A German critic, whom I have no reason to regard as specially favourable to me or it, makes the following remark on the style of the book: "This passion [for the mountains] tempts him frequently to reveal more of his Alpine wanderings than is necessary for his demonstrations. The reader, however, will not find this a disagreeable interruption of the course of thought; for the book thereby gains wonderfully in vividness." This, I would say, was the express aim of the breaks referred to. I desired to keep my companion fresh as well as instructed, and these interruptions were so many breathing-places where the intellectual tension was purposely relaxed and the mind of the pupil braced to fresh action. Of other criticisms, flattering and otherwise, I forbear to speak. As regards some of them, indeed, it would be a reproach to that manliness which I have sought to encourage in my pupil to return blow for blow. If the reader be acquainted with them, this will let him know how I regard them; and if he be not acquainted with them, I would recommend him to ignore them, and to form his own judgment of this book. No fair-minded person who reads it will dream that I, in writing it, had a thought of acting otherwise than justly and generously towards my predecessors, the last of whom, to the grief of all who knew him, has recently passed away. John Tyndall. _April, 1874._ CONTENTS. Cloud-banner of the Aiguille du Dru _Frontispiece_ PAGE § 1, 2. Clouds, Rains, and Rivers 1, 6 3. The Waves of Light 8 4. The Waves of Heat which produce the Vapour of our Atmosphere and melt our Glaciers 11 5. Experiments to prove the foregoing statements 14 6. Oceanic Distillation 19 7. Tropical Rains 23 8. Mountain Condensers 27 9. Architecture of Snow 29 10. Atomic Poles 32 11. Architecture of Lake Ice 35 12. The Source of the Arveiron. Ice Pinnacles, Towers, and Chasms of the Glacier des Bois. Passage to the Montanvert 38 13. The Mer de Glace and its Sources. Our First Climb to the Cleft Station 43 14. Ice-cascade and Snows of the Col du Géant 46 15. Questioning the Glaciers 48 16. Branches and Medial Moraines of the Mer de Glace from the Cleft Station 51