108 Mr. R. S. Brough 07i a Case of Light ningf. 



with a lofty metallic rod, discharge is more likely to occur 

 than if the rod were away. In proof of this I may refer to 

 the case reported by Mr. Pidgeon in ' Nature/ and subse- 

 quently discussed before the Society of Telegraph Engineers 

 (Proceedings, May 12, 1875), in which the flagstaff acted the 

 part of an ordinary '' lightning-protector." 



Professor Clerk-Maxwell observed, in his paper recently read 

 before the British Association at Glasgow, that such lightning- 

 protectors are designed rather to relieve the charged cloud 

 than to protect the threatened building. In fact lightning- 

 rods are legitimately employed for this very purpose in the 

 vineyards, where the object in view is to relieve charged 

 clouds and prevent disruptive discharges and the consequent 

 showers of hail. 



Under ordinary circumstances, however, the noise and light 

 of the lightning-flash must be regarded as a very harmless, if 

 disagreeable, way of getting rid of some of the potential 

 energy of electrical separation. 



The protection of cities on the same principle, even if ne- 

 cessary or desirable, would be too expensive and unsightly 

 ever to be put in practice. But Faraday has proved that if 

 our houses were made of metal, they would constantly remain 

 at the potential of the earth, we should virtually be '^ under- 

 ground," and live within them in perfect security. The iron 

 churches occasionally employed in Europe fulfil this condition 

 exactly. It is not, of course, usually practicable to live in 

 metal houses ; but we can live in almost equally effective 

 metal cages formed by running conductors connected to earth 

 along the summit, eaves, and corners of our houses*. 



The usual rod protectors appear to be only suitable to such 

 structures as themselves determine lines of maximum induc- 

 tion, e. g. church-spires, factory-chimneys, flagstafts, &c. 



The case of lightning referred to at the beginning of this 

 paper is of peculiar interest, because we know precisely the 

 mechanical effect produced by the flash, and from this we can 

 work back and estimate roughly the potential and quantity of 

 the electrical discharge. 



In the first place we can calculate the force required to 

 burst the cylindrical portion of the porcelain insulator into 

 which the iron stalk is cemented. 



Let r = radius of the inside of the cylinder, 



11= „ outside „ 



and F = the resistance to bursting ; 



* This portion of the paper was written before the Meeting- of the Bri- 

 tish Association at Glasgow. 



