The variation of the magnetick needle in the great
Æthiopick and American sea, beyond
_the æquator_.
Discourse 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 {179} 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.
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