AIR PRESSURE 



379 



FIG. 247. Water rises 

 in the tube when the 

 air is withdrawn. 



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 rises to a height of 34 feet approxi- 

 mately. 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. When, however, the pressure 



within the tube is removed, the liquid rises 



because of the unbalanced pressure which 



acts on the water in the cistern. 



If water were twice as heavy, just half 



as high a column would be supported by the atmosphere. 



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 atmos- 

 phere. This can easily be demon- 

 strated. Fill a glass tube about a 

 yard long with mercury, close the 

 open end with a finger, and quickly 

 insert the end of the inverted tube in 

 a dish of mercury (Fig. 248). When 

 The air supports a the finger is removed, the mercury 

 column of mercury 3 o inches falls somew hat, leaving an empty 



space, or vacuum, in the top of the 

 If we measure the column in the tube, we find its height 



FIG. 248. 



tube. 



is nearly 30 inches, or about one thirteenth of 34 feet, exactly 



