poles of the stone are at the starting points of the
quadrants, then the versorium lies straight, as if in aequilibrio, 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 lon