For a generation longer the Portuguese continued to venture farther and farther along the coast, in the hope of finding it coming to an end, so that they might make their way by sea to India. At last, in 1486, Diaz rounded the Cape of Good Hope. Twelve years later (1498) Vasco da Gama, spurred on by Columbus' great discovery, after sailing around the Cape of Good Hope and northward beyond Zanzibar, steered straight across the Indian Ocean and reached Calicut, in Hindustan, by sea. [Sidenote: The spice trade.] These adventurers were looked upon with natural suspicion by the Mohammedan spice merchants, who knew very well that their object was to establish a direct trade between the spice islands and western Europe. Hitherto the Mohammedans had had the monopoly of the spice trade between the Moluccas and the eastern ports of the Mediterranean, where the products were handed over to Italian merchants. The Mohammedans were unable, however, to prevent the Portuguese from concluding treaties with the Indian princes and establishing trading stations at Goa and elsewhere. In 1512 a successor of Vasco da Gama reached Java and the Moluccas, where the Portuguese speedily built a fortress. By 1515 Portugal had become the greatest among maritime powers; and spices reached Lisbon regularly without the intervention of the Italian towns, which were mortally afflicted by the change. [Sidenote: Importance of spices in encouraging navigation.] There is no doubt that the desire to obtain spices was the main reason for the exploration of the globe. This motive led European navigators to try in succession every possible way to reach the East--by going around Africa, by sailing west in the hope of reaching the Indies, before they knew of the existence of America; then, after America was discovered, by sailing around it to the north or south, and even sailing around Europe to the north. It is hard for us to understand this enthusiasm for spices, for which we care much less nowadays. One former use of spices was to preserve food, which could not then as now be carried rapidly, while still fresh, from place to place; nor did our conveniences then exist for keeping it by the use of ice. Moreover, spice served to make even spoiled food more palatable than it would otherwise have been. [Illustration: The Voyages of Discovery] [Sidenote: Idea of reaching the spice islands by sailing westward.] It inevitably occurred to thoughtful men that the East Indies could be reached by sailing westward. The chief authority upon the form and size of the earth was still the ancient astronomer, Ptolemy, who lived about A.D. 150. He had reckoned the earth to be about one sixth smaller than it is; and as Marco Polo had given an exaggerated idea of the distance which he and his companions had traveled eastward, it was supposed that it could not be a very long journey from Europe across the Atlantic to Japan. [Sidenote: Columbus discovers America, 1492.]