86 



AIR 



80. The Weight of the Air. We have seen that the pres- 

 sure of the atmosphere at any point is due to the weight of the 

 air column which stretches from that point far up into the 

 sky above. This weight varies slightly from time to time and 

 from place to place, but it is equal to about 15 pounds to the 

 square inch as shown by actual measurement. It comes to 

 us as a surprise sometimes that air actually has weight ; for 

 example, a mass of 12 cubic feet of air at average pressure 

 weighs i pound, and the air in a large assembly hall weighs 

 more than I ton. 



We are practically never conscious of this really enormous 

 pressure of the atmosphere, which is exerted over every inch 

 of our bodies, because the pressure is exerted equally over 

 the outside and the inside of our bodies ; the cells and tissues 

 of our bodies containing gases under atmospheric pressure. 

 If, however, the ringer is placed over the open end of a tube 

 and the air is sucked out of the tube by the mouth, the flesh 

 of the finger bulges into the tube because the pressure within 



the finger is no longer equalized 

 by the usual atmospheric pressure 

 (Fig. 47)- 



Aeronauts have never ascended 

 much higher than 7 miles ; at that 

 height the barometer stands at 7 

 inches instead of at 30 inches, and 

 the internal pressure in cells and 

 tissues is not balanced by an equal 

 external pressure. The unequal- 

 ized internal pressure forces the 

 blood to the surface of the body 



and causes rupture of blood vessels and other physical diffi- 

 culties. 



81. Use of the Barometer. Changes in air pressure are 



FIG. 47. The flesh bulges out. 



