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 Joao 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.