Unpublished letters, on magnetic variation and poles from John Macdonald to Sir Humphry Davy A collection of letters from Macdonald to Davy regarding his reseach on magnetic variation and his quest to find the location of the northwest and southeast magnetic poles. The letters are dated 28 September 1825, 19 October 1825, 29 September 1825, 26 June 1826, 30 November 1826, and 18 January 1827. Subject: Geodesy Written by Macdonald in Summerlands, Exeter [Devon, England]. September 1825 - January 1827