Chapter 6

CLEOMEDES.--A large oblong enclosure, 78 miles in diameter, with massive walls, varying in altitude from 8000 to 10,000 feet above the interior. The most noteworthy features in connection with the circumvallation are the prominent depressions on the W. wall. Under a rising sun, when about one-fourth of the floor is in shadow, three of these can be easily distinguished, each resembling in form the analemma figure. There are two other curious depressions at the S.W. end of the formation. On the dark steel-grey floor are two irregular dusky areas, and a narrow but bright central mountain, on which, according to Schmidt, stand two little craters. There are two ring-plains on the S.W. quarter, and a group of three associated craters on the N. side, one of which (A) Schroter believed came into existence after he commenced to observe the formation, a supposition that has been shown by Birt and others to be very improbable.

TRALLES.--A large irregular crater, one of the deepest on the visible surface of the moon, situated on the N.E. wall of Cleomedes. There is a crater on its N. wall, and, according to Schmidt, some ridges and three closely associated craters on the floor.

BURCKHARDT.--This object, situated on an apparent extension of the W. wall of Cleomedes, is 35 miles in diameter, with a lofty border, rising on the E. to an altitude of nearly 13,000 feet. It has a prominent central mountain and some low ridges on the floor, which, together with two minute craters on the S.W. wall, I have seen under a low angle of morning illumination. It is flanked both on the E. and W. by deep irregular depressions, which present the appearance of having once been complete formations.

GEMINUS.--A fine regular ring-plain, 54 miles in diameter, nearly circular, with bright walls, rising on the E. to a height of more than 12,000 feet, and on the opposite side to nearly 16,000 feet above the floor. Their crest is everywhere very steep, and the inner slope is much terraced. There is a small but conspicuous mountain in the interior; N. of which I have seen a long ridge, where Schmidt shows some hillocks. Two fine clefts are easily visible within the ring, one running for some distance on the S.E. side of the floor, mounting the inner slope of the S.W. border to the summit ridge (where it is apparently interrupted), and then striking across the plain in a S.W. direction. Here it is accompanied for a short distance by a somewhat coarser companion, running parallel to it on the N. The other cleft occupies a very similar position on the N.W. side of the floor at the inner foot of the wall. On several occasions, when observing this formation and the vicinity, I have been struck by its peculiar colour under a low evening sun. At this time the whole region appears to be of a warm light brown or sepia tone.

BERNOUILLI.--A very deep ring-plain on the W. side of Geminus. Under evening illumination its lofty W. wall, which rises to a height of nearly 13,000 feet above the floor, is conspicuously brilliant. This formation exhibits a marked departure from the circular type, being bounded by rectilineal sides. The inner slope of the W. wall is slightly terraced. The border on the S. is much lower than elsewhere, as is evident when the formation is on the evening terminator. On the N. is the deep crater Messala _a_.

NEWCOMB.--The most prominent of a group of formations standing in the midst of the Haemus Mountains. Its crest is nearly 12,000 feet above the floor, on which there are some hills.

MESSALA.--This fine walled-plain, nearly 70 miles in diameter, is, with its surroundings, an especially interesting object when observed under a low angle of illumination. Its complex border, though roughly circular, displays many irregularities in outline, due mainly to rows of depressions. The best view of it is obtained when the W. wall is on the evening terminator. At this phase, if libration is favourable, the manifold details of its very uneven and apparently convex floor are best seen. On the S.W. side is a group of large craters associated with a number of low hills, of which Schmidt shows five; but I have seen many more, together with several ridges between them and the E. wall. I noted also a cleft, or it may be a narrow valley, running from the foot of the N.W. wall towards the centre. On the floor, abutting on the N.E. border, is a semicircular ridge of considerable height, and beyond the border on the N.E. there is another curved ridge, completing the circle, the wall forming the diameter. This formation is clearly of more ancient date than Messala, as the N.E. wall of the latter has cut through it. Where Messala joins Schumacher there is a break in the border, occupied by three deep depressions.

SCHUMACHER.--A large irregular ring-plain, 28 miles in diameter, associated with the N. wall of Messala, and having other smaller rings adjoining it on the E. and N. The interior seems to be devoid of detail.

HOOKE.--Another irregular ring-plain, 28 miles in diameter, on the N.E. of Messala. There is a bright crater of considerable size on the S.W., which is said to be more than 6000 feet in depth, and, according to Neison, is visible as a white spot at full. There is a smaller crater on the slope of the N.W. wall.

SHUCKBURGH.--A square-shaped enclosure on the N. of the last, with a comparatively low border. It has a conspicuous crater at its N.W. corner.

BERZELIUS.--A considerable ring-plain of regular form, with low walls and dark interior, on which there is a central peak, difficult to detect.

FRANKLIN.--A ring-plain, 33 miles in diameter, which displays a considerable departure from the circular type, as the border is in great part made up of rectilineal sections. Both the W. and N.E. wall is much terraced, and rises about 8000 feet above the dark floor, on the S. part of which there is a long ridge. There is a bright little isolated mountain on the plain E. of the formation, and a conspicuous craterlet on the N.W. An incomplete ring, with a very attenuated border, abuts on the S. side of Franklin.

CEPHEUS.--A peculiarly shaped ring-plain, 27 miles in diameter. The E. border is nearly rectilineal, while on the W., the wall forms a bold curve. There is a very brilliant crater on the summit of this section, and a central mountain on the floor. The W. wall is much terraced. W. of Cepheus, close to the brilliant crater, there is a cleft or narrow valley running N. towards Oersted.

OERSTED.--An oblong formation with very low walls, scarcely traceable on the S.E., except when near the terminator. There is a conspicuous crater on the N.W. side of the floor, and a curious square enclosure, with a crater on its W. border, abutting on the N.E. wall.

CHEVALLIER.--An inconspicuous object enclosed by slightly curved ridges. It includes a deep bright crater. On the N. is a low square formation and a long ridge running N. from it. Just beyond the N.E. wall is the fine large crater, Atlas A, with a much smaller but equally conspicuous crater beyond. A has a central hill, which, in spite of the bright interior, is not a difficult feature.

ATLAS.--This, and its companion Hercules on the E., form under oblique illumination a very beautiful pair, scarcely surpassed by any other similar objects on the first quadrant. Its lofty rampart, 55 miles in diameter, is surmounted by peaks, which on the N. tower to an altitude of nearly 11,000 feet. It exhibits an approach to a polygonal outline, the lineal character of the border being especially well marked on the N. The detail on the somewhat dark interior will repay careful scrutiny with high powers. There is a small but distinct central mountain, south of which stands a number of smaller hills, forming with the first a circular arrangement, suggestive of the idea that they represent the relics of a large central crater. Several clefts may be seen on the floor under suitable illumination, among them a forked cleft on the N.E. quarter, and two others, originating at a dusky pit of irregular form situated near the foot of the S.E. wall, one of which runs W. of the central hills, and the other on the opposite side. A ridge, at times resembling a light marking, extends from the central mountain to the N. border. During the years 1870 and 1871 I bestowed some attention on the dusky pit, and was led to suspect that both it and the surrounding area vary considerably in tone from time to time. Professor W.H. Pickering, observing the formation in 1891 with a 13 inch telescope under the favourable atmospheric conditions which prevail at Arequipa, Peru, confirmed this supposition, and has discovered some very interesting and suggestive facts relating to these variations, which, it is hoped, will soon be made public. On the plain a short distance beyond the foot of the _glacis_ of the S.E. wall, I have frequently noted a second dusky spot, from which proceeds, towards the E., a long rill-like marking. On the N. there is a large formation enclosed by rectilineal ridges. The outer slopes of the rampart of Atlas are very noteworthy under a low sun.

HERCULES.--The eastern companion of Atlas, a fine ring-plain, about 46 miles in diameter, with a complex border, rising some 11,000 feet above a depressed floor. There are few formations of its class and size which display so much detail in the shape of terraces, apparent landslips, and variation in brightness. In the interior, S.E. of the centre, is a very conspicuous crater, which is visible as a bright spot when the formation itself is hardly traceable, two large craterlets slightly N. of the centre, and several faint little spots on the east of them. The latter, detected some years ago by Herr Hackel of Stuttgart, are arranged in the form of a horse-shoe. There are two small contiguous craters on the S.E. wall, one of which, a difficult object, was recently detected by Mr. W.H. Maw, F.R.A.S. The well-known wedge-shaped protuberance on the S. wall is due to a large irregular depression. On the bright inner slope of the N. wall are manifest indications of a landslip.

ENDYMION.--A large walled-plain, 78 miles in diameter, enclosed by a lofty, broad, bright border, surmounted in places by peaks which attain a height of more than 10,000 feet above the interior, one on the W. measuring more than 15,000 feet. The walls are much terraced and exhibit two or three breaks. The dark floor appears to be devoid of detail. Schmidt, however, draws two large irregular mounds E. of the centre, and shows four narrow light streaks crossing the interior nearly parallel to the longer axis of the formation.

DE LA RUE.--Notwithstanding its great extent, this formation hardly deserves a distinctive name, as from the lowness of its border it is scarcely traceable in its entirety except under very oblique light. Schmidt, nevertheless, draws it with very definite walls, and shows several ridges and small rings in the interior. Among these objects, a little E. of the centre, there is a prominent peak.

STRABO.--A small walled-plain, 32 miles in diameter, connected with the N. border of the last.

THALES.--A bright formation, also associated with the N. side of De la Rue, adjoining Strabo on the N.E. Schmidt shows a minute hill in the interior.

There are several unnamed formations, large and small, between De la Rue and the limb, some of which are well worthy of examination.

WEST LONGITUDE 40 deg. TO 20 deg.

MASKELYNE.--A regular ring-plain, 19 miles in diameter, standing almost isolated in the Mare Tranquilitatis. The floor, which includes a central mountain, is depressed some 3000 feet below the surrounding surface. There are prominent terraces on the inner slope of the walls. Schmidt shows no craters upon them, but Madler draws a small one on the E., the existence of which I can confirm.

MANNERS.--A brilliant little ring-plain, 11 miles in diameter, on the S.E. side of the Mare Tranquilitatis. There appears to be no detail whatever in connection with its wall. It has a distinct central mountain. About three diameters distant on the S.W. there is a bright crater, omitted by Madler and Neison.

ARAGO.--A much larger formation, 18 miles in diameter, N. of the last, with a small crater on its N. border, and exhibiting two or three spurs from the wall on the opposite side. The inner slopes are terraced, and there is a small central mountain. There are two curious circular protuberances on the Mare E. of Arago, which are well seen when the W. longitude of the morning terminator is about 19 deg., and a long cleft, passing about midway between them, and extending from the foot of the E. wall to a small crater on the edge of the Mare near Sosigenes. Another cleft, also terminating at this crater, runs towards Arago and the more northerly of the protuberances.

CAUCHY.--A bright little crater, not more than 7 or 8 miles in diameter, on the W. side of the Mare Tranquilitatis, N.E. of Taruntius. It has a peak on its W. rim considerably loftier than the rest of the wall, which is visible as a brilliant spot at sunrise long before the rest of the rampart is illuminated. On the S. there are two bright longitudinal ridges ranging from N.E. to S.W. These stand in the position where Neison draws two straight clefts. The Cauchy cleft, however, lies N. of these, and terminates, as shown by Schmidt, among the mountains N.E. of Taruntius. I have seen it thus on many occasions, and it is so represented in a drawing by M.E. Stuvaert (_Dessins de la Lune_). There is a number of minute craters and mounds standing on the S. side of this cleft, and many others in the vicinity.

JANSEN.--Owing to its comparatively low border, this is not a very conspicuous object. It is chiefly remarkable for the curious arrangement of the mountains and ridges on the S. and W. of it. There is a bright little crater on the S. side of the floor, and many noteworthy objects of the same class in the neighbourhood. The mountain arm running S., and ultimately bending E., forms a large incomplete hook-shaped formation terminating at a ring-plain, Jansen B. The ridges in the Mare Tranquilitatis between Jansen B. and the region E. of Maskelyne display under a low sun foldings and wrinklings of a very extraordinary kind.

MACLEAR.--A conspicuous ring-plain about 16 miles in diameter. The dark floor includes, according to Madler, a delicate central hill which Schmidt does not show. Neison, however, saw a faint greyish mark, and an undoubted peak has been subsequently recorded. I have not succeeded in seeing any detail within the border, which in shape resembles a triangle with curved sides.

ROSS.--A somewhat larger ring-plain of irregular form, on the N.W. of the last. There are gaps on the bright S.W. border and a crater on the S.E. wall. The central mountain is an easy feature.

PLINIUS.--This magnificent object reminds one at sunrise of a great fortress or redoubt erected to command the passage between the Mare Tranquilitatis and the Mare Serenitatis. It is 32 miles in diameter, and is encompassed by a very massive rampart, rising at one peak on the E. to more than 6000 feet above the interior, and displaying, especially on the S.E., and N., many spurs and buttresses. The exterior slopes at sunrise, and even when the sun is more than 10 deg. above the horizon, are seen to be traversed by wide and deep valleys. The S. _glacis_ is especially broad, extending to a distance of 10 or 12 miles before it runs down to the level of the plain. The shape of the circumvallation, when it is fully illuminated, approximates very closely to that of an equilateral triangle with curved sides. There are two bright little craters on the outer slope, just below the summit ridge on the S.E., and another, larger, on the N. wall, in which it makes a prominent gap. The interior is considerably brighter than the surface of the surrounding Mare, and, a little S. of the centre, includes two crater-like objects with broken rims. These assume different aspects under different conditions of illumination, and it is only when the floor is lighted by a comparatively low morning sun, that their true character is apparent. On the N.W. quarter of the interior are two smaller distinct craters, and a square arrangement of ridges. On the N.E. there are some hillocks and minor elevations. The Plinius rills form an especially interesting system, and under favourable conditions may be seen in their entirety with a good 4 inch refractor, about the time when the morning terminator passes through Julius Caesar. They consist of three long fissures, originating amid the Haemus highlands, on the S. side of the Mare Serenitatis, and diverging towards the W. The most southerly commences S.S.E. of the Acherusian promontory (a great headland, 5000 feet high, at the W. termination of the Haemus range), and, following a somewhat undulating course, runs up to the N. side of Dawes. Under a low evening sun, I have remarked many inequalities in the width of that portion of it immediately N. of Plinius, which appear to indicate that it is here made up of rows of inosculating craters. The cleft north of this originates very near it, passes a little S. of the promontory, and runs to the E. edge of the plateau surrounding Dawes. The third and most northerly cleft begins at a point immediately N. of the promontory, cuts through the S. end of the well-known Serpentine ridge on the Mare Serenitatis, and, after following a course slightly concave to the N., dies out on the N. side of the plateau. This cleft forms the line of demarcation between the dark tone of the Mare Serenitatis and the light hue of the Mare Tranquilitatis, traceable under nearly every condition of illumination, and prominent in all good photographs.

DAWES.--A ring-plain 14 miles in diameter, situated N.W. of Plinius, on a nearly circular light area. Its bright border rises to a height of 2000 feet above the Mare, and includes a central mountain, a white marking on the E., and a ridge running from the mountain to the S. wall. There are two closely parallel clefts on the N. side of the plateau running from E. to W., that nearer Dawes being the longer, and having a craterlet standing upon it about midway between its extremities. At its W. termination there is a crater-row running at right angles to it. The light area appears to be bounded on the E. by a low curved bank.

VITRUVIUS.--A ring-plain 19 miles in diameter with bright but not very lofty walls, situated among the mountains near the S.W. side of the Mare Serenitatis. It is surrounded by a region remarkable for its great variability in brightness. There is a large bright ring-plain on the W., with a less conspicuous companion on the S. of it.

MARALDI.--A deep but rather inconspicuous formation, bounded on the W. by a polygonal border. A small ring-plain with a central mountain is connected with the S.W. wall; and, running in a N. direction from this, is a short mountain arm which joins a large circular enclosure with a low broken border standing on the N. side of the Mare Tranquilitatis.

LITTROW.--A peculiar ring-plain, rather smaller than the last, some distance N. of Vitruvius, on the rocky W. border of the Mare Serenitatis. It is shaped like the letter D, the straight side facing the W. There is a distinct crater on the N. wall. On the N.W. it is flanked by three irregular ring-plains, and on the S.E. by a fourth. Neison shows two small mountains on the floor, but Schmidt, whose drawing is very true to nature, has no detail whatever. A fine cleft may be traced from near the foot of the E. wall to Mount Argaeus, passing S. of a bright crater on the Mare E. of Littrow. It extends towards the Plinius system, and is probably connected with it.

MOUNT ARGAEUS.--There are few objects on the moon's visible surface which afford a more striking and beautiful picture than this mountain and its surrounding heights with their shadows a few hours after sunrise. It attains an altitude of more than 8000 feet above the Mare, and at a certain phase resembles a bright spear-head or dagger. There is a well- defined rimmed depression abutting on its southern point.

ROMER.--A prominent formation of irregular outline, 24 miles in diameter, situated in the midst of the Taurus highlands. It has a very large central mountain, a crater on the N. side of the floor, and terraced inner slopes. Some distance on the N. is another ring, nearly as large, with a crater on its S. rim, and between this and Posidonius is another with a wide gap on the S. and a crater on its N. border. One of the most remarkable crater-rills on the moon runs from the E. side of Romer through this latter ring, and then northwards on to the plain W. of Posidonius. Under suitable conditions, it can be seen as such in a 4 inch achromatic. It is easily traceable as a rill in a photograph of the N. polar region of the moon taken by MM. Henry at the Paris Observatory, and recently published in _Knowledge_.

LE MONNIER.--A great inflection or bay on the W. border of the Mare Serenitatis S. of Posidonius. Like many other similar formations on the edges of the Maria, it appears at one time or other to have had a continuous rampart, which on the side facing the "sea" has been destroyed. In this, as in most of the other cases, relics of the ruin are traceable under oblique light. A fine crescent-shaped mountain, 3000 feet high, stands near the S. side of the gap, and probably represents a portion of a once lofty wall. It will repay the observer to watch the progress of sunrise on the whole of the W. coast-line of the Mare up to Mount Argaeus.

POSIDONIUS.--This magnificent ring-plain is justly regarded as one of the finest telescopic objects in the first quadrant. Its narrow bright wall with its serrated shadow, the conspicuous crater, the clefts and ridges and other details on the floor, together with the beautiful group of objects on the neighbouring plain, and the great Serpentine ridge on the E., never fail to excite the interest of the observer. The circumvallation, which is far from being perfectly regular, is about 62 miles in diameter, and, considering its size, is not remarkable for its altitude, as it nowhere exceeds 6000 feet above the interior, which is depressed about 2000 feet below the surrounding plain. Its continuity, especially on the E., is interrupted by gaps. On the N., the wall is notably deformed. It is broader and more regular on the W., where it includes a large longitudinal depression, and on the N.W. section stand two bright little ring-plains. On the floor, which shines with a glittering lustre, are the well-marked remains of a second ring, nearly concentric with the principal rampart, and separated from it by an interval of nine or ten miles. The most prominent object, however, is the bright crater a little E. of the centre. This is partially surrounded on the W. by three or four small bright mountains, through which runs in a meridional direction a rill-valley, not easily traced as a whole, except under a low sun. There is another cleft on the N.E. side of the interior, which is an apparent extension of part of the inner ring, a transverse rill-valley on the N., a fourth _quasi_ rill on the N.W., and a fifth short cleft on the S. part of the floor. Between the principal crater and the S.E. wall are two smaller craters, which are easy objects. Beyond the border on the N., in addition to Daniell, are four conspicuous craters and many ridges.