Venetian Monopoly of Trade.--Thus all interest centered upon the southern route. By treaty with the sultan or ruler of Egypt, Venice secured a monopoly of the products which came over this route. Goods from the East now came in fleets up the Red Sea, went through the hands of the sultan of Egypt, who collected a duty for them, and then were passed on to the ships of the wealthy Venetian merchant princes, who carried them throughout Europe. Although the object of intense jealousy, it seemed impossible to wrest this monopoly from Venice. Her fleet was the strongest on the Mediterranean, and her rule extended along the Adriatic to the Grecian islands. All eager minds were bent upon the trade with the East, but no way was known, save that which now Venice had gained. Extent of Geographical Knowledge.--The Maps of this Period.--To realize how the problem looked to the sailor of Genoa or the merchant of Flanders at that time, we must understand how scanty and erroneous was the geographical knowledge of even the fifteenth century. It was believed that Jerusalem was the center of the world, a belief founded upon a biblical passage. The maps of this and earlier dates represent the earth in this way: In the center, Palestine, and beneath it the Mediterranean Sea, the only body of water which was well known; on the left side is Europe; on the right, Africa; and at the top, Asia--the last two continents very indefinitely mapped. Around the whole was supposed to flow an ocean, beyond the first few miles of which it was perilous to proceed lest the ship be carried over the edge of the earth or encounter other perils. Ideas about the Earth.--The Greek philosophers before the time of Christ had discovered that the world is a globe, or ball, and had even computed rudely its circumference. But in the Middle Ages this knowledge had been disputed and contradicted by a geographer named Cosmas, who held that the world was a vast plane, twice as long as it was broad and surrounded by an ocean. This belief was generally adopted by churchmen, who were the only scholars of the Middle Ages, and came to be the universal belief of Christian Europe. The Renaissance revived the knowledge of the writings of the old Greek geographers who had demonstrated the earth's shape to be round and had roughly calculated its size; but these writings did not have sufficient circulation in Europe to gain much acceptance among the Christian cosmographers. The Arabs, however, after conquering Egypt, Syria and northern Africa, translated into their own tongue the wisdom of the Greeks and became the best informed and most scientific geographers of the Middle Age, so that intercourse with the Arabs which began with the Crusades helped to acquaint Europe somewhat with India and China.