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air, water. If Mars be capable of supporting life, there must be water upon his surface; for, to all forms of life, water is as vital a matter as air. On the question of habitability, therefore, it becomes all-important to know whether there be water on Mars.

To the solution of this inquiry, also, the planet's polar cap turns out to hold the key. For just as the fact of change in the cap proves the presence of air, so the manner of that change implies the presence of water. It not only does this; it turns out to do a deal more. For to the whole water question it appears to play the part not only of occasion but of cause. In more senses than one, it is in that great glistening white patch that our water problem takes its rise.

On the 3d of June, 1894, the south polar cap stretched, almost one unbroken waste of white, over about 55 degrees of latitude. A degree on Mars measures 37 miles; consequently the