180' HYDRAULIC.-,. 



must be under the surface of the water in the well. 

 When the piston is pushed down, the water below 

 it not being able to go downwards, on account of 

 the valve a, raises the valve of the piston, and gets 

 above it ; and when the piston is drawn up, it lifts 

 all the water above it ; while the pressure of the 

 atmosphere causes more water to supply its place, 

 by lifting the valve a. When the piston is moved 

 down again, the same thing is repeated, and more 

 water gets above the piston. In this manner, by 

 successive motions of the piston, the water is at 

 last got to the top, and discharged into the head, 

 from whence it runs out by the spout D. In this 

 pump there is always a column of water lifted, 

 whose base is equal to the top of the piston, and 

 whose height is equal to the distance from the 

 piston to the head. It is evident, that this weight 

 will not be made less by diminishing the diameter 

 of the barrel above the piston; because fluids press 

 in proportion to their bases and perpendicular 

 altitudes. 



This pump is much used in great water-works; 

 it is the simplest of all in its operation. 



The forcing-pump, represented in Fig. 3., con- 

 sists of a barrel, A B, and a piston, or forcer, C. 

 There are also two fixed valves in this kind of 

 pump ; one in some convenient part of the suck- 

 ing-pipe, as at D, the other in the branching or 

 forcing-pipe, as S. These ought, in like manner, 

 to be air-tight, and so disposed, -as to let the water 

 freely rise, but absolutely to hinder its return. 



When the forcer is first moved upwards in the 

 barrel, the air between that and the water below, 

 having room to dilate, by its natural spring, will of 

 course be rarefied; the pressure of the atmosphere 

 being intercepted by the force of the barrel A B, 



