e is sensibly rather
imperfect, so also is it in others whole and perfect, when often one part
of the stone has stronger external parts, which nevertheless do not
disclose themselves manifestly to the senses. In such a terrella the
demonstration of the variation and the discovery of the stronger parts is
on this wise. [Illustration] * Let A be the pole, B the place of the
variation, C the stronger regions; then the horizontal versorium at B
varies from the pole A toward C: so that both the variation is shown and
the stronger places of the loadstone recognized. The stronger surface is
also found by a fine iron wire of the length of two barleycorns: for since
at the pole of the terrella it rears up perpendicularly, but in other
places inclines toward the aequator, if in one and the same parallel circle
it should be more erect in one place than in another; where the wire is
raised more upright, there the part and surface of the terrella is
stronger. Also when the iron wire placed over the pole inclines more to one
part than to another. [Illustration] * {157} Let the experiment be made by
means of a fine iron wire of three digits length placed over the pole A, so
that its middle lies over the pole. Then one end is turned away from B
toward C, and is not willing to lie quietly toward B; but on a terrella
which is perfect[220] all round and even it rests on the pole directed
toward any point of the aequator you please. Otherwise, let there be two *
[Illustration] meridians meeting in the poles A B, let iron wires be reared
just at the ends D and C of the equal arcs D A and C A; then the wire at D
(the stronger region) will be more raised up than that at C, the weaker.
And thus the sounder and stronger part of the loadstone is recognized,
which otherwise would not be perceived by the touch. In a terrella which is
perfect, and even, and similar in all its parts, there is, at equal
distances from the pole, no variation[221]. Variation is shown by means of
a terrella, a considerable part of which, forming a surface a little higher
than the rest, does, although it be not decayed and broken, allure the
versorium from the true * direction (the whole terrella co-operating).

A terrella uneven in surface.

[Illustration] {158} It is shown by a small spike placed over a terrella or
by a small versorium; for they are turned by the terrella toward the mass
that stands out and toward the large eminences. In the same way on the
earth the v