ATMOSPHERIC ELECTRICITY. 359 



a horizontal flash of lightning and the commencement of the report, 

 and he found that the angle emhraced by the two extremities of this 

 flash was 75°. By means of these data he calculated the length of 

 the flash observed to be more than 8,932 metres — that is to sa}^, more 

 than 2 leagues. M. Arago,* in his notice of thunder, points out a 

 very ingenious method of oljtaining if not the true lengths of the flashes, 

 at least the limit within which these lengths must be contained ; 

 but this method rests on the hypothesis that the duration of the 

 peal of thunder is the time which its sound requires to pass through a 

 space equal to the diflerence in length of the two lines, drawn from 

 the ear of the observer to the two extremities of the flash. In apply- 

 ing it to some observations, M. Arago found for certain flashes the 

 lengths of at least 3.3, of 3.4, and 3.8 leagues. So great a length, 

 naturally leads us to admit that the electricity vrhich forms the flash 

 is incomparably greater than that which can be accumulated in the 

 most powerful batteries. But M. day Lussacf observes that we cannot 

 really judge of the relative intensity of the electricity accumulated on 

 our conductors and in a thunder cloud by the length of the sparks, since 

 the former are very good conductors of electricity, and are surrounded 

 by air, which in its usual state has little conducting power, while the 

 clouds are tolerably good conductors, and the surrounding air, rarified 

 and saturated Avith moisture, nossesses in a considerable degree the 

 power of conducting electricity. The portion of vapor with which 

 the air enveloping the cloud is probably more or less charged appeared 

 to M. Pouillet| capable of serving as a medium to produce a discharge, 

 in the same manner that metallic powders, spread on woolen or silk 

 cloth, cause the spark of our machines to be projected to a greater dis- 

 tance. In order to explain the length of the discharge of lightning he 

 supposes that in the course v/hich it is to take there are patches of vapor, 

 and possibly even those of air, already electrified by the contrary 

 induction of the electricities which have a tendency to rueh against 

 each other, and that at a given moment the equilibrium is finally 

 broken without there having been any transfer of the fluid of one 

 cloud to another^ but simply the successive transfer of the electricity 

 of one patch to another over the whole path of the discharge. 



M. Lara6§ explains the length of the flashes of lightning by supposing 

 that it may be the result of the small excess of the conductibility of the 

 clouds over that of the air between them, and consequently the free 

 electricity of feeble tension, diffused at their surface, may be discharged 

 between two clouds at several points at once, and successively between, 

 different portions of each one of these clouds. 



It is known that the zig-zag form is common to the flash of light- 

 ning and to the electric spark. Helvig]| sought to determine the exact 

 form by means of a camera lucida, and, according to this observer, it 



~' Anmiaire pour 1838, p. 459. 



t Annales de Chimic et dc Physique, torn. XXIX, p. 105. 1825. 



J Elemeuts dc rh}-«i(|ue ct dc Mctcorologie, torn. II, p. 808. 



§ Cours de Physicpie do Ecole Folytechnir[ue, torn. II, 2i partie, p. 82. 



Ij Annalcs de Gilbert, torn. LI, p. I'd'J. 



