Clavius, June 7, 1889, 10 p.m., 2⅝ inch.
3. The Mountain Ranges.—These are comparatively few in number, and are never of such magnitude as to put them, like the craters, beyond terrestrial standards of comparison. The most conspicuous range is that known as the Lunar Apennines, which runs in a north-west and south-east direction for a distance of upwards of 400 miles along the border of the Mare Imbrium, from which its mass rises in a steep escarpment, towering in one instance (Mount Huygens) to a height of more than 18,000 feet. On the western side the range slopes gradually away in a gentle declivity. The spectacle presented by the Apennines about first quarter is one of indescribable grandeur. The shadows of the great peaks are cast for many miles over the surface of the Mare Imbrium, magnificently contrasting with the wild tract of hill-country behind, in which rugged summits and winding valleys are mingled in a scene of confusion which baffles all attempt at delineation. Two other important ranges—the Caucasus and the Alps—lie in close proximity to the Apennines; the latter of the two notable for the curious Alpine Valley which runs through it in a straight line for upwards of eighty miles. This wonderful chasm varies in breadth from about two miles, at its narrowest neck, to about six at its widest point. It is closely bordered, for a considerable portion of its length, by almost vertical cliffs thousands of feet in height, and under low magnifying powers appears so regular as to suggest nothing so much as the mark of a gigantic chisel, driven by main force through the midst of the mountain mass. The Alpine Valley is an easy object, and a power of 50 on a 2-inch telescope will show its main outlines quite clearly. Indeed, the whole neighbourhood is one which will well repay the student, some of the finest of the lunar craters, such as Plato, Archimedes, Autolycus, and Aristillus, lying in the immediate vicinity (Plates [XIII.] and [XVII.]).
Region of Theophilus and Altai Mountains. Yerkes Observatory.
Among the other mountain-ranges may be mentioned the Altai Mountains, in the south-west quadrant (Plate [XVI.]), the Carpathians, close to the great crater Copernicus, and the beautiful semicircle of hills which borders the Sinus Iridum, or Bay of Rainbows, to the east of the Alpine range. This bay forms one of the loveliest of lunar landscapes, and under certain conditions of illumination its eastern cape, the Heraclides Promontory, presents a curious resemblance, which I have only seen once or twice, to the head of a girl with long floating hair—'the moon-maiden.' The Leibnitz and Doerfel Mountains, with other ranges whose summits appear on the edge of the moon, are seldom to be seen to great advantage, though they are sometimes very noticeably projected upon the bright disc of the sun during the progress of an eclipse.[*] They embrace some of the loftiest lunar peaks reaching 26,000 feet in one of or two instances, according to Schröter and Mädler.
FIG. 23.
Aristarchus and Herodotus, February 20, 1891, 6.15 p.m., 3⅞ inch.
4. The Clefts or Rills.—In these, and in the ray-systems, we again meet with features to which a terrestrial parallel is absolutely lacking. Schröter of Lilienthal was the first observer to detect the existence of these strange chasms, and since his time the number known has been constantly increasing, till at present it runs to upwards of a thousand. These objects range from comparatively coarse features, such as the Herodotus Valley (Fig. 23), and the well-known Ariadæus and Hyginus clefts, down to the most delicate threads, only to be seen under very favourable conditions, and taxing the powers of the finest instruments. They present all the appearance of cracks in a shrinking surface, and this is the explanation of their existence which at present seems to find most favour. In some cases, such as that of the great Sirsalis cleft, they extend to a length of 300 miles; their breadth varies from half a mile, or less, to two miles; their depth is very variously estimated, Nasmyth putting it at ten miles, while Elger only allows 100 to 400 yards. In a number of instances they appear either to originate from a small crater, or to pass through one or more craters in their course. The student will quickly find out for himself that they frequently affect the neighbourhood of one or other of the mountain ranges (as, for example, under the eastern face of the Apennines, Plate [XVII.]), or of some great crater, such as Archimedes. They are also frequently found traversing the floor of a great walled-plain, and at least forty have been detected in the interior of Gassendi (Plate [XIX.], No. 90). Smaller instruments are, of course, incompetent to reveal more than a few of the larger and coarser of these strange features. The Serpentine Valley of Herodotus, the cleft crossing the floor of Petavius, and the Ariadæus and Hyginus rills are among the most conspicuous, and may all be seen with a 2½-inch telescope and a power of 100.