# On the Magnet/V-8

[ 200 ]
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.

n the more elaborate diagram a circle of rotations and a circle of declinations are adjusted to the body of the earth or terrella, with a first, a last, and a middle arc of rotation and declination. Now from each fifth division of the arc which limits all the arcs of rotation (and which are understood[238] as divided into 90 equal parts) arcs are drawn to the pole, and from every fifth degree of the arc limiting the quadrants of declination, quadrants are drawn to the centre; and at the same time a spiral line is drawn, indicating (by the help of a movable quadrant) the declination in every latitude. Straight lines showing the direction of the needle are drawn from those degrees which are marked on the meridian of the earth or a terrella to their proper arcs and the corresponding points on those arcs.

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
.

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 declinatorium (by its versatory nature) indicates the degree of declination.

In a declination instrument the magnetick needle, which
in a meridional position dips, if turned
along a parallel hangs perpendicularly.

In a proper position a magnetick needle, while by its rotatory nature conformed to the earth, dips to some certain degree below the horizon on an oblique sphere. But when the plane of the instrument is moved out of the plane of the meridian, the magnetick needle (which tends toward the pole) no longer remains at the degree of its own declination, but inclines more toward the centre; for the force of direction is stronger than that of declination, and all power of declination is taken away, if the plane of the instrument is on a parallel. For then the magnetick needle, because it cannot maintain its due position on account of the axis being placed transversely, faces down perpendicularly to the earth; and it remains only on its own meridian, or on that which is commonly called the magnetick meridian.

The page and line references given in these notes are in all cases first to the Latin edition of 1600, and secondly to the English edition of 1900.

238 ^  Page 200, line 12. Page 200, line 11. subintelligūtur.—This is printed subintelligitur, and is altered in ink in all copies of the folio edition. The editions of 1628 and 1633 read subintelliguntur. Similarly in line 14 the word ducit has had a small r added in ink, making it read ducitur, as also the other editions.