Magnetism and the Trade Winds, 279 



tropical sea, it would, somewhere to the east of these islands, 

 arrive at the belt of equatorial calms which always divides 

 the NE. from the SE. trade winds. 



Here depositing a portion of its vapour as it ascends, it 

 would with the residuum take, on account of diurnal rotation, 

 a course in the upper regions of the atmosphere to the SE., 

 as far as the calms of Capricorn. Here it descends and con- 

 tinues on towards the coast of South America in the same 

 direction, appearing now as the prevailing NW. wind of the 

 extra- tropical regions of the southern hemisphere. 



.Travelling on the surface from warmer to colder regions, 

 it must, in this part of its circuit, precipitate more than it 

 evaporates. 



Now this is the route by which, on account of the land in 

 the northern hemisphere, the NE. trade winds have the fairest 

 sweep over that ocean. This is the route by which they are 

 longest in contact with an evaporating surface ; the route by 

 which all circumstances are most favourable to complete 

 saturation ; and this is the route by which they can pass over 

 into the southern hemisphere, most heavily laden with vapours 

 for the extra-tropical regions of that half of the globe ; and 

 this is the supposed route which the NE. trade winds of the 

 Pacific take to reach the equator, and to pass from it. 



Accordingly, if this process of reasoning be good, that 

 portion of South America between the calms of Capricorn 

 and Cape Horn, upon the mountain ranges of which this part 

 of the atmosphere, whose circuit I am considering as a type, 

 first impinges, ought to be a region of copious precipitation. 

 I accordingly turned to the hyetography of Berghaus and 

 Johnson, and find it stated on the authority of Captain King, 

 that upwards of 12* feet of rain fell there in 41 days ! 



Passing the snow-clad summits of the Andes, this same 

 wind tumbles down upon the eastern slopes of the range, and 

 then traverses the almost rainless and barren regions of 

 Patagonia and South Buenos Ayres. 



These conditions, the direction of the prevailing winds 

 and the amount of precipitation were regarded as evidence 



* 153 inches. 



t2 



