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Rh meridian throughout it as to bring the cap on the nearer side of the pole, and therefore show it to best advantage. What had certainly been there on the 12th was not there on the 13th. The ice-cap had disappeared.

No such occurrence has ever been chronicled before. It is the first time since man began to observe the planet that the ice-cap has completely disappeared. Hitherto it has been seen to diminish to a minimum of from 7° to 4°, and then begin to increase again. This last autumn, for the first time, it vanished entirely. The date of this occurrence was, in Martian chronology, about July 20. Evidently, for some reason unknown to us, it was a phenomenally hot season in the southern hemisphere of the planet.

Practically it never reappeared again during the season. That it did return occasionally, as a very small speck, was from time to time suspected, and doubtless did take place. Certainly it left for some time behind it a glimmer where it had been, due presumably to the moisture from its melting, still tarrying on the ground or lingering in the air. Otherwise, to all intents and purposes, where the polar ice-cap and polar sea had been was now one ochre stretch of desert.

Having thus followed to its vanishing point the polar cap, we will now return to it in the