From David Rittenhouse
[ca. 20 Aug. 1791]
If we consider the same thing in the manner following we shall immediately perceive that the proposition is not new. Since the hole in the vessel may be of any size it may be equal to the intire bottom. The vessel then becomes a Cylinder and the problem is reduced to this, To find the lenghth of a perpendicular Cylinder in which a fluid will descend from the top to the bottom in a given time—that is, the space thro’ which a heavy body descends in the Same time, which brings it immediately to the Case of a Pendulum. Yours &c.
RC (DLC); addressed: “Mr. Jefferson”; undated, but endorsed by TJ as received 20 Aug.  and so recorded in SJL.
No communication from TJ to Rittenhouse to which the above was a response has been found, but clearly the two were again discussing the use of the rod pendulum as a unit of measure as they had done the previous summer when TJ was preparing his report on weights and measures. This renewed discussion may have been prompted by TJ’s having received from Condorcet on 19 July 1791 the report of the Academy of Sciences recommending a unit of measure of which he thoroughly disapproved (see note to Condorcet to TJ, at 3 May 1791, and TJ to Condorcet, 30 Aug. 1791).