Page:Physical Geography of the Sea and its Meteorology.djvu/104

78 equal power for the mastery. (8.) Crossing 40°, the counter-trades (1),—the north-west winds of the southern hemisphere,—become the prevailing winds, and so remain, as far as our observations at sea extend towards the south pole.

Such are the most striking movements of the winds at the surface of the sea. But, in order to treat of the general system of atmospherical circulation, we should consider where those agents reside which impart to that system its dynamical force. They evidently reside near the equator on one side, and about the poles on the other. Therefore, if, instead of confining our attention to the winds at the surface, and their relative prevalence from each one of the four quarters, we direct our attention to the upper and lower currents, and to the general movements back and forth between the equator and the poles, we shall be enabled the better to understand the general movements of this grand machine.

205. The trade-wind belts.—Thus treating the subject, observations show that from the parallel of about 30° or 35° north and south to the equator, we have, extending entirely around the earth, two zones of perpetual winds, viz., the zone of north-east trades on this side, and of south-east on that. With slight interruptions, these winds blow perpetually, and are as steady and as constant as the currents of the Mississippi River, always moving in the same direction (Plate I.) except when they are turned aside by a desert or a rainy region here and there to blow as monsoons, or as land and sea breezes. As these two main currents of air are constantly flowing from the poles toward the equator, we are safe in assuming that the air which they keep in motion must return by some channel to the place toward the poles whence it came in order to supply the trades. If this were not so, these winds would soon exhaust the polar regions of atmosphere, and pile it up about the equator, and then cease to blow for the want of air to make more wind of.

206. The return current.—This return current, therefore, must be in the upper regions of the atmosphere, at least until it passes over those parallels between which the trade-winds are usually blowing on the surface. The return current must also move in the direction opposite to that wind the place of which it is intended to supply. These direct and counter currents are also made to move in a sort of spiral or loxodronic curve, turning to the west as they go from the poles to the equator, and in the