In the attempt to prevent your doing him any harm by thus throwing boxes of explosives at him, the enemy clothes the sides of his most valuable and important ships with thick steel plates, wherefore you have to make your shell strong and tough so that it shall not splinter against the armour but shall on the contrary bore its way through, finally exploding in the interior of the ship.

If it is not a ship that you are attacking but, say, an earthwork or an arrangement of trenches, then you do not need to penetrate steel armour and your shell can be thinner and of lighter construction. It still needs to be strong, however, for it has another function besides simply carrying the explosive. It must hold the force of the explosion in for a moment while it gathers force so that when the hour comes the pent-up energy may strike all round with the utmost violence. Even the most powerful explosives are comparatively feeble if they go off in the open. By holding them in check for a moment and then letting their force loose suddenly you get a much more forceful blow.

Shells which contain only an explosive are called common shells or high-explosive shells. Shrapnel shells constitute another type in which the force of the explosion is simply employed to release a number of round bullets, which strike mainly because of the velocity which they derive from the original motion of the shell. These are above all things man-killing shells, for their result is akin to a volley of bullets at close range.

We can thus sum up the chief types of shell as follows: the naval shell which has to be capable of penetrating armour: the high-explosive shell which must be able to break up earthworks and blow down the walls of trenches: and the shrapnel shell which scatters a shower of bullets and is most useful in attacks upon bodies of men rather than upon material structures.

Some shells have their propellent explosive combined with them just as the familiar rifle cartridge contains the propellant combined with the bullet. In the larger sizes, however, it is much more convenient to have the propellant in a separate cartridge, which can be handled separately and loaded into the gun separately.

As has been already explained, the propellant is a "powder" which gives a steady push rather than a destructive blow: moreover, it is practically smokeless, so as not to "give away" the position of the gun to the enemy. The "high explosive," however, shatters and usually makes a dense smoke, so that the observers can see where it fell and report to the gunners whether or not they have got the range. Soldiers' letters have told us of the "black Marias" and "coal boxes" used by the Germans, those terms being simply soldiers' nick-names arising no doubt from the fact that certain particular shells are filled with "tri-nitro-toluene" which gives a black smoke. Clearly, smoke, which is most objectionable in the propellant, is a positive advantage in the bursting charge.

And now let us take a glimpse at the manufacture of one of these terrible missiles. An ingot of shell-steel is first cast as described in an earlier chapter. Since impurities are apt to rise, while the metal is liquid, the top of the ingot is always cut off and discarded. This waste material is used for many other purposes, in which a chance flaw would not be a serious matter, under the title of "shell-discard" steel.

The lower part is then heated and passed through a rolling mill, a machine very similar in principle to the domestic mangle, the rollers being of iron with suitable grooves cut in them. A few passages through this machine transforms the ingot into a thick round bar. This is then sawn into short pieces called billets, each of which is the right size to form a shell. Again heated, a powerful press drives a pointed bar through the softened steel, thereby converting the short billet into a rough tube. Another press then slightly closes in one end, making it resemble a bottle without a bottom and with the neck broken off.

The rough forging is then ready to be machined, an operation which is performed in a lathe. The outside is made perfectly round and smooth and of precisely the right size. The inside is also bored out to the correct diameter and finished off to an exceeding smoothness so as to avoid the possibility of any rough places irritating the explosive which in due time will be filled into it. For the same reason, the inside, when finished, is varnished in a certain way and with a certain varnish. The formation of this varnish is one of those little thought of but highly important services which alcohol renders to us, as mentioned elsewhere.

The smaller end (that which has already been partially squeezed in) is bored out and screwed for the reception of the nose-bush, while the other end is recessed for the reception of the plate which forms the bottom.

Most of these operations have to be very accurately carried out and, to ensure that that is so, gauges are continually employed to check the work. These gauges are based upon a very simple principle, known as the "limit" principle. This is both interesting and important, sufficiently so to merit a more detailed reference.

It must first be realized that no two things are alike and no measurement is perfectly correct. When we lightly speak of two things being "alike" we really mean that for the purpose contemplated they are nearly enough alike. Two things might be "alike" for one purpose and yet be so unlike as to be useless for another.

What the authorities do in the case of shells, therefore, and what is done nowadays in many branches of engineering, is to recognize this fact and at the same time overcome the difficulty by stating what difference is permissible. In other words, instead of saying that a thing must be a certain size, it is required to fall between two limits: it must not be more than one or less than the other.

For example, suppose a hole is required to be nominally an inch in diameter it may be specified that it shall not exceed an inch plus one-thousandth or fall short of an inch minus one-thousandth. In such a case a variation of a thousandth of an inch either way is permitted. The permitted variation may be more than that, or it may be less and be measured in ten-thousandths, it all depends upon circumstances. Clearly in every case it is desirable to permit as large a variation as is consistent with a good result.

Now to make measures with the degree of accuracy just mentioned is not easy. One can just about see through a crack a thousandth of an inch wide if held up to a bright light. How then can dimensions such as these be dealt with easily and quickly in the rough conditions of a large workshop?

Let us again think of that one-inch hole and we shall see how simply and easily it is done. The gauge in such a case would be shaped somewhat like a dumb-bell, one end being the "go" end and the other the "not-go" end. The former is made to agree as nearly as possible with the lower limit, the other with the higher limit, and all the inspector has to do is to try first one end in the hole and then the other. One must "go" in and the other must "not-go." So long as that happens he knows that the hole is correct within the prescribed limits. If, on the other hand, both go in, then he knows the hole is too large, or if neither goes in he knows it is too small. It may be urged by some acute reader that the gauges themselves cannot be correct, and that is quite true, but it is possible, by great care and laborious methods, to produce gauges which are correct to within far narrower limits than those mentioned.

In the case of outside dimensions the gauges take the form of a thumb and finger capable of spanning the object to be measured, and in that case also two are used, one of which must "go" and the other "not-go."

By methods such as these the shells are measured and examined.

One of the most important features of a shell is its driving band. In the old days of round cannon balls it is said that the gunners used to wrap greasy rag round each so as to make it fit the cannon and to prevent the force of the explosion to some extent wasting itself by blowing past the ball. That is one of the functions of the driving band. It is made of copper which is comparatively soft, and it forms a fairly tight fit in the bore of the gun, so that while the shell is free enough to slide out of the gun it is tight enough to prevent the loss of any of the driving force of the explosive.

Its second purpose is to give the necessary spinning action to the shell. The old cannon ball suffered from the fact that it offered a considerable surface to the air in proportion to its weight. The idea arose, therefore, of making projectiles cylindrical and with a pointed nose, so that while the weight might be increased the resistance to the air might be even reduced. But it was clearly no use doing this unless the thing could be made to travel point foremost. Now for some rather mysterious reason, if you shoot a cylindrical object out of a gun, it will turn head over heels in the air, unless you give it a spinning motion. This motion, however, because of a gyroscopic effect, keeps the shell point first all the time.

It has another effect, too, known as "air-boring." A spinning shell seems actually to bore its way through the air. Probably this is due to a centrifugal action, the spinning shell throwing the air outwards from itself and so to some extent sucking the air away out of its own path. Whether that be the true explanation or not, the fact remains that the spinning shell makes its way through the air better than a non-spinning one would do.

The gun, therefore, has formed in its bore a very slow screw-thread called "rifling," from a French word meaning a screw. And it is the second function of the copper band to catch this rifling and by it be turned as the shell proceeds along the barrel. The soft copper conforms to the shape of the rifling and so itself becomes in a sense a screw engaging with the rifling.

This band is situated near the base of the shell, lying in a groove turned in the shell for its reception. To prevent the band turning round without turning the shell there is a wavy groove turned in the bottom of the larger groove, and the band, being put on hot, is squeezed into the latter by a powerful press.

The nose-bush is a little fitting of brass which screws into the smaller end of the shell and it has a hole in its centre into which another brass fitting, the nose itself, is screwed.

The base of the shell is closed with a little disc of steel plate. People sometimes wonder why the original forging is not made solid at the bottom so as to save the necessity for this disc, but the reason is that if that were done defects might very possibly arise in the steel in the centre which, since it is the very spot whereon the propellant acts, might let some of the heat or force of the propellant through, causing a premature explosion of the charge inside the gun itself instead of among the ranks of the enemy.

In the case of naval shells, the nose is not of brass but of a soft kind of steel. One might expect it to be of the very hardest steel, since it has to pierce the hard armour, but experience has shown that the soft nose is better than a hard one. The reason probably is that a hard nose splinters, whereas a soft one spreads out on striking the armour and then acts as a protection to the body of the shell behind it. In these shells, too, the fuse which explodes the charge is placed in the base. In the others it is in the nose, but clearly it could not be so placed in the armour-piercing shell.

It is interesting to mention that the propellent "powder" has combined in it some vaseline or other greasy matter which acts as a lubricant between the gun and the shell when firing takes place.

Shrapnel is so different from the other types of shell that it merits a short paragraph or two to itself. Instead of being filled, as the others are, solely with explosive, the front part of it accommodates a considerable number of small round bullets, behind which comes a charge of gunpowder. The front half of the shell is separate from the back part, the two being connected by rivets of soft iron wire, so that a sudden shock can rend them apart. The shell is fired from the gun and comes flying along: suddenly, owing to the action of the fuse, the gunpowder explodes: the case then flies in two, the bullets are liberated and fall in a shower. In the South African War, where fortifications were few, these shells were very effective, but against fortifications, and particularly against trenches and barbed wire, big explosive shells are of much greater value.

CHAPTER XI
WHAT SHELLS ARE MADE OF

Just think what a shell is and what it has to do. It is a metal case filled with explosive. It is thrown from a gun and is intended to blow itself to pieces on arrival at its destination. It is that self-destruction which carries destruction to all around as well. It is necessary, in order to obtain the best result, that an appreciable time should elapse between the ignition of the explosive and the bursting of the case. The force of the most sudden explosion is not really developed at once, but takes an appreciable time. After ignition, therefore, as the explosion gradually becomes complete, the pressure inside the shell is growing, and too weak a shell would go to pieces before the maximum pressure had been attained. Thus much of the energy of the explosion would simply be liberated into the air instead of being employed in hurling the fragments of shell with enormous force.

That is, of course, not a complete explanation of the whole action of a high-explosive shell, but it indicates generally the reason why a special quality of steel is required in order to get the best results.

Steel having been dealt with in another chapter, we will pass to the other metals which play important if not essential parts in the production of modern projectiles. So important are several of these that the lack of one or two of them would, under modern conditions, mean certain defeat for a nation.

Let us first of all take copper, of which is made the driving bands of the shells and which in combination with zinc forms brass of which noses and other important parts are made.

Its ore is found in many parts of the world, notably in the United States, Chile and Spain. The ores are of several kinds, the simpler ones to deal with being oxides and carbonates of copper, meaning compounds of copper with oxygen and with oxygen and carbon respectively.

It will be remembered that ores of iron are usually of the same nature, namely, oxides and carbonates, and consequently we find that the method of obtaining copper from these ores resembles the methods employed to obtain iron from its ores.

The ore is thrown into a large furnace, like the blast furnaces of the ironworks, and in the heat of the fire the bonds between copper and oxygen are loosened and the superior attractions of the carbon in the fuel entice the oxygen away, leaving the metal comparatively pure.

Unfortunately, however, copper is found most plentifully in combination with sulphur with which it forms what is termed sulphide. This copper sulphide is called by miners "copper pyrites." Another trouble is that mixed with the copper pyrites there is usually more or less of iron pyrites, or sulphate of iron, so that to obtain the copper not only has the sulphur to be got rid of but also the iron. This complicates the operations very much, the ore having to be subjected to repeated roastings and meltings during which the sulphur passes off in the form of sulphur dioxide (a material from which sulphuric acid can be obtained), leaving oxygen in its place. Thus the copper sulphide becomes copper oxide, after which the oxygen is carried away by carbon, leaving the relatively pure metal. Moreover, at each operation various substances are thrown into the furnace called fluxes, which do not mingle with the metal but float on the top in the form of slag, and into the slag the iron passes, so that finally the copper is obtained alone.

Zinc is another important material for shell-making. Its ores used to be found in great plenty in Silesia, but the chief source of supply is now Australia. It is what is called "zinc blende," and consists of zinc sulphide, or zinc and sulphur in combination. In all these names, it may be interesting to mention, at this point, the termination "ide" indicates a compound of two substances, so that we can safely conclude that the "ides" consist of the two elements named in their titles and no others. Thus zinc sulphide is zinc and sulphur and nothing else, iron sulphide is iron and sulphur, copper oxide is copper and oxygen, and so on.

The blende is first roasted in huge furnaces specially built for the purpose. To ensure its being thoroughly treated it has to be "rabbled" or turned over and over, since otherwise all of it might not be brought into contact with the necessary oxygen. At one time done by men with rakes, it is now generally accomplished by mechanical means.

A description of one such furnace will be of interest. It consists of a long rectangular building of brickwork bound together with steel framework. Inside it is divided up into low chambers, the roof of each forming the floor of the one above.

At intervals along its length mighty shafts of iron pass up from underneath right through all the floors, emerging finally above the topmost, while along underneath the furnace there runs a shaft the action of which turns the vertical shafts slowly round and round.

Attached to the vertical shafts are long strong arms of iron, one arm to each floor, and upon the arms are placed rabbles, as they are termed, pieces of iron shod sometimes with fireclay, resembling most of any familiar objects a small ploughshare.

As the arms slowly revolve, at the rate of once or twice per minute, the arms are carried round and round and the rabbles plough up and turn over and over the layer of ore lying upon the floor.

There are arms on the top of the furnace, too, sometimes, where the ore is first laid so that it may be dried by the heat escaping from the furnace beneath, an interesting example of economy effected by utilizing heat which would otherwise be wasted.

The whole of the furnace, from end to end and on every floor, is thus swept continually by the rotating arms with their dependent rabbles, and the latter are cunningly shaped so that they not only turn the ore over and over, but gradually pass it along the different floors or hearths. It is fed automatically by a mechanical feeder which pushes it on, a small quantity at a time, to the drying hearth on the top. Then the rabbles take charge of it and gradually pass it from the area swept by one shaft to that of the next until it has passed right along the top and has become thoroughly dried. Arrived there it falls through a hole on to the topmost hearth or floor, along which it travels by the same means but in the contrary direction until it again falls through a hole on to the top floor but one. And so it goes on until at last, fully roasted, it falls from the bottom floor of the furnace into trucks or other provision for carrying it away.

Some kinds of ore require to be heated by means of gas which is generated in a "gas-producer" near by. In others, however, the sulphur in the ore acts as the fuel, and so the furnace, having been once started, can be kept up for long periods without the expenditure of any coal at all. Very little attention is needed by furnaces such as these, so that with no fuel to pay for and very little labour, they are extremely economical.

Owing to the great heat, too, the arms would stand a very good chance of getting melted were they not kept cool by a continual stream of water flowing through the shafts and arms. This furnishes a continual supply of hot water which is sometimes used for other purposes in the works.

The process of roasting, whether carried on in furnaces such as these or not, results in the formation of oxide instead of sulphide; in other words, the sulphur is turned out and oxygen takes its place. The dislodged sulphur then joins up with some more oxygen and forms sulphur dioxide, which can be led away to the sulphuric acid plant and there, by union with water, turned into that extremely valuable substance, sulphuric acid.

We cannot, however, treat zinc oxide as we would iron oxide or copper oxide, for zinc is volatile, and so, instead of accumulating in the bottom of a blast furnace as the iron and copper do, would pass off up the chimney.

The oxide is therefore mixed with coal or some other form of carbon and placed in retorts made of fireclay. These retorts are fixed in rows one above the other like the retorts at a gasworks, and hot gases from a gas-producer down below pass around and among them. To the mouth of each retort is fitted a condenser, also made of fireclay.

Now what happens in the retorts is this: the heat loosens the bonds between the zinc and the oxide, the latter passing into union with some carbon from the coal. The zinc at the same time becomes vapour and passes into the condenser, the lower temperature of which turns it into a liquid which the workmen remove at intervals in ladles. On being poured into moulds and allowed to solidify this metal is called by the name of "spelter," which bears to zinc the same relation that pig-iron does to the more highly developed forms of iron. Spelter is simply zinc in its crudest form.

Tin, although less important in war than copper and zinc, plays a not unimportant part. It has been found for centuries in Cornwall. The Romans used to trade with the natives of Britain for tin. Although considerable quantities of it is still obtained from there, the greatest tin-producing country of all at present is the Federated Malay States. Australia also furnishes ore, as does Bolivia and Nigeria.

In Cornwall the ore occurs as rock in veins or lodes filling up what must once have been fissures in granite rocks. That near the surface has long been taken, so that to-day the mines are very deep and costly to work. Some can only afford to operate when the market price of tin is above a certain limit. Much of the ore from the newer districts—the Malay States, for example—is in small fragments mixed with gravel in beds near the surface. Such is called alluvial or stream tin, since the deposits were undoubtedly put in their present position by streams or rivers. So long as they last these easily accessible alluvial deposits will always be cheaper to work than the deep mines. On the other hand, they may give out, and recent explorations underground seem to indicate that there is still much valuable ore not only of tin but of other metals too, to be obtained from the old mines of Cornwall.

The ore of tin, like so many other ores, is generally oxide. It is first roasted to expel sulphur and arsenic which are often present as impurities, and then it is melted in a reverberatory furnace such as that described for the manufacture of wrought iron. As usual, the oxygen combines with carbon, the impurities form slag which floats on the top, and the pure metal falls to the bottom of the furnace from whence it can be drawn off.

Mixed with or in the neighbourhood of tin ore there is sometimes found another mineral called wolfram, which plays an extremely important part in modern warfare, so much so that the British and other Governments engaged in the war were at times hard put to it to find enough. Its value resides in the fact that it contains tungsten, an element which has wonderful powers in hardening steel.

It consists of tungsten and oxygen, but is not an oxide since there is also iron in the partnership. This fact is very useful, however, since it enables the particles of wolfram to be picked out from the mass of other stuff among which they are found by a magnet.

There are some very wonderful machines called magnetic separators, made for this express purpose. In one, with which I am familiar, there is an endless band stretched horizontally upon two rollers. One of the rollers being driven round the belt travels along so that the mineral being fed on to it in a stream is carried along under several magnets. These magnets are very different from the ordinary magnet, inasmuch as they are revolving. We might almost describe them as small magnetized flywheels. As they spin round they pick up slightly the particles of ore which contain iron, but have no effect at all upon those which do not contain iron. They do not actually lift the particles up on to themselves: they just exercise a slight pull upon them, and by virtue of the fact that they are revolving, pull them off the band and throw them to one side. The wheels can be set closer or farther from the belt at will so as to make them act more or less strongly, and thus the most magnetic particles can be separated from those less magnetic, these latter being still kept separate from the wholly non-magnetic particles. Thus by simple and purely mechanical means are the precious bits of wolfram obtained from the other less valuable or worthless minerals with which they are mixed.

The same method is used with other minerals besides wolfram: it can be applied to all those which exhibit in some small degree the magnetic properties which we usually associate with iron.

This sorting out of one mineral from others continually crops up in connection with nearly all the metals except iron. Iron is practically the only one whose ore occurs in vast masses which need simply to be dug up and thrown into the furnace. The others, where they occur as rock in veins, have to be crushed to detach what is wanted from what is not wanted, and then the two have to be sorted in some way. Magnetic separation is but one of these ways. Another takes advantage of the fact that we seldom find two things together which have precisely the same specific gravity. Consequently, if we throw the mixture on to a shaking table the heavier particles will behave differently from the lighter ones and the two will separate. The same result can be obtained by throwing the mixture into a stream of water, the water acting differently upon the lighter and upon the heavier particles. Another way which may be mentioned is founded upon the fact that some things can be readily wetted with oil while others throw the oil off and refuse to be wetted by it. If a mixture of these two sorts of thing be stirred violently in a suitable oily liquid the former will be found eventually in the froth, while the latter will sink to the bottom. All these different methods are employed, as they are found necessary in preparing the ores of the various metals to which we have been referring.

Except in the case of alluvial ores which have been broken already by the action of ancient streams of water, nearly all ores (except iron) have to be crushed before the ores can be separated out. Some of this work is done by the very simplest contrivances, showing how in some cases invention has almost come to a stop through the machines having been reduced to their simplest form. A notable instance of this is the stamp mill, in which heavy timbers are lifted up by machinery and then allowed to slide down upon the ore, just like gigantic pestles. More elaborate grinding machines are sometimes used, however, but it is impossible to mention them all here.

The action of sorting out the fragments of ore from the miscellaneous assortment of crushed rocks is termed "concentrating," and the sorted ores are called "concentrates."

Another metal which has proved itself of immense importance in war is aluminium, and it fittingly comes at the close of the list since it is dealt with in a manner peculiar to itself. Practically all the others are obtained from their ores by means of heat and heat alone. Aluminium is obtained by electricity acting in the process called electrolysis.

It is surprising to learn that aluminium is one of the very commonest things on the face of the earth. Clay and many common rocks are very largely made of it. Clay, to be precise, is a silicate of alumina, a term which is interesting when it is explained. Silica is the name given to oxide of silicon. Sand is mostly silica. Alumina, too, is oxide of aluminium. Silicate of alumina is a combination of the two.

Any clay, therefore, could be used as an ore from which to obtain aluminium, but of course there are certain minerals specially suitable for the purpose, since in them the metal is plentiful and easily extracted.

In another chapter reference is made to the production of caustic soda from a solution of common salt by electrolysis. The same principle, precisely, is used to obtain the metal aluminium from its ore, which is generally an oxide.

Common salt, let me remind you, is sodium and chlorine combined. When you dissolve it in water it becomes ionized, which means that each molecule of salt splits up into two ions one of which is electrically positive and the other electrically negative. Then, when we introduce two electrodes into the solution and connect them to a battery or dynamo, all the positive ions go to one electrode and all the negative ions to the other.

We cannot dissolve aluminium ore in water, but we can in a bath of molten cryolite, and for some reason or other, whether because of the heat or not we cannot say, the ore becomes ionized, the aluminium atoms being one sort and the oxygen atoms the other sort. These ions then sort themselves out, the oxygen ions being taken into combination with the carbon rod which forms the positive electrode, while the metal ions collect upon the negative electrode. Since this latter is a slab of carbon which forms the bottom of the vessel in which the process is carried on, the result is that pure aluminium gradually accumulates in the bottom of the vessel and can be drawn off from time to time.

Aluminium is always produced in places where electric power can be obtained cheaply, such as near waterfalls.

CHAPTER XII
MEASURING THE VELOCITY OF A SHELL

People often ask how quickly electricity travels, as if when we sent a telegraph signal along a wire a little bullet, so to speak, of electricity were shot along the wire like the carriers of the pneumatic tubes in the big drapers' shops. That is quite a misconception, for in reality the circuit of wire is more like a pipe full of electricity, and when we set a current flowing what we do is to set the whole of that electricity moving at once. If we think of a circular tube full of water with a pump at one spot in the circuit, we see that as soon as the water begins to move anywhere it moves everywhere. Moreover, if it stops at one point it stops simultaneously at every other point. While practically this is the case it is theoretically not quite so, for the inertia of the water when it is suddenly started or stopped no doubt causes a slight distortion of the tube itself resulting in a very slight (quite imperceptible) retardation of the movement of the water. Electricity also has a property comparable to the inertia which we are familiar with in the objects around us, and there is also a property in every conductor which to a certain extent resembles the elasticity of the water-pipe, whereby it may for a moment be bulged out. In a short wire, however (up to a mile or so), particularly if the flow and return parts of the circuit be twisted together, this electrical inertia practically vanishes and consequently we may say that for all practical purposes the current starts or stops, as the case may be, at precisely the same moment in every part of the circuit.

That fact is of great value when, as in the case we are now discussing, we want to compare very exactly two events occurring very near together as to time but far apart as to place.

We need to compare the time when the shell leaves the gun with the time when it passes another point, say, one hundred yards away, and then again another point, say one hundred yards further on still. Supposing, then, a velocity of 3,000 feet per second, the time interval between the first point and the second and between the second and third will be somewhere about a tenth of a second. So we shall need a timepiece of some sort which will not only measure a tenth of a second, but will measure for us a very small difference between two periods, each of which is only about a tenth of a second and which will be very nearly alike. That represents a degree of accuracy exceeding even what the astronomers, those princes of measurers, are accustomed to.

This exceedingly delicate timepiece is found in a falling weight. So long as the thing is so heavy that the air resistance is negligible, we can calculate with the greatest nicety how long a weight has taken to fall through a given distance.

Near the muzzle of the gun there is set up a frame upon which are stretched a number of wires so close together that a shell cannot get past without breaking at least one of them. These wires are connected together so as to form one, and through them there flows a current of electricity the action of which, through an electro-magnet in the instrument house, holds up a long lead weight.

At some distance away, say one hundred yards, there is a similar frame also electrically connected to an electro-magnet in the same instrument house. This second magnet, when energized by current from the frame, holds back a sharp point which, under the action of a spring, tends to press forward and scratch the lead weight. The third frame is likewise connected to a third magnet controlling a point similar to the other.

To commence with, current flows through all three frames so that all three magnets are energized. The gun is then fired and immediately the shell breaks a wire in the first frame, cutting off the current from the first magnet and allowing the weight to fall. Meanwhile, the shell reaches the second frame, breaking a wire there, with the result that the second magnet loses its power, lets go the point which it has been holding back and permits it to make a light scratch upon the falling weight. This action is followed almost immediately by a similar action on the part of the third magnet, resulting in a second scratch on the lead weight.

The position of these two scratches on the weight and their distance apart gives a very accurate indication of the time taken by the shell to pass from the first screen to the second and from the second to the third. From those times it is possible to calculate the initial velocity of the shell and the speed at which it will move in any part of its course. Indeed, with those two times as data, it is possible to work out all that it is necessary to know about the behaviour of the shell.

This is rendered practicable by the fact that the moment the wire is cut the magnet lets go, no matter what the distance of the screen from the instrument may be. But for the instantaneous action of the current, allowance of some sort would have to be made for the fact that one screen is farther than another and the whole problem would be made much more complicated.

Even as it is, someone may urge that the magnets themselves possess inertia and will not let go quite instantaneously, but that can be overcome by making the magnets all alike so that the inertia will affect all equally. It is only necessary to have a switch which will break all the three circuits at the same moment (quite an easy thing to arrange) and then adjust all three magnets so that when this is operated they act simultaneously. After that they can be relied upon to do their duty quite accurately.

Thus by a method which in its details is quite simple is this seemingly impossible measurement taken.

CHAPTER XIII
SOME ADJUNCTS IN THE ENGINE ROOM

As everyone knows, there is in every ship (except those few which are propelled by paddles) a long steel shaft, called the tail-shaft, which runs from the engine situated somewhere near amidships to the propeller at the stern. Many ships, of course, have several propellers, and then there are several shafts. Now each of these shafts is a thick strong steel rod supported at intervals in bearings. If anyone were told that, in working, that shaft became more or less twisted, he would be tempted to think he was being made fun of. Yet such is literally the case. The thick strong massive bar becomes actually twisted by the turning action of the engine at one end and the resistance of the propeller at the other. And the amount of that twisting is a measure of the work which the engine is doing. The puzzle is how to measure it while the engine is running, for of course the twist comes out of it as soon as the engine stops.

A space on the shaft is selected, between two bearings, for the fixing of the apparatus. Near to each bearing there is fitted on to the shaft a metal disc with a small hole in it. On one of the bearings is fixed a lamp and on the other a telescope. When the engine is at rest and there is no twist in the shaft, all these four things—the lamp, the two holes, and the telescope—are in line. Consequently, on looking through the telescope the light is visible. But when the engine is at work and the shaft is more or less twisted one of the holes gets out of line and it becomes impossible to see the light through the telescope. A slight adjustment of the telescope, however, brings all four into line again, which adjustment can be easily made by a screw motion provided for the purpose. And the amount of adjustment that is found necessary forms a measure of the amount of the twisting which the shaft suffers and that again tells the number of horse-power which the engine is putting into its work.

But it is also necessary to know how fast the engine is working. There are many devices which will tell this, of which the speedometer on a motor-car is a familiar example. Most of those work on the centrifugal principle, the instrument actually measuring not the speed but the centrifugal force resulting from the speed, which amounts to the same thing. There is one instrument, however, which operates on quite a different principle, because of which it is specially interesting. It consists of a nice-looking wooden box with a glass front. Through the glass one sees a row of little white knobs. If this be placed somewhere near the engine while it is at work immediately one of the knobs commences to move rapidly up and down, so that it looks no longer like a knob but is elongated into a white band. There is no visible connection between the instrument and the engine, yet the number over that particular knob which becomes thus agitated indicates the speed of the engine.

Let us in imagination open the case and we shall find that the knobs are attached to the ends of a number of light steel springs set in a row. The springs are all precisely alike except for their length, in which respect no two are alike. Indeed, as you proceed from one side of the instrument to the other each succeeding one is a little longer than the previous one. Now a spring has a certain speed at which it naturally vibrates and other things being equal that speed depends upon its length. You can, of course, force any spring to vibrate at any speed if you care to take the trouble, but each one has its own natural speed at which it will vibrate under very slight provocation.

Every engine is, of course, made to run as smoothly as possible. All revolving or reciprocating parts are for this reason carefully balanced and in turbines the whole moving part, since it is round and symmetrical, naturally approaches a condition of perfect balance. Hence every engine ought to run perfectly smoothly. As a matter of fact, however, no engine ever does. There are certain limitations to man's skill and at the high speed of a fast-running engine, such as is to be found on a destroyer, for example, some little irregularity is sure to make itself felt by a slight vibration in the floor. It may be hardly perceptible to the senses, but to a spring whose natural frequency happens to be just that same speed or nearly so, it will be very apparent and in a few seconds that spring will be responding quite vigorously.

It is another example of the principle of resonance, which is employed so finely in making wireless telegraph apparatus selective. Every wireless apparatus is made to have a certain natural frequency of its own and it therefore picks up readily those signals which proceed from another station having the same frequency while ignoring those from others. In just the same way a reed or spring in this speed-indicator picks up and responds to impulses derived from the engine only when they are of a frequency corresponding with its own natural frequency. Hence, one spring out of the whole range responds to the vibrations of the engine while the others remain almost if not entirely unaffected.

In another form, the springs are actuated electrically. A magnet, or a series of magnets, is arranged so that as the engine turns the magnets pass successively near to a coil of wire, thereby inducing currents in that wire. They form, in fact, a small dynamo or generator, generating one impulse per revolution or two or three or whatever number may be most convenient. Then the current from this is led round the coil of a long electro-magnet placed just under the free ends of all the springs. The magnet therefore gives a series of pulls, at regular intervals, and the rapidity of those pulls will depend upon the speed of the engine, while the frequency of them will be registered by the movement of one or other of the springs.

This instrument can also be employed to determine the speed of aeroplane motors and, in fact, any kind of engine, especially those whose speed is very high.

CHAPTER XIV
ENGINES OF WAR

In these latter machines there are a number of cylinders with closed ends and with very smooth interiors, in each of which slides a disc-like object called a piston. The steam enters a cylinder first at one end and then at the other, thus pushing the piston to and fro. The movement of the piston is communicated to the outside by means of a rod which passes through a hole in the cover at one end of the cylinder, the to and fro motion being converted into a round and round motion by a connecting-rod and crank just as the up and down motion of a cyclist's knees is converted into a round and round motion by the lower leg and the crank. The lower part of a cyclist's leg is, indeed, a very accurate illustration of what the connecting-rod of a steam-engine is.

As is evident to the hastiest observer, some arrangement has to be made whereby the steam shall be led first into one end and then into the other end of the cylinder: also that provision shall be made for letting the steam out again when it has done its work. Moreover, such arrangements must be automatic. Hence, every reciprocating engine has special valves for this purpose and such valves need rods and cranks (or something equivalent) to operate them. Further, to get the best results the steam must not simply be passed through one cylinder but through several in succession. Engines where the steam goes through only one cylinder are called "simple," where it goes through two they are "compound," where three "triple-expansion," where four "quadruple-expansion." Generally speaking, each cylinder has its own connecting-rod and crank, also its own set of rods, etc., for working its valves. Hence, a high-class marine reciprocating engine is of necessity a complicated mass of cylinders, rods, cranks and other moving parts continually swinging round or to and fro at considerable speeds, all needing oiling and attention and all liable at times to give trouble.

And now compare that with the turbine, which has TWO parts, only one of which moves. That part, moreover, is tightly shut up inside the other one, being thereby protected from any chance of damage from outside and likewise rendered unable to inflict any damage upon those in attendance upon it.

At first sight it seems very strange that the turbine should be the newer of the two, for it is simply an improved form of the old time-honoured picturesque windmill which used to top every hill and grind the corn for every village and hamlet.

The old windmill had four sails against which the wind blew, driving the whole four round as everyone knows. The new turbine has a great many sails, only we now call them blades, and the steam blows them round. The old windmill had to have another smaller set of sails at the back for the purpose of keeping the main sails always in that position in which they would catch the full force of the breeze. In the turbine we need not do that, for we shut the windmill up in a kind of tunnel and cause the steam to blow in at one end and out at the other.

The difference between the various kinds of turbine lies simply in the manner in which the steam is guided in its passage through the machine.

After that general description we can take a more detailed view of the Parsons turbine. The casing or fixed part is a huge iron box suitably shaped for standing firmly and rigidly upon the floor of the engine-room. It is made in two halves, the upper of which can be easily lifted off when necessary. Often, indeed, this upper half is hinged to the lower, so that it can be opened like the lid of a box.

Inside, the casing is cylindrical, comparatively small at one end but increasing by steps till it is very much larger at the other end. At each end is a bearing or support in which the rotor or moving part is held and in which it can turn freely.

The rotor or part which rotates is a strong steel forging shaped somewhat to follow the lines of the inside of the casing. It does not entirely fill the casing but leaves a space all round and all the way along, which space is intended to accommodate the blades. The ends of the rotor are smaller than the body since they are intended to fit into the bearings, and one of the ends is prolonged so as to be available for coupling to the propeller-shaft of the ship.

At one end of the casing, the smaller one, is the steam inlet and the steam after emerging from it passes along till it finds its way out at a very large outlet formed at the bigger end. On its way it has to pass thousands of small blades so that the progress of each individual particle of steam is not a straight line but a continual zigzag. There are rings of blades round the rotor, tightly fixed to its surface. There are likewise rings of blades affixed to the inner surface of the casing, the rings upon the casing coming in the spaces between the rings on the rotor.

Let us imagine that we can see through the casing of a turbine at work and that looking down upon it from above we can trace the progress of a particle of steam. It rushes in from the inlet and at once makes straight for the outlet at the further end. Suddenly, however, it encounters one of the guide blades (those on the case) and by it is deflected to one side, we will suppose the left. That causes it to rush straight at one of the blades upon the rotor against which it strikes violently, giving that blade a distinct and definite push to the left. Rebounding, it then comes back towards the right but quickly is caught by another guide blade and by it hurled back upon a second rotor blade, giving it a leftward push just as it did to the first. Thus it goes zigzagging from one set of blades to the other until, tired out, so to speak, it finally flows away forceless and feeble through the outlet, having given up all its energy to the blades of the rotor against which it has struck in its course.

That, then, is the journey of one single particle. Multiply that by an unknown number of millions and you have a description of what takes place in the interior of a steam turbine. The blades are so proportioned, so arranged and so placed that it is very difficult indeed for a particle of steam to creep past without doing its share of work. Practically every one is made use of and while, of course, the action of a single particle of steam would have but a negligible effect, the vast number engaged cause the rotor to be powerfully blown round.

The reason why the casing and rotor are made larger and larger as one proceeds from the inlet towards the exhaust or outlet is that the steam must, if all its energy is to be extracted, expand as it goes and the enlargement provides room for this expansion.

One of the great advantages of the turbine is that the steam is always entering at the same end. In the cylinder of a reciprocating engine the steam enters alternately. It comes in hot but as it does its work and finally goes out it becomes very much cooler: the next lot of steam which enters, therefore, is chilled by the cool walls of the cylinder which have just been cooled by the departure of the previous lot of steam: so heat is wasted. Wasted heat means fuel lost, and as any given ship can only carry a limited quantity of fuel, wasted heat means less range and more frequent returns to the base to coal or to "oil."

Also let me remark again upon the simplicity of the turbine as opposed to the other sort. The latter consists of a mass of moving and swaying rods and cranks, to work among which, as the engineers have to do, is a terrifying and nerve-racking experience. The turbine, on the other hand, has its only working part enclosed. It is difficult to tell, by looking at it, whether a turbine is at work or not, so silent and still is it, so self-contained. The reciprocating engine-room is noisy and full of turmoil: the turbine room is weirdly still by comparison.

On the whole, too, it makes better use of the steam which it uses, but it has one decided drawback. It will not reverse, which the other type of engine does readily.

This means that two turbines have to be coupled together, one with the blades so set that the steam drives it round correctly to produce motion ahead and the other set the opposite way so that it drives the vessel astern. The steam can be sent through either turbine at will and so motion can be obtained in either direction. Whichever turbine is in use the other revolves idly.

Unfortunately it is impossible to make a turbine to go slowly and yet be efficient. Consequently, slow steamers cannot use turbines, but for warships, which are nearly all fast boats, it has almost displaced the older type of engine.

The Curtiss turbine is different from the Parsons in that the steam encounters periodically, in its passage through, a partition perforated with funnel-shaped holes. Between the partitions it passes blades upon which it acts just as already described. The chief effect of this is to permit the machine being made of a rather more convenient shape and size. Other varieties of turbine are more or less combinations of the two ideas underlying these two.

When we look at a locomotive in motion we always see steam coming out of the funnel, but we never see that in the case of a steamer. That is because all the energy of the steam is taken and used in the latter case, while in the former much valuable energy goes off up the funnel, making a puffing noise instead of doing useful work.

On the steamship the steam is led not to the open air but to a vessel called a condenser the walls of which are kept cool by a continual circulation of cold water. The steam on entering the condenser at once collapses into water, leaving a vacuum. A pump called the "air-pump" removes the water (which was once steam) from the condenser and also any air which might get in, with the result that the engine is always discharging its steam into a vacuum. Thus to the pressure of the steam is added the suction of the vacuum.

In turbine ships the cooling water for the condensers is circulated by powerful centrifugal pumps driven by subsidiary engines.

The steam is obtained from boilers of that special variety known as "water-tube."

The boilers with which most people are familiar are either Lancashire or Cornish, both sorts being large steel cylinders with two steel flues in the former and one in the latter running from back to front. The fire is made in the front part of the flue and the hot gases from it pass to the back and then along the sides and underneath through flues formed in the brickwork in which the boiler is set. Locomotive boilers, however, have no flues, but the hot gases from the fire in the fire-box pass through tubes which run from end to end through the cylindrical shell, each tube starting from the fire-box behind and terminating in the smoke-box in front. Thus we have tubes with fire inside and water outside: hence such boilers are called "fire-tube" boilers.

On many ships of the merchant type cylindrical boilers are used which combine the features, to some extent, of the Cornish and the fire-tube, since there is a flue running from front to back in which the fire is made and the hot gases return from back to front through a number of tubes which occupy the space above the fire. Arrived at the front the gases pass upwards to the chimney.

Water-tube boilers are different from all of these, since in them the water is inside the tubes while the fires play around the outside. This enables steam to be got up very quickly, a matter of much importance for a warship which may be called upon to undertake some operation at a moment's notice.

The boilers are fed with water from the condensers, so that the same water is used over and over again. When coal is burnt it is put on the fires by hand, for although mechanical stoking is a great success on land, there are special difficulties which prevent its use at sea. It is becoming more and more the fashion now to burn oil instead of coal in several types of ships and in those cases the oil is blown in the form of spray into the furnace. This has many advantages, some of which are exemplified on a small scale by the difference between using a coal fire and a gas stove. Like the latter, the oil spray can be quickly lit when needed and as quickly extinguished. It can be regulated and adjusted with equal facility. Oil can be taken on board too through a pipe, silently and quickly and without the terrible dirt and the exhausting labour involved in coaling a big ship. Oil, too, can be taken on board at sea, from a tank steamer, almost as easily as it can be taken in ashore, whereas the difficulty of coaling at sea despite many ingenious efforts has never been solved quite satisfactorily. Finally, oil can be stowed anywhere, for the stokers do not need to dig it out with a shovel. Therefore it can be carried in those spaces between the inner and outer bottoms which have to be there in order to give strength to the ship's hull but which would be quite useless for carrying coal. The advantages of oil fuel, therefore, are many and no doubt it will be used more and more as time goes on.

For Great Britain, oil fuel has the disadvantage that it has to be imported whereas the finest steam coal in the world is found in abundance in South Wales, but the difficulty may eventually be overcome by distilling from native coal an oil which will serve as well as that which is now imported.

So much for the turbine, the engine of the big ships: now for the Diesel oil-engine which drives the submarines. It belongs to that family of engines called "internal-combustion" since in them the fuel is burnt actually inside the cylinder and not under a separate contrivance such as a boiler. There have been oil-engines, so called, for many years, but they were really gas-engines since the oil was first heated till it turned into vapour and then that vapour was used as a gas. The Diesel engine, however, actually burns oil in its liquid state.

To understand how it works let me ask you to conjure up this little picture before your mind's eye. A hollow iron cylinder is fixed in a vertical position: its upper end is closed but its lower end is open: inside it is a piston, free to slide up and down: by means of a connecting-rod hinged to it and passing downwards through the open lower end the piston is connected to a crank and flywheel. At the upper end of the cylinder are certain openings which can be covered and uncovered in succession by the action of suitable valves.

Now let us assume that that engine is at work, the piston going rapidly up and down in the cylinder. As it goes down it draws in a quantity of air through a valve which opens to admit the air at just the right moment. The moment the piston reverses its movement and starts to go up again that valve closes and the air is entrapped. The piston continues to rise, however, with the result that the air becomes compressed in the upper part of the cylinder.

Now it is necessary to remind you at this point that compressing air or indeed any gas, raises its temperature. This air, therefore, which was drawn in at the temperature of the outer atmosphere, by the time the piston has reached the top of its stroke has attained a temperature well above the ignition point of the oil fuel.

The piston, having arrived at the top of its stroke, the upper part of the cylinder is filled with hot compressed air: the next moment the piston commences its descent, but at precisely that same moment a valve opens and there is projected into the cylinder a spray of oil. Instantly it bursts into flame, heating the air still more, so that as the piston descends the air, expanding with the heat, pushes strongly and steadily upon it. The amount of that push can be varied by varying the duration of the jet. The longer the jet is injected the more heat is generated and the more sustained is the push. On the other hand, if the jet is cut off very quickly the push is only a gentle one.

The power of the engine can thus be adjusted to suit varying circumstances by a slight variation in the valve which controls the jet.

The piston having thus been driven down to the limit of its stroke, it commences another upward movement, at which moment another valve opens and lets out the hot waste gases which have resulted from the burning of the oil. Thus the cylinder is cleaned out ready for a fresh supply of pure air to be drawn in on the next ensuing downstroke.

The engine thus works upon a series or cycle of operations which are repeated automatically over and over again. First comes a downstroke, drawing in air: then an upstroke, compressing it: then a second downstroke, during which the fuel burns and the power is generated: and, finally, a second upstroke during which the waste products of the burning are ejected. Power, it will be noticed, is only developed in one out of the four strokes: the other movements having, in single cylinder engines, to be performed by the momentum of the flywheel.

In most cases, however, the engine has several cylinders in which the cycles are arranged to follow in succession. Thus, if there are four cylinders, there is always power being developed by one of them.

The valves are operated automatically by the engine itself just as is the case with steam-engines. The engine also works a small pump which provides the very highly compressed air necessary to blow the oil jet into the cylinder.

Arrangements are often provided whereby the engine when working stores up a reserve of compressed air which can be used to start it. From the very nature of its working such an engine cannot develop power until it has accomplished at least four strokes or two revolutions, so that it cannot possibly start itself. If, however, compressed air be admitted to the cylinders to give it a vigorous push or two and so get it going, it can then take up its own work and go on indefinitely.

In some cases this is not necessary and that of an engine in a submarine is one of them. In that instance, the electric motor, which drives the boat when submerged, can be made to give the engine a start.

By altering the rotation in which the valves act the direction can be reversed. A very simple mechanism can be made to effect this change, so that reversing is quite easy.

Aircraft are mostly, if not entirely, driven by petrol engines, some of which are very little different from those of a motor-car or motor-cycle.

These motor-car engines are so well known that little need be said about them. It may be well to explain, however, that they, like the Diesel engines, work on a cycle of four strokes, as follows:—