Letter, from Edward Sabine to Sir John Herschel, dated at 12 Park Place Autograph letter signed by sender. On discrepancy of data from G. B. Airy and Humphrey Lloyd. Captain Boileau and John Caldecott exchange instruments to normalize data. Will send letters from Airy, [A. T.] Kupffer, Lloyd, Edward Sabine, and John Frederick 24 February 1841