SCIENCE OF THRESHING. 



107 



main features do not otherwise differ. The essen- 

 tial parts consist of a barrel in which a plunger works, 

 an inlet valve to the barrel, and an outlet valve. 



The plunger must have an air tight packing around 

 it at the end of the barrel which it enters, to exclude 

 the air, so that when it is drawn out, the attempt of 

 the outer air to get in will force the water in through 

 the inlet valve. When the plunger is forced back in, 

 the water is pushed out through the outlet valve 

 which permits its escape but closes against its return. 

 The figure shows a pump having a piston, A, barrel, 

 B, inlet valve, C, and outlet valve, D. There is an 

 air chamber, E, opening into the pipe beyond the 

 outlet valve, so as to relieve anv undue sudden pres- 



