more miles are
traversed on the parallel circle C D, near C, in order that the versorium
may deviate by one degree from the pole A, than near D. So also in order
that the variation may be diminished from D toward E more miles are
required near D than near E. Thus the deviations become equal in unequal
courses, whether the variation be increasing or decreasing; and yet the
variation decreases by lesser intervals than it increases. There intervene,
however, many other causes which perturb this proportion.

* * * * *

{184} [Illustration]

BOOK FIFTH.

_CHAP. I._

ON DECLINATION.

In due course we have now come to that notable experiment, and remarkable
motion of magnetick bodies dipping below the horizon by their own rotatory
nature; by the knowledge of which is revealed a unity, a concordancy, and a
mutual agreement between the terrestrial globe and the loadstone (or the
magnetick iron), which is wonderful in itself, and is made manifest by our
teaching. This motion we have made known in many striking experiments, and
have established its rules; and in the following pages we shall demonstrate
the causes of it, in such a way that no sound, logical mind can ever
rightly set at nought or disprove our chief magnetick principles.
Direction, as also variation, is demonstrated in a horizontal plane, when a
balanced magnetick needle comes to rest at some definite point; but
declination is seen to be the motion of a needle, starting from that point
of the horizon, first balanced on its own axis, then excited by a
loadstone, one end or pole of it tending toward the centre of the earth.
And we have found that it takes place in proportion to the latitude of each
region. But that motion arises in truth, not from any motion from the
horizon toward the centre of the earth, but from the turning of the whole
magnetick body toward the whole of the earth, as we shall show hereafter.
Nor does the iron dip from the horizontal in some oblique sphere, according
to the number of degrees of elevation of the pole in the given region, or
by an equal arc in the quadrant, as will appear hereafter. {185}

Instrument of the Declination

[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' compas