PREFACE.

IN the following pages I have endeavoured to give a full and concise statement of the facts and arguments adduced in support of the theory of Secular Changes of the Earth's Climate. Considerable portions of the volume have already appeared in substance as separate papers in the Philosophical Magazine and other journals during the past ten or twelve years. The theory, especially in as far as it relates to the cause of the glacial epoch, appears to be gradually gaining acceptance with geologists. This, doubtless, is owing to the greatly increased and constantly increasing knowledge of the drift-phenomena, which has induced the almost general conviction that a climate such as that of the glacial epoch could only have resulted from cosmical causes.

Considerable attention has been devoted to objections, and to the removal of slight misapprehensions, which have naturally arisen in regard to a subject comparatively new and, in many respects, complex, and beset with formidable difficulties.

I have studiously avoided introducing anything of a hypothetical character. All the conclusions are based either on known facts or admitted physical principles. In short, the aim of the work, as will be shown in the introductory chapter, is to

prove that secular changes of climate follow, as a necessary effect, from admitted physical agencies, and that these changes, in as far as the past climatic condition of the globe is concerned, fully meet the demand of the geologist.

The volume, though not intended as a popular treatise, will be found, I trust, to be perfectly plain and intelligible even to readers not familiar with physical science.

I avail myself of this opportunity of expressing my obligations to my colleagues, Mr. James Geikie, Mr. Robert L. Jack, Mr. Robert Etheridge, jun., and also to Mr. James Paton, of the Edinburgh Museum of Science and Art, for their valuable assistance rendered while these pages were passing through the press. To the kindness of Mr. James Bennie I am indebted for the copious index at the end of the volume, as well as for many of the facts relating to the glacial deposits of the West of Scotland.

JAMES CROLL.

EDINBURGH, March, 1875.

CONTENTS.

CHAPTER I.

INTRODUCTION.

The Fundamental Problem of Geology.-Geology a Dynamical Science.—

The Nature of a Geological Principle.-Theories of Geological Climate.

-Geological Climate dependent on Astronomical Causes.-An Impor-

tant Consideration overlooked.-Abstract of the Line of Argument pur-

sued in the Volume

PAGE

CHAPTER II.

OCEAN-CURRENTS IN RELATION TO THE DISTRIBUTION OF HEAT OVER

THE GLOBE.

The absolute Heating-power of Ocean-currents.-Volume of the Gulf-

stream.-Absolute Amount of Heat conveyed by it.-Greater Portion

of Moisture in Inter-tropical Regions falls as Rain in those Regions.-

Land along the Equator tends to lower the Temperature of the Globe.

-Influence of Gulf-stream on Climate of Europe.-Temperature of

Space.-Radiation of a Particle.-Professor Dove on Normal Tem-

perature.-Temperature of Equator and Poles in the Absence of

Ocean-currents.-Temperature of London, how much due to Ocean-

currents

23

CHAPTER III.

OCEAN-CURRENTS IN RELATION TO THE DISTRIBUTION OF HEAT OVER

THE GLOBE.Continued.)

Influence of the Gulf-stream on the Climate of the Arctic Regions.-Abso-

lute Amount of Heat received by the Arctic Regions from the Sun.-

Influence of Ocean-currents shown by another Method.-Temperature

of a Globe all Water or all Land according to Professor J. D. Forbes.

-An important Consideration overlooked.-Without Ocean-currents

the Globe would not be habitable.-Conclusions not affected by Imper-

fection of Data

45

CHAPTER IV.

OUTLINE OF THE PHYSICAL AGENCIES WHICH LEAD TO SECULAR

CHANGES OF CLIMATE.

Eccentricity of the Earth's Orbit; its Effect on Climate.-Glacial Epoch

not the direct Result of an Increase of Eccentricity.-An important

Consideration overlooked.-Change of Eccentricity affects Climate only

indirectly. Agencies which are brought into Operation by an Increase

of Eccentricity.-How an Accumulation of Snow is produced.-The

Effect of Snow on the Summer Temperature.-Reason of the Low

Summer Temperature of Polar Regions.-Deflection of Ocean-currents

the chief Cause of Secular Changes of Climate.-How the foregoing

Causes deflect Ocean-currents.-Nearness of the Sun in Perigee a

Cause of the Accumulation of Ice.-A remarkable Circumstance re-

garding the Causes which lead to Secular Changes of Climate.-The

primary Cause an Increase of Eccentricity.-Mean Temperature of

whole Earth should be greater in Aphelion than in Perihelion.-Pro-

fessor Tyndall on the Glacial Epoch.-A general Reduction of Tem-

perature will not produce a Glacial Epoch.-Objection from the present

Condition of the Planet Mars

PAGE

54

CHAPTER V.

REASON WHY THE SOUTHERN HEMISPHERE IS COLDER THAN THE

NORTHERN.

Adhémar's Explanation.-Adhémar's Theory founded upon a physical Mis-

take in regard to Radiation.-Professor J. D. Forbes on Underground

Temperature. Generally accepted Explanation.-Low Temperature

of Southern Hemisphere attributed to Preponderance of Sea.-Heat

transferred from Southern to Northern Hemisphere by Ocean-current

the true Explanation.-A large Portion of the Heat of the Gulf-stream

derived from the Southern Hemisphere

81

EXAMINATION OF THE

CHAPTER VI.

GRAVITATION THEORY OF OCEANIC CIRCULA-

TION. LIEUT. MAURY'S THEORY.

Introduction.-Ocean-currents, according to Maury, due to Difference of

Specific Gravity.-Difference of Specific Gravity resulting from Dif-

ference of Temperature.-Difference of Specific Gravity resulting from

Difference of Saltness.-Maury's two Causes neutralize each other.---

How, according to him, Difference in Saltness acts as a Cause

95

CHAPTER VII.

EXAMINATION OF THE GRAVITATION THEORY OF OCEANIC CIRCULA-

TION.-LIEUT. MAURY'S THEORY.-(Continued.)

Methods of determining the Question.-The Force resulting from Difference

of Specific Gravity.-Sir John Herschel's Estimate of the Force.-

Maximum Density of Sea-Water.-Rate of Decrease of Temperature

of Ocean at Equator.-The actual Amount of Force resulting from

Difference of Specific Gravity.-M. Dubuat's Experiments.

CHAPTER VIII.

PAGE

EXAMINATION OF THE GRAVITATION THEORY OF OCEANIC CIRCU-

LATION. DR. CARPENTER'S THEORY.

Gulf-stream according to Dr. Carpenter not due to Difference of Specific

Gravity.-Facts to be Explained.-The Explanation of the Facts.-

The Explanation hypothetical.-The Cause assigned for the hypo-

thetical Mode of Circulation.-Under Currents account for all the

Facts better than the Gravitation Hypothesis.-Known Condition of

the Ocean inconsistent with that Hypothesis

CHAPTER IX.

115

122

EXAMINATION OF THE GRAVITATION THEORY OF OCEANIC CIRCULA-

TION. THE MECHANICS OF DR. CARPENTER'S THEORY.

Experimental Illustration of the Theory.-The Force exerted by Gravity.

-Work performed by Gravity.-Circulation not by Convection.-

Circulation depends on Difference in Density of the Equatorial and

Polar Columns.-Absolute Amount of Work which can be performed

by Gravity. How Underflow is produced.-How Vertical Descent at

the Poles and Ascent at the Equator is produced. The Gibraltar

Current.-Mistake in Mechanics concerning it.-The Baltic Current. 145

CHAPTER X.

CIRCULA-

EXAMINATION OF THE GRAVITATION THEORY OF OCEANIC

TION. DR. CARPENTER'S THEORY.-OBJECTIONS CONSIDERED.

Modus Operandi of the Matter.-Polar Cold considered by Dr. Carpenter

the Primum Mobile.-Supposed Influence of Heat derived from the

Earth's Crust.-Circulation without Difference of Level.-A Confusion

of Ideas in Reference to the supposed Agency of Polar Cold.-M.

Dubuat's Experiments.-A Begging of the Question at Issue.-

Pressure as a Cause of Circulation

172