[Illustration]
{186} Now how much it dips at every horizon may be ascertained in the first
place by a contrivance, which, however, is not so easily made as is that in
dials for measuring time, in which the needle turns to the points of the
horizon, or in the mariners' compass. From a plank of wood let a smooth and
circular instrument be prepared, at least six digits in diameter, and affix
this to the side of a square pillar, which stands upright on a wooden base.
Divide the periphery of this instrument into 4 quadrants: then each
quadrant into 90 degrees. At the centre of the instrument let there be
placed a brass peg, at the centre of the end of which let there be a small
hollow, well polished. To this wooden instrument let a brass circle or ring
be fixed, about two digits in width, with a thin plate or flat rod of the
same metal, representing the horizon, fixed across it, through the middle
of the circle. In the middle of the horizontal rod let there be another
hollow, which shall be exactly opposite the centre of the instrument, where
the former hollow was made. Afterward let a needle be fashioned out of
steel, as versoria are accustomed to be made. Divide this at right angles
by a thin iron axis (like a cross) through the very middle and centre of
the wire and the cross-piece. Let this dipping-needle be hung (with the
ends of the cross resting in the aforesaid holes) so that it can move
freely and evenly on its axis in the most perfect æquilibrium, so
accurately that it turns away from no one point or degree marked on the
circumference more than from another, but that it can rest quite easily at
any. Let it be fixed upright to the front part of the pillar, whilst at the
edge of the base is a small versorium to show direction. Afterward touch
the iron, suspended by this ingenious method, on both ends with the
opposite ends of a loadstone, according to the scientifick method, but
rather carefully, lest the needle be twisted in any way; for unless you
prepare everything very skilfully and cleverly, you will secure no result.
Then let another brass ring be prepared, a little larger, so as to contain
the former one; and let a glass or a very thin plate of mica be fitted to
one side of it. When this is put over the former ring, the whole space
within remains inclosed, and the versorium is not interfered with by dust
or winds. Dispose the instrument, thus completed, perpendicularly on its
base, and with the small versorium horizontal, in such a way that, while
standing perpendicularly, it may be directed toward the exact magnetical
point respective. Then the end of the needle which looks toward the north
dips below the horizon in northern regions, whilst in southern regions the
end of the needle which looks toward the south tends toward the centre of
the earth, in a certain proportion (to be explained afterward) to the
latitude of the district in question, from the æquator on either side. The
needle, however, must be rubbed on {187} a powerful loadstone; otherwise it
does not dip to the true point, or else it goes past it, and does not
always rest in it. A larger instrument may also be used, whose diameter may
be 10 or 12 digits; but in such an instrument more care is needed to
balance the versorium truly. Care must be taken that the needle be of
steel; also that it be straight; likewise that both ends of the cross-piece
be sharp and fixed at right angles to the needle, and that the cross-piece
pass through the centre of the needle. As in other magnetical motions there
is an exact agreement between the earth and the stone, and a correspondence
manifestly apparent to our senses by means of our experiments; so in this
declination there is a clear and evident concordance of the terrestrial
globe with the loadstone. Of this motion, so important and so long unknown
to all men, the following is the sure and true cause. A magnet-stone is
moved and turned round until one of its poles being impelled toward the
north comes to rest toward a definite point of the horizon. [231]This pole,
which settles toward the north (as appears from the preceding rules and
demonstrations), is the southern, not the boreal; though all before us
deemed it to be the boreal, on account of its turning to that point of the
horizon. A wire or versorium touched on this pole of the stone turns to the
south, and is made into a boreal pole, because it was touched by the
southern terminal of the stone. So if the cusp of a versorium be excited in
a similar manner, it will be directed toward the southern pole of the
earth, and will adjust itself also to it; but the cross (the other end)
will be southern, and will turn to the north of the earth (the earth itself
being the cause of its motion); for so direction is produced from the
disposition of the stone or of the excited iron, and from the verticity of
the earth. But declination takes place when a magnetick is turned round
toward the body of the earth, with its southern end toward the north, at
some latitude away from the æquator. For this is certain and constant, that
exactly under the coelestial æquator, or rather over the æquator of the
terrestrial globe, there is no declination of a loadstone or of iron; but
in whatever way the iron has been excited or rubbed, it settles in the
declination instrument precisely along the plane of the horizon, if it were
properly balanced before. Now this occurs thus because, when the magnetick
body is at an equal distance from either pole, it dips toward neither by
its own versatory nature, but remains evenly directed to the level of the
horizon, as if it were resting on a pin or floating free and unhindered on
water. But when the magnetick substance is at some latitude away from the
æquator, or when either pole of the earth is raised (I do not say raised
above the visible horizon, as the commonly imagined pole of the revolving
universe in the sky, but above the horizon or its centre, or its proper
diameter, æquidistant from the plane of the visible horizon, which is the
true elevation of the terrestrial pole), {188} [Illustration] then
declination is apparent, and the iron inclines toward the body of the earth
in its own meridian. Let A B, for example, be the visible horizon of a
place; C D the horizontal through the earth, dividing it into equal parts;
E F the axis of the earth; G the position of the place. It is manifest that
the boreal pole E is elevated above the point C by as much as G is distant
from the æquator. Wherefore, since at E the magnetick needle stands
perpendicularly in its proper turning (as we have often shown before), so
now at G there is a certain tendency to turn in proportion to the latitude
(the magnetick dipping below the plane of the horizon), and the magnetick
body intersects the horizon at unequal angles, and exhibits a declination
below the horizon. For the same reason, if the declinatory needle be placed
at G, its southern end, the one namely which is directed toward the North,
dips below the plane of the visible horizon A B. And so there is the
greatest difference between a right sphere[232] and a polar or parallel
sphere, in which the pole is at the very Zenith. For in a right sphere the
needle is parallel to the plane of the horizon; but when the coelestial
pole is vertically overhead, or when the pole of the earth is itself the
place of the region, then the needle is perpendicular to the horizon. This
is shown by a round stone. Let a small dipping-needle, of two digits length
(rubbed with a magnet), be hung in the air like a balance, and let the
stone be carefully placed under it; and first let the terrella be at right
angles, as in a right sphere, and as in the first figure; for so the
magnetick needle will remain in equilibrium. But in an oblique position of
the terrella, as in an oblique sphere, and in the second figure, the needle
dips obliquely at one end toward the near pole, but does not rest on the
pole, nor is its dip ruled by the pole, but by the body and mass of the
whole; for the {189} dip in higher latitudes passes beyond the pole. But in
the third position of the terrella the needle is perpendicular; because the
pole of the stone is placed at the top, and the needle tending straight
toward the body reaches to the pole. The cross in the preceding figures
always turns toward the boreal pole of the terrella, having been touched by
the boreal pole of the terrella; the cusp of the needle, having been
touched by the southern pole of the stone, turns to the south. Thus one may
see on a terrella the level, oblique, and perpendicular positions of a
magnetick needle. *
[Illustration]
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