Note 142 — Chapter IV · 1492-1493.

“On September 13, 1492, he had reached far enough to the westward to come from a previously eastern declination within a region of westerly declination, and that on September 17 it amounted to a whole point (11¼°).” This constitutes his well-known discovery of a part of a line of no-declination. “Two hundred and twenty-four leagues or, near enough for our purpose, 672 nautical miles, west of the island of Gomera would place him on September 13, in latitude 28° 06´ north, and in longitude 12° 42´ + 17° 08´ = 29° 50´, according to Bowditch, or if we take the position of the harbor of Sebastian near the eastern point of Gomera Island, according to admiralty chart No. 1873, viz.: latitude 28° 05´ 5 and longitude 17° 06´ 3 and considering that 11° 12´ correspond to 12° 42´ of difference of longitude in that latitude, we have for a point in the line of no-declination the latitude of 28° 05´ and longitude 29° 48´. In E. Walker’s treatise on Terrestrial and Cosmical Magnetism, Cambridge (England), 1866, p. 300, we read: ‘The history of this line dates from the 13th of September, 1492, when Columbus observed the needle pass from the east to the west of the meridian, in latitude 28° N. longitude 28° W. (probably roughly adding 11° of difference of longitude to 17° for longitude of Gomera). According to my computation of the daily position of the Admiral’s flagship, and based upon his log-book, he was on September 13 in latitude 28° 21´ longitude 29° 16´.... According to my computation of the daily track, Columbus was on September 17, 1492, in latitude 27° 38´ and in longitude 36° 30´, when he noted 11° west declination.”—An inquiry into the variation of the compass off the Bahama Islands, at the time of the landfall of Columbus in 1492. By Charles A. Schott. United States Coast and Geodetic Survey. Appendix No. 19. Report for 1880. Washington, 1882. p. 5.

“Christopher Columbus has not only the merit of being the first to discover a line without magnetic variation, but also of having excited a taste for the study of terrestrial magnetism in Europe, by means of his observations on the progressive increase of western declination in receding from that line.”—Humboldt: Cosmos. Otté’s trans. vol. ii. p. 656.