§ 295, 296. EAINS AND RIVERS. n^ 



rivers, and as they prevail to a much greater extent at sea, and 

 have exposed to their action about twice as much ocean as the 

 northeast trade-winds have, we might expect, according to this 

 hypothesis, more rains in the northern — and, consequently, more 

 and larger rivers — than in the southern hemisphere. 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 verifi- 

 cation by any more than the rudest approximations ; for the great- 

 er extent of southeast trades on one side, and of high mountains 

 on the other, must each of necessity, and independent of other 

 agents, have their effects. ISTevertheless, this estimate gives as 

 close an approximation as we can make out from our data. 



295. The Bainy Seasons, how caused. — The calm and trade- wind 

 The Rainy SEAsoNa rcgious or bclts movc up and down the earth, an- 

 nually, in latitude nearly a thousand miles. In July and August 

 the zone of equatorial 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 Ore- 

 gon it rains every month, but about five times more in the winter 

 than in the summer months. The winter there is the summer of 

 the southern hemisphere, when this steam-engine (§ 24) is work- 

 ing 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 85° or 40° north, where it descends and appears 

 on the surface with the southwest winds of those latitudes. Driv- 

 ing upon the highlands of the continent, this vapor is condensed 

 and precipitated, during this part of the year, almost in constant 

 showers, and to the depth of about thirty inches in three months. 



296. In the winter the calm belt of Cancer approaches the equa- 



The rainy seasons of tor. This wholc SVStCm of ZOUCS, viz., of tradcS, 

 California and Pan- . ^ i-inni i 



ama. calms, aud westerly winds, follows the sun ; and 



they of our hemisphere are nearer the equator in the winter and 

 spring months than at any other season. The southwest winds 



* See the Trade-wind Chart. 



