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( 19 ) always assuming that no practical difficulties arose in the pouring of so ponderous a casting and none from contraction in the cooling, that is to say, assuming the 12-inch thick casting were as trustworthy as the 6-inch thick.

It can readily be understood from a consideration of the foregoing, how it may be possible to extend, under the strain of firing, the skin of the bore, to a point where rupture will begin, while the metal towards the exterior should be so little elongated that it afforded no sufficient aid to this skin.

It is with the view of curing these defects, that the system of putting an initial compressive strain on the inner metal, by means of an initial tensile strain on the outer metal, has been devised and adopted. By this system it is possible to much more justly distribute the strain, and to greatly reduce that on the immediate wall of the bore.

Attempts have been made to obtain this external initial compression, in large cast-iron guns, by casting them hollow, on a metal core, and by passing, as soon as the casting had been made, streams of water through the centre, with the object of compelling the outside when it cooled down, to set in a state of tension upon an already solidified centre. A considerable number of guns were made, in the United States, on this construction, but the results were not trustworthy, as many of them broke up spontaneously from the effect of the internal strains.

The attempt to obtain the desired result, by such treatment of the metal of a casting while cooling, has, for the present at all events, been given up, and the system of shrinking on hoops is pursued.

To fulfil the condition of obtaining the very best effect from the metal, the hoops should be very numerous and very thin; but in this, as in many cases, it is better to sacrifice a small percentage of effect, to obtain simplicity of manufacture, and thus in a gun built up of turned and bored hoops, it is found on the whole desirable, to make these of such dimensions, that from two to five thicknesses of hoops, depending on the size of the gun, are employed.