On Lightning and Gunpowder Magazines. 369 



knobbed conductors as effectually answering the purpose of 

 " conveying away the lightning safely, without that tendency 

 to increase or invite it/' that belongs to points. Wilson pub- 

 lished a long defence of his view of the matter, and was ably 

 answered by Nairne and Henley. Franklin admitted that 

 " points drew electricity at a greater distance in the gradual 

 silent way ; but knobs will draw at the greatest distance a 

 stroke." 



Wilson found many supporters of his views, and the time 

 of the Society was long occupied with discussions as to the 

 respective merits of sharp and blunt conductors. Some even 

 adopted the idea, which has survived even to our own time, 

 that lightning-rods attracted the lightning to the building 

 they were intended to protect, and thus occasioned loss instead 

 of protection. This idea was started by the Abbe Nollet, who 

 was for some time an electrical authority in France. In his 

 Memoire sur les effets du Tonnerre he expressed his opinion in 

 the following terms : — " Toutes ces pointes de fer qu'on dresse 

 en Fair sont plus propres a nous attirer le feu du ton- 

 nerre qu'a nous en preserver." 



The Board of Ordnance, however, resolved to adopt the 

 Report of the Committee, supported as it was by such power- 

 ful names, and accordingly proceeded to erect pointed con- 

 ductors on the powder-magazines. The Report is dated 21st 

 August, 1772. Now it so happened that on the 12th May, 1777, 

 the Board-house at Purfleet was struck by lightning at a point 

 upwards of 40 feet from the conductor. The damage was 

 but slight, consisting chiefly of a few stones fastened by iron 

 cramps being thrown down. A similar accident had occurred 

 on the 17th June, 1774, when the chimney of a house at 

 Tenterden was struck by lightning although another chimney 

 50 feet distant was furnished with a conductor. Such cases 

 as these were made the most of by Wilson in favour of his 

 knobbed conductors, although the whole contest between 

 the sharps and the blunts involved a sort of fallacy, since, 

 as Snow Harris puts it, tl any termination which can con- 

 veniently be given to a conductor, even if it were a ball 

 a foot in diameter, would be in relation to, say, a thousand 

 acres of cloud, virtually, a pointed conductor/' The mistake 

 made by both parties in this controversy was in supposing 

 that a lightning-conductor attracted the lightning ; whereas it 

 is as passive as the rain-pipes destined to carry off the rain. 

 Should a building or a ship form part of the line of least re- 

 sistance between a thunder-cloud and the earth or the sea, 

 the building or the ship will be struck ; but if they are furnished 

 with properly arranged conductors, the lightning will pass 



Phil. Mag. S. 5. Vol. 28. No. 174. Nov. 1889. 2 E 



