ATMOSPHERIC PRESSURE 



37 



those who use the barometer. The measure of atmos- 

 pheric pressure is commonly expressed by the height 

 (in inches) of the mercury column. 



47. Lifting Pump. Water, being much lighter than 

 mercury, can be held to a height of over thirty feet by 

 atmospheric pressure. Clearly, then, water may be raised 

 from a well thirty feet deep by the force of the air, if 







FIG. 22 



FIG. 23 



we can only arrange to cause a vacuum in the upper 

 end of the pipe. The work of a common pump, then, 

 is to cause a vacuum in a pipe and to allow the water 

 to run out above it. Figs. 22 and 23 will help to 

 explain how this is done. 



Fig. 22 shows the downstroke of a piston p moving in 

 a pump barrel. A valve v swings freely on a hinge. As 

 the piston moves down, air in the space s pushes the 

 valve open and escapes above it. If now the piston is 



