I had the honor duly to receive your letter of the 12th of october. I now take the liberty Sir, to enclose you an Extract from some observations which I made in 1811. (shortly after my return from the white Mountains,)1 upon the Method of calculating Heights by means of the Barometer—together with the Rule I used for that purpose. This will shew you the manner in which my calculations were made, and may, perhaps assist you in determining what degree of credit ought to be attached to them. I hope therefore it will not be unacceptable to you. The calculations of Genl. Williams were made from a table, constructed by himself and Mr Patterson. This table The Genl was afterwards convinced, was incorrect—he accordingly in 1810 requested me to take his data, and make the calculations over again by the Rule I had used. This I did, and obtained Results considerably different, from those he had previously obtained. These were published,2 by his direction, in a periodical3 work, Edited in New york. A Copy of those Altitudes is enclosed, as they stand corrected by my calculation, and also as they were first published in the 4th volume of the American Philosophical Transactions. I shall feel myself much obliged to you for your calculations of the Heights of the Peaks of Otter as I wish to compare them with the Altitudes of our Northern4 Mountains. I shall take the liberty Sir, in the course of a few days to enclose you some tables containing Meteorological observations5 which I have made during, and Subsequent to the year 1810, and which I trust will not be unacceptable to you— I have the honor to be with