Page:Mars - Lowell.djvu/55

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33
ATMOSPHERE

planet's surface at that time was tripartite. Upon the top part of the disk, round what we know to be the planet's pole, appeared to be a great white cap. This was the planet's south polar cap. The south lay at the top, because all astronomical views are, for optical reasons, upside down; but, inasmuch as we never see the features otherwise, to have them right side up is not vital to the effect. Below the white cap lay a region chiefly bluish-green, interspersed, however, with portions more or less reddish-ochre. Below this, again, came a vast reddish-ochre stretch.

The first sign of change occurred in the polar cap. It proceeded slowly to dwindle in size. Such self-obliteration it has, with praiseworthy regularity, been seen to undergo once every two years since it was first seen by man. For nearly two hundred years now, it has been observed to wax and wane with clock-like precision, a precision timed to the change of season in the planet's year. During the spring, these snow-fields, as analogy at once guesses them to be, and as beyond doubt they really are, stretch in the southern hemisphere, the one presented to us at this last opposition, down to latitude sixty-five south and even further, covering thus more than the whole of the planet's frigid zone. As summer comes on, they dwindle gradually away, till by early au-