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Title: Marvels of Scientific Invention - An Interesting Account in Non-technical Language of the - Invention of Guns, Torpedoes, Submarine Mines, Up-to-date - Smelting, Freezing, Colour Photography, and many other - recent Discoveries of Science
Author: Corbin, Thomas W.
Language: English
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Copyright Status: Not copyrighted in the United States. If you live elsewhere check the laws of your country before downloading this ebook. See comments about copyright issues at end of book.

*** Start of this Doctrine Publishing Corporation Digital Book "Marvels of Scientific Invention - An Interesting Account in Non-technical Language of the - Invention of Guns, Torpedoes, Submarine Mines, Up-to-date - Smelting, Freezing, Colour Photography, and many other - recent Discoveries of Science" ***

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Transcriber's note: For this text version passages in italics are
indicated by _underscores_. Small caps have been replaced by ALL CAPS
and subscripts are denoted as _{2}.


       *       *       *       *       *

[Illustration: cover]


[Illustration: _By permission of Messrs. Chance Bros, and Co., Ltd._

                        A HUGE LAMP

The marvellous arrangement of lenses and prisms which enables the
lighthouse to send out its guiding flashes, with the mechanism for
turning it. Made for "Chilang" Lighthouse, China  _Frontispiece_]



MARVELS OF
SCIENTIFIC INVENTION

AN INTERESTING ACCOUNT IN NON-TECHNICAL LANGUAGE
OF THE INVENTION OF GUNS, TORPEDOES, SUBMARINES
MINES, UP-TO-DATE SMELTING, FREEZING, COLOUR
PHOTOGRAPHY, AND MANY OTHER RECENT
DISCOVERIES OF SCIENCE

BY

THOMAS W. CORBIN

AUTHOR OF
"ENGINEERING OF TO-DAY," "MECHANICAL INVENTIONS
OF TO-DAY," "THE ROMANCE OF SUBMARINE
ENGINEERING," _&c., &c._

WITH 32 ILLUSTRATIONS & DIAGRAMS

PHILADELPHIA
J. B. LIPPINCOTT COMPANY
LONDON: SEELEY, SERVICE & CO. LTD.
1917



CONTENTS


      CHAPTER                                             PAGE

        I. DIGGING WITH DYNAMITE                             9

       II. MEASURING ELECTRICITY                            22

      III. THE FUEL OF THE FUTURE                           42

       IV. SOME VALUABLE ELECTRICAL PROCESSES               55

        V. MACHINE-MADE COLD                                67

       VI. SCIENTIFIC INVENTIONS AT SEA                     78

      VII. THE GYRO-COMPASS                                 90

     VIII. TORPEDOES AND SUBMARINE MINES                    98

       IX. GOLD RECOVERY                                   109

        X. INTENSE HEAT                                    123

       XI. AN ARTIFICIAL COAL MINE                         137

      XII. THE MOST STRIKING INVENTION OF RECENT TIMES     149

     XIII. HOW PICTURES CAN BE SENT BY WIRE                176

      XIV. A WONDERFUL EXAMPLE OF SCIENCE AND SKILL        191

       XV. SCIENTIFIC TESTING AND MEASURING                198

      XVI. COLOUR PHOTOGRAPHY                              212

     XVII. HOW SCIENCE AIDS THE STRICKEN COLLIER           220

    XVIII. HOW SCIENCE HELPS TO KEEP US WELL               231

      XIX. MODERN ARTILLERY                                236

    APPENDIX                                               245

    INDEX                                                  247



LIST OF ILLUSTRATIONS


    A Huge Lamp                  _Frontispiece_

                                    FACING PAGE

    First Effect of the Dynamite             16

    A Fine Crop                              24

    Apple-tree planted by Spade              48

    Machine-made Ice                         72

    A Cold Store                             80

    Dassen Island Lighthouse                 88

    Measuring Heat                          128

    The Telewriter                          184

    A Miners' Rescue Team                   208

    Pneumatic Hammer Drill                  216

    An Artificial Coal Mine                 224

    Sectional view of a 60-pounder Gun      232

    Rifles of different Nations             240


DIAGRAMS

     FIG.                                         PAGE

     1. Principle of Galvanometer                   30

     2. String Galvanometer                         31

     3. Duddell Thermo-Galvanometer                 39

     4. Construction of a Voltmeter                 64

     5. The Working of a Refrigerating Machine      70

     6. Hertz's Machine                            155

     7. Hertz "Detector"                           156

     8. 9. 10. Wireless Waves                      158

    11. A Wireless Antenna                         164

    12. Poulsen's Machine                          166

    13. 14. How Pictures are sent by Wire          177

    15. Message received by Telewriter             189



MARVELS OF SCIENTIFIC

INVENTION



CHAPTER I

DIGGING WITH DYNAMITE


Most people are afraid of the word explosion and shudder with
apprehension at the mention of dynamite. The latter, particularly,
conjures up visions of anarchists, bombs, and all manner of wickedness.
Yet the time seems to be coming when every farmer will regard
explosives, of the general type known to the public as dynamite, as
among his most trusty implements. It is so already in some places. In
the United States explosives have been used for years, owing to the
exertions of the Du Pont Powder Company, while Messrs Curtiss' and
Harvey, and Messrs Nobels, the great explosive manufacturers, are busy
introducing them in Great Britain.

It will perhaps be interesting first of all to see what this
terror-striking compound is. One essential feature is the harmless gas
which constitutes the bulk of our atmosphere, nitrogen. Ordinarily one
of the most lazy, inactive, inert of substances, this gas will, under
certain circumstances, enter into combination with others, and when it
does so it becomes in some cases the very reverse of its usual peaceful,
lethargic self. It is as if it entered reluctantly into these compounds
and so introduced an element of instability into them. It is like a
dissatisfied partner in a business, ready to break up the whole
combination on very slight provocation.

And it must be remembered that an explosive is simply some chemical
compound which can change _suddenly_ into something else of much larger
volume. Water, when boiled, increases to about 1600 times its own volume
of steam, and if it were possible to bring about the change suddenly
water would be a fairly powerful explosive. Coal burnt in a fire
changes, with oxygen from the atmosphere, into carbonic acid gas, and
the volume of that latter which is so produced is much more than that of
the combined volumes of the oxygen and coal. When the burning takes
place in a grate or furnace we see nothing at all like an explosion, for
the simple reason that the change takes place gradually. That is
necessarily so since the coal and oxygen are only in contact at the
surface of the former. If, however, we grind the coal to a very fine
powder and mix it well with air, then each fine particle is in contact
with oxygen and can burn instantly. Hence coal-dust in air is an
explosive. It used to be thought that colliery accidents were due
entirely to the explosion of methane, a gas which is given off by the
coal, but it has of recent years dawned upon people that it is the
coal-dust in the mine which really does the damage. The explosion of
methane stirs up the dust, which then explodes. The former is
comparatively harmless, but it acts as the trigger or detonator which
lets loose the force pent up in the innocent-looking coal-dust. Hence
the greatest efforts in modern collieries are bent towards ridding the
workings of dust or else damping it or in some other way preventing it
from being stirred up into the dangerous state.

So the essential feature of any explosive is oxygen and something which
will burn with it. If it be a solid or liquid the oxygen must be a part
of the combination or mixture, for it cannot get air from the
surrounding atmosphere quickly enough to explode; and, moreover, it is
generally necessary that explosives should work in a confined space away
from all contact with air. So oxygen, of necessity, must be an integral
part of the stuff itself. But when oxygen combines with anything it
usually clings rather tenaciously to its place in the compound and is
not easily disturbed quickly, and that is where the nitrogen seems to
find its part. It supplies the disturbing element in what would
otherwise be a harmonious combination, so that the oxygen and the
burnable substances readily split up and form a new combination, with
the nitrogen left out.

Of all the harmless things in the world one would think that that sweet,
sticky fluid, glycerine, which most of us have used at one time or
another to lubricate a sore throat, was the most harmless. As it stands
in its bottle upon the domestic medicine shelf, who would suspect that
it is the basis of such a thing as dynamite?

Such is the case, however, for glycerine on being brought into contact
with a mixture of sulphuric and nitric acids gives birth to
nitro-glycerine, an explosive of such sensitivity, of such a furious,
violent nature, that it is never allowed to remain long in its primitive
condition, but is as quickly as possible changed into something less
excitable.

Glycerine is one of those organic compounds which is obtained from
once-living matter. Arising as a by-product in the manufacture of soap,
it consists, as do so many of the organic substances, of carbon and
hydrogen, the atoms of which are peculiarly arranged to form the
glycerine molecule. To this the nitric acid adds oxygen and nitrogen,
the sulphuric acid simply standing by, as it were, and removing the
surplus water which arises during the process. So while glycerine is
carbon and hydrogen, nitro-glycerine is carbon, hydrogen, nitrogen and
oxygen. In this state they form a compact liquid, which occupies little
space.

The least thing upsets them, however. The carbon combines with oxygen
into carbon dioxide, commonly called carbonic acid gas, the hydrogen and
some more oxygen form steam, while the nitrogen is left out in the cold,
so to speak. And the total volume of the gases so produced is about 6000
times that of the original liquid. It is easy to see that a substance
which is liable suddenly to increase its volume by 6000 times is an
explosive of no mean order.

But the fact that it is liable to make this change on a comparatively
slight increase in temperature or after a concussion makes it too
dangerous for practical use. It needs to be tamed down somewhat. This
was first done by the famous Nobel, who mixed it with a fine earth known
as kieselguhr, whereby its sensitiveness was much decreased. This
mixture is dynamite.

It will be seen that the function of the "earth" is simply to act as an
absorbent of the liquid nitro-glycerine, and several other things can be
used for the same purpose. Moreover, there are now many explosives of
the dynamite nature but differing from it in having an active instead of
a passive absorbent, so that the decrease in sensitivity is accompanied
by an increase in strength. For example, gelignite, which is being used
for agricultural purposes in Great Britain, consists of nitro-glycerine
mixed with nitro-cotton, wood-meal and saltpetre. The wood-meal acts as
the absorbent instead of the kieselguhr, while the nitro-cotton is
another kind of explosive and the saltpetre, one of the ingredients in
the old gunpowder, provides the necessary oxygen for burning up the
wood-meal. Nitro-cotton is made in much the same way as nitro-glycerine,
except that cotton takes the place of the glycerine. Cotton is almost
pure cellulose, another organic substance, like glycerine insomuch as it
is composed of carbon and hydrogen, but, unlike it, containing also
oxygen. Treated with nitric acid it also forms a combination of carbon,
hydrogen, oxygen and nitrogen, which is called nitro-cotton,
nitro-cellulose, or gun-cotton.

It may be asked, why, if these two substances are thus similar, need
they be mixed? The answer is that although alike to a certain degree
they are not exactly the same, and the modern manufacturer of explosives
in his strife after perfection finds that for certain purposes one is
the best, and for others another, while for others again a combination
may excel any single one.

For some work another kind of explosive altogether is to be preferred.
This is based upon chlorate of potash, a compound very rich in oxygen,
which it is prepared to give up readily to burn any other suitable
element which may be at hand. A well-known explosive of this class is
that known as cheddite, since it was first made at a factory at Chedde,
in Savoy.

For the sake of simplicity, however, I propose in the following
descriptions to refer to all these explosives under the common term
"dynamite," since that will probably convey to the general public an
idea of their nature better than any other term or terms which I could
choose.

So now we come to the great question, how can the modern farmer benefit
by the use of high explosives such as these? The answer is, in many
ways. Let us take the most obvious one first.

A farmer has been ploughing his land and growing his crops upon it for
years. Perchance his forefathers have been doing the same for
generations. Every year, for centuries possibly, a hard steel
ploughshare has gone over that ground, turning over and over the top
soil to a depth of six to eight inches. Each season the plants, whatever
they may be, grow mainly in that top layer. They take the goodness or
nourishment out of it and it eventually becomes more or less sterile. By
properly rotating his crops he mitigates this to a certain extent, in
addition to which he restores to the land some of its old nitrogenous
constituents by the addition of manure. Yet, do what he will, this thin
top layer is bound to become exhausted. And all the while a few inches
lower down there is almost virgin soil which has scarcely been disturbed
since the creation of the world.

Nay, more, that virgin soil, with all its plant food still in it, is not
only doing little for its owner, it is positively doing him harm. For
every time his plough goes over it it tends to ram it down flat; every
time a man walks over it the result is the same; every horse that
passes, everything that happens or has happened for centuries in that
field, tends to make that soil just below the reach of the ploughshare a
hard, impervious mass, through which only the roots of the most strongly
growing plants can find a way, and which tends to make the soil above
it wet in wet weather and dry in dry weather. Thus roots have to spread
sideways instead of downwards; or, growing downwards with difficulty,
each plant has to expend vital energy in forcing its roots through the
hard ground which it might better employ in producing flowers or fruits.
And there is no natural storage of water. A shower drenches the ground.
In time it dries, through evaporation into the air, and then when the
drought comes all is arid as the Sahara.

That hard subsoil is known by the term "hard-pan," and, as we have seen,
it is produced more or less by all that goes on in the field. Even worse
is the case--a very frequent one too--wherein there is a natural stratum
of clay or equally dense waterproof material lying a few feet down.

Beyond the reach of any plough, this hard stratum can be broken up by
the use of dynamite. The usual method is to drive holes in the ground
about fifteen to twenty feet apart and about three or four feet deep,
right into the heart of the hard layer. At the bottom of each hole is
placed a cartridge of dynamite with a fuse and a detonator. This latter
is a small tube containing a small quantity of explosive which, unlike
the dynamite, can be easily fired, and initiates the detonation of the
cartridge.

When these miniature earthquakes have taken place all over a field a
very different state of things prevails. The "hard-pan" has been broken.
The explosive used for such a purpose has a sudden shattering power,
whereby it pulverises the ground in its vicinity rather than making a
great upheaval at the surface. The sudden shock makes cracks and
fissures in all directions, through which roots can easily make their
way. Moreover, it permits air to find an entrance, thereby aerating the
soil in such a way as to increase its fertility. The heat, or else the
chemical products of the explosion, seem to destroy the fungus germs in
the ground. Finally a natural storage of water is set up. Heavy rain,
instead of drenching the upper soil, simply moistens it nicely, while
the surplus water descends into the newly disturbed layers, there to
remain until the roots pump it up in time of drought.

It is stated that an acre of hay pumps up out of the soil 500 tons of
water per annum, so it is easy to see what an important feature this
natural water-storage is.

Farmers say that their crops have doubled in value after thus dynamiting
the subsoil.

This operation has been spoken of as a substitute for ploughing, but
that may be put down to "journalistic licence," for while it truly
conveys the general idea, it is hardly correct. The ordinary plough
turns over about eight inches, the special subsoil plough reaches down
to about eighteen inches, but the dynamite method loosens the ground to
a depth of six or seven feet. Corn roots if given a chance will go
downwards from four to eight feet. Potatoes go down three feet, hops
eight to eighteen feet and vines twenty feet, so it is easy to see how
restricted the plants are when their natural rooting instincts are
restrained by a hard layer at a depth of eighteen inches or so.

The holes are made by means of a bar or drill. A great deal depends, of
course, upon the hardness of the soil. Sometimes a steel bar has to be
driven in by a sledge-hammer. At others a pointed bar can be pushed down
by hand. In some cases it will be found that the best tool to employ is
a "dirt-auger," a tool like a carpenter's auger, which on being turned
round and round bores its way into the earth. However it may be done,
one or more cartridges of dynamite are lowered into the finished hole,
one of them being fitted with the necessary detonator and fuse. Then a
little loose earth or sand is dropped into the hole until it is filled
to a depth of six inches or so above the uppermost cartridge. Above that
it is quite safe to fill the hole with earth, ramming it in with a
wooden rammer. This is called "tamping," and it is necessary in order to
prevent the force of the explosion being wasted in simply blowing up the
hole. What is wanted is that the explosion shall take place within an
enclosed chamber so that its effect may be felt equally in all
directions. The holes are generally about an inch and a half or an inch
and three-quarters in diameter.

There are two ways of firing the charges. One is by means of fuses. The
detonator is fastened to one cartridge and a length of fuse is attached
to the detonator, which passing up the hole terminates above the ground.
The fuse is a tube of cotton filled with gunpowder, and it burns at the
rate of about two feet a minute. Thus if three feet of fuse be used the
man who lights it has a minute and a half in which to find a place of
safety from falling stones.

The other way is by electricity. In this case an electric fuse is
attached to the cartridge and two wires are led up the hole. These are
connected to an electrical machine, which causes a current to pass down
into the fuse, where, by heating a fine platinum wire, it fires the
detonating material with which it is packed. This detonating material in
turn fires the dynamite.

The advantage of the electrical method is that twenty or thirty holes
being simultaneously connected to the same machine can all be fired at
once.

And now let us think of another kind of farming, in which fruit trees
are concerned. With a large tree the need of plenty of underground space
for its roots would seem to be more important even than in the case of
annual plants like wheat. Yet we know very well that the usual procedure
is to dig a small hole just about big enough to accommodate the roots of
the sapling when it is planted, while the ground all round is left
undisturbed. The assumption is that the tree will, in time, be able to
push its roots through anything which is not actually solid rock. So
much is this the case that one authority has thought fit to warn
tree-growers in this picturesque fashion. "When planting a tree," he
says, "forget what it is you are doing, and think that you are about to
bury the biggest horse you know." How many people when planting any tree
dig a hole big enough to bury a horse? It is fairly safe to reply, only
those who do it by dynamite.

[Illustration: _By permission of Dupont Powder Co., Wilmington, Delaware_

                     FIRST EFFECT OF THE DYNAMITE

       Clearing a field of tree stumps by blowing them up with
                        dynamite.--_See_ p. 16]

The method of working is to bore a hole nearly as deep as the hole you
want to blast. At the bottom place a powerful charge, far stronger than
you would use for "subsoiling," as just described. That will not only
blow a hole big enough for you to put your tree in, but it will loosen
the ground all around the hole for yards. The main debris from the hole
will fall back into it, but that will not matter much, since, being all
loose, it is an easy matter to remove as much as is necessary to plant
the young tree. The advantages are the same as those enumerated in the
previous case--namely, the loosened ground gives more scope for the
roots--apple-tree roots want twenty feet or so--the ground holds
moisture better, and the explosion kills the fungus germs. In addition
to these there is the advantage that to blast a hole like this is
cheaper than digging it.

And that the advantages are not merely theoretical is shown by the fact
that trees so planted actually do grow stronger, bigger and quicker than
precisely similar ones under the same conditions, but set in the
ordinary way with a spade.

And not only do new trees thus benefit; old trees can be helped by
dynamite. Many an existing orchard has been improved by exploding
dynamite at intervals between the rows of trees. Care has to be taken to
see that the disturbance is not so violent or so close as to damage the
trees, but that can be easily arranged, and then the result is that the
soil all around the trees is loosened, the roots are given more freedom
and the water-storing properties of the ground are greatly improved.

Again, how often a farmer is troubled with a pond or a patch of marshy
ground right in the midst of his fields. It is of no use, and simply
serves to make the field in which it occurs more difficult to plough and
to cultivate--besides being so much good land wasted. Now the reason for
the existence of that pond or marsh is that underneath the surface there
is an impervious layer in which, as in a basin, the water can collect.
Make a hole in that and it will no more hold water than a cracked jug
will. And to make that hole with dynamite is the easiest thing in the
world.

If the pond be merely a collection of water which occurs in wet weather,
but which dries up quickly, there simply needs to be drilled a deep hole
and a fairly strong explosion caused at the bottom of it. How deep the
hole must be depends upon the formation of the earth at that point, and
how low down is the stratum which, being waterproof, causes the water to
remain. It is that, of course, which must be broken through, and so the
explosion must be caused at a point near the under side of that layer.
With a little experience the operator can judge the position by the feel
of the tool with which he makes the hole. If the pond is permanent but
shallow, men can wade to about the centre, there to drill a hole and
fire a shot. If it be permanent and deep, then the work must be done
from a raft, which, however, can be easily constructed for the purpose.
Once broken through, the water will quickly pass away below the
impervious stratum and useless land will become valuable.

The same may be done on a larger scale by blasting ditches with
dynamite. This is in many cases much cheaper than digging them. A row of
holes is put down, or even two or three rows, according to the width of
the proposed ditch. In depth they are made a little less than the depth
of the ditch that is to be. And for a reason which will be apparent they
are put very close together, say three feet or so apart. Preparations
may thus be made for blasting a ditch hundreds of feet long and then all
are fired together. The earth is thrown up by a mighty upheaval, a ditch
being produced of remarkable regularity considering the means by which
it is made. The sides, of course, take a nice slope, the debris is
thrown away on both sides and spread to a considerable distance, so
that, given favourable conditions and a well-arranged explosion, there
is constructed a finished ditch which hardly needs touching with spade
or other tool.

It not being feasible to fire a lot of holes electrically, the limit
being about thirty, the simultaneous explosion of perhaps hundreds has
to be brought about in some other manner, and usually it is accomplished
by the simple device of putting the holes fairly near together and
firing one with a fuse. The commotion set up by this one causes the
nearest ones to "go off," they in turn detonating those farther on, with
the result that explosion follows explosion all along the line so
rapidly as to be almost instantaneous.

A farmer who is troubled by a winding stream passing through his land,
cutting it up into awkward shapes, can straighten it by blasting a ditch
across a loop in the manner just described. In the case of low-lying
land, however, ditches are obviously no use, since water would not flow
away along them. In that case the principle suggested just now for
dealing with an inconvenient pond can sometimes be used, for if the
subsoil be blasted through at several points it is very likely that
water will find a way downwards by some means or other.

And the list of possible uses is by no means exhausted yet. A man
opening up virgin land often finds old tree stumps his greatest bother.
He can dig round them and then pull them out with a team of horses, but
by far the simpler way is with a few well-placed dynamite cartridges,
for they not only throw up the stump for him, but they break it up,
shake the earth from it, and leave it ready for him to cart away or to
burn.

Boulders, too, can be blown to pieces far more easily than one would
think. The charges may be put underneath them as with the tree stumps,
but in many cases that is not necessary, all that is needed being some
dynamite laid upon the top of the rock and covered with a heap of clay.
So sudden is the action of the explosive that its shock will break up
the stone underneath it. Yet another way, perhaps the most effective of
all, is to drill a hole into the stone and fire a charge inside it. It
behoves the onlooker then to keep away, for the fragments may be thrown
three or four hundred feet, a fair proof that the stone will be very
thoroughly demolished.

Even in the digging of wells explosives may be useful. In that case the
holes are made in a circle, and they slant downwards and inwards, so
that their lower ends tend to meet. The result of simultaneously
exploding the charges in these holes is to cut out a conical hole a
little larger in size than the ring and a little deeper than the point
at which the explosion took place. The bottom of that hole can be
levelled a little and the operation repeated, and so stage by stage the
well will proceed to grow downwards.

The thought that naturally occurs to one is this. All the operations
described may be very well, the cost may be low, and the effect good,
but are they sufficient to compensate for the risks necessarily
dependent upon the use of explosives? The doubt implied in that
question, natural though it be, is based upon prejudice, with which we
are all more or less afflicted. The art of making these explosive
substances has been brought to such a pitch that with reasonable care
there is no risk whatever. The greatest possible care is used in the
factory to see that all explosives sent out are what they are meant to
be, and that they can therefore be relied upon to behave according to
programme and not to play any tricks. That is the first step, and what
with competition between makers, Government inspection, and searching
inquiry into the slightest accident, and the desire of each maker to
keep up the credit of his name, it is safe to say that modern explosives
may be relied upon to do their duty faithfully. The second step in the
process of securing safety is that the powerful explosive, the one that
does the work, is made very insensitive, so that it is really quite hard
to explode it. With reasonable care, then, it will never go off by
accident. On the other hand, the sensitive material, which is easy to
"let off," is in very small quantities, so small that an accident with
it would not, again with reasonable precautions, be a serious matter.

Fuse, too, is very reliable nowadays. The man who lights the fuse may be
absolutely sure that he will have that time to get to a place of safety
which corresponds to the length of fuse which he employs. With
electrical firing, too, it is quite easy to arrange that the final
electrical connection shall not be made until all are at a safe
distance, so that a premature explosion is impossible.

In many of the cases described, the shock takes place almost entirely
within the earth and there is very little debris thrown about.

Indeed the only danger which is to be feared with these operations is
about on a par with that which every farm hand runs from the kick of a
horse. Any careful, trustworthy man could be quite safely taught to do
this work, and with the assistance of a labourer he could do all that is
necessary. Given a fair amount of intelligence, too, he would take but
little teaching. Altogether there is no doubt that the use of explosives
is going to have a marked effect upon farming operations in the near
future.



CHAPTER II

MEASURING ELECTRICITY


There are many people whose acquaintance with electricity consists
mainly in paying the electric light bill. To such the instruments
whereby electricity is measured will make a specially interesting
appeal.

Current is sold in Great Britain at so much per Board of Trade Unit. To
state what that is needs a preliminary explanation of the other units
employed in connection with electric currents.

The public electricity supply in any district is announced to be so many
volts, it may be 100, 200 or perhaps 230, but whatever it be, it is
always so many "volts." Then the electrician speaks lightly of numbers
of "amperes," he may even talk of the number of "watts" used by the
lamps, while occasionally the word "ohm" will leak out. Among these
terms the general reader is apt to become completely fog-bound. But
really they are quite simple if once understood, and, as we shall see in
a moment, there are some very remarkable instruments for measuring them,
some of which exhibit a delicacy truly astonishing.

It is well at the outset to try and divest ourselves of the idea that
there is anything mysterious or occult about electricity. It is quite
true that there are many things about it very little understood even by
the most learned, but for ordinary practical purposes it may be regarded
as a fluid, which flows along a wire just as water flows along a pipe.
The wire is, electrically speaking, a "hole" through the air or other
non-conducting substance with which it is surrounded. A water-pipe being
a hole through a bar of iron, so the copper core of an electrical wire
is, so far as the current is concerned, but a hole through the centre of
a tube of silk, cotton, rubber or whatever it be. Electricity can flow
through certain solids just as water can flow through empty space.

Water will not flow through a pipe unless a pressure be applied to it
somewhere. In a pipe the ends of which are at the same level water will
lie inert and motionless. Lower one end, however, and the pressure
produced by gravity--in other words, the weight of the water--will cause
it to move. In like manner pressure produced by the action of a pump
will make water flow. On the other hand, when it moves it encounters
resistance, through the water rubbing against the walls of the pipe.

Similarly, an electrical pressure is necessary before a current of
electricity will flow. And every conductor offers more or less
resistance to the flow of current, thus opposing the action of the
pressure. Before current will flow through your domestic glow-lamps and
cause them to give light there must be a pressure at work, and that
pressure is described as so many volts.

A battery is really a little automatic electrical pump for producing an
electrical pressure. And the volt, which is a legal measure, just as
much as a pound or a yard, is a certain fraction of the pressure
produced by a certain battery known as Clark's Cell. It is not necessary
here to say exactly what that fraction is, but it will give a general
idea to state that the ordinary Leclanche or dry cell, such as is used
for electric bells, produces a pressure of about one and a half volts.

Thus we see the volt is the electrical counterpart of the term "pound
per square inch" which is used in the case of water pressure.

A flow of water is measured in gallons per minute. An electrical current
is measured in coulombs per second. Thus the coulomb is the electrical
counterpart of the gallon. But in this particular we differ slightly in
our methods of talking of water and electricity. Gallons per minute or
per hour is the invariable term in the former case, but in the latter
we do not speak of coulombs per second, although that is what we mean,
for we have a special name for one coulomb per second, and that same is
ampere. One ampere is one coulomb per second, two amperes are two
coulombs per second, and so on.

There is no recognised term to denote the resistance which a water-pipe
offers to the passage of water through it, but in the similar case with
electricity there is a term specially invented for the purpose, the ohm.
Legally it is the resistance of a column of mercury of a certain size
and weight. A rough idea of it is given by the fact that a copper wire a
sixteenth of an inch thick and 400 feet long has a resistance of about
one ohm.

The three units--the volt, ampere and ohm--are so related that a
pressure of one volt acting upon a circuit with a resistance of one ohm
will produce a current of one ampere.

A current can do work; when it lights or heats your room or drives a
tramcar it is doing work; and the rate at which a current does work is
found by multiplying together the number of volts and the number of
amperes. The result is in still another unit, the watt. And 1000 watts
is a kilowatt. Finally, to crown the whole story, a kilowatt for one
hour is a Board of Trade unit.

So for every unit which you pay for in the quarterly bill you have had a
current equal to 1000 watts for an hour. To give a concrete example, if
the pressure of your supply is 200 volts, and you take a current of five
amperes for an hour, you will have consumed one B.T.U.

Perhaps it will give added clearness to this explanation to tabulate the
terms as follow:--

     _Volt_ = The unit of pressure, analogous to "pounds per square
     inch" in the case of water.

     _Coulomb_ = The measure of quantity, analogous to the gallon.

     _Ampere_ = The measure of the "strength" of a current, meaning one
     coulomb per second.

     _Watt_ = The unit denoting the power for work of any current. It is
     the result of multiplying together volts and amperes.

     _Kilowatt_ = 1000 watts.

     _Board of Trade Unit_ = A current of one kilowatt flowing for one
     hour.

[Illustration:                  _By permission of Dupont Powder Co._

                            A FINE CROP

           Celery grown on soil tilled by dynamite.--_See_ p. 24]

In practice the measurements are generally made by means of the
connection between electricity and magnetism. A current of electricity
is a magnet. Whenever a current is flowing it is surrounded by a region
in which magnetism can be felt. This region is called the magnetic
field, and the strength of the field varies with the strength that is
the number of amperes in the current. If a wire carrying a current be
wound up into a coil it is evident that the magnetic field will be more
intense than if the wire be straight, for it will be concentrated into a
smaller area. Iron, with its peculiar magnetic properties, if placed in
a magnetic field seems to draw the magnetic forces towards itself, and
consequently, if the wire be wound round a core of iron, the magnetism
due to the current will be largely concentrated at the ends of the core.
But the main principle remains--in any given magnet the magnetic power
exhibited will be in proportion to the current flowing.

The switchboard at a generating station is always supplied with
instruments called ammeters, an abbreviation of amperemeters, for the
purpose of measuring the current passing out from the dynamos. Each of
these consists of a coil of wire through which the current passes. In
some there is a piece of iron near by, which is attracted more or less
as the current varies, the iron being pulled back by a spring and its
movement against the tension of the spring being indicated by a pointer
on a dial.

In others the coil itself is free to swing in the neighbourhood of a
powerful steel magnet, the interaction between the electro-magnet, or
coil, and the permanent magnet being such that they approach each other
or recede from each other as the current varies. A pointer on a dial
records the movements as before.

In yet another kind the permanent magnet gives way to a second coil, the
current passing through both in succession, the result being very much
the same, the two coils attracting each other more or less according to
the current.

Another kind of ammeter known as a thermo-ammeter works on quite a
different principle. It consists of a piece of fine platinum wire which
is arranged as a "shunt"--that is to say, a certain small but definite
proportion of the current to be measured passes through it. Now, being
fine, the current has considerable difficulty in forcing its way through
this wire and the energy so expended becomes turned into heat in the
wire. It is indeed a mild form of what we see in the filament of an
incandescent lamp, where the energy expended in forcing the current
through makes the filament white-hot. The same principle is at work when
we rub out a pencil mark with india-rubber, whereby the rubber becomes
heated, as most of us have observed. The wire, then, is heated by the
current passing through it, and accordingly expands, the amount of
expansion forming an indication of the current passing. The elongation
of the wire is made to turn a pointer.

A simple modification makes any of these instruments into a voltmeter.
This instrument is intended to measure the force or pressure in the
current as it leaves the dynamo.

A short branch circuit is constructed, leading from the positive wire
near the dynamo to the negative wire, or to the earth, where the
pressure is zero. In this circuit is placed the instrument, together
with a coil made of a very long length of fine wire so that it has a
very great resistance. Very little current will flow through the branch
circuit because of the high resistance of the coil, but what there is
will be in exact proportion to the pressure. The voltmeter is therefore
the same as the ammeter, except that its dial is marked for volts
instead of for amperes, and it has to be provided with the resistance
coil.

Instruments of the ammeter type can also be used as ohmmeters. In this
case what is wanted is to test the resistance of a circuit, and it is
done by applying a battery, the voltage of which is known, and seeing
how much current flows.

All the voltmeters and ohmmeters mentioned owe their method of working
to what is known as Ohm's law. One of the greatest steps in the
development of electrical science was taken when Dr Ohm put forward the
law which he had discovered whereby pressure, current and resistance are
related. The reader will probably have noticed from what has already
been said about the units of measurement--the volt, the ampere and the
ohm--that the current varies directly as the pressure and inversely as
the resistance. That is the famous and important "Ohm's law" and anyone
who has once grasped that has gone a long way towards understanding many
of the principal phenomena of electric currents.

But the instruments so far referred to are of the big, clumsy type,
suitable for measuring large currents and great pressures. They are like
the great railway weigh-bridges, which weigh a whole truck-load at a
time and are good enough if they are true to a quarter of a
hundredweight. The instruments about to be described are more comparable
with the delicate balance of the chemist, which can detect the added
weight when a pencil mark is made upon a piece of paper. Indeed beside
them such a balance is quite crude and clumsy. They may be said to be
the most delicate measuring instruments in existence.

We will commence with the galvanometer. The simplest form of this is a
needle like that of a mariner's compass very delicately suspended by a
thin fibre in the neighbourhood of a coil of wire. The magnetic field
produced by the current flowing in the wire tends to turn the needle,
which movement is resisted by its natural tendency to point north and
south. Thus the current only turns the needle a certain distance, which
distance will be in proportion to its strength. The deflection of the
needle, therefore, gives us a measure of the strength of the current.

But such an instrument is not delicate enough for the most refined
experiments, and the improved form generally used is due to that prince
of inventors, the late Lord Kelvin. He originally devised it, it is
interesting to note, not for laboratory experiments, but for practical
use as a telegraph instrument in connection with the early Atlantic
cables.

Before describing it, it may sharpen the reader's interest to mention a
wonderful experiment which was made by Varley, the famous electrician,
on the first successful Atlantic cable. He formed a minute battery of a
brass gun-cap, with a scrap of zinc and a single drop of acidulated
water. This he connected up to the cable. Probably there is not one
reader of this book but would have thought, if he had been present, that
the man was mad. What conceivable good could come of connecting such a
feeble source of electrical pressure to the two thousand miles of wire
spanning the great ocean; the very idea seems fantastic in the extreme.
Yet that tiny battery was able to make its power felt even over that
great distance, for the Thomson Mirror Galvanometer was there to detect
it. Two thousand miles away, the galvanometer felt and was operated by
the force generated in a battery about the size of one of the capital
letters on this page.

This wonderful instrument consisted of a magnet made of a small fragment
of watch-spring, suspended in a horizontal position by means of a thread
of fine silk, close to a coil of fine wire. When current flowed through
the coil the magnetic field caused the watch-spring magnet to swing
round, but when the current ceased the untwisting of the silk brought it
back to its original position again.

So far it seems to differ very little from the ordinary galvanometer
previously mentioned, but the stroke of genius was in the method of
reading it. With a small current the movement of the magnet was too
small to be observed by the unaided eye, so it was attached to a minute
mirror made of one of those little circles of glass used for covering
microscope slides, silvered on the back. The magnet was cemented to the
back of this, yet both were so small that together their weight was
supported by a single thread of cocoon silk. Light from a lamp was made
to fall upon this mirror, thereby throwing a spot of light upon a
distant screen. Thus the slightest movement of the magnet was magnified
into a considerable movement of the spot of light. The beam of light
from the mirror to the screen became, in fact, a long lever or pointer,
without weight and without friction.

The task of watching the rocking to and fro of the spot of light was
found to be too nerve-racking for the telegraph operators, and so Lord
Kelvin improved upon his galvanometer in two ways. He first of all
managed to give it greater turning-power, so that, actuated by the same
current, the new instrument would work much more strongly than the older
one. Then he utilised this added power to move a pen whereby the signals
were recorded automatically upon a piece of paper. The new instrument is
known as the Siphon Recorder.

The added power was obtained by turning the instrument inside out, as it
were, making the coil the moving part and the permanent magnet the fixed
part. This enabled him to employ a very powerful permanent magnet in
place of the minute one made of watch-spring. The interaction of two
magnets is the result of their combined strength, and that of the coil
being limited by the strength of the minute current the only way to
increase the combined power of the two was to substitute a large
powerful magnet for the small magnetised watch-spring. This large magnet
would, of course, have been too heavy to swing easily and therefore the
positions had to be reversed.

So now we have two types of galvanometer, both due originally to the
inventions of Lord Kelvin. For some purposes the Thomson type (his name
was Thomson before he became Lord Kelvin) are still used, but in a
slightly elaborated form. Its sensitiveness is such that a current of a
thousandth of a micro-ampere will move the spot of light appreciably.
And when one comes to consider that a micro-ampere is a millionth part
of an ampere this is perfectly astounding.

But there is a more wonderful story still to come, of an instrument
which can detect a millionth of a micro-ampere, or one millionth of a
millionth of an ampere. It is not generally known that we are all
possessors of an electric generator in the form of the human heart, but
it is so, and Professor Einthoven, of Leyden, wishing to investigate
these currents from the heart, found himself in need of a galvanometer
exceeding in sensitiveness anything then known. Even the tiny needles or
coils with their minute mirrors have some weight and so possess in an
appreciable degree the property of inertia, in virtue of which they are
loath to start movement and, having started, are reluctant to stop. This
inertia, it is easy to see, militates against the accurate recording of
rapid variations in minute currents, so the energetic Professor set
about devising a new galvanometer which should answer his purpose. This
is known as the "String Galvanometer."

[Illustration: FIG. 1.-This shows the principle of this wonderful
Galvanometer invented by Lord Kelvin in its latest form. Current enters
at _a_, passes round the coils, as shown by the arrows, and away at _b_.
A light rod, _c_, is suspended by the fine fibre, _d_, so that the eight
little magnets hang in the centres of the coils--four in each. The
current deflects these magnets and so turns the mirror, _m_, at the
bottom of the rod. At _e_ are two large magnets which give the little
ones the necessary tendency to keep at "zero."]

[Illustration: FIG. 2.--Here we see the working parts of the "String
Galvanometer," by which the beating of the heart can be registered
electrically. The current flows down the fine silvered fibre, between
the poles, _a_ and _b_, of a powerful magnet. As the current varies, the
fibre bends more or less.]

The main body of the instrument is a large, powerful electro-magnet, in
shape like a large pair of jaws nearly shut. Energised by a strong
current, this magnet produces an exceedingly strong magnetic field in
the small space between the "teeth" as it were. In this space there is
stretched a fine thread of quartz which is almost perfectly elastic. It
is a non-conductor, however, so it is covered with a fine coating of
silver. Silver wire is sometimes used, but no way has yet been found of
drawing any metallic wire so thin as the quartz fibre, which is
sometimes as thin as two thousandths of a millimetre, or about a
twelve-thousandth of an inch. A hundred pages of this book make up a
thickness of about an inch, so that one leaf is about a fiftieth of an
inch. Consequently the fibre in question could be multiplied 240 times
before it became as stout as the paper on which these words are printed.

The current to be measured, then, is passed through the stretched fibre
and the interaction of the magnetic field by which the fibre is then
surrounded, with the magnetic field in which it is immersed, causes it
to be deflected to one side. Of course the deflection is exceedingly
small in amount, and as it is undesirable to hamper its movements by the
weight of a mirror, no matter how small, some other means of reading the
instrument had to be devised. This is a microscope which is fixed to one
of the jaws, through a fine hole in which the movements of the fibre can
be viewed. Or what is often better still, a picture of the wire can be
projected through the microscope on to a screen or on to a moving
photographic plate or strip of photographic paper. In the latter case a
permanent record is made of the changes in the flowing current.

An electric picture can thus be made of the working of a man's heart. He
holds in his hands two metal handles or is in some other way connected
to the two ends of the fibre by wires just as the handles of a shocking
coil are connected to the ends of the coil. The faint currents caused by
the beating of his heart are thus set down in the form of a wavy line.
Such a diagram is called a "cardiogram," and it seems that each of us
has a particular form of cardiogram peculiar to himself, so that a man
could almost be recognised and distinguished from his fellows by the
electrical action of his heart.

The galvanometer has a near relative, the electrometer, the astounding
delicacy of which renders it equally interesting. It is particularly
valuable in certain important investigations as to the nature and
construction of atoms.

The galvanometer, it will be remembered, measures minute currents; the
electrometer measures minute pressures, particularly those of small
electrically charged bodies.

Every conductor (and all things are conductors, more or less) can be
given a charge of electricity. Any insulated wire, for example, if
connected to a battery will become charged--current will flow into it
and there remain stationary. And that is what we mean by a charge as
opposed to a current.

Air compressed into a closed vessel is a charge. Air, however
compressed, flowing along a pipe would be better described as a current.

Imagine one of those cylinders used for the conveyance of gas under
pressure and suppose that we desire to find the pressure of the gas with
which it is charged. We connect a pressure-gauge to it, and see what the
finger of the gauge has to say. What happens is that gas from the
cylinder flows into the little vessel which constitutes the gauge and
there records its own pressure.

And just the same applies with electrometers. Precisely as the
pressure-gauge measures the pressure of air or gas in some vessel, so
the electrometer measures the electrical pressure in a charged body.

Further, some of the charged bodies with which the student of physics is
much concerned are far smaller than can be seen with the most powerful
microscope. How wonderfully minute and delicate, therefore, must be the
instrument which can be influenced by the tiny charge which so small a
body can carry.

It will be interesting here to describe an experiment performed with an
electrometer by Professor Rutherford, by which he determined how many
molecules there are in a centimetre of gas, a number very important to
know but very difficult to ascertain, since molecules are too small to
be seen. This number, by the way, is known to science as "Avogadro's
Constant."

Everyone has heard of radium, and knows that it is in a state which can
best be described as a long-drawn-out explosion. It is always shooting
off tiny particles. Night and day, year in and year out, it is firing
off these exceedingly minute projectiles, of which there are two kinds,
one of which appears to be atoms of helium.

Some years ago, when radium was being much talked about and the names of
M. and Madame Curie were in everyone's mouth, little toys were sold, the
invention, I believe, of Sir William Crookes, called spinthariscopes.
Each of these consisted of a short brass tube with a small lens in one
end. Looking through the lens in a dark room, one could see little
splashes of light on the walls of the tube. Those splashes were caused
by a tiny speck of radium in the middle of the tube, the helium atoms
from which, by bombarding the inner surface of the tube, produced the
sparks.

Now if we can count those splashes we can tell how many atoms of helium
are being given off per minute. And if then we reckon how many minutes
it takes to accumulate a cubic centimetre of helium we can easily reckon
how many atoms go to the cubic centimetre. But the difficulty is to
count them.

So the learned Professor called in the aid of the electrometer. He could
not count all the atoms shot off, so he put the piece of radium at one
end of a tube and an electrometer at the other. Every now and then an
atom shot right through the tube and out at the farther end. And since
each of these atoms from radium is charged with electricity, each as it
emerged operated the electrometer. By simply watching the twitching of
the instrument, therefore, it was possible to count how many atoms shot
through the tube--one atom one twitch. And from the size and position of
the tube it was possible to reckon what proportion of the whole number
shot off would pass that way.

The result of the experiment showed that there are in a cubic centimetre
of helium a number of atoms represented by 256 followed by seventeen
noughts. And as helium is one of the few substances in which the
molecule is formed of but one atom, that is also the number of
molecules.

And now consider this, please. A cubic centimetre is about the size of a
boy's marble. That contains the vast number of molecules just mentioned.
And the electrometer was able to detect the presence of those _one at a
time_. Need one add another word as to the inconceivable delicacy of the
instrument.

In its simplest form the electrometer is called the "electroscope." Two
strips of gold-leaf are suspended by their ends under a glass or metal
shade. As they hang normally they are in close proximity. Their upper
ends are, in fact, in contact and are attached to a small vertical
conductor. A charge imparted to the small conductor will pass down into
the leaves, and since it will charge them both they will repel each
other so that their lower ends will swing apart. Such an instrument is
very delicate, but because of the extreme thinness of the leaves it is
very difficult to read accurately the amount of their movement and so to
determine the charge which has been given to them.

In a more recent improvement, therefore, only one strip of gold-leaf is
used, the place of the other being taken by a copper strip. The whole of
the movement is thus in the single gold-leaf, as the copper strip is
comparatively stiff, and it is possible to arrange for the movement of
this one piece of gold-leaf to be measured by a microscope.

The other principal kind of electrometer we owe, as we do the
galvanometers, to the wonderful ingenuity of Lord Kelvin. In this the
moving part is a strip of thin aluminium, which is suspended in a
horizontal position by means, generally, of a fine quartz fibre. Since
it is necessary that this fibre should be a conductor, which quartz is
not, it is electro-plated with silver. Thus a charge communicated to the
upper end of the fibre, where it is attached to the case, passes down to
the aluminium "needle," as it is called. Now the needle is free to swing
to and fro, with a rotating motion, between two metal plates carefully
insulated. Each plate is cut into four quadrants, the opposite ones
being electrically connected, while all are insulated from their nearest
neighbours. One set of quadrants is charged positively, and one set
negatively, by a battery, but these charges have no effect upon the
needle until it is itself charged. As soon as that occurs, however, they
pull it round, and the amount of its movement indicates the amount of
the charge upon the needle, and therefore the pressure existing upon the
charged body to which it is connected. The direction of its movement
shows, moreover, whether the charge be positive or negative.

A little mirror is attached to the needle, so that its slightest motion
is revealed by the movement of a spot of light, as in the case of the
mirror galvanometers. Instruments such as these are called "Quadrant
Electrometers."

My readers will remember, too, the "String Galvanometer" already
mentioned. The same idea has been adapted to this purpose. A fine fibre
is stretched between two charged conductors while the fibre is itself
connected to the body whose charge is being measured. The charge which
it derives from the body causes it to be deflected, which deflection is
measured by a microscope.

In all cases of transmission of electricity over long distances for
lighting or power purposes the currents are "alternating." They flow
first one way and then the other, reversing perhaps twenty times a
second, or it may be two hundred, or even more times in that short
period. Some electric railways are worked with alternating current, and
it is used for lighting quite as much as direct current and is equally
satisfactory.

In wireless telegraphy it is essential. In that case, however, the
reversals may take place _millions_ of times per second. Consequently,
to distinguish the comparatively slowly changing currents of a
"frequency" or "periodicity" of a few hundreds per second from these
much more rapid ones, the latter are more often spoken of as electrical
oscillations. And these alternating and oscillating currents need to be
measured just as the direct currents do. Yet in many cases the same
instruments will not answer. There has therefore grown up a class of
wonderful measuring instruments specially designed for this purpose, by
which not only does the station engineer know what his alternating
current dynamos are doing, but the wireless operator can tell what is
happening in his apparatus, the investigator can probe the subtleties of
the currents which he is working with, and apparatus for all purposes
can be designed and worked with a system and reason which would be
impossible but for the possibility of being able to measure the
behaviour of the subtle current under all conditions.

One trouble in connection with measuring these alternating currents is
that they are very reluctant to pass through a coil.

One method by which this difficulty can be overcome has been mentioned
incidentally already. I refer to the heating of a wire through which
current is passing. This is just the same whether the current be
alternating or direct.

One of the simplest instruments of this class has been appropriated by
the Germans, who have named it the "Reiss Electrical Thermometer,"
although it was really invented nearly a century ago by Sir William Snow
Harris. It consists of a glass bulb on one end of a glass tube. The
current is passed through a fine wire inside this bulb, and as the wire
becomes heated it expands the air inside the bulb. This expansion moves
a little globule of mercury which lies in the tube, and which forms the
pointer or indicator by which the instrument is read. As the temperature
of the wire rises the mercury is forced away from the bulb, as the
temperature falls it returns. And as the temperature is varied by the
passage of the current, so the movement of the mercury is a measure of
the current.

Another way is to employ a "Rectifier." This is a conductor which has
the peculiar property of allowing current to pass one way but not the
other. It thus eliminates every alternate current and changes the
alternating current into a series of intermittent currents all in the
same direction. Rectified current is thus hardly described by the term
continuous, but still it is "continuous current" in the sense that the
flow is always in the same direction, and so it can be measured by the
ordinary continuous current instruments. The difficulty about it is that
there is some doubt as to the relation between the quantity of rectified
current which the galvanometer registers and the quantity of alternating
current, which after all is the quantity which is really to be measured.
How the rectification is accomplished will be referred to again in the
chapter on Wireless Telegraphy.

But to return to the thermo-galvanometers, as those are termed which
ascertain the strength of a current by the heat which it produces, the
simple little contrivance of Sir William Snow Harris has more elaborate
successors, of which perhaps the most interesting are those associated
with the name of Mr W. Duddell, who has made the subject largely his
own. Besides their interest as wonderfully delicate measuring
instruments, these have an added interest, since they introduce us to
another strange phenomenon in electricity. We have just noted the fact
that electricity causes heat. Now we shall see the exact opposite, in
which heat produces electrical pressure and current. And the feature of
Mr Duddell's instruments is the way in which these two things are
combined. By a roundabout but very effective way he rectifies the
current to be measured, for he first converts some of the alternating
current into heat and then converts that heat into continuous current.

If two pieces of dissimilar metals be connected together by their ends,
so as to form a circuit, and one of the joints be heated, an electrical
pressure will be generated which will cause a current to flow round the
circuit. The direction in which it will flow will depend upon the metals
employed. The amount of the pressure will also depend upon the metals
used, combined with the temperature of the junctions. With any given
pair of metals, however, the force, and therefore the volume of current,
will vary as the temperature. Really it will be the difference in
temperature between the hot junction and the cold junction, but if we so
arrange things that the cold junction shall always remain about the
same, the current which flows will vary as the temperature of the hot
one. The volume of that current will therefore be a measure of the
temperature. Such an arrangement is known as a thermo-couple, and is
becoming of great use in many manufacturing processes as a means of
measuring temperatures.

In the Duddell Thermo-galvanometers, therefore, the alternating current
is first led to a "heater" consisting of fine platinised quartz fibre or
thin metal wires. Just above the heater there hangs a thermo-couple,
consisting of two little bars, one of bismuth and the other of antimony.
These two are connected together at their lower end, where they nearly
touch the heater, but their upper ends are kept a little apart, being
joined, however, by a loop formed of silver strip. This arrangement will
be quite clear from the accompanying sketch, and it will be observed
that the loop is so shaped that the whole thing can be easily suspended
by a delicate fibre which will permit it to swing easily, like the coil
in a mirror galvanometer.

It is indeed a swinging coil of a galvanometer formed with a single turn
instead of the many turns usual in the ordinary instruments, and it will
be noticed from the sketch that there is a mirror fixed just above the
top of the loop.

This coil, then, with the thermo-couple at its lower extremity, is hung
between the ends of a powerful magnet much as the fibre of the Einthoven
Galvanometer is situated. The alternating current to be measured comes
along through the heater. The heater rises in temperature. That warms
the lower end of the thermo-couple. Instantly a steady, continuous
current begins to circulate round the silver strip which forms the coil,
and that, acting just as the current does in the ordinary galvanometer,
causes the coil to swing round more or less, which movement is indicated
by the spot of light from the mirror. A current as small as twenty
micro-amperes (or twenty millionths of an ampere) can be measured in
this way.

Mr Duddell has also perfected a wonderful instrument called an
Oscillograph, for the strange purpose of making actual pictures of the
rise and fall in volume of current in alternating circuits.

[Illustration: FIG. 3.--The "Duddell" Thermo-galvanometer.

In this remarkable instrument _alternating_ current enters at _a_,
passes through the fine wire and leaves at _b_. In doing this it heats
the wire, which in turn heats the lower end of the bismuth and antimony
bars. This generates _continuous_ current, which circulates through the
loop of silver wire, _c_, which, since it hangs between the poles, _d_
and _e_, of a magnet, is thereby turned more or less. The amount of the
turning indicates the strength of the _alternating_ current.]

To realise the almost miraculous delicacy of these wonderful instruments
we need first of all to construct a mental picture of what takes place
in a circuit through which alternating current is passing. The current
begins to flow: it gradually increases in volume until it reaches its
maximum: then it begins to die away until it becomes nil: then it begins
to grow in the opposite direction, increases to its maximum and dies
away once more. That cycle of events occurs over and over again at the
rate it may be of hundreds of times per second. Now for the actual
efficient operation of electrical machinery working on alternating
current it is very necessary to know exactly how those changes take
place--do they occur gradually, the current growing and increasing in
volume regularly and steadily, or irregularly in a jumpy manner?
Engineers have a great fancy for setting out such changes in the form of
diagrams, in which case the alternations are represented by a wavy line,
and it is of much importance to obtain an actual diagram showing not
what the changes should be according to theory, but what they really are
in practice. It is then possible to see whether the "wave-form" of the
current is what it ought to be.

Once again we must turn our thoughts back to the string galvanometer. In
that case, it will be remembered, there is a conducting fibre passing
between the ends or poles of a powerful magnet, the result of which
arrangement is that as the current passes through the fibre it is bent
by the action of the magnetic forces produced around it. If the current
pass one way, downwards let us say, the fibre will be bent one way,
while if it pass upwards it will be bent the opposite way. Suppose then
that we have two fibres instead of one, and that we send the current up
one and down the other. One will be bent inwards and the other outwards.
Then suppose that we fix a little mirror to the centre of the fibres,
one side of it being attached to one fibre and the other to the other.
As one fibre advances and the other recedes the mirror will be turned
more or less. Consequently, as the current flowing in the fibres
increases or decreases, or changes in direction, the mirror will be
slewed round more or less in one direction or the other.

The spot of light thrown by the mirror will then dance from side to side
with every variation, and if it be made to fall upon a rapidly moving
strip of photograph paper a wavy line will be drawn upon the paper which
will faithfully represent the changes in the current.

In its action, of course, it is not unlike an ordinary mirror
galvanometer, but its special feature is in the mechanical arrangement
of its parts which enable it to move with sufficient rapidity to follow
the rapidly succeeding changes which need to be investigated. It is far
less sensitive than, say, a Thomson Galvanometer, but the latter could
not respond quickly enough for this particular purpose.



CHAPTER III

THE FUEL OF THE FUTURE


We now enter for a while the realm of organic chemistry, a branch of
knowledge which is of supreme interest, since it covers the matters of
which our own bodies are constructed, the foods which we eat and the
beverages which we drink, besides a host of other things of great value
to us.

Although the old division of chemistry into inorganic and organic is
still kept up as a matter of convenience, the old boundaries between the
two have become largely obliterated. The distinction arose from the fact
that there used to be (and are still to a very great extent) a number of
highly complex substances the composition of which is known, for they
can be analysed, or taken to pieces, but which the wit of man has failed
to put together. Consequently these substances could only be obtained
from organic bodies. The living trees, or animals, could in some
mysterious way bring these combinations about, but man could not. The
molecules of these substances are much more complicated than those with
which the inorganic chemist deals. The important ingredient in them all
is carbon, which with hydrogen, nitrogen and oxygen almost completes the
list of the simple elements of which these marvellous substances are
compounded. In some cases there appear to be hundreds of atoms in the
molecule.

If one takes a glance at a text-book on organic chemistry the pages are
seen to be sprinkled all over with C's and O's, N's and H's, with but an
occasional symbol for some other element.

Another feature of this branch which cannot fail to strike the casual
observer is the queer names which many of the substances possess.
Trimethylaniline, triphenylmethane and mononitrophenol are a few
examples which happen to occur to the memory, and they are by no means
the longest or queerest-sounding.

Another peculiarity about these organic substances is that a number of
them, each quite different from the others, can be formed of the same
atoms. Certain atoms of hydrogen, sulphur and oxygen form sulphuric
acid, and under whatever conditions they combine they never form
anything else. On the other hand, there are sixty-six different
substances all formed of eight of carbon, twelve of hydrogen and four of
oxygen. This can only mean that in such cases as the latter the atoms
have different groupings and that when grouped in one way they form one
thing, in another way some other thing, and so on. This explains the
extreme difficulty which the chemist finds in building up some of these
organic substances.

Every now and again we are startled by some eminent man stating that the
time will come when we shall be able to make living things, when the
laboratory will turn out living cows and sheep, birds and insects, even
man with a mind and soul of his own. Yet one cannot but feel that such
men, no matter how great their authority, are simply "pulling the
public's leg," to use a colloquial expression. For they hopelessly fail
to make many of the commonest things. In many cases where they wish to
produce an organic substance they have to call in the aid of some living
thing to do it for them, even if it be but a humble microbe. For the
microbes perform wonderful feats in chemistry, far surpassing those of
the most eminent men. Hence the latter very sensibly use the microbe,
employ it to work for them, just set things in order and then stand by
while the microbe does the work.

Thus most things can be analysed--that is to say, taken to pieces--while
many things can now be synthesised--that is to say, built up from their
constituent atoms--but still a great many remain, and among them the
most important, the synthesis of which completely baffles man. One of
the most useful and widespread substances, for example, cellulose, is,
at present at least, utterly beyond us. We do not even know how many
atoms there are in the cellulose molecule. The molecules may, for all we
know, contain thousands of atoms. Indeed many of these organic matters
have very large molecules.

And even if the chemist were able to make all kinds of organic matter,
he would still be as far off as ever from making _living_ matter. Indigo
used to be derived entirely from plants of that name. One of the
greatest triumphs of the organic chemist was when he produced artificial
or synthetic indigo. But he is as far off as ever from making the indigo
plant. It is claimed that "synthetic" rubber is exactly the same as
natural rubber, although some users say it is not quite the same. Still,
if it be so, it is dead rubber, not the living part of the plant. The
time, then, is infinitely far distant when the chemist will be able to
make anything with the characteristics of life--namely, to grow by
accretion from within and to reproduce its kind. The most wonderful
product of the laboratory is dead. At most it simply resembles something
which _once_ was alive.

But that is somewhat of a digression. This dissertation on organic
chemistry was simply intended to lead up to the question of liquid
fuels, all of which are organic.

In the life of to-day one of the most important things is petroleum.
This is a kind of liquid coal. Just how it was formed down in the depths
of the earth is not clear. One idea is that it is due to the
decomposition of animal and vegetable matter. Another is that certain
volcanic rocks which are known to contain carbide of iron might, under
the influence of steam, have in bygone ages given off petroleum, or
paraffin, to use the other name for the same thing.

In many parts of the world these deposits of oil are obtained by sinking
wells and pumping up the oil. In others the liquid gushes out without
the necessity of pumping at all. This is believed to be due to the fact
that water pressure is at work. Artesian wells, from which the water
rushes of its own accord, are quite familiar, and are due to the fact
that some underground reservoir tapped by the well is fed through
natural pipes, really fissures in the rock, from some point higher than
the mouth of the well. Now supposing that a reservoir of oil were also
in communication with the upper world in the same way, the descending
water would go to the bottom, underneath the lighter oil, and would thus
lift it up, so that on being tapped the oil would rush out.

Another source of mineral oil is shale, such as is to be found in vast
deposits in the south-east of Scotland. This shale is mined much as coal
is: it is then heated in retorts as coal is heated at the gas-works: and
the vapour which is given off, on being condensed, forms a liquid like
crude petroleum.

In all these cases the original oil is a mixture of a great number of
grades differing from each other in various ways. They are all
"hydro-carbons," which means compounds of carbon and hydrogen, and they
extend from cymogene (the molecules of which contain four atoms of
carbon and ten of hydrogen) to paraffin wax, which has somewhere about
thirty-two of carbon to sixty-six of hydrogen. For practical purposes
their most important difference is the temperature at which they boil,
or turn quickly into vapour.

This forms the means by which they are sorted out. In a huge still, like
a steam-boiler, the crude or mixed oil is gradually heated, and the gas
given off is led to a cooling vessel where it is chilled back into
liquid. The lightest of all, cymogene, is given off even at the
freezing-point of water. That is led into one chamber and condensed
there. Then, as the temperature rises to 18° C., rhigolene is given off:
that is collected and condensed in another vessel. Between 70° and 120°
petroleum ether and petroleum naphtha are produced, and they together
constitute what is commonly called petrol. Between 120° and 150°
petroleum benzine arises. All the foregoing taken together constitute
about 8 to 10 per cent. of the whole crude oil. Then between 150° and
300° there comes off the great bulk of the oil, nearly 80 per cent., the
kerosene or paraffin which we burn in lamps. Above 300° there is
obtained another oil, which is used for lubrication, also the invaluable
vaseline, and finally, when the still is allowed to cool, there remains
a solid residuum known as paraffin wax. This process is known as
fractional distillation, and it will be noticed that it consists
essentially in collecting and liquefying separately those vapours which
are given off at different ranges of temperature. For our purpose in
this chapter we are mainly concerned with the petrol and the kerosene.

Many efforts have been made in times gone by to use kerosene for firing
the boilers of steam-engines. In naval vessels a great deal is so used
at the present time. But the chief method of employing oil for
generating power is to use it in an internal combustion-engine. These
machines have been dealt with at length in _Engineering of To-day_ and
_Mechanical Inventions of To-day_ and so must be simply mentioned here.
They consist of two types. In one, which is exemplified by the ordinary
car or bicycle motor, the oil is gasified in a vessel called a
carburetter or vaporiser and then led into the cylinder of the engine,
together with the necessary air to enable it to burn. At the right
moment a spark ignites the mixture, which burns suddenly, causing a
sudden expansion, in other words, an explosion. Thus the power of the
engine is derived from a succession of explosions. If the fuel be petrol
it vaporises at the ordinary temperature of the engine and needs no
added heat. With kerosene, however, heat has to be employed in the
vaporiser to make it turn readily into a gas.

The other method is employed in engines of the new "Diesel" type, in
which the cylinder of the engine, being already filled with hot air, has
a jet of oil sprayed into it. The heat of the air causes it to burst
into flame, causing an expansion which drives the engine.

An important feature in the latter type of engine is that the oil is
very completely burnt, so that very heavy oils can be used, oils which,
if employed in an engine of the other kind, would choke up the cylinder
with soot. In other words, the range of oils which can be used in this
new kind of engine is much wider than is possible in the others. The
latter may be likened to a fastidious man who is very particular about
his food, while the former resembles the man of hearty appetite who can
eat anything. And just as a man of the latter sort is more easily
provided for by the domestic authorities, so the Diesel engine makes the
problem of the provision of liquid fuel much simpler.

For it must never be forgotten that the provision of liquid fuel for the
world is by no means a simple matter, since the supply is by no means
adequate. The output runs into thousands of millions of gallons, and the
whole world is being searched for new fields of oil, and yet it is all
swallowed up as fast as it can be produced, while the coal mines do not
feel the competition. A year or so ago the United States and Russia
between them (and they are the greatest producers) obtained
5,000,000,000 gallons of oil, seemingly an enormous quantity. But, on
the other hand, Great Britain alone produces over 250,000,000 _tons_ of
coal per annum. If, therefore, liquid fuel is to displace coal, as some
people lightly think it is going to do, the supply will have to be
multiplied many times. In the amount of heat which it is capable of
giving the coal of Great Britain alone beats the oil produced by the
whole world.

And another thing to be borne in mind is that as the coal miner goes
down to the seam and sees for himself what is there, while the oil
producer simply stays at the surface and draws it up with a pump, the
coal man knows far more as to how much there is still left than the oil
man does. We know that the coal deposits will last for many years to
come, even if the production go on increasing, whereas the oil supply
may fall off in the near future instead of increasing.

And in both cases we are using up capital. Coal is not being made on the
earth now, at any rate in any appreciable quantity. The stage of the
earth's history favourable to the formation of coal measures has long
gone by. And the same probably applies to oil.

It is interesting in this connection to note that coal itself is to a
certain extent, or can be at all events, a source of oil. When coal is
heated in order to make it give up its gas, or to turn it into coke,
vapours are given off which on cooling become coal-tar. At one time
regarded only as a crude sort of paint, this is now the source from
which many chemical substances are obtained, varying from photographic
chemicals to saccharine, a substitute for sugar. So valuable are these
products that there is a brisk demand for the tar, in other directions
than the manufacture of oils, but oils of various kinds are also
obtained from it.

The first step in the operations is fractional distillation, after the
manner just described for petroleum. The first "fraction" is "coal-tar
naphtha." Then follows "carbolic oil," after that "heavy" or "creosote
oil," anthracene oil, and finally there remains in the still on cooling
a solid residue known as coal-pitch. The naphtha, on being distilled
again, gives, among other things, benzine, from which the famous aniline
dyes are made, and which is useful in many industries. Creosote is
largely employed as a preservative for wood, being forced into the
timber under high pressure, so that it penetrates right into it and
tends to prevent rotting, no matter how wet it may be. Railway sleepers
are thus treated, small truck-loads of them being run into a cast-iron
tunnel which is then sealed at both ends, while the creosote is forced
in by powerful pumps. After such treatment they can lie nearly buried in
the damp ballast for a long time without any deterioration.

These coal-tar substances are all very similar to petroleum and its
products, hydro-carbons, compounds of hydrogen and carbon in various
proportions. Many of them could be used for fuel.

[Illustration: _By permission of Dupont Powder Co._

                    APPLE TREE PLANTED WITH A SPADE

This apple tree was planted in the ordinary way with a spade. Compare
     its size with that in following illustration at p. 48.]

But since they are based upon the supply of coal, which is itself
limited, they cannot, however they may be used, do more than stave off
the evil day when the supply will be exhausted.

Quite different is it with alcohol, which it seems likely may be the
fuel of the future. Some people will be inclined to exclaim "What a pity
to burn it!" since to many the word conveys ideas of another sort
altogether. There are many nowadays, however, who, like the writer, have
none but a scientific interest in it. To such whisky, for example, is
but "impure" alcohol, and it is without the "impurities" that it may
become of vast use to the world, thereby possibly repaying man for some
of the harm which in the past it has inflicted upon him.

Alcohol, again, is a hydrocarbon. It is really more correct to speak of
it in the plural, as "alcohols," since there is a large group of
substances all of the same name. Two of these are of the greatest
importance, methyl alcohol and ethyl alcohol. The former is obtained
from wood, hence it is sometimes called wood spirit. Wood is strongly
heated in an iron still, and the methyl alcohol is given off in the form
of vapour, which on being collected and cooled condenses into liquid. It
is exceedingly unpleasant to the taste: if it were the only kind there
would be no consumption of alcohol as a drink.

The second kind mentioned is obtained by the agency of germs or
microbes, and the story of its production is so interesting as to demand
a little space.

We will commence with the maltster. He performs the first part of the
operation. Starting with ordinary barley, by the action of heat, aided
by natural growth, he produces the raw material on which the brewer may
work. Now barley, like all grain, is largely made up of starch, and
although starch will not make alcohol, it can be turned into sugar,
which will. So the task of the maltster is to commence the change of the
starch in the grain into sugar.

First of all it is soaked in water and spread upon floors and heated
until it begins to sprout. There is a little part in each grain called
the endosperm, which is the embryonic plant, and the starch is really
the food provided by nature to nourish the growing endosperm until such
time as it shall be strong enough to draw its nourishment from the soil.
In order that it may not be washed away prematurely, the starch is
locked up by nature in closely fastened cells, and, moreover, it is
insoluble, so that water cannot carry it away. The endosperm, however,
has at its disposal certain substances known as enzymes (and it
increases its store of these as it grows), one of which is able to
dissolve away the walls of the cells, to unlock the treasures, as it
were, while the other turns the insoluble starch into soluble matter, in
which state the growing organism is able to make use of it as food.

So as the grain sprouts upon the maltster's floor this process is going
on--the cells are being opened and their contents converted from starch
into soluble matters. Then, when the growth has gone far enough, the
grain is transferred to a kiln, where it is subjected to heat, by which
the growth is stopped. The living part of the grain is, in fact, killed.
That is mainly to stop the young plant from eating up the altered
starch, which it would do if allowed time, but which the brewer wants to
be kept for his own use.

The maltster's task is now finished, and we come to the brewer's. The
first thing he does with the malt is to crush it between rolls, thereby
liberating thoroughly those substances which have been formed from the
starch and which he intends to turn into sugar. Having crushed it, he
places it in the "mash tun," a large tank of wood or iron, in which it
is mixed with water and subjected to heat. While in this vessel the
enzymes become active again and turn the soluble starch, or a part of
it, into a kind of sugar.

The liquid drawn off from the mash tun, containing, of course, the
sugar, is subsequently boiled, numerous flavouring matters (including
hops) are added, and then it is cooled again, ready for the final
process--fermentation.

This takes place in a large vat or "tun" and is brought about by the
agency of yeast which is added to the liquid.

Now yeast is a multitude of microscopic plants round in shape and about
one three-thousandth of an inch in diameter. Though so small, this
little organism is really quite complicated in its structure, and within
its little body there are carried on complicated chemical changes which
baffle entirely the most learned chemist to imitate. Further, he has yet
to find out how the little yeast plant does it. He not only cannot
imitate the process, he does not know what the process is. These little
organisms multiply mainly by the process of "budding." A new one grows
out of the side of each old one, rapidly reaches maturity, breaks away
and commences an independent existence. No sooner is it free than it in
turn gives birth to another. Indeed so great is its hurry to propagate
itself that sometimes the new cell begins to throw out a bud before it
has itself separated from its parent. It is therefore easy to see that
yeast increases in quantity by what some call "leaps and bounds," but
which the mathematically minded know as geometrical progression.

The particular form of sugar with which we are concerned here is known
as "dextro-glucose." This the yeast splits up into alcohol and carbonic
acid gas. The latter bubbles up to the surface, and escapes into the
air, while the alcohol becomes dissolved in the watery liquid. It is
believed that the yeast performs this operation not directly, but by the
production of certain enzymes, which in their turn act upon the sugar.

The liquid so formed is beer. But since it is alcohol with which we are
concerned, and not beer, many details connected with its manufacture
have been omitted. Enough has been said, however, to show that by
comparatively simple processes grain of all sorts, in fact, anything
which contains starch, and such things are to be found in worldwide
profusion, can be turned into alcohol. All the really intricate chemical
functions are performed readily and cheaply by living organisms. All
man has to do is to set up the conditions under which the organisms can
work.

In the process just described only a portion of the starch in the grain
is converted into sugar, hence the percentage of alcohol in beer is
comparatively small. If all the starch be converted a liquid much
stronger in alcohol is produced, and if that be distilled, so as to
separate the spirit from the water with which it is mixed, there results
whisky. Brandy, likewise, is the spirit distilled from wine, rum from
molasses, and so on. In all these familiar beverages the essential
feature is this same alcohol, of the variety known as ethyl alcohol.

It will be noticed that in the making of beer the alcohol is actually
formed in water. There is a sugary water which under the action of the
yeast becomes an alcoholic water. And this indicates a very useful
feature about the liquid when used for industrial purposes. A tank full
of petrol is extremely dangerous, so much so that the storage of petrol
is hedged about by all manner of precautions. The danger is that it
gives off an inflammable vapour and that if it once begin to burn there
is practically no possibility of putting it out. Being lighter than
water, it simply clothes with a layer of fire any water which may be
thrown on to it. The water in such circumstances simply serves to spread
the naming petrol about and so to make matters worse. Now alcohol, with
its partiality for the companionship of water, behaves quite
differently. True, it also may give off an inflammable vapour, but if a
quantity of it catch fire it can be extinguished in the usual way by a
fire-engine. The water and alcohol immediately combine--the alcohol
becomes dissolved in the water just as sugar may do, and as soon as the
percentage of water in the mixture becomes considerable the burning
stops.

It may be that some readers will have discovered this fact for
themselves without knowing precisely what it was. It is a common dodge
with amateur photographers if they want to dry a negative quickly to
immerse it in methylated spirit. The spirit seems to take the water out
of the film and, itself drying quickly, leaves the negative in a
perfectly dry condition in a few minutes. Now after using spirit in that
way it is useless to put it in a spirit stove or lamp. It will not burn.
Methylated spirit is alcohol, and the reason why it has such a quick
drying action is that it and the water in the wet film quickly mix.
After immersion the film is wet, not with water merely, but with a
mixture of a lot of spirit and a little water. Hence the speed with
which it evaporates. And the non-inflammability of the mixture is due to
the presence of the water.

Methylated spirit only differs from the alcohol in alcoholic beverages
in that something is added to make it undrinkable. Owing to the craving
for it, which is so widespread, and the doubtful effect which it has on
certain citizens, most states regard it as pre-eminently a subject for
taxation, thereby on the one hand bringing in a good revenue, and on the
other discouraging its too free use. But those considerations apply only
to drinkable alcohol. That which is to be used for industrial purposes
is not in any way a legitimate object for taxation. Hence the problem
arises of making a form of alcohol which shall answer all the needs of
the industries which use it, and at the same time be so repulsive to the
senses that no one can possibly drink it. This result is achieved by
adding some of the methyl alcohol derived from the vapour given off by
wood when heated. Commonly known as "wood spirit," this is so unpleasant
that it renders the mixture of no use for drinking, and so it can safely
be freed from taxation.

Unfortunately this spirit has less heating value than petrol. That means
that a given quantity of each liquid will produce more heat in the case
of petrol than in the case of alcohol. Indeed the difference is about
two to one. Hence an engine to give out a certain horse-power would need
to have its cylinders twice as big if it were to use alcohol instead of
the other fuel. There is a certain compensation, however, in the fact
that alcohol is very easily compressible. In modern internal
combustion-engines much of the efficiency is due to the explosive charge
which is drawn into the cylinder being compressed into a small space
before it is fired. It was the discovery of the value of compressing the
gas which made the gas-engine so formidable a rival to the steam-engine,
and the wonderful performances of the Diesel engines are due very
largely to the fact that the air is compressed in the cylinder to a very
high pressure. The jet of oil burns in highly compressed air. And
because of the facility with which alcohol can be compressed it is said
to be more effective as a source of motive power than would be expected
from its comparatively feeble heat.

Thus we may sum up the possibilities of the future. Coal, petroleum and
their derivatives exist in limited quantities in the world, and so far
as we can see the vast drafts which we are taking from them are not
being replaced, indeed at this stage of the earth's development cannot
be replaced, by any more. Sooner or later we must come to an end of
them. Is it not comforting, therefore, to know that there is another
source of fuel at hand, inexhaustible, since it can be produced as
needed. We have only to set the sun and the ground to work to produce
grain, rice, potatoes, or any of the myriad substances which contain
starch, and from that, by simple and well-known processes, we can obtain
a cheap, safe and reliable fuel. Indeed there seems nothing but the
ultimate loss of sunlight, countless millions of years hence, which can
ever check the supply of this valuable commodity. What has doubtless, in
many cases, been a curse in the past may turn out to be the great boon
of the future.



CHAPTER IV

SOME VALUABLE ELECTRICAL PROCESSES


Students of that branch of science known as physics are coming to the
conclusion that electricity plays a much more important part in the
universe than was supposed. They are led to believe that electrical
attraction is the cement which binds together those exceedingly minute
particles out of which everything is built up. Whether electricity binds
them together or not, it is certain that electrical action can in some
cases _separate_ those particles, and this process of separation
provides a means of carrying on some very remarkable and useful
industrial processes.

Let us imagine a vessel filled with water to which has been added a
little sulphuric acid, while suspended in it are two strips of platinum.
There is a space between the strips, so that when their upper ends are
suitably connected to a source of electric current that current flows
from one strip to the other _through the liquid_.

That is an example of the apparatus for carrying out this electrical
separation in its simplest form, and it will facilitate the further
description if the names of various parts are enumerated.

The process itself is electrolysis; the liquid is the electrolyte, while
the strips are the electrodes. The individual electrodes, again, have
special names, that by which the current enters being the anode and that
by which it leaves the cathode. It is not difficult to remember which is
which if we bear in mind that the current traverses them in alphabetical
order. Since, however, it may not be easy for the general reader to
carry all these terms in his mind, we will, when it is necessary to
differentiate between the two electrodes, call one the in-electrode and
the other the out-electrode.

Returning now to our imaginary apparatus, let us turn on the current. At
first nothing seems to be happening, although suitable instruments would
show that current was flowing. Soon, however, little bubbles appear upon
the electrodes, and these grow larger and larger, until they detach
themselves from the platinum to which they have been adhering, float up
to the surface and burst. The question which naturally arises is, What
do those bubbles consist of? Are they air?

If we take means to collect the gases which formed them we get an
unmistakable answer. The bubbles which arise from the in-electrode are
oxygen, those from the other hydrogen. If we allow our apparatus to work
for some time, and collect all the gas which arises, we shall find that
there is twice as much hydrogen as oxygen. We shall also find, as the
process goes on, that the quantity of water diminishes.

Perhaps I may be allowed at this point to remind my readers that water
is a collection of minute ultra-microscopic particles called
"molecules," each of which is formed of three smaller particles still
called "atoms." Of the three atoms two are hydrogen and one oxygen.
Water therefore consists of hydrogen and oxygen, there being twice as
much of the former as there is of the latter.

We see, therefore, that electrolysis gives us hydrogen and oxygen in
exactly those proportions in which they occur in water, and since we
also see that as these gases appear the water itself disappears, we are
led to conclude that the current is splitting up the water into the
gases of which it is formed.

But the strange thing is that this will not work with pure water. We
have to add something to it. In the case of our imaginary experiment it
was sulphuric acid. What part does that play?

This is not fully understood, but we may be able to form a mental
picture of what is believed to happen as follows.

The in-electrode is surrounded by a vast assemblage of these tiny
molecules, most of them those of water, but a few those of the acid. The
latter are more complex in their structure than the former, but they too
contain hydrogen. Current flows into the electrode and instantly
hydrogen atoms from the _acid_ molecules crowd round it, like boatmen at
the seaside anxious to secure a passenger. Each takes on board a
quantity of electricity and with it darts across the intervening space
to the other electrode. Arrived there, it gives up its load and, its
work done, remains lying upon the electrode until enough others like
unto itself have gathered there to form a bubble and so escape. These
hydrogen atoms are thought to be the _craft which carry the current
through the liquid_ and enable it to pose, as it were, as a conductor of
electricity, which in reality it is not.

But where does the oxygen come from?

To find the answer to that we must add a second chapter to our story.
When the hydrogen "boats" took on board their load of electricity they
left their former associates, and these forthwith "set upon"
neighbouring water molecules and with the audacity of highwaymen stole
from them enough hydrogen atoms to take the place of those they had
lost. Thus the acid molecules became complete once more, while the scene
of the conflict near the in-electrode was strewn with the remains of the
water molecules from which the hydrogen atoms had been stolen. These
remains, of course, would be oxygen, and they, collecting together on
the electrode, would eventually be in numbers sufficient to form bubbles
and so escape.

Thus it may be the acid which really does the work, yet because of its
subsequent raid upon the water it is the latter which disappears, and it
is the materials of the latter which are bought to the surface in the
bubbles.

And there we see the mechanism whereby, so it is believed, electric
current can pass through otherwise non-conducting liquids. And the
important point, as far as practical utility is concerned, is that the
passage of the current is accompanied by a splitting up of something or
other, either the water or something in it, the materials of which are
deposited, one on one electrode and the other on the other.

And now we can proceed to those useful applications of electrolysis, the
commonest of which, perhaps, is electro-plating.

We have seen how electrolysis causes hydrogen, probably out of the acid,
to be deposited upon one electrode. Suppose that, instead of an acid, we
put in the water one of those substances known to chemists as a "salt,"
the commonest example of which is ordinary table salt. This well-known
condiment is caused by the interaction of hydrochloric acid and the
metal sodium and will serve to illustrate what all salts are.

All acids are compounds of hydrogen and something else, and their biting
action is due to the readiness with which the "something else" evicts
the hydrogen and takes in a metal in its place. Thus hydrochloric acid,
given the opportunity, gets rid of its hydrogen and takes in sodium,
thereby forming chloride of soda or common salt.

Another example is the gold chloride familiar to photographers. This is
the result of the action of certain acids upon gold, wherein the acids
throw out their hydrogen and take in gold instead.

To sum up, then, a salt is just the same sort of thing as an acid, like
the sulphuric acid which we used in our "experiment," except that some
metal has taken the place of the hydrogen.

It is not surprising, then, to find that if we put a salt in the
electrolyte instead of an acid we get a similar result. In the one case
hydrogen is deposited upon the out-electrode, in the other the metal. In
the former case, since hydrogen is a gas, it forms bubbles and floats
away, but in the latter the solid metal remains a thin, even coating
upon the electrode. That is the principle of electro-plating.

The electrolyte consists of a suitable solution containing a salt of the
metal to be deposited, and it is placed in an insulating vessel or vat.
The articles to be plated form the out-electrode, so that they have to
be suspended in some convenient way from a metal conductor by conducting
wires. Of course they are entirely immersed in the liquid. The
in-electrode is sometimes a plate of platinum (the reason that expensive
metal is used being that it is unaffected by the chemicals) or else a
plate of the metal being deposited. In the former case, the solution
becomes weaker as the work proceeds, and more salt has to be added. In
the latter, however, the strength of the solution remains unchanged, for
by an interesting interchange the in-electrode adds to it just what it
loses by deposition upon the other one. The effect is therefore just as
if the current tore off particles from the one and placed them upon the
other.

This is believed to be due to the agency of the oxygen which in the case
of the electrolysis of water becomes free, but which in this case forms
with the metal electrode a layer of oxide upon its surface, this oxide
being then dissolved away by the liquid. Thus as fast as the metal is
deposited upon the out-electrode its place is taken by more metal from
the in-electrode.

In some processes it is desired to deposit metal upon a non-conducting
surface, and it is evident that such cannot be used as an electrode. Nor
is it any use to attempt to deposit upon anything except an electrode.
The only thing to do, then, is to make the object a conductor by some
means. Models in clay, wax and plaster, once-living objects like small
animals, fruit, flowers or insects, can, however, have a perfect replica
made of them by electrical deposition, by the simple method of coating
the surface to be plated with a thin layer of plumbago. This skin,
although extremely thin, is a sufficiently good conductor to make the
process possible. Process blocks for printing are copied in this way, so
that a particularly delicate example of the blockmaker's art need not be
worn down by much pressing, copies or "electros" being made off it for
actual use in the press.

The original block is a plate of copper on which the picture is
represented by minute depressions and prominences. On this a layer of
soft wax is pressed, so as to obtain a perfect but reversed copy. Having
been coated with plumbago, this is then put into a vat containing a
solution of copper salts and is used as the out-electrode, the other
being a plate of copper. When the current is turned on the copper is
thus deposited on the wax until a thin sheet of copper is formed which
is an exact but reversed copy of the wax, a direct copy, that is, of the
original block.

The back of this thin sheet is then covered with molten lead or type
metal to fill up any depressions and to give it sufficient strength.
Anyone who has seen one of these "half-tone" blocks covered with minute
depressions so slight that they can scarcely be seen, yet so perfect
that a beautiful print can be obtained from them, will realise the
wonderful power of this electrolytic process, the marvellous accuracy
with which the original is copied, and the unerring way in which the
electric current carries the particles of copper into every one of the
myriad recesses in the wax.

Another specimen of the marvellous work of this system is the wax
cylinder of the phonograph. The sound is produced by a needle trailing
along a groove of varying depth cut in the surface of the cylinder. This
groove forms a spiral, passing round and round like the thread of a
screw, and it encircles the cylinder one hundred times in every inch of
its length. Consequently, at any point one may take, there is but one
one-hundredth of an inch from the centre of one turn to the centre of
the turn on either side of it. And at its deepest the groove is less
than one-thousandth of an inch deep. The phonograph itself cuts the
first "master" record, as it is termed, and the problem is to take a
number of casts off this model of such delicacy and accuracy that every
variation in that exceedingly fine groove shall be faithfully
reproduced. Such a task might well be given up as hopeless, but with
the help of electrolysis it is accomplished easily and cheaply.

To attempt to press anything upon the surface of the "master" would but
smooth out the soft wax and obliterate the groove altogether. To apply
anything softened by heating would be to melt it. But electrolysis,
without tending in any way to distort or damage the delicately cut
surface, deposits upon it a surface of metal from which thousands of
casts can be made. The gentle fingers of the electricity overlay the
soft wax with the hard, strong metal with a minute perfection almost
beyond belief.

To commence with, the master record is placed upon a sort of turntable
in a vacuum and turned round in the neighbourhood of two strips of
gold-leaf strongly electrified. By this means the gold is vaporised and
a perfect coating of gold is laid upon the wax. This is far too thin to
be of any use, except to render the cylinder a conductor, for the
coating is so fragile that it is no stronger than the wax itself. It
enables the cylinder, however, to be electro-plated with copper until it
is surrounded by a strong metallic shell a sixteenth of an inch thick.
It takes about four days to deposit this thickness. The copper shell is
then turned smooth in a lathe and fitted tightly into a brass jacket. A
little cooling causes the wax record to shrink sufficiently to free it
from the copper shell and allow it to be lifted out. A copper mould is
thus formed in which any number of additional records can be cast. The
molten wax is simply introduced into the inside, and allowed to set; the
inside is bored out in a lathe, and then with a little cooling it
shrinks and can be withdrawn, a completely finished record, every tiny
depression or swelling in the original master being reproduced with an
accuracy almost incredible.

Another valuable use to which this process is put is the purification of
metals. The electro-chemical action works with unerring precision: it
never mistakes an atom of iron for an atom of copper, for example.
Passing through a solution of copper salt, the current deposits only
copper.

For modern electrical machinery and apparatus copper is required of the
utmost possible purity, for every impurity adds to its electrical
resistance, in other words, diminishes its value as a conductor.
Consequently thousands of tons of "electrolytic" copper, as it is
termed, are produced every year. The electrodes used are plates of
ordinary copper. A coating of pure metal is deposited by electrolysis
upon the out-electrode from the other one. When the deposit is thick
enough the out-electrode is taken out and the deposit torn off it, the
union between the two being sufficiently imperfect for this to be done
without difficulty. The metal of which the in-electrode is made has
already been purified by other processes, until it contains but one per
cent. of foreign matter, and by this means even that small percentage is
entirely got rid of. The impurities fall to the bottom of the vessel in
the form of "slime," which is periodically removed.

And not only is electrolysis thus unerring in picking out certain atoms
from among a mixture, but there is an exact relation between the work
done and the quantity of current used. Consequently it forms a very
exact method of measuring currents. The method of measuring current by
the strength of the magnetic field which it produces has been mentioned
already, and such measurements can be checked by electrolysis. Thus the
practical definition of the ampere is "that current which when passed
through a solution of silver nitrate in water will deposit silver at the
rate of ·001118 gramme per second."

The electric accumulator or secondary battery, one of the most useful
appliances, is the result of electrolysis reversed. Many large
electric-lighting plants have in addition to their generating machinery
a large battery of secondary cells, which, being kept charged, are able
to help the machinery in times of heavy demand, or even to supply the
whole current needed for, say, half-an-hour, so that the whole of the
machinery could, in the event of an accident, be shut down for that time
and the supply maintained from the batteries. This would be sufficient
in many cases for fresh machinery to be brought into action or emergency
arrangements to be made.

It may be that this book is being read by someone seated serenely in his
arm-chair while engineers and workmen at the generating station are
working in frantic haste to set right some sudden breakdown before the
batteries are run down. The batteries may have saved the town
half-an-hour's darkness.

Large telegraph offices are fitted with secondary batteries. Many
motorists owe the ignition which keeps their engines at work to
secondary batteries. It is secondary batteries which keep the wireless
apparatus at work on a wrecked vessel after the engines have stopped.
Indeed secondary batteries are one of the most beneficent inventions.
And if only they could be made in a lighter form than is possible at
present their value would be infinitely increased.

We have seen how the passage of current through acidulated water
produces hydrogen and oxygen. If those gases be collected in closed
vessels over the water, so that they remain in contact with the water,
as soon as the current is stopped a reverse action sets in. The gases
tend to recombine with the electrolyte and in so doing to give back a
current equal to that which formed them. Fig. 4 shows the construction
of what is called a voltameter, in which the gases arising from the
electrodes are collected in little glass vessels placed just above them.
Such an apparatus enables us to see easily how the accumulator works.
The picture shows the battery which is effecting the separation of the
oxygen and hydrogen. If that be disconnected, and the wires joined, as
shown by the dotted line, a current will flow back until the oxygen and
hydrogen have returned into the solution again. The apparatus will, in
fact, work like an ordinary battery, except that instead of a plate or
rod of zinc a mass of hydrogen will form the essential part.

An appliance such as a voltameter is not of much use for the practical
purpose of storing large quantities of electrical energy, because the
surfaces of the electrodes are so small and the surfaces where liquid
and gases are in contact are small too. It is clear that the larger the
electrodes are the wider will be the passage for the current, just as a
wide road can accommodate more traffic than a narrow path. We may regard
the electrodes as like gateways through which the current passes. By
making them large, therefore, we enable a large current to pass, and
consequently permit electrolysis to take place with great comparative
rapidity.

[Illustration: FIG. 4.]

The "plates," as the electrodes in a secondary battery are termed, are
generally large metal plates. Experiment has shown that lead is the best
for this purpose. It has also been found that it can be improved by
making it porous, since the inner surfaces of the pores are so much
added surface through which current can pass into the electrolyte. There
are various ways of producing this porosity, which need not trouble us
here, however. It will suffice for our purpose to understand that an
ordinary secondary cell consists of two lead plates, with the largest
possible surface, immersed in a liquid, generally a dilute solution of
sulphuric acid in water.

To charge the battery, current is sent to one plate, through the liquid
to the other plate, and so away. A thin film of hydrogen is thus formed
upon the outgoing plate, while oxygen is formed at the incoming one.
Since the hydrogen is spread over such a large area, it does not
accumulate sufficiently for much of it to rise to the surface. Most of
it remains adhering to the plate. The oxygen combines with the lead of
its plate and so is safely stored up there in the form of oxide of lead.
This storage of hydrogen upon the one plate and oxygen on the other
cannot go on indefinitely, and so as soon as the limit is reached the
cell is fully charged. Passage of further current is then simply waste.

The dynamo or primary batteries which are used for charging having been
disconnected, the two plates can be connected together through lamps,
motors, or in any other desired way, and the current will then flow out
again, the opposite way to that in which it entered, just as a stone
thrown up in the air returns the opposite way. The current which comes
out is, in fact, a sort of reflex action arising from that which went
in, the mechanism by which it is produced being the reabsorption of the
oxygen and hydrogen into the electrolyte.

Whether a cell is fully charged or not is ascertained by weighing the
electrolyte, an operation which at first sight seems to have nothing
whatever to do with the matter. It arises from the difference in weight
between water and sulphuric acid, the latter being the heavier. We have
seen that while a little acid must be added to water before it can be
electrolysed, it is the water which is ultimately resolved into its
constituent gases. Hence the result of electrolysis is to increase not
the amount, but the proportion of acid. Therefore it increases the
weight of the electrolyte. This weight is ascertained by means of a
"hydrometer," a glass tube, stopped, and loaded with some small shot at
its lower end. On the upper part is engraved a graduated scale, so that
the exact depth to which it sinks can be easily read. This depth will,
of course, vary with the specific gravity of the liquid, and so the
depth recorded by the scale will be an indication of the proportion of
acid, and that in turn will show how far the process of charging has
progressed.

Accumulators are, or have been hitherto at any rate, very troublesome
things. They are apt to lose their power. If not properly charged they
are easily damaged. Too rapid charging or too rapid discharging,
standing for a while only partly charged--all these things have a bad
effect, in extreme cases even destroying them altogether. Because of the
use of lead they are terribly heavy too, so much so that for traction
purposes they are of very little use, for a large amount of the energy
stored in the accumulators is then used up in hauling them about.

Yet what a field there is for the successful accumulator! Take the one
instance of the electrification of a railway. If good light and
efficient accumulators were to be had, no alteration at all would be
necessary to the permanent way. The engines or motor carriages would
need to go periodically to a depot to be re-charged, but that could
easily be arranged. Such things as conductor rails, overhead conductors
and so on would be needless.

The world has therefore been interested for years in the rumour that T.
A. Edison was engaged upon this problem, and at last he has produced his
accumulator, by which he has removed many of the difficulties, if not
all. Instead of a case of glass or celluloid, as is usual with the older
cells, his cells are enclosed in strong boxes of nickel steel. The
positive plate consists of nickel tubes filled with alternate layers of
nickel hydroxide, while the negative plate is formed of prepared oxide
of iron in a nickel framework. The electrolyte is a solution of
potassium hydroxide. The chemical action and the electrical reaction is,
of course, on the same principle precisely as in the older cells, but it
is claimed that the Edison cells are "fool-proof"--that is to say, they
cannot be damaged by careless handling, and they appear to be a little
lighter. Thus the problem is partly solved, and with that as a fresh
starting-point someone may sooner or later give us a secondary battery
which is light as well as strong.

If any would-be scientific inventor reads these words there is a
suggestion for a promising line of investigation.



CHAPTER V

MACHINE-MADE COLD


One of the most remarkable adaptations of scientific knowledge is the
"manufacture of cold." At first that phrase seems strange, but it is
really quite legitimate. There are machines at work at this moment which
are turning out cold as if it were any other manufactured article. It is
not that they manufacture cold water or cold air, it is the cold itself
which they produce.

Of course, cold has no real existence, since it is simply a negative
quantity, an absence of heat, yet its effects are so real that we are in
the habit of talking of it as if it were a reality, and in that sense we
can regard it as a product of manufacture.

Moreover, we see in this a conspicuous instance of the interdependence
of invention and science, for scientific principles were first adapted
to produce cold, and then artificial cold was employed in scientific
investigations, whereby the rare gases of the atmosphere have been
discovered, as we shall see presently.

In _Mechanical Inventions of To-day_ I have dealt with the uses which
can be made of heat as a motive power. Here we have in some sense a
reversal of the process. In the heat-engine the expenditure of heat
produces motion. In the refrigerating machine motion produces heat, on
the face of it a strange way of producing cold. Yet it is by the
production of heat in the first instance that we are ultimately able to
obtain the cold.

One way to make a thing cold is to place it in contact with ice. But
that process suffers from severe limitations. In the first place, we may
not be able to procure ice when we want it. And in the second place, we
may want to produce a temperature much lower than that of ice.

Now a machine can produce any degree of coldness, almost down to the
"absolute zero," the point at which a body is absolutely devoid of any
heat whatever, the condition in which its molecules are absolutely
still. That point is 274° C. _below_ freezing-point. Freezing-point on
that scale is "zero," and so this _absolute_ zero is _minus_ 274°. Or,
to put it another way, freezing-point is 274° _absolute_ temperature.
The absolute zero has never been reached, and there is reason to believe
that it never can be quite reached, but by methods about to be described
a temperature within a few degrees of it has been attained. And all of
this can be done without any cooling agent colder than water at an
ordinary temperature.

There are several systems, but the one which illustrates the principle
most simply is that in which carbonic acid gas is the "working fluid."
This is a very compressible gas, and so is well fitted for the purpose.
First of all a pump or compressor compresses it. That has the effect of
heating it. Such we might expect from the fact that heat is molecular
activity: when by compressing the gas we force the molecules closer
together, they naturally hit each other and the sides of the containing
vessel harder than they did before, and the increased activity is
manifested as increased heat. So the first effect, as was remarked just
now, is to produce, apparently, increased heat.

But then the hot compressed gas, by being passed through a coil of pipe
surrounded by cold water, can be robbed of that heat. According to the
speed at which it traverses the coil it will be more or less cooled: by
causing it to travel slowly it can be brought down almost to the
temperature of the water. So we start with the gas at atmospheric
pressure and at somewhere about atmospheric temperature too. This we
convert into compressed gas at a high temperature. After cooling it we
have compressed gas at a moderate temperature.

Then, to complete the process, we let the gas expand again. Now just as
compressing a gas heats it, letting it expand cools it. If we compressed
it and then expanded it again we should be just as we were to commence
with. But since, in between the two operations we extract a quantity of
heat by means of the cooling water, we get at the end a very much lower
temperature than that with which we started.

We cannot cool the gas without compressing it, because heat will only
flow from one body into another when the second is cooler than the
first. But by making the gas hot temporarily by compression we enable
the water to draw some heat from it, and then, allowing it to sink back
to its original state, we get practically the old temperature, less what
the water has extracted. The principle is really absurdly simple when
one once gets to understand it. The application is not so simple as far
as the designer of the machine is concerned, for he has to adjust the
various parts to exactly the right shape and dimensions, so that they
may work well with one another and produce the desired result with the
minimum expenditure of power.

To the observer, however, and to the user too, the finished machine is
wonderful in its simplicity. The principle is illustrated
diagrammatically in Fig. 5.

In the centre is the compressor. Its action forces the gas along the
pipe to the right and down into the condenser. As it flows downwards
through the coil there cold water enters at the bottom of the tank,
flows upward past the coil and escapes again at the top. Thus the coil
is kept in contact with _cold_ water.

Passing then through the bottom of the tank the gas travels from right
to left through the "regulating valve" and into an arrangement almost
exactly similar to the condenser but called the evaporator. Here the gas
expands and suffers a great fall in temperature. This cold is
communicated to liquid circulating in the tank which forms a part of the
evaporator, and this liquid can be circulated through pipes into any
rooms to be cooled or around vessels of water which it is desired to
freeze. This liquid, which acts as the carrier of the cold, is called
"brine," and is water to which is added calcium chloride to keep it from
freezing.

[Illustration: FIG. 5.--This diagram shows the working of the
Refrigerating Machine. The pump compresses the gas and drives it through
the coil in the condenser, where it is cooled by water. It passes thence
through the coil in the evaporator, where it expands and cools the
surrounding brine.]

Now the observant reader may have noticed that there is no apparent
reason for the name of the left-hand vessel. It will be quite clear,
however, when I explain that although I have spoken of the working fluid
all along as gas, I have only done so to avoid bringing in too many
explanations at once. It is actually liquid for a good part of its
journey. Carbonic acid gas liquefies at a very moderate temperature and
pressure, and so while it leaves the compressor as a gas it becomes
liquid in the condenser and remains so until it has passed the
regulating valve. Then it begins to expand into gas once more, and in
that state it passes back to the compressor.

There is a pressure-gauge on the pipe leaving the compressor and another
on the one entering it. A comparison of the readings on these two tells
how the apparatus is working. The difference between them indicates how
much compression is being given to the gas. Assuming that the compressor
is working at a constant speed, this compression can be regulated to a
nicety by the valve: close it a little and the compression will
increase: open it a little and the compression will decrease. By this
means the degree of cold produced can be varied at will.

This is the way in which many ships are enabled to carry cargoes of
frozen meat. The chambers in which the meat is stowed are
insulated--that is to say, their walls are made as impervious as
possible to heat. Then the brine is carried into the chambers in pipes,
cooling them much as the hot-water pipes heat an ordinary public
building.

Or another method is to carry the pipe which constitutes the evaporator
into the chamber to be cooled. A third way is to dispense with brine and
to blow air through the coils of the evaporator, whereby the air is made
to carry away the cold to wherever it is needed.

Ice can be made easily in moulds of metal or wood around which brine
circulates. If made of ordinary water the ice is likely to be cloudy and
opaque, which is quite good enough for many purposes. In cases where it
is desired that it should be clear, the water is agitated during
freezing, or else distilled water is used. To enable the blocks to be
got out of the moulds it is sometimes arranged to circulate warm brine
for a few moments.

Ice skating rinks are formed by making, first, an insulating layer of
sawdust, slag-wool or something of that sort (those by the way, being
the materials generally used for insulating cold chambers) underneath
the floor. The floor, too, is made waterproof and then upon it is laid
as closely as possible a series of iron pipes. Water is flooded on to
the floor until the pipes are covered to a depth of several inches, and
then brine is pumped through the pipes. In time the water freezes, and
so long as the brine circulates it remains so.

But although the "CO_{2} process" described above is the simplest
illustration of the principle, there are other systems. In one very
popular form ammonia gas is the "working fluid." This is liquefied by
pressure and cooling with water, being subsequently expanded just as
described above.

Another much-used system is the "ammonia-absorption" process, in which
the ammonia is not liquefied, but when under pressure is absorbed by
water, returning to gas again when the pressure is released.

But the degree of cold attained in these commercial machines is as
nothing to the extremely intense cold generated on the same principles
in the liquid-air machine, which is found in every well-equipped
physical laboratory.

Briefly, this consists of a coil of many turns of small tube enclosed in
a small double vessel, the space between the inner and outer skins of
which is packed with insulating material. A compressor pumps air in at
the top of the coil at a pressure of from 150 to 200 atmospheres. An
"atmosphere," it may be remarked, is a unit often used in scientific
matters, meaning the normal pressure of the atmosphere, which is,
roughly speaking, 15 lb. per square inch. Hence 200 atmospheres is about
3000 lb. per square inch.

Of course air so highly compressed as that is hot, but after it has
passed down the coil and has escaped from the valve which liberates it
at the bottom it is much cooler. But that is only the beginning of the
operation. The expanded, and therefore cooled, air finds its way upward
through the turns of the coil down which the following air is coming.
That, expanding in its turn, is colder still, because of the cooling
action of the first air, and so the process goes on.

[Illustration: _By permission of Messrs. J. and E. Hall, Ltd., London
and Dartford_

                        MACHINE-MADE ICE

Here we see a huge block of ice being lifted (it may be on a hot summer
             day) from the mould in which it has been made]

This is perhaps easier to understand if we imagine that the air comes
through the coil in gusts and we notice what happens to each succeeding
gust. The first comes down, expands, cools and ascends, thereby cooling
the second gust as it comes down. The second then, after expansion, will
be cooler than the first was. That in its turn will cool the third, and
so the third after expansion will be cooler than the second. And that
will go on, each succeeding gust being cooler than the one before. And
although the flow of air is continuous, and not in gusts, the result is
just the same: it goes on getting cooler and cooler until at last the
air comes out in its liquid form. This liquid collects in a little
chamber formed at the bottom of the vessel which contains the coil and
can be drawn off when desired.

Air in its liquid state looks very much like water. In fact it is
difficult to get chance observers to believe that it is not water. It
boils at a temperature far below the freezing-point of water, so that
liquid air if placed in a cup made of ice will boil furiously. Ice is so
much the hotter that it behaves towards liquid air as a very hot fire
does to water.

The feature of the above machine, it will be noticed, is that no cooling
water is required, as in the refrigerating machine, although the
principle of the two is the same. The coil is the "condenser" and the
vessel in which it is enclosed is the "evaporator," and so the cold air
produced by the process in the evaporator cools the coil of the
condenser. Thus it is "self-intensive," as the makers call it.

Hydrogen can be liquefied in a similar machine, except that it needs a
little preliminary cooling with liquid air. Liquid hydrogen is the
coolest thing known approaching the region of absolute zero.

And now we can turn to the wonderful discoveries which have followed
upon the manufacture of liquid air.

To make the story complete we need to go back to the time of Priestly
and Cavendish, early in last century. They investigated the atmosphere
and showed that it consisted of oxygen and nitrogen in certain
invariable proportions, with under certain conditions a small proportion
of carbonic acid. These facts were so well authenticated, and they
seemed to explain everything so satisfactorily, that it was quite
thought almost up to the end of the nineteenth century that there was
nothing more to learn about the atmosphere.

Nevertheless there was an idea in the minds of some scientists that
there must be another group of elements somewhere, the existence of
which was then undiscovered, but it was never dreamed that these were in
the air.

Soon after the weights of the atoms had been found a medical student
named Prout in an anonymous essay called attention to the fact that
there were curious numerical relationships between them. Speculation on
the subject went on for many years, until in 1865 the great Russian
chemist Mendeléeff published his conclusions. He had arranged the
elements in the form of a table _in the order of their atomic weights_.
The table consisted of twelve rows of names forming eight vertical
columns, and the remarkable thing was that all those elements which fell
into any particular column, although their atomic weights were very
widely different, had similar properties. This enabled him to _predict_
the discovery of certain new elements, for the table contained a number
of blank spaces. Three elements _have been found_ since, and their
atomic weights and properties are just such as to fill three of the
blank spaces. One blank space, it is thought, may be filled some day by
the gas coronium, which like helium has been discovered in the sun, but
unlike it has not yet been detected here. When it is, there is the place
in the table which it may fill. The table then commenced with what is
still called Group 1, but for reasons too complicated to explain here it
appeared as if there must be a group before that, a group the chief
characteristic of which would be the inactivity of the elements included
in it. These were expected to be of various atomic weights, but these
weights, it was anticipated, would so occur in the intervals between the
others that they would all fall into a new column to the left of "Group
1."

In the year 1892 Lord Rayleigh was investigating the question of the
density of a number of different gases, including, so it happened,
nitrogen. Now there are several ways of procuring nitrogen. One is to
get it from the atmosphere by ridding it of the oxygen with which it is
normally mixed. Another way is to split up some compound, such as
ammonia, of which it forms a part, in such a way as to catch the
nitrogen and leave the other elements with which it was combined
elsewhere.

Lord Rayleigh tried both ways, and he found that the nitrogen from the
atmosphere was denser than that derived from ammonia. Sir William Ramsey
then carried the matter a step further. He heated atmospheric nitrogen
in the presence of magnesium, under which conditions some of the
nitrogen combines with the latter element to form nitride of magnesium.
That, it was found, made the remaining nitrogen denser still. The
explanation then seemed obvious. Suppose we imagine a mixture of sawdust
and iron filings: it will be heavier than an equal quantity of pure
sawdust. And if we contrive to take away some of the sawdust from the
mixture we shall find that what is left is heavier still, when compared
with an equal bulk of pure sawdust. For it is clear that as we take away
sawdust we thereby increase the proportion of the heavier iron filings
and so we make the mixture heavier.

Applying a similar process of reasoning to these discoveries, the
conviction grew that the nitrogen of the air was not pure, but that it
had mixed with it a small proportion of some other gas of greater
density. They soon succeeded in isolating this denser gas, to which they
gave the name of argon. Its atomic weight was found, and, wonderful to
relate, it was such that argon fell into a new column to the left of
Group 1, as had been anticipated.

The discovery of argon was announced in 1894. The next year Sir William
Ramsey, investigating a gas which had been discovered locked up in the
interstices of a mineral called clevite, was able to state that it was
helium, the element which had been previously noticed by the
spectroscope in the sun. Like argon, it was found to be extremely
inactive, and its atomic weight turned out to be such that it too fell
into the "Zero Group."

In 1898 Professors Ramsey and Travers found two more gases in the air,
krypton and neon, and a little later still, there was found mixed with
the krypton a further new gas, xenon. All of these had their atomic
weights found, and fell into that new column in the periodic table.

But what has all this got to do with liquid air? The two subjects are
closely related, for it is by liquid-air machines that these rare gases
are now obtained, and it was from liquid air that the last three were
first discovered.

For air, as we well know, is a mixture of gases, and when extreme cold
and pressure are applied these gases liquefy, each behaving according to
its own nature. They do not all liquefy at the same time, nor on being
relieved from the pressure and heated do all evaporate again at the same
temperature. Although they emerge from the liquid-air machine in the
form of a single liquid, it is really a mixture of liquids, each with
its own boiling-point.

In an earlier chapter we saw how petroleum can be separated into its
various constituents, such as petrol, by fractional distillation,
advantage being taken of the difference in the "boiling-point" of the
various "fractions." The boiling-point of a liquid is, of course, the
temperature at which it turns freely into vapour, and just as petroleum
when heated gives off first cymogene, next rhigolene, then petrol,
benzine, kerosene and so on, in the order named, so liquid air, when it
is evaporated, gives off its different constituents in order. Nitrogen,
oxygen, argon, helium, krypton, neon and xenon can all be separated each
from the others in this way, by "fractional distillation." The heat from
the surrounding objects is allowed to get at the liquid, and the gases
are then given off in the order of their boiling-points.

And thus we see how the mechanical production of cold has assisted in
the pursuit of pure science. The newly-found gases are not of any great
use at present. They are so inactive that possibly they never will be,
with one exception, and that is neon. If an electric discharge be made
to pass through a tube filled with this gas, a beautiful glow is the
result, and it is just possible that neon tubes may become the electric
light of the future. That is only a prediction, however, and a
hesitating one at that.

The inactive elements may become of value in explosives. We have seen
how important nitrogen is in these dangerous substances, the chief
feature of which is their instability--their readiness, that is, to
change into something else--which instability is due to the reluctance
with which nitrogen enters into them. Now nitrogen, though inactive, is
much less so than these others, and if a way should ever be found of
inducing them to enter into a compound, that compound will probably be
an extremely powerful explosive.



CHAPTER VI

SCIENTIFIC INVENTIONS AT SEA


The safety of our fellow-creatures has always been a strong stimulus to
our inventive faculties. The occurrence of a bad railway accident, and,
roughly, its nature, can be inferred from the files of the Patent
Office, for such an event brings men's thoughts to devising ways and
means of preventing a recurrence, and an avalanche of such inventions
descends upon the patent department in consequence. In like manner a
particularly distressing accident to a lifeboat some years ago brought
out many inventions for the improvement of those romantic craft. Many of
the inventions which arise under these conditions are, of course,
utterly worthless, but some of them "come to stay."

It is not surprising, therefore, when we think of the almost innumerable
wrecks which happen, even with modern shipping, that human ingenuity has
been extremely busy in devising ways for bringing more of safety and
less of risk into the lives of those who go down to the sea in ships. Of
these perhaps none is more fascinating than the modern lighthouse, with
its tall tower, its brightly flashing light, standing undisturbed in the
wildest storm, quietly and persistently sending forth its guiding rays,
no matter how the elements may be buffeting it. There is something
specially attractive in this perfect embodiment of quiet strength and
devotion to duty.

Of course, its origin is very ancient. One of the earliest inventions,
no doubt, was the bright thought of a very primitive man who lit a fire
on a hill to serve as a guide to some belated friends out in their
fishing canoes. From some such beginning the modern lighthouse, a
magnificent product of the science of civil engineering and the science
of optics, has arisen.

Of the difficulties encountered in the construction of lighthouse towers
on outlying rocks much has been written. The historic Eddystone, for
example, has quite a voluminous literature of its own. Of the light
itself, however, much less is known.

It will be interesting first to note the different purposes for which a
light may be required, and then see how the apparatus of the lighthouse
is made to serve these purposes.

There is the "making" light, perched, if possible, upon some high
eminence, deriving its name from the fact that the sailor sights it as
he is "making" the land. Vessels approaching England from the south-west
by night first see the light at the Lizard. The transatlantic vessels
know they are approaching land by catching sight of the Fastnet Rock
light off the coast of Ireland. Cape Race light serves in the same way
for those about to enter the St Lawrence and Navesink for the entrance
to New York harbour. All such as these have to be of the greatest power
practicable, so that they may be visible not only at the longest
possible distance, but also under unfavourable conditions, such as haze
and slight fog. No light, of course, can penetrate thick fog, but in
light fog and haze a powerful light can be seen at considerable
distances. For the same reason these lights must be high up, or the
curvature of the ocean's surface will limit their range. A light
elevated 100 feet above the sea-level will be visible nearly 16 miles
away, but if only 50 feet up it will be invisible at 13 miles. To be
seen 40 miles away it must be as high as 1000 feet.

But then again height is in some cases a disadvantage, for sometimes fog
hovers a little distance above the sea, while below it the air is clear,
and the higher a light may be the more likely is it to have its lantern
immersed in a floating cloud of fog. Many readers familiar with the
south coast of Britain will remember that the light which used to show
on the summit of Beachy Head is there no more, but has been replaced by
a tower at the foot of the cliffs, the reason being that it may be below
the clouds of fog which are prevalent at that point.

But the mention of Beachy Head introduces us to another class of lights,
known as "coasting" lights, since they are intended to lead the mariner
on from point to point along a coast. It will be seen at once that in
many cases they do not need to be visible at such great distances as the
making lights. When the mariner has sighted the Lizard, for example, he
knows where he is. In order that he may learn that important fact as
soon as possible it is desirable that that light should have the
greatest possible range, but having thus located himself, when he begins
to feel his way along the English Channel he is guided by the coasting
lights, and so long as they are of such range that he will never be out
of sight of one or two of them that will be sufficient. Thus the Beachy
Head light, in its present low position, has a sufficient range for its
purpose, with the added advantage of more freedom from obscuration by
fog. Thus we see how the local conditions and the purpose of each
particular light have to be taken into consideration in determining its
position and power.

The Eddystone, again, is an example of a further class. It simply serves
to denote the position of a group of dangerous rocks. Its function is
not so much guidance, although no doubt it often serves for that, but
for warning. The Lizard light beckons the on-coming ship to the safety
of the English Channel; the Eddystone warns it away from danger. The
latter, therefore, and similar lights are "warning" lights.

[Illustration: _By permission of Messrs. J. and E. Hall, Ltd._

                              A COLD STORE

Interior of a cold store, in which meat and poultry are kept good and
         fresh by the use of machine-made cold.--_See_ p. 67]

Right at the entrance to the English Channel, that greatest of all
highways for shipping, there lie the Scilly Isles. This group comprises
some few islands of fair size from which we draw those plentiful
supplies of beautiful spring flowers, but it also includes a large
number of rocky islets which have sent many a strong ship to its doom.
On one of the islets, therefore, the Bishop's Rock, there now stands a
very powerful light which exemplifies many whose purpose is the
double one of welcoming the mariner as he approaches our shores and at
the same time warning him of a local danger. Such are both making and
warning lights.

Of no less importance, though less impressive, are the guiding lights,
which guide the ships into and out of harbours and through narrow
channels. These are generally arranged in pairs, one of the pair being a
little way behind and above the other. Thus when the sailor sees them
both, one exactly over the other, he knows he is on the right course.

Sometimes lighthouses have subsidiary lights as well as the main light,
to mark a passage between two dangers, or to give warning of some
danger. The subsidiary lights are often coloured, and they are generally
"sectors" showing not all round a complete circle, or even a
considerable portion of one, but just in one certain direction. They are
generally shown from a window in the tower lower down below the main
light.

Finally, it is important to remember that every light must be
distinguishable from its neighbours. Hence every one in any given
locality has a different "character" from all the others. This character
is given to it by means of flashes. Instead of showing, as the primitive
lights did, a steady light, the modern lighthouse exhibits a series of
flashes, the duration of which, together with the intervals between,
give it its distinctive character. This flashing arrangement has a
further advantage over the steady light. Each flash can be made more
powerful than a steady light could be. But of that more later.

The actual source of light varies with circumstances. The electric arc
is, as we all know, a very powerful light, in fact it can be made the
most powerful of all, but its light is decidedly bluish. Now the time
when a light is most of all needed is when the weather is thick. Fogs
varying from a slight haze to a thick pall of darkness are of very
common occurrence, and the lighthouse light must be able as far as
possible to penetrate them.

As a matter of fact clean fog, such as one gets at sea, is not by any
means opaque. The black fogs of the great cities are another matter, but
they are not the sort which afflict the mariner. On a foggy day in the
open country or by the sea it is often particularly light; indeed the
light is of a peculiarly diffuse nature which gives a nice even
illumination to everything. Thus we see that fog is really transparent,
but it diffuses the light. It does not stop the light rays, but simply
bends them about and scatters them in all directions. Thus we can see
nothing through the fog, yet a flood of light reaches us through it. In
its effect it is like that "crinkled" glass which is often used for
partitions between rooms, which lets light through, but which cannot be
_seen_ through.

We see, then, that the effect which a fog produces is mainly to refract
the light rays. Each little drop of water (for it must be remembered
that fog is myriads of tiny drops of liquid; it is not vapour) acts like
a minute lens, and bends the rays which pass through it. And the more
blue a ray is the more it is bent. On the contrary, the more red it is
the less is it bent. When a beam of light is analysed in the
spectroscope the red rays are bent least and the blue rays most, so that
the red rays fall at one end of the spectrum and the blue at the other.

Now we only _see_ a thing when light rays proceeding from every part of
it fall straight (or nearly so) upon our eyes. Consequently, since red
rays are bent and scattered by the fog less than blue rays are, a red
light will be more easily seen through a fog than a blue one. It might
seem from this that a red glass put in front of a light would make it
better for this purpose, but that is not the case, for the simple reason
that filtering the light through red glass does not really make it any
redder than it was before: it simply makes it look redder by extracting
from the original light all except the red. But a source of light which
is _naturally_ reddish is so because it is more plentifully endowed with
red rays, while a bluish light like the electric arc is naturally
deficient in red rays. Consequently we should be inclined to expect from
theory that the electric arc would not be a good light for a lighthouse,
since it would lack penetrating power in foggy weather. Some readers may
have noticed themselves, in towns where electric lights and gas lamps
are in use near each other, that the latter, though relatively feebler
under normal conditions, seem to give more light in fog. And experiments
show that this is really the case. So although there are some
lighthouses with electric arc lights, that which is now believed to be
the best is an oil lamp of special design, using a mantle of the
Welsbach type.

The oil is stored in strong steel reservoirs into which air is pumped by
means of a pump not unlike those used to inflate bicycle tyres. By this
means a pressure is maintained upon the oil of about 65 lb. per square
inch. This forces the oil up a pipe and drives it in a jet into a
vaporiser, a tube heated from the outside so that in it the oil is
turned into gas. This gas then rises to the burner and heats the mantle,
just as the gas does in the ordinary incandescent gas light. Indeed in
the case of lights on the mainland near a town the gas from the town
main is often utilised. But this simple arrangement for using vaporised
oil, as will readily be seen, can be employed anywhere. A little of the
gas produced is led through a branch pipe and burnt to heat the
vaporiser. To start the apparatus the vaporiser is heated with a little
methylated spirit. Thus everything is quite self-contained and so simple
that there is little to get out of order. The largest size of lamp will
give 2400 candle-power, with an expenditure of 2-1/4 pints of oil per
hour, just common oil, too, of the kind used with ordinary wick lamps.

Having got a source of powerful light, the next thing is to collect that
light and throw it in the direction required. For the light proceeds
from the lamp in all directions (practically), and much of it would be
entirely wasted could it not be collected and guided in the required
direction.

The earliest attempt at this was to use a reflector of bright polished
metal. In the most improved form these were made to that peculiar curve
known as a parabola. This is a curve obtained by cutting a cone in a
certain way, wherefore it is one of the "conic sections," and its
particular appropriateness for this work resides in the fact that if a
light be placed at a certain point known as the "focus" all the
diverging rays which fall upon the reflector will be reflected in the
same direction, parallel to each other. An ordinary spherical mirror
would reflect them either back to the lamp or in diverging directions.

At any distance the beam from the parabolic reflector will be more
intense than that from the spherical one, since the rays will be closer
together. But even with the parabolic one there is some diffusion, for
the simple reason that whereas the focus is a mathematical point
(position without magnitude) the most concentrated form of light known
has a considerable magnitude. Hence the rays proceeding from the centre
of the mantle are reflected as per the theory, but those from the
outlying parts of it are somewhat diffused. This difficulty cannot
possibly be overcome, and hence even in the finest examples of
lighthouse architecture the flashes are not quite sharp and clear-cut.
There is a central moment, so to speak wherein the flash is almost
blinding in its intensity, but it is preceded by a period of growing
brightness and succeeded by one of decreasing light.

In the modern apparatus, however, metallic mirrors are entirely
dispensed with, their place being taken by reflecting prisms of glass.
The metallic ones had to be continually rubbed to keep them clean, and
this soon dulled their brightness, while the glass prisms need only to
be wiped carefully, which operation has little effect upon their
surface.

It may come as a surprise to some that reflecting prisms are possible.
The idea of refraction through a prism is quite familiar. Such forms the
essential principle of the spectroscope. Refraction is explained to
every school child in order to account for the rainbow. But _reflection_
by a piece of the clearest glass seems a contradiction in terms almost.
Yet it is only a question of shape. In some prisms the light is simply
bent as it passes through. In others it is bent twice, so that it leaves
the prism just as if it had been reflected off a mirror. Both devices
are used in the lighthouse. Let us see how they are combined so as to
perform the work to be done.

Take first of all the case of a light upon an isolated rock where the
warning is needed equally all round. All that is necessary here is to
pick up those rays which, if left to themselves, would fall upon the
water near the foot of the tower, and those which would waste themselves
skywards, and then to gather all the rays into several bundles or beams.
We will suppose a simple case in which the light is supposed to give
flashes at regular intervals.

We are in the topmost room of the lighthouse, the lantern, as it is
called. In the centre there stands the murette or pedestal. In this
several columns support a circular platform on the top of which there
moves what we might call a turntable, which in turn bears a frame of
gun-metal into which are fitted a maze of glass bars triangular in
section and curved to form concentric circles. The whole structure,
possibly, is of great size. From the floor to the platform is as high as
an ordinary man. Indeed around the turntable there is a gallery which
forms a roof over our heads, so that it is only after mounting some iron
steps on to this gallery that we are able to examine the glass part.

As we ascend we notice that the walls of the chamber as far up as the
gallery are formed of iron plates, while above that there is a metal
framework filled in with glass panes, and above all a dome-shaped roof.

Having reached the platform we proceed to examine the glass, and we find
that the metal framework forms a cage with four sides, each
approximately flat, but really slightly spherical. Each of these sides
is called a "panel." In the centre of each is a lens. Peeping through
the interstices between the prisms, we perceive that the lamp is inside
this structure, exactly in the centre, so that its light shines
directly through the central lens or bull's eye. Around this bull's-eye
are many circles of glass bar, forming refracting prisms. Around this
again are more bars in the form of segments, which together form
circles, some being refracting prisms and others reflecting prisms. All
the light rays from the lamp which fall on any one prism are deflected,
so that they proceed approximately in the same direction. Those prisms
in the upper part lay hold of the rays which would otherwise go up into
the sky. Those at the bottom collect those which would fall near the
foot of the tower. So scarcely any are lost. But for the fact that the
lamp itself is comparatively large and not a theoretical point, as
already explained, the beam from this panel would be perfectly straight,
parallel, and of uniform density everywhere. As it is, it widens
slightly as it proceeds, but, practically speaking, we might call it a
solid beam of light.

Each of the panels sends forth such a beam, so that they strike out in
four directions from the central lamp much as four spokes from the hub
of a wheel.

Then descending once more to the floor from which we started, we see
that among the columns there is a large clockwork arrangement, the
purpose of which is to drive round the turntable and all that it
carries--in the language of the lighthouse engineer the "optical
apparatus" or, more briefly, "the apparatus." And as this turns the
radiating beams of light sweep round the horizon and in succession
strike into the eyes of any mariner who may be within range. Each time a
beam strikes him he sees a flash. If the apparatus revolve once a minute
he will see four flashes every minute, one from each panel.

Let us consider, then, the advantages of this wonderful mechanism, with
its cunning arrangement of prisms. It is these latter, of course, which
are the important thing. The rest, the mechanical portion, is simply for
the purpose of holding them and turning them at the proper speed. In the
first place, the contrivance gives us flashes instead of a steady light;
it gives the lighthouse its "character." Then again it enhances the
brightness of the light. Instead of shining all round, the light is
concentrated in four special directions, and the light which would be
wasted upwards or downwards is saved and brought into use.

But suppose that the lighthouse we are considering be near the shore, so
that there is no need for it to throw any light in one--the
landward--direction. Then we should see inside the revolving framework
with its prisms a fixed frame with reflecting prisms which would catch
any rays going from the lamp in the direction of the land and simply
hurl them, as it were, back into the flame. Thus the intensity of the
flame becomes increased by those rays thrown back which would else have
been wasted.

Or suppose that the character of the light is such that the flashes have
to be at irregular intervals. Then the framework, instead of being
symmetrically four-sided, would be of an irregular shape.

And that brings us to a beautiful feature of the mechanism of the
apparatus. We have been discussing a four-panel arrangement. Suppose
that we were to reduce it to three. Then, since all the light would be
concentrated into three beams instead of four, each beam would be more
intense. We should thereby have increased the range of our apparatus
without any increase in the cost of oil--for nothing, as it were. But to
get the same number of flashes per minute we should have to drive it
round so much the faster. But increased speed means increased burden on
the keepers who have to wind up the heavy weights which operate the
clockwork. So there is a limit to the speed which can be attained.

But if friction can be almost eliminated the apparatus can revolve at a
high speed without throwing undue burden upon the men. But how can
friction thus be got rid of? Messrs Chance Bros., the great lighthouse
constructors, of Birmingham, have done it, almost entirely, by floating
the apparatus on mercury. The turntable has on its under side a large
ring which nearly fits a cast-iron trough on the top of the pedestal.
In this trough there is mercury, so that upon the liquid metal the
apparatus floats as if upon a circular raft. The table with its lenses,
prisms and other fittings may weigh six or seven tons, yet it can be
pushed round by one finger.

The various sizes of optical apparatus are known as "orders." One of the
"first order" has a focal distance of 920 millimetres. This means that
there is that distance between the centre of the lamp and the
bull's-eye. They descend by successive stages down to the sixth order,
with a focal distance of 150 millimetres, while the most important
lights are of an order superior even to the so-called "first," termed
the "hyper-radial," the focal distance of which is 1330 millimetres.

A recent example of a hyper-radial light is at the well-known Cape Race
in Newfoundland. It revolves once every 30 seconds, giving a flash of 3
seconds every 7-1/2 seconds. The optical apparatus weighs seven tons.

[Illustration:
_By permission of Messrs. Chance Bros. and Co., Ltd., Birmingham_

               DASSEN ISLAND LIGHTHOUSE, CAPE OF GOOD HOPE

     This lighthouse, 80 feet high, is built of cast-iron plates,
                           bolted together]

Most lighthouses are fitted with fog signals of some kind which have a
distinctive character the same as the lights. Some are horns blown at
intervals by compressed air often obtained from a special air-pump
driven by an oil-engine. Another thing is to let off detonators at
stated intervals. But perhaps the most interesting of all is the
submarine telephone. The trouble with audible signals is that they are
apt to vary as the conditions of the atmosphere change. For, strange
though it may appear, the air which is the natural medium by which
sounds are carried to our ears is really a very bad substance for the
purpose. Water is much superior. A swimmer who cares to try the
experiment of lying upon the water with his ears immersed while a friend
beats a gong under the water some distance off will be astounded at the
result. So many modern ships are fitted with under-water ears,
waterproof telephone receivers, really. One is fixed each side of the
vessel, the wires from them being led to telephone receivers near the
bridge. Many lighthouses and lightships in like manner are fitted
with under-water bells which can be rung at intervals. The sounds so
conveyed through the water are always the same. Atmospheric or similar
changes have no effect upon them. And, moreover, the officer can tell
which side of his ship the bell is. If it be on his port-side it sounds
louder in his port telephone, and vice versa. By turning his ship until
he hears them equally he knows that he is pointing directly to or from
the bell. Thus if the bell belong to a warning light he can steer
confidently right away from the danger even in the thickest fog.

But science has not only provided the mariner with lights of marvellous
power and of strange distinctive characters, and reliable sound-signals
for foggy weather, it has also found him a reliable compass, but that is
worthy of a chapter to itself.



CHAPTER VII

THE GYRO-COMPASS


The magnetic compass has been for ages the mariner's guide over the
trackless waters. In cloudy weather it has been his only means of
knowing the direction in which his craft was heading. Indeed, it is not
too much to say that the maritime commerce of the world was based upon
the behaviour of that little piece of magnetised steel.

It has always, however, been subject to certain faults. To commence
with, it points, not to the geographical north, but to the "magnetic
pole," a point some distance from the geographical pole, and one,
moreover, which is not quite permanent. The fact that the magnetic pole
varies its position is impressively shown by the fact that a special
department at Greenwich Observatory is continually employed, by the aid
of delicate self-recording instruments, watching and setting down its
fluctuations. And the premier observatory of the world, it should be
remembered, exists primarily, not in the interests of pure science, but
as a department of the British Admiralty in order to study matters of
interest to navigation. Thus we have testimony to the importance of
these little vagaries on the part of the magnetic compass.

But in addition to these inherent faults there is a new source of error
in the magnetic compass which man has introduced himself by making his
ships of iron instead of wood. Every ship of the present day is a huge
magnet. A piece of iron left in the same position for a length of time
becomes polarised, which is to say that it acquires the properties of a
magnet; and two magnets always exert an influence upon each other.
Consequently the ship, after lying for perhaps a year in one position,
during the period of building, becomes itself magnetic and interferes
with its own compass.

Then, again, our methods of ship construction aggravate this trouble. It
is believed that every molecule of iron is itself a minute magnet with a
north and south pole of its own. These lying in confusion in the mass of
unmagnetised iron neutralise each other, so that the mass, taken as a
whole, does not exhibit any magnetic power. But if by some means the
whole of the millions of millions of molecules can be set the same
way--with all their north poles in one direction, and their south poles
in the opposite direction--then they will all act together. Instead of
neutralising each other they will then help each other, and under those
conditions the mass of iron will possess that peculiar power which is
distinctive of a magnet. So long as a piece of iron is left in the same
position the magnetism of the earth is thus acting upon the molecules.
Just as it tends to place the compass needle north and south, so it does
with every molecule in the iron mass. And if, while lying still, the
iron be hammered, the shaking of the molecules due to the hammering
loosens them as it were and assists the earth's power in pulling them
into position.

One has only, then, to watch the riveting up of a ship, and to see the
vigorous way in which the riveters wield their hammers, to realise that
when the thousands or even millions of rivets have all been finished the
material of that ship will have had the very best possible chance of
becoming magnetic.

To make matters worse still, ships are often loaded with great weights
of iron among their cargo. That, too, may affect the compass. On
warships there are the heavy guns, each weighing, with its turret,
hundreds of tons, and they move, so that their effect upon the compass
is not always the same, but may vary from time to time. And finally one
may mention the electrical machinery in a modern ship consisting largely
of powerful magnets.

Altogether, then, it is not surprising that the old magnetic compass is
somewhat unreliable. It has to be coaxed into doing its duty. Pieces of
iron and magnets have to be disposed about it to counteract these
disturbing influences with which it is surrounded. Before a voyage
experts have to come on board to adjust the compasses, and even then
there is reason to believe that the instrument sometimes plays the ship
false.

It is not to be wondered at, then, that the naval authorities in
particular throughout the world have welcomed the advent of a new
compass which appears to possess none of these drawbacks. It points to
the geographical north, to the actual pivot, if one may so speak, upon
which the earth turns. It is non-magnetic, so that the presence of iron
or magnets even in its immediate neighbourhood has little or no effect
upon it. On the other hand, it has to be driven by a current of
electricity, and it seems just possible that in some great crisis it
might fail, although every provision is made for alternative sources of
supply in case of one failing, and there is always the possibility of
falling back upon the old magnetic compass should the new one go wrong.

In principle the improved compass is, like its older brother, simplicity
itself. The latter is but a small piece of iron magnetised; the former
is nothing more than a spinning-top.

It is rather strange that although the spinning object has been a
familiar toy for years, and that, moreover, its behaviour has been the
subject of investigation by some very eminent scientific men, it is only
of recent years that its principles have been put to practical use.

Everyone is familiar with the fact that a round block of wood will
support itself upon a comparatively tall peg so long as it is rapidly
rotating. And that is but one of the curious things which a rotating
body will do. For example, imagine a wheel mounted upon an axle the ends
of which are supported inside a ring, while the ring again is supported
on pivots between the two prongs of a fork, the fork being free to
swivel round in a socket. The wheel is then free to move in any
direction. Technically, it is said to have "three degrees of freedom."
It can spin round, its axle can turn over and over with the pivoted ring
inside which it is fixed, while it can also swing round and round as the
fork turns in its socket. Assuming that the joints are all perfectly
free, that the pivots move in their sockets with perfect freedom--which,
of course, they do not--then a wheel so mounted could move in any
direction under the influence of any force that might act upon it. Now a
wheel so mounted if left alone remains in precisely the same position so
long as it goes on rotating. If it be turning sufficiently quickly its
tendency to remain will be strong enough to overcome the friction of any
ordinarily well-made instrument. Consequently a wheel of that
description has been used to demonstrate the rotation of the earth, it
remaining still (except, of course, for its rotating movement) while the
earth has moved under it.

Could we entirely eliminate the effects of friction that might be used
as a compass, for it could be set, say with its axle pointing north and
south, at the commencement of the voyage, and it would remain so despite
all the evolutions through which the ship might go.

But there is a better scheme even than that, based upon the peculiar
behaviour of a revolving wheel when it has only two degrees of freedom.
Suppose that we dispense with the ring employed in the previous
arrangement, pivoting the ends of the axle between the prongs of the
fork. The wheel is then free to rotate, and its axle can slew round
through a complete circle by the turning of the fork in its socket, but
there can be no tilting of the axle. Being thus deprived of one of its
movements the gyroscope with three degrees becomes a gyroscope with two
degrees of freedom, and in that form it supplies the need for an
efficient and reliable compass.

The secret of the whole thing is the curious fact that a gyroscope with
two degrees of freedom exhibits a keen desire to place its axis parallel
with the axis of the earth. Owing to the shape of the earth, a device
such as has been described, with its fork standing up vertically, cannot
possibly have its axis really parallel with that of the earth, except on
the Equator. Still it gets as nearly parallel as possible. To be
scientifically accurate, we ought to say that it places it own axis "in
the same plane" as that of the earth.

To understand this we need to realise that all movement is relative. In
ordinary language, when we say a thing is still we mean that it is still
in relation to the surface of the earth, but since the earth is moving
the stillest thing, apparently, is really travelling at enormous speed.

Saint Paul's Cathedral in London, or a tall sky-scraper in New York,
would usually be regarded as supreme instances of immobility. It would
be hard to find better examples of stationariness, as we ordinarily look
at things. Each stands, firm and strong, upon a horizontal base. Yet
each is really turning a somersault every twenty-four hours. The plateau
upon which St Paul's stands, though it seems still and motionless
beneath our feet, is continually tilting; its eastern edge is
continually going downwards and its western edge upwards, as the earth
performs its daily spin. It is only a north and south line which does
not share in some degree this continual tilting action. Every plane,
large or small, so long as it remains horizontal, is being tilted thus,
down at the eastern edge and up at the western. And the plane in which
the axle of a gyroscope with "two degrees" is free to move is a
horizontal plane. Owing to its being held between the prongs of the
fork, while it can swing round to point north, south, east or west, or
towards any point between them, it cannot deviate from the horizontal
plane. Therefore such axle is always being tilted by the motion of the
earth, _except when it happens to be lying exactly north and south_.

Now for a reason which is too complex to go into here a gyroscope
strongly objects to having its axle tilted in this manner. If it be
compelled by superior force to submit to tilting, it tries to wrench
itself round sideways. Anyone who has a gyroscope top and cares to try
the experiment will feel this action quite easily. Hold the spinning-top
in your hand and turn it over so as to tilt the axle, when it will, if
you are not careful, twist itself out of your grasp.

So a gyroscope of the kind we are considering, when the motion of the
earth tilts its axis, turns itself round in its socket until at last it
reaches the north and south position, when the tilting, and therefore
the twisting, ceases. Hence the axle of the gyroscope if left to itself
(the rotation of the wheel being maintained the while) will place itself
in a north and south direction. And, moreover, it will keep in that
direction. It will take some force to slew it round into any other. And
if moved into any other by some extraneous means it will restore itself
to the old position again.

Hence a wheel thus arranged has all the attributes which we need for a
mariner's compass. But unfortunately there are mechanical difficulties
in the way of using such a simple contrivance for that purpose.

Chief of all these is the fact that it is not what engineers call
"dead-beat." That means that it will not go to the proper position and
then remain there quite still. Instead, it will first slightly overshoot
the mark, which being followed by the reverse action, it will come back
and overshoot it just as far in the opposite direction. Instead,
therefore, of a steady pointing, always in the same direction precisely,
it will oscillate more or less, the exact north and south line being the
mean or average position, the centre of the oscillations.

It would of course be possible to damp this, to apply a break as it
were, if the apparatus were to remain stationary. For example, if the
whole concern were immersed in water the resistance of the liquid would
restrain any quick movement of the axle, yet it would not prevent it
from slowly finding its true position. Thus the oscillations would be
reduced to such a small range as to be for practical purposes
negligible. But the drawback to a device of that kind, applied to a
gyroscope on board ship, would be that the axle would be carried round
to some extent every time the ship turned. As she changed direction it
would more or less carry round the water with it; that in turn would
carry the gyroscope, and so the direction of the latter would be for a
time untrue. It would in course of time regain its accuracy, but in the
meantime it would be leading the ship astray.

Consequently the application of this, in itself wonderfully simple,
idea, to this extremely important purpose was accompanied with a
difficulty which was for a long time insuperable.

But all was overcome at last by the genius of Dr Anschutz, of Hamburg,
whose firm were the first to turn out the practicable article. Taking
advantage of another movement of the gyroscope when arranged as has been
described, and using the revolving wheel itself as a centrifugal fan, he
was able to make the wheel blow air "against itself," as it were, when
in any position other than north and south. Thus, if it deviates towards
the east, this jet of air tends to blow it back; if it turns westwards
the jet again comes into operation, tending to bring the erring gyro
back to its proper place; and so the tendency to oscillate is checked.

The finished instrument as it is installed on the latest warships is, of
course, quite different in detail from the simple contrivance which we
have been considering so far, although it is the same precisely in
principle. The essential part is a heavy metal wheel combined with which
is an electric motor which keeps it rotating at a speed of 20,000 or so
times per minute.

The bearings of the wheel are supported upon a metal ring which floats
upon the surface of a trough of mercury. Thus friction is brought down
almost to the irreducible minimum. The only place where the wheel and
its supports touch anything solid is at one delicately made pivot which
serves to keep the floating mechanism in the centre of the mercury
basin, and to prevent it from rubbing against the side of it. The
current which drives the motor reaches it through this pivot and leaves
through the mercury. Thus arranged, although the floating part is of
considerable weight, a very slight force indeed is enough to move it;
while, looking at it the other way, we can see that the ship might turn
rapidly to right or to left, carrying round the mercury bowl with it,
without turning the floating part at all. Thus the gyroscopic action is
very free indeed to exercise its function of keeping the contrivance
pointing always in the one way.

The float has mounted upon it a compass card much like that of the
ordinary magnetic instrument, and the sailor reads it in precisely the
same way. To outward appearance there is little essential difference; in
one case there is a magnet under the card to keep it still, in the other
there is the float with the revolving wheel mounted upon it.

It is customary to have one "master compass" of this kind on a ship,
with an electrical repeater in each of the steering positions. As the
"master" turns in its casing it sends a rapid series of currents to all
the others, causing them to turn in unison with it. The "master" is
fitted in some safe part of the ship where it is least likely to be the
victim of any accidental damage.



CHAPTER VIII

TORPEDOES AND SUBMARINE MINES


It is sad to think how much scientific skill and learning has, during
the Great War, been devoted to killing people. It used to be thought
that one day a great scientific invention would arise, of such deadly
power that for ever afterwards war would be unthinkable; its horrors
would be such that all nations would shrink from it. That prophecy,
however, has not been fulfilled, nor are there any signs of it. On the
contrary, each scientific achievement in the realm of warfare is quickly
countered by another: so much so that with all our science in the
manufacture of weapons, and our skill in using them, warfare in the
twentieth century is if anything less deadly in proportion to the
numbers engaged than it used to be.

There are, however, two weapons which in this war have reached a deadly
efficiency which they did not seem to possess before, and to which
satisfactory antidotes have not yet appeared.

These two are the submarine mine and the torpedo. The latter,
particularly, had been a dismal failure previously, but as the weapon of
the submarine it has now established itself. It is, however, only in
connection with the submarine that it has achieved any measure of
success, and, as there are strong indications that very soon the
submarine itself will be robbed of its terrors, it is quite likely that
the reign of the torpedo will be brief.

Although it has only just made itself felt seriously in warfare, the
torpedo is a fairly old idea. In fact we can trace the general idea of
it back to very ancient times. The modern weapon, however, dates from
the year 1864, when an Austrian inventor approached an English engineer
named Whitehead with a request to take up his idea. Mr Whitehead had at
that time a works at Fiume, on the Adriatic, and it was really his
genius that developed the crude idea into a practicable invention.

Thus there came into existence the Whitehead Torpedo, now used in a
great many navies, and also the Schwartzkopff, which may be regarded as
the German variety of the same thing.

Speaking generally, it may be described as a small automatic submarine
boat. Externally, it naturally follows somewhat the lines of a fish.
Deriving its name from that curious fish which is able to give electric
shocks from its snout, it likewise carries on its nose that appliance
whereby it gives a shock, not electric it is true, but equally deadly,
to anything which it may touch.

Since no man-made mechanism can approach the marvellous action of the
fish's fins and tail, the propulsion is achieved by a propeller like
that of a steamboat, but of course on a very small scale. A single
propeller, however, would tend to turn the torpedo over and over in the
water, and so it has two, one behind the other, driven in opposite ways,
so that the turning tendency of one is neutralised by that of the other.
The blades of the propellers are, however, set in opposite ways, so that
although rotating in different directions they both push the torpedo
along.

Behind the propellers, again, there are rudders for steering. One steers
to right or left, as does that of an ordinary ship, while two others are
so placed that they can steer upwards and downwards.

So there we have the general picture of the outside: a smooth, fish-like
body with a "sting" in its nose, propellers at the rear to drive it
along, and rudders to guide it.

Inside are various chambers. One contains the explosive which blows up
when the nose strikes something. This "head," as it is termed, is
detachable, so that it can be left off until it is really required for
war. The peace-head, which is of the same size, shape and weight as the
war-head, is what the torpedo carries during its earlier career. With
this it can be tried and tested in safety, the war-head being
substituted when the real business of the torpedo begins.

Another chamber contains the compressed air which furnishes the motive
power. This also serves to give buoyancy.

Another chamber, again, contains the engines, beautiful little things of
the finest workmanship almost exactly like the finest steam-engine, but
of course very small in comparison.

In the early stages the range of the torpedo was limited by the amount
of compressed air which it could carry. At first sight there seems no
reason why any limit should be placed upon this, but in practice there
are often limitations in engineering matters which are not apparent on
the surface. For example, to increase the air chamber would mean
enlarging the whole torpedo, calling for more propulsive power and
larger engines, and these larger engines would call for more air, thus
defeating the object in view. Forcing more air in by using a higher
pressure, in a similar way would necessitate a thicker chamber, to
resist the higher pressure. This would add weight, calling for more
buoyancy. Thus there seemed to be a practical limit beyond which it was
impossible to go.

The difficulty was overcome, however, in a very cunning way. When the
engines have used some of the air, and the store is somewhat exhausted,
chemicals come into action which generate heat, which is imparted to the
air which is left. This heat expands the air, producing in effect a
larger supply of it, and enabling the torpedo to make a longer journey.

Steering in a horizontal direction--that is to say, to left or right--is
done by a gyroscope. The action of a rotating wheel is discussed in the
last chapter, and it is not necessary here to say more than this: a
rotating wheel always tries to keep its axle pointed in the same
direction. Just at the moment of starting such a wheel is set going
inside the torpedo, and its arrangement is such that, should the torpedo
swerve to the left, the gyroscope operates the rudder and steers it
back. In the same way, if it tends to turn to the right, the
ever-watchful gyroscope brings it to its true course once more. The
effect of the gyroscope, therefore, acting upon the rudder, is to keep
the torpedo faithfully to the direction upon which it is started.

The up and down rudders are likewise controlled quite automatically, but
in a different way. Their function, clearly, is to keep the thing at a
certain uniform level. Without such control a torpedo would be equally
likely to jump out of the water altogether, or to go downwards
vertically and bury its nose in the mud. The depth at which it is to
move is determined beforehand, certain necessary adjustments are made,
and the torpedo then pursues its even way, neither coming to the surface
nor driving beneath its target.

For this purpose there is first of all a "hydrostatic valve." This
little appliance, which is open to the action of the water, responds to
changes in pressure. The pressure at any point under water is exactly
proportional to the depth. At ten feet, for example, it is precisely ten
times what it is at one foot. So the hydrostatic valve is adjusted to
set the rudders straight when the water-pressure upon it is a certain
amount. If, then, it dives downwards the pressure increases and the
valve operates the rudders so as to bring it upwards, while if it rise
too high the decrease of pressure causes it to be guided downwards.

This action, however, is too sudden and violent, so that with it alone
the torpedo would proceed by leaps and bounds. After being low it would
come up too suddenly, overshoot the mark, only to be steered downwards
again equally suddenly.

The valve, therefore, is combined with a pendulum, whose action tends to
restrain these too sudden changes, with the result that under the
influence of the two things combined the torpedo keeps fairly well to
an even course, only varying upwards or downwards to an extent which is
negligible.

Finally, there is an interesting little feature about the firing
mechanism which merits a description. The actual firing is caused by the
driving in of a little pin which projects at the nose of the torpedo.
Suppose that, in the process of pointing the torpedo and launching it
upon its course, that pin were to be knocked accidentally, an awful
disaster would result. It must be provided against, therefore, and the
method adopted is beautiful in its certainty and simplicity.

Normally, the firing-pin is fixed by a screw so securely that no
accidental firing is possible. There is, however, a little
propeller-like object associated with it, which is driven round by the
water as the torpedo is pushed through it, and this unscrews, and
thereby releases the pin. The little "fan" has to rotate a certain
number of times before the pin is released, and it is quite impossible
for this number to be accomplished before the torpedo has proceeded to a
safe distance from the ship which fires it. On board the ship,
therefore, and so long as it is near the ship, it is quite safe, but by
the time it reaches its target it is ready to explode.

As far as is known, the foregoing description gives a true general
description of the torpedoes now in use. Those of different powers may
vary in detail, but, broadly, they are as just described.

There are others, however. The Brennan, for instance, was once adopted
and largely used by the British for harbour defence. This was controlled
from the shore by wires. It was driven, so to speak, with wire reins,
and thus guided it could fairly hunt down its prey, turning to right and
to left as required.

Of greater scientific interest, perhaps, still, is the "Armor1" wireless
controlled torpedo. This is the invention of two gentlemen, Messrs
Armstrong and Orling, whose first syllables combine to form the title of
the torpedo.

Of this, two very interesting features may be mentioned. Firstly, the
wireless control. In the chapter on Wireless Telegraphy there is
described the coherer, a simple little apparatus which we might describe
as a door which is opened by the "waves" which travel through the ether
from the sending apparatus. Whenever the key of the sending apparatus is
depressed these waves travel forth, and when they fall upon the coherer
it "opens." Normally, the coherer is shut, but when acted upon by the
incoming waves it opens and lets through current from a battery, which
current can be caused to perform any duty which we may wish. Thus,
ignoring the intermediate steps, we get this: whenever the sending key
is depressed current flows through the coherer and performs whatever
duty is set before it.

And now picture to yourself a tooth wheel with four teeth. A catch
normally holds one of the teeth, but when the catch is lifted for a
moment it lets that tooth slip and the next one is caught. At every
lifting of the catch the wheel turns a quarter of a turn. Then imagine
that that catch is operated by an electro-magnet energised by the
current which passes through the coherer. We see, then, that every time
the sending key is depressed the wheel turns a quarter turn.

Attached to the wheel is a little crank which turns with it, and the pin
of this crank fits in a slot in the end of a bar like the tiller of a
boat. Suppose that, to commence with, the tiller is straight, so as to
steer the boat straight. Depress the key, the wheel turns a quarter turn
and the tiller is set so as to steer to one side, say the left. Another
pressure upon the key and a second quarter turn brings the tiller
straight again. Yet another pressure, another quarter turn, and the
tiller is steering to the right. Thus by simply pressing the key the
correct number of times the torpedo can be made to travel in any desired
direction.

The second ingenious feature of this weapon is the means by which it is
made visible to the man who is controlling it from the shore or ship.
Probably the reason why these torpedoes are not used more is that the
man who guides them is of necessity himself visible. He has to be posted
somewhere where he can follow its course, or he has no idea how to steer
it. Consequently, he would be an object for attack by the enemy. Such a
torpedo would be useless in a submarine, for the submarine would need to
come to the surface in order that the observer might get a sufficiently
good view to be able to steer the torpedo, and we all know that when
upon the surface a submarine is a very vulnerable craft.

But that is by the way. The point is how to make the torpedo very
clearly visible while it is still under water. A short mast might be
used, but that would be liable to be shot away. The inventor had a happy
inspiration when he made it blow up a jet of water, like a whale does.
This jet is quite easy to see, yet no shot can destroy it. Compressed
air blows up this tell-tale jet which the observer can see, and by its
means he can guide the torpedo at will.

A submarine mine may be regarded as a stationary torpedo. It consists of
a metal case filled with a powerful charge of explosive which floats
harmlessly in the water until some unfortunate vessel strikes against
it, when it blows up with sufficient force to make a hole in the
stoutest ship.

There are two classes of mine: one which is laid in peace time, to
protect harbours and channels; and the other, which is laid during
actual warfare.

The former are anchored in a more or less permanent way. The services of
divers are used to place them in position. In some cases they float well
down in the water, out of the way of passing ships, but come up nearer
the surface when needed. This result is achieved by having an anchor
chain of such a length that when fully extended the mine floats a little
way under the surface, just high enough to be struck by a passing ship,
together with what is called an "explosive link." The link is used to
loop together two parts of the chain, and so, in effect, to reduce its
length. Wires pass from the link to the shore, and when an electric
current is sent along these wires the link bursts asunder, liberates
the chain, and the mine floats up to the full length of its chain.

Another plan is to let the mines float high up always, but to fire them,
not by the touch of the ship but by electricity from the shore. In this
way a safe channel is kept for friendly vessels, while an enemy can be
destroyed.

Necessarily, those mines which are hurriedly laid in war time are very
different from these. To be of much use, a mine must be concealed below
the surface. If it floats upon the water it will be visible, and can be
avoided, or, at all events, easily picked up. It is practically
impossible to set a floating object at a certain depth in the water,
except by anchoring it to another, heavier, object, which will lie at
the bottom. Therefore mines have to be anchored in some way.

But the sea varies in depth, so that the length of the anchor chain must
be varied, or else some of the mines will be on the surface, thereby
advertising the presence of the mine-field, while others will be below
the depth of even the biggest ship. In warfare, however, mines need to
be laid quickly. There is no time to sound for the depth and then to
adjust the length of cable accordingly. Hence the mine must be so made
as to set itself correctly at a pre-determined depth.

Possibly some readers may think that such things might be made to float,
of themselves, at the right depth. It is a fact, however, that a thing
either floats upon the surface of water or falls to the bottom. Water is
practically incompressible, so that the water at the bottom of the sea
is no heavier than that near the surface. The conditions which prevail
in air and allow a balloon to float at any desired height do not apply.
The only thing, in this case, is to have an anchor chain or rope of the
right length.

So let us picture a mine-laying ship steaming along, probably in the
dead of night, surreptitiously laying mines in the hope that the enemy
will run into them on the morrow.

Along the deck of the ship are small railway lines, and on these lines
stand what appear to be trains of small trucks, each truck having small
wheels to run on, and each bearing a large round metal ball. As the ship
travels along, the crew, handling these deadly things quite freely, as
if they were innocent of any danger, propel them along to the stern, and
at regular intervals push one overboard. That is all.

The freedom with which the men handle them is not folly, for they are
then quite harmless. Nor need they trouble about the length of rope, for
that adjusts itself. Just tumble the things overboard, and in due time
they anchor themselves at the right depth and set themselves in the
right condition for blowing up any ship which may get amongst them.

The truck-like object upon wheels is not the mine itself: it is the
sinker which lies at the bottom of the sea. The round ball which it
bears is the mine, and the two are connected together by a wire rope. To
commence with, this rope is coiled upon a drum in the sinker, which drum
is either held tightly or is free to revolve according to the position
of a catch. That catch is held open, so that the drum is free, by a
weight at the end of a short rope. Let us assume that that rope is ten
feet long.

Then, when the whole thing is tumbled into the water, the weight sinks
first ten feet below the sinker, which, being more bulky in proportion
to its weight, follows downwards more slowly. While sinking, the weight
is pulling upon its rope and holding open that catch, so that the drum
pays out its rope and the mine lies serenely upon the surface. As soon
as the weight touches bottom, however, the pull on the short rope
ceases, the catch grips the drum, no more rope is paid out, and the
sinker, in settling down its last ten feet, has to drag the mine down
too. Thus, quite automatically, by what is really a beautifully simple
arrangement, the mine becomes automatically anchored at a depth below
the surface equal to the length of the short rope. By making that rope
the desired length, the depth of the mine under the water can be fixed.

There are various methods of firing these mines, all of which work
perforce by the concussion of the ship itself. In some cases the sudden
tilting over causes an electric contact to be made, and permits a
battery in the mine to cause the explosion. Another way is to furnish
the mine with projecting horns of soft metal, inside which are glass
vessels containing chemicals. The ship, striking a horn, bends it,
breaks the glass, and liberates the chemicals which cause the explosion.

In the type of mine largely used by the British Navy there is a
projecting arm pivoted on the top of the mine and projecting from it
horizontally. The mine itself rolls along the side of the passing ship,
but the arm simply trails or scrapes along. Thus the mine turns in
relation to the arm, and a trigger is thereby released, which fires the
mine.

In this, be it noted, the ship only pulls the trigger, so to speak, and
releases a hammer which does the work, just as the trigger of a gun
releases the hammer. The motive force which makes the hammer do its work
when the trigger is "pulled" is the pull on the anchor rope. That
arrangement has a virtue which is not apparent at first sight.

Since it is the pull on the anchor rope which actually fires the mine,
it follows that if such a mine break away from its moorings it instantly
becomes harmless.

Safety for the men who lay the mines is secured in several ways. One is
by the use of a hydrostatic valve. The firing mechanism is locked until
the pressure of water releases it, and that pressure does not exist
until the mine is several feet under water. Another way is to seal up
the firing mechanism with a soluble seal made of some substance such as
sal-ammoniac. The mine cannot then explode until it has been under water
long enough for the seal to be melted.

It now remains to relate how these mines are swept up and removed, yet
there is very little really to tell, for the process is so exceedingly
simple. So far as is generally known, no method has been found that is
superior to the primitive plan of dragging a rope along between two
ships so as to catch the anchor ropes. The vessels employed are usually
of very light draft, so that they stand a good chance of passing over
the mines themselves, and the rope used is as long as possible, so that
a mine, if exploded by being caught in the loop of the rope, explodes so
far away as to do no harm.

When dragged to the surface the mines are exploded from a distance by
shots from a small gun, or even from a rifle. In the case of those mines
which have horns, a blow from a bullet is enough to break the glass and
cause explosion, and in all cases mines seem sooner or later to succumb
to a sharp blow. Thus they are destroyed, by their own action, at a safe
distance from the sweepers. Accidents happen, however, and mine-sweeping
is no job for anyone but the bravest.

It has been somewhat difficult to crowd a description of torpedoes and
mines into the small space of one chapter, and so many details have had
to be omitted, but the above descriptions give the broad, general
principles underlying practically all forms of these terrible weapons.



CHAPTER IX

GOLD RECOVERY


There has always been something very fascinating about gold. Even in
ancient times it was prized above all other things, and apparently it
was comparatively plentiful. It is estimated, for example, that King
Solomon possessed over £4,000,000 worth of it, while the little gift
which the Queen of Sheba brought him was of the handsome value of
£600,000, so that she too must have been plentifully supplied with it.

Probably it was more easily come by in those days, owing to the richness
of the primitive deposits, the best of which, perchance, have been
worked out. In one respect gold differs from all other metals (with the
single exception of platinum, which is scarcer still) in that it appears
naturally as gold, not as ore. The little pieces of gold lie in the mine
ready to be picked out, and so if the deposit in which it occurs be near
the surface, and the particles be of any considerable size, they are
sure to be found. A savage may be, and often is, very anxious to secure
weapons and tools of iron, little knowing that the very ground upon
which he stands is possibly of iron ore. He covets the single article of
iron, and in some cases is willing to give much gold for it, or ivory,
or some such treasure, while thousands or millions of tons of iron lie
at his feet, only he does not recognise it, nor would he know how to
utilise it if he did.

For iron, like all other metals except the two just referred to, is
found naturally in combination with something else, generally oxygen,
and the combination bears no resemblance at all to the metal. The red
rust so familiar to us on iron is a combination of iron and oxygen, and
it is fairly typical of the kind of state in which iron is found in the
earth. Nor would anyone recognise copper ore, lead ore, tin ore, or any
of the ores, any better than iron ore. All are difficult to recognise.
It is said that the highest compliment that a Cornish miner--the finest
metalliferous miners in the world come from Cornwall, or are the product
of Cornish influence--the highest compliment that such a man can pay to
another is to say that "he knows tin," meaning that he can tell tin ore
when he sees it.

Contrasted with these other metals, gold is easy to find. It does, it is
true, under certain conditions, form chemical compounds with other
things, as, for instance, in gold chloride, which is present in
sea-water, but it does not oxidise as the others do, and so when it is
in the earth it is in the bright yellow grains such as (if they be large
enough) can easily be recognised at sight.

And it is often found in beds of loose gravel, alluvial deposits, as
they are termed. In such cases the gold is to be had simply for the
picking up. Sometimes a lucky find occurs in the form of a big nugget,
but more often the metal lies in tiny grains at long distances apart, so
that a ton of gravel has to be sorted over to find a paltry ounce or so
of gold. Yet so desired is it that gold will always fetch its price, and
an ounce to the ton (even less) is sometimes worth getting.

But in the early history of the world there were possibly particularly
generous deposits with plenty of gold in good-sized pieces, and such
would be quickly discovered and worked by primitive man. No doubt the
chieftains of those days took much, if not all, of the gold that their
people found, and more powerful chiefs and kings would, in turn, either
by force or in trade, take it from the weaker, so that it is not
surprising to learn that some of the mighty kings and potentates of long
ago were well supplied with gold.

Yet there are few things more useless. Its value in the first instance
was probably entirely due to its beautiful colour, and the fact that it
does not easily tarnish. For this reason, coupled with the fact that it
was by no means plentiful, men liked to deck themselves with it, not
only adding to their "beauty" by so doing, but advertising to their
fellows the fact that they were men of wealth, men who possessed what
few others had, or at all events possessed it more abundantly. These
three basic facts about gold, its beauty, its freedom from deterioration
and its comparative scarcity, give it its peculiar status among the
commodities of commerce, in that for it, and for it alone, there is a
continuous and universal demand. No gold-mining company ever shut down
its properties because of the falling off in the demand for gold. No one
ever had to hawk gold about to find a purchaser; it is always saleable.

And hence its value to humanity as the great medium of exchange. When a
tailor wants bread, as has been pointed out by a great political
economist, he does not go searching for a baker who happens to need a
coat. If he did, he might starve before he found one. Instead, he gives
his coat to anyone who needs one, no matter what his trade may be,
taking gold in exchange. Then he goes with confidence to the baker,
knowing full well that he, in turn, will be perfectly ready to give
bread in exchange for gold. That is the principle upon which gold, and
in a few cases silver, has become the foundation of trade. We use it for
toning photographs and a few other things, but, practically speaking, it
is useless stuff, yet certain special circumstances have given it a
special function in civilised society, and so governments now make it up
into little flat discs, putting their own special stamp upon them as a
guarantee of size and quality, and it is by handing those little discs
about that we carry on our trade. Or even where we use no actual disc,
we pretend that we do, and use a piece of paper the value of which we
say is so many discs, but that value depends entirely upon the fact that
someone has guaranteed, on demand, to give so many discs for it.

And the strange thing about it is that although this usefulness of gold
depends upon its rarity, we lose no opportunity of looking for new
sources of supply, and so diminishing that rarity. As has been said,
gold is present in sea-water, although no one knows how to get it out,
except at a cost which makes it not worth while. But suppose that some
genius found a way, and gold thus became twice as plentiful as it is
now, the world would be no better off. Everything would cost twice as
much as it does now; that is all. A pound is merely so much gold. If
gold be twice as plentiful people will want twice as much of it in
exchange for what they have to sell. Yet, all the same, the man who
could solve that problem of getting gold from sea-water, or from
anywhere else, in fact, would be hailed as a benefactor, and for a time
at least he would reap a generous harvest.

Even as it is, science has done much for the production of gold. Not, as
in other metals, in finding ways for extracting it from its ores, for,
strictly speaking, it has none, but in finding ways of catching the tiny
particles of metal from the "gangue," as it is called, the rock or earth
in which they are embedded. The trouble is that they are so small, so
infinitesimally small, almost.

There are two great types of place where gold is found. In the alluvial
deposits, the beds of old rivers, the gold is quite loose. The
convulsions of ages ago have, in many cases, elevated these beds, until
now they are on the sides of mountains. In such cases the loose,
gravelly stuff of which they are composed is washed down by a powerful
stream of water from a huge hose-pipe terminating in a nozzle called a
"monitor." This process, called "hydraulicing," brings down everything
into a pond formed at the foot of the hill, and in some cases a boat or
raft is floated upon the pond with machinery on board for dredging up
the material. Often a powerful centrifugal pump sucks up the water
through a pipe reaching to the bottom of the pond, bringing gravel and
gold with it. Arrived in this way upon the raft, it all goes on to
separating tables, by which the gold, being heavier, is divided from the
gravel, which is lighter. These tables will be referred to again later.

In non-alluvial workings the gold is embedded in rock of some kind, such
as that called quartz. This is hard, somewhat of the nature of granite,
and before the gold can be liberated it has to be crushed to the
likeness of fine sand, so that the tiny grains of gold can be captured.
The quartz is found in veins or lodes, fissures, evidently, in the
original crust of the earth, produced probably as the earth cooled.
These have been gradually filled up by hot volcanic streams of water,
which carried not only the gold in solution but also the materials of
which the quartz is formed. It used to be thought that the veins were
the result of hot liquids forced up from below by volcanic action, the
rock and metal being themselves in the liquid state through intense
heat. It is now more generally held that water was the vehicle by which
the materials were brought in, and the vein formed. The gold in the
alluvial deposits, too, is now thought to have come there in solution in
water, and not by the erosion and washing down of rocks higher up the
original river.

However that may be, and it is the subject of discussion among
geologists and metallurgists, there the gold is to-day, firmly fixed in
the hard rock, and the problem which confronts the metallurgist is to
get it out with the least expense. The old historic way of breaking up
the quartz rock is with what are called "stamps," pestles and mortars on
a huge scale. There are a number of vertical beams of wood, each shod
with iron, fixed in a wooden frame, so that they are free to slide up
and down. Running along behind these stamps is a horizontal shaft with
projections upon it called cams. There is one cam for each stamp, and as
the shaft turns slowly round this projection catches under a projection
on the stamp, and after lifting it up a short distance drops it
suddenly. Thus, as the machine works, the stamps are lifted and dropped
in rapid succession. The rock is fed into a box into which the feet of
the stamps fall, and thus it is pounded until it is quite small.
Meanwhile a stream of water flows through the box and carries away the
finely broken particles through a kind of sieve which forms the front
of the box, and which allows the fine, small pieces to escape, while
holding back the larger ones and keeping them until they too have been
crushed.

An average stamp will weigh 600 to 700 lb., and the repeated blows of
such a hammer are enough to pulverise the hardest rock.

Machines such as these have been employed since the sixteenth century,
at all events, and the improvements of modern times are only as regards
details. It may well be wondered, then, why such an old device is still
in use and how it comes about that it has not been displaced by
something newer and better. The answer, which is an instructive one,
well worth bearing in mind by many inexperienced inventors, is that it
is so simple. It can be shipped in comparatively small parts, and so
taken cheaply to any outlandish place. A good deal of it can be made
roughly of wood, so that if native timber is available it can be made
partly at the mine, and carriage costs saved. Finally, it is so easy to
work and to understand that the most inexperienced workman can handle
it, and there is so little that can go wrong that the most careless
attendant cannot damage it.

In the bottom of the boxes there is placed some mercury, for which gold
has a curious affinity. If a particle of gold once gets into contact
with the surface of the mercury it will not get away again easily. Thus
the mercury catches and holds many of the gold particles which are
liberated when the rock is broken up.

As it reaches the required fineness, then, the crushed rock escapes from
the stamp machine and flows away in the stream of water, and although
much gold is caught by the mercury, it is by no means all. The stream is
therefore directed over tables formed of copper sheets coated with
mercury, so that additional opportunities are given to mercury to catch
the grains of gold. Moreover, the table, which, by the way, is placed at
a slight incline, is broken at intervals by little troughs of mercury
called riffles, which assist in the depositing and catching of the metal
particles.

But even then all the gold is not captured. The crushed rock is now like
sand, and some of the grains still contain gold, which has not been
detached by the crushing. The gold, however, makes such grains slightly
heavier than the others, and because of that they can be separated. The
old way is to use a blanket table, a table, that is, covered with coarse
flannel or baize, the hairs of which catch these heavier particles as
the water stream carries them along, the lighter particles escaping. The
grains so caught form what are known as "concentrates," since in them
the gold is concentrated.

The concentrates are subsequently treated as we shall see later.

Now we can see how modern scientific methods have supplemented the old
ways. Take first the case of the stamp mill or stamp battery. In spite
of that prime virtue of simplicity which has kept it at work almost
unchanged for centuries, it has its weaknesses, and no doubt for some
purposes crushing mills are better. Of these there are a great variety,
several of which depend for their action upon centrifugal force, or, as
it is more correctly termed, "centrifugal tendency." In these crushing
mills there is a ring, generally of steel, inside which are suspended
one or more heavy iron rollers. The shafts which carry these rollers are
attached by their upper ends to the driving mechanism on the top of the
mill, and when that is set in motion the rolls are carried round and
round inside the ring. Because of the centrifugal tendency, they swing
outwards, pressing heavily against the inner surface of the ring. The
rock is fed in in such a way that the rollers, as they roll round the
inside of the ring, repeatedly travel over it and crush it.

In another type of mill, called the ball mill, the principle is
different. There you have a cylinder of steel which turns upon a
horizontal axis. This cylinder is partly filled with steel balls of
various sizes, and as the mill turns, the rock, being mixed with these
balls, is pounded and broken up. As the mill turns over and over the
balls fall upon the pieces of rock, thus producing a fine powder. Other
mills, again, are but refined editions of the common mortar mill so
often seen where building operations are going on, in which heavy iron
rollers travel over the material to be crushed as it lies in a round
pan.

The blanket table, too, gives place at the modern mine to the "vanner,"
of which there are several varieties. Essentially they are much the
same, and a description of two will serve to give an idea of them all.
Let us take the "Record" vanner.

Imagine a large table formed of wood, the upper surface covered with
linoleum. It is fixed on slides so that it can move to and fro endwise.
It is given a slight slope in the direction at right angles to its
length--that is to say, one edge is a little lower than the other. The
material is fed on at one end, at the higher edge, and naturally tends
to run down and off at the lower edge. It is restrained somewhat from
doing this by the presence of rows of riffles or ridges running
lengthwise. Nevertheless it does in a short time find its way off the
table at the lower end. But all the time that it is at work the table is
being slidden backwards and forwards on the slides. By a simple but
curious mechanism it is arranged so that it moves quickly in one
direction and slowly in the other, with the result that the heavier
particles of sand--those which contain gold--are carried to the farther
end of the table. Thus, as has been said, all the stuff is fed on to the
higher edge and carried down by the water, until it falls off at the
lower edge, but during the journey from edge to edge the peculiar motion
of the table causes the different kinds of sand to separate themselves,
so that the concentrates fall off near one end, and the rest near the
other end.

Another interesting example of ingenuity is the well-known "Frue"
vanner. In this the table is a broad, endless band of india-rubber,
extended upon two rollers, one of which is slightly higher than the
other. The stream of water and crushed ore flows on at the upper end,
and runs down to the lower, the lighter particles being carried down
and dropped off at the _lower_ end, while the heavier rest upon the
band. Meanwhile the turning of the rollers carries the band slowly
along, so that the heavier particles gradually ascend and are carried
over at the _upper_ end. To assist in the separation, the whole concern
is given a side-to-side shaking motion while it is at work.

We have seen so far how the ore is crushed, and the coarser grains of
gold got out of it by the aid of mercury. The mixture of mercury and
gold is termed amalgam, and the process of extracting gold by mercury is
called amalgamation. The gold is actually dissolved in the mercury, and
so when the amalgam has been (as it is periodically) collected from the
plant, it has to be filtered and then evaporated in a retort. The
mercury vapour is caught and condensed back into a liquid, while the
gold is left in the retort. In fact the amalgam is distilled in order to
separate the gold and mercury.

But when all that is done we still have the concentrates from the
vanners, or whatever be used, to deal with. Mercury is useless with
them, for the gold is covered probably with a coating of the other
substances, whatever they may be, with which it has been associated, or
else there is mixed with the gold some substances which make
amalgamation impossible, or at least difficult.

Often roasting is necessary before anything more can be done. If arsenic
or sulphur be present, for example, they interfere with the recovery of
the gold, and roasting will disperse them. So the concentrates are
passed through great furnaces, in which they are heated in contact with
air until these objectionable matters have been oxidised or burnt.

Then finally we come to some process by which the remaining gold is
dissolved out from its admixtures in some solvent liquid from which it
can be subsequently precipitated. This is rather interesting, because it
means that man has adopted, to recover this gold from the ore, the very
method which it is believed nature employed to put it there. As already
said, the latest idea is that the gold was carried into and deposited in
the lodes where it is now to be found by water--that the gold was
actually dissolved in water at the time. But, of course, gold in its
metallic state will not dissolve in water. Salts of gold, however (the
meaning of the term salt, as applied to a metal, has been explained
earlier), will dissolve in water, as every photographer who makes up his
own toning solution knows from experience. Gold will not dissolve in
water, but chloride of gold will. And so the gold must have been carried
to its resting-place as a salt, and converted into the metallic form
after arrival. In the same way, to recover these finest particles of
all, it has to be converted back into a salt; then that salt must be
dissolved and drained away from the other stuff; and, finally, the gold
must be thrown out of solution again in some way. The great example of
this operation is the familiar "cyanide" process.

The word familiar is appropriate to this matter in only one way,
however. Holders of shares in mining companies, for example, may hear
about it repeatedly at shareholders' meetings and in prospectuses, but
very few have any clear idea as to what it is. So I cannot be accused of
telling an oft-told tale if I devote a short space to its consideration.

The combination of one atom of carbon and one atom of nitrogen is called
cyanogen.

If cyanogen be given the chance it will take unto itself an atom of
hydrogen, producing the deadly hydrocyanic or prussic acid.
Alternatively, if potassium be brought into combination with it, there
results potassium cyanide, which, with the assistance of water and
oxygen, can dissolve gold.

In applying this scientific fact to the purpose of recovering gold from
the concentrates, the latter are placed in vats with a weak solution of
the cyanide in water. The time during which they are allowed to remain
depends upon the size of the gold particles. If they be comparatively
large, it stands to reason that it must be longer than if they be
small, for they will take longer to dissolve. After the proper time,
which is found by experiment, the liquid is drawn off, and in some cases
the concentrates are given a second dose to ensure that the gold shall
be thoroughly removed and none left undissolved. If the material being
operated upon be very fine, as it often is, forming what the mining
people call "slimes," then mechanical stirrers have to be used in the
vats to keep the stuff moving, as otherwise the cyanide would not get to
all the particles and some would not be acted upon.

The liquid, having been the appropriate time in the vat, is drawn off,
placed in wooden tanks or boxes, and fine shreds of zinc are added to
it. Discs of sheet zinc are put into a lathe and a fine shaving taken
off them, and it is these fine shavings which are used. Now zinc, as we
know from the fact that it is the essential part in electric batteries,
has very pronounced electrical properties, and it is believed that these
come into play here. At all events the gold becomes deposited upon the
zinc, while the zinc itself is to a certain extent eaten away by the
solution. The result is (_a_) a solution weaker than it was before,
(_b_) the remains of the shavings, and (_c_), at the bottom of the box
in which this process takes place, _a dark mud_. That black mud, on
being heated, produces the bright metallic gold, and the object of the
whole operation is achieved. The solution is then led to another tank,
brought up to its proper strength again and is ready to be used once
more, while the remains of the shavings are used for the next batch of
material to be treated.

In some cases the crushed ore straight from the crushing mill is
cyanided, in others it is simply the remains left over from the previous
amalgamating process which is thus treated. All depends upon the nature
of the material in question.

There are other chemical methods besides the cyaniding, but it is the
chief. It has been found specially useful with the Johannesburg ores,
and to it the South African goldfields owe a great deal of their
success.

There is a more modern form of it, although the whole process is quite
novel, having been introduced only in the nineties of last century. This
development, it is almost wearying to repeat, is electrical. Instead of
the zinc shavings being used to precipitate the gold out of the
solution, the process is electrolytic. A lead anode is used while the
process is carried on in a box the bottom of which is covered with
mercury, which forms the cathode. The precipitated gold is thus
amalgamated, the amalgam being removed at intervals, retorted, and the
gold recovered.

The idea of recovering gold from the waters of the sea is certainly a
most attractive one. To some, it is true, the suggestion may bring
thoughts the reverse of pleasant, for there have been several partially
successful attempts to delude the public with specious promises of vast
dividends to be gathered in the form of pure gold from the inexhaustible
sea. Still, there is something in it, and some day the dreams may be
realised.

The quantity of gold dissolved in sea-water is so small that in 200
cubic centimetres it is impossible to detect it, even by the most
delicate tests known. The quantity needs to be multiplied threefold
before the quantity of gold becomes even detectable, to say nothing of
being recoverable.

A writer in _Cassier's Magazine_, a few years ago, related how he had
actually obtained gold from the water of Long Island Sound. But whereas
he got two dollars' worth, it cost him over 4000 dollars to do it. No
company will ever be floated on results such as that. From the mud of a
creek near New York, however, he did a little better, for there ten
dollars' worth of gold only cost 379 dollars. A company promoter would
still look askance at even that comparatively successful undertaking.

As usual, authorities differ, but there is a consensus of opinion that
in every ton of sea-water there is from one-half to one grain of gold,
besides silver and iodine.

It seems as if the water were able to dissolve that amount and no more.
If, as has been suggested earlier in this chapter, all the gold which is
now found in mines and in gravel beds was carried there in water, it is
probable that the sea obtains its gold from the same original sources,
and that, just as the hot ocean of ages ago carried its burden of gold
in solution, so the colder water of to-day has its share, the cold water
naturally carrying less than the hot did.

It is quite likely, then, that, could we find out how to rob the sea of
its precious metal, it could replenish its store from some secret hoard
of its own. But even if it could not, it would make little difference to
us, since what it holds is far more than we could ever use. Put it at
half-a-grain per ton: there are 4205 million tons in every cubic mile of
ocean, and 300 million cubic miles of water in the ocean. If all the
gold that man has ever handled were to be dissolved in the sea, no
chemist would be able to discover the fact. On the other hand, if that
half-grain per ton which we believe to be in the ocean now were to be
recovered we should have about 40,000 million tons of gold, a prospect
which is enough to make the political economist turn pale with
apprehension.

What is required is some substance which, on being added to sea-water,
will combine with the gold, and then be precipitated--that is to say,
fall to the bottom. The precipitate--that which falls to the
bottom--would need to be heavy, so that it would fall quickly and not
necessitate the water being left standing for long periods. It would
need to be cheap, too, or easily recoverable, so that it could be used
over and over again. And, finally, it would need to be such that the
gold, having been captured by it, could be easily obtained from it.

Given such a precipitant, the process of recovering the gold would be
simple and cheap. Tanks would be formed in sheltered bays and inlets. At
every tide these would be filled, and when full the precipitant would be
added. The tide falling, the water would run out again and leave the
precipitate on the floor of the tanks, whence it could be removed by
scraping. Simple treatment would release the gold from its partner,
which would then be returned to the tanks to act as the precipitant once
more. Thus by simple means, the tide itself assisting, the gold could
be obtained from the sea.

And there is nothing inherently impossible about this suggestion. The
necessary precipitant may exist, awaiting discovery. A large works
operating in this manner would produce, it is estimated, about thirteen
tons of gold per annum. It looks as if it would be a bad day for the
Rand when that discovery is made.

And there is yet another possibility, though less alluring than what has
just been described. The American writer mentioned a little while back
got a better return from the mud of a creek than from the water itself.
In all probability this is due to the action of organic matter carried
down by streams, or in some other way introduced into the waters of the
creek whence the mud was obtained. This organic matter would possibly
have an effect as a precipitant upon the dissolved gold, causing it to
be thrown out of solution and deposited in the mud. Thus the mud around
our shores, and particularly in the creeks and estuaries, may be
potential gold mines whence in time to come we may draw supplies of the
precious metal. The cyanide or some similar process may be needed in
order that we may extract the metal from its enclosing mud, but the time
may not be so very far distant when dredging for gold may be a regular
occupation at, for example, the mouths of the Thames and the Hudson.



CHAPTER X

INTENSE HEAT


Many of the useful and interesting manufacturing processes of to-day are
based upon the intense heat which science has taught the manufacturer
how to produce. Tasks which our forefathers dreamed of, but were unable
to accomplish, are easy to-day because of the facility with which great
heat can be generated. The "burning fiery furnace" "seven times heated"
is as nothing to some of the temperatures which are now obtained in the
ordinary course of things.

The greatest heat of all is that of the electric arc. Two conductors,
generally rods of carbon, are placed with their ends touching, and the
current is turned on so that it passes from one to the other. Then they
are gradually drawn apart. As the gap widens the current experiences
more and more difficulty in passing over this non-conducting gap, and
great electrical energy has to be employed to keep it going. Now that
wonderful law of the Conservation of Energy decrees that no energy can
ever be lost. It can only be changed from one form into another.
Therefore the energy expended upon the arc is not lost, but is converted
into heat. It is that heat, acting upon the small particles of carbon
which are torn off the ends of the rods, which gives us the arc light.

As a matter of fact nearly all artificial light (and natural light too
for that matter[1]) is due to heat. The heat sets the molecules in
violent agitation, which, acting upon the corpuscles in the atoms, sets
them in violent motion too, so that light is often the companion of
heat. Some substances give light more readily than others, under the
influence of heat, and we may reasonably believe that they are those
whose corpuscular arrangements are such that they can be readily
accelerated by the molecular action.

[1] The glow-worm is an example of the few exceptions.

To take a familiar instance, coal-gas is mainly "methane," one of the
many combinations of carbon and hydrogen, and when it is burnt in air
the hydrogen and oxygen combine, liberating heat, which causes the
carbon liberated at the same time to glow. As each methane molecule
breaks up the carbon atoms are thrown out, forming solid particles of
carbon, and it is they really which give the light. It is therefore the
combustible gas heating the solid particles of carbon which forms the
luminous part of the gas flame. The non-luminous part of the flame, near
the burner (I am now speaking of the old-fashioned burner), is the
burning gas before the carbon particles have had time to heat up.

And the old gas flame, as we know, is now being rapidly displaced by the
incandescent mantle, the reason being simply that Von Welsbach
discovered how certain rare minerals gave a more brilliant light when
heated than particles of carbon do. In other words, it is easier to
accelerate the motion of the corpuscles in ceria, thoria and the other
ingredients of the mantle, than it is those of carbon. Consequently,
they sooner reach that degree of agitation which will send forth
electro-magnetic waves of the high frequency necessary to produce the
sensation of light.

For this reason the mantle heated by gas gives as bright a light as the
carbon particles in the electric arc, although the latter are subjected
to a much more intense heat.

But the arc can be, and often is, used as a source of heat, apart
altogether from the light which it gives. In Sweden, for example, where
coal is rare, but water-power plentiful, the power of the waterfalls is
made to smelt iron. Hence the waterfalls are sometimes termed the "white
coal" of that country. Needless to say, it is the ubiquitous electricity
which performs the change from the force of falling water into heat.

The furnaces are in shape much like those in which iron is smelted with
coal--namely, tall chimney-like structures at the bottom of which is the
fire. In the "arc furnaces" there are, passing in through the side, near
the bottom, a number of electrodes, and between these a series of arcs
are formed. Coke and ironstone are thrown in from the top into this
region of intense heat, and there the iron is liberated from the oxygen
with which it is combined in the ore. Liberated, it flows out through a
spout at one side of the furnace.

But the question will arise in the reader's mind: Why is coke needed in
an electric furnace? It is for metallurgical reasons. The heat of the
arc loosens the bonds between the iron and oxygen, but it needs the
presence of some carbon to tempt the oxygen atoms away. Therefore coke,
as the most convenient form of carbon, has to be there. It is there,
however, in much smaller quantity than it would be in an ordinary
furnace. It is not there as fuel, but simply as the "counter-attraction"
to draw the oxygen atoms away from their old love.

The arc is also used for welding pieces of iron together, for which
purpose it is eminently suitable, since what is wanted is intense heat
at a particular point. But perhaps the reader will be wondering by this
time what the heat of the arc is. It has been repeatedly referred to as
"intense," but something more definite may be demanded. In theory it is
unlimited. Apply more pressure--more volts, that is--thereby driving
more current across, and the temperature will rise. It is only a
question of making dynamos large enough, and driving them fast enough,
and any temperature is possible. But there are practical difficulties
which limit the degree of heat. One is the melting-point of the furnace
itself. Fire-clay melts at about 1700° to 1800° C. So in a furnace which
has to be lined with fire-clay that is about the limit.

In welding two pieces of iron together, the iron, of course, defines
what the limit shall be. It needs to be heated to "welding heat" and no
more--that is, a little short of melting--so that the parts to be joined
are soft, and, with a little hammering, will join thoroughly together.
If too much heat were to be applied the parts would melt away. But the
heat of the arc can be controlled by simply varying the current, and so
the right heat can be applied at the right place, than which little more
is wanted.

One very simple way of doing this is for the workman to hold one of the
"electrodes"--a rod of carbon suitably insulated--in his hand. The
current is led to it through a flexible wire. The iron itself is made
the other electrode by being gripped in a vice which is itself insulated
but connected to the source of current. Thus on bringing the point of
his rod near to the part to be heated the man causes an arc to be
created there. By moving the rod he can move the arc about, heating one
part more than another, distributing his heat if he wants to do so over
a larger area, or keeping it to a small one, just as he wills. On
reaching the right heat the rod is withdrawn, the arc destroyed, and the
iron can be hammered just as if it had been heated in a fire.

Yet another way still is known as "resistance" welding. In it an
enormous current at an extremely low voltage is used. The fundamental
principle is the same, since the heat is formed by forcing current past
a point over which it is reluctant to pass. That point of poor
conductivity is the ends of the two bars to be joined. They are placed
just touching, but since an imperfect contact like that always offers
considerable resistance to the flow of a current, the passing current
needs only to be made large enough for great heat to be generated.

This is exceedingly pretty to watch. We will suppose that the article to
be operated upon is the tyre of a wheel. The bar of iron has already
been bent by rollers into the correct curve and the two ends are
touching. Brought to the machine, it is gripped, each side of the
junction, in the jaws of an insulated vice and the current is turned on.
In a few seconds the place where the two ends are just touching begins
to glow. Rapidly it increases in brightness until in about half-a-minute
it is at welding heat. Then one vice, which is movable, is forced along
a little by a screw, so that the ends are pressed firmly together, a
little judicious hammering meanwhile helping to complete the job. Then
the current is switched off and the complete tyre taken out of the
machine. The current used has a force comparable with that which
operates domestic electric bells, but in volume it is thousands of
amperes. Alternating current is used, and it is obtained from a
transformer or induction coil. In such a case the primary part of the
coil is made of many turns of fine wire, so that little current passes
through it, while the secondary part is but one or two turns of thick
bar. Thus the voltage generated in the secondary is very little, but
since the secondary has an almost negligible resistance the current
caused by that small voltage is enormous. Such an arrangement is in
industrial realms generally called a transformer, the term induction
coil being employed more for those things of a similar nature intended
for the laboratory. The one just described is, moreover, a "step-down"
transformer, since it lowers the voltage, to distinguish it from
"step-up" transformers, which raise the voltage.

And the "resistance" principle is also applied in another way to large
furnaces, such as those for refining iron. In these the resistance of
the iron itself is utilised to generate the heat. Of course, it should
be well understood, heat is always generated in everything through which
current flows. There is no perfect conductor, and so every conductor is
more or less heated by the passage of current through it. Some energy
needs to be expended to drive current, even along large copper wires,
and that energy must be turned into heat in the wires. If the same
volume of current be forced along iron wires of the same size, the heat
will be greater, since iron is but a poor conductor compared with
copper, the relation being about as one to six. And if the iron be hot
the resistance will be still more, for it stands to reason that when
heated the molecules, being farther apart, will be the less easily able
to exchange corpuscles. We have the best reasons for believing, as has
been suggested already, that a current of electricity is but a flow of
corpuscles, and so we are not surprised to hear that, as a general rule,
the hotter a thing is the less does it conduct electricity.

[Illustration:

_By permission of Cambridge Scientific Inst. Co., Ltd., Cambridge, Eng._

                   MEASURING HEAT AT A DISTANCE

This wonderful instrument, the Fery Radiation Pyrometer, although itself
some distance away from the furnace, is telling the temperature of its
                           hottest part.]

So imagine a circular trough of fire-clay or other heat-resisting
material filled with fragments of iron, or, it may be, with iron barely
above melting-point, which has come from another furnace, where it
underwent the previous process. Circling inside or outside this trough
is an enormous coil of wire through which currents of electricity are
alternating. That is the "primary" of a transformer, and the "secondary"
is--the iron itself, in the trough. If it be, as it often is, in the
form of scrap, or broken pieces, the heat will begin to show itself
where the pieces touch each other. The currents generated in the trough,
by the coil outside, will, of course, pass from piece to piece and the
points of contact, since they offer the greatest resistance, will show
signs of heat. This will increase until the pieces begin to melt. As the
separate fragments merge into the molten mass the resistance will in one
way decrease, for the imperfect contacts between the pieces will give
place to the perfect contact throughout the mass of liquid metal. But
for another reason--namely, the increase in heat--the resistance will
increase. And all the while the alternations in the primary coil will be
pumping currents, as it were, round and round the ring of molten iron.
Whether the resistance increase or decrease, the current will do the
opposite, so that heat will be generated whatever happens. For as
resistance decreases current increases, and vice versa. And the
slightest variation in the strength of the primary current will have its
effect upon the secondary, and therefore on the heat generated. So, by
simply regulating the primary current, the temperature of the metal can
be controlled to a nicety. And such furnaces have the immense advantage
that there is no possibility of deleterious substances in the fuel
getting into and spoiling the metal, a thing which may very easily
happen during the manufacture of high-class steels, alloys of iron in
which the exact quantities, purity and proportions of the ingredients
are of the utmost importance.

Hence these "induction furnaces," as they are called, are frequently
used quite apart from any question of utilising water-power. And they
will probably be used still more as time goes on.

For one thing, they may become valuable adjuncts to the older form of
iron and steel furnaces, from which they will obtain their power free,
gratis and for nothing. In districts such as Middlesbrough they could
generate more electricity than they have any use for. The ordinary iron
furnaces belch forth flames which are really good useful gas (carbon
monoxide) burning to waste. Many of the furnaces are covered in at the
top, and this gas is led away to heat boilers for the steam-engines or
to drive large gas-engines, but in a large works there is more of this
waste gas than they know what to do with. Now that could, and probably
will ere long, be turned into electricity by means of gas-engines and
the current used for making steel in induction furnaces.

It will probably surprise many to know that these enormous currents
which can thus heat great masses of metal until they melt are no danger
at all to the men who work with them. A man might dip an iron rod into
the trough of metal and he would scarcely feel the shock. And the same
is true of the welding machine, which can be touched in any part without
fear. The reason, of course, is that, broadly speaking, it is volume of
current which does harm, and the resistance of the human body is so
great that with the small voltages used, the volume which can pass is
negligible. It should be mentioned, however, that the volume of current
in lightning is also small, but we know that it is capable of inflicting
terrible injury. Lightning, however, is in a class by itself. Our
terrestrial voltages are baffled by an air-gap of a few inches, but
lightning springs across a gap miles wide. Its voltage must, therefore,
amount to millions, and the ordinary rules relating to earthly currents
do not apply.

But other sources of heat besides electricity are at the disposal of our
manufacturers nowadays. Pre-eminently there is the flame of some gas
burning with pure oxygen. The oxyhydrogen jet has been known for many
years as the best means of producing the light for a magic lantern. Such
a jet impinging upon a pencil of lime causes the latter to glow with a
dazzling white light.

But the oxyhydrogen jet is now employed in many factories for the
welding of metals. This is known as fusion welding, since the two parts
are actually reduced to liquid. The usual way to go about this work is
to bevel off the ends or edges to be joined. Suppose, for instance, that
we wanted to weld two pieces of brass pipe together. We should first
file or otherwise trim the edges to be joined until when put together
they form a groove practically as deep as the metal is thick. Then with
a stick of brass wire in the left hand, and an oxyhydrogen blowpipe in
the right, we should direct the flame from the pipe on to the metal
until, at one point, the sides of the groove were beginning to melt.
Then, inserting the point of the wire into the groove, we should melt a
little off it. Thus we should work all round the joint, melting the
sides of the groove and filling in with melted metal from the wire,
until the whole groove had been filled up and the metal added had been
thoroughly amalgamated with that on either side.

As a matter of fact, if it were brass which we were working on we should
probably use the cheaper though less pure form of hydrogen--coal-gas--so
that it would really be "oxycoal-gas" that we should use and not
oxyhydrogen. The latter is used, however, notably for the fusion-welding
of lead, or "lead-burning," as it is termed.

The blowpipe is a brass tube about a foot or eighteen inches long, with
two passages in it, one for the oxygen and the other for the other gas.
The gases are brought to one end of it through rubber pipes, while at
the other end there is a nozzle in which the gases mingle and from
which they emerge in a fine jet.

The oxyhydrogen flame has a temperature of about 2000° C., hot enough to
melt fire-clay. That does not matter in the case of welding, however,
since the molten metal is very small in quantity at any given moment,
and is allowed to cool before it can run away. It would be an awkward
temperature to deal with, nevertheless, in a furnace. It seems strange
that it does not burn the nozzle of the blowpipe, but the fact that it
does not is, it is believed, explained by the fact that the expansion of
the gas, as soon as it emerges from the hole out of which it shoots,
causes a comparatively cool space just there, shielding it from the
intense heat farther on.

An exceedingly interesting use of the oxyhydrogen flame is in the
manufacture of artificial rubies. These stones are made in Paris by a
very simple means. The necessary chemicals are prepared and ground to an
exceedingly fine powder. This is then allowed to fall through an
oxyhydrogen flame. Thus there is no need for a crucible capable of
withstanding this high temperature, since the melting takes place as the
particles are in the act of falling. When they reach the support
prepared to catch them they have cooled somewhat. Stones so called are
real rubies--artificial, but not shams. They possess every property of
the ruby from the mine.

Another product of the oxyhydrogen flame is the quartz fibres which are
used for suspending the needles in the finest galvanometers. The quartz
is melted, in this case a crucible being employed. An arrow is then
dipped in the liquid quartz and immediately "fired" into the air. The
thick treacly liquid is thus drawn out into a thread of such fineness
that a microscope is necessary to find it with.

Hotter even than oxyhydrogen is the oxyacetylene flame, which at its
hottest point reaches nearly 3500° C. The gas, which is another of the
combinations of carbon and hydrogen (its molecules containing two atoms
of each), is easily made by allowing water to come into contact with
calcium carbide. The latter, which is CaC_{2}, is made by heating coke
and lime together in the intense heat of an electric furnace. This
accounts largely for the great heating power of acetylene, for since
great heat is necessary to cause the elements to combine great heat is
given out by them when they ultimately separate. Here again is the
conservation of energy. The heat energy of the electric furnace is
largely expended in forcing these two elements into partnership. They
are, as it were, given a large amount of capital in the form of heat. It
ceases to be sensible heat, becoming latent in the compound, but still
it is there. So a lump of calcium carbide, with which many readers are
familiar, has vast stores of heat locked up within it. When water comes
into contact with the carbide the partnership is broken, but the heat is
not liberated then, since another partnership is formed, which still
retains the old heat-capital. The calcium in the carbide is displaced by
the hydrogen from the water, and so C_{2}H_{2} comes into being, while
the rejected calcium consoles itself by entering into combination with
the equally forsaken oxygen from the water, forming CaO, which is but
another name for lime.

Then the acetylene (C_{2}H_{2}) is mixed with oxygen in the blowpipe and
burnt, under which conditions the pent-up heat, borrowed originally from
the electric furnace, is brought into play. With this flame the harder
metals can be fused and welded. Wrought iron, cast-iron, steel in all
its forms, all can be melted by the oxyacetylene flame, almost as easily
as snow by a hot iron. The fusion welding of these metals is then
carried on just as already described for brass.

By means of a special blowpipe, wherein an excess of oxygen is
introduced at the hot point, hard steel plates can be cut to pieces
almost as easily as a grocer cuts cheese. Even thick, hard armour-plate
can thus be cut, almost the only way, indeed, in which it can be cut.

And for purposes such as welding and cutting this flame has an
interesting and peculiar advantage over all other kinds of heat. When a
metal is heated in the air there is usually trouble from oxidation. The
domestic poker, for example, after it has been left to get red-hot in
the fire is seen to be coated, in the part which has been heated, with
scales which will flake off if the thing be struck. Those scales are
oxide of iron, caused by the union of iron and oxygen when the poker was
hot. But if the heat be applied by the oxyacetylene flame that will not
happen. The oxygen and the carbon from the acetylene will burn, and if
the supply of the former be properly regulated it will be entirely used
up in the process. The hydrogen from the acetylene is, strange to say,
unable to unite with oxygen at such a high temperature as that of the
oxygen and carbon, so that it passes on beyond the oxygen-carbon flame
and ultimately burns on its own account with the oxygen from the
atmosphere in a second flame surrounding the first. Thus there is a
double flame: inside, a little pointed cone of white flame, that is the
oxygen and carbon; and outside that a bluish flame, the hydrogen and the
atmospheric oxygen. The latter flame forms a kind of jacket entirely
enveloping the former. And so when one melts metal by means of the white
cone the hydrogen jacket shields the molten metal from oxygen and
prevents the oxidation. Only one who knows the bother caused by
oxidation whenever metals are heated can realise the wonderful advantage
of this.

And now we can turn to even another source, also quite modern, of high
temperature.

If the oft-quoted "man in the street" were asked the two commonest
things on earth he might possibly name oxygen as one, and so far he
would be right, but the chances are much against his naming aluminium as
the second. If he did not, however, he would be wrong. Aluminium and
oxygen form alumina, of which are constituted the sapphire, the ruby and
other precious stones, but alumina is most commonly found in combination
with silica, or silicon and oxygen. This compound is called silicate of
aluminium, and of it are formed clay and many rocks. The reason why the
metal aluminium was until recently rare and expensive was because of the
great difficulty of disentangling the metal from this rather complex
combination. And these two commonest elements have, under certain
conditions, a rare affinity for each other. They join forces with such
energy that great heat is given out in the process. This, again, we may
regard as an example of the conservation of energy. Heat had to be used
up, apparently, in separating the aluminium and oxygen as they were
found together in the natural state. And that heat reappears when they
combine together again. This is a most useful principle, for if heat has
disappeared anywhere in the course of some operation, we know that in
all probability, if we go about it the right way, we can get that heat
back again, perhaps in a more convenient form. That is so in this case
at all events.

Now aluminium will not readily combine with atmospheric oxygen, but it
will readily do so with oxygen from the oxide of a metal. So if we put
into a vessel some oxide of iron and some finely powdered aluminium, and
give it some heat at one point, just to set the process going, the whole
mass will burn with intense heat. And when the burning is finished the
crucible will be found to contain (1) some molten iron, the oxide of
iron with the oxygen gone, and (2) some oxide of aluminium or alumina,
in the form which we call corundum, a very hard substance which in a
powdered form is used for grinding hard metals. We start, you will
notice, with a pure metal and an oxide. We finish with a pure metal and
an oxide, only the oxygen has changed its quarters, having passed from
the iron to the aluminium. And in the course of the change a vast amount
of pent-up heat has been liberated. Aluminium is thus a fuel, strange
though it may seem to say so, just as coal is. Coal, however, is willing
to pair off with oxygen from the air, while aluminium, more fastidious,
will only accept it as partner when it can steal it from another
combination.

But the practical result is eminently satisfactory, for the action of
the aluminium and iron oxide is to leave us with a crucible full of
molten iron at a very high temperature. And this can be used in various
ways.

Tramway rails, for example, can be joined together by it. A mould is
formed around the ends of two rails, where they "butt" together, and
into this mould a quantity of the melted iron can be poured. So hot is
it that it partially melts the ends of the rails, and then, amalgamating
with them, it forms a perfectly homogeneous connection between them.

The same method can be applied to the repair of iron structures of all
kinds. The propeller shaft of a ship, for example, sometimes breaks on a
voyage. Such a catastrophe is fraught with the most serious
consequences, unless it can be quickly repaired. Thermit, as this
process is called, is perhaps the only means whereby, under certain
conditions, this can be accomplished.

The extraordinary heat of the metal produced in this way is demonstrated
by the fact that if it be poured on to an iron plate an inch thick it
goes clean through it. It melts its way through instantly.

But although such high temperatures are at the command of the modern
manufacturer, there are some things--indeed many things--which still
baffle him, the diamond, for example. It is true that diamonds of small
size have been made, but larger ones have so far defied all efforts.

One very interesting fact about this may be mentioned in concluding this
chapter. Sir Andrew Noble, a member of the great firm of Armstrong,
Whitworth & Co., of Elswick, tried the experiment of exploding some
cordite, a high explosive, inside a steel vessel of enormous strength.
He thus produced what is believed to be the highest temperature ever
produced on earth. It is reckoned to have been 5200° C., and the
pressure at the same time was, it is calculated, 50 tons per square
inch. His intention was not to make diamonds, but Sir William Crookes
predicted that diamonds would be the result. For the cordite consisted
mainly of carbon, which, as is well known, is the material of which the
diamond is formed, and the combination of high temperature and high
pressure is just what is needed, so it is believed, to bring the carbon
into this particular form. And true enough, on the iron being examined
after the explosion, there were seen tiny diamonds. For larger ones even
higher temperatures and greater pressures are, no doubt, necessary, and
as the diamond, like gold, has a peculiar fascination for mankind, so
the efforts to manufacture it will continue. In years to come the means
may be found of creating these extreme conditions of temperature and
pressure, and so another of the problems of the ages will be solved.

[Illustration:

              _By permission of the British Aluminium Co_

             A STRIKING FEATURE OF MODERN ALUMINIUM WORKS

For the production of aluminium water power is required. Water is stored
at a high level and is then brought down to the factory in pipes. The
illustration shows the pipe track recently laid down for this purpose at
Kinlochleven in Argyleshire. The six pipes, each of which is thirty-nine
inches in diameter, run down the hillsides for one mile and a quarter]



CHAPTER XI

AN ARTIFICIAL COAL MINE


Those countries which are blessed with a plentiful supply of coal are
periodically shocked and saddened by a terrible calamity--an explosion
in one of the mines, in which often scores of poor fellows lose their
lives, and hundreds of widows and orphans find themselves without a
breadwinner. One has only to recall that heart-rending calamity of the
Courrières mines in France, where over a thousand lives were lost, to
realise how important is the question of the cause and the cure of the
colliery explosion.

It used to be thought a settled matter that these were due to the
accidental ignition of a gas called, scientifically, "methane," but by
the miners "fire-damp." This undoubtedly does collect in many mines, and
since it is much the same as the domestic coal-gas (indeed methane forms
the bulk of coal-gas) it is not surprising that the explosions were
attributed to it. At times shots were fired, to blast down the coal, and
although the greatest precautions are taken to prevent any accident
resulting, it seems certain that explosions have occasionally followed
the firing of shots. But still more dangerous is the adventurous miner
who, for some reason, opens his safety lamp. It is lit for him before he
enters the workings, and locked up, so that, theoretically, he cannot
tamper with it; but it has to be a cleverly devised lock that cannot be
picked in some way, and with the carelessness born of long immunity from
accident these are got open sometimes, with, it may be, disastrous
results.

Even a spark struck from a miner's pick may ignite the gas; or a spark
from some electrical machine used in the mine. That is one of the
reasons why electrical apparatus is suspect in colliery matters and
machines worked by the less convenient and more costly means of
compressed air are preferred.

In some such manner the fire-damp is ignited, and then there follows the
fiery blast, which, sweeping through the narrow galleries and passages
which constitute the workings, simply licks up the life of the men whom
it encounters. Others, in byways and sheltered corners, escaping the
burning cloud of flame, are poisoned by the deadly fumes of carbon
monoxide which it leaves when its force is spent. While others,
perchance the most unfortunate of all, are saved for a time, but, being
imprisoned by falls from the roof and walls, die a lingering death of
hunger and slow suffocation. A colliery explosion is one of the
ghastliest events imaginable, the only relief from which is the noble
heroism with which the survivors, from the mine managers to the humblest
workmen, crowd round the pit-mouth, eager to risk their own lives for
the faint chance of saving some below. Not infrequently these brave
volunteers only share the fate of the men they would rescue.

Now all that, as I have said, used to be put down to the effect of the
fire-damp. But it dawned upon men's minds some years ago that the damage
seemed to be out of proportion to the power of the gas. Modern mines are
well ventilated by large fans, which impel great volumes of air through
all the workings. The air currents are cunningly guided by partitions or
"brattices," so that every nook and corner shall be scoured out by the
plentiful draught of pure fresh air. Consequently the amount of
explosive gas which can collect in any one place is but small. How,
then, can so small a volume of gas do so large an amount of damage?

Coupled with this was the fact that explosions take place in flour
mills, where there is no gas, and experimenters had found in their
laboratories that almost any burnable substance, _if ground up finely
enough_ and blown into a cloud, would explode. Coal-dust would naturally
do this. Indeed anyone throwing the dust from the bottom of the
coal-shovel upon a fire will see for himself how, quickly such dust
will burn, and, as has been pointed out in an earlier chapter, an
explosion is but rapid burning.

So the blame was largely transferred from the shoulders of the fire-damp
to those of the clouds of coal-dust which collect throughout the
workings of a mine.

But then a difficulty arose from the fact that there is dust in all
mines, yet some districts are quite free from explosions. And such
districts are those where there is little or no fire-damp. These two
facts seem to be explainable in one way, and in one way only. It must be
that the gas first of all explodes feebly, and so, stirring up the dust
lying along the roads and passages, prepares the way for the powerful,
deadly explosion of coal-dust which follows.

But that was only a guess, and the matter was of such importance that it
needed something more certain than mere assumption. So the Mining
Association of Great Britain decided to have a series of experiments
which should settle once and for all what part the coal-dust played in
these catastrophes, and how best they could be prevented.

It was at first thought that an old mine might be utilised for the
experiments, but there was the difficulty that such always become wet
after work has ceased in them, and so the dust would not behave
normally. Moreover, the work would be extremely dangerous and the
results difficult to observe. Then a culvert was suggested built of
concrete, partly buried in the ground, but that too was dismissed.
Finally it was decided to make an imitation mine of steel, using old
boiler shells with the ends taken out.

The sum of £10,000 was subscribed for the purpose by the coal-owners of
Great Britain, and the great work was carried out at Altofts, in
Yorkshire, close to a colliery where a terrible disaster occurred in
1886.

Here the great tube or gallery was built. Roughly the shape of a letter
L, one leg is over 1000 feet long, while the other is 295
feet. The longer leg is 7-1/2 feet in diameter and the shorter 6 feet.
At the end of the shorter part a large fan is installed which can force
50,000 to 80,000 cubic feet of air per minute through the structure, so
producing the conditions of a well-ventilated mine. The shorter length
has several sharp turns in it for the purpose of breaking the force of
the explosion along that part, and so shielding the fan from damage,
while a tall chimney is provided there, so that, the door being shut to
cut off the fan, the gases from the explosion can find a harmless way
out.

Inside the tube, shelves are fixed along the sides so as to reproduce
the effect of the timbering in a real mine, upon the beams of which the
dust finds lodgment. Props were put up too, just as they would be in the
real mine. Everything, in fact, was done to make the place as perfect a
replica as possible of actual underground workings.

And then, added to this huge and costly structure, was an outfit of
scientific instruments worthy of the important investigations which were
to be carried on.

To grasp the purpose and working of these we need to remind ourselves of
the aims and intentions of the experiments. First of all it was desired
to find out how various quantities and qualities of coal-dust behaved.
The dust was laid along the floor of the tube and along the shelves. A
small gun fired at some point in the tube raised a cloud of this dust
just as the gas explosion in the real mine would do. Then another gun
was fired to explode the dust-cloud. So far all is quite simple and
easy. But to do that would be of no value without the means of finding
out exactly what resulted from the explosion. And that is the function
of the instruments.

To commence with, there is the great wave or tide of force or pressure
which surges along the gallery immediately the cloud bursts into flame.
How fast does that wave travel? How long is it after the explosion
before the shattering effects of it are felt a hundred yards away? To
solve that problem electrical contact-breakers are fixed at intervals of
fifty yards along the gallery. Each of these consists of a cylinder with
a piston inside it something like, shall we say, a cycle pump. The
piston, held down normally by a spring, is blown upwards by the force of
the explosion. The spring is adjustable, and so it can be arranged that
the feeble force of the gun cannot lift the piston, but the more
powerful coal-dust explosion which follows can.

Thus when the explosion takes place these contact-breakers are operated
in succession. The one nearest the seat of the disturbance is operated
first; next the one fifty yards farther away; then the one a hundred
yards away, and so on. The moments when they work will tell the speed at
which the blast travels along the gallery. But it travels with great
speed, and so to measure and record the exact moment when each
contact-breaker is moved is a matter of no little difficulty.
Electricity, however, makes this, like so many other things,
comparatively easy.

There is an apparatus used in astronomical observatories called a
chronograph, which registers, within a small fraction of a second, the
moment when a star seems to pass across a wire in the "transit circle,"
the telescope by which the positions of stars are determined and the
exact time kept. The observer sits with his eye to the telescope,
watching the apparent movement of the star. In his hand he holds a small
"push," pressure on which by his fingers operates a minute pricker,
which acts upon a moving strip of paper. The paper travels along with
the utmost steadiness and regularity, while a clock drives a sharply
pointed pricker on to it every two seconds. Thus the clock marks out the
paper into lengths, each of which represents two seconds. But the other
pricker, worked electrically by the observer's hand, also makes its mark
upon the paper, and so, while the regular marks indicate intervals of
two seconds, each irregular one marks the time of a transit or passing
of a star across the wire. An examination of the strip subsequently
enables the times of a transit to be seen with great accuracy, from the
position of the corresponding mark between two of the _regular_ marks.

And the same principle was applied to the circuit-breakers of this
artificial mine. Normally, current flows through the circuit-breaker,
but the lifting of the piston breaks the circuit (whence the name of the
contrivance), and that breaking of the circuit and consequent cessation
of the current operates the chronograph. By a cleverly constructed
device, the details of which are too complicated to set out here, each
circuit-breaker in turn makes its mark on the same strip, so that the
distances apart of these marks show the time taken by the force of the
explosion to travel fifty yards. Meanwhile the clock goes on making its
regular marks (in this case every half-second), so that they form a
scale by which the other intervals can be measured very exactly.

The chronograph used here is more accurate than that in use at Greenwich
Observatory, the reason being that in this case the recording currents
are sent mechanically by the contact-breakers operated by the explosion
itself, while in the case of the astronomer the human element comes in.
To watch a moving speck of light and to tell exactly when it crosses a
fine line is by no means easy, and so to tell the time within a tenth of
a second, is about the limit of possible accuracy. The instrument we
have been referring to, however, can register the time which a gaseous
wave moving 3000 feet per second takes to travel fifty feet. In other
words, the circuit-breakers can be operated so fast that when only a
sixtieth of a second intervenes between the action of one and that of
the next the chronograph can duly record the fact.

The records of the chronograph can be made in two ways: one by a pen on
a piece of paper tape, and the other by a scratch on a piece of smoked
paper.

So by that means the progress of the "force" of the explosion can be
measured. It is necessary also to time the movement of the "heat" of the
explosion, for the two may not travel together, and the difference
between them may let in some light as to the nature and behaviour of the
explosion. So for this second purpose a second set of circuit-breakers
are used. Each of these consists of a strip of thin tinfoil stretched
across the gallery. Being placed edgeways to the moving current of gas,
the force of the explosion has no effect upon it, but the heat instantly
melts it. Normally, current flows through the strip, and so the melting
is signalised by the cessation of the current, which event is recorded
by the chronograph.

Thus the speeds at which the force and the heat of the explosion travel
are ascertained. Another important fact which needs to be found is the
amount of the force, or the pressure, at different points. For this
purpose pressure-gauges can be connected to the gallery at the desired
spots by means of flexible tubes. This flexible tube is necessary in
order that the vibration of the steel shell, due to the explosion, shall
not be communicated to the instrument. The pressure, finding its way
along the flexible pipe, raises a piston against the force of a spring,
and the distance to which it is raised forms, of course, a measure of
the pressure inside the gallery at the point to which the tube is
connected. The pressure is recorded by the action of the piston in
moving a style which just touches against the surface of a moving paper.
There are three styles in all marking this paper. The first is the one
just mentioned. The second is held down on to the paper by an
electro-magnet energised by current flowing through a fine wire
stretched across the gallery just where the explosion originates. This
fine wire is broken at the moment of the explosion, whereby the current
is cut off and the style raised. It therefore makes its mark until the
moment the explosion occurs, and then leaves off. The end of that line,
therefore, shows the time of the explosion. Meanwhile the first style is
drawing a straight line, but as soon as the pressure begins to be felt
by the pressure recorder this style moves and the line slopes upward.
Upward it goes as the pressure increases, until it has reached its
height, after which it descends, until the style is drawing a straight
line once more. Thus the rise and fall of the line represents the rise
and fall of the force of the explosion.

Then comes the matter of time. How soon after the explosion occurred did
the pressure begin to be felt? How long did it take to reach its maximum
and how long to die out again? These questions need answers which the
apparatus so far described does not give. True, the speed of the paper
may be known approximately, but all that I have described will occur
within the space of a fraction of a second, and it is difficult to tell
the speed of the paper with sufficient accuracy. Therein we see the
purpose of the third style. It is attached electrically to the
"tenth-of-a-second time-marker." This consists of a weight suspended at
a height. The force of the explosion lets it drop. The moment it starts
to fall it causes the style to make a mark on the paper. When it has
fallen a certain distance the style makes another mark. And the distance
that the weight falls between the making of the two marks is so adjusted
that the space between them on the chart represents exactly a tenth of a
second. Thus a scale is formed upon the chart by which the other times
can be measured. There is the line terminating at the moment of
explosion; the straight line changing into an up-and-down curve,
representing the time and the variation of the pressure; finally there
are the two marks representing a tenth of a second by which the other
marks recorded upon the chart can be interpreted.

But the mere pressure and velocity of the explosion form but a part of
the knowledge desired. How the explosion is formed, whether or not the
coal-dust is burnt up entirely, whether, indeed, it be the dust itself
which burns or coal-gas given off by the dust under the heat of the
preliminary explosion, what the gas is which is left by the explosion at
various stages--these are important things to be known, and they can
only be ascertained by taking samples of the gases in the gallery at
different moments during and after the explosion. To obtain these
samples bottles are used, but the question is how to get them filled at
just the right time. Into the shell of the gallery holes are drilled,
and to these the metal bottles or flasks are screwed, a pipe leading
from the mouth of each bottle well in towards the centre of the gallery.
The end of this tube is closed by a cap of glass above which there
stands poised a little hammer. Controlling the hammer is an electrical
device called a "contact-maker," so arranged that just at the desired
moment the hammer falls, breaking the glass, and admitting a sample of
the gas in the gallery, the bottle and its tube having previously had
the air exhausted from them, so that on the glass being broken the gas
is sucked in.

At the same moment a weight falls, attached to the end of a cord, and
this, on reaching the end of its tether, closes the end of the tube, and
the sample is imprisoned until such time as the bottle can be
disconnected and taken away to the laboratory for its contents to be
analysed.

The contact-makers are of two kinds. In one the pressure of the
explosion raises a piston which completes a circuit allowing current to
flow through the very fine wire which prevents the fall of the hammer.
This fine wire being fused by the current, the hammer falls and does its
work. The other kind, which are used when the force of the explosion is
not enough to raise a piston, is operated by one of the tinfoil
circuit-breakers. A magnet, being energised by current passing through
the foil, holds up a curved bar over two cups of mercury. Broken by the
heat of the explosion, the foil cuts off this current, de-energises the
magnet, and allows the bar to fall with its ends in the mercury. This
completes another circuit, permitting current to pass to the fine wire,
whereby the hammer is released. By connecting a bottle to a
contact-maker at a distance the sample can be obtained at any desired
period of the explosion. If, for instance, the sample is to represent
the immediate products of combustion, it is placed near to the
contact-maker. Then the sample is drawn in practically at the moment of
explosion. If, on the other hand, it is the after-damp that is to be
sampled, then the bottle would be connected to a contact-maker a long
way from the seat of the explosion, with the result that its glass cap
would not be broken until some considerable time had elapsed after the
explosion has passed the bottle. The time also during which the bottle
is drawing in its sample can be adjusted by varying the length of the
cord to which the weight is attached.

And last of all must be mentioned the employment of a kinematograph,
capable of taking twenty-two photographs per second, for observing the
effects at the ends of the gallery (see illustrations).

Thus records are obtained of the force and heat of the explosion, its
mechanical and thermal effects upon the walls of the gallery, or, if it
were in a real pit, the effects which it would have in shaking and in
heating the workings, and the men labouring in them. This and the
analysis of the gases producing and produced by the explosion, derived
from the contents of the bottles, give sound data upon which can be
built up reliable theories as to the nature of colliery explosions and
the way to prevent them, results which could be obtained in no other
way. No one can help being struck with the thoroughness and ingenuity of
the means adopted to these ends, and it is no exaggeration to say that
it is a splendid example of thoroughly scientific methods applied to an
important industrial investigation. It will be interesting to conclude
this account with a brief mention of some of the results to which these
painstaking efforts have led.

First in importance the fact is placed beyond doubt that coal-dust,
which in bulk will only burn slowly, will, when well mixed with air,
explode. And no combustible gas need be present to aid in the explosion.

The dust-raising gun, by blowing some dust into a cloud which was
ignited by the second gun, caused an explosion powerful enough to do all
the damage experienced in the most disastrous natural explosions. So it
is practically certain that the function of the gas is but that of the
first gun, to raise the cloud of dust.

A typical experimental explosion may be briefly described. On the
cloud-raising gun being fired a small cloud of dust was driven out of
the ends of the gallery, even that end at which the fan was blowing air
_in_. In other words, the current of air was checked, even reversed, by
the preliminary shock. This cloud was, of course, shown by the
kinematograph.

Then when the second gun was fired, and the real coal-dust explosion
occurred, there was first a cloud of dust shot out larger than the other
one, to be followed by a cloud of flame 180 feet long. These also were
recorded by the kinematograph. The sound was heard seven miles away.

Pressures as high as 92 lb. per square inch were recorded, and the force
of the blast was found to travel well over 2000 feet per second.

In many cases, strange to say, the effects were very slight at the seat
of the disturbance, the force seeming to increase as the wave travelled
along the gallery. Probably the dust had not time to burn completely but
only partially at the first onset. Where props or timbers checked the
flow of the flaming gases there the damage was most, for no doubt the
eddies caused the air and coal to be particularly well mixed at such
points. An encrustation of coke was found on the sides and the timbers
after all was over, probably because there was not sufficient air to
burn all the dust, and some was only heated into coke to be deposited on
the nearest surface, where the tarry matters would make it stick.

Finally, the most important, perhaps, of all, it was demonstrated that
an admixture of stone-dust with the coal-dust made it non-inflammable.
If a small zone were treated in this way, stone-dust being mingled with
the other, the explosion became stifled at that point. True, the
poisonous after-damp swept on beyond, so that men there might have been
poisoned by it, but the stone zone would certainly save them from the
direct effects of the blast. If, however, stone-dust be mingled with
coal-dust all along the gallery, then no explosion at all would occur,
again proving that it is the coal-dust which does the damage.

In the colliery adjoining the experimental gallery this plan had been in
use for years. Soft shale is ground to fine powder, and is sprinkled
wherever coal-dust has collected. It is just strewn by hand, giving the
workings the appearance of having been roughly whitewashed. And since
that has been done there has been no explosion in that pit. The
experiments showed beyond doubt that that was no chance occurrence. They
showed that in some way not thoroughly understood this addition of
stone-dust renders the coal-dust harmless. It may be that it merely
dilutes it. It may be that in some way it takes some of the heat and so
prevents the coal particles becoming hot enough. It may be that, being a
little heavier, it checks the formation of the dust-cloud. However that
may be, there is no doubt now that stone-dust is the salvation of the
miner so far as explosions are concerned.

Water sprinkled upon the coal-dust, by laying it and keeping it from
forming a cloud, has the same effect, but it is less convenient, for the
simple reason that water evaporates, while stone-dust stays where it is
put.



CHAPTER XII

THE MOST STRIKING INVENTION OF RECENT TIMES


Probably no invention has made such a sensation during recent years as
wireless telegraphy. And since it is the direct outcome of the most
abstruse, purely scientific investigations, there could be no more
appropriate subject for a place in this book.

For many years there has been a belief in the existence of a mysterious
something to which has been given the name of "The Ether." Totally
different, it should be noted, from the chemical of the same name, it is
entirely a creature of the intellect. None of our senses give us the
slightest direct indication of its existence. No one has either seen,
felt, heard, smelt or tasted it. Yet we feel that it must exist, for the
simple reason that some things which our senses do tell us of are
utterly inexplicable without it.

It was originally thought of in connection with light. Standing at night
upon the top of a hill, we see the lights of a town a mile away. How is
it that those distant gas or electric lamps affect our eyes? They are a
mile away; and the idea that one object can affect another _at a
distance_ is one which the human mind refuses to accept. We feel
compelled to believe that there is something in contact with the source
of light which is affected first, and through which the disturbance,
whatever it may be, is conveyed to our eyes, with which it must also be
in contact. We feel that there must be a something stretching from our
eyes to the distant objects, by which the light is carried. Of course
the air fills the space referred to, but that cannot be the carrier of
light, for if we look through a glass vessel from which the air has been
exhausted we see distant objects undimmed. We also have good reason to
believe that the air belongs specially to our globe, and does not extend
upwards for more than a few miles. Consequently it cannot be air which
brings sunlight and starlight. We are forced to fall back, therefore,
upon the belief in something, of which we have no other knowledge, which
must fill all the vacant spaces in the whole universe, passing, even,
between the particles of which ordinary matter is composed, reaching as
far as the remotest star, able to penetrate everything, and consequently
not excludable from the most perfect vacuum. It is something so
different from anything of which we have any direct knowledge that one
is tempted sometimes to doubt whether there must not be some other
explanation of light. In order to transmit light at the speed at which
we find that it does in fact travel, the ether must be more rigid than
the hardest substance we know of. Many, many thousand times more rigid,
indeed. Yet it seems to offer no resistance to the passage of the
planets through it. Still, there is no other alternative, so far as men
can conceive, and we are compelled, therefore, to believe in the
existence of the ether.

The first things discovered by the telescope were the larger satellites
of Jupiter. With that precision for which astronomers are noted, they
soon drew up time-tables, showing not only the past movements of these
bodies, but also their future ones. They were soon puzzled, however, by
the obvious fact that the moons of Jupiter were not working according to
schedule, to use a railway expression. They got later and later for a
time, and then gradually quickened up until they got too fast. Then they
slowed down again. This repeated itself, and is going on still, with
this difference, however, that the cause has been discovered and the
schedules amended accordingly. The solution of the puzzle was that when
the earth and the great planet are on the same side of the sun they are
some 186 millions of miles nearer together than when they are on
opposite sides of the sun. The evolutions of the satellites are quite
regular, according to the astronomers' calculations, but they seemed to
the earthly astronomers to vary, because of the time which light took to
traverse that 186 millions of miles. When the two bodies were nearest
together the occurrences seemed to happen about 1000 seconds (16
minutes) earlier than when they were farthest apart. Consequently it
became evident that light took 1000 seconds to travel 186 million miles,
or that, in other words, it moved at the prodigious speed of 186
thousand miles per second. That discovery was, of course, many years
ago, but experiments since have proved the figure mentioned to be about
right.

It put beyond question the fact that the action of a distant light upon
the eye was not an "action at a distance," for such action, were it
possible, would take effect at once. Seeing that light passed from the
distant satellites at a definite velocity, and took a certain time to
reach us, it was evident that it was, during that time, passing through
a medium of some sort, and that medium must be the ether, for no
alternative explanation will suffice.

So it became recognised that light really consists of waves or
undulations of some sort in the ether; that a distant, luminous body set
these waves going; that they travelled with a definite velocity, and
then, striking our eyes, produced the sensation known as light. Many
things were found out about light in the years which followed the
discovery of its velocity. The lengths of the waves were
ascertained--that is to say, the distance from the crest of one to the
crest of the next. The different lengths were sorted out and found to
give rise to different colours, while longer waves, which produced no
sensation of light, were found to carry heat, thereby explaining how the
heat reaches us from a distant fire, or from the sun.

Of the actual nature of the waves, however, little was known, although
there was a vague idea that they were connected in some way with
electricity, at which point in the story there comes in the famous name
of James Clerk Maxwell, a professor of Cambridge University, who in
1864 produced before the Royal Society the explanation of the nature of
the waves and their connection with electricity and magnetism. That in
itself was a wonderful achievement, but far more wonderful still is the
fact that he truly predicted the existence of longer waves than any then
known, which no one knew how to cause, or how to detect if caused. That
prediction has since been fulfilled. The long waves have been found; we
know how to make them and how to perceive their presence. They are the
messengers which carry our wireless messages.

The discovery of these, at that time unknown waves, on paper, by simply
calculating and reasoning about them, is more marvellous even than the
feat of Adams and Le Verrier in discovering a planet on paper before
anyone had seen it. It established Maxwell among the heroes of science
for all time.

A magnet acts upon a piece of iron some distance away. The pull must be
transmitted through some kind of ether. A current of electricity behaves
in the same way, acting precisely as a magnet, with power to affect
things at a distance. Again an ether is necessary. A dynamo works by
moving a magnet past a wire which it does not touch, thereby generating
current in it. There again an ether is necessary to transmit the effect
from the one to the other.

Taking, then, the known magnetic effects of an electric current and the
electrifying effects of magnets, he was able to show that the same ether
accounted for all, and for the transmission of light as well, that, in
fact, there was but one ether which performed all these various duties.

He proved from the known facts about electricity and magnetism that
waves such as he imagined would, in fact, move with the speed of light.
And once knowing the nature of the waves, he asserted that in all
probability there were others of which men had then no practical
knowledge.

Maxwell's theory soon set experimenters searching for the means of
producing the long waves which he had predicted would be found.

Several authorities had before then stated their belief that the current
derived from a Leyden jar was not simply a flow in one direction. They
suggested, and gave grounds for the belief, that the current surged to
and fro for some time before it settled down; that it swung to and fro,
indeed, like a pendulum.

There may be some of my readers who are unacquainted with this
interesting piece of electrical apparatus the Leyden jar. It is a
convenient form of what is called an electrostatic condenser. This is
two conductors, generally in the form of two plates with an insulator
between them. In the Leyden jar the insulator is a glass jar, while the
"plates" are coatings of tinfoil, one inside and the other outside. On
connecting one coating to one pole of a battery, and the other to the
other pole, they become charged, one positively and the other
negatively. One, that is, acquires an excess of electricity, while the
other becomes deficient to an exactly similar extent. When the two are
afterwards connected by a wire the surplus on one flashes through it to
make good the deficiency on the other.

Rushing first of all from positive coating to negative, electrical
inertia causes it to overshoot the mark and to recharge the jar with the
charges reversed. Then current begins to flow back again, doing the same
several times over, until at last equilibrium is established.

The power to absorb and hold a charge of electricity, which is the
characteristic of a condenser, is called "capacity."

What, then, is "electrical inertia"? I have already referred to the
effect which the creation of a magnetic field around a current has upon
neighbouring conductors. It also has an effect upon itself. As soon as
the current begins to flow it builds up the magnetic field, and in the
process some of its energy is exhausted. On the original current
ceasing, however, the magnetic field collapses back on to the conductor
once more and in so doing restores that energy. This occurs whenever
current flows, but it is specially noticeable in long conductors, like
submarine cables. In them the battery has to act for a considerable time
before any current reaches the farther end. It is in the meantime
employed in building up the magnetic field around the wire. Then when
the battery has ceased to act the current still comes flowing out at the
farther end--the magnetic field is giving back the energy expended upon
it. Thus a current is reluctant to start flowing through a conductor,
and, having started, is disinclined to stop. This is called
"inductance," and it has exactly the same effect upon the current that
inertia has upon a body. What inertia is to a material body inductance
is to an electric current.

And lastly, the resistance which the conductor offers to the passage of
the current is precisely analagous to the friction of the water in a
pipe.

So, we see, the "capacity" of the two coatings of the jar and the
inductance which occurs in the connecting wire cause the current to
oscillate to and fro for a while when the jar is discharged, which
surging or oscillation is ultimately stopped by the resistance of the
wire. The two coatings and the wire form what is called an oscillatory
circuit.

We can now resume our story.

After much experimenting Hertz, of Carlsruhe, discovered the fact that
when a discharge was taking place in an oscillatory circuit tiny sparks
passed between the ends of a curved wire held some distance away. His
apparatus is illustrated in Figs. 6 and 7. The former, which is termed
nowadays a "Hertz Oscillator," is simply two metal discs almost
connected by a thick wire. The wire is broken, however, at the centre,
and the two halves terminate in two metal balls. Each ball is connected
to one terminal of an induction coil. Now the current comes from an
induction coil in a series of spurts. It is not an alternating current
exactly (since every alternate current is so feeble as to be
negligible), but is practically an intermittent current always in the
same direction. Thus we may call one the positive end of the coil and
the other the negative. A short current comes along with every backward
movement of the little vibrating arm which forms a part of the
apparatus. This breaking of the "primary" circuit may take place perhaps
fifty times per second, so that the intermittent "secondary" currents
will succeed each other at intervals of a fiftieth of a second, or even
less. The brain reels at the attempt to think of a fiftieth of a second,
but it is really quite a long interval as these things go, and during
that interval quite a lot happens. For the current first of all charges
the two plates as a condenser.

[Illustration: FIG. 6.--The apparatus by which Hertz made his
discoveries, hence called the Hertz Oscillator. _a a_ are metal plates;
_d_ is the spark-gap between the two metal balls; _b_ is the battery,
and _c_ the induction coil.]

When they are as full as they will hold the current overflows, as it
were, across the gap between the two balls.

Now an air-gap--a gap that is filled with air, between two
conductors--is a very strong insulator. But when current has once broken
through it it becomes a fairly good conductor. Hence as soon as the
first spark has passed between the two knobs the plates become connected
almost as if a wire were passed from one to the other. And there we have
quite a good oscillatory circuit. There is capacity at each end and a
fairly long length of wire to provide the inductance. Consequently that
breakdown of the insulation of the air in the spark-gap is followed by
electrical oscillations which take place with inconceivable rapidity.
Yet because of the resistance of the spark-gap, which is considerable
even after it has been broken through, the oscillations do not continue
for long. They have died away long before the lapse of a fiftieth of a
second, when the next impulse comes along from the coil. In the meantime
the air-gap regains its insulating properties, and so, on the arrival of
the next impulse, the whole thing occurs once more.

Thus a little train of oscillations is produced for every impulse from
the coil. Every train causes a corresponding disturbance in the ether,
and sends off a train of electro-magnetic waves, and these, falling upon
the distant wire, generate in it a train similar to that which brought
them into being. These trains, in Hertz' simple apparatus, manifested
themselves in the form of minute sparks leaping across the small gap
between the ends of the curved wire (Fig. 7).

[Illustration: FIG. 7.--Hertz "Detector." It was with this simple
apparatus that Hertz discovered how to detect the "wireless waves."]

It was in 1888 that Hertz made this discovery of a way to detect long
electric waves. He subjected the matter to many more experiments and
found that the waves have many points in common with light rays. He
found that they were reflected from certain surfaces, just as light is
reflected from the surface of a mirror. He made prisms which were able
to bend them as light waves are bent by a prism of glass. Some things
appeared to be transparent to them, as clear glass is to light, while
others are opaque. It does not follow that the same things which reflect
light waves reflect electric waves, and so on. The latter can pass
through a brick wall, for example. But the same divergence is to be
observed between light and radiant heat, of which the action of glass is
a familiar example. Clear glass will let light through almost undimmed,
yet we use it for fire-screens to shield us from too much radiant heat.
The important fact is that all three--light, radiant heat and Hertzian
waves--in addition to travelling at the same speed, are reflected,
absorbed or refracted, according to precisely the same principles. This
is almost perfect testimony to their essential identity.

The difference between them, as has been said already, is the distance
from crest to crest of the waves--the "wave-length," that is. And the
reader will wonder by what manner of means this mysterious dimension can
be ascertained. In spite of its seeming mystery the method is very
simple.

It is based upon the fact that two sets of similar waves travelling at
the same speed in opposite directions interfere with one another in a
peculiar way. Suppose that one set of waves travel along to a reflector
and strike it vertically; then another set will travel back from the
reflector exactly similar to the first, except that their direction will
be opposite. And the result will be that at certain intervals they will
exactly neutralise each other, so that at those points there will be no
wave-action appreciable at all. Those points where no action is to be
perceived are called "nodes," and they are exactly half a wave-length
apart.

This will be quite easily understood from the accompanying diagrams. In
each of these diagrams the set of waves marked _a_ are supposed to be
moving from left to right, while those denoted by _b_ are reflected back
and are moving from right to left. It will be noticed that each wavy
line has a straight line drawn through it, dividing it into alternate
crests and hollows, which line is known as the axis of the waves.

Now notice that in Fig. 8 there are points marked x, where
the _a_ waves are just as much above the axis as the _b_ waves are below
it, and vice versa. Hence at those points the two sets of waves will
neutralise each other.

Now turn to the next figure, which, be it remembered, shows the same
waves a moment later, when they have moved a little farther on in their
respective journeys, and it will be seen that there, too, are places
marked x where the two sets of waves neutralise each other. And the same
with the third diagram.

And finally observe that the places marked x are always
the same in all the diagrams--that is to say, they are always the same
distance from the line on the right-hand side, which denotes the
reflector. It will be clear, too, that each node is half a wave-length
from the next.

Thus it can be shown that at every moment, and not merely at the three
indicated in the diagrams, the two sets neutralise each other at the
nodes, that the nodes are always in the same places and half a
wave-length apart.

[Illustration: FIGS. 8, 9 and 10.--These diagrams help us to see how the
"wireless waves" are measured. The _a_ waves are supposed to be moving
from left to right and the _b_ waves from right to left. At the points
marked x they neutralise each other. It is then easy to
discover those points and the distance apart of any two adjacent ones is
half the "wave-length."

_N.B._--In Fig. 10 the _b_ waves fall exactly on top of the _a_ waves.]

Everywhere else, except at the nodes, there is action more or less
energetic, but _there_ is perpetual calm.

But how can we tell where the nodes are? When we recollect that they are
points at which no wave-motion at all takes place it is easy to see that
we shall at those points get no spark in our detector. So what Hertz did
was to set his oscillator going so that it threw waves upon a reflecting
surface and then move his detector to and fro in the neighbourhood until
he found the nodes. Between the nodes, as will be seen by an inspection
of the curves once more, there are other points at which the wave-action
will be twice as great as with the single wave, and so at those points
the response of the detector would be especially energetic.

This mutual action between an incident wave and a reflected wave is
termed "interference," and by it the wave-lengths of all the ethereal
waves have been measured. The plan used in the case of light waves,
although the same in principle, is somewhat different because of the
extreme shortness of the waves.

So the experiments of Hertz not only showed that long electric waves
existed, but that they were in all essentials similar to light, and
their wave-lengths were ascertained. On that basis has been built up
modern wireless telegraphy.

It may be interesting to mention at this point a very curious, and in a
sense pathetic, incident. Professor Hughes, whose name is associated
with certain well-known instruments for ordinary telegraphy, nine years
before Hertz' discovery noticed that a microphone was affected by the
action of an induction coil some distance away. He himself attached some
importance to the matter, but he allowed himself to be dissuaded from
following up the discovery by other scientists, more eminent than
himself at the time, who thought that it was not a promising field for
investigation. But for the influence of these friends he would possibly
be the hero of this story in place of Hertz.

Professor Silvanus Thomson has said that he too noticed the sparks
produced at a distance when a Leyden jar was discharged, but he makes no
claim to precedence over Hertz, since, seeing the phenomenon, he did not
perceive its real meaning, while Hertz, though a little later in time,
realised the profound significance of it.

Hertz himself in his account of his experiments is generous enough to
assert that, had he not discovered the waves when he did, he is quite
certain that Sir Oliver Lodge would have done so.

Before proceeding to describe the principal apparatus used in the
wireless station I should like to devote a little space to the
explanation of a term which will come up again and again, and which
represents that which is responsible, in the main, for the marvellous
advances which the art of sending wireless messages has achieved in the
last few years. I refer to "resonance."

It will be a great help if the reader will try for himself a simple,
inexpensive little experiment. Stretch a string horizontally across a
room and on to it tie two other strings so that they hang down
vertically a little distance apart. To the ends of the two strings tie
some small objects--a cotton reel on each will answer admirably. They
will thus form two pendulums, and, to commence with, they should be just
the same length. Having rigged all this up, give one pendulum a good
swing. It will impart motion of a to-and-fro variety to the supporting
string, which in its turn will pass that motion on to the other
pendulum. In a very short time, then, the second pendulum will be
vibrating like the first. Indeed the _whole_ motion of the first will
shortly become transferred to the second, so that the second will be
swinging and the first still. Then the second will re-transfer its
energy back to the first, and so they will go on until the original
energy given to the first pendulum is exhausted. The point to be
observed is the quickness with which one pendulum responds to the
impulses given it by the other, and the ease with which the energy of
the one passes to the other.

Now reduce the length of one pendulum. On setting the first in motion a
certain irregular spasmodic action is to be observed in the second, but
it is very different from the "whole-hearted" response in the previous
instance. In the former case the second one responded naturally and
readily to the first. Now its response is reluctant in the extreme. It
moves somewhat because it is forced to, but it is apparently unwilling.
Energy has to be _impressed_ upon it. There is no readiness, because
there is no sympathy between them.

That sympathy between the two equal pendulums is "resonance." The same
occurs between two violin or piano strings when they are "in tune."

The explanation is that a pendulum has a certain natural frequency which
depends upon its length. Another pendulum of the same length, arranged
as just described, therefore imparts impulses to it at just the
frequency which is natural to it. Consequently the effect is a
cumulative one, and it responds quickly. Impulses at any other frequency
tend more or less to neutralise each other. In the same way a string, of
a certain length and a certain tension, has a frequency peculiarly its
own, and it will respond to another similar string because the other
gives its impulses at its own natural frequency.

It is on record that an engine in a factory happened to run at precisely
the same speed as the natural frequency of the building, with the result
that after a little time the structure shook so much that it collapsed.

Now electrical circuits in which currents oscillate have a natural
frequency of their own. That frequency depends upon the two electrical
properties of the circuit: capacity and inductance. And if you want to
set up an electrical oscillation in any circuit you can best do it by
giving it impulses at intervals which agree with its natural frequency.

Sir Oliver Lodge seems to have been the first to appreciate fully the
effects of resonance in wireless telegraphy. It is strange that in
England the work of this eminent man in "wireless" matters is not more
fully recognised. When wireless telegraphy reached the point at which
the public became interested, Marconi was just coming to the front and
so, for ever, will his name be foremost in the public estimation. Indeed
more than foremost, for in the minds of many he monopolises the credit
for this invention. Many people are under the impression that he is the
one and only, or at any rate the original, inventor of wireless
telegraphy.

Now Marconi has done exceedingly valuable work in this field. Moreover,
he has been the means of placing the affair on a good commercial
footing. But all the same he is by no means the original or only
inventor. While admitting that he is a remarkable man, who has done
wonders, it is only common justice to refer to the others whose
contributions to the solution of the problem are possibly of equal
value. And, of these, few can compare with Sir Oliver Lodge.

But to return to the question of resonance. At first the distances over
which messages could be sent were but small. Now a marconigram can be
flung across a hemisphere. At first little could be done by day, work
had to be done mainly at night. Now communication passes by day and
night alike. Yet in principle, and in many details, the instruments are
unaltered from what they were several years ago. The main source of all
this improvement is the use of resonance.

To enumerate broadly the apparatus used for the dispatch and receipt of
messages the following list will be useful:--

_Transmitting End_

     (1) An Antenna, consisting of a number of wires raised to a
     considerable height above the ground.

     (2) A Spark-gap, consisting of a series of metal balls with gaps
     between them, the outer ones being connected to the antenna and to
     the induction coil.

     (3) A powerful Induction Coil with batteries or other source of
     current to work it.

     (4) A Telegraph Key, by which the induction coil can be started and
     stopped at will.

_Receiving End_

     (1) An Antenna precisely similar to the other.

     (2) A Coherer or other "oscillation detector."

     (3) A Receiving Instrument which may be a writing telegraph
     instrument, a telephone, any of a number of ordinary telegraph
     instruments, or a galvanometer.

Transmitting and sending instruments are, of course, installed at both
ends and either of them can be connected to the antenna at will by the
simple movement of a switch.

The antenna plays the part of one of the metal plates in the Hertz
oscillator. Early experiments were made with Hertz apparatus, but the
range of such a contrivance is very limited. For one thing, it neglects
to take advantage of the earth. It is little realised what an important
part the earth plays in the carrying of wireless messages. A very great
step was taken when Marconi dispensed with one of the plates of Hertz,
and used the earth instead; while the other plate gave place to the
elevated wires, the most familiar part of the apparatus to most people.

The condenser is thus formed by the earth as one plate, the elevated
wires as the other, and the intervening air as the insulator. The
"capacity" must be exceedingly small in such an apparatus, but it is
sufficient; while the long lines of electrical force stretching from the
high antenna to the earth produce waves of great carrying power. Lastly,
when the earth forms a part of the condenser the waves cling to it, so
that instead of being largely dissipated into space, they move along the
surface of the earth. The advantage of this is obvious.

At first it was customary to place the spark-gap in the wire leading
from the antenna to the earth, as in the accompanying sketch. Later,
however, it was found better to place the coil and spark-gap in a local
circuit in which the oscillations are first produced. These oscillations
pass through a coil which is interwound with another one connected to
the antenna and to earth, and thus the local oscillations, as we might
call them, induce similar oscillations in the antenna, just as the
fluctuations in one part of an induction coil induce fluctuations in the
other. Indeed the coil in the local circuit and the one in the antenna
circuit actually constitute an induction coil.

The advantage of this is that by introducing condensers the capacity of
which can be varied, and coils the inductance of which can be varied,
into the oscillation circuit it becomes possible to "tune" the circuits
effectively. Thus resonance comes into play and the power expended can
be made to produce the maximum effect.

Some attempts have been made to displace the induction coil in wireless
telegraphy altogether by a specially made dynamo. These machines can
produce either alternating or continuous currents, in fact the
alternating current dynamo is really simpler than the more familiar
continuous-current machine. The difficulty is, however, to run it
sufficiently fast to produce sufficiently rapid alternations. Nicola
Tesla made an alternator (to give the alternating current dynamo its
short title) which could produce 1500 alternations per second, while Mr
W. Duddell made one which produced 120,000, but neither was satisfactory
for the work in question. Could such a machine be made, it would be
invaluable, for it will be apparent that a continuous succession of
waves would be formed by it and not a succession of short trains of
waves such as is produced by the induction coil and spark-gap. The
difficulties are not electrical, but mechanical. It seems doubtful if a
machine will ever be made to run with sufficient rapidity which would
not knock itself to pieces in a very short time.

[Illustration: FIG. 11.--The simplest form of wireless antenna.]

Small alternators are used sometimes, however, to supply alternating
current to the primary of an induction coil, or transformer, as it is
more often called in its larger sizes. The interrupter is only needed
when the primary current is continuous--from batteries, for example.
Alternating current needs no interrupter, and so that bother is removed.
The alternations of a hundred or so per second, which are quite the
common thing with alternators, are just what is needed to excite an
induction coil. Consequently small machines of this kind are to be found
in many stations.

A Danish inventor, Valdemar Poulsen, has adopted an altogether different
method of producing electrical oscillations, which method is the
distinctive feature of his mode of telegraphy. He takes advantage of a
curious effect of passing current between two rods, one of which is
carbon, so as to form an arc such as we see in arc lamps.

My readers are already familiar with the term "shunt" in connection with
electrical matters, and so will perceive at once what is meant when a
second circuit is said to be arranged as a shunt to the arc. The
accompanying diagram will in any case make the matter clear.

The current comes along from the battery or continuous-current dynamo to
a hollow rod of copper which, to prevent it being melted, has cold water
continually circulating inside it. Thence the current jumps across to a
carbon rod, forming an arc between the two rods, and returns whence it
came. In its journey it traverses the coils of an electro-magnet, the
poles of which are one each side of the arc. This tends to blow the arc
out, as a puff of wind blows out a candle, an effect which a magnet
always has upon an electric arc.

The shunt consists of a wire leading from the copper to the carbon rod
with a condenser and an inductance coil inserted in it. The latter coil
also forms one part of that coil by which the oscillations in the local
circuit are transferred to the antenna.

The electrical explanation of what happens when the current is turned on
to an arrangement like this is rather too complex to set out here. It
depends upon a curious behaviour of the arc. It is really a conductor,
yet it does not behave as ordinary conductors do, and the result is that
the continuous current flowing through the arc is accompanied by an
oscillating current in the shunt circuit. And the important feature of
the arrangement is that these oscillations are continuous, in one long
train, not in a succession of trains. The advantage of this has already
been referred to.

One other feature of the apparatus just described should be mentioned,
since it will seem curious to the general reader. For it to work
properly it is necessary that the arc should be enclosed in a chamber
filled with hydrogen or a hydro-carbon gas. Coal-gas is generally used.

Hertz' original discovery was that small sparks could be seen to pass
between the ends of a curved wire when the electric waves fell upon it.
Such "spark detectors," as they are called, are useful in the
laboratory, but not for practical telegraphy.

[Illustration: FIG. 12.--Diagram (simplified) showing how Poulsen
generates oscillations. Current from a dynamo flows through the arc,
whereupon currents oscillate through the condenser and coil (as
described in the text).]

Several people seem to have noticed in years gone by that a mass of
loose metal filings, normally a very bad conductor of electricity,
became a much better conductor when an electrical discharge of some sort
occurred near by. The demand for a wireless receiver had not then
arisen, however, and so the discoveries were not followed up.
Consequently it remained to be rediscovered by Branly, of Paris, in
1890. He placed some metal filings in a glass tube, the ends of which
he closed with metal plugs. Lying loosely together the filings would not
conduct the current of a small battery from one plug to the other, but
when a spark occurred not far away they suddenly became conductive and
allowed it to pass. Several years after this Sir Oliver Lodge took up
the idea as a receiver for wireless messages, and believing that its
action was due to the waves causing the filings to cling together, he
christened it "Coherer."

Marconi succeeded in making a very delicate form of this, although
working on strictly the same lines.

The trouble with a coherer is that when once it becomes conductive it
remains so unless the filings be shaken apart. Lodge therefore arranged
for the tube to be continually struck by clockwork or by a mechanism
like that of an electric bell. Marconi effected a further improvement by
making the current passing through the coherer control the striking
mechanism, so that the latter is normally quiet but administers one or
two taps at just the right moment.

Sir Oliver Lodge and Dr Muirhead devised another detector which, though
quite different in form, is really much the same in principle. A steel
disc with a sharp knife-like edge is made to rotate above a vessel of
mercury. The edge just touches the mercury but no more. On the top of
the mercury there floats a thin layer of oil, a bad conductor. Now as
the disc revolves it picks up on its edge a film of oil, which it
carries down into the mercury. The film adheres so tightly that it
prevents the moving disc from actually touching the liquid metal. Thus,
under normal conditions, the two are electrically insulated from each
other by the film of oil and no current can pass from mercury to disc.
Oscillations, however, caused by incoming electric waves, are able to
break through the oil film and so bring disc and mercury into contact,
whereupon the current flows. The constant movement of the disc restores
the oil-film as soon as the oscillations cease.

The reason why these detectors act as they do is not quite understood.
One suggested explanation is that the oscillating currents heat the
particles and so partially weld them together. Another is that adjacent
particles become charged as the plates of a minute condenser, and so are
drawn tightly together as the plates in an electrostatic voltmeter are
drawn towards each other. Supposing that the original non-conductivity
of the loose filings be due to the film of air which may surround them,
either of these things would account for the film being broken or
squeezed out, resulting in better contact and improved conducting power.
But both suggestions seem to be contradicted by the fact that if the
pieces in contact be of certain substances the coherer works the
opposite way. Under those conditions the conductivity is normally good,
but the influence of the incoming waves causes it to become bad.

In 1896 Professor Rutherford, now of Manchester, described some
discoveries which he had made as to the magnetic effects of
oscillations. A simple little contrivance which he had constructed was
operated by the discharge of a coil half-a-mile away, at that time a
great performance. This detector was simply an electro-magnet with a
steel core instead of the usual soft iron core. The reason the latter is
used in the ordinary magnet is that it loses its magnetism the moment
the current ceases to pass through the coil with which it is surrounded,
while a steel core retains its magnetism. For most purposes a steel core
would render an electro-magnet useless, but in this case it was desired
that the core should be permanently magnetised. So a current was first
passed through the coil to magnetise the core, and then the coil was
connected to a simple form of antenna while a swinging magnet was
brought near so that the magnetic power of the core would be indicated
and any change made apparent. The effect of the discharge half-a-mile
away was to _de_magnetise the core slightly. This was shown by the
movement of the swinging magnet, and so the first "magnetic detector"
was found.

But here, perhaps, I ought to explain the use of the antenna at the
receiving station--its function at the sending end has already been
made clear. The electro-magnetic waves, coming from the distant
transmitter, strike the receiving antenna and in so doing _set up in it
oscillations such as those which set them in motion_. For every
oscillation in the sending antenna there will be another, similar in
every respect except that it will be feebler, in the receiving antenna.
And the oscillations are here led to the detector, of whatever form it
may be, and in it they make their presence felt.

In some few cases a Duddell thermo-galvanometer has been employed as the
detector, in which the oscillating currents report themselves directly.
In coherers the detector works by causing the oscillating currents to
control a continuous current from a battery and it is the latter which
actually gives the signal, but there are a number of extremely
interesting means which have been invented to detect the oscillating
currents by their heating effect.

R. A. Fessenden, for instance, has perfected one which is a marvel of
delicate workmanship. He depends upon the heating of a wire by the
currents passing through it. Such heating is the result of the
electrical force acting against resistance, and the difficulty is that
if the resistance be great it will almost entirely kill the faint
oscillating forces in the receiving antenna, while if, on the other
hand, it be small, the rise in temperature will be inappreciable. So he
encloses a fine thread of platinum in a glass bulb from which the air is
exhausted. The platinum wire is first of all embedded in a wire of
silver: the silver wire is given a core of platinum, in fact. Then the
compound wire is drawn down until it is so thin that the platinum core
is only one and a half thousandths of an inch in diameter. A short
length of this compound wire is then bent into a U-shaped
loop and its ends connected to thicker wires. Finally the bottom of the
loop is immersed in nitric acid, which eats away the silver at that
point and leaves the bare platinum. Thus is produced a very short length
(a few millimetres) of exceedingly thin platinum wire supported at its
ends by comparatively thick wires.

Being so short, this wire does not offer much resistance, and
consequently does not materially check the oscillations. At the same
time, since it is so fine, it does offer some resistance, and finally,
since what heat is generated will be in an exceedingly small space, it
will be appreciable there. A telephone is arranged so that its current
also passes through the fine wire, and every slight variation in the
temperature of the platinum wire, by varying its resistance, varies the
current through the telephone. And exceedingly slight variations can be
detected by sound in the telephone. Thus the oscillations generated in
the antenna affect the heat in the wire; that affects its resistance;
and that again affects the telephone, which, finally, affects the ear of
anyone who is listening to it. It must be understood, however, that this
is not a wireless telephone, for the sounds heard are not articulate but
merely long and short sounds, representing the dots and dashes of the
"Morse Code."

Electrolysis provides us with another form of detector. An exceedingly
small platinum wire forms one electrode and a large lead plate the
other, and both are immersed in dilute acid. The passage of current from
a local battery sets up electrolysis, and so stops itself by forming a
film of oxygen on the small electrode. This film, however, is broken by
the oscillating currents from the antenna, so that as long as they are
coming the battery current can flow, but as soon as they cease the
battery current stops itself again. Thus the flowing and stopping of the
oscillating currents is exactly copied by the current from the battery,
which current is led through a telephone or a sensitive galvanometer.

It may occur to readers to inquire why the oscillating currents are not
passed direct to a galvanometer. The answer is that because they are
oscillating a very sensitive galvanometer is not possible.

True, the Duddell thermo-galvanometer has been mentioned in this
connection, but although it is a beautiful instrument it cannot compare
for delicacy with the direct-current galvanometers. The latter are
easily a _hundred thousand times_ more sensitive. But the trouble can be
overcome by "rectifying" the oscillating currents, by passing them
through a "unidirectional" conductor--one, that is, which passes current
one way only. These remind one of a turnstile as installed at certain
public places, which let you out but will not let you in unless you pay.
In fact they will not let you _in_ at all. In like manner "rectifiers"
will only allow those currents to pass which are flowing in one
direction, and so they cut out every alternate oscillation, thus
producing something very like continuous current, which can be detected
by the very delicate galvanometers which are usable where continuous
currents are concerned, or more often by a telephone receiver. The
rectifying conductors are in many cases crystals, hence these detectors
are called "Crystal Detectors." Carborundum is a favourite for this
purpose.

And that brings us to the important question of the secrecy of wireless
communication, and the measures taken to prevent confusion from the
number of independent messages flying through the air at the same time.

This can be largely achieved by the aid of resonance. Trains of waves
flung out by one antenna may strike several other antennæ, but unless
the latter are in tune with the sending apparatus they will probably not
be affected appreciably. Let one of them, however, be in tune, and it
will pick up easily the message which is not noticed by the others. It
is as if three people watching a distant lamp were affected by a form of
colour-blindness which rendered them practically blind to all colours
except one. Suppose one could see red only, the other blue and the third
yellow. A light sent through a blue glass being robbed of all rays
except the blue ones would be visible only to the man who could see
blue. The man who could see blue would, in like manner, be quite blind
to light sent through red or yellow glass. Each of them, in fact, could
be signalled to quite independently of the others by simply sending him
rays of the colour to which his eyes were sensitive. In precisely the
same way each wireless receiver is or can be made most sensitive to
waves of a particular length and practically blind to all others. The
operator can adjust his apparatus for certain prearranged wave-lengths,
and so he can communicate with secrecy to stations whose wave-length he
knows. The change, of course, is made by altering the capacity, or
inductance, or both. The instruments can be so calibrated that it is
quite easy to make the alteration.

Then, antennæ can be so constructed that messages can be received with
most readiness from one particular direction. In others, they can be
received from any direction, but the direction can be discovered. This,
it will be easy to see, is of great value to ships in a fog.

Antennæ made with a short vertical part and a long horizontal part
radiate best in the direction away from which their horizontal part
points. This is of great advantage in stations which are built specially
to communicate with other particular stations. In such cases the antenna
is carefully built, so as to point in the required direction. Such
antennæ also receive more readily those signals which come from the
direction away from which they are pointing.

Reference has been made already to the interesting fact that wireless
communication is easier at night than in the daytime. That is probably
because of the "ionisation" of the atmosphere by the action of sunlight.
Along with the visible sunlight there comes to us from the sun a
quantity of light known as "ultra-violet," since it makes its effect
known in the spectrum of sunlight beyond the violet, which is the limit
of visibility at one end of the spectrum. We cannot see it but it
affects photographic plates powerfully. It has energetic chemical
powers, and it has the ability to make the air more conductive than it
is ordinarily. Comparatively little of it penetrates our atmosphere, but
it must exercise a good deal of influence a little higher up. Now
readers will remember that the process by which electro-magnetic waves
are propagated is checked when the waves strike a conductor. The energy
in the waves is then employed in causing currents in the conductor
instead of forming more waves. And so partially conductive air forms a
partial barrier to the waves. The effect is not appreciable in the case
of the tiny waves of light and heat, but it is in the case of the long
"wireless waves." Everyone has seen the waves of an advancing tide
coming up a sandy beach, and has noticed how the dry sand (a good
conductor of water) sucks up and destroys the foremost ripples. In like
manner are the wireless waves "sucked up" by the partially conductive
atmosphere. But the effect of the ultra-violet light does not last long,
and so, at night-time, it disappears. Therefore messages can be sent
better at night than by day.

For wireless _telephony_ what is wanted is a continuous uninterrupted
train of waves, such as those from the "Poulsen arc," and a receiver of
the magnetic type. The coherer is no good for this purpose, since it
either stops the current entirely or lets it flow copiously. The
magnetic detectors, however, respond to the variations in the strength
of the incoming waves. As the latter increase or decrease in strength so
does the magnetic detector give out stronger or weaker signals. So a
telephone transmitter of the ordinary type is made to vary the strength
of the oscillations at the sending end, while an ordinary telephone
receiver is placed in series with the detector at the receiving end.
Thus every slight variation corresponding to sound waves spoken into the
transmitter is reproduced in the receiver.

It is strange that wireless telephony has not made greater progress, for
it may be said, on the word of one of the greatest authorities, that
wireless telephony is simpler and easier than telephony through a
submarine cable. In the latter there are almost insuperable obstacles
caused by the capacity and inductance of the circuit, while in the
wireless method there is very little difficulty.

There are, of course, several so-called "systems" of wireless telegraphy
in use. There is the Marconi in Great Britain; the secret Admiralty
system in the British Navy; the De Forest in the United States; the
Telefunken in Germany, not to mention the promising Poulsen system. And
there are still others. But it would be futile to attempt to explain
how they differ from one another in a work like this. In principle they
are alike. The precise forms of instrument used may vary, but even there
there is much in common between them. As time goes on there will
inevitably be a tendency to more and more uniformity. That is always the
case, for some things are inherently better than others, and rival
systems, although each is working along its own lines, always come to
very much the same result in the end. Without making any comparisons, it
is safe to say that if the Telefunken system, for example, has any
points of superiority over the Marconi, the latter will sooner or later
find out the fact, and will modify their apparatus accordingly. In all
probability this will operate both ways, and some things which the
German system is now using will give place to those which the British
have in operation.

In another very modern industry this is very apparent. Having attended
and carefully studied several annual exhibitions of flying machines, I
have noticed with great interest how the varying types of a few years
ago are merging into the more or less uniform types of to-day. And it
has been the same with wireless telegraphy, and will be still more so in
the future.

The best means of generating the waves and the best means of detecting
them at a distance--that is the whole problem, and all the workers in it
will sooner or later come to much the same conclusions as to which are
the best ways.

Patents may do a little to delay this, but not much. For one thing,
patents only last a few years. For another, a patent only covers a
particular way of doing a particular thing. A machine that is termed
"patent" is often the subject of a hundred patents, each covering a
particular little point. It is well-nigh impossible to patent a whole
machine. A general principle cannot be patented, only a particular
application of that principle, and so there are in a great many cases
little variations of a patented method which are quite as good as the
patented one, and which can be used freely. So even patents will not
have much effect, in all probability, upon this unification process.

But, however that may be, there is no doubt that the whole world owes a
deep debt of gratitude to the men who have worked out this most
beneficent of inventions. It is difficult to think of a single one which
has ever brought such a load of benefits to poor, struggling humanity as
this has. The ship in distress, the lighthouse man on his lonely islet,
the explorer in the Polar regions, the pioneer settler in the new
lands--in fact, just those who most need some connecting link with their
fellows--are the people to whom the wireless telegraph brings aid and
comfort. All honour to the men who have done it.



CHAPTER XIII

HOW PICTURES CAN BE SENT BY WIRE


The sending of a message by telegraph is easily understandable. Various
combinations of two simple signs, such as short sounds and long sounds,
can readily be made to indicate letters by which the words can be spelt
out.

Nor does the sending of sound over a wire make a very great demand upon
the credulity. We all know that sound consists of innumerable little
waves in the air, and by the simplest of devices these can be converted
into variations in an electric current, which variations, by means
equally simple, can be made to re-convert themselves back into sound
waves at the other end.

But to transmit a picture is another matter altogether. It seems barely
possible in the case of a drawing such as a pen-and-ink sketch, which
consists of a comparatively small number of definite lines; but with a
shaded sketch or a photograph, with its gradations of light and
shadow--to transmit such would seem to be beyond the bounds of
possibility, did we not know that it has been done. The description of
the methods will therefore constitute a not uninteresting subject for a
chapter.

It is worthy of remark that an attempt along these lines was made many
years ago by a man named Caselli, and a description of this pioneer
apparatus will form a good introduction to the later developments.

In Fig. 13 we see a square which represents a sheet of tinfoil, upon
which is drawn, in non-conductive ink, a simple geometrical figure. The
ink may be grease, or shellac varnish, indeed there are many substances
which are available for use as an insulating ink. Across the square
there are a number of parallel dotted lines, but these, it must be
understood, are not actually drawn upon the foil--their purpose will be
apparent in a moment.

Suppose that we connect the foil to one pole of a battery, and the other
pole by a flexible wire to a metal pen or stylus. If we place the point
of the pen in contact with the foil, we make a complete circuit, through
which, of course, current will flow. But if, with it, we touch one of
the non-conductive lines, there will be no current.

[Illustration: FIG. 13]

[Illustration: FIG. 14]

Taking a ruler, then, let us draw the point of the stylus across the
foil in a series of parallel straight lines. It is these excursions of
the stylus which the dotted lines are intended to represent. For nearly
the whole of the time current will be flowing; but whenever the stylus
is crossing one of the lines of non-conductive ink there will be a
momentary cessation. Thus, the reader will begin to perceive, we obtain
what we may call an electrical representation of the figure drawn upon
the foil.

And now let us turn to Fig. 14. There, too, is a square, but in this
case it is not foil, but paper which has been soaked in prussiate of
potash. The reason for introducing this chemical is that it is
susceptible to electrical action. Wherever current passes through it, it
becomes changed into Prussian blue, so that if we place the point of a
pen upon the paper, and cause current to flow out of that point through
the paper, there we get a blue dot. If, while the current is flowing, we
draw the pen along, we get a blue line.

Fig. 13 therefore represents in principle the sending apparatus of
Caselli's writing telegraph, while Fig. 14 represents the receiving
instrument. The two pens are connected together by the main wire, in
such a manner that, when the point of the one is in contact with the
bare foil current flows out of the other and into the paper; but as the
former crosses an ink line all current ceases.

If, then, while the sending pen is drawn line by line across the foil,
the other is drawn at the same speed, line by line, across the
chemically prepared paper, we shall get on the latter a series of lines
as shown in Fig. 14 almost continuous, but broken here and there. Each
breakage represents a passage of the sending pen across a line, and
taken together, as will be seen, they constitute a reproduction of the
geometrical figure drawn upon the foil. As shown, the lines are rather
far apart, and so the reproduction is not a very good one. They are only
drawn so, however, in order that the principle may be shown the more
clearly. They may be drawn so that they overlap, and then the effect is
very much better, the received picture being almost an exact
reproduction of the other.

It will be noticed that an essential to the success of this method is
that the two pens should move in perfect unison, and that was the great
difficulty. Caselli used an arrangement of pendulums, the best thing
available at the time.

The reproduction is, in photographic language, a negative, a somewhat
unsatisfactory feature of the method. A simple modification, however, of
the electrical connections will reverse that, so that the reproduction
shall be a positive. There are two ways of cutting off a current from
any particular circuit. One is to interpose a resistance, through which
current cannot pass in an appreciable quantity, and the other is to
provide a second path for the current so much easier than the first that
practically all the current will pass that way, leaving the first
circuit, to all intents and purposes, free. It is as if a farmer wished
to stop people passing across a certain field. Two methods would be open
to him: one to put up a high gate over which no one would dare to climb,
and the other to provide a short cut so much more pleasant and
convenient than the old path that no one having the choice of the two
ways would think of going the old way.

What the farmer would call a short cut the electrician calls a short
circuit, and a short circuit is often a more convenient way of cutting
off a current than a switch which interposes resistance. At all events,
in a case like this, a short circuit enables that to be accomplished
which would be very difficult by any other means.

In the apparatus as already described the battery had to drive the
current along a long wire, terminating at the distant receiving
instrument, whence the current returned via the earth. The foil and pen,
acting as a kind of electrical "tap," controlled this. When foil and pen
touched, the tap was open and current flowed. When the line of
non-conductive ink interposed itself, the tap was off and the flow
ceased.

But connect the battery directly to the wire, and place the foil and pen
in a short branch circuit, and the whole thing is reversed. Then the
opening of the "tap" sent current to the other end; now the opening of
the tap causes it to flow round the short branch and leave the main
wire. Then the closing of the tap stopped the current reaching the
farther end; now it causes it to do so. In fact, the entire action of
the apparatus is completely reversed, and the bare parts of the foil
become represented by blank paper, while the insulating lines produce
the marks. In short, a positive results instead of a negative.

Such was the scheme of Caselli years ago. It is mentioned here at some
length, since the principle of it is largely re-used in an improved
form in the most successful of modern apparatus for a like purpose.

It undoubtedly was a very excellent scheme, simple and effective, which
ought to have succeeded; but it did not do so, for the sufficient reason
that at that time knowledge of electricity and skill in constructing
delicate mechanism were not so highly developed as they are to-day. For
success, as has already been said, one thing was essential, and that
thing very difficult to obtain--a perfect synchronism between one stylus
and the other. If the one were but the slightest degree "out of step"
with the other, failure followed inevitably.

So the electrical transmission of sketches dropped for the time being.
More recently a perfectly successful solution of the problem has come in
another way altogether. This apparatus, at first called the
telautograph, but now known as the telewriter, it will be more
convenient to refer to later.

Of modern systems for the transmission of pictures the most successful,
probably, are the Korn telautograph and the Thorn-Baker telectrograph.

Both of these are able to transmit very fair reproductions of
photographs besides line drawings. The difficulty with photographs is,
of course, that many parts of them are not of equal blackness or
whiteness, but shade off gradually from one into the other. Take the
case of a simple portrait. Part of the subject's face will be pure
white, while the side in shadow will be comparatively dark. There is no
hard and fast line between the two, but by a gradation through an
infinite number of shades the one tones into the other. How can it be
possible to convey that, more or less mechanically, over a wire? The
solution is due to the fact that the eye will blend together a number of
distinctly different shades, if properly arranged, into a gradual
change. Really the change is step by step, but the effect is apparently
quite continuous. This can be seen in the "half-tone" illustrations in
this book. Close examination will show that such a picture is cut up
into small squares. In the pure white part the squares are invisible,
while in the perfectly black parts, if there be any, they are so merged
into one another as to be inseparable. But everywhere else in the
picture it will be seen that there are squares each with a dot in the
middle. In the darker parts the dots are large; in the lighter ones they
are small. We get the effect almost of colour, although the picture is
done entirely in black ink. The eye does not see the individual dots
when we are just looking at the picture; we have to examine it very
closely to find them. Yet they are there all the time, and it is simply
the peculiar action of the eye which sees beautiful half-tones, shading
imperceptibly one into another, whereas in real fact there are only a
vast number of equidistant dots, all equally black.

We see, therefore, that it is possible to split up a picture of any kind
into a number of small squares and to treat each square as being of
equal darkness throughout. Then, if we can communicate by wire that
particular degree of darkness to a distant station, where the small
parts can be put together in their proper order and given their correct
shade, the picture as constructed at the receiving end will be something
like that at the sending end. And we have only to make the size of each
separate square small enough to obtain a copy which will resemble the
original very closely indeed.

In the early days it was actually proposed to telegraph pictures by
ordinary telegraphy, using this principle. The suggestion was to agree
upon a code of twenty-six shades, each called by a letter of the
alphabet. One shade was to be _a_, the next _b_, and so on. Then the
picture was to be divided up into squares, and the particular shade of
each square telegraphed by means of the corresponding letter. The shades
thus communicated were to be put together at the receiving end, on a
prearranged system, and so the picture was to be built up. Given plenty
of time, that scheme might be moderately successful, but to get a really
good reproduction the subdivision needs to be so minute, and the number
of squares, therefore, so immense, that it would be quicker to send the
picture by train than to telegraph it by such laborious means. In a
fairly coarse half-tone block the squares are, say, 2500 to the square
inch. That number of letters would therefore have to be telegraphed for
every square inch of picture transmitted, to say nothing of the
difficulty of building up a picture of such a great number of parts and
giving to each the desired shade. That idea, abortive though it is in
its crude form, illustrates very clearly the fundamental principle on
which this work is done.

The problem is really to devise a machine which will do that same thing
rapidly and automatically divide up the original into a large number of
squares, and then send an electric current to represent each square,
such current by its strength to indicate the shade of the square: and
finally a similar instrument is needed to act as receiver, and to
reproduce those squares in the proper order, giving to each its correct
shade.

In practically all of them the mechanism is rotatory, the original being
placed upon a drum which turns round under a stylus, or its equivalent,
while the stylus gradually travels along from end to end after the
manner of the needle of a phonograph, or else the same result being
achieved by the drum itself having an endwise movement as well as a
rotative one. The receiving instrument is of similar form, and both must
start together, move at the same speed and indeed preserve a perfect
correspondence with each other.

If the distance be great between the two there may be difficulties due
to the "retardation" of the currents passing between them. Electricity
does not pass through long wires, particularly if they be under the sea,
with anything like the quickness which we are apt to think. Over a short
line and under favourable circumstances the receipt of a telegraph
signal at the farther end is practically instantaneous, but on long
lines, and under certain conditions, that is far from being the case.

Then something has to be done to quicken the action of the current, or
else the receiving drum must be made to lag behind the sending drum by
the requisite amount. In some cases, too, the transmitting apparatus
loses a little time in sending off the currents, and that, too, has to
be allowed for, so that, all things considered, the reader will see that
the successful solution of this problem is hedged about with many subtle
difficulties which are probably only appreciated by those who are well
acquainted by sad experience with the little vagaries of both
electricity and mechanical devices. Neither of them does quite what we
want it to do; each suffers from little faults, which in the case of a
delicate problem like this, where a difference of a hundredth of a
second would be fatal to success, introduce difficulties almost
insuperable.

To transmit line drawings, Professor Korn uses a sending instrument very
like that of Caselli. The picture is placed, either by hand or
photographically, upon a sheet of copper foil, which is fixed round the
rotating cylinder, the lines being formed of non-conducting material.
The foil being electrified and the stylus connected to the "line" or
main wire, currents pass to the farther end just as in the old
apparatus.

At the receiving end the drum is covered with photographic paper and
enclosed in a light-tight box. Through a hole in this box a fine pencil
of light passes from a lamp, suitable lenses being used to ensure that
the pencil shall have, as it were, a very fine point, producing a very
small spot of light upon the paper. If the light remains quite steady,
the drum meanwhile rotating, a line will be drawn by it upon the paper
which will be visible when the latter is developed. Since the drum not
only turns upon its axis, but also moves endwise one hundredth of an
inch at every revolution, this line will be a spiral, the turns of which
will be one hundredth of an inch apart. Thus the paper will be blacked,
practically uniformly, all over. Should the intensity of the light vary,
however, the line will at times be lighter than at others, while, should
it be cut off altogether for a moment, then there will be a
corresponding gap in the line, and it is easy to see that if these
lighter parts or gaps occur in the correct places they will form a
picture. In other words, by controlling that light we can build up a
picture upon the paper. The question is how to control it.

Professor Korn uses a form of the Einthoven galvanometer already
described. Instead of the silvered fibre generally employed in this
instrument, a silver wire is fitted, the movement of which partly or
entirely cuts off the pencil of light.

The Korn transmitter for photographs is quite different, although the
receiver is practically the same as what has just been described. The
basis of it is a peculiar power possessed by the metal selenium when in
a certain state. This, like all metals, is a conductor of electricity,
but of course offers resistance in some degree. Now the special feature
of selenium is that its resistance is reduced if light shine upon it.
Suppose, then, that current be flowing through a mass of selenium and
that the latter be suddenly illuminated brightly, the resistance will at
once fall and the current increase. On the other hand, should the light
falling upon the selenium diminish, its resistance will increase and the
current flowing through it will decrease. In short, given a suitable
arrangement, the current flowing in a circuit of which a selenium "cell"
forms a part will increase or decrease with the increase or decrease in
the light falling upon the cell.

A while ago the papers were telling striking stories of a way by which
blind people, so it was said, were to be recompensed for the loss of
their sight--a new sense, as it were, was to be given them by which they
could "hear" light, even if they could not see it. All this had
reference to this curious property of selenium, it being, of course, an
undoubted fact that it will vary an electric current in accordance with
the variations in the light, and if that current be led through a
telephone receiver a man, by holding that to his ear, could, in a sense,
hear the variations in the light.

[Illustration:         THE TELEWRITER

This remarkable instrument transmits actual writing and drawings, the
receiving pen copying precisely the movements of the sending pen]

In the Korn transmitter for photographs selenium is employed as
follows:--A transparent photograph is made, on a celluloid or gelatine
film, and this is fixed upon a glass cylinder mounted as already
described. A pencil of light falls upon this in much the same way as in
the case of the receiver just described, and, as the cylinder revolves,
describes a fine spiral line all round and round it.

Moreover, the light passes right through the photograph and falls upon a
mirror inside, off which it is reflected on to a selenium cell. At every
moment, then, the light is falling upon some small part of the
photograph, and the amount of it which gets through and ultimately
reaches the selenium depends upon the density of that part.

Current, meanwhile, is flowing from a battery through the selenium, and
thence over the main wire to the distant station. As the light pencil
traces its spiral path over the rolled up photograph every variation in
the density of the picture is reproduced as a variation in the current
through the selenium. This, at the remote end, operates the Einthoven
galvanometer, the movements of which vary the shade of the spiral line
being drawn upon the photographic paper.

This process takes place with remarkable celerity, so that in a few
minutes the innumerable variations constituting a complete photograph
can be transmitted and faithfully recorded at the distant end of the
wire.

But perhaps the most successful of these methods is that known as the
telectrograph. It is surprisingly like the scheme of Caselli in
principle, and forms another example of the fact that good ideas often
fail through lack of the proper means to carry them out. Mr
Thorne-Baker, the inventor of the telectrograph, has had at his disposal
accumulated stores of knowledge and skill which did not exist in
Caselli's time. Consequently the former has made a brilliant success
where his predecessor produced only an interesting but somewhat
ineffective attempt.

Reference has been made already to the half-tone blocks wherein a host
of small dots of varying sizes make up a picture. Now instead of
parallel rows of dots parallel lines of varying thickness will give very
much the same result. The former are made by photographing the picture
through a sheet of glass ruled with two sets of lines at right angles to
each other. The latter can be made by using a screen with lines one way
only instead of two ways. It is therefore quite easy for a blockmaker to
produce a "process block" wherein lines are used instead of dots. For
this particular purpose, however, it is not an ordinary block that is
needed, although it is in essentials very similar. The picture to be
transmitted is photographed through a screen as if a half-tone block
were to be made. The negative so obtained is then printed by the gum
process on to a sheet of soft lead and, after washing, the picture
remains upon the lead in the form of lines of insoluble gum on a
background of bare lead. A squeeze in a press drives the gum into the
lead, and so gives the whole sheet a smooth surface over which a stylus
will ride easily, but which is, nevertheless, made up of conductive
parts and non-conductive parts, the latter forming the picture.

The lead sheet is then put upon a revolving cylinder and turned under a
moving stylus in the manner with which we are now familiar. The sheet is
placed with the lines lengthwise of the cylinder so that current passes
to the stylus except as it passes over the breadth of the lines, and so
similar lines are built up at the distant end.

The receiving mechanism is of the electro-chemical type which Caselli
used. The current passes from the receiving stylus to the paper, and
there makes its mark in a way that will be understood from the
description of the earlier apparatus.

The supreme advantage of this method of working, over that of Professor
Korn, is that the operator can see what he is doing. To obtain good
results, a number of electrical adjustments have to be made, and whether
he has got them right or wrong can be seen as soon as the picture begins
to grow upon the receiving paper. If a little readjustment be needed the
operator sees it and can set things right before the really important
part of the picture begins to appear, whereas with the Korn apparatus he
does not know what is happening at all, since he can see nothing until
the picture is finished and the photographic paper has been developed.

It will be apparent, too, to anyone who has carefully considered the
wireless telegraphy chapters, that it ought to be possible to make the
sending stylus or its equivalent control a wireless transmitter and a
wireless receiver to operate the receiving stylus, so as to be able to
send pictures by "wireless." Experiments to this end have been made with
some measure of success, and sooner or later we are almost sure to hear
that the difficulties, which are by no means small, have been overcome.

But we cannot conclude this chapter without a fuller reference to that
marvellous invention, the telewriter.

In this a man makes a sketch with a pen on a piece of paper, or maybe he
writes a message, and simultaneously a pen, hundreds of miles away if
need be, does precisely the same thing. The receiving instrument draws
the sketch line by line, or it transcribes the message in the actual
handwriting of the sender. A little touch, almost weird in its
naturalness, is that every now and then the receiving pen leaves the
paper and dips itself into a bottle of ink, after which it resumes its
work at the very spot where it left off.

Now how the complicated lines and curves, the strokes and dots which
make up a written language, even the little shakes and defects which
give each man's writing a personality of its own, how all these can be
sent over a wire is at first sight very difficult to understand. The
inventor of this apparatus has discovered an extremely simple way of
doing it.

But even he does not attempt to do it with one wire, it should be said,
for he uses two. This is no drawback when, as is often the case, it is
used in conjunction with a telephone, for the latter, to be effective,
also requires two wires. Years ago single wires were employed for
telephones as for telegraphs, the circuit being completed through the
earth. But the difficulty arose that every wire through which currents
flow is apt to induce currents in neighbouring wires--the induction coil
is based upon that fact--and so messages in one wire were overheard on
others, or, what was perhaps more annoying still, the dots and dashes
passing in a telegraph wire would produce loud noises in a telephone
wire that happened to be near. The use of two wires, however, entirely
removes that trouble, for the neighbouring current then induces two
currents instead of one, one in each, and it so happens that these are
opposed to each other, so that they neutralise each other. So every
telephone wire now is double and therefore is ready, as it were, to have
the telewriter fitted to it.

But even with two wires the difficulty seems insuperable until we
remember that the most complex of curves can be resolved into two simple
movements.

The sending pen, with which the original writing or drawing is done, is
attached to the junction of two light rods. The farther end of each rod
is attached to the end of a light crank fixed so that it can rotate or
oscillate, after the manner of cranks, in the plane of the desk upon
which the paper lies. All the joints mentioned are of the hinge nature,
so that as the pen is moved about the rods turn, more or less, one way
or the other, the two cranks. This simple mechanism, it will be
observed, carries out very effectively the principle just mentioned, for
it resolves the motion of the pen, no matter how complicated it may be,
into a simple rotating motion of the two cranks.

So the cranks turn this way or that as the draughtsman makes his
picture, and it is very easy to arrange that their movement shall vary
the strength of two electric currents, whereby we obtain electric
currents varying in accordance with the movement of the cranks.

This is done by making each crank operate a variable resistance or
rheostat. When in its extreme position on one side the crank permits
current to flow freely, but as it moves over to the other extreme
position the resistance in the path of the current is increased. Such an
arrangement is a common feature in electrical apparatus.

So current from a battery flows to the two wires leading to the distant
station, each passing through the rheostat connected to one of the
cranks. We may think of the rheostats as taps which can be turned on or
off by the action of the cranks. Let us imagine that crank _a_ is in the
position when the current flows freely--when the electrical "tap" is
fully open; then a strong current will flow along wire _a_, returning to
the sending battery via the earth. As that crank is moved the current
will gradually be reduced, until, if it be moved right over to the other
extreme, the current will be at its feeblest.

[Illustration: FIG. 15.--A Message received by Telewriter.]

Arrived at the other end, this current passes to a device which we may
describe simply as a magnet so arranged that its action pulls round a
crank against the restraining action of a spring.

Now the stronger the current the more does that magnet pull and the
farther does the receiving crank turn. The sending crank varies the
resistance, the resistance varies the current, the current varies the
strength of the receiving magnet, and the magnet varies the position of
the receiving crank. Properly adjusted, then, the motion of the crank at
the one end is communicated through that long chain of causes and
effects, until at last it is repeated _exactly_ by the movement of the
crank at the other end.

The same thing occurs simultaneously over each of the two wires, crank
_a_ at the sending end communicating over wire _a_ to crank _a_ at the
other end, while crank _b_ communicates its motion over wire _b_ to the
other crank _b_. Each sending crank is closely imitated in its every
action by the corresponding one at the distant station.

The two receiving cranks are connected by light rods to the receiving
pen in precisely the same way that the sending pen is connected.
Consequently, not only are the separate movements of the two cranks
repeated at the remote station but the complex movements of the sending
pen, which gave rise to the actions of the cranks, are also conveyed to,
and repeated by, the recording pen. The movements of the first pen are
resolved into rotating motions by the two cranks, these are transferred
to the other cranks, and their movements are in turn converted back into
the written curves.

Thus as the pen in the artist's hand draws his sketch, so does the
automatic hand at the other place, it may be at a great distance, repeat
faithfully his work, and the sketch grows line by line simultaneously at
both ends.

There is not space here to detail how, by another current superposed
upon those referred to already, the receiving-pen is made to dip itself
periodically into the inkwell at the will of the sender. By a cunning
use of alternating current this is done without in any way interfering
with the action of the cranks as described above.

But of course there is a severe limitation to the usefulness of this
machine, inasmuch as the drawing has to be made at the time of
transmission, and it can only be "put on the wire" by the hand of the
artist himself.



CHAPTER XIV

A WONDERFUL EXAMPLE OF SCIENCE AND SKILL


In the preceding chapter reference was made to the fact that for the
successful sending of pictures "by wire" one thing was necessary above
all others. That one thing consists in making two machines, perhaps
hundreds of miles apart, start working together, stop together and, when
working, turn at exactly the same speed. Let the reader just picture the
problem to himself, and ask himself how such an arrangement can be
possible. Let him think of a town two hundred miles away and then
meditate on the possibility of making a machine working in his own room
and another in that distant town maintain perfect unanimity in their
movements. The result of such reflection will probably be the assertion
that such a thing is beyond the bounds of possibility. Then he will find
the following description of how it is done extremely interesting.

In the first place it must be understood that each machine is driven by
an electric motor. The motors are designed to run at 3000 revolutions
per minute, and they drive the cylinders of the machines through gearing
so arranged that the latter turn at 50 revolutions per minute.

Now of all machines perhaps the most docile and easily managed is the
direct-current electric motor. Each such machine is made with a view to
its working at a certain speed, but that can be varied within certain
limits, by simply varying the force of the current which drives it. And
that force can be very easily varied by the use of an instrument called
a "rheostat" or variable resistance. We are all familiar with the way in
which the engine-driver regulates the speed of a locomotive, by means of
a valve in the steam-pipe. The opening and closing, more or less, of
the valve enables the speed to be changed at will and adjusted to a
nicety. The rheostat is to the electric current what the valve is to the
steam; it can be opened and closed, more or less, as necessary. By it
the current driving the motor can be made stronger or weaker, and as
that change is made so does the speed of the motor change accordingly.
Thus we see that there is at hand the means of setting a motor to work
at any desired speed.

The difficulty, however, is to tell when the desired speed has been
attained. One can count the revolutions of a machine at two or three
revolutions per minute with a certain amount of accuracy, but fifty
revolutions per minute are more than one could count correctly. Still
less could we count the 3000 revolutions every minute of the motors.
Thus, even if we had the two motors side by side, we should have extreme
difficulty in making them work at the same speed exactly. One might be
doing 3000 while the other did 2990 or 3010 and we should be none the
wiser. And when we separate the two by a distance of many miles, the
task of synchronising them is even worse.

But fortunately there is a simple contrivance by which we can tell very
accurately the speed of a motor. The reader has already been
familiarised, in previous chapters, with the difference between direct
or continuous electric currents and alternating ones. It is the
continuous sort which is used to drive these motors, but a slight
addition to the machine will make it so that while direct current is put
in, to drive it, alternating current can be drawn out of it. Two little
insulated metal rings are fitted on to the spindle of the machine, and
these are connected in certain ways to the wires of the motor; then
against these rings, as they turn, there rub two little metal arms,
called, because of their sweeping action, brushes; and from these
brushes we can draw the alternating current.

For our present purpose the importance of this lies in the fact that the
rate at which that current will alternate depends upon the speed of the
motor. As the motor increases or decreases in speed, so will the rate of
alternation increase or decrease. So that if we can measure the rate at
which the current drawn from the motor is alternating, we shall know
from that the rate at which the machine is working.

This we can do by the aid of a "frequency meter." The working of this is
based upon the acting of a tuning-fork. Everyone knows that a given
tuning-fork always gives out the same note. The note depends upon the
rate at which the fork vibrates, and the reason that one fork always
gives the same note is because it always vibrates at the same rate. That
rate, in turn, depends upon its length. If one were to file a little off
the end of a tuning-fork, its note would be raised, because its rate of
vibration would become faster. Similarly, lengthening the fork would
result in a lower note being given. Thus, a tuning-fork, or any bar of
steel held by one end, and free to vibrate at the other, gives us a
standard of speed which is very reliable. And it so happens that we can
easily use a set of such forks to test the rate of alternation of an
alternating current.

Generally speaking, alternating current is no use for energising a
magnet. The chief reason for that is that the current tends to get
choked up, as it were, in the coil. Alternating current traverses a coil
very reluctantly indeed. It is, however, possible to make an electric
magnet of special design which will work sufficiently well with
alternating current to answer our present purpose. And it will be clear
that just as the alternating current itself consists of a series of
short currents, so the force of the magnet will be intermittent; it will
give not a steady pull, as is usually the case with magnets, but a
succession of little tugs. There will, in fact, be one tug for every
alternation of the current.

A simple form of motor fitted up as just described, and rotating at 3000
revolutions per minute, would give out 100 alternations per second. If,
then, such current were employed to energise a magnet, that magnet would
give 100 tugs per second.

So a small steel bar of the right length to give 100 vibrations per
second can be fixed with its free end nearly touching such a magnet, and
when the current is turned on it will very soon be vibrating vigorously.
For the tugs of the magnet will agree with the natural rate of vibration
of the bar. And just as the two pendulums described in Chapter XII.
responded readily to each other, so the bar responds readily to the
pulls of the magnet. But increase or decrease the rate of alternation
ever so slightly, and that sympathy between magnet and bar is destroyed.
The bar will not then respond. It will only answer when the pulls of the
magnet and the natural rate of vibration of the bar exactly correspond.

So it is usual to place five or six such bars with their ends near the
one magnet. The lengths of the bars vary slightly, so that the rates of
vibration are, say, 98, 99, 100, 101, 102 respectively.

Let us, in imagination, adjust the speed of a supposititious motor until
we get that which corresponds to 100 alternations.

We switch on the current and at first, possibly, we get no response from
any of the vibrating bars. Just a touch to the handle of the rheostat
and we notice that bar 102 shows signs of life. We see then that our
first speed was much too fast, and that reducing it has brought it down
to 102, which is still a little too fast. Just a little more movement of
the handle, and 102 begins to relapse into quiet, while 101 shows
animation. A little more movement and 101 gives place to 100, and then
we know that our motor is working at the desired speed. If our motor had
been too slow to commence with, it would have been 98 which first got
into action, but the method of adjustment would have been precisely the
same.

And thus we see the whole scheme. We regulate the speed by the rheostat,
and meanwhile that tell-tale stream of alternating current comes flowing
out of the motor to indicate to us what the speed is, while the
"frequency meter," with its various vibrating bars, interprets to us
the message which the alternating current brings to us. So by watching
the meter we know when we have got the speed that we desire.

But even that is only half the battle. We have seen how to make a
machine turn at any desired speed, and so we can adjust any two, so that
they revolve at the same speed, but we have not seen how to start and
stop the two machines at the same time.

First of all, it must be understood that in the case of the receiving
machine there is a friction clutch, as it is termed, between the motor
and the cylinder which it is driving. That means that while, under
ordinary circumstances, the motor drives the cylinder round, we can, if
we like, hold the latter still without stopping the motor. When we do
so, the connection between the two simply slips.

So if we fit a catch on the cylinder which is capable of holding it from
rotating, we can still start the motor, and the latter will work. Then,
the moment the catch is released the cylinder will begin to turn too.
The commonest form of "friction drive" is the flat leather belt upon two
pulleys, which everyone has seen at some time or other in a factory. And
it will be quite easy to conceive how, if one of the driven machines
were to stick, the belt might simply slip upon one of the pulleys, yet,
as soon as the machine became free again, it would rotate just as it did
before. It is just the same with what we are considering. The motor
works continuously at its proper speed, but the cylinder can be stopped
when desired by the catch.

Combined with the catch is an electro-magnet, and through its coils
there flows the current of electricity which is engaged in printing the
picture on the cylinder. If a magnet be arranged to attract another
magnet, it will do so only when the energising current flows one way.
When it flows the other way, it does not attract. Therefore it is easy
to arrange matters so that the printing current, though passing through
the coil of the magnet, shall not pull open the catch. But if that
current be _reversed_ in direction for a moment the magnet gives a pull,
open flies the catch, and away goes the cylinder upon its revolution.

Thus, we see, all that is necessary to start the receiving cylinder is
to reverse the current for a moment.

And now let us turn our attention to the sending machine. Upon its
cylinder there is an arrangement which automatically reverses the
current flowing to the main wire once in every revolution. Normally the
current flows to the wire as described in the last chapter, carrying by
means of its variations the details of the picture for reproduction by
the receiving machine at the other end. But for an instant once in every
revolution that current is interrupted and a current sent in the
opposite direction instead. This the sending machine does of itself,
quite automatically.

And now the reader knows of all the apparatus; it remains only to see
how the different parts work in combination.

Standing by the sending machine we first of all turn on the current,
which goes coursing along the wire to the distant station. Then we set
the motor to work and the cylinder begins to rotate. Before it has
completed a single revolution the "reverser" is operated, and just for a
moment the reverse current goes to the wire. On arrival at the other end
that lifts the catch and the receiving cylinder starts. That first
partial revolution of the sending cylinder counts for nothing. Real
business begins when the reverser first acts, and that is the moment
when the receiving cylinder also begins to move. Similarly, when the
sending cylinder stops it sends no more reversed currents, and so the
receiving cylinder is caught by the catch and not released.

So starting and stopping are quite automatic. The same arrangement
enables a continual readjustment of the relative speed of the two
cylinders to take place. With all the best devices, the tuning-forks and
the rest, it is still impossible to attain perfect unanimity, but the
variation in a single revolution cannot be enough to matter; it is only
when the error in one revolution goes on multiplying itself that serious
difference might arise, and that is prevented in the following
beautifully simple way.

The motor which drives the receiving drum is so regulated that it
travels _slightly faster_ than does the other. Thus the receiving
cylinder completes every revolution slightly in advance of the other,
and consequently it is stopped and held by the catch every time. The
catch retains it, of course, until the reverse current arrives and
releases it. Thus not only does the sending cylinder start the other
when the operations first commence, but it does so every revolution.
Every revolution, therefore, the two cylinders start together.

So the two cylinders are set, according to the frequency meter, at as
nearly as possible exactly the correct speeds, and the action of the
reverser, the reverse current and the catch, ensures quite automatically
that at the commencement of every revolution there shall be perfect
agreement between the two. No accumulation of errors can possibly occur,
and the problem, though apparently so difficult, if not insuperable, at
first sight, is surmounted.



CHAPTER XV

SCIENTIFIC TESTING AND MEASURING


Science, whether it be of the pure variety, that which is pursued for
its own sake--for the mere greed for knowledge--or applied science, the
purpose of which is to assist manufacture, is based entirely upon
accurate testing and measuring. It is only by discovering and
investigating small differences in size, weight or strength that some of
the most important facts can be brought to light. There are some
problems, too, that defy theory, since they are too complicated; they
involve too many theories all at once, and such can only be solved by
accurate tests. And all these necessitate the use of very ingenious and
often costly devices.

Electrical measuring instruments were of sufficient importance and
interest to warrant a chapter of their own, but there are many others of
great value, and not without interest to the general reader.

For example, some years ago there was a collision in the Solent, just
off Cowes, between the cruiser _Hawke_ and the giant liner _Olympic_.
The cause of this was a subject of dispute and of litigation; the
theorists theorised; some reached the conclusion that the _Hawke_ was to
blame, and others the _Olympic_; and where doctors disagree who shall
decide? It was wisely decreed that tests should be made to settle the
question.

The main point was this. The officers of the _Hawke_, by far the smaller
vessel, averred that they were drawn out of their course by suction
caused by the movement of so large a ship as the _Olympic_ in the
comparatively narrow and shallow waters of the Solent; in other words,
that the _Olympic_ in moving through the water caused a swirling,
eddying motion in the water, tending to draw a lighter vessel towards
itself. And that is just one of those problems with which theory is
unable to deal. So it was transferred to the National Physical
Laboratory at Teddington, near London, for investigation by experiment.

At this institution, which is a semi-national one, there is a tank
constructed for purposes such as this. The word tank leads us to
underestimate its size somewhat, for it is 494 feet long and 30 feet
wide. It is solidly constructed of concrete, with a miniature set of
docks at one end, and a sloping beach at the other.

On either side are rails upon which run trollys which support the ends
of a bridge which spans the whole. This bridge can be propelled along,
by means of electric motors operating the wheels of the trollys, from
one end of the tank to the other, at any desired speed, within, of
course, reasonable limits, and from it may be towed any model which it
is desired to test.

The models used are usually made of wax, by means of a machine specially
designed for the purpose. It should be explained that the plans of a
ship consist of a series of curves, each of which represents the contour
of the vessel at one particular height. For example, if you can imagine
a ship cut horizontally into slices of uniform thickness, then each
slice could be shown on the drawing (the "shear plan," as it is termed)
by a curved line. Near the keel the lines would, of course, be almost
straight, but they would bulge more and more as they occur higher up.
And what this machine is required to do is to make, quickly and
economically, a wax model which shall be an exact reproduction, on a
small scale, of the vessel under discussion. It may be--it most often
is--a ship as yet unbuilt, the behaviour of which it is desired to test.
Or it may be an existing vessel, as it was in the case mentioned just
now. However that may be, the model is made from the drawings.

A block of wax rests upon a table, while the drawing is spread upon a
board near by. A pointer is moved by hand along one of the lines, and
its movement is repeated by a rapidly revolving cutter which cuts away
the wax to a similar curve. By suitable adjustments the cutter can be
made to magnify or reduce the size, so as to produce any desired scale.
Thus every line is gone over and a similar curve cut in the wax at the
correct height. Of course this only produces a lump of wax shaped _in
steps_, as it were, but it is then quite easy to trim it down by hand,
so as to produce a smooth model of the ship, perfectly accurate in its
shape, and a copy on a small scale of the vessel portrayed on the
drawing.

It can also be hollowed out, ballasted with weights inside, and so made
to sink to any desired level, thereby representing the vessel when fully
loaded, half loaded and so on. All sorts of unequal loading can be
produced if needed, indeed every condition of the real ship can be
imitated in the model.

It can then be towed to and fro in the tank by the travelling carriage
described above. The speed of towing can be varied by changing the speed
of the motors which drive it. The force needed to pull the model through
the water is measured by means of a dynamometer which registers the pull
on the towing apparatus.

A matter very often needing investigation is the shape and size of the
wave thrown up by the bow of the vessel, and of that left behind her,
known as the "bow wave" and the "stern wave" respectively. These waves
represent wasted energy, for they are no use and are produced actually
by the power of the engines of the ship as they drive her along. The
ideal ship would cause no waves, but since that is a degree of
perfection impossible even to hope for, the shipbuilder has to content
himself by so designing his ships that these waves shall be as small as
possible.

The waves are recorded photographically, in some cases by the
kinematograph.

Some of the large shipbuilders have their own tanks, and so have the
naval authorities of the great naval Powers. The one at Teddington was
established through the munificence of a famous British shipbuilder, Mr
Yarrow, who not only defrayed the cost of construction, but gave an
endowment to assist in its upkeep. It is intended to serve the needs of
the smaller builders who have not tanks of their own, and also for the
investigation of matters of general interest to shipbuilders, and for
such tests as that relating to the _Hawke_ and _Olympic_. In this
last-named case, of course, two models were made, one to represent each
ship, and they were towed along in such a way as to imitate very closely
the movements of the ships at the time when they collided. It was as the
result of these tests that the _Olympic_ was ordered to pay damages to
the Admiralty, it being held that she was the cause of the accident.

A very interesting investigation of this kind was recently carried out
in the tank at the United States Navy Yard. The port of New York
consists very largely of jetties projecting out from the banks of the
river. With the growth of the Atlantic liner the old jetties had become
too short, and questions arose as to the elongation of them. If it were
done, how would it effect the current in the river, and the handling of
shipping generally? If, on the other hand, it were not done, what would
be the effect of the ships lying with their ends projecting out into the
stream unprotected by a jetty.

To determine these points the experimental tank was converted into a
model of the New York Harbour, or at all events of that part in
connection with which these questions arose.

A false floor was put in, so as to make the depth exactly right in
proportion to the width. Little model jetties were arranged to represent
exactly the real ones, while against them were moored model vessels, so
that the effect upon them could be observed as the model of the large
vessel was towed past.

In addition to this, special appliances were arranged for finding out
what the disturbance might be which the movement of a giant liner
produces under the surface as well as above it. For this purpose buoyant
balls were employed, moored at various distances below the surface, from
which thin rods projected upwards, the movement of which rendered
visible the movements of the submerged balls and therefore the effects
of the under-water currents.

All these things had to be observed at one and the same time--the moving
model itself, the models alongside the jetties, the commotion on the
surface, the swayings to and fro of the rods attached to the submerged
floats--all, or most of which, at all events, it was impossible to make
self-recording. Yet, seeing that it was of the utmost importance that
the relations between all these things should be observed, and recorded
from time to time as the model was towed along, it is evident that
something must be done, and a cunning use of the kinematograph solved
the problem quite easily. At various points commanding a good view of
the model harbour and its shipping these machines were placed, and so
several series of photographs were obtained, by the study of which all
the different movements could be seen and compared. A large dial too was
rigged up upon the travelling carriage by which the model was towed, a
finger on which denoted the distance which the carriage had travelled at
any moment. This large dial came into each photograph, of course, and so
each picture bore upon itself a clear record of that particular moment
in the voyage of the model to which it referred.

Thus we see an instance of how the very latest and most up-to-date
methods of amusement are sometimes applied to serve very practical
purposes.

Akin to the experiments upon ships are aerial experiments to determine
matters connected with the navigation of the air. At Barrow-in-Furness
the great firm of Vickers, shipbuilders and armament manufacturers, and
latterly builders of aerial craft for the British Admiralty, have
erected a machine for testing the efficiency of aerial propellers and
other things of a kindred nature. Upon the top of a tall tower there is
pivoted a long arm of light iron framework. To the end of this a
propeller can be fixed, so that as the arm revolves there is produced
almost exactly the same conditions as those which prevail when a
propeller drives an aeroplane or steerable balloon.

By means of suitable mechanism the propeller can be turned at any
desired speed, with the result that it drives the arm round and round
upon its pivot on the top of the tower. The force which the propeller
thus exerts can easily be measured, and so can be determined such
questions as the most efficient speed for each type of propeller, the
power which any particular one can develop, the best form for each
particular need, and so on.

Materials, too, require the most careful testing, in order that they may
be put to the best possible use in modern machinery and structures. For
example, anyone can measure the strength of a spring, but what do we
know as to its lasting power? Springs often have to form part of a
machine in which they are stretched and compressed millions of times,
and the question arises as to what is the best shape and material for
the purpose. It may be that the spring which works best a few times will
be the first to become "weary," for with repeated strain such things as
steel get tired, just as the human frame does. Now that is a matter
which will yield to no calculation, the only way to determine it is
actual test. So a mechanism has to be employed which will extend and
compress the spring over and over again, just as it will be in actual
use, with a counter of the nature of a cyclometer to count how many
times it has been subjected to this distortion. Then the apparatus is
set going and left to itself for hours, or even for days, during which
time it may work the spring millions of times. This may go on until it
breaks, or else it may be done a prearranged number of times, and then
the spring taken out and tested by other means to see how its strength
has been affected.

Metal bars are often subjected to sudden blows, light in themselves but
oft repeated. The point to be determined then is how many times the blow
may fall before permanent injury is done to the bar. To investigate such
matters we have the "repeated-impact" machine. The bar is held in a
suitable holder, under a hammer which gives it a blow, the force of
which can be easily regulated, at regular intervals, the number of blows
being counted by a suitable recording mechanism. Ultimately the bar
breaks, under a blow the like of which it can endure singly without any
apparent strain at all. The machine, by the way, can be caused to turn
the bar round to some degree after each blow, so that it is struck from
all directions in succession.

The microscope, too, has established its place in the testing
laboratory. It is a very valuable adjunct to chemical and mechanical
tests.

Suppose, for example, that a bar of steel is being investigated; it can
be put into a machine and pulled until it breaks in two. The machine
registers the amount of the pull which was applied. Or a small piece can
be put under a press and compressed to any desired degree. It can also
be tested by impact or even pulled apart by a sudden blow, as described
in _Mechanical Inventions of To-day_. The bar can be supported by its
ends and loaded or pulled down in the centre, so that its power of
resisting bending can be determined. It can be judged, too, from its
chemical composition. Steel, in particular, depends for its properties
very largely upon its chemical composition. The difference between
cast-iron, wrought-iron and steel, also the differences between the
innumerable varieties of steel, are due almost entirely to the admixture
of a certain percentage of carbon with the metal. This can be
ascertained by chemical analysis. This form of inquiry has the advantage
over the more purely mechanical methods in that the latter, for the most
part, have to be applied to the bar as a whole, whereas the quality may
vary in different parts, the surface in particular being liable to
differ from the interior. In such cases, one analysis can be made of a
piece cut from the surface and another of a piece from the centre.

And it is here, too, that microscopical analysis comes in. For this
purpose a piece is sawn off the bar, and the end ground perfectly
smooth. This is then washed in a suitable chemical, such as a mild acid,
which acts differently upon the different materials of which the "metal"
is built up, thereby rendering them visible one from another. A
photograph taken through a microscope then shows the structure of the
metal; how the different constituents are built together.

This is known as metallographic testing, and its advantage as compared
with chemical analysis is that the latter shows, as we might say, what
are the bricks of which the thing is built, while the former shows how
the bricks are arranged. Indeed it is hardly correct to speak of the
advantage or superiority of one over the other, since each is the
complement of the other, supplying the information which the other fails
to give.

And there are other mechanical tests which have not yet been mentioned.
There are machines which twist a bar so as to discover its power to
resist torsion, there are others which apply a downward pressure on one
part of the bar and an upward one on an adjacent part, so as to show its
capabilities in withstanding shearing strain.

Moreover, many of these tests are nowadays, in a well-equipped
testing-house, carried out in conjunction with the use of heat. It
stands to reason that a part of a machine which will have to work under
considerable heat may have to be of different material from a part which
works under a normal temperature. In some cases the bar is surrounded by
a spiral wire through which electric current is passing, and by the
regulation of this current any desired temperature can be set up in the
bar. Or it may be placed in a bath of hot oil in such a way that the bar
shall be raised to any temperature required, without interfering with
the machinery which exerts the tension or pressure, or whatever it be.

Years ago such elaborate tests as these were never thought of. There are
certain well-known figures, to be found in all engineering text-books,
which give what stresses different materials ought to be able to stand,
and these were, and are still, to a large extent, relied upon, it being
taken for granted that the material used will be up to the average
standard. In large and important works, however, the testing has been
developed upon scientific lines, so that it is known from actual
experiment what each particular thing is capable of. This not only means
security but economy, for it is sometimes found that a substance is
stronger than it is thought to be, and so things made of it can be
designed to give the requisite strength lighter and cheaper than they
would have been otherwise.

Some of the machines employed are of enormous strength, capable of
exerting a pull or a compression of, it may be, 100 tons or more. They
are often made, too, with self-recording appliances, whereby the course
of the test is set down automatically upon a chart. For example, when a
bar is being tested for tension, it is desirable to know not only the
actual pull under which it came in two, but the behaviour of the test
piece during the period before that. It begins to stretch as soon as the
tension is applied, theoretically at all events, and if the metal were
perfectly ductile it would stretch continuously as the load increases,
until at last the breaking stress is reached. But in actual practice it
probably stretches somewhat by fits and starts, and a record of that
fact will be of great value in estimating the strength of the material
in actual work. For such, an automatically made record, which can be
studied at leisure, is of the utmost importance.

But perhaps the finest instance of scientific methods in manufacture is
to be found in the methods by which standard parts of machines are
measured, so as to ensure that they shall be interchangeable.

It may surprise the casual reader to be told that an absolutely exact
measurement is an impossibility. It is safe to say that out of a
million similar articles--articles made with the intention that they
shall be exactly alike--there are no two which are, in fact, absolutely
similar. They may be made with the same machines and the same tools,
handled by the same man, but machines and tools wear or get out of
adjustment, while man's liability to err is proverbial. Astronomers are
the greatest experts in the art of measurement, and they recognise the
possibility, nay, the probability, of error so frankly as to make every
measurement several times over; if it be an important one they make it,
if possible, a great many times over, and then take the average of the
results. By this means they eliminate, to a certain extent at any rate,
the error which cannot be avoided. That process is to allow for errors
on the part of their instruments, for the most part. To deal with
personal errors another method is used as well, for it is known that
some observers have a natural tendency to err on one side more or less,
while others tend to make mistakes in some degree on the other side.
This tendency to err is known as the "personal equation" of the
observer, and there are machines and tests by which the personal
equation of each man can be determined, or perhaps it would be more
correct to say estimated, so that in all observations made by him the
proper allowance can be added or deducted.

But of course it would be extremely difficult to apply such methods in a
workshop. It would never do to have to measure everything several times
over, hoping that the average would come out in such manner as to
indicate that the thing being measured was the size required. Instead,
therefore, of wasting time seeking an accuracy which is known to be
unattainable, the manufacturing engineer adopts a scientific system of
measurement wherein a certain amount of inaccuracy is determined upon as
permissible, and then simple appliances are used to see that it does, in
fact, fall within those limits. For instance, a round bar is to be made,
say, an inch in diameter. Now we know from what has just been said that,
when made, we have no means of telling whether the bar is really and
truly an inch in diameter or not. We consider, then, what it is for, and
decide, say, that it will be near enough so long as we are sure that it
is not larger than one inch plus one thousandth, nor less than one inch
minus one thousandth. So long as it does not exceed or fall short of its
reputed size by more than one thousandth of an inch, then we know that
it will answer its purpose.

Now, having come to that decision, we can build up a system upon which
any intelligent workman can proceed, with the result that all the inch
bars which he makes will be the same size within the limits of 1/1000
over or under, so that the greatest possible difference between any two
will be 1/500.

This system involves the use of two gauges for every size. The man
employed upon making one-inch bars has a plate with a hole in it
1-1/1000 inches in diameter and another hole 999/1000 of an inch in
diameter. One of these is the "go in" gauge; the other is the "not go
in." So that all he has to do, in order to be quite sure that his work
is right, is to see that it can be poked through one of these holes, but
not through the other. No trouble at all, it will be observed, adjusting
fine measuring appliances, simply a plate with two holes in it, and the
workman can be sure that he is turning out articles every one of which
is practically correct, with no variation beyond a slight inequality too
small to matter.

And probably at some other part of the factory there is a man making
articles each of which has a hole in it, into which this bar must fit.
How does he manage? He is provided with a gauge somewhat the shape of a
dumb-bell, one end of which is slightly larger than the other. One is
the "go in" end, the other the "not go in" end. If the hole which he
makes will permit the former to enter, but will refuse admittance to the
latter, then he knows that that hole is sufficiently near its reputed
size to answer its purpose.

[Illustration: _By permission of The Mining Engineering Co., Sheffield_

                        A MINERS' RESCUE TEAM

These men are equipped with breathing apparatus which enables them to
pass safely through the deadly fumes after an explosion, to rescue their
                        unfortunate comrades]

In the instances mentioned, a thousandth of an inch either way has been
mentioned as the limit of inaccuracy, or the "tolerance," as it is
sometimes termed, but often the limits are much narrower than that. The
gauges themselves are a case in point, for they must be true within,
say, a ten-thousandth, or even less. And they too are checked by master
gauges of a finer degree of accuracy still, being made by the most
laborious methods, and checked over and over again, so as to reach the
utmost limits in the way of correctness.

So this methodical "scientific" system of "limit gauges" is based upon
the principle of having one gauge limiting the error one way and another
defining it in the other. Anything simpler or more effective it would be
impossible to conceive. It is due very largely to this system that many
manufactured articles are now so much cheaper than they used to be. For
it enables each individual part to be made wholesale on a large scale,
by machines specially adapted to the work, operated by men specially
trained to work them, with the practical certainty that these parts when
assembled together will fit each other.

In conclusion, there is another very interesting instrument which was
first made for a purely utilitarian use--namely, the investigation of
the methods of making coloured glass--but which has since been applied
to some interesting problems in pure science. It is called the
"ultra-microscope."

It must first be pointed out that there is a limit to the power of the
ordinary microscope, beyond which the skill of the optician cannot go.
He is baffled at that point not because of any lack of ability on his
own part, but because of the nature of light itself. An opaque object,
unless it be self-luminous, which few things are, can only be seen by
reflected light. Generally speaking, we see things because they reflect
in some degree the light which falls upon them. But light consists of
waves, and when we reach an object so minute that its diameter is about
half the wave-length of light, then we cannot see it because it is
unable to reflect the light on account of its smallness. We can see
this any day by the seaside, or by a river or large pond. There it is
evident that the waves and ripples are reflected by such things as large
stones, wood posts or anything of any size which come in their way; but
when a wave encounters an object much smaller than itself it simply
swallows it up, as it were, flows all over it or around it, without
being in any way reflected by it. And it is just the same with the waves
of light; they are unaffected by obstacles below a certain size, and so
are not reflected by them. For this reason things smaller than about a
seven-thousandth of a millimetre cannot possibly be seen by a microscope
in the ordinary way.

But if an object can be made self-luminous, then it can be seen,
whatever its size, if the magnifying power of the microscope be great
enough. So this ultra-microscope, as it is called, is really an ordinary
microscope of the highest power possible, with an added apparatus for
making the tiny particles which are being sought for self-luminous. This
is done by directing upon them a pencil of light of exceeding intensity.
Generated by powerful arc lamps, the light is concentrated by a system
of lenses until it is of an almost incredible brightness, after which it
falls upon the object.

Now at first sight this seems to be no different from the usual
procedure with a microscope, and there appears to be no reason why it
should be more successful, but the explanation is this: light is a form
of energy, and the waves of this very intense beam, falling upon the
object, throw it into a state of violent agitation, by virtue of which
it shines, not with reflected light, but with light of its own. It is
not that the waves are reflected, but that they so shake up the particle
that it gives off light waves itself. And thus it comes within the range
of human vision.

In this way, not only have the very small particles of colouring matter
in glass been seen individually, but it is thought that the actual
molecules of matter have been seen, or if not the molecules
individually, little groups of molecules, dancing and capering about,
just as scientific people for years have believed them to be doing,
although they could not see them. So here we have an instance in which
manufacture has aided science--an inversion of the usual order of
things.



CHAPTER XVI

COLOUR PHOTOGRAPHY


Photography has introduced many of the general public to a branch of
practical science which otherwise they would never have cared much
about. The action of light upon certain chemicals, the subsequent action
upon the same of other chemicals, such as developers, toning solutions
and so on, form a very well-known region of the domain of science. And
this is, too, a branch of chemistry in which the practical inventor has
been very busy. The efforts, therefore, which have been made to invent
ways of producing photographic pictures which shall give to the objects
their natural colours, will probably be of special interest in a book
like this.

Of these there are two very well-known systems, and to them we will
mainly confine our attention.

It should first be pointed out, however, that what we are discussing is
quite different from the simple "orthochromatic" plates which are used
by many photographers. These latter are coated somewhat differently from
other plates, with a view to their giving a more realistic picture, but
the result is still in one colour. They are, in fact, a little more
sensitive to differences in colour than ordinary plates, so that colours
which appear, when the latter are used, very much the same, appear, when
orthochromatic plates are employed, a little different. But the
difference in colour in the object photographed is only, even then,
represented by a difference in shade in the picture. The object is, it
may be, in many colours, in all the colours, very likely, but the
picture is only in one.

And the step from that to a coloured picture is a very long one. True,
the solution of the problem is very simple in principle, yet the
practical difficulties are so great that even now they have not been
entirely overcome.

Let us first of all examine the principle. Sunlight, by which
photographs are usually taken, appears to the eye white and colourless.
It is not really so, however, as can be proved by analysing it with the
spectroscope. In this instrument a flat beam of light, having passed
through a narrow slit, falls upon a prism of glass, from which it
emerges as a broad band, known as the "spectrum." This band can be seen
upon a screen, or can be examined through a telescope. So far from being
white and colourless, it consists of the most lovely colours. At one end
of the spectrum is a beautiful red, which, as the eye travels along,
imperceptibly merges into orange, which in turn merges into yellow,
after which we find green, blue, indigo and violet, in the order named.
These seven are known as the "primary colours," but it is quite a
mistake to suppose that there are seven clearly defined and distinct
colours. The colours so change, one into another, that their number is
really infinite. The seven names indicate seven points in the spectrum,
whereat the colours are sufficiently distinct from others to warrant a
separate name being given to them. We call the starting colour red, for
example, and as we pass our eyes along we perceive a constant change,
and when that change has become sufficiently pronounced to justify our
doing so, we call the new colour "orange." Continuing, we find the
orange changing into something else, and when it has gone far enough, we
bring in a third name, yellow, and so on to the violet. Thus we see the
division into seven colours is arbitrary, and only for our own
convenience, since the whole number of colours is innumerable.

Passing through a prism is not, however, the only means by which white
light can be split up. When the sun shines upon a blue flower, for
instance, the blue petals perform a partial separation; they reflect the
blue part of the sunlight, and absorb all the rest. A red flower
likewise reflects the red part of the sunlight and absorbs the rest. It
is because things can thus discriminate, reflecting some kinds of light
and absorbing the remainder, that we perceive things in different
colours.

It follows, therefore, that when we look upon a landscape, or a field of
flowers, we receive into our eyes an enormous variety of coloured
lights. The white sunlight furnishes each thing we see with a flood of
white light, and each thing according to its nature, reflects more or
less. A white flower reflects the whole, a pure black object reflects
none, but the great majority of things reflect some part or other of
that infinite variety of which white light really consists.

So a view at all varied sends to our eyes a variety of colours, almost
as manifold as the colours of the spectrum, which, as has been said, are
infinite. And the task of reproducing them, or even of producing a
similar general effect, upon a piece of paper seems at first sight
beyond the bounds of possibility.

But fortunately there is a way by which we can produce, approximately at
all events, the intermediate colours by mixtures of the others. The
second colour of the spectrum, for example, orange, can be obtained by
mixing its neighbours on either hand--namely, red and yellow. We can,
indeed, imitate very closely the imperceptible change from red to yellow
through orange, by skilful mixture of red and yellow pigments. First
there is the pure red, then just a suggestion of yellow is added; more
and more yellow brings us to orange; after which by gradually
diminishing the amount of red we reach the pure yellow. Next, by
introducing blue pigment, we can gradually change the yellow into green,
and further manipulation of the same two colours will lead us on to pure
blue. Indeed by mixtures of red, yellow and blue we can obtain almost
all the perceptible varieties of colour.

And it must be remembered that when, by mixing blue and yellow pigments,
we get the effect of green, that is only the result of an optical
illusion. The particles of which the yellow pigment is made remain
yellow, and the particles of blue remain blue. The one sort reflect
yellow light to our eyes, the other sort reflect blue light, and owing
to what in one sense may be called a defect in our vision, these two
mingling together look as if the whole were green. In the spectrum we
see real green light; from green paint made by mixing yellow and blue,
we only see an imitation or artificial green. If the particles were
large enough, we should see the yellow and the blue ones quite separate,
but since they are too small for us to see at all, except in the mass,
our eyes blend the whole together into the intermediate colour.

Thus we see that, although the variety of colours is infinite, we can
for practical purposes reproduce as much difference as our eyes can
perceive by the judicious blending of three--namely, red, yellow and
blue.

And there is a further fortunate fact--we can filter light. The red
glass with which the photographer covers his dark-room lamp looks red,
and throws a red light into the room, because it is acting as a filter
to the light proceeding from the lamp behind it. The lamp is sending out
light of many colours, but the glass is only transparent to the red. It
holds up all the others but lets the red pass freely. So if we were to
take a photograph through a red screen, we should get on the plate only
those parts which were more or less red in colour. For example, if we
thus photographed a group of three flowers, one red, one orange and one
yellow, the red one would come out prominently, the orange one would
come out faintly, and the yellow one not at all.

Then suppose we took the same picture again through a yellow screen. In
that case the yellow flower would be prominent, the orange would again
be faint, but the red would be absent.

Having got, in imagination, two such negatives, let us make two carbon
prints, one off each. And let the print off the first negative be red,
while that off the second is yellow. Let each be, in fact, of the same
colour as the screen through which the picture was taken. Finally, let
the two films be placed in contact one upon the other. On holding the
two up to the light, what should we see?

We should see a red flower, for there would be a red flower clearly
defined upon one film coinciding with a blank transparent space upon the
other film. We should see, too, a yellow flower, for a clearly defined
yellow flower on the second film would coincide with a clear space upon
the first. We should see also an orange-coloured flower, for there would
be a faint red image of it, and a faint yellow image of it, one on each
film, lying one over the other, producing the same effect as a mixture
of yellow and red pigments. Thus by taking two negatives through two
coloured screens, and then colouring the prints to correspond, we can
obtain three colours in the finished picture.

By taking a third negative, through a blue screen, we could add
immensely to the range of colours obtainable. Indeed, with three films,
red, yellow and blue respectively, made through three screens of the
same colour, a variety of colours practically infinite can be obtained.

So the principle is quite simple; the difficulty is in carrying it out.
For the three kinds of light have not the same photographic power, and
so to avoid upsetting the "balance" of the colours different exposures
would be required for each. Then there is the difficulty of so
manipulating the films as to get them one over another exactly. Anyone
who has tried the handling of carbon prints will readily realise how
difficult this would be. It is possible and has been done, but the
process is too uncertain and too laborious to be of general use.

But the same result can be attained more or less automatically, as the
following descriptions will show.

Let us turn to the Lumière autochrome process, by which the results
desired can be in a large measure attained by methods of manipulation
comparatively simple.

[Illustration: _By permission of The Mining Engineering Co., Ltd.,
                          Sheffield_

                   PNEUMATIC HAMMER DRILL

This tool is used by miners for making holes in hard rock, preliminary
to blasting. Note the spray of water, which prevents the stone dust
     rising and getting into the miner's lungs.--_See_ p. 220]

The plates used for this are of a very special nature. In the first
place, there is the basis of glass, but upon that there is laid what we
might term the selective screen. This is a layer of starch grains, of
exceeding smallness. The size of them is as little as a half a
thousandth of an inch and there are about four millions of them on every
square inch of plate. Next, upon the screen of starch grains is a layer
of waterproof varnish, while over that is the ordinary sensitive
emulsion such as forms the essential part of the usual non-colour plate.

Now the starch grains which form the screen are, before they are laid
on, stained in three colours. Some are blue, some red, and some a
yellowish-green, which experience shows is preferable to pure yellow.
The differently coloured grains are well mixed, and when the screen is
held to the light and looked through the effect is almost that of clear
glass. That is because red rays from the red grains, and green and blue
rays from the grains of those colours, all proceed to the eye mingled
together.

This plate is placed in the camera differently from the usual way, since
the glass side is turned towards the lens. The light, therefore, after
entering the camera, passes through the glass, then through the screen,
and finally falls upon the sensitive film.

Suppose, then, that the camera were pointed to a red wall; red light
would fall upon the plate and, passing through the red grains, would act
upon the sensitive film behind them. The blue and green grains, on the
other hand, would stop those rays which fell upon them, and so those
parts of the sensitive film which they cover would remain unaffected by
light. Then, if that plate were to be developed, a dark, opaque spot
would be produced upon the film under each red grain, the film under the
other grains remaining transparent. Hence, when held up to the light and
looked through, the plate would appear a greenish-blue, for all the red
grains would be covered up.

In like manner, if the wall were blue instead of red, a greenish-red
plate would result, while if it were green, the plate would be a purple,
the result of the combination of red and blue.

But this, it will be seen, is a topsy-turvy effect, the exact opposite
of what we want, so that it is fortunate that by a simple chemical
method we can set it right. After a first development in the ordinary
way the plate is placed in another bath and exposed to strong daylight,
with the result that those parts which were darkened by the first
development become clear and the parts which were clear become opaque.
Thus, after this twofold development of the photograph of the red wall,
we find ourselves in possession of a red plate, in which only the red
grains are visible, since all the others are covered up by opaque parts
of the sensitive film. The photograph of the blue wall will also, after
it has been subjected to the double development, show blue only, and the
same with the green.

But suppose that instead of a red wall or a blue wall we focus our
camera upon one which is half red and half blue. Then it is easy to
perceive that we shall get a plate which is half one colour and half the
other. Moreover, it follows that a wall covered with a mosaic of red,
blue and green would give us a plate duly coloured in the same way.

But when we go a step further and photograph, say, a landscape, which
may contain a vast range of colours, we find a difficulty in believing
that they can all be rendered by the simple process of covering or
leaving uncovered grains either blue, red or green. It can be done,
however, since the other colours may be made up of two or more of these
three in varying proportions. For example, should there be something in
the landscape of a darker, more blue, shade of green than the green
grains, then the light proceeding from that object, while passing freely
through the green grains upon which it falls, will slightly penetrate
the neighbouring blue ones as well, and so at that point on the plate
there will be not only green grains visible, but some of the blue grains
partly visible also. The light from the blue grains will enter the eye
along with that from the green grains, and by so doing will add just
that amount of blue to the green as to give it the right shade.

After this manner is the whole picture built up. It is, of course,
really a mosaic, consisting entirely of little coloured patches, but
since they are so small none can be seen individually, all merging
together in the eye so as to form a picture in which colours change
imperceptibly from one into another.

To sum up, then, what happens is this. We start with a layer of coloured
grains; the action of taking and developing the photograph covers up
some of these grains and leaves others exposed, and the action of the
light is such that those which are left visible produce a picture
closely resembling the original, not only in form but in colour.

But there is one other interesting point about this process which
deserves mention. The differently coloured lights are not of the same
power photographically. Red light, as we know well, is very weak in this
respect, wherefore, we use it in the dark-room. A faint red light will
have no perceptible effect upon a plate unless it be exposed to it for
some time. Blue light, on the other hand, is very active, and were the
blue and red lights to be allowed to act equally on the autochrome
plate, the result would be much too blue. It is therefore necessary to
handicap the blue light, as it were, by placing a "reddish-yellowish"
screen either just in front of, or just behind, the lens to cut off a
proportion of the blue rays.

The other very successful process is known as the Dufay dioptichrome
process. It differs very little from the Lumière except in detail, the
selective screen being formed of small coloured squares instead of by a
mass of little grains.

In both, it will be noticed, the result is a single positive. It is not,
as in ordinary photography, a negative off which any desired number of
positive prints can be made. And, moreover, it is a transparency: it
cannot be viewed except by light shining through it. The results are,
however, extremely beautiful, when well done, and anyone who cares to
try either of these methods of working will be well repaid for the
trouble involved.



CHAPTER XVII

HOW SCIENCE AIDS THE STRICKEN COLLIER


Nothing is more characteristic of the present age than the care which
is, quite rightly, expended upon the comfort and safety of those who do
the manual labour of the community. The stores of scientific knowledge
and skill are drawn upon freely for this end, and some very interesting
examples can be given of the truly scientific methods which have been
evolved, not only for preventing injuries of any kind, but for
succouring those who may, despite those precautions, fall victims to
disease or accident.

An example has already been given of the scientific investigation into
the nature of colliery explosions and the best means of preventing them.
We have seen there how expense has been poured out lavishly in fitting
up the experimental gallery or artificial pit, and how the most cunning
mechanical and electrical devices have been pressed into the service in
order to find out just what happens when an explosion occurs. It has
been related how these investigations have revealed with certainty the
true cause of the explosions and thereby led the way to their
prevention.

But with it all there is still an occasional disaster, occurring,
sometimes, at the best and most carefully managed collieries. And
therefore it is still necessary to provide for rescuing the unfortunate
men who are affected.

It is worth remark, here, that colliery explosions are, all things
considered, a very rare occurrence. Because of their dramatic
suddenness, and the number of lives which are commonly lost in a single
disaster, we are apt to magnify their severity in our minds and to
picture the life of the miner as a very hazardous one. In point of fact,
the expectation of life, as the insurance people call it, is quite as
great among the coal-miners as among any class of manual labour. And of
those who do meet an untimely end there are more lost through isolated
accidents, involving one or two men, than in the great disasters.

To meet these isolated cases science is almost powerless. For the most
part, they are due to falls of material from the roof of the mine, or
some simple accident of that kind, caused by an error of judgment or
lack of care on the part of fellow-workmen, and the only safeguard
against such is the most careful and systematic supervision, which, in
Great Britain at all events, is rigidly applied. The underground staff
are very carefully organised with this end in view, and the whole is
supervised by Government inspectors. No amount of scientific
investigation or invention will help much in these matters.

With the explosion or fire, however, it is different, for there subtle
forces and strange chemical influences come into play with which science
is specially well fitted to deal.

To a great many people the first news of organised, trained and
scientifically equipped rescue parties came at the time of the terrible
Courrières disaster in France, when over 1000 men lost their lives. For
then a party with apparatus hurried from Germany and played a prominent
part in the rescue operations. But unfortunately the glamour of their
performance was somewhat dimmed by the fact that after they had done all
they could, and had gone home again, more men were rescued. Many,
reading of that fact, were inclined to scoff at the "new-fangled" ideas,
thinking that after all the old way of working with a party of brave but
untrained and often ignorant volunteers was better than the new way of
working with equipped and trained men. It certainly did seem as if the
former had succeeded where the latter had failed. But that was quite a
mistake, as subsequent events have shown, and in all probability it was
due to the fact that the uninstructed party were local men, thoroughly
familiar with the mine in which they were working, its geography and
its special local conditions, whereas the trained men came from far
away.

At all events the pioneer work of the Germans in the matter of rescue
teams has been amply justified by the fact that other people have copied
them, and none more thoroughly than the mining authorities of Great
Britain. Indeed we see here another instance of the remarkable way in
which the British people, though a little slow to take up a new idea, do
take it up when it has once been established, and in such a way that
they are soon among the foremost in its use. The Germans, all honour to
them, started the rescue teams, but at this moment there are rescue
teams and stations for their training in Britain second to none in the
world. Of these there is a splendid example in the Rhondda Valley, in
South Wales, supported and worked by the owners of the pits in that
district, besides others at Aberdare, in the same neighbourhood, at
Mansfield, to serve the collieries in Derbyshire and Nottinghamshire;
indeed rescue stations are now dotted throughout the mining districts.

The general idea of these stations is as follows. The building is
centrally situated in the district which it is intended to serve, and in
it are kept an ample supply of the necessary appliances, in the shape of
breathing apparatus, which enables men to walk unhurt through poisonous
gas, reviving apparatus, by which partially suffocated men can be
brought round again by the administration of oxygen, together with
quantities of that valuable gas in suitable portable cylinders.
Everything which forethought can suggest as even possibly useful in an
emergency is kept in a constant state of readiness. And all the while a
swift motor car stands ready to carry them to the scene of operations.

But the appliances are of little use without men to work them, who know
them and can trust them. The case of David, who felt able to do better
work with his sling and stone than in all the panoply of Saul's armour,
because he "had not proved it," is typical of a universal human
instinct. A man feels safer unarmed, or simply armed, than he does with
the most elaborate weapons in which he has not learned to have
confidence. And therefore the men who may be called upon to work this
apparatus are first taught to have confidence in it. Each station has
its instructor, who is usually also the general superintendent of the
station, and "galleries" in which the instruction can be carried out.

Volunteers are called for in each colliery and a number of the most
suitable men are chosen to undergo training, preference being given,
very naturally, to those who are already trained, as fortunately so many
workmen are nowadays, in ambulance work.

These chosen men then repair at intervals to the station to undergo a
proper course of instruction. The instructor, often an ex-non-commissioned
officer in the Royal Engineers, accustomed, therefore, to engineering
matters, and also to systematic discipline, there puts them through a
course of drill the object of which is to teach them to work together as
a squad under the orders of a properly constituted chief. Thus when called
upon in some emergency there will be no confusion, but each man will know
what to do, and a few short words of command from the chief will serve
better than the long explanations which would be necessary with an
undisciplined body. It welds the individual men, as it were, into a
smoothly working machine, thereby increasing the efficiency of the whole.
And arrangements are made whereby, should the leader fail, another man
steps into his place of authority at once and without question.

Then, having thus brought them under a suitable discipline, the
instructor takes his men into the experimental gallery. This may be
described as a long, low, narrow shed, in which are timber props and
beams, rails on the floor, heaps of coal, all things, in fact, which may
tend to make it closely resemble the actual workings of a coal-mine
after they have been shaken and shattered by the force of an explosion.

The great difficulty, in a real disaster, arises from what are known as
"falls." The roof of the mine is normally supported by timbers, and
these the explosion moves, so that in places many tons of the earth of
which the roof of the mine consists will fall and block completely the
"roads" or tunnels which communicate from the shaft to the places where
the men are at work. These, of course, have to be removed or burrowed
through before the men imprisoned in the distant workings can be
reached. The rescue party do not, of course, wait to clear away the
whole of this debris, only just enough to enable them to crawl through
or over it, but even then it often represents the waste of precious
hours, and the expenditure of great exertions, to get past a "fall." So
at intervals "falls" are made in the gallery, in order that men may be
practised in dealing with them.

[Illustration: _By permission of W. E. Garforth, Esq., Pontefract_

                        AN ARTIFICIAL COAL MINE

These two photographs show the clouds of flame and smoke issuing from
the mouth of the "Artificial Coal Mine" during the experiments described
                            in the text]

It may be interesting to give a brief statement of the training
undergone by the men at the Mansfield Rescue Station. In that case, it
should be stated, the gallery is made double, so that men can go one way
and return the other back to their starting-point. Having donned their
breathing apparatus, they enter the gallery, which, by the way, is
filled with smoke and foul gas. Passing along it, they encounter two
falls, which they must get over or through; then they have to set twelve
timber props as might be necessary to maintain the safety of a damaged
road in the mine; all that they do three times over. Then they are
required to bring up and lay 250 bricks, a thing which might also be
necessary in an actual emergency, after which they have to fix up
"brattice cloth" in a part of the gallery. One of the first duties, of
course, for a rescue party is to restore the circulation of air in the
mine, and brattice cloth is a rough kind of cloth which is put to guide
the air currents. That done, they have to take a dummy representing a
man of 14 stone, put it on a stretcher, and carry it round the gallery
and over the falls. Finally, they restore the timber, bricks and cloth,
and their turn of work is done. The total time required for this is two
hours, and during the whole of that period they are, of course,
breathing not the natural air, but the artificial atmosphere provided
for them by the apparatus with which each man is provided. The chief
point of this part of the training, as has been remarked already, is to
accustom the men to the wearing of the apparatus and to doing work in
it. By this means they gain confidence in it, and get to know that it
will not fail them in the time of trial.

The course of instruction consists of ten drills such as has been
described, after which the men are called up twice a year, just to
refresh their memories.

One side of the gallery is glazed, so that the instructor can watch his
men at work without of necessity being inside himself, and there are
emergency doors as well, which can be opened to let a man out should the
ordeal be too much for him. The necessary "fumes" are generated in a
stove and driven into the gallery by a fan. The stations are beautifully
fitted up, with baths for the men to wash after their somewhat dirty
experience in the gallery, and everything is done for their convenience
and welfare.

The advantage of this systematic training of a great number of men is
that there are men at each colliery who can be called upon when needed.
The team of strangers, as has been remarked, partially failed at
Courrières, largely because they were strangers, but when every colliery
has a team ready, composed of its own men, then clearly there is the
greatest chance of success. The central station of the district is the
training-ground where the men go from all the collieries to get the
experience and instruction, and where a reserve store of appliances is
kept. In many cases, of course, the collieries have their own
appliances, so that work can be begun at once, without having to wait
for that from the rescue station, but the latter forms a reserve in case
of need, and, being kept under the care of an expert, it is naturally
always in the best possible working order.

To give an idea of the cost of these stations, it may be stated that the
one at Porth, in the Rhondda Valley, cost, including equipment, £7000,
while the one at Mansfield cost £3000. This first cost and the expense
of maintenance is borne by the collieries of the district in proportion
to the quantity of coal which they raise.

And now we can turn to the apparatus itself, without which the
organisation already described would be of little value.

There are several makes of these, but a description of the particular
apparatus used at the two stations mentioned will serve as an
illustration. The purpose, of course, is to give the wearer an
atmosphere of his own, which he can carry about with him, and which will
render him quite independent of the ordinary atmosphere and quite
indifferent to the poisonous nature of the gases around him. To this end
his mouth and nostrils must be cut off from the outer world altogether.
There are two ways of doing this. In the one there is used a helmet, or
perhaps mask would be the better term. This fits right over the man's
face, an air-tight joint being made between the helmet and his head by
means of a rubber washer which can be inflated with air. The inflation
is accomplished by squeezing a rubber ball on the right-hand side of the
helmet. In the centre is a glass window through which he can see easily,
and since this is apt to become clouded by the dampness of his breath
there is a wiper inside, which can be turned by a knob on the outside,
so that by simply turning his knob with his hand he can clean the window
at any time that may be necessary. Two soft pads inside the helmet bear
one on the man's forehead and the other on his chin, and these, working
in conjunction with a strap which passes right round the back of his
head, keep the thing firmly in position. In addition there is combined
with the helmet a leather skull-cap which, being continued down behind,
gives good protection to the head and neck.

The other form of apparatus consists of a mouth-piece and nose-clip. The
mouth-piece, as its name implies, fits in the man's mouth, being
supported and kept in position by a strap passing behind the back of his
head. Combined with it is a little screw clip which closes his nostrils.
The man also wears a leather skull-cap, from which straps depend to
bear the weight of the mouth-piece and its attached tubes, so that the
weight does not fall upon his mouth.

Either of these arrangements, it is clear, cuts him off from
communication with the outer air, but that is only half the problem, for
he must be given a substitute or he will be suffocated.

This part of the appliance he carries, knapsack fashion, upon his back.
First there is a rectangular case, called the regenerator, with, below
it, two small cylinders of compressed oxygen. A suitable arrangement of
pipes connects these together, and to the helmet or mouth-piece as the
case may be.

When the man exhales, as we all know, the air which he then discharges
from his lungs is deficient in oxygen and instead contains carbonic acid
gas. The latter must be got rid of and replaced by pure oxygen. The
exhaled air is therefore led down a pipe to the regenerator, where it
comes into contact with several trays of caustic soda, a chemical which
has a great affinity for carbonic acid. The result is that the latter
gas is extracted from the impure air, finding a more congenial home in
the caustic soda. It is then necessary to restore the normal quantity of
oxygen, and so, as the air passes on, it meets, in a little apparatus
known as an injector, a spray of pure oxygen from the cylinders. Thus,
after being purified and re-oxygenated, the air passes on through more
pipes to the helmet or mouth-piece, to be breathed once more. The
apparatus contains sufficient oxygen and caustic soda for this to go on
for a space of two hours.

But during times of extra exertion a man needs more air than at others,
for which provision has to be made, and so on his chest the rescuer
carries a flexible bag divided into two compartments. Through one of
these the exhaled air passes on its way to the regenerator, while
through the other the oxygenated air flows on its way to the man's
mouth. When he is breathing hard, then, during a moment of extra
exertion, and when, therefore, he is turning out bad air faster than it
can be purified, and drawing in pure air faster than it can be
produced, this bag comes to his aid. From the store of oxygenated air in
one side of it he draws the extra which he requires, while the other
side stores up temporarily the excess of vitiated air, until the
regenerator is able to overtake its work. Thus at all times, whether
breathing ordinarily or heavily, the apparatus can respond to his
demands.

The spray of oxygen as it escapes from the cylinders into the injector
has the effect of driving the air along, so that the circulation through
the tubes and the regenerator is automatic, and the foul air flows away
from the man's mouth and the new air comes back to him quite without
effort on his part. As time goes on, of course, and the stored oxygen
becomes used up, the pressure in the cylinders falls, which fall, shown
upon a little pressure-gauge, tells the man how much longer time he has
before he must return for fresh supplies of oxygen and soda. Fresh
cylinders of oxygen can be connected up very quickly in place of the
empty ones, while a fresh regenerator can be put in, or new caustic soda
supplied, in a very short time.

The superintendent of the Mansfield station has invented what is termed
a "self-rescue" apparatus, to be used in conjunction with that which has
been described above. It is simpler and lighter than the rescue
apparatus, and will not keep a man supplied with air for more than an
hour or an hour and a quarter. Moreover, it is not automatic, since the
flow of oxygen has to be controlled by the man himself. Since, however,
it consists only of a mouth-piece, a breathing-bag and a cylinder of
oxygen, it is very portable, and may well be carried by a rescue party
for the use of any men who may be discovered alive beyond the danger
zone. It may well happen, indeed it often has happened, that a remote
part of a mine, although cut off from the shaft by passages full of
"after-damp," as the foul gases caused by the explosion are termed, may
itself contain fairly pure air in which men can live for a long time. If
such men be reached, the difficulty is to get them through the passages
containing the bad air. Consequently a rescue party which carried one
or two of these light forms of apparatus could equip such men with them
and then they could pass out with safety.

Another use, the one, in fact, from which the appliance draws its name,
is the facility with which, by its aid, a man could set right a chance
defect in his ordinary rescue apparatus. Suppose, for example, that a
fully equipped man found something wrong, whereby he was prevented from
getting his proper supply of purified air. Then, if the party had one of
the self-rescue sets with them, he could slip off his helmet or
mouth-piece, quickly replacing it, for a time, with the self-rescue
mouth-piece. This might enable him to reach safety, or even to put the
other apparatus right and then don it once more. The whole thing can be
packed up into a small tin case which can be slung over one shoulder,
and with the oxygen cylinder slung over the other one the complete
outfit can be carried quite easily by a man in addition to what he is
wearing himself.

Still another form of breathing appliance may well be taken on these
rescue expeditions, and that is the reviving apparatus, for use upon
those who have apparently ceased to breathe. In this case a mask is put
over the sufferer's mouth and nose, and then the turning of a lever into
a certain position causes oxygen to escape from a cylinder in such a way
as to cause a suction which empties the man's lungs of the bad gases
which have laid him low. That done, another movement of the lever and a
deep breath of oxygen flows into his lungs in their place. Thus by
alternating the positions of the lever an artificial respiration is set
up far more effective than can possibly be attained by the ordinary
method of moving the man's arms and pressing his chest. Indeed there are
cases, such as when his arms or ribs are injured, when the ordinary
method is impossible, but it is hard to imagine an instance when this
beneficent apparatus could not be used, and so long as there be any
spark of life left in the poor fellow there seems to be every reason to
expect a complete revival as the result of its use.

Of course there are many other places where poisonous gases are likely
to be met with, such as gas-works, chemical-works, limeworks, and so on,
where this apparatus may be kept with advantage, in case of accident.

Indeed all that has been described above has its use apart from colliery
explosions, although they are the outstanding opportunities for its
employment. Old workings, tunnels which have been empty for a time,
sewers--all these have, on occasion, to be entered, not to mention
houses full of smoke, or factories full of chemical fumes, all of which
form cases in which the rescue apparatus would find useful employment.



CHAPTER XIX

HOW SCIENCE HELPS TO KEEP US WELL


One branch of science--medical science--concerns itself almost entirely
with health, but it would be out of place to refer to such matters here,
even if the present writer were capable of doing justice to the subject.
A new medicine or a new method of operating upon a suffering patient
would be quite correctly described as a scientific marvel, but it is not
of such that this chapter deals, but rather with those great works by
which the engineer, often taught by the medical man, promotes the health
of a whole community.

Most important of these, perhaps, is the provision of pure water. Some
places are more fortunately situated than others in this respect, being
near streams flowing down from mountains clear and unpolluted, which can
be drunk after the minimum of purification. Others have to make use of
the waters of a moderately clean river, as London does those of the
Thames and Lea, in which cases the greatest care has to be exercised in
the filtration of the liquid before it can be sent out through the mains
for domestic consumption.

In this particular domain invention has been comparatively slow. There
are novel pumps, it is true, for handling the water, such as the
Humphrey Gas Pump, which the Metropolitan Water Board (London) have
installed for filling their great reservoirs at Chingford. In these an
explosion of gas is the motive force. Water flows by gravitation into a
huge iron pipe closed at the top but open at the bottom. It is so
arranged that a quantity of gas shall be entrapped in the upper end,
which, being exploded by an electric spark, drives the mass of water
out. Some of it, together with a quantity of fresh water, presently
comes surging back, entrapping a fresh supply of gas and causing a new
explosion; and so it goes on over and over again. The particular pumps
at the waterworks referred to discharge about fourteen tons of water at
each explosion, of which there are nine every minute.

The special effect of these machines, however, is not to improve the
public health so much as to relieve the public pocket, for their chief
feature is that they work more economically than any other kind of pump.

The filters, by which the water is purified, are simply layers of sand,
much the same as have been in use for many years, although in some cases
chemistry is brought in and the work of the filters aided by the action
of precipitants. These are substances which combine in some way with the
impurities in the water, and carry them to the bottom of the tank or
reservoir, while the pure water remains to be drawn off from the top.

This is also the most usual method by which water is softened. Hardness
in water is due to the presence of certain salts which are dissolved out
of the ground as the water percolates through it, and which are absent
from rain-water. To get rid of these the hard water has chemicals mixed
with it in a tank, from which it flows slowly through another tank. The
effect of the added chemicals is to convert the soluble salts in the
water into insoluble particles, which then tend to fall down to the
bottom of the containing vessel. The slow passage through the second
tank is intended to give the particles time to settle.

[Illustration: SECTIONAL VIEW OF HYDRAULIC BUFFER AND RUNNING-OUT
               PRESSES OF A 60-POUNDER GUN]

Finally, to make sure that these have been all got rid of, the water
traverses a filter, and then it is for all practical purposes as soft as
rain-water. Some people are frightened of this artificially softened
water, on the ground that chemicals have been added to it, supposing,
apparently, that when they use such water they are really employing a
chemical solution. That is quite wrong, however, for the added
chemicals, combining with the "hardness," form substances which are
quite easily extracted from the water altogether. If we liken the
hardness to a number of pickpockets in a crowd, and the added chemicals
to a number of policemen who come in to arrest the said pickpockets,
finally leaving the crowd free from both pickpockets and policemen, we
get a simple illustration of what takes place.

But almost equally important as the provision of pure water is the
effective dealing with the drainage of a large town. Much offensive
matter flows under the streets of our towns and cities, and if it is not
to become a nuisance it must be scientifically dealt with.

Years ago the drains of London simply emptied themselves into the
Thames, until, in 1864, two large drains were constructed, one on each
side of, and approximately parallel with, the river, to intercept the
old drains and to carry their contents to points many miles down towards
the sea. Even that, however, by no means abated the evil, for it simply
transferred it to a new place. The river was as foul as ever.

William Morris, in _News from Nowhere_, pictures the catching of salmon
in the Thames off Chelsea, while one of London's prominent citizens,
referring to what was being done in the direction of purifying the
river, jocosely promised the members of Parliament a little fly-fishing
at Westminster. Equally remote, it is to be feared, from actual
accomplishment, these two prophecies do certainly indicate the tendency
of events, for science has enabled the authorities to relieve the
long-suffering river of much of the pollution which they used to thrust
into it.

The first great step was the introduction, in 1887, of a treatment in
principle very like that just described for softening water. The liquid
from the drains is gathered into large reservoirs, where chemicals are
added to it, causing the heavier matter to be precipitated in the form
known as "sludge."

The liquid portion, or "effluent," as it is called, which is left is
discharged into the river just as the tide is ebbing, so that it is
carried right away, and, being comparatively inoffensive, it pollutes
the river very little indeed. The sludge, on the other hand, is pumped
into special steamers, which carry it down to a certain spot off the
Thames Estuary, where they drop it into the sea. The currents at the
particular spot chosen are such that none of it returns to the river.

For a similar purpose electrolysis has been employed. In this process
the sewage is made to flow between two iron plates which are connected
up to a source of electric current so that they form electrodes, while
the sewage is the electrolyte. The current decomposes the liquid sewage,
causing chlorine and oxygen to be deposited upon that plate which forms
the anode. This deodorises and purifies the sewage, in addition to which
iron salts are formed on the iron plates, the effect of which is to
precipitate the solid particles. Thus the same result is achieved as
when chemicals are used, the main difference being that instead of
chemicals being added, they are produced by the passage of the current.

But, from the scientific point of view, the most interesting process of
all is that in which bacteria or microbes are brought into the service.
The fact is familiar to most people that there are certain minute
organisms which cause terrible diseases. It is not so well known that
there are still more of them whose action is extremely beneficent. The
writer has seen these minute living things described in a popular book
as "insects," but they really belong to a low order of plant life, and,
as has been said in an earlier chapter, in spite of the lowliness of
their status in the order of creation, they are able to accomplish
certain chemical processes which baffle the cleverest men. They are
particularly good, or some of them are at any rate, at forming compounds
in which nitrogen forms a part. Further, they can be divided into two
classes, the aerobic and the anaerobic. The former work best in air,
while the latter need an absence of air while they perform their
functions. After which preliminary explanation we can proceed to
describe how they are induced to carry on this valuable work for
mankind.

The sewage flows first of all into a tank from which light and air are
excluded as far as possible. There the anaerobic microbes flourish and
multiply, and in the course of their life work they convert the sewage
into an inoffensive liquid. After an appropriate interval the liquid
passes to filter-beds, where it trickles over and through beds of coke,
the effect of which is to aerate it very thoroughly, whereby the aerobic
microbes come into action, completing the good work, so that nothing is
left except a clean, colourless and odourless liquid. Indeed it is more
than that, for the microbes have turned the offensive matter into
nitrogenous compounds which, as we have seen in a previous chapter, are
the best fertilisers. Hence this effluent, if placed upon the soil, is
of great value.

The advantage of this to towns which are not blessed, like London, with
a broad river and the sea near at hand needs no explanation.

The bacteria necessary to carry on the process are always present in
sewage, and after any particular plant has been in operation for a
little while there results an accumulation of them, so that the process
becomes more and more active as time goes on. Mechanical ingenuity has
so arranged matters that a sewage disposal plant on this system can be
made quite automatic, requiring little or no attention for months
together, the raw sewage flowing in at one end, while the odourless,
harmless effluent pours out at the other.

And, moreover, so powerful is the action of these beneficent bacteria
that should disease germs come down in the sewage they soon destroy
them. No chemicals are needed, for the bacteria replenish themselves. No
sludge is left, everything being turned into the harmless effluent. And,
it may be said once more, disease germs are destroyed. Of all the
valuable inventions of modern times this is surely not one of the
least.



CHAPTER XIX

MODERN ARTILLERY


Even as late as the time of the Crimean War guns, even the largest, were
made of that extremely common material, cast-iron. In fact, so far as
material went, there was no difference between a gun and a water-pipe.

It was the need for some material possessing strength comparable with
that of steel combined with the ease of production of cast-iron which
led Sir Henry Bessemer to experiment in the manufacture of steel. Out of
those experiments came Bessemer steel and its near relative, Siemens
steel, two materials of universal application at the present time, so
that to the needs of the artilleryman we owe two inventions which have
proved of infinite value in peace as well as in war.

If any particular piece of ordnance can be said to be the prime
favourite with the English-speaking peoples, it is the big naval gun.
With both British and Americans the navy takes pride of place; both
nations are given to contemplating with pleasure the number of
dreadnoughts which they possess, and the distinguishing feature of a
dreadnought is the large number of big guns which it carries.

Of the latest of these gigantic weapons one may not speak, but much is
already public property concerning the 12-inch gun which the original
_Dreadnought_ carried, and which is probably followed in its general
features by the still greater guns of the most recent ships.

A gun is spoken of by its "calibre," which means the inside diameter,
or, to use another expression, the size of the "bore." So the "12-inch"
naval gun is 12 inches in the bore. Its length is in some cases 45
calibres and in others 50 calibres. In other words, some are 45 feet
long and others 50 feet.

Why the difference? someone may ask. The answer is that the longer ones
are an improved type. The extra length gives longer range and harder
hits, as is quite apparent after a little thought. The explosive "goes
off" and forthwith commences to drive the shell towards the muzzle. So
long as it is in the gun the shell is being pushed faster and faster,
but so soon as it leaves the muzzle the pushing ceases and the shell is
left to pursue its course with its own momentum. Therefore, generally
speaking, one may say that the longer the gun the faster will be the
speed of the shell as it leaves the muzzle, the farther will it go and
the harder will be the blow at a given range.

Incidentally this explanation reveals the need for different kinds of
explosive. The propellant whose function it is to drive the shell out of
the gun is different from that with which the shell is itself filled.
The former needs to act comparatively slowly, so that it may continue
its pushing action during the whole time that the shell is travelling
along the gun. It might be ever so powerful, but were its action too
sudden it would simply tend to burst the gun, without imparting very
much speed to the shell. On arrival at its destination, however, the
shell needs to burst suddenly and violently.

Another interesting question arises at this point. Seeing how fast is
even the slowest speed at which a projectile travels, how can it be
possible to measure the rate at which a shell issues from one of these
monster guns. Needless to say, it is electricity which makes a thing
apparently so difficult really quite easy.

Near the gun is set up a frame with a wire zigzagging to and fro across
it, in such a manner that when the gun is fired the shell is bound to
cut the wire. Electric current is made to pass through this wire on its
way to a suitable house in which are recording instruments, where it
energises a magnet and so holds something up. Now it is easy to see
that as soon as the shell cuts the wire the current will stop, the
magnet will "let go" and the "something" will drop.

At a certain distance farther on there is a second frame with wires upon
it, through which passes a second current, which is also led to the
instrument house, where it again operates a second magnet.

When the first magnet releases its hold it drops something, to wit, a
long lead weight. When the second magnet lets go it permits a second
weight to fall against the first and make a dent or scratch upon it. The
longer the interval between the action of the two magnets the higher up
upon the lead weight will the scratch be. The apparatus, in short, will
register the distance fallen through by the lead weight between the
breaking of the wire in the first frame and the breaking of the wire in
the second frame.

Now a falling object, if only it has such weight that the resistance of
the air is negligible, falls according to a well-understood law, which
law it obeys with the utmost accuracy. Therefore the distance fallen by
the weight between the passage of the shell through two points gives a
very accurate record of the time taken to travel from one to the other.
Of course several such frames can be used if desired in the same way.

But to return to the gun itself. It is not merely one piece of metal but
several tubes beautifully fitted one inside another. Moreover, in the
British gun at all events, between two of the tubes there is a space
filled with "wire."

This wire is perhaps better described as steel tape, and is of the
finest material for the purpose, flexible and tremendously strong. It is
wound round and round one of the tubes until there are many miles of it
on a single gun. It is wound tightly, too, by means of special
machinery.

The purpose of the wire is to resist cracking. The solid steel tubes may
crack, and, as is the way with all cracks, these will tend to grow
longer and longer. The many turns of wire, however, will not crack. Even
if a few turns should break, the damage will not spread, and the gun
can probably go on as if nothing had happened.

The material of which these guns are made is nickel chrome gun steel.
Steel is ordinarily an alloy of iron and carbon, but this metal also
contains traces of nickel and chromium, which make it specially suitable
for its special purpose.

Each of the tubes of which the gun is formed start as an ingot, a mere
lump of metal, but roughly shaped. The requisite mixture is obtained in
a furnace and the molten metal is run out into a mould. The ingot is
heated again and pressed under enormous hydraulic presses until it is
approximately the shape required. This pressing not only produces the
desired shape, it also improves the quality of the metal.

The rough forging is then bored out, to make it into a tube. One is
inclined to wonder why the ingot is not cast hollow to commence with,
and so save the labour of boring it all out later. The explanation of
this is that certain impurities are always present in the metal and
these always gather together in the part which sets last. Now in a solid
block or ingot it is clear that the centre is the part which will set
last, and hence that is the part where the impurities will congregate.
Then, when the centre part is all bored out the impurities are entirely
removed.

The tube is shaped externally by being turned in a lathe.

The innermost tube is not simply smooth. There is a spiral groove,
called the "rifling," running round and round, screw fashion, inside it.
The purpose of this is to give the shell a spinning action which causes
it to keep point foremost throughout its flight. But for this the shell
would tend to turn over and over, resulting in uncertain and inaccurate
flight.

The shell is a little smaller than the bore of the gun, but near its
base it has an encircling band of soft copper, which band is a tight fit
in the gun. The soft copper crushes into the "rifling," whereby the
shell obtains its spinning action.

The large guns are mounted in pairs, each pair on a turntable, by the
movement of which to right or left they are trained upon the distant
target. The turntable is surrounded by a wall of thick armour and is
covered by an iron hood or roof.

In addition to being turnable to right or left, there is, of course,
provision for raising or depressing the direction in which each gun is
pointing. They need always to point more or less upwards, and the
particular angle depends upon the range or distance of the object aimed
at. This is ascertained by range-finding instruments and communicated to
the officers in the turrets, as the covered turntables are called. The
guns are then elevated or depressed to suit the range.

Each gun rests upon a cradle which is itself fitted upon a slide. When
it is fired it "kicks" backwards, against the force of a buffer of
springs, or a hydraulic or pneumatic cylinder. Thus after each shot the
gun moves backwards upon the slide, but the hydraulic apparatus brings
it back again into position for firing almost instantaneously.

In naval guns all the movements, including that of the turntable, are by
power, either hydraulic or electric, or a combination of the two. The
loading is also by power.

The shells and ammunition are kept well down towards the bottom of the
ship, under each turret. Lifts bring them up from there to a chamber
just beneath the turntable, known as the working chamber. Here a small
quantity only is kept, and that for as short a time as possible before
it is sent up by other hoists straight to the guns themselves. The
hoists are so arranged that, no matter how they may be elevated or
depressed, the ammunition is delivered exactly opposite the breech, as
the rear end of a gun is termed. Then a mechanical rammer pushes it
straight in.

[Illustration:        RIFLES OF DIFFERENT NATIONS
                                                     (_See_ Appendix)]

The breech of the gun is closed by a beautiful piece of mechanism called
the breech-block. It is really a huge plug which securely closes the end
of the gun, a partial turn after it is in place fixing it firmly enough
to resist all the force of the explosion. Yet it can be freed and
swung back upon hinges in a few seconds. At the same moment that it is
opened a jet of air blows into the gun, clearing out all effects of the
recent explosion.

The process of firing one of these guns may thus be summarised. The
turntable is swivelled to right or left until the gunners, looking
through the sights, which are really telescopes, see the object straight
in front of them. Meanwhile the sights have been set according to the
range--that is to say, they have been so set in relation to the gun
itself that when they point directly at the target the gun will be
pointed upwards at exactly the right angle for that range. The whole
thing, therefore, gun and sights combined, is tilted upwards or
downwards as may be necessary until the sights point directly at the
object aimed at. Then at a signal the gun is fired by electricity. The
shock causes the gun to slide backwards upon its supporting slide, but
the buffers, having taken the shock automatically, return it to its
position again; the aim is thus undisturbed and it is ready for the next
shot. These enormous guns can be fired at the rate of one shot every
fifteen seconds.

Field guns are in principle very similar to these, only, of course, they
are much smaller and are mounted upon carriages, so that they can be
quickly moved from place to place. It must be borne in mind, however,
that there are in the case of land guns two distinct types. Field guns,
like naval guns, fire straight at their target; howitzers or mortars
fire upwards with a view to letting the shell fall on the target from
above. The latter are, generally speaking, short, fat, stumpy guns, as
compared with the long, slender field guns.

In the field all guns have to be loaded by hand. The elaborate system of
hoists which enables the great naval guns to be loaded with such
rapidity is obviously impossible. That has to be compensated for by the
skill and quickness of the gunners themselves, and it is indeed
astonishing to see with what deftness they can handle the heavy and
dangerous projectiles.

With all guns, of whatever kind, range-finding is of the utmost
importance. No projectile, however fast it may travel, really moves in a
straight line. It must be fired more or less upwards in order to
compensate for the downward pull of gravity. If the elevation be
insufficient the shell will fall short; if it be too much it may go
beyond the mark, or it may fall short, according to circumstances. Just
the right elevation is absolutely essential for good shooting. And for
that to be achieved the range must be known with the utmost possible
accuracy.

There are various systems and instruments used for this purpose, but all
depend upon the same principle. It is the principle underlying all
surveying and all astronomy; indeed it is the only possible principle
for measuring a distance when you cannot actually go and lay a measure
upon it or by it.

It is based upon a peculiar property of a triangle. In the case of every
triangle which has straight sides, if we know the size of two of the
angles and the length of one of the sides we can easily calculate all
that there is to be known about that triangle. We unconsciously use the
principle when we judge a distance with our eyes. We focus each eye
separately upon the object which we are looking at. In other words, each
of our eyes looks along a straight line terminating in the object. Those
two lines, together with a line joining our two eyes, form a triangle.
The line between our eyes is the "base," the line of which we know the
length, while the directions in which we point our eyes give us the
angles at each end of the base. From this we are able to judge the
distance of the object. Of course there is probably not one of us who
knows the length of that natural "base" in inches, but that does not
matter in this case, since it is always the same whatever we may look
at, and so the mere inclination of the eyes gives us a means of
comparing distances. When we judge by the eye alone, what we really do
is to draw upon our experience and consciously or unconsciously compare
the distance which we are estimating with some others which we already
know.

In surveying, a telescope is set up at one end of a base-line and
pointed first at the other end of the base-line and then at the distant
object. A scale with which the instrument is provided gives us the size
of the angle between the two. Then the same thing is done at the other
end of the "base" and the similar angle there is obtained. The length of
the base being known, the distance of the remote object can then be
calculated.

In the same way two observations can be made, one at each end of a ship,
the length of the ship forming the base-line. Or an instrument can be
made by which two observations can be made simultaneously by the same
man.

This is done by means of mirrors which are turned so that the same
object is seen in both of them, apparently in a straight line. The
extent to which one of them has to be turned gives the angle, and the
instrument forms the base.

Anyone with the slightest geometrical experience will perceive at once
that the best results are obtained when the base-line is of considerable
length, and hence small portable range-finding instruments such as can
be easily carried and used by one man are necessarily less accurate than
an arrangement such as has been suggested above, where two observers
work simultaneously from the two ends of a ship.

In many cases, however, the self-contained instrument is the only one
which it is possible to use, and when the instrument is well made and in
experienced hands the results are surprisingly good.

As used in surveying, for example, where the base-line may be anything,
according to circumstances, and the angles may likewise vary at both
ends, elaborate trigonometrical calculations have to be performed to
arrive at the desired result. If, however, the base-line be always the
same, and one of the angles be always a right angle, the distance of the
distant object will vary with the remaining angle. Indeed the scale by
which that angle is measured can be made to give not degrees, but the
distance of the object. Portable range-finders, therefore, in many cases
have one reflector set for a right angle and only one of the reflectors
movable. The instrument then shows the distance of the object at a
glance.

This is impossible in the case of two separate observations on a ship.
In that case the base is always the same, but since the ship cannot be
set at right angles to the object whenever a range has to be found, both
angles have to be measured. There is, however, a beautifully simple
little mechanism in which two pointers are set one to each of the two
angles, and the distance is then shown instantly.



APPENDIX

A DESCRIPTION OF THE RIFLES SHOWN AT PAGE 240


THE GERMAN MAUSER can fire forty rounds a minute--more than any other
rifle used in the war. The rifle is of the 1898 pattern, weighs 9 lb. 14
oz. with bayonet fixed, and is sighted from 219 to 2187 yards. The
magazine holds five cartridges, packed in chargers. As the rifle is not
provided with a cut-off, it cannot be used as a single-loader. With its
long barrel and long bayonet it gives a stabbing length of 5 ft. 9
in.--8 in. longer than the British.

THE AUSTRIAN RIFLE is the Mannlicher. This rifle is very fast in action
as a snap back and forth of the wrist is sufficient to operate it. It
is, however, more trying for prolonged work, owing to the throwing of
the strain only on the wrist. Without the bayonet the rifle weighs only
8 lb. 5 oz., the lightest of all, yet the bullet--244 grains--is the
heaviest used by any of the belligerents. The rifle is sighted from 410
to 2132 yards, and the barrel has a four-groove rifling.

THE BRITISH LEE-ENFIELD--MARK III--is the outcome of the South African
War. It is not too long for horseback and is yet quite efficient for
infantry. The barrel is 25 in. long and has five grooves in the rifling.
It is sighted from 200 to 2800 yards. The rifle is fitted with a
magazine which holds ten cartridges packed in chargers, each of which
contains five rounds, so that the magazine is filled with ten rounds in
two motions. The rifle is also fitted with a cut-off, which enables it
to be used as a single-loader. It is altogether a most efficient
weapon.

THE FRENCH LEBEL is of the 1886-1893 pattern, and with bayonet fixed is
longer than any other rifle. It weighs, without bayonet, 9 lb. 3-1/2 oz.
The tube magazine under the barrel contains eight cartridges; it takes,
of course, longer to charge than a magazine loaded with a charger. It
does not fire as many shots a minute as some of the other rifles in the
field. The position of the magazine is indicated by the crosses. The
rifle is sighted from 273 to 2187 yards, and the bullet weighs 198
grains.

THE BELGIAN ARMY uses the 1889 pattern Mauser, which weighs just over 8
lb. and is sighted from 547 to 2187 yards. The magazine holds five
cartridges carried in clips; not having a cut-off, the rifle cannot be
used as a single-loader. It has four grooves in its rifling and measures
4 ft. 2-1/4 in., or, with the bayonet, 4 ft. 11-3/4 in. The bayonet is
short and flat.

THE "3 LINE" NAGANT of Russia is 1/4 lb. heavier than the British rifle
and is over 7 in. longer. The triangular bayonet is always fixed and
never removed from the rifle. The magazine of the rifle is of the box
type and holds five cartridges. The rifle is capable of discharging
twenty-four bullets to the minute. A useful feature is the interrupter,
which prevents jamming of two cartridges.

THE ITALIAN MANNLICHER-CARCANO is of the 1891 pattern. It weighs,
without bayonet, just over 8 lb. 6 oz. and measures 50-3/4 in. The
barrel, 30-3/4 in. long, has a four-groove rifling. The box magazine,
fixed under receiver without cut-off, holds six cartridges. The magazine
holds six rounds, and the rifle is capable of discharging fifteen rounds
a minute.



INDEX


  A

  Accumulators or secondary batteries, 65

  Aerial craft experiments, 202

  Aerobic and Anaerobic bacteria, 234

  Afterdamp, 228

  Alcohol as a fuel, 49

  Alternating current, 35, 193

  Altofts, artificial coal mine at, 139

  Aluminium, 133

  Amalgam, 117

  Ammeters, 25

  Ammonia in making ice, 72

  Ammunition for big guns, 240

  Amperes, 22, 24

  Analysis and synthesis, 43

  Anode, 55

  Anschutz, Dr, 96

  Antennæ, 162, 171

  Anthracene oil, 48

  Arc, the, in wireless, 165

  Argon, the gas, 75

  Artesian wells, 45

  "Atmosphere," a unit of measure, 72

  Atoms, 56

  "Avogadro's Constant," 33


  B

  Bacteria, beneficent, 234

  Ball mill, the, 115

  Battery, electrical, 23

  Benzine, 45, 48

  Bessemer, Sir H., 236

  Blowpipe, oxyhydrogen, 120

  Board of Trade Unit, the, 22

  Boiling water, 10, 76

  Bore of a gun, 236

  Boulders, blasting, 20

  Branly, 166

  "Brattice cloth," 224

  Breech of a big gun, 240

  Brennan torpedo, the, 102

  Brewing, 50

  "Brine" in machine-made cold, 70

  "Budding" of yeast, the, 51


  C

  Calibre of a gun, 236

  "Capacity," 153

  Capacity and inductance, electrical properties, 161

  Carbolic oil, 48

  Carbon, 11

  Carbonic acid gas, 10

  Carburetter, the, 46

  Cardiograms, 32

  Caselli, 176

  Cathode, 55

  Cavendish, investigations of, 73

  Cellulose, 12, 44

  Centrifugal tendency, 115

  "Character" of a lighthouse, 86

  Charge and current, 32

  Cheddite, 13

  Chemicals in waterworks, 232

  Chemistry, organic and inorganic, 42

  Chlorate of potash, 12

  Chloride of soda, 58

  Chronograph, the, 141

  Clark's Cell, 23

  Coal and oil, 47

  Coal, burnt, 10

  Coal-dust an explosive, 10

  Coal-dust, explosions from, 139

  Coal-pitch, 48

  Coal-tar, 48

  "Coasting" lights, 80

  Coherer, the, 103, 162, 167

  Coke in smelting, 125

  Colliery explosions, 137

  Colliery explosions, rescue apparatus, 226

  Colours of the spectrum, 213

  Colours of flowers, 213

  Compass, a ship's, 91

  Compressed air in torpedoes, 100

  "Concentrates," 115

  Condensers in wireless, 163

  Conservation of energy, 132

  Contact makers, 145

  Coronium, the gas, 74

  Corundum, 134

  Coulombs, 23

  Courrières colliery disaster, 221

  Creosote, 48

  Creosote oil, 48

  Crooks, Sir W., 33

  Crushing mills, 115

  Crystal detectors, 171

  Curie, M. and Mme., 33

  Curtis and Harvey, 9

  Cyanide process, the, 118

  Cyanogen, 118

  Cymogene, 45


  D

  Detectors, 167

  Detonator, the, 14

  Dextro-glucose, 51

  Diamonds, 135

  Diesel engines, 46

  Direct-current electric motor, 191

  "Dirt-auger," the, 15

  Ditches, blasting, 18

  Drainage, 233

  Du Pont Powder Company, 9

  Duddell, W. H., 37

  Dufay dioptichrome process, 219

  Dynamite, what it is, 9, 12;
    in agriculture, 13;
    firing a charge, 16;
    fruit trees, 16;
    marshy ponds, 17;
    ditches, 18;
    tree stumps, 19;
    boulders, 19;
    wells, 20

  Dynamo, the, 65


  E

  Eddystone Lighthouse, 80

  Edison's accumulator, 66

  Einthoven, Prof., 30

  Electric arc, the, 123

  Electric furnace, 125

  Electric fuse, the, 16

  "Electrical Inertia," 153

  Electrical battery, 23;
    pressure, 23;
    cells, 23;
    measure, 24;
    magnetism, 25

  Electricity, 22;
    the current, 56;
    electro-plating, 58;
    purification of metals, 61;
    secondary batteries, 62

  Electrode, 55

  Electrolysis, 55, 170;
    in drainage, 234

  Electrolyte, 55

  Electrometer, the, 32, 34

  Electro-plating, 58

  Electros, 60

  Electroscope, the, 34

  Endosperm, the, 50

  Engines driven by oil fuel, 46

  Enzymes, 50

  Ether, 45, 149

  Ethyl alcohol, 49

  Explosions, 9;
    in mines, 137

  Explosive link, the, 104

  Explosives for guns, 237


  F

  "Falls" in a coal mine, 223

  Fermentation, 50

  Fessenden, R. A., 169

  Field guns, 241

  Filters in waterworks, 232

  Fire-damp, 137

  Firing-pin of torpedo, 102

  Flashing lights, 81

  Fog, effects of, 82

  Fog signals, 88

  "Fractional distillation," 76

  "Frequency," 36

  Frequency meter, 193

  Friction clutch, 195

  "Frue" vanner, the, 116

  Fruit trees and dynamite, 16

  Fuses, firing, 20


  G

  Galvanometer, the, 27, 170

  "Gangue," the, 112

  Gauges, 208

  Gelignite, 12

  Glycerine in explosives, 11

  Gold, 110

  Guiding lights, 81

  Gyroscope, the, 93, 100


  H

  Half-tone illustrations, 181

  "Hard-pan," 14

  Harris, Sir W. S., 36

  _Hawke_ and _Olympic_, collision between, 198

  "Head" of the torpedo, 99

  Heat and electricity, 37

  Heat of the electric arc, 123

  Heat, testing by, 205

  Helium, 33, 75

  Hertz, 154

  Howitzers, 241

  Hughes, Prof., 159

  Humphrey Gas Pump, 231

  Hydraulicing, 112

  "Hydro-carbons," 45

  Hydrogen, liquid, 73

  Hydrometer, the, 65

  Hydrostatic valve of torpedo, 101

  "Hyper-radial" apparatus, 88


  I

  Ice, machine-made, 71

  Indigo, synthetic, 44

  Inductance, 154

  Induction coil for wireless, 162

  Induction furnaces, 129

  Insulating ink, 177

  "Interference" of light waves, 159

  Ionisation of the atmosphere, 172

  Iron, 109


  J

  Jupiter's moons, 150


  K

  Kelvin, Lord, 28

  Kerosene, 46

  Kieselguhr, 12

  Kilowatt, the, 25

  Kinematograph in coal mine experiments, 146

  Korn, Prof., 183

  Krypton, the gas, 75


  L

  Leclanche cell, the, 23

  Leyden jar, the, 153

  Light, speed of, 151

  Light waves, 151

  Lighthouse, the, 78

  Lighthouse lamp, the, 83

  Limit gauges, 209

  Liquid air, 73

  Lodge, Sir O., 159, 161

  Lumière autochrome process, 216


  M

  Magnetic detector, the first, 168

  Magnetic pole, the, 90

  Magnetism, 25

  Magnets, 25

  "Making" light, the, 79

  Maltster, the, 50

  Mansfield Rescue Station, the, 224

  Marconi, 161

  Marshy ponds, to remove by dynamite, 17

  Mash tun, the, 50

  "Master compass," the, 97

  "Master" records, 60

  Maxwell, J. C., 152

  Measuring by electrolysis, 62

  Mendeluff's table, 74

  Mercury, 114

  Metallographic testing, 205

  Metals, testing, 204

  Methane gas, 10, 124

  Methyl alcohol, 49, 53

  Microbes, their use, 43

  Mine-laying, 105

  Mine-sweeping, 107

  Molecules, 56

  Morris, William, 233

  Mud, gold from, 122

  Muirhead, Dr, 167

  Murette or pedestal of lighthouse lamp, 85


  N

  Naphtha, 45

  National Physical Laboratory, 199

  Natural frequency, 161

  Neon, the gas, 75

  Nickel chrome gun steel, 239

  Nitric acid, 11

  Nitro-cotton, 12

  Nitro-glycerine, 11

  Nitrogen gas, 9

  Nobel, inventor of dynamite, 12, 135

  Nodes, 157


  O

  Ohm, the, 22, 24

  Ohmmeter, the, 27

  Ohm's law, 27

  Oil, mineral, 44

  Oil-producing countries, 47

  Optical apparatus of lighthouse, 86

  "Orders" of lighthouse apparatus, 88

  Ores, 110

  Orthochromatic plates, 212

  Oscillations, electrical, 36

  Oscillatory circuit, 154

  Oscillograph, Duddell's, 39

  Oxide of iron, 133

  Oxyacetylene flame, the, 131

  Oxygen gas, 10

  Oxyhydrogen jet, 130


  P

  Paraffin wax, 45

  Patents, 174

  "Periodicity," 36

  "Personal equation," the, 207

  Petrol, 45, 52

  Petroleum, 44

  Phonograph, the, 60

  Plans of a ship, 199

  Plates of the secondary battery, 64

  Platinum, 184

  Plumbago in plating, 59

  Poulsen arc, the, 173

  Poulsen, Valdemar, 165

  Pressure gauges, 143

  Priestly, investigations of, 73

  Primary colours, 213

  Prisms, reflection of, 85

  Process blocks, 186

  Projectiles, velocity of, 237

  Propellers of the torpedo, 99

  Propellers, testing aerial, 203

  Prout's anonymous essay, 74

  Prussiate of potash, 177

  Purification of metals, 62


  Q

  Quadrant electrometer, the, 35

  Quartz, 113;
    fibre, 31, 131


  R

  Radium, 33

  Ramsey, Sir W., 75

  Range-finding, 240, 242

  Rayleigh, Lord, 74

  Receiving instruments for wireless, 162

  "Record" vanner, the, 116

  "Rectifier," the, 37, 171

  Red rays of light, 82

  Reflection by prisms, 84

  Reflectors, lighthouse, 84

  Reiss electrical thermometer, 36

  Repeated-impact testing machine, 204

  Rescue teams for colliery accidents, 221, 222

  Resistance welding, 126

  "Resonance," an experiment, 160

  Reviving apparatus for coal mines, 229

  Rheostat, the, 188, 191

  Rhigolene, 45

  Rifling of a gun, 239

  Rubber, synthetic, 44

  Rubies, artificial, 131

  Rudders of a torpedo, 100

  Rutherford, Prof., 33, 168


  S

  Saccharine, 48

  Saltpetre, 12

  Schwartzkopff torpedo, the, 99

  Scilly Island lighthouse, 80

  Sea, gold in the, 120

  Secondary battery, the, 62

  "Sectors," 81

  Selenium, 184

  "Self-rescue" apparatus, a, 228

  Shale, oil from, 45

  Shells for guns, 239

  Ships, testing by models, 200

  Short circuit, 179

  "Shunt," the, 165

  Sighting a big gun, 241

  Silica, 133

  Skating rinks, ice in, 71

  "Sludge" and "effluent" of drainage, 233

  Spark detectors, 166

  Spark-gap, 162

  Spectrum, the, 213

  Spinthariscopes, 33

  Spirits, 52

  Springs, testing, 203

  Stamps for crushing quartz, 113

  Starch grains in colour photography, 217

  "Step-down" and "step-up" transformers, 127

  "String galvanometer," the, 30

  Submarine mines, 104

  Submarine telephone, 88

  Sulphuric acid, 11, 43

  Sunlight, composition of, 213

  Synchronism, difficulties of, 182, 191

  Synthetic substances, 44


  T

  "Tamping," 15

  Tank for testing at Teddington, 201;
    New York harbour, 201

  Telautograph, the, 180

  Telectograph, the, 180, 185

  Telegraph key for wireless, 162

  Telewriter, the, 187

  Temperature, measuring, 38

  Tesla, Nicola, 164

  Testing by heat, 205

  Testing machines, 206

  Thermit, 135

  Thermo-couple, the, 38

  Thermo-galvanometer, the, 37

  Thomson Mirror Galvanometer, the, 28

  Thomson, Prof., S., 159

  Torpedo, the, 98

  Training station at Porth, 225

  Transformer, the, 127

  Transmitting instruments, 163

  Travers, Prof., 75

  Tree stumps, blasting, 19

  Tuning-fork a standard of speed, 193

  Turret of a battleship, 240


  U

  Ultra-microscope, the, 209

  Ultra-violet rays, 172


  V

  Varley and the Atlantic cable, 28

  Vaseline, 46

  Veins or lodes, 113

  Vickers, 202

  Voltmeter, the, 26

  Volts, 22, 24


  W

  Water a source of heat, 124

  Water, soft and hard, 232

  Watt, the, 24

  Waves caused by ships, recording, 200

  Wax models of ships, 199

  Welding by electricity, 125

  Wells, blasting, 20

  Welsbach mantle, the, 124

  Whitehead, 99

  Wire guns, 238

  Wireless telegraphy, 161, 173

  Wireless torpedo, the, 102

  Wood-meal in explosives, 12

  Wood spirit, 49

  "Working fluid," the, 68


  Y

  Yeast, 51


  Z

  Zero, 68

  Zinc in gold recovery, 119


       *       *       *       *       *

THE RIVERSIDE PRESS LIMITED, EDINBURGH

                    1917





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