on the Specific Gravity of aqueous Vapours. 195 



be observed that the formula I have given would bring out 

 results conformable to the indications of an air thermometer. 

 This is of little importance in temperatures beneath 212°; but 

 iia very high ranges, in which at least one experiment ought 

 to be made, it would be of vital consequence. 



I am, dear sirs, 



Your obedient humble servant, 



J. Herapath. 



XXXV. On simultaneous Thunder-Stoi-ms. Bi/Dr.T. Forster. 



To the Editors of the Philosophical Magazine andjourmal. 

 J Hartwell, Sept. 3, 1822. 



Gentlemen, — 1 have already stated my opinion that the 

 foiunation of thunder-storms, and other electrical phaenomena, 

 frequently took place simultaneously in very distant parts of 

 the atmosphere. I have lately had an opportunity of con- 

 firming this opinion, and of witnessing several of the most vio- 

 lent instances of storms which have ever been recorded in the 

 memory of the inhabitants of those districts where they oc- 

 curred ; and of comparing them with sunilar phaenomena in 

 distant parts of Europe. 



On Monday evening, July tlie 29th, while ti'avelling fix)ni 

 Gex to Nyon, on the Lake of Geneva, we were overtaken by 

 one of the most violent storms I ever remember to have seen : 

 it formed very rapidly, for at half-past four o'clock tlie atmo- 

 sphere was clear. I observed however that on descending 

 Mount Jura, the Alps (which form a beautiful back-ground to 

 the view of the lake below) were intersected with clouds : the 

 cirroslratus seemed to unite the summits of the range of 

 mountains by forming long lines of cloud stretched across 

 from one hill to another, while the more elevated gi'ound of 

 Mont Blanc was involved in cumidostratus. The air very sud- 

 denly became totiilly obscure, and before 5 o'clock a violent 

 shower of rain and hail came down in torrents, accompanied 

 by thunder and lightning: it cleared ofi" very suddenly, but as 

 tjuickly returned again ; and for the space of nearly an hour 

 there was scarcely an interval of two minutes between the most 

 vivid flashes of lightning. I distinctly noticed the two sorts 

 nu'nlioned by M. Van Mons; namely, the fidguntf ion followed 

 quickly by a short and loud clap; and ihc f/diiiitialion fol- 

 lowed at a longer interval by rolling thunder, llic fulgura- 

 lion or foiki'd liglilning darted into the lake; from several di- 

 .""lant j)ortions of cloud at once, and was al times of a bright 

 IJ b 2 blue 



