CHAP. 2. 11. OF CLOUDS. 71 



SECTION XL 



Of Thunderstorms. 



THE paper of M. B. P. Van Mons, reprinted 

 in Nicholson's Phil. Jour. Sept. 1809, induced 

 me to observe accurately the two different kinds 

 of lightning therein mentioned. I will not 

 venture to speculate on their causes, referring 

 for them to his paper, but shall proceed to state 

 the difference. One kind is a vivid flash, shortly 

 afterwards followed by a loud clap of thunder, 

 resembling the sound of the discharge of a 

 mortar or cannon. This is found to be the mis- 

 chievous kind, and is attributed to the discharge 

 of the fluid analogous to the flying off of the 

 electric spark. The other kind, ascribed by 

 M. Van Mons to the combustion of the gases of 

 water, and is not so vivid, but has more lititude 

 of light and is followed by rolling thunder.* 

 These two sorts often alternate in the same 

 Storm. But it is often the case in other 

 Storms that none but the rolling thunder is 

 distinguished. The vespertine fulgurations, 



* Two kinds of lightning are mentioned by several ancient 

 writers. Consult Seneca, Nat. Quaes. lib. ii. sc. 16. 20. 



