thing reminiscent of them in
our terrestrial volcanoes, nevertheless the volcanic theory seems to
receive more favour than the others.

In addition to the craters there are two more features which demand
notice, namely, what are known as _rays_ and _rills_. The rays are long,
light-coloured streaks which radiate from several of the large craters,
and extend to a distance of some hundreds of miles. That they are mere
markings on the surface is proved by the fact that they cast no shadows
of any kind. One theory is, that they were originally great cracks which
have been filled with lighter coloured material, welling up from
beneath. The rills, on the other hand, are actually fissures, about a
mile or so in width and about a quarter of a mile in depth.

The rays are seen to the best advantage in connection with the craters
Tycho and Copernicus (see Plate XI., p. 204). In consequence of its
fairly forward position on the lunar disc, and of the remarkable system
of rays which issue from it like spokes from the axle of a wheel, Tycho
commands especial attention. The late Rev. T.W. Webb, a famous observer,
christened it, very happily, the "metropolitan crater of the moon."

[Illustration: PLATE XI. THE MOON

The systems of rays from the craters Tycho, Copernicus and Kepler are
well shown here. From a photograph taken at the Paris Observatory by
M.P. Puiseux.

(Page 204)]

A great deal of attention is, and has been, paid by certain astronomers
to the moon, in the hope of finding out if any changes are actually in
progress at present upon her surface. Sir William Herschel, indeed, once
thought that he saw a lunar volcano in eruption, but this proved to be
merely the effect of the sunlight striking the top of the crater
Aristarchus, while the region around it was still in shadow--sunrise
upon Aristarchus, in fact! No change of any real importance has,
however, been noted, although it is suspected that some minor
alterations have from time to time taken place. For instance, slight
variations of tint have been noticed in certain areas of the lunar
surface. Professor W.H. Pickering puts forward the conjecture that these
may be caused by the growth and decay of some low form of vegetation,
brought into existence by vapours of water, or carbonic acid gas, making
their way out from the interior through cracks near at hand.

Again, during the last hundred years one small crater known as Linne
(Linnaeus), situated in the Mare Ser