The Magnetick Horizon.
Horizon is the name given to the great circle, separating the things which
are seen from those which are not seen; so that a half part of the heaven
always is open and easily seen by us, half is always hidden. This seems so
to us on account of the great distance of the star-bearing orbe: yet the
difference is as great as may arise from the ratio of the semi-diameter of
the earth compared with the semi-diameter of the starry heaven, which
difference is in fact not perceived by our senses. We maintain, however,
that the magnetick horizon is a plane level throughout touching the earth
or a terrella in the place of some one region, with which plane the
semi-diameter, whether of the earth or of the terrella, produced to the
place of the region, makes right angles on every side. Such a plane is to
be considered in the earth itself and also in the terrella, for magnetick
proofs and demonstrations. For we consider the bodies themselves only, not
the general appearances of the world. Therefore not with the idea of
outlook (which varies with the elevations of the lands), but taking it as a
plane which makes equal angles with the perpendicular, we accept in
magnetick demonstrations a sensible horizon or boundary, not that which is
called by Astronomers the rational horizon.
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{81} CHAP. XIII.
On the Axis and Magnetick Poles.
Let the line be called the axis which is drawn in the earth (as in a
terrella) through the centre to the poles. They are called [Greek: poloi]
by the Greeks from [Greek: polein], to turn, and by the Latins they are
also called _Cardines_ or _Vertices_; because the world rotates and is
perpetually carried around them. We are about to show, indeed, that the
earth and a terrella are turned about them by a magnetick influence. One of
them in the earth, which looks towards the Cynosure, is called Boreal and
Arctic; the other one, opposite to this, is called Austral and Antarctic.
Nor do these also exist on the earth or on a terrella for the sake of the
turning merely; but they are also limits of direction and position, both as
respects destined districts of the world, and also for correct turnings
among themselves.
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