ATMOSPHEEIC ELECT2ICITY. 365 



decreases in proportion as the electrical charge increases, and because 

 the expansion of the electrical clouds is always such tint their own 

 elastic force, added to the expansion of the electric fluid, is found to be 

 in every point equal to the external atmospheric pressure. Now, if in 

 this state an electric discharge bursts from a cloud, the external air 

 being no longer retained by the expansive force of the electric fluid 

 which produces the equilibrium will rush from all parts towards the 

 cloud. There will be a condensation and a driving back of the sur- 

 rounding particles, and consequently a report. Such would be, accord- 

 ing to M. Tessan, the explanation of thunder, and the shower that 

 follows it. 



A question, the solution of wdiich possibly may not be without 

 some importance in the explanation of thunder, is the existence of 

 flashes of lightning without noise. Many observations tend to prove 

 its existence ; and first, do we not often perceive, even in a clear sky, 

 in the beautiful nights of summer, gleams more or less vivid which 

 are termed heat lightning, and which are not distinguished from 

 the flashes of ordinary lightning except that they are not attended 

 by thunder ? It is true some philosophers regard these flashes of 

 heat lightning as the reflection on atmospheric strata more or less 

 elevated, of the flashes of common lightning produced in thunder clouds 

 hidden by the convexity of the earth. Experiments mentioned by M. 

 Arago* prove tliat the feeble light which results from the kindling of 

 several ounces of powder is reflected very perceptibly by the atmo- 

 sphere ; we may then admit that the reflection, infinitely more bright, 

 of distant flashes of lightning must produce similar effects. Moreover, 

 we may observe that there exist observations which directly demon- 

 strate the possibility of this explanation, We find in De Saussure'sf 

 work that on the night of the 10th or 11th of July, 1783, this philo- 

 sopher, finding himself at the hospital of the Grimsel, with a calm and 

 clear sky, saw near the horizon, in the direction of Geneva, several 

 bands of clouds from which darted forth flashes of lightning that pro- 

 duced no noise. Now, the same night and at the same moment the 

 city of Geneva experienced a most terrific storm. But if heat light- 

 ning appears sometimes to derive its origin from atmospheric reflec- 

 tion, observation shows that in many cases we cannot attribute it to 

 distant storms. Among others, Schiiblerl states that in some parts 

 of Wurtemberg, over an extent of more than 400 square leagues, there 

 were observed on the night of the 26th of August, 1823, from nine to 

 eleven o'clock, flashes of lightning in a sky perfectly clear, without the 

 slighest storm appearing in that whole extent of coimtry. Besides, the 

 fact that heat lightning is sometimes seen entirely around the horizon, 

 may alone suffice to demonstrate that it is impossible always to explain 

 it thus; for it would be necessary to suppose that storms exist at the 

 same instant and in all directions, from the centre at which the ob- 

 server is placed in a calm and clear atmosphere, a supposition which 

 is scarcely admissible. We may remark again that if heat lightning 

 ordinarily shows itself on the edge of the horizon, it is also sometimes 



»• Annuaire pour 18o8, p. 429. f Voyages dans les Alpes, torn. Ill, vj 1700, p. 470. 

 J Journal de Schweiggcr, torn. XXXI, p. 3'J. 1821. 



