But within a few years there has been a reaction from this extreme view of the lifelessness of the moon.  Observers tell us of clouds suddenly appearing and then melting to invisibility over volcanic craters; of evidences of an atmosphere, rare as compared with ours, yet manifest in its effects; of variations of color witnessed in certain places as the sunlight drifts over them at changing angles of incidence; of what seem to be immense fields of vegetation covering level ground, and of appearances indicating the existence of clouds of ice crystals and deposits of snow among the mountainous lunar landscapes.  Thus, in a manner, the moon is rehabilitated, and we are invited to regard its silvery beams not as the reflections of the surface of a desert, but as sent back to our eyes from the face of a world that yet has some slight remnants of life to brighten it.

The suggestion that there is an atmosphere lying close upon the shell of the lunar globe, filling the deep cavities that pit its face and penetrating to an unknown depth in its interior, recalls a speculation of the ingenious and entertaining Fontenelle, in the seventeenth century—­recently revived and enlarged upon by the author of one of our modern romances of adventure in the moon—­to the effect that the lunar inhabitants dwell beneath the surface of their globe instead of on the top of it.

Now, because of this widespread and continually increasing interest in the subject of other worlds, and on account of the many curious 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 Pluralite des Mondes Habites, 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?”