connected with these peculiar principles of their distribution
is the remarkable fact that spots in different latitudes do not indicate
the same values for the period of rotation of the sun. By watching a
spot near the sun's equator Carrington found that it completed a
revolution in twenty-five days and two hours. At a latitude of 20 deg. the
period is about twenty-five days and eighteen hours, at 30 deg. it is no
less than twenty-six days and twelve hours, while the comparatively few
spots observed in the latitude of 45 deg. require twenty-seven and a half
days to complete their circuit.

As the sun, so far at least as its outer regions are concerned, is a
mass of gas and not a solid body, there would be nothing incredible in
the supposition that spots are occasionally endowed with movements of
their own like ships on the ocean. It seems, however, from the facts
before us that the different zones on the sun, corresponding to what we
call the torrid and temperate zones on the earth, persist in rotating
with velocities which gradually decrease from the equator towards the
poles. It seems probable that the interior parts of the sun do not
rotate as if the whole were a rigidly connected mass. The mass of the
sun, or at all events its greater part, is quite unlike a rigid body,
and the several portions are thus to some extent free for independent
motion. Though we cannot actually see how the interior parts of the sun
rotate, yet here the laws of dynamics enable us to infer that the
interior layers of the sun rotate more rapidly than the outer layers,
and thus some of the features of the spot movements can be accounted
for. But at present it must be confessed that there are great
difficulties in the way of accounting for the distribution of spots and
the law of rotation of the sun.

In the year 1826 Schwabe, a German astronomer, commenced to keep a
regular register of the number of spots visible on the sun. After
watching them for seventeen years he was able to announce that the
number of spots seemed to fluctuate from year to year, and that there
was a period of about ten years in their changes. Subsequent
observations have confirmed this discovery, and old books and
manuscripts have been thoroughly searched for information of early
date. Thus a more or less complete record of the state of the sun as
regards spots since the beginning of the seventeenth century has been
put together. This has enabled astronomers to fix the