arth's, take
the round stone in hand, and place upon the stone a needle or wire of iron:
the ends of the iron move upon their own centre and suddenly stand still.
Mark the stone with ochre or with chalk where the wire lies and sticks:
move the middle or centre of the wire to another place, and so on to a
third and a fourth, always marking on the stone along the length of the
iron where it remains at rest: those lines show the meridian circles, or
the circles like meridians on the stone, or terrella, all of which meet as
will be manifest at the poles of the stone. By the circles thus continued
the poles are made out, the Boreal as well as the southern, and in the
middle space betwixt these a great circle may be drawn for an aequator,
just as Astronomers describe them in the heavens and on their own globes,
or as Geographers do on the terrestrial globe: for that line so drawn on
this our terrella is of various uses in our demonstrations and experiments
magnetical. Poles are also found in a round stone by a versorium, a piece
of iron touched with a loadstone, and placed upon a needle or point firmly
fixed on a foot so as to turn freely about in the following way:[63]

[Illustration]

On the stone A B the versorium is placed in such a way that the versorium
may remain in equilibrium: you will mark with chalk the course of the iron
when at rest: Move the instrument to another spot, and again make note of
the direction and aspect: do the same thing in several places, and from the
concurrence of the lines of direction you will find one pole at the point
A, the other at B. A versorium placed near the stone also indicates the
true pole; when at right angles it eagerly beholds the stone and seeks the
pole itself directly, and is turned in a straight line through the axis to
the {14} centre of the stone. For instance, the versorium D faces toward A
and F, the pole and centre, whereas E does not exactly respect * either the
pole A or the centre F[64]. A bit of rather fine iron wire, of the length
of a barley-corn, is placed on the stone, and is moved over the regions and
surface of the stone, until it rises to the perpendicular[65]: for it
stands erect at the actual pole, whether Boreal or austral; the further
from the pole, the more it inclines from the vertical. The poles thus found
you shall mark with a sharp file or gimlet.

* * * * *

CHAP. IIII.

Which pole of the stone is the Boreal: & ho