Now the air of the equator, moving with the earth's rotary motion, has a greater velocity than the earth itself at high northern or southern latitudes, and consequently appears to gain an eastward motion in its progress toward the poles. Without friction, this relative eastward motion would increase as the air moves toward the poles, and diminish at the same rate as the air returns, till at the equator the velocity of the earth and of the air would again be equal; but friction reduces the motion of the returning air to that of the earth, at or near the calms of the tropics; so that the air, passing the tropics, gains a relative westward motion in its further progress through the torrid zone. The southwestward motion thus produced between the tropic of Cancer and the equator is the well-known trade-wind. Now, according to this theory, the prevailing winds of our temperate latitudes ought to have a southeastward motion as far as the calms of Cancer or "the horse latitudes." Moreover, instead of these calms, there should still be a southward motion. But observation has shown, that though the prevailing lower winds of our latitude move eastward, still their motion is toward the north rather than the south; so that they appear to contradict the theory by which the trade-winds are explained. To account for these anomalies, Lieut. Maury has invented a very ingenious hypothesis, which is published in his "Physical Geography of the Sea." He supposes that the air, which passes from the equator toward the poles in the upper regions of the atmosphere, is brought down to the surface of the earth beyond the calms of the tropics, and that it thence proceeds with an increasing eastward motion, appearing in our northern hemisphere as the prevailing northeastward winds. Approaching the poles with a spiral motion, the air there rises, according to this hypothesis, in a vortex, and returns toward the equator in the upper atmosphere, gradually acquiring a westward motion; till, returning to the tropics, it is again brought down to the earth, and thence proceeds, with a still increasing westward motion, as the trade-winds. At the equator the air rises again, and, according to Lieut. Maury, crosses to the other side, and proceeds through a similar course in the other hemisphere. The rising of the air at the equator is supposed to cause the equatorial rains; and the drought of the tropics is also explained by that descent of the air, in these latitudes, which this hypothesis supposes.