Page:Physical Geography of the Sea and its Meteorology.djvu/147

 Rh of equatorial calms, the yapours which make the rains that feed the rivers in the extra-tropical regions of the southern hemisphere. 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 hemisphere 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 XIII. 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 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 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 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 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 greatest 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,