From about 1600 to 1800 scientists and mariners made increasingly sophisticated attempts to understand the Earth's magnetic field and use it in navigation. Europeans had long understood the difference between the magnetic and true north, but why did it vary as one traversed the sea? Could this variation be used to pinpoint longitude? Drawing on a wealth of unpublished sources - including manuals, treatises, sailing directions, and logbooks in a half-dozen languages - A. R. T. Jonkers explores these early efforts both for what they reveal about the history of science and navigation and as a unique record of the actual changes in the Earth's magnetic field. The result, a combination of science and history, will appeal to a broad audience of specialists as well as general readers.