The distinctness of the lunar features shows at once that the Moon is in an altogether different condition from that of the Earth. Here the sky is continually being hidden by cloud, and hence the details of the surface of the Earth as viewed from any other planet must often be invisible, and even when actual cloud is absent there is a more permanent veil of dust, which must greatly soften and confuse terrestrial outlines. The clearness, therefore, with which we perceive the lunar formations proves that there is little or no atmosphere there. Nor is there any sign upon it of water, either as seas or lakes or running streams. Yet the Moon shows clearly that in the past it has gone through great and violent changes. The gradation is so complete from the little craterlets, which resemble closely, in form and size, volcanic craters on the Earth, up to the great ring-plains, like "Copernicus" or "Tycho," or formations larger still, that it seems natural to infer not only that the smaller craters were formed by volcanic eruption, like the similar objects with which we are acquainted on our own Earth, but that the others, despite their greater sizes, had a like origin. In consequence of the feebler force of gravity on the Moon, the same explosive force there would carry the material of an eruption much further than on the Earth. The darker, low-lying districts of the Moon give token of changes of a different order. It is manifest that the material of which the floors of these plains is composed has invaded, broken down, and almost submerged many of the ring-formations. Sometimes half {61} of a ring has been washed away; sometimes just the outline of a ring can still be traced upon the floor of the sea; sometimes only a slight breach has been made in the wall. So it is clear that the Moon was once richer in the great crater-like formations than it is to-day, but a lava-flood has overflowed at least one-third of its area. More recent still are the bright streaks, or rays, which radiate in all directions from "Tycho," and from some of the other ring-plains. It is evident from these different types of structure on the Moon, and from the relations which they bear to each other, that the lunar surface has passed through several successive stages, and that its changes tended, on the whole, to diminish in violence as time went on; the minute crater pits with which the surface is stippled having been probably the last to form. But the 300 years during which the Moon has been watched with the telescope have afforded no trace of any continuance of these changes. She has had a stormy and fiery past; but nothing like the events of those bygone ages disturbs her serenity to-day.