In the manufacture of heavy guns the United States for some time led the world. In 1860, General Rodman, of the Ordnance Department, produced the first 15-inch gun ever made. This gun was made of cast-iron, and was cast on a hollow core, cooled by a stream of water passing through it, by which means the metal nearest the bore was made the hardest and most dense, and the tendency towards bursting was thus reduced to a minimum. General Rodman was also the inventor of the hollow cake powder, which consisted of cakes perforated with numerous small holes for the passage of the flame, thus enabling the powder to be progressively consumed, and causing the amount of gas at the last moments of the discharge to be greater than at the instant of ignition. A large-grain powder, known as “mammoth powder,” was afterwards devised by him to produce the same results. It will be seen later that this invention has rendered possible the powerful ordnance of the present day; and it is perhaps not too much to say, that Rodman is really thus the father of the modern high-power guns. At the beginning of the War of Secession the heaviest gun in the United States was the 15-inch Rodman, the projectile of which weighed 320 lbs., the charge of powder weighing 35 lbs. Next to this was the 10-inch Columbiad, which fired a 100-lb. shell with a charge of 18 lbs. of powder. The effective range of these guns was a little less than three miles. The heaviest mortar was of 13-inch caliber, fired a 200-lb. shell, with a charge of 20 lbs. of powder, and had a range of 4325 yards. This mortar was, like all others then in use, manipulated by means of handspikes, and not only was much less powerful, but was much more clumsy than the admirable mortar of the present day. [Illustration: OLD SMOOTH-BORE MORTAR.]