Of Discords between pieces of Iron upon the same pole
of a loadstone, and how they can agree and
_stand joined together_.
Suppose two iron wires or a pair of needles stuck on the pole of a
terrella; though they ought to stand perpendicularly, they mutually repel
one another at the upper * end, and produce the appearance of a fork; and
if one end be forcibly impelled toward the other, the other declines and
bends away from association with it, as in the following figure.
[Illustration] {133} A and B, iron spikes, adhære obliquely[207] upon the
pole on account of their nearness to one another; either alone would
otherwise stand erect and perpendicular. For the extremities A B, being of
the same verticity, mutually abhor and fly one another. For if C be the
northern pole of the terrella, A and B are also northern ends; but the ends
which are joined to and held at the pole C are both * southern. But if
those spikes be a little longer (as, for example, of two digits length) and
be joined by force, they adhære together and unite in a friendly style, and
are not separated without force. For they are magnetically welded, and
there are now no longer two distinct ends, but one end and one body; no
less than a wire which is doubled and set up perpendicularly. But here is
seen also another subtile point, that if those spikes were shorter, not as
much as the * breadth of one digit, or even the length of a barleycorn,
they are in no way willing to harmonize or to stand straight up at the same
time, because naturally in shorter wires the verticity is stronger in the
ends which are distant from the terrella and the magnetick discord more
vehement than in long ones. Wherefore they in no way admit of an intimate
association and connection.
[Illustration]
Likewise if those lighter pieces of iron or iron wires be suspended,
hanging, as A and B, from a very fine silk thread, not twisted * but
braided, distant from the stone the length of a single barleycorn, then the
opposing ends, A and B, being situated within the orbe of virtue above the
pole, keep a little away from one another for the same reason; except when
they are very near the pole of the stone C, the stone then attracting them
more strongly toward one end.
* * * * *
{134} CHAP. IX.
Figures illustrating direction and showing varieties
_of rotations_.
[Illustration]
Passing from the probable cause of motion toward fixed points (according to
magnetick laws and principles), it remains for us to indicate those
motions. Above a round loadstone (whose poles are A, B) let a versatory
needle be placed whose cusp has been excited by the pole A; that cusp is
certainly directed toward A, and is strongly attracted by A; because,
having been touched by A, it is in true harmony with A, and combines with
it; and yet it is called contrary, because when the versorium is separated
from the stone, it is seen to be moved toward the opposite part of the
earth to that toward which the pole A of the loadstone is moved. For if A
be the northern pole of the terrella, the cusp is the southern end of the
needle, of which the other end (namely, the cross) is pointed to B; so B is
the southern pole of the loadstone, but the cross is the northern end of
the versorium. So also the cusp is attracted by E, F, G, H, and by every *
part of a meridian, from the æquator toward the pole, by the faculty
disponent; and when the versorium is on the same parts of the meridian, the
cusp is directed toward A. For it is not the point A that turns the
versorium toward it, but the whole loadstone; as also the whole earth does,
in the turning of loadstones to the earth.
[Illustration]
_Figures illustrating magnetick directions in a right sphere[208] of stone,
and in the right sphere of the earth, as well as the polar directions to
the perpendicular of the poles._ All these cusps have been touched by the
pole A; all the cusps are turned toward A, excepting that one which is
repelled by B.
{135}
[Illustration] _Figures illustrating horizontal directions above the body
of a loadstone._ All the cusps that have been made southern by rubbing on
the boreal pole, or some place round the northern pole A, turn toward the
pole A, and turn away from the southern pole B, toward which all the
crosses look. I call the direction horizontal, because it is arranged along
the plane of the horizon; for nautical and * horological instruments are so
constructed that the iron hangs or is supported in æquilibrium on the point
of a sharp pin, which prevents the dipping of the versorium, about which we
intend to speak later. And in this way it is of the greatest use to man,
indicating and distinguishing all the points of the horizon and the winds.
Otherwise on every oblique sphere (whether of stone or the earth) versoria
and all magnetick substances would have a dip by their own nature below the
horizon; and at the poles the directions would be perpendicular, which
appears in our discussion _On Declination_.
[Illustration]
_A round stone (or terrella) cut in two at the æquator;_ and all the cusps
have been touched by the pole A. The points at the centre of the earth, and
between the two parts of the terrella which has been cut in two through the
plane of the æquator, {136} are directed as in the present[209] diagram.
This would also happen in the same way if the division of the stone were
through the plane of a tropick, and the mutual separation of the divided
parts and the interval between them were the same as before, when the
loadstone was divided through the plane of the æquator, and the parts
separated. For the cusps are repelled by C, are attracted by D; and the
versoria are parallel, the poles or the verticity in both ends mutually
requiring it.
[Illustration]
_Half a terrella by itself and its directions, unlike the directions * of
the two parts close to one another as shown in the figure above_. All the
cusps have been touched by A; all the crosses below except the middle one
tend toward the loadstone, not straight, but obliquely; because the pole is
in the middle of the plane which before was the plane of the æquator. All
cusps touched by places distant from the pole move toward the pole (exactly
the same as if they had been rubbed upon the pole itself), not toward the
place where they were rubbed, wherever that may have been in the undivided
stone in some latitude between the pole and the æquator. And for this
reason there are only two distinctions of regions, northern and southern,
in the terrella, just {137} as in the general terrestrial globe, and there
is no eastern nor western place; nor are there any eastern or western
regions, rightly speaking; but they are names used in respect of one
another toward the eastern or western part of the sky. Wherefore it does
not appear that Ptolemy did rightly in his _Quadripartitum_, making eastern
and western districts and provinces, with which he improperly connects the
planets, whom the common crowd of philosophizers and the superstitious
soothsayers follow.
* * * * *