On the proportion of declination to latitude[236], and
_the cause of it_.
Concerning the making of an instrument for finding declination, the causes
and manner of declination, and the different degrees of rotation in
different 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 æquinoctial 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 æquator 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
æquator the magnetick needle remains in æquilibrio 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 æquinoctial G toward the pole B, rotates on its own
centre, and halfway on the progress of its centre[237] from the æquator to
the pole B it is pointing toward the æquator 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 æquator, namely, from A to L; but more tardy in the later
degrees from L to B, when facing from the æquator at F to C. But if the
declination were equal to the latitude (_i.e._, always just as many degrees
from the horizon, as the centre of the versorium has receded from the
æquator), then the magnetick needle would be following some potency and
peculiar virtue of the centre, as if it {198} were a point operating by
itself. But it pays regard to the whole, both its mass, and its outer
limits; the forces of both uniting, as well of the magnetick versorium as
of the earth. *
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