340 HAIL STORMS. 



3. They usually run in veins of limited extent, 

 and are most frequent in level districts, especi- 

 ally such as are hemmed in by mountains, as in 

 the South of France, where they are very de- 

 structive to the crops. 



4. They are often attended with the sudden 

 precipitation of immense quantities of ice, that 

 descends in large globular masses ; and always 

 with a great reduction of temperature. 



Several of the above facts are exceedingly 

 difficult to explain, without admitting the hypo- 

 thesis of Franklin in regard to the cause of 

 thunder gusts, that is, a sudden descent of cold 

 air from the upper regions, by which the vapour 

 of the lower atmosphere is rapidly condensed 

 and congealed into globules of ice. 



In opposition to this view of the subject, it is 

 maintained by Dr. Thomson, that the upper air 

 cannot descend without undergoing condensa- 

 tion, and giving out a portion of its latent caloric, 

 which he thinks would prevent it from cooling 

 the atmosphere. (Treatise on Heat and Elec- 

 tricity, p. 129.) Whatever may be the just weight 

 of this objection, the fact is certain, that aqueous 

 vapour is suddenly and rapidly congealed by 

 very cold air ; by which the temperature of the 

 lower atmosphere is often reduced from 80 or 

 90 down to 40 F. within a few minutes, during 

 extensive and violent hail storms. 



