. This is primarily
occasioned by the diurnal revolution of the earth upon its axis from west
to east; but whether through the operation of the sun, proceeding
westward, upon the atmospheric fluid, or the rapidity of revolution of
the solid body, which leaves behind it that fluid with which it is
surrounded, and thereby causes it virtually to recede in a contrary
direction; or whether these principles cooperate, or unequally oppose
each other, as has been ingeniously contended, I shall not take upon me
to decide. It is sufficient to say that such an effect appears to be the
first general law of the tropical winds. Whatever may be the degree of
the sun's influence upon the atmosphere in his transient diurnal course,
it cannot be doubted but that, in regard to his station in the path of
the ecliptic, his power is considerable. Towards that region of the air
which is rarefied by the more immediate presence of the heat, the colder
and denser parts will naturally flow. Consequently from about, and a few
degrees beyond, the tropics, on either side, the air tends towards the
equator; and, combining with the general eastern current before
mentioned, produces (or would, if the surface were uniform) a north-east
wind in the northern division, and a south-east in the southern; varying
in the extent of its course as the sun happens to be more or less remote
at the time. These are denominated the trade-winds, and are the subject
of the second general observation. It is evident that, with respect to
the middle space between the tropics, those parts which at one season of
the year lie to the northward of the sun, are, during another, to the
southward of him; and of course that an alteration of the effects last
described must take place, according to the relative situation of the
luminary; or in other words, that the principle which causes at one time
a north-east wind to prevail at any particular spot in those latitudes
must, when the circumstances are changed, occasion a south-east wind.
Such may be esteemed the outline of the periodical winds, which
undoubtedly depend upon the alternate course of the sun northwards and
southwards; and this I state as the third general law. But although this
may be conformable with experience in extensive oceans, yet, in the
vicinity of continents and great islands, deviations are remarked that
almost seem to overturn the principle. Along the western coast of Africa
and in some parts of the Indian seas