earance one can hardly
reject his conclusion that Linne is an active volcanic vent, but
variable in its manifestations. This is only one of a number of similar
instances among the smaller craters of the moon. The giant ones are
evidently entirely extinct, but some of the minor vents give occasional
signs of activity. Nor should it be assumed that these relatively slight
manifestations of volcanic action are really insignificant. As Professor
Pickering shows, they may be regarded as comparable with the greatest
volcanic phenomena now witnessed on the earth, and, speaking again of
Plato, he says of its evidences of volcanic action:

"It is, I believe, more active than any area of similar size upon the
earth. There seems to be no evidences of lava, but the white streaks
indicate apparently something analogous to snow or clouds. There must be
a certain escape of gases, presumably steam and carbonic acid, the
former of which, probably, aids in the production of the white
markings."[19]

[Footnote 19: Annals of Harvard College Observatory, vol. xxxii, part
ii, 1900.]

To Professor Pickering we owe the suggestion that the wonderful rays
emanating from Tycho consist of some whitish substance blown by the
wind, not from Tycho itself, but from lines of little volcanic vents or
craters lying along the course of the rays. This substance may be
volcanic powder or snow, in the form of minute ice crystals. Mr. Elger
remarks of this theory that the "confused network of streaks" around
Copernicus seems to respond to it more happily than the rays of Tycho
do, because of the lack of definiteness of direction so manifest in the
case of the rays.

As an encouragement to amateur observers who may be disposed to find out
for themselves whether or not changes now take place in the moon, the
following sentence from the introduction to Professor Pickering's
chapter on Plato in the Harvard Observatory Annals, volume xxxii, will
prove useful and interesting:

"In reviewing the history of selenography, one must be impressed by the
singular fact that, while most of the astronomers who have made a
special study of the moon, such as Schroeter, Maedler, Schmidt, Webb,
Neison, and Elger, have all believed that its surface was still subject
to changes readily visible from the earth, the great majority of
astronomers who have paid little attention to the subject have quite as
strenuously denied the existence of such changes."

In regard to the lun