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.
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