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'clock
in the evening, when it becomes nearly constant for the rest of
the night. In the southern hemisphere the direction of the
movement is reversed.

When the pointing of the needle is compared with the direction of
the moon, it is found that there is a similar change. But, instead
of following the moon in its course, it goes through two periods
in a day, like the tides. When the moon is on the meridian,
whether above or below us, the effect is in one direction, while
when it is rising or setting it is in the opposite direction. In
other words, there is a complete swinging backward and forward
twice in a lunar day. It might be supposed that such an effect
would be due to the moon, like the earth, being a magnet. But were
this the case there would be only one swing back and forth during
the passage of the moon from the meridian until it came back to
the meridian again. The effect would be opposite at the rising and
setting of the moon, which we have seen is not the case. To make
the explanation yet more difficult, it is found that, as in the
case of the sun, the change is opposite in the northern and
southern hemispheres and very small at the equator, where, by
virtue of any action that we can conceive of, it ought to be
greatest. The pointing is also found to change with the age of the
moon and with the season of the year. But these motions are too
small to be set forth in the present article.

There is yet another class of changes much wider than these. The
observations recorded since the time of Columbus show that, in the