In order that any advantage may be gained from an oblong projectile, it must move through the air in the direction of its length; numerous unsuccessful attempts have been made to ensure accuracy in its flight when fired from a smooth-bored piece. One of the sim plest plans for this purpose is to place the center of gravity, or inertia, in advance of the centre of figure, or resistance. As these points should be in the longer axis of the projectile, the force of inertia and the resistance of the air, acting along the same right line and in opposite directions, will tend to preserve the line of light. This was tried on a hollow, pointed projectile in the time of Louis XIV; the cavity was divided into two compartments; the front one was filled with leaden balls and powder, and the rear one with powder only. The flight of these projectiles was uncertain and irregular, some of them bursting in the air, and others striking the object sidewise. Another plan of this kind, proposed by Thiroux, is to make the projectile very long, with its rear portion of wood, and its point of lead or iron, somewhat after the manner of an arrow.
Attempts have also been made to give an elongated projectile a motion of rotation around its longer axis: (1) By cutting spiral grooves on the base for the action of the gas from the charge; (2) by forming such grooves on the forward part for the action of the air; (3) combining the preceding methods in the same projectile; (4) by causing the air to enter a cavity at the front end, pass through nearly the length of the cylinder, and escape by radical openings at the sides. None of these plans have succeeded in practice, for the reason, per haps, that the projectile naturally turns over end for end, and the charge and the air do not act with sufficient promptness, energy, and cer tainty to prevent it. An oblong projectile, thrown under a high angle and with a moderate velocity, can have rotation about its shortest axis arrested by attaching to its rear portion a light body by means of a chain or cord; the resistance which this body experiences from the air will cause the projectile to move point foremost. Projectil? with wide flanges or wings, operated by springs by which they were extended after the shot left the piece, have been tried, but without success.
Projectiles may be further classified ac cording' to their construction •and mode of operation, as solid, hollow and case shot. Solid projectiles produce their effect by im pact ; they are used in guns and in small-arms; those for guns are known as solid shot or shot, and those for small-arms as bullets. Such pro jectiles are required when great range, ac curacy and penetration are sought; they must, therefore, possess great strength and density, and be fired with large charges of powder.
Shells are hollow shot which act both by impact and explosion, for which purpose they contain an explosive and a fuse to ignite it at the proper time. As they have less strength, they are fired with smaller charges of powder than solid projectiles, and are used against animate objects and such inanimate ones as will not cause them to•break on striking. The thicker the sides of a shell, the greater its ability to resist the shock of discharge, and the greater the penetration and accuracy; on the other hand, a shell should be capable of con taining sufficient explosive or incendiary ma terial to accomplish the proposed end. The number of pieces resulting from an explosion varies with the brittleness of the metal, and is increased by giving to the interior the form of a regular polyhedron. A dodecahedral form
has been found advantageous. The most rapid and violent explosive practicable should be employed; the size of the fuse-hole should be as small as possible, and should diminish with the size of the cavity, to prevent the loss of too great an amount of gas. Case-shot act only by impact; they consist of a collection of small projectiles enclosed in a case or envelope. The envelope is intended to be broken, either in the piece by the shock of discharge, or at any point of its flight, by a charge of powder, inclosed within it; in either case, the contained projectiles continue to move on after the rup ture, but cover a large surface, and attain a greater number of objects. These projectiles can be used with effect only against animate objects situated at a short distance from the point of rupture; they are divided into grape, canister and shrapnel. Grenades are projectiles that are commonly thrown by hand, propelled by the rifle or otherwise. Larger projectiles of a similar type, designated as bombs, are pro pelled at low velocities by mortars or other forms of bomb throwers, or are rolled down the slopes of a work. They are designed to act only by the force of their own explosion. Hand grenades are thrown against troops in mass; for this purpose any spherical projectiles filled with powder only and weighing not over six pounds are suitable; these can be thrown from 20 to 30 yards; they are provided with a short fuse which is ignited with a match, or, in the act of throwing, by a special device. Projectiles have been designated especially for this service. Rampart grenades are intended to be rolled down a breach in its defense, or to be thrown over the rampart, etc. Spherical shells of any size will answer for this purpose; those unfit for firing may thus be utilized. Shells are fired from guns, from howitzers and from mortars. They are made of and their calibre is expressed in the same manner as solid shot or an equal diameter, the thick ness of metal in spherical shells is about one sixth of the diameter, and their weight, when empty, is generally about of that of the corresponding solid shot. In the United States service there are two kinds of spherical shells; one for guns and another for mortars. Each consists of the sides, the cavity, the fuse hole and the ears; and, in gun-shells, the rein force. The sides are thicker in gun-shells than in mortar shells of the same size, to withstand the high charge of powder with which they are fired. The fuse-hole is used for inserting the charge and to hold the fuse for communicating fire to it. All shells of eight inches or more in diameter have to receive the used in lifting the projectile to the muzzle of the piece in loading. The reinforce of metal about the fuse-hole of the gun-shell gives a greater bearing surface to the fuse and prevents it from being driven in by the force of the dis charge; this reinforce also serves, in a meas ure, to compensate for the metal taken out of the fuse-hole, and thereby render the shell more nearly concentric. In some services shells have in the upper hemisphere, a placed at an angle of 45° with the fuse-hole, through which the charge is poured immediately before the shell is used and after the fuse has been inserted. This is not necessary with the fuses used mostly in the United States service as the powder and fuse can be readily intro duced at the moment of loading.