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114 the whole planet, the particular character of different parts of the surface alters the action there to some extent.

For an appreciation of the meaning of the changes, it is to be borne in mind throughout that the vernal equinox of Mars’ southern hemisphere occurred on April 7, 1894; the summer solstice of the same hemisphere on August 31; and its autumnal equinox on February 7, 1895.

On the 31st of May, therefore, it was toward the end of April on Mars. The south polar cap was, as we have seen, very large, and the polar sea in proportion. That the polar sea was the darkest and the bluest marking on the disk implies that it was, at least, the deepest body of water on the planet, whether the so-called seas were seas or not. But from the fact that it was quite wide,—350 miles,—and that it all eventually vanished, it can hardly have been very deep. Its relative appearance, therefore, casts a first doubt upon the fact that the others were seas at all. This polar sea plays deus ex machina to all that follows.

So soon as the melting of the snow was well under way, long straits, of deeper tint than their surroundings, made their appearance in the midst of the dark areas. I did not see them come, but as I afterward saw them go it is evident that they must have come. They were