Unpublished diagram, 'Figure showing an experimental mode of finding the depth of the magnetic pole' by John Macdonald Figure showing a section of the Earth through the north west and north magnetic poles. Labelled as 'Figure showing an experimental mode of finding the depth of the magnetic pole "P", within the Earth, provided care be taken to free the dip at "a" or "r", from such local attraction as may be found at either place'. 5 February 1831