es or as astounding discoveries!"

"Often the case, friend Ardan, too often the case," said Barbican.

"In the question under consideration, however," continued the Frenchman,
"my conjecture has this advantage over some others: it explains why
these rills appear and seem to disappear at regular intervals."

"Let us hear the explanation," said the Captain.

"They become invisible when the trees lose their leaves, and they
reappear when they resume them."

"His explanation is not without ingenuity," observed Barbican to
M'Nicholl, "but, my dear friend," turning to Ardan, "it is hardly
admissible."

"Probably not," said Ardan, "but why not?"

"Because as the Sun is nearly always vertical to the lunar equator, the
Moon can have no change of seasons worth mentioning; therefore her
vegetation can present none of the phenomena that you speak of."

This was perfectly true. The slight obliquity of the Moon's axis, only
1-1/2 deg., keeps the Sun in the same altitude the whole year around. In the
equatorial regions he is always vertical, and in the polar he is never
higher than the horizon. Therefore, there can be no change of seasons;
according to the latitude, it is a perpetual winter, spring, summer, or
autumn the whole year round. This state of things is almost precisely
similar to that which prevails in Jupiter, who also stands nearly
upright in his orbit, the inclination of his axis being only about 3 deg..

But how to account for the _grooves_? A very hard nut to crack. They
must certainly be a later formation than the craters and the rings, for
they are often found breaking right through the circular ramparts.
Probably the latest of all lunar features, the results of the last
geological epochs, they are due altogether to expansion or shrinkage
acting on a large scale and brought about by the great forces of nature,
operating after a manner altogether unknown on our earth. Such at least
was Barbican's idea.

"My friends," he quietly observed, "without meaning to put forward any
pretentious claims to originality, but by simply turning to account some
advantages that have never before befallen contemplative mortal eye, why
not construct a little hypothesis of our own regarding the nature of
these grooves and the causes that gave them birth? Look at that great
chasm just below us, somewhat to the right. It is at least fifty or
sixty miles long and runs along the base of the _Apennines_ in a line
almost perfectly str