In the make-up of modern guns the breech-loading mechanism is of essential importance, it being necessary that the breech should be capable of rapid opening for the insertion of the charge into the loading chamber, as rapidly closed and firmly secured to prevent it being forced open by the reaction of the discharge. It also must fit with such tightness as to prevent any escape of the gas in that direction, and force it to exert all its impelling power upon the ball. Various methods are used for this purpose, with the result that loading and firing can be very quickly and effectively performed. In the case of guns in fortifications, the disappearing carriage is a highly important invention of recent date. By its aid the gun is quickly lifted to fire over the walls of the fort and is driven backward by the force of its discharge, sinking to a place of safety behind the walls. This saves the gun and its crew from injury by return fire. We may say in conclusion that the great European war was notable for the use of artillery to an extent far surpassing its employment in any previous war. This great conflict, indeed, was very largely a contest of gun fire, in which the opposing fields of the battling armies were so swept with shells and other explosives as to render life impossible on the open land, trench digging being one of the main employments of the embattled hosts. Never before had the supreme value of gunnery in warfare been so fully demonstrated. [Illustration: GEAR WHEEL AND DRUM FOR COAL HOISTING PLANE Diameter of wheel, 20 feet 9-1/2 inches; face, 43-1/2 inches; diameter of hub, 26 inches; number of teeth, 128; pitch, 6-1/8 inches; pitch diameter, 249.554 inches; shipping weight, 108,873 pounds. _Courtesy of the Bethlehem Steel Co._] [Illustration: SIX-INCH RIBBED CAVITY ARMOR-PIERCING SHELL Projectile was loaded with two pounds of black charcoal powder and fused with magazine fuse. Fired at six-inch Krupp hard-faced armor plate. Shell burst about eight feet to rear of plate after penetrating the same. Weight of largest fragment recovered 10-1/4 pounds. Average weight of fragments, 2-5/16 ounces. Total number of pieces recovered, 650. _Courtesy of the Bethlehem Steel Co._] AMMUNITION. (See page 402.) Made-up ammunition, with brass cartridge cases, and cast-iron and forged steel shells and armor-piercing projectiles. The rounds shown are as follows: Rounds with forged steel shell for one-pounder gun, for three-pounder gun and for six-pounder gun respectively; round with cast-iron shell for three-inch field gun; round with capped armor-piercing shell for three-inch fifty-caliber rapid-fire gun; round with forged steel shell for four-inch forty caliber rapid-fire gun; round with capped armor-piercing projectiles for the four-inch and twelve-centimeter fifty-caliber rapid-fire guns respectively, and round with forged shell for six-inch gun. TWO-HANDED ELEVATING GEAR. (See page 402.)