If Mars is
inhabited, and if the people of that planet have equal powers with
ourselves, the problem of merely producing an illumination which
could be seen in our most powerful telescope would be beyond all
the ordinary efforts of an entire nation. An unbroken square mile
of flame would be invisible in our telescopes, but a hundred
square miles might be seen. We cannot, therefore, expect to see
any signs of the works of inhabitants even on Mars. All that we
can do is to ascertain with greater or less probability whether
the conditions necessary to life exist on the other planets of the
system.
The moon being much the nearest to us of all the heavenly bodies,
we can pronounce more definitely in its case than in any other. We
know that neither air nor water exists on the moon in quantities
sufficient to be perceived by the most delicate tests at our
command. It is certain that the moon's atmosphere, if any exists,
is less than the thousandth part of the density of that around us.
The vacuum is greater than any ordinary air-pump is capable of
producing. We can hardly suppose that so small a quantity of air
could be of any benefit whatever in sustaining life; an animal
that could get along on so little could get along on none at all.
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