Magellan stopped at the Plate in the beginning of 1520, and explored the estuary to make sure that it did not afford the passage he was seeking. In October he reached the mouth of the strait that bears his name, and, wonderfully favoured by wind and weather, threaded his way to the Pacific in five weeks. Subsequent wayfarers were not so fortunate and the strait never became a practicable commercial route until after the introduction of steam navigation. In the succeeding hundred years not half a dozen ships reached the Pacific around South America. Practically, the Pacific was accessible only over the Isthmus or by the immensely long journey around the Cape of Good Hope. Nevertheless, the importance of this epoch-making voyage has not been overestimated. The Pacific became, in a sense, a Spanish lake, in which she could maintain at will a naval preponderance. She occupied the Philippines and secured control at leisure of the Pacific coast of America. However, the scientific results were more important. Thereafter, the thorough exploration of all the shores of the South Sea was only a question of time. Magellan's voyage made geography an exact science. He sketched the map of the world with broad and sure strokes and left nothing for subsequent explorers except the filling-in of details. The occupation of the Philippines and Moluccas gave rise to new disputes between Spain and Portugal as to their rights under the Treaty of Tordesillas. The imperfect instruments of those days left the line doubtful on the eastern South American coast, as well as on the other side of the world. In 1526, Sebastian Cabot was sent by the Spanish government to determine astronomically the location of the line in America, and then to follow Magellan's track to western Asia. At the mouth of the Plate he heard rumours among the Indians of silver mines on the river's banks and of the existence of a great and wealthy empire at its headwaters. This was Peru--not yet reached by the Castilians on their way south from the Isthmus, but the coast Indians showed Cabot silver ornaments which had been passed from hand to hand from the highlands of Peru and Bolivia down the river to the Atlantic.