form an eleventh moon but was prevented by the nearness
of Saturn itself. There is no evidence of life on Saturn.

THE MOON

Mars and Venus are therefore the only planets, besides the earth, on
which we may look for life; and in the case of Venus, the possibility is
very faint. But what about the moons which attend the planets? They
range in size from the little ten-miles-wide moons of Mars, to Titan, a
moon of Saturn, and Ganymede, a satellite of Jupiter, which are about
3,000 miles in diameter. May there not be life on some of the larger of
these moons? We will take our own moon as a type of the class.

A Dead World

The moon is so very much nearer to us than any other heavenly body that
we have a remarkable knowledge of it. In Fig. 14 you have a photograph,
taken in one of our largest telescopes, of part of its surface. In a
sense such a telescope brings the moon to within about fifty miles of
us. We should see a city like London as a dark, sprawling blotch on the
globe. We could just detect a Zeppelin or a Diplodocus as a moving speck
against the surface. But we find none of these things. It is true that a
few astronomers believe that they see signs of some sort of feeble life
or movement on the moon. Professor Pickering thinks that he can trace
some volcanic activity. He believes that there are areas of vegetation,
probably of a low order, and that the soil of the moon may retain a
certain amount of water in it. He speaks of a very thin atmosphere, and
of occasional light falls of snow. He has succeeded in persuading some
careful observers that there probably are slight changes of some kind
taking place on the moon.

[Illustration: FIG. 17.--A MAP OF THE CHIEF PLAINS AND CRATERS OF THE
MOON

The plains were originally supposed to be seas: hence the name "Mare."]

[Illustration: FIG. 18.--A DIAGRAM OF A STREAM OF METEORS SHOWING THE
EARTH PASSING THROUGH THEM] [Illustration: _Photo: Royal Observatory,
Greenwich._

FIG. 19.--COMET, September 29, 1908

Notice the tendency to form a number of tails. (See photograph below.)]

[Illustration: _Photo: Royal Observatory, Greenwich._

FIG. 20.--COMET, October 3, 1908

The process has gone further and a number of distinct tails can now be
counted.]

But there are many things that point to absence of air on the moon. Even
the photographs we reproduce tell the same story. The edges of the
shadows are all hard and black. If there had been an appreciable
atm