lar caps on Mars. They seem to melt in the
spring, and the dark fringe round them grows broader.

Other astronomers, however, say that they find no trace of water-vapour
in the atmosphere of Mars, and they think that the polar caps may be
simply thin sheets of hoar-frost or frozen gas. They point out that, as
the atmosphere of Mars is certainly scanty, and the distance from the
sun is so great, it may be too cold for the fluid water to exist on the
planet.

If one asks why our wonderful instruments cannot settle these points,
one must be reminded that Mars is never nearer than 34,000,000 miles
from the earth, and only approaches to this distance once in fifteen or
seventeen years. The image of Mars on the photographic negative taken in
a big telescope is very small. Astronomers rely to a great extent on the
eye, which is more sensitive than the photographic plate. But it is easy
to have differences of opinion as to what the eye sees, and so there is
a good deal of controversy.

In August, 1924, the planet will again be well placed for observation,
and we may learn more about it. Already a few of the much-disputed
lines, which people wrongly call "canals," have been traced on
photographs. Astronomers who are sceptical about life on Mars are often
not fully aware of the extraordinary adaptability of life. There was a
time when the climate of the whole earth, from pole to pole, was
semi-tropical for millions of years. No animal could then endure the
least cold, yet now we have plenty of Arctic plants and animals. If the
cold came slowly on Mars, as we have reason to suppose, the population
could be gradually adapted to it. On the whole, it is possible that
there is advanced life on Mars, and it is not impossible, in spite of
the very great difficulties of a code of communication, that our "elder
brothers" may yet flash across space the solution of many of our
problems.

Sec. 2

Jupiter and Saturn

Next to Mars, going outward from the sun, is Jupiter. Between Mars and
Jupiter, however, there are more than three hundred million miles of
space, and the older astronomers wondered why this was not occupied by a
planet. We now know that it contains about nine hundred "planetoids," or
small globes of from five to five hundred miles in diameter. It was at
one time thought that a planet might have burst into these fragments (a
theory which is not mathematically satisfactory), or it may be that the
material which is scatte