*
An indicatory instrument, showing by the virtue of a
_stone the degrees of declination from the horizon_
of each several latitude.
{192} [Illustration] {193} _Description of the Instrument, and its use._
Take a terrella of the best strong loadstone, and homogeneous throughout,
not weakened by decay or by a flaw in any parts; let it be of a fair size,
so that its diameter is six or seven digits; and let it be made exactly
spherical. Having found its poles according to the method already shown,
mark them with an iron tool; then mark also the æquinoctial circle.
Afterwards in a thick squared block of wood, one foot in size, make a
hemispherical hollow, which shall hold half of the terrella, and such that
exactly one half of the stone shall project above the face of the block.
Divide the limb close to this cavity (a circle having been drawn round it
for a meridian) into 4 quadrants, and each of these into 90 degrees. Let
the terminus of the quadrants on the limb be near the centre of a quadrant
described on the block, also divided into 90 degrees. At that centre let a
short, slender versorium (its other end being rather sharp and elongated
like a pointer) be placed in æquilibrio on a suitable pin. It is manifest
that when the poles of the stone are at the starting points of the
quadrants, then the versorium lies straight, as if in æquilibrio, over the
terrella. But if you move the terrella, so that the pole on the left hand
rises, then the versorium rises on the meridian in proportion to the
latitude, and turns itself as a magnetick body; and on the quadrant
described on the flat surface of the wood, the degree of its turning or of
the declination is shown by the versorium. The rim of the cavity represents
a meridional circle, to which corresponds some meridian circle of the
terrella, since the poles on both sides are within the circumference of the
rim itself. These things clearly always happen on the same plan on the
earth itself when there is no variation; but when there is variation,
either in the direction or in the declination (a disturbance, as it were,
in the true turning, on account of causes to be explained later), then
there is some difference. Let the quadrant be near the limb, or have its
centre on the limb itself, and let the versorium be very short, so as not
to touch the terrella, because with a versorium that is longer or more
remote, there is some error; for it has a motion truly proportionate to the
terrella only on the surface of the terrella. But if the quadrant, being
far distant from the terrella, were moved within the orbe of virtue of the
terrella toward the pole on some circle concentrick with the terrella, then
the versorium would indicate the degrees of declination on the quadrant, in
proportion to and symmetrically with that circle, not with the terrella.
* * * * *
{194} CHAP. IIII.
Concerning the length of a versorium convenient
_for declination on a terrella_.
Declination being investigated on the earth itself by means of a
declination instrument, we may use either a short or a very long versorium,
if only the magnetick virtue of the stone that touches it is able to
permeate through the whole of its middle and through all its length. For
the greatest length of a versorium has no moment or perceptible proportion
to the earth's semi-diameter. On a terrella, however, or in a plane near a
meridian of a terrella, a short versorium is desirable, of the length, say,
of a barleycorn; for longer ones (because they reach further) dip and turn
toward the body of the terrella suddenly and irregularly in the first
degrees of declination. [Illustration] For example, as soon as the long
versorium is moved forward from the aequator A to C, it catches on the
stone with its cusp (as if with a long extended wing), when the cusp
reaches to the parts about B, which produce a greater rotation than at C.
And the extremities of longer wires also and rods turn irregularly, just as
iron wires and balls of iron and other orbicular loadstones are likewise
turned about irregularly by a long non-orbicular loadstone. Just so
magneticks or iron bodies on the surface of a terrella ought not to have
too long an axis, but a very short one; so that they may make a declination
on the terrella truly and naturally proportionate to that on the earth. A
long versorium also close to a terrella with difficulty stands steady in a
horizontal direction on a right sphere, and, beginning to waver, it dips
immediately to one side, especially the end that was touched, or (if both
were touched) the one which felt the stone last.
* * * * *
{195} CHAP. V.
That declination does not arise from the attraction
of the loadstone, but from a disposing and
_rotating influence_.
In the universe of nature that marvellous provision of its Maker should be
noticed, whereby the principal bodies are restrained within certain
habitations and fenced in, as it were (nature controlling them). For this
reason the stars, though they move and advance, are not thrown into
confusion. Magnetical rotations also arise from a disposing influence,
whether in greater and dominating quantity, or in a smaller, and compliant
quantity, even though it be very small. For the work is not accomplished by
attraction, but by an incitation of each substance, by a motion of
agreement toward fixed bounds, beyond which no advance is made. For if the
versorium dipped by reason of an attractive force, then a terrella made
from a very strong magnetick stone would cause the versorium to turn toward
itself more than one made out of an average stone, and a piece of iron
touched with a vigorous loadstone would dip more. This, however, never
happens. Moreover, an iron snout placed on a meridian in any latitude does
not raise a spike more toward the perpendicular than the stone itself,
alone and unarmed; although when thus equipped, it plucks up and raises
many greater weights[234]. But if a loadstone be sharper toward one pole,
toward the other blunter, the sharp end or pole allures a magnetick needle
more strongly, the blunt, thick end makes it rotate more strongly; but an
orbicular stone * makes it rotate strongly and truly, in accordance with
magnetick rules and its globular form. A long stone, on the other hand,
extended from pole to pole, moves a versorium toward it irregularly; for in
this case the pole of the versorium always looks down on the pole itself.
Similarly also, if the loadstone have been made in the shape of a circle,
and its poles are on the circumference, whilst the body of it is plane, not
globular, if the plane be brought near a versorium, the versorium does not
move with the regular magnetick rotation, as on a terrella; but it turns
looking always toward the pole of the loadstone, which has its seat on the
circumference of the plane. Moreover, if the stone caused the versorium to
rotate by attracting it, then in the first degrees of latitude, it would
attract the end of a short versorium toward the body itself of the
terrella; yet it does not so attract it that they are brought into contact
and unite; but the versorium rotates just so far as nature demands, as is
clear from this example. {196} [Illustration] * For the cusp of a versorium
placed in a low latitude does not touch the stone or unite with it, but
only inclines toward it. Moreover, when a magnetick body rotates in
dipping, the pole of the versorium is not stayed or detained by the pole of
the earth or terrella; but it rotates regularly, and does not stop at any
point or bound, nor point straight to the pole toward which the centre of
the versorium is advancing, unless on the pole itself, and once only
between the pole and the æquator; but it dips as it advances, according as
the change of position of its centre gives a reason for its inclination in
accordance with rules magnetical. The declination of a magnetick needle in
water also, as demonstrated in the following pages, is a fixed
quantity[235]; the magnetick needle does not descend to the bottom of the
vessel, but remains steady in the middle, rotated on its centre according
to its due amount of declination. This would not happen, if the earth or
its poles by their attraction drew down the end of the magnetick needle, so
that it dipped in this way.
* * * * *