nes
are permanently displaced on the surface of our planet. These must be
attributed to changes of temperature in the interior of the globe, and
to the direction in the progress of subterranean fires, which it may
also be expected will change the isogeothermal lines. But there are
changes, which although of long period, are yet periodic, one of which
is obviously due to the revolution of the lunar nodes in eighteen and a
half years, and the revolution of the apogee in nine years. The first is
continually changing the obliquity of the axis of the vortex, and they
both tend to limit the vortices in their extreme latitudes; but the
planet Jupiter has an indirect influence, which is probably equal, if
not greater, than the action of the moon, in changing the magnetic
declination.

From the investigations of Lamont, it would appear, that the period of
the variations of magnetic declination is about 10 1/3 years, while,
more recently, R. Wolfe has suggested the connection between this
variation and the solar spots, and assigns a period of 11.11 years, and
remarks, that it "corresponds more exactly with the variations in
magnetic declination than the period of 10 1/3 years established by
Lamont. The magnetic variations accompany the solar spots, not only in
their regular changes, but even in their minor irregularities: this
latter fact is itself sufficient to prove definitely the important
relations between them."[28]

As the planet Jupiter exerts the greatest influence on the sun, in
forcing the centre from the mechanical centre of the system, the
longitude of the sun will in a great measure depend on the position of
this planet; and, in consequence, the sun will generally revolve around
this centre in a period nearly equal to the period of Jupiter. The
sidereal period of Jupiter is about twelve years, but the action of the
other planets tend to shorten this period (at least, that has been the
effect for the last twenty or thirty years), and bring it nearly to the
period assigned by M. Wolfe to the variations in the magnetic
declinations. As this has its influence on the radial stream, and the
radial stream on the declination, we see at once the connection between
them. When we come to a consideration of the solar spots, we shall
exhibit this influence more fully.

AURORA BOREALIS.

Let us now examine another phenomenon. The Aurora Borealis has been
generally considered to be in some way connected with the magnetism of