184 HYDRAULICS. 



weight. Draw the bucket C up from D to C ; it 

 is evident, that as it fits the pipe exactly, a vacuum 

 will be formed in that part ; consequently the air 

 in the rest of the pipe under the piston will open 

 the fixed valve D, and fill the part which had been 

 exhausted. But it will be rendered rarer than 

 before ; consequently its spring, not being equiva- 

 lent to the pressure of the atmosphere upon the 

 water in which the pump is immersed, the water 

 will therefore be forced up into the suction-pipe 

 until the air within be as dense as before, and then 

 it will stop. Upon depressing the piston C a 

 second time, the same effect is repeated, until at 

 last the water lifts the valve D, and comes into the 

 barrel. When the piston now descends, it is forced 

 through the water, which cannot be pushed down 

 through the valve D, because it opens upwards 

 only. The water,tlierefore, gets above the piston 

 by passing through its valve C ; and when it is next 

 raised, all the water above it will be lifted up, and 

 will run off by the pipe E. 



Thus, by alternately raising and depressing the 

 piston, the effect is produced. Every time the 

 bucket is raised, the valve D rises, and the valve 

 C falls ; and at every time the bucket is depressed, 

 the valve D falls, and C rises ; and thus a stream 

 will be produced in some degree continual, but 

 very unequal. This is inconvenient in many cases; 

 in a pump for domestic uses, for instance, such an 

 irregular stream would make it troublesome to fill 

 a bucket. It is, therefore, usual to have at the 

 head of the pump a cistern, A, and to make the 

 spout small. By this means the water, brought up 

 by the successive strokes of the piston, rises to 

 such a height in the cistern, as to produce an efflux 

 by the spout, nearly equable. 



