UntitledNos. 1 and 2 show the effect of the planet's rotation. Nos. 3 and 4depict quite different sections. Note the change in the polar snow-capsin the last two.][Illustration: FIG. 16.--THE MOON, AT NINE AND THREE-QUARTER DAYSNote the mysterious "rays" diverging from the almost perfectly circularcraters indicated by the arrows (Tycho, upper; Copernicus, lower), andalso the mountains to the right with the lunar dawn breaking on them.]We turn to Mars; and we must first make it clear why there is so muchspeculation about life on Mars, and why it is supposed that, if there_is_ life on Mars, it must be more advanced than life on the earth.Is there Life on Mars?The basis of this belief is that if, as we saw, all the globes in oursolar system are masses of metal that are cooling down, the smaller willhave cooled down before the larger, and will be further ahead in theirdevelopment. Now Mars is very much smaller than the earth, and must havecooled at its surface millions of years before the earth did. Hence, if
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