Part 8 (2/2)

The 12-inch gun of fifty calibres weighs 69 tons and fires a projectile weighing 850 lbs. which it hurls from its muzzle at a velocity of about 3000 feet per second.

More recently the size has grown to 13, 14 and even as great as 15 inches calibre, but we may for the moment take the 12-inch gun as typical of all these large guns and have a look at its construction.

It is made of a special kind of steel known as nickel-chrome gun steel, formed by adding certain proportions of the two rare metals nickel and chromium to the mixture of iron and carbon which we ordinarily call steel. The metal is made after the manner described in another chapter and is cast into the form of suitably-sized ingots which are afterwards squeezed in enormous hydraulic presses into the rough shape required.

Besides giving the metal the desired form this action has the effect of improving its quality. Since a gun is necessarily a tube it may be wondered why the steel is not cast straight away into that shape instead of into a solid block and the reason why that is not done is very interesting. It is found that any impurities in the metal--and it is impossible to make it without some impurities--collect in that part which cools last and obviously that part of a block which cools last is the centre. Thus the impurities gather together in the centre of the ma.s.s whence they are removed when that centre is cut away, whereas if the first casting were a tube they would collect in a part which would remain in the finished gun.

The ingot, then, is cast and pressed roughly to shape. Then it is put into a lathe where it is turned on the outside and a hole bored right through the centre.

But that is by no means all of the troubles through which this piece of steel has to pa.s.s. It undergoes a very stringent heat treatment, being alternately heated in a furnace to some precise temperature and then plunged into oil, whereby the exact degree of hardness required is attained.

Moreover, this is only one of the tubes which go to make up the gun, which is a composite structure of four tubes placed one over another with a layer of tightly wound wire as well.

First, there is the innermost tube, the whole length of the gun, then a second one outside that, usually made in two halves. Both are carefully made to fit, and then the outer is expanded by heat to enable it to be slidden over the inner one, after which on cooling it contracts and fits tightly. Outside this second tube is wound the wire, or more strictly speaking tape, for it is a quarter of an inch wide and a sixteenth thick. It is so strong that a single strand of it could sustain a ton and a half. It is carefully wound on; first several layers running the whole length of the gun and then extra layers where the greatest stresses come, that is to say, near the breech, for that has to withstand the initial shock of the explosion. Altogether about 130 miles of wire go on a single gun.

The advantages of this form of construction are many. For one thing, a wire or strip can be examined throughout its whole length and any defect is sure to be found, whereas in a solid piece of steel, no matter how carefully it may be made, there may lurk hidden defects. Moreover, if a solid tube develops a crack anywhere it is liable to spread, whereas a few strands of wire may be broken without in any way affecting the rest.

It has been found that even if a sh.e.l.l burst while inside one of these guns no harm is done to the men in the turret where it stands, a thing which cannot be said for guns composed entirely of tubes, so that the merit rests with the wire. A third advantage is that the wire can be wound on to the tube beneath it at precisely that tension which is calculated to give the best result, whereas in shrinking one tube on to another this cannot always be attained.

Over the wire there come two more tubes not running the whole length but meeting and overlapping somewhat near the middle, so that at one point there are actually four concentric tubes besides the wire.

At the rear end a kind of cap called the breech-piece covers over the ends of all the tubes, itself having a central hole into which fits the breech-block, one of the triumphs of modern engineering, of which more in a moment.

While we have in mind the wire-wound form of construction it is interesting to note that something similar but in a crude form was practised sixty years or more ago. The guns of that era were some of them even of cast iron while the more refined consisted of a steel tube strengthened with coils of wrought iron. This iron was first rolled into flat bars, then it was made hot, and wound on spirally round an iron bar the same size as the tube. A little hammering converted this spiral into a tube which was then fitted round the steel tube. Thus, although very different there is still a distinct resemblance between this old method and the up-to-date wire-wound weapon.

The manufacture of guns, it may be remarked, owes more to one man than to any other, namely, Mons. Gustave Canet, a French engineer who, having fought in the Franco-German War, decided to devote his engineering talents to developing the artillery of his native land. He spent many years in England but later established works at Havre for the manufacture of guns upon improved methods, finally merging his interests into those of the great French armament firm of Schneider of Creusot.

By French and English artillerists at all events the name of Canet is regarded with reverence.

But to get back to our naval gun. It will be clear that operations such as have been described, involving the handling of great tubes fifty feet or more in length, heating them as required, dipping them in oil while hot and so on, can only be carried out at works specially designed for the purpose.

The furnaces where the tubes are heated are well-like formations in the ground, deep enough to take the tube vertically. To lift them in and out there have to be tall travelling cranes capable of catching the tube by its upper end and lifting it right out of the furnace so that its lower end clears the ground. To accomplish this with a little to spare the cranes need to be seventy feet or so high.

Then there are deep pits full of oil so that a tube can be heated in a furnace, drawn out by a crane and quickly dropped into the adjacent oil bath. Likewise there have to be pits of a third kind wherein a cold tube can be set up while a hot one is dropped over it for the purpose of shrinking the latter on.

Then, of course, there have to be lathes of gigantic dimensions capable of taking a length of nearly sixty feet and of swinging an object weighing anything up to fifty tons. But of those machines we can only pause to make mention, for we must pa.s.s on to the breech-block, in some ways the most interesting part of the gun.

When it was first suggested to leave the back end of the gun open so that the powder and projectiles could be put in that way instead of through the muzzle, people at once foresaw how much would depend upon the arrangements for stopping up the hole while the gun was fired. For, of course, the force of the explosion is exerted equally in all directions, backward just as much as forward, so that unless very securely fixed the stopper closing the breech would be liable to become a projectile travelling in the wrong direction. To fix such a thing securely enough to avoid accidents would surely take up too much time and so largely neutralize any advantage arising from its use. These fears were, indeed, to some extent justified by accidents which actually occurred with the early examples of breech-loading guns, and for that reason our own authorities for a time looked askance at breech-loaders.

Now let us take a look at the breech-block of the 12-inch naval gun of to-day, which never blows out, not even when 350 lbs. of cordite go off just the other side of it. The explosion hurls an 850-pound sh.e.l.l at the rate of 8000 feet per second but it never stirs the breech-block. Yet it can be opened and closed so quickly, including the necessary fastening-up after closing, that shots can be fired from the gun at the rate of one every fifteen seconds.

The breech-block partakes of the nature of a plug and also of a door. It swings upon hinges like the latter but its shape more resembles the former. If we want to make such a thing very secure we usually make it in the form of a screw with many threads, but that entails turning it round many times and that takes time. Given plenty of time to screw the breech-block into its place and there would never have been any anxiety as to the possibility of its blowing out, but there is not time. The problem, therefore, was to get the strength of a screw combined with quickness of action.

This dilemma is avoided in the following simple manner. The breech-block is given a screw thread on its exterior surface, and the hole in the breech-piece is given a similar screw-thread on its inner surface, just as if the one were to be laboriously screwed into the other after the manner of an ordinary screw in machinery. Then four grooves are cut right across the threads on the block and similarly on the breech-piece, so that at four different places there is no thread left. In other words, instead of the thread running round and round continuously, each turn is divided up into four sections with sections of plain unthreaded metal in between. Thus in a certain position the block can be pushed into the hole without any threads engaging at all, for each strip of threaded block pa.s.ses over an unthreaded strip in the hole and vice versa, in other words, the threads on the one part miss those on the other part. Yet an eighth of a turn serves to make all the threads engage and the thing is held almost as securely as if it were just an ordinary screw with threads its whole length.

The block is carried upon a hinged arm so that although it can be turned in this manner it can also be swung back freely when necessary.

Combined with the breech-block is a pneumatic contrivance which blows a powerful jet of air through the gun every time the breech is opened, thereby cleaning away the effects of the last explosion.

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