Explanation of the diagram of the rotation of
_a magnetick needle_.
[Illustration]
Suppose A C D L to be the body of the earth or of a terrella, its centre M,
Æquator A D, Axis C L, A B the Horizon, which changes according to the
place. From the point F on a Horizon distant from the æquator A by the
length of C M, the semi-diameter of the earth or terrella, an arc is
described to H as the limit of the quadrants of declination; for {199} all
the quadrants of declination serving the parts from A to C begin from that
arc, and terminate at M, the centre of the earth. The semi-diameter of this
arc is a chord drawn from the æquator A to the pole C; and a line produced
along the horizon from A to B, equal to that chord, gives the beginning of
the arc of the limits of arcs of rotation and revolution, which is
continued as far as G. For just as a quadrant of a circle about the centre
of the earth (whose beginning is on the horizon, at a distance from the
æquator equal to the earth's semi-diameter) is the limit of all quadrants
of declination drawn from each several horizon to the centre; so a circle
about the centre from B, the beginning of the first arc of rotation, to G
is the limit of the arcs of rotation. The arcs of rotation and revolution
of the magnetick needle are intermediate between the arcs of rotation B L
and G L. The centre of the arc is the region itself or place in which the
observation is being made; the beginning of the arc is taken from the
circle which is the limit of rotations, and it stops at the opposite pole;
as, for example, from O to L, in a latitude of 45 degrees. Let any arc of
rotation be divided into 90 equal parts from the limit of the arcs of
rotation toward the pole; for whatever is the degree of latitude of the
place, the part of the arc of rotation which the magnetick pole on or near
the terrella or the earth faces in its rotation is to be numbered similarly
to this. The straight lines in the following larger diagram show this. The
magnetick rotation at the middle point in a latitude of 45 degrees is
directed toward the æquator, in which case also that arc is a quadrant of a
circle from the limit to the pole; but previous to this all the arcs of
rotation are greater than a quadrant, whilst after it they are smaller; in
the former the needle rotates more quickly, but in the succeeding positions
gradually more slowly. For each several region there is a special arc of
rotation, in which the limit to which the needle rotates is according to
the number of degrees of latitude of the place in question; so that a
straight line drawn from the place to the point on that arc marked with the
number of degrees of latitude shows the magnetick direction, and indicates
the degree of declination at the intersection of the quadrant of
declination which serves the given place. Take away the arc of the quadrant
of declination drawn from the centre to the line of direction; that which
is left is the arc of declination below the horizon. As, for example, in
the rotation of the versorium N, whose line respective proceeds to D, from
the quadrant of declination, S M, take away its arc R M; that which is left
is the arc of declination: how much, that is, the needle dips in the
latitude of 45 degrees.
* * * * *
{200} CHAP. VIII.
Diagram of the rotation of a magnetick needle,
_indicating magnetical declination in all latitudes, and_
from the rotation and declination, the
_latitude itself_.
In the more elaborate diagram a circle of rotations and a circle of
declinations are adjusted to the body of the earth or terrella, with a
first, a last, and a middle arc of rotation and declination. Now from each
fifth division of the arc which limits all the arcs of rotation (and which
are understood[238] as divided into 90 equal parts) arcs are drawn to the
pole, and from every fifth degree of the arc limiting the quadrants of
declination, quadrants are drawn to the centre; and at the same time a
spiral line is drawn, indicating (by the help of a movable quadrant) the
declination in every latitude. Straight lines showing the direction of the
needle are drawn from those degrees which are marked on the meridian of the
earth or a terrella to their proper arcs and the corresponding points on
those arcs.
To ascertain the elevation of the pole or the latitude of a place
anywhere
_in the world, by means of the following diagram, turned into
a magnetick instrument, without the help of the coelestial
bodies, sun, planets, or fixed stars, in fog
and darkness_.
[Illustration]
We may see how far from unproductive magnetick philosophy is, how
agreeable, how helpful, how divine! Sailors when tossed about on the waves
with continuous cloudy weather, and unable by means of the coelestial
luminaries to learn anything about the place or the region in which they
are, with a very slight effort and with a small instrument are comforted,
and learn the latitude of the place. With a declination instrument the
degree of declination of the magnetick needle below the horizon is
observed; that degree is noted on the inner arc of the quadrant, and the
quadrant is turned round about the centre of the instrument until that
degree on the quadrant touches the spiral line; then in the open space B at
the centre of the quadrant the latitude of the region on {201} the
circumference of the globe is discerned by means of the fiducial line A B.
Let the diagram be fixed on a suitable flat board, and let the centre of
the corner A of the quadrant be fastened to the centre of it, so that the
quadrant may rotate on that centre. But it must be understood that there is
also in certain places a variation in the declination on account of causes
already mentioned (though not a large one), which it will be an assistance
also to allow for on a likely estimate; and it will be especially helpful
to observe this variation in various places, as it seems to present greater
difficulty than the variation in direction; but it is easily learnt with a
declination instrument, when it dips more or less than the line in the
diagram.
[Illustration]
_To observe magnetick declination at sea_.
Set upon our variation instrument a declination instrument; a wooden disc
being placed between the round movable {202} compass and the declination
instrument: but first remove the versorium, lest the versorium should
interfere with the dipping needle. In this way (though the sea be rough)
the compass box will remain upright at the level of the horizon. The stand
of the declination instrument must be directed by means of the small
versorium at its base, which is set to the point respective of the
variation, on the great circle of which (commonly called the magnetick
meridian), the plane of the upright box is arranged; thus the declinatorium
(by its versatory nature) indicates the degree of declination.
In a declination instrument the magnetick needle, which
_in a meridional position dips, if turned
along a parallel hangs perpendicularly._
In a proper position a magnetick needle, while by its rotatory nature
conformed to the earth, dips to some certain degree below the horizon on an
oblique sphere. But when the plane of the instrument is moved out of the
plane of the meridian, the magnetick needle (which tends toward the pole)
no longer remains at the degree of its own declination, but inclines more
toward the centre; for the force of direction is stronger than that of
declination, and all power of declination is taken away, if the plane of
the instrument is on a parallel. For then the magnetick needle, because it
cannot maintain its due position on account of the axis being placed
transversely, faces down perpendicularly to the earth; and it remains only
on its own meridian, or on that which is commonly called the magnetick
meridian.
* * * * *