om the pole as H, are bad. These have been placed
so that they might indicate the distinct forces of a round stone. But
mechanicians very often have a stone tending more to a cone shape, and more
powerful on account of that shape since the pole, on which they rub their
wires, is at the apex of the projecting part. Sometimes the stone has on
the top and above its own pole an artificial acorn or snout made of steel
for the sake of its power. Iron needles are rubbed on the top of this;
wherefore they turn toward the same pole as if they had been prepared on
that part of the stone with the acorn removed. Let the stone be large
enough and strong; the needle, even if it be rather long, should be
sufficiently thick, not very slender; with a moderate cusp, not too sharp,
although the virtue is not in the cusp itself only, but in the whole piece
of iron. A strong large stone is not unfit for rubbing all needles on,
excepting that sometimes by its strength it occasions some dip and
disturbance in the iron in the case of longer needles; so that one which,
having been touched before, rested in equilibrium in the plane of the
horizon, now when touched and excited dips at one end, as far as the
upright pin on which it turns permits it. Wherefore in the case of longer
versoria, the end which is going to be the Boreal, before it is rubbed,
should be a little lighter, so that it may remain exactly in aequilibrio
after it is touched. But a needle in this way prepared does its * {149}
work worse the farther it is beyond the aequinoctial circle. Let the
prepared needle be placed in its capsule, and let it not be touched by any
other magneticks, nor remain in the near vicinity of them, lest by their
opposing forces, whether powerful or sluggish, it should become uncertain
and dull. If you also rub the other end of the needle on the other pole of
the stone, the needle will perform its functions more steadily, especially
if it be rather long. A piece of iron touched by a loadstone retains the
magnetick virtue, excited in it even for ages[217], firm and strong, if it
is placed according to nature meridionally and not along a parallel, and is
not injured by rust or any external injury from the surrounding medium.
Porta wrongly seeks for a proportion between the loadstone and the iron:
because, he says, a little piece of iron will not be capable of holding
much virtue; for it is consumed by the great force of the loadstone. A
piece of iron receives