THE ATMOSPHERE. 
77 
ot the world that it actually has at this moment, what would take 
place, supposing the uniform temperature to be a mean between 
that at the equator and that at the poles ? Why, this would take 
place: a swelling up of the atmosphere about the equator by the 
expansive force of inter-tropical heat, and a contraction of it about 
the poles in consequence of the cold. These two forces, consid- 
ering them under their most obvious effects, would disturb the 
supposed atmospherical equilibrium by altering the level of the 
great aerial ocean ; the expansive force of heat elevating it about 
the equator, and the contracting powers of cold depressing it about 
the poles. And forthwith two systems of winds would commence 
to blow, viz., one in the upper regions from the equator toward 
the poles, and as this warm and expanded air should flow toward 
either pole, seeking its level, a wind would blow on the surface 
from either pole to restore the air to the equator which the upper 
current had carried off. 
These two winds would blow due north and south ; the effects 
of heat at the equator, and cold at the poles, would cause them so 
to do. Now suppose the earth to commence its diurnal rotation ; 
then, instead of having these winds north and south winds, they 
will, for reasons already explained 97), approach the equator 
on both sides with easting in them, and each pole with westing. 
111. The circumference of the earth measured on the parallel 
of 60° is only half what it is when measured on the equator. 
Therefore, supposing velocity to be the same, only half the vol- 
ume of atmosphere 109) that sets off from the equator as an 
upper current toward the poles can cross the parallel of 60° north 
or south. The other moiety has been gradually draw^n in and 
carried back 108) by the current which is moving in the oppo- 
site direction. 
Such, and such only, would be the extent of the power of the 
sun to create a polar and equatorial flow of air, were its power 
confined simply to a change of level. But the atmosphere has 
been invested with another property which increases its mobility, 
and gives the heat of the sun still more power to put it in motion, 
and it is this : as heat changes the atmospherical level, it changes 
also the specific gravity of the air acted upon. If, therefore, the 
level of the great aerial ocean were undisturbed by the sun's rays, 
and if the air were adapted to a change of specific gravity alone, 
