Page:Physical Geography of the Sea and its Meteorology.djvu/378

352 winds, and crossing the belt after a delay in it of only a few hours instead of days.

652. It varies with the strength of the trade-winds.—Hence we infer that the position of the equatorial calm belt is determined by the difference of strength between the north-east and south-east trade-winds, which difference, in turn, depends upon difference of barometric pressure (§ 642), and upon difference in temperature between them in corresponding latitudes north and south. In it the air which they bring ascends. Now if we liken this belt of calms to an immense atmospherical trough, extending, as it does, entirely around the earth, and if we liken the north-east and south-east trade-winds to two streams discharging themselves into it, we shall see that we have two currents perpetually running in at the bottom, and that, therefore, we must have as much air as these two currents bring in at the bottom to flow out at the top. What flows out at the top is carried back-north and south by these upper currents, which are thus proved to exist and to flow counter to the trade- winds.

653. Precipitation in it.—Captain Wilkes, of the Exploring Expedition, when he crossed this belt in 1838, found it to extend from 4° north to 12° north. He was ten days in crossing it, and during those ten days rain fell to the depth of 6.15 inches, or at the rate of eighteen feet and upwards during the year. In its motions from south to north and back, it carries with it the rainy seasons of the torrid zone, always arriving at certain parallels at stated periods of the year; consequently, by attentively considering Plate VIIL, one can tell what places within the range of this zone have, during the year, two rainy seasons, what one, and what are the rainy months for each locality.

654. The appearance of the calm belts from a distant planet.—Were the north-east and the south-east trades, with the belt of equatorial calms, of different colours, and visible to an astronomer in one of the planets, he might, by the motion of these belts or girdles alone, tell the seasons with us. He would see them at one season going north, then appearing stationary, and then commencing their return to the south. But, though he would observe (§ 295) that they follow the sun in his annual course, he would remark that they do not change their latitude as much as the sun does his declination; he would therefore discover that their extremes of declination are not so far asunder as the tropics of Cancer and Capricorn, though in certain seasons the