and the gun becomes dangerously weak after but little use. Nevertheless, this method of construction did not begin to receive modification of any great importance until about fifty years ago. In 1846 these smooth-bore, cast-iron columbiads varied in caliber from eight inches to twenty inches, and in weight from four tons to fifty-seven tons. The projectiles were spherical iron balls, from sixty-eight to one thousand pounds in weight, the charge of powder never exceeding one sixth of the weight of the ball.
Between 1850 and 1860 Major Rodman, of the United States Army, conducted an epoch-making series of experiments on the improvement of gunpowder and the method of casting iron guns. Dahlgren, about the same time, modified the form of gun, giving it great thickness at the breach and as far as the trunnions, with 
The Tsar Cannon at Moscow.
Caliber, thirty inches. Seventeenth century. rapidly diminishing diameter thence to the muzzle. This form has often been compared to that of a champagne bottle. The contrast between this and the older forms is well shown by comparing the "Tsar cannon," a thirty-inch gun of the seventeenth century, now in the arsenal at Moscow, with the United States fifteen-inch Columbiads, as improved by Dahlgren. Accepting the proportions thus established, Rodman devised the method of "hollow casting" and cooling from the interior. The melted iron is poured into a vertical mold, the axis of which is occupied by a hollow core. Through a pipe in this cold water is conveyed to the bottom and conducted away at the top after being warmed by the surrounding hot metal. The hardening of this begins thus at the inner surface where the greater strength is needed. The exterior surface of the mold is at first strongly heated from without and this heat gradually diminished, while the flow of water is continued many hours or even days. The cast iron thus goes through a process much like the tempering and annealing of steel. As the metal gradually cools the inner surface becomes strongly compressed, and the outer surface is left in a state of tension. The condition is the exact reverse of that brought about by the older process of solid casting and subsequent boring. The great improvement in strength secured by this process is indicated by Rodman's testing of two columbiads of the same size, material, and form, made at the same time, the one by hollow casting, the other by solid casting. The solid-cast gun burst at
