Pressure of the Atmosphere. 



559 



perature much higher than it could be with this gas absent ; 

 and Chamberlin has proposed the working hypothesis that 

 long period changes in the amount of carbon dioxide in 

 the atmosphere may be the cause of the recurrent glacial 

 periods to which the earth has been subjected. 



711. Pressure of the Atmosphere. The air, like all other 

 substances, has weight, and this weight causes it to exert 

 pressure proportional to the amount above a place. Its 

 mean pressure at sea level is equal to 14.73 pounds per 

 square inch. A cubic foot of air at this pressure and at a 

 temperature of 62 F. weighs about .08 pounds, 100 cubic 

 feet would weigh 8 pounds, and 10,000 cubic feet 807.28 

 pounds. The air of a stable 50x50 feet, 10 feet high, 

 weighs a ton. 



As the hight increases above sea level the amount of air 

 to exert pressure is less, the weight of a cubic foot becomes 

 less and it is necessary to breathe a larger volume to supply 

 the system with the same amount of oxygen. In the next 

 table are given in round numbers the bights above the sea 

 at which the pressure would fall from 30 to 16 inches and 

 the hight to which these pressures would sustain a column 

 of water, could a perfect vacuum be maintained. 



712. Applications of Atmospheric Pressure. The most 

 general application of atmospheric pressure by the animal 

 world is in bringing air into their respiratory organs. 

 Where animals are constituted so as to take advantage of 

 this, a reduction of pressure is made about the lungs, as in 

 raising the ribs and lowering the diaphragm, and then the 

 greater pressure of tho afr outside expands them and causes 

 a fresh supply to enter and fill the space. 



