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 but 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 them 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 Static Method, consists in determining with a press the tarage (or the law connecting known pressures with observed permanent 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 deformed by
making a cut, which is Genera] Rodman's plan; or by compressing a cylinder 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 Alayevski (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 he advantageously determined from the acceleration of the gun 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 Bomfo•d, 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.