EVOLUTION OF THE MODERN HEAVY GUN. 151 



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 pow- 

 der -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 

 rapidly diminishing di- 

 ameter thence to the muz- 

 zle. 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 seven- 

 teenth century, now in THE TgAK 



the arsenal at MOSCOW, Caliber, thirty inches. Seventeenth century. 



with the United States 



fifteen-inch columbiad, 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 indi- 

 cated 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 



