THE ATMOSPHERE. 75 



atmosplierical cii'ciilation, we sliould consider where those agents 

 reside which impart to that system its d^Tiamical force. They e\i- 

 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 smface, and then' relative prevalence from each one of 

 the four quarters, we direct" our attention to the upper and lower 

 cm-rents, and to the ^e;iera/ movements lach and forth betvv^een 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, obseiTations show that from the 

 The trade-wiDd belts. parallel of *about 30^ or 35^ north and south to the 

 equator, we have, extending entkely around the earth, two zones 

 of perpetual v/inds, 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 t^^o main cmTents of air are constantly flowing 

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

 the au" 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. 



206. This retm^n cm-rent, therefore, must be in the upper re- 

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

 those parallels between w^hich the trade-winds are usually blowing 

 on the sm-face. The return cm-rent must also move in the direc- 

 tion opposite to that wind the place of which it is intended to sup- 

 ply. These dii^ect and counter cmTents are also made to move in 

 a sort of spiral or loxodronic cmwe, turning to the WTst as they go 

 from the poles to the equator, and in the opposite direction as they 

 move from the equator towards the poles. This tm^ning 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- ^e imao^iue a particle of atmosphere at the north 



tation on the course -, ,o l , . i ■*■ , • 



of the trade-winds, polc, wbere it IS at Tost, to DO put m motion m a 

 straight hne towards the equator, we can easily see how this par- 

 ticle of au', coming fi'om the very axis of dim'nal rotation, where 

 it did not partake of the dimmal motion of" the earth would, in 



