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
course of centuries, the variation of the compass, at any one
point, changes very widely. It is well known that in 1490 the
needle pointed east of north in the Mediterranean, as well as in
those portions of the Atlantic which were then navigated. Columbus
was therefore much astonished when, on his first voyage, in mid-
ocean, he found that the deviation was reversed, and was now
towards the west.
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