Columbus was not the first great successful explorer of this century that we have called by his name. Many daring navigators, particularly during the half century preceding the discovery of America, had braved the perils of the ocean, so literally trackless for them, in order to add to man's knowledge. A great stimulus to the spirit of navigation and exploration came with the rediscovery of the Cape Verde Islands by the Portuguese in 1447. Men dared after this to sail with the definite purpose of finding hitherto unknown land, and their bravery was rewarded in 1460 by the discovery of Sierra Leone. Prince Henry of Portugal then realized that the future of his country, hemmed in as it was in Europe, would largely depend upon the success of her navigators. He gathered together and systematized all the knowledge obtainable in nautical matters, and well deserves the name of Henry the Navigator. It was under his inspiration that the coast of Africa and the Senegal and the Gambia were explored. Probably no one more than he helped to remove the imagined terrors of the deep and gave men courage to venture ever farther and farther in exploration. His great purpose was the spread of Christianity, and to this he brought every incentive from patriotism and every possible help that could be obtained from science in any way. His name gloriously opened Columbus' Century. It is possible that the old tradition that Henry established a college of navigation and even, as some have declared, an astronomical observatory at Sagres, near Cape St. Vincent, with the special purpose of making observations on the declination of the sun so as to secure more accurate nautical tables, may be a pious exaggeration of ardent admirers. Undoubtedly, however, he did a great deal for the scientific {263} development of navigation and established a tradition that was well followed in Portugal. John II of Portugal appointed a commission on navigation consisting of Roderick and Joseph, his physicians, and Martin of Bohemia. They invented the astrolabe, though the cross staff continued to be used for some time by navigators and was one of the few instruments possessed by Columbus and Vasco da Gama. Martin Cortez described the astrolabe and shows how much more convenient it is than the cross staff for taking altitudes.