148 ELEMENTARY SCIENCE 



fifteen pounds to the square inch. You have learned that 

 this pressure is used by man in the performance of cer- 

 tain kinds of work. You have learned that this atmos- 

 pheric pressure is less on mountain tops than it is at sea- 

 level, and that this lessening of the pressure is gradual, 

 being so definitely related to altitude that by means of a 

 pressure recorder (the aneroid barometer) we may esti- 

 mate the altitude wherever we are. 



Density of Atmosphere. This last fact about atmos- 

 phere indicates that the higher one goes the thinner it gets. 

 What you know about the kinetic theory (see page 125) 

 makes you realize that this thinning out of the atmos- 

 phere as you go up means that the molecules of the 

 gases which compose it get farther and farther apart; 

 the higher you go the fewer the molecules to the cubic 

 centimeter, until presently, as is well known from the 

 experiences of balloonists and mountain climbers, the air 

 becomes so rarefied that it is difficult to breathe. In 

 balloon ascents made for height, oxygen in metal con- 

 tainers is taken along for breathing purposes. 



Balloons without passengers have gone up as high as 

 eighteen miles, showing that even at that height the 

 atmosphere is sufficiently dense to support them. The 

 altitude record for balloons with passengers is between six 

 and seven miles. At that height, air for breathing may be 

 artificially supplied, but great discomfort results from the 

 intense cold. 



Extent of Atmosphere. Other and very important evi- 

 dence of the upward extent of the atmosphere is obtained 

 from computing the height of meteors (shooting-stars). 



