necessary to the highest forms of life, we still have reason to
believe that these same conditions prevail on thousands of other
worlds. The fact that we might find the conditions in millions of
other worlds unfavorable to life would not disprove the existence
of the latter on countless worlds differently situated.

Coming down now from the general question to the specific one, we
all know that the only worlds the conditions of which can 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.