Marvels of Scientific Invention Part 20

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 sh.e.l.l towards the muzzle. So long as it is in the gun the sh.e.l.l is being pushed faster and faster, but so soon as it leaves the muzzle the pushing ceases and the sh.e.l.l 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 sh.e.l.l 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 sh.e.l.l out of the gun is different from that with which the sh.e.l.l is itself filled.

The former needs to act comparatively slowly, so that it may continue its pushing action during the whole time that the sh.e.l.l 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 sh.e.l.l. On arrival at its destination, however, the sh.e.l.l 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 sh.e.l.l 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 sh.e.l.l is bound to cut the wire. Electric current is made to pa.s.s 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 sh.e.l.l 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 pa.s.ses 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 pa.s.sage of the sh.e.l.l 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 s.p.a.ce 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 sh.e.l.l a spinning action which causes it to keep point foremost throughout its flight. But for this the sh.e.l.l would tend to turn over and over, resulting in uncertain and inaccurate flight.

The sh.e.l.l 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 sh.e.l.l 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 sh.e.l.ls 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 quant.i.ty 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.

[Ill.u.s.tration: 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 sh.e.l.l 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 sh.e.l.l will fall short; if it be too much it may go beyond the mark, or it may fall short, according to circ.u.mstances. 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 circ.u.mstances, 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.