68 R- S. Browgh— Oil a Case of Li ffhtiimg. [Feb. 



stratum increased ; and a French writer has referred the possibility of the 

 occurrence of lightning discharges several kilometres in length to this 

 cause. Sir W. Thomson's earlier experiments certainly shewed this unex- 

 pected result, probably due to the minute distances at which he was opera- 

 ting, but a later series of experiments, made at larger distances, shew^ed this 

 result in a much less marked degree ; and Sir W. Thomson himself says, 

 " It seems most probable that at still greater distances the electromotive 

 force will be found to be sensibly constant, as it w^as certainly expected to 

 be at all distances*." 



Another assertion of the text-books is that the metallic rods now 

 employed as lightning-protectors on buildings do not " attract" lightning. 

 This statement is literally true, according to the meaning of the word 

 *' attract", but is untrue in effect. For such a rod-lightning-protector 

 determines a line of maximum induction, and a discharge is more likely to 

 occur at the place than if the protector were not there. Prof. Clerk Max- 

 well does not appear to hold this opinion ; but it seems to me unquestion- 

 able that if a charged thunder- cloud, driving before the wind, is carried 

 over a building furnished 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 subsequently discussed 

 before the Society of Telegraph Engineers (Proc. 12th May, 1875), in which 

 the Flag-staff acted the part of an ordinary " lightning-protector." 



Prof. Clerk Maxwell observed in his paper recently read before the 

 British Association at Glasgow, that such lightning-protectoi's 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 \ine3'ards, where the object in view is to relieve charged clouds and pre- 

 vent 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 princi23le, even if necessary or de- 

 sirable 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 secm-ity. The iron 

 churches occasionally employed in Europe fulfil this condition exactly. I 

 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 con-l 

 nected to earth along the summit, eaves, and corners of our housesf. 



* Papers on Electrostatics and Magnetism, p. 2o9. 

 t This portion of this paper was written before the meeting of the British Asso 

 ciation at Glasgow. 



