or the earth over
which it passes, may not be _continually condensing its moisture_, and
thereby _enveloping the earth in a perpetual mist_; but so that it may
part with it at _intervals_, making _cloudy_ and _clear days_; and part
with it in _portions_, so that a _regular_ and _necessary supply_ may be
furnished to the _entire hemisphere_, even up to the geographical poles.
Is there such an agent? There is, precisely and perfectly adapted to the
ends to be attained, ever there and ever active, and that agent is
_magnetism_.

[Illustration: Fig. 12.]

The earth is a magnet. It has its magnetic poles, and they are distinct
from its geographical ones; and there are two in each hemisphere. They are
situated from 17 deg. to 19 deg. distant from the geographical poles; and ours is
not far from longitude 97 deg. W. from Greenwich, and 71 deg. north latitude.
Navigators have gone north and north-west of it, and found its situation
by the declination of the needle. From these poles, lines of magnetic
intensity extend to the opposite and corresponding pole of the other
hemisphere, and upon or near those lines the needle points north without
variation; and toward these lines of no variation the needle every where,
on either side declines. The foregoing diagram shows the situation of our
magnetic pole and line of no variation, the dip of the needle by the
arrows, and the magnetic equator.

Recent discoveries have shown that the magnetic force is exerted in lines
and currents; that such currents, as physical lines of force, surround
magnets, and currents of electricity. Doubtless such lines of force exist
around the earth and the magnetic poles. There are also _longitudinal_
lines of force existing and active, between the poles, and extending from
one side of the center to the other, occupying nearly one third of the
magnet. If you take a large needle thoroughly magnetized, place it upon
paper and drop filings of iron upon it, they will become arranged about it
in circular and perpendicular, and also in _longitudinal lines_,
conforming to the currents.

[Illustration: Fig. 13.]

This experiment is illustrated in all our books on natural philosophy.

The foregoing diagram, copied from Olmstead's Philosophy, does not show as
accurately as Faraday's projection of the lines upon a globe-magnet the
comparative distance from the poles of the needle, at which the
longitudinal currents commence and terminate, and _where the