be made the
subject of observation are the planets which revolve around the sun,
and their satellites. The question whether these bodies are inhabited
is one which, of course, completely transcends not only our powers of
observation at present, but every appliance of research that we can
conceive of men devising. 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.

But the proof of the absence of life is yet stronger when we consider
the results of actual telescopic observation. An object such as an
ordinary city block could be detected on the moon. If anything like
vegetation were present on its surface, we should see the changes which
it would undergo in the course of a month, during one portion of which
it would be exposed to the rays of the unclouded sun, and during
another to the intense cold of space. If men built cities, or even
separate buildings the size of the larger ones on our earth, we might
see some signs of them.

In recent times we not only observe the moon with the telescope, but
get still more definite information by photography. The whole visible
surface has been repeatedly photographed under the best conditions. But
no change has been established beyond question, nor does the photograph
show the slightest difference of s