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Full text of "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 
literary men, to University Professors, or to Extension Lecturers 
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 P ar * 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 rn f 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 
i s> _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- 
placinent 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 /S ilicon 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.T atic ref l action v 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, 
ir y m S 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^cftSS; ; 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 j ng once i n t h e same time as it 

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

secutively to the centre. same s i de to t h e 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 T ature 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 ^ 
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 20 ooo 1 ooooTT ^ tne s k y ' 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 ' G bread,I of"fh e e 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 . . 


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 stars v 
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," u an 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 
y 1 ^ 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 



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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 10 20 30 40 50 60 s 


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


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i 




/ 




/ 




10 

- / 


/ 

7& 

/ 


*) 




/ 









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 



i 3 6 



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 








/ 


^ 


"X, 


\ 
















/ 








^ 


\ 












/ 




^ 






-^ 


^Sy 












T\ 


ER 


K ft 


WA 


NS 


\ 




^ 







\ 




+0 

w'^s 


J 


LOFCT 


'ON 


mean 

ISL^S 


tern 


p. ( 


/ 








mdan temp 


40 J F 




\ 


*S^_ 



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 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 y 1 ^ 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 P er 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 n K Q r vptinnc: 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 t and 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 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 .-sio P es of the Gulf of Guinea, 
ing its gradient. These The X^tV^V 5 , 40 times the 1 i ori " 

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. 4 i.-Murray's Theory of the origin of P ol 7P S 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. 5 o.-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 


5 80 


23,800 


117 


Michigan 


55 55 


5 80 


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 


I 5'3 


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 80and 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 



2 9 



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 



3 i6 



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 / Ne g ro MONGOL- / Kalmuck 

N \Bantu TATAR \ Kirghiz 

NEGRITO TIBETO-CHINESE 

EASTERN -f? a P uan ,.._ FINNO- f? skimo 



Kelt 
Teuton 
Slav 
Hindu 

Arab 



/Malay TT AA/rrrTr f 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 the s 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 



34 2 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 
L 30 
^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- 



35 2 



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, 

345-35 1 
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 



3 6 9 



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 



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