But there are also irregular fluctuations in the direction of the
magnetic needle. These depend on the moon, and are caused by the passage
of the vortices over or near to the place of observation. The action of
these vortices is proved to be of variable force, whether arising from
atmospheric conditions, or due to an increased activity of the ethereal
medium throughout the whole system, is at present immaterial. They do
vary, and sometimes the passage of a vortex will deflect the needle a
whole degree. At other times, there are magnetic storms extending over a
great part of the earth's surface; but there is reason to suppose, that
the extent of these storms has been over estimated. Thus, on the 25th of
September, 1841, a magnetic storm was observed in Toronto, and at the
same time there was one felt at the Cape of Good Hope. There is no great
mystery in this. If we suppose the axis of the central vortex, for
instance, to have passed Toronto in latitude 43d 33' north, in ordinary
positions of the moon, in her orbit, the southern portion of the axis
would be in 33d or 34d south latitude, and consequently would have
passed near the Cape of Good Hope on the same night. Now, we certainly
could not expect the northern portion of the vortex to be intensely
active, without the southern portion being in the same state of
activity. That this is the true explanation is proved by magnetic storms
in the same hemisphere being comparatively limited in extent; as,
according to Gauss and Weber, magnetic storms which were simultaneously
felt from Sicily to Upsala, did not extend from Upsala to Alten. Still
it would not be wonderful if they were felt over a vast area of
thousands of miles as a consequence of _great_ disturbance in the
elasticity of the ether in the terral vortex; as the solid earth must be
permeable to all its motions, and thus be explicable on the general
principles we have advanced.
But besides these variations which we have mentioned, there are changes
steadily going on, by which the isodynamic, isogonic and isoclinic lines
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.