of a place
anywhere
_in the world, by means of the following diagram, turned into
a magnetick instrument, without the help of the coelestial
bodies, sun, planets, or fixed stars, in fog
and darkness_.

[Illustration]

We may see how far from unproductive magnetick philosophy is, how
agreeable, how helpful, how divine! Sailors when tossed about on the waves
with continuous cloudy weather, and unable by means of the coelestial
luminaries to learn anything about the place or the region in which they
are, with a very slight effort and with a small instrument are comforted,
and learn the latitude of the place. With a declination instrument the
degree of declination of the magnetick needle below the horizon is
observed; that degree is noted on the inner arc of the quadrant, and the
quadrant is turned round about the centre of the instrument until that
degree on the quadrant touches the spiral line; then in the open space B at
the centre of the quadrant the latitude of the region on {201} the
circumference of the globe is discerned by means of the fiducial line A B.
Let the diagram be fixed on a suitable flat board, and let the centre of
the corner A of the quadrant be fastened to the centre of it, so that the
quadrant may rotate on that centre. But it must be understood that there is
also in certain places a variation in the declination on account of causes
already mentioned (though not a large one), which it will be an assistance
also to allow for on a likely estimate; and it will be especially helpful
to observe this variation in various places, as it seems to present greater
difficulty than the variation in direction; but it is easily learnt with a
declination instrument, when it dips more or less than the line in the
diagram.

[Illustration]

_To observe magnetick declination at sea_.

Set upon our variation instrument a declination instrument; a wooden disc
being placed between the round movable {202} compass and the declination
instrument: but first remove the versorium, lest the versorium should
interfere with the dipping needle. In this way (though the sea be rough)
the compass box will remain upright at the level of the horizon. The stand
of the declination instrument must be directed by means of the small
versorium at its base, which is set to the point respective of the
variation, on the great circle of which (commonly called the magnetick
meridian), the plane of the upright box is arranged; thus the declin