46 PHILOSOPHY OF STORMS. 



is shown to be the fact by Mr. Redfield, as to all great storms 

 which travel any considerable distance in the West Indies, 

 And in the Philosophical Transactions, Lathrop'sAbridg- 

 ment, volume 2, page 107, it is said that hurricanes in the 

 West Indies begin from the north west, and terminate with 

 a south east wind. 



76. It is quite reasonable to suppose that these spouts 

 sometimes meet with a middle current, moving in a differ- 

 ent direction from the uppermost, which will account for 

 the exceptions to the general rule ; for the spouts will, in 

 such case, certainly lean, and, of course, move in the di- 

 rection of the middle current. 



77. These three storms all occurred in the day, and two 

 of them in the afternoon; and M. Pouillet says that many 

 more occur in the day than in the night. Now, this is pre- 

 cisely what the theory would lead us to suppose, and the 

 explanation of this fact affords me an opportunity of ex- 

 plaining the very commencement of those spouts which 

 occur during the day. The sun, during the day, and espe- 

 cially in the afternoon, heats up the surface of the earth, and 

 the air in contact with that surface, many degrees above the 

 air, a few hundred feet above the earth. This heated air 

 below, and cold air above, will form an unstable equili- 

 brium, and a very slight agitation will cause to be formed 

 upward vortices of the light air below. Now, if the dew 

 point is not more than ten degrees below the temperature of 

 the air in contact with the soil, the air of the upward vortex 

 will not ascend much above one thousand yards, before the 

 refrigeration,"caused by expansion, will cause a beginning of 

 condensation of vapor; and the moment this occurs, the 

 velocity of upward motion is rapidly increased, from the 

 expanding effect caused by the evolution of latent caloric, 

 as before explained. 



If the dew point of the air at this elevation should be al- 

 most identical with its temperature, the cloud of the upward 



