The one special feature of the Moon, which never fails to attract the attention of everybody who looks at our satellite for the first time through a telescope, are the crater mountains, which indeed constitute an immense majority of all the lunar mountains. Their outline almost always conforms, more or less, to that of the circle, but when seen near either limb of the Moon they often appear considerably oval simply because they are then seen considerably foreshortened. In their normal form they exhibit a basin bounded by a ridge, with a conical elevation in the centre of the basin, the basin and the cone together being evidently the result of an uprush of gases breaking through the outer crust of the Moon and carrying with them masses of molten lava. This lava, with perhaps the materials in fragments, projected in the first instance up into the air, fell back on to the Moon forming first of all the outer edge of the basin, and subsequently, as the eruptive force became weakened, the small central accumulation, which took, as it naturally would do, a conical shape. An experimental imitation of the process thus inferred was carried out some years ago by a French physicist, Bergeron, who acted upon a very fusible mixture of metals known as Wood’s alloy by forcing through it a current of hot air. The success of this experiment was complete, and Bergeron considered that his experiments, taken as a whole, were calculated to throw much light on the past history of the Moon. Several observers at various times have fancied they have seen signs that the lunar mountain Aristarchus was an active volcano even up to the present century; but it admits of no doubt that this idea is altogether a misconception, and that what they saw as a faint illumination of the summit of Aristarchus was no more than an effect of earth-shine. On the general question of volcanic action on the Moon, Sir John Herschel summed up as follows:—“Decisive marks of volcanic stratification arising from successive deposits of ejected matter, and evident indications of lava currents, streaming outwards in all directions, may be clearly traced with powerful telescopes. In Lord Rosse’s magnificent Reflector the flat bottom of the crater called Albategnius is seen to be strewed with blocks not visible in inferior telescopes, while the exterior ridge of another (Aristillus) is all hatched over with deep gulleys radiating towards its centre.” The valleys and clefts or rills visible on the Moon’s surface constitute another remarkable feature in the topography of our satellite. The valleys, properly so-called, require no particular comment, because they are just what their name implies—hollows often many miles long and several miles wide. The clefts or rills, however, are more mysterious, by reason of their great length and remarkable narrowness. One is almost led to infer that they are naught else but cracks in the lunar crust, the result of sudden cooling, how caused is of course not known.