THE NON-LIVING WORLD 161 



Central Asia, is a rainless desert because the vaporous 

 winds from the Indian Ocean are drained of their mois- 

 ture ere they reach it over the Himalaya, while the 

 mountains of China drain the humid winds which pass 

 to it from the Pacific. 



It is evident that districts far from the sea, and desti- 

 tute of mountains, must possess less aqueous vapour in 

 their atmosphere than others in the vicinity of the ocean, 

 of seas, or of large lakes. This alone must make the 

 climate of South America more humid than are the 

 central regions of the Old World which are so much more 

 distant from the shores of the ocean. 



It is from rain that all the rivers of the world have 

 their origin. For the springs from which so many arise 

 are but the outcrop of the rain which has penetrated the 

 soil till, having come to an impervious stratum, it can 

 descend no longer, but must issue forth at the lowest 

 level of the upper surface of such impervious stratum. 

 In rain, all fresh-water lakes also have their origin, since 

 they are produced by impediments in the rapid progress 

 of rivers. There are, however, other lakes such as 

 the intensely salt Dead Sea which have another 

 origin, being remnants left behind by seas which have 

 receded. 



The largest rivers are in the New World ; the 

 Amazon runs a course of 3000 miles, and drains 

 1,500,000 square miles of country. The Mississippi of 

 North America is almost if not quite as long, but 

 drains a less extent of land. The Yangtse-kiang of 

 China runs a course of 2500 miles; the Nile, the Ganges, 

 and the Indus are also of great extent. 



Water in the form of rain, rivers, streams and sea 

 waves, is continually modifying the earth's surface, by 

 destroying its more elevated parts. This modifying 



