400 THE RAINLESS REGION OF SOUTH AMERICA. 



is from the eastward, and these mountains, as we 

 have already explained, cause the precipitation of vapour 

 carried by trade winds in invisible and impalpable form 

 from the Atlantic ocean, to take place almost entirely 

 on their opposite or eastern slopes, and so the desic- 

 cation of the western seaboard is brought about. 



" Although (says the Encyclopedia Britannica in its article 

 upon the Himalayas) the loftiest mountains when compared 

 with the earth's diameter are insignificant; and the irregularities 

 of surface would hardly be perceptible on any sphere however 

 large, that could be made to represent the earth ; yet heights 

 such as those reached by the Himalayas, introduce modifications 

 of climate that are not surpassed by those observed in moving 

 from the equator to the poles. One half of the total mass 

 of the atmosphere, and three quarters of the water suspended 

 in it, lie below their average elevation; and of the residue, 

 one half the air, and virtually almost all the vapour, come 

 within the influence of the highest peaks." 



The truth of this hypothesis is beautifully illustrated 

 by the condition of the Pacific seaboard of South 

 America, and it is hardly too much to say that it renders 

 the meteorology of the Andes region perhaps the most 

 remarkable and instructive of any other in the world. 



And not only does this mountain range exercise 

 this commanding influence upon the rainfall of the 

 country, but it also, by reason of its great elevation, 

 creates upon its slopes a range of climate which em- 

 braces every possible variation of temperature. The 

 principal part of this range passes directly through the 

 whole breadth of the southern tropical regions and 

 also extends through a considerable portion of those 

 lying to the northward of the equator so that it 



'* Encycl. Brit., gth Edition, Vol. xi, p. 829 (Article "Himalaya"). 



