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. 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 Tristao, 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
down by the Prince's cartographers on portulanos, and just before
his death the King of Portugal sent to a Venetian monk, Fra Mauro,
details of all discoveries up to that time, to be recorded on a
_mappa mundi_, a copy of which still exists (p. 77).

The impulse thus given by Prince Henry's patient investig