[Footnote 44: Columbus not only has, incontestably, the merit of first discovering the line where there is no declination of the needle, but also of first inducing a study of terrestrial magnetism in Europe, by his observations concerning the increasing declination as he sailed in a westerly direction from that line. It had been already easily recognized in the Mediterranean, and in all places where, in the twelfth century, the declination was as much as eight or ten degrees, even though their instruments were so imperfect that the ends of a magnetic needle did not point exactly to the geographical north or south. It is improbable that the Arabs or Crusaders drew attention to the fact of the compass pointing to the northeast and northwest in different parts of the world, as to a phenomenon which had long been known. The merit which belongs to Columbus is, not for the first observance of the existence of the declination, which is given, for example, upon the map of Andrew Bianca, in 1436, but for the remark which he made on the 13th of September, 1492, that about two degrees and a half to the east of the Island of Corvo the magnetic variation changed, and that it passed over from northeast to northwest. This discovery of a magnetic line without any variation indicates a remarkable epoch in nautical astronomy. It was celebrated with just praise by Oviedo, Casas, and Herrera. If with Livio Sanuto we ascribe it to the renowned mariner Sebastian Cabot, we forget that his first voyage, which was undertaken at the expense of some merchants of Bristol, and which was crowned with success by his touching the main-land of America, falls five years later than the first expedition of Columbus.--Humboldt's _Cosmos_, vol. ii., p. 318; Las Casas, _Hist. Ind._, lib. i., cap. 6.] [Footnote 45: "In sailing toward the West India Islands birds are often seen at the distance of two hundred leagues from the nearest coast."--Sloane's _Nat. Hist. of Jamaica_, vol. i., p. 30. Captain Cook says, "No one yet knows to what distance any of the Oceanic birds go to sea; for my own part, I do not believe that there is any one of the whole tribe that can be relied on in pointing out the vicinity of land."--_Voyage toward the South Pole_, vol. i., p. 275. The Portuguese, however, only keeping along the African coast and watching the flight of birds with attention, concluded that they did not venture to fly far from land. Columbus adopted this erroneous opinion from his early instructors in navigation.]