BALLISTIC Pendulum

, an ingenious machine invented by Benjamin Robins, for ascertaining the velocity of military projectiles, and consequently the force of fired gun-powder. It consists of a large block of wood, annexed to the end of a strong iron stem, having a cross steel axis at the other end, placed horizontally, about which the whole vibrates together like the pendulum of a clock. The machine being at rest, a piece of ordnance is pointed straight towards the wooden block, or ball of this pendulum, and then discharged: the consequence is this; the ball discharged from the gun strikes and enters the block, and causes the pendulum to vibrate more or less according to the velocity of the projectile, or the force of the blow; and by observing the extent of the vibration, the force of that blow becomes known, or the greatest velocity with which the block is moved out of its place, and consequently the velocity of the projectile itself which struck the blow and urged the pendulum.

The more minute and particular description may be seen in my Tracts, vol. 1, where are given all the rules for using it, and for computing the velocities, with a multitude of accurate experiments performed with cannon balls, by means of which the most useful and important conclufions have been deduced in military projectiles and the nature of physics. I have also since that publication, made many other experiments of the same kind, by discharging cannon balls at various distances from the block; from which have resulted the | discovery of a complete series of the resistances of the air to balls passing through it with all degrees of velocity, from o up to 2000 feet in a second of time.

Other writers on this subject are Euler, Antoni, Le Roy, Darcy, &c. See also Robins's Mathematical Tracts.