bered similarly
to this. The straight lines in the following larger diagram show this. The
magnetick rotation at the middle point in a latitude of 45 degrees is
directed toward the aequator, in which case also that arc is a quadrant of
a circle from the limit to the pole; but previous to this all the arcs of
rotation are greater than a quadrant, whilst after it they are smaller; in
the former the needle rotates more quickly, but in the succeeding positions
gradually more slowly. For each several region there is a special arc of
rotation, in which the limit to which the needle rotates is according to
the number of degrees of latitude of the place in question; so that a
straight line drawn from the place to the point on that arc marked with the
number of degrees of latitude shows the magnetick direction, and indicates
the degree of declination at the intersection of the quadrant of
declination which serves the given place. Take away the arc of the quadrant
of declination drawn from the centre to the line of direction; that which
is left is the arc of declination below the horizon. As, for example, in
the rotation of the versorium N, whose line respective proceeds to D, from
the quadrant of declination, S M, take away its arc R M; that which is left
is the arc of declination: how much, that is, the needle dips in the
latitude of 45 degrees.

* * * * *

{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_.

In 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