ON RECOIL. 187 



proper centre of gravity of the pendulum. In common prac- 

 tice various contrivances, called eprouvettes, are employed. 

 One of these consists of a small pistol with the muzzle closed 

 by a valve, acted on by a spring. This valve is in con- 

 nexion with a ratchet-wheel, which marks the extent to 

 which it is opened by the explosion of the powder, but the 

 spring varying in strength, the test is imperfect. The ordi- 

 nary plan adopted by large sellers and purchasers is to load 

 a small mortar with a given weight of powder, and then a 

 ball, being accurately ground to fit it, in proportion to the 

 strength of the powder will be the distance to which the 

 ball is thrown. The eprouvette of the French Government 

 is a small mortar capable of containing three ounces of 

 powder, and this is expected to throw a copper ball of sixty 

 pounds weight 300 feet. Sometimes the amount of recoil is 

 made the test, and then a little mortar is suspended and 

 loaded with ball, when the extent of the arc of the circle 

 which it describes marks the strength of the powder. 



RESISTANCE TO THE MOTION OF PROJECTILES THROUGH 

 THE AIR, 



In a state of rest the air offers considerable resistance to 

 the passage of balls and shot through it, and when moving 

 in a direction different from that of the course of either, it is 

 still more perceptible. It is calculated that this resistance 

 increases nearly in a duplicate proportion to the velocity of 

 the resisted body; that is, it is four times as much when 

 the resisted body moves with twice the velocity; nine times 

 as much when it moves with three times the velocity, and 

 so on. In addition to this resistance there is also the force 

 of gravity constantly tending to bring the body towards the 

 centre of the earth. 



ON RECOIL. 



Tlie expansion of gunpowder wJien converted into gas being 

 in all directions, it follows that at the moment of firing a 

 blow is given to the breech end of the gun equal to that on 

 the projectile which is to be moved. In proportion to the 

 difficulty of moving the charge up the barrel will this force 



