It is hardly necessary to
add that none of the fluctuations in terrestrial magnetism can be
explained on the hypothesis that either the moon or the sun acts
as a magnet. In such a case the action would be substantially in
the same direction at the same moment the world over.
Such being the case, the question may arise whether the action
producing a magnetic storm comes from the sun at all, and whether
the fluctuations in the sun's activity, and in the earth's
magnetic field may not be due to some cause external to both. All
we can say in reply to this is that every effort to find such a
cause has failed and that it is hardly possible to imagine any
cause producing such an effect. It is true that the solar spots
were, not many years ago, supposed to be due in some way to the
action of the planets. But, for reasons which it would be tedious
to go into at present, we may fairly regard this hypothesis as
being completely disproved. There can, I conclude, be little doubt
that the eleven-year cycle of change in the solar spots is due to
a cycle going on in the sun itself. Such being the case, the
corresponding change in the earth's magnetism must be due to the
same cause.
We may, therefore, regard it as a fact sufficiently established to
merit further investigation that there does emanate from the sun,
in an irregular way, some agency adequate to produce a measurable
effect on the magnetic needle.
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