Before the adoption of smokeless powder, the cake powder invented by General Rodman had been highly developed and improved in the form of “cocoa powder.” This was made in hexagonal prisms, each perforated longitudinally, so as to have a hollow core. These grains were carefully arranged in the cartridges so as to have this core continuous from one grain to another, in order that upon ignition the combustion would begin in the interior and produce a constantly increasing volume of gas as the exterior surface of the grain was reached. Though the time of combustion was too rapid to be appreciated by the ordinary senses, it was, nevertheless, quite different from the practically instantaneous combustion of the old small-grain powder, and was susceptible of accurate measurement. Much difficulty was experienced in overcoming the detonating tendencies of the smokeless powders, but at last the requisite slow-burning properties were obtained. The smokeless powder for large guns is made in cartridges composed of bundles of strips or cords, or in the same prismatic form as the cocoa powder, and the process of combustion is the same. [Illustration: MORTAR ON REVOLVING HOIST.] The form of the gun is dependent entirely upon the nature of the powder used. As the pressure of the gas constantly increases with the burning of the powder, the maximum force will be reached at the moment the combustion is complete. The length of the bore should, therefore, be just sufficient to enable the powder to be entirely consumed at the exact instant the projectile leaves the muzzle of the piece. A shorter bore would cause much of the powder to be thrown out unconsumed, while a much greater length would retard the projectile by subjecting it to the friction of the bore after the maximum force of the powder had been reached. This accounts for the greatly increased length of the modern cannon. A change in the method of gun construction has accordingly become necessary. Guns are no longer made of cast iron, but are “built up” of steel. The explosion of the powder is, of course, exerted in every direction, against the bore and sides of the piece as well as against the base of the projectile. This produces two strains; a longitudinal strain which is exerted in the direction of the axis of the piece, and a transverse strain which tends to burst the gun. It is necessary, therefore, to have the piece so strong, especially at the points of first explosion, as to counteract these strains, and thus cause the entire force to be exerted upon the projectile in the direction of the “least resistance.” This strength, or “initial tension,” is obtained by shrinking cylinders of steel over the original cylinder of the piece, each outer cylinder or jacket being a few thousandths of an inch smaller in its interior diameter than the outer diameter of the cylinder which it incloses, and being expanded by heating to a sufficient degree to enable it to be slipped over the latter.