THE UNITED STATES WEATHER BUREAU 



509 



The pressure of the air is constantly changing. When 

 the pressure becomes greater, the mercury in the tube rises ; 

 and when the pressure becomes less the mer- 

 cury falls. Thus a change in the height of the 

 mercury shows a change in the pressure of the 

 air. This pressure is measured in terms of the 

 height of the mercury column in inches. Thirty 

 inches is the average pressure at sea level. 

 This means that the air exerts a pressure of 

 about fifteen pounds on every square inch on 

 which it rests, or a pressure of about a ton on 

 every square foot. The reason that buildings 

 and other objects are not crushed by this 

 weight is because the air pressure beneath and 

 within hollow objects is equal to the pressure 

 bearing down, and so the two balance. 



Use for measuring heights. As one goes up 

 a mountain or in a balloon, the amount of air 

 above one becomes less and so bears down on 

 the mercury of a barometer less heavily ; there- 

 fore the column of mer- 

 cury falls. Because of 

 this, the barometer can 

 be used to determine 

 heights of mountains 

 and other elevations. 

 The higher one goes, 

 the lower the mercury FIG. 202. A 

 drops. For the first 

 mile the mercury drops 

 one inch for every nine hundred 



FIG. 203.-Aneroid barometer. or thousand feet At a he i gh t of 



two miles the mercury drops to twenty inches. 



Aneroid barometer. As the mercurial barometer is awk- 

 ward to carry, another form called the aneroid barometer 



standard ba- 

 rometer. 



