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LEFT ARTILLERY 286 ARTILLERY a spherical shot or shell weighing per- haps 10 per cent, of the weight of the elongated projectile of a modern gun of the same caliber. About 1850, General T. J. Rodman, of the United States Army, conceived the idea of increasing the strength of guns by casting them with a hollow core through which a stream of cold water was kept flowing. The effect of this was to jjroduce a varying tension in the successive layers of metal from the inside out, so that the walls of the gun acted as a whole in resisting the pressure of the powder when the gun was fired. This principle, of "varying initial tensions," is applied in modern guns by building up the walls in successive layers of steel hoops, shrunk, one upon another, over a central tube. Figure 1, Plate A. The tube is pierced throughout its length to form the bore, which is rifled by grooves running spir- ally from breech to muzzle. The projec- tile, which is cylindrical with a pointed head, carries at its base a ring of soft copper, which, when the gun is fired, is forced into the grooves of the rifling and sets the projectile spinning with great velocity as it is driven along the bore. It is this spinning that keeps the pro- jectile true in flight and makes possible the long ranges and great accuracy at- tained by modern artillery. The gun is fired by a primer through a vent in the breech plug. The practical development of the rifled gun of large caliber as an actual and important factor in warfare dates from about 1855, although the principle involved had long been familiar to ar- tillerists and had been applied experi- mentally as early as 1745. Built-up guns came into use at about the same time (1850-1860), and the combination of these two factors resulted in the de- velopment of the high-powered, built- up, rifled gun, which, in the last half of the 19th century, practically revolu- tionized artillery, especially naval artil- lery, and, in association with smokeless powder — perfected about the end of the century — may be held to have practi- cally revolutionized warfare. The first built-up guns were of wrought iron, but this was soon re- placed by steel, the manufacture of which was greatly improved through the demands of artillerists for a combina- tion of elastic and tensile strength never before considered possible. The leader in the manufacture of the new guns on a commercial scale was the English met- allurgist Sir William Armstrong. In Germany, Krupp used steel at an early date, but was behind Armstrong in adopting the built-up system. The first modern gun in the United States was built about 1880, but the manu- facture on a large scale was not under- taken until several years later, when the great steel plant at Bethlehem was established through the influence, and to meet the demands, of the navy. By 1917, when the United States entered the World War, the manufacture of ord- nance by Bethlehem and other establish- ments and especially by the Naval Gun Factory at Washington, had so far pro- gressed that more than three thousand guns of all calibers, with their mounts and ammunition, were turned out in little more than a year. As used on shipboard, guns of the larger calibers — from 10 to 16 inches — are mounted in turrets with elaborate mechanism for handling ammunition, loading, pointing, firing, and controlling the recoil. On shore, these calibers are mounted in fortifications, often on "dis- appearing" carriages by which the gun is held behind the shelter of a parapet until ready for firing, when it is lifted, fired, and automatically returned to shelter by the force of the recoil. The smaller calibers, as used by the army, are mounted on mobile carriages, drawn by men or horses or by automobile trac- tors, constituting what is technically known as "field" and "siege" artillery. Projectiles. — In the long struggle be- tween the penetrative power of projec- tiles and the resisting power of armor, the projectile appears to have estab- lished a permanent advantage, at least under proving-ground conditions, which are admittedly unfavorable to armor. The 16-inch guns with which the 1920 class of United States battleships are armed have power enough at fighting ranges to drive their 2,100-pound pro- jectiles through the thickest armor carried by any ship that has up to the present time been proposed. The Powder Charge. — Modern gun- powder, composed of gun-cotton and nitroglycerine, is as different from the powder of fifty years ago as modern guns are from the guns of the same period. Although commonly called "smokeless," it is far from being so, but it produces decidedly less smoke than the old "black" powder, and its force is many times as great. Its essential fea- ture, apart from its force, lies in its "progressiveness," by which is meant that, instead of burning all at once, with a violent explosion, it burns slowly and gives off its gases gradually, thus giv- ing the projectile a push rather than a blow — but a push that becomes more and more pronounced as the projectile moves toward the muzzle. In this way it gives a higher velocity to the projectile with much less strain on the gun.