Draft letter, from Sir John Herschel to James South Autographed draft letter. John Frederick William Herschel's attempts with Edward Sabine to measure the difference in longitude between Greenwich and the Paris Observatory. Supplies, for Alexander von Humboldt, John Frederick William Herschel's measurement of Mount Etna's height. Gives message for François Arago concerning paper on magnetism published by John Frederick William Herschel and Charles Babbage. 2 August 1825