322 PUMPS, AND THEIR VALUE TO MAN 



manner. It consists of a piston or plunger which moves back 1 

 and forth in an air-tight cylinder, and which contains an out- 

 ward-opening valve through which water and air can pass. 

 From the bottom of the cylinder a tube runs down into the well 

 or reservoir, and water from the well has access to the cylinder 

 through another outward-moving valve. In practice the tube 

 is known as the suction pipe, and its valve as the suction valve. 



In order to understand the action of a pump, we will sup- 

 pose that no water is in the pump, and we will pump until 

 a stream issues from the spout. The various stages are repre- 

 sented diagrammatically by Figure 194. In (i) the entire 

 pump is empty of water but full of air at atmospheric pressure 

 and both valves are closed. In (2) the plunger is being raisec 

 and is lifting the column of air that rests on it. The air anc 

 water in the inlet pipe, being thus partially relieved of down- 

 ward pressure, are pushed up by the atmospheric pressure on 

 the surface of the water in the well. When the piston moves 

 downward, as in (3), the valve in the pipe closes by its own 

 weight, and the air in the cylinder escapes through the valve 

 in the plunger. In (4) the piston is again rising, repeating thi 

 process of (2). In (5) the process of (3) is being repeated, but 

 water instead of air is escaping through the valve in the plunger 

 In (6) the process of (2) is being repeated, but the water has 

 reached the spout and is flowing out. 



After the pump is in position (6), motion of the plungei 

 is followed by a more or less regular discharge of water through 

 the spout. The quantity of water which gushes forth depends 

 upon the speed with which the piston is moved. A strong man 

 giving quick strokes can produce a large flow; a child, on the 

 other hand, is able to produce only a thin stream. Whoever 

 pumps must exert sufficient force to lift the water from the 

 surface of the well to the spout exit. For this reason the pump 

 has received the name of lifting pump. 



