THE SUCTION-PUMP 293 



lever with unequal arms, S, usually called the pump- 

 handle. The lower part of the pipe is formed by a 

 somewhat narrower tube, R, called the suction-pipe. To 

 the top of this pipe a clack-valve v l , usually called the 

 suction-valve, is attached, represented in the figure as 

 being opened. Another valve, v$ (the pressure-valve), 

 is placed within the piston, which has an aperture for 

 its reception, while the rod is forked at the end so as 

 to afford space for the valve to open. (The relative 

 proportions of the various parts of the figure are not 

 quite correct). 



When the piston is raised, the air which fills the space 

 below the piston will expand, and consequently, by 

 Mariotte's law, its pressure will decrease, so as to 

 become less than the atmospheric pressure. No 

 air can enter this space from above, because the valve 

 v> 2 opens only upwards: but air enters through the 

 valve Vi from the suction-pipe, the air in which 

 therefore also expands and has less pressure than 

 the external air which presses upon the surface of 

 the water in which the suction-pipe is immersed. 

 The external pressure thus forces a column of water 

 into the suction-pipe. When the piston descends, 

 the air below it is compressed, but since the valve v l is 

 low closed, no air can enter the suction-pipe, and when 

 lie pressure of the air enclosed in the space between 

 lie two valves becomes greater than that of the exter- 

 lal air, the valve v 2 is opened by the excess of pressure, 

 md the air escapes. At each stroke of the piston the 

 lescribed effect is evidently repeated ; a portion of the 

 ir below the piston is removed, the excess of the ex- 

 ernal over the internal pressure of the air increases, 

 nd the column of water raised by this excess becomes 



