74 THE PHYSICAL GEOGRAPHY OF THE SEA. 



great volume wliicli the mariner at sea as well as the philosopher 

 on the mountain each sees spread out before him. 



121. Of its circulation. — We have seen (§ 31) that there 

 are constant currents in the ocean ; we shall now see that there 

 are also regular currents in the atmosphere. 



122. From the parallel of about 30° north and south, nearly to 

 the equator, we have, extending entirely around the earth, two 

 zones of perpetual winds, viz., the zone of northeast trades on this 

 side, and of southeast on that. With slight interruptions, they 

 blow perpetually, and are as steady and as constant as the cur- 

 rents of the Mississippi River, always moving in the same direc- 

 tion (Plate I.) except when they are turned aside by a desert here 

 and there to blow as monsoons, or as land and sea breezes. As 

 these two main currents of air are constantly fowing from the 

 poles toward the equator, we are safe in assuming that the air 

 which they keep in motion must return by some channel to the 

 place toward the poles whence it came in order to supply the 

 trades. If this were not so, these winds would soon exhaust the 

 Polar regions of atmosphere, and pile it up about the equator, and 

 then cease to blow for the want of air to make more wind of. 



123. 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 toward the poles. This turning is caused by the ro- 

 tation of tlie earth on its axis. 



124. 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 diurnal rotation, 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. 



