CHAPTER III 



AIR AND FIRE 



38. The Atmosphere. As we already know, the earth 

 has a diameter of about 7,900 miles. By means of wells, 

 of cracks in the earth, and of deep mines the solid surface 

 has been studied to a depth of about 6,000 feet. This 



outer layer is called the crust to 

 distinguish it from the unknown 

 interior, or core. Filling the great 

 hollows of the crust is a layer of 

 water, which, if it were spread 

 evenly over the crust, would 

 cover all the land to a depth of 

 about two miles. Finally, sur- 

 rounding both land and sea is 

 the ocean of gas the atmos- 

 phere (Fig. 30) . The atmosphere 

 extends outward from the crust 

 an unknown distance, but most of it is within 50 miles 

 of the earth's surface. Half of it is within four miles. 

 The substance that makes up the atmosphere is air. 



Because air is a gas, and therefore difficult to handle, 

 men looked upon it for a long time as a mysterious, un- 

 knowable part of the earth. Only when men became used 

 to the idea that air and other kinds of gaseous matter are 

 not really different from solid and liquid matter was it 



38 



Fia. 30. 



Ideal Section of the Earth. Thick- 

 ness of the outer layers greatly 

 exaggerated. 



