ON PNEUMATIC MACHINES. 351 



one third. When a 24 pounder is fired with two thirds of its weight of powder, it 

 may be thrown almost four miles, the resistance of the air reducing the 

 distance to about one fifth of that which it would describe in a vacuum. 



r 

 Bullets of all kinds are usually cast in separate moulds: shot are granu- 

 lated by allowing the lead, melted wilh a little arsenic, to pass through 

 perforations in the bottom of a vessel, and to drop in a shower into water. 

 The patent shot fall in this process through a height of 120 feet: the round- 

 est are separated by rolling them down an inclined plane slightly grooved, 

 those which are of an irregular form falling off at the sides. 



Condensed air may also be employed for propelling a bullet by means of 

 an air gun, an instrument of considerable antiquity, but of little utihty. It 

 is obvious that no human force can so far increase the density of air as to 

 make its elasticity at all comparable to that of the fluid evolved by 

 fired gunpowder, and even if it were reduced to such a state, its effects 

 would still be far inferior to those of gunpowder; for the utmost velocity, 

 with which it could expand itself, would not exceed 1300 feet in a second, 

 and it would, therefore, be incapable of imparting to a ball a velocity even 

 as great as this, while the vapour of gunpowder impels a heavy ball with a 

 velocity of more than 2000 feet in a second. When, however, it is consi- 

 dered that by far the greatest part of such a velocity as this is uselessly em- 

 ployed, and that the mechanical power which is practically obtained from 

 gunpowder is much more expensive than an equivalent exertion of any of the 

 ordinary sources of motion, it must be allowed that the force of condensed 

 air may possibly be applied in some cases, with advantage, as a substitute for 

 that of gunpowder. (Plate XXIV. Fig. 339.) 



