Unpublished letter, regarding magnetic variation and structure of the earth from John Macdonald to Sir Humphry Davy Macdonald writes to correct an error in a letter sent on 12 June 1826 (AP/12/15/6). Davy discusses observations by explorers of magnetic variation as well as theories of the internal structure of the earth. Includes a figure showing the presumed location of the northwest magnetic pole in relation to northeast line of no variation. Minor corrections appear in ink. Below the date, an ink inscription reads 'N. T.Y. [Thomas Young]'. Subject: Geodesy Written by Macdonald in Summerlands, Exeter [Devon, England]. 30 November 1826