From Thomas Jefferson to David Rittenhouse, 8 June 1792
To David Rittenhouse
June 8. 92.
Th: Jefferson incloses to Mr. Rittenhouse the first criticism which has come to his hands on the rod-pendulum as a standard of measure. It is from a clergyman of Scotland. The author’s language is so lax, that it is difficult to know with precision what idea he means to express. It is particularly so in the following sentence at the bottom of the 1st. page. ‘It is therefore impossible to fix an invariable, or nearly equal measure from the rod of such a cyclindrical pendulum—for, measured from the extremities, a second’s pendulum is of all possible lengths from 39 to 58 ½ inches.’—In a preceding passage where he says that ‘the length of the pendulum has nothing to do with the whole length of the rod,’ he cannot possibly mean what he says. That they are the same thing, no body ever pretended; but that, in theory, they have a determinate relation of 2 to 3. to one another, nobody can deny.—He says again pa. 2. ‘no philosopher who has studied the doctrine of the Pendulum ever measured it from it’s extremities.’ Why this observation? Th:J. had made the difficulty, in a bob-pendulum of finding the center of oscillation, from which it was to be measured, one reason for rejecting the bob-pendulum, and adopting the rod-pendulum which admits of being measured from the extremities, because in this1 one extremity may be made the center of motion.—What does he mean by saying that the difference between the cubic2 foot proposed by Th:J. and the English cubic3 foot (which Th:J. had stated to be 1/14 as Mr. Skene does) ‘is a monstrous error?’—If Mr. Rittenhouse can find out what Mr. Skene means to object and will favor Th:J. with his thoughts on it, he will thank him.
PrC (DLC). Tr (DLC); 19th-century copy.
The enclosure was George Skene Keith’s manuscript commentary on TJ’s 4 July 1790 Report on Weights and Measures that Keith had sent in his 14 Jan. 1792 letter to Washington and that he intimated the President should pass on to TJ (DLC: Washington Papers). TJ’s citations to this commentary, which has not been found, indicate that it was distinct from Keith’s essay on a uniform system of weights and measures published in his Tracts on Weights, Measures, and Coins (London, 1791), and from a manuscript copy of the same essay that Keith forwarded to the President in 1790. The Secretary of State had received copies of the Tracts from Keith and James Somerville (see Keith to TJ, 1 July 1791; TJ to Somerville, 1 Dec. 1791; No. 3766). Keith also sent copies of the pamphlet to Washington with letters of 1 July 1791 and 14 Jan. 1792 (DLC: Washington Papers). The manuscript copy of the essay, written in Keith’s hand and entitled “Synopsis of a System of Equalization of Weights & Measures Or The Theory and Practice of a System of Equalization,” is dated 12 Jan. 1790 and contains subjoined an unpublished postscript and observation by the author (MS in DLC: TJ Papers, 236: 42278–89; see also Keith to TJ, 22 June 1801). The President acknowledged receipt of all these items in a 22 June 1792 letter to the Scottish clergyman, supplementing a more perfunctory note to Keith written by Tobias Lear on 7 May 1792 ( xxxii, 72). Rittenhouse’s own objections to TJ’s proposed use of the rod-pendulum are analyzed in Brooke Hindle, David Rittenhouse (Princeton, 1964), 312–16.
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