and 132 degrees west
long., and consequently in the middle of the present closed isogonal system.
(Hansteen, 'Magnet. der Erde', 1819 ?ยค 28.) It must not be forgotten, in
the midst of all these considerations, that we can only follow the direction
of the magnetic lines in their progress as they are projected upon the
surface of the Earth.
One would almost be inclined to regard this singular configuration of
closed, almost concentric, lines of declination as the effect of a local
character of that portion of the globe; but if, in the course of centuries,
these apparently isolated systems should also advance, we must suppose, as
in the case of all great natural forces, that the phenomenon arises from
some general cause.
The horary variations of the declination, which, although dependent upon
true time, are apparently governed by the Sun, as long as it remains above
the horizon, diminish in angular value with the magnetic latitude of place.
Near the equator, for instance, in the island of Rawak, they scarcely amount
to three or four minutes, while they are from thirteen to fourteen minutes
in the middle of Europe. As in the whole northern hemisphere the north
point of the needle moves from east to west on an average from 8 1/2 in the
morning until 1 1/2 at mid-day, while in the southern hemisphere the same
north point moves from west to east,* attention has recently been drawn,
with much justice, to the fact that there must be a region of the Earth
between the terrestrial and the magnetic equator where no horary deviations
in the declination are to be observed.
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