TO THE PRESIDENT OF THE MASSACHUSETTS INSTITUTE OF TECHNOLOGY TO MY COLLEAGUES THERE AND TO ITS STUDENT BODY TO WHOSE INTEREST AND ATTENTION THESE LECTURES ARE INDEBTED THEY ARE APPRECIATIVELY INSCRIBED “Si je n’étais pas devenu général en chef et l’instrument du sort d’un grand people, j’aurais couru les bureaux et les salons pour me mettre dans la dépendance de qui que ce fût, en qualité de ministre ou d’ambassadeur? Non, non! je me serais jeté dans l’étude des sciences exactes. J’aurais fait mon chemin dans la route des Galilée, des Newton. Et puisque j’ai réussi constamment dans mes grandes entreprises, eh bien, je me serais hautement distingué aussi par des travaux scientifiques. J’aurais laissé le souvenir de belles découvertes. Aucune autre gloire n’aurait pu tenter mon ambition.” —NAPOLEON Iᴱᴿ, QUOTED BY ARAGO. The substance of the following pages was written and presented in a university course of lectures before the Massachusetts Institute of Technology—in February and March of this year. The kind interest with which the lectures were received, not only by the students and professional bodies, but by the public, was followed by an immediate request from The Macmillan Company to issue them in book form, and as such they now appear. PERCIVAL LOWELL. BOSTON, MASS., May 29, 1909. CONTENTS CHAPTER PAGE I. BIRTH OF A SOLAR SYSTEM 1 II. EVIDENCE OF THE INITIAL CATASTROPHE IN OUR OWN CASE 31 III. THE INNER PLANETS 58 IV. THE OUTER PLANETS 94 V. FORMATION OF PLANETS 127 VI. A PLANET’S HISTORY—SELF-SUSTAINED STAGE 155 VII. A PLANET’S HISTORY—SUN-SUSTAINED STAGE 182 VIII. DEATH OF A WORLD 213 NOTES 1. METEOR ORBITS 241 2. DENSITIES OF THE PLANETS 243 3. VARIATION IN SPECTROSCOPIC SHIFT 243 4. ON THE PLANETS’ ORBITAL TILTS 244 5. PLANETS AND THEIR SATELLITE SYSTEMS 245 6. ON THE INDUCED CIRCULARITY OF ORBITS THROUGH COLLISION 250 7. CAPTURE OF SATELLITES 251 INDEX 253 LIST OF ILLUSTRATIONS PLATES