M. Arago oil Lightning. 8S 



summed up, in a few words, the opinions more or less dis- 

 similar in form, but very analogous in fact, of the naturalists 

 of antiquity, concerning the origin of lightning : " Fire is pro- 

 duced by the percussion of flint and steel, and by the friction 

 of two pieces of wood ; it may happen, therefore, that the 

 clouds, hurried away by the wind, are likewise inflamed by 

 means of percussion and friction."" (Ques. N. liv. ii. § 22). I 

 must request those who are disposed to treat with contempt the 

 certainly far-fetched analogy we have just read, to consider 

 how many great gaps two thousand years have still left in the 

 explication of the phenomena which the celebrated author of 

 the Qucestiones Naturales had in view. 



The fulminating material, whatever the velocity of its pro- 

 pagation may lead us to suppose, does not move with an un- 

 defined freedom in solid bodies. This seems evidently proved 

 by the violent ruptures to which they are exposed, and the 

 wide dispersion of their fragments. What, then, more natural 

 than to suppose, that, in traversing the atmospheric air, this 

 matter pushes the particles of which it is composed violently 

 before it, whence compression successively results throughout 

 the entire line of projection ? Smart compressions, as proved 

 by the pneumatic syringe, are always accompanied with a dis- 

 engagement of light ; hence, the direction pursued by the ful- 

 minating matter ought to be marked by a luminous streak. 



This line of argument seems quite consistent, whilst, at the 

 same time, it is liable to several objections. For, first, if in 

 each point of the line which the lightning traces, it be necessary 

 for the disengagement of the light that certain considerable 

 volumes of air be sensibly compressed, then there is difficulty 

 in conceiving how all these displacements of particles can be 

 reconciled with the extraordinary velocity of lightning, as de- 

 monstrated by the experiments of Mr Wheatstone. Again, 

 the analogy derived from the pneumatic syringe, does not sup- 

 ply all the support that is desirable. In this beautiful instru- 

 ment, the atmospheric air is not the only element which plays 

 a part ; for the experiments of M. Thenard, in fact prove, that, 

 if the cylinder of the syringe be perfectly clean, and the felt of 

 the piston be moistened with water, and not with any greasy or 

 oily matter^ then the compression is not accompanied vriih the 



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