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 channels; for they generally commence as very fine grooves, and, becoming broader and broader, join them at an acute angle.  An attempt again has been made to compare the lunar clefts with those vast gorges, the marvellous results of aqueous action, called canyons, which attain their greatest dimensions in North America; such as the Great Canyon of the Colorado, which is at least 300 miles in length, and in places 2000 yards in depth, with perpendicular or even overhanging sides; but the analogy, at first sight specious, utterly breaks down under closer examination.  Some selenographers consider them to consist of long-extending rows of confluent craters, too minute to be separately distinguished, and to be thus due to some kind of volcanic action.  This is undoubtedly true in many instances, for almost every lunar region affords examples of crater-rows merging by almost imperceptible gradations into cleft-like features, and crater-rows of considerable size resemble clefts under low powers.  Still it seems probable that the greater number of these features are immense furrows or cracks in the surface and nothing more; for the higher the magnifying power employed in their examination, the less reason there is to object to this description.  Dr. Klein of Cologne believes