We have seen that Ptolemy, with true scientific caution, had left undefined the extent of Africa to the south; but Eratosthenes and many of the Roman geographers, even after Ptolemy, were not content with this agnosticism, but boldly assumed that the coast of Africa made a semicircular sweep from the right horn of Africa, just south of the Red Sea, with which they were acquainted, round to the north-western shore, near what we now term Morocco. If this were the fact, the voyage by the ocean along this sweep of shore would be even shorter than the voyage through the Mediterranean and Red Seas, while of course there would be no need for disembarking at the Isthmus of Suez. The writers who thus curtailed Africa of its true proportions assumed another continent south of it, which, however, was in the torrid zone, and completely uninhabitable. Now the north-west coast of Africa was known in Prince Henry's days as far as Cape Bojador. It would appear that Norman sailors had already advanced beyond Cape Non, or Nun, which was so called because it was supposed that nothing existed beyond it. Consequently the problems that Prince Henry had to solve were whether the coast of Africa trended sharply to the east after Cape Bojador, and whether the ideas of the ancients about the uninhabitability of the torrid zone were justified by fact. He attempted to solve these problems by sending out, year after year, expeditions down the north-west coast of Africa, each of which penetrated farther than its predecessor. Almost at the beginning he was rewarded by the discovery, or re-discovery, of Madeira in 1420, by João Gonsalvez Zarco, one of the squires of his household. For some time he was content with occupying this and the neighbouring island of Porto Santo, which, however, was ruined by the rabbits let loose upon it. On Madeira vines from Burgundy were planted, and to this day form the chief industry of the island. In 1435 Cape Bojador was passed, and in 1441 Cape Branco discovered. Two years later Cape Verde was reached and passed by Nuno Tristão, and for the first time there were signs that the African coast trended eastward. By this time Prince Henry's men had become familiar with the natives along the shore and no less than one thousand of them had been brought back and distributed among the Portuguese nobles as pages and attendants. In 1455 a Venetian, named Alvez Cadamosto, undertook a voyage still farther south for purposes of trade, the Prince supplying the capital, and covenanting for half profits on results. They reached the mouth of the Gambia, but found the natives hostile. Here for the first time European navigators lost sight of the pole-star and saw the brilliant constellation of the Southern Cross. The last discovery made during Prince Henry's life was that of the Cape Verde Islands, by one of his captains, Diogo Gomez, in 1460--the very year of his death. As the successive discoveries were made, they were jotted