le of a compass, or a mariners' compass, but also a
terrella in its boat, as well as * iron ore, iron stones, and magnetick
earths, properly prepared, are drawn aside and deviate towards some point
of the Horizon very near to the meridian. For they with their poles
frequently face termini away from the meridian. This variation {152}
(observed by means of instruments or a nautical variation compass) is
therefore the arc of the horizon between the common point of intersecion of
it with the true meridian, and the terminus of the deflecion on the horizon
or projection of the deviating needle. That arc varies and differs with
change of locality. To the terminus of the variation is commonly assigned a
great circle, called the circle of variation, and also a magnetick meridian
passing through the zenith and the point of variation on the horizon. In
the northern regions of the earth this variation is either from the north
toward the east or from the north toward the west: similarly in the
southern regions it is from the south toward the east or toward the west.
Wherefore one should observe in the northern regions of the earth * that
end of the versorium or compass which turns toward the North; but in the
southern regions the other end looking to the south--which seamen and
sciolists for the most part do not understand, for in both regions they
observe only the boreal lily of the compass (that which faces North). We
have before said that all the motions of the magnet and iron, all its
turning, its inclination, and its settlement, proceed from bodies
themselves magnetical and from their common mother the earth, which is the
source, the propagatrix, and the origin of all these qualities and
properties. Accordingly the earth is the cause of this variation and
inclination toward a different point of the horizon: but how and by what
powers must be more fully investigated. And here we must at the outset
reject that common opinion of recent writers concerning magnetick
mountains, or any magnetick rock, or any phantasmal pole distant from the
pole of the earth, by which the motion of the compass or versorium is
controlled. This opinion, previously invented by others, Fracastorio
himself adopted and developed; but it is entirely at variance with
experience. For in that case in different places at sea and on land the
point of variation would change toward the east or west in proportion and
geometrical symmetry, and the versorium would always