Page:The Moon (Pickering).djvu/67

Rh means, of course, is that if formed by the action of water, as seems from their appearance probable, the lake flowed into the river, and not the river into the lake. Nor, when we come to think of the matter, is this result surprising. The pressure of the lunar atmosphere must always have been slight, and evaporation from the sunny side of the Moon extremely rapid. Indeed, several of the South American and also some of our own rivers, near the west coast, grow smaller and finally disappear after entering the desert regions of that portion of the continent. The only surprising feature seems to be that the sources of the lunar streams should have been so large. But the rills from the smaller sources cannot be detected, so that only the larger ones are seen, and they probably were not all flowing at the same time.

With so much volcanic activity upon the Moon, it is not surprising that a considerable amount of water should have been expelled from its interior in the form of hot springs or geysers. The illustration (Fig. 3) represents a characteristic riverbed, which was classed by Schmidt as a rill, and which takes its origin on the Mount Hadley range, in the Apennines, upon the eastern slopes of the peak known as 5. Plate 7A [2.4, 5.3]. Its course lies a little west of north, and at the point of disappearance it is fifty miles in a straight line from its source, its length, measured around the curves, being about sixty-five miles. Its breadth at the point of exit from the craterlet is perhaps 2,000 feet; but it soon narrows to between 500 and 1,000 feet, although the stream itself was probably much narrower. This drawing was constructed from a series of sketches made February 6 and 25, March 5 and 27, 1893, and each curve and bend has been reproduced with all possible care and accuracy, so that the drawing represents, as nearly as may be, exactly what was seen and no more.

Whether the forks shown near the bottom of the sketch on the right represent] successive positions occupied by the stream, or whether they merely represent a couple of small rills of the ordinary type occurring by chance at that place, it is impossible to determine.

With one exception, this is the largest riverbed foimd upon the Moon. This exception is Schröter's Valley, which we have just been studying. The pear-shaped craterlet from which it starts is well shown in the various sketches (Plate B), while its general course is shown in Figure 4 and also on Plate 15E [0.7, 4.1]. At its