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.
Aurora Borealis, or Northern Polar Light
Dr. Halley was the first to give (Phil. Trans., No. 347) a distinct history of this phenomenon, which has certainly an electric as well as magnetic origin, and to which Gassendi originally gave the name it now bears, as has been stated at A.D. 1632.
According to Dr. Lardner (“Lectures,” Vol. I. p. 137), Prof. Eberhart, of Halle, and Paul Frisi, of Pisa, first proposed an explanation of the aurora founded upon the following: 1. Electricity transmitted through rarefied air exhibits a luminous appearance, precisely similar to that of the aurora borealis. 2. The strata of atmospheric air become rarefied as their altitude above the surface of the earth is increased, a theory which has since been countenanced by many scientists. It has been observed, notably by Dalton, of Manchester, that the primitive beams of the aurora are constantly in a direction parallel to that of the dipping needle, and that the latter appears most affected when the aurora is the brightest. Arago noticed that the changes of inclination amounted, upon one occasion to 7’ or 8’. The discovery that the magnetic needle was agitated during the presence of an aurora has been ascribed to Wargentin (Am. Journal Sc., Vol. XXX. p. 227), though it is claimed by the friends of Olav Hiörter (see A.D. 1740), that it was independently ascertained by the latter during the year 1741.
The well-known Swiss chemist Auguste Arthur De la Rive has made many important observations upon the electric character of the aurora, the experiments carried on by him in the mountains of Finland being thus described: “We surrounded the peak of a mountain with copper wire, pointed at intervals with tin nibs. We next charged the wire with electricity, and nearly every night during our stay produced a yellowish white light on the tin points, in which the spectroscope analysis revealed the greenish yellow rays so characteristic of the aurora borealis. On the peak of Pietarintumturi we were especially successful, an auroral ray making its appearance directly over and about 150 yards above the copper coil.”
A complete list of all auroras appearing prior to 1754 is to be found in Jean Jacques d’Ortons de Mairan’s, Paris, 1731, “Traité Physique de l’Aurore Boréale,” and a catalogue of auroræ observed, 1800–1877, has been made up by M. Zenger (Sci. Am. Supp., p. 10915). One of the most interesting displays is known as the purple aurora, alluded to in the Annals of Clan-mac-noise as having appeared A.D. 688 (Biot “Note sur la direction,” etc., Comptes Rendus, Tome XIX for 1844, p. 822). Between September 19, 1838, and April 8, 1839, Lottin, Bravais, Lilliehöök and Siljeström observed 160 auroras at Bossekop (69° 58’ N. lat.) in Finmark and at Jupvig (70° 6’ N. lat.); they were most frequent during the period the sun remained below the horizon, that is, from November 17 to January 25. During this night of 70 times 24 hours there were 64 auroras visible (Comptes Rendus, Tome X. p. 289; Martin, “Météorologie,” 1843, p. 453; Argelander, in the “Vorträgen geh. in der Königsberg Gesellschaft,” Bd. I. s. 259).
A Finnish physicist, named S. Lenström, who had been attached to the Nordenskjold Polar Expedition of 1868, visited Lapland in 1871, and, after a series of important observations, constructed an apparatus that permitted him to “artificially reproduce the light of the aurora.” The intensity of this light is so great at times that Lowenörn perceived the coruscations in bright sunshine on the 29th of January, 1786, and Parry saw the aurora throughout the day during the voyage of 1821–1823.