r 



{ 518 



THUNDER-STORMS. 





) De L'Isle also observed the varying intensity of the sound in each roll. In 

 i some cases the clap is loudest at the commencement, and afterward declines 

 ) gradually until it ceases to be heard. Sometimes it commences with a low and 

 barely audible sound, which augments in force until it attains a maximum loud- 

 ness, after which it diminishes gradually in intensity until it becomes inaudible. 

 These changes were carefully observed and recorded on several occasions by 

 De L'Isle. The following examples will serve to illustrate the phenom- 

 enon : 



1712 Seconds. 



17th of June, Lightning flashed. 



3 Thunder feebly audible. 



12 Thunder loudest. 



19 Thunder became gradually inaudible. 



21st of July, Lightning flashed. 



16 Thunder feebly heard. 



20 Thunder loudest. 



32 Thunder became gradually inaudible. 



8th of July, Lightning flashed. 



11 Thunder feebly heard. 



12 Thunder loudest. 



38 Loudest thunder began to decrease in force. 

 47 Thunder became gradually inaudible. 



8th of July, Lightning flashed. 



11 Thunder feebly heard. 



12 Thunder became loudest. 



38 Thunder began to decrease in loudness. 

 47 Thunder became gradually inaudible. 



8th of July, Lightning flashed. 



10 Thunder feebly heard. 



13 Thunder became loud. 



20 Thunder broke with redoubled force. 

 35 Thunder began to lose its force. 



39 Thunder became gradually inaudible. 



It appears from these observations that the durations of the loudest part of 

 each roll varied from twenty, to thirty seconds. 



The degree of loudness is also very various. On the 2d of March, 1769, 

 the tower of the church at Buckland Brewer was struck by lightning, followed 

 by a clap of thunder described by an ear-witness as equal to the simultaneous 

 report of one hundred pieces of cannon. 



The most violent thunder sometimes follows ball-lightning. When the ship 

 Montague was struck, on the 4th of November, 1749, the captain (Chalmers) 

 declared that the sound produced by the explosion was equal to the simulta- 

 neous discharge of several hundred pieces of ordnance, but that it did not last 

 above half a second. 



The interval of time which elapses between the flash, of lightning and the 

 thunder which succeeds it is an important element in the theoretical investiga- 

 tion of the atmospheric conditions which produce these phenomena. It is es- 

 pecially useful to ascertain the major and minor limits of this interval. The 

 observations of this kind collected by M. Arago are arranged in the following 

 table : 



