more or less heated, there is a call—a pulling back, if you please—upon these trades to turn about and restore the equilibrium which the deserts destroy. There being few or no such regions in the rear of the south-east trades, the south-east trade-wind force prevails, and carries them over to the northern hemisphere.
631. Diurnal rotation.—We see by the plate that the two opposing currents of wind called "the trades," are so unequally balanced that the one recedes before the other, and that the current from the southern hemisphere is larger in volume; i. e., it moves a greater zone or belt of air. The south-east trade-winds discharge themselves over the equator—i. e., across a great circle—into the region of equatorial calms, while the north-east trade-winds discharge themselves into the same region over a parallel of latitude, and consequently over a small circle. If, therefore, we take what obtains in the Atlantic as the type of what obtains entirely around the earth, as it regards the trade-winds, we shall see that the south-east trade-winds keep in motion more air than the north-east do, by a quantity at least proportioned to the difference between the circumference of the earth at the equator and at the parallel of latitude of 9° north. For if we suppose that those two perpetual currents of air extend the same distance upward from the surface of the earth, and move with the same velocity, a greater volume from the south should, as has already been shown (§ 343), flow across the equator in a given time than would flow from the north over the parallel of 9° in the same time ; the ratio between the two quantities would be as radius to the secant of 9°. Besides this, the quantity of land lying within and to the north of the region of the north-east trade-winds is much greater than the quantity within and to the south of the region of the south-east trade-winds. In consequence of this, the mean level of the earth's surface within the region of the north-east trade-winds is, it may reasonably be supposed, somewhat above the mean level of that part which is within the region of the south-east trade-winds. And as the north-east trade-winds blow under the influence of a greater extent of land surface than the south-east trades do, the former are more obstructed in their course than the latter by the forests, the mountain ranges, unequally heated surfaces, and other such like obstacles.
632. The land in the northern hemisphere.—That the land of the northern hemisphere does assist to turn these winds is rendered
