PRIMER OF PRIMERS. 195 



More rain falls in the equatorial regions than 

 elsewhere ; more falls in the temperate than in the 

 polar regions. More rain falls on the coasts of 

 continents than in the interior. 



Where the temperature is sufficiently high, as in 

 the equatorial regions, rain may be caused by the 

 chilling produced by ascending currents. 



Mountains cause a heavy rainfall on account of 

 the air being chilled when forced to ascend their 

 cold slopes. 



Nearly all the great rivers of the world rise in 

 mountainous districts. 



The rain that falls on the earth either runs di- 

 rectly off the surface or sinks slowly into the 

 ground. The part that runs directly off the sur- 

 face collects in streams that discharge directly into 

 the rivers. The part that sinks into the ground 

 collects in underground basins, from which it 

 slowly emerges as springs. 



When the mountains are covered by forests, the 

 rain which falls on their slopes, for the greater 

 part, drains slowly into the ground. When the 

 mountains are denuded of their forests most of 

 the rain drains rapidly off the surface. The de- 

 struction of the forests on mountain-slopes is, 

 therefore, apt to cause floods. 



