RAINS AND RIVERS. 121 



of equatorial calms, the yapours whicli make the rains that feed 

 the rivers in the extra-tropical regions of the southern hemi- 

 sphere. Upon this supposition, then, two-thirds only of the 

 northern trade-winds are fully charged with moisture, and only 

 two-thirds of the amount of rain that falls in the northern hemi- 

 sphere should fall in the southern; and this is just about the 

 proportion (§ 292) that observation gives. In like manner, the 

 south-east trade-winds take up the vapours which make our 

 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 

 north-east 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 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 hemispheres is one which is not 

 capable of verification by an}^ more than the rudest approxi- 

 mations : 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 Bainy Seasons, Jioiv caused. — The calm and trade-wind 

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

 nearl}^ a thousand miles. In July and August, the zone of equa- 

 torial 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 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 working with the gi'eatest pressure. 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, 

 * See the Trade-wind Cliai-t. 



