The arc of variation is not changed equally
_in proportion to the distance of places_.
In the open sea, when a vessel is borne by a favourable wind along the same
parallel, if the variation be changed by one degree in the course of one
hundred miles, the next hundred miles do not therefore lessen it by another
degree; for the magnetick [needle] varies erratically as respects position,
form, and vigour of the land, and also because of the distance. As, for
example, when a course from the Scilly Isles to Newfoundland has proceeded
so far that the compass is directed to the true pole, then, as the vessel
proceeds, in the first part of the course the variation increases toward
the north-west[222], but rather indistinctly and with small difference:
thence, after an equal distance, the arc is increased in a greater
proportion until the vessel is not far from the continent: for then it
varies most of all. But before it touches actual land or enters port, then
at a certain distance the arc is again slightly diminished. But if the
vessel in its course should decline greatly from that parallel either
toward the south or the north, the magnetick [needle] will vary more or
less, according to the position of the land and the latitude * of the
region. For (cæteris paribus) the greater the latitude the greater the
variation.
* * * * *
{161} CHAP. V.
An island in Ocean does not change the variation[223], as
_neither do mines of loadstone_.
Islands, although they be more magnetick than the sea, yet do not change
the magnetick directions or variations. For since direction is a motion
derived from the power of the whole earth, not from the attraction of any
hill but from the disposing and turning power of the whole; so variation
(which is a perturbation of the direction) is an aberration of the real
turning power arising from the great inequalities of the earth, in
consequence of which it, of itself, slightly diverts movable magneticks
toward those which are the largest and the more powerful. The cause now
shown may suffice to explain that which some so wonder at about the Island
of Elba (and although this is productive of loadstone, yet the versorium
(or mariners' compass) makes no special inclination toward it whenever
vessels approach it in the Tyrrhenian sea); and the following causes are
also to be considered, viz.: that the virtue of smaller magnetick bodies
extends scarcely or not at all of itself beyond their own mines: for
variation does not occur because of attraction, as they would have it who
have imagined magnetick poles. Besides, magnetick mines are only agnate to
the true earth, not innate: hence the whole globe does not regard them, and
magneticks are not borne to them, as is demonstrated by the diagram of
eminences.
* * * * *