Copy letter, from Sir John Herschel to Edward Sabine, dated at Collingwood Copy of letter, copied as part of the correspondence project led by Colonel John Herschel FRS following the death of his father. Plans to confer with [J. T.] Walker and F. A. T. Winnecke from Pulkovo to learn of Russian pendulum experiments before reporting to R.S.L. council. J. H. Pratt's measurement of polar axis and theory about earth's center of gravity. 11 June 1864