h 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
in ordinary weather dead-reckoning is very accurately calculated by the
number of revolutions of the propellers recorded in the engine-room; and
a device not unlike this was known to the Romans in the time of the
Republic. They attached small wheels about four feet in diameter to the
sides of their ships; the passage of the water turned the wheels, and a
very simple gearing was arranged which threw a pebble into a tallypot at
each revolution. This device, however, seems to have been abandoned or
forgotten in Columbus's day, when there was no more exact method of
estimating dead-reckoning than the primitive one of spitting over the
side in calm weather, or at other times throwing some object into the
wate