ATMOSPHERE 23 



CHAPTER IV.— The Atmosphere 



19. Extent of the Atmosphere. — The earth is com- 

 pletely surrounded by a mixture of several gases, which 

 is called the atmosphere, or air. The quantity of air 

 becomes less and less as the distance from the earth's 

 Mir face increases, until finally there is no more air, and 

 what is called* outer space begins. No one knows just 

 how far the atmosphere extends above the earth, but it 

 is variously estimated at from 200 to 500 miles. Per- 

 sons climbing high mountains all notice how the quan- 

 tity of air diminishes as they ascend, and how the cold 

 increases. In the thin air of high mountain tops 

 breathing becomes difficult, and finally, owing to a, lack 

 of air ami to the intense cold, a point is reached I 

 which man cannot climb. It is probable that more than 

 90 per cent of the atmosphere a included in the first 

 fifteen milcv ahove the earth. 



While the thin upper air will not furnish breath for 

 man or beast, it serves the useful purpose of protecting 



:rth from the fast number of meteors — shooting 

 ailed — which are constantly falling 

 into it. The numher of meteors striking the upper air 

 fan twenty-four hours has been 1 at two mil- 



lion. Meteors travel with enormous speed, and when 

 they strike the upper air the friction produced is so 

 great that it causes enough heat to burn them up. Most 

 meteors are small bodies, and are burned up so far 



