roads, the latter have another disadvantage, the effects of which
have probably been exaggerated by those who are opposed to this
application of steam-power. One of the [Pg421] laws of adhesion
long since developed by experiment, and established as a principle
of practical science, is that the adhesion is greater between
surfaces of the same than between surfaces of a different kind. Thus
between two metals of the same kind, the adhesion corresponding to
any given pressure is greater than between two metals of different
kinds; between two metals of any sort the adhesion is greater than
between metal and stone, or between metal and wood. Hence, the
wheels of steam-carriages running on a railroad have a greater
adhesion with the road, and therefore offer a greater resistance to
slip round without the advance of the carriage, than wheels would
offer on a turnpike road; for on a railroad the iron tire of the
wheel rests in contact with the iron rail, while on a common road
the iron tire rests in contact with the surface of stone, or
whatever material the road may be composed of. Besides this, the
dust and loose matter which necessarily collect on a common road,
when pressed between the wheels and the solid base of the road, act
somewhat in the manner of rollers, and give the wheels a greater
facility to slip than if the road were swept clean, and the wheels
rested in immediate contact with its hard surface. The truth of this
observation is illustrated on the railroads themselves, where the
adhesion is found to be diminished whenever the rails are covered
with any extraneous matter, such as dust or moist clay. Although the
adhesion of the wheels of a carriage with a common road, however, be
less than those of the wheels of a steam-carriage with a railroad,
yet still the actual adhesion on turnpike roads is greater in amount
than has been generally supposed, and is quite sufficient to propel
carriages drawing after them loads of large amount.
The relative facility with which carriages are propelled on
railroads and turnpike-roads equally affects any moving power,
whether that of horses or steam engines; and whether loads be
propelled by the one power or the other, the railroad, as compared
with the turnpike-road, will always possess the same proportionate
advantage; and a given amount of power, whether of the one kind or
the other, will always perform a quantity of work less in the same
proportion on a [Pg422] turnpike-road than on a rail-road. But, on
the other hand, the expense of original construction, and of
maintaining the repairs of a rail-road, is to be placed against the
certain facility which it offers to draught.
In the attempts which have been made to adapt locomotive engines to
turnpike-roads, the projectors have aimed at the accomplishment of
two objects: first, the construction of lighter and smaller engines;
and, secondly, increased power. These ends, it is plain, can only be
attained, with our present knowledge, by the production of steam of
very high temperature and pressure, so that the smallest volume of
steam shall produce the greatest possible mechanical effect. The
methods of propelling the carriage have been in general similar to
that used in the railroad engines, viz. either by cranks placed on
the axles, the wheels being fixed upon the same axles, or by
connecting the piston rods with the spokes of the wheels. In some
carriages, the boiler and moving power, and the body of the carriage
which bears the passengers, are placed on the same wheels. In
others, the engine is placed on a separate carriage, and draws after
it the carriage which transports the passengers, as is always the
case on railways.
The chief difference between the steam engines used on railways, and
those adapted to propel carriages on turnpike roads, is in the
structure of the boiler. In the latter it is essential that, while
the power remains undiminished, the boiler should be lighter and
smaller. The accomplishment of this has been attempted by various
contrivances for so distributing the water as to expose a
considerable quantity of surface in contact with it to the action of
the fire: spreading it in thin layers on flat plates; inserting it
between plates of iron placed at a small distance asunder, the fire
being admitted between the intermediate plates; dividing it into
small tubes, round which the fire has play; introducing it between
the surfaces of cylinders placed one within another, the fire being
admitted between the alternate cylinders,—have all been resorted to
by different projectors.
(204.) First and most prominent in the history of the application of
steam to the propelling of carriages on turnpike roads stands the
name of Mr. Goldsworthy Gurney, a medical [Pg423] gentleman, and
scientific chemist, of Cornwall. In 1822, Mr. Gurney succeeded Dr.
Thompson as lecturer on chemistry at the Surrey Institution; and, in
consequence of the results of some experiments on heat, his
attention was directed to the project of working steam-carriages on
common roads; and he subsequently devoted his exertions in
perfecting a steam-engine capable of attaining the end he had in
view.
The mistake which so long prevailed in the application of
locomotives on railroads, and which, as we have shown, materially
retarded the progress of that invention, was shared by Mr. Gurney.
Without reducing the question to the test of experiment, he took for
granted, in his first attempts, that the adhesion of the wheels with
the road was too slight to propel the carriage. He was assured, he
says, by eminent engineers, that this was a point settled by actual
experiment. It is strange, however, that a person of his quickness
and sagacity did not inquire after the particulars of these "actual
experiments." So, however, it was; and, taking for granted the
inability of the wheels to propel, he wasted much labour and skill
in the contrivance of levers and propellers, which acted on the
ground in a manner somewhat resembling the feet of horses, to drive
the carriage forward. After various fruitless attempts of this kind,
the experience acquired in the trials to which they gave rise at
last forced the truth upon his notice, and he found that the
adhesion of the wheels was not only sufficient to propel the
carriage heavily laden on level roads, but was capable of causing it
to ascend all the hills which occur on ordinary turnpike-roads. In
this manner it ascended all the hills between London and Barnet,
London and Stanmore, Stanmore Hill, Brockley Hill, and mounted Old
Highgate Hill, the last at one point rising one foot in nine.
[Illustration: _Fig._ 114.]
[Illustration: _Fig._ 115.]
The boiler of Mr. Gurney's engine is so constructed, that there is
no part of it in which metal exposed to the action of the fire is
out of contact with water. If it be considered how rapidly the
action of an intense furnace destroys metal when water is not
present to prevent the heat from accumulating, the advantage of
this circumstance will be appreciated. In the boiler of Mr.
Gurney, the grate-bars [Pg424] themselves are tubes filled with
water, and form, in fact, a part of the boiler itself. This boiler
consists of three strong metal cylinders placed in a horizontal
position one above the other. A section, made by a perpendicular
or vertical plane, is represented in _fig._ 114. The ends of the
three cylinders just mentioned are represented at D, H, and I. In
the side of the lowest cylinder D are inserted a row of tubes, a
ground plan of which is represented in _fig._ 115. These tubes,
proceeding from the side of the lowest cylinder D, are inclined
[Pg425] slightly upwards, for a reason which I shall presently
explain. From the nature of the section, only one of these tubes
is visible in _fig._ 114. at C. The other extremities of these
tubes at A are connected with the same number of upright tubes,
one of which is shown at E. The upper extremities G of these
upright tubes are connected with another set of tubes K, equal in
number, proceeding from G, inclining slightly upwards, and
terminating in the second cylinder H.
[Illustration: _Fig._ 116.]
An end view of the boiler is exhibited in _fig._ 116., where the
three cylinders are expressed by the same letters. Between the
cylinders D and H there are two tubes of communication B, and two
similar tubes between the cylinders H and I. From the nature of
the section these appear only as a single tube in _fig._ 114. From
the top of the cylinder I proceeds a tube N, by which steam is
conducted to the engine.
It will be perceived that the space F is enclosed on every side by
a grating of tubes, which have free communication with the
cylinders D and H, which cylinders have also a free communication
with each other by the tubes B. It follows, [Pg426] therefore,
that if water be supplied to the cylinder I, it will descend
through the tubes, and first filling the cylinder D and the tubes
C, will gradually rise in the tubes B and E, will next fill the
tubes K and the cylinder H. The grating of water-pipes C E K forms
the furnace, the pipes C being the fire-bars, and the pipes E and
K being the back and roof of the stove. The fire-door, for the
supply of fuel, appears at M, fig. 116. The flue issuing between
the tubes F is conducted over the tubes K, and the flame and hot
air are carried off through a chimney. That portion of the heat of
the burning fuel, which in other furnaces destroys the bars of the
grate, is here expended in heating the water contained in the
tubes C. The radiant heat of the fire acts upon the tubes K,
forming the roof of the furnace, on the tube E at the back of it,
and partially on the cylinders D and H, and the tubes B. The draft
of hot air and flame passing into the flue at A acts upon the
posterior surfaces of the tubes E, and the upper sides of the
tubes K, and finally passes into the chimney.
As the water in the tubes C E K is heated, it becomes specifically
lighter than water of a less temperature, and consequently
acquires a tendency to ascend. It passes, therefore, rapidly into
H. Meanwhile the colder portions descend, and the inclined
positions of the tubes C and K give play to this tendency of the
heated water, so that a prodigiously rapid circulation is
produced, when the fire begins to act upon the tubes. When the
water acquires such a temperature that steam is rapidly produced,
steam-bubbles are constantly formed in the tubes surrounding the
fire; and if these remained stationary in the tubes, the action of
the fire would not only decompose the steam, but render the tubes
red hot, the water not passing through them to carry off the heat.
But the inclined position of the tubes, already noticed,
effectually prevents this injurious consequence. A steam-bubble,
which is formed either in the tubes C or K, having a tendency to
ascend proportional to its lightness as compared with water,
necessarily rushes upwards; if in C towards A, and if in K towards
H. But this motion of the steam is also aided by the rapid
circulation of the water which is continually maintained [Pg427]
in the tubes, otherwise it might be possible, notwithstanding the
levity of steam compared with water, that a bubble might remain in
a narrow tube without rising. To bring the matter to the test of
experiment, I have connected two cylinders, such as D and H, by a
system of glass tubes, such as represented at C E K. The rapid and
constant circulation of the water was then made evident: bubbles
of steam were formed in the tubes, it is true; but they passed
with great rapidity into the upper cylinder, and rose to the
surface, so that the glass tubes never acquired a higher
temperature than that of the water which passed through them.
Every part of the boiler being cylindrical, it has the form which,
mechanically considered, is most favourable to strength, and
which, within given dimensions, contains the greatest quantity of
water. It is also free from the defects arising from unequal
expansion, which are found to be most injurious in tubular
boilers. The tubes C and K can freely expand in the direction of
their length, without being loosened at their joints, and without
straining any part of the apparatus; the tubes E, being short, are
subject to a very slight degree of expansion; and it is obvious
that the long tubes, with which they are connected, will yield to
this without suffering a strain, and without causing any part of
the apparatus to be loosened.
When water is converted into steam, any foreign matter which may
be combined with it is disengaged, and is deposited on the bottom
of the vessel in which the water is evaporated. All boilers,
therefore, require occasional cleansing, to prevent the crust thus
formed from accumulating; and this operation, for obvious reasons,
is attended with peculiar difficulty in tubular boilers. In the
case before us, the crust of deposited matter would gather and
thicken in the tubes C and K, and if not removed, would at length
choke them. But besides this, it would be attended with a still
worse effect; for, being a bad conductor, it would intercept the
heat in its transit from the fire to the water, and would cause
the metal of the tube to become unduly heated. Mr. Gurney of
course foresaw this inconvenience, and contrived an ingenious
chemical method of removing it, by occasionally injecting [Pg428]
through the tubes such an acid as would combine with the deposit,
and carry it away. This method was effectual; and although its
practical application was found to be attended with difficulty in
the hands of common workmen, Mr. Gurney was persuaded to adhere to
it by the late Dr. Wollaston, until experience proved the
impossibility of getting it effectually performed, under the
circumstances in which boilers are commonly used. Mr. Gurney then
adopted a method of removing the deposit by mechanical means.
Opposite the mouths of the tubes, and on the other side of the
cylinders D and H, are placed a number of holes, which, when the
boiler is in use, are stopped by pieces of metal screwed into
them. When the tubes require to be cleaned, these stoppers are
removed, and an iron scraper is introduced through the holes into
the tubes, which, being passed backwards and forwards, removes the
deposit.
In these engines the draught through the furnace was produced by
projecting the waste steam up the chimneys as is practised in
railway engines; a method so perfectly effectual, that it is
unlikely to be superseded by any other. The objection which has
been urged against it in locomotive engines, working on
turnpike-roads, is, that the noise which it produces has a
tendency to frighten horses.
In the engines on the Liverpool road, the steam is allowed to pass
directly from the eduction pipe of the cylinder to the chimney,
and it there escapes in puffs corresponding with the alternate
motion of the pistons, and produces a noise, which, although
attended with no inconvenience on the railroad, would perhaps be
objectionable on turnpike-roads. In the engine used in Mr.
Gurney's steam-carriage, the steam which passes from the cylinders
is conducted to a receptacle, which he calls a blowing box. This
box serves the same purpose as the upper chamber of a smith's
bellows. It receives the steam from the cylinders in alternate
puffs, but lets it escape into the chimney in a continued stream
by a number of small jets. Regular draught is by this means
produced, and no noise is perceived. Another exit for the steam is
also provided, by which the conductor is enabled to increase or
diminish, or to suspend altogether, the draught [Pg429] in the
chimney, so as to adapt the intensity of the fire to the
exigencies of the road. This is a great convenience in practice;
because on some roads a draught is scarcely required, while on
others a powerful blast is indispensable.
Connected with this blowing box is another apparatus of
considerable practical importance. The pipe through which the
feeding water is conducted from the tank is carried through this
blowing box, within which it is coiled in a spiral form, so that
an extensive thread of the water is exposed to the heat of the
waste steam which has escaped from the cylinders, and which is
enclosed in this blowing box. In passing through this pipe the
feeding water is raised from the ordinary temperature of about 60°
to the temperature of 212°. Fuel is thus economised and weight
diminished; but there is another still greater advantage attending
this process. The feeding water in the worm just mentioned, while
it takes up the heat from the surrounding steam in the blowing
box, condenses a part of the waste steam, which is thence
conducted to the tank, from which the feeding water is pumped.
When steam is generated so rapidly as is necessarily the case in
locomotive boilers, it rises with great violence in numerous
bubbles from the bottom of the boiler to the surface of the water,
and puts the liquid into a state of foaming turbulence not unlike
the sea in a storm. As the steam rushes from the surface into the
upper part of the boiler, under these circumstances, it carries
with it a spray by which water is scattered in minute subdivision
among the steam, and floats there like the spray which rises from
the base of a cascade. If the steam be conducted immediately to
the cylinder from the boiler in this state, it will carry with it
the water which is thus suspended in it, which will pass through
the cylinder, and finally be driven into the atmosphere upon the
returning stroke of the piston. The hot water thus carried off
possesses none of the mechanical properties of steam, and is
wholly inefficient as a moving power, and is therefore an
extensive source of the waste of heat. In every boiler, some means
should be provided for the separation of the water thus suspended
in the steam, before the steam is conducted to the cylinder. In
ordinary boilers, the large space which [Pg430] remains above the
surface of the water serves this purpose. The steam being there
subject to no agitation or disturbance, the water mechanically
suspended in it descends by its own gravity, and leaves pure steam
in the upper part. In the small tubular boilers, this has been a
matter, however, of greater difficulty. The contracted space in
which the ebullition takes place causes the water to be mixed with
the steam in a greater quantity than could happen in common
boilers; and the want of the same steam-room renders the
separation of the water from the steam a matter of some
difficulty. These inconveniences have been attempted to be
overcome by various contrivances. I have already described the
rapid and regular circulation effected by the arrangement of the
tubes. By this a regularity in the currents is established, which
has a tendency to diminish the mixture of water with the steam. In
addition to this, a method of separation is provided in the vessel
I, which is a strong iron cylinder of some magnitude, placed out
of the immediate influence of the fire. A partial separation of
the steam from the water takes place in the cylinder H; and the
steam with the water mechanically suspended in it, technically
called moist steam, rises into the _separator_ I. Here, being free
from all agitation and currents, and being, in fact, quiescent,
the particles of water fall to the bottom, while the pure steam
remains at the top. This separator, therefore, serves all the
purposes of the steam-room above the surface of the water in the
large plate boilers. The dry steam is thus collected and ready for
the supply of the engine through the tube N, while the water,
which is disengaged from it, is collected at the bottom of the
separator, and is conducted through the tube T to the lowest
vessel D, to be again circulated through the boiler.
The pistons of the engine work on the axles of the hind wheels of
the carriage which bears the engine, by cranks, as in the
locomotives on the Manchester railway, so that the axle is kept in
a constant state of rotation while the engine is at work. The
wheels placed on this axle are not permanently fixed or keyed upon
it, as in the Manchester locomotives; but they are capable of
turning upon it in the same manner as ordinary carriage wheels.
Immediately within [Pg431] these wheels there are fixed upon the
axles two projecting spokes or levers, which revolve with the
axle, and which take the position of two opposite spokes of the
wheel. These may be occasionally attached to the wheel or detached
from it; so that they are capable of compelling the wheels to turn
with the axle, or leaving the axle free to turn independently of
the wheel, or the wheel independent of the axle, at the pleasure
of the conductor. It is by these levers that the engine is made to
propel either or both of the wheels. If both pairs of spokes are
thrown into connexion with the wheels, the crank shaft or axle
will cause both wheels to turn with it, and in that case the
operation of the carriage is precisely the same as those of the
locomotives already described upon the Liverpool and Manchester
line; but this is rarely found to be necessary, since the adhesion
of one wheel with the road is generally sufficient to propel the
carriage, and consequently only one pair of these fixed levers are
used, and the carriage propelled by only one of the two hind
wheels. The fore wheels of the carriage turn upon a pivot similar
to those of a four-wheeled coach. The position of these wheels is
changed at pleasure by a pinion and circular rack, which is moved
by the conductor, and in this manner the carriage is guided with
precision and facility.
The force of traction necessary to propel a carriage upon common
roads must vary with the variable quality of the road, and
consequently the propelling power, or the pressure upon the pistons
of the engine, must be susceptible of a corresponding variation; but
a still greater variation becomes necessary from the undulations and
hills which are upon all ordinary roads. This necessary change in
the intensity of the impelling power is obtained by restraining the
steam in the boiler by the throttle-valve, as already described in
the locomotive engines on the railroad. This principle, however, is
carried much further in the present case. The steam in the boiler
maybe at a pressure of from 100 to 200 lbs. on the square inch;
while the steam on the working piston may not exceed 30 or 40 lbs.
on the inch. Thus an immense increase of power is always at the
command of the conductor; so that when a hill is encountered, or a
rough piece of road, [Pg432] he is enabled to lay on power
sufficient to meet the exigency of the occasion.
The two difficulties which have been always apprehended in the
practical working of steam-carriages upon common roads are, first,
the command of sufficient power for hills and rough pieces of
road; and, secondly, the apprehended insufficiency of the adhesion
of the wheels with the road to propel the carriage. The former of
these difficulties has been met by allowing steam of very great
pressure to be constantly maintained in the boiler with perfect
safety. As to the second, all experiments tend to show that there
is no ground for the supposition that the adhesion of the wheels
is in any case insufficient for the purposes of propulsion. Mr.
Gurney states, that he has succeeded in driving carriages thus
propelled, up considerable hills on the turnpike roads about
London. He made a journey to Barnet with only one wheel attached
to the axle, which was found sufficient to propel the carriage up
all hills upon that road. The same carriage, with only one
propelling wheel, also went to Bath, and surmounted all the hills
between Cranford Bridge and Bath, going and returning.
A double stroke of the piston produces one revolution of the
propelling wheels, and causes the carriage to move through a space
equal to the circumference of those wheels. It will therefore be
obvious, that the greater the diameter of the wheels, the better
adapted the carriage is for speed; and, on the other hand, wheels
of smaller diameter are better adapted for power. In fact, the
propelling power of an engine on the wheels will be in the inverse
proportion of their diameter. In carriages designed to carry great
weights at a moderate speed, smaller wheels will be used; while in
those intended for the transport of passengers at considerable
velocities, wheels of at least 5 feet diameter are most
advantageous.
(205.) Among the numerous popular prejudices to which this new
invention has given rise, one of the most mischievous in its
effects and most glaring in its falsehood, is the notion that
carriages thus propelled are more injurious to roads than
carriages drawn by horses. This error has been successfully
exposed in the evidence taken before the committee of the [Pg433]
House of Commons upon steam carriages. It is there demonstrated,
not only that carriages thus propelled do not wear a turnpike road
more rapidly than those drawn by horses, but that, on the other
hand, the wear by the feet of horses is far more rapid and
destructive than any which could be produced by the wheels of
carriages. Steam carriages admit of having the tires of the wheels
broad, so as to act upon the road more in the manner of rollers,
and thereby to give consistency and firmness to the material of
which the road is composed. The driving wheels being proved not to
slip upon the road, do not produce any effects more injurious than
the ordinary rolling wheels; consequently the wear occasioned by a
steam carriage upon a road, is not more than that produced by a
carriage drawn by horses, of an equivalent weight and the same or
equal tires; but the wear produced by the pounding and digging of
horses' feet in draught is many times greater than that produced
by the wear of any carriage. Those who still have doubts upon this
subject, if there be any such persons, will be fully satisfied by
referring to the evidence which accompanies the report of the
committee of the House of Commons, printed in October, 1831.
The weight of machinery necessary for steam carriages is sometimes
urged as an objection to their practical utility. Mr. Gurney
states, that, by successive improvements in the details of the
machinery, the weight of his carriages, without losing any of the
propelling power, may be reduced to 35 cwt., exclusive of the
load, and fuel and water: but thinks that it is possible to reduce
the weight still further.
A steam carriage constructed by Mr. Gurney, weighing 35 cwt.,
working for 8 hours, is found, according to his statement, to do
the work of about 30 horses. He calculates that the weight of his
propelling carriage, which would be capable of drawing 18 persons,
would be equal to the weight of 4 horses; and the carriage in
which these persons would be drawn would have the same weight as a
common stage coach capable of carrying the same number of persons.
Thus the weight of the whole—the propelling carriage and the
carriage for passengers taken together—would be the same [Pg434]
with the weight of a common stage coach, with 4 horses inclusive.
There are two methods of applying locomotives upon common roads to
the transport of passengers or goods; the one is by causing the
locomotive to carry, and the other to draw the load; and different
projectors have adopted the one and the other method. Each is
attended with its advantages and disadvantages. If the same
carriage transport the engine and the load, the weight of the
whole will be less in proportion to the load carried; also a
greater pressure may be produced on the wheels by which the load
is propelled. It is also thought that a greater facility in
turning and guiding the vehicle, greater safety in descending the
hills, and a saving in the original cost, will be obtained. On the
other hand, when the passengers are placed in the same carriage
with the engine, they are necessarily more exposed to the noise of
the machinery and to the heat of the boiler and furnace. The
danger of explosion is so slight, that, perhaps, it scarcely
deserves to be mentioned; but still _the apprehension_ of danger
on the part of the passengers, even though groundless, should not
be disregarded. This apprehension will be obviously removed or
diminished by transferring the passengers into a carriage separate
from the engine; but the greatest advantage of keeping the engine
separate from the passengers is the facility which it affords of
changing one engine for another in case of accident or derangement
on the road, in the same manner as horses are changed at the
different stages: or, if such an accident occur in a place where a
new engine cannot be procured, the load of passengers may be
carried forward by horses, until it is brought to some station
where a locomotive may be obtained. There is also an advantage
arising from the circumstance, that when the engines are under
repair, or in process of cleaning, the carriages for passengers
are not necessarily idle. Thus the same number of carriages for
passengers will not be required when the engine is used to draw as
when it is used to carry.
In case of a very powerful engine being used to carry great loads,
it would be quite impracticable to place the engine [Pg435] and
loads on four wheels, the pressure being such as no turnpike road
could bear. In this case it would be indispensably necessary to
place a part of the load at least upon separate carriages to be
drawn by the engine.
In the comparison of carriages propelled by steam with carriages
drawn by horses, there is no respect in which the advantage of the
former is so apparent as the safety afforded to the passenger.
Steam power is under the most perfect control, and a carriage thus
propelled is capable of being guided with the most admirable
precision. It is also capable of being stopped almost suddenly,
whatever be its speed: it is capable of being turned within a
space considerably less than that which would be necessary for
four-horse coaches. In turning sharp corners, there is no danger,
with the most ordinary care on the part of the conductor. On the
other hand, horse power, as is well known, is under very imperfect
control, especially when horses are used adapted to that speed
which at present is generally considered necessary for the
purposes of travelling. "The danger of being run away with and
overturned," says Mr. Farey, in his evidence before the House of
Commons, "is greatly diminished in a steam coach. It is very
difficult to control four such horses as can draw a heavy stage
coach ten miles an hour, in case they are frightened or choose to
run away; and, for such quick travelling, they must be kept in
that state of courage that they are always inclined to run away,
particularly down hill, and at sharp turns in the road. Steam
power has very little corresponding danger, being perfectly
controllable, and capable of having its power reversed, to retard
in going down hill. It must be carelessness that would occasion
the overturning of a steam carriage. The chance of breaking down
has been hitherto considerable, but it will not be more than in
stage coaches when the work is truly proportioned and properly
executed. The risk from explosion of the boiler is the only new
cause of danger, and that I consider not equivalent to the danger
from horses."
That the risk of accident from explosion is extremely slight, may
be proved by the fact that the railway between Liverpool and
Manchester has now been in operation for about ten [Pg436] years,
and that other railways more extensive in length have been worked
for a considerable time, and that no instance has ever yet
occurred of an accident to passengers from the explosion of a
boiler. Generally these machines, when they fail, are attended
with no other effect than the extinction of the fire, by the water
of the boiler flowing in upon it. I am not aware of more than one
instance, in which a serious accident has been produced by
explosion; and in that instance, the sufferers were only the
engineer and stoker. In the steam-engine of Mr. Gurney, the
carriage is drawn after the engine, as represented in _fig._ 117.
[Illustration: _Fig._ 117.]
[Illustration: _Fig._ 118.]
(206.) In the boiler to be used in the steam carriage projected by
Mr. Walter Hancock, the subdivision of the water is accomplished
by dividing a case or box by a number of [Pg437] thin plates of
metal, like a galvanic battery, the water being allowed to flow
between every alternate pair of plates, at E, _fig._ 118., and the
intermediate spaces H forming the flue through which the flame and
hot air are propelled.
In fact, a number of thin plates of water are exposed on both
sides to the most intense action of flame and heated air; so that
steam of a high pressure is produced in great abundance and with
considerable rapidity. The plates forming the boiler are bolted
together by strong iron ties, extending across the boiler, at
right angles to the plates, as represented in the figure. The
distance between the plates is two inches.
There are ten flat chambers of this kind for water, and
intermediately between them ten flues. Under the flues is the
fire-place, or grate, containing six square feet of fuel in vivid
combustion. The chambers are all filled to about two thirds of
their depth with water, and the other third is left for steam. The
water chambers, throughout the whole series, communicate with each
other both at top and bottom, and are held together by two large
bolts. By releasing these bolts, at any time, the chambers fall
asunder; and by screwing them up they may be all made tight again.
The water is supplied to the boiler by a forcing-pump, and the
steam issues from the centre of one of the flues at the top.
These boilers are constructed to bear a pressure of 400 or 500
lbs. on the square inch; but the average pressure of the steam on
the safety valve is from 60 to 100. There are 100 square feet of
surface in contact with the water exposed to the fire. The stages
which such an engine performs are eight miles, at the end of which
a fresh supply of fuel and water are taken in. It requires about
two bushels of coke for each stage.
The steam carriage of Mr. Hancock differs from that of Mr. Gurney
in this—that in the former the passengers and engine are all
placed on the same carriage. The boiler is placed behind the
carriage; and there is an engine-house between the boiler and the
passengers, the latter being placed in the fore part of the
vehicle; so that all the machinery is behind them. The carriages
are adapted to carry 14 [Pg438] passengers, and weigh, exclusive
of their load, about 3-1/2 tons, the tires of the wheels being
about 3-1/2 inches in breadth. Mr. Hancock states, that the
construction of his boiler is of such a nature, that, even in the
case of bursting, no danger is to be apprehended, nor any other
inconvenience than the stoppage of the carriage. He states that,
while travelling about nine miles an hour, and working with a
pressure of about 100 lbs. on the square inch, loaded with
thirteen passengers, the carriage was suddenly stopped. At first
the cause of the accident was not apparent; but, on opening one of
the cocks of the boiler, it was found that it contained neither
steam nor water. Further examination proved that the boiler had
burst. On unscrewing the bolts, it was found that there were
several large holes in the plates of the water-chamber, through
which the water had flowed on the fire, but neither noise nor
explosion, nor any dangerous consequences, ensued.
(207.) Mr. Nathaniel Ogle of Southampton obtained a patent for a
locomotive carriage, and worked it for some time experimentally;
but as his operations do not appear to have been continued, I
suppose he was unsuccessful in fulfilling those conditions,
without which the machine could not be worked with economy and
profit. In his evidence before a committee of the House of
Commons, he has thus described his contrivance:—
"The base of the boiler and the summit are composed of cross
pieces, cylindrical within and square without; there are holes
bored through these cross pieces, and inserted through the whole
is an air tube. The inner hole of the lower surface, and the under
hole of the upper surface, are rather larger than the other ones.
Round the air tube is placed a small cylinder, the collar of which
fits round the larger aperture on the inner surface of the lower
frame, and the under surface of the upper frame-work. These are
both drawn together by screws from the top; these cross pieces are
united by connecting pieces, the whole strongly bolted together;
so that we obtain, in one tenth of the space, and with one tenth
of the weight, the same heating surface and power as is now
obtained in other and low-pressure boilers, with incalculably
[Pg439] greater safety. Our present experimental boiler contains
250 superficial feet of heating surface in the space of 3 feet 8
inches high, 3 feet long, and 2 feet 4 inches broad, and weighs
about 8 cwt. We supply the two cylinders with steam, communicating
by their pistons with a crank axle, to the ends of which either
one or both wheels are affixed as may be required. One wheel is
found to be sufficient, except under very difficult circumstances,
and when the elevation is about one foot in six to impel the
vehicle forward.
"The cylinders of which the boiler is composed are so small as to
bear a greater pressure than could be produced by the quantity of
fire beneath the boiler; and if any one of these cylinders should
be injured by violence, or any other way, it would become merely a
safety valve to the rest. We never, with the greatest pressure,
burst, rent, or injured our boiler; and it has not once required
cleaning, after having been in use twelve months."
Dr. Church of Birmingham has obtained a succession of patents for
contrivances connected with a locomotive engine for stone roads;
and a company, consisting of a considerable number of individuals,
possessing sufficient capital, has been formed in Birmingham, for
carrying into effect his designs, and working carriages on his
principle. The present boiler of Dr. Church is formed of copper.
The water is contained between two sheets of copper, united
together by copper nails, in a manner resembling the way in which
the cloth forming the top of a mattress or cushion is united with
the cloth which forms the bottom of it, except that the nails or
pins, which bind the sheets of copper, are much closer together.
The water, in fact, seems to be "quilted" or "padded" in between
two sheets of thin copper. This double sheet of copper is formed
into an oblong rectangular box, the interior of which is the
fire-place and ash-pit, and over the end of which is the
steam-chest. The great extent of surface exposed to the immediate
action of the fire causes steam to be produced with great
rapidity.
Various other projects for the application of steam engines on
common roads were in a state of progressive improvement, [Pg440]
when the greater advantages attending railways were considered so
manifest, that considerable doubts were raised, whether, supposing
the problem of the application of the steam engine on common roads
to be successfully solved, it could ever be attended with the same
economy and effect, as by the adoption of a railway. Among the
projects which promised a successful issue, may be mentioned the
locomotive engines contrived by Messrs. Maudslay and Field, by
Colonel Maceroni, and by Mr. Scott Russell. These and others have,
however, been abandoned, mainly, we believe, from the impression,
that wherever traffic can exist, sufficiently extensive to render
the application of steam power profitable, a railway must always
supersede a common road; and that, even in the limited traffic to
be expected on branches to the great railways, horse power applied
to railways would be attended with more economy than steam power
applied on stone roads.
[Illustration]
[Pg441]
[Illustration]
CHAP. XIII.
STEAM NAVIGATION.
FORM AND ARRANGEMENT OF MARINE ENGINES. — EFFECTS OF SEA WATER
IN BOILERS. — REMEDIES FOR THEM. — BLOWING OUT. — INDICATORS
OF SALTNESS. — SEAWARD'S INDICATOR. — HIS METHOD OF BLOWING
OUT. — FIELD'S BRINE PUMPS. — TUBULAR CONDENSERS APPLIED BY
MR. WATT. — HALL'S CONDENSERS. — COPPER BOILERS. — PROCESS OF
STOKING. — MARINE BOILERS. — MEANS OF ECONOMISING FUEL. —
COATING MARINE BOILERS WITH FELT. — NUMBER AND ARRANGEMENT OF
FURNACES AND FLUES. — HOWARD'S ENGINE. — APPLICATION OF THE
EXPANSIVE PRINCIPLE IN MARINE ENGINES. — RECENT IMPROVEMENTS
OF MESSRS. MAUDSLAY AND FIELD. — HUMPHRYS' ENGINE. — COMMON
PADDLE-WHEEL. — FEATHERING PADDLES. — MORGAN'S WHEELS. — THE
SPLIT PADDLE. — PROPORTION OF POWER TO TONNAGE. — IMPROVED
EFFICIENCY OF MARINE ENGINES. — IRON STEAM-VESSELS. —
STEAM-NAVIGATION TO INDIA.
(208.) Among the many ways in which the steam-engine has
ministered to the advancement of civilisation and the social
progress of the human race, there is none more [Pg442] important
or more interesting than its application to navigation. Before it
lent its giant powers to the propulsion of ships, locomotion over
the waters of the deep was attended with so much danger and
uncertainty that, as a common proverb, it became the type and the
representative of every thing which was precarious and perilous.
The application, however, of steam to navigation has rescued the
mariner and the voyager from many of the dangers of wind and
water; and even in its present state, putting out of view its
probable improvement, it has rendered all voyages of moderate
length as safe, and very nearly as regular, as journeys over-land.
As a means of transport by sea, the application of this power may
be considered as established; and it is now receiving improvements
by which its extension to the longest class of ocean voyages is a
question not of practicability, but merely of profit.
The manner in which the steam-engine is rendered an instrument for
the propulsion of vessels must in its general features be so
familiar to every one as to require but short explanation. A shaft
is carried across the vessel, being continued on either side
beyond the timbers: to the extremities of this shaft, on the
outside of the vessel, are fixed a pair of wheels constructed like
undershot water-wheels, having attached to their rims a number of
flat boards called _paddle-boards_. As the wheels revolve, these
paddle-boards strike the water, driving it in a direction contrary
to that in which it is intended the vessel should be propelled.
The moving force imparted to the water thus driven backwards is
necessarily accompanied by a re-action upon the vessel through the
medium of the paddle-shaft, by which the vessel is propelled
forwards. On the paddle-shaft two cranks are constructed, similar
to the cranks already described on the axle of the driving wheels
of a locomotive engine. These cranks are placed at right angles to
each other, so that when either is in its highest or lowest
position the other shall be horizontal. They are driven by two
steam-engines, which are placed in the hull of the vessel below
the paddle-shaft. In the earlier steam-boats a single steam-engine
was used, and in that case the unequal action of the engine on the
crank was equalised by a fly-wheel. This, however, has been long
[Pg443] since abandoned in European vessels, and the use of two
engines is now almost universal. By the relative position of the
cranks it will be seen, that when either crank is at its dead
points, the other will be in the positions most favourable to its
action, and in all intermediate positions the relative efficiency
of the cranks will be such as to render their combined action very
nearly uniform.
The steam-engines used to impel vessels may be either condensing
engines, similar to those of Watt, and such as are used in
manufactures generally, or they may be non-condensing and
high-pressure engines, similar in principle to those used on
railways. Low-pressure condensing engines are, however, universally
used for marine purposes in Europe and to some extent in the United
States. In the latter country, however, high-pressure engines are
also in pretty general use, on rivers where lightness is a matter of
importance.
The arrangement of the parts of a marine engine differs in some
respects from that of a land engine. The limitation of space,
which is unavoidable in a vessel, renders greater compactness
necessary. The paddle-shaft on which the cranks to be driven by
the engine are constructed being very little below the deck of the
vessel, the beam and connecting rod could not be placed in the
position in which they usually are in land engines, without
carrying the machinery to a considerable elevation above the deck.
This is done in the steam-boat engines used on the American
rivers; but it would be inadmissible in steam-boats in general,
and more especially in sea-going steamers. The connecting rods,
therefore, instead of being presented downwards towards the cranks
which they drive, must, in steam-vessels, be presented upwards,
and the impelling force received from below. If, under these
circumstances, the beam were in the usual position above the
cylinder and piston-rod, it must necessarily be placed between the
engine and the paddle-shaft. This would require a depth for the
machinery which would be incompatible with the magnitude of the
vessel. The beam, therefore, of marine engines, instead of being
above the cylinder and piston, is placed below them. To the top of
the [Pg445] piston-rods cross pieces are attached of greater
length than the diameter of the cylinders, so that their
extremities shall project beyond the cylinders. To the ends of
these cross pieces are attached by joints the rods of a parallel
motion: these rods are carried downwards, and are connected with
the ends of two beams below the cylinder, and placed on either
side of it. The opposite ends of these beams are connected by
another cross piece, to which is attached a connecting rod, which
is continued upwards to the crank-pin, to which it is attached,
and which it drives. Thus the beam, parallel motion, and
connecting rod of a marine engine, is similar to that of a land
engine, only that it is turned upside down; and in consequence of
the impossibility of placing the beam directly over the
piston-rod, two beams and two systems of parallel motion are
provided, one on each side of the engine, acted upon by, and
acting on the piston-rod and crank by cross pieces.
The proportion of the cylinders differs from that usually observed
in land engines, for like reasons. The length of the cylinder of
land engines is generally greater than its diameter, in the
proportion of about two to one. The cylinders of marine engines
are, however, commonly constructed with a diameter very little
less than their length. In proportion, therefore, to their power
their stroke is shorter, which infers a corresponding shortness of
crank and a greater limitation of play of all the moving parts in
the vertical direction. The valves and the gearing by which they
are worked, the air-pump, the condenser, and other parts of the
marine engines, do not materially differ from those already
described in land engines.
[Illustration: _Fig._ 119.]
These arrangements of a marine engine will be more clearly
understood by reference to _fig._ 119.[35], in which is
represented a longitudinal section of a marine engine with its
boiler as placed in a steam-vessel. The sleepers of oak,
supporting the engine, are represented at X, the base of the
engine being secured to these by bolts passing through them
[Pg446] and the bottom timbers of the vessel; S is the steam-pipe
leading from the steam-chest in the boiler to the slides _c_, by
which it is admitted to the top and bottom of the cylinder. The
condenser is represented at B, and the air-pump at E. The hot well
is seen at F, from which the feed is taken for the boiler; L is
the piston-rod connected by the parallel motion _a_ with the beam
H, working on a centre K, near the base of the engine. The other
end of the beam I drives the connecting rod M, which extends
upwards to the crank which it works upon the paddle-shaft O. Q R
is the framing by which the engine is supported. The beam here
exhibited is shown on dotted lines as being on the further side of
the engine. A similar beam similarly placed, and moving on the
same axis, must be understood to be at this side connected with
the cross head of the piston in like manner by a parallel motion,
and with a cross piece attached to the lower end of the connecting
rod and to the opposite beam. The eccentric which works the slides
is placed upon the paddle shaft O, and the connecting arm which
drives the slides may be easily detached when the engine requires
to be stopped. The section of the boiler, grate, and flues, is
represented at W U. The safety-valve _y_ is enclosed beneath a
pipe carried up beside the chimney, and is inaccessible to the
engine-man; _h_ are the cocks for blowing the salted water from
the boiler; and I I the feed-pipe.
The general arrangement of the engine-room of a steam-vessel is
represented in _fig._ 120.
The nature of the effect required to be produced by marine engines
does not render either necessary or possible that great regularity
of action which is indispensable in a steam-engine applied to the
purposes of manufacture. The agitation of the surface of the sea
will cause the immersion of the paddle-wheels to be subject to
great variation, and the resistance produced by the water to the
engine will undergo a corresponding change. The governor,
therefore, and other parts of the apparatus, contrived for giving
to the engine that great regularity required in manufactures, are
omitted in nautical engines, and nothing is introduced save what
is [Pg447] necessary to maintain the machine in its full working
efficiency.
[Illustration: _Fig._ 120.]
[Illustration: _Fig._ 121.]
To save space, marine boilers are constructed so as to produce the
necessary quantity of steam within the smallest possible
dimensions. With this view a more extensive surface in proportion
to the capacity of the boiler is exposed to the action of the
fire. The flues, by which the flame and heated air are conducted
to the chimney, are so constructed that the heat may act upon the
water on every side in thin oblong shells or plates. This is
accomplished by constructing the flues so as to traverse the
boiler backwards and forwards several times before they terminate
[Pg448] in the chimney. Such an arrangement renders the expense of
the boilers greater, but their steam-producing power is
proportionally augmented, and experiments made by Mr. Watt, at
Birmingham, have proved that such boilers with the same
consumption of fuel will produce, as compared with common land
boilers, an increased evaporation in the proportion of about three
to two.
[Illustration: _Fig._ 122.]
[Illustration: _Fig._ 123.]
The form and arrangement of the water-spaces and flues in marine
boilers may be collected from the sections of the boilers used in
some of the government steamers, exhibited in _figs._ 121, 122,
123. A section made by a horizontal plane passing through the
flues is exhibited in _fig._ 121. The furnaces F communicate in
pairs with the flues E, the air following the course through the
flues represented by the arrows. The flue E passes to the back of
the boiler, then returns to the front, then to the back again, and
is finally carried back to the front, where it communicates at C
with the curved flue B, represented in the transverse vertical
section, _fig._ 122. This curved flue B finally terminates in the
chimney A. There are in this case three independent boilers, each
worked by two furnaces communicating with the same system of
flues; and in the curved flues B, _fig._ 122., by which the air is
finally conducted through the chimney, are placed three
independent [Pg449] dampers, by means of which the furnace of
each boiler can be regulated independently of the other, and by
which each boiler may be separately detached from communication
with the chimney. The letters of reference in the horizontal
section, _fig._ 121., correspond with those in the transverse
vertical section, _fig._ 122., E representing the commencement of
the flues, and C their termination.
[Illustration: _Fig._ 124.]
A longitudinal section of the boiler made by a vertical plane
extending from the front to the back is given in _fig._ 123.,
where F, as before, is the furnace, G the grate-bars sloping
downwards from the front to the back, H the fire-bridge, C the
commencement of the flues, and A the chimney. An elevation of the
front of the boiler is represented in _fig._ 124., showing two of
the fire-doors closed, and the other two removed, displaying the
position of the grate-bars in front. Small openings are also
provided, closed by proper doors, by which access can be had to
the under side of the flues between the foundation timbers of the
engine for the purpose of cleaning them.
Each of these boilers can be worked independently of the others.
By this means, when at sea, the engine may be worked by any two of
the three boilers, while the third is being cleaned and put in
order. In all sea-going steamers multiple boilers are at present
provided for this purpose.
In the boilers here represented the flues are all upon the same
level, winding backwards and forwards without passing one above
the other. In other boilers, however, the flues, [Pg450] after
passing backwards and forwards near the bottom of the boiler, turn
upwards and pass backwards and forwards through a level of the
water nearer its surface, finally terminating in the chimney. More
heating surface is thus obtained with the same capacity of boiler.
The most formidable difficulty which has been encountered in the
application of the steam-engine to sea-voyages has arisen from the
necessity of supplying the boiler with sea-water instead of pure
fresh water. The sea-water is injected into the condenser for the
purpose of condensing the steam, and it is thence, mixed with the
condensed steam, conducted as feeding water into the boiler.
(209.) Sea-water holds, as is well known, certain alkaline
substances in solution, the principal of which is muriate of soda,
or common salt. Ten thousand grains of pure sea-water contain two
hundred and twenty grains of common salt, the remaining
ingredients being thirty-three grains of sulphate of soda,
forty-two grains of muriate of magnesia, and eight grains of
muriate of lime. The heat which converts pure water into steam
does not at the same time evaporate those salts which the water
holds in solution. As a consequence it follows, that as the
evaporation in the boiler is continued, the salt, which was held
in solution by the water which has been evaporated, remains in the
boiler, and enters into solution with the water remaining in it.
The quantity of salt contained in sea-water being considerably
less than that which water is capable of holding in solution, the
process of evaporation for some time is attended with no other
effect than to render the water in the boiler a stronger solution
of salt. If, however, this process be continued, the quantity of
salt retained in the boiler having constantly an increasing
proportion to the quantity of water, it must at length render the
water in the boiler a saturated solution—that is, a solution
containing as much salt as at the actual temperature it is capable
of holding in solution. If, therefore, the evaporation be
continued beyond this point, the salt disengaged from the water
evaporated instead of entering into solution with the water
remaining in the boiler will be precipitated in the form of
sediment; and if the process be continued in the [Pg451] same
manner, the boiler would at length become a mere salt-pan.
But besides the deposition of salt sediment in a loose form, some
of the constituents of sea-water having an attraction for the iron
of the boiler, collect upon it in a scale or crust in the same
manner as earthy matters held in solution by spring-water are
observed to form and become incrusted on the inner surface of
land-boilers and of common culinary vessels.
The coating of the inner surface of a boiler by incrustation and
the collection of salt sediment in its lower parts, are attended
with effects highly injurious to the materials of the boiler. The
crust and sediment thus formed within the boiler are almost
non-conductors of heat, and placed, as they are, between the water
contained in the boiler and the metallic plates which form it,
they obstruct the passage of heat from the outer surface of the
plates in contact with the fire to the water. The heat, therefore,
accumulating in the boiler-plates so as to give them a much higher
temperature than the water within the boiler, has the effect of
softening them, and by the unequal temperature which will thus be
imparted to the lower plates which are incrusted, compared with
the higher parts which may not be so, an unequal expansion is
produced, by which the joints and seams of the boiler are loosened
and opened, and leaks produced.
These injurious effects can only be prevented by either of two
methods; first, by so regulating the feed of the boiler that the
water it contains shall not be suffered to reach the point of
saturation, but shall be so limited in its degree of saltness that
no injurious incrustation or deposit shall be formed; secondly, by
the adoption of some method by which the boiler may be worked with
fresh water. This end can only be attained by condensing the steam
by a jet of fresh water, and working the boiler continually by the
same water, since a supply of fresh water sufficient for a boiler
worked in the ordinary way could never be commanded at sea.
(210.) The method by which the saltness of the water in the boiler
is most commonly prevented from exceeding a certain [Pg452] limit
has been to discharge from the boiler into the sea a certain
quantity of over-salted water, and to supply its place by
sea-water introduced into the condenser through the injection-cock
for the purpose of condensing the steam, this water being mixed
with the steam so condensed, and being, therefore, a weaker
solution of salt than common sea-water. To effect this, cocks
called _blow-off cocks_, are usually placed in the lower parts of
the boiler, where the over-salted, and therefore heavier, parts of
the water collect. The pressure of the steam and incumbent weight
of the water in the boiler force the lower strata of water out
through these cocks; and this process, called _blowing out_, is,
or ought to be, practised at such intervals as will prevent the
water from becoming over salted. When the salted water has been
blown out in this manner, the level of the water in the boiler is
restored by a feed of corresponding quantity.
This process of blowing out, on the due and regular observance of
which the preservation and efficiency of the boiler mainly depend,
is too often left at the discretion of the engineer, who is, in most
cases, not even supplied with the proper means of ascertaining the
extent to which the process should be carried. It is commonly
required that the engineer should blow out a certain portion of the
water in the boiler every two hours, restoring the level by a feed
of equivalent amount; but it is evident that the sufficiency of the
process founded on such a rule must mainly depend on the supposition
that the evaporation proceeds always at the same rate, which is far
from being the case with marine boilers. An indicator, by which the
saltness of the water in the boiler would always be exhibited, ought
to be provided, and the process of blowing out should be regulated
by the indications of that instrument. To blow out more frequently
than is necessary is attended with a waste of fuel; for hot water is
thus discharged into the sea while cold water is introduced in its
place, and consequently all the heat necessary to produce the
difference of the temperatures of the water blown out and the feed
introduced is lost. If, on the other hand, the process of blowing
out be observed less frequently than is necessary, then more or less
incrustation and deposit [Pg453] may be produced, and the injurious
effects already described ensue.
As the specific gravity of water holding salt in solution is
increased with every increase of the strength of the solution, any
form of hydrometer capable of exhibiting a visible indication of
the specific gravity of the water contained in the boiler would
serve the purpose of an indicator, to show when the process of
blowing out is necessary, and when it has been carried to a
sufficient extent. The application of such instruments, however,
would be attended with some practical difficulties in the case of
sea-boilers.
The temperature at which a solution of salt boils under a given
pressure varies considerably with the strength of the solution;
the more concentrated the solution is, the higher will be its
boiling temperature under the same pressure. A comparison,
therefore, of a steam-gauge attached to the boiler, and a
thermometer immersed in it, showing the pressure and the
temperature, would always indicate the saltness of the water; and
it would not be difficult so to graduate these instruments as to
make them at once show the degree of saltness.
If the application of the thermometer be considered to be attended
with practical difficulty, the difference of pressures under which
the salt water of the boiler and fresh water of the same
temperature boil, might be taken as an indication of the saltness
of the water in the boiler, and it would not be difficult to
construct upon this principle a self-registering instrument, which
would not only indicate but record from hour to hour the degree of
saltness of the water. A small vessel of distilled water being
immersed in the water of the boiler would always have the
temperature of that water, and the steam produced from it
communicating with a steam-gauge, the pressure of such steam would
be indicated by that gauge, while the pressure of the steam in the
boiler under which pressure the salted water boils might be
indicated by another gauge. The difference of the pressures
indicated by the two gauges would thus become a test by which the
saltness of the water in the boiler would be measured. The two
pressures might be made to act on opposite ends of the same column
of [Pg454] mercury contained in a siphon tube, and the difference
of the levels of the two surfaces of the mercury would thus become
a measure of the saltness of the water in the boiler. A
self-registering instrument founded on this principle formed part
of the self-registering steam-log which I proposed to introduce
into steam-vessels some time since.
(211.) The Messrs. Seaward of Limehouse have adopted, in some of
their recently constructed engines, a method of indicating the
saltness of the water, and of measuring the quantity of salted
water or brine discharged, by blowing out. A glass-gauge, similar
in form to that already described in land engines (156.), is
provided to indicate the position of the surface of the water in
the boiler. In this gauge two hydrometer balls are provided, the
weight of which in proportion to their magnitude is such that they
would both sink to the bottom in a solution of salt of the same
strength as common sea-water. When the quantity of salt exceeds
5/32 parts of the whole weight of the water, the lighter of the
two balls will float to the top; and when the strength is further
increased until the proportion of salt exceeds 6/32 parts of the
whole, then the heavier ball will float to the top. The actual
quantity of salt held in solution by sea-water in its ordinary
state is 1/32 part of its whole weight; and when by evaporation
the proportion of salt in solution has become 9/32 parts of the
whole, then a deposition of salt commences. With an indicator such
as that above described, the ascent of the lighter hydrometer ball
gives notice of the necessity for blowing out, and the ascent of
the heavier may be considered as indicating the approach of an
injurious state of saltness in the boiler.
[Illustration: _Fig._ 125.]
The ordinary method of blowing out the salted water from a boiler
is by a pipe having a cock in it leading from the boiler through
the bottom of the ship, or at a point low down at its side.
Whenever the engineer considers that the water in the boiler has
become so salted that the process of blowing out should commence,
he opens the cock communicating by this pipe with the sea, and
suffers an indefinite and uncertain quantity of water to escape.
In this way he discharges, according to the magnitude of the
boiler, from two to six tons [Pg455] of water, and repeats this
at intervals of from two to four hours, as he may consider to be
sufficient. If, by observing this process, he prevents the boiler
from getting incrusted during the voyage, he considers his duty to
be effectually discharged, forgetting that he may have blown out
many times more water than is necessary for the preservation of
the boiler, and thereby produced a corresponding and unnecessary
waste of fuel. In order to limit the quantity of water discharged,
Messrs. Seaward have adopted the following method. In _fig._ 125.
is represented a transverse section of a part of a steam-vessel; W
is the water-line of the boiler, B is the mouth of a blow-off
pipe, placed near the bottom of the boiler. This pipe rises to A,
and turning in the horizontal direction, A C is conducted to a
tank T, which contains exactly a ton of water. This pipe
communicates with the tank by a cock D, governed by a lever H.
When this lever is moved to D′, the cock D is open, and when it is
moved to K, the cock D is closed. From the same tank there
proceeds another pipe E, which issues from the side of the
[Pg456] vessel into the sea governed by a cock F, which is
likewise put in connection with the lever H, so that it shall be
opened when the lever H is drawn to the position F′, the cock D′
being closed in all positions of the lever between K and F′. Thus,
whenever the cock F communicating with the sea is open, the cock D
communicating with the boiler is closed, and _vice versâ_, both
cocks being closed when the lever is in the intermediate position
K. By this arrangement the boiler cannot, by any neglect in
blowing off, be left in communication with the sea, nor can more
than a ton of water be discharged except by the immediate act of
the engineer. The injurious consequences are thus prevented which
sometimes ensue when the blow-off cocks are left open by any
neglect on the part of the engineer. When it is necessary to blow
off, the engineer moves the lever H, to the position D′. The
pressure of the steam in the boiler on the surface of the water W
forces the salted water or brine up the pipe B A, and through the
open cock C into the tank, and this continues until the tank is
filled: when that takes place, the lever is moved from the
position D′ to the position F′, by which the cock D is closed, and
the cock F opened. The water in the tank flows through the pipe E
into the sea, air being admitted through the valve V, placed at
the top of the tank, opening inwards. A second ton of brine is
discharged by moving the lever back to the position D′, and
subsequently returning it to the position F′; and in this way the
brine is discharged ton by ton, until the supply of water from the
feed which replaces it has caused both the balls in the indicator
to sink to the bottom.
(212.) A different method of preserving the requisite freshness of
the water in the boiler has been adopted by Messrs. Maudslay and
Field, and introduced with success into the Great Western and
other steam-vessels. Pumps called _brine-pumps_ are put into
communication with the lower part of the boiler, and so
constructed as to draw the brine therefrom, and drive it into the
sea. These brine-pumps are worked by the engine, and their
operation is constant. The feed-pumps are likewise worked by the
engine, and they bear such a proportion to the brine-pumps that
the quantity of salt discharged in a given time in the brine is
equal to the quantity of salt [Pg457] introduced in solution by
the water of the feed-pumps. By this means the same actual
quantity of salt is constantly maintained in the boiler, and
consequently the strength of the solution remains invariable. If
the brine discharged by the brine-pumps contains 5/32 parts of
salt while the water introduced by the feed-pumps contains only
1/32 part, then it is evident that five cubic feet of the feeding
water will contain no more salt than is contained in one cubic
foot of brine. Under such circumstances the brine-pumps would be
so constructed as to discharge 1/5 of the water introduced by the
feed-pumps, so that 4/5 of all the water introduced into the
boiler would be evaporated, and rendered available for working the
engine.
To save the heat of the brine, a method has been adopted in the
marine engines constructed by Messrs. Maudslay and Field similar
to one which has been long practised in steam-boilers, and in
various apparatus for the warming of buildings. The current of
heated brine is conducted from the boiler through a tube which is
contained in another, through which the feed is introduced. The
warm current of brine, therefore, as it passes out, imparts a
considerable portion of its heat to the cold feed which comes in;
and it is found that by this expedient the brine discharged into
the sea may be reduced to a temperature of about 100°.
This expedient is so effectual that when the apparatus is properly
constructed, and kept in a state of efficiency, it may be regarded
as nearly a perfect preventive against the incrustation, and the
deposition of salt in the boilers, and is not attended with any
considerable waste of fuel.
(213.) About the year 1776, Mr. Watt invented a tubular condenser,
with a view to condense the steam drawn off from the cylinder
without the process of injection. This apparatus consisted of a
number of small tubes connecting the top and bottom of the
condenser, arranged in a manner not very different from that of
the tubes which traverse the boiler of a locomotive engine. These
tubes were continually surrounded by cold water, and the steam, as
it escaped from the cylinder passing through them, was condensed
by their cold surfaces, and collected in the form of water in a
reservoir below, from [Pg458] whence it was drawn off by a pump
in the same manner as in engines which condensed by injection. One
of the advantages proposed by this expedient was, that no
atmospheric air would be introduced into the condenser, as is
always the case when condensation by injection is practised. Cold
water, which is injected, has always combined with it more or less
common air. When this water is mixed with the condensed steam, the
elevation of its temperature disengages the air combined with it,
and this air circulating to the cylinder, vitiates the vacuum. One
of the purposes for which the air-pump in condensing steam-engines
was provided, and from which it took its name, was to draw off
this air. If, however, a tubular condenser could be made to act
with the necessary efficiency, no injection water would be
introduced for condensation, and the pump would have no other duty
except to remove the small quantity of water produced by the
condensed steam. That water being subsequently carried back to the
boiler by the feed-pumps, a constant system of circulation would
be maintained, and the boiler would never require any fresh supply
of water, except what might be necessary to make good the waste by
leakage and other causes.
This contrivance has been of late years revived by Mr. Samuel Hall
of Basford, near Nottingham, with a view to supersede in marine
engines the necessity of using sea-water in the boilers. Mr. Hall
proposes to make marine boilers with fresh water to condense the
steam without injection, by a tubulated condenser, and to provide
by the distillation of sea-water the small quantity of fresh water
which would be necessary to make good the waste. These condensers
have been introduced into several steam-vessels: in some they have
been continued, and in others abandoned, and various opinions are
entertained of their efficacy. I have not been able to obtain the
results of any satisfactory experiments on them, and cannot
therefore form a judgment of their usefulness. Mr. Watt abandoned
these condensers from finding that the condensation of the steam
was not sufficiently sudden, and that consequently at the
commencement of the stroke the piston was subject to a resistance
which [Pg459] injuriously diminished the amount of the moving
power, whereas condensation by jet was almost instantaneous, and
the efficiency of the piston throughout the entire stroke was more
uniform.
Mr. Watt also found that a fur collected around the tubes of the
condenser, so as to obstruct the free passage of heat from the
steam to the water of the cold cistern; and that, consequently,
the efficiency of the condenser was gradually impaired, and could
only be restored by frequent cleansing.
It is stated by Mr. Hall that a vacuum is preserved in his
condensers as perfect as that which is maintained in the ordinary
condensers by injection. It is objected, on the other hand, that
without the injection water and the air which accompanies it being
introduced into his condensers, Mr. Hall uses as large and
powerful an air-pump as those which are used in engines of equal
power condensing by injection; that, consequently, the vacuum
which is maintained is produced, not as it ought to be altogether
by the condensation of steam, but by the air-pump drawing off the
uncondensed steam. To whatever extent this may be true, the
efficacy of the machine, as indicated by the barometer-gauge, is
only apparent; since as much power is necessary to pump away any
portion of uncondensed vapour as is obtained by the vacuum
produced by the absence of that vapour.
A tubular condenser of the form proposed by Mr. Hall is represented
in _fig._ 126.; _a_ is the upper part of the condenser to which
steam is admitted from the slide after having worked the piston; _k_
is the section of a thin plate, forming the top of the condenser,
perforated with small holes, in which the tubes are inserted so as
to be steam-tight and water-tight. Water is admitted to flow around
these tubes between the top _k_ and the bottom _d_ of the condenser,
so as to keep them constantly at a low temperature. The steam passes
from _a_ through the tubes to the lower chamber _f_ of the
condenser, where it is reduced to water by the cold to which it has
been exposed. A supply of cold water is constantly pumped through
the condenser, so as to keep the tubes at a low temperature. The
air-pump _g_ is of the usual construction, having valves in the
piston opening upwards, and [Pg460] similar valves in the cover of
the pump also opening upwards. The water formed by the condensed
steam in _f_ is drawn through the foot-valve, and after passing
through the piston-valves, is discharged by the up-stroke of the
piston into the hot well. Any air, or other permanent gas, which may
be admitted by leakage through the tubes of the condenser, or by any
other means, is likewise drawn out by this pump, and when drawn into
the hot well is carried from thence to the feeding apparatus of the
boiler, to which it is transferred by the feed-pump.
[Illustration: _Fig._ 126.]
A provision is likewise made by which the steam escaping at the
safety-valve is condensed and carried away to the feeding cistern.
(214.) One of the remedies proposed for the evil consequences
arising from incrustation is the substitution of copper for iron
boilers. The attraction which produces the adhesion of the
calcareous matter held in solution by salt water to the surface of
iron has no existence in copper, and all the saline and other
alkaline matter precipitated in the boiling water in [Pg461]
copper boilers is suspended in a loose form, and carried off by
the process of blowing out.
Besides the injury arising from the deposition of salt and the
incrustation on the inner surface of boilers, an evil of a
formidable kind attends the accumulation of soot mixed with salt
in the flues, which proceeds from the leaks. In the seams of the
boiler there are numerous apertures, of dimensions so small as to
be incapable of being rendered stanch by any practicable means,
through which the water within the boiler filters, and the salt
which it carries with it mixes with the soot, forming a compound
which rapidly corrodes the boilers. This process of corrosion in
the flues takes place not less in copper than in iron boilers. In
cleansing the flues of a copper boiler, the salt and soot which
was thrown out upon the iron-plates which formed the flooring of
the engine-room, having remained there for some time, left behind
it a permanent appearance of copper on the iron flooring, arising
from the precipitation of the copper which had combined with the
soot and salt in the flues.[36] In this case the leaks from whence
the salt proceeded were found, on careful examination, so
unimportant, that the usual means to stanch them could not be
resorted to without the risk of increasing the evil.
(215.) In the application of the steam-engine to the propulsion of
vessels in voyages of great extent, the economy of fuel acquires
an importance greater than that which appertains to it in
land-engines, even in localities the most removed from coal-mines,
and where its expense is greatest. The practical limit to
steam-voyages being determined by the greatest quantity of coals
which a steam-vessel can carry, every expedient by which the
efficiency of the fuel can be increased becomes a means, not
merely of a saving of expense, but of an increased extension of
steam-power to navigation. Much attention has been bestowed on the
augmentation of the duty of engines in the mining districts of
Cornwall, where the question of their efficiency is merely a
question of economy, but far greater care should be given to this
subject when the practicability of maintaining intercourse by
steam between distant points of the globe will perhaps depend on
the effect produced by a given quantity [Pg462] of fuel. So long
as steam-navigation was confined to river and channel transport,
and to coasting voyages, the speed of the vessel was a paramount
consideration, at whatever expenditure of fuel it might be
obtained; but since steam-navigation has been extended to
ocean-voyages, where coals must be transported sufficient to keep
the engine in operation for a long period of time without a fresh
relay, greater attention has been bestowed upon the means of
economising it.
Much of the efficiency of fuel must depend on the management of
the fires, and therefore on the skill and care of the stokers.
Formerly the efficiency of firemen was determined by the abundant
production of steam, and so long as the steam was evolved in
superabundance, however it might have blown off to waste, the duty
of the stoker was considered as well performed. The regulation of
the fires according to the demands of the engine were not thought
of, and whether much or little steam was wanted, the duty of the
stoker was to urge the fires to their extreme limit.
Since the resistance opposed by the action of the paddle-wheels of
a steam-vessel varies with the state of the weather, the
consumption of steam in the cylinders must undergo a corresponding
variation; and if the production of steam in the boilers be not
proportioned to this, the engines will either work with less
efficiency than they might do under the actual circumstances of
the weather, or more steam will be produced in the boilers than
the cylinders can consume, and the surplus will be discharged to
waste through the safety-valves. The stokers of a marine engine,
therefore, to perform their duty with efficiency, and obtain from
the fuel the greatest possible effect, must discharge the
functions of a self-regulating furnace, such as has been already
described: they must regulate the force of the fires by the amount
of steam which the cylinders are capable of consuming, and they
must take care that no unconsumed fuel is allowed to be carried
away from the ash-pit.
(216.) Until within a few years of the present time the heat
radiated from every part of the surface of the boiler was allowed
to go to waste, and to produce injurious effects on those parts of
the vessel to which it was transmitted. This evil, [Pg463]
however, has been lately removed by coating the boilers,
steam-pipes, &c. of steam-vessels with felt, by which the escape
of heat from the surface of the boiler is very nearly, if not
altogether, prevented. This felt is attached to the boiler-surface
by a thick covering of white and red lead. This expedient was
first applied in the year 1818 to a private steam-vessel of Mr.
Watt's called the _Caledonia_, and it was subsequently adopted in
another vessel, the machinery of which was constructed at Soho,
called the _James Watt_.
The economy of fuel depends in a considerable degree on the
arrangement of the furnaces, and the method of feeding them. In
general each boiler is worked by two or more furnaces communicating
with the same system of flues. While the furnace is fed, the door
being open, a stream of cold air rushes in, passing over the burning
fuel and lowering the temperature of the flues: this is an evil to
be avoided. But, on the other hand, if the furnaces be fed at
distant intervals, then each furnace will be unduly heaped with
fuel, a great quantity of smoke will be evolved, and the combustion
of the fuel will be proportionally imperfect. The process of coking
in front of the grate, which would insure a complete combustion of
the fuel, has been already described (147.). A frequent supply of
coals, however, laid carefully on the front part of the grate, and
gradually pushed backwards as each fresh feed is introduced, would
require the fire-door to be frequently opened, and cold air to be
admitted. It would also require greater vigilance on the part of the
stokers than can generally be obtained in the circumstances in which
they work. In steam-vessels the furnaces are therefore fed less
frequently, fuel introduced in greater quantities, and a less
perfect combustion produced.
When several furnaces are constructed under the same boiler,
communicating with the same system of flues, the process of
feeding, and consequently opening one of them, obstructs the due
operation of the others, for the current of cold air which is thus
admitted into the flues checks the draft and diminishes the
efficiency of the furnaces in operation. It was formerly the
practice in vessels exceeding one hundred horse-power, to place
four furnaces under each boiler, communicating with the same
system of flues. Such an arrangement [Pg464] was found to be
attended with a bad draft in the furnaces, and therefore to
require a greater quantity of heating surface to produce the
necessary evaporation. This entailed upon the machinery the
occupation of more space in the vessel in proportion to its power;
it has therefore been more recently the practice to give a
separate system of flues to each pair of furnaces, or, at most, to
every three furnaces. When three furnaces communicate with a
common flue, two will always be in operation, while the third is
being cleared out; but if the same quantity of fire were divided
among two furnaces, then the clearing out of one would throw out
of operation half the entire quantity of fire, and during the
process the evaporation would be injuriously diminished. It is
found by experience, that the side plates of furnaces are liable
to more rapid destruction than their roofs, owing, probably, to a
greater liability to deposit. Furnaces, therefore, should not be
made narrower than a certain limit. Great depth from front to back
is also attended with practical inconvenience, as it renders
firing tools of considerable length, and a corresponding extent of
stoking room necessary. It is recommended, by those who have had
much practical experience in steam-vessels, that furnaces six feet
in depth from front to back should not be less than three feet in
width, to afford means of firing with as little injury to the side
plates as possible, and of keeping the fires in the condition
necessary for the production of the greatest effect. The tops of
the furnaces almost never decay, and seldom are subject to an
alteration of figure, unless the level of the water be allowed to
fall below them.[37]
(217.) A form of marine engine was some years since proposed and
patented by Mr. Thomas Howard, possessing much novelty and
ingenuity, and having pretensions to a very extraordinary economy
of fuel, in addition to the advantages claimed by Mr. Hall. In Mr.
Howard's engines, the steam, as in Mr. Hall's, is constantly
reproduced from the same water, so that pure or distilled water
may be used; but Mr. Howard dispenses altogether with the use of a
boiler.
A quantity of mercury is placed in a shallow wrought-iron
vessel over a coke fire, by which it is maintained at a [Pg465]
temperature varying from 400° to 500°. The surface exposed to the
fire was computed at three fourths of a square foot for each
horse-power. The upper surface of the mercury was covered by a
very thin plate of iron in contact with it, and so contrived as to
present about four times as much surface as that exposed beneath
the fire. Adjacent to this a vessel of water was placed,
maintained nearly at the boiling point, and communicating by a
nozzle and valve with the chamber immediately above the mercury.
At intervals corresponding to the motion of the piston a small
quantity of water was injected from this vessel, and thrown upon
the plate of iron resting upon the hot mercury. From this it
received not only the heat necessary to convert it into common
steam, but to give it the qualities of highly superheated steam.
In fact, the steam thus produced had a temperature considerably
above that which corresponded to its pressure, and was, therefore,
capable of being deprived of more or less of its heat without
being condensed. (94.) The quantity of water injected into the
steam-chamber was regulated by the power at which the engine was
intended to be worked. The fire was supplied with air by a blower
subject to exact regulation. The steam thus produced was conducted
to a chamber surrounding the working cylinder, and this chamber
itself was enclosed by another space through which the air from
the furnace passed before it reached the flue. By this contrivance
the air imparted its redundant heat to the steam, as the latter
passed to the cylinder, and raised its temperature to about 400°,
the pressure, however, not exceeding 25 lbs. per square inch. The
valves, governing the admission of steam to the piston, were
adapted for expansive action.
The vacuum on the opposite side was maintained by condensation in
the following manner:—The condenser was a copper vessel placed in
a cistern of cold water, and the steam was admitted to it from the
cylinder by an eduction pipe in the usual way. A jet was
introduced from an adjacent vessel filled with distilled water,
and the condensing water and condensed steam were pumped from the
condenser as in common engines. The warm water thus pumped out of
the [Pg466] condenser was drawn through a copper worm, carried
with many coils through a cistern of cold water, so that when it
arrived at the end of this pipe it was reduced nearly to the
temperature of the atmosphere. The pipe was thus brought to the
vessel of distilled water already mentioned, and the water
supplied by it replaced. The water admitted to the condenser
through the condensing jet being purged of air, a small air-pump
was sufficient, since it had only to exhaust the condenser and
tubes at starting, and to remove the air which might be admitted
by leakage. Mr. Howard stated that the condensation took place as
rapidly and perfectly as in the best engines of the common kind.
An engine of this construction was in the spring of 1835 placed in
the government steamer called the _Comet_. It was stated, that
though the machinery was not advantageously constructed, a part of
the engine being old, and not made expressly for a boiler of this
kind, the vessel performed a voyage from Falmouth to Lisbon, in
which the consumption of fuel did not exceed a third of her former
consumption when worked by Boulton and Watt's engines, the former
consumption of coals being about eight hundred pounds per hour,
and the consumption of Mr. Howard's engine being less than two
hundred and fifty pounds of coke per hour.
The advantages claimed for this contrivance were the following:
_first_, the small space and weight occupied by the machinery,
arising from the absence of a boiler; _second_, the diminished
consumption of fuel; _third_, the reduced size of the flues;
_fourth_, the removal of the injurious effects arising from
deposit and incrustation; _fifth_, the absence of smoke.
(218.) The method by which the greatest quantity of practical effect
can be obtained from a given quantity of fuel must, however, mainly
depend on the extended application of the expansive principle. This
has been the means by which an extraordinary amount of duty has been
obtained from the Cornish engines. The difficulty of the application
of this principle in marine engines has arisen from the objections
entertained in Europe to the use of steam of high pressure under the
circumstances in which the engine must be worked at sea. To apply
the expansive principle, it is necessary that the moving power at
the commencement of the stroke shall considerably exceed the
[Pg467] resistance, its force being gradually attenuated till the
completion of the stroke, when it will at length become less than
the resistance. This condition may, however, be attained with steam
of limited pressure, if the engine be constructed with a sufficient
quantity of piston-surface. This method of rendering the expansive
principle available at sea, and compatible with low-pressure steam,
has recently been brought into operation by Messrs. Maudslay and
Field. Their improvement consists in adapting two steam-cylinders in
one engine, in such a manner that the steam shall act simultaneously
on both pistons, causing them to ascend and descend together. The
piston-rods are both attached to the same horizontal cross-head,
whereby their combined action is applied to one crank by means of a
connecting rod placed between the pistons.
[Illustration: _Fig._ 127.]
A section of such an engine, made by a plane passing through the
two piston-rods P P′ and cylinders, is represented in _fig._ 127.
The piston-rods are attached to a cross-head C, [Pg468] which
ascends and descends with them. This cross-head drives upwards and
downwards an axle D, to which the lower end of the connecting rod
E is attached. The other end of the connecting rod drives the
crank-pin F, and imparts revolution to the paddle-shaft G. A rod H
conveys motion by means of a beam I to the rod K of the air-pump
E.
(219.) Connected with this, and in the same patent, another
improvement is included, consisting of the application of a hollow
wrought-iron framing carried across the vessel above the machinery,
to support the whole of the bearings of the crank-shaft. A plan of
this, including the cylinders and paddle-wheel, is represented in
_fig._ 128. The advantages proposed by these improvements are
simplicity of construction, more direct action on the crank, economy
of space and weight of material, combined with increased area of the
piston, whereby a given evaporating power of the boiler is rendered
productive, by extended application of the expansive principle, of a
greater moving power than in former arrangements. Consequently,
under like circumstances, greater power and economy of fuel is
obtained, with the further advantage at sea, that when the engine is
reduced in its speed, either by the vessel being deeply laden with
coal, as is the case at the commencement of a long sea voyage, or by
head winds, more steam may be given to the cylinders, and
consequently more speed imparted to the vessel, all the steam
produced in the boiler being usefully employed.
(220.) Another improvement, having the same objects, and analogous
to the preceding, has been likewise patented by Messrs. Maudslay
and Field. This consists in the adoption of a cylinder of greater
diameter, having two piston-rods P P′, as represented in _fig._
129., of considerable length, connected at the top by a cross-head
C. From this cross-head is carried downwards the connecting rod D,
which drives the crank-pin E, and thereby works the paddle-shaft
S. In this case the paddle-shaft is extended immediately above the
piston, and the double piston-rod has sufficient length to be
above the paddle-shaft when the piston is at the bottom of its
stroke. This improvement is intended to be applied more
particularly for engines for river navigation, the advantages
resulting from [Pg469] it being that a paddle-shaft placed at a
given height from the bottom of the vessel will be enabled to
receive a longer stroke of piston than by any other arrangement
now in use. A more [Pg470] compact and firm connection of the
cylinder with the crank-shaft bearings is effected by it, and a
cylinder of much greater diameter may be applied by which the
expansive action of steam may be more fully brought into play; and
a more direct action of the steam-power on the crank with a less
weight of materials and a greater economy of space may be obtained
than by any of the arrangements of marine engines hitherto used.
[Illustration: _Fig._ 128.]
[Illustration: _Fig._ 129.]
(221.) Mr. Francis Humphrys has obtained a patent for a form of
marine engine, by which some simplification of the machinery is
attained, and the same power comprised within more limited
dimensions. In this engine there is attached to the piston of the
cylinder, instead of a piston-rod, a hollow casing D D (_fig._
130.), which moves through a stuffing-box G, constructed in a
manner similar to the stuffing-box of a piston-rod. In the figure,
this casing is presented in section, but [Pg471] its form is that
of a long narrow slit, or opening, rounded at either end as
exhibited in the plan (_fig._ 131) of the cylinder-cover. The
crank C is driven by the other end of the connecting rod H, the
crank-shaft being immediately above the centre of the piston and
the connecting rod passing through the oblong opening D, and
descending into the hollow piston-rod it is attached to an axis I
at the bottom of the piston. A box or cover K K encloses the
cross-piece or axis I with its bearings, and is [Pg472] attached
so as to be steam-tight to the bottom of the piston. A hollow
space L L is cast in the bottom of the cylinder for the reception
of the box K K, when the piston is at the bottom of the cylinder.
[Illustration: _Fig._ 130.]
[Illustration: _Fig._ 131.]
By this arrangement the force by which the piston is driven in its
ascent and descent is communicated to the connecting rod, not, as
usual, through the intervention of a piston-rod, but directly from
the piston itself by the cross-pin I, and from thence to the crank
C, which it drives without the intervention of beams, cross-heads,
or any similar appendage.
The slide-valves regulating the admission and eduction of steam
are represented at _a_; the rod of the air-pump is shown at _d_,
being worked by a crank placed on the centre of the great crank
shaft.[38]
(222.) To obtain from the moving power its full amount of
mechanical effect in propelling the vessel, it would be necessary
that its force should propel, by constantly acting against the
water in a horizontal direction, and with a motion contrary to the
course of the vessel. No system of mechanical propellers has,
however, yet been contrived capable of perfectly accomplishing
this. Patents have been granted for many ingenious mechanical
combinations to impart to the propelling surfaces such angles as
appeared to the respective contrivers most advantageous. In most
of these the mechanical complexity has formed a fatal objection.
No part of the machinery of a steam-vessel is so liable to become
deranged at sea as the paddle-wheels; and, therefore, that
simplicity of construction which is compatible with those repairs
which are possible on such emergencies is quite essential for safe
practical use.
[Illustration: _Fig._ 132.]
The ordinary paddle-wheel, as has been already stated, is a wheel
revolving upon a shaft driven by the engine, and carrying upon its
circumference a number of flat boards, called paddle-boards, which
are secured by nuts and braces in a fixed position; and that
position is such that the planes [Pg473] of the paddle-boards
diverge nearly from the centre of the shaft on which the wheel
turns. The consequence of this arrangement is that each
paddle-board can only act in that direction which is most
advantageous for the propulsion of the vessel when it arrives near
the lowest point of the wheel. In _fig._ 132. let O be the shaft
on which the common paddle-wheel revolves; the position of the
paddle-boards are represented at A, B, C, &c.; X, Y represents the
water line, the course of the vessel being supposed to be from X
to Y; the arrows represent the direction in which the paddle-wheel
revolves. The wheel is immersed to the depth of the lowest
paddle-board, since a less degree of immersion would render a
portion of the surface of each paddle-board mechanically useless.
In the position A the whole force of the paddle-board is efficient
for propelling the vessel; but as the paddle enters the water in
the position H, its action upon the water, not being horizontal,
is only partially effective for propulsion: a part of the force
which drives the paddle is expended in depressing the water, and
the remainder in driving it contrary to the course of the vessel,
and, therefore, by its re-action producing a certain propelling
effect. The tendency, however, of the paddle entering the water at
H, is to form a hollow or trough, which the water, by its ordinary
property, has a continual tendency to fill up. After passing the
lowest point A, as the paddle approaches the position B, where it
[Pg474] emerges from the water, its action again becomes oblique,
a part only having a propelling effect, and the remainder having a
tendency to raise the water, and throw up a wave and spray behind
the paddle-wheel. It is evident that the more deeply the
paddle-wheel becomes immersed, the greater will be the proportion
of the propelling power thus wasted in elevating and depressing
the water; and if the wheel were immersed to its axis, the whole
force of the paddle-boards, on entering and leaving the water,
would be lost, no part of it having a tendency to propel. If a
still deeper immersion take place, the paddle-boards above the
axis would have a tendency to retard the course of the vessel.
When the vessel is, therefore, in proper trim, the immersion
should not exceed nor fall short of the depth of the lowest
paddle; but for various reasons it is impossible in practice to
maintain this fixed immersion: the agitation of the surface of the
sea, causing the vessel to roll, will necessarily produce a great
variation in the immersion of the paddle-wheels, one becoming
frequently immersed to its axle, while the other is raised
altogether out of the water. Also the draught of water of the
vessel is liable to change, by the variation in her cargo; this
will necessarily happen in steamers which take long voyages. At
starting they are heavily laden with fuel, which as they proceed
is gradually consumed, whereby the vessel is lightened.
(223.) To remove this defect, and economise as much as possible
the propelling effect of the paddle-boards, it would be necessary
so to construct them that they may enter and leave the water
edgeways, or as nearly so as possible; such an arrangement would
be, in effect, equivalent to the process called feathering, as
applied to oars. Any mechanism which would perfectly accomplish
this would cause the paddles to work in almost perfect silence,
and would very nearly remove the inconvenient and injurious
vibration which is produced by the action of the common paddles.
But the construction of feathering paddles is attended with great
difficulty, under the peculiar circumstances in which such wheels
work. Any mechanism so complex that it could not be easily
repaired when deranged, with such engineering implements and
skill [Pg475] as can be obtained at sea, would be attended with
great objections; and the efficiency of its propelling action
would not compensate for the dangers which must attend upon the
helpless state of a steamer, deprived of her propelling agents.
Feathering paddle-boards must necessarily have a motion
independently of the motion of the wheel, since any fixed position
which could be given to them, though it might be most favourable to
their action in one position would not be so in their whole course
through the water. Thus the paddle-board when at the lowest point
should be in a vertical position, or so placed that its plane, if
continued upwards, would pass through the axis of the wheel. In
other positions, however, as it passes through the water, it should
present its upper edge, not towards the axle of the wheel, but
towards a point above the highest point of the wheel. The precise
point to which the edge of the paddle-board should be directed is
capable of mathematical determination. But it will vary according to
circumstances, which depend on the motion of the vessel. The
progressive motion of the vessel, independently of the wind or
current, must obviously be slower than the motion of the
paddle-boards round the axle of the wheel; since it is by the
difference of these velocities that the re-action of the water is
produced by which the vessel is propelled. The proportion, however,
between the progressive speed of the vessel and the rotative speed
of the paddle-boards is not fixed: it will vary with the shape and
structure of the vessel, and with its depth of immersion;
nevertheless it is upon this proportion that the manner in which the
paddle-boards should shift their position must be determined. If the
progressive speed of the vessel were nearly equal to the rotative
speed of the paddle-boards, the latter should so shift their
position that their upper edges should be presented to a point very
little above the highest point of the wheel. This is a state of
things which could only take place in the case of a steamer of a
small draught of water, shallop-shaped, and so constructed as to
suffer little resistance from the fluid. On the other hand, the
greater the depth of immersion, and the less fine the lines of the
[Pg476] vessel, the greater will be the resistance in passing
through the water, and the greater will be the proportion which the
rotative speed of the paddle-boards will bear to the progressive
speed of the vessel. In this latter case the independent motion of
the paddle-boards should be such that their edges, while in the
water, shall be presented towards a point considerably above the
highest point of the paddle-wheel.
A vast number of ingenious mechanical contrivances have been
invented and patented for accomplishing the object just explained.
Some of these have failed from the circumstance of their inventors
not clearly understanding what precise motion it was necessary to
impart to the paddle-board: others have failed from the complexity
of the mechanism by which the desired effect was produced.
(224.) In the year 1829 a patent was granted to Elijah Galloway
for a paddle-wheel with movable paddles, which patent was
purchased by Mr. William Morgan, who made various alterations in
the mechanism, not very materially departing from the principle of
the invention.
[Illustration: _Fig._ 133.]
This paddle-wheel is represented in _fig._ 133. The contrivance
may be shortly stated to consist in causing the wheel which bears
the paddles to revolve on one centre, and the radial arms which
move the paddles to revolve on another centre. Let A B C D E F G H
I K L be the polygonal circumference of the paddle-wheel, formed
of straight bars, securely connected together at the extremities
of the spokes or radii of the wheel which turns on the shaft which
is worked by the engine; the centre of this wheel being at O. So
far this wheel is similar to the common paddle-wheel; but the
paddle-boards are not, as in the common wheel, fixed at A B C,
&c., so as to be always directed to the centre O, but are so
placed that they are capable of turning on axles which are always
horizontal, so that they can take any angle with respect to the
water which may be given to them. From the centres, or the line
joining the pivots on which these paddle-boards turn, there
proceed short arms K, firmly fixed to the paddle-boards at an
angle of about 120°. On a motion given to this arm K, it will
therefore give a corresponding angular motion to the paddle-board,
so as to make it turn on its pivots. At [Pg477] the extremities
of the several arms marked K is a pin or pivot, to which the
extremities of the radial arms L are severally attached, so that
the angle between each radial arm L and the short paddle-arm K is
capable of being changed by any motion imparted to L; the radial
arms are connected at the other end with a centre, round which
they are capable of revolving. Now, since the points A B C, &c.,
which are the pivots on which the paddle-boards turn, are moved in
the circumference of a circle, of which the centre is O, they are
always at the same distance from that point; consequently they
will continually vary their distance from the other centre P.
Thus, when a paddle-board arrives at that point of its revolution
at which the centre round which it revolves lies precisely between
it and the centre O, its distance from the former centre is less
than in any other position. As it departs from that point, its
distance from that centre gradually increases until it arrives at
the opposite point of its revolution, where the centre O is
exactly between it and the former centre; then the distance of the
paddle-board from the former centre is greatest. [Pg478] This
constant change of distance between each paddle-board and the
centre P is accommodated by the variation of the angle between the
radial arm L and the short paddle-board arm K; as the paddle-board
approaches the centre P this gradually diminishes; and as the
distance of the paddle-board increases, the angle is likewise
augmented. This change in the magnitude of the angle, which thus
accommodates the varying position of the paddle-board with respect
to the centre P, will be observed in the figure. The paddle-board
D is nearest to P; and it will be observed that the angle
contained between L and K is there very acute; at E the angle
between L and K increases, but is still acute; at G it increases
to a right angle; at H it becomes obtuse; and at K, where it is
most distant from the centre P, it becomes most obtuse. It again
diminishes at K, and becomes a right angle between A and B. Now
this continual shifting of the direction of the short arm K is
necessarily accompanied by an equivalent change of position in the
paddle-board to which it is attached; and the position of the
second centre P is, or may be, so adjusted that this paddle-board,
as it enters the water and emerges from it, shall be such as shall
be most advantageous for propelling the vessel, and therefore
attended with less of that vibration which arises chiefly from the
alternate depression and elevation of the water, owing to the
oblique action of the paddle-boards.
(225.) In the year 1833, Mr. Field, of the firm of Maudslay and
Field, constructed a paddle-wheel with fixed paddle-boards, but
each board being divided into several narrow slips arranged one a
little behind the other, as represented in _fig._ 134. These
divided boards he proposed to arrange in such cycloidal curves
that they must all enter the water at the same place in immediate
succession, avoiding the shock produced by the entrance of the
common board. These split paddle-boards are as efficient in
propelling when at the lowest point as the common paddle-boards,
and when they emerge the water escapes simultaneously from each
narrow board, and is not thrown up, as is the case with common
paddle-boards.[39]
[Illustration: _Fig._ 134.]
[Pg479] The theoretical effect of this wheel is the same as that
of the common wheel, and experience alone, the result of which has
not yet been obtained, can prove its efficiency. The number of
bars, or separate parts into which each paddle-board is divided,
has been very various. When first introduced by Mr. Galloway each
board was divided into six or seven parts: this was subsequently
reduced, and in the more recent wheels of this form constructed
for the government vessels the paddle-boards consist only of two
parts, coming as near to the common wheel as is possible, without
altogether abandoning the principle of the split paddle.
(226.) To obtain an approximate estimate of the extent to which
steam-power is applicable to long sea-voyages, it would be
necessary to investigate the mutual relation which, in the
existing state of this application of steam-power, exists between
the capacity or tonnage of the vessel, the magnitude, weight, and
power, of the machinery, the available stowage for fuel, and the
average speed attainable in all [Pg480] weathers, as well as the
general purposes to which the vessel is to be appropriated,
whether for the transport of goods or merchandise, or merely for
despatches and passengers, or for both of these combined. That
portion of the capacity of the vessel which is appropriated to the
moving power consists of the space occupied by the machinery and
the fuel. The distribution of it between these must mainly depend
on the length of the voyage which the vessel must make without
receiving a fresh supply of coals. If the trips be short, and
frequent relays of fuel can be obtained, then the space allotted
to the machinery may bear a greater proportion to that assigned to
the fuel; but in proportion as each uninterrupted stage of the
voyage is increased, a greater stock of coals will be necessary,
and a proportionally less space left for the machinery. Other
things being the same, therefore, steam-vessels intended for long
sea-voyages must be less powerful in proportion to their tonnage.
It will be apparent that every improvement which takes place in
the application of the steam-engine to navigation will modify all
these data on which such an investigation must depend. Every
increased efficiency of fuel, from whatever cause it may be
derived, will either increase the useful tonnage of the vessel, or
increase the length of the voyage of which it is capable. Various
improvements have been and are still in progress, by which this
efficiency has undergone continual augmentation, and voyages may
now be accomplished with moderate economy and profit, to which a
few years since marine engines could not be applied with permanent
advantage. The average speed of steam-vessels has also undergone a
gradual increase by such improvements. During the four years
ending June, 1834, it was found that the average rate of steaming
obtained from fifty-one voyages made by the Admiralty steamers
between Falmouth and Corfu, exclusive of stoppages, was seven
miles and a quarter an hour direct distance between port and port.
The vessels which performed this voyage varied from 350 to 700
tons measured burden, and were provided with engines varying from
100 to 200 horse-power, with stowage for coals varying from 80 to
240 tons. The proportion of the power to the [Pg481] tonnage
varied from one horse to three tons to one horse to four tons.
Thus the MESSENGER had a power of 200 horses and measured 730
tons; the FLAMER had a power of 120 horses, and measured 500 tons;
the COLUMBIA had a power of 120 horses, and measured 360 tons. In
general it may be assumed that for the shortest class of trips,
such as those of the Channel steamers, the proportion of the power
to the tonnage should be about one horse for every two tons; but
for the longer class of voyages, the proportion of power to
tonnage should be about one horse-power to from three to four tons
measured tonnage. These data, however, must be received as very
rough approximations, subject to considerable modifications in
their application to particular vessels. We have already stated
that the nominal horse-power is itself extremely indefinite; and
if, as is now customary in the longer class of voyages, the steam
be worked expansively, then the nominal power almost ceases to
have any definite relation to the actual performance of the
vessel. It is usual to calculate the horse-power by assuming a
uniform pressure of steam upon the piston, and, consequently, by
excluding the consideration of the effect of expansion. The most
certain test of the amount of mechanical power exerted by the
machinery would be obtained from the quantity of water actually
transmitted in the form of steam from the boiler to the cylinder.
But the effect of this would also be influenced by the extent to
which the expansive principle has been brought into operation.
From the reported performances of the larger class of steam-ships
within the last few years, it would appear that the average speed
has been increased since the estimate above mentioned, which was
obtained in 1834; and on comparing the consumption of fuel with
the actual performance, it would appear that the efficiency of
fuel has also been considerably augmented. No extensive course of
accurate experiments or observations have, however, been obtained
from which correct inferences may be drawn of the probable limits
to which steam-navigation, in its present state, is capable of
being extended. The jealousy of rival companies has obstructed the
inquiries of those who, solicitous more [Pg482] for the general
advancement of the art than for the success of individual
enterprises, have directed their attention to this question; and
it is hardly to be expected that sufficiently correct and
extensive data can be obtained for this purpose.
(227.) Increased facility in the extension and application of
steam-navigation is expected to arise from the substitution of iron
for wood, in the construction of vessels. Hitherto iron steamers
have been chiefly confined to river-navigation; but there appears no
sufficient reason why their use should be thus limited. For
sea-voyages they offer many advantages; they are not half the weight
of vessels of equal tonnage constructed of wood; and, consequently,
with the same tonnage they will have less draught of water, and
therefore less resistance to the propelling power; or, with the same
draught of water and the same resistance, they will carry a
proportionally heavier cargo. The nature of their material renders
them more stiff and unyielding than timber; and they do not suffer
that effect which is called _hogging_, which arises from a slight
alteration which takes place in the figure of a timber vessel in
rolling, accompanied by an alternate opening and closing of the
seams. Iron vessels have the further advantage of being more proof
against fracture upon rocks. If a timber vessel strike, a plank is
broken, and a chasm opened in her many times greater than the point
of rock which produces the concussion. If an iron vessel strike, she
will either merely receive a dinge, or be pierced by a hole equal in
size to the point of rock which she encounters. Some examples of the
strength of iron vessels were given by Mr. Macgregor Laird, in his
evidence before the Committee of the Commons on Steam Navigation,
among which the following may be mentioned:—An iron vessel, called
the ALBURKAH, in one of their experimental trials got aground, and
lay upon her anchor: in a wooden vessel the anchor would probably
have pierced her bottom; in this case, however, the bottom was only
dinged. An iron vessel, built for the Irish Inland Navigation
Company, was being towed across Lough Derg in a gale of wind, when
the towing rope broke, and she was driven upon rocks, on which she
bumped for a considerable time [Pg483] without any injury. A wooden
vessel would in this case have gone to pieces. A further advantage
of iron vessels (which in warm climates is deserving of
consideration) is their greater coolness and perfect freedom from
vermin.
Iron steam-vessels on a very large scale are now in preparation in
the ports of Liverpool and Bristol, intended for long sea-voyages.
The largest vessel of this description which has yet been
projected is stated to be in preparation for the voyage between
Bristol and New York, by the company who have established the
steam-ship called the Great Western, plying between these places.
Several projects for the extension of steam-navigation to voyages
of considerable length have lately been entertained both by the
public and by the legislature, and have imparted to every attempt
to improve steam-navigation increased interest. A committee of the
House of Commons collected evidence and made a report in the last
session in favour of an experiment to establish a line of
steam-communication between Great Britain and India. Two routes
have been suggested by the committee, each being a continuation of
the line of Admiralty steam-packets already established to Malta
and the Ionian Isles. One of the routes proposed is through Egypt,
the Red Sea, and across the Indian Ocean to Bombay, or some of the
other presidencies; the other across the north part of Syria to
the banks of the Euphrates, by that river to the Persian Gulf, and
from thence to Bombay. Each of these routes will be attended with
peculiar difficulties, and in both a long sea-voyage will be
encountered.
In the route by the Red Sea it is proposed to establish steamers
between Malta and Alexandria (eight hundred and sixty miles). A
steamer of four hundred tons' burden and one hundred horse-power
would perform this voyage, upon an average of all weathers
incident to the situation, in from five to six days, consuming ten
tons of coal per day. But it is probable that it might be found
more advantageous to establish a higher ratio between the power
and the tonnage. From Alexandria the transit might be effected by
land across the isthmus to Suez—a journey of from four to five
days—by caravan and camels; or the transit might be made either
[Pg484] by land or water from Alexandria to Cairo, a distance of
one hundred and seventy-three miles; and from Cairo to Suez,
ninety-three miles, across the desert, in about five days. At Suez
would be a station for steamers, and the Red Sea would be
traversed in three runs or more. If necessary, stations for coals
might be established at Cosseir, Judda, Mocha, and finally at Aden
or at Socatra—an island immediately beyond the mouth of the Red
Sea, in the Indian Ocean; the run from Suez to Cosseir would be
three hundred miles—somewhat more than twice the distance from
Liverpool to Dublin. From Cosseir to Judda, four hundred and fifty
miles; from Judda to Mocha, five hundred and seventeen miles; and
from Mocha to Socatra, six hundred and thirty-two miles. It is
evident that all this would, without difficulty, in the most
unfavourable weather, fall within the present powers of
steam-navigation. If the terminus of the passage be Bombay, the
run from Socatra to Bombay will be twelve hundred miles, which
would be from six to eight days' steaming. The whole passage from
Alexandria to Bombay, allowing three days for delay between Suez
and Bombay, would be twenty-six days: the time from Bombay to
Malta would therefore be about thirty-three days; and adding
fourteen days to this for the transit from Malta to England, we
should have a total of forty-seven days from London to Bombay, or
about seven weeks.
If the terminus proposed were Calcutta, the course from Socatra
would be one thousand two hundred and fifty miles south-east to
the Maldives, where a station for coals would be established. This
distance would be equal to that from Socatra to Bombay. From the
Maldives, a run of four hundred miles would reach the southern
point of Ceylon, called the Point de Galle, which is the best
harbour (Bombay excepted) in British India: from the Point de
Galle, a run of six hundred miles will reach Madras, and from
Madras to Calcutta would be a run of about six hundred miles. The
voyage from London to Calcutta would be performed in about sixty
days.
At a certain season of the year there exists a powerful physical
opponent to the transit from India to Suez: from [Pg485] the
middle of June until the end of September, the south-west monsoon
blows with unabated force across the Indian Ocean, and more
particularly between Socatra and Bombay. This wind is so violent
as to leave it barely possible for the most powerful steam-packet
to make head against it, and the voyage could not be accomplished
without serious wear and tear upon the vessels during these
months.
The attention of parliament has therefore been directed to another
line of communication, not liable to this difficulty: it is
proposed to establish a line of steamers from Bombay through the
Persian Gulf to the Euphrates.
The run from Bombay to a place called Muscat, on the southern shore
of the gulf, would be eight hundred and forty miles in a north-west
direction, and therefore not opposed to the south-west monsoon. From
Muscat to Bassidore, a point upon the northern coast of the strait
at the mouth of the Persian Gulf, would be a run of two hundred and
fifty-five miles; from Bassidore to Bushire, another point on the
eastern coast of the Persian Gulf, would be a run of three hundred
miles; and from Bushire to the mouth of the Euphrates, would be one
hundred and twenty miles. It is evident that the longest of these
runs would offer no more difficulty than the passage from Malta to
Alexandria. From Bussora, near the mouth of the Euphrates, to Bir, a
town upon its left bank near Aleppo, would be one thousand one
hundred and forty-three miles, throughout which there are no
physical obstacles to the river-navigation which may not be
overcome. Some difficulties arise from the wild and savage character
of the tribes who occupy its banks. It is, however, thought that by
proper measures, and securing the co-operation of the pacha of
Egypt, any serious obstruction from this cause may be removed. From
Bir, by Aleppo, to Scanderoon, a port upon the Mediterranean,
opposite Cyprus, is a land-journey, said to be attended with some
difficulty, but not of great length; and from Scanderoon to Malta is
about the same distance as between the latter place and Alexandria.
It is calculated that the time from London to Bombay by the
Euphrates—supposing the passage to be successfully [Pg486]
established—would be a few days shorter than by Egypt and the Red
Sea.
Whichever of these courses may be adopted, it is clear that the
difficulties, so far as the powers of the steam engine are
concerned, lie in the one case between Socatra and Bombay, or
between Socatra and the Maldives, and in the other case between
Bombay and Muscat. This, however, has already been encountered and
overcome on four several voyages by the HUGH LINDSAY steamer from
Bombay to Suez: that vessel encountered a still longer run on
these several trips, by going, not to Socatra, but to Aden, a
point on the coast of Arabia, near the Straits of Babel Mandeb,
being a run of one thousand six hundred and forty-one miles, which
she performed in ten days and nineteen hours. The same trip has
since been repeatedly made by other steamers; and, in the present
improved state of steam navigation, no insurmountable obstacles
are opposed to their passage.
[Illustration]
FOOTNOTES:
[35] This cut is taken from the plate of the engine of the Red
Rover, manufactured by Boulton and Watt, given in the last
edition of _Tredgold on the Steam Engine_.
[36] Appendix I., _on Marine Boilers, by J. Dinnen; Tredgold_
_on the Steam Engine_, second edition.
[37] _Tredgold on the Steam Engine_, Appendix, I. p. 171.
[38] Engines on a very large scale constructed upon this
principle are said to be in process of construction for an
iron steam-vessel of great tonnage, which is in preparation
for the New York passage. It is said that the cylinders of
these engines will be one hundred and twenty inches in
diameter.
[39] A patent was subsequently taken out for these by Mr.
Galloway. Mr. Field did not persevere in its use at the time
he invented it. It has, however, been more generally adopted
since the date of Galloway's patent.
[Pg487]
[Illustration]
CHAP. XIV.
AMERICAN STEAM NAVIGATION.
STEAM NAVIGATION FIRST ESTABLISHED IN AMERICA. — CIRCUMSTANCES
WHICH LED TO IT. — FITCH AND RUMSEY. — STEVENS OF HOBOKEN. —
LIVINGSTONE AND FULTON. — EXPERIMENTS ON THE SEINE. — FULTON'S
FIRST BOAT. — THE HUDSON NAVIGATED BY STEAM. — EXTENSION AND
IMPROVEMENT OF RIVER NAVIGATION. — SPEED OF AMERICAN STEAMERS.
— DIFFERENCE BETWEEN THEM AND EUROPEAN STEAMERS. — SEA-GOING
AMERICAN STEAMERS. — AMERICAN PADDLE-WHEELS. — LAKE STEAMERS.
— THE MISSISIPPI AND ITS TRIBUTARIES. — STEAMERS NAVIGATING
IT. — THEIR STRUCTURE AND MACHINERY. — NEW ORLEANS HARBOUR. —
STEAM TUGS.
(228.) The credit of having afforded the first practical solution
of the problem to apply the steam engine to the propulsion of
ships, undoubtedly belongs to the people of the United States of
America. The geographical character of their vast country, not
less than the sanguine and enterprising spirit of the nation,
contributed to this. A coast of four thousand miles in extent,
stretching from the Gulf of St. Lawrence to the embouchures of the
Mississippi, indented and [Pg488] serrated in every part with
natural harbours and sheltered bays, and fringed with islands
forming sounds—capes, and promontories enclosing arms of the sea,
in which the waters are free from the roll of the ocean, and take
the placid character of lakes,—rivers of imposing magnitude,
navigable for vessels of the largest class, for many hundreds and
in some instances for many thousands of miles, affording access to
the innermost population of an empire, whose area vastly exceeds
the whole European continent,—chains of lakes composed of the
most extensive bodies of fresh water in the known world,—and this
extensive continent peopled by races carrying with them the habits
and feelings together with much of the skill and knowledge of the
most civilized parts of the globe, endowed also with that
inextinguishable spirit of enterprise which ever belongs to an
emigrant people,—form a combination of circumstances more than
sufficient to account for the fact of this nation snatching from
England, the parent of the steam engine, the honour of first
bringing into practical operation one of the most important—if
indeed it be not altogether the most important—of the many
applications of that machine to the uses of life.
The circumstances which rendered these extensive tracts of inland
and coast navigation eminently suited to the application of steam
power, formed so many obstructions and difficulties to the
application of other more ordinary means of locomotion on water.
The sheltered bays and sounds which offered a smooth and
undisturbed surface to the action of the infant steamer argued the
absence of that element which gave effect to the sails and rigging
of the wind-propelled ship, and the rapid currents of the gigantic
streams formed by the drainage of this great continent, though
facilitating access to the coast, rendered the oar powerless in
the ascent.
(229.) The first great discovery of Watt had scarcely been
realized in practice by the construction of the single-acting
steam-engine, when the speculative and enterprising Americans
conceived the project of applying it as a moving power in their
inland navigation. So early as the year 1783 [Pg489] Fitch and
Rumsey made attempts to apply the single-acting engine to the
propulsion of vessels, and their failure is said to have arisen
more from the inherent defects of that machine in reference to
this application of it, than from any want of ingenuity or
mechanical skill on their parts. In 1791, John Stevens of Hoboken
commenced his experiments on steam navigation, which were
continued for sixteen years; during a part of this period he was
assisted by Livingstone (who was subsequently instrumental in
advancing the views of Fulton), and by Roosevelt. These projectors
had, at that time also, the assistance and advice of Brunel, since
so celebrated for the invention of the block machinery, and the
construction of the Thames Tunnel. Their proceedings were
interrupted by the appointment of Livingstone as American Minister
at Paris, under the Consular Government.
At Paris, Livingstone met Fulton, who had been previously engaged
in similar speculations, and being struck with his mechanical
skill, and the soundness of his views, joined him in causing a
series of experiments to be made, which were accordingly carried
on at Plombières, and subsequently on a still more extensive scale
on the Seine, near Paris. Having by this course of experiments
obtained proofs of the efficiency of Fulton's projects, sufficient
to satisfy the mind of Livingstone, he agreed to obtain for Fulton
the funds necessary to construct a steam boat on a large scale, to
be worked upon the Hudson. It was decided, in order to give the
project the best chance of success, to obtain the machinery from
Bolton and Watt. In 1803, Fulton accordingly made drawings of the
engines intended for this first steamer, which were sent to Soho,
with an order for their construction. Fulton, meanwhile, repaired
to America, to superintend the construction of the boat. The
delays incidental to these proceedings retarded the completion of
the boat and machinery until the year 1807, when all was
completed, and the first successful experiment made at New York.
The vessel was placed, for regular work, to ply between New York
and Albany, in the beginning of 1808; and, from that time to the
present, this river has been the theatre of the most [Pg490]
remarkable series of experiments on locomotion on water which has
ever been presented in the history of navigation.
(230.) The form and arrangement of this first marine engine was,
in many respects, similar to that which is still generally used
for marine purposes. The cold water cistern was abandoned, and an
increased condensing power obtained by enlarging the condenser. It
was usual to make the condenser half the diameter of the cylinder,
and half its length, and therefore one eighth of its capacity. The
condenser, however, was now made of the same diameter as the
cylinder, being still half its length; its capacity therefore,
instead of being only an eighth, was half of the cylinder; the
condensing jet was admitted by a pipe passing through the bottom
of the vessel. As in the present marine engines, two working beams
were provided, one at either side of the cylinder; but in order to
provide against the difficulties which might arise in the
adaptation of machinery made at Birmingham to a vessel made at New
York, beams were constructed in the form of an inverted ┻, the
working arms being twofold, one horizontal and the other vertical,
so that the connecting rod might be carried from the crank, either
downwards, to the end of the horizontal arm, or horizontally, to
the end of the vertical arm. In fact there was a choice, to use
either a straight beam, or a bell-crank. The latter was that which
was adopted in this instance. The paddle-shaft, driven by the
crank, passed across the vessel, and had the paddle-wheels keyed
upon it as at present; and in order to equalise the effect of the
engine spur wheels were also placed on the paddle-shaft, by which
pinions were driven, placed upon an axle, which carried a
fly-wheel.
The speed attained by this steam boat, when it first began to ply
upon the river, did not exceed four miles an hour, but by a series
of improvements its rate of motion was soon increased to six miles
an hour. In the steam boats subsequently constructed by Fulton a
greater speed was attained; but in the latest vessels built by him
he did not exceed a speed of nine miles an hour, which he
considered to be the greatest that could be advantageously
obtained.
While Fulton was making his plans, and engaged in the [Pg491]
construction of his first boat, Mr. Stevens of Hoboken, already
mentioned, was engaged in a like project, and completed a vessel,
to be propelled by a steam engine, within a few weeks after the
first successful voyage of Fulton. Stevens was likewise completely
successful; but the exclusive privilege of navigating the Hudson
by steam having been granted to Fulton by an act of Congress,
Stevens was compelled to select another theatre for his
operations, and he accordingly sent his steam boat by sea to
Philadelphia, to navigate the Delaware, thus securing for himself
the honour of having made the first sea voyage by steam.
Fulton did not long retain the monopoly of the steam navigation of
the Hudson. Fortunately for the progress of steam navigation, the act
conferring upon him that privilege was declared unconstitutional;
and the navigation of that noble river was thrown open to the spirit
and enterprise of American genius. The number of passengers conveyed
upon it became enormous beyond all precedent, and inducements of the
strongest kind were accordingly held out to the improvement of its
navigation. The distance between New York and Albany, ascertained by
a late survey to be one hundred and twenty-five geographical miles by
water, had been performed by Fulton's boats occasionally in fifteen
or sixteen hours, being at the rate of about eight miles an hour,
including stoppages. It became a great object to increase the speed
of this trip, so that it might at all times of the year be performed
between sunrise and sunset. Robert L. Stevens, the son of the person
of that name already mentioned, immediately after the abolition
of Fulton's monopoly, placed on the river a vessel which had been
built for the Delaware, which easily performed the passage in twelve
hours, being at the rate of nearly ten and a half geographical miles
an hour. By this increase of speed the improved boats so entirely
monopolised the day work upon the river, that the former steamers
were either converted into steam tugs to draw barges laden with
goods, or used for night trips between New York and Albany. In the
night trips the saving of one or two hours was immaterial, it being
sufficient that the vessel which left the one port at night should
reach the other in the morning. [Pg492]
The river Hudson rises near Lake Champlain, the easternmost of the
great chain of lakes or inland seas which extend from east to west
across the northern boundary of the United States. The river
follows nearly a straight course southwards for two hundred and
fifty miles, and empties itself into the sea at New York. The
influence of the tide is felt as far as Albany, above which the
stream begins to contract. Although this river in magnitude and
extent is by no means equal to several others which intersect the
States, it is nevertheless rendered an object of great interest by
reason of the importance and extent of its trade. The produce of
the state of New York and that of the banks of the great Lakes
Ontario and Erie are transported by it to the capital; and one of
the most extensive and populous districts of the United States is
supplied with the necessary imports by its waters. A large fleet
of vessels is constantly engaged in its navigation; nor is the
tardy but picturesque sailing vessel as yet excluded by the more
rapid steamers. The current of the Hudson is said to average
nearly three miles an hour; but as the ebb and flow of the tide
are felt as far as Albany, the passage of the steamers between
that place and New York may be regarded as equally affected by
currents in both directions, or nearly so. The passage therefore,
whether in ascending or descending the river, is made nearly in
the same time.
(231.) The prevalence of smooth water navigation, whether on the
surfaces of rivers or in sheltered bays and sounds, has invested
the problem of steam navigation in America with conditions so
entirely distinct and different from those under which the same
problem presents itself to the European engineer, that any
comparison of the performance of vessels, whether with regard to
speed or the absorption of power in the two cases, must be utterly
fallacious. In Europe a steamer is almost invariably a vessel
designed to encounter the agitated surface of an open sea, and is
accordingly constructed upon principles of suitable strength and
stability. It is likewise supplied with rigging and with sails, to
be used in aid of the mechanical power, and manned and commanded
by experienced seamen; in fact, it is a combination of a nautical
and mechanical structure. In America, on the other hand, [Pg493]
with the exception of the vessels which navigate the great
northern lakes, the steamers are structures exclusively
mechanical, being designed for smooth water. They require no other
strength or stability than that which is sufficient to enable them
to float and to bear a progressive motion through the water. Their
mould is conceived with an exclusive view to speed; they are
therefore slender and weak in their build, of great length in
proportion to their width, and having a very small draught of
water. In fact, they approach in their form to that of a Thames
wherry on a very large scale.
The position and form of the machinery is likewise affected by
these conditions. Without the necessity of being protected from a
rough sea, it is placed on the deck in an elevated position. The
cylinders of large diameter and short stroke invariably used in
Europe are unknown in America, and the proportions are reversed, a
small diameter and stroke of great length being invariably
adopted. It is rarely that two engines are used. A single engine,
placed in the centre of the deck, with a cylinder from forty to
sixty inches' diameter, and from eight to ten foot stroke, drives
paddle-wheels from twenty-one to twenty-five feet in diameter,
producing from twenty-five to thirty revolutions per minute. The
great magnitude of the paddle-wheels and the velocity imparted to
them enable them to perform the office of fly-wheels, and to carry
the engine round its centres, not however without a perceptible
inequality of motion, which gives to the American steamer an
effect like that of a row boat advancing by starts with each
stroke of the piston. The length of stroke adopted in these
engines enables them to apply with great effect the expansive
principle, which is almost universally used, the steam being
generally cut off at half stroke.
The steamers which navigate the Hudson are vessels of considerable
magnitude, splendidly fitted up for the accommodation of
passengers; they vary from one hundred and eighty to two hundred
and forty feet in length, and from twenty to thirty feet in width
of beam. In the following table is given the particulars of nine
steamers plying on this river, taken from [Pg494] the work of Mr.
Stevenson, and from the paper of Mr. Renwick, inserted in the last
edition of Tredgold:—
—————————————————————————————————————————————————————————————
| Length | Breadth | Draft | Drain | Length
Names. | of | of | of | of | of
| Deck. | Beam. | Water. | Wheel. | Paddles.
—————————————————————————————————————————————————————————————
| Ft. | Ft. | Ft. | Ft. | Ft.
Dewit Clinton | 230 | 28 | 5·5 | 21 | 13·7
Champlain | 180 | 27 | 5·5 | 22 | 15
Erie | 180 | 27 | 5·5 | 22 | 15
North America | 200 | 30 | 5 | 21 | 13
Independence | 148 | 26 | —— | —— | ——
Albany | 212 | 26 | —— | 24·5 | 14
Swallow | 233 | 22·5 | 3·75 | 24 | 11
Rochester | 200 | 25 | 3·75 | 23·5 | 10
Utica | 200 | 21 | 3·5 | 22 | 9·5
—————————————————————————————————————————————————————————————
—————————————————————————————————————————————————————————————————————
| Depth | Number | Drain | Length| Number | Part of
Names. | of | of | of | of | of | Stroke
| Paddles| Engines| Cylinder| Stroke| Revs | at which
| | | | | | it is
| | | | | | cut off.
—————————————————————————————————————————————————————————————————————
| In. | | In. | Ft. | |
Dewit Clinton | 36 | 1 | 65 | 10 | 29 | 3/4
Champlain | 34 | 2 | 44 | 10 | 27·5 | 1/2
Erie | 34 | 2 | 44 | 10 | 27·5 | 1/2
North America | 30 | 2 | 44·5 | 8 | 24 | 1/2
Independence | —— | 1 | 44 | 10 | |
Albany | 30 | 1 | 65 | —— | 19 |
Swallow | 30 | 1 | 46 | —— | 27 |
Rochester | 24 | 1 | 43 | 10 | 28 |
Utica | 24 | 1 | 39 | 10 | |
—————————————————————————————————————————————————————————————————————
None of these vessels have either masts or rigging, and
consequently never derive any propelling power except from the
engines: they are neither manned nor commanded by persons having
any knowledge of navigation: the works that are visible above
their decks are the beam and framing of the engine, and the
chimneys.
The engines used for steamers on the Hudson, and other great
rivers and bays on the eastern coast of America, are most commonly
condensing engines, but they nevertheless work with steam of very
high pressure, being seldom less than twenty-five pounds per
square inch, and sometimes as much as fifty. By reference to the
preceding table it will be seen, that the velocity of the piston
greatly exceeds the limit generally observed in Europe. It is
customary in European marine engines to limit the speed of the
piston to about two hundred and twenty feet per minute. Even the
piston of a locomotive engine does not much exceed the rate of
three hundred feet per minute. In the American steamers, however,
the pistons commonly move at the rate of from five to six hundred
feet per minute, while the circumference of the paddle-wheels are
driven at the rate of from twenty to twenty-two miles an hour.
[Pg495]
[Illustration: _Fig._ 135.]
The hulls of these boats are formed with a perfectly flat bottom
and perpendicular sides, rounded at the angles, as represented in
_fig._ 135. At the bow, or cutwater, they are made very sharp, and
the deck projects to a great distance over the sides. The weight
of the machinery is distributed over an extensive surface of the
bottom of this feeble structure, by means of a frame-work of
substantial carpentry to which it is attached.
At the height of from four to six feet above the water-line is
placed the deck, which is a platform, having the shape of a very
elongated ellipse. The extremities of its longer axis are
supported by the sternpost and the cutwater, and its sides expand
in gentle curves on either hand to a considerable distance beyond
the limits of the hull; those parts of the deck thus overhanging
the water are called the wheel guards.
Beneath the first deck is the saloon, or dining-room, which also,
as is usual in European steamers, forms the gentlemen's
sleeping-room. It usually extends from end to end of the vessel.
The middle of the first deck is occupied by the engine, boilers,
furnaces, and chimneys, of which latter there are generally two.
Between the chimneys and the stern, above the first deck, is
constructed the ladies' cabin, which is covered by the second
deck, called the promenade deck. The great length of these boats
and the elevation of the cabins render it impossible for a
steersman at the stern to see ahead, and they are, consequently,
steered from the bow; the wheel placed there communicating with
the helm at the stern, by chains or rods carried along the sides
of the boat. Until a recent period, the wheel was connected with
the stern by ropes, but some fatal accidents, produced by fire,
[Pg496] in which these ropes were burnt, and the steersman lost
all power to guide the vessel, caused metal rods or chains to be
substituted.
(232.) The paddle-wheels universally used in American steam-boats
are formed, as if by the combination of two or more common
paddle-wheels, placed one outside the other, on the same axle, but
so that the paddle boards of each may have an intermediate
position between those of the adjacent one, as represented in
_fig._ 136.
[Illustration: _Fig._ 136.]
The spokes, which are bolted to cast-iron flanges, are of wood.
These flanges, to which they are so bolted, are keyed upon the
paddle shaft. The outer extremities of the spokes are attached to
circular bands or hoops of iron, surrounding the wheel; and the
paddle boards, which are formed of hard wood, are bolted to the
spokes. The wheels thus constructed, sometimes consist of three,
and not unfrequently four, independent circles of paddle boards,
placed one beside the other, and so adjusted in their position,
that the boards of no two divisions shall correspond.
The great magnitude of the paddle-wheels, and the circumstance of
the navigation being carried on, for the most part, in smooth
water, have rendered unnecessary, in America, the adoption of any
of those expedients for neutralising the effects of the oblique
action of the paddles, which have been tried, but hitherto with so
little success, in Europe.
(233.) Sea-going steamers are not numerous in America, the chief
of them being those which ply between New York and Providence, and
between New York and Charleston. These vessels, however, do not
resemble the sea-going steamers of Europe as closely as might be
expected; and to those who are accustomed to the latter, the
sea-going [Pg497] steamers of America can hardly be regarded as
safe means of transport.
In the following Table is given the dimensions of five of these
vessels, all plying between New York and Providence:—
——————————————————————————————————————————————————————————————
| Length | Breadth | Draft | Diameter | Length
Names. | of | of | | of | of
| Deck. | Beam. | | Wheel. | Paddles.
——————————————————————————————————————————————————————————————
| Ft. | Ft. | Ft. | Ft. | Ft.
Providence | 180 | 27 | 9 | —— | ——
Lexington | 207 | 21 | —— | 23 | 9
Narragansett | 210 | 26 | 5 | 25 | 11
Massachusetts | 200 | 29·5 | 8·5 | 22 | 10
Rhode Island | 210 | 26 | 6·5 | 24 | 11
——————————————————————————————————————————————————————————————
—————————————————————————————————————————————————————————————————————
| Depth | Number | Diameter| Length| Number | Part of
Names. | of | of | of | of | of | Stroke
| Paddles| Engines| Cylinder| Stroke| Revs | at which
| | | | | | it is
| | | | | | cut off.
—————————————————————————————————————————————————————————————————————
| In. | | In. | Ft. | |
Providence | —— | 1 | 10 | 65 | |
Lexington | 30 | 1 | 11 | 48 | 24 |
Narragansett | 30 | 1 | 60 | 12 | 2 | 1/2
Massachusetts | 28 | 2 | 44 | 8 | 26 |
Rhode Island | 30 | 1 | 11 | 60 | 21 |
—————————————————————————————————————————————————————————————————————
The Narragansett, the finest of these vessels, is built of oak,
strengthened by diagonal straps or ties of iron, by which her
timbers are connected; she is driven by a condensing engine, and
has two boilers, exposing about three thousand square feet of
surface to the fire. The steam is maintained at a pressure of from
twenty to twenty-five lbs. per square inch: the cylinder is
horizontal.
The cabins of these sea-boats are of great magnitude, and afford
excellent accommodation for passengers, containing generally four
hundred berths. In the Massachusetts the chief cabin is one hundred
and sixty feet long, twenty-two feet wide, and twelve feet in
height, its vast extent being uninterrupted by pillars or any other
obstruction. "I have dined," says Mr. Stevenson, "with one hundred
and seventy-five persons in this cabin, and, notwithstanding this
numerous assembly, the tables, which were arranged in two parallel
rows, extending from one end of the cabin to the other, were far
from being fully occupied, the attendance was good, and every thing
was conducted with perfect regularity and order. There are one
hundred and twelve fixed berths ranged round this cabin, and one
hundred temporary berths can be erected in the middle of the floor:
besides these there are sixty fixed berths in the ladies' cabin, and
several temporary sleeping [Pg498] places can be erected in it
also. The cabin of the Massachusetts is by no means the largest in
the United States. Some steamers have cabins upwards of one hundred
and seventy-five feet in length. Those large saloons are lighted by
Argand lamps, suspended from the ceiling, and their appearance, when
brilliantly lighted up and filled with company, is very remarkable.
The passengers generally arrange themselves in parties at the
numerous small tables into which the large tables are converted
after dinner, and engage in different amusements. The scene
resembles much more the coffee-room of some great hotel than the
cabin of a floating vessel."
(234.) Nothing has excited more surprise among engineers and
others interested in steam navigation in Europe, than the
statements which have been so generally and so confidently made of
the speed attained by American steamers. This astonishment is due
to several causes, the chief of which is the omission of all
notice of the great difference between the structure and operation
of the American steamers and the nature of the navigation in which
they are engaged, compared with the structure and operation of,
and the navigation in which European steamers are employed: as
well might the performance of a Thames wherry, or one of the
fly-boats on the northern canals, be compared with that of the
Great Western, or the British Queen. The statements alluded to all
have reference to steamers navigating the Hudson between New York
and Albany, the form and structure of which we have already
described; and doubtless the greatest speed ever attained on the
surface of water has been exhibited in the passages of these
vessels.
Mr. Stevenson states, that exclusive of the time lost in
stoppages, the voyage between New York and Albany is usually made
in ten hours. Dr. Renwick, however, who has probably more
extensive opportunities of observation, states, that the average
time, exclusive of stoppages, is ten hours and a half. The
distance being 125·18 geographical miles, the average rate would
therefore be 11-9/10 miles per hour. If it be observed that the
average rate of some of the best sea-going steamers in Europe
obtained from experiments [Pg499] and observations made by
myself, more than three years ago, showed a rate of steaming
little less than ten geographical miles per hour, and that since
that time considerable improvements in steam navigation have been
made, and further, that these performances were made under
exposure to all the disadvantages of an open sea, the difference
between them and the performance of the American river steamers
will cease to create astonishment.
Dr. Renwick states that he made, in a boat called the "New
Philadelphia," one of the most remarkable passages ever performed.
He left New York at five in the afternoon, with the first of the
flood, and landed at Catskill, distant 95·8 geographical miles
from New York, at a quarter before twelve. Passengers were landed
and taken in at seven intermediate points: the rate, including
stoppages, was therefore 14·2 miles per hour; and if half an hour
be allowed for stoppages, the actual average rate of motion would
be fifteen miles and three quarters an hour. As the current, which
in this case was with the course of the vessel, did not exceed
three miles and a half an hour, the absolute velocity through the
water would have been somewhat under twelve miles an hour. This
speed is nearly the same as the speed obtained from taking the
average time of the voyages between New York and Albany at ten
hours and a half; it would therefore appear that the great speed
attained in this trip must have been chiefly, if not altogether,
owing to the effect of the current.
(235.) The steamers which navigate the great northern lakes differ
so little in their construction and appearance from the European
steam-boats, that it will not be necessary here to devote any
considerable space to an account of them. These vessels were
introduced on the lakes at about the same time that steamers were
first introduced on the Clyde. These steamers are strongly built
vessels, supplied with sails and rigging, and propelled by
powerful engines. The largest in 1837, when Mr. Stevenson visited
the States, was the _James Madison_. This vessel was one hundred
and eighty-one feet in length on the deck, thirty feet in breadth
of beam, and twelve feet six inches in depth of hold: her draught
of water was ten feet, and her measured capacity seven hundred
[Pg500] tons. She plyed between Buffalo on Lake Erie and Chicago
on Lake Michigan, a distance of nine hundred and fifty miles.
The severe storms and formidable sea encountered on the lakes
render necessary for the navigation, vessels in all respects as
strong and powerful as those which navigate the open ocean.
(236.) By far the most remarkable and important of all the
American rivers is the Mississippi and its tributaries. That part
of the American continent which extends from the southern shores
of the great northern lakes to the northern shores of the Gulf of
Mexico, is watered by these great streams. The main stream of the
Mississippi has its fountains in the tract of country lying north
of the Illinois and east of Lake Michigan, in latitude forty-three
degrees. At about latitude thirty-nine degrees, a little north of
St. Louis, it receives the waters of the Missouri, and further
south, at the latitude of thirty-seven degrees, the Ohio flows
into it, after traversing five degrees of longitude and four of
latitude, and winding its way from the Alleghany range through
several of the states, and forming a navigable communication with
numerous important towns of the Union, among which may be
mentioned Pittsburg, Cincinnati, Frankfort, Lexington, and
Louisville. The main stream of the Mississippi, after receiving
the waters of the Arkansas, and numerous other minor tributaries,
flows into the Gulf of Mexico by four mouths. The main stream of
the Mississippi, independently of its tributaries, forms an
unbroken course of inland navigation for a distance of nearly two
thousand three hundred miles. Its width, through a distance of one
thousand one hundred miles from its mouth, is not less than half a
mile, and its average depth a hundred feet. The Ohio, its chief
eastern tributary, flowing into it at a distance of about a
thousand miles from its mouth, traverses also about the same
extent of country, and is navigable throughout the whole of that
extent. This river also has several navigable tributaries of
considerable extent, among which may be mentioned the Muskingum,
navigable for one hundred and twenty miles; the Miami, navigable
for seventy-five miles; the Scioto, navigable for one hundred and
twenty [Pg501] miles; the Tennessee, navigable for two hundred
and fifty miles; the Cumberland, navigable for four hundred and
forty miles; the Kentucky, navigable for one hundred and thirty
miles; and the Green River, navigable for one hundred and fifty
miles. The total length of the Ohio and its tributaries is
estimated at above seven thousand miles.
(237.) Steam-boats were introduced on the Mississippi about the
year 1812, the period of their first introduction in Europe; and
their increase has been rapid beyond all precedent. In the year
1831 there were one hundred and ninety-eight steamers plying on
its waters; and the number in 1837 amounted to nearly four
hundred. These vessels are built chiefly on the banks of the Ohio,
at the towns of Pittsburg and Cincinnati, at distances of about
two thousand miles from the mouth of the river they are intended
to navigate.
(238.) These steamers, which are decidedly inferior to those which
navigate the eastern waters, are generally of a heavy build,
fitted to carry goods as well as passengers, and vary from one
hundred to seven hundred tons burthen. Their draught of water is
also greater than that of the eastern river steamers—varying from
six to eight feet. The hull, at about five feet from the water
line, is covered with a deck, under which is the hold, in which
the heavy part of the cargo is stowed. About the middle of this
deck the engines are placed, the boilers and furnaces occupying a
space nearer to the bow, near which two chimneys are placed. The
fire-doors of the furnaces are presented towards the bow, and
exposed so as to increase the draught. That part of the first deck
which extends from the machinery to the stern is the place
allotted to the crew and the deck passengers, and is described as
being filthy and inconvenient in the extreme. A second deck is
constructed, which extends from the chimneys near the bow to the
stern of the vessel. On this is formed the great cabin or saloon,
which extends from the chimneys to within about thirty feet of the
stern, where it is divided by a partition from the ladies' cabin,
which occupies the remaining space. These principal cabins are
surrounded by a gallery about three feet in width, from which, at
convenient [Pg502] places, an ascent is supplied by stairs to the
highest deck, called the hurricane or promenade deck.
(239.) The engines by which these boats are propelled are totally
different from the machinery already described as used in the
eastern steamers. They are invariably non-condensing engines,
worked by steam of extremely high pressure; the boilers are
therefore tubular, and the cylinders small in diameter, but
generally having a long stroke.
The pressure of steam used in these machines is such as is never
used in European engines, even when worked on railways. A pressure
of one hundred pounds per inch is here considered extremely
moderate. The captain of one of these boats, plying between
Pittsburg and St. Louis, told Mr. Stevenson that "under ordinary
circumstances his safety valves were loaded with a pressure equal
to one hundred and thirty-eight pounds per square inch, but that
the steam was occasionally raised as high as one hundred and fifty
pounds to enable the vessel to pass parts of the river in which
there is a strong current;" and he added, by way of consolation,
that "this pressure was never exceeded except on _extraordinary
occasions_!"
The dimensions and power of the Mississippi steamers may be
collected from those of the St. Louis, a boat which was plying on
that river in 1837. That vessel measured two hundred and fifty
feet on deck, and had twenty-eight feet breadth of beam. Her
draught of water was eight feet, and her measured capacity one
thousand tons. She was propelled by two engines with thirty-inch
cylinders, and ten feet stroke; the safety valve being loaded at
one hundred pounds per square inch.
The paddle wheels of these vessels are attached to the paddle
shaft, in such a manner as to be thrown into and out of gear, at
discretion, by the engineer, so that the paddle shaft may revolve
without driving the wheels: by this expedient the power of the
engine is used to feed the boilers while the vessel stops at the
several stations. The vessel is therefore stopped, not, as is
usually the case, by stopping the engines, but by throwing the
wheels out of connection with the paddle shaft. The engines
continue to work, but their [Pg503] power is expended in forcing
water into the boiler. By this expedient the activity of the
engines may, within practical limits, be varied with the
resistance the vessel has to encounter. In working against a
strong current, the feed may be cut off from the boilers, and the
production of steam, and consequently the power of the engines,
thereby stimulated, while this suspension of the feed may be
compensated at the next station.
The stoppages to take in goods and passengers, and for relays of
fuel, are frequent. "The liberty which they take with their
vessels on these occasions," says Mr. Stevenson, "is somewhat
amusing: I had a good example of this on board a large vessel,
called the Ontario. She was steered close in shore amongst stones
and stumps of trees, where she lay for some hours to take in
goods: the additional weight increased her draught of water, and
caused her to heel a good deal; and when her engines were put in
motion, she actually _crawled_ into the deep water on her paddle
wheels: the steam had been got up to an enormous pressure to
enable her to get off, and the volume of steam discharged from the
escapement pipe at every half stroke of the piston made a sharp
sound almost like the discharge of fire-arms, while every timber
in the vessel seemed to tremble, and the whole structure actually
groaned under the shocks."
Besides the steamers used for the navigation of the Mississippi,
innumerable steam tugs are constantly employed in towing vessels
between the port of New Orleans and the open sea of the Gulf of
Mexico. Before the invention of steam navigation, this southern
capital of the United States laboured under the disadvantage of
possessing almost the only bad and inconvenient harbour in the
vast range of coast by which the country is bounded. New Orleans
lies at a distance of about one hundred miles from the Gulf of
Mexico. The force of the stream, the frequency of shoals, and the
winding course of the channel rendered it scarcely possible for a
sailing vessel to pass between the port and the sea with the same
wind. The anchorage was every where bad, and great difficulty and
risk attended the mooring of large vessels to the banks. The steam
engine has, however, overcome all [Pg504] these difficulties, and
rendered the most objectionable harbour of the Union a safe and
good seaport, perfectly easy of approach and of egress at all
times; a small steam tug will take in tow several large ships, and
carry them with safety and expedition to the offing, where it will
dismiss them on their voyage, and take back vessels which may have
arrived.
[Illustration: GREAT WESTERN OFF NEW YORK.]
[Pg505]
APPENDIX.
_On the Relation between the Temperature, Pressure, and Density_
_of Common Steam._
There is a fixed relation between the temperature and pressure of
common steam, which has not yet been ascertained by theory.
Various empirical formulæ have been proposed to express it,
derived from tables of temperatures and corresponding pressures
which have been founded on experiments and completed by
interpolation.
The following formula, proposed by M. Biot, represents with great
accuracy the relation between the temperature and pressure of
common steam, throughout all that part of the thermometric scale
to which experiments have been extended.
Let
a = 5·96131330259
log. a_{1} = 0·82340688193 − 1
log. b_{1} = −·01309734295
log. a_{2} = 0·74110951837
log. b_{2} = −·00212510583
The relation between the temperature t with reference to the
centesimal thermometer, and the pressure p in millimètres of
mercury at the temperature of melting ice, will then be expressed
by the following formula:—
log. p = a − a_{1}b_{1}^{20 + t} − a_{2}b_{2}^{20 + t}. (1.)
Formulæ have, however, been proposed, which, though not applicable
to the whole scale of temperatures, are more manageable in their
practical application than the preceding.
For pressures less than an atmosphere, Southern proposed the
following formula, where the pressure is intended to be expressed
[Pg506] in pounds per square inch, and the temperature in
reference to Fahrenheit's thermometer,—
p = 0·04948 + ((51·3 + t) / 155·7256)^{5·13} |
|. (2.)
t = 155·7256 ((p − 0·04948) − 51·3)^{1/(5·13)} |
The following formula was proposed by Tredgold, where p expresses
the pressure in inches of mercury:—
p = ((100 + t) / 177)^{6}.
This was afterwards modified by Mellet, and represents with
sufficient accuracy experiments from 1 to 4 atmospheres. Let p
represent pounds per square inch, and t the temperature by
Fahrenheit's thermometer,—
p = ((103 + t) / 201·18)^{6} |
|. (3.)
t = 201·18 p^{1/6} − 103 |
M. de Pambour has proposed the following formula, also applicable
through the same limits of the scale:—
p = ((98·806 + t) / 198·562)^{6} |
|. (4.)
t = 198·562 p^{1/6} − 98·806 |
MM. Dulong and Arago have proposed the following formula for all
pressures between 4 and 50 atmospheres:—
p = (0·26793 + 0·0067585 t)^{5} |
|. (5.)
t = 147·961 p^{1/5} − 39·644 |
It was about the year 1801, that Dalton, at Manchester, and
Gay-Lussac, at Paris, instituted a series of experiments on
gaseous bodies, which conducted them to the discovery of the law
mentioned in art. (96.), p. 171. These philosophers found that all
gases whatever, and all vapours raised from liquids by heat, as
well as all mixtures of gases and vapours, are subject to the
_same quantity of expansion_ between the temperatures of melting
ice and boiling water; and by experiments subsequently made by
Dulong and Petit, this uniformity of expansion has been proved to
extend to all temperatures which can come under practical
inquiries.
Dalton found that 1000 cubic inches of air at the temperature of
melting ice dilated to 1325 cubic inches if raised to the
temperature of boiling water. According to Gay-Lussac, the
increased volume was 1375 cubic inches. The latter determination
has been subsequently found to be the more correct one.[40]
[Pg507] It appears, therefore, that for an increase of temperature
from 32° to 212°, amounting to 180°, the increase of volume is 375
parts in 1000; and since the expansion is uniform, the increase of
volume for 1° will be found by dividing this by 180, which will
give an increase of 208-1/3 parts in 100,000 for each degree of
the common thermometer.
To reduce the expression of this important and general law to
mathematical language, let v be the volume of an elastic fluid at
the temperature of melting ice, and let nv be the increase which
that volume would receive by being raised one degree of
temperature under the same pressure. Let V be its volume at the
temperature T. Then we shall have
V = v + nv (T − 32) = v (1 + n (T − 32)).
If V′ be its volume at any other temperature T′, and under the
same pressure, we shall have, in like manner,
V′ = v (1 + n (T′ − 32)).
Hence we obtain
V/V′ = (1 + n (T − 32)) / (1 + n (T′ − 32)); (6.)
which expresses the relation between the volumes of the same gas
or vapour under the same pressure and at any two temperatures. The
co-efficient n, as explained in the text, has the same value for
the same gas or vapour throughout the whole thermometric scale.
But it is still more remarkable that this constant has the same
value for all gases and vapours. It is a number, therefore, which
must have some essential relation to the gaseous or elastic state
of fluid matter, independent of the peculiar qualities of any
particular gas or vapour.
The value of n, according to the experiments of Gay-Lussac, is
0·002083, or 1/480.
To reduce the law of Mariotte, explained in (97.) p. 171., to
mathematical language, let V, V′ be the volumes of the same gas or
vapour under different pressures P, P′, but at the same
temperature. We shall then have
VP = V′P′. (7.)
If it be required to determine the relation between the volumes of
the same gas or vapour, under a change of both temperature and
pressure, let V be the volume at the temperature T and under the
pressure P, and let V′ be the volume at the temperature T′ and
under the pressure P′. Let v be the volume at the temperature T
and under the pressure P′.
By formula (7.) we have
VP = vP′;
[Pg508] and by formula (6.) we have
(V′/v) = (1 + n(T′ − 32)) / (1 + n(T − 32))
Eliminating v, we shall obtain
(V/V′) = (P′/P) · (1 + n(T − 32)) / (1 + n(T′ − 32));
or,
(VP/V′P′) = (1 + n(T − 32)) / (1 + n(T′ − 32)); (8.)
which is the general relation between the volumes, pressures, and
temperatures of the same gas or vapour in two different states.
To apply this general formula to the case of the vapour of water,
let T′ = 212°. It is known by experiment that the corresponding
value of P′, expressed in pounds per square inch, is 14·706; and
that V′, expressed in cubic inches, the water evaporated being
taken as a cubic inch, is 1700. If, then, we take 0·002083 as the
value of n, we shall have by (8.),
VP = 1700 × 14·706 × (1 + 0·002083 (T − 32)) / (1 + 0·002083 × 180)
= 18183(1 + 0·002083 (T − 32)). (9.)
If, by means of this formula (9.), and any of the formulæ (1.),
(2.), (3.), (4.), (5.), T were eliminated, we should obtain a
formula between V and P, which would enable us to compute the
enlargement of volume which water undergoes in passing into steam
under any proposed pressure. But such a formula would not be
suitable for practical computations. By the formulæ (1.) to (5.),
a table of pressures and corresponding temperatures may be
computed; and these being known, the formula (9.) will be
sufficient for the computation of the corresponding values of V,
or the enlargement of volume which water undergoes in passing into
steam.
In the following table, the temperatures corresponding to
pressures from 1 to 240 lbs. per square inch are given by
computation from the formulæ (2.) to (5.), and the volumes of
steam produced by an unit of volume of water as computed from the
formula (9.).
The mechanical effect is obtained by multiplying the pressure in
pounds by the expansion of a cubic inch of water in passing into
steam expressed in feet, and is therefore the number of pounds
which would be raised one foot by the evaporation of a cubic inch
of water under the given pressure. [Pg509]
——————————————————————————————————————————————————————————
| | Volume of | Mechanical
Total pressure| | the Steam | Effect of
in Pounds | Corresponding| compared to | a Cubic Inch
per Square | Temperature. | the Volume | of Water
Inch. | | of the | evaporated
| | Water that | in Pounds
| | has | raised One
| | produced it.| Foot.
——————————————+——————————————+—————————————+—————————————
1 | 102·9 | 20868 | 1739
2 | 126·1 | 10874 | 1812
3 | 141·0 | 7437 | 1859
4 | 152·3 | 5685 | 1895
5 | 161·4 | 4617 | 1924
6 | 169·2 | 3897 | 1948
7 | 175·9 | 3376 | 1969
8 | 182·0 | 2983 | 1989
9 | 187·4 | 2674 | 2006
10 | 192·4 | 2426 | 2022
11 | 197·0 | 2221 | 2036
12 | 201·3 | 2050 | 2050
13 | 205·3 | 1904 | 2063
14 | 209·1 | 1778 | 2074
15 | 212·8 | 1669 | 2086
16 | 216·3 | 1573 | 2097
17 | 219·6 | 1488 | 2107
18 | 222·7 | 1411 | 2117
19 | 225·6 | 1343 | 2126
20 | 228·5 | 1281 | 2135
21 | 231·2 | 1225 | 2144
22 | 233·8 | 1174 | 2152
23 | 236·3 | 1127 | 2160
24 | 238·7 | 1084 | 2168
25 | 241·0 | 1044 | 2175
26 | 243·3 | 1007 | 2182
27 | 245·5 | 973 | 2189
28 | 247·6 | 941 | 2196
29 | 249·6 | 911 | 2202
30 | 251·6 | 883 | 2209
31 | 253·6 | 857 | 2215
32 | 255·5 | 833 | 2221
33 | 257·3 | 810 | 2226
34 | 259·1 | 788 | 2232
35 | 260·9 | 767 | 2238
36 | 262·6 | 748 | 2243
37 | 264·3 | 729 | 2248
38 | 265·9 | 712 | 2253
39 | 267·5 | 695 | 2259
40 | 269·1 | 679 | 2264
41 | 270·6 | 664 | 2268
42 | 272·1 | 649 | 2273
43 | 273·6 | 635 | 2278
44 | 275·0 | 622 | 2282
45 | 276·4 | 610 | 2287
46 | 277·8 | 598 | 2291
47 | 279·2 | 586 | 2296
48 | 280·5 | 575 | 2300
49 | 281·9 | 564 | 2304
50 | 283·2 | 554 | 2308
51 | 284·4 | 544 | 2312
52 | 285·7 | 534 | 2316
53 | 286·9 | 525 | 2320
54 | 288·1 | 516 | 2324
55 | 289·3 | 508 | 2327
56 | 290·5 | 500 | 2331
57 | 291·7 | 492 | 2335
58 | 292·9 | 484 | 2339
59 | 294·2 | 477 | 2343
60 | 295·6 | 470 | 2347
61 | 296·9 | 463 | 2351
62 | 298·1 | 456 | 2355
63 | 299·2 | 449 | 2359
64 | 300·3 | 443 | 2362
65 | 301·3 | 437 | 2365
66 | 302·4 | 431 | 2369
67 | 303·4 | 425 | 2372
68 | 304·4 | 419 | 2375
69 | 305·4 | 414 | 2378
70 | 306·4 | 408 | 2382
71 | 307·4 | 403 | 2385
72 | 308·4 | 398 | 2388
73 | 309·3 | 393 | 2391
74 | 310·3 | 388 | 2394
75 | 311·2 | 383 | 2397
76 | 312·2 | 379 | 2400
77 | 313·1 | 374 | 2403
78 | 314·0 | 370 | 2405
79 | 314·9 | 366 | 2408
80 | 315·8 | 362 | 2411
81 | 316·7 | 358 | 2414
82 | 317·6 | 354 | 2417
83 | 318·4 | 350 | 2419
84 | 319·3 | 346 | 2422
85 | 320·1 | 342 | 2425
86 | 321·0 | 339 | 2427
87 | 321·8 | 335 | 2430
88 | 322·6 | 332 | 2432
89 | 323·5 | 328 | 2435
90 | 324·3 | 325 | 2438
91 | 325·1 | 322 | 2440
92 | 325·9 | 319 | 2443
93 | 326·7 | 316 | 2445
94 | 327·5 | 313 | 2448
95 | 328·2 | 310 | 2450
96 | 329·0 | 307 | 2453
97 | 329·8 | 304 | 2455
98 | 330·5 | 301 | 2457
99 | 331·3 | 298 | 2460
100 | 332·0 | 295 | 2462
110 | 339·2 | 271 | 2486
120 | 345·8 | 251 | 2507
130 | 352·1 | 233 | 2527
140 | 357·9 | 218 | 2545
150 | 363·4 | 205 | 2561
160 | 368·7 | 193 | 2577
170 | 373·6 | 183 | 2593
180 | 378·4 | 174 | 2608
190 | 382·9 | 166 | 2622
200 | 387·3 | 158 | 2636
210 | 391·5 | 151 | 2650
220 | 395·5 | 145 | 2663
230 | 399·4 | 140 | 2675
240 | 403·1 | 134 | 2687
——————————————+——————————————+—————————————+—————————————
[Pg511] In the absence of any direct method of determining the
general relation between the pressure and volume of common steam,
empirical formulæ expressing it have been proposed by different
mathematicians.
The late Professor Navier proposed the following:—Let S express
the volume of steam into which an unit of volume of water is
converted under the pressure P, this pressure being expressed in
kilogrammes per square mètre. Then the relation between S and P
will be
S = a/(b + mP),
where a = 1000, b = 0·09, and m = 0·0000484.
This formula, however, does not agree with experiment at pressures
less than an atmosphere. M. de Pambour, therefore, proposes the
following changes in the values of its co-efficients:—Let P
express the pressure in pounds per square foot; and let
a = 10000 b = 0·4227 m = 0·00258,
and the formula will be accurate for all pressures. For pressures
above two atmospheres the following values give more accuracy to
the calculation:—
a = 10000 b = 1·421 m = 0·0023.
In these investigations I shall adopt the following modified
formula. The symbols S and P retaining their signification, we
shall have
S = a/(b + P) (10.)
where
a = 3875969 b = 164.
These values of a and b will be sufficiently accurate for practical
purposes for all pressures, and may be used in reference to
low-pressure engines of every form, as well as for high-pressure
engines which work expansively.
When the pressure is not less than 30 pounds per square inch, the
following values of a and b will be more accurate:—
a = 4347826 b = 618.
_On the Expansive Action of Steam._
The investigation of the effect of the expansion of steam which
has been given in the text, is intended to convey to those who are
not conversant with the principles and language of analysis, some
notion of the nature of that mechanical effect to which the
advantages attending the expansive principle are due. We shall
now, however, explain these effects more accurately. [Pg512]
The dynamical effect produced by any mechanical agent is expressed
by the product of the resistance overcome and the space through
which that resistance is moved.
Let
P = the pressure of steam expressed in pounds per square foot.
S = the number of cubic feet of steam of that pressure
produced by the evaporation of a cubic foot of water.
E = the mechanical effect produced by the evaporation of a
cubic foot of water expressed in pounds raised one foot.
Then we shall have E = PS; and if W be a volume of water
evaporated under the pressure P, the mechanical effect produced by
it will be WPS.
By (10.) we have
SP = a − bS.
Hence, for the mechanical effect of a cubic foot of water
evaporated under the pressure P we have
E = a − bS. (11.)
Let a cubic foot of water be evaporated under the pressure P′, and
let it produce a volume of steam S′ of that pressure. Let this
steam afterwards be allowed to expand to the increased volume S
and the diminished pressure P; and let it be required to determine
the mechanical effect produced during the expansion of the steam
from the volume S′ to the volume S.
Let
E′ = the mechanical effect produced by the evaporation of
the water under the pressure P′ without expansion.
E″ = the mechanical effect produced during the expansion
of the steam.
E = the mechanical effect which would be produced by
the evaporation under the pressure P without expansion.
_E_ = the total mechanical effect produced by the evaporation
under the pressure P′ and subsequent expansion.
Thus we have
_E_ = E′ + E″.
Let s be any volume of the steam during the process of expansion,
p the corresponding pressure, and e″ the mechanical effect
produced by the expansion of the steam. We have then by (10.)
p = (a/s) − b;
∵ de″ = (ads/s) − bds.
Hence by integrating we obtain
e″ = a log. s − bs + C;
[Pg513] which, taken between the limits s = S′ and s = S, becomes
E″ = a log. S/S′ − b(S − S′). (12.)
But by (11.) we have
E′ = a − bS′,
E = a − bS;
∵ E′ − E = b(S − S′);
∵ E″ = a log. S/S′ − E′ + E;
∵ _E_ = E″ + E′ = a log. S/S′ + E. (13.)
Or,
_E_ = a (1 + log. S/S′) − bS. (14.)
Hence it appears that the mechanical effect of a cubic foot of
water evaporated under the pressure P may be increased by the
quantity a log. S/S′, if it be first evaporated under the greater
pressure P′, and subsequently expanded to the lesser pressure P.
The logarithms in these formulæ are hyperbolic.
To apply these principles to the actual case of a double acting
steam engine,
Let
L = the stroke of the piston in feet.
A = the area of the piston in square feet.
n = the number of strokes of the piston per minute.
∵ 2n AL = the number of cubic feet of space through which
the piston moves per minute.
Let cLA = the clearage, or the space between the steam valve
and the piston at each end of the stroke.
∵ The volume of steam admitted through the steam valve
at each stroke of the engine will be 2n AL(1 + c).
Let
V = the mean speed of the piston in feet per minute,
∵ 2nL = V.
The volume of steam admitted to the cylinder per minute will
therefore be VA (1 + c), the part of it employed in working the
piston being VA.
Let
W = the water in cubic feet admitted per minute in the form
of steam through the steam valve.
S = the number of cubic feet of steam produced by a cubic
foot of water.
[Pg514] Hence we shall have
WS = VA (1 + c);
∵ S = (VA(1 + c))/W. (15.)
Since by (10.) we have
P = a/S − b;
∵ P = [Wa/(VA(1 + c))] − b. (16.)
By which the pressure of steam in the cylinder will be known, when
the effective evaporation, the diameter of the cylinder, and speed
of the piston, are given.
If it be required to express the mechanical effect produced per
minute by the action of steam on the piston, it is only necessary
to multiply the pressure on the surface of the piston by the space
per minute through which the piston moves. This will give
VAP = W(a/(1 + c)) − VAb; (17.)
which expresses the whole mechanical effect per minute in pounds
raised one foot.
If the steam be worked expansively, let it be cut off after the
piston has moved through a part of the stroke expressed by e.
The volume of steam of the undiminished pressure P′ admitted per
minute through the valve would then be
VA (e + c);
and the ratio of this volume to that of the water producing it
being expressed by S′, we should have
S′ = (VA(e + c))/W.
The final volume into which this steam is subsequently expanded
being VA(1 + c), its ratio to that of the water will be
S = (VA (1 + c))/W.
The pressure P′, till the steam is cut off, will be
P′ = [Wa / (VA(e + c))] − b. (18.)
The mechanical effect E′ produced per minute by the steam of full
pressure will be
E′ = P′AVe = [Wae / (e + c)] − AVbe;
and the effect E″ per minute produced by the expansion of the
steam will by (12.) be [Pg515]
E″ = Wa log.[(1 + c) / (e + c)] − bVA(1 − e).
Hence the total effect per minute will be
_E_ = Wa [(e/(e + c)) + log.([1 + c]/[e + c])] − bVA. (19.)
If the engine work without expansion, e = 1;
∵ _E′_ = ( Wa/(1 + c)) − bVA, (20.)
as before; and the effect per minute gained by expansion will
therefore be
_E_ − _E′_ =
Wa [(e/(e + c)) − (1/(1 + c)) + log.([1 + c]/[e + c])]; (21.)
which therefore represents the quantity of power gained by the
expansive action, with a given evaporating power.
In these formulæ the total effect of the steam is considered
without reference to the nature of the resistances which it has to
overcome.
These resistances may be enumerated as follows:—
1. The resistance produced by the load which the engine is
required to move.
2. The resistance produced by the vapour which remains
uncondensed if the engine be a condensing engine, or of the
atmospheric pressure if the engine do not condense the steam.
3. The resistance of the engine and its machinery, consisting
of the friction of the various moving parts, the resistances
of the feed pump, the cold water pump, &c. A part of these
resistances are of the same amount, whether the engine be
loaded or not, and part are increased, in some proportion
depending on the load.
When the engine is maintained in a state of uniform motion, the
sum of all these resistances must always be equal to the whole
effect produced by the steam on the piston. The power expended on
the first alone is the _useful effect_.
Let R = the pressure per square foot of the piston surface, which
balances the resistances produced by the load.
mR = the pressure per square foot, which balances that part of the
friction of the engine which is proportional to the load.
r = the pressure per square foot, which balances the sum of all
those resistances that are not proportional to the load.
The total resistance, therefore, being R + mR + r, which, when the
mean motion of the piston is uniform, must be equal to the mean
pressure on the piston. The total mechanical effect [Pg516] must
therefore be equal to the total resistance multiplied by the space
through which that resistance is driven. Hence we shall have
[R(1 + m) + r]VA = Wa[(e/(e + c)) + log.([1 + c]/[e + c])] − VAb;
∵ RVA(1 + m) = Wa[(e/(e + c)) + log.([1 + c]/[e + c])] − VA(b + r).
For brevity, let
e′ = a[(e/(e + c)) + log.([1 + c]/[e + c])];
∵ RVA(1 + m) = We′ − VA(b + r). (22.)
By solving this for VA, we obtain
VA = We′/(R(1 + m) + b + r);
∵ RVA = We′R/(R(1 + m) + b + r). (23.)
This quantity RVA, being the product of the resistance RA, of the
load reduced to the surface of the piston, multiplied by the space
through which the piston is moved, will be equal to the load
itself multiplied by the space through which it is moved. This
being, in fact, the useful effect of the engine, let it be
expressed by U, and we shall have
U = We′R/(R(1 + m) + b + r). (24.)
Or by (22.),
U(1 + m) = We′ − VA(b + r). (25.)
The value of the useful effect obtained from these formulæ will be
expressed in pounds, raised one foot per minute, W being the
effective evaporation in cubic feet per minute, A the area of the
piston in square feet, and V the space per minute through which it
is moved, in feet.
Since a resistance amounting to 33,000 pounds moved through one
foot per minute is called one-horse power, it is evident that the
horse power H of the engine is nothing more than the useful effect
per minute referred to a larger unit of weight or resistance; that
is to 33,000 pounds instead of one pound. Hence we shall have
H = U/33000. (26.)
Since the useful effect expressed in (24.) and (25.) is that due
to a number of cubic feet of water, expressed by W, we shall
obtain the effect due to one cubic foot of water, by dividing U by
W. If, therefore, U′ be the effect produced by the effective
evaporation of a cubic foot of water, we shall have [Pg517]
U′ = U/W. (27.)
If the quantity of fuel consumed per minute be expressed by F, the
effect produced by the unit of fuel, called the DUTY of the
engine, will, for like reason, be
D = U/F. (28.)
If the fuel be expressed in hundredweights of coal, then D will
express the number of pounds' weight raised one foot by a
hundredweight of coal.
By solving (24.) and (25.) for W, we obtain
W = [U(R(1 + m) + b + r)]/Re′, (29.)
W = (1/e′)[U(1 + m) + VA(b + r)]. (30.)
By eliminating U, by (26.), we shall have
W = [33000 H(R(1 + m) + b + r)]/Re′, (31.)
W = (1/e′)[33000 H(1 + m) + VA(b + r)]. (32.)
The evaporation necessary per horse power per minute will be found
by putting H = 1 in these formulæ.[41]
It will be observed that the quantities A and V, the area of the
cylinder and the speed of the piston, enter all these formulæ as
factors of the same product. Other things, therefore, being the
same, the speed of the piston will be always inversely as the area
of the cylinder. In fact, VA is the volume of steam per minute
employed in working the piston, and if the piston be increased or
diminished in magnitude, its speed must be inversely [Pg518]
varied by the necessity of being still moved through the same
number of cubic feet by the same volume of steam.
It has been already stated in the text, that no satisfactory
experiments have yet been made, by which the numerical value of
the quantity r can be exactly known. In engines of different
magnitudes and powers, this resistance bears very different
proportions to the whole power of the machine. In general,
however, the larger and more powerful the engine, the less that
proportion will be.
That part of this resistance which arises from the reaction of the
uncondensed vapour on the piston is very variable, owing to the
more or less perfect action of the condensing apparatus, the
velocity of the piston, and the magnitude and form of the steam
passages. M. de Pambour states, that, by experiments made with
indicators, the mean amount of this resistance in the cylinder is
2-1/2 lbs. per square inch more than in the condenser, and that
the pressure in the latter being usually 1-1/2 lb. per square
inch, the mean amount of the pressure of the condensed vapour in
the cylinder is about 4 lbs. per square inch. Engineers, however,
generally consider this estimate to be above the truth in
well-constructed engines, when in good working order.
In condensing low pressure engines of forty horse power and
upwards, working with an average load, it is generally considered
that the resistance produced by the friction of the machine and
the force necessary to work the pumps may be taken at about 2 lbs.
per square inch of piston surface.
Thus the whole resistance represented by r in the preceding
formulæ, as applied to the larger class of low pressure engines,
may be considered as being under 6 lbs. per square inch, or 864
lbs. per square foot, of the piston. It is necessary, however, to
repeat, that this estimate must be regarded as a very rough
approximation; and as representing the mean value of a quantity
subject to great variation, not only in one engine compared with
another, but even in the same engine compared with itself at
different times and in different states.
In the same class of engines, the magnitude of the clearage is
generally about a twentieth part of the capacity of the cylinder,
so that c = 0·05.
That part of the resistance which is proportional to the load, and
on which the value of m in the preceding formulæ depends, is still
more variable, and depends so much on the form, magnitude, and the
arrangement of its parts, that no general rule can be given for
its value. It must, in fact, be determined in every particular
case.
In the practical application of the preceding formulæ in
condensing engines we shall have [Pg519]
a = 3875969 b = 164 c= 0·05;
e′ = 3875969([e/(e + 0·05)] + log.[1·05/(e + 0·05)]).
In engines which work without condensation, and therefore with
high pressure steam, we shall have
a = 4347826 b = 618 c = 0·05
e′ = 4347826([e/(e + 0·05)] + log.[1·05/(e + 0·05)])
To facilitate computation, the values of e′ corresponding to all
values of e, from e = ·10 to e = ·90, are given in the following
table:—
———————————————————————————————————————————————————————————————
|Condensing|Non-condensing|| |Condensing|Non-condensing
| Engines | Engines || | Engines | Engines
e | e′. | e′. || e | e′. | e′.
————+——————————+——————————————++————+——————————+———————————————
·10 | 10126265 | 11359029 ||·51 | 5966367 | 6692708
·11 | 9956867 | 11169008 ||·52 | 5903837 | 6622565
·12 | 9793136 | 10985344 ||·53 | 5842288 | 6553525
·13 | 9634926 | 10807875 ||·54 | 5781693 | 6485552
·14 | 9482029 | 10636364 ||·55 | 5722024 | 6418619
·15 | 9334219 | 10470560 ||·56 | 5663251 | 6352693
·16 | 9191251 | 10310186 ||·57 | 5605353 | 6287745
·17 | 9052888 | 10154978 ||·58 | 5548297 | 6223742
·18 | 8918896 | 10004675 ||·59 | 5492064 | 6160662
·19 | 8789043 | 9859014 ||·60 | 5436628 | 6098478
·20 | 8663120 | 9717760 ||·61 | 5381969 | 6037166
·21 | 8540918 | 9580682 ||·62 | 5328065 | 5976699
·22 | 8422242 | 9447559 ||·63 | 5274896 | 5917057
·23 | 8306916 | 9318193 ||·64 | 5222444 | 5858219
·24 | 8194770 | 9192396 ||·65 | 5170684 | 5800159
·25 | 8085644 | 9069984 ||·66 | 5119605 | 5742860
·26 | 7979392 | 8950796 ||·67 | 5069186 | 5686304
·27 | 7875870 | 8834674 ||·68 | 5019410 | 5630469
·28 | 7774952 | 8721468 ||·69 | 4970263 | 5575340
·29 | 7676514 | 8611048 ||·70 | 4921727 | 5520894
·30 | 7580447 | 8503284 ||·71 | 4873790 | 5467121
·31 | 7486640 | 8398056 ||·72 | 4826434 | 5414000
·32 | 7394990 | 8295250 ||·73 | 4779648 | 5361519
·33 | 7305407 | 8194760 ||·74 | 4733417 | 5309659
·34 | 7217807 | 8096496 ||·75 | 4687728 | 5258408
·35 | 7132097 | 8000352 ||·76 | 4642569 | 5207751
·36 | 7048206 | 7906249 ||·77 | 4597928 | 5157676
·37 | 6966058 | 7814100 ||·78 | 4553794 | 5108170
·38 | 6885585 | 7723832 ||·79 | 4510155 | 5059218
·39 | 6806720 | 7635365 ||·80 | 4466999 | 5010808
·40 | 6729408 | 7548642 ||·81 | 4424317 | 4962931
·41 | 6653578 | 7463580 ||·82 | 4382096 | 4915569
·42 | 6579187 | 7380132 ||·83 | 4340332 | 4868720
·43 | 6506174 | 7298230 ||·84 | 4299010 | 4822368
·44 | 6434491 | 7217822 ||·85 | 4258120 | 4776500
·45 | 6364099 | 7138858 ||·86 | 4217658 | 4731113
·46 | 6294944 | 7061285 ||·87 | 4177613 | 4686192
·47 | 6226989 | 6985058 ||·88 | 4137974 | 4641728
·48 | 6160190 | 6910126 ||·89 | 4098737 | 4597713
·49 | 6094510 | 6836450 ||·90 | 4059893 | 4554140
·50 | 6029916 | 6763992 || | |
————+——————————+——————————————++————+——————————+———————————————
[Pg520] In engines which work without expansion we have
e′ = a/(1 + c).
For condensing engines without expansion, we shall then have
e′ = 3875969/1·05 = 3691399; (33.)
and for non-condensing engines,
e′ = 4347826/1·05 = 4140787. (34.)
As the diameters of the cylinders of engines are generally
expressed in inches, the corresponding areas of the pistons
expressed in square feet are given in the following table, so that
the values of A may be readily found:—
——————————————————————————————————————————————————————————————————————
Diam. | Area. | Diam. | Area. | Diam. | Area. | Diam. | Area.
———————+—————————+———————+—————————+———————+—————————+———————+————————
Inches.| Sq.feet.|Inches.| Sq.feet.|Inches.| Sq.feet.|Inches.| Sq.feet.
10 | 0·545 | 48 | 12·566 | 86 | 40·339 | 124 | 83·863
11 | 0·660 | 49 | 13·095 | 87 | 41·283 | 125 | 85·221
12 | 0·785 | 50 | 13·635 | 88 | 42·237 | 126 | 86·590
13 | 0·922 | 51 | 14·186 | 89 | 43·202 | 127 | 87·970
14 | 1·069 | 52 | 14·748 | 90 | 44·179 | 128 | 89·361
15 | 1·227 | 53 | 15·321 | 91 | 45·166 | 129 | 90·763
16 | 1·396 | 54 | 15·904 | 92 | 46·164 | 130 | 92·175
17 | 1·576 | 55 | 16·499 | 93 | 47·173 | 131 | 93·599
18 | 1·767 | 56 | 17·104 | 94 | 48·193 | 132 | 95·033
19 | 1·969 | 57 | 17·721 | 95 | 49·224 | 133 | 96·479
20 | 2·182 | 58 | 18·348 | 96 | 50·265 | 134 | 97·935
21 | 2·405 | 59 | 18·986 | 97 | 51·318 | 135 | 99·402
22 | 2·640 | 60 | 19·635 | 98 | 52·382 | 136 | 100·880
23 | 2·885 | 61 | 20·295 | 99 | 53·456 | 137 | 102·369
24 | 3·142 | 62 | 20·966 | 100 | 54·542 | 138 | 103·869
25 | 3·409 | 63 | 21·648 | 101 | 55·638 | 139 | 105·380
26 | 3·687 | 64 | 22·340 | 102 | 56·745 | 140 | 106·901
27 | 3·976 | 65 | 23·044 | 103 | 57·863 | 141 | 108·434
28 | 4·276 | 66 | 23·758 | 104 | 58·992 | 142 | 109·977
29 | 4·587 | 67 | 24·484 | 105 | 60·132 | 143 | 111·532
30 | 4·909 | 68 | 25·220 | 106 | 61·283 | 144 | 113·097
31 | 5·241 | 69 | 25·967 | 107 | 62·445 | 145 | 114·674
32 | 5·585 | 70 | 26·725 | 108 | 63·617 | 146 | 116·261
33 | 5·940 | 71 | 27·494 | 109 | 64·801 | 147 | 117·859
34 | 6·305 | 72 | 28·274 | 110 | 65·995 | 148 | 119·468
35 | 6·681 | 73 | 29·065 | 111 | 67·201 | 149 | 121·088
36 | 7·069 | 74 | 29·867 | 112 | 68·417 | 150 | 122·719
37 | 7·467 | 75 | 30·680 | 113 | 69·644 | 151 | 124·361
38 | 7·876 | 76 | 31·503 | 114 | 70·882 | 152 | 126·013
39 | 8·296 | 77 | 32·338 | 115 | 72·131 | 153 | 127·676
40 | 8·727 | 78 | 33·183 | 116 | 73·391 | 154 | 129·351
41 | 9·168 | 79 | 34·039 | 117 | 74·662 | 155 | 131·036
42 | 9·621 | 80 | 34·907 | 118 | 75·944 | 156 | 132·732
43 | 10·085 | 81 | 35·785 | 119 | 77·236 | 157 | 134·439
44 | 10·559 | 82 | 36·674 | 120 | 78·540 | 158 | 136·157
45 | 11·045 | 83 | 37·574 | 121 | 79·854 | 159 | 137·886
46 | 11·541 | 84 | 38·485 | 122 | 81·180 | 160 | 139·626
47 | 12·048 | 85 | 39·406 | 123 | 82·516 | 161 | 141·377
—————+—————————+———————+—————————+———————+—————————+———————+————————
[Pg521] The practical application of the preceding formulæ will be
shown by the following examples.
EXAMPLES.
1. _A 36-inch cylinder with 5-1/2 feet stroke is supplied by a
boiler evaporating effectively 60 cubic feet of water per hour,
and the piston makes 20 strokes per minute without expansion;--
what is the power of the engine and the pressure of steam
in the cylinder?_
Let it be assumed that r = 6 × 144 = 864 and m = 0·1. Since the
engine is a condensing engine, we have b = 164 and e′ = 3691399.
By the formulæ (25.) and (26.) we have
H = [We′ − VA(b + r)]/[33000(1 + m)];
and since by the data we have
W = 1 A = 7·069 V = 2nL = 40 × 5·5 = 220,
the formula, by these substitutions, becomes
H = (3691399 − 220 × 1028 × 7·069) / (33000 × 1·1);
∵ H = 57·6.
Since e = 1, the pressure P of steam in the cylinder, by (18.), is
P = (We′/VA) − b.
Therefore
P = (3691399/1555·18) − 164 = 2210;
which being the pressure in pounds per square foot, the pressure
per square inch will be 15-1/3 lbs.
2. _To find the effective evaporation necessary to produce a power
of 80 horses with the same engine. Also, find the pressure of
steam in the cylinder, the speed of the piston being the same._
By the formula (32.), with the above substitutions, we have
W = (33000 × 80 × 1·1 + 220 × 7069 × 1028)/3691399 = 1·22.
The evaporating power would therefore be only increased 22 per
cent., while the working power of the engine would be increased
nearly 40 per cent.
The pressure P in the cylinder will be given, by (18.), as before.
P = [(1·22 × 3691399)/1555·18] − 164 = 2732;
which is equivalent to 19 lbs. per square inch. [Pg522]
3. _What must be the diameter of a cylinder to work with a power
of a hundred horses, supplied by a boiler evaporating effectively
70 cubic feet of water per hour, the mean speed of the piston
being 240 feet per minute, and the steam being cut off at half
stroke? Also, what will be the full pressure of steam on the
piston?_
Taking, as in the former examples, m = 0·1, b = 164, and r = 864,
we shall have
H = 100 W = 7/6 V = 240,
and by the column for condensing engines, in table, p. 519, we
have e′ = 6029916, where e = 0·50. Making these substitutions in
We′ = 33000 H (1 + m) + VA (b + r),
we shall have
(7/6) × 6029916 = 3300000 × 1·1 + 240 × 1028 × A.
Whence we find
A = 13·8;
and by the table, p. 520, the corresponding diameter of the
cylinder will be 50-1/3 inches.
If P′ be the full pressure of the steam, we shall have, by (18.),
P′ = (Wa/VA(e + c)) − b.
Making in this the proper substitutions, we have
P′ = ((7/6) × 3875969) / (240 × 13·8 × 0·55) − 164 = 2318;
which being in pounds per square foot, the pressure per square
inch will be 16-1/10 lbs.
FOOTNOTES:
[40] M. de Pambour states that the increased volume is 1364
cubic inches.
[41] Formulæ equivalent to some of the preceding are given,
with numerous others, by M. de Pambour, in his Theory of the
Steam Engine. These mathematical details contain nothing new
in principle, being merely the application of the known
principles of general mechanics to this particular machine. M.
de Pambour objects against the methods of calculating the
practical effects of steam engines generally adopted by
engineers in this country. Their estimates of the loss of
power by friction, imperfect condensation, and other causes,
are, as I have stated in this volume, vague, and can be
regarded at best as very rough approximations; but, subject to
the restrictions under which their methods of calculation are
always applied, they are by no means so defective as M. de
Pambour supposes. He proves what he considers to be their
inaccuracy, by applying them in cases in which they are never
intended to be applied by English engineers. Those who desire
to reduce to general algebraical formulæ the effects of the
different kinds of steam engines will, however, find the
volume of M. de Pambour of considerable use.
[Pg523]
INDEX.
Air, elasticity of, 28;
May be partially expelled from a vessel by the application of
heat, 44.
America, steam navigation first established in, 487;
Circumstances which led to it, 488;
Fitch and Rumsey, their attempts to apply the single-acting
engine to the propulsion of vessels, 489;
Stevens of Hoboken commences experiments on steam navigation,
489;
Experiments of Livingstone and Fulton, 489;
Fulton's first boat, 490;
The Hudson navigated by steam, 491;
Extension and improvement of river navigation, 492;
American steamers, 494;
Difference between them and European steamers, 494;
Steamers on the Hudson, 494;
American paddle-wheels, 495;
Sea-going American steamers, 496;
Speed attained by American steamers, 497;
Lake steamers, 499;
The Mississippi and its tributaries, 499;
Steam-boats navigating it, 500;
Their structure and machinery, 500;
New Orleans Harbour, 503;
Steam tugs, 503.
Atmosphere, 38;
Weight of, 39.
Atmospheric air, mechanical properties of, 38;
Composition of, 253.
Atmospheric engine, Thomas Newcomen the reputed inventor of, 62;
Description of, as first constructed by Newcomen, 67;
The operation of considered, 69;
Not unfrequently used in preference to the modern steam
engine, 72;
Advantages which it possessed over Savery's, 73;
Considerably improved by Beighton, 75;
John Smeaton investigates this machine, 76;
Brindley obtains a patent for improvements in, 76;
Applied by Champion of Bristol to raise water, 181;
Possessed but limited power of adaptation to a varying load,
151;
Expedient to remedy this, 151;
Working-beam, cylinder, and piston applied to by Newcomen,
322.
Atmospheric pressure rendered available as a mechanic agent by
Denis Papin, 38;
Means of measuring the force of, 39;
The idea of using against a vacuum or partial vacuum to work
a piston in a cylinder, suggested by Otto Guericke, 73.
Barometer gauge, 272.
Barton's piston, 248.
Beighton, his improvement of the atmospheric engine, 75.
Black, Dr., his doctrine of latent heat, 93.
Blasco de Garay, his contrivance to propel vessels, 16;
The contrivance of, probably identical with that of Hero, 17.
Blinkensop, his locomotive engine, 337.
Blowing-box, 429.
Blowing out, Seaward's method of, 454.
Blow-off cocks, 452.
Boiler, forms of, most convenient, 255;
The waggon boiler adopted by Watt, 255;
Furnace, 256;
Method of feeding, 257;
Combustion of gas in flues, 260;
Mr. Williams's method of consuming the unburned gases which
escape from the grate, and are carried through the flues,
260;
Construction of grate and ash-pit, 261;
Magnitude of heating surface of boiler, 262;
Capacity of, must be proportioned to the quantity of water to
be evaporated, 263;
Water-space and steam-space in boiler, 263;
Proportion of water-space in the boiler, how to be regulated,
264;
Position of flues, 264;
Method of feeding, 265;
The magnitude of the feed should be equal to the quantity of
water evaporated, 265;
Different methods for indicating the level of the water in
the boiler, 266;
Level guages, 266;
Self-regulating feeder, 267;
Another method of arranging, 269;
Steam gauge, 270;
Thermometer gauge, 271;
Barometer gauge, 272;
The indicator to measure the mean efficient force of the
piston invented by Watt, 274;
The counter contrived by Watt, 278;
Safety valve, 279;
Fusible plugs used in high pressure boilers, 280;
Self-regulating damper, 281;
Self-regulating furnace invented by Brunton, 283;
Duty of a boiler, 294;
Boilers of locomotive engines, 351;
Construction of the boiler of Gurney's steam carriage, 423;
All boilers require occasional cleansing, 427;
Gurney's method of removing crust of deposited matter in
boilers, 427;
The boiler of Dr. Church's engine formed of copper, 439;
Boilers in marine engines, 449;
Effects of sea-water in, 450;
Remedies for them, 451;
Substitution of copper for iron, 460;
Expedient of coating boilers with felt, applied by Watt, 463.
Booth, Mr., his report on locomotive engines, 361.
Boulton and Watt's experiments on the horse power of engines,
288.
Branca, Giovanni, his machine for propelling a wheel by a blast
of steam, 22.
Brindley (James) obtains a patent for improvements in
atmospheric engine, 76;
Undertook to erect an engine at Newcastle-under-Lyne, 76;
Discouraged by the obstacles thrown in his way, 76.
Brougham, Lord, his sketch of Watt's character, 313;
Inscription from the pen of, on Watt's monument in Westminster
Abbey, 320.
Buffers, 404.
Cartwright's engine to use the vapour of alcohol to work the
piston, 245;
His piston, 247.
Cawley and Newcomen obtain a patent for the atmospheric engine,
64.
Champion applies atmospheric engine to raise water, 181.
Chapman, Messrs., their locomotive engine, 337.
Chlorine introduced in bleaching by Watt, 310.
Church, Dr., his steam engine, 439;
The boiler formed of copper, 439.
Coals, the virtues and powers which steam has conferred upon,
6;
The amount of labour a bushel of performs by means of the
steam engine, compared with horse power, 7;
Constituents of, 252;
Process of combustion, 252.
Coal mines, apprehensions as to the possibility of the
exhaustion of groundless, 8.
Cocks, friction on, 240.
Cocks and valves, 227.
Combustion of gas in flues, 260.
Condensation by injection, accidental discovery of, 69.
Condensation in the cylinder incompatible with a due economy
of fuel, 120.
Condensing principle, circumstance which led to Savery's
discovery of, 47.
Condensing pipe in Savery's engine, 52.
Condensing out of the cylinder, 120.
Condensing jet, 191.
Conical steam valves, 228.
Conversion of ice into water, 103;
Of water into steam, 105.
Copying press invented by Watt, 302.
Cornish system of inspection, 297.
Cornish engines, improvement of, 298;
Historical detail of the duty of, 299.
Cylinders, Wilkinson's machine for accurately boring the
insides of, 149.
D valve, 230.
Dalton and Gay-Lussac, law of, relating to the pressure of
elastic bodies, 171.
Dixon, Mr. The substitution of brass for copper tubes in
locomotive engines ascribed to him, 370.
Double clack-valve, 228.
Eccentric, 225;
Two expedients to reverse the position of, 379.
Effect of an engine, 285.
Elastic fluids. The law according to which the pressure of,
increases with their temperature, discovered by Dalton and
Gay-Lussac, 171.
Evaporation of water and other liquids, physical and
mechanical principles connected with, 97.
Expansion of common steam, effects of, 173.
Expansive action of steam, 159;
Stated by Watt in a letter to Dr. Small, 157;
Its principle explained, 158;
Mechanical effect resulting from it, 161;
Computed effect of cutting off steam at different portions
of the stroke, 162;
Involves the condition of a variation in the intensity of
the moving power, 163;
Expedients for equalising the power, 164;
The expansive principle in the engines constructed by
Boulton and Watt, limited, 165;
Its more extensive application in the Cornish engines, 165;
Methods of equalising, 174;
Description of Hornblower's engine for this purpose, 174.
Expansive principle, application of in marine engines, 466.
Farey on the steam engine, quotation from, relative to
Savery's engine, 58;
His evidence before the House of Commons, 435.
Field, construction of his split paddle, 478.
Fitch and Rumsey, their attempts to apply the single-acting
engine to the propulsion of vessels, 489.
Flues, position of, 264.
Fluids, of two kinds, 25;
Mechanical properties of, 25;
Elastic, 27;
Experimental proof that they press equally in all
directions, 41.
Fly-wheel, 205.
Four-way cock, 239;
Disadvantages of, 240.
Fuel, means of economising, in marine furnaces, 463.
Fulton and Livingstone, their experiments in steam navigation,
489.
Fulton's first boat, 490.
Furnace, self-regulating, invented by Brunton, 283.
Fusible plugs used in high-pressure boilers, 280.
Galloway, his paddle-wheel described, 476.
Gas, elasticity of, 28.
Gay-Lussac and Dalton, law of, relating to the pressure of
elastic bodies, 171.
Governor, adaptation of, 209.
Gradients, restrictions on, 411;
Disposition of, should be uniform, 415.
Great Western Railway, Dr. Lardner's experiments on, 408.
Griff, proposals to drain a colliery at, mentioned by
Desaguliers, 64.
Gurney's steam carriage, 423;
Construction of the boiler of, 423;
His method of removing crust of deposited matter in boilers,
427;
His experiments on common roads, 432.
Hall, his condensers described, 458.
Hancock, his steam carriage, 436;
In what manner it differs from that of Gurney, 437.
Harris, Dr., mentions Savery's engine in his "Lexicon
Technicum," 56.
Heat, effects of upon water, 29;
Waste of in atmospheric engine, 89;
An examination of the analogous effects produced by the
continued application of, to water in the liquid state,
102;
Radiation of, 254.
Heating by steam brought forward by Watt, 303.
"Hecla," experiments with the, 412.
Hero of Alexandria, description of his machine, 12.
High pressure engines described, 321;
One of the earliest forms of the steam engine, 322;
Obscurely described in the "Century of Inventions," 322;
Construction of the first, by Messrs. Trevethick and Vivian,
324.
Hooke exposes the fallacy of Papin's project, 64.
Horse carriages compared with steam, 435.
Horse power of steam engines, 288;
Smeaton's estimation of, 288;
Boulton and Watt's experiments on, 288.
Howard's description of his marine engine, 464.
Hudson, the, navigated by steam, 491.
Hull, Jonathan, his application of the steam engine to water
wheels, 180.
Humphrey. His marine engine described, 470.
Huskisson, Mr., death of, 329.
Hydrogen, 253.
India, steam navigation to, 483.
Indicator invented by Watt, 274.
Jeffrey, Lord; his sketch of the character of Watt, 315.
Kinneal, description of Watt's experimental engine at, 131.
Lake steamers, 499.
Lardner's, Dr., experiments on the Manchester Railway in 1832,
357;
His experiments in 1838, 406;
Experiments on the Great Western Railway, 408.
Leupold's engine, description of, 323.
Level gauges, 266.
Linen, machine for drying by steam, invented by Watt, 303.
Liverpool and Manchester railroad, effects of the introduction
of steam transport on, 329;
Want of experience in the construction of the engines, 329;
Death of Mr. Huskisson, 329;
Proceedings of the directors, 342;
Premium offered by them for the best engine, 344;
Experimental trial, 344.
Livingstone and Fulton, experiments of in steam navigation,
489.
Locomotive engine, history of, 328;
Blinkensop's engine, 337;
Chapman's engine, 337;
Walking engine, 337;
Mr. Stephenson's engine at Killingworth, 339;
Defect of, 341;
Description of the "Rocket," 345;
The "Sanspareil," 347;
The "Novelty," 349;
Superiority of the "Rocket," 350;
Subsequent improvements in the locomotive engine, 352;
Table, showing the economy of fuel gained by subdividing the
flue into tubes, 354;
Engines constructed in the form of the "Rocket" subject to
two principal defects, 354;
These defects remedied, 355;
Improved by the adoption of a more contracted blast pipe,
356;
Dr. Lardner's experiments in 1832, 357;
Adoption of brass tubes, 361;
Mr. Booth's report, 361;
Detailed description of the most improved locomotive
engines, 364;
Substitution of brass for copper tubes ascribed to Mr.
Dixon, 370;
Mr. Stephenson constructed the driving wheels without
flanges, 383;
Pressure of steam in the boiler limited by two safety-
valves, 402;
Buffers, 404;
Steam whistle, 404;
Water tank, 404;
Power of locomotive engines, 405;
Evaporation of boilers, 406;
Dr. Lardner's experiments in 1838, 406;
Resistance to railway trains, 407;
Dr. Lardner's experiments on the Great Western Railway, 408;
Restriction on gradients, 411;
Experiment with the "Hecla," 412;
Disposition of gradients should be uniform, 415;
Method of surmounting steep inclinations, 415;
Steam carriages on common roads, 419;
Difference between steam engines on railways and those used
to propel carriages on turnpike roads, 422;
Gurney's steam carriage, 423;
Construction of the boiler of, 423;
Escape of steam from the engines on the Liverpool road, 428;
Blowing-box, 429;
Separator, 430;
Difficulties in the practical working of steam carriages
upon common roads, 432;
Gurney's experiments on common roads, 432;
Prejudice against locomotive engines on common roads, 432;
Not more destructive to roads than carriages drawn by
horses, 433;
Report of the committee of the House of Commons, 433;
Weight of steam carriages, 433;
Two methods of applying locomotives upon common roads, 434;
Horse carriages compared with, 435;
Farey's evidence before the House of Commons, 435;
Risk of accident from explosion extremely slight, 435;
Hancock's steam carriage, 436;
In what manner it differs from that of Gurney, 437;
Ogle's steam carriage, 438;
His evidence before the House of Commons, 439;
Dr. Church's steam engine, 439;
The boiler of formed of copper, 439.
Lunar Society, Boulton and Watt leading members in, 302.
Marine engines, form and arrangement of, 441;
Difference between marine and land engines, 443;
Engine-room, arrangement of, 446;
Boilers in, 449;
Effects of sea-water on boilers, 450;
Remedies for them, 451;
Blow-off cocks, 452;
Indicators of saltness, 452;
Seaward's indicator, 454;
His method of blowing out, 454;
Method of Maudslay and Field to preserve freshness of water
in the boiler, 456;
Brine pumps, 457;
Tubular condensers applied by Mr. Watt, 457;
Hall's condensers, 458;
Substitution of copper for iron boilers, 461;
Process of stoking, 462;
Marine furnaces, 463;
Expedient of coating boilers with felt applied by Watt, 463;
Means of economising fuel, 463;
Description of Howard's engine, 464;
Application of the expansive principle in marine engines,
466;
Recent improvements of Messrs. Maudslay and Field, 467;
Humphrey's engine, 470;
Common paddle-wheel, 472;
Defect of, 474;
Feathering paddles, 474;
Galloway's paddle-wheel, 476;
Field's split paddle, 478;
Proportion of power to tonnage, 480;
Iron steam vessels, 482.
Mariotte's law relating to pressure, 171.
Maudslay and Field, their method to preserve the requisite
freshness of water in the boiler, 456;
Brine pumps, 457;
Recent improvements of in marine engines, 466.
Metallic pistons, 244;
Cartwright's engine, 245;
An improved form given to by Barton, 248.
Mill work, Stewart's application of the steam engine to, 182.
Mines, the drainage of, Watt endeavours to bring to perfection
the application of the steam engine to, 178.
Mississippi and its tributaries, 499;
Steam-boats on, 500;
Their structure and machinery, 500.
Morland, Sir Samuel, his application of steam to raise water,
34;
The reputed inventor of several ingenious contrivances, 34;
His work in French upon the raising of water, 35;
Extract from it, 35;
Evelyn's account of his visit to, 36.
Murray's slide-valve, 229.
Newcomen, Thomas, the reputed inventor of the atmospheric
engine, 62;
His acquaintance with Dr. Hooke, 62;
Acquainted with Papin's writings, 64;
The merits of his engine ascribed principally to its
mechanism and combinations, 73;
Obtains with Cawley a patent for the atmospheric engine, 64;
Resumes the old method of raising water from mines by
ordinary pumps, 65;
The means proposed to effect this, 66;
First conception of the atmospheric engine, 66;
Description of his construction of atmospheric engine, 67;
Suggestion of a better method of condensation than the
application of cold water on the external surfaces of the
cylinder, 69;
He abandons the external cylinder, 69;
Applied the working-beam, cylinder, and piston to the
atmospheric engine, 322.
New Orleans Harbour, 503.
"Novelty," description of the, a locomotive engine, 349.
Ogle, his steam carriage, 438;
His evidence before the House of Commons, 439.
Otto Guericke, his suggestion relative to atmospheric
pressure, 73.
Oxley made the first attempt to drive water-wheels by the
steam engine, 182.
Paddle-wheel described, 472;
Defect of, 474;
Feathering paddles, 474;
Galloway's paddle-wheel, 476;
Field's split paddle, 478.
Paddle-wheels of American steamers, 495.
Papin, Denis, conceived the idea of rendering atmospheric
pressure available as a mechanical agent, 37;
Description of his contrivance, 37;
His discovery of condensation of steam, 45;
Quotation from his work relative to this discovery, 45;
Explanation of this important discovery, 46;
Discovers the method of producing a vacuum by the
condensation of steam, 178;
His projected applications of the steam engine, 178;
His proposition for the construction of an engine working by
atmospheric pressure, 62;
Abandons the project when informed of the principle and
structure of Savery's engine, 62;
His engine described, 62;
This project nothing more than a reproduction of the Marquis
of Worcester's engine, 63;
The fallacy of his project exposed by Hooke, 64;
His project for producing a vacuum under a piston by
condensing the steam, published in the "Actæ Eruditorum,"
64.
Parallel motion, 195.
Physical science, the rapid progress of, 8.
Pistons, 242;
The common hemp-packed, 242;
Woolf's method of tightening the packing of, without
removing the lid of the cylinder, 244;
This method further simplified, 244;
Metallic, 244;
Cartwright's engine, 245;
Cartwright's piston, 247;
Invention of the indicator by Watt to measure the mean
efficient force of, 274.
Piston rod and beam, methods of connecting in the double-
acting engine, 193.
Pneumatic institution at Clifton, Watt one of the founders of,
310.
Potter, Humphrey, his contrivance for working the valves, 71;
Improved by the substitution of a plug-frame, 72.
Power, proportion of, to tonnage in marine engines, 480.
Power and duty of steam engines, 287.
Priestley, Watt's letter to, relative to the composition of
water, 307.
Pump, an illustration of force attained by a vacuum, 43.
Puppet clacks, or button valves, 144.
Rack and Sector, 194.
Railways, speed of coaches on, compared with that of stage-
coaches on a common road, 7.
Railway transport, effects of, 328. 330.
Railways and stone roads compared, 420.
River navigation, extension and improvement of, 492.
"Rocket," description of the, a locomotive engine, 345;
Engines constructed in the form of, subject to two principal
defects, 354;
These defects remedied, 355;
Improved by the adoption of a more contracted blast-pipe, 356.
Roebuck, Dr., Watt's partnership with, 130.
Rotatory motion, method of producing by sun and planet wheels,
187.
Safety-valve not adopted by Savery, 57;
Invented by Papin, 57;
Description of, 57;
First applied to Savery's engine by Desaguliers, 58.
"Sanspareil," description of the, a locomotive engine, 347.
Savery, Thomas, obtains a patent for an engine to raise water,
47;
Circumstance which led to his discovery of the condensing
principle, 47;
An account of his engine, 49;
Description of the working apparatus in which the steam is
used as a moving power, 51;
His engine described in a work entitled "The Miner's Friend,"
56;
Mentioned by Dr. Harrison in his "Lexicon Technicum," 56;
Quotation from his address to the Royal Society, 56;
Quotation from his address to the Miners of England, 57;
Mentioned by Bradley in his "Improvements of Planting and
Gardening," 57;
The safety-valve not adopted by him, 57;
The safety-valve first applied to his engine by Desaguliers,
58;
Farey on the steam engine quoted, 58;
Further Improvements made by Desaguliers, 58;
Defects of his engine, 59;
His engine applied to the drainage of mines, 59;
Further defects of, 60;
The first to suggest the method of expressing the power of
an engine with reference to that of horses, 61;
Failure of his engine in the work of drainage, 61;
The tendency of high pressure to weaken and gradually destroy
the vessels, 72;
The power of his engines restricted, 73;
The atmospheric engine superior to, 73;
The boiler, guage-pipes, and regulator borrowed from his
engine, 73;
Proposes to apply his engine as a prime mover for all sorts
of machinery, 180.
Scott, Sir Walter, his sketch of the character of Watt, 314.
Sculpture, Watt's invention of machine for copying, 318.
Sea-going American steamers, 496.
Sea-water, effects of upon boilers, 450.
Seaward's slides, 235;
Indicator of saltness, 454;
His method of blowing out, 454.
Self-regulating damper, 281;
Furnace, 283.
Separator, 430.
Single-acting engine, description of Watt's, 133. 144.
Single clack-valve, 227.
Single cock, 238.
Slide-valves, 229;
That contrived by Mr. Murray, 229.
Smeaton, John, investigates the atmospheric engine, 76;
Applies himself to the improvement of wind and water mills,
181;
His estimate of the horse power of engines, 288.
Solomon De Caus, description of the apparatus of, 17;
M. Arago claims for him a share of the honour of the
invention of the steam engine, 21;
Republished, with additions, the work of Isaac De Caus, 22.
Somerset, Edward, Marquis of Worcester. Invention of the steam
engine ascribed to him, 23;
Description of his contrivance, 23;
His "Century of Inventions," 24;
Brief account of his engine described in this work, 31;
His contrivance compared with that of De Caus, 33;
Many of his inventions have been reproduced and brought into
general use, 34.
Steam cannot be applied _immediately_ to any useful purpose,
but requires the interposition of mechanism, 11;
Elastic force of, recognised by the ancients only in vague
and general terms, 14;
The power of, formerly made to minister to the objects of
superstition, mentioned by Arago, 15;
Anecdote showing the knowledge which the ancients had of the
mechanical force of, 15;
The discovery of the condensation of, by Papin, 45;
Mechanical power obtained from the direct pressure of the
elastic force of, suggested by De Caus and Lord Worcester,
73;
Latent heat of, 107;
The mechanical force of considered, 115;
Watt's early experiments on, 87;
Discovery of the expansive action of, 157;
Expansive action of stated by Watt in a letter to Dr. Small,
157;
Its principle explained, 158;
Mechanical effect resulting from it, 161;
Properties of, 168;
Common and super-heated steam, 168;
Pressure and temperature of, 171;
Relation between the temperatures of common steam and its
pressure and density, 172;
Effects of the expansion of common steam, 173;
Mechanical effects of, 173;
Methods of equalising the varying force of expanding steam,
174;
Method of producing a vacuum by the condensation of,
discovered by Papin, 178;
Applied to move machinery, 179;
Steam guage, 270;
Heating by steam brought forward by Watt, 303;
A machine for drying linen by, invented by Watt, 303;
Mode of escape of, from the engines on the Liverpool road,
429.
Steam case or jacket, invented by Watt, 124.
Steam engine, a subject of popular interest, 3;
The effects which it has produced upon the well-being of the
human race considered, 4;
Presents peculiar claims upon the attention of the people of
Great Britain, 5;
The exclusive offspring of British genius, 5;
The virtues and powers which it has conferred upon coals, 6;
Water the means of calling these powers into activity, 6;
Used in the drainage of Cornish mines, 7;
Comparison of its power with human labour, 8;
Investigation of the origin of, 10;
A combination of a great variety of contrivances and the
production of several inventions, 12;
Before the discoveries of James Watt was of extremely
limited power, 12;
Invention of, ascribed to the Marquis of Worcester, 23;
Account of Savery's, 49;
Farey quoted, 58;
Improvements made by Desaguliers, 58;
Applied to the drainage of mines, 59;
Humphrey Potter's contrivance, 72;
Advantages of the atmospheric engine over that of Savery, 73;
Progress of the atmospheric engine, 75;
Description of Papin's engine, 62;
Smeaton's improvements, 76;
First experiments of Watt and subsequent improvements, 83;
Watt's experiments on the force of steam at high pressure,
83;
Watt discovers the great defects of the atmospheric engine,
85;
Waste of heat in atmospheric engine, 89;
Dr. Black's theory of latent heat, 93;
Description of Watt's experimental engine at Kinneal, 131;
Description of his single-acting engine, 133;
Disadvantages of the atmospheric compared with the old
engine, 150;
Expedients to force the atmospheric engines into use, 152;
Watt's exertions to improve the manufacture of, at Soho, 155;
Efficiency of fuel in the new engines, 156;
Hornblower's engine, 175;
Woolf's engine, 176;
Watt endeavours to bring to perfection the application of,
to the drainage of mines, 178;
Papin's projected application of, 178;
Savery proposed to apply his steam engine as a prime mover
for all sorts of machinery, 180;
Jonathan Hull's application of, to water-wheels, 180;
Steam engine used for driving water wheels, 182;
First attempt of this kind made by Oxley, 182;
Stewart's application of, to mill work, 182;
Wasbrough's application of the fly-wheel and crank, 183;
Reasons why Watt's single-acting engine was not adapted to
produce continuous uniform motion of rotation, 184;
Watt's second patent, 186;
Valves of double-acting engine, 189;
Condensing jet, 191;
Methods of connecting the piston-rod and beam in the double-
acting engine, 193;
Rack and sector, 194;
Parallel motion, 195;
Connecting rod and crank, 202;
Fly-wheel, 205;
Throttle-valve, 207;
Adaptation of the governor, 209;
Double-acting engine considered as a whole, 216;
Process of its operation investigated, 217;
The eccentric, 225;
Cocks and valves, 227;
Single clack-valve, 227;
Double clack-valve, 228;
Conical steam-valves, 228;
Slide-valves, 229;
Murray's slide-valve, 229;
D valve, 230;
Seaward's slides, 235;
Single cock, 238;
Four-way cock, 239;
Pistons, 242;
Gross effect and useful effect of engines, 285;
Power and duty of, 287;
Horse power of, 288;
The means whereby mechanical power is expended in working
the engines enumerated, 290;
Common rules followed by engine makers, 292;
Duty of engines, 294;
Duty distinguished from power, 295;
Proportion of stroke to diameter of cylinder, 295;
Cornish system of inspection, 297;
Improvement of the Cornish engines, 298;
Historical detail of the duty of Cornish engines, 299;
High-pressure engines, 321;
Leupold's engine described, 323;
Construction of the first high-pressure engine by Messrs.
Trevethick and Vivian, 324;
First application of the steam engine to propel carriages
on railroads, 328;
Computation of how much corn could be saved by the
substitution of steam engines for horse power, 332;
Marine engines, form and arrangement of, 441;
Difference between marine and land engines, 443;
Mr. Howard's patent engine described, 464;
Humphrey's engine described, 470.
Steam navigation to India, 483;
First established in America, 487;
Circumstances which led to it, 488;
Attempts of Fitch and Rumsey to apply the single-acting
engine to the propulsion of vessels, 489;
Stevens of Hoboken commences experiments in, 489;
Experiments of Livingstone and Fulton, 489;
Fulton's first boat, 490;
The Hudson navigated by steam, 491;
Extension and improvement of river navigation, 492;
American steamers, 494;
Difference between them and European steamers, 494;
Steamers on the Hudson, 494;
Sea-going American steamers, 496;
Speed attained by American steamers, 497;
Lake steamers, 499;
Steam-boats on the Mississippi, 500.
Steam tugs, 503.
Steep inclinations, method of surmounting, 415.
Stephenson, his locomotive engine at Killingworth, 339;
Defect of, 341;
Constructed the driving wheels without flanges, 383.
Stevens, of Hoboken, commences experiments on steam navigation,
489.
Stewart, his application of the steam engine to mill work, 182.
Stoking, process of, 462.
Stuffing-box, contrivance of, 147.
Sun and planet wheels, method of producing rotatory motion,
187.
Thermometers, the process of filling described, 44;
Explanation of the principle of, 98;
Construction of mercurial thermometer, 98;
Method of graduating, 99.
Thermometer gauge, 270.
Throttle-valve, description of, 207.
Tredgold, his remark relative to Newcomen's engine, 73.
Trevethick and Vivian's engine described, 325.
Vacuum, force obtained by a, 43;
The pump an illustration of this, 43.
Valves of double-acting engine, 189.
Wasbrough, his application of the fly-wheel and crank, 183.
Water, a pint of, the mechanical force produced by its
evaporation, 6;
The alternate decomposition and recomposition of, by
magnetism and electricity, analogous to vaporisation and
condensation, 8;
The fixed temperature which it assumes in boiling subject to
variation, 108;
Experiments to illustrate this, 109;
Table to show the temperature at which it will boil under
different pressures of the atmosphere, 113;
Mechanical force of a cubic inch of, converted into steam,
118;
Discovery of the composition of, 303;
The merit of this discovery shared between Cavendish,
Lavoisier, and Watt, 305;
Latent heat of, 101;
Conversion of ice into, 103.
Water tank, 404.
Water-wheels, steam engine used for turning, 182.
Watt (James), birth of, 77;
His infancy, 78;
Anecdotes respecting, 78;
His boyhood, 79;
Goes to London, 80;
Returns to Glasgow, 80;
Appointed mathematical instrument-maker to the university,
81;
Adam Smith one of his earliest friends and patrons, 81;
Also Black and Robert Simson, 81;
Extract from an unpublished manuscript of Robison respecting
the character of, 82;
His first experiments on steam, 83;
Observes defects of atmospheric engine, 84;
His first attempt to improve it, by using a wooden instead
of an iron cylinder, 85;
His method to ascertain the temperatures at which water would
boil under pressures less than that of the atmosphere, 86;
His early experiments on steam, 87;
His notice of the waste of heat in atmospheric engines, 89;
His experiments to determine the extent to which water
enlarged its volume when it passed into steam, 90;
Discovers the latent heat of steam, 91;
Learns the theory of latent heat, 93;
His letter to Dr. Brewster, explaining the circumstances
which led to the error that a large share of the merit of
his discoveries were due to Black, 93;
Finds that condensation in the cylinder is incompatible with
a due economy of fuel, 120;
Conceives the notion of condensing out of the cylinder, 120;
Discovers separate condensation, 121;
Invents the air-pump, 122;
Substitutes steam pressure for atmospheric pressure, 123;
Invents the steam case or jacket, 124;
His first experiments to realise these inventions, 125;
His experimental apparatus, 126;
Difficulties of bringing the improved engines into use, 128;
Practises as a civil engineer, 129;
Makes a survey of the river Clyde, 129;
His partnership with Dr. Roebuck, 130;
His first patent, 130;
Description of his experimental engine at Kinneal, 131;
Removes to Soho, 131;
Abstract of the act of parliament for the extension of his
patent, 132;
Description of his single-acting engine, 133-144;
His condenser worked by an injection, 146;
Objections attending condensation by surface, 146;
Improvements in construction of piston, 147;
Effected by a contrivance called a stuffing-box, 147;
Method of packing, 148;
Improved methods of boring the cylinder, 149;
His letter to Smeaton on this subject, 149;
Used black-lead dust for the purpose of lubrication, 149;
This found to wear the cylinder, 149;
Disadvantages of the atmospheric compared with the old
engines, 150;
Greatly increased economy of fuel, 151;
Expedients to force the atmospheric engines into use, 152;
His correspondence with Boulton, 153;
His correspondence with Smeaton, 154;
Exertions to improve the manufacture of engines at Soho, 155;
Efficiency of fuel in the new engines, 156;
Endeavours to bring to perfection the application of the
steam engine to the drainage of mines, 178;
The reasons why his single-acting engine was not adapted to
produce continuous uniform motion of rotation, 184;
His notes upon Dr. Robison's article on the steam engine,
184;
His second patent, 186;
His third patent, 189;
His application of the fly-wheel, 205;
His application of the throttle-valve, 207;
His adaptation of the governor, 209;
His double-acting engine considered as a whole, 216;
Investigation of the process of its operation, 217;
Eccentric, 225;
Cocks and valves, 227;
Single clack-valve, 227;
Double clack-valve, 228;
Conical steam-valve, 228;
Slide-valves, 229;
The waggon boiler adopted by him, 225;
Invents the indicator, 274;
The counter contrived by him, 278;
The Lunar Society in which Watt and Boulton were leading
members, 302;
Invents the copying press, 302;
His friends and associates at Birmingham, 302;
Method of heating by steam brought forward by him, 303;
His invention of a machine for drying linen by steam, 303;
His share in the discovery of the composition of water, 303;
His letter to Priestley on this subject, 307;
Anecdote of his inventive genius, 309;
Introduces the use of chlorine in bleaching, 310;
One of the founders of the Pneumatic institution at Clifton,
310;
His first marriage, 310;
Private life of, 311;
Death of his first wife, 311;
His second marriage, 311;
He retires from business, 311;
Death of his younger son, 311;
Extracts from his letters, 312;
His death, 313;
Character of, by Lord Brougham, 313;
By Sir Walter Scott, 314;
By Lord Jeffrey, 315;
Occupation of his old age, 318;
Invention of machine for copying sculpture, 318;
His last days, 318;
Monuments, 319;
Inscription on the monument in Westminster Abbey from the
pen of Lord Brougham, 319;
His application of tubular condensers, 457;
His expedient for coating boilers with felt, 463.
Wilkinson, his machine for accurately boring the insides of
cylinders, 149.
Williams's method of consuming the unburned gases which escape
from the grate, and are carried through the flues, 260.
Woolf's engine, 176;
Woolf's piston, 243.
[Illustration: RICHMOND BRIDGE.]
LONDON:
Printed by A. SPOTTISWOODE,
New-Street-Square.
* * * * *
Transcriber's endnote:
Original spelling and grammar has mostly been retained. For
example, the forms "Cyclopoedia", "cyclopædia", "Encyclopædia",
"Encyclopoedia", "guage", and "gauge" are all retained. Figures
were moved from within paragraphs to between paragraphs.
Footnotes were re-indexed and moved to the ends of chapters.
An entry for the INDEX was inserted into the Table of Contents.
In the Table of Contents, changed "MM. Dulong and Arrago" to "MM
Dulong and Arago". Also "Blinkinsop" to "Blinkensop". Also
"Wasborough's" to "Wasbrough's".
Figs. 4, 5 and 6 are all in one image. Two tubes in Fig. 4
were incorrectly labeled T'; one of these has been crossed out
and changed to T. Both tubes in Fig. 6 were incorrectly
labeled G. One of these was crossed out and replaced by G'.
Note also that Figs. 4, 5, 6 are repeated in the text on
different pages; this feature has been retained.
Page 10: "it s already" to "it is already".
Page 43: "Thu if heat" changed to "Thus if heat".
Page 45: "had a diameter of only one square foot" changed to
"had a diameter of only one foot".
Page 47: "immedate" to "immediate".
Page 51: "a a level" to "a level". Also, comma removed from "A
gauge, pipe is inserted".
Page 53: "proportionably" to "proportionally".
Page 79: A paragraph beginning "He was not fourteen" contains three
double quotation marks; this is presumably an error. Possibly there
should be two double quotation marks and two single quotation
marks.
Page 80: "S'. Gravesande" is retained, although this probably
refers to a person known as "'s Gravesande".
Page 103: "gases n general" to "gases in general".
Page 122: comma removed from "process may, be continued".
Page 123: "two thin pipes F G of tin" to "two thin pipes F, G
of tin".
Page 172: "empyrical" to "empirical".
Page 187, Fig. 32.: The text refers to "end I of the
connecting rod", but this was labeled L on the Figure. This L
has been crossed out and replaced by I.
Page 285: In "surrounding the boiler with iron-conducting
substances", changed "iron-" to "non-".
Page 308: "exeitement" to "excitement".
Page 362: "acomplish" to "accomplish".
There were several extended quotations, for example beginning on
page 312, in which each line began with a quotation mark, with
ending quotation marks at the end of each paragraph. In this
edition, these passages have been marked by indentation, and all
but the first and last quotation marks from each paragraph were
removed.
Page 366: Figs. 97-104 appeared originally between pages 385
and 399, as full-page prints. Numerically, however, they
belong between Figs. 96 and 105--therefore between pages
366 and 369. Therefore, they have been moved to a location
between two paragraphs on page 367.
Page 368: "rivetted" to "riveted".
Page 419: "TREVITHECK'S INVENTION" changed to "TREVETHICK'S
INVENTION", in the chapter heading. However, the references to
Trevethick occur in a previous chapter, around page 324.
Page 468: Period added to end sentence "[...] piston is at the
bottom of its stroke".
Page 490: Period added to end sentence "[...] therefore one
eighth of its capacity".
Pages 494, 497: large data tables were split into two pieces
each.
Page 505: The logarithm originally given as log x =
"[=1]·82340688193", where "[=1]" represents a numeral one with
a horizontal line over it, is herein changed to log x =
"0·82340688193 - 1", as that is the meaning of this convention.
Page 513 "formulæ are hyberbolic" to "formulæ are hyperbolic".
In the Appendix, pp 505-522, mathematical variables such as
"a", "p", "t", etc. were originally italicized. In these text
file versions, italicized variables have been removed from this
section of the book. This rule has two unfortunate exceptions:
E, _E_, E', and _E'_ on pp 512-515 are different variables, and
have been retained. Italics have been removed from tables
throughout the work.
The tables on page 494 and 497 were divided into two parts,
better to fit the width constraints of this format. Most of the
tables will not look good unless viewed with a monospace font,
such as Courier New or Lucida Console.
*** End of this LibraryBlog Digital Book "The Steam Engine Explained and Illustrated (Seventh Edition) - With an Account of its Invention and Progressive Improvement, and its Application to Navigation and Railways; Including also a Memoir of Watt" ***
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