diagram of the rotation of a magnetick needle, indicating magnetical declination in all latitudes, and from the rotation and declination, the latitude itself.
i 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 understood238 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.
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.
set upon our variation instrument a declination instrument; a wooden disc being placed between the round movable 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 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.
