48 NOTES OF A NATURALIST. 



domestic animals that it is possible to keep in such a 

 place. 



The problems suggested by the singular climatal 

 conditions of this region of South America have not, 

 I think, been as fully discussed as they deserve to be, 

 and I here venture on some remarks as a contribution 

 to the subject. 



The existence of the so-called rainless zone on the 

 west coast of South America is usually accounted for 

 by two agencies whose union is necessary to produce 

 the result. The great range of the Andes, it is said, 

 acts as a condenser on the moisture that is constantly 

 carried from the Atlantic coasts by the general west- 

 ward drift of the atmosphere in low latitudes. The 

 copious rainfall thus produced on the eastern slopes 

 of the great range leaves the air of the highlands of 

 Peru and Bolivia relatively dry and cool, so that any 

 portion that may descend to the coast on the western 

 declivity tends to prevent rather than to cause fresh 

 aqueous precipitations. Meanwhile the branch of 

 the Antarctic Ocean current known as the Humboldt 

 current, which sets northward along the sea-board 

 from Western Patagonia, is accompanied by an aerial 

 current* or prevailing breeze, which keeps the same 

 direction. The cold air flowing towards the equator, 

 being gradually warmed, has its capacity for holding 

 vapour in suspension constantly increased, and is thus 

 enabled to absorb a large portion of the vapour con- 

 tained in the currents that occasionally flow inland 

 from the Pacific, so that the production of rain is a 

 rare event, recurring only at long intervals. Admit- 

 ting the plausibility of this explanation, a first diffi- 



