This work by William Scoresby (1789-1857) was edited by Archibald Smith (1813-1872) and published posthumously in 1859. It is the account of Scoresby's final voyage and last scientific study, which took place between February and August 1856. Scoresby made his Australian voyage on board the Royal Charter, owned by the Liverpool and Australia Steam Navigation Company. He wished to observe the changes that take place in the magnetic state of iron ships travelling on a north-to-south magnetic latitude, and to assess how magnetic changes affect the working of a compass so that he could discover the most reliable location for it on board ship. The first part of the work is an exposition of magnetic principles, followed by the results and conclusions of Scoresby's experiments. The second part contains a travel account of the actual voyage. It is a key work of nineteenth-century navigation science.