Copy letter, from Sir John Herschel to Jacques Babinet, 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. Suggestions on improving Jacques Babinet's gravimetric balance, constructed on principle of 'Bifilar suspension.' Refers to 1861 report of similar invention by J. A. Brown and another article on this topic. 19 February 1863