Lightning-Conductors in Ships. 307 



passive ; that is to say, the removal of resistance : indeed, in the 

 case of a vacuum, or rather a very finely exhausted medium, 

 which is found to answer the same purpose as a conducting 

 body, since the electric discharge is freely transmitted through 

 it, we must necessarily admit the truth of the above principle ; 

 the conducting power here evinced must arise solely from the 

 removal of a resisting medium ; for what is equivalent, in a com- 

 parative point of view, to the absence of all substance, cannot 

 be supposed to be endowed with any peculiar or positive quali- 

 ty. Now, the circumstances attending the conducting power 

 being precisely the same, whether we suppose the latter to be 

 peculiar to a void, or to a positive substance, it is a legitimate 

 deduction, and not contradicted by any known fact, that, in 

 either case, the conducting power is dependent on the same 

 cause, and is therefore a negative quality. In further confir- 

 mation of this notion, we find that an artificial discharge will 

 rather jump over an interval of air than pervade a very exten- 

 sive circuit of metallic wire ; that is to say, when the resistance 

 of the metal becomes greater than that offered by the interval 

 of air, the electric matter will no longer pass in the best conduc- 

 tor, for it is no longer the line of least resistance. 



29. With respect to the actual quantity of electric matter 

 which may possibly be discharged in a thunder-storm, and the 

 effect likely to be produced on lightning-rods ; that must alto- 

 gether be determined by experience. It is by no means con- 

 tended that lightning-conductors operate as a charm or 7iostrumj 

 but that they are a useful means of defence against such cases 

 of damage as come within the experience of mankind, not 

 against convulsions of nature, when it would not be of great con- 

 sequence whether we had lightning-rods or not. It is therefore 

 against such cases of damage as may be reasonably expected to 

 occur, that we purpose to employ lightning-rods. Now, we 

 have the experience of nearly a century to guide us in this ; 

 and from which we have every reasonable demonstration that 

 our proposed conductor is more than fully efficient. We do 

 not find in any case of damage by lightning at sea that a quan- 

 tity of metal has been melted equal to that contained in a cop- 

 per bolt of half an inch diameter and six inches long, or other- 

 wise an equivalent quantity of any other metal more easily 



