CHAP. III.] 



PUMPS. 



51 



upwards through the piston. When the piston rises the valve which 

 it contains closes, the fluid above it is lifted up and a vacuum is pro- 

 duced in the barrel which is immediately filled by the fluid in the 

 pipe raising the lower valve and rising into the barrel. Theoretically, 

 the water ought to rise in the suction-pipe to a height of 10*33 

 metres when the barometric pressure is 760 millimetres; but, in 

 reality, the rise is much less, as the apparatus does not act with 

 the perfection which is necessary. There are escapes at the joints, 



FIG. 30. Suction-pump. 



FIG. 31. Suction and Force-pump. 



moreover, the water contains air in solution, in the form of bubbles, 

 which destroys the vacuum. The movement of the water itself, the 

 friction of the liquid against the sides and its disturbances, causes 

 losses of power, and the height to which it can be brought is very 

 often reduced to the 7 or 8 metres of which we have just spoken. 



If the depth of the well is greater, the suction-pump is not 

 sufficient ; its action is completed by an arrangement which forces 

 the water to a greater height, and thus conducts it from the point 



E 2 



