On the Magnet/II-12

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On the Magnet
by William Gilbert
Book II. Chap. 12.
CHAP. XII.
The Magnetick Horizon.


Gilbert De Magnete IlloH.jpg

orizon 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.