ferent places, the inclination of the stone, and concerning an
instrument indicating by the influence of a stone the degree of declination
from any horizon we have already spoken. Then we spoke about needles on the
meridian of a stone, and their rotation shown for various latitudes by
their rise toward the perpendicular. We must now, however, treat more fully
of the causes of the degree of that inclination. Whilst a loadstone and a
magnetick iron wire are moved along a meridian from the aequator toward the
pole, they rotate toward a round loadstone, as also toward the earth with a
circular movement. On a right horizon (just as also on the aequinoctial of
{197} the stone) the axis of the iron, which is its centre line, is a line
parallel to the axis of the earth. When that axis reaches the pole, which
is the centre of the axis, it stands in the same straight line with the
axis of the earth. The same end of the iron which at the aequator looks
south turns to the north. For it is not a motion of centre to centre, but a
natural turning of a magnetick body to a magnetick body, and of the axis of
the body to the axis; it is not in consequence of the attraction of the
pole itself that the iron points to the earth's polar point. Under the
aequator the magnetick needle remains in aequilibrio horizontally; but
toward the pole on either side, in every latitude from the beginning of the
first degree right up to the ninetieth, it dips. The magnetick needle does
not, however, in proportion to any number of degrees or any arc of latitude
fall below the horizon just that number of degrees or a similar arc, but a
very different one: because this motion is not really a motion of
declination, but is in [Illustration] reality a motion of rotation, and it
observes an arc of rotation according to the arc of latitude. Therefore a
magnetick body A, while it is advancing over the earth itself, or a little
earth or terrella, from the aequinoctial G toward the pole B, rotates on
its own centre, and halfway on the progress of its centre[237] from the
aequator to the pole B it is pointing toward the aequator at F, midway
between the two poles. Much more quickly, therefore, must the versorium
rotate than its centre advances, in order that by rotating it may face
straight toward the point F. Wherefore the motion of this rotation is rapid
in the first degrees from the aequator, namely, from A to L; but more tardy
in the later degrees from L to B, when fac