Westward from the middle of an imaginary line joining Aristillus and Cassini is the much smaller crater Theætetus. Outside the walls of this are a number of craterlets, and a French astronomer, Charbonneaux, of the Meudon Observatory, reported in December, 1900, that he had repeatedly observed white clouds appearing and disappearing over one of these small craters.
South of the Mare Vaporum are found some of the most notable of those strange lunar features that are called "clefts" or "rills." Two crater mountains, in particular, are connected with them, Ariadæus at the eastern edge of the Mare Tranquilitatis and Hyginus on the southern border of the Mare Vaporum. These clefts appear to be broad and deep chasms, like the cañons cut by terrestrial rivers, but it can not be believed that the lunar cañons are the work of rivers. They are rather cracks in the lunar crust, although their bottoms are frequently visible. The principal cleft from Ariadæus runs eastward and passes between two neighboring craters, the southern of which is named Silberschlag, and is noteworthy for its brightness. The Hyginus cleft is broader and runs directly through the crater ring of that name.
The observer will find much to interest him in the great, irregular, and much-broken mountain ring called Julius Cæsar, as well as in the ring mountains, Godin, Agrippa, and Triesnecker. The last named, besides presenting magnificent shadows when the sunlight falls aslant upon it, is the center of a complicated system of rills, some of which can be traced with our five-inch glass.
We next take up [Lunar Chart No. 2], and pay a telescopic visit to the southwestern quarter of the lunar world. The Mare Tranquilitatis merges through straits into two southern extensions, the Mare Fecunditatis and the Mare Nectaris. The great ring mountains or ringed plains, Langrenus, Vendelinus, Petavius, and Furnerius, all lying significantly along the same lunar meridian, have already been noticed. Their linear arrangement and isolated position recall the row of huge volcanic peaks that runs parallel with the shore of the Pacific Ocean in Oregon and Washington—Mount Jefferson, Mount Hood, Mount St. Helen's, Mount Tacoma—but these terrestrial volcanoes, except in elevation, are mere pins' heads in the comparison.
In the eastern part of the Mare Fecunditatis lies a pair of relatively small craters named Messier, which possess particular interest because it has been suspected, though not proved, that a change of form has occurred in one or other of the pair. Mädler, in the first half of the nineteenth century, represented the two craters as exactly alike in all respects. In 1855 Webb discovered that they are not alike in shape, and that the easternmost one is the larger, and every observer easily sees that Webb's description is correct. Messier is also remarkable for the light streak, often said to resemble a comet's tail, which extends from the larger crater eastward to the shore of the Mare Fecunditatis.
Goclenius and Guttemberg, on the highland between the Mare Fecunditatis and the Mare Nectaris, are intersected and surrounded by clefts, besides being remarkable for their broken and irregular though lofty walls. Guttemberg is forty-five miles and Goclenius twenty-eight miles in diameter. The short mountain range just east of Guttemberg, and bordering a part of the Mare Nectaris on the west, is called the Pyrenees.
The Mare Nectaris, though offering in its appearance no explanation of its toothsome name—perhaps it was regarded as the drinking cup of the Olympian gods—is one of the most singular of the dark lunar plains in its outlines. At the south it ends in a vast semicircular bay, sixty miles across, which is evidently a half-submerged mountain ring. But submerged by what? Not water, but perhaps a sea of lava which has now solidified and forms the floor of the Mare Nectaris. The name of this singular formation is Fracastorius. Elger has an interesting remark about it.
"On the higher portion of the interior, near the center," he says, "is a curious object consisting apparently of four light spots, arranged in a square, with a craterlet in the middle, all of which undergo notable changes of aspect under different phases."
Other writers also call attention to the fine markings, minute craterlets, and apparently changeable spots on the floor of Fracastorius.
We go now to the eastern side of the Mare Nectaris, where we find one of the most stupendous formations in the lunar world, the great mountain ring of Theophilus, noticeably regular in outline and perfect in the completeness of its lofty wall. The circular interior, which contains in the center a group of mountains, one of whose peaks is 6,000 feet high, sinks 10,000 feet below the general level of the moon outside the wall! One of the peaks on the western edge towers more than 18,000 feet above the floor within, while several other peaks attain elevations of 15,000 to 16,000 feet. The diameter of the immense ring, from crest to crest of the wall, is sixty-four miles. Theophilus is especially wonderful on the fifth and sixth days of the moon, when the sun climbs its shining pinnacles and slowly discloses the tremendous chasm that lies within its circles of terrible precipices.