74

the force of

CHAP. IV.

Of the Initial Velocity of Projectiles, andthe Law under which they are Impelledfrom Fire-arms of cylindric Bores.

159. T^HERE are three methods of determining theinitial velocity of projectiles: by deducing it from a know-ledge of the line they describe in their flight; by measuringit instrumentally near the muzzle of the gun; or by a thirdmode, which tnay be adopted whenever the thickness ofmetal is proportional throughout the length of the cylinderto the pressure of the fluid. The first method has beenalready treated on (Philos. Instit.) and there will be occasionto mention it again in the following chapter; the second andthird will be for the present the objects of consideration.

160. Benjamin Robins, an English Engineer, was thefirst person who thought of applying instruments to measurethe initial velocity of a ball of small calibre, near the mouthof the gun. His method consists in firing against a pendu-lum, moveable upon its axis of suspension, which describesan arch proportionate to the shock of the ball. The exactmeasure of the chord of the arch, the weight of the ball andpendulum, the centre of gravity and oscillation, and thedistance from the axis of motion to the point of impact, shewthe absolute velocity with which the ball impinged on thependulum.

Fig. 16. To apply this instrument C E for measur-ing the initial velocities of balls from mufquets or wall-pieces; it should be made of iron, and hung at rightangles to the axis of suspension A B, which must beabout a foot in length; to D E is screwed a plank ofabout nine inches square, and of such a thickness thatthe ball cannot penetrate to the iron. For the ex-periment, the pendulum is suspended (Fig. 17.) froma triangular frame; underneath it, is placed a piece ofwood cut in the arch of a circle, of which the circum-ference is described from the centre C with a radiusgreater than C E ; and in the plane of the vibrations of

the