THEBIT.—A fine ring-plain, 32 miles in diameter, on the mountainous W. margin of the Mare Nubium, N.E. of Purbach. Its irregular rampart is prominently terraced, and its continuity on the N.E. interrupted by a large deep crater (Thebit A), at least 9 miles in diameter, which has in its turn a smaller crater, of about half this size, on its margin, and a small central mountain within, which was once considered a good optical test, though it is not a difficult object in a 4 inch achromatic, if it is looked for at a favourable phase. The border of Thebit rises at one place on the N.W. to a height of nearly 10,000 feet above the interior, which includes much detail. The E. wall of Thebit A attains the same height above its floor, which is depressed more than 5000 feet below the Mare.
BIRT.—This ring-plain, about 12 miles in diameter, is situated on the Mare Nubium, some distance due E. of Thebit. It has a brilliant border, surmounted by peaks rising more than 2000 feet above the Mare, and a very depressed floor, which does not appear to contain any visible detail. A bright crater adjoins it on the S.W., the wall of which at the point of junction is clearly very low, so that under oblique light the two interiors appear to communicate by a narrow pass or neck filled with shadow. I have frequently seen a break in the N.W. wall of Birt, which seems to indicate the presence of a crater. There is a noteworthy cleft on the E., which can be traced from the foot of the E. wall to the hills on the N.E. It is a fine telescopic object, and, under some conditions, the wider portion of it resembles a railway cutting traversing rising ground, seen from above. It is visible as a white line under a high light.
THE STRAIGHT WALL.—Sometimes called "the railroad," is a remarkable and almost unique formation on the W. side of Birt, extending for about 65 miles from N.E. to S.W. in a nearly straight line, terminating on the south at a very peculiar mountain group, the shape of which has been compared to a stag's horn, but which perhaps more closely resembles a sword-handle,—the wall representing the blade. When examined under suitable conditions, the latter is seen to be slightly curved, the S. half bending to the west, and the remainder the opposite way. The formation is not a ridge, but is clearly due to a sudden change in the level of the surface, and thus has the outward characteristics of a "fault" Along the upper edge of this gigantic cliff (which, though measures differ, cannot be anywhere much less than 500 feet high) I have seen at different times many small craterlets and mounds. Near its N. end is a large crater, and on the W. is a row of hillocks, running at right angles to the cliff. No observer should fail to examine the wall under a setting sun when the nearly perpendicular E. face of the cliff is brilliantly illuminated.
NICOLLET.—A conspicuous little ring-plain on the E. of Birt, and somewhat smaller. Between the two is a still smaller crater, from near which runs a low mountain range, nearly parallel to the straight wall, to the region S.E. of the Stag's Horn Mountains. Here will be found three small light-surrounded craters arranged in a triangle, with a somewhat larger crater in the middle.
PURBACH.—An immense enclosure of irregular shape, approximating to that of a rhomboid with slightly curved sides. It is fully 60 miles across, and the walls in places exceed 8000 feet in altitude, and include many depressions, large and small. On the E. inner slope are some fine terraces and several craters. The continuity of the circumvallation is broken on the N. by a great ring-plain, on the floor of which I have seen a prominent cleft and a crater near the S. side. There is a large bright crater in the interior of Purbach, S. of the centre, two others on the W. half of the floor, and a few ridges.
REGIOMONTANUS.—A still more irregular walled-plain, of about the same area, closely associated with the S. flank of Purbach, having a rampart of a similar complex type, traversed by passes, longitudinal valleys, and other depressions. Schmidt alone shows the especially fine example of a crater-row, which is not a difficult object, in connection with the S.E. wall. Excepting one crater, nearly central, and some inconspicuous ridges, I have seen no detail on the floor. Schmidt, however, records many features.
WALTER.—A great rhomboidal walled-plain, 100 miles in diameter, with a considerably depressed floor, enclosed by a rampart of a very complex kind, crowned by numerous peaks, one of which, on the W., rises 10,000 feet above the interior. If the formation is observed when it is close to the morning terminator, say, when the latter lies from l deg. to 2 deg. E. of the centre of the floor, it is one of the most striking and beautiful objects which the lunar observer can scrutinize. The inner slope of the border which abuts on Regiomontanus, examined at this phase under a high power, is seen to be pitted with an inconceivable number of minute craters; and the summit ridge, and the region towards Werner, scalloped in a very extraordinary way, the engrailing (to use an heraldic term) being due to the presence of a row of big depressions. The floor at this phase is sufficiently illuminated to disclose some of its most noteworthy features. Taking its area to be about 8000 square miles, at least 1200 square miles of it is occupied by the central mountain group and its adjuncts, the highest peak rising to a height of nearly 5000 feet (or nearly 600 feet higher than Ben Nevis), above the interior, and throwing a fine spire of shadow thereon. In the midst of this central boss are two deep craters, one being about 10 miles in diameter, and a number of shallower depressions. In association with the loftiest peak, I noted at 8 h., March 9, 1889, two brilliant little craters, which presumably are not far from the summit. Near the E. corner of the floor there is another large deep crater, and, ranging in a line from the centre to the S.E. wall, three smaller craters.
LEXELL.—On the E. of Walter extends an immense plain of irregular outline, which is at least equal to it in area. Though no large formation is found thereon; many ridges, short crater-rows, and ordinary craters figure on its rugged superficies; and on its borders stand some very noteworthy objects, among them, on the S., the walled-plain Lexell, about 32 miles in diameter, which presents many points of interest. Its irregular wall, rising, at one point on the S.W., to a height of nearly 8000 feet, is on the N.W. almost completely wanting, only very faint indications of its site being traceable, even under a low morning sun. On the opposite side it is boldly terraced, and has a large crater on its summit. The interior, the tone of which is conspicuously darker than that of the region outside, contains a small central hill, with two craters connected with it. The low N.W. margin is traversed by a delicate valley, which, originating on the N. side of the great plain, crosses the W. quarter of Lexell and terminates apparently on the S.W. side of the floor.
HELL.—A prominent ring-plain, about 18 miles in diameter, on the E. side of the great plain. There is a central mountain and many ridges within.
BALL.—A somewhat smaller ring-plain on the S.E. edge of the great plain, with a lofty terraced border and a central mountain more than 2000 feet high. There are two large irregular depressions on the W. of the formation, a crater on the S., and a smaller one on the N. wall.