276 Proximate Causes of certain Winds and Storms. 



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 witli 

 what is here stated, it is observed 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 lightning 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. 



The causes assigned, by Daniell, for the infrequency of rain with- 

 in the limits of the trades, are strange and unsatisfactory. He re- 

 marks first, that it being then only that the aqueous vapor attains its 

 highest elasticity and rises into the upper current of the atmosphere, 

 it must flow off along with the equatorial wind into the temperate 

 zones on either hand. Grant that it is so, we may answer ; the 

 language implies what is known to be a fact ; that there is no defi- 

 ciency of vapor within the limits of the trades ; that the whole tract 

 is in truth a great ocean of vapor ; why is it not precipitated ? why 

 is there so little rain ? Because, says the author, " the temperature 

 being remarkably steady, seldom varying more than two or three de- 

 grees, precipitation can but seldom occur." But why this steadiness 

 of temperature ? Precipitation, evaporation, heat and cold, stand to 

 each other in the relation of reciprocal cause and effect, which pro- 

 duce and re-produce each other in endless succession. Why are 

 there not within the limits of the trades the vicissitudes of the regions 

 beyond ? To say that precipitation seldom occurs there, because 

 the temperature is remarkably steady, is very little more than rea- 

 soning in a circle. 



(g.) We seem to witness in the appearances described in the fol- 

 lowing extract from Humboldt's account of his voyage across the 

 Atlantic in 1799, the effects of a succession of vortices moving west- 

 ward over the ocean, creating a cloud by a mixture of the upper and 

 lower strata of the atmosphere, and a breeze, by which the vessel was 

 for a short time driven rapidly forward, and then subsiding into a 

 calm. 



" The wind fell gradually the farther we removed from the Afri- 

 can coast ; it was sometimes smooth water for several hours, and 

 these short calms were regularly interrupted by electrical phenom- 

 ena. Black thick clouds, with strona; outlines, rose in the east, and 



