How the deviation of the versorium is augmented and
_diminished by reason of the distance of places_.
In the middle of great and continent lands there is no variation. Nor,
generally, in the middle of very great seas. On the margin of those lands
and seas the variation is often ample, yet not so great as at a little
further distance on the sea. As, for example, near Cape St. Augustine the
compass varies; but at 50 miles from land toward the East it varies more;
and 80 miles off it varies still more; and yet still more at a distance of
100 miles. But from a distance of 100 miles the diminutions of deviation
are slower, when they are navigating toward the mainland, than at a
distance of 80 miles, and at a distance of 80 miles than at 50: for the
deviations change and are diminished rather more swiftly the more they
approach and draw near land than when at a great distance off. As, for
instance, navigating toward Newfoundland the change of variation is more
rapid (that is, it decreases a degree in a smaller arc of the course on the
parallel) when they are not far from land than when they are a hundred
miles distant: but when travelling on land toward the interiors of regions
the changes are slower in the first parts of the journey than when they
come more into the interior.
The ratio of the arcs on a parallel circle, when a versorium is moved
toward continents which extend to the pole, corresponds with the degrees of
variation. Let A be the pole; B the eminences of the dominant lands; at C
there is no variation caused by B, for it is too far away; at D the
variation is very great because the versorium is allured or turned by the
whole earth toward the eminent {183} land B; and moreover it is not
hindered, or restrained or brought back to the pole by the verticity of the
earth; but, tending of its own nature to the pole, it is nevertheless
deflected from it by reason of the site, or position, and convenient
distance of the dominant and high lands.
[Illustration]
Now from C toward D the variation increases; the versorium, however, does
not deviate so rapidly in the first spaces as near D: for more miles are
traversed on the parallel circle C D, near C, in order that the versorium
may deviate by one degree from the pole A, than near D. So also in order
that the variation may be diminished from D toward E more miles are
required near D than near E. Thus the deviations become equal in unequal
courses, whether the variation be increasing or decreasing; and yet the
variation decreases by lesser intervals than it increases. There intervene,
however, many other causes which perturb this proportion.
* * * * *
{184} [Illustration]