zones. Yet even in this more diffuse part of the phenomenon one can
detect the presence of submerged curves bearing more or less
resemblance to those about the poles. Just what part electricity or
electro-magnetism plays in the mechanism of the solar radiation it is
impossible to say, but on the assumption that it is a very important
part is based the hypothesis that there exists a direct solar
influence not only upon the magnetism, but upon the weather of the
earth. This hypothesis has been under discussion for half a century,
and still we do not know just how much truth it represents. It is
certain that the outbreak of great disturbances on the sun,
accompanied by the formation of sun-spots and the upshooting of
eruptive prominences (phenomena which we should naturally expect to be
attended by action), have been instantly followed by corresponding
``magnetic storms'' on the earth and brilliant displays of the auroral
lights. There have been occasions when the influence has manifested
itself in the most startling ways, a great solar outburst being
followed by a mysterious gripping of the cable and telegraph systems
of the world, as if an invisible and irresistible hand had seized
them. Messages are abruptly cut off, sparks leap from the telegraph
instruments, and the entire earth seems to have been thrown into a
magnetic flurry. These occurrences affect the mind with a deep
impression of the dependence of our planet on the sun, such as we do
not derive from the more familiar action of the sunlight on the growth
of plants and other phenomena of life depending on solar influences.

Perhaps the theory of solar magnetic influence upon the weather is
best known in connection with the ``sun-spot cycle.'' This, at any
rate, is, as already remarked, closely associated with the corona. Its
existence was discovered in 1843 by the German astronomer Schwabe. It
is a period of variable length, averaging about eleven years, during