THE ATMOSPHERE. 
71 
96. This return current, therefore, must be in the upper regions 
of the atmosphere, at least until it passes over those parallels be- 
tween which the trade-winds are always blowing on the surface. 
The return current must also move in the direction opposite to 
that wind the place of which it is intended to supply. These di- 
rect and counter currents are also made to move in a sort of spiral 
or loxodromic curve, turning to the west as they go from the poles 
to the equator, and in the opposite direction as they move from 
the equator to the poles. This turning is caused by the rotation 
of the earth on its axis. 
97. The earth, we know, moves from west to east. Now if we 
imagine a particle of atmosphere at the north pole, where it is at 
rest, to be put in motion in a straight line toward the equator, we 
can easily see how this particle of air, 'coming from the very axis 
of the pole, where it did not partake of the diurnal motion of the 
earth, would, in consequence of its vis inertice, find, as it travels 
south, the earth slipping from under it, as it were, and thus it 
would appear to be coming from the northeast and going toward 
the southwest ; in other words, it would be a northeast wind. 
The better to explain, let us take a common terrestrial globe 
for the illustration. Bring the island of Madeira, or any other 
place about the same parallel, under the brazen meridian ; put a 
finger of the left hand on the place ; then, moving the finger down 
along the meridian to the south, to represent the particle of air, 
turn the globe on its axis from west to east, to represent the diur- 
nal rotation of the earth, and when the finger reaches the equator, 
stop. It will now be seen that the place on the globe under the 
finger is to the southward and westward of the place from which 
the finger started ; in other words, the track of the finger over 
the surface of the globe, like the track of the particle of air upon 
the earth, has been from the northward and eastward. 
98. On the other hand, we can perceive how a like particle of 
atmosphere that starts from the equator, to take the place of the 
other at the pole, would, as it travels north, in consequence of 
its vis inerticB, be going toward the east faster than the earth. 
It would, therefore, appear to be blowing from the southwest, and 
going toward the northeast, and exactly in the opposite direction 
to the other. Writing south for north, the same takes place be- 
tween the south pole and the equator. 
