New York D. Appleton and Company 1901 Copyright, 1901, by D. Appleton and Company. TO THE MEMORY OF WILLIAM JAY YOUMANS. PREFACE The point of view of this book is human interest in the other worlds around us. It presents the latest discoveries among the planets of the solar system, and shows their bearing upon the question of life in those planets. It points out the resemblances and the differences between the earth and the other worlds that share with it in the light of the sun. It shows what we should see and experience if we could visit those worlds. While basing itself upon facts, it does not exclude the discussion of interesting probabilities and theories that have commanded wide popular attention. It points out, for instance, what is to be thought of the idea of interplanetary communication. It indicates what must be the outlook of the possible inhabitants of some of the other planets toward the earth. As far as may be, it traces the origin and development of the other worlds of our system, and presents a graphic picture of their present condition as individuals, and of their wonderful contrasts as members of a common family. In short, the aim of the author has been to show how wide, and how rich, is the field of interest opened to the human mind by man's discoveries concerning worlds, which, though inaccessible to him in a physical sense, offer intellectual conquests of the noblest description. And, finally, in order to assist those who may wish to recognize for themselves these other worlds in the sky, this book presents a special series of charts to illustrate a method of finding the planets which requires no observatory and no instruments, and only such knowledge of the starry heavens as anybody can easily acquire. G.P.S. BOROUGH OF BROOKLYN, NEW YORK CITY, _September, 1901._ CONTENTS CHAPTER I _INTRODUCTORY_ 1 Remarkable popular interest in questions concerning other worlds and their inhabitants--Theories of interplanetary communication--The plurality of worlds in literature--Romances of foreign planets--Scientific interest in the subject--Opposing views based on telescopic and spectroscopic revelations--Changes of opinion--Desirability of a popular presentation of the latest facts--The natural tendency to regard other planets as habitable--Some of the conditions and limitations of the problem--The solar system viewed from outer space--The resemblances and contrasts of its various planets--Three planetary groups recognized--The family character of the solar system CHAPTER II _MERCURY, A WORLD OF TWO FACES AND MANY CONTRASTS_ 18