a great plain and some typical craters. There are thousands of
these craters, and some theories of their origin are explained on page
34.]

[Illustration: FIG. 15.--MARS

1} Drawings by Prof. Lowell to accompany actual photographs of Mars
showing many of the
2} canals. Taken in 1907 by Mr. E. C. Slipher of the Lowell Observatory.
3 Drawing by Prof. Lowell made January 6, 1914.
4 Drawing by Prof. Lowell made January 21, 1914.

Nos. 1 and 2 show the effect of the planet's rotation. Nos. 3 and 4
depict quite different sections. Note the change in the polar snow-caps
in the last two.]

[Illustration: FIG. 16.--THE MOON, AT NINE AND THREE-QUARTER DAYS

Note the mysterious "rays" diverging from the almost perfectly circular
craters indicated by the arrows (Tycho, upper; Copernicus, lower), and
also 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 much
speculation 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 our
solar system are masses of metal that are cooling down, the smaller will
have cooled down before the larger, and will be further ahead in their
development. Now Mars is very much smaller than the earth, and must have
cooled at its surface millions of years before the earth did. Hence, if
a story of life began on Mars at all, it began long before the story of
life on the earth. We cannot guess what sort of life-forms would be
evolved in a different world, but we can confidently say that they would
tend toward increasing intelligence; and thus we are disposed to look
for highly intelligent beings on Mars.

But this argument supposes that the conditions of life, namely air and
water, are found on Mars, and it is disputed whether they are found
there in sufficient quantity. The late Professor Percival Lowell, who
made a lifelong study of Mars, maintained that there are hundreds of
straight lines drawn across the surface of the planet, and he claimed
that they are beds of vegetation marking the sites of great channels or
pipes by means of which the "Martians" draw water from their polar
ocean. Professor W. H. Pickering, another high authority, thinks that
the lines are long, narrow marshes fed by moist winds from the poles.
There are certainly white po