erticity is perturbed by great continents, which are mostly
elevated above the depths of the seas and make the versorium deviate
sometimes from the right tracks (that is, from the true meridians). On a
terrella it is thus demonstrated: the end of the versorium A is not
directed straight to the pole P, if there be a large protuberance B on the
terrella; so also the cusp C deviates from the pole because of the eminence
F. In the middle between the two eminences the versorium G collimates to
the true pole because, being at equal distances from the two eminences B
and F, it turns aside to neither, but observes the true meridian,
especially when the protuberances are of equal vigour. But the versorium N
on the other side varies from the pole M toward the eminences H, and is not
held back, stopped, or restrained by the small eminence O on the terrella
(as it were, some island of land in the ocean). L, however, being
unimpeded, is directed to the pole M. The variation is demonstrated in
another way on a terrella, just as on the earth. Let A be the pole of the
earth, B the equator, C the parallel circle of latitude of 30 degrees, D a
great eminence spread out toward the pole, E another eminence spread out
from the pole toward the aequator. It is manifest that in the middle of D
the versorium F {159} does not vary; while G is very greatly deflected: but
H very little, because it is further removed from D. Similarly also the
versorium I placed directly toward E does not deviate from the pole: but L
and M turn themselves away from the pole A toward the eminence E.

[Illustration]

* * * * *

CHAP. III.

The variation in any one place
_is constant_.

[Illustration]

Vnless there should be a great dissolution of a continent and a subsidence
of the land such as there was of the region Atlantis of which Plato and the
ancients tell, the variation will continue perpetually immutable; the arc
of the variation remains the same in the same place or region, whether it
be at sea or on land, as in times past a magnetick body has declined toward
the East or the West. The constancy of the variation and the pointing of
the versorium to a definite point on the horizon in individual regions is
demonstrated by a small versorium placed over a terrella the surface of
which is uneven: for it always deviates from the meridian by an equal arc.
It is also shown by the inclination of a versorium toward a second magne