WATER PRESSURE 



119 



A 



necessary to fill the tube with the 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 pres- 

 sure 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 

 b'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 a'd. Evidently 



the siphon will cease to oper- F IG . 63. The siphon 



ate 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 atmos- 

 pheric pressure. This is about 33 feet in the case of water 

 and about 30 inches in the case of mercury. 



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

 common pump which stands on the 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. 64). The essential part of 

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



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 



