Modern heavy guns are made of medium open-hearth carbon steel, forged as stated. The liner and tube are then placed upright in an assembling pit, the jacket and hoops shrunk on, and the finishing work done, as above said, the breech mechanism being finally fitted. Within recent years there has been a steady increase in the size and range of cannon, until an immense size and weight have been attained. For naval purposes the 14-inch gun is the largest now used in American battleships, but in the United States coast defense forts, 16-inch guns are installed. England has equipped several of her latest battleships with 15-inch guns and other nations are following in the same direction. In recent great battleships four turrets are used, each carrying three of these great guns, giving a broadside of twelve of these monster weapons of war. Of the three guns, the middle one is raised above the line of the others. A battleship thus armed is able to fire six guns ahead and six astern by raising the second and third turrets so as to fire over the others. [Illustration: FOUR-INCH FIFTY CALIBER RAPID-FIRE GUN ON PEDESTAL MOUNT Extraction of cartridge case by opening of breech mechanism. Weight of gun, 6,170 pounds. Length of gun, 205 inches (51.2 calibers). Weight of projectile, 33 pounds. Travel of projectile in bore, 165.6 inches (41.4 calibers). Weight of charge, 15 pounds of smokeless powder. Muzzle velocity, 2,900-foot seconds. Muzzle energy, 1,928-foot tons. Weight of mount with shield, 9,470 pounds. Thickness of shield, 2 inches of nickel steel. Gun equipped with telescopic and night sights and with electric and percussion pull-off firing gear. _Courtesy of the Bethlehem Steel Co._] Military cannon are divided into three classes, based upon the length of caliber, and technically known as guns, mortars and howitzers. In guns the length is relatively great, in mortars relatively small, compared to their calibers. Howitzers form a class between guns and mortars in length. The field guns of the American army are the 3.6-inch breech-loading mortars, and the 3.6-inch heavy and 3.2-inch light guns. The siege guns in the service are the 5-inch siege guns, the 7-inch howitzer, and the 7-inch mortar. The coast defense artillery consists of the 8-, 10-, 12- and 16-inch guns and the 12-inch mortars. In the recent European war very heavy cannon were used for field service, pieces of the size usually placed in forts being drawn to the field by powerful tractors, set on concrete platforms and used in attacks on fortified cities. It was through the use of such ordnance that the German army so easily reduced the strongly fortified Belgian cities. [Illustration: FLUID COMPRESSION PLANT