RAIN. 103 



wind behind it to slowly creep up the slopes of the 

 mountain, and becomes chilled in the colder re- 

 gions which lie near the summit. If this lowering 

 of temperature be sufficiently great, the moisture 

 will be precipitated from the air, no matter from 

 what direction the wind may come. 



Mountains may, therefore, cause rain to fall 

 from any wind that is forced to blow over them, 

 provided they are sufficiently high to cause the 

 necessary amount of cooling. When a mountain 

 reaches sufficiently far upward into the air to cause 

 the temperature to fall below the freezing-point 

 of water, the condensed moisture falls as snow. 



The reason so many rivers rise in mountains is 

 to be found in the fact that the mountains act to 

 chill the winds, and so rob the air of its moisture, 

 no matter in from what direction the wind, which 

 is forced to ascend their slopes, may happen to 

 blow. 



Nearly all the rivers of the world rise in moun- 

 tainous districts. 4-8 a rule, the largest rivers of 

 the world rise in the highest mountains. This is 

 because the higher the mountain the colder its 

 slopes, the cold mountain slopes acting, as ex- 

 plained, to deprive the air of its moisture. 



The rain that falls on a mountain's slopes, like 



