The earliest placing of a magnetic pole seems due to the voyage of Nicholas of Lynn, whose narrative was presented to Edward III. of England. This account is no longer known,[371] though the title of it, Inventio fortunata, is preserved, with its alleged date of 1355. Cnoyen, whose treatise is not extant, is thought to have got his views about the regions of the north and about the magnetic pole from Nicholas of Lynn,[372] while he was in Norway in 1364; and it is from Cnoyen that Mercator says he got his notion of the four circumpolar islands which so long figured in maps of the Mercator and Finæus school. In the Ruysch map (1508) we have the same four polar islands, with the magnetic pole placed within an insular mountain north of Greenland. Ruysch also depended on the Inventio fortunata. Later, by Martin Cortes in 1545, and by Sanuto in 1588, the pole was placed farther south.[373]
Ptolemy, in the second century, accepting the generally received opinion that the world as known was much longer east and west than north and south, adopted with this theory the terms which naturally grew out of this belief, latitude and longitude, and first instituted them, it is thought, in systematic geography.[374]
Pierre d’Ailly, in his map of 1410,[375] in marking his climatic lines, had indicated the beginnings, under a revival of geographical inquiry, of a systematic notation of latitude. Several of the early Ptolemies[376] had followed, by scaling in one way and another the distance from the equator; while in the editions of 1508 and 1511 an example had been set of marking longitude. The old Arabian cartographers had used both latitude and longitude; but though there were some earlier indications of the adoption of such lines among the European map-makers, it is generally accorded that the scales of such measurements, as we understand them, came in, for both latitude and longitude, with the map which Reisch in 1503 annexed to his Margarita philosophica.[377]
Ptolemy had fixed his first meridian at the Fortunate Islands (Canaries), and in the new era the Spaniards, with the sanction of the Pope, had adopted the same point; though the Portuguese, as if in recognition of their own enterprise, had placed it at Madeira,—as is shown in the globes of Behaim and Schöner, and in the map of Ruysch. The difference was not great; the Ptolemean example prevailed, however, in the end.[378]
In respect to latitude there was not in the rude instruments of the early navigators, and under favorable conditions, the means of closely approximate accuracy. In the study which the Rev. E. F. Slafter[379] has made on the average extent of the error which we find in the records of even a later century, it appears that while a range of sixty geographical miles will probably cover such errors in all cases, when observations were made with ordinary care the average deviation will probably be found to be at least fifteen miles. The fractions of degrees were scarcely ever of much value in the computation, and the minute gradation of the instruments in use were subject to great uncertainty of record in tremulous hands. It was not the custom, moreover, to make any allowance for the dip of the horizon, for refraction or for the parallax; and when, except at the time of the equinox, dependence had to be placed upon tables of the sun’s declination, the published ephemerides, made for a series of years, were the subjects of accumulated error.[380]
REGIOMONTANUS’ ASTROLABE.
This cut follows the engravings in Ruge’s Geschichte des Zeitalters der Entdeckungen, p. 106, and in Ghillany’s Ritter Behaim, p. 40. Cf. Von Murr, Memorabilia bibliothecarum Norimbergensium, i. 9.