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+Project Gutenberg's Marvels of Scientific Invention, by Thomas W. Corbin
+
+This eBook is for the use of anyone anywhere at no cost and with
+almost no restrictions whatsoever.  You may copy it, give it away or
+re-use it under the terms of the Project Gutenberg License included
+with this eBook or online at www.gutenberg.org
+
+
+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: Thomas W. Corbin
+
+Release Date: November 18, 2011 [EBook #38045]
+
+Language: English
+
+Character set encoding: ASCII
+
+*** START OF THIS PROJECT GUTENBERG EBOOK MARVELS OF SCIENTIFIC INVENTION ***
+
+
+
+
+Produced by Chris Curnow, Julia Neufeld and the Online
+Distributed Proofreading Team at https://www.pgdp.net (This
+file was produced from images generously made available
+by The Internet Archive)
+
+
+
+
+
+
+
+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 Small caps have been replaced by ALL CAPS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 deg. C., rhigolene is given off:
+that is collected and condensed in another vessel. Between 70 deg. and 120 deg.
+petroleum ether and petroleum naphtha are produced, and they together
+constitute what is commonly called petrol. Between 120 deg. and 150 deg.
+petroleum benzine arises. All the foregoing taken together constitute
+about 8 to 10 per cent. of the whole crude oil. Then between 150 deg. and
+300 deg. there comes off the great bulk of the oil, nearly 80 per cent., the
+kerosene or paraffin which we burn in lamps. Above 300 deg. 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 deg. C. _below_ freezing-point. Freezing-point on
+that scale is "zero," and so this _absolute_ zero is _minus_ 274 deg. Or,
+to put it another way, freezing-point is 274 deg. _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 Mendeleeff 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 L4,000,000 worth of it, while the little gift
+which the Queen of Sheba brought him was of the handsome value of
+L600,000, 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 deg. to 1800 deg. 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 deg. 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 deg. 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 deg. 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
+Courrieres 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 L10,000 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 antennae, 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, antennae 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.
+
+Antennae 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
+antennae 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 Lumiere 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 Lumiere 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
+Courrieres 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
+Courrieres, 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, L7000,
+while the one at Mansfield cost L3000. 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
+
+  Antennae, 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
+
+  Courrieres 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
+
+  Lumiere 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
+
+
+
+
+
+End of the Project Gutenberg EBook of Marvels of Scientific Invention, by
+Thomas W. Corbin
+
+*** END OF THIS PROJECT GUTENBERG EBOOK MARVELS OF SCIENTIFIC INVENTION ***
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