Page:Mars - Lowell.djvu/163

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With the familiar melting of the snow-cap begins the yearly round of the planet’s life. With the melting of our own arctic or antarctic cap might similarly be said to begin the earth’s annual activity. But here at the very outset there appears to be one important difference between the two planets. On the earth the relation of the melting of the polar snows to the awakening of surface activity is a case of post hoc simply; on Mars it seems to be a case of propter hoc as well. For, unlike the earth, which has water to spare, and to which, therefore, the unlocking of its polar snows is a matter of no direct economic value, Mars is apparently in straits for the article, and has to draw on its polar reservoir for its annual supply. Upon the melting of its polar cap, and the transference of the water thus annually set free to go its rounds, seem to depend all the seasonal phenomena on the surface of the planet.

The observations upon which this deduction is based extend over a period of nearly six months, from the last day of May to the 22d of November. They cover the regions from the south pole to about latitude forty north. That changes analogous to those recorded, differing, however, in details, occur six Martian months later in the planet’s northern hemisphere, is proved by what Schiaparelli has seen; for though the general system is, curiously, one for