Unpublished letter, regarding magnetic variation in Cape Horn from John Macdonald to Sir Humphry Davy Macdonald writes about an east variation of the magnetic needle observed near Cape Horn [Tierra del Fuego, Chile] on its west, south, and east sides, which he believes is caused by the local attraction of magnetic strata. Includes a figure illustrating this phenomenon. John Frederick William Herschel's initials appear in graphite 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 at Lady Chambers', Richmond [London]. Received by the Royal Society on 19 January 1827 from Davy. 18 January 1827