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 aequator 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 aequilibrium 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 s