Why at the Pole itself the Coition is stronger than in
_the other parts intermediate between the æquator and the pole;_
and on the proportion of forces of the coition in
_various parts of the earth and of the terrella_.
Observation has already been made that the highest power of alluring exists
in the pole, and that it is weaker and more languid in the parts adjacent
to the æquator. And as this is apparent in the declination, because that
disponent and rotational virtue has an augmentation as one proceeds from
the Æquator towards the poles: so also the coition of magneticks grows
increasingly fresh by the same steps, and in the same proportion. For in
the parts more remote from the poles the loadstone does not draw magneticks
straight down towards its own viscera; but they tend obliquely and they
allure obliquely. For as the smallest chords in a circle differ from the
diameter, so much do the forces of attracting differ between themselves in
different parts of the terrella. {82} For since attraction is coition
towards a body, but magneticks run together by their versatory tendency, it
comes about that in the diameter drawn from pole to pole the body appeals
directly, but in other places less directly. So the less the magnetick is
turned toward the body, the less, and the more feebly, does it approach and
adhære. [Illustration] Just as if A B were the poles and a bar of iron or a
magnetick fragment C is allured at the part E; yet the end laid hold of
does not tend towards the centre of the loadstone, but verges obliquely
towards the pole; and a chord drawn from that end obliquely as the
attracted body tends is short; therefore it has less vigour and likewise
less inclination. But as a greater chord proceeds from a body at F, so its
action is stronger; at G still longer; longest at A, the pole (for the
diameter is the longest way) to which all the parts from all sides bring
assistance, in which is constituted, as it were, the citadel and tribunal
of the whole province, not from any worth of its own, but because a force
resides in it contributed from all the other parts, just as all the
soldiers bring help to their own commander. Wherefore also a slightly
longer stone attracts more than a spherical one, since the length from pole
to pole is extended, even if the stones are both from the same mine and of
the same weight and size. The way from pole to pole is longer in a longer
stone, and the forces brought together from other parts are not so
scattered as in a round magnet and terrella, and in a narrow one they agree
more and are better united, and a united stronger force excels and is
preeminent. A much weaker office, however, does a plane or oblong stone
perform, when the length is extended according to the leading of the
parallels, and the pole stops neither on the apex nor in the circle and
orbe, but is spread over the flat. Wherefore also it invites a friend
wretchedly, and feebly retains him, so that it is esteemed as one of an
abject and contemptible class, according to its less apt and less suitable
figure.
* * * * *
{83} CHAP. XV.
*
The Magnetick Virtue which is conceived in Iron is
more apparent in an iron rod than in a piece of iron that
_is round, square, or of other figure_.
Duly was it said before that the longer magnet attracts the greater weight
of iron[167]; so also in a longish piece of iron which has been touched the
magnetick force conceived is stronger when the poles exist at the ends. For
the magnetick forces which are driven from the whole in every part into the
poles are not scattered but united in the narrow ends. In square and other
angular figures the influence is dissipated, and does not proceed in
straight lines or in convenient arcs. Suppose also an iron globe have the
shape of the earth, yet for the same reasons it drags magnetick substances
less; wherefore a small iron sphere, when excited, draws another piece of
iron more sluggishly than an excited rod of equal weight.
* * * * *