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Monticello, November 10, 1811.

Dear Sir,—I write this letter separate, because you may perhaps think something in the other of the same date, worth communicating to the committee.

I accept, willingly, Mr. Voigt's offer to make me a time-piece, and with the kind of pendulum he proposes. I wish it to be as good as hands can make it, in everything useful, but no unnecessary labor to be spent on mere ornament. A plain but neat mahogany case will be preferred.

I have a curiosity to try the length of the pendulum vibrating seconds here, and would wish Mr. Voigt to prepare one which could be substituted for that of the clock occasionally, without requiring anything more than unhanging the one and hanging the other in its place. The bob should be spherical, of lead, and its radius, I presume, about one inch. As I should not have the convenience of a room of uniform temperature, the suspending rod should be such as not to be affected by heat or cold, nor yet so heavy as to effect too sensibly the centre of oscillation. Would not a rod of wood not larger than a large wire, answer this double view? I remember Mr. Rittenhouse told me he had made experiments on some occasion, on the expansibility of wood lengthwise by heat, which satisfied him it was as good as the gridiron for a suspender of the bob. By the experiments on the strength of wood and iron in supporting weights appended to them, iron has been found but about six times as strong as wood, while its specific gravity is eight times as great. Consequently, a rod of it of equal strength, will weigh but three-fourths of one of iron, and disturb the centre of oscillation less in proportion. A rod of wood of white oak, e. g. not larger than a seine twine, would probably support a spherical bob of lead of one inch radius. It might be worked down to that size I suppose, by the cabinet-makers, who are in the practice of preparing smaller threads of wood for inlaying. The difficulty would be in making it fast to the bob at one end, and scapement at the other, so as to regulate the length with ease and accuracy. This Mr. Voigt's ingenuity can supply, and in all things I would submit the whole matter to your direction to him, and be thankful to you to give it. Yours affectionately.