364 ATMOSPHERIC ELECTRICITr. 



This explanation is in accordance with the violent showers which 

 ordinarily attend the loud peals of thunder, but numerous objections 

 have been made to it whicli are difficult to answer in a satisfactory 

 manner. We do not see, for example, why the rapid condensation of 

 vapor into rain should not always be attended by thunder. On the 

 other hand, it has been established that there is sometimes formed, in 

 a perfectly clear sky, a single cloud, the appearance of which is imme- 

 diately followed by the rapid formation of several other clouds, which 

 spread in a short time over the whole extent of the sky without thunder 

 being heard. 



Other philosophers have sought for the cause of thunder in the 

 vacuum generated by the lightning in its passage through the air. 

 Kinnersley's thermometer and many electrical effects prove that the 

 electricity, in traversing the air, drives aside the portions which it 

 meets, producing a momentary void, into which immediately after the 

 surrounding air rushes, wi! n a violence which depends on the intensity 

 of the electricity. Hencj the prolonged noise of thunder is the report 

 from all points of the discharge. This explanation was combatted 

 by M. Pouillet,* who remarked that, if this were the cause of thunder, 

 the passage of a cannon ball in the air ought to produce a similar 

 noise, while there is heard from it only a sort of hissing. This 

 philosopher contends that when the spark passes between two bodies 

 the electric fluid experiences no movement of transfer like that of the 

 projectiles of ponderable matter, but that there are decomposition and 

 recompositions of electricity along all the path of the discharge, and 

 consequently there is a vibration more or less violent in the pon- 

 derable matter — a vibration which M. Pouillet compares to a kind of 

 tearing apart, or quick separation, as, for example, in the experiment 

 of the perforated card. According to him, the noise is the result of 

 this vibration which afterwards is propagated in all the surrounding 

 mass. 



On the subject of this explanation M. Becquerelf observes that 

 there is no proof of the agitation of the particles of the air being 

 sufficient to produce the noise of thunder ; and as for the objection 

 drawn from the motion of the ball, as the circumstances are not the 

 same, the velocity of the ball being appreciable, while that of elec- 

 tricity is not so, he thinks that the effects ought not to be alike in the 

 two cases. The velocity of electricity being nearly infinite compared 

 to that of the ball, the vibration of the air must be vastly more in- 

 tense, and there is nothing to prevent the contractions and expan- 

 sions of the air from producing detonations with reverberation. 



The explanation which attributes the sound of the thunder to the 

 sudden re-entrance of the air into the void produced by the lightning is 

 regarded by M. TessanJ as insufficient, and we owe to this obicrver some 

 new ideas on the formation of this sound. According to him a gaseous 

 conducting body, such as a cloud, changes its form and volume, and 

 which its surface is externally subjected from the surrounding dry air 

 consequently its density, when it is electrified, because the pressure to 



'' EMments de Physique et de Me'tcorologie, torn. II, page 809. 

 J Traite de rElectricite et dii Magnetisme, torn. IV, page 129. 

 :j: Comptes Rendus, torn. XII, p. 792. 1841. 



