MECHANICAL USES OF WATER AND AIR 129 



liquid. The liquid will flow through the tube if there is a 

 greater force acting in one direction than in the other. In 

 the experiment shown in the illustration the upward pressure 

 in the short arm of the tube is due to the pressure of the 

 air. In the tube at b this pressure is equal to the atmospheric 

 pressure minus the downward pressure due to the weight of 

 the column of liquid ab. The 

 upward pressure in the tube 

 at b' is the atmospheric pres- 

 sure minus the downward 

 pressure due to the column 

 I'd. The force tending to 

 drive the liquid from A to D 

 will therefore be greater than 

 that tending to drive it in the 

 opposite direction. It will be 

 greater by an amount equal 

 to the difference in weight 

 of the two columns of liquid, 

 corresponding to the height 

 old. Evidently the siphon will 

 cease to operate if d is at the 

 level of aa' or if the distance 

 ab is greater than the distance 

 the liquid will be raised by 

 the atmospheric pressure. This is about 33 feet in the case 

 of water and about 30 inches in the case of mercury. 



148. The lift pump. Everyone has seen examples of the 

 common pump which stands on a platform built over a well. 

 When the handle is operated, a stream of water is said to be 

 " lifted " from the well. The structure of a pump is best 

 learned from an examination of the pump itself or, if that is 

 not possible, from a diagram (fig. 66). The essential part of 

 the pump is the cylinder, which is hollow, usually two or 

 three inches in diameter and possibly eight or ten inches in 



FIG. (55. The siphon 



Water flows from A into D when d is be- 

 low the line aa', if the tube is first filled 

 with water. The water is lifted to the 

 level bb' by atmospheric pressure 



