THE EFFECTS OF LIGHTNING. 



71 



original place ; one of its ends was transported nine and the other four feet. 

 This wall thus raised and transported, was composed of seven thousand bricks, 

 which, independent of the mortar by which they were cemented together, 

 would have weighed about twenty-six tons. This wall was eleven feet high 

 and three feet thick, and its foundation was about a foot below the level of the 

 ground. Above this coal-shed was a cistern, which, at the' time of the phe- 

 nomenon, contained a quantity of water, and the shed contained about a ton of 

 coals. 



If these mechanical effects could be explained by supposing them to be pro- 



duced by the moving force of the electric fluid itself impinging on the bodies 



which are struck, no difficulty would arise from the extreme lightness and 



tenuity of the electric fluid, for the momentum of a body depends as much on 



its velocity as on its weight, and however subtle the electric fluid may be, it is 



possible to imagine a velocity by which it may acquire any proposed moving 



force. There are, however, circumstances among the observed effects, which 



cannot be explained by the mere impact of any fluid upon the bodies struck. 



One of those is, that the fragments of bodies struck by lightning are usually 



dispersed in all directions, and this is the case even when the fragments are 



large and heavy masses. If the pinnacle of the church at Breag had been. 



struck by the mechanical force of a body moving in a determinate direction, it 



could not have happened that two large and heavy masses of stone would be 



driven, one to a distance of sixty yards south, and the other four hundred yards 



north. If the circumstances attending bodies struck by lightning be attentively 



considered, it will be apparent that they are such as would be produced by a 



I force suddenly called into action, and directed outward from the internal di- 



1 mensions of the body, so as to burst it in pieces. If the approach of lightning 



! could be shown to be capable of producing, instantaneously, within a body, a 



| highly elastic fluid, such a fluid, in exerting an outward pressure, would burst 



! the body, exactly as the explosion of gunpowder forces out the ball, or failing 



| to do so, bursts the gun. 



From what has been established respecting the action of free electricity, it 

 1 is evident that lightning will decompose the natural electricities of any bodies 

 ! which it approaches, drawing toward itself the fluid of its own name, and re- 

 ' pelling to the more remote parts the contrary fluid. If the body be a conduc- 

 I tor, this decomposition will take place, and the free electricities of opposite 

 [ names will be accumulated on opposite sides of it, and when their tensions ex- 

 ! ceed that of the atmosphere, they will escape. If it be not a conductor, then 

 [ the natural electricities, being forced asunder by the inductive action of the 

 ! lightning, may produce the effect of a confined elastic fluid, and a separation 

 | of the parts of the body will be the consequence. 



The hypothesis proposed by M. ARAGO, to explain the mechanical effects 

 I of lightning refers their origin to the water, or other fluids contained in the 

 pores of the body on which the lightning acts. Lightning is proved by obser- 

 J vation to evolve heat sufficiently intense to reduce metallic wires suddenly to a 

 state of incandescence. M. Arago argues, that it may therefore be reasonably 

 { inferred that it may also produce a like effect on the minute threads of water 

 $ which pervade the interstices of certain bodies. By the experiments of MM. 

 { Dulong and Arago, the elasticity of steam at the temperature of 500 degrees 

 J Fahr. amounts to 45 atmospheres. But this temperature is much less than 

 \ that of red-hot iron. It may therefore be inferred that any small portions of 

 > water contained in the pores of bodies, which suddenly acquire as much b 



< as would render iron red-hot, must acquire an elastic force so enormous as t 

 } be capable of producing any of the mechanical effects which have ensued 



< lightning. In foundries, where a small quantity of water has accidentally b< 



