In the Gun Factory Departments the Armstrong guns themselves are represented by several mounted, and unmounted pieces, large and small, and by the well-known gun trophy, at the centre of the class of military engineering. In the latter the Armstrong gun is shown in all its parts, and in all stages, from the black forgings, up to the completed gun; and several forms of the beautiful rifling bars to which we have already alluded in describing Mr. Anderson’s rifling machine, of which they may be considered parts. There are also shown in the interior of a single wrought-iron tube, four different tours de force, demonstrating what may be done if called for, in the way of great gun rifling, and showing that in fact, any form of groove, lande or rib, uniform, increasing, or decreasing in spiral, taper in width or depth, or even zig-zag (which, we take it, will not be called for), may be produced at will. Here also are shown many of the steel gauges, and other measuring instruments of great precision, employed in the operations of mutual adaptation and adjustment of these built up guns. The value of precision of artillery itself, as a completed tool generally for very rough use, may be greatly overrated; but the precision of operation in its manufacture, as here shown, can never be so. In fact, without this precision carried as far as human skill and senses can carry it, the production as a manufacture, of any built up guns, is simply impossible; and it is owing in part to the want of any such systematized and disciplined guagings and measurings, that so few built up or even hooped guns, made as occasional productions by private firms, are worth anything, or to be in the least degree trusted beforehand.
In giving the briefest possible sketch of the mode of manufacture of the Armstrong guns, we must assume our readers acquainted with the principles upon which a vast accession of strength against bursting, over and above that which is due to the mere accumulation of matter to the same extent in one solid thickness, is obtained by building up a gun, of whatever material, in superimposed rings or hollow cylinders, each grasping those within, with an initial or constant elastic tension.
Those who are not acquainted with these principles, we must refer to our own work “On the Physical Conditions involved in the Construction of Artillery,” 4to, Longman, 1856, where the method was first publicly promulgated in English, and the mathematical theory of its laws given by Dr. Hart, their first discoverer.
The iron employed for the Armstrong guns is of a carefully selected quality, combining strength, hardness, toughness, fibre, and the capability of being welded, in certain proportions, which experience has proved to be best. It is, in fact, the nearest approach to steel that can be obtained without sacrificing fibre, toughness, and the capability of being soundly welded without blistering in the fire. The usual specification to makers for it prescribes “a tenacity (ultimate) of about 26 tons per square inch, not over 27 nor under 25 tons; elongation not to become permanent under 13 tons tension per square inch, nor compression to become permanent under 14 to 15 tons pressure on like surface.”
Whenever welding, which obliges the keeping down the steely character of the iron, shall be totally abolished in built up guns, the standard of tenacity and of hardness may be still further raised; the higher both are, the better for rifled guns, indeed for any gun.