Chapter IV: Part 4

CASSINI.--This remarkable ring-plain, about 36 miles in diameter, is very similar in character to Posidonius. It has a very narrow wall, nowhere more than 4000 feet in height, and falling on the E. to 1500 feet. Though a prominent and beautiful object under a low sun, its attenuated border and the tone of the floor, which scarcely differs from that of the surrounding surface, render it difficult to trace under a high angle of illumination, and perhaps accounts for the fact that it escaped the notice of Hevel and Riccioli; though it is certainly strange that a formation which is thrown into such strong relief at sunrise and sunset should have been overlooked, while others hardly more prominent at these times have been drawn and described. The outline of Cassini is clearly polygonal, being made up of several rectilineal sections. The interior, nearly at the same level as the outside country, includes a large bright ring-plain, A, 9 miles in diameter and 2600 feet in depth, which has a good-sized crater on the S. edge of a great bank which extends from the S.W. side of this ring-plain to the wall. On the E. side of the floor, close to the inner foot of the border, is a bright deep crater about two- thirds of the diameter of A, and between it and the latter Brenner has seen three small hills. The outer slope of Cassini includes much detail. On the S.W. is a row of shallow depressions just below the crest of the wall, and near the foot of the slope is a large circular shallow depression associated with a valley which runs partly round it. The shape of the _glacis_ on the W. is especially noteworthy, the S.W. and N.W. sides meeting at a slightly acute angle at a point 10 or 12 miles W. of the summit of the ring. On the outer E. slope is a curious elongated depression, and on the N. slope two large dusky rings, well shown by Schmidt, but omitted in other maps. Most of these details are well within the scope of moderate apertures. Perhaps the most striking view of Cassini and its surroundings is obtained when the morning terminator is on the central meridian.

ALEXANDER.--A large irregularly shaped plain, at least 60 miles in longest diameter, enclosed by the Caucasus Mountains. On the S.W. and N.W. the border is lineal. It has a dark level floor on which there is a great number of low hills.

EUDOXUS.--A bright deep ring-plain, about 40 miles in diameter, in the hilly region between the Mare Serenitatis and the Mare Frigoris, with a border much broken by passes, and deviating considerably from circularity. Its massive walls, rising more than 11,000 feet above the floor on the W., and about 10,000 feet on the opposite side, are prominently terraced, and include crater-rows in the intervening valleys, while their outer slopes present a complicated system of spurs and buttresses. There is a bright crater on the N. _glacis_, and some distance beyond the wall on the N.W. is a small ring-plain, and on the S.E. another, with a conspicuous crater between it and the wall. Neison draws attention to an area of about 1400 square miles on the N.E. which is covered with a great multitude of low hills. E. of Eudoxus are two short crossed clefts, and on the N. a long cleft of considerable delicacy running from N.E. to S.W. It was in connection with this formation that Trouvelot, on February 20, 1877, when the terminator passed through Aristillus and Alphonsus, saw a very narrow thread of light crossing the S. part of the interior and extending from border to border. He noted also similar appearances elsewhere, and termed them _Murs enigmatiques_.

ARISTOTELES.--A magnificent ring-plain, 60 miles in diameter, with a complex border, surmounted by peaks, rising to nearly 11,000 feet above the floor, one of which on the W., pertaining to a terrace, stands out as a brilliant spot in the midst of shadow when the interior is filled with shadow. The formation presents its most striking aspect at sunrise, when the shadow of the W. wall just covers the floor, and the brilliant inner slope of the E. wall with the little crater on its crest is fully illuminated. At this phase the details of the terraces are seen to the best advantage. The arrangement of the parallel ridges and rows of hills on the N.E. and S.W. is likewise better seen at this time than under an evening sun. A bright and deep ring-plain, about 10 miles in diameter, with a distinct central mountain, is connected with the W. wall.

EGEDE.--A lozenge-shaped formation, about 18 miles from corner to corner, bounded by walls scarcely more than 400 feet in height. It is consequently only traceable under very oblique illumination.

THE GREAT ALPINE VALLEY.--A great wedge-shaped depression, cutting through the Alps W. of Plato, from W.N.W. to E.S.E. It is more than 80 miles in length, and varies in breadth from 6 miles on the S. to less than 4 miles on the N., where it approaches the S. border of the Mare Frigoris. For a greater part of its extent it is bounded on the S.W. side by a precipitous linear cliff, which, under a low evening sun, is seen to be fringed by a row of bright little hills. These are traceable up to one of the great mountain masses of the Alps, forming the S.W. side of the great oval-shaped expansion of the valley, whose shape has been appropriately compared to that of a Florence oil-flask, and which Webb terms "a grand amphitheatre." On the opposite or N.E. side, the boundary of the valley is less regular, following a more or less undulating line up to a point opposite, and a little N. of, the great mountain mass, where it abuts on a shallow _quasi_ enclosure with lofty walls, which, projecting westwards, considerably diminish the width of the valley. South of this lies another curved mountain ring, which still farther narrows it. This curtailment in width represents the neck of the flask, and is apparently about 16 or 17 miles in length, and from 3 to 4 miles in breadth, forming a gorge, bordered on the W. by nearly vertical cliffs, towering thousands of feet above the bottom of the valley; and on the E. by many peaked mountains of still greater altitude. At the entrance to the "amphitheatre," the actual distance between the colossal rocks which flank the defile is certainly not much more than 2 miles. From this standpoint the view across the level interior of the elliptical plain would be of extraordinary magnificence. Towards the S., but more than 12 miles distant, the outlook of an observer would be limited by some of the loftiest peaks of the Alps, whose flanks form the boundary of the enclosure, through which, however, by at least three narrow passes he might perchance get a glimpse of the Mare Imbrium beyond. The broadest of these aligns with the axis of the valley. It is hardly more than a mile wide at its commencement on the S. border of the "amphitheatre," but expands rapidly into a trumpet-shaped gorge, flanked on either side by the towering heights of the Alps as it opens out on to the Mare. The bottom, both of the "amphitheatre" and of the long wedge-shaped valley, appears to be perfectly level, and, as regards the central portion of the latter, without visible detail. Under morning illumination I have, however, frequently seen something resembling a ridge partially crossing "the neck," and, near sunset, a tongue of rock jutting out from the E. flank of the constriction, and extending nearly from side to side. At the base of the cliff bordering the valley on the S.W., five or six little circular pits have been noted, some of which appear to have rims. They were seen very perfectly with powers of 350 and 400 on an 8 1/2 inch Calver reflector at 8 h. on January 25, 1885, and have been observed, but less perfectly, on subsequent occasions. The most northerly is about 10 miles from the N.W. end of the formation, and the rest occur at nearly regular intervals between it and "the neck." In the neighbourhood of the valley, on either side, there are several bright craters. Three stand near the N.E. edge, and one of considerable size near the N.W. end on the opposite side. A winding cleft crosses the valley about midway, which, strange to say, is not shown in the maps, though it may be seen in a 4 inch achromatic. It originates apparently at a bright triangular mountain on the plain S.W. of the valley, and, after crossing the latter somewhat obliquely, is lost amid the mountains on the opposite side. That portion of it on the bottom of the valley is easily traceable under a high light as a white line. The region N. of the Alps on the S.W. side of the valley presents many details worthy of examination. Among them, parallel rows of little hills, all extending from N.W. to S.E. There is also a number of still smaller objects of the same type on the E. side. The great Alpine valley, though first described by Schroter, is said to have been discovered on September 22, 1727, by Bianchini, but it is very unlikely that an object which is so prominent when near the terminator was not often remarked before this.

ARCHYTAS.--A bright ring-plain, 21 miles in diameter, on the edge of the Mare Frigoris, due N. of the Alpine Valley, with regular walls rising about 5000 feet above the interior on the N.W., and about 4000 feet on the opposite side. It has a very bright central mountain. Several spurs radiate from the wall on the S., and a wide valley, flanked by lofty heights, forming the S.W. boundary of W.C. Bond, originates on the N side. There is also a crater-rill running towards the N.W. On the Mare, S.W. of Archytas, is a somewhat smaller ring-plain, Archytas A (called by Schmidt, PROTAGORAS), with lofty walls and a central hill.

CHRISTIAN MAYER.--A prominent rhomboidal-shaped ring-plain, 18 miles in diameter, associated on every side, except the N., with a number of irregular inconspicuous enclosures. It has a central peak. Madler discovered two delicate short clefts, both running from N.W. to S.E., one on the W. and the other on the E. of this formation.

W.C. BOND.--A great enclosed plain of rhomboidal shape on the N. of Archytas, the bright ring-plain Timaeus standing near its E. corner, and another conspicuous but much smaller enclosure with a smaller crater W. of it on the floor at the opposite angle. The interior, which is covered with rows of hillocks, is very noteworthy at sunrise.

BARROW.--There are few more striking or beautiful objects at sunrise than this, mainly because of the peculiar shape of its brilliant border and the remarkable shadows of the lofty peaks on its western wall. There is a notable narrow gap in the rampart on the W., which appears to extend to the level of the floor. The walls, especially on the S., are very irregular, and include two large deep craters and some minor depressions. If the formation is observed when its E. wall is on the morning terminator, a fine view is obtained of the remarkable crater-row which winds round the N. side of Goldschmidt. Barrow is about 40 miles in diameter. According to Schmidt, there is one crater in the interior, a little S.E. of the centre.

SCORESBY.--A much fore-shortened deep ring-plain, 36 miles in diameter, between Barrow and the limb. It has a central mountain with two peaks, which are very difficult to detect.

CHALLIS.--A ring-plain adjoining Scoresby on the N.E. It is of about the same size and shape.

MAIN.--A very similar formation, on the N. of the last, much too near the limb to be well observed.

SECOND QUADRANT

EAST LONGITUDE 0 deg. TO 20 deg.

MURCHISON.--A considerable ring-plain about 35 miles across on the E., where it abuts on Pallas. It is a pear-shaped formation, bounded on the N. by a mountainous region, and gradually diminishes in width towards the S.E., on which side it is open to the plain. The walls are of no great altitude, but, except on the N.W., are very bright. At the S. termination of the W. wall there is an exceedingly brilliant crater, Murchison A, five miles in diameter and some 3000 feet deep; adjoining which on the N.W. is an oval depression and a curious forked projection from the border. The only objects visible in the interior are a few low ridges on the E. side, and a number of long spurs running out from the wall on the N. towards the centre of the floor. Murchison A is named CHLADNI by Lohrmann.

PALLAS.--A fine ring-plain, about 32 miles in diameter, forming with Murchison an especially beautiful telescopic object under suitable illumination. Its brilliant border, broken by gaps on the W., where it abuts on Murchison, has a bright crater on the N.E., from which, following the curvature of the wall, and just below its crest, runs a valley in an easterly direction. There is a large bright central mountain on the floor, with a smaller elevation to the S. of it, and a ridge extending from the N. wall to near the centre. On the W., a section of the border is continued in a N. direction far beyond the limits of the formation; and on the S. it is connected with a small incomplete ring; on the E. of which, near the foot of the wall, is a somewhat smaller and much duskier enclosure.

BODE.--A brilliant ring-plain, 9 miles in diameter, situated on the N. side of Pallas. Its walls rise about 5000 feet above the interior, which is considerably depressed, and includes, according to Schmidt and Webb, a mountain or ridge. There are two parallel valleys on the W., which are well worth examination.

SOMMERING.--An incomplete ring-plain, 17 miles in diameter, situated on the lunar equator. It has rather low broken walls and a dark interior.

SCHROTER.--A somewhat larger formation, with a border wanting on the S. Schmidt draws a considerable crater on the S.W. side of the floor. It was in the region north of this object, which abounds in little hills and low ridges, that in the year 1822 Gruithuisen discovered a very remarkable formation consisting of a number of parallel rows of hills branching out (like the veins of a leaf from the midrib) from a central valley at an angle of 45 deg., represented by a depression between two long ridges running from north to south. The regularly arranged hollows between the hills and the longitudinal valley suggested to his fertile imagination that he had at last found a veritable city in the moon--possibly the metropolis of Kepler's _Subvolvani_, who were supposed to dwell on that hemisphere of our satellite which faces the earth. At any rate, he was firmly convinced that it was the work of intelligent beings, and not due to natural causes. This curious arrangement of ridges and furrows, which, according to Webb, measures about 23 miles both in length and breadth, is, owing to the shallowness of the component hills and valleys, a very difficult object to see in its entirety, as it must be viewed when close to the terminator, and even then the sun's azimuth and good definition do not always combine to afford a satisfactory glimpse of its ramifications. M. Gaudibert has given a drawing of it in the _English Mechanic_, vol. xviii. p. 638.

GAMBART.--A regular ring-plain, 16 miles in diameter, with a low border and without visible detail within; situated nearly on the lunar equator, about 130 miles S.S.W. of Copernicus, at the N.W. edge of a very hilly region. A prominent pear-shaped mountain, with a small crater upon it, stands a short distance on the S.W., and further in the same direction, a large bright crater with two much smaller craters on the N. of it. The rough hilly district about midway between Copernicus and Gambart is remarkable for its peculiar dusky tone and for certain small dark spots, first seen by Schmidt, and subsequently carefully observed by Dr. Klein. The noteworthy region where these peculiar features are found represents an area of many thousand square miles, and must resemble a veritable _Malpais_, covered probably with an incalculable number of craters, vents, cones, and pits, filled with volcanic _debris_. It is among details of this character that the true analogues of some terrestrial volcanoes must be looked for. Under a low angle of illumination the surface presents an extraordinarily rough aspect, well worthy of examination, but the dusky areas and the black spots can only be satisfactorily distinguished under a somewhat high sun. I have, however, seen them fairly well when the W. wall of Reinhold was on the morning terminator.

MARCO POLO.--A small and very irregularly-shaped enclosure (difficult to see satisfactorily) on the S. flank of the Apennines. It is hemmed in on every side by mountains.

ERATOSTHENES.--A noble ring-plain, 38 miles in diameter; a worthy termination of the Apennines. The best view of it is obtained under morning illumination when the interior is about half-filled with shadow. At this phase the many irregular terraces on the inner slope of the E. wall (which rises at one peak 16,000 feet above an interior depressed 8000 feet below the Mare Imbrium) are seen to the best advantage. The central mountain is made up of two principal peaks, nearly central, from which two bright curved hills extend nearly up to the N.W. wall,--the whole forming a V-shaped arrangement. On the S. there is a narrow break in the wall, and the S.W. section of it seems to overlap and extend some distance beyond the S.E. section. The border on the S.W. is remarkable for the great width of its _glacis_. Eratosthenes exhibits a marked departure from circularity, especially on the E., where the wall consists of two well-marked linear sections, with an intermediate portion where the crest for 20 miles or more bends inwards or towards the centre. From the S.E. flank of this formation extends towards the W. side of Stadius one of the grandest mountain arms on the moon's visible surface, rising at one place 9000 feet, and in two others 5000 and 3000 feet respectively above the Mare Imbrium. If this magnificent object is observed when the morning terminator falls a little E. of Stadius, it affords a spectacle not easily forgotten. I have often seen it at this phase when its broad mass of shadow extended across the well-known crater-row W. of Copernicus, some of the component craters appearing between the spires of shade representing the loftiest peaks on the mountain arm. There is a prominent little crater on the crest of the arm between two of the peaks, and another on the plain to the west.

STADIUS.--An inconspicuous though a very interesting formation, 43 miles in diameter, W. of Copernicus, with a border scarcely exceeding 200 feet in height. Hence it is not surprising that it was for a long time altogether overlooked by Madler. Except as a known object, it is only traceable under very oblique illumination, and even then some attention is required before its very attenuated wall can be followed all round. It is most prominent on the W., where it apparently consists of a S. extension of the Eratosthenes mountain-arm, and is associated with a number of little craters and pits. This is succeeded on the S.W. by a narrow strip of bright wall, and on the S. by a section made up of a piece of straight wall and a strip curving inwards, forming the S. side. On the E. the border assumes a very ghostly character, and appears to be mainly defined by rows of small depressions and mounds. On the N.E., N., and N.W. it is still lower and narrower; so much so, that it is only for an hour or so after sunrise or before sunset that it can be traced at all. On every side, with the exception of the curved piece on the S., the wall consists of linear sections. The interior contains a great number of little craters and very low longitudinal mounds. Ten craters are shown in Beer and Madler's map. Schmidt only draws fifteen, though in the text accompanying his chart he says that he once counted fifty. In the monograph published in the _Journal_ of the Liverpool Astronomical Society (vol. v. part 8), forty-one are represented. They appear to be rather more numerous on the S. half of the floor than elsewhere. Just beyond the limits of the border on the N., is a bright crater with a much larger obscure depression on the W. of it. The former is surrounded by a multitude of minute craters and crater-cones, which are easily seen under a low sun. Though almost every trace of Stadius disappears under a high light, I have had little difficulty in seeing portions of the border and some of the included details when the morning terminator had advanced as far as the E. wall of Herodotus, and the site was traversed by innumerable light streaks radiating from Copernicus. At this phase the bright crater, just mentioned, on the N. edge of the border was tolerably distinct.

COPERNICUS.--This is without question the grandest object, not only on the second Quadrant, but on the whole visible superficies of the moon. It undoubtedly owes its supremacy partly to its comparative isolation on the surface of a vast plain, where there are no neighbouring formations to vie with it in size and magnificence, but partly also to its favourable position, which is such, that, though not central, is sufficiently removed from the limb to allow all its manifold details to be critically examined without much foreshortening. There are some other formations, Langrenus and Petavius, for example, which, if they were equally well situated, would probably be fully as striking; but, as we see it Copernicus is _par excellence_ the monarch of the lunar ring-mountains. Schmidt remarks that this incomparable object combines nearly all the characteristics of the other ring-plains, and that careful study directed to its unequalled beauties and magnificent form is of much more value than that devoted to a hundred other objects of the same class. It is fully 56 miles in diameter, and, though generally described as nearly circular, exhibits very distinctly under high powers a polygonal outline, approximating very closely to an equilateral hexagon. There are, however, two sections of the crest of the border on the N.E. which are inflected slightly towards the centre, a peculiarity already noticed in the case of Eratosthenes. The walls, tolerably uniform in height, are surmounted by a great number of peaks, one of which on the W., according to Neison, stands 11,000 feet above the floor, and a second on the opposite side is nearly as high. Both the inner and outer slopes of this gigantic rampart are very broad, each being fully 10 miles in width. The outer slope, especially on the E., is a fine object at sunrise, when its rugged surface, traversed by deep gullies, is seen to the best advantage. The terraces and other features on the bright inner declivities on this side may be well observed when the sun's altitude is about 6 deg. Schmidt, whose measures differ from those of Neison, estimates the height of the wall on the E. to be 12,000 feet, and states that the interior slopes vary from 60 deg. to 50 deg. above, to from 10 deg. to 2 deg. at the base. The first inclination of 50 deg., and in some cases of 60 deg., is confined to the loftiest steep crests and to the flanks of the terraces. There are apparently five bright little mountains on the floor, the most easterly being rather the largest, and a great number of minute hillocks on the S.E. quarter. S.W. of the centre is a little crater, and on the same side of the interior a curious hook-shaped ridge, projecting from the foot of the wall, and extending nearly halfway across the floor. The region surrounding Copernicus is one of the most remarkable on the moon, being everywhere traversed by low ridges, enclosing irregular areas, which are covered with innumerable craterlets, hillocks, and other minute features, and by a labyrinth of bright streaks, extending for hundreds of miles on every side, and varying considerably both in width and brilliancy.

The notable crater-row on the W., often utilised by observers for testing the steadiness of the air and the definition of their telescopes, should be examined when it is on the morning terminator, at which time Webb's homely comparison, "a mole-run with holes in it," will be appreciated, and its evident connection with the E. side of Stadius clearly made out. There is another much more delicate row running closely parallel to this object; it lies a little W. of it, and extends farther in a northerly direction.

ARCHIMEDES.--Next to Plato the finest object on the Mare Imbrium. It is about 50 miles in diameter. The average height of its massive border is about 4000 feet above the interior, which is only depressed some 500 or 600 feet below the Mare, the highest peak (about 7000 feet) being on the S.E. The walls are terraced, and include much detail, both within and without. The most noteworthy features in connection with this formation are the crater-cones, craterlets, pits, white spots, and light streaks which figure on the otherwise smooth interior. Mr. T.P. Gray, F.R.A.S., of Bedford, who, with praiseworthy assiduity, has devoted more than ten years to the close scrutiny of these features, Mr. Stanley Williams, and others, have detected four crater-cones on the E. half of the floor, and about fifty minute craters and white spots, also probably volcanic vents, and a very curious and interesting series of light streaks, mostly traversing the formation from E. to W. A little E. of the centre is a dusky oval area about 6 miles across, and S.W. of this is another, much smaller. Under some conditions of illumination the two principal light markings may be traced over the W. wall, and for some distance on the plain beyond.

On the southern side of Archimedes is a very rugged mountain region, extending for more than 100 miles towards the south: on the W. of this originates a remarkable rill-system, best seen under evening illumination. The two principal clefts follow a nearly parallel course up to the face of the Apennines near Mount Bradley, crossing in their way, almost at right angles, other clefts which run at no great distance from the E. foot of this range and ramify among the outlying hills. Archimedes A is a brilliant little ring-plain on the S.E. of Archimedes. It casts an extraordinary shadow at sunrise, and has a well-marked crater-row on the E. of it, and two long narrow valleys, one of which appears to be a southerly extension of the row.

BEER.--A very bright little crater, with an unnamed formation of about the same size adjoining it on the N., with which is associated a curious winding ridge that appears to traverse a gap in its N. wall.

TIMOCHARIS.--A fine ring-plain, 23 miles in diameter (the centre of a minor ray-system). It stands isolated on the Mare Imbrium (below the level of which it is depressed some 3000 feet), about midway between Archimedes and Lambert. Its walls, rising about 7000 feet above the floor, are conspicuously terraced, and on their W. outer slopes exhibit some remarkable depressions. There is a distinct break on the N., and a bright little crater on the N.W., connected with the foot of the _glacis_ by a prominent ridge. On the bright central mountain, Schmidt, in 1842, detected a crater, which is easily seen under a moderately high light. Timocharis and the neighbourhood, especially the peculiar shape of the terminator on the E. of the formation, is well worth examination at sunrise.

PIAZZI SMYTH.--A conspicuous little ring-plain, 5 or 6 miles in diameter, depressed about 1500 feet below the Mare Imbrium, with a border rising about 2000 feet above it. With the curious arrangement of ridges, of which it is the apparent centre, it is a striking object under a low sun.

KIRCH.--A rather smaller object of the same class on the S.E.

PLATO.--This beautiful walled-plain, 60 miles in diameter, with its bright border and dark steel-grey floor, has, from the time of Hevelius to the present, been one of the most familiar objects to lunar observers. In the rude maps of the seventeenth century it figures as the "Lacus Niger Major," an appellation which not inaptly describes its appearance under a high sun, when the sombre tone of its apparently smooth interior is in striking contrast to that of the isthmus on which the formation stands. It will repay observation under every phase, and though during the last thirty years no portion of the moon has been more diligently scrutinised than the floor; the neighbourhood includes a very great number of objects of every kind, which, not having received so much attention, will afford ample employment to the possessor of a good telescope during many lunations.

The border of Plato, varying in height from 3000 to 4000 feet above the interior, is crowned by several lofty peaks, the highest (7400 feet) standing on the N. side of the curious little triangular formation on the E. wall. Those on the W., three in number, reckoning from N. to S., are respectively about 5000, 6000, and 7000 feet in altitude above the floor. The circumvallation being very much broken and intersected by passes, exhibits many distinct breaches of continuity, especially on the S. There is a remarkable valley on the S.W., which, cutting through the border at a wide angle, suddenly turns towards the S.E., and descends the slope of the _glacis_ in a more attenuated form. Another but shorter valley is traceable at sunrise on the W. On the N.W., the rampart is visibly dislocated, and the gap occupied by an intrusive mountain mass. This dislocation is not confined to the wall, but, under favourable conditions, may be traced across the floor to the broken S.E. border. It is probably a true "fault." On the N.E., the inner slope of the wall is very broad, and affords a fine example of a vast landslip.

The spots and faint light markings on the floor are of a particularly interesting character. During the years 1869 to 1871 they were systematically observed and discussed under the auspices of the Lunar Committee of the British Association. Among the forty or more spots recorded, six were found to be crater-cones. The remainder--or at least most of them--are extremely delicate objects, which vary in visibility in a way that is clearly independent of libration or solar altitude; and, what is also very suggestive, they are always found closely associated with the light markings,--standing either upon the surface of these features or close to their edges. Recent observations of these spots with a 13 inch telescope by Professor W.H. Pickering, under the exceptionally good conditions which prevail at Arequipa, Peru, have revived interest in the subject, for they tend to show that visible changes have taken place in the aspect of the principal crater-cones and of some of the other spots since they were so carefully and zealously scrutinised nearly a quarter of a century ago. The gradual darkening of the floor of Plato as the sun's altitude increases from 20 deg. till after full moon may be regarded as an established fact, though no feasible hypothesis has been advanced to account for it.

On the N.E. of Plato is a large bright crater, A; and, extending in a line from this towards the E., is a number of smaller rings, the whole group being well worth examination. On the N. there is a winding cleft, and some short crossed clefts in the rugged surface just beyond the foot of the wall, which I have seen with a 4 inch achromatic. The region on the W., imperfectly shown in the maps, includes much unrecorded detail. On the Mare Imbrium S. of Plato is a large area enclosed by low ridges, to which Schroter gave the name "Newton." It suggests the idea that it represents the ruin of a once imposing enclosure, of which the conspicuous mountain Pico formed a part.

TIMAEUS.--A very bright ring-plain, 22 miles in diameter, with walls about 4500 feet in height, on the coast-line of the Mare Frigoris, and associated with the E. side of the great enclosed plain W.C. Bond. Schmidt shows a double hill, nearly central, and Neison a crater on the S.W. wall.

BIRMINGHAM.--A large rhomboidal-shaped enclosure, defined by mountain chains and traversed by a number of very remarkable parallel ridges. It is situated nearly due N. of Plato on the N. edge of the Mare Frigoris, and lies on the S.E. side of W.C. Bond, to which it bears a certain resemblance. This region is characterised by the parallelism displayed by many formations, large and small. It is more apparent hereabouts than in any other part of the moon's visible surface. When favourably placed under a low morning sun, Birmingham is a striking telescopic object.

FONTINELLE.--A fine ring-plain, 23 miles in diameter, on the N. margin of the Mare Frigoris, N.N.E. of Plato, with a wall rising on the E., 6000 feet above a bright interior. I find its border indistinct and nebulous, excepting under very oblique light, though three of the little craters upon it are bright and prominent. One stands on the S., another on the N.W., and a third on the E. Schmidt shows only the first of these, and Neison none of them. Fontinelle has a low central mountain which is easily distinguished. Fontinelle A, an isolated mountain on the S., is more than 3000 feet high. On the N. there is a curious mountain group, also of considerable altitude, and on the W. an irregular depression surrounded by a dusky area. North of Fontinelle, extending towards Goldschmidt and the limb, Schroter discovered a very wide irregular valley which he named "J.J. Cassini." It is really nothing more than a great plain bounded by ridges. At 9 h. October 15, 1888, when Philolaus was on the morning terminator, I had a fine view of it, and, as regards its general shape, found that it agreed very closely with Schroter's drawing.

EPIGENES.--A remarkable ring-plain, about 26 miles in diameter, abutting on a mountain ridge running parallel to the E. flank of W.C. Bond. It is a notable object under a low morning sun. There are several elevations on the floor.

GOLDSCHMIDT.--A great abnormally-shaped enclosure with lofty walls between Epigenes and the limb. Neison mentions only two crater-pits within. I have seen the rimmed crater shown by Schmidt on the W. side and three or four other objects of a doubtful kind.

ANAXAGORAS.--A brilliant ring-plain of regular form, 32 miles in diameter, adjoining Goldschmidt on the E. It is a prominent centre of light streaks, some of which traverse the interior of Goldschmidt. On the north a peak rises to the height of 10,000 feet. There is a long ridge on the floor, running from E. to W.

GIOJA.--A ring-plain about 26 miles in diameter, near the north pole.

EAST LONGITUDE 20 deg. TO 40 deg.

REINHOLD.--A prominent ring-plain, 31 miles in diameter, with a lofty border, rising at a peak on the W. to more than 9000 feet above the floor. Its shape on the W. is clearly polygonal, the wall consisting of three rectilineal sections, and on the E. it is made up of two straight sections connected by a curved section. The inner slope includes a remarkably distinct and regular terrace, the E. portion of which is well seen when the interior is about half illuminated by the rising sun. At this phase also the great extent of the _glacis_ on the S.W., and the deep wide gullies traversing it on the E. are observed to the best advantage. The central mountain, though of considerable size, is not prominent. Close to Reinhold on the N.W. stands a noteworthy little formation with a low and partially lineal wall, exhibiting a gap on the north. There is a distinct crater on the S. side of its floor.

GAY-LUSSAC.--A very interesting ring-plain, 15 miles in diameter, situated in the midst of the Carpathian Highlands N. of Copernicus, with a smaller but brighter and deeper formation (Gay-Lussac A) on the S.W. of it, and a conspicuous little crater, not more than 2 or 3 miles in diameter, between the two. The interior of Gay-Lussac is traversed by two coarse clefts, lying nearly in a meridional direction. The more easterly runs from the foot of the S. wall, near the little crater just mentioned, across the floor to the low N. border, which it apparently cuts through, and extends for some distance beyond, terminating in a great oval expansion. The other, which is not shown in the maps, is closely parallel to it, and can be traced up to the N. border, but not farther. Schmidt represents the first as a crater-row, which it probably is, as it varies considerably in width. From the S.E. side of the formation extends a long cleft, terminating at the end of a prominent spur from the S. side of the Carpathians. There are also two remarkable rill-like valleys, commencing on the N. of Gay-Lussac A, which curve round the W. side of Gay-Lussac.

HORTENSIUS.--This brilliant crater, about 10 miles in diameter, is remarkable for its depth, and as being a ray-centre surrounded by a nimbus of light. It has a central mountain, and Schmidt shows a minute crater on the outer slope of the S. wall. The former is a test object.

MILICHIUS.--Is situated on the N.N.E. of Hortensius. It is fully as bright, but rather smaller. Its floor, apparently devoid of detail, is considerably depressed below the surrounding surface.

TOBIAS MAYER.--Like Gay-Lussac, a noteworthy ring-plain associated with the Carpathian Mountains. It is 22 miles in diameter, and has a wall which rises on the W. to a height of nearly 10,000 feet above the floor; on the latter there is a conspicuous central mountain, and on the E. side a crater, and some little hills. Schmidt shows a smaller crater on the W. side, which I have not seen. Adjoining the formation on the W. is a ring- plain of about one-fourth its area, which is a bright object. Tobias Mayer and the neighbouring Carpathians form an especially beautiful telescopic picture at sunrise.

KUNOWSKY.--An inconspicuous ring-plain, about 11 miles in diameter, standing in a barren region in the Mare Procellarum, W.S.W. of Encke. The central mountain is tolerably distinct.

ENCKE.--A regular ring-plain, 20 miles in diameter, with a comparatively low border, nowhere rising more than 1800 feet above the interior, which is depressed some 1000 feet below the surrounding Oceanus Procellarum. A lofty ridge traverses the floor from S. to N., bifurcating before it reaches the N. wall. There is a bright crater on the W. wall, and a depression on the opposite wall, neither of which, strange to say, is shown on the maps. Encke is encircled by ridges, which, when it is on the morning terminator, combine to make it resemble a large crater surrounded by a vast mountain ring.

KEPLER.--One of the most brilliant objects in the second quadrant,--a ring-plain about 22 miles in diameter, with a lofty border; a peak on the E. attaining an altitude of 10,000 feet above the surface. The wall is much terraced, especially the outer slope on the W., where a narrow valley is easily traceable. Though omitted from the maps, there is a prominent circular depression on the W. border, which forms a distinct notch thereon at sunrise. On the N., the wall exhibits a conspicuous gap. There is a central hill on the floor. Immediately E. of Kepler is a bright plateau, bounded on the N. by a very straight border, with two small craters on its edge. Both these objects are incomplete on the N., as if they had been deformed by a "fault," which has apparently affected the N. end of Kepler also. Kepler is the centre of one of the most extended systems of bright streaks on the moon's visible surface.

BESSARION.--A bright little ring-plain, about 6 miles in diameter, in the Oceanus Procellarum N. of Kepler. There is a smaller and still brighter companion on the N. (Bessarion E), standing on a light area. Bessarion has a minute central hill, difficult to detect.

PYTHEAS.--A small rhomboidal-shaped ring-plain, 12 miles in diameter, standing in an isolated position on the Mare Imbrium between Lambert and Gay-Lussac. Its bright walls, rising about 2500 feet above the Mare, are much terraced within, especially on the E. There is a bright little crater on the N. outer slope, with a short serpentine ridge running up to it from the region S. of Lambert, and another winding ridge extending from the S. wall to the E. of two conspicuous craters, standing about midway between Pytheas and Gay-Lussac. The former bears a great resemblance to the ridge N. of Madler, and, like this, appears to traverse the N. border. The interior of Pytheas, which is depressed more than 2000 feet below the Mare, includes a brilliant central peak.

LAMBERT.--A ring-plain, 17 miles in diameter, presenting many noteworthy features. The crest of its border stands about 2000 feet above the Mare Imbrium, and more than double this height above the interior. The wall is prominently terraced both within and without; the outer slope on the W. exhibiting at sunrise a nearly continuous valley running round it. When near the morning terminator, the region on the N. is seen to be traversed by some very remarkable ridges and markings; one cutting across the N. wall appears to represent a "fault." On the S. is a large polygonal enclosure formed by low ridges. On the W., towards Timocharis, is a brilliant mountain 3000 feet high, a beautiful little object under a low sun.

LEVERRIER.--The more westerly of a pair of little ring-plains on the N. side of the Mare Imbrium, and S.W. of the Laplace promontory. It is about 10 miles in diameter, with walls rising some 1500 feet above the Mare, and more than 6000 feet above the interior, which seems to be without a central mountain or other features. Schmidt shows the crater on the N. rim and another on the S.E. slope, both of which are omitted by Neison, though they are easy objects when Helicon is on the morning terminator. About 20 miles on the S.E. there is a very bright little crater on a faint light area.

HELICON.--The companion ring-plain on the E. It is 13 miles in diameter, and is very similar, though not quite so deep. There is a crater on the S.E. wall, and, according to Neison, another on the outer slope of the N. border. Webb records a central crater. If Helicon is observed when on the morning terminator, it will be seen to be traversed by a curved ridge which cuts through the walls, and runs up to a bright crater S.E. of Leverrier. It appears to be a "fault," whose "downthrow," though slight, is clearly indicated by an area of lower ground on the E. There is a great number of small craters in the neighbourhood of this formation.

EULER.--The most easterly of the row of great ring-plains, which, beginning on the W. with Autolycus, and followed by Archimedes, Timocharis, and Lambert, extends almost in a great circle from the N.W. to the S.E. side of the Mare Imbrium. It is about 19 miles in diameter, and is surrounded by terraced walls, which, though of no great height above the Mare, rise 6000 feet above the floor. There is a distinct little gap in the S. wall, easily glimpsed when it is close to the morning terminator, which probably represents a small crater. Euler has a bright central mountain, and is a centre of white silvery streaks.

BRAYLEY.--A very conspicuous little ring-plain E.S.E. of Euler, with two smaller but equally brilliant objects of the same class situated respectively E. and W. of it.

DIOPHANTUS.--Forms with Delisle, its companion on the N., a noteworthy object. It is about 13 miles in diameter, with a wall, which has a distinct break in its continuity on the N., rising about 2500 feet above the Mare. A rill-valley runs from the E. side of the ring towards the W. face of a triangular-shaped mountain on the E. of a line joining the formation with Delisle. North are three bright little craters in a line, the middle one being much the largest. From the most easterly of these objects a light streak may be traced under a high sun, extending for many miles to another small crater on the N.W. of Diophantus, and expanding at a point due N. of the formation into a spindle-shaped marking. At sunrise, the W. portion of the streak has all the appearance of a cleft, with a branch about midway running to the S. side of Delisle. Under the same phase a broad band of shadow extends from the N.E. wall to the triangular mountain just mentioned, representing a very sudden drop in the surface--resembling on a small scale the well-known "railroad" E. of Thebit. Diophantus has no central mountain.

DELISLE.--A larger and more irregularly-shaped object than the last, 16 miles in diameter, with loftier and more massive walls, and an extensive but ill-defined central hill. There is an evident break in the northern border. A triangular mountain on the S.E. and a winding ridge running up to the N. wall are prominent features at sunrise, as are also the brilliant summits of a group of hills some distance to the E.N.E.

CARLINI.--A small but prominent and deep little crater about 5 miles in diameter on the Mare Imbrium about midway between Lambert and the Sinus Iridum. There are many faint light streaks in the vicinity, one of which extends from Carlini to Bianchini, on the edge of the Sinus,--a distance of 300 miles. Schmidt shows a central peak.

CAROLINE HERSCHEL.--A bright and very deep ring-plain about 8 miles in diameter on the Mare Imbrium, some distance E.N.E. of the last. On the S.E. lies a larger crater, Delisle B, which has a small but obvious crater on its N. rim, and casts a very prominent shadow at sunrise. Caroline Herschel stands on a long curved ridge running N.E. from Lambert towards the region E. of Helicon, and, according to Schmidt, has a central peak. On the E. is a bright mountain with two peaks; some distance N. of which is a large ill-defined white spot, with another spot of a similar kind on the W. of it, nearly due N. of Caroline Herschel.

GRUITHUISEN.--This ring-plain, 10 miles in diameter, is situated on the Mare Imbrium on the N.E. of Delisle. It is associated with a number of ridges trending towards the region N. of Aristarchus and Herodotus.

THE LAPLACE PROMONTORY.--A magnificent headland marking the extreme western extremity of the finest bay on the moon's visible surface, the Sinus Iridum; above which it towers to a height of 9000 feet or more, projecting considerably in front of the line of massive cliffs which define the border of the Sinus, and presenting a long straight face to the S.E. Near its summit are two large but shallow depressions, the more easterly having a very bright interior. At a lower level, almost directly below the last, is a third depression. All three are easy objects under a low sun. The best view of the promontory and its surroundings is obtained when the E. side of the bay is on the morning terminator. Its prominent shadow is traceable for many days after sunrise.

THE HERACLIDES PROMONTORY.--The less lofty but still very imposing headland at the E. end of the Sinus Iridum, rising more than 4000 feet above it. It consists of a number of distinct mountains, forming a triangular-shaped group running out to a point at the S.W. extremity of the bay, and projecting considerably beyond the shore-line. There is a considerable crater on the E. side of the headland, not visible till a late stage of sunrise. It is among the mountains composing this promontory that some ingenuity and imagination have been expended in endeavouring to trace the lineaments of a female face, termed the "Moon- maiden."

BIANCHINI.--A fine ring-plain, about 18 miles in diameter, on the N.E. side of the Sinus Iridum, surrounded by the lofty mountains defining the border of the bay. Its walls, which are prominently terraced within, rise about 7000 feet on the E., and about 8000 feet on the W. above the floor, which includes a prominent ridge and a conspicuous central mountain. There is a distinct crater on the S. wall, not shown in the maps. Between this side of the formation and the bay is a number of hills running parallel to the shore-line: these, with the intervening valleys, will repay examination at sunrise.

MAUPERTUIS.--A great mountain enclosure of irregular shape, about 20 miles in diameter, in the midst of the Sinus Iridum highlands, N. of Laplace. The walls are much broken by passes, and the interior includes many hills and ridges.

CONDAMINE.--A rhomboidal-shaped ring-plain, about 23 miles in diameter, N. of Maupertuis, with lofty walls, especially on the E., where they rise some 4000 feet above the interior. There are three large depressions on the outer N.W. slope, and at least three minute craters on the crest of the wall just above. Though neither Neison nor Schmidt draw any detail thereon, there is a prominent ridge on the N. side of the floor, and a low circular hill on the S. On the S.E. four long ridges or spurs radiate from the wall, and on the N.E. are three remarkable square-shaped enclosures. On the edge of the Mare Frigoris, N.W. of Condamine, are many little craters with bright rims and a distinct short cleft, running parallel to the coast-line. The winding valleys in the region bordering the Sinus Iridum, and other curious details, render this portion of the moon's surface almost unique.

BOUGUER.--A bright regular little ring-plain, about 8 miles in diameter, N. of Bianchini.

J.F.W. HERSCHEL.--A vast enclosed plain, about 90 miles across, bounded on the W. by a mountain range, which here defines the E. side of the Mare Frigoris, on the S. by massive mountains, and on the other sides by a lofty but much broken wall, intersected by many passes. Within is a large ring-plain, nearly central, and a large number of little craters and crater-pits. The floor is traversed longitudinally by many low ridges, lying very close together, which at sunrise resemble fine grooves or scratches of irregular width and depth.

HORREBOW.--A ring-plain of remarkable shape, resembling the analemma figure, standing at the S. end of the mountain range bounding J.F.W. Herschel on the W. Schmidt shows a crater on the W. wall, near the constriction on this side, and a second at the foot of the slope of the E. wall.

PHILOLAUS.--A ring-plain 46 miles in diameter, on the N.E. of Fontinelle. Its bright walls rise on the W. to a height of nearly 12,000 feet above the floor (on which there is a conspicuous central mountain), and exhibit many prominent terraces. Philolaus is partially encircled, at no great distance, by a curved ridge, on which will be found a number of small craters.

ANAXIMINES.--A much foreshortened ring-plain, about 66 miles in diameter, on the E. of Philolaus. One peak on the E. is nearly 8000 feet in height. Schmidt shows four craters on the W. side of the floor, and a fifth on the S.E. side. There is a bright streak in the interior, which extends southwards for some distance across the Mare Frigoris.

EAST LONGITUDE 40 deg. TO 60 deg.

REINER.--A regular ring-plain, 21 miles in diameter, in the Mare Procellarum, S.S.E. of Marius, with a very lofty border terraced without and within, and a minute but conspicuous mountain standing at the N. end of a ridge which traverses the uniformly dark floor in a meridional direction. A long ridge extends some way towards the S. from the foot of the S. wall, and at some distance in the same direction lie six ill- defined white spots of doubtful nature. On the E.N.E. there is a large white marking, resembling a "Jew's harp" in shape, and farther on, towards the E., a number of very remarkable ridges. On the W. will be found many bright little craterlets. A ray from Kepler extends almost up to the W. wall of Reiner.