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