90 Sheet Lightnivg, 



Are they white, or rather, are they of the same shade as the 

 lightning itself? Then we may certainly conclude that we 

 have seen direct light, which has not reached the eye in the way 

 of reverberation ; in a word, that the light has emanate dj'rom 

 that part of the atmosphere which is situated above the horizon. 

 On the contrary, is it that the two disks appeared coloured ? 

 This is a proof that the light enclosed by the crystals in the 

 tube has undergone a kind of analysis, and is reflected light ; 

 in short, that it proceeds from lightrmig which has been playing 

 below the visible horizon. By measuring the intensity of the 

 colour exhibited by the disks, we shall be able, without much 

 difficulty, to decide which region of the atmosphere is the seat 

 of this kind of lightning. But here I must guard against en-, 

 tering into minute details ; I have said enough to shew how, 

 by means of a very simple observation, we may dispel all the 

 doubts which the question concerning sheet lightning has ori- 

 ginated. 



If little attention is at the present day paid to that noiseless 

 lightning which is produced amo?igthe clouds, it arises from this 

 circumstance, that the only explanation which has been given 

 of that lightning, how improbable soever it may be, implies the 

 production of sound, at least as inevitably as it does that of 

 light, as the result of the physical agencies it brings into play. 

 Moreover, there is no expediency in having recourse to the 

 enormous distances of the thunder-clouds above alluded to, 

 whilst we have yet to learn why sometimes we hear absolutely 

 no detonation after very dazzling flashes of lightning. There 

 is, in truth, no justification for the introduction of these im- 

 mensely distant sources, which in no case will explain the obser- 

 vation made hy Deluc in which flashes of the same intensity and 

 emanating from the same clouds, were followed, some of them, by 

 stunning thunder-claps, and others with absolute silence. In 

 addition, should any one require a proof that every noise in the 

 atmosphere is not necessarily accompanied with the production 

 of light, he may have it in the fact that zvater-spouts are some- 

 times the focus of very brilliant lightnings. On the 4th of 

 June 1814, M. Griswold found himself very near (within 430 

 yards) one of these meteors in the territory of Illinois. Light- 

 ning which was almost continuous, and of incomparable briU 



