I08 SCIENCE OF THRESHING. 



sure in the pipe, while a check valve, F, beyond the 

 chamber prevents any back pressure from the boiler. 

 A very essential feature of the pump is an air relief 

 cock, G, by which the air ahead of the piston and in 

 the passages of the pump can be allowed to escape 

 when the pump is first started, the cock being kept 

 open until water passes out of it. 



When the piston, A, is forced ahead into the bar- 

 rel, E y it pushes the water in the barrel out through 

 the outlet valve, D. At each stroke of the pump, the 

 barrel is alternately filled and emptied, thus imparting 

 an intermittent motion to the column of water going 

 into the boiler. To partially overcome this starting 

 and stopping of the water so suddenly and frequently, 

 an air chamber, E, communicating with the feed pipe 

 is placed between the pump and the boiler. When 

 the water is suddenly forced out of the pump by the 

 plunger, a part of it enters the air chamber, com- 

 pressing the air; when the plunger reaches the end of 

 its stroke and ceases to force the water, the air in the 

 air chamber expands and causes the water to continue 

 to flow into the boiler after the valve, D, has closed, 

 thus lengthening the pulsations of the water and 

 acting as a spring or cushion for the water column. 

 Care should be taken that the air chamber is tight so 

 that the air cannot leak out and spoil the cushioning 

 of the water, as it is hard on valves, packing and 

 joints of the pump if the plunger is obliged to set the 

 whole column of water from the pump to the boiler 

 into motion as suddenly as it would do if the air 

 cushion were not there. 



