than the land maps of the period. There were no soundings or currents
marked on the old charts, which were drawn on a plane projection; and
they can have been of little--practical use to navigators except in the
case of coasts which were elaborately charted on a large scale. The
chart of Columbus, in so far as it was concerned with the ocean westward
of the Azores, can of course have contained nothing except the
conjectured islands or lands which he hoped to find; possibly the land
seen by the shipwrecked pilot may have been marked on it, and his failure
to find that land may have been the reason why, as we shall see, he
changed his course to the southward on the 7th of October. It must be
remembered that Columbus's conception of the world was that of the
Portuguese Mappemonde of 1490, a sketch of which is here reproduced.
This conception of the world excluded the Pacific Ocean and the continent
of North and South America, and made it reasonable to suppose that any
one who sailed westward long enough from Spain would ultimately reach
Cathay and the Indies. Behaim's globe, which was completed in the year
1492, represented the farthest point that geographical knowledge had
reached previous to the discoveries of Columbus, and on it is shown the
island of Cipango or Japan.

By far the most important element in the navigation of Columbus, in so
far as estimating his position was concerned, was what is known as
"dead-reckoning" that is to say, the computation of the distance
travelled by the ship through the water. At present this distance is
measured by a patent log, which in its commonest form is a
propeller-shaped instrument trailed through the water at the end of a
long wire or cord the inboard end of which is attached to a registering
clock. On being dragged through the water the propeller spins round and
the twisting action is communicated by the cord to the clock-work
machinery which counts the miles. In the case of powerful steamers and