RIDGES.--Among the Maria which exhibit the most remarkable arrangement of ridges is the Mare Humorum, in the south-eastern quadrant. Here, if it be observed under a rising sun, a number of these objects will be seen extending from the region north of the ring-mountain Vitello in long undulating lines, roughly concentric with the western border of the "sea," and gradually diminishing in altitude as they spread out, with many ramifications, to a distance of 200 miles or more towards the north. At this stage of illumination they are strikingly beautiful in a good telescope, reminding one of the ripple-marks left by the tide on a soft sandy beach. Like most other objects of their class, they are very evanescent, gradually disappearing as the sun rises higher in the lunar firmament, and ultimately leaving nothing to indicate their presence beyond here and there a ghostly streak or vein of a somewhat lighter hue than that of the neighbouring surface. The Mare Nectaris, again, in the south-western quadrant, presents some fine examples of concentric ridges, which are seen to the best advantage when the morning sun is rising on Rosse, a prominent crater north of Fracastorius. This "sea" is evidently concave in cross-section, the central portion being considerably lower than the margin, and these ridges appear to mark the successive stages of the change of level from the coast-line to the centre. They suggest the "caving in" of the surface, similar to that observed on a frozen pond or river, where the "cat's ice" at the edge, through the sinking of the water beneath, is rent and tilted to a greater or less degree. The Mare Serenitatis and the Mare Imbrium, in the northern hemisphere, are also remarkable for the number of these peculiar features. They are very plentifully distributed round the margin and in other parts of the former, which includes besides one of the longest and loftiest on the moon's visible surface--the great serpentine ridge, first drawn and described nearly a hundred years ago by the famous selenographer, Schroter of Lilienthal. Originating at a little crater under the north- east wall of great ring-plain Posidonius, it follows a winding course across the Mare toward the south, throwing out many minor branches, and ultimately dies out under a great rocky promontory--the Promontory Acherusia, at the western termination of the Haemus range. A comparatively low power serves to show the curious structural character of this immense ridge, which appears to consist of a number of corrugations and folds massed together, rising in places, according to Neison, to a height of 700 feet and more. The Mare Imbrium also affords an example of a ridge, which, though shorter, is nearly as prominent, in that which runs from the bright little ring-plain Piazzi Smyth towards the west side of Plato. The region round Timocharis and other quarters of the Mare are likewise traversed by very noteworthy features of a similar class.