revelations that we owe to modern telescopes and other improved means of
investigation, it is certainly to be desired that the most important and
interesting discoveries that have lately been made concerning the
various globes which together with the earth constitute the sun's
family, should be assembled in a convenient and popular form--and that
is the object of this book. Fact is admittedly often stranger and more
wonderful than fiction, and there are no facts that appeal more
powerfully to the imagination than do those of astronomy. Technical
books on astronomy usually either ignore the subject of the habitability
of the planets, or dismiss it with scarcely any recognition of the
overpowering human interest that it possesses. Hence, a book written
specially from the point of view of that subject would appear calculated
to meet a popular want; and this the more, because, since Mr. Proctor
wrote his Other Worlds than Ours and M. Flammarion his Pluralité des
Mondes Habités, many most important and significant discoveries have
been made that, in several notable instances, have completely altered
the aspect in which the planets present themselves for our judgment as
to their conditions of habitability.

No doubt the natural tendency of the mind is to regard all the planets
as habitable worlds, for there seems to be deeply implanted in human
nature a consciousness of the universality of life, giving rise to a
conviction that one world, even in the material sense, is not enough for
it, but that every planet must belong to its kingdom. We are apt to say
to ourselves: "The earth is one of a number of planets, all similarly
circumstanced; the earth is inhabited, why should not the others also be
inhabited?"

What has been learned of the unity in chemical constitution and
mechanical operation prevailing throughout the solar system, together