296 On Lightning Conductors to Powder Magazines. [No. 99. 



was exactly what it ought not to have been, and the explosion 

 which occurred, by no means invalidates the position, that the 

 well constructed magazine has but an infinitely small chance of 

 being struck by lightning. 



8. The questions now arise — First, Would even this minute 

 contingency be obviated thoroughly by a lightning conductor 

 being attached to the magazine, on the method advised by the 

 Honorable the Court of Directors ? Secondly, Can the conductor 

 itself by possibility become a source of collateral danger ? 



9. I will take up each of these questions in detail. I grant 

 in the first place, as the foundation of the argument, that 

 metallic conductors have the power, when properly placed, of 

 silently drawing off considerable accumulations of electricity 

 from the clouds ; and, secondly, of guiding away to the earth 

 considerable direct explosive discharges or flashes of lightning, 

 without permitting the electric matter, whatever it be, to im- 

 pinge directly on any adjacent bodies. 



10. The extent to which the protecting influence of a con- 

 ductor extends laterally, has long been a subject of attention 

 and discussion. Leroy, in 1783, asserted that a rod four to 

 five metres high, above the roof of an ordinary building, de- 

 fended a circle of sixteen metres radius, or more than three 

 times the distance of its own elevation above the roof. 



11. The Academy of Sciences in 1823, in a report to the 

 Minister of War, adopted the opinion of M. Charles, that the 

 circle protected was of a radius double the total elevation of 

 the conductor above the roof. This opinion seems to have 

 been generally adopted, but must be modified in consideration 

 of the facts which M. Arago has collected, and some which 

 have come under my own observation. 



12. If masses of metal of any kind enter into the construc- 

 tion of a building, the protecting influence does not extend to 

 the distance above mentioned. The powder magazine of Pur- 

 fleet, provided with a conductor erected by Franklin and 

 Cavendish, was struck by lightning twenty-four feet from the 



