Professor Mitchell on the Trade-WiJids. 37 



tions in consequence of its mingling with the current that is pro- 

 ceeding southward, a northwest wind, which may be regarded 

 as the natural wind of the parts of the globe lying on the north 

 side of the equator beyond the 30th parallel. The same reason- 

 ing applies to the other hemisphere. As, however, the natural 

 and gentle flow of the air in this direction is interrupted by 

 evaporation, condensation, and other causes, the result is simply 

 a predominance in those latitudes of winds from the west, and 

 the direction of the pole over those from the opposite quarters. 



Two different causes, therefore, must exert an influence in 

 the production of the trade-winds. One is the permanent eleva- 

 tion of the temperature of the parallels lying near the equator 

 over those more remote from it. Its action is indirect and most 

 energetic in the northern parts of the temperate zone. The 

 other is the diurnal increase of the temperature of the earth in 

 all latitudes, in consequence of the passage of the sun over the 

 meridian. Its action is direct within the limits of the trades. 

 That it is adequate to the creation of a considerable wind is 

 proved by the fact, that it is upon this that the other or perma- 

 nent temperature depends, and that it is what determines the ex- 

 istence of two winds ; the land and sea breezes blowing in oppo- 

 site directions every twenty-four hours. By attending to the phe- 

 nomena of the trade-winds in different parts of the globe, we may 

 form a tolerable conjecture respecting the one of the two causes 

 which must be supposed to exert a predominant influence in the 

 production of the total effect. In the immediate neighbourhood 

 of the equator, or at a small distance on the north side of it, 

 the cause assigned by Halley, acts almost by itself, and the 

 wind prevailing there appears to be from the east, but much 

 less constant and violent than at some distance on either side. 

 At those greater distances the two causes conspire, and a com- 

 mensurate effect is observed. It is there that the trade-winds 

 rush onwards with the greatest velocity. Between the latitudes 

 of 30° and 60°, the two causes act in opposition ; that assigned 

 by Halley prevails, and there is a predominance of winds from 

 the west. 



That the trade-winds are in fact produced by a circulation 

 within their own limits, carried on by' vortices in which the 

 motion is westward below, and eastward above, is rendered pro- 

 bable by a number of separate considerations. 



