72 



THE EFFECTS OF LIGHTNING. 



deposited in the mould in which the liquid metal is poured, the most terrible 

 explosions have taken place at the moment the metal comes in contact with 

 the water. Admit that humidity is found in the fissures and cells of the blocks 

 of stone which form a building, and if the thunder strikes this stone, the sud- 

 den production of vapor within it would break it, and its fragments would be 

 projected in all directions. In like manner, the sudden formation of vapor in 

 the ground beneath the foundations of the walls of a house would be sufficient 

 to raise the walls in a mass, and transport them to a distance. The circum- 

 stances attending the action of lightning on trees are still more easily explica- 

 ble by M. Arago's hypothesis, since the sap and vegetable juices, being placed 

 in lines parallel to the direction of the fibres, the vapor which would be formed 

 would split them in pieces exactly in the manner in which trees are observed 

 to be split by lightning. 



This explanation, ingenious as it is, is not free from objection. That water 

 may be suddenly and strongly heated by lightning when the body which con- 

 tains it is a conductor of heat, may be admitted. But when lightning strikes 

 a large block of stone, the heat must penetrate its dimensions before it can 

 reach the water which may be contained within them ; but stone being almost 

 a non-conductor of heat, its surface might be fused, while its internal dimen- 

 sions would not suffer a sensible elevation of temperature, especially when the 

 stone is exposed to the source of heat only for an instant. Wood is also a bad 

 conductor of heat, yet M. Arago's hypothesis seems to require the admission, 

 that a tree struck by lightning is heated sufficiently to produce aqueous vapor 

 of enormous elasticity, without producing the combustion, or even the carbon- 

 ization of the wood itself. The soil, or earthy matter at the surface of the 

 ground, is also almost a non-conductor of heat, yet M. Arago requires the 

 admission, that the lightning acting on it produces a vapor from water below it 

 of sufficient pressure to lift the wall of a house and project it to a distance. 



None of these difficulties appear to attend the supposition that the natural 

 electricities of non-conducting bodies, being forcibly decomposed by the prox- 

 imity of the electric fluid which forms the lightning, and which may be con- 

 ceived to have an almost infinite intensity, their violent separation resisted, as 

 it would be, by the non-conducting quality of the bodies themselves, would be 

 attended with all the effects which M. Arago ascribes to the sudden formation 

 of vapor, without any of the difficulties or objects which are involved in that 

 supposition. 



If the electricity projected from the thunder-cloud be supposed to be positive, 

 that of the ground which it approaches will necessarily be negative, and more 

 intensely negative the more intensely positive is the electricity coming from 

 the cloud and the more nearly it approaches the ground. 



Whatever hypothesis may be adopted to explain the facts, the terms ascend- 

 ing and descending lightning may be allowed, if they be understood to refer to 

 the direction in which the electricity is propagated, as manifested by its effects. 

 Facts are not wanting to indicate the progress of the electric influence upward. 



On the 24th of February, 1774, lightning struck the steeple of the village of 

 Rouvroi, to the northwest of Arras. A pavement composed of large blue 

 stones, which was laid under the steeple, was violently raised upward. 



In the summer of 1787, lightning struck two persons who took refuge under 

 a tree near the village of Tacon in Beaujolois. Their hair was driven upward 

 and found upon the top of the tree. A ring of iron which was upon the shoe of 

 one of these persons was found afterward suspended on one of the upper branches. 



On the 29th of August, 1808, lightning struck a small building near the hos- 

 pital of Salpetriere in Paris. A laborer who was in it was killed, and, after 

 the event, the pieces of his hat were found incrusted on the ceiling of the room. 



