years during which
the sun is never without them. The period from minimum to maximum
[Page 91] of spots is about eleven years. We might look for them
again and again in vain this year (1878). They will be most numerous
in 1882 and 1893. The cause of this periodicity was inferred to be
the near approach of the enormous planet Jupiter, causing
disturbance by its attraction. But the periods do not correspond,
and the cause is the result of some law of solar action to us as yet
unknown.

These spots may be seen with almost any telescope, the eye being
protected by deeply colored glasses.

Until within one hundred years they were supposed to be islands of
scoriae floating in the sea of molten matter. But they were depressed
below the surface, and showed a notch when on the edge. Wilson
originated and Herschel developed the theory that the sun's real
body was dark, cool, and habitable, and that the photosphere was
a luminous stratum at a distance from the real body, with openings
showing the dark spots below. Such a sun would have cooled off in
a week, but would previously have annihilated all life below.

The solar spots being most abundant on the two sides of the equator,
indicates their cyclonic character; the centre of a cyclone is
rarefied, and therefore colder, and cold on the sun is darkness.
M. Faye says: "Like our cyclones, they are descending, as I have
proved by a special study of these terrestrial phenomena. They
carry down into the depths of the solar mass the cooler materials
of the upper layers, formed principally of hydrogen, and thus produce
in their centre a decided extinction of light and heat as long as
the gyratory movement continues. Finally, the hydrogen set free
at the base of the whirlpool becomes reheated at this [Page 92]
great depth, and rises up tumultuously around the whirlpool, forming
irregular jets, which appear above the chromosphere. These jets
constitute the protuberances. The whirlpools of the sun, like those
on the earth, are of all dimensions, from the scarcely visible pores
to the enormous spots which we see from time to time. They have,
like those of the earth, a marked tendency, first to increase and
then to break up, and thus form a row of spots extending along the
same parallel."

[Illustration: Fig. 36.--Solar spot, by Langley.]

A spot of 20,000 miles diameter is quite small; there was one 14,816
miles across, visible to the naked eye for a week in 1843. This
particular s