Lesson xxn.] THUNDER 8c LIGHTNING. 139 



f 



stored between the two places of the earth. The 

 Lightning throws before it the narts of conducting 

 bodies, and distributes them along the resisting 

 medium through which it must force its passage ; 

 the longest flashes seem to be made, by forcing 

 in the way of the electric matter, part of the va- 

 pours of the air. 



The claps of Thunder which accompany the 

 flashes of Lightning, seem to be occasioned by the 

 filling of the vast vacuum made by the passage of 

 the electric matter: for, although the air collapses 

 the moment after the matter has passed, and the 

 vibration on which the sound depends commences 

 at the same moment : yet when the flash is di- 

 rected towards the person who hears the report, the 

 vibrations excited at the nearer end of the track 

 will reach his ear much sooner than those from the 

 remote end ; and the sound will, without any echo 

 or repercussion, continue, till all the vibrations 

 have successively reached him. 



It must be confessed, however, that the question, 

 how it happens that particular parts of the earth or 

 the clouds come into the opposite states of positive 

 and negative electricity, is not absolutely deter- 

 mined: among the numerous conjectures, the one 

 which supposes the electric matter then in the 

 clouds to be generated by the fermentation of sul- 

 phureous vapours with mineral or acid ones, seems 

 to have a great share of probability, especially 

 when it is recollected that Thunder-storms generally 

 happen when the air is in a sultry state. 



With 



