st altogether within thirty
degrees of the equator of the sun, a field corresponding approximately
to the tropical belt of the earth, there appear from time to time the
curious disturbances which are termed spots. These appear to be
uprushes of matter in the gaseous state, the upward movement being
upon the margins of the field and a downward motion taking place in
the middle of the irregular opening, which is darkened in its central
part, thus giving it, when seen by an ordinary telescope, the aspect
of a black patch on the glowing surface. These spots, which are from
some hundred to some thousand miles in diameter, may endure for
months before they fade away. It is clear that they are most abundant
at intervals of about eleven years, the last period of abundance being
in 1893. The next to come may thus be expected in 1904. In the times
of least spotting more than half the days of a year may pass without
the surface of the photosphere being broken, while in periods of
plenty no day in the year is likely to fail to show them.

[Illustration: Fig. 6.--Ordinary Sun-spot, June 22, 1885.]

It is doubtful if the closest seeing would reveal the cause of the
solar spots. The studies of the physicists who have devoted the most
skill to the matter show little more than that they are tumults in the
photosphere, attended by an uprush of vapours, in which iron and other
metals exist; but whether these movements are due to outbreaks from
the deeper parts of the sun or to some action like the whirling storms
of the earth's atmosphere is uncertain. It is also uncertain what
effect these convulsions of the sun have on the amount of the heat and
light which is poured forth from the orb. The common opinion that the
sun-spot years are the hottest is not yet fully verified.

Below the photosphere lies the vast unknown mass of the unseen solar
realm. It was at one time supposed that the dark colour of the spots
was due to the fact that the photosphere was broken through in those
spaces, and that we looked down through them upon the surface of the
slightly illuminated central part of the sphere. This view is
untenable, and in its place we have to assume that for the eight
hundred and sixty thousand miles of its diameter the sun is composed
of matter such as is found in our earth, but throughout in a state of
heat which vastly exceeds that known on or in our planet. Owing to its
heat, this matter is possibly not in either the solid or the f