Page:The New International Encyclopædia 1st ed. v. 02.djvu/495

BALLISTICS. mounted, with its axis vertical, upon a solid stone foundation, and having a projecting axial vent with no outside opening. The charge was fired by applying a red-hot ball provided with a hole,' into which the projecting vent-tube was inserted, there being no gas lost through the vent by this arrangement. The muzzle was closed by a stopper, upon which rested a weight used for confining the elastic fluid resulting from the combustion of the powder. The charges were increased a grain at a time from 1 grain to 18 grains, and for each charge a weight was found by experiment just sufficient to retain the gaseous product. From the mean of all the observed pressures, Rumford found the maximum pressure — that is, the pressure when the powder entirel.y filled the space into which it is fired — to be 29,178 atmospheres, about four and one-half times the maximum pressure found by Noble and .bel, in an en- tirely similar set of experiments, nearly a century later. The result found by Noble and Abel ((5554 atmospheres, or 45 tons per .square inch) is accepted by all writers on interior bal- listics as being very near the truth. Various other scientists did original work of more or less value to the development of interior ballis- tics prior to the year 1860. Consult the works of Gay-Lussac, Chcvruil, Graham, Cavalli, Wayevski, Otto, Neumann, etc.

Gen. T. .J. Rodman, of the United States Ordnance Department, made a series of im- portant experiments (1857-61) relating to the pressure and muzzle velocity, due to the varia- tion in the size of the powder grains and the densities of loading. General Rodman invented what he called the 'indenting apparatus,' which is now known and extensively used all over the world, under the name of "Rodman's Pressure Gauge.' He was also the first person to suggest the proper shape for powder grains in order to relieve the initial strain and to increase the volume of gas as the space behind the projectile became greater, thereby distributing the pres- sure more uniformly along the bore. His theo- retical charge, which he called a 'perforated cake cartridge,' was composed of disks of com- pressed powder from one to two inches thick and of a diameter to fit the bore. General Rodman built the best smooth-bore gun ever made, and experimented with this powder in this gun, fully confirming his theory; but for many reasons it was found to be far more convenient to build up a cartridge of pierced hexagonal prisms, about an inch in diameter, instead of having tliem the diameter of the bore. This prismatic powder, slightly modified, is now used by all military nations. There are two general methods for measuring the pressure — viz. the Statical, in which the pressure is placed in equilibrium with a known resistance, a method which is open to objection on account of the magnitude of the force to be measured, and the rapidity of its variation, but which is valuable for determining the maxi- mum pressure at the point where the measure- ment is made ; and the Kinetic, in which the pressure at any instant is deduced from the ac- celeration given to a known mass: the objec- tions to this method relating to the minute- ness of the times to be measured, and to the consequences of small errors in measuring the spaces.

The process of deformation, which is the present Htalic Method, consists in determining with a press the tarage (or the law connecting known pressures with observed penuanent de-' formations of similar metallic specimens), and comparing the deformation observed in a similar specimen, which has been exposed to the action of powder gases, with the results of these ex- periments. The specimen may be defonned by making a cut, which is General Rodman's plan; or by compressing a cj'linder between flat sur- faces, which is Captain Noble's plan; both methods being adaptable, either within or with- out the bore. Copper is generally used for the specimen to be deformed, the pressures being exerted through a freely moving piston. The internal gauge is tied to the bottom of the car- tridge, or recessed into the face of the breech- block, or into the face of the projectile. In using the Kinetic Method, we determine the rate of change of the pressure from the change in rate of motion of some body (the projectile, the gun, or an auxiliary projectile or piston), the mass of which is known. The Russian General Mayevski (1867) experimented with this method. He used a projectile, to the base of which was attached a rod passing through the breech of the gun and arranged so as to rupture electric currents placed at varying distances along the path of this rod. This ap- paratus was subject to many errors, but his experiments gave reasonably close approxima- tions to the results obtained by the statical method. The pressure may be advantageously determined from the acceleration of the gua and its "recoil, and a great many instruments have been devised for this purpose. General Rodman invented the first, which consisted of a cylinder, rotating with a known and uniform velocity about an axis parallel to that of the gun and close to it. A pointer fastened to the gun, which was hung as a pendulum oscillat- ing in the plane of fire, traced upon the cylin- der during the recoil a line the development of which gave the successive accelerations of the recoil. Colonel Sebert (French) replaced the rotary cylinder by a broad steel tape, smoked on one side, against which rested the tines of a tuning-fork set vibrating by the recoil. Colonel Bomford, of the United States Ordnance De- partment (1841), bored small holes through the walls of a cannon, at right angles to its axis, and placed in each hole a bullet, the velocity of which was measured, thereby furnishing the data for the determination of the pressure at the various points along the bore.

. In exterior ballistics, we determine the path of the projectile, knowing its shape, calibre, weight, its initial velocity of translation and rotation, the resistance it meets from the air, and the action of gravity. To this branch of the science the following defini- tions would apply:

The Trajectory is the curve described by a point on the axis of a projectile in its passage through the air. It is divided into ascending and descending branches, the highest point of flight being called the summit.

By Rigidity 'of Trajectory is meant the important principle, proven analytically, that the whole trajectory, with its chord, may be revolved a certain distance about a horizontal axis passing through the centre of the muzzle.