The elements of efficiency of the modern rapid-fire breech-loading rifle are to be found in the following features: First, in the increased length of the gun, which, for a 6-inch gun is now as much as 25 feet, the increased length lending a longer period of expansion for the explosion of the powder charge, and imparting a correspondingly higher momentum; secondly, in the fixed ammunition, which means a cartridge case in which a metallic shell encloses the powder charge, and is connected with the projectile, and third, in the great improvement and rapidity of action of the breech mechanism, which permits as many as eight rounds per minute to be fired. In Fig. 269 is shown a 6-inch rapid-fire gun of the United States Navy, loaded, and being sighted for fire. Rapid-fire guns of this class represent the most effective form of modern ordnance. It was largely such rapid fire batteries of Admiral Dewey’s squadron that swept the Spanish fleet out of existence at Manila, and that demolished the fleet of Cervera at Santiago by the awful hail of shells poured into his ships. These relatively small guns throw a shell six miles, and the striking energy of their projectiles at the muzzle is equal to the penetration of iron plate 21 inches thick, or 16 inches of steel. When the gun is loaded, it is held in the forward position by coil springs, inclosed in cylinders and holding a recoil seat for the trunnions, and also has two pistons traveling in cylinders filled with glycerine. When the gun is fired, the recoil causes it to slide back, carrying the pistons, and the recoil is checked by the resistance of the glycerine traveling through an opening past the pistons. After full recoil, the gun is automatically returned to its forward position by the action of the coil springs, which are compressed during the recoil. The gun crew is protected by Harveyized steel plate 4 inches thick, and the gun is so delicately mounted on ball bearings that its great weight of 7½ tons responds readily to the slight pressure in training the same. [Illustration: FIG. 270.--RANGE OF SIXTEEN-INCH GUN.]