an ingenious machine, invented by Benjamin Robins, for ascertaining the velocity of military projectiles, and consequently the force of fired gunpowder. It consists of a large block of wood, affixed to the end of a strong iron stem, having at the other end a cross steel axis, 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; in consequence of which the ball strikes, 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; so that by observing the extent of the vibration, or the chord of the arc—which is ascertained by means of a ribbon fixed to the lower extremity of the pendulum, and passed through a couple of steel edges something in the manner of a drawing pen,—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 which struck the blow and urged the pendulum.

See GUNNERY.

BALISTRARIA, cruciform apertures in the walls of a stronghold, through which the cross-bowmen discharged their bolts. It also signified a projecting turret, otherwise called a bartizan, such as is commonly seen in the castles of the Border.