DEAR SIR,--According to promise I send you our observations of the solar eclipse of September 17th. We had, you know, a perfect observation of the passage of the sun over the meridian, and the eclipse began so soon after as to leave little room for error from the time-piece. Her rate of going, however, was ascertained by ten days' subsequent observation and comparison with the sun, and the times, as I now give them to you, are corrected by these. I have no confidence in the times of the first and ultimate contacts, because you know we were not early enough on the watch, deceived by our time-piece which was too slow. The impression on the sun was too sensible when we first observed it, to be considered as the moment of commencement, and the largeness of our conjectural correction (18´´) shows that that part of the observation should be considered as nothing. The last contact was well enough observed, but it is on the forming and breaking of the annulus that I rely with entire confidence. I am certain there was not an error of an instant of time in either. I would be governed, therefore, solely by them, and not suffer their result to be affected by the others. I have not yet entered on the calculation of our longitude from them. They will enable you to do it as a college exercise. Affectionately yours. First contact, 0h. 13´ 54´´ Annulus formed, 1h. 53´ 0´´ }central time of }central time of Annulus broken, 1h. 59´ 25´´}annulus, }the two contacts, Ultimate contact, 3h. 29´ 2´´ 1h. 56´ 12½´ 1h. 51´ 28´´ Latitude of Monticello, 38° 8´ TO DOCTOR ROBERT PATTERSON. MONTICELLO, November 10, 1811. DEAR SIR,--Your favor of September 23d came to hand in due time, and I thank you for the nautical almanac it covered for the year 1813. I learn with pleasure that the Philosophical Society has concluded to take into consideration the subject of a fixed standard of measures, weights and coins, and you ask my ideas on it; insulated as my situation is, I am sure I can offer nothing but what will occur to the committee engaged on it, with the advantage on their part of correction by an interchange of sentiments and observations among themselves. I will, however, hazard some general ideas because you desire it, and if a single one be useful, the labor will not be lost. The subject to be referred to as a standard, whether it be matter or motion, should be fixed by nature, invariable and accessible to all nations, independently of others, and with a convenience not disproportioned to its utility. What subject in nature fulfils best these conditions? What system shall we propose on this, embracing measures, weights and coins? and in what form shall we present it to the world? These are the questions before the committee.