s north end being now turned downwards. Thus we
learn that on the one side of the magnetic equator the north end of
the needle dips; on the other side the south end dips, the dip varying
from nothing to 90 deg.. If we go to the equatorial regions of the earth
with a suitably suspended needle we shall find there the position of
the needle horizontal. If we sail north one end of the needle dips;
if we sail south the opposite end dips; and over the north or south
terrestrial magnetic pole the needle sets vertical. The south
magnetic pole has not yet been found, but Sir James Ross discovered
the north magnetic pole on June 1, 1831. In this manner we establish
a complete parallelism between the action of the earth and that of an
ordinary magnet.

The terrestrial magnetic poles do not coincide with the geographical
ones; nor does the earth's magnetic equator quite coincide with the
geographical equator. The direction of the magnetic needle in London,
which is called the magnetic meridian, encloses an angle of 24 deg. with
the astronomical meridian, this angle being called the Declination of
the needle for London. The north, pole of the needle now lies to the
west of the true meridian; the declination is westerly. In the year
1660, however, the declination was nothing, while before that time it
was easterly. All this proves that the earth's magnetic constituents
are gradually changing their distribution. This change is very slow:
it is therefore called the secular change, and the observation of it
has not yet extended over a sufficient period to enable us to guess,
even approximately, at its laws.

Having thus discovered, to some extent, the secret of the earth's
magnetic power, we can turn it to account. In the line of 'dip' I
hold a poker formed of good soft iron. The earth, acting as a magnet,
is at this moment constraining the two fluids of the poker to
separate, making the lower end of the poker a north pole, and the
upper end a south pole. Mark the experiment: When the knob is
uppermost, it attracts the north end of a magnetic needle; when
undermost it attracts the south end of a magnetic needle. With such a
poker repeat this experiment and satisfy yourselves that the fluids
shift their position according to the manner in which the poker is
presented to the earth. It has already been stated that the softest
iron possesses a certain amount of coercive force. The earth, at this
moment, finds in this force