nd, the directive force is stronger where
coition is weaker and accordingly, verticity is most prominent at the
equator.[161]

[153] M: pp. 184-186, 190, 232. This is not quite the same
argument as that the powers of the loadstone are identical
with those of the earth. See footnote 78.

[154] M: pp. 125, 180.

[155] M: p. 151.

[156] M: pp. 121, 150.

[157] M: pp. 115, 151, 165.

[158] M: pp. 106, 118, 151, 191, 205, 221, 243.

[159] M: pp. 116, 117, 119, 131, 183, 188, 221.

[160] M: p. 31.

[161] M: pp. 116, 151, 200.

The strength of a loadstone depends upon its shape and mass. A bar
magnet has greater powers than a spherical one because it tends to
concentrate the magnetic powers more in the ends.[162] For a given
purity and shape, the heavier the loadstone, the greater its
strength.[163] A loadstone has a maximum degree of magnetic force that
cannot be increased.[164] However, weaker ones can be strengthened by
stronger ones.[165] Similarly, the shape and weight of the iron
determine the magnetic force in coition.[166]

[162] M: pp. 131, 132, 153-158.

[163] M: pp. 141, 152, 153, 158, 161, 191, 222.

[164] M: p. 146.

[165] M: p. 165.

[166] M: p. 153.

The formal forces of a loadstone emanate in all directions from
it,[167] but there is a bound to it that Gilbert called the "orbis
virtutis."[168] The shape of this "orbis virtutis" is determined by
the shape of the stone.[169] This insensible effusion is analogous
to the spreading of light that reveals its presence only by opaque
bodies.[170] Similarly, the magnetic forms are effused from the
stone,[171] and can only reveal their presence by coition with
another loadstone or by "awakening" magnetic bodies within the
"orbis virtutis."[172] Unmagnetized iron that comes within the "orbis
virtutis" is altered, and the magnetic virtue renews a form that is
already potentially in the iron.[173] The formal energy is drawn not
only from the stone but from the iron.[174] This is not generation, or
alteration in the sense of a new impressed quality, but alteration in
the sense of the entelechy or the activation of a form potentially
present.[175] Those bodies magnetized by coming within the "orbis
virtutis" have in turn an efflux of their own.[176] Iron can also
receive verticity directly from the earth without the intervention of
an ordinary loadstone.[177] Such verticity can be expelled