On the Varying Ratio of Strength, and of the Motion
_of coition, within the orbe of virtue_.
Should a very large weight, which at a very small distance is drawn towards
a loadstone, be divided into ever so many equal parts, and should the
radius of the orbe of magnetick attraction be divided into the same number
of parts, the like named parts of the weight will correspond to the
intermediate parts of the radius.
The orbe of virtue extends more widely than the orbe of motion of any
magnetick; for the magnetick is affected at its extremity, even if it is
not moved with local motion, which effect is produced {104} by the
loadstone being brought nearer. A small versorium also is turned when a
good distance off, even if at the same distance it would not flow towards
the loadstone, though free and disengaged from impediment.
The swiftness of the motion of a magnetick body to a loadstone is dependent
on either the power of the loadstone, on its mass, on its shape, on the
medium, or on its distance within the magnetick orbe.
A magnetick moves more quickly towards a more powerful * stone than towards
a sluggish one in proportion to the strength, and [as appears] by a
comparison of the loadstones together. A lesser mass of iron also is
carried more quickly towards a loadstone, just as also one that is a little
longer in shape. The swiftness of magnetick motion towards a loadstone is
changed by reason of the medium; for bodies are moved more quickly in air
than in water, and in clear air than in air that is thick and cloudy.
By reason of the distance, the motion is quicker in the case of bodies near
together than when they are far off. At the limits of the orbe of virtue of
a terrella a magnetick is moved feebly and slowly. At very short distances
close to the terrella the moving impetus is greatest.
A loadstone which in the outmost part of its orbe of virtue * hardly moves
a versorium when one foot removed from it, doth, if a long piece of iron is
joined to it, attract and repel the versorium more strongly with its
opposite poles when even three feet distant. The result is the same whether
the loadstone is armed or unarmed. Let the iron be a suitable piece of the
thickness of the little finger.
For the vigour of the loadstone excites verticity in the iron and proceeds
in the iron and through the iron much further than it extends through the
air.
The vigour proceeds even through several pieces of iron (joined * to one
another end to end), not so regularly, however, as through one continuous
solid.
Dust of steel placed upon paper rises up when a loadstone is moved near
above it in a sort of steely hairiness; but if the loadstone is placed
below, such a hairiness is likewise raised.
Steel dust (when the pole of a loadstone is placed near) is cemented * into
one body; but when it desires coition with the loadstone, the mass is split
and it rises in conglomerated parts.
But if there is a loadstone beneath the paper, the mass is split in the
same way and many portions result, each of which consists of very many
parts, and remains cemented together, as individual bodies. Whilst the
lower parts of these pursue greedily the pole of the loadstone placed
directly beneath, even they also are raised up as magnetick wholes, just as
a small iron wire of the length of a grain or two grains of barley is
raised up, both when the loadstone is moved near both beneath and above.
* * * * *
{105} CHAP. XXXIIII.
Why a Loadstone should be stronger in its poles
in a different ratio; as well in the Northern
_regions as in the Southern_.
The extraordinary magnetick virtue of the earth is * remarkably
demonstrated by the subtility of the following magnetical experiment. Let
there be given a terrella of no contemptible power, or a long loadstone
with equal cones as polar extremities; but in any other shape which is not
exactly round error is easy, and the experiment difficult. In the Northern
regions, raise the true North pole of the terrella above the horizon
straight toward the zenith; it is demonstrable that it raises up a larger
iron spike on its North pole, than the South pole of the same terrella is
able to raise, when turned in the same way toward the highest point of the
sky. The same thing is shown by a small terrella placed in the same way
above a larger.
[Illustration]
Let _a b_ be the earth or a somewhat large terrella, also _a b_ a smaller
terrella. There is set up above the Northern pole of the smaller terrella a
spike larger than the pole _b_ of the smaller terrella can raise, if it is
turned round to the higher parts. And the pole _a_ of the {106} smaller
terrella has its strength from the larger, declining from the Zenith to the
plane of the horizon or to the level. But now, if, * leaving the terrella
disposed in the same way, you bring a piece of iron to the lower and
Southern pole, it will attract and retain a greater weight than the Boreal
pole could, if it were turned round to the lower parts. Which thing is
demonstrated thus: let A be the earth or a terrella; E the Boreal pole or
some place in some great latitude; B a rather large terrella above the
earth or a smaller terrella on the top of a larger; D its Southern pole. It
is manifest that D (the Southern pole) attracts a larger piece of iron, C,
than F (the Boreal pole) will be able to, if it is turned round downward to
the position D, toward the earth or the terrella in the Northern regions.
[Illustration]
Magneticks acquire strength through magneticks, if they are properly placed
according to their nature, in near neighbourhood and within the orbe of
virtue. Wherefore when a terrella is placed on the earth or on a terrella,
so that its Southern pole is turned round toward the Northern pole, its
Northern pole, however, turned away from the Northern pole, the influence
and strength of {107} its poles are increased. And so the Northern pole of
a terrella in such a position lifts up a larger spike than the Southern
pole, if the Southern pole is turned away. Similarly the Southern pole in a
proper and natural arrangement, acquiring strength from the earth or from a
larger terrella, attracts and retains larger rods of iron. In * the other
part of the terrestrial globe toward the South, as also in the Austral
portion of a terrella, the reasoning is converse; for the Southern pole of
the terrella being turned away is more robust, as also the Northern pole
when turned round. The more a region on the earth is distant from the
æquinoctial (as also in a larger terrella), the larger is the accession of
strength perceived; near the æquator, indeed, the difference is small, but
on the æquator itself null; at the poles finally it is greatest.
* * * * *