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These examples, very far from being exhaustive, will be sufficient to show that the walled-plains exhibit noteworthy differences in other respects than size, height of rampart, or included detail. Still another peculiarity, confined, it is believed, to a very few, may be mentioned, viz., convexity of floor, prominently displayed in Petavius, Mersenius, and Hevel.
TO 90 deg. LOHRMANN. This ring-plain, with Hevel and Cavalerius on the N. of it, is a member of a linear group, which, but for its propinquity to the limb, would be one of the most imposing on the moon's surface. Lohrmann, about 28 miles in diameter, is surrounded by a bright wall, which, to all appearance, is devoid of detail.
No map shows this cleft, though it is obvious enough; and, when the E. wall of Hevel is on the morning terminator, the notches made by it in the border of Lohrmann are easily detected. Capt. Noble, F.R.A.S., aptly compares two of the crossed clefts to a pair of scissors, the craters at which they terminate representing the oval handles.
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.
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 more striking of these systems were recognised and drawn at a very early stage of telescopic observation, as may be seen if we consult the quaint old charts of Hevel, Riccioli, Fontana, and other observers of the seventeenth century, where they are always prominently, though very inaccurately, portrayed.
HEVEL. A great walled-plain, 71 miles in diameter, adjoining Lohrmann on the N., with a broad western rampart, rising at one peak to a height above the interior of nearly 6000 feet, and presenting a steep bright face to the Oceanus Procellarum. There are three prominent craters near its crest, and one or two breaks in its continuity.
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