ing from the aequator 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 aequator), 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. *

* * * * *

CHAP. VII.

Explanation of the diagram of the rotation of
_a magnetick needle_.

[Illustration]

Suppose A C D L to be the body of the earth or of a terrella, its centre M,
Aequator A D, Axis C L, A B the Horizon, which changes according to the
place. From the point F on a Horizon distant from the aequator A by the
length of C M, the semi-diameter of the earth or terrella, an arc is
described to H as the limit of the quadrants of declination; for {199} all
the quadrants of declination serving the parts from A to C begin from that
arc, and terminate at M, the centre of the earth. The semi-diameter of this
arc is a chord drawn from the aequator A to the pole C; and a line produced
along the horizon from A to B, equal to that chord, gives the beginning of
the arc of the limits of arcs of rotation and revolution, which is
continued as far as G. For just as a quadrant of a circle about the centre
of the earth (whose beginning is on the horizon, at a distance from the
aequator equal to the earth's semi-diameter) is the limit of all quadrants
of declination drawn from each several horizon to the centre; so a circle
about the centre from B, the beginning of the first arc of rotation, to G
is the limit of the arcs of rotation. The arcs of rotation and revolution
of the magnetick needle are intermediate between the arcs of rotation B L
and G L. The centre of the arc is the region itself or place in which the
observation is being made; the beginning of the arc is taken from the
circle which is the limit of rotations, and it stops at the opposite pole;
as, for example, from O to L, in a latitude of 45 degrees. Let any arc of
rotation be divided into 90 equal parts from the limit of the arcs of
rotation toward the pole; for whatever is the degree of latitude of the
place, the part of the arc of rotation which the magnetick pole on or near
the terrella or the earth faces in its rotation is to be num