[Sidenote: Navigation]
Along with this general knowledge of the situation of continents, the
sailors of the fifteenth century had learned a good deal about
navigation. The compass had been used first by Italian navigators in
the thirteenth century, mounted on the compass card in the fourteenth.
Latitude was determined with the aid of the astrolabe, a device for
measuring the elevation of the pole star above the horizon. With maps
and accurate sailing directions (_portolani_), seamen could lose
sight of land and still feel confident of their whereabouts. Yet it
undoubtedly took courage for the explorers of the fifteenth century to
steer their frail sailing vessels either down the unexplored African
coast or across the uncharted Atlantic Ocean.
[Sidenote: The Portuguese Explorers]
In the series of world-discoveries which brought about the Commercial
Revolution and which are often taken as the beginning of "modern
history," there is no name more illustrious than that of a Portuguese
prince of the blood,--Prince Henry, the Navigator (1394-1460), who,
with the support of two successive Portuguese kings, made the first
systematic attempts to convert the theories of geographers into proved
fact.
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