206 ON THE GALES OF 



may be as much rarer by the diminution of pressure 

 consequent to its altitude, as denser by the cold, 

 consequent to its remoteness from the earth, and an- 

 other may be as much denser by the increased pres- 

 sure arising from its proximity to the earth, as rarer 

 by being warmer. Hence when unequally heated, 

 different strata of the atmosphere do not always dis- 

 turb each other. Yet after a time, the rarefaction in 

 the lower stratum, by greater heat, may so far ex- 

 ceed that in an upper stratum attendant on an infe- 

 rior degree of pressure, that this stratum may pre- 

 ponderate, and begin to descend. Whenever such a 

 movement commences, it must proceed with increas- 

 ing velocity ; for the pressure on the upper stratum 

 and of course its density and weight, increases as it 

 falls; while the density and weight of the lower 

 stratum, must lessen as it rises. Hence the change 

 is, at times, so much accelerated, as to assume the 

 characteristics of a tornado, squall or hurricane. In 

 like manner may we suppose, the predominant gales 

 of our climate to originate. Dr. Franklin long ago 

 noticed, that north-eastern gales are felt in the south- 

 westernmost portions of the continent first, the time 

 of their commencement beins; found later, as the 

 place of observation is more to the leeward. This 

 need not surprise us, as it is evident that a current 

 may be produced either by a pressure from behind, 

 or by a hiatus consequent to a removal of a portion 

 of the fluid from before. 



The Gulf of Mexico is an immense body of water, 

 warm in the first place by its latitude, in the second 



