78 PHYSICAL GEOGRArHY OF THE SEA, AND ITS METEOllOLOGY. 



equal power for the mastery. (8.) Crossing 40°, the counter- 

 trades (1), — the north-west winds of the southern hemisphere, — ■ 

 become the prevailing winds, and so remain, as far as our obser- 

 vations at sea extend towards the south pole. 



Such are the most striking movements of the winds at the 

 surface of the sea. But, in order to treat of the general system 

 of atmospherical circulation, we should consider where those 

 agents reside which impart to that system its d^mamical force. 

 They evidently 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 preva- 

 lence from each one of the four quarters, we direct our attention 

 to the upper and lower currents, and to the general movements 

 haclc and forth between the equator and the poles, we shall be 

 enabled the better to understand the general movements of this 

 grand machine. 



205. Tlie trade-wind belts. — Thus treating the subject, obser- 

 vations show that from the parallel of about 30° or 35° north 

 and south to the equator, we have, extending entirely around the 

 earth, two zones of perpetual winds, viz., the zone of north-east 

 trades on this side, and of south-east on that. With slight 

 interruptions, these winds blow perpetually, and are as steady 

 and as constant as the currents of the Mississippi Eiver, always 

 moving in the same direction (Plate I.) except when they are 

 turned aside by a desert or a rainy region 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 w^hich they 

 keep in motion must return by some channel to the place toward 

 the i)oles 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. 



206. TJie return current. — This return current, therefore, must 

 be in the upper regions of the atmosphere, at least until it passes 

 over those parallels between which the trade-winds are usually 

 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 direct and counter currents are also 

 made to move in a sort of spiral or loxodronic curve, turning to 

 the west as they go from the poles to the equator, and in the 



