THE SMITHSONIAN INSTITUTION. 199 



and sea breezes. The latent caloric evolved by the condensation of 

 vapor is the moving power in this class of phenomena, and the differ- 

 ence betv^een the temperature of the land and the sea is the disturbing 

 force in the other. I have frequently attempted to show that many of 

 the European storms were to be ascribed to a modification of these 

 principles. Since reaching Washington it has been pointed out to me 

 by Professor Henry that Professor Mitchell, of North Carolina, gave in 

 Silliman's Journal a similar explanation of some of the American 

 storms as far back as 1831. I had never seen this paper, and was a 

 little surprised to find so great a similarity in our views, more especial- 

 ly as he had actually selected the same passages I have quoted from 

 Humboldt as a common starting-point. But I have since found that 

 Volney more than half a century ago accounts for one American snow- 

 storm on the same principle. 



I must, in the first place, draw your attention to a feature which ap- 

 pears to be common to the hurricanes of the West India Islands, and 

 to the local squalls in the region of the trade winds, viz : that there is 

 usually a calm before these terrific disturbances of the atmosphere. 

 All who have examined these meteors are of opinion that the current 

 above is from the southeast, and bears the hurricane clouds over the 

 still and highly heated air ; this being surcharged with moisture is in a 

 most unstable state of equilibrium, which is liable to be overturned by 

 the shghtest cause that conspires to produce an up-moving volumn 

 of air. To illustrate this I will merely read to you Captain Marry atl's 

 graphic description of the coming on of a West Indian hurricane as 

 given in "Peter Simple:" "What a hot day this has been ; not a cat's 

 paw upon the water, and the sky all of a mist. Only look at the sun, 

 how lie goes down, puffed out to three times his size, as if he were in 

 a terrible passion. I suspect we shall have a land breeze off strong. 

 The heat was excessive and unaccountable, not the slightest breath of 

 wind moved in the heavens or below; no clouds to be seen, and the 

 stars were obscured by a sort of mist : there appeared a total stagna- 

 tion in the elements. We had not pulled long before a low moaning 

 was heard in the atmosphere, now here, now there, and we appeared 

 to be pulling through solid darkness ; I looked, and dark as it was, it 

 appeared as if a sort of black wall was sweeping along right towards 

 us. The moaning gradually increased to a stunning roar, and then at 

 once it broke upon us with a noise to which no thunder can bear com- 

 parison. The sea was perfectly level, but boiling and covered with a 

 white foam, so that we appeared in the night to be floating on milk." 



Professor Mitchell, in the paper to which I have already referred, 

 expresses the opinion that thunderstorms must be much less grand and 

 imposing in Europe than in America, because poets have not considered 

 them worthy of particular attention. Shakspeare, however, has not 

 neglected our thunder storms, and with his usual philosophical dis- 

 crimination has portrayed some of their more important peculiari- 

 ties: 



" We often see, against some storm, 

 A silence in the heavens, the rack stand still, 

 The bold winds speechless, and the orb below 

 As hush as death : anon the dreadful thunder 

 Doth rend the region." — Hamlet. 



The formation and progress of thunder storms are very peculiar in 



