In their search for new trade-routes to the lands of silk and spice, these peoples of southwestern Europe were not as much in the dark as sometimes we are inclined to believe. Geographical knowledge, almost non-existent in the earlier middle ages, had been enriched by the Franciscan friars who had traversed central Asia to the court of the Mongol emperor as early as 1245, and by such merchants and travelers as Marco Polo, who had been attached to the court of Kublai Khan and who subsequently had described that potentate's realms and the wealth of "Cipangu" (Japan). These travels afforded at once information about Asia and enormous incentive to later explorers. Popular notions that the waters of the tropics boiled, that demons and monsters awaited explorers to the westward, and that the earth was a great flat disk, did not pass current among well-informed geographers. Especially since the revival of Ptolemy's works in the fifteenth century, learned men asserted that the earth was spherical in shape, and they even calculated its circumference, erring only by two or three thousand miles. It was maintained repeatedly that the Indies formed the western boundary of the Atlantic Ocean, and that consequently they might be reached by sailing due west, as well as by traveling eastward; but at the same time it was believed that shorter routes might be found northeast of Europe, or southward around Africa. [Sidenote: Navigation] Along with this general knowledge of the situation of continents, the sailors of the fifteenth century had learned a good deal about navigation. The compass had been used first by Italian navigators in the thirteenth century, mounted on the compass card in the fourteenth. Latitude was determined with the aid of the astrolabe, a device for measuring the elevation of the pole star above the horizon. With maps and accurate sailing directions (_portolani_), seamen could lose sight of land and still feel confident of their whereabouts. Yet it undoubtedly took courage for the explorers of the fifteenth century to steer their frail sailing vessels either down the unexplored African coast or across the uncharted Atlantic Ocean. [Sidenote: The Portuguese Explorers] In the series of world-discoveries which brought about the Commercial Revolution and which are often taken as the beginning of "modern history," there is no name more illustrious than that of a Portuguese prince of the blood,--Prince Henry, the Navigator (1394-1460), who, with the support of two successive Portuguese kings, made the first systematic attempts to convert the theories of geographers into proved fact. A variety of motives were his: the stern zeal of the crusader against the infidel; the ardent proselyting spirit which already had sent Franciscan monks into the heart of Asia; the hope of reëstablishing intercourse with "Prester John's" fabled Christian empire of the East; the love of exploration; and a desire to gain for Portugal a share of the Eastern trade.