Hydraulics. 55 



able, and fills the bore of the pump. D is tli 

 rod (which is generally of iron) by which the 

 piston is raised. When, therefore, by drawing up 

 the rod B the piston or bucket is raised from B to 

 C, the valve and pisjton being perfectly or nearly 

 air-tight, it is obvious that a vacuum is created, 

 that is, there is a space from B to C, from which 

 the air is drawn out. This, however, is in some 

 measure supplied by the air from below, which 

 enters through the valve b, which it opens by its 

 force. It is evident, however, that this air must 

 be exceedingly dilated, by its now occupying so 

 much more space than it did before. The force or 

 spring of the air, within the pump, is so much 

 weakened, that it is not able to resist the pres- 

 sure of the external air upon the water. The ex- 

 ternal air, therefore, pressing upon tjbe surface of 

 the water, forces it to ascend through the notched 

 foot of the pump A, perhaps as high as e in the 

 body or bore of the pump. By another stroke of 

 the piston G, or by causing it to descend, the 

 upper valv,e a is again opened by the force or 

 spring of the air, and the valve below (b) is shut 

 by the same pressure. Thus by the descent of 

 the piston, all the air which was included be- 

 tween the box H and the space C, to which the 

 piston was before raised, will rise above the valve 

 a in the piston, and by drawing it up, the valve 

 a will again be shut, and a second vacuum 

 created as before, which again will be filled by 

 the air from below, ascending through the lower 



