Another fact worthy of note during these earlier years was his vocation during the intervals between his voyages. He seems to have interlarded his more or less piratical expeditions on the sea with the gentle experiences of a bookseller and map-maker on the land. The art of printing had but recently been invented, and few books had been issued from the press; but there was some trade in books for all that. There is abundant evidence that this youthful enthusiast, at the period of his life between fifteen and twenty-four, availed himself of whatever knowledge came in his way in regard to the subject that was beginning to fill and monopolize his mind. During the fifteenth century, as hereafter we shall have occasion to see, a large number of books on geography became generally known. Many of the classics, after lying dormant for a thousand years, sprang suddenly into life; and it is quite within the scope of a reasonable historical imagination to conjecture that, even during his years at Genoa, many of the leisure hours of what could hardly have been a very absorbing vocation as a bookseller were spent in gaining such knowledge as was possible concerning the shape and size of the earth. It would be out of place in this connection to consider details; it is enough to know that even in his earliest writings on the subject, he alluded freely to the geographical writers whose works he had read. At some time between 1470 and 1473, Columbus changed his abode from Genoa to Lisbon. There were two facts that made this transfer of his activities both natural and beneficial. The first was that during the early part of the fifteenth century Portugal had placed herself far in advance of other nations, by her maritime expeditions and achievements. Prince Henry, with a courage and enterprise that have secured for him imperishable renown, had pushed out the boundaries of geographical knowledge, and had awakened an enthusiastic zeal for further discoveries. The fleets of Portugal had made themselves at length familiar with the west coast of Africa; and the bugbear of a tropical sea whose slimy depths were supposed to make navigation impossible, had been dispelled. The interest of every geographical explorer had been aroused and excited. Lisbon was the centre of this new ferment. The second consideration of importance was the fact that Bartholomew, a younger brother of Columbus, had established himself at the Portuguese capital as a maker and publisher of maps and charts. For the products of this handicraft there had been created an active demand. Nothing was more natural, then, than that this young enthusiast, in whom there were already welling up all kinds of maritime ambitions, should remove to that centre of geographical knowledge and interest, and ally himself with his brother in so congenial and promising a vocation.