Monticello Sep. 19, 05
In exploring the rivers of Louisiana which extend such immense distances through countries uninhabited & without resources, I was sensible that we hazarded the main object of our mission, the taking accurately the geography of those rivers, in relying for the longitude on the lunar observations taken as at sea with the aid of a time piece. possessing only the theory of astronomy, my vocations having never permitted me to become familiar with it’s practice, I thought that a meridian, which we can always have at land, might enable us to use lunar observations without a time piece; so that our missionaries, if their time pieces failed, might not be without resource in so important an object. the method you have seen occurred to me. I communicated it to several, who were better judges of it’s practicability. mr Dunbar has been so kind as to favor me with his observations on it, in which, admitting it’s correctness in theory, he points out the many difficulties & intricacies of calculation which it involves in practice. I am therefore the more pleased with your communication, as it clears the process of those difficulties. I shall immediately take the liberty of handing it on to mr Dunbar who directs our mission up the Red river, in hopes it will reach him before the party sets out; and I pray you to accept my thanks for this aid to our undertaking, with my salutations & respects.
DLC: Papers of Thomas Jefferson.