The realm of nature : an outline of physiography

EOb

REE SE*LIBR ARY

OF THE UNIVERSITY
OF CALIFORNIA

UNIVERSITY EXTENSION MANUALS
EDITED BY PROFESSOR KNIGHT

THE REALM OF NATURE

EDITORS PREFACE

This Series is primarily designed to aid the University Extension
Movement throughout Great Britain, and to supply the need so
widely felt by students, of Text-books for s-tudy and reference, in
connection with the authorised Courses of Lectures.

The Manuals differ from those already in existence in that they
are not intended for School use, or for Examination purposes ; and
that their aim is to educate, rather than to inform. The statement
of details is meant to illustrate the working of general laws, and the
development of principles ; while the historical evolution of the
subject dealt with is kept in view, along with its philosophical
significance.

The remarkable success which has attended University Extension
in Britain has been partly due to the combination of scientific treat-
ment with popularity, and to the union of simplicity with thorough-
ness. This movement, however, can only reach those resident in the
larger centres of population, while all over the country there are
thoughtful persons who desire the same kind of teaching. It is for
them also that this Series is designed. Its aim is to supply the
general reader with the same kind of teaching as is given in the
Lectures, and to reflect the spirit which has characterised the move-
ment, viz. the combination of principles with facts, and of methods
with results.

The Manuals are also intended to be contributions to the Literature
of the Subjects with which they respectively deal, quite apart from
University Extension; and some of them will be found to meet a
general rather than a special want.

They will be issued simultaneously in England and America.
Volumes dealing with separate sections of Literature, Science,
Philosophy, History, and Art have been assigned to representative
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connected with Oxford, Cambridge, London, and the Universities of
Scotland and Ireland.

A list of the works in this. Series will be found at the end of the
volume.

THE

REALM OF NATURE

AN OUTLINE OF PHYSIOGRAPHY

BY

HUGH ROBERT MILL

D.Sc. EDIN.

FELLOW OF THE ROYAL SOCIETY OF EDINBURGH ', OXFORD UNIVERSITY
EXTENSION LECTURER

WITH 19 COLOURED MAPS AND 68 ILLUSTRATIONS

LONDON

JOHN MURRAY, ALBEMARLE STREET
i 892

AUTHOR'S PREFACE

IT is the aim of this volume to illustrate the principles
of science by applying them to the world we live in,
and to explain the methods by which our knowledge of
Nature has been acquired and is being daily enlarged.
An attempt is made to define the place of physical science
in the sphere of human knowledge, and to show the
interrelations of the various special sciences. The
greater part of the book is occupied by an outline of the
more important facts regarding the structure of the
Universe, the form, material, and processes of the Earth,
and the relations which they bear to Life in its varied
phases. Such descriptions must necessarily be brief,
and they are consequently apt to appear more dogmatic
than the discoveries of science warrant ; but care has
been taken to minimise this result. References to
original memoirs are given in cases where the facts
or theories are not yet fully accepted, and the student
is urged whenever it is practicable to read and study
these works.

The Fahrenheit scale of temperature and the British
system of weights and measures are used throughout,
as these are most familiar to the class of readers
expected.

The division into numbered paragraphs is intended
to facilitate the frequent cross-references, which are neces-
sary in order to bring out the interdependence of the
various departments of Nature. The illustrations are

vi Preface

meant to elucidate the text rather than to serve as
pictures. With the exception of those marked in the
list with an asterisk (*), which are adapted from other
sources, they were designed and drawn by the author
and Mrs. H. R. Mill for this book. The maps have
been specially compiled by Mr. J. G. Bartholomew, who
has spared no pains to make them accurate and complete.

The book has been planned and written with the
constant advice and assistance of Professor Knight,
editor of the series, to whom the author desires to record
hearty thanks. Mr. Murray, the publisher of the series,
has also made valuable suggestions, and the title of the
book, The Realm of Nature, is due to him.

Thanks are given to many of the author's teachers and
friends who have kindly revised the proofs of chapters
referring to the departments in which they are authorities,
especially to Professor P. G. Tait, Professor R. Cope-
land (the Astronomer-Royal for Scotland), Dr. A. Buchan,
Dr. John Murray of the Challenger, Professor James
Geikie, Mr. H. M. Cadell, Mr. J. Arthur Thomson, and
Mr. A. J. Ramsay.

H. R. M.

HERIOT-WATT COLLEGE,

EDINBURGH, Atigust 1891. '

CONTENTS

I. THE STUDY OF NATURE

Definition and scope of Physiography . §§ 1-23, p. I

II. THE SUBSTANCE OF NATURE

Propertiesof matterand measurement of space §§ 24-48^. 15

III. ENERGY, THE POWER OF NATURE

Work — Wave-motion — Light — Heat — Electricity — Mag-
netism §§ 49-80, p. 30

IV. THE EARTH A SPINNING BALL

Figure of the Earth — Results of rotation : polarity,
direction, latitude, longitude, time, terrestrial mag-
netism §§ 81-99, p. 49

V. THE EARTH A PLANET

The Moon— Tides— Earth's orbit— The Sun— The Earth's
share of sun-heat . . . §§ 100-125, p. 65

VI. THE SOLAR SYSTEM AND UNIVERSE

Planets — Comets — Meteors — Stars — Nebular and Meteor-
itic hypotheses ....§§ 126-144, p. 84

VII. THE ATMOSPHERE

Air, composition and properties . §§ 145-162, p. 98

VIII. ATMOSPHERIC PHENOMENA

Warmth in air — Dew, mist, clouds, rain, snow and hail
— Lightning — Circulation of atmosphere — Permanent
and seasonal winds . . . §§ 163-185, p. ill

IX. CLIMATES OF THE WORLD

Configuration and climate — Isotherms — Isobars — Warmth
and winds of January and July — Climate of British
Islands — Storms— Weather forecasts §§ 186-213, p. 131

viii Contents

CHAP.

X. THE HYDROSPHERE

Land and Water — Oceans and Seas — Tides — River and
sea-water — Temperature of water — Oceanic currents

§§ 214-250, p. 157
XL THE BED OF THE OCEANS

Divisions of the Lithosphere — Mean sphere level — Abys-
mal and Transitional Areas — Beach-formation — Marine
deposits — Coral islands . . §§ 251-282, p. 188

XII. THE CRUST OF THE EARTH

Rocks — Temperature of the Crust — Volcanoes — Earth-
quakes— Origin of Mountains . §§283-304^.214

XIII. ACTION OF WATER ON THE LAND

Weathering of Rocks— Springs — Rivers — Mountains of
circumdenudation — Lakes — Glaciers §§ 305-340, p. 234

XIV. THE RECORD OF THE ROCKS

Fossils — Classification of rocks — Evolution of continents

§§ 341-353, P- 262
XV. THE CONTINENTAL AREA

Form of the continents, their mountain and river systems
— Configuration of the British Islands §§ 354-392, p. 274

XVI. LIFE AND LIVING CREATURES

Classification and functions of plants and animals — Floral
zones and Faunal realms . . §§ 393-417, p. 307

XVII. MAN IN NATURE

Civilisation and environment — Races of Mankind — Geo-
graphy— Man's power in Nature . §§ 418-436, p. 326

APPENDICES

I. Some Important Instruments

Weights and measures — Mariner's compass — Barometers —
Thermometers — Hygrometers — Anemometers — Deep -
sea soundings . §§ 437~443> P- 343

II. Curves and Maps

Graphic representations — Map - projections — Contour -
lines §§ 444-446, p. 350

III. Derivations of Scientific Terms . . . . -355
INDEX 361

LIST OF MAPS

(Compiled by J. G. BARTHOLOMEW, F.R.G.S.)

PAGE

1. Magnetic Conditions of the Earth, after Admiralty

Chart, 1885 .... . . 62

2. Earthquake Regions and Volcanoes . . . . 92

3. Isotherms for January, after A. Buchan . . . 104

4. Isotherms for July, after A. Buchan . . . . 112

5. Isobars and Winds for January, after A. 'Buchan . 120

6. Isobars and Winds for July, after A. Buchan . . 128

7. Permanent Winds, Calms, and Storms . . .136

8. Mean Annual Rainfall of Land and Salinity of Ocean,

after Loomis, J. Y. Buchanan, and others . . 144

9. British Islands, Isotherms for January, after A. Buchan 152

10. British Islands, Isotherms for July, after A. Buchan . 152

11. Configuration of the Globe . . . . .192

12. Equidistant Coastal Lines ..... 208

13. Drainage Areas of Continents and Co-tidal Lines of

Oceans, after J. Murray and others . . . 262

14. Evolution of Continents, after J. Geikie „ . . . 272

15. Ocean Surface Isotherms, Coral Reefs, Rising and

Sinking Coasts, after A. Buchan, H. B. Guppy, and

others 288

1 6. British Islands, Physical Configuration . . . 304

17. British Islands, Mean Annual Rainfall, and Co-tidal

Lines, after A. Buchan and Charts . . . 304

1 8. Vegetation Zones of Continents and Oceanic Currents,

after Engler and others ..... 320

19. Faunal Realms, after Sclater and A. R. Wallace . 336

LIST OF ILLUSTRATIONS

FIG. SECT. PAGE

1. Interrelation of the Sciences . . . . 21 12

2. Four right angles ....... 31 18

3. Angular measurement of distance . . . . 33 20
4.*Inverse Squares ....... 36 22

5. Swing of a pendulum ...... 54 33

6. Wave-motion 57 35

7. Prismatic Refraction ...... 62 37

8. Diagram of the Solar Spectrum . . . 63 39

9. Curvature of the Earth . . . . . . 81 49

10. Direction of rotation round North Pole . . 88 53

11. Direction of rotation round South Pole . . 88 54

1 2. *Diagrammatic section of the Earth . . . 93 58

13. Revolution of a non-rotating body .... 102 67

14. Revolution of a body rotating in the same time . 102 67

15. Problem of the Earth and Sun . . . .106 70
i6.*Ellipse representing Earth's Orbit . . . . 109 72

i7.*Cause of the Seasons 121 80

1 8. Angle of Light Rays 124 82

i9.*Zones of Climate . . . . . . . 125 83

List of Illustrations xi

FIG. SECT. PAGE

20. Mercurial Barometer . . . . . 146 99

21. Atmospheric Refraction . . . . . 150 101

22. ^Theoretical Circulation of the Atmosphere . . 177 122

23. *Daily Range of Atmospheric Temperature . 182 126

24. *Daily Range of Atmospheric Pressure . . 183 127

25. Sea-breeze . . . . . . . 184 128

26. Land-breeze . . . . . . . 184 128

27. Distribution of Atmospheric Temperature in lati-

tude 187 132

28. Curves of monthly mean temperature . . . 191 136

29. *Isobars of Anticyclone 205 147

30. *Isobars of Cyclone . ...... 207 149

3 1. *Proportion of land and sea in different latitudes . 214 157

32. Curves of temperature in river entrances . . . 232 171

33. *Curves of temperature in the Ocean . . . 235 173

34. Temperature section of Red Sea and Indian Ocean. 236 175

35. Circulation of Water by Wind .... 240 178

36. Section across Atlantic Ocean 20° N. 258 194

37. Steep slopes 260 195

38. *Slopes of the Gulf of Guinea ..... 263 197
39.* Formation of a Beach . . . . . . 265 199

40. *Darwin's Theory of Coral Islands . . •. . 281 211

41. Murray's Theory of Coral Islands .... 282 212

42. Illustration of Rock structures .... 290 218

43. *Ideal Section of a Volcano . . . . . 295 224

44.*Earthquake Wave ....... 300 227

45. Anticline and Syncline ...... 302 230

46.* Production of thrust-planes in rocks . . . 302 231

xii List of Illustrations

FIG. SECT. PAGE

47-*Section of the Alps ...... 303 232

48. Mellard Reade's Theory of Mountain Origin . . 304 233

49. *The origin of Springs . . . . . -314 240

50. * Artesian Wells . . . . . . .314 240

51. Origin of River Windings ..... 323 247

52. Embankment of a River . . ... . 324 248

53. *Ideal Section of Falls of Niagara .... 330 252

54. Map of a Glacier 337 258

55. Section of Loch Goil ...... 339 260

56. Typical Section of a Continent .... 356 277

57. Section across South America in 18° S. . . . 359 280

58. Section across North America in 36° N. . . . 363 282

59. Section along North America in 90° W. . . 367 285

60. Section across Australia in 26° S. . . . . 370 287

61. Section across Africa on the Equator . . . 374 290

62. Section across Asia in 90° E. . . . . • . 380 295

63. *Climate and Vegetation in latitude and altitude . 405 316

64. Photographic Barograph ..... 439 345

65. Mercurial Thermometer ...... 440 346

66. Cylindrical Projection ...... 445 352

67. Conical Projection. ...... 445 353

68. Contour-lines ....... 446 353

CHAPTER I

THE STUDY OF NATURE

1. Physiography means literally the description of
Nature. In order to describe anything we must know
something about it, and in order to know something about
anything we must study it. Knowledge obtained by the
best method of study is science, 'and it differs from know-
ledge otherwise obtained in being so clear and definite that
every step leading to the final result may be recalled and
tested, if any doubt should arise as to its trustworthiness.
Hence description based on science is clear and full, and
this is the kind of description required in Physiography.

2. Nature means all creation ; not only all created
things but also all the changes they undergo. The scope
of Physiography is thus immense but not unlimited. It
includes everything of which we can gain knowledge in the
Earth and beyond it, and every change now happening or
of which a record has been left, together with the causes of
all these changes. It is, however, customary to exclude the
First Cause of all from consideration in connection with the
account of facts and immediate causes. Theology — the
study of the Creator — is in itself an immense field of science,
and although it accounts for the origin of Nature, it may
be readily separated from the study of natural facts and
phenomena. The chief reason for separating Theology from
Physiography is that authorities are greatly divided as to
the right means of studying the former science, while every
one is agreed as to the right method of studying Nature.

B

2 The Realm of Nature CHAP.

A description of the steam-engine which did not refer to
Watt and other inventors and give something of their
biography would . not be held satisfactory unless some ex-
planation of the omission were given, such as the desire to
avoid controversy. For the same reason in a general
description of Nature it is necessary to insist on the relation
of Nature to God, and explain why this relation is not more
fully dealt with.

3. Science is organised and accurate knowledge, and con-
sequently its range has no limits ; it is equally necessary in
order to understand Nature and the supernatural. Science
deals with everything, and its first duty is to classify by
observing resemblances and differences.

4. Comparison and Description. — Suppose that we
were comparing the tastes of different kinds of fruit in
a garden. It is not enough to pluck bunches of red
currants, black currants, gooseberries, and raspberries off the
bushes and eat them. Each bunch must be classified into
berries and leaves or stems ; the former are to be tasted, the
latter to be thrown away and thought no more of. Without
this precaution the taste of gooseberries might be compared
with that of black currant leaves, and different tasters would
give irreconcilable reports. When we compare the various
things around us, a preliminary classification is quite as
much required to ensure that we compare things that are
comparable. If we were to take into account mountains,
pain, rivers, happiness, air, beauty, and motion, the
description would be very confused and puzzling. When
several people who have had the same opportunity of seeing,
describe an event, the descriptions are almost sure to differ
among themselves. This is because a different impression
is produced on each mind, and the various subjective
sensations of interest, or fear, or pleasure, or surprise, are
confused to a greater or less degree with the objective facts.
A scientific description should be as free as possible from
all subjective colouring ; a mountain must not be described
as impressive in its grandeur or beautiful in its colouring,
but as of so many feet in height and composed of such and
such materials. Nature presents us with so many pheno-

i The Study of Nature 3

mena to observe, and these are to all rightly constituted
minds so full of wonder, beauty, and charm that we are apt
to be dazzled and distracted, and even if our attention is
roused it is too often satisfied by the first superficial im-
pressions. It is only by putting aside these and looking at
bare facts and abstract principles that we can truly under-
stand our natural surroundings and so fully appreciate "all the
wonder and wealth " of the Universe in its deepest meaning.

5. Real Tilings. — The first classification of things is into
(a) Things that exist only in our own minds ; (£) things
that exist outside of us and independent of us. Emo-
tions, feelings, tastes, and beliefs belong to the former
class and are termed subjective things. Facts and pheno-
mena which exist whether we know of them and under-
stand them or not, are termed objective or real things.
The real things of Nature are the objects of physical
science, and they alone fall to be considered here. The
one test of reality in Nature is permanence. Only those
things are real which can neither be created nor put out of
existence by human power. Subjective things, such as pain,
happiness, beauty, may be very readily produced and de-
stroyed, hence however vivid the impression of them may be
they are not real in our sense of the word and form no part
of Physiography.

6. Definition of Physiography. — Physiography is an
account of physical science as a whole. It describes the
substance, form, arrangement, and changes of all the real
things of Nature in their relations to each other, giving
prominence to comprehensive principles rather than to
isolated facts. This definition of the term Physiography is
simply a definite statement of the meaning of the word
Physiography (§§ I, 12).

7. Use of the Senses. — Our senses are windows of
knowledge through which alone information enters the
mind, and through which alone we are able to study the
things outside us. Instruments and apparatus of various
kinds are of value only in making the evidence of the senses
more precise or more appropriate to the object of study.
All the senses — sight, hearing, touch, and the less used

4 The Realm of Nature CHAP.

smell and taste — are limited in their scope, and liable to get
out of order through disease or neglect. But even when
in full health and within their own range they are not fully
trustworthy. If an object present different appearances
when looked at through different windows, we are justified
in supposing that the windows are not equally trustworthy.
A few simple experiments show us that this is the case with
the windows of knowledge. Every one is familiar with
optical illusions proving the imperfection of the sense of
sight. A coin spinning quickly looks like a hazy sphere,
but we know it to be a flat disc. Strobic circles which seem
to whirl rapidly when the card on which they are printed
is moved slightly, and designs appearing in their com-
plementary colours on looking at a blank wall have been
made familiar by their use as advertisements. Mountains
always look steeper than they really are ; in a slight haze
on a wide moor a rabbit close at hand may be mistaken for
a distant deer, and the most familiar object is often passed
unrecognised if in an unusual place. One well-known
experiment shows that touch is as fallacious as sight. When
a pea or small ball is rolled on a table by the middle finger
crossed over the forefinger of the same hand, so that both
fingers touch the object, the impression produced is that
there are two peas, not one. Similarly if one hand has been
held in hot water and the other in cold water, and then
both are plunged into a mixture of hot and cold, the
mixture will be pronounced cold by the heated hand and
hot by the chilled one. The deceitfulness of the senses
may impose upon the most acute and practised mind if
taken unawares. When Sir Humphry Davy discovered
potassium he showed a piece of it to Dr. Wollaston, one of
the most accurate observers who ever lived. Wollaston
saw the silvery lustre of the new metal, weighed it in his
hand and said, " How ponderous it is ! " Davy in reply
threw the metal into a basin of water, where it floated
lightly on the surface. Wollaston's illusion is the more
striking because at that time he was the only man who
was in the habit of handling platinum, a metal which, bulk
for bulk, is twenty-five times heavier than potassium.

i The Study of Nature 5

8. Use of Reason. — In spite of such cases of deception,
we trust our senses and are rarely deceived by them.
Reason, man's supreme gift, examines, weighs, extends, and
judges the evidence of the senses. It requires a course of
reasoning to let us know that a tall man far off on a straight
road is not a dwarf close at hand, or that the Moon rising
behind a wood is not a yellow plate hung in the trees.
Long practice has made the operation of reason so swift
and smooth that we are seldom conscious of an interval
between seeing and understanding. Reason makes the
senses satisfactory means for acquiring knowledge, although
reason alone can give no information about natural things.
Just as the senses may be greatly aided by instruments and
apparatus, reason may be greatly aided by mathematics.
And as accurate measurements, on which the value of all
scientific observations depend, can only be made by means
of suitable apparatus, sometimes of a very elaborate nature,
so accurate reasoning, which is essential in all scientific
discussions, can only be fully carried out by mathematical
processes which are sometimes difficult and complicated.

9. Common Sense is the name which practical people
give to the best and easiest way of doing their work, and
the simplest and completest way of gaining knowledge or
explaining any difficulty. Common sense consists of reason-
ing on the evidence of the senses, but without keeping
account of the process. When this common-sense method
is made precise and accurate, it becomes the Scientific
Method of gaining knowledge. The two guardians of
thought in science are Accuracy and Definiteness. The
scientific man deals with phenomena as the banker
does with money, counting and recording* everything with
scrupulous exactness. The student should remember that
for the practical purposes of life the knowledge of what are
called scientific facts is unimportant compared with the
power of using the scientific method. It is really more
scientific to repeat a quotation from a political speech
correctly, or to pass on a story undistorted, than it is to
know of the rings of Saturn or the striation of diatoms.

// " 10. Accuracy in observation usually takes the form of

6 TJie Realm of Nature CHAP.

correct measurement of mass, space, or time, by means of
suitable instruments. Accuracy is always to be striven
for, but it can never be attained. This fact is only fully
realised by scientific workers. The banker can be accurate
because he only counts or weighs masses of metal which
he assumes to be exactly equal. The Master of the Mint
knows that two coins are never exactly equal in weight,
although he strives by improving machinery and processes
to make the differences as small as possible. When the
utmost care is taken the finest balances which have been
constructed can weigh i Ib. of a metal with an uncertainty
less than the hundredth part of a grain. In other words,
the weight is not accurate but the inaccuracy is very small,
and its greatest possible amount is known to 'less than
To o~o o 0 Par* °f ^e mass weighed. In weighing out tea
or sugar a grocer is content if the inaccuracy is not more
than about ^-^ of the mass. No person is so stupid as
not to feel sure that the height of a man he sees is between
3 ft. and 9 ft. ; some are able by the eye to estimate
the height as between 5 ft. 6 in. and 5 ft. 8 in. ; measure-
ment may show it to be between 5 ft. 6| in. and 5 ft. 7
in., but to go closer than that requires many precautions.
Training in observation and the use of delicate instruments
thus narrow the limits of approximation. Similarly with
regard to space and time, there are instruments with which
one-millionth of an inch, or of a second, can be measured, but
even this approximation, although far closer than is ever
practically necessary, is not accuracy. In the statement of
measurements there is no meaning in more than six signifi-
cant figures, and only the most careful observations can be
trusted so far. The height of Mount Everest is given as
29,002 ft. ; but here the fifth figure is meaningless, the
height of that mountain not being known so accurately
that two feet more or less would be detected. Similarly
the radius of the Earth is sometimes given as 3963-295833
miles, whereas no observation can get nearer the truth than
3963-30 miles.

11. Definiteness in thought and description does not
require perfect accuracy in observation. We must always

i The Study of Nature 7

be definite in order to be clear. If he wishes to be definite
in thought the student must never rest content with the
dubious "I think" or the vague "about," but endeavour
after the clear " I know " and the precise " with a prob-
able error of." Vagueness and indecision are utterly
foreign to the scientific method. It often happens that
there is no definite knowledge concerning some fact ; then
all that the scientific method of description permits is
to say, " There is no information," and to wait until the
scientific method of observation has found out something.
The difficulty is not overcome by guessing, or by calling
the unknown unknowable. There is a place for specula-
tion and imagination in the scientific method (§ 18), but it
is a place apart, which must be shut off, for if speculations
are not kept in strict quarantine they are certain to infect
our conceptions of facts with their own fatal vagueness.

1 2. Scientific Terms. — Definite words are necessary for
the expression of definite ideas, hence scientific terms
have to be employed. A term has one definite meaning
which does not change with time. The rush of affairs
drifts words from their original meanings, as ships drag
their anchors in a gale, but terms sheltered from common
use hold to their moorings for ever. The word let, for
example, has drifted in 200 years from meaning hinder
until now it means permit ; but the term bisect has remained
unaltered in significance for twenty centuries. Many
scientific terms are derived from the Greek and have an
unfamiliar appearance ; a list of all those employed in this
book, together with their derivation, is given in Appendix III.

1 3. Classification of the facts and processes of Nature
is necessary before we can form definite 'ideas concerning
them ; but the definiteness of classification is an artificial
restriction. In Nature one thing merges into another by
imperceptible degrees, and although, for example, we can
readily class typical metals and non-metals, typical igneous
and sedimentary rocks, typical plants and animals, there are
in each of these pairs of classes many cases which cannot
be referred with certainty to either side of the dividing
line. Nature is discrete only within certain limits, and its

8 The Realm of Nature CHAP.

classes are never so definite as to isolate one from another,
the unity of Nature being as marked as its diversity.

14. Natural Law is the order in which things have
been observed to happen. The fact that there is order
and not chance in the way things happen is one of the
chief discoveries of science. It is the discovery on which
all science depends, because knowledge could never be
definite and accurate if it were not based on orderly
phenomena. It is impossible that there can be any ex-
ception to a law of Nature, or any contradiction of it.
Much has been written as to the impossibility of miracles
because they would be breaches of the laws of Nature. If
there is evidence, however, that a miracle did happen, the
law of Nature it- appears to contravene must be restated
so as to take account of the new phenomenon. It is be-
cause the law expands to admit apparent exceptions that
we say there can be no exceptions. We have, strictly
speaking, no right to assume that things will continue
to happen in the order in which they have happened
hitherto. Nothing in; time past has been more regular
and uniform in its recurrence than the appearance of the
Sun rising and setting. This regular order is a natural
law, yet we cannot say certainly that the Sun will rise
to-morrow ; merely that its rising is very highly probable.
The law of gravitation, the laws of heat, light, sound, and
of all other observed facts, are similarly the summary of
observations in the past ; and although each new verification
increases the probability that the laws will continue to hold
good, that probability never becomes certainty.

15. Probability. — The probability of 7,000,000 to i
is so great that all but very cautious people think of it as
certainty. It represented the chance of a passenger arriv-
ing alive at the end of a railway journey in the United
Kingdom in the year 1890. The probability that the Sun
will rise to-morrow is far greater than this, because no
failure has ever been recorded in the past. The laws of
Nature, although only expressions of very high probability
as regards the future, may be assumed as quite certain for
all the practical purposes of life.

i The. Study of Nature 9

1 6. Cause and Effect. — The relation of Cause and Effect
is the fundamental law of Nature. There is no recorded
instance of an effect appearing without a previous cause, or
of a cause acting without producing its full effect. Every
change in Nature is the effect of some previous change and
the cause of some change to follow ; just as the movement
of each carriage near the middle of a long train is a result
of the movement of the one in front and a precursor of the
movement of the one behind. Facts or effects are to be
seen everywhere, but causes have usually to be sought for.
It is the function of science or organised knowledge to
observe all effects, or phenomena, and to seek for their
causes. This twofold purpose gives richness and dignity
to science. The observation and classifying of facts soon
becomes wearisome to all but the specialist actually engaged
in the work. But when reasons are assigned, and classifi-
cation explained, when the number of causes is reduced
and the effects begin to crystallise into essential and clearly
related parts of one whole, every intelligent student finds
interest, and many, more fortunate, even fascination in the
study.

17- Inductive and Deductive Reasoning. — Reason
may be applied to the study of facts in two different ways.
Inductive Reasoning is the arduous process of finding the
meaning of phenomena by collecting and classifying facts
and thinking out their causes. Deductive Reasoning is the
shorter operation of finding what effects must result from
the operation of a known cause. It is often supposed that
since we can observe facts alone the inductive method of
reasoning is the only one which can be employed in study-
ing Nature, but the number of facts even in one small
department is so great that life is not long enough for the
labour of collecting, classing, and discussing them all.

1 8. The Scientific Method of discovering the causes of
phenomena involves the use of both inductive and deductive
reasoning linked together by imagination, a mental power
which is as essential to the scientific discoverer as it is to
the poet. After observing a considerable number of facts
the investigator imagines a possible cause or explanation,

io The Realm of Nature CHAP.

and this possible explanation is termed an hypothesis. Then
he reasons deductively from the assumed explanation, usually
employing mathematics for the purpose, and so arrives at a
number of additional facts which must exist if the hypo-
thesis be true. These predicted facts may not be familiar
or may not occur naturally at all. In the latter case it is
necessary to seek them by making experiments, and so
important is this aid in some cases that the expression Ex-
perimental Science is often used in the sense of physical
science. If the facts predicted to exist in certain circum-
stances by hypothesis are not found, and if others which the
hypothesis could not account for appear, the hypothesis
is proved to be erroneous, or, at least, incomplete. Renewed
inductive reasoning from the wider basis of ascertained facts
must then furnish material for a fresh effort of imagination
and a new hypothesis to be similarly tested, and, if neces-
sary, rejected in turn. Should the facts agree with those
deduced from the hypothesis there is a probability of its
being true, but a great many tests must be thought of,
applied, and found realised before the hypothesis is accepted
as a true and complete explanation. An explanation of
facts found, tested, and proved to be true and complete
in this way is called a theory, and when a theory is con-
firmed by a great number of observations it is accepted as
a Law of Nature.

19. Proof of a Theory. — The process of testing an
hypothesis requires great caution in order to prevent mis-
takes. A long time and the labour of many observers are
often necessary to perfect a theory or demolish an incorrect
hypothesis. When Newton imagined the hypothesis of
universal gravitation, according to which the force that
causes a stone to fall to the ground also controls the motion
of the Moon round the Earth and of the Earth round the
Sun, he deduced from the hypothesis that the Moon in its
orbit should fall toward the Earth 15 feet in a minute.
Careful observation of the Moon's motion showed that it
was only bent toward the Earth 1 3 feet in a minute, and
therefore Newton abandoned his hypothesis as untrue.
Thirteen years later a new measurement of the size of the

i The Study of Nature u

Earth, and consequently of the distance of the Moon, gave
him more accurate data, and applying these to his hypothesis
and to the observations, he found that the discrepancy
vanished. This assured him of the truth of his hypothesis,
which has ever since taken rank as a theory and a law of
Nature.

20. Test of a Law of Nature. — A law of Nature has
no exceptions (§ 14) ; the only test by which a theory can be
accepted as of this rank is the successful prediction of future
effects. The theory of gravitation enables astronomers to
calculate the relative position of the Sun, Moonj planets, and
stars as seen from all parts of the Earth's surface. This is
regularly done by a government office in London, and the
positions for stated times each day are published three years
in advance in the Nautical Almanac. From the tables of
this work the captains of ocean-going vessels are able to
work out their exact place on the ocean by observations
of the positions of the heavenly bodies (§ 92). The
smallest deviation from truth in the expression of the law of
gravitation would throw the results into confusion and lead
to almost certain shipwreck. No such confusion has ever
occurred, and every successful sea-voyage is one proof more
that the law of gravitation was fully understood in the past,
and holds in the present. The appointments made for
the appearance of the Sun, Moon, and planets amongst
special groups of stars at definite times, in the Nautical
Almanac are analogous to the appointments_for the arrival
of trains at stations made in official railway time-tables.
Observation of the fulfilment of time-table predictions very
soon demonstrates that the hypothesis in* accordance with
which they are framed is not exact, and cannot be depended
upon for timing watches or determining our position on the
Earth.

21. Magnitude of Nature. — The Scientific Method is
applicable to the acquisition of knowledge of any kind, but
it has been most used in the study of Nature. It is neces-
sary that each scientific investigator should confine himself
to one department of Nature in which he finds the facts and
tries to reason out the theories connecting them. Thus we

ff OF rnf T>»>

K UNlVEBsixy

12 The Realm of Nature CHAP.

are apt to form the impression that Physics, Chemistry,
Astronomy, Geology, Geography, Meteorology, Biology are
definite sciences, distinct from each other, dealing with dif-
ferent orders of facts which are accounted for by independ-
ent theories. These sciences do not completely cover the

FIG. i. — Inter-relation of the Sciences.

field of Nature as the coloured blocks of counties do the map
of England. They traverse the field rather like the railway
lines which radiate from London. The main line of each
science is distinct and easily followed, but the branches
interlace with one another in a very complex manner, and
though the network is very comprehensive, a mere fraction of
the vast surface is after all covered with the lines of definite
knowledge. The inter-relations of the sciences are shown in

i The Study of Nature 13

Fig. i by representing each as a circle cutting all the others,
for on the outskirts of every science there are regions in
which another science shares the explanation of phenomena.
Chemistry, for example, is called in to aid astronomy in inter-
preting the spectra of the stars, to aid geology in explaining
the composition of rocks, to aid biology in determining the
changes of substance in living creatures. Physics or Natural
Philosophy in a sense includes every other branch of physical
science, although portions of Biology and Geography extend
beyond its limits.

22. Physiography and the Special Sciences. — By
division of labour the various parts of a watch are con-
structed by different workmen, and by the specialisation of
science the different realms of Nature are explored by
different investigators. In order to have a watch, however,
the results of divided labour must be combined, and in
order to have a just conception of the Universe the results
of specialised research must be fitted harmoniously together.
This is the function of Physiography, which has consequently
a unique value in mental training, being at once an in-
troduction to all the sciences and a summing up of their
results. • It enables a beginner to obtain a quicker insight
into any of the special sciences and a fuller grasp of it,
while at the same time a student versed in any one special
science is enabled to appreciate far more fully than an
unversed one its relation to all others and to the system of
the Universe.

23. Physiography and Nature. — The natural Universe
may be compared to a gorgeous carpet of rich design. In
order to understand such a web we might follow out the
pattern thread by thread. Selecting first a red thread of
the weft, we notice how it passes above and below the
threads of the warp, across the fabric and back again, to
and fro until the end. Next a blue thread may be followed
in the same way, and so with all the separate colours.
The course of each thread has explained something, but the
results of all must be brought together to give a complete
explanation. In some such way each special science un-
ravels one of the threads of the Universe, but that thread is

14 The Realm of Nature CHAP, i

so interwoven with the clues of other sciences that a general
knowledge of them all is necessary to understand the effect
produced by one. Pursuing the simile a step farther, we
may note how one observer sees in the rich world-carpet
nothing but a number of coloured threads intricately inter-
woven ; the taste of another is so much gratified by the
colour and design that he enjoys the beauty without think-
ing of the parts or the process ; while a third loses sight of
material and beauty alike in admiration of the genius of
the designer and the skill of the craftsman. Thus the typi-
cal man of science, poet, and theologian look differently
on the multiform unity of Nature, which has a true though
different meaning for each.

BOOKS OF REFERENCE

T. H. Huxley, Science Primers — Introductory. Macmillan and
Co.

W. S. Jevons, Principles of Science. Macmillan and Co.

CHAPTER II

THE SUBSTANCE OF NATURE

24. Matter. — Diverse and innumerable as the things
around us seem to be, the number of kinds is reduced greatly
when they are tested by trying to destroy them. Only what
cannot be annihilated is real, according to our definition
(§ 5). Tested thus, air, wood, marble, vinegar, to take a
few random examples, appear unreal, for they can be
produced and destroyed. Closer study shows that though
burning destroys both wood and air it produces at the same
time other things — ashes, water, carbonic acid, nitrogen —
exactly equal in amount though different in properties.
Vinegar and marble are both destroyed by mixing them,
but other things — calcium acetate, carbonic acid, water —
appear in exactly the same amount. So with all the things
we see or feel, their properties and appearance can be
completely changed, but the amount of substance that
exists in them cannot be increased or diminished by any
power which man has learned to wield. Substance is thus
a real thing, of which air, wood, marble, Vinegar and the
rest are kinds. The term Matter is applied to every-
thing, however diverse in appearance, which we see and
touch, as Man is the term used to include every human
being in the world. The difference between some kinds
of matter is as slight and superficial as that between soldiers
and chimney-sweeps ; between other kinds it may be com-
pared to that which separates Europeans from Negroes.

25. Energy. — There is another real thing which does

1 6 TJie Realm of Nature CHAP.

not appeal so directly to our senses as matter does ; fifty
years ago it was unknown and a long course of reasoning
was necessary to convince investigators of its existence and
reality. Nothing appears more readily produced or destroyed
than motion, heat, or light. Motion is destroyed in a
railway train by applying the brake, in a bullet by contact
with the target. Heat can be destroyed by using it up in
a steam-engine ; the visible motion of an engine can be
destroyed in turning a dynamo -electric machine ; electric
currents can be destroyed in an incandescent lamp ; light can
be destroyed by allowing it to fall on a black surface. Hence
none of these things is real in itself. But when motion is
stopped in a train heat is invariably produced, the wheels
sometimes becoming red-hot. When heat is destroyed in
a steam-engine, visible motion is produced ; when motion
is destroyed in a dynamo -electric machine, electricity is
produced ; when electricity is destroyed in a lamp, light is
produced ; and when light is destroyed by falling on a black
surface, heat is produced. More than this, the amount of
heat, motion, electricity, light produced is the precise
equivalent of what is destroyed in producing it. All are
capable of doing work of some kind, and this power of
doing work can neither be created nor destroyed, its amount
can neither be increased nor diminished. Energy is the
name given to this real thing.

26. Matter and Energy in Nature. — Besides matter
and energy nothing has been proved to have an independent
existence. The whole of Nature consists of the two grand
parts, that which works and that which is worked on. The
two are quite inseparable, for work of every kind has been
proved to necessarily involve motion through a large or a
small space in straight or curved lines, and motion is in-
comprehensible except as some piece of matter moving. It
is only through matter that we recognise energy, and only
through energy that we recognise matter. It has been
proved in some cases, and is possibly true in all, that the
properties which distinguish different kinds of matter from
each other are due to the different amounts of energy with
which they are associated.

ii The Substance of Nature 17

27. Matter is that which occupies space. This defini-
tion is in many ways the most satisfactory ; but although
attempts to say what matter is have been made by philo-
sophers in all ages, no really sufficient definition has ever
been arrived at. Matter is often defined as that which can
be perceived by the senses.

28. Mass is the term used to denote quantity of matter.
Thus when the mass of the Sun is spoken of as being
300,000 times that of the Earth, it is meant that the Sun
contains 300,000 times as much matter as the Earth con-
tains. Mass is usually measured out by the balance,
and it is common to speak of the mass of any portion of
matter as its weight (§ 38), although on the same principle
we might speak of a man's health as his appetite. The
unit of mass in British Possessions and the United States
is the pound ; in almost all other civilised nations it is the
kilogram. (See § 437.)

29. Volume and Density. — Volume is the amount of
space occupied by a body, and if matter were of one kind and
always in the same state, the same mass would always fill
the same volume. But matter exists in many forms, and
if, for example, we compare together charcoal, lithium, coal,
granite, arsenic, lead, and platinum, we find that the same
volume contains very different quantities of matter. Indeed,
the mass of a cubic inch of platinum is twice that of a cubic
inch of lead, four times that of arsenic, eight times that of
granite, sixteen times that of coal, thirty-two times that of
lithium, and sixty-four times that of charcoal. So that these
parcels of matter are packed with different degrees of
tightness, as much as is present in 64 -cubic inches of
charcoal being packed within the limits of I cubic inch of
platinum. The amount of matter in a unit of volume is
called its density ; thus in the list given above the density
of each substance mentioned is twice that of the preceding.
The unit of density universally employed is that of water,
and calling this I the densities given above run : —

Charcoal. Lithium. Coal. Granite. Arsenic. Lead. Platinum.

0-34 0-59 1-33 2-70 5-96 11-36 21-53

i8

The Realm of Nature

CHAP.

The density of each kind of matter is very distinctive ; that
of quartz, for example, is 2-6, that of the diamond 3-5 ; and
by means of this difference diamond buyers at once detect
any attempts at fraud. The term specific gravity is often
used to express the ratio of the density of substances to
that of water.

30. Form. — The form which different kinds of matter
assume varies greatly, and can be easily changed. Pure
kinds of matter, i.e. elements and compounds (§§ 42, 45),
when allowed to solidify or separate out of solution fre-
quently assume a shape of beautiful symmetry — metallic
bismuth, alum, or quartz, for example — and these definite
forms are spoken of as crystals. Mixtures, and sometimes
pure kinds of matter, have no special form naturally, but
occur as they were moulded in the cavity or vessel contain-
ing them, or as they were broken off from larger pieces.
These are often spoken of as amorphous or formless. The
forms of crystals are so characteristic that the minutest
trace of some compounds may be recognised by their
appearance under the microscope.

31. Angular Measurement. — In considering the form

and position of bodies re-
gard must be had to the
properties of space, and
especially to the nature
and use of angles. An
angle is the inclination of
two lines which meet at a
point and may be measured
by a certain definite amount
of turning done by a line ;
the angle APB in Fig. 2
is the amount of turning
in a line from the position
PA to the position PB.
If the line PA were drawn

FIG. 2. — Four right angles.

on a piece of card pivoted to a table by a pin through P, and
if it were made to turn completely round, as shown by the
arrow, until it came back to its original position, the end A

ii Tlie Substance of Nature 19

would have pointed in turn all round the room ; or if the
table were in the open air, all round the horizon. The space
of a whole turn is divided into 4 equal quadrants or quarters,
each of which is called a right angle, and the amount of
turn in a right angle is divided into 90 equal steps called
degrees (°), each degree being the 36oth part of a whole
turn. Every degree is subdivided into 60 equal parts called
minutes ( ' ), and each minute into 60 parts called seconds
("). An angle of i" is thus simply a short name for "the
i, 296,oooth part of a whole turn," and small though this
is> _ij. of a second or less can be measured in fine instru-
ments. The amount of turning from the horizon or sky-
line of a plain to the zenith or point directly overhead is
one-quarter of a complete turn or a right angle, i.e. 90°.
Degrees, minutes, and seconds are thus simply fractions of
the unit which is a turn ; and a turn is the same whether
the turning line sweeps round the horizon, the Earth's
equator, or a watch dial.

32. Position by Angles. — By fixing points from which
to begin the reckoning it is evident that two sets of
angles will enable one to define the position of any object
on a sphere, such as the sky. In the case of a star, for
example, by taking the north point of the horizon as a zero,
one first measures the number of degrees, minutes, and
seconds of turn until directly under the star, noting in
which direction (toward east or west) the turn is taken.
Then from the horizon at that point one measures the
number of degrees, minutes, and seconds of turn toward the
zenith to the star. Angular distance round the horizon is
called the azimuth of a point ; angular dis-tance toward the
zenith from the horizon is called its altitude. The instru-
ment which is most convenient and most generally used for
measuring angles of all kinds is the Sextant.

33. Measurement of Distances by Angles. — Every
one must have noticed in passing a church clock that if,
when standing directly opposite it, he sees the long hand
pointing exactly to XII, yet from a little distance on one
side he sees it to be a few minutes before, and from the
same distance on the other side a few minutes past the

20 The Realm of Nature CHAP.

hour. This is because we look at the hand from different
points of view and at different angles with the direction in
which we are going. By measuring the angles, and the
distance between the two points of observation, it is possible
to calculate the distance of the object by trigonometry — the
part of mathematics dealing with triangles. Suppose that
in Fig. 3 the distance from A to B, which is called the base-
line, is 100 yards, and that by means of a sextant the angles
PAB and PBA are measured, then since all the angles of a
triangle are equal to two right angles, the angle at P can be
got by a simple subtraction, and an easy calculation would
give us the distance of P from the eye. The more nearly

FIG. 3. — Angular measurement of distance. AB, base-line, P, Q, vertical angles.

equal the three angles of APB, the more accurately can this
distance be found. For example, from the same base-line
the angles to the hand of a much more distant clock would
scarcely differ from right angles ; the angle at Q would be
so minute that the least mistake in measuring the two large
angles would put the calculation all wrong. The more
nearly the base-line is equal to the other sides of the triangle,
the more exact is the trigonometrical measurement of
distance. In this illustration the angle at P might be
measured without a sextant by noting the amount of dis-
plac£inent of the long hand of the clock on the dial. In
the distant clock the displacement would be too slight for
the eye to detect.

34. Exclusiveness is a term descriptive of the way in
which matter occupies space. It means that when one
portion of matter is in a certain space no other portion of
matter can be in the same space. The fact that a quantity
of water can be absorbed by a sponge without much in-
creasing the volume is no argument against this statement,

ii The Substance of Nature 21

for the water occupies only the cavities between the sponge
fibres. The particles of many kinds of matter are packed
loosely together so that vacant spaces or pores occur.
Porous bodies, like unglazed earthenware, sandstone, and
charcoal, apparently allow air or water to pass through
them ; really, however, the fluid passes through the other-
wise empty pores. The exclusiveness of the space-occupa-
tion thus holds good for the smallest particles of matter
only (§ 48). The term impenetrability is often used for
this property.

35. Stresses and Strains. — When the form or volume
of a body is altered the body is said to be strained, and
the set of forces which produce a strain is called a stress.
Stresses act always in two opposite directions, either as a
push or a pull. Rigidity is the resistance that a solid body
offers to shearing stress. Extremely rigid substances, such
as steel, require the action of powerful stresses in order to
change their form ; while less rigid substances may be
readily deformed or strained, as a rod of lead is bent or
a piece of sandstone pounded into dust. When uniform
pressure is applied all solid substances, and still more all
liquids and gases, are reduced in volume, the matter in them
being compressed into smaller space and the density being
of course increased. The amount of compression which the
same pressure effects is called Compressibility and it differs
in various kinds of matter, being greatest of all in gases
(§§ 72, 148). The tendency of a body to recover from
strain and return to its previous form and volume when the
stress ceases to act is termed Elasticity. A steel watch-
spring is said to be elastic, because afte-r being coiled up
tight it returns to its former size and shape. Air is said to
be elastic, because when it has been compressed and the
pressure is removed it returns at once to its previous
volume.

36. Gravitation. — Every portion of Matter attracts or
tends to approach every other portion of Matter in the
Universe ivith a force proportional to the masses and
inversely as the square of the distance. This is Newton's
Law of Universal Gravitation, and is established beyond

22

The Realm of Nature

CHAP.

doubt (§§ 19, 20), yet no one understands what gravitation
is nor how it produces its remarkable effects. The greater
the mass of two bodies, the more strongly do they attract ;
if the total mass is doubled the attraction is doubled. The
nearer they are the more strongly do they attract in the

proportion that halving the
distance increases the at-
traction fourfold, reducing
the distance to one-third in-
creases the attraction nine-
fold. Fig. 4 illustrates the
law of inverse squares as
applied to central forces.

37. The meaning of
"Down." — If two distant
bodies equal in mass could

FIG. 4. — Inverse Squares. The gravita-
tional force of O acting on the square
at i, is spread over four times the area
at 2, and nine times the area at 3, so

be left free to follow the
attraction of gravitation, they would approach each other and
meet midway. But if one of the distant bodies had a much
larger mass than the other it would move a shorter distance,
because the result of attraction is to give the same amount
of motion or momentum to each (§ 50). If one body is
very large and the other very small, the small body seems
to fall to the larger, while the latter does not apparently
leave its place. This is the case of a stone outside the
Earth's surface. It falls directly toward the centre, and the
word " down " is used to designate this direction. The
movement of the Earth to meet the stone is so slight that it
cannot be detected, nor very easily expressed by figures.
Still the attraction of gravitation is equal and opposite, the
stone attracting the Earth as much as the Earth attracts the
stone.

38. Weight. — The attraction of the Earth would draw
an external body down to the centre, but the rigidity of the
Earth's crust resists distortion. Those parts of the surface
which possess no rigidity (the oceans) allow any body
denser than water to pass through, or sink in obedience to
the pull of gravity until it reaches the solid crust below.
The pull of gravity which is counteracted by the push of

ii The Substance of Nature 23

rigidity is of course greater for greater masses, and the
amount of the pull in any case may be measured by
pulling against it. Weight is the name given to the pull
of the Earth upon some other body. At any definite
distance from the Earth's centre the weight of a body is
proportional to its mass, and hence it is that when we want
one pound mass of tea we ask for one pound weight. If
any mass is removed to a greater distance from the Earth's
centre the pull upon it is diminished, or, in other words, its
weight is less ; if it is brought nearer the centre (without
passing inside the Earth) the pull upon it is increased,
or the weight is greater. Weight, or " Earth-pull," is
measured by means -of the spring -balance or by the
pendulum. On account of the uniform pull of the Earth's
gravitation, liquids, which have no rigidity, assume a level
surface, or rather a surface parallel to that of the Earth. One
of the necessary conditions for equilibrium in a liquid is that
all points in the same plane are subject to the same pressure,
hence the level of water in a series of connected vessels is
always the same. Hence also if the height or the density
of a column of liquid is altered equilibrium is destroyed,
and the liquid moves under the influence of gravity until
it again becomes homogeneous and of level surface (see
§ 238). Gravitation is a property which affects every kind
of matter alike, and it binds together the great masses of
the Universe into a firm and flexible whole.

39. Cohesion. — When the distance between particles of
matter is very minute — too small to be measured — the force
of attraction is very great, and binds the particles together
very firmly. In this case it is called cohesion. It is by the
powerful attraction of particles of matter at very minute
distances that a stone is wetted or covered with a thin liquid
film when dipped in water. These forces are also shown at
work when a liquid rises in a narrow tube, or in a porous
body like a sponge, a lump of sugar, or a piece of sand-
stone. This raising of liquids is called capillarity because
it is best seen in tubes whose bore will just admit a
hair, but it is quite visible on the sides of a tumbler.
Another manifestation of the same force is seen in surface

24 The Realm of Nature CHAP.

tension, or the tendency all liquid surfaces have to become
as small as possible. A small portion of a liquid when
thrown off as a drop shrinks into a little sphere, because
a . sphere has the smallest surface possible containing a
given volume. A soap-bubble blown on the wide end
of a glass funnel contracts and creeps up to the narrowest
part of the tube when left to itself. Surface tension
accounts for such phenomena as the rapid spreading of a
film of oil over a wide surface of water, and the extra-
ordinary gyrations of a piece of camphor floating on clean
water.

40. Analysis and Synthesis. — If we wish to find out
for ourselves of what parts a piece of mechanism, such
as a watch, is composed, we must begin by unloosening
the parts from one another and taking the watch to pieces.
So when we wish to find of what parts a piece of matter,
such as a rock, is made up, we must unloosen its parts and
take it to pieces. This process is called by the Greek
name of analysis. There is another process sometimes
employed : we might imagine a watch so strongly made
that it could not be taken to pieces, but if we had seen
the parts put together to .make it, we would know of what it
was composed. This putting together is called synthesis,
and the process is sometimes used for investigating kinds
of matter.

41. Mixtures. — We may take a piece of granite as
typical of a pure kind of matter which is easily recognised
by its characteristic appearance. On examining it with
the eye we see that it is made up of three different
substances. One of these is clear and glassy, breaking
with a sharp edge, and hard enough to scratch glass. It
is called quartz. Another is milky and opaque, whitish
or pinkish in colour, too soft to scratch glass, and when it
is broken it splits into regular smooth -sided blocks of
similar shape. It is called felspar. The third ingredient
is silvery or black in appearance ; it forms flakes which are
soft enough to be scratched by the nail, and flexible, split-
ting up into thin transparent scales. It is called mica.
Granite, then, is a mixture of quartz, felspar, and mica,

ii The Substance of Nature 25

and the proportion of each ingredient varies in different
specimens. In a mixture each ingredient retains all
its own properties, and so can readily be recognised and
separated. A mixture of sand, salt, and sawdust, for
example, could be separated by throwing it into water, in
which the sawdust would float, the sand sink, and the salt
dissolve.

42. Compounds. — Quartz, felspar, and mica may be
examined as closely as the most powerful microscope allows,
but no sign of any of them being a mixture will appear.
Every one part of quartz is exactly like every other. Quartz,
which is also called silica, can be separated into two sub-
stances by means of certain processes explained by the
science of chemistry. One of these substances is a brown
opaque solid called silicon^ the other an invisible odourless
gas named oxygen. Silica is not called a mixture but a
compound, the distinction of which is that the components
lose all their characteristics and unite to form a homo-
geneous substance, different in its properties from any of
the components. For example, the metal magnesium is a
tough lustrous solid ; oxygen is an invisible gas present in
the air ; the compound resulting from their union is a soft
snow-white powder. The composition of compounds is
always exactly the same, the same proportion of each
component being always present. Silica is invariably
composed of 1 4 parts by mass of silicon and 1 6 of oxygen ;
magnesia always contains 24 parts of magnesium and 16
of oxygen.

43. Analysis of Granite. — Felspar may be analysed
into silica, alumina, lime, and potash, each one of which
is in itself a compound ; and Mica can be analysed into
silica, alumina, magnesia, potash, water, and iron oxide,
all of which are compounds. The ultimate components
are termed elements, of which some, such as oxygen and
silicon, are classed as non-metals, the others as metals.
Thus :—

26 The Realm of Nature CHAP.

GRANITE

'QUARTZ FELSPAR MICA"

SILICA /Silicon S!LICA
\ Oxygen

MAGNESIA (gpi-

IRONOXIDE{ Oxygen
WATER (Hydrogen
I Oxygen

44. Acids and Bases. — Two classes of compounds
require to be specially mentioned. The non-metal oxygen
when it unites with a metal produces a compound called
a basic oxide^ and this is the case whether we consider the
gaseous metal hydrogen, the liquid metal mercury, or any
of the solid metals such as magnesium, calcium, or potas-
sium. When oxygen unites with another non-metal, such as
carbon, silicon, or sulphur, it produces an acid oxide. The
main characteristic of basic oxides and acid oxides is that
when brought together they unite to form more complicated
compounds called salts. A certain amount of each acid
oxide unites with a certain amount of each basic oxide to
form a compound showing neither acid nor basic properties,
but in many cases an additional definite amount of acid or
of basic oxide takes part in the compound which then
shows a more or less distinct acid or basic nature. Other
non-metals, such as sulphur and chlorine, unite with metals
to form compounds or salts termed sulphides and chlorides.
Energy in the form of light or heat is given out when
elements combine, and a precisely equal amount of energy
must be used up on the resulting compound in order to
decompose it. When much energy is involved in the trans-
action the compound is said to be a firm one.

45. Elements. — The process of analysis ceases when
we come to oxygen, silicon, aluminium, etc., for no

ii The Substance of Nature 27

method yet attempted has been successful in breaking

up any of these substances into other kinds of matter,

hence they are called the

simple substances or ele- ELEMENTS OF THE EARTH'S

ments. There are about CRUST.

seventy elements known to

Oxygen . . . 50-0
Silicon . . .25-0

Aluminium . . 10-0

Calcium . . . 4-5

Magnesium . . 3-5

Sodium and Potassium 3 >6
Carbon, Iron, Sulphur, 1

and Chlorine

All others

Total i oo-o

chemists, but those which
have been enumerated, to-
gether with carbon, appear
to make up by far the greater
part of the mass of the Earth.
Professor Prestwich gives
the accompanying estimate
of the proportion in which
each of the common elements
occur in the Earth's crust.

46. Transmutation of Elements. — For centuries the
alchemists firmly believed that one element could be
turned into another, and hundreds of men spent their
fortunes and their lives in seeking the " Philosopher's
Stone" which would bring about the magic change of
lead to gold. In more recent times, as the knowledge of
the properties of matter has increased, the possibility of
such a change has been generally conceded ; but although
several modern chemists have believed that they got evidence
of transmutation, the fact has never been proved. The re-
arrangement of the particles with regard to each other in
one kind of matter produces great changes in the outward
properties. Charcoal and diamond are simply forms of
pure carbon, and each has been changed into the other by
the action of energy in certain ways. "Hence it appears
possible that the separate elements may themselves be
simply different groupings of the one real thing we call
matter, associated with different amounts of the other real
thing we call energy.

47. The Periodic Law. — Elements are roughly classed
into metals and non-metals, but there are intermediate ones
which it is not easy to assign to either division. A more
natural grouping was discovered by Mr. Newlands in England,

28 The Realm of Nature CHAP.

and Professor Mendelejeff in Russia, and is known as the
Periodic Law. This states that if the elements are arranged
in the order of the mass of their smallest particles, Le. their
atomic weight, they will fall into eight groups of about twelve
elements each, and the first, second, third, etc., element of
each group bears a strong family resemblance to the first,
second, third, etc., of each of the other groups. Some of
the groups have many gaps, only seventy elements being as
yet known ; but the atomic mass, the density, the melting
temperature, the colour and the nature of the compounds it
would form with known elements can be calculated and
predicted for each of the elements which are absent. Names
have even been given to these hypothetical elements, and in
at least two cases the elements were subsequently discovered
by chemists and found to correspond very closely to the
prophetic description. This fact was the strongest con-
firmation of the truth of the Periodic Law. If the figures
known to chemists as " atomic weights " really correspond
to the mass of the atoms of each element, as there is reason
to believe that they do, the chief difference between the
elements may consist in the fact that their smallest particles
contain different amounts of matter ; the extreme cases are
uranium and hydrogen, the mass of the atom of the former
being 240 times that of the latter. We could imagine a great
rock to be quarried into blocks of ninety-six definite sizes,
the smallest being only -^Q- of the largest, and ship-loads of
these cut and squared stones might be sent to a nation
where tools were unknown. These people might use the
stones in building houses, but would be unable to change
any one size into another until they invented the proper tools.
They might be supplied only with sixty or seventy of the
sizes, but by studying the weights of these and seeing the
order in which they ran they might predict the existence of
intermediate sizes. As they could not in the absence of
tools change the form or size of the blocks, though recognis-
ing their unity of composition, they would look on them as
unalterable elements in their building. Similarly modern
chemistry has enabled us to understand how it is possible
that the elements are merely separate parcels of matter which

ii The Substance of Nature 29

may be broken up and rearranged when the proper tools
are found.

48. Structure of Matter. — Any element or compound
appears perfectly homogeneous under the most powerful
microscope, but the investigations of scientific men prove
that there is a limit to homogeneity. The smallest particles
of which matter consists are far too minute ever to become
visible — the smallest visible speck is calculated to contain
more than 50,000,000 of them. By careful experi-
ments and ingenious reasoning Sir William Thomson has
shown that matter is made up of particles so small that
if a little cube I inch in the side were magnified until it
was 8000 miles in the side, neighbouring particles would be
i inch apart ; in other words, there are about 500,000,000
particles in the length of an inch. The study of chemistry
has shown that each particle must, in almost every case,
consist of at least two, but probably many, parts called
atoms which cannot exist separately but always form groups.
The atoms of every element are different from those of every
other element ; but each atom of any element is exactly
like all the other atoms of that element. Sir John Herschel
compared the immense numbers of exactly similar atoms of
hydrogen or of iron or of oxygen that are found on the
Earth, in the Sun, and in the remotest regions of space,
to manufactured articles all turned out by the same process
and all trimmed to exactly the same size and pattern. Sir
William Thomson has shown that it is possible to explain
the structure of matter as made up of myriads of minute
vortex rings or whirlpools set up in a perfect fluid which
fills all space.

BOOKS OF REFERENCE

P. G. Tait, Properties of Matter. A. and C. Black.
H. E. Roscoe, Lessons in Elementary Chemistry. Macmillan
and Co.

CHAPTER III

ENERGY, THE POWER OF NATURE

49. Energy is the power of doing work. Work, in the
scientific sense, is any change brought about in the position
of portions of matter against resistance. Change of position
implies motion, and thus work may be spoken of as the
moving of matter. Lifting water from a well by means of
a bucket and rope is work against the resistance of gravity ;
tearing a piece of paper is work against the resistance of
cohesion ; pulling a piece of iron from a magnet is work
against the resistance of magnetic attraction, and so on.
Work is measured by the resistance overcome, and the
distance through which it is overcome ; the resistance
usually chosen for this purpose is weight or the pull of the
Earth on matter in consequence of gravitation (§ 38). In
English-speaking countries the unit of work usually adopted
is the foot-pound, the amount of work necessary to raise I
Ib. weight to . the height of I ft. The work of raising I o
Ibs. i ft. is 10 foot-pounds, and the work of raising i Ib.
10 ft. is 10 foot-pounds also. The work a man of 150
Ibs. weight does in climbing to the top of a mountain 10,000
ft. high is 1,500,000 foot-pounds, as much work as lifting
170 tons of coal from the ground up to carts 4 ft. high.

50. Newton's first Law of Motion expresses the property
of Matter called Inertia, thus : All bodies remain in a state
of rest or of uniform motion in a straight line except when
compelled by some external power to change that state. On
the Earth friction is always at work retarding motion. A

ISOBARS AND Wlf

After P

160 180 160 140 120 1OO

E&ttbrn-gh. Geographical Institute

S FOR JANUARY.

uchan.

120 140

Tint indicates J'ressru-e teliow 30 Inr.lie

2O 4O

1OO 120

CHAP, in Energy ', the Power of Nature 31

train moving at 60 miles an hour on a smooth level railway
only requires the engine to give out enough energy to over-
come the resistance of the air and the rails ; when that is done
the train, however great its mass, continues to move with
undiminished speed. When it has to be stopped quickly,
shutting off steam from the engine is not enough ; great
resistance has to be introduced by means of brakes which
convert the energy of motion rapidly into heat. The energy
expended in setting a mass in motion is preserved in the
moving mass when there is no external resistance, and
returned unaltered in quantity when the motion is stopped.
The amount of motion in a moving body is called its
momentum, and is measured by the mass and the velocity
.together. A mass of I Ib. moving with a velocity of 1000
ft. per second has the same momentum as a mass of I ooo
Ibs. moving at i ft. per second.

5 1 . The Gyroscope illustrates the first law of motion.
It consists of a heavy leaden wheel turning on an axle in a
brass ring. The inertia of the fly-wheel requires to be
overcome by imparting a considerable amount of energy to
it by means of a cord and a strong pull of the arm ; once
set in motion it would never stop but for the friction of its
axle and of the air. A gyroscope in rotation behaves
differently from one at rest. When the experimenter takes
it by the stand and attempts to change the direction of its
axis of rotation it seems to have a will of its own ; it strongly
resists any change of position, although when the fly-wheel
is at rest its axis may be easily turned in any direction. In
the fly-wheel itself there is a struggle going on ; the particles
tend to move in straight lines, and it is only the attraction
of cohesion that compels them to move in a circle. In
factories grindstones are sometimes made to rotate so fast
that they burst ; the tendency of the parts to move in straight
lines is too great for the cohesion of the stone to counter-
balance. The tendency for bodies to move in a straight
line, unless compelled by some power to follow a curve, is
often called centrifugal force.

52. Work against Gravity. — In employing energy to
overcome weight there seems at first sight to be a real loss

32 The Realm of Nature CHAP.

unlike the case of inertia (§ 50). An exhausted mountaineer,
on reaching the summit referred to in § 49, might ask,
" Where are my million and a half foot-pounds of energy ? —
are they not lost for ever ? " If the mountain were precipitous
on one side the climber could answer his question by an
experiment, not on his own person, but on a block of stone
of equal weight (150 Ibs.) Such a block in virtue of its
elevated position has acquired the power of doing work.
The attraction of the Earth draws the stone downward, and
once allowed to fall it moves faster and faster until it strikes
the ground with enough energy of motion to do 1,500,000
foot-pounds of work. This energy in a real case would be
expended partly in heating the air during descent, and
partly in shattering the stone and heating the fragments
and the ground. The amount of energy expended and the
ultimate form assumed are the same if the stone rolls down
a slope as if it falls vertically.

53. Energy of Motion. — The faster a body is moving
the more work it can do, i.e. the more energy it contains.
A leaden bullet thrown against a man by the hand might
inflict a painful blow, projected from a sling at the same
distance it would produce a serious bruise, but fired out of
a gun it would pass right through the victim. The greater
the velocity of the bullet the greater is its power of doing
work. But a small bullet striking a steel target is stopped,
while a cannon ball, though moving at the same speed, breaks
its way through ; hence the greater the mass in motion the
greater is its energy. When the mass of a moving body is
doubled its energy is doubled, but when the velocity of a
moving body is doubled the energy is increased fourfold.
For example, a small river flowing at 6 miles an hour could
do as much work in turning mills as a river four times the
volume flowing at the rate of 3 miles an hour. This is
expressed in the form of a Law — Energy of motion is pro-
portional to the moving mass and to the square of the
velocity.

54. Potential and Kinetic Energy. — Energy of position
may be termed an expectant, energy of motion an active
power of doing work ; or, to use the usual terms, the former

in Energy, the Poiver of Nature 33

is potential, the latter kinetic. The raised weight or coiled
spring of a clock contains potential energy, which is gradu-
ally converted into the kinetic energy of moving wheels and
hands. The simple Pendulum consists of a heavy ball hung
by a thin cord. Its practical value depends on the fact that
if the length of the cord does not
change, the ball swings from one
side to the other in exactly the
same time through any small arc.
If the ball is pulled to one side to
A (Fig. 5), since the cord does not
stretch A is more distant from the
Earth's centre than is B, and when
let go its weight makes it swing
back toward B. At A the pendulum FIG. 5.— Swing of a pendulum.
has a certain amount of potential &u£. ^west point. °
energy on account of its raised

position, and as it falls it loses that potential energy,
gaining instead kinetic energy, so that it passes the point
B in the full swing of its active movement. The power
immediately begins to do work against gravity in raising the
ball to C, and the ball rises more and more slowly as its
kinetic energy is being used up until at C it comes to rest.
Here it possesses as much potential energy as it did at A, and
so swings back again. The swings are shorter and shorter
and finally it comes to rest only because the friction of the
air and of the cord on its point of attachment gradually
change all the energy into heat.

55. Conservation of Energy is the term employed to
denote the fact that the total amount of energy in Nature, as
in the case of a frictionless pendulum in a perfect vacuum,
never varies ; that energy like matter can neither be created
nor destroyed. Many clever mechanicians have endeavoured
to find the Perpetttal Motion, by which a machine when
once wound up and set agoing would not only go on for ever,
but would do work as well. In January 1890 an advertise-
ment in the Times stated that the discovery had been
made, and the inventor wanted pecuniary help to com-
plete it. Knowledge of the laws of energy would have

D

34 The Realm of Nature CHAP.

saved the advertiser much lost time and useless trouble.
We know that if a machine could run without resistance
it would go on for ever at the same rate in virtue of
inertia if energy is once imparted to it. But if a machine
could not only keep going but set looms in motion as
well, energy must be created at every turn, and experiment
proves that this has never taken place. If energy be a real
thing the Perpetual Motion is impossible. Energy is always
undergoing transformation, visible motion, magnetism, elec-
tricity, heat, and light being a few of the many forms which
it assumes. But Nature says sternly and unmistakably,
" Nothing for nothing." No form of energy can be obtained
without paying an exact equivalent in some other form.

56. Invisible Energy. — Work can be done and potential
energy stored in separating atoms (§ 44) as well as in climb-
ing mountains ; and the union of the separated atoms recon-
verts potential to kinetic energy as truly as the downward
rush of an avalanche. When a stone strikes the ground its
energy of motion as a whole is changed into energy of
motion of its parts, which we recognise as heat. Three
kinds of motion occur both on the great scale, perceptible
to the eye, and on the small scale, discoverable by observa-
tion and reason. These are simple translation, like the
movement of falling stones or of the darting particles of
gases ; 'wave motion, like the undulations of the sea or the
vibrations producing light ; and vortex motion, like whirl-
pools in tidal streams or the disturbances we recognise as
magnetism.

57. Wave -motion. — Every elastic substance (§ 35)
can propagate wave-motion. This motion consists in one
particle moving through a comparatively short path and
returning to its previous position, after passing on its energy
of motion to another particle which also moves a short
distance and returns. Waves of to -and -fro or up-and-
down motion occur in solids and liquids ; and waves of
alternate compression and expansion occur in gases.
Waves are measured by the distance between similar parts
of successive waves. The distance between crest and crest
(CC in Fig. 6) or between trough and trough (TT) of

in Energy ', tJie Power of Nature 35

waves in water, or between succeeding maxima of com-
pression or of rarefaction in waves of air, is spoken of as the

wave-length. The am- ,

plitude of a wave is the /^^^ y
height from crest to / ^

trough (CT), or the V i

difference in degree of ^"^

Compression and dilata- FlG- 6. -Wave-motion CC, crests ; TT,

troughs.

tion.

58. Sound. — When a wave of alternate compression and
rarefaction of air strikes the ear, it produces the sensation
of sound ; the more rapid the vibration and shorter the
wave-length the shriller is the sound, but neither very short
rapidly vibrating waves of air nor very long slowly vibrat-
ing ones affect the ear at all. The greater the ampli-
tude of an air -wave, the louder is the sound. Waves
of compression and rarefaction pass through the air at
the rate of about 1 1 oo feet per second when the tempera-
ture is 32° F., and travel 2 feet per second faster for
every degree that the air is warmer. Sound-waves pass
through water with four times the velocity, and through
solids with many times the velocity of their passage through
air. Air is set into wave-motion by any substance that
is vibrating as a whole, such as a tuning-fork, a stretched
string, or a column of air in a pipe. A tuning-fork when
made to vibrate sets up air-waves that produce the sensation
of a particular musical note in the ear ; if that tuning-fork
is at rest, and air-waves of the same kind as those it can
set up strike it, they transfer their energy to the fork and
start its vibrations. All other air-waves, longer and shorter
alike, pass by with but slight and transitory effects, and,
stated generally, the law holds that Bodies absorb vibrations
of the same period as those which they give out. When
certain notes are sung, or struck on a piano, the gas globes
in a room absorb the particular waves which they would set
up if struck, and ring in response to them.

59. Molecular Vibrations are the minute movements
of the smallest particles of bodies, either as a quivering
of the particle itself or as quick oscillations to and fro.

36 The Realm of Nature CHAP.

As long as there is any kinetic energy associated with a
portion of matter the particles will be in motion. The
amplitude of the oscillations in solids is very slight, not
sufficient to overcome the resistance of cohesion (§ 39).
However large a body may be, its particles will in time
come to oscillate at the same rate throughout if not inter-
fered with, any more quickly-moving particles passing on
some of their energy to their more slowly -moving neigh-
bours. The process of passing on and equalising the rate
of molecular vibration is called conduction, and takes place,
although more slowly, in liquids and gases as well as in
solids.

60. Radiant Energy. — As the vibrations of bodies, as
a whole, set up waves of various length in air which may
travel to a distance, and some of which are capable of
impressing the ear, so the invisible vibration of the particles
of bodies sets up waves of radiant energy which travel to
a distance, and some of which impress the senses. The
quiverings of particles are very complex, and the particles
of each kind of matter seem to quiver and oscillate in a
way of their own, setting up waves which, although ex-
cessively minute, are far more complex than those of sound.
There is much difficulty in understanding how the waves of
radiant energy travel, and it is assumed that a very remark-
able kind of matter called the Ether fills all space, and
penetrates freely between the particles of ordinary matter.
It is so fine that it offers no perceptible resistance to the
movement of the planets through it, or to the movements
of the particles of matter ; but it is so elastic that it passes
on the smallest and swiftest undulations. The undulations
travel in straight lines through the ether at the rate of
nearly 186,000 miles per second, and all amplitudes of
these undulations travel at the same rate, about a million
times as fast as the waves of sound in air.

6 1. Reflection and Refraction.— When the waves of
radiant energy reach a surface through which they cannot
pass, they are turned into a new path, either directly back-
ward or at a definite angle to their former direction.
Sound-waves meeting an obstacle are reflected in the same

in Energy, the Power of Nature 37

way, giving rise to echoes, and so are the little ripples of a
water surface on meeting a straight line of cliffs. When
the ripples of the sea pass among a number of half-covered
stones their onward path is changed in direction, each little
undulation being bent from its course by the obstacle it
meets. Similarly, when a ray of radiant energy passes from
one medium into a denser, from the ether into air, or from
air to glass, for example, the undulations are diverted by
the particles of matter, and the path of the ray is bent or
refracted. Radiant energy is made up of many different
vibrations ; some are comparatively long and are slow in
their vibration, others are very short and much more
rapid. The short quickly -vibrating waves are most bent
from their straight path by passing into a different medium,
and are therefore said to be most refrangible. It is evident
that if a beam of radiant energy, in which each ray corre-
sponds to a definite wave-length, travelling straight on,
enters a denser medium, the separate rays will be spread
out like the ribs of a fan, those of the shortest waves being
most turned from the straight line, those of the longest
waves least.

62. The Spectrum. — When the undulations which come
from an intensely vibrating solid enter a triangular glass
prism (Fig. 7, P) through a narrow slit, they are spread
out by refraction and arranged side by side in perfect order
from those of shortest
wave-length, -z/, to those
of longest, r forming a
spectrum. The waves
shorter than -g-yj^ of
an inch have a peculiar
power of affecting certain
substances and produc-
ing chemical changes, FlG- 7--Pf.Tatic reflactionv RR', straight

5. ' path of light ray ; Rvr, refracted path.

but they have no effect

on the senses. The waves between -QJ-^-^ and -jj-g-g-g-g- of
an inch in length (vr) affect the sense of sight through the
eye, producing the sensations of light and colour, hence
they are termed light-waves. Waves of longer wave-length

38 The Realm of Nature CHAP.

set the particles of bodies in vibration when they fall on
them ; they are invisible to the eye and are known as heat-
waves. The shortest of the light- waves (v} produce the
effect of violet light, longer ones (b} blue, still longer (g)
green, longer yet (j) yellow, and the longest that produce
any effect on the eye (r) red. Thus when one looks at a
glowing solid body through a spectroscope, an instrument
containing one or more prisms, the colours red, yellow,
green, blue, violet are seen ranged in a row as in the rain-
bow (Fig. 8, which gives a detailed view of the range vr
of Fig. 7), but the eye sees nothing of the short wave-length
rays beyond the violet, nor of the relatively long wave-length
rays beyond the red. Still longer waves can be detected
by their electro-magnetic action. In fact, all radiation is
essentially electro-magnetic.

63. Radiation and Absorption. — The different wave-
lengths of sound in air correspond to different musical notes,
the different wave-lengths of light in the ether to different
colours. The molecules of each of the elements vibrate
in a way of their own when set in motion, and produce
waves in the ether of one or more definite lengths only.
Sodium vapour, for example, when intensely heated sets up
only rays the wave-length of which is 4 3 Q o o °* an mcn>
and these produce the sensation of yellow light in the
eye. A spectroscope sorting out the light from glowing
sodium shows only a strong double yellow line (D in
Fig. 8). The molecules of calcium vapour produce several
distinct kinds of quivering, originating rays corresponding
to definite colours of light. The same is true of all the
other elements ; the spectra of the radiant energy sent
out from them are distinctive in every case. But, as in
the case of sound, bodies absorb the same kind of radia-
tions as they emit. If a beam of white light, which
includes rays of all wave-lengths, is passed through sodium
vapour, the particles of sodium are set vibrating by the
waves -j-^jo^- of an inch in length, and the energy of these
waves is absorbed, so that when the beam is examined by
the spectroscope, and the rays are spread out side by side,
the peculiar double yellow ray is missing and in its place

in Energy, the Power of Nature 39

there is a blank or black line. The same is true with the
vapours of all the other elements, the particular waves
absorbed differing in each case. Spectrum Analysis is a
term used to describe the discovery of the elements whose
vibrations give out a certain kind of light. It is not only
analysis or unloosening ; it is also a method of seeing

FIG. 8. — Diagram of the solar spectrum, showing the order of colour and the
position of the principal absorption lines.

through a compound when taken apart by the action of heat.
However distant a body may be, if it gives out light, the
light tells its own tale as to the matter whose quiverings
sent waves through the ether, and as to any other kinds
of matter which may have exercised absorption on it in
intermediate space.

64. Light and Colour. — White light is produced when
waves of radiant energy corresponding to all or nearly all
the wave-lengths that affect vision strike the eye together.
When waves of light fall upon any object, some of them are
absorbed and the others are reflected ; the report these
reflected rays convey through the optic nerve to the brain
names the colour of the object. Thus when sunlight falls
on grass the rays whose vibrations produce the effect of
red, yellow, blue, and violet are almost all absorbed, their
energy being set to do work in the plant (§ 399), and only
those which produce the sensation of green are sent back
to the eye. Similarly when light falls on a sliced beetroot
the yellow, green, blue, and violet-producing vibrations are
absorbed and only the red-producing rays sent back. When
light falls on a piece of charcoal it is all absorbed, and as
none is reflected the body appears devoid of light, or black.
A sheet of paper, on the other hand, absorbs very little of
the light and reflects white light as white. The fact that

40 The Realm of Nature CHAP.

colour comes from the light, not from the object, may be
illustrated by sprinkling salt on the wick of a burning spirit-
lamp. The sodium of the salt gives out light of one wave-
length only, producing the sensation of yellow. Objects
which reflect all kinds of light and those that reflect yellow
appear yellow, but such things as beetroots and grass absorb
all the yellow light and appear black, like charcoal, which
absorbs all light whatever, and the most brilliant painting
appears in tones of black and yellow only.

65. Heat and Temperature. — The action on matter
of radiant energy, particularly of the comparatively long
and slowly vibrating waves known as heat, is to make
the particles oscillate more rapidly. When the particles
of matter vibrate rapidly they send out waves of radiant
energy, and thus a heated body radiates heat. Two
bodies are said to be at the same temperature when
each communicates the same amount of heat to the other
as it receives from it. If one body by conduction (§ 59)
or radiation (§ 63) gives to another body more heat than
it receives from it, the former is said to be at a higher
temperature. The hand plunged into water (§7) lets us
know whether the water is at a higher or lower temperature
than the hand. If the water is at a higher temperature,
heat passes into the hand which feels warmth, if the water
is at a lower temperature heat passes out of the hand which
feels cold. The amount of heat which gives a small body
a great rise of temperature imparts to a large body a much
smaller rise of temperature. Heat is the total amount of
molecular motion in the mass, while temperature depends
on the rate of that motion. The unit of heat used in this
volume is the amount required to raise the temperature of
I Ib. of water i° F. Temperature is measured by the
thermometer (§ 440).

66. Capacity for Heat. — Heat bears to temperature
exactly the same relation as volume of a liquid does to
level. When a large quantity of liquid must be poured
into a vessel to raise the level one inch, we say that the
vessel has great capacity ; while if only a few drops are
required to raise the level one inch, the vessel is said to

in Energy \ tJie Power of Nature 41

have small capacity. It is level alone that decides the
direction in which the liquid will flow when two vessels are
connected by a pipe. Similarly there are some kinds of
matter one pound of which requires a great deal of heat to
raise its temperature by one degree, while an equal mass of
others is raised in temperature to the same amount by very
little heat. The former class of substances are said to have
a great capacity for heat, or, as it is sometimes called, a high
specific heat. Thirty times as much heat is required to
raise the temperature of i Ib. of water i° as to raise the
temperature of the same mass of mercury by the same
amount. Water, indeed, has the greatest capacity for heat
of any substance known. On the same fire, if other con-
ditions are the same, mercury becomes as hot in a minute
as an equal mass of water does in half an hour ; but then as
a necessary consequence heated mercury cools as much in
a minute as an equal and equally heated mass of water does
in half an hour.

67. Expansion by Heat. — When the temperature of
matter is raised the oscillations of the particles are not only
more rapid but of greater amplitude. Each particle occupies
a greater space in its longer swing, and consequently the
volume occupied by the matter is increased and the density
diminished. Expansion of volume by heat takes place in
solids, liquids, and gases alike, though its amount is different
in each kind of matter and is always greater for gases and
liquids than for solids. The lengthening of a bar of iron
when heated or its contraction when cooled takes place with
nearly irresistible force. The rails of the railway 400 miles
long between London and Edinburgh are nearly 1000
feet longer on a summer afternoon than on a winter
night. The expansion of a metal rod is often used as a
measure of temperature ; but thermometers (see § 440) are
usually constructed by taking advantage of the greater
expansion of liquids or gases. If heat is applied to the
lower part of a vessel containing liquid the layer next the
source of heat is raised in temperature, expands, and becom-
ing less dense rises to the surface, allowing the denser
liquid above to subside to the bottom and get heated in its

42 . The Realm of Nature CHAP.

turn, thus setting up complete circulation throughout the
mass. This transmission of heat by the translation of
heated portions is called convection, and in consequence of
it the temperature of a liquid heated from beneath becomes
much more rapidly uniform than that of a solid. The
conduction (§ 59) of heat in liquids is very slow, and when
the upper layer is heated the vibrations of its particles are
passed on by conduction to the mass below very slowly
indeed (§ 229), as the expanded upper layer tends to remain
in its position.

68. States of Matter. — If the particles of any kind of
matter were absolutely at rest, that is to say if they possessed
no kinetic energy, it is usually assumed that the body would
be absolutely cold, or at the absolute zero of temperature.
This total absence of heat has never been actually observed.
The difference between the same substance in the solid,
liquid, and gaseous states is due to the rate of motion of the
particles alone, and the work of moving the particles may
be readily expressed in terms of heat. Thus in solids which
contain relatively little heat the particles move so slowly
that cohesion confines them to excessively minute paths, and
the substance possesses rigidity (§ 35). In liquids there is
much more internal movement or heat, and the particles
having a longer path and greater rapidity of motion partly
overcome cohesion and show the property of fluidity. Gases
contain so much heat that their particles are in very rapid
motion through comparatively long paths and the power of
cohesion is quite overcome. When the pressure remains
the same, every additional degree of temperature makes the
particles of a gas move more quickly through a longer path,
and the volume occupied by the gas is increased by ~-^
(_L g. for each centigrade degree). A fall of i ° reduces the
volume by -^\1S. Hence a fall of 490° of temperature in
a gas at o° should reduce its volume to nothing, which
is impossible ; hence it is believed that no gas or liquid
can exist at — 490° F. In other words the particles of
solid matter would be motionless, that is, absolutely without
heat or at the Absolute Zero of temperature.

69. Action of Heat on Ice. — We may follow the action

in Energy, the Power of Nature 43

of heat on matter by supposing radiant heat to be supplied
to a mass of I Ib. of ice at o° F. Each unit of heat raises
the temperature of the mass by 2° (hence the capacity for
heat of ice is only half that of water), and by the time 1 6
units of heat have been absorbed, the mass of ice has ex-
panded considerably, and its particles are vibrating with
increased energy so that the temperature is 32°. The next
144 units of heat which enter the mass produce no effect
on the temperature, which remains at 32°. But the energy
is doing other work, for when the 144 units have been
absorbed we are dealing with water, not ice. Those 144
units have been expended in work against cohesion and are
stored up as potential energy. The heat employed in doing
this work of separating particles is sometimes said to
become latent, and the latent heat of water, i.e. the amount
of heat necessary to change I Ib. of the solid into i Ib. of
the liquid substance, is 144 F. heat-units. This is higher
than the latent heat of any other substance known.

70. Action of Heat on Water. — The volume of i Ib.
of water at 32° is 8 per cent less than the volume of i Ib.
of ice. This is a very significant fact, for almost all
other substances occupy a greater volume in the liquid than
in the solid state. When 7 heat-units are absorbed by i Ib.
of water at 32° the temperature rises to 39°, but the volume
continues to diminish, a state of things which appears to
show that in water, unlike almost all other liquids, the faster
moving particles fit in a smaller space. But after 39° is past
each fresh unit of heat raises the temperature by about i°,
and the volume of the liquid increases faster and faster.
From 32° the addition of 180 heat-units raises the tempera-
ture to 212° at ordinary atmospheric pressure; but here
another change takes place, and the water is said to boil.
No less than 967 units of heat must be supplied before the
temperature of i Ib. of water rises above 212°, and at the
end of that operation there is not water but i Ib. of steam
or water- vapour at 212°. A real experiment would not
proceed so regularly, because at all temperatures water,
and even ice, are partly converted into vapour, to produce
which a certain amount of heat is used up.

44 The Realm of Nature CHAP.

7 1 . Action of Heat on Water- vapour. — The work done
by 967 heat-units on I Ib. of water at 212° was done once
more against cohesion. The vibrating particles have been
enabled to increase the amplitude of their oscillations to a
great extent, the volume of the gaseous steam being 1700
times as great as that of the water from which it was derived,
and every particle of the water-vapour is darting with the
speed of nearly i mile per second. When heat is supplied
to steam every unit raises the temperature by 2° (its specific
heat being only half that of water) ; the rise of temperature
means increase in the velocity of the darting particles and
brings about an increase of volume by ±\-§ part for each
degree if the pressure upon the vapour remains the same, or
a corresponding increase of pressure on the sides of the
containing vessel if expansion is prevented. When water-
vapour is raised to a very high temperature the heat begins
to do the work of breaking up the molecules of water into
its components oxygen and hydrogen, thus doing work
against chemical attraction and storing up potential energy
in the separated gases.

72. Pressure and Change of State. — Under pressure
ice melts at a lower temperature than 32°, and the few other
bodies which contract when they liquefy also have their
melting-points lowered by pressure. Bodies which expand
when they liquefy — like mercury, rocks, and most other
substances — have their melting temperatures raised by
pressure so that more heat is required to liquefy them. The
effect of pressure on the temperature at which the change
from liquid to gas takes place is much more marked. In
every case an increase of pressure delays complete vaporisa-
tion or boiling until a higher temperature is reached. Water,
for example, cannot be heated in the liquid state to a greater
temperature than 68° if the atmospheric pressure is one-
fortieth of its average amount, but to 176° at half the usual
pressure, and to 250° if the usual pressure is doubled. The
boiling-point of a liquid may thus be used to measure
atmospheric pressure.

73. Heat-energy. — The changes which take place when
heat is withdrawn from matter are the exact opposite of

in Energy, the Power of Nature 45

those accompanying the application of heat. When oxygen
and hydrogen unite, the potential energy of separation is
changed into kinetic heat -energy, as already explained
(§ 44). When i Ib. of hot water- vapour radiates out its
heat -energy its temperature falls gradually to 212° at
ordinary pressure ; but then, in assuming the liquid state,
967 heat-units are given out as the particles rush together
under the influence of cohesion. One pound of steam at
212° if passed into 4 Ibs. of water at 32° gives out heat
enough in liquefying to warm up the whole 5 Ibs. of water
to 212°; hence the great value of condensing steam as
a heating agent. One pound of water cooling from 212° to
32° gives out 1 80 heat-units, and as the particles come
fully under the influence of cohesion and group themselves
into solid crystals of ice, the energy that held them apart
is changed into 144 units of heat.

74. Mechanical Equivalent of Heat. — The great task
of measuring the quantity of heat-energy which is equal to a
certain amount of work (§§ 25, 49), and so of comparing
the invisible motion of molecules with the visible motions
of masses, was attempted and triumphantly accomplished
by Joule in 1843, when the modern theory of energy was
founded. He showed that I heat -unit was equal to 772
foot-pounds. In other words, if a mass of i Ib. were to be
pulled down by gravity through 772 feet, and the whole of
its kinetic energy changed into heat in i Ib. of water at 32°,
the temperature of the water would be thereby raised to
33°. Thus we can measure the work done by heat in
melting i Ib. of ice at 32° (§ 69) and find it to be equal to
i n,ooo foot-pounds, while that done in evaporating i Ib. of
water at 212° (§ 70) is 747,000 foot-pourids. It appears
that the heating of ij Ibs. of ice at 32° until it becomes
steam at 212° requires as much energy as the feat of
mountain-climbing described in §§ 49, 52.

75. Degradation of Energy. — It is always possible and
easy to change work or electricity or light into heat, and
772 foot-pounds of work will always yield the full heat-
unit. The inverse operation is different, and from i unit of
heat the best machine it is possible to imagine could only

46 The Realm of Nature CHAP.

obtain a small fraction of its equivalent of work. As water
tends to flow to the lowest level, so in Nature energy of
every kind tends to assume the least available form, which
is that of heat. This process is called the degradation of
energy, and in course of time, if it continues to act, all the
energy of the Universe will be reduced to the form of heat-
vibrations in one uniform mass of matter at one uniform
temperature, and although present in full amount quite
unavailable for doing work. Viewing the past of the
Universe in the light of the degradation of energy, Sir
William Thomson has shown that there was a time when
the distribution of heat was such as could not have been
derived from any conceivable previous distribution ; in other
words, that there was a beginning or a creation and that ever
since the Universe has been like a machine running down.

76. Electrical Energy is not yet sufficiently understood
to admit of its nature being simply explained. It seems
to be the energy of any form of stress or motion of the
ether. Electricity is often spoken of as a fluid, but this
is simply the survival of a more dense ignorance of its
nature. Electrical energy appears to take part in nearly
every change of matter as to composition or state. It has
the power of decomposing many chemical compounds which
resist the action of every other form of energy, and it can
also make some elements combine together which do not
unite by any other means. As heat is transmitted from
matter at a high temperature to matter at a low tempera-
ture, so electricity passes from matter at a high electrical
potential to matter at a lower potential. This passage of
electricity is called an electric current.

77- Conductors and non - Conductors. — Electricity
passes readily through some substances, such as copper,
silver, metals of every kind, sea- water, damp earth, etc.,
and these are called conductors. Other substances, such
as dry air, glass, sealing-wax, allow it to pass with such
difficulty that they are called non-conductors. There is no
perfect conductor, nor any absolute non-conductor. Even
copper and silver offer a certain resistance to the passage
of electricity, and if the difference of potential is sufficiently

in Energy, the Poiver of Nature 47

great, electricity will overcome the greatest resistance of
glass or air. The energy expended by electricity in over-
coming resistance is changed directly into heat or light
vibrations, as in the case of an electric glow-lamp.

78. Disruptive Discharge. — When the amount of
electricity on the surface of a small body increases, the
potential rapidly rises, and a transference of electricity
takes place along the path that offers least resistance.
With high potential, electricity can force its way across an
interval of air, and as the resistance of air is very great
much of the electrical energy is transformed into heat in
the process, and the particles of air are set in such violent
vibration that they become luminous. Such a transfer-
ence is called a disruptive discharge, or when it occurs in
Nature a flash of lightning.

79. Magnetism. — An oxide of iron which exists natur-
ally in considerable quantities has the power of attracting
to itself pieces of iron, this attractive force being much
more powerful than gravitation. When a bar of this
mineral is cut, and so uniformly shaped that no difference
in appearance can be found between its two ends, the ends
still differ, much as the right hand differs from the left. If
the bar be balanced on a pivot it will turn and come to
rest with one end pointing toward the north. On this
account the mineral is called the lodes tone. If two similar
bars are balanced in this- way the north-seeking end of
each can be found and marked. The effect of one such
lodestone on another emphasises the difference between
the two ends. If the north-seeking end of a lodestone
is brought near the south-seeking end of another which is
balanced the latter is strongly attracted,* but if brought
near the north-seeking end of the balanced lodestone there
is as strong repulsion. The property of two-endedness in
bodies outwardly similar is called polarity, and the ends
are termed poles. The rule of magnetic attraction and
repulsion is very simple — Unlike poles attract, like poles
repel. The lodestone imparts all its properties to steel
when rubbed upon a bar of that metal, and such steel bars
are then termed magnets.

48 The Realm of Nature CHAP, in

80. Electro-magnetism. — The properties of magnets
would be inexplicable had not an accidental discovery shown
the close relation of magnetism and electricity. It was
found that when electric energy is passing through a wire
placed above a balanced magnetic needle, the needle
swings round and sets itself at right angles to the wire.
It was found later that when a coil of copper wire traversed
by electricity surrounds a bar of iron, the iron becomes a
powerful magnet and retains its properties of polarity and
attraction as long as the electricity passes, losing them the
instant the current ceases. A coil of copper wire without
any iron in the centre was subsequently found to possess
polarity, and to exert attraction and repulsion as long as an
electric current flowed through it. Hence magnetism can
be produced by electricity, and the reverse also holds good.
A magnet placed inside a coil of common wire generates a
momentary current of electricity. By merely making a coil
of wire move in the field of a powerful magnet electricity
can be produced in the wire, and thus work can be changed
directly into electric currents.

In Nature nothing is so simple as has been represented
in this and the last chapter. We do not know how particles
vibrate and oscillate, and only guess at the real nature of
the forms of matter and energy. Authorities differ in their
interpretation of many of the facts, and we have only pre-
sented a few of the simpler conclusions in order to assist
the student who does not know much of physics and
chemistry to follow the chapters which come after.

BOOKS OF REFERENCE

Balfour Stewart, Elementary Physics. Macmillan and Co.
P. G. Tait, Recent Advances in Physical Science. Macmillan
and Co.

CHAPTER IV

THE EARTH A SPINNING BALL

8 1 . The Earth a Sphere. — The field of view at sea or
on a level plain is always bounded by an unbroken circle
called the horizon ; and in all parts of the Earth when one
watches a receding object at sea or on a level plain the
horizon appears slowly to swallow it up, and it disappears
like a traveller over a hill. In all parts of the Earth if the
eye is placed 5 feet above sea -level the lower 5 feet of
any object are concealed when 4 miles away. Across a
lake 4 miles wide, two men of ordinary height standing
erect and looking at each other with telescopes can see only
the head and hands of the other apparently floating on the
water, their bodies being entirely concealed from view (Fig.
9). So from the sea-shore the hull of a ship 10 feet above
the water vanishes at 5 miles' distance, and its masthead

FIG. 9. — Curvature of the Earth, exaggerated 400 times.

100 feet high sinks out of sight at 12 miles. Since the
same length of an object is concealed by the horizon at the
same distance from the observer in all parts of the Earth, it
is evident that the dip of the horizon, as it is termed, is
practically the same everywhere, and that the surface of the
Earth is uniformly curved in a convex form. The only
figure which has uniform convex curvature is a sphere, and

E

50 The Realm of Nature CHAP.

the Earth is hence generally spoken of as being a sphere or
globe. From 5 feet above sea-level the horizon is only
3 miles distant ; from a height of 4000 feet it is 80 miles,
so that an observer can see to a distance of 80 miles
all round j while from 24,000 feet it is more than 200
miles distant, and in each case a perfect circle.

82. The Earth an Ellipsoid. — If the Earth were a
perfect sphere its size could be measured by measuring the
length, in miles or yards, of the arc of a great circle (i.e. a
circle the centre of which is at the centre of the Earth) sub-
tending one degree, and multiplying by 360 to give the cir-
cumference, for each degree subtends an equal arc on a
sphere. It is easy by observations of the stars (§ 92) to tell
exactly how many degrees one has advanced along a great
circle ; and parts of great circles (arcs of the meridian) have
been measured in many parts of the Earth with much exact-
ness. In Great Britain i° was found to be almost exactly
365,000 feet long; but in Peru i° was found not quite
363,000 feet in length, and in the north of Sweden i° was
found to measure about 366,000 feet. These measurements
are undoubtedly correct to within a few feet ; and the only
conclusion that can be drawn from them is that the Earth
is not a sphere, but a figure the curvature of which is less
than that of a sphere in some parts and greater in other
parts. It resembles a sphere slightly compressed along one
diameter, and correspondingly bulged out in the direction at
right angles. The length of the shortest diameter has been
calculated as 7899-6 miles (about 500,000,000 inches), and
the diameter at right angles as 7926-6 miles. The circum-
ference is about 24,000 miles. The form is very nearly
that known as an ellipsoid, or oblate spheroid of revolution
— a figure that could be made in a turning-lathe, with the
axis of rotation in the lathe as the shortest diameter.

83. The Earth a Ball. — The most exact measurements
which have been made, show that the figure of the Earth is
not a true ellipsoid. It appears to be compressed to a slight
extent at right angles to the shortest diameter, so that the
equatorial diameters vary in length by one or two miles.
The exact form of the Earth is being gradually discovered

iv The Earth a Spinning Ball 5 1

by very careful measurements of the force of gravity (§§38,
252) by means of a pendulum or fine spring-balance. The
weight of a given mass on the Earth's surface depends only
on its distance from the centre, and thus as the strength of
gravity at different places is found, the figure of the Earth is
gradually felt out. The form of the Earth is termed by
mathematicians a geoid or earth-like figure ; and it is more
accurate to speak of it as a ball than as an ellipsoid or
sphere. Yet the difference in shape is so slight that if a
geoid or ball, exactly like the Earth, an ellipsoid and a
sphere were made each a foot in diameter, it would be
quite impossible to tell which was which by the eye or
touch.

84. Structure of the Earth. — The Earth is a structure
composed of three divisions — (i) a vast stony ball termed
the lithosphere with an irregular surface, part of which forms
the dry land ; (2) a liquid layer resting in the hollows of the
lithosphere, a great part of which it covers ; this is termed
the hydrosphere or water-shell ; and (3) a complete envelope
of gas surrounding the whole to a considerable height and
known as the atmosphere or air.

85. Mass and Density of the Earth. — To weigh the
Earth, all that is necessary is to measure the attraction
of gravity between a large block of metal and a small
block set at a measured distance. Then (making allow-
ance for^the distance of the small block from the Earth's
centre) the attraction of the large block on the small one
bears to the weight of the small one, i.e. the attraction
of the Earth on it, the same proportion as the mass of the
large block bears to the mass of the Earth. Cavendish,
who first carried out this experiment a hundred years ago,
employed a cumbrous apparatus in which the large attracting
mass took the shape of two leaden balls a foot in diameter.
The small block consisted of two small leaden balls fixed to
the ends of a light rigid rod, which was hung by a fine silver
wire. This arrangement is termed a torsion balance, because
when the small spheres were attracted by the large ones
and moved slightly toward them the wire was slightly
twisted, and the force required to twist the wire to that

52 The Realm of Nature CHAP.

extent having been found by experiment, was a measure of
the attraction between the small and large spheres. The
weight of the small balls is the measure of the attraction of
the Earth upon them, and as the distance of the small balls
from the centre of the Earth is known, the mass of the
Earth can be calculated from the known mass of the large
leaden spheres. Mr. Vernon Boys has recently succeeded
in making a very fine elastic thread of quartz which acts
as an extremely sensitive spring, and can be used to measure
the force of attraction between bodies as small as ordinary
bullets.1 As the result of several independent methods,
the mass of the Earth has been found to be the same as if
it were a globe of homogeneous substance 5 J times as dense
as water ; the mean density of the Earth is thus said to be

5-5-

86. The Earth in Motion. — On a clear morning the
bright disc of the Sun appears somewhere on the eastern
horizon, rises slowly and wheels round the sky, then, as
slowly sinking, it disappears somewhere on the western
horizon. When the Sun is visible its light fills the whole sky,
which appears as a bright blue dome unless clouds interrupt
our view of it. Sometimes a glimpse may be had of the
Moon, as a ghostly white broken disc like a little fleecy cloud ;
very rarely, indeed, the bright light of a planet is visible, or
the weird form of a comet. At night the curtain of the
Sun's excessive light is dropped, and we see that the whole
sky is really gemmed over with bright points or stars, as if
a dome or hollow sphere of black paper pricked with in-
numerable holes had been wheeled between us and the Sun.
This star -dome appears to revolve round the Earth, the
various marks on it preserving an unaltered arrangement.
The stars have been grouped into fanciful constellations,
which are easily recognised and serve as a rough-and-ready
way of naming any definite part of the sky. By a curious
mixture of guessing and of reasoning on the observations
which they made, Copernicus and Galileo and their followers
came to the conclusion that the regular changes in the
appearance of the sky from hour to hour and month to
month could only be accounted for by the Earth having at

iv The Earth a Spinning Ball 53

least two different kinds of motion. The first convincing
proof of the Earth's motion was the discovery that a weight
dropped from the top of a high tower did not reach the
Earth's surface perpendicularly under the point from which
it was let go, but always a little to the east (§ 93).

87. Rotation of the Earth. — The old difficulty that the
Earth could not be moving because we do not feel it, and
that the star-dome could not be fixed because we see it
move, no longer troubles people who are familiar with the
imperceptible motion of a well-started train, and the apparent
gliding away of the platform in the opposite direction. The
Earth spins uniformly and regularly from west to east, as
may be inferred from the uniform and regular apparent
rotation of the starry sky at night. The first Law of Motion
(§ 50) enables us to understand how the rotation of the
Earth has been actually proved, and what the immediate
consequences of rotation are. The French physicist
Foucault showed how a large pendulum once set swinging
changed the plane of its swing slowly and regularly. If
started, for instance, swinging above a table from north to
south, at the end of twelve hours it would be found swinging
from east to west, and in twenty-four hours it would have
changed its plane still farther and be

swinging from south to north again.

Since the only force which could act on

a moving pendulum hung from the solid

roof of a building is the rotation of the

Earth, this change in the direction of

the pendulum proves it. The pendulum

does not really change its direction of

swinging in space ; it remains in a state

of uniform motion, and the apparent FIG. 10.— Direction of

twisting is produced by the house and Kfe'and^^ctS'nS-

the whole Earth turning while the pen- deviation of moving

dulum marks out its invariable line. Sphere. S°Uthem

88. Polarity. — A ball at rest has no

ends or natural points from which to reckon position, but as
soon as the ball is made to spin two opposite points on its
surface become different from all others, although there may

54 The Realm of Nature CHAP.

be no visible mark or sign of the fact. These points, which
are called ends or poles, are relatively at rest like the centre
of a wheel, and the rate at which a point on the surface of
a spinning ball moves is greater in proportion to its distance
from them. A body spinning uniformly turns round the axis
(NS in Fig. 12) or line joining its poles as a wheel spins
round an axle. The two poles of a spinning body are distin-
guished from each other by the apparent direction of rotation
about them. Looking down on the Earth from above one
pole, an observer would see the surface rotating in a direction
opposite to that of the hands of a watch, as shown by the
thick arrow (Fig. I o), while if he were to look down similarly
on the other pole the surface would appear to rotate in
the same direction as the hands of a watch do (thick arrow,
Fig. 1 1). The end first mentioned is called the North Pole,
and the opposite is named the South Pole. The Earth
always rotates in one direction, from west to east (arrows
in Fig. 12); the apparent difference at the poles is due to our
looking from opposite sides. The arrow of Fig. I o appears
turning to the left in its flight, that of Fig. i I appears
turning to the right, but on holding the page up to the light
they are seen to be one and the same. The student should,
if possible, make himself familiar with
these facts by actual observations on a
terrestrial globe.

89. Ferrel's Law. — On a steamer
at rest or moving steadily straight for-
ward a passenger has no difficulty in
walking in a straight line parallel to the
planks of the deck, or in any other
direction. But if the steamer is turning
rapidly to the right, the promenader,
irymS to keep in a straight line, has
deviation of moving the greatest difficulty in preventing him-
self from deviating to the left and
running against the bulwarks ; or if the
steamer is turning to the left he can hardly help de-
viating to the right with reference to the planking. The
passenger tends to continue walking in a straight line

iv The Earth a Spinning Ball 55

with regard to objects outside the ship all the while, and the
real motion of the deck toward the right gives an apparent
motion of the passenger toward the left. The same thing
is true of everything moving rapidly on the surface of the
rotating Earth, whether the moving body be a shot from a
cannon, a railway train, a river, or simply wind. This fact
is thus stated by the American meteorologist, Professor
Ferrel : If a body moves in any direction on the EartWs
surface, there is a deflecting force arising from the Earttts
rotation, which deflects it to the right in the northern
hemisphere, but to the left in the southern hemisphere.
The moving body has a tendency to keep on in a straight
line ; it is the Earth that changes its direction, as in Foucault's
pendulum experiment. Fig. 10 represents the apparent
deviation of a body moving in the southern hemisphere,
Fig. 1 1 that in the northern — the thin arrow showing the
original direction, the thick arrow the deviation.

90. Position of the Axis. — The axis of the Earth about
which it rotates is the shortest diameter (§ 82). If the
Earth was once much hotter than now and in a semi-fluid
condition (as we shall" subsequently see reasons to believe),
the mere fact of its rotation would make it bulge out along
the line farthest from the poles, and that to the precise degree
which is found to be the case. As in the case of the rapidly
spinning gyroscope (§ 51), and for the same reason, the axis
of the Earth preserves its direction practically unchanged in
space ; and consequently the ends of the axis always point
to opposite parts of the starry sky. As the Earth rotates,
these points — the poles of the heavens — appear to be at
rest, while the sky with its constellations Appears to revolve
round them from east to west. The north pole of the Earth
points very nearly to a bright star which has received the
name of the Pole Star or Polaris, and is of the greatest
importance as a guide to direction and position on the
Earth in the northern hemisphere.

9 1 . Direction on the Earth. — On account of the Earth's
rotation it is possible to fix direction and position on its
surface. The line which we may imagine to be traced
round the Earth equally distant from both poles is termed

5 6 The Realm of Nature CHAP.

the Equator, and it is the only great circle the plane of
which cuts the axis at right angles. The half of the globe
in which the north pole is situated is termed the northern
hemisphere ; the half whose centre is the south pole is the
southern hemisphere. Great circles running through the
poles, and therefore having a north and south direction,
are called meridians. The direction toward which the Earth
turns is called the east, that from which it turns the west.
East and west thus indicate merely a direction of turning,
and do not refer to fixed points. Small circles traced round
the Earth, their planes cutting the axis at right angles,
have thus an east and west direction and are called parallels.
They are, of course, smaller and smaller as the poles are
approached. The equator, meridians, and parallels are
well shown on the map of the world in hemispheres (Plate
XIV).

92. Latitude is the name given to the angular distance
at the centre of any point on the Earth's surface from
the equator measured toward the poles. The equator is
chosen as o° of latitude, and as the distance of the poles
is a quarter turn or right angle (§ 31) the north pole
has latitude 90° N., the south pole latitude 90° S. The
latitude of any place, except the poles, merely refers to the
distance from the equator of a small circle, or parallel of
latitude, passing through the place in question. Latitude
is always measured astronomically by observing the altitude
of the pole of the heavens, directly or indirectly. The
altitude of the pole, or its angular distance above the
horizon of an observer, is equal to the angular distance of
the observer from the Earth's equator. Standing on the
equator an observer (if the effects of refraction are not
considered) would see the north pole of the heavens close
to the pole star on the northern horizon, and the south pole
of the heavens on the southern horizon, while all the stars
would appear to rise in the eastern half of the sky, to
describe vertical semicircles, and sink on the western side.
If the observer were to journey farther north he would lose
sight of the south pole of the heavens, while the north pole
would rise higher and higher above the horizon. By the

iv The Earth a Spinning Ball 57

time he had got half-way from the equator to the pole (45°
N., at O Fig. 12) the pole star would appear to have
risen half-way from the northern horizon toward the zenith,
an elevation of 45°. All the stars within 45° of the pole
would remain in sight all night, never rising or setting, but
circling round the pole ; a star exactly 45° from the pole
would describe a circle, passing through the zenith at its
highest point, and touching the northern horizon at the
lowest. Stars beyond that limit would rise in the eastern
part of the sky, describe oblique arcs, and set in the
western; while stars more than 135° from the north pole
of the heavens would never become visible. Finally, if it
were possible to reach the north pole of the Earth, the pole
of the heavens would appear in the zenith (altitude of
90°). All the stars within 90° of the pole would be visible,
but no others. They would ne\ er rise nor set, but always
wheel round in horizontal circles, once in twenty-four hours.
Measuring with a sextant the altitude of the pole of the
heavens above the horizon thus gives the latitude of the
observer. In practice the altitude of some bright star or of
the Sun when at the highest point of its daily apparent
path is observed, and the relative position of the Sun
or star being given with proper corrections in the Nautical
Almanac, it is easy to calculate the latitude. Thus the
position of an observer on the Earth with respect to the
poles can be found by observations of the stars without
any measuring of distances on the surface, and the position
of a degree of the meridian can be fixed. A degree of the
meridian varies a little in length (§ 82) but averages 69-09
miles ; the sixtieth part of this, or one minute of latitude,
measures nearly 6000 feet, and is called a sea-mile, or
nautical mile ; the second of latitude measures about 100
feet.

93. Angular and Tangential Velocity of Rotation.
—The Earth turns on its axis uniformly, and the rate of
turning or angular velocity is the same at all parts. A
line drawn perpendicularly from the equator to the Earth's
axis at C describes a whole turn in the same time as a
line drawn perpendicular to the Earth's axis at A from a

The Realm of Nature

CHAP.

point B in 60° latitude. But the line CE is nearly 4000
miles long, while the line AB is not 2000 miles ; therefore
during the time of one rotation the point E is carried through
more than 24,000 miles, while the point B is carried through
little over 12,000 miles, and the points N and S are at rest.

The rate of movement of the
Earth's surface by rotation is
called its tangential velocity,
and diminishes from over 1000
miles an hour at the equator
to 500 miles an hour at 60°,
and o at the poles. A body
resting on the Earth's surface
has a tendency to fly away at
a tangent, like a stone in a
sling, and the force of gravity
is partly employed in prevent-
ing this. The centrifugal force

FIG. 12.— Diagrammatic Section of (§ 51) makes bodies weigh

CN,S^cft£SS;;N^'; less at the equator than at the
AB, perpendicular to axis; o, a poles, reinforcing the change

point in 45 N. lat. ; B, B, points J

m 60° lat. due to the fact that the equator

is more distant than the

poles from the Earth's centre (§38). If the Earth rotated
seventeen times more rapidly than it does the centrifugal
force would be equal to gravity, and if it rotated in the least
faster the equatorial part of the Earth would split off like
the edge of a burst grindstone. The increase of tangential
velocity with length of radius enabled the fact of the Earth's
rotation to be proved in the seventeenth century by dropping
a weight from the Leaning Tower of Pisa, and observing
the distance of its fall to the east of the perpendicular line.
The weight was moving eastward on the top of the tower
more rapidly than the base of the tower, and retained its
original motion in consequence of inertia.

94. Measurement of Rotation. — The period which
elapses between the Sun crossing the meridian or north
and south line of a place on two successive occasions is
called a day, and is divided into 24 equal parts or hours ;

iv The Earth a Spinning Ball 59

this is the apparent time occupied by the Earth in making
one rotation. It is in many ways more convenient, and
also more exact (§ 1 1 1), to determine the period of rotation
of the Earth by observing the successive transits of con-
spicuous stars. By this means the exact period of the
Earth's rotation has been fixed as 23 hours, 56 minutes,
4 seconds. The name Sidereal Day is given to the rota-
tion period of the Earth as measured by the stars, and
astronomers divide it into 24 hours, subdivided into minutes
and seconds of sidereal time.

95. Time. — The uniform rotation of the Earth is the
only standard of time which is practically employed, and
for common purposes the solar day of 24 hours is every-
where used as the unit. The Sun crosses the meridian of
any place midway between its hour of rising and of setting,
and the name meridian (mid-day) was given to the north
and south line on this account. Mid-day or noon can be
determined exactly by measuring the altitude of the Sun
by a sextant or transit circle, or roughly by watching the
shadow cast by a stick or a pillar. As the Sun is rising the
shadow gradually becomes shorter, and at noon the Sun
being at its highest the shadow is at its shortest, and
marks out on the ground the north and south line or
meridian of the place. The movement of mechanism
actuated by a falling weight or an uncoiling spring, and
regulated by a pendulum or a balance-wheel, is uni-
versally employed for time -measuring ; but all clocks,
watches, and chronometers must be adjusted according to
astronomical determinations of the rotation period of the
Earth.

96. Local Time. — As the Earth turns, the Sun appears
successively on every meridian. It is always noon some-
where, but it can never be noon on two meridians at the
same moment. The rate of angular rotation is 360° in 24
hours, or 15° in I hour, or i° in 4 minutes. Thus when
the Sun is on the meridian of Greenwich it is 1 2 hours since
it shone on the meridian of the Fiji Islands (180°), where
it is consequently midnight. Two towns 15° apart differ I
hour in their local noon, so that it is necessary in describing

60 TJie Realm of Nature CHAP.

the time of any occurrence to specify by what meridian the
time is regulated. The local time in different parts of the
world at Greenwich noon is shown on Plate XIX. Greenwich
time is used throughout all Great Britain, although at
Greenwich noon it is 12-7 local time in the east of Norfolk
and 1 1-37 in the west of Cornwall. In Ireland, Dublin time
is employed, the clocks there showing 11-35 at Greenwich
noon. Throughout the United States and Canada the time
is changed by I hour at every 15° of longitude ; so that in
each belt of that width the same time is shown on all the
clocks, and between the Atlantic and Pacific there are five
changes of this kind. Travelling eastward or toward the sun-
rising has the effect of making the Sun rise earlier each day
and set earlier each night ; passengers on an eastward-bound
steamer in the North Atlantic have their meals 20 minutes or
half an hour earlier each day according to the speed of the
vessel, and the clock appears to go slow. Going right round
the world in an easterly direction the few minutes cut off each
day by meeting the Sun before the complete rotation of the
Earth amount to one whole day extra, so that, for example,
in i oo Earth rotations the traveller has seen I o i noons, and
recorded the doings of 101 days (each i per cent shorter
than a day at home) in his diary. Similarly going in a
westerly direction the rising and setting of the Sun are
delayed by an equal interval of time, and on going round
the world westerly in 100 Earth rotations there have been
only 99 noons and the doings of only 99 days recorded,
each " day " of course being i per cent longer than a day
at home. In order to keep the dates right a day is dropped
out of the reckoning of all vessels sailing eastward when
they cross the meridian of 1 80° from Greenwich, and a day
is added on to the reckoning when they cross the same
meridian bound westward.

97. Longitude. — The longitude of a place is the angular
distance of its meridian from some prime meridian, that of
Greenwich being usually adopted. In order to find the longi-
tude of a place from the meridian of Greenwich it is only
necessary to know the local time and Greenwich time at the
same moment. Local noon is easily ascertained by direct

iv The Earth a Spinning Ball 61

observation of the Sun, or by observing when the Sun attains
equal altitudes, before or after crossing the meridian, and
halving the interval of time. To get Greenwich time in
remote places is more difficult. Accurate chronometers,
very carefully regulated and rated, are usually relied on, the
average time shown by two or three instruments being taken
as correct. If at noon local time, when the Sun is on the
meridian, the chronometer shows that it is 1 1 A.M. Green-
wich time, it is evident that an hour must elapse before the
Earth has turned sufficiently far toward the east to bring
the meridian of Greenwich under the Sun. The interval
between the local meridian and that of Greenwich is there-
fore I hour's turning or 15° ; and since the Earth is turning
toward the east the local meridian must lie 15° E. of that of
Greenwich. If at local noon in another place the chrono-
meter showed 2 P.M. Greenwich time, it is evident that the
Earth has been turning for 2 hours toward the east since
Greenwich was under the meridional sun, and the place of
observation lies 2 hours of turning or 30° W. The apparent
position of the Moon on the star-dome at successive intervals
of Greenwich time is given in \heNautical A lmanac,\he Moon
thus serving as a clock-hand pointing to the hour. But seen
from different parts of the surface of the Earth the Moon is
displaced to one side or another, and it is necessary to
calculate the angular distance of the Moon from certain stars
as it would appear if measured from the centre of the Earth,
just as correct time is only shown by a clock when the
observer stands in front of it (§ 33). When this correction
for parallax, as it is termed, is made, the lunar distances
give the Greenwich time by a simple calculation and the
longitude can be found at once. Since the great circle of
the equator, the circle of only half the size of the parallel of
60°, and the minute circle immediately surrounding the pole
are all divided into 360° of longitude, it is evident that
while the arc subtending I ° on the equator is equal to that of
a degree of latitude, a little over 69 miles, the arc subtend-
ing i° of longitude at the parallel of 60° is only 34^ miles,
and that close to the pole only a few feet or inches. The
parallels of latitude are equidistant from each other, but

62 The Realm of Nature CHAP.

the meridians of longitude converge and all meet at the
poles.

98. Terrestrial Magnetism. — The rotation of the Earth
is probably the cause which confers on the globe as a whole
the properties of a great magnet (§ 79). The poles of the
Earth-magnet are near the poles of rotation, but do not
coincide with them ; the north magnetic pole lies in 70° 51'
N. 96° 46' W. and the south about 73° S. 146° E. (see map
Plate I.) When a small straight magnet is hung by a fine
thread so that it can move freely in all directions, it takes
up a position which in most parts of the world is nearly
north and south, hence its use in the mariner's compass
(§ 438) as a ready means of finding directions. A
suspended magnet when free from any disturbing attraction
points due north and south in all places, marked in the map
by the curves of o° or agonic lines. The angle between
the meridian and the direction of a suspended magnetic
needle is called the declination, or by sailors the variation
of the needle. Between the agonic lines over almost all
Europe, Africa, the Atlantic and Indian Oceans, the needle
points west of north, the lines in the magnetic chart
showing the number of degrees in different places. In the
north-west of Greenland the declination is 90°, or the needle
points due west ; while northward of the magnetic pole it is
1 80°, or the north-seeking pole turns due south. Over most
of Asia, America, the Pacific and Indian Oceans, the declina-
tion is to the east of north. After a freely suspended steel
needle, balanced so as to rest horizontally upon its pivot, is
magnetised one end is found to be drawn downward by the
magnetic attraction of the Earth. This phenomenon is
called the Dip of the needle. Along a certain line on the
Earth's surface there is no dip ; this line is termed the
magnetic equator and is shown in the map. North of it
the north-seeking pole dips more and more until at the north
magnetic pole it points vertically downward. South of the
magnetic equator the south -seeking end of a suspended
magnetic needle dips downward. The intensity of magnetic
force varies from place to place, being nearly proportional to
the dip. In certain regions the rocks beneath the surface

I
cc
u

UJ

LL
O

go,

02

tl

Q E

<

8 £ S

iv The Earth a Spinning Ball 63

of the Earth exercise a powerful attraction on a suspended
magnet (§ 348).

99. Periodical Magnetic Changes. — In 1576, when the
declination of the magnetic needle was first measured in
London, the north-seeking pole pointed 1 1 ° east of north,
but the easterly declination gradually diminished until in
1652 the needle pointed due north, and, the change still
continuing, in 1815 it pointed 24^° west of north. Since
then the declination has gradually diminished, being only
17° W. at London in 1891, and decreasing about 9' per
annum. The dip is subject to a similar slow change. These
changes were formerly accounted for by supposing that the
magnetic poles changed their position on the Earth's surface.
Recent observations indicate that this is not the case ; they
rather suggest that the alteration of declination and dip
may be produced by geological changes taking place in the
Earth's crust. Commander Creak, as the result of the
" Challenger " observations, states that the change is most
rapid at several points in a line drawn from the North Cape
along the Atlantic to Cape Horn, and that the British
Islands are situated in the region where the rate of change
is greatest of all.2 Regular changes of shorter period also
occur, the needle daily swinging perhaps 5' or 6' to E.
and W. of its average position and back again ; and there
is a yearly periodicity as well. Irregular variations of much
greater extent, sometimes amounting to one or two degrees,
are called magnetic storms, and are closely connected with
the appearance of the aurora (§ 174). Auroras and
magnetic storms are most frequent at intervals of about 1 1
years, corresponding to the periods of greatest frequency of
sun-spots. It is remarkable that whenever a great uprush
of heated gas takes place in the Sun, producing solar
prominences (§ 1 1 6), there is a simultaneous disturbance
of all the delicately - hung magnetic needles on the
Earth. Thus it appears that while the Earth's magnetism
resides in the massive rocks of its crust, and is probably
produced and maintained by the Earth's rotation, the
Sun's energy exercises a regulating or disturbing influence
upon it.

64 The Realm of Nature CHAP, iv

REFERENCES

1 See Nature, xl. p. 65 (1889).

2 Summary of Creak's Report on "Challenger" Magnetic
Observations, Nature, xli. p. 105 (1889).

BOOKS OF REFERENCE
See end of Chapter V.

CHAPTER V

THE EARTH A PLANET

ioo. The Moon. — So far we have looked on the
heavenly bodies as convenient marks blazoned on the hollow
dome of space around the spinning Earth. In § 97 it was
implied, however, that the Moon at least was free to change
its position on the star-dome. The Moon appears to move
amongst the stars, from west to east, so fast that if we
observe it rising due east at the same moment as a star, it
will be seven times its own diameter behind the star on cross-
ing the meridian, and the star will have set about half an
hour before the Moon reaches the western horizon. The
Moon often passes between us and a star, and occasionally it
passes in front of the Sun, causing an eclipse. These facts
prove that the Moon revolves round the Earth from west to
east, and that it is the nearest of all the heavenly bodies.
The diameter of the Earth affords a sufficiently long base-
line (§ 33) to measure the distance of the Moon accurately,
the vertical angle at the Moon of the triangle of which
the radius (or semi -diameter) of the Earth is the base
being 5 7'. This angle is called the horizontal parallax of the
Moon, and shows that the diameter of the Earth as seen from
the Moon would be i° 54'. The parallax varies somewhat
during a month, showing that the distance of the Moon is
not always the same ; but from its average value the average
distance of the Moon is found to be 238,793 miles, or in
round numbers 240,000. The apparent, or angular,
diameter of the Full Moon as seen from the Earth is about

66 The Realm of Nature CHAP.

30' ; that is to say, 1 80 full moons, one above another,
would extend from the horizon to the zenith. The diameter
of a body subtending this angle at a distance of 240,000
miles must be about 2000 miles, or, to be exact, 2153 miles.
The mass of the Moon has been estimated to be -^ of that
of the Earth ; its mean density is about 3 times that of water.

10 1. The Moon's Surface. — The Full Moon appears
to be diversified with patches of unequal brightness, but
observations with powerful telescopes prove that it is simply
a lithosphere surrounded by neither water nor air. Ring-
shaped mountains closely resembling volcanic craters may
be easily seen by using an ordinary field-glass, especially
when the Moon is so placed that sunlight illuminates only
part of the surface. The Moon shines by reflecting sunlight,
and even when most brilliant its light is so feeble that if the
whole visible sky (a surface equal to 105,000 moons) were
to shine as brightly the effect on the Earth would only be
equal to one-fifth that of the Sun. As the Moon revolves
round the Earth we see the side turned toward us wholly lit
by the Sun once a month and call it Full Moon ; a fortnight
later the Sun is shining only on the side turned from us and
we see the Moon dark, calling it New Moon. Between these
periods the illuminated area wanes or dwindles down to a
crescent, and again waxes or grows into the full round.

102. Period of the Moon. — The Moon revolves round
the Earth in 27 days, 7 hours, 43 minutes; but the in-
terval of time between successive new moons or full moons
(the lunar month) is rather more than two days longer. The
Moon always presents the same aspect to the Earth — only
one half, and always the same half, is to be seen, although
now and again slight irregularities in its motion reveal a
narrow additional strip at one edge or another. The fact that
no one has seen the other half proves that the Moon rotates
on its axis in exactly the same time as it revolves round the
Earth. If it had no rotation we should see all round it. To
prove this, pass a loop of thread over a drawing-pin fixed in
a horizontal board or table and the other end of the loop
round a pencil. Keep the cord stretched, and, holding the
pencil between the finger and thumb facing in the direction

v The Earth a Planet 67

of the arrows (Fig. 1 3), trace a circle without allowing the
hand to rotate. The diagram shows that the drawing pin, A,
if endowed with vision, would see all sides of the pencil (re-
presented by the arrows) in succession. Next trace a similar
circle, holding the pencil firmly but keeping one side of it, say
that covered by the thumb, toward the centre, so that the

FIG. 14. — Revolution of a body rotat-

FIG. 13.— Revolution of a non-rotating jng once in the same time as it

body; presenting all sides con- revolves; presenting always the

secutively to the centre. same side to the centre.

drawing-pin can only see the thumb-nail (arrow-head in Fig.
14). When the circle is complete the cramped position of
the hand will prove that there has been rotation at the wrist.
The fact of rotation is shown in the diagram by the arrow
pointing successively in every direction.

103. Differential Attraction and Tides. — Since at-
traction varies inversely as the square of the distance
between the attracting bodies (§ 36), it follows that the
Moon must exert a greater attractive power on the side of
the Earth which is nearest to it than on that which is 8000
miles farther away. In consequence of this, the Earth is
subjected to a stress tending to lengthen it out toward the
Moon. The rigid lithosphere is not perceptibly strained ;
the gaseous atmosphere is so readily disturbed by other
causes acting irregularly that only the slightest effect from
this cause can be detected in it ; but the liquid hydrosphere
responds readily and swells into a long low wave, the crests

68 The Realm of A Tature CHAP.

of which are on opposite sides of the Earth, and equal troughs
between them. As the Earth rotates, high water and low water
succeed each other regularly, from east to west, as the crest
and trough of the wave pass at intervals of about 6|- hours.
Without mathematical reasoning it is impossible to explain
how the tidal wave, pulsating round the world, is related to
the actual position of the Moon in its orbit and in the sky
(§ 218). On account of the formation of tidal currents, the
hydrosphere is very gently pressed like a brake on the
lithosphere by the differential attraction of the Moon ; and as
the energy of the currents comes from the Earth's rotation,
the rate of rotation at the end of each century is slower by
the fraction of a second, and the time of rotation, or day, is
longer in the same minute proportion.

104. The Tidal Romance of the Moon. — Millions of
years ago the Earth must have rotated much more rapidly
than now, when it suffers from long application of the brake.
At that remote epoch the Moon was much nearer than
now, for it is a property of revolving bodies, which cannot
be explained here, that any reduction in the rate of the
Earth's rotation is necessarily accompanied by an increase
in the Moon's distance. The nearer Moon must have raised
far greater tides than those we now know, in the more
extensive and denser hydrosphere of those ancient days.
In the remotest past on which this argument casts light
the Moon must have been close to the Earth, whirling
round its little orbit in the same time as the Earth spun
round on its axis, which was then only a few hours. The
Moon, indeed, seems to have been originally part of the
semi-fluid Earth whirled off by the furious rotation (§93)
of the earliest times. As the Moon receded from the Earth
in its slowly widening spiral path it also had a hydrosphere
in which the Earth's differential attraction raised tides, the
friction of which gradually brought the rapid rotation of our
satellite to correspond with its period of revolution round
the Earth.

105. The Sun even more conspicuously than the Moon,
separates itself from the other heavenly bodies, which are
dim by contrast with its brilliance, and when the Sun

v The Earth a Planet 69

rises vanish from sight like tapers when an electric arc is
turned on. The altitude of the Sun at noon, observed at
any place, varies throughout the year, increasing day by
day until a certain maximum is reached, and then decreasing
gradually to a minimum. The period from highest Sun to
highest Sun, as observed in regions outside the tropics,
is about 365 days. The angular diameter of the Sun
when measured daily is found to gradually increase
from a minimum of 31' 32" to a maximum of 32' 36", and
then to diminish again to its former value, and this change
also takes place in about 365 days. Unless with the aid
of a very powerful telescope we cannot see the constellations
in daylight so as to be able to tell amongst what group of
stars the Sun appears at noon ; but we know that these
stars are just opposite those which cross the meridian at
midnight. In the course of 365 days all the constellations
of the star-dome successively cross the meridian at midnight,
and from this fact we know that the Sun, like the Moon,
moves amongst the stars from west to east, although in a
year instead of a month.

1 06. Problem of the Earth and Sun. — The most
natural explanation of the Sun's annual path amongst the
stars is that the Sun, like the Moon, revolves round the
Earth, but in a year instead of in a month. Another
hypothesis, that the Earth revolves round the Sun, would
also explain the facts. In Fig. 15 both hypotheses are illus-
trated. S represents the sun, E the earth, the arrow ESN
shows where the Sun appears amongst the stars at noon,
and the arrow EM shows what stars cross the meridian at
midnight. The dark circle is the hypothetical orbit of the
Earth round the Sun, the lighter circle the hypothetical
orbit of the Sun round the Earth. The arena is so vast
that the gyrating pair of globes are practically at the same
distance from the amphitheatre of stars. Whether we
assume that the arrow ESN, passing through the Sun,
turns round the centre E, or that the arrow ESN, passing
through the Earth, turns round the centre S, the arrow
would point successively to the same parts of the star-dome,
and observation of the stars would not decide which is the

The Realm of Nature

CHAP.

correct hypothesis. The law of gravitation explains that two
revolving bodies circle round the centre of gravity of the
pair. In the case of the Earth and Moon the centre of
gravity of the system lies within the Earth, hence the Moon

*********** ' •"

FIG. 15. — Problem of the Earth and Sun. Showing how observation of the Sun's
place amongst the stars cannot tell whether the Earth (E) goes round the
Sun (S), or the Sun goes round the Earth.

appears to revolve round it. It remains to inquire where
the centre of gravity of the Earth and Sun lies ; in other
words, whether, and by how much, the Earth or the Sun is
the greater body.

107. The Sun's Distance and Mass. — The horizontal
parallax (§ i oo) of the Sun is not quite 9" ; and being so
minute it is not easily measured accurately. Since the Sun's
parallax is about ^i^ of the Moon's (§ 100), it follows that
the Sun must be about 380 times more distant from the
Earth than is the Moon. Accurate determinations give
the average distance as 92,700,000 miles. Since the Sun
subtends as large an angle to our eye (about 30') as the
Moon does, it follows that the Sun, being 380 times as
distant, must have a diameter 380 times as great as that of
the Moon, that is to say, about 800,000 miles. The Sun's
volume is thus more than 1,200,000 times that of the Earth.
By the attractive force it exerts the Sun's mass is proved to

v The Earth a Planet 71

be more than 300,000 times that of the Earth. The centre
of gravity of the Earth-Sun System must, indeed, lie within
the Sun, and it is therefore as certain that the Earth goes
round the Sun as that a weight of 50 Ibs. will cause i grain
to fly up if the two are placed in the opposite scales of a
balance.

1 08. Proof of Revolution. — If a man, sitting in a dog-
cart on a dead-calm day while a steady downpour of rain
is falling, finds the raindrops driving against his face instead
of falling straight upon his hat, he concludes correctly that
this aberration or wandering of the raindrops from their
normal path is due to the fact that the dogcart is not at
rest but in rapid motion. By estimating the angle at which
the rain strikes he may even calculate the rate at which
he is being carried along. The astronomer, sitting in his
observatory, detects a similar aberration in the light -rays
from each of the stars. He finds the light reach him at a
different angle at various times of the year, so that each star
traces out a minute annual curve on the sky the greatest
radius of which is about 20". No other cause can account
for this aberration of the starlight except the fact that the
observatory and the Earth itself are rushing with tremendous
velocity through space in a closed curve which takes one year
to complete. The rate of motion can be calculated from the
angle of aberration, when the velocity of light is known.

109. The Earth's Orbit. — The regular change in the
angular diameter of the Sun seen from the Earth (§ 105)

.proves that the annual orbit is not a circle, as the two
bodies are sometimes nearer and at other times farther apart.
The form is an ellipse (Fig. 16), of which the Sun occupies
one focus (S) ; but the ellipse is very like a circle, the
ratio of the longest to the shortest diameters being as
100,000 to 100,014. Indeed, if a circle 3 inches diameter
were drawn with a very sharp pencil making a line 5 ^ 0
of an inch thick, it would represent the orbit correctly,
the difference between the ellipse and the circle being con-
cealed by the thickness of the line. The place of the Sun
would, however, require to be represented —^ of an inch
from the centre of the circle. Certain slow changes take

The Realm of Nature

CHAP.

place in the form of the orbit on account of the per-
turbation of the Earth by other planets. The eccentricity,
or distance of the Sun from the centre, increases to a very
marked degree, diminishes until the orbit becomes almost
a circle, and then begins to increase again. The time
elapsing between successive maxima of eccentricity is
about half a million years. The Earth moves round
this orbit with varying speed, moving fastest when nearest
the Sun (or in perihelion, ^), and slowest when most remote
(or in aphelion, a) ; the average velocity is about i8j miles

FIG. 16. — Ellipse, representing the Earth's orbit enormously exaggerated in
ellipticity and eccentricity. S, the sun ; a, aphelion ; p, perihelion.

per second or 66,000 miles an hour. Before Newton
proved that the power of gravity would produce precisely
this effect, Kepler had discovered the nature of the motion,
and expressed ' it in his " Second Law " thus : The radius
vector, or line joining the centres of the Earth and
Sun, sweeps through equal areas in equal times.
In Fig. 16 the figure SAB is equal in area to the
triangle SCD ; S being the sun, SA, SB, SC, SD, being
successive positions of the Earth's radius vector. Hence,
since the radius vector sweeps through the angle SAB in
the same time as it takes to sweep through SCD, the Earth
traverses the long part of its orbit from A to B through
perihelion in the same time as it traverses the much shorter
distance from C to D through aphelion.

no. The -Year. — The period in which the Earth

v The Earth a Planet 73

accomplishes one revolution round the Sun is called a year,
and is the unit for long intervals of time. The unit for
shorter intervals of time is the solar day or apparent period
of the Earth's rotation. Unfortunately these two natural
units are incommensurable ; the revolution period of the
Earth with regard to the stars is not made up of an
even number of rotation periods or of solar days, but
consists of 365 days, 6 hours, 9 minutes, 9^ seconds.
The tropical year or time of apparent revolution is 365
days, 5 hours, 48 minutes, 46 seconds ; and it is in order
to fit in the extra 5 hours and odd minutes that the plan
of having ah extra day every fourth year (leap year), and
omitting it o^§ a century, is adopted. If this were not
done, the same period of the year would not occur in the
same part of the Earth's orbit at each successive revolution.

in. Solar and Sidereal Time. — The revolution of the
Earth round the Sun once in a year accounts for the interval
between two successive transits of the Sun across the
meridian, the solar day being nearly 4 minutes greater than
the Earth's rotation period or sidereal day. While the
Earth is turning once round on its axis it advances so
far upon its orbit that nearly 4 minutes of turning more
than a complete rotation are necessary to bring the Sun
once more on the meridian. Since the Earth moves with
unequal velocity in different parts of its course, and its axis
is not perpendicular to the plane of its orbit, the day, as
measured from noon to noon, varies slightly in its length
throughout the year. The average solar day is taken in
order to calculate the solar mean time which is always used
in ordinary affairs.

112. The Ecliptic. — The Earth's orbit lies always nearly
in the same plane, because there is no force competent to
change its direction. That is to say, the Earth goes round
the Sun in limitless space as a boat sails round a ship on the
surface of a calm sea. We may imagine the plane to extend
beyond the Earth's orbit through all space so that it inter-
sects the dome of stars. The line of intersection is the
apparent yearly path of the Sun amongst the stars, and is
called the ecliptic ; the constellations it traverses are the

74 The Realm of Nature CHAP*

well-known twelve " signs of the zodiac." The plane of the
ecliptic in space serves as a standard level, to which other
directions may be referred for comparison. It seems most
natural that the Earth's axis should be perpendicular to the
plane of the ecliptic, but, as has been said, this is not the case.
The axis is inclined about 23 J° from the perpendicular. We
have thus to picture the Earth sailing round the Sun, not " on
even keel" but with a list or inclination of 23^°, and with
the north end of the axis always pointing toward the same
bright star on the celestial dome. This inclination is not
absolutely constant, but like the eccentricity of the orbit is
subject to slight increase and diminution in long periods.

113. Eclipses. — Instead of saying that the Earth re-
volves round the Sun we should, in order to be accurate,
say that " the Earth-Moon System " does so ; for the Moon
shares the annual revolution of the Earth as a point on the
tire of a wheel shares the onward movement of the centre.
If the Moon's orbit lay in the plane of the ecliptic, the Moon
would pass between the Earth and Sun once every month,
and a fortnight later the Earth would cut off the sunlight
from the Moon. In other words, at every New Moon there
would be an eclipse of the Sun, at every Full Moon there
would be an eclipse of the Moon. But the Moon's orbit is
inclined at an angle of about 5° to the ecliptic, and it is only
when the Moon happens to be at one of the nodes, or points
on the orbit where its plane intersects the ecliptic, that an
eclipse can take place. From this fact the ecliptic gained its
name. Eclipses of the Moon are common occurrences, for
they happen several times in a year and are visible from a
large area of the Earth's surface, as the Earth's shadow is
wide compared with the angular diameter of the Moon.
Eclipses of the Sun are more frequent, but are more seldom
seen at a given place, being visible only for a comparatively
short time and over a limited tract of the Earth's surface,
since the Moon's shadow thrown by the Sun is a com-
paratively narrow cone. When the Moon is at its nearest
point to the Earth, in the course of its elliptical orbit, its
angular diameter is great enough to entirely conceal the
Sun, and the eclipse is said to be total. But when the Sun

v The Earth a Planet 75

is at its nearest, its disc appears larger than that of the
Moon at its farthest ; and if an eclipse occurs in such con-
ditions it is said to be annular, the black disc of the Moon
being surrounded by a narrow bright ring of the Sun, like
a penny lying on a half-crown.

1 1 4. Solar Tides. — The differential attraction of the Sun
on the opposite sides of the Earth has a tide-raising power
like that of the Moon (§ 103). But the Sun is so distant that
in spite of its vast mass the difference in its attracting power
on opposite sides of the Earth, due to the distance of 8000
miles, is only two-fifths as great as the difference in the
attracting power, of the nearer Moon. At New Moon and
at Full Moon the tide-raising power of Sun and Moon is
exerted in the same direction, and produces Spring-tides in
the ocean ; the tidal wave rises highest and sinks lowest
or has its greatest amplitude. At the quarters, on the
other hand, the Sun is raising high water where the
Moon is producing low water, and consequently the ampli-
tude is much less, the tidal wave not rising to the average
height nor sinking to the average depth. These are called
neap-tides, and represent the difference, as spring-tides
represent the sum, of the tide-raising power of Sun and
Moon, the relative amplitudes being as 3 to 7.

115. Precession of the Equinoxes. — The tropical year
or apparent time of the Sun's circuit of the heavens is 20
minutes shorter than the Earth's revolution period (§ no);
in other words, if the Sun starts from that point of the
ecliptic known as the vernal equinox it will reach it again
20 minutes before completing the annual circuit of the
heavens. Thus the equinox seems to be moving slowly along
the ecliptic to meet the Sun, and so every year it precedes
or comes before its former position, the phenomenon being
known as the precession of the equinoxes. The star-dome,
not sharing the movement, appears to rotate about an axis
at right angles to the plane of the ecliptic, but so slowly that
2 5,000 years are required for a single turn. Consequently the
constellations on the zodiac have ceased to correspond with
the " signs " of 30° each which formerly included them. This
apparent movement of the heavens must be produced by a

76 The Realm of Nature CHAP.

real rotation of the Earth in 25,000 years round an axis per-
pendicular to its orbit. The axis of diurnal rotation thus
describes a slow conical motion like the mast of a boat which
is pitching and rolling equally, and the north pole, instead of
pointing steadily to the pole star, traces out a circle on the
star-dome about 47° in diameter in the course of 25,000 years.
The horizontal axis of a gyroscope at rest is at once drawn into
a perpendicular position by attaching a lightweight to one end.
But if the fly-wheel is in rapid rotation, the angle which the
axis makes with the perpendicular remains constant, and
the weight attached merely sets up a slow rotation of the gyro-
scope about the perpendicular, the axis of spinning tracing
out a circular cone (§ 51). The differential attraction of the
Sun and Moon on the protuberant region about the Earth's
equator (§82) exerts a force tending to pull the equator
into the plane of the ecliptic and make the axis of diurnal
rotation perpendicular. Rotation sets up resistance as in
the gyroscope, and the attempt to make the Earth sit up-
right results in the very slow rotation about the perpendic-
ular, to which the axis of diurnal rotation preserves the
nearly constant angle of 23^°.

1 1 6. The Sun's Surface. — The bright disc of the Sun
which we see is termed the Photosphere, and although it
appears uniform in texture to the eye, the telescope shows that
it is finely mottled with brilliant granules separated by a less
luminous network. The Sun rotates in about 25 days, but
not like a solid globe, and the fact that marks on different
parts of the surface move at different rates proves that the
photosphere is the surface of a dense and intensely heated
atmosphere in which the bright granules are vast luminous
clouds. During a total solar eclipse red flames of fantastic
form are usually seen projecting beyond the black disc of the
Moon, and these Prominences may also be observed without
an eclipse by an ingenious arrangement of the spectroscope.
They consist of great outbursts of intensely heated gas,
mainly hydrogen. Prominences have been seen rising to
the height of 400,000 miles above the Sun's surface in a
few hours, against gravity 27 times as powerful as that of
the Earth. This gives us some idea of the terrific violence

v The Earth a Planet 77

of the manifestations of solar energy. Down -rushes of
comparatively cool gases from the upper regions of the
Sun's atmosphere are believed to be the cause of black
marks which are often seen on the photosphere and termed
sun-spots^ although sometimes many thousand miles in
diameter. Though apparently black, compared with the
intense glow of the rest of the surface, sun-spots really shine
with a light brighter than that of the electric arc lamp.
Photographs of the Sun's disc are taken daily in some
observatories in order to preserve a record of the number
and movements of sun-spots, and in this way much inform-
ation has been obtained on the subject. It has been
observed that spots usually originate at some distance on
either side of the Sun's equator, and for a time they increase
in size ; then beginning to diminish they travel toward the
equator and gradually vanish, being succeeded by others,
which are smaller and fewer. Finally, after about twelve
years or so, the whole set fades away, and a new series of
larger size appear and go through the same changes.
Periods when sun-spots are at a maximum succeed each
other at intervals of about eleven years, and relations have
been traced between them and the influence of the Sun's
radiant energy on the Earth. During total eclipses a halo
of silvery light, sometimes circular, sometimes spreading
out like great wings, surrounds the Sun. It is called the
corona, and is probably composed of fine particles of dust
either thrown off by the Sun or being attracted toward it
and shining, in part at least, by reflected light.

117. The Spectrum of Sunlight is a continuous band of
colour crossed by an immense number of .black lines (the
more conspicuous of which are named in Fig. 8, § 63),
showing that the light from some glowing solid or liquid
has reached us after traversing an expanse of cooler vapour.
Every year more of the lines in this spectrum are identified,
and those which are produced in the Earth's atmosphere
are being distinguished from those due to the Sun's. The
lines produced by absorption of light in the Earth's atmo-
sphere are best recognised by comparing the spectrum of
the Sun low in the sky, when they are strongest, with that

78 The Realm of Nature CHAP.

at noon, when they are faint. When a body giving out
light is in rapid motion toward the observer, the wave-length
of the light is apparently shortened and the lines of its
spectrum are shifted toward the violet end. In the light
of a rapidly receding body the lines are similarly shifted
toward the red end. At its equator the Sun's surface is
moving 70 miles a minute, toward an observer on one side
— from him on the other. By causing a small image of the
solar disc to flit across the slit of the spectroscope several
times in a second, an observer analyses in quick succession
the light from the approaching and receding edges. Con-
sequently the most distinct solar absorption lines are seen
to oscillate slightly from side to side, being displaced alter-
nately toward the red and toward the violet, while the lines
produced in the Earth's atmosphere remain motionless
and can be readily distinguished. The elements which have
been detected in the Sun are identical with those found in
the Earth, but the spectrum shows that they are at an
enormously high temperature, so much so that some of the
solar lines not yet identified may be due to matter of
a simpler form than any elements known on the Earth

(§ 47).

1 1 8. The Heat of the Sun. — The temperature of the
Sun is higher than any that has been produced on Earth,
and it does not perceptibly differ from year to year. If the
Sun were a heated solid or liquid globe it would be falling
in temperature as it radiated heat, unless the supply were
kept up in some way. There is no external source of heat
that is sufficient to account for the vast solar expenditure.
The collision of meteorites and many other theories have
been suggested, tested, and rejected, and we must look to
the Sun itself for an explanation. Sir William Thomson *
and Professor von Helmholz have shown that as the solar
atmosphere loses its heat the power of gravity draws its
particles closer together, and this shrinking transforms the
potential energy of separation (§§ 54, 56) into heat, which
is sufficient to maintain the diminished volume at the same
or even a higher temperature. The process will go on,
loss of heat being compensated, or more than compensated,

v The Earth a Planet 79

by shrinkage, as long as the Sun remains mainly gaseous.
If this theory is correct, Sir William Thomson estimates
that twenty million years ago the substance of the Sun was
so diffused and cool that it had not begun to give out light
such as we now enjoy, and that five or six million years
hence the sphere will have grown solid, cold, and dark.

1 1 9. The Earth's Share of Sun-heat. — Since the Sun's
parallax is less than 9" it follows that, viewed from the Sun,
the Earth only occupies 20ooo1ooooTT °^ tne sky' or a disc
1 8" in diameter. The Earth consequently receives less
than ^g-g.^.^-^^ of the radiant energy sent out by the Sun.
If the Sun were expending, instead of energy, money at the
rate of ;£ 18,000,000,000 a year, the Earth's annuity would
be only £9. This endowment, however, is payable con-
tinuously, and at the same rate throughout the year, in the
proportion of 6d. every day or ^d. every hour. Minute as
the energy which reaches the Earth appears in view of what
streams away into space, it is stupendous when compared
with the power of the greatest steam-engine ever constructed,
and is, indeed, the source of all the work and all the wealth
of the world actual and prospective.

120. Effects of Inclined Axis. — If the Earth's axis of
rotation were perpendicular to the plane of the ecliptic
the Sun's radiant energy would be dispensed for an equal
time each day over the whole surface — every place would
always have 1 2 hours of daylight and 1 2 hours of darkness.
The Sun would always be in the zenith at noon on the
equator, but never elsewhere ; at the poles the Sun would
always be half above the horizon, and at every inter-
mediate point the meridian altitude would always be
(as in fact it is at the equinoxes) the complement of the
latitude, i.e. 90° minus the latitude. In consequence of the
inclination of the axis the distribution of radiant energy on
the Earth is unequal and varies at different times of the
year, giving rise to the difference of the seasons.

121. Vernal Equinox. — The position on 2ist March
(Fig. 17) is such that the equator lies in the plane of the
Earth's orbit as viewed from the Sun, ana!" the Sun appears
in the zenith at noon viewed from the equator. Sunlight

80 TJie Realm of Nature CHAP.

reaches both poles simultaneously, and as the Earth rotates,
every place on the surface is lighted up for twelve hours
and plunged in darkness for the other twelve, day and night
being equal everywhere. This period is therefore called
the vernal or spring equinox, and happens at that point in

FIG. 17.— Diagram illustrating the cause of the seasons.

the Earth's orbit from which the Sun appears projected on
the star-dome in the sign of Aries. This season is spring
in the northern and autumn in the southern hemisphere.

122. Summer Solstice. — In three months, the Earth
having advanced along one quarter of its path, the equator
dips 23^-° S. of the plane of the ecliptic when viewed from
the Sun, hence the Sun viewed from the Earth appears at
noon in the zenith on the parallel of 23 *° N. ; and as at
this time the Sun is projected on the star-dome in the
sign of Cancer, this parallel is called the Tropic of Cancer.
This is the highest northern latitude for a vertical Sun, and
is called a tropic because the Sun appears to turn south-
ward after reaching it. Sunlight reaches 23^-° beyond the
north pole, and falls short of the south pole by 23^°. As
the Earth rotates the whole region for 23.^° round the
north pole keeps in sight of the Sun, the whole region round
the south pole rests in darkness, and the period of daylight
diminishes while that of darkness increases over the world

v The Earth a Planet 81

from north to south, being 1 2 hours each at the equator.
The Sun being vertical at noon, 23^° north of the equator,
its meridian altitude from the south point of the horizon in
the northern hemisphere is equal to the complement of the
latitude plus 231°. In the southern hemisphere the Sun's
greatest altitude is equaljto the complement of the latitude
minus 23!°. This period is termed the summer solstice^ as
the Sun stops in its northern path. It is the middle of the
northern summer and of the southern winter. The parallels
of 66^-° (23^° from the poles) are termed the Arctic and
Antarctic Circles, and these are the lowest latitudes in which
sunlight or darkness can last for 24 hours at a time.

123. Autumnal Equinox and Winter Solstice. — In
three months more it is the autumnal equinox ; the equator
comes again into the plane of the Earth's orbit, day and
night are equal from pole to pole, and the Sun's meridian
altitude is again equal to the complement of the latitude.
The Sun is projected on the star-dome in the sign of Libra,
and it is the autumn of the northern hemisphere and the
spring of the southern. Another period ,of three months
brings the Earth into such a position that the equator is
23|-° N. of the Sun's place in the ecliptic, and con-
sequently the Sun is seen vertically overhead at noon from
the parallel of 23^° S., which is termed the Tropic of
Capricorn after the sign in which the Sun is projected on
the star-dome. This is the highest south latitude for a
vertical Sun. The Sun is visible everywhere within the
antarctic circle, but all within the arctic circle is in day-
long darkness. . In all parts of the southern hemisphere
the Sun's meridian altitude above the north point of the
horizon is 23*° greater than the complement of the latitude ;
in the northern hemisphere it is 23^-° less, and the days
grow shorter and the nights longer from south to north,
day and night being equal on the equator. This is the
winter solstice, midwinter in the northern hemisphere and
midsummer in the southern.

124. Altitude of the Sun. — The altitude of the Sun
and duration of daylight are described above for a globe
without an atmosphere. On account of refraction (§ 150)

G

82 The Realm of Nature CHAP.

the Sun always appears higher in the sky than its true
position ; the period of daylight is thus increased and the
period of darkness diminished, the effect being greatest in
high latitudes.

LENGTH OF THE LONGEST DAY.

Latitude o° TO° 20° 30° 40° 50° 60° 70° 80° 90°

Hours 12 i2h. 35 130. 12 1311.56 1411.51 i6h. 19 i8h. 30 65 days i6id. i86d.

(Refraction slight and (Refraction

not allowed for) allowed for)

Between the tropics the Sun is vertical in every latitude
twice in the year ; outside the tropics never. Even in

summer the altitude of the
Sun is low in high latitudes ;
it can never be more than
23^° at the poles, nor more

ABODE

The amount of radiant

F'Gbread,I of"fhee Cea^E £$. J™ eneigy falling on the surface

as that of AB, but striking at an angle varies with the altitude of
of 30° the length CE is twice the length ,1 c 17 ' T y V,

AB where the rays fall perpendicularly. tne ^Un. Fig, I 8 Shows

that the same beam of light

which, falling vertically, covers I sq. ft. of surface, will,
when falling at an angle of 30° cover 2 sq. ft., and so
produce on each square foot only one half of the effect of
vertical light ; at a lower angle the heating effect of sun-
light is very slight. Oblique rays of light also pass through
a thicker layer of the Earth's atmosphere, and so are more
absorbed than vertical rays.

125. Zones of Climate. — It follows that the region
between the tropics receives most of the solar energy,
higher latitudes sharing it in smaller and smaller pro-
portions. The Earth has consequently been divided into
zones of climate — a word originally meaning inclination of
the Sun's rays. The areas within the polar circles, poorest
in radiant energy, are termed the Frigid Zones, those
between the polar circles and the tropics, where there is a
tolerable abundance of radiation, the Temperate Zones, and
the wide belt between the tropics which is overflowing with

The Earth a Planet

solar wealth the Torrid Zone (Fig. 19). If the Earth were
a smooth lithosphere, either free from water or surrounded
by a continuous hydrosphere and atmosphere, this unequal
distribution of solar energy would
give rise to a regular system
of redistribution by currents
streaming from the equator to
the poles in the upper regions
of the atmosphere, and from the
poles to the equator in the lower,
their paths curved in consequence
of the rotation of the Earth ; and
in this way the tropical warmth
would be distributed with some
approach to uniformity over the
whole surface. The actual re-
distribution is much more complicated (§178 and following).

FIG. 19. — Zones of Climate on the
Earth.

REFERENCE

1 Sir Wm. Thomson on "The Sun's Heat," Nature^ vol. xxxv. p.
297 (1887).

BOOKS OF REFERENCE

(The

J. F. W. Herschel, Astronomy, Cabinet Cyclopaedia,
most perfect description of simple mathematical astronomy. )

R. S. Ball, Time and Tide: A Romance of the Moon. S.P.C.K.

James Nasmyth, The Moon considered as a Planet, a World, and a
Satellite. John Murray. (Unique illujJfS&era <3f.£fi5*S$tirface of the
Moon.) Xfe1e>

See also list at end of Chapter

CHAPTER VI

THE SOLAR SYSTEM AND UNIVERSE

126. The Solar System. — The Sun and Moon are not
the only celestial bodies which pass between our eyes and
the dome of stars. Several bright objects, which, unlike
the stars, shine without twinkling by light reflected from
the Sun and show a distinct disc in the telescope, were
long ago called planets^ or wanderers, for they pursue a
devious track among the constellations, changing in posi-
tion on the star-dome from night to night. All the planets
are related to each other, as their wanderings are all
confined to the belt of sky termed the zodiac, extending
only a few degrees on each side of the ecliptic. The
distances of these bodies from the Earth have been
measured, and it has been proved that like the Earth
they all rotate and revolve round the Sun in elliptical
orbits, the planes of which are, as a rule, only slightly
inclined to the plane of the ecliptic. Some of the statistics
of the members of the solar system are given in the following
table.

127. Inner Planets. — The four planets next the Sun
are often called the inner planets. Mercury and Venus
are never seen very far from the Sun, and Mercury is
rarely visible to the naked eye. Venus, visible sometimes
as the evening star shortly after sunset, and at other times
as the morning star shortly before sunrise, is a magnificent
object, its light being often strong enough to throw a
distinct shadow. These two planets exhibit phases like

CHAP, vi The Solar System and Universe

the Moon, those of Venus being clearly visible by the aid
of an opera-glass. Signor Schiaparelli has recently proved
that the period of rotation of Mercury is equal to its period
of revolution round the Sun ; and this is probably true of
Venus also. Solar tidal friction has evidently acted on
these planets as the tidai- friction of the Earth has acted
)

THE PLANETS.

;s m

Mean Dis-
tance from
Sun. Mil-
lion Miles.

Periodic
Time.
Solar
Days.

Diameter
of Planet.
Miles.

Rotation
Period.

Satel-
lites.

Days.

Mercury . $

35-9

88

2,992

88

Venus . . ?

67-0

224-7

7,660

224-7

Hrs. Min.

Earth . . 0

92.7

365-3

7,918

23 56

I

Mars . . <J

141

687

4,200

24 37

2

ASTEROIDS \ ...

Jupiter . .

K

482

4,332

85,000

9 55 4

Saturn . . ^

884 j 10,759

7I,OOO

10 14

8

Uranus . . B[

1780 130,687

31,700

4

Neptune . *j?

2780 60,127

34,500

...

i

on the Moon ; and it is interesting that the two planets
nearest to the Sun, and receiving enormously more heat
and light than the Earth, have perpetual day in one
hemisphere, and perpetual night with a cold approaching
the absolute zero in the other. Mercury and Venus
occasionally pass between us and the Sun, the planet
appearing to pass across the solar disc lilce a small black
spot. A -transit of Venus affords, the best opportunity of
measuring the solar parallax, and hence the Sun's distance,
by noticing how far the path of the planet across the disc
is altered when viewed from distant parts of the Earth.

128. Mars, the first planet beyond the Earth, most
resembles it. The rotation period is nearly the same, and
the surface is diversified by marks which evidently indicate
continents and seas, while at each pole a gleaming white
patch increases and decreases as the planet wheels round

86 The Realm of Nature CHAP.

the Sun, suggesting the forming and melting of great areas
of snow. Until 1877 Mars was supposed to have no
satellites, but in that year Professor Hall of Washington
discovered two. One is very small, very near the planet,
and races round it, from west to east, in little more than
7 hours, making three complete revolutions whilst the planet
rotates once ; the other, farther away, revolves in 30 hours.
129. Asteroids. — It had been observed even before
Kepler's time that there is a certain symmetry in the
placing of the planets. This relation was subsequently
formulated by the German astronomer Bode in the end of
the eighteenth century, and has since been termed Bode's
Law. It is as follows : If 4 be added to each member of
the numerical series —

o 3 6 12 24 48 96
we get- -

4 7 10 16 28 52 100

Mercury. Venus. Earth. Mars. Jupiter. Saturn.

These figures represent very nearly the relative distance
of the planets from the Sun, e.g. Saturn is I o times farther
than the Earth. There is a gap between Mars and Jupiter,
and although no physical reason was, or is, known for this
arrangement, the whole system seemed so orderly that
Kepler supposed this gap to represent the place of a
missing planet. Bode and several other astronomers were
so impressed by the gap in this law that they agreed
to examine the sky very minutely for the missing planet.
While their search was in progress the Italian Piazzi (who
was not one of the number) discovered on the first night
of the nineteenth century a small planet occupying exactly
the position prescribed by this law, and gave it the name
of Ceres. Next year another little planet was discovered,
and when half the century had elapsed no less than fifteen
had been found. A more systematic search was then
commenced by many astronomers, and the small stars
made visible only by powerful telescopes were followed
individually night after night, with the result that a great
many were found to have no fixed place on the star-dome,

vi The Solar System and Universe 87

and to show the movements of planets. They are so like
stars that the name Asteroid (star-like) is usually given
them. No. 311 was discovered on nth June 1891.
These minor planets are all very small, the largest being
probably only 300 miles in diameter; the orbits of some
are very long ellipses, and lie far out of the plane of the
ecliptic (see § 132).

130. Outer Planets. — Beyond the asteroid ring the
giants of the solar system, each attended by a train of
satellites, rotate with amazing speed, and are surrounded
by thick atmospheres loaded with heavy clouds. Jupiter,
the largest of all, with four satellites, has a temperature so
high that dense layers of cloud, arranged in belts parallel
to the equator by its rapid rotation, completely obscure the
body of the planet. The spectrum of its light shows some
dark bands which are not due to reflected sunlight, and it is
generally assumed that Jupiter is only now cooling down
from being a self-luminous body. Saturn, although some-
what smaller, is unique in being accompanied by a series
of rings or thin flat discs surrounding its globe parallel to
the equator, and reflecting sunlight like the planet itself.
These rings can only be accounted for on the assumption
that they are composed of orderly crowds of innumerable
minute satellites. Outside the rings there are eight separate
satellites of various sizes, one being larger than the Moon.

131. Uranus and Neptune. — Uranus has been known
as a planet since 1781, when it was discovered by Herschel.
One astronomer had observed it previously twelve times,
and only the careless way in which he kept his notes pre-
vented him from recognising it as a new member of the
solar system. This remote body is remarkable for its four
satellites revolving in apparently circular orbits in a plane at
right angles to that of the planet's orbit, and from east to
west, whereas the satellites of all planets nearer the Sun
revolve, like the Moon, from west to east. The movements
of planets in their orbits under solar attraction is calculated
from Kepler's Laws (§ 109), but allowance has always
to be made for the perturbations or deviations produced by
the attraction of other planets. After all possible allowances

88 The Realm of Nature CHAP.

were made, and the path of Uranus along the star-dome
calculated, it was found that the planet did not keep to its
time-table. The English astronomer Adams and the French
Leverrier made calculations on the assumption that this
irregularity was produced by an unknown planet beyond
Uranus. In 1846 their work was finished almost simultane-
ously, and each predicted the position of the hypothetical
planet in the sky. The very day that the information from
Leverrier reached the observatory of Berlin, the German
astronomer Galle turned his telescope to the part of the sky
indicated, and there discovered the new planet which was
named Neptune. Like Uranus it had previously been
recorded as a star, and it was only by mistrusting his
observations that an earlier astronomer failed to detect its
true nature. One satellite has been observed which re-
volves, like those of Uranus, from east to west.

132. Comets. — Occasionally a luminous body appears
in the sky, brighter in some cases than the planets, and usu-
ally enswathed in a long flowing tail of gauzy texture, from
which peculiarity it is called a comet. Many comets have
been found to travel in elliptical orbits, much more elongated
than those of the planets, but like them with the Sun in one
focus. As a comet pursues its path, it approaches the Sun
with increasing velocity, sweeps round and sometimes almost
touches the solar surface, and then flies on with ever
diminishing speed to its aphelion. Halley's comet was the
first the regular return of which was noticed ; its period
is 76 years, and it should next return to perihelion in
1910. It will then pass within the Earth's orbit, but its
aphelion lies outside the orbit of Neptune. Several
comets have their farthest points from the Sun near the
orbit of Neptune ; others show a similar relation to Uranus
and to Saturn, while quite a number of comets of short
period are associated with the orbit of Jupiter. Many of
the grandest comets that have been seen pursued a path
shaped like a parabola or hyperbola, and after passing the
Sun swept out of the solar system for ever. It is supposed
that the orbits of comets are naturally parabolas, but when
the comet happens to pass near enough to a planet the

vi The Solar System and Universe 89

path is changed by attraction either into a closed curve — an
ellipse — or into a hyperbola. Comets are thus viewed as
the carriers of new stores of matter and energy into the
solar system from remoter realms of space. Halley's comet
is believed to have been captured by the attraction of
Neptune when it was sweeping through the solar system,
and the other periodic comets are similarly the slaves of the
great planets. The planes of the orbits of comets show no
relation to that of the ecliptic, sometimes indeed being
perpendicular to it. To revert to a former simile (§ 112),
if the Sun be compared to a large ship, and the ecliptic to
the surface of the ocean, steam-launches manoeuvring round
the ship represent the planets, all nearly in the same plane,
though the swell of the ocean causes them to be above the
mean level at one part of their evolutions and beneath it at
another. A comet would be represented by a diving bird
going round the ship by diving under the keel and flying
above the deck.S

133. Nature of Comets. — The tail of a comet, some-
times several million miles long, is greatest when near the
Sun, away from which it points whether the comet is
approaching or receding. Comets shine, according to the
spectroscope, partly with reflected sunlight and partly with
the light of glowing vapour. The density of their substance
is very slight, and they were long supposed to consist of
masses of glowing gas. Recent observations, however, make
it almost certain that they are swarms of very small solid
bodies far enough apart to let starlight pass between them,
and these when heated by approach to the Sun give off
vapour at first composed of a compound of carbon and
hydrogen, latterly, as the temperature is higher, of metals
such as sodium and iron. The particles which make up
comets may be only a few inches, or possibly only the
fraction of an inch in diameter, and they are known as
meteorites.

134. Meteors. — Attentive observers may see a few
meteors or " falling stars " on any clear night. A star
apparently detaches itself from its neighbours on the star-
dome and silently glides downward, sometimes leaving an

90 The Realm of Nature CHAP.

evanescent track of light. At certain times, particularly
about loth August and I3th November, this phenome-
non is so common that showers of shooting - stars are
seen. At those dates the Earth crosses the orbits of
two comets. The November shower is sometimes mar-
vellously magnificent, and the grandest displays recur
at intervals of about 33 years. The last is still re-
membered in 1866, and a similarly fine spectacle may be
looked forward to in 1899. Meteors are not falling stars,
for the stars are as numerous after a meteor shower as before.
They are produced by small solid bodies, on the average
perhaps as large as a pea, which enter the Earth's atmosphere
with enormous velocity. The energy of motion is converted
into heat by the friction of the air, and the solid is im-
mediately driven into vapour and vanishes, being con-
densed into fine invisible dust (§§ 1 6 1 , 277). Meteors usually
begin to glow at the height of about 80 miles above the
Earth's surface, and die out at the height of at least 50
miles.

135. Meteorites. — It has occasionally happened that
meteoric masses of considerable size, weighing several
pounds or even hundredweights, have fallen on the Earth,
and in about a dozen cases this has happened in the sight
of intelligent witnesses. Meteorites, as such masses are
termed, are of at least two classes, either metallic composed
mainly of iron and nickel, or stones resembling volcanic
rock, although frequently associated with minerals not
known in terrestrial rocks. They often contain carbon,
and almost always considerable quantities of various gases
absorbed in their pores. When a powdered meteorite is
heated in a tube from which the air has been exhausted,
and through which an electric current is passed, it glows
with a faint light, the spectrum of which is very like that of
comets, strongly confirming the meteoric theory of those
bodies (§ 133). The close relation of meteors and comets
was proved very forcibly in 1861 when the Earth dashed
through the tail of a comet; again in 1872, and in 1885
when Biela's comet was calculated to cross the Earth's orbit
close to the Earth's place at the time. The only sign of the

vi The Solar System and Universe 91

collision on these occasions was a fine shower of shooting-
stars, through which the Earth sailed as safely as a locomotive
passes through a cloud of dust. Meteorites of all sizes,
from an invisible granule to masses of several tons
and moving in various directions, seem to be scattered
in infinite numbers through all space, and occasional denser
swarms moving together form comets.

136. The Stars. — The Sun, surrounded by its orderly
family of planets and an irregular host of attendant comets
and meteorites, is practically alone in the centre of the star-
sphere, forming one system isolated by inconceivable ex-
panses of space from the fixed stars. But the Sun and its
train are sweeping with tremendous velocity in the direc-
tion of the constellation Hercules. The number of stars
or fixed points of light on the star-dome which are visible
at any one time to the unaided eye of an observer on the
Earth is about 3000. More people in fact assemble to
hear a popular concert than there are stars in the heavens,
so far as our vision can tell. By the aid of an opera-glass
more than 120,000 stars, too feeble in their light to be seen
by the unaided eye, spring into sight. A million may be seen
through a small telescope ; in a large telescope the number
is enormously increased, and with every instrumental im-
provement smaller specks of light crowd in myriads on the
view. Some stars, invisible in the most powerful telescopes
to the eye, have been discovered by their effect on a
sensitive photographic plate. Altogether the existence of
something like 100,000,000 stars has been ascertained.
The telescope, no matter how powerful, fails to make even
the brightest star appear as a disc ; but it often shows that
what we see as a single star is actually double, triple,
quadruple, or multiple. In some cases this is an accidental
result of stars, perhaps very distant from one another, lying
nearly in the same line as seen from the Earth ; but there
are many " physical doubles " the associated stars of
which are seen to revolve round one another. This dis-
covery proves that these stars are subject to gravitation.
Several stars vary in their brightness at definite intervals, at
one time blazing out with extraordinary brilliance and then

92 The Realm of Nature CHAP.

fading down to invisibility. This happens so regularly in
some as to suggest that a dark body revolving round the
star comes between it and us. In other stars the increase
in brightness is accompanied, according to the spectroscope,
by a change in chemical constitution and a great increase
of temperature, as if perhaps swarms of meteorites flying in
opposite directions had come into collision.

137. Distance of the Stars. — The stars are so remote
that when corrected for aberration (§ 108) there is, as a rule,
no apparent parallax. This means that the displacement
of our eye by 186,000,000 miles from one side of the
Earth's orbit to the opposite does not alter their apparent
position on the star-dome. In several cases a minute
parallax has been measured. The largest, barely i", was
found in the case of a Centauri, one of the brightest stars
visible in the southern hemisphere. The parallax of Sirius,
the brightest star in the sky, is \ of a second, that of the
Pole Star only -^ of a second. Light which travels at
186,000 miles per second requires 8 minutes to flash from
the Sun to the Earth, and would require 9 hours to traverse
the diameter of Neptune's orbit. Yet the light from a Cen-
tauri, the nearest star, has been more than 3 years on its
way to us. We see Sirius by the rays sent out more than
1 7 years ago, and for nearly half a century the light-waves
which are now arriving from the Pole Star have been shooting
with lightning speed across the awful voids of space. Other
stars are perhaps a hundred or a thousand times more
remote than these. Although the star-dome may be spoken
of as a vastly remote whole with reference to the solar
system, it is really made up of remotely isolated objects
placed at different distances and seen by us at different
dates. For all our sight can tell us to the contrary, every
star that shines placidly in the sky may have grown cold
years or centuries ago, and snapped the thread of light
the end of which may now be fast approaching our Earth.

138. Stars as Suns. — For classifying the stars the
spectroscope has entirely superseded the telescope. By its
means great differences have been detected in the chemical
composition and physical states of various stars, and the

vi The Solar System and Universe 93

classification now viewed with most favour is of a bio-
graphical character, referring the star to its position in the
long evolution or series of changes through which our Sun
is passing (§ 118). In arranging the stars in the order of
their evolution their state at the period their light left them
is of course referred to. Stars of youth, or the earlier stages,
are comparatively cool and diffused agglomerations of
matter gradually condensing and rising in temperature.
Stars of middle life, or the central stages, are intensely hot,
invested with a glowing atmosphere of gas which gives
bright lines in the spectra of their light. Stars of old age,
or the later stages of evolution, have survived the period of
maximum temperature and are steadily consolidating and
cooling down. There is reason to believe that many stars
are invisible to us because they have ceased to glow. We
may infer, from their general similarity to our Sun, that stars
of the central and later stages at least are accompanied by
systems of planets. Some double stars present much the
same appearance as the Sun would have done at a similar
distance when Jupiter was still brilliantly incandescent.
Many of the stars have a rapid motion through space as
shown by the displacement of their spectral lines. This is
termed their proper motion, to distinguish it from the various
apparent movements, but though it is inconceivably swift it
has produced very little change in the appearance of the
constellations in 2000 years.

139. Charting the Heavens. — Although the constella-
tions remain of the same form as when first described by
astronomers, some change must be taking place. Common
star-maps fail to let the changes appear, but a series of
large photographic charts of the sky would probably show a
definite alteration of position amongst the stars on account
of their proper motion in a few years. An International
Astronomical Congress held in 1891 decided that in several
observatories such photographs should be taken with the
ultimate object of completing a photographic survey of the
entire star-dome. In order to prevent confusion from chance
specks and to detect asteroids, a device has been suggested
by which the photographic plate is exposed in the telescope

94 The Realm of Nature CHAP.

in three long stages with a slight shift of position in each.
Each star thus prints itself as a little triangle of three points,
while in consequence of its relative motion an asteroid pre-
sents its record in one little blurred streak and can thus be
readily detected.

140. Form of the Universe. — On a clear moonless
night a luminous gauzy band called the Milky Way may be
seen spanning the sky like a wide but ragged and colourless
rainbow. As this is visible from all parts of the Earth it
evidently forms a complete girdle round the star-dome. A
telescope of moderate power shows that the Milky Way is
really a dense pavement of stars. There is no reason to
believe that any two of these stars are nearer each other
than the Sun and a, Centauri, and the apparent crowding
is simply an optical effect due to their great number. If
we were led blindfolded into a regular pine plantation, and
on looking round found that to east and west the tree trunks
stood out sharply against the sky, affording a glimpse of
diversified country beyond them, while to north and south
the trunks were crowded so closely that they formed merely
a reddish mist under the dark green leafage, we would natur-
ally conclude that the wood was planted in a long narrow
belt running north and south. So from our station in the Uni-
verse the Milky Way appears as the direction in which the
extent of star-sown space is greatest ; the widely strewn stars
indicate the sides on which it is least. The form of the
Universe, if this mode of reasoning be correct, is that of
a vast disc, the edge of which, as shown by a division in
the Milky Way, is partially split and bent back. Within
this expanse the great family of 100,000,000 or more stars is
supposed to be clustered together, and separated by incalcul-
able distances of vacancy from other universes which may
exist.

141. Star-clusters. — As one might catch glimpses of
other forests through the tree trunks on either side of the
long plantation, so we catch glimpses of remote universes
through the thinly star -sown regions remote from the
Milky Way. These are faint patches of light, which were
long called Nebula from their cloudy appearance. Genera-

vi The Solar System and Universe 95

tions of astronomers have laboured to discover the nature
of these cloudy tracts, and in many cases they have succeeded
in showing them to be clusters of immeasurably remote starsv
The forms of these star -clusters or remote universes are
in many cases wonderfully beautiful — ring-shaped, oval,
rod -like, or resembling dumb-bells or spirals of much
complexity.

142. Nebulae. — The old observers were accustomed to
find that many nebulae which their telescopes only showed
as a gauzy cloud were resolved into star-clusters when a
more powerful instrument was brought to bear on them.
Consequently it was long believed that all unresolved nebulas
were simply star-clusters that larger telescopes could make
plain. When Mr. Huggins first succeeded in observing
the spectra of the unresolved nebulas in 1864 he detected
bright lines unlike those of stars, and doubtless coming from
intensely heated gases. The nebulas were therefore sup-
posed to be distant masses of glowing gas. Professor Nor-
man Lockyer has recently suggested a somewhat different
explanation of the spectrum. He points out that the spectra
of nebulae and of comets' tails and of meteorites in a vacuum
tube (§§ 133, 135) are all so much alike that they are prob-
ably produced by the same materials. Following an earlier
suggestion of Professor P. G. Tait, he views a nebula as a
vast swarm of meteorites moving in different directions, and
by dashing against each other producing heat enough to
drive a part of their substance into luminous vapour.

143. The Nebular Hypothesis. — The Prussian philo-
sopher Kant and subsequently the French astronomer
Laplace accounted for the origin of the solar system by sup-
posing that at one time in the remote past it consisted
merely of a vast nebula or cloud of intensely hot gas
extending far beyond the orbit of the outermost planet.
As this cloud cooled and contracted it acquired a whirling
motion from west to east, and formed a rotating gaseous
disc which gradually condensed at the centre to form the
embryo Sun. The edge of the whirling disc was thrown
off as a ring by centrifugal force, and the ring ultimately
condensed into the planet Neptune. The gaseous disc

96 The Realm of Nature CHAP.

continuing to contract and to spin more rapidly threw off
another ring which gave rise to Uranus, and so on with the
other planets, which themselves by a similar process threw
off rings to persist like those of Saturn or to condense into
satellites. The ring thrown off after the formation of
Jupiter, instead of condensing into one planet, consolidated,
perhaps on account of perturbation by its great neighbour,
into separate bodies — the asteroids. The residue of the
original nebula remained as the great globe of the Sun.

144. Meteoritic Hypothesis. — Recently Norman Lock-
yer has pieced together the facts discovered by modern astron-
omers, and he believes them to countenance the theory that
originally all space was filled with matter in its simplest
or primary form, that this matter commenced to aggregate
under the influence of gravity and chemical affinity, produc-
ing a fine moving dust of the elements and latterly of their
compounds. This dust further condensed and gave rise to
meteorites in great moving swarms separated by tracts of
empty space. As the meteoritic swarms shrank by gravity,
collisions between the individual meteorites became more
frequent, and some of their energy of motion was changed
to heat which partly vaporised them, giving rise to the bodies
we recognise as nebulas or as variable stars. These swarms
of moving meteorites present many of the properties of a
gas on a very large scale, and the motion and equilibrium
of a meteoritic nebula would be very similar to those of a
gaseous one. Meteoritic material is supposed to pass from
the nebular state into that of separate and much denser
suns surrounded by families of planets somewhat in the
manner Laplace suggested. Lockyer differs from Laplace
in making gravitation and molecular attraction the primary
cause rather than heat, and so including in the theory the
heating up as well as the cooling down of the Universe.

BOOKS OF REFERENCE

J. Stuart, A Chapter of Science, S.P.C.K. (A thoroughly
scientific and attractively simple explanation of the movements of
th'e solar system. )

vi The Solar System and Universe 97

S. P. Langley, The New Astronomy. Boston : Ticknor and
Co. (Remarkable for its fine illustrations.)

R. S. Ball, Star-Land. Cassell and Co. (Simple and racy
sketch of elementary astronomy.) The Story of the Heavens.
Cassell and Co.

J. N. Lockyer, The Meteoritic Hypothesis. Macmillan and Co.

See also list at end of Chapter V.

CHAPTER VII

THE ATMOSPHERE

145. The Ocean of Air. — We live and move at the bottom
of a shoreless ocean of invisible fluid to the surface of which
we are powerless to rise. The existence of this ocean is
revealed to us by its power of exercising pressure, but the
substance composing it was long supposed to have no
weight, and the phrases "light as air," uan airy nothing"
are remnants of that idea. The simple experiment of
inverting a tumbler over a cork floating in a basin of water
shows that air can exert pressure and that it occupies space.
.By means of the air-pump a glass vessel can be nearly
emptied of air, and on weighing it before and after empty-
ing, it is ascertained that a pint of air has the mass of about
10 grains, or a cubic foot that of i^- ounce.

146. The Barometer. — Torricelli, an Italian mathema-
tician of the seventeenth century, when investigating the
action of the common sucking-pump, made a discovery which
laid the foundations of scientific knowledge of the atmo-
sphere. He took a tube closed at one end and about 33
inches long, filled it with mercury, and placing his thumb on
the open end inverted it (Fig. 20) in a basin of mercury.
The column of mercury in the tube sank gradually and
stood just 30 inches above the level of the mercury in the
basin. Mercury placed in a tube open above and below
and set in the same manner would immediately run out by
its own weight. Torricelli argued that the only difference
in the mercury in the closed tube was that the weight of

CHAP, vii The Atmosphere 99

the atmosphere could not press upon it. He knew that in
a liquid at rest every point in the same horizontal plane
must be at the same pressure, so he argued that every point
in the line a b (Fig. 20) must be at the same pressure.
The points between c and d were pressed
upon by the weight of 30 inches of mercury,
but were free from the weight of the air,
while the points from a to c and d to b
were free from the weight of mercury, but
subject to the pressure of the weight of
the air. Thus the pressure of the atmo-
sphere on a given area is equal to the
weight of 30 inches of mercury, or 14^
pounds on a square inch. This reasoning
proved that the atmosphere presses as
heavily on the Earth's surface as if it were
an ocean of mercury 30 inches deep, or,
since mercury is about 13^ times denser FIG. 20. - Mercurial
than water, an ocean of water 34 feet deep. Barometer and

. 11 yard measure.

Exact observation shows that the column
of mercury balanced by the atmosphere at sea-level over
the whole Earth averages 29.9 inches, and it is calculated
from this that the whole mass of the atmosphere is 5500
million millions of tons. Since the mercury tube enables
one to measure the weight of the atmosphere it has been
called the Barometer (see also § 439).

147. Pressure of the Atmosphere. — Torricelli's experi-
ment made it clear that the piston of a common suction-
pump lifts the atmosphere from above the piece of water in
which the pipe dips, and that the pressure, of the atmosphere
on the rest of the surface forces up the water over that
space until the weight of the column is equal to the pressure
on an equal area of the free surface : this height never exceeds
about 34 feet, which is the limit of lifting power in a pump.
Air, and fluids generally, exert pressure equally in all
directions ; and on account of this uniform pressure of the
air all round us and through the tissues of our bodies, we
do not feel the pressure to which we are always subjected
of 1 4|- pounds on every square inch, or 1 4 tons for the

i oo The Realm of Nature CHAP.

whole body of a man of ordinary size. A common limpet
/ weighing perhaps half an ounce sticks to a smooth level
rock as if its weight were from I o to 15 pounds, because the
soft tough foot is planted so closely on the stone as to
exclude all air from below and the pressure comes from the
outside only. The limpet sticks as firmly to a vertical
or an inverted surface as to a level one. The vacuum
brake is a powerful illustration of the pressure of air, for by
it the pressure of the atmosphere applied to a very small
part of the surface of a rapidly moving train brings it to a
stand in a very few minutes.

148. Density of Air. — The mass of the air has been
measured with great accuracy, but the height to which it
extends, the depth of our aerial ocean, is difficult to estimate.
If the density of the air ocean were uniformly the same as
it is at the Earth's surface (about -§\-§ of the density of
water), its height would be five miles. That this is not the
case was proved by Mr. Glaisher, who once ascended more
than seven miles in a balloon and still found air around him,
though of much less density than at the Earth's surface.
But the fact was known by theory two centuries earlier.
Boyle, in 1662, announced the discovery of the law known
by his name : —

The density of any gas is proportional to the pressure it
supports.

The pressure of the atmosphere produced by its own
weight is greatest on the Earth's surface or in a mine, where
the density is accordingly greatest also. As one ascends
in the atmosphere the pressure falls steadily, because less
air remains above, and the density of the remaining air is
consequently less. Thus the barometer can be used to
measure heights : near sea-level a fall of one inch in the
barometer corresponds to a rise of I ooo feet. One half of
the atmosphere lies beneath the height of 3^ miles, or 1 8,500
feet, from the Earth's surface, and the half which is above this
height can exert a pressure only equal to about 1 5 inches of
mercury at that level. Another rise of 3 J miles (to 7 miles)
leaves half of the half atmosphere below, and only one
quarter above, the pressure being equal to 7^ inches. At

vii The Atmosphere 101

iol miles above the Earth's surface J, at 14 miles ^, at 17^
miles ^ , and at 2 1 miles only ^ of |rfe atmosphere lies at
a higher level : at 2 1 miles the barometer would stand
at half an inch. Thus, if Boyle's law holds good the
atmosphere has no definite limit, but extends with diminish-
ing density throughout infinite space. It has however been
proved that this law does not hold for gases of very small
density, which behave like very light liquids and have a
definite surface, so that the atmosphere has an upper limit,
beyond which the particles of gas do not stray.

149. Height of the Atmosphere. — Observations of
twilight (§ 162) show that the atmosphere is not less than
45 miles high. The aurora, which is produced in the
upper atmosphere (§ 174) has been measured at more
than 100 miles above the Earth, and meteors (§ 134)
sometimes become visible at 200 miles. Hence it is prob-
able that the atmosphere extends at least 200 miles beyond
the Earth's surface ; but in consequence of its compres-
sibility nearly three quarters of the air lies between sea-level
and the summit of the loftiest mountain.

150. Atmospheric refraction. — When light from any
of the heavenly bodies enters the atmosphere, it traverses
denser and denser layers, and is consequently bent down-
ward from a straight line

as it approaches the sur-
face (§ 6 1 ). The amount
of this bending or refrac-
tion is proportional to the
obliqueness of the rays of
light — thus when the light
falls perpendicularly from
the zenith there is none, FlG- "--Atmospheric Refraction. A, ob-

' server; b, true position; S. apparent

but when it COmeS parallel position of Sun. The density of the

to the horizon the refrac- JSSbS! ™ ™*™^ by the c?oseness
tion is great. A person

always refers an object to the direction from which the
light enters the eye. When the Sun is near the horizon
its light is bent into the curve SA (Fig. 21) and as the light
reaches the eye of an observer at A from the direction S'A,

102 The Realm of Nature CHAP.

he sees the Sun's image at S', considerably higher in the
sky than it really is. In astronomical observations it is
necessary to correct this error, and tables of refraction at
every altitude of a star and for different temperatures of the
air have been compiled. The atmosphere, by raising the
apparent position of the Sun, thus serves to lengthen the
period of sunlight by about four minutes on the equator, and
by several hours and even days in high latitudes (§ 124).
For the same reason the midnight sun is visible in places
where it would not appear above the horizon if there
were no atmosphere. Thus at Archangel in lat. 64° 32',
nearly 2° south of the Arctic Circle, there is perpetual sun-
light for several days at midsummer. When from unequal
heating or other causes the distribution of density in the
atmosphere becomes irregular, light is reflected and refracted
by the layers of air in such a way as to make objects at a
great distance visible as if near at hand. This effect, which
is most marked in deserts and at sea, is called mirage.
All our knowledge of the outer regions is obtained by
looking through the window pane of air which encloses the
world, and allowance must always be made for its im-
perfections.

151. Composition of Air. — The experiments of Priestley,
Black, and Rutherford at the close of the eighteenth century
proved that common air is a mixture of several different
airs or gases, and at that date it ceased to be considered
an element. Innumerable analyses of air have since been
made which show that in all parts of the world the atmo-
sphere has almost the same composition. Traces of nearly
every gas which exists naturally, or is produced artificially
in large quantities, have been found in air, but the main
constituents are few. A rough analysis of air may be
made thus : — (a) A large tightly -corked flask of warm
air when chilled by being covered with snow or
ice is seen to become dewed with liquid drops on the
inside. These drops are water, and their appearance
proves that water -vapour is a constituent of air. When a
person wearing spectacles steps from the frosty night into
a warm room he is the victim of an irritating variation of

vii The Atmosphere 103

this experiment, for the cold glasses immediately condense
a blinding film of dew-drops from the air, (b) When a
little clear lime-water is shaken in a flask of air the liquid
becomes milky from the formation of solid carbonate of
lime, a compound of carbonic acid with lime. Hence,
carbonic add is one of the constituents of air. (c) When
a candle, or a piece of charcoal, or of phosphorus is allowed
to burn in a limited quantity of air under a tumbler or bell-
jar inverted in water, the flame soon goes out, and another
bit of burning charcoal, or phosphorus is extinguished the
moment it is introduced ; moreover, the water rises until it
fills about one-fifth of the jar, showing that about one-fifth
of the atmosphere is a gas which is consumed by burning
substances. This gas .is oxygen, (d) The residue from
which burning phosphorus has extracted the oxygen is a
gas with no striking properties called nitrogen, (e) When
a sunbeam traverses a darkened room, or when strong sun-
light streams through an opening in a thick cloud, immense
multitudes of motes may be seen dancing in the light.
Thus dust is an ingredient of the atmosphere. The amount
of water-vapour is variable, and the amount of dust is still
more uncertain ; but the other constituents occur always
very nearly in the proportions : —

Nitrogen .
Oxygen
Carbonic acid .

By weight.
76-80

23-I4
0-06

By volume.
79-00 or i
20-96 or %
0-04 or -5-sV

Total 100-00 100-00 or i

152. Nitrogen. — The most abundant gas of the atmo-
sphere has no colour, no taste, no smell, no tendency to
combine with other elements, no poisonous effect on living
creatures, and no power to keep them alive. From the
last circumstance it is sometimes called Azote. Its service
in the atmosphere is mainly to dilute the other ingredients,
and to produce mechanical effects. Most of the pressure
of the atmosphere, the strength of wind, the refraction of
light, and the buffer -action which breaks the force of
meteorites and drives them into dust, are due to nitrogen.

104 The Realm of Nature CHAP.

When an electric discharge passes through air, a small
quantity of nitrogen is always caused to combine with
hydrogen and oxygen to form salts of ammonia.

153. Oxygen was originally known as Vital Air, for it
is the ingredient of the atmosphere which sustains life, and
by its ready combination with other elements supports com-
bustion. The oxygen of the atmosphere is a great store of
potential energy when taken into account with the uncom-
bined substances in the Earth (§§ 56, 44). Oxygen in the
pure state combines very energetically with carbon, hydrogen,
and almost all the other elements ; but when it is diluted
with four times its volume of inert nitrogen, combustion is
slower and quieter, although the same amount of energy is
ultimately set free as would be the case if no nitrogen
were present. Under the influence of electric dis-
charge, and of the growth of some trees, oxygen is partly
changed into a condensed form called ozone, and partly
combined with water to form peroxide of hydrogen. These
substances exist in the air in very minute proportions, but
when either of them is present it is believed to increase
the healthfulness of a neighbourhood. Oxygen in small
quantities is a colourless and transparent gas, but in the
atmosphere it absorbs a good deal of sunlight, giving
broad black bands in the red part of the spectrum. The
blue tint of the sky may be due in part to the true colour
of oxygen. The proportion of oxygen in the free air of the
country is a very little greater than in crowded towns.

154. Carbonic Acid, though present in small amount,
has an important part to play in the economy of the atmo-
sphere. Green plants in sunlight absorb it, decompose it,
retain the carbon to build up in their own substance, and
breathe back the oxygen into the air. Animals and also
plants (§§ 399, 400) breathe in air, absorb the oxygen, which
is ultimately combined with carbon and breathed out as car-
bonic acid. There is a large proportion of carbon in coal, oil,
wood, fat, and almost all combustible substances, which thus
produce carbonic acid as the principal result of their union
with oxygen. The amount present in the atmosphere
varies considerably ; 3 parts in 10,000 is the proportion in

ISOTHERMS

After /

Land Surface from
600-6000 Ft- Elevation.

iR JANUARY,

lichan.

MO 120

fl» Temperature Ww32° FaLr. cdonredBhie

Land Surface Above p
6000 Ft Elevation. 1

The Atmosphere 105

the open country, 5 parts is common in towns, and as much
as 30 parts of carbonic acid in 10,000 of air may be found
in badly-ventilated overcrowded rooms. More than this
proportion acts poisonously on animal life. Carbonic acid is
the most soluble of the atmospheric gases, water at 60° F.
and under ordinary pressure absorbing its own volume.

155. Mixture of Gases. — One consequence of the
nature of gases is that when -two or more different kinds
are mixed, each one acts as if it alone were present. This
is known as Dalton's Law. Thus there is an atmosphere
of nitrogen surrounding the globe, and exerting the pres-
sure of its weight upon the Earth's surface, and an atmo-
sphere of oxygen pressing upon the surface with its weight,
which is rather less than one quarter of the pressure exerted
by nitrogen, and a very thin atmosphere of carbonic acid
exerting a very feeble pressure. There is also an atmo-
sphere of water-vapour pressing with its independent weight
on the Earth's surface, and all these partial pressures together
make up the pressure exerted by the whole atmosphere.
The particles of the different gases pass each other freely,
without interfering, like crowds moving in different directions
across a market-place. Thus it is that the composition of
the atmosphere as a whole remains constant so far as
regards the three gases, nitrogen, oxygen, carbonic acid, and
the proportion of each of them is the same at all heights.

156. Water -vapour. — Next to oxygen, water- vapour
is the most important ingredient of the atmosphere. The
other gases are a long way above their liquefying point, so
that the addition or withdrawal of heat only affects their
temperature and their volume. But water-vapour in the
atmosphere is near the temperature at which it becomes
liquid or solid, and is nearly always in the presence of liquid
water, hence changes of temperature greatly affect the
amount of vapour present. Let us suppose for a moment
that the atmosphere consisted of water -vapour only, and
that the hydrosphere covered the Earth uniformly with a
liquid layer. The amount of this atmosphere, and con-,
sequently its pressure, would depend upon the temperature.
Evaporation takes place from cold water, or even ice, but

io6 The Realm of Nature CHAP.

at every temperature when the vapour exerts a certain
definite pressure upon the liquid, evaporation is stopped,
and the vapour is said to be saturated at that tempera-
ture.

157. Water-vapour and Temperature. — At the freezing-
point (32°) water-vapour is saturated, i.e. presses sufficiently
to stop evaporation, when its pressure is equal to that of
o- 1 8 inch of mercury ; at 50° it must exert twice this pressure,
or 0-36, before evaporation ceases ; at 70° it must exert a
pressure of 0-73, and at 90° a pressure of 1-45 inches, in
order to be saturated. These figures show that at 50°
twice as much vapour is required to form a saturated
atmosphere as at 32°, and at 70° twice as much as at 50°,
and at 90° twice as much as at 70°. If an atmosphere of
water- vapour saturated at 50° is warmed up to 70°, evapora-
tion is at once allowed to commence and will continue
until the amount of vapour present above the water is
doubled. Then the vapour will exert pressure sufficient to
stop further change, and will be saturated. Again, if the
temperature of the saturated vapour is reduced from 70° to
50°, half the vapour must return to the liquid state or
become condensed in order that the pressure may fall to
that which is just sufficient to prevent further evaporation.
Hence it is plain that every rise of temperature is ac-
companied necessarily by evaporation, every fall of temper-
ature is accompanied necessarily by condensation, until the
vapour exerts the pressure proper to its new temperature.
Precisely the same thing happens, as explained by Dalton's
law, when there are atmospheres of nitrogen, oxygen, and
carbonic acid surrounding the Earth. The pressure of
saturated water-vapour at 50° is still equal to 0-36 inches
of mercury, — the only difference is that it takes a longer
time for the pressure to readjust itself to a change of
temperature, as a party of excursionists crossing a broad
railway platform reach their carriages, whether the platform
is left to themselves or is thronged by crowds moving in
different directions, only in the latter case the transference
takes a longer time. On account of the low temperature at
great elevations, water- vapour, although its density is only half

.

vii The Atmosphere 107

that of air, is almost entirely confined to the lowest region of
the atmosphere.

/ 158. Vapour Pressure and Humidity. — The fraction
of atmospheric pressure exerted by the water-vapour it con-
tains is often termed vapour tension, but preferably vapour
pressure. The amount of water-vapour in the atmosphere at
any place as measured by the hygrometer (see § 441), and ex-
pressed in the pressure it exerts in inches of mercury or by
the number of grains weight in a cubic foot of atmosphere,
is called the absolute humidity. In the case of saturated
vapour this depends only on the temperature. The vapour
in the atmosphere has seldom an opportunity to become
saturated, for the air is never at rest. Suppose, for example,
,. that air containing water-vapour saturated at 32°, and there-
fore exerting a vapour pressure of 0-18 inches, is carried
inland to a waterless place and heated up to 50°. Or
suppose simply that its temperature is raised so rapidly
that the somewhat slow process of evaporation has not had
time to produce its full effect. The absolute humidity or
vapour pressure is consequently only o- 1 8 inches, but
evaporation could continue if time and opportunity were
given until the amount of vapour would be doubled. Hence
this portion of the atmosphere has only one half, or 50 per
cent, of the water-vapour it could contain at its temperature.
If the same portion of air were cooled without other change
to 32° it would contain all the vapour possible at that
temperature, or 100 per cent, and have no tendency to
evaporate more. If it were heated to 70° it would contain
only one quarter, or 2 5 per cent, of what might be present
at that temperature, and evaporation would go on rapidly
from free surfaces of water. The term relative humidity
is applied to the percentage of the whole possible amount of
water-vapour which is present at any particular temperature.
When the relative humidity is low the atmosphere is "drying"
or has a tendency to raise more vapour from water or damp
soil • when on the other hand the relative humidity is high,
there is little tendency to evaporation, and a slight fall
of temperature leads to saturation and condensation.

159- Thermal Changes in Evaporation and Con-

io8 The Realm of Nature CHAP.

densation. — The change of a pound of water into a
pound of vapour requires the same expenditure of energy
(§ 70), whether it takes place in a kettle boiling on a fire,
or over the surface of a freezing pond. The work of
evaporation uses up heat, and produces a lowering of
temperature. On the other hand, when vapour is condensed
to the state of water, the potential energy stored up is
reconverted into heat ; thus condensation produces a rise
of temperature (§§ 70-73). When air resting over water is
heated by the Sun's rays, evaporation begins actively and
diminishes the rate of rise of temperature in the air.
On the other hand, when a portion of the atmosphere
containing saturated vapour is cooling down by radiation,
the vapour begins to condense, giving out heat, and so
retarding the rate of fall of temperature. In both cases
the tendency is toward moderation and slowness of change.
The cooling of air containing unsaturated vapour goes on
unchecked until the temperature of saturation is reached.

1 60. Absorptive Power of Air. — The water-vapour of
the atmosphere is not transparent to all light ; it absorbs
certain rays from sunlight, producing black lines or bands in
the spectrum, particularly a set in the yellow known as the
rain-band (TT in Fig. 8). The rain-band in the spectrum
increases in width and darkness as the amount of vapour in
the slice of atmosphere looked through increases, and the
probability of rain occurring within a certain time may
be judged from the darkness of the band. The heat rays
of the Sun pass readily into the atmosphere, but heat
does not so readily pass out through the air into space.
The atmosphere thus acts toward the Earth as a great
blanket, or rather a heat-trap allowing radiant heat to enter
freely but greatly retarding its escape. Water-vapour has
usually been considered the chief heat-entrapping agent,
because the chilling by radiation at night is always
greatest when the proportion of water-vapour in the air
is least. But there is now reason to believe that condensed
water and solid dust-motes are more powerful in producing
the effect.

1 6 1. Dust. — Solid dust is always present in the at-

VII

The Atmosphere 109

mosphere throughout its whole depth. Twenty million
meteorites are calculated to reach the Earth every day, and
most of these are broken up by the friction of the air,
furnishing a supply of Cosmic dust (§ 134), which being
excessively fine, and even invisible, settles down very slowly.
Terrestrial dust is carried into the atmosphere by ascending
currents of air and is of • many kinds, resulting from the
wearing down of rocks, from volcanic explosions (§ 297),
from flowers in the form of pollen, from minute organisms
either plants or animals (§ 401), from burning fuel, from
factories, mines, flour-mills, and from the spray of the sea.
The number of motes is almost incredible. Every puff of
smoke from a cigarette contains about 4000 million separate
granules of dust. Dust appears to float in the atmosphere,
and the motes of a sunbeam seem to be rising as often as
falling. This is, however, a result of currents of air. In
still air, dust always falls, but the large motes fall most
rapidly under the pull of gravitation, and against the
resistance of the friction of the air. When a cube of
stone one inch in the side is falling, its mass drags it
down, and the friction of the air on its six square inches of
surface resists the fall. If the cube were cut into ten slices
y1^ of an inch thick, each of these into ten bars, and
each of these into ten cubes T^ of an inch in the side, there
would result 1000 little cubes drawn down by the same
force as had acted on the one ; but the atmosphere would
now have sixty square inches of surface to act on. If each
of these little cubes were cut into 1000, the downward
attraction of the Earth on the whole million would be the
same as for the one-inch cube, but the air-brake would be
applied to no less than 600 square inches of surface, so
that their fall must be very slow indeed. The average dust-
motes of the air are much smaller than the^e, hence it is
not surprising that even the stillest air is never free from
dust.

162. Quantity of Dust in Air. — Mr. John Aitken, the
discoverer of the importance of dust in Nature, invented an
ingenious piece of apparatus by which he was able to
count the number of invisible dust-motes in any sample of

no The Realm of Nature CHAP, vn

air.1 His numerous experiments show that in one cubic
centimetre of the air of great cities there are hundreds of
thousands of motes ; in the air of small villages there are
thousands, and there are hundreds even in the open
country far from towns or factories. The purest air
met with was on one occasion on the summit of Ben Nevis
where one cubic centimetre contained only one dust-mote,
the mean of ten observations. These minute motes catch-
ing and scattering the sunlight are the agents by which the
whole atmosphere is so illuminated that not even the bright-
est of the stars is visible by day. If the air were free from
dust we should probably see the Sun shining from a perfectly
black star-filled sky, and one side of a house would be dazz-
lingly illuminated, the other in a shadow of absolute darkness.
The blue colour of the clear sky (§ 153) is largely due to
the scattering of sunlight by the dust-motes of the higher
layers. The red tints produced at sunrise and sunset
(§ 297) and the lingering twilight of high latitudes have a
similar origin. Twilight is produced when light from the
Sun, while still below the horizon, strikes on the upper
atmosphere, too obliquely for refraction (§ 150) to bend
the rays down to the surface ; then the illuminated dust-
motes of the upper air light up the sky for hours with a
soft shimmer.

REFERENCE

1 J. Aitken, "On the Number of Dust Particles in the Atmo-
sphere." Transactions Roy. Soc. Edin. xxxv. p. I (1888).
See also Nature, xxxvii. 428 (1888) and xli. 394 (1890).

BOOKS OF REFERENCE

R. Angus Smith. — Air and Rain. Longmans.
See also lists at end of Chapters VIII. and IX.

CHAPTER VIII

ATMOSPHERIC PHENOMENA

163. Solar Energy in the Atmosphere. — All the

changes in the atmosphere are directly or indirectly due to
the radiant energy received from the Sun (§§ 119-125), the
whole of which must pass through the air before reaching
the Earth's surface. Thermometers placed in specially
contrived screens are employed to measure the temperature
of air. On lofty mountains, where the atmosphere contains
little water-vapour and few dust-motes, the air is heated
so slightly by the Sun's rays passing through, that it
remains bitterly cold, although the Sun's direct heat-
blisters the traveller's face and hands. At an eleva-
tion of 11,000 feet, water has even been boiled by
exposing it in a blackened bottle to the sunshine. On
account of the low pressure of the air at great heights, air
from sea-level rising as a heated current expands greatly,
as explained by Boyle's law (§ 148). But the work of
expansion against the attraction of gravity consumes heat,
and the temperature of the expanded air, if unsaturated,
falls i° for every 180 feet of ascent. When cold
air from a great altitude is carried toward the Earth's
surface by a descending current, the pressure upon it is
continually increasing, and its volume is being reduced.
The work thus done on the air by gravity is changed into
heat, and the temperature of the air rises i° for each
1 80 feet it descends. The actual rate of change
of temperature in the air near the Earth's surface is not

ii2 The Realm of Nature CHAP.

so great as this, for the Sun has a certain heating-
effect. Several years of continuous observations on the
summit of Ben Nevis, and at sea-level at Fort William
have shown that the actual falling off of temperature with
height is i° for every 270 feet of ascent. Thus, whatever
the temperature may be at sea- level, there is a certain
height where the air has an average temperature of 32° F.,
no matter how much sun-heat passes through ; and snow
which falls above that height does not melt. This limit
is termed the snow-line. It is sea-level in the extreme arctic
regions, about 5000 feet at latitude 62° in Norway, about
9000 feet in latitude 46° in Switzerland, and ^bove 16,000
feet at the equator (see figure 63 and section in Plate VIII.)

164. Heating and Cooling of Air. — Near sea-level
the dense air is charged with water-vapour and dust which,
during the day, absorb solar radiant energy and pass on
the heat to the air. The ground also is rapidly heated, as
its specific heat is only about one quarter that of water, and
its temperature therefore rises four times as much as water
does for the same amount of heat. Once heated, the
ground is effectual in heating up the air in contact with
it. In the case of water, the Sun's rays penetrate to a
great depth, the temperature of the surface is very slightly
raised, and transfers little heat to the air over it. Hence
in sunshine a land surface heats air greatly, a sea surface
heats it only slightly. After sunset the hot land radiates
its heat through the atmosphere, and falls to a low tem-
perature, thereby chilling the air in contact with it, and
were it not for the dust-motes and condensed water catch-
ing and retaining most of this heat (§ 160) the radiation
of a single clear night would chill down the land far more
than the solar energy received during the day could heat it
up. The temperature of the dust-motes is also lowered by
radiation from the particles at night, and this is not fully
compensated by the heat radiated from the earth, so that
the air temperature falls greatly. From a water surface
heat is radiated slowly at night, and the air over water is
not greatly chilled.

165. Dew and Hoar Frost. — On a 'clear night, when

ISOTHERIN

After

TLeZ&utrargk Grofrojiical last-tut*

Land Surface from
600-6000 Ft- Elevation.

"OR JULY,
uchan.

20 40

ifliTeiiq>erature tdo*r 32° Fahr. coloured Bhie-i

I l I I

20 40 6O

100 120 140

Land Surface Above
6000 Ft Elevation.

viii Atmospheric Phenomena 113

the temperature of the land surface falls to the point at
which the water-vapour present becomes saturated, moisture
is deposited on all exposed objects in the form of drops of
dew or as small crystals of ice, called hoar-frost. The tem-
perature of saturation of water-vapour is hence called the
dew-point. The deposition of dew, or of hoar-frost, liberates
heat (§ 159), and so diminishes the subsequent fall of tem-
perature. In last century, Dr. Wells made a number of
experiments on the cause of dew. He showed that it was
only deposited when the sky was clear, and on objects
which had become greatly cooled by radiation, and he
proved that these in turn chilled the air below the dew-
point, and so condensed the water-vapour on their surfaces.
On a cloudy night radiation is checked, the water spherules
of the clouds retaining and radiating back the heat lost by
the Earth, so that dew is not formed. Mr. John Aitken
has recently shown that though the chilling by radiation
of exposed objects is certainly the cause of dew, only a
small part of the moisture is extracted from the air. Indeed,
on a still night when there is no wind the air resting over
a cabbage, for example, could never have contained the
quantity of water found on the leaves in the morning.
This is really condensed from the water- vapour always
being breathed out by plants. On a gravelled road also,
the under side of the gravel and not the upper, is often wet
with dew, the stones chilled by radiation condensing the
water-vapour which is always rising from the ground.1

1 66. Condensation and Dust. — It is remarkable that
water -vapour never condenses except .upon a solid
substance. In air quite free from dust, water- vapour
has been cooled far below the dew-point without condensa-
tion ; but the instant a puff of common dust-laden air is
admitted, each dust-mote becomes a nucleus, and a globule
of water is formed upon it. All condensation of water-
vapour in the air, whether it appears as rain, mist, fog,
cloud, or snow, takes place on a nucleus of dust.

167. Fog and Mist. — When dust-motes are very
numerous, and the temperature of air falls suddenly below
the dew-point, each mote can receive only a small coating

I

ii4 The Realm of Nature CHAP.

of water. The minute globules formed in this way fall
very slowly, and in the absence of wind may remain sus-
pended in the air for a long time. This accounts for the
black winter fogs of great cities where the specks of soot
are very numerous and are only thinly coated with water.
Over the open sea, when a broad stream of warm air carrying
saturated water-vapour crosses a cold current of water or
meets an iceberg, the sudden cooling of the vapour necessi-
tates an enormous condensation, and the dust which is
abundant even far from land, enables the condensation to
take place in the form of a bank of mist. The famous
" fogs " of Newfoundland are so produced. Fog differs
from mist in not wetting solid objects with which it
comes in contact. The light mists formed at night
over low - lying meadows or valleys are usually very
thin sheets, and as soon as the Sun appears the water
particles are heated up and evaporated again, so that the
mist clears quickly away. When a mass of warm air rises
in the atmosphere its temperature falls (§ 163). On
reaching a certain height the vapourjDecomes saturated, and
as it still rises, and the temperature continues to fall, the
vapour condenses upon the dust -motes forming a mist.
Clouds, which are mists at high altitudes, often hang over
a mountain or sail slowly through the -air for hours. In
such a case, though the form of the cloud does not change,
the water globules composing it are always falling as fast
as the friction of the air allows (§ 161) ; when they reach
the warmer air below they are evaporated again and vanish,
while new globules are as rapidly condensed on the dust
above.2

1 68. Classes of Clouds. — The differences between
clouds arise mainly from the height of the layer of mist
composing them. Three types of cloud are distinguished
by characteristic forms and by their usual elevation, and all
other kinds may be classed as a mixture of two or more of
them. The highest form of cloud is a mist of minute ice-
specks, usually forming at a height of about 5 J miles above
sea-level. It appears like tufts or curls of snow-white hair,
and is named Cirrus. In certain conditions this cloud

vin Atmospheric PJienomena 115

gives rise to halos, wide faintly-coloured rings which appear
to surround the Sun or Moon. The name Mare's tail is
sometimes applied to it. Little rounded tufts which often
cover the whole sky in summer and are familiarly called
mackerel scales belong to a class of cloud which floats
about 3 miles above the Earth's surface, and may be looked
upon as half-way between cirrus and the next type ; they
are termed cirro-cumulus. Cumulus is the cloud type
which comprises the great white billowy clouds common in
summer. They are usually flat on their under surface, and
rise above into rounded forms often of wonderful beauty.
The base of cumulus cloud is usually about ^ of a mile
above the Earth's surface, while the summits may rise as
high as 2 or 3 miles. These clouds are formed by the
condensation of vapour in ascending currents of air, and
each mass of cumulus has been likened to a grandly carved
capital topping the invisible column of rising heated air.
The lowest clouds are sheets of fog floating within half a
mile of the Earth's surface, and being so low they are
usually seen edgeways when at any distance, and so appear
as long layers parallel to the horizon. This arrangement
gave rise to their name of Stratus, A cloud, presenting a
dark gray or black colour and a ragged stormy appearance,
from which rain usually falls is called Nimbus, or simply
rain-cloud. It forms at the elevation of about a mile, and
is described as a mixture of cumulus and stratus. The
upper clouds act as floats, by the study of which much has
been learned as to the movements of the upper atmosphere.
The lower clouds are of great value as heat curtains, pre-
venting the Sun's heat from being excessive by day, and
almost entirely Checking the loss of heat by radiation from
the Earth at night.

169. Rain. — Sometimes the temperature of air remark-
ably free from dust falls below the dew-point, and a large
quantity of water-vapour must condense, while there are
very few solid motes to act as nuclei. Each mote conse-
quently gets a very heavy coating of water, and drops are
formed which are too large to be much checked by friction
of the air as they fall. Thus a shower of rain may fall

n6 The Realm of Nature CHAP.

from a cloudless sky. Rain more often originates in clouds.
The upper part of a very deep layer of cloud is less dust-
laden than the lower ; the motes accordingly form larger
water-drops, and these descend comparatively quickly,
overtaking and embodying smaller globules as they fall,
until they emerge from the cloud as large drops of water.
If the cloud floats very high above warm air, the vapour of
which is unsaturated, the raindrops will evaporate as they
fall and may vanish before reaching the Earth. But if the
cloud is low or the vapour in the air traversed by the rain-
drops is nearly or quite saturated, there is so little evapora-
tion that they reach the surface undiminished or even in-
creased in size. When much water-vapour is rapidly con-
densed near the surface or over air which is fully charged
with vapour, there must be a great fall of rain. Herice,
when a hot vapour-laden sea-wind blows against the side of
a mountain, the air rises, and growing cold in consequence
(§ 163), the dew-point is reached and passed, and deluges
of rain fall, while dark masses of clouds fill the sky. On
the other hand, when wind blows over a mountain range
and descends on the other side, it grows warmer as it sinks,
evaporates all the cloud it carries, and becomes a drying
wind upon the low ground. Rainfall is measured by the
rain gauge, and its amount is stated in the number of
inches of water which would accumulate on a level surface
if the rain of a year were to rest where it fell.

1 70. Snow is produced when water-vapour condenses at
a temperature below the freezing-point. The water forms
small clear spicules of ice which always cross at an angle
of 60°, so that snow-crystals usually have six rays uniformly
arranged about a centre ; but the variety of forms is very
great. A number of crystals getting hooked or felted
together form a snow-flake, and the fluttering showers of
flakes rest lightly on the ground, sometimes covering it to
the depth of several feet. One foot of snow is, roughly
speaking, equivalent to one inch of rain. The whiteness of
snow is produced by the reflection and refraction of light
again and again amongst the numerous small crystals.
The real colour is bluish or greenish like a block of ice.

viii Atmospheric Phenomena 117

A great quantity of air is entangled between the spicules of
snowflakes, and this makes a covering of snow act as a non-
conductor of heat — almost as perfectly as a covering of
feathers — preventing radiation from the Earth at night, and
so keeping the ground from freezing in cold weather.
Under heavy pressure snow is compacted into solid ice.

171. Hail. — In winter there are often showers of tightly
packed little snowballs about the size of small shot or
rarely as large as peas. This is called soft hail, and it
appears to be formed by the larger ice particles in a deep ice
cloud overtaking and adhering to the smaller ones. True
hail is a different thing, which only occurs in warm weather
usually as an accompaniment of thunderstorms (§ 173) or
tornadoes (§ 209). True hailstones are lumps of ice which
sometimes weigh several ounces, and occasionally as much
as 3 Ibs. A shower of such masses is very destructive,
breaking windows, cutting down standing crops, and
often killing animals or even people. The, hailstone
when cut across usually shows alternate layers of clear
ice and of compact snow. According to Ferrel such a
hailstone is produced by an ordinary soft hailstone formed
at a great height falling into a rain-cloud, where it gets
a coating of water, and then being carried by an ascending
current into a high cold region, where the water is frozen
into clear ice and a deposit of snow takes place outside.
The same hailstones may be caught in ascending and
descending currents several times in succession, thus getting
alternate coats of ice and snow. This theory accounts
for true hailstones only occurring in summer, for it is
only in hot weather that powerful ascending currents of
air are formed.

172. Electrification of the Atmosphere. — Every
change in the atmosphere, particularly evaporation, con-
densation and wind, gives rise to some disturbance in the
distribution of electricity. As electricity resides on the
surface of a body, it follows that when the minute particles
of a cloud are uniting to form rain-drops, their electrical
potential (§ 76) is rapidly rising, because the surface of a
large rain-drop is smaller than the total surfaces of the

nS The Realm of Nature CHAP.

small water globules which combine to form it. A heavy
shower of rain rapidly carries off the electricity, reducing the
potential of a cloud to that of the Earth. In certain states
of the atmosphere which are not yet thoroughly understood,
silent electric discharge takes place between pointed bodies,
such as flagstaff's or the masts of ships, and the air. This
is accompanied by a pale brush-shaped light, which goes
by the name of St. Elmo's fire. Air which is almost free
from water-vapour is a nearly perfect non-conductor (§ 77),
and in the dry climates of mountain observatories and high
latitudes in winter, electricity produced by friction is not
immediately conducted away to the Earth as it is in damp
air. In Canada one can often light a gas-jet by an electric
spark from the finger, produced by shuffling the feet on the
carpet ; and at Pike's Peak observatory in the United States
the friction of opening a drawer or shutting a door often
gave rise to electricity enough to give a severe shock.

173. Lightning and Thunder. — When the electric
potential of a cloud becomes much higher than that of the
Earth or another cloud, a disruptive discharge takes place
between them through the air (§ 78). The .electrical
energy is mainly converted into heat by the resistance of
the~aif, the particles of which become instantaneously white
hot ; but the passage of the electric current is so rapid that
only a brilliant flash is visible. The intensely heated air
expands suddenly, and then as suddenly contracts, setting
up a succession of air waves (§ 58) all along the line of the
flash. These reach the ear as a prolonged growl or roar,
or as a sharp rattling explosion, according to the distance
of the observer and to the direction of the flash. The
sound is prolonged by echoes from the Earth's surface and
hills, or from clouds. The electric discharge follows the
path of least resistance, and as vegetable juices offer less
resistance to it than air, trees are often traversed by the
current. The sap between the wood and the bark is so
heated by the discharge that steam is formed with explosive
violence, splitting off the bark, tearing away branches, and
ploughing deep furrows in the solid wood, as if the tree
had been struck by a solid spear hurled with gigantic

viii Atmospheric Phenomena 119

strength. An animal or a human body may form part of
the path of least resistance and so be " struck," but this will
never happen if there is a better conductor near. The im-
pressiveness of a thunderstorm is largely due to the majestic
roar of the thunder, the darkness of the sky, the lurid glare
of the clouds, and the ominous stillness of the air ; but apart
from these the presence of highly electrified bodies produces
an indescribable effect on the nerves of many people.
Lightning-conductors attached to buildings serve to equalise
the potential of the Earth and clouds, and thus tend to prevent
a disruptive discharge from taking place. Thunderstorms
occur most frequently in the tropics, and usually during the
day ; in polar regions they occur very rarely, and then only
at night.

174. The Aurora. — In the north polar regions, where
thunderstorms are practically unknown, beautiful luminous
effects are produced at night by the Aurora borealis or
Northern Lights (see small map on Plate XIV.) A
similar appearance in the south polar regions is called
Aurora australis. The Aurora forms an arch or ring of
coloured light over the magnetic pole (§ 98) at a
great height in the atmosphere, from 50 to 150 miles.
Coloured fringes and streamers shoot from this arch
in all directions, sometimes spreading over the whole sky,
and again shrinking back with a pulsing motion. The
Aurora appears to be caused by electrical discharges in
rare air, as it very closely resembles the glow seen when
a current traverses a "vacuum tube" containing a little
highly rarefied air. This theory was recently confirmed by
the Finnish physicist Prof. Lemstrom, who covered the top
of Mount Oratunturi in the north of Finland with a network
of wires and found a true Aurora produced when he sent a
current of electricity from these wires to the Earth.3

175. Wind. — When air is heated at the Earth's surface
it expands, and becoming less dense, rises and flows away
in the upper regions of the atmosphere. The pressure of
the air over the region where expansion has taken place
thus becomes less than that of the surrounding atmosphere,
and air is accordingly driven in from all sides until equili-

120 The Realm of Nature CHAP.

brium of pressure is restored. Moving air is known as
wind, and always blows from regions where the pressure
is higher to those where it is lower. The greater the differ-
ence of pressure, or rather the gradient^ that is difference
of pressure in a definite distance, the stronger is the wind.
In English-speaking countries gradient is measured by
the number of hundreths of an inch difference in the read-
ing of two barometers at a distance of 1 5 nautical miles
(17 miles). For example, if the barometer at one place
read 29. 14, and at another 34 miles away it read 29.00, the
difference is 14 hundredths of an inch in 34 miles, or 7 in
17, and the gradient is spoken of as 7. The same gradient
would result from a barometric difference of only 3.5
hundredths of an inch if the stations were only 8J miles
apart. The strength of wind is proportional to the gradient
as the following table shows : —

Gradient 0.5 3 7 15

Velocity of wind) 8o

in miles per hour J

Wind Light breeze. Fresh breeze. Gale. Hurricane.

Wind ceases to blow as soon as the difference of pres-
sure ceases to exist. While blowing, currents of air move
spirally from areas of high pressure to areas of low pressure,
as is explained by Ferrel's law, deviating toward the right
hand in the northern hemisphere and toward the left hand
in the southern (§ 89). The strength of wind is measured
by anemometers (§ 442), and is expressed either in terms
of its velocity or of the pressure it exerts. Wind is
named by the direction from which it blows, a wind blow-
ing from east to west being called an East wind.

176. Circulation of the Atmosphere. — In order to
understand the movements of the atmosphere as a whole,
it is convenient first to consider the Earth as smooth and
entirely surrounded by the hydrosphere. The air between
the tropics, and especially over the equator, is always being
heated by strong solar radiation, and it consequently expands
and rises, through the rest of the air, as oil would rise through
water. This region forms the furnace which furnishes motive
power for the whole system of circulation. The cooler and

vni A tmospheric Phenomena 121

denser air from the neighbouring temperate zones flows to-
ward the equator along the surface to take the place of the
ascending air, and is in turn heated and forced to rise. The
polar regions receive little heat from the Sun at any time, and
in the long dark winters radiate heat away into space. The
air over them consequently becomes chilled, grows denser,
.and descends toward the surface. Thus by equatorial
heating and polar cooling the air is constantly being raised
at the equator, carried in the upper regions north and
south to the poles, brought down there to the surface and
drawn back toward the equator. The upper current blows
spirally as a wind from the west-south-west in the northern
hemisphere, and from the west-north-west in the southern
hemisphere (as explained by Ferrel's Law), while the winds
from the poles would blow from north-east in the northern
hemisphere and from south-east in the southern.

177. Ferrel's Theory of Circulation. — The result of
this arrangement, according to Professor Ferrel, is that
in the upper layers of the atmosphere the pressure is
highest above the equator and lowest over the poles.
But the rush of air at a lower level from the poles toward
the equator tends to carry the mass of the atmosphere in
that direction, while the movement of the upper air toward
the poles tends but more feebly to carry the mass of the
atmosphere in the opposite direction. The two tendencies
balance each other between latitudes 20° and 30° north and
south, and the pressure of the lower strata of the atmosphere
is thus greatly increased in the neighbourhood of the tropics.
This is shown in Fig. 22 by the boundary line of the por-
tion of the atmosphere shown being drawn nearest the
surface at the equator and poles, farthest from it at the
tropics. The arrangement of pressure at the surface is thus
— Two belts of air at high pressure girdle the Earth a little
poleward of the northern and southern tropics, a ring of air
at lower pressure lies along the equator, and great regions
where the atmospheric pressure is low surround the north
pole and the south pole. The tropical zones of high
pressure give rise to surface winds toward the equator,
strengthening the north-east and south-east winds of the

122

The Realm of Nature

CHAP.

lower atmosphere. They also produce air currents
toward the poles in the opposite direction as south-
west and north-west winds, which gradually die away
about the polar circles, where the equator-seeking winds
meet, check, and rise above them. Hence in the tem-
perate zones the surface winds should be parallel to the

FIG. 22. — Theoretical Circulation of the Atmosphere, after Fen-el. The arrows
show the directions of the winds over the surface and of the vertical move-
ments of air.

pole-seeking upper winds, while between the two are the
equator-seeking middle winds. In the tropics and the polar
circles there are only the lower equator-seeking winds and
the upper polar-seeking winds, as shown in the diagram.

178. Zones of Winds and Calms. — This theoretical
circulation divides the Earth's surface into zones, which
roughly correspond to those of solar climate (§ 125). In
the tropical belts of high pressure, from which surface winds

vin Atmospheric Phenomena 123

blow poleward and equatorward, there is a calm. Since the
upper air, which contains little vapour, is always descending,
these regions are cloudless and the scene of enormous
evaporation. The Temperate zones of poleward surface
winds receive the hot vapour-laden tropical air and conduct
it to colder regions, where much of its vapour is condensed.
They are thus windy cool regions of moderate cloudiness
and rainfall. The polar regions of low pressure are
practically calm, and as most of the air descends from above
they are relatively dry. The tropical regions swept by the
equator-seeking winds are windy, hot, cloudless, but the
scene of great evaporation from the hot sea surface. The
narrow equatorial belt of low pressure into which the
equator-seeking winds blow from north and south is also
a region of calm. The air as it ascends here expands,
cools, and the enormous supply of vapour swept in from
the tropics condenses into the heaviest cloud, and falls
as deluges of never-ceasing rain. The heat liberated
by the condensation of so much vapour strengthens the
equatorial up-draught. The equatorial belt of low pressure
always lies nearly under the vertical Sun, consequently in
the northern summer (§§ 122, 123) it swings to the north,
and in the southern summer it swings to the south, dis-
placing the belts of tropical high pressure northward and
southward alternately. For reasons which cannot be
explained here, this displacement is comparatively slight,
extending over only five or six degrees of latitude. In
the North Atlantic, for example, the equatorial low
pressure belt never moves farther south than 5° N. All
parts of the Earth's surface that the equatorial rain-belt
traverses in its annual movement, experience a rainy season
as it lies over them, and a dry season all the rest of the
year, when swept by the equator-seeking winds. Near the
equator, where the narrow rain-belt crosses a tract of the
Earth both in its northward and in its southward swing,
there are two wet and two dry seasons in the year. The
theoretical circulation of the air and its resulting climates
are affected by two causes, unequal heating of the air by
land and sea surfaces (§ 164), and the deflection of the

124 The Realm of Nature CHAP.

prevailing winds by plateau edges and mountain ranges.
Regular zones of surface winds and climates consequently
are found only in great expanses of ocean, and do not
appear in narrow seas or on land (see Plates V. VI. VII.)

179. Trade Winds and Doldrums. — When the Spanish
and Portuguese explorers of the i6th century found that
north-easterly winds blew steadily all the year round on
the Atlantic between 30° and 5° N. and enabled them
to make quick voyages to the West Indies, they gave
the name of Trade Winds to the favouring breezes. The
name has since been extended to include all the permanent
winds which blow from the tropical toward the equatorial
calms. In the winter half of the year (November to April)
the north-east trades of the Atlantic are felt as far north as
25° N. and reach southward to 5° N. ; and in the Pacific
they sweep over the range of sea between 28° N. and 8° N.,
and the tropical calms reach as far north as 40°. The
south-east trade winds at the same season are experienced
in the Atlantic between a line drawn from the Cape of
Good Hope to Rio de Janeiro, and the equator. In the
eastern Pacific they reach farther north, crossing the equator
to at least 5° N. The equatorial belt of calms and rains
lies entirely to the north of the equator ; its width varies
from 1 20 to 200 miles in the Atlantic, and is about 300
miles in the Pacific. This calm belt, called by sailors the
Doldrums, was greatly dreaded in the days of sailing
ships, on account of the absence of wind, which often kept
a vessel rolling helplessly for weeks, while the close damp
air made the men dispirited and ill. Thunderstorms of
terrific violence are very common in it. It was consequently
of the greatest importance for a captain to know where the
narrowest part of the belt could be found at each season, in
order that he might pass quickly from the clea^r bright skies
and fresh invigorating winds of the north-east trades to the
equally pleasant and favourable region of the south-east
trades. During the summer half-year (May to October)
the rain-belt of the Doldrums with its calms moves farther
north, and widens to from 300 to 500 miles. The north-
east trades then begin in about 30° N. and die off about

vin Atmospheric Phenomena 125

12° N., while the south-east trades do not extend so far
south, but cross the equator, blowing as far as 5° or even 8°
N. The calm equatorial zone of rains always lies north of
the equator, on account of the heating influence of the greater
mass of land in the northern hemisphere. (Plate VII.)

1 80. The Roaring Forties is a name given by sailors
to the belt of ocean between 40° and 50° S. in which the
" Brave West Winds " blow all the year round, as regularly
as the trades and more strongly. This belt is more nearly
covered with a uniform stretch of ocean than any other
part of the Earth, and exhibits the theoretical circulation of
the atmosphere in great perfection. The prevailing wind is
produced by the high pressure of the south tropical calm
'belt and the remarkably low pressure which surrounds the
south pole. The strength and constancy of the brave west
winds enable sailing vessels to compete with steamers in
trading with New Zealand going by the Cape of Good Hope
and returning by Cape Horn.

1 8 1. The Northern Anti-trades. — The south-west
winds of the northern hemisphere, which blow from the
northward edge of the north tropical zone of high pressure
to the north polar region of low pressure, are sometimes
called the Anti-trades ; but they are much less constant and
more variable in strength than the trade winds or the winds
of the Roaring Forties. The trade winds blowing into the
Gulf of Mexico in the summer months from the east or
south-east are deflected by the edge of the great table-
lands of Mexico into south-westerly winds, which blow up the
Mississippi valley and sweep across the Atlantic, reinforcing
the somewhat uncertain anti-trades.

182. Daily Temperature Changes. — The circulation of
the atmosphere which has just been described was deduced
by mathematical reasoning from a few simple data, and then
proved by observation to be correct so far as disturbing
causes allow. But the changes in the atmosphere which
take place from hour to hour throughout the day were first
observed in thousands of cases, and their cause has been
subsequently ascertained by inductive reasoning (§ 17).
Solar radiation goes on from sunrise to sunset, but the

126

The Realm of Nature

CHAP.

temperature of the air reaches its maximum about 2 P.M.
local time, or about 2 hours after the Sun has passed the
meridian. Cooling then sets in, and the temperature reaches
a minimum about 5 A.M., or shortly before sunrise. These
hours apply to the tropics and vary slightly in different parts
of the world, but the air is always coldest in the early
morning and always warmest in the early afternoon. Sir
David Brewster discovered, by comparing a long series of

M 2 4 6 8 10 N S

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FIG. 23. — Daily Range of Atmospheric Temperature in temperate
and tropical climates (after A. Buchan).

hourly observations, that the average temperature at any
pair of hours of the same name (e.g. 9 A.M. and 9 P.M.) was
almost exactly the average temperature for the whole day.
Fig. 23 shows the range of temperature above and below
the average for the day, the hours being marked along the
top and the temperature in degrees above and below the
average on the side (see § 444). The solid curve refers to a
station in the tropics, the lighter curve to a temperate region.
183. Daily Pressure Changes. — The pressure of the
atmosphere is least about 4 A.M. and 4 P.M. and greatest
about 10 A.M. and 10 P.M. In Fig. 24 the diurnal range
of the barometer above and below its mean value is given,
the range in fractions of an inch being marked on the side,
the hours from noon to noon along the top. The solid
curve shows the typical range in the tropics, the lighter
curve that in a temperate region. This regular increase
and decrease of pressure twice daily, was for a long time
supposed to be a tidal effect caused by the Moon's differen-
tial attraction, but Dr. A. Buchan in discussing the
barometric observations made on the Challenger Expedi-

VIII

A tmospheric Phenomena

127

tion proved that it really depends on the changes of
atmospheric temperature, and so is a result of the radiant
energy of the Sun. The Morning Minimum of pressure
about 4 A.M. results from the cooled dust-motes condensing
upon themselves most of the water-vapour contained in the
air, the vapour pressure is greatly reduced, and the total
observed reduction of atmospheric pressure is thus accounted
for. When the Sun appears, the dust-motes are warmed up,
the vapour returns to the atmosphere, and the temperature
of the air rapidly increasing, produces the Forenoon Maxi-
mum of pressure about 10 A.M. When the temperature of

M246810N24

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^

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s

5

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FIG. 24. — Daily Range of Atmospheric Pressure in temperate
and tropical climates (after A. Buchan).

a gas is raised it must either expand or press more strongly
on the vessel containing it ; and in the forenoon the heated
air is prevented from expanding for a time by the resistance
of the cooler layers of atmosphere above, against which it
presses with increasing force, and the barometer rises.
After 10 o'clock the continued heating enables the air to
overcome the resistance, and ascending currents set in, the
air rises and, meeting the west winds of the upper atmo-
sphere, is carried away to the eastward. The density of the
whole column of atmosphere is diminished by the removal
of the ascending air, and the Afternoon Minimum of
pressure occurs about 4 P.M. As the surface air cools
in the evening it grows denser and sinks, the upper air
welling over from the heated regions to the west, where it
is still only early afternoon, flows in above, cools and sinks,
so raising the pressure to produce the Evening Maximum
about 10 P.M. Thus the morning minimum and maximum

128 The Realm of Nature CHAP.

are caused by the action of condensing and evaporating
water in the atmosphere ; the afternoon minimum is caused
by a bodily removal of air upward and to the east, the
evening maximum results from the sinking down and piling
up of air from the west.

184. Land and Sea Breezes. — The different heating
and cooling of land and sea (§ 164) produces a regular
change in the daily winds of tropical coasts and islands, and
in very calm clear weather similar effects may be observed
in all latitudes. An island or strip of coast when heated
by the Sun gives rise to ascending currents of air (Fig. 25).
About 10 A.M. these ascending currents, having carried the

FIG. 25.— Sea-Breeze during sunshine. FIG. 26. — Land-Breeze, at night.

air into the upper regions, produce a fall of pressure over the
land compared with that over the cooler sea, and a sea-
breeze sets in, at first as a very gentle air, but gradually
increasing in force until about 3 P.M., when the land surface
is most highly heated. After that hour the land cools down
more quickly than the sea, and as the atmospheric pressure
becomes equalised the sea-breeze dies away. The air over
the land continues to cool down and to sink ; more air con-
sequently flows in above, and the pressure over the land
thus becomes greater than that over the sea. A surface
land-breeze (Fig. 26) sets in about 8 P.M., often with sudden
squalls, which are dangerous to boats. It gradually
increases in strength as the land grows cooler until it
reaches a maximum about 3 A.M. In the trade-wind

ISOBARS AND V\

After A

160 180

JJ8O 180 16O 14O 120

3 FOR JULY.

an.

40 60

100 120

t indicates Pressure teltnv 30 Inches

viii Atmospheric Phenomena 129

regions the land and sea-breezes are often not strong enough
to reverse the direction of the prevailing winds, and merely
alter the strength. On the south-east coasts of the Fiji
Islands, for example, the prevailing south-east trade wind
is intensified during the day and much reduced at night,
while on the north-west coasts the wind is reduced through
the day and strengthened at night. Land and sea-breezes are
always light on a low flat island or coast ; but when a range
of mountains rises near the sea very strong winds are pro-
duced, the mountain slope acting like a flue, aiding the
ascent of the hot air by day and the descent of cold air by
night. On account of the lofty backbone of the Blue
Mountains the sea-breeze in Jamaica is the strongest known.
185. Monsoons. — Over the centre of continents far
removed from the ocean the range of air -temperature is
greatest, the great dryness (§ 164) favouring radiation and
producing very high temperatures in summer and very low
temperatures in winter. Over the sea the range of tempera-
ture is le'ast. The continents by heating the air in summer
set up ascending currents which last for months, so that the
pressure of the air is greatly lowered, and surface winds blow
in toward the continent from the surrounding seas. In
winter the air being cooled by the continents produces de-
scending currents ; the pressure becomes much higher than
that over the less chilled seas, and consequently surface winds
blow outward from the continents during the winter months.
These winds changing with the seasons are called Monsoons.
They are produced exactly like land and sea-breezes, only
with a period of a year instead of a day. Just as in the
former case, monsoon winds may be too feeble to reverse
the direction of the prevailing winds, and may succeed only
in modifying their force (see Plate VII.) The monsoon effect
of most continents is comparatively insignificant, and is con-
fined to a small part of the coast. In the southern continents
these winds are slightly developed, because in the widest part
of South America and Central Africa the annual range of
temperature is very small, and in the narrower part farther
south the influence of the vast expanses of the neighbouring
oceans predominates all the year round. In Australia the

K

130 The Realm of Nature CHAP, vni

monsoons are well-marked but not very strong, although the
range of temperature is considerable ; but with an equally
great range the Sahara region of North Africa has a very
much slighter monsoon -raising power. The flatness of
these expanses of land and their low elevation partly account
for this ; the disturbing influence on atmospheric pressure
of the expanses of sea to the north is also important.
On the west coast of North America there are distinct
monsoons, but it is in Asia with its steep mountain slopes
rising from the sea that the monsoon blows with greatest
power, and in India the name was first applied.

REFERENCES

1 J. Aitken, "On Dew," Transactions R.S.E. xxxiii. p. 9
(1885); also Nature, xxxiii. p. 256 (1885).

2 J. Aitken, "Dust, Fogs, and Clouds," Transactions R.S.E.
(i 88 1) ; also Nature, xxiii. p. 195 (1881).

3 See note on Lemstrom's Aurora Experiments in Nature,
xxxv. p. 433 (1887).

BOOKS OF REFERENCE

W. Ferrel, Popular Treatise on the Winds. Macmillan & Co.
(An admirable discussion, but not easy reading.)

R. Abercromby, Weather. International Scientific Series.

CHAPTER IX

CLIMATES OF THE WORLD

1 86. Configuration and Climate. — In passing from the
theoretical system of atmospheric circulation sketched in
last chapter to the actual conditions of the atmosphere in
different parts of the world, the disturbing influence of the
land must be taken into account. The student should
therefore read §214 and Chapter XV. as far as it refers to
the configuration of the continents, and study Plate XL, as
well as the maps illustrating atmospheric conditions. Sur-
face winds are altered in their direction in a very marked
way by mountain ranges and the edges of plateaux. At
the same time, sloping land differs from level ground by
setting up a local vertical circulation, acting exactly as a
chimney does in increasing a draught. ' In hot climates
mountaineers find a strong wind sweeping up the slope by
day helping their ascent, and on the summit the ascending
air-current from opposite sides rises straight* up, and is often
strong enough to carry off hats and notebooks. At night
the effect is reversed, and strong winds blow down the
slopes. The same effects are produced in a more intense
degree in narrow steep mountain valleys, the furious day
and night winds of which make travelling difficult and
dangerous in some of the Himalayan passes. Experienced
hunters on the Rocky Mountains build their fires just below
their tent, knowing that the night-wind will carry the smoke
down the valley. In still winter weather the air, chilled as
a thin layer on mountain sides, grows dense as its tempera-

132

The Realm of Nature

CHAP.

ture falls, and flows gently down into the valleys, filling them
to a certain level with intensely cold air. The peasants in
many valleys of the Alps perch their wooden cottages on
knolls or rocks, not so much for the picturesqueness of the
site, but in order to stand above the surface of the flood of
icy air which streams through the valley in winter. Rain-
fall is still more intimately connected with configuration.
Meteorologists, in speaking of the climate of a place, mean
the average state of the atmosphere with regard to warmth,
wind, rain, and all other variable conditions.

187. Atmospheric Temperature in different latitudes.
— The excess of land in the northern hemisphere, compared

LATN 70 60 50 40 30 20 10 0 10 20 30 40 50 60s

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=^-jjr

F.

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lobe

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FIG. 27.— Distribution of Atmospheric Tempera
ture in latitude, for January, July, and the year

.0 /

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i

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with the southern, alters the distribution of the solar energy
shed equally on both, and prevents the simple astronomical
climate zones (§ 125) from corresponding on the two sides
of the equator. Fig. 27 shows by means of curves the
mean temperature of the year, as calculated by Professor
Ferrel for each 10° of latitude from 80° N. to 60° S.
Latitude is marked along the top and temperature up
the side of the diagram, the three curves of which cor-

ix Climates of the World 133

respond to the average temperatures of January, July,
and of the whole year. The curve of average tempera-
ture for the year shows that the warmth of the air at
80° N. is only 5° F., that in 4° N. there is the maximum
temperature of 81° F., and that in 60° S. it is 35° F., the
northern hemisphere being as a whole a little colder than
the southern. The mean surface air temperature of the
whole Earth is about 59-5°. The curves for January and
July show a great annual range of temperature in the
northern hemisphere, increasing toward the north where
the land preponderates, and a slight annual range in the
southern hemisphere, decreasing toward the south where
the sea influence prevails. The student should study this
diagram, comparing the temperature at each season at
various latitudes in the two hemispheres. To do this follow
the vertical line of latitude until it cuts the curve ; the point
on the thermometer scale in a horizontal line with this inter-
section is the temperature at that particular latitude.

1 88. Isotherms. — If the temperature of every place in
the world at some one instant were marked in figures on a
large map the result would be very confusing to look at.
But if all the figures except those showing a difference of
i o degrees were blotted out the map would be much simpler.
Near the equator the number 80 would occur frequently,
farther north and south there would be rows of 70, still
farther strings of 50, and so on. A line might be drawn
through all figures 80, and the figures themselves might
then be blotted out, except one left to mark the line, and
the same might be done for 70, 60, and the rest, greatly
simplifying the map. Such lines are termed isotherms, or
lines of equal warmth, as they pass through all the places
where the air temperature is the same. In interpreting
the maps (Plates III. and IV.) it is usually assumed that
the temperature between two isotherms is proportional to
the distance. For example, in the January map (Plate III.)
the line of 70° temperature in Central America is one inch
from the line of 80° in South America, so that between
them one-tenth of an inch on the map corresponds to a
change of i° of temperature. The lines of 40° and 50° in

134 The Realm of Nature CHAP.

North America approach at one place on the same map to
within one-tenth of an inch of each other, so that between
them one-hundredth of an inch corresponds to a change of
i°. The space between the isotherms is coloured to bring
out the difference of temperature, the hottest regions being
shown in deepest red, the coldest in deepest blue. Isotherms
are constructed to refer to sea-level, so in order to find from
the maps the actual temperature at any place a deduction
of i° for every 270 (or for convenience say 300) feet must
be made (§ 163). For every place on the contour-line of
600 feet of elevation 2° must be deducted, and for every
place on the 6000 foot contour-lines 20° must be deducted
from the isothermal temperatures. Those two contour lines
are marked on the maps as a guide to the interpretation of
the results.

The maps in this volume are reduced from the most
recent set of isotherms, compiled in connection with the
scientific reports of the Voyage of the Challenger^ by Dr.
A. Buchan, and they give the average temperatures for the
fifteen years from 1870 to 1884.

189. Air Temperature in January. — January is the
midsummer of the southern hemisphere. The map (Plate
III.) shows that the region with a temperature over 70° lies
south of the Tropic of Cancer on land, and the only places
warmer than 90° are under the Tropic of Capricorn in
Africa and Australia, the land being more heated than the
water by the nearly vertical Sun. The eastern sides of the
southern continents are warmer than the western ; thus on
the Tropic of Capricorn, the east coasts of Africa and South
America have a temperature of 80°, and the west coasts less
than 70°. This is explained by the prevailing winds and
ocean currents (§ 243). The isotherm of 32° in the southern
hemisphere occurs about 64° S., and its direction is nearly
east and west, being uninfluenced by any land. Farther
north, the direction of the isotherms becomes more ir-
regular on account of the increasing interference of land
in altering the temperature of the air. In the northern
hemisphere, where January is midwinter, the sea as a rule
is warmer than the land in the same latitude, and the

ix Climates of the World 135

coldest regions are the centres of the great continents.
The coldest place where observations have ever been
made, is the Siberian village of Verkhoyansk just within
the Arctic Circle (see Plate VII.) On account of the
Arctic Sea being frozen across in winter, this village lies
close to the centre of the northern continental mass. The
mean January temperature at this station is 61° below zero,
Fahrenheit ; and the absolutely lowest temperature ever
experienced by human beings occurred there in January
1886, which was —89° F. The powerful influence of the
warm surface-water of the Gulf Stream (§ 244) on the air
is shown by the temperature of the Lofoten Islands, in the
same latitude as Verkjflpyansk, being above 32°, the differ-
ence between the two being more than 93°. The coldest
point in the American continent lies a little north of the
magnetic pole (§ 98), and has a temperature of — 40°. In
order to appreciate the effect of land and sea in modifying
climate the student should carefully follow the isotherms of
30° and 40°, noting carefully the latitude at which these
temperatures prevail near the coast and in the heart of
continents. To make this exercise still more instructive,
the lines might be traced on the contoured map (Plate XL),
and the actual surface temperatures calculated.

190. Air Temperature in July. — The lapse of six
months brings round the northern summer and southern
winter. The Sun now vertical near the Tropic of Cancer
beats down upon a far greater breadth of land surface than
in January, and so the area with a temperature exceeding
90° in North America, North Africa, and Asia extends far
to northward of the tropic. The sea now exercises a cool-
ing influence on the air of the middle latitudes in the
northern hemisphere. The isotherm of 70° F., for example,
runs far to the north over the continents, reaching 55° N. in
North America, and 58° N. in Eastern Asia, but it scarcely
gets north of 40° N. in the Atlantic, and is carried south
to 25° N. by the Pacific. In higher north latitudes the
slight north-eastward trend of the isotherms shows that
some warming effect is still due to south-west winds and
currents. In July the Lofoten Islands, having warmed up

i36

The Realm of Nature

CHAP.

only by 20° on account of the sluggish heat transactions of
water, are at the same temperature, 55° F., as Verkhoyansk,
where, however, the air has been heated no less than 116°
since January, this being the greatest annual range known.
The purely continental character of Verkhoyansk is
modified by the fact that in summer it is not far from
the shore of the cold Arctic Sea, whence cool monsoon
winds blow. In the southern hemisphere the tempera-
ture of the land has fallen by radiation a little below
that of the sea. The prevailing winds, however, are so
powerful, and the oceanic influence predominates so greatly,
that temperatures below 70° are found north of 20° S. only
on the west sides of the continents which are cooled by the
increasing upwelling of ocean water due to the trade winds.
The extreme tip of South America is the only southern
continental land which has a winter temperature below 40°
in July, and the isotherm of 32° encircles the globe in 55°
S., not touching any land at all, and showing but a slight
range from its winter position.

191. Land and Sea Climates. — The comparatively
cool summers and mild winters of the extremities of the

southern continents

60

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compared with those
of the same latitudes
in the northern
hemisphere is a
direct result of the
arrangement of land
and sea on the
globe. A land
is every-
where extreme, a
sea climate is al-
ways mild ; but an

of Fig. 27 will show that the average
for the year is nearly the same in both

FIG. 28. — Curve of monthly mean temperature for a r

typical land -climate (dark) and sea -climate Climate
(light). The dark horizontal line marked 0 re-
presents mean annual temperature ; the figures
show number of degrees above and below the
mean.

examination
temperature

hemispheres. Fig. 28 shows the annual range of tem-
perature in degrees above and below the mean for the
year in typical continental and oceanic climates. The

PERMANENT WINDS,

rade/|TTijid.s

Rfigiim of &aie "Vftads during ITardieni STinniiiBE |"

Limit, of Trade, Winds ilurStq FarOum. Jflnter '

Eegiom cf Calms chniag Isacthfim STimiaflr
Winter

160 180 160 140 120 1OO

The DARK BLUE TINT indicates the PRESENT Distribution of Glaciei

LMS, AND STORMS.

the. yardum, Slimmer tktMonsoons art, •from^te, SW.& SJS
d from. Ae-JVTf. &.TW during &eMrAer7,,Winter.

The LIGHT BLUE TINT indicates the PAST Distribution of Glaciers *•

ix Climates of the World 137

solid curve shows the range at Verkhoyansk, the finer line
that at the Lofoten Islands. In continental or land climates
the range of temperature is great and the rainfall very
small, in oceanic or sea climates the range of temperature
is small and the rainfall great. Prevailing winds carry an
oceanic climate for a considerable distance inland on the
west coasts of northern continents, and they carry a con-
tinental climate a considerable distance seaward on the
west shores of northern oceans. The student should verify
these statements by making a detailed comparison of the
two isothermal maps, and the map of rainfall (Plate VIII.)
192. Isobars. — The invisible differences of atmospheric
pressure may be laid down on a map in the same way as
the invisible differences of temperature. Lines running
through places over which the atmospheric pressure, as
measured by the barometer and reduced to sea-level, is the
same, are called isobaric lines, or shortly isobars. Those
shown on the maps (Plates V. and VI.) express the pres-
sure in inches of mercury at intervals of every tenth of an
inch, and the spaces between them are coloured, so that
the regions of highest pressure are deep red, and those of
lowest pressure deep blue. When adjacent isobars are
drawn far apart on the map the barometric gradient (§ 175)
between them is slight, and the wind set up is consequently
gentle ; but when the isobars are crowded closely together,
a steep gradient is indicated, giving rise to furious wind.
A gradient of 0-5 corresponds to a difference of pressure
of 0-20 inches in one degree of latitude ; a gradient of
15, giving rise to a hurricane, corresponds to a difference
of pressure equal to 0-60 inches in one degree of latitude.
In the maps the arrows are represented as flying with the
wind. The shortest path from a region of high to one of
low pressure is at right angles to the isobars, but in con-
sequence of the rotation of the Earth the actual path of the
wind is that stated in Ferrel's Law (§ 89). The deviation
is proportional to the latitude, so that in the far north and
south, wind blows nearly parallel to the isobars. Dr. Buys
Ballot, the late eminent Dutch meteorologist, independently
discovered the Law of the Winds, which Dr. Buchan has

138 The Realm of Nature CHAP.

put in this form : — " Stand with the Low pressure on
your Left hand and the high pressure on your right ; then
in the Boreal (northern) hemisphere the wind is Blowing
on your Back ; but in the southern hemisphere in your
face." The student should impress these statements by
studying Plates V. and VI.

193- Winds of the Southern Hemisphere in January.
— The theoretical arrangement of atmospheric pressure
and winds (§§ 178-181) is changing from hour to hour in
response to the changes of day and night (§ 1 84) and summer
and winter (§185). The two maps (Plates V. and VI. — which
should be referred to continually in reading what follows)
are reduced from Buchan's Challenger maps, and give
the average conditions of the atmosphere in January and
July for the fifteen years 1870 to 1884. In the map for
January the equatorial zone of low pressure, as limited by
the isobars of 29-90 inches, is narrow over the ocean but
widens greatly over the three southern continents, where the
heat of summer causes the air to ascend, flow away, and
reduce the pressure over the land. Another consequence
of high te'mperature over the continents is, that the south
tropical belt of high pressure is broken into three isolated
portions lying altogether south of the tropic in the three
oceans. The southern area of low pressure and of
steep gradient, as limited by the isobar of 29-90 inches,
occupies the whole surface south of 40° S., the isobars
running nearly straight east and west. From the three
south oceanic regions of high pressure, surface winds
blow outward, forming the south-east trades on the
northern margin toward the equatorial low pressure, and
the brave west winds on the southern toward the great
south polar low pressure. The portions of the equatorial
low-pressure zone extended to the south by the continents
produce monsoons, or an indraught of surface air toward
the land. On the north-west coast of Australia the Australian
low-pressure area draws the trade wind round to form a
north-west monsoon. On the west coast of Africa the
south-east trade is drawn in to form a light south-west
monsoon, and in the Gulf of Guinea it is drawn in strongly

ix Climates of the World 139

from the west. The same action is seen on the west coast
of South America, but there the uniform face of the Andes
deflects the wind back again. On the eastern shores of
the southern continents the monsoon effects strengthen the
prevailing trade winds.

194. Winds of the Northern Hemisphere in January.
—The north tropical zone of high pressure forms a con-
tinuous belt round the world, narrow over the oceans, but
extending right up to the polar regions over the two
northern continents. Over these continents, air is continu-
ally descending from the upper atmosphere, where its place
is being taken by the air driven up from over the northern
oceans and southern continents. The Arctic low-pressure
area is cut up into two comparatively small depressions,
one with its centre between Iceland and Greenland, the
other in the North Pacific Ocean. In consequence of this
arrangement the north-east trade winds of the Atlantic blow
into the Carribean Sea and against the coast of South
America beyond the equator, and under the influence of the
South American low-pressure area, unite with the south-
east trades, blowing up the valley of the Amazon, and
obliterate the belt of calms along the coast. The North
Atlantic low-pressure area, while maintaining the south-west
winds of western Europe, draws in cold north-east winds
on the east coast of North America. The high pressure
over North America gives rise to monsoon winds which
attain considerable force as north-westers along the west
coast of Central America, and also sets up prevailing
northerly winds down the Mississippi valley.

195. Winter Monsoon of Indian Ocean. — About the
month of October, when the pressure over the great Asiatic
continent becomes higher than that over the ocean, light
northerly winds set in in the Bay of Bengal and the
Arabian Sea, gradually changing to north-east winds at
sea, where they represent the trade winds, but rarely
attaining great force, and often broken by calms. Along
the base of the Himalayas in the plain of the Ganges the
wind is north-westerly. This state of matters lasts for
several months, coming to a climax in January. Over

140 The Realm of Nature CHAP.

most of India it is a dry season, as the air of the North-
East or Winter Monsoon has descended from the upper
region of the atmosphere, and contains little water-vapour.
On the east coast of the Indian peninsula and of Ceylon,
the north-east wind having traversed the Bay of Bengal,
sweeps along a considerable amount of vapour, which is
precipitated on the Eastern Ghats and the eastern side of
the Ceylon hills, winter being their rainy season.

196. Winds of the Southern Hemisphere in July.—
Notwithstanding the change from summer to winter in the
southern hemisphere, the southern region of low pressure is
practically unaltered in position, but the gradient southward
is reduced, and the winds of the Roaring Forties blow with
slightly diminished strength. The south tropical belt of high
pressure has reunited in consequence of the cooling down of
the southern continents, and it now stretches far north of the
tropic. In consequence of the small range of temperature
and slight winter cooling of the southern continents, the
highest pressure in the southern hemisphere is over the
oceans even in winter, and this fact accounts for the per-
manence and steadiness of the brave west winds. The
south-east trades blow across the equator far to the north
in all the oceans. In the Indian Ocean the calm belt is
completely obliterated on account of the great suction over
Asia which draws supplies from the southern hemisphere
and turns the south-east trade winds to feed the south-west
monsoon. At the same time pressure is high over the
continent of Australia, from which monsoon winds blow
outward.

197- Winds of the Northern Hemisphere in July.—
The equatorial belt of low pressure extends over the whole
land surface of the northern hemisphere and unites with
the North Polar region of low pressure, centres of lowest
pressure lying in the south-west of Asia and in the west of
North America. The north tropical belt of high pressure
is broken into two great isolated high-pressure areas, which
occupy the North Atlantic and the North -West Pacific,
keeping up the north-east trade winds in those oceans ;
and giving rise to south-westerly winds over eastern North-

ix Climates of the World 141

America and North -Western Europe. The monsoon
influence of North America is very slight, on account of the
position of these two high-pressure areas, and that of North
Africa is also remarkably feeble (§ 185). The winds of the
Indian Ocean and Western Pacific are completely dominated
by the vast furnace flue of Western Asia, which attains its
maximum effect in July, and destroys the theoretical atmo-
spheric circulation of the northern hemisphere.

198. Summer Monsoon of the Indian Ocean. — Round
the coast of Asia the north-east wind falls off in February,
and gradually shifts to the south as the winter high pressure
over the continent is reduced, and gives place to the summer
low pressure. March and April are characterised by
variable winds and frequent storms. By May the north-
east wind has died away, and in its place south-west winds,
usually spoken of in India as The Monsoon, blow strongly
across the Arabian Sea and Bay of Bengal, and wheel round
along the foot of the Himalayas, blowing up the Ganges
valley as south-east winds. This state of matters lasts until
August or later. As the wind blows for a long distance over
the heated surface of the ocean it reaches land laden with
vapour, and, rising up the steep and almost unbroken slopes
of the Western Ghats, condenses in tremendous showers.
The first deluges of rain are known as the bursting of the
monsoon. A heavier rainfall reaches the western edge of
the Indo-Chinese peninsula, and the heaviest of all is
found in the converging valley of Assam, at one place over
500 inches a year. After August the south-west monsoon
diminishes in force and gradually dies away as the pressure
over the land increases. The monsoon "owes much of its
strength to the energy set free by the condensation of the
vapour it carries. On the coast of China the summer
monsoon blows from the south-east, and the winter monsoon
from the north-west.

199. Yearly Swing of the Atmosphere. — The disturb-
ing effect of land and sea on the normal arrangement and
movements of the atmosphere may be put briefly thus. In
winter the chilled land draws down the blanket of air which
the less-cooled sea is tossing off upward. In summer the

142 The Realm of Nature CHAP.

heated land throws off as much of its air- covering as
possible, piling it upon the colder sea which eagerly
draws it down. While the land is throwing off the air
above, which descends upon the sea, the sea commences to
return it to the land along the surface more slowly than it
receives it at first, more rapidly afterwards. When the land
has drawn down on itself from above a greater supply of
air at the opposite season the sea gradually draws it off
along the surface. There is thus a constant effort to restore
the equilibrium of atmospheric covering between land and
sea, disturbed by the rapid radiation of the land. The
prevailing winds of the year, disregarding minor seasonal
changes, are shown in Plate VII.

200. Rainfall and Evaporation. — A continual circula-
tion of water takes place between the hydrosphere and
atmosphere. Sea winds blow water -vapour against the
land and ascending currents carry it into the upper atmo-
sphere, where it condenses and returns either directly as
rain or through springs and rivers to the sea. The amount of
evaporation at sea, and of rain falling on land depend mainly
on temperature and winds. Dr. John Murray has calculated
that nearly 130 million million tons of water, or about -^ of
the whole mass of the atmosphere, are transferred from the
sea surface to the land, and find their way back again in
rivers every year. More than half of the rain falls between
the tropics, and probably not more than — of it reaches
ground as snow beyond the polar circles.1 The average
rainfall of the globe is about thirty -three inches. A
calculation has been made that one quarter of the land
surface has a rainfall less than one foot in a year, one
quarter has a rainfall between one and two feet, one quarter,
of which the British Islands form part, has a rainfall of
between two and four feet, and over the remaining quarter
the rainfall exceeds four feet in a year. In all regions not
reached by sea winds the rainfall is very slight, and
evaporation preponderates, a nearly rainless area containing
dwindling salt lakes occupying part of the interior of each
continent (§ 356).

201. Distribution of Rainfall. — Plate VIII., the data

ix Climates of the World 143

of which were mainly compiled by the American meteor-
ologist Professor Loomis, shows the rainfall on the land by
deepening blue tints according to the number of inches
which fall at each place in a year. It also shows, mainly
from the data of Mr. Buchanan of the Challenger, the
salinity of the ocean by deepening red tints ; salt areas in
the sea are produced by evaporation of* the water which
supplies the rainfall of the land, and they may be termed
the comparatively dry regions of the sea. They correspond
very closely with the centres from which the trade
winds blow. The lightest blue colour on the map denotes
regions where the rainfall is under ten inches per annum.
These correspond exactly with the regions of extreme range
of temperature, lying as a rule in the interior of continents.
The regions of greatest rainfall coloured in deepest blue are
all steep land slopes exposed to a sea wind. In North
America, for example, the trade winds blowing round the
Gulf of Mexico, and the south-west winds beating on the
coast of Oregon and British Columbia, ensure heavy rain-
fall. South America shows a very interesting relation. In
the trade- wind region vapour is carried up the flat valley of
the Amazon and condensed on the eastern slope of the
Andes, the western slope of which is rainless. In the
south of the continent the west winds of the Roaring
Forties dash perpetual showers against the western face of
the Andes, and descending sweep as drying winds across
Patagonia. In India and the Malay Archipelago the
heavy rains are produced entirely by the summer monsoons.
Attentive study of the rainfall map, along with those of
winds and configuration, will bring out similar reasons for
the local distribution of rainfall in all parts of the Earth.

202. Winds of the British Islands. — The British
Islands are usually covered by the edge of the North
Atlantic area of low pressure. The pressure being lowest
in the north-west, and highest in the south-east, corresponds
to prevailing south-westerly winds. In January the isobars
are closely crowded together ; in that month the average
gradient over the British Islands is steeper and the winds
are consequently stronger than in any other part of the

144 The Realm of Nature CHAP.

world. From January onward the atmospheric pressure
increases rapidly in the north and much more slowly in the
south, so that in the month of April the gradient, though
still for westerly winds, is very slight. A small temporary
rise of pressure in the north may thus reverse the gradient,
and as soon as the pressure in the north becomes higher
than that in the south, east wind sets in. A similar state
of matters occurs again in November, on account of the
pressure in the south falling more rapidly than that in the
north, and the months of April and November are famed
for bitter east winds in all parts of Britain.2

203. Temperature of the British Islands. — The
temperature of the British Islands on the average for the
year is about 48°, increasing from 45° in Shetland to 53° in
Scilly, or an average rise of temperature of i ° for every i oo
miles toward the south. In winter the temperature has no
relation to the latitude, the islands grow colder from
west to east. The isotherms of January (Plate IX.) run
from north-west to south-east. A broad strip of country from
Caithness to Lincoln has an air temperature of 38° or less.
Shetland, Orkney, Ayr, Liverpool, Oxford, and London
are traversed by the isotherm of 39°. The points of Kerry,
Cork, and Scilly are at 45°. The south of England is mild
in winter, not because it is the south, but because it runs
so far to the west. By the month of April the isotherms
run nearly east and west ; the temperature is 42° in
Shetland, 45° from Skye to Aberdeen, and 48° from
Erris Head through Dublin and Liverpool to Harwich.
In this month land and sea have practically the same
temperature. In July the land has heated up more than
the sea, so that the south-west wind now has a cooling
effect, and the isotherms (Plate X.) run roughly from S.W.
to N.E. Shetland is at 54°, the line of 58° runs from
Malin Head, near Rothesay and Inverness, to Peterhead, and
that of 60° from Killarney across Ireland, through Lough
Neagh southward, north through Whitehaven to Selkirk,
and then south to Newcastle. The hottest region is round
London, where the temperature averages 64°. As autumn
advances the air cools down most rapidly on the east coast,

MEAN ANNUAL RAINFALL ON

After Loomis. J. Y

160 180 160 140 120

DIAGRAMM
SHOWING HEIGHT OF THE LINE OF PI

^L Seograjlical In.stitatB

Land Ra.infall in Inches, - Reference to Colouring

I Below 10 [ 10 to 15 I 25 to 50 [50 to 75

D, AND SALINITY OF OCEAN,
tanan, and others.

EC SECTION

TUAL SNOW AT DIFFERENT LATITUDES

Relative Density of Surface Water, - Reference to Colouring

I 025 j I 025 to I 026J7026 to H

ix Climates of the World 145

and in September the isotherms run west and east once
more, and the temperature varies from 52° in Orkney to
58° along a line from Pembroke through Bristol and
Reading to Lowestoft. The way in which proximity to the
sea and exposure to the prevailing winds influences the
range of temperature is shown in the following tables : —

Shetland Rothesay Plymouth Inverness Edinburgh London
Jan. temp. 39.5 39.5 43.0 37.5 38.0 39.0

July temp. 53.5 58.0 62.5 58.0 59.5 64.0

Annual range 14.0 18.5 19.5 20.5 21.5 25.0

If it were not for the warm south-westerly winds the
January temperature would be 7-5° in Shetland, 12-5° at
Edinburgh, and 22° at London, and the sea all round the
islands would be frozen.

204. The Rainfall of British Islands has been studied
by Mr. G. J. Symons, who publishes an annual volume on
British Rainfall. The rainfall is greatest on the west
and least on the east coast, warmth always going with
wetness (Plate XVII., compare also Plate XVI.) In
Ireland, on account of the mountains forming irregular
isolated groups, the rainfall is remarkably uniform over the
whole island, averaging about forty inches in the year. In
Great Britain the low outer Hebrides have a rainfall of
about fifty inches, but the high mountains of Skye and the
Western Highlands condense more than eighty inches in
the year over an area stretching from Skye to Loch Lomond.
The mountains of Cumberland and Wales and the high
land of Cornwall have also a large rainfall ; but the whole
east of Britain has less than forty inches. Most of the
district between the H umber and the Thames, the driest
part of the British Islands, receives less than twenty-five
inches of rain per annum. Contrary to the usual opinion,
November is nowhere the rainiest month in the British
Islands. The heaviest rainfall in the west and north of
Ireland and the west of Scotland takes place in December
and January. In England and the east of Scotland it
occurs in October, except in the very dry region between
the Thames and Humber, where most rain falls in August.
In the south of England the least rainy month is March, in

L

146 The Realm of Nature CHAP.

the north of England and south of Scotland it is April, in
the Scottish highlands it is May, and in Orkney it is June.
The average distribution of climate shown in the maps,
although correct on the whole, cannot be depended upon to
hold good at any special place for any particular month.
Such maps are of great value in choosing a place to reside
in, but of very little use for planning a pleasure trip. The
conditions of weather are somewhat complicated, but appear
to depend mainly on the distribution of atmospheric pressure,
which may be classified into certain well-marked types.

205. Anticyclones. — An anticyclone is a portion of the
atmosphere in which the pressure is highest at the centre,
and diminishes nearly uniformly in all directions. The
wind in an anticyclone blows spirally outward, as is illus-
trated in the high-pressure regions shown in the Isobaric
maps. In the northern hemisphere the circulation of surface
wind round the edge of an anticyclone is in the same
direction as the hands of a watch move, in the southern hemi-
sphere in the opposite direction, as explained by FerrePs or
Buys Ballot's Law. An anticyclone when once formed is
a very steady arrangement of pressure, and usually lasts
for many days or even weeks at a time. This being so,
it is evident that a supply of air must be continuously
renewed from above to take the place of that passing out
as surface winds. Air in fact passes through an anti-
cyclone much as grain does through a pair of mill-stones,
though of course without suffering any physical change.
In the upper regions of the atmosphere air must be moving
inward and sinking downward to maintain the anticyclone,
and the pressure in the upper region of the atmosphere
must thus be least above the spot where it is greatest on
the Earth's surface. This deduction has been proved to
be true by observations at mountain meteorological stations.
The surface winds of an anticyclone are usually light and
variable. As the air is descending from above, it contains
very little water- vapour, and no clouds are formed. Hence
in summer, anticyclonic weather is brilliant, hot, and calm,
with haze at night or heavy deposits of dew, on account of
great cooling by radiation. In winter an anticyclone is

IX

Climates of the World

calm and clear, and by intense radiation the land cools
down greatly at night, and the temperature of the air falls.
This is the condition required for long spells of frost, and
in large towns and over lakes and estuaries it produces
dense, low-lying fogs. The low temperature tends to in-
crease the density of the lower air in an anticyclone,
and until very recently was viewed as the main cause of
the formation of this arrangement of pressure. Fig. 29

FIG. 29. — Isobars of an Anticyclone. (After the Hon. Ralph Abercromby.)
Direction of wind shown for the northern hemisphere. The prevailing
weather in winter is shown on the left side, that in summer on the right side
of the diagram.

shows the form of isobars and the kind of weather in a
typical anticyclone, which may be summarised as a very
steady and nearly stationary descending eddy or whirl of
banked-up air crowded into one place by the converging
currents of the upper atmosphere as they flow toward the
poles.

206. Cyclones. — An area of low atmospheric pressure
which has the lowest pressure in the centre was called a
Cyclone, because the early observers believed that the wind
blew round it in circles. We now know that wind blows
in toward the centre of low pressure in a spiral curve with

148 The Realm of Nature CHAP.

a strength proportional to the gradient. The circulation of
winds in a cyclone is — following Ferrel's or Buys Ballot's
law — in the direction opposite to the movement of watch-
hands in the northern hemisphere, and in the same direc-
tion as the hands of a watch in the southern. As the
centre of a cyclone remains at the lowest pressure in spite
of surface winds pouring in from every side, the air must
rise in the centre and flow out above. A cyclone is thus
an inward and upward whirl or eddy of the atmosphere.
The air above has its normal pressure raised by the upflow,
and consequently gives rise to outward-flowing upper winds ;
the cyclone in fact is necessarily crowned by an anticyclone.
The cyclone is not a stationary eddy, such as is represented
by the low-pressure areas on the charts of average atmo-
spheric conditions, but is always moving. In the northern
hemisphere the direction of its motion is westward near
the equator, gradually turning to the right and becoming
north-eastward in high latitudes. In the southern hemi-
sphere it is westward near the equator, turning toward the
left and becoming south-eastward in high latitudes. The
rate of motion of the centre is from 15 to 30 miles an hour
in the temperate zones, but only from 3 to 8 miles an hour
in the tropics. The actual particles of air do not move
forward, but the diminution of pressure is passed on like
a wave (§ 57) through the air. Cyclones usually form
on the edge of the permanent regions of high pressure,
and travel away along their margins. Professor Hann, the
great German meteorologist, has concluded that the cyclones
and anticyclones of the temperate zone are true eddies and
bankings-up formed in the great streams of air which set
poleward from the equator.3

207. Cyclonic Weather. — There are certain changes
of weather associated with a cyclone which result from the
fact that it is an eddy of ascending surface air. The air
on rising near the centre is cooled by expansion, and the
vapour condenses into cloud, and ultimately falls as rain.
Hence, when the cyclone is approaching an observer and
condensation has just begun to take place in the upper
regions, a halo produced by reflection from the condensed

IX

Climates of the World

149

particles of ice is commonly seen round the Sun or Moon.
Later the sky becomes gloomy, the air feels warm and
oppressive even in winter, thick clouds form, and there is
heavy rain, while the barometer is all the time falling, and
the wind shifting its direction. As soon as the barometer
begins to rise, the centre of the cyclone has passed ; and
as the atmospheric pressure increases in the rear of the
depression the sky clears, the wind freshens, and the air
feels peculiarly exhilarating. Fig. 30 shows the form

Cirrus

;RONT

REAR

FIG. 30. — Isobars of a Cyclone. (After the Hon. Ralph Abercromby.) Direction
of wind and distribution for weather shown for the north temperate zone.

of the isobars, direction of wind, and the' different condi-
tions of weather in various parts of a cyclone. It shows
what changes an observer would notice according as he
was to the north or south of the cyclone as it passed. The
long arrow shows the direction in which it moves. In a
typical cyclone, such as that represented, the gradients are
always steeper in the rear than in the front, so that the
strongest winds are experienced after the barometer has
begun to rise. The succession of weather is the same
in every cyclone ; but the intensity of it depends on the

150 The Realm of Nature CHAP.

gradient of pressure. If the cyclone is of great size and
the diminution of pressure in the centre very slight, gentle
winds and light showers only will be produced. But if,
on the other hand, the cyclone is of small dimensions, less
than a hundred miles across, and the diminution of pressure
in the centre is great, terrific winds and deluges of rain
result. The centre of a cyclone is always calm, although a
gale may be blowing round it in every direction a few
miles distant. The weather of the northern hemisphere
is to a very great degree determined by passing cyclones of
large size.

208. Hurricanes and Typhoons. — Small cyclones of slow
motion but with steep gradients, and therefore accompanied
by very severe winds, are common in the tropics at certain
seasons. Unlike the cyclones of the temperate zone they
appear to originate from local heating of the air. Among
the West Indian Islands such storms are liable to occur
during the months from July to October, and their terrific
violence has given wide currency 'to their native name
of Hurricanes. In the Bay of Bengal at the changing of
the monsoons, and along the east coast of Africa, similar
storms, to which the name Cyclones was first applied, are
experienced. In the China Sea they are common from
July to November, and are known as Typhoons. These
tropical storms differ from the less violent cyclones of the
temperate zone in always having a patch of clear blue sky
over the central calm where the barometer is lowest ; this
is called the eye of the storm. Although the calm centre
of a cyclone is referred to poetically as " the whirlwind's
heart of peace," it is the part most dreaded by sailors.
There is no wind to move a sailing ship, and a terrible
chaos of heavy waves is driven in by the winds raging on
every side. A ship -captain in the season when these
storms are prevalent is always on the watch for them,
and as their approach is heralded by a fall in the barometer
and the way in which the wind changes, he can find the
direction of the centre. A steamer in many cases can then
by changing its course let the storm pass harmlessly.

The commonest Cyclone tracks of the tropics and the

IX

Climates of the World 151

usual direction of motion of the storm-centre are represented
on Plate VII.

209. Whirlwinds. — Eddies of ascending air which are
of small diameter compared with their height, and move
rapidly forward over the Earth's surface, are called Whirl-
winds. They are often set up by the sudden heating of
the lower layers of the atmosphere. The dreaded Simoom
of the Sahara is a whirlwind which raises great gyrating
clouds of sand, and sweeps forward with irresistible force, dark-
ening the sky at midday. The Tornado of North America is
even more destructive. It is most often formed in the south-
east side of a slowly moving cyclone, and usually acquires its
full force suddenly in sultry summer afternoons. The origin
of a tornado has given rise to much controversy, but is
usually believed to be the rapid heating by the Sun of a lofty
column of air fully charged with water-vapour. The heated
air expands upward and rotates as it rises, drawing the
surface air in from all sides. The water-vapour, condensing
as the air cools in ascending, adds to the heat-energy (§ 159)
of the whirl, and helps to produce a tremendous reduction
of pressure in the centre. Surface winds rush into this
partial vacuum, and whirl with terrific violence up the
central hollow as if it were a furnace chimney. In conse-
quence of their force the tornado cuts a clean path through
forests or towns that lie in its path. The breadth of the
zone of destruction is seldom more than a quarter of a
mile. Houses are not simply knocked down but burst up
when a tornado passes over them. The low pressure of
the centre creates a partial vacuum, and the air inside a
house consequently expands so rapidly, that the roof is
blown off and the walls thrown outward. Sheep and fowls
when caught up are completely plucked of wool or feathers
by the fierce whirls of wind before they are dropped. After
about an hour the heated vapour-laden air that originates
the tornado is dispersed, and as the whirl travels at the
rate of 30 miles an hour the track of destruction is usually
30 miles long, although instances of papers being carried
45 miles are on record. Tornadoes are most common in
the United States east of 100° W. ; but it is only in a

152 The Realm of Nature CHAP.

small district of Kansas on the Missouri River, and in the
south-west of Illinois, near the Mississippi and Ohio, that
more than 50 have been recorded in the last hundred
years.

210. Waterspouts and Cloudbursts. — The rapid con-
densation of water-vapour in the axis of a tornado, or in
the comparatively harmless whirlwinds that sometimes occur
in all parts of the world, produces a dark funnel-shaped
cloud tapering downward to the Earth. Such a cloud
occupying the centre of an ascending eddy of air is called
a waterspout. When it strikes the ground the heavy fall
of rain on a very small area sometimes produces great
destruction. At sea, or in passing over a lake or river,
the low pressure of the whirling air of a waterspout often
sucks up a column of water and whirls it on for consider-
able distances. In this way shoals of fish or swarms of
frogs are sometimes raised high in the air, carried for miles
inland, and dropped as showers of fish or frogs to the
wonder of country people. It often happens that the upward
rush in a tornado is strong enough to prevent the condensed
water from falling until a great quantity has accumulated ;
then it descends not as rain but like a river, and the
phenomenon is spoken of as a Cloudburst. On mountain
slopes cloudbursts have been known to hollow out deep
ravines in a few minutes. Hail as well as rain may be
similarly accumulated, and the worst hailstorms occur dur-
ing the passage of a tornado.

211. Weather-charts. — The gradual growth of know-
ledge about the atmosphere showed that the barometer
could be used for predicting changes of weather in certain
cases. Most barometers have a series of words from "Set
fair" to "Stormy" engraved on the scale, as if high or
rising pressure always means calm and fine weather, and
low or falling pressure always foretells wind and rain. A
few weeks' observation will in most cases convince any
one that this is a mistake, and that a single barometer is
of little value for forecasting the weather. Fig. 30 shows
that it is not the actual height of the barometer at one place,
but the difference in the height of many barometers at con-

ISOTHERMS FOR JANUARY.
After A. Buchan.

I /

BRITISH ISLES

Reference to Colour! n

[ Below 38* [ 38* to 40* ~| 40*

ISOTHERMS FOR JULY.
After A. Buchan.

10

; Temperature in Deg. Fahr

10' ]~ 60' to 65° j Above 65* j

J.<J.B*rtkolom<Jvr

IX

Climates of the World 153

siderable distances apart, that can throw light on the state
of the atmosphere and the associated weather. About
thirty years ago the first synoptic charts showing the
isobars of a country were introduced as an aid to the study
of weather, and such weather-charts enable storms to be
foreseen in many cases. In nearly every country there is
now a number of meteorological stations where observa-
tions of barometer, thermometer, wind, etc., are made at
the same hour morning and evening, and telegraphed to a
Central Meteorological Office maintained by Government.
Here charts are prepared showing at one glance the state
of the atmosphere both as regards pressure and tempera-
ture (corrected to their value at sea-level) over the whole
country and surrounding districts. If the student will take
the trouble of tracing in red ink on thin paper the figures
of a cyclone and anticyclone given above (Figs. 29 and
30), and will then lay this tracing over the map of the
British Islands (Plate IX.), he will see exactly how the
weather varies in different parts of the country according
to the distribution of these types of atmospheric pressure.

212. Weather Forecasts. — Several arrangements of
isobars besides those into cyclones and anticyclones may
occur. Isobars drawn from actual observations may be
straight, showing that they form part of neither cyclone nor
anticyclone ; sometimes they are sharply curved, forming
V-shaped areas of low pressure or wedge-like areas of high
pressure lying between adjacent anticyclones or cyclones ;
and they very often form loops, showing the existence of a
small secondary cyclone inside a larger. Each type of pres-
sure-distribution corresponds to a special 'kind of weather,
and the relation between isobars and weather has been
carefully studied and is well known to practical meteoro-
logists. The commonest weather in the British Islands is
that produced by the passage of cyclones eastward from
the Atlantic, and this may be taken as a characteristic
example to illustrate weather forecasts. If the student
places a tracing of Fig. 30 on Plate X. so that the large
arrow points north-east and its head is on the south-west of
Ireland, and then moves the tracing gradually north-east-

154 The Realm of Nature CHAP.

ward, he will see how the weather varies in all parts of the
islands as the cyclone passes along its path. By shifting
the centre to north or south, and changing the direction
of passing (but always moving the tracing as the arrow
flies), the effect on the student's own locality of cyclones
passing in any direction may be followed. Remembering
that as isobars of successively lower value are passing the
barometer is falling, and that as isobars of higher value
are passing the barometer is rising, it will be found possible
to identify the actual movements of a cyclone by watching
the barometer and the .changes of wind and weather.
In order to predict on Monday the kind of weather and
direction of wind on Tuesday when a cyclone is passing,
it is necessary to know where the centre is, at what
speed, and in what direction it is moving, so that a map
of the conditions expected on Tuesday can be drawn up
from the data supplied by Monday's observations. But
in order to predict the intensity of the weather and the
force of the wind, it is necessary to know whether the
cyclone is "deepening" or "rilling up," that is, whether
the gradient of pressure from circumference to centre is
growing greater or less. Only experience and practice can
guide a forecaster in these matters, and the success of the
predictions issued daily by all civilised governments depends
on the knowledge and skill of the men who make them.
It often happens that a cyclone does not follow the usual
path, or that the pressure at the centre increases when the
forecaster thought it would diminish, or that a secondary
depression suddenly forms in an unexpected place, and of
course in all such cases the forecast made is a failure. Yet on
the whole more than 80 per cent of the predictions issued
in Britain and America are successful. The British Islands
are divided for purposes of forecasting the weather into
eleven districts. At 10 A.M. and 8.30 P.M. forecasts for
the next 24 hours of the weather in each of these districts
are published at the Meteorological Office in London from
observations made all over the country at 8 A.M. and 6 P.M.
The weather-charts, reports, and forecasts in a daily and
weekly form are sent out to subscribers by the Meteoro-

ix Climates of the World 155

logical Office. The Reports and Forecasts are published
in all the morning and evening newspapers, but only the
Times prints a daily weather-chart of the British Islands.

213. Storm Warnings. — A sudden fall of the barometer
at any of the special British meteorological stations is at once
telegraphed to London, and if it is found to indicate the
discovery or sudden deepening of a ' cyclone crossing the
islands which is likely to cause a dangerous storm at sea,
warnings are telegraphed to all the important harbours and
fishing stations on the coast, where signals are immediately
hoisted to give notice to fishermen and sailors. Such
signals are most valuable on the east coast because the
disturbances usually come from the west. Farmers profit
by weather predictions as well as sailors, particularly in the
hay and harvest seasons. The escape of gas in coal-mines
and consequent risk of explosions has been found to de-
pend largely on variations of atmospheric pressure, and
miners' warnings are now regularly issued when any serious
change of pressure over the coal-mining regions is anticipated.
In many ways the British Islands are in the worst position for
forecasting the weather as they lie in the most disturbed region
of the atmosphere. The most westerly observing station is
on Valentia Island in the south-west of Ireland, which often
does not give time to warn the country before a storm
appears, and affords very little opportunity of tracing the
probable path in which it will travel. A floating station
in the Atlantic, west of Ireland, would be an enormous help
in framing British forecasts, and would undoubtedly save
many lives and much money. On the continent of Europe
forecasting is comparatively easy, as the British stations give
early notice of all changes. Similarly, in a broad stretch of
land like North America, Australia, or India, where the
stations are widely distributed and well equipped, there are
great advantages for the prediction of weather. In the United
States the Weather Bureau of the Agricultural Department
has charge of meteorological observations, and the forecasts
are not only distributed as in Britain, but in the thinly peopled
districts the trains are fitted with special signals so that the
farmers along the railway have only to look out as the train

156 The Realm of Nature CHAP, ix

passes in order to know what weather to expect for the day.
The attempt to time the arrival on the coast of Europe of
cyclones whose path across America has been tracked out
is rarely successful, as most depressions either fill up or
change their path or rate of moving on the way across the
Atlantic. There are many prognostics or signs, such as the
appearance of halos, of mist on hill -tops, great clearness of
the atmosphere, exceptionally bright reflections in water, the
movements of animals, by which experienced people can
foretell the weather of their own district with marvellous
correctness. Indeed, for any mountain valley or seaside
town the opinion of an observant old shepherd or fisher-
man on the approaching weather is likely to be more
correct than the somewhat general Meteorological Office
forecast.

REFERENCES

1 J. Murray, "On the Total Annual Rainfall," etc., Scot. Geog.
Mag. iii. 65 (1887).

2 A. Buchan, " Climate of British Islands, Pressure and Tem-
perature, " Journ. Scot. Met. Soc. for 1882. "Rainfall," ibid, for
1885.

3 H. F. Blanford, "Cause of Anticyclones and Cyclones,"
Nature, xliii. 15 (1890). "The Genesis of Tropical Cyclones,"
Nature, xliii. 81 (1890).

BOOKS OF REFERENCE

Challenger Reports, Physics and Chemistry, Circulation of the
Atmosphere, by A. Buchan. (A unique collection of isobaric and
isothermal maps for every month of the year. )

A. Buchan, Art. " Meteorology," Encyclopedia Britannica.

R. H. Scott, Elementary Meteorology. International Scientific
Series.

H. F. Blanford, Climates and Weather of India. Macmillan
and Co.

N. S. Shaler, Aspects of the Earth, pp. 197-257. Smith,
Elder, and Co.

Consult also the publications of the Royal Meteorological
Society, the Scottish Meteorological Society, and of the Meteoro-
logical Office, 116 Victoria Street, London, S.W.

CHAPTER X

THE HYDROSPHERE

214. Land and Water. — The hydrosphere does not
completely cover the globe, because the lithosphere which
supports it is diversified by great heights and hollows.
The portion of the heights projecting above the water
surface forms land, which is estimated at the present time
to cover 28 per cent or a little more than one quarter of
the globe. Most of the hydrosphere is retained in the
great world-hollows forming the ocean, which covers about
7 2 per cent of the surface ;
but on account of evapora-
tion and condensation a
small part is always present
as vapour in the air, and
a larger amount rests as
lakes in hollows of the land
or flows across the surface
in rivers. The proportion
of land and water in differ-
ent latitudes is represented
in Fig. 31, where the land
area is indicated by shad-
ing. The largest propor-
tion of land is in the
northern hemisphere, where

FIG. 31. — Proportion of land and sea in
different latitudes. Land area shaded

(after Krummel).

it occupies about 42 per cent of the surface, while water
largely predominates in the southern hemisphere, where

158 The Realm of Nature CHAP.

about 1 7 per cent of the surface is dry land. The fine curve
in the figure shows the average distribution of 28 per cent
of land in all latitudes. All the great land masses of the
globe are widest in the north, and taper to a point toward the
south. Only a few small islands lie beyond 56° S. if the
unexplored Antarctic region is excepted. The inequality
of the distribution of land and water appears greatest in
the hemisphere having its centre near New Zealand, which
comprises two-thirds of the entire ocean surface and only
one-eighth of the land ; and in the opposite hemisphere
(with its centre in the English Channel) which contains only
one-third of the ocean and seven-eighths of the land of the
Earth. In the water hemisphere the proportion of land
is about y1^ or 8 per cent ; in the land hemisphere it is
about J or 50 per cent, the areas of land and sea being
equal (see small maps on Plate XI.)

215. Divisions of the Hydrosphere. — The Caspian Sea
is the only large sheet of water which is cut off by land
from the rest of the hydrosphere, and its separation from
the ocean is comparatively recent (§ 335). Otherwise the
hydrosphere is a connected whole, made up of four wide
open expanses called Oceans, from which smaller portions'
called Seas are more or less distinctly marked off by the land.
It is a matter of opinion where to draw the line between
oceans and seas ; the expanse of water within the Arctic
Circle, for example, is by some authorities considered the
smallest ocean, and by others with more show of reason it
is held to be the largest sea. Seas may be classed in three
groups— (a) Inland Seas, entirely surrounded by land, of
which the Caspian is the only example ; (b) Enclosed
Seas, nearly surrounded by land but connected with the
ocean or with another sea by one channel, which is narrow
and shallow compared with the general breadth and depth ;
(c) Partially Enclosed Seas, which (a) have two or more
entrances, or (/3) are marked off from the ocean by a line
of islands, or (y) by an entirely submerged barrier.

216. The Oceans. — No natural boundaries mark off the
hydrosphere sharply into separate parts, but it is convenient
to distinguish four divisions called oceans, the positions

x The Hydrosphere 159

of which are shown on Plate XIII. The Southern Ocean
may be characterised as the shoreless ocean, for it extends
round the Earth from 40° S. to the Antarctic ice, only a
portion of South America, the islands of Tasmania and
South New Zealand, and some smaller ones projecting
into it. Its area is about 30,000,000 square miles. The
Pacific Ocean, with an area of 55,000,000 square miles, as
large as all the land of the globe, is well called the Great
Ocean by the Germans. It contains many islands and
partially enclosed seas, the names of which are given in
the following table. The Pacific is the only ocean parts of
which lie more than 2500 miles from the nearest continent
(see Plate XII.) The Indian Ocean is entirely enclosed
by land on the north, and has an area of 17,000,000 square
miles. The Atlantic, with an area of 33,000,000 square
miles, has a more indented shore than any other, and may
be called the ocean of enclosed seas. The largest of these,
often itself termed an ocean, is the Arctic. More than half
the land of the globe sends rivers into the Atlantic and its
associated seas.

OCEANS AND SEAS
ATLANTIC. PACIFIC. INDIAN.

Enclosed. Partially Enclosed. Partially Enclosed. Partially

Enclosed. Enclosed. Enclosed.

Mediterranean Arctic Yellow Bering Red Andaman

Black Kara Okhotsk

Adriatic Norwegian * Gulf of Japan Persian^-

Baltic North California China Gulf

White Caribbean Celebes

Hudson Bay Banda

Gulf of Java

Mexico Sulu

Arafura.

217. Ocean Tides. — If the hydrosphere were continuous,
or if the land were arranged in narrow strips from east to
west, a double tidal wave (§§ 103, 114) would travel round
the globe every day, the velocity of this free wave form
being thus about I ooo miles an hour at the equator, and its
length half the circumference of the Earth. If the land of

160 The Realm of Nature CHAP.

the globe were arranged in strips from north to south,
cutting up the hydrosphere into a series of narrow com-
partments, there would be no appreciable tidal effect. By
the actual arrangement of land there is a free water ring
in the Southern Ocean only ; there is one long comparatively
narrow compartment, the Atlantic Ocean ; another wider
and shorter, the Indian Ocean ; while the rest of the hydro-
sphere forms the wide open surface of the Pacific extending
half-way round the globe at the equator. In the Pacific
and the adjacent Southern Ocean alone the tidal wave has
full room to form, and from them the wave passes westward,
being deflected northward into the other oceans. Co-tidal
lines on a map (Plate XIII.) show the places which the
same phase of the tidal wave reaches at the same hour.
Starting from 1 2 the position of the crest of the wave at
each successive hour is marked by i, 2, 3, up to 12.
The tidal wave travels most rapidly, and is longest and of
least amplitude in deep water ; in the central Pacific the
range between High Water and Low Water (the amplitude
of the tidal wave) is less than 2 feet, and no current is
produced.

218. Tidal Currents. — When the tidal wave enters
shallow water it becomes shorter and moves more slowly.
The under side of the wave becoming more retarded than
the top, the surface water is carried forward as a true
current, the energy of which is derived from the Earth's
rotation. In this way shoals or submarine peaks convert
the simple up and down movement of the tide in the open
ocean into rapid currents, usually for a very short distance
but sometimes extending to a great depth. These are
more definite along the shores. The usual tidal effects
observed on a broad gently-shelving shore are the gradual
rise of the level of the water, the submergence of the beach
and advance of the sea on the land ; then after the highest
point has been attained, the gradual lowering of level with
corresponding uncovering of the beach and retreat seaward
of the sea-margin. At New and Full Moon, when spring-
tides (§ 114) occur, the rise and fall is at the greatest, and
then, at any one place, high water occurs at the same

VEGETATION ZONES OF CONT!

After Engle

165 18O 165 150 155 12O 105 90 ,

_±^^

to Monsoon Drifts &c.

In. the, Indian, Ooean,,the- China, Sea,, and the West Coast of Mexico
and Central Amervca,, the Currents duznge, vith, tfue, 3fonsoon#.

Th& sijnpl& arrows >• sTww ~t}te 5-"WT and 5.£". JfoTwoort 2}riftf

abusing the- Northern, Summer. The arrows marked -thus
aha* die Jf.W. and KE. Monsoon Drift during

165 180 165 150 155 120 105 9O 75 6O

Sea-Weed;

"Warm Currents coLtmred Red

The, directioTt, of the. Cv

AND OCEANIC CURRENTS,
d others.

18

75 90 105 120 155

be to XazicL Surface Characteristics

rultivatei Lancts Arctic Region o£ Snow & Ice

76 9O 105 120 135

Cold Currents coloured Blue Limit of Paak !»

if shown, "bv thje. arr-ows.

x The Hydrosphere 161

hour. Admiralty charts show the tidal data for each sea-
port, thus, e.g., " High Water, Full and Change, X. rise 10
feet." This means that on the day of Full Moon and of
Change or New Moon high water occurs at 10 A.M., and the
rise of the sea between low water and high water is 10
feet. Each successive high tide after Full Moon occurs at
an interval of about 12^ hours, rises to a somewhat less
height and falls to a somewhat less depth, thus covering
and laying bare a narrower strip of the beach until the
Moon's phase is the third quarter, when the time of morn-
ing high water is 4 A.M. and neap-tide occurs. After this
the tides increase in amplitude again until the period of
Change or New Moon, when the time of morning high water
is once more 10 o'clock. The time during which tidal
currents run in one direction and in the opposite bears
little relation to the hours of high water and low water,
depending largely on the form of the coast. In partially
enclosed seas a branch of the tidal wave usually enters by
each channel, as shown in the co-tidal map of the British
Islands (Plate XVII.)

219. Tides in Bays and Estuaries. — When the tidal
wave of the ocean enters a narrowing bay or sea inlet, the
depth of which diminishes rapidly, the tidal currents become
rapid and tumultuous and the water is heaped up to a great
depth against the land. At the entrance of the Bay of Fundy
the tide rises 8 or 9 feet, but at the head the rise at spring-
tides is more than 70 feet, the greatest tidal range known.
The highest spring-tide at Cardiff docks rises 42 feet, and
the lowest neap-tides 20 feet, while at the mouth of the
Bristol Channel the rise of spring-tide is only about 10
feet. The tidal wave rushes up some rivers with great
violence, forming a bore or wall of foaming water stretching
right across the stream, and often producing much destruc-
tion to shipping in the Amazon and Yang-tse-kiang. A
tidal current sweeping through a narrow irregular channel
gives rise to eddies or whirlpools sometimes of great size,
like that of the Maelstrom in the Lofoten Island group.

220. Properties of Water. — In order to understand
the action of solar energy on the hydrosphere, we must

M

162

The Realm of Nature

CHAP.

know something of its composition and physical properties.
The hydrosphere is composed almost entirely (about 96-5
per cent) of water, and the total amount of this substance
which exists upon the Earth is estimated at about 335
million cubic miles or 1,500,000 million million tons. The
mass of the hydrosphere is thus about 300 times as great
as that of the atmosphere, but its volume is at least 100
times less. Pure water is a chemical compound of oxygen
and hydrogen united together in the proportion of one-ninth
hydrogen and eight-ninths oxygen by mass. Intense heat
(§ 71), the action of some heated metals, or the passage
of an electric current, separate these constituents, giving to
water in some rare circumstances the character of an ex-
plosive (§ 294). The student should read again §§ 66-73.
Water, on account of its singularly high specific heat and
latent heat, is better fitted than any other fluid for the part
it plays in transmitting and regulating energy in Nature.
Water is capable of dissolving all natural substances, although
some, such as glass or silica, are taken up in minute propor-
tions. Natural water is consequently never pure ; however
clear it appears, it contains various gases and solids in
solution.

SALTS OF RIVER- WATER

Calcium Carbonate .

42-90 ^Carbonates

Magnesium Carbonate

14-80 / -57-70

Silica

9.90

Calcium Sulphate
Sodium Sulphate

7*X 1 Sulphates

A"2O r*

Potassium Sulphate .

2-70 J [I>4°

Sodium Nitrate

3-5o

Sodium Chloride

2-2O

Iron Oxide and )

•2. fin

Alumina )

3-OO

Other Salts .

1-30

Organic Substances .

10-40

Total

100-00

221. River- water contains salts of many kinds in solution
derived from the surface over which it flows. The amount

The Hydrosphere

of dissolved solids in river-water may vary from about 2
grains in the gallon where a river flows over granite rocks,
to more than 50 grains per gallon where the streams traverse
a limestone country ; the average salinity of river-water is
about 12 grains per gallon or 0-018 parts in 100. The
composition of the dissolved solids is different for each
river on account of the different rocks traversed, but the
accompanying table gives the composition of 100 parts by
weight of the dissolved salts of an average sample of river
or lake water. The large proportion of carbonates and of
silica and the small proportion of common salt (sodium
chloride) present are characteristic.

222. Sea -water. — The water of the ocean contains
nearly 200 times as much dissolved solids as the water
of the land. Sea-water, indeed, is at once recognised by
taste as salt, while rivers are pronounced fresh. Although
the salinity of sea-water varies from place to place and from
time to time within certain narrow limits, the composition
of the dissolved solids remains almost the same everywhere.
In other words, water collected in any part of the great
oceans, and boiled down with suitable precautions so as to
leave the solids behind, yields "salt" of almost exactly the

SALTS OF SEA-WATER

Sodium Chloride . 77-70
Magnesium Chloride 10-80
Magnesium Sulphate
Calcium Sulphate .
Potassium Sulphate .
Calcium and Mag- \
nesium Carbonate j
Magnesium Bromide 0-20
Other Salts , 0-20

Jglsulphates
2-50 J =IO'8°

0.30

Total 100-00

same composition which is shown in the accompanying
table. The only exception which has been proved to this
statement is that at great depths there is a slightly greater

164 The Realm of Nature CHAP.

proportion of calcium or magnesium carbonate than near
the surface. It is remarkable that more than three-quarters
of the whole is made up of common salt, while the propor-
tions of carbonates and of silica are very minute. Silica in
carefully filtered sea- water never appears to exceed I part in
250,000 or 0-0004 Per cent. The proportion of sulphates is
nearly the same as in the salts of river-water. Some geo-
logists suppose that the sea consists merely of concentrated
river-water ; and even on the more probable assumption
that sea-water contained salts in solution derived from the
primeval atmosphere, it is evident that some agent must
be at work withdrawing silica and carbonates from river-
water as it enters the sea. That agency is known to be
the power of living creatures — plants and animals — to make
themselves shells or skeletons of silica or of calcium carbon-
ate secreted from the water (§ 273). Sea-water is slightly
alkaline, probably on account of its containing bicarbonates
in solution. It dissolves carbonate of lime, especially when
subjected to great pressure.

223. Salinity. — The salinity of sea-water is the amount
of dissolved salts contained in 100 parts. One hundred
pounds of average sea- water contain about 3-5 pounds of
dissolved salts, and thus the average salinity is said to be
3-5 per cent. It is difficult to measure salinity directly, as
some of the salts decompose when the water is boiled down.
The density of sea-water, however, depends on its tempera-
ture and on the salinity, so that if the density is always
measured at the same standard temperature, or corrected
to it, the differences of density are due to differences of
salinity alone. For example, if a bottle contains exactly
1000 grains of pure water at the temperature of 60° F. it
would contain 1013 grains of sea-water which held 1-75
per cent of salts in solution, and 1026 grains of water
holding 3-5 per cent of salts. Density (specific gravity) is
measured most easily by means of a delicate hydrometer,
but most accurately by weighing a carefully measured por-
tion of the water. The standard temperature to which
density of sea- water is calculated is usually 32° F. or 60°
F. in English-speaking countries, and o° C. or 17-5° C. on

x The Hydrosphere 165

the continent of Europe. The density at 60° F. corre-
sponding to various degrees of salinity is as follows : —

Salinity o-oo i-oo 2-00 3-00 3-25 3-50 3-75 4-00
Density i-oooo 1-0058 1-0138 1-0220 1-0240 1-0260 1-0280 1-0300

224. Salinity of the Ocean. — As a rule the surface
water of the ocean is salter than that lying beneath, the
fresher water below being denser in its position, because its
temperature is much lower and the pressure upon it greater.
In those parts of the ocean where the rainfall is heavy the
surface water is always being freshened, and its salinity is
consequently lowered. The map (Plate VIII.) shows the
freshened regions by a lighter tint of pink, the figures
referring to the density. There is one band of compara-
tively fresh water in the rainy equatorial region of each
ocean, and fresh zones around the melting ice of the Arctic
and Antarctic coasts. Seas and ocean shores situated in
regions of great rainfall, or receiving large rivers, are also
usually fresher than the average. The saltest water occurs
in the regions of greatest evaporation and least rainfall,
pre-eminently the Mediterranean and Red Sea, and in the
trade -wind regions of the open oceans. The track of
fresher water along the west coast of Africa and of South
America is probably produced by upwelling in consequence
of off-shore winds (§ 240, 241). The way in which the very
salt water extends close to shore along the coast of South
America, between the mouths of the rivers Amazon and La
Plata, is accounted for by the westward trade-wind drift of
surface water. All the salts dissolved and invisible in the
whole ocean would suffice to form a solid crust 170 feet
thick over the entire sea surface. J& '

225. Absorbed Gases in Sea- water. — All atmospheric
gases are to some extent dissolved by sea- water. The
amount absorbed depends conjointly on the pressure of the
gas (being greater as the pressure is greater), the tempera-
ture of the water (being greater as the temperature is lower),
and the nature of the gas itself. Under the same pressure
oxygen is nearly twice as soluble in water as nitrogen ; but
nitrogen exerts on the sea surface four-fifths, and oxygen only

1 66 The Realm of Nature CHAP.

one-fifth, of the whole atmospheric pressure ; thus sea-water
in contact with air absorbs twice as much nitrogen as oxygen.
Still the proportion of oxygen in the air which is breathed in
the water by sea creatures is twice as great as that in the
atmosphere. At the average pressure and 32° F., 100 parts
of water by volume absorb from air 1-56 parts of nitrogen
and 0-82 of oxygen ; at 70° F. the quantities absorbed are
i-oo part of nitrogen and 0-52 of oxygen, and so on in
inverse proportion to the temperature. The amount of ab-
sorbed nitrogen in sea-water does not change after it has
sunk below the surface ; thus by finding how much nitrogen
is dissolved in any part of the ocean one can calculate the
temperature the water originally had at the surface, and also
the amount of oxygen which must have been absorbed at
the same time. The creatures living in the sea, and dead
animals and plants decaying, diminish the amount of oxygen,
so that the full quantity which was absorbed by the sea-
water is hardly ever found in samples taken from a con-
siderable depth. If any part of the ocean were quite
stagnant, and never renewed from the surface, the dissolved
oxygen would in time become exhausted. The chemists of
the Challenger and of other deep-sea expeditions have
never found a sample of sea-water free from oxygen, and
this is a sure indication that all parts of the ocean are
moving, however slowly. Very little carbonic acid is ab-
sorbed from the air, on account of the small proportion of
that gas in the atmosphere ; but the oxygen, when used
up as described above, is changed in great part into car-
bonic acid, which remains in the sea- water chemically
combined with the carbonates.

226. Pressure and Sea -water.— Professor Tait has
found by experiment that sea -water is very slightly com-
pressed by its own weight. Under the surface the pressure
increases about I ton per square inch for every mile of
depth. At the bottom of the deepest part of the ocean the
vast pile of water exerts a pressure more than 500 times
that of the atmosphere on the surface, or about 4 tons to
the square inch. At this depth 11,000 cubic feet of sea-
level air would be squeezed into 22 cubic feet ; but 1 1,000

x The Hydrosphere 167

cubic feet of sea -water would only be reduced to about
10,000 cubic feet, the density being only slightly increased.
If sea -water were absolutely incompressible the oceans
would be about 200 feet deeper than they actually are.
Sea-water is perfectly elastic. When pressure is removed
from a portion it returns at once to its original volume.

227. Heat and Sea-water. — When sea- water is warmed
it expands, steadily diminishing in density as the tempera-
ture rises. The specific heat is less than that of fresh
water, for while 100 units of heat (§65) are needed to
raise 100 Ibs. of pure water from 32° to 33°, 93-5 units can
raise the temperature of 100 Ibs. of sea -water (density
1.0260) through the same range. Sea-water conducts heat
better than fresh water, so that the heat of the surface
penetrates to a greater depth in the sea than in a deep lake
in the same time. When heat is removed from sea-water,
i.e. when it is cooled down, its density increases steadily,
for its maximum density occurs below the freezing point.
The chilled surface layer in contact with a very cold atmo-
sphere always sinks, unless it is much fresher than the
lower layers, which only happens in polar regions or near
shore. Sea-water freezes about 28° F., or at a temperature
4° lower than fresh water, and in the process of freezing
most of the salts separate out, so that the ice formed is
nearly fresh, while the water yielding it is left much salter.
All the salts are not excluded equally, the ice retaining a
larger proportion of sulphates than of chlorides.1 Sea-water
ice has a soft and spongy texture, full of cavities containing
residues of unfrozen brine, and the water produced by
melting it is consequently bitter and unwholesome.

228. Circulation of Deep Fresh Lakes. — When the
Sun shines on a deep lake in summer the upper layer of
water is warmed, and expanding maintains its position,
heat being passed on to the lower layers by the slow pro-
cess of conduction. There is no tendency to transmit the
heat by descending hot currents as in the sea. When
winter sets in, the surface water cools rapidly by radiation,
and contracting, it becomes denser and sinks allowing
warmer water from beneath to take its place. This process

1 68 The Realm of Nature CHAP.

goes on just as in the sea, until the lake cools down to
39° F., but at that temperature fresh water attains its
maximum density, and the similarity to the cooling of the
sea ceases. Further cooling of the upper layer makes the
water expand, and therefore it remains at the surface until
the temperature falls to 32°, when it solidifies to form a
sheet of ice. Ice is not formed as long as any of the water
in the lake is warmer than 39°. The heat from the water
under the ice is conducted upward very slowly, so that the
whole mass of water can only become solid in very shallow
lakes when the winter is long and severe. A deep fresh-
water lake in a region where the summers are warm is
rarely altogether cooled down below 39° during winter,
unless the season is very severe, hence the common
observation that deep lakes do not freeze. In calm
weather the study of the Swiss lakes, carried on by Pro-
fessor Forel and others, shows that the upper 5 fathoms
of water may be affected by the diurnal range of air tem-
perature between day and night, but the annual change of tem-
perature between summer and winter exerts some influence
to a depth of from 50 to 80 fathoms. Beneath that depth the
temperature remains unchanged all the year round at 39°.
A steady wind blowing in the direction of the length of a
long narrow lake (§ 240) may, however, mix the water so
thoroughly that the temperature is made practically uniform
from surface to bottom at any season of the year.

229. Phenomena of Sea-lochs. — Fjords or sea-lochs
are miniature enclosed seas of great depth, surrounded on
all sides but one by lofty mountains, and barred off from
the deep sea outside by a sill rising to within a few fathoms
of the surface, as shown in Fig. 55, § 339. The sea-lochs
of Scotland have been studied in some detail by Dr. John
Murray and the author of this book.2 The lochs are filled
with sea-water much freshened on the surface by numerous
small mountain torrents, but scarcely less salt at the bottom
than the open sea. In summer the surface temperature is
greatly raised, but at the bottom, which is cut off from
tidal influence, the temperature falls steadily, and comes
to a minimum when the surface is warmest. As winter

x The Hydrosphere 169

advances the surface cools rapidly, and since the water is
comparatively fresh it continues, in spite of its increasing
density, to float on the warmer sea-water below, and some-
times freezes, while at a depth of a few fathoms the tem-
perature of the salt water may be more than 45°. The
heat of summer is conducted downward so slowly that the
highest temperature of the year is reached at the bottom
when the surface is at its coldest in January or February ;
the seasons at the bottom of Upper Loch Fyne or Loch
Goil, for example, being six months behind those at the
surface. In the far deeper basins of the fjords of Norway
seasonal changes of temperature penetrate to about 200
fathoms, but no farther.

230. River and Sea- water. — When a large swift river
flows directly into the sea it spreads out over the surface for
many miles, floating on the salt water, which it freshens
superficially. The form of the fresh stream may often be
traced by the contrast of its colour with the clear blue of the
ocean. Off the mouths of the Amazon and the Orinoco, for
example, muddy fresh water is found floating on the surface
of the sea several hundred miles from land. The Sun's heat
rapidly evaporates the floating fresh water, and salt from
below diffuses up and increases its density, thus enabling
it to mix with the mass of the ocean, a process assisted
by wind and waves. When rivers pour directly into a sea
affected by tides it may happen that the current of fresh
water is only slackened, but not reversed, by the rising tide.
In the Spey, which is the swiftest river in Britain, salt sea-
water is forced, like a dense fluid wedge, for a considerable

'distance up the bed of the river by the rising tide, and
lifts the fresh stream to a higher level, so that perfectly
fresh water is found on the surface, separated by a brackish
layer a foot or two thick from the salt water below. The
salt wedge is withdrawn by the ebb-tide, and the river
current resumes its rapid flow to the sea.3 Rivers which
enter the sea directly have little influence on the salinity
and temperature of the deeper layers of sea-water.

231. Estuaries and Firths. — In the La Plata, the
Thames, the Severn, the Forth, the Tay, the Garonne, and

1 70 The Realm of Nature CHAP.

other rivers where the fresh water meets the sea gradually
in a narrow inlet, the wedge-like action of the salt water at
high tide is scarcely perceptible. The effect of the tidal
currents sweeping to and fro in the funnel-shaped channel
is to mix the river and sea-water together as if they were
being shaken in a bottle. In such an inlet as that of the
Thames or the Firth of Tay, where the river is large, the
water is found to grow rapidly salter from river to sea, the
surface is much fresher than the lower layers, and the
change of salinity between high and low tide is very marked.
This form of river entrance is appropriately called an Estuary.
When, however, the inlet is very large compared with the
river, and when there is no bar at the opening, the estuar-
ine character is only shown at the upper end. In the Firth
of Forth, for example, the landward half is an estuary, but
in the seaward half the water has become more thoroughly
mixed, the salinity is almost uniform from surface to bottom,
and increases very gradually toward the sea. The result is
that the river-water meets the sea diffused uniformly through
a deep mass of water scarcely fresher than the sea itself, so
that the two mix uniformly, and the sea becomes slightly
freshened throughout its whole depth for many miles from
land.

232. Temperature in River Entrances. — The tempera-
ture of a river in the temperate zone follows that of the land
over which it flows, and is thus subject to considerable
variations between day and night. River-water, unless it
flows very rapidly, can never become colder than 32°; but
in summer its temperature may be raised to a very high
degree if there is little rain and strong sunshine. Rain
lowers the temperature of rivers in summer, especially
when it floods torrents descending from cold mountains.
Such rivers are warmer than the sea in summer and cooler
than the sea in winter. In an estuary or firth in summer
the temperature is highest on the surface and in 'the river,
diminishing at first very rapidly, but afterwards more slowly
as the sea is approached. In autumn, on account of the
more rapid chilling of the land, the temperature becomes
nearly uniform in river, estuary, and sea, and from surface

The Hydrosphere

171

to bottom. In winter the water is coldest on the surface
and in the river, growing warmer, at first rapidly, and then
more gradually, toward the sea. In spring, on account of
the land heating up more rapidly, the temperature becomes
once more uniform throughout.4 Fig. 32 shows the actual

FIG. 32. — Temperature of surface water at different seasons along the middle line
of the Firth of Forth. Distances from Alloa are shown in miles horizontally
from left to right ; temperature in degrees Fahrenheit is shown vertically.

distribution of temperature along the Firth of Forth, from
Alloa to the sea, at four typical seasons, on the surface.

233. Surface Temperature of the Ocean. — The iso-
thermal lines on the ocean in Plate XV. represent the
average temperature of the surface water for the year. Al-
though more easily heated than fresh water (§ 227), the sea
surface has a less range of temperature than that of fresh
lakes. This results in part from the greater clearness of
sea-water, in part from its distance from heated land. The
average temperature of the surface of the open ocean varies
less than i° between day and night, but between summer
and winter there is a range of from 5° to 10°. Along a
line, drawn from Newfoundland to Iceland, the annual'
change of temperature between the colclest rnonth, February,
and the hottest month, August, is as much as 20°; but this
is due less to the heating and cooling of water than to a
seasonal change in direction of warm and cold currents
(§ 242). In the tropical zone the sea surface has a temperature
higher than 80° for the whole year. This zone of very hot
water is widest in the Indian Ocean and narrowest in the
Atlantic ; and in all three oceans it is wider on the western
than on the eastern shores. The temperature falls very
uniformly toward the south, reaching 40° F. about latitude
48° S. south of Africa, but not until latitude 58° S. south of

172 The Realm of Nature CHAP.

New Zealand. In the Southern Ocean there is practically
no annual change of temperature, the water growing steadily
colder toward the Antarctic ice at all seasons. Toward the
north the ocean grows cooler more gradually, 40° being
found in summer only in the Arctic Sea, but in winter between
New York and the Lofoten Islands, and between Japan and
Alaska. As a general rule the sea surface on the west
coasts of the southern continents is colder, and on the west
coasts of the northern continents warmer, than on the east
coasts in the same latitudes (§ 241). The northern half of
each ocean is also warmer than the southern half at all
seasons. Enclosed tropical seas have the highest tempera-
ture of any water surfaces in the world. In the Red Sea
readings of from 90° to 100° F. have been reported.

234. Polar Seas. — The Arctic Sea, lying in the coldest
region of the globe, appears to be frozen over every
winter, and the ice, measuring from 2 to I o feet in thickness,
is only partially dissipated in summer. Ice first forms
along the shore-line, remaining attached to the land as a
flat shelf, termed the ice-foot, which is often strewn with
boulders and shattered rocks from the cliffs that tower
above it. Thence the surface gradually freezes across.
When the winter covering of the ocean breaks up, ice-
islands, or floes, some of which have been seen 60 miles
long, drift away with the wind. Open lanes and wide
expanses of water thus appear in summer across the Arctic
Sea, but these are liable to be closed at any time by a
change of wind driving the floes together. Two floes in
collision present a grand and terrifying scene, the ice crack-
ing and rending with a noise louder than thunder, while the
shattered sheets are piled up one above another to a great
height, forming irregular hummocks or ice-hills. Sir George
Nares, in the last great North Polar expedition, found the
ice-floes in what he called the Palasocrystic Sea more than
150 feet thick, and he estimated that some of them were
500 years old. The water in which the floes float has the
temperature of melting sea ice (about 28°), and the lower
layers are usually considerably warmer. Indeed, in Polar
regions there are often alternate layers of cold and warm

x The Hydrosphere 173

water, one above another, the greater salinity of the warmer
water making its density greater than the colder but fresher
water above. A temperature curve of such a region
("Atlantic 71° N. lat.") is shown in Fig. 33. The ends of
great glaciers reaching to the sea break off in the water
and float away in summer as icebergs (§ 338). In the
Arctic regions the icebergs are lofty pinnacled masses,
often resembling cathedrals or castles several hundred feet
in height, with a covering of dazzlingly white snow, but
showing the true ice-colour of intense blue in their cracks
and caves. Lofty as these icebergs are, we know that as
ice has a density of about 0-9, only one-ninth of its volume
floats above water. The Antarctic icebergs are usually
flat-topped and table-like, but are far larger and of a deeper
blue colour than those of the Arctic regions. 35^"

235. Temperature of Ocean Depths. — The hot surface
water in the tropical zone is merely a film covering a vast
depth of cold water. Even although the surface is at 70°
or 80°, temperature of 40° or less is found at the depth of from
300 to 400 fathoms in almost all parts of the ocean. The

FIG. 33. — Curves of vertical distribution of Temperature in the ocean. Tempera-
ture is shown along the top ; depth down the side. The middle curve, for
example, shows 80° at the surface, 60° at 100 fathoms, 50° at 200 fathoms, 40°
at 600 fathoms, and 35° at the bottom.

fall of temperature is consequently very rapid from the sur-
face down to 400 fathoms in the tropics ; but much less
abrupt in the cooler regions to the north and south. Below
400 fathoms the fall of temperature to the bottom is every-
where very slight and gradual (Curve, "Pacific 5° N. lat.,"
Fig. 33), and no matter how great the depth may be the

174 The Realm of Nature CHAP.

bottom temperature of the open ocean remains near the
freezing point of fresh water. Five-sixths of the mass of the
ocean has a temperature under 40° F., so that taken as a
whole the hydrosphere is a body of cold water, its average
temperature being probably about 38° or 39°. The prevail-
ing low temperature of the hydrosphere is explained mainly
by the great surface of water exposed in the Southern
Ocean to the influence of the cold Antarctic ice-continent,
and in less degree to the still colder winter weather within
the Arctic Circle. The surface drift of warm salt water
carried into the Southern Ocean from the north grows
gradually cooler and therefore denser, and sinks about
latitude 50° S. About this latitude also the comparatively
fresh and cold water drifting northward from the Antarctic
regions grows salter and sinks on account of the consequent
increase of density. The sinking water appears to be
drawn back by slow and massive movements to north and
south, thus maintaining the circulation of the ocean to its
greatest depths.

236. Temperature in Enclosed Seas. — Except in
polar regions the temperature at the bottom of the deep
ocean is much lower than the average winter temperature
of the air at sea-level ; but this is not the case for deep
enclosed seas. The common form of enclosed seas is
that of a basin, often descending to oceanic depths, but
barred off from the ocean by a sill. The Red Sea, for
example, is separated from the Indian Ocean at the Strait
of Bab-el-Mandeb by a sill rising to within 200 fathoms of
the surface, while it attains a depth of 1200 fathoms near
the centre, and the Indian Ocean in the Gulf of Aden is
still deeper. In the Red Sea the temperature at the surface
varies from over 85° in summer to about 70° in winter.
At the hottest season the rate of cooling is comparatively
rapid to a depth of 200 fathoms, where the temperature is
70°, and from that level right down to the bottom the
temperature remains uniform all the year through. The
basin of the Red Sea is thus filled up to the lip with
uniformly warm water, whereas, as shown in Fig. 34, the
water of the Indian Ocean, nearer the equator, and with

x The Hydrosphere 175

the same surface temperature, sinks to 70° at about 200
fathoms, and falls as low as 37° at 1200, where it is pre-
vented from entering the Red Sea basin by the ridge. The
surface water in the Red Sea is densest when its tempera-
ture is lowest in winter, and the dense layers at 70° tem-
perature sink to the bottom, so that the whole basin below
the level of the barrier assumes and maintains the lowest
average winter temperature of the air above. The hotter
water in summer being less dense on account of its expan-
sion, though it contains more salt, remains floating on the

Indian Ocean

Red Sea

FIG. 34. — Temperature Section of the Red Sea and Indian Ocean ; showing the
action of a barrier in separating bodies of water at different temperatures.
The shading is darker as the temperature is lower. Not drawn to scale.

surface, and its heat passes down only by the slow process
of conduction. The Mediterranean furnishes another in-
stance of the same distribution of temperature. The sill
separating its basin from the Atlantic is 190 fathoms below
the surface, and the water on it is at 55°, a temperature
which prevails to the bottom, of the Mediterranean, while
in the Atlantic the temperature falls as low as 35° at the
same depth.

237. Circulation of Seas by Concentration. — The
great evaporation in the Red Sea raises the density of its
surface water (at 60° F.) to 1-0300, and the salinity is 4
per cent. The level of the sea is lowered by evaporation
to the extent of from 10 to 25 feet a year, and a surface
current of the fresher but equally hot water of the Indian
Ocean is consequently always pouring in. If there were no
return current of dense salt water, it is calculated that the

176 The Realm of Nature CHAP.

Red Sea would become a mass of solid salt in less than
2000 years. Since there is no. perceptible change in its
salinity, it is certain that a deep undercurrent of salt water
passes out through the Strait of Bab-el-Mandeb sufficient to
carry back to the Indian Ocean all the salt received from
it. The circulation of the Mediterranean is carried on in
the same way, as the rainfall it receives is only equal to
about one-quarter of the evaporation from its surface, and
its water, although of higher salinity than the neighbour-
ing Atlantic, is not growing salter. The outflowing current
through the Strait of Gibraltar underneath the inflowing
fresher current has been observed, and the deep water of
the Atlantic in that neighbourhood is perceptibly wanned
and increased in saltness by the outflow.

238. Circulation of Seas by Dilution. — The Black
Sea is a deep basin cut off from the Mediterranean by the
shallow Bosphorus, the Sea of Marmora, and the Dar-
danelles. This sea contains only about 2 per cent of salts,
its water being very much freshened by the Don, Danube,
and other great rivers which flow into it, supplying more
water than is removed by evaporation, and raising its level
about 2 feet higher than that of the Mediterranean. A
steady surface outflow of brackish water from the Black Sea
consequently sets through the Bosphorus ; but a slower
stream of very salt Mediterranean water forces its way
along the bottom into the Black Sea, so that the sea is not
permanently freshened. The cause of the undercurrent of
salt water between seas of different salinity is that in order
to produce equilibrium the pressure exerted by two adjacent
columns of a fluid must be the same. A column of salt
water exerts the same pressure as a column of fresh water
higher in proportion to the difference of salinity. But (§38)
water cannot stand at a higher level beside water at a lower
level, and the fresher water pours over the surface of the
salter column, upon which the pressure is thereby increased
and the undercurrent is produced in order to equalise
matters. As long as the supply of fresh water is kept up
there can be no equality, and thus the circulation con-
tinues. The Baltic Sea has a somewhat similar circulation.

x The Hydrosphere 177

239. Wind- waves. — Difference of barometric pressure
over a large sheet of water causes a slight change of level
and sets up a to-and-fro surge, known as a seiche in the
Swiss lakes, without the action of wind. The air, being
more mobile, obeys the direct touch of solar energy much
more readily and rapidly than water, to which motion is,
however, imparted by wind. Part of the water surface
yields to the stress of wind striking it obliquely, and is
depressed, thereby ridging up the neighbouring portions
and originating a wave, the form of which advances as a
line of rollers before the wind. OnJyJtheJform advances,
for while the particles of water in the crest of a wave are
moving rapidly forward, those in the trough move back to
almost exactly the same extent. Thus rollers merely lift
and lower the vessels that float upon them. Water being
an elastic substance continues to swing up and down as a
swell after the wind which produced the motion has died
away, just as a pendulum continues to swing after the hand
setting it in motion is withdrawn. Waves may be trans-
mitted from a great distance, and as wind is always blowing
somewhere the surface of the ocean is never quite at rest.
When a wave enters gradually shallowing water the lower
part is retarded by friction, and the upper part sweeps for-
ward more rapidly. The wave becomes steeper and shorter,
and finally the top curves over in a hollow sheet of clear
water, which breaks with a roar into foam and spray, the
roller becoming a breaker. Sailors are in the habit of
speaking of waves as " mountains high," but this is only a
metaphor. The highest wind-waves that have been mea-
sured have an amplitude of only 50 feet from trough to
crest, and a length of about a quarter of a mile between
successive crests. Earthquakes raise waves of much greater
height and destructive power than either tide or wind. The
wave form travels over the sea at a rate depending on the
size of the wave and the depth of the water, the maximum
speed being about 80 miles an hour. At the depth of 100
fathoms the greatest waves produce a movement too slight, as
a rule, to affect anything but the finest mud, and probably wave-
motion never penetrates to as great a depth as 5 oo fathoms.

N

178 The Realm of Nature CHAP.

240. Circulation of Water by Wind. — Apart from
producing waves, the wind slips the top layer of water
before it as one might slip a card from the top of a
pack, and although it can act only on a very thin film
a new surface is constantly exposed, and a steady breeze
causes a great surface drift. Mr. J. Aitken appears to have
been the first to point out the importance of this action in
disturbing the deeper layers of water. Dr. Murray, by a

series of temperature ob-
,A servations on Loch Ness,
showed how rapidly wind
acting in this manner on
the surface of a deep lake
could completely alter the
distribution of the water.5

1*IG. 35. — Circulation of water by wind. . .

The light lines and figures show dis- The explanation of his

tribution of temperature before, and nKQ«rvptinnc: cppmc rr» \\e-

the dark lines and figures distribution Observations S

of temperature after, the wind has been as follows I On a Calm

blowing in the direction of the long summer day ^ ]ake QQn_

tains a surface layer about

1 5 fathoms deep, the temperature of which is from 60° to
50°, floating upon 100 fathoms of water, the temperature
of which is from 50° to 40°. When strong wind blows
steadily along the length of the lake from A to B the
surface water is driven toward B, where the wind heaps it
up, but the greater pressure of the heaped-up water causes
the lower layers at B to move off toward A, and thus the
whole end of the lake-basin at B is filled with the warm
water that had been resting on the surface, while the cold
water formerly filling the depths rises against the shore
at A, as represented by the arrows. If the wind lasts long
enough the water will be thoroughly mixed and the tempera-
ture made uniform throughout (§ 228).

241. Effect of On-shore and Off-shore Winds. —
Bathers know that in summer the sea is colder when the
wind is blowing from the land than when it is blowing from
the sea. The reason is that the wind blowing from the sea
(an " on-shore " wind) drives the surface water, which has
been heated over a wide area, in toward the shore, on which

x The Hydrosphere 179

warm water becomes banked up to a considerable depth,
displacing the cold lower water, which slips seaward as an
undercurrent (B, Fig. 35). During a prevailing sea-wind
the water along the shore assumes what may thus be called
an on-shore condition, just as by blowing steadily across
a milk dish one might drive the cream to one side, and
even blow it up on the shelving lip, completely displacing the
milk on that shallow coast. A wind from the land in like
manner drives the warm surface water seaward, and colder
water from a great depth wells up to take its place (A, Fig.
35), this being characteristic of an off-shore condition.
This enables us to understand how the permanent winds of
the Earth which blow steadily off shore (like the trade winds
from the west coasts of Africa and South America, § 179)
cause cold water to well up from great depths. The up-
welling off the coast of south-western Africa and off the
coast of Morocco explains the exceptionally low sea surface
and air temperatures observed in these neighbourhoods,
and similar conditions are found on the west coasts of
Australia and South America. Where the prevailing winds
blow against the land, as on the north-east of South
America into the Caribbean Sea, and toward Western
Europe, the sea assumes a permanent on-shore condition,
the warm surface water from the tropics being piled up
against the land, while the colder deep water natural to
the locality slips away seaward. The effect of the pre-
vailing winds of the world is to set up a general skimming
of the ocean from the equator poleward, sweeping the
warm surface water away to one side tand allowing cold
water from the depths to rise up, completing the vertical
circulation.

242. Wind and Ocean Currents. — In a strong gale the
wind blows off the crests of the waves in spray or spindrift,
and even a moderate breeze sweeps forward a thin layer of
surface water over the ridged surface of the sea, giving rise
to what is called a surface drift. The currents of the Indian
Ocean and of the sea off the west coast of Central America
change twice a year with the changing of the monsoons, and
it is recognised that these currents are produced solely by the

180 The Realm of Nature CHAP.

wind. Ocean currents are very different from surface drift.
They are usually narrow tracts of the sea surface, the water
of which flows steadily and strongly in a definite direc-
tion, passing through the rest of the sea without appreciable
mixing, as a river runs through a meadow. Some of these
ocean rivers flow steadily in a constant direction at the rate
of nearly 4 miles an hour ; thus it is matter of importance
to sailors to map out the ocean so that they may avoid
or take advantage of the currents in making a passage.
Solar energy in one form or another is undoubtedly the
power that keeps the whole system of oceanic circulation in
motion, and the rotation of the Earth (§ 89) together with
the form of the coast-lines of the continents direct the flow
of currents. Sun-power acts on the hydrosphere (a) by
"raising the temperature in the tropical regions far above
that in the polar zones, thus causing expansion and altering
the level ; (£) by causing great evaporation in the tropical
regions, great rainfall in equatorial regions and moderate
rainfall in the temperate zone, thus altering both level and
density; (<r) by setting up the whole system of winds. Some
difference of opinion exists as to the chief cause of oceanic
movement, but it is usually allowed that the most powerful
is the wind. All three, however, act together and reinforce
each other. If the student compares the map of ocean
currents (Plate XVIII.) with those of temperature, of salinity,
and of prevailing winds (Plates XV. VIII. V. VI.), he will
see that the currents circulate in the same way as the winds
and around nearly the same centres, which lie close to the
regions of maximum sea - temperature and salinity. All
ocean currents are more or less irregular in form and speed ;
they usually flow as parallel streams separated by spaces of
still water, and vary in position and strength, as the winds
do, with the time of year. Plate XVIII. should be specially
referred to in reading the following paragraphs.

243. Equatorial Currents of the Atlantic. — The trade
winds blowing from the west coast of Africa drive the
surface water before them in rapid currents. The North
Equatorial Current, sweeping along the north-west coast
of Africa past the Canary Islands, turns toward the west

x The Hydrosphere 181

about the latitude of Cape Verde, and while part of it is
driven by the north-east trade winds into the Caribbean
Sea, most of the current sweeps north-westward (as ex-
plained in Ferrel's Law), outside the West Indies, toward the
coast of North America. The South Equatorial Current,
originating in the Benguela Current of cool water, flows
northward at first. In the latitude of the Congo it sweeps
westward across the ocean and divides into two branches
off the wedge-shaped front of South America. One branch
(as explained by Ferrel's Law for the southern hemi-
sphere) turns southward along the coast, and is known as
the Brazil Current ; and getting within reach of the brave
west winds it is drifted east again to rejoin the Benguela
Current. The other branch continues on its westerly
direction and is driven northward by the south-east trades,
most of it flowing into the Caribbean Sea. Along the
north-east coast of South America there is a heaping up of
water, produced by the convergence of the two great equa-
torial currents, and this does not appear to be fully com-
pensated for by vertical circulation. Some of the banked-
up water escapes eastward on the surface along the rainy
zone of the equatorial calms, forming a narrow counter-
current between the west-flowing Nor.th and South Equa-
torials. Near the coast of Africa this Counter Equatorial
Current^ consisting of extremely hot water of slight salinity,
and known as the Guinea Current, sweeps along the north
shore of the Gulf of Guinea, and is deflected southward
by the coast to rejoin the South Equatorial.6

244. The Gulf Stream. — The level of the Caribbean
Sea and Gulf of Mexico is raised considerably by the hot
surface water continually pouring in from the south-east.
Off the mouth of the Mississippi it is about 3 feet higher
than off New York — an effect which may, however, be due
in part to the attraction of the land (§ 252). The Gulf
Stream forced out of this reservoir through the Strait of
Florida is a river of salt and very warm water (surface
temperature 81°), 50 miles wide, 350 fathoms deep, and
flowing at the rate of 5 miles an hour. On emerging from
the Strait it is swept to the north close along the American

1 82 The Realm of Nature CHAP.

coast by the branch of the North Equatorial Current, which
had passed outside the West Indies and through the
Bahamas. The Gulf Stream sweeps the bottom clear of
mud not only in the Strait but for a considerable distance
northward. As it flows on, it grows wider and shallower ;
off Cape Hatteras it curves away from the American coast
and coming within the range of the prevailing south-
westerly winds, it is carried eastward across the Atlantic,
spreading out like a fan and growing cooler as it flows.
The Gulf Stream passes to the south of the Grand Banks
of Newfoundland with a velocity of about ij miles per
hour, and its rate gradually diminishes to about 4 miles a
day in the general North Atlantic drift. This drift of
comparatively warm water forks into three, diverging toward
the coast of Spain, the British Islands, Norway, and the
south-eastern coast of Iceland, stranding driftwood on that
treeless island. The surface water of t*he tropics is thus
being steadily poured into the temperate North Atlantic,
where it drives the cold deep water toward the south, and
gives rise to the highest temperature at great depths found
in any part of the open ocean. The temperature of 40°
occurs as deep as 900 fathoms off the west of Scotland,
and seldom deeper than 300 fathoms in the tropics. This
is the source which supplies the south-west wind with
heat and moisture to modify the climate of Western
Europe.

245. Polar Currents of the Atlantic. — Careful study
of the drifting of ice-floes in the Arctic Sea gives some
ground for believing that a current sets straight across from
near the New Siberian Islands on the coast of Asia toward
Arctic North America. Dr. Nansen has resolved to set
out in 1892 on an expedition to the North Pole, believing
that this current will drift his vessel to the point which so
many explorers have hitherto attempted in vain to reach.
A cold current, carrying icebergs in summer, when the
frozen sea breaks up, flows south from the Arctic Sea
between Spitzbergen and Greenland, strengthened by a
cold drift from the north coast of Asia. It passes along
the north coast of Iceland, where it strands driftwood

x The Hydrosphere 183

from the Siberian rivers, and as the East Greenland
Current flows more rapidly, under the influence of prevail-
ing north-easterly winds, along the east shore of Green-
land, causing that side of the great ice-covered peninsula
to be much colder and less accessible than the western.
The Labrador Current is a more important cold stream,
driven also by the northerly winds induced by the northern
low-pressure region of the atmosphere (§§ 194, 197), and
flowing southward along the west side of Baffin Bay,
past the coasts of Labrador and Newfoundland. It
meets the northern edge of the Gulf Stream off the Grand
Banks of Newfoundland. Many geologists believe that
this encounter led to the formation of the Banks, for
the icebergs carried by the Labrador Current are melted
on entering the Gulf Stream, and drop the stones and
mud which were frozen up in them. The mingling of
cold and warm currents undoubtedly produces the fogs
for which this region is famous. Being comparatively
fresh, the density of the cold Labrador Current is not
greater than that of the Gulf Stream, by which it appears
to be deflected along the coast of North America, where it
is known as the Cold Wall. It disappears from the surface
off Cape Hatteras, having partly mixed with the Gulf Stream
and in part sunk under the less dense because warmer
water. Recent observations point to the possibility that
the cold current cuts horizontally through the Gulf Stream,
like a paper-cutter through the leaves of a book, and mixes
with the mass of Atlantic water. The limits reached by
icebergs drifted from the north and south are shown on
Plate XVIII., illustrating how the warm currents off
Northern Europe keep the sea clear from this danger.
The cool water of the Benguela Current is partly supplied
by upwelling from beneath, but the steady flow of the
current is maintained by cold streams sweeping north-
eastward from the Antarctic regions.

246. Circulation of the Atlantic. — The Gulf Stream
is often spoken of as if it were a phenomenon by itself ; but
it is really only part of a great system of surface circulation,
the water whirling as if stirred in the direction of the hands

184 The Realm of Nature CHAP.

of a watch in the northern Atlantic, and as if stirred in the
opposite direction in the southern part of the ocean. The
centre of each whirl is nearly at rest, and immense quan-
tities of floating sea-weed accumulate, especially in the North
Atlantic, where the calm weed-hampered water is known as
the Sargasso Sea. Mr. A. W. Clayden has devised an
interesting model, in which a current of air sets up real
currents on a water surface formed like the Atlantic. So
far as can be gathered from the imperfect data more water
is driven poleward by this circulation than returns in surface
currents. Much of the surface water sinks off the British
Islands (§ 244) south of the Wyville- Thomson Ridge
(§258). Over this ridge the Atlantic water streams so
strongly that the bottom is swept clear of mud to the depth
of 500 fathoms. North of the Ridge the basin of the
Norwegian Sea is filled up to its lip with ice-cold water from
the Arctic region which finds no exit southward.

247. Currents of the Pacific Ocean.— The Pacific
Ocean, on account of its vast extent and its remoteness
from great trade routes until within recent years, has not
been so carefully studied as the Atlantic. It is known,
however, that the general system of its circulation is the
same, and the map should be carefully studied in order to
recognise the similarities. The Bight of Panama, extend-
ing along the west coasts of Central America and of
the north of South America, serves, like the Gulf of
Guinea, as the starting-place of the great equatorial current
system. The south-east trade wind produces the Peru
Current as a stream of cool water raised by the off-shore
winds, precisely like the Benguela Current of the Atlantic.
This stream, deflected westward by the Peruvian outcurve
of the coast, sweeps as a South Equatorial Current past
the Galapagos Islands on the equator, giving them a cooler
climate than any other equatorial land. Setting westward
before the steady trade winds, it sends off branches to the
south, which wind amongst innumerable island groups, and
ultimately reunite under the influence of the brave west
winds, and drift eastward to rejoin the Peru Current. The
main branch of the South Equatorial Current splits at New

x The Hydrosphere 185

Guinea, a small part passes through Torres Strait to the
Indian Ocean, but the main body streams through the Malay
Archipelago toward the Philippine Islands. Toward this
goal the North Equatorial Current is also driven by the
north-east trade wind, and as in the Atlantic, the_j>ilmg
up of warm surface water against the chain of islands gives
rise to a strong Counter Equatorial Current, which sets
straight eastward across the Pacific, along the line of
equatorial calms, into the Gulf of Panama. The South
Equatorial Current streams from the South China Sea into
the Indian Ocean in winter, when the north-east monsoon
is blowing, and mixes with a cold current flowing south
from the Yellow Sea. But in summer, during the south-
west monsoon, the pressure of water in the China Sea is
increased by tributary currents from the Indian Ocean, and
acts in many respects like the Gulf of Mexico. The
extremely hot water (surface temperature 85°) escapes
between Luzon and Formosa as a broad salt river. As it
sweeps past the east coast of Japan, and begins to widen
and thin out, the name Kuro Siwo or Black Stream is
given it, from the deep colour of its clear water. The
Kuro Siwo comes into range of the prevalent south-west
winds, and, like the Gulf Stream, is carried at a diminishing
rate eastward across the ocean, merging into a general
surface drift, which washes the coast of Alaska and British
Columbia. The North Pacific has its temperature increased
throughout a great depth in this way. Cold currents
resembling those of Greenland and Labrador, but much
smaller in volume, set south from Bering Sea along the
coast of Kamchatka and Sakhalin, passing between Japan
and the Kuro Siwo like a cold wall. This cold wall is
greatly increased by the north-eastern monsoon, and seems
to prevent the oceanic part of the north equatorial current
from entering the China Sea, by turning it aside to supply
the Kuro Siwo, which would otherwise cease to flow at that
season.

248. Currents of the Indian Ocean. — The south-east
trade wind blows the surface water westward off the coast
of Western Australia, causing an upwelling of colder water

1 86 The Realm of Nature CHAP.

similar to the Benguela and Peru Currents. The South
Equatorial Current of the Indian Ocean is reinforced by
affluents from that of the Pacific between Australia and
Java, which give to the eastern shore of the ocean a par-
tially on-shore character. Turning as it flows west, the
South Equatorial Current washes the east coast of Mada-
gascar, and turns south in several branches, which are drifted
back to the West Australian Current by the brave west
winds. A strong drift of warm water passing southward
along the Mozambique Channel is known as the Agulhas
Current off the south of Africa, from the fact that the
Agulhas Bank turns the bulk of the stream from its south-
westward direction back to the east. A narrow stream of
the Agulhas Current rounds the Cape and joins the Ben-
>guela Current in the Atlantic. In winter, when the north-
east monsoon is blowing, a North Equatorial Current
appears, eddying westward round the Bay of Bengal and
Arabian Sea, and setting southward along the coast of
Africa to join the Agulhas Current. At this season there
is also a well-marked Counter Equatorial Current across the
ocean from Zanzibar to Sumatra, rather to the south of the
equator. During the south-west monsoon the currents in
the northern part of the Indian Ocean are reversed. The
Somali coast assumes an off-shore condition (§ 241), with
strong upwelling of cold water, and the currents flow in
eddies eastward round the Arabian Sea and Bay of Bengal
in the same direction as the Counter Equatorial Current, the
force of which is increased.

249. Currents in the Southern Ocean. — The westerly-
winds of the Roaring Forties carry a continuous surface
drift of water in an easterly direction round the world, thus
serving to mix the surface waters of the three great oceans.
In many parts of the Southern Ocean slow drift currents of
small volume set northward, and this is particularly the
case toward the west coasts of the southern continents.
Drift ice is rarely found farther north than the latitude of
42° or 43°, but south of that line Antarctic icebergs are
frequently met with.

250. Functions of the Sea. — The hydrosphere regulates

x The Hydrosphere 187

the distribution of energy, acting as a great fly-wheel to the
world machine. Solar energy directly or indirectly is the
cause of all its movements. The sea carries nearly half of
the sun-heat falling in the tropical zone to higher latitudes,
and from the high latitudes of the south it tempers the
tropical climates of the western shores of the continents
by cold updraughts. By the solution and restoration of
carbonic acid, it helps to maintain the uniform composition
of the atmosphere, and by its comparatively slow changes
of temperature, it keeps up land and sea breezes and
monsoons. It is an unfailing reservoir for supplying water-
vapour to the atmosphere, and rain for the lakes and rivers.
The smooth and level surface of the ocearis allow the
normal system of atmospheric circulation (§ 177) to be
developed to a far larger extent than is possible on the
land, and produces the steady winds which dominate the
climate of the whole globe. In the sea also the material
brought down by rivers from the land is redistributed and
worked up into new forms.

REFERENCES

1 J. Y. Buchanan, "On Ice and Brines," Proc. Roy. Soc. Ed.
xiv. 129 (1887) ; or Nature, xxxv. 608, and xxxvi. 9.

2 H. R. Mill, "On the Physical Conditions of the Clyde Sea
Area," Proc. Phil. Soc. Glasgow, xviii. 332 (1887) ; or Nature,
xxxvi. 37, 56 (1887). Also Trans. Roy. Soc. Ed. (1891).

3 H. R. Mill and T. M. Ritchie, "On the Physical Conditions
of Rivers entering a Tidal Sea," Proc. Roy. Soc. Ed. xiii. 460.

4 H. R. Mill, "On the Salinity and Temperature of the Firth of
Forth," Proc. Roy. Soc. Ed. xiii. 29 (1885);* and xiii. 157; also
Nature, xxxi. 541 (1885) ; Scot. Geog. Mag. ii. 20.

5 J. Murray, " Effects of Wind on Distribution of Temperature,"
Scot. Geog. Mag. iv. 345 (1888).

6 J. Y. Buchanan, " Physical Exploration of the Gulf of Guinea,"
Scot. Geog. Mag. iv. 177, 233 (1888). "On Similarities in the
Physical Geography of the Great Oceans," Proc. Roy. Geog. Soc.
vu'i- 753 (1886) ; also Nattire, xxxv. 33, 76.

CHAPTER XI

THE BED OF THE OCEANS

251. The Lithosphere. — The wide smooth expanse of
the hydrosphere is apt to give one a wrong idea of the
surface of the Earth by veiling the true topography of the
great hollows. Serious attempts to find out the whole
form of the lithosphere only began when the vast hidden
region acquired commercial value as a bed for telegraph
cables. Since the commencement of submarine telegraphy
accordingly the process of taking deep - sea soundings
(§ 443) has been rapidly perfected, and hundreds of
accurate measurements of depth have been made in all the
oceans. During the magnificent expedition of the Chal-
lenger in. 1872-76, many deep soundings were taken for a
purely scientific purpose in parts of the oceans never likely
to be visited by telegraph ships. In recent years numerous
smaller expeditions fitted out by the British government
and by the governments of the United States, Norway,
Germany, France, and Austria -Hungary, have made de-
tailed studies of parts of the sea-bed. The form of the
floor of the ocean has thus been gradually felt out point by
point, and though quite in the dark as to the scenery of the
veiled part of the lithosphere, we are now able to compare
its general features with the smaller portion which is open
to the light of day. If the Earth, like the Moon, had lost
its hydrosphere, and could be viewed from a distance, the
surface would appear to be made up of two great and
roughly uniform regions, both convex, following the curva-

CHAP, xi The Bed of the Oceans 189

ture of the globe, but one about 3 miles higher than the
other. The lower and larger is composed of broad gently
undulating plains rising into gentle ridges, and broken by
some abrupt peaks. It is divided into bay-like expanses by
the higher region, the slopes up to which are almost every-
where steep and often precipitous. The higher region is
smaller and more diversified, rising into numerous terraced
plateaux and rugged peaks. The whole of the low-lying
region and the lower slopes of the higher region are
entirely covered by the hydrosphere, only the plateaux and
peaks of the latter project above the water surface and
form the land.

252. Sea-Level. — The surface which naturally presents
itself for purposes of comparison in describing the con-
figuration of the Earth is that of the Ocean. This surface
is usually considered to be level, that is to say it is looked
on as having the exact form of the geoid (§ 83) and being
concentric with it. The level of the sea at any place is
always varying on account of waves and tides. In con-
structing charts, all soundings of depth are corrected to
their value for a calm sea at the average low water of
spring-tides for the place** in question. Heights -on land
are measured from a datum-level, which differs in different
countries, but is usually the average height of the sea
at some selected place. The heights marked on an Ord-
nance Survey map of Great Britain are quite accurate with
regard to the datum-level (that of mean tide^ at Liverpool),
but are 8 inches too high compared with the average sea-
level round the island, and in certain plages are as much as
2 feet too high or too low compared with actual mean sea-
level. Many reasons exist for those small permanent dif-
ferences of level, such, for example, as heavy local rainfall,
or evaporation, the direction of prevailing winds or currents.
The greatest distortion of the sea-surface is, however, due
to the mobility of water and its readiness to yield to the
attraction of gravity. If the surface of the lithosphere were
smooth and its interior of uniform density, this property of
water would ensure a truly similar surface in the ocean.
The Elevated Regions projecting to unequal heights far

190 The Realm of Nature CHAP.

above the general level of the Earth, and composed of sub-
stances of different density, attract the water by gravity
toward themselves, and thus prevent the uniform action
of the central force, much as the sides of a tumbler attract
the contained water by cohesion and heap it up slightly at
the edges. The amount of distortion in the hydrosphere is
as difficult to determine as the form of the Earth itself
(§ 83), and must be found in the same way. It was shown
by the survey of India that the sea-surface is 300 feet nearer
the centre of the Earth at Ceylon than it is at the Indus
delta, where the attraction of the Himalayas comes into
play. According to Professor Hull's estimate, the attraction
of the Andes is sufficient to raise the level of the sea more
than 2000 feet higher on the west coast of South America
than at the Sandwich Islands. The rocks beneath the bed
of the ocean are, however, believed to be of greater density
than those composing continents, and therefore their attrac-
tion on the sea should to a large extent counter-balance
that of the land. In any case the sea -surface is un-
doubtedly not level in any strict sense, and all comparisons
of height and depth of distant places are shadowed by
uncertainty.

253. Volume of Oceans and Continents. — The most
logical datum-level is the mean surface of the lithosphere,
the surface which would be produced if the heights were all
smoothed down and the hollows rilled up uniformly to pro-
duce the geoid. The amount of distortion of the sea-
surface must be ascertained, more soundings must be made
in many parts of the ocean, and the yet unknown regions
surrounding the north and south poles must be explored
and surveyed before the position of this ideal surface can
be found with certainty. A fair approximation to it has,
however, been made in an exhaustive estimate by Dr.
John Murray of the area of all the land and of all the oceans
lying between certain limits of height and depth.1 From
these areas he calculated the total volume of the land which
projects above, and of the oceanic hollows which extend
beneath sea -level. The land is estimated to occupy
55,000,000 square miles, and its average height is

xi The Bed of tJie Oceans 191

about 2 200 feet above sea-level, while the sea covering
the remaining 141,000,000 square miles of surface has
an average depth of 12,600 feet, or 2100 fathoms (§ 355).
The loftiest point of the land, Mount Everest in the
Himalayas, reaches to 29,000 feet above sea-level, and
the deej^st parts of the Pacific Ocean descend to a depth
of Y2. 8, 200 feet below sea-level. The whole vertical range
on the surface of the lithosphere is thus about 60,000 feet,
nearly 12 miles, which is only -^—^ of the Earth's diameter.
The narrow crest of the Elevated Region forming the
visible land has only ^ of the volume of the ocean hollows,
and thus the average level of the solid Earth evidently lies
beneath the sea-surface, and the summits of the land rise
higher above the mean level than the depressions of the
ocean sink below it.

254. Mean Sphere Level. — From Murray's figures, the
position of the mean surface of the lithosphere (mean sphere
level) was calculated by the author to be about 10,000 feet
(1700 fathoms) below the present sea-level, or more than
half-way down the slope which separates the two great
regions. If we imagine a transparent shell, similar in
form to the Earth and concentric with it, to cut this slope
at the level indicated, the volume of all the elevations
projecting above the shell would be precisely equal to the
volume of all the depressions extending below it. By a
remarkable coincidence, one-half of the area of the Earth's
surface is above mean sphere level and one -half below.
The line of mean sphere level traced on a map (PI. XIV.)
thus serves to divide the surface of the ^lithosphere into a
depressed and an elevated half.2

255. Three Areas of the Lithosphere. — The depressed
half of the lithosphere is called by Dr. Murray the Abysmal
Area, all parts of which are always covered by water more
than 10,000 feet deep. The upper part of the elevated
half of the lithosphere forms the Continental Area, which
is always above water, and occupies rather more than one-
quarter (28 per cent) of the surface. The remainder of
the surface, measuring somewhat less than one-quarter
(22 per cent), and always covered by water less than

192 The Realm of Nature CHAP.

10,000 feet deep, is called the Transitional Area. The
Abysmal Area, or group of World Hollows, is capacious
enough to contain exactly the whole volume of the group
of World Ridges made up of the Transitional and Conti-
nental Areas. The position of the coast-line or boundary
between the Transitional and Continental Areas obviously
depends on the volume of the hydrosphere. It is con-
venient for most purposes to class the Abysmal and Tran-
sitional Areas together as the Bed of the Oceans. In
originally proposing this division of the Earth's surface, Dr.
Murray took the boundary line between the Transitional and
Abysmal areas at the arbitrary depth of 1000 fathoms, or
6000 feet below sea-level.

256. Elevated Half of the Lithosphere. — The eleva-
tions and depressions of the Earth, although irregular in
form and distribution, are arranged with a certain rough
symmetry about the poles. A small detached elevation
occupying about one-twelfth of the area of the elevated
half has its centre within the Antarctic circle, and slopes
down gradually on all sides to mean sphere level. The
surface of the northern hemisphere is as a whole more
elevated than that of the southern. A great Northern
Plateau surrounding the pole to a distance of 2000 miles,
and broken only by one depression (that of the Norwegian
and Arctic Seas), is the centre of a continuous mass com-
prising fully nine -tenths of the whole elevated half, and
extending toward the south irv two vast World Ridges of
unequal size. In reading the following paragraphs the
student should refer constantly to the map (Plate XL), and
to Plate XIV. on which the line of mean sphere level is
depicted. The Western World Ridge stretches from 60° N.,
where the Polar plateau splits, in a south-easterly direction
to the equator, and thence southward, rapidly narrowing,
to 60° S. The ridge, nowhere of great width, is narrowest
between the Tropic of Cancer and the equator, where three
small isolated depressions (the basins of the Caribbean Sea
and Gulf of Mexico) nearly sever it. The crest of this
ridge forms the connected continents of America. The
Eastern World Ridge is of much greater size, and has

CONFIGURATION

165 ISO 165 l&O 135 120 105 9O

wsrn

•vw

165 ISO 165 150 135 120 105

75 60 *5

3F THE GLOBE.

11

15 O 15 50 46 60 75

105 120 155

15 0 15 SO

75 9O 1O5 12O 155

xi The Bed of the Oceans 193

somewhat the form of a horse-shoe, the toe to the north.
The western limb rises very steeply from the depressed
area, and tapers southward to a point in 40° S. ; it is
crowned by the continent of Africa, and marked off from
the European portion by two small depressions forming
the deep basins of the Mediterranean. The eastern limb,
marked off from the solid mass, which is the foundation of
Asia, by a great series of deep depressions (the basins of
the seas of the Malay Archipelago), runs south-eastward as
a comparatively narrow ridge bearing Australia, and ends
at 55° S. in two great spurs from which Tasmania and
New Zealand rise. This limb lies exactly on the opposite
side of the globe to the Western World Ridge. '

257. The Depressed half of the Lithosphere or
Abysmal Area forms a hollow ring round the south polar
elevation, and runs northward in the form of nearly flat-
bottomed troughs between the steep slopes of the World
Ridges to the edge of the North Polar Plateau. It is ridged
by long gentle rises and abrupt mountain-like peaks, and
grooved by depressions infinitely various in size and form.
Distinct hollows or basins of the Abysmal Area correspond
to each ocean, and the slopes of the world ridges rising from
them usually run parallel to the shore line which bounds
the various oceans (§ 216). The basins of the Pacific,
Atlantic, and Indian Oceans extend southward into the
Southern Ocean, which has not a separate basin of its own.
A typical section studied in conjunction with the map will
impress the general form on the student's mind, although
the scale of depth is necessarily exaggerated.

258. The Atlantic Basin, extending between the eastern
edge of the Western and the western edge of the Eastern
World Ridge, is long and comparatively narrow. It is
deepest near the walls (Fig. 36) forming in fact two
long sinuous troughs separated by the Dolphin Ridge along
the centre, which reaches on the average to mean sphere
level. The Azores, St. Paul Rocks near the equator, and
Ascension all spring from this ridge, while the lonely islets
of Tristan d'Acunha mark its southern extremity. Four
great hollows or groups of -hollows, the floors of which

O

194

The Realm of Nature

CHAP.

descend to more than 3000 fathoms below sea-level, occur
symmetrically, two in each of the lateral troughs, one north
and one south of the equator. One of the north-western
groups of hollows known as International Deep, contains
in 20° N., just north of the Virgin Islands, the deepest
sounding in the Atlantic, 4561 fathoms below sea-level, or
nearly 18,000 feet below mean sphere level (see Fig. 36).
The lateral troughs unite south of the Dolphin Ridge, and

FIG. 36. — Section across Atlantic Ocean in 20° N. lat. The vertical scale is about
300 times greater than the horizontal ; the slopes are thus shown 300 times as
steep as they really are.

appear to form one vast abyss which deepens toward the south
and extends far into the Southern Ocean. The deep basins
of the Caribbean Sea, Gulf of Mexico, and Mediterranean
communicate with the main Atlantic Basin over sills which
rise nearly to sea-level. In the north the Wyville-Thomson
Ridge, from an extension of which the Faroe Islands and
Iceland rise, shuts off the deep basin of the Norwegian and
Arctic Seas (§ 246).

259. Pacific Basin. — The Pacific Basin is far more vast
than that of the Atlantic, and is still to a great extent un-
explored ; but the survey for a telegraph cable from Canada
to New Zealand is at present (1891) revealing a chain of
new and most important facts regarding it. The Pacific
Basin appears to form one grand hollow extending from 60°
N. to 60° S., between the western edge of the Western
World Ridge, and the eastern edge of the Eastern. From
50° N. to 50° S. and right up to the steep walls to
east and west, the depth is .greater than 2000 fathoms,

xi The Bed of the Oceans 195

and close under the edge of the Western World Ridge, off
the west coast of South America, hollows more than 4000
fathoms below sea -level have recently been discovered.
The map shows the nature of the slopes of the Pacific Basin
to east and west, and brings out the fact that the Pacific
and Indian Oceans are connected by shallow water across
the top of a steep ridge pitted with small sea-basins of great
depth. The floor of the basin slopes up very gradually in
the south to form the gently swelling Antarctic Elevation.
Numerous groups of long narrow ridges and isolated peaks,
rising close to or above the surface of the water, with
depressions of various forms between them, stretch roughly
parallel to each other from south-east to north-west across
the basin, becoming more numerous toward the west.

260. The Tuscarora Deep. — In the extreme north-west
the steepest part of the bounding wall of the Pacific Basin
rises abruptly, barring off the seas of Japan and Okhotsk,
and bearing the chain of Japanese and Kurile Islands. In
front of it lies the deepest abyss in the Earth's crust, the
Tuscarora Deep. It extends from 20° N. to 50° N. in a
crescent-shaped curve, deepening toward the steep slope of
the World Ridge to the north-west, where a mighty gully
1000 miles long and 20 wide lies at a depth greater than
4000 fathoms (see Fig. 37). Here the United States survey-
ing ship Tuscarora, obtained at least one sounding of
almost 4700 fathoms below the surface, or 20,000 feet below

FIG. 37. — Steep slopes. The diagram is divided into squares representing 10 miles
in the side. The upper black figure shows the true average slope from the
summit of Mount Everest to sea-level ; the lower shows the true average
slope from sea-level to the bottom of the Tuscarora Deep.

mean sphere level. H.M.S. Egeria obtained an equally
deep sounding in a very small depression south-east of the
Friendly Islands ; but there is no satisfactory proof of greater
depths existing in any ocean.

196 The Realm of Nature CHAP.

261. The Indian Basin.— The Indian Basin, protected
on three sides by the inner edges of the great Eastern
World Ridge into which it penetrates, is only half the size
of the Atlantic, and one-third of the Pacific, to which it
bears some resemblance. The greatest depth, over 3000
fathoms, is found in the eastern angle between the coasts
of north-west Australia and Java. The basin grows
gradually shallower toward the south, most gradually toward
the south-east. The western half is greatly diversified by
narrow ridges running north-eastward from Madagascar to
Ceylon, and rising in numerous groups of low islands above
the surface of the water.

262. Islands and Shoals. — Those islands which are
merely parts of the crests of the World Ridges separated by
shallow water from the mainland, and composed of similar
rocks, are termed Continental Islands. Oceanic Islands are
those which rise from the depressed half of the Earth and
have no geological relation to the neighbouring land.
Many of them are composed of volcanic rocks, and must
be viewed simply as the summits of ridges or submerged
mountains. Others are built up of the remains of living
creatures, and rise only a few feet above the surface of the
water. These (§§ 280-282) require a foundation before they
can be formed, and the foundation is usually a submarine
peak or ridge. A submerged peak, rising within a few
hundred feet of the surface, is called an oceanic shoal. It
was supposed at one time that very few shoals of this kind
existed, the bed of the ocean being looked upon as an
almost unbroken plain, but the recent explorations of
telegraph ships have revealed a large number of shoals in
all the oceans, in some cases rising precipitously from vast
depths.3 Probably many more remain undiscovered, for
unless the lines of soundings across an ocean are run at
very close intervals, they might be passed over.

263. The Transitional Area. — From mean sphere level
the upward slope of the World Ridges is at first gentle, but
after a certain height in almost all places it becomes com-
paratively steep, in rare cases even forming a succession of
rocky precipices. Fig. 37 shows that the average slope

XI

The Bed of the Oceans

197

from the summit of Mount Everest to sea-level is very little
steeper than the slope from sea-level to the bottom of the
Tuscarora Deep ; about i in i 5 or nearly 4°. The steep-
ness of sloping land almost always appears greater to
the eye than it actually is. Only precipices of bare
rock have an angle of slope greater than 45° or a
gradient of i in i, and the steep slope of the lower part
of the world ridges probably rarely exceeds 35°, which
on land would be felt a very steep hill to climb, a
gradient of i in ij. The steepest hill on a well-made
road is i in 20 or an angle of 3°. Mr. J. Y. Buchanan
found that in some cases where the slope was comparatively
slight the original rocky wall had been covered by a
mound of sediment brought down from the neighbouring
land by great rivers (§§ 325, 326). In nearly all cases at
the top of the acclivity, usually at the point where the
depth of water is about 100 fathoms, the slope suddenly
becomes much more gentle, and continues very gradual up
to the coast line. This gentle slope has been termed the
Shore Flat, or the Continental Shelf. The typical profile of the
transitional area is given in
Fig. 38, which represents
the slope of part of the
Gulf of Guinea. The outer
curve shows the slope at
a part of the coast where
a pile of river -mud has
been thrown down like an
embankment in front of the
ridge face, thereby reduc- YlG 38.-sioPes of the Gulf of Guinea,
ing its gradient. These The X^tV^V5,40 times the 1iori"

zontal. Solid black shows average slope
Slopes are represented forty of the coast edge ; the shaded part

times steeper than they are SgSt*?6 modified by river-borne

in order to bring out the

change of gradient, the vertical scale being forty times the

horizontal.

264. The Continental Shelf. — The world ridges form-
ing the walls of the ocean-basins are flattened at the top
like the rim of a pudding-dish, and beyond the flat edge

198 The Realm of Nature CHAP.

the continent itself rises. The breadth of the continental
shelf varies greatly. In the map (Plate XI.) the area of the
shelf is left white, and it will be seen to attain its maximum
breadth off Western Europe where the British Islands stand
upon it, off south-eastern America where it bears the Falk-
land Islands, around Florida, at intervals along the east
coast of Asia, and off the north of Australia. Along the
east and west coasts of Africa, and along the west coast of
America, it is very narrow, and around some volcanic islands
it is entirely absent. The total area of the continental
shelf, covered with water less than 100 fathoms deep, is
10,000,000 square miles. This includes the whole of
many shallow seas, such as the North Sea, the Baltic, the
White Sea, Hudson Bay, and the Yellow Sea, and unites
all the great continental islands, except Madagascar,
Celebes, and New Zealand, to their nearest continent. The
land bordering the coast-line is in most places a low un-
dulating plain, which rises gradually inland until it attains
an elevation of about 600 feet above the sea, and then rises
more abruptly to much greater heights. The low plains
(under 600 feet in elevation) measure altogether about
12,000,000 square miles. From the margin of the con-
tinental shelf to the end of the low plains there is therefore
an expanse of 22,000,000 square miles, the level of which
differs by only 1200 feet. Except possibly on the floor of
the Abysmal Area there is no other part of the Earth's
surface where so wide an expanse possesses such a
slight range of elevation ; and it is significant that
the coast-line at present almost bisects it, occupying the
only position in which a rise of 600 feet would submerge,
and a fall of 600 feet would enable it to lay bare so large
an area.-^tr

265. Beach Formation. — The upper margin of the
Transitional Area is a region of great activity and rapid
change. Tide and wind together urge the water against
the land and withdraw it, dragging back the solid material
it has seized. If the land is a low plain of very gentle
slope the waves gradually encroach upon it, drawing the
sand or soil seaward at every tide and building up the

xi The Bed of the Oceans 199

continental shelf nearly to sea -level for a considerable
distance, as, for example, along the east coast of India.
Sandbanks or bars, sometimes locking in lagoons of salt
water and forming a lace-like margin to the land, are pro-
duced where river deposits are brought down to the coast —
for instance, on the south-east coast of North America, and in
the vast mangrove-grown mudbanks of many parts of South
America and Africa. Where the land is high and rocky the
broken-off stones are rolled and rounded by the waves and
used as battering-rams to break away the land ; finally they
are swept out to sea and spread in sheets over the bottom,
the level of which is raised and the slope reduced. In this
way a beach is formed, the upper part of it being quarried
out of the solid land, and forming a notch or ledge (ABC,
Fig. 39) on which the sea is always encroaching, while the
lower part forms an embankment (CDE) built up of the ex-
cavated material which
is laid down in flat beds
one above another. The
name Beach is restricted
to the strip of land
covered and laid bare by

the tides. On a typical FIG. 39. — Formation of a Beach. AD, original
slope of land ; ABC. notch cut out by wave

beach large Stones are action; CDE, embankment of sand, etc.
Usually found heaped Up (worn-down rock); BC, gravel resting on

near high-water mark ;

smaller pebbles, rounded by the sea, form a steeply sloping
bank at a lower elevation, and are rattled to and fro, ground
against each other, reduced in size to. fine shingle, and
raked nearer the sea by every tide. Next there is a wide
stretch of sand, which usually consists of quartz grains,
resulting from the breaking down of the pebbles, the quartz
being the densest and hardest ingredient of the rocks.
Nearest the sea, and often only uncovered at the lowest
spring ebbs, there are banks of mud formed of the softest
ingredients of the rocks, which were ground to the finest
powder and carried to the greatest distance. Sometimes
perpendicular cliffs occur, at the base of which the rushing
tides permit no fragments to accumulate.

200 The Realm of Nature CHAP.

266. Wave Action. — The measurements of Mr. Thomas
Stevenson on the coast of Scotland show that during severe
storms the waves may exert a force equal to 3 tons on
eVery square foot of the cliffs they beat against. A force
of i ton per square foot is commonly exerted by the waves
of the Atlantic in winter, and 600 Ibs. on the square foot in
summer. This ponderous surge of the waves tears off
loose pieces of rock, and the deluge of spray and pebbles
which the breakers toss into the air has been known to
break the windows of a lighthouse 300 feet above the sea.
When a wave swells up against a cliff it powerfully com-
presses the air in all the cracks of the rock, thus striking a
sudden blow throughout the whole mass. An explosive
expansion of the air follows when the wave subsides, and
the loosened fragments are sucked out along the lines of
bedding or jointing (§ 290). This action and the bombard-
ment by pebbles are the chief agents in forming sea-caves,
of which one of the finest examples is Fingal's Cave in Staffa,
carved out of columnar basalt As the cave extends into the
cliff it grows narrower, and finally a long diagonal tunnel may
be drilled out, opening on to the upland far from the shore.
Such openings or blow-holes are common along all cliff-
girdled coasts, and throw up columns of spray during
storms often with a noise resembling the outburst of a
geyser. Blow -holes naturally widen as the sides are
weathered (§31 o), and form deep isolated pools where the
tidal water rises and falls at the bottom of a nearly vertical
rocky shaft. When softer and harder rocks alternate along
a coast, the former are in time cut back by the waves and
form bays, while the latter project as headlands. Currents,
or tidal eddies, attacking a narrow headland on both sides,
and driving the pebbles against one part of the cliff, often
break a cave right through, which when wide forms a
tunnel, when high and narrow a natural bridge. Atmo-
spheric erosion may cut as rapidly above as the waves do
below, and the headland become separated from the main-
land as an isolated rock or stack, round the base of which
the water sweeps. Some of the finest examples of such
cliff scenery occur on the north coast of Scotland and in

xi The Bed of the Oceans 201

Orkney, where the Old Man of Hoy is a magnificent stack
450 feet in height.

267. Origin of the Continental Shelf. — The action of
waves and tidal currents usually ceases to be perceptible at
the depth of 100 fathoms. Beach deposits swept seaward
by the waves assist in scooping out and deepening the
shore, the final result being, possibly, to eat inward along
the top of the wall of the world ridge until a depth of 100
fathoms is attained. The continental shelf is widest on the
margins of the oldest continents exposed to the heaviest
waves, and may be compared to the line which some
chemical solutions etch on the glass bottles containing
them. Harder masses resisting the attacks of the waves
remain as islands or shoals on the continental shelf.
Where currents sweeping mud and sand to and fro are
checked by some inflection of the coast -line, sandbanks
are formed. In many cases it is possible that the con-
tinental shelf is the end of a low plain submerged by
subsidence ; in others a low plain may be an upheaved
continental shelf, and probably wave action is only one of
the factors at work. Long furrows of great depth cross it
in some places. These grooves and submarine canons
(§ 326) have a peculiar interest, because they seriously
detract from the usefulness of the continental shelf as a
guide to sailors groping their way to land by means of the
sounding-line in foggy weather.

268. Marine Deposits. — Immense quantities of sedi-
ment are carried down by rivers into the sea (§331). M.
de Lapparent calculates the amount as 33 times greater than
all the sand, gravel, and pebbles worn off by tidal and
solar energy acting through waves and currents on the
coasts. Countless myriads of plants and animals living in
the water affect the substance in solution (§ 222), forming
shells or skeletons which at their death fall to the bottom,
producing various kinds of deposits. Sea-water acts chemi-
cally on substances exposed to it, producing a further series
of changes. In all parts of the ocean not precipitous nor
swept by strong currents, the original rock is covered with
a mantle of deposits of various thickness, to which the

2O2

The Realm of Nature

CHAP.

gently-rounded contour of the ocean-beds is largely due.
MM. Murray and Renard in their report on the deposits
collected during the Challenger expedition have adopted
the following classification : —

MARINE DEPOSITS

i. Deep-Sea Deposits

(beyond I oof at horns}.

I. PELAGIC DEPOSITS,
formed in deep water
remote from land.

II. TERRIGENOUS DE-
POSITS, formed in
deep and shallow
water close to land

Red Clay
Radiolarian Ooze
Diatom Ooze
Globigerina Ooze
Pteropod Ooze

Blue Mud
Red Mud
Green Mud
Volcanic Mud
Coral Mud

2. Shallow- Water Deposits (in less than
100 fathoms}, sands, gravels, muds, etc.

3. Littoral Deposits (betiveen high and
low water marks}, sands, gravels, muds,
etc.

269. Terrigenous Deposits. — Sediment, such as fine
clayey mud, requires a very long time to settle to the bottom
of fresh and still more of running water, but in sea-water,
especially when the temperature is high, it settles out much
more rapidly. The smaller a particle of mud and the
deeper the sea, the farther from land will the particle be
carried by currents before it falls to the bottom. As a rule,
however, land-derived material all reaches the bed of the
ocean within 100 or 200 miles of the shore ; only in excep-
tional circumstances does it extend to a greater distance
than 300 miles. The line of 250 miles from the coast
shown on Plate XII. is practically the boundary of terri-
genous deposits. Very large and swift muddy rivers like
the Congo and Amazon (§ 230) form such exceptions.
Congo mud has been found 600 miles from shore. The
Arabian Sea and Bay of Bengal are carpeted for nearly
1000 miles from land by the mud of the Indus and Ganges
river systems. Other exceptions result from icebergs,

xi The Bed of the Oceans 203

which drop land -derived stones and mud all along the
path of the ocean currents, which drift them into warm
seas. Wind also blows sand or dust far out to sea. Volcanic
eruptions throw up quantities of fine dust, which are
carried far and wide by the winds and scattered over the
whole sea surface. Pumice-stone, being porous, floats for
months and probably years, and may be drifted to any part
of the ocean before it becomes waterlogged and sinks. All
terrigenous deposits, although soft and sticky when wet,
fall int'o a loose powder on being dried, hence the term Mud
is specially applied to them. Such deposits are characteristic
of enclosed seas and of the upper margin of the Transitional
Area, which they clothe much as snow clothes a tropical
mountain, most thickly on the upper part of the slope. It is
estimated that terrigenous deposits cover one-fifth of the
area of the oceans, and it is distinctive of these deposits
that they are made up of fragments of continental rocks,
such as compact limestone, quartz, schist and gneiss.

270. Blue Mud. — The littoral deposits or shore forma-
tions sometimes extend in the form of sand or bars of
fine gravel, enclosing hollows filled with mud, right across
shallow seas. As a rule, however, deep enclosed seas,
margins of islands and of continents for 200 or 300 miles
from land, are carpeted with extremely fine mud, containing
small grains of sand and the remains of shells and of
marine plants. Where the material is derived mainly from
rivers it assumes the form of a blue mud, which is the most
characteristic of terrigenous deposits in every ocean, and is
found at all depths. Blue mud owes its dark blue or slaty
colour to chemical changes produced by decomposing vege-
table and animal substances, in presence of the sulphates of
sea- water, which appear to be reduced to sulphides, and
decompose the ferric oxide abounding in all deposits into
sulphide of iron and ferrous oxide. When there is much
iron in the state of ferric oxide, as in the ochrey muds that
redden the water of the Amazon, there may not be sufficient
organic matter to reduce it all, and the mud retains its red
colour. Blue mud contains variable quantities of carbonate
of lime according to the abundance of shell-producing

204 The Realm of Nature CHAP.

creatures living in the water where it is deposited, but it
accumulates so rapidly that shells as a rule form a very
small proportion of the whole.

271. Green Mud. — Along cliff- bound coasts in which
few rivers open, terrigenous deposits form very slowly, and
to a distance only of 100 miles, or less, from land. The
finely-ground particles of rocks are thus exposed for a long
time to the action of sea- water and undergo extensive
chemical changes. A greenish mineral called Glanconite
is thus produced, which fills up the interior of dead calcar-
eous shells, forming casts of the interior which remain when
the shells themselves are dissolved away by weak acids.

272. Volcanic and Coral Muds and Sands. — Oceanic
islands of volcanic origin are surrounded by Volcanic Muds
or Sands, formed by the wearing down of volcanic rock and
its subsequent partial decomposition by the chemical action
of sea-water, the fragments of shells which are present being
often coated with peroxide of manganese derived from the
rocks. Islands of Coral origin are in a similar way sur-
rounded by Coral Muds or Sands which consist almost
entirely of carbonate of lime. The remains of calcareous
marine plants (chiefly corallines) often make up a large part
of this deposit.

273. Siliceous and Calcareous Organisms. — Certain
minute moving organisms or living creatures, rarely visible
except by means of the microscope, and possessed of the
power of secreting silica from solution in sea-water, are
found in the surface layers of all oceans, especially where the
salinity is slight. One kind, known as Diatoms, abounds in
cold seas and in estuaries, forming delicate cases or shells
exquisitely marked. They probably obtain some of their
silica by decomposing the clayey mud of rivers. Radio-
larians, another class of silica-secreting organisms, frequent
warmer water and are not found in estuaries ; they form
a minute framework or skeleton of glassy spicules often
arranged in very complex and beautiful groups. The
chief pelagic molluscs living on the surface far from land
are a few kinds called Heteropods and Pteropods and they
inhabit tropical seas. Their shells are thin papery cases

xi The Bed of the Oceans 205

of carbonate of lime, varying in length from half an inch
downward. Innumerable forms of the simplest and smallest
of living creatures abound in the surface water. They are
most numerous in warm regions, and gradually disappear
toward the poles. One class of these is called Foramini-
fera, as they construct dense microscopic shells of carbonate
of lime pierced with innumerable little holes, through which
the soft substance of the animal projects during life. The
most common, is a kind called Globigerina, on account of
its globular form, the largest shells of which are about the
size of a small pin's head. It has been proved that an
animal requiring a shell of carbonate of lime can manu-
facture it out of any salt of lime, the carbonic acid coming
from the creature itself, hence all the lime of sea-water
(§ 222) is available to be drawn upon.4 The death of
countless millions of minute creatures produces a steady
though invisible snowfall of dead bodies falling from the
surface layers crowded with ever-renewed life, and gradually
subsiding through the cold still depths of water. This
takes place over every part of the hydrosphere, but within
reach of terrigenous deposits the shells are covered over
and buried in the rapidly increasing pile, of which they form
a small proportion. Deposits of organic remains are more
coherent and plastic than the muds, and have received the
general name of Ooze. Living creatures, such as sponges
which make skeletons of silica, calcareous sea-urchins,
crabs, and corals, exist on the bed of the ocean to all
depths, although they are incomparably more abundant
in the shallow water near shore.

274. Pteropod Ooze is formed of the shells of all surface-
living organisms in tropical seas, and contains a consider-
able proportion of pteropods, whence its name. It is never
found below mean sphere level, but abounds on submarine
ridges rising to within 1000 fathoms of the surface. The
reason of this distribution appears to be that the delicate
shells of pteropods expose a very large surface to the sea-
water as they fall through it, and are dissolved away before
they reach the bottom when the depth is great.

275. Globigerina Ooze. — The small dense shells of the

206 The Realm of Nature CHAP.

Globigerina can fall through a far greater depth than the
thin pteropods before they are dissolved. Globigerina ooze
accordingly covers a far greater part of the ocean bed.
It does not occur in enclosed seas, nor under the cold cur-
rents of the north-east Atlantic, nor in the Southern Ocean
south of 5 5° S. ; but otherwise it is practically universal within
certain limits of depth. Under the Gulf Stream its deposit
is carried far to the north, as the surface water of that current
swarms with globigerinse. The ooze is a white or pinkish
substance, which when dried is seen to have a fine granular
structure, due to the little round shells of which it is composed.
It varies in composition with the depth, that which has formed
in the deepest water containing only the stronger and denser
species, and the shells of these even being much corroded.
The percentage of carbonate of lime varies from 30 to over
80, sometimes reaching 95 ; and if the carbonate is dissolved
by a weak acid, the residue consists of a fine clayey sub-
stance mixed with the cases of diatoms and the spicules of
radiolarians. At depths exceeding 2500 fathoms, with rare
exceptions, none of this ooze occurs, the proportion of car-
bonate of lime in the deposit being reduced almost to the
vanishing point. Globigerina ooze borders the upper zone
of the Abysmal Area, and thins away toward the great
depths (see Fig. 41).

276. Radiolarian and Diatom Oozes. — The siliceous
skeletons of radiolarians and diatoms are present in small
amount in almost every deposit. Silica is not nearly so
soluble as carbonate of lime in sea-water ; hence when the
depth is greater than 2500 fathoms, and radiolarians abound
on the surface, their spicules form a large proportion of the
deposits reaching the bottom. The name of Radiolarian
Ooze is given when they amount to more than 25 per cent.
Radiolarian ooze is spread over a considerable part of the
central Pacific, and the east of the Indian Ocean where
the maximum depression occurs, but it is not found in
the Atlantic or the Southern Ocean. Diatom Ooze con-
tains about 50 per cent of diatom skeletons, mixed with
from 10 to 20 per cent of carbonate of lime. It is the
distinctive deposit of the Southern Ocean, where it occurs

XI

The Bed of the Oceans 207

at all depths ; the small number of foraminifera living in
the cold and comparatively fresh surface water accounts for
the small quantity of carbonate of lime in the deposits of
that region. The whole Southern Ocean is within the limits
of icebergs drifting from the Antarctic region, and the Diatom
ooze often contains a considerable proportion of terrigenous
deposit, the nature of which proves the existence of conti-
nental rocks, and thus of an unexplored continent near
the south pole.

277. Red Clay. — The deepest parts of every ocean are
covered with a stiff clay of a deep brown or red colour,
containing little or no carbonate of lime. Red clay is the
distinctive deposit of the Abysmal Area, toward the upper
margin of which it passes very gradually into Globigerina
ooze ; and where radiolarians abound on the surface the
accumulation of their spicules gives to it the name of
Radiolarian ooze. It covers more than half the area of
the Pacific Ocean. Red clay is exactly like the residue
of Globigerina ooze after the carbonate of lime has been
removed. The snowfall of calcareous shells from the sur-
face of the open ocean melts into solution before reaching
the abysmal depths, but the horny remnants of those shells,
siliceous relics of life, waterlogged pumice-stone, wind-borne
dust from deserts and volcanoes, ultimately settle down and
accumulate on the bottom. The rate of deposit is incom-
parably slower than that at which any of the oozes form.
Microscopic examination has revealed as one of the con-
stituents of Red clay cosmic dust from meteorites (§ 134),
which falling uniformly over the Earth's surface is concealed
by the rapid changes going on in every other region but the
still Abysmal Area. The red colour of the clay is due to
the formation of ferric oxide and peroxide of manganese
from decomposing volcanic material. These oxides also
become deposited upon any hard objects lying on the sea-
floor, and form nodules composed of layer above layer
and often attaining the size of a large potato, to which their
usual shape is very similar. Manganese nodules were
dredged up in great numbers by the Challe?iger, and in
every case the nucleus on which they had formed was

208 The Realm of Nature CHAP.

found to be a piece of pumice, or the hard teeth or bones
of the larger creatures inhabiting the sea. Sharks' teeth
are very numerous, and also bits of the hardest bones of
whales. Red clay also contains in certain localities small
but perfectly formed crystals of the class of minerals known
as zeolites (§ 286), which have evidently resulted from
chemical changes in the material of the clay.

278. Permanence of Elevated and Depressed Regions.
— From the scanty supply of materials out of which Red clay
is elaborated, it is evident that if the deposit has attained
any great thickness it must have been a very long time in
course of formation. There is no evidence as to the thick-
ness of the Red clay, but the teeth and bones found embedded
in its nodules are known in many cases to belong to species
of sharks which no longer live in the ocean, and must have
been extinct for an immense period of time. Moreover, if
the Abysmal and Continental Areas had ever changed places,
some rocks would almost certainly be found on the land
resembling a consolidated Red clay. None such have ever
been discovered unless in volcanic oceanic islands that have
been recently upheaved. Accordingly the existence of the
Red clay is a strong argument that the elevated and
depressed halves of the lithosphere have occupied their
present positions during past geological ages.

279. Corals. — Many oceanic islands and reefs are com-
posed of the stony framework of carbonate of lime which is
secreted by animals known generally as coral polyps. These
polyps belong to the same class as the sea-anemone, and
are of many different species, each characterised by some
peculiarity in the form of its calcareous support. Some
secrete a wide disc, the surface of which is starred with
their groups of waving tentacles ; others form little cups on
which they grow, these cups being either separate, as in the
deep-sea corals, or united by a solid stony stem forming
many branches. The branching corals of various species
are of most importance in reef-building. The distribution
of coral islands over the oceans depends on the suitability
of the water for the life of the polyps and the existence of
good foundations. The polyps flourish best in very salt,

EQUIDISTANT

1BO 180

ISO 180 160 140 12O 1OO 8O 6O 40

ypg

OT Z5U milCS fllOTIlt, Iffi

)ASTAL LINES.

12

80 100

8O iOO 12O 140

Seaward from nearest coast. — The Figures indicate the number of Miles.

xi The Bed of the Oceans 209

clear, and warm water ; and, although they may live,
they do not form reefs where the temperature is less
than 70°, or has a yearly range greater than 12°, or a
depth greater than about 20 fathoms. They are par-
ticularly active on the margin of the Red Sea, where the
conditions of salinity, temperature, and depth are most
favourable. The distribution of reef-building corals is given
in the map of Plate XV. Corals are never found near the
mouths of great rivers on account of the water being fresh
and muddy. They do not build on the west coasts of the
tropical continents because of the cold upwelling water
(§241). The part of the Somali coast in the Indian Ocean
against which the south-west monsoon raises cold water
(§ 248) is free from corals on account of the great annual
range of temperature which results. ' Corals are confined to
the centre and western sides of tropical oceans, except in
warm currents such as the Gulf Stream, which enables them
to live luxuriantly far into the temperate zone, the Bermuda
islands, in 32^° N., having the highest latitude where coral
islands are now forming. There the polyps appear able
to form reefs at a temperature as low as 68°, but these reefs
are largely composed of calcareous sea-weeds and worm-
tubes.

280. Coral Reefs and Islands. — The Gulf of Mexico
and the west coast of Florida, the western Indian Ocean,
and in particular the western Pacific, are the seats of very
active and typical coral growth. There are three distinctive
forms of coral structure, (a) The fringing reef, which
closely surrounds the shore, forming on .the seaward slope
of the land in shallow water, and as it grows older gradually
widening toward the sea. (b) The barrier reef, which
usually lies at a distance from the land, running parallel
to the coast, and on its seaward side often springing
abruptly from great depths. On the landward side a
shallow lagoon of still water is shut in by the reef, which
is always broken by one or more narrow channels, allowing
boats or even large vessels to enter. Innumerable volcanic
islands in the Pacific, such as the Solomon Islands, the Fiji
group, and Tahiti, are encircled with fringing and barrier

P

2io The Realm of Nature CHAP.

reefs. The great barrier reef of Australia, stretching for
1 200 miles along the east coast of Queensland, is the finest
example known, (c) The atoll^ which is a reef in the form
of a closed curve with no land in the centre. The lagoon
encircled by an atoll is usually shallow, and th,e bed of it
composed of coral which is either dead or not in vigorous
life. Typical examples of the true coral islands or atolls
are found in the Maldives, Laccadives, and Chagos groups
in the west of the Indian Ocean. These reefs are usually
very narrow compared with their length, and their surface
never rises higher than from 10 to 20 feet above the
sea. In most instances only a portion of the reef rises
above the surface, giving the appearance of a chain of low
islands separated by very shallow water. The coral polyp
dies when it reaches sea-level, but blocks of coral are broken
off by the waves and thrown on the reef, where they get
broken down into sand, and this becoming compacted
amongst the branches of living coral is raised by degrees
until it forms dry land. Water percolating through the
coral rock and sand gradually converts the whole into a
solid mass of coral limestone, part of the carbonate of lime
being dissolved and re-deposited in a crystalline form in the
crevices. Drifting pumice strands on the beach and
weathers into clay (§ 311) for the formation of soil.
Ultimately the seeds of trees and other plants get drifted
to the islands and take root, birds visit them, and the coral
island becomes habitable.

281. The Formation of Coral Islands. — During the
famous voyage of H.M.S. Beagle the naturalist Darwin
made a detailed examination of several coral formations,
and he came to the conclusion that the three typical forms
were closely related to each other. He recognised that it
was possible for atolls to form if they had a submarine
mountain, the top of which was less than 20 fathoms below
the surface, as a foundation, but he did not know that such
peaks often occurred. He found also that the walls of
coral rock on the seaward face of reefs sometimes rose from
an enormous depth, and since coral polyps can only live and
build in the warm surface layer, he concluded that the

XI

The Bed of the Oceans

211

corals had built in that layer, but that the foundations had
been gradually sinking. Thus he supposed a fringing reef
(I, Fig. 40) to form round a volcanic island, and as the
island slowly subsided the corals built the reef higher and
higher, keeping pace with
the subsidence. In time,
as the outer edge of the
coral grows fastest on
account of the greater
abundance of oxygen in
the breakers, the reef
would widen and grow
higher seaward, forming a
barrier reef by the time
subsidence has brought
the sea-level to the posi-
tion (II). Finally, sub-
sidence submerges the
whole mountain below
the surface, and the barrier

FIG. 40. — Darwin's theory of the origin of
Coral Islands. I, II, and III show suc-
cessive levels of the sea brought about
by subsidence of a volcanic island (solid
black). The corresponding coral forma-
tions, respectively fringing reef, barrier
reef, and atoll, are shown by ^different
shading.

reef grows up to form an
atoll (III) peeping above sea-level. In recent years a num-
ber of objections to this widely accepted theory have been-
made by many investigators. It has been pointed out that
atolls are as common in areas which are being gradually
elevated as in those that are subsiding. Dr. Guppy, in the
course of a study of the Solomon Islands, where many reefs
have been elevated far above sea-level, also found that the
coral limestone is never of greater thickness than about 120
feet, and he thus casts doubt on the existence of vast sub-
merged walls of coral. He found that the cake of coral rock
rested either on volcanic rock or on rocks formed by the
consolidation of pteropod or globigerina ooze.

282. Murray's Coral Island Theory. — During the cruise
of the Challenger Dr. John Murray formed another theory,
which has been strikingly confirmed by the observa-
tions of Dr. Guppy and others. He believes that the
foundation for coral reefs is in every case supplied by
submarine peaks. Some of these may have been formed

212 The Realm of Nature CHAP.

by volcanic upheaval and then reduced below sea-level by
the eroding action of waves, and some may have existed
originally at a suitable height. Others may have been
raised to the coral zone by ages of submarine sedimentation,

being covered first by glob-
igerina ooze, then as the
depth was gradually dim-
inished by pteropod ooze,
and finally brought com-
paratively rapidly within
reach of reef- builders by
the accumulation of the
remains of sea-urchins, star-
fish, deep - sea corals, and
the like. The reef-building

FIG. 4i.-Murray's Theory of the origin of Pol7PS raise a flat table °f

Coral Islands. The central volcanic solid rock, which, as it ap-
rock (solid black) is shown covered by 1,^*1, (

deep-sea deposits which build it up to proacnes tne SUrlace, grOWS

the reef-building zone where an atoll is more rapidly on the circum-
formed. '

ference on account of the

abundance of food supplied by ocean currents. The rim fin-
ally reaches the surface and cuts off the supply of food from
the polyps in the interior, which die, and the dead coral is
partly dissolved by the water, partly scoured out by tide and
waves, and so a lagoon is gradually hollowed. The outer
slope of the reef is alive, and ever growing outward. As it
becomes steep and wall-like, masses broken off by the waves
roll down to the bottom and form a more gentle slope or talus
on which the active corals continue to build seaward, always
increasing the diameter of the atoll. Meanwhile the sea-water
in the lagoon is at work dissolving and removing coral from
the inner edge, and the island does not increase in width
although its circle is continually widening. An atoll is thus
supposed to grow like a " fairy-ring " in the grass. Fringing
reefs growing seaward in the same way ultimately form
barrier reefs, in which the same process of active growth
seaward, and decay on the landward side has been observed.5
In some cases barrier reefs have grown up directly far from
the island on the edge of a wide and shallow continental

xi The Bed of the Oceans 213

shelf, which is formed when a loose volcanic upheaval, such
as Graham Island, which recently appeared in the Pacific,
is rapidly worn away by the waves. Most geologists now
recognise the greater probability and wider application of
the solution theory of coral reefs over the subsidence theory.

REFERENCES

1 J. Murray, " On the Height of the Land and the Depth of the
Ocean," Scot. Geog. Mag. iv. I (1888).

2 H. R. Mill, " On the Vertical Relief of the Globe," Scot. Geog.
Mag. vi. 182 (1890); and "On the Mean Level of the Surface of
the Solid Earth," Proc. Roy. Soc. Ed. xvii. 185 (1890).

3 J. Y. Buchanan, "On Oceanic Shoals," Proc. Roy. Soc. Ed.
xiii. 428 (1884); also Nature, xxxvii. 452 (1888).

4 R. Irvine and G. S. Woodhead, " On the Secretion of Carbonate
of Lime by Animals," Proc. Roy. Soc. Ed. xv. 308 ; xvi. 324.

J. Murray and R. Irvine, "On Coral Reefs, etc." Proc. Roy.
Soc. Ed. xvii. 79 (1889).

5 J. Murray, " Structure, Origin, and Distribution of Coral Reefs
and Islands," Proc. Roy. Inst. (1888); also Nature, xxxix. 424
(1889).

BOOKS OF REFERENCE

M. F. Maury, Physical Geography of the Sea. (The earliest and
most interesting book on Oceanography, although the facts and
theories are now out of date.) Edition 1883, T. Nelson and Sons.

Charles Darwin, Coral Reefs and Islands. (Several cheap editions
recently published.)

J. D. Dana, Coral Islands. Sampson Low and Co.

H. B. Guppy, The Solomon Islands : Geology. Swan, Sonnen-
schein, 1887.

C. Wyville Thomson, The Depths of the Sea. Macmillan and Co.

J. J. Wild, Thalassa. Marcus Ward and Qo.

"Challenger" Reports, Narrative, Physics and Chemistry.
(Several volumes.)

Reports of the Norwegian North Atlantic Expedition (Physics
and Chemistry}.

A. Agassiz, Three Cruises of the "Blake.''' Cambridge, U.S.,
1888.

Numerous papers on Oceanography will be found in recent
volumes of Nature, the Scottish Geographical Magazine, and in pub-
lications of the Royal Societies of London and of Edinburgh, the
Royal Geographical Society, and the Fishery Board for Scotland.

CHAPTER XII

THE CRUST OF THE EARTH

283. Lithospheric Changes. — In the Abysmal Area
the hydrosphere protects the solid rock beneath, by the
extremely slow formation of a covering of red clay or ooze.
In the Transitional Area, where the hydrosphere is stirred
more forcibly by solar energy, the formation of deposits is
accompanied by the wearing away of rocks. All our know-
ledge of the substance and structure of the lithosphere is
obtained by studying the processes of change going on in
the Continental Area, which alone is open to our inspection.
It is subject to much greater changes than the other areas on
account of the strong action of solar energy, which through
various agents is always crumbling down the heights and
carrying the resulting detritus to the sea-margin. In the
course of time this action, termed erosion, would, if not
counterbalanced, reduce the whole Continental Area below
sea-level.

284. Elevation and Subsidence. — The attention of
tourists along the steep coasts of Norway and Scotland is
often attracted by lines of horizontal terraces running par-
allel to each other at various heights above the shore. These
when examined are found to be shelves or notches cut out
of hard rock or soft ground, sometimes covered with pebbles
and sand often containing sea-shells. Behind the terrace
the cliffs are sometimes perforated by caves, which show
every mark of having been excavated by wave action (§ 266).
The terraces are in fact raised beaches, and their position

CHAP, xii The Crust of the Earth 215

proves that the surface of the sea must have sunk, or the
land must have risen since the waves eroded them. In the
south of Scandinavia and the south of England there are
many places where the sea now flows over what was dry
land even during historical time. This encroachment cannot
be due to erosion, as in some cases trunks of trees and walls
of buildings may be seen still standing under the shallow
water, and the necessary conclusion is that either the level
of the sea has risen or the land has sunk. It is difficult
to believe that for thousands of years the sea-level has been
slowly sinking around Scotland and Norway, and at the
same time slowly rising round England and Sweden, and the
only satisfactory explanation of the facts is that the land
must be undergoing gradual elevation in the north, and
gradual subsidence in the south of Britain and Scandinavia.
The regions of recent elevation and subsidence are marked
on Plate XV. Since the average height of the land is much
above sea-level, it is obvious that upheaval has been more
rapid on the whole than erosion, and more general in its
action over the Continental Area than subsidence. The
interpretation of the appearances of the Earth's crust, and
the utilisation of these to throw light on the past history of
the planet, is the subject-matter of geology.

285. Rocks. — The word rock is usually restricted to
the hard stony masses of cliffs and mountains, but the term
rock has a wider meaning. Geologists class as rocks all
substances which occur on or in the crust of the Earth and
have not been recently formed by the decay of living
creatures. Thus the term rock includes soil, sand, stones,
etc., but not bones nor dead leaves. . Some rocks are
uniform in structure like white marble or flint, but in most
cases they appear to be built up of small separate portions
which may be broken or rounded grains as in sandstone,
large crystals of different compounds as in granite (§ 43), or
minute crystals so tightly packed as to be indistinguishable
by the unaided eye as in basalt. The grains of sandstone
or clay are merely fragments of older rocks that have been
broken and worn down before becoming cemented together
again ; but the regularly formed crystals are portions of pure

2i6 The Realm of Nature CHAP.

substances, sometimes elements, although usually com-
pounds, and they are known as minerals. While the term
mineral is restricted to the pure constituents of rocks, the
word mineral is often used to include everything useful
found in the Earth's crust.

286. Rock-forming Minerals. — The crystalline mine-
rals which make up many rocks must have formed slowly
by the combination of their elements or the decomposition
of other compounds. Some were evidently deposited from
solution in water, as, for example, rocksalt and gypsum
(calcium sulphate) ; in both these and in some other unim-
portant instances rocks may be composed of only one
mineral. Other rocks have evidently crystallised from
a state of fusion ; in basalt, for example, the small
crowded and imperfect crystals bear evidence to rapid
cooling and solidification. Rocks, like obsidian, which show
a vitreous or glassy texture, quite smooth, and it may be
free from any appearance of crystals, have evidently been
cooled still more quickly, so that crystallisation could not
take place. After a rock has been formed its minerals may
undergo chemical changes. The process of weathering, or
slow alteration of rocks in air (§ 3 1 o), affects some minerals
more than others. Many new kinds of mineral result from
chemical change brought about by the absorption of oxygen
(oxidation), by the absorption of water (hydration) pro-
ducing zeolites, etc., or by the formation and removal of
some product (decomposition). Mineralogists recognise
about 800 different minerals, most of which, however, occur
in very small quantities. Sixty or seventy only can be
considered important as rock formers. Indeed the bulk of
the rocky crust may be said to be composed of the following
minerals, and those resulting from their alteration — felspar,
quartz, mica, amphibole, pyroxene, and iron oxides.

287. Igneous Rocks, as a class, include all that have
solidified from a state of fusion or have been formed by the
accumulation of fragments thrown out by volcanoes. Most
of them are dense and hard ; they have a glassy or crystal-
line texture, and the minerals of which they are composed
are almost invariably silicates or silica. Silica as flint,

xii The Crust of the Earth 217

agate, and chalcedony is also deposited from solution in
water, but in that case its form is not crystalline. The
way in which igneous rocks occur, whether poured out as
lava on the surface or forced as intrusive sheets between
beds of other rocks, greatly influences the part they take
in determining the scenery of a country.

288. Sedimentary Rocks result from the consolidation
of sediment deposited in lakes or on the margin of the
sea. They are easily recognised by their structure, being
built up of worn rock-fragments of all sizes. Fine muds
consolidate into shales, sand into sandstone, gravel or
pebbles into fine or coarse conglomerates, sometimes
cemented together by the deposition of silica or carbonate
of lime. In consequence of their formation in lakes or on
the sea -shore sedimentary rocks show marks of bedding,
the layers or strata having been laid down horizontally or
nearly so. The beds of rock are not of uniform thickness
throughout, but thin away as the original sediment formed
a thinner layer of deposit far from the land. This class
also includes rocks formed by the accumulation of remains
of animal or plant life, such as decayed vegetation forming
coal, and the shells of mollusca or of foraminifera, the
skeletons of corals and other lime - secreting creatures
giving rise to chalk and limestone.

289. Metamorphic Rocks. — Changes are produced by
heat, pressure, and Earth movements so that it is difficult
in many cases to decide the origin of rocks. It is con-
venient to class all such doubtful cases as metamorphic or
changed. There is much difference of opinion amongst
geologists as to the exact way in which metamorphism
occurs, and we can only indicate here how some of the
changes may take place. Limestone subjected to heat under
pressure crystallises and forms marble ; a bed of clay under
similar influences is altered into slate. The temperature of
rocks deeply buried under a mass of newer sediment is
greatly raised (§ 291), and, as the pressure of the upper
layers is extreme, changes of chemical composition and of
structure are necessarily produced. When great Earth
movements fold over and thrust forward masses of rock, the

218 The Realm of Nature CHAP.

friction produces heat enough to soften the substance which
is rolled out, so that the original structure disappears, the
minerals are altered chemically, and the rock acquires a
flaky texture and is known as a schist. The change may
produce a crystalline structure very similar to that of granite
as in the rock called gneiss. Local or contact metamorphism
is brought about by an intrusive sheet of liquid igneous rock
forcing its way between other strata and altering their com-
position and physical state. The edges of sandstone may
thus be fused into glassy quartzite, and soft clay beds baked
into a hard porcelain-like mass.

290. Dip, Cleavage, Joints, and Faults. — Sedimentary
rocks are sometimes raised by upheaval so steadily and
uniformly that the strata remain horizontal, but far more
commonly the strata are inclined in a particular direction.
The inclination of a bed of rock to the horizon is called
its dip, and is measured by the angle FAB (Fig. 42).
Rocks are found dipping at all angles, sometimes as high

FIG. 42. — Illustration of rock structures. AB, horizontal line ; FF, fault ; S,
slaty cleavage ; J, joints. The long parallel lines mark planes of bedding
making an angle of 19° with the horizontal.

as 90° ; then the strata stand upright. The stresses which
elevate rocks usually act horizontally as a thrust from two
sides, and the particles of the rock sometimes yield and are
flattened out. When this happens the rocks split up more
readily along the flattened sides of the particles than along
its original planes of bedding, and are said to have acquired
cleavage-planes (oblique lines S in Fig. 42). The cleavage-

xii The Crust of the Earth 219

planes of slate, by means of which thin slabs can be split
off, are sometimes at right angles to the planes of bedding.
In more rigid rocks the strain during upheaval is relieved by
the strata cracking more or less nearly at right angles to the
planes of bedding. When these cracks, which are origin-
ally extremely narrow, sometimes invisible, simply traverse
the rock without any distortion (fine lines JJ) they are
termed joint-planes, and it is on account of the existence of
joint-planes in all rocks that the quarrying of stones is
possible without continual blasting. Igneous rocks show
joints, probably the result of contraction in cooling after
solidifying. The fine hexagonal columns of basalt cliffs are
outlined by joint-planes produced by the uniform cooling of
a great mass of rock, the interior of which is brought into
a state of tremendous tension by contraction until relieved
by cracking into columns. A layer of wheat starch on
drying is strained in exactly the same way by contraction
throughout the mass, and similarly cracks into many-sided
columns. The same phenomenon has been observed in
partially solidified beds of moist sand and clay. When the
rocks on one side slip along a crack so that the strata no
longer correspond (F, Fig. 42) it is termed a fault ; the
lower side is called the downthrow, the upper the upthrow.
Parallel lines 'of faults usually mark the borders of regions
where upheaval has taken place and the strata preserve a
low dip. When a fault shows at the surface no sudden rise
of level marks the upthrow side, as the action of erosion is
continually smoothing away such inequalities perhaps as
rapidly as they form.

291. Temperature of the Earth's Crust. — Whatever
be the nature of the surface rocks, the Sun's heat penetrates
them slightly and slowly. By observations in Britain with
thermometers fixed at various depths beneath the surface of
the land, it has been proved that the difference of day and
night temperatures vanishes at about 3 feet, and that the
greater and more regular difference between summer heat
and winter cold becomes less and less perceptible as the
distance increases, and dies away within 40 feet. The
average temperature shown by the rock thermometers on

220 The Realm of Nature CHAP.

the Calton Hill at Edinburgh during the eight years 1880-
1887 were, at the depth of 2 feet, 39°-4 in February and
53°-o in July, an annual range of i3°-6, with the minimum
and maximum in winter and summer respectively ; and at
the depth of 20 feet, 46°-9 in January and 45°-4 in July, a
range of only i°-5, with the maximum temperature in
winter and the minimum in summer, showing that it
requires six months for the conduction of heat from the
surface to the depth of 20 feet (compare § 229). A
zone of invariable temperature lies beyond the reach of
solar heat and is found at different depths in different
places, being deeper in regions of great annual range of
temperature. Beneath the invariable zone temperature
increases with depth in all parts of the world. In deep
mines the air is always oppressively hot, and the water from
deep Artesian wells is warm in proportion to their depth.
The Underground Temperature Committee of the British
Association, after collecting all the observations of tempera-
ture at great depths which have been taken in mines and
deep borings in all parts of the world, concluded that the
rate at which temperature increases downward averages i°
in each 5 5 feet ; Professor Prestwich, after a full discussion,
puts it at i° in 45 feet.1 In some instances the increase is
more rapid, in others less so, according to the conducting
power of the rocks. The temperature I mile beneath the
surface must be about 100° higher than that of the invari-
able layer, and at the depth of 30 miles the temperature
must be high enough to melt all known substances. At
greater depths than this the rate of increase of temperature
must diminish, in accordance with calculations from the
condition of small heated bodies. Professor Tait calculates,
from the gradient of temperature and the conductivity of
rocks, that through every square- foot of surface the interior
of the Earth is losing heat at the rate of 230 units (§65)
per annum, or sufficient to warm I J Ibs. of water from the
freezing to the boiling point.

292. Interior of the Lithosphere. — Surface rocks have
an average density of 2-5, and the deep-seated igneous
rocks a density of about 3-0, while the mean density of the

XII

The Crust of the Earth 221

Earth, as a whole, is 5-5 (§ 85). Unless the enormous
internal pressure of the weight of the Earth's mass were
counteracted the rock substance would be compressed into
less space, and the mean density of the Earth would be
greatly raised. The high temperature of the interior causes
the rock substance to expand against the pressure of gravity,
and so maintains the comparative low mean density which is
actually found. The great pressure in its turn counteracts
the effects of high temperature by raising the melting-point
of the rock substance (§ 72), and so preventing it from
assuming the liquid state. Astronomical observations show
that the Earth behaves as if it were a solid ball, and Sir
William Thomson has calculated, from the imperceptible
tidal effect produced in the lithosphere, that it must be as
rigid as if it were composed throughout of solid flawless
steel.

293. Volcanic Action. — Volcanoes are conical moun-
tains in communication with openings in the Earth's
crust, which continually or occasionally throw out steam,
hot stones, or white-hot melted rock called lava. Professor
Prestwich believes that there are hollows in the lower part
of the Earth's crust full of molten rock, which is squeezed
out by the pressure exerted by the Earth's crust contract-
ing slightly as it cools. Mr. Mallett, on the other hand,
thinks that the heated interior of the Earth in cooling con-
tracts more rapidly than the crust, shrinking away from
it and leaving hollows, into which the solid rocks subside
with much straining and crushing. The motion of the
rocks converted into heat melts some of them, and the
cracked crust allows the hot fluid to escape. Other author-
ities point out that since the lithosphere is solid only on
account of the pressure of the crust upon it (§ 292), any
relief of pressure produced by the shrinking in of the central
mass, or by the cracking of the strata above, must allow the
rock substance to liquefy suddenly and with explosive vio-
lence. All volcanic activity is accompanied by the emission
of great quantities of steam, to the expansion of which
geologists believe the great power of volcanic explo-
sions is due. It is probable that a good deal of under-

222 The Realm of Nature CHAP.

ground water (§ 313) creeps down by capillarity deep into
the heated layers under the crust, there combining chemi-
cally with the rock under pressure, but always ready to
resume the form of steam if the pressure is relaxed.

294. Volcanic Materials. — In addition to water- vapour
volcanoes throw out other gases in great abundance. Hydro-
gen and oxygen, resulting from the dissociation of water at
high temperature (§§ 7 1, 220), combine as they rush out, pro-
ducing violent explosions and great flames. These flames,
together with the reflection of glowing liquid rock on the
overhanging vapour, gave to volcanoes the popular name of
burning mountains. Sulphurous acid, sulphuretted hydrogen,
nitrogen, carbonic acid, hydrochloric acid, and the vapour
of boracic acid, also occur very frequently, being produced
by the chemical action of heat and water-vapour on minerals
in the volcano. Lava, or molten rock, is the most im-
portant of all volcanic products. Welling over the cup-like
hollow at the summit it flows down the sides of the mountain
in white-hot streams, which gradually solidify on the outside,
and advance like a glacier of slow- moving viscous rock,
ultimately hardening into crystalline igneous rocks, such as
basalt and trachyte. Pumice is a sponge-like glassy rock
which forms over the surface of certain lavas, being frothed
up by the vapours which are continuously given off. Scoria
are the rough cindery upper portions of very viscous lavas
formed in the same way. During eruption immense quan-
tities of these crusts of lava, together with stones torn from
the throat of the volcano, are thrown out. The finer grained
loose materials are known as dust or volcanic sand. A light
gray powder, known from its appearance as ash, is the
solidified spray of molten rock similarly thrown into the air
by the explosion of escaping vapours.

295. Volcanic Mountains. — Wherever a crack or
fissure of the Earth's crust allows volcanic activity to assert
itself the material driven out from below accumulates and
solidifies on the foundation of the surface rocks, which are
usually sedimentary, and a cone or mountain of accumula-
tion (contrast §§ 303, 329) is thus piled up. If the lava is
very fluid and escapes from a long fissure it may flood

XII

The Crust of the Earth 223

extensive tracts of land with nearly level sheets. Such lava
floods now occur very rarely, although they were common in
past ages. Volcanoes are usually connected with their
subterranean lava-stores by a comparatively narrow pipe, in
which the lava wells up and overflows on all sides. A very
hot and fluid lava forms a hill of gentle slope ; a cooler or
viscous lava, which solidifies before it flows far, builds
a steeper mound. In either case the centre is formed by a
trumpet-shaped hollow called the crater, the rim of which
is raised by each successive outflow. In some instances
cones are built up round the orifice of a volcano before the
flow of lava commences, and are composed of volcanic
ashes, pumice, and broken stones, etc., the ejection of
which is the prelude to an eruption. When compacted by
the pressure of its own weight, and cemented together by
the chemical action of rain, such a deposit forms the rock
known as volcanic tuff. When fluid lava rises in the pipe
of a tuff cone the pressure it exerts frequently bursts an
opening in the side, through which a stream escapes.
When the force of the eruption is small and the walls of
the cone strong, the ascending lava may cool down in
the funnel and seal the volcano by solidifying. The most
common form of volcanic mountain is of composite struc-
ture, being built up of alternate layers of tuff and flows of
lava. Such a cone grows slowly, and, as represented in
Fig. 43, is the outcome of several periods of activity and
quiescence. The explosions which herald a new eruption
shake the mountain, and cracking the walls allow tongues
of lava to penetrate in all directions from the central shaft.
These sometimes force a way to the exterior and form small
cones on its slopes, from which streams of lava flow.
Sometimes they harden as dykes or walls in the fissures into
which they were injected. The cone cac is represented as
formed by a late outflow of lava, and occupies the middle
of an old crater which had become plugged up and was
then partially destroyed by an explosion.

296. Volcanic Eruptions. — Volcanoes are often classed
as active, dormant, and extinct. Stromboli, in the Mediter-
ranean, is the type of a continuously and moderately active

224 The Realm of Nature CHAP.

volcano. It serves as a natural lighthouse and also as an
automatic storm warning, as its activity is always greatest
when the atmospheric pressure is low and gales may be
expected, while the violence of its eruptions is much reduced
when the barometer rises. Volcanoes from which no erup-

FIG. 43. — Ideal Section of a volcano. SS, stratified rocks of crust ; bb, old lava
solidified in throat of volcano and in dykes ; aa, new outburst of lava ; cc,
old crater ; a, new crater. (After J. Geikie.)

tion has ever been recorded are called extinct ; those which
break out at intervals are said to be dormant during their
periods of tranquillity, but the distinction can hardly be
drawn with confidence. Vesuvius is the type of volcanoes
which are occasionally dormant and sometimes supposed
to be extinct. The commencement of activity after a
dormant period is usually preceded by earthquakes and
subterranean noises, indicating that pressure is accumulat-
ing in the heart of the mountain. Hot springs break out
on the slopes, and gases and hot vapour rise in increasing
volume from the crevices in the crater. Then a terrific
explosion occurs, shattering the solid lava plug and perhaps
destroying the entire cone ; volumes of water-vapour shoot
up into the air, mixed with clouds of dust that darken the
sky and fall like snow over the mountain slopes and sur-
rounding country. Flashes of lightning dart from the over-
hanging cloud, the friction of dust and vapour on the air
causing great electrical disturbance, and the noise of thunder
is added to the roar of the escaping steam and volcanic

xii The Crust of the Earth 225

explosions. The clq^d reflects the fierce glare of the lava
welling up in the crater, from which the explosions and
bombardment of heated stones become more frequent, until
finally the molten rock surges up to the lip and pours over
as a river of fire. The vast quantities of water -vapour
meanwhile condense into floods of rain, which convert the
dust-strewn slopes into torrents of hot mud, more voluminous
and often more important in obliterating the surface features
of the scenery than the lava itself. Such a mud deluge
destroyed the Roman town Herculaneum when the first
recorded eruption of Vesuvius took place in the year 79.
Snow-clad volcanoes like Etna and Cotopaxi send down still
more serious floods on account of the sudden melting of their
snow.

297. Krakatoa. — On 27th August 1883 the volcano of
Krakatoa, a small island in the middle of the Strait of
Sunda, terminated a set of comparatively quiet eruptions
by the most terrific explosions which have ever been wit-
nessed. A great crater had been previously formed, and
sea -water gained access to the crater full of molten lava-
as the mountain walls were gradually broken down. The
result was a temporary reduction of activity as the cold
water chilled the surface, and then the grand explosion
shot out a column of dust and vapour 20 miles high with
a roar that was heard at Rodriguez 3000 miles distant,
and attracted attention over one-thirteenth of the surface of
the globe. The concussion caused by this explosion was
severe enough to break windows and crack walls in
Batavia I oo miles away, and the disturbance of the air was
shown by the records of barographs to have expanded as
an air-wave from Krakatoa until it spread round a great
circle 180° in diameter, then contracted to the antipodes of
Krakatoa, whence it was reflected back, and so continued
pulsing round the world four times from the centre of dis-
turbance to the antipodes, and three times back again.
Two -thirds of the island were blown away, most of the
material being deposited in the Strait of Sunda, where
several new islands formed of piles of tuff and ashes ap-
peared, and after a few months were washed away by the

Q

226 The Realm of Nature CHAP.

waves. For weeks fields of floating pumice made naviga-
tion very difficult. The disturbance in the sea pro-
duced a wave more than 100 feet in height, which rushed
upon the neighbouring coasts, overwhelming lighthouses
and towns, and stranding ocean steamers in mountain
valleys. More than 36,000 people were washed away and
drowned. Part of Krakatoa was scattered as the finest
dust through the air and carried to every part of the Earth,
its presence being detected in rain, and by the magnificent
red sunsets (§ 162) that were visible everywhere during the
autumn and winter of 1883 and i884.2

298. Distribution of Volcanoes. — Volcanoes are usually
found in the line of great mountain chains and near the sea
coast. They form a " ring of fire " round the Pacific Ocean,
being very numerous in the Andes, and more widely spaced
along the plateau of Central America, the coast ranges of
North America, and the Aleutian Islands. Thence they
increase in frequency along the island festoons of Asia, and
come to a maximum in the Malay Archipelago and New
Zealand. The West Indies, many of the small Atlantic
islands, the Mediterranean coasts, Iceland, and Jan Meyen,
also contain active volcanoes, but none are known with
certainty in the heart of continents. The distribution of
active volcanoes is shown in Plate II.

299. Earthquakes. — The crust of the Earth is elastic
and readily transmits wave -motion. Any cause which
produces a local disturbance of the crust sets up a series
of waves, which may become apparent on the surface
in the quick up-and-down or to-and-fro shaking of the land
called an Earthquake. Earthquakes of considerable severity
accompany volcanic action, and are accounted for by the
jarring of the Earth's crust by successive explosions, but
they are by no means confined to volcanic regions. The
falling-in of underground caverns may give rise to earth-
quakes of slight intensity. Very severe shocks accompany
the elevation of land when that process takes place in
sudden steps of a few inches or a few feet at a time, in
consequence probably of the strata, subjected to the power-
ful stresses set up by the contracting Earth, snapping under

XII

The Crust of the Earth

227

the strain. Every large fault found in rocks must have
given rise to earthquakes. Professor Milne points out that
most shocks originate along the lower part of the slopes of
the world ridges. This coincides with the lines along which
the process of elevation is going on most rapidly, and where
the strata are consequently subject to accumulating stresses.
The regions in which earthquakes are common are
coloured light blue on Plate II. and those where they
are very severe and frequent are coloured in a darker shade.
Many geologists believe that sea-water filtering through the
bed of the ocean, or buried to a great depth in the lower
layers of terrigenous deposits, causes explosions in the in-
tensely heated region below, and that all great earthquakes
originate from this cause and are essentially volcanic ;
the upheavals accompanying earthquakes would thus be
reckoned as ..heir consequences, not their causes.

300. Propagation of Earthquakes. — If the crust of the
Earth were perfectly uniform in substance, and a shock were
communicated to it at any point by a sudden yielding to
stress, a wave would spread in concentric spherical shells
from that centre like the sound-wave from a vibrating bell
in air (§ 58). In the rock the wave travels more rapidly
than in air, and the to-and-fro movement of each particle
passing it on is very small. If the shock is given at A
(Fig. 44) the circles
I. II. III. show the
position of the crest of
the wave at intervals
of i, 2, 3 seconds.
The wave is shown
reaching the surface
at B, directly over the
centre of disturbance,
in 3 seconds ; there
it strikes perpendicu-
larly from beneath, although the force of the shock
is greatest at a little distance from B. A second later
the wave reaches the surface along a circular path
(IV.-IV.) and strikes obliquely upward; at the posi-

FIG. 44. — Earthquake wave, illustrating Mallet's
method of finding the depth at which an
earthquake originates.

228 The Realm of Nature CHAP.

tion reached in the next second, the stroke is still more
oblique along a wider circle, and is more feeble on account
of loss of energy due to friction among the rock particles.
The distance of the centre of disturbance beneath the surface
may be calculated by observing the angle from which the
shock comes at different points and constructing a diagram
somewhat like the above. It appears from many observa-
tions recorded by Mallet and others that the depth of
origin rarely or never exceeds 35 miles. Although the
crust of the Earth is probably homogeneous at a consider-
able depth, it is very far from being so in its upper part, and
the earth-wave consequently travels at an unequal rate in
different directions as it nears the surface. A thick bed of
sand or loosely compacted and inelastic stones (S in Fig. 44)
greatly retards and may entirely absorb the wave by fric-
tion between the particles, so that no shock would be felt
on the surface, while houses built on the hard rock all round
would be shaken severely. On the other hand, a small
deposit of sand or alluvial soil occupying a shallow hollow
would be jarred by confused earth-waves from every side
and buildings on it damaged most severely.

301. Earthquake Shocks. — The area of the surface
shaken depends on the intensity of the original shock and the
nature of the Earth's crust at the place where it occurs. The
memorable earthquake that destroyed Lisbon in 1755 shook
a space four times as large as Europe, and probably made
the whole Earth tremble ; and that which damaged Charles-
town in 1886 was felt over 3,000,000 square miles, from
Cuba to Canada, and from Bermuda to the west of Missouri
State. By the use of delicate seismometers the dying
tremor of an earthquake-wave may be detected at a great
distance, beyond the limit of unaided observation. Thus
the tremor of an earthquake on the Italian Riviera in 1887
was distinctly recorded by instruments in Greenwich Obser-
vatory. The shaking of the Earth's crust throws down any
slenderly supported rock masses like perched blocks, natural
bridges, and earth pillars, and when such structures are
conspicuous features of the scenery the district may be
reckoned free from risk of serious shocks. Landslips, the

xii The Crust of the Earth 229

opening of great fissures, and other surface changes often
result from earthquakes, which may thus alter the course
of rivers and form or drain lakes. But the occasional
destruction of cities and houses, and the peculiar sensation
of terror and helplessness which earthquakes produce in
most minds, are apt to give an erroneous and much ex-
aggerated idea of the power of such shocks in forming the
scenery of the globe. The researches of Professor Milne
and other scientific men in Japan, and the extensive
use of seismometers or earthquake measurers, have
thrown much light on the nature of shocks and tremors.
The to-and-fro or up-and-down motion of the Earth in a
shock severe enough to throw down houses is probably not
much more than an inch. A model constructed by Pro-
fessor Sekiya (the professor of Earthquake Phenomena in
Tokyo) of the path described by a particle during the pas-
sage of an earthquake shock resembles a tangled hank of
twine.3 It is the shaking produced by such a complex dis-
turbance rather than the actual lifting of the surface that
produces destructive effects. Some of the tremors detected
by seismometers are not produced by the internal energy of
the Earth. It has been proved in Italy that changes of
atmospheric pressure jar the elastic and sensitive crust ; and
in Japan a gale blowing against a range of mountains has
been found to set the greater part of the island quivering.

302. Wrinkling of the Earth's Crust. — The Earth
necessarily contracts as it cools, and the crust composed of
stratified rocks falls into wrinkles in order to adapt itself to
the reduced area of the globe, just as the skin of an apple
gradually becomes wrinkled in adapting ftself to the drying
and shrinking fruit. Reasons have already been given
(§ 278) for believing that from a very early period the
Abysmal and Continental Areas have occupied their present
position, and probably they represent the troughs and crests
of the earliest Earth wrinkles. The primitive furrows
themselves must have disappeared as the crests were worn
away by erosion, and the resulting sediment was deposited
on the upper slopes of the hollows, to be consolidated in
turn and form part of a new set of wrinkles, which shared

230

The Realm of Nature

CHAP.

the same fate and passed on the process. Some geologists
believe that as denudation lightens the ridges and loads the
hollows, the Earth's crust is strained by the redistribution
of the pressure on it ; that consequently the strata snap with
a succession of earthquake shocks, and the parts loaded
with deposits sink, while those lightened by the effect of
erosion are upraised. Other geologists take an opposite
view of the result of sedimentation (§ 304). The typical
form of an Earth wrinkle is a gentle ridge, A, accompanied
by a gentle hollow, S (Fig. 45). The curved strata of the

FIG. 45. — Strata bent into anticline A and syncline S.

ridge are said to form an anticline, because at the summit A
the strata, as shown by the arrows, dip or incline away from
each other. The curved strata of the trough are similarly
said to form a syncline, as at S the strata dip together
or toward each other. Even although the wrinkled crust
should be worn smooth by erosion to form the surface ss',
it is still easy to tell by observing the dip of the strata
where the ridge and the hollow were situated. Thus rock
structure is not concealed by surface change. Synclines
and anticlines are ridged up in consequence of the lateral
pressure or tangential thrust produced by the downsink-
ing of part of the crust. The tremendous lateral pressure
effected by a great subsidence throws the strata on both
sides into sharp anticlines and synclrnes, while at a greater
distance from the origin the wrinkles are low and uniform.
The Geological Survey of Scotland has brought to light
many remarkable proofs of the intensity of the thrust
which ridged up the western margin of Europe in ancient
times. Sometimes the compressing force was so violent
that the strata, instead of puckering up into anticlines and

XII

The Crust of the Earth

231

FIG. 46. — Production of thrust-planes.
The strata represented are layers of
clay and sand separated by cloth ;
they were laid down horizontally,
and ridged into the position shown
by a thrust acting in the direction
of the arrow.

synclines, cracked, and allowed one part to be lifted up and
thrust bodily over the other, in certain cases for a distance
of ten miles or more. The
consequent crushing, faulting,
and folding produced a very
confused arrangement of the
rocks, and extensive meta-
morphism. The structure of
the region was extremely
puzzling until Messrs. Peach
and Home traced out the
thrust-planes along which the
sliding movement took place.
Figure 46 represents the pro-
duction of thrust-planes, A,
in a series of experiments on mountain structure recently
carried out by Mr. H. M. Cadell.4

303. Mountains of Elevation. — When lateral com
pression of the Earth's crust takes place the strata pucker
up along the line where they are weakest, and are thrown
into a series of anticlines and synclines growing sharper
and higher toward the central line. The rocks in the
interior of the mass and those occupying the hollows of the
synclines are necessarily compressed, heated, and altered,
while those on the outer curve of the anticlines are stretched
and split in the process. A mountain range is formed in
this way, with anticlines as ridges and synclines as longi-
tudinal valleys between them, the slopes of the surface
corresponding to the dip of the strata. The true mountain
ranges of the world are all of this character, the Alps,
Himalayas, and Andes being, typical examples, and it is
significant that all such ranges are situated near the edge
of great depressions, the subsidence of which probably
accounts for their uplifting. Rocks of recent sedimentary
origin always form the first gentle undulations on the slope
of a mountain range, but toward the main ridge the strata
are of greater age and more contorted, while in the centre
there are masses of schistose or igneous rocks, probably pro-
duced either by the rolling and compression of the uplifted

232

The Realm of Nature

CHAP.

strata or by volcanic action from below. Figure 47 repre-
sents a section across the chain of the Alps from north
to south, the dotted lines indicating the anticlinal arch.

Mont Blanc

FIG. 47. — Section of the Alps, a, Tertiary rocks ; b, secondary and primary
rocks ; c, central core of schistose and igneous rocks.

Erosion by solar energy probably accompanies the whole
process of ridging up a mountain range, and after the
elevation is complete the aspect of its scenery, the form of
its slopes and valleys, are increasingly due to this cause.
Streams flowing down opposite sides of the slope of the
long mountain ridges hollow transverse valleys, and so cut
the ridge into peaks. Two transverse valleys meeting in a
col or pass allow of easy access between the longitudinal
valleys which lie between the ridges. Anticlines are much
more rapidly eroded than horizontal strata, even when the
surface may have the same slope, for the direction of the
joint planes and the dip of the rocks favour the formation
of landslips. An anticlinal mountain may be viewed as
geologically unstable, like a pile of inverted saucers. In
many cases the low mountains of the Scottish Highlands,
which in remote ages excelled the Alps in height, are now
carved out by erosion (§ 329) from synclinal strata — a form
of structure which gives great stability, like a pile of saucers
set one within another right side up.

304. Theories of Mountain Origin. — The theory most
generally held is that horizontal strata subjected to great
thrusting stresses have wrinkled up along a line of weakness
in the Earth's crust, by which the whole crumpling is con-
fined to a narrow area, the actual lifting power being
derived from the contraction of the heated interior of the
Earth. Mr. Mellard Reade has brought forward another

XII

The Crust of the Earth

233

B

theory of great ingenuity. Observing that all mountains of
elevation are of comparatively recent formation and are
ridged up out of thick sheets of sedimentary rock, he sup-
poses that the accumulation of sediment produces the
mountains. He points out
that if a large and deep
hollow in the Earth's crust
is rilled up with sediment
to the line AB (Fig. 48) at
the ordinary surface tem-
perature, say 60°. the mass
now forming part of the

FIG. 48.— Mellard Reade's Theory _ of
Mountain Building. Light shading
shows original crust of the Earth,
dark shading sediment ; dark lines
original isotherms, fine lines isotherms
after deposition of sediment.

Earth's crust will grow
warmer until, if the surface
temperature remains at 60°,
that at the depth of 1200 feet at 80°, and so on (dark lines in
figure), the covering in of the cavity raises the tempera-
ture throughout by preventing the loss of heat through
the crust, the new positions of the temperatures of 60° and
80° being shown by fine lines in figure. The warmed up
strata necessarily expand, and as they cannot expand side-
ways or downward on account of the solid walls of the depres-
sion, they must expand upward, and the surface of the sheet
of sediment is thrown into a series of ridges, true synclines
and anticlines, like the surface of a cake as it rises in being
baked. In Jhis theory also the energy which does the
work of elevating the mountain range is derived from the
interior of the Earth.

REFERENCES

1 J. Prestwich, " On Underground Temperature," Proc. Roy.
Soc. xli. (1886).

2 The Eruption of Krakatoa, edited by G. J. Symons. Triibner
and Co., 1888.

3 "Model of an Earthquake," Nature, xxxvii. 297 (1888).

4 H. M. Cadell, " Experimental Researches on Mountain Build-
ing," Tr&ns. Roy. Soc. Ed. xxxv. 337 (1888) ; or Nature^ xxxvii.
488.

BOOKS OF REFERENCE

See end of Chapter XIV.

CHAPTER XIII

ACTION OF WATER ON THE LAND

305. Land Sculpture. — The crests of the world ridges
upheaved by the internal energy of the cooling Earth in gently
undulating strata, or in the sharp broken anticlines of moun-
tain ranges, are subjected to erosion by solar energy acting
through various agencies. Earth energy is continually at
work raising the level of the elevated half of the globe, and
depressing the Abysmal Area. Sun energy acts as a leveller,
continually cutting down the high places and building up the
hollows with the resulting detritus or crushed fragments. The
process of uncovering old rocks by erosion of newer ones is
termed denudation. The rate at which it proceeds depends
to a very large extent on the chemical composition of the
rocks, on their tenacity, their dip, and joints (§ 290), and
it is to the variety of these conditions that the* great variety
and character of the existing scenery of every part of the
world is due.

306. Work of direct Sun-heat. — One unit of heat (§65)
when absorbed by one pound of an average rock raises
its temperature about 4°, compared with i° in the case of
water. In consequence of this low specific heat, although
the heat does not penetrate far (§ 291), it greatly heats and
expands the superficial layer. At night the temperature
falls quickly by radiation and the chilled rock con-
tracts. In dry tropical regions the alternate heating and
chilling causes the surface layers to split off in angular
pieces or thin sheets, which, when the face of the rock is

CHAP, xiii Action of Water on the Land 235

steep, slip down toward the base and form a talus or slope
of detritus.

307. Work of Wind. — Air in motion (§ 175) is a
powerful vehicle of energy for eroding rocks, sweeping away
the fragments loosened by sun-heat in the tropics, and
keeping the hard rock surface exposed to destructive radia-
tion. The Sahara and some other deserts bear undoubted
traces of having once formed the beds of shallow seas, so
that their sand is partly of marine origin ; but the amount
of sand is always increasing by wind action. Clouds of sand,
driven by the wind like showers of hard angular hailstones
against the face of the bare rock, cut into the surface as the
artificial sandblast etches glass. In Kerguelen, situated in
the Roaring Forties, all the exposed rocks are chiselled into
grooves from west to east by wind-driven sand. Dunes, or
wave-like ranges of sandhills, are piled up by the wind on
deserts or broad sea-beaches, and attain the height of about
60 feet round the North Sea, and sometimes over 600 feet
in the Sahara. The Bermuda Islands owe their configura-
tion entirely to dunes of coral sand, some of which are 250
feet high, and have been hardened into a kind of limestone
by the percolation of water.

308. Wind-borne Deposits. — Sand driven by the wind
is an important ingredient in deep-sea deposits (§ 269), and
rivers flowing across arid regions are kept charged with
sand and dust in the same way. When the prevailing \vind
blows inland and the rainfall is scanty, sand and dust may
be carried far before being deposited. The remains of
many ancient cities in Egypt, Mesopotamia, and Central
Asia have been covered by such dust, a"nd their sites are
now uninhabited deserts. The name loess is given to a
deposit of very fine clay found first to the north of the Alps
and amongst the Carpathians, where it often fills up valleys
and covers large areas of ground at various levels. It is
much more abundant in the north of China, where it covers
thousands of square miles as a dense yellow earth to the
depth of more than 1000 feet. The loess of Europe and
of North America (Mississippi basin) is believed by most
geologists to be the sediment of the greatly swollen rivers

236 The Realm of Nature CHAP.

of the glacial period (§ 352) subsequently modified by wind
and other agencies. The great German geologist, Professor
von Richthofen, who studied the deposit in China, came to
the conclusion that there it resulted from the gradual
accumulation of the fine dust carried by wind from Central
Asia, and brought to the ground by the moister air near
the coast.

309. Water as a Sculpture Tool. — Water is the agent
by which the Sun's energy is usually brought to bear upon
the land. The process consists in the Sun's heat evapora-
ting the surface of the hydrosphere and depositing it as
snow or rain on the land. The work done against gravity
in raising water-vapour to the height at which it condenses
to the liquid state, as rain, is converted into potential energy,
all of which would be restored in heat to the hydrosphere
if the rain fell without friction back to the sea again. Rain
evaporated before it reaches the sea has a new store of
potential energy imparted to it, like a clock wound up
before it has run down. The height to which a quantity of
water is raised by the Sun's heat is a measure of the dynamic
power which the water can exert in its descent (§ 49).
This power in the case of raindrops is expended in heating
the air they fall through, and in friction against the channel
down which the water flows, in breaking off portions of
rock against the power of cohesion, and in dragging stones
or gravel along. The expended energy finally takes the
form of diffused heat in the water and rocks. The chemical
properties of water and its effects as a solvent are also
brought into action by sun-heat, which separates it from the
salts in the sea, shakes it with the gases of the atmosphere,
and pours this powerfully solvent and oxidising solution over
the rocks. The hydrosphere might be compared to a bee-
hive, whence the sunlight .attracts swarms of workers in the
form of raindrops, which after a longer or shorter journey
return laden with spoil from the land.

310. Weathering. — Rain, assisted by the dissolved
gases and surrounding air, acts chemically on rock surfaces,
producing changes known as weathering. Next to beds of
rock-salt and gypsum (calcium sulphate), limestone is the

xiii Action of Water on the Land 237

rock which is dissolved most readily. The waste of the
hard and massive surface is often shown only by the way
in which it becomes studded with less soluble nodules or
fossils originally hidden in its substance. Sir Archibald
Geikie has calculated that by the acid-laden rain of towns
one-third of an inch is removed from the surface of marble
monuments in a century. Insoluble sulphides, such as
that of iron, are rapidly oxidised by air in the presence of
moisture to form soluble sulphates, and when this process
goes on in the pores of a rock the expansion of the crystal-
lised salt splits the block into thin layers. This action is
the basis of the common way of making alum. In the case
of granite (§ 43) and most other rocks the process of
weathering is more complicated. Some of the minerals are
decomposed. In felspar, for instance, the silicates of potash,
soda, and lime are changed to carbonates which are washed
away, while the silica and the more resisting silicate of
alumina remain as a soft crust of kaolin or china clay,
valuable for making porcelain. Granite has been found
weathered in this way in South America to the depth of 600
feet. Rocks containing, iron usually become brown or
reddish in colour, although the freshly broken rock may be
white or gray. The lines of stratification and joints (§ 290)
of rocks are sometimes etched out by weathering, so that
the face of a cliff assumes the appearance of a gigantic wall
of masonry. The crumbling of rocks in rainy regions is
assisted by the action of the Sun in drying and warming
the surface, which may then be splintered into flakes by a
shower of cold rain. Rain soaking by capillary attraction
(§ 39) through the weathered crust and into the pores of
the solid rock is frozen in cold weather, and the ice, ex-
panding as it forms, acts like a multitude of minute wedges
driven simultaneously in all directions. When the thaw
comes, the bases of cliffs and banks are strewn with
weathered crusts and stones, often of a great size, broken
off in this way.

311. Soil. — Weathered rock is the basis of soil, which
accumulates to the greatest depth on level or slightly- inclined
land. When the rocks yield only angular grains of quartz

238 The Realm of Nature CHAP.

or silicates, the soil is pure sand, which allows water to drain
away so rapidly that in a dry region no moisture is retained.
When only the finely divided silicate of alumina results from
weathering, the soil is a pure clay, forming when wet a
sticky paste through which water does not easily pass. In
rainy places clay land is consequently always wet and stiff.
Sand and clay are both produced from the decay of most
rocks, and the mixture of these constituents forms loams,
which, according to the proportion of sand and clay, are
either moderately porous or moderately retentive of moisture.
Almost all rocks contain smaller or larger quantities of car-
bonate of lime, iron, and sulphates or phosphates of the
alkalies potash and soda, all of which form part of the
resulting soil. Rain contributes salts of ammonia (§ 152),
partly derived from the air, partly from decomposing animal
matter, and these are ultimately oxidised (§ 401) to nitric
acid, which forms nitrates. Plants pulverise the rock frag-
ments of the lower layers or sub-soil by their roots pene-
trating the crevices and acting as wedges. The decay of
vegetation finally produces vegetable mould. Earth-worms
have been shown by Darwin to assist in the formation of
soil by dragging decaying vegetation into their burrows
and by swallowing the earth, which is thrown out again on
the surface as extremely finely -powdered worm -castings.
Professor Henry Drummond points out that a similar ser-
vice is rendered by the termites or white ants of tropical
Africa.

312. Work of Rain. — Rain is the chief agent engaged
in the slow but continuous moving on of particles of broken-
up rock-crust and soil from high ground to low ground, and
from low ground to the sea. When rain falls on beds of
clay or soft rock mixed up with harder pebbles or boulders
it washes away the softer material, except where it happens
to be protected by a stone, which in course of time remains
capping a pedestal. The largest examples of such earth
pillars are those of the Sawatch region of North America,
which attain a height of 400 feet. Mount Roraima, in
north-eastern South America, a nearly perpendicular moun-
tain of soft sandstone capped with hard conglomerate, and

xiii Action of Water on the Land 239

rising 5000 feet above the plain, is believed by Mr. Im
Thurn, who first succeeded in reaching its summit, to be
simply a rain-wrought earth pillar on a gigantic scale ; the
soft sandstone, when freshly exposed, being rapidly washed
away by the torrents of one of the rainiest regions of the
world, while the harder conglomerate resists erosion and
protects the rock beneath.

313. Underground Water. — Of the rain which falls
upon the surface of the Earth in a region like Great Britain
it is estimated that one -third is returned to the air by
evaporation, one-third flows off over the surface, and one-
third sinks into the ground. Where the rocks are imper-
meable by water, such as shales and stiff clays, more flows
off over the surface, but where they are permeable, like
sandstone, gravel, or many limestones, a greater proportion
soaks through. The movement of water underground is
slow or rapid, according to the facility with which the rocks
allow it to work its way through them. In time some water
undoubtedly filters downward, until, under the influence of
great pressure and high temperature, it combines chemically
with the rock substance (§ 293), but the greater part of it
returns to the surface at a level lower than that it started
from. Each variety of rock can absorb by capillarity (§ 39)
a certain definite proportion of water, which remains in it as
in a sponge, until enough accumulates to overcome friction,
when it percolates through. The rate of percolation is
often greatly increased by the presence of cracks or joints.
Soft porous rocks becoming saturated may give rise to
landslips, especially in cases where they, rest on beds of
stiff clay that become lubricated and slippery when wet.
As the percolating water dissolves out narrow crevices
between the grains of rock, the pressure of the strata above
forces them together again, thus producing a slow general
settling down of the land-surface.

314. Wells and Springs. — When a thick layer of per-
meable rock rests on an impermeable bed, water accumulates
until the pressure of the liquid suffices to force a way
between the rocks and so reach the surface on the slope of a
hill or the side of a valley. This outflow of underground

240

The Realm of Nature

CHAP.

water is termed a spring, and its origin is indicated at s
(Fig. 49). If a pit is dug through the upper rock, as at
W, deep enough to pass below the limit of saturation /,

FIG. 49.— The origin of springs. (After Prestwich.) The darker shading repre-
sents rocks impervious to water, the light shading shows permeable rocks.
W, a surface well ; the curves on the shaded part show different positions
of the limit of saturation ; s's, springs ;f, fault.

water will ooze in from all sides, and a surface well will be
formed from which water may be lifted by a bucket or
pump. The limit of saturation rises in wet weather, but
sinks in a dry season. When it rises from / to /' the water
in the well deepens, when it sinks to the lowest curve
shown, the well becomes dry, and if the height is not
sufficient to overcome the resistance of capillarity the
springs also cease to flow. When layers of permeable
and impermeable rocks occur one above another, the
water which soaks into the permeable rocks at the surface
filters down along the junction with the impermeable
layer, and if a fissure or fault occurs (/ in the figure)
so that the permeable layer is brought against an im-
permeable wall, the water
will be forced up along the
crack and will reach the
surface as a fault-spring if
the ground -level is below
that of the limit of satura-
tion. Artificial bores driven

FIG. 5o.-Artesian wells, pp, permeable through an impermeable
rocks ; L, /, limits of saturation, show- stratum of rock to reach the

ing level beyond which water from the , . , ,

bores aaa cannot rise. water-bearing strata below

are termed Artesian wells,

from the old name of part of the north of France where
they were largely used. By this means a copious water-

xiii Action of Water on the Land 241

supply may often be obtained even in rainless deserts, as
the deep layer of permeable rock may come to the surface
at a great distance in a rainy region (Fig. 50).

315. Thermal and Mineral Springs. — When the dip of
the permeable strata carries them far down into the Earth's
crust the water is greatly heated (§ 291), and if it is brought
back to the surface its high temperature entitles the outflow
to the name of a thermal spring. Hot springs also abound
in volcanic regions and along the slopes of recently up-
heaved mountains, in which cases they are not necessarily
deep-seated (§ 289). Hot water dissolves much more of
the rock substance than cold, and if it has traversed beds of
very soluble salts, such as the sulphates, carbonates, or
chlorides of the alkali metals or magnesium, it rises to the
surface as a mineral spring, often possessed of valuable
medicinal properties. When charged with carbonate of
lime, dissolved in the presence of carbonic acid under pres-
sure, the heated water on evaporating at the outlet deposits
carbonate of lime in large quantities. Calcareous deposits
from such springs often clothe whole hillsides with fantastic
sheets of rock, which under the name of tufa or travertine
furnish one of the most valuable building-stones in Italy.

316. Geysers. — Very hot water under high pressure
decomposes the silicates in granite and similar rocks, dis-
solving large quantities of silica, which are deposited as a
crust, termed siliceous sinter, when the heated water evapo-
rates on the surface. Some of the most fairylike scenery
in the world has been formed by such deposits of silica in
New Zealand, where the dazzling pink and white terraces
near Lake Tarawera were famous show- places until they
were destroyed by an earthquake in 1886. Many hot
springs depositing silica show the characteristic action of
geysers — an Icelandic name expressive of the violent and
explosive gushes of steam and boiling water which alternate
with periods of quietness. At the bottom of the shaft of a
geyser the temperature is far above 212°, but the water is
kept from boiling by the pressure of the column above, and
the uppermost layer is cooled by the air below the boiling-
point. After a time the surface water gets sufficiently

R

242 The Realm of Nature CHAP.

heated from below to begin to boil (§ 72) ; this relieves the
pressure on the layers beneath, which flash into vapour in
a series of explosions, throwing up a column of water and
steam with a terrific roar. The geyser remains quiescent
until it fills up again, when the same process is repeated.
In the Yellowstone region of North America (§ 364) the
Giantess Geyser throws up a stately column of steam and
water 250 feet high in each outburst, after which several
weeks of tranquillity elapse ; and " Old Faithful," throwing
a column of 150 feet, explodes with wonderful regularity
at intervals of about an hour.

317. Caverns. — Since the masses of tufa or sinter
formed round hot springs have been taken from the rocks
beneath, hollows or caverns must be left in the Earth's crust.
These are usually enlargements of the natural crack or fault
which allowed the spring to reach the surface. In limestone
regions caverns are very numerous and often of great size,
on account of the solvent action of rain-water charged with
carbonic and other acids on the joints and faults of the
strata. The roofs of caverns sometimes sink in, leaving a
funnel-shaped hollow on the surface called a sink or swallow-
hole, in which, if rubbish blocks up the outlet below, small
isolated lakes may form. Part of a cavern roof may
remain standing as a natural tunnel or bridge after the
debris of the fallen portion has been carried away by rivers.
Caverns are usually very picturesque on account of the
formation by the dripping water of fantastic stalactites,
white or tinted icicle-like appendages of carbonate of lime,
hanging from the roof. Where the water-drop falls from
the stalactite to the floor more carbonate of lime is de-
posited, and a stalagmite grows upward, and the two
ultimately form a natural pillar. Small stalactites formed
by the, percolation of rain-water through the mortar may be
seen hanging from the arches of bridges. The most
extensive limestone caverns are those of Adelsberg in
Austria, the Mammoth Cave in Kentucky (which comprises
more than 150 miles of passages), and the Jenolan Caves
in New South Wales. Some of these caverns contain
lakes tenanted by blind fish, and underground rivers

xni Action of Water on the Land 243

flow through them. In all limestone regions rivers
disappear beneath the surface, and although most of
them, like the Guadiana in Spain and the Poik in the
Adelsberg caves, reappear on land, several vanish alto-
gether and ultimately well up through the salt water of the
sea, sometimes from depths of 100 fathoms or more.

318. Surface Water. — During a shower, and for some
time after it has ceased, little runnels of water flow down
the steeper slopes of the land, uniting where opposed slopes
meet to form streams, which ultimately- converge in rivers
and flow on to lakes or to the sea. If the land were com-
posed of impermeable rock the whole of the rain-water not
lost by evaporation would run off over the surface, and
rivers would flow only during and immediately after the
fall of rain ; this is in fact the case in many mountainous
regions where the smooth rock walls are too steep to allow
soil to form upon them. On gentler slopes the rain first
soaks into the soil, and the streamlets swell gradually and
are kept flowing long after the rain stops by the subsequent
oozing of moisture. About one-half of the water in large
rivers enters them from springs which have pursued an
underground course from higher levels, and being inde-
pendent of local fluctuations of rainfall these give perma-
nence to the flow. When the melting of snow takes place at
one period of the year, or when heavy rains occur at definite
seasons, the springs are replenished as a store to be drawn
on gradually, and the increased supply of surface water pro-
duces a regular periodical rise in the level of the river. The
Ganges always rises and overflows its banks in summer,
when the melting snow of the Himalayas and the rains of
the south-west monsoon fill its higher tributaries. Similarly
the Nile (§ 375), after the monsoon rainfall of Abyssinia,
overflows its channel in the rainless land of Lower Egypt
every autumn, covering a narrow strip on each side with
soft and fertile mud. The Amazon (§ 361), on the other
hand, is almost always high, as the rainy seasons of its
southern and northern tributaries occur at opposite times
of the year with the shifting of the trade winds (§ 178),
but its floods are greatest in June. Dr. John Murray

244 The Realm of Nature CHAP.

calculates that of 29,350 cubic miles of rain falling on the
land every year, only 6520 cubic miles reach the sea as the
discharge from rivers, the remainder being re-evaporated or
absorbed in the Earth's crust.

319. River Systems. — The connected streams which
unite to form a river constitute a river system. The
series of convergent slopes down which a river system flows —
in other words the land which it drains — is called its basin,
and is separated by a watershed or water-parting from the
basins of neighbouring river systems. A watershed is
always the meeting-place of the highest part of two diverg-
ing slopes. This is sometimes a mountain range, but often
only the crest of a gently rising ground, on which the line of
water-parting is difficult to trace (§§ 360, 362). It is usual to
name a river system after the river into which the water is
collected from the whole basin, the other streams being
called tributaries or affluents. The basins of all river
systems draining into one ocean are known collectively
as the drainage area of that ocean. The beginning of a
river is called its source, and must necessarily be the
highest part of its course. When a large river flows from
a lake it is often difficult to decide which of the short
streams entering the lake is to be viewed as the ultimate
source. The name of the main river in a great system, such
as that of the Amazon or the Mississippi, is given by some
geographers to the tributary which has the most direct
course, by others to that of greatest length or to that with
the highest source. This diversity of opinion accounts to
some extent for the great difference in length assigned to
rivers by different authorities. The area of the basins or
the volume of discharge is a better measure of the size of a
river. It is interesting to notice in the following table of
the five greatest rivers that although the Nile basin receives
one-third more rain than the Mississippi, its discharge is only
one-fifth, on account of the great evaporation in crossing
the desert. The Yang-tse-Kiang, Yenesei, Amur, afid Mac-
kenzie are intermediate in length between the Amazon and
Congo, and the Yang-tse-Kiang and Orinoco have a dis-
charge equal to the Mississippi.

Action of Water on the Land

245

Name.

Area of
Basin.
Square Miles.

Rainfall of
Basin.
Cubic Miles.

Average
Annual
Discharge.
Cubic Miles.

Length of
Chief Rivers.

Amazon

2,230,000

2834

528

3060

Congo

I,54O,OOO

1213

419

2900

Nile .

I,29O,OOO

892

24

4000*

Mississippi

1,285,000

673

126

4200f

La Plata .

995,000

905

189

2000

* Including Lake Victoria and its longest tributary,
t From Missouri source.

320. Torrential Track. — On account of the forms of
the land-slopes (see sections of continents, Figs. 56-62)
the course of a typical river falls into three natural divisions :
the Torrential Track, with a slope usually exceeding 50
feet in a mile ; the Valley Track, with a slope rarely greater
than 10 feet, and often less than 2 feet ; and the Plain
Track, in which the change of level is only a few inches in
a mile. Some rivers have only one or two of these charac-
teristic divisions. Torrents dash down the mountain-sides
with tremendous speed, often exceeding 20 miles an hour,
leaping in cataracts from rock to rock and foaming through
ravines. Little soil forms on the steep slopes, hence as a
rule torrents swell quickly during rain and dwindle away to
a mere thread of water at other times. The work of a river
in its torrential track is purely destructive. When wholly
immersed in water, rocks are practically reduced in weight
from one-half to one-third, and are therefore moved with
much less expenditure of energy than would be required in
air. Huge boulders are thus hurled along by the flooded
stream, and hammer out the hollows in which the water
flows. The chips struck off at every concussion get broken
into smaller pieces, forming pebbles, gravel, sand, and
mud, or, to use a general term, detritus, which is swept
away to lower levels. As the ravines are deepened, tribu-
tary torrents leaping down the rugged slopes carve out
tributary ravines and increase the volume of water and of
detritus in the river.

321. Valley Track. — The valley track of a river lies

246 The Realm of Nature CHAP.

over the more gentle slopes that separate mountains from
plains, and the velocity of the stream rarely reaches 5
miles an hour, and is usually not more than 2 miles. The
work of a river in this part of its course is at the same time
destructive and constructive. A stream dashing along at
8 miles an hour can drag boulders 4 feet in diameter ; at 2
miles an hour stones as large as a hen's egg are rattled
along; at ij mile an hour the current can just roll pebbles
i inch in diameter ; when gliding at half a mile an hour
gravel as large as peas is swept forward ; while at a quarter
of a mile an hour a river cannot disturb fine sand. In the
slackening current of the valley track heavy stones brought
down by the torrent cannot be stirred, and the pebbles,
gravel, and sand are successively deposited as the slope
decreases ; and, since a river is retarded by friction with
the sides and bottom and flows slowest at the edges, the
deposit of stones and sand takes place chiefly at the sides,
where they form a shore or terrace. This is the constructive
work of a valley river, and the terraces built up are termed
alluvial deposits. The stones stranded in these terraces
gradually get weathered and crumble to pieces ; and during
floods the river sweeps away the fragments which are
readily broken by friction into sand or mud, and are
deposited in new terraces farther down stream. The
material swept along the bed of the river acts like coarse
sand-paper, scouring the hard clay or rock which forms the
river-bed ; and as the stream sinks in its deepening channel
it leaves its old terraces lining the valley at higher levels.
The river also attacks the banks, pressing now against one
side, now against the other, undermining cliffs and carrying
away the fallen fragments, thus widening the flat bottom
of the valley. Other conditions being the same, a valley cut
through horizontal strata is equally steep on both sides ; but
if the strata dip across the stream, the bank toward which
they dip becomes much less steep than the other on
account of the greater erosive action of springs and
percolating rain along the bedding planes.

322. Plain Track. — On the almost imperceptible slope
of its plain track the work of a river becomes entirely con-

xin Action of Water on the Land 247

structive. Water in this case ceases to carve and com-
mences to model the surface of the land. The alluvial
deposits are composed of the finest sands, and finally of
mud, which assist to raise the level of a wide area as the
river wanders over the plain. The alluvial plains of the
Mississippi cover 50,000 square miles, a space equal to all
England. Remains of dead animals and plants swept away
by the river in time of flood become embedded and buried
in the alluvial deposits on the margin of rivers or in the
mud and sand carried into lakes and seas, where they
either decay away or are preserved by various processes.
The work of a river has been compared to that of a mill
which "grinds slowly, but grinds exceeding small," rough
angular blocks being supplied in the torrential hopper, and
the most finely powdered material poured into the great
sack of the ocean.

323. River Windings. — When a swift -flowing river
laden with sediment is checked by any obstacle the sedi-
ment is deposited, and a sandbank or mudbank is formed.
When an obstruction of this kind is formed on the left bank
of a river at A (Fig. 51) the current of the river is deflected
from the straight line and strikes against the right bank,
rapidly undermining it at the
point, while the velocity of the
stream is checked opposite on
the left side, which becomes
built up by the deposit of sedi-
ment. The current is reflected
back to the left side at C, and |»»|t ; \ ratting in on right

7. bank.. Ihe arrows show the

SO the process goes On, Until the direction of the stream.

straight river forms a series of

winding loops as shown by the dotted line. The same
effect is produced by the unequal hardness of parts of the
bank, the softer being worn away and the harder left as
obstacles deflecting the current. The windings once begun
are perpetuated by the set they give to the current always
against the concave side, which is made more concave, while
the deposit of sediment adds to the convexity of the convex
side. The narrow neck of land between two concave curves

248 The Realm of Nature CHAP.

may ultimately be cut through by the river, which establishes
a short direct passage, leaving an island ; or the ends of
the cut-off portion may be silted up, converting it into a
crescent-shaped lake.

324. Embanking of Rivers on Plains. — During a flood
the swift, muddy stream rises, and, overflowing the banks,
immediately widens out on the level land ; the current is
checked at once, and most of the sediment is deposited
close to the banks in the form of broad bars of alluvial soil.
When the amount of mud in the water is very great, as in
the Mississippi, the Po in Northern Italy, and still more the
Yellow River (Hoang Ho) which traverses the loess deposits of
China, the land on both sides of the stream is raised rapidly.
The river-bed also gets silted up, and the great muddy
river ultimately flows along the top of a gently sloping
embankment, many feet above the level of the plain (Fig. 52).
The natural mud walls, called levees, on the lower Missis-
sippi are strengthened arti-
ficially in order to protect the
dwellers on the fertile borders

-c, „„ of the river. Floods frequently

FIG. 52.— Embankment of a river. BB, *

original slope of valley. The light make a breach in the wall, and

shading shows successive layers of . r->\\^(\ a havnii in

deposit; AA, level of river. a stream, called a bayou in

Louisiana, escapes, winding

over the low plain, either to rejoin the main river at a lower
level or to reach the sea independently. The Yellow River
of China has repeatedly changed its course by the high banks
bursting. One such disaster occurred in 1852, when the
embankments burst about 500 miles from the sea, and the
great stream, half a mile wide, formed a new channel,
entering the Gulf of Pechili several hundred miles from
its former mouth. In 1887 the banks burst again near the
same place, leading to the most fatal catastrophe recorded
in history, as the river, inundating hundreds of towns and
villages, drowned several millions of people.

325. Bars, Banks, and Deltas. — When rivers enter a
tidal sea directly, the effect of the salt water is to cause a
rapid precipitation of sediment, which may accumulate at
the mouth of the river and form a bar. Bars are often purely

xiii Action of Water on tJie Land 249

marine formations consisting of shingle or pebbles ridged
by the waves, but most of them are due to a combination
of river and sea action. When rivers enter a tidal sea by a
comparatively wide shallow estuary, such as the Tay, Mersey,
or Thames, sandbanks are formed, the size, position, and
shape of which depend on the amount of sediment brought
down and the form of the coast -lines which guide the tidal
currents. Professor Osborne Reynolds, in a series of beauti-
ful experiments, shows how, in a small flat -bottomed model
of an estuary, the floor of which was strewn with fine sand, it
was possible, by causing mimic tides to stream to and fro
in rapid succession, to rearrange the sand in banks with
channels between', precisely like those of the real estuary
represented.1 In lakes and seas not subject to strong tides,
such as the Baltic, Black Sea, Mediterranean, and Gulf of
Mexico, the sediment thrown down by rivers is not swept
away, but accumulates like a railway embankment in course
of formation until it rises to the level of the sea. The
action of waves piles up the deposited mud into low islands
on which vegetation takes root and assists to raise the level
by forming vegetable mould. These islands split the river
into numerous branches, which interlace with one another
sometimes in a very complicated way. The typical delta of
the Nile originated the name, for below Cairo the river
splits into two main branches which enclose a triangular
piece of land like the Greek letter A (delta) in form, the
broad growing edge of the delta, 180 miles long on the
Mediterranean, being the base of the triangle. The
Mississippi delta grows much more rapidly than that of
the Nile. It forms a long narrow peninsula spreading out
into a series of branches, each traversed by an arm of
the river and all constantly varying in size and position.
When the amount of sediment is very great, deltas are formed
even in tidal seas, as, for example, where the Ganges and
Brahmaputra meet at the head of the Bay of Bengal.
The Adriatic Sea is being filled up so rapidly by the
sediment of rivers descending from the Alps and Apen-
nines that the coast is lined by a broad belt of new land
interposing a stretch of 14 miles between the present

250 The Realm of Nature CHAP.

coast and the port of Adria, which originally gave its name
to the sea.

326. Submarine Canons. — Mr. Buchanan points out that
along the margin of the Gulf of Guinea the soft mud
brought down by the Niger and the Congo builds up the
slope of the transitional area, diminishing its steepness ; but
that right under the broad, swift, and deep current of the
Congo there is a deep submarine gully or canon walled
by the soft mud, but kept clear from deposit by a strong
counter-current of sea-water setting along the bottom up
the estuary. This counter-current is due to the same cause
as that through the Bosphorus (§ 238). Professor Forel
has pointed out a similar sub -lacustrine ravine under the
impetuous Rhone as it enters the Lake of Geneva laden
with glacier mud.

327. River Work on Dry Plateaux. — When a river
flows across an elevated plateau it wears out a channel for
itself, the form of which depends on the nature and arrange-
ment of the rocks and on the rainfall over the surface of
the region. The result of dip has already been referred to
(§321). Lines of faults frequently mark out the sites of
valleys and affect their formation. For the sake of sim-
plicity and contrast, it will suffice to explain the extreme cases
of river action on arid and on rainy plateaux composed of
horizontally stratified rocks. In a dry plateau the river
flowing from a snow -topped mountain range, over the
steepest slope, receives few and small tributaries as it pro-
ceeds, and the action of the water loaded with wind-borne
sediment is to wear its channel down through the rocks.
Cutting now on one side, now on the other, it makes
rapid progress through the softer strata, forming banks of
comparatively gentle slope, and slower progress through the
harder which are cut into steeper cliffs. The walls of the
valley retain the original slope as the detritus, instead of
accumulating in a talus, is swept away as it is formed, and
weathering takes place very slowly in the dry atmosphere.
The valley becomes eroded in a somewhat V-shaped curve,
and forms a gorge narrow compared with its depth and
sunk far below the level of the plain. Such gorges occur

xni Action of Water on the Land 251

on a magnificent scale in the plateaux west of the Rocky
Mountains, where they have received the name of Canons.
The most wonderful example is the Grand Canon of the
Rio Colorado about 400 miles in length, in many parts from
4000 to 7000 feet beneath the level of the plateau, and
with very steep terraced sides that strike the eye as vertical
walls.

328. River Work on Rainy Plateaux. — A river flowing
over a rainy plateau cannot form a canon or V-shaped
gorge because of the number of small tributaries it receives,
each of which helps to reduce the slope of the valley walls.
The action of rain on the cliffs leads to occasional landslips,
forming a gently sloping talus which protects the lower
rocks from erosion and gives the valley a U-shaped section.
Only in places where the rocks are hard and vertically
jointed and the river strong can the talus be swept away as
it is being formed, and a steep-sided gorge result. The
valleys excavated across a plateau in rainy regions become
wider as they grow older ; and according as the rate of
denudation over the whole area is nearly equal to, quite equal
to, or more rapid than the deepening of the river-bed, the
apparent depth of the valley increases very slowly, remains
unchanged, or actually diminishes.

329. Mountains of Circumdenudation. — To a traveller
ascending the Colorado River the sides of the canon
appear like lofty and precipitous mountain ranges, and
where a tributary canon enters, the appearance of the two
meeting slopes is exactly that of a mountain. On the
summit instead of a peak there is a vast plateau stretching
out as a boundless plain, broken by massive buttes, the
remnants of more resisting rocks left as monuments of
denudation. In a rainy region the valleys of adjacent rivers
cut up the plateaux into rounded blocks of elevated land, the
exact form of which depends on the composition and
arrangement of their rocks. Most geologists believe that
the mountains of Scotland and of Norway have been carved
out in this way from a solid plateau of great height by the
agency of rain, streams, springs, and ice, guided by the
durability and structure of the rocks (contrast §§ 295, 303).

252 The Realm of Nature CHAP.

330. Rivers and the Land Surface. — When a river is
fairly established in its valley it is the most permanent
feature of a land surface. Upheaval, which acts very
slowly, may even elevate a range of mountains across its
course, while the river cutting its way downward remains
at the same absolute level. The Uintah mountains were
elevated in this way across the course of the Green River,
one of the tributaries of the Colorada (§ 364). The
range in such a case rises divided, like a bar of soap
pressed upward against a horizontal wire. Where a
river crosses soft and regularly placed rocks its valley
is comparatively wide, the sides of gentle slope, and the
gradient of the stream uniform ; but where a strip of
hard rocks is encountered the valley narrows into a
steep-sided gorge, and the gradient of the river will be
suddenly changed. In such circumstances the hard rock
is cut through more slowly, and above it the gradient is
reduced to what is termed the base -level of erosion, where
no destructive action can take place but alluvial deposits
are formed. The softer rock farther down stream being
eroded more rapidly, a waterfall is formed over the hard
ledge, which is worn through in time, and a line of rapids
formed in the short portion of steep slope. Eventually the
gradient of the bed becomes uniform and the rapids also
disappear. The great waterfall of Niagara is caused by
thick beds of hard limestone (black in Fig. 53) resting on
soft shale. The river flowing over the cliff formed by the

FIG. 53. — Ideal Section of Falls of Niagara.

edge of the limestone cuts away the soft shale from below
and so produces occasional slips of the overhanging rock,
causing the falls steadily to recede. The falls are now at
the head of a gorge 7 miles from the escarpment of the
limestone cliff, where the rock is being eroded much less
rapidly by weathering. From recent surveys it is stated

xiii Action of Water on the Land 253

that the " American " falls have receded 30 feet, and the
"Horse-Shoe" falls 104 feet in the last 48 years. If the
structure of the rocks is the same all the way even at this
rate the time, geologically speaking, is close at hand when
the river-bed will be lowered along its whole length and
Lake Erie will be drained. If the Niagara River had been
muddy instead of being exceptionally clear, its erosive power
would have been greater, and the falls would have been
worked out long ago. The falls of St. Anthony on the
Mississippi, for example, have been cut back about 900 feet
since they were discovered in 1680.

331. The Work of Rivers. — The amount of sediment
and of dissolved solids in the water of rivers gives a clue
to their average effect in lowering the whole surface of their
basins. From calculations of this kind it appears that in
order to lower the average level of their basins by i foot
the Danube must work for nearly 7000 years, the Missis-
sippi for 6000 years, the Yellow River for 1500 years, the
Upper Ganges for 800 years, and the Po only for 700
years. Dr. John Murray calculates that in 6,000,000 years
river erosion at the present rate would reduce all the land
of the globe to sea-level, and M. de Lapparent, observing that
the deposit of sediment at the same time raises the level of
the ocean, shows that at the present rate of surface erosion
4,500,000 years would suffice to equalise the level of land and
sea.

332. Lakes are bodies of water occupying hollows of
the land. As contrasted with rivers (§ 330) they are
transitory features of a region, being subject to considerable
fluctuations in extent and destined ultimately to disappear.
Lakes often originate in the obstruction of a river valley.
If blocked at a narrow gorge by drifting ice or an avalanche
the river-bed below runs dry, and the water above rises,
flooding the valley until it reaches the lip of the ice-wall.
Ultimately the pressure of the accumulated water bursts
the ice-barrier, and a terrific flood suddenly desolates the
valley below. The famous parallel roads of Glen Roy in
Scotland are believed to be beaches etched out at succes-
sive levels by the water of a glacieji^festryGt^d lake, the

^ftESE Us

254

The Realm of Nature

CHAP.

barrier of which gave way in successive steps separated
by long intervals of time. A landslip, the melting of a
glacier, or the flow of a lava stream, sometimes obstructs
a valley by forming a barrier of earth, moraine stuff, or
solid rock, through which the issuing stream cuts very
slowly, and the lake so formed is permanent as far as
the observations of a lifetime can discover. Hollows
produced by the irregular deposit of boulder -clay left
by the melting of an ice-sheet form lakes in regions where
rainfall exceeds evaporation. Rock-basins (§ 339) con-
tain the typical Alpine lakes of mountainous regions. Slow
upheaval of the end of a valley, subsidence of a plain, or the
collapse of caverns, are also processes of lake formation, and
the craters of extinct volcanoes often collect a large quantity
of rain, forming lakelets with neither inflow nor outflow.

333. Great Lakes of the World. — The Caspian Sea is
the largest lake, and a typical example of a hollow isolated
by upheaval of surrounding land. Lake Superior comes
next in size and is the largest fresh lake. Lake Baikal in
Asia, at an elevation of 1360 feet above the sea, is the
deepest known lake, the maximum sounding obtained in it
being almost 800 fathoms. The highest lake yet measured
is Askal Chin in Tibet, 16,600 feet, and the lowest is the
Dead Sea, 1290 feet below sea-level.

THE LARGEST LAKES

Name.

Situation.

Height
above Sea.

Area.
Square Miles.

Depth.
Max. Fms.

Caspian

Eurasia

—90

17O,OOO

500

Superior

North America

600

31,200

1 68

Victoria

Africa

3300

26,900

...

Aral

Asia

150

26,2OO

37

Huron

North America

580

23,800

117

Michigan

55 55

580

22,400

H5

334. Function of Lakes. — When water begins to flow
over a new land surface, either freshly upheaved from the

xin Action of Water on the Land 255

bed of the sea or remodelled by the deposit of boulder-clay,
it necessarily forms a series of lakelets which overflow into
one another by streams. As the river system cuts its
channels more deeply the smaller hollows are either drained
or filled up and remain as meadows along its course. The
abundance of fresh lakes is a testimony to the comparative
newness of the land surface and to the early stage of evolu-
tion of its rivers. A river issuing from a lake cuts down the
lip it flows over very slowly, except when the barrier is soft
clay, as all the sediment which gives to running water the
properties of a file is dropped on entering the lake. Lakes
thus act as filters for rivers. The exquisite deep blue
colour for which the lakes of Northern Italy and Switzer-
land are famous is due to the scattering of light from the
fine flakes of mica brought in by glacier rivers and suspended
in the water. This deposit tends to gradually fill up
the lake. The fans of alluvial deposit laid down by
each inflowing stream grow into deltas; and flat meadows
encroach on the water so rapidly that lawsuits are oc-
casionally required to determine the ownership of the
new land. Lakes regulate the flow of rivers by keeping
up their supply in times of drought, and checking floods
during rain. For example, if a river -^ of a mile wide
passes through a lake of 100 square miles in area,
10 miles from the sea, and a flood takes place in the
upper stream which, if passed on directly, would raise the
level of the lower 10 miles by 25 feet, and so produce a
disastrous flood, the immediate effect is to raise the level of
the lake 3 inches, causing a very slight increase of the lower
stream.

335. Salt Lakes. — In arid regions, where evaporation
is in excess of rainfall, rivers flowing into hollows of the
Earth's crust may fail to fill them up to the brim, and lakes
will thus be formed with no outlet. These are necessarily
salt, on account of the evaporation of the river-water, and
the salts contained differ (§ 221) from those of the sea.
Analysis of the water of salt lakes shows this to be usually
the case ; but the salts of the Caspian are very similar to
those of ocean water, indicating that it is part of the sea

256 The Reahn of Nature CHAP.

cut off by a geologically recent elevation of the land. Yet its
salinity is less than 2 per cent, while that of the sea averages
3. 5. This is because the shelving shores, and particularly the
wide shallow inlet of Kara-Baghas, act as natural salt-pans,
evaporating the thin layer of water covering them and
causing a deposit of crystalline salt, which is thus being
gradually withdrawn from solution, while the evaporation is
made good by a continual supply of fresh river-water. On
account .of the excess of evaporation the surface of the
Caspian is now about 90 feet below sea-level, and its shores
form a sunk plain. The Jordan Valley ^ in an equally rain-
less region, is a still more remarkable instance of a sunk
plain. The Sea of Galilee is a small lake 600 feet below
sea-level, and from it the Jordan flows for 100 miles along
the line of a great fault in a valley averaging 7 miles in
width, and enters the Dead Sea at a level 1290 feet below
that of the Mediterranean.

336. Ice Action. — The snow-fields lying on the high
parts of mountain ranges above the snow-line (§ 163) con-
tinually increase by the condensation of vapour from the
atmosphere. The weight of the accumulating mass of
snow compresses the lower layers, squeezing out the air,
and forming compact ice, which, although one of the most
brittle substances to a blow, is plastic when subjected to
steady pressure. Glaciers or streams of ice flowing down
the slopes prevent an excessive accumulation of snow
on high mountains. The cause of the plasticity of ice
under pressure is usually considered, following Professor
J. Thomson's theory, to be that pressure lowers the melting-
point (§ 72), allowing the lower layer of the mass to liquefy
and adapt itself to the surface it rests on ; the relief of
pressure thus afforded allows the water to solidify again,
and the process is repeated continually. But since ice at
very low temperatures is plastic, though in a less degree,
the theory of melting by pressure is not a sufficient explana-
tion. Messrs. M'Connel and Kidd have recently made
experiments which show that while crystals of ice are
individually rigid and brittle, a mass of them frozen together
is plastic even at low temperatures, the crystals apparently

xiii Action of Water on the Land 257

sliding over each other.2 Since glacier ice is known to
consist of grains or lumps (from the size of a pea to that
of a melon), each of which is a single crystal, the flowing of
glaciers can be readily explained. Part of the accumulated
snow on a mountain slope is got rid of by avalanches or
snow-slips, which are powerful erosive agents, breaking
through everything in their path.

337. Glaciers, although solid, flow like rivers, the centre
and surface moving nearly twice as fast as the sides, which
are retarded by friction with the valley. Compared with
rivers their motion is very small. The Mer de Glace, the
most famous glacier in Switzerland, creeps at the rate of
about an inch an hour in the centre during summer, and
only half as fast in winter. Some of the great glaciers of
Greenland move much faster, advancing from 50 to 60 feet
in a day, although 20 feet is a more common rate. The
thickness of glaciers in the Alps often exceeds 1000 feet,
and their length averages about 5 miles ; the longest is the
Aletsch Glacier, which measures 1 5 miles, including the
parent snow -field. As a glacier descends along the valley,
stones, clay and sand loosened by erosion fall from the slopes,
and rest as huge heaps of rubbish, called lateral moraines,
along each side of the ice. When two glaciers traverse con-
vergent valleys the lateral moraines on one side of each coal-
esce to form a medial moraine (see Fig. 54) down the centre
of the united ice-flow. In time a great glacier carrying the
ice of many tributaries becomes roughened with numerous
parallel ridges of rock rubbish along its length. The heat
of the Sun in summer continuously melts the ice, except
where it is protected by the overlying moraines, which thus
stand out prominently on the surface. Isolated blocks of
stone similarly protect and remain perched on ice pillars,
while the general surface is being lowered. As a glacier
forces its way along an irregular valley the ice is severely
strained, and cracks or crevasses result, which are narrow
and close at first, but gradually widen out in consequence
of the centre moving more rapidly than the sides. Huge
clefts are thus formed extending through the ice from surface
to bottom, and swallowing up masses of moraine rubbish.

s

258

The Realm of Nature

CHAP.

Some change in the channel alters the stresses, and as time
goes on the old crevasses close up and new ones open.

FIG. 54. — Map of a Glacier showing the formation of medial moraines, by
union of tributary glaciers. The arrows show the direction of flow, and the
lines radiating from the edges represent crevasses.

The regions where glaciers occur are coloured dark blue on
Plate VII.

338. Glacial Work. — Glaciers work both by transport-
ing the moraine material that falls upon them and by
powerfully eroding the ground they pass over. Moraine
rubbish falling down crevasses gets wedged in the ice,
which presses the angular stones firmly against the bed-rock
as the glacier slides forward, the action exactly resembling
the cutting of glass by a diamond. Immense quantities of
sand and clay result from the grinding down of rock and
stones, and are carried along the bed of the glacier, forming
the ground or bottom moraine or boulder clay. When the
climate of a glacial region grows warmer, as the Alpine district
has been doing for the last twenty years, the glaciers melt
away at the lower end, which shrinks up the valleys, while the
boulders which may have been carried far by the ice are
deposited on the slopes amongst rocks of an entirely
different nature, and sometimes in very precarious positions.
Such travelled and perched blocks are called erratics, or
simply boulders. The rocks of the valley uncovered by the
ice are seen to be deeply grooved or striated by the stones
dragged over them, the run of the striae showing the direc-
tion in which the glacier was moving. The surface scratched

xin Action of Water on the Land 259

by sharp stones is at the same time finely polished by the
clay, and thus acquires a highly characteristic appear-
ance. The general aspect of the smoothed and rounded
rocks is supposed to resemble the backs of sheep, hence the
peasants named them roches moutonnees^ i.e. sheep rocks.
The stones which took part in the polishing action, and
remain embedded in the clay, are themselves scratched and
smoothed in a similar way. The descent of a glacier in
a steep valley is believed by some geologists to give it an
impetus which causes the mass of ice to dig like a gouge
when it enters suddenly on flatter ground. To this gouge
action, strongest at the first shock, and then gradually
diminishing, the peculiar form of the rock -basins of
alpine lakes and fjords is usually ascribed. The deep
weathered crust (§ 310) which forms on granite and other
hard rocks is readily scooped out, and its presence doubtless
helped in the formation of deep rock -basins. When the
climate admits of glaciers reaching the sea they give rise to
icebergs (§ 234), and distribute their deposits far over the
bed of the ocean. At the end of a glacier on land the
ground moraine forms a ridge of boulder clay, and the
various moraine heaps carried along on the surface of the
ice are thrown down above it, producing what is called the
'terminal moraine. A diminishing glacier in a climate that
is growing warmer strews the whole valley, up which it
has retreated, with consecutive terminal moraines made up
of low hills of detritus. From the melting end of a glacier a
rapid stream of ice-cold water flows away, milky with mud,
which imparts to it great erosive power. The amount of
sediment removed by the Isortek River in Greenland from
the base of its parent glacier is calculated at 4,000,000 tons
a year.

339. Rock -basins are usually long and narrow, and
attain a maximum depth, often of several hundred fathoms, at
a point about one-third of the distance from the head of the
basin, as shown in Fig. 5 5. The lakes occupying rock-basins
are characteristic of the valleys on the lower slopes of all
mountains which once bore great glaciers. By subsidence
of the coast-lands they form fjord-basins (§ 229) filled with

260 The Realm of Nature CHAP.

sea-water. On the west coast of Scotland Loch Morar, a
fresh-water lake 178 fathoms deep, with its surface 30 feet
above sea-level, is connected with the sea by a short river.

Loch Etive, exactly simi-
lar in configuration but
filled with sea- water, and
only 80 fathoms deep,
has its sill so near the

FIG. ss.-Section of Loch Goil, a typical rock Surface that, although it

basin, the slopes exaggerated 10 times. The is in free Communication
upper line shows by its varying thickness the . , , , . .. . ,

true slope of the bed of the basin. With the SCa at high tide,

the current rushing out

at low tide forms a veritable waterfall. Loch Nevis, with a
depth of 70 fathoms, has its sill 8 fathoms below the surface.
The gigantic Sogne Fjord in Norway, more than 100 miles
in length, is a rock-basin with a maximum depth of 700
fathoms.

340. Ice-caps. — In very cold climates, where the snow-
line approaches sea-level, the whole surface of an extensive
region may be covered by snow to such a depth that it is
compacted into ice, filling up all the valleys and standing
high over the mountains ; such a covering is called an ice-
cap. Greenland is covered with an ice-cap presenting a
shield-shaped surface, which Dr. Nansen in his adventurous
journey across the peninsula in 1888 found to be about
10,000 feet above sea-level, and nearly flat in the interior,
sloping rapidly to the sea on each side. The weight of this
shield of ice is always squeezing out its edges in the form of
glaciers to the sea, and there is probably a constant though
very slow outward movement of the ice from the centre over
the hills and valleys of the deeply buried land. The Ant-
arctic continent appears to be covered with a still larger and
probably thicker ice-cap, regarding which little information
has been obtained, except that the glaciers from it give rise
to fleets of immense flat-topped icebergs (§§ 234, 276).

REFERENCES

1 Osborne Reynolds, " On Model Estuaries," British Association
Reports, 1889, p. 328, and 1890, p. 512.

xni Action of Water on the Land 261

2 J. C. M'Connel and D. A. Kidd, "The Plasticity of Glacier
and other Ice," Proc. Roy. Soc. (1888), xliv. 331, also (1890)
xlviii. 259 ; Nature^ xxxix. 203.

BOOKS OF REFERENCE
See end of Chapter XIV.

CHAPTER XIV

THE RECORD OF THE ROCKS

341. Looking Backward. — Two opposed agencies now
at work on the Earth's surface — internal energy ridging up
the crust, and solar energy cutting down the heights — are
sufficient, if they have been long enough in action, to
account for all the features of the land. The Uniformi-
tarian School of geologists holds that the Earth has attained
its present condition after passing through vast ages of
change so slow as to be almost imperceptible. The other
school, sometimes called that of the Catastrophists, affirms
that the processes at work in past time were quite different
from those of the present, being much more violent and
uncertain in their action. They look on valleys as rent in
the solid rock by Earth movements of titanic strength, and
on mountain ranges as elevated to their full height in a
single stupendous heave of the strata. Erosion is con-
sidered only to trim off the broken edges, as a plane
smooths down the signs of the rough rending of a saw.
Modern research shows that the truth lies between the two
extremes. The Earth, like any other cooling body, must be
cooling less and less rapidly as time goes on. When the
crust was first formed its high temperature must have con-
siderably increased the erosive power of water. So, too,
tidal friction, now insignificant, must once have been a
tremendously powerful agent in shaping the surface (§ 104).
Thus, while the processes at work have been the same in
kind as the Uniformitarians prove, the energy available for

DRAINAGE AREAS OF CONTINENT!

After J. Muri

1BO ISO 160 140 12O 1OO 8O 6O 4O

THE DRAINAGE AREAS are coloured according to the Oceans which they drain into. REGIONS OF INL/

ND CO-TIDAL LINES OF OCEANS.

and others.

20 4O 60 SO 000 2BO

6O 8O WO 120

LINAGE shown thus

The Figures on the CO-TIDAL LINES denote the Time in Hours.

CHAP, xiv The Record of the Rocks 263

the work in a given time was once much greater than now,
as the Catastrophists maintain. Reasoning from the rate
of cooling of lava, Sir William Thomson estimated that
living creatures such as now exist could not have inhabited
the Earth more than 100,000,000 years ago ; and Professor
Tait, calculating from the rate at which the Earth is losing
heat (§ 291) and its present temperature, concludes that
20,000,000 years is more nearly the truth, while even
10,000,000 years may include the whole range of possible
life on the globe.

342. Reading the Rock Story. — If exactly the same
areas of the Earth's surface were always subject either to
elevation or depression, we could not discover from the
rocks laid bare on the surface any record of the process
of their formation. The sedimentary rocks would remain
in the subsiding hollows, the older layers being successively
covered by newer ones. But it happens that the margins
of the world ridges on which sediment is deposited are sub-
ject to frequent elevation and depression (§ 303), and the
sedimentary rocks which are exposed bear traces of these
changes which it is the special study of geologists to inter-
pret. Where rocks are very much crumpled and folded, it
often happens that the strata have been inverted, the bottom
beds of a series having been folded back upon the upper
beds. When a stratum occurs resting on a different sort
of rock, which dips in a different direction or bears signs
of ancient erosion, the two are said to be unconformable.
This structure is clearly indicative of some time having
elapsed since the formation of the older series, and
before the accumulation of the overlymg younger beds.
The stratified rocks are like the sheets of an unbound
book, some of which have been printed over a second time
with a later part of the work ; many have been crumpled,
torn, and rubbed so that they are illegible ; the numbering
of all the pages except the last one has been destroyed,
and there are evidently places where several pages together
have been dropped out. By reading the legible portions of
such a book one could find hints of the development of
events if the mutilated work were a history, or of the unfold-

264 The Realm of Nature CHAP.

,ing of the plot if it were a novel. A few consecutive pages
found in their proper order would give a key to arranging
the rest, and although uncertainty as to the precise sequence
of some parts of the narrative would remain, the patient
reader could in time obtain a fair idea of the nature and
order of the contents. If it is possible to find a narrative
showing a regular development of events written in charac-
ters with which we are familiar on the sheets of rock, the
order and circumstances in which these rocks were formed
can be got at, however confusedly they may now lie. Sedi-
mentary rocks are full of picture-writings recording the
history of successive races of living creatures, and the
writings are very legible, being the actual mummies or
casts of the creatures themselves.

343. Fossils. — All remains and traces of living creatures
preserved in rocks are called fossils. Some of the traces
are only footprints, or worm-tracks that have been impressed
on an ancient surface of clay or wet sand, and after harden-
ing have been filled in by finer sediment. Plants and
animals are usually represented only by their hardest
parts, such as bark, shells, teeth, or bones. But often the
whole organism was surrounded by compact sediment, in
which, as it decayed away, a hollow was left exactly corre-
sponding with its outer surface. This mould became filled
in turn with fine sediment, or impregnated with carbonate
of lime or silica deposited from solution in the water which
percolated through, and thus a perfect cast or model has been
produced. The most complete fossils preserve not only the
external form but the minutest internal structure, every
part being individually turned into stone by the exchange
of animal or vegetable substance, molecule by molecule, for
some mineral such as pyrite (sulphide of iron), calcite
(carbonate of lime), or one of the many forms of silica.
Other fossils are simply shells or skeletons closely com-
pacted together, such as chalk, made up of foraminifera
like the deep-sea oozes (§ 275), coral limestone (§ 280),
and siliceous earth composed of the cases of diatoms,
Sometimes organic substance undergoes only partial de-
composition while retaining much of its original form.

xiv The Record of the Rocks 265

Coal, for example, is the residue of partially decomposed
vegetation.

344. Interpretation of Fossils. — As a general rule it
is assumed that the creatures whose remains occur in the
rocks were similar in their habits to those now living, and
were in an equal degree dependent on the climate. Rocks
formed of the sediment of lakes and rivers may, by the
greater abundance of land creatures amongst their fossils,
be distinguished from those composed of marine deposits.
These inferences are often confirmed by the nature of the
rocks themselves, the fine mud of estuaries naturally yield-
ing a shale, while the pebbles of an exposed seashore are
compacted into a conglomerate. Rocks containing the
remains of the same species of creatures have evidently
been formed under similar physical conditions, and possibly
at the same time ; hence they are said to belong to the
same geological horizon.

345. Divisions of Sedimentary Rocks. — There is so
much scope for individual opinion in interpreting the record
of the rocks that no minute classification of them meets the
approval of all competent geologists, but a few compre-
hensive divisions are generally accepted. The most ancient
sedimentary rocks are allowed to be those containing fossils
exclusively of the simplest form of life. The variety and
complexity of the organisms found, usually increase as the
more recent strata are approached. The greatest thickness
of a bed of sedimentary rock may in some cases give a
rough measure of the shortest time it could have taken
in formation, but all attempts at fixing a definite geological
chronology have as yet been unsatisfactory. The great
divisions of rocks and their more important subdivisions
are given below in the order of antiquity, and some typical
forms of life are mentioned.

QUATERNARY

RECENT — Now forming.

PLEISTOCENE — All modern plants and animals. Man.

TERTIARY

PLIOCENE— Most modern plants. Elephant, Ox.

266 The Realm of Nature CHAP.

MIOCENE — Tropical plants. Ape, Antelope.

OLIGOCENE — Tropical plants.

EOCENE — Tropical plants. Palceotheriiim, Lemur.

SECONDARY

CRETACEOUS — Flowering plants. Foraminifera, Marsupials,

Toothed Birds.
JURASSIC — Ferns. Saurians, Marsupials, Archaopteryx, Corals,

Ammonites, Cuttlefish.
TRIASSIC — Cycads. Ammonites, Reptiles.

PRIMARY

PERM IAN — Amphibians.

CARBONIFEROUS — Lycopods. Tree-ferns, Conifers, Crinoids,

Fishes, Amphibians.
DEVONIAN AND OLD RED SANDSTONE — Fishes, Brachiopods,

Lycopods.

SILURIAN — Sea-weeds. Graptolites, Trilobites, Fishes.
CAMBRIAN — Trilobites, Sponges.
ARCHAEAN — No forms of life known with certainty.

346. Older Primary Rocks. — The primary division is
called the Palceozoic, as in it the fossils of the earliest living
creatures are preserved. The Archaean, which forms the
foundation rocks, consists mainly of crystalline schists.
Wherever these appear on the surface we know that the
land is of extreme antiquity, for it must either have remained
above the sea while all the other formations were being
deposited elsewhere, or if it was upheaved after being
covered with younger rocks, the period must yet be suffi-
ciently remote to have allowed all the more recent strata to
be eroded away. No fossils are known with certainty in
Archaean rocks. The Cambrian, Silurian, and Devonian
systems, named after the districts in south-western Britain
where they were first studied, were formed in successive
periods. Fossils of sea creatures are abundant in these
rocks ; a peculiar crustacean called the trilobite swarmed in
the Silurian seas, and seems to have become altogether
extinct before the end of the Primary period. The earliest
land-plants, which were cryptogams, leave a record in the
Upper Silurian rocks. In the Old Red Sandstone rocks

xiv The Record of the Rocks 267

which were laid down as sediment in fresh-water lakes in the
Devonian period, fossils of fishes clad in enamelled bone
and of scorpion-like creatures appear.

347. The Carboniferous System is composed of thick
beds of limestone, which must have been deposited at the
bottom of a clear shallow sea, of sandstones laid down on
ancient beaches, and of shales which represent the solidified
mud of estuaries. The name Carboniferous comes from the
beds of coal which result from the decay of bark, fronds
and spores of club-mosses, and tree-ferns of giant size, on
the swampy margin of the ancient sea. Clay-beds usually
underlie coal-seams, and represent the soil in which the
carboniferous plants grew, being often full of the fossil roots.
The formation of coal is an interesting example of chemical
decomposition. The action of heat and pressure on veget-
able matter in the absence of air is to drive out more and
more of the oxygen, nitrogen, and hydrogen it contains,
combined with very little carbon. The following table
gives the average composition (omitting the ash) of dry
wood ; peat, which results from vegetation decaying in
recent formations ; lignite, a woody form of coal found in
tertiary rocks ; true coal ; and anthracite, which is apparently
derived by heating coal. It is conjectured that the final
product of this process is the diamond, which is pure
crystallised carbon.

CARBONIFEROUS MINERALS

Wood. Peat. Lignite. Coal. Anthracite.

Carbon ... 50 60 67 85 94

Hydrogen 6 6 5-5 3

Oxygen and Nitrogen 44 34 28 10 3

100 100 100 100 100

The great limestone beds of the Carboniferous period are
composed of the remains of crinoids, mollusca, and many
other marine creatures. Amphibians mostly small, but some
of great size crawled through the marshes, but the only true
land animals preserved are of the nature of scorpions, insects,
and snails.

268 The Realm of Nature CHAP.

348. Newer Primary Rocks. — In the Permian period,
named after the Russian government of Perm, where the
rocks of this age are greatly developed, plant life appears
to have been less abundant and varied than in Carboniferous
times, but remains of great amphibians abound, and those of
true reptiles appear for the first time. Palaeozoic rocks some-
times exert a considerable local influence on a freely sus-
pended magnet (§ 98). In the course of a magnetic survey
of the British Islands, Professors Thorpe and Riicker recently
found a line of magnetic disturbance running across the
comparatively recent strata of southern England, coincident
with a deeply buried mass of Palaeozoic formation running
from the old mountains of Wales toward the Carboniferous
region of the continent of Europe, the existence of which
had previously been inferred from geological evidence.1 In
1890 this conclusion was strikingly confirmed by the dis-
covery of coal in a very deep boring through the tertiary
rocks of eastern Kent.

349- Secondary Rocks. — The secondary rocks are
termed Mesozoic, because they contain evidence of the
existence of living creatures intermediate between those of
the Primary period and of the present time. In the Trias
there are signs of gigantic amphibians, reptiles of the croco-
dile kind, and of the simplest forms of mammals, the mar-
supials. The Jurassic system takes its name from the Jura
Mountains, and is sometimes known as Oolitic (egg-stone),
from the granular limestones resembling the structure of a
fish-roe, by which it is characterised. Many beds of lime-
stone of this period are fossil coral-reefs. The most abun-
dant mollusca were the ammonites, with wonderful rolled
shells, and cuttle-fishes. Saurians — reptile-like animals —
grew in those days to an enormous size, and inhabited air,
sea, and land. The Pterodactyls were small reptiles with
wings not unlike those of a bat in appearance. Ichthyo-
saurus and Plesiosaurus were swimming reptiles, some-
times 40 feet in length, and the land reptiles were probably
the hugest animals that ever inhabited the globe — the
remains of the Atlantosaurus, discovered in North America,
indicating a length of 100 feet and a height of 30 feet.

XIV

The Record of the Rocks 269

Archaeopteryx, the first bird-like creature, appears in the
Jurassic period. The Cretaceous or chalky rocks are
largely composed of solidified globigerina oozes, and in-
numerable shell-bearing sea creatures occur amongst them.
Fishes like the herring and salmon appear for the first
time, and huge reptiles and birds with teeth were common.
Traces of the flowering plants also appear amongst the
prevailing ferns.

350. Tertiary Rocks. — A great gap generally separates
the period of the Mesozoic rocks from that of the Cainozoic
or Tertiary. During the interregnum the great reptiles and
ammonites became extinct, and forms of life appeared more
closely resembling those of the present day. The divisions
of tertiary rocks — Eocene, Oligocene, Miocene, and Pliocene
— were originally arranged in the order of the abundance of
the fossils of mollusca, resembling those now existing. As
the period progressed plants and animals which approached
more and more closely to those we now know appeared on
the Earth. Foraminifera attained a great size and were
extremely numerous, one being the large coin -shaped
nummulite which makes up many of the limestones. Mol-
lusca like the oyster and snail began to predominate over
those of the cuttle-fish kind. Amongst the mammals the
marsupials became less numerous, and many transition
forms of the Eocene approach the carnivorous type. Later,
gigantic ant - eaters, the elephant - like Mastodon, pig-
like animals, antelopes, and apes appeared. A succession
of animals of increasing size, approaching nearer and nearer
the nature of the horse, runs through the series, culminating
in the true horses of the Pliocene age. The fossils of these
large animals are never so complete as those of mollusca or
ferns, some teeth, or a few shattered bones, being all that
is usually found. The Tertiary period was characterised by
great volcanic activity in all parts of the world, and the
existing scenery of many lands is due to the effects of
denudation on the basalt sheets and lava dykes of the old
volcanoes.

351. Quaternary Rocks. — The post-Tertiary or Quater-
nary rocks are the least ancient of all. They are rarely

270 The Realm of Nature CHAP.

even consolidated, consisting chiefly of clays and sands.
The Pleistocene formation in Northern Eurasia and America
consists almost entirely of boulder clay, the result of ice-
action, and the period has been termed the Great Ice Age.
Many exposed rock surfaces on the mountain-tops as well
as in valleys, in places where glaciers have never been seen,
closely resemble the roches moutonnees of Switzerland (§ 338).
Perched blocks are scattered thickly over all parts of Northern
Europe and America, and from their nature many of them
are known to be far travelled. The conclusion is irresist-
ible that after the formation of the last tertiary rocks these
lands were subject to ice-action. Great and wide -spread
subsidence, and subsequent elevation of the land took
place during this period. Some writers, among whom
is the Duke of Argyll, maintain that the boulder clay,
perched blocks, and ice-scratchings were brought about by
this subsidence permitting fleets of icebergs sailing south-
ward to strand or rub against surfaces which were afterwards
elevated. To most geologists, however, the evidence of
true glacier action having occurred over the whole area is
overpowering, although the period is so remote that atmo-
spheric erosion has in many cases obliterated the work of ice.
352. The Great Ice Age. — Glaciation probably oc-
curred on the grandest scale, the ice marching over mountain
and valley with little regard to the form of the surface.
In the Glacial period it appears that all Northern Europe
and Northern America (see light blue tint on Plate VII.),
were covered by vast ice-caps, thicker than that now over-
spreading Greenland, which polished and smoothed off the
mountains, and covered the valleys and plains with layers of
boulder clay. The ice seems to have spread beyond the
margin of the land, to have hollowed out deep furrows
across the Continental shelf, and sometimes even to have
ploughed up the shallow sea-bed and scattered the sand and
shells on the coast-lands. Professor James Geikie points out
that the Great Ice Age was divided into periods during which
the climate was very severe, while between them a genial
climate prevailed, and interglacial beds of peat were formed
containing a varied vegetation and the remains of insects

xiv The Record of the Rocks 271

and mollusca. The cause of changes of climate, sufficient
to produce such effects, has been the subject of much specu-
lation. The late Dr. Croll, whose theory is now most widely
received, pointed out that the changes in the eccentricity
of the Earth's orbit (§ 109) combined with the precession of
the equinoxes (§ 115), must have produced a severe climate
in the northern hemisphere at the period when aphelion
occurred in the northern winter, and the eccentricity was at
a maximum. If Croll's theory is true, cold periods must
have occurred in all geological epochs. Erratic blocks and
glaciated stones found in many different formations seem to
confirm it, but no sign has been found of such extensive
ice-action as characterised the Pleistocene. This may be
accounted for by the probable absence in those remote
periods of continental areas sufficiently extensive to support
a great ice-sheet. Some geologists account for the changes
of level during this period by supposing that the great ice-
sheet depressed the elastic strata by its weight, producing
extensive subsidence, followed by upheaval when the ice-
cap melted. Others explain raised beaches (§ 284) on the
assumption that the land remained rigid and the mass of
ice raised the level of the ocean by attraction (§ 2 52). In the
river and cave accumulations of the Pleistocene age the first
undoubted signs of the human race appear in the form of
coarse chipped stone implements and rough etchings on bone
of extinct or no longer indigenous animals.

353. Evolution of Continents. — Rocks of Archaean and
Palaeozoic age cover a greater area on the Earth than those of
Mesozoic age, which are in turn more extensive in their dis-
tribution than those of the Tertiary system. This shows that
more of the elevated half of the globe was covered by the sea,
in which sediment aocumulated, in Palaeozoic than in Meso-
zoic, and in Mesozoic than in Tertiary times. It is pointed
out by Professor J. Geikie that the elevated and depressed
halves of the World have been growing more and more
distinct throughout geological ages, and as the Abysmal Area
has grown deeper and the World Ridges higher the superficial
extent of the hydrosphere has been steadily diminishing,
although its volume remains the same.2 This change must

272 The Realm of Nature CHAP.

be looked on as a general result of innumerable minor
elevations and depressions. The following hypothesis of
the growth of continents is not to be looked on as an
established theory, but as a probable conjecture of the
relative order in which the various land - masses were
formed. Plate XIV., adapted from Professor J. Geikie's
maps, shows in the deepest tint the areas of the
World Ridges that are believed (although the evidence
is far from complete) to have projected above the
hydrosphere during the greater part of the period when
Palaeozoic rocks were being formed. They composed groups
of great islands clustered on the northern and scattered over
the southern parts of the World Ridges, between which warm
ocean currents would flow from the equatorial seas, and an
equable climate would reign over the whole land. In the
Mesozoic period the lands (shown in the second tint) were
far more extensive, but insular conditions still prevailed.
The deepened Abysmal Area drained the oceans from the
summits of the World Ridges, and the up-ridging of the
Continental Area raised wide tracts far above the sea. The
western and eastern edges of the great Eastern World Ridge
were clearly outlined, but the sea spread across its central
portion from east to west, and from north to south. The
Western World Ridge was developed similarly, land extend-
ing along its western and eastern edges in North America,
separated by a wide sea-channel from south to north, while in
the South American portion the central part of the existing
continent had appeared running almost from north to
south. In the Tertiary period there was an enormous
increase of upheaval over the World Ridges, and the
crests of them (lightest brown on map) everywhere emerged.
The sea still swept over the central* part of the Eastern
World Ridge from north to south and south-west, so that the
Indian Ocean was united with the Arctic Sea, and through
the wide Mediterranean with the Atlantic. Africa and
Australia were almost as extensive as at present. Britain
was separated from Scandinavia, and the south of Europe
formed a mountainous archipelago, amongst the islands
of which the Alps and Balkans were conspicuous. The

xiv The Record of the Rocks 273

Indian peninsula was still an island, and the Himalayas
were beginning to appear. The Western World Ridge was
nearer completion, North America was almost all above
water, and the line of the Andes was commencing to
give outline to South America. By the close of the Ter-
tiary period the elevation of the continents had been prac-
tically completed.

REFERENCES

1 Thorpe and Riicker, " Magnetic Survey of British Islands,"
Philosophical Transactions, vol. clxxxi. (1890, A), 53. See also
Good Words, 1890.

2 J. Geikie, "The Evolution of Climate," Scottish Geographical
Magazine, vi. 57 (1890).

BOOKS OF REFERENCE

A. Geikie, Text-Book of Geology. Macmillan and Co. (A com-
plete discussion of Geology from the modified Uniformitarian stand-
point, with references to important original papers.)

J. Prestwich, Geology. Clarendon Press. Two volumes. (An
admirable treatise from the modified Catastrophist standpoint.)

J. Geikie, Outlines of Geology. Stanford.

A. Geikie, Class-Book of Geology. Macmillan and Co.

A. H. Green, Physical Geology (second edition).

A. Geikie, Scenery of Scotland. Macmillan and Co. (A fascinat-
ing account of the origin of surface features. )

J. Geikie, Great Ice Age, and Prehistoric Europe. Stanford.

N. S. Shaler, Aspects of the Earth. (Suggestive essays.) Smith,
Elder and Co.

A. J. Jukes-Browne, Building of the British Islands. Bell
and Son.

J. W. Judd, Volcanoes. International Science Series.

J. Milne, Earthquakes. International Science Series.

T. Mellard Reade, Origin of Mountains. Taylor and Francis.

J. Croll, Climate and Time. A. and C. Black.

CHAPTER XV

THE CONTINENTAL AREA

354. Crest of the World Ridges. — (Read §§ 214, 251,
255, 256.) The five largest islands or peninsulas in which
the crests of the World Ridges break through the uniform
covering of the hydrosphere are termed continents, and
designated by the names Eurasia, Africa, North America,
South America, and Australia. They are distinguished from
other islands and peninsulas by size alone, Australia being
ten times larger than New Guinea, and Africa ten times
larger than Arabia, these being the greatest island and
peninsula not called continents. The elevated region round
the South Pole is crowned by the unexplored and scarcely
discovered continent of Antarctica. The land mass of
Eurasia is conveniently supposed to consist of the two
" continents " of Europe and Asia, and if this be allowed,
we find that the six known continents group themselves into
three pairs. North and South America share the Western
World Ridge ; Asia and Australia, on the eastern limb of the
Eastern World Ridge, lie diametrically opposite ; while
Europe and Africa occupy the western limb of the Eastern
World Ridge, diametrically opposite the great Pacific basin.
Until the Tertiary period, when the heights of Central Asia
were upheaved, the Indian Ocean stretched to the Arctic
Sea ; and even in Quaternary times Europe and Asia were
separated by a broad channel of water between the Medi-
terranean and the Arctic Sea. The prevailing continental
form is a south-pointing triangle. In each pair of continents

CHAP, xv The Continental Area 275

the northern has a wide extension from east to west, a deeply
indented coast, and a great group of islands on the south-
east stretching toward the unindented coast of the southern
member, which, as a rule, extends from north to south, and
has an island or island group lying to the south-east.

355. Comparison of the Continents. — By studying the
.maps (Plates XI. XII. and XIII.) and the following tables
the student will be able to compare the characteristics of the
separate continents. The average heights in Table A are
those calculated by Dr. John Murray, from whose figures
also the relative areas at various elevations (Table C) are
derived.1 The distance from the sea of the continental
centre or position farthest from the coast is that calculated
by the Russian, General von Tillo ; the figure for Europe is
not strictly comparable 'with the others, since Europe is
widest at its junction with Asia. Professor Kriimmel, a
leading German oceanographer, has calculated the percent-
age of surplus coast given in Table A. Since a circle has
the smallest boundary of any figure of the same area, if we
imagine the coast-line stripped off a continent like braid off
a coat, and the continent moulded into a circular outline
without change of area, a smaller length of coast would serve
to surround it. The length of coast left over, is expressed as
percentage of the original length, and serves as a measure of
the surplus available for bordering peninsulas and bays. In
the three northern continents, it will be noticed, more than
two-thirds of the coast-line are thus available ; in the three
southern continents less than one-third. Table B, calcu-
lated by Dr. Rohrbach,2 gives the percentage of each conti-
nent lying within certain zones of distance from the coast,
and is thus a measure of their accessibility from the sea
(compare Plate XII.) The chief mountain ranges of each
continent are marked by red lines on Plate XVIII. ; this
should be compared with the orographical map (Plate XL),
on which plains and plateaux are more clearly shown.

276 The Realm of Nature

COMPARISON OF THE CONTINENTS

CHAP.

CONTINENT.

2

I

N. America.

S. America.

1

Australia.

All Land.

TABLE A. — AREA, ELEVATION AND COAST-LINE

Area (million

16-4

ii-i

7-6

6-8

3-7

3'°

55-°

sq. miles)

Average height

3000

2000

1900

2OOO

940

800

2IOO

(feet)

Highest point

29.000

18,800

18,200

22,400

18,500

7200

29,000

(feet)

•

Surplus coast

61-7

28-3

64-6

32-6

87-6

30-6

(per cent)

Distance of

1616

I-II9

1057

1057

810

591

continental

centre (miles)

TABLE B. — PERCENTAGE OF CONTINENTAL AREAS WITHIN

EQUIDISTANT ZONES FROM COAST

0-125 miles

22-9

18-4

31-6

24-1

46-4

35-7

25.8

125-250 „

14-8

16-2

2I-I

20-1

21-0

25-5

17.9

250-375 „

11-7

14-5

15-7

15-5

14-2

21-7

14-4

375-500 „
Mean distance

9-3

I3'i

"•5

12-4

7-7

13-5

II-I

from coast
(miles)

U85

420

295

345

210

215

380

Percentage of

60

53

58

56

62

55

area under

mean dis-

tance

Do. over mean

40

47

42

44

38

45

distance

TABLE C. — PERCENTAGE OF CONTINENTAL AREAS WITHIN

ZONES OF EQUAL ALTITUDE ABOVE SEA

Below sea-level

1.4

O.I

0.05

0.0

1.8

0.0

0.6

0-600 feet

23-3

12.5

32-25

40.0

53-8

29.8

26.7

600-1500 ,,

16.0

34-8

32.1

26.8

27.0

64-3

27.8

1500-3000 „

21.7

27.6

13-3

16.8

IO.O

4.1

19.3

3000-6000 ,,

21.8

21.8

13.2

7-0

5-5

1.5

17.0

6000-12,000 ,,

IO.O

2.8

8.4

5-o

0-3

6.0

Above 12,000

5-8

0.4

0.7

4-4

0.2

0.0

2.6

feet

xv The Continental Area 277

356. Continental Slopes. — The simplest conceivable
continent would consist of two land-slopes meeting, like the
roof of a house, along a central line or axis, so that a
section across it would resemble A, Fig. 56. The axis of
a continent is usually formed by a mountain range of eleva-

FIG. 56. — Typical Section of a Continent. In BCD the short slope is shown
to the left, the long slope to the right.

tion (§ 303), which most frequently occurs near the edge of
the slope of the world ridge, and consequently near one
side of the continent, so as to produce a short slope on one
side and a long slope on the other, giving a section like B.
A mountain chain is rarely single, and is about equally steep
on both sides. It occupies a narrow strip of a continent ; so
while the short slope of the continent is nearly uniform to the
sea, the long slope is broken into a steep and a gentle portion,
giving the section C. But since both sides of a continent have
been ridged up, a lower and broken mountain range usually
intervenes between the long slope and the sea, converting the
central part of the continent into a wide valley, and forming
a second short slope to the seaward side, as shown in section
D. The various slopes form parts of river-basins (§ 319),
and the course of rivers in an ordinary map serves to mark
out the direction of the slopes. Where there are no rivers,
or when rivers flow into a salt lake, a region of internal
drainage results. Such regions occur in every continent
wherever the arrangement of the heights cuts off rainfall
and allows full scope to the action of evaporation. One-
quarter of the Earth's land surface is thus situated.
The long slopes of all the continents are directed toward
the Atlantic Ocean and its seas, which thus receive the
drainage of more than half the land (Plate XIII.) All the
continents turn their backs, so to speak, on the Indian and
Pacific Oceans. The following table is calculated by Dr.
John Murray.3 The small area draining into the Southern
Ocean is added, in the table, to those of the Atlantic and
Pacific Oceans.

278 The Realm of Nature CHAP.

PERCENTAGE AREA OF CONTINENTS SLOPING TO EACH OCEAN

1

J

Eurasia.

Africa.

N. America.

1

1

World.

1

C/3

Atlantic, in-

13-91

49.0

36-0^1

86-4

...

34-31

cluding

Mediter-

i-37-9

1-76-5

Uo-8

ranean

1

|

|

Arctic Sea

24-0 J

40- 5 J

I6-5J

Pacific .

19-6

20-3

'6-3

9-3

14.4

Indian

I5'3

20-0

40-0

12-8

Inland

27-2

31-0

3-2

7-3

50-7

22-0

100-0

100-0

100-0

IOO-O

IOO-O

100-0

357. South America being the most typical continent
may be first described. The triangular outline is modified
by a large outcurve of the northern half of the west coast
north of 20° S., and on the middle of the east coast by a
more prominent outcurve culminating in Cape San Roque.
Its greatest length, nearly along the meridian of 70° W., is
4800 miles, from Point Gallinas on the Caribbean Sea in
13° N. to Cape Horn on the Southern Ocean in 56° S.
The greatest breadth from west to east is 3300 miles along
the parallel of 5° S., between Point Parina (82° W.) and
Cape San Roque (35° W.) A group of rocky islands, the
Chonos Archipelago, runs for 1200 miles close to the fjord-
grooved west coast at its southern extremity, and a tortuous
channel separates the south-eastern tip, Tierra del Fuego,
from the mainland. The average elevation of the continent
is almost exactly that of the whole continental area.

358. The Andes. — The main axis of South America lies
close to the west coast along the crest of the Andes, which
form the longest mountain system, unbroken by passes of
low elevation, in the world. The short slope to the Pacific
varies from 30 to 150 miles in breadth ; the long slope to
the Atlantic is in parts 3000 miles wide. A mountain
system is not a ridge, but a region showing diversities of

xv The Continental Area 279

structure and scenery from point to point. The highest peak
of the Andes is Aconcagua, 22,400 feet, in 33° S. ; but at
least thirteen other summits rise more than 19,000 feet above
the sea. Many of the passes, which mark the meeting of
the heads of transverse valleys of opposite slopes, are
elevated more than 14,000 feet, and the lowfest in a stretch
of 4000 miles is 11,400 feet above sea-level. Tertiary
sedimentary rocks form the slopes of the Andes, and are
overspread in many places by sheets of volcanic rock, while
the loftiest volcanic cones in the world shoot up in solitary
grandeur above the ridges. The Andes are young moun-
tains, geologically speaking, and are still growing. Every
little step of upheaval is accompanied by earthquakes (§ 299),
which occur more frequently along the western margin of
South America than anywhere else. South of Aconcagua the
system consists of a single rugged ridge, which gradually
diminishes in height and in steepness toward the south,
where the sea has invaded its valleys forming the Chonos
Archipelago. From Aconcagua northward to the equator
the system forms two mountain ranges, one keeping close
by the Pacific coast, the other sweeping inland. Where
they diverge most widely the two mountain walls ericircle
a high plateau of internal drainage, which is as large as
Ireland, and its lowest part, 12,000 feet above the sea, is
occupied by the great Lake of Titicaca. Converging at
the northern extremity of the Titicaca Plateau the two
ranges wall in a longitudinal valley of great length, sloping
northward and traversed by rivers which escape by wild
gorges through the eastern ridge. From the equator north-
ward the ridges of the Andes diminish in height, unite in
the " Knot of Pasto," and then branch' into three spurs,
separated by the long valleys of the Magdalena and
Cauca sloping to the north. The eastern spur sweeps round
the north coast of South America, completing the framework
of the continent. Along its whole length the eastern ridge
of the Andes slopes down to the central low plain by a
succession of great terraces, and sends out many short
diverging mountain buttresses. Ores of silver, mercury,
and copper abound in these mountains, and coal-beds occur

280

The Realm of Nature

CHAP.

in the south. On the rainless short slope in the centre
nitrate of soda forms extensive deposits.

359. Eastern Mountains and Low Plains. — The long
slope of South America from the base of the Andes forms
one vast low plain stretching from north to south, the portion
of which, at a less elevation than 600 feet, is equal to two-
fifths of the continent. It is broken into three divisions
by two very gentle ridges stretching eastward from the
Andes. The northern and smaller swells up into the
High Plain of Guiana, which is cut into lines of heights,
known as the Sierra Parima, the Sierra Pacarai, cul-
minating in Roraima (§ 312), and the Sierra Acaray.
The larger or High Plain of Brazil fills the whole eastern
outcurve. It is an upheaval of very ancient rock, which
has been cut by the valleys of numerous great rivers into a
medley of mountain masses, few of which exceed 3000 feet
in height. The Sea Range, under many names, runs along
the coast from 10° S. to 30° S., forming the steep seaward
slope of the High Plain. The eastern mountains contain

FIG. 57.— Section across South America on parallel of 18° S. Vertical scale
300 times the horizontal. Sea-level marked O.

deposits of gold and of diamonds, and are covered in many
parts by fertile soil. Fig. 57 gives an idea of the form of
the slopes of South America on the parallel of 18° S.

360. Orinoco Basin. — The northern division of the Low
Plain is known as the Llano, and forms the basin of. the
Orinoco River, which is kept supplied with water by tribu-

xv The Continental Area 281

taries descending from the mountain borders. In the
rainy season, June to August, the plains are flooded, driving
the inhabitants to take refuge in houses built in the trees.
The Orinoco, from its source on the south-west of the
Guiana High Plain, flows along the watershed which parts
its basin from that of the Amazon. One branch, retaining
the name Orinoco, eventually flows down the northern slope
and sweeps east to the sea, while another, known as the
Casiquiare, breaks away down the southern slope and flows
rapidly into the Rio Negro, a tributary of the Amazon.
The two great river systems are thus connected by a natural
canal.

361. Amazon Basin. — At a distance of 1900 miles
from the sea the vast central plain only reaches an
elevation of 600 feet, and the basin of the Amazon pre-
sents the gentlest land-slope in the world. Nearly the whole
plain is covered with dense tropical forests, and it is there-
fore called the Selvas or Woods. On each side the Amazon
and its tributaries overflow in the rainy season (§ 318),
covering the land for 20 or 30 miles from the banks,
so that the forests appear to be growing in the water ;
and depositing fine alluvial soil which, over the whole region,
does not contain a stone as large as a pea. Numerous
great tributaries, many exceeding 1000 miles in length,
converge to the main river from the slopes and high valleys
of the Andes. Of these the Maranon is generally con-
sidered the head stream, although the Ucayali is longer.
Other rivers flow in like veins joining a leaf-stem, from
the Guiana High Plain in the north and the Brazil
High Plain in the south. Two of the- largest rivers of
the latter region flow north in wide valleys but do not
reach the Amazon : one, the Tocantins, enters the sea
close to its mouth, and the other, the Rio San Fran-
cisco, curving sharply to the east, reaches the Atlantic
about 10° N.

362. La Plata River System. — From the temporary
lake which forms west of the flat low plateau of Matto Grosso
in the rainy season, and gives origin to some of the southern
tributaries of the Amazon, the river Paraguay flows south

282

The Realm of Nature

along the low plain, receiving numerous tributaries from
the Andes slopes on the west, and the great River Parana
from a southern valley of the Brazilian High Plain on the
east. The united river swerves eastward and enters the
wide shallow estuary termed the Rio de la Plata at 34° S.
The undulating grassy plain of its lower track is called the
Pampas, and is one of the flattest low plains in the
world. South of the La Plata several rivers flow to
the Atlantic from the Andes ; all are subject to floods on
account of the abrupt change of slope at the base of the
mountains, the inclination of the low plain toward the east
being too slight to let the water drain away when the
torrential track is flooded. Patagonia, the southern ex-
tremity of the continent, is for the most part a desert of
shingle, and much of it is an area of internal drainage on
account of the drying of the brave west winds by the Andes

(§201).

363. North America presents the typical form and
configuration of a continent, but it resembles South America
passed through a mangle, being larger, wider, lower, with
less contrast between its heights and plains, and a much
more broken coast-line. Fig. 58, a section across the
continent on the parallel of 36° N., and Fig. 59, on the

FIG. 58. — Section across North America in 36° N. Vertical scale
300 times the horizontal. Sea-level marked O.

meridian of 90° W. along the central low plain, are on
the same scale as that of South America. The total length
of the continent, nearly on the meridian of 100° W., is

xv The Continental Area 283

4000 miles from the ice-bound Parry Islands in 75° N.
to the tropical isthmus of Tehuantepec in 17° N. The
greatest breadth, on the parallel of 52° N., is 3000 miles.
In the extreme north-west Cape Prince of Wales on Bering
Sea comes within 40 miles of the north-eastern extremity
of Asia ; and on the north-east Greenland is bound to
America by continuous ice .in winter. The west coast
and northern part of the east coast of North America are
high and rocky, but the south-east presents the longest
stretch of gently shelving shore in the world.

364. Western Heights of North America. — From
Tehuantepec to Alaska the axis of the continent runs along
the Rocky Mountains. This range is often considered to
be a continuation of the Andes, but it is less lofty, the
passes across it are lower, and the two slopes into which it
divides the continent are more nearly equal than those of
South America. The average distance of the range from
the west coast is about 400 miles, except where, the great
Pacific outcurve increases the distance to almost 1000 miles.
Mount Brown, near 52° N., is the highest peak, 16,000
feet; and Pake's Peak (14,200 feet), in 39° N., is one of
the next in elevation. Midway between these summits one
of the grandest portions of the range has been set apart as
a permanent museum of physical geography on a grand
scale, under the name of the Yellowstone National Park
(§ 316). On the east the Rocky Mountains slope down in
wide terraces comparatively gently to the central low plain.
On the west their slope is abrupt but short, terminating in
a wide plateau, averaging 5000 feet in height, which runs
along the entire length of the continent, and is buttressed
on the west by a less continuous series of ranges. The
Sierra Madre is the western buttress of the plateau in the
south, where it forms the watershed, and near the point
where it diverges from the Rocky Mountains the volcanic
peaks of Orizaba (18,200 feet) and Popocatepetl (17,500
feet) rise as majestic summits, which with Mount
Wrangel (17,500 feet) in Alaska are the loftiest in North
America. Farther north the plateau te supported by the
rugged snow-clad Sierra Nevada^ which presents a very

284 The Realm of Nature CHAP.

steep front to the west, cut into by rugged transverse
valleys, with scenery of the wildest grandeur. Its highest
peak is Mount Whitney (14,900 feet), and at Mount Shasta
it passes into the Cascade Range, which runs northward,
diminishing in height, to Alaska, its chief summit being
Mount St. Elias (13,900 feet). Between latitudes 35°
and 40° N. a lower mountain ridge, the Coast Range, joined
to the Sierra Nevada on the north and the south, encloses
a remarkable low plain, the Californian Valley, the rivers of
which find access to the sea through an abrupt gap near
the middle of the range. The eastern part of the centre
of the plateau between the Rocky Mountains and the
parallel Wahsatch Range, in longitude 112° W., forms
the most elevated region, and is crossed by the Uintah
Mountains, running from west to east. Cutting right
through the Uintah range, and southward across the
plateau to the Gulf of California, the great Colorado River
and its tributaries lay bare the structure of the rocks,
showing the horizontal sedimentary strata, interspersed with
outflows of basalt, based on a bed of Archaean gneiss. The
other great river of the Pacific slope is the Columbia, the
tributaries of which converge from all parts of the Rocky
Mountains, from near Mount Brown in the north to the
Wahsatch Range. In the north-west, where the low border-
ing ranges* spread out, the great Yukon flows down the
northern slope of the diminished plateau into Bering Sea.
Gold, and the ores of silver, lead, mercury, and copper,
occur very abundantly in the valleys and mountains of the
plateau.

365. The Great Basin.— Between the Wahsatch Moun-
tains and the Sierra Nevada the plateau sinks slightly into
a vast triangular area of internal drainage, known as the
Great Basin. It is most depressed near the sides, and
rises in the middle in a series of mountain ridges. In the
Quaternary period a wide sheet of water — called Lake
Bonneville — occupied the eastern depression, and its
shrunken remnant now forms the Great Salt Lake, at the
base of the Wahsatch Mountains. A smaller expanse —
Lake Lahontan — filled the western depression, which is

xv The Continental Area 285

now dotted by a series of small salt lakes under the eastern
slope of the Sierra Nevada. The soil of the Great Basin
is encrusted with borax and other alkaline salts deposited
by the shrinking lakes. In recording their researches on
this region, the officers of the United States Geological
Survey have produced a series of the most fascinating
memoirs on physical geography. The volumes on the
exploration of the Colorado River by Major Powell, and on
Lake Lahontan by Mr. Russell, are especially interesting.

366. The Appalachian Mountains, running parallel to
the east coast, form a broad chain of moderate height,
Mitchell's Peak, 6700 feet above sea-level, being the loftiest.
They are true mountains of elevation, the alternate anti-
clines and synclines forming parallel ridges and longitudinal
valleys, and their rocks are much more ancient than those
of the western heights. In the south, Carboniferous strata
and coal seams are laid bare in the transverse valleys, and
the extension north of the St. Lawrence, in the broad low
ridge of the Laurentides, is composed mainly of Archaean
rock. The Appalachians, which are sometimes called the
Alleghanies, form a complete minor axis, giving the east of
North America a short slope to the Atlantic and a long slope
westward. The watershed follows the eastern ridge of the
chain in the south, and the western ridge in the north ; the
Hudson River, however, cuts right across the entire chain.

367. Mississippi Basin. — One great valley, formed by
the meeting of the long slopes of the two mountain axes,
occupies the whole centre of North America. The southern
and northern halves of this valley dip in opposite directions

FIG. 59. — Section of North America on the meridian of '90° W. Vertical scale
300 times the horizontal. Sea-level marked O.

from a broad flat transverse ridge of very slight elevation in
48° N. The southern south-sloping half of the valley forms
the basin of the Mississippi River. The Mississippi rises
on the crest of the gentle transverse slope, and after a

286 The Realm of Nature CHAP.

winding course of more than 1000 miles receives on its
right bank the Missouri, a river of much greater length,
formed by the union of tributaries from 900 miles along
the Rocky Mountain Range. Farther south the Arkansas,
another long river, flows in from the Rocky Mountains.
The steep eastern slope of this range, unlike that of the
Andes, stops at an elevation of nearly 6000 feet above the
sea, and thence the rivers flowing to the Mississippi cross a
slope so gentle that the land is spoken of as the Great
Plains. As the elevation diminishes the slope decreases
also, and the lowlands of the basin become known as the
Prairies. The Ohio River, flowing down the slope of the
Appalachians, is the largest tributary reaching the Mis-
sissippi on its left bank.

368. Arctic Basins. — In the northern half of North
America several nearly level terraces, of from 200 to 300
miles in breadth, separated by narrow zones of steeply
sloping land, descend from the Rocky Mountains toward
Hudson Bay. The lower terraces are covered with
boulder clay, and the terminal moraine of the great
Pleistocene ice -sheet has been traced in the form of a huge
ridge called the Grand Coteau des Prairies. This ridge
turns the Missouri River to the south, and the Saskatche-
wan, flowing from near Mount Brown in the Rocky Moun-
tains, to the north, thus separating the northern and
southern slopes. Upon the lowest terrace, where the
glacial remains are thickest, a line of wide shallow lakes
stretches from 49° N. to the Arctic Sea. Lake Winnipeg
in the south receives the Saskatchewan, and has an outlet
by the Nelson River to Hudson Bay. This lake is the
centre of a great but ill-defined drainage area, some of the
hundreds of small lakes surrounding it being connected
with several river systems, on account of the confused
ridges left by the melting ice-sheet. Traces remain of a
much larger ancient body of water, called Lake Agassiz,
which included Lake Winnipeg, and many smaller lakes
and river -valleys. The Athabasca, rising near Mount
Brown, flows north-eastward to Lake Athabasca, which has
an outlet northward to Great Slave Lake, whence the wide

xv The Continental Area 287

Mackenzie River flows parallel to the Rocky Mountains to
the Arctic Sea, receiving the outflow from Great Bear Lake
on the Arctic circle.

369. St. Lawrence System. — The gentle transverse
ridge separating the northern and southern slopes of North
America is nowhere higher than 2000 feet, and it only
attains this elevation in the east. Its surface is slightly
concave, the northern eidge, called the Height of Land,
being a continuation westward of the Archaean plateau of
the Laurentides ; while the southern edge, known as the
Great Divide, is a prolongation toward the east of the
moraine heaps of the Cot'eau des Prairies. The central
hollow contains a remarkable group of lake basins, which
are claimed, with some probability, to contain half of the
fresh water in the world. Before the Ice Age they were
probably in connection with the Mississippi river system,
and from ancient raised beaches surrounding them they
were evidently at one time much more extensive than now.
The western group — Lakes Superior, Michigan, and Huron
— are closely connected, and their surface stands about 600
feet above sea-level. From the south of Lake Huron they
discharge into Lake Erie, whence the Niagara River
(§ 330) leads northward into Lake Ontario, from which the
broad St. Lawrence sweeps, on to the Atlantic.

370. Australia, the onlj^knoww continent entirely in the
southern hemisphere, is 2300 miles in extreme length along
the parallel of 26° S. (see section Fig. 60). The greatest

FIG. 60. — Section across Australia in 26° S. Vertical scale 300 times
the horizontal. Sea-level marked O.

breadth is 2000 miles along the meridian of 143° E. from
Cape York in 11° S., which is the most northerly point, to
Cape Otway in 39° S. Incurves of the north and south

288 The Realm of Nature CHAP.

coasts reduce the width to 1 1 oo miles in the narrowest part
of the continent, while both the east and west coasts form
bold outcurves. Tasmania rests on the Continental Shelf
to the south, and New Guinea to the north. The average
height of the land, as far as can be judged from the imper-
fect exploration of the interior, is about 800 feet. In spite
of this low elevation the proportion of land less than 600
feet above the sea is small, while the proportion between
600 feet and 1500 feet in elevation is greater than for any
other continent.

371- Configuration of Australia. — The continent is
apparently one low plateau, rising into a line of hills along
the west coast, and ridged irregularly here and there by
mountains in the nearly unknown interior. It sinks in the
south-east to an extensive low plain (the Australian Basin)
less than 600 feet above the sea. Half of Australia is
made up of areas of internal drainage. The Great
Dividing Range, forming the axis of the continent, rises
along the eastern edge. It sweeps round the south-east
coast under the name of the Australian Alps, and cul-
minates in Mount Townsend or Kosciusko (7300 feet).
Thence it runs northward under different names as a
chain of short ranges, scored by deep transverse valleys,
sending short full rivers to the Pacific. Diminishing in
height toward the north, it merges into the general eleva-
tion of the plateau. The ranges were, as a rule, ridged up
out of primary rocks, the Silurian system being now most
prominent in the south, and the Carboniferous, with thick
seams of coal cropping out, farther north. Gold, and the
ores of silver, tin, and lead occur in great abundance.

372. River Basins. — The southern part of the Dividing
Range slopes down very steeply westward to the low plain
of the Australian Basin. The Murray River flows westward
across the Basin from its source near Mount Townsend, and
after receiving the Lachlan and Darling it swerves to the
south and enters the sea. Many long rivers are marked on
maps converging from the east and north to the Australian
Basin, but most of these are stony channels only occupied
by water after rain, and many of the streams dry up as they

OCEAN SURFACE ISOTHERMS, CORA

After A. Buchan, H

180 180 14O 12O

60 40

Edioburgk &»ogriqihical 3asti.tu.te

RISING COASTS coloured BLUE — SINKING COAST

[*EEFS, RISING AND SINKING COASTS.
Guppy, and others. 15

*0 60 ~ 8O 10O 120 1*0

OCEAN SURFACE ISOTHERMS
es on the Ocean are the Mean Annual Isotherms .. .
ace Water -The Figures indicate the Temperature
Fahr -The Red Tint shows Areas where the
is 80°and upwards.

- ' '""' I J

2O 40 60 8O 1OO 120 14O

oured YELLOW — CORAL REEFS coloured DARK RED.-<

xv The Continental Area 289

flow. The Basin is divided by the Flinders Range west of
the Murray, and its western part forms a depression scarcely
raised above sea-level, in which lie Lake Torrens and Lake
Eyre — salt lakes with no outlet. The whole depression is
rimmed round with coral limestone of Tertiary age, and
appears to have formed a wide shallow bay long after the
rest of the continent was upheaved. The plateau to the
west is a great desert not fully explored, and composed of
the rock known as desert sandstone, fringed to the south
by grand cliffs of tertiary limestone which line the Great
Australian Bight as a wall about 400 feet high, unbroken
by a single river for 1000 miles.

373. Africa presents a typical triangular outline resem-
bling that of South America, but the north-western outcurve
is much more pronounced, while the north-eastern outcurve
is broken by the depression of the Red Sea. Round Africa
the Continental Shelf is extremely narrow, and the islands it
bears are few and small, while the coast-line is less indented
than that of any other continent. The greatest length,
nearly 5000 miles, lies along the central meridian of 20°
E., and the" greatest breadth, 4500 miles, is on the parallel
of 10° N. Africa is the only continent crossed by both
tropics, the equator passing nearly through the centre.
The average elevation of Africa is nearly that of all the
land ; but no other continent has such a small proportion
of land below 600 feet in height (one-eighth of its area),
and none has so great an extent (nearly two-thirds) between
the heights of 600 and 3000 feet.

374- Slopes of Africa. — A section drawn across the con-
tinent, along the equator (Fig. 61) hardly shows how com-
pletely the typical continental structure is departed from, as
Mount Kenia is only an isolated peak, not part of a range.
All the rivers pursue singularly curved courses, unlike those
of any other continent, and where they drop over the edges
of the plateaux form great cataracts. The watersheds are
not dominated by mountain ranges, but by the broad backs
of plateaux, out of which the main features of the land-
slopes have been carved by erosion. The Atlas moun-
tains run along the coast in the north-west and rise

U

290

The Realm of Nature

CHAP.

into a succession of snow -crowned peaks, the loftiest of
which was estimated by Mr. Joseph Thomson to be 15,500
feet above the sea. All round the coast, except in the
north and north-west, the edges of the plateau present a

FIG. 61. — Section across Africa on the equator. Vertical scale 300 times
the horizontal. Sea-level marked O.

mountainous aspect, and several great volcanic summits
risel from their highest levels. Kenia, Kilima-njaro, and
Ruwenzori reach heights approaching 19,000 feet above
the sea. The loftiest elevated belt, which may be termed
the Great Plateau^ runs from the Red Sea southward and
westward across the continent, and may be looked on as
forming the main axis. Its greatest elevation is in the
rugged valley-riven plateau of Abyssinia, and it continues
highest on its eastern side. A strip of eastward-sloping
land, down which the Zambesi pursues a cataract-broken
course to the Indian Ocean, separates the Great Plateau
from a smaller plateau which fills the southern extremity of
the continent. This Southern Plateau sinks to the sea in
steep terraces bordered on the south and east by curved
mountain ranges, the most important of which is the
Drakenberg. It dips to the west and is drained by the
Orange River, a rapid stream flowing through a deep
canon far below the general level. The Great Plateau
sends off three long branches of high land toward the
north-west, which cannot be clearly traced on a map unless
the contour-line of 1500 feet is shown. The first or Coast
Ridge runs round the west coast and descends to sea-level in
terraced mountain slopes. It bears the high Cameroon

XV

The Continental Area 291

Mountains near the angle of the Gulf of Guinea, and slopes
down very gradually inland. Its western extension is pierced
by the great River Niger, flowing into the Gulf of Guinea.
The second or Central Ridge runs from the equator toward
the Atlas Range across the northern high plain. Uniting
with the Coast Ridge in latitude 5° N. and again in 20° N.
it forms two great basins, of which the southern] or equa-
torial is, on the average, higher, and the northern lower,
than 1000 feet. The third or Red Sea Ridge runs along
the Red Sea coast from the northern extremity of the
Great Plateau. A very remarkable hollow furrows the
whole length of the Great Plateau for nearly 2000 miles
from north to south, and contains a succession of four great
lakes connected with three distinct river systems. These
are Lakes Albert and Albert Edward draining to the Nile
in the north, Lake Tanganyika attached to the Congo in
the centre, and Lake Nyasa united to the Zambesi in the
south.

375- Nile River System. — Lake Albert collects the
head-waters of the Nile, receiving the Semliki River from
Lake Albert Edward lying at the base of Ruwenzori, and
fed by the ceaseless torrents from that mountain. It also
receives at the northern extremity the outflow of the largest
lake in Africa, the Victoria Nyanza, which is situated on a
higher part of the plateau east of the Great Hollow at an
elevation of 3300 feet. This branch, the Victoria Nile, is
broken by a succession of falls as it descends the steep
edge of the plateau. From Lake Albert the White Nile
flows northward to the Mediterranean . across the desert
which stretches between the slopes of the Red Sea Ridge
and the Central Ridge, receiving many tributaries from both.
The rainy heights of Abyssinia send down the Blue Nile
and the Atbara, on which the periodical flooding of the
Nile depends, but after the junction of the latter stream the
Nile flows in three great bends across the parched low plain
to its delta (§ 325) without receiving another drop of water,
and subject to continual evaporation (§ 318). The six
famous cataracts which occur in its lower course are pro-
duced by its bed crossing bars of hard rock, and they thus

292 The Realm of Nature CHAP.

differ in their nature from the cataracts of the plateau rivers
of the south.

376. Congo Basin. — Shut in between the Central and
West Coast Ridges, the equatorial basin was probably at one
time a great inland sea several times larger than the Caspian.
Its waters found an outlet across a comparatively low part of
the West Coast Ridge, which they eroded into a deep gorge
and so drained the lake into the Atlantic, leaving a basin
of fertile soil now covered in great part with dense forests.
Rivers flow into the circular basin from the high ground on
every side and become tributaries to the giant Congo.
This river descends from the Great Plateau at the equator
foaming over the cataracts of Stanley Falls, sweeps through
the basin in a magnificent curve as a navigable stream for
1000 miles, and bursts in a far grander chain of cataracts
over the plateau edge through the gorge of Yellala. The
source of the Congo lies somewhere in the Great Plateau
about 13° S., Lake Bangweolo, 4000 feet above the sea,
serving as a reservoir to collect the head- waters. In its
northward course the river is joined by the Lukuga from
Lake Tanganyika in the centre of the Great Hollow, 2600 feet
above the sea ; but it is only when the level of that lake is
raised considerably above its average height that it overflows.
Tanganyika, like most continental lakes, was once much
larger, and appears to be shrinking into a basin of internal
drainage, destined ultimately to become a small salt lake.

377. Tsad Basin and Sahara. — The northern basin
enclosed by the West Coast and Central Ridges is even
larger than that of the Congo, and so far as this very
inaccessible region has been explored it appears to have
no outlet. Lake Tsad, a sheet of shallow water varying in
size from 4000 to 10,000 square miles according to the
rainfall, and 800 feet above the sea, receives a number of
great rivers from the south, and overflows in the rainy season
to a much lower enclosed basin in the north-west, where
excessive evaporation leaves only a crust of salt upon the
ground. To the north nearly the whole breadth of Africa
forms the internal drainage area of the Sahara, a sandy
high plain broken by the rugged mountains of the Tibesti

xv The Continental Area 293

Range which cap the Central Ridge, and dipping in the
west and north to a low plain with some small depressions,
called shotts^ below sea-level.

v 378. Eurasia, containing one-third of the land of the
globe and occupying the central part of the Eastern World
Ridge, when looked at largely, shows the typical features of
a triangular outline and a mountainous axis giving a long
and a short slope to the land, and supporting a plateau of
internal drainage. It is the least tropical of the continents,
only the three south-eastern peninsulas crossing the tropic
of Cancer. The greatest length of Eurasia is about 7000
miles, from Cape Roca in 9° W. to East Cape on Bering
Sea in 170° W., the continent extending more than half-
way round the Earth. The greatest breadth is about 5000
miles, along the meridian of 105° E., from Cape Chelyuskin
in 77^° N. to Cape Buru in ij° N. at the extremity of
the Malay Peninsula. More than one-quarter of this vast
area slopes together, forming basins of internal drainage,
and almost a quarter slopes north toward the Arctic Sea,
giving a peculiarly inaccessible character to half the conti-
nent and tending to increase the severity of its continental
climate. The low plain of Eurasia forms a great triangle
with its base along the Arctic Sea. This is divided into a
smaller western and a larger eastern portion by the low belt
of the Ural Mountains in 60° E., and maybe taken as form-
ing the boundary between Europe and Asia. A section
of the continent, along the meridian of 90° E. (Fig. 62),
gives a general idea of the structure. The main features of
the west coast of Europe correspond on a smaller scale
with the east coast of Asia — the Scandinavian peninsula
answering to Kamchatka, the Baltic to the Sea of Okhotsk,
the British Islands and North Sea to the islands and Sea
of Japan. Similar resemblances connect the south coasts.
Spain and Arabia are both square and massive plateaux ;
Italy and India are both separated from the continent by a
low plain under a lofty mountain wall, and taper southward,
ending in a large island ; and the Balkan peninsula, like
Indo-China, is mountainous, deeply indented, and termi-
nates in an archipelago.

294 The Realm of Nature CHAP.

379. Asia, the highest as well as the largest of the
continents, has an average elevation of more than 3000
feet. The zone of heights between 600 and 1500 feet is
narrower than in any other continent, and more than one-
sixth of the surface stands more than 6000 feet above the
sea. The orographical centre of Eurasia is formed by the
lofty plateau of Pamir (in 38° N. and 73° E.), as large as
Ireland, and rising to 25,800 feet above the sea in its
highest summit, while its lowest point is 9000 feet ; it is
called by the dwellers in the region "The Roof of the
World." From this centre, mountain chains spread out
like the ribs of a fan to the east and to the west. The
lofty range of the Hindu Kush — cleft by a few snow-blocked
passes and rising into summits 24,000 feet high — runs south-
west from the Pamir, separating the low plain of India
from the low plain of Northern Asia. It branches in lower
ridges to the south and west, enclosing the internal drainage
area of Iran (Persia), which lies at an average height of
3000 feet. The northern mountain ridge, sweeping round
the south shore of the Caspian as the Elburz Range, merges
into the broken Plateau of Asia Minor. Here the southern
ranges also converge, walling the Plateau of Iran from the
low plain down which the Tigris and Euphrates pour into
the Persian Gulf. Mount Ararat, 17,000 feet above the
sea, is the grandest summit in Asia Minor. The plateau
spreading southward occupies Arabia, most of which is an
internal drainage area. One of the most perfect types of a
mountain chain of elevation is presented by the Caucasus,
which runs from the Black Sea to the Caspian as a magnifi-
cent barrier between the high plateau of Asia Minor and the
low, level plain of Europe, and culminates in Mount Elbruz,
18,500 feet high. In the calculation of elevation in the
tables of § 355 this chain is assigned to Europe.

380. Eastern Asiatic Mountain System. — The moun-
tain chains which radiate eastward from the Pamir con-
verge at two centres, one near the north of the Indo-China
peninsula, the other near the Sea of Okhotsk. Between
these three knotting points the long mountain ranges seem
on the map to droop in graceful folds. They define an

XV

The Continental A rea

295

area which is as large as South America, and is occupied by
the highest and most extensive plateaux in the world. The
southern front of the whole system is the triple chain of the
Himalaya, sweeping in a noble curve south-eastward from the
Pamir, rising from the plain in stately slopes and ridges,
and crowned by innumerable snowy summits, amongst them

FIG. 62. — Section across Asia on the meridian of 90° E. Vertical
scale 300 times the horizontal. Sea-level marked O.

Mount Everest (29,000 feet), the culminating point of the
Earth's surface. It is cleft by no passes less than 15,000
feet above the sea. The Karakorum, a short but very
lofty range (its chief summit, Dapsang, is 28,700 feet above
the sea), runs parallel to the Himalaya from the Pamir.
Thence also the long and lofty range known as the Kuen
Lun stretches east, and sends off the Altyn Tagh and
Nan-shan range in a north-easterly curve. Between the
Himalaya and the Kuen Lun extends the high plateau of
Tibet, 13,000 feet above the sea, and measuring 2000
miles from east to west, and 1700 miles from south to
north. The plateau slopes downward to the east, and
the mountains and valleys which ridge its surface converge
into a series of close parallel ranges at the Indo-China
knotting point. Thence some ranges diverge southward into
the peninsula, some descend eastward toward the plain, and
some sweep north-eastward to the Okhotsk knotting point
as the Khingan Chain. Most of the Tibet high plateau is
free from snow in summer owing to the extreme dryness of
the air, and is a region of internal drainage. The great

296 The Realm of Nature CHAP.

rivers Indus and Brahmaputra rise in the most northerly
longitudinal valley of the Himalaya, and break a way round
the northern and southern extremities of the range to the
southern plain. Other rivers, amongst them the Irawadi
and the Mekong, flow south in the longitudinal valleys of
the Indo-China Peninsula. Rising on the eastern margin
of the plateau, the Yellow River (Hoang Ho> § 324) sweeps
north-eastward until it breaks a passage through the Khingan
Range and turns south again over the eastern plain. The
Yang-tse-Kiang rises close to the Yellow River ; at first it
rushes southward through one of the longitudinal valleys,
but making a gap through the bordering mountain, and
piercing in turn several parallel chains, it swerves north-
ward and emerges from its gorges on the plain, to once
more approach the Yellow River near its mouth.

381. Tarim and Gobi Basins. — Standing with one foot, as
it were, on the northern edge of the Tibet High Plateau, the
Kuen Lun and Altyn Tagh reach down with the other to the
much lower High Plain of the Tarim River and the Gobi
Desert, which averages a little more than 3000 feet above
the sea. The vast range of the Tian Shan (" The Moun-
tains of Heaven "), with some summits 24,000 feet above the
sea, stretches north-eastward from the Pamir, and walls in the
northern side of the Tarim basin. Many rivers from the
slopes of the amphitheatre, formed by the converging
mountains, unite in the Tarim, which flows east for 1300
miles to dry up in the swampy salt lake of Lob Nor. The
Tian Shan is continued north-eastward by a number of
ranges, including the Altai, the Sajan, and the plateaux of
Vitim and Aldan, all of which rise much higher than the
Gobi, and are separated from each other by mighty valleys
sloping into the northern low plain. They are united by
the Yablonoi and Stanovoi Ranges to the great Khingan
Chain at the Okhotsk knotting point, and continue in the
diminishing Stanovoi Range to East Cape. There is
abundant evidence that the Gobi High Plain, now covered
in most parts with drifting sand, was once a vast inland
sea, discharging its surplus waters by the great valleys
between the northern heights into the Arctic Sea. Now

xv The Continental Area 297

the few rivers which flow into it from the surrounding
mountains end in the sand or in small salt lakes, and the
process of desiccation seems still going on. Rising on the
western slope of the Khingan with tributaries from the
eastern slope of the Yablonoi, the great Amur River cuts
through the Khingan Chain and several parallel ranges,
and finds its way into the Pacific.

382. Northern Low Plain. — The Tian Shan and
other northern mountains descend in terraces to a narrow
belt of undulating land about 1000 feet in elevation, which
sinks into the wide low plain less than 600 feet above the
sea. Lake Balkash, without outlet and intensely salt,
occupies a depression north of the Tian Shan, from which
it receives several rivers, but its area is steadily diminishing
by evaporation. Lake Baikal (§ 333), to the north-east
at a higher level, receives much water from the Altai and
surrounding heights, but its outflow is comparatively
trifling. The northern plain bears evidence in its gravel
beds of having emerged from the sea in the Quaternary
period, and the gradual desiccation of Asia probably dates
from the time when its upheaval cut off from the interior
the tempering influence of the sea. Three vast rivers, only
matched for length of course and area of basin by the
giant streams of Africa and America, flow from the moun-
tains across the plain to the Arctic Sea during the few
months when they remain unfrozen. The Lena, farthest east,
rising near Lake Baikal, terminates in a wide delta. The
Yenisei flows due north from the Sajan Mountains, and re-
ceives no considerable tributaries on its left bank, but the
Angara, from Lake Baikal, and the two Tunguskas flowing
from the east, join on its right bank. The Ob and Irtish
flowing north from the Altai unite, and after receiving tribut-
aries from the eastern slope of the Urals, enter the head of a
long narrow gulf of the Arctic Sea. From the Pamir the Amu
Daria and Syr Daria (Oxus and Jaxartes) flow across the
desert low plain, rapidly dwindling by evaporation, to Lake
Aral, the area of which is shrinking. In the time of the
early Greek geographers the Oxus swerved to the west and
entered the Caspian, and its old bed, from which it seems

298 The Realm of Nature CHAP.

'to have been diverted by sand-dunes (§ 307), may still be
traced. At a more remote period the Aral Lake was part
of a large sea which covered the Caspian basin and com-
municated with the Mediterranean, and in Quaternary
times spread over the low watershed of the Ob to the
Arctic Sea.

383. Indian Peninsula. — A great low plain extends
along the base of the Himalaya, separated by a gentle
ridge into a south-western slope traversed by the Indus ^n
its way to the Arabian Sea, and a gentler eastern slope,
along which the Ganges (§ 318) flows to the vast delta
which it shares with the Brahmaputra. An ancient
and much denuded plateau largely built up of volcanic
rocks fills the southern part of the peninsula. This plateau
is loftiest on its western edge, where it sinks in abrupt
terraces to the sea, presenting a mountain-like wall known
as the Western Ghats. The more gentle slope to the east
has been cut by numerous rivers into wide valleys, and the
broken plateau edge forms a lower and less regular line of
heights more remote from the sea, called the Eastern
Ghats. The coast-line on both sides is remarkable for
its unbroken character and the gentle shelving of the
beach.

384. Europe, a bunch of peninsulas thrust out into the
'Atlantic Ocean, is the only great land-mass not crossed by
one of the tropics, and from its well-marked sea-climate it
may be appropriately termed the Temperate Continent.
An axis of true mountains of elevation runs through
Southern Europe, and another forms the low belt of the
Urals on the boundary with Asia. A rim of ancient
plateaux worn into mountains of denudation marks the
north-western border in Scotland and Scandinavia.
Within this elevated frame the land is a wonderfully
uniform low plain, fully half of the continent being less
than 600 feet above the sea. Only one-sixth of the
surface has an elevation greater than 1500 feet. The
lines of elevation have a comparatively slight share in
determining the slopes, which exhibit none of the typical
continental simplicity.

xv The Continental Area j 299

385. Southern Mountain System. — The Alps, the
most thoroughly studied mountain system in the world,
form the orographical centre of Europe. The main chain
(§ 3° 3) runs east and west in a series of ridges separated
by longitudinal valleys and cleft by transverse valleys
into distinct mountain blocks. Mont Blanc (15,800 feet)
is the loftiest summit. On the south the main range
slopes down steeply to the low plain of Lombardy, which
is enclosed to the south by the Apennines, an exten-
sion of the western Alps. The northern range of the Alps
descends to a plateau sloping gently to the north and east,
and buttressed by the limestone ridges of the Jur-a. To
the east the system runs southward through the' Balkan
Peninsula as the Dinaric Alps, also a limestone chain, full
of the characteristic scenery wrought by erosion and sub-
terranean solution. The Balkan Range stretches east and
west across the peninsula, sloping down to the low plain of
the Danube in the north. The granite heights of the Black
Forest Mountains run north of the Jura, and are continued
by a broad ridge of Palaeozoic rock, which dips down into
the northern plain in an outcrop of the coal-measures. A
broken hill country extends north-eastward from the Alpine
plateau, sinking in elevation toward the north, and terminat-
ing in the Harz Mountains in 52° N. The hilly region
rises in the east into the steep heights of the Bohemian
Forest, which runs north-west from the eastern extremity
of the Alps. The Bohemian Forest Range turns sharply
north-eastward as the Erzgebirge or Ore Mountains, the
rocks of which are traversed by veins of many metallic
ores, and these in turn run eastward as the Sudetic Range.
Supported between the three ranges the irregular plateau
of Bohemia rises toward the south, and is terminated by the
higher land of Moravia. Eastward the Sudetic Range
adjoins the fine curve of the Carpathian Mountains, which
sweep steeply round the low Hungarian plain, and sink
down gradually to north and east into the great Northern
Plain. The Carpathian Range terminates in the Tran-
sylvanian Alps, which first run parallel to the Balkans,
and then converge in the west until they almost

300 The Realm of Nature CHAP.

meet that range. West of the Alps the Vosges Moun-
tains run northward, separated by a wide flat valley
from the parallel range of the Black Forest, and terminat-
ing in the same belt of ancient rock. Separated by the
narrow valley of the Rhone on the west, the Auvergne
plateau, studded with extinct volcanic cones, rises in a
steep terraced slope known as the Cevennes Mountains, and
sinks more gently to the low plain on the north and west.
The rugged high plain of the Iberian Peninsula is shut off
from Northern Europe by the straight line of the Pyrenees,
one of the steepest mountain ranges, and presenting some
of the finest examples of erosion in the form of cirques or
round valleys.

386. Elvers of Western Europe. — In Western Europe
the main watershed (see Plate XIII.) lies, as a rule, nearer the
south coast than the north, following roughly the Pyrenees, the
Cevennes, the Vosges, the Alps, the Black Forest, the Fran-
conian Jura, the Moravian Plateau, and the Northern Car-
pathians. Thus the northern slope is long, and the southern
slope short. In Eastern Europe the watershed is nearer the
north coast, crossing the low plain on a ridge of very slight
elevation, which stretches from the Carpathians north-east-
ward to the Urals, and swells up into the Valdai Hills about
the centre. This gives a comparatively short slope to the
north and a long slope to the south. The rivers of Western
Europe, the Guadiana, Tagus, and Douro in the Iberian
Peninsula, and the Garonne, Loire, and Seine from the
Auvergne high plain, flow to the Atlantic Ocean directly.
The rivers of Central Europe all originate in the Alps and
its connected ranges. The Rhone and Rhine flow in
opposite directions along the great longitudinal valley which
bisects the Alps. The Rhone, descending from its source
near the great central mass of the St. Gothard, enters the
Lake of Geneva, escapes westward between the Alps and
Jura, and sweeps south to the Mediterranean, beneath the
steep front of the Cevennes. Flowing east, the Rhine turns
northward into Lake Constance, passes out westward between
the Alps and the Black Forest, turns north through the wide
valley between the Black Forest and the Vosges, crosses the

xv The Continental Area 301

ancient rock plateau by a series of grand gorges, and, flow-
ing over the low plain, oozes into the North Sea along
several branches embanked above the sunk plain of Holland.
The Elbe drains the Bohemian plateau, and breaking
through the mountain barrier in " the Saxon Switzerland,"
between the Erzgebirge and the Sudetic Range, winds
across the low plain north-westward to the North Sea.
The Oder and Vistula, from the northern slopes of the
Sudetic Range and Carpathians, flow northward to the
Baltic. The Danube is remarkable for its disregard of
mountain barriers. It rises on the eastern slope of the
Black Forest, flows eastward across the plateau north of
the Alps, and finds a way between the Alps and the
Bohemian Forest Range. After penetrating some smaller
ranges it turns south in several parallel channels across the
flat plain of Hungary, which plain was probably once a
great lake. It is joined by the Drave and Save from the
Alps, and the Theiss from the Carpathians, as it crosses
the nearly level plain. The narrow channel of the Iron
Gate, between the opposed ranges of the Carpathians and
Balkans, allows the Danube to enter the open plain, across
which it flows to a delta on the Black Sea.

387. Rivers of Eastern Europe. — The long southern
slope of Eastern Europe is traversed by the great rivers
Dnieper and Don, flowing through gorges cut in the low
plain to the Black Sea. The still greater Volga (§ 89) rising
in the Valdai Hills winds eastward and southward, always
encroaching on its right bank, which is high and steep, and
always leaving successive alluvial terraces on its low left
bank. The Oka is the most important' of its many tribu-
taries on the right, and on the left the Kama, flowing from
the Ural Mountains, is the largest. When the Volga reaches
sea-level its course is directed south-westward, parallel to that
of the Don and very near that river, but the great stream
turns sharply south-eastward, splitting into numerous chan-
nels, and finally enters the closed Caspian Sea (§ 335) by a
great delta. The short northern slope of Eastern Europe is
occupied by the basins of the Pechora flowing to the Arctic
Sea, and the Northern Dwina to the White Sea.

302 The Realm of Nature CHAP.

388. Lake District of Northern Europe. — North of
the Baltic the long slope of the peninsular mass of land,
including Scandinavia and Finland, is toward the south.
The great Lake Ladoga, which discharges its overflow by
the short swift Neva into the Baltic, receives the drainage
of a vast lake district — Lake Onega on the north, Lake
Ilmen on the south, Lake Saima and innumerable con-
nected lakelets on the west, all draining to it. At Imatra,
on the river joining Lakes Saima and Ladoga, the most
impressive cataract in Europe is formed in a nearly flat
country by the water pouring through a narrow and steep
bed of hardest granite, which converts the course for more
than a mile into a thunderous mass of feathery foam and
leaping yellow waves. All the lake-basins of this district are
due to glacial action, and date from the same period as those
of North America. They are, as a rule, shallow, some
having been scooped out of a flat floor of crystalline rock,
while others are formed by the irregular accumulation of
glacial detritus (§ 332). About one-sixtieth of the area
of Europe is covered with lakes, but in the district of Fin-
land the proportion is one-tenth.

389. The British Islands. — An upheaval of 300 feet
would convert the bed of the North Sea, south of a line
drawn from St. Abb's Head to the Skaw, into a low plain
continuous with that of England and of Northern Europe.
During the evolution of Europe elevation and subsidence
have repeatedly raised the whole region into land and again
lowered it under water. Viewed as a whole, the island of
Great Britain is higher toward the west than the east
(see Plate XVI.) The watershed lies near the west coast,
giving a long east slope traversed by the longest rivers.
The east coast is comparatively smooth, with occasional
wide funnel-like estuaries and scarcely any islands ; while
the west coast is very deeply indented by winding fjords or
sea-lochs, and many groups of large and often lofty islands.
No true mountain ranges can now be traced in the British
Islands (§§ 303, 329). Glacial action has been traced over all
the British Islands except the extreme south of England, and
the existing configuration has thus been modified in most

xv The Continental Area 303

places. Mountains, below the height of 3000 feet at least,
have acquired a more or less flowing outline through glacial
grinding ; and the low land has been largely enveloped in
boulder clay and similar accumulations.

390. Scotland. — On the map of vertical relief (Plate
XVI.), the northern part of Great Britain is seen to be
divided into three natural regions stretching across the
island from north-east to south-west. Most of the area
north-west of a line drawn from the Firth of Clyde to
near Aberdeen is occupied by the Highlands. This is
an old plateau, largely composed of crystalline schistose
rocks, and pierced by many granite -like masses. The
heights, separated by deep valleys, are rugged and
often precipitous, crowned by crests of splintered rock.
The Highlands are divided into a northern and a south-
ern group by the Great Glen which unites the Moray
Firth with the Firth of Lome, and contains Loch Ness and
Loch Lochy, two long narrow fresh -water lakes. The
highest point of Great Britain is the mass of Ben Nevis
(4400 feet), near the south-western extremity of the Great
Glen, but a greater area of scarcely lower elevation occurs
round Ben Macdhui. South of the Highlands stretches a
broad low plain — the Midland Valley — diversified by lines
of hills like the Pentlands, Ochils, and Sidlaws, and isolated
precipitous crags such as those occupied by the castles of
Dumbarton, Stirling, and Edinburgh. These abrupt heights
are due to masses of hard volcanic rocks formed in the Car-
boniferous period or later, and now exposed by the more
rapid erosion of the softer strata which had buried them.
Along the border of the Highlands there is a strip of Old
Red Sandstone sharply separated from the crystalline schists,
slates, etc., by the Great Fault which runs from the Firth of
Clyde to near Aberdeen. Along the southern edge of the
plain a similar strip of the same formation is terminated
by a line of faults stretching from near Ayr to near D unbar.
Carboniferous strata — with the coal-measures cropping out
in several places — occupy the centre of the Lowland Valley.
The Southern Uplands, which form the third division, are
a group of rounded grassy and often peat-topped hills, lower

304 The Realm of Nature • CHAP.

than the Highlands, divided from each other by gently slop-
ing valleys, and composed mainly of Silurian rocks, although
the Cheviot Hills on its southern boundary are largely of
igneous origin.

391. England. — The mountainous Lake District of north-
western England has been carved by erosion from great masses
of Silurian rock, but numerous outbursts of ancient volcanic
material have given ruggedness and grandeur to many of
the summits. The mountains of North Wales culminating
in Snowdon (3570 feet) generally resemble those of the
Lake District in their geological structure. They slope in
steep terraces to the sea on the west, and dip down more
gently to the low plain of England on the east. In South
Wales the mountains of circumdenudation are lower, and
the Silurian rocks give place to Old Red Sandstone. This
is in turn covered by a great expanse of Carboniferous
rocks in the south, where the coal-measures come to the
surface. Ancient Primary rocks, especially lower Carboni-
ferous and Devonian strata, build up the peninsula of
Devon and Cornwall, but great intrusions of igneous rock
form the hard framework which the sea has wrought into a
coast-line vying in grandeur with that of the north-west of
Scotland. The band of high land in the north of England
known as the Pennine Chain slopes to the sea on the east,
adjoins the Lake District on the west, and to the south-
west and south gradually spreads out and sinks into the low
plain. The hills and dales of this region are carved out of
a great anticline of Carboniferous rocks, comprising lime-
stones, coal-measures, and grits or coarse sandstones. The
crest of the anticline has been denuded down to the grits,
while the coal-measures and limestones crop out on the
slopes, forming extensive coal-fields. All the rest of England
to east and south is occupied by a great low plain built up of
Secondary and Tertiary rocks, the elevation of which scarcely
anywhere exceeds 600 feet. From this plain the rugged
heights of Primary rocks in the west and north rise as from
a sea, the whole character of their scenery contrasting
with its gentle ridges and low undulations. An irregular
line of heights forming a steep escarpment to the west and

PHYSICAL CONFIGURATION.
After Ordnance Survey.

MEAN ANNUAL RAINFALL AND CO-TIDAL LINES.
After A. Buchan and Charts.

xv The Continental Area 305

a gentle slope to the south-east overlooks the Severn Valley as
the Cots wold Hills. It is continued north-eastward to the
Humber and thence on the other side of the estuary north-
ward, where it swells up into the Yorkshire Moors, and
terminates in a line of cliffs along the coast. This edge is
the outcrop of a great belt of relatively hard oolitic limestone
(Jurassic period) which dips gently to the south-east, and is
separated by a line of older but less durable Secondary rocks
from the Primary system in the north and west. A similar but
more broken escarpment is formed farther south by an outcrop
of Cretaceous rocks, which also dip gently to the south-east.
This Chalk ridge reaching its greatest height in Salisbury
Plain, the Marlborough Downs, and the Chiltern Hills, is
continued in the lower East Anglian heights running north-
eastward through Norfolk. It appears north of the Wash
as the Wolds of Norfolk, and north of the Humber as the
Yorkshire Wolds, terminating in Flamborough Head. From
Salisbury Plain two low chalk ridges diverge : one runs east-
ward as the North Downs, the other south-eastward as the
South Downs, and both end in the Chalk Cliffs of Kent.
The River Thames rising on the southern slope of the Oolitic
ridge, flows through the Chalk ridge between the Marlborough
Downs and Chiltern Hills, and turns eastward to the North
Sea. Its triangular valley between the Chiltern Hills on
the north and the North Downs on the south is occupied
by tertiary rocks, consisting of clay, sands, and marls of
Eocene age.

392. Ireland. — The east coast of Ireland is compar-
atively low and unindented, while the west coast is cut into
many long inlets lined by lofty cliffs and fringed with islands.
The configuration of Ireland is entirely different from that
of Britain. A low plain occupies the whole interior, and
its elevation is so slight that a subsidence of 250 feet would
unite the Irish Sea and the Atlantic across the island.
Isolated groups of lofty mountains rise at irregular intervals
round the outer edge, the highest being Cam Tual (3400
feet) in the south-west. The Shannon, the largest river,
flows southward along the centre of the plain, and turns
westward into the Atlantic. Geologically the low plain of

x

306 The Realm of Nature CHAP, xv

Ireland is composed of a vast expanse of the Carboniferous
formation, in which the coal-measures are only slightly
developed. The mountains are islands of more ancient
rock, Silurian and Old Red Sandstone, with metamorphic
schist and gneiss, like those of the Highlands, in the
north-west. Great masses of volcanic rock occur in the
north-east, where the basaltic columns of the Giant's Cause-
way form one of the wonders of the world. These harder
rocks are prominent on account of their resistance to the
erosion which planed down the soft strata into a uniform
surface. The centre of Ireland is full of shallow lakes
surrounded by peat-bogs, formed by the decay of vegetation
in the wet climate on ground too flat to allow of natural
drainage.

REFERENCES

1 J. Murray, " On the Height of the Land and the Depth of the
Ocean," Scot. Geog. Mag. iv. I (1888).

2 Rohrbach, "Continental Distances." See Scot. Geog. Mag. vii.
213 (1891).

3 J. Murray, "Drainage Areas of the Continents," Scot. Geog.
Mag. ii. 548 (1886).

BOOKS OF REFERENCE

Longmans' New Atlas.

J. G. Bartholomew, Macmillan's School Atlas.

H. R. Mill, Elementary General Geography. Macmillan and Co.

For explorations in little-known regions see Proceedings of the
Royal Geographical Society.

For papers and references regarding Physical Geography see
Scottish Geographical Magazine.

j J

CHAPTER XVI

LIFE AND LIVING CREATURES

393. The World without Life.— The World as a whole
may be compared to a great house. Geology describes its
materials, records the process of building, and keeps account
of the alterations which are always being carried out. Ocean-
ography has to do with the currents of water interchanged
between the tropical boilers fired by the central furnace of
the Sun and the polar refrigerators. It explains the arrange-
ments by which those rooms most exposed to the furnace are
cooled down by iced water, whilst those more remote have
their temperature raised by copious hot streams. Geology
records many past contests between the furnace and ice-
house in controlling the heating arrangements, and many
changes in the direction of the hot and cold water-pipes.
Meteorology discusses the still more complicated and vari-
able methods of ventilation in use in various rooms, depend-
ing as they do on the circulation of water and on the structure
of the buildings. Astronomy has something to say as to
the arrangements for lighting the great house, explaining
how each room is illuminated with a certain brilliancy for
a special time. Astronomy also supplies reasons for the
changes in the strength of furnace and refrigerators in the
past. Geography concerns itself with the plan of the house
so far as it is completed, showing the dominant style of
architecture and tracing the modifications adopted in the
several parts, and gives a general view of all the arrange-
ments.

308 The Realm of Nature CHAP.

394. Life in the World. — Geology and Oceanography
bear evidence of changes in structure which cannot be
explained by the laws of matter and energy. These laws
enable us to understand that water should in certain condi-
tions dissolve carbonate of lime and silica. But they cannot
account as yet for the opposite process which is at work in
exactly the same physical conditions. Carbonate of lime and
silica separate out from solution and assume the solid form,
not with the uniform sharp angles and smooth faces of crystals,
but with curved and varied outlines decorated with delicately-
etched designs of infinite variety (§ 273). Fossils are evid-
ently due to a similar temporary reversal of ordinary chemical
and physical change. (These reversed processes are recog-
nised as the characteristic result of life. Geology may be said
to present us with a view of the world as a vast cemetery
full of monuments of past generations of living creatures.
When we look around us in the open country our eye is
not, as a rule, attracted by bare rocks or soil, but by a
covering of grass, flowers, and trees, amongst which beasts
and birds and insects are moving. These are the living
inhabitants of the great World House. Between them and
the rooms they inhabit there is a close and ever-varying
relation, the comprehension and description of which is the
central aim of Physiography.

395. Classification of Living Creatures. — Every one
can tell at a glance that a bush and a cow belong to widely
different classes ; indeed a close observer might fail to find
anything in common between them. It is easy and natural
to class trees, bushes, herbs, grass, and even seaweeds, as
essentially similar, and to recognise them all as Plants.
Similarly, although four-footed beasts, birds, reptiles, insects,
and fishes, differ a good deal amongst themselves, they are

«sed together, almost without a thought, as Animals. A
it gulf seems to separate the Vegetable and Animal
kingdoms, to use the names given by Linnaeus who laid the
foundations of the modern classification of creatures. Plants
are rooted in the soil ; animals are free to move over the
land, through the water or air. When carefully studied
both of the great kingdoms are found to fall into a number

xvi Life and Living Creatures 309

of natural groups, the members of which show a regular
advance in complexity of structure. Between the simplest
groups of each kingdom it is difficult and often impossible
to trace any difference. All living creatures are termed
organisms, and the science which takes account of them
with special regard to their common characteristics is
termed Biology (literally Life-lore). The classification and
life-history of plants are the objects of the department of
Biology known as Botany, while the department known as
Zoology is similarly occupied with the study of animals.

396. Classes of Plants. — Botanists group plants into
sub-kingdoms, classes, natural orders, genera, and species.
A species includes all the individual plants, which are so
much alike as to make it certain that they are descended
from the same stock and which are mutually fertile. A
genus includes a group of species closely related to each
other. A group of related genera forms a family, a number
of allied families forms an order, and the orders are them-
selves grouped in classes. Thus, for example, in the class
of Dicotyledons there is an order called Ranunculacese,
which includes several families and many genera, amongst
others that of Ranunculus, which in turn includes many
distinct species. Following the suggestion of Linnaeus, each
species, that is each separate kind of plant, is known to
botanists by the name of its genus, followed by a specific
name. One particular kind of buttercup is thus termed
Ranunculus acris. The classes of plants, with a typical
example of each, are as follows : —

I. THALLOPHYTES (no stem],
PROTOPHYTA— Bacteria.
ZYGOSPORE^S — Diatoms.
OOSPORE^E — Fucus.
CARPOSPORE^E — Most Seaweeds and Fungi.

II. MUSCINE.E.

HEPATIC^E — Liverworts.
Musci — Mosses.

III. VASCULAR CRYPTOGAMS.

EQUISETINE/E — Horsetails.
FILICINE^E — Ferns.
LYCOPODINE/E— Club-mosses.

310 The Realm of Nature CHAP.

IV. PHANEROGAMS (flvwering plants).
GYMNOSPERMS — Pines and Firs.
MONOCOTYLEDONS — Lilies.
DICOTYLEDONS — Buttercups.

397. Classes of Animals.— Animals are more numerous
and varied in their kinds than plants, and their classification,
according to resemblances and differences, is in consequence
more complex. Species, genera, families^ and orders are
distinguished much in the same way as with plants, and
animals also are named after both genus and species. The
great groups into which they are divided (and the classes of
the last group), with typical examples, are as follows : —

PROTOZOA — Radiolarian, Foraminifera, Amoeba, etc.
PORIFERA — Sponge.

CCELENTERATA — Jellyfish, Sea-anemone, Coral.
ECHINODERMATA — Starfish, Crinoid, Sea-urchin.
VERMES — Worms.

ARTHROPODA — Lobster, Barnacle, Millipede, Spider, Insects.
MOLLUSCA— Oyster, Snail, Pteropod, Cuttlefish.
PRIMITIVE VERTEBRATES — Tunicate, Lancelet.
VERTEBRATA — Fishes — Flounder, Salmon, Shark.
. Amphibians — Frog, Newt.

Reptiles— Turtle, Serpent, Lizard.

Birds — Eagle, Ostrich, Sea-gull, Sparrow.

Mammals — Kangaroo, Lion, Ox, Whale, Ape,
Man.

398. Functions of Living Creatures. — The simplest
organism or the unit-mass of any living creature is merely
a jelly-like speck made visible by means of the microscope.
Part of the jelly-like substance may form a darker nucleus in
the interior, and in some cases a tougher film is seen to sur-
round and contain the whole. The organism is said to consist
of a single cell. The jelly-like substance called protoplasm
is a complex kind of matter, the precise nature of which is
unknown, but it consists mainly of carbon, oxygen, and
hydrogen, with minute quantities of nitrogen, sulphur, and
phosphorus. Living protoplasm is continually undergoing
two opposite sets of changes — building up or renewal,
and breaking down or decay. The process of build-
ing up, which is distinctive of living creatures alone,

xvi Life and Living Creatures 311

involves nutrition or the taking in of food -substance,
digestion or the elaboration of food, and assimilation or
absorption into protoplasm. While this process goes on the
organism grows by the assimilation of unlike substances,
which are transformed into protoplasm and added to the
mass from within and throughout. The simultaneous
breaking -down process, on the most commonly accepted
theory, is brought about by respiration or the absorption of
oxygen. Protoplasm is an extremely unstable compound,
always ready to combine with oxygen and break up into
carbonic acid, water, and a very small proportion of a few
other stable compounds. The living protoplasm is purified
by the process of excretion, which is simply the thrusting
out of the burnt products (carbonic acid, water, etc.) and
of those parts of the food which escape digestion. When
life ceases, protoplasm ceases to grow, oxidation continues
unchecked, and the organism breaks up and decays away
by slow combustion. In the process of growth, matter
which is not living may be built into the substance. For
example diatoms and radiolarians, which are single-celled
organisms, form coats or skeletons of silica, and foramini-
fera, also consisting of one cell, secrete hard shells of
carbonate of lime (§ 273). All organisms, except the
protozoa and the simplest plants, consist of many cells
containing protoplasm, built up into organs set apart for
special purposes. These cells are usually supported in a
framework of matter such as wood or bone, elaborated
by the living organism and sharing its life for a time, but
becoming practically lifeless as they grow older. When a
cell grows, it increases in size to a certain limit and then
divides into two cells, the process being termed reproduc-
tion. In the protozoa the division of a cell is complete
separation, producing two individuals ; but in higher organ-
isms a single cell, termed an ovum or egg-cell, is separated
from the rest, and grows by subdivision into a separate
many-celled organism similar to the parent form. Most
often, both in plants and animals, this liberated cell is unable
to develop until it unites with a cell of another kind (termed
a male cell) from the same species. Thus the continuance

, UNIVERSITY

312 The Realm of Nature CHAP.

of the species is secured in spite of the death of the
individual.

399. Constructive Plant Life. — Plants alone are able
to raise inorganic substances, such as water, oxygen, carbonic
acid, into the sphere of life-wrought or organic material.
They cause the elements to combine into proteids^ the raw
material of protoplasm. This power in its entirety is con-
fined to those plants which possess green leaves, and is
exercised by them only when the energy of sunlight falls
on the green colouring matter known as chlorophyll. Then
the leaf is able to break up carbonic acid derived from
the atmosphere, to restore the oxygen to the air, and
cause the carbon to combine with the elements of water,
forming starch which is at first stored up amongst the
cells of the leaf. Subsequently the starch is transformed
into sugar, which dissolves in the sap and is carried through
the whole plant. On meeting the nitrates, sulphates, phos-
phates, and other salts of lime or potash, absorbed from the
soil by the roots, the sugar combines with them, producing
proteids and various waste products in a manner not yet
discovered. The influence of green leaves on the air in
sunlight is to unburn or decompose (§ 44) the carbonic acid.
The solar energy used up in this work is converted into
potential energy of chemical separation, which is restored
to the kinetic form when wood or coal (§ 347) unites with
oxygen. The oxygen given out by the action of chlorophyll
in the leaf laboratory is more than enough to supply the
ceaseless respiration of the plant in daylight and darkness
so that, on the whole, green plants diminish the proportion of
carbonic acid and increase that of oxygen in the air.

400. Destructive Animal Life. — Contrasted with the
constructive processes of plants, changing lifeless into living
matter and kinetic into potential energy, animals are wholly
destructive. They cannot utilise solar energy, but derive
all their power of doing work from oxidation of their own
substance. They cannot manufacture proteids, so that all
their food has to be prepared for them by plants. Animal
life would indeed be impossible if plant life did not precede
it. In their respiration animals are always removing oxygen

xvi Life and Living Creatures 313

and increasing the amount of carbonic acid in the atmo-
sphere (§ 154). Those plants which do not contain chloro-
phyll, such as the fungi, moulds, and bacteria, are as powerless
as animals to manufacture food from carbonic acid and water.
But unlike animals they have the power of manufacturing
proteids if they obtain starch or sugar and the various salts
amongst their food. Thus fungi — all the mushroom kind
— grow abundantly only in decaying vegetable substance,
which supplies plenty of starch. To sum up in a metaphor,
the green plant, like a coal-laden steamer, conveys solar
energy — using up some in the process — to the animal, which
like a stationary steam-engine converts it into work.

401. Micro-organisms. — Many minute one-celled organ-
isms, probably plants allied to the fungi and moulds, known
as bacteria, bacilli, microbes, or classed together as micro-
organisms, play a very important part in the course of their
life-history. One of these, known as the nitrifying ferment ',
changes the salts of ammonia derived from the atmosphere,
or from decomposing animal matter, into nitric acid in the
soil, thereby greatly facilitating the growth of plants (§ 311).
Another known as yeast, when cultivated in a weak solution
of sugar, uses up some of the sugar, and changes the rest into
carbonic acid and alcohol, hence it is extensively used in rais-
ing bread and in making wine and beer. A different micro-
organism changes alcohol in the presence of air into vinegar,
and is extensively cultivated for that purpose. The spores,
or young undeveloped cells, of many kinds of micro-organisms
form a considerable part of the dust in air (§ 161), and are
present everywhere. When these find a suitable place to
grow in — for example, the blood or the tissues of a person not
in strong health — they develop and multiply, producing by
their vital processes certain poisons, which give rise to disease.
Different species of micro-organisms have been detected as
the cause of cholera, consumption, diphtheria, and other
maladies. The recognition q£ this cause of disease has led
within the last few years to the greatest modern advances
in medical treatment.

402. Evolution. — As the Earth, like other members of
the solar system, is the result of a slowly unfolding series of

314 The Realm of Nature CHAP.

changes ; as the continents have by long and gradual degrees
come to their present form, and are still undergoing altera-
tion,— so also living creatures display a progressive evolution.
The classifications of plants (§ 396) and of animals (§ 397)
are ascending scales, showing in each group a more com-
plex structure and organs more distinctly set apart for
special purposes. Amongst animals, for example, the pro-
tozoa have no organs at all ; the single cell acts as a whole in
every function. In the echinoderms, eyes and a separate
stomach appear ; in the arthropoda, limbs adapted for walk-
ing ; and an internal skeleton connected to a backbone, and
supporting the framework of the body, is only found in
the vertebrata. Similar progressive advancement is to be
found within each group, and even in the same species in-
dividuals vary so much that a regular gradation may often
be traced into other species making it difficult to draw
the dividing line. Transition types, such as Archasopteryx
(§ 349)5 a bird partly resembling a reptile, and the
Australian duck-bill, which although a mammal has a beak
like a bird and lays eggs, connect the different classes of
animals or of plants. When this regular order of succes-
sion from lower to higher forms in plants and animals
became apparent to biologists they were convinced that
different species had not been created separately in different
places, but had gradually developed in the course of ages
from a common parent form. The late Charles Darwin
and Mr. A. R. Wallace almost simultaneously framed a
theory to account for organic evolution — the gradual un-
folding of the progressive design of plant and animal life ;
and the period of most rapid advance in modern biology
dates from the publication of Darwin's Origin of Species in
1859. The original views of Darwin and Wallace are
gradually being modified as new facts are encountered and
the general principles of evolution stand out more clearly.

403. Heredity and Environment. — Darwin explained
the origin of different species of living creatures by the
two great influences of heredity or likeness to parents and
environment or surrounding circumstances. As a rule the
young of plants and animals resemble their parents, but no

xvi Life and Living Creatures 315

two are precisely like each other. General similarity is
associated with small variations of structure. Sometimes
these variations produce no influence on the life of the
organism, and may pass unnoticed. But when they happen
to make one individual better fitted for obtaining food or
escaping danger than the others, that one has a better chance
of living, thriving, and handing on its fortunate peculiarities
to its descendants. If the variation of structure throws an
individual out of harmony with its environment, making it
weakly or stupid, that individual has a smaller chance of
surviving and leaving offspring. According to Darwin's
view the constant struggle for life is always weeding out the
weak and improving the position of the strong, leading by a
process of natural selection to the survival of the fittest.
But climate, and even the outline and configuration of the
land, are not constant ; hence organisms, hitherto victorious
in the struggle for existence, have to contend with an altered
environment, and their development, according to natural
selection, must after a time take place in a new direction
with great sacrifice of life, and possibly the extinction of
some species. This subject is far from simple, many of the
facts have still to be discovered, and none of the hypotheses
as yet can compare for certainty with theories that admit of
mathematical proof. An excellent idea of the difficulties
and the fascinating interest of biological facts and theories
will be obtained from Professor Geddes's Modern Botany^
and Mr. J. Arthur Thomson's Animal Life, in this series.

404. Conditions of Plant Distribution. — Plant life, as
a rule, is most luxuriant where there is abundant sunlight,
high temperature, copious rainfall, and soil abounding in the
soluble salts necessary for nutrition. In the course of the
ages plants have gradually been modified, so as to adapt
themselves to their environment. Thus not only the com-
parative luxuriance, but also the species of plants, depends
to a large extent on the conditions of their growth. Where
natural conditions change abruptly, as, for example, on the
sea-coast, on the slopes of a snow-clad mountain, or the
edge of a desert, the kinds of creatures inhabiting the two
regions differ in a very marked way. If such barriers are

3i6

The Realm of Nature

CHAP.

developed in a region formerly of uniform configuration and
climate, similar plants may become separated by quite
different species produced by the new conditions. While
all animals are absolutely dependent on plants for food,
some plants are in great part dependent on animals for their
continued existence, and bright flowers, perfumes, and honey
have an important office in attracting them. Insects especi-
ally carry pollen from one flower to another, and so secure
cross fertilisation, which greatly improves the strength of
the seed.

405. Floral Zones. — Speaking widely, the luxuriance
and variety of vegetation decrease from the equator to the
poles, and from sea-level toward the summit of mountains.
Fig. 63, adapted from Smirnoff's Russian Physical Geo-

V. COOL TEMPERATE

__— —

IV. WARM TEMPERAT

III. SUBTROPICAL

II. TROPICAL

I. EQUATORIAL
FIG. 63. — Zones of climate and vegetation in latitude and altitude (after Smirnoff ).

graphy, represents a quadrant of the Earth's surface divided
into climate zones at sea-level. The Eqitatorial zone corre-
sponds to the region of maximum heat and rainfall ; the
Tropical to the region of maximum heat and small rainfall.
The Subtropical, Warm Temperate and Cool Temperate
zones show a gradual transition to the Subarctic, in which

xvi Life and Living Creatures 317

long cold winters produce a dwarfing effect on vegetation.
The Arctic zone of stunted plants leads to the Frigid, and
that to the unchanging ice-deserts of the Polar zone. The
vertical columns represent slices of 2000 feet of mountain-
sides from the region above the snow-line (shown at the top
of each column) down to sea-level. The horizontal rows
show by their connecting lines at what average height the
climate and vegetation corresponding to each of the sea-
level zones is attained. Dr. Oscar Drude divides the Earth
according to the affinities of its vegetation into three great
divisions — the Boreal or Northern, the Tropical, and the
Austral or Southern. In each one of these the species of
plants are closely allied to each other, but distinct from
those inhabiting the other divisions. The Austral Group
includes the parts of the three southern continents south of
the tropic of Capricorn, and falls naturally into an American,
African, and Australian division. The flora of Australia
is unlike all the others ; there are trees, such as the
eucalyptus or gum-tree, which are evergreen but shed their
bark yearly ; the wattle (a kind of acacia) and the beef-tree,
which bears long green branchlets instead of leaves. The
Tropical Group extends from the tropic of Capricorn
northward to the Tropic of Cancer in America, to the centre
of the Sahara in Africa, and to the Himalaya in Asia. It
also contains three main divisions. Cinchona, mahogany,
and the cactus are characteristic of the American section ;
the oil-palm, baobab, and giant euphorbias of the African;
and teak, banyan, and sandal -wood of the Oriental.
The Boreal Group is remarkable for the wide range of
plants of similar species, such as the pine, birch, and oak,
over the Northern division in the three continents — in
America north of the Great Divide, in Europe north of the
southern peninsulas, in Asia the whole northern slopes. The
other divisions of this group are the Eastern Asiatic ; the
Central Asiatic, comprising the vast plateau region ; the
Mediterranean lands, where the olive, mulberry, chestnut,
orange, and cork -oak flourish ; and the Central North
American, the natural home of maize, tobacco, and the
giant pines of California.

318 The Realm of Nature CHAP.

406. Deserts. — In many respects Plate XVIII. gives the
most interesting division of the world according to its vege-
tation. It shows three great barren zones forming broken
girdles round the Earth, and covering, according to Mr.
Ravenstein's calculation, more thari 4,000,000 square
miles. Ice deserts surround the north pole, and are suc-
ceeded in the north of Europe, Asia, and America by a
belt of frozen land called the Tundra, which thaws on the
surface in summer and supports a thin growth of moss and
stunted grass. Arid deserts occur in all areas of great heat
and very small rainfall. A northern zone includes the vast
Sahara, the interior of Arabia, and Central Asia, terminating
in the dreary Gobi, and the Great Basin of North America.
Horny cactuses, the saxaul with foliage like wire, and the
dull-gray sage-bush, are characteristic of the scanty plant
life. A similar set of smaller deserts appears in the southern
hemisphere, near the cooler but drier western sides of the
continents, the Kalahari in Africa, the great Victoria Desert
in Australia, and the small salt desert of Atacama in South
America, forming links in the chain. Solar energy here falls
on barren land, and, not being absorbed by plants, does the
work of heating air and maintaining the permanent winds of
the globe, which carry rain to more favoured regions. Thus
in a sense the existence of fertile lands is a consequence of
deserts. Treeless plains are common in all regions of scanty
rainfall and great range of temperature, such as the borders
of deserts. They occupy about 14,000,000 square miles of
surface, covered with rich grass during part of the year,
transformed into deserts of driving dust in the dry season,
and flooded or covered with snow, according to the climate,
during the rainy season or winter. The fertile prairies of
North America, the llanos and pampas of South America,
and the steppe-lands of Russia and Central Asia, are examples
of such semi-deserts.

407. Tropical Forests. — When the grassy plains sur-
rounding the tropical deserts on the equatorial side begin
to receive a larger rainfall, bushes first break their mono-
tony, and then great forests are formed, the trees standing
well apart, but growing closer as the heavy rains of the

xvi Life and Living Creatures 319

equatorial zone are approached. The densest forests
naturally extend on both sides of the equator, where heat
and rainfall unite to produce a paradise for plants. The
Selvas of the Amazons, the darkest forests of the Congo
and its tributaries, the forests of the Western Ghats of
India, of the west coast of the Malay Peninsula, and of the
islands of the Malay Archipelago, vie with each other as types
of the utmost wealth of vegetation. Soft leafy canopies borne
'by lofty evergreen trees meet and intercept the light, so that
no grass can grow in the dark depths of the woods, but
climbing and twining plants innumerable, with stems like
ropes or cables, force their way up on the trunks of their
stouter rivals, and push on to expand their crown of leaves
in the sunlight. The decaying vegetation below supplies
abundant nourishment for pale -coloured parasitic plants,
which, deprived of sunlight, have lost their chlorophyll and
the power to manufacture food, and therefore live on their
fellows.

408. Temperate Forests. — On the temperate side of the
tropical deserts, the plains reaching into regions of moderate
warmth and moderate rainfall become covered with less
luxuriant but very extensive forests. These are most
developed around the great lakes of North America, in
Scandinavia, and as a broad belt from the Carpathians
north-eastward to the Baltic, eastward to the Ural Moun-
tains, and beyond them across Asia north of 50° to the
Pacific Ocean. In Western Europe the ancient forests—-
which appear to have once formed an unbroken belt across all
the northern continents — have been cut down and the land
cultivated. The warm temperate forests are composed of
deciduous trees, that is, trees whose leaves wither and drop
each winter, the leaf laboratories being shut up in the com-
paratively sunless months. " Oak, beech, elm, ash, lime, and
many other kinds of forest tree, are found in their greatest
luxuriance in this zone. Toward the pole, where the
winters are longer and more severe, the deciduous trees
vanish, the hardy birch, with its silvery bark, reaching
farthest north. Pines and firs, clad in small, hard, needle-
shaped leaves, can alone resist the climate, and vast forests

320 The Realm of Nature CHAP.

of these characterise the subarctic zone and the higher slopes
of mountains.

409. Animals and their Life Conditions. — The life
conditions of some of the marine animals of most import-
ance from the physiographical standpoint, have already
been touched on (§§ 273, 279). Amongst all animals the
struggle for life is harder, or at least more apparent, than
with plants, the stronger hunting down and devouring the
weaker. Animals in their native haunts should therefore
be inconspicuous if they are not to attract the attention of
their enemies, or to arouse the suspicion of their prey.
Almost all fishes, and many caterpillars, rapidly assume the
colour of their surroundings. The hare and ptarmigan,
living amongst the brown heather of northern hillsides in
summer, are brown in fur or plumage, but in winter, when
the land is white with snow, their colour also changes to
white, and they remain inconspicuous in their new sur-
roundings. This periodical adaptation to environment,
which is common in Arctic animals, is one of the causes
which has led to the preservation of the race. Some
insects are so like withered leaves or twigs that even an
experienced eye is often deceived by them. Strange
resemblances have also been traced out between entirely
different species of animals ; and since the similarity is
always brought about by the weaker or inferior type assum-
ing the appearance of the stronger or superior, almost as if
of purpose to impose on enemies, it is called mimicry.

410. Faunal Realms. — Animals exhibit more marked
peculiarities of distribution than do plants. {.Similar forms
are usually, though by no means always, found in like con-
ditions. ) The fauna, or collection of animals, of each one of
the northern continents bears a close resemblance to that of
the others ; while the fauna of the three southern continents
are similar in a much less degree, and, as a rule, totally
unlike that of any of the northern. The most generally
accepted division of the Earth into realms occupied by
different faunae is that suggested by Dr. Sclater, shown
in Plate XIX. The names adopted for these divisions
or realms are — the Palcearctic or Old Northern, the

xvi Life and Living Creatures 321

Ethiopian, the Oriental, the Nearctic or New Northern,
the Neotropical or New Tropical, and the Australian.
Professor Heilprin, another eminent authority, prefers to
class the Palasarctic and Nearctic realms together, on
account of their general similarity, as the Holarctic or
Entire Northern. He also recognises a region of transi-
tion to the Neotropical realm occupying the south of North
America, and another of much greater extent forming a
transition to the Ethiopian and Oriental realms and in-
cluding the whole Mediterranean region.

411. Northern Realms. — In both the Old and the New
Northern realms the white polar bear frequents the northern
snow-deserts. Farther south occur reindeer and elks,
bears — black, brown, or grizzly, — foxes, wolves, beavers,
hares and squirrels, and the bison, now almost extinct in
Europe and rapidly being exterminated in America. The
representatives of various families become more unlike each
other toward the southern border. Moles, rats and mice,
badgers, sheep and goats, the camel and the yak, are con-
fined naturally to the Palaearctic realm. On the other
hand, the musk-ox, skunk, prairie dog, racoon, and jump-
ing mouse, are exclusively restricted to the Nearctic realm.
Compared with the southern realms, those of the north are
remarkable for the high place in the scale of development
occupied by their most common animals. But the very
complete study of the fossil forms of life preserved in the
rocks shows that in past ages the northern lands were
inhabited by a gradually developing series of more primi-
tive types, from which the existing creatures are evidently
descended.

412. Ethiopian Realm. — -Africa, south of the Sahara,
and Arabia, contain Jew or none of the animals which make
their home round the Mediterranean at the present time.
There are no wolves, foxes, bears, or tigers, the flesh-eating
animals being represented by the lion, " the king of beasts,"
the leopard, panther,, hyaena, and jackal. This purely tropical
realm is the exclusive home of the hippopotamus and the
giraffe, tallest of living animals. The elephant and rhino-
ceros are common also to the Oriental realm. Swift-footed,

Y

322 The Realm of Nature CHAP.

graceful, and fantastically striped zebras and quaggas fre-
quent the grassy plains. Of all African animals the most
widespread and characteristic are the antelopes, which
gallop in vast herds over the plains, and, ranging in size
from an ox to a rabbit, inhabit bush, forest, and desert as
well. Apes — narrow-nosed, tailless creatures of the monkey
kind — are very common in all parts of the continent. The
forests are the chosen home of the most highly developed
and fiercest, the gorillas and chimpanzees. The ostrich,
the largest bird in the world, is typical of Africa, being
found in all the open plains and deserts both in the north
and south. The adjacent island of Madagascar contains
very few of the animals common in the Ethiopian realm,
but abounds in lemurs, a kind of half-monkey.

413. Neotropical Realm. — South America is richer in
varieties of animal life than any other realm, and it is also
peculiar for the very large number of species which are
found nowhere else. The true monkeys are confined to the
great forests, where they swarm in amazing numbers. They
differ from the African and Oriental apes mainly in having
a broad nose and a long prehensile tail, by which they
swing from branch to branch. Vampires and others of the
leaf-nosed bats, the rabbit -like chinchilla of the Andes
slopes, the beaver-like coypu rat of the plains, and the little
agouti, allied to the guinea-pig, are all exclusively South
American. So are the more peculiar sloths which swing
back downward from the trees, the great bushy-tailed ant-
eaters with long slimy tongues specially modified to lick up
ants, and the curious armour-clad armadilloes resembling in
their habits the hedgehogs of Europe. Although no bears,
foxes, or wolves penetrate south of the transition zone, the
jaguar, resembling in many respects the tiger of the Oriental
realm, ranges over the entire continent, and the puma or
American lion even extends far into North America. The
llama, alpaca, and vicuna, confined to the upper slopes of
the Andes, are closely allied to the camel family, which
inhabits only the Palaearctic realm. Neotropical birds are
numerous and distinctive, ranging in size from the huge
unsightly condor to gem -like humming-birds, which are

xvi Life and Living Creatures 323

smaller than many insects. The rhea of the southern
plains belongs to the ostrich family, but, as a whole, the
bird-fauna of South America is more allied to the Oriental
than to the Ethiopian.

414. Oriental Realm. — Animals common in the Palae-
arctic and the Ethiopian regions meet together in the
Oriental realm, and give it a characteristically mixed
fauna. Lions, leopards, rhinoceroses, and elephants, almost
or quite identical with those of Africa, are found along with
bears, wild dogs, foxes, and the true deer so distinctive of
Northern Eurasia. Lemurs akin to those of Madagascar
are abundant in the south, and the mixture is completed by
tapirs and many birds with strong South American affinities.
The tiger is peculiar to the Oriental realm, but ranges from
Java northward within the borders of the Palaearctic as far
as Sakhalin, and is curiously enough absent from Ceylon
and Borneo. This realm abounds in squirrels, mice, and
bats, and, together with some Ethiopian forms of apes, it
affords a home in Borneo to the man -like oran-outan.
Although to north and west the Oriental merges gradually
into other realms it has a sharp boundary to the south-
east, where Wallace in his exploration of the Malay Archi-
pelago found the Oriental species, even of birds and insects,
stop at a line drawn between the small islands of Bali
and Lombok, and thence between Borneo and Celebes
south of the Philippines. Celebes, however, seems to be
occupied by a transition fauna.

415. Australian Realm. — So peculiar and distinctive
is the fauna of Australia and the surrounding islands that
many naturalists class it as a main division opposed to all
the rest of the globe. Except the dingo or native dog,
which may have been introduced by man, the flying foxes
(of the bat family), and some birds, none of the animals of
other realms occur in it. Their place is taken by the least
developed of mammals, the monotremes, of which the duck-
bill is the type, and the marsupials, represented by the kan-
garoo. Opossums, living in trees, are the only Australian
form of animals, and indeed the only marsupial, found in
other continents, a few species occurring in America. The

324 The Realm of Nature CHAP.

emu and cassowary are allied to the ostrich family ; the
bower-bird, which delights in laying out the ground in front
of its nest like a garden ornamented with pebbles and flowers,
cockatoos, and the black swan, are characteristic birds.
Australian animals are found in all the islands of the Archi-
pelago northward and westward to Celebes and Lombok.

416. Island Life. — From Wallace's researches in the
Malay Archipelago it appears that an entirely different
fauna and a largely different flora live on adjacent islands
in identical physical conditions. Hence he concludes that
the islands on the Australian side of the dividing line have
not been united with those on the Asiatic side since the
fossil marsupials of the northern hemisphere were alive.
It is equally evident that the islands of Lombok and
Celebes have been connected with Australia, and that Bali
and Borneo have been connected with Asia by land which
has been submerged so recently that the organisms have
not yet had time to be much modified from the type
of their continental contemporaries. Similarity of faunae
between the Malay Archipelago and South America, and
many resemblances in the flora of the three southern conti-
nents, indicate the probability of a former Antarctic land
connection right round the world, which is not contradicted
by the configuration of the bed of the Southern Ocean.
Purely oceanic islands are usually inhabited only by species
which might have been conveyed by sea from the nearest
continent, and often contain very remarkably modified
forms.

4 1 7. Action of Living Creatures on the Earth. — The
processes of erosion by which the continents are carved
into their present form are largely modified by the action
of living creatures. Corals and other marine organisms
are powerful agents in rock-making (§ 280). Forests, and
the growth of vegetation generally, bind the soil together,
preventing denudation on mountain slopes, reclaiming
alluvial terraces in rivers, and often putting a stop to the
drift of sand-dunes. Vegetation also affects climates, pro-
ducing a uniform rainfall, checking evaporation, and regu-
lating the flow of rivers by absorbing the water of heavy

xvi Life and Living Creatures 325

rain, saving sudden floods, and by keeping up continu-
ous oozing in dry weather, preventing the streams from
dwindling away. Disintegrating action is on the whole more
frequent. The roots of plants and the little root-like fibres
of lichens serve as wedges, splitting up rocks and aiding
the formation of soil. Earthworms, termites, and ants
(§ 311) aid largely in mixing and pulverising the in-
gredients of the soil. Boring molluscs drive long narrow
holes into the rocks below sea -level, and enable the
breakers to produce a much more rapid disintegration of
the cliffs than would be possible otherwise. Cray-fishes,
burrowing under the banks of rivers, are important agents
in causing changes in the direction of the stream and
the position of its bed. Beavers have a strange instinct
of felling trees and constructing dams across streams
to provide an expanse of water in which to build their
"lodges." These dams serve to accumulate a head of
water, and when burst by a flood the destructive force
of the current works great changes on surface scenery.
There is no living creature, large or small, which does
hot leave some trace of its life-work impressed upon the
solid globe, and although the individual result of the action
of most creatures may be little, the sum of the life of the
globe is a very potent factor in the evolution of the con-
ditions which ultimately determine it.

BOOKS OF REFERENCE
(In addition to those mentioned in the text)

Charles Darwin, Origin of Species, Insectivorous Plants, Forma-
tion of Vegetable Mould, and other books.

A. R. Wallace, The Malay Archipelago and Island Life.
Macmillan and Co.

A. Heilprin, The Distribution of Animals. International
Science Series.

CHAPTER XVII

MAN IN NATURE

4 1 3- Man as an Animal. — One genus of the animal
kingdom separates itself from the rest in a manner so com-
plete as to require special consideration. It is the genus
to which we ourselves belong, and it contains the one
species — Mankind. Varieties of this species differ so much
amongst themselves physically that there is nearly as great
a gap between the most highly developed and the most de-
graded as between the latter and some of the most developed
apes. Organic evolution seems liable to no exception at
this point. Mankind is subject to the same natural con-
ditions as other animals, being dependent on plant life
for food, and always under the control of heredity and
environment. In some respects the human species is in-
ferior to the less developed animals, particularly in the
possession of a thin skin without fur or feathers, and in the
absence of claws, tusks, or any natural weapons of offence.

419. Man as Man. — The differences between Man and
the lower animals are so numerous, definite, and distinctive
that until within the present century they obscured, even
in scientific minds, the full significance of the similarities.
'(There are no limits to the geographical distribution of the
species."^ Men live in all the continents, and from the
equator to 84° of latitude, but there is no reason to believe
it impossible to support life at the poles when they can be
reached. Although the contrast between Man and other
animals becomes more distinct amongst the higher mem-

CHAP, xvn Man in Nature 327

bers of the human species, it may be traced in all. It is
less of degree than of kipd, and is rather intellectual and
spiritual than physical, {uhe use of reason with the asso-
ciated power of language^ the recognition of a Creator, and
as a necessary consequence the ^ense of religious duty, are
distinctively human attributes. I As these powers become
developed, strengthened, and purified, Man advances in the
scale of being, independently of his physical development.
Heredity and environment acquire new importance, and
indeed their existence and potency were first recognised by
the way in which birth and education determine the higher
powers of the mind. The intellectual as well as the
physical unity of the human species is strikingly shown by
the fact that even amongst the most advanced peoples
there are individuals who exhibit the untamed instincts of
the savage, while in the most degraded tribes individuals
with some higher powers and finer feelings occasionally rise
far above the level of the rest. (By the use of reason men
are able to modify or choose their environment^) and thus,
consciously or unconsciously, to direct the course of their
own development toward advancement or degradation.
This power gives to the individual man far greater influ-
ence and independence than is exercised by individuals of
any other species.

420. Civilisation may be defined as the result of men
using the power of changing their natural surroundings, and
regulating their natural wishes or impulses in order to
increase the wellbeing of the community to which they
belong. Each variety of the human species appears to be
capable of attaining a certain degree of mastery over them-
selves and their surroundings, this degree being much
higher in the case of some varieties than in others.
The position occupied by different groups of the human
species with respect to civilisation is intimately connected
with their conceptions of religion. Tribes of the lowest
civilisation live, as a rule, in a state of vague fear of evil
spirits and of the ghosts of their ancestors, which they try
to appease by worship and sacrifices. They believe that
the 'spirits dwell in rude idols or fetishes, to which they

328 The Realm of Nature CHAP.

accordingly pay great respect. More civilised peoples,
reasoning on the appearances of Nature, are Polytheists^i
believers in many separate gods, to whom the creation of
different parts of Nature is ascribed. Pantheism (illustrated
by Buddhism) is a development of Polytheism, from which
it differs in conceiving God to be present everywhere, and
all existing things, Man included, to form part of Him.
The highest and most civilised races are Monotheists,
v recognising one G6d, who created the World and directs its
'•processes of endless evolution. Three forms of Monotheism
are prevalent — the Jewish, in which the Old Testament is
held as a divine revelation ; the Christian, all sections of
which accept also the teachings of the New Testament ; and
the Mohammedan, following the Koran, a book compiled
from the Jewish and Christian Scriptures by Mohammed.

421. Environment and Man. — External conditions do
much to determine Man's position in the scale of civilisation.
It is matter of dispute whether the different races of man-
kind result merely from the different conditions in which
they have developed, or if changes consequent on moral
advancement or degradation have had a large share in pro-
ducing them. ^JThe races lowest in civilisation are most
completely slaves to their environmentj exercising only the
purely animal powersT) (Where the climate makes clothing
unnecessary, and abundant fruit-bearing plants supply the
means of life without labour or forethought, as in tropical
forests, mankind is found in the least developed or most
degraded form.) On the other hand, when natural condi-
tions are very nard, the climate severe, and the means of
life only to be obtained by chance success in hunting or
fishing, the development of intelligence appears to stop
short when the prime necessities — food, clothing, and
shelter — are secured. The fur-clad Eskimo, feeding on
blubber in his ingeniously- constructed house of ice, is
certainly an advance on the naked homeless savage of the
tropics, who satisfies his hunger with fruits and insects.
But both are so exclusively fitted to their environment that
the Eskimo pines by the Mediterranean, and the forest
Pygmy sickens and dies in the sunlit grass-lands, f Intel-

XVII-

Man in Nature 329

lectual development appears to be stimulated by conditions
which make life neither too easy nor too hard.) In temper-
ate regions, necessitating shelter and warm clothing, where
there is a regular succession of seasons, forethought and
thrift are encouraged by the need of providing in summer
for the coming winter. Ingenuity has to be exercised in
evading the effects alike of heat and cold, and the skill thus
acquired finds additional employment in providing orna-
ments and luxuries to gratify an awakened and cultivated
taste. Strength and self-reliance come from the successful
struggle with adverse conditions, and many of the charac-
teristics of nations are due as much to the nature of the
land they dwell in as to the inherent qualities of the race.
Mountaineers of every race are hardier, more independent,
and more attached to their native land than the dwellers on
low plains, who, on the other hand, work more, excel in
perseverance, and are as a rule more successful in obtaining
a sufficiency of the means of life. Seafaring peoples, com-
pelled to be continually watching for signs of change in
weather, and often called upon to decide quickly and act
promptly in circumstances of danger, acquire a distinc-
tive steadiness of nerve and quickness of resource which
lead to a general advance in civilisation. (^ Climate and
scenery exercise a powerful influence on moral as well as
on physical conditionsj By contrasting the stolid earnest-
ness and ceaseless activity of the dwellers in Northern
Europe with the passionate vivacity and general listless-
ness of Southerners, an ingenious author once went so
far as to say that Character is a function of latitude.
The poetry and the religious system's of all peoples are
closely connected with the nature of their land. Patriotism
also is a quality derived from the same source, and is shown
most intensely by peoples long settled on small but clearly
characterised natural regions. The tendency of civilisation
is gradually to modify the influence of environment, widen-
ing the field of view from that of the family or tribe to that
of all mankind, and merging love of country into cosmo-
politanism.

422. Races of Man. — Certain distinct types of mankind

33° The Realm of Nature CHAP.

may be easily recognised, but the transition between them
is so gradual that it is almost impossible to draw the divid-
ing line. Students of Ethnology form classes of mankind
partly by taking account of physical resemblance and
difference, partly by considering the nature of the languages
spoken. Following Professor Keane, we may group man-
kind around three main centres, corresponding respectively
to the Black, Yellow, and White types of humanity. The
table expresses some of the larger groups, with a selection
of illustrative races : —

BLACK YELLOW WHITE

WFSTFRN / Negro MONGOL- / Kalmuck

N \Bantu TATAR \ Kirghiz

NEGRITO TIBETO-CHINESE

EASTERN -f?aPuan,.._ FINNO- f?skimo

Kelt
Teuton
Slav
Hindu

Arab

/Malay TTAA/rrrTrf Berber
LN \Maori HAMITIC \Somali

MALAYO-

POLYNESIAN
AMERICAN CAUCASIC

423. Black Type. — This represents the least civilised
peoples, and around it is grouped about one-seventh of the
World's population. As the name implies, the complexion
is black or dark brown. The hair, also black, is woolly
or frizzled, and each hair has an extremely characteristic
form, resembling a minute flat ribbon. Most of the people
of the Black type are tall and powerful, often with well-
formed bodies, but with wide flat noses, thick lips, and
projecting jaws. They are sensual and unintellectual ;
like children they are usually happy, light-hearted, and
careless, but are subject to moods of depression and out-
bursts of appalling cruelty. They inhabit the tropics
exclusively, except when removed as slaves to warm tem-
perate regions. As a rule, in their own lands they go nearly
unclothed, living by hunting or by cattle-rearing, and, in
rare cases, following a primitive agriculture. The religion
professed is usually a low form of Nature-worship, character-
ised by fetishism and the practice of witchcraft. Moham-

xvii Man in Nature 331

medanism makes rapid headway amongst some of the tribes,
but Christianity seems less adapted to the nature of the
Black type. The Negro tribes occupying the Sudan region
of Africa are the most typical examples. The brown-
skinned Bantus inhabiting the whole of the Great African
Plateau are best known as the Zulu nation of the South.
The eastern division of the Black type includes the frizzly-
headed Papuans, or natives of New Guinea, and the Aus-
tralian Aborigines, who, while probably the lowest race in
point of civilisation, differ from the typical Black and
approach the White in possessing abundant wavy hair and
a full beard. The Negritoes, or " Little Negroes," are diffi-
cult to classify. They are usually small of stature and of
slight mental power. The best representatives are the
Pygmies of the Central African forests, the Bushmen of
South Africa, and the diminutive natives of the Andaman
Islands.

424. Yellow Type. — People grouped around the Yellow
type make up considerably more than one-third of the
World's inhabitants. Their complexion varies from clear
yellow to coppery brown, and typically they have a small
nose, frequently upturned, and narrow slit-like eyes. Their
hair is black, coarse, and straight, and each hair forms a
minute circular tube. They are usually under the middle
height, and although of slight physical strength they have
great powers of endurance, and are as a rule very laborious
workers. Intellectually they show a fair degree of civili-
sation, and in many instances have attained consider-
able success in science and in art. Conceit and apathy
are characteristic mental qualities. They are usually
Polytheists and worshippers of ancestors ; many are
Buddhists, and a considerable number Mohammedans.
Finns and Magyars, inhabiting Finland and Hungary,
are included under the Yellow type only on account of the
nature of their language ; physically and intellectually they
are indistinguishable from the highest members of the White
type. The Tib'eto- Chinese are possessed of an ancient
civilisation, and the Japanese, a race of this group, show
great aptitude in following modern western ways of life and

33 2 The Realm of Nature CHAP.

thought. The greater part of Asia is peopled by tribes of
the Yellow type of a relatively high civilisation. Except
where seafaring has called forth their powers, the people
inhabiting the tropical Malay Archipelago are as a rule
ignorant and uncivilised, although far above the level of the
degraded peoples of the Black type. The Maoris of New
Zealand, belonging to the Malayo-Polynesian section, con-
trast strongly with the Australian blacks. The American
section shows some well-marked differences from the other
representatives of the Yellow type. Their coppery com-
plexion won for them the name of Red Indians in the days
when the first Europeans reached America and believed it
to be part of Asia. From the Arctic Circle to Cape Horn
the race is, in its essential features, the same, although the
degree of civilisation attained varies. In the hot forests of
the Amazon they range as tribes of naked savages, as low
in the scale as the African blacks. On the northern prairies
they form nations of hardy warriors, brave in battle and
inconceivably cruel to their captured foes, living by hunting,
but scorning work, and rapidly dwindling away before the
white settlers. The highest native American civilisation
had its seat on the plateau of Mexico and in the Andes
valleys ; and although the strongly organised native empires
of the Aztecs and the Incas were destroyed by the Spaniards
in the sixteenth century the " Indian " element has always
remained of importance, and appears now to be rapidly
gaining ground in the countries of that region.

425. White Type. — The leading physical peculiarities
of this type are a prominent and highly arched forehead,
and abundant wavy hair, the cross-section of which is oval.
Dark skins, almost approaching those of the Black type,
occur in the Hamitic section, but the complexion of the
White races is usually fair and ruddy. The White type
is the centre of a more numerous group of mankind than
either of the others, and intellectually it is the most
advanced. Religion has its fullest and purest forms of
expression, science has been- studied to best purpose,
the fine arts have been raised to the highest perfection
amongst them. Enterprise in commerce and valour in war

XVII

Man in Nature 333

are equally pronounced, and at the present time the White
type, particularly the Aryan races spreading from Western
Eurasia, dominate the whole world. No other peoples
have ever succeeded in establishing democratic govern-
ment. The classification given in the table is founded
mainly on affinities of language. The Aryan group, for
example, includes the speakers of the Aryan or Indo-
Germanic languages, all of which contain many words of
common derivation, notwithstanding the differences between
English, German, Danish, Spanish, French, Italian, Latin,
Greek, Russian, Persian, and Sanscrit. Consequently it is
assumed by Professor Max Miiller and other philologists
that the races using these languages are also descended
from a common ancestry, and much ingenuity has been
applied to the discovery of the original seat of this primi-
tive people — the Aryans.

426. People of Europe. — Professor Huxley has recently
shown that, so far as the peoples of Europe are concerned,
it is impossible to reconcile the linguistic with the physical
classification. He points out that the difference between
the Teutonic-speaking nations of Britain, Germany, Holland,
and Scandinavia ; the Romanic-speaking people of Spain,
Portugal, Italy, France, and Rumania ; the Slavonic-speak-
ing people of Servia, Bulgaria, and Russia ; and even the
Magyar and Finnish-speaking people of Hungary, Finland,
and Lapland, do not warrant a scientific classification by
language. He recognises two extreme types of Europeans,
which are rarely found pure, and occur mixed together in
varying proportions in all parts of the continent. The
first type is that of tall men, averaging about 5 feet 8 inches
in height, with long heads, fair complexions, yellow or light
brown hair, and blue eyes. Such people are most abun-
dant in the north, round the coasts of the Baltic, and their
character is typically solid, trustworthy, persevering, and
deliberate. The second type is that of a shorter race,
averaging about 5 feet 5 inches in height, with rojindgd
heads, swarthy complexions, black or dark brown hair and
eyes. They are most numerous in the south bordering on
the Mediterranean. Their prevailing character is impulsive

334 The Realm of Nature CHAP.

and enthusiastic ; they are passionate, inconstant, and fond
of ease. These two types evidently represent different
races ; but they have mingled so thoroughly that any
attempt at exact classification is now impossible, although
some indefinite but very interesting subdivisions have been
made out.1

427. Distribution of the Human Race. — The esti-
mated population of the world is 1,470,000,000 people.
These are all dependent for their means ofTIFe on the
land, and the densest population, that is the greatest
number living on a given area, is necessarily found where
the land is richest in useful productions. Deserts are
practically unpeopled ; the few inhabitants live on the pro-
duce of thes date-trees of the oases and on the aid given
them by passing caravans, to which the oases afford in-
valuable halting -places. Steppe -lands can carry more
inhabitants, who as a rule are wandering shepherds feeding
their flocks on the best grass they can find, and moving on
to " pastures new " when the ground is cropped bare.
Well -watered lands, when naturally treeless, or after the
trees have been cleared, yield to agriculture abundance of
food and material for clothing, hence such countries can
support many inhabitants. The crowded Nile delta, the
river-plains of China, and the valley of the Ganges are the
most densely peopled parts of the Earth, on account of the
fertility of the ever-renewed soil allowing large crops of
food-plants to be raised at moderate expense. The question
of the production of food is the most important in order to
find how many people a given country can support. Mr.
Ravenstein calculates that with proper treatment of the land
about 6,000,000,000 inhabitants should be comfortably pro-
vided for on the Earth, a number which, if the present
rate of increase continues, will be attained in less than 200
years.2 There are other wants besides food, and by the divi-
sion of labour made possible by the organisation of civilised
life, a large population may be engaged in working mines or
carrying on manufactures in regions where sufficient food
for them cannot be grown. The supply of bread and meat
is kept up by trade with their fellow- workers on lands

xvii Man in Nature 335

yielding a superabundance. Western Europe has a dense
population on this account. Traffic, or carrying commodi-
ties to and fro, gives rise, at points where a change of
routes or means of conveyance occurs, to a local concen-
tration of population, and thus trading towns arise at
harbours, fords, and the intersections of roads or rail-
ways.

428. Centrifugal Migrations. — In a primitive state of
society the migrations of tribes are not unlike the migra-
tion of the lower animals, being directed from regions in
which the means of life no longer suffice for the in-
habitants. They are of the nature of evictions. A much
larger population formerly resided in Central Asia, the
margin of the Gobi (§ 381) being lined with remains of
ruined cities ; but the desiccation of the continent drove
the people outward into whatever lands afforded food for
their cattle or plunder on which to live. The people
against whom the hordes of wanderers were driven were
in turn dispersed in all directions, and the disturbance
spread throughout every part of Eurasia. Overcrowding
in countries of dense population also necessitates migration
to more thinly peopled regions ; but here as a rule the
human power of discrimination and choice regulates the
resulting movement. Lands are sought out which afford
similar natural conditions to those in which the emigrants
have formerly lived, and promise an easier or more pros-
perous life than the overcrowded country could offer.
Thus the people of North-western Europe, and particu-
larly of the British Islands, have thronged in millions to
North America, South Africa, and Australia ; while num-
bers of the people of Southern Europe have migrated to
South America and Northern Africa. Another form of
Centrifugal Migration is the voluntary exile of people
persecuted for holding particular religious or political
opinions. The settlement of New England by the Puri-
tans, of Maryland by Irish Catholics, and of Utah by the
Mormons, illustrates the action of this principle.

429. Centripetal Migrations have exercised an extra-
ordinary influence in modern times. They are the result

336 The Realm of Nature CHAP.

of attraction rather than repulsion, and take place toward,
and not from, a special region. The most potent magnet
is gold. This led the Spaniards to Mexico and South
America on the discovery of the new continent. In 1849
the discovery of gold in California caused a rush of fortune-
seekers from all parts of the world, and led to the very
rapid settlement of the Pacific coast of North America.
Victoria was the scene of a similar rush in 1850, and
tropical South Africa presents the same phenomenon,
though in a less intense form, at the present time.
Diamonds have had a like effect in attracting a large
population to Kimberley, in Cape Colony. In each case
many of the people attracted by the abundance of precious
and portable products remained after these ceased to be
readily available, in order to develop the agricultural
resources of the land. Coal-fields and regions where
petroleum or natural gas abound now rapidly attract a
large population, on account of the facilities afforded for
carrying on manufactures of every kind. Rich agricultural
lands such as those of Dakota and Manitoba also give rise
to concentration of population from all sides, when means
are provided by railways or rivers to carry the wheat or
other farm products to a profitable market.

430. Geography takes account of the relations between
regions and races. Physiography is concerned with the
study of Man in relation to the Earth, while Geography
treats of the Earth in its relation to Man% The branch of
geography dealing with the useful or desirable things which
occur in or on the Earth's crust, and the effects which the
discovery, production, transport, and exchange of these
have on mankind, is known as Commercial Geography.
Communities of civilised people associated together under
one government form nations, and the definite region of
the Earth's surface occupied by a nation is called a country.
Countries have sometimes arisen from the centrifugal or
centripetal migration of peoples under natural influences ;
but more commonly their limits have acquired their present
position by the conquest or loss of territory in struggles
against neighbouring nations. Wars carried on by kings

xyn Man in Nature 337

or governments, usually without the consent of the people
concerned, have drawn most of the boundary lines on
" political " maps. Historical Geography concerns itself
with tracing out the changes in the extent of territory
exclusively occupied or controlled by each nation at
different times.

431. Man's Power in Nature. — Man more than any
other animal leads a destructive life. The use of wood in
construction and for fuel enables him to destroy forests so
rapidly that in comparison the depredations of beavers and
all other animals are insignificant. The need for com-
munication between distant parts of the Earth has pro-
duced considerable changes in the configuration of coasts
and in the distribution of land and water. Plants and
animals also have been modified by cultivation, and their
natural limits of distribution entirely altered. Much of
Man's power in Nature is evasive. It consists in devising
methods of utilising natural phenomena for the purpose of
escaping uncomfortable consequences. Thus the invention
of the umbrella and of the sun -helmet give a certain
amount of independence of the weather ; still more the
methods of heating, cooling, and lighting houses. Light-
ning conductors reduce the risk to which life and property
are exposed in a thunderstorm ; knowledge of the laws of
cyclone - motion often enables sailors to escape the
fury of a storm. Steam-engines on land and sea, and
above all the electric telegraph, deprive wide tracts of the
Earth's surface of their natural influence as barriers. But
in every case natural powers are not overcome ; they are
merely utilised.3

432. Geographical Changes. — When land becomes
valuable it is often profitable to reclaim ground from the
sea. This is done along the flat coasts of most civilised
countries, and to an unequalled extent in Holland, where
most of the people actually live below sea-level. The sea
is kept out by a grand system of artificial dykes and regu-
lated sand-dunes, while continual pumping by steam or wind
power keeps the water-tight compartments of the reclaimed
land dry. On the other hand, there are many projects for

z

338 The Realm of Nature CHAP.

flooding the sunk plains of arid regions, so as to provide
new sea-routes, or modify desert climate by the presence of
a sheet of water. Examples of these are the proposal to
admit the Mediterranean and Red Sea to the great Jordan
Valley (§ 335), in order to open a new sea-route to India ;
and the suggestion of admitting the Mediterranean water to
some of the shotts of the northern Sahara (§ 377). Land-
masses necessitating long sea-routes have frequently been
severed by artificial channels, of which the Suez Canal is
the most remarkable example. A German canal for large
vessels across Jutland, from the North Sea to the Baltic, is
in progress ; a French canal to admit war-vessels is about
to join the Bay of Biscay and the Mediterranean north of
the Pyrenees ; a Greek canal has severed the isthmus of
Corinth. Several attempts have been made to cut the
isthmuses of Central America and have hitherto failed, not
because the task is impossible, but on account of financial
or political bungling. Rivers are continually being inter-
fered with, their mouths deepened into harbours, their
course levelled into canals, the current split up into irriga-
tion channels, or diverted bodily to prevent floods, or to
furnish a route for railways. The greatest project of river
diversion ever proposed is that of a Russian engineer to
restore the Oxus to its ancient bed (§ 382) and bring it
into the Caspian once more, thus affording a water-way
from Europe into Central Asia. Tunnels such as those
through the Alps, through the Khojak Hills in North-western
India, and under the Andes in South America, are other
examples of geographical changes wrought by human power.
So too are the subsidences which follow mining operations,
and sometimes alter the direction of streams.

433. Biological Changes. — By diligent cultivation and
careful selection the food-grains of the modern farmer have
been produced from various species of wild grasses, which
naturally had small and innutritious seeds. In like manner
many varieties of animals have been obtained by careful
breeding, which are specially fitted for the use of man.
Without his interference they would never have existed,
and in many cases, if left to their own devices, they would

xvii Man in Nature 339

be unable to make a living. Savage or useless creatures
have been exterminated over wide areas, and useful forms
of life introduced in their place. Sheep are now far more
numerous in Australia and temperate South America than
any indigenous species of mammal ever was. Human
interference can never overcome, but only take advantage of,
natural conditions ; and the rabbits accidentally introduced
to Australia happened to be so much in harmony with their
new surroundings that they have thriven and multiplied, so
as to be an intolerable plague in some districts. By human
agencies the horse, dog, sheep, and cow are no longer con-
fined to any faunal realm, and the useful plants of each of
the continents have been transplanted wherever suitable
conditions are found in all the others. Maize and tobacco
brighten the fields of Southern Europe, while wheat, sugar-
cane, and coffee spread over vast expanses of America.
The American cinchona and the Australian eucalyptus are
now invaluable to the fever-haunted lands of India, and the
latter tree flourishes in the swampy lowlands of the Medi-
terranean, while the vine and olive gladden the heart of the
Australians.

434- Meteorological Changes. — The regulating effect
of vegetation on rivers (§ 417) is accompanied by an actual
increase in the rainfall of wooded as compared with barren
regions. This is so clearly recognised that in many of the
treeless plains of North America and Australia tree-planting
is encouraged by the institution of an annual holiday called
Arbour Day, on which each citizen is expected to plant a
tree. In Russia the cutting of trees is prohibited in the
whole belt of forests which covers the Ural -Carpathian
ridge, whence all the rivers of Eastern Europe flow to north
and south. Palestine presents a very striking example of
climate altered by human action. In the days of the
Israelites the steep mountain slopes were terraced artifi-
cially by walls supporting a narrow strip of soil, on which
grain, vines, olives, and fruit-trees of many kinds were
grown. The rainfall was regular and gentle ; and after
percolating through the terraces, formed perennial springs
at the foot of the slopes, feeding the brooks which rippled

34° The Realm of Nature CHAP.

through the valleys. Now by neglect the terraces have been
broken down, and the soil is all swept into the valleys.
The mountain-sides, being bare and rocky, allow the occa-
sional heavy showers to dash down in impetuous torrents
to flood temporary streams, which, when the rain passes,
give place to channels of dry stones. The land becomes
baked in the fierce rays of the sun by day, and chilled by
intense radiation through the clear dry air at night, the
range of temperature having increased as the rainfall
diminished.

435- Man and the Degradation of Energy. — Men are
continually at work altering the distribution of matter and
energy on the Earth. Gold is sought for in all lands, and
accumulated in enormous quantities in London, Paris,
Berlin, and other towns. Diamonds are more numerous in
Amsterdam than in Africa, India, or Brazil ; and so with
other mineral commodities. The salts of the soil on which
its fertility depends are being removed by every crop of wheat,
to be ultimately cast as useless sewage into the sea. Land
deprived of its salts ceases to yield crops ; the natural process
of restoration by weathering (§310) is too slow, and manures,
which every year are becoming scarcer, must be sought far
and near to replace them. No animal but man is so im-
provident. All others restore the mineral constituents to the
land from which they gathered their food, and so insure a con-
tinuous supply. The potential energy laboriously stored in
growing trees is destroyed by reckless timber-cutting, and
the use of wood as fuel. The accumulated savings of energy
stored up in coal are being expended in every industrial
occupation, and coal is rapidly becoming scarcer. Every
consumption of energy, except that of the regular income of
solar radiation (§ 119), is impoverishing the Earth, and
accelerating the natural process of the degradation of
Energy (§ 75). The great steamer, driving its giant bulk
across the ocean at 20 miles an hour, consumes as much
potential energy in every revolution of the propeller as
served in former days for the stately clipper, rising and
dipping over the crests of the sea under the impulse of the
sun -driven winds, to make the whole journey. Tidal

Man in Nature 341

power, already utilised to some extent, and likely to be
made use of increasingly, simply does work off the energy
of the Earth's rotation (§ 103), and, although in a very
minute degree, its employment hastens the time when
Earth and Moon will have the same period of rotation.
Similarly, all processes now proudly being increased in
power and speed dissipate ever faster the wealth of poten-
tial energy that Nature lays up at an ever diminishing rate.
Wind and water power and the Earth's store of internal
heat are the only non-wasteful sources of work. Nothing
is given for nothing, and even the knowledge revealed by
the scientific study of Nature, that the power for effecting
these processes will not last for ever, has been dearly^
bought. Since the true part played by energy has been
understood in fact, though possibly not in name, the
governments of all civilised nations have exerted themselves
to encourage the most economical processes of manufacture,
the most satisfactory systems of agriculture, the most
intelligent methods of sewage disposal, and particularly to
ensure the continuance, and if possible the increase, of the
forests of the world, on which its prosperity, and even its
habitability, largely depend.

436. Man's Place in Nature. — The grand distinction/I
between Man and other creatures is that he can take
advantage of his environment, so as to modify his develop-
ment in any desired direction. He need not, except wil-
fully, drift before the wind of natural changes, but can
sail close up to it like a well -handled ship. Man's
higher nature can, and in many cases does, completely
control his lower or animal existence. The sense of moral
duty overcomes even the first law of animal nature — the
preservation of life ; it reverses the struggle for existence by
substituting the principle of self-sacrifice, on which the
stronger protects, instead of destroys, the weaker. Man,
when most truly human, or in the highest attained stage of
the evolution of civilisation, ceases to be in harmony
with the system of Nature in the sense true of the lower
animals —

342 The Realm of Nature CHAP, xvn

" Know, man hath all which Nature hath, but more,
And in that more lie all his hopes of good.
Nature is cruel, man is sick of blood ;
Nature is stubborn, man would fain adore ;

" Nature is fickle, man hath need of rest ;
Nature forgives no debt, and fears no grave ;
Man would be mild, and with safe conscience blest.

" Man must begin, know this, where Nature ends ;
Nature and Man can never be fast friends,
Fool, if thou canst not pass her, rest her slave ! "

V
REFERENCES

1 T. H. Huxley, "On the Origin of the Aryans," Nineteenth
Century, November 1890.

2 E. G. Ravenstein, "Lands of the Globe still available for
European Settlement," Proc. Roy. Geog. Soc. xiii. 27 (1891).!

3 H. R. Mill, " Scientific Earth Knowledge as an Aid to Com-
merce," Scot. Geog. Mag. v. 302 (1889). "The Influence of Man
on Nature," Madras Christian College Magazine, August 1888.

BOOKS OF REFERENCE

E. B. Tylor, Anthropology.

H. T. Buckle, History of Civilisation, vol. i. Longmans.

Keith Johnstone, Physical, Historical, Political, and Descriptive
Geography, revised by E. G. Ravenstein. Stanford.

G. P. Marsh, The Earth as modified by Human Action. New
York: Scribners.

G. G. Chisholm, Manual of Commercial Geography. Long-
mans.

H. R. Mill, Elementary Commercial Geography. Cambridge :
Pitt Press Series.

APPENDIX I

SOME IMPORTANT INSTRUMENTS

437. Weights and Measures. — Standard masses called "weights "
are used in a balance in order to find the mass of any body of con-
venient size by weighing it, that is by finding how many of the
standard masses are attracted by the Earth with the same force as
the body of unknown mass is attracted. The standard masses may
be of any size or form, provided they can be easily 'obtained, and
new ones exactly equal to them made if the originals be lost.
Grains of seed were once used for this purpose, but now the standards
are always made of dense metal of a kind which does not alter in
the air. When a standard is once accepted it does not matter how
it originated, as copies are always made by actual weighing. The
British unit mass or pound avoirdupois is divided into 7000 grains
or 1 6 ounces, and 2240 pounds are called a ton. In the United
States the same unit pound is used, but 2000 of them are called a
ton. In English-speaking countries the way in which masses are
calculated is very contradictory and puzzling ; but almost all other
civilised nations employ a uniform system called the metric, the
unit mass of which is the kilogramme (equal to about 2^ Ibs.)
divided into 1000 grammes, and the gramme is similarly divided
into 10 decigrammes, or 100 centigrammes, or 1000 milligrammes.
These standards of mass are used by scientific men in every country,
although the results have often to be translated into pounds and
grains to make them popularly intelligible. The unit of length
amongst English-speaking people is the yard, divided into 3 feet
of 12 inches each, and 1760 yards are called a mile, although the sea-
mile or mean minute of latitude contains rather more than 2000 yards.
Measures on the metric system are like the weights subdivided

344 The Realm of Nature

decimally. The unit is the metre (about 39^ inches), divided into
10 decimetres or 100 centimetres or 1000 millimetres ; and for
measuring long distances 1000 metres are called a kilometre. It is
convenient to remember that 25 millimetres are nearly equal to I
inch, or, more exactly, that 33 centimetres are equal to 13 inches, .
and that 8 kilometres are equal to 5 miles. The measures of volume
fluid ounces, pints, gallons, bushels, cubic inches, cubic feet, used
in English-speaking countries are as confused as the other standards,
while the unit volume of I litre (about if pints) divided into 1000
cubic centimetres is as convenient as the other parts of the metric
system. The only connection between the British systems of
weights and measures is that the gallon is fixed as the volume of 10
Ibs. of pure water at 60° F. Relations of a much more intimate
kind pervade the metric system. It is true that the metre is not
quite the length originally intended, which was •nro'troooo °f a
quadrant of the Earth's meridian, but the litre is a cube I decimetre
in the side, and the kilogramme is the mass of I litre of pure water
at 4° C. , the gramme being similarly equal to the mass of I cubic
centimetre of water at the maximum density point. Notwithstand-
ing the simplicity and convenience of the metric system, it was
considered advisable in this book to make use of the familiar British
units in order to present the facts of science in the manner most
easily grasped by English-speaking people.

438. The Mariner's Compass consists of a magnetised steel needle,
or a series of such needles fixed parallel to each other, delicately
pivoted in a box, which is loaded with lead and hung so as to remain
horizontal in spite of the tossing of a ship. A light circular card is
fixed above the needles and moves with them. The point over the
north-seeking end of the needle is marked as the North, the opposite
point is marked South, and the ends of the diameter at right angles
East and West. The edge of the card is divided into 360 degrees,
there being 90 in each quadrant, i.e. from N. to E. or from E. to S.
The exact direction or bearing of a distant object may be stated as
N. 45° E. if it appears midway between the north and east points of
the horizon as estimated from the card. Sailors have another way
of expressing direction. They divide the edge of the card into
thirty-two "points," each containing u| degrees, but divided into
halves and quarters. For each point they have a special name ; thus
the quadrant from north to east is divided into North, North by
East, North- North- East, North-East by North, North-East, North-
East by East, East- North- East, East by North, East ; and so on

Appendix I

345

round the card. (See compass in Plate I., where each alternate
point is named.) The indications of the compass require to be
corrected for variation (§ 98), and also for the local attraction of
the vessel, in order to be as free as possible from which the standard
compass is usually carried on the top of a high pole rising above the
highest part of the deck.

439. Barometers and Barographs. — The simple mercury-tube
(§ 146) mounted in a metallic case is the most accurate form of
barometer. The height of the mercury in the tube is measured
either to the fiftieth of a millimetre or to the thousandth of an inch
by means of an arrangement called a vernier, due allowance being
made for the change of level in the cistern as well as in the tube
of mercury. In comparing atmospheric pressure at different stations
it is necessary to correct the reading to some standard temperature
(always 32° F. or o° C.), because when mercury is heated it expands,
its density becomes less, and a slightly higher column would be
supported by the same atmospheric pressure. A correction for grav-
ity, or rather for gravity and centrifugal force combined (§§ 38, 93),
must also be made, as a column of mercury weighs less at the equator
than near the poles. For popular purposes a barometer is some-
times made to show its rise or fall by the movement of a pointer
round a dial, the change of quarter of an inch in level of the mercury
being thus magnified on the dial to an inch or so. Glycerine baro-
meters are in use in some places, and as the liquid is only about
one-twelfth of the density of mercury, the tube has to be over 30 feet
in length, and the fluctuations are shown in feet instead of in inches.
The readings of a glycerine barometer are recorded daily on a
diagram in the Times. Self-recording barometers are used in
observatories. The simplest in principle (Fig. 64) produces a photo-
graphic record by a beam of par-
allel light from a lamp passing
through the upper part of the tube
ac, and falling on a cylinder a'c'
covered with photographic paper,
and revolving once in twenty-four
hours by means of clock-work.
The paper opposite the clear space
is blackened by the light, and
Fig. 64 shows the sort of record
left by a barometer rising irregularly, the height of which at any
given moment can be estimated by seeing how much of the paper b'c'

346

The Realm of Nature

was shielded from light by the mercury be in the tube. The portable
Aneroid barometer consists essentially of a metal box with an elastic
top and exhausted of air. When the atmospheric pressure increases
the top is forced in, when it diminishes the top curves out, and this
movement is transmitted by suitable mechanism to a hand moving
round a dial, or to a lever carrying a pen which records the fluctua-
tions of pressure in a curve drawn in ink on a rotating
cylinder. " Inches" and fractions are marked round
the dial by comparison of the aneroid with a mercurial
barometer, and a scale of heights is usually added,
for aneroids are of most value in hill-climbing.

440. Thermometers are instruments for measuring
temperature by means of the difference of expansion of
a gas or liquid and the glass containing vessel. Mer-
cury is usually employed as the liquid, because it has
a low specific heat, great conducting power, expands
considerably when heated, has a low melting-point and
a high boiling-point. A mercurial thermometer con-
sists of a globular or cylindrical bulb (Fig. 65), and
a long tube of extremely small bore, which has been
sealed while filled with boiling mercury, so that, after
cooling, the bulb and part of the tube contain mercury
and the remainder is a vacuum. The freezing and
boiling points of any liquid depend only on the pres-
sure, and if the pressure remains unchanged the liquid
always freezes at one definite temperature, and always
boils at one definite temperature. Thermometers are
graduated by plunging them bodily into melting ice
and after the mercury has contracted to the full, mark-
ing its position by a scratch on the glass ; then by
hanging them in the vapour of boiling water at ordinary
atmospheric pressure, and when the mercury has ex-
panded to the full, marking its new position by a
scratch. Between the two fixed points any kind of
FIG. 65.— Mer- subdivision might be made, but only three ways of
curial Ther- dividing the space into ' ' degrees " or steps are in use.
On the centigrade scale (often erroneously named after
Celsius) the freezing-point is marked o, the boiling-point 100, and the
space between is divided into 100 equal degrees, which are con-
tinued above 100 and below o as far as may be necessary (C, Fig. 65).
On Fahrenheit's scale, used popularly in English-speaking countries,

F

230-
220

212

200-
190-

130-
120-
110-
100-

32,

r110

-100
-90
^80
•70
-60

50
h4C
L30
^20

ho

ho

r20

30

Appendix I 347

the freezing-point is called 32, the boiling-point 212, the space be-
tween being divided into 180 equal degrees, which are continued
downward and upward (F, Fig. 65). On the Reaumur scale, used
popularly in Germany and Russia, the space between freezing and
boiling point is divided into 80 degrees. The centigrade scale is
used in scientific work all over the world, except for meteorological
observations in English-speaking countries, for which the Fahrenheit
scale presents too many advantages to be discarded. It is convenient
to remember a quick way of translating centigrade into Fahrenheit
degrees. Miiltiply by 2, subtract one-tenth of the result, and add 32.
For example, to translate 15° C., 15x2 = 30, subtracting one-tenth
30 - 3 = 27, adding, 27 + 32 = 59° F. Since mercury freezes at - 40 (a
temperature which happens to be expressed by the same figure
on both centigrade and Fahrenheit scales), alcohol thermometers are
used for measuring lower temperatures, such as those of the winter at
Verkhoyansk. No two common thermometers read exactly alike,
and those employed for accurate observations are always compared
with standard instruments (those of Kew Observatory for the United
Kingdom), and have their error ascertained and allowed for.
Thermographs are constructed on the principle of the barograph,
to furnish a continuous record of changes of temperature. Deep-sea
thermometers require to be protected against the pressure at great
depths by surrounding the bulb by a glass sheath partly filled with
mercury or other liquid. They are constructed either to leave an
index sticking in the tube at the points of highest and lowest
temperature encountered while submerged, or to be inverted by
appropriate mechanism, and so caused to register the temperature
at any given point. (See article "Thermometer" in Encyclopedia
Britannica, 9th edition.)

441. Hygrometers measure the amount of water -vapour in the
atmosphere by finding either at what rate the air is taking up
vapour by evaporation at its actual temperature, or how far the air
must be cooled in order that its vapour may be saturated. The
commonest form consists of two thermometers placed side by side,
the bulb of one being left dry, while that of the other is covered by
a piece of fine muslin, and kept wet by a thread dipping into a
vessel of water. The farther the vapour of the air is from satura-
tion the more rapid is the evaporation from the wet bulb, and since
evaporation withdraws heat (§§71, 157), the temperature shown by the
wet-bulb thermometer is lower than that shown by the dry. The
greater the difference between the readings of the two, the smaller is

348 The Realm of Nature

the relative humidity of the atmosphere, the exact value of which for
each difference of temperature has been calculated and recorded in
tables by Glaisher. Dew-point hygrometers, in various forms, in-
vented by Regnault, Daniel, Dynes, and others, consist of a polished
surface, the temperature of which can be lowered by evaporating a
liquid, or by a current of iced water, until a film of moisture is con-
densed from the air. The temperature at which condensation takes
place is that of the dew-point, at which the vapour of the air becomes
saturated, and a table of the vapour-pressure of saturated vapour at
different temperatures gives the absolute humidity (§ 158).

442. Anemometers, or instruments for measuring the force of the
wind, are constructed either to record velocity or pressure. To
show velocity a series of hollow metal cups, mounted on a light
pivoted frame, are caused to revolve by the wind, and each revolu-
tion is registered by an arrangement like that of a gas-meter.
Experiment shows what ratio the speed of the revolving cups bears
to that of the wind. In pressure anemometers the wind blows
against a large flat surface, the pressure exerted on which is indi-
cated by the tension of spiral springs. These instruments, like all
others for measuring phenomena subject to constant variation, can
be made to write a continuous record on a revolving cylinder, from
which the exact direction, force, and velocity of the wind may be
ascertained at any moment.

443. Deep-sea Soundings. — The depth of calm water, when less
than 200 fathoms, can easily be found by letting down a lead
weighing 7 Ibs. by a line marked at regular intervals. The impact
of the lead on the bottom may usually be felt, and the line ceases to
run out, or at any rate, if too much line is let out, a sudden increase
in weight is felt when, on hauling it in, the lead is lifted off the
bottom. At great depths a very heavy sinker must be used : its
impact on the bottom cannot be felt, and the line runs out steadily.
In making a deep sounding, the line — usually a fine steel wire — is
marked at every 100 or 50 fathoms, and the intervals of time at
which each mark disappears in the water are carefully noted. On
account of the increasing resistance of the water on the lengthening
line the time interval lengthens gradually and uniformly ; but when
the sinker reaches the bottom there is an abrupt increase in the
time taken for the next 50 fathoms to run out, which is sufficient to
assure the officer in charge that bottom is reached. From depths of
3000 or 4000 fathoms no ordinary line or wire is strong enough to
haul up the heavy sinkers, which accordingly are so constructed as

Appendix I 349

to detach themselves after driving the brass "sounding tube" to
which they were attached deep into the floor of the ocean, where it
is filled with mud, and whence it can readily be raised to the sur-
face. The process of making deep-sea researches of every kind is
full of interest, and the student should, if possible, read the descrip-
tions in the works referred to at the end of Chapter XL

APPENDIX II

CURVES AND MAPS

444. Graphic Representations. — Self-recording instruments,
like the barograph and thermograph, write their changes as con-
tinuous curves, which present to the eye a vivid picture of the nature
and extent of these changes. The daily and annual changes of tem-
perature and pressure are represented in the form of curves in Figs.
23, 24, and 28. When any one of the conditions under consideration
varies uniformly, the curve form of expression can be used ; thus
Fig. 27 shows temperature at different latitudes, where position
on the Earth varies uniformly, and Fig. 33 shows temperature at
various depths in the sea, where depth varies uniformly. The highest
point of a curve or any convex bend is called a maximum the lowest
point, or any concave bend, a minimum ; and a line drawn horizontally,
so that the curve cuts off an equal area above and below, is called its
mean. It is simply a matter of convenience that the space represent-
ing a degree of temperature, and that representing an hour, a day, a
fathom, or a degree of latitude, should have the same length in a
diagram. In the sections of oceans and continents there is a
natural relation between heights and lengths ; but if on a section of
Asia 100 miles of length were represented by an inch, the greatest
height of the continent would be shown by one-twentieth of an inch,
and would scarcely be visible. Accordingly heights are drawn on a
much larger scale, and the steepness of the slope is exaggerated in
the same proportion, while the positions and relative amounts of
change of level are brought vividly before the eye. It would be an
excellent exercise for the student to reduce these sections to a true
scale, either by reducing the heights on the paper to one-three-hun-
dredth of their height (but this is scarcely possible), or by keeping

Appendix II 351

them unchanged, and lengthening the whole section, or a part of it,
three hundred times. This would give the true average slopes of
the continents and oceans.

445. Maps. — The plan or map of a room is simply an exact drawing
of the outline of the floor, and the spaces occupied by each article of
furniture, drawn so that one inch or any other definite length on the
paper corresponds to one foot on the floor. The ratio of the lengths
is called the scale of the map ; thus the scale of a map in which
one inch represents one foot is I : 12 ; the maps of counties on the
Ordnance Survey of the United Kingdom are drawn on the scale of
six inches to one mile, or I : 10,560 ; those of the country generally,
in which one inch stands for one mile, are on the scale of I : 63,360 ;
Plates IX. and X. represent the British Islands on the scale of
I : 7,500,000; and Plates I II. -VI II., etc., show the Earth on the
scale of I : 200,000,000 along the equator. In the case of the plan
of a room, the map, if increased twelve times in length and breadth,
would make a carpet accurately fitting the floor, with spaces marked
for the furniture to rest on ; but if the map of the British Islands
were magnified 7,500,000 times each way, it would not fit the
country exactly, because the Earth's surface is curved, and a flat
sheet cannot lie smoothly on a curved surface without being folded
or stretched. In the case of the Earth as a whole, this difficulty of
representing the whole surface in its true form and proportions is
much greater. The surface of the sphere cannot be spread out flat,
and many devices — termed projections — are adopted to represent it
with as little distortion as possible. On Mercator's projection, shown
in Plates I. and II., the parallels of latitude are shown as straight
lines, the equator being unbent from a ring into a rod, so that we can
see all round it at one glance. The other parallels are not only unbent,
but stretched to the same length as the equator, so that the meridians
become parallel straight lines, and, in latitude 60°, are just twice as
far apart as they should be. In order to preserve the correct out-
line of the land, and to make the directions measured on the map
correct, the parallels are not placed equidistant, but stretched out
toward the poles, the degrees of latitude increasing in length in the
same proportion as the degrees of longitude. Thus different parts
of the map are on different scales ; one square inch including Green-
land, for example, represents only one-tenth of the area which one
square inch including India comprises. It resembles a cylindrical
projection, which may be supposed to be drawn on a great sheet
wrapped round the globe, as shown in Fig. 66. Mercator's projec-

352

The Realm of Nature

tion, although much less distorted than the true cylindrical projection
of Fig. 66, is useless for comparing areas. But it is of unique value,

;. 66. — Cylindrical Projection. Lines drawn from
the centre of the globe through the parallels and
meridians are produced until they meet the surface
of the bounding cylinder, on which each parallel is
represented by an equal circle and each meridian
as a straight line. When the cylinder is unrolled
the mode in which the surface of the globe is
represented on the flat sheet is evident.

because a line drawn between any two points cuts all the meridians
at the true angle, and it is therefore much used in navigation.
Plate III. and most of the other maps of the world shown are
drawn on Gall's stenographic projection, which does not distort the
areas so much, and does distort the angles considerably. Plate XII.
shows beautifully the amount of distortion of area in this projection,
the 250 mile coast belt appearing nearly three times as broad round
Greenland as round Africa where the distortion is least. In maps
of the world in hemispheres the meridians are shown converging to
the poles, and there is an infinite number of projections employed
for special purposes. Lambert's equivalent area projection (Plate
XIV.) is valuable because, although it distorts angles greatly, it pre-
serves the equality of areas ; a square inch measured on any part of
the map represents exactly the same number of square miles. The
calculations of Dr. John Murray, referred to in previous chapters,
were made by measuring areas on large-scale maps constructed on
this projection. Maps of a small area can be more accurately shown

Appendix II

353

on a conical projection. Those of the British Islands (Plates IX. X.
etc.), for example, are on a conical projection; the meridians con-
verging to the proper degree and the parallels being arcs of circles.
If a cone of transparent paper were placed over an artificial globe
(Fig. 67) and the lines traced through, a map of this kind would
result ; the distortion being greatest at the greatest distances from the

FIG. 67. — Conical Projection. The left hand figure represents a cone placed on
the globe, the surface features of which are projected, as in Fig. 66, by lines
drawn from the centre. The right hand figure shows the cone unrolled
showing the parallels as semicircles and meridians radiating from a centre.
The double lines show a map cut from the developed cone.

parallel along which the cone touched. When the cone is supposed
to cut the globe along two parallels, the resulting map is much more
accurate. In actual map-making the distance and curvature of the
parallels and meridians for each projection are ascertained by mathe-
matical calculations.

446. Contour -lines are drawn on maps to express differences of
level in an exact manner.
They express the height of
the land in the same way as
isotherms express the tem-
perature. Each contour-line
represents a string of figures
of elevation having the same
value. The sea -coast is a
natural contour - line, and
raised beaches are natural
contour-lines etched on the

hill -sides. Every contour-
line represents the coast-
line that would result, if

FIG. 68. — Contour-lines. The Line BC re-
presents sea-level, each of the inner lines
represents a level 100 feet higher than that
next to it on the outside, the line round A
being 500 feet above the sea. The scale
below refers to horizontal distance.

the sea rose to that level. When

contour-lines are far apart the gradient or slope is gentle ; for ex-
ample, along AB (Fig. 68) we could advance nine divisions of the
2 A

354 The Realm of Nature

scale before the elevation became 500 feet lower, but along AC this
difference of height is reached in three divisions of the scale, or the
slope is three times as steep and the contour-lines are much closer.
The student, if residing in the United Kingdom, should procure and
carefully study the Ordnance Survey maps (contoured) on the one-
inch and six-inch scales for his own locality. He might advan-
tageously follow the lines in pencil to make them more prominent,
and then paint the map in successive washes, deepening the colour
within the higher contour-lines as in Plates XI. and XVI. He will
thus produce a pictorial relief map, on which all the features of hill
and dale will stand out with great distinctness. The Ordnance maps
for England and Wales are to be had from Mr. Edward Stanford,
55 Charing Cross, London, S.W. ; those for Scotland from Messrs.
John Menzies and Co., 12 Hanover Street, Edinburgh; and those
for Ireland from Messrs. Hodges, Figgis, and Co., 104 Grafton Street,
Dublin. Mountains and watersheds are frequently represented on
maps by shading in certain conventional ways, so as to bring out the
general appearance of the surface. One of these systems combined
with contour-lines is shown in the map of a glacier in Fig. 54.

APPENDIX III

DERIVATIONS OF SCIENTIFIC TERMS

ABERRATION, L. ab, from ; erro, to wander

Absorption, L. ah, from ; sorbeo, to suck in

Agglomeration, L. ad, to ; glomus, a ball /

Agonic, Gr. a, not ; gonia, a corner or angl£

Amorphous, Gr. a, not ; morphe, form

Amplitude, L. amplitude, large

Analysis, Gr. ana, up ; /«<?, to loosen

Anemometer, Gr. anemos, wind ; metron, measure

Aneroid, Gr. a, not ; neros, liquid

Annular, L. annuhis, a ring

Anticline, Gr. anti, against ; klino, to lean or incline

Anticyclone, Gr. anti, opposite to, and CYCLONE

Aphelion, Gr. apo, from ; helios, the sun

Approximation, L. ad, to ; proximus, nearest

Aqueous, L. aqua, water

Arc, L. arcus, a bow

Archaean, Gr. archaios, ancient

Archseopteryx, Gr. archaios, ancient ; pteryx, wing

Arthropoda, Gr. arthros, a joint ; pous, foot

Asteroid, Gr. aster, star ; eidos, form

Atmosphere, Gr. atmos, air ; sphaira, a sphere

Aurora, L. , the goddess of dawn

Austral, L. auster, the south wind, southern

Axis (pi. axes), L., an axle

Azote, Gr. a, not ; zao, to live

BAROGRAPH, Gr. baros, weight ; grapho, to write
Barometer, Gr. baros, weight : metron, measure

356 The Realm of Nature

Biology, Gr. bios, life ; logos, a discourse

Bisect, L. bis, twice ; seco, to cut (to divide into two equal parts)

Boreal, L. boreas, the north wind, northern

Botany, Gr. botane, herb or plant

CAINOZOIC, Gr. kainos, recent ; zoe, life

Calcareous, L. calcarms, chalky

Capillarity, L. capillus, hair

Carposporese, Gr. karpos, fruit ; sporos, seed

Centrifugal, L. centrum, centre ; fugio, to flee from

Centripetal, L. centrum, centre ; peto, to seek

Chlorophyll, Gr. chloros, pale green ; ptmllon, leaf

Chromosphere, Gr. chroma, colour ; sphaira, a sphere

Chronometer, Gr. chronos, time ; metron, measure

Cirrus, L. cirrus, a curl

Ccelenterata, Gr. koilos, hollow ; enteron, bowel

Cohesion, L. co, together ; h&reo, to stick

Comet, Gr. kometes, long-haired

Complement, L. complementum, that which fills up

Concentric, L. con, with ; centrum, centre (having the'same centre)

Conduction, L. con, together ; duco, to lead

Constellation, L. con, together ; Stella, a star

Convection, L. con, together ; veho, to carry

Cretaceous, L. creta, chalk

Cryptogam, Gr. kruptos, concealed ; gamos, marriage

Cumulus, L. cunmhts, a heap

Cyclone, Gr. kuklos, a circle

DATUM (pi. data), L. datum, given
Deciduous, L. decidttus, falling off
Desiccation, L. desicco, to dry up
Detritus, L. de, off; tero, tritus, to rub
Devitrification, L. de, from ; vitrum, glass ; facio, to make
Diameter, Gr. dia, through ; metrein, to measure
Dicotyledon, Gr. dis, two ; kotuledon, a cup( -shaped leaf)
Discrete, L. discretus, separate

ECHINODERMATA, Gr. echinos, hedgehog (spiny) ; derma, skin
Elasticity, Gr. elaso, to drive

Electricity, Gr. elektron, amber (by rubbing which electric phen-
omena were first observed)
Ellipsoid, Gr. en, in ; leipo, to leave ; eidos, form

Appendix III 357

Eocene, Gr. eos, dawn ; kainos, recent

Equator, L. aquus, equal

Equisitinese, L. equus, horse ; seta, bristle

Erosion, L. e, away ; rodo, to gnaw

Escarpment, Fr. escarper, to cut down steeply

Estuary, L. cesttiare, to boil up, i.e. tumultuous tides

Ethnology, Gr. ethnos, a nation ; logos, a discourse

Evolution, L. e, out ; volvo, to roll

Experiment, L. experior, to try thoroughly

FAUNA, native animals supposed to be protected by the Fauns, or

rural gods

FilicinesB, L. filicis, a fern
Flora, L. flos, a flower
Foraminifera, L. foramina, openings ; fero, to carry

GENUS (//. genera), L. genus, birth (related by birth, of one kin)

Geography, Gr. ge, the earth ; grapho, to describe

Geoid, Gr. ge, the earth ; eidos, form

Geology, Gr. ge, the earth ; logos, a discourse

Glacier, Fr. glace, ice

Glauconite, Gr. glaukos, bluish gray

Gravitation, L. grams, heavy

Gymnosperm, Gr. gumnos, naked ; sperina, seed

HEMISPHERE, Gr. hemi, half ; sphaira, a sphere
Hepaticae, Gr. hepatos, the liver
Homogeneous, Gr. homos, one ; genos, kind
Horizon, Gr. horizo, to bound
Humidity; L. humidus, moist
Hydrosphere, Gr. hudor, water ; sphaira, a sphere
Hygrometer, Gr. htigros, wet ; metron, measure
Hyperbola, Gr. huper, beyond ; ballo, to throw

ICHTHYOSAURUS, Gr. ichthos, fish ; saiira, lizard

Igneous, L. z^mV, fire

Indigenous, L. indu, in ; geneo, to produce

Inverse, L. inverto, to turn round

Isobaric, Gr. isos, equal ; &z?w, weight

Isothermal, Gr. isos, equal ; therme, heat

LATERAL, L. talus, a side

358 The Realm of Nature

Latitude, L. latitudo, breadth

Lithosphere, Gr. lithos, stone ; sphaira, a sphere

Littoral, L. littus, the shore

Longitude, L. longitude, length

MEDIUM, L. medius, middle (anything coming between)

Meridian, L. meridies, mid -day

Mesozoic, Gr. mesas, middle ; zoe, life

Meteorite, Gr. meteoron, suspended beyond ; lithos, a stone

Meteorology, Gr. meteoron, suspended beyond ; logos, a discourse

Miocene, Gr. meion, less ; kainos, recent

Mollusca, L. mollis, soft

Monocotyledon, Gr. monos, alone ; kotuledon, a cup(-shaped leaf)

Monsoon, Malay musim, a season

Musci, L. muscus, moss

NEBULA, L. nebula, a little cloud

Nimbus, L. nimbus, a rain-cloud

Nitrogen, Gr. nitron, nitre ; gennao, to produce

Node, L. nodus, a knot

Normal, L. norma, a rule

OBLATE, L. oblatus, carried forward

Oligocene, Gr. oligos, few ; kainos, recent

Oolite, Gr. oon, an egg ; lithos, a stone

Oosporese, Gr. oon, an egg ; sporos, seed

Orbit, L. orbis, a ring

Oriental, L. orior, to rise ; hence' the east

Orographical, Gr. oros, a mountain ; grapho, to describe

Oxygen, Gr. oxus, acid ; gennao, to produce

Ozone, Gr. ozo, to smell

PALJEOCRYSTIC, Gr. palaios, ancient ; krustallos, ice

Palaeozoic, Gr. palaios, ancient ; zoe, life

Parabola, Gr. para, beside ; ballo, to throw

Parallax, Gr. para, beside ; alasso, to change

Parallel, Gr. para, beside ; attelon, one another

Pelagic, Gr. pelagos, the sea

Perihelion, Gr. peri, near ; helios, the sun

Perturbation, L. per, thoroughly ; tttrbo, to disturb

Phanerogam, Gr. phaino, to bring to light ; gamos, marriage

Phenomenon, Gr. phainomenon, an appearance

Appendix III 359

Philology, Gr. philos, loving ; logos, word (the study of language)

Photosphere, Gr. phos, light ; sphaira, a sphere

Physiography, Gr. phusis, nature ; grapho, to describe

Plane, L. plamts, even, smooth

Planet, Gr. planetes, a wanderer

Pleistocene, Gr. pleistos, most ; kainos, recent

Plesiosaurus, Gr. plesios, near to ; saura, a lizard

Pliocene, Gr. pleion, more ; kainos, recent

Porifera, L. porus, a pore ; fero, to carry

Potential, L. potens, being able

Proteid, Gr. protos, first

Protophyta, Gr. protos, first ; phuton, plant

Protoplasm, Gr. protos, first ; plasma, form

Protozoa, Gr. protos, first ; zoon, animal

Pterodactyl, Gr. pteron, wing ; daktulos, finger

Pteropod, Gr. pteron, wing ; podes, feet

RADIATION, L. radio, to radiate
Radius, L. radius, a rod, ray
Rarefaction, L. rarus, rare ; facio, to make
Reflection, L. ;-£, back ; flecto, to bend
Refraction, L. re, back ; frango, to break
Rotation, L. 7-0/0, to turn

SATELLITE, L. satelles, an attendant

Saurian, Gr. saura, a lizard

Secretion, L. secretus, from ^, apart ; cerno, to separate

Sedimentary, L. sedimentum, from j*£0, to sit, to settle

Sequence, L. sequor, to follow

Sidereal, L. «V/#.y, a star

Solstice, L. sol, the sun ; sto, to stand

m J L' •r^^' to look (that which is seen)
Spicule, L. spicuhun, a point
Stalactite, Gr. stalaktos, dropping
Stalagmite, Gr. stalagtnos, a dropping

StratuM//. strata), } L' 5te™> ^^^ to s?read out
Subtend, L. ^w^, under ; tendo, to stretch
Syncline, Gr. sun, together ; klino, to lean or incline
Synoptic, Gr. sun, with ; opsis, a view

360 The Realm of Nature

TALUS, Fr. talus, a slope

Tangent, L. tango, to touch

Terrigenous, L. terra, the earth ; geneo, to produce

Thallophyte, Gr. thallos, a twig ; phuton, a plant

Thermometer, Gr. therme, heat ; metron, a measure

Transit, L. trans, across ; eo, to go

Trias, Gr. trias, union of three

Trigonometry, Gr. trigonon, triangle ; metron, a measure

Tropic, Gr. tropos, a turning

UNIVERSE, L. unus, one ; verto, to turn

VACUUM, L. vacuum, empty
Vermes, L. vermis, a worm
Vernal, L. ver, spring
Vertebrata, L. vertebra, a joint
Vertical, L. vertex, the top
Vibration, L. vibro, to quiver
Vortex, L. vorto, to turn or whirl

ZENITH, Arabic, semt-ur-ras, way of the head
Zero, Arabic, sifr, nothing (a starting-point)
Zone, Gr. zone, a girdle
Zoology, Gr. zoon, an animal ; logos, a discourse

INDEX

The Figures refer to the sections.

ABERRATION of light, 108
Absolute Zero, 68
Absorption and radiation, 63
Absorptive power of air, 160
Abysmal area, 255, 257, 277
Accuracy, 10
Acids, 44

Adelsberg caves, 317
Adriatic Sea, 216, 325
Africa, 373'377. 412
Agassiz, Lake, 368
Agonic lines, 98
Agulhas current, 248
Air, 151 ; in sea- water, 225 ;
temperature of, 187, 189, 190
Albert, Lake, 375
Aletsch glacier, 337
Alluvial deposits, 322
Alps, 303, 385
Altai Mountains, 381
Altitude of the Sun, 124
Amazon, 219, 230, 269, 318,

319. 36i
America, see North, and South

America.

American race, 424
Amu Daria (Oxus), 382, 432
Amur River, 319, 381
Analysis, 40 ; spectrum, 63
Andes, 201, 353, 358
Anemometer, 442
Aneroid barometer, 439
Angles, 31 et seq.

Angular measurement, 31-33

Animals, 397, 400, 409-417

Antarctic circle, 122

Antarctic continent, 276, 340

Anthracite, 347

Anticlines, 302, 303

Anticyclones, 205

Anti-trade winds, 181

Apennines, 385

Appalachian Mountains, 366

Approximation, 10

Arabia, 379, 412

Aral, Lake, 333, 382

Arbour Day, 434

Archaean rocks, 346

Arctic circle, 122 ; sea, 216, 234

Aryans, 425

Ash, volcanic, 294

Asia, 379-383

Asteroids, 129

Atlantic Ocean, 216, 243-246,

258 ..

Atmosphere, 84, 145-213
Atmospheric electricity, 172
Atolls, 280
Atoms, 47, 48
Aurora, 99, 174
Austral group of plants, 405
Australia, 353, 370-372
Australian people, 423 ; realm,

4i5

Autumnal equinox, 123
Avalanches, 336

362

The Realm of Nature

The Figures refer to the sections.

Axis of continents, 356 ; of the

Earth, 90
Azote, see Nitrogen

BAIKAL, Lake, 333, 382

Balkan Range, 385

Balkash, Lake, 382

Baltic Sea, 216, 238, 325, 388

Bangweola, Lake, 376

Banks, submarine, 325

Bantu race, 423

Barograph, 439

Barometer, 146, 439

Barrier reefs, 280

Bars of rivers, 325

Basalt, 290, 294

Bases, 44

Basin, Australian, 371, 372

Basin, the great, 365

Basins, river, 319 ; see also Ocean

Bayous, 324

Beaches, formation, 265 ; raised,

284

Benguela current, 243, 245
Bermuda Islands, 279, 307
Biela's comet, 135
Black Forest Mountains, 385
Black Sea, circulation, 238
Black type of mankind, 423
Blow-holes, 266
Blue mud, 270
Bode's Law, 129
Bohemian Forest Range, 385
Boiling, 70, 72
Bonneville, Lake, 365
Bore, the tidal, 219
Boreal group of plants, 405
Boulder clay, 338, 368, 389
Boyle's Law, 148, 163
Brahmaputra River, 380, 383
Brazil, High Plain, 359
British Islands, climate, 202-204 ;

surface, 389-392
Buys Ballot's Law, 192

CAINOZOIC rocks, 350
Calcareous organisms, 273

Cambrian rocks, 346

Canals, 432

Canons, 328 ; submarine, 326

Capacity for heat, see Specific

heat

Capillarity, 39, 310, 314
Carbonic acid, 154, 294, 317,

399, 400
Carboniferous rocks, 347, 366,

37i. 390. 39i

Carpathian Mountains, 385

Cascade Range, 364

Casiquiare River, 360

Caspian Sea, 333, 335, 382, 387

Caucasus Mountains, 379

Cause and effect, 16

Cavendish experiment, 85

Caverns, 317

Cells, 398

Centigrade scale, 440

Centrifugal and centripetal migra-
tions, 428, 429

Centrifugal force, 51

Chalk Ridge, 391

Challenger expedition, 183, 188,
251, 268

Charlestown earthquake, 301

Chlorophyll, 399

Circulation, atmospheric, 176,
177 ; of deep lakes, 228 ; of
enclosed seas, 237, 238 ; of
water by wind, 240-242

Cirrus cloud, 168

Civilisation, 420

Classification, 4, 13 ; of animals,
395, 397 ; of elements, 47 ; of
plants, 395, 396 ; of stars, 138

Clay, 311

Cleavage of rocks, 290

Climate, 125 ; of British Islands,
202-204; of Earth, 186-201

Cloud-bursts, 210

Clouds, 167, 168

Coal, 29, 347

Coast Range, 364

Cohesion, 39

Colorado River, 327, 329, 364

Index

363

The Figures refer to the sections.

Colour, 64
Columbia River, 364
Comets, 132, 133
Common-sense, 9
Comparison, 4
Compass, mariner's, 438
Compounds, 42
Compressibility, 35
Condensation, 70-73, 159, 166
Conduction, 59
Conductors of electricity, 77
Configuration and climate, 186
Congo, 269, 319, 326, 376
Conical projection, 445
Continental Area, 255, 354-392
Continental Shelf, 263, 264, 267
Continents, evolution of, 353 ;

statistics of, 355
Contour-lines, 446
Convection, 68
Coral Islands, '280-282
Coral mud and sand, 272
Corals, 279
Corona, solar, 116
Cosmic dust, 161, 277
Cotswold Hills, 391
Counter equatorial currents, 243,

247, 248

Cretaceous rocks, 349, 391
Crevasses, 337
Crystals, 30
Cumulus cloud, 168
Currents of the ocean, 242-249
Curvature of the Earth, 81
Curves, use of, 444
Cyclones, 206-208

DAILY range of temperature, 1 82 ;

of pressure, 183
Dalton's Law, 155
Danube River, 331, 386
Day, longest, 124 ; period of,

94

Dead Sea, 333, 335
Declination, magnetic, 98
Deductive reasoning, 17
Deep-sea soundings, 443

Definiteness, n

Degradation of energy, 75, 435

Degree, angular, 31 ; of latitude,
92; of longitude, 97; of tem-
perature, 440

Deltas, 325

Density, 29 ; of air, 148 ; of the
Earth, 85 ; of sea- water, 223

Denudation, 305

Deposits in the ocean, 268-277

Deserts, 406

Devonian rocks, 346

Dew, 165

Diatom ooze, 273, 276

Differential attraction, 103

Dip of horizon, 81 ; of a magnet,
98 ; of rocks, 290

Direction on the Earth, 91

Disruptive discharge, 78

Distance of stars, 137

Distribution of animals, 409 ; of
mankind, 427; of plants, 410

Doldrums, 179

Dolphin Ridge, 258

Drainage areas of ocean, 356

Dust, 134, 151, 161, 162, 297

Dykes, volcanic, 295

EARTH, the, 81 et seq.\ and

Moon, 104; orbit, 109
Earthquakes, 299-301
Eclipses, 113
Ecliptic, 112
Elasticity, 35 ; of Earth's crust,

299

Elbe, river, 386
Elburz Mountains, 379
Electrical energy, 76
Electrification of the atmosphere,

172

Electro-magnetism, 80
Elements, 45
Elevation and subsidence of land,

284
Energy, 25, 49, 53-56, 60, 163,

250, 283, 304, 305, 399, 435
England, 391

364

The Realm of Nature

The Figures refer to the sections.

Environment, 403, 421

Eocene rocks, 350

Equator, 91

Equinoxes, 121, 123; precession
of, 115

Erie, Lake, 330, 369

Erratics, 338

Eruptions, volcanic, 296, 297

Erzgebirge, 385

Estuaries, 231

Ether, the, 60

Ethiopian Realm, 412

Etive, Loch, 339

Eurasia, 378

Europe, 384-392 ; people of, 426

Evaporation, 70, 159

Evolution, organic, 402 ; of con-
tinents, 353

Exclusiveness, 34

Expansion by heat, 67, 68

Experiments, 18

Eyre, Lake, 372

FAHRENHEIT scale, 440
Faults, 290
Faunal Realms, 410
Felspar, 41, 286, 310
Ferre!' s Law, 89
Figure of the Earth, 82
Fingal's Cave, 266
Firths, 231
Fjords, 229
Flinders Range, 372
" Floating " of dust in air, 161
Floes, ice, 234
Floods in rivers, 324
Floral Zones, 405
Fog, 167
Foraminifera, 273
Forests, 407, 408, 417
Form, 30
Fossils, 343-345
Foucault's pendulum, 87
Frigid Zones, 125
Fringing reefs, 280
Function of lakes, 334 ; of living
creatures, 398 ; of the sea, 250

GALL'S Projection, 445

Ganges River, 269, 318, 331,

383

Genus, 396
Geography, 430
Geoid, 83

Geological theories, 341
Geysers, 316

Ghats, Eastern and Western, 383
Giant's Causeway, 392
Glacial action, 351, 352
Glaciers, 336-338
Globigerina ooze, 273, 275
Gobi, desert, 381, 406, 428
Gradient (barometric), 175'
Granite, 29, 41, 43, 310
Graphic representations, 444
Gravitation, 19, 36-38, 52
Great Basin, 365, 406
Great Divide, 369
Great Dividing Range, 371
Great Fault of Scotland, 390
Great Lakes, 333, 369
Great Plateau of Africa, 374
Great Salt Lake, 365
Greenland, 340, 363
Green mud, 271
Guiana, High Plain, 359
Guinea, current, 243
Gulf Stream, 244, 279
Gyroscope, 51

HAIL, 171

Halley's comet, 132

Heat, 65-74; in air, 163, 164;

in rocks, 306 ; of the Sun, 118 ;

in water, 227
Height of land, 369
Heredity, 403

Highlands of Scotland, 390
Himalaya Mountains, 303, 380
Hindu Kush, 379
Hoang Ho, see Yellow River
Hoar-frost, 165
Humidity, 158
Huron, Lake, 369
Hurricanes, 208

Index

365

The Figures refer to the sections.

Hydrosphere, 84, 214-282
Hygrometer, 441
Hypothesis, 18

ICE, 69, 336-340

Ice Age, 351, 352

Icebergs, 234

Ice-caps, 340

Ice deserts, 406

Igneous rocks, 287

Impenetrability, 34

Indian Ocean, 216, 248, 261

Indian peninsula, 383

Inductive reasoning, 17

Indus River, 380

Inertia, 50

International Deep, 258

Inverse squares, 36

Ireland, 392

Islands, 262; life on, 416

Isobars, 192

Isotherms, 188

JENOLAN Caves, 317
Joints in rocks, 290
Jupiter, 127, 130
Jura Mountains, 385
Jurassic rocks, 349

KALAHARI desert, 406
Karakorum Mountains, 380
Kepler's second Law, 109
Krakatoa, eruption of, 297
Kuen Lun Mountains, 380
Kuro Siwo, 247

LADOGA, Lake, 388

Lahontan, Lake, 365

Lake district of Europe, 388 ; of

North America, 368
Lakes, 332-335, 339; circulation

of water in, 228
Lambert's equal-area projection,

445

Land and sea breezes, 184
Land and sea climates, 191
Land and water, 214

Land sculpture, 305
La Plata River System, 362
Latent heat, 69
Latitude, 92
Lava, 294

Laws of Nature, 14, 20
Lena River, 382
Life in the world, 394
Light, 62, 64
Lightning, 173
Lignite, 347

Limit of saturation in rocks, 314
Lisbon earthquake, 301
Lithosphere, 84, 251 et seq.\ in-
terior of, 292
Llanos, 360, 406 •
Loam, 311
Loess, 308
Longitude, 97
Low plains, 264
Lukuga River, 376

MAGNETISM, 79 ; terrestrial, 98
Malay Archipelago, 407, 414-

416

Malayo- Polynesian race, 424
Mammoth Cave, 317
Man, 418-436
Maps, 445
Mars, 127, 128
Mass, 28
Mathematics, 8
Matter, 24, 27, 48
Mean sphere level, 254
Measures, 437

Mechanical equivalent of heat, 74
Mekong River, 380
Mercator's projection, 445
Mercury (metal), 66, 146, 440
Mercury (planet), 127
Mer de Glace, 337
Meridians, 91
Mesozoic rocks, 349
Metamorphic rocks, 289
Meteorites, 135
Meteoritic hypothesis, 144
Meteors, 134

366

The Realm of Nature

The Figures refer to the sections.

Mica, 41, 286

Michigan, Lake, 369

Micro-organisms, 401

Midnight Sun, 122, 150

Migration of peoples, 428, 429

Milky Way, 140

Mimicry, 409

Minerals, 285, 286

Miocene rocks, 350

Mirage, 150

Mississippi, 319, 322, 324, 331,

367

Mist, 167
Mixture, 41; of gases, 155; of

rivers and sea, 230
Molecular vibrations, 59
Momentum, 50
Monsoons, 185, 195, 198
Moon, 100 et seq.
Moraines, 337
Morar, Loch, 339
Motion, first law of, 50; energy

of, S3
Mountains, of accumulation, 295;

of circumdenudation, 329 ; of

elevation, 303
Muds, oceanic, 269
Murray River, 372

NATURAL law, 14

Nature, 2, 13, 21, 23

Nautical Almanac, 20, 92

Neap tides, 114

Nearctic Realm, 411

Nebulas, 141, 142

Nebular hypothesis, 143

Negrito race, 423

Negro race, 423

Neotropical realm, 413

Neptune, 127, 131

Nevis, Ben, 163, 390; Loch, 339

Niagara Falls, 330

Nile, 318, 325, 375

Nimbus cloud, 168

Nitrifying ferment, 401

Nitrogen, 151, 152

North America, 363-369, 411

North equatorial currents, 243,
247, 248

OB River, 382

Objective things, 5

Oceanic currents, 242-249

Oceans, 215, 216

Old Red Sandstone, 346, 390

Oligocene rocks, 350

Onega, Lake, 388

On-shore and off-shore winds, 241

Ontario, Lake, 369

Oolitic limestone, 349

Oolitic Ridge, 391

Oozes, oceanic, 273

Orange River, 374

Ordnance Survey maps, 446

Organic evolution, 402

Oriental Realm, 414

Orinoco basin, 360

Oxus, see Amu Daria

Oxygen, 151, 153

Ozone, 153

PACIFIC Ocean, 216, 247, 259

Palaearctic Realm, 411

Palaeocrystic sea, 234

Palaeozoic rocks, 346

Palestine, 434

Pamir, the, 379, 380

Pampas, 406

Paraguay River, 362

Parallax, 97

Parallels of latitude, 91

Parana River, 362

Peat, 347

Pendulum, 54

Pennine Chain, 391

People, see Races ; of Europe, 426

Periodic law, 47

Permian rocks, 348

Perpetual motion, 55

Peru current, 247

Phases of the Moon, 101

Philosopher's Stone, 46

Photosphere, 116

Physiography, i, 6, 22, 23

Index

367

The Figures refer to the sections.

Planets, 126 et seq.

Plants, 396, 399, 404, 417

Pleistocene rocks, 352

Pliocene rocks, 350

Polar currents, 245 ; seas, 234

Polarity, 79, 88

Pole star, 90, 137

Poles, of the Earth, 88 ; magnetic,
98 ; of a magnet, 79

Post Tertiary, see Quaternary

Prairies, 367, 406

Precession of trie equinoxes, 115

Pressure, and change of state, 72 ;
of atmosphere, 147; and sea-
water, 226

Primary rocks, 346-348

Probability, 15

Prominences (solar), 116

Proteids, 399

Protoplasm, 398

Pteropod ooze, 274

Pumice, 294

Pyrenees, 385

QUARTZ, 30, 41
Quaternary rocks, 351

RACES of man, 422-427

Radiant energy, 60

Radiation and absorption, 63

Radiolarian ooze, 273, 276

Rain, 169, 312

Rain-band, 160

Rainfall, 200, 318 ; of world,

201 ; of British Islands, 204
Raised beaches, 284, 352
Reason, 8

Reaumur scale, 440
Red Clay, 277, 278
Red Sea, 279, 373; circulation,

237; temperature, 233, 236
Reflection and refraction, 61
Refraction, atmospheric, 150
Religion, 420
Rhine River, 386
Rhone River, 386
Rivers, 318-331 ; water, 221

" Roaring Forties," 180
Roches Moutonndes, 338, 351
Rock-basins, 332, 338, 339
Rocks, 285, 287-290
Rocky Mountains, 364
Roraima, Mount, 312
Rotation of the Earth, 87, 93, 94 ;
of the Moon, 102

SAHARA desert, 377, 406

Saima, Lake, 388

St. Anthony Falls, 330

St. Elmo's fire, 172

St. Lawrence River System, 369

Salinity of the ocean, 223, 224

Salt Lakes, 335

Salts, 44 ; of river -water, 221

of sea-water, 222
Sargasso Sea, 246
Saturn, 127, 130
Scale of maps, 445
Science, 3

Sciences, scope of, 21, 22
Scientific method, 9, 18
Scoriae, 294
Scotland, 390
Seas, classes of, 215 ; level, 252;

water, 222-227
Seasons, 120-123
Secondary rocks, 349
Sedimentary rocks, 288, 304,

345-351
Seiche, 239
Seismometers, 301
Selvas, 361, 407
Senses and their use, 7
Shoals, 262
Shooting-stars, 134
Sidereal time, 94, in
Sierra Madre, 364 ; Nevada,

364

Siliceous organisms, 273
Silurian rocks, 346, 390, 391
Simoom, 209
Slopes of continental edges, 263 ;

of land, 356
Snow, 170

;68

The Realm of Nature

The Figures refer to the sections.

Snow line, 163

Sogne Fjord, 339

Soil, 311

Solar spectrum, 117; system, 126

et seq. ; time, in; tides, 114
Solstices, 122, 123
Sound, 58

Soundings, deep-sea, 443
South America, 357-362, 413
South equatorial currents, 243,

247, 248

Southern Ocean, 216, 249
Southern Uplands of Scotland,

39°

Species, 396
Specific gravity, 29
Specific heat, 66, 227, 306
Spectrum, 62 ; analysis, 63 ; of

comets, 133; of stars, 138; of

Sun, 117
Springs, 314, 315
Spring tides, 114
Stars, 136 et seq.
States of matter, 68
Steam, 71
Steppe-lands, 406
Storm-warnings, 213
Strain, 35
Stratus cloud, 168
Stress, 35

Subjective things, 5
Subsidence and elevation of land,

284

Summer solstice, 122
Sun, 105 et seq.
Sunspots, 116
Superior, Lake, 333, 369
Synclines, 302, 303
Synthesis, 40
Syr Daria (Jaxartes), 382

TANGANYIKA, Lake, 376
Tarim basin, 381
Temperate forests, 408
Temperate zones, 125
Temperature, 65 ; of air, 187-191 ;
of British Islands, 203 ; of

Earth's crust, 291 ; of lakes,

228 ; of ocean, 233, 235 ; of

river entrances, 232 ; of seas,

236

Tension, surface, 39
Terms, 12
Terraces, pink and white, 316 ;

river, 321

Terrigenous deposits, 269
Tertiary rocks, 350
Theory, 18, 19
Thermograph, 440
Thermometers, 440
Thrust-planes, 302
Thunder, 173
Tian Shan Mountains, 381
Tibet Plateau, 380
Tibeto-Chinese race, 424
Tides, 103, 114; in bays, etc.,

219 ; currents, 218 ; oceanic,

217

Time, 95, 96, in
Tornado, 209
Torrens, Lake, 372
Torrents, 320
Torrid zone, 125
Trade winds, 179
Transitional area, 255, 263
Trias rocks, 349
Trigonometry, 33
Tropical, group of plants, 405 ;

forests, 407
Tropics, 122
Tsad basin, 377
Tuff, volcanic, 295
Tundras, 406
Tunnels, transalpine, 432
Tuscarora Deep, 260
Typhoons, 208

UNCONFORMABILITY, 342
Underground water, 313 ; tern

perature, 291
Ural Mountains, 384
Uranus, 127, 131

VALLEYS, 321, 327, 328

Index

369

The Figures refer to the sections.

Vapour pressure, 157, 158

Variation, magnetic, 98 ; bio-
logical, 403

Venus, 127

Vernal equinox, 121

Victoria desert, 406

Victoria Nyanza, 375

Volcanic action, 293 ; eruptions,
296 ; materials, 272, 294

Volcanoes, 295, 298

Volga River, 387

Volume, 29

WATER, 66, 69-71, 220; work

of, in Nature, 293, 309 et seq. ;

313. 3i8
Watershed, 319
Waterspouts, 210
Water-vapour, 71, 156, 157
Wave-length, 62 ; motion, 57,

61 ; sea waves, 239, 265
Weather, 207 ; charts, 211 ;

forecasts, 212
Weathering of rocks, 310
Weight, 38
Weights, 437
Wells, Artesian, 314
Whirlpools, 219
Whirlwinds, 209

White type of mankind, 425

Wind, 175 ; in British Islands,
202 ; and currents, 242 ; pre-
vailing, 193 - 198 ; work of,
307. 308

Windings of rivers, 323

Winnipeg, Lake, 368

Winter solstice, 123

Work, 49, 52 ; of rivers, 327-331;
of wind, 307, 308

World ridges, 256

Wrinkling of Earth's crust, 302

Wyville-Thomson ridge, 246

YANG - TSE - KIANG, 219, 319,

380

Year, no
Yeast, 401

Yellow River, 324, 331, 380
Yellowstone Park, 316, 364
Yellow type of mankind, 424
Yenisei, river, 382
Yukon, river, 364

ZAMBESI, river, 374

Zodiac, 112

Zones, of climate, 125 ; of vege-
tation, 405 ; of winds and
calms, 178

THE END

Printed by R. & R. CLARK, Edinburgh

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