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62 acquainted with that gives practically the same result, is glass paper, used in the arts for polishing, and he thinks that this may he made to explain the corona encircling the moon in total eclipses. This, after all, does not give a very definite notion of the nature of the surface, but it dissipates the idea that Arago had lately revived, that these dark plains were really seas, so shallow that the unevenness of the bottom might be detected through the transparent water.

More recently Mr Warren De la Rue has been led to suspect that the dark regions owe their darkness to vegetation. These regions are much darker in the photograph than in the moon itself, and he thinks this feebleness of actinic action can only be due to the foliage, with which the plains may be clothed. In the absence of any other indications of an atmosphere, this view must be received with caution.

Let us direct our steps to the lunar Alps, a very lofty range of mountains skirting the Mare Imbrium. In passing along the plain, we come to interruptions like the crevasses in a glacier, only they are much wider, more regularly formed, and of unfathomable depth. They present exactly the appearance a trapdyke would do, if quarried out. These rents, or rills, as they are termed, cross each other in such a way as to produce fantastic forms. The first observers imagined that they were roads or canals. The most probable explanation is, that they are rents in the moon's surface; which have never been filled no with