This letter poses a remarkable problem of dating, and we cannot solve it. Henly’s date is all too clear, and could not have been a slip: April 18 was a Sunday in 1773. Yet he makes a number of references that seem inapplicable to that year, and are definitely applicable to 1774. They were all related to the continuing controversy with Benjamin Wilson over lightning rods. Franklin had withdrawn from the fray in late 1772, and Henly had taken his place as chief protagonist.4 Developments thereafter are the background of the dating problem.

In March, 1773, Wilson published his Observations upon Lightning, which was an outgrowth of the arguments the year before about pointed versus blunt rods.5 He subsequently developed his idea that the quantity of charge in a cloud could not be known, and might well be greater than any rods could conduct in safety to the ground. To test this possibility he suggested constructing a giant metal ring, sharpened on the upper edge, and placing it horizontally on top of St. Paul’s, to see whether it would not draw more electricity from a thunderhead than any one dreamed the cloud could hold. He challenged whoever was skeptical-meaning Franklin-to try the experiment on a smaller scale: “I would ask the philosopher, whether he would chuse to put a crown of this kind upon his own head, and venture to walk or ride with it on, over Salisbury-plain, in the midst of a very violent thunderstorm; notwithstanding there should drag behind him a communication of metal from the crown itself to the earth, during the whole of his journey.” Wilson, as far as we know, first publicized his idea and made this jibe at Franklin, without naming him, in a paper that he says he read before the Royal Society on February 24, 1774, and that he published unaltered as a pamphlet the following May.6 On May 5 Henly responded, again before the Royal Society, to the wild idea of the ring by pointing out that a similar one existed, and had for more than a century without incident, on top of the Monument to the Great Fire.7 When he subsequently obtained and read Wilson’s pamphlet, he contented himself with scathing marginalia; the public controversy ended there.

This background suggests why the present letter is puzzling. In it Henly speaks of a paper read before the Royal Society the previous evening, April 17 (when the Society’s records do not show any meeting), and virtually says that the paper was Wilson’s and contained the jibe at Franklin. Wilson himself refers only to the later paper of February, 1774, which became his second pamphlet. Among Henly’s marginalia in that pamphlet is one, on the taunt to Franklin, that is similar to what he says here.8 The letter, in short, could not have been written when it was: it refers to Wilson’s argument before the Royal Society and contains some of Henly’s response; yet the paper was not presented until more than ten months later, according to its author, and Henly’s response, in his paper and his marginalia, was later still. We have two alternative explanations, both implausible. One is that the final phase of the controversy that became public in the spring of 1774 was in rehearsal a year earlier. The other is that Henly dated his letter in the grip of some aberration.