And no doubt he
is right.
"What, do I suppose, is the cause of these spots in the sun?
Well, that is a very difficult question to answer. Changes are
constantly going on at the sun's surface, or, I may rather say,
in the sun's interior, and making themselves apparent at the
surface. Sometimes they go on with enormous activity; at other
times they are more quiet. They recur alternately in periods of
seven or eight weeks, while these again are also subject to a
period of about eleven years--that is, the short recurring
outbursts go on for some years, when they attain a maximum, from
which they go on decreasing. I may say that we are now (August
1883) at, or very near, a maximum epoch. There is no doubt that
this period has an intimate connection with our auroral displays;
but I don't think that the influence sun-spots have on light or
heat is perceptible. Whatever influence they possess would be
felt alike on the whole terrestrial globe. We have wet, dry,
cold, and warm years, but they are never general. The kind of
season which prevails in one country is often quite reversed in
another perhaps in the adjacent one. Not so with our auroral
displays. They are universal on both sides of the globe; and
from pole to pole the magnetic needle trembles during their
continuance. Some authorities are of opinion that these
eleven-year cycles are subject to a larger cycle, but sun-spot
observations have not existed long enough to determine this
point.
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