Under the heads of GCNNERT and RIFLE we have considered the points to be attended to In the formation of rifle anus and projectiles. These are equally applicable to ordnance. But when we come to the manufacture of very large guns, either smooth bore or rifled, we meet with a difficulty in making them sufficiently strong to resist the effect of the explosion of the charge. This difficulty arises as we increase the diameter of the bore much sooner in rifled pieces than in smooth bores, because in the former there is a greater resistance to the motion of the projectile in the bore, while at the same time the elongated pro jectile employed is of greater proportional weight. It has been how ever found very difficult to make solid shot guns—that is, when heavy charges are employed—of a greater diameter of bore than 8 inches (the 68-pounder); and in the .war with Russia it was found that the 13-inch mortars, even the immensely heavy sea service ones, constantly buret after heavy firing, as at Sweaborg.
This can be easily understood when we remember that, in guns of the ordinary construction, it is useless to increase the thickness of metal beyond a certain limit, and that to produce equal initial velo cities in two similar projectiles of different diameters—as, for instance, two spherical shot of 2 and 4 inches diameter—the pressure or strain on the metal of the gun increases in a ratio much higher than the ratio of the surfaces of the bores. To make the first point clear, if we take the transverse section of a gun, which is an annulus, we may consider it composed of a series of concentric rings, and the strain of the explosion a statical force, though it is of the nature of impact. That the metal of the gun is, as might be supposed, extended by the action of the powder at the moment of explosion (some portion of which extension, from want of elasticity, is permanent), may be proved experimentally ; that is to say, the diameter of the annulus or trans verse section is increased, but the area of the annulus must remain the same, and therefore the width of the annulus must diminish, or, which is the same thing, the circumference of the inner circle must increase in a greater ratio than the outer. Hence the greater the distance of any of the rings composing the annulus from the centre or axis of the gun, the less will it be stretched, and the less will be the strain on it, and the resistance to a pressure of any two rings will be inversely as the squares of their distances from the axis.
Now iron extends with a tensile strain, and the extension is more than proportionate to the strain; that is, the resistance to extension is only at first nearly proportionate to the extension. It is found, by
experiment, that for wood and wrought-iron the resistance to eaten aim in each fibre Is at first nearly proportional to the extension. Mr. IlealgkInson found that in cast-iron the resistance to extension was leas than a quantity proportional to the extension by a quantity nearly propectional to the square of the extension. If so represent resistance to extension, and e the corresponding extension, lessee—be; It is therefore evident that at a certain thickness of metal the cohesion cc tensile strength of the inner ring may be overcome, and the ring Wept before the outer ring receives any strain, And therefore it is useless to increase the thickness of metal beyond the point at which the force exerted on the surface of the bore is sufficient to rupture it before the strain acts to any extent on the exterior. With respect to the second point, taking two spherical bullets of 2 and 4 inches diameter, or kris yeritese. the pressure required to give them the same initial velocities will be as their weights. that ia,141 the cubes of their diameters, but the surfaors of the bores will only increase as the squares of the diameters, and therefore with larger calibres the strain., on the guns will be greatly increased.
In order to obtain the requisite strength in ruled ordnance (espe cially In these of a large calibre), where the strain is again increased by the resistance to motion offered by the rifling, it is necessary to adopt some means of equalling the strains through the metal. Different nays of effecting this have been susaseeted. The object of all of them Is to give a certain isithil tension to the outer rings—the exterior haring the greatest, and the tensions decreasing and resulting iu com m.:glen in the Interior. The rings can be shrink on to an interior cylinder er core, that is, put on hot and then cooled, or they may be put on by hydraulic pressure, or the core or tube may be bound round with wire, gradually increasing the strain of each layer. Captain IllakoPy; ILA., who appears to have investigated the subject fully, made a gun in this last manner, which stood some most severe trials at Shoe 'Airspeed, in 1856 and 1S57. The ancient built-up guns, such as Mons Meg, were often made with longitudinal bars, kept together with rings shrunk on. This process was evidently adopted on account of the imprfect stato of manufacturing power, as there could be no use in not having the interior a homogeneous cylinder. A huge mortar of 36 inches diameter was, however, mule in this way a few years ago by Mr. Mallet, and failed.