II. —THE PROBLEM STATED 21 III. —THE FIRE-MIST 39 IV. —NEBULÆ—APPARENT AND REAL 52 V. —THE HEAT OF THE SUN 75 VI. —HOW THE SUN’S HEAT IS MAINTAINED 95 VII. —THE HISTORY OF THE SUN 112 VIII. —THE EARTH’S BEGINNING 122 IX. —EARTHQUAKES AND VOLCANOES 158 X. —SPIRAL AND PLANETARY NEBULÆ 191 XI. —THE UNERRING GUIDE 207 XII. —THE EVOLUTION OF THE SOLAR SYSTEM 246 XIII. —THE UNITY OF MATERIAL IN THE HEAVENS 261 AND THE EARTH XIV. —THE FIRST CONCORD 294 XV. —THE SECOND CONCORD 308 XVI. —THE THIRD CONCORD 324 XVII. —OBJECTIONS TO THE NEBULAR THEORY 337 XVIII. —THE BEGINNING OF THE NEBULA 348 XIX. —CONCLUDING CHAPTER 361 APPENDICES 369 INDEX 382 ------------------------------------------------------------------------ LIST OF ILLUSTRATIONS FIG. PAGE An English Sunset tinged by Krakatoa _Frontispiece_ (colour) 1. Immanuel Kant (from an old print) 7 2. A Faint Diffused Nebulosity 17 3. The Crab Nebula 19 4. Jupiter 25 5. Nebulous Region and Star-cluster 33 6. The Great Nebula in Orion 41 7. The Dumb-bell Nebula 45 8. The Crossley Reflector 49 9. The Cluster in Hercules 53 10. Spectra of the Sun and Capella 62 11. Spectrum of Nebula in Orion and Spectrum 64 of White Star 12. Solar Spectra with Bright Lines and Dark 69 Lines during Eclipse 13. The Nebulæ in the Pleiades 71 14. The Sun 81 15. I. Spectrum of the Sun. II. Spectrum of 85 Arcturus 16. Brooks’ Comet and Meteor Trail 89 17. Argus and the surrounding Stars and 103 Nebulosity 18. Trifid Nebula in Sagittarius 105 19. To illustrate the History of the Sun 113