“This method of navigating permitted them to make voyages from place to place; but when they wished to sail in the open sea, losing sight of the coast and standing out on the wide ocean, they perceived the numerous errors they had made in calculating and judging the day’s run, for they had been accustomed to allow so much way to the ship in the twenty-four hours on account of the currents and the other mysteries of the sea, the facts of which are clearly demonstrated by navigating by altitude. But as necessity is the teacher of all arts, in the time of King John II., the matter of navigation was assigned by him to Master Roderic, and Master Joseph, a Jew, (who were his physicians,) and to one Martin of Bohemia, a native of those parts, who boasted of being a pupil of John of Monteregius, a famous astronomer among the professors of that science,[100] and these devised the way of navigating by the sun’s altitude, and they made tables of his declination such as are now used by navigators, now more complete than they were at the beginning when the great wooden astrolabes were first used.”[101] This novel and serviceable nautical instrument, first made of wood and of a triangular shape, was soon in general use on Portuguese vessels. The astrolabe was improved from time to time. At the beginning of the seventeenth century, when the French seaman, Samuel de Champlain, was exploring the St. Lawrence River and its tributaries, it was a disk of brass, having one of its faces marked with degrees and minutes. A brass rule, called a label, with sight-holes, turning upon a pivot at the centre of the instrument, moved over the marked circumference of the disk. At the zenith part of the instrument there was a small ring by which it could be suspended from the thumb of the person taking an observation. When used the astrolabe was turned to the sun, so that his rays could freely pass through the two sight-holes of the label. In this position the altitude of the sun was indicated on the divided limb of the instrument. Opposite the zenith point was a small eyelet from which a weight could be suspended to keep the instrument from shaking when used. [Illustration: A representation of the astrolabe found in 1867 in the county of North Renfrew, province of Ontario, Canada, supposed to have been lost by Champlain on his way to Ottawa in 1613. The diameter of the instrument is “five inches and five-eights.” _Vide_ “Champlain’s astrolabe.” By A. J. Russell. Montreal, 1879.]