origin to some veritable cause in the sun itself, nor
shall we find it hard to explain what that cause must be. As we shall
have occasion to mention further on, the velocities with which the
glowing gases on the sun are animated must be exceedingly great. Even in
the hundredth part of a second (which is about the duration of the
exposure of this plate) the movements of the solar clouds are
sufficiently great to produce the observed indistinctness.

[Illustration: Fig. 12.--Photograph of the Solar Surface. (_By
Janssen._)]

Irregularly dispersed over the solar surface small dark objects called
sun-spots are generally visible. These spots vary greatly both as to
size and as to number. Sun-spots were first noticed in the beginning of
the seventeenth century, shortly after the invention of the telescope.
Their general appearance is shown in Fig. 13, in which the dark central
nucleus appears in sharp contrast with the lighter margin or penumbra.
Fig. 16 shows a small spot developing out of one of the pores or
interstices between the granules.

[Illustration: Fig. 13.--An Ordinary Sun-spot.]

The earliest observers of these spots had remarked that they seem to
have a common motion across the sun. In Fig. 14 we give a copy of a
remarkable drawing by Father Scheiner, showing the motion of two spots
observed by him in March, 1627. The figure indicates the successive
positions assumed by the spots on the several days from the 2nd to the
16th March. Those marks which are merely given in outline represent the
assumed positions on the 11th and the 13th, on which days it happened
that the weather was cloudy, so that no observations could be made. It
is invariably found that these objects move in the same
direction--namely, from the eastern to the western limb[3] of the sun.
They complete the journey across the face of the sun in twelve or
thirteen days, after which they remain invisible for about the same
length of time until they reappear at the eastern limb. These early
observers were quick to discern the true import of their discovery. They
deduced from these simple observations the remarkable fact that the sun,
like the earth, performs a rotation on its axis, and in the same
direction. But there is the important difference between these rotations
that whereas the earth takes only twenty-four hours to turn once round,
the solar globe takes about twenty-six days to complete one of its much
more deliberate rotations.

[Illust