[18] Bauer, L. A. Principal Facts Relating to the Earth’s Magnetism. (In: United States Magnetic Declination Tables and Isogonic Charts for 1902. Washington, 1902.) Printed also as a separate; Wolkenhauer, A. Beiträge zur Geschichte der Kartographie und Nautik des 15 bis 17 Jahrhundert. München, 1904; Hellmann, G. Ueber die Kentniss der magnetischen Deklination vor Christopher Columbus. (In: Meteorologische Zeitschrift. Braunschweig, 1906.) Gilbert, W. De Magnete. London, 1600, and reissued in translation in 1893. This work is one of great significance in its treatment of magnetism and electricity. See especially Bk. IV on Variation, Pedro Medina in his Art de Navigar, Valladolid, 1545, contended that the magnetic needle always points to the true north; Stevenson, E. L. Early Spanish Cartography of the New World. (In: Proceedings of the American Antiquarian Society. Worcester, 1909.) Attention is called in this paper to certain errors in early Spanish maps, probably due to a failure to note properly the declination of the magnetic needle. [19] Baudet, op. cit.; same author. Nachscrift, 1872; same author. Notice sur la part prise par Willem Jansz. Blaeu dans la détermination des longitudes terrestres. Utrecht, 1875. Stevenson, E. L. Willem Janszoon Blaeu (1571-1638), a sketch of his life and work with an especial reference to his large world map of 1605 with facsimile. New York, 1914; Aa, op. cit., Vol. II. pp. 578-580; Dozy, C. M. Willem Janszoon Blaeu. (In: Tijdschrift van het Nederlandsch Aardrijkskundig Genootschap. Amsterdam, 1887. pp. 206-215.); Tiele, P. A. Leven en werken van Willem Jansz. Blaeu door P. J. Baudet. (In: De Gids. Amsterdam, 1872. Dorde Serie, Vol. I, pp. 356-367.); Tiele, P. A. Nederlandsche Bibliographie van Land- en Volkerkunde. Amsterdam, 1884. See this work for a bibliography of the works of Blaeu. [20] Baudet, op. cit., pp. 77-114. [21] See reference to Tycho Brahe, I, 183. [22] See I, 184. [23] Pictures of these instruments may be found in Le grand Atlas. [24] Kepler, J. Astronomia nova ... De Motibus Stellae Martis. Prague, 1609; Allgemeine deutsche Biographie, “Kepler, Johann”; Wolf, Geschichte der Astronomie, pp. 281-310. [25] In his earliest maps and charts Blaeu clearly had as his main purpose that of being of service to navigators. [26] Blaeu, J. Le grand Atlas ou Cosmographie Blaviane. Amsterdam, 1663-1671. 12 Vols. Practically the same work in the Latin, the Dutch, and the Spanish languages. A bibliographical list of Blaeu’s principal geographical publications is given in Stevenson, op. cit., pp. 65-67, in Phillips, op. cit., and in Tiele, op. cit. [27] Stevenson, op. cit., p. 25. [28] Génard, P. M. N. J. Les globes de Guillaume Blaeu. (In: Bulletin Société Géographie d’Anvers. Anvers, 1883. Vol. VIII, pp. 159-160.); Baudet, op. cit., pp. 35-52; Stevenson, op. cit., pp. 15, 43-50. [29] The Mercator globe has a diameter of 41 cm. and the Van Langren a diameter of 32 cm.