George Biddell Airy invented the first successful mechanical system of compass correction in 1838, at a time when iron ship-building, especially for steam-driven vessels, had become firmly established. One serious drawback to iron ships was the difficulty in the management of the magnetic compass on board due to the magnetic condition of the ship. The introduction to this paper, which outlines the early history of ship magnetism, is followed by a brief account of Airy's mechanical system. The main purpose of (...) the paper, however, is to comment on the detailed correspondence relating to the adjustment of the compass, by the Astronomer Royal himself, of the first iron sailing vessel, the Ironside, in November 1838; and to consider the degree of success, at least for the Ironside, of the new plan which was readily adopted for ships of the Mercantile Marine. (shrink)