rections, 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 be