It will be understood, that, as long as we imagine the globe at rest while this circulation is going on, the course of the lower air along the surface would be directly towards the equator, from due north in one hemisphere, and from due south in the other; while in the upper regions the currents would follow the opposite directions, and stream towards the poles.  But the instant we conceive the earth put into rotatory motion from west to east, a change would take place in the course of these aerial currents, both above and below.  It must be recollected that a volume of air, when once put in motion, will move on, like any other body, by the mere force of its own momentum, till that motion is destroyed by its friction against the substances along or through which it is impelled.  Any one who has observed the ring of smoke sometimes projected from the mouth of a cannon will be sensible that this is true.

It may likewise be of use, before going further, to consider, that, if the globe, instead of being unequally heated; were equally heated at all parts, from pole to pole, and being surrounded by an atmosphere, were then made to revolve on its axis, it would carry the atmosphere round with it exactly at the rate at which it was itself going.  That portion of the air in contact with the equator would move about 1000 miles in one hour, while that in latitude 90 deg. would be as motionless as the poles themselves.

From this it will be seen, that, while the equator moves at the rate of 1000 miles an hour, the district about the latitude 30 deg. moves only 860, or 140 miles slower.  The average whirling velocity of the earth’s easterly motion, in the space between the equator and latitude 30 deg., may be stated at 950 miles an hour; and that of the belt lying between 30 deg. and 40 deg., at about 800 miles.

In the hypothetical case, above suggested, of the whole surface being equally heated, and consequently the whole atmosphere at the same temperature, there would be a universal calm, whatever might be the rotatory motion impressed upon the earth.  If, however, we next suppose, what really is the case, that the air over the tropical region is more heated than that which is farther from the equator, this rarefied air will instantly ascend, and occupy a place above the colder and denser air, which will flow in from the belts lying beyond the tropics.

When the comparatively slow-moving air of the temperate zone, lying beyond the tropics, first comes in contact with those quicker-moving parts of the earth forming the tropical edges of the torrid zone, the apparent motion of the air from the east, caused by the relative difference of the rotatory velocity between the air and earth, is great, compared to the other motion of the air, caused by its being drawn directly towards the equator, to supply the place of the heated portions raised into the sky.  Consequently, at the tropical borders of both Trades the wind is found to blow very nearly from the east point.