THE ATMOSPHERE 149 



Meteors are solid bodies which come from the heavenly 

 spaces beyond the earth's atmosphere. They move through 

 space with great rapidity, as does the earth itself. If they 

 approach near enough to the earth to be affected by its 

 mass attraction (gravitation), they may be drawn into our 

 atmosphere. When they enter it they are intensely cold; 

 the temperature of the space from which they come has 

 been estimated to be about 459 F. But as they plunge 

 downward at tremendous speed they become red hot 

 through friction, for now they are travelling through real 

 substance instead of through empty space. So they glow 

 and we see them as shooting-stars. Usually they are con- 

 sumed by heat before they reach the solid surface of 

 earth. 



Meteors have been observed that were as high up as 

 two hundred miles. So the outer limits of the atmosphere 

 must extend beyond even that height, for meteors evi- 

 dently do not begin to glow, and thereby become visible, 

 until they have already passed far within the outer limits. 



Now perhaps we can form a mental picture of this 

 great "envelope" of earth, the atmosphere. We see that 

 it is really a part of earth ; that we live at the bottom of a 

 great sea of air which stretches for many miles above us. 

 Yet we know that most of the atmosphere, computed by 

 its density, lies close to earth; half of it, by weight, lies 

 under a height of 3.6 miles above sea-level, while three- 

 fourths of it lies under 6.8 miles. Above that level, it 

 thins out gradually to nothingness, and we may conceive 

 of its outer limits as like the fine spray from the top of 

 a fountain; its particles bound off into space, and are 

 then drawn back again by force of gravity. Beyond these 

 vague outer limits of our atmosphere are the enormous 



