Page:Mars - Lowell.djvu/90

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only account for irregularities on the terminator, but they account also for irregularity in the plus or minus character of these irregularities. Clouds, therefore, are capable of explaining the case before us, although mountains are not.

From what we have just shown let us mark now just what clouds are here required to account for what we see. The clouds that cause depressions are those within the terminator,—those, that is, that form before sunset or after sunrise; while those that cause projections are those that gather after sunset or before sunrise. As the observed projections in this case exceed the depressions in number, we infer, then, that there are more clouds after nightfall than before it, and similarly more before daybreak than after it; next, as the average depression is greater than the average projection, we likewise infer that the day clouds lie at a higher altitude. Now, this is precisely what we should expect would be the case, just as it is the case on the Earth.

Of the other class of irregularities, the long and low, there were observed 196 projections and 346 depressions. The projections averaged 0”.136 in height; the depressions, 0”.125 in depth. Here, then, we have an opposite state of things from that with which we were confronted in the short and sharp class. Here, as compared with the projections, instead of relatively few