366 ATMOSPHEBIC ELECTEICITY. 



seen at the zeuitli. A number of physisists likewise regard tliis phe- 

 nomenon as existing by itself indepgndently of sforms.* 



Besides these flashes of heat lightning, frequently in cloudy weather 

 very bright flashes may be seen to pass through the clouds without 

 being followed by any noise. f Perhaps it might bo said that the storm 

 is at too great a distance from the observers to hear the thunder, since, 

 according to the calculations of M. Aragot a distance of three, four or 

 five leagues is sufficient wholly to deaden the noise of the detonations 

 of the lightning. But to this it may be answered, that during the 

 same storm and in the same clouds frequent flashes of lightning are 

 observed to gleam forth, a small number of which only are accom- 

 panied b}'' thunder. It is true that M. Delez3nne§ has given, in a 

 similar case, an explanation, based on the phenomenon of interference 

 of the waves of sound ; but if this explanation is applicable to the case 

 in which a flash of lightning without thunder is followed by another 

 accompanied with noise, we cannot see how it can be admitted when 

 numerous flashes of lightning succeed each other in silence. Now if 

 it were Avell established that there are flashes of lightning without 

 thunder, would not their existence be irreconcilable with the expla- 

 nation which regards the sudden re-entrance of the air into the void 

 left by the flash of lightning as the physical cause of thunder? Obser- 

 vations on cases of lightning supposed to be witliout thunder, giving 

 the heights and distances of the clouis, in which they appear, might 

 therefore throw some light on the question of the theory of thunder. 



Another fact which, if it were confirmed by new observations, 

 would be no less important in regard to this theory, is that of 

 the hissing noise by which a flash of lightning was attended, in a 

 storm whicli raged on the road from Avignon to Remoulin on the night 

 of the 18th or 19th of September, 1840. "A flash of lightning," 

 says M. Tessan|| in reporting this observation, "'caused a loud hissing 

 sound, attended by a terrific crash of thunder. The flash, the hissing, 

 and the crash appeared to me simultaneous, as well as to two persons 

 v/ho were with me in the front part of the stage coach ; I, however, 

 believe that it was rather in the order just mentioned in which the 

 sensations succeeded each other." 



Whatever explanation of thunder we adopt, we must not neglect 

 to take into account two remarkable circumstances, which are its 

 long duration and the successive decrease as well as increase of intensity 

 which are so frequently renewed during the reverberations of the same 

 peal. In discussing the point as to what effect echoes may be considered 



•~ Schiibler thinks tliat the flashes of heat lightning are only irradiations of insulated 

 clouds, surcharged with electricity. M. Mattenci has sought for the cause in the electricity 

 accumulated at the surface of cert.ain portions of the earth which have become insidated, 

 either bj' the nature of the soil which compose them, or by the effect of the evaporation 

 which has dried them up. Finally, Bnmdes believed that the phenomenon of heat light- 

 ning has some relation to the falling stars. — See Lehrbuch der ileteorologie von Kaemtz, 

 torn. II, p. 48'i ; Eiblioth&que Universelle, Sciences et Arts, tom. XLII, p. 9. l->29 ; and 

 Beitrage zur Witterungskunde, p. 354. 



-j- Annuaire pour 1838, p. 295 ; Aunales de Poggendorf, tom. XLIII, p. ^31; 1S38, &c. 



JAnnuaire pour 1833, p. 442. 



§Traite de Mcteorologie, cu Physique du Globe, par Garnier, tom. I. p. 369. 



IIComptesKendus, tomiXII, page 702. 



