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 eastern velocity than those parts of the earth which form the southern shores of Mexico, a westerly wind must be produced by the relative difference in these two motions.

At that particular season of the year when the sun is in high southern declination, Mexico is not exposed to his perpendicular rays.

The equatorial regions are then more heated than Mexico, and accordingly we actually find north-easterly breezes nearly as they would be if Mexico were out of the way, and quite in accordance with our theory.
In like manner, in the Atlantic, when the sun is far to the north, the great deserts of the western angle, or shoulder of Africa, become as vehemently heated, or more so, perhaps, than Mexico, and this draws the air from the equator, so as to produce the south-westerly winds I have already spoken of in the troublesome range called the Variables.
Finally, the great monsoons of the Indian ocean and China sea contribute to establish this theory of Hadley, though I am not aware that he ever brought it to bear on these very interesting phenomena.
They are eminently deserving of such notice, however, from being periodical Trade-winds of the highest order of utility in one of the busiest commercial regions of the world.

Their periodical or shifting character is the circumstance upon which their extensive utility in a great measure depends, amongst nations where the complicated science of navigation is but in a rude state.

Myriads of vessels sail from their homes during one monsoon before the wind, or so nearly before it, that there is no great skill required in reaching all the ports at which they wish to touch; and when the wind shifts to the opposite quarter, they steer back again, in like manner, with a flowing sheet.
Thus, with an exceedingly small portion of nautical skill, they contrive to make their passages by means of what we blue-jackets call "a soldier's wind, there and back again." It will sometimes happen that these rude navigators miscalculate their time, or meet with accidents to retard them till the period of change has gone past, and then they have no resource but to wait for half-a-year till the monsoon shifts.
Experienced sailors, in like circumstances, acquainted with the varieties of winds prevailing in those seas, would speedily get their vessels out of this scrape, into which the lubberly Chinese junks sometimes fall.