CHAP. XV.
The variation of the magnetick needle in the great
Æthiopick and American sea, beyond
the æquator.
iscourse hath already been had of the mode
and reason of the variation in the great Atlantick Ocean: but when one has advanced beyond the æquator off the east coast
of Brazil the magnetick needle turns aside toward the mainland, namely, with that end of it which points to the south; so
that with that end of the versorium it deviates from the true meridian toward the west; which navigators observe at the
other end and suppose a variation to occur toward the east. But throughout the whole way from the first promontory on the
east of Brazil, by Cape St. Augustine and thence to Cape Frio, and further still to the mouth
of the Strait of Magellan, the variation is always from the south toward the west with that end of the versorium which
tends toward the antarctick pole. For it is always with the accordant end that it turns toward a continent. The
variation, however, occurs not only on the coast itself, but at some distance from land, such as a space of fifty or
sixty German miles or even more. But when at length one has progressed far from land, then the arc begins to diminish:
for the magnetick needle turns aside the less toward what is too far off, and is turned aside the less from what is
present and at hand, since it enjoys what is present. In the Island of St. Helena (the longitude of which is less than is
commonly marked on charts and globes) the versorium varies by one degree or nearly two. The Portuguese and others taught
by them, who navigate beyond the Cape of Good Hope to the Indies, set a course toward the Islands of Tristan d'Acunha, in
order that they may enjoy more favourable winds; in the former part of their course the change of variation is not great;
but after they have approached the islands the variation increases; and close to the islands it is greater than anywhere
else in the whole course. For the end of the versorium tending to the south (in which lies the greatest source of the
variation) is caught and allured toward the south-west by the great promontory of the southern land. But when they
proceed onward toward the Cape of Good Hope the variation diminishes the more they approach it. But on the prime meridian
in the latitude of 45 degrees, the versorium tends to the south-east: and one who navigates near the coast from Manicongo
to the tropick, and a little beyond, will perceive that the versorium tends from the south to the east, although not
much. At the promontory of Agulhas it preserves slightly the variation which it showed near the islands of d'Acunha,
which nevertheless is very much diminished because of the greater remoteness from the cause of variation, and
consequently there the southern end of the versorium does not yet face exactly to the pole.