Page:Mars - Lowell.djvu/89

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67
CLOUDS

sions which averaged 0.368 seconds of arc in depth. It will be noticed then, first, that the projections of this character exceeded in number the depressions of the same; secondly, that the average depth of the depressions exceeded the average height of the projections. Now, if the appearances had been due to mountains, both the number and size of the projections and of the depressions should have been substantially the same. They were emphatically neither. Consequently mountains fail to explain them. But there is another possible set of phenomena that will; namely, clouds. For, in the first place, clouds would cause apparent depressions and projections, since the light would linger on them as it does on mountain tops, and they would cast shadows as mountains do. But furthermore their two effects, of extending or curtailing the limit of vision along the terminator, would not necessarily be equal, as would be the case with hills. Because it is a peculiarity of mountains that they are attached to the soil, and are commonly permanencies; while clouds are not. The latter form and dissipate, dissipate and re-form, and their metamorphoses are phenomena depending upon the time of day. Consequently they may appear in one place at one time, in another the next; and what is no less important, they may form at different heights at different times. They therefore not