They and their pupils wrote books giving their reasons for believing this; but as printing was not invented till seventeen hundred years later, these works were known only to a few learned men. Most of the European scholars then lived in Greece or Constantinople, and kept these precious manuscripts in monasteries or private libraries, or in palaces and schools. In the twelfth century a Spanish-A-ra´bi-an philosopher read some of these Greek manuscripts, and then wrote a book, saying that he not only believed that the earth is round, but that he thought it would be possible to sail around it! This statement, so natural and simple now, seemed so absurd to the people who heard it then that they began to make all manner of fun of it. They asked how it would be possible for a ship to sail uphill, even supposing it did not tumble off the earth when it reached the edge, which they called the jumping-off place. They also asked how the trees on the other side of the earth could grow with their roots up in the air, and inquired whether the rain and snow flew up instead of falling down. These questions, which seem so odd now, were very natural, for people did not then know, what your teachers have explained to you, that the earth is like a big magnet. It holds fast everything on its surface, and nothing can fall off, even though it spins around and around, and whirls through space much faster than the fastest express train can travel. The Turks, who were not Christians, besieged the city of Constantinople in 1453, and when they became masters of it, and of the surrounding country, the learned men all fled, taking with them many of their precious manuscripts. Deprived of their quiet homes, and in many cases forced to teach to earn their living, these wise men settled in various cities, where they imparted to others all they knew. As printing had just been invented, books, instead of being worth a fortune, soon became so much cheaper that almost everybody could afford to have one or two volumes. The precious manuscripts the wise men had saved from the Turks were therefore printed, and people soon began to talk about the strange things they read in them, and longed to know more. VII. PRINCE HENRY THE NAVIGATOR. Among the first books printed were the accounts of the travels of two daring men, Mar´co Po´lo and Sir John Man´de-ville. These men had visited many of the countries of the East, and the first had even gone to China, which was then called Ca-thay´. The stories these travelers told were so interesting and exciting that people became anxious to visit these strange countries, and especially to trade there and thus grow rich.