1. Gilbert William, De magnete (London, 1600), trans. by Thompson Sylvanus P. (New York, 1958), 212. Gilbert devoted Book VI to the discussion of terrestrial magnetism.
2. Cosmology and the Magnetical Philosophy, 1640–1680
3. Kepler Johannes, Gesammelte Werke, ed. by von Dyck Walther and Caspar Max (Munich, 1937–75), xvi, 86. Letter to Johann Georg Brengger of 30 November 1607.
4. Kepler Johannes, Astronomia nova seu Physica coelestis (Prague, 1609), c. 23, p. 176.