380 PHILOSOPHICAL TRANSACTIONS. [aNNO 1773. 



letter related them to me. Mr. Delaval and mvself attended, about a week 

 afterwards, to observe them, and their particular situations, with the circum 

 stances attending them ; when we were very well satisfied with his account, not- 

 withstanding it had rained in the interim for 3 days together. 



It is worthy of note, that those conductors did not terminate in a point, nor 

 was any point put upon the cross at the top. And yet Dr. Franklin was of that 

 committee. If points are so essential to our safety, why was not the reason 

 enforced at the committee, for having them on that capital edifice? For my part, 

 I think it was a happy circumstance that there was no point fixed on the top of 

 the church, to solicit a greater quantity of lightning at that moment, than what 

 fell on the conductors, circumstanced as they were : as that quantity was great 

 enough to heat so considerably a bar of iron, near 4 inches broad, and about half 

 an inch thick. 



This powerful effect reminds me of another instance still more extraordinary, 

 which happened in Martinico, and is related by Captain Dibden, where a bar of 

 iron, one inch in diameter, was by a violent shock of lightning reduced in one 

 part of it to the thickness of a slender wire only. See Phil. Trans., vol. liv. p. 

 251.* Since then we are at all times ignorant of the quantity of lightning in 

 the earth and its atmosphere ; and the difference in the effects, between blunted 

 and pointed ends, in causing a discharge in our electrical experiments, appears 

 to be as 1 to 12 ; it is easy to comprehend the very great danger this noble fabric 

 has probably escaped, by having no pointed apparatus upon it. 



From the above observations, I am naturally led to consider a part of the 

 proceedings of the committee, respecting the magazines at Purfieet; when a 

 certain number of conductors, with tapering points at the top, were resolved on, 

 as necessary to protect the several buildings Where the powder is deposited. For 

 it was agreed ori at the same meeting, that the board-house, which is a large 

 building for the use of the board officers, and which stands considerably higher 

 than the magazines, as was observed above, did not require any point at the 

 top : because it was apprehended to be perfectly secure, by reason of the copings 

 on the roof, the gutters and pipes to carry off the water being all of lead : and 

 further, because those pipes communicated with two wells, which always con- 

 tained water. 



I was not a little surprized at this last resolution, which appeared to be so 



observed, that the conducting iron did not touch the lead. We likewise observed, that there was a 

 very thick coat of rust all over that part of tlie iron ; particularly at the end next the lead, where the 

 water entered the trunk. As tlie necessity of attending to these circumstances will be obvious to any 

 one, who is but in the least degree acquainted with these researches, the danger of neglecting them 

 will be seen in the strongest light, by tlie gentlemen of the committee who recommended the conduc- 

 tors for the security of that cathedral. — Orig. 



* Abridgment, vol. xii. p. 149. 



