82 M. Arago on Lightn'mg. 



When, on the one hand, I consider the observations of Mr 

 Wheatstone as completely established, and when I direct my 

 attention to the incomparable velocity with which the thunder- 

 bolt traverses the aerial regions, and the solid bodies which 

 propagate it at the surface of the earth, I feel myself but little 

 inclined to regard it as composed of an agglomeration of mate- 

 rial molecules, or a mass of minute projectiles, and the idea 

 of undulation seems most readily to comport with such extra- 

 ordinary velocities. But no sooner, on the other hand, have I 

 reached this conclusion, than the prodigious mechanical effects 

 produced by thunder storms, and the transport of the heavy 

 substances thereby effected, rush to my recollection. And when 

 to this I add, that, despite of all the delicacy of the methods 

 employed by operating upon levers suspended in vacuum, with 

 threads of gossamer, with light concentrated to a focus by the 

 largest mirrors and most powerful lenses, the slightest devia- 

 tions have not been produced, then all my uncertainty returns, 

 and the undulations of the thufiderbolt appear enveloped in ten 

 thousand difficulties. We hasten, therefore, to a rapid examina- 

 tion of the principal phenomena brought under review. 



Of' Lightning, 



The Etrusci, whom all antiquity has celebrated for their 

 knowledge of the subject of lightning, divided it into three kinds. 

 The first, according to them, was lightning of information or 

 advice ; the second produced a certain degree of mischief; and 

 the third was composed of destructive fire, which struck par- 

 ticular individuals, ravaged kingdoms, and left nothing which 

 it encountered in its previous condition. Jupiter, they say, 

 sported with the first at his w'ill ; the second left not his hand 

 except at the direction of a council composed of twelve great 

 deities ; and finally, the third imperiously required a decree of 

 all the superior gods. It is not easy to imagine how a people 

 who entertained such ideas should ever thinii of inquiring how 

 thunder was engendered in the clouds, how it produced light, 

 and was accompanied with noise. Yet, notwithstanding, these 

 inquiries occupied a large space in the works of Aristotle, in 

 the poem of Lucretius, in the writings of Pliny, and in the 

 Qiiaestiones Naturales of Seneca. This last philosopher has 



