Copy letter from Christian Huygens, the Hague, to Robert Moray Discusses Wren's and Frenicle's theory of Saturn; Boyle's 'Sceptical Chemist'; the universal measure; has seen the observations of Jonathon Horrocks of Venus in the Sun; of Hevelius at Gdansk on Mercury; the length of a pendulum for half a second for the universal measure; the cylinder and its vibrations. 30 December 1661