t although the common properties of the
loadstone were known, and even studied, during the dark ages, its
directive power, or that of a needle touched or rubbed by it, seems to be
the discovery of modern times, notwithstanding the claims of the Chinese
and Arabians to an early acquaintance with this peculiarity.

There is no doubt that the mariner's compass was known in the twelfth
century, for several authors of that period make special allusion to it;
but centuries elapsed before its variation from pointing precisely to the
poles became noticed. If a magnet be suspended by a thread, in such a
manner as to enable it to move freely, it will, when all other magnetic
bodies are entirely removed from it, settle in a fixed position, which, in
this country, is about 25 deg. to the west of north; this deviation of the
needle from the north is called its variation. Again, if, in place of
suspending a magnetized needle, making it move horizontally on a pivot, we
balance it upon a horizontal axis, as the beam of a pair of scales, we
shall find that it no longer remains horizontal, but that one end will
incline downward, or, as it is called, _dip_, and this dip or inclination
from a horizontal line is about 70 deg. in this country.

Thus we are presented with two distinct magnetical phenomena: 1. The
variation or declination of the needle; 2. Its dip or inclination; and to
these we may add the intensity or force which draws the needle from
pointing to the north, and which varies in different latitudes. These
phenomena constitute what has been called terrestrial magnetism.

Recent writers, and among them the great philosopher Humboldt, have shown
that in all probability the declination or variation of the magnet was
known as early as the twelfth century; but this important discovery has
been generally ascribed to Columbus. His son Ferdinand states that on the
14th September 1492, his father, when about 200 leagues from the island of
Ferro, noticed for the first time the variation of the needle. "A
phenomenon," says Washington Irving, "that had never before been
remarked." "He perceived," adds this author, "about nightfall that the
needle, instead of pointing to the north star, varied half a point, or
between five and six degrees, to the northwest, and still more on the
following morning. Struck with this circumstance, he observed it
attentively for three days, and found that the variation increased as he
advanced. He at first mad