Page:Mars - Lowell.djvu/83

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61
CLOUDS

to appear again at the next opposition. Still smaller bright spots, apparently more fugitive, have been seen this year by Professor W. H. Pickering, notably just north of the Mare Sirenum. None of the phenomena look distinctively like cloud. There are, however, phenomena that do.

Toward the end of August there were seen several times, first by Professor Pickering and then by me, strange flocculent collections of white patches, about fifteen degrees from the pole, in the place where the snow-cap had been, the cap itself having retreated farther south. In look they were unlike the snow-cap; and also unlike the land. But they did have very much the look of clouds. Possibly they were clouds, formed from the vapor left in the air by the melting of the cap. it was then but a few days to the summer solstice.

But the most marked instance of variability was detected in September last by Mr. Douglass in the western part of Elysium. On September 22 and 23 he found this blissfully bright region, as usual, equally bright throughout. But on September 24 he noticed that the western half of it had suddenly increased in brightness, and far outshone the eastern half, being almost as brilliant as the polar cap. When he looked at it again the next night, September 25, the effect of the night before