t;
although in reality it is by the turning power of the whole, whether in the
earth or in a terrella. Place upon a plane a versorium whose cusp is
directed toward the north A: place beside it a loadstone, B, at such a
distance that the versorium may turn aside toward B to the point C, and not
beyond. Then move the needle of the versorium as often as you will (the box
and the loadstone not being moved), and it will certainly always return to
the point C. In the same manner, if you {160} placed the stone so that it
may be truly directed toward E, the cusp always reverts to E, and not to
any other point of the compass. Accordingly, from the position of the land
and from the distinctive nature of the highest parts of the earth (certain
terrene and more magnetick eminences of the regions prevailing), the
variation indeed becomes definite in one and the same place, but diverse
and unaequal from a change of place, since the true and polar direction
originating in the whole terrestrial globe is diverted somewhat toward
certain stronger eminences on the broken surface.

* * * * *

CHAP. IIII.

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 (caeteris paribus) the greater the latitude the greater the