that lateral cause is not
_greater than has hitherto been observed, having been_
rarely seen to reach two points of the mariners'
_compass, except near the pole_.

The earth, by reason of lateral eminences of the stronger globe, diverts
iron and loadstone by some degrees from the true pole, or true meridian.
As, for example, with us English at London it varies eleven degrees and
1/3: in some other places the variation is a little greater, but in no
other region is the end of the iron ever moved aside very much more from
the meridian. For as the iron is always directed by the true verticity of
the earth, so the polar nature of the continent land (just as of the whole
terrene globe) acts toward the poles: and even if that mass divert
magnetick bodies from the meridian, yet the verticity of those lands (as
also of the whole earth) controls and disposes them so that they do not
turn toward the East by any greater arc. But it is not easy to determine by
any general method how great the arc of variation is in all places, and how
many degrees and minutes it subtends on the horizon, since it becomes
greater or less {164} from diverse causes. For both the strength of true
verticity of the place and of the elevated regions, as well as their
distances from the given place and from the poles of the world, must be
considered and compared; which indeed cannot be done exactly: nevertheless
by our method the variation becomes so known that no grave error will
perturb the course at sea. If the positions of the lands were uniform and
straight along meridians, and not defective and rugged, the variations near
lands would be simple; such as appear in the following figure.

[Illustration]

This is demonstrated by a long loadstone the poles of which are in the ends
A B; let C D be the middle line and the aequinoctial, and let G H and E F
(the lines) be for meridians on which versoria are disposed, the variations
of which are greater at a greater distance from the aequator. But the
inequalities of the maritime parts of the habitable earth, the enormous
promontories, the very wide gulfs, the mountainous and more elevated
regions, render the variations more unequal, or sudden, or more obscure;
and, moreover, less certain and more inconstant in the higher latitude.

* * * * *

{165} CHAP. VIII.

On the construction of the common mariners'
compass[225], and on the diversity of the compasses
_of different n