HYD'IIAULICS. 187 



on one hand, and by the upper valve at S in the 

 branching-pipe, on the other, the water will rise 

 from the spring into A B, for the reason already 

 given ; and repeated strokes of the piston will 

 fetch up the fluid to the forcer, and fill the cavity 

 of the pipes between the fixed valves D and S. 

 The water in this manner raised, being hindered 

 from going down again by the lower valve, will be 

 pressed by the forcer every time it descends, and 

 be thereby obliged to make its way where there is 

 least resistance, viz. through the upper valve at S. 

 And whenever, on the rising of the forcer, this 

 pressure intermits, the valve at S will immediately 

 close under the weight of the upper water, and 

 prevents its return that way, while the piston is 

 rising with a fresh supply ; and this is repeated at 

 every stroke of the forcer. 



It is evident, that the operation of a pump 

 is by starts, and that the water in the main remains 

 at rest, pressing on the valve during the time that 

 the piston is withdrawn from the bottom of the 

 working-barrel. It is- in most cases desirable to 

 have this motion equable, and in some cases it is 

 absolutely necessary. Thus, in the engine for ex- 

 tinguishing fires, the spout of water, going by jerks, 

 could never be directed with a certain aim, and 

 half of the water would be lost by the way ; be- 

 cause a body at rest cannot in an instant be put in 

 rapid motion ; and the first portion of every jerk 

 of water would have but a small velocity. A very 

 ingenious., contrivance has been fallen upon, for 

 obviating this inconvenience, and procuring a 

 stream nearly equable. At any convenient part 

 of the rising-pipe beyond the valve S, there is 

 annexed a strong and capacious vessel, U, closed 

 -at top by a small pipe, T, fixed into it, which 



