smooth within, and sometimes broken with small peaks and craters or hilly ridges—­are to be found scattered over almost all parts of the moon.  If our satellite was ever an inhabited world like the earth, while its surface was in its present condition, these valleys must have presented an extraordinary spectacle.  It has been thought that they may once have been filled with water, forming lakes that recall the curious Crater Lake of Oregon.

[Illustration:  THE MOON AT FIRST AND LAST QUARTER (WESTERN AND EASTERN HEMISPHERES).  Photographed with the Lick Telescope.]

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 found anywhere except on the asteroids.  Employing the same method of comparison that was made in the case of Mars, we compute that a man on the moon could attain a height of thirty-six feet without being relatively more unwieldy than a six-foot descendant of Adam is on the earth.