Ramsden, and in many other similar positions. Rugged highlands also are often traversed by them, as in the case of those lying west of Le Monnier and Chacornac, and in the region west of the Mare Humorum. It may be here remarked, however, as a notable fact, that the neighbourhood of the grandest ring-mountain on the moon, Copernicus, is, strange to say, devoid of any features which can be classed as true clefts, though it abounds in crater-rows. The intricate network of rills on the west of Triesnecker, when observed with a low power, reminds one of the wrinkles on the rind of an orange or on the skin of a withered apple. Gruithuisen, describing the rill-traversed region between Agrippa and Hyginus, says that "it has quite the look of a Dutch canal map." In the subjoined catalogue many detailed examples will be given relating to the course of these mysterious furrows; how they occasionally traverse mountain arms, cut through, either completely or partially (as in Ramsden), the borders of ring-plains and other enclosures, while not unfrequently a small mound or similar feature appears to have caused them to swerve suddenly from their path, as in the case of the Ariadaeus cleft, and in that of one member of the Mercator-Campanus system. Of the actual nature of the lunar rills we are, it must be confessed, supremely ignorant. With some of the early observers it was a very favourite notion that they are artificial works, constructed presumably by Kepler's _sub-volvani_, or by other intelligences. There is perhaps some excuse to be made for the freaks of an exuberant fancy in regard to objects which, if we ignore for a moment their enormous dimensions, judged by a terrestrial standard, certainly have, in their apparent absence of any physical relation to neighbouring objects, all the appearance of being works of art rather than of nature. The keen-sighted and very imaginative Gruithuisen believed that in some instances they represent roads cut through interminable forests, and in others the dried-up beds of once mighty rivers. His description of the Triesnecker rill-system reads like a page from a geographical primer. A portion of it is compared to the river Po, and he traces its course mile by mile up to the "delta" at its place of disemboguement into the Mare Vaporum. From the position of some rills with respect to the contour of the surrounding country, it is evident that if water were now present on the moon, they, being situated at the lowest level, would form natural channels for its reception; but the exceptions to this arrangement are so numerous and obvious, that the idea may be at once dismissed that there is any analogy between them and our rivers. The eminent selenographer, the late W.K. Birt, compared many of them to "inverted river-beds" from the fact that, as often as not, they become broader and deeper as they attain a higher level. The branches resemble rivers more frequently than the main