Unpublished letter, regarding southeast line of no variation from John Macdonald to Sir Humphry Davy Macdonald discusses his desire to find the southeast line of no variation and the different locations for this line previously provided by various explorers. He then discusses possible relations between solar heat, polar attraction, and magnetism. Includes a diagram showing the locations of poles and magnetic meridians. John Frederick William Herschel's initials appear in red ink next to the date received. An ink inscription at top of first page reads 'N. T.Y. [Thomas Young]'. Subject: Geodesy Written by Macdonald in Summerlands [Devon, England]. Received by the Royal Society on 15 November 1826. 12 June 1826