Accepting it as a fact that agencies do exist which travel from
sun to planet and from star to star with a speed which beggars all
our previous ideas, the first question that arises is that of
their nature and mode of action. This question is, up to the
present time, one which we do not see any way of completely
answering. The first difficulty is that we have no evidence of
these agents except that afforded by their action. We see that the
sun goes through a regular course of pulsations, each requiring
eleven years for completion; and we see that, simultaneously with
these, the earth's magnetism goes through a similar course of
pulsations. The connection of the two, therefore, seems absolutely
proven. But when we ask by what agency it is possible for the sun
to affect the magnetism of the earth, and when we trace the
passage of some agent between the two bodies, we find nothing to
explain the action. To all appearance, the space between the earth
and the sun is a perfect void. That electricity cannot of itself
pass through a vacuum seems to be a well-established law of
physics. It is true that electromagnetic waves, which are supposed
to be of the same nature with those of light, and which are used
in wireless telegraphy, do pass through a vacuum and may pass from
the sun to the earth.
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