On the Potency of the Magnetick Virtue, and on
its nature capable of spreading out into an orbe.
From about a magnetical body the virtue magnetical is poured out on every
side around in an orbe; around a terrella; in the case of other shapes of
stones, more confusedly and unevenly. But yet there exists in nature no
orbe or permanent or essential virtue spread through the air, but a magnet
{77} only excites magneticks at a convenient distance from it. And as light
comes in an instant (as the opticians teach), so much more[165] quickly is
the magnetick vigour present within the limits of its strength; and because
its activity is much more subtile than light, and does not consent with a
non-magnetick substance, it has no intercourse with air, water, or any
non-magnetick; nor does it move a magnetick with any motion by forces
rushing upon it, but being present in an instant, it invites friendly
bodies. And as light strikes an object, so a loadstone strikes a magnetick
body and excites it. And just as light does not remain in the air above
vapours and effluvia, and is not reflected from those spaces, so neither is
the magnetick ray held in air or water. The appearances of things are
apprehended in an instant in mirrors and in the eye by means of light; so
the magnetick virtue seizes upon magneticks. Without the more intangible
and shining bodies, the appearances of things are not seized or reflected;
so without magnetical objects the magnetick power is not perceived, nor are
the forces thus conceived sent back again to the magnetick substance. In
this, however, the magnetick power excels light, in that it is not hindered
by any opaque or solid substance, but proceeds freely, and extends its
forces on every side. In a terrella and globe-shaped loadstone the
magnetick power is extended outside the body in an orbe; in a longer one,
however, not in an orbe, but it is extended in an ambit conformably to the
shape of the stone. As in the somewhat long stone A, the vigour is extended
to the ambient limit F C D, equidistant on every side from the stone A.
[Illustration]
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{78} CHAP. VIII.
On the geography of the Earth,
_and of the Terrella_.
Desiring that what follows may be better understood, we must now say
something also about magnetick circles and limits. Astronomers, in order to
understand and observe methodically the motion of the planets and the
revolution of the heavens, and to describe with more accuracy the celestial
attire of the fixed stars, settled upon certain circles and definite limits
in the sky (which geographers also imitate), so that the varied face of the
earth and the beauty of its districts might be delineated. But we, in a way
differing from them, recognize those limits and circles, and have found
very many fixed by nature, not merely conceived by the imagination, both in
the earth and in our terrella. The earth they mark out[166] chiefly by
means of the æquator and the poles; and those limits indeed have been
arranged and marked out by nature. The meridians also indicate straight
paths from pole to pole through distinct points on the æquator; by which
way the magnetick virtue directs its course and moves. But the tropics and
arctic circles, as also the parallels, are not natural limits placed on the
earth; but all parallel circles indicate a certain agreement of the lands
situated in the same latitude, or diametrically opposite. All these the
Mathematicians use for convenience, painting them on globes and maps. In
like manner also in a terrella all these are required; not, however, in
order that its exterior appearance may be geographically delineated, since
the loadstone may be perfect, even, and uniform on all sides. And there are
no upper and lower parts in the earth, nor are there in a terrella; unless
perchance some one considers those parts superior which are in the
periphery, and those inferior which are situated more towards the centre.
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{79} CHAP. IX.
On the Æquinoctial Circle of the Earth
_and of a Terrella_.
As conceived by astronomers the æquinoctial circle is equidistant from both
poles, cutting the world in the middle, measures the motions of their
_primum mobile_ or tenth sphere, and is named the zone of the _primum
mobile_. It is called æquinoctial, because when the sun stands in it (which
must happen twice in the year) the days are equal to the nights. That
circle is also spoken of as _æquidialis_, wherefore it is called by the
Greeks [Greek: isêmerinos]. In like manner it is also properly called
Æquator, because it divides the whole frame of the earth between the poles
into equal parts. So also an æquator may be rightly assigned to a terrella,
by which its power is naturally divided, and by the plane of which
permeating through its centre, the whole globe is divided into equal parts
both in quantity and strength (as if by a transverse septum) between
verticities on both sides imbued with equal vigour.
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