The well-known fact that the continents, and especially the mountainous and denuded districts of our globe, reflect much more light than the surfaces covered by water, has led astronomers to suppose that the dark spots on Mars are produced by a liquid strongly absorbing the rays of light, like the liquids on the surface of the Earth. According to this theory the dark spots observed are supposed to be lakes, seas, and oceans, similar to our own seas and oceans, while the reddish and whitish surfaces separating these dark spots, are supposed to be islands, peninsulas, and continents. This supposition seems certainly to have a great deal of probability in its favor, although some of the lighter markings may have a different origin, and perhaps be due to vegetation; but no observer has yet seen in them any of the changes which ought to result from change of seasons. Some of the changes in the dark spots might also be attributed to the flooding or drying up of marshes and low land. The change which I have observed lately might be attributed to such a cause, especially as my observation was made shortly after the spring equinox of the northern hemisphere of Mars, which occurred on December 8th. Besides the dark spots just described, there are markings of a different character and appearance. Among the most conspicuous are two very brilliant white oval spots, which always occupy opposite sides of the planet. These two bright spots, which correspond very closely with the poles of rotation of Mars, have been called "polar spots." On account of the inclination of the axis of rotation of this planet to the ecliptic, it is rare that both of these spots are visible on the disk at the same time; and when this occurs, they are seen considerably foreshortened, as they are then both on the limb of the planet. Usually only one spot is visible, and it appears to its best advantage when the region to which it belongs attains its maximum of inclination towards the Earth.