Note 97 — MEMOIRS OF THE LIFE OF DAVID RITTENHOUSE; ANTERIOR TO HIS SETTLEMENT IN PHILADELPHIA.
Reading edition
Besides the testimony of so distinguished an astronomer as Mr. de Zach, already given, respecting the very great accuracy to which time-keepers have been brought, the following translation, taken from what the celebrated Lalande has said in his treatise Des Horloges Astronomiques, (in the second volume of his Astronomie,) furnishes some curious and interesting facts on that subject.
“Short (the mathematical instrument maker,) upon the occasion of the transit of mercury over the sun observed in 1753, assures us that he had found by many observations, that his clock had not varied more than one second, from the 22d of February to the 6th of May (Philos. Trans. 1753, p. 200;) so that, with a like pendulum, it is possible to obtain an exactness which, till this time, was thought incredible. There are English astronomers who have assured me,” continues Lalande, “that pendulum-clocks have been made which did not vary more than five″ in a year:[97a] but that does not appear to me to be yet established as a fact; the oils that one is obliged to use in them are sufficient, by the change of consistency they undergo, to prevent such preciseness. The count de Bruhl, a great amateur and a perfect connoisseur also, on the subject of time-pieces, shewed me in London a diary of the going of two pendulums of Mudge, one of the most celebrated clock-makers in London: in one, there was a difference of half a second a day, between winter and summer; and in the other a second. Mr. Aubert has a pendulum made by Shelton, which varies also nearly a second in the day, in extreme seasons. Picard, in 1671, had a clock which did not lose a second in two months. But, whatever may have been, since that period, the skill of the clock-makers of Paris, we cannot obtain such exactness, but by mere accident and an equality of temperature in the atmosphere that is very rare: now, the correctness of our clocks is a necessary consequence of their principles; but these do not go so far. Mr. Emery has observed two clocks beat the same second, during three months; they were, however, very near to each other, and probably had some influence on one another by means of their foot-board or support.”