MAGNETISM AND CIRCULATION OF THE ATMOSPHERE. I35 



341. Meeting no land in this long oblique track over the tepid 

 waters of a tropical sea, it would, if such were its route, arrive 

 somev/hsre about the meridian of 140° or 150° west, at the belt 

 of equatorial calms, which always divides the northeast from the 

 southeast trade-winds. Here, depositing a portion of its vapor as 

 it ascends, it would, with the residuum, take, on account of diurnal 

 rotation, a course in the upper region of the atmosphere to the 

 southeast, as far as the calms of Capricorn. Here it descends 

 and continues on toward the coast of South America, in the same 

 direction, appearing now as the prevailing northwest wind of the 

 extra-tropical regions of the southern hemisphere. Traveling on 

 the surface from warmer to colder regions, it must, in this part of 

 its circuit, precipitate more than it evaporates. 



342. 'Now it is a coincidence, at least, that this is the route by 

 which, on account of the' land in the northern hemisphere, the 

 northeast 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 favorable to complete saturation ; and this is the route by 

 which they can pass over into the southern hemisphere most 

 heavily laden with vapors for the extra-tropical regions of that 

 half of the globe ; and this is the supposed route which the north- 

 east trade-Avinds of the Pacific take to reach the equator and to 

 pass from it. 



343. 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 im- 

 pinges, ought to be a region of copious precipitation. 



344. Now let us turn to the works on Physical Geography, 

 and see what we can find upon tliis subject. In Berghaus and 

 Johnston — department Hyetography — it is stated, on the authority 

 of Captain King, R. N., that upward of twelve feet (one hundred 

 and fifty-three inches) of rain fell in forty-one days on that part 

 of the coast of Patagonia which lies within the sweep of the winds 

 just described. So much rain falls there, navigators say, that they 

 sometimes find the water on. the top of the sea fresh and sweet. 



