After the writers mentioned in the last chapter a long interval elapsed without any progress being made in the knowledge of the shape or configuration of the earth. From the fall of the Roman Empire, whose colonies themselves gave a certain knowledge of geography, down to the fifteenth century, when the great impetus was given to discovery by the adventurous voyagers of Spain and Portugal, there was nothing but servile copying from ancient authors, who were even misrepresented when they were not understood. Even the peninsula of India was only known by the accounts of Orientals and the writings of the Ancients until the beginning of the fifteenth century. Vague notions, too, were held as to the limits of Africa, and even of Europe and Asia--while of course they knew nothing of America, in spite of their marking on their maps an antichthonal continent to the south. Denys, the traveller, a Greek writer of the first century, and Priscian, his Latin commentator of the fourth, still maintained the old errors with regard to the earth. According to them the earth is not round, but leaf-shaped; its boundaries are not so arranged as to form everywhere a regular circle. Macrobius, in his system of the world, proves clearly that he had no notion that Africa was continued to the south of Ethiopia, that is of the tenth degree of N. latitude. He thought, like Cleanthus and Crates and other ancient authors, that the regions that lay nearest the tropics, and were burnt by the sun, could not be inhabited; and that the equatorial regions were occupied by the ocean. He divided the hemisphere into five zones, of which only two were habitable. "One of them," he said, "is occupied by us, and the other by men of whose nature we are ignorant." Orosus, writing in the same century (fourth), and whose work exercised so great an influence on the cosmographers of the middle ages and on those who made the maps of the world during that long period, was ignorant of the form or boundaries of Africa, and of the contours of the peninsulas of Southern Asia. He made the heavens rest upon the earth. S. Basil, also of the fourth century, placed the firmament on the earth, and on this heaven a second, whose upper surface was flat, notwithstanding that the inner surface which is turned towards us is in the form of a vault; and he explains in this way how the waters can be held there. S. Cyril shows how useful this reservoir of water is to the life of men and of plants. Diodorus, Bishop of Tarsus, in the same century, also divided the world into two stages, and compared it to a tent. Severianus, Bishop of Gabala, about the same time, compared the world to a house of which the earth is the ground floor, the lower heavens the ceiling, and the upper, or heaven of heavens, the roof. This double heaven was also admitted by Eusebius of Caesaraea.