190 PUMPS AND THEIR VALUE TO MAN 



through a straw (Section 77). The atmosphere presses with 

 a force of 1 5 pounds upon every square inch of water in the 

 large vessel, and forces some of it into the space left vacant 

 by the retreating piston. The common pump works in a simi- 

 lar manner. It consists of a piston or plunger which moves 

 back and forth in an air-tight cylinder, and 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 dip-grammatically by Figure 133. In (i) the entire 

 pump is empty of water but full of air at atmospheric pres- 

 sure, and both valves are closed. In (2) the plunger is be- 

 ing raised and is lifting the column of air that rests on it. 

 The air and water in the inlet pipe, being thus partially re- 

 lieved of downward 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 the 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 condition (6), motion of the plunger 

 is followed by a more or less regular discharge of water 

 through the spout, and the quantity of water which gushes 



