Benjamin Franklin, continuing his mission to provide his London landlady’s young daughter MARY “POLLY” STEVENSON (later HEWSON) instruction in science and philosophy wrote a long letter on 13 September 1760 about tides and rivers. It ends with this passage.

. . . . I have made this Letter longer than I intended, and therefore reserve for another what I have farther to say on the Subject of Tides and Rivers. I shall now only add, that I have not been exact in the Numbers, because I would avoid perplexing you with minute Calculations, my Design at present being chiefly to give you distinct and clear Ideas of the first Principles.
After writing 6 Folio Pages of Philosophy to a young Girl, is it necessary to finish such a Letter with a Compliment? Is not such a Letter of itself a Compliment? Does it not say, she has a Mind thirsty after Knowledge, and capable of receiving it; and that the most agreable Things one can write to her are those that tend to the Improvement of her Understanding? It does indeed say all this, but then it is still no Compliment; it is no more than plain honest Truth, which is not the Character of a Compliment. So if I would finish my Letter in the Mode, I should yet add something that means nothing, and is merely civil and polite. But being naturally awkward at every Circumstance of Ceremony, I shall not attempt it. I had rather conclude abruptly with what pleases me more than any Compliment can please you, that I am allow’d to subscribe my self Your affectionate Friend
B Franklin

Polly wrote a note of thanks by return mail from Draycot in Wiltshire where she was staying with her aunt and a friend. She had asked Franklin why the water pumped at Bristol seemed warmer than it was when pumped from the spring at its source. She apologizes for accepting the generally accepted temperature of water at Bristol and for not confirming it herself.

I implore your pardon, Dear Sir, for asking you the Reason before I could assure you of the Fact. . . . I confess it was not from my own observation I told you the Water at Bristol, though cold at the Spring, became warm by pumping, I had only heard that it was so.
. . . . It is I own great Assurance in me to say so much but I hope it will not offend my dear and honour’d Friend. The familiar agreable manner in which you deliver Instruction renders it easy and pleasant; but you must bear patiently with me if I do not always comprehend things as clearly as might be expected. . . . I hope soon to have the pleasure of seeing you, or if I cannot have that happiness I shall take an opportunity of writing to you again, therefore I will not add to the length of this Letter. I could not forbear returning my earliest Thanks for the charming Letter I receiv’d yesterday; and am always ready to lay hold of the Privilege you give me of subscribing myself (though I acknowledge it is too presumptuous) Your sincerely affectionate Friend
M Stevenson

“From Benjamin Franklin to Mary Stevenson, 13 September 1760,” and “To Benjamin Franklin from Mary Stevenson, 16 September 1760,” Founders Online, National Archives, version of January 18, 2019, https://founders.archives.gov/documents/Franklin/01-09-02-0058 and 0059. [Original source: The Papers of Benjamin Franklin, vol. 9, January 1, 1760, through December 31, 1761, ed. Leonard W. Labaree. New Haven: Yale University Press, 1966, pp. 212–217, 217-218.] Credit photograph: Carrie Vonderhaar/Ocean Futures Society/National Geographic Creative.