In order to test Halley’s theory, the English Government permitted him to make three voyages in the Atlantic Ocean (1698, 1699, 1702), in vessels of which he had the command as post-captain. Humboldt states that these were the first expeditions equipped by any government for the establishment of a great scientific object--that of observing one of the elements of terrestrial force on which the safety of navigators is especially dependent. The result of these voyages was the construction of the first accurate Magnetic Chart, whereon the points at which navigators have found an equal amount of variation were connected together by curved lines. This was the model of all charts of a similar nature since constructed. Halley remarked upon its completion: “The nice determination of the variation, and several other particulars in the magnetic system, is reserved for a remote posterity. All that we can hope to do is to leave behind us observations that may be confided in, and to propose hypotheses which after-ages may examine, amend or refute.” See copy of his chart in Vol. I. No. I of “Terrestrial Magnetism,” also in Musschenbroek’s “Essais de Physique,” or, preferably, in Bouguer’s “Traité de Navigation,” where the lines for 1700 are in red ink, while those for 1744 are traced in black, thus readily indicating the changes in the declination. REFERENCES.--Cavallo, “Magnetism,” and “Nat. or Exp. Phil.,” Vol. II. p. 273; Lloyd, “Treatise on Magnetism,” 1874, p. 102; _Sci. Am. Suppl._, No. 224, pp. 3570, 3571; Whewell, “Hist. of the Inductive Sciences,” 1859, Vol. I. pp. 396–8, 435–7, 450, 451, 480, 481, and Vol. II. p. 225; Giambattista Scarella, “De Magnete,” 1759, Vol. II; also G. Casali, “Sopra la Grandine,” etc., 1767; “The Phil. Hist. and Mem. of the Roy. Ac. of Sciences at Paris,” London, 1742, Vol. I. p. 245; Vol. II. pp. 240–244, 270, 349; “Magnetic Results of Halley’s Expedition (1698–1700)” in “Terrestrial Magnetism,” September 1913, pp. 113–132; Houzeau et Lancaster, “Bibl. Gén.,” Vol. II. pp. 156–7; Dr. G. Hellmann “Neudrucke von schriften,” Nos. 4 and 8; Humboldt, “Cosmos,” 1859, Vol. V. pp. 59–60; John Wallis’s letters to Halley, London (_Phil. Trans._ for 1702–1703), p. 106; _Phil. Trans._ for 1667, 1683, 1692; “Memoirs of the Roy. Soc.,” 1739, Vol. II. p. 195; “A Bibliography of Dr. Edmund Halley,” by Alex. J. Rudolph, in the “Bulletin of Bibliography” for July 1905; “Old and New Astronomy,” by Richard A. Proctor, 1892, pp. 37–38; _Phil. Trans._ Vol. XIII for 1683, No. 148, p. 208; Vol. XVII. p. 563; Vol. XXIII. p. 1106; Vol. XXIX. p. 165; Vol. XLII. p. 155; Vol. XLVIII. p. 239, also the following abridgments: Hutton, Vol. II. p. 624; Vol. VI, pp. 99, 112; J. Lowthorp, Vol. II. p. 285; Reid and Gray, Vol. VI. p. 177; Eames and Martyn, Vol. VI. pp. 28, 286; Baddam, 1745, Vol. II. pp. 195–202; Vol. III. pp. 25–32.