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.
Since the cool air of the temperate and comparatively slow-moving zones beyond the tropics is thus drawn towards the equator, and comes successively in contact with parallels of latitude moving faster and faster towards the east, there must be gradually imparted to it, by the increased friction, a considerable degree of the increased rotatory velocity belonging to the low latitudes it has now reached; that is to say, there will be less and less difference of velocity between the easterly motion of this temperate air and the easterly motion of the earth; and, consequently, the wind, as it approaches the equator, will appear to blow less and less directly from the eastward.

But, while the earth's rotation within the tropics is thus acting on the slower-moving air which has travelled to it from beyond the tropics, with increased friction at every successive moment, there has been no such powerful counteracting influence in operation to diminish the meridional motion impressed on the air in question; for, although in proceeding from the tropics towards the equator, the wind might, at first sight, be supposed to have its speed somewhat lessened by friction along the earth's surface, the retardation due to this cause, if there be any at all, must be inconsiderable, compared to that which affects the motion caused by the difference in the rotatory velocity of the earth at the different parallels.