If the Southern Portion of a Stone be lessened,
something is also taken away from the power
_of the Northern Portion._
Now although the southern end of a magnetick iron is attracted by a
northern end, and repelled by a southern, yet the southern portion of a
stone does not diminish, but increases the potency of the boreal part.
Wherefore if a stone be cut in two and divided through the arctick circle,
or through the tropick of Cancer or the æquator, the southern portion does
not attract magnetick substances so strongly with its pole as before;
because a new whole arises, and the æquator is removed from its old
position and moves forward on account of that cutting of the stone. In the
former condition, since the opposite portion of the stone increases the
mass beyond the plane of the æquator, it strengthens also the verticity,
and the potency, and the motion to unity.
* * * * *
{147} CHAP. XVII.
On the Use and Excellence of Versoria: and how iron
_versoria used as pointers in sun-dials, and the fine needles_
of the mariners' compass, are to be rubbed, that
_they may acquire stronger verticity._
Versoria prepared by the loadstone subserve so many actions in human life
that it will not be out of place to record a better method of touching them
and exciting them magnetically, and a suitable manner of operating. Rich
ores of iron and such as yield a greater proportion of metal are recognized
by means of an iron needle suspended in æquilibrium and magnetically
prepared; and magnetick stones, clays, and earths are distinguished,
whether crude or prepared. An iron needle (the soul of the mariners'
compass), the marvellous director in voyages and finger of God, one might
almost say, indicates the course, and has pointed out the whole way around
the earth (unknown for so many ages). The Spaniards (as also the English)
have frequently circumnavigated (by an immense circuit) the whole globe by
aid of the mariners' compass. Those who travel about through the world or
who sit at home have sun-dials. A magnetick pointer follows and searches
out the veins of ore in mines. By its aid mines are driven in taking
cities; catapults and engines of war are aimed by night; it has been of
service for the topography of places, for marking off the areas and
position of buildings, and for excavating aqueducts for water under ground.
On it depend instruments designed to investigate its own dip and variation.
When iron is to be quickened by the stone, let it be clean and bright,
disfigured by no rust or dirt, and of the best steel[216]. Let the stone
itself be wiped dry, and let it not be damp with any moisture, but let it
be filed gently with some smooth piece of iron. But the hitting of the
stone with a hammer is of no advantage. By these means let their bare
surfaces be joined, and let them be rubbed, so that they may come together
more firmly; not so that the material substance of the stone being joined
to the iron may cleave to it, but they are rubbed gently together with
friction, and (useless parts being rubbed off) they are intimately united;
whence a more notable [Illustration] virtue arises in the iron that is
excited. A is the best way of touching a versorium when the cusp touches
the pole and faces it; B is a moderately good way, when, though facing it,
it is a little way {148} distant from the pole; also in like manner C is
only moderately good on account of the cusp being turned away from the
pole; D, which is farther distant, is hardly so good; F, which is prepared
crosswise along a parallel, is bad; of no virtue and entirely irresponsive
and feeble is the magnetick index L, which is rubbed along the æquator;
oblique and not pointing towards the pole as G, and oblique, not pointing
toward but turned away from 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 æquilibrio
after it is touched. But a needle in this way prepared does its * {149}
work worse the farther it is beyond the æquinoctial 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 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 æquator 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.
When a needle is being excited by a loadstone, begin in the middle, and
draw the needle toward its end; at the end let the application be continued
with a very gentle rubbing around the end for some time; that is to say,
for one or two minutes; do not repeat the motion from the middle to the end
(as is frequently done) for in this way the verticity is injured. Some
delay is desirable, for although the power is imparted instantly, and the
iron excited, yet from the vicinity of the loadstone and a suitable delay,
a more steady verticity arises, and one that is more firmly durable in the
iron. Although an armed stone raises a greater weight of iron than an
unarmed one, yet a needle is not more strongly excited by an armed stone
than by an unarmed one. Let there be two iron wires of the same length,
wrought from the same wire; let one be excited by an armed end, the other
by an unarmed end; it is manifest that the same needles have a beginning of
motion or a sensible inclination at equal distances from the same armed and
unarmed loadstone; this is ascertained by measuring with a longish reed.
But objects which are more powerfully excited move more quickly; those
which are less powerfully excited, more feebly, and not unless brought
rather close; the experiment is made on water with equal corks.
* * * * *
{151} [Illustration]