In England the Armstrong gun was introduced about 1855, and was covered by British patents No. 401, of 1857; No. 2,564, of 1858; No. 611, of 1859, and No. 743, of 1861. This originally consisted of an internal tube of wrought iron or gun metal, with cylindrical casings of wrought iron shrunk on. It was afterwards improved in what was known as the Fraser gun. In Germany the operations of Krupp as a gun maker began to be notable about this period. In the United States, Colonel Rodman devised a means of casting guns of large calibre, by having a hollow core through which water was circulated to rapidly cool and harden the metal in the vicinity of the bore, and to relieve the unequal strain in cooling. He obtained patent No. 5,236, August 14, 1847, for the same. The Dahlgren gun was patented August 6, 1861, Nos. 32,983, 32,984, and 32,985, by Admiral Dahlgren, U. S. N. The improvement covered the adjustment of the thickness of the metal at the breech of the gun to the varying pressure strains along the bore. These guns were distinguishable by the smooth bulbous breech of great thickness and curvilinear contour. The Parrott gun, patented October 1, 1861, No. 33,401, and May 6, 1862, No. 35,171, comprehended a form of hooped ordnance in which the breech was re-enforced by an encompassing hoop or sleeve, which was shrunk on. [Illustration: FIG. 267.--THE KRUPP BREECH MECHANISM.] _Breech-Loading Ordnance._--While the breech-loading cannon is several centuries old, and was, in fact, one of the first forms of that firearm, it is to this principle of action that the rapid fire and great execution of the modern weapon are chiefly due. The earliest of existing forms of breech mechanism is that which comprehends the channeling of the breech transversely to receive a tapered plug, which permits the charge to be placed in the open rear end of the gun in front of the channel, and the transverse plug then closed behind the charge. This is described in Hadley’s British patent No. 577, of 1741; was first patented in the United States in a modified form by Wright and Gould, No. 22,325, December 14, 1858, and afterwards came to be known as the Broadwell system, being developed by him and covered in patents No. 33,876, of December 10, 1861; No. 43,553, July 12, 1864, and No. 55,762, June 19, 1866. This general principle is still employed by Krupp in some of his guns, and as used by him is shown in Fig. 267. The transverse channel through the breech is tapered, and the sliding breech block X is slightly wedge-shaped to fit tightly therein. When the breech block is withdrawn for loading, as shown, a sleeve S, shown in dotted lines, is temporarily arranged in alignment with the bore and gives smooth passage way to the charge to a position in front of the breech block. This sleeve is then withdrawn, the breech block forced in, and is there locked by a turn of the threads of a locking screw _b_ into the corresponding recesses _a_ in the breech.