The Materials > Figures 1 – 14 > Mr. Mallet’s Discoveries
We will now go on to see how cast iron, in the light of Mr. Mallet’s discoveries, is an unsuitable material for artillery. First of all, its greatest strength is against a compressive strain, and it is therefore not suited for resisting the tensile strains that occur when a charge of gunpowder is exploded in the chamber of a gun. Secondly, the process of casting is not only liable to a great many crystalline changes, which depend upon the shape of the casting and the manner in which the heat leaves it (Figs. 6, 7, 8, 10), but it is incapable of being subjected to the pressure in certain directions, which has already been compared to the carding process in a woollen mill, and which is essential to obtaining the greatest strength in the direction of a tensile strain. The first person who has turned to practical account the important influence of pressure upon the strength of molten metal is Sir Joseph Whitworth, whose “fluid compressed steel” is the strongest material known, but being in itself of a malleable character, and so afterwards capable of being subjected to pressure, it cannot be looked upon as affording any hope that cast iron can be similarly improved in its subsequent treatment after the process of casting. The inventor of this process has proved, however, that the new material of “fluid compressed steel” is thoroughly suited to the construction of artillery. The principles of the process are exactly those which are laid down by Mr. Mallet as essential to the construction of reliable guns; and at some no distant date Sir Joseph Whitworth’s process may assume the position of a great industry; meantime we will confine ourselves to the making of guns as practised at Woolwich, based as it is on the labours of Mr. Mallet, and because it may be said to be the only scientific process conducted upon a large scale in this or any other country.
Such work as that which we have attempted to explain, although unrecognised by those persons whose position ought to enable them to accord an ungrudging acknowledgment to those to whom honour is due, could not long remain unappropriated in such a practical age as the one in which we live. Accordingly, we find that the principles laid down by Mr. Mallet were not long in abeyance, but soon were interpreted by an engineer who has proved himself to be one of the ablest masters of the art of applying practical means to a scientific end that has ever bestowed his services on this country. We refer to Mr. Fraser, of Woolwich Arsenal. Of the difficulties which lay in his way against having his plans carried out, the obstructions of nature formed a comparatively small proportion. Fortunately, however, for Mr. Fraser and for the country, an officer was at the head of the executive at Woolwich for a length of time that sufficed for his intelligent appreciation of Mr. Fraser’s views to be put upon its trial. The result has been the development of an industry at Woolwich which is distinguished not only for its consistency with scientific principles, but which is carried out in all its details in a manner that stands unrivalled by the efforts of private enterprise. In the next chapter we hope to give a description of the many ingenious devices which have been adopted in order to attain this result; but in order to understand to some extent the revolution that has been accomplished, it is necessary to consider the state of the industry even so recently as the time of the Crimean war. At that time Woolwich Arsenal presented the appearance of a factory, the extent of which was rivalled by hundreds of private establishments through the length and breadth of the country; now it has assumed the appearance of a town.
No doubt this advance has been rendered possible only by the improvements which have been made in the appliances necessary to the manipulation of iron upon a great scale; but in the department of big guns at Woolwich it has been above all things necessary to appreciate the nature and capabilities of these improvements, which of itself requires intelligence of the highest order. There is no one at all familiar with the great factory who is not aware of the almost instinctive readiness on the part of Mr. Fraser and his assistants to adopt and assimilate anything and everything that is good in the world of invention; and so it has come to be the case that the department of big guns at Woolwich may in all respects be looked upon as a model establishment, both with regard to efficiency and economy. Every process is conducted with an amount of precision that to the onlooker appears to give it an outward appearance of easiness, although he well knows that the acquisition of that ease is simply the perfection of mechanical and manipulative skill. Huge masses of white hot iron, which it would take a team of a hundred horses to drag along the ground, are removed from place to place with a care that might appear to be better bestowed upon the labours of the statuary, but which is essential to the crystalline stability, and to the ultimate strength of the whole structure. Every blow of the steam hammer, although applied with apparent indifference, has been studied in relation to the effects of its pressure upon the mass; and at certain stages of the work they are regarded as necessary evils, for which a substitute would at once be employed if the result of welding could be attained by other means.
Comparing the past with the present, it is by going back to the period of the Crimean war, which is still, a vivid memory among the middle-aged persons of our own day, that we find the rapid progress that has been made. At that time the labours of the forge had been almost entirely abandoned in the construction of artillery, and the art which produced Mons Meg and the other great guns referred to in the previous chapter had become well-nigh extinct. No trustworthy system had been substituted. Rule of thumb, in the choice of metal and in the manner of casting it, was the only safeguard against disastrous explosions, and when it failed – as too often happened – science, when appealed to, was found unable to give satisfactory replies. The, phenomenon of the drooping muzzle and many other effects of repeated firing, which are now explained by the effects of heat upon the material of the gun, were accounted for by reasoning upon data that have been proved to be fallacious.
Upon referring to Figs. 4 and 5, some idea may be formed of the defects of a system which the scientific investigations referred to in this chapter have rendered obsolete. These show sections of cast iron of a cruciform contour, in which the reader will notice the loose crystalline structure towards the centre of the castings. This is an inherent condition in all castings that have been “poured” in in the ordinary way, and under the old system of casting guns this species of defect occurred at the breech, a part that is subjected to the greatest strain. Referring to Figs. 14 and 9B, it will be seen that where the form was square the crystalline weakness extended all round the back of the breech, and rendered it liable to be blown out in one piece, ruder ordinary circumstances the spongy character of the breech was a constant source of trouble, but instead of its being put down to its proper cause, it was almost universally attributed to a chemical action which the gases of the exploded gunpowder exercise upon the metal. Again turning to Fig. 1 (we are indebted to Mr. Mallet’s exhaustive work upon the construction of artillery for our illustrations), which shows the lines of fracture in a burst cast-iron cannon, we find an example of what must, from the very fact of the fractures being where they are, be lines of least strength or greatest weakness. The invariable manner in which these fractures followed certain lines remained for a long time inexplicable, but in the light of the explanations we have attempted to give, they are easily interpreted. The planes of crystallisation under ordinary conditions of casting group themselves perpendicularly to the surfaces of external contour, and this law enables us not only to account for the appearance of the fractures as shown in Fig. 1, but also to predict the lines of greatest weakness in the mass before it has been subjected to an excessive or dangerous strain. The gun, rupturing first in the interior, bursts asunder, and in doing so folds over, as in Fig. 3, at the bottom of the dotted line, which forms a sort of hinge; or, as in Fig. 2, at the bottom and right-hand side. Not only, then, is cast iron unsuited for the purposes of artillery, from its chemical and molecular nature, but from the uncertainty which must always attend its crystalline structure during the process of cooling. These objections were of themselves sufficient to lead to the abandonment of cast metal of all sorts, whether iron or bronze; but it is almost unnecessary to add that the adoption of malleable iron as a substitute would have been attended with few, if any advantages, if its use had not been regulated so as to take advantage of the greatest strength of which it is capable. It has been attempted in the present chapter to explain the principles of crystalline adaptability, upon which the superiority of wrought iron chiefly depends; in the next we will describe how these principles are carried into practice at Woolwich.