Tilt of its Spectral Lines 89 Orbits of the Outer Planets 95 Drawing of Jupiter showing its Ellipticity 103 Two Drawings of Jupiter and its Wisps 105 Photograph of Jupiter, 1909 107 Diagram of Saturn’s Rings 113 The Tores of Saturn 114 Chart showing increasing Tilts of the Major Planets 131 Orbital Tilts and Eccentricities of Satellites 133 Masses of Planets and Satellites 136 Two Drawings of Jupiter and its “Great Red Spot” 164 Sun Spots 165 Photograph of a Sun Spot 166 The Volcano Colima, Mexico, March 24, 1903 169 Jukes Butte, a Denuded Laccolith, as seen from the Northwest 170 Ideal Section of a Laccolith 170 Earth as seen from above.—Photographed at an Altitude of 5500 Feet 183 Tracks of Sauropus Primævus 188 Adventures of a Heat Ray 193 Polar Caps of Mars at their Maxima and Minima 198 Glacial Map of Eurasia 200 Map showing the Glaciated Area of North America 201 Photograph of the Moon 205 Petrified Bridge, Third Petrified Forest, near Adamana, Arizona 210 Three Views of Venus, showing Agreement at Different Distances 220 Diagram of Libration in Longitude due to Rotation 222 Moon,—Full and Half 225 Diagram illustrating Molecular Motion in a Gas 227 Distribution of Molecular Velocities in a Gas 229 THE EVOLUTION OF WORLDS CHAPTER I BIRTH OF A SOLAR SYSTEM Astronomy is usually thought of as the study of the bodies visible in the sky. And such it largely is when the present state of the universe alone is considered. But when we attempt to peer into its past and to foresee its future, we find ourselves facing a new side of the heavens—the contemplation of the invisible there. For in the evolution of worlds not simply must the processes be followed by the mind’s eye, so short the span of human life, but they begin and end in what we cannot see. What the solar system sprang from, and what it will eventually become, is alike matter devoid of light. Out of darkness into darkness again: such are the bourns of cosmic action.