e.]

It is not my intention to give a complete description of the various
lunar features, and I mention but one other--the "clefts" or "rills,"
which are to be seen running across the surface like cracks. One of the
most remarkable of these is found in the _Oceanus Procellarum_, near the
crater-mountain Aristarchus, which is famed for the intense brilliance
of its central peak, whose reflective power is so great that it was once
supposed to be aflame with volcanic fire. The cleft, or crack, in
question is very erratic in its course, and many miles in length, and
it terminates in a ringed plain named Herodotus not far east of
Aristarchus, breaking through the wall of the plain and entering the
interior. Many other similar chasms or canons exist on the moon, some
crossing plains, some cleaving mountain walls, and some forming a
network of intersecting clefts. Mr. Thomas Gwyn Elger has this to say on
the subject of the lunar clefts:

"If, as seems most probable, these gigantic cracks are due to
contractions of the moon's surface, it is not impossible, in spite of
the assertions of the text-books to the effect that our satellite is now
a 'changeless world,' that emanations may proceed from these fissures,
even if, under the monthly alternations of extreme temperatures, surface
changes do not now occasionally take place from this cause also. Should
this be so, the appearance of new rills and the extension and
modification of those already existing may reasonably be looked for."

Mr. Elger then proceeds to describe his discovery in 1883, in the
ring-plain Mersenius, of a cleft never noticed before, and which seems
to have been of recent formation.[15]

[Footnote 15: The Moon, a Full Description and Map of its Principal
Features, by Thomas Gwyn Elger, 1895.

Those who desire to read detailed descriptions of lunar scenery may
consult, in addition to Mr. Elger's book, the following: The Moon,
considered as a Planet, a World, and a Satellite, by James Nasmyth and
James Carpenter, 1874; The Moon, and the Condition and Configurations of
its Surface, by Edmund Neison, 1876. See also Annals of Harvard College
Observatory, vol. xxxii, part ii, 1900, for observations made by Prof.
William H. Pickering at the Arequipa Observatory.]

We now return to the question of the force of lunar gravity. This we
find to be only one sixth as great as gravity on the surface of the
earth. It is by far the smallest force of gravity that we have f