egional latitude of 50
degrees, so also the axis of a stone (of a spherical stone, of course) is
depressed below the horizon, and its natural austral pole falls, and its
boreal pole is raised on the {191} south toward the Zenith. In the same way
also a circular disc of iron behaves, which has been carefully touched at
opposite parts on its circumference; but the magnetical experiments are
less clear on account of the feebler forces in round pieces of iron.

_Variety in the declinations of iron spikes at various latitudes of a
terrella._

[Illustration]

The declination of a magnetick needle above a terrella is shown by means of
several equal iron wires, of the length of a barleycorn, arranged along a
meridian. The wires on the aequator are directed by the virtue of the stone
toward the poles, and lie down upon its body along the plane of its
horizon. The nearer they are brought to the poles, the more they are raised
up by their versatory nature. At the poles themselves they point
perpendicularly toward the very centre. But iron spikes, if they are of
more than a due length, are not raised straight up except on a vigorous
stone.

* * * * *

CHAP. III.

*

An indicatory instrument, showing by the virtue of a
_stone the degrees of declination from the horizon_
of each several latitude.

{192} [Illustration] {193} _Description of the Instrument, and its use._

Take a terrella of the best strong loadstone, and homogeneous throughout,
not weakened by decay or by a flaw in any parts; let it be of a fair size,
so that its diameter is six or seven digits; and let it be made exactly
spherical. Having found its poles according to the method already shown,
mark them with an iron tool; then mark also the aequinoctial circle.
Afterwards in a thick squared block of wood, one foot in size, make a
hemispherical hollow, which shall hold half of the terrella, and such that
exactly one half of the stone shall project above the face of the block.
Divide the limb close to this cavity (a circle having been drawn round it
for a meridian) into 4 quadrants, and each of these into 90 degrees. Let
the terminus of the quadrants on the limb be near the centre of a quadrant
described on the block, also divided into 90 degrees. At that centre let a
short, slender versorium (its other end being rather sharp and elongated
like a pointer) be placed in aequilibrio on a suitable pin. It is manifest
that when the