Meanwhile King John, carrying on Prince Henry’s work, kept sending his discovery voyages farther down the west African coast. Diogo Cão reached the mouth of the Congo (1480-84), and began the customs of setting up stone markers (padrões), surmounted by a cross, as proof of first discovery and tokens of Christian faith.

These advances down the coast brought new rumors of the famous but still unseen Prester John. While Prince Henry’s first objective was to move into the unknown, another objective, his chronicler Zurara reported, was “to know if there were in those parts any Christian princes, in whom the charity and the love of Christ was so ingrained that they would aid him against those enemies of the faith.” This conjectural potential ally must have been Prester John, whose “letter,” as we have seen, had been circulating in Europe for two centuries. By this time the locale of the legendary priest-king had been transferred from “furthest Asia” to Ethiopia. Whenever one of Prince Henry’s voyages found another great river – the Senegal, the Gambia, the Niger – debouching on the west coast, he found new hope that this at last might be the “Western Nile” that would lead to Prester John’s Ethiopian Kingdom. […]

“The Portuguese Discoverers”, from “The Discoverers”, Daniel J. Boorstin, The National Board for the Celebration of Portuguese Discoveries, Lisbon, 1987