Thomas Jefferson Papers

From Thomas Jefferson to Robert Patterson, 16 November 1805

Washington Nov. 16. 05

Dear Sir

When Capt Lewis’s mission was under contemplation, and it’s principal object the obtaining a correct map of the Missouri, I recommended to him the making himself thoroughly acquainted with the practice of the Lunar observations for the longitude. but fearful that the loss or derangement of his watch, on which these were to depend, might lose us this great object of his journey, I endeavored to devise some method of ascertaining the longitude by the moon’s motion without a time piece. I thought that a meridian, always to be had at land, tho’ not at sea, might supply the want of the time piece, and proposed a method of determining by the aid of a meridian. having never been a practical astronomer, and a life far otherwise spent having even rendered me unfamiliar with the detailed theory of the lunar observations, I suggested my idea to mr Ellicot by Capt. Lewis who was going on to him to take some lessons. however he never communicated to me what he thought of my suggestion. I have since mentioned it to mr Briggs, mr Dunbar & some others, and I am led by them to believe that the calculations would be very complicated & liable to error. mr Freeman told me he had put my paper into your hands, and, as you have taken the trouble to think of it I will ask the favor of your information as to it’s practicability. mr Dunbar promises to send me another method of attaining the same object without a time piece. mr Moore of this place has proposed one to me which I now inclose for your consideration & opinion the paper to be returned as I retain no copy. I persevere in this enquiry because we have a mission now preparing for the Red river; on which mr Freeman goes, and shall send others up other rivers hereafter. will you be so good as to give the inclosed letter to mr Freeman who was to set out this morning and will call on you. Accept my friendly salutations & assurances of great esteem & respect.

Th: Jefferson


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