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