CHAPTER XV 

 ATMOSPHERIC PRESSURE 



The downward pressure (or weight) of the air has already been 

 stated to be about 15 pounds to the square inch at sea-level. It 

 is convenient to have some simple method of measuring and 

 recording atmospheric pressures. The instrument by which the 

 pressure of the atmosphere is measured is the barometer. 



The barometer. The principle of the barometer is as follows: 

 A tube more than 30 inches long, closed at one end, is filled with 

 mercury. The open end of the tube is then placed in a dish 

 of mercury (Fig. 221). The mercury in the tube will sink until 

 its upper surface reaches a level about 30 inches above the level 

 of the mercury in the dish, if the place of the experiment is at 

 sea-level. The mercury remains at this level in the tube because 

 the pressure of the air on the mercury in the dish is enough to 

 balance the weight of the mercury in the tube. Since the pressure 

 of the air at sea-level holds the mercury in the tube up about 30 

 inches (or 760 millimeters), the pressure of the air at sea-level is 

 said to be 30 inches (or 760 millimeters). 



At elevations above sea-level the pressure is less, and the higher 

 the rise the less the pressure, as shown in the following table : 



Altitude above Barometric pressure 



sea-level, in feet. in inches. 



30 



1,800 28 



3,800 26 



5,900 24 



8,200 22 



10,600 20 



The decrease of pressure with increasing height being known, 

 the altitude of a place above sea-level may be measured by means 



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