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 Maury will now be superseded by a new theory of atmospheric movements, an account of which was presented by its author, Mr.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. [1]
[Footnote 1: A fuller discussion of this theory the author reserved for the Royal Society. The _London Athenaeum_ gives a brief abstract of his paper, in its report of the proceedings of the Association.]
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 air tends to move,) into the small circle of the latitude, in which the air actually does move. The force of this deflection, measured by the centrifugal force of the air as it circulates around the pole, retards the movement from the equator, and finally wholly suspends it; so that the upper air circulates around in the higher latitudes as water may be made to circulate in a pail; and the air is drawn away from the polar regions as this circulatory motion is communicated to it, and tends to accumulate in the middle latitudes, as the circulating water is heaped up around the sides of the pail. Hence, in the middle latitudes there is a greater weight of air than at the poles, and this tends to press the lower air to higher latitudes. Centrifugal force, however, balances this pressure, so long as the lower air moves with the velocity of the upper strata; but as the friction of the earth retards its motion and diminishes its centrifugal force, it gradually yields to the pressure of the air above it, and moves toward the poles. Near the polar circles it is again retarded by its increasing centrifugal force, and it returns through the middle regions of the atmosphere.