40 Professor Mitchell on the Trade- Winds. 



questionably movements of the air in vortices revolving eastward 

 below, and westward above*. Their existence proves nothing 

 absolutely, but lends a degree of probability to the accuracy of the 

 views advanced in this paper. Why may not the eastern and 

 western, or trade-winds, resemble each other in their causes, ef- 

 fects, and all the circumstances of their progress ? 



(e) The coolness and freshness of the air, within the limits 

 of the trades, so much exceeding what might be expected from 

 the latitude, is a proof that it is affected by currents flowing down 

 from above, and altogether incompatible with the idea that they 

 are ground currents, of which the cold returning upper current 

 flows off towards the poles. 



"• Nothing equals the beauty and mildness of the equinoctial 

 region on the ocean •f-.''' " In these winds there is something so 

 exhilarating that one with difficulty believes so much vapour ex- 

 ists as the hygrometer indicates J." " The climate of these 

 (the Sandwich islands) is far more cool than might be supposed, 

 judging from their latitude §.'' He attributes the circumstance 

 to the prevalence of the north-east trade-winds. 



fj") The unfrequency of rain within the limits of the trades 

 is another proof of the mixture of the upper and lower strata 

 of the air, by ascending and descending currents. Rain is pro- 

 duced by the sudden mixture of the air of very different tem- 

 peratures charged with moisture, effected, as there is good reason 

 to believe, by the establishment of a vortex or horizontal whirl- 

 wind upon the spot where it falls ; but the trade-winds, keeping 

 up a constant circulation and intermixture of the upper and lower 

 strata, there is no opportunity for those sudden changes which 

 produce rain. In accordance with what is here stated, it is ob- 

 served that such tracts of the intertropical ocean, as from any 

 cause are not swept by the regular trade-winds, are subject to 

 violent rain storms, accompanied by hghtning and wind. So long 

 as the monsoons blow regularly in either direction, the same effect 

 is produced by them in the same way as by the trade- winds, but 

 the period of their change is characterized by most violent storms. 



• See an Account of the Land and Sea Breezes. 



t Humboldt ; see also his remarks on the temperature of the air, which 

 are too long to be extracted. 

 X Caldcleugh's Observations in Brazil and on the Equator. 

 § Stewart's Journal 



