The ancient Chinese invented simple compasses; the Phoenician navigators used them; Prince Henry of Portugal sponsored research and development; and later Europearl navigators developed declination charts, mapping lines of magnetic force throughout the world. Better maps required better compasses, and so followed improvements on the magnetic compass. . . even to the non-magnetic gyrocompass used by space explorers. This is mostly the history of successive devices, reminiscent of the same publishers' How Did We Find Out series by Asimov--but less informative on the science of magnetism than we'd expect Asimov to be, and less skilled at casting the different steps as an ongoing development. Lackluster.