46 THE rUINCirLKS OF A(iHICUi;rLKE. 



i-('tain all its moisture. A portion is lil)crat(Ml. accu- 

 inulatcs in (li'oj)S, and falls to the earth This is in 

 all cases the explanation of i-ain. 



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

 IS produced, are : — 



1. Bij llisbuj into the Upper lle<jlons. — The hitrher we 

 ascend from the earth's surface, the co(jler we find the 

 atmosphere. Mountains are sometimes clothed with 

 grass and flowci'S at the foot, while their sunnnits are 

 covei'ed with perpetual snow. 



The atmosphere is warmed chieflv by the warm earth. 

 The heat of the sun's rays is accunudated 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 si)ace, it becomes 

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

 comes cooler. As the air ascends, the ]n"essare 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. Bij Passing over Moinitaitis. — Hills and mountains 

 are sometimes called " rain condensers." As the rising 

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

 j)ands and is coo1(m1, and parts with its surplus moisture. 

 Hilly I'cgions are more abundantly suppli(Ml with rain 

 than level tracts. That side of a range of mountains 



