In 1854 Mr. Brunel took up warmly the question of improvement in large guns, which was then attracting the attention of several scientific men.
The friendship which had for some years previously existed between him and Mr. (now Sir William) Armstrong, of Newcastle-on-Tyne, gave Mr. Brunel opportunities for discussing these matters, with a view to their being carried into practical effect.
The following extracts from a letter written by Mr. Brunel, in April 1855, to Mr. James Nasmyth, explain generally his opinions at that time upon the construction of large guns:—
From what I have observed of the operation of fractures under sudden, quick-acting forces—such as bursting of guns, and fractures under blows, as in our railway smashes—I have arrived deliberately by observation at the conclusion, which every mechanical-minded man arrives at more or less by intuition, that homogeneity and equality of tension and of elasticity in the parts are necessary for strength to resist a violent strain applied suddenly in its full force, which I will call a blow. My impression, from the result of observation, is, that this operates much more than is generally assumed.
If you suppose a bar, say an axle of uniform section and uniform quality in every respect, it will bear bending into extraordinary forms even by a blow; and if you assume that portions of it become more tenacious and stronger, but remain equally elastic, the ultimate strength of this bar will not, I think, be materially increased or diminished; but if you suppose the elasticity of these portions either increased or diminished, I believe the ultimate strength of the bar under a blow is diminished. In like manner, I imagine that in the section of a gun barrel, if portions are more or less elastic than others, or at all different in their character, not only many points of fracture may be determined on, but that the whole may be rendered much less able to resist the violent explosion. The strain produced by the explosion and the plane of fracture is almost certain to be in a plane passing through the longitudinal axis, and therefore I had assumed that one would avoid as much as possible having any variation of quality which fagotting must produce to some extent in planes in this direction. To attain this end, I had endeavoured to scheme some way of welding up ‘cheeses’ or discs, which might be hammered up splendidly homogeneous of the full diameter and of a considerable thickness, and I wish that you would scheme the best way of welding them together. I should suppose that the centre surface might be welded, and wedges welded in all round, or some other mode devised, bearing in mind that the strain in any plane transverse to the axis is small, only that arising from the recoil of the breech and the friction of the shot.
I have also an impression that something harder than ordinary wrought iron is wanted for the inner surface to resist the explosion. This you might give probably in fagotting up. I am trying the effect—as much for the amusement of the thing as with any great expectation—of a cylinder of hardish material wrapped round with iron wire, laid on with a certain amount of tension proportioned to the diameter. Such a barrel ought to be strong—whether practically successful is another thing.
The scheme of making a gun with the barrel wrapped round with wire, which is referred to in this letter, was one which Mr. Brunel and Mr. Armstrong were very desirous of making the subject of actual experiment. Whether or not it would under their hands have become practically successful, could not be ascertained, as they were obliged to abandon the project, in consequence of the wire covering being patented, in May 1855, by Mr. Longridge.
The following letter to Mr. Armstrong relates to the same subject, and is interesting not only as showing Mr. Brunel’s correct appreciation of a principle which is the essence of the coil system of constructing guns, but as further illustrating his objections to the patent laws:—
June 8, 1855.
Have you ever done anything towards my experiment of the wire gun? I have been anxious for some time past to learn about it, but have waited to see you; to-day I learn that Longridge is taking out a patent for it. I daresay it is his own idea, and I only regret it, as I suppose it will now prevent my pursuing it; and I think it likely that with your assistance we should have succeeded in making at least as good a gun as he will. The principle I am disposed to think good; the success would depend upon the practical application, and but for these patents, the more competitors the better for the public. As it is, competition is destroyed. Let me know if you had done anything. Pray let me know also what you are doing about your own, in which I feel equally interested.