74 THE PHYSICAL GEOGRAPHY OF THE SEA. 



reside which impart to that system its dynamical force. They evi- 

 dently reside near the equator on one side, and about the poles on 

 the other. Therefore, if, instead of confining our attention to the 

 winds at the surface, and their relative prevalence from each one 

 of the four quarters, we direct our attention to the upper and 

 lower currents, and to the general movements hack and forth be- 

 tween the equator and the poles, we shall be enabled the better 

 to understand the general movements of this grand machine. 



205. Thus treating the subject, observations show that from the 

 The trade-wind belts, parallel of about 80° or 35° north and south 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, these winds 

 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 flowing 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 suppl}^ 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 



206. This return current, therefore, must be in the upper re- 

 The return current, gions of the atmosphcrc, at Icast until it passes over 

 those parallels between which the trade- winds are usually blow- 

 ing 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 direct 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 di- 

 rection as they move from the equator toward the poles. This 

 turning is caused by the rotation of the earth on its axis. 



207. The earth, we know, moves from west to east. Now if 

 Effect of diurnal ro- wc imagine a particle of atmosphere at the north 



tation on the course , , . . , . . . 



of the trade-winds, polc, wherc it IS at rcst, to DC put m motion in a 

 straight line toward the equator, we can easily see how this par- 

 ticle of air, coming from the very axis of diurnal rotation, where 



