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12 logic, and there is nothing in the world or beyond it to prevent, so far as we know, a being with gills, for example, from being a most superior person. A fish doubtless imagines life out of water to be impossible; and similarly, to argue that life of an order as high as our own, or higher, is impossible, because of less air to breathe than that to which we are locally accustomed, is, as Flammarion happily expresses it, to argue, not as a philosopher, Lea 2 Ot but as a fish. To sum up, now, what we know about the atmosphere of Mars: we have proof positive that Mars has an atmosphere; we have reason to believe that this atmosphere is very thin,- -thinner at least by half than the air upon the summit of the Himalayas,--that in constitution it does not differ greatly from our own, and that it is relatively heavily 1 toot Percival Lowell. [1] Both Jupiter and Saturn are ruddier than is commonly stated. In the air of Flagstaff, Arizona, the site of my observations, both of them show conspicuously red between their 4. belts. MARS. AFTER air, water. If Mars be capable of supporting life, there must be water upon his surface; for to all forms of life water is as vital a matter as air. To all organisms water is absolutely essential. On the question of habitability, therefore, it becomes all-important to know whether there be water on Mars. Any one looking through a telescope at the planet, early last summer, would at once have been struck by the fact that its surface was diversified by markings in three colors,-- white, blue-green, and reddish-ochre; the white lying in a great oval at the top of the disc. The white oval was the south polar ice-cap. In this polar cap our water problem takes its rise. On the 31st of May, 1894, the south polar cap stretched, practically one unbroken waste of ice, over about fifty degrees of latitude; that is, it covered nearly the whole frigid zone. Although due, in all probability, to successive depositions of frost rather than snow, the result, both in appearance and in behavior, makes striking counterpart to the antarctic ice- sheet of our own earth. Its visible contour was almost perfectly elliptical, showing it to be, in truth, nearly circular. That it was already in active process of melting was evident from its slowly lessening size. It was the most interesting feature on the disc, being peculiarly well placed for observation, owing to the tilt of the polar axis; for the Martian south pole was at the time bowed toward the earth at an angle of 24°, a southern charged with water vapor. In the next paper I shall take up the question of water upon the planet. Il. THE WATER PROBLEM.