For properly observing these objects, a power of not less than 300 on telescopes of large aperture is needed; and in studying their minute and delicate details, we are perhaps more dependent on atmospheric conditions than in following up any other branch of observational astronomy. Few indeed are the nights, in our climate at any rate, when the rough, irregular character of the steep interior of even the coarser examples of these immense chasms can be steadily seen. We can only hope to obtain a more perfect insight into their actual structural peculiarities when they are scrutinised under more perfect climatic circumstances than they have been hitherto. When observing the Hyginus cleft, Dr. Klein noticed that at one place the declivities of the interior displayed decided differences of tint. At many points the reflected sunlight was of a distinctly yellow hue, while in other places it was white, as if the cliffs were covered with snow. He compares this portion of the rill to the Rhine valley between Bingen and Coblentz, but adds that the latter, if viewed from the moon, would probably not present so fresh an appearance, and would, of course, be frequently obscured by clouds. Since the erection of the great Lick telescope on Mount Hamilton, our knowledge of the details of some of the lunar clefts has been greatly increased, as in the case of the Ariadaeus cleft, and many others. Professor W.H. Pickering, also, at Arequipa, has made at that ideal astronomical site many observations which, when published, will throw more light upon their peculiar characteristics. A few years ago M.E.L. Trouvelot of Meudon drew attention to a curious appearance which he noted in connection with certain rills when near the terminator, viz., extremely attenuated threads of light on their sites and their apparent prolongations. He observed it in the ring-plain Eudoxus, crossing the southern side of the floor from wall to wall; and also in connection with the prominent cleft running from the north side of Burg to the west of Alexander, and in some other situations. He terms these phenomena _Murs enigmatiques_. Apparent prolongations of clefts in the form of rows of hillocks or small mounds are very common. FAULTS.--These sudden drops in the surface, representing local dislocations, are far from unusual: the best examples being the straight wall, or "railroad," west of Birt; that which strikes obliquely across Plato; another which traverses Phocylides; and a fourth that has manifestly modified the mountain arm north of Cichus. They differ from the terrestrial phenomena so designated in the fact that the surface indications of these are destroyed by denudation or masked by deposits of subsequent date. In many cases on the moon, though its course cannot be traced in its entirety by its shadow, yet the existence of a fault may be inferred by the displacement and fracture of neighbouring objects.