Compared to Mars the Moon offers a scene of far greater desolation. On Mars, we observe at least some considerable changes such as the disappearance of the white pole-caps at midsummer when at the same time a dark ring appears to surround them; then the “lakes” and the “canals” come into view, beginning close to the ring mentioned, later on nearer the equator, and finally on its other side, while the opposite pole-cap puts on its winter hue. Again, we have the sudden appearance, and equally hasty disappearance, of white spots, particularly in the neighbourhood of the lakes, and the sand storms which hide the surface of Mars and often fill its canals. The abruptness of the changes indicate that they are confined to a very thin surface layer. The formation, on the other hand, of canals, for many years unobserved, must be ascribed to a volcanic activity which, while feeble, yet must be seated in the deeper portions of the planet. In addition, a stunted vegetation of low forms is not unthinkable in the polar regions. As against this, the Moon is undoubtedly a stellar body entirely insusceptible of surface change. Near its centre, it is probably not completely solidified and an extremely slow growth of the firm crust is therefore likely. Gases are no doubt set free during this process, but they are unable to penetrate the enclosing thick armour and remain therefore as bubbles in the hardening magma. As a matter of fact, no changes have, with certainty, been detected on the Moon’s surface. It is true that the great Wm. Herschel, known as an excellent observer, believed that he discovered, in 1873, mountains which had not existed before that time, and Schröter, who diligently studied the lunar surface, was of the opinion that he too had discerned numerous changes. These discoveries, however, were doubted by careful critics, and after the publication of Mädler’s great work about the Moon (1837) the complete stagnation on that body was taken for granted. Nevertheless, there are several astronomers, such as Schmidt in Athens (1866) and lately W. H. Pickering in Cambridge, Mass., who think that they have discerned considerable modifications. The former held that the crater Linné (Fig. 27) had vanished since the publication of Mädler’s work. In 1867, Mädler, himself, proclaimed that it had the same appearance as before. Pickering, again, reports periodic changes of “snow” and “vegetation.” (Compare Fig. 27 taken from Pickering’s Moon-atlas.) Closer analysis, however, indicates that the phenomena are probably only apparent and depend on the angle of illumination at each particular time of observation. For some time, rather more than a quarter of a century, photography has been pressed into the service of lunar investigation with far more objective results than would be possible through direct ocular inspection alone. During this period, which, it must be admitted, is not very long, no distinct signs of