nd raised up. The necessary consequence of this is, to
produce a rapid motion in the air from the west over the earth's
surface; and this, combined with the other motion of the same portion
of air, or that which has driven it from the equatorial regions,
produces this remarkable prevalence of south-westerly winds in the
northern hemisphere, and north-westerly winds in the southern
hemisphere, in those districts lying between the latitudes of 30 deg. and
60 deg..

In all that has been said above it has been assumed that the
quickest-moving or equatorial belt of the earth is also the hottest,
and consequently that over which the air has the greatest tendency to
rise. But, although this is generally true, it is not, by any means,
universally so. The variations, however, which are observed to occur
in those places where the circumstances form an exception to the
general rule, tend strongly to confirm the theory of Hadley. The
monsoons of India, as I shall presently show, are examples of this;
but the most striking instance with which I am personally acquainted
occurs in the Pacific Ocean, between the Bay of Panama and the
Peninsula of California, from latitude 8 deg. to 22 deg. north. If the huge
continent of Mexico were taken away, and only sea left in its place,
there can be no doubt but the ordinary phenomena of the Trade-winds
would be observable in that part of the Pacific above mentioned. Cool
air would then be drawn from the slow moving parallels lying to the
northward, towards the swift moving latitudes, near the equator, in
order to supply the place of the rarefied air removed to the higher
regions of the atmosphere, and, of course, north-easterly breezes
would be produced. But when the sun comes over Mexico, that vast
district of country is made to act the part of an enormous heater, and
becomes a far more powerful cause of rarefaction to the superincumbent
air than the ocean which lies between it and the equator. Accordingly,
the air over Mexico, between the latitudes of 10 deg. and 30 deg., is more
heated than that which lies over the sea between the line and latitude
20 deg.; and as the coolest, or least heated, that is, the most dense
fluid, always rushes towards the place lately occupied by the hottest
and most buoyant, the air from the equator will be drawn towards the
coast of Mexico, the great local source of heat and rarefaction.

But as this equatorial air is of course impressed with a more rapid
easte