THUNDER-STORMS. 



5i; 



M. Arago states, as the general impression on his memory, that he has often 

 observed the thunder follow the flash after an interval so brief as half a second. 



In the early part of June, 1841, being in the reading-room of the Alfie- 

 iianm at Philadelphia, I witnessed a vivid flash of lightning which was suc- 

 ceeded by the loudest clap of thunder I ever recollect to have heard. The in- 

 terval was, by my estimation, a very small fraction of a second. An ordinary 

 observer would have said that the flash and the sound were simultaneous 



The occurrence of thunder not preceded by lightning has not been proved 

 by evidence as clear and satisfactory as that by which the existence of silent 

 lightnings have been established. No example is found of it in any of the me- 

 teorological registers kept at observatories in Europe. Tkibanlt de Chanvalun, 

 already quoted, mentions in the register of his observations made at Martinique, 

 that in October, 1751, there were two days on which thunder was heard with- 

 out the appearance of lightning ; and that on one day in November there were 

 three loud claps of thunder without lightning. 



On the 19th of March, the vessel in which Bruce the traveller had embarked 

 on the Red sea, near Cosseir encountered a clap of thunder so violent as to 

 strike the seamen with terror. There was no lightning. 



The occurrence of thunder when the firmament is cloudless has been doubted. 

 SENKBIER speaks of thunder on clear days as a known fact, but does not state 

 whether such was the result of his own observations. VOLXEY states, that on 

 the 12th of July, 1788, at six o'clock in the morning, the sky being unclouded, 

 he heard at Pont Chartrain, a place four leagues from Versailles, four or five 

 claps of thunder. At a quarter past seven clouds began to rise in the south- 

 west, and in some minutes the heavens were covered. Soon afterward hail- 

 stones fell as large as a man's fist. 



The noise which often attends earthquakes is similar to thunder, and by an 

 acoustic deception not yet clearly explained, it is heard as if it proceeded irom 

 the upper regions of the air. Observations, therefore, of supposed thunder 

 with a clear sky, in places subject to earthquakes, cannot safely be received as 

 evidence of real thunder. 



THE ATTEMPTS TO EXPLAIN THE PHENOMENA OF THUNDER AND LIGHTNING. 



Although the investigations of Franklin removed all doubts respecting the 



identity of lightning and artificial electricity, still, in the great variety of atmo- < 



spheric phenomena developed in the disturbances of electrical equilibrium which ( 



are produced on so grand a scale in the vast regions of the air, much remained / 



