He obtained his wishes, and being furnished with a caravel, he embarked his merchandise in it, and set off on a voyage of discovery.

There was now, for the first time, an intelligent man on board one of these vessels, giving us his own account of the voyage.
TRADE WITH THE ARABS.
From Ca da Mosto the reader at once learns the state of things with regard to the slave-trade.

The Portuguese factory at Argnim was the headquarters of the trade.

Thither came all kinds of merchandise; and gold and slaves were taken back in return.

The "Arabs" of that district (Moors, the Portuguese would have called them) were the middle men in this affair.
They took their Barbary horses to the negro country, and "there bartered with the great men for slaves," getting from ten to eighteen slaves for each horse.