THEORY DEDUCED FROM PHYSICAL LAWS. 21 



wind assailed him with such violence, that he was obliged 

 to throw himself on his body, and stick his hands and feet 

 in the snow, to prevent himself from being hurled over the 

 tremendous slope which threatened his instant destruction. 

 The cloud having passed, the air became calm, and he im- 

 mediately descended. Now this is just the effect which 

 would be produced by a large mass of air moving upwards 

 rapidly by being of less specific gravity, than the surround- 

 ing air ; which is too plain to need demonstration. 



43. Gay-Lussac, on entering a cloud with his balloon, 

 found his thermometer rise several degrees above what it 

 had been in the sun's rays. 



44. The Reverend W. B. Clarke says, in the Magazine of 

 Natural History, volume 7, page 300, that Mr. Kelsall, who 

 was an eye witness of the great eruption of ./Etna, in 1809, 

 writes thus: " At fifteen minutes past nine, A. M., April 1st, 

 a quantity of dense smoke proceeded from two rents, which, 

 raised to a considerable height in the atmosphere, before 

 serene, was dilated, and formed a black cloud above two thou- 

 sand paces in diameter, which presently discharged a co- 

 pious shower of large hail stones, on the red hot lava." 



In the same page he says that " during the eruption of a 

 volcano in Iceland, in 1793, not only did rain fall in torrents, 

 but also hail in showers." 



45. Mr. J. R. Jackson, in his Aide-Memoire Du Voyageur, 

 says, " I have seen, in the plains of Agra, Hindoostan, lati- 

 tude 27, enormous columns of sand, sometimes thirty at a 

 time, several feet in diameter, rising perpendicularly out of 

 sight, and followed frequently by a shower of large hail 

 stones, containing such a quantity of sand, in large grains, 

 that in filling a goblet with this hail when it was melted there 

 was a sediment of sand almost half an inch thick." From 

 these accounts it is manifest that hail is sometimes produced 

 by an upward motion of a column of air both with and 

 without volcanic agency ; and it is confidently believed that 



