CHAPTER XXXI.

THE PHYSICAL CAUSE OF THE MOTION OF GLACIERS.—THE MOLECULAR THEORY.

Present State of the Question.—Heat necessary to the Motion of a Glacier.—Ice does not shear in the Solid State.—Motion of a Glacier _molecular_.—How Heat is transmitted through Ice.—Momentary Loss of Shearing Force.—The _Rationale_ of Regelation.—The Origin of “Crevasses.”—Effects of Tension.—Modification of Theory.—Fluid Molecules crystallize in Interstices.—Expansive Force of crystallizing Molecules a Cause of Motion.—Internal molecular Pressure the chief Moving Power.—How Ice can excavate a Rock Basin.—How Ice can ascend a Slope.—How deep River Valleys are striated across.—A remarkable Example in the Valley of the Tay.—How Boulders can he carried from a lower to a higher Level 514

APPENDIX.

I. Opinions expressed previous to 1864 regarding the Influence of the Eccentricity of the Earth’s Orbit on Climate 528

II. On the Nature of Heat-Vibrations 544

III. On the Reason why the Difference of Reading between a Thermometer exposed to direct Sunshine and One Shaded diminishes as we ascend in the Atmosphere 547

IV. Remarks on Mr. J. Y. Buchanan’s Theory of the Vertical Distribution of Temperature of the Ocean 550

V. On the Cause of the Cooling Effect produced on Solids by Tension 552

VI. The Cause of Regelation 554

VII. List of Papers which have appeared in Dr. A. Petermann’s _Geographische Mittheilungen_ relating to the Gulf-stream and Thermal Condition of the Arctic Regions 556

VIII. List of Papers by the Author to which Reference is made in this Volume 560

INDEX 563

LIST OF PLATES.

EARTH’S ORBIT WHEN ECCENTRICITY IS AT ITS SUPERIOR LIMIT _Frontispiece._

PLATE

I. SHOWING AGREEMENT BETWEEN THE SYSTEM OF OCEAN-CURRENTS AND WINDS _To face page_ 212

II. SHOWING HOW OPPOSING CURRENTS INTERSECT EACH OTHER 219

III. SECTION OF MID-ATLANTIC 222

IV. DIAGRAM REPRESENTING THE VARIATIONS OF ECCENTRICITY OF THE EARTH’S ORBIT 313

V. SHOWING PROBABLE PATH OF THE ICE IN NORTH-WESTERN EUROPE 449

VI. SHOWING PATH OF ICE ACROSS CAITHNESS 453

VII. MAP OF THE MIDLAND VALLEY (SCOTLAND), SHOWING BURIED RIVER CHANNELS 471