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The Materials > Figures 1 – 14 > Mr. Mallet’s Discoveries
It was shown in the chapter upon the history of big guns that improvements in their construction have closely followed the development of the metal trades. First of all they were made of bronze, because a knowledge of the processes necessary for its production preceded the discovery of gunpowder, and was at once available for the purposes of artillery. So soon, however, as sufficient skill had been acquired in the manipulation of wrought-iron as to render it practicable, the blacksmiths of the Middle Ages turned their attention to this material, and the gigantic pieces of ordnance previously referred to were the result of their labours. Even when the greatest care was bestowed upon their construction they were liable to accidents, and the Governments of these early days were therefore eager to discover some other system, not only on this account, but with the object of avoiding the costliness of malleable iron guns as well. The art of casting in iron, which had already made considerable advances in France, was developed in England during the reign of Henry the Eighth by a Frenchman, named Peter Baud, whose labours have already been spoken of, but in this country at least the Government was almost wholly dependent upon private enterprise for its supplies of artillery until the close of last century.
But although this country had become possessed of a national arsenal, it was many years before anything approaching to a perfect system was introduced as a substitute for castings of iron and brass. It appears strange that a knowledge of the scientific principles upon which the construction of big guns ought to be conducted, and upon which their safety altogether depends, should never have been acquired until very recently, and the more so, as an acquaintance with the structure of iron and the manner in which it is affected by the processes of manufacture is really all the information that is necessary. Even now, though iron is employed so universally for the every-day purposes of life, there is a great amount of ignorance of the principles upon which it ought to be manufactured.
It may be said that the difficulties which our armies had to overcome at the siege of Sebastopol, in throwing shells into the city from a distance that had never previously been contemplated, led to efforts on the part of our home Government which were not only followed by great improvements in the construction of artillery, but by the discovery of the principles which it is one object of this chapter to explain(*). It is to Mr. Robert Mallet that the chief credit for these advances is due, and it is upon his scientific labours that the practical construction of the Fraser guns, generally known as Woolwich Infants, is based.
In many great industries the raw materials with which the manufacturer has to deal come to his hands as nature made them, and his business is simply to convert them into marketable commodities. This is the case with the textile industries that form so large a part of our trade and national wealth. The fibres from which cotton cloth is woven, or the hairs of woollen fabrics, however they may be treated by cleansing and spinning and weaving, remain unaltered as far as their own nature is concerned. It is different with the fibres of iron. The stability of an iron structure depends just as much upon the strength of the fibres of which it is composed as the durability of a piece of cloth depends upon the strength of its threads. Not only is this the case, but just as a piece of cloth is capable of bearing a great strain in one direction, and a very small one in another, so is a piece of iron. The great difference between the two materials is, that while the fibres of the cloth remain the same, the fibres of the iron are very readily affected by the processes which are necessary for converting it to useful purposes. The essential conditions referred to are heat and pressure. Now, until Mr. Robert Mallet studied the subject we were practically in the dark as to how iron was affected by these two conditions, and the consequence of this ignorance led to a great many accidents, the wonder being that there were not a great many more. In the case of a woollen fabric, it is very easy, with the aid of a magnifying glass, to trace all the details of the structure. It is quite possible to find out whether or not the threads have been uniformly spun and received just the proper amount of twisting to give them the greatest strength, and an expert is equally well able to judge of the quality of work in the weaving. In iron no such means of ascertaining its internal structure are available, and it is therefore necessary to fall back upon a knowledge of cause and effect, and in this way, knowing that certain causes will produce certain effects, avoid everything in the construction of a big gun which we know will have the effect of making it weaker, and do all those things which we know, from previous experiment, will make it stronger.
Taking the analogy of a piece of cloth once more, and referring again to the familiar fact that it will generally tear more easily in one direction than it will in another, a very good illustration of what happens to a piece of iron when heat is incautiously applied to it may be given by supposing that the piece of cloth when strained to its utmost in the direction of its greatest strength had its fibrous structure suddenly turned round in an opposite direction; the effect of this, of course, would be that it could no longer be strong enough to bear the strain, and would at once be torn to pieces. So far Mr. Mallet only took up the work of others, for many experiments had previously been made which showed that heat had an effect upon the structure of iron and other metals, but he was probably the first to state broadly the law upon which these changes depend. In a very general way, it may be said that the direction which heat takes either in entering or leaving a crystalline body, such as iron, will be the direction which the fibres will ultimately assume when all the heat has left it; or, taking the illustration of the piece of cloth, if the heat followed the direction of its length, then, as in the case of the cloth, its greatest strength would be in that direction, and be capable of bearing a longitudinal strain, but if it crossed the cloth its greatest strength would be against a transverse strain, because its fibres would then lie in that direction. The fact of the cloth being a fibrous substance, and iron a crystalline one, renders this illustration not very suitable from a scientific point of view, but roughly it is sufficient to explain what really does happen.
The reader will now be better able to appreciate what amount of safety was likely to be obtained from a system of constructing artillery which ignored, because it was ignorant of, these important facts, and by which iron was treated almost as unwisely as if a rope were fastened along its side, and torn asunder by the strain, or as if a woollen or cotton manufacturer supplied his customers with felt instead of cloth.
But while the effect of heat upon iron and the manner in which it altered its crystalline structure was apparent, there remained the disturbing influence of pressure, which necessarily occurred in the process of manufacture. This is evident in many of the operations of the iron industry, and in none more so than that of rolling. If a plate of zinc about one inch in thickness, which has been rolled, and has its greatest strength in the direction of its length, as in the case of a piece of fibrous wrought iron, is placed upon a red-hot plate, the heat passing through the thickness of the plate will have the effect of reversing the planes of crystallisation and turning them through an angle of 90”. The effect of this upon the metal will be to render it so brittle that a small fraction of the strain it was previously able to resist will be sufficient to tear it asunder. If the plate, however, were again heated and subjected to the process of rolling, it would be found that the fibrous structure had been restored to it, and that the crystals had been again turned through a further angle of 90°.
Mr. Robert Mallet, generalising from these facts, found that all we know of heat and pressure leads to the conclusion that the effects produced by these two forces are really dependent upon the same cause – viz., that the crystals arrange themselves in the direction of least pressure, which in the case of the rolling-mill will be at right angles to the pressure or along the length of the bar, and in the case of heat entering or leaving a piece of iron in the direction in which the heat enters or leaves it most readily.
It must now have become clear to the reader that an explanation of this somewhat obscure subject is necessary to an understanding of the industry of big gun making. Just as a description of a woollen manufactory would not be complete unless some account were given of how the rough fleece, in which the fibres of the wool are lying in every direction, is carded and arranged so as to give the greatest amount of strength after it has been spun into threads, so in an account of the making of big guns an explanation is necessary of the means employed to arrange the fibres of the iron, and these are, as already stated, heat and pressure.
(*) “The necessity for increased range for siege-artillery,” writes Mr. Mallet, “was first rendered evident at the bombardment of the Fort of Matagorda, when bombarded by the French from across the Estuary at Cadiz – a range not before achieved. It was attained there by lead-loaded shells thrown from the howitzers now in St. James’s Park.”