gnet to various parts of
the world, the magnetic force can be determined at every point where a
proper support for the magnet is obtainable. The intensity thus found
is called the horizontal force. This is not really the total force,
because the latter depends upon the dip; the greater the dip, the less
will be the horizontal force which corresponds to a certain total
force. But a very simple computation enables the one to be determined
when the value of the other is known. In this way it is found that, as
a general rule, the magnetic force is least in the earth's equatorial
regions and increases as we approach either of the magnetic poles.

When the most exact observations on the direction of the needle are
made, it is found that it never remains at rest. Beginning with the
changes of shortest duration, we have a change which takes place every
day, and is therefore called diurnal. In our northern latitudes it is
found that during the six hours from nine o'clock at night until three
in the morning the direction of the magnet remains nearly the same. But
between three and four A.M. it begins to deviate towards the east,
going farther and farther east until about 8 A.M. Then, rather
suddenly, it begins to swing towards the west with a much more rapid
movement, which comes to an end between one and two o'clock in the
afternoon. Then, more slowly, it returns in an easterly direction until
about nine at night, when it becomes once more nearly quiescent.
Happily, the amount of this change is so small that the navigator need
not trouble himself with it. The entire range of movement rarely
amounts to one-quarter of a degree.

It is a curious fact that the amount of the change is twice as great in
June as it is in December. This indicates that it is caused by the
sun's radiation. But how or why this cause should produce such an
effect no one has yet discovered.

Another curious feature is that in the southern hemisphere the
direction of the motion is reversed, although its general character
remains the same. The pointing deviates towards the west in the
morning, then rapidly moves towards the east until about two o'clock,
after which it slowly returns to its original direction.

The dip of the needle goes through a similar cycle of daily changes. In
northern latitudes it is found that at about six in the morning the dip
begins to increase, and continues to do so until noon, after which it
diminishes until seven or eight o'cl