M . Arago on Thunder. 8l 



rocks, events wliich must have taken place prior to the exist- 

 ence of our records. There is doubtless no difficulty in also ex- 

 plaining these phenomena through the agency of steam. Elie 

 de Beaumont,* however, is of opinion, that these elevations are 

 a consequence of the inequality between the cooling of the in- 

 terior and exterior of the earth. We shall examine this subject, 

 after pointing out the laws that prevail during the cooling of 

 large masses of fused matter. 



(To he continued.) 



On Thunder and Lightning. By M. Arago. 



M. Arago has lately published a valuable essay on thunder^ 

 for the Annuaire du Bureau des Lojigitiides, in which he 

 gives a masterly historical sketch of the real facts which have 

 hitherto been accumulated, and from these deduces the infer- 

 ences, scientific and practical, which may legitimately be drawn. 

 With the view of supplying as much of the valuable informa- 

 tion there coUectecf as our space will allow, we shall in this 

 number give the ge^j ral results of that elaborate review, from 

 which, as the authoi anticipated, various truths have been dis- 

 covered, which the examination of the isolated facts could never 

 have revealed. 



From the earliest times it has been known that sound wa9 

 not material. Thus, Aristotle knew perfectly that it was pro- 

 duced by simple undulations of the common air. At the pre- 

 sent time, this result may, without scruple, and with a single 

 modification, be extended to light ; for light also is the conse- 

 quence of the undulatory motion, not of air, but of a certain 

 very rare and very elastic medium which pervades the universe, 

 and which it has been agreed shall receive the name of ether. 



Are we to range the thunderbolt, whose presence is almost 

 always manifested simultaneously with light and sound, in the 

 same category ? Though free to declare myself the decided 

 partizan of the theory of the undulation of light, I must avow 

 I remain completely undecided on this other point. 



* Poggendorflf's Aunal. vol. xxv. p. 55. 

 VOL. XXVI. NO. LI. JANUARY 1839. F 



