CHAPTER II 

 THE ATMOSPHERE 



The atmosphere or air forms an invisible envelope surrounding 

 and resting upon the earth. It's exact thickness is unknown, for 

 it blends gradually with the imperceptible ether which fills inter- 

 planetary space. While its functions are less apparent than 

 those of water and soil, it nevertheless bears important relations 

 to agricultural life and industries. 



Weight of the air. The resistance which air offers to rapidly 

 moving bodies, its own motion as wind and the support of clouds 

 and other bodies are evidences of its mass. The pressure by 

 which it forces water into the vacuum of a pump or balances a 

 column of mercury in the barometer is a measure of its weight, 

 which is approximately 15 pounds per square inch at sea level, 

 or 41,300 tons for each acre of the earth's surface. Were the air 

 of uniform density throughout, its height could be easily meas- 

 ured. The barometer falls, however, with decreasing rapidity as 

 it is raised from the earth, thus proving that the air decreases 

 in density with increase in height. 



Height of the air. The band of haze attending the earth's 

 shadow at lunar eclipse, the twilight period upon the earth, the 

 time of falling meteors and other phenomena dependent upon the 

 atmosphere give means of estimating its approximate height as 

 at least 200 miles. 



Air essential to life. If an animal be enclosed with a supply 

 of food in a perfectly tight chamber but with a limited supply of 

 air it will finally suffocate. This occurs as a result of exhausting 

 the greater part of a constituent of the air known as oxygen. 

 This element is absolutely essential to the processes by which food 

 is assimilated and waste matter is expelled from the animal body. 

 So too, if a plant be similarly enclosed, it will finally cease to grow 

 and prematurely die. This is because it exhausts the limited sup- 



