louds, the lunar astronomer in looking at our earth
would often see hardly anything but water in one form or other. Very
likely he would come to the conclusion that our globe was only fitted to
be a residence for amphibious animals.

But when we look at the moon with our telescopes we see no direct
evidence of water. Close inspection shows that the so-called lunar seas
are deserts, often marked with small craters and rocks. The telescope
reveals no seas and no oceans, no lakes and no rivers. Nor is the
grandeur of the moon's scenery ever impaired by clouds over her surface.
Whenever the moon is above our horizon, and terrestrial clouds are out
of the way, we can see the features of our satellite's surface with
distinctness. There are no clouds in the moon; there are not even the
mists or the vapours which invariably arise wherever water is present,
and therefore astronomers have been led to the conclusion that the
surface of the globe which attends the earth is a sterile and a
waterless desert.

Another essential element of organic life is also absent from the moon.
Our globe is surrounded with a deep clothing of air resting on the
surface, and extending above our heads to the height of about 200 or 300
miles. We need hardly say how necessary air is to life, and therefore we
turn with interest to the question as to whether the moon can be
surrounded with an atmosphere. Let us clearly understand the problem we
are about to consider. Imagine that a traveller started from the earth
on a journey to the moon; as he proceeded, the air would gradually
become more and more rarefied, until at length, when he was a few
hundred miles above the earth's surface, he would have left the last
perceptible traces of the earth's envelope behind him. By the time he
had passed completely through the atmosphere he would have advanced only
a very small fraction of the whole journey of 240,000 miles, and there
would still remain a vast void to be traversed before the moon would be
reached. If the moon were enveloped in the same way as the earth, then,
as the traveller approached the end of his journey, and came within a
few hundred miles of the moon's surface, he would meet again with traces
of an atmosphere, which would gradually increase in density until he
arrived at the moon's surface. The traveller would thus have passed
through one stratum of air at the beginning of his journey, and through
another at the end, while the main portion of