Africa hovered behind his eyes, a dream carried through the day. Its massive wildnerness called out to him, luring him with its uncharted mysteries.    The Venetian Captain diUmbria had sailed to the wild continent many times of course. From when he was just starting out, south from the Adriatic; in those days he had feared the brutal pirates that roamed the waters around Tunis, Algiers, or Alexandria. When he had braved the Strait of Gibraltar he had soon turned south, like Hanno of old, and pushed his way as far as he dared. Little by little, to Arguin, to Sierra Leone, to Sao Tome- to the Cape itself, over…