Page:The American Cyclopædia (1879) Volume III.djvu/720

 710 CANNON are afterward welded on, and the piece bored and turned to the proper shape. The 3-pdr. Parrott rifle gun, which is also employed in the FIG. 9. Parrott Rifle Gun. United States, is a cast-iron piece, reinforced by a band of wrought iron shrunk upon the first reenforce. Before the general introduction of rifle guns the Napoleon gun, taking its name from the late emperor of the French, was adopted by nearly all nations. It was designed to take the place of both light and heavy pieces, and thus simplify field artillery. Its exterior is characterized by the absence of mouldings and ornament; it has no chamber, and its weight is 1,200 Ibs., or 100 times the weight of the projectile. Mountain and prai- rie cannon are designed for use in regions des- titute of roads, and must therefore be made sufficiently light to be carried on pack animals. They are not very effective, and are only used when nothing heavier can be brought up in time. Rifle cannon are. of various sizes and pat- terns, and are constructed with spiral grooves, or rifles, cut into the surfaces of the bore ; the purpose of which is to communicate a rotary motion to oblong projectiles, without which they would turn over during their flight, and present a varying surface to the air. This rotary motion, if effectively communicated to the projectile, keeps its point constantly in the direction of its flight, and gives it increased range and steadiness, and also increased accu- racy. Great attention has been given to the method of rifling cannons with the object of se- curing the surest and safest means of causing the projectile to follow the grooves, as it passes along the bore of the piece, under the action of the powder. A great variety of methods have been devised, but they may be classed under the following heads : by means of flanges upon the projectile, or by giving it a peculiar shape by causing some part of it to expand, or by causing some part to be compressed. Solid flanges projecting from the projectile, and so shaped as to fit the rifling of the bore, were the first means used; but as these projections were of the same hard and unyielding substance as the body of the projectile, they frequently led to the bursting of the piece. Buttons of zinc, copper, or bronze were afterward adopted by nearly all European powers. Whitworth and Lancaster adopted the idea of making cannon with the bore cut in a spiral form, having a polygon or curve for its base, and giving the projectile a corresponding shape. This method secures the rifle motion with much certainty, and permits the use of bolt-like projectiles of great strength, but which bring heavy strains upon the cannon, accompanied by a correspond- ing tendency to deterioration of the piece. Projectiles which take the rifles by expansion are composed of cast iron, with a cup, band, or other arrangement of soft metal at the base, which is expanded and forced into the grooves by the explosion of the charge. Projectiles of this class are generally used in the United States service, because they are easily forced to the bottom of the bore, and do not clog the grooves or overstrain the piece. There are many varieties of this class, the most noted of which were invented by Parrott, Hotchkiss, Schenkle, and Dyer. Expanding projectiles cannot be fired with such heavy charges as some others, for fear of breaking, nor are they so sure to receive the rifle motion. Projectiles which take the rifles by compression are prin- cipally used in breech-loading cannon. They are generally covered with soft metal, so as to be slightly larger than the bore of the piece, but still not too large to enter the chamber easily. The explosion of the charge sets the projectile in motion, compressing the soft metal into the grooves, and compelling the projectile to follow the direction of the rifling. The form of a rifle groove is determined by the angle which a tangent to any point of it makes with the corresponding element of the bore. If the angles be equal at all points, the groove is said to be uniform ; if they increase from the breech to the muzzle, the grooves are called increas- ing; and if the reverse, decreasing. Grooves are cut by moving a rod armed with a cutter backward and forward in the bore, and giv- ing it a rotary motion at the same time. The width of a groove depends upon the diame- ter of the bore and the peculiar form of pro- jectile to be used. Rifled cannon for flanged projectiles generally have four grooves, while those for expanding projectiles generally have an odd number, from five upward. The pro- cesses used in manufacturing cast-iron or bronze cannon are moulding, casting, cooling, and fin- ishing. Moulding consists in forming a cav- ity of the proper shape in moulding sand, by using wooden or cast-iron patterns, slightly larger than the finished piece is to be. The pattern is divided into several pieces ; the first consists of the body of the piece, from the base ring to the chase ring ; the second, of the swell of the muzzle and the sprue or dead head ; the third, of the breech ; the fourth and fifth, of the trunnions. The sprue, or head, is an additional length given to the piece for the purpose of re- ceiving the scoria of the metal as it rises to the surface, and furnishing the extra metal to pro- vide for shrinkage, and extra weight to increase the density of the lower portions of the piece. The best material for the mould is dry, hard, refractory sand, which must be moistened with clay water to make it adhesive. The mould is formed in a casing of cast iron, called a box or flask. The pattern for the sprue and muzzle, previously coated with pulverized char- coal or coke, moistened with clay water to prevent adhesion, is placed upon the ground,