PROJECTILES (from project. OF. projccter, ptojctcr, Fr. projekr. to project, from Lat. pro jcetarc, to thrust forth, frequentative of proji erre, to throw forward. from pro, before, for + jacere, to throw; connected with Gk. Itirretp, iaptein, to throw). Objects thrown forward by an impulse of short duration. Projectiles have been a form of offensive weapon from the earliest days of warfare when a stone o• similar missile was thrown from the hand or from some simple de vice. The development of guns (see ARTILLERY; CiUNS, NAVAL) necessitated suitable forms of projectiles. The earliest were of stone, some times merely bags of round pebbles. The larger stone projectiles were made to fit the gun loosely and were generally, though not always, spherical and often very neatly and smoothly cut. A few stone elongated projectiles are known to have been used, but they were not common. Iron pro jectiles came into general use in Europe in the fifteenth century, though stone was used more or less for some centuries after this. The (Infi rmity of tightly closing the breech caused the dis use of breech-loading cannon and prevented the early development of the heavy rifled gun. Rifled small arms (q.v.) using a spherical bullet have been in more or less use for three centuries. Smooth-bore guns almost invariably used spher ical projectiles very slightly smaller than the bore of the gun, the difference being termed the windage.
The resistance to the movement of a projectile through the air is proportional to the cross section perpendicular to the line of flight, and the power of the projectile to over come the resistance is proportional to its weight multiplied by the square of the velocity. For a given velocity, therefore, we may say that the ratio of power of resistance varies roughly as where W is the weight and D the diameter expressed in the usual units. This is not strictly correct, for there are other factors (shape, den site of atmosphere, etc.) which enter into a precise calculation, but the statement is approxi mately true and the expression is called the ballistic 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 the resistance of the air and thereby add to its range and accu racy. The elongated projectile evidently fills the desired condition, and the advantages of its use are at once apparent. Robin, published his treatise on Ordnance in 1742. and in it he stated very clearly the advantages of rifled guns and elongated projectiles (see Gras, NAVAL), but his work was far beyond the comprehension of his contemporaries. (See PROJECTILES, MOTION 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 deigned 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 shell: have formed the greater part of projectiles above a pound in weight.
About the middle of the nineteenth century 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 projectile's for muzzle loader: necessarily did not fit closely, and this decreased their accuracy. The revolution of an elongated projectile about its axis is to keep tie axis steady and prevent the projectile from 'tumbling' or pitching end over end, which would be fatal to power and accuracy. This was effected in muzzle-loaders by means of various rotating devices such as studs, ridges, expanding ,e rings or bands. or a bore of special cross section such as that of a polygon or ellipse.