46 THE PRINCIPLES OF AGRICULTURE. 



retain all its moisture. A portion is liberated, accu- 

 mulates in drops, and falls to the earth. This is in 

 all cases the explanation of rain. 



The different ways in which the air is cooled, and rain 

 is produced, are : 



1. By Rising into the Upper Regions. The higher we 

 ascend from the earth's surface, the cooler we find the 

 atmosphere. Mountains are sometimes clothed with 

 grass and flowers at the foot, while their summits are 

 covered with perpetual snow. 



The atmosphere is warmed chiefly by the warm earth. 

 The heat of the sun's rays is accumulated upon the 

 surface of the earth. The earth then, like a heated 

 stove, throws out or radiates its heat, and warms the 

 atmosphere. So, as in case of the stove, the atmosphere 

 nearest it is always the warmest. 



As the air in any locality becomes heated in this 

 way, it is pressed upward by cooler and heavier air from 

 other sections, where it is cooled; and, if it contains 

 sufficient moisture, rain is produced. 



Ascending air is also cooled by expanding. When air 

 is compressed so as to occupy a smaller space, it becomes 

 warmer. On the other hand, when air expands, it be- 

 comes cooler. As the air ascends, the pressure upon it 

 from the air above becomes less, and consequently it 

 expands and becomes cooler. This, of itself, is a com- 

 mon cause of rain. 



2. By Passing over Mountains. Hills and mountains 

 are sometimes called " rain condensers." As the rising 

 currents of air pass over mountainous regions, the air ex- 

 pands and is cooled, and parts with its surplus moisture. 

 Hilly regions are more abundantly supplied with rain 

 than level tracts. That side of a range of mountains 



