EFFECTS OF LIGHTNIKG. 



It is considered generally that the range of protection of a 

 paratonnerre is a circle round its base, whose radius is two or 

 three times its length. 



22. The effects of lightning, like those of electricity evolved 

 by artificial means, are threefold, physiological, physical, and 

 mechanical. 



When lightning kills, the parts where it has struck bear the 

 marks of severe burning ; the bones are often broken and crushed 

 as if they had been subjected to violent mechanical pressure. 

 When it acts on the system by induction only, which is called the 

 secondary or indirect shock, it does not immediately kill, but 

 inflicts nervous shocks so severe as sometimes to leave effect* 

 which are incurable. 



The physical effects of lightning produced upon conductors is- 

 to raise their temperature. This elevation is sometimes so great 

 that they are rendered incandescent, fused, and even burned. 

 This happens occasionally with bell- wires, especially in exposed 

 and unprotected positions, as in courts or gardens. The drops of 

 molten metal produced in such cases set fire to any combustible- 

 matter on which they may chance to fall. Wood, straw, and such 

 non-conducting bodies are ignited generally by the lightning 

 drawn through them, by the attraction of other bodies near them 

 which are good conductors. 



The mechanical effects of lightning, the physical cause of which 

 has not been satisfactorily explained, are very extraordinary.. 

 Enormous masses of metal are torn from their supports, vast 

 blocks of stone are broken, and massive buildings are razed to the 

 ground. 



23. ETo theory or hypothesis which has commanded general 

 acceptation, has yet been suggested for the explanation of the 

 Aurora Borealis. All the appearances which attend the pheno- 

 menon are, however, electrical ; and its forms, directions, and 

 positions, though ever varying, always bear a remarkable relation 

 to the magnetic meridians and poles. Whatever, therefore, be 

 its physical cause, it is evident that the theatre of its action is the 

 atmosphere ; and that the agent to which the development is due, is 

 electricity, influenced in some unascertained manner by terrestrial 

 magnetism. In the absence of any satisfactory theory for the 

 explanation of the phenomenon, we shall confine ourselves here to 

 a short description of it, derived from the most extensive and 

 exact series of observations which have been made in those 

 regions, where the meteor has been seen with the most marked 

 characters and in the greatest splendour. 



24. The Aurora Borealis is a luminous phenomenon which 

 appears in the heavens, and is seen in high latitudes in both 



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