It was found that the inner surface of the tube stretched more than the outer surface, and that after the inner surface had been stretched to its limit of elasticity the outer part failed to add to its strength, so that further thickness was of no benefit. To do away with this condition cannon were constructed on the principle of varying elasticity, the metal with the greatest elongation within its elastic limit being placed next to the bore, yet in high-powered guns this system failed to yield the result desired and it was replaced by what is known as the initial tension system. This comprised two methods: the plain built-up gun and the wire-wound gun. In the latter certain parts of the gun were wrapped with wire in the form of a ribbon. [Illustration: RAPID-FIRE GUN Six-inch rapid-fire gun equipped with patented two-handed elevating gear, consisting of two hand wheels on opposite ends of the same shaft, the handles being 180 degrees apart. The pointer uses both hands in elevating and depressing the gun. The electric firing trigger _A_ is worked by the index finger of the right hand without releasing the handle. There is a second firing handle _B_ attached to the slide, for firing either electrically or percussively. _Courtesy of the Bethlehem Steel Co._] Built-Up and Wire-Wound Guns. A built-up gun is made of several layers of forged steel. The parts of such a gun are known as the liner, the tube, the jacket and the hoops. The liner is a single piece which extends the length of the bore and is intended to contain the rifling and the powder chamber. This is inclosed by the tube, which is also in one piece, surrounding the liner throughout its length. Outside this is the jacket, made in two pieces and shrunk on the tube. Over the jacket lie the hoops, six or seven of these being used in a big gun. Like the jacket, these also are shrunk on. All these parts are made of the finest quality of open-hearth steel. [Illustration: FIVE-INCH NAVAL GUN AND MOUNT The latest type of gun used in the U. S. Naval Service in the secondary batteries on a battleship.] [Illustration: COAST DEFENSE GUN A modern 14-inch coast defense gun at Sandy Hook. The gun is mounted on a disappearing carriage, which lowers it out of sight behind the breastworks after firing. This is one of the most powerful guns in the world, firing a projectile which would pierce the armor of a battleship more than five miles away.] [Illustration: _Copyright by Underwood & Underwood, N. Y._ A BATTERY OF 12-INCH COAST DEFENSE MORTARS These powerful weapons fire a projectile which weighs from 700 to 1,046 pounds, depending on the range desired, and which is capable of piercing the deck armor of any battleship. They have a range of 20,000 yards with the 700-pound projectile. The gun is 16-2/3 feet long and is fired only at elevations between 45° and 65°.] [Illustration: GIANT GUNS--THEIR MUZZLE-ENERGY, PROJECTILES, AND PENETRATING POWERS