The moon has been deposed from the executive chair, though she still has her supporters and advocates; and an innumerable host of minor causes are found to constitute, upon strictly republican principles, the ruling power of the winds and the rain. That regularity, however complicated, which reason still demands, and expects even from the weather, is not found to be so simple as our rules and signs of the weather indicate; for the operation of these innumerable causes is so complicated, that the repetition of similar phenomena or similar combinations of causes, to any great extent, is the most improbable of events. Perhaps the meteorologist will ultimately find that Nature has succeeded, in what seems, indeed, to be her aim, in completely retracing her steps, and reducing the operation of that simple and regular system of causes, which she brought out of chaos, back to a confusion of detail, from which all law and regularity are obliterated.
Meteorological observations have, however, determined many regular and constant causes and a few regular phenomena. The method pursued in these investigations is, for the most part, the elimination, by general averages, of limited and temporary changes in the elements of the weather, and the determination of those changes which depend upon the constant influences of locality, of season, and of constant or slowly varying causes. These constant influences constitute the climate; and the study of climates is thus the first step towards the solution of the problem of the weather. Climates, in their changes and distribution, are very important elements in the determination of the movements of the weather, and are to the meteorologist what the elements of the planetary orbits are to the astronomer; but, unlike planetary perturbations, the weather makes the most reckless excursions from its averages, and obscures them by a most inconsequent and incalculable fickleness.
Whether mechanical science will hereafter succeed in calculating these perturbations of climate, as we may style the weather, or will find the problem beyond its capacity, it will yet, doubtless, account for much that is now obscure, as observation brings the facts more distinctly to view. We propose to give a brief general survey of the mechanics of the atmosphere in its present state, and to indicate the nature and limits of our knowledge on this subject.
Among the first noticed and most remarkable features of regularity in atmospheric changes are constant, periodic, and prevailing winds. The most remarkable instances of these are the trade-winds of the torrid zone, the monsoons of the Indian Ocean, and the prevailing southwest wind of our northern temperate latitudes. Of these, the trade-winds are the most important to science, as furnishing the key to that general explanation of the winds which was first advanced by the distinguished Halley.
In Halley's celebrated theory, the trade-winds are explained as the effects of the unequal distribution of the sun's heat in different latitudes. The air of the equator, heated more than the northern or southern air, expands more, and overflows, moving in the upper regions of the atmosphere toward the poles; while the lower, colder air on both sides moves toward the equator to preserve equilibrium. Thus an extensive circulation is carried on. The air that moves from the equator in the upper atmosphere, gradually sinking to the surface of the earth, finally ceases to move toward the poles, and returns as an undercurrent to the equator, where it again rises and moves toward the poles.
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.
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]