24 THUNDER-STORMS. 



either hat the tlmnder heard by Volney on that occasion was produced in the 

 elf ar atmosphere, or that it proceeded from a cloud not more than thirty-three 

 yards from the ground, at a distance of about fifteen miles from the observer. 



It has been elsewhere stated that the explanation proposed and universally 

 received as accounting for the phenomena, is a sudden displacement of the air. 

 produced by the electrical discharges, in which lightning is evolved. Since 

 all sound must proceed from an agitation of the air, and since lightning and 

 electricity are identified, this explanation consists of little more than a state- ( 

 merit of the facts. A more rigoro.us account, however, must be exacted from 

 those who would propound an adequate theory of thunder. 



Some have explained the origin of thunder, by supposing that the electric 

 fluid, in passing with great velocity through the air, leaves behind it a vacuum ; 

 that the air rushing suddenly into this vacuum produces a detonation like that 

 which takes place in the common experiment in which a vacuum being pro- 

 duced under a bladder extended tigh'ly over the mouth of a receiver, the blad- 

 der is broken by the pressure of the external air. To make this explanation 

 valid, it would be necessary to show how the vacuum is produced, or that it is, 

 injact, produced, otherwise the explanation is reduced to a mere conjecture. 



It is also explained by supposing that the electric fluid in passing through 

 the air, compresses successively the air lying before it, whence there results 

 a displacement of those masses of air which are contiguous, and consequently 

 a series of contractions and dilatations, which, extending to a distance, produce 

 long-continued reverberations. 



M. Pouillet rejects these hypotheses as insufficient to explain the phe- 

 nomenon. He considers that if such were the cause of thunder, the passage 

 of a cannon-ball through the air ought to produce a like effect. M. Pouillet 

 maintains that when an electric discharge takes place between two bodies 

 charged with opposite electricities, the fluid does not actually pass from the one 

 body to the other, but that the effect is produced by a series of decompositions 

 and recompositions of the natural electricities of the molecules of the inter- 

 vening medium, precisely similar to that which takes place in a liquid solution 

 in which the poles of the Voltaic arrangement are immersed. He argues that 

 there must thence result vibrations more or less violent in the ponderable mat- 

 ter of that medium, which would be sufficient to explain the sound. 



The rolling of thunder has by some been ascribed to the effect of echo. That 

 echo has in some cases a share in the production of the phenomena cannot be 

 doubted by any one who has ever witnessed an Alpine storm. A multitude of 

 causes affecting the loudness, the reverberation, and the continuity of the peals, 

 are quite apparent. The question is whether echo is the only cause of the 

 rolling thunder. 



It has been shown that the duration of the thunder-roll amounts sometimes 

 to forty-five seconds. Whether the echoes of any sound ever have such dura- 

 tion, can only be determined by observation. The example of the often-re- 

 iterated echo at a certain island on the lake of Killarney, is known to all travel- 

 lers. Mr. Scoresby observed on a particular occasion its duration, and found 

 it about thirty seconds. The original sound is usually produced by the dis- 

 charge of a small piece of cannon. 



It would seem that on the occasion of Mr. Scoresby's observations, a pistol 

 was used. It is argued by M. Arago, that if a cannon had been used, the du- 

 ration would have been much greater, and probably equal to the continuance 

 of the longest roll of thunder. 



During the experiments made to determine the velocity of sound in June, 

 1822, MM. Humboldt, Bouvard, Gay-Lussac, and Emile de Laplace, heard 

 the echo of a cannon discharged near them during twenty-five seconds. 



