Professor Mitchell oil the Trade-Winds. 41 



The causes assigned by Daniell for ihe^unfrequency 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 vapour at- 

 tains 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 deficiency of vapour within the limits of 

 the trades ; that the whole tract is in truth a great ocean of va- 

 pour ; 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 degrees, precipi- 

 tation can but seldom occur."" But why this steadiness of tem- 

 perature ? Precipitation, evaporation, heat and cold, stand to 

 each other in relation of cause and effect, which produce and 

 reproduce 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 

 reasoning in a circle. 



{g) We seem to witness in the appearance described in the 

 following extract from Humboldf s account of his voyage across 

 the Atlantic in 1799, the effects of a succession of vortices mov- 

 ing westward 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 

 African coast ; it was sometimes smooth water for several hours, 

 and these short calms were regularly interrupted by electrical 

 phenomena. Black thick clouds, with strong outlines, rose out 

 in the east, and it seems as if a squall would have forced us to 

 hand our top-sails ,• but the breeze freshened anew, there fell a 

 few large drops of rain, and the storm was dispersed without 

 our hearing any thunder.'* — It is by means of these squalls, 

 which alternate with dead calms, that the passage from the 

 Canary Islands to the Antilles or southern coasts of America, 

 is made in the month of June and July*. 



• Personal Narrative, vol. ii. p. 5. 



