William Macintosh had a restless mind and a restless pen, and the networks within which his letters and ideas circulated were large and complex. For those reasons I was always slightly surprised that I had not been able to find any evidence that he corresponded with Joseph Banks, who was, in many ways, right at the centre of late-Enlightenment networks of epistolary exchange. It seemed almost inconceivable, given the flow of letters across Banks’s desk, and Macintosh’s penchant for firing off letters to the great and the good, that there was not at least one from Macintosh. There was, of course; I had just failed to spot it.

Yesterday I noticed that the Royal Society holds a 1782 letter from Macintosh to Banks, in which he submitted to the Society’s attention a model of a rope pump. According to the letter, the machine had been shown to Macintosh in Paris the previous winter by its inventor, a man “in poor circumstances”. Macintosh claimed to have encouraged the inventor to send a model of the machine to Britain, “where the ingenuity of an alien would meet with encouragement and reward”, but not having done so, Macintosh felt himself “justified in having the model made and improving upon it”.

This letter immediately explains the presence in Macintosh’s archive of a set of calculations (above) describing the “Quantity of Water raised by the Rope-pump as improved by Mackintosh [sic]”. For years I have wondered about that document—why it was there, what it related to—and the letter to Banks finally offers an explanation. I look forward to reading the letter in full when the Royal Society’s library reopens, but, for now, I can tick off one of the thousand-or-so outstanding questions I have about Macintosh, his world, and its traces.