Page:The Atlantic Monthly Volume 1.djvu/281

1858.] 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