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.
ersoria 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
steel216. 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
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 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 * 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 ages217, 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 
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.