SOME CHARACTERISTICS OF THE AIR 



9 



the air above. It must be plain that if we ascend in a balloon, 

 we are leaving some of the air below us, and the pressure of 

 the air at our elevation will be less by a proportionate amount. 

 In the case of our bladder-glass experiment, if the experiment 

 were conducted at a place 1000 feet up in the air, as in a 

 balloon, the rubber would be pressed down by a force equal 

 to the weight of the air above it; but there would be 1000 feet 



FIG. 10. Air pressures at different elevations 



The illustration shows Mt. Rainier from 5500 feet to the summit. Figures are placed 

 on the illustration to show the elevations at different points and the air pressures 

 at these points. Note the decrease of air pressure with the increase of elevation 



less depth of air above it than there would be if it were at the 

 earth's surface, and the pressure would be correspondingly less. 

 The same thing would be true on a mountain 1000 feet high, 

 as has been noticed by those who have ascended mountains. 

 It is this fact which makes it possible to use the barometer to 

 measure altitudes (fig. 10). 



It is not possible to state any exact rule as to the pressure 

 which will be found at any given altitude, but at elevations 

 of less than a mile above sea level the pressure decreases by 



