In examining—a few days ago—the Calculations of the heights I last sent you, I found I had a mistake in the elevations of Connecticutt river at Windsor and Orford-Bridges above tide-Water, and which consequently led to some other Errors. Whether this mistake arose from an inaccuracy in the table of logarithms I then used, or from an error committed by myself in making the Calculations which were done in some haste—I know not, but being very unwilling to propagate Error, I hasten to rectify it. The following results therefore, I think may be relied upon as nearly correct, (viz)

I have endeavored to ascertain as accurately as possible the elevation of my residence at this Place above tide-Water, as by having a station so far in the interior the altitude of which above tide-Water Was accurately determined, I should be enabled to determine with greater Accuracy the altitudes of the mountains and Eminences in its vicinity.1 About Six years ago I made the first calculation for this purpose, from a single observation made here, compared with some made at West Point, and found my Elevation above tide water at that Place, to be 903 feet. I also at the same time calculated from one observation, at each place, My elevation above Connecticutt River at Hanover Bridge, which is about two miles East from this, and found it to be 629 feet. A few days ago I repeated my Calculations for determining my elevation above tide Water. For this purpose I took a mean between a series Of Barometrical and thermometrical observations which I made at West Point, between the Spring of 1810, and December 1814, amounting to 2763 in number, and another series which I made at this Place in 1811, and 1812, amounting to 794 in number[.] The result gave an Elevation of 886⅓ feet, which I believe May be relied upon as very nearly correct. I have also lately determind my elevation here above Connecticutt river at Hanover Bridge, from2 two observations to be 640 feet differing from my former result 11 feet.—This I believe to [be]3 sufficiently Accurate. Hanover Bridge is about midway between Windsor and Orford (Bridges),4 being 18 miles from each. Between Hanover and Windsor however the descent of the river is much greater—on account of falls—Than between Hanover and Orford, where it is smooth and the Current Gentle. I will now, Sir, take the liberty to state to you a Project which has occupied my mind Occasionally for several years—It is the undertaking to determine from a series of Barometrical and thermometrical observations, The altitudes above tide Water of a number5 of stations, Commencing from tide-water on the East side of the Alleghany mountains and extending across either to the ohio river or perhaps to the Missisippi. By making those Stations sufficiently numerous an accurate Profile of The Country extending from the atlantic to the Western Waters might be made Which would be a useful appendage to a general map of the Union, whenever such an6 one is Completed, and which appears to be contemplated from the trigonometrical Survey that has commenced under the direction of Mr Hasler. I should be pleased, Sir, to know your opinion of the practicability, and probable utility of this Project if executed, and also the most proper Place to cross the Mountains. I have thought of two or three different routes but am not well enoughe acquainted with the Country to determine upon the best one for the purpose. I do not know that circumstances Will ever permit me to undertake the Execution of this Plan, but should they, it would be of great importance to undertake it in the best manner and for this purpose information would be very useful—I must request, Sir, you Will have the Goodness to excuse me for the trouble I have given you, and