The Crusades had been a lesson in the liberal art of travelling. But very few people had ever ventured beyond the well-known beaten track which led from Venice to Jaffe. In the thirteenth century the Polo brothers, merchants of Venice, had wandered across the great Mongolian desert and after climbing mountains as high as the moon, they had found their way to the court of the great Khan of Cathay, the mighty emperor of China. The son of one of the Polos, by the name of Marco, had written a book about their adventures, which covered a period of more than twenty years. The astonished world had gaped at his descriptions of the golden towers of the strange island of Zipangu, which was his Italian way of spelling Japan. Many people had wanted to go east, that they might find this gold-land and grow rich. But the trip was too far and too dangerous and so they stayed at home. [Illustration: MARCO POLO] Of course, there was always the possibility of making the voyage by sea. But the sea was very unpopular in the Middle Ages and for many very good reasons. In the first place, ships were very small. The vessels on which Magellan made his famous trip around the world, which lasted many years, were not as large as a modern ferryboat. They carried from twenty to fifty men, who lived in dingy quarters (too low to allow any of them to stand up straight) and the sailors were obliged to eat poorly cooked food as the kitchen arrangements were very bad and no fire could be made whenever the weather was the least bit rough. The mediæval world knew how to pickle herring and how to dry fish. But there were no canned goods and fresh vegetables were never seen on the bill of fare as soon as the coast had been left behind. Water was carried in small barrels. It soon became stale and then tasted of rotten wood and iron rust and was full of slimy growing things. As the people of the Middle Ages knew nothing about microbes (Roger Bacon, the learned monk of the thirteenth century seems to have suspected their existence, but he wisely kept his discovery to himself) they often drank unclean water and sometimes the whole crew died of typhoid fever. Indeed the mortality on board the ships of the earliest navigators was terrible. Of the two hundred sailors who in the year 1519 left Seville to accompany Magellan on his famous voyage around the world, only eighteen returned. As late as the seventeenth century when there was a brisk trade between western Europe and the Indies, a mortality of 40 percent was nothing unusual for a trip from Amsterdam to Batavia and back. The greater part of these victims died of scurvy, a disease which is caused by lack of fresh vegetables and which affects the gums and poisons the blood until the patient dies of sheer exhaustion.