se for taking the altitude of the sun. The
astrolabe, it will be remembered, had been greatly improved, by Martin
Behaim and the Portuguese Commission in 1840--[1440 D.W.]; and it was
this instrument, a simplification of the astrolabe used in astronomy
ashore, that Columbus chiefly used in getting his solar altitudes. As
will be seen from the illustration, its broad principle was that of a
metal circle with a graduated circumference and two arms pivoted in the
centre. It was made as heavy as possible; and in using it the observer
sat on deck with his back against the mainmast and with his left hand
held up the instrument by the ring at the top. The long arm was moved
round until the two sights fixed upon it were on with the sun. The point
where the other arm then cut the circle gave the altitude. In
conjunction with this instrument were used the tables of solar
declination compiled by Regiomontanus, and covering the sun's declination
between the years 1475 and 1566.

The compass in Columbus's day existed, so far as all essentials are
concerned, as it exists to-day. Although it lacked the refinements
introduced by Lord Kelvin it was swung in double-cradles, and had the
thirty-two points painted upon a card. The discovery of the compass, and
even of the lodestone, are things wrapt in obscurity; but the lodestone
had been known since at least the eleventh century, and the compass
certainly since the thirteenth. With the compass were used the sea
charts, which were simply maps on a rather larger and more exact scale
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