1840.] On Lightning Conductors to Powder Magazines. 307 



a shock will in greater quantity cause a spark, and if the spark 

 be but the tenth of an inch in length, it can inflame powder, 

 which is the great matter we have to guard against. 



37. In connexion with this subject M. Arago gives us some 

 very useful hints. 



A few detached sentences may be quoted to shew M. Ara- 

 go's ideas : — " Lightning once engaged in a metallic bar of suf- 

 ficient dimensions, and well constructed, does not quit it to 

 strike the materials of which buildings are usually constructed, 

 but in such small quantity that no injury can arise, nor even 

 any appreciable effect." 



38. M. Arago is here writing of ordinary buildings. But 

 what would produce no appreciable effect on these, would cause 

 the explosion of a magazine. 



39. M. Arago proceeds to ask, ^Should conductors be placed 

 within, or external to, buildings ?" The Board will see how this 

 bears on Mr. Harris' ship conductors, which run through the 

 after magazine. " I confess," says M. Arago, " that on this 

 point I would be much less affirmative. Voltaire used to say, 

 * there are some great lords not to be approached without ex- 

 treme precaution, and lightning is one of them.' I think the 

 illustrious author is perfectly right, especially when I recol- 

 lect the case of Mr. Raven's house, already alluded to. 

 Doubtless the conductor was not sufficiently thick ; but here 

 is an occurrence in which all was apparently in good or- 

 der, the conductors acting as well as could be desired, and 

 nevertheless there was a deviation of the electric matter. * * * 



"The 31st July 1829, in the Jail of Charlestown, at the 

 moment of an immense thunder clap, 300 persons received a 

 violent shock, the effects of which lasted for some seconds. * * 



" The jail had three good conductors, eighteen feet apart, the 

 building was untouched by the lightning." * * * (See the 

 Annuaire for 1838). 



40. How did the inmates receive this shock ? M. Arago 

 refers it to the large quantity of iron the building contained. 



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