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

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 Important Consideration

overlooked.—Abstract of the Line of Argument pursued in the

Volume.

_The Fundamental Problem of Geology._—The investigation of the

successive changes and modifications which the earth’s crust has

undergone during past ages is the province of geology. It will be

at once admitted that an acquaintance with the agencies by means of

which those successive changes and modifications were effected, is of

paramount importance to the geologist. What, then, are those agencies?

Although volcanic and other subterranean eruptions, earthquakes,

upheavals, and subsidences of the land have taken place in all ages,

yet no truth is now better established than that it is not by these

convulsions and cataclysms of nature that those great changes were

effected. It was rather by the ordinary agencies that we see every day

at work around us, such as rain, rivers, heat and cold, frost and snow.

The valleys were not produced by violent dislocations, nor the hills

by sudden upheavals, but were actually carved out of the solid rock,

silently and gently, by the agencies to which we have referred. “The

tools,” to quote the words of Professor Geikie, “by which this great

work has been done are of the simplest and most every-day order—the

air, rain, frosts, springs, brooks, rivers, glaciers, icebergs, and the

sea. These tools have been at work from the earliest times of which

any geological record has been preserved. Indeed, it is out of the

accumulated chips and dust which they have made, afterwards hardened

into solid rock and upheaved, that the very framework of our continents

has been formed.”[1]

It will be observed—and this is the point requiring particular

attention—that the agencies referred to are the ordinary meteorological

or climatic agencies. In fact, it is these agencies which constitute

climate. The various peculiarities or modifications of climate result

from a preponderance of one or more of these agencies over the rest.

When heat, for example, predominates, we have a hot or tropical

climate. When cold and frost predominate, we have a rigorous or arctic

climate. With moisture in excess, we have a damp and rainy climate;

and so on. But this is not all. These climatic agencies are not only

the factors which carved out the rocky face of the globe into hill

and dale, and spread over the whole a mantle of soil; but by them are

determined the character of the _flora_ and _fauna_ which exist on

that soil. The flora and fauna of a district are determined mainly by

the character of the climate, and not by the nature of the soil, or

the conformation of the ground. It is from difference of climate that

tropical life differs so much from arctic, and both these from the life

of temperate regions. It is climate, and climate alone, that causes

the orange and the vine to blossom, and the olive to flourish, in the

south, but denies them to the north, of Europe. It is climate, and

climate alone, that enables the forest tree to grow on the plain, but

not on the mountain top; that causes wheat and barley to flourish on

the mainland of Scotland, but not on the steppes of Siberia.

Again, if we compare flat countries with mountainous, highlands with

lowlands, or islands with continents, we shall find that difference of

climatic conditions is the chief reason why life in the one differs

so much from life in the other. And if we turn to the sea we find

that organic life is there as much under the domain of climate as on

the land, only the conditions are much less complex. For in the case

of the sea, difference in the temperature of the water may be said

to constitute almost the only difference of climatic conditions.

If there is one fact more clearly brought out than another by the

recent deep-sea explorations, it is this, that nothing exercises so

much influence on organic life in the ocean as the temperature of the

water. In fact, so much is this the case, that warm zones were found

to be almost equivalent to zones of life. It was found that even the

enormous pressure at the bottom of the ocean does not exercise so much

influence on life as the temperature of the water. There are few, I

presume, who reflect on the subject that will not readily admit that,

whether as regards the great physical changes which are taking place on

the surface of our globe, or as regards the growth and distribution of

plant and animal life, the ordinary climatic agents are the real agents

at work, and that, compared with them, all other agencies sink into

insignificance.

It will also be admitted that what holds true of the present holds

equally true of the past. Climatic agents are not only now the most

important and influential; they have been so during all past geological

ages. They were so during the Cainozoic as much as during the present;

and there is no reason for supposing they were otherwise during the

remoter Mesozoic and Palæozoic epochs. They have been the principal

factors concerned in that long succession of events and changes which

have taken place since the time of the solidification of the earth’s

crust. The stratified rocks of the globe contain all the records which

now remain of their action, and it is the special duty of the geologist

to investigate and read those records. It will be at once admitted that

in order to a proper understanding of the events embodied in these

records, an acquaintance with the agencies by which they were produced

is of the utmost importance. In fact, it is only by this means that we

can hope to arrive at their rational explanation. A knowledge of the

agents, and of the laws of their operations, is, in all the physical

sciences, the means by which we arrive at a rational comprehension

of the effects produced. If we have before us some complex and

intricate effects which have been produced by heat, or by light, or by

electricity, &c., in order to understand them we must make ourselves

acquainted with the agents by which they were produced and the laws of

their action. If the effects to be considered be, for example, those of

heat, then we must make ourselves acquainted with this agent and its

laws. If they be of electricity, then a knowledge of electricity and

its laws becomes requisite.

This is no mere arbitrary mode of procedure which may be adopted in

one science and rejected in another. It is in reality a necessity of

thought arising out of the very constitution of our intellect; for the

objective law of the agent is the conception by means of which the

effects are subjectively united in a rational unity. We may describe,

arrange, and classify the effects as we may, but without a knowledge of

the laws of the agent we can have no rational unity. We have not got

the higher conception by which they can be _comprehended_. It is this

relationship between the effects and the laws of the agent, a knowledge

of which really constitutes a science. We might examine, arrange, and

describe for a thousand years the effects produced by heat, and still

we should have no science of heat unless we had a knowledge of the

laws of that agent. The effects would never be seen to be necessarily

connected with anything known to us; we could not connect them with

any rational principle from which they could be deduced _à priori_.

The same remarks hold, of course, equally true of all sciences, in

which the things to be considered stand in the relationship of cause

and effect. Geology is no exception. It is not like systematic botany,

a mere science of classification. It has to explain and account for

effects produced; and these effects can no more be explained without

a knowledge of the laws of the agents which produced them, than can

the effects of heat without a knowledge of the laws of heat. The only

distinction between geology and heat, light, electricity, &c., is,

that in geology the effects to be explained have almost all occurred

already, whereas in these other sciences effects actually taking place

have to be explained. But this distinction is of no importance to

our present purpose, for effects which have already occurred can no

more be explained without a knowledge of the laws of the agent which

produced them than can effects which are in the act of occurring. It

is, moreover, not strictly true that all the effects to be explained

by the geologist are already past. It falls within the scope of his

science to account for the changes which are at present taking place on

the earth’s crust.

No amount of description, arrangement, and classification, however

perfect or accurate, of the facts which come under the eye of the

geologist can ever constitute a science of geology any more than a

description and classification of the effects of heat could constitute

a science of heat. This will, no doubt, be admitted by every one who

reflects upon the subject, and it will be maintained that geology,

like every other science, must possess principles applicable to the

facts. But here confusion and misconception will arise unless there be

distinct and definite ideas as to what ought to constitute a geological

principle. It is not every statement or rule that may apply to a great

many facts, which will constitute a geological principle. A geological

principle must bear the same characteristics as the principles of those

sciences to which we have referred. What, then, is the nature of the

principles of light, heat, electricity, &c.? The principles of heat

are the laws of heat. The principles of electricity are the laws of

electricity. And these laws are nothing more nor less than the ways

according to which these agents produce their effects. The principles

of geology are therefore the laws of geology. But the laws of geology

must be simply the laws of the geological agents, or, in other words,

the methods by which they produce their effects. Any other so-called

principle can be nothing more than an empirical rule, adopted for

convenience. Possessing no rationality in itself, it cannot be justly

regarded as a principle. In order to rationality the principle must be

either resolvable into, or logically deducible from, the laws of the

agents. Unless it possess this quality we cannot give the explanation

_à priori_.

The reason of all this is perfectly obvious. The things to be explained

are effects; and the relationship between cause and effect affords the

subjective connection between the principle and the explanation. The

explanation follows from the principle simply as the effect results

from the laws of the agent or cause.

_Theories of Geological Climate._—We have already seen that the

geological agents are chiefly the ordinary climatic agents.

Consequently, the main principles of geology must be the laws of the

climatic agents, or some logical deductions from them. It therefore

follows that, in order to a purely scientific geology, the grand

problem must be one of geological climate. It is through geological

climate that we can hope to arrive ultimately at principles which will

afford a rational explanation of the multifarious facts which have

been accumulating during the past century. The facts of geology are

as essential to the establishment of the principles, as the facts of

heat, light, and electricity are essential to the establishment of the

principles of these sciences. A theory of geological climate devised

without reference to the facts would be about as worthless as a theory

of heat or of electricity devised without reference to the facts of

these sciences.

It has all along been an admitted opinion among geologists that the

climatic condition of our globe has not, during past ages, been

uniformly the same as at present. For a long time it was supposed that

during the Cambrian, Silurian, and other early geological periods, the

climate of our globe was much hotter than now, and that ever since

it has been gradually becoming cooler. And this high temperature of

Palæozoic ages was generally referred to the influence of the earth’s

internal heat. It has, however, been proved by Sir William Thomson[2]

that the general climate of our globe could not have been sensibly

affected by internal heat at any time more than ten thousand years

after the commencement of the solidification of the surface. This

physicist has proved that the present influence of internal heat on

the temperature amounts to about only 1/75th of a degree. Not only

is the theory of internal heat now generally abandoned, but it is

admitted that we have no good geological evidence that climate was much

hotter during Palæozoic ages than now; and much less, that it has been

becoming _uniformly_ colder.

The great discovery of the glacial epoch, and more lately that of a

mild and temperate condition of climate extending during the Miocene

and other periods to North Greenland, have introduced a complete

revolution of ideas in reference to geological climate. Those

discoveries showed that our globe has not only undergone changes of

climate, but changes of the most extraordinary character. They showed

that at one time not only an arctic condition of climate prevailed in

our island, but that the greater part of the temperate region down

to comparatively low latitudes was buried under ice, while at other

periods Greenland and the Arctic regions, probably up to the North

Pole, were not only free from ice, but were covered with a rich and

luxuriant vegetation.

To account for these extraordinary changes of climate has generally

been regarded as the most difficult and perplexing problem which has

fallen to the lot of the geologist. Some have attempted to explain

them by assuming a displacement of the earth’s axis of rotation in

consequence of the uprising of large mountain masses on some part

of the earth’s surface. But it has been shown by Professor Airy,[3]

Sir William Thomson,[4] and others, that the earth’s equatorial

protuberance is such that no geological change on its surface could

ever possibly alter the position of the axis of rotation to an extent

which could at all sensibly affect climate. Others, again, have tried

to explain the change of climate by supposing, with Poisson, that the

earth during its past geological history may have passed through hotter

and colder parts of space. This is not a very satisfactory hypothesis.

There is no doubt a difference in the quantity of force in the form of

heat passing through different parts of space; but space itself is not

a substance which can possibly be either cold or hot. If, therefore,

we were to adopt this hypothesis, we must assume that the earth during

the hot periods must have been in the vicinity of some other great

source of heat and light besides the sun. But the proximity of a

mass of such magnitude as would be sufficient to affect to any great

extent the earth’s climate would, by its gravity, seriously disarrange

the mechanism of our solar system. Consequently, if our solar system

had ever, during any former period of its history, really come into

the vicinity of such a mass, the orbits of the planets ought at the

present day to afford some evidence of it. But again, in order to

account for a cold period, such as the glacial epoch, we have to assume

that the earth must have come into the vicinity of a cold body.[5]

But recent discoveries in regard to inter-glacial periods are wholly

irreconcilable with this theory.

A change in the obliquity of the ecliptic has frequently been, and

still is, appealed to as an explanation of geological climate. This

theory appears, however, to be beset by a twofold objection: (1), it

can be shown from celestial mechanics, that the variations in the

obliquity of the ecliptic must always have been so small that they

could not materially affect the climatic condition of the globe; and

(2), even admitting that the obliquity could change to an indefinite

extent, it can be shown[6] that no increase or decrease, however great,

could possibly account for either the glacial epoch or a warm temperate

condition of climate in polar regions.

The theory that the sun is a variable star, and that the glacial

epochs of the geologists may correspond to periods of decrease in the

sun’s heat, has lately been advanced. This theory is also open to two

objections: (1), a general diminution of heat[7] never could produce

a glacial epoch; and (2), even if it could, it would not explain

inter-glacial periods.

The only other theory on the subject worthy of notice is that

of Sir Charles Lyell. Those extraordinary changes of climate are,

according to his theory, attributed to differences in the distribution

of land and water. Sir Charles concludes that, were the land all

collected round the poles, while the equatorial zones were occupied by

the ocean, the general temperature would be lowered to an extent that

would account for the glacial epoch. And, on the other hand, were the

land all collected along the equator, while the polar regions were

covered with sea, this would raise the temperature of the globe to an

enormous extent. It will be shown in subsequent chapters that this

theory does not duly take into account the prodigious influence exerted

on climate by means of the heat conveyed from equatorial to temperate

and polar regions by means of ocean-currents. In Chapters II. and III.

I have endeavoured to prove (1), that were it not for the heat conveyed

from equatorial to temperate and polar regions by this means, the

thermal condition of the globe would be totally different from what it

is at present; and (2), that the effect of placing all the land along

the equator would be diametrically the opposite of that which Sir

Charles supposes.

But supposing that difference in the distribution of land and water

would produce the effects attributed to it, nevertheless it would not

account for those extraordinary changes of climate which have occurred

during geological epochs. Take, for example, the glacial epoch.

Geologists almost all agree that little or no change has taken place

in the relative distribution of sea and land since that _epoch_. All

our main continents and islands not only existed then as they do now,

but every year is adding to the amount of evidence which goes to show

that so recent, geologically considered, is the glacial epoch that the

very contour of the surface was pretty much the same then as it is at

the present day. But this is not all; for even should we assume (1),

that a difference in the distribution of sea and land would produce the

effects referred to, and (2), that we had good geological evidence to

show that at a very recent period a form of distribution existed which

would produce the necessary glacial conditions, still the glacial

epoch would not be explained, for the phenomena of warm inter-glacial

periods would completely upset the theory.

_Geological Climate depending on Astronomical Causes._—For a good many

years past, an impression has been gradually gaining ground amongst

geologists that the glacial epoch, as well as the extraordinary

condition of climate which prevailed in arctic regions during the

Miocene and other periods, must some way or other have resulted from

a cosmical cause; but all seemed at a loss to conjecture what that

cause could possibly be. It was apparent that the cosmical cause must

be sought for in the relations of our earth to the sun; but a change

in the obliquity of the ecliptic and the eccentricity of the earth’s

orbit are the only changes from which any sensible effect on climate

could possibly be expected to result. It was shown, however, by Laplace

that the change of obliquity was confined within so narrow limits that

it has scarcely ever been appealed to as a cause seriously affecting

climate. The only remaining cause to which appeal could be made was

the change in the eccentricity of the earth’s orbit—precession of the

equinoxes without eccentricity producing, of course, no effect whatever

on climate. Upwards of forty years ago Sir John Herschel and a few

other astronomers directed their attention to the consideration of this

cause, but the result arrived at was adverse to the supposition that

change of eccentricity could greatly affect the climate of our globe.

As some misapprehension seems to prevail with reference to this, I

would take the liberty of briefly adverting to the history of the

matter,—referring the reader to the Appendix for fuller details.

About the beginning of the century some writers attributed the lower

temperature of the southern hemisphere to the fact that the sun remains

about seven days less on that hemisphere than on the northern; their

view being that the southern hemisphere on this account receives

seven days less heat than the northern. Sir Charles Lyell, in the

first edition of his “Principles,” published in 1830, refers to this

as a cause which might produce some slight effect on climate. Sir

Charles’s remarks seem to have directed Sir John Herschel’s attention

to the subject, for in the latter part of the same year he read a

paper before the Geological Society on the astronomical causes which

may influence geological phenomena, in which, after pointing out the

mistake into which Sir Charles had been led in concluding that the

southern hemisphere receives less heat than the northern, he considers

the question as to whether geological climate could be influenced by

changes in the eccentricity of the earth’s orbit. He did not appear at

the time to have been aware of the conclusions arrived at by Lagrange

regarding the superior limit of the eccentricity of the earth’s orbit;

but he came to the conclusion that possibly the climate of our globe

may have been affected by variations in the eccentricity of its orbit.

“An amount of variation,” he says, “which we need not hesitate to

admit (at least provisionally) as a possible one, may be productive

of considerable diversity of climate, and may operate during great

periods of time either to mitigate or to exaggerate the difference of

winter and summer temperatures, so as to produce alternately in the

same latitude of either hemisphere a perpetual spring, or the extreme

vicissitudes of a burning summer and a rigorous winter.”

This opinion, however, was unfortunately to a great extent nullified

by the statement which shortly afterwards appeared in his “Treatise

on Astronomy,” and also in the “Outlines of Astronomy,” to the effect

that the elliptic form of the earth’s orbit has but a very trifling

influence in producing variation of temperature corresponding to the

sun’s distance; the reason being that whatever may be the ellipticity

of the orbit, it follows that equal amounts of heat are received

from the sun in passing over equal angles round it, in whatever part

of the ellipse those angles may be situated. Those angles will of

course be described in unequal times, but the greater proximity of

the sun exactly compensates for the more rapid description, and thus

an equilibrium of heat is maintained. The sun, for example, is much

nearer the earth when he is over the southern hemisphere than he is

when over the northern; but the southern hemisphere does not on this

account receive more heat than the northern; for, owing to the greater

velocity of the earth when nearest the sun, the sun does not remain

so long on the southern hemisphere as he does on the northern. These

two effects so exactly counterbalance each other that, whatever be

the extent of the eccentricity, the total amount of heat reaching

both hemispheres is the same. And he considered that this beautiful

compensating principle would protect the climate of our globe from

being seriously affected by an increase in the eccentricity of its

orbit, unless the extent of that increase was very great.

“Were it not,” he says, “for this, the eccentricity of the orbit

would materially influence the transition of seasons. The fluctuation

of distance amounts to nearly 1/30th of its mean quantity, and

consequently the fluctuation in the sun’s direct heating power to

double this, or 1/15th of the whole. Now the perihelion of the orbit is

situated nearly at the place of the northern winter solstice; so that,

were it not for the compensation we have just described, the effect

would be to exaggerate the difference of summer and winter in the

southern hemisphere, and to moderate it in the northern; thus producing

a more violent alternation of climate in the one hemisphere, and an

approach to perpetual spring in the other. As it is, however, no such

inequality subsists, but an equal and impartial distribution of heat

and light is accorded to both.”[8]

Herschel’s opinion was shortly afterwards adopted and advocated by

Arago[9] and by Humboldt.[10]

Arago, for example, states that so little is the climate of our globe

affected by the eccentricity of its orbit, that even were the orbit to

become as eccentric as that of the planet Pallas (that is, as great as

0·24), “still this would not alter in any appreciable manner the mean

thermometrical state of the globe.”

This idea, supported by these great authorities, got possession of the

public mind; and ever since it has been almost universally regarded

as settled that the great changes of climate indicated by geological

phenomena could not have resulted from any change in the relation of

the earth to the sun.

There is, however, one effect that was not regarded as compensated. The

total amount of heat received by the earth is inversely proportional

to the minor axis of its orbit; and it follows, therefore, that the

greater the eccentricity, the greater is the total amount of heat

received by the earth. On this account it was concluded that an

increase of eccentricity would tend to a certain extent to produce a

warmer climate.

All those conclusions to which I refer, arrived at by astronomers, are

perfectly legitimate so far as the direct effects of eccentricity are

concerned; and it was quite natural, and, in fact, proper to conclude

that there was nothing in the mere increase of eccentricity that could

produce a glacial epoch. How unnatural would it have been to have

concluded that an increase in the quantity of heat received from the

sun should lower the temperature, and cover the country with snow and

ice! Neither would excessively cold winters, followed by excessively

hot summers, produce a glacial epoch. To assert, therefore, that the

purely astronomical causes could produce such an effect would be simply

absurd.

_Important Consideration overlooked._—The important fact, however, was

overlooked that, although the glacial epoch could not result _directly_

from an increase of eccentricity, it might nevertheless do so

_indirectly_. Although an increase of eccentricity could have no direct

tendency to lower the temperature and cover our country with ice, yet

it might bring into operation physical agents which would produce this

effect.

If, instead of endeavouring to trace a direct connection between a high

condition of eccentricity and a glacial condition of climate, we turn

our attention to the consideration of what are the physical effects

which result from an increase of eccentricity, we shall find that a

host of physical agencies are brought into operation, the combined

effect of which is to lower to a very great extent the temperature of

the hemisphere whose winters occur in aphelion, and to raise to nearly

as great an extent the temperature of the opposite hemisphere, whose

winters of course occur in perihelion. Until attention was directed to

those physical circumstances to which I refer, it was impossible that

the true cause of the glacial epoch could have been discovered; and,

moreover, many of the indirect and physical effects, which in reality

were those that brought about the glacial epoch, could not, in the

nature of things, have been known previously to recent discoveries in

the science of heat.

The consideration and discussion of those various physical agencies are

the chief aim of the following pages.

_Abstract of the Line of Argument pursued in this Volume._—I shall

now proceed to give a brief abstract of the line of argument pursued

in this volume. But as a considerable portion of it is devoted to the

consideration of objections and difficulties bearing either directly

or indirectly on the theory, it will be necessary to point out what

those difficulties are, how they arose, and the methods which have been

adopted to overcome them.

Chapter IV. contains an outline of the physical agencies affecting

climate which are brought into operation by an increase of

eccentricity. By far the most important of all those agencies, and the

one which mainly brought about the glacial epoch, is the _Deflection_

of Ocean-Currents. The consideration of the indirect physical

connection between a high state of eccentricity and the deflection

of ocean-currents, and also the enormous influence on climate which

results from this deflection constitute not only the most important

part of the subject, but the one beset with the greatest amount of

difficulties.

The difficulties besetting this part of the theory arise mainly from

the imperfect state of our knowledge, (1st) with reference to the

absolute amount of heat transferred from equatorial to temperate and

polar regions by means of ocean-currents and the influence which the

heat thus transferred has on the distribution of temperature on the

earth’s surface; and (2nd) in connection with the physical cause of

ocean circulation.

In Chapters II. and III. I have entered at considerable length into

the consideration of the effects of ocean currents on the distribution

of heat over the globe. The only current of which anything like

an accurate estimate of volume and temperature has been made is

the Gulf-stream. In reference to this stream we have a means of

determining in absolute measure the quantity of heat conveyed by it.

On the necessary computation being made, it is found that the amount

transferred by the Gulf-stream from equatorial regions into the North

Atlantic is enormously greater than was ever anticipated, amounting

to no less than one-fifth part of the entire heat possessed by the

North Atlantic. This striking fact casts a new light on the question

of the distribution of heat over the globe. It will be seen that to

such an extent is the temperature of the equatorial regions lowered,

and that of high temperate, and polar regions raised, by means of ocean

currents, that were they to cease, and each latitude to depend solely

on the heat received directly from the sun, only a very small portion

of the globe would be habitable by the present order of beings. This

being the case, it becomes obvious to what an extent the deflection

of ocean currents must affect temperature. For example, were the

Gulf-stream stopped, and the heat conveyed by it deflected into the

Southern Ocean, how enormously would this tend to lower the temperature

of the northern hemisphere, and raise the temperature south of the

equator.

Chapters VI., VII., VIII., IX., X., and XIII., are devoted to the

consideration of the physical cause of oceanic circulation. This has

been found to be the most difficult and perplexing part of the whole

inquiry. The difficulties mainly arise from the great diversity of

opinion and confusion of ideas prevailing in regard to the mechanics

of the subject. There are two theories propounded to account for

oceanic circulation; the one which may be called the _Wind_ theory, and

the other the _Gravitation_ theory; and this diversity of opinion and

confusion of ideas prevail in connection with both theories. As the

question of the cause of oceanic circulation has not only a direct and

important bearing on the subject of the present volume, but is further

one of much general interest, I have entered somewhat fully into the

matter.

The Gravitation theories may be divided into two classes. The first of

these attributes the Gulf-stream and other sensible currents of the

ocean to difference of specific gravity, resulting from difference

of temperature between the sea in equatorial and polar regions. The

leading advocate of this theory was the late Lieutenant Maury, who

brought it so much into prominence in his interesting book on the

“Physical Geography of the Sea.” The other class does not admit that

the sensible currents of the ocean can be produced by difference of

specific gravity; but they maintain that difference of temperature

between the sea in equatorial and polar regions produces a general

movement of the upper portion of the sea from the equator to the

poles, and a counter-movement of the under portion from the poles

to the equator. This form of the gravitation theory has been ably

and zealously advocated by Dr. Carpenter, who may be regarded as

its representative. The Wind theories also divide into two classes.

According to the one ocean currents are caused and maintained by the

impulse of the trade-winds, while according to the other they are

due not to the impulse of the trade-winds alone, but to that of the

prevailing winds of the globe, regarded as a general system. The former

of these is the one generally accepted; the latter is that advocated in

the present volume.

The relations which these theories bear to the question of secular

change of climate, will be found stated at length in Chapter VI. It

will, however, be better to state here in a few words what those

relations are. When the eccentricity of the earth’s orbit attains a

high value, the hemisphere, whose winter solstice occurs in aphelion,

has, for reasons which are explained in Chapter IV., its temperature

lowered, while that of the opposite hemisphere is raised. Let us

suppose the northern hemisphere to be the cold one, and the southern

the warm one. The difference of temperature between the equator and

the North Pole will then be greater than between the equator and the

South Pole; according, therefore, to theory, the trades of the northern

hemisphere will be stronger than those of the southern, and will

consequently blow across the equator to some distance on the southern

hemisphere. This state of things will tend to deflect equatorial

currents southwards, impelling the warm water of the equatorial regions

more into the southern or warm hemisphere than into the northern or

cold hemisphere. The tendency of all this will be to exaggerate the

difference of temperature already existing between the two hemispheres.

If, on the other hand, the great ocean currents which convey the warm

equatorial waters to temperate and polar regions be not produced by

the impulse of the winds, but by difference of temperature, as Maury

maintains, then in the case above supposed the equatorial waters would

be deflected more into the northern or cold hemisphere than into the

southern or warm hemisphere, because the difference of temperature

between the equator and the poles would be greater on the cold than

on the warm hemisphere. This, of course, would tend to neutralize or

counteract that difference of temperature between the two hemispheres

which had been previously produced by eccentricity. In short, this

theory of circulation would effectually prevent eccentricity from

seriously affecting climate.

Chapters VI. and VII. have been devoted to an examination of this form

of the gravitation theory.

The above remarks apply equally to Dr. Carpenter’s form of the theory;

for according to a doctrine of General Oceanic Circulation resulting

from difference of specific gravity between the water at the equator

and at the poles, the equatorial water will be carried more to the

cold than to the warm hemisphere. It is perfectly true that a belief

in a general oceanic circulation may be held quite consistently with

the theory of secular changes of climate, provided it be admitted

that not this general circulation but ocean currents are the great

agency employed in distributing heat over the globe. The advocates of

the theory, however, admit no such thing, but regard ocean currents

as of secondary importance. It may be stated that the existence of

this general ocean circulation has never been detected by actual

observation. It is simply assumed in order to account for certain

facts, and it is asserted that such a circulation must take place as

a physical necessity. I freely admit that were it not that the warm

water of equatorial regions is being constantly carried off by means

of ocean currents such as the Gulf-stream, it would accumulate till,

in order to restoration of equilibrium, such a general movement as is

supposed would be generated. But it will be shown that the warm water

in equatorial regions is being drained off so rapidly by ocean currents

that the actual density of an equatorial column differs so little

from that of a polar column that the force of gravity resulting from

that difference is so infinitesimal that it is doubtful whether it is

sufficient to produce sensible motion. I have also shown in Chapter

VIII. that all the facts which this theory is designed to explain are

not only explained by the wind theory, but are deducible from it as

necessary consequences. In Chapter XI. it is proved, by contrasting

the quantity of heat conveyed by ocean currents from inter-tropical to

temperate and polar regions with such an amount as could possibly be

conveyed by means of a general oceanic circulation, that the latter

sinks into insignificance before the former. In Chapters X. and XII.

the various objections which have been advanced by Dr. Carpenter and

Mr. Findlay are discussed at considerable length, and in Chapter IX.

I have entered somewhat minutely into an examination of the mechanics

of the gravitation theory. A statement of the wind theory is given in

Chapter XIII.; and in Chapter XIV. is shown the relation of this theory

to the theory of Secular changes of climate. This terminates the part

of the inquiry relating to oceanic circulation.

We now come to the _crucial test_ of the theories respecting the cause

of the glacial epoch, viz., Warm Inter-glacial Periods. In Chapters

XV. and XVI. I have given a statement of the geological facts which

go to prove that that long epoch known as the Glacial was not one

of continuous cold, but consisted of a succession of cold and warm

periods. This condition of things is utterly inexplicable on every

theory of the cause of the glacial epoch which has hitherto been

advanced; but, according to the physical theory of secular changes of

climate under consideration, it follows as a necessary consequence.

In fact, the amount of geological evidence which has already been

accumulated in reference to inter-glacial periods may now be regarded

as perfectly sufficient to establish the truth of that theory.

If the glacial epoch resulted from some accidental distribution of sea

and land, then there may or may not have been more than one glacial

epoch, but if it resulted from the cause which we have assigned, then

there must have been during the geological history of the globe a

succession of glacial epochs corresponding to the secular variations

in the eccentricity of the earth’s orbit. A belief in the existence

of recurring glacial epochs has been steadily gaining ground for many

years past. I have, in Chapter XVIII., given at some length the facts

on which this belief rests. It is true that the geological evidence of

glacial epochs in prior ages is meagre in comparison with that of the

glacial epoch of Post-tertiary times; but there is a reason for this in

the nature of geological evidence itself. Chapter XVII. deals with the

geological records of former glacial epochs, showing that they are not

only imperfect, but that there is good reason why they should be so,

and that the imperfection of the records in reference to them cannot be

advanced as an argument against their existence.

If the glacial epoch resulted from a high condition of eccentricity, we

have not only a means of determining the positive date of that epoch,

but we have also a means of determining geological time in absolute

measure. For if the glacial epochs of prior ages correspond to periods

of high eccentricity, then the intervals between those periods of high

eccentricity become the measure of the intervals between the glacial

epochs. The researches of Lagrange and Leverrier into the secular

variations of the elements of the orbits of the planets enable us

to determine with tolerable accuracy the values of the eccentricity

of the earth’s orbit for, at least, four millions of years past and

future. With the view of determining those values, I several years

ago computed from Leverrier’s formula the eccentricity of the earth’s

orbit and longitude of the perihelion, at intervals of ten thousand and

fifty thousand years during a period of three millions of years in the

past, and one million of years in the future. The tables containing

these values will be found in Chapter XIX. These tables not only give

us the date of the glacial epoch, but they afford, as will be seen

from Chapter XXI., evidence as to the probable date of the Eocene and

Miocene periods.

Ten years ago, when the theory was first advanced, it was beset by

a very formidable difficulty, arising from the opinions which then

prevailed in reference to geological time. One or two glacial epochs in

the course of a million of years was a conclusion which at that time

scarcely any geologist would admit, and most would have felt inclined

to have placed the last glacial epoch at least one million of years

back. But then if we assume that the glacial epoch was due to a high

state of eccentricity, we should be compelled to admit of at least two

glacial epochs during that lapse of time. It was the modern doctrine

that the great changes undergone by the earth’s crust were produced,

not by convulsions of nature, but by the slow and almost imperceptible

action, of rain, rivers, snow, frost, ice, &c., which impressed so

strongly on the mind of the geologist the vast duration of geological

periods. When it was considered that the rocky face of our globe had

been carved into hills and dales, and ultimately worn down to the

sea-level by means of those apparently trifling agents, not only once

or twice, but many times, during past ages, it was not surprising that

the views entertained by geologists regarding the immense antiquity of

our globe should not have harmonised with the deductions of physical

science on the subject. It had been shown by Sir William Thomson and

others, from physical considerations relating to the age of the sun’s

heat and the secular cooling of our globe, that the geological history

of our earth’s crust must be limited to a period of something like

one hundred millions of years. But these speculations had but little

weight when pitted against the stern and undeniable facts of subaërial

denudation. How, then, were the two to be reconciled? Was it the

physicist who had under-estimated geological time, or the geologist

who had over-estimated it? Few familiar with modern physics, and who

have given special attention to the subject, would admit that the sun

could have been dissipating his heat at the present enormous rate for

a period much beyond one hundred millions of years. The probability

was that the amount of work performed on the earth’s crust by the

denuding agents in a period so immense as a million of years was, for

reasons stated in Chapter XX., very much under-estimated. But the

difficulty was how to prove this. How was it possible to measure the

rate of operation of agents so numerous and diversified acting with

such extreme slowness and irregularity over so immense areas? In other

words, how was it possible to measure the rate of subaërial denudation?

Pondering over this problem about ten years ago, an extremely simple

and obvious method of solving it suggested itself to my mind. This

method—the details of which will be found in Chapter XX.—showed that

the rate of subaërial denudation is enormously greater than had been

supposed. The method is now pretty generally accepted, and the result

has already been to bring about a complete reconciliation between

physics and geology in reference to time.

Chapter XXI. contains an account of the gravitation theories of the

origin of the sun’s heat. The energy possessed by the sun is generally

supposed to have been derived from gravitation, combustion being

totally inadequate as a source. But something more than gravitation

is required before we can account for even one hundred millions of

years’ heat. Gravitation could not supply even one-half that amount.

There must be some other and greater source than that of gravitation.

There is, however, as is indicated, an obvious source from which far

more energy may have been derived than could have been obtained from

gravitation.

The method of determining the rate of subaërial denudation enables us

also to arrive at a rough estimate of the actual mean thickness of the

stratified rocks of the globe. It will be seen from Chapter XXII. that

the mean thickness is far less than is generally supposed.

The physical cause of the submergence of the land during the glacial

epoch, and the influence of change in the obliquity of the ecliptic on

climate, are next considered. In Chapter XXVI. I have given the reasons

which induce me to believe that coal is an inter-glacial formation.

The next two chapters—the one on the path of the ice in north-western

Europe, the other on the north of England ice-sheet—are reprints of

papers which appeared a few years ago in the _Geological Magazine_.

Recent observations have confirmed the truth of the views advanced

in these two chapters, and they are rapidly gaining acceptance among

geologists.

I have given, at the conclusion, a statement of the molecular theory of

glacier motion—a theory which I have been led to modify considerably on

one particular point.

There is one point to which I wish particularly to direct

attention—viz., that I have studiously avoided introducing into the

theories propounded anything of a hypothetical nature. There is not,

so far as I am aware, from beginning to end of this volume, a single

hypothetical element: nowhere have I attempted to give a hypothetical

explanation. The conclusions are in every case derived either from

facts or from what I believe to be admitted principles. In short, I

have aimed to prove that the theory of secular changes of climate

follows, as a necessary consequence, from the admitted principles of

physical science.

CHAPTER II.

OCEANS-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

Temperature.—Temperature of Equator and Poles in the Absence

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

Ocean-currents.

_The absolute Heating-power of Ocean-currents._—There is perhaps no

physical agent concerned in the distribution of heat over the surface

of the globe the influence of which has been so much underrated as that

of ocean-currents. This is, no doubt, owing to the fact that although

their surface-temperature, direction, and general influence have

obtained considerable attention, yet little or nothing has been done

towards determining the absolute amount of heat or of cold conveyed by

them or the resulting absolute increase or decrease of temperature.

The modern method of determining the amount of heat-effects in absolute

measure is, doubtless, destined to cast new light on all questions

connected with climate, as it has done, and is still doing, in every

department of physics where energy, under the form of heat, is being

studied. But this method has hardly as yet been attempted in questions

of meteorology; and owing to the complicated nature of the phenomena

with which the meteorologist has generally to deal, its application

will very often prove practically impossible. Nevertheless, it is

particularly suitable to all questions relating to the direct thermal

effects of currents, whatever the nature of these currents may happen

to be.

In the application of the method to an ocean-current, the two most

important elements required as data are the volume of the stream and

its mean temperature. But although we know something of the temperature

of most of the great ocean-currents, yet, with the exception of the

Gulf-stream, little has been ascertained regarding their volume.

The breadth, depth, and temperature of the Gulf-stream have formed the

subject of extensive and accurate observations by the United States

Coast Survey. In the memoirs and charts of that survey cross-sections

of the stream at various places are given, showing its breadth and

depth, and also the temperature of the water from the surface to the

bottom. We are thus enabled to determine with some precision the

mean temperature of the stream. And knowing its mean velocity at any

given section, we have likewise a means of determining the number of

cubic feet of water passing through that section in a given time. But

although we can obtain with tolerable accuracy the mean temperature,

yet observations regarding the velocity of the water at all depths have

unfortunately not been made at any particular section. Consequently we

have no means of estimating as accurately as we could wish the volume

of the current. Nevertheless, since we know the surface-velocity of the

water at places where some of the sections were taken, we are enabled

to make at least a rough estimate of the volume.

From an examination of the published sections, I came to the conclusion

some years ago[11] that the total quantity of water conveyed by the

stream is probably equal to that of a stream fifty miles broad and

1,000 feet deep,[12] flowing at the rate of four miles an hour,

and that the mean temperature of the entire mass of moving water is

not under 65° at the moment of leaving the Gulf. But to prevent the

possibility of any objections being raised on the grounds that I may

have over-estimated the volume of the stream, I shall take the velocity

to be _two_ miles instead of four miles an hour. We are warranted,

I think, in concluding that the stream before it returns from its

northern journey is on an average cooled down to at least 40°,[13]

consequently it loses 25° of heat. Each cubic foot of water, therefore,

in this case carries from the tropics for distribution upwards of

1,158,000 foot-pounds of heat. According to the above estimate of the

size and velocity of the stream, which in Chapter XI. will be shown

to be an under-estimate, 2,787,840,000,000 cubic feet of water are

conveyed from the Gulf per hour, or 66,908,160,000,000 cubic feet

daily. Consequently the total quantity of heat thus transferred per day

amounts to 77,479,650,000,000,000,000 foot-pounds.

This estimate of the volume of the stream is considerably less by

one-half than that given both by Captain Maury and by Sir John

Herschel. Captain Maury considers the Gulf-stream equal to a stream

thirty-two miles broad and 1,200 feet deep, flowing at the rate of five

knots an hour.[14] This gives 6,165,700,000,000 cubic feet per hour

as the quantity of water conveyed by this stream. Sir John Herschel’s

estimate is still greater. He considers it equal to a stream thirty

miles broad and 2,200 feet deep, flowing at the rate of four miles

an hour.[15] This makes the quantity 7,359,900,000,000 cubic feet

per hour. Dr. Colding, in his elaborate memoir on the Gulf-stream,

estimates the volume at 5,760,000,000,000 cubic feet per hour, while

Mr. Laughton’s estimate is nearly double that of mine.

From observations made by Sir John Herschel and by M. Pouillet on the

direct heat of the sun, it is found that, were no heat absorbed by the

atmosphere, about eighty-three foot-pounds per second would fall upon

a square foot of surface placed at right angles to the sun’s rays.[16]

Mr. Meech estimates that the quantity of heat cut off by the atmosphere

is equal to about twenty-two per cent. of the total amount received

from the sun. M. Pouillet estimates the loss at twenty-four per cent.

Taking the former estimate, 64·74 foot-pounds per second will therefore

be the quantity of heat falling on a square foot of the earth’s surface

when the sun is in the zenith. And were the sun to remain stationary in

the zenith for twelve hours, 2,796,768 foot-pounds would fall upon the

surface.

It can be shown that the total amount of heat received upon a unit

surface on the equator, during the twelve hours from sunrise till

sunset at the time of the equinoxes, is to the total amount which

would be received upon that surface, were the sun to remain in the

zenith during those twelve hours, as the diameter of a circle to half

its circumference, or as 1 to 1·5708. It follows, therefore, that

a square foot of surface on the equator receives from the sun at

the time of the equinoxes 1,780,474 foot-pounds daily, and a square

mile 49,636,750,000,000 foot-pounds daily. But this amounts to only

1/1560935th part of the quantity of heat daily conveyed from the

tropics by the Gulf-stream. In other words, the Gulf-stream conveys as

much heat as is received from the sun by 1,560,935 square miles at the

equator. The amount thus conveyed is equal to all the heat which falls

upon the globe within thirty-two miles on each side of the equator.

According to calculations made by Mr. Meech,[17] the annual quantity

of heat received by a unit surface on the frigid zone, taking the

mean of the whole zone, is 5·45/12th of that received at the equator;

consequently the quantity of heat conveyed by the Gulf-stream in one

year is equal to the heat which falls on an average on 3,436,900

square miles of the arctic regions. The frigid zone or arctic regions

contain 8,130,000 square miles. There is actually, therefore, nearly

one-half as much heat transferred from tropical regions by the

Gulf-stream as is received from the sun by the entire arctic regions,

the quantity conveyed from the tropics by the stream to that received

from the sun by the arctic regions being nearly as two to five.

But we have been assuming in our calculations that the percentage of

heat absorbed by the atmosphere is no greater in polar regions than

it is at the equator, which is not the case. If we make due allowance

for the extra amount absorbed in polar regions in consequence of the

obliqueness of the sun’s rays, the total quantity of heat conveyed by

the Gulf-stream will probably be nearly equal to one-half the amount

received from the sun by the entire arctic regions.

If we compare the quantity of heat conveyed by the Gulf-stream with

that conveyed by means of aërial currents, the result is equally

startling. The density of air to that of water is as 1 to 770, and

its specific heat to that of water is as 1 to 4·2; consequently the

same amount of heat that would raise 1 cubic foot of water 1° would

raise 770 cubic feet of air 4°·2, or 3,234 cubic feet 1°. The quantity

of heat conveyed by the Gulf-stream is therefore equal to that which

would be conveyed by a current of air 3,234 times the volume of the

Gulf-stream, at the same temperature and moving with the same velocity.

Taking, as before, the width of the stream at fifty miles, and its

depth at 1,000 feet, and its velocity at two miles an hour, it follows

that, in order to convey an equal amount of heat from the tropics by

means of an aërial current, it would be necessary to have a current

about 1¼ mile deep, and at the temperature of 65°, blowing at the

rate of two miles an hour from every part of the equator over the

northern hemisphere towards the pole. If its velocity were equal to

that of a good sailing-breeze, which Sir John Herschel states to be

about twenty-one miles an hour, the current would require to be above

600 feet deep. A greater quantity of heat is probably conveyed by the

Gulf-stream alone from the tropical to the temperate and arctic regions

than by all the aërial currents which flow from the equator.

We are apt, on the other hand, to over-estimate the amount of the heat

conveyed from tropical regions to us by means of aërial currents. The

only currents which flow from the equatorial regions are the upper

currents, or anti-trades as they are called. But it is not possible

that much heat can be conveyed directly by them. The upper currents of

the trade-winds, even at the equator, are nowhere below the snow-line;

they must therefore lie in a region of which the temperature is

actually below the freezing-point. In fact, if those currents were

warm, they would elevate the snow-line above themselves. The heated air

rising off the hot burning ground at the equator, after ascending a

few miles, becomes exposed to the intense cold of the upper regions of

the atmosphere; it then very soon loses all its heat, and returns from

the equator much colder than it went thither. It is impossible that

we can receive any heat directly from the equatorial regions by means

of aërial currents. It is perfectly true that the south-west wind, to

which we owe so much of our warmth in this country, is a continuation

of the anti-trade; but the heat which this wind brings to us is not

derived from the equatorial regions. This will appear evident, if we

but reflect that, before the upper current descends to the snow-line

after leaving the equator, it must traverse a space of at least 2,000

miles; and to perform this long journey several days will be required.

During all this time the air is in a region below the freezing-point;

and it is perfectly obvious that by the time it begins to descend it

must have acquired the temperature of the region in which it has been

travelling.

If such be the case, it is evident that a wind whose temperature

is below 32° could never warm a country such as ours, where the

temperature does not fall below 38° or 39°. The heat of our south-west

winds is derived, not directly from the equator, but from the warm

water of the Atlantic—in fact, from the Gulf-stream. The upper current

acquires its heat after it descends to the earth. There is one way,

however, whereby heat is indirectly conveyed from the equator by the

anti-trades; that is, in the form of aqueous vapour. In the formation

of one pound of water from aqueous vapour, as Professor Tyndall

strikingly remarks, a quantity of heat is given out sufficient to melt

five pounds of cast iron.[18] It must, however, be borne in mind that

the greater part of the moisture of the south-west and west winds is

derived from the ocean in temperate regions. The upper current receives

the greater part of its moisture after it descends to the earth, whilst

the moisture received at the equator is in great part condensed, and

falls as rain in those regions.

This latter assertion has been so frequently called in question

that I shall give my reasons for making it. According to Dr. Keith

Johnston (“Physical Atlas”) the mean rainfall of the torrid regions

is ninety-six inches per annum, while that of the temperate regions

amounts to only thirty-seven inches. If the greater part of the

moisture of the torrid regions does not fall as rain in those regions,

it must fall as such beyond them. Now the area of the torrid to that

of the two temperate regions is about as 39·3 to 51. Consequently

ninety-six inches of rain spread over the temperate regions would give

seventy-four inches; but this is double the actual rainfall of the

temperate regions. If, again, it were spread over both temperate and

polar regions this would yield sixty-four inches, which, however, is

nearly double the mean rainfall of the temperate and polar regions. If

we add to this the amount of moisture derived from the ocean within

temperate and polar regions, we should have a far greater rainfall for

these latitudes than for the torrid region, and we know, of course,

that it is actually far less. This proves the truth of the assertion

that by far the greater part of the moisture of the torrid regions

falls in those regions as rain. It will hardly do to object that the

above may probably be an over-estimate of the amount of rainfall in

the torrid zone, for it is not at all likely that any error will ever

be found which will affect the general conclusion at which we have

arrived.

Dr. Carpenter, in proof of the small rainfall of the torrid zone,

adduces the case of the Red Sea, where, although evaporation is

excessive, almost no rain falls. But the reason why the vapour raised

from the Red Sea does not fall in that region as rain, is no doubt

owing to the fact that this sea is only a narrow strip of water in a

dry and parched land, the air above which is too greedy of moisture

to admit of the vapour being deposited as rain. Over a wide expanse

of ocean, however, where the air above is kept to a great extent in a

constant state of saturation, the case is totally different.

_Land at the Equator tends to Lower the Temperature of the Globe._—The

foregoing considerations, as well as many others which might be stated,

lead to the conclusion that, in order to raise the mean temperature of

the whole earth, _water_ should be placed along the equator, and not

_land_, as is supposed by Sir Charles Lyell and others. For if land is

placed at the equator, the possibility of conveying the sun’s heat from

the equatorial regions by means of ocean-currents is prevented. The

transference of heat could then be effected only by means of the upper

currents of the trades; for the heat conveyed by _conduction_ along the

solid crust, if any, can have no sensible effect on climate. But these

currents, as we have just seen, are ill-adapted for conveying heat.

The surface of the ground at the equator becomes intensely heated by

the sun’s rays. This causes it to radiate its heat more rapidly into

space than a surface of water heated under the same conditions. Again,

the air in contact with the hot ground becomes also more rapidly

heated than in contact with water, and consequently the ascending

current of air carries off a greater amount of heat. But were the

heat thus carried away transferred by means of the upper currents to

high latitudes and there employed to warm the earth, then it might to

a considerable extent compensate for the absence of ocean-currents,

and in this case land at the equator might be nearly as well adapted

as water for raising the temperature of the whole earth. But such is

not the case; for the heat carried up by the ascending current at the

equator is not employed in warming the earth, but is thrown off into

the cold stellar space above. This ascending current, instead of being

employed in warming the globe, is in reality one of the most effectual

means that the earth has of getting quit of the heat received from the

sun, and of thus maintaining a much lower temperature than it would

otherwise possess. It is in the equatorial regions that the earth loses

as well as gains the greater part of its heat; so that, of all places,

here ought to be placed the substance best adapted for preventing the

dissipation of the earth’s heat into space, in order to raise the

general temperature of the earth. Water, of all substances in nature,

seems to possess this quality to the greatest extent; and, besides, it

is a fluid, and therefore adapted by means of currents to carry the

heat which it receives from the sun to every region of the globe.

These results show (although they have reference to only one stream)

that the general influence of ocean-currents on the distribution of

heat over the surface of the globe must be very great. If the quantity

of heat transferred from equatorial regions by the Gulf-stream

alone is nearly equal to all the heat received from the sun by the

arctic regions, then how enormous must be the quantity conveyed from

equatorial regions by all the ocean-currents together!

_Influence of the Gulf-stream on the Climate of Europe._—In a paper

read before the British Association at Exeter, Mr. A. G. Findlay

objects to the conclusions at which I have arrived in former papers

on the subject, that I have not taken into account the great length

of time that the water requires in order to circulate, and the

interference it has to encounter in its passage.

The objection is, that a stream so comparatively small as the

Gulf-stream, after spreading out over such a large area of the

Atlantic, and moving so slowly across to the shores of Europe, losing

heat all the way, would not be able to produce any very sensible

influence on the climate of Europe.

I am unable to perceive the force of this objection. Why, the very

efficiency of the stream as a heating agent necessarily depends upon

the slowness of its motion. Did the Gulf-stream move as rapidly along

its whole course as it does in the Straits of Florida, it could produce

no sensible effect on the climate of Europe. It does not require much

consideration to perceive this. (1) If the stream during its course

continued narrow, deep, and rapid, it would have little opportunity of

losing its heat, and the water would carry back to the tropics the heat

which it ought to have given off in the temperate and polar regions.

(2) The Gulf-stream does not heat the shores of Europe by direct

radiation. Our island, for example, is not heated by radiation from a

stream of warm water flowing along its shores. The Gulf-stream heats

our island _indirectly_ by heating the winds which blow over it to our

shores.

The anti-trades, or upper return-currents, as we have seen, bring no

heat from the tropical regions. After traversing some 2,000 miles

in a region of extreme cold they descend on the Atlantic as a cold

current, and there absorb the heat and moisture which they carry to

north-eastern Europe. Those aërial currents derive their heat from the

Gulf-stream, or if it is preferred, from the warm water poured into the

Atlantic by the Gulf-stream.

How, then, are these winds heated by the warm water? The air is heated

in two ways, viz., by direct _radiation_ from the water, and by

_contact_ with the water. Now, if the Gulf-stream continued a narrow

and deep current during its entire course similar to what it is at

the Straits of Florida, it could have little or no opportunity of

communicating its heat to the air either by radiation or by contact. If

the stream were only about forty or fifty miles in breadth, the aërial

particles in their passage across it would not be in contact with the

warm water more than an hour or two. Moreover, the number of particles

in contact with the water, owing to the narrowness of the stream,

would be small, and there would therefore be little opportunity for

the air becoming heated by contact. The same also holds true in regard

to radiation. The more we widen the stream and increase its area, the

more we increase its radiating surface; and the greater the radiating

surface, the greater is the quantity of heat thrown off. But this is

not all; the number of aërial particles heated by radiation increases

in proportion to the area of the radiating surface; consequently, the

wider the area over which the waters of the Gulf-stream are spread,

the more effectual will the stream be as a heating agent. And, again,

in order that a very wide area of the Atlantic may be covered with the

warm waters of the stream, slowness of motion is essential.

Mr. Findlay supposes that fully one-half of the Gulf-stream passes into

the south-eastern branch, and that it is only the north-eastern branch

of the current that can be effectual in raising the temperature of

Europe. But it appears to me that it is to this south-eastern portion

of the current, and not to the north-eastern, that we, in this country,

are chiefly indebted for our heat. The south-west winds, to which we

owe our heat, derive their temperature from this south-eastern portion

which flows away in the direction of the Azores. The south-west winds

which blow over the northern portion of the current which flows past

our island up into the arctic seas cannot possibly cross this country,

but will go to heat Norway and northern Europe. The north-eastern

portion of the stream, no doubt, protects us from the ice of Greenland

by warming the north-west winds which come to us from that cold region.

Mr. Buchan, Secretary of the Scottish Meteorological Society, has

shown[19] that in a large tract of the Atlantic between latitudes 20°

and 40° N., the mean pressure of the atmosphere is greater than in any

other place on the globe. To the west of Madeira, between longitude

10° and 40° W., the mean annual pressure amounts to 30·2 inches, while

between Iceland and Spitzbergen it is only 29·6, a lower mean pressure

than is found in any other place on the northern hemisphere. There

must consequently, he concludes, be a general tendency in the air to

flow from the former to the latter place along the earth’s surface.

Now, the air in moving from the lower to the higher latitudes tends

to take a north-easterly direction, and in this case will pass over

our island in its course. This region of high pressure, however,

is situated in the very path of the south-eastern branch of the

Gulf-stream, and consequently the winds blowing therefrom will carry

directly to Britain the heat of the Gulf-stream.

As we shall presently see, it is as essential to the heating of our

island as to that of the southern portion of Europe, that a very large

proportion of the waters of the Gulf-stream should spread over the

surface of the Atlantic and never pass up into the arctic regions.

Even according to Mr. Findlay’s own theory, it is to the south-west

wind, heated by the warm waters of the Atlantic, that we are indebted

for the high temperature of our climate. But he seems to be under the

impression that the Atlantic would be able to supply the necessary

heat independently of the Gulf-stream. This, it seems to me, is the

fundamental error of all those who doubt the efficiency of the stream.

It is a mistake, however, into which one is very apt to fall who does

not adopt the more rigid method of determining heat-results in absolute

measure. When we apply this method, we find that the Atlantic, without

the aid of such a current as the Gulf-stream, would be wholly unable to

supply the necessary amount of heat to the south-west winds.

The quantity of heat conveyed by the Gulf-stream, as we have seen,

is equal to all the heat received from the sun by 1,560,935 square

miles at the equator. The mean annual quantity of heat received from

the sun by the temperate regions per unit surface is to that received

by the equator as 9·08 to 12.[20] Consequently, the quantity of heat

conveyed by the stream is equal to all the heat received from the sun

by 2,062,960 square miles of the temperate regions. The total area of

the Atlantic from the latitude of the Straits of Florida, 200 miles

north of the tropic of Cancer, up to the Arctic Circle, including also

the German Ocean, is about 8,500,000 square miles. In this case the

quantity of heat carried by the Gulf-stream into the Atlantic through

the Straits of Florida, is to that received by this entire area from

the sun as 1 to 4·12, or in round numbers as 1 to 4. It therefore

follows that one-fifth of all the heat possessed by the waters of the

Atlantic over that area, even supposing that they absorb every ray that

falls upon them, is derived from the Gulf-stream. Would those who call

in question the efficiency of the Gulf-stream be willing to admit that

a decrease of one-fourth in the total amount of heat received from the

sun, over the entire area of the Atlantic from within 200 miles of

the tropical zone up to the arctic regions, would not sensibly affect

the climate of northern Europe? If they would not willingly admit

this, why, then, contend that the Gulf-stream does not affect climate?

for the stoppage of the Gulf-stream would deprive the Atlantic of

77,479,650,000,000,000,000 foot-pounds of energy in the form of heat

per day, a quantity equal to one-fourth of all the heat received from

the sun by that area.

How much, then, of the temperature of the south-west winds derived from

the water of the Atlantic is due to the Gulf-stream?

Were the sun extinguished, the temperature over the whole earth

would sink to _nearly_ that of stellar space, which, according to

the investigations of Sir John Herschel[21] and of M. Pouillet,[22]

is not above −239° F. Were the earth possessed of no atmosphere, the

temperature of its surface would sink to exactly that of space, or to

that indicated by a thermometer exposed to no other heat-influence than

that of radiation from the stars. But the presence of the atmospheric

envelope would slightly modify the conditions of things; for the

heat from the stars (which of course constitutes what is called the

temperature of space) would, like the sun’s heat, pass more freely

through the atmosphere than the heat radiated back from the earth, and

there would in consequence of this be an accumulation of heat on the

earth’s surface. The temperature would therefore stand a little higher

than that of space; or, in other words, it would stand a little higher

than it would otherwise do were the earth exposed in space to the

direct radiation of the stars without the atmospheric envelope. But,

for reasons which will presently be stated, we may in the meantime,

till further light is cast upon this matter, take -239° F. as probably

not far from what would be the temperature of the earth’s surface were

the sun extinguished.

Suppose now that we take the mean annual temperature of the Atlantic

at, say, 56°.[23] Then 239° + 56° = 295° represents the number of

degrees of rise due to the heat which it receives. In other words,

it takes all the heat that the Atlantic receives to maintain its

temperature 295° above the temperature of space. Stop the Gulf-stream,

and the Atlantic would be deprived of one-fifth of the heat which

it possesses. Then, if it takes five parts of heat to maintain a

temperature of 295° above that of space, the four parts which would

remain after the stream was stopped would only be able to maintain a

temperature of four-fifths of 295°, or 236° above that of space: the

stoppage of the Gulf-stream would therefore deprive the Atlantic of an

amount of heat which would be sufficient to maintain its temperature

59° above what it would otherwise be, did it depend alone upon the heat

received directly from the sun. It does not, of course, follow that

the Gulf-stream actually maintains the temperature 59° above what it

would otherwise be were there no ocean-currents, because the actual

heating-effect of the stream is neutralized to a very considerable

extent by cold currents from the arctic regions. But 59° of rise

represents its actual power; consequently 59°, minus the lowering

effect of the cold currents, represents the actual rise. What the rise

may amount to at any particular place must be determined by other means.

This method of calculating how much the temperature of the earth’s

surface would rise or fall from an increase or a decrease in the

absolute amount of heat received is that adopted by Sir John Herschel

in his “Outlines of Astronomy,” § 369^a.

About three years ago, in an article in the _Reader_, I endeavoured

to show that this method is not rigidly correct. It has been shown

from the experiments of Dulong and Petit, Dr. Balfour Stewart,

Professor Draper, and others, that the rate at which a body radiates

its heat off into space is not directly proportionate to its absolute

temperature. The rate at which a body loses its heat as its temperature

rises increases more rapidly than the temperature. As a body rises

in temperature the rate at which it radiates off its heat increases;

the _rate_ of this increase, however, is not uniform, but increases

with the temperature. Consequently the temperature is not lowered in

proportion to the decrease of the sun’s heat. But at the comparatively

low temperature with which we have at present to deal, the error

resulting from assuming the decrease of temperature to be proportionate

to the decrease of heat would not be great.

It may be remarked, however, that the experiments referred to were

made on solids; but, from certain results arrived at by Dr. Balfour

Stewart, it would seem that the radiation of a material particle may

be proportionate to its absolute temperature.[24] This physicist found

that the radiation of a thick plate of glass increases more rapidly

than that of a thin plate as the temperature rises, and that, if we go

on continually diminishing the thickness of the plate whose radiation

at different temperatures we are ascertaining, we find that as it grows

thinner and thinner the rate at which it radiates off its heat as its

temperature rises becomes less and less. In other words, as the plate

grows thinner and thinner its rate of radiation becomes more and more

proportionate to its absolute temperature. And we can hardly resist the

conviction that if we could possibly go on diminishing the thickness

of the plate till we reached a film so thin as to embrace but only one

particle in its thickness, its rate of radiation would be proportionate

to its temperature. Dr. Balfour Stewart has very ingeniously suggested

the probable reason why the rate of radiation of thick plates increases

with rise of temperature more rapidly than that of thin. It is this:

all substances are more diathermanous for heat of high temperatures

than for heat of low temperatures. When a body is at a low temperature,

we may suppose that only the exterior rows of particles supply the

radiation, the heat from the interior particles being all stopped by

the exterior ones, the substance being very opaque for heat of low

temperature; while at a high temperature we may imagine that part

of the heat from the interior particles is allowed to pass, thereby

swelling the total radiation. But as the plate becomes thinner and

thinner, the obstructions to interior radiation become less and less,

and as these obstructions are greater for radiation at low temperatures

than for radiation at high temperatures, it necessarily follows that,

by reducing the thickness of the plate, we assist radiation at low

temperatures more than we do at high.

In a gas, where each particle may be assumed to radiate by itself, and

where the particles stand at a considerable distance from one another,

the obstruction to interior radiation must be far less than in a

solid. In this case the rate at which a gas radiates off its heat as

its temperature rises must increase more slowly than that of a solid

substance. In other words, its rate of radiation must correspond more

nearly to its absolute temperature than that of a solid. If this be the

case, a reduction in the amount of heat received from the sun, owing to

an increase of his distance, should tend to produce a greater lowering

effect on the temperature of the air than it does on the temperature of

the solid ground. But as the temperature of our climate is determined

by the temperature of the air, it must follow that the error of

assuming that the decrease of temperature would be proportionate to the

decrease in the intensity of the sun’s heat may not be great.

It may be observed here, although it does not bear directly on this

point, that although the air in a room, for example, or at the earth’s

surface is principally cooled by convection rather than by radiation,

yet it is by radiation alone that the earth’s atmosphere parts with its

heat to stellar space; and this is the chief matter with which we are

at present concerned. Air, like all other gases, is a bad radiator;

and this tends to protect it from being cooled to such an extent as it

would otherwise be, were it a good radiator like solids. True, it is

also a bad absorber; but as it is cooled by radiation into space, and

heated, not altogether by absorption, but to a very large extent by

convection, it on the whole gains its heat more easily than it loses

it, and consequently must stand at a higher temperature than it would

do were it heated by absorption alone.

But, to return; the error of regarding the decrease of temperature

as proportionate to the decrease in the amount of heat received, is

probably neutralized by one of an opposite nature, viz., that of taking

space at too high a temperature; for by so doing we make the result too

small.

We know that absolute zero is at least 493° below the melting-point

of ice. This is 222° below that of space. Consequently, if the heat

derived from the stars is able to maintain a temperature of −239°,

or 222° of absolute temperature, then nearly as much heat is derived

from the stars as from the sun. But if so, why do the stars give so

much heat and so very little light? If the radiation from the stars

could maintain a thermometer 222° above absolute zero, then space must

be far more transparent to heat-rays than to light-rays, or else the

stars give out a great amount of heat, but very little light, neither

of which suppositions is probably true. The probability is, I venture

to presume, that the temperature of space is not very much above

absolute zero. At the time when these investigations into the probable

temperature of space were made, at least as regards the labours of

Pouillet, the modern science of heat had no existence, and little or

nothing was then known with certainty regarding absolute zero. In this

case the whole matter would require to be reconsidered. The result of

such an investigation in all probability would be to assign a lower

temperature to stellar space than −239°.

Taking all these various considerations into account, it is probable

that if we adopt −239° as the temperature of space, we shall not be far

from the truth in assuming that the absolute temperature of a place

above that of space is proportionate to the amount of heat received

from the sun.

We may, therefore, in this case conclude that 59° of rise is probably

not very far from the truth, as representing the influence of the

Gulf-stream. The Gulf-stream, instead of producing little or no effect,

produces an effect far greater than is generally supposed.

Our island has a mean annual temperature of about 12° above the normal

due to its latitude. This excess of temperature has been justly

attributed to the influence of the Gulf-stream. But it is singular

how this excess should have been taken as the measure of the _rise

resulting from the influence of the stream_. These figures only

represent the number of degrees that the mean normal temperature of

our island stands above what is called the normal temperature of the

latitude.

The mode in which Professor Dove constructed his Tables of normal

temperature was as follows:—He took the temperature of thirty-six

equidistant points on every ten degrees of latitude. The mean

temperature of these thirty-six points he calls in each case the

_normal_ temperature of the parallel. The excess above the normal

merely represents how much the stream raises our temperature above

the mean of all places on the same latitude, but it affords us no

information regarding the absolute rise produced. In the Pacific, as

well as in the Atlantic, there are immense masses of water flowing

from the tropical to the temperate regions. Now, unless we know how

much of the normal temperature of a latitude is due to ocean-currents,

and how much to the direct heat of the sun, we could not possibly,

from Professor Dove’s Tables, form the most distant conjecture as

to how much of our temperature is derived from the Gulf-stream. The

overlooking of this fact has led to a general misconception regarding

the positive influence of the Gulf-stream on temperature. The 12°

marked in Tables of normal temperature do not represent the absolute

effect of the stream, but merely show how much the stream raises the

temperature of our country above the mean of all places on the same

latitude. Other places have their temperature raised by ocean-currents

as well as this country; only the Gulf-stream produces a rise of

several degrees over and above that produced by other streams in the

same latitude.

At present there is a difference merely of 80° between the mean

temperature of the equator and the poles;[25] but were each part of the

globe’s surface to depend only upon the direct heat which it receives

from the sun, there ought, according to theory, to be a difference of

more than 200°. The annual quantity of heat received at the equator is

to that received at the poles (supposing the proportionate quantity

absorbed by the atmosphere to be the same in both cases) as 12 to 4·98,

or, say, as 12 to 5. Consequently, if the temperatures of the equator

and the poles be taken as proportionate to the absolute amount of heat

received from the sun, then the temperature of the equator above that

of space must be to that of the poles above that of space as 12 to 5.

What ought, therefore, to be the temperatures of the equator and the

poles, did each place depend solely upon the heat which it receives

directly from the sun? Were all ocean and aërial currents stopped,

so that there could be no transference of heat from one part of the

earth’s surface to another, what ought to be the temperatures of the

equator and the poles? We can at least arrive at a rough estimate

on this point. If we diminish the quantity of warm water conveyed

from the equatorial regions to the temperate and arctic regions, the

temperature of the equator will begin to rise, and that of the poles

to sink. It is probable, however, that this process would affect the

temperature of the poles more than it would that of the equator; for as

the warm water flows from the equator to the poles, the area over which

it is spread becomes less and less. But as the water from the tropics

has to raise the temperature of the temperate regions as well as the

polar, the difference of effect at the equator and poles might not, on

that account, be so very great. Let us take a rough estimate. Say that,

as the temperature of the equator rises one degree, the temperature of

the poles sinks one degree and a half. The mean annual temperature of

the globe is about 58°. The mean temperature of the equator is 80°, and

that of the poles 0°. Let ocean and aërial currents now begin to cease,

the temperature of the equator commences to rise and the temperature

of the poles to sink. For every degree that the temperature of the

equator rises, that of the poles sinks 1½°; and when the currents are

all stopped and each place becomes dependent solely upon the direct

rays of the sun, the mean annual temperature of the equator above that

of space will be to that of the poles, above that of space, as 12 to

5. When this proportion is reached, the equator will be 374° above

that of space, and the poles 156°; for 374 is to 156 as 12 is to 5.

The temperature of space we have seen to be −239°, consequently the

temperature of the equator will in this case be 135°, reckoned from the

zero of the Fahrenheit thermometer, and the poles 83° below zero. The

equator would therefore be 55° warmer than at present, and the poles

83° colder. The difference between the temperature of the equator and

the poles will in this case amount to 218°.

Now, if we take into account the quantity of positive energy in the

form of heat carried by warm currents from the equator to the temperate

and polar regions, and also the quantity of negative energy (cold)

carried by cold currents from the polar regions to the equator, we

shall find that they are sufficient to reduce the difference of

temperature between the poles and the equator from 218° to 80°.

The quantity of heat received in the latitude of London, for example,

is to that received at the equator nearly as 12 to 8. This, according

to theory, should produce a difference of about 125°. The temperature

of the equator above that of space, as we have seen, would be 374°.

Therefore 249° above that of space would represent the temperature

of the latitude of London. This would give 10° as its temperature.

The stoppage of all ocean and aërial currents would thus increase the

difference between the equator and the latitude of London by about

85°. The stoppage of ocean-currents would not be nearly so much felt,

of course, in the latitude of London as at the equator and the poles,

because, as has been already noticed, in all latitudes midway between

the equator and the poles the two sets of currents to a considerable

extent compensate each other—the warm currents from the equator

raise the temperature, while the cold ones from the poles lower it;

but as the warm currents chiefly keep on the surface and the cold

return-currents are principally under-currents, the heating effect very

greatly exceeds the cooling effect. Now, as we have seen, the stoppage

of all currents would raise the temperature of the equator 55°; that

is to say, the rise at the equator alone would increase the difference

of temperature between the equator and that of London by 55°. But the

actual difference, as we have seen, ought to be 85°; consequently the

temperature of London would be lowered 30° by the stoppage of the

currents. For if we raise the temperature of the equator 55° and lower

the temperature of London 30°, we then increase the difference by

85°. The normal temperature of the latitude of London being 40°, the

stoppage of all ocean and aërial currents would thus reduce it to 10°.

But the Gulf-stream raises the actual mean temperature of London 10°

above the normal. Consequently 30° + 10° = 40° represents the actual

rise at London due to the influence of the Gulf-stream over and above

all the lowering effects resulting from arctic currents. On some parts

of the American shores on the latitude of London, the temperature is

10° below the normal. The stoppage of all ocean and aërial currents

would therefore lower the temperature there only 20°.

It is at the equator and the poles that the great system of ocean and

aërial currents produces its maximum effects. The influence becomes

less and less as we recede from those places, and between them there

is a point where the influence of warm currents from the equator and

of cold currents from the poles exactly neutralize each other. At

this point the stoppage of ocean-currents would not sensibly affect

temperature. This point, of course, is not situated on the same

latitude in all meridians, but varies according to the position of the

meridian in relation to land, and ocean-currents, whether cold or hot,

and other circumstances. A line drawn round the globe through these

various points would be very irregular. At one place, such as on the

western side of the Atlantic, where the arctic current predominates,

the neutral line would be deflected towards the equator, while on

the eastern side, where warm currents predominate, the line would be

deflected towards the north. It is a difficult problem to determine the

mean position of this line; it probably lies somewhere not far north of

the tropics.

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.—Absolute 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

Imperfection of Data.

_Influence of the Gulf-stream on the Climate of the Arctic

Regions._—Does the Gulf-stream pass into the arctic regions? Are the

seas around Spitzbergen and North Greenland heated by the warm water of

the stream?

Those who deny this nevertheless admit the existence of an arctic

current. They admit that an immense mass of cold water is continually

flowing south from the polar regions around Greenland into the

Atlantic. If it be admitted, then, that a mass of water flows across

the arctic circle from north to south, it must also be admitted that an

equal mass flows across from south to north. It is also evident that

the water crossing from south to north must be warmer than the water

crossing from north to south; for the temperate regions are warmer than

the arctic, and the ocean in temperate regions warmer than the ocean in

the arctic; consequently the current which flows into the arctic seas,

to compensate for the cold arctic current, must be a warmer current.

Is the Gulf-stream this warm current? Does this compensating warm

current proceed from the Atlantic or from the Pacific? If it proceeds

from the Atlantic, it is simply the warm water of the Gulf-stream.

We may call it the warm water of the Atlantic if we choose; but this

cannot materially affect the question at issue, for the heat which

the waters of the Atlantic possess is derived, as we have seen, to

an enormous extent from the water brought from the tropics by the

Gulf-stream. If we deny that the warm compensating current comes from

the Atlantic, then we must assume that it comes from the Pacific. But

if the cold current flows from the arctic regions into the Atlantic,

and the warm compensating current from the Pacific into the arctic

regions, the highest temperature should be found on the Pacific side of

the arctic regions and not on the Atlantic side; the reverse, however,

is the case. In the Atlantic, for example, the 41° isothermal line

reaches to latitude 65°30′, while in the Pacific it nowhere goes beyond

latitude 57°. The 27° isotherm reaches to latitude 75° in the Atlantic,

but in the Pacific it does not pass beyond 64°. And the 14° isotherm

reaches the north of Spitzbergen in latitude 80°, whereas on the

Pacific side of the arctic regions it does not reach to latitude 72°.

On no point of the earth’s surface does the mean annual temperature

rise so high above the normal as in the northern Atlantic, just at

the arctic circle, at a spot believed to be in the middle of the

Gulf-stream. This place is no less than 22°·5 above the normal, while

in the northern Pacific the temperature does not anywhere rise more

than 9° above the normal. These facts prove that the warm current

passes up the Atlantic into the arctic regions and not up the Pacific,

or at least that the larger amount of warm water must pass into the

arctic regions through the Atlantic. In other words, the Gulf-stream is

the warm compensating current. Not only must there be a warm stream,

but one of very considerable magnitude, in order to compensate for the

great amount of cold water that is constantly flowing from the arctic

regions, and also to maintain the temperature of those regions so much

above the temperature of space as they actually are.

No doubt, when the results of the late dredging expedition are

published, they will cast much additional light on the direction and

character of the currents forming the north-eastern branch of the

Gulf-stream.

The average quantity of heat received by the arctic regions as a whole

per unit surface to that received at the equator, as we have already

seen, is as 5·45 to 12, assuming that the percentage of rays cut off by

the atmosphere is the same at both places. In this case the mean annual

temperature of the arctic regions, taken as a whole, would be about

−69°, did those regions depend entirely for their temperature upon the

heat received directly from the sun. But the temperature would not even

reach to this; for the percentage of rays cut off by the atmosphere in

arctic regions is generally believed to be greater than at the equator,

and consequently the actual mean quantity of heat received by the

arctic regions will be less than 5·45−12ths of what is received at the

equator.

In the article on Climate in the “Encyclopædia Britannica” there is

a Table calculated upon the principle that the quantity of heat cut

off is proportionate to the number of aërial particles which the rays

have to encounter before reaching the surface of the earth—that, as

a general rule, if the tracts of the rays follow an arithmetical

progression, the diminished force with which the rays reach the ground

will form a decreasing geometrical progression. According to this Table

about 75 per cent. of the sun’s rays are cut off by the atmosphere

in arctic regions. If 75 per cent. of the rays were cut off by the

atmosphere in arctic regions, then the direct rays of the sun could

not maintain a mean temperature 100° above that of space. But this is

no doubt much too high a percentage for the quantity of heat cut off;

for recent discoveries in regard to the absorption of radiant heat by

gases and vapours prove that Tables computed on this principle must be

incorrect. The researches of Tyndall and Melloni show that when rays

pass through any substance, the absorption is rapid at first: but the

rays are soon “sifted,” as it is called, and they then pass onwards

with but little further obstruction. Still, however, owing to the dense

fogs which prevail in arctic regions, the quantity of heat cut off

must be considerable. If as much as 50 per cent. of the sun’s rays

are cut off by the atmosphere in arctic regions, the amount of heat

received directly from the sun would not be sufficient to maintain a

mean annual temperature of −100°. Consequently the arctic regions must

depend to an enormous extent upon ocean-currents for their temperature.

_Influence of Ocean-currents shown by another Method._—That the

temperature of the arctic regions would sink enormously, and the

temperature of the equator rise enormously, were all ocean-currents

stopped, can be shown by another method—viz., by taking the mean annual

temperature from the equator to the pole along a meridian passing

through the ocean, say, the Atlantic, and comparing it with the mean

annual temperature taken along a meridian passing through a great

continent, say, the Asiatic.

Professor J. D. Forbes, in an interesting memoir,[26] has endeavoured

by this method to determine what would be the temperature of the

equator and the poles were the globe all water or all land. He has

taken the temperature of the two meridians from the tables and charts

of Professor Dove, and ascertained the exact proportion of land and

water on every 10° of latitude from the equator to the poles, with the

view of determining what proportion of the average temperature of the

globe in each parallel is due to the land, and what to the water which

respectively belongs to it. He next endeavours to obtain a formula for

expressing the mean temperature of a given parallel, and thence arrives

at “an approximate answer to the inquiry as to what would have been the

equatorial or polar temperature of the globe, or that of any latitude,

had its surface been entirely composed of land or of water.”

The result at which he arrived is this: that, were the surface of

the globe all water, 71°·7 would be the temperature of the equator,

and 12°·5 the temperature of the poles; and were the surface all

land, 109°·8 would be the temperature of the equator, and −25°·6 the

temperature of the poles.

But in Professor Forbes’s calculations no account whatever is taken

of the influence of currents, whether of water or of air, and the

difference of temperature is attributed wholly to difference of

latitude and the physical properties of land and water in relation to

their powers in absorbing and detaining the sun’s rays, and to the laws

of conduction and of convection which regulate the internal motion of

heat in the one and in the other. He considers that the effects of

currents are all compensatory.

“If a current of hot water,” he says, “moderates the cold of a Lapland

winter, the counter-current, which brings the cold of Greenland to the

shores of the United States, in a great measure restores the balance of

temperature, so far as it is disturbed by this particular influence.

The prevalent winds, in like manner, including the trade-winds, though

they render some portions of continents, on the average, hotter or

colder than others, produce just the contrary effect elsewhere. Each

continent, if it has a cold eastern shore, has likewise a warm western

one; and even local winds have for the most part established laws of

compensation. In a given parallel of latitude all these secondary

causes of local climate may be imagined to be mutually compensatory,

and the outstanding gradation of mean or normal temperature will

mainly depend, 1st, upon the effect of latitude simply; 2nd, on

the distribution of land and water considered in their primary or

_statical_ effect.”

It is singular that a physicist so acute as Professor Forbes should,

in a question such as this, leave out of account the influence of

currents, under the impression that their effects were compensatory.

If there is a constant transference of hot water from the equatorial

regions to the polar, and of cold water from the polar regions to the

equatorial (a thing which Professor Forbes admitted), then there can

only be one place between the equator and the pole where the two sets

of currents compensate each other. At all places on the equatorial

side of this point a cooling effect is the result. Starting from this

neutral point, the preponderance of the cooling effect over the heating

increases as we approach towards the equator, and the preponderance of

the heating effect over the cooling increases as we recede from this

point towards the pole—the cooling effect reaching a maximum at the

equator, and the heating effect a maximum at the pole.

Had Professor Forbes observed this important fact, he would have

seen at once that the low temperature of the land in high latitudes,

in comparison with that of the sea, was no index whatever as to

how much the temperature of those regions would sink were the sea

entirely removed and the surface to become land; for the present

high temperature of the sea is not due wholly to the mere physical

properties of water, but to a great extent is due to the heat brought

by currents from the equator. Now, unless it is known how much of

the absolute temperature of the ocean in those latitudes is due to

currents, we cannot tell how much the removal of the sea would lower

the absolute temperature of those places. Were the sea removed,

the continents in high latitudes would not simply lose the heating

advantages which they presently derive from the mere fact of their

proximity to so much sea, but the removal would, in addition to this,

deprive them of an enormous amount of heat which they at present

receive from the tropics by means of ocean-currents. And, on the other

hand, at the equator, were the sea removed, the continents there

would not simply lose the cooling influences which result from their

proximity to so much water, but, in addition to this, they would have

to endure the scorching effects which would result from the heat which

is at present carried away from the tropics by ocean-currents.

We have already seen that Professor Forbes concluded that the

removal of the sea would raise the mean temperature of the equator

30°, and lower the temperature of the poles 28°; it is therefore

perfectly certain that, had he added to his result the effect due to

ocean-currents, and had he been aware that about one-fifth of all the

heat possessed by the Atlantic is actually derived from the equator by

means of the Gulf-stream, he would have assigned a temperature to the

equator and the poles, of a globe all land, differing not very far from

what I have concluded would be the temperature of those places were all

ocean and aërial currents stopped, and each place to depend solely upon

the heat which it received directly from the sun.

_Without Ocean-currents the Globe would not be habitable._—All these

foregoing considerations show to what an extent the climatic condition

of our globe is due to the thermal influences of ocean-currents.

As regards the northern hemisphere, we have two immense oceans, the

Pacific and the Atlantic, extending from the equator to near the north

pole, or perhaps to the pole altogether. Between these two oceans lie

two great continents, the eastern and the western. Owing to the earth’s

spherical form, far too much heat is received at the equator and far

too little at high latitudes to make the earth a suitable habitation

for sentient beings. The function of these two great oceans is to

remove the heat from the equator and carry it to temperate and polar

regions. Aërial currents could not do this. They might remove the heat

from the equator, but they could not, as we have already seen, carry

it to the temperate and polar regions; for the greater portion of the

heat which aërial currents remove from the equator is dissipated into

stellar space: the ocean alone can convey the heat to distant shores.

But aërial currents have a most important function; for of what avail

would it be, though ocean-currents should carry heat to high latitudes,

if there were no means of distributing the heat thus conveyed over the

land? The function of aërial currents is to do this. Upon this twofold

arrangement depends the thermal condition of the globe. Exclude the

waters of the Pacific and the Atlantic from temperate and polar regions

and place them at the equator, and nothing now existing on the globe

could live in high latitudes.

Were these two great oceans placed beside each other on one side of the

globe, and the two great continents placed beside each other on the

other side, the northern hemisphere would not then be suitable for the

present order of things: the land on the central and on the eastern

side of the united continent would be far too cold.

_The foregoing Conclusions not affected by the Imperfection of the

Data._—The general results at which we have arrived in reference to the

influence of ocean-currents on the climatic condition of the globe are

not affected by the imperfection of the data employed. It is perfectly

true that considerable uncertainty prevails regarding some of the data;

but, after making the fullest allowance for every possible error, the

influence of currents is so enormous that the general conclusion cannot

be materially affected. I can hardly imagine that any one familiar

with the physics of the subject will be likely to think that, owing to

possible errors in the data, the effects have probably been doubled.

Even admitting, however, that this were proved to be the case, still

that would not materially alter the general conclusion at which we

have arrived. The influence of ocean-currents in the distribution of

heat over the surface of the globe would still be admittedly enormous,

whether we concluded that owing to them the present temperature of the

equator is 55° or 27° colder than it would otherwise be, or the poles

83° or 41° hotter than they would be did no currents exist.

Nay, more, suppose we should again halve the result; even in that case

we should have to admit that, owing to ocean-currents, the equator

is about 14° colder and the poles about 21° hotter than they would

otherwise be; in other words, we should have to admit that, were it not

for ocean-currents, the mean temperature of the equator would be about

100° and the mean temperature of the poles about −21°.

If the influence of ocean-currents in reducing the difference between

the temperature of the equator and poles amounted to only a few

degrees, it would of course be needless to put much weight on any

results arrived at by the method of calculation which I have adopted;

but when it is a matter of two hundred degrees, it is not at all likely

that the general results will be very much affected by any errors which

may ever be found in the data.

Objections of a palæontological nature have frequently been urged

against the opinion that our island is much indebted for its mild

climate to the influence of the Gulf-stream; but, from what has already

been stated, it must be apparent that all objections of that nature

are of little avail. The palæontologist may detect, from the character

of the flora and fauna brought up from the sea-bottom by dredging and

other means, the presence of a warm or of a cold current; but this can

never enable him to prove that the temperate and polar regions are

not affected to an enormous extent by warm water conveyed from the

equatorial regions. For anything that palæontology can show to the

contrary, were ocean-currents to cease, the mean annual temperature

of our island might sink below the present midwinter temperature of

Siberia. What would be the thermal condition of our globe were there no

ocean-currents is a question for the physicist; not for the naturalist.

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 regarding

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.—Professor Tyndall on the Glacial Epoch.—A

general Reduction of Temperature will not produce a Glacial

Epoch.—Objection from the present Condition of the Planet

Mars.

_Primary cause of Change of Eccentricity of the Earth’s Orbit._—There

are two causes affecting the position of the earth in relation to

the sun, which must, to a very large extent, influence the earth’s

climate; viz., the precession of the equinoxes and the change in the

eccentricity of the earth’s orbit. If we duly examine the combined

influence of these two causes, we shall find that the northern and

southern portions of the globe are subject to an excessively slow

secular change of climate, consisting in a slow periodic change of

alternate warmer and colder cycles.

According to the calculations of Leverrier, the superior limit of the

earth’s eccentricity is 0·07775.[27] The eccentricity is at present

diminishing, and will continue to do so during 23,980 years, from the

year 1800 A.D., when its value will be then ·00314.

The change in the eccentricity of the earth’s orbit may affect

the climate in two different ways; viz., by either increasing or

diminishing the mean annual amount of heat received from the sun, or

by increasing or diminishing the difference between summer and winter

temperature.

Let us consider the former case first. The total quantity of heat

received from the sun during one revolution is inversely proportional

to the minor axis.

The difference of the minor axis of the orbit when at its maximum and

its minimum state of eccentricity is as 997 to 1000. This small amount

of difference cannot therefore sensibly affect the climate. Hence we

must seek for our cause in the second case under consideration.

There is of course as yet some little uncertainty in regard to the

exact mean distance of the sun. I shall, however, in the present volume

assume it to be 91,400,000 miles. When the eccentricity is at its

superior limit, the distance of the sun from the earth, when the latter

is in the aphelion of its orbit, is no less than 98,506,350 miles;

and when in the perihelion it is only 84,293,650 miles. The earth is

therefore 14,212,700 miles further from the sun in the former position

than in the latter. The direct heat of the sun being inversely as the

square of the distance, it follows that the amount of heat received

by the earth when in these two positions will be as 19 to 26. Taking

the present eccentricity to be ·0168, the earth’s distance during

winter, when nearest to the sun, is 89,864,480 miles. Suppose now that,

according to the precession of the equinoxes, winter in our northern

hemisphere should happen when the earth is in the aphelion of its

orbit, at the time when the orbit is at its greatest eccentricity; the

earth would then be 8,641,870 miles further from the sun in winter than

at present. The direct heat of the sun would therefore be one-fifth

less during that season than at present; and in summer one-fifth

greater. This enormous difference would affect the climate to a very

great extent. But if winter under these circumstances should happen

when the earth is in the perihelion of its orbit, the earth would then

be 14,212,700 miles nearer the sun in winter than in summer. In this

case the difference between winter and summer in the latitude of this

country would be almost annihilated. But as the winter in the one

hemisphere corresponds with the summer in the other, it follows that

while the one hemisphere would be enduring the greatest extremes of

summer heat and winter cold, the other would be enjoying a perpetual

summer.

It is quite true that whatever may be the eccentricity of the earth’s

orbit, the two hemispheres must receive equal quantities of heat per

annum; for proximity to the sun is exactly compensated by the effect of

swifter motion—the total amount of heat received from the sun between

the two equinoxes is the same in both halves of the year, whatever the

eccentricity of the earth’s orbit may be. For example, whatever extra

heat the southern hemisphere may at present receive from the sun during

its summer months owing to greater proximity to the sun, is exactly

compensated by a corresponding loss arising from the shortness of the

season; and, on the other hand, whatever deficiency of heat we in the

northern hemisphere may at present have during our summer half year

in consequence of the earth’s distance from the sun, is also exactly

compensated by a corresponding length of season.

It has been shown in the introductory chapter that a simple change in

the sun’s distance would not alone produce a glacial epoch, and that

those physicists who confined their attention to purely astronomical

effects were perfectly correct in affirming that no increase of

eccentricity of the earth’s orbit could account for that epoch. But

the important fact was overlooked that although the glacial epoch

could not result directly from an increase of eccentricity, it might

nevertheless do so indirectly. The glacial epoch, as I hope to show,

was not due directly to an increase in the eccentricity of the earth’s

orbit, but to a number of physical agents that were brought into

operation as a result of an increase.

I shall now proceed to give an outline of what these physical agents

were, how they were brought into operation, and the way in which they

led to the glacial epoch.

When the eccentricity is about its superior limit, the combined

effect of all those causes to which I allude is to lower to a very

great extent the temperature of the hemisphere whose winters occur in

aphelion, and to raise to nearly as great an extent the temperature of

the opposite hemisphere, where winter of course occurs in perihelion.

With the eccentricity at its superior limit and the winter occurring

in the aphelion, the earth would be 8,641,870 miles further from the

sun during that season than at present. The reduction in the amount

of heat received from the sun owing to this increased distance would,

upon the principle we have stated in Chapter II., lower the midwinter

temperature to an enormous extent. In temperate regions the greater

portion of the moisture of the air is at present precipitated in the

form of rain, and the very small portion which falls as snow disappears

in the course of a few weeks at most. But in the circumstances under

consideration, the mean winter temperature would be lowered so much

below the freezing-point that what now falls as rain during that season

would then fall as snow. This is not all; the winters would then not

only be colder than now, but they would also be much longer. At present

the winters are nearly eight days shorter than the summers; but with

the eccentricity at its superior limit and the winter solstice in

aphelion, the length of the winters would exceed that of the summers by

no fewer than thirty-six days. The lowering of the temperature and the

lengthening of the winter would both tend to the same effect, viz., to

increase the amount of snow accumulated during the winter; for, other

things being equal, the larger the snow-accumulating period the greater

the accumulation. I may remark, however, that the absolute quantity

of heat received during winter is not affected by the decrease in the

sun’s heat,[28] for the additional length of the season compensates

for this decrease. As regards the absolute amount of heat received,

increase of the sun’s distance and lengthening of the winter are

compensatory, but not so in regard to the amount of snow accumulated.

The consequence of this state of things would be that, at the

commencement of the short summer, the ground would be covered with the

winter’s accumulation of snow.

Again, the presence of so much snow would lower the summer temperature,

and prevent to a great extent the melting of the snow.

There are three separate ways whereby accumulated masses of snow and

ice tend to lower the summer temperature, viz.:—

_First._ By means of direct radiation. No matter what the intensity of

the sun’s rays may be, the temperature of snow and ice can never rise

above 32°. Hence the presence of snow and ice tends by direct radiation

to lower the temperature of all surrounding bodies to 32°.

In Greenland, a country covered with snow and ice, the pitch has been

seen to melt on the side of a ship exposed to the direct rays of the

sun, while at the same time the surrounding air was far below the

freezing-point; a thermometer exposed to the direct radiation of the

sun has been observed to stand above 100°, while the air surrounding

the instrument was actually 12° below the freezing-point.[29] A similar

experience has been recorded by travellers on the snow-fields of the

Alps.[30]

These results, surprising as they no doubt appear, are what we ought

to expect under the circumstances. The diathermancy of air has been

well established by the researches of Professor Tyndall on radiant

heat. Perfectly dry air seems to be nearly incapable of absorbing

radiant heat. The entire radiation passes through it almost without any

sensible absorption. Consequently the pitch on the side of the ship may

be melted, or the bulb of the thermometer raised to a high temperature

by the direct rays of the sun, while the surrounding air remains

intensely cold. “A joint of meat,” says Professor Tyndall, “might be

roasted before a fire, the air around the joint being cold as ice.”[31]

The air is cooled by _contact_ with the snow-covered ground, but is not

heated by the radiation from the sun.