BALLISTIC :HACIIINES. There are two ways in which the velocity of a projectile may be determined experimentally—by measuring the projectile's momentum, and by actually measur ing the time required for the projectile to pass over a given space. The first method is the older; and many years ago, when guns and pro jectiles were small and velocities were low, the results were sufficiently accurate for practical purposes. Modern gunnery is much indebted to the now discarded ballistic pendulum and glen pendu-lum, which have long since been sup planted by the cheaper and more accurate ma chines using the second method.
The Ballistic Pendulum was invented about 1740, by Benjamin Robins, who was the first to make a systematic and intelligent series of ex periments to determine the velocity of pro jectiles. The whir/big-machine, for determining the resistance of the air to slowly moving bodies of different forms, is also attributed to Robins. The principle of the ballistic pendulum, as well as of Count Rumfo•d's gun-pendulnm, is the transformation of the elements of the momen tum of the small mass and high velocity of the projectile to the more easily measured large mass and low velocity of the pendulum. The original ballistic pendulum of Robins consisted of a block of wood bolted to a plate of iron, the whole revolving freely about a horizontal axis, the block, when struck by a bullet, recoiling through a certain arc, which was measured by the length of ribbon wound off the reel. This light pendulum could stand the impact of mus ket-balls only, but with it Robins began the science of gunnery by determining the relations which should exist between the calibre, length of barrel, and charge of powder. Dr. Hutton (1775-91), Dr. Gregory (1814), and Major (later Colonel) Mordecai, of the United States Ordnance Department (1842-45), made many experiments with improved ballistic pendulums.
Major Mo•decai used, also, the gun-pendu lum, his apparatus being briefly as follows: A ballistic pendulum weighing 9358 pounds, with a cast-iron block made in the shape of a hollow frustrum of a cone, with hemispherical bottom and filled with sand-bags, to receive the pro jectiles, which were from 6 to 32 pounds in weight. By very delicate bearings and means of mounting, this pendulum, if set. in motion through an are of 12 degrees, would come to rest only after 30 hours. The recoil was meas ured by a sliding pointer on a brass line. In front of this pendulum was constructed a screen of 2-inch plank, designed to intercept the blast from the discharge, a 12-inch circular aperture being left for the passage of the pro jectile. The gun itself was mounted as a pen
dulum, weighing 10.500 pounds, by suspending it from a horizontal axis by means of rods. The remarkable closeness of the results from these two pendulums furnished a conclusive proof of the theoretical correctness of the method em ployed.
In using the second method, a great variety of ballistic machines have been invented for measuring the very short interval of time re quired for the projectile to pass over the short space between two given points of its trajectory. In 1767, an Italian instrument-maker, named Alattei, invented a very simple machine for this purpose, consisting of a large paper cylinder revolving around a vertical axis. The principle was this: If a musket-ball be tired horizontally through the axis of the cylinder at rest, the two bullet-holes will be 180 degrees apart on the surface of the cylinder; if, however, the cylinder be revolving at a known rate, the two bullet-holes will be more, or less, than 180 de grees apart, depending on the direction in which the cylinder is revolving, the difference measur ing the distance a point on the surface of the cylinder has moved, while the projectile was traveling a. distance equal to the diameter of the cylinder. A simple arithmetical calcula tion gives the desired velocity. This machine was also extensively used, in improved forms, by subsequent investigators with good results. With it General d'Antoni determined the initial velocity of musket and rifle balls with consid erable accuracy; the French Colonel Grobert (1803) constructed a modified form of the ma chine, which was used by members of the French Institute deputized to experiment with the muzzle velocity of musket-balls; Dr. Gregory (1818) gave the machine a more rapid and regu lar motion by introducing coiled springs and cog-wheels. Colonel Dobooz, of the French Ar tillery, invented (1818) a machine for measur ing velocity depending upon the principle of the law of falling bodies. A movable screen sus pended by a cord which passed over two pulleys and down immediately in front of the muzzle, was the falling body; near the movable screen was a fixed screen; two marks were made on the screens at the same height from the ground. When the gun was fired and the cord was cut at the muzzle, the movable screen fell, and both screens were pierced by the bullet (almost) simultaneously. The space through which the screen had fallen when the bullet reached it was shown by the two bullet-holes relatively to the two marks on the screens.