MODE OF DETERMINING VELOCITY OF PROJECTILE. 183 



even two other balls are added. It is now generally ad- 

 mitted, in accordance with the experiments made by a com- 

 mittee of the Royal Society in 1742, that Robins was wrong 

 in his theory on this point ; and not only is it shown that 

 the combustion of all the grains in a charge of powder is 

 not simultaneous, but it is pretty well ascertained that 

 coarse-grained powder burns more slowly than fine, and for 

 that reason it is preferred for mining charges. Indeed, so 

 satisfied are practical miners of the importance of slow com- 

 bustion, that they mix sawdust with their powder, for the 

 purpose of producing it. In the old flint gun combustion 

 was too slow, and for that reason very fine powder answered 

 the best ; but in detonators a coarser grain is preferred, and 

 in needle-guns, or in those cartridges when the cap itself is 

 introduced into the charge of powder, a coarser grain still 

 is adopted. By the use of coarse powder, also, the pro- 

 jectile in front of it being more gradually moved, the recoil 

 is diminished, and thereby a larger quantity can be used 

 with comfort to the shooter and safety to his barrels. 



MODE OF DETERMINING THE VELOCITY OF THE PROJECTILE. 



In order to determine the velocity with which a ball moves 

 at any distance from the piece, a simple plan, now commonly 

 known as the ballistic pendulum, was 

 invented by Robins, and has never 

 yet been improved upon, though, from 

 the difficulty of hitting its centre, it 

 can only be used at short ranges. A 

 square plate of iron faced with wood 

 (Fig. 7, a) is suspended like a pen- 

 dulum from a tripod ; and to the 

 lower part of two of the legs of this 

 a cross bar (6) is attached. Then 

 fixing a piece of tape to the lower 

 edge of the pendulum, and letting it 

 slide through a notch in a brass plate 

 fixed upon the cross bar, the extent to 

 which the pendulum, when struck by 

 the ball, draws the tape, shows the 

 force of the blow, and the velocity 



