116 PHYSICAL GEOGRAPHY OF THE SEA, AND ITS METEOROLOGY. 



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

 rivers — than in the southern hemisphere. A glance at Plate YIII. 

 will show how very much larger that part of the ocean over which 

 the south-east trades prevail is than that where the north-east 

 trade-winds blow. This estimate as to the quantity of rain in the 

 two hemisj^heres is one which is not capable of verification by any 

 more than the rudest approximations ; for the greater extent of 

 south-east trades on one side, and of high mountains on the other, 

 must each of necessity, and independent of other agents, have their 

 effects. Nevertheless, this estimate gives as close an approximation 

 as we can make out from our data. 



295. The Bcmiy Seasons, Jioiv caused. — The calm and trade-wind 

 The raiky Seasons, rcgious or bclts movo up and down the earth, an- 

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

 the zone of equatorial calms is fomid 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 pressm'e. The vapour that is taken up by 

 the south-east trades is borne along over the region of north-east 

 trades to latitude 35° or 40° north, where it descends and appears 

 on the surface with the south-west winds of those latitudes. Driving 

 upon the highlands of the continent, this vapour 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. Tilis wholc SVStcm of ZOnCS, viz., of tradcS, 

 California and Pa- , -, , i • -\ o m n t 



nama. calms, and westerly wmds, lollows the sun; and 



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

 spring months than at any other season. The south-west mnds 

 commence at this season to prevail as far down as the lower part 

 of CalifoiTiia. In winter and spring the land in California is cooler 

 than the sea au', and is quite cold enough to extract moisture from 

 it. But in summer and autumn the land is the warmer, and can- 

 not condense the vapours of water held by the air. So the same 

 * See the Trade-wind Chart. 



