Page:The Atlantic Monthly Volume 1.djvu/282

274 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.

Now although this hypothesis explains the phenomena, it has still met with great opposition. The motions which Lieut. Maury supposes can hardly be accounted for without resorting, as is usual in such cases, to electricity or magnetism,—to some occult cause, or some occult operation of a known cause. Moreover, it has been difficult for the mechanical philosopher to understand how the winds manage to cross each other, as Lieut. Maury supposes them to do, at the equator and the tropics, without getting into "entangling alliances." If this hypothesis were advanced, not as a physical explanation of the phenomena, but, like the epicycles and eccentrics of Ptolemy, "to save the appearances," its ingenuity would be greatly to its author's credit; but, like the epicycles and eccentrics, though it represents the phenomena well enough, it contradicts laws of motion, now well known, which ought to be familiar to every physical philosopher. But these speculations of Lieut. Maury will now be superseded by a new theory of atmospheric movements, an account of which was presented by its author, Mr. J. Thompson, at the recent meeting of the British Association for the Advancement of Science.

Mr. Thompson's theory takes account of forces, hitherto unnoticed, which are generated by the eastward circulation of the atmosphere in high latitudes. He shows that these forces cause the prevailing northeastward under-current of our latitudes, while above this, yet below the highest northeastward current, the air ought still to move southward according to Halley's theory.

This under-current is not the immediate effect of differences of temperature, but a secondary effect induced by the friction of the earth's surface and the continual deflection of the air's eastward motion from a great circle, (in which the