ks truly to the north; nor on the
meridian of Plymouth is the variation in other places 13 deg. 24 min.--nor
again in other parts of the meridian of Helmshuda does it point to the true
pole. For on the meridian passing through Plymouth in Latitude 60 degrees
the North-easterly variation is greater: in Latitude 40 deg. much less; in
Latitude 20 deg. very small indeed. On the meridian of Corvo, although
there is no variation near the {168} island, yet in Latitude 55 degrees the
variation is about 1/2 a rumbe to the North-west; in Latitude 20 deg. the
versorium inclines 1/4 of a rumbe toward the East. Consequently the limits
of variation are not conveniently determined by means of great circles and
meridians, and much less are the ratios of the increment or decrement
toward any part of the heavens properly investigated by them. Wherefore the
rules of the abatement or augmentation of Northeasting or Northwesting, or
of increasing or decreasing the magnetick deviation, can by no means be
discovered by such an artifice. The rules which follow later for variation
in southern parts of the earth investigated by the same method are
altogether vain and absurd. They were put forth by certain Portuguese
mariners, but they do not agree with the observations, and the observations
themselves are admitted to be bad. But the method of haven-finding in long
and distant voyages by carefully observed variation (such as was invented
by Stevinus, and mentioned by Grotius) is of great moment, if only proper
instruments are in readiness, by which the magnetick deviation can be
ascertained with certainty at sea.

* * * * *

CHAP. X.

Why in various places near the pole the variations
are much more ample than in a
_lower latitude_.

Variations are often slight, and generally null, when the versorium is at
or near the earth's aequator. In a higher Latitude of 60, 70 or 80 deg.
there are not seldom very wide variations. The cause of this is to be
sought partly from the nature of the earth and partly from the disposition
of the versorium. The earth turns magnetick bodies and at the aequator
directs them strongly toward the pole: [227]at the poles there is no
direction, but only a strong coition through the congruent poles. Direction
is therefore weaker near the poles, because by reason of its own natural
tendency to turn, the versorium dips very much, and is not strongly
directed. But since the force of those ele