From William Henly
ALS: American Philosophical Society
This letter poses a problem of dating that we are unable to solve. The internal evidence is as ample as it is conflicting: some of it points to 1772 and some to 1773. The whole tone of the opening paragraph suggests that the Purfleet magazine had not yet been protected by lightning rods, and that Benjamin Wilson’s “scheme” for doing so was being actively considered. His recommendation of a base of brickwork for the rods was mentioned only in his report to the Board of Ordnance of May 4,9 which we assume was the scheme. Henly must have been writing after that date and, to judge by the way he wrote, before the Purfleet committee overruled Wilson in August.1 Yet the conclusion of the letter refers to an exploded story about a house in Jamaica; Franklin, as far as we know, did not hear this story until autumn, or learn that it was false until the following March.2 The letter patently belongs to the period between May and August of 1772, in other words, and just as patently to the spring of 1773. We have tried elaborate lines of conjecture to resolve this paradox, but they lead nowhere. Only one fact is clear, that Henly could not have written the letter before Wilson’s report.
[After May 4, 1772]
I shall take it as a particular favour if you will be so obliging as to give the bearer leave to copy out for me the Account of the damage done by the powder Magazine which was fired by the Lightening—ever since you mention’d it to me I cannot get it out of my thoughts.3 I spoke of it to Mr. Nairne on Saturday, repeating what you observed to me concerning that at Erith,4 says he I really shudder at the thought. The more I think of Mr. Wilsons scheme for a Conductor the more I dislike it. If I understand the plan aright, it seems not at all unlikely that he would invite a stroke upon his Iron Bar in its full force, and as a Base of Brick work may in dry seasons be looked on as a kind of imperfect insulation, if the Electricity should not be properly dissipated at the bottom who can foresee the consiquence?5 Should mischief ensue from such an improper apparatus, it might contribute to bring them into disrepute, and so prevent the many good effects which may by proper management be always expected from them.
Mr. Nairne observes to me that the mark made by the spontaneous discharge of a positively charged bottle is only on the inside of the uncoated part of the glass—if the bottle be charged negatively the mark is on the outside only. I intend to try this soon with 2 new Bottles which I shall mark previously to my making the experiment, tis however perfectly consonant to all my experiments made on the surfaces of Glass Ivory &c.
I have written to Dr. Priestley and contradicted the Report of the House &c. in Jamaica. I am Sir faithfully yours
Addressed: Dr. Franklin
9. See the headnote on BF to Dawson below, May 29. Wilson did not mention brickwork, as far as we have been able to discover, in either of his subsequent pamphlets on lightning.
1. See its report below, Aug. 21.
3. Henly is undoubtedly referring to the explosion at Brescia, discussed in the headnote on the letter to Dawson; accounts of the disaster are cited there. Although Henly might have been asking for one of the brief notices in the press in 1769, he would have been more interested in the first detailed description, which was in Wilson’s pamphlet. If that was what he wanted, he was writing after its publication in March, 1773.
4. For the use of Erith as a synonym for Purfleet see Robertson to BF below, July 14. During the summer, as Purfleet became more familiar, the synonym tended to disappear; its use here suggests an early date for the letter.
5. BF had long been aware that lightning running through an inadequately grounded conductor might, particularly in dry conditions, produce an explosion. See above, X, 50–1, 57–9; XIV, 263.