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15 everywhere but a few feet deep; which again is a most singular characteristic for seas that cover hundreds of thousands of square miles in extent,--seas, that is, as big as the Bay of Bengal. The Martian surface would have to be amazingly flat for this to be possible. We know it to be relatively flat, but to be as flat as all this would seem to pass the bounds of credible simplicity. Here also Professor W. H. Pickering's polariscope investigations come in with effect, for he found the light from the supposed seas to show no trace of polarization. Hence these were probably not water. In parenthesis we may here take notice of the absence of a certain phenomenon whose presence, apparently, should follow upon water surfaces such as the so-called seas would offer us. Although its absence is not perhaps definitive as to their marine character, it is certainly curious, and worth nothing. If a planet were covered by a sheet of water, that water surface would, mirror-like, reflect the sun in one more or less definite spot. Looked at from a distance, this spot would, were it bright enough, be seen as a high light on the dark background of the ocean about it. It would seem to be a fixed star at a certain point on the disc, the surface features rotating under it. The necessary position is easily calculated, and this shows that parts of the so-called seas, especially at oppositions like the last one, pass under the point. There remains merely the question of sufficient brilliancy in the spot for visibility; but as in the case of Mars its brilliancy should be equal to that of a star of the third magnitude, it would seem brilliant enough to be seen. No such starlike effect in such position has ever been noticed coming from the blue-green regions. From this bit of negative evidence, to be taken for what it is worth, we return again to what there is of a positive sort. Not only do different parts of the so-called seas contrast in tint with one another, but the same part of the same sea varies in tint at different times. Schiaparelli noticed that, at successive oppositions, the same sea showed different degrees of darkness, and he suggested that the change in tone was dependent in some way upon the Martian seasons. see the tints occur consecutively. In consequence, we know not only that changes take place on the surface of Mars other than in the polar cap, and very conspicuous ones too, but that these are due to the changing seasons of the planet's year. We will now see what al 22 ¥ tate they look like. To the transubstantiation of changes of the sort it is a prime essential that the drawings from whose comparison the contrast appears should all have been made by the same person, at the same telescope, under as nearly as possible the same atmospheric conditions, since otherwise the personal equation of the observer, the impersonal inequalities of instruments, and the special atmosphere of the station play so large a part in the result as to mask that other factor in the case, any change in the planet itself. How easily this masking is accomplished appears from drawings made by different observers of the same Martian features at substantially the same moment. Several interesting specimens of such personal peculiarities may be seen by the curious in Flammarion's admirable thesaurus, La Planete Mars. In some of these likenesses of the planet it is pretty certain that Mars would never recognize himself. Observations at Flagstaff have demonstrated this to be the case, for it has been possible to a ee et Te cee ee da