September 3, 1891] 



NATURE 



425 



will prove this to you. The delineation of Italy, on the so-called 

 Catalan map, is surprisingly correct ; whilst Gastaldo, whose 

 map of Italy is nearly two hundred years younger, has not yet 

 been able to emancipate himself from the overpowering authority 

 of Ptolemy. And in this he did not sin alone, for Italian and 

 other cartographers of a much later time still clung pertina- 

 ciously to the same error. 



There were others, however, who recognized the value of 

 these charts, and embodied them in maps of the entire world. 

 Among such were Marino Sanuto (1320) and Fra Mauro (1453), 

 both of whom made their maps the repository of much informa- 

 tion gathered from the Arabs or from their own countrymen who 

 had seen foreign parts. Fra Mauro, more especially has trans- 

 mitted to us a picture of Abys-inia marvellously correct in its 

 details, though grossly exaggerated in its dimensions. 



Another step in the right direction was taken when the 

 cartographers and pilots of Portugal and Spain returned to the 

 crude projection of Dica^arch, Eratosthenes, and Marinus, 

 which enabled them to lay down places according to latitude 

 and longitude upon their "plane charts," 



Germany, debarred from taking a share in the great maritime 

 discoveries of the age, indirectly contributed to their success by 

 improvements in mathematical geography and the introduction 

 of superior instruments. The navigators of the early middle 



Lalande formed the basis of all astronomical calculations during a 

 century, that more exact results were obtained. The suggestion 

 to determine longitude by meansof lunar distances or occultations 

 of stars bore no fruit at that time, as the knowledge of the com- 

 plicated motion of the moon was still very imperfect. Still less 

 was known about the movements of the satellites of Jupiter, 

 which Galileo had first espied in 1610 when looking at that 

 planet through his telescope. They became available only after 

 tables of their revolutions and eclipses had been published by 

 Cassini in 1668. 



Another suggestion for the determination of longitude was 

 made by Gemma Frisius in 1530 — namely, that a clock or time- 

 keeper should be employed for the purpose. One of Huygens's 

 pendulum clocks was actually carried by Holmes to the Gulf of 

 Guinea, but the results obtained were far from encouraging. 



The difficulties which still attended the determination of 

 longitude in the sixteenth century are conspicuously illustrated 

 by the abortive attempts of a Congress of Spanish and Portuguese 

 navigators who met at Badajoz and Yelves in 1524 for the purpose 

 of laying down the boundary line, which Pope Alexander Vf. 

 had drawn at a distance of 370 Spanish leagues to the west of 

 Cape Verde Islands, to separate the dominions of Spain from 

 those of Portugal. Not being able to agree either as to the 

 length of a degree, nor even as to that of a league, they separated 

 without settling the question placed before them. 



ages still made use of an astrolabe when they desired to deter- 

 mine a latitude, but this instrument, which in the hands of an 

 expert observer furnished excellent results on land, was of little 

 use to a pilot stationed on the unsteady deck of a vessel. 

 Regiomontanus consequently conferred an immense service upon 

 the mariners of his time when, in 1471, he adapted to their use 

 an instrument already known to the ordinary surveyors. It 

 was this cross-staff which Martin Behaim introduced into the 

 Portuguese navy, and which quickly made its way among the 

 navigators of all countries. Most observations at sea were 

 made with this simple instrument, variously modified in the 

 course of ages, until it was superseded by Hadley's sextant. In 

 the hands of the more skilful navigators of the seventeenth 

 century, such as Baffin, James, and Tasman, the results obtained 

 with the cross-staff were correct within two or three minutes. 



Far greater difficulties were experienced in the observations of 

 longitudes. Lunar eclipses were most generally made use of, 

 but neither the ephemerides of Regiomontanus, for the years 

 1474 to 1506, which Columbus carried with him on his voyages, 

 nor those of Peter Apianus, for 1521-70, were sufficiently ac- 

 curate to admit of satisfactory results, even though the actual 

 observation left nothing to be desired. Errors of 30° in longi- 

 tude were by no means rare, and it was only when Kepler had 

 published his " Rudolphine Tables" (1626), which according to 



So uncertain were the results of observations for longitude 

 made during the sixteenth and seventeenth centuries, that it was 

 thought advisable to trust to the results of dead-reckoning rather 

 than to those of celestial observations. But the method of dead- 

 reckoning is available only when we have a knowledge of the 

 size of the earth, and this knowledge was still very imperfect, 

 notwithstanding the renewed measurement of an arc of the 

 meridian by Snellius, the Dutch mathematician (1615). This 

 measurement, however, is remarkable on account of its having 

 for the first time applied the exact method of triangulation to a 

 survey. 



The problem of measuring the ship's way had been attempted 

 by the Romans, who dragged paddle-wheels behind their ships, 

 the revolutions of which enabled them to estimate the distance 

 which the ship had travelled. But time, the strength of the 

 wind, and the pilot's knowledge of the qualities of his ship, still 

 constituted the principal elements for calculations of this kind, 

 for the "catena a poppa" (which Magellan attached to the 

 stern of his ship was merely intended to indicate the ship's 

 leeway, and not the distance which it had travelled. The 

 log, which for the first time enabled the mariner to carry 

 out his dead-reckoning with confidence, is first described in 

 Bourne's "Regiment for the Sea," which was published in 

 1577. 



NO. I 140, VOL. 44] 



