[CHAP. VIII].
Diagram of the rotation of a magnetick needle,
indicating magnetical declination in all latitudes, and
from the rotation and declination, the
latitude itself.
To ascertain the elevation of the pole or the latitude of a place anywhere
in the world, by means of the following diagram, turned into
a magnetick instrument, without the help of the cœlestial
bodies, sun, planets, or fixed stars, in fog
and darkness.
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 cœlestial 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
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.
To observe magnetick declination at sea.
Set upon our variation instrument a declination instrument; a wooden disc being placed between the round movable