tro-heliograph show that spots are not the primary
phenomenon of solar activity, but are simply the outcome of processes
going on constantly in the sun which result in spots only in special
regions and on special occasions. It does not, therefore, necessarily
follow that a spot does cause a magnetic storm. What we should conclude
is that the solar activity which produces a spot also produces the
magnetic storm.

When we inquire into the possible nature of these relations between
solar activity and terrestrial magnetism, we find ourselves so
completely in the dark that the question of what is really proved by
the coincidence may arise. Perhaps the most obvious explanation of
fluctuations in the earth's magnetic field to be inquired into would be
based on the hypothesis that the space through which the earth is
moving is in itself a varying magnetic field of vast extent. This
explanation is tested by inquiring whether the fluctuations in question
can be explained by supposing a disturbing force which acts
substantially in the same direction all over the globe. But a very
obvious test shows that this explanation is untenable. Were it the
correct one, the intensity of the force in some regions of the earth
would be diminished and in regions where the needle pointed in the
opposite direction would be increased in exactly the same degree. But
there is no relation traceable either in any of the regular
fluctuations of the magnetic force, or in those irregular ones which
occur during a magnetic storm. If the horizontal force is increased in
one part of the earth, it is very apt to show a simultaneous increase
the world over, regardless of the direction in which the needle may
point in various localities. 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 man