THE ATMOSPHERE. 85 



fall in the southern, and this is just about the proportion (^ 124) 

 that observation gives. 



In like manner, the southeast trade-v^^inds take up the vapors 

 which make our rivers, and as they prevail to a much greater ex- 

 tent at sea, and have exposed to their action about three times as 

 much ocean as the northeast trade-winds have, we might expect, 

 according to this hypothesis, more rains in the northern — and, con- 

 sequently, more and larger rivers — than in the southern hemi- 

 sphere. A glance at Plate VIII. will show how very much larger 

 that part of the ocean over which the southeast trades prevail is 

 than that where the northeast trade-winds blow. 



This estimate as to the quantity of rain in the two hemispheres 

 is one which is not capable of verification by any more than the 

 rudest approximations ; for the greater extent of southeast trades 

 on one side, and of high mountains on the other, must each of ne- 

 cessity, and independent of other agents, have their effects. Nev- 

 ertheless, this estimate gives as close an approximation as we can 

 make out from any other data. 



129. The rainy seasons, how caused. — The calm and trade-wind 

 regions or belts move up and down the earth, annually, in latitude 

 nearly a thousand miles. In July and August the zone of equato- 

 rial calms is found between 7° north and 12° north; sometimes 

 higher ; in March and April, between latitude 5° south and 2° north. 

 With this fact and these points of view before us, it is easy to 

 perceive why it is that we have a rainy season in Oregon, a rainy 

 and dry season in California, another at Panama, two at Bogota, 

 none in Peru, and one in Chili. 



In Oregon it rains every month, but more in the winter months. 

 The winter there is the summer of the southern hemisphere, 

 when this steam-engine is working with the greatest pressure. 

 The vapor that is taken up by the southeast trades is borne along 

 over the region of northeast trades to latitude 35° or 40° north 

 (§ 124), where it descends and appears on the surface with the 

 southwest winds of those latitudes. Driving upon the highlands 

 of the continent, this vapor is condensed and precipitated, during 

 this part of the year, almost in constant showers. 



130. In the winter, the calm belt of Cancer approaches the 

 equator. This whole system of zones, viz., of trades, calms, and 

 westerly winds, follows the sun ; and they of our hemisphere are 



