Unpublished letter, regarding magnetic variation from John Macdonald to Sir Humphry Davy Macdonald writes about his research for a paper on magnetic variation to be published in the Encylopaedia Metropolitana, explaining that he had published previous papers on the topic and hoped to encourage others to carry out experiments in India, but that he had not yet received any new data. He writes about his theory relating to magnetic poles and diurnal variation. Includes a diagram illustrating magnetic variation in relation to the north pole. Corrections appear throughout in ink. Subject: Geodesy Written by Macdonald in Summerlands, Exeter [Devon, England]. 29 September 1825