PROJECTILES. 437 PROJECTILES. sitj of atmosphere, etc.) which enter into a precise calculation, but the statement is approxi- W mately true and the expression j^ is called the lallistic coefficient. It is apparent by inspection that if we have means of increasing" the weight without increasing the diameter (that is to say the area of cross-section) we shall increase the power of the projectile to overcome tlie resistance of the air and thereby add to its range and accu- racy. The elongated projectile evidently fills the desired conditions and the advantages of its use are at once apparent. Robins published his treatise on Ordnance in 1742, and in it he stated very clearly the advantages of rifled guns and elongated projectiles (see Guxs, Naval), but his work was far beyond the comprehension of his contemporaries. (See Projectiles, Motiox OF, below.) The use of smooth-bore guns and spherical projectiles continued, and the next im- portant improvement was the development of the shell gun. Incendiary shells designed to burst and scatter balls and fragments of the case had been in use for a long time, but up to 1820 these had very thin walls and were fired solely from mortars. In 1821 the Paixhans shell gun was de- signed in France, and thereafter explosive shells have formed the greater part of projectiles above a pound in weight. About the middle of the nineteenth centurs- the rifled gun established itself firmly. Difficulties in the breech mechanism caused some nations to develop muzzle-loading systems of construction, but this false step lasted nowhere more than twenty-five years. The projectiles for muzzle- loaders necessarily did not fit closely, and this decreased their accuracy. The revolution of an elongated projectile about its axis is necessary to keep t^ie axis steady and prevent the projectile from "tumbling" or pitching end over end, which would be fatal to power and accuracy. This was efl'ected in muzzle-loaders by means of various rotating devices such as studs, ridges, expanding base rings or bands, or a bore of special cross- section such as that of a polygon or ellip.se. STUDDED WHITWORTH BUTLER EUREKA MrZZLE-LOADISG PEOJECTILES. The difficulties connected with developing an effective breech mechanism were soon surmount- ed and modern projectiles quickly took on very nearly their present oblong shape. With breech- loaders the band could be made larger than the bore (being inserted from the rear) and forced in. the lands or grooves of the rifling cutting their way through the soft metal band. Projectiles for breech-loading rifles were first made of plain cast iron and wrought iron, but the advances in armor brought forth first the chilled east-iron armor-piercing shot and shell, and then the steel shell. For a long time common shells not in- tended to pierce armor were made of cast iron. and some countries still use cast-iron common shell: but in the United States Xa'y all shells are of forged steel. The common shells have hardened points and are intended to pierce armor two-thirds of a calibre in thickness, while the ABMOB-PIEHCING PBOJECTILE SHOKISa STEEL CAP. armor-piercing shells are designed to pass through any thickness through which they can be driven, not only without breaking up by the shock, but without deformation. Armor-piercing projectiles of a calibre exceeding six inches are not gen- erally loaded, as their walls are so strong that gimpowder charges will not burst them. Several high explosives have been successfully emploved for these shells, but none has been definitely adopted for shells of all calibres. (See Explo- .SIVE.S.) To further increase the penetrative power of armor-piercing shells, they are fitted with a soft steel cap over the point. This cap is cylindrical and about a quarter of the calibre of the shell in diameter, while its height on the outside is about equal to its diameter ; inside, the point of the shell extends well up into it. UTien the projectile strikes a hard-faced armor plate the force of the blow, acting through the cap, slightly dishes the face of the plate and thus strains the metal: while under this strain the point of the projectile passes through the cap and easily ruptures the strained material. In the United States and British navies shells are about 3ll> calibres in length, and their weight is approximately equal to one-half of the cube of the calibre (expressed in inches). In the United States Army guns, the weights of pro- jectiles are about one-sixth greater. In addition to the two kinds of shell mentioned, shrapnel are used in modern rifled guns, espe- cially in the modern field guns, and in some pieces canister is occasionally employed. Shrap- nel are shaped like shell, but have thinner walls and are filled with lead or iron bails from half an inch to an inch in diameter held in a resin Copper bar UNITED STATES ABMY 3.2- NCH SHBAPNEL. matrix or by metal rings. A small bursting charge, located either at the front or rear, breaks up the case in the air and the balls scatter and fly onward, spreading out like shot from a shotgun. In canister the balls, larger than those in shrapnel, are sunk in soft-wood disks piled up to form a cylinder, and the whole cov- ered with a tin case: or. in small calibres, the balls are simplv packed in sawdust and inclosed in a cylindrical tin case with wooden ends. Tn the days of smooth-bore guns many kinds of projectiles were used, as bar shot, chain shot.
