Yet it is well to remember the possibility that changes may at times take place in the nearer planets—the terrestrial planets, as they are commonly called, Mars, Venus, and Mercury—such as telescopic observation under favourable conditions might detect. Telescopists have, indeed, described apparent changes, lasting only for a short time, in the appearance of one of these planets, Mars, which may fairly be attributed to disturbances affecting its surface in no greater degree than the great Peruvian earthquakes have affected for a time the surface of our earth. For instance, the American astronomer Mitchel says that on the night of July 12, 1845, bright polar snows of Mars exhibited an appearance never noticed at any preceding or succeeding observation. In the very centre of the white surface appeared a dark spot, which retained its position during several hours: on the following evening not a trace of the spot could be seen. Again the same observer says that on the evening of August 30, 1845, he observed for the first time a small bright spot, nearly or quite round, projecting out of the lower side of the polar spot. “In the early part of the evening,” he says, “the small bright spot seemed to be partly buried in the large one. After the lapse of an hour or more my attention was again directed to the planet, when I was astonished to find a manifest change in the position of the small bright spot. It had apparently separated from the large spot, and the edges alone of the two were now in contact, whereas when first seen they overlapped by an amount quite equal to one-third of the diameter of the small one. This, however, was merely an optical phenomenon, for on the next evening the spots went through the same apparent changes as the planet went through the corresponding part of its rotation. But it showed the spots to be real ice masses. The strange part of the story is that in the course of a few days the smaller spot, which must have been a mass of snow and ice as large as Novaia Zemlia, gradually disappeared. Probably some great shock had separated an enormous field of ice from the polar snows, and it had eventually been broken up and its fragments carried away from the Arctic regions by currents in the Martian oceans. It appears to me that the study of our own earth, and of the changes and occasional convulsions which affect its surface, gives to the observation of such phenomena as I have just described a new interest. Or rather, perhaps, it is not too much to say that the telescopic observations of the planets derive their only real interest from such considerations.