its own virtue fully, even if it be only of the
weight of one scruple, whilst the mass of the loadstone is a thousand
pounds. It is also useless to make the needle rather flat at the end that
is touched, so that it may be better and more perfectly magnetick, and that
it may best receive and hold certain magnetick particles; since hardly any
part will stick on a sharp point; because he thought that it was by the
adhesion of parts of the loadstone (as it were, hairs) that the influence
is imparted and conserved, though those particles are merely rubbed off by
the rubbing of the iron over the softer stone, and the iron none the less
points toward the North and South, if after it is touched it be scoured
with sand or emery powder, or with any other material, even if by long
rubbing of this kind the external parts of it are lessened and worn away.
When a needle is being rubbed, one should always leave off at the end;
otherwise, if it is rubbed on the loadstone from the point toward the
middle, less verticity is excited in the iron, sometimes none at all, or
very little. For where the last contact is, there is the pole and goal of
verticity. In order that a stronger verticity may be produced in the iron
by rubbing on the loadstone, one * ought in northern lands to turn the true
northern pole of the loadstone toward the highest part of the sky; on this
pole that end of the needle is going to be rubbed, which shall afterwards
turn toward the north of the earth; whilst it will be an advantage for the
other end of the needle to be rubbed on the southern pole of the terrella
turned toward the earth, and this being so excited will incline toward the
south. In southern regions beyond the aequator the plan is just the
contrary. The reason of this dissimilarity is demonstrated, Book II., chap,
xxxiv., in which it is shown (by a manifest combination of a terrella and
the earth) why the poles of a loadstone, for different reasons, are one
stronger than the other. If a needle be touched between the mutually
accordant * poles of two loadstones, equal in power, shape, and mass, no
strength {150} [Illustration] is acquired by the needle. A and B are two
loadstones attracting one another, according to nature, at their dissimilar
ends; C, the * point of a needle touched by both at once, is not excited
(even if those loadstones be connected according to nature), if they are
equal; but if they are not equal, virtue is acquired from the stronger.