Chapter 3

Some years ago Dr. Klein of Cologne called attention to some very interesting types of crater-cones, which may be found on certain dark or smoky-grey areas on the moon. These, he considers, may probably represent active volcanic vents, and urges that they should be diligently examined and watched by observers who possess telescopes adequate to the task. The most noteworthy examples of these objects are in the following positions:--(1) West of a prominent ridge running from Beaumont to the west side of Theophilus, and about midway between these formations; (2) in the Mare Vaporum, south of Hyginus; (3) on the floor of Werner, near the foot of the north wall; (4) under the east wall of Alphonsus, on the dusky patch in the interior; (5) on the south side of the floor of Atlas. I have frequently described elsewhere with considerable detail the telescopic appearance of these features under various phases, and have pointed out that though large apertures and high powers are needed to see these cones to advantage, the dusky areas, easily traced on photograms, might be usefully studied by observers with smaller instruments, as if they represent the _ejecta_ from the crater-cones which stand upon them, changes in their form and extent could very possibly be detected. In addition to those already referred to, a number of mysterious dark spots were discovered by Schmidt in the dusky region about midway between Copernicus and Gambart, which Klein describes as perforated like a sieve with minute craters. A short distance south-west of Copernicus stands a bright crater-cone surrounded by a grey nimbus, which may be classed with these objects. It is well seen under a high light, as indeed is the case with most of these features.

CRATERLETS, CRATER-PITS.--To a great extent the former term is needless and misleading, as the so-called craters merge by imperceptible gradations into very minute objects, as small as half a mile in diameter, and most probably, if we could more accurately estimate their size, still less. The crater-pit, however, has well-marked peculiarities which distinguish it from all other types, such as the absence of a distinguishable rim and extreme shallowness. They appear to be most numerous on the high-level plains and plateaus in the south-western quadrant, and may be counted by hundreds under good atmospheric conditions on the outer slopes of Walter, Clavius, and other large enclosures. In these positions they are often so closely aggregated that, as Nasmyth remarks, they remind one of an accumulation of froth. Even in an 8 1/2 inch reflector I have frequently seen the outer slope of the large ring-plain on the north-western side of Vendelinus, so perforated with these objects that it resembled pumice or vesicular lava, many of the little holes being evidently not circular, but square shaped and very irregular. The interior of Stadius and the region outside abounds in these minute features, but the well-known crater-row between this formation and Copernicus seems rather to consist of a number of inosculating crater-cones, as they stand very evidently on a raised bank of some altitude.

MOUNTAIN RANGES, ISOLATED MOUNTAINS, &c.--The more massive and extended mountain ranges of the moon are found in the northern hemisphere, and (what is significant) in that portion of it which exhibits few indications of other superficial disturbances. The most prominently developed systems, the _Alps_, the _Caucasus_, and the _Apennines_, forming a mighty western rampart to the Mare Imbrium and giving it all the appearance of a vast walled plain, present few points of resemblance to any terrestrial chain. The former include many hundred peaks, among which, Mont Blanc rises to a height of 12,000 feet, and a second, some distance west of Plato, to nearly as great an altitude; while others, ranging from 5000 to 8000 feet, are common. They extend in a south-west direction from Plato to the Caucasus, terminating somewhat abruptly, a little west of the central meridian, in about N. lat. 42 deg. One of the most interesting features associated with this range is the so-called great Alpine valley, which cuts through it west of Plato. The _Caucasus_ consist of a massive wedge-shaped mountain land, projecting southwards, and partially dividing the Mare Imbrium from the Mare Serenitatis, both of which they flank. Though without peaks so lofty as those pertaining to the Alps, there is one, immediately east of the ring-plain Calippus, which, towering to 19,000 feet, surpasses any of which the latter system can boast. The _Apennines_, however, are by far the most magnificent range on the visible surface, including as they do some 3000 peaks, and extending in an almost continuous curve of more than 400 miles in length from Mount Hadley, on the north, to the fine ring-plain Eratosthenes, which forms a fitting termination, on the south. The great headland Mount Hadley rises more than 15,000 feet, while a neighbouring promontory on the south-east of it is fully 14,000 feet, and another, close by, is still higher above the Mare. Mount Huygens, again, in N. lat. 20 deg., and the square-shaped mass Mount Wolf, near the southern end of the chain, include peaks standing 18,000 and 12,000 feet respectively above the plain, to which their flanks descend with a steep declivity. The counterscarp of the Apennines, in places 160 miles in width from east to west, runs down to the Mare Vaporum with a comparatively gentle inclination. It is everywhere traversed by winding valleys of a very intricate type, all trending towards the south-west, and includes some bright craters and mountain-rings. The _Carpathians_, forming in part the southern border of the Mare Imbrium, extend for a length of more than 180 miles eastward of E., long. 16 deg., and, embracing the ring-plain Gay- Lussac, terminate west of Mayer. They present a less definite front to the Mare than the Apennines, and are broken up and divided by irregular valleys and gaps; their loftiest peak, situated on a very projecting promontory north-west of Mayer, rising to a height of 7000 feet. Notwithstanding their comparatively low altitude, the region they occupy forms a fine telescopic picture at lunar sunrise. The _Sinus Iridum highlands_, bordering the beautiful bay on the north-east side of the Mare Imbrium, rank among the loftiest and most intricate systems on the moon, and, like the Apennines, present a steep face to the grey plain from which they rise, though differing from them in other respects. They include many high peaks, the loftiest, in the neighbourhood of the ring- plain Sharp, rising 15,000 feet. There are probably some still higher mountains in the vicinity, but the difficulties attending their measurement render it impossible to determine their altitude with any approach to accuracy.

_The Taurus Mountains_ extend from the west side of the Mare Serenitatis, near Le Monnier and Littrow, in a north-westerly direction towards Geminus and Berselius, bordering the west side of the Lacus Somniorum. They are a far less remarkable system than any of the preceding, and consist rather of a wild irregular mountain region than a range. In the neighbourhood of Berselius are some peaks which, according to Neison, cannot be less than 10,000 feet in height.

On the north side of the Mare Imbrium, east of Plato, there is a beautiful narrow range of bright outlying heights, called the _Teneriffe Mountains_, which include many isolated objects of considerable altitude, one of the loftiest rising about 8000 feet. Farther towards the east lies another group of a very similar character, called the _Straight Range_, from its linear regularity. It extends from west to east for a distance of about 60 miles, being a few miles shorter than the last, and includes a peak of 6000 feet.

_The Harbinger Mountains_.--A remarkable group, north-west of Aristarchus, including some peaks as high as 7000 feet, and other details noticed in the catalogue.

The above comprise all the mountain ranges in the northern hemisphere of any prominence, or which have received distinctive names, except the _Hercynian Mountains_, on the north-east limb, east of the walled plain Otto Struve. These are too near the edge to be well observed, but, from what can be seen of them, they appear to abound in lofty peaks, and to bear more resemblance to a terrestrial chain than any which have yet been referred to.

The mountain systems of the southern hemisphere, except the ranges visible on the limb, are far less imposing and remarkable than those just described. The _Pyrenees_, on the western side of the Mare Nectaris, extend in a meridional direction for nearly 190 miles, and include a peak east of Guttemberg of nearly 12,000 feet, and are traversed in many places by fine valleys.

_The Altai Mountains_ form a magnificent chain, 275 miles in length, commencing on the outer eastern slope of Piccolomini, and following a tolerably direct north-east course, with a few minor bendings, to the west side of Fermat, where they turn more towards the north, ultimately terminating about midway between Tacitus and Catherina. The region situated on the south-east is a great table-land, without any prominent features, rising gently towards the mountains, which shelve steeply down to an equally barren expanse on the north-west, to which they present a lofty face, having an average altitude of about 6000 feet. The loftiest peak, over 13,000 feet, rises west of Fermat.

_The Riphaean Mountains_, a remarkably bright group, occupying an isolated position in the Mare Procellarum south of Landsberg, and extending for more than 100 miles in a meridional direction. They are most closely aggregated at a point nearly due west of Euclides, from which they throw off long-branching arms to the north and south, those on the north bifurcating and gradually sinking to the level of the plain. The loftiest peaks are near the extremity of this section, one of them rising to 3000 feet. Two bright craters are associated with these mountains, one nearly central, and the other south of it.

_The Percy Mountains_.--This name is given to the bright highlands extending east of Gassendi towards Mersenius, forming the north-eastern border of the Mare Humorum. They abound in minute detail--bright little mountains and ridges--and include some clefts pertaining to the Mersenius rill-system; but their most noteworthy feature is the long bright mountain-arm, branching out from the eastern wall of Gassendi, and extending for more than 50 miles towards the south-east.

The principal ranges on the limb are the _Leibnitz Mountains_, extending from S. lat. 70 deg. on the west to S. lat. 80 deg. on the east limb. They include some giant peaks and plateaus, noteworthy objects in profile, some of which, according to Schroter and Madler, rise to 26,000 feet. The _Dorfel Mountains_, between S. lat. 80 deg. and 57 deg. on the eastern limb, include, if Schroter's estimate is correct, three peaks which exceed 26,000 feet. On the eastern limb, between S. lat. 35 deg. and 18 deg., extend the _Rook Mountains_, which have peaks, according to Schroter, as high as 25,000 feet. Next in order come the _Cordilleras_, which extend to S. lat. 8 deg., and the _D'Alembert Mountains_, lying east of Rocca and Grimaldi, closely associated with them, and probably part of the same system. Some of the peaks approach 20,000 feet. In addition to these mountain ranges there are others less prominent on the limb in the northern hemisphere, which have not been named.

ISOLATED MOUNTAINS are very numerous in different parts of the moon, the most remarkable are referred to in the appendix. Many remain unnamed.

CLEFTS OR RILLS.--Though Fontenelle, in his _Entretiens sur la Pluralite des Mondes_, informs his pupil, the Marchioness, that "M. Cassini discovered in the moon something which separates, then reunites, and finally loses itself in a cavity, which from its appearance seemed to be a river," it can hardly be supposed that what the French astronomer saw, or fancied he saw, with the imperfect telescopes of that day, was one of the remarkable and enigmatical furrows termed clefts or rills, first detected by the Hanoverian selenographer Schroter; who, on October 7, 1787, discovered the very curious serpentine cleft near Herodotus, having a few nights before noted for the first time the great Alpine valley west of Plato, once classed with the clefts, though it is an object of a very different kind. Between 1787 and 1797 Schroter found ten rills; but twenty years elapsed before an addition was made to this number by the discoveries of Gruithuisen, and, a short time after, by those of Lohrmann, who in twelve months (1823-24) detected seventy. Kinau, Madler, and finally Schmidt, followed, till, in 1866, when the latter published his work, _Ueber Rillen auf dem Monde_, the list was thus summarised:--

In the 1st or N.W. quadrant 127 rills In the 2nd or N.E. quadrant 75 rills In the 3rd or S.E. quadrant 141 rills In the 4th or S.W. quadrant 82 rills

or 425 in all. Since the date of this book the number of known rills has been more than doubled; in fact, scarcely a lunation passes without new discoveries being made.

The significance of the word _rille_ in German, a groove or furrow, describes with considerable accuracy the usual appearance of the objects to which it is applied, consisting as they do of long narrow channels, with sides more or less steep, and sometimes vertical. They often extend for hundreds of miles in approximately straight lines over portions of the moon's surface, frequently traversing in their course ridges, craters, and even more formidable obstacles, without any apparent check or interruption, though their ends are sometimes marked by a mound or crater. Their length ranges from ten or twelve to three hundred miles or more (as in the great Sirsalis rill), their breadth, which is very variable within certain limits, from less than half a mile to more than two, and their depth (which must necessarily remain to a great extent problematical) from 100 to 400 yards. They exhibit in the telescope a gradation from somewhat coarse grooves, easily visible at suitable times in very moderately sized instruments, to striae so delicate as to require the largest and most perfect optical means and the best atmospheric conditions to be glimpsed at all. Viewed under moderate amplification, the majority of rills resemble deep canal-like channels with roughly parallel sides, displaying occasionally local irregularities, and fining off to invisibility at one or both ends. But, if critically scrutinised in the best observing weather with high powers, the apparent evenness of their edges entirely disappears, and we find that the latter exhibit indentations, projections, and little flexures, like the banks of an ordinary stream or rivulet, or, to use a very homely simile, the serrated edges and little jagged irregularities of a biscuit broken across. In some cases we remark crateriform hollows or sudden expansions in their course, and deep sinuous ravines, which render them still more unsymmetrical and variable in breadth. With regard to their distribution on the lunar surface; they are found in almost every region, but perhaps not so frequently on the surface of the Maria as elsewhere, though, as in the case of the Triesnecker and other systems, they often abound in the neighbourhood of disturbed regions in these plains, and in many cases along their margins, as, for example, the Gassendi-Mersenius and the Sabine-Ritter groups. The interior of walled plains are frequently intersected by them, as in Gassendi, where nearly forty, more or less delicate examples, have been seen; in Hevel, where there is a very interesting system of crossed clefts, and within Posidonius. If we study any good modern lunar map, it is evident how constantly they appear near the borders of mountain ranges, walled-plains, and ring-plains; as, for instance, at the foot of the Apennines; near Archimedes, Aristarchus, Ramsden, and in many other similar positions. Rugged highlands also are often traversed by them, as in the case of those lying west of Le Monnier and Chacornac, and in the region west of the Mare Humorum. It may be here remarked, however, as a notable fact, that the neighbourhood of the grandest ring-mountain on the moon, Copernicus, is, strange to say, devoid of any features which can be classed as true clefts, though it abounds in crater-rows. The intricate network of rills on the west of Triesnecker, when observed with a low power, reminds one of the wrinkles on the rind of an orange or on the skin of a withered apple. Gruithuisen, describing the rill-traversed region between Agrippa and Hyginus, says that "it has quite the look of a Dutch canal map." In the subjoined catalogue many detailed examples will be given relating to the course of these mysterious furrows; how they occasionally traverse mountain arms, cut through, either completely or partially (as in Ramsden), the borders of ring-plains and other enclosures, while not unfrequently a small mound or similar feature appears to have caused them to swerve suddenly from their path, as in the case of the Ariadaeus cleft, and in that of one member of the Mercator-Campanus system.

Of the actual nature of the lunar rills we are, it must be confessed, supremely ignorant. With some of the early observers it was a very favourite notion that they are artificial works, constructed presumably by Kepler's _sub-volvani_, or by other intelligences. There is perhaps some excuse to be made for the freaks of an exuberant fancy in regard to objects which, if we ignore for a moment their enormous dimensions, judged by a terrestrial standard, certainly have, in their apparent absence of any physical relation to neighbouring objects, all the appearance of being works of art rather than of nature. The keen-sighted and very imaginative Gruithuisen believed that in some instances they represent roads cut through interminable forests, and in others the dried-up beds of once mighty rivers. His description of the Triesnecker rill-system reads like a page from a geographical primer. A portion of it is compared to the river Po, and he traces its course mile by mile up to the "delta" at its place of disemboguement into the Mare Vaporum. From the position of some rills with respect to the contour of the surrounding country, it is evident that if water were now present on the moon, they, being situated at the lowest level, would form natural channels for its reception; but the exceptions to this arrangement are so numerous and obvious, that the idea may be at once dismissed that there is any analogy between them and our rivers. The eminent selenographer, the late W.K. Birt, compared many of them to "inverted river-beds" from the fact that, as often as not, they become broader and deeper as they attain a higher level. The branches resemble rivers more frequently than the main channels; for they generally commence as very fine grooves, and, becoming broader and broader, join them at an acute angle. An attempt again has been made to compare the lunar clefts with those vast gorges, the marvellous results of aqueous action, called canyons, which attain their greatest dimensions in North America; such as the Great Canyon of the Colorado, which is at least 300 miles in length, and in places 2000 yards in depth, with perpendicular or even overhanging sides; but the analogy, at first sight specious, utterly breaks down under closer examination. Some selenographers consider them to consist of long-extending rows of confluent craters, too minute to be separately distinguished, and to be thus due to some kind of volcanic action. This is undoubtedly true in many instances, for almost every lunar region affords examples of crater- rows merging by almost imperceptible gradations into cleft-like features, and crater-rows of considerable size resemble clefts under low powers. Still it seems probable that the greater number of these features are immense furrows or cracks in the surface and nothing more; for the higher the magnifying power employed in their examination, the less reason there is to object to this description. Dr. Klein of Cologne believes that rills of this class are due to the shrinkage of parts of the moon's crust, and that they are not as a rule the result of volcanic causes, though he admits that there may be some which have a seismic origin. No good reason has as yet been given for the fact that they so frequently cross small craters and other objects in their course, though it has been suggested that the route followed by a rill from crater to crater in these instances may be a line of least surface resistance, an explanation far from being satisfactory.

Whether variations in the visibility of lunar details, when observed under apparently similar conditions, actually occur from time to time from some unknown cause, is one of those vexed questions which will only be determined when the moon is systematically studied by experienced observers using the finest instruments at exceptionally good stations; but no one who examines existing records of observations of rills by Gruithuisen, Lohrmann, Madler, Schmidt, and other observers, can well avoid the conclusion that the anomalies brought to light therein point strongly to the probability of the existence of some agency which occasionally modifies their appearance or entirely conceals them from view.

The following is one illustration out of many which might be quoted. At a point in its course, nearly due north of the ring-plain Agrippa, the great Ariadaeus cleft sends out a branch which runs into the well-known Hyginus cleft, reminding one, as Dr. Klein remarks, of two rivers connected in the shortest way by a canal. This uniting furrow was detected by Gruithuisen, who observed it several times. On some occasions it appeared perfectly straight, at others very irregular; but, what is very remarkable, although two such accurate observers as Lohrmann and Madler frequently scrutinised the region, neither of them saw a trace of this object; and but for its rediscovery by Schmidt in 1862, its existence would certainly have been ignored by selenographers as a mere figment of Gruithuisen's too lively imagination. Dr. Klein has frequently seen this rill with great distinctness, and at other times sought for it in vain; though on each occasion the conditions of illumination, libration, and definition were practically similar. I have sometimes found this cleft an easy object with a 4 inch achromatic. Again, many rills described by Madler as very delicate and difficult to trace, may now be easily followed in "common telescopes." In short, the more direct telescopic observations accumulate, and the more the study of minute detail is extended, the stronger becomes the conviction, that in spite of the absence of an appreciable atmosphere, there may be something resembling low-lying exhalations from some parts of the surface which from time to time are sufficiently dense to obscure, or even obliterate, the region beneath them.