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
aequator, 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, aequidistant 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 aequator. 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 sp