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 w^est 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 inei'ticB, 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. 



