Proximate Causes of certain Winds and Storms. 277 



it seemed as if a squall would have forced us to hand our topsails ; 

 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 months of June and July."* 



Other arguments of less weight might be added to the above, but 

 if these fail of producing conviction, I have no great hope that the 

 others would be regarded as satisfactory, and shall therefore omit 

 them. These vortices may be supposed to be either stationary or 

 moveable, regular or irregular, few in number, and having their hori- 

 zontal much greater than their vertical diameter, or numerous, and 

 rolling in rapid succession across the ocean. The points where there 

 is either a remission of the breeze, or a calm, will of course mark the 

 separation of an individual from that which succeeds it. 



The theory here advocated, requires the prevalence in the lati- 

 tude of the United States, of a westerly, or rather north-westerly 

 wind, proceeding from the higher regions of the atmosphere. That 

 westerly winds predominate over the easterly in both hemispheres, 

 between the 30th and 60lh parallels, is shewn in a preceding page. 

 That the north-west winds of the United States, descend from the 

 higher regions of the atmosphere, is proved by President Dwight, 

 with his usual ability, in a passage copied from his travels, into the 

 eighth volume of this Journal, to which the reader is referred. The 

 progress of scientific discovery, and especially the discovery of the 

 immense power of radiation to cool the surface of the earth, has de- 

 prived some of his arguments of a part of their value, but the weight 

 and force of the greater number remain unimpaired. 



Of the Causes of Thunderstorms. 



From the foregoing principles, facts and arguments, and the con- 

 clusions we have derived from them, there flows a theory of thunder- 

 storms, which is capable however of being established on independ- 

 ent evidence of its own. It has appeared that the effect of the daily 

 passage of the sun is to create, some time after mid-day, but how 

 long after we have no means of accurately determining, a current of 



^ Personal Narrative, Vol. ii, page 5. 



Vol. XIX.— No. 2. 36 



