AIR PRESSURE 



the tube when the air 

 is withdrawn. 



cistern of water, the water in the pipe does not rise above the 



level of the water in the cistern. If, how- 

 ever, the pressure in the tube is removed, 



the water in the tube will rise to a height 



of 32 feet approximately. If now the air 



pressure in the tube is restored, the water 



in the tube sinks again to the level of that 



in the cistern. The air pressing on the 



liquid in the cistern tends to push some 



liquid up the tube, but the air pressing on 



the water in the tube pushes downwards, 



and tends to keep the liquid from rising, 



and these two pressures balance each other. FJG ^^vvater rises in 



When, however, the pressure within the 



tube is reduced, the liquid rises because of 



the unbalanced pressure which acts on the water in the cistern. 

 The column of water which can be raised this way is ap- 

 proximately 32 feet, sometimes a trifle more, sometimes a 



trifle less. If water were twice as 

 heavy, just half as high a column 

 could be supported by the atmos- 

 phere. Mercury is about thirteen 

 times as heavy as water and, there- 

 fore, the column of mercury sup- 

 ported by the atmosphere is about 

 one thirteenth as high as the column 

 of water supported by the atmosphere. 

 This can easily be demonstrated. 



FIG. 43 . The air supports a Fill a glass tube about a yard long 



column of mercury 29 inches ^^ mercuryj dose the open end with 



a finger, and quickly insert the end of 



the inverted tube in a dish of mercury (Fig. 43). When the fin- 

 ger is removed, the mercury falls somewhat, leaving an empty 



