er Geological Library
 
 
 presented to the 
 UNIVERSITY LIBRARY 
 UNIVERSITY OF CALIFORNIA 
 SAN DIEGO 
 
 by 
 
 DR. & MRS. W L GARTH
 
 ELEMENTARY LESSONS 
 
 PHYSICAL GEOGRAPHY
 
 ELEMENTARY LESSONS 
 
 IN 
 
 ( (i . 
 
 PHYSICAL GEOGRAPHY 
 
 BY 
 
 ARCHIBALD GEIKIE, LL.D., F.R.S. 
 
 Director-General of the Geological Survey of the United Kingdom, and 
 
 Director of the Museum of Practical Geology, Jermyn Street, 
 
 London ; formerly Murchison Professor of Geology and 
 
 Mineralogy in the University of Edinburgh 
 
 ILLUSTRATED WITH WOODCUTS AND TEN PLATES 
 
 HottDon 
 MACMILLAN AND CO. 
 
 AND NEW YORK 
 1892 
 
 The Right of Translation and Reproduction is Reserved.
 
 312575 
 
 First Edition prated and elcctrotyped 1877. 
 Reprinted 1878, 1880, 1882, August and October \yi,z, 
 Thoroughly revised and clectroty fed July 1884. 
 Reprinted 1886, 1887, 1889, 1890,
 
 PREFACE. 
 
 A SIMPLE but systematic and yet interesting descrip- 
 tion of the familiar features of the Earth's surface may 
 be regarded as a natural introduction to the teaching 
 of science. Treated from this point of view, Physical 
 Geography may be made a valuable instrument of 
 education. To give it such importance, the most 
 advantageous method is to make use of the common 
 knowledge and experience of the pupils, and, starting 
 from this groundwork, to train them in habits of 
 observation and in scientific modes of thought and 
 inquiry among every -day phenomena. From the 
 very outset the instruction should be as far as pos- 
 sible practical. A shower of rain, the growth and 
 disappearance of a cloud, the flow of a brook, the 
 muddy water of a river, the shape of a cliff, the out- 
 lines of a mountain, the undulations of a plain 
 these and the thousand other common features of 
 landscape should be eagerly seized by the teacher and 
 used as vivid illustrations of the broad fundamental 
 principles which it will be his main object that his
 
 viii PREFACE. 
 
 pupils should thoroughly master. Thus employed, 
 Physical Geography is not learnt as an ordinary task, 
 but rather becomes a delightful recreation, in which, 
 however, the observing faculty is exercised, the power 
 of induction cultivated, and the imagination kept 
 constantly active. 
 
 Having been long convinced that such a method 
 of instruction would place this branch of science upon 
 a firmer and broader footing in our educational system, 
 and would moreover prove of great service in foster- 
 ing a spirit of observation and reflection even among 
 children, I projected many years ago the Primer of 
 Physical Geography, published in the series of Science 
 Primers. The continued sale of large impressions of 
 that little work encourages the hope that the method 
 advocated has been found successful in practice. 
 
 The present volume may be regarded as a further 
 development of the same plan of instruction. As its 
 title implies, it still deals mainly with the broad ele- 
 mentary questions of Physical Geography. It would 
 have been impossible to find a place within its Lessons 
 for the treatment of every branch of the wide subject, 
 and as impossible, had it even been desirable, to 
 bestow equal fulness upon every branch for which 
 room has been made. I have devoted most space to
 
 PREFACE. ix 
 
 those aspects of the science which, in my own ex- 
 perience, have been found best suited for practical 
 instruction. While as much general information as 
 may be feasible should be communicated to them, 
 young pupils cannot of course be expected to find the 
 same interest in all divisions of the subject. It is of 
 far more consequence to awaken in them a taste for 
 such pursuits, and lead them to carry on the study of 
 their own accord, than to try to charge their memories 
 with dry facts and figures which, in the absence of 
 intelligent and suggestive teaching, are too commonly 
 meaningless and repulsive. 
 
 While, therefore, adhering to a systematic treat- 
 ment, I have been led to dwell, for example, on the 
 phenomena of the atmosphere at much greater length 
 than is usual in elementary class-books. These 
 phenomena are among the most familiar and uni- 
 versal features of the globe ; examples of them 
 can be constantly adduced, and they may thus be 
 used with singular advantage to illustrate how the 
 facts of science are observed and its laws are 
 deduced. 
 
 I acknowledge with pleasure my obligations to my 
 friend Mr. Buchan, who not only kindly allowed me 
 to make use of his Charts of Atmospheric Pressure
 
 x PREFACE. 
 
 and Temperature, but who also read over the proof- 
 sheets of the first two chapters and gave me valuable 
 suggestions on subjects regarding which he is so high 
 an authority. 
 
 The present Edition has been carefully revised 
 throughout. Two new Plates have been added, one 
 (from the " Challenger " results) showing the form of 
 the great sea-basins as revealed by recent deep-sea 
 research ; the other (from Professor Loomis' Chart) 
 indicating the general distribution of rain over the 
 globe. 
 
 May 1884.
 
 CONTENTS. 
 
 PAGE 
 
 INTRODUCTION . . . . i 
 
 CHAPTER I. THE EARTH AS A PLANET. 
 
 LESSON I. The Earth's Form 8 
 
 ,, II. The Earth's Motions n 
 
 ,, III. The Earth and the Sun . . . .17 
 
 ,, IV. Measurement and Mapping of the Earth's 
 
 Surface . . . . . .24 
 
 ,, V. A General View of the Earth ... 32 
 
 CHAPTER II. AIR. 
 
 LESSON VI. Its Composition 38 
 
 VII. The Height of the Air . . . -45 
 ,, VIII. The Pressure of the Air .... 47 
 ,, IX. The Temperature of the Air ... 54 
 
 ,, X. The Moisture of the Air .... 64 
 
 ,, XL The Movements of the Air . . 83 
 
 CHAPTER III. THE SEA. 
 
 LESSON XII. The Great Sea-basins . . . .103 
 
 ,, XIII. The Saltness of the Sea . . 113 
 
 ,. XIV. The Depths of the Sea . . . 119 
 
 ,, XV. The Temperature of the Sea . . -125 
 
 XVI. The Ice of the Sea 129 
 
 ,. XVII. The Movements of the Sea . . . 137 
 
 XVIII. The Offices of the Sea . . . .152
 
 CONTENTS. 
 
 CHAPTER IV. THE LAND. 
 
 PAGE 
 
 LESSON XIX. Continents and Islands . . . 162 
 ,, XX. The Relief of the Land Mountains, 
 
 Plains, and Valleys . . 173 
 
 ,, XXI. The Composition of the Earth . .182 
 
 ,, XXII. Volcanoes ...... 196 
 
 XXIII. Movements of the Land . . . 210 
 XXIV. The Waters of the Land Part I. 
 
 Springs and underground Rivers . 222 
 ,, XXV. The Waters of the Land Part II. 
 
 Running Water Brooks and Rivers . 244 
 XXVI. The Waters of the Land Part III. 
 
 Lakes and Inland Seas . . . 258 
 XXVII. The Waters of the Land Part IV. 
 
 The Work of Running Water . . 272 
 XXVIII. The Waters of the Land Part V. 
 
 Frost, Snow-fields, Glaciers . . 293 
 ,, XXIX. The Sculpture of the Land . . .314 
 
 CHAPTER V. LIFE. 
 
 LESSON XXX. The Geographical Distribution of Plants 
 
 and Animals 328 
 
 XXXI. The Diffusion of Plants and Animals- 
 Climate Migration and Transport 
 Changes of Land and Sea . . 337 
 
 INDEX ..... . 357
 
 LIST OF ILLUSTRATIONS. 
 
 PLATES. 
 
 'TO FACE PAGE 
 
 PLATE I. The World in Hemispheres 24 
 
 ,, II. Pressure of the Atmosphere over the Globe in January . 46 
 
 ,, III. Pressure of the Atmosphere over the Globe in July . 50 
 
 ,, IV. Isothermal Lines over the Globe for January . . 54 
 
 ,, V. Isothermal Lines over the Globe for July ... 60 
 
 VI. Map showing the mean Annual Rainfall of the Globe . 76 
 
 ., VII. The Ocean Basins ...... . 103 
 
 ., VIII. Ocean Currents ........ 144 
 
 JX. Distribution of Earthquakes and Volcanoes over the 
 
 Globe .......... 200 
 
 ,, X. Zoological Provinces of the Globe 328 
 
 WOOD-CUTS. 
 
 FIG. PAGE 
 
 Ocean-Waves Frontispiece 
 
 1. Curvature of the Earth's Surface, as shown by ships at sea . . 9 
 
 2. The Earth and Moon as seen from space jo 
 
 3. The Earth's path round the Sun 14 
 
 4. Plan of Volcanic Hills and Craters in the Bay of Naples . . 19 
 
 5. A part of the Surface of the Moon, showing Volcanic Craters . 20 
 
 6. Measurement and Mapping of a Country by means of Triangu- 
 
 lation 31 
 
 7. What is seen after some raindrops collected in a town are evapor- 
 
 ated, and the residue is placed below a microscope ... 40
 
 xiv LIST OF ILLUSTRATIONS. 
 
 FIG. PACK 
 
 8. Diagram showing the influence of the varying thickness of the 
 
 atmosphere in retarding the Sun's heat ..... 57 
 
 9. Clouds condensed by the Cliff of the Noss Head, Shetland, with 
 
 a clear sky and a S. E. wind ....... 73 
 
 10. Snow-flakes ........... 80 
 
 11. Position and height of the Snow-line between Equatorial Africa 
 
 and the North Polar Seas 81 
 
 12. Sand-dunes ridges of dry sand blown inland off the shore by the 
 
 wind 100 
 
 13. What is seen when a drop of concentrated sea-water is evaporated 
 
 under a microscope 115 
 
 14. Some of the Ooze of the Atlantic floor, magnified . . . . 124 
 
 15. Iceberg at Sea 130 
 
 16. The Birthplace of the Arctic Icebergs 132 
 
 17. Scene among the disrupted ice of the frozen Arctic Sea . . 134 
 
 1 8. The Ice-foot of Greenland . . . . . . . .136 
 
 19. Diagram illustrating the origin of the tides . . < . .149 
 
 20. Diagram showing the relation of the beach to the lines of high and 
 
 low water 152 
 
 21. View of the sea-cliffs south of the river Tyne (magnesian limestone 
 
 worn away by the waves, and the isolated fragments left) . . 159 
 
 22. Steep shore descending abruptly into deep water .... 167 
 
 23. Low shore shelving into shallow water 167 
 
 24. Section of one side of a Continent . . . . . . -175 
 
 25. Section to show the formation of soil and subsoil from rotting away 
 
 of rock underneath 183 
 
 26. Bedded arrangement of rocks 185 
 
 27. Piece of shale containing portion of a fossil fern .... 186 
 
 28. Piece of limestone, showing how the stone is made up of animal 
 
 remains 187 
 
 29. Some of the grains of a piece of chalk 188 
 
 30. A piece of granite, showing the composition of a Crystalline Rock 188 
 
 31. View of the geysers of Iceland. Great Geyser in eruption . . 193 
 
 32. Plan of the Peak of Teneriffe, showing the large crater, partly 
 
 effaced, and smaller craters with lava currents issuing from 
 
 them ............ 198 
 
 33. View of a Street in Pompeii 199 
 
 34. View of Vesuvius as seen from Naples during the eruption of 1872 201 
 
 35. Mount Vesuvius as seen from the sea, with the remaining part of 
 
 the old crater of Somma behind 202
 
 LIST OF ILLUSTRATIONS. 
 
 FIG. I'AGE 
 
 36. Houses surrounded and partly demolished by the lava of Vesuvius, 
 
 1872 204 
 
 37. View of the extinct volcanoes of Central France, taken from the 
 
 Puy de Pariou 208 
 
 38. House rent by earthquake (Mallet) ...... 215 
 
 39. Diagram-section to illustrate the propagation of an earthquake- 
 
 wave and the mode of calculating the depth of its origin . . 215 
 
 40. View of an old sea-terrace or raised beach, with sea-worn caves 
 
 on its inner margin . . . . . . . . .217 
 
 41. Raised sea-terraces of the Alien Fjord, Norway .... 218 
 
 42. Section of an island with a fringing coral reef .... 219 
 
 43. Section of an island with a Barrier reef 219 
 
 44. Section of an Atoll or coral island built over submerged land . 220 
 
 45. View of an Atoll or coral island 220 
 
 46. Section across a valley to show how the simplest kinds of springs 
 
 arise ............ 228 
 
 47. Section to show how deep-seated springs arise . - . . . 229 
 
 48. Section to show the intricate underground drainage which issues 
 
 in a deep-seated spring . 230 
 
 49. Section to the position of the water-bearing rocks below the clay 
 
 at London . 231 
 
 50. Section across the cliff and landslip of Antrim . . . 242 
 
 51. View of part of the cliffs and landslip of Antrim . . . 242 
 
 52. Delta of the Nile 246 
 
 53. Windings of the Mississippi 249 
 
 54. Part of the Island of Lewis, illustrating the abundant lakes of the 
 
 north-west of Europe 259 
 
 55. Section of a lake-basin excavated in solid rock .... 260 
 
 56. Section of a lake-basin lying in a hollow of superficial detritus . 261 
 
 57. Section of a lake dammed up by a barrier of earth or gravel . 261 
 
 58. Prints made in soft mud or moist sand by rain-drops . - . 273 
 
 59. Earth pillars of the Tyrol ........ 274 
 
 60. View of ravines cut by streams out of a table-land . . . 277 
 
 61. Cascade and pot-holes of a water-course ..... 278 
 
 62. Section of a waterfall and ravine ....... 279 
 
 63. Terraces of gravel, sand, and mud, left by a river . . . 287 
 
 64. Delta of the Mississippi ......... 292 
 
 65. Snow-field and glaciers of Holands Fjord, Arctic Norway . . 301 
 
 66. View of a glacier, with its lines of rubbish (moraines) and the 
 
 river which escapes from its end 302
 
 LIST OF ILLUSTRATIONS. 
 
 FIG. PAGE 
 
 67. Plan of the Mer-de-Glace of Chamouni and its tributary glaciers, 
 
 showing the way in which lateral moraines become medial . 305 
 
 68. Glacier table a pillar of ice supporting a block of stone . . 308 
 
 69. The Pierre-a-Boat, near Neufchatel 309 
 
 70. Glacier descending to the sea. Head of Jokuls Fjord, Arctic 
 
 Norway 310 
 
 71. Stone polished and striated under glacier ice .... 313 
 
 72. Quarry in flat stratified rocks ........ 316 
 
 73. Section across a mountain-chain to show how the level rocks of 
 
 the plains are bent and inverted along the flanks of the moun- 
 tain, while the lowest and oldest rocks are made to form the 
 central and highest point of the chain 318 
 
 74. Section across a mountain-chain, showing two successive periods 
 
 ofuplift ........... 319 
 
 75. Section across a mountain-chain showing three successive periods 
 
 ofuplift ... 320 
 
 76. Scene on the Coast of Caithness 322 
 
 77. Portion of the west front of Salisbury Crag, Edinburgh . . 323 
 
 78. Vertical distribution of climate on mountains .... 343
 
 ELEMENTARY LESSONS 
 
 IN 
 
 PHYSICAL GEOGRAPHY. 
 
 INTRODUCTION. 
 
 1. At night, when the sky is clear, the largest stars 
 seem to stand out in front, with others less in size and 
 brightness crowding behind them. As we gaze into these 
 depths, still remoter and feebler twinkling points appear, 
 until at last our eyes can no longer shape out any distinct 
 specks of light. Such a sight impresses our minds, as 
 nothing else can do so vividly, with the vastness of the 
 Universe. We feel how comparatively small must, after 
 all, be the distance which we can see into that " star- 
 dust " which has been sown through the regions of space. 
 And even when, with the aid of a good telescope, we 
 return to these same skies, it is to find more cause than 
 ever to acknowledge how immeasurably vast is that part 
 of the Universe which man can thus explore ; but at the 
 same time, to meet again with a vague limit, beyond 
 which we cannot see, not because we have reached the 
 utmost verge of creation, but because our instruments 
 can carry our vision no farther. Far beyond that limit, 
 it may be that the regions of space contain other stars 
 and systems, though too remote ever to be brought into 
 <E B
 
 PHYSICAL GEOGRAPHY. 
 
 view eyei) by the most powerful telescopes which human 
 skill could construct. 
 
 Astronomers have calculated the distance of some 
 of tite "largest and nearest stars. But their figures, ex- 
 pressing sums of many millions of miles, are too vast to 
 carry* any definite ideas to our minds. When we reflect 
 
 'that each of those stars, from the brightest to the faintest 
 twinkling point, is really a sun, many of them, no doubt, 
 
 .far vaster in size than our sun, but dwindled into such 
 seeming feebleness by reason of their inconceivable dis- 
 tance, we cannot but feel what a little speck of dust, in 
 comparison, must this dwelling-place of ours really be 
 which we call the Earth. 
 
 2. It is useful to get this comparative insignificance 
 of the Earth firmly realised by our minds. And in no 
 way can this be done so well as by watching the starry 
 sky, and learning what has been discovered regarding 
 the motions, sizes, and distances of the heavenly bodies. 
 What, then, is the Earth in relation to these bodies ? 
 Has it always been in the same condition as now, or has 
 it perhaps passed through long ages of change and pro- 
 gress ? Mankind has had a long and varied history. 
 May not the Earth itself have had one also ? If so, can 
 we learn anything about the story of the Earth ? 
 
 3. Again, when, on the other hand, we look upon the 
 face of the Earth by day, how boundless and varied it 
 seems ! From the district in which we may chance to 
 live, we can pass in thought to the country at large, then 
 to other countries, and then to the idea of the whole wide 
 globe, with its continents and oceans, its mountains, 
 valleys, and plains, and all that wonderful diversity of 
 form and colour which makes its surface so unceasingly 
 beautiful. 
 
 4. This variety is everywhere associated with life 
 and movement. Consider, for instance, the unvarying 
 succession of day and night ; the orderly march of the 
 seasons ; the constant or fitful blowing of the winds ; the
 
 INTRODUCTION. 
 
 regular circling of the ocean tides ; the ceaseless flow of 
 rivers ; the manifold growth and activity of plant and 
 animal life ! Surely it was no strange thought when 
 men in old times pictured this world as a living being. 
 And even though we cannot look on the earth as a living 
 thing in the sense in which a plant or animal is so called, 
 yet in view of all that multitudinous movement which is 
 ever in progress upon its surface, and on which, indeed, 
 we know that our own existence depends, there is evi- 
 dently another sense in which we may speak of the life 
 of the Earth. 
 
 5. Now this Life of the Earth is the central thought 
 which runs through all that branch of science termed 
 Physical Geography. 1 The word geography, as ordin- 
 arily used, means a description of the surface of the 
 earth, including its natural sub -divisions, such as con- 
 tinents and oceans, together with its artificial or political 
 sub-divisions, such as countries and kingdoms. But 
 Physical Geography is not a mere description of the 
 parts of the earth. It takes little heed of the political 
 boundaries except in so far as they mark the limits of 
 different races of men. Nor does it confine itself to a 
 mere enumeration of the different features of the surface. 
 It tries to gather together what is known regarding the 
 Earth as a heavenly body, its constitution, and probable 
 history. In describing the parts of the earth air, land, 
 and sea it ever seeks so to place them before our 
 minds as to make us realise, not only what they are in 
 themselves, but how they affect each other, and what 
 part each plays in the general system of our globe. Thus 
 Physical Geography endeavours to present a vivid picture 
 of the mechanism of that wonderfully complex and har- 
 monious world in which we live. 
 
 6. Many of the facts with which this branch of science 
 deals are familiar to every one. With nothing, for exam- 
 
 1 This term as here used is synonymous with Physiography, which has 
 been proposed in its stead.
 
 PHYSICAL GEOGRAPHY. 
 
 pie, have we better acquaintance than with the air which 
 surrounds us. We have breathed it every moment of 
 our lives ; we know it at rest as well as in storms ; we 
 have watched the growth in it of mists and clouds, as 
 well as the fall of dew and rain. At first there might 
 seem hardly anything more perfectly known to us, and 
 therefore regarding which we should have so little new 
 to learn. But Physical Geography, taking up the sub- 
 ject as one great whole, strives to show how all the 
 different conditions and changes of the air are connected 
 together, to explain their causes, and to point out the 
 essential part which the air takes in the great move- 
 ments that affect both sea and land. One of the great 
 advantages of this study lies in the very commonplace 
 character of the phenomena of which it treats. We can- 
 not go anywhere without meeting with illustrations of 
 some of the great lessons which it enforces. When we 
 have once practically learnt these lessons, therefore, we 
 carry an additional source of pleasure in every walk and 
 journey. We get into the habit of using our eyes, and 
 noting an endless variety of things that would other- 
 wise have been passed by unseen, and this habit of 
 observation, besides the pleasure its exercise brings 
 with it, will be found of no slight service in the ordinary 
 affairs of life. 
 
 7. In completing its picture of the Life of the Earth, 
 Physical Geography must necessarily draw its illustra- 
 tions from all parts of the world. It thus brings before 
 us phenomena of which we may have no experience in 
 our own country, and which, very probably, we may 
 never have any opportunity of seeing for ourselves. In 
 the study of it we journey, so to speak, all over the world, 
 and learn in a short time far more about the world than 
 we should be able to do from reading a few ordinary 
 books of travel. Indeed, a good treatise on Physical 
 Geography may be regarded as a condensed and well- 
 arranged book of travels in all countries, with this dis-
 
 INTRODUCTION. 
 
 tinction, that although it has no personal adventures to 
 describe, it enables us to understand how one region of 
 the earth differs from another, and it explains these differ- 
 ences by connecting them all together and showing their 
 relation to the general principles or laws on which they 
 depend. So that, although a man may never have been 
 in India or Africa, or the Arctic regions, he may, from 
 the study of Physical Geography, have a far better notion 
 of the general features of these countries, and why they 
 differ from each other so much in climate, than many other 
 men who have travelled to, or even have lived for years 
 in them. It is a matter of no little encouragement for 
 all of us to know that the more we watch what takes place 
 around us in our own country, and the more thoroughly 
 we understand it, the more easily do we realise what goes 
 on in other and distant parts of the world. 
 
 8. By taking up the study of Physical Geography, not 
 merely as a subject to be learnt from books, but as a 
 practical pursuit to be followed out by our own observa- 
 tions, as opportunity offers in the course of our daily 
 occupations, we make most progress in it and get the 
 largest amount of pleasure out of it. This is the spirit 
 in which the following chapters are written. They are 
 not meant merely to describe the different parts of the 
 earth in such a way that these may most easily be learnt 
 by heart, but rather to incite the learner to use his own 
 eyes, and to examine, compare, and contrast what he sees 
 to take place from day to day. They are so arranged as 
 to begin, where practicable in each sub-division, with our 
 common knowledge. Then they point out what can be 
 ascertained on the subject by our own simple observation 
 and experiment. Lastly, they present such further in- 
 formation as may be acquired from the observations and 
 travels of men who have given much time and thought 
 to the collection and investigation of the facts. In the 
 case of the Air, for example, starting from what each one 
 of us knows by everyday experience, we proceed to con-
 
 PHYSICAL GEOGRAPHY. 
 
 sider what we can ourselves easily find out about the 
 air, and from this basis of knowledge we follow what has 
 been still further discovered by prolonged investigation 
 in all parts of the world. 
 
 9. At the outset it may be well to group together in 
 due order the different subjects which, coming within the 
 scope of Physical Geography, will have to be attended 
 to in these Lessons. 
 
 10. First of all we shall consider what the Earth is, 
 as a heavenly body, how it is related to other heavenly 
 bodies, and specially to the Sun, as the source of light 
 and heat. 
 
 11. Secondly, looking at the Earth in itself, we find it 
 wrapped round with an outer envelope of Air, which will 
 next deserve our attention. What this envelope con- 
 sists of, and the part it takes in the phenomena of the 
 Earth's surface, furnish materials for much interesting 
 inquiry. 
 
 12. Thirdly, beneath the surrounding shell of air, and 
 covering the greater part of the surface of the solid 
 globe, lies that vast expanse of water known as the Ocean. 
 We shall follow its tides and currents, and trace the 
 vapour which, ascending from its surface, is carried 
 through the air until it falls as rain and snow upon the 
 land, whence it is borne by rivers back again into the 
 ocean. The wonderful beauty and high importance of 
 this circulation will claim our attention. 
 
 13. Fourthly, the solid Land will be considered, with 
 its continents and islands, its mountains and valleys, its 
 earthquakes and volcanoes. The evidences of continual 
 change on the surface of the land will lead us back to 
 the action of the air and of water, and we shall find how 
 largely the forms even of " the everlasting hills " have 
 been determined by that action. 
 
 14. Fifthly, we shall inquire what Climate is, how 
 different kinds of climate are distributed over the globe, 
 and whether any causes can be assigned to account for
 
 INTRODUCTION. 
 
 such differences. This will lead us to note that as there 
 is a geographical distribution of climates, so likewise is 
 there one of plants and animals. Each great region of 
 the earth's surface which has a peculiar climate of its 
 own, has also its own distinguishing assemblage of pecu- 
 liar plants and animals. Even in the way in which the 
 races of man are grouped over the earth there is evi- 
 dence of the same connection between the distribution 
 of climates and of life. Thus the geographical distri- 
 bution of Life over the earth's surface will form the 
 concluding part of these Lessons.
 
 PHYSICAL GEOGRAPHY. [LESS. 
 
 CHAPTER I. 
 
 THE EARTH AS A PLANET. 
 
 LESSON I. The Earth's Form. 
 
 1. What then is this Earth on which we live ? The 
 answer to this question must be chiefly given by astro- 
 nomy, but there are some familiar features in the earth 
 itself which, if rightly considered, help to make the 
 answer more vividly realised. Evidently there is much 
 advantage in obtaining a clear idea of what the earth is 
 as a whole, before we begin to trace the events which 
 take place upon its surface. 
 
 2. In the first place, then, the earth, like the sun and 
 moon, is a globe. This may be proved in various ways. 
 
 (1) If we set sail from England, and steer westward 
 without turning back, we shall eventually find ourselves 
 in England again. This is called Circumnavigating 
 the world. It would not be practicable unless the 
 earth were really a globe. 
 
 (2) Standing by the margin of the sea, and looking 
 seawards, we observe that a ship which is moving away 
 from us, by and by seems to sink into or under the sea. 
 (Fig. i.) The hull first disappears, and then by degrees 
 the sails. So, on the other hand, when a vessel ap- 
 proaches us, we can first, with a telescope, make out the 
 tops of the sails and masts just peeping above the dis- 
 tant surface of the ocean. Little by little the sails rise 
 as it were out of the water, until at last the hull and the 
 whole vessel come into sight. This could not happen
 
 I.] 
 
 THE EARTH'S FORM. 
 
 unless the surface of the sea instead of being flat were 
 really curved, that is, part of the curved surface of a 
 globe. 
 
 (3) If the surface of the earth were flat, as men once 
 supposed it to be, the sun would rise at the same time at 
 all places on the earth. But this is not the case ; sun- 
 rise takes place later or earlier as we travel west or east. 
 Again, on ascending a mountain we should see the whole 
 of the earth's surface if that surface were really a plain. 
 But the extent to which we see depends on the height to 
 
 FIG. i.. Curvature of the Earth's Surface, as shown by ships at sea 
 
 which we climb. This shows that the earth must be a 
 globe. 
 
 (4) When the earth comes directly between the sun 
 and moon, so as to cut off the light of the former from 
 the latter, the moon is said to be eclipsed. Now, the 
 earth's shadow as it creeps over the moon's surface, is 
 seen to be circular, and hence the real form of our planet 
 must be that of a globe. 
 
 3. Were it possible for us to quit the earth and look 
 back upon it from a distance of a few millions of miles, 
 it would appear as a large bright moon, hanging in space, 
 with the stars twinkling on all sides of it. Its surface 
 would shine like that of the moon, with the light falling 
 on it from the sun, and would be seen to be marked by
 
 PHYSICAL GEOGRAPHY. 
 
 [LESS. 
 
 irregular bright and dark strips and patches. There 
 would be two specially bright parts on opposite sides of 
 the earth's body, marking the great regions of snow and 
 ice at the north and south poles. Between these two 
 areas certain bright irregular bands would indicate the 
 
 FIG. 2. The Earth and Moon as seen from space. 
 
 position of the dry land, while the intervening darker 
 portions would show the extent of the seas (Fig. 2). It 
 we could transport ourselves still farther away ; if, for 
 instance, we could get as far off as the sun, that is to 
 say more than ninety-two millions of miles from where 
 we are living at this moment, our earth would be seen 
 shining merely as a bright star. And were it possible
 
 ii.] THE EARTH'S MOTIONS. n 
 
 for us to reach one of the nearest fixed stars, our globe 
 would be no longer visible, while the sun itself, if seen 
 at all, would appear merely as a twinkling star. 
 
 4. But the earth is not a perfectly regular sphere. 
 In particular it is somewhat flattened on two opposite 
 sides, so as to resemble an orange in shape. How this 
 is known and measured will be seen from Lesson IV. 
 A line from the middle of one of these flattened sides 
 passing through the centre of the planet to the middle 
 of the opposite flattened side, is called the Axis, and 
 the points at which it reaches the surface are known 
 respectively as the North Pole and South Pole. A 
 line drawn midway between the poles round the bulging 
 part of the earth is termed the Equator. 
 
 LESSON II. The Earth's Motions. 
 
 1. So long as men in early times looked upon the 
 earth as a great plain placed in the centre of the uni- 
 verse, with sun, moon, and stars moving over and under 
 it every day and night, they could not for a moment 
 imagine that the Earth itself is moving. But the truth 
 in this matter has now been so long known, that we 
 have no difficulty in seeing how the apparent movements 
 of the sun and stars are explained by the real move- 
 ments of the earth. Of these movements the two chief 
 are called Rotation and Revolution. 
 
 2. Rotation. The most obvious movement of our 
 earth is its rotation on its axis, to which the succession 
 of day and night is due. A complete rotation is per- 
 formed in about twenty-four hours. During that time 
 each part of the earth's surface is alternately presented 
 to, and turned away from, the sun. As the sun rises in 
 the east to give us day, and appears to travel westward 
 across the sky, the real motion of the earth must be in 
 the reverse direction, or from west to east.
 
 12 PHYSICAL GEOGRAPHY. [LESS. 
 
 3. It is evident that as the earth rotates, the rate of 
 motion and the distance travelled will vary for different 
 parts of the surface according to their nearness to or dis- 
 tance from the axis. When a cart-wheel, for example, 
 is made to spin round on its axle, a mark made on its 
 rim must move faster and describe a much larger circle 
 than one made near the axle. Places situated on the 
 earth's equator must have the maximum velocity and 
 travel over the greatest distance, while at either pole the 
 velocity must be nil. The circle described by a place 
 on the equator is indeed the whole circumference of the 
 globe. So that when the number of miles in the cir- 
 cumference is divided by the number of hours required 
 for a complete rotation, we obtain in figures the rate 
 of motion. At the equator this rate is found to be 
 upwards of 500 yards in a second ; it gradually dimin- 
 ishes to the poles. 
 
 4. Now if the earth is rotating with such speed, why 
 are we not jerked off its surface ? When a stone is 
 thrown into the air, why does it not immediately rush 
 away into space instead of falling back again to the 
 ground ? Because what is called the earth's attrac- 
 tion is far stronger than this tendency to fly off. Every- 
 thing in and on the surface is pulled down towards the 
 centre of the earth by this attracting force, which is 
 called gravity. But it will be seen that the influence 
 of rotation is nevertheless unmistakably marked upon 
 the great atmospheric currents, since these have their 
 direction modified by it (Lesson XI. Art. 13). 
 
 5. Revolution. The earth revolves round the sun 
 and takes about 365 days to perform the circuit. What 
 we call a year is simply the length of time taken by the 
 earth to make one complete revolution round the sun. 
 The path which it follows, called its orbit, has been 
 ascertained not to be a perfect circle but an ellipse, so 
 that our globe is at one part of its course nearer to the 
 sun than at another, its mean distance being computed
 
 n.] THE EARTH'S MOTIONS. 13 
 
 at 92,800,000 miles. - From this number and from the 
 time taken for a single revolution it is easy to find the 
 average rate at which our globe must rush along its 
 orbit. That rate is about 68,000 miles in an hour. 
 
 6. There are certain circumstances connected with the 
 movement of revolution which serve to explain why the 
 days and nights throughout the year are not all of the 
 same length, and why, instead of perpetual summer or 
 winter, there is the regular succession of the Seasons. If 
 the axis of the earth were perpendicular to the plane of 
 the orbit, that is, if it moved along in a perfectly upright 
 position, there would be equal day and night all the year 
 round all over the globe. But it is really inclined to the 
 path of the planet round the sun at an angle of about 
 66 and remains always parallel to itself, that is, always 
 points to the same star. In the summer of the northern 
 half of the globe, the North Pole is turned towards the 
 sun, in winter it is turned away from it, the South Pole 
 being at the same times in the opposite positions. 
 Hence while midway between the two poles the day 
 and night remain each of twelve hours' duration, they 
 depart more and more from this uniformity towards the 
 poles, until there we have a day lasting for one half of 
 the year, and a night lasting for the other half. 
 
 7. About the 22d of March, and again about the 22d 
 of September, the earth is so placed in its orbit that the 
 sun is exactly vertical over the equator. The line be- 
 tween the sun-lit half and the dark half passes through 
 the poles. At these times, therefore, day and night are 
 each of twelve hours' duration over the whole globe ; and 
 these points in the year are accordingly called Equi- 
 noxes, that is, "equal nights." After the March equi- 
 nox, the northern parts of the globe, as the earth moves 
 round in its orbit, come more and more into the sun- 
 light ; in other words, the days get longer and longer 
 until, at the North Pole and for a certain space all round 
 it, within what is called the Arctic Circle (see Plate
 
 PHYSICAL GEOGRAPHY. 
 
 [LESS. 
 
 I.), the sun does not set at midsummer at all, and there 
 is continuous daylight The farther north we go the 
 longer are the days in June, so that a summer-day in 
 England differs very much in length from one in India, 
 and one in the north of Scandinavia from one in England. 
 
 FIG. 3. The Earth's path round the Sun. 
 
 On the other hand, when the earth has reached the 
 September equinox, the North Pole begins to point away 
 from the sun, and the days in the northern part of the 
 earth begin to grow shorter, till in midwinter, at the pole 
 and over the ground within the Arctic Circle the sun 
 does not rise, and there is continued night. The farther 
 north we go in December, therefore, the longer are the
 
 ii.] THE EARTH'S MOTIONS. 15 
 
 nights ; so that while a summer-day in London is not so 
 long as a summer-day in the north of Scotland, a winter- 
 day is considerably longer. 
 
 8. It is evident that when the North Pole is enjoying 
 perpetual daylight, the South Pole must be in continuous 
 night. It is only at the two equinoxes that their day 
 and night are equal. As the days about the North Pole 
 shorten, those at the opposite pole lengthen, and then 
 the reverse takes place, and so on from year to year. 
 
 9. The alternation of the Seasons depends, in like 
 manner, upon the inclination of the earth's axis in its 
 yearly orbit. At one part of the year, people in Europe 
 and North America see the sun comparatively low in the 
 sky : his rays are then but feeble ; the climate is cold, 
 and then comes the familiar sight of the frost and snow 
 of winter. Six months later the sun appears much 
 higher in the sky ; that is, more directly overhead : his 
 rays are then warm -even scorching, and we find our- 
 selves in the midst of summer. 
 
 10. In reality, however, it is not the sun which has 
 changed place, but our planet, which has reached dif- 
 ferent parts of its orbit, and consequently has presented 
 itself in different positions towards the sun. In summer 
 the days in the Northern Hemisphere (as the half 
 of the globe between the equator and the North Pole is 
 called) are long, because the North Pole is turned to- 
 wards the sun. That hemisphere is consequently warmed. 
 The rays of the sun come more directly down upon it ; 
 the long days allow much more heat to be received from 
 the sun, and the short nights permit much less heat to 
 be given off from the earth than in winter. While this 
 state of things in the Northern Hemisphere brings sum- 
 mer there, precisely the opposite effects are taking place 
 in the Southern Hemisphere. There, in proportion as 
 the days lengthen and the warmth increases in the North, 
 the days shorten and the heat decreases, until at the mid- 
 summer of the Northern half of the globe, the people at
 
 16 PHYSICAL GEOGRAPHY. [LESS. 
 
 the Antipodes, that is, on the opposite side, or Southern 
 Hemisphere, are in their midwinter. Christmas, which 
 in Europe and North America is always associated with 
 the frost and cold of midwinter, happens at the mid- 
 summer of countries in the Southern Hemisphere, like 
 Australia and New Zealand. 
 
 11. It is evident that the contrast between summer 
 and winter must be most marked in the region round 
 each pole, where indeed the year may be looked upon 
 as consisting of only two seasons, one of daylight and 
 summer, the other of night and winter. In the equato- 
 rial belt, too, that is on either side of the equator as far 
 as the lines called the tropics, there is often a striking 
 difference between the seasons, according to the position 
 of the sun in the sky. At each equinox the sun appears 
 vertical over the equator. From March to June he 
 seems to be travelling northward until about the 22d of 
 the latter month, when he shines vertically over a line 
 called the Tropic of Cancer, 23! north of the equator. 
 He then turns southward, is again vertical above the 
 equator at the September equinox, after which he travels 
 southward until about the 22d of December, when he 
 shines directly overhead along the line known as the 
 Tropic of Capricorn, 23^ south of the equator. The 
 word tropic means " a turning " and is applied because 
 the sun after appearing to travel away from the equator 
 begins to turn at these two limits on each side. The 
 sun seems to be constantly travelling to and fro across 
 the sky between the two lines of the tropics. This ap- 
 parent movement of the sun is of course due to the real 
 movement of the earth, and the limit within which the 
 sun travels north and south of the equator is fixed by the 
 angle of inclination of the earth's axis (66^). 
 
 12. In the belt between the tropics the sun appears 
 vertical in the sky twice in each year. With each of 
 these periods comes a rainy season, and between them 
 a dry and hot season. So that if no other influences
 
 in.] THE EARTH AND THE SUN. 17 
 
 came into operation the equatorial regions would have 
 two wet and two dry periods in each year. Owing, 
 however, to the way in which the wind currents are in- 
 terfered with by the masses of high land, it is in the 
 oceanic regions that this arrangement of the seasons is 
 most completely carried out. 
 
 LESSON III. The Earth and the Sun. 
 
 1. Long before astronomy or any science had arisen, 
 the early races of man, seeing how greatly the earth is 
 dependent on the sun, worshipped that luminary as the 
 great parent of all the light, and heat, and life of the 
 world. And surely of all the forms of idolatry none is so 
 natural as this. Before they knew what the sun really is, 
 men might well be content to prostrate themselves before 
 it as a great and good Being, who when he- rose at dawn, 
 lighted up and warmed the world, and when he sank at 
 dusk, left it in darkness and chill. 
 
 2. But how or why is it that our light and heat come 
 from the sun ? We know that the earth revolves round 
 the sun, but why should this be ? Is it possible to know 
 anything as to the real relation of the earth to the great 
 luminary ? 
 
 3. Let us try to find an answer to these questions by 
 putting down some of the facts which have been dis- 
 covered about the earth and the sun. In doing this we 
 shall see how step by step the evidence grows, that the 
 dependence of the earth upon the sun is so close because 
 in reality the two bodies originally formed part of one 
 great rotating mass of matter. 
 
 4. To begin with the earth. When a deep bore is 
 made into the ground in search of water, as has been 
 done round London and Paris, sometimes to depths 
 of more than a quarter of a mile, the water which 
 rises from so great a distance is found to be warm.
 
 i8 PHYSICAL GEOGRAPHY. [LESS. 
 
 In like manner the air of deep mines is often so hot 
 that the miners have to work with hardly any clothes on 
 them. In many parts of the world springs of hot and 
 even boiling water rise to the surface, and have been 
 doing so for hundreds of years without becoming per- 
 ceptibly cooler. Again, in all quarters of the globe vol- 
 canoes, or burning mountains, are found from which 
 steam, hot gases, and molten rock are from time to time 
 given out, often with prodigious force and in enormous 
 quantity (see Lesson XXII.). 
 
 5. From all these facts which have been observed 
 throughout the world, the inference has naturally been 
 drawn, that the inside of the earth must be intensely hot. 
 And since the depth of even the deepest bores or mines 
 is not, in comparison to the whole mass of the earth, so 
 much as the thickness of varnish is to an ordinary school 
 globe, it would seem that the earth must be, on the whole, 
 a hot globe with a cool outer skin or crust. 
 
 6. Since its surface is cold, the earth cannot give out 
 any light of its own. And yet it undoubtedly shines in 
 the sky, as the moon does, because like that orb it re- 
 ceives and reflects light from the sun. 
 
 7. Passing from the earth to the nearest of the 
 heavenly bodies the moon, which as the earth's satel- 
 lite or attendant, revolves round our globe, while the 
 latter is revolving round the sun, we find some wonderful 
 evidence that the earth is not the only orb that shines 
 with light borrowed from the sun, and though cool on the 
 surface, bears traces of the influence of internal heat. 
 
 8. The nature of this evidence may be followed by 
 comparing the two drawings, Figs. 4 and 5. In Fig. 4 
 a sketch or plan is given of part of the neighbourhood 
 of Naples. The numerous detached conical hills are 
 volcanoes, either now active as Vesuvius is, or cold and 
 silent like the group of cones to the west of Naples. 
 The circular pits placed on the tops of these hills are the 
 " craters " or vents from which dust, cinders, steam, and
 
 III.] 
 
 THE EARTH AND THE SUN. 
 
 molten rock, have at different times been copiously dis- 
 charged. In Lesson XXII. some account will be given 
 of volcanoes, and it will there be shown how the streams 
 of molten rock now and then run down the outside of 
 the cone, while vast clouds of steam and hot dust, cast 
 up with great violence, bear further witness to the intense 
 heat of the interior. 
 
 FIG. 4. Plan of Volcanic Hills and Craters in the Bay of Naples. 
 
 9. Now, with this little piece of the earth's surface 
 compare the drawing in Fig. 5, which shows a portion 
 of the surface of the moon. We observe in it a series 
 of large " craters," with steep walls in the inside, and 
 numerous smaller ones, sometimes in close -set rows. 
 Several of the larger craters overlap each other, and in 
 some cases their interior is crowded with the smaller 
 ones, or the latter have broken through the ridges sepa- 
 rating the great basins. So close is the resemblance of 
 this scene to parts of the earth's surface, like that shown 
 in Fig. 4, that there seems no reason why we should not 
 call these conical elevations on the moon volcanoes, and 
 look on their well-marked " craters" as really the open-
 
 20 
 
 PHYSICAL GEOGRAPHY. 
 
 [LESS. 
 
 ings whence molten rock and other hot materials have 
 been thrown out, as in terrestrial volcanoes. So far as 
 can be seen they abound over most of the moon's surface. 
 Indeed, the lunar volcanoes have been measured by 
 astronomers, and found to be far more numerous, and 
 more gigantic than the terrestrial ones. We may fairly 
 infer that they mark a much more intense kind of vol- 
 canic action than anything we know of on the earth. 
 
 FIG. 5. A part of the Surface of the Moon, showing Volcanic Craters. 
 
 So that though the chill surface of the moon now gives 
 out no light of its own, but only throws back to us that 
 which strikes upon it from the sun, the interior of that 
 globe must once have been intensely hot, with a harden- 
 ing crust through which the internal molten liquid was 
 poured by those hundreds of craters and rifts. 
 
 1O. No telescope has yet detected any actual volcanic 
 eruption going on in the moon. It would seem, indeed, 
 that the once prodigious volcanic action there has spent 
 itself, and that the inside of the moon, though perhaps 
 still warmer than the outside, has not heat enough left
 
 ill.] THE EARTH AND THE SUN. 21 
 
 for further outbursts. Evidently the moon has cooled 
 very greatly since the time when its craters were active 
 volcanoes. 
 
 11. The earth must be cooling likewise. The mere 
 difference of temperature between its surface and interior 
 indicates this fact. The outer shell, or crust, would not 
 be kept colder than the inner mass unless there were a 
 continual loss of heat from the earth into space. Heat is 
 steadily passing outwards through the crust and surface, 
 but it does not make the ground on which we walk 
 sensibly warmer, because as fast as it reaches the surface 
 it is radiated into space. 
 
 12. Thus we see that the earth cannot have been 
 always as it is now. A million of years ago it must 
 have had considerably more heat than it possesses to-day. 
 A hundred millions of years ago it was, perhaps, in the 
 condition of a globe of molten matter, with no land or 
 sea, and of course with no life of any kind upon its sur- 
 face. The present flattening of its form at the poles is 
 just the kind of shape which such a liquid globe would 
 necessarily assume under the influence of rotation, and 
 probably t)ie permanent flattening dates from that time. 
 Earlier still the earth may have existed merely in the 
 state of vapour. 
 
 13. That this has really been its history has been 
 thought by many philosophers to be in the highest de- 
 gree probable, when, turning from the evidence which 
 the earth itself presents, attention is given to that fur- 
 nished by the sun. That the sun is hot has long been 
 familiar knowledge, but only in recent years has any 
 adequate notion been formed of how intense its heat 
 actually is. One fundamental difference between the 
 sun on the one hand, and the earth and moon on 
 the other, is that its light is given out by itself, while 
 theirs is only borrowed sun-light. When sun-light is 
 examined by means of the spectroscope, it is found 
 to indicate a temperature in the sun so enormously
 
 22 PHYSICAL GEOGRAPHY. [LESS. 
 
 high that nothing can exist there except in the form of 
 gas or vapour. The light and heat which we receive 
 from the sun proceed from glowing vapours, among 
 which have been detected those of some of the metals 
 found on the earth only as solid bodies very difficult to 
 melt. Probably most or all of the simple substances 
 composing the earth exist also in the sun, but in the 
 form of vapour. Were our globe thrown into the sun 
 it would immediately be dissipated into the same glow- 
 ing vapour. 
 
 14. Observation of the sun's surface has shown that 
 spots which appear there are carried steadily round from 
 west to east. This points to a movement of rotation, 
 which, however, is slower than that of the earth. The 
 sun, or at least its outer luminous envelop which we see, 
 takes about twenty-five of our days to turn round once 
 on its axis ; but the movement is in the same direction 
 as that of the earth. 
 
 15. Besides the earth and its attendant moon, a small 
 number of other heavenly bodies has been found to revolve 
 round the sun. Noting carefully the positions of the 
 stars in the sky at night, we find that, though the whole 
 sky seems to travel slowly westward, each star keeps the 
 same place with reference to the other stars. But we 
 may observe a few which look at first like stars, but 
 which, when watched attentively from time to time, may 
 be seen to shift their place and to travel across the other 
 stars. These were called by the ancients, planets, 
 that is, wanderers. They are now known to be really 
 revolving round our sun at different distances and in 
 various periods of time. So far as can be judged from 
 what the telescope reveals, they closely resemble the 
 earth in, many points. They rotate on their axes. Some 
 of them have a group of attendant moons. In some 
 there are indications of an atmosphere with clouds and 
 air-currents, and in one of them, called Mars, there 
 appears to be a cap of snow and ice at each pole, as in
 
 in.] THE EARTH AND THE SUN. 23 
 
 the earth. Some are much larger, others much smaller 
 than the earth; some are nearer to the sun than we, 
 others greatly farther away. This whole series of 
 planets (including, of course, the earth), which revolves 
 round the sun as its centre, is known as the Solar 
 System. 
 
 16. Now let us sum up all these facts, and see how 
 they bear upon the present condition and probable history 
 of our own globe. In the centre of the solar system 
 stands the sun an enormous globe of incandescent gas 
 and vapour, rotating on its axis, and sending forth heat 
 and light far and wide through space. Round this 
 central luminary, and depending on it for light and 
 heat, a number of planets revolve in the same general 
 plane, sometimes with minor planets or satellites revolv- 
 ing round them, as the moon does round the earth. The 
 planets and their attendant satellites or moons likewise 
 rotate on their axes. The earth is one of the planets. 
 Its present condition points to its once having been 
 hotter than it now is, and probably to its existence in a 
 liquid or even gaseous state. 
 
 17. The nebular theory, which has been proposed 
 to connect and explain these facts, maintains that at first 
 the whole solar system existed only in the state of 
 vapour, like one of the faint cloud-like nebulas disclosed 
 by the telescope among the stars ; that this nebula 
 gradually condensed and threw off from its hot mass 
 successive portions, which, by subsequent condensation 
 and cooling, became planets, and that the present sun is 
 the remaining incandescent, but still slowly condensing and 
 cooling nucleus of the whole, round which the various 
 detached portions continue to revolve. 
 
 18. This theory, which the discoveries of modern 
 science go far to confirm, gives an intelligible reason for 
 the intimate way in which the earth and the other 
 planets are related to the sun. It furnishes a satis- 
 factory explanation of the fact that the inside of the
 
 24 PHYSICAL GEOGRAPHY. [LESS. 
 
 earth is still hot, though slowly cooling. This internal 
 heat, manifested in every deep bore and mine, as well 
 as in hot springs and volcanoes, appears thus as a 
 residue of the original heat of the great nebula out of 
 which the planets and the sun have been condensed. 
 
 LESSON IV. Measurement and Mapping of the 
 Earth's Surface. 
 
 1. It would not have been possible for geography to 
 make much progress without some means of accurately 
 ascertaining the positions of places on the earth's surface. 
 On a small scale we can measure with great exactness 
 by means of carefully-constructed chains or rods, how 
 far one place is distant from another. But evidently 
 this laborious method could have been of little use in 
 laying down the great features of the earth's surface 
 continents, islands, oceans, and the rest or in discover- 
 ing the actual dimensions of the planet itself. It was 
 needful to obtain some other method which could be 
 easily used, and would give the means of determining 
 with precision the position of every part of the surface of 
 the globe. Such a method is supplied by observing the 
 position of the sun and different stars. 
 
 2. If we note the height of the sun in the sky at noon, 
 we shall find that an hour after that time he seems to 
 have moved a certain distance westward. In the course 
 of another hour he will have traversed another similar 
 space, and so on hour after hour till nightfall. Next 
 morning, if we again watch his progress, we detect a 
 similar advance hour by hour, until at noon he once more 
 stands in the same position as at noon of the day before. 
 In this interval the earth has made one entire rotation. 
 
 3. Every circle is divided into 360 equal parts, or 
 degrees, and as we travel round our terrestrial circle in 
 twenty- four hours, we must pass over fifteen degrees
 
 FIG. 5. 
 
 FIG. 3.
 
 IV.] MAPPING THE EARTH'S SURFACE. 25 
 
 every hour. Suppose that some place, say the Green- 
 wich Observatory, is fixed on from which to count the 
 degrees. It is clear that all places lying to the east of 
 Greenwich have their noon earlier, and all places to the 
 west have it later than at Greenwich itself. As the sun 
 takes an hour to travel 15 of the circle, any place 
 which has its noon exactly one hour after noon at Green- 
 wich must lie 15 to the west. 
 
 4. To determine, therefore, how far we are to the east 
 or west of Greenwich, we may try to find out the differ- 
 ence between Greenwich time and the local time at the 
 place where we may be. This has been done for short 
 distances by flashing mirrors, exploding gunpowder, or in 
 any other way communicating an instantaneous signal 
 from one point to another. The most efficacious method 
 is by electric telegraph. But for long journeys, especially 
 sea-voyages, where no such communication is possible, 
 carefully-constructed clocks called chronometers are used, 
 which show Greenwich time. By comparing the local 
 time, as fixed by taking the sun's position, with the time 
 shown in the chronometer, it is easily seen how far a 
 place lies to the east or west of Greenwich. 
 
 5. But as even the most accurately finished clock is 
 apt to gain or lose, there is still another and more reliable, 
 though less convenient kind of observation that of deter- 
 mining the position in the sky of the moon or some of the 
 planets. The places which these bodies will have with 
 reference to each other and to the fixed stars at any 
 moment is calculated for a long time beforehand at the 
 Greenwich Observatory, and tables showing these posi- 
 tions are printed. By referring to these tables the tra- 
 veller finds the precise instant of Greenwich time, for 
 each position of the heavenly bodies ; and the difference 
 between that time and the time he observes at the place 
 where he is shows him whether and how far he is to the 
 east or west of Greenwich. This is called " finding 
 the Longitude" of a place.
 
 26 PHYSICAL GEOGRAPHY. [LESS. 
 
 6. If each of the 360 degrees in the circumference of 
 the globe were marked by a line passing from pole to 
 pole, each line would be found to run due north and 
 south. A little reflection will show us that every place 
 on the earth's surface lying along the course of the same 
 line must have its noon at the same moment. These 
 lines, supposed to be traced on the earth's surface and 
 actually placed upon maps, are termed meridians 
 (Plate I. Fig. i). Of course we may fix on any point 
 from which to begin to count them. In England and in 
 English-speaking countries they are reckoned from Green- 
 wich Observatory. In France they are counted from 
 the Observatory at Paris. But this is immaterial, 
 though it would be a great convenience if all countries 
 would agree to count from the same starting - point. 
 There is reason to hope that this may before long be 
 accomplished, and that Greenwich will be selected. The 
 meridian line passing through Greenwich is marked by 
 us as the zero or o. When it is twelve o'clock at 
 Greenwich it is precisely the same time at every place 
 through which the meridian of Greenwich passes. All 
 the meridians lying west are West Longitude, those to the 
 east are East Longitude, so that the two series meet on 
 the other side of the globe, exactly opposite to Green- 
 wich, at 1 80, or half the number (360) into which the 
 whole circle of the globe is divided. Having then fixed 
 on our meridian as the starting-point, it is not difficult to 
 see how the relative distances of places east or west of 
 that meridian are determined and expressed. We say 
 that a place which lies five degrees east or west from 
 Greenwich is in 5 East or West longitude. Each degree 
 is further divided into sixty minutes, and each minute 
 into sixty seconds. Paris is situated two degrees, twenty 
 minutes, and nine seconds east from Greenwich, which is 
 abbreviated thus : 2 20' 9" E. 
 
 7. One degree of longitude is equal to four minutes of 
 time. When we travel eastwards our watches seem to
 
 iv.] MAPPING THE EARTH'S SURFACE. 27 
 
 be going slower at the rate of four minutes for every such 
 degree, because we are getting into longitudes where the 
 time is earlier than at home, while, on the other hand, 
 when we travel westwards, our watches seem to be going 
 too fast by the same intervals. 
 
 8. But this effect of difference of longitude is brought 
 out in by far the most striking manner in the sending of 
 messages by electric telegraph. Though two places may 
 be thousands of miles apart, a word sent from one is 
 almost instantaneously received at the other. When a 
 clerk in London telegraphs at noon to Calcutta (which 
 lies in 88 30' E. longitude) his words, though they go 
 with the speed of lightning, arrive about six o'clock in 
 the evening by the clock in India ; or should he send a 
 telegram at the same hour to New York, which is situ- 
 ated in 74 W. it will find the clocks there pointing to 
 seven o'clock in the morning. 
 
 9. But the determination of the longitude of a place 
 would not be enough. We must be able to tell where- 
 abouts that place lies on its meridian. To do this is 
 what is termed "finding the Latitude." The first 
 thing to strike us in this problem is the fact that we do 
 not require to fix on any arbitrary point from which to 
 begin our reckoning. The axis of the earth gives us 
 two definite points at each pole, and midway between 
 these lies the line of the equator. To determine the 
 latitude of a place, therefore, we have to ascertain how 
 far it lies to the north or south of the equator. Here 
 again we must have recourse to the heavenly bodies. 
 
 10. If the axis of the earth were prolonged from the 
 North Pole, it would reach a point in the heavens close 
 to the pole-star, called the Celestial Pole, round which 
 in the northern hemisphere the stars, by reason of the 
 earth's rotation, seem to revolve. So that though we 
 cannot measure from the North Pole itself we can deter- 
 mine our distance from it by observing how much the 
 point of the heavens directly above us (that is, the
 
 28 PHYSICAL GEOGRAPHY. [LESS. 
 
 zenith) is removed from the pole-star or from the celes- 
 tial pole. 
 
 In the same way if the line of the equator were pro- 
 longed it would traverse the heavens as a great circle. 
 Knowing exactly its position in the sky, or the distance 
 of any heavenly body from it, we observe how far our 
 zenith is removed from it, and thereby learn our distance 
 from the terrestrial equator. 
 
 11. Now the distance from either pole to the equator 
 is exactly a quarter of a circle, or 90. If we mark off 
 the degrees by a series of lines on the surface of the 
 earth, these circles will run round the globe parallel to 
 the equator and to each other, but diminish in diameter 
 as they advance to the pole. Such lines are called 
 parallels of latitude (Plate I. Fig. 2). They are counted 
 from the equator, which is the zero, or o ; and each degree 
 is divided into minutes and seconds like the degrees of 
 longitude. 
 
 12. If we find by observation that a place is situated 
 fifteen degrees north and another twenty degrees south 
 of the equator, we say that the one is in Latitude 15 
 North, and the other is 20 South. Each pole is hence 
 in latitude 90. Since the figures expressing the degrees 
 increase in amount as they recede from the equator, it 
 has become common to speak of " low latitudes," that 
 is, regions or places lying towards the equator, and 
 " high latitudes," that is, tracts situated near either 
 pole. 
 
 13. The degrees of latitude and longitude, then, are de- 
 fined by two sets of lines, one running north and south, the 
 other running east and west, by which the surface of the 
 globe is supposed to be traversed as by a regular network. 
 By means of these lines it is clear that we can fix exactly 
 the relative positions of places on the earth's surface. 
 But we must still determine what is the absolute length 
 of one of those degrees in miles before we can ascertain 
 the true distances of places and the real area which any
 
 iv.] MAPPING THE EARTH'S SURFACE. 29 
 
 country, or continent, or sea covers. When this is done 
 we have the materials for estimating the total bulk of our 
 planet, and making a correct survey of its surface. 
 
 14. Since the lines expressing the meridians of longi- 
 tude converge to each pole, their distance from each other 
 must vary according to latitude ; the parallels of latitude 
 necessarily diminish in circumference as they recede 
 from the equator. But were the globe a perfectly sym- 
 metrical sphere, the length of each meridian from equator 
 to pole should be exactly alike. The accurate measure- 
 ment, therefore, of the length of one degree of a meridian 
 should give us the means of easily computing the sum of 
 the whole, and thence of ascertaining the true size of the 
 globe. 
 
 15. This measurement of a degree of the meri- 
 dian, as it is called, has been made with great care in 
 different parts of the world. In India the length of a 
 degree was found to be 362,956 English feet, or rather 
 more than 68 statute miles ; a degree in Sweden mea- 
 sured 365,744 feet, or, in round numbers, about 69^ 
 miles. It has been found that, besides little irregularities 
 which show that the shape of the planet is slightly dis- 
 torted, there is a progressive increase in the length of the 
 degrees towards the poles, as these two measurements in 
 India and Sweden show. This could only take place by 
 a flattening of the globe at the poles. 
 
 16. The sum of all the observations gives for the 
 polar diameter of our globe a length of 7,899-17 
 statute miles, and for the mean equatorial diameter 
 a length of 7,925-65 statute miles. As the difference is 
 about 26^ miles, each pole must be compressed to the 
 extent of 1 3^- miles. 
 
 It is not easy at once to grasp the full value of these 
 figures. An express train travelling at the rate of thirty 
 miles an hour would take about a month to go completely 
 round the earth at the equator ; and if it could go through 
 the earth between the poles, it would take about eleven
 
 30 PHYSICAL GEOGRAPHY. [LESS. 
 
 days to perform the journey. Upon the surface of a 
 globe of such dimensions, the highest mountains and the 
 deepest oceans are far less in proportion than the rough- 
 nesses upon the rind of an orange. 
 
 17. Astronomy has taught us not only the size of our 
 own planet, but has computed the dimensions of the 
 others, whence we learn the comparative place of the 
 earth in the solar system. The planet Jupiter, for 
 example, is 1400 times the size of the Earth. On the 
 other hand, our planet is seventeen times larger than 
 Mercury, and greatly larger than certain small bodies 
 called Asteroids. Again, the Earth is neither nearest to 
 nor farthest removed from the sun ; its mean distance, 
 as already stated, is computed to be nearly 93 millions 
 of miles. But Mercury, at its greatest distance, is only 
 about 44|- millions of miles away from the sun, while 
 Neptune revolves at the enormous mean distance of 
 2862 millions of miles. The sun itself, the great centre 
 of all the movement of the solar system, would contain 
 1,400,000 globes as large as ours. 
 
 18. Having found a means of accurately fixing the 
 position of any place on the earth's surface, and of deter- 
 mining the distance of places from each other, men would 
 nevertheless have been able to carry in their minds but 
 a vague notion of the general features of that vast and 
 varied surface, had they not devised a way of putting 
 down these features upon paper so as to show their rela- 
 tive positions and shapes. Such a delineation of the 
 whole or part of the earth's surface is called a Map. 
 The map of any country or continent may be seen to be 
 crossed by two sets of lines, one of which, running from 
 the top to the bottom of the paper, marks the degrees of 
 longitude, while the other set, running from side to side, 
 shows the parallels of latitude. It is usual in such maps 
 to represent the ground as it might be supposed to 
 appear could we ascend to a great height in the air and 
 take in the whole area at one view, our heads being
 
 IV.] 
 
 MAPPING THE EARTH'S SURFACE. 
 
 turned to the north. Hence the top of the map is north, 
 the right hand being the east side, and the left hand the 
 west. 
 
 19. When maps of a country are wanted on a large 
 scale to express the features of the ground in great 
 
 FIG. 6. Measurement and mapping of a country by means of triangulation. 
 
 detail, another process of measurement called triangu- 
 lation is used. First a base-line, a few miles in 
 length, is accurately measured with rods or chains as 
 from A to B in the figure. Then from A an observation 
 is taken with an instrument called a theodolite, to the 
 point C, which may be a hill-top or church tower, or any 
 distinct object, and the angle C A B is carefully noted. A 
 similar observation is taken at B to determine the angle
 
 32 PHYSICAL GEOGRAPHY. [LESS. 
 
 C B A. We can now construct our first triangle, and 
 having found by actual measurement the length of its 
 base, we find by calculation the length of its two sides, 
 and consequently the exact position of C and its distance 
 from A and B respectively. From such a measured 
 base a system of triangles is observed all over the 
 country, and the true positions of all the main land- 
 marks are fixed by triangulation without the necessity 
 of further actual measurement. 
 
 LESSON V. A General View of the Earth. 
 
 1. We proceed now to take a first general view of the 
 parts of the earth with which further acquaintance is to 
 be made in the following lesso'ns. Our progress will 
 probably be found to be more easy and pleasant, if we 
 can carry with us from the outset such a broad but clear 
 notion of the earth as a whole, as will enable us to fol- 
 low the necessary references to parts with which we may 
 still be unfamiliar 
 
 2. Beneath the outer envelope of Air, with its winds, 
 clouds, rain, and snow, the surface of the globe presents 
 the two clearly marked, but very irregular divisions of 
 Sea and Land. As the result of surveys and obser- 
 vations taken in all parts of the globe, it is ascertained 
 that the sea covers nearly three-fourths, and the land 
 rather more than one-fourth, of the whole surface of the 
 planet ; or, more exactly, there are 275 parts of water to 
 i oo parts of land. The total area of the earth's surface 
 is computed at 196,712,000 square statute miles, of 
 which 144,712,000 are assigned to the ocean, and 
 52,000,000 to the land. 
 
 3. For the sake of aiding our conceptions of such 
 statements as these, globes and maps are prepared, of 
 which Plate I. may serve as an illustration. Each of 
 the two large circles there represents one side of the
 
 v.J GENERAL VIEW OF THE EARTH. 33 
 
 earth, and shows the manner in which sea and land are 
 grouped. It will be observed that, while the amount of 
 sea greatly exceeds that of land, the difference becomes 
 more marked in the southern half or hemisphere, which 
 is almost all water, while most of the land lies on the 
 north side of the equator. But a school globe may be 
 so placed, as to show nearly the whole of the land at 
 one view, and the greater part of the sea at another. 
 For this purpose, turn the globe so that the south-west 
 of Britain appears directly in the centre of the globe as 
 you look at it (Fig. 3, Plate I.). Britain is thus seen to 
 lie in the centre of the habitable part of the earth. Now 
 turn the globe so that the point on the other side, ex- 
 actly opposite to the south-west of Britain, shall be the 
 centre. The islands of New Zealand are then not far 
 from that position, and all round them lies the water 
 hemisphere, with comparatively few detached masses of 
 land (Fig. 4, Plate I.). 
 
 4. While the great body of the land lies on the north 
 side of the equator, it will be seen from Plate I. that it 
 forms there two well-marked portions separated by two 
 great tracts of sea. The larger of these portions includes 
 Europe, Asia, and Africa, that is, all the regions which 
 have been longest settled by a human population. Hence 
 it is often spoken of as the Old World. The other por- 
 tion embraces America, and is called the New World. 
 As the Old World lies to the east and the other to the 
 west, they are frequently referred to as the Eastern 
 Hemisphere and Western Hemisphere. 
 
 5. One difference between the distribution of sea and 
 land cannot fail to arrest attention at the outset. The 
 land is much broken up. Even in the Eastern Hemi- 
 sphere, where it presents the most compact mass, long 
 arms and inlets of the sea intersect it, and cut off large 
 portions from the rest. The sea, on the other hand, is 
 evidently one continuous whole. Even when penetrat- 
 ing farthest into the land its most distant arms retain 
 
 D
 
 34 PHYSICAL GEOGRAPHY. [LESS. 
 
 their connection with the main body. A vessel may 
 sail over every sea and into every far recess and inlet, 
 without ever needing to be dragged over an intervening 
 mass of land. But no coach or form of carriage could 
 visit every part of the land, without requiring to be trans- ' 
 ported across intervening tracts of sea. There are no 
 isolated areas of sea completely encircled by land, an- 
 swering to the abundant portions of the land which, 
 encircled entirely by sea, receive the name of islands. 1 
 
 6. Though the sea is one continuous liquid mass it 
 has been, for the sake of convenience in description, 
 divided into different areas, termed Oceans. The 
 limits of these are for the most part indicated by the 
 position of the masses of land. Thus the largest, under 
 the name of the Pacific Ocean, fills up the vast area be- 
 tween the western edge of America and the eastern 
 margin of Asia and Australia. The line of the equator 
 divides it into the North Pacific and the South Pacific 
 Ocean. Another longer but much narrower belt of sea, 
 intervenes between the eastern coasts of America and 
 the western coasts of Europe and Africa ; divided in 
 the same way by the equator into the North Atlantic 
 and the South Atlantic Ocean. Owing to the broad 
 mass of land forming Europe and Asia, no corresponding 
 belt of sea can run there from pole to pole. But south 
 of that land lies the wide Indian Ocean, between Africa 
 and Australia. It is usual to call all that part of the 
 sea within the Arctic Circle the Arctic Ocean, and the 
 corresponding tract in the southern hemisphere, the 
 Southern or Antarctic Ocean. 
 
 7. Besides these main sub-divisions there are minor 
 tracts of sea, more or less surrounded by land. Such 
 names as sea, gulf, strait, channel, are given to them, 
 according to their size or the shape of the land-outlines 
 by which they are bounded. 
 
 1 Reference will be afterwards made to one or two interesting exceptions 
 to this rule, the Caspian Sea being the chief.
 
 v.] GENERAL VIEW OF THE EARTH. 35 
 
 8. The surface of the sea forms a sharp, well-marked 
 platform, which is called the sea-level. It serves as 
 the line from which the heights of the land and the 
 depths of the oceans are measured. Slight differences 
 of level have been detected between different oceans or 
 seas, as, for instance, between the Atlantic and Pacific, 
 on the two sides of the narrow part of America, and be- 
 tween the Mediterranean and the Red Sea. But such 
 differences do not amount to more than a few feet, so 
 that for most practical purposes we may assume the sea- 
 level to be uniform. 
 
 9. Passing next to the land, we observe from the map 
 that while massed on the whole in the northern half of 
 the globe, it is far from forming there one solid block ; 
 on the contrary, it is intersected by branches from the 
 sea so as to be easily grouped into a few great sub- 
 divisions. These are termed Continents. Strictly 
 speaking, there are only two continents, the Old World 
 and the New World (Art. 5). More usually, however, 
 they are grouped in three pairs, the first pair consisting 
 of North and South America, the second of Europe and 
 Africa, and the third of Asia and Australia. 
 
 10. Now, in considering the general arrangement of 
 the land on the globe, we soon see that one of the most 
 marked general features of the continents is their ten- 
 dency to be massed together towards the north, and to 
 taper away towards the south to about the parallel of 
 45 S. Look, for example, at the way in which the 
 mass of Africa dwindles away southward to the Cape of 
 Good Hope, and how the huge bulk of the Asiatic Con- 
 tinent and its prolongation in Australia tapers away to 
 the headlands of Tasmania. Still more remarkable is 
 the attenuation of the American Continent and its sharp- 
 ened termination at Cape Horn. 
 
 11. Again, not only is the land much intersected by 
 inlets, such as the great Mediterranean Sea and the Red 
 Sea, but unlike the sea (Art. 5) large parts are com-
 
 36 PHYSICAL GEOGRAPHY. [LESS. 
 
 pletely cut off from the main mass, as in Australia, New 
 Zealand, Japan, Britain, and the host of islands, large 
 and small. Other parts nearly isolated are termed 
 peninsulas. Of these Africa may be taken as the 
 most conspicuous illustration. It is joined only by the 
 little connecting neck or isthmus of Suez to the con- 
 tinent of Asia, so that were that strip of lowland to be 
 cut through or sunk beneath the sea, Africa would 
 become an island. This, indeed, has been in some 
 sense accomplished by man in the making of the Suez 
 Canal. 
 
 12. Another feature of the land marks it off in striking 
 contrast from the sea. The surface of the latter, though 
 liable to be roughened by ripples and waves, preserves 
 everywhere the character of one vast plain. But the 
 surface of the land abounds in unevenness. Some parts, 
 indeed, are flat, but most of it undulates into hills and 
 valleys, and some portions mount up into vast ranges of 
 rugged and pointed mountains. 
 
 13. This irregular distribution and inequality of sur- 
 face give the land a peculiar character and influence in 
 the physical geography of the earth's surface. Without 
 inquiring at present how this influence shows itself, we 
 can readily see that if either in the air or in the sea 
 there is any movement of circulation, the currents, both 
 of air and sea, must be greatly affected by the position 
 and form of the continents and islands which may lie in 
 their course. A current in the sea moving westward, 
 for instance, across the middle of the Atlantic, will strike 
 against the long ridge of America, and be turned aside 
 either to right or left, or both. A current of air, on the 
 other hand, if it should take its rise from some centre in 
 the ocean region, will be turned aside when it meets a 
 mass of high land, and may be driven to ascend the 
 slopes, discharge its moisture, and flow on at a much 
 greater height, and at a very different temperature (Les- 
 son X. Art. 31). The vaiying influence of sea and
 
 v. ] GENERAL VIEW OF THE EARTH. 37 
 
 land upon the air profoundly affects the different climates 
 of the earth. 
 
 14. It has been already stated (Lesson III. Art. 4) 
 that at many parts of our planet's surface, pipes or 
 funnels exist, which descending deep into the earth, cast 
 out from time to time steam, hot vapours, dust, stones, 
 and melted rock, so as in the end to form conical hills 
 or mountains called volcanoes. It is observed that 
 orifices of this kind are apt to occur in long lines, and 
 especially along the back-bones of the continents, and 
 in long chains of islands (see Plate IX.). Over many 
 parts of the globe, too, and particularly in those regions 
 where volcanoes abound, the ground is frequently shaken 
 by earthquakes, and sometimes permanently lifted 
 above its previous level. Such appearances as these 
 afford indications, not only of the nature of the earth's 
 interior, but also of the way in which the interior affects 
 the surface. They help us to understand how it is that 
 the land has been ridged up above the general level of 
 the sea. 
 
 15. Into these matters we shall enter more fully in 
 later Lessons. Meanwhile, carrying with us this general 
 outline of the several parts of the earth, we may proceed 
 to consider these parts one by one in detail.
 
 38 PHYSICAL GEOGRAPHY. [LESS. 
 
 CHAPTER II. 
 
 AIR. 
 
 LESSON VI. Its Composition. 
 
 1. Above and around us, to what part soever of the 
 earth's surface we may go, at the top of the highest 
 mountain, as well as at the bottom of the deepest mine, 
 we find ourselves surrounded by the invisible ocean of 
 gas and vapour which we call AIR. It therefore wraps 
 the whole planet round as an outer envelope. Considered 
 in this light it receives the distinctive name of the ATMO- 
 SPHERE, that is, the vapour-sphere. the region of clouds, 
 rain, snow, hail, lightning, breezes, and tempests. In 
 the study of the earth as a great habitable globe this 
 outer encircling ocean of air is the first thing to be con- 
 sidered. What is it ? and what purposes does it serve 
 in the general plan of the earth ? 
 
 2. In early times men regarded the air as one of the 
 four elements out of which the world had been made. 
 It is not so very long since this old notion disappeared. 
 But now it is well known that the air is not an ele- 
 ment, but a mixture of two elements viz. the gases 
 called Nitrogen and Oxygen. It is easy to prove this 
 by burning a piece of another element, phosphorus, in a 
 closed jar. We thereby remove the oxygen, which unites 
 with the phosphorus to form a compound substance, and 
 the nitrogen is left behind. 1 In various ways chemists 
 have analysed or decomposed air into its component 
 
 * See Rescue's Chemistry Primer, p. 21.
 
 vr] COMPOSITION OF THE AIR. 39 
 
 elements, but the result is always the same, viz., that 
 in every hundred parts of ordinary air there are by 
 weight about seventy-nine of nitrogen and twenty-one of 
 oxygen. 
 
 3. Air, when carefully tested, is always found to con- 
 tain something else than nitrogen and oxygen. Solid 
 particles, with various gases and vapours, are invariably 
 present, but always in exceedingly minute, though most 
 irregular quantities, when compared with the wonderfully 
 constant proportions of the two chief gases. Some of 
 these additional components of air are not less important 
 than the nitrogen and oxygen. That they exist may be 
 easily proved, and some light may thereby be thrown on 
 the nature and uses of the air. 
 
 4. The presence of vast numbers of solid particles 
 in the air may be shown by letting a beam of sunlight, 
 or of any strong artificial light, fall through a hole or 
 chink into a dark room. Thousands of minute motes 
 are then seen driving to and fro across the beam as the 
 movements of the air carry them hither and thither. 
 Such particles are always present in the air, though 
 usually too small to be seen, unless when, as in the 
 darkened room, they are made visible against surround- 
 ing darkness by the light which they reflect from their 
 surfaces when they cross the path of any strong light- 
 rays. They are quite as abundant in the dark parts ot 
 the room, though for want of light falling upon them they 
 are not seen there. 
 
 5. Could we intercept these dancing motes and exam- 
 ine them with a strong microscope, we should find them 
 to consist chiefly of little specks of dust. But among 
 them there sometimes occur also minute living germs, 
 from which, when they find a fitting resting-place, lowly 
 forms of plants or animals spring. Various diseases 
 arise from such infinitesimal germs, which are so small 
 as to pass with the air into our lungs, and thus to reach 
 our blood, where they rapidly multiply.
 
 40 PHYSICAL GEOGRAPHY. [LESS. 
 
 6. It is difficult to catch these tiny motes from a sun- 
 beam, but rain does this admirably for us. One great 
 office of rain is to wash the air and free it from im- 
 purities. Hence, when rain-water is carefully collected, 
 especially in large towns, it is found to contain plenty 
 of solid particles, which it has brought down with it in 
 its fall through the air. The accompanying drawing, 
 
 for example, shows what is seen when 
 a small quantity of rain, gathered from 
 an open space in a town, is evaporated 
 to dryness, and the residue is placed 
 under a microscope. Abundant par- 
 ticles of dust or soot may be observed, 
 mingled with minute crystals of such 
 
 FIG. 7. What is seen su bstanC6S as Sulphate Of SOda and COm- 
 after some raindrops , ... 
 
 collected in a town mon salt - Hence we learn that, besides 
 
 are evaporated, and the solid particles, there must be floating 
 
 the residue is placed j n t h e a j r t h e vapours or minute par- 
 
 scope ' tides of various soluble substances 
 
 which are caught by the rain and carried down with it 
 
 to the soil. In seizing these impurities and taking them 
 
 with it to the ground, the rain purifies the air and makes 
 
 it more healthy, while at the same time it supplies the 
 
 soil with substances useful to plants. 
 
 7. But far more important than these solid ingredients 
 are three invisible substances, ozone, carbonic acid gas 
 (carbon dioxide), and water-vapour. After a thunder- 
 storm the air may sometimes be perceived to have a 
 peculiar smell, like that which is given off from an elec- 
 tric machine. This is ozone, which is believed to be 
 oxygen gas in a peculiar and very active condition. It 
 promotes the rapid decomposition of decaying animal 
 or vegetable matter, uniting with the noxious gases, and 
 thus disinfecting and purifying the air. It is most 
 abundant where sea-breezes blow, and least in the air 
 of the crowded parts of towns. The healthiness or un- 
 healthiness of the air seems to depend much on the
 
 vi.] COMPOSITION OF THE AIR. 41 
 
 quantity of ozone, which may be estimated by the amount 
 of discolouration produced by the air within a certain 
 time upon a piece of paper prepared with starch and 
 iodide of potassium. 
 
 8. When a piece of coal is set on fire it burns away 
 until nothing but a little ash is left behind. Or when a 
 candle is lighted it continues to burn until the whole is 
 consumed. Now, what has become of the original sub- 
 stance of the coal and the candle ? It seems to have 
 been completely lost ; yet in truth we have not destroyed 
 one atom of it. We have simply, by burning, changed 
 it into another and invisible form, but its component 
 elements are just as really existent as ever. We cannot 
 put these back into the form in which they appeared in 
 the coal and candle, but we can at least show that they 
 are present in the air, where they pass mainly into water- 
 vapour and other gaseous compounds, of which the most 
 important is carbonic acid gas or carbon dioxide. 
 
 9. The substance of a piece of coal or of a candle is 
 composed of different elements, one of which is called 
 carbon. This element forms one of the main ingredients 
 out of which the substance of all plants and animals is 
 built up. Our own bodies, for example, are in great part 
 made of it. In burning a bit of coal, therefore (which 
 is made of ancient vegetation compressed and altered 
 into stone), or a candle (which is prepared from animal 
 fat), we set free its carbon, which goes off at once to 
 mix with the air. Some of it escapes in the form of 
 little solid particles of soot, as we may show by holding 
 a plate over the candle flame, when the faint column 
 of dark smoke at once begins to deposit these minute 
 flakes of carbon as a black coating of soot on the cool 
 plate. The black smoke issuing from chimneys is another 
 similar illustration of the way in which solid particles are 
 conveyed into the air. 
 
 10. But the largest part of the carbon does not go off 
 in smoke. In the act of burning it is seized by the
 
 42 PHYSICAL GEOGRAPHY. [LESS. 
 
 oxygen of the air, with which it enters into chemical 
 combination, to form the invisible carbonic acid gas. 
 It is, indeed, this very chemical union which constitutes 
 what is called burning or combustion. The moment we 
 prevent the flame from getting access of air, it drops 
 down and soon goes out, because the supply of oxygen 
 is cut off. 1 All ordinary burning substances, therefore, 
 furnish carbonic acid gas to the atmosphere. 
 
 11. The amount of this gas which the atmosphere 
 thus obtains is, of course, comparatively small, for the 
 quantity of vegetable or animal substance, burned either 
 by man or naturally, must be but insignificant when the 
 whole mass of the atmosphere is considered. A vastly 
 larger quantity is furnished by living air-breathing 
 animals. In breathing we take air into our lungs, where 
 it reaches our blood. A kind of burning goes on there, 
 for the oxygen of the air unites with the carbon of the 
 blood ; carbonic acid is produced, and comes away with 
 the exhausted air, which we exhale again before taking 
 the next breath. Just as a burning candle is extinguished 
 when a glass is so placed over it as to exclude the air, 
 so a living animal is killed when it is shut into a confined 
 space from which the access of the outer air is cut off. 
 When we reflect that every air-breathing animal is con- 
 tinually supplying carbonic acid gas to the atmosphere, 
 we perceive how important this source of supply must be. 
 
 12. Living plants in the presence of sunlight have the 
 power of abstracting from the carbonic acid of the air 
 the carbon of which their framework is so largely made. 2 
 When they die their decay once more sets loose the 
 carbon, which, uniting again with oxygen, eventually 
 
 1 For some simple experiments on the nature and production of carbonic 
 acid, see Roscoe's Chemistry Printer. 
 
 2 The familiar practice of growing cress-seed on moist cloth shows this 
 power of living plants. Kept in the light the seeds soon begin to grow, and 
 yield a crop of cress, the carbon of whicli the grown plants consist being 
 derived, not from the water, nor from the cloth, but from the air. (Roscoe's 
 Ci'teiitistry Primer.')
 
 vi.] COMPOSITION OF THE AIR. 43 
 
 becomes carbonic acid gas, and is carried by rain into 
 the soil, or is taken up by the air. All decaying plants 
 and animals, freely exposed to air and moisture, furnish 
 this gas. 
 
 13. Lastly, in many parts of the world, particularly 
 in volcanic regions, this same gas is given out in large 
 quantities from the ground. From all these various 
 sources, then, the atmosphere is continually replenished 
 with carbonic acid gas to supply the loss caused by the 
 enormous demands of the vegetable world for carbon. 
 
 14. Nevertheless, the quantity of this gas present in 
 the air is very small compared with the volume of the 
 nitrogen and oxygen. It has been found to amount to 
 no more in ordinary pure air than about three parts in 
 every ten thousand of air. Yet this small proportion 
 suffices to support all the luxuriant growing vegetation 
 of the earth's surface. 
 
 15. By the term water-vapour, or aqueous-vapour, 
 is meant the invisible steam always present in the air. 
 Every one is familiar with the fact, that when water 
 is heated it passes into vapour, which becomes invisibly 
 dissolved in the air. A vessel of water, for instance, 
 may be placed on a table in the middle of a room, heated 
 by means of a spirit lamp till it boils, and kept boiling 
 till the water is entirely driven off into vapour or eva- 
 porated. The air in the room shows no visible change, 
 though it has had all this water-vapour added to it. But 
 it may be easily made to yield back some of the vapour. 
 Let an ice-cold piece of glass, metal, or any other sub- 
 stance be brought into the room. Though perfectly dry 
 before, its surface instantly grows dim and damp. And 
 if it is large and thick enough to require some minutes 
 to get as warm as the air in the room, the dimness or 
 mist on its surface will pass into trickling drops of water. 
 The air is chilled by the cold glass, and gives up some 
 of its moisture. Cold air cannot retain so much dif- 
 fused vapour as warm air, so that the capacity of the
 
 44 PHYSICAL GEOGRAPHY. [LESS. 
 
 air for vapour is regulated by its temperature. (See 
 Lesson X.) 
 
 16. It is not needful to boil water in order to get 
 water-vapour enough in the air of a room to be capable 
 of being caught and shown in this way. In a warm 
 sitting-room, where a few persons are assembled, there 
 is always vapour enough to be made visible on a cold 
 glass. In frosty weather the windows may be found 
 streaming with water inside, which has been taken out of 
 the air by the ice-cold window-panes. Whence came 
 this moisture ? It has been for the most part breathed 
 out into the air by the people in the room. 
 
 17. Each of us is every moment breathing out water- 
 vapour into the air. As a rule, we do not see it, because 
 the air around us is dry enough to dissolve it at once. 
 But anything which chills our breath will make the 
 vapour visible, such as breathing on a cold piece of glass, 
 or metal, when a film of mist at once appears on the 
 object ; or walking outside on a cold or very damp day, 
 when the vapour of each breath becomes visible as a 
 little cloud of mist in the air. 
 
 18. No matter how dry the air may appear to be, 
 more or less of this invisible water -vapour is always 
 diffused through it. Every mist or cloud which gathers 
 in the sky every shower of rain, snow, or hail, which 
 falls to the ground every little drop of dew which at 
 nightfall gathers upon the grass, bear witness to its 
 presence. 
 
 19. The importance of this ingredient of the atmo- 
 sphere in the general plan of our world, can hardly be 
 over-estimated. It is to the vapour of the atmosphere 
 that we owe all the water-circulation of the land rain, 
 springs, brooks, rivers, lakes on which the very life of 
 plants and animals depends, and without which, as far 
 as we know, the land would become as barren, silent, and 
 lifeless as the surface of the moon. It is, likewise, to 
 the changes in the supply of this same invisible, but ever
 
 vii.] HEIGHT OF THE AIR. 45 
 
 present substance, that the rise of winds and storms is 
 largely due (Lesson XI.). 
 
 2O. The quantity of water-vapour in the air, always 
 comparatively small in amount, constantly varies from 
 day to day, and from hour to hour. It is affected by 
 each change of temperature, and by every condensation 
 and evaporation that takes place. The air is never 
 quite free from vapour, even in the driest climates. The 
 proportion of vapour increases with temperature, as ex- 
 plained in Lesson VIII. 
 
 LESSON VII. The Height of the Air. 
 
 1. Though no actual upper limit has been found to the 
 atmosphere corresponding at all to the sharply-defined 
 surface of the sea, we cannot suppose the air to extend 
 indefinitely outwards from the earth. The atmospheric 
 envelope clasps the planet firmly, and moves along with 
 it, both in rotation and revolution. Were this not the 
 case, it is plain that the earth's movement through the 
 air would be far more rapid than the most furious 
 hurricane. No loose object could appear on the surface 
 of the globe without being instantly whisked off. But 
 the attraction of the earth retains the atmosphere in 
 its place, so that it is carried with the rest of the planet 
 through space. 
 
 2. There must then be some upper limit to the atmo- 
 sphere. Beyond it lies the ether, which is supposed to 
 fill all space, and through which all the heavenly bodies, 
 and the rays of light proceeding from them are moving. 
 How can it be known how far the atmosphere extends 
 above us ? 
 
 3. There are different ways of trying to answer this 
 question. Let us look for a moment at one of them. 
 On clear, dark nights, shooting-stars or meteors may be 
 seen, sometimes in considerable numbers. They sud-
 
 46 PHYSICAL GEOGRAPHY. [LESS. 
 
 denly appear, and after making a train or tail of light, 
 quickly vanish. Sometimes they have been actually 
 heard to explode in the sky, and fragments of them have 
 been picked up. They have been carefully watched by 
 astronomers. By observing their positions and directions 
 of movement from different stations, it has been possible 
 to determine how high they are above us, by a process 
 very similar to that by which distances are estimated on 
 the earth by taking angles to any object from the two 
 ends of a measured base-line. They have thus been 
 found to begin to be visible at from 70 to 500 miles from 
 the earth's surface. 
 
 4. In themselves they are little fragments, usually not 
 more than a few ounces or a few pounds in weight ; but 
 they revolve round the sun with the velocity of planets 
 or of comets. While still following their usual orbit, 
 they are in themselves cold, 1 opaque bodies. The light 
 which they emit, and by which they become visible, 
 arises from the fact that, drawn out of their course by 
 the attraction of our earth, and rushing into our atmo- 
 sphere with an enormous velocity, they rapidly get 
 heated by friction against the air, as well as by the heat 
 liberated from the compression of the air in front of 
 them. They soon become white-hot, and in most cases 
 so intense is the heat to which they are raised that they 
 are dissipated into vapour, which shows as a tail or 
 train of light, gradually fading out of the sky. From 
 the height at which these shooting-stars begin to glow, 
 it is inferred that the atmosphere must extend at least 
 to 500 miles above the general solid surface of the earth, 
 and may even reach to 600 miles. 
 
 5. But the air at these great heights must be very 
 different in many respects from the air next the earth. 
 We could not breathe it. When, for instance, travellers 
 ascend lofty mountains, they often find an increasing 
 
 1 They probably have nearly the temperature of space, which is supposed 
 to be about 271 Fahr. below the freezing-point of water.
 
 180 160 140 120 100 
 
 DISTRIBUTION OI 
 
 ATMOSPHERIC PRESSURE 
 
 over the Surface of the 
 
 Globe. 
 
 - laobaric Lines shewing mean 
 
 pressure for JANUARY. 
 The Arrows shew the prevalent 
 
 Winds. 
 
 (From the Charts of Mr Buchan.)
 
 PLATE II. 
 
 20 40 60u~ E 80.d-Gr~JOO 120 140 160 fS 180
 
 viii.] PRESSURE OF THE AIR. 47 
 
 difficulty in breathing as they advance. In a similar 
 manner persons who have gone up to great heights in 
 balloons have become insensible, and have nearly died 
 from the difference between the air below and that above. 
 The chief difference is in density, the air getting continu- 
 ally lighter or thinner as it recedes from the sea-level. 
 Our bodies cannot endure the difference between the 
 close, heavy air to which we are accustomed next the 
 earth, and the thinner air farther up though when the 
 transition is made gradually, the lungs may be trained to 
 breathe the air at considerable heights on lofty moun- 
 tains. Breathing, however, becomes impossible at a 
 greater height than six or seven miles. Beyond that 
 distance the air must get less and less dense, until it 
 reaches the extremest attenuation at the farthest out- 
 skirts of the atmosphere. 
 
 LESSON VIII. The Pressure of the Air. 
 
 1. Though invisible to us, and so light that we live 
 and move in it without thinking of its existence, the 
 atmosphere nevertheless presses upon every part of the 
 earth. The lower layers must be pressed down by the 
 weight of all the mass of air above them. This is what 
 is usually spoken of as atmospheric pressure, which 
 is due, not merely to the weight, but to other properties 
 of the gases and vapours which form the air. 
 
 2. To prove the pressure of the air, take a little 
 glass phial, and putting it to the mouth, suck out the air 
 as well as possible, taking care to let the tongue press 
 instantly back upon the opening. You feel the tongue 
 driven into the phial, and perhaps even with pain, owing 
 to the pressure of the air outside, and the absence of 
 corresponding pressure from air inside. An effect of 
 this kind is capable of being accurately measured. 
 Accordingly, observation shows that, at the sea -level,
 
 48 PHYSICAL GEOGRAPHY. [LESS. 
 
 this pressure is as much as about I4| pounds upon 
 every square inch. Every one of us, therefore, bears a 
 weight of 1 2 or 14 tons of air. Yet we do not feel this 
 pressure, because it is exerted equally on all sides, and 
 because the air within our bodies has the same pressure 
 outwards which the air outside has inwards. If we 
 could withdraw the air from all the cavities and passages 
 in a human body, the weight of the air outside would 
 crush the body in and cause immediate death. 
 
 3. Bearing in mind that each part of the atmosphere 
 has to bear the weight of all the air which lies above it, 
 we can understand why, as we ascend a mountain, and 
 have a decreasing mass of air above us (Lesson VII. 
 Art. 5), we should encounter air of less and less density. 
 Atmospheric pressure diminishes with altitude. 
 
 4. The pressure of the atmosphere is measured with 
 the instrument called the Barometer. The principle 
 of this instrument is that the weight of the atmosphere 
 will balance the weight of a column of any other fluid, 
 the height of that column being determined by the 
 relative weight, or what is called the specific gravity, of 
 the fluid employed. A glass tube about thirty- three 
 inches long, closed at one end, is filled with mercury, 
 and then inverted with its open end in a cup of the same 
 fluid metal. The mercury is observed to fall in the tube 
 until, if near the sea-level, it stands at a height of about 
 thirty inches above the surface of the mercury in the cup. 
 The column of mercury, thirty inches in height, is balanced 
 and kept from descending farther by the pressure of the 
 overlying column of the atmosphere upon the surface of 
 the mercury in the cup. The more the atmosphere 
 presses upon the latter, the farther does the mercury rise 
 in the tube, while on the other hand the less it presses, 
 the more the mercury sinks in the tube. 
 
 5. With such an instrument, variations in atmospheric 
 pressure may be detected, even when so small and slow 
 that we should never otherwise have been sensible of
 
 viii.] PRESSURE OF THE AIR. 49 
 
 them. If the height of the mercury in the tube is 
 accurately noted at starting, and the barometer is carried 
 to a higher level, the mercury will be observed to fall 
 because of the diminished pressure, while it will rise 
 again when the instrument is brought back to lower 
 ground. So regular and delicate is this action that the 
 barometer is often employed for measuring heights. 
 Were there no other cause for variations in the pressure 
 of the atmosphere than merely elevation, the barometer 
 would be most useful for that purpose, but, as we shall 
 immediately see, would be of no service in the way it is 
 now chiefly employed. 
 
 6. The diminution of atmospheric pressure according 
 to height in the air is regular and constant. But besides 
 this, the pressure is liable to continual changes at every 
 level, sometimes sudden and great, at other times slow 
 and slight. We are made sensible of these variations 
 when they are accompanied by changes of weather. But 
 they are most accurately measured by the movements of 
 the barometer. If from any cause the pressure should 
 decrease, the mercury will fall ; should it increase, the 
 mercury will rise ; the rapidity or slowness of the move- 
 ment in the column of mercury affording us, as it were, 
 a mirror of the amount and the rate of change in the 
 balancing atmospheric column. 
 
 7. Suppose, by way of illustration, that on looking at 
 the barometer some morning, we should find the mercury 
 to have fallen a whole inch during the night ; the mer- 
 cury column would thus indicate that during a few hours 
 it had lost a thirtieth part of its whole length, and we 
 should be justified in believing that, in some way or other, 
 the column of air that presses on the mercury in the 
 cup, had in like manner lost a thirtieth part of its pres- 
 sure or weight. Some of the upper portions of the 
 atmosphere must have flowed over into surrounding 
 regions so as to diminish the pressure to this extent. 
 No such sudden and great change, however, could fail to 
 
 E
 
 50 PHYSICAL GEOGRAPHY. [LESS. 
 
 produce a violent hurricane. The fall of the barometer 
 in almost every case occurs in time to prepare us for the 
 coming storm. 
 
 8. The barometer tube is divided into inches, and 
 these into tenths, hundredths, and thousandths, so that 
 the position of the mercury is noted to the thousandth 
 of an inch. When the pressure of the atmosphere just 
 balances a column of mercury thirty inches in height, the 
 barometer is said to mark or stand at 30-00. If the 
 mercury falls half an inch, the barometer stands at 29-50. 
 If it then rises a tenth of an inch, it is read as 29-60, a 
 further rise of a hundredth of an inch would make 29-61. 
 The height of the mercury in the barometer at the level 
 of the sea has been found to be very nearly thirty inches 
 as a mean over the whole globe. In different regions, 
 however, the actual average height for the year varies 
 considerably from that mean. In the Pacific Ocean, for 
 instance, some distance to the westward of California, the 
 mercury stands on the average at 30-30 inches. On the 
 other hand, in the north-west of Iceland it stands at a 
 mean height of 29-66 inches, while within the Antarctic 
 circle the average is even lower. When the mercury 
 has fallen below its average height, it indicates a low 
 pressure, when it rises above the average it marks a 
 high pressure. 
 
 9. The importance of noting carefully the variations 
 in atmospheric pressure will be apparent from the fact, 
 which has now been proved by observation in all parts 
 of the world that it is differences of pressure which give 
 rise to winds, storms, and in short all the movements of 
 the air which are intimately connected with changes of 
 weather. 
 
 But what is the cause of these variations in pressure ? 
 Why should the air be liable to such increasing and often 
 sudden, as well as great changes ? The answer to these 
 questions is, that the pressure is affected, 1st, by tem- 
 perature, and 2d, by aqueous vapour.
 
 140 120 /OO 
 
 DISTRIBUTION OT 
 
 ATMOSPHERIC PRESSURE 
 
 over the Surface of the 
 
 Globe. 
 
 . iBobaric Lines shewing mean 
 
 pressure for JOLT. 
 The Arrows shew the prevalent 
 
 Winds. 
 
 (From the Chartt of Mr Buckan.) 
 
 ISO 160 140 120 100i~*v80.r<~60 40
 
 PLATE III. 
 
 20 40 60i-ti. 80-Gr 100 120 140 160 ^ 180 
 
 ncfli and Landau .
 
 viii.] PRESSURE OF THE AIR. 51 
 
 10. (4) Temperature. It is not difficult to see 
 how this influence acts. When air is heated it expands, 
 when cooled it contracts, behaving in this respect like 
 other substances. Cold air is denser than warm air, so 
 that the latter ascends, while the former descends. The 
 ascent of warm air must necessarily diminish atmospheric 
 pressure. When a broad tract of the earth's surface, 
 such for instance as the centre of Asia, is greatly heated 
 by the sun's rays, the hot air in contact with the ground 
 rises and flows over into the surrounding regions. Hence 
 the atmospheric pressure there is lowered during the hot 
 months of the year. 
 
 11. (2) Aqueous Vapour is, however, found to be 
 a still more important agent in affecting the pressure of 
 the air. We have already considered how universally 
 present this invisible vapour is, and how easily it may 
 usually be made visible by cooling the air ; when it is 
 at once changed into visible water. In Lesson X. an 
 account is given of the vapour of the atmosphere ; how 
 it is continually passing into the air, and as constantly 
 passing out of it again into some visible form of water. 
 Let us consider here how this unceasing process affects 
 the pressure of the atmosphere. 
 
 12. Suppose that we take two empty glass vessels, 
 each capable of containing exactly one cubic foot of any 
 substance. By means of an air-pump we remove the 
 air from them as completely as possible ; one of them 
 we fill with water-vapour, at a temperature of, say 50 
 Fahr. (Lesson IX. Art. i) ; the other, at precisely the 
 same temperature, we fill with perfectly dry air, that is, 
 air from which the water-vapour has been removed as 
 thoroughly as possible. We then weigh them both, and 
 after allowing for the weight of the glass-vessel in each 
 case, we find that the vapour weighs only 4-10 grains, 
 while the air weighs as much as 546*81 grains. 
 
 13. Now, without entering into the question how far 
 atmospheric pressure is due to mere weight or to other
 
 52 PHYSICAL GEOGRAPHY. [LESS. 
 
 causes, what conclusion must we draw from this experi- 
 ment ? Evidently, that water-vapour is vastly lighter, or 
 has less pressure, than air. At the temperature of 50 it 
 is about 133 times lighter, and though the difference 
 would be rather less at higher temperatures, still, under 
 all the temperatures to which the atmosphere is ordin- 
 arily subject, the weight or pressure of the air is always 
 far greater than that of the vapour. 
 
 14. Suppose, again, that we take six glass vessels, 
 each holding exactly a cubic foot, and fill them as fol- 
 lows : three with air saturated with vapour, each at a 
 different temperature, the first, say at that of freezing- 
 water (32), the second at that of an ordinary English 
 spring morning (50), and the third at that of a warm 
 English summer noon (80), and three with perfectly dry 
 air at the same three temperatures. In the first three, 
 the air of each vessel contains as much vapour as its 
 temperature will allow it to retain, and as we have seen 
 that cold air cannot hold so much as warm air, we know 
 that in the warmest vessel there must be a good deal 
 more vapour than in the coldest. We weigh them all 
 carefully as before, and find that the ice-cold moist air 
 weighs about a grain and a quarter (1-27 grain) less than 
 the perfectly dry air at the same temperature, that the 
 moist air at the medium temperature is about two and a 
 half grains lighter than the dry air, and that the warmest 
 moist air is about six and a half grains lighter than the 
 warmest dry air. 
 
 15. What lesson does this experiment teach ? Plainly 
 that the addition of water-vapour makes air lighter or 
 diminishes its pressure, and that this change is greater 
 the warmer the air, for more vapour can be dissolved in 
 warm than in cold air. 
 
 The vapour which rises so abundantly from sea and 
 land into the atmosphere diffuses itself through the air, 
 pushing the air atoms aside ; and having far less weight 
 than the air, it necessarily lessens the density of the
 
 viii.] PRESSURE OF THE AIR. 53 
 
 atmosphere, or in other words, lowers the atmospheric 
 pressure. A mixture of air and vapour is lighter than 
 the same volume of dry air would be, and, of course, the 
 larger the proportion of vapour, the greater will this 
 difference be. 
 
 16. The quantity of vapour in the atmosphere is con- 
 stantly varying from day to day, and from season to 
 season (Lesson X.). We perceive, therefore, that this 
 cannot fail to be at least one grand cause of the ceaseless 
 oscillations of pressure which the barometer reveals. 
 The addition of a large volume of vapour to the atmo- 
 sphere lowers the atmospheric pressure, and the mercury 
 in the barometer therefore falls. The removal of this 
 vapour either by condensation into rain, or otherwise, 
 restores the pressure, and the mercury rises again. Some- 
 times these fluctuations are very slow, extending over 
 days or weeks, sometimes a great oscillation may take 
 place within a few hours. 
 
 17. How it is that these vast accumulations of vapour 
 in any part of the atmosphere are brought about is still 
 unknown. It is well ascertained, however, that with the 
 condensations that follow, they determine movements of 
 the air. When sudden and extensive, they are accom- 
 panied by storms of rain and wind. When less violent, 
 they still show their influence upon the winds and 
 weather. 
 
 18. All movements of the atmosphere arise from 
 differences of pressure, caused, as far as we can tell, by 
 variations in temperature, and water-vapour. We shall 
 consider these in the next two lessons. 
 
 19. Observations with the barometer carried on for 
 many years in all quarters of the globe, have enabled 
 meteorologists to prepare charts showing, by means of 
 lines of equal pressure called Isobars, the general distri- 
 bution of atmospheric pressure over the earth's surface 
 for each month or season, or for the whole year. Plates 
 II. and III. are examples of such charts. It is found
 
 54 PHYSICAL GEOGRAPHY. [LESS. 
 
 that, taking a very broad view of the subject, there are 
 three great areas of low pressure. One extending as a 
 broad belt round the equatorial regions, the other two 
 lying about each pole ; and two areas of high pressure 
 extending parallel to the equatorial belt on either side, 
 and separating it from the polar area of low barometer. 
 These areas are most continuous in the southern hemi- 
 sphere ; but even there, and therefore still more in the 
 northern, they are broken up into detached portions 
 owing to the irregular distribution of sea and land. 
 They change position too with the seasons, as is well 
 shown by comparing the distribution of pressure in 
 January with that in July, as is done in Plates II. and 
 III. 
 
 LESSON IX. The Temperature of the Air. 
 
 1. As in the study of the pressure of the air so in that 
 of its temperature, it would not be possible to make 
 much progress without some means of accurately mea- 
 suring the fluctuations, for it is only the more marked of 
 these of which we are sensible from the comfort or dis- 
 comfort they bring to us. Happily, in this case too, 
 the instrument for accurate measurement is exceedingly 
 simple both in its construction and use. It is called 
 the Thermometer, or heat-measure, and consists of a 
 small glass tube closed at both ends, the lower of which 
 is expanded into a bulb. The tube, previously deprived 
 as completely as possible of air, is partially filled with 
 mercury or spirits of wine, and attached to a flat piece 
 of ivory, wood, or other substance, on which is placed a 
 graduated scale. Under the influence of heat the fluid 
 in the tube expands and rises ; when the heat is with- 
 drawn it contracts and descends. The temperature is 
 expressed by the figure on the scale of degrees, opposite 
 to which the upper end of the mercury column stands. 
 The thermometer scale in most common use in this
 
 DISTRIBUTION OF 
 
 TEMPERATURE 
 
 over the Surface of the 
 
 Globe. 
 
 Isothermal Lines shewing the 
 mean temperature of JANUARY. 
 (From the Chartt of Mr Buchan.) 
 
 140 
 
 120
 
 PLATE IV. 
 
 20 40 aot-s:eo**_Joo 120 140 ieo *s iso
 
 ix.] TEMPERATURE OF THE AIR. 55 
 
 country, called after the original maker, Fahrenheit's, is 
 so graduated that when the instrument is placed in 
 melting ice or in water just about to freeze, the mercury 
 marks 32 . 1 This is the freezing-point of fresh water. 
 On a pleasant summer day in England the mercury may 
 mark 70. On a hot noon in India it would rise to 90 
 or more, while on the burning sands of an African desert, 
 at the hottest time of the day, it might sometimes stand 
 as high as 1 50, or even higher. Under ordinary at- 
 mospheric pressure the thermometer indicates 212 as 
 the temperature at which water boils. When the mer- 
 cury is low in the tube, it marks cold, or what is called 
 a low temperature ; when it stands high in the tube 
 it indicates heat, or a high temperature. 
 
 2. By means of the thermometer it is possible to 
 measure very minute changes of temperature, and to 
 compare the range of temperature in different places. 
 Observations of this kind have now been carried on for 
 many years in all parts of the world, with the result of 
 making known the general distribution of temperature 
 over the globe. To show this distribution it is usual 
 to construct maps on which lines are drawn through 
 all places having the same temperature (see Plates IV. 
 and V.). Such lines have received the name of Iso- 
 therms, Isothermal lines, or lines of equal tempera- 
 ture. Each of them is named after the degree of the 
 thermometer which it expresses, as, for example, the 
 isotherm of 60, which shows that all the places through 
 which it is drawn on the map have the average tempera- 
 ture of 60. 
 
 3. Whence does the earth receive its heat, and why 
 does the temperature of one part of its surface vary so 
 much from that of another ? 
 
 1 The zero (o) of Fahrenheit's scale is 32 below the freezing-point of fresh 
 water. When the temperature sinks below zero it is marked by prefixing 
 the minus sign, thus - 5, - 10 : that is five, ten degrees below the zero- 
 point.
 
 56 PHYSICAL GEOGRAPHY. [LESS. 
 
 4. Although, as we have seen (Lesson III.), our 
 planet was once probably a molten globe, and retains 
 even now a vast amount of heat in its interior, its sur- 
 face temperature is not materially affected thereby. Were 
 it left without any other source of heat than its own, its 
 surface would become so intensely cold as to be utterly 
 uninhabitable by at least the races of plants and animals 
 which now live upon it. 
 
 5. It is from the Sun that our supply of heat comes ; 
 and, according to the greater or less amount of this 
 heat which different parts of the earth receive, they vary 
 in temperature. Were there no water-vapour in the 
 atmosphere the heat-rays would pass through with no 
 appreciable diminution ; but the presence of this vapour 
 partially arrests them, and thereby increases the tem- 
 perature of the air, but only to a very limited extent. 
 Hence in dry climates radiation (Art. 6) is most active, 
 the days being warmer and the nights colder than in 
 moist climates. 
 
 6. There are three processes whereby temperature is 
 interchanged, (i) The surface of the land, being warmed 
 by the sun's heat, warms the layer of air that lies upon 
 it. This is called conduction. (2) The warmed air 
 rises, while colder air descends, to be in turn warmed and 
 made to ascend. Currents in the atmosphere are thus 
 produced, and the heat carried along by them (or by 
 currents in the sea) is said to be diffused by convection. 
 (3) Heat is continually passing to and fro between ob- 
 jects freely exposed to each other. This is known as 
 radiation. The sun, for example, is unceasingly radiat- 
 ing heat in all directions, and so likewise is our own 
 globe. During the day the illuminated side of the earth 
 receives much more heat from the sun than it gives off 
 itself, and is consequently warmed. At night, on the 
 other hand, the dark side of the earth radiates into space 
 more heat than it receives from planets and stars, and is 
 therefore cooled. It is when the sun's rays have heated a
 
 IX.] 
 
 TEMPERATURE OF THE AIR. 
 
 57 
 
 part of the earth's surface that the air resting upon that 
 surface becomes heated by conduction, and the currents 
 arise which diffuse the heat by convection. 
 
 7. The heating power of the sun's ray? has been as- 
 certained to be dependent upon the angle at which they 
 reach the surface of our planet. Wherever they fall 
 vertically upon the earth, as at B in Fig. 8, their heating 
 power is greatest. This power diminishes as the direc- 
 tion of the rays recedes more and more from the vertical, 
 until, when they become horizontal, as at A and C, it 
 reaches its lowest point. Hence, though powerful at 
 
 FIG. 8. Diagram showing the influence of the varying thickness of the 
 atmosphere in retarding the Sun's heat. A. Line of the Sun's rays at 
 sunrise. B. Line of the rays at noon. c. Line of the rays at sunset. 
 
 noon, they are comparatively feeble in the morning and 
 in the evening. 
 
 8. In considering, then, the manner in which the 
 temperature of the atmosphere is distributed over the 
 globe, we see at the outset that those countries must 
 necessarily be warmest where the solar rays are vertical 
 or most nearly so, and that those regions must be coldest 
 where the rays are most oblique. At each place be- 
 tween the tropics (Lesson II. Art. 1 1) the sun is vertical 
 twice a year. That belt of the globe must needs there- 
 fore have the highest temperature. Round the poles 
 the sun does not shine for six months in winter, and 
 never gets very high in the sky even in summer. These 
 must consequently be the coldest regions. Hence the
 
 58 PHYSICAL GEOGRAPHY. [LESS. 
 
 first conclusion we may draw regarding the temperature 
 of the atmosphere is that it depends upon distance from 
 the equator, or, expressed more shortly (i) tempera- 
 ture is regnlated by latitude. 
 
 9. If no counteracting influence came into play there 
 would be a regular diminution of temperature from the 
 equator to the poles. Every latitude would then have 
 its own temperature, so that a determination of the 
 average temperature of a place would give us the lati- 
 tude ; or, knowing the latitude, we should know also 
 what must be its temperature. 
 
 10. But this correspondence holds only to a very 
 limited degree. The places which may chance to lie on 
 the same parallel most frequently do not enjoy the same 
 temperature. London, for example, coincides in latitude 
 with Labrador. But in England the summers are not 
 hot, and the winters are not very cold, while in Labrador 
 the summers are mild and the winters rigorously cold. 
 There must, therefore, be some other cause at work 
 modifying the influence of latitude. What is this 
 secondary cause ? 
 
 11. When Isothermal lines are drawn round the 
 globe, so as to connect together all the places which 
 have the same mean temperature, these lines, instead 
 of following the parallels of latitude, are found to bend 
 up and down, and these bendings are observed to have 
 reference to the positions of the continents and oceans. 
 Any temperature may be chosen. We may select, for 
 example, a temperature of 32 Fahr. or of 40, 50, 60, 
 or any other arbitrary point on the thermometer, it being 
 understood that over every part through which the line 
 is drawn the mean temperature during the year or one 
 of the seasons or months has been found by prolonged 
 observation to be the same, and amounts to the number 
 of degrees fixed upon. In this way we may represent 
 the mean temperature of the whole year, or that of 
 winter or of summer.
 
 IX.] TEMPERATURE OF THE AIR. 59 
 
 12. By referring to the maps in Plates IV. and V., 
 which represent the distribution of temperature over the 
 globe for January and for July, it will be seen how these 
 lines are drawn and how simply they tell the story of the 
 arrangement of temperature upon the earth. They arc 
 least irregular in the southern hemisphere over the wide 
 expanse of ocean ; they show the greatest deflections 
 across North America, the Atlantic, Europe, and Asia. 
 In this way they indicate that temperature is more 
 uniform and more directly dependent on latitude in the 
 oceanic parts of the globe than in the continental, or in 
 the regions where the oceanic and continental come 
 together, as they do in the basin of the Atlantic. 
 
 13. In order to see the full use and meaning of these 
 isothermal lines, let us take by way of illustration the 
 line in the northern hemisphere marking a mean annual 
 temperature of 50 Fahr. Traced over Britain, this line 
 runs from about London, across the centre of England 
 and the north of Wales : that is to say, all the parts of 
 the country lying along that line have a mean annual 
 temperature of 50, while the districts to the north-east 
 are a little colder and those to the south-west a little 
 warmer. The line bends south-westward and crosses to 
 the western shores of Ireland. If now we turn to the 
 opposite side of the Atlantic to see where places are to 
 be found having the same average temperature for the 
 year, we do not meet with them on the same parallel of 
 latitude as in the British Islands. They lie much farther 
 south, so that the line or isotherm of 50 makes a bend 
 in crossing the ocean, and reaches the American coast 
 not far from New York. The mean annual temperature 
 of London and New York is the same. And yet New 
 York is about as far south from London as Madrid. 
 
 14. This isothermal line of 50 between Europe and 
 America, and still more the other lines lying to the 
 north of it, illustrate how far the belts of equal heat are 
 from coinciding with the parallels of latitude. It will
 
 60 PHYSICAL GEOGRAPHY. [LESS. 
 
 be seen from the maps how this divergence is deter- 
 mined by the way in which the areas of land and sea are 
 grouped. 
 
 15. Land is sooner heated by the sun's rays than 
 the sea, and also gives off its heat again sooner. The 
 sea, though it does not become so hot as the land does, 
 retains its heat longer, and is enabled, by virtue of its 
 liquidity and motion, to diffuse it. Hence, the influence 
 of the sea tends to mitigate both the heat and the cold 
 of the land. Its warm currents heat the air resting on 
 them, and so give rise to warm winds which blo'v upon 
 the land, while its colder waters in like manner temper 
 the air, which reaches the land in cooling breezes, or it 
 may be in cold damp winds and fogs. Thus, in the 
 basin of the North Atlantic, a warm ocean-current called 
 the Gulf Stream issues from the Gulf of Mexico, and, 
 augmented by the surface-drift of warm water which is 
 driven onward by the prevalent south-west winds, flows 
 across the Atlantic to the shores of Britain and even of 
 Spitzbergen. It brings with it the supplies of heat which 
 make the climate of the west of Europe so much less 
 cold than it would naturally be. On the other hand, an 
 icy stream of water, coming out of Davis Strait, bears 
 a chill to the coasts of Labrador and Newfoundland. 
 The ocean, therefore, by its cold currents is depressing 
 the temperature in America, along the same latitudes 
 where, in Europe, by its warm currents, it is raising it. 
 
 16. A large body of land in high latitudes, by accu- 
 mulating snow and ice, gives rise to a lower temperature, 
 and a similar mass of land in low latitudes, by exposing 
 a broad surface to the tropical rays of the sun, produces 
 a higher temperature than would be found if the region 
 were occupied by the sea In illustration of this state- 
 ment it will be seen from the map (Plate IV.) that the 
 January isothermal lines marking temperatures below 
 the freezing-point, come a good way southwards over 
 Northern Asia, and again over Greenland and North
 
 180 160 140 120 100 
 
 DISTRIBUTION OF 
 
 TEMPERATURE 
 
 over tbe Surface of the 
 Globe. 
 
 Isothermal Lines shewing the 
 
 mean temperature of JULY. 
 
 (From the Charts of Mr Buchan.) 
 
 I00irfir 80rfG-.60
 
 PLATE. V. 
 
 20 40 60i-tz.80.rfe~.IOO 120 140 160
 
 ix.] TEMPERATURE OF THE AIR. 61 
 
 America, though in the water passage between these 
 regions they stretch a long way northwards. The coldest 
 winter temperature on the globe is that of north-eastern 
 Siberia, where the thermometer falls to 5 5-8 (nearly 
 56 below zero or 88 below the freezing-point). The 
 map of the July temperature (Plate V.) shows that over 
 the equatorial parts of America and the great mass of 
 Africa and Southern Asia, the space enclosed between 
 the isothermal lines of 80 swells out greatly, so as to 
 include a much larger area than where these lines cross 
 the oceans. The hottest climates are found in the dry 
 regions that stretch from the African deserts through 
 Arabia to India. 
 
 17. It by no means follows that two places which 
 have the same average yearly temperature enjoy the 
 same climate. For instance, Reykjavik, in the south of 
 Iceland (Lat. 64 40' N.) has a mean annual tempera- 
 ture of about 40 Fahr., while the city of Quebec has 
 one of about 39 ; but in July the average temperature 
 in the former locality is 52, in the latter it is 67; while 
 in the one case the January temperature is 30, and the 
 other 10 ; so that in winter Quebec is usually so intensely 
 cold as to have 22 of frost, while the south of Iceland 
 is often without frost. In summer, on the other hand, 
 Quebec is i 5 warmer than the south of Iceland. Canada 
 is chilled by the cold land and sea lying to the north and 
 north-east of it. Iceland is warmed by the ocean-cur- 
 rent (Art. i 5) which, summer and winter, sweeps past 
 its shores. 
 
 18. In order to compare the climates of two places, it 
 is necessary to know how the temperature is distributed 
 through the different seasons. To aid comparisons of 
 this kind, charts are prepared like those in Plates IV. and 
 V., showing the average distribution of temperature for 
 each month, or for summer and winter, as well as charts 
 constructed to show the mean temperature of each part 
 of the globe for the whole year. From the facts
 
 62 PHYSICAL GEOGRAPHY. [LESS. 
 
 embodied on such charts, gathered as they have been 
 from all parts of the world, we conclude that (2) 
 temperature is regulated by the distribution 
 of sea and land. 
 
 19. The influence of atmospheric pressure likewise 
 comes into play as powerfully affecting the distribution 
 of temperature. Thus the low pressure in the region of 
 Iceland during winter (Plate II.) causes the air to move 
 from the south-westwards over north-western Europe, and 
 from the north-westwards over north-eastern America. 
 In the one case the winds come from a warm ocean and 
 raise the temperature ; in the other they travel from the 
 cold Arctic lands and seas and therefore depress the 
 temperature. Again, the high pressure area of Central 
 Asia in winter is marked on the western side by a flow 
 of warmer air from the south south-west over Russia and 
 Western Siberia, and on the eastern side by a flow of 
 colder air from the north-west. Thus, places on the same 
 latitude and equally continental in position may differ as 
 much as 40 in their mean winter temperature. We 
 may conclude from such facts that (3) temperature is 
 materially affected by the prevailing winds. 
 
 20. The form of the land likewise greatly modifies 
 climate. Thus, a range of lofty mountains which stretches 
 across the track of a prevailing wind, by causing a pre- 
 cipitation of rain (or snow), makes the air drier. Conse- 
 quently, in the regions beyond the mountains, the effects 
 of radiation are increased, the summers being hotter 
 and the winters colder than they would otherwise be. 
 
 21. There is another way in which the form of 
 the land influences temperature. It is a familiar fact 
 that on low-lying land the air is warmer than on the 
 tops of hills. Even en many of the British mountains, 
 which are, comparatively speaking, low, the summits are 
 so cold that in their sheltered crevices, screened from the 
 sun and wind, snow remains unmelted through the sum- 
 mer. In the upper parts of the Alps, the Himalaya, the
 
 ix.] TEMPERATURE OF THE AIR. 63 
 
 Andes, and all the higher mountains of the globe, the 
 cold is such that the snows of winter never wholly dis- 
 appear, but remain as a perpetual covering. A gradual 
 cooling of the air is perceptible as we rise above the sea- 
 level in every part of the world. The rate at which this 
 fall in temperature takes place varies much, but it is 
 commonly taken to be on an average i Fahr. for every 
 300 feet. Within the tropics, while the low lands are 
 lying under a burning heat, the mountains, if lofty enough 
 to reach the upper cold air, have their summits covered 
 with snow : hence, an elevation of a few thousand feet 
 above the sea gives as great a change of temperature 
 as a journey of many thousand miles away from the 
 equator would do. From such examples we are taught 
 that (4) temperature is influenced by the form 
 of the land and especially by its height above 
 the sea. 
 
 22. A further familiar but important fact in the dis- 
 tribution of temperature deserves to be noticed here. All 
 over the globe there is a daily oscillation of temperature, 
 the highest reading of the thermometer being reached 
 shortly after noon, and the lowest between four and six 
 o'clock in the morning. The time of occurrence of the 
 maximum and minimum and the amount of difference 
 between them vary at different places, and even at the 
 same place, according to the season of the year. The 
 daily range of temperature generally increases towards 
 the equator and diminishes towards the poles ; it like- 
 wise increases in proportion to distance from the sea, 
 and is augmented by the dryness of the air. 1 
 
 23. Since the earth is continually receiving a supply 
 of heat from the sun, we may be tempted to ask whether 
 it is not getting warmer. So far as observations of 
 temperature have yet been made, they do not indicate 
 any sensible permanent increase or diminution of heat. 
 It would seem that the earth radiates back into space as 
 
 1 See Buchan : Art. " Meteorology," Encyc. Britann., 9th edit.
 
 64 PHYSICAL GEOGRAPHY. [LESS. 
 
 much heat as it absorbs. The quantity of heat received 
 from the sun may be looked upon as remaining, on the 
 whole, constant from year to year ; though careful obser- 
 vations of the sun's surface, and especially of the recur- 
 rence of the black spots visible there, may yet show that 
 the quantity varies from time to time, and that the varia- 
 tion affects the temperature and climate of our planet. 
 It has been satisfactorily established that there is a coin- 
 cidence between rainfall, including storms, and those 
 periods when the sun's face is most marked with spots. 
 
 24. Radiation from the earth's surface is felt most 
 at night, and especially when the sky is clear. At such 
 times we may learn how rapidly the heat of the day is 
 passing away from the earth into cold stellar space, and 
 how entirely our globe depends on the sun for its present 
 surface temperature. Objects, which in the day were 
 warm to the touch, now grow more and more chilly. The 
 air grows colder by contact with the cooled ground, and 
 our own bodies, like everything else around, are at the 
 same time radiating heat, and adding to our sensation 
 of cold. 
 
 LESSON X. The Mgistnre of the Air. 
 
 1. One of the constantly present ingredients in air 
 described in Lesson VI. was water- vapour. We have 
 found how important this component is in determining 
 differences of pressure, and consequently in giving rise 
 to changes of weather. It may now be considered 
 rather more at length in reference to its sources of 
 supply, and the different forms in which it is taken out 
 of the air and restored again to land and sea. 
 
 2. First, then, let us ask whence does this widely- 
 diffused and all-important vapour come ? It is all 
 evaporated, or given off in an invisible form, from the 
 surface of every sea, lake, river, and spring ; in short, 
 from every water-surface on the face of the earth, and
 
 x.] THE MOISTURE OF THE AIR. 65 
 
 even from ice and snow. Nothing is more familiar than 
 the rapidity with which water dries up on streets and 
 roads after rain. Every accumulation of water, indeed, 
 which is freely exposed to the air, and is not replenished 
 with water, is seen to diminish, and finally to disappear. 
 It is not that the water merely sinks into the ground. 
 Part of it no doubt does so, but we find the water to 
 disappear even from a saucer or other vessel where there 
 can be no loss underneath. 
 
 3. The air is usually busy absorbing vapour. When 
 it can hold no more it is said to be saturated, or to reach 
 its point of saturation, and then evaporation ceases. 
 This limit varies according to temperature, warm air, as 
 was proved in Lesson VIII., being able to contain more 
 vapour than cold air. Evaporation is greatly helped by 
 wind. Damp places and pools of water, for instance, 
 dry up in a breeze sooner than they do in still air, 
 because the wind removes the vapour as it is formed, 
 and brings other and drier air to drink up and carry 
 away renewed supplies of vapour. 
 
 4. Evaporation, therefore, takes place chiefly during 
 the day, especially the warm parts of the day, and more 
 actively in summer than in winter. It is feeble in 
 amount when the air is moist and still, but goes on 
 briskly when a fresh wind blows. It takes place far 
 more copiously in warm tropical regions than in those 
 with a temperate or polar climate. 
 
 6. It has been calculated that the amount of water 
 which is annually condensed out of the atmosphere upon 
 the surface of the earth would, if collected together into 
 one mass, cover to the depth of one mile an area of 
 about 200,000 square miles, or a space nearly equal to 
 the size of the whole of France. This vast liquid mass 
 is, as it were, pumped out of the sea and out of the 
 waters of the land by the sun's heat. But the amount 
 of water that passes off into vapour in the atmosphere is 
 best realised when we consider the enormous quantity 
 F
 
 66 PHYSICAL GEOGRAPHY. [LESS. 
 
 discharged into the sea by rivers. All over the world, 
 rivers, great and small, are continually pouring their bur- 
 den of water into the ocean. All the water thus supplied 
 is obtained by the rivers from the atmosphere, either 
 directly by rains and snows, or indirectly by springs. 
 Vast though the amount of water is which is daily dis- 
 charged into the sea by rivers, it is obviously less than 
 the quantity actually received by the rivers themselves, 
 for while they are coursing down from the mountains to 
 the sea, vapour continually rises from their surfaces, and 
 their volume consequently grows less. 
 
 6. If, then, from every ocean, lake, and river on the 
 surface of the globe water-vapour is continually passing 
 into the air, what becomes of all this vapour, and why- 
 do not the waters of the globe sensibly diminish ? The 
 reason is, that the conversion of water from its common 
 visible liquid form into the invisible gaseous state is 
 only one-half of a gigantic system of circulation. The 
 vapour is not allowed to accumulate indefinitely in the 
 atmosphere ; it is changed back again or condensed into 
 water, and then appears in such forms as dew, clouds, 
 rain, hail, or snow. 
 
 7. The two processes of evaporation and condensation 
 balance each other ; that is, so far as the larger features 
 of the earth's system are concerned, as much water is 
 returned to the sea and land as is taken from them by 
 the atmosphere. It is from this circulation of water 
 that all the manifold phenomena of clouds, rain, snow, 
 rivers, glaciers, and lakes arise. But more than this, if 
 we reflect that now evaporation and now condensation 
 must from time to time be predominant at any place, we 
 perceive how greatly these processes must affect the 
 pressure of the air ; and since variations in pressure 
 determine the various movements of the atmosphere 
 (Lesson XL), it is obvious how all-important this water- 
 vapour must be in the present arrangements of our 
 globe.
 
 x.] THE MOISTURE OF THE AIR. 67 
 
 8. So far as we can tell, though the quantity of vapour 
 may sometimes be greatly reduced, it never disappears 
 wholly from the air at any place. On the other hand, 
 the air round about us is comparatively seldom so satu- 
 rated with moisture as not to be able to hold any more, 
 though in damp weather we may see from the extreme 
 slowness with which wet places dry up, how little appetite 
 the air then has for further vapour. 
 
 9. One office of the vapour of the atmosphere is to 
 keep the earth much warmer than it would be were the 
 air quite dry. On the one hand, it interposes as an 
 invisible screen between the earth and the rays of the 
 sun, which would otherwise be intensely hot. On the 
 other hand, the same screen, often condensing into visible 
 form as clouds, prevents the earth at night from giving 
 off its heat too rapidly into cold space. If it could be 
 removed for a little from around us, we should be burnt 
 up by day and frozen hard at night. Clouds would cease 
 to form, rain to fall, and streams to flow, and the condi- 
 tion of the planet as a habitable globe would be brought 
 to a stand. 
 
 10. When evaporation takes place, heat is abstracted 
 by the vapour from the surface which evaporates. If 
 you spread a drop of water over the back of the hand 
 you feel the skin to be chilled a little, because the water 
 in passing into vapour abstracts heat from the hand. It 
 is a common practice to cool liquids by placing moist 
 flannel round the vessels in which they are contained. 
 The moisture of the flannel evaporates, and in so doing 
 takes heat away from the vessel inside. The invisible 
 vapour which rises into the air so abundantly carries 
 heat along with it. This heat, not being sensible so long 
 as the vapour remains uncondensed, has been termed 
 latent. 
 
 11. Again, when condensation goes on, the heat which 
 the vapour had held in its grasp is given out again and 
 becomes sensible, as the vapour passes into water. It
 
 68 PHYSICAL GEOGRAPHY. [LESS. 
 
 has been pointed out, for example, that every pound of 
 water which is condensed from vapour liberates heat 
 enough to melt five pounds of cast-iron. We can well 
 understand, therefore, that when the process of conden- 
 sation takes place in nature on a large scale, the con- 
 version of vapour back again into the state of water 
 materially warms the air. 
 
 12. The act of condensation always occurs when air 
 is cooled down to, or rather just below, its dew-point 
 (Art. 1 4) ; but this does not always happen at the same 
 temperature, nor does it appear in the same forms. 
 Sometimes it issues in a light mist, or beads of dew, or 
 drops of rain, or, if the temperature should be low enough, 
 in hoar-frost, flakes of snow, or pellets of hail. 
 
 13. The substance which we call water is thus shown 
 to exist in three forms, according to temperature. At 
 ordinaiy temperatures, or from 32 to 212 Fahr., it is 
 abundant as a liquid, and this is, of course, its most 
 familiar condition. If at any temperature, even in the 
 condition of ice or snow, it is allowed to stand exposed to 
 the air, or if by having its temperature sufficiently raised, 
 it boils, it passes off into invisible vapour. If again the 
 temperature be depressed to 32, the water begins to 
 become solid. It crystallises into the brittle, colourless 
 substance called ice. This act of crystallisation has 
 received the name of freezing, and the temperature (32) 
 at which it takes place, is called the freezing-point. 
 According therefore to the temperature at which con- 
 densation takes place, the invisible water-vapour assumes 
 a liquid or a solid form. 
 
 14. Dew. On a summer evening, if the sky is clear, 
 condensation takes place on the leaves of plants, on 
 stones and other objects exposed to the sky. These 
 become covered with fine drops of water, which is known 
 by the name of Dew. If the sky is cloudy, dew either 
 does not form or only in small quantity, since on cloudy 
 nights the air is not so cold as on those which are
 
 x.] THE MOISTURE OF THE AIR. 69 
 
 clear. The cause of the appearance of this moisture is 
 the same as that of the mist on a tumbler of very cold 
 water brought into a warm room. The dew comes out 
 of the vapour of the atmosphere, and not out of the 
 objects on which it forms. When the sky is cloudless, 
 radiation goes on rapidly from the earth, and the sur- 
 faces of those substances which part with their heat 
 most readily become much colder than the air. Grass, 
 for example, cools about twice as much as common 
 garden-soil. As the process advances, the air next the 
 cooling surfaces is so chilled as to be no longer able to 
 keep all its vapour, part of which is condensed and 
 appears as dew. Hence the grass soon becomes quite 
 wet with the copious gathering of dew upon its blades. 
 The temperature at which this condensation takes place 
 is the point of saturation or dew-point. 
 
 15. It has been pointed out (Art. 10, n) that the 
 vapour of the atmosphere contains a large amount of 
 latent heat, and that when the vapour is condensed this 
 heat becomes sensible. The formation of a film of dew 
 upon the surface of the earth lets heat escape back again 
 into the air. But as radiation proceeds, the warmed air, 
 lying on the cooling surfaces, is again chilled down to 
 the dew-point, then more dew is abstracted, and more 
 heat given out. In this way the nights are kept from 
 getting colder than the temperature of the dew-point. 
 But sometimes, when the temperature is low, as in win- 
 ter, or when the dew-point is low and radiation is great, 
 the surface of the ground is so chilled that the dew 
 freezes in forming, and appears as the white rime, or 
 hoar-frost, which we see at early morning on the grass. 
 Clouds hinder dew from forming because they check the 
 passage of heat from the earth into space while they 
 themselves radiate heat towards the earth ; hence it is 
 that cloudy nights are warmer than those which are 
 clear and starry. 
 
 16. Mist and Fog. When a mass of warm and
 
 70 PHYSICAL GEOGRAPHY. [LESS. 
 
 moist air meets colder air, or comes into contact with 
 cold ground or in any other way is cooled down below 
 its dew-point, the excess of vapour, which it can no 
 longer retain, condenses into minute particles, and is 
 made visible in the form of Mist or Fog. In winter, we 
 have a familiar illustration of this phenomenon in the 
 condensation of our breath into mist, as it passes from 
 the mouth into the cold air. In summer, mists fre- 
 quently form in the evening over rivers and sheets of 
 still water. As radiation goes on, the ground round the 
 water becomes several degrees colder than the water 
 itself, and the vapour rising from the water is conse- 
 quently chilled by the cold overlying air and is condensed 
 into banks or sheets of fog. Again, when a warm wind 
 strikes upon a hill or mountain, and is forced to ascend 
 the slopes, its temperature begins to fall, and if this cool- 
 ing is carried below the temperature above which it can 
 retain its vapour, that is its dew-point, the excess of 
 vapour takes the form of a mist. 
 
 17. Clouds. Dew and mist are formed on or near 
 the ground, whether low plains or high mountains. 
 Vapour carried up into the cold upper parts of the at- 
 mosphere, condenses and becomes visible there in 
 Clouds. A cloud is only a mist hanging in the air, in- 
 stead of resting on the ground. When the ground rises 
 into the upper air, as it does in mountains, it reaches or 
 even surmounts the layers of the atmosphere where 
 clouds are commonly formed. We see wreaths of cloud 
 gathering on the sides or summits of mountains, and 
 yet if we climb up to these clouds we find them to be 
 mere masses of mist, such as are formed down among 
 the valleys. 
 
 18. Owing to the constant great changes in tempera- 
 ture and evaporation at the earth's surface, currents of 
 air must always be somewhere ascending and carry- 
 ing vapour up with them. But besides this vertical 
 movement, the atmosphere, as far upward as can be
 
 x.] THE MOISTURE OF THE AIR. 71 
 
 observed, includes many different layers or horizontal 
 currents one above another, moving in various and 
 even opposite directions. We may often notice the 
 existence of these high aerial currents by watching the 
 movements of the clouds. A lower layer of thick, woolly 
 clouds may, for example, be rolling along, but through 
 the rifts, a far higher layer of thin, light, white cloudlets 
 may be drifting in a contrary direction. People who 
 have made ascents in balloons, have found abundant 
 evidence of the great number and various motions of 
 these atmospheric currents. 
 
 19. When an ascending current of warm, moist air 
 rises high above the surface of the earth, it meets with 
 air much colder than itself, and it likewise loses sensible 
 heat from the expansion consequent on the diminished 
 atmospheric pressure at these elevations. Chilled down 
 to its dew-point, it forms a cloud in the air. The cloud 
 may be thought to remain perfectly motionless and un- 
 changed in form, yet when watched attentively, it may 
 be seen to alter, somewhat in the same way as a wreath 
 or cap of mist on a mountain, and for the same reason, 
 as explained in Art. 23. 
 
 20. It is often easy to watch how clouds grow and 
 disappear. During summer, for instance, when the sky 
 has been quite clear in the morning, we may observe 
 that as the day advances white clouds make their ap- 
 pearance, small at first, but growing by degrees from a 
 more or less level base into huge piles with every variety 
 of fantastic outline. As evening comes on, these masses 
 diminish slowly. At sunset, only a few thin flaky cloud- 
 lets are perhaps to be seen, and at last when night 
 sets in, the sky is once more clear and cloudless. In 
 such cases, it is to the warming of the earth by the sun 
 during the day, and the consequent ascent of moist air- 
 currents, that the clouds are due. Each growing mass 
 of cloud forms the top or capital, as it were, of a column 
 of up-streaming warm air laden with vapour. The air,
 
 72 PHYSICAL GEOGRAPHY. [LESS. 
 
 expanding and cooling as it rises, at last reaches a point 
 where it can no longer retain its vapour, and there the 
 cloud begins to form. After the heat of the day is over, 
 and the warm moist air no longer streams upward, the 
 cloud ceases to grow, and then begins to sink earthwards, 
 as radiation goes on, until coming into warmer air, it 
 gradually melts away and leaves the sky clear and starry. 
 
 21. As the atmosphere is traversed in all directions 
 by currents of air of various temperatures and degrees 
 of moisture, the meeting of these currents must often 
 give rise to clouds, and likewise dissolve clouds already' 
 formed. A warm moist wind, for example, coming in 
 contact with a cold wind will part with some of the 
 vapour as cloud. On the other hand, a belt of cloud 
 invaded by warm dry air will be evaporated and 
 disappear. As a rule, ascending clouds increase in 
 size, while descending clouds diminish, because in the 
 one case they rise into colder air, and in the other, sink 
 into warmer air. It is the continual movements of the 
 atmosphere which produce the never-ending comings 
 and goings of the cloud-world above us. 
 
 22. When clouds enter or are formed in one of the 
 upper steady air-currents they are borne along, sometimes 
 for great distances and at a great rate. On a breezy 
 spring day, they may be seen sailing across the sky at 
 what may seem a leisurely pace, which, however, by the 
 rate at which their swiftly-moving shadows fly across hill 
 and plain, is proved to be often more than 80 or even 
 1 20 miles an hour. They can be watched continually 
 changing shape and size as they move along, rolling in 
 huge folds over each other, sometimes lessening and 
 sometimes increasing, and in all these movements testi- 
 fying to the ceaseless turmoil of the atmosphere in which 
 they are suspended. 
 
 23. In hilly countries, another curious feature of cloud- 
 formation may often be seen. When a strong wind blows, 
 sending leaves and dust high into the air, a cloud may
 
 x.] THE MOISTURE OF THE AIR. 73 
 
 be observed to remain persistently on a mountain-top. 
 Fragments of the cloud may now and then be torn off 
 by the wind, but these, after travelling some way, gradu- 
 ally disappear. In such cases, the wind comes along 
 with its warm vapour, which is quite invisible until it 
 strikes against the colder hill-side and undergoes expan- 
 sion, as it is forced up the slope into a higher level in the 
 atmosphere. Chilled there, it passes into a fine mist, 
 which, seen from a distance, takes the shape of a cloud 
 
 FIG. 9. Clouds condensed by the cliff of the Noss Head, Shetland, with 
 a clear sky and a S. E. wind. Sketched on June I4th, 1876. 
 
 capping the summit. This cloud is stationary, but the 
 little particles of which it consists are not. The wind 
 continues to blow up and over the mountain-top, and 
 the vapour which it carries along is made visible as mist 
 or cloud, while it sweeps over the chill ground. When 
 it has passed the mountain and mixes again with the 
 warmer air beyond, the visible cloud is redissolved and 
 therefore melts away on the leeward side of the hill as 
 fast, perhaps, as it formed on the windward side. The 
 cloudlets, too, which are occasionally detached and 
 carried away from the mountain -top by the wind, 
 being gradually absorbed into the air again, disappear. 
 (Fig. 9-)
 
 74 PHYSICAL GEOGRAPHY. [LESS. 
 
 24. Considerable variety exists in the forms which 
 clouds assume, from the thin flaky cloudlets seen in the 
 high upper air down to the huge rolling rain-clouds that 
 descend low upon the hills, and the dull gray sheet of 
 cloud that overspreads the whole sky. To these various 
 forms special names have been given which, however, 
 it is not necessary to enumerate here. Each kind of 
 cloud is formed in particular conditions of the atmo- 
 sphere, and hence a study of the clouds furnishes valu- 
 able information regarding the weather. This subject 
 properly belongs to the study of Meteorology. 
 
 25. The chief function to be considered in the me- 
 chanism of the clouds is the way in which they supply 
 the earth with moisture. The vast amount of water 
 raised into the atmosphere as invisible vapour, returns 
 to the surface of the earth, to feed the springs and rivers 
 of the land and to replenish the sea. In this constant 
 circulation the clouds act the part of condensers. They 
 collect in a visible form the ever-present invisible vapour 
 of the air, and allow it to fall back again upon the earth. 
 
 26. Rain. By far the larger part of the vapour of 
 the atmosphere descends to the earth in the form of rain. 
 The little water-particles of which cloud is composed 
 run together as the condensation proceeds. As the 
 drops thus formed increase, they become too heavy to 
 float in the air, and begin to fall earthwards as rain. 
 At first they are very small, as we may feel if we happen 
 to be on a mountain where mist is gathering into rain- 
 cloud. But as they descend through the air they in- 
 crease in size until they reach the ground in well-marked 
 rain-drops. 
 
 27. Rain, therefore, is a further stage in the conden- 
 sation of mist or cloud. If the chilling of the cloud is 
 prolonged, rain falls from it. This may be caused in 
 several ways. For instance, where a warm moisture- 
 laden wind comes against a range of high mountains, 
 and is consequently forced to ascend, it may not only
 
 x.] THE MOISTURE OF THE AIR. 75 
 
 be condensed internist (Art. 19), but if cooled still more, 
 will drop its moisture as rain. Or a cold wind which 
 is heavy and keeps next to the ground, may wedge itself 
 in below a warm, moist layer of air, and so chill it as to 
 form cloud and bring down rain. 
 
 28. The fall of rain, being dependent upon the amount 
 of evaporation, is greatest in tropical regions, where the 
 largest supplies of vapour pass into the air, and decreases 
 with the gradual sinking of the temperature towards the 
 poles. But this general law is subject to some import- 
 ant qualifications arising from the distribution of sea and 
 land, and the direction of the great currents of the 
 atmosphere. 
 
 29. (i.) Although evaporation is more abundant from 
 the surface of the sea than of the land, condensation is 
 more active over land than over sea. Hence the rain- 
 fall is also greater on land than at sea, and over the 
 northern hemisphere, much of which is land, than over 
 the southern hemisphere, most of which is water. 
 
 (2.) As the ocean furnishes most of the vapour of the 
 atmosphere, the condensation of vapour into rain upon 
 the land is greatest near the coast-line. The sea-board 
 of a country may be rainy while the interior is compara- 
 tively dry. 
 
 (3.) The fall of rain is greatly affected by the form of 
 the surface of the land. Mountains act as condensers 
 (Art. 23), and are consequently much wetter than plains. 
 It has likewise been ascertained by rain-gauges that the 
 amount of rain which falls at the surface of the ground 
 is greater than at a height of even a few feet above it. 
 
 (4.) Places which lie in the path of any of the regular 
 air-currents are wet when they cool the current, and dry 
 when they warm it. Hence winds blowing towards the 
 equator, since they come into warmer latitudes, are not 
 usually wet winds ; but when winds blow towards the 
 poles, they reach colder latitudes, are chilled, and there- 
 fore become rainy.
 
 76 PHYSICAL GEOGRAPHY. [LESS. 
 
 30. Some of these laws are well illustrated in the 
 British islands, where the rains are chiefly brought by 
 the south-westerly winds from the Atlantic. The coast- 
 line facing that ocean is more rainy than the east side 
 looking to the narrow North Sea. On the western sea- 
 board, the amount of rain which falls in a year would, if 
 collected together, have a depth of from thirty to forty-five 
 inches. On the east side, however, the average annual 
 rainfall does not exceed twenty to twenty-eight inches. 
 Where the west coast happens to be mountainous, the 
 rainfall becomes still more copious ; hence the wetness 
 of the climate along many parts of the north-west coast 
 of Scotland, and in the Lake district of England, where 
 the annual rainfall ranges from eighty to one hundred 
 and fifty, and sometimes even more than two hundred 
 inches. 
 
 31. Great differences are observed at different points 
 of the earth's surface in the annual amount of rainfall. 
 Between the tropics, where rapid and copious evapora- 
 tion raises a constant stream of vapour into the atmo- 
 sphere, rain is heavy and frequent, so much so that this 
 belt of the earth's surface is known as the Zone of 
 Constant Precipitation (Lesson XI. Arts. I o, 1 1 ). 
 Where, in this rainy region, a high mass of land lies in 
 the path of the warm moist air-currents, the rainfall is 
 still further increased. Thus, in India, the range of the 
 Khasi hills stretches across the course taken by the 
 winds called the south-west monsoons, which bring up 
 their burden of warm vapour from the Bay of Bengal 
 (Lesson XL Art. 34). The result is, that the winds as 
 they slant up the hills into the higher and cooler air 
 have their moisture at once precipitated as rain, of which 
 as much as 493 inches fall there in a year. 
 
 32. Again, a tract of country situated behind a high 
 mass of land to which vapour-laden winds blow, may 
 have little or no rain. Thus, in India, while the range 
 of the Western Ghats, lying in the pathway of the warm
 
 MEAN ANNUAL RAINFALL 
 OP THE GLOBE. 
 
 Less than 10 inches 
 
 10 to 25 
 
 26 ,,50 
 
 60 ,,76,, 
 
 Orer-76 
 
 (From Chart of Professor B. Lornn.it. 
 Yale College.)
 
 PLATE VI. 
 
 20 40 60i.r t. 80rf&-IOO 120 140 160 'S 180
 
 x.] THE MOISTURE OF THE AIR. 77 
 
 moist monsoon from the Indian Ocean, intercepts a 
 heavy rainfall, amounting on the tops of the range to 
 260 inches annually, the country on the east or lee side 
 receives comparatively little rain. Poona, for instance, 
 lying at the foot of the hills, has a yearly rainfall of only 
 30 inches. 
 
 33. In South America the high chain of the Andes 
 takes out the last of the moisture which blows from the 
 east across the Continent, and the winds descend upon 
 Peru so dry that rain is almost unknown there. Another 
 and much larger rainless tract lies in the deserts that 
 stretch from North Africa across Arabia far into the 
 heart of Asia. In these regions, the dry, sandy soil is 
 raised to an intense heat during the day. There is little 
 or no water to evaporate. The hot, dry air ascends, 
 but the winds which blow in upon the deserts cannot 
 deposit any moisture, for instead of being cooled, they 
 are heated and driven up in the ascending currents. 
 
 34. In some countries (Lesson XI. Art. 21), the 
 wind blows for part of the year in one direction, and for 
 the rest in the opposite direction. These periodical 
 winds are generally accompanied with rain when they 
 come from warm to cold, and with dry weather when 
 they come from cool to warmer regions. They conse- 
 quently are accompanied with rainy seasons and dry 
 seasons. During June and July, for example, the south- 
 erly wind, referred to in Art. 31, brings the "rains" 
 that refresh the surface of India after the scorching heat 
 of April and May. During November, December, and 
 January, on the other hand, a cool dry wind gently 
 streams down from the northern mountains into the 
 plains of Hindostan, and brings calm, dry, and settled 
 weather. In north-western Europe, and generally in 
 those parts of the earth which lie in temperate and arctic 
 regions, great irregularity of rainfall occurs. In the 
 western parts of Europe the chief rainfall is in winter, 
 but eastwards it comes in summer.
 
 78 PHYSICAL GEOGRAPHY. [LESS. 
 
 35. Rain falls to the earth as water nearly pure. It 
 is, indeed, natural distilled water. But nevertheless it is 
 never quite pure, and sometimes it contains a noticeable 
 amount of impurities. (Lesson VI. Art. 6.) Thus, it ab- 
 sorbs a little air, but the proportions of the atmospheric 
 gases which it dissolves are not the same as in the at- 
 mosphere itself, there being a higher percentage of oxygen 
 and carbonic acid than in air. Other gases and vapours 
 are also occasionally present. All these, when dissolved 
 in the falling rain, are brought down to the earth together 
 with the particles of dust floating in the air. The decay 
 of animals and plants furnishes a large supply of infinit- 
 esimal particles of organic matter, while over and above 
 these, millions of microscopic living organisms float all 
 through the lower parts of the atmosphere. In ordinary 
 good air, these various impurities are, of course, in ex- 
 ceedingly small proportions. They are least abundant 
 in the clear air of the mountains, and most in the bad 
 air of towns. Now, the rain, as it were, washes these 
 out of the air, and thus purifies it, and makes it healthier, 
 while at the same time it carries down into the soil sub- 
 stances such as ammonia, which are diffused in the air, 
 and are of service to the growth of plants. So that 
 in addition to all the benefits which rain confers upon 
 us by moistening and fertilising the soil, filling the rivers, 
 replenishing the springs, and thus keeping the face of 
 nature fresh and green, it cleanses for us the very air 
 which we breathe. 
 
 36. Snow. When from any cause and in any place, 
 whether on the land, or in the air, water is cooled down 
 to 32 Fahr., it usually no longer remains in the liquid 
 form, but passes into ice (Art. 13). Ice is formed in 
 the air by the freezing of the particles which are con- 
 densed from the water- vapour. It may descend to 
 the ground as Snow, Sleet, or Hail, according to the 
 circumstances in which it is formed, or the condition 
 of the successive layers of air through which it has
 
 x.] THE MOISTURE OF THE AIR. 79 
 
 to pass in its descent. Since the temperature falls in 
 proportion as the air recedes from the surface of the 
 earth, the freezing-point will be reached at no great 
 height above the ground. We might, indeed, imagine a 
 line passing through the atmosphere from pole to pole, 
 and marking the isotherm of 32, that is, the limit 
 above which the water-vapour must condense into ice, 
 and below which it will condense into water. Such a 
 line would be liable to oscillate up and down, accord- 
 ing to the latitude, the season of the year, and the vary- 
 ing currents of the air. In England, for instance, it 
 would descend to the very ground in winter when the 
 ponds and streams are crusted with ice during the cold 
 period, called a frost, while in summer it would be a 
 mile and a half above our heads. In India it would be 
 three miles overhead. 
 
 37. Let us imagine, then, such a variable line or 
 limit in the air, and consider the forms in which the 
 frozen moisture exists there, or descends thence to the 
 ground. It is highly probable that many of the delicate 
 white cloudlets which we see far up in the air in summer 
 are made of fine snow. At a lower level, flakes of snow 
 are formed and fall earthwards, though from their 
 feathery forms and lightness they are liable to be driven 
 about by every feeble breath of wind. 
 
 38. A snow-flake formed in calm air will be found 
 on examination to possess a regular structure. It is, in 
 fact, built up of minute needles or crystals of ice sym- 
 metrically arranged into a star with six rays, each of 
 which has a feathery surface, from the abundance of 
 minute crystals of ice ranged along its sides. The 
 accompanying woodcut (Fig. 10) represents different 
 varieties of snow-flakes, each of which, however, is only 
 a modification of the same six-rayed star. The rays 
 strike off from each other at an angle of 60, and no 
 matter how complicated the form of the snow-flake may 
 be, this angle is always maintained between all the rays.
 
 8o PHYSICAL GEOGRAPHY. [LESS. 
 
 All ice, even in the solid sheets which form on rivers 
 and lakes, during the winter of cold climates, is built up 
 of particles which tend to range themselves in hexagonal 
 
 FIG. 10. Snow-flakes. 
 
 crystals, although it is only in the snow-flake that this 
 structure is usually seen. 
 
 39. Snow is white, but if one of the flakes be taken 
 by itself it will .be seen to be a little crystal or group of 
 crystals of transparent ice, glittering with the prismatic 
 colours. So that the white hue of a mass of snow arises 
 from a combination of all these colours reflected from 
 the immense number of minute surfaces of ice. In the 
 same way a dish of salt appears white, and yet any one 
 of the little crystals of which it is made up will be found 
 to be transparent and colourless. 
 
 40. When the air is intensely cold, that is, much 
 below the freezing-point, any snow which may then fall 
 appears in the form of minute flakes, like a white powder. 
 The largest flakes appear when the temperature is nearly 
 at the freezing-point. The heaviest snow-falls do not 
 take place during severe frosts, but before or after them. 
 This arises from the fact that in proportion as the air 
 grows colder it loses the capacity to retain watery vapour, 
 so that as it becomes intensely cold it grows also com- 
 paratively dry. 
 
 41. Over by much the larger part of the globe snow 
 never falls. It can only appear when the temperature 
 sinks nearly to 32 at the surface of the earth. Again, 
 it appears on those elevated parts of the land which rise 
 into higher layers of the atmosphere, where the same low 
 temperature prevails. Thus the Himalaya mountains,
 
 x.] THE MOISTURE OF THE AIR. 81 
 
 though they stand in one of the hottest quarters of the 
 globe, are yet so lofty that their upper portions tower far 
 into the cold upper air, and are therefore covered with 
 snow. On the south side of that high range the lower 
 limit of the snow descends about 4000 feet lower than 
 on the north side, because the cold mountain slopes con- 
 dense into snow the moisture which is blown from the 
 Indian Ocean, and allow the air to pass comparatively 
 dry over to the north side, and because the dry air from 
 the heated plains of Thibet evaporates the snow on 
 the north side. 
 
 42. The enow-line, or limit of perpetual snow, is the 
 
 FIG. n. Position and height of the Snow-line between Equatorial Africa 
 and the North Polar Seas. 
 
 name given to a line below which the summer heat suffices 
 to melt all the snow, but above which more snow falls 
 than the warmth of the summer months can thaw (Fig. 
 1 1 ). We may picture it to ourselves as a great invisible 
 arch, of which the centre rises high over the equatorial 
 regions while the extremities come down to the sea-level 
 within the Arctic and Antarctic circles. Under the centre 
 of this arch the temperature is so high that snow is only 
 seen on the loftiest mountains, about a height of i 5,000 
 to 20,000 feet over the sea-level, while in the far north 
 and south where the temperature is greatly lower, the 

 
 82 PHYSICAL GEOGRAPHY. [LESS. 
 
 snow remains not wholly melted, even down to the 
 margin of the sea. 
 
 43. Snow is of great use in winter, as it protects 
 vegetation from being nipped by severe frost. Being a 
 bad conductor of heat, it keeps the soil and plants on 
 which it lies from parting readily with their warmth. 
 During a frost, therefore, the soil and plants lying under a 
 few inches of snow will be found soft and uninjured, while 
 on those places where the snow has been blown away 
 the soil is frozen stiff, sometimes to a depth of eighteen 
 inches. 
 
 44. Snow that accumulates above the snow-line is 
 pressed into ice, and creeps down into the valleys in the 
 form of Glaciers. But this further stage in its history 
 will be considered in a later Lesson in connection with 
 the circulation of water over the land. 
 
 45. Sleet. When snow is much driven about by the 
 wind, its delicate fretwork of crystals is greatly broken. 
 If this takes place when the temperature is rising, or if 
 the driving snow falls through a warmer layer of air, it 
 begins to melt, and in this half-melted state reaches the 
 ground as Sleet. 
 
 46. Hail is the name given to pellets of snow and 
 pieces or concretions of ice which fall from clouds. 
 Usually hailstones are small, white, rounded, conical, 
 or irregular. Sometimes, though rarely, they assume 
 more definite crystalline shapes. They are occasionally 
 as large as eggs, and sometimes, when several of these 
 come together in their descent through the air, they 
 freeze into large irregular lumps of ice. Hail is more 
 frequent in summer than in winter, and in hot than in 
 cool weather. Hail frequently accompanies thunder- 
 storms, and is thus probably connected with electrical 
 changes in the atmosphere. 
 
 47. Hailstorms are sometimes remarkably destructive. 
 When the hailstones which fall are of large size they 
 break branches from trees, destroy growing crops, maim
 
 XL] THE MOVEMENTS OF THE AIR. 83 
 
 and kill cattle and human beings, and injure buildings. 
 The course of one of these storms can thus be traced 
 across a country by the devastation which it causes. 
 
 LESSON XI. The Movements of the Air. 
 
 1. How rarely does the air seem to be perfectly motion- 
 less ! Even when we say that "not a leaf is stirring," 
 that " the chimney-smoke mounts up straight," or that 
 " the clouds are at rest," we can find, if we look for it, 
 proof enough that the air is not so stagnant as it seems, 
 but is ever moving and mixing. Usually the motion is 
 easily recognised, sometimes in a mere light air or gentle 
 breeze, sometimes in a strong wind or boisterous gale 
 or destructive tempest 
 
 2. Why should the air be so restless ? We have 
 already found the answer to this question : because, 
 owing to the unequal heating of the earth's surface by 
 the sun, and the ever-varying amount of water-vapour 
 poured into or withdrawn from the air, the density or 
 pressure of the atmosphere never remains long stationary 
 at any one place. All movements of the air arise out of 
 differences of pressure. The law governing the direction 
 of these movements may be stated thus : Air always 
 
 flows in spirally from areas of high pressure to areas of 
 low pressure. That this must be their direction will 
 be apparent if we reflect that low pressure indicates a 
 deficiency, and high pressure a surplus of air. The 
 column of air is heavier in the latter case than in the 
 former. Consequently, obeying the universal law of 
 gravitation, the heavier column must necessarily flow out 
 at the base to supply the deficiency in the lighter one. 
 The air does not flow from all sides straight into a low- 
 pressure area. It is drawn towards and inwards upon 
 it, and is then drawn up and passes away into higher 
 regions of the atmosphere. This inward advance of the
 
 84 PHYSICAL GEOGRAPHY. [LESS. 
 
 air is termed a cyclone or cyclonic movement ; while the 
 outward flowing from a region of high pressure is called 
 an anticyclone or anticyclonic movement. The rate of 
 motion of the air is usually much higher in a cyclone 
 than in an anticyclone. Storms of wind and rain are 
 associated with cyclones ; while calm air or light breezes 
 accompany anticyclones. 
 
 3. We are accustomed to think only of the horizontal 
 movement of the wind along the surface of the earth. 
 But to form a proper idea of the circulation of the air, 
 we must look upon that horizontal movement as only one 
 part of a much wider system. Each cyclone or area of 
 low pressure has at its centre an ascending current of 
 air ; each anticyclone or area of high pressure has a 
 descending current, and it is the position and extent of 
 these upward and downward currents which govern the 
 direction of the winds at the surface. 
 
 4. The rate of motion of the surface-currents or winds 
 must evidently be determined in each case mainly by the 
 amount of difference in the pressure, and by the distance 
 between the centres of the two areas of higher and lower 
 pressure. The greater the difference of pressure and the 
 shorter the distance within which it occurs, the more 
 rapid will be the flow of air. This is referred to again 
 in connection with storms in Arts. 24-29. 
 
 5. As there is so constant a relation between atmo- 
 spheric pressure and atmospheric movement, a knowledge 
 of the distribution of pressure over the globe will supply 
 the clue to the disposition of all the great movements of 
 the atmosphere. When the average readings of the 
 barometer over a continent or over the whole surface of 
 the globe for any given period are known, it is easy to 
 tell what must be the direction of the chief aerial move- 
 ments at any particular place. Plates II. and III. 
 illustrate this relation between the distribution of atmo- 
 spheric pressure over the globe and the general circula- 
 tion of the atmosphere.
 
 XL] THE MOVEMENTS OF THE AIR. 85 
 
 6. The principal causes of differences in atmospheric 
 pressure were stated, in Lesson VIII. Art. 18, to be 
 temperature and water-vapour. There seems to be at 
 present no method of ascertaining how far any particular 
 movement of the air is due to the one or the other of 
 these causes, or to both combined. Directly or indirectly, 
 indeed, everything might be referred to temperature, for 
 evidently the process of evaporation, on which the addition 
 of water-vapour to the air depends, is mainly regulated 
 by temperature, being greatest when the temperature is 
 high, and least when it is low. But it is useful to dis- 
 tinguish between the effect produced by the mere heating 
 and cooling of the air and that due to the changes in the 
 amount and distribution of its vapour. 
 
 7. Let us take some simple illustrations of the influence 
 of temperature in producing movements in the air. A 
 fire, whether in a room or out of doors, furnishes an 
 excellent example on a small scale of what takes place in 
 nature. When a fire is kindled in a grate, the air over- 
 lying it is heated, and ascends ; but at the same time 
 there is an in-draught of air at the bottom of the grate. 
 A circulation is established. The air from all parts of the 
 room and from outside, through the crevices of the doors 
 and windows, is drawn towards the fire, warmed, and 
 driven up the chimney. If in any way this free circula- 
 tion is interrupted, the fire does not burn well, while if it 
 is stopped, the fire goes out. Again, when a house or any 
 large building, or a prairie or forest, takes fire, the heat 
 may be so great as to cause the rapid ascent of a con- 
 siderable mass of air above the burning materials. This 
 is necessarily accompanied by a flow of air inwards from 
 all sides along the ground towards the fire as a centre, 
 and this in-draught may amount even to the rapidity and 
 force of a violent wind. Rushing round towards the 
 blazing centre, this wind feeds the flames, and, as it gets 
 heated, ascends with great vigour, carrying clouds of 
 smoke, sparks, and even, it may be, large fragments of 
 burning wood along with it.
 
 86 PHYSICAL GEOGRAPHY. [LESS. 
 
 8. Though the sun's heat is not concentrated upon a 
 limited spot like a burning building, or a forest on fire, 
 yet the heating of a mass of land during the day pro- 
 duces similar effects, where the heated land lies near 
 some other area (of water, for example) which has not 
 been heated so much. 
 
 Nowhere can this be more instructively seen than by 
 the seaside, when the days are warm and the nights cool. 
 During the day, the land surface becomes much warmer 
 under the sun's rays than the sea does, and the air resting 
 on it becomes warmer than that resting upon the sea. The 
 result of this difference of temperature (and therefore of 
 pressure) is to set in motion a light breeze, which moves 
 from the sea to the land, in order to take the place of the 
 hot air that is always streaming up from the heated 
 surface of the land. This is a sea-breeze. It dies 
 away towards evening. When the nightly radiation 
 begins, however, the land parts with its heat more readily 
 than the sea does, and consequently cools the air more. 
 The cooler and denser air above the land moves seaward 
 to take the place of the warmer and lighter air that rises 
 from the sea. Hence arises a land-breeze, which, in- 
 creasing in force, as the difference of temperature between 
 sea and land is augmented by continued radiation, may 
 rise to a good stiff breeze. 
 
 In mountainous countries, where the higher ground 
 rises far up into the colder layers of the atmosphere, 
 another beautiful illustration of these changes of move- 
 ment in the air may be watched. During the day the 
 air, warmed on the bare mountain-sides, ascends, and a 
 breeze blows up the valleys towards the heights. At 
 evening, when the mountain-slopes begin to cool rapidly, 
 the cold heavy air lying on them flows down as a cool 
 breeze into the valleys. 
 
 9. As regards the influence of the water-vapour on 
 movements of the atmosphere, it may be expected to be 
 most marked where the temperature is greatest. The
 
 XL] THE MOVEMENTS OF THE AIR. 87 
 
 broad belt of low atmospheric pressure between the 
 tropics (Lesson VIII. Art. 19), should be the part of 
 the earth's surface where this influence may be best 
 studied. There the sun's heat is greatest and the eva- 
 poration most abundant. The tropical belt, indeed, 
 with its wide ocean surfaces, may be looked upon as the 
 great evaporating cauldron of the globe, whence comes 
 most of the moisture that is distributed by the winds as 
 rain and snow. Air, from north to south, must be 
 constantly pouring into it, because it is a vast area of 
 low pressure. Were there no interrupting masses of 
 land the regularity of this inflow of air would be con- 
 tinuous all round the globe. A constant wind would 
 blow towards the equator from either side, and between 
 the two opposite currents there would be a zone, in 
 which the currents would meet and ascend as an up- 
 streaming mass of air, along the centre of the tract of 
 low pressure. The position of this zone would vary 
 according to the position of the sun. In July it would 
 be about the position of the Tropic of Cancer. Thence, 
 following the sun in his southward passage, it would 
 travel to the line of the Equator in October, and to the 
 Tropic of Capricorn in January. 
 
 By the present arrangement of land and water, how- 
 ever, this regularity is greatly interfered with. It is 
 best shown in the wide Pacific Ocean, where there is 
 least land. Also, it is more distinct in January than in 
 July, because the belt of low pressure then lies nearly 
 along the line of the equator with more sea in its track 
 than in July, when it goes north of the equator, spreading 
 over Asia and the northern part of Africa. 
 
 1O. Putting aside for the present these irregularities, 
 let us think of the air round the tropical belt of the globe 
 as constantly receiving an enormous volume of water- 
 vapour from the oceans. From this great influx of 
 vapour, as well as from the high temperature along that 
 belt, there is a continued ascent of heated and humid
 
 88 PHYSICAL GEOGRAPHY. [LESS. 
 
 air into the higher regions of the atmosphere, and a con- 
 sequent in-draught from north and south. This tract of 
 the earth's surface, extending in the Atlantic region from 
 lat. 1 1 N. in August to lat. i N. in February, and having 
 a breadth of from three to eight degrees of latitude, is 
 called the Belt of Equatorial Calms a somewhat 
 misleading name, because, although there are no constant 
 winds, the air is in ceaseless disturbance, as it is heated 
 and streams upward into the cloudy sky. 
 
 11. Constant "Winds and Aerial Currents. 
 The hot moist air that mounts up from the equatorial 
 belt expands and cools as it ascends, and in so doing 
 parts with much of its moisture, which condenses and 
 falls as rain. Hence the atmospheric pressure is much 
 reduced. Moreover, the belt is distinguished not only 
 by its great heat, but, in many parts, by its almost 
 continual rains and thunderstorms, so as to be spoken of 
 as the Zone of Constant Precipitation (Lesson X. Art. 
 31). Reaching higher parts of the atmosphere the 
 equatorial hot air divides, one part flowing northward, 
 the other southward, and each as an upper current 
 taking a direction opposite to that of the in-draught 
 at the surface. These upper currents must be many 
 thousand feet above the sea at the equatorial belt. They 
 travel pole-wards until, entering the belt of high pressure 
 they descend, and at last reach the surface in temperate 
 latitudes. Thence the air streams again outward, part 
 of it back along the surface to the equator and another 
 portion onwards into low pressure areas in higher lati- 
 tudes. 
 
 12. Here, then, we have the fundamental aerial circu- 
 lation of the globe a great system of (i) surface currents, 
 continually streaming out of the areas of high atmo- 
 spheric pressure towards those of low pressure, and of 
 (2) upper currents, constantly flowing away from above 
 the low pressure areas. Considerable difference is observ- 
 able in the distribution of the winds according to the
 
 xi.] THE MOVEMENTS OF THE AIR. 89 
 
 season of the year (Plates II., III.), and this difference is 
 manifestly dependant on the shifting of the areas of high 
 and low pressure. In winter, for example, the winds of 
 the northern hemisphere flow out of the great centres 
 of high pressure that lie over Asia, the western terri- 
 tories of America, and a tract of the Atlantic westwards 
 from Northern Africa, and stream into the low pressure 
 equatorial belt on the one side and into the low pressure 
 tracts of the North Atlantic and North Pacific on the 
 other. In summer these directions are on the whole 
 reversed, because the high pressure areas have in great 
 part become areas of low pressure. 
 
 13. It might be supposed that currents flowing to 
 and from the equator should have a direct north and 
 south course. And so they would, were it not that the 
 earth, instead of being at rest, is always rotating on 
 its axis. Owing to the motion of rotation, an object 
 at the equator is carried along with a higher speed than 
 an object nearer the pole, just as the rim of a wheel 
 moves through more space in the same interval of 
 time than the parts near the axle. Hence, when a 
 current of air travels away from the equator northward 
 or southward, it moves from a region where the velocity 
 of the surface is greater to one where it is less. At the 
 equator the air, having the same rate of rotation as the 
 rest of the surface, is borne along with the rest of that 
 surface, and if it blows eastward or westward, rotation 
 does not affect its direction. But when it blows north- 
 ward or southward from the equatorial tracts, it carries 
 with it part of its equatorial rapidity of rotation, and 
 moves a little faster than the surfaces over which it 
 successively moves. Now, as the earth rotates from 
 west to east, all the air that flows from the equator 
 towards the pole is bent towards the east. On the other 
 hand, as the air which travels from higher latitudes 
 towards the equator is always getting into regions where 
 the speed of rotation increases, it seems to lag behind,
 
 90 PHYSICAL GEOGRAPHY. [LESS. 
 
 and instead of flowing straight north and south, is 
 deflected towards the west. 
 
 14. The lower or surface currents, turned more and 
 more westwards as they advance, become on the north 
 side of the equatorial belt the North-east Trade- 
 winds, and on the south side the South-east Trade- 
 winds. These are constant winds. They always 
 stream towards the equator because of the lower atmo- 
 spheric pressure there. They received the name of 
 " Trade-winds " in the early days of navigation, when it 
 was found that they blew so steadily in one direction 
 that they could always be reckoned on for the purposes 
 of commerce. 
 
 The Trade-winds are chiefly felt between each tropic 
 and the equator, but they begin in some places consider- 
 ably beyond the tropics (Art. 10), their constancy being 
 better exhibited over the oceans than over the land, see- 
 ing that the more marked heating and cooling of the land 
 by day and night (Art. 8) tend to disturb their regularity. 
 Beyond the limits of the Trade-wind region, along the belt 
 of high atmospheric pressure, there is in each hemisphere 
 a belt of calms and variable winds, that on the north side 
 being called the Calms of Cancer, that on the south side 
 the Calms of Capricorn. These Calms, unlike those of 
 the equatorial belt of low atmospheric pressure, are 
 characterised by generally bright weather. 
 
 15. Out of each high pressure belt the winds blow 
 towards the equator on the one side and towards the pole 
 on the other. As the direction of the Trade-winds is 
 deflected to the west by the influence of the earth's 
 rotation, so the winds which emerge from the polar side 
 of the tropical calms acquire an easterly direction. In 
 the northern hemisphere therefore they come from the 
 south-west. In Britain, and the west of Europe, which 
 lie considerably to the north of the Calms of Cancer, the 
 prevalent winds are the familiar south-west winds. In 
 these countries the large towns commonly grow towards
 
 xi.] THE MOVEMENTS OF THE AIR. 91 
 
 the west, and the term " west-end " usually means the 
 quarter where the newest and best streets or dwelling- 
 houses are situated. The reason is evidently to be 
 sought in the direction of the prevalent winds, for the 
 west and south-west are the quarters that escape most 
 of the town-smoke, which is blown eastward. 
 
 16. Much interesting information may often be gathered 
 respecting the movements of the high upper currents of 
 the atmosphere, as well as of lower aerial currents, by 
 watching the clouds. Fine light fleecy clouds may 
 be noticed at a great height, sometimes with other 
 heavier clouds lying below them. They may be seen 
 to move and change their shape, affording by these 
 movements valuable prognostications of coming weather. 
 
 17. But the fact that there are upper currents in the 
 atmosphere is now and then brought home to us still 
 more strikingly. Volcanoes have been already referred 
 to as mountains from which melted rock, steam, and gas 
 are given out, and from which fine dust is sometimes 
 thrown for an enormous height up into the air. With 
 such force is this dust or ash ejected, that it sometimes 
 enters a high aerial current blowing steadily and strongly 
 in one direction. It is then borne along to vast dis- 
 tances, and descends to the earth, perhaps in regions 
 where nobody ever saw such a thing as a volcano. In 
 this way, the inhabitants of the northern parts of the 
 British Islands have more than once been visited with 
 showers of dust that came from Iceland. In the year 
 1783, during a great eruption of Skaptar Jokul, one 
 of the volcanoes of that island, the fine impalpable dust 
 fell in such quantities between the Orkney and Shetland 
 Islands during north - west winds, that vessels sailing 
 there required to have the deposit shovelled off their 
 decks every morning. In that season, too, the crops 
 along the north coast of Caithness proved a failure, from 
 the quantity of volcanic material which fell on the ground. 
 The inhabitants still speak of the year of "the ashie."
 
 92 PHYSICAL GEOGRAPHY. [LESS. 
 
 The distance from Iceland to the coast of Caithness is 
 upwards of 600 miles. Similar volcanic dust has been 
 frequently carried from the Icelandic volcanoes to Scan- 
 dinavia. 
 
 18. Instances are on record where the erupted ashes 
 have been borne over a much greater space. In the 
 year 1835, Coseguina, a volcano in Guatemala, broke 
 out in eruption, and though the regular east wind was 
 then blowing, the fine ashes were carried eastward by 
 an upper current, and fell on the island of Jamaica, 800 
 miles from the point of emission. They had been four 
 days on the journey, and their rate of progress must have 
 been 2 oo miles a day. Again, in 1815, during a calamitous 
 volcanic outbreak in the island of Sumbawa, lying to the 
 east of Java, the ash was carried for 800 miles to the 
 east, as far as the islands of Amboyna and Banda, 
 although the south-east wind was then at its height. 
 
 19. In countries bordering the Mediterranean, and 
 even as far west as the Cape de Verde and Canary 
 Islands, the air is occasionally filled with a peculiar 
 reddish, or brown dust, which sometimes falls in such 
 quantities as to cover the decks of ships at sea, though 
 far out of sight of land. When rain falls at the same 
 time it brings down this dust with it, and having then a 
 peculiar colour, is known as " blood-rain." It seems 
 that in most cases this "red-fog," "sea -dust," or 
 " sirocco-dust," as it is called, has come from the hot 
 deserts of Africa, whence, swept up by whirlwinds from 
 the burning soil in great clouds, it has been borne 
 away by the overflow of air at the top of the hot ascend- 
 ing column, and after being carried for hundreds of 
 miles by the upper currents, has at last descended to 
 the surface again. 
 
 20. Clouds, ashes, and dust, therefore, sometimes give 
 important evidence as to the direction in which the upper 
 aerial currents are moving, and help to prove how regular 
 and constant is the circulation of the atmosphere.
 
 XL] THE MOVEMENTS OF THE AIR. 93 
 
 21. Seasonal or Periodic Winds. When the two 
 charts of the world in Plates II. and III. are compared, 
 it may be seen that the larger masses of land in the 
 northern hemisphere interfere a good deal with that 
 regular distribution which, as shown by the southern 
 hemisphere, is promoted by a broad unbroken expanse 
 of ocean. In January, for instance, the high and cold 
 table-lands of Central Asia become the centre of a vast 
 area over which the pressure of the air is high. Conse- 
 quently from that elevated region the wind issues on all 
 sides. In China and Japan it appears as a north-west 
 wind. In Hindostan it comes from the north-east. In 
 the Mediterranean it blows from the east and south-east. 
 But in July matters are reversed, for then the centre of 
 Asia, heated by the hot summer sun, becomes part of a 
 vast region of low-pressure, which includes the north- 
 eastern half of Africa and the east of Europe. Into that 
 enormous basin the air pours from every side. Along 
 the coasts of Siberia and Scandinavia it comes from the 
 north. From China, round the south of the continent 
 to the Red Sea, it comes from the Indian Ocean, that is, 
 from south-east, south, or south-west. Across Europe 
 it flows from the westward. Hence, according to the 
 position of any place with reference to the larger masses 
 of sea and land, the prevalent direction of its winds may 
 be estimated. 
 
 22. On the shores of the Indian Ocean the summer 
 and winter winds are known as Monsoons an Arabic 
 word signifying any part or season of the year but now 
 generally applied to all winds which have a markedly 
 seasonal character. Since the air is drawn in towards 
 the heart of Asia in summer, and comes out from that 
 centre in winter, the direction of the monsoon at any 
 place depends upon geographical position. In India the 
 winter wind is the N.E. Monsoon, which corresponds to 
 the N.E. Trades of the North Atlantic and North Pacific 
 Oceans ; the summer wind is the S.W. Monsoon, which
 
 94 PHYSICAL GEOGRAPHY. [LESS. 
 
 is a complete reversal of the natural course of the Trade- 
 wind owing to the enormous in-draught caused by the low 
 summer pressure over Asia. On the Chinese coast the 
 winter wind is a N.W. monsoon, and the summer wind 
 a S.E. monsoon. Similar but not quite so strongly 
 contrasted monsoons occur in North America. In the 
 Southern States, for instance, the winter wind comes from 
 the north-east, the summer wind from the south-west. 
 Even in Euorpe the prevalent winds partake of the 
 character of Monsoons. During winter, when the atmos- 
 pheric pressure is low in the north and north-west, they 
 blow from the land into the North Atlantic and Arctic 
 Oceans. In summer, on the other hand, when the low 
 pressure is shifted to the great Asiatic land, the winds 
 blow from the sea, as the prevalent west and south-west 
 winds, bringing the moisture of the Atlantic to condense 
 it in rain-showers as they pass eastwards. 
 
 23. Local Winds. Many winds, often of a destruc- 
 tive character, occur in different countries or in different 
 districts of the same country, to which local names are 
 given. When they come from tracts where the pressure 
 is high and the temperature low to where the pressure is 
 lower and the temperature higher, they are felt as cold 
 blasts, whereby the humidity of the air in the low pres- 
 sure area is condensed into torrents of rain. One of the- 
 best known examples of such a wind is that known as 
 the Mistral, which descends from the high plateaux and 
 plains of Central and Eastern France, and is felt as 
 a cold and sometimes tempestuous wind along the shores 
 of the Mediterranean. When a low atmospheric pres- 
 sure happens to arise on the borders of a hot desert 
 region like those of Africa, Arabia, or the interior of 
 Australia, it draws in towards it the hot air lying 
 over the burning sands, which in the countries where 
 it blows is extremely unhealthy. In Italy such a 
 wind is known as the Sirocco a hot moist wind which 
 raises a haze in the air, and produces a sensation of
 
 XI.] THE MOVEMENTS OF THE AIR. 95 
 
 extreme languor both in man and beast. In Spain, where 
 it receives the name of the Solano, it sometimes comes 
 across the narrow part of the Mediterranean laden with 
 fine hot dust from the vast African deserts. In Africa 
 and Arabia it appears as the dreaded Simoom a hot 
 suffocating wind, which sometimes rushes across the 
 desert with such violence as to raise clouds of sand, and 
 sweep them in whirling masses for many miles. It thus 
 heaps up vast mounds of sand, under which caravans of 
 travellers may be completely buried. One of the armies 
 of Cambyses, 50,000 in number, is said to have been 
 engulphed in the sand when on its way to attack the oasis 
 and temple of Jupiter Ammon. Again on the coast of 
 Guinea, during December, January, and February, a hot 
 wind, called the Harmattan, blows from the interior out 
 to sea. The north-west provinces of India have likewise 
 their hot-winds, which sometimes produce violent whirl- 
 winds sweeping up the dust and carrying it in tall whirl- 
 ing columns into the upper air, whence it gradually finds 
 its way to the earth again. 
 
 24. Storms. But besides the winds and currents 
 already noticed, most of which blow continuously for 
 weeks or months, sudden and violent commotions arise 
 in the atmosphere, and are often very disastrous in their 
 effects. These may be classed under the general name 
 of Storms. Let us see whether they can be accounted 
 for by the general principle of aerial circulation, which 
 holds good for the constant and periodic winds. 
 
 25. By careful observation of the barometer, and more 
 particularly by having observing stations at many different 
 places from which the readings of the barometer can be 
 telegraphed at frequent intervals to some central office, 
 it is possible to forecast the weather and to detect the 
 approach of storms before they actually arrive. A system 
 of weather-forecasts has now been in operation for some 
 years in Western Europe and in the United States. 
 When a violent storm bursts upon a country the
 
 96 PHYSICAL GEOGRAPHY. [LESS. 
 
 wind rises rapidly into a gale and continues to rush 
 along at a rate of often 70 or 80, sometimes in gusts, 
 even of 120 and 150 miles an hour. After some hours 
 it slackens its speed, and may even die away as rapidly 
 as it rose. But again it may spring up from a different 
 or even from the opposite quarter and possibly equal, 
 if it does not exceed, its first fury and destructiveness. 
 All parts of a country have not the storm simultane- 
 ously. In Europe, for instance, storms usually come 
 from the westward. Britain and the western shores of 
 France and Portugal catch the first fury of the tempest, 
 which then travels eastward or north-eastward, and not 
 unfrequently dies away in Russia. The rate at which the 
 storm moves from place to place does not exceed on 
 an average from fifteen to thirty miles an hour a much 
 more leisurely rate than the rapid and destructive rush 
 of the wind would have led any one to expect. 
 
 26. Suppose that such a storm has passed over 
 Europe, and that after it is gone, we obtain information 
 as to the readings of the barometer over all the countries 
 traversed by the storm as well as those around its track. 
 We find that the centre of the storm has been an area 
 of very low atmospheric pressure, 600 mites or more in 
 breadth, and that the pressure has been much higher 
 immediately outside of that area. The great law of 
 atmospheric movement, therefore, holds good here also. 
 The air has moved as a cyclone inward from surrounding 
 regions of high pressure upon a central area of low 
 pressure. 
 
 27. Observations of this kind have proved, that the 
 greater the amount of difference between the low pressure 
 in the centre of the storm and that of the high pressure 
 outside, and the nearer the extremes of pressure lie to 
 each other, the more violent is the wind. Suppose, for 
 example, that while the barometer stood at 30 inches at 
 one side of a country, it should rapidly fall to 28 inches 
 at some district on the other side, 300 miles away : so
 
 XL] THE MOVEMENTS OF THE AIR. 97 
 
 great a difference in so short a space would certainly be 
 accompanied by a violent gale. The air in such circum- 
 stances rushes in with an in-moving spiral and ascending 
 motion. It turns inwards upon the storm-ring, and is 
 carried upward in the central vortex as a vast up-stream- 
 ing current, which overflows at the top and passes off 
 into other regions. 
 
 28. Hence it is evident why, though the wind rushes 
 so furiously, the whole body of the storm does not travel 
 so fast in many cases as an ordinary passenger train. 
 There are two motions in the storm, that of the whirling 
 air as it is borne inwards, and that of the whole rotating 
 mass or storm. When the storm-ring bursts upon a 
 place the wind may be blowing from the south-east. It 
 dies down when the centre of the storm reaches the 
 place, but rises again from a different quarter when the 
 remaining side of the storm-ring advances, and continues 
 to blow until the whole whirling mass of air has gone by. 
 
 29. A good illustration of this vorticose movement of 
 a storm is afforded by the little dust-whirlwinds often 
 seen upon a dusty road in dry weather. The air in each 
 of these, as shown by the movements of the dust, takes 
 a rapid inflowing spiral course, drawing in air on all sides 
 below and whirling it rapidly round and upward, till it 
 expands at the top and flows over into the surrounding 
 air. 
 
 30. Office of the "Winds. There are two kinds of 
 work done by the winds and currents of the atmosphere 
 (i) the distribution of temperature ; and (2) the distri- 
 bution of moisture. This work has been referred to 
 already, but may again be stated in a summary way 
 here. 
 
 31. (i.) When a wind blows from a warm or mild 
 quarter, it raises the temperature of the places to which 
 it comes. The south-westerly winds of Britain, for in- 
 stance, wanned by the heated waters of the North Atlantic, 
 keep the air of that part of Europe much milder than 
 
 H
 
 9 8 PHYSICAL GEOGRAPHY. [LESS. 
 
 from its latitude it ought to be. When, on the other hand, 
 a wind blows from a cold to a warm region it lowers the 
 temperature as it moves along. Thus, owing to the vast 
 expanse of cold land in Asia during winter and the high 
 pressure there, the outflowing winds are cold. A care- 
 ful study of the charts in Plates II. and III. will show 
 this more clearly than a description in words can do. 
 By noting the areas of low and high pressure, we learn 
 in which main direction the wind must blow in any part 
 of the globe ; then a reference to the isothermal charts 
 will indicate to us the temperature of the high pressure 
 tracts from which the wind comes, and therefore whether 
 the wind in any particular case must be cold or warm. 
 
 32. (2.) The winds are the great agents by which the 
 moisture of the atmosphere is distributed over the globe. 
 Were it not for their help, the condensed vapour would 
 be discharged over the same tracts from which it had 
 been evaporated. And therefore, as by far the most of 
 the vapour rises from the ocean, it would fall back again 
 into the ocean, and the land would only get back the 
 comparatively small quantity evaporated from rivers and 
 lakes. But since the winds bear the vapour along with 
 them, and since areas of low atmospheric pressure often 
 pass over continents, the vapour is carried away from the 
 sea, and condensed into rain over the land. On the 
 whole, therefore, the wetness or dryness of any place will 
 depend on the direction from which its prevalent winds 
 come (Lesson X. Art. 29-34). If they blow from a wide 
 and warm expanse of sea they will be laden with moisture, 
 and be ready to drop it when chilled by passing over the 
 land. If, on the other hand, they blow from the interior 
 of a continent, they will be dry their heat or cold 
 depending upon the temperature of the region whence 
 they come. 
 
 33. In short, winds are wet when they travel from a 
 warm vapour-laden tract to a colder one, because they 
 are cooled below their dew-point and must let fall the
 
 xi.] THE MOVEMENTS OF THE AIR. 99 
 
 surplus moisture. Winds are dry when they travel from 
 a cold to a warm tract, because, instead of parting with 
 their vapour, they are ready to take up more ; they are 
 dry, too, when they issue from a hot and dry region, and 
 before they have an opportunity of crossing any sea and 
 greedily drinking up vapour from its surface. 
 
 34. When a mass of high land stretches across the 
 track of a warm moist wind, it forces the wind to flow up 
 and over its ridges. The air is thereby expanded and 
 cooled, and as a consequence deluges of rain descend. 
 The most notable instance of this kind is that of the 
 Khasi Hills already mentioned (Lesson X. Art. 31), 
 which front the Bay of Bengal, and interpose as a great 
 steep buttress against the inland advance of the warm 
 vapour-laden south-west monsoon. Driven up the slopes, 
 the wind has to discharge its moisture in rain, of which, 
 during the seven months, when that monsoon blows, a 
 depth of as much as 493 inches, or about 41 feet, falls to 
 the ground. But, after passing over the hills, the wind, 
 having lost so much of its moisture, is comparatively dry. 
 Hence the rainfall rapidly diminishes northward. As the 
 wind journeys in that direction, however, it is once more 
 forced to ascend into higher regions of the atmosphere 
 by the giant chain of the Himalaya. Its moisture is 
 further wrung out of it, as rain on the lower slopes, and 
 as snow among the higher ridges. And thus it descends 
 on the north side as a cool dry wind into the wide plains 
 of Thibet. 
 
 35. In countries like India, therefore, the dry season 
 and rainy season depend upon the changes of the monsoon 
 and the quarter whence the monsoon blows, whether 
 dry land or vapour-giving sea. 
 
 36. In regions where the winds are irregular the rains 
 are found to be the same. In the west of Europe, for 
 example, the wind shifts continually, but the prevalent 
 winds are from the west and south-west, though during 
 part of the year east winds become frequent. The
 
 PHYSICAL GEOGRAPHY. 
 
 [LESS. 
 
 westerly winds come from the warm Atlantic, laden with 
 vapour, which they discharge plentifully on the western 
 parts of Britain and the Continent, leaving the eastern 
 districts comparatively dry. The easterly winds on the 
 other hand come from the heart of the Old World, and 
 are so dry as often to wither up vegetation as much as a 
 hard frost or even a scorching fire would do. 
 
 37. There is yet another kind of work performed by the 
 winds, which, though of very limited and local character 
 
 FIG. 12. Sand-dunes ridges of dry sand blown inland off the shore by 
 the wind. 
 
 compared with their other services, deserves notice here. 
 Storms by sweeping over the surface of the land some- 
 times produce great changes there, uprooting trees and 
 even prostrating large woods, destroying fields and houses, 
 and generally carrying ruin with them in their track. But 
 even where the wind does not rush with such fury over 
 the land it sometimes effects remarkable changes there 
 when its prevalent direction is from the sea to the shore. 
 On sandy beaches exposed to winds which usually blow 
 from the sea, the dried sand is often driven inland, where 
 it forms mounds and ridges called sand-dunes, sometimes 
 60 or 100 feet high, fronting the beach for, it may be, 
 many miles. A small streamlet, flowing behind these
 
 XL] THE MOVEMENTS OF T*IE AIR. 
 
 ridges may arrest their advance by carryirig.Aw'ay the loose 
 sand as fast as it creeps on. But where no'Sach check 
 exists, the sand may gradually extend inland; n^d com- 
 pletely bury the cultivated soil over whole farms or. parishes. 
 
 38. Many examples of these effects are to tye found 
 along the coasts of the British Islands. Within the last 
 few hundred years, thousands of acres of valuable .land 
 have been overwhelmed by the drifting of sand ffpiri 
 the sea. Along the shores of the Moray Firth and of, 
 Aberdeenshire, for example, several parishes have been 
 wholly or in great part buried. On the coasts of Nor- 
 folk and Suffolk, villages and thousands of acres of land 
 have been covered with blown sand during the last two 
 centuries, and on the Cornish coast similar inroads have 
 taken place. The shores of the Bay of Biscay furnish 
 still more striking instances of the power of the winds to 
 alter the surface of a district. The dunes which are 
 there heaped up by the westerly gales, march inland at 
 a rate of 60 or 70 feet in a year. No barrier, natural 
 or artificial, is able to withstand their progress. Fields, 
 woods, and villages are buried in succession. Nor is 
 this all. The sand ridges interrupt the drainage from the 
 interior, and the water collects among and in front of the 
 ridges. Ponds and lakes are formed, which, unable to 
 find an exit, are driven inland along with the sand barriers 
 which dam them up. Large tracts are thus first inundated 
 with water, and then finally overwhelmed under the 
 advancing sand. Roman Roads, and many villages 
 which existed in the Middle Ages, have disappeared. 
 And the same destruction is going on still. 
 
 39. It is not always along the sea-margin that these 
 effects of the action of wind are to be traced. In the 
 interior of many countries there are wide tracts of barren 
 sand or Deserts (Art. 23), over which the wind raises the 
 sand into ridges, as it does along the shore. Such, for 
 instance, are the great desert of Sahara and much of the 
 interior of Arabia.
 
 102 PHYSICAL GEOGRAPHY. [LESS. 
 
 4O. In dry climates vast quantities of dust are trans- 
 ported by' Wind. Thus in Central Asia, the air is often 
 thick vitjra'fine yellow dust, the sun being sometimes so 
 obscure 'af mid-day that lamplight is required to read. 
 The dust settles down as a fine sediment, covering every- 
 thing and increasing the soil. In some countries, ancient 
 cities and ruins have been gradually more or less buried 
 under the gradual accumulation of wind-borne dust, aided 
 "by vegetation which intercepts the sediment and grows 
 -over it. Nineveh, Babylon, and other historic sites 
 appear to have been covered up in this manner.
 
 140 120 100 <t ^ 
 
 AP.,rick? T^',, * v ^ J 
 
 OCEAN BASINS. 
 
 Depths down to 1000 fathoms 
 
 uncoloured. 
 1000 to 2000 fathoms 
 2000 to 8000 
 
 3000 to 4000 
 Over 4000 
 
 180 160 140 120 lOOi^w 80rfc~..60 40 20
 
 PLATE VII. 

 
 XII.] THE GREAT SEA-BASINS. 103 
 
 CHAPTER III. 
 THE SEA. 
 
 LESSON XII. The Great Sea-basins. 
 
 1. From the outer envelope of air which encloses the 
 earth we now pass to the underlying envelope of water 
 called the Sea. At the outset some obvious differences 
 between these two coverings may be noticed. For ex- 
 ample, while the atmosphere completely wraps round the 
 whole planet, rising to a height of many miles above its 
 general surface, the water envelope is pierced in many 
 places by masses of the underlying solid part of the earth, 
 which rise above it to form land. As already mentioned 
 (Lesson V. Art. 2), the sea covers not quite three quarters, 
 and the land a little more than one quarter, of the entire 
 surface of the earth. 
 
 2. Again, we know nothing about the upper surface of 
 the atmosphere, and cannot say precisely how far it lies 
 above us, but the surface of the sea forms a great plain, 
 and the line between it and the air is sharply defined. 
 Though we speak of the sea as forming a plain, we know 
 this apparent plain to be really curved, and that from its 
 wide extent and its freedom from inequalities, it shows 
 the curvature of the earth's surface better than can be 
 seen on land. (Lesson I. 2.) The amount of the curva- 
 ture is about eight inches in a statute mile ; that is, an 
 object eight inches high above the sea-level sinks out of 
 sight when looked at from the same level at a distance 
 of more than a mile. The line of meeting between the
 
 104 PHYSICAL GEOGRAPHY. [LESS. 
 
 sky and the surface of the earth is termed the horizon. 
 Its distance from us evidently depends upon the elevation 
 at which we may happen to stand. Thus, at the sea- 
 shore with our eyes exactly six feet above the sea-level, 
 our horizon out to sea is three miles off. If we ascend 
 so that our eyes are about ten feet and a half above the 
 sea-level, our horizon is extended to four miles. If we 
 climb to some adjoining height, say to a lighthouse top, 
 about ninety-six feet above the sea, our horizon is increased 
 to a distance of twelve miles. 1 
 
 3. Another evident contrast between the air and the 
 sea lies in the fact that while every inhabitant of the earth 
 is familiar with the one, only a comparatively small part 
 of mankind has ever seen the other. Even in an island 
 like Britain, a large proportion of the inland population 
 has never been within sight of the sea. On the con- 
 tinents the proportion is necessarily much greater, for, 
 except the people dwelling along the sea-margin, the 
 great mass of the inhabitants, having little or no com- 
 munication with the coasts, have no acquaintance with 
 any larger sheet of water than their own native river or 
 lake. 
 
 4. One who has not seen it can hardly realise what 
 the sea is from descriptions in books. Let us suppose, 
 however, that some intelligent dweller in the inland 
 regions were taken for the first time to the sea-coast, and, 
 after recovering from his first impression of wonder and 
 admiration, were to begin to look attentively at those 
 features which would be most likely to attract his notice. 
 He would observe that the solid ground, with which he 
 had been familiar all his life, gives place to a seemingly 
 boundless plain of water, at first sight level and motion- 
 
 1 In estimating the extent of the visible horizon, however, when the dis- 
 tance exceeds half a mile, we need to take into account the effect of atmo- 
 spheric refraction which tends to make distant objects seem higher than they 
 are. The allowance to be made for this effect varies from day to day ; it 
 commonly requires a deduction of about one-seventh from the apparent 
 height of an object.
 
 xii.J THE GREAT SEA-BASINS. 105 
 
 less, but soon found to be in a state of perpetual unrest, 
 and answering to all the movements of the air above, 
 heaving or rippling when the air is gently stirred, but 
 rising into waves and foam-crested breakers along the 
 shore when the wind blows strongly. Should he taste 
 some of this blue sparkling water he would find it salt 
 and undrinkable, even though clearer perhaps than his 
 own river at home. Gathering the shells and other 
 remains of the living things of the sea from the sands of 
 the shore, he would find every one of them different from 
 anything he had ever found on the land or in fresh water. 
 Were he to watch by the shore from day to day, he would 
 notice that twice every day the water advances slowly and 
 as slowly retreats, and that this regular movement takes 
 place whether the water be smooth or rough. Were he 
 to set sail upon the seemingly boundless plain of water, 
 he would watch the land behind him gradually sinking, as 
 it were, into the water, till at last its highest point had 
 disappeared, and the same long level line of meeting be- 
 tween sea and sky would then sweep around him on every 
 side. With no land in sight and no other vessel perhaps 
 to be descried ; with only the sky overhead and the 
 heaving water beneath and around, he would learn better 
 than from any map or description the vastness and 
 solitude of the great deep. And yet from the deck, or 
 even from the mast-head of his ship, only a comparatively 
 small patch of the sea could be seen at once. The 
 horizon or sky-line, which he thinks so immeasurably 
 distant, is only a few miles off (Art. 2). And he might 
 sail for weeks together, passing over thousands of miles, 
 with all the time the same limited horizon and the 
 same monotony of sea and sky. 
 
 5. The traveller who finds himself for the first time 
 on the open sea is not only impressed by the vastness of 
 its surface, but by a vague notion of its profound depth. 
 He has read of the " unfathomable abysses of ocean,' 
 and naturally feels some awe at the thought that they
 
 io6 PHYSICAL GEOGRAPHY. [LESS. 
 
 are actually lying beneath him. In recent years, how- 
 ever, Expeditions have been fitted out to measure the 
 depth of the sea in all parts of the globe, and the result 
 has been to show that even the deepest of the so-called 
 abysses probably does not much exceed five miles, while 
 the average depth of the sea may be taken at about half 
 that amount. 
 
 6. Had the earth been a perfectly smooth homogene- 
 ous globe, like a ball of polished steel, we may suppose 
 that the sea would have completely covered its surface. 
 There would then have been an outer shell of air and an 
 inner shell of water, within which the solid planet itself 
 would have lain. The average depth of such an univer- 
 sal sheet of water, as may be inferred from what has 
 been made known by numerous soundings in all parts 
 of the present sea, would have been about a mile and 
 a half. It is almost certain, however, that the water 
 envelope never completely wrapped the globe round in 
 this way. 
 
 7. Looking at a map of the world, we may be disposed 
 to ask whether any reason can be given for the shape 
 which the great ocean -basins have taken ; whether, for 
 example, the sea, by the force of its own waves and 
 currents, could have hollowed out the great depressions 
 in which it now lies. That the sea could have had 
 little to do with the formation of its basins will be 
 evident if we reflect that it is only the upper parts of the 
 sea which eat away the solid part of the earth (see Les- 
 son XVIII.); the water in the deep abysses does not 
 even stir the fine mud, which, like dust in a deserted 
 room, slowly settles down upon the bottom. Where the 
 waves wear down the land, the materials which they 
 remove are not destroyed, but are carried away into some 
 still part of the sea, where they accumulate on the sea- 
 floor. So that even if we could suppose it possible that 
 the sea might have scooped its basins out of the solid 
 surface of the globe, it could only remove the debris from
 
 xii.] THE GREAT SEA-BASINS. 107 
 
 one part of its bed to another, and, in short, fill up its 
 hollows after it had formed them. 
 
 8. Two causes may be assigned for the present distri- 
 bution of the oceans. In the first place there must be 
 an excess of density in the southern hemisphere, so that 
 the larger mass of water is attracted to that half of the 
 globe. In the second place, the surface of the solid 
 globe has been irregularly depressed and ridged up. 
 Consider what was said in Lesson III. about the prob- 
 able history of the earth. If our planet has slowly 
 cooled from a condition of intense heat, it must have 
 gradually contracted in bulk. We may compare it to 
 an apple which is kept a long while, until it begins to 
 dry up, and consequently to shrink. The apple does 
 not diminish equally all round. Some parts of the skin 
 sink down, others are puckered into wrinkles and fold- 
 ings, so that in the end the once smooth surface becomes 
 shrivelled and uneven. In the case of the earth, the 
 contraction has likewise been irregular. Some portions 
 have sunk down, others which shrank up soonest have 
 become more and more wrinkled, and rise far above the 
 rest. The depressed parts have formed basins for the 
 oceanic waters. Those portions of the larger wrinkles 
 which rise above the sea-level form continents and islands. 
 
 9. Until a comparatively few years ago, the abysses 
 of the sea were as unknown as the interior of the earth. 
 Their depth was only guessed at, for the few attempts 
 to measure it had led to great exaggeration. The 
 nature of their floor was equally a matter of conjecture. 
 It was supposed, indeed, that the water at these pro- 
 found depths lay dead and motionless, that no living 
 thing could possibly exist at the bottom, and that no 
 change went on there save the slow accumulation of 
 fine mud, carried by ocean currents from far- distant 
 land. 
 
 10. But much of this uncertainty has now been dis- 
 pelled. Expeditions to measure the depth and explore
 
 io8 PHYSICAL GEOGRAPHY. [LESS. 
 
 the bottom of the sea have been sent to all parts of the 
 world, and have collected such a mass of information 
 that we now know more about the depths of the ocean 
 than about some parts of the continents. In particular, 
 the ship Challenger was, in 1872, sent round the world by 
 the British Government to ascertain the depths, tem- 
 perature, and other features of the great ocean basins. 
 Similar expeditions have since been organised by the 
 United States and Germany. Three chief instruments 
 have been used in these researches : the Sounding-line, 
 the Dredge, and the Thermometer. 
 
 11. The Sounding-line consists essentially of a rope 
 or wire with a weight at the end, so arranged that, when 
 dropped from the side of a vessel to the sea-bottom, the 
 amount of line which runs out from the reel is accurately 
 registered, and at once indicates the depth. The weight 
 may have its bottom covered with lard or other soft 
 material, which, when it strikes against the bottom, will 
 bring up some of the mud, sand, gravel, or other indica- 
 tion of the nature of the ground below. The Dredge 
 consists of an iron frame with a strong net or bag 
 attached to it ; the object of this instrument being to 
 bring up a larger quantity of the materials that form the 
 sea-floor, as well as any of the living or dead plants and 
 animals to be found there. The Thermometer has been 
 used to such good purpose that the temperature of the 
 vast Atlantic and Pacific Oceans has been determined 
 at many points, from the surface down to the deepest 
 parts of the bottom. By means of these instruments of 
 research the first beginnings have been made towards a 
 knowledge of the geography of the ocean-basins. The 
 general results, so far as at present known, are expressed 
 in Plate VII., which shows instructively how these basins 
 are broken up by submarine ridges, and how connected 
 the continental areas are in spite of the inland seas 
 which divide them. 
 
 12. At present the tract of sea which has been most
 
 xii ] THE GREAT SEA-BASINS. 109 
 
 thoroughly explored is the Atlantic Ocean. As shown 
 upon Plate VII., this Ocean runs as a long and wind- 
 ing belt of water between the New World and the Old. 
 Towards the north, as shown by recent soundings, it is 
 closed in by a submarine ridge, which, extending from the 
 north-west of Scotland through the Faroe Islands and 
 Iceland to Greenland, separates it from the Arctic basin. 
 Southward from the equator, the Atlantic Ocean gradu- 
 ally widens out and merges into the wide Antarctic 
 Ocean. It has been estimated that the whole area of 
 the Atlantic Ocean amounts to 35,165,000 square miles, 
 which is nearly a fifth part of the entire surface of the 
 globe. 
 
 13. Stretching from the Arctic to the Antarctic water, 
 the Atlantic Ocean crosses the various zones of tempera- 
 ture which girdle the globe. Its central parts lave the 
 shores of equatorial America and Africa. Beyond these, 
 it reaches the temperate climates of North America and 
 Europe. At the southern end it enters the icy Polar 
 regions. 
 
 14. Again, owing to the arrangement of the contin- 
 ents which bound it on the west and east, the Atlantic 
 receives a far larger river-drainage than any other ocean. 
 The map shows that down the whole length of America 
 all the large rivers flow eastward, and therefore fall into 
 the Atlantic the St. Lawrence, Mississippi, Orinoco, 
 Amazon, and La Plata. In Europe and Africa, if we 
 include the rivers which enter the Mediterranean and 
 Black Sea, by far the largest proportion of the drainage 
 finds its way into this ocean by such important rivers as 
 the Rhine, Rhone, Danube, Dnieper, Nile, Niger, and 
 Congo. The Atlantic basin is thus the great reservoir 
 into which the largest rivers of the globe discharge their 
 waters. 
 
 15. That the floor of the Atlantic cannot be a vast 
 plain like its surface is shown by the islands which here 
 and there rise even from the middle of the ocean.
 
 i io PHYSICAL GEOGRAPHY. [LESS. 
 
 From the numerous soundings which have been taken 
 throughout its entire length and breadth, its form can be 
 laid down with considerable accuracy upon a map. The 
 wider parts of the Atlantic have- a depth of from 2000 to 
 3000 fathoms, or from about 2 to 3^ miles. They form a 
 vast undulating plain, crossed by a ridge, of which the 
 Azores form the highest elevation. This ridge may be 
 regarded as starting from the western edge of the great 
 plateau on which Britain stands. Passing southwards 
 by the Azores it forms what is known as the Dolphin Rise, 
 which, at its southern end, about latitude 30 N., ascends 
 to within 400 fathoms from the surface. The ridge then 
 strikes south-westward at a depth of less than 2000 
 fathoms to the coast of Guiana, whence it turns south- 
 eastward across the ocean, coming to the surface under 
 the equator in the lonely St. Paul's Rock, and turning 
 southward as a long ridge from which the volcanic islet 
 of Ascension and Tristan d'Acunha rise. 
 
 16. To the west of the British Isles for 230 miles, the 
 slope of the Atlantic bottom is very gentle, being only six 
 feet in the mile. But beyond that distance the ground 
 descends more rapidly, for in the next 20 miles there is a 
 fall of 9000 feet, or a descent of 450 feet in the mile, 
 down to the level of the great submarine plain, which 
 stretches for hundreds of miles to the west, with little 
 variety of surface. This plain ascends slowly towards 
 the north till it forms the great plateau which, culmina- 
 ting in the Faroe Islands and Iceland, separates the 
 deeper water of the Atlantic from that of the Arctic 
 Ocean. The Newfoundland banks prolong the North 
 American continental mass far into the ocean. The 
 Florida peninsula and West Indian Islands separate 
 the deep Atlantic waters from the basins of the Mexican 
 Gulf and Caribbean Sea, which are obviously submerged 
 mediterranean seas (Art. 20). 
 
 17. In the deeper abysses of the Atlantic lying on 
 both sides of the central ridge, soundings of between 3000
 
 xii.] THE GREAT SEA-BASINS. ill 
 
 and 4000 fathoms have been found. About 100 miles 
 north from the Island of St. Thomas the Challenger 
 obtained a sounding of 3875 fathoms, or rather less 
 than 4^ miles, in what appears to be a vast hollow 
 running north-eastwards from the end of the ridge 
 on which the West Indian Islands rise. This is the 
 greatest depth yet found in the Atlantic Ocean. 
 
 18. Recent soundings in the Pacific Ocean have re- 
 vealed the general contour of its bottom, of which indi- 
 cations are supplied by the distribution of the islands. 
 In the South Pacific a submerged plateau extends west- 
 wards from Chili and Patagonia, and beyond it, to the 
 west, a group of submarine peaks and ridges rises into 
 scattered islets which culminate in Tahiti (7000 feet). 
 New Zealand, New Caledonia, the Feejee Islands, and 
 other groups, are based on a vast submarine platform 
 between 1000 and 2000 fathoms below the surface, which, 
 merging into that from which Australia rises, is prolonged 
 towards the north-west through New Guinea, Borneo, and 
 the Malay Archipelago into the great Asiatic area. An 
 irregular winding ridge, supporting the Caroline, Marianne, 
 and Bonin Islands, appears also to connect this platform 
 with Japan. The oceanic islands in the North Pacific, 
 including the Marshall, Gilbert, and Sandwich groups 
 rise from similar submarine banks and ridges, reaching 
 their highest point in the volcanic peaks of Hawaii 
 (13,760 feet). It is worthy of notice that while the 
 large islands on the prolongation of the Asiatic and 
 Australian plateau (New Caledonia, New Zealand, and 
 others) are composed mainly of non-volcanic rocks such 
 as those of which the continents chiefly consist, the 
 scattered oceanic islands, where they present any other 
 material than coral-rock, reveal a volcanic origin. They 
 have probably been originated by the piling up of vol- 
 canic rocks from submarine eruptions. 
 
 19. It is likewise deserving of remark that according 
 to recent soundings the floor of the Pacific Ocean, like
 
 U2 PHYSICAL GEOGRAPHY. [LESS. 
 
 that of the Atlantic, has an average depth of between 
 2000 and 3000 fathoms, but sinks here and there into 
 deep basins, even among the plateaux and islands. The 
 deepest abysses have been found between the Caroline 
 Archipelago and the Aleutian Islands. In particular, 
 immediately to the east of Japan, lies a long trough, of 
 which the bottom descends to between 4000 and 5000 
 fathoms. There the Tuscarora took the deepest sound- 
 ing yet obtained in any part of the globe, 465 5 fathoms. 
 In the Caroline Archipelago the Challenger found the 
 depth to be 4475 fathoms. 
 
 20. The map of the world shows that although the 
 great water envelope of our planet may, for the sake of 
 convenience, be parcelled out into separate oceans, these 
 are all united into one vast continuous sheet of water. 
 Here and there, however, owing to the way in which the 
 land has been ridged up, portions of the water have been 
 almost separated from the main mass. These are called 
 mediterranean, the best example being that which has 
 long been known as the Mediterranean Sea. The Black 
 and Baltic Seas in Europe, Hudson's Bay and the Gulf 
 of Mexico in North America, the Red Sea, Persian Gulf, 
 and Seas of Japan and Okhotsk in Asia, are other illus- 
 trations. It will be observed from Plate VII. that such 
 inland seas are comparatively shallow, and really belong 
 not to the oceanic but to the continental areas of the 
 earth's surface. Though their sites are now occupied by 
 the sea, they may once have been land, and might be 
 raised into land again without greatly disturbing the 
 present order of things. 
 
 21. But sometimes the uprise of the land has taken 
 place in such a way as to cut off completely some outlying 
 parts of the oceans. These are termed inland seas, ofwhich 
 the Caspian Sea and Sea of Aral are the most import- 
 ant instances. The former sea covers a larger space 
 than the British Isles. Its surface is about 85 feet below 
 sea-level, and its greatest depth amounts to nearly 3000
 
 xiii.] THE SALTNESS OF THE SEA. 113 
 
 feet. Its waters are tenanted by seals and other animals 
 that elsewhere inhabit the ocean. That a much larger area 
 in that region was once submerged is shown by the fact 
 that in the tracts of land which now inclose the Caspian 
 and Sea of Aral and separate them from the Black Sea on 
 the one hand, and from the Arctic Ocean on the other, beds 
 of dead sea-shells are found. The sea has been excluded 
 by the rise of its bottom into land. Reference will be 
 made in Lesson XXIII. to the proofs that the land along 
 the coast of Siberia has in comparatively recent times 
 been raised out of the sea. It has even been conjec- 
 tured that the Arctic Ocean formerly extended in a long 
 arm between Europe and Asia as far as the hill-range 
 which is now cut through by the narrow channel of the 
 Bosphorus, but did not communicate with the present 
 Mediterranean Sea, and that by the rise of the land 
 towards the north all that part of this vast inlet lying 
 to the south of the parallel of 50 or 52 N. was cut off 
 from the main ocean. The present abundant salt lakes 
 and marshes, as well as the two large basins of the 
 Caspian and Aral, have been regarded as the mere 
 shrunk remnants of this old Mediterranean Sea. The 
 Black Sea has been separated from the waters of the 
 Caspian region, and the intervening ridge between it 
 and the Mediterranean Sea has been cut through, so 
 that the Black Sea now communicates through the Bos- 
 phorus and Sea of Marmora with the Mediterranean. 
 There seems also to be less rain or more evaporation 
 now than formerly in the region of the Caspian and 
 Aral, so that these sheets of water are still further 
 shrinking. 
 
 LESSON XIII. The Saltness of the Sea, 
 
 1. The aqueous vapour of the atmosphere is in great 
 part derived from the evaporation of the surface of the 
 
 I
 
 ii 4 PHYSICAL GEOGRAPHY. [LESS. 
 
 sea, and as the sea does not become sensibly lower in level, 
 notwithstanding the enormous volume of moisture that 
 is annually driven off from it into the air, it must receive 
 back again from the rivers that descend to its shores, 
 and from the rain that falls upon its surface, as much 
 water as it supplies. There is thus a ceaseless coming 
 and going of water between the sea, the air, and the 
 land. 
 
 2. When the water, however, is examined in different 
 parts of its passage, it shows great varieties. Though 
 rain, being water condensed from natural distillation, is 
 nearly pure, it sometimes has a distinct taste, and when 
 evaporated leaves behind various salts and particles of 
 dust, which it has taken out of the air (Lesson VI.). 
 The water of springs and rivers gives a still larger pro- 
 portion of substances after evaporation, though usually 
 quite fresh and tasteless. But sea- water is always 
 strongly salt to the taste, and this is true of the sea in 
 all quarters of the globe. 
 
 3. Another distinction between the water of the air or 
 of springs and rivers and that of the sea is shown in 
 their different weights. A bottle filled with sea-water 
 weighs more than when filled with fresh -water. Per- 
 fectly pure water being taken as the unit, sea-water has 
 an average relative weight or specific gravity of about 
 i '026 ; in other words, if a certain quantity of perfectly 
 pure water weighs 1000, the same quantity of sea- water 
 will weigh 1026. The water of the different oceans 
 varies slightly in density. That of the Atlantic, for 
 example, varies more and is generally heavier than that 
 of the Pacific. In the path of the trade-winds in the 
 North Atlantic, where evaporation must be comparatively 
 rapid, some of the heaviest and of course saltest sea- 
 water occurs, its specific gravity being 1-02781. In the 
 Antarctic Ocean, on the other hand, among the broken 
 ice, the specific gravity sinks to I-O24I8. 1 
 
 1 Buchanan, Proc. Royal Society (1876), vol. xxiv.
 
 XIII.] 
 
 THE SALTNESS OF THE SEA. 
 
 A striking proof of the greater lightness of fresh water 
 may sometimes be seen after heavy rain has fallen in still 
 weather upon the sea. The rain floats on the surface of 
 the sea, and so does the water brought down by streams 
 from the land. Fresh drinkable water may then be taken 
 from the surface of the salt-water which lies below, until, 
 unless more quickly mingled by wind and waves, the 
 fresh-water slowly diffuses itself downward. 
 
 4. What is it that makes the sea-water heavy and 
 salt ? To answer this question let a small quantity o/ 
 
 FIG. 13 What is seen when a drop of concentrated sea-water is evaporated 
 under a microscope. 
 
 sea-water be evaporated till only a little of it remains, 
 which tastes extremely salt and bitter. Stirring this 
 remaining liquid, so as to mix it completely, put a drop 
 or so upon a piece of thin glass under a microscope, and 
 watch what takes place in this simple and beautiful 
 experiment. The drop continues to pass off into 
 vapour, but the substances which gave the water its 
 salt and bitter taste cannot do so. They remain behind, 
 and as the water leaves them they are seen shooting 
 into symmetrical crystals. Wonderfully interesting is it 
 to watch how the little particles rush to each other, and 
 unite to form those exactly proportioned outlines. First
 
 ii6 PHYSICAL GEOGRAPHY. [LESS. 
 
 there appear some oblong, pointed forms (gypsum, or 
 sulphate of lime), which probably began to form before 
 the drop was taken out of the concentrated salt-water ; 
 then, as the drop rapidly evaporates, a crowd of little 
 squares marshal themselves all round the edges, and 
 then become fixed and motionless. The water has been 
 driven off into vapour, and what remains on the glass is 
 the salts that give the sea-water its characteristic taste 
 and weight. Those who live far from the sea may see 
 a part of this experiment by taking a drop of water into 
 which as much common salt has been put as it will dis- 
 solve, and placing it under the microscope. The square 
 crystals will be seen growing round each other with great 
 vigour and in perfect symmetry. 
 
 5. While the different forms of crystals indicate the 
 different saline materials present in the sea-water, each 
 substance which crystallises keeping its own form of 
 crystals, the relative abundance of each form shows in a 
 general way the proportions in which the substances 
 occur in the water. We see, for example, that by much 
 the most abundant are the square or cubical crystals. 
 These belong to common salt, or chloride of sodium. 
 They make their appearance after the gypsum has 
 crystallised, whence it is clear that they can remain 
 longer dissolved in the water. They are therefore said 
 to be more soluble. Now while the dry film left by the 
 drop is still below the microscope, breathe gently on it, 
 and watch what takes place. The moisture of your 
 breath dissolves the crystals one by one, until, if you 
 have supplied moisture enough, they wholly disappear 
 in the little drop of water which now replaces them. 
 The cubical crystals of salt vanish first. Should there 
 be much vapour in the air, you may not be able to 
 keep the crystals from attracting it and dissolving, unless 
 you heat the glass before a fire. This tendency to attract 
 moisture and pass into solution is called diliquescence. 
 
 6. When sea-water is analysed it is found to contain
 
 xiii.] THE SALTNESS OF THE SEA. 117 
 
 about three and a half parts by weight of salts in every 
 hundred parts of water. As indicated by the drop eva- 
 porated in the experiment, common salt forms by much 
 the largest proportion, at least three-fourths, of the whole 
 amount of salts. The other saline substances are chlor- 
 ides of magnesium and potassium, sulphates of lime 
 (gypsum) and magnesia (Epsom salts), and some others 
 in still smaller quantity. The proportions do not greatly 
 differ throughout the whole extent of the ocean. As 
 the sea receives the rivers that descend from the land, 
 its waters must contain in solution the various sub- 
 stances carried in river water. There is perhaps no 
 kind of mineral matter in the outer portion of the earth 
 which does not occur dissolved in sea-water, though the 
 proportions of them are usually so minute as to escape 
 detection by the ordinary process of analysis. 
 
 7. The next question that arises is, Whence does the 
 sea get its salt ? This is not quite so easily answered. 
 We cannot tell what the earliest condition of the sea may 
 have been. Probably its waters have always been salt, 
 ever since they condensed out of the first atmosphere of 
 gas and vapour that enveloped the earth. The saline 
 vapours, which were no doubt diffused abundantly 
 through that atmosphere, would be carried down in 
 solution into the primeval ocean. But even if the sea had 
 been quite fresh at the beginning, it would have been 
 perceptibly salt now. In Lesson XXVII. an account will 
 be given of the way in which the fresh-waters that flow 
 off the land invariably carry saline substances to the sea. 
 As this process continues without ceasing, and in every 
 quarter of the globe where land exists, no one can fail 
 to understand how vast must be the yearly tribute of 
 dissolved mineral matter received by the sea from the 
 land. If the methods of analysis were delicate enough, 
 it would be possible to detect in sea-water traces of every 
 substance dissolved in the waters of the springs, rivers, 
 and lakes of the land. Some ingredients, for example,
 
 ii8 PHYSICAL GEOGRAPHY. [LESS. 
 
 which are present in too minute proportion to be 
 detected by chemical analysis in a small quantity of 
 sea-water, are absorbed by marine plants and animals, 
 and found in their ashes or skeletons. The copper 
 bottom of a ship which has been sailing for months 
 gathers out some of the dissolved arsenic or other 
 metals which are diffused through the waters of the sea, 
 and these substances are likewise found in the crust 
 which forms inside the boilers of ocean-steamers where 
 salt-water is used. 
 
 8. Among the ingredients present in sea-water in such 
 minute proportions as almost to escape notice, two are 
 of such importance that their names should be noted 
 carbonate of lime and silica. The former of these 
 is the material of which the hard parts of shells, corals, 
 sea-urchins, star-fishes, and other familiar sea-creatures 
 are mainly built up. When embodied by these animals 
 as part of their skeletons, it becomes the white chalk- 
 like substance which we recognise as the material com- 
 posing the common shells of the sea-shore. Silica is 
 secreted by lowly forms of plant and animal life in the 
 sea. The hard compact stone called flint is a form of 
 ancient sea-grown silica. 
 
 9. Sea-water likewise contains air. That this must 
 be the case is evident from what takes place in windy 
 weather at sea. The surface of the water is tossed up 
 into waves, and these are blown into spray. The sea 
 becomes a sheet of white foam, which is a mixture of air 
 and water. Some of the air remains dissolved in the 
 water after the waves have disappeared, and is slowly 
 diffused even to the depths of the ocean. One important 
 function of wind is thus to aerate the water of the sea. 
 The quantity of air present in sea-water varies from time 
 to time; sometimes it has been found to amount to no 
 more than one-hundredth of the volume of water, at other 
 times to as much as one-twentieth. 
 
 10. These may seem small proportions, yet we cannot
 
 xiv.] THE DEPTHS OF THE SEA. 119 
 
 but look upon them with interest, for it is this included 
 air which enables the animals of the ocean to breathe. 
 They inhale it, and exhale carbonic acid. Hence that 
 gas, besides the proportion of it in the dissolved air, is 
 likewise present in sea- water. In the higher parts, which 
 come under the influence of wind-waves, carbonic acid 
 is partly removed, and fresh supplies of air are furnished 
 in its place. 
 
 11. One other substance in sea-water deserves notice 
 here, called organic matter, or protoplasm. This 
 is the material which serves the simpler forms of marine 
 creatures as food. It is derived from the substance of 
 dead plants and animals, which live in the sea, and of 
 those which are carried by rivers from the land. 
 
 12. 1 1 appears, then, that the sea is salt, partly probably 
 from the vapours of the original atmosphere from which 
 it condensed, partly because many different salts are con- 
 tinually carried into it by brooks and rivers ; that it con- 
 tains in solution more or less of all terrestrial substances 
 which water will keep dissolved ; that it is aerated by 
 the waves, the air that is diffused through its mass 
 serving for the respiration of marine animals ; and that 
 from the dead bodies of animals and the remains of 
 plants, both on land and sea, the fundamental substance 
 or protoplasm of which plant and animal forms are built 
 is diffused through the whole mass of the ocean. 
 
 LESSON XIV. The Depths of the Sea. 
 
 1. From what has been said in Lesson XII. it is plain 
 that the bottom of the sea must be uneven and undulating, 
 like many parts of the surface of the land ; stretching out 
 in some parts into vast plains, rising here and there into 
 broad and long ridges, shooting up into vast slopes, like 
 that of the Atlantic floor to the west of Britain, sinking 
 into deep hollows, which descend far below its general
 
 120 PHYSICAL GEOGRAPHY. [LESS. 
 
 level, as lake-bottoms do below the general level of the 
 land, and traversed by mountain ranges, of which the 
 oceanic islands are only the unsubinerged peaks. But 
 what is the nature of the surface of the sea-floor? Is it 
 solid rock, or soft mud, or barren sand, or are the sub- 
 marine plains covered with an oceanic vegetation, as the 
 plains of the land are with grass and herbage ? 
 
 2. Standing by the margin of the sea we observe that 
 the water breaks upon sand, gravel, mud, or shells, and 
 that these materials pass down beneath it. If the shore 
 is rocky, pools of the salt-water may be noticed, from 
 which some idea may be formed of the nature of the 
 bottom of at least the shallower parts of the sea. Each 
 of these pools forms as it were a miniature sea. Its sides, 
 hung with tufts of delicate sea -weeds, are bright with 
 clusters of sea-anemones, while many a limpet and peri- 
 winkle rests fixed to the stone, or creeps cautiously over 
 its surface. The bottom of the water abounds in shady 
 groves of algae, through which many tiny forms of marine 
 creatures dart or crawl. As we look into one pool after 
 another, we find them all to be more or less full of 
 plant and animal life. 
 
 3. Turning from these shore pools to the edge of the 
 sea itself when the tide is low, we mark that the ledges 
 of rock or sheets of sand and shingle support a thick 
 growth of coarse dark green or brown tangles and sea- 
 wrack, among which, if the water is still enough, tiny 
 crabs, sea-urchins, jelly-fish, and other bright-coloured 
 marine animals may be seen. If the water is examined 
 from a boat, this forest-belt of large dark sea-weed is 
 found not to extend to a greater depth than a few 
 fathoms. Beyond it the bottom, whether rocky, sandy, 
 or muddy, can be seen through the clear water, or may 
 be examined by means of the dredge. Delicate scarlet 
 sea-weeds with corallines and deeper-water shells inhabit 
 these tracts. The sea-weed belt which fringes the land 
 has an average breadth of about a mile. Beyond it, as
 
 xiv,] THE DEPTHS OF THE SEA. 121 
 
 we gradually get into deeper water, the common plants 
 and animals of the shore are found one by one to dis- 
 appear, and other kinds to take their place. The dredge 
 may be dragged along some parts of the sea-floor and 
 bring up only sand or mud, while at a short distance off 
 it may come up full of many and varied forms of marine 
 life, thus showing that there must be bare tracts of sand, 
 mud, or stone on the sea-floor, and other patches where 
 plants and animals are crowded together. 
 
 4. Down to a depth of 100 fathoms, the waters of 
 the ocean round the margin of the land abound in plants 
 and animals. This upper and marginal belt supports 
 indeed by far the most abundant and varied marine 
 population. Down to depths of 200 or 300 fathoms 
 there is still a plentiful assemblage of animals. But as 
 the depth and distance from land increase, not only do 
 the types of the shallower waters diminish, but other 
 forms make their appearance which are universally dif- 
 fused over the deeper parts of the ocean. Until recently, 
 naturalists supposed, that as life becomes less prolific in 
 the deeper water, the great abysses of the sea must be 
 barren of life. They even fixed the limit at 300 fathoms, 
 below which, as light ceases to penetrate farther, they 
 thought the waters of the ocean must be lifeless. 
 
 5. But within the last few years, researches, carried 
 on across the Atlantic and other oceans, have shown 
 that no such limit really exists, that, on the contrary, an 
 assemblage embracing representatives of all the great 
 types of animal life exists on the sea-floor at depths of 
 more than two miles, where total darkness must prevail, 
 and where the temperature is very low. The species of 
 animals obtained from these depths, however, differ from 
 those of the shallower tracts. They present no great 
 variety of form, nor generally a great abundance of 
 individuals. Plant-life is much more restricted in its 
 range. Sea-weeds abound from high-water mark down 
 to about I 5 fathoms ; they become rare beyond a depth of
 
 122 PHYSICAL GEOGRAPHY. [LESS. 
 
 50 fathoms, and are not found at all in water deeper than 
 200 fathoms, although some other kinds of marine plants 
 occur at still greater depths. But from the bottom of the 
 profounder abysses plants seem to be wholly absent 
 
 6. In such comparatively shallow water as that which 
 surrounds the British Islands, where the action of sea- 
 currents extends to the bottom, banks of sand, with 
 patches of broken shells and gravel, and wide flats of mud, 
 cover much of the sea-floor. Some of these sand-banks, 
 where oysters, clams, and other shell-fish live, furnish 
 good feeding-ground for cod, haddock, turbot, and other 
 familiar fishes, and are therefore well-known to fishermen. 
 They seem to retain their place and form for many years. 
 But in the shallower parts of these seas they are apt to 
 be altered by storms. Some of them come so near the 
 surface that vessels are often driven upon them and 
 wrecked. 
 
 7. The mud, sand, and gravel, strewn over these 
 tracts of the sea-bottom, all consist of water-worn par- 
 ticles of rock. If we could trace back the history of 
 these deposits, we should find that each grain of sand 
 and each pebble of gravel once formed part of the solid 
 rocks of the land. It will be pointed out in Lesson 
 XVIII. that these materials are not produced on the 
 sea-bottom, but are brought thither from the land. It 
 is along the margin of the land, where waves can act 
 upon the rocks, that the sea does nearly all its erosive 
 work. Every stream, too, is busy carrying and pushing 
 mud, sand, and gravel to the sea. Thus, whether the 
 materials have come from the surf-beaten shore, or from 
 the hills and valleys of the land, they are strewn over 
 those tracts of the sea- bottom which lie nearest to land. 
 The sea is, indeed, the vast receptacle into which all 
 the materials derived from the crumbling of the surface 
 of the land must ultimately find their way. Wherever 
 there is movement of the bottom water, sand, and even 
 fine gravel, may be pushed along. But, throughout most
 
 \'iv.] THE DEPTHS OF THE SEA. 123 
 
 of the great sea-basins, the water that lies upon the bot- 
 tom, though not quite motionless, has too little motion 
 to be able to grind down rocks into sand and mud, and 
 is probably too sluggish in its flow even to disturb the 
 fine sediment which must be the only kind of deposit 
 that can be slowly accumulated there. 
 
 8. Much light has been thrown upon the condition 
 of the deep-sea bottom by the soundings of the Challenger 
 expedition. In comparatively shallow water near land, 
 whether islands or continents, variously coloured deposits 
 of mud and sand occur, evidently in great part formed 
 from the finer sediment which has been worn away 
 from the surface of the land and carried out to sea. 
 These deposits of land-derived sediment may extend in 
 exceptional cases to 300 miles from the continents, though 
 usually confined to much narrower limits. But away 
 from land, in the wide and deep depressions of the 
 ocean basins, other characteristic deposits are laid down. 
 The most universal of these are fine red and gray clays, 
 in which fragments of the volcanic rock called pumice, 
 may be recognised. Remains of minute organisms 
 (Globigerina and Radiolaria) occur in them, sometimes 
 abundantly. Most of the bottom of the Pacific and 
 Atlantic basins, at depths of more than 2000 fathoms, 
 as far as yet explored, is covered with these clays. 
 
 9. There seems little reason to doubt that these fine 
 clays are derived from the decomposition of volcanic 
 detritus slowly drifted across the ocean. This detritus 
 may come mainly from submarine eruptions ; but part of 
 it is no doubt supplied by the pumice which is sometimes 
 discharged from volcanic islands in prodigious quantities, 
 and which, being light and porous, may float away to 
 vast distances before becoming water-logged and sinking 
 to the bottom. The extreme slowness with which the 
 clays gather on the ocean-floor is strikingly shown by the 
 fine iron dust of falling stars which has been recognised 
 in an appreciable quantity in the clays brought up in the
 
 124 PHYSICAL GEOGRAPHY. [LESS. 
 
 Challenger soundings, and by the finding of 600 sharks' 
 teeth, 100 ear- bones of whales, and other bones of 
 whales, in a single haul of the dredge. These objects 
 have accumulated with almost inconceivable slowness on 
 the sea-bottom, yet have not been covered up in the still 
 more slowly gathering clays. 
 
 1O. Other wide-spread oceanic formations are derived 
 from the accumulation of the remains of minute plants 
 or animals. In the Southern Ocean, towards the icy 
 Antarctic land, a pale, straw-coloured deposit has been 
 dredged up from depths of nearly 2000 fathoms. 
 
 FIG. 14. Some of the Ooze of the Atlantic floor, magnified. 
 
 It consists chiefly of the remains of minute siliceous 
 plants called diatoms. Another kind of siliceous deposit, 
 formed from the minute animal forms called Radiolaria, 
 covers a considerable space in the intertropical regions 
 of the Pacific. The best known of these deep-sea forma- 
 tions is the ooze which was brought to light when the 
 earliest soundings were made for the telegraphic cable 
 between Britain and America. It covers the floor of 
 the Atlantic for thousands of square miles as a fine, 
 soft, chalky deposit. A sounding-lead dropped upon 
 the bottom sinks deeply into it. When the dredge 
 goes down, it plunges into this light, fleecy mud, and 
 comes up full of it. The upper layer is a yellowish, 
 creamy, somewhat sticky substance, full of entire and 
 broken shells of such minute animal organisms (Globi- 
 gerina) as are shown in Fig. 14, with fragments of 
 sponges and other forms. Underneath this surface layer
 
 xv.] THE TEMPERATURE OF THE SEA. 125 
 
 the ooze shows the shells in a more crumbling state, and 
 no doubt in the deeper parts of the deposit, which the 
 dredge cannot reach, the substance may consist only of 
 a kind of fine chalk, formed almost entirely of the mould- 
 ered remains of these lowly forms of life. Living sponges, 
 star-fishes, crinoids, and various shells have been brought 
 up by the dredge with the ooze. It would seem that 
 while this widely extended deposit may contain the re- 
 mains of such forms of life as inhabit deep water, it is 
 derived mainly from the Globigerina, which lives in im- 
 mense numbers in the surface waters, and which, when 
 dead, sinks through the depths to the bottom. 
 
 LESSON XV. The Temperature of the Sea. 
 
 1. Since the temperature of the air and of the land 
 varies in different latitudes, we may expect to find that 
 of the sea to show corresponding variation. There are 
 indeed climates in the sea as well as on the land. The 
 temperature of the surface water of the sea varies with 
 the seasons, and with the direction of the great ocean 
 currents. Thus, on the west side of Britain, the super- 
 ficial sea-temperature is about 49 Fahr., and over a 
 great part of the North Atlantic it ranges between 44 
 and 54. Further south, in the equatorial part of that 
 ocean, it rises to from 80 to 83-5. In the North Pacific 
 Ocean the mean surface temperature is about 70, and 
 in the South Pacific about 677. In Polar seas, away 
 from the influence of warm currents, the water does not 
 rise much above the temperature of melting ice. On 
 the other hand, in land-locked parts of the ocean within 
 the tropics, where the water is confined, and exposed to 
 great solar heat, the temperature sometimes rises to that 
 of a warm bath. Thus, in the Red Sea, it has been 
 noted at 94, and in different parts of the Indian Ocean 
 at from 88 to 91.
 
 126 PHYSICAL GEOGRAPHY. [LESS. 
 
 2. The importance of the temperature of the sea is 
 seen in the influence which it exerts upon the climate of 
 the maritime tracts of land. The temperature of these 
 regions depends greatly upon that of the winds which 
 come from the sea. The air resting upon the ocean 
 surface acquires the temperature of that surface, and 
 carries it over the land. Save for this convection, the 
 sea would have comparatively little influence upon the 
 climates of the land. But by warming or cooling the 
 air over tracts thousands of square miles in extent, it 
 becomes the great distributor and regulator of tempera- 
 ture. This has been partly explained in Lesson XL, and 
 will be further treated of in Lesson XVIII. 
 
 3. The surface temperature of the sea varies with the 
 latitude and with the season of the year. Over the 
 wider oceans the isothermal lines for the surface-water 
 do not greatly vary from the direction of the parallels 
 of latitude ; but in the North Atlantic they depart widely 
 from that direction, for the reasons already explained 
 (Lesson IX. Art. 13). The surface-waters of the sea 
 in the northern hemisphere are coldest in February and 
 warmest in August ; in the southern hemisphere they 
 are coldest in August and warmest in February. More- 
 over, the surface temperature varies at each place during 
 every day, perhaps to the amount of rather less than one 
 degree Fahr., the lowest temperature being reached in 
 the morning and evening, and the highest between two 
 and four o'clock in the afternoon. Recent observations 
 of the Challenger expedition, discussed by Mr. Buchan, 
 indicate that the daily oscillation of the temperature of 
 the air over the sea is 3*21 Fahr., or nearly four times 
 greater than that of the water on which it lies. 
 
 4. It is evident, however, that the readings of a 
 thermometer immersed merely in the surface oceanic 
 water give no more indication of the real temperature 
 of the great mass of the sea than if we tried to esti- 
 mate the temperature of the whole atmosphere merely
 
 xv.] THE TEMPERATURE OF THE SEA. 127 
 
 from observations with the thermometer at the sea- 
 level. By taking observations at intervals up to the 
 mountain-tops, the gradual lowering of the tempera- 
 ture referred to in Lesson IX. Article 20, has been 
 determined. In like manner, by means of a specially 
 arranged kind of thermometer, the temperature of the 
 sea at any depth can be ascertained. The instrument 
 is protected in such a way as to be unaffected by the 
 pressure of the water. It is let down with a cord, allowed 
 to remain long enough at each depth to acquire the tem- 
 perature of the surrounding water, and then pulled up. 
 
 5. In this way the temperature of the Atlantic and 
 Pacific Oceans has been determined at intervals from 
 the surface down to the deepest depressions of the bottom. 
 Temperature-soundings have likewise been made in the 
 Indian Ocean. As the result of such observations all 
 over the world, it has been discovered that in general 
 the surface-water is the warmest part of the sea, and that 
 the water gets colder towards the bottom. In the open 
 sea, indeed, communicating freely with the Polar regions, 
 the deep bottom-water has everywhere been found to have 
 very nearly the same temperature as that of the cold 
 Polar Seas. Sometimes the thermometer has come up 
 with readings as low as 29-5 ; that is, two and a half 
 degrees colder than the freezing-point of fresh water. 
 
 6. Soundings taken by the Challenger showed every- 
 where a progressive lowering of the temperature from 
 the surface downwards, with variations in the rate of 
 decrease. In the North Atlantic, water having a tem- 
 perature above 40 forms the upper layer of the ocean 
 to a depth of from 750 to 1000 fathoms. Under that 
 warmer sheet the temperature slowly sinks, until in the 
 deep abysses, more than 3000 fathoms down, it reaches 
 34-4. Hence the main mass of this ocean consists of 
 chilly water. In the South Atlantic the warm upper 
 layer is only about 300 fathoms thick. Below it the 
 temperature of the water falls from 40 to 32-9, so that
 
 128 PHYSICAL GEOGRAPHY. [LESS. 
 
 the body of chill water is colder and comes nearer to 
 the surface than in the North Atlantic. In the equatorial 
 part of the ocean, while the surface temperature is 76- 
 80, the cold water under 40 is found at a depth of 
 only 300 fathoms. The bottom layer of water is at less 
 than 35 for a thickness of 600 fathoms, while some of 
 its deeper cavities have the chilly temperature of 32-4. 
 
 7. In the North Pacific Ocean a general bottom tem- 
 perature below 35 Fahr. has been observed. Some- 
 times the body of water at this low temperature exceeds 
 2000 fathoms in depth. In the South Pacific Ocean, 
 beyond Cape Otway, Australia, the bottom temperature 
 sinks to 32-5, while farther south (Lat. 53 55' S.), the 
 body of icy water below 35 comes within less than 100 
 fathoms from the surface. Below a depth of 1000 
 fathoms the water is as cold as ice (32) down to the 
 bottom, where the thermometer marked 3 1 at a depth 
 of 1950 fathoms. 
 
 8. The bottom water may be regarded as remaining 
 constantly at an almost icy temperature, with a remark- 
 ably small variation of only about 7, or from about 31 
 to 38 Fahr. This low temperature of all but the upper 
 parts of the sea shows that as the deep water could not be 
 cold unless it came from cold latitudes, there must be a 
 continual transference of water from the cold Polar tracts 
 towards the equator. The heavy cold water sinks down, 
 and creeps on below the upper warmer layers, which 
 move towards the poles to supply its place. When the 
 cold water reaches the equatorial regions, it must rise 
 towards the surface, where, being gradually warmed, it 
 moves away again as a surface layer towards the poles. 
 Hence soundings with the thermometer in the oceans 
 not only reveal the various temperatures of the sea, but 
 bring before us evidence of vast slow movements, by 
 which the oceanic waters are constantly mingled and 
 kept from stagnation. 
 
 O. In the North Atlantic, between Scotland and the
 
 xvi.] THE ICE OF THE SEA. 129 
 
 Faroe Islands, below a uniform surface temperature of 
 5o-53 Fahr., two parallel belts of water have been ob- 
 served lying side by side and having nearly the same depth. 
 In the southern of these the bottom temperature was found 
 to be 42-7, in the northern 29-6. Two very different 
 sea-climates were thus ascertained to lie close together, 
 one with the mild character of the latitude, the other 
 with the chilly temperature of the icy Arctic sea. It was 
 at first supposed that two parallel currents here impinged 
 against each other, one coming from the icy north and 
 the other from the warmer Atlantic. But recent re- 
 searches have shown that the warm and cold areas 
 are separated from each other by a submarine ridge 
 which completely prevents any flow of polar water into 
 that part of the North Atlantic. So that the cold water 
 with its characteristic forms of plant and animal life is 
 banked up against the north side of the ridge, while the 
 warmer Atlantic water, with its distinctive organisms, 
 lies at corresponding depths on the south side. 
 
 LESSON XVI. Tfie Ice of the Sea. 
 
 1. When sailing across the North Atlantic between 
 ports in Europe and in the United States, or when 
 traversing the South Pacific to round the southern pro- 
 montory of the American or African continent, vessels 
 occasionally encounter floating masses of solid ice, termed 
 Iceberg's. These are of all sizes up to huge mountain- 
 like islands rising several hundred feet out of the water. 
 Ice being lighter than water, an iceberg projects above 
 the sea-level, yet has about eight times more of its bulk 
 under water than above. Hence, when the mean height 
 of one of these floating masses is 300 feet above the 
 surface of the sea, its bottom must be some 2400 feet 
 below. The bergs vary infinitely in shape as well as in 
 size. Sometimes, as in the Antarctic Ocean, they take 
 K
 
 130 
 
 PHYSICAL GEOGRAPHY. 
 
 [LESS. 
 
 the form of vast square blocks with vertical walls ; some- 
 times they bristle into peaks and pinnacles, with deep 
 cliffs and gullies between. At a distance they look like 
 snow-covered islands. Seen closer, they gleam with all 
 the intensity of colour white, green, and blue so 
 characteristic of the ice of glaciers upon the land. For 
 the most part, nothing but ice in different forms is seen 
 upon their surface. Now and then a block of stone or 
 
 FIG. 15. Iceberg at Sea. 
 
 some dark earthy rubbish may be noticed. As they 
 drift onwards into warmer latitudes they melt away both 
 under water and above. Cascades of water tumble down 
 their thawing slopes and fall into the waves below. It 
 often happens that, as melting advances under water, 
 the centre of gravity of an iceberg is altered, so that the 
 mass shifts its position, or, becoming top-heavy, turns 
 completely over. 
 
 2. Such are the drifting icebergs which in summer 
 cross the navigation track across the Atlantic between
 
 xvi.] THE ICE OF THE SEA. 131 
 
 Newfoundland and Britain. They form a serious source 
 of danger to vessels, for they may be encountered at any 
 hour of the voyage. Many a ship has struck against one 
 in the dark and gone to the bottom. They chill the air 
 around them, and thus often give rise to dense fogs. A 
 sudden fall of temperature is regarded by sailors as an 
 indication of their approach to an iceberg, and a warning 
 to be on the outlook. 
 
 3. But icebergs are not formed in the open sea, where 
 they float until they are entirely melted. To reach their 
 birthplace we must travel to the cold regions which 
 surround each pole. In North Greenland, for example, 
 the country is covered with ice, which, creeping slowly 
 down the slopes, gathers in the valleys into wide and 
 deep masses that not only come down to the sea-level, 
 but actually push their way out to sea in vast walls of 
 ice, rising 300 or 400 feet above the water, and stretching 
 sometimes for 60 miles along the coast. From time 
 to time portions of these great tongues of ice break off 
 and float out into the open sea. These are icebergs. 
 They consist, therefore, not of frozen sea-water, but of 
 true land-ice. Each berg which may be met with, float- 
 ing and melting far away in mid-ocean, had its origin 
 among the snows of the Polar lands. (Lesson XXVIII.) 
 
 4. Since so large a proportion of their bulk is under 
 water, we may expect to find that floating icebergs are 
 comparatively little influenced by winds and waves, for 
 these touch only their upper parts. The larger bergs 
 move with the ocean-currents or drifts in which so much 
 of their substance is immersed. Hence they may some- 
 times be seen moving steadily and even rapidly along, 
 right in the face of a strong gale. 
 
 5. So long as the icebergs sail over deep water, they 
 move freely about, as the currents or winds may drive 
 them. But when they get into water shallow enough to 
 allow their bottoms to grate along the sea-floor, they tear 
 up the mud or sand there, until they are at last stranded.
 
 132 
 
 PHYSICAL GEOGRAPHY. 
 
 [LESS.
 
 xvi.] THE ICE OF THE SEA. 133 
 
 The coast of Labrador is often fringed with such grounded 
 bergs, some so small as to be driven to the beach, others 
 so large as to run aground while still a good way from 
 the shore. Chill fogs consequently hang over that 
 desolate region all through the summer. 
 
 6. Besides icebergs, other kinds of ice are found 
 on the sea in high latitudes, derived from the freezing 
 of the water of the sea itself. As voyagers advance 
 into the narrow seas about the pole, they encounter 
 great sheets of ice, which, at first in irregular, scat- 
 tered fragments, become larger in size and more abun- 
 dant until at last the whole expanse of the sea, as far 
 as the eye can reach, is covered with ice. This is 
 known as Floe -ice or Field -ice, and is due to the 
 freezing of the surface-water of the sea during the intense 
 frost of Polar winters. When the sea- water freezes, the 
 salt is left behind by the ice, which, except for the little 
 salt vesicles entangled in it, is fresh, and, when thawed, 
 quite drinkable. In early summer, the ice-field, or sheet 
 of ice, which, stretching outwards from the land, may be 
 regarded as continuous for hundreds of miles during 
 winter, begins to break up and float away. If the sea 
 remained motionless it would, by the end of winter, be 
 covered with a sheet of ice about eight feet thick. But 
 owing to the pressure arising partly from the movements 
 of tides and currents in the water, partly from changes 
 of ternperature and atmospheric pressure, including winds 
 and storms, the ice-sheet is liable to continual disruption. 
 Cracks are formed abundantly through it, and portions 
 are squeezed up or broken into innumerable huge frag- 
 ments, which are pushed over each other until the frozen 
 sea becomes like a heap of ruins. These movements 
 are accompanied with sharp reports, as of cannon, and 
 with loud growling noises, as if some monsters of the deep 
 were engaged below in fierce and angry warfare. Some 
 notion of the appearance of this broken-up ice may be 
 formed from Fig. 17. This represents a scene in the
 
 134 
 
 PHYSICAL GEOGRAPHY. 
 
 [LESS. 
 
 memorable Arctic voyage of the Austrian vessel Teget- 
 thoff, which, after almost incredible endurance, was even- 
 tually abandoned by the courageous crew, who, taking to 
 
 boats or sledges drawn by dogs, pushed across the frozen 
 sea and reached Novaya Zemlya. 1 
 
 1 For a graphic account of this and other Arctic voyages, see New Lands 
 Within the Arctic Circle, by Julius Payer.
 
 xvi.] THE ICE OF THE SEA. 135 
 
 7. In consequence of the breaking up of the vast sheet 
 of ice, water-channels or lanes are opened out between 
 the separated fragments, and form passages, by which 
 vessels make their way through the ice-pack. But as the 
 great sheets are carried against each other in the general 
 movement, it sometimes happens that a ship is caught 
 between them, and pushed up on the floe, or crushed so 
 effectually that it goes to the bottom as soon as the frag- 
 ments of ice separate again. 
 
 8. Except where piled in heaps by pressure, which 
 breaks up its surface as well as its outer edges, floe-ice 
 occurs in level sheets, the surface of which rises but little 
 above the level of the sea. It never rivals the height and 
 grandeur of true icebergs, though it covers a much wider 
 space of sea. Nor does it travel so far from the regions 
 of its birth. When the ice-field breaks up in summer 
 the portions next the land may remain there, and of 
 these, indeed, some parts may continue unmelted for 
 generations. But other parts farther from shore, and 
 sometimes many hundreds of square miles in extent, are 
 loosened and carried away by the sea-currents which 
 drift from the pole. Vessels frozen into the ice-field have 
 in this way been carried many hundreds of miles, until 
 disengaged by the disruption and thawing of the ice. 
 Some of the icebergs from Greenland, however, travel 
 much farther south, and may be met with now and then 
 even as far south as lat. 37, that is, about the same 
 parallel as Richmond in Virginia, and Cape St. Vincent 
 in the south of Spain. 
 
 9. When the sea freezes along the margin of the land, 
 as it does in a remarkable way in North Greenland, it 
 forms a cake of ice which, rising with the tide, is frozen 
 to the shore. By degrees a shelf of ice, called the Ice- 
 foot, rising from twenty to thirty feet above the general 
 level of the floe, and having a width of 1 20 feet or more, 
 forms along the coast and clings to it all winter. Immense 
 quantities of earth and stones, dislodged from the coast
 
 136 
 
 PHYSICAL GEOGRAPHY. 
 
 [LESS. 
 
 cliffs by the severe frost of the Arctic winter, fall upon 
 the ice-foot, so that its surface becomes in some places 
 a waste of rubbish which completely covers and conceals 
 the ice underneath. When the summer storms arise, 
 this shore-terrace of ice is broken up, and large pieces of 
 it, laden with the waste of the cliffs, are floated away out 
 to sea, among the fleets of bergs and broken sheets of 
 
 FIG. 18. The Ice-foot of Greenland. 
 
 floe-ice. Some portions are driven ashore again, others 
 are caught and frozen fast into the floe-ice of next winter, 
 while others succeed in escaping into the more open sea, 
 where they gradually melt and tumble their load of earth 
 and stones on the sea-bottom. 
 
 1O. Floe-ice and the ice-foot are formed by the freezing 
 of the surface of the sea. There is yet another way in 
 which some of the ice of the sea takes its origin, viz., by 
 the freezing of the water lying on the sea-bottom. This
 
 xvii.] THE MOVEMENTS OF THE SEA. 137 
 
 is known as Ground-ice. It is probably formed only 
 in enclosed and shallow seas and inlets, and is of little 
 consequence compared with the thick and wide sheets of 
 floe-ice. It is well known in the Baltic Sea. In still 
 weather, before the surface of that sea is frozen, little thin 
 cakes of ice may be observed floating about, sometimes 
 with portions of sand or scattered pebbles imbedded in 
 them. These are formed on the bottom, from which 
 they break away and rise to the surface. They form a 
 source of some danger, at least to small boats, for they 
 sometimes appear suddenly in such numbers as to cover 
 the sea, the surface of which is then apt to freeze too, so 
 that the boats are in danger of being nipped between the 
 detached ice-rafts, or of being beset and frozen into the 
 united cake of ice. Sometimes large blocks of rock as 
 well as masses of seaweed are borne away from the sea- 
 floor and carried to the surface by the ascending sheets 
 of ground-ice. In the rivers of cold countries, as for 
 instance, in the St. Lawrence, similar ground-ice is formed 
 in winter, sometimes round iron chains or anchors, which, 
 if the enclosing ice is thick enough, may actually be lifted 
 by it. Hence it is known as Anchor-ice. 
 
 LESSON XVII. The Movements of the Sea. 
 
 1. The restlessness of its water is one of the features 
 of the sea which first impress the onlooker. Even in a 
 calm day, when not a leaf appears to be stirring on the 
 earth, and the clouds seem perfectly motionless in the 
 sky, the sea may be seen heaving, or curling into faint 
 ripples, or rolling in broad undulations which break into 
 foam when they reach the shore. On the other hand, in 
 stormy weather the unrest of the air is fully equalled by 
 the tumultuous uproar of the sea, when clouds of driving 
 rain and salt sea-spray are swept along by the tempest, 
 until it almost seems as if sea and sky were commingled.
 
 138 PHYSICAL GEOGRAPHY. [LESS. 
 
 2. We do not at first perceive that the movements of 
 the sea are not all by any means so fitful and capricious 
 as they appear to be, but that, on the contrary, they are 
 regulated by easily understood laws, and can, to a great 
 extent, be foretold and, if need be, provided against. 
 Nor do we at once realise to what different causes these 
 movements are due. 
 
 3. Let us suppose that, placed in some favourable 
 position for observing the motions of the sea, we care- 
 fully register the facts that come under our notice. The 
 margin of any of the great oceans would be suitable for 
 the purpose. Such a coast-line as that of the west of 
 Ireland, for example, forms an excellent field of observa- 
 tion, allowing opportunities of watching some of the most 
 characteristic movements of the water of a great ocean 
 basin. 
 
 4. The first point to be remarked is the obvious close 
 dependence between the motion of the air and that of 
 the water. As a rule, when the air is still the water is 
 smooth, though on some days we should find that even 
 with no wind, a good deal of agitation might be observ- 
 able in the sea, large waves rolling heavily to shore. 
 In such cases, however, we may often learn that a gale, 
 blowing somewhere out at sea, has raised the waves, 
 of which the final undulations now reach the land. 
 Every gradation of ripple, wave, and billow may be 
 traced on the water, answering doubtless to gentler or 
 stronger movements of the air. ' In this respect the sur- 
 face of the sea shows the same sensitiveness as that of 
 any lake or pond on the land, but its movements are more 
 marked, because it is so much larger, and allows the 
 accumulated effects of the continued pressure of the wind 
 to make themselves far more apparent. 
 
 5. But we could not long attend to the aspects of the 
 sea without observing traces of other movements inde- 
 pendent of the daily or hourly variations in the state of 
 the atmosphere. On the Irish coast, which we have
 
 xvn.] THE MOVEMENTS OF THE SEA. 139 
 
 taken by way of illustration, as on other parts of the sea- 
 board of north-western Europe, scattered leaves and fruits 
 are occasionally washed ashore from the wide ocean, 
 which differ greatly from the vegetable productions of any 
 European country. These are in reality West Indian 
 plants. They have been drifted across the Atlantic, and 
 hence they indicate the existence of some general drift 
 or current which sets across that ocean from west to east 
 or north-east. In like manner, bottles containing pieces 
 of writing, which have been thrown overboard from ships 
 at sea, have been picked up on the same shores, after 
 safely traversing hundreds of miles of sea. 
 
 6. But by far the most constant and regular movement 
 is one which, if we have never witnessed it before, cannot 
 but fill our minds with wonder. Twice every day we 
 find the edge of the sea to rise and fall, or to advance 
 and retreat, upon the land. On a line of steep coast- 
 cliff the surface of the sea may be observed slowly to sink 
 during about six hours, and then for a corresponding 
 interval slowly to rise again. On a flat or gently shelv- 
 ing shore this vertical movement manifests itself in an- 
 other way. The fall of the water lays bare wide stretches 
 of rocks, sand, or mud. During the retreat of the sea, 
 we notice that successive ripples or waves advance less 
 and less upon the beach. Step by step they seem to be 
 drawn back, until in some large bays many square miles 
 of flat ground are laid bare. During the advance of the 
 water, on the other hand, wave after wave comes a little 
 farther up the shore until all the uncovered flats are once 
 more under the sea, and boats can sail over places 
 which two or three hours before could be crossed on foot. 
 This rise and fall of the sea is known as the Tides. 
 
 7. A little observation enables us to make quite sure 
 that these daily movements have no connection with 
 those of the air. When a strong gale is blowing, and 
 driving the surface of the sea into foaming waves, it does 
 not arrest the advance or retreat of the tide. The waves
 
 I 4 o PHYSICAL GEOGRAPHY. [LESS. 
 
 continue to creep up or down the shore even in the face 
 of the wind. 
 
 8. Again, we cannot but remark the wonderful regu- 
 larity of the movements. Each advance of the water 
 occupies rather more than six hours, and each retreat 
 requires the same interval, so that the time of high or 
 low water may be foretold for years in advance. From 
 an early period of human history, and long before the 
 real cause of the tides was understood, it was observed 
 that there is a relation between them and the state 
 of the moon. In the course of the observations which 
 we have supposed ourselves to make, we find that the 
 time of high water corresponds with the passing of the 
 moon across the meridian, and that the highest and 
 lowest tides occur at the time of new and full moon. 
 
 9. Putting now the sum of these observations together, 
 we have ascertained the existence of three distinct kinds 
 of movement in the water of the sea. First, wind-waves ; 
 second, surface -drifts or currents ; and third, tides. 
 From what was said in Lesson XV. it is clear that a 
 fourth form of motion must be added to these, viz. a 
 general creep or movement of the cold polar water along 
 the bed of the sea towards the equator, and a flow of 
 the upper waters towards the poles. We must consider 
 each of these movements a little further. 
 
 10. (i.) Wind- waves. These, like the ripples we 
 send along the surface of a trough of water by blowing 
 briskly at one end, are due to the pressure of the wind. 
 Viewed from the top of a high cliff, or from the mast of 
 a ship at sea, they may be seen to succeed each other in 
 long parallel lines, travelling regularly and rapidly across 
 the surface of the sea. Though the waves move forward, 
 the water itself in the open deep sea shares but slowly in 
 this movement. It rises and falls with a slight oscilla- 
 tion as each wave passes, though, if the wind continues 
 to blow for a time, the surface water is gradually pushed 
 onward. A field of corn or tall grass may often be
 
 xvii.] THE MOVEMENTS OF THE SEA. 141 
 
 observed to be thrown into Waves as a smart breeze 
 sweeps over it, yet each stalk and blade retains its place, 
 and merely bends up and down and to and fro with a 
 kind of motion like that of the particles of water in a 
 wave. 
 
 11. When the sea has been thrown into violent agita- 
 tion by a storm, the waves do not at once subside when 
 the storm is over, nor do they remain confined to that 
 region only through which the storm has passed. So 
 sensitive is the great body of oceanic water that the waves 
 are propagated far beyond the area of the storm in vast 
 undulations, or ground-swell, as this heaving is called. 
 In the deep water of an open ocean the only trace of 
 this movement is seen in the broad undulations which, 
 like a great pulse, keep the surface regularly rising and 
 falling. A ship sailing over this surface alternately 
 mounts on the top of a swell and sinks into the trough 
 between two of them. But where the sea shallows to- 
 wards land, the up-and-down movement passes into a 
 true onward rush of the water. The upper part of the 
 undulation travelling faster than the lower parts, which 
 are impeded by the friction of the bottom, begins to 
 assume the aspect of a wave, curling over as it advances, 
 like a huge wall of green water, to burst into foam against 
 the land. 
 
 12. The motions of waves and ground-swell must be 
 insensible at the bottom, in the deeper parts of the sea. 
 In reference to the great body of the sea, they are merely 
 surface agitations, probably seldom extending sensibly 
 more than a few hundred feet downwards. So that when 
 a hurricane is raising the surface of an ocean into the 
 most violent commotion, we must think of the deep 
 abysses below as dark, silent, and calm. And this is a 
 matter of some importance when we consider the various 
 offices of the sea, as will be done in the following Lesson. 
 
 13. Since the longer the space of deep sea over which 
 the wind has to blow, the larger are the waves, we may
 
 I 4 2 PHYSICAL GEOGRAPHY. [LESS. 
 
 expect to meet with the grandest waves in the great 
 oceans. In the accounts of storms at sea it is common 
 to meet with such expressions as that " the waves were 
 mountains high." The size of waves is apt, however, to 
 be much exaggerated. Thus, a series of measurements, 
 taken during a voyage across the North Atlantic, gave 
 forty-three feet as the extreme height of the waves in 
 stormy weather. In the narrow British seas, which are 
 in a great measure barred off from the Atlantic, the 
 largest waves are less than half the maximum size which 
 they attain in that ocean. 
 
 14. No one can watch one of these huge waves as it 
 nears the land without being impressed by the great 
 force with which it beats upon the coast. The scene 
 represented in the frontispiece of this volume may give 
 some idea of the grandeur of a stormy sea. Each long 
 undulation which rolls in from the outer sea mounts 
 higher as it approaches the shore. At every step it 
 becomes a more and more marked ridge of water, which 
 begins to curl into a green -crested wave with a steep 
 concave front towards the land. By degrees the upper 
 advancing crest topples over, and the immense mass of 
 water plunges with all its weight upon the rocks or sand 
 of the coast-line. When the broken water has spent 
 itself in rushing up the beach, it runs rapidly down 
 again under the next advancing wave. In its recoil it 
 drags back the sand and gravel with a loud rattle or 
 hoarse roar, sometimes audible for many miles, as the 
 stones of the shingle are ground over each other. 
 
 15. The force with which such breakers, as they are 
 termed, fall upon the land has been measured and calcu- 
 lated. An undulation, or " roller," of the ground-swell 
 twenty feet high has been computed to fall with a 
 pressure of about a ton on every square foot of surface 
 exposed to its reach. In summer the average force of 
 the Atlantic breakers on the west coast of Britain amounts 
 to about 6 1 1 Ibs. on the square foot ; in winter the force
 
 xvn.J THE MOVEMENTS OF THE SEA. 143 
 
 is more than three times as great. On some occasions 
 a pressure of as much as three tons and a half has been 
 recorded. Some of the effects of the action of waves 
 upon the land will be referred to in the next Lesson. 
 
 16. (ii.) Currents. Since the sea responds so sensi- 
 tively to every movement of the air, we may expect to 
 find that as there is a regular system of circulation in 
 the atmosphere, so there must be a corresponding system 
 in the ocean. That this is really true has now been 
 made clear by observations in all parts of the world. 
 The sea has been found to be in continual circulation. 
 Such evidence as that given in Art. 5 makes the re- 
 ality of the surface-movement readily apparent. Those 
 superficial parts of the sea which have a regular, con- 
 tinuous, onward motion, are termed "currents." They 
 are called superficial because seldom more than 500 feet 
 deep, which is but a small fraction of the depth of the 
 total liquid mass over which they move. 
 
 17. The current -system of the sea presents in its 
 leading features great simplicity. It arises from the 
 action of the great atmospheric currents, which, steadily 
 blowing on the surface of the sea, drive its waters before 
 them. So that if we clearly follow the course of the 
 circulation of the air, we need have little difficulty in 
 understanding that of the sea. The general features of 
 the circulation of the oceans are shown in Plate VIII. 
 
 18. The trade-winds set the surface-water of the sea 
 in motion, so that it acquires a gradual tendency towards 
 the equator. On the north side it comes from the north- 
 east, on the south side from the south-east. Uniting along 
 the equatorial belt, it necessarily assumes a westerly 
 direction, and crosses both the Atlantic and Pacific 
 Oceans as the Equatorial Current. In the former 
 ocean it appears not to exceed about 300 feet in depth, 
 and to move with a surface rate of not more than 
 eighteen miles a day. 
 
 19. Were there no land to interfere with its flow,
 
 144 PHYSICAL GEOGRAPHY. [LESS. 
 
 this Equatorial Current would pass round the globe as 
 a continuous stream of warm water. But owing to the 
 position of the continents across its path, it is broken up 
 and made to diverge in different directions, each of the 
 independent branches receiving a distinct name. Thus 
 in the Atlantic Ocean, setting out from the African coast, 
 it crosses to the American side, where, striking against 
 the projection of Cape St. Roque, it splits into two. 
 The smaller branch, turning southward to form the Brazil 
 Current, skirts the coast as far as the mouth of the La 
 Plata, whence bending eastward, it once more crosses 
 the Atlantic towards the Cape of Good Hope, and turns 
 northward along the west coast of Africa, until drawn 
 again into the great equatorial current. The larger 
 branch sweeps round the northern coast-line of South 
 America into the Caribbean Sea and the Gulf of Mexico, 
 whence it issues through the Florida Strait as the warm 
 and rapid ocean-river well known by the name of the 
 Gulf Stream. After emerging from the narrows, with 
 a maximum surface temperature of 80 Fahr. and a 
 rate of 70 to 120 miles a day, this current runs par- 
 allel with the coast of the United States, but separated 
 from it by a cold current, which descends from the 
 Arctic seas. It gradually lessens in speed, and begins 
 to spread out and to turn north-eastward. One portion, 
 getting thinner and cooler, but yet retaining a higher 
 temperature than that which is proper to the latitudes, 
 and joining the general surface drift of the Atlantic water 
 towards the north-east, extends to the coasts of Britain 
 and Norway, and is said even to be traceable beyond 
 Spitzbergen ; the other turns southward between the 
 Azores and the coast of Spain, bends round the north- 
 west shores of Africa, and then coming once more within 
 the influence of the trade-winds, is again sent across the 
 Atlantic. 
 
 2O. Within the wide circuit embraced by the second 
 or south-eastern branch of the Gulf Stream lies a vast
 
 WfcJLXJaknetemJ
 
 j;h and Laadi
 
 xvn.] THE MOVEMENTS OF THE SEA. 145 
 
 area of comparatively still water, over the surface of 
 much of which dense masses of sea-weed grow. This 
 tract is known by the name of the Sargasso Sea. 
 
 21. In the Pacific Ocean the equatorial current has 
 more room to develop itself. Starting from the western 
 coast of the American continent, it streams westward 
 across the whole breadth of that vast ocean, encounter- 
 ing on its way only the obstruction of scattered islands 
 and submarine banks. It reaches the eastern margin 
 of Asia where that continent is flanked by the islands of 
 the Malay Archipelago. Hemmed in there, it divides 
 into two main branches, one of which turns northward 
 as the warm, rapid, and river-like Japan Cttrrent, and 
 sweeps along the east coast of Asia as the Gulf Stream 
 does along that of North America. The other and larger 
 branch forces its way into the Indian Ocean, and joins 
 the westerly equatorial current there. 
 
 22. These movements of the equatorial waters could 
 not be carried on without drawing the water of the nor- 
 thern and southern hemispheres into the gereral circu- 
 lation. Thus in the Pacific Ocean, which is open to the 
 south, there is a general northerly set of the cold southern 
 water. A broad current or drift pours into the hollow 
 of the western coast of South America and, passing north- 
 wards, joins the equatorial movement. Part of it, how- 
 ever, strikes the promontory of Cape Horn, round which 
 it sweeps as a well-marked current. Since the Pacific 
 basin is almost closed at its northern margin, no great 
 current of warm water can pass northwards through 
 Behring's Strait, and on the other hand no considerable 
 body of cold water can escape from the Arctic Sea, al- 
 though a cold current does issue from the Strait and 
 pass down the west side of North America. In the 
 Atlantic basin, while the prolongation of the Gulf Stream 
 bears its warmth far within the Arctic circle, the cold 
 water of the Northern Ocean makes its way southward 
 in return currents down each side of Greenland. The 
 
 L
 
 146 PHYSICAL GEOGRAPHY. [LESS. 
 
 more important of these currents descends from Davis 
 Strait and pursues its course southward along the Ameri- 
 can coast, interposing between the land and the warm 
 Gulf Stream, under which it partially passes. Icebergs 
 have been carried across the Gulf Stream as far as lat. 
 36 S., which would seem to prove that the cold Arctic 
 water continues to flow steadily with a sensible motion 
 for a long way southward. The soundings of the Chal- 
 lenger show that the Gulf Stream between New York and 
 Bermuda, with a temperature of 70 Fahr. and upwards, 
 does not exceed about 70 fathoms in depth, and that 
 below a depth of 600 fathoms the temperature of the 
 water is always lower than 40 Fahr. Between the Gulf 
 Stream and New York this cold water rises to within 
 300 fathoms from the surface, while farther north, along 
 the shores of Nova Scotia, it comes quite to the surface, 
 forming there the surface Arctic current out of Davis 
 Strait. 
 
 23. But besides these surface -currents, which are 
 always more or less readily perceptible, the waters of 
 the ocean basins are affected likewise by a slower mo- 
 tion, which extends to the deepest abysses and from the 
 equator to the poles. The existence of this general 
 " creep," or slow diffusion, has been made clear by the 
 temperature soundings described in Lesson XV. The 
 very remarkable fact has been proved that even under 
 the equator the main mass of the sea is cold, except a 
 comparatively shallow layer at the surface warmed by 
 the sun, and that at the bottom the temperature is as 
 low as in the Antarctic and Arctic Seas. Were there 
 no transference of cold polar water towards the equator, 
 the temperature there ought to be comparatively high. 
 But the presence of cold water even within 300 fathoms 
 of the surface shows that there must be a deep and gen- 
 eral movement from the polar regions to the equator, 
 under the surface-currents, which, especially in the North 
 Atlantic, carry the warm equatorial water into the cold
 
 xvii.] THE MOVEMENTS OF THE SEA. 147 
 
 polar tracts. The cause of this movement of the cold 
 water is not yet well understood. 
 
 24. (iii.) Tides. One of the most remarkable and 
 regular of the motions of the sea was alluded to in Art. 
 6, as that alternate rise and fall of the water called the 
 tides, the coincidence of which with the position of the 
 moon could not but be noticed from the earliest times. 
 To understand this motion, we must bear in mind that 
 while all the members of the solar system mutually at- 
 tract each other, there are two which specially influence 
 our earth the sun by reason of its vast size, and the 
 moon on account of its nearness. Each of these two 
 luminaries exercises a strong attractive force upon our 
 planet, the tendency of which is to pull out the side of 
 the earth which is opposite to it. The solid part of the 
 globe does not sensibly yield to the strain, but the liquid 
 ocean, unable to withstand it, is drawn outwards so as 
 to be heaped up on that side where the attraction is 
 exerted. 
 
 25. On the other side, too, where the distance from 
 the attracting body is greatest, and the force of attrac- 
 tion therefore least, the water is not attracted so much 
 as the earth, which is, as it were, pulled away from the 
 water. Hence on that side the ocean likewise rises into 
 another vast but less prominent swelling, the summit 
 of which is exactly opposite to that on the near side of 
 the earth. Were there no land to interfere with these 
 motions, the distortion produced by this cause would 
 give the form of an ellipsoid to the liquid mass covering 
 our planet, its longer axis pointing to the centres of the 
 earth and the moon. Were the earth and the other 
 members of the solar system at rest, the two swellings 
 of the ocean surface would remain stationary. But owing 
 to rotation, these protuberant masses of water are forced 
 to travel rapidly round the globe. 
 
 26. Let us fix our attention on that side of the earth 
 which happens at the time to be facing the moon. Owing
 
 r 4 8 PHYSICAL GEOGRAPHY. [LESS. 
 
 to the earth's rotation, combined with the moon's own 
 motion, our satellite appears to be revolving round the 
 earth in about twenty-five hours. The outward bulging 
 of the ocean-surface, which is caused by the moon's at- 
 traction, must therefore follow the moon, and run com- 
 pletely round the earth in about twenty-five hours, and 
 the swelling on the opposite side of the earth must take 
 the same course. Each of these two uprisings is felt, at 
 every place which it passes, as a broad undulation or 
 wave which appears once a day, or more exactly in every 
 24 hours 54 minutes, that being the length of the interval 
 between each appearance of the moon on the meridian. 
 Thus in that interval there are two times of high-water 
 or flood, and two times of low-water or ebb. 
 
 27. It is not, however, the moon alone by which the 
 water of the sea is affected in this way. The sun, too, 
 draws the ocean -surface outward ; but this influence, 
 which has about one-third of the force of the moon's, is 
 combined with the latter in the production of the broad 
 tidal undulation. When the sun and moon are in the 
 same line, which happens at new and full moon, their 
 combined attraction must evidently most powerfully draw 
 out the surface of the sea, while at the quarters of the 
 moon their respective influences partially neutralise each 
 other. 
 
 The accompanying figure (Fig. 19) shows how this 
 effect is accomplished. The highest rise and, of course, 
 the lowest fall, of the tidal undulation must occur at new 
 and full moon. These are called spring tides. The 
 least rise and fall take place at the time of the moon's 
 first and last quarters. These are known as neap-tides. 
 
 28. Since the movements of the tidal undulation de- 
 pend upon those of the earth, sun, and moon, they can 
 be measured and predicted a long while beforehand. 
 Observations have likewise been made in all parts of 
 the world regarding the times of high -water and the 
 height of the tide, so that the length of time taken by
 
 xvn.] THE MOVEMENTS OF THE SEA. 149 
 
 the tide -wave to travel from point to point has been 
 carefully determined. The rate of motion depends upon 
 the depth of water and the absence or presence of land, 
 being most rapid where the water is deepest and has no 
 obstructions in its course. In the equatorial parts of 
 the Atlantic Ocean it exceeds 500 geographical miles 
 
 SUN 
 
 SPRING-TIDES 
 
 HlUHTlDC 
 
 -SUN 
 
 NEAP-TIDES 
 
 FIG. 19. Diagram illustrating the origin of the tides. 
 
 in the hour. It is estimated to take fourteen or fifteen 
 hours to come from the south of Africa to the south- 
 west of Europe, where it gets into comparatively shallow 
 water and requires to struggle through many narrow 
 passages. Striking the south-west of Ireland, it branches 
 out, one part turning north along the west coast of Ire- 
 land and Scotland, then bending south along the east 
 coast, while the other part turns to the east up the Eng- 
 lish Channel. Now the former of these branches takes 
 nineteen hours to circle round the British Islands as far
 
 150 PHYSICAL GEOGRAPHY. [LESS. 
 
 as the mouth of the Thames, where it meets and mingles 
 with the wave which started twelve hours later, and 
 worked its way in about seven hours through the Eng- 
 lish Channel. 
 
 29. Though the tidal undulation is called a wave, 
 and travels with great speed across the deep ocean- 
 basins, the water itself does not partake of this onward 
 motion. Each particle of water merely oscillates as the 
 rapid pulsation passes, like the stalks of corn in a field, 
 (Art. 10). But when the undulation enters a narrow 
 and shallow sea, it becomes jammed between the con- 
 verging coasts, and experiences increasing friction against 
 the bottom. Its rate of motion consequently slackens, 
 but in proportion as this takes place, the undulation 
 gathers height and force, and becomes a true current. 
 
 30. In the deep ocean-basins, the passing of the broad 
 tidal undulation probably produces little or no sensible 
 effect upon the bottom. The surface of the water merely 
 rises gently for about six hours, and then gently falls 
 again, the total height of the undulation, from low-water 
 to high-water, not being more than a few inches. Thus, 
 in the middle of the Pacific Ocean, the rise is sometimes 
 less than a foot ; in the Atlantic, round St. Helena, 
 about three feet. But where the undulation meets with 
 the resistance of converging masses of land and a shal- 
 lowing bottom, it is heaped up, sometimes, as in the 
 Bay of Fundy, to a height of seventy feet, and rushes 
 along as a great wave or as a surging and foaming 
 ocean-river. In the Bristol Channel, which opens to- 
 wards the west, the tide rolls up a rapidly-narrowing 
 channel, and during spring-tides attains a height of forty 
 feet in the estuary of the Severn. The advance of the 
 tide is shown by a wave nine feet high, which rushes in 
 front, succeeded by smaller ones, until the full tide has 
 filled the estuary. This wave, or bore as it is called, 
 forms a marked feature of the flowing tide in many bays 
 and estuaries which open out towards the quarter whence
 
 xvii.] THE MOVEMENTS OF THE SEA. 151 
 
 the tidal undulation comes. Thus, on the west coast of 
 Europe it occurs in the Elbe, Weser, Seine, Dordogne, 
 and Garonne. At the mouth of the Seine the tidal 
 wave enters with a speed of fifteen to twenty feet per 
 second and a height of six and a half to ten feet. The 
 bore on the Hooghly River rushes up with such force 
 as to do great damage to shipping unprepared for its 
 approach. 
 
 31. When the converging shores are not closed at 
 the end, as in the case of an estuary, but open out again 
 into a wider sea, the tidal undulation takes the form of a 
 rapid, river-like current, or race. Heaped up on the 
 one side, it attains a higher level than on the other side 
 of the narrow passage, through which, therefore, it rushes 
 with great force and speed. The British Islands, lying, 
 as they do, on the margin of the Atlantic Ocean, pre- 
 sent many examples, of which the strait called the Pent- 
 land Firth, between the Orkney Islands and the north 
 of Scotland, may be taken as one of the best. Standing 
 at high or low water on a headland overlooking that 
 narrow passage, one looks down upon a strip of blue 
 sea about six or eight miles broad, opening westward 
 into the Atlantic and eastward into the North Sea. As 
 soon as the tide begins to flow or to ebb, this smooth 
 belt of water becomes more and more troubled, until, 
 when the motion of the tide is at its height, it sweeps 
 past at a rate of eleven miles in the hour, boiling up 
 and foaming along like some vast river. Here and 
 there, where sunken rocks lie in the path of the current, 
 the tumult of the water is strongest, while, should a high 
 wind be blowing against the tide, huge waves and sheets 
 of foam are tossed high into the air, and the whole sur- 
 face of the firth becomes white with breakers. No small 
 vessel can then attempt to force its way through the 
 strait. 
 
 32. In other narrow passages between islands where 
 the tide is thrown from side to side against sunken rocks,
 
 152 PHYSICAL GEOGRAPHY. [LESS. 
 
 or where two opposing currents meet each other, the 
 water forms -whirlpools. Such are found in the well- 
 known Corryvreckan between the islands of Jura and 
 Scarba on the west coast of Scotland, where the current 
 runs alternately east and west at a speed of eleven miles 
 in the hour. The famous Maelstrom, at the southern 
 end of the Lofodden Islands on the Norwegian coast, is 
 another illustration. 
 
 33. The strip of sand, gravel, or mud which is alter- 
 nately covered and laid bare by the rise and fall of the 
 tidal undulation is called the beach. When the tide is 
 at the full, the sea reaches to the upper limit of the beach 
 or high-water mark, when the tide is at the ebb the 
 
 Fig. 20. Diagram showing the relation of the beach to the lines of high and 
 low water. 
 
 sea does not come farther than the lower limit of the 
 beach or low-water mark. The distance between 
 these two limits must evidently depend partly upon the 
 height of the tides, and partly upon the slope of the 
 beach. It is, of course, greatest where the largest rise 
 and fall of the tides is combined with the gentlest in- 
 clination of the shore. 
 
 LESSON XVIII. The Offices of the Sea. 
 
 1. In the foregoing Lessons the more important pur- 
 poses which the sea serves in the general life of the 
 earth have been described. Let us here consider them 
 in a brief summary.
 
 xviii.] THE OFFICES OF THE SEA. 153 
 
 2. (i.) The sea supplies most of the moisture 
 of the atmosphere. Were the air overlying the land 
 dependent for its moisture solely upon the evaporation 
 from the land, rain and dew would almost cease, and 
 neither vegetable nor animal life could flourish. The 
 clouds that overspread the sky and discharge their 
 rain -showers upon the ground, the springs that feed 
 the brooks, the innumerable streams that go to swell 
 the bulk of the great rivers, the snows that overspread 
 the higher mountains, the dews that refresh the surface 
 of the earth even in tracts where no rain falls, all come 
 directly or indirectly from the sea. In spite of the 
 enormous volume of fresh water continually poured into 
 the sea by innumerable streams, no change of the sea- 
 level is perceptible. The water is evaporated again, 
 rises into the air and is borne along by the winds, till 
 it is condensed and once more discharged upon the 
 land. 
 
 3. The influence of the sea upon the moisture of the 
 air is well shown by the much greater rainfall of mari- 
 time than of inland tracts (Lesson X. Art. 29). The air 
 lying over the sea is probably for the most part not far 
 from the point of saturation, so that when it moves away 
 landwards it is ready to part with some of its vapour as 
 soon as it meets with a mass of air or of land colder 
 than itself. Near the coast of India the evaporation 
 from the sea is said to amount to about three-quarters 
 of an inch in the twenty-four hours, or nearly twenty- 
 three feet in the year. A rough estimate gives a depth 
 of fifteen feet as annually evaporated from the trade-wind 
 region of the oceans. 
 
 4. In the torrid zone, where the evaporation is greatest, 
 much of the moisture raised from the sea undoubtedly 
 falls back into it in those heavy and continual rains 
 which characterise that belt of the earth's surface. Some 
 of it goes to supply the copious rainfall of such regions 
 as the Khasi Hills (Lesson X. Art. 31), the Himalaya,
 
 154 PHYSICAL GEOGRAPHY. [LESS. 
 
 and the high lands of Abyssinia. Very little of this 
 equatorial vapour, however, can pass over to the tem- 
 perate regions on either side, because the air which 
 contains it, after discharging abundant showers in the 
 zone of constant precipitation, ascends into the high 
 regions of the atmosphere as a cold and comparatively 
 dry current, travelling away from the equator, and reach- 
 ing the surface of the earth again, ready rather to take 
 up moisture than to part with it. 
 
 5. In Europe the rains are supplied chiefly by the 
 moist winds that drink up the vapour of the warm sur- 
 face of the Atlantic ; west, and especially south-west 
 winds are wet, east and north-east winds are dry. The 
 vast cauldron of the Indian Ocean furnishes the rains 
 of the south of Asia and the east of Africa. When the 
 south-west monsoon blows, it bears the vapour of that 
 ocean to India and China, and discharges it in deluges 
 of rain. The large rivers of western Africa are supplied 
 by the monsoons, which bear the vapour of the Atlantic 
 into the interior of that continent. In North America 
 the mountainous western sea-board derives its moisture 
 from the moist winds that blow from the south-west 
 across the Pacific Ocean. In South America the south- 
 east trade-winds carry the vapour of the South Atlantic 
 across the continent, until they discharge the last of it 
 on the slopes of the Andes, and pass over to the Pacific 
 coast as dry, rainless winds. 
 
 6. (ii.) The sea regulates the distribution of 
 temperature. Its currents carry the heat of the tropics 
 to warm the regions lying towards the pole ; while, on 
 the other hand, they bring the cold of the poles to temper 
 the heat of lower latitudes. 
 
 7. This general influence of the sea, well illustrated 
 by the charts of isothermal lines, has been referred to 
 in Lesson IX. Art. i 5, and the cold and warm currents 
 of the North Atlantic ocean were given as examples. 
 The heated water of the Gulf Stream turns north-east-
 
 xviii.] THE OFFICES OF THE SEA. 155 
 
 ward and crosses the Atlantic, spreading out over a 
 larger surface and losing heat as it advances, yet dis- 
 tinctly traceable by its warmth even into the Arctic 
 seas. The average winter temperature of the sea round 
 the British Islands is considerably higher than that of 
 the land. But as already remarked (Lesson XV. Art. 2) 
 it is not directly by contact with the land that the sea 
 tempers the climate, but by convection currents of air. 
 It warms the air overlying it, which then passes on 
 and carries the warmth across the land. And here we 
 see the advantage to the west of Europe in that spread- 
 ing out of the Gulf Stream water which has been referred 
 to. Were this current to pass across the ocean with 
 the same breadth and speed which it has when it issues 
 from the Florida Strait, even though it should retain its 
 highest temperature, it would be too narrow to produce 
 much effect on the climate of Western Europe. But by 
 widening out over a far larger area and moving much 
 more slowly, it exposes a much greater surface to the 
 south-westerly breezes, which, warmed and moistened by 
 it, pass over to Europe. 
 
 8. The extent of this amelioration of the climate in 
 Western Europe may best be seen by comparing the 
 winter temperature of places on the same latitude on 
 opposite sides of the Atlantic. Hammerfest, the most 
 northerly seaport in the world, is open even in January, 
 while to the west, on the same parallel of latitude, the 
 east coast of Greenland is covered with snow and ice, 
 and the sea remains frozen in vast floes even during 
 summer. Still farther south, the harbours of Glasgow 
 and Liverpool are not only never frozen, but enjoy a 
 mean annual temperature of 47 51 Fahr. On the 
 corresponding latitudes in North America the coast-line 
 of Labrador is cumbered with ice all the year. Even at 
 St. John's, Newfoundland, which is two degrees farther 
 south than Liverpool, the harbour has been closed with 
 ice in the month of June.
 
 156 PHYSICAL GEOGRAPHY. [LESS. 
 
 9. These contrasts, which are probably the most striking 
 of the kind anywhere to be found upon the globe, do not 
 depend wholly upon the influence of the warm Atlantic 
 current in raising the temperature of Western Europe. 
 They are partly caused also by the influence of the cold 
 Arctic current, already described as descending from 
 Davis Strait, keeping close to the American coast and 
 depressing the temperature there. Even as far south as 
 lat. 44, that is, on the same parallel as the south of 
 France and north of Italy, the water that washes the 
 shores of Nova Scotia is not more than five or six degrees 
 above the temperature of thawing ice. A further cause 
 of the low temperature of Labrador and adjacent parts 
 of North America is to be sought in the low atmospheric 
 pressure over the North Atlantic, and the consequent 
 flow of cold air from the north-west. 
 
 10. In the southern hemisphere, where sea so largely 
 predominates, the climates are distributed with com- 
 parative regularity according to latitude. In the northern 
 hemisphere, however, where the equalising influence of 
 the sea is opposed by the existence of large masses of 
 land, which are alternately heated and chilled, no such 
 regularity exists. And yet even there, the tempering 
 action of the sea makes itself sensible along the borders 
 of the land, where neither are the winters so intensely 
 cold nor the summers so hot as they are inland. Hence 
 a continental climate is one of extremes great heat 
 in summer, severe cold in winter ; an insular climate 
 is more equable a distinction well brought out by com- 
 paring the summer and winter temperatures of Ireland 
 with those of regions on the same latitudes in the midst 
 of the continent, such as the heart of Russia. (See 
 Lesson XXXI.) 
 
 11. (iii.) The sea wears away its shores, and 
 thus tends to reduce the area of the land. No 
 one can watch the action of the waves (Lesson XVII. 
 Art. 14) without being well able to realise how powerful
 
 xviii.] THE OFFICES OF THE SEA. 157 
 
 it must be in the destruction of even the most rocky 
 coast-line. The mere weight of so many tons of water, 
 as are suddenly thrown by a huge wave against the land 
 during a storm, suffices to loosen and detach fragments 
 of rock. Even the hardest rocks are apt to be split up 
 by the action of the atmosphere (Lesson XXIX.), and 
 consequently the waves are aided in their work of 
 demolition by the cracks and lines of weakness which 
 the atmosphere has prepared for them. 
 
 12. Fragments of stone detached from a sea -cliff 
 become powerful implements in the further destruction 
 of the coast-line. They are caught up and hurled forward 
 by the waves, battering down the cliffs, and at the same 
 time being further broken up themselves. Ground to 
 and fro by the advance and recoil of the waves, they 
 assume the smoothed and rounded forms so familiar in 
 the gravel and shingle of the sea-shore. But even then, 
 their demolition continues, for they are still rolled back- 
 wards and forwards, until they become at last mere sand 
 and mud, in which condition they may be swept away 
 out to sea, and laid down in the quiet depths there. 
 
 13. There is yet another way in which the sea loosens 
 and demolishes the rocks on its margin. Those who 
 live at a seaport, on any coast exposed to storms, must 
 often have noticed that after a violent gale portions of 
 the solid masonry of the pier-walls and breakwaters have 
 been started from their places. Even with the utmost 
 care in the building and repair of these works, it is 
 hardly possible to avoid injury of this kind ; and unless 
 constant vigilance is used in cementing every crack and 
 opening in the masonry, the destruction of the pier or 
 bulwark may be rapid and complete. Now it is not by 
 the mere weight of water thrown against the building, 
 nor by the impact of any blocks of stone hurled forward 
 by the waves, that the stones are started out of a piece of 
 well-built masonry. When a large wave falls upon such 
 a surface, the air in every cranny of the wall is driven
 
 158 PHYSICAL GEOGRAPHY. [LESS. 
 
 inward. When the wave recoils again, a great suction 
 takes place behind it, and the compressed air rushes out 
 of the masonry. Moreover, where the water is driven in 
 with immense force so as to fill crevices and openings 
 within the masonry, it exerts upon their walls the same 
 pressure as the wave of which it is for the moment a 
 part, on the principle of the hydraulic press ; and this 
 pressure may amount to three tons on every square foot. 
 After a time some weak part of the work is discovered ; 
 one or more stones are loosened and then pulled out at 
 the recoil of some large wave, so that a breach is made, 
 which, if the storm continues, may be rapidly widened. 
 The same kind of influence is exerted upon cliffs. Every 
 mass of solid rock is more or less traversed by crevices, 
 which afford scope for the combined action of compressed 
 air and waves. When the sea has drilled a cave at the 
 base of a cliff, the action of the air, alternately com- 
 pressed and released as the waves rush in and out, 
 loosens the rock at the end and roof of the passage, so 
 that if the cliff be not too high, or the rock too solid, an 
 opening is actually made between the end of the cave 
 and the ground behind the top of the cliff. Through 
 this opening, called a blow-hole, the sea in storms sends 
 clouds of spray. Sometimes the mouth of the open- 
 ing lies a short distance from the edge of the sea. It 
 is singular to find on a hill-side, or in the middle of a 
 field or moor, a dark cauldron-like hole, with the sea 
 ebbing and flowing at the bottom. 
 
 14. Since even cliffs of the most solid rocks are worn 
 down by the incessant attacks of the waves, we may 
 readily believe that where the material is of less durability 
 the rate of destruction must be still more rapid. On 
 some parts of the eastern shores of England, where the 
 waves beat against cliffs of crumbling clay, the rate of 
 waste occasionally reaches to as much as the loss 
 of a yard (here and there even five yards) of land in the 
 year. The sites of some of the former ports and towns
 
 XVIII.] 
 
 THE OFFICES OF THE SEA. 
 
 159 
 
 of Yorkshire now lie beneath the restless waters of the 
 North Sea, a mile or more from the present shore. 
 
 15. We must not suppose, however, that the sea is 
 everywhere cutting away the margin of the land. It 
 does so chiefly on those coasts which lie exposed to 
 prevalent storms, and where the form of the shore allows 
 
 FlG. 21. View of the sea-cliffs south of the river Tyne (magnesian lime- 
 stone worn away by the waves, and the isolated fragments left). See 
 also Fig. 76. 
 
 the waves to act with effect. But on every coast-line 
 there are sheltered places where the waves either do not 
 encroach at all, or at least do so very slowly. For 
 example, if the sea is shallow for a long distance out- 
 ward, with a flat, sandy beach and a low coast-line, the 
 largest waves may roll in and spend their force merely 
 in rushing along the flat beach, and may do no damage 
 to the shore. But in many places the sea, instead of 
 removing material, casts it up on the land. The finer 
 sediment swept away from one part of the coast and 
 carried off by the currents is sometimes thrown ashore 
 again at no great distance. When that takes place, the
 
 160 PHYSICAL GEOGRAPHY. [LESS. 
 
 land may gain there as much, or nearly as much, as it 
 loses from that part of its margin whence the sand and 
 mud are derived. In this way the flat shores of Lincoln- 
 shire encroach upon the sea, because they continually 
 receive some of the material removed from the coast of 
 Yorkshire. In many parts of the world, for example 
 along the coast of North America from the borders of 
 Mexico to those of Virginia, and on both sides of the 
 Madras Presidency, so much mud and sand are brought 
 down by the rivers from the land that bars are formed, 
 by which the land behind them is protected from the 
 waves. 
 
 16. (iv.) The sea receives and preserves the 
 materials out of -which new land -will in course 
 of time be formed. We shall trace in a later Lesson 
 how the surface of the land undergoes continual decay. 
 Even its hardest rocks crumble down, and their remains 
 are swept out by brooks and rivers into the quiet depths 
 of the ocean, where, at no great distance from the shores, 
 the sand and mud derived from the decay of the surface 
 of the land are laid down. There they remain undis- 
 turbed, slowly accumulating and spreading, until some 
 future movement of the earth's crust shall raise them 
 above the level of the sea. 
 
 17. Many of the rocks of which the present dry land 
 is formed contain abundant remains of coral, shells, 
 fishes, and other marine creatures. These animals lived 
 in the sea, and when they died their harder parts, en- 
 closed in the sediments of the sea-bottom, were preserved 
 there ; thereafter these sediments, slowly consolidated 
 into rock, have been upraised so as to form the land on 
 which we live. This has happened frequently in the past, 
 and will probably continue in the future. 
 
 18. Besides the deposits formed of the fine sand and 
 mud carried away from the land, vast spaces of the 
 bottom of the ocean are likewise covered with a slowly- 
 gathering deposit, derived from the remains of minute
 
 xvm.] THE OFFICES OF THE SEA. 161 
 
 forms of life (Lesson XIV.). Some of the rocks of the 
 land, the chalk of England and France for example, have' 
 been laid down in much the same way, by the gradual 
 accumulation of the tiny shells of foraminifera and other 
 inhabitants of the sea. 
 
 19. It is evident that while the surface of the land is 
 subject to continual decay and removal, that portion of 
 the solid earth lying below the comparatively shallow 
 depth (a few hundred feet at most) to which the influence 
 of waves extends, is preserved. By far the largest part 
 of the destruction which the sea works is done between 
 tide marks. (See Fig. 21.) Waves, as we have seen, 
 do not stir the deeper parts of the ocean, which are 
 therefore left in undisturbed repose. When we reflect 
 how large a proportion of the earth's surface is covered 
 with sea, we perceive that although the waves are always 
 beating against the shores, the destruction which they 
 effect takes place only on the mere margin of the land, 
 while the vast spaces under the sea are not only pre- 
 served from waste, but are continually receiving fresh 
 deposits. So that if we consider the action of the sea 
 as a whole, we find it to be far more preservative than 
 destructive.
 
 i6*2 PHYSICAL GEOGRAPHY. [LESS. 
 
 CHAPTER IV 
 
 THE LAND. 
 LESSON XIX. Continents and Islands. 
 
 1. From the two envelopes of air and sea, in which 
 our earth is enclosed, we now pass to the consideration 
 of what lies beneath or within them the solid mass of 
 the planet. One of the first points to be noticed here, is 
 that while both air and sea admit of being penetrated 
 and examined all over the world, only a very small pro- 
 portion of the solid framework of the globe rises out of 
 the sea to form land, and of that comparatively insigni- 
 ficant part little more than the mere surface or skin is 
 known to us. We cannot at will pierce the land as we 
 can sound the sea. Evidently, therefore, all that can be 
 discovered by actual observation regarding the constitu- 
 tion of the solid globe must be derived from what may 
 be seen at or near the surface of the land. 
 
 2. What then, to judge from all that can be learnt from 
 its visible parts, is this solid globe on which we live ? 
 We are familiar with the earth and soil of the surface, 
 and we have seen the rocks beneath it in one country 
 clays and limestones, in another sand and gravel, in 
 another sandstones and shales, in still another granites 
 and other crystalline formations. How have these various 
 materials arisen ? Can we learn anything regarding their 
 origin, and do they in any way throw light upon the 
 history of the earth ? Why, for example, should the land 
 rise into lofty ridges at one place and sink into low plains
 
 xix.] CONTINENTS AND ISLANDS. 163 
 
 in another ; and have these ridges and plains been always 
 the same as now ? 
 
 3. No matter in what part of the world we may live, 
 such questions as these must often arise in our minds. 
 The more prominent the features of the land, the more, 
 of course, do these questions press for an answer. In a 
 mountainous country, for example, or one from which a 
 distant range of mountains can be seen, the high peaks 
 and ridges of the land form so striking a contrast to the 
 level plains below, that we cannot help asking how and 
 when these vast elevations took their rise. We watch 
 the clouds on the far summits as they gather into thunder- 
 storms, or dissolve into showers of rain among the valleys, 
 or powder with fresh snow the white shoulders and crests 
 of the mountains. There seems to be a constant tur- 
 moil in the air of these heights. What effect must it 
 have upon the forms of the mountains ? Do these ever 
 recurring tempests, these frequent rains and snows, 
 which fill the torrents and feed the great rivers, leave 
 the outlines of the mountains unchanged ? 
 
 4. But even where the landscape is least marked by 
 any striking feature, an observant eye cannot fail to 
 notice much that is suggestive of inquiry. No scene, 
 for instance, can be more monotonous than that presented 
 by the wide plains at the mouths of large rivers. And 
 yet, when the mind is once opened to perceive the in- 
 terest of such subjects as these, can the nature of the 
 soil of these flat lands escape notice, and fail to be 
 connected with the mud brought down by the river ? 
 Whence does the mud come ? How long has the river 
 been busy in strewing it over these plains ? 
 
 5. Such are some of the questions that are now to 
 come before us in the Lessons contained in this chapter. 
 In entering upon them it is well to remember that, as 
 far as possible, we should try to find illustrations of them 
 in the district wherein we may chance to live. We may 
 not be able to do so in all, nor even perhaps in most cases.
 
 164 PHYSICAL GEOGRAPHY. [LESS. 
 
 Yet the search for examples will always be of the utmost 
 use in making us better acquainted with the physical 
 geography of our country, and in accustoming us to the 
 observant use of our eyes. The features of a mountain- 
 chain, a plain, a table-land, a watershed, a spring, a 
 brook, a river, a lake, and of every other part of the 
 land, should be studied on the ground, wherever we can 
 find examples of them. In all such cases, too, we should 
 not rest content until we have put to the proof the state- 
 ments regarding these features in the Lessons, and seen 
 how far they are borne out, or can be extended by what 
 we see in nature for ourselves. 
 
 6. As the land consists of those portions of the solid 
 globe which happen to project above the level of the sea, 
 its distribution shows the position of the large wrinkles 
 into which the surface of the planet has been folded, 
 while its form and its materials must evidently furnish 
 our chief sources of information regarding the composi- 
 tion and the history of the general mass of the earth. 
 First, the distribution of the land over the globe may 
 be noted ; then its horizontal outlines or coast-lines 
 where it meets the sea ; next, its vertical outlines or 
 relief, as it rises up into the air. 
 
 7. Distribution of the Land. The total area of 
 dry land on the surface of the globe has been com- 
 puted to amount to 52,000,000 of square miles. By 
 much the larger part of this land lies in the northern 
 hemisphere. (Lesson V.) Within the Arctic Circle 
 (Plate I. Fig. 5) it forms an almost continuous ring round 
 the north polar regions, whence it extends southwards 
 in long irregular masses which taper away into points 
 (Lesson V. Art. 10). Each of these masses of land or 
 continents may be looked upon as consisting of a 
 northern and southern division. In the western hemis- 
 phere they are united by a narrow neck of land or isth- 
 mus. In the eastern hemisphere, Europe and Africa are 
 separated by the narrow water-channel or inland sea
 
 xix.] CONTINENTS AND ISLANDS. 165 
 
 called the Mediterranean. Europe and Asia form, indeed, 
 one united and continuous continent, which is prolonged 
 southwards by Africa on the one side, and by a vast 
 archipelago, or clusters and chains of islands, into 
 Australia, on the other. In a general view, the land 
 may be regarded as forming three pairs of continents : 
 the most regular being North and South America, and 
 the least regular being Asia and its insular prolongations 
 into Australasia. 
 
 8. Looking now at the position of these continental 
 masses we may observe that there is among them a 
 general tendency to run in a north-west and south-east 
 direction. This is specially noticeable in the case of 
 America. It is seen, too, in the bend of the islands 
 from the south-eastern margin of Asia to Tasmania and 
 New Zealand. Even in the European and African pair 
 of continents it may be traced, in the plateau or ridge 
 that connects Greenland with Iceland, the Faroe and 
 British Islands and the main mass of Europe. An uneven 
 and broken ridge of land is thus seen to extend from the 
 polar circle south-eastwards to the Cape of Good Hope. 
 Hence the shape of the Atlantic ocean is determined as 
 that of a deep long channel or trough between the Old 
 and New Worlds. It will be noticed too that the central 
 submerged ridge of this ocean runs in a general sense 
 parallel to the land on either side. Owing to the expan- 
 sion of Asia to the north-east, the American and Asiatic 
 continents almost meet at Behring Strait, thus enclosing 
 the vast Pacific basin on the north. 
 
 9. In order to have a clear notion of the main 
 heights and hollows on the surface of the solid globe we 
 cannot merely consider the trend of the continents. Be- 
 sides these chief masses of land, innumerable smaller 
 patches project above the surface of the oceans, in the 
 form of scattered groups or clusters of islands. As these 
 are only the tops of submerged ridges or mountains, their 
 position serves to indicate the direction of the submarine
 
 166 PHYSICAL GEOGRAPHY. [LESS. 
 
 elevations of the earth's surface, and the submerged parts 
 of the continental areas. Much information regarding 
 the form of the sea-bottom and its relation to the land 
 has been gained from deep-sea sounding, as already 
 remarked (Lesson XII.). Such a map as that on Plate 
 VII. is hardly less suggestive in regard to the continental 
 than to the oceanic areas. Turning now more particu- 
 larly to the land itself, we observe it to be defined by 
 two limits, one where it meets the sea that is, its coast- 
 lines, the other where it is surmounted by the air, giving 
 its vertical relief or contour. 
 
 10. Coast-lines of the Continents. Though the 
 limit between land and sea is always sharply drawn, the 
 map of the world shows how varied and irregular it is. 
 But even the largest and most detailed map can show 
 only the more prominent curvings and windings of the 
 coast-line. When such a map is carried along the edge 
 of the land, and compared with the actual boundaries 
 of the sea, many little projections and recessions of the 
 coast are seen which, from their small size, could not 
 find a place upon the map. Comparatively seldom does 
 the coast continue in a straight line for more than a 
 brief space. It curves to and fro, jutting out into head- 
 lands, capes, or promontories, and retiring into wide 
 gulfs and bays, or into narrower firths, creeks, and 
 inlets, 
 
 11. Besides these abundant changes in its horizontal 
 extension or plan, the coast-line of the land presents con- 
 tinual variety in its vertical elevation or relief. Some- 
 times, for example, it rises abruptly out of the sea, in 
 steep and lofty cliffs, against which the waves are ever 
 beating. Elsewhere, it shelves gently into the water. 
 The slope of the land above the surface of the sea is, for 
 the most part, prolonged under sea-level. Hence, where 
 the coast-line is precipitous, the water is usually deep 
 (Fig. 22), while, on the other hand, a low shore com- 
 monly indicates shallow water (Fig. 23). This relation
 
 XIX.] 
 
 CONTINENTS AND ISLANDS. 
 
 167 
 
 between form of ground and depth of water is so general, 
 that in navigation a vessel will usually be kept well away 
 from a low shore, though brought without hesitation 
 close under a high headland. 
 
 FIG. 22. Steep shore descending abruptly into deep water. 
 
 12. As a rule the projecting and higher parts of a 
 coast-line consist of harder materials, the receding and 
 lower parts of softer materials. This is true on a small 
 
 FIG. 23. Low shore shelving into shallow water. 
 
 scale of any exposed piece of shore, and likewise of the 
 larger irregularities of the coast all over the world. 
 Wherever a bold headland stands well out to sea, we may 
 infer that it consists of some hard rock which can offer 
 a stout resistance to the waves. Wherever, on the other 
 hand, the land is shown on the map as retiring into wide 
 unindented bays, it may be presumed to be low and to be
 
 168 PHYSICAL GEOGRAPHY. [LESS. 
 
 formed of soft clays or other materials that have yielded 
 more easily to the encroachments of the sea. In the 
 great majority of cases these inferences, drawn merely 
 from the examination of a map, would be correct. 
 
 13. Compare now the coast-lines of the three pairs of 
 continents. A remarkable contrast is at once observable 
 between those of the northern and southern divisions. 
 The northern continents are marked by coast-lines so 
 indented that the oceans penetrate far inland in many 
 bays, creeks, inlets, firths, and inland seas. The southern 
 continents, on the other hand, are distinguished by long 
 monotonous stretches of shore, unbroken by bay or creek. 
 The following calculation has been made of the relative 
 proportions of coast -line to extent of surface among 
 continents. 
 
 FIRST PAIR 
 
 N. America has i geograph. mile of coast-line to 265 sq. miles of surface. 
 S. America i ,, ., ,, 434 ,, 
 
 SECOND PAIR 
 
 Europe ,, t ,, ,, ,, 143 ., ,, 
 
 Africa ,, i 895 
 
 THIRD PAIR 
 Asiainclud-") 
 
 ing the / ,, i ,, ., 469 
 
 Islands J 
 Australia ,, i ,, ,, ,, 332 ,, 
 
 14. It will be seen that the contrast is most marked 
 between Europe and Africa. The former continent has 
 six times more coast-line in proportion to its superficial 
 extent than Africa has. There can be little doubt that 
 this contrast has had a powerful influence on the civilisa- 
 tion of the two continents. In the one case abundant 
 bays and arms of the sea have offered ready facilities for 
 .discovery, conquest, and commerce. Nation has been 
 brought into contact with nation ; the arts of peace and 
 war have spread into every country ; no region, however 
 remote, has lain wholly beyond the reach of communica- 
 tion with the rest ; and hence all the communities of 
 Europe, whether slowly or rapidly, have been carried
 
 xix.] CONTINENTS AND ISLANDS. 169 
 
 forward in the general progress of mankind. But con- 
 trast this abundant intercourse and steady advance with 
 the condition of Africa, its vast expanse of hitherto 
 inaccessible and unopened territories, peopled by races 
 having no intercourse with other tribes save their im- 
 mediate and usually hostile neighbours, and remaining 
 from time immemorial in the same stationary condition 
 of barbarism. The noble rivers and lakes of Africa, 
 however, will doubtless eventually become highways of 
 civilisation into the very heart of the continent, and be 
 made to serve the purpose which in old times was 
 accomplished in Europe by the far-reaching arms of the 
 sea. 
 
 15. General Belief of the Continents. The 
 horizontal outlines or coasts of the land are greatly sur- 
 passed in variety and interest by its vertical outlines or 
 relief. It is easy to show the former upon a map, but 
 much more difficult adequately to express the latter. 
 Hence even the best maps convey a most imperfect idea 
 of the external aspect of the land. Leaving details for 
 the next Lesson we may notice at present some general 
 features which are illustrated in all parts of the globe. 
 
 16. Even the loftiest peak of land rises but a small 
 way above the average level of the earth's surface. Think, 
 for instance, of the proportion between the height of even 
 the highest summit of the Himalaya Mountains and the 
 total bulk of the earth. The top of Mount Everest 
 stands 29,000 feet, or 5^ miles, above the level of the 
 sea, an enormous height when compared with the little 
 undulations of the plains or even with the heights of 
 many hilly countries. And yet it is only y^g-^th of the 
 polar diameter of the earth ; so that on a globe of ten 
 feet in diameter the highest mountain in the world would 
 be represented by a little projection only one-twelfth of 
 an inch in length. 
 
 17. If, then, the most stupendous mountains are 
 really so small, it is evident that the whole mass of dry
 
 170 PHYSICAL GEOGRAPHY. [LESS. 
 
 land must form but an insignificant part of the entire 
 bulk of the earth. For by far the largest part of the 
 land is not mountainous. Calculations have been made 
 regarding the probable average height of the land, if the 
 mountains could be levelled down and the valleys filled 
 up so as to reduce the whole to one general level. This 
 has been variously estimated to be from 1000 to 1450 
 feet. The continents, however, differ much from each 
 other in this respect. Europe has been computed to 
 have an average elevation of 97 3 '6 feet. That of Africa 
 must be more than twice as great. Vast mountain-chains 
 do not form such a large proportion of the mass of the 
 land as they might be supposed to do. It has been 
 calculated, for instance, that if the whole of the Alps 
 could be ground down and spread over Europe, the 
 surface of that continent would not be raised more than 
 about 2 1 feet. 
 
 18. In no respect does the land stand out in more 
 marked contrast with the sea than in the endless variety 
 of its relief. While the surface of the oceans is one vast 
 level, that of the land exhibits every contrast of form, from 
 the jagged peaks and precipices of the mountains down 
 to the level flats of the meadows and plains. We are apt 
 to see no order in this variety. Mountain and valley 
 seem to succeed each other, and to change their shapes 
 at random. And yet an attentive study of them shows 
 that they have not been stuck down on the earth's 
 surface at mere hazard, but have an intelligible mean- 
 ing and history, and a traceable connection with each 
 other. 
 
 19. First of all it is to be noticed that each of the 
 continents is traversed by a line or axis from which the 
 ground slopes on either side to the sea. This axis does 
 not necessarily coincide with the highest parts of the 
 land : these may lie now to the one side of it, now to the 
 other ; but it is the line of average highest elevation, as 
 is shown by the way in which the rivers flow from each
 
 xix.] CONTINENTS AND ISLANDS. 171 
 
 side of it. Nor is the axis placed down the centre of the 
 continent ; more usually it is much on one side. In 
 America, for instance, it lies close to the Pacific sea- 
 board. In Europe it runs from Cape Finisterre through 
 the chain of the Pyrenees, the Alps, the Carpathians, and 
 the Caucasus to the shores of the Caspian Sea. 
 
 20. The position of the axis determines the general 
 slope on either side. When it runs along the centre of 
 a continent the average angle of slope on either side will be 
 the same. When it lies close to one side the angle must 
 be higher on that side than on the other. Each continent 
 or country, with an axis lying far from the true centre of 
 the region, has therefore a short and steep slope on one 
 side, and a long and gentle slope on the other. South 
 America presents the most remarkable example of this 
 feature. The axis, with an elevation of perhaps 8000 or 
 10,000 feet, runs down the line of the Andes at a distance 
 of only from 50 to 100 miles from the Pacific, but 2000 
 miles from the Atlantic Ocean. On a much smaller scale 
 the same character is shown by Scandinavia, where the 
 axis lies close to the Atlantic, and where consequently the 
 ground descends rapidly into that ocean from the snow- 
 fields of Norway, but slopes gently eastward across 
 Sweden into the Gulf of Bothnia. In the British Islands, 
 too, the western slope is short and steep in the northern 
 half of Scotland and in Wales, while the eastern slope is 
 long and gentle. 
 
 21. But observe how the axial line runs through the 
 great continents. On the whole it is so placed that the 
 steep and short slope shall face the larger ocean. Look 
 again at America, with its vast line of heights towering 
 steeply out of the Pacific on the one hand, and inclining 
 gently for hundreds of miles into the Atlantic on the 
 other. On the opposite side of the Pacific the lofty high- 
 lands of Asia rise as a great mountainous girdle which 
 stretches from Kamtschatka to Arabia, and is thence 
 prolonged through Africa to the Cape. The slope to the
 
 172 PHYSICAL GEOGRAPHY. [LESS. 
 
 Pacific and Indian Oceans from that range of high ground 
 is comparatively short, while on the opposite side to the 
 Arctic and Atlantic Oceans it is gentle and long. It 
 would seem that on the average the gentler slope of 
 a continent is four or five times longer than the steep 
 one. 
 
 22. Moreover, it will be noticed that subordinate to 
 the main axis a minor axis or ridge is apt to occur on 
 the other side of a continent fronting the smaller ocean. 
 This likewise may be seen most conspicuously in America. 
 The line of the Alleghany and White Mountains lies near 
 the Atlantic border in North America, and the Sierras of 
 Brazil rise close to the sea in South America. These 
 dominant lines cannot be mere accidents ; they must be 
 due to the same causes which have ridged up the whole 
 of the solid land. 
 
 23. Again, it is evident that when the continents are 
 flanked with high ground on either side they must contain 
 wide plains, or at least vast spaces of lower ground, in 
 their centre. America illustrates this aspect by the great 
 plains of Canada, of the Mississippi, Orinoco, Amazon, 
 and La Plata. The greater part of Europe is a low 
 plain bounded by the chain of the Alps, Carpathians, 
 and Caucasus on the south, the Ural Mountains on the 
 east, and the high grounds of Scandinavia and Britain on 
 the west. 
 
 24. In most cases the upheaval of the continents has 
 been accomplished in such a way that these central plains 
 or lower grounds slope seawards. The water which falls 
 upon them, therefore, is not retained there, but flows off 
 to the ocean. In the crumpling and erosion of the surface 
 of the earth, however, depressions have been formed upon 
 each of the continents below the level of the surrounding 
 country. Into these the water flows, filling them up till 
 they overflow and discharge their surplus by means of 
 streams. But in desert regions the rivers gradually dis- 
 appear in the sandy wastes, or if they flow into a lake,
 
 XX.] THE RELIEF OF THE LAND. 173 
 
 there is no outflow. In such places the evaporation is 
 at present so great as to balance the supply of water. 
 On the high central platform of Asia a region of this 
 kind, full of salt-lakes, lies enclosed between the Hindu 
 Kush and Thian Shan Mountains, and stretches through 
 Turkestan and Mongolia for a distance of about 2000 
 miles. In North America a much smaller basin, shut in 
 among the ridges to the west of the Rocky Mountains, 
 receives its drainage into lakes without outlets, of which 
 the Great Salt Lake of Utah is the most important. In 
 the south-east of Europe the depression of the Caspian 
 descends far beneath the level of the sea ; even the 
 surface of that inland sheet of salt water being about 
 eighty-four feet below the general level of the oceans. 
 The surface of the Dead Sea, which fills a small detached 
 hollow shut in by high grounds, is 1298 feet below the 
 level of the Mediterranean. 
 
 We have now traced the position and broad general 
 outlines of the great ridges on the surface of the earth. 
 The following Lesson deals with some of the more pro- 
 minent features of the surface of the land. 
 
 LESSON XX. The Relief of the Land Mountains, 
 Plains, and Valleys. 
 
 1. Although the surface of the land is very uneven, 
 rising sometimes, as in the Himalaya, five and a half 
 miles above, and sometimes, as in the hollow of the 
 Caspian (3000 feet deep), sinking nearly three-quarters 
 of a mile below the sea-level, its inequalities are not 
 altogether disposed at random. Each continent has its 
 own system of heights and hollows, and these are grouped 
 in relation with the general axis. 
 
 2. The main features of the continents are the lines 
 of mountain-chains. It is along these lines that the 
 elevation of the land has been greatest. They are, as
 
 174 PHYSICAL GEOGRAPHY. [LESS. 
 
 it were, the crests of the great waves into which the 
 surface of the solid globe has been thrown. They 
 govern the trend of the chief valleys, they determine the 
 position of the plains, they regulate the climate, the 
 winds, rains, and rivers of the land. They ought first 
 to be considered, therefore, in any description of the 
 general aspect of a country : we may then pass to the 
 valleys, plains, and table-lands. 
 
 3. Mountains. The term " mountain " is rather 
 vaguely used to describe any large and lofty elevation 
 rising conspicuously above the region which surrounds 
 it. Sometimes a single conical mountain towers above 
 a plain or shoots up from the sea. Solitary cones of 
 this kind are usually volcanoes, such as Etna, Vesuvius, 
 and the Peak of Teneriffe. More frequently a mountain 
 is merely a more prominent part of a long lofty ridge, 
 and is joined at the sides to other similar mountains, 
 into which the ridge is divided by the valleys that cross 
 it. Such a connected series of mountains is called a 
 mountain-range. It often happens that two or more 
 such lines of mountain run parallel with each other as 
 one vast and continuous mountain-chain, or moun- 
 tain-system. Let us consider for a little the aspect 
 of some leading mountain-chain as an illustration of this 
 feature of the surface of the land. 
 
 4. The Alps of Europe have been so long familiar to 
 mankind that their name has passed into use as an 
 appellation for the main mountain system of any country. 
 To one who approaches it from the north, that noble 
 chain of mountains only reveals its character step by 
 step. Crossing the plains of France or Germany into 
 the district of the Jura, the traveller finds the ground 
 begin to rise and sink in long ridges and valleys, which 
 follow each other in parallel lines, like undulations on 
 the surface of a great ocean. The smooth valleys are 
 green and fertile ; the ridges, for the most part also 
 smooth, bear their slopes of rich pasture and shaggy
 
 XX.] THE RELIEF OF THE LAND. 175 
 
 wood, but often show along their crests long narrow 
 valleys or amphitheatres bounded by walls of rock. 
 Here and there deep transverse gorges cross the ridges 
 and carry the drainage out into the plains beyond. 
 Threading his way through these transverse valleys, the 
 traveller finds the ridges on either hand to grow higher, 
 and the hollows between to become steeper and deeper, 
 until he reaches one of the last and loftiest of these 
 outer elevations. From its top he can see on the one 
 side, as in a map, the succession of wave-like folds of 
 
 MMRH 
 FIG. 24. Section of one side of a Continent. 
 
 hill and valley across which he has journeyed ; on the 
 other, beyond a broad plain that lies at his feet, his eye 
 may take in the whole panorama of the Alps a vast 
 array of mountains crowding behind each other along 
 the sky-line, their higher slopes and crests white with 
 snow, and hardly at times to be distinguished from the 
 white clouds that rest upon them. 
 
 5. On a nearer view this great line of heights is found 
 to be flanked with minor ridges, rising into loftier and 
 bolder masses than the Jura, as they approach the main 
 mass of the mountains, but still retaining their covering 
 of forest, save on the steeper or more rugged slopes, 
 where the naked rock forms broken crags or steep bare 
 precipices. Beyond these outer bulwarks rise the central 
 ranges, with flat meadows and corn-fields or deep still 
 lakes at their base, dark pine-forests and green hollows 
 of pasture on their sides, but rearing their shoulders 
 above the limit at which tree or grass can live, and 
 raising their heads far up into the region of eternal 
 snow. 
 
 6. It is not until we have gained some commanding 
 point of view above the snow-line that we can form any
 
 i;6 PHYSICAL GEOGRAPHY. [LESS. 
 
 proper conception of the vastness of the scale on which 
 these mountain ridges have been piled up. The point 
 which we have reached after some hours of laborious 
 climbing seemed the very summit of a mountain when 
 we stood in the valley far below. And yet we now find 
 a vast sweep of much higher ground all around. Peak 
 rises behind peak, crest above crest, with infinite variety 
 of outline, and with a wild grandeur which often suggests 
 the tossing and foaming breakers of a stormy ocean. 
 Over all the scene, if the air be calm, there broods a 
 stillness which makes the majesty of the mountains yet 
 more impressive. No hum of bee or twitter of bird is 
 heard so high. No brook or waterfall exists up among 
 these snowy heights. The usual sounds of the lower 
 ground have disappeared. Now and then a muttering 
 like distant thunder may be caught, as some loosened 
 mass of snow or ice falls with a crash down the slopes ; 
 or as the wind brings up from below in fitful gusts the 
 murmur of the streams that wander down the distant 
 valley. 
 
 7. It is seldom that these high summits are free from 
 cloud during at least some part of the day. They rise 
 so far into the air, that often for weeks at a time they 
 are wholly wrapped in mist. The traveller who climbs 
 them finds sometimes that the higher points rise above 
 the cloud-belt into the clear upper sunshine, and he may 
 even now and then look down upon a thunder-storm 
 raging along the lower crests of the mountains and send- 
 ing torrents of rain upon the valleys, while he has a clear 
 blue sky above him. 
 
 8. Another feature that strongly impresses the mind 
 is the contrast between the climate and vegetation of the 
 plains and those of the mountains. Among the Swiss 
 valleys, for instance, corn-fields, gardens, orchards, and 
 vineyards cover the lower grounds, while thick woods 
 climb the slopes. As one ascends, the plants of the 
 valleys gradually disappear, others of species peculiar to
 
 xx.] THE RELIEF OF THE LAND. 177 
 
 the higher and colder air of the mountains take their 
 place ; the pine-trees, after a time, grow fewer, and at 
 last cease, until after farther ascent, the line of permanent 
 snow is reached, at a height of about 9000 feet above 
 the sea (Fig. 78). Beyond that line the climate has 
 quite an Arctic character, with few traces of life, either 
 vegetable or animal. Within the torrid zone the moun- 
 tain contrasts are still more marked. The sultry plains 
 of the Ganges, for example, show a luxuriant tropical 
 vegetation ; on the middle slopes of the Himalaya the 
 climates and the vegetation resemble those characteristic 
 of the temperate zone, while the higher summits are like 
 polar latitudes in their temperature and partly in their 
 plants. 
 
 9. Every mountain-chain is traversed by a system of 
 valleys. Those which run along the line of the chain 
 and separate its ridges, or ranges, are called longitu- 
 dinal ; those which run across the ridges and divide 
 them into independent mountains are termed transverse. 
 Thus the Swiss Alps contain the two parallel lines of 
 the Bernese Oberland and the Pennine range, separated 
 by the longitudinal valleys of the Rhone and Rhine. 
 The former of these ranges is cut through by many 
 transverse valleys and passes, such as the Rawyl, Gemmi, 
 and Grimsel, between which rise massive snowy moun- 
 tains. The latter is likewise deeply trenched by valleys 
 and passes, separating some of the monarchs of the 
 Swiss mountains Mont Blanc, the Matterhorn, Monte 
 Rosa, and the St. Gothard. 
 
 10. The direction of the great mountain-chains of the 
 globe has evidently a close connection with that of the 
 continents. Of this connection America, which in many 
 respects is a typical continent, furnishes an admirable 
 illustration. The long, continuous line of mountain- 
 chain that extends from the southern spur of the Andes 
 to the most northerly swell of the Rocky Mountains, a 
 distance of some 9000 English miles, coincides with the 
 
 N
 
 1 78 PHYSICAL GEOGRAPHY. [LESS. 
 
 general trend of the continent, and forms the axis or 
 back-bone of that vast tract of land. This main line of 
 elevation in the New World runs in a general north-west 
 and south-east direction. In the Old World a vast 
 mountain-system, with many branches, stretches from 
 the north-east of Asia across the centre of that continent 
 and the south of Europe to the north-western headlands 
 of Spain -a distance of about 12,000 miles. Here the 
 general trend is on the whole east and west. These 
 are the two great mountain ridges of the world. 
 
 11. Reserving the structure and origin of mountains 
 for a later Lesson (Lesson XXIX.), after the nature and 
 arrangement of the materials of the solid earth have 
 been examined, we may for the present consider the 
 other leading forms of terrestrial outline already referred 
 to plains and table-lands. 
 
 12. Plains. The chief flat lands of the globe lie 
 between parallel mountain ridges ; to a smaller extent 
 they occur as narrow belts or fringes, flanking the sea- 
 ward slopes of mountains or uplands. Take again the 
 American continent by way of illustration. In North 
 America, a vast plain stretches between the Rocky 
 Mountains on the west, and the Alleghany and White 
 Mountains on the east from the Gulf of Mexico up to 
 the Arctic Ocean. Of course this wide expanse of land 
 is not a mere dead level. It abounds with ridges of low 
 hills and lines of valley, but over many thousands of 
 square miles neither the heights nor the hollows are 
 sufficiently marked to destroy its general level character. 
 Westward it slowly ascends until its surface reaches a 
 level of from 4000 to 5000 feet above the sea, and forms 
 a vast table-land from which rise the Rocky Mountains 
 and other parallel ranges farther west. The enormous 
 plains watered by the Mississippi and its tributary rivers 
 are generally fertile grass-covered prairies. So flat 
 are they that vessels can sail into them up the rivers 
 for a distance of about 4000 miles from the sea.
 
 xx.] THE RELIEF OF THE LAND. 179 
 
 13. More than half the surface of Europe is a vast 
 plain bounded by the heights of Scandinavia, Scotland, 
 and Wales on the north-west, by the ranges of the Ural 
 mountains on the east, and by the chains of the Pyrenees, 
 Alps, Carpathians, and Caucasus on the south. From 
 the west of England the plain sweeps eastward across 
 the north of France and Germany, spreads out over most 
 of Russia, and sinking into the depression of the Caspian 
 Sea, passes into Asia, where it bends northward between 
 the Ural and Central Asian mountains, covering there a 
 belt of territory 1000 to 1500 miles broad, and nearly 
 4000 miles long. In the " Black Lands " of the Russian 
 plain, the soil is a dark rich fertile loam yielding abundant 
 corn-crops. Farther south, where it sinks below the 
 sea-level towards the Caspian, it becomes full of salt and 
 brackish lakes, and assumes a bare and barren aspect. 
 Round the Caspian Sea and northward into Asia the 
 clayey or sandy soil is covered with grass in spring, but 
 is dried up and withered by the scorching summer, and 
 frozen by the severe winter. These tracts are the 
 Steppes of Russia. Within 400 or 500 miles of the 
 Arctic Ocean trees and grass disappear from their sur- 
 face and are replaced by moss ; the soil is frozen to 
 a considerable depth, and only thawed for a few inches 
 down in the height of summer. These northern wastes 
 are the Tundras of Siberia. 
 
 14. Some plains are so placed as to remain mere bare 
 sandy wastes, without vegetation, and incapable of ever 
 being inhabited. The north of Africa, for example, pre- 
 sents to us the widest barren region or Desert on the 
 surface of the globe, that of the Sahara. This is a long 
 broad plain, rising here and there into hills and table- 
 lands ; but at its eastern end actually sinking to a depth 
 of from loo to 150 feet below the sea-level. Where 
 scattered springs come to the surface, little spots of vege- 
 tation called " Oases " make their appearance, but with 
 these sparse exceptions, the vast plain is a region of dry
 
 i8o PHYSICAL GEOGRAPHY. [LESS. 
 
 barren shifting sand, where no rain falls, and where the 
 sun's heat is more intensely felt than anywhere else in 
 the world. 
 
 15. In the Sahara Desert, as well as in many other 
 plains both in the Old World and in the New, traces of 
 shells are found belonging to species which must have 
 lived in the sea. It can be shown that they have not 
 been transported from the sea to where they now occur, 
 but have lived and died there. Hence they serve to 
 prove that these wide flat or undulating tracts of country 
 must at one time have lain below the waters of the sea. 
 This is an interesting fact which will be afterwards 
 referred to, in its bearings upon the origin of the present 
 contours of the surface of the land. 
 
 16. The tracts of comparatively level land are not 
 always low plains. Sometimes they lie at a considerable 
 elevation. When their height exceeds about 1000 feet 
 above the level of the sea, they are usually called table- 
 lands or plateaux. But, as in the case of the country 
 west of the Mississippi, no hard line can be drawn be- 
 tween plains and table-lands. They often merge into each 
 other. A table-land is only an elevated plain, and, like a 
 plain, it undulates into heights and hollows, is sometimes 
 deeply trenched by valleys and river-gorges, and here 
 and there rises into lofty ranges of hills, 
 
 17. A table -land may be enclosed between parallel 
 ranges of mountains. Of this arrangement the most 
 stupendous example is the vast plateau of Central Asia, 
 10,000 feet above the sea, with its mountain barriers of 
 the Thian Shan on the north, and the Kuen Lun and 
 Himalaya on the south. Another remarkable instance 
 is the vast plateau in Western America, 4000 to 5000 
 feet high, from which the Rocky Mountains rise, and 
 which stretches westward to the Sierra Nevada and 
 Pacific Coast ranges. It is diversified with numerous 
 parallel mountain ranges, which, like the Rocky Moun- 
 tains, have a general northerly trend. Its waters descend
 
 xx.] THE RELIEF OF THE LAND. 181 
 
 each side of the continent, and it also includes the " Great 
 Basin," which has no outlet, and where the lowest hollows 
 are occupied by salt and bitter lakes. 
 
 18. Other table-lands rise out of the sea or from low 
 plains bordering the sea. The African continent, for 
 example, is one vast table-land with but a narrow fringe 
 of low plain skirting its shores. Spain and Portugal 
 form another example of a table-land rising steeply from 
 the sea. This well-marked peninsula consists of a plateau 
 about 2600 feet high, crossed by five hill-ranges (in 
 Spanish Sierras) and trenched more or less deeply by 
 river-valleys the Douro, Ebro, Tagus, Guadalquiver, 
 and others. 
 
 19. Here and there a table-land has been so exten- 
 sively worn down into valleys that its former character 
 becomes almost lost. Scandinavia is an illustration 
 of this result. Central parts of the table -land still 
 remain a vast undulating plateau between 4000 and 
 5000 feet high. But the outer portions are so traversed 
 by lines of valley as to be cut into narrow ridges. The 
 Highlands of Scotland have been still more thoroughly 
 trenched by valleys until the original table- land can 
 scarcely be any longer traced (Lesson XXIX.). 
 
 20. Having noted now the leading forms of surface 
 by which the land is marked, we are led to ask whether 
 any explanation can be given of them. How have the 
 vast mountain- chains been formed ? Have they existed 
 from the beginning of things, and have their crests and 
 pinnacles, their precipices, ravines, and valleys, always 
 been as we see them now ? An answer to these ques- 
 tions can be given with considerable confidence and ful- 
 ness, but to be able to understand it we must attend with 
 some care to two aspects of the physical geography of 
 our globe, ist. The nature of the earth's interior and 
 the reaction of the interior upon the surface ; and 2d, 
 the circulation of water upon the land and the other ex- 
 ternal processes by which the surface of the land is
 
 182 PHYSICAL GEOGRAPHY. [LESS. 
 
 affected. The next eight Lessons will be devoted to 
 these two subjects, and we shall then return, in Lesson 
 XXIX., to the consideration of the origin of the present 
 features of the land. 
 
 LESSON XXI. The Composition of the Earth. 
 
 1. Of what materials does the solid land consist ? 
 How are these materials arranged so as to form the vast 
 mass of the globe ? Man cannot pierce beyond the mere 
 outer skin of the planet ; can he then ever hope to learn 
 the probable constitution of its interior ? These are 
 some of the questions to which we must now turn. In 
 endeavouring to find answers for them let us begin with 
 what is nearest and most familiar. 
 
 2. No matter in what part of the world we may live, 
 the uppermost layer or covering of the land is almost 
 always one of vegetation ; in one place grass, in another 
 forest, in another thickets of jungle or brushwood. Here 
 and there, indeed, the surface is formed of shifting sand, 
 where no plants may be able to take root, or of rough 
 mountainous ground, where ledges and crags of bare rock 
 shoot up into the air. But even in such places as these 
 straggling weeds or shrubs may be found striving to bind 
 the loose sand together, or to find a lodgment among 
 the crannies of the rock. 
 
 3. Beneath the cover of vegetation we see the layer 
 of soil on which the plants grow, sending their roots 
 down into it and extracting from it the soluble mineral 
 materials which are needed for their framework. Soil 
 varies greatly in colour and composition, being some- 
 times a stiff, gray clay, sometimes a soft, black, crum- 
 bling loam, sometimes a brown or yellow sand, and 
 sometimes so stony as to be little else than a sheet 
 of gravel. In all cases, however, it will be found to 
 consist of small particles or larger fragments, which
 
 xxi.] THE COMPOSITION OF THE EARTH. 
 
 183 
 
 appear broken or worn, and have been evidently derived 
 from some solid rocks. Looked at with a magnifying- 
 glass this broken and derivative character appears still 
 more clearly. Roots of plants descending through the 
 soil open it up and permit rain and air to pass into it, 
 while the common earth-worm likewise aids in this work 
 by swallowing the fine parts of the loam, voiding them 
 above ground, and thus bringing up lower portions of the 
 soil to the surface. 
 
 4. Besides the grains of sand, earth, or clay, the soil 
 
 FIG. 25. Section to show the formation of (3) soil and (2) subsoil from 
 rotting away of (i) rock underneath. 
 
 contains more or less organic matter derived from the 
 mouldering remains of plants and animals. This ingredi- 
 ent is essential to the fertility of the soil, and is made use 
 of by living plants. Where arable land has been culti- 
 vated for a long time, each successive crop has taken so 
 much soluble mineral materials and organic matter out of 
 the ground that the soil becomes exhausted, and needs 
 to be renovated by the introduction of various kinds of 
 manures before it will continue to yield good crops. In 
 some countries, where the soil is deep and fertile, many 
 centuries may elapse before such exhaustion takes place. 
 5. The soil varies greatly in depth. A common thick-
 
 1 84 PHYSICAL GEOGRAPHY. [LESS. 
 
 ness is three or four feet, but on sheets of rock it may 
 not be so much as an inch, while in rich plains it may 
 be several yards. The next layer below it is called the 
 subsoil. It consists of the same materials as the soil, 
 but less finely broken up, and with little organic matter. 
 Indeed, the soil is simply the upper part of the subsoil 
 which has crumbled away and been mixed up with the 
 decayed remains of plants and animals. Usually only 
 the longer roots of the larger plants, such as forest-trees, 
 reach down into the subsoil. But as the soil is washed 
 off by rain from the surface of the ground, the upper 
 parts of the subsoil, coming in consequence nearer to the 
 surface, are more exposed to air, percolating rain, plant- 
 roots, and earth-worms, so that they gradually pass into 
 soil. 
 
 6. Beneath the subsoil lies the rock, from the break- 
 ing up and decay of which the subsoil is formed. The 
 character of the soil thus essentially depends upon the 
 nature of the rock underneath. We shall trace in subse- 
 quent Lessons how universally and strikingly the surface 
 of the land is crumbling down, even when it consists of 
 the most solid rocks, such as granite, limestone, or sand- 
 stone. For the present we may bear in mind that the 
 covering of soil, so generally distributed over the land, 
 and on which the verdure and fertility of a country 
 depend, has been formed by the gradual decay of the 
 rocks and of the remains of the successive generations 
 of plants and animals that have lived upon the surface. 
 
 7. Beneath the decaying soil and subsoil, then, we 
 arrive at the undecomposed rock. In most countries of 
 the world a great variety of rocks occurs. To describe 
 these and to trace their origin and history forms the sub- 
 ject of the science of Geology. But a little consideration 
 may here be given to some of their more important char- 
 acters, as it will enable us to understand better how the 
 materials of the land have been put together, and what 
 is the probable constitution of the interior of the planet.
 
 xxi.] THE COMPOSITION OF THE EARTH. 185 
 
 8. In the first place, then, the most cursory observa- 
 tion suffices to show that most of the rocks of the land 
 have, like the soil, been formed out of worn fragments or 
 particles of older rocks. Sandstone, for instance, out of 
 which so much of the solid framework of the plains, hills, 
 and mountains has been built, consists of mere sand com- 
 pacted into solid stone. Shale and slate are only so 
 much hardened clay or mud. Conglomerate or pudding- 
 stone is nothing but a mass of solidified gravel. In all 
 these cases the materials of which the rocks are com- 
 posed have been worn away from still older rocks, and 
 have been rolled to and fro in water, as gravel, sand, and 
 mud are produced at the present day. 
 
 FIG. 26. Bedded arrangement of rocks, (a) conglomerate ; (t>) sandstone ; 
 (c) shale. 
 
 9. Rocks formed of these water-worn materials have 
 been accumulated to depths of many thousand feet. 
 They now frequently form lofty ridges of mountains. 
 They spread also over the lower grounds, and form the 
 wide plains of the world. One of their most obvious 
 features is their arrangement in layers, beds, or strata, 
 which, varying in thickness from less than an inch to 
 several feet or yards, are piled over each other. Hence 
 a cliff of these rocks has a striped or banded look
 
 186 PHYSICAL GEOGRAPHY. [LESS. 
 
 (Fig. 26). They are known as bedded or stratified 
 rocks. 
 
 1O. Together with these there often occur other rocks 
 formed wholly or in great measure of the remains of plants 
 or animals. For instance, amid a mass of sandstone and 
 hardened clay, traces of the fronds of ferns (Fig. 27), 
 and the seed-cones, leaves, stems, .and roots of other 
 plants, may occasionally be observed. These vegetable 
 remains have in some places been so crowded together 
 
 FIG. 27. Piece of shale containing portion of n fossil fern. 
 
 as to form black or brown seams of coal, and in that 
 condition supply much of the fuel which man consumes 
 at the present day. 
 
 11. Again, many rocks are composed mainly or entirely 
 of broken shells, corals, and other animal remains. 
 Limestones, for example, commonly have this composi- 
 tion, as shown in Fig. 28, which represents a piece of 
 limestone made up of the crowded joints of the encrinite 
 or stone-lily a marine animal of which there are some 
 small modern kinds still living. Chalk is a variety of 
 limestone consisting mainly of the broken remains of
 
 xxi.] THE COMPOSITION OF THE EARTH. 187 
 
 minute forms of marine animal life (Fig. 29) like those 
 that compose the modern ooze of the Atlantic sea-bottom 
 (Fig. 14). 
 
 12. Limestone, made up entirely of the crowded 
 remains of corals, shells, sea-urchins, and other sea- 
 creatures, is extensively formed at the present time upon 
 submerged banks in the tropical seas. Rock of a similar 
 kind, and anciently formed in like manner on the sea- 
 
 Fic. 28. Piece ofTimestone, showing how the stone is made up of animal 
 remains. 
 
 floor, covers thousands of square miles in many countries, 
 and not only underlies parts of the plains, but, like sand- 
 stones and shales, rises up even into lofty mountains. 
 The outer ridges of the Alps, Himalaya, Rocky Moun- 
 tains, and Andes, for example, are in large measure built 
 up of limestones and other rocks, formed mainly or in 
 part of animal remains. 
 
 13. It appears, then, that most of the land, as far as 
 we can see it, is made of rocks, which are either hard-
 
 1 88 
 
 PHYSICAL GEOGRAPHY. 
 
 [LESS. 
 
 ened gravel, sand, and clay, or have been formed out of 
 the broken and compressed remains of once living plants 
 and animals. From these facts we conclude first, that 
 
 FIG. 29, Some of the grains of a piece of chalk. 
 
 what is now land must have been under water ; secondly, 
 that as the limestones and other strata contain chiefly 
 marine forms of life, the water in which most of them 
 were deposited must have been the sea ; thirdly, that 
 the materials out of which the land has mainly been 
 formed were laid down in the sea ; and fourthly, that by 
 some means these ancient deposits of the deep have been 
 upheaved into dry land. 
 
 14. In the second place, besides the various stratified 
 rocks, which have been chiefly derived from the wearing 
 
 FIG. 30. A piece of granite, showing the composition of a Crystalline Rock. 
 
 down of rocks older than themselves, others may be seen 
 in which no bedded or stratified arrangement exists. 
 These are not made up of fragments derived from pre- 
 viously-formed rocks, but, for the most part, are crystal-
 
 xxi.] THE COMPOSITION OF THE EARTH. 189 
 
 line, that is, are made up of crystals, either felted together 
 or imbedded in a glass or base. Granite, porphyry, and 
 basalt are examples of such crystalline rocks. Instead 
 of spreading out in vast sheets over the continents, as the 
 stratified series does, the crystalline rocks occur in bands 
 or bosses, and rise through the stratified rocks sometimes 
 in huge disruptive masses, sometimes in veins and 
 variously -shaped intrusions. They are particularly to 
 be noticed along the central portions of mountain-chains, 
 where they may be seen coming up through the very 
 oldest of the stratified rocks. They likewise appear in 
 the eruptions of volcanoes the molten rock known as 
 lava being one form of the crystalline rocks. 
 
 15. There can be little doubt that, as a rule, the rocks 
 of the crystalline series have come from below, and have 
 been thrust in a melted state among the other rocks, or 
 have been poured out at the surface as lava. Hence we 
 must infer that beneath the outer layer of stratified or 
 derivative rocks, though it may be many thousands of 
 feet in thickness, there must be an inner layer or mass 
 of crystalline material, which has here and there been 
 squeezed through the stratified rocks, as in the axis of 
 mountain-chains, or has communicated with the surface 
 by means of the openings of volcanoes. 
 
 16. So far these conclusions, being founded on what 
 can be actually seen, may be regarded in the light of 
 established truths. We have traced the materials of the 
 solid earth from the thin outer layer of surface-soil down 
 through the thick piles of stratified sandstones, clays, 
 limestones, and other rocks into the still lower granite, 
 lava, and other crystalline masses. We have seen that 
 most of the materials of the land have been raised up 
 out of the sea, and that portions of the underlying crys- 
 talline rocks have actually been pushed up into the 
 very heart of the mountains. Can we descend any far- 
 ther and trace still deeper layers in the structure of our 
 planet ? Not directly ; no distinct lower portions have
 
 190 PHYSICAL GEOGRAPHY. [LESS. 
 
 come through the crystalline rocks. Nevertheless, evi- 
 dence enough may be gathered to indicate with some 
 probability what lies still farther down. 
 
 17. Most of the rocks at the earth's surface weigh 
 from twice to thrice as much as water. In other words, 
 their specific gravity is from 2-0 to 3-0, that of pure 
 water being reckoned as i -o. Experiments made with 
 the pendulum and the plumb-line on the earth's attrac- 
 tion, indicate that the weight of the planet is about twice 
 as much as that of the rocks at the surface, or, more 
 shortly, the density of the earth is about 5-5. Obviously 
 the interior of the earth consists of materials heavier 
 than those that form the land. 
 
 18. Owing to the increase of the force of gravity, the 
 density of every substance becomes greater in propor- 
 tion as it approaches the centre of the earth. But we 
 know very little of the rate of compressibility in the high 
 temperature that exists within the earth. If the increase 
 of density continued unchecked, air would be as heavy 
 as water at a depth of 34 miles, and water would be as 
 heavy as mercury at a depth of 362 miles. But the effect 
 of heat is to expand bodies and thus to diminish their 
 density, and doubtless the earth's internal heat prevents 
 the density of the whole planet from being so great as it 
 would otherwise be. 
 
 19. The deepest mines do not reach one mile in 
 depth, or aa 1 62 of the distance from the surface to the 
 centre of the earth. Man can never, indeed, hope to 
 penetrate far into the interior of the planet on which he 
 lives. There are yet several kinds of evidence open to 
 him which go to demonstrate our globe's high internal 
 temperature. These are: ist, mines, wells, and borings; 
 2d, hot-springs ; and 3d, volcanoes. 
 
 20. (i.) It has long been known that the air of deep 
 mines is warmer than that above ground, and that, as 
 a rule, the deeper the mine the warmer is the air in it. 
 A coal - mine sunk at Rose Bridge near Wigan, for
 
 xxi.] THE COMPOSITION OF THE EARTH. 191 
 
 example, to the great depth of 2445 feet has a tempera- 
 ture at the bottom of 94 Fahr., while the temperature 
 at the surface is only 50 Fahr., which is a mean increase 
 of i Fahr. for every 55 feet of descent. But the rate 
 varies greatly at different places and even at different 
 levels at the same place. The water that rises from deep 
 borings is warm. Thus from a well sunk to a depth of 
 1798 feet at Crenelle, near Paris, water issues with a 
 temperature of 81-7 Fahr. As the results of observa- 
 tions made all over the globe it appears that an increase 
 of temperature always occurs, and that though it varies 
 in amount in different kinds of rock, its average rate is 
 about I Fahr. for every 60 feet of descent. 
 
 21. If this rate should continue, it is plain that at a 
 comparatively short depth even the most refractory sub- 
 stance would be at its melting-point, though the effect of 
 the enormous pressure within the earth may prevent it 
 from actually passing into a molten condition. Water 
 at 12,000 feet would be as hot as boiling water at the 
 surface. At a depth of about 24 miles, if its component 
 gases could remain united, it would be as hot as melting 
 gold. 
 
 22. (ii.) In almost all countries of the world springs 
 of warm water rise from the ground. In volcanic districts 
 the water is often even at the boiling-point, and remains 
 permanently so. Warm springs often occur, however, at 
 a distance from any active volcano. Those of Bath, for 
 example, rise with a temperature of 1 20 Fahr., in the low- 
 lands of the south-west of England, more than i ooo miles 
 from the burning mountains of Iceland on the one hand 
 and more than 1 1 oo from Vesuvius on the other. At Bux- 
 ton in Derbyshire the springs have a temperature of 82. 
 In Germany the wells of Wiesbaden throw out water at a 
 temperature of 158, and those of Carlsbad at 167, 
 while in the north-west of Spain some of the springs have 
 a temperature as high as even 192. All these places 
 are remote from an active volcano, but it may often be
 
 192 PHYSICAL GEOGRAPHY. [LESS. 
 
 noticed that hot springs rise along mountain-chains, or 
 at least on lines where the rocks have been intensely 
 crumpled and where they may have been greatly heated 
 during the crumpling. Here and there, as will be 
 pointed out in the next Lesson, the remains of volcanoes 
 occur which are now quite cold and silent, and from 
 which no eruption has taken place within the memory 
 of man. Yet some of these extinct volcanoes are still 
 associated with hot springs. In the ancient volcanic 
 district of Central France, for example, numerous springs 
 occur, sometimes with a temperature of as much as 
 174. 
 
 23. By far the most remarkable kinds of hot springs, 
 as showing the great heat of at least some parts of the 
 earth's interior, are those which throw out their contents 
 with violence. They occur in volcanic districts, and are 
 called geysers (that is, gushers), that being the local 
 name borne by those of Iceland, which were first de- 
 scribed. The part of Iceland where they chiefly occur 
 is a space about two miles square in a low, wide valley 
 among volcanic rocks. In this limited tract the ground 
 is pierced with abundant openings from which jets of 
 steam and hot water escape. These openings, said to 
 be nearly 100 in number, are usually surrounded with 
 basin-shaped rims or mounds of white, gray, and vari- 
 ously coloured incrustations of silica called sinter, depo- 
 sited from solution in the hot water. They vary in size 
 from mere tiny bowls, a few inches across, up to huge 
 cauldrons, like the Great Geyser, which rises 1 5 feet 
 above the ground, and measures 56 feet in diameter. 
 In the centre of the basin of the Great Geyser a pipe or 
 funnel, eight feet wide, descends into the earth. From 
 this opening boiling water is constantly ascending into 
 the basin and flowing over the lowest part of its lip into 
 the plain. At intervals of a few hours a loud rumbling 
 is heard to proceed from the funnel, the water in the 
 basin begins to boil up, and jets of it together with clouds
 
 xxi.] THE COMPOSITION OF THE EARTH. 193 
 
 of steam are spurted up for a few feet. After these in- 
 termittent eruptions have continued for about a day, 
 they are succeeded by a much more violent explosion. 
 The ground trembles slightly, as the rumbling sound in- 
 creases in violence, until a huge column of boiling water, 
 150 or 200 feet high, is thrown up into the air, with 
 
 FK;. 31. View of the geysers of Iceland. Great Geyser in eruption. 
 
 loud explosions and clouds of steam. In this way the 
 basin and pipe are emptied of water, but they gradually 
 fill again, the rumblings and jets of water and steam 
 begin anew, until next day another grand outburst emp- 
 ties the geyser. It has been found by observation that 
 the lower parts of the subterranean column of hot water 
 o
 
 194 PHYSICAL GEOGRAPHY. [LESS. 
 
 are sometimes actually raised to a temperature of 261 
 Fahr., or 49 degrees above the common boiling-point. 
 Hence when this superheated water rises to near the 
 surface, and is thereby relieved of the pressure of the 
 column of water that previously lay above it, it flashes 
 into steam with a loud roar, and throws out jets of boil- 
 ing water and steam. 
 
 24. The Yellowstone National Park, in the Territories 
 of the United States, contains several hundreds of gey- 
 sers scattered over a volcanic tract of country, some 
 exceeding the Great Geyser of Iceland in size and in the 
 height and volume of water and steam which they dis- 
 charge when in eruption. One of the most remarkable, 
 known as " Old Faithful," emits a lofty jet of hot water 
 and steam, with great regularity, about once an hour. 
 The sites of the underground foci appear to shift their 
 positions from time to time. Some of the vents have 
 evidently been only recently opened, for the trees invaded 
 by their deposits are hardly dead yet. Others are no 
 longer active geysers, but mere quiet pools of hot water, 
 or mounds, whence only steam rises. But many are 
 quite extinct ; their rims and basins of siliceous incrust- 
 ations, standing above the ground, mark where the 
 boiling water once rose, and the surrounding forest is 
 beginning to encroach upon their decaying heaps of 
 sinter. 
 
 25. Another interesting series of geysers occurs in 
 New Zealand, where also active and extinct volcanoes 
 are met with. The spouting hot springs of Orakei- 
 korako gush forth in numbers on either side of a river- 
 valley, while at Tetarata the water is so largely charged 
 with silica that it has formed a series of terraces and 
 basins down the precipitous bank from near the top of 
 which it takes its rise. 
 
 26. (iii.) Volcanoes form so interesting a part of 
 the earth that they will be described in some detail in 
 the following Lesson. We need only note here that
 
 xxr.] THE COMPOSITION OF THE EARTH. 195 
 
 being openings from which steam and other hot vapours 
 as well as streams of melted rock are discharged, and 
 occurring in many parts of the world, they prove that 
 within the earth the temperature must be at least as high 
 as the white heat at which lava issues at the surface. 
 
 27. It is plain, then, from such evidence that the 
 earth cannot but have an enormously high internal tem- 
 perature. Much dispute has arisen as to whether the 
 interior is liquid or solid. Some have maintained that 
 the earth consists of a ball of molten material with an 
 exterior crust which has been variously estimated at 
 from 20 to 1000 miles in thickness. Others have in- 
 sisted that a planet so constituted could not rotate as 
 the earth does, and that our globe must be solid to the 
 centre. 
 
 28. If the heat goes on increasing at the same rate as 
 it is observed to do in the comparatively shallow borings 
 which man can make into the earth, the ordinary melt- 
 ing-point of even the most refractory substance would, no 
 doubt, be met with at a comparatively short depth 
 beneath the surface (Art. 21). But it does not follow, 
 therefore, that the materials of the interior should be 
 actually in a state of fusion. Pressure is believed to 
 have the effect of raising the melting-point of such sub- 
 stances as those which form most rocks, that is, it keeps 
 them from melting until a still greater heat is applied. 
 The pressure at great depths within the earth must be 
 enormous. Hence below a depth of a few miles, where 
 the temperature reaches the ordinary melting-point of 
 most rocks at the surface, each successive layer of the 
 earth's substance may not be actually fused, but may be 
 just at the melting-point proper to its depth. The whole 
 globe might thus be solid, but the least diminution of 
 pressure at any point would allow the parts so relieved 
 to melt at once. This, so far as can be judged, seems to 
 be the most probable condition of the interior of the earth. 
 
 29. The internal heat being so great, there seems no
 
 196 PHYSICAL GEOGRAPHY. [LESS. 
 
 reason why the innermost parts of the planet may not 
 consist of metal, such as iron or gold. And indeed 
 there is some reason to infer that they are really metallic. 
 Cracks have been made abundantly through the rocks 
 forming the land, and in many of these occur metallic 
 ores which are believed to have not improbably come 
 up from a metallic region below. Researches into the 
 constitution of the sun and of the other planets have 
 tended to confirm this view of the metallic composition 
 of the central parts of the earth. 
 
 3O. Summing up what has been said in this and pre- 
 vious Lessons regarding the constitution of the globe, 
 we conclude that beneath the outer envelopes of the 
 atmosphere and the sea, the solid earth has an upper 
 layer of loose, crumbled materials gravel, sand, and 
 mud on the sea-floor, soil upon the land under which 
 lies a thick series of stratified and derivative rocks ; that 
 still farther down masses of crystalline rocks, which have 
 in many places been forced through the overlying series, 
 descend to an unknown depth, and that within them 
 there may be a metallic central core. We see, more- 
 over, that since the heat rapidly increases as we descend, 
 the innermost parts of the globe must have a tempera- 
 ture, probably more than sufficient to melt every known 
 substance, but that the pressure of gravity may be great 
 enough to retain the general mass of the globe in a solid 
 state, except at such places as supply the streams of 
 molten lava that flow from volcanoes. 
 
 LESSON XXII. Volcanoes. 
 
 1. The word " volcano " is derived from Vulcanus, 
 the name of the Roman god of fire, who was supposed 
 to have his subterranean forges at the roots of the moun- 
 tain Etna. It is applied to any conical mound, hill, or 
 mountain, formed of materials which have been erupted
 
 xxn.] VOLCANOES. 197 
 
 in a molten or solid condition from beneath the surface. 
 A volcano is active when it ejects with violence gases, 
 steam, water, mud, dust, stones, or molten rock from its 
 summit, or from fissures or other openings on its sides. 
 The term volcanic action is used to describe all the 
 kind of work done by a volcano. A volcano may be 
 dormant when it remains a long time without any erup- 
 tion ; and it is said to be extinct when, though its 
 external form may remain, it does not seem to have 
 been in eruption for a great many centuries, and gives 
 none of the usual signs of volcanic action. 
 
 2. The size of a volcano varies from a mere little 
 mound, a few yards in diameter, like some of the mud- 
 volcanoes around the Caspian Sea, up to a giant moun- 
 tain like Cotopaxi, which rises among the Andes to a 
 height of 18,887 f eet above the sea, its upper 4000 feet 
 forming a smooth snow-covered cone, with an orifice at 
 the top, whence hot ashes and stones are scattered far 
 and wide over the surrounding country. 
 
 3. At the top of a volcano lies a basin-shaped hollow 
 called the crater, from the bottom of which the pipe 
 or shaft descends, whereby the volcanic products are 
 brought to the surface. Showers of fine dust and 
 stones are frequently thrown out from volcanic craters. 
 These materials falling down the slopes of a cone 
 gradually increase its diameter and height. In like 
 manner streams of molten rock called lava, issuing 
 either from the lowest part of the lip of a crater or from 
 some fissure or orifice on the outer declivity, pour down 
 the slope and harden there, thus still further augmenting 
 the bulk of the hill. 
 
 4. As a volcano increases in size, and cracks are 
 formed in weak parts of the cone, smaller cones are 
 piled up on its flanks by the emission of dust, stones, 
 and lava from these fissures. A large volcanic moun- 
 tain, like Etna, or the Peak of Teneriflfe, is thus some- 
 times loaded with small volcanoes which may reach a
 
 PHYSICAL GEOGRAPHY. 
 
 [LESS. 
 
 height of five or six hundred feet. In the drawing (Fig. 
 32) a plan is shown of one of these great volcanoes 
 with its groups of minor cones. 
 
 5. At the beginning of a volcanic eruption rumblings 
 are heard, like the muttering of distant thunder, while 
 the ground is felt to tremble slightly. These noises and 
 
 FIG. 32. Plan of the peak of Teneriffe, showing the large crater, partly 
 effaced, and smaller craters with lava currents issuing from them. 
 
 tremors increase in intensity, successive loud explosions 
 take place in the pipe of the volcano, and at last clouds 
 of fine dust and steam are hurled with prodigious force 
 far up into the air. The steam rapidly condenses into 
 rain, which falls in torrents down the outer slopes of the 
 mountain. The fine dust is sometimes given out in such 
 quantities as to darken the sky for many miles around. 
 In the famous eruption of Vesuvius, which in the year
 
 XXII.] 
 
 VOLCANOES. 
 
 199 
 
 79 destroyed the Roman cities, Herculaneum, Pompeii, 
 and Stabice, the air was as dark as midnight for twelve 
 or fifteen miles round, and a thick deposit of fine ashes 
 and stones fell on the whole district. Some notion of 
 how completely this eruption of ashes covered the country 
 may be obtained from the accompanying drawing, which 
 
 FIG. 33. View of a Street in Pompeii. 
 
 represents one of the streets of Pompeii as it now ap- 
 pears, after the volcanic deposit under which the city 
 lay buried for sixteen centuries has been cleared off. 
 At the farther end of the street where the excavations 
 have stopped, the dark layer on the top of the walls, and 
 on which the pine-tree is growing, represents the thick- 
 ness of the general covering of volcanic ashes. In some 
 volcanic eruptions the finer dust, carried up into a strong
 
 200 PHYSICAL GEOGRAPHY. [LESS. 
 
 upper current of air, has been known to be transported 
 for hundreds of miles before descending to the ground. 
 Illustrations of this kind of transport were cited in Lesson 
 XI. to prove the existence of these upper currents in the 
 atmosphere. 
 
 6. Besides the light dust, vast numbers of white-hot 
 or red-hot blocks and smaller fragments of stone are 
 ejected from the crater. Many of these strike against 
 each other as they rise and fall, producing at night a 
 wonderful spectacle as their sparks and flashes light up 
 the darkness. Of the force with which the stones are 
 occasionally discharged some notion may be formed 
 from the feats of the volcano Cotopaxi, which is said to 
 have thrown a block, estimated to weigh 200 tons, to a 
 distance of nine miles. The volcano of Antuco in Chili 
 is reported to have sent stones flying to a distance of 
 thirty-six miles. 
 
 7. It is not difficult to understand why, in the earlier 
 stages of a violent eruption, so vast an amount of frag- 
 mentary materials should be ejected. During the suc- 
 cessive explosions, the walls of the crater and the 
 accumulated lava and other volcanic ejections, which 
 had gradually choked up the funnel, are rent and finally 
 blown into fragments. It is possible that at the deep 
 roots of a volcano the temperature is so high as to keep 
 the elements of water dissociated. Where these elements 
 can still remain combined, under enormous pressure, 
 and at a temperature far above the boiling-point, the 
 water, which has perhaps saturated the molten and solid 
 rocks in the pipe of the volcano, instantly explodes, when, 
 in its upward ascent, it can overcome the influence of 
 the superincumbent pressure. So violent is the conver- 
 sion of such superheated water into steam that the molten 
 lava is actually blown into the finest dust. It is the 
 liberation of successive portions of this highly-heated 
 water into the gaseous state that produces the explosions 
 which form so magnificent a part of a great volcanic
 
 EARTHQUAKES 
 
 AND 
 
 VOLCANOES. 
 
 The Earthquake districts are 
 l_ coloured blue, the frequency 
 1 and force of the shocks being 
 indicated by the intensity of the 
 tint. Active, dormant, and re- 
 | cently extinct Volcanoes are 
 shewn by Bed dote. , %
 
 PLATE IX. 
 
 20 40 60i^.i 80*<~~IOO 120 140 160
 
 XXII.] 
 
 VOLCANOES. 
 
 201 
 
 eruption. At each explosion a vast ball of steam shoots 
 up into the air, at once condensing into white clouds 
 and rolling over in huge folds (Fig. 34), which either 
 dissolve and float away in the higher atmosphere or 
 are further condensed and fall as rain. 
 
 8. The upper part of a cone, during the early part of 
 
 FIG. 34. View of Vesuvius as seen from Naples during the eruption of 
 1872. 
 
 a tremendous eruption, sometimes disappears, because 
 it is shattered by the explosions and blown up into frag- 
 ments, which fall back either into the crater or down the 
 outer slopes of the mountain. Mount Vesuvius has sup- 
 plied some excellent illustrations of this kind of destruc- 
 tion. Prior to the first century of the Christian era that 
 mountain was a dormant volcano, from which no ex-
 
 PHYSICAL GEOGRAPHY. 
 
 [LESS. 
 
 plosions had ever been known to come, but which had 
 an enormous crater on its summit, overgrown then with 
 brushwood and wild vines, like the crater of Astroni and 
 some of the other extinct volcanoes near Naples (see 
 Fig. 5). Suddenly in the year 79, when the great 
 eruption took place which destroyed Pompeii, the south- 
 western side of the cone was blown away and a new 
 cone of much smaller dimensions was formed inside the 
 circuit of the former crater. In the drawing of Vesu- 
 
 FIG. 35. Mount Vesuvius as seen from the sea, with the remaining part of 
 the old crater of Somma behind. 
 
 vius (Fig. 35) as seen from the sea, the crescent-shaped 
 half of the old crater appears behind the modern dimin- 
 ished cone. In August 1883 the volcanic cone forming 
 the island of Krakatau in the Sunda Strait was almost 
 entirely destroyed by one of the most colossal volcanic 
 explosions of modern times. 
 
 9. The enormous expansive force of imprisoned water 
 and steam drives the molten lava up the pipe of the 
 volcano. After the first explosions, during which a 
 column of molten rock has been propelled upwards in
 
 xxii.] VOLCANOES. 203 
 
 the funnel, lava is seen to flow either from the top or 
 from one or more points on the side of the cone. Should 
 the sides of the mountain be solid enough to resist the 
 enormous pressure of the ascending lava, the latter will, 
 of course, find no escape until it fills up the crater to 
 the level of the lowest part of its rim, over which it will 
 pour down the mountain. More usually, however, there 
 are weak parts, such as rents, caused by the previous 
 explosions, through which the lava finds egress to the 
 outer slopes of the cone. 
 
 10. Few sights in nature are more terrible than that 
 of a lava torrent as it pours down the side of a mountain. 
 At first, glowing with a white light, it flows freely like 
 melted iron ; but rapidly grows red and darkens. Its 
 surface soon hardens into a black or brown crust that 
 breaks up into rough cinder-like pieces, beneath which, 
 as may be seen at the rents, the main mass still remains 
 red-hot. A short way from the point of emission, the 
 lava-stream looks like a river of rugged blocks of slag, 
 grinding against and over each other, with a harsh 
 metallic sound, and revealing, every here and there, a 
 glimpse of the fiery flood underneath, over which they 
 are floating. Clouds of steam and hot vapours rise from 
 all parts of the moving mass. Its rate of march varies 
 much according to the slope of the ground, distance 
 from the point of exit, and other causes. Thus, in the 
 year 1805, a current of lava ran down the first three 
 miles of the slope of Vesuvius in four minutes, yet took 
 three hours to reach its farthest point, which was only 
 six miles. In 1840 one of the more liquid lavas of 
 Mauna Loa, in the Sandwich Islands, was observed to 
 run eighteen miles in two hours. 
 
 11. The front of a slowly advancing lava-current seems 
 to be a huge mound of rubbish, somewhat resembling 
 one of the piles of debris at ironworks, or a new railway 
 embankment, with its raw earth and stones. The rugged 
 blocks of lava tumble over each other as the mound
 
 204 
 
 PHYSICAL GEOGRAPHY. 
 
 [LESS. 
 
 slowly advances. A breeze coming from the lava is hot 
 and stifling, for it carries with it some of the steam and 
 acid vapours that stream out abundantly from the molten 
 rock. Trees, walls, gardens, and vineyards are suc- 
 cessively buried under the burning flood. A house or 
 range of houses may form a temporary barrier ; but 
 
 FIG. 36. Houses surrounded and partly demolished by the lava of 
 Vesuvius, 1872. 
 
 eventually these buildings are pushed over like mere 
 houses of card and engulfed (Fig. 36), or the lava is 
 piled up against, and passes round them, meeting again 
 below so as to leave their roofs, perhaps, projecting from 
 the middle of a rugged sea of lava. It occasionally 
 happens that the still liquid rock inside bursts through 
 the hardened cnist at the side or in front, and pours
 
 xxii.] VOLCANOES. 205 
 
 down in a new direction. This is always one of the 
 risks to be considered in such populous and cultivated 
 tracts as those along the slopes of Vesuvius. 
 
 12. The distance to which a torrent of lava may flow 
 can never be foretold, even when it is seen pouring down 
 from a mountain at a great velocity. At Vesuvius many 
 of the lava-streams have reached the sea ; others have 
 not travelled farther than a few hundred yards from their 
 point of emission. The most tremendous floods of lava 
 ever known to proceed from a volcano were those of 
 Skaptar Jokul in Iceland, in the years 1783-5. Vast 
 torrents of molten rock were then poured over that 
 island, filling up river-courses, ravines, and lakes, and 
 completely destroying the surface for many hundreds of 
 square miles. The lava reached to an extreme distance 
 of forty-five miles in one direction, and of fifty miles in 
 another. In some level places, where the lava spread 
 out, the stream reached a breadth of fifteen miles, and a 
 thickness of 100 feet, but it accumulated in narrow 
 valleys sometimes to a depth of 600 feet. It has been 
 computed that the total mass of lava poured forth dur- 
 ing that series of eruptions would form a mountain equal 
 in bulk to Mont Blanc. 
 
 13. The hardened crust on a lava -stream forms an 
 excellent non-conducting layer between the still molten 
 mass underneath and the air. One can walk on that 
 crust, though, through the fissures, the red, glowing lava 
 may be seen to lie only a few inches below. It is evi- 
 dent that the inner parts of such a current, even at a 
 considerable distance from the point of emission, may 
 retain a very high temperature, long after the current 
 itself has come to rest. Thus the lava of the eruption 
 of Vesuvius in 1858 continued to give out steam and hot 
 vapours in 1873, and still retained so much heat that 
 one could not keep one's hand in some of the fissures 
 for more than a few seconds, although the surface of the 
 current was everywhere quite cold.
 
 206 PHYSICAL GEOGRAPHY. [LESS. 
 
 14. When the lava begins to flow freely from a vol- 
 cano, the violence of the eruption usually abates. The 
 showers of ashes gradually cease, or at least do not ex- 
 tend beyond the crater and its immediate vicinity. The 
 explosions and subterranean rumblings disappear, and 
 except for the cloud which always hangs over the summit 
 of the mountain and marks how constantly and abun- 
 dantly steam still continues to rise from the crater, nothing 
 at last remains to indicate from a distance that volcanic 
 action is not extinguished, but merely quiescent for a 
 time. Yet a visit to the summit of the cone at such an 
 interval would show that hot vapours and gases still keep 
 streaming out from the summit and sides of the mountain. 
 Occasionally after an eruption of Vesuvius it is observed 
 that a large destruction takes place among hares and 
 birds on the hill-sides, owing to the plentiful emanation 
 of the poisonous carbonic acid gas. Many centuries 
 after a volcanic district has ceased to be subject to any 
 eruptions this gas continues to rise, sometimes in large 
 quantities, either bubbling through the water of springs 
 or coming out of crevices in the ground. 
 
 15. The active volcanoes on the surface of the globe 
 are not distributed at random, but follow certain lines, 
 as shown in the map on Plate IX. It will be observed 
 that these lines generally coincide with some of the ridges 
 on the surface of the earth already referred to (Lesson 
 XIX.). Almost all volcanoes are ranged close to one or 
 other of the great ocean basins. The continental and 
 island barriers which encircle the Pacific Ocean form 
 one vast ring of active volcanoes. Beginning on the 
 east side we find in the giant chain of the Andes a series 
 of active volcanoes, some of them the loftiest on the 
 globe, running along the western margin of South 
 America. This series is continued through Guatemala 
 and Mexico into North America, and stretches through 
 the Aleutian and Kurile Islands, forming a close -set 
 fringe to the northern Pacific Ocean. The line is
 
 xxii.] VOLCANOES. 207 
 
 prolonged down Japan, the Formosa and Philippine 
 Islands, to the Malay Archipelago, where it divides into 
 two branches. One of these, turning south-eastward by 
 New Guinea and the New Hebrides into New Zealand, 
 is prolonged, but with wide gaps, across the Pacific basin 
 in the volcanoes of the Friendly, Society, Marquesas, 
 and Easter Islands, towards the coast-line of South 
 America, thus completing the vast volcanic ring. Even 
 along the centre of that basin on the submarine ridge 
 alluded to (Lesson XII. Art. 18), magnificent active 
 volcanoes appear in the Sandwich Islands, some of those 
 in Hawaii exceeding 1 3,000 feet in height. 
 
 16. Returning to the Malay Archipelago we observe 
 that the second branch of the volcanic line turns north- 
 westward through Java and Sumatra, where more active 
 and dormant volcanoes are crowded into a shorter space 
 than anywhere else in the world, and where the eruptions 
 have sometimes been on a colossal scale (Art. 8). It is 
 prolonged through the islands off the west coast of Bur- 
 mah. After a wide interval it reappears in Mantchouria, 
 then on the southern borders of the Caspian Sea, whence 
 it may be traced by the Greek Archipelago, Vesuvius, and 
 the Italian Islands, to the Azores, Canaries, and Cape 
 Verde Islands. 
 
 17. Besides these main lines, however, scattered vol- 
 canoes occur on other distant islands or on the edges 
 of the continents. In the Arctic Ocean lie the active 
 craters of Iceland and Jan Mayen. On the west side 
 of the basin of the Indian Ocean we find small vol- 
 canoes on the Red Sea and in the Isle of Bourbon. 
 The line of volcanoes in Terra del Fuego at the southern 
 apex of America seems to be continued in the chain of 
 the South Shetland Islands, and to extend even into the 
 supposed Antarctic continent, where, amid the vast snow- 
 fields of that region, Sir James Ross in 1841 discovered 
 an active volcanic cone 12,369 feet high. 
 
 18. But besides active volcanoes there is a still larger
 
 208 
 
 PHYSICAL GEOGRAPHY. 
 
 [LESS.
 
 xxii.] VOLCANOES. 209 
 
 number which are either dormant or extinct. It would 
 seem that few large tracts of land exist where evidence 
 may not be obtained of former volcanic action. Lava- 
 streams and consolidated beds of volcanic dust may 
 be found in almost all countries. Sometimes, indeed, as 
 in Central France (Fig. 37) the cones are still as fresh 
 as if they had been thrown up only recently ; and yet 
 no record remains that they have ever been in eruption 
 within the times of human history. Hence, if to the 
 present long list of active volcanoes be added those 
 which are now extinct, the whole surface of the land 
 will be found to be studded over with points of volcanic 
 eruption. 
 
 19. When we consider that each of these points marks 
 an orifice at which highly-heated materials are emitted 
 now, or at least have been emitted at some former period, 
 and that they are so widely distributed over the earth's 
 surface, we see how important is their evidence as to the 
 high internal temperature of the earth (Lesson XXI. 
 Art. 27). 
 
 20. It was pointed out (Lesson XXI. Art. 29), that 
 while the interior of the earth is probably solid as a 
 whole, each portion of it, beyond a depth of a few miles, 
 is probably at the melting-point and ready to pass into a 
 liquid condition when any diminution of the pressure 
 takes place. The ridges on the surface of the earth, 
 formed, as they have been, during the contraction and 
 consequent general subsidence of the outer parts of the 
 planet, have doubtless by their uprise relieved the pres- 
 sure upon the parts underneath them. This relief has 
 probably allowed portions of the interior to pass into the 
 state of fusion. Observe how the active volcanoes of 
 the globe are mostly arranged along such lines of eleva- 
 tion, whether on a continent, as in South America, or in 
 chains of islands, as on the western and northern sides of 
 the North Pacific Ocean. This arrangement can hardly 
 be accidental. It helps to connect the elevation of the 
 
 p
 
 210 PHYSICAL GEOGRAPHY. [LESS. 
 
 land and the phenomena of volcanoes by showing that 
 long spaces of melted rock should be expected to lie 
 under those very regions where active volcanoes occur. 
 Volcanoes do not pierce every mountain-chain, however, 
 though in many cases they can be shown to have once 
 existed there, but to have been long extinct. 
 
 21. Reservoirs of lava may exist underneath without 
 giving rise to actual volcanic explosions, so long as no 
 passage is opened to the surface, and nothing occurs to 
 determine volcanic excitement. The vapours absorbed 
 in lava may be partly original constituents of the earth's 
 interior, partly derived from water which, supplied by 
 rain, rivers, lakes, or the sea, filters through the upper 
 rocks. In the deep, intensely hot regions, whether its 
 gases remain combined or not, the absorbed water must 
 exist at an enormously high temperature, and in a con- 
 dition that would be impossible save under enormous 
 pressure. Portions of this superheated water that may 
 succeed in effecting their escape and in blowing out an 
 opening in the earth's crust, relieve the pressure on 
 the deep-seated mass of molten rock which is then im- 
 pelled upwards to the surface. With a loud explosion 
 the liquid rock is blown into dust as its water flashes 
 into steam, and lower portions then begin to flow out 
 tranquilly as streams of lava. 
 
 LESSON XXIII. Movements of the Land. 
 
 1. Along certain lines, or at certain points, where 
 communication has been opened between the surface 
 and intensely hot materials below, showers of dust and 
 stones, and streams of melted rock, have been emitted 
 in such quantity as to form huge mountains like Etna, 
 the Peak of Teneriffe, and Cotopaxi. But other notable 
 effects show the influence of the internal condition of the 
 globe upon its surface. The solid earth is subject to
 
 xxiii.] MOVEMENTS OF THE LAND. 211 
 
 movements either sudden and violent, or slow and im- 
 perceptible. It is sometimes convulsed and rent open, 
 sometimes one tract is gradually raised up above the 
 sea-level, while another is step by step depressed. We 
 shall consider these movements under the three divisions 
 of ist, Earthquakes ; 2d, Upheaval ; 3d, Subsidence. 
 
 2. (i.) Earthquakes. These, as the word denotes, 
 are tremblings or concussions of the ground. They vary 
 in intensity from mere slight tremors, like that caused by 
 a loaded waggon moving along a street, up to such 
 violent catastrophes as those in which the ground is 
 thrown into undulations whereby rocks are loosened, 
 trees are shaken out of the soil, and villages and cities 
 are levelled to the ground. 
 
 3. It is chiefly the terrible devastation of life and 
 property which is usually dwelt upon as the main feature 
 of importance in earthquakes. In a moment, without 
 the least warning of any kind, a rumbling like that of 
 distant thunder or the firing of cannon is heard or felt, 
 and before men have had time to ask each other what 
 the tremor can be, the earthquake conies upon them. 
 The ground rises and falls under their feet ; houses rock 
 to and fro until they are rent from top to bottom, or fall 
 with a crash into ruins ; the earth here and there opens 
 and closes again. In a few seconds a whole city is de- 
 molished, and hundreds or thousands of its inhabitants 
 are dead or dying. The Lisbon earthquke in 1755, for 
 instance, destroyed 60,000 human beings. The Cala- 
 brian earthquake a generation later proved fatal to not 
 less than 40,000. Nor is the destruction confined to 
 one limited district. Frequently it spreads over thou- 
 sands of square miles, carrying death and havoc far and 
 wide through different provinces and states. 
 
 4. When, however, we regard earthquakes as part of 
 the machinery of nature, it is not so much their influence 
 upon human life and property as their permanent effects 
 upon the surface of the land which claim our notice.
 
 212 PHYSICAL GEOGRAPHY. [LESS. 
 
 An earthquake is not felt simultaneously over the whole 
 region affected by it. It begins at one side or end, and 
 travels rapidly to the other, or it is experienced first and 
 most violently over a certain central space, from which 
 it spreads with diminishing intensity in all directions. 
 The sound of its approach, like that of distant thunder, 
 artillery, or rumbling waggons, may precede by a few 
 seconds the advent of the earthquake itself. When the 
 shock comes, the ground is felt to be alternately raised 
 and depressed, with more or less violence, as if an undula- 
 tion like the ground-swell of the sea were passing beneath. 
 Several shocks in succession, but commonly less destruc- 
 tive than the first, may follow within a few seconds. 
 
 5. The earthquake is really of the nature of a wave 
 passing through the substance of the solid earth. Just 
 as the masts of ships at rest in a harbour are seen to 
 rock to and fro, as the swell of the sea rolls under them, 
 so trees and other tall objects have been observed to 
 sway backwards and forwards during an earthquake. 
 
 6. On steep banks and cliffs, large masses of clay or 
 rock are sometimes disengaged by an earthquake shock, 
 and are sent rolling down into the water-course or valley 
 below. The streams are thus choked up, temporary lakes 
 are formed, until, by the bursting of the barrier of fallen 
 debris, the water rushes out with great force and carries 
 another kind of devastation down the valley. 
 
 7. On many occasions the ground has been seen to 
 be rent open. The fissures thus formed sometimes 
 swallow up trees, houses, or other objects on the surface, 
 and close up again. Sometimes they remain open and 
 are subsequently deepened and widened by running 
 water into ravines and valleys. 
 
 8. Again, the ground has been found to have been 
 permanently raised above or depressed below its former 
 level. Thus in the year 1835, during a violent earth- 
 quake, which convulsed the coast of Chili, the ground at 
 the island of Santa Maria was suddenly upheaved from
 
 MOVEMENTS OF THE LAND. 213 
 
 eight to ten feet above high-water mark, so that the 
 gaping sea-shells still adhering to the rock were exposed 
 to the air, where they soon began to putrefy. The valley 
 of the Mississippi was the scene of a succession of earth- 
 quakes, from the end of the year 1811 to 1813, at the 
 close of which time, the ground was in many places left 
 with huge yawning fissures, and in certain districts sank 
 down, so as to be converted into lakes, some of which 
 are fifty miles in circumference. Part of this submerged 
 land is called " the Sunk Country." And even now, large 
 trees of walnut, white oak, mulberry, and cypress may be 
 seen ten or twenty feet or more below the water, and a 
 canoe may be paddled among their submerged branches. 
 9. When the earthquake shock takes its rise under 
 the sea and travels thence towards the land, the greatest 
 amount of destruction is caused. Not only does the 
 solid land rock to and fro, but the waters of the sea are 
 shaken, and sent with terrible force against the margin of 
 the land. From the point of origin of the shock a long, 
 low undulation is propagated over the surface of the sea 
 in all directions. When it reaches shallow water, its 
 front, like that of the bore of a tidal wave in an estuary 
 (Lesson XVII. Art. 30), becomes steep and advances 
 with great rapidity. To those on shore the approach of 
 this wave is seen to be preceded by a retreat of the sea 
 from the beach. Spaces never bare before are now 
 exposed to view as the water retires, but only for a few 
 seconds. The wave, gathering up the retreating water, 
 rushes forward with a front sometimes sixty feet or more 
 in height, not only covering the beach but even sweeping 
 far in upon the land. 
 
 1O. This sea-wave does not travel across the ocean so 
 fast as the earthquake- shock. Hence it arrives at a coast 
 some time later and completes the destruction. This 
 was the case in the famous earthquake of 1755, by which 
 Lisbon was destroyed. Again, on the I3th of August 
 1868, when Peru and Ecuador were devastated by a
 
 214 PHYSICAL GEOGRAPHY. [LESS. 
 
 disastrous earthquake, a great sea-wave inundated and 
 overthrew Arica, the principal port in the south of Peru, 
 with such rapidity and completeness, that in a few minutes 
 every vessel in the harbour was either ashore, wrecked, 
 or floating bottom upwards. A man-of-war was swept 
 inland for a quarter of a mile. Another vessel disap- 
 peared, and no vestige of it was ever seen again. Lastly, 
 in the great eruption of Krakatau, in August 1883, a sea- 
 wave, said to have been as much as i oo feet high, swept 
 across the Sunda Strait, and is supposed to have killed 
 between 30,000 and 40,000 people. 
 
 11. The true cause of earthquakes is not yet well 
 understood. Probably they take their rise from more 
 causes than one. Thus, they may sometimes be due to 
 the giving way of the roofs of the cavities which no doubt 
 exist in the interior of the earth, especially in volcanic 
 countries ; sometimes to the sudden fracture of rocks 
 under great strain ; or to the sudden generation or escape 
 of steam. Whatever be the cause of their origin, some 
 sudden blow within the interior of the earth will account 
 for the phenomena of earthquakes. The point directly- 
 above that from which the blow is given suffers the 
 severest shock, and the intensity diminishes outwards 
 from this centre. The effect may be compared to that 
 produced on a surface of still water into which a stone 
 is thrown. Where the stone strikes, a series of rings is 
 formed which are propagated outwards in ever-widening 
 circles, but become less marked and farther apart as they 
 recede from the point of origin. 
 
 12, If, after an earthquake has passed, the point can 
 be ascertained where the shock was vertical, and if, by 
 further observations upon the direction of the rents in 
 walls and other evidence (Fig. 38), it can be determined 
 at what angle the earth-wave emerged at the surface in 
 one or more places beyond the centre, an indication may 
 be obtained as to the probable depth at which the shock 
 took its rise. Let v in Fig. 39 be the point at which the
 
 MOVEMENTS OF THE LAND. 
 
 215 
 
 shock was felt to have come up vertically, and A a village 
 or town where, from the prevalent direction of the rents 
 in the buildings, the shock appears to have come up 
 
 FIG. 38. House rent by earthquake (Mallet). The arrow shows the direction 
 from which the earthquake-wave must have reached the surface. 
 
 obliquely. If we prolong the path indicated by these 
 rents until it meets the vertical, we obtain at F the 
 probable focus of disturbance. Calculations of this kind 
 
 FIG. 39. Diagram-section to illustrate the propagation of an earthquake- wave 
 and the mode of calculating the depth of its origin. 
 
 were made by the late Mr. Mallet, who concluded that, 
 on the whole, earthquakes are not deep-seated, but pro- 
 bably never arise at a greater depth than thirty geogra-
 
 2i6 PHYSICAL GEOGRAPHY. [LESS. 
 
 phical miles. As implied in the diagram, the successive 
 waves, like the rings on the pool of water (Art. 1 1 ), are 
 closer and stronger at the focus, whence they spread 
 in all directions through the solid earth, becoming less 
 and less violent as they recede. 
 
 13. Earthquakes, as represented in Plate IX., are 
 most frequent in volcanic districts, though not by any 
 means confined to them. The great earthquake region 
 of the Old World stretches from the Azores along the 
 basin of the Mediterranean into the heart of Asia. In 
 the New World the western border of the Continent 
 suffers most from earthquakes, especially from Guatemala 
 southwards through Ecuador, Peru, and Chili. 
 
 14. Most frequently the more striking effects of earth- 
 quakes are comparatively local, though pulsations in 
 lakes and on the sea may be noticed at long distances 
 from the centre of disturbance. At the time of the 
 Lisbon earthquake some of the lakes of Scotland were 
 agitated, and the sea-wave that was generated travelled 
 across the Atlantic, and was felt on the American coast. 
 Occasionally the actual vibration of the ground is sensibly 
 felt for a great distance, as in the case of the earth- 
 quake of the 1 3th August 1868, which was felt in Peru 
 for 2000 miles. 
 
 15. (ii.) Slow Upheaval. After an earthquake has 
 ceased, the ground is sometimes found to have been per- 
 manently raised above or depressed beneath its previous 
 level. But besides such sudden movements others have 
 affected different parts of the earth in so slow and quiet 
 a manner as not to be perceptible at the time. As a 
 rule, they can only be detected by careful observation of 
 their effects along the margin of the land. 
 
 16. Old harbours and sea-walls are sometimes found 
 to stand now at some height above even the highest 
 tide. Islands once separated by a water-channel from 
 the land are now joined to it. Caves, evidently hollowed 
 out by the sea, may be seen far above the reach of the
 
 XXIII.] 
 
 MOVEMENTS OF THE LAND. 
 
 217 
 
 waves. Barnacles and sea-shells are found still adhering 
 to rocks, at heights of several hundred feet above the 
 level where they lived and died. Terraces of sand and 
 gravel, quite like the present beach, and containing sea- 
 shells, occur at different heights above the sea (Figs. 40 
 and 41). These terraces are old beaches, each of which 
 marks a former level of the sea, before the land rose to 
 
 FIG. 40. View of an old sea-terrace or raised beach, with sea-worn caves 
 on its inner margin. 
 
 its present height. Many such terraces skirt the shores 
 of Great Britain. They form a marked feature of the 
 coast in the north of Norway (Fig. 41). On the western 
 margin of South America they occur in great perfection, 
 reaching sometimes a height of 1 300 feet above the sea, 
 where sea-shells, still in position, attest the amount of 
 uprise. 
 
 17. By evidence of these various kinds it has been 
 ascertained that many long tracts of coast-line are slowly
 
 218 
 
 PHYSICAL GEOGRAPHY. 
 
 [LESS. 
 
 rising from the sea. Thus the shores of Sweden at 
 Stockholm and northwards appear to be upheaved at a 
 rate varying from six or ten inches to two and a half 
 feet in a hundred years. Farther north, the island of 
 Spitzbergen is fringed with raised beaches up to a height 
 of 147 feet. The coast-line of northern Russia and 
 Siberia for hundreds of miles has been recently elevated 
 out of the sea, as is shown by raised beaches with marine 
 
 FIG. 41. Raised sea-terraces of the Alien Fjord, Norway. 
 
 shells. The shores of the Mediterranean afford local 
 illustrations of uprise, while the great sandy desert of 
 the Sahara, containing here and there scattered sea- 
 shells, up to a height of 900 feet, is a case of -the recent 
 elevation of a wide tract of sea-bottom. 
 
 18 (iii.) Slow Subsidence. While in some regions 
 the slow, imperceptible movement of the ground is an up- 
 ward one, in others it is downward. This may be shown 
 by different kinds of evidence. Thus, some seaport 
 towns in the south of Sweden contain, under their pre- 
 sent streets, traces of older structures which are now
 
 XXIII.] 
 
 MOVEMENTS OF THE LAND. 
 
 219 
 
 below the sea-level. At different parts of the coast of 
 Scotland and the south-west of England, stumps of trees, 
 with their roots still imbedded in the soil on which they 
 grew, are to be seen actually under the water of the sea. 
 19. Until recently the most striking evidence of the 
 subsidence of large portions of the earth's surface was 
 believed to be supplied by the Coral Reefs of the Pacific 
 
 FIG. 42. Section of an island (L), with a Fringing coral reef (R) 
 
 and Indian Oceans. These are reefs or submarine banks 
 of a kind of lime-stone, formed entirely by the growth of 
 coral polypes. Three kinds of reef were recognised by 
 Mr. Darwin. The first, called Fringing Reefs, skirt the 
 margin of the land at a distance of one or two miles, 
 with a shallow-water channel or lagoon inside. The out- 
 side of the reef is its highest part, and there the large 
 strong kinds of coral live which delight in the dash and 
 play of the waves ; the finer and branched kinds prefer 
 the stiller water of the lagoon. Reefs of this kind skirt 
 the east coast of Africa and occur also in the West Indies 
 and fronting the shores of Brazil. The accompanying 
 
 figure (Fig. 42) shows a section of an island where the 
 land (L) is skirted by a fringing reef (R). 
 
 2O. The second kind of reef is called a Barrier Reef 
 (Fig. 43). It may skirt a long tract of coast, as in the
 
 220 
 
 PHYSICAL GEOGRAPHY. 
 
 [LESS. 
 
 north-east of Australia, where the great reef fronts the 
 land for 1000 miles ; or it may encircle an island, as at 
 Tahiti. Its slope to the sea is in the upper part nearly 
 precipitous, but beneath this steep face the bottom slopes 
 away gently into the surrounding sea-floor. Inside, the 
 reef is separated from the land by a deep lagoon channel. 
 
 FIG. 44. Section of an Atoll or coral island (R R) built over submerged 
 land (L). 
 
 There may sometimes be more than one island inside a 
 barrier reef. 
 
 21. The third form of reef has received the name 
 of Atoll or coral island (Figs. 44 and 45). It is a 
 ring of coral rising out of a deep ocean, and having a 
 
 FIG. 45. View of an Atoll or coral island. 
 
 breadth of about a quarter of a mile. The outer face 
 towards the sea is precipitous in the upper part, like a 
 barrier reef. Inside lies a lagoon of comparatively shallow 
 water, full of the more delicate branching kinds of coral. 
 The waves break off fragments from the outer edge of
 
 xxin.] MOVEMENTS OF THE LAND. 221 
 
 the reef, and pile these up above the ordinary limits to 
 which the sea reaches. By degrees a narrow ring of land 
 is formed on the reef. Seeds are carried to it and take 
 root, and at last a humap population arrives and finds 
 shelter and the means of subsistence. 
 
 22. It has been ascertained that the reef-building 
 corals cannot live in deep water, that indeed they do 
 not go down farther than about twenty fathoms. Con- 
 sequently they cannot have grown up from the deep 
 bottom on which the atolls are planted. As they could 
 not have begun to build more than about twenty fathoms 
 from the surface, Mr. Darwin inferred that the bottom has 
 been gradually sinking, while the little coral-builders have 
 kept pace with the subsidence, and have maintained 
 their reef at the sea-level. The three sections here given 
 (Figs. 42, 43, and 44) illustrate the stages in this sup- 
 posed subsidence. First comes the fringing reef, with 
 its shallow lagoon and not very deep water outside. Then 
 this shore reef passes into the barrier reef, with its deeper 
 lagoon channel and much deeper sea outside. The land 
 inside becomes less in height and extent as it settles 
 down beneath the sea-level. At last it sinks out of sight 
 altogether, and the barrier reef remains as an atoll. 
 
 23. This theory of Mr. Darwin's has been so long 
 accepted, and presents so impressive a picture of vast sub- 
 sidence in the ocean-basins, that it still holds its ground. 
 More recent observations, however, by Messrs. Semper, 
 Murray, Rein, and Agassiz, have brought to light many 
 facts that were not known to Mr. Darwin, and which show 
 that the growth of coral-reefs and islands can be satisfac- 
 torily explained without the supposition of any depression 
 of the sea-bottom. In particular, it has been ascertained 
 that in the surface water and on the bottom of tropical 
 seas animal life is so prodigiously abundant, that the 
 remains of the calcareous organisms accumulate on the 
 bottom as a growing deposit of limestone. By the con- 
 tinued growth of such limestone the tops of submarine
 
 222 PHYSICAL GEOGRAPHY. [LESS. 
 
 peaks and ridges may eventually be brought up to the 
 limit of depth within which reef- building corals can 
 thrive. Having once established themselves on the 
 platform prepared for them, these creatures grow upward 
 until they reach the level of low- water. But, though 
 unable to grow higher, they continue to thrive vigorously 
 on the outer face of the reef in the full play of the surf, 
 which brings them their supplies of food. The breakers 
 tear off huge blocks of solid coral and strew these down 
 the slope below, piling them here and there even into a 
 precipitous wall. Meanwhile the living coral continues 
 to build outward on the top of the blocks which, by the 
 chemical action of the sea-water and the settling of coral- 
 sand into their interstices, are cemented together into 
 a compact mass. In this way, the steep sea- ward front 
 of barrier reefs and atolls may be explained. In the 
 inner parts of a reef the corals, being removed from con- 
 tact with the open sea, from which they get their food, 
 dwindle and die. Hence the coral rock does not grow 
 in the lagoons. On the contrary it appears to be gradu- 
 ally eaten away by the solvent action of the sea-water, 
 whereby the lagoons are probably widened and deepened. 
 
 LESSON XXIV. The Waters of the Land Part I. 
 Springs and Underground Rivers. 
 
 1. From every water-surface on the globe invisible 
 vapour is ascending into the air, where it is condensed 
 into clouds, and whence it is returned to the surface of 
 the earth again in rain, dew, snow, hail, or sleet. Dis- 
 charged upon the land, the water partly sinks under- 
 ground to rise again in the form of springs, partly flows 
 off in rivers into the sea, whence, once more evaporated, 
 it enters the atmosphere to begin again the same cycle 
 of change (Lesson X.)
 
 xxiv.] SPRINGS. 223 
 
 2. There is thus a continual circulation of water be- 
 tween the atmosphere, the land, and the sea. The 
 more this circulation is considered the more importance 
 will be assigned to it in the general plan of the earth. 
 The clouds form and melt and form again. Day by 
 day, or season by season, the rain-showers reappear, to 
 moisten the parched soil. The brook and waterfall, ever 
 rushing downwards, are yet continually fed with renewed 
 supplies from above. The river still bears its broad 
 breast of waters to the sea, as it has done ever since 
 the earliest tribes of men settled upon its banks. The 
 sea, though receiving the surplus drainage of all the 
 continents, is not thereby raised in level, but yields to 
 the air those abundant vapours which, borne back to the 
 land, and condensed into running water, once more 
 renew their downward journey to the sea. This con- 
 tinual coming and going of water may be looked upon 
 as the pulsation of the very life-blood of our globe as a 
 habitable planet. 
 
 3. Let it be remembered, too, that water enters 
 largely into the composition of the bodies both of plants 
 and animals. If its circulation were arrested, our earth 
 would cease to be the green, populous planet which it is 
 at present. Were evaporation and condensation to cease, 
 clouds, springs, and rivers would disappear. Scorched 
 by the fierce heat of the sun during the day, and frozen 
 by the intense cold of radiation at night, the land would 
 become lifeless and silent. 
 
 4. The moisture of the air returns to the land either 
 in the liquid form, as water, or in the solid form, as ice 
 (Lesson X.) In the former state it chiefly appears as 
 Rain ; in the latter as Snow. The all-important part 
 which, under each of these conditions, water takes in the 
 daily operations of nature, deserves the most attentive 
 study. In this and the following Lessons we shall look 
 more in detail at the circulation of water over the land, 
 taking first Rain and its consequences, and then Snow.
 
 224 PHYSICAL GEOGRAPHY. [LESS. 
 
 5. When rain reaches the surface of the land, 
 part of it sinks into the soil, and the rest flows off into 
 brooks and rivers, by which it is carried back to the sea. 
 These two portions of the rainfall have each an inde- 
 pendent history, and may be conveniently considered 
 separately. In the present Lesson, therefore, we shall 
 deal with the underground course, and in the following 
 Lesson with the course above ground. 
 
 6. It might naturally be supposed that the portion of 
 the rain which sinks into the earth is permanently with- 
 drawn from the circulation of water on the earth's surface. 
 But a little reflection will convince us that if this were 
 really the case, the amount of water flowing over the 
 land would be diminished. Rivers and lakes would 
 shrink in size, or dry up altogether. Yet as this does 
 not happen, there must obviously be some way in which 
 the water that sinks into the ground is restored to the 
 surface again. This restoration takes place by means 
 of springs, which are the outflow of the subterranean 
 water from openings in the ground. 
 
 7. The intimate connection between ordinary springs 
 and rainfall is familiar to every one. We know that in 
 a season of drought many springs and wells give a 
 limited supply of water, or fail altogether, while, as wet 
 weather sets in, they fill again. They obviously derive 
 their supplies from rain-water, which has percolated 
 through the rocks beneath the surface. Such springs as 
 have a deep-seated origin are less affected by rainy or 
 dry seasons, because they gather their stores from a 
 wider area of subterranean drainage, where the effects 
 of a scarcity of rain take longer to make themselves felt 
 than is the case near the surface. 
 
 8. All rocks, even the hardest, are porous, and there- 
 fore pervious to water. The channels of brooks and 
 rivers, the beds of lakes, and the floor of the sea, are all 
 more or less cracked, so as to present openings for the 
 descent of water. Rain-water, therefore, not only soaks
 
 xxiv.] UNDERGROUND WATER. 225 
 
 through the soil, but, sinking lower still, finds its way 
 through the pores and joints of rocks underneath. Water 
 is likewise supplied by lakes, rivers, and the sea, either 
 oozing through the pores of rocks or entering open cracks, 
 and carrying with it sand and other impurities. 
 
 9. In the sinking of deep wells in some districts of 
 France, leaves and other parts of plants have come up 
 with the first gush of water from a depth of nearly 400 
 feet. These organic remains were comparatively fresh, 
 and were supposed to have travelled in underground 
 channels from hills I 50 miles distant, and to have been 
 three or four months on their subterranean journey. The 
 same phenomenon has been observed in other places ; 
 sometimes even live fish have been brought up in borings 
 from depths of 1 70 feet. 
 
 10. As the result of this constant percolation and 
 descent of water from the surface, the rocks, for some 
 way down, are in many places charged with moisture. 
 Proofs of the almost constant presence of water are fur- 
 nished in quarries, pits, and mines, in short, in nearly 
 every place where any considerable cutting is made 
 through rock. It is this underground water which forms 
 one of the greatest obstacles in quarrying and mining 
 operations. Before the introduction of steam machinery 
 many coal-pits, after having been worked to a certain 
 depth, had to be abandoned, from the impossibility of 
 getting rid of the water. They were then said, in the 
 expressive language of the miners, to be "drowned." The 
 powerful pumping engines, now to be seen everywhere 
 in the coal-fields, point to the abundance of water below 
 ground, and to the labour and cost which are necessary 
 for removing it. 
 
 11. Another and familiar illustration of the way in 
 which water everywhere pervades the soil and rocks is to 
 be seen in the sinking of wells. These are artificial 
 cavities, dug out to serve as receptacles wherein the 
 water that is soaking through the rocks may be collected. 
 
 Q
 
 226 PHYSICAL GEOGRAPHY. [LESS. 
 
 Wells may be successfully made even in places where it 
 could hardly be supposed that water would be found. 
 Thus, on the borders of the African deserts, where little 
 or no rain falls, and where, therefore, there can be but a 
 scant supply of water from the surface, serviceable wells 
 are dug. The French colonists of Algeria sink what are 
 known as "Artesian wells" (Art. 18) on the northern 
 margin of the great desert of Sahara, and the sandy 
 tracts between Cairo and Suez yield water even so near 
 the surface as at a depth of fifty feet. The existence of 
 those fertile green patches called Oases, in the midst of 
 the barren deserts of Africa and Arabia, is due to the 
 rise of springs. Again, in the valley of the Mahanadi 
 and other Indian rivers, where, in the dry season, little or 
 no rain falls, a hole dug out to the depth of thirty or 
 forty feet is sure even then to be partly filled with water. 
 
 Hence we may conclude that the springs of a district 
 do not always or necessarily obtain their water from the 
 rainfall of the immediate neighbourhood. If that were 
 the case there could hardly be perennial springs and 
 wells in the African deserts, where rain is exceedingly 
 rare. 
 
 12. To what depth the water will descend must depend 
 greatly upon the nature and condition of the rocks at 
 each locality. Very deep mines are often without water. 
 When the Alps were pierced in making the railway 
 tunnel between France and Italy, the rocks at a depth 
 of more than 5000 feet below the summit of Mont 
 Cenis were quite dry. We need not suppose, therefore, 
 that the water generally sinks to a very great depth. 
 But here and there it no doubt does find its way down 
 even into the intensely hot internal regions of the earth, 
 whence it reissues in those vast clouds of steam that play 
 so important a part in the arrangements of active vol- 
 canoes (Lesson XXII.). It is probable that in spite of 
 the plentiful discharge of steam at these volcanic open- 
 ings, a part of the water which descends so far may be
 
 xxiv.] UNDERGROUND WATER. 227 
 
 lost by absorption into the molten rock, or by being 
 decomposed, and forced to enter into chemical combina- 
 tion. If this be the case, then the earth must be losing 
 its superficial water ; slowly and insensibly, indeed, but 
 yet if continuously, with the probable result of ultimately 
 reducing our planet to the dry and sterile condition of 
 the moon. 
 
 13. Though the water which falls upon the land is 
 distributed over the surface as rain, when it reappears at 
 the surface it does not ooze everywhere from the soil. 
 Sinking underground, it finds its way along cracks and 
 hollows of the rocks below, until it comes out again to 
 the surface at certain points. Just as in the subaerial 
 course of the fallen rain, the water at once runs off into 
 brooks and larger streams until it finally enters the sea, 
 so in a somewhat similar way, the underground drainage, 
 collected from many branching channels, is brought out 
 to the surface in springs. 
 
 14. A difficulty may be felt in understanding how the 
 water, having once sunk down, can ever be driven up 
 again. But we must remember that the springs, which 
 form its points of escape at the surface, lie at a lower 
 level than the ground from which the original supplies 
 of rain have been drawn. A little reflection on this sub- 
 ject will convince us that the underground circulation 
 must be effected in one or other of two ways ; either by 
 simple gravitation, as in what may be called Surface 
 Springs, or by hydrostatic pressure, as in Deep-seated 
 Springs. 
 
 15. (I.) In the case of Surface Spring's the water, 
 which has been steadily flowing downward as well as 
 onward in its underground course, comes to a point where, 
 owing to some depression of the surface, it finds itsfcii 
 again in the open air. The subjoined woodcut will 
 explain how such springs arise. A porous bed of rock 
 (b), or one traversed with cracks or joints, lies nearest 
 the surface, and allows the rain water to soak through it,
 
 PHYSICAL GEOGRAPHY. 
 
 [LESS. 
 
 down to a stiff impervious layer (a), by which the descent 
 of the water is arrested. Unable, therefore, to sink 
 farther downward, the water flows along the surface of 
 this lower bed. If a valley should happen to cut through 
 these rocks, there will be a spring or line of springs (s) 
 on the junction of the two rocks along the side of the 
 valley. In the same way, the rain which falls upon a 
 mountain may sink underground, and gush out in springs 
 at the foot of the mountain. In springs of this kind, the 
 water merely descends in the ordinary way by gravita- 
 tion, and issues where the surface happens to sink below 
 the level of the subterranean water-channel. In heavy 
 rain a good deal of water soaks through the soil into 
 the nearest brooks without ever forming actual springs. 
 
 FIG. 46. Section across a valley to show how the simplest kinds of 
 springs arise. 
 
 Wherever water accumulates among underground 
 rocks, whether in the pores or in the open crevices, 
 so as to convert the rocks into subterranean reservoirs, 
 it will rise in them up to the lowest levels at which it 
 can find outlets to the surface, where it will appear in 
 these surface- or land-springs. This " water-level " must 
 be reached before a well can catch a supply of water. 
 
 16. (II.) In Deep-seated Springs, on the other hand, 
 the water has in its journey sunk to a lower level than 
 its point of escape, and has risen again by hydrostatic 
 pressure, as in a syphon. Evidently the arms of a 
 syphon may be as long as we choose to make them, yet 
 while the one is longer than the other, and is supplied 
 with water at the top, the water will continue to flow out 
 from the top of the shorter arm. In like manner the 
 subterranean channel of a deep-seated spring may descend
 
 xxiv.] UNDERGROUND WATER. 229 
 
 for hundreds of feet, yet the water which fills it will not 
 cease to flow and to rise again to the surface. Having 
 reached its greatest depth, often far below the level of 
 the sea, the water accumulates there, saturating such 
 porous rocks as lie in its way, until the pressure of the 
 column of water behind it forces it up any fissure which 
 may allow of its escape to the surface, and there it bub- 
 bles up as a spring. The rain which falls on the high 
 grounds, and is absorbed by the rocks and soils, flows 
 down more or less permeable rocks which may be 
 arranged in different beds or strata (a in Fig. 47). 
 Taking advantage of the cracks that may opportunely 
 
 X 
 
 *^ 
 
 FIG. 47. Section to show how deep-seated springs arise. 
 
 present themselves, as much of the water as these open- 
 ings will admit rises through them to form springs, as at 
 s, s. At f the strata are broken across by a great frac- 
 ture or dislocation, which brings them against a hard 
 massive rock (g). Here a considerable body of water 
 may escape to the top at s'. Hence the direction of a 
 great dislocation of the rocks is often traceable at the 
 surface by a line of springs. 
 
 17. In nature, the course which water takes under- 
 ground is in reality, for the most part, much more com- 
 plicated than is shown in the diagram (Fig. 47). All 
 rocks are abundantly traversed by divisional planes called 
 "joints ; " they are likewise full of cracks, and they pre- 
 sent many changes in texture. So that subterranean 
 water finds a most intricate network of passages through 
 which it must flow. It may possibly come many times
 
 230 PHYSICAL GEOGRAPHY. [LESS. 
 
 near to the surface, and then sink down again through 
 other fissures, performing in this way a zigzag up-and- 
 down journey, before it finally issues in springs. A 
 rough representation of this kind of circulation is given 
 in Fig. 48. 
 
 FIG. 48. Section to show the intricate underground drainage which issues 
 in a deep-seated spring. The numerous joints and cracks in the rocks 
 lead the water at last into a main channel, by which it reascends to the 
 surface as a spring at J. 
 
 18. On further reflection we perceive that there must 
 necessarily be many underground rocks which are per- 
 manently saturated with water. These, if they can be 
 reached, will furnish an abundant and constant supply of 
 water. Advantage is taken of this kind of knowledge to 
 sink deep wells, called "Artesian," from the old French 
 province of Artois, where they have long been in use. 
 The principle on which these wells are made is as fol- 
 lows : When an impervious rock covers a porous one 
 over a considerable district of country, the water that 
 soaks into the lower bed will tend to accumulate there 
 as in a great reservoir. If now a hole be drilled through 
 the upper retentive bed, the water will rise in it at once,
 
 xxiv.] UNDERGROUND WATER. 23! 
 
 as it would do if a natural fissure existed there, and may 
 ascend even above the surface of the ground. The water 
 occasionally issues from these bore-holes with such force 
 as to form a jet, rising to a height of thirty feet or more 
 above the soil. In the north of France the force of the 
 jet has been made use of to drive mills. Many wells 
 on this principle have been sunk in London and its 
 neighbourhood. The section (Fig. 49) across the dis- 
 trict in which London lies, shows roughly how the water 
 which falls on the high grounds to the north and south, 
 filters through the sands and gravels (b) at the top of 
 the chalk (a), and is imprisoned by the retentive London 
 clay (c). 1 When wells are sunk down to this water- 
 bearing zone, water ascends in abundance. So many 
 
 FIG. 49. Section to the position of the water-bearing rocks below the 
 clay at London. 
 
 wells, however, have now been made, that the level of 
 the water in them has gradually sunk year by year, the 
 consumption being thus somewhat more rapid than the 
 supply to the underground reservoirs. 
 
 19. In trying to picture to ourselves the amount of 
 water which is always circulating under ground, we ought 
 not to measure it merely by what is seen coming out at 
 the surface in well-marked springs. For, in the first 
 place, we must bear in mind that the abundance of 
 springs is really much greater than might at first sight 
 appear. Much of the water which rises from under 
 ground does not bubble up in the form of well-marked 
 springs. When it reaches the surface, it soaks through 
 the soil or trickles over it in tiny runnels. On unculti- 
 
 1 In the diagram the curve of the beds has necessarily been greatly exag. 
 gerated, there being in reality hardly a basin at all.
 
 232 PHYSICAL GEOGRAPHY. [LESS. 
 
 vated land such places are marked by greener patches 
 of vegetation, or by tufts of rushes and a swampy soil. 
 Sometimes, too, we may see them exposed, even on 
 ploughed fields, during dry weather in spring-time. From 
 want of rain the bare soil becomes dry and light in 
 colour. Here and there, however, it is diversified with 
 dark brown patches, which point to places where the 
 water is oozing out from below, and soaking through the 
 soil, in spite of the drain-pipes of the farmer. In these 
 and similar cases, the water, after coming up to the 
 surface, sinks down again into the rocks, and commences 
 another underground journey. 
 
 20. In the second place, we must remember that the 
 natural flow of water from a higher to a lower level must 
 carry much of the underground drainage to the rocks 
 under the sea, so that, if the water rises to the surface, 
 it will do so on the sea-floor, instead of under the open 
 air. Springs along the beach within tide -marks are 
 sufficiently common, and many cases have been observed 
 in the Mediterranean where powerful springs or even 
 underground rivers rise to the surface of the sea at some 
 distance from the shore, so that fresh water may actually 
 be obtained at these places for the supply of ships. 
 
 21. In some countries, where there are no rivers, and 
 little or no rain, springs and artificial wells are the only 
 sources from which a supply of water can be obtained by 
 the inhabitants. But even in regions where both rain 
 and rivers abound, the usefulness of springs is hardly less 
 apparent. Think for a moment what would happen if 
 all the rain were immediately to run off" the surface of the 
 land, without allowing any portion to sink under ground. 
 Except where fed from melting snow, the brooks and 
 rivers would flow only after showers. Their channels 
 would dry up in the absence of rain. It is the under- 
 ground circulation which, by means of springs, replenishes 
 the water-courses of the land, even in drought, and thus 
 preserves the surface of the land fresh and green.
 
 xxiv.] UNDERGROUND WATER. 233 
 
 22. Rain is water nearly in a state of chemical purity ; 
 but in its descent it takes up a little air and some of the 
 impurities which may be floating in the air (Lesson VI.). 
 These admixtures, however, form only a minute propor- 
 tion in rain-water, especially at a distance from the smoke 
 and vapours of towns and manufactories. But the water 
 of any spring, even the clearest and most sparkling, 
 though it sank into the ground as pure or almost pure 
 water, reappears again with an impregnation of various 
 substances. These are often so abundant as to be made 
 readily visible after the water has been boiled and evapor- 
 ated, when they remain behind as a film on the bottom of 
 the vessel. They are chemically dissolved in the water, 
 and do not interfere with its clearness and brilliancy, nor 
 in most cases do they impart to it any taste. They may 
 be found in the water of every spring, but their quantity 
 varies greatly. Sometimes they occur in such minute 
 proportions as fifty parts in every million parts of water ; 
 in what are called mineral waters they may rise to as 
 much as 32,700 parts in the million. 
 
 23. Three questions naturally arise in reference to this 
 remarkable impregnation of all spring-water : firstly, what 
 are the substances present in the water ? Secondly, how 
 does the water obtain them ? and, thirdly, what is the 
 result of their constant removal by the water ? 
 
 24. (i.) Substances contained in spring- water. 
 If a glass of bright sparkling spring- water be allowed to 
 stand for a time, minute bubbles may be observed adher- 
 ing to the inner surface of the glass. These are air, or 
 gas, which has been dissolved in the water. They rise 
 by degrees and escape, and when they have done so, the 
 water ceases to have the same fresh brisk taste which it 
 had when first drawn from the spring. In like manner, 
 if the water is boiled it acquires an unpleasant insipid 
 taste, not because anything has been put into it, but 
 because the air or gas which it contained has been driven 
 out. Hence it appears that at least one cause of the
 
 234 PHYSICAL GEOGRAPHY. [LESS. 
 
 pleasantness of spring-water to the taste is the presence 
 of gaseous materials. 
 
 25. Chemists have carefully examined these materials, 
 and have found that they consist mainly of oxygen and 
 nitrogen, that is, the ordinary gases of the air. Pure 
 water when saturated with air will contain by volume 
 17-95 parts of air in every 1000 parts of water. The 
 proportions of the two atmospheric gases, oxygen and 
 nitrogen, in this dissolved air are considerably different 
 from those in the atmosphere, being in every hundred 
 parts 65-09 of nitrogen and 34-91 of oxygen. The 
 latter gas being more soluble exists in considerably larger 
 proportions in water than in the air (Lesson X. Art. 35). 
 Spring water also contains carbonic acid, and sometimes 
 sulphuretted hydrogen, marsh gas, or other gases. 
 
 26. The fine white film of mineral substance left 
 behind, when a quantity of spring-water has been boiled 
 down and driven off" into vapour, is found to vary greatly in 
 composition in different springs. It commonly consists 
 of carbonates or sulphates of lime, soda, magnesia, and 
 often includes chloride of sodium or common salt. In 
 limestone districts, owing to the large proportion of lime 
 or magnesia in the water, much soap is needed in wash- 
 ing, because the mineral constituents unite with the fatty 
 acid of the soap to form a curdy insoluble compound. 
 Such water is said to be hard. When the hardness is 
 due to the presence of carbonates, it disappears when 
 the water is boiled. A hard crust is deposited on the 
 inside of the kettles or boilers in which the water is 
 boiled. If the hardness arises from the presence of 
 sulphates it cannot be removed by boiling. Where the 
 percentage of mineral matter is small, the water is called 
 soft. The softest water that can be had for domestic 
 purposes is of course rain, for it contains little or no 
 mineral admixture. 
 
 27. While all spring-water holds more or less mineral 
 matter in solution, some springs contain so much that it
 
 xxiv.] UNDERGROUND WATER. 235 
 
 affects the taste or becomes visible as the water flows 
 over the ground. These are known as Mineral Springs. 
 The chief mineral substance in solution is carbonate of 
 lime. When the water begins to evaporate, as it trickles 
 away from the spring, the lime is sometimes deposited as 
 a white crust upon stones or other objects lying in its way. 
 This is particularly apt to occur on certain kinds of moss 
 which prefer water containing much lime. A white limey 
 crust gathers round every little fibre and completely en- 
 closes it, so that as the plant dies the white crust remains 
 and retains the outward form of the original moss. These 
 calcareous springs, as they are called, occur abundantly 
 in limestone countries. In other springs, carbonate or 
 sulphate of iron is the principal substance contained in 
 the water, which, as it flows along, deposits iron oxide 
 as a yellow or brown scum on the sides of its channel. 
 Springs of this nature are known as ferruginous or chaly- 
 beate. Common salt is the characteristic of some springs, 
 and when they contain a large proportion of it they re- 
 ceive the name of brine-springs. Sometimes they are 
 nearly or quite saturated with salt. Many springs con- 
 taining a considerable admixture of mineral ingredients 
 are useful in certain diseases, either in the form of 
 draughts or of baths. They are termed Medicinal 
 springs. For example, the waters of Bath contain car- 
 bonate of lime, sulphate of soda, sulphate of lime, chloride 
 of sodium, silica and carbonic acid ; those of Harrogate, 
 carbonates of magnesia and lime, sulphate of magnesia, 
 chlorides of sodium, magnesium, and lime, nitrogen, car- 
 bonic acid and sulphuretted hydrogen. The waters of 
 Vichy are alkaline and acidulous from the quantity of 
 soda, potash and carbonic acid which they contain. 
 Those of Carlsbad abound in sulphates, those of Wies- 
 baden in chlorides. 
 
 28. In mountainous and snow-covered ground, spring- 
 water may be met with scarcely above the freezing-point. 
 From that extreme, every degree of temperature may be
 
 236 PHYSICAL GEOGRAPHY. [LESS. 
 
 noted in different springs, up to such boiling fountains as 
 the Geysers. Evidently, the temperature of the spring 
 must depend upon that of the rocks from which the water 
 rises. If the water comes from melting snow and ice, it 
 will necessarily be cold. If it sinks deep within the earth, 
 so as to come within the influence of the internal heat, it 
 will be warm. So that there seems good reason to regard 
 the temperature as affording some indication of the 
 probable depth from which the water has risen. 
 
 29. Hot water has a greater power of dissolving most 
 substances than cold water. Consequently, warm or 
 thermal springs are often largely impregnated with 
 mineral matter. Many medicinal wells are warm. Those 
 of Vichy, for example, have a temperature of 1 1 1 
 Fahr., those of Carlsbad, 165 Fahr., and those of 
 Chaudes Aigues, 178 Fahr. One of the substances 
 particularly soluble in hot water is silica, which has been 
 already alluded to as occurring in the water of boiling 
 springs, and as being deposited in the form of a hard 
 crust round the orifices from which the water escapes, 
 as in the Geysers of Iceland, the Yellowstone Park and 
 New Zealand (Lesson XXI. Art. 24). 
 
 SO. The proportion of mineral matter held in solution 
 differs widely in different springs. In common spring- 
 water it may range from 50 to 400 or 500 parts in every 
 million parts of water. But in districts where the water 
 is "hard" the proportion may rise to 2000 parts in every 
 million. In mineral springs, the proportion is of course 
 very much greater. Thus, in the Vichy waters, the solid 
 contents are ^VoVoo' tf> ose f Seidlitz contain I QOOOOO ; 
 those of Piillna, in Bohemia, J^OOOOQ- The proportion 
 in the Atlantic Ocean water, is about JQOOOOO* wn '" e m 
 the Dead Sea, it is iVooVoV 
 
 31. When the discharge of water, as well as the pro- 
 portion of dissolved mineral matter, is large, the quantity 
 of material dissolved out of the rocks below ground and 
 carried up by springs, must be enormous. Thus, one of
 
 xxiv.] UNDERGROUND WATER. 237 
 
 the hot springs of Leuk, in Switzerland, having a tempera- 
 ture of 144 Fahr., is estimated to bring to the surface 
 every year nearly 9,000,000 of pounds of gypsum. If 
 this mass of mineral could be collected it would form a 
 square column, upwards of 650 feet high, and twenty- 
 seven feet on each side. The brine spring of Neusalz- 
 werk, near Minden, has been found to yield in one year 
 enough of brine to form a cube of solid salt, measuring 
 seventy-two feet. The wells of Bath, in like manner, are 
 computed to yield annually enough of mineral matter to 
 form a square pillar ten feet on the side, and eighty feet 
 high. 
 
 32. (ii.) Origin of the Substances dissolved in 
 Spring-water. Rain in falling dissolves a little air, but 
 takes considerably more oxygen and carbonic acid gas than 
 the normal proportions of these gases in the atmosphere 
 (Lesson VI. Art. 35). The proportion of carbonic acid 
 gas indeed is between thirty and forty times greater. In 
 sinking through the soil rain obtains, from decomposing 
 plant and animal remains, more carbonic acid and vari- 
 ous organic acids. In old volcanic districts, as Auvergne 
 in Central France, and the Eifel in Rhenish Prussia, 
 carbonic acid is given off in great quantities from sub- 
 terranean sources, and is here and there brought up by 
 springs, which like the Bad Tonnistein in the valley of 
 Brohl, are so full of it that it escapes in copious bubbles 
 when the water comes to the surface. 
 
 33. The oxygen and carbonic acid carried by perco- 
 lating rain downward into subterranean rocks are of 
 great importance in aiding the water to decompose 
 rocks. The presence of carbonic acid enables it to 
 dissolve large quantities of various mineral substances. 
 How this solution takes place is admirably illustrated 
 above ground by what happens at the arches of many 
 bridges. On the under side of an arch, or the vaulted 
 roof of a cellar, each line of mortar between the courses 
 of masonry is often marked by a sort of fringe of
 
 238 PHYSICAL GEOGRAPHY. [LESS. 
 
 slender pendent white stalks or pencils. At the point 
 of each of these stalks, a clear drop of water may be 
 noticed, which in time falls to the ground, and is slowly 
 replaced by another drop. Should the ground beneath 
 the arch be undisturbed by passing footsteps, where the 
 drops fall upon it, a white deposit, like that of the roof 
 above, grows up in little mounds, each of which is kept 
 wet by the constant drip of the water. These white 
 incrustations may be seen to increase in size from year 
 to year. It is evident that they have come out of the 
 masonry and that trickling water has had something to 
 do with their formation. 
 
 34. Rain-water containing, as it does, carbonic acid, 
 has the power of dissolving lime, and holding it in solu- 
 tion in the form of what is known as carbonate of lime. 
 The mortar which binds the masonry together consists 
 mainly of lime. Being usually more porous than the 
 stone or brick, which it cements, it allows some rain 
 to sink through the seams and joints of the masonry. 
 The water in its passage from the upper to the under sur- 
 face of the archway, takes out a little lime, and carries it 
 off in solution. As each drop appears upon the roof and 
 hangs for a time before falling, it is diminished by eva- 
 poration, and losing part of its carbonic acid, cannot hold 
 so much lime as at first. It is therefore compelled to 
 deposit the surplus as a white film upon the roof. The 
 drop then falls, and is succeeded by the next, which goes 
 through the same stages, and thus the original ring of 
 lime, left round the edges of the first drop, grows into a 
 long slender hollow tube or stalk, like an icicle of stone. 
 If it is undisturbed, it may lengthen until it actually 
 reaches the floor, and its sides may be added to by fur- 
 ther trickling water until it becomes a stout rod, or even 
 a pillar seeming to support the roof. These hanging 
 stalks or columns of lime are called stalactites, 
 
 35. But the drops do not leave all their lime behind 
 them on the roof. They carry some of it with them to
 
 xxiv.] UNDERGROUND WATER. 239 
 
 the ground, where, on further evaporation, they deposit it 
 as a white solid crust called stalagmite. This process 
 of removal and re-deposit, seen so well in a small way 
 in arches of masonry, occurs sometimes on a magnificent 
 scale in vast limestone caverns (Arts. 39-41). 
 
 36. If, now, in sinking through a few feet of masonry, 
 rain-water can work such great changes, what may we 
 not expect to take place under ground, where the water 
 has to traverse vast masses of rock, and where its solvent 
 power may be increased by the earth's internal heat ! 
 
 37. (iii.) Results of the removal of materials 
 from below in spring-water. In tracing the results 
 of the universal percolation of water through underground 
 rocks, and of the removal and transport to the surface 
 of so much of their solid substance, we may consider 
 first the effects on the surface, and secondly the effects 
 below ground. 
 
 38. With regard to the influence exerted on the earth's 
 surface by the solution of subterranean rocks, we may 
 reflect that springs supply rivers and lakes, and thus, 
 indirectly, the sea, with lime, iron, soda, and other soluble 
 ingredients. In fresh water and in the sea, there are 
 large tribes of animals which obtain the materials of their 
 shells or skeletons from the lime or other minerals pre- 
 sent in the water. If these mineral substances could be 
 removed for a time, the result would be to destroy a large 
 part of the denizens of our rivers, lakes, and seas. Man 
 himself would suffer, not only by losing some of the most 
 valuable of his supplies of food molluscs (oysters, etc.), 
 crustaceans (lobsters, crabs, etc.), and fishes, but from 
 the mere absence in his drinking water of mineral sub- 
 stances, useful in keeping his body healthy. The failure 
 of medicinal springs, too, would deprive him of an im- 
 portant element in the curative treatment of many 
 diseases. 
 
 39. It is in its underground operations, however, and 
 the effect of these upon the surface, that the effects of the
 
 240 PHYSICAL GEOGRAPHY. [LESS. 
 
 removal of mineral matter by springs are most striking. 
 Since every spring is continually dissolving and carrying 
 up to the surface some of the solid substance of the earth's 
 interior, and since the amount so carried by many springs 
 even in a single year would, if it were collected in the 
 solid form, make considerable hills, the result must be 
 that cavities arise among the subterranean rocks. That 
 this is really the case is best seen in limestone countries. 
 Limestone is liable to be dissolved and removed by per- 
 colating rain-water, in the same way as the mortar of an 
 archway (Art. 33). It is full of chinks and joints, by 
 which water finds its way downward. Each of these 
 passages is gradually widened by the solution and re- 
 moval of the rock at its sides. Hence in districts where 
 limestone forms the uppermost rock, the ground is some- 
 times full of holes, which have been eaten out of the 
 solid rock by trickling rain-water, and which lead down 
 into numerous branching tunnels and chambers under- 
 ground excavated in the same way. 
 
 4O. One of the most remarkable examples of this kind 
 of scenery is that of the Karst in Carniola on the flanks 
 of the Julian Alps. It is a table-land of limestone, so full 
 of holes as to resemble a sponge. All the rain which 
 falls upon it is at once swallowed up and disappears in 
 underground channels, where, as it rushes among the 
 rocks, it can be heard even from the surface. Some of 
 the holes which open to daylight lead downward for 
 several hundred feet. Some turn aside and pass into 
 tunnels in which the collected waters move along as large 
 and rapid subterranean rivers, either gushing out like the 
 Timao at the outer edge of the table-land, or actually 
 passing for some distance beyond the shore, and finding 
 an outlet below the sea. Here and there the labyrinths 
 of honeycombed rocks expand into a vast chamber, 
 with stalactites of snowy crystalline lime hanging from 
 the roof, or connecting it by massive pillars and parti- 
 tions with the floor. Such is the famous grotto of Adels-
 
 xxiv.] UNDERGROUND WATER. 241 
 
 berg near Trieste a. series of caverns and passages with 
 a river rushing across them. 
 
 41. Still more extensive is the Mammoth Cave of 
 Kentucky a cavern about 10 miles long, but with many 
 ramifying passages which are said to have a united length 
 of more than 200 miles. In the Island of Antiparos, a 
 famous grotto lies 600 feet below ground, forming a 
 spacious hall 300 feet wide and 240 feet high. 
 
 42. Partly from the solvent action of descending 
 water upon the sides of chinks of the limestone, and 
 partly from the falling in of the roofs of underground 
 passages, the surface of the ground in some limestone 
 countries is so full of holes, and at the same time so 
 bare of soil, as to become a kind of barren and dry 
 desert. All its rain, its springs and its rivers are with- 
 drawn from the surface. In the course of time the 
 ground has here and there subsided to such an extent 
 as to form hollows in which the water collects into lakes. 
 These, however, have no outlet at the surface. The water 
 issues from openings in the rocks to fill them, and flows 
 away by other openings of the same kind. The Zirknitz 
 See in Carniola is a good instance of a lake of this kind. 
 It is about five miles long and from one to two miles 
 broad, but usually not more than from six to ten feet 
 deep. Its bottom is said to be perforated with about 
 400 funnels or pipes through which the water ascends. 
 In wet weather it rises to three times its ordinary height. 
 But even at high water, it is so surrounded with high 
 ground, that it cannot find any outlet at the surface, and 
 has to discharge its surplus waters down some of the 
 innumerable caverns in the limestone (Lesson XXVI. 
 Art. 13). 
 
 43. In some parts of the world, therefore, and more 
 especially in limestone regions, the surface of the ground 
 is greatly changed by the chemical action of subterranean 
 water. But there is yet another way in which the cir- 
 culation of water below ground affects the form of the 
 
 R
 
 242 
 
 PHYSICAL GEOGRAPHY. 
 
 [LESS. 
 
 surface. Where rain sinks through a porous sloping 
 bed of soil or rock, it sometimes forms a loose watery 
 layer underneath, which, by destroying the support of 
 the overlying mass, allows the latter to slip down the 
 
 FIG. 50. Section across the cliff and landslip of Antrim. 
 
 slope and tumble into fragments below. This is called 
 a landslip. Changes of this kind can, of course, only 
 occur on the sides of mountains, cliffs, ravines, or steep 
 
 FIG. 51. View of part of the cliffs and landslip of Antrim. 
 
 slopes, where movement by gravity from a higher to a 
 lower level is possible. They are common along the 
 sea-coast, many parts of the shore-line of the British 
 islands being fringed with old landslips. When the 
 slipped mass is large in extent and becomes covered
 
 xxiv.] UNDERGROUND WATER. 243 
 
 with vegetation, it forms a strip of broken and picturesque 
 ground in front of the higher cliff behind. Such is the 
 undercliff of the Isle of Wight, and the long line of rough 
 crags and grassy mounds flanking the steep cliffs of 
 Antrim. In the latter case (Figs. 50, 51) a great table- 
 land of ancient hard lava beds (b) rises from the coast 
 in a line of noble cliffs, resting upon layers of much softer 
 and more porous rocks (s). Owing to the loosening of 
 the support of the upper part of the cliff by the trickling 
 of water between the beds in the lower half, huge slices 
 of the heavy solid lava-rocks have been launched down 
 to the low ground. Many of these fallen fragments are 
 themselves large enough to be called hills. 
 
 44. In hilly countries subject to heavy rains, land- 
 slips are of frequent occurrence. The earth and upper 
 layers of rock, saturated with water, are loosened, and 
 slide down the slopes, carrying trees and fields to the 
 valleys below, and piling up vast heaps of ruin there. In 
 Sikkim, and other districts to the south of the Himalayan 
 chain, the surface of the ground is being altered from 
 this cause after every heavy fall of rain, vast spaces of 
 mountain-slope, many acres in extent, being detached 
 so as to sweep down, with their covering of forest, into 
 the lower ground. Sometimes these fallen masses of 
 earth and rock are thrown across a valley, so as to bar 
 back the river and form a lake. But as the barrier con- 
 sists only of loose rubbish, it is apt to give way to the 
 pressure of the accumulating waters, which then pour 
 down the valley with great force, sweeping everything 
 before them, and desolating the district for many miles 
 along their course. 
 
 45. When landslips take place in well-peopled and 
 cultivated valleys, they sometimes cause great destruc- 
 tion of life and property. Thus, in the valley of Goldau, 
 Switzerland, in the year 1 806, after a continuance of 
 wet weather, a bed of rock, 100 feet thick, resting on 
 saturated sandy layers, slipped down. The whole side
 
 244 PHYSICAL GEOGRAPHY. [LESS. 
 
 of the mountain of the Rossberg seemed to be in motion. 
 In a few minutes the descending mass had, with a terrible 
 noise, rushed across the valley, burying five villages and 
 about 500 people under a mass of ruined rocks 100 to 
 250 feet high. 
 
 LESSON XXV. The Waters of t/ie Land Part II. 
 Running water. Brooks and Rivers. 
 
 1. Having in the last lesson followed the course of 
 that portion of the rainfall which disappears into the 
 ground, we have now to trace what becomes of the rest. 
 Since the surface of the land is higher than the sea, 
 and slopes downward to the sea-level, the water which 
 falls upon it from the sky cannot remain there, but must, 
 in obedience to gravity, seek the lowest level. This it 
 can only do by moving downward, till it flows into the 
 sea. If the land sloped evenly from a central ridge, 
 like the roof of a house, the rain might run off in sheets 
 of water. But instead of such uniformity it everywhere 
 presents the most irregular surface. Even on what we 
 might suppose to be a perfectly level and smooth piece 
 of ground, there are innumerable little heights and hol- 
 lows, which become at once apparent during a shower 
 of rain, for then the hollows are marked by tiny runnels 
 of water which course along them to a lower level. 
 
 2. "Water -courses and River-basins. Owing, 
 therefore, to the unevenness of the surface of the land, 
 the surplus rain runs off into the hollows, down which it 
 flows until it can descend no farther. These hollows or 
 channels, which receive and conduct the drainage of the 
 land, are termed water-courses. They vary in size, 
 from the rut that holds the merest rill or gutter, up to 
 the bed of a broad river which carries the drainage of 
 half a continent to the sea.
 
 xxv.] BROOKS AND RIVERS. 245 
 
 3. Take the map of any large country, or of a con- 
 tinent, and notice in the arrangement of the water-courses 
 upon it, that they are grouped somewhat like the branches 
 and stem of a tree. On the low grounds, towards the 
 sea, a river, in one single trunk, bends to and fro across 
 the land. But farther inland it divides into separate 
 limbs, these again into smaller branches, and so on as 
 far as the upper limit of the region drained by the river. 
 This is the common plan on which the drainage of the 
 land is arranged, innumerable little runnels in the 
 heart of a country, coalescing more and more as they 
 descend, until they all finally unite into a few main 
 streams. 
 
 4. Let us in imagination trace the course of a large 
 river, from its beginning in the midst of a continent, to 
 its end in the sea. Among the "far mountains, where 
 the sources of the river must be sought, the higher sum- 
 mits are probably covered, or at least streaked, with 
 snow, while long tongues of snow and ice may be seen 
 creeping down the upper parts of the valleys. Perhaps 
 the river issues from the melting end of a " glacier " 
 (Lesson XXVIII.). If so, it springs up at once as a 
 tumultuous torrent of muddy water, and rushes down the 
 valley, receiving from either side innumerable minor 
 torrents and runnels, which descend the rugged slopes, 
 either from the melting edge of the snow, or from 
 abundant clear bubbling springs. Or perhaps the river 
 takes its origin in some single spring, not larger, it may 
 be, than many others in its neighbourhood, but which 
 has been fixed upon from early times by the human 
 population of the district as the true fountain of the river. 
 Such a spring, either welling quietly from the ground or 
 gushing out copiously, supplies the first little stream 
 which dashes down its rocky channel, receiving from 
 each side, as it descends, many tributary torrents, until 
 after leaping from rock to rock in foaming cascades, and 
 working its way through deep gullies, it reaches a more
 
 246 
 
 PHYSICAL GEOGRAPHY. 
 
 [LESS. 
 
 level part of the valley. In this first or torrent part of 
 its course, the infant river is only one of many such 
 streams by which the sides of the higher mountains are 
 channelled. 
 
 5. But when it gains the valley it enters on a second 
 and distinct portion of its journey. Its flow is less rapid, 
 its channel less steep and uneven. It winds to and fro, 
 in many bends and loops, across the flat parts of the 
 valley, and rushes through the narrow gorges that occur 
 
 FIG. 52. Delta of the Nile. 
 
 at intervals. Growing, by the addition of many smaller 
 streams from either side, it becomes more and more like 
 a true river, as it rolls along. This valley part is by far 
 the longest and most important in its course. Here it 
 receives most of its water, and puts on its distinguishing 
 characters as a river. Its tributaries are no longer mere 
 torrents or brooks, but rivers, sometimes as large as 
 itself. As these increase in size, however, they become 
 fewer in number, so that in the last stages of its journey, 
 the main stream receives few or no affluents.
 
 xxv.] BROOKS AND RIVERS. 247 
 
 6. When at last, quitting the valley which has con- 
 ducted it through the hills and the lower undulating 
 country, the river reaches the low plains towards the sea, 
 it enters upon the final section of its course, that of the 
 delta. Hitherto it has always been receiving, but never 
 giving off branches. But now, reaching the low level 
 land of the delta, it begins to divide, sending off many 
 arms which wind to and fro, and again divide among the 
 swampy flats. So much does the river ramify in this 
 part of its course, that it may enter the sea by many 
 channels, or mouths, often so nearly of the same size, 
 that it may be hard to say which of them should be 
 called the chief. Moreover, these intricate channels are 
 constantly shifting their position as the river- water moves 
 seaward. Hence, what was once the chief mouth is now 
 perhaps half-filled up, and the principal discharge takes 
 place by another channel at some distant part of the 
 delta. 
 
 7. It will be noticed on a map, that except where 
 flowing along some straight valley among mountains, most 
 rivers do not run in a straight line for more than a short 
 distance. This is the case in every part of their course. 
 The channel is continually winding from side to side. 
 On a small scale, the same arrangement of curving water- 
 lines may be observed when the rain, during a heavy 
 shower, is running down a sloping piece of roadway. 
 At the upper part, the runnels, though always rushing 
 downwards, cannot, owing to the unevenness of the 
 ground, descend in straight lines, but are deflected, now 
 to one side, now to another, by little pebbles or bits of 
 clay or other roughnesses on the road. Where straight 
 ruts have been made for them by cart wheels, they take 
 advantage of these, and flow then in straight courses, as 
 a river does in a longitudinal valley. But they escape 
 before long, and resume their winding way as before, 
 joining each other in the descent, and swelling the main 
 runnel which sweeps down the road, and eventually finds
 
 248 PHYSICAL GEOGRAPHY. [LESS. 
 
 its tvay into some neighbouring ditch. What the peb- 
 bles, ruts, and other roughnesses on the road are to 
 rain, the uneven surface of the country is to brooks and 
 rivers. In either case, the water seeks the readiest 
 path of escape to lower levels. But this path is seldom 
 the shortest. Every obstacle which the water cannot 
 surmount or remove, serves to turn it aside, and, as such 
 obstacles abound, the flow of the water is a continuous 
 series of turnings and windings. Ridges and hollows, 
 heights and valleys, turn the streams, now to one side, 
 now to another, until, after a journey which may be many 
 times longer than the direct distance from their source, 
 the waters find at last a rest in the great sea. The ser- 
 pentine curves or meanderings of running water form 
 one of its most characteristic features. They are ex- 
 hibited by tiny brooks flowing in shallow channels across 
 level meadows, and by mighty rivers that wind in deep 
 gorges over vast table-lands. The curves of a portion 
 of one of the large rivers of the globe are represented 
 in Fig. 53 ; but a closely similar drawing might be 
 made from the course of many a little runnel or beck. 
 
 8. Amidst its bendings, a river will here and there 
 eventually cut through the narrow part of a loop, and 
 thus shorten and straighten its channel. The loop being 
 gradually shut off from the river by an accumulation of 
 sand and mud, becomes a crescent-shaped pool or la- 
 goon of stagnant water. Instances of this kind occur 
 commonly along the courses of streams which flow through 
 flat land, as in the case of the Mississippi (Fig. 53). 
 In the delta of the Rhone they are called " Aigues 
 Mortes," or dead waters. 
 
 9. In consequence of the frequent shifting of the 
 direction of a river-course, what is at one place the east 
 side, may be successively the north, south, and west sides 
 in the space of a few miles. It would therefore lead to 
 much confusion were we to speak of the east or west side, 
 or the north or south bank of a river. Accordingly, it
 
 XXV.] 
 
 BROOKS AND RIVERS. 
 
 249 
 
 is usual to call one side the right bank, and the other 
 the left bank, the observer being supposed to be 
 looking down the river in the direction of its flow, so that 
 no matter how much the river may wind, the terms 
 right and left are always correctly descriptive. 
 
 FIG. 53. Wind 
 
 Mississippi. The shaded part marks the 
 alluvial plain. 
 
 1O. As may be readily understood from a map, each 
 large river is the natural drain for a wide region. For 
 instance, all the surplus rain and the discharge from 
 melted snow and springs over by far the largest part of 
 North America, find an outlet to the sea by a single 
 river the Mississippi. The space drained by this river
 
 250 PHYSICAL GEOGRAPHY. [LESS. 
 
 is computed to be 1,244,000 square miles. This is 
 termed the drainage-basin or catchment-basin of 
 the river. The drainage-basin of the Ganges is estimated 
 at 432,480 square miles ; that of the Rhine at 75,000 ; 
 that of the Severn 8580 ; of the Thames at 6960 ; of the 
 Shannon at 6946 ; and of the Tay at 2250 square miles. 
 
 11. A pencil line traced on the map of North America, 
 round the sources of all the streams which are tributary 
 to the Mississippi, will represent what is called the 
 water-shed, 1 water-parting', or divide of the Mis- 
 sissippi basin. To the north of it lie the basins of the 
 Mackenzie and of the rivers that drain into Hudson's 
 Bay, to the east the St. Lawrence and the smaller rivers 
 of the Eastern States, while to the west the Fraser, 
 Columbia, Sacramento, Colorada, and many lesser rivers 
 carry the drainage of the Rocky Mountain slopes into 
 the Pacific Ocean. 
 
 12. The water-shed of a country or continent is thus 
 a line which divides the flow of the brooks and rivers on 
 two opposite slopes. On many maps it is marked as if 
 it were a ridge or mountain-chain. But in reality it does 
 not necessarily coincide with the highest ground. Trace 
 on the map of Europe, for example, a pencil line between 
 the streams which drain to the Atlantic, Baltic, and 
 North Sea on the one side, and those which drain to the 
 Mediterranean, Black, and Caspian Seas on the other. 
 Such a line will mark the general water-shed or divide of 
 the continent, but you will observe that the line, instead 
 of running along, and coinciding with, any central moun- 
 tain-chain, crosses all the great mountains, table-lands, 
 and plains. Beginning at Gibraltar, it traverses the table- 
 
 1 Sir John Herschel proposed to write this word water-scJud, meaning 
 " separation of the waters, not water-iAfrf, the slope down which the waters 
 run " (Physical Geography, p. 120). But the original meaning of "shed" 
 was to dividt*, or part, and this use of the word still holds in Scotland, where 
 a girl is said to shed her hair in the middle. The idea of disjunction is a 
 secondary one, which has gradually come to be the common usage of the 
 word.
 
 xxv.J BROOKS AND RIVERS. 251 
 
 land of the Spanish Peninsula, crosses obliquely the chain 
 of the Pyrenees, passes athwart the plateau of Central 
 France, on the right bank of the Rhone, runs through and 
 across the ranges of the Alps, the Black Forest, and the 
 Carpathians, and then descends into the vast plains of 
 Russia, across which it winds in a general north-easterly 
 course to the chain of the Urals. That the water-shed 
 need not be a high ridge, may often be noticed even on 
 low or gently undulating ground, where the same valley 
 may have a stream at either end, flowing in opposite 
 directions, the water- shed being a quite imperceptible 
 rise of the surface between them. 
 
 13. Some important lessons as to the form of a country 
 may be learnt by noting the line of its water-shed upon a 
 map. That line seldom runs down the centre of a region 
 like the ridge on the roof of a house. Very commonly 
 it lies much to one side, and winds in great curves as it 
 traverses the land. Now the position of the water-shed, 
 like the axis described in Lesson XIX. Art. 25, suffices 
 to indicate the relative slopes of the two sides of a 
 continent or country, especially where these two sides 
 descend to the sea-level. Take South America by way 
 of illustration. The water-shed of the whole continent 
 lies near the western coast-line. The slope facing the 
 Pacific must thus be very much steeper than that which 
 looks towards the Atlantic. A heavy shower of rain 
 falling on the water-shed will of course run off partly to 
 the west and partly to the east. The westward portion, 
 starting from a height of perhaps 1 0,000 feet, will reach 
 the Pacific after a journey of not more than seventy miles 
 in direct distance ; while the other half, setting out from 
 the same elevation, will have a journey of about 2000 
 miles in a straight line before it can enter the Atlantic. 
 The water-shed of Hindostan, south of the Gulf of Cam- 
 bay, is another example of this one-sided position. On 
 a smaller scale, Scandinavia, Great Britain, and Spain 
 illustrate the same feature.
 
 252 PHYSICAL GEOGRAPHY. [LESS. 
 
 14. When a water-shed runs close to one edge of a 
 continent, there is no room for large rivers on that side ; 
 these must flow on the opposite slope. America again 
 furnishes an admirable instance of this obvious arrange- 
 ment. In South America, for example, there is no river 
 of large size flowing down the short slope into the Pacific, 
 but on the east side, the largest rivers of the world bear 
 the drainage of the continent into the Atlantic. 
 
 15. Sources of Rivers. Every shower of rain that 
 falls, and every spring that rises, within the drainage 
 basin of a river, may be regarded as one of the sources 
 of the river. In many parts of the world indeed, such, 
 for example, as the central and southern regions of India, 
 where there are dry and rainy seasons, and where the 
 rivers do not take their rise in high snowy mountainous 
 ground, the water that floods the streams during the wet 
 months is mostly derived from the rain which runs off 
 the saturated soil. In common language, however, the 
 source of a river is understood to mean the point from 
 which the head-waters of the main branch of the river 
 take their rise. It is often hard to say which of the 
 branches of a large river should be called the chief. 
 One may be largest in volume of water, another greatest 
 in length of course. One of the branches has usually 
 been selected by the people of the country, and called by 
 them the main stream. 
 
 16. Large rivers rise from various sources springs, 
 rains, melted snows, or the ends of glaciers (Lesson 
 XXVI 1 1.) and lakes. A great proportion of them may 
 be traced up till their farthest little tributary brook is 
 found gushing out as a spring from the side of some hill 
 or mountain. In limestone countries, as stated in the 
 foregoing Lesson, large rivers sometimes issue from the 
 caverns by which the underground rocks are there per- 
 forated. Occasional or periodical rain directly supplies 
 much of the water of many rivers. The Nile, for example, 
 owes its annual rise to the heavy rainfall of the wet
 
 xxv.] BROOKS AND RIVERS. 253 
 
 season among the mountains of Abyssinia. The snow- 
 fields of the higher mountains furnish unfailing nourish- 
 ment to many of the largest rivers of the globe. Thus 
 in Europe, the Rhine and the Rhone take their rise 
 from the melted snow and ice of the Alps. In Asia, the 
 rivers of Northern India descend from the snows and 
 glaciers of the giant chain of the Himalaya. In North 
 America, the abundant patches of snow which mark the 
 higher parts of the Rocky Mountains supply part of the 
 water which drains from the west into the great valley 
 of the Mississippi. Here and there, among the higher 
 summits of the land, little hollows arrest the first runnels 
 of melted snow or of springs, and form little lakes, out 
 of which the infant waters of important rivers flow. Or 
 a lake on lower ground at the confluence of several tribu- 
 taries fills a vast hollow of the land, whence the united 
 drainage escapes by one large river, like the Rhine from 
 the Lake of Constance, or the Rhone from the Lake of 
 Geneva. 
 
 17. Proportion of the Rain-fall carried to sea 
 by Rivers. Since the size of rivers depends upon the 
 amount of rain -fall or snow-fall within their drainage- 
 basins, we may naturally inquire how much of the total 
 moisture discharged from the air upon the land is actu- 
 ally returned to the sea by rivers. The proportion be- 
 tween rain-fall and river-discharge has never been very 
 satisfactorily determined, but is said to vary from ^ to ^ ; 
 that is to say, only about a third or a fourth part of the 
 water which falls upon the land as rain or snow is carried 
 off by streams. The greater portion is returned to 
 the air again by evaporation. From the moistened soil, 
 from every surface of snow or water, from each spring, 
 brook, lake, and river, vapour is continually passing 
 into the air. The rivers, therefore, do not bear to the 
 sea even all the water poured into them, for they are 
 continually losing water by evaporation from their 
 surface.
 
 254 PHYSICAL GEOGRAPHY [LESS. 
 
 18. The diminution of rivers, the drying up of brooks, 
 and the cessation of springs, during seasons of drought, 
 show how dependent is the flow of water over the land 
 upon its circulation through the air. In countries like 
 Britain, where heavy rains may occur at any time of the 
 year, the rivers are subject to irregular increase. In 
 those regions, however, where a wet and dry season suc- 
 ceed each other at certain intervals, the rivers have their 
 periodical rise and fall. The most familiar example of 
 this regularity is that shown by that historical river the 
 Nile. Egypt, through which this river flows, is a singu- 
 larly dry country, wherein rain seldom falls, and yet every 
 year, and with such regularity that almost the very day 
 of the change may be foretold, the river begins to rise, 
 and continues to do so until the low plains on either side 
 are overflowed. It then slowly subsides, leaving a film 
 of fine mud over the ground, and resumes its former 
 channel. This remarkable feature greatly puzzled the 
 ancients. They accounted for it by supposing that the 
 Nile rose among snowy mountains far to the south, and 
 that the inundations were caused by the melting of the 
 snows. But in recent years the true cause has been 
 ascertained. The high and rocky table-land of Abyssinia 
 is visited by heavy rains during the months of March 
 and April. The numerous gullies and gorges which 
 intersect that rugged country, and which were previously 
 quite dry, are then filled with torrents of water, which 
 rush down and swell that branch of the great river known 
 as the Blue Nile. It is these annual rains in the far 
 highlands of Abyssinia, therefore, which cause the regular 
 inundation of the rainless plains of Egypt. 
 
 19. In countries liable to heavy periodical rains, the 
 relation between the flow of rivers and the rain-fall is 
 made strikingly clear. The Mahanadi River in Central 
 India, for instance, has its basin within the area to which 
 the south-west monsoon brings the copious rains alluded 
 to in Lesson X. Falling upon high rocky ground, the
 
 xxv.] BROOKS AND RIVERS. 255 
 
 rain at once rushes down by innumerable channels to 
 swell the main river, which, unable to find in its channel 
 room for all this water, overflows the surrounding country, 
 producing great havoc in its course. But in the dry 
 season, from February to May, the river is low, because, 
 as little or no rain then falls, it depends for its water 
 upon the supplies which it receives from springs. 
 
 20. Another illustration of the relation between rain- 
 fall and rivers is supplied by those singular dry ravines 
 and gravel-tracks, so common in Syria, Arabia, and the 
 Valley of the Euphrates, called " wadys." Those which 
 lie within tracts where there is a rainy season, are turned 
 into water-courses during the wet part of the year. But 
 over a great part of the region where they occur, little or 
 no rain falls, so that they remain constantly dry. That 
 they were once the channels of brooks and rivers cannot 
 be doubted. It is apparent that a change has taken 
 place in the climate of these districts, perhaps partly 
 caused or at least aggravated by the destruction of the 
 ancient woods and forests, and the abandonment of the 
 cultivation of the soil. The rains that once refreshed 
 the land have ceased to fall, the river-beds which carried 
 the surplus water to the sea are now dry, and the valleys 
 are parched and barren. 
 
 21. In a country which is subject to heavy rains at 
 different and uncertain times of the year, the rivers are 
 liable to be swollen on any day ; and as heavy rain often 
 succeeds dry weather, the rivers may pass rapidly from 
 a state of low water to full flood. Where, as in the case 
 of the Nile, the rain comes regularly at the same season, 
 the river swells and falls again with remarkable slowness 
 and uniformity. But there is another cause of the regular, 
 as well as irregular, flooding of rivers. Where a river 
 draws its supplies of water in great part from the melting 
 of snow among the mountains, it will evidently have a 
 larger volume in summer and autumn than in winter and 
 spring. The Rhine and Rhone, for example, which take
 
 256 PHYSICAL GEOGRAPHY. [LESS. 
 
 their rise among the snows and glaciers of the Alps, fill 
 their channels with water during the dry hot weather of 
 July and August, and shrink in size during the cold and 
 often wet months of the year. Besides this annual in- 
 crease and diminution, the upper course and higher or 
 mountain tributaries of such snow-fed rivers are liable to 
 occasional and sometimes disastrous floods in summer, 
 not caused by heavy rains, but by dry and warm weather. 
 When the warm south wind called Fbhn blows upon the 
 snowy slopes of the Alps, it causes a rapid thaw. Tor- 
 rents of milky snow-water pour down the sides of the 
 mountains and feed the brooks, these soon swell and rush 
 forward into the rivers, which rapidly rise and sweep 
 down their valleys, often carrying away bridges, inundat- 
 ing meadows and strewing them over with gravel, or even 
 breaking down parts of villages and towns built upon 
 their banks. 
 
 22. Flow of Rivers. Standing by the side of a 
 broad rushing river, trying to estimate the rate at which 
 the water is moving, and how much passes us in an hour 
 or day, we should probably, in most cases, guess the rate 
 of motion to be faster than it is. The broad stream, 
 with its eddies and gurglings of water, impresses us with 
 the idea of rapidity as well as of volume. The rate of 
 flow is determined mainly by the angle of slope, partly 
 also by the volume of water, and the form of the chan- 
 nel. The average angle of slope of river- channels is 
 considerably less than we might have expected it to be, 
 that of the larger rivers of the continents probably not 
 exceeding two feet in the mile. The Missouri has a 
 mean fall of 28 inches in the mile ; but the Volga does 
 not slope more than 3 inches in the mile. To be easily 
 navigable, a river should not have a mean declivity of 
 more than 10 inches in the mile or i in 6336. Of course, 
 in their mountain-track, streams have much higher angles 
 of descent. That of the Arve at Chamounix is r in 6 1 6, 
 and that of the Durance varies from i in 467 to I in
 
 xxv.] BROOKS AND RIVERS. 257 
 
 208. But such rapid declivities are those of mountain 
 torrents rather than of rivers. 
 
 23. A moderate rate of the flow of a river is about 
 I i mile in the hour ; that of a rapid torrent does not 
 exceed 18 or 20 miles in the hour. The larger rivers of 
 Britain have a velocity varying from about i mile to about 
 3 miles in the hour. Hence we may walk by the margin 
 of a river and easily outstrip the rate of its current. 
 
 24. But the water of a river does not flow in every 
 part of the channel at the same rate. Owing to friction 
 against its bed, the river moves slower at the sides and 
 bottom than in the middle. On a small stream, we can 
 easily prove this by throwing in pieces of wood, and 
 watching how much faster those travel which fall in the 
 middle, than those which have lighted near the edge. 
 Evidently, therefore, the addition of more water to the 
 same channel will increase the rate of motion of the 
 stream. When two rivers join into one, the united stream 
 may occupy a channel no broader than one of them did 
 before, but, without acquiring any increased slope, it 
 runs faster, because the water has now to overcome the 
 friction of only one channel instead of two. For the 
 same reason, a river confined in a deep narrow gorge 
 runs faster than where, with a like declivity, it spreads 
 over a broad gravelly bed. When this cause is kept in 
 mind, we can understand the meaning of the curious fact 
 that a river is sometimes not increased in breadth, even 
 by the influx of large tributaries. Thus the Mississippi 
 becomes no wider, even after it receives its largest 
 affluents. Eighteen hundred miles above its mouth it is 
 5000 feet, or nearly a mile, broad. But at New Orleans, 
 where it enters the Gulf after having been successively 
 fed by the vast streams of the Missouri, Ohio, Arkansas, 
 and Red Rivers, it is only 2470 feet broad, or less than 
 half a mile, and yet in that comparatively narrow channel 
 the drainage of nearly half a continent passes out to 
 the sea. 
 
 s
 
 258 PHYSICAL GEOGRAPHY. [LESS. 
 
 25. Volume of Water discharged by Rivers. 
 Measurements and estimates have been made of the 
 amount of water carried annually into the sea, or hourly 
 past certain places by different rivers. The Mississippi, 
 for example, has been found, after a careful survey of its 
 operations, to discharge annually into the Gulf of Mexico 
 no less than 21,300,000,000,000 cubic feet of water, 
 enough to make a lake as large as the whole of England 
 and Wales and twelve feet deep. The Danube sends 
 into the Black Sea 207,000 cubic feet of water every 
 second. The Tay, though not the largest river in the 
 British Islands, brings down more water into the sea 
 than any other, its annual discharge being estimated at 
 144,020,000,000 cubic feet. 
 
 LESSON XXVI. The Waters of the Land Part III. 
 Lakes and Inland Seas. 
 
 1. Owing to inequalities of the surface, the water that 
 falls upon the land cannot always flow off at once into 
 the sea. Hollows occur which intercept the drainage, 
 until it fills them and flows over from the lowest parts of 
 their margins, or otherwise escapes. Such water-filled 
 basins are called lakes, but when they are of large size 
 and are occupied by salt water, they receive the name of 
 inland seas. 
 
 Lakes. 
 
 2. At first lakes might be expected to lie indifferently 
 on any part of the earth's surface. Any good general 
 map of the world, however, shows that they are not dis- 
 tributed altogether at random, but are chiefly developed 
 in certain regions. An attentive study of this develop- 
 ment throws some light upon the problem of the origin
 
 XXVI.] 
 
 LAKES AND INLAND SEAS. 
 
 259 
 
 of the hollows, and why they should be where we now 
 find them. 
 
 3. (i) First, then, in the northern hemisphere, lakes 
 may be observed to be scattered in extraordinary abun- 
 dance over the northern parts of Europe, down to about 
 
 FIG. 54. Part of the Island of Lewis, illustrating the abundant lakes 
 of the north-west of Europe. 
 
 the 52d parallel of latitude, and in America to about the 
 42d parallel. In some districts within these regions, 
 there seems to be almost as much water as land. In 
 Finland, for example, nearly a third of the country is 
 covered with lakes and marshes, and as there are no
 
 260 PHYSICAL GEOGRAPHY. [LESS. 
 
 mountain-ridges, and no dominant lines of valley, the 
 undulating surface looks like that of a land which has 
 been half submerged. In the north-west of Scotland 
 similar scenery occurs ; from some of the hill-tops there, 
 the undulating surface of the low country is seen to be 
 plentifully strewn with lakes. In the accompanying 
 figure (Fig. 54) a representation, on the scale of one inch 
 to a mile, is given of a portion of the surface of Lewis, 
 one of the islands of the Outer Hebrides, from which 
 some idea may be formed of the abundance and irregular 
 distribution of these sheets of water. Again in North 
 America, the British possessions and a large part of the 
 
 FIG. 55. Section of a lake-basin excavated in solid rock. 
 
 north-eastern States are plentifully dotted over with lakes, 
 varying in size from such vast basins as that of Lake 
 Superior down to mere pools or tarns. 
 
 4. In these northern regions, as the lakes are not con- 
 fined to valleys, but lie indiscriminately over the surface, 
 they may be found at any height, from the sea-level to 
 near the crest of the land, which is generally undulating 
 and hilly rather than mountainous. The abundance of 
 these lakes has been associated by many geologists with 
 the fact that the land in the northern hemisphere has 
 been covered with sheets of ice which, grinding down 
 the surface, have left it strewn with clay, gravel, and 
 sand (Lesson XXVIII.). Many of the lakes lie in hollows 
 excavated out of the solid rock (Fig. 55), which still 
 retains abundantly the scratches and groovings made
 
 XXVI.] 
 
 LAKES AND INLAND SEAS. 
 
 261 
 
 upon it by the movements of the ice. Other sheets of 
 water are enclosed among heaps of debris, left on the land 
 when the ice melted (Fig. 56). 
 
 
 FIG. 56. Section of a lake-basin lyi 
 
 5. (2) A further feature in the distribution of lakes, 
 which may be observed on maps, is the more or less abun- 
 dant occurrence of these sheets of water among mountains. 
 Take Europe as an illustration. Even in the comparatively 
 low mountain groups of Scotland, Cumberland, and Wales, 
 lakes abound, forming one of the great charms of the 
 well-known scenery of these districts. Among the Alps a 
 series of large lakes occurs on each side of the main axis 
 of the chain, and innumerable minor sheets of water 
 
 FIG. 57. Section of a lake dammed up by a barrier of earth or gravel. 
 
 occur scattered at all heights among the central moun- 
 tains, up even to the borders of the snow -line. All 
 mountain systems, indeed, have not the same abundance 
 of water-filled hollows ; in some there are few or none. 
 Lakes among mountains may be, in some cases, hollows 
 formed during the elevation of the mountains (Lesson 
 XXIX.) ; in other examples, like those referred to in 
 Art. 4, they may either have had their basins scooped 
 out by glaciers or formed by the irregular piling up of
 
 262 PHYSICAL GEOGRAPHY. [LESS. 
 
 ice-borne debris (Fig. 57). In most volcanic districts, 
 lakes occur in cavities which have been formerly blown 
 open by explosions from below. 
 
 6. (3) A third series of lakes may be observed to 
 occupy depressions on table-lands. The most remarkable 
 examples are furnished by the great lakes of Equatorial 
 Africa. Of these the Victoria Nyanza lies at a height of 
 about 3300 feet above the sea, and is believed to cover 
 an area of not less than 30,000 square miles. On the 
 same continent a vast depression, with many small lakes, 
 extends westward from the Nile valley, and stretches 
 between the southern limits of Barbary and the country 
 drained by the River Niger. Another smaller hollow lies 
 in the southern part of the continent, and contains some 
 small lakes, of which Lake Ngami (2900 feet above the 
 sea) is the largest. On the great table-land of Asia 
 numerous lakes occur over Thibet, Turkestan, and 
 Mongolia. 
 
 7. It will be noticed that many of these table-land 
 lakes have no outlet, but lie in hollows below the general 
 level of the surrounding country. They receive supplies 
 of water from the streams which drain into them, but 
 no river escapes even from the lowest parts of their 
 borders. Now almost all lakes of this kind are filled 
 with salt water. That they should be salt, will be found 
 on reflection to be a necessary consequence of the 
 solvent action of running water on the land (Lesson 
 XXIV. Arts. 22-36). Water flowing over or through 
 rocks dissolves out of them some of their soluble in- 
 gredients. Among the substances so removed, common 
 salt, sulphate and carbonate of lime, and sulphate of 
 magnesia, are of common occurrence. These dissolved 
 salts are carried down by rivers, and in most cases find 
 their way ultimately into the sea. But in the depressions 
 of the great table-lands, the hollows into which the water 
 drains and finds no outlet may be compared to great 
 evaporating vats or troughs, like those in which sea-water
 
 xxvi.] LAKES AND INLAND SEAS. 263 
 
 is boiled down in the manufacture of salt. Water is con- 
 stantly poured into them and none flows out ; yet their 
 level is not rising, because, as fast as the water enters, it 
 passes off again in invisible vapour into the atmosphere. 
 The supply of water and atmospheric evaporation just 
 balance each other, and any cause that diminishes the 
 one or the other would make the level of the lakes rise or 
 fall. But the vapour which rises from the inland basins 
 leaves behind in the water the various saline solutions 
 carried into them by rivers. Hence year by year these 
 lakes and pools become salter. When they dry up, they 
 leave a crust of salt upon the ground which they once 
 covered. The soil is in such places so impregnated with 
 salt that plants will not grow upon it, and its arid sandy 
 surface stretches for leagues as an inhospitable desert. 
 
 8. Where a fresh-water lake does occur in these 
 regions it will be found to have some outlet by which its 
 surplus water is removed, so as to prevent increase of 
 saltness. Lake Chad in Central Africa, for example, 
 lies in a hollow from which no river escapes to the sea, 
 and it was believed in consequence that this fresh-water 
 lake had no outflow, and was thus an exception to the 
 general rule. More recently, however, a river has been 
 found opening from its north-eastern margin and carry- 
 ing the overflow along a wide valley, in which the water 
 is finally dried up amid sandy wastes. 
 
 9. In the eastern countries bordering the Mediter- 
 ranean many salt-lakes occur, as well as ground incrusted 
 or impregnated with salt. The naturally-formed salt has 
 been used from time immemorial by the dwellers there. 
 A little reflection on the mode of its formation and its 
 composition explains the meaning of a curious passage 
 in the Bible which is not in itself very intelligible. It 
 occurs in the Sermon on the Mount. " If the salt have 
 lost his savour, wherewith shall it be salted ? it is thence- 
 forth good for nothing, but to be cast out, and to be 
 trodden under foot of men." (St. Matthew v. 13.)
 
 264 PHYSICAL GEOGRAPHY. [LESS. 
 
 Were the substance here spoken of our common salt 
 (chloride of sodium), it would be difficult to explain how it 
 could possibly lose its taste without ceasing to exist at all. 
 Its taste is to us as essential a character as its chemical 
 constitution. But no doubt the substance referred to was 
 the white incrustation obtained in the East from the sides 
 of salt lakes and the bottoms of dried-up saline pools. 
 Now this incrustation, besides common salt, contains also 
 carbonates or sulphates of magnesia, lime, soda, and other 
 ingredients. Of these various components, common salt 
 is one of the most soluble (Lesson XIII. Art 5), that is 
 to say, it is among the last ingredients to appear when 
 the water is evaporated, and among the first to disappear 
 when moisture is supplied again. We can see, therefore, 
 that the white natural crust used by the people of Syria 
 for salt, being kept for a time and exposed to damp or 
 rain, might lose all its salt. The more insoluble residue, 
 consisting of gypsum, carbonate of lime, etc., though in 
 appearance unchanged, yet having little or no taste, 
 would be quite useless for the purposes for which the 
 salt had been gathered. The question, therefore, might 
 well be asked " If the salt have lost his savour (or 
 taste), wherewith shall it be salted ? " 
 
 10. (4) A fourth series of lakes occurs along many 
 parts of the margin of the land where the ground is low, 
 and consists of soft sandy, clayey, or gravelly materials. 
 These maritime sheets of water are known by the name 
 of lagoons. In Europe they fringe all the Prussian 
 shores of the Baltic, reappear on the west of Denmark, 
 Holland, and Belgium, and are found at intervals along 
 the northern coasts of the Mediterranean Sea, from the 
 east of Spain to the western shores of Greece. In Asia 
 they extend for hundreds of miles along the eastern and 
 western sides of the peninsula of Hindostan. In America 
 a long line of them skirts the Atlantic sea-board of the 
 United States. 
 
 11. Lagoons along the sea-margin are, for the most
 
 xxvi.] LAKES AND INLAND SEAS. 265 
 
 part, shallow and narrow, running parallel with the 
 coast, from which they are separated by a strip of low land 
 formed of sand, gravel, or other loose materials. When 
 the sea flows into them, the waters are salt or brackish. 
 When they communicate with the sea only by a narrow 
 outlet, or when they have no outflow, but soak through 
 a porous bar which banks them out from the sea, they 
 are fresh. 
 
 12. Most lakes derive their water from streams that 
 flow into them. In Fig. 54, for example, this connection 
 is well shown, though there the feeders of the smaller 
 lakes are too minute to be shown upon the map. Many 
 valleys contain chains of lakes along their course. A 
 river flowing in one of these valleys, appears alternately 
 to contract and expand, having a comparatively rapid 
 flow where it takes its own river-form, and losing itself 
 in still water when it enters a lake. It would seem that 
 this arrangement was formerly commoner than now, for 
 in many mountainous districts the rivers wind to and 
 fro across flat meadows where no doubt lakes once lay. 
 The meadows are, in fact, the alluvial plains formed by 
 the gradual filling up of the lakes, owing to the deposit 
 of sediment brought down by the inflowing streams 
 (Lesson XXVII. Art. 30). 
 
 13. But lakes likewise derive their supplies partly 
 from springs which rise beneath them. In some cases, 
 indeed, the whole supply comes from such underground 
 sources. The Lake Zirknitz near Trieste (Lesson XXIV. 
 Art. 42) affords an excellent illustration of this feature, 
 for it comes and goes with the seasons. After long 
 drought it disappears. When heavy rains fall on the 
 surrounding mountains of Carniola, which are formed of 
 remarkably honeycombed limestones, the water at first 
 sinks out of sight, but after filling the underground pas- 
 sages, it comes out with a roaring noise from the fun- 
 nels and caverns which open upward into the hollow 
 of Zirknitz, which is then converted into a lake. No
 
 266 PHYSICAL GEOGRAPHY. [LESS. 
 
 stream flows out of it, since both its supply and its 
 overflow pass away by underground channels. This 
 example shows how lakes, like rivers, depend ultimately 
 upon the rainfall for their water, and are apt to vary in 
 level with the wetness or dryness of the seasons. In 
 Northern Africa also, the Sebka-el-Faroon, a hollow loo 
 miles in length, lying to the South of Tunis, at a level 
 of several feet below the Mediterranean, is in winter 
 covered with water to the depth of two or three feet. 
 Having no outlet, and undergoing rapid evaporation 
 during the parching summer of that hot climate, the 
 lake disappears and leaves a salt-crusted floor. 
 
 14. Some of the largest fresh -water lakes in the 
 world are those of North America, Lake Superior alone 
 covering an area of 23,000 square miles, its surface 
 being 627 feet above the level of the Atlantic, and 
 its average depth nearly 1000 feet. Another series of 
 vast sheets of fresh-water, already (Art. 6) referred to 
 as lying on the tableland of equatorial Africa, forms the 
 source whence the Nile and the Congo take their rise. 
 In the heart of Asia, Lake Baikal stands at a height of 
 1363 feet above the sea, and covers a space 370 miles 
 in length by from 20 to 70 in breadth. On these vast 
 sheets of water storms arise on a scale hardly inferior 
 to those on the sea itself. Fresh-water, being lighter, is 
 more easily stirred by the wind than salt-water. It is 
 soon raised into ripples, and when deep and wide, and 
 driven onward under the pressure of a continuous high 
 gale, it rises into large waves, which roll across and 
 burst in foam against the windward shore, heaping up 
 gravel and sand, or cutting down the cliffs, as is else- 
 where done by the waves of the sea (Lesson XVIII. 
 Art. n). 
 
 15. The depth of lakes varies almost within as wide 
 and indefinite limits as their size. The form of the land 
 round a lake generally affords some indication of the 
 probable depth of the basin, as the depth of the sea
 
 xxvi.] LAKES AND INLAND SEAS. 267 
 
 near the land may be inferred from the contours of the 
 shores. Thus, if the ground is low and slopes gently 
 into the water, we may infer with some confidence that 
 the lake must be shallow. If, on the other hand, the 
 ground rises steeply out of the water, as when a lake 
 fills a valley between two precipitous mountain-ridges, 
 we may be prepared to find that the lake is deep. 
 Several of the Alpine lakes attain a great depth. Thus 
 Lake Como is nearly 2000 feet deep, and the Lago Maggi- 
 ore 2800 feet. In both of these lakes the bottom sinks 
 below the level of the sea, that of the Lago Maggiore 
 being 2149 and that of Como 1318 feet below the sur- 
 face of the Mediterranean. But they all have outlets, 
 and their waters are fresh. In Scotland, Loch Ness 
 forms a remarkable feature in the great valley which 
 cuts the Highlands in two, its surface being about 70 
 feet above the sea, and its greatest depth 810 feet. 
 The bottom is therefore not only below the sea-level, 
 but actually a good deal deeper than any part of the 
 North Sea between Scotland and Denmark. 
 
 16. Little has yet been done in exploring the depths 
 of these profound hollows on the land. Some of them, 
 like Loch Ness, were at a comparatively recent period 
 filled by the sea, and though probably all trace of the 
 salt-water has long since been removed, perhaps some 
 lingering forms of life may have survived the change, 
 and, though possibly somewhat modified by the changed 
 conditions in which they have lived, may exist still in 
 the undisturbed abysses, somewhat in the same way that 
 certain common shore-plants, such as the coast-plantain 
 {Plantago marittma), are met with on the tops of the 
 Scottish mountains. 
 
 17. It would be interesting also to know the distribu- 
 tion of temperature in the water of these deep lakes. 
 According to observations made in Loch Lomond, in 
 Scotland, the depth of which is about 600 feet and its 
 surface 25 feet above the sea, it appears that a tolerably
 
 268 PHYSICAL GEOGRAPHY. [LESS. 
 
 constant temperature of about 42 Fahr. characterises 
 the lowest stratum of water for 100 feet or so above 
 the bottom. The cold water of winter must sink down, 
 and as the sun's rays can sensibly heat the water only 
 for a short way beneath the surface, the temperature of 
 the deeper parts of such lakes must be kept permanently 
 low. In the Lake of Geneva in autumn, while the sur- 
 face water shows a temperature of 78 Fahr., the 
 bottom water, at a depth of 950 feet, marks 41 '7 . 
 Similar observations on the other deep lakes of Switzer- 
 land and Northern Italy show that they all have a per- 
 manent mass of cold water at the bottom. Farther 
 south the Lago Sabatino, near Rome, was found to have 
 a temperature of 77 at the surface, but one of 44 at a 
 depth of 490 feet. 
 
 18. One great and useful office of lakes is to exercise 
 an important equalising influence on temperature, pre- 
 venting the air around them from being so much heated 
 in summer and so much cooled in winter as would other- 
 wise happen. (Lesson XXXI. Art. 20.) The mean 
 annual temperature of the surface water of the Lake of 
 Geneva as it issues into the Rhone is nearly 4 warmer 
 than that of the air. A second and still more important 
 function of lakes is to regulate the flow of the rivers that 
 issue from them. They receive the water discharged by 
 heavy rains and rapidly-melted snows, spread it over a 
 large surface, and allow it gradually to escape by the out- 
 flowing river. In this way they prevent the occurrence of 
 those sudden and destructive floods, which, in the absence 
 of such natural reservoirs, are apt to occur in all countries 
 subject to copious rain, or where large masses of snow 
 may be quickly thawed. A third part played by lakes is 
 that of arresting the gravel, sand, and mud brought down 
 by the streams which flow into them. These materials, 
 which so often discolour the tributary brooks and rivers, 
 fall to the bottom when the currents are checked by 
 entering the still lake-water. Lakes in this way filter
 
 xxvi.] LAKES AND INLAND SEAS. 269 
 
 the rivers. The Rhone, for instance, is a muddy stream 
 where it enters the Lake of Geneva, but at the lower 
 end, where it quits the lake, its water is as pure and 
 limpid as that of a spring. Its sediment has been 
 dropped upon the bottom of the lake, which must conse- 
 quently be slowly rising. The Lake of Geneva is being 
 gradually filled up in the manner already referred to in 
 Art. 12. 
 
 Inland Seas. 
 
 19. From what has been said in Arts. 6-9 the learner 
 will have no difficulty in tracing out upon a map those 
 areas of the earth's surface where the lakes must be salt. 
 In every tract of land into which rivers flow without an 
 outlet, and where the surplus water passes off by evapora- 
 tion, brackish or salt lakes may be looked for. A salt 
 lake need not necessarily have been once connected with 
 the main ocean. So constantly are salts present in fresh- 
 water that any fresh-water lake, where the only escape 
 for the water is by evaporation, will eventually become 
 salt. The salt lakes on the table-lands of Asia and 
 Africa were, no doubt, fresh at first, and have gradually 
 grown salt as the process of evaporation has been con- 
 tinued century after century. In North America, the 
 Great Salt Lake at Utah, lying at a height of 4200 feet 
 above the sea, and covering a space of 7 5 miles in length 
 by from 15 to 40 in breadth, together with many other 
 smaller salt lakes in the same region, afford most inter- 
 esting evidence of the process whereby fresh-water lakes, 
 by change of climate, leading to diminished rainfall and 
 increased evaporation, become saline and even intensely 
 bitter. 
 
 20. But in some parts of the world there exist sheets 
 of salt-water which are either still connected, or can be 
 shown to have been once connected, with the main body 
 of the ocean, from which they have been separated by
 
 270 PHYSICAL GEOGRAPHY. [LESS. 
 
 subterranean movements. Inland seas, now com- 
 pletely isolated, may have their surface below the level 
 of the main ocean, or they may have been carried 
 up together with the land around them, so as to lie 
 above that level. By far the most remarkable of these 
 sheets of water is the chain of inland seas and salt lakes 
 which extends from the Black Sea and Sea of Azov east- 
 ward into the basin of the Caspian and Aral Sea. At 
 one time the Arctic Ocean extended for some way south- 
 ward across what are now the " tundras " or frozen 
 plains of Siberia, while the Black, Caspian, and Aral 
 Seas formed a united mediterranean sea, though it is not 
 ascertained that this sea was once united to the Arctic 
 Ocean. Europe and Asia were thus, in some measure 
 at least, separated from each other by arms of the sea. 
 Owing to underground movements this separation has 
 been in great measure obliterated. But in the deeper 
 cavities of the ancient depression portions of the sea 
 still remain, retaining even yet the marine shells, fishes, 
 and seals which abounded in the water before the 
 elevation of its bed. Of these relics the largest and 
 most important is the Caspian Sea, which lies 84 feet 
 below the level of the Black Sea, is from 2000 to 3000 
 feet deep in the central parts, and covers an area of 
 about 180,000 square miles. It receives the drainage 
 of the whole of the south-east of Russia in Europe 
 by such important rivers as the Volga and the Ural, 
 but it has no outlet. So large is the mass of fresh- 
 water poured into the Caspian, that the saltness of the 
 greater part of that sea is not more than about one-third 
 that of ordinary sea-water (Lesson XIII. Art. 6). But 
 along the shore numerous lagoons occur, where, in the 
 dry and hot weather of summer, so much evaporation 
 goes on that the water becomes intensely bitter and salt, 
 and saline incrustations form at the bottom and on the 
 shores. On the east side of the sea, lies the wide but 
 shallow Karaboghas Bay, which may be looked upon as
 
 xxvi. LAKES AND INLAND SEAS. 271 
 
 a vast evaporating basin. A current is always passing 
 in through the narrow opening, but there is said to be no 
 compensating under -current outwards, so that, as the 
 water-level does not rise, all this constant inflow must be 
 supplying the loss by evaporation. The bay, therefore, 
 grows every year salter. A solid layer of salt forms on 
 the bottom of some of the shallower parts of its shores, 
 and the water is there so saline that a cord on being let 
 down into it and pulled up again is immediately crusted 
 over with salt. Seals, which used to flourish there, are 
 said to have been driven away by the increasing saltness 
 of the water. 
 
 21. The Sea of Aral fills another of the hollows in the 
 same ancient depression between the European and 
 Asiatic high-grounds. It is a lake of brackish water 265 
 miles long and 145 broad. It is said to be at a height 
 of only 33 feet above the level of the sea. On its 
 southern side it receives the Oxus, which carries into it 
 the drainage from the northern slopes of the great chain 
 of the Hindu Kush mountains. It likewise obtains sup- 
 plies of water and mud from the Jaxartes, which takes 
 its rise among the lofty Thian-Shan mountains. Yet the 
 Sea of Aral, like the Caspian, has no river flowing out of 
 it. It loses by evaporation as much water as it receives. 
 Indeed, the loss from this cause would seem at present 
 to be greater than the supply of water, for the sea is said 
 to be sensibly decreasing in size. 
 
 22. The valley of the Dead Sea is remarkable as 
 being the most depressed on any part of the land of the 
 globe. The surface of that sheet of water is 1298 feet 
 below the level of the Mediterranean Sea. The water, 
 so intensely salt as to be a kind of brine, contains in 
 every 100 parts rather more than 24 parts by weight of 
 salts, or about seven times the proportion in ordinary 
 sea-water.
 
 272 PHYSICAL GEOGRAPHY. [LESS. 
 
 LESSON XXVII. The Waters of the Land Part IV. 
 The Work of Running "Water. 
 
 1. In the two previous Lessons we have followed the 
 circulation of running water over the surface of the land 
 in an elaborate network of branching water- courses, 
 which, stretching from the slopes of the central mountains 
 down to the sea-shore, carry back to the ocean the sur- 
 plus drainage of the land. We have seen how in the 
 hollows of the land the water gathers into lakes, yet that 
 it does not accumulate indefinitely there, since it either 
 overflows and again takes the form of streams, or passes 
 off by evaporation. Delivered to the sea, and mingling 
 once more with that great body of water, it is anew raised 
 by the sun's heat into invisible vapour, and carried by 
 winds across the land to begin again the same circulation. 
 
 2. So vast a body of water, ceaselessly moving over 
 the land, slowly, but in the end, extensively, modifies the 
 forms of mountains, hills, valleys, and plains on which 
 it falls and flows. We shall now consider the nature of 
 the change thus effected, beginning with the tiny rain- 
 drop, and tracing the operations of running water down 
 to the mouths of the great rivers as they enter the sea. 
 
 3. Rain. The action of rain in washing the air was 
 described in Lesson X. Art. 35. Its further influence 
 in decomposing rocks underneath the surface of the 
 ground was traced in Lesson XXIV. Similar changes 
 take place upon the surface of the land. Rain-water, by 
 means of the carbonic acid which it takes out of the 
 air and soil, or the organic acids which it absorbs from 
 decomposing vegetable matter, attacks rocks exposed to 
 the air, dissolving and removing the more soluble parts 
 of them, thereby loosening their cohesion and causing 
 them gradually to crumble down. Calcareous rocks, like 
 marble, suffer much from this kind of waste ; but even
 
 xxvii.] THE WORK OF RUNNING WATER. 273 
 
 hard rocks like granite do not escape. The solvent action 
 of the acids upon some of the ingredients of the stone is 
 a chemical process. But when the outer layer or crust 
 of rock has, in this manner, been loosened, heavy rain 
 may wash off the disintegrated particles, and thereby 
 expose a new surface to further decay. The action now 
 becomes mechanical. These two combined processes 
 powerfully influence the scenery of the land (Lesson 
 XXIX.). 
 
 4. The little prints which the rain-drops leave upon 
 a surface of moist clay or sand offer the simplest instance 
 of the mechanical action of rain (Fig. 58). From such 
 apparently trivial effects many stages may be traced, until 
 
 FIG. 58. Prints made in soft mud or moist sand by rain-drops. 
 
 we reach huge pillars, like those shown in Fig. 59, which 
 have been carved out by the blows of innumerable rain- 
 drops. The material of these columns is a stiff earth or 
 clay, stuck full of stones and large blocks of rock, and 
 readily crumbling down under the influence of the weather. 
 The large blocks, remaining of course unwasted, serve 
 each to protect the portion of the earth lying underneath 
 it, while the surrounding clay is washed down. Thus 
 the block becomes, as it were, the capital of a pillar 
 which seems to rise slowly out of the rest of the earthy 
 mass. Each pillar stands as a monument of the con- 
 tinuous waste, somewhat in the same way as the columns 
 of rock or clay, left by the workmen in a railway-cutting 
 or quarry, show the extent of material which has been 
 removed. On reaching the more level ground, the run- 
 nels of rain drop the particles of earth, washed by them 
 T
 
 274 
 
 PHYSICAL GEOGRAPHY. 
 
 [LESS. 
 
 from the slopes and steep faces, and spread them out 
 over the soil. 
 
 5. Here then, in the action of the rain-drops, a sort 
 
 FIG. 59. Earth-pillars of the Tyrol (from a photograph). 
 
 of type may be seen of the work of all the great rivers 
 of the globe. It is threefold. First we have Erosion, 
 in the loosening of the particles of earth or rock.
 
 xxvii.] THE WORK OF RUNNING WATER. 275 
 
 Secondly, Transport, in the removal of these loosened 
 particles, and the consequent exposure of fresh surfaces 
 to waste ; and thirdly, Deposit, in the laying down of a 
 new stratum formed out of the removed materials. Let 
 us now see how these three kinds of action are manifested 
 over the surface of the land by rivers. 
 
 6. River-Erosion. Every runnel, brook, and river, 
 in short every current of water, no matter how small, which 
 moves over the land is busy removing part of the soil or 
 rock over which it flows. This work, like that of the 
 rain-drops, is twofold. In the first place, the rain and 
 the water of the streams, dissolving certain parts of the 
 solid substance of the land, and carrying them away in 
 chemical solution, effect a considerable amount of waste 
 in countries where the rocks consist of limestone, or con- 
 tain a marked proportion of soluble substances, although 
 the running water is not visibly affected in colour or 
 transparency. Some idea of the extent of the loss thus 
 sustained by the surface of the land may be formed from 
 the amount of dissolved mineral matter which is found in 
 the water of rivers (Art. 14). 
 
 7. In the second place, the solvent action of rain and 
 the disintegrating effects of frost (Lesson XXVIII.) cause 
 the surface of exposed rocks to crumble down into loose 
 clay, sand, and angular rubbish, even the hardest rocks 
 being split into fragments. This debris of the land, washed 
 away by the rain and brooks, becomes the instrument of 
 still further destruction. As it is hurried along, its par- 
 ticles, ground against each other, are reduced still further 
 in size, and at the same time are worn smooth and round. 
 They thus acquire that familiar water-worn character 
 which we recognise as the most obvious and distinguish- 
 ing feature of the detritus in the channel of a stream. 
 So constant is this character, that when, at any point in 
 the course of a river, sharp-edged fragments appear on 
 the banks, we naturally conclude that they cannot have 
 lain very long in the current, nor have travelled very far.
 
 276 PHYSICAL GEOGRAPHY. [LESS. 
 
 The fragments, the longer they are rolled about, and the 
 farther they are carried, grow smaller in size, until at the 
 far end of the river they may be found as mere fine sand 
 and mud. The source of this fine sediment must be 
 sought among the rocks of the far distant mountains, in 
 the higher part of the river-basin. In many rivers it 
 may be traced upward through every gradation of sand, 
 gravel, shingle, and boulders, until its origin is found in 
 the huge blocks and abundant angular rubbish loosened 
 from the parent cliffs, which in the course of ages have 
 supplied a constant and abundant tribute of detritus to 
 the river. In some respects a river may be compared 
 to an enormous grinding-mill, where large pieces of stone 
 go in at one end, and only fine sand and mud are seen 
 to emerge at the other. 
 
 8. But the loose materials swept away by the streams 
 not only wear each other down, they likewise erode the 
 sides and bottom of the water-courses. The water-worn 
 character is thus not confined to the loose sand, gravel, 
 and stones, but is as marked upon the solid rocks over 
 which these materials are driven. Even the hardest kinds 
 of stone cannot resist constant friction. They become 
 smooth and polished, though, where out of the scour of the 
 water, they may present a rough surface and sharp edges. 
 The upper limit of the grinding action of the flooded stream 
 is in this way well defined along the sides of a rocky ravine. 
 
 9. In the course of time the stream grinds out a 
 channel for itself through even the hardest rocks. This 
 channel, however, is seldom a mere deep straight trench. 
 Since rocks offer many varying degrees of resistance 
 to erosion, they are worn down unequally, being scooped 
 out where more easily worn away, and left projecting 
 where more durable. The stream, thrown from side to 
 side, dashes along, sweeping onward the sand and mud 
 which it drives over its rocky bed, and excavating those 
 winding picturesque ravines which are such familiar fea- 
 tures in water-courses.
 
 xxvn.] THE WORK OF RUNNING WATER. 
 
 277 
 
 1O. Along the walls of ravines when the water is low, 
 curious round cauldron-shaped cavities with smooth sides 
 may often be observed. These, known as pot-holes (Fig. 
 61), are formed by the grinding action of loose stones, 
 which, caught in eddies of the water, are kept in rapid 
 rotation. Of course the stones, in excavating the holes, 
 are themselves reduced to sand or fine gravel, but other 
 stones are swept in to supply their place. On the sides 
 
 VlG. 60. View of ravines cut by streams out of a table-land. 
 
 of many a narrow gorge through which a stream forces 
 its way, traces of old pot-holes may be seen high above 
 the present water-line. These mark former levels of the 
 stream, and show how in course of time the rocky bed 
 has been gradually dug out. 
 
 11. Another way in which a stream erodes its channel 
 is by means of "waterfalls (Fig. 62). It is not always 
 possible to tell from what cause a particular waterfall 
 may have been formed at first. Some original cliff or 
 steep bank, when the stream began to flow, or some
 
 2 7 8 
 
 PHYSICAL GEOGRAPHY. 
 
 [LESS. 
 
 harder mass of rock, encountered in the erosion of the 
 ravine, may have determined it. But in what way 
 soever it may have begun, a waterfall gradually creeps 
 up stream, carving out a ravine between its original 
 
 FIG. 61. Cascade and pot-holes of a water-course. 
 
 point of commencement and the point which it has 
 now reached. This excavation is done by the recoil of 
 the water and spray, whereby the rocks behind the bot- 
 tom of the fall are loosened and precipitated into the
 
 xxvii.] THE WORK OF RUNNING WATER. 
 
 279 
 
 whirlpool or rapid below. So long as these crumble 
 down more easily than those at the top of the fall, the 
 cliff over which the water dashes will remain precipitous. 
 Slice after slice being cut off its face, the precipice over 
 which the water tumbles will shift its place farther and 
 farther up the stream, carrying with it the waterfall 
 which, though slowly moving up the gorge, may appear 
 stationary, because it will preserve from year to year the 
 same general appearance. If, however, from any differ- 
 
 FIG. 62. Section of a waterfall and ravine. 
 
 ence in the nature or position of the rocks, the rate of 
 waste should become more rapid at the top of the cliff 
 than at the bottom, the cliff, instead of overhanging, will 
 then begin to retire at its upper part. The waterfall 
 will now gradually grow less marked, until it will pass 
 into the condition of rapids, that is, a shoot of water 
 over a steep and rough part of the bed. Finally, these 
 rapids may themselves be worn down, and all trace of 
 the original fall will then disappear, except the gorge which 
 it excavated during its recession.
 
 280 PHYSICAL GEOGRAPHY. [LESS 
 
 12. Almost every large river, flowing through a hillj 
 or mountainous region, illustrates these features of the 
 erosive action of running water. Perhaps the most 
 stupendous example is that of the Niagara River. The 
 famous Falls of Niagara consist of two vast cascades 
 separated by a small island, and having a united breadth 
 of 950 yards and a height of 140 to 160 feet. It has 
 been computed that 670,000 tons of water are poured 
 every minute over these falls into the foaming torrent 
 below, from which vast clouds of spray rise up into the 
 air. Originally the falls stood at Queenstown, where the 
 limestone forms a cliff above a great plain. Since that 
 period, however, the cataract has slowly receded for 
 about seven miles to its present position. That it is 
 still moving upwards is shown by the large slices of rock 
 that from time to time fall from the cliff over which the 
 water rushes. The present rate of retreat has been com- 
 puted at about one foot in the year. Probably this is 
 an exaggerated estimate ; if it be taken as an average 
 for the past work of the river, somewhere about 35,000 
 years must have been required to excavate the ravine 
 between the present Niagara Falls and Queenstown. 
 
 13. Every stream, then, which drives along sand and 
 gravel on its bottom, is busy with the work of erosion. 
 If even in ordinary weather this action may be perceived, 
 how much more stupendous must it be in floods, when 
 every little runnel is swollen, when earth, sand, and 
 stones are swept by rain off the ground, and when the 
 rivers, rising high above their ordinary level, and 
 acquiring from the increase of their volume augmented 
 velocity of flow, rush over the land and bear their vast 
 burden of detritus down to the sea. A river may do far 
 more of its erosive work in a few hours of flood than in 
 many days or weeks of its ordinary flow. Hence this 
 kind of river-action reaches its maximum when the river 
 attains its greatest body of water and highest velocity. 
 
 14. River-Transport. The loose materials acquired
 
 xxvir.] THE WORK OF RUNNING WATER. 281 
 
 in the process of erosion are removed and variously dis- 
 posed of by the streams. As long as the water has 
 velocity enough, it keeps the sediment moving, and con- 
 veys it sometimes to great distances. Any check to the 
 velocity causes some of the sediment to fall to the bot- 
 tom. In considering the nature and amount of work 
 done by rivers in the transport of mineral materials 
 from the land, we must bear in mind that these mate- 
 rials exist not only in visible form, such as gravel, sand, 
 and mud, but invisibly dissolved in the water. As 
 every spring is busily employed in bringing up to the 
 surface mineral substances which the water has dissolved 
 out of underground rocks, and as rain and streams are 
 similarly engaged above ground, a vast quantity of 
 dissolved material must be conveyed into the sea. It 
 is not difficult to make an approximate estimate of 
 the amount of invisible mineral substance thus carried 
 by a river. The amount of water discharged by the 
 river must be ascertained, likewise the average propor- 
 tion of mineral ingredients contained in a gallon or other 
 given quantity of the water. The one sum multiplied by 
 the other will give the required result. The celebrated 
 chemist, Bischoff, calculated that the Rhine carries past 
 Emmerich every year enough carbonate of lime, chemi- 
 cally dissolved in its water, to form 332,539 millions of 
 oysters of the usual size. If all these oysters could be 
 put together they would form a cube measuring 560 
 feet in the side. The river Rhone is estimated to carry 
 past Avignon every year 8,290,464 tons of dissolved 
 salts in its water. The annual discharge of the Thames 
 past Kingston, which stands a few miles above London, 
 is estimated at 548,230 tons of mineral matter, two-thirds 
 of which is carbonate of lime. It has been computed 
 that the rivers of England and Wales carry every year 
 into the sea 8,370,630 tons of solids in solution. If this 
 quantity were entirely dissolved away from the surface, 
 it would be equivalent to a general lowering of the sur-
 
 282 PHYSICAL GEOGRAPHY. [LESS. 
 
 face of the country at the rate of one foot in 12,978 
 years. 
 
 15. But by far the largest amount of mineral matter 
 borne by rivers from the land is in the form of mechanical 
 sediment gravel, sand, and mud. Every river is more 
 or less muddy. After heavy rain even the clearest brook 
 is discoloured by the earth it carries down. Mere dis- 
 coloration, therefore, is a proof of the constant trans- 
 port of sediment by running water. The amount of 
 material thus transported depends partly, of course, upon 
 the carrying power of the river, which is regulated by 
 its volume and velocity; partly upon the nature of the 
 soil and rocks of its drainage-basin, whether they happen 
 to be earthy and easily worn away, or the reverse ; partly 
 upon the distribution of the rain-fall, whether it is spread 
 over all the seasons of the year, or crowded into a few 
 weeks or months, so as to produce a swollen and muddy 
 torrent while it lasts ; and partly, where the river takes 
 its rise from a glacier, upon the quantity of mud which 
 escapes from the melting end of the ice. (Lesson 
 XXVIII.) 
 
 16. A stream having a current of about half a mile in 
 the hour, which is a comparatively feeble flow, can carry 
 along ordinary sandy soil suspended in the water. With 
 a velocity of twelve inches in a second, which is about 
 two-thirds of a mile in the hour, it can roll along fine 
 gravel, while, when the rate rises to three feet in a second, 
 or a little more than two miles in the hour, it can sweep 
 away slippery angular stones as large as an egg. We 
 can readily understand that in torrents, with a steep 
 slope and high velocity (Lesson XXV. Art. 22), the 
 power of transport must be enormous. Huge masses of 
 rock, as large as a house, have been known to be moved 
 during heavy floods. 
 
 17. It is evident, therefore, that in a rapid river or 
 brook, the mud which discolours its water represents 
 only a part of the sediment carried down. A great deal
 
 xxvn.] THE WORK OF RUNNING WATER. 283 
 
 of sand and gravel, or even coarse shingle, is at the same 
 time being pushed along the bottom. This material 
 cannot, indeed, be seen, but the large stones may some- 
 times be heard rattling against each other, as they are 
 rolled onward by the current. 
 
 18. Measurements and estimates have been made of 
 the proportion of sediment in the water of different rivers. 
 This proportion varies, of course, in different seasons, 
 being greatest during floods, and least when the rivers 
 are low. The Ganges, during its four months of flood, 
 is stated to contain one part of sediment in every 428 
 parts by weight of water, while the mean average for the 
 year is one part in 510. In the water of the Irrawaddy 
 the proportion was found to be one part in 1700 by 
 weight of water during floods, and one part in 5725 
 during the dry season. In the water of the Mississippi 
 the average proportion was determined to be one part 
 in i 500 by weight, or one part in every 2900 by volume 
 of water. The Danube has been found to contain a 
 mean proportion of one part in 3060 by weight of sus- 
 pended matter, and during extraordinary floods discharges 
 into the Black Sea as much as 2,500,000 tons of silt in 
 twenty-four hours. 
 
 19. In any adequate estimate of the total discharge, 
 the coarse heavy sediment which is pushed along the 
 bottom must also be allowed for. In the case of the 
 Mississippi, this moving layer has been estimated to de- 
 liver into the Gulf of Mexico the vast amount of 
 750,000,000 cubic feet of earth, sand, and gravel every 
 year. 
 
 20. Having ascertained the average quantity of mine- 
 ral matter suspended in the water, or pushed along the 
 bottom, and having estimated the average amount of 
 water carried by a river into the sea, we may easily 
 obtain, by multiplication, the total quantity of sediment 
 removed from the land by that river in a year. Thus 
 the Rhone is estimated to carry into the Mediterranean
 
 284 PHYSICAL GEOGRAPHY. [LESS. 
 
 every year rather more than 600,000,000 of cubic 
 feet of sediment. The discharge of the Danube into 
 the Black Sea has been determined to be 67,760,000 
 tons of silt annually. The mean yearly amount of solid 
 matter carried in suspension by the Mississippi into the 
 Gulf of Mexico is estimated to be about 362,723,000 
 tons ; and this, including the coarse sand and gravel 
 which are pushed along the bottom, would make a 
 column one mile square and 268 feet high. We may 
 form some notion of this amount of material by suppos- 
 ing that 1000 merchantmen, each laden with 1000 tons 
 of it, were to arrive every day for a whole year at the 
 mouth of the Mississippi and discharge their cargoes into 
 the sea. They would little more than equal as carriers 
 the work of this single river. 
 
 21. But many rivers greatly exceed the Mississippi in 
 the proportion of solid matter which they transport. 
 During the rainy season in India the streams become 
 torrents of mud. Dr. Livingstone in his African travels 
 came upon " sand -rivers " currents of sand, moving 
 along with a comparatively small amount of water. In 
 trying to ford them, he felt thousands of particles of sand 
 and pebbles of gravel striking against his legs, even in 
 dry weather, and he saw that after the rains the quantity 
 of detritus removed by these streams must be enormous. 
 
 22. The amount of sediment carried down by a river 
 to the sea in a year represents the extent of loss which 
 the region drained by the river has sustained within that 
 time. Knowing the quantity of sediment and the area 
 of country from which it has been derived, we can de- 
 termine the amount by which the general surface of the 
 river-basin has been lowered. Thus at its present rate 
 of work, the Mississippi reduces the general level of its 
 drainage area g^o of a foot annually, or one foot in 
 6000 years. Could this rate of denudation be continu- 
 ally kept up over the surface of the land, which is com- 
 puted to rise to an average level of about 1000 feet
 
 xxvii.] THE WORK OF RUNNING WATER. 285 
 
 above the sea, a whole continent might be reduced to 
 the sea level in about 6,000,000 years. Such calcu- 
 lations are of importance in showing that the present 
 surface forms of the land must be continually changing, 
 and cannot, therefore, be of comparatively high antiquity. 
 
 23. River-Deposit. All rivers, then, are constantly 
 busy grinding down and transporting gravel, sand, and 
 mud over the surface of the land. To ascertain what 
 becomes of all this material, let us in imagination again 
 follow the course of a river, from the mountains to the 
 sea, and watch how the sediment is disposed of by the 
 way. 
 
 24. When running water has its velocity checked, it 
 loses some of its power to transport sediment, which 
 then partly sinks to the bottom. This may happen 
 when a stream enters upon a gentle slope or plain, where 
 it must move more slowly, or when it joins a larger and 
 more gently-flowing current, or when it falls into still 
 water, like that of a lake, or into the sea. Hence, dur- 
 ing its course, as well as at its termination, a river 
 necessarily encounters many obstacles to its progress, 
 by which it is compelled to slacken its pace and to drop 
 some of its sediment. The general name of alluvium 
 or "alluvial deposits " is given to accumulations of detritus 
 by running water. 
 
 25. Beginning among the mountains, we meet with 
 abundant examples of this arrest of the detritus which 
 the torrents have been sweeping down the declivities. 
 A steep slope is often deeply trenched with gullies that 
 have been torn out of the soil or rock by torrents. Where, 
 at the bottom of these ravines, a strip of more level ground 
 stretches in front of them, they each heap up a pile of rub- 
 bish, which the headlong brooks, checked in their flow by 
 the change in their angle of descent, have been forced to 
 throw down. Among mountains, so numerous may be 
 the torrents, and so vast the heaps of gravel and stones 
 which they tear out of the hill-sides and strew over the
 
 286 PHYSICAL GEOGRAPHY. [LESS. 
 
 slopes and valleys, that it is in many places difficult to 
 maintain roadways, for these are liable at one point to 
 be buried under huge masses of debris, and at another 
 to be swept away by a flooded stream. 
 
 26. But only the coarser kind of sediment is usually 
 arrested at the foot of such mountain slopes. The finer 
 parts, and even some of the rougher gravel, are carried 
 farther into the valley, where the various brooks unite 
 into one main stream. At every point where this stream 
 is checked, an accompanying deposit of sediment occurs. 
 Among its many serpentine windings the current, though 
 rushing briskly round the convex side of a bend, is thrown 
 into an eddy or slack water on the concave side, and 
 there a bank of sand or shingle will generally be found. 
 
 27. When the stream is flooded, it not only fills its 
 ordinary channel, but rises and overflows the flat meadows 
 on either side. These tracts of level ground, by dimin- 
 ishing the velocity of the overflowing water, compel the 
 stream to drop some of its sand and mud upon them. 
 Should they be covered with herbage or brushwood, the 
 leaves and branches act as filters, and clear the water by 
 retaining the sediment. So that when the stream has 
 subsided, the inundated ground is found to have received 
 a coating of fine silt, or even, it may be, of coarse gravel. 
 The flat land which lies on either side of a river, and is 
 liable to inundation when the water rises, is termed the 
 flood-plain. 
 
 28. If, then, each flood adds to the height of the 
 flood-plain, the time will come when, even at the high- 
 est flood, the water will not be able to overspread the 
 plain. This result arises not only from the height- 
 ening of the plain by deposit of sediment, but also from 
 the gradual deepening of the stream-channel by the 
 scour of the current. As the channel is deepened, the 
 current continues to eat into its banks, curving from side 
 to side, and forming a new plain at a lower level. This 
 process has been in progress for a long time in most
 
 xxvil.] THE WORK OF RUNNING WATER. 287 
 
 river-valleys. A succession of terraces marking former 
 levels of the river-floods may be seen rising even up to 
 heights of several hundred feet above the present rivers. 
 The accompanying figure (Fig. 63), representing a sec- 
 tion drawn through such a river valley (s. S.), shows the 
 relation which the low level terrace or present flood-plain 
 (3) bears to the higher, and, of course, older ones (2, i). 
 In some of the latter, remains of primitive man, such as 
 chipped stone spear-heads and other implements, have 
 been met with in different parts of Europe, showing that 
 when the rivers flowed at the level of these higher ter- 
 races, a rude human population already existed in coun- 
 tries wherein the river -valleys have undergone hardly 
 any appreciable change within historic times. 
 
 RIVER 
 
 FIG. 63. Terraces of gravel, sand, and mud, left by a river. 
 
 29. While still in the mountainous or hilly part of its 
 course, a river may have to traverse one or more lakes. 
 Each of these sheets of still water, by arresting the cur- 
 rent, compels it to drop its burden of sediment. Lakes 
 filter river-water, which, leaving on their floors its sand 
 and mud, issues at the lower end quite clear (Lesson 
 XXVI.). The chief deposit takes place where the river 
 enters the lake. By degrees that portion of the lake is 
 filled up and converted into a plain, which, after being 
 gradually heightened by the river-floods, at last comes to 
 lie above the flood-limit. In this way the upper end of 
 the Lake of Geneva has been so diminished by the de- 
 posits of the Rhone, that a Roman port, still called Port 
 Valais, is now nearly two miles from the edge of the 
 lake, the intervening space consisting of meadows and 
 marshes.
 
 288 PHYSICAL GEOGRAPHY. [LESS. 
 
 30. Lakes must evidently in course of time be filled 
 up by the earth and sand washed into them. This has 
 already been the fate of many. The once united lakes 
 of Thun and Brienz in Switzerland have been separated 
 by a tract of land, formed by the deposits brought down 
 from either side by the streams at Interlaken. In Great 
 Britain, as well as generally throughout Northern Europe, 
 every stage in the disappearance of lakes is abundantly 
 shown, from the mere tongue of sand encroaching upon 
 the edge of a deep mountain tarn up to the flat moss or 
 fertile meadow, which marks where a former lake has 
 been silted up. 
 
 31. When a river enters the lowlands, the diminished 
 slope of its course causes it to flow with a weaker current, 
 and therefore to drop some of its burden of mud and sand 
 as it moves along. Winding to and fro, it cuts down its 
 banks at one part, and heaps up sediment at another, so 
 that in course of time, the whole of a wide plain comes, 
 piece by piece, to be levelled by the shifting stream. 
 The plain is formed, indeed, out of sediment carried 
 down by the water from higher ground. A well or pit, 
 sunk anywhere over its surface, shows that beneath the 
 soil there lie layers of water-worn silt, sand, or gravel, 
 like the materials now being transported and deposited 
 by the river. 
 
 32. When its plain is long and the seaward inclina- 
 tion slight, the flow of a river may be so lazy that, instead 
 of scouring out its channel, it may at last be unable to 
 prevent the sediment from sinking to the bottom and 
 actually heightening the bed of the stream. During 
 floods, the chief deposit of silt takes place on the banks. 
 These are consequently raised in level more than the 
 plain beyond, which, not being so liable to inundation, 
 receives less " sediment. In this manner, the river 
 gradually comes to flow at a higher level, between broad 
 embankments of its own building. From time to time 
 it breaks through the lower or weaker parts of these
 
 xxvii.] THE WORK OF RUNNING WATER. 289 
 
 embankments, and inundates the plain, perhaps even 
 scouring out here and there a new bed. In cultivated 
 regions, like those watered by the courses of the Po and 
 Adige, in the plain of Lombardy, and by the lower 
 portion of the Mississippi, much care is needed to 
 strengthen and heighten the banks, with the view of 
 averting inundations. Some rivers have so heightened 
 their channels that their surface flows during floods at a 
 higher level than the streets of towns on their banks. 
 
 33. The most famous example of such a broad plain, 
 formed by the accumulation of sediment brought from the 
 interior of the land and laid down by the long-continued 
 flow and overflow of a river, is Lower Egypt, which even in 
 ancient times was recognised to be a "gift of the Nile." 
 At midsummer every year, this river begins to rise and 
 to cover the flat land on either side of its course. The 
 inundation is at its height in about three months, and 
 then the waters, after remaining stationary for nearly a 
 fortnight, retire to their former level. This annual rise 
 corresponds to the rainy season in the mountainous table- 
 land of Abyssinia (Lesson XXV. Art. 1 8). When the 
 monsoon blows from the Indian Ocean it brings torrents 
 of rain, which rush down the rugged slopes of that country 
 and sweep along a prodigious amount of mud. Thousands 
 of swollen and muddy streams are at last united in the 
 Blue Nile, which carries this vast body of discoloured 
 water down into Egypt. After the inundation has ceased, 
 the overflowed ground is found to be covered with a thin 
 coating of rich fertile mud. It has been estimated that 
 the thickness of this annual deposit does not exceed that 
 of a thin sheet of pasteboard, so that a depth of only two 
 or three feet of the soil of Lower Egypt represents the 
 continuous deposits of a thousand years. The increase of 
 the Egyptian plain evidently takes place at the expense 
 of the high grounds of Abyssinia. It is the finer particles, 
 worn away from the rocks of these uplands, which form 
 the mud spread over Egypt. Here we see how the erosion, 
 U
 
 290 PHYSICAL GEOGRAPHY. [LESS. 
 
 transport, and deposit of materials by running water com- 
 bine to build up a plain and renew its fertility. 
 
 34. The great plains of India furnish, likewise, admir- 
 able illustrations of the way in which the debris of the 
 mountains is spread out on low grounds by rivers. The 
 valleys of the Indus, Ganges, and Brahmaputra have 
 been filled up by the accumulations carried down from 
 the Himalaya chain by these great rivers. The Tigris 
 and Euphrates have combined to fill up the upper half of 
 the valley of which the Persian Gulf is the still remaining 
 lower half. On the American continent, also, this pro- 
 cess is exhibited on the most stupendous scale. Much 
 of the eastern borders of the United States is a plain 
 formed by the deposit of sediment washed off the land. 
 Many thousands of square miles of nearly level land in 
 the valleys of the Mississippi and its tributaries are under- 
 laid by alluvial deposits. The valley of the Amazon, 
 with its vast forests or silvas, forms so long and so level 
 a plain that ships can sail up the river to the very foot 
 of the Andes, a distance of 2000 miles inland from the 
 sea. 
 
 35. Lastly, it remains to inquire how the sediment 
 borne down by rivers is disposed of when it reaches the 
 sea. Many rivers have, at their mouths, what is called a 
 bar, that is, a ridge of gravel or sand stretching across 
 the channel, and always tinder water. From what has 
 been said already in this Lesson, the origin of this bar 
 will now be understood. It is evidently due mainly to 
 arrest of the sediment, where the river current is checked 
 by coming in contact with the sea. The coarser gravel 
 and sand, pushed along the bottom of the river channel, 
 now encounter the opposition of the salt-water, over which 
 the lighter fresh water of the river flows onward. The 
 sea piles up fresh materials upon the bar from the outside, 
 while, on the other hand, the river when flooded drives 
 its bar farther seaward. Hence this barrier to navigation 
 at the mouth of a river is continually shifting its position
 
 xxvn.] THE WORK OF RUNNING WATER. 291 
 
 and altering its shape and size, according as the action 
 of the river or that of the sea predominates. 
 
 36. Some of the larger rivers of the globe exhibit at 
 their mouths on a vast scale the same operations that 
 take place where streams enter lakes. The sediment 
 deposited by them in the sea has gradually filled up 
 bays or gulfs and converted them into flat land, or has 
 pushed its way out to sea. This advancing tract of 
 river-formed ground is usually triangular in form, the 
 apex pointing up the river. It was this resemblance to 
 the Greek letter A that suggested the name of delta, as 
 applied to these accumulations at the mouths of rivers. 
 Down to the head of its delta a river (Fig. 52) is usually 
 augmented by tributaries from either side, and does 
 not branch, except for a limited space, as where it 
 encircles an island in its course. Its current thus be- 
 comes more and more ample. But when it reaches the 
 delta it begins to subdivide, and continues to branch 
 out, until in some cases the flat plains and marshes are 
 traversed by innumerable tortuous channels of water. 
 (Lesson XXV. Art. 6.) By this ramification over the flat 
 ground the velocity of the current is diminished and the 
 deposit of sediment is facilitated, consequently the chan- 
 nels are being continually filled up, while new ones are 
 cut through the soft alluvial earth and silt. Two or 
 more main branches of the river carry out the chief part 
 of the water and sediment to sea, and at their mouths 
 the increase of the delta is most apparent. 
 
 37. The general form of a typical delta is shown by 
 that of the Nile (Fig. 52) which enters the tideless 
 Mediterranean. The Mississippi, on the other hand 
 (Fig. 64), brings down such an amount of sediment that 
 not only has it filled up its valley, but it now pushes 
 tongues of alluvial land far out into the Gulf of Mexico. 
 The average rate of advance of this delta has been 
 estimated to amount to 86 yards in the year. The Tiber 
 throws forward its delta at the annual rate of about 1 2
 
 292 
 
 PHYSICAL GEOGRAPHY. 
 
 [LESS. 
 
 to 1 3 feet. The delta of the Po has increased at such a 
 rate that the port of Adria, which stood on it and was 
 so important in Greek and Roman times as to give its 
 name to the Adriatic sea, is now 14 miles inland. 
 
 38. The deltas of some of the larger rivers of the 
 globe are of enormous size. That of the Mississippi 
 embraces an area of about 40,000 square miles. That 
 
 OF 
 
 Fro. 64. Delta of the Mississippi. 
 
 of the Ganges and Brahmaputra is as large as the whole 
 of England and Wales. Their vast superficial extent 
 indicates the high antiquity of these deltas. But we 
 must remember also that, as they have been formed by 
 the gradual filling up of gulfs of the sea, a prolonged 
 period of time, where these gulfs were deep, would be 
 occupied in bringing up the level of the bottom to that
 
 xxviii.] FROST, SNOW-FIELDS, GLACIERS. 293 
 
 of the surface, before any advance of the plain of the 
 delta could take place. Where the tides and currents of 
 the sea interfered in such a way as to sweep away much 
 of the sediment as it arrived, the time required would be 
 still further increased. The delta of the Ganges has 
 been bored into at Calcutta, and its deposits, consisting 
 of sand, gravel, silt, and layers of vegetation, were found 
 to be more than 400 feet thick. 
 
 39. In cases where the form of the coast is unfavour- 
 able to the formation of deposits, or where the rivers flow 
 with sufficient velocity to carry out their mud to sea, or 
 where marine currents sweep past the river-mouths and 
 carry away the sediment, deltas do not occur. The 
 Amazon and La Plata, for example, form no deltas. But 
 they nevertheless pour a vast quantity of fine silt into the 
 Atlantic. Even for a distance of 300 miles from the 
 mouth of the Amazon the sea is said to be perceptibly 
 discoloured by the mud of that river. 
 
 LESSON XXVIII. The Waters of the Land Part V. 
 Frost, Snow-fields, Glaciers. 
 
 1. The moisture of the air, when the temperature sinks 
 to the freezing-point, passes into ice (Lesson X. Art. 36). 
 We have now to follow the course of this ice when it falls 
 from the air to the land. We must further consider how 
 the waters of the land are affected when cooled down 
 below the freezing-point ; for, changed into solid ice, they 
 cannot but acquire new and different powers from those 
 which have been the subject of the last four Lessons. 
 What part, then, does the frozen water of the land play in 
 the general plan of the globe ? Some answers to this 
 question may be found in the study of the behaviour of 
 three forms of ice on the land Frost, Snow-fields, and 
 Glaciers.
 
 294 PHYSICAL GEOGRAPHY. [LESS. 
 
 2. Frost. Most substances suffer contraction from 
 cold, and consequently increase in density. A cubic foot 
 of pure water, for example, at a temperature of 40 Fahr. 
 weighs more than the same quantity at 60 : or, in other 
 words, a vessel exactly filled with water at 60 would be 
 found to be not quite full were the water cooled down to 
 40. 
 
 3. But a remarkable change takes place during the 
 further cooling of the water. When the temperature 
 reaches 39-1 Fahr., contraction ceases. This is, there- 
 fore, the temperature at which pure water is heaviest, and 
 is called the point of maximum density of fresh-water. 
 Below this point, the water expands, and instead of sink- 
 ing, remains on the surface where it is at last converted 
 into ice when the temperature has fallen to 32. Hence 
 it is that sheets of water are not frozen to the bottom. 
 The ice being lighter than water, floats on the surface. 
 As the cold continues, the first thin crust of ice is thick- 
 ened by additions to its under side, until after a severe 
 winter it may be a foot or two in thickness. In this 
 way, during a long and hard frost, rivers and lakes be- 
 come so firmly frozen over that caits and heavy waggons 
 may be drawn across them, though the water with its 
 living inhabitants remains still liquid underneath. Canals 
 and rivers which, for most part of the year, are high- 
 ways for boats and barges, become, in such cold winters, 
 thoroughfares for carriages and sledges, as well as pass- 
 engers on skates or on foot. 
 
 4. The parts of the earth's surface where this curious 
 transformation may be ordinarily seen in winter are indi- 
 cated on Plates IV. and V. by the position of the isotherm 
 of 32. In the northern hemisphere, over all the ground 
 lying to the north of that isotherm, and in the southern 
 hemisphere, on all the ground lying to the south of it, the 
 waters are frozen during winter. But even in much more 
 temperate latitudes, the cold is occasionally severe enough 
 to make the rivers and canals passable on foot, and what
 
 xxvni.] FROST. 295 
 
 is more remarkable, actually to encrust the sea with ice. 
 Thus in the year 401, and again in 642, the winter was 
 so intensely cold in Southern Europe that the Black Sea 
 was entirely frozen over ; in 850 the Adriatic was covered 
 with ice ; while in 1233 and in 1314 the rivers in northern 
 Italy were likewise frozen. In the year 1205 a severe 
 frost, lasting for three months, bound up the soil of Eng- 
 land so that it could not be ploughed, and froze the 
 canals, rivers, and ponds. The last time that the Thames 
 at London was passable on ice was in the year 1814. 
 Thousands of passengers crossed the river on foot between 
 London and Blackfriars Bridges, and numerous wooden 
 booths and tents were erected on the ice, together with 
 merry-go-rouads and other amusements, so that the busy 
 concourse of people was called " Frost Fair." 
 
 5. The freezing of sheets of water on the land may 
 take place so equably, and the ice may disappear again 
 so quietly, that after the frost has passed away, no trace 
 may be left of its having occurred at all. But should a 
 frozen lake break up under a storm of wind, large masses 
 of ice will be driven ashore, and may push up sand, 
 gravel, and stones lying at the water's edge. Such heaps 
 of ice often take some weeks to thaw, and after they 
 have melted, the stones and sand which they had driven 
 ashore are found scattered over the ground, as may be 
 seen on a great scale on the shores of the large Canadian 
 lakes. In the lakes and rivers of Canada also, another 
 result of severe frosts may be observed. Blocks of stone 
 lying in shallow water are frozen into the ice, and when 
 the spring thaws set in are actually lifted from their 
 places by the fringes of ice which have formed round 
 them, and are floated away to some other part of the 
 shore, or may even be driven out into the deeper water, 
 so as at last to be dropped there. This transport of 
 stones has been particularly observed on the shores of 
 the St. Lawrence. 
 
 6. An indirect result of the freezing of rivers, but one
 
 ig6 PHYSICAL GEOGRAPHY. [LESS. 
 
 of great importance, is the accumulation of the ice in bars, 
 whereby disastrous floods are produced. When the ice 
 breaks up in a rapid thaw, huge blocks of it, borne down 
 by the current, and piled upon each other, may be driven 
 together in such a way as to dam up the river. The 
 water accumulates behind the ice -barrier, and at last 
 bursts through it, sweeping with prodigious force down 
 the valley, and carrying destruction far and wide in its 
 course. 
 
 7. The effects of frost, however, are to be traced not 
 merely on sheets of water on the land. Rain, falling 
 upon the land, soaks through soil and rocks, which con- 
 sequently contain abundant water in their pores and 
 cavities. When severe frost sets in, this 'water freezes. 
 The soil becomes hard, so that even muddy places, where 
 one would have sunk deep in mire, can now be safely and 
 easily traversed on foot. A jug of water exposed to severe 
 frost is apt to be split. In insufficiently warmed dwell- 
 ing houses, water-pipes in like manner burst during frost. 
 The reason of these mishaps is to be sought in the ex- 
 pansion of water in the act of freezing. When passing 
 from the liquid to the solid state, water undergoes a 
 sudden and remarkable expansion, amounting to about 
 one-tenth of its volume. At the moment of change it 
 exerts great force upon the sides of the vessel or cavity 
 containing it, and if it cannot readily escape it will do its 
 best to burst these sides, that it may find room for its 
 increase in bulk. Now what takes place in a water-jug 
 or pipe, goes on also among the little particles of water 
 enclosed between the grains of soil and rocks. Frost 
 by expanding the water, pushes these grains aside. On 
 a winter morning, after a night of sharp frost, we may 
 notice that the expansion has been great enough to force 
 the little pebbles of a roadway out of their places. So 
 also, in countries like Canada, where the winters are 
 extremely cold, wooden fences are, in the course of a 
 year or two, twisted out of the ground.
 
 xxvui.] FROST. 297 
 
 8. It is not until a time of thaw that we are made fully 
 aware of what the frost has done. So long as the cold 
 continues, the separated particles of soil are kept together 
 by the ice, which binds them into a hard solid mass. 
 But when this ice melts, the grains of sand and earth 
 become loosened from each other. Walking on a road 
 or over a ploughed field after frost, we find that this 
 loosening has been carried so far that the ground has 
 become coated with mud. Indeed the millions of little 
 ice crystals, which frost wedges in among the grains of 
 soil, have much the same effect as if the earth were 
 ground down in a grinding-mill or mortar. They pulver- 
 ise it, that is, reduce it to powder, and thus lay it more 
 open to the branching roots of plants, which obtain so 
 much of their sustenance from the soil. Farmers are in 
 the habit of ploughing their land before cold weather 
 sets in, so as to leave the upturned loam exposed to the 
 beneficial effects of succeeding frosts. 
 
 9. The powerful mechanical effects of frost are so well 
 seen in soil, because of the abundant moisture retained 
 among the particles of which soil is composed. But 
 any porous rock which contains sufficient water, and is 
 exposed to a great enough cold, may show the same kind 
 of disintegration. Hence in countries where the winters 
 are severe, ordinary building-stones and mortar are found 
 to peel off in successive crusts, or to crumble down into 
 powder, after frost has given way to milder weather. 
 Even in the comparatively mild winters of Britain, this 
 may be constantly seen ; in the severer climate of North 
 America it is a serious and costly evil, since it prevents 
 the use of many kinds of building-stone, which in the 
 absence of frost would be very valuable. 
 
 10. Passing now from the water retained and frozen 
 among the pores of soil and of rocks, let us consider 
 further the result of the formation of ice in the larger 
 cracks and crevices of cliffs and crags. To realise how 
 abundant these natural lines of division are in all rocks,
 
 298 PHYSICAL GEOGRAPHY. [LESS. 
 
 look at any exposed face of rock such as a sea-cliff, the 
 ravines of a river, or the precipitous sides of a mountain. 
 No matter what may be the kind of rock, it is traversed 
 by many parallel and intersecting divisional planes or 
 "joints," which serve as channels, wherein the surface 
 water descends underground and re -ascends to form 
 springs (Lesson XXIV. Art. 17). When frost becomes 
 so severe as to penetrate beyond the mere outer crust of 
 rock, the water contained in the external parts of the 
 joints freezes. This must often take place in cavities 
 where there is little room for expansion, and where, there- 
 fore, the ice exerts its great force in pushing the walls 
 asunder. Winter after winter the process is repeated, 
 until at last the portion of rock on the outside gets so far 
 wedged off from the rest, that it loses its balance and 
 falls to the bottom of the cliff. In all countries subject 
 to intense frosts, the bases of cliffs and crags may be 
 seen to be strewn with large rough blocks, which have in 
 this manner been loosened from their places above. 
 In the valleys among mountains that rise high up above 
 the snow- line, the operations of frost are powerful in 
 splintering the crags and pinnacles, and giving them 
 the sharp spiry forms they so often assume. (Lesson 
 XXIX.) 
 
 11. Snow-fields. Wherever the land rises above the 
 snow-line, it is buried under a permanent sheet of snow, 
 from which only the higher and steeper mountain peaks 
 project. In some regions, as on the table-land of Nor- 
 way, a broad and tolerably level plateau allows the snowy 
 covering to spread in a vast undulating sheet, three or 
 four thousand feet above the level of the valleys. Standing 
 upon one of the heights at the edge of such an expanse 
 of snow, we see what looks like a white frozen plain, with 
 no limit save the line where it meets the sky. In other 
 parts of the globe, where rugged groups of mountains 
 tower above the snow-line, and there are consequently 
 no such level tracts, the snow accumulates in the hollows
 
 xxvin.] SNOW-FIELDS. 299 
 
 and on the higher slopes. The semicircular ranges of 
 mountain cliffs and crests often enclose vast basin-shaped 
 depressions, each of which becomes a gathering place for 
 the snow. To all the permanent sheets of snow, whether 
 occurring on table-lands or in the hollows of mountains, 
 the general term of snow-fields is given. 
 
 12. In these regions, since the moisture falls from the 
 air as snow rather than rain, and the heat of summer is 
 insufficient to melt it all, the quantity of snow would 
 increase indefinitely were there no provision for the 
 removal of the surplus. The thickness of snow on some 
 snow-fields must amount to many hundred feet. Green- 
 land, for example, is almost wholly buried under one vast 
 snow-field, so deep as to cover over the inequalities of 
 the surface of the land almost as completely as the 
 furrows of a ploughed field are lost beneath a heavy 
 snow-wreath. The Antarctic regions are believed to be 
 buried under a mantle of snow passing down into solid 
 ice, having a thickness of 10,000 feet or more. 
 
 13. There are two ways, besides melting and evapora- 
 tion, in which the snow-fields get rid of their excess of 
 snow ; these are avalanches and glaciers. Where 
 the edges of a snow- field overhang steep slopes, por- 
 tions of the more or less consolidated snow from time to 
 time break off, and rush with a noise like thunder 
 and a terrific force into the valleys, tearing up the soil, 
 sweeping down loose blocks of stone, uprooting or break- 
 ing trees, and carrying destruction as far as they reach. 
 These snow-falls are called avalanches. In Alpine 
 countries, forests which lie in the pathway of such de- 
 scending masses of snow are carefully preserved as 
 barriers to protect the meadows and villages from ruin. 
 Roads which pass along the base of snowy mountains 
 require in some places to be covered over with a strong 
 archway of masonry to protect them from the disasters 
 caused by frequent snow-falls, which would not only 
 sweep away every traveller and carriage in their course,
 
 300 PHYSICAL GEOGRAPHY. [LESS. 
 
 but would even tear up the road itself, or bury it under 
 heaps of earth and stones. 
 
 14. Glaciers, however, are the chief means whereby 
 the superabundant snow above the snow-line is removed. 
 While the snow forming the surface of a snow-field is 
 loose and open in texture, like that which covers the 
 ground in winter and disappears in spring, the under 
 portions become more and more close and firm under 
 the pressure of the overlying mass. This compacted 
 snow (nez>J or firn, as it is called in Switzerland) passes 
 by degrees into clear blue ice, as the imprisoned air is 
 more and more completely squeezed out of it. If the 
 snow-fields lay on perfectly level table-lands, there would 
 be no general movement of the snow except along the 
 edges of the plateaux, where the gathering snow-sheet 
 would break or slide off into the valleys. But since the 
 surface of the land has a more or less marked inclination 
 from its axis or water-shed, the snow, by virtue of the 
 action of gravity, must slide downward even upon a 
 very gentle slope. It is chiefly during this movement 
 that the air is pressed out, whereby the loose, white, 
 opaque snow is converted into solid blue, transparent 
 ice. Having acquired a slow, sliding motion, the mass 
 of snow seeks the lowest levels. It therefore moves 
 downward into the heads of the valleys that ascend into 
 the snow-fields. Each of these hollows becomes a kind 
 of reservoir in which the snow, pressing downwards from 
 each side as well as from behind, accumulates to a great 
 depth, and is so jammed up between the mountain 
 slopes as to take the form of solid ice from bottom 
 almost to top. Driven onwards by the pressure of the 
 advancing mass behind and by its own gravity, this ice 
 fills up each valley sometimes to a depth of several 
 hundred feet, and for a distance of many miles. Such 
 a tongue of ice, proceeding from a snow-field above and 
 descending below the snow-line, is termed a glacier. 
 In the accompanying drawing (Fig. 65) two glaciers are
 
 XXVIII.] 
 
 GLACIERS. 
 
 301 
 
 seen descending from one of the great snow-fields of 
 Arctic Norway. In one case, the ice almost reaches 
 the sea ; in the other the glacier is smaller, and does not 
 proceed so far from its parent snow. 
 
 15. A glacier, then, represents the escaping drainage 
 of the snow-fall, above the limit of perpetual snow, as a 
 river does the surplus drainage of the rainfall. Its size 
 must depend upon that of its enclosing valley, and on the 
 
 FIG. 65. Snow-field and glaciers of Holands Fjord, Arctic Norway. 
 
 extent and declivity of the gathering ground of snow from 
 which it issues. In the Alps, for example, the Great 
 Aletsch Glacier extends about fifteen miles down its 
 valley. Sometimes a little glacier fills up a high recess 
 on the flank of a mountain, and never reaches even to 
 the nearest valley. In other cases the glacier descends 
 far below the snow-line, even into the region of meadows, 
 corn-fields, and gardens. The lower glacier of Grindel- 
 wald, in the Bernese Oberland, reaches to. a point about 
 3500 feet below the snow-line on the north side of the 
 Alps. 
 
 16. To realise as clearly as possible the general
 
 302 
 
 PHYSICAL GEOGRAPHY. 
 
 [LESS. 
 
 appearance presented by a glacier, let us suppose our- 
 selves placed at the lower end of one of those among 
 the Alps. On either side of the valley the slopes are 
 clothed with pine. Patches of green pasture catch the 
 sunlight in the hollows and on the lower projecting hills ; 
 while around us lie scattered cottages and bright mea- 
 dows. In front stands the abrupt end of the glacier a 
 
 FIG. 66. View of a glacier, with its lines of rubbish (moraines) and the 
 river which escapes from its end. Ice-worn hummock of rock and trans- 
 ported stones are shown in the foreground. 
 
 steep, craggy face of ice, from the base of which issues 
 a river of pale muddy water. Numerous large blocks 
 of rock are scattered about on the valley-bottom below 
 the ice. The ground there, indeed, is mainly composed 
 of coarse shingle, like that which forms the bed of the 
 present river. Even on the ice itself we may see heaps 
 of stones, some, perhaps, poised just on the verge of the
 
 xxviii.] ' GLACIERS. 303 
 
 last steep slope of the glacier, whence they must soon 
 roll down to join the crowd of others that have preceded 
 them. Looking into one of the deeper rents in the ice, 
 we see it to be of wonderful purity, and of the most ex- 
 quisite blue colour. And yet most of its outer surface 
 may be so obscured by earth and stones, that at first, 
 perhaps, we can hardly be persuaded that it is ice at all. 
 
 1*7. We ascend to the surface of the glacier, either by 
 mounting among the broken cliffs of ice forming its 
 abrupt front, or by choosing a safer and easier path up 
 the slope on one side of the valley. The ice is now 
 seen to lie as a great sheet, filling the bottom of the 
 valley from side to side, and stretching far up into the 
 heart of the mountains. Its surface has at first a gentle 
 slope, and is comparatively smooth, though many glaciers 
 even at the lower end present a marvellously rugged 
 aspect, like that of a tempestuous sea suddenly frozen. 
 Ascending the valley, we notice that the surface accumu- 
 lations of earth, gravel, and stones are especially abun- 
 dant along the sides of the glacier, and likewise in one or 
 more ridges along its centre. During the day, when 
 the sun shines out warmly, the surface of the ice is 
 thawed, and consequently abundant little runnels of 
 water flow over it. At night, when these are frozen, the 
 glacier becomes once more silent. 
 
 18. In several respects a glacier resembles a river ; 
 and this resemblance is closer than might at first appear. 
 If the position of any prominent object on the surface of 
 the ice be observed with relation to some fixed point on 
 the bank, or if, as in the original survey of the Mer-de- 
 Glace by the late Principal Forbes, a transverse series 
 of stakes be driven into the ice across the breadth of the 
 glacier, and their position be observed at intervals from 
 the bank, the ice is found to be slowly moving down the 
 valley. This motion is faster in the centre than at the 
 sides, because the friction of the sides impedes the flow 
 of the ice. The average rate of movement on the Mer-
 
 304 PHYSICAL GEOGRAPHY. [LESS. 
 
 de-Glace was thus ascertained to be, during summer and 
 autumn, from 20 to 27 inches in the twenty-four hours 
 at the centre, and from 13 to 19^ inches at the side. 
 
 19. Bending from side to side, and travelling over an 
 uneven, rocky floor, the glacier is split across by rents 
 called crevasses, which often open and form wide and 
 deep chasms, extending sometimes down to the very 
 bottom of the glacier. Stones and earth lying on the 
 surface of the ice frequently fall into these fissures, and 
 thus reach the floor of the valley beneath the glacier, or 
 are imprisoned in the ice, when the sides of the yawning 
 crevasses are pressed together again as the glacier moves 
 onward. 
 
 20. When a river reaches a steep and rocky part of 
 its bed it forms a rapid ; when it comes to a cliff, it 
 takes the shape of a waterfall. Ice, not having the 
 same mobility as running water, cannot so easily adapt 
 itself to the irregularities of its channel. But it shows 
 these irregularities in a very marked way. In the course 
 of the glacier which we have supposed ourselves to be 
 visiting, after, perhaps, some miles of slow ascent, we 
 come to a precipitous slope, where the ice, completely 
 shattered by innumerable fissures, rises into pinnacles 
 and sharp crests of many fantastic forms. Could we watch 
 that tumultuous descent of broken ice for a long enough 
 time, we should find it to be all in slow motion down- 
 wards. It is an icefall, and answers in the mechanism 
 of a glacier to the waterfall in that of a river. Under- 
 neath it, the bottom of the valley is steep or precipitous, 
 and the ice, unable to descend the declivity in one un- 
 broken sheet, is cracked and splintered in this wonderful 
 way. But just as a river, no matter how much it may 
 have been tossed into foam by its descent in a waterfall, 
 speedily takes its usual shape and rolls onward as if no 
 sudden plunge had so recently disturbed its current, so 
 a glacier, though it may have been reduced, as it were, 
 to fragments at one of these icefalls, soon reunites at the
 
 XXVIII.] 
 
 GLACIERS. 
 
 305 
 
 bottom, and again pursues its course as a solid and con- 
 tinuous sheet of ice. If the glacier is a large one it may 
 
 FIG. 67. Plan of the Mer-de-Glace of Chamouni and its tributary glaciers, 
 showing the way in which lateral moraines became medial. 
 
 receive tributary glaciers from valleys on either side. 
 
 The Mer-de-Glace of Chamouni, for example, is formed 
 
 X
 
 306 PHYSICAL GEOGRAPHY. [LESS. 
 
 by the united mass of several glaciers, as shown in Fig. 
 67. Each of these, as well as the main stream, may be 
 traced upward until it is found insensibly merging into 
 the snows which fill up the hollows in the higher part of 
 the mountains. 
 
 21. One cannot trace the course of a glacier, and 
 realise on the ground the size and character of those 
 vast tongues of ice which carry off the drainage of the 
 snow-fields, without wishing to know what task is allotted 
 to them in the general economy of nature. The rivers 
 which bear away the surplus water of the land are busy 
 in a stupendous work, wearing down the mountains and 
 valleys, and strewing their debris over the plains, or 
 sweeping it out to sea (Lesson XXVII.). Glaciers, too, 
 are engaged in a similar task. They transport the 
 waste of the mountains down to lower levels, and they 
 erode the sides and bottoms of the valleys in which they 
 move. 
 
 22. Transport. Whence come the earth and stones 
 which so darken and obscure the surface of the ice, and 
 which, far below the snow-line, where the glacier melts 
 almost at the edge of the meadows and gardens, lie piled 
 in heaps on every side ? From a good point of obser- 
 vation above the glacier we may notice that the heaps 
 are not scattered wholly at random across the surface of 
 the ice ; but that there are long lines of stones that keep 
 apart from but parallel to each other, and run along the 
 length of the glacier. They wind to and fro, with the 
 varying curves of the glacier in its course, until they are 
 lost in the distance. In the drawing of a glacier in Fig. 
 66, for example, some of these lines of stones are seen 
 both in the centre and towards either side. Following 
 the central line on a glacier (sometimes there are several 
 lines down the middle), we find at last that, at some 
 higher part of the valley, it brings us to a point where 
 two branches of the glacier join, and where the line 
 of stones either continues up the side of one of the
 
 xxviii.] GLACIERS. 307 
 
 branches, or divides into two, one portion keeping to the 
 right-hand side of one of the tributary glaciers, the other 
 remaining on the left-hand side of the other branch (Fig. 
 67). In every case it will be observed, if the glacier is 
 followed far enough up its valley, that a line of stones in 
 the middle of the ice comes really from the side, and is 
 due to the confluence of two branches of the glacier. 
 Owing to the irregularities in-the slope and breadth of 
 the glacier-channel, and the manner in which the ice is 
 consequently driven to adapt itself to these, as well as 
 owing to the melting of the surface of the glacier (Art. 25), 
 the lines of stones are apt to lose their distinctness as they 
 advance down the valley, until, after some great icefalls 
 and abundant crevasses, the rubbish comes to be spread 
 more or less over the whole of the glacier's surface, as 
 it is towards the lower end of the Mer-de-Glace (Fig. 
 
 67). 
 
 23. These heaps of stones earth, and gravel lying on 
 the ice are known by the name of moraines. When 
 on the side of the glacier, they are termed lateral mo- 
 raines ; on the centre, they are called medial; at the 
 end, where the glacier throws down its burden as the ice 
 melts, they are spoken of as terminal. 
 
 24. In all cases, then, these moraine-heaps can be 
 followed up the glacier to the base of some cliff or 
 craggy mountain-slope, whence the blocks of rock have 
 been derived. In such positions one sees how the frag- 
 ments have been loosened by the severe and prolonged 
 frosts of these high grounds, until, wedged off from the 
 face of the cliffs, they have rolled down to find a resting- 
 place upon the glacier below. Once on the ice, they are 
 slowly borne down the valley, and dropped among the 
 heaps of rubbish at the far end of the glacier. Much 
 of the rubbish, however, falls down the numerous rents 
 or crevasses in the ice, and reaches the bottom of the 
 glacier, there to aid the ice in accomplishing anothei 
 important part of its work (Art. 28).
 
 308 PHYSICAL GEOGRAPHY. [LESS. 
 
 25. Watching the progress of one of the large blocks 
 as it travels with the ice, we discover what might not 
 otherwise be so evident, that the surface of the glacier is 
 continually being lowered by melting and evaporation. 
 Take the case of a large flat stone, which, bounding 
 from some high crag, has found a lodgment upon the 
 ice. The portion of the ice lying below the stone is 
 screened from loss by thawing and evaporation, but the 
 surrounding parts of the glacier, not so protected, are 
 insensibly wasted away. Consequently the stone begins, 
 as it were, to rise out of the glacier. Its pedestal of ice 
 
 FIG. 68. Glacier table a pillar of ice supporting a block of stone. 
 
 continues to increase in height, but being exposed on the 
 sides to sun and air, is lessened in diameter, until it 
 becomes too slim to support the heavy burden of stone, 
 which then tumbles down upon the surface of the glacier 
 (Fig. 68). But as the general waste of the surface of 
 the ice continues, the new position of the stone is soon 
 marked by the rise of a new pillar of ice as before. The 
 same block of rock may thus, in the course of its journey 
 down the glacier, become the capital of several succes- 
 sive ice-columns. The same kind of testimony to the 
 remarkable lowering of the surface of the glacier is 
 shown by the long parallel moraine mounds. Looking 
 at one of these ridges, and even climbing and standing
 
 XXVIII.] 
 
 GLACIERS. 
 
 309 
 
 on it, we might suppose it to consist of fragments of 
 stone throughout. But by pulling down some of the 
 loose blocks, we find that solid ice lies immediately 
 below. It is in fact a ridge of ice with a coating of 
 debris which has protected it from the general waste, as 
 the detached stones screen the ice - pedestals beneath 
 them. 
 
 26. In most valleys with glaciers in them, large blocks 
 of rock may be observed above the present limits of the 
 
 FIG. 69. The Pierre-a-Bot, near Neufchatel (J. D. Forbes). 
 
 ice, but in places to which at one time the ice evidently 
 reached. They occur poised sometimes in the most 
 precarious positions, as if a man's strength would be 
 sufficient to dislodge and send them down the slope. 
 These perched blocks, as they are called, furnish 
 good proof of the former extent of the glaciers. By 
 their means, for instance, combined with other evidence, 
 it can be proved that the glaciers of the Alps at one 
 period filled up the Swiss valleys, and even spread over 
 the broad plain of Switzerland, between the Bernese
 
 PHYSICAL GEOGRAPHY. 
 
 [LESS. 
 
 Oberland and the Jura. On these vast ice-rivers blocks 
 of granite from the Mont Blanc group of mountains were 
 transported across what is now the valley of the Rhone 
 and the Lake of Geneva, and were stranded high on 
 the sides of the Jura range. The accompanying figure 
 (Fig. 69) by Principal Forbes, representing one of these 
 travelled blocks, erratics, or, as the Swiss call them, 
 foundlings, shows the great size which some of them 
 attain. 
 
 FIG. 70. Glacier descending to the sea. 
 Arctic Norway. 
 
 ; Fjord, 
 
 27. We conclude then that one important part of the 
 work of glaciers is the transport of the materials 
 of the mountains from higher to lower levels. In 
 the case of such a mountain chain as the Alps or the 
 Himalaya, the glaciers melt long before they can reach 
 the outskirts of the high grounds. They, therefore, do 
 not bear their burden beyond the mountain region, 
 though, as we have just seen, they once carried it much 
 farther than they do now. But in Arctic and Antarctic
 
 xxviii.] GLACIERS. 311 
 
 regions the glaciers actually reach the sea-level, and, even 
 pushing their way out to sea, break off into icebergs 
 (Lesson XVI.). The accompanying drawing (Fig. 70) 
 represents the little glacier at the head of the Jokuls 
 Fjord in the north of Norway, which descends into the 
 sea. A few transverse crevasses may be observed in it 
 near the base. From time to time the outer portions 
 break away, and fragments of ice, which are true minia- 
 ture icebergs, float slowly on the current of fresh water 
 moving down the inlet. Pieces of stone may now and 
 then be seen upon these little bergs. In Greenland vast 
 glaciers, like the Humboldt glacier, which measures 60 
 miles in width, descend to the sea-level, and extend for 
 some distance from the shore, till large sections of their 
 seaward ends split off and float away as icebergs (Fig. 
 1 6). Occasional blocks of stone have been noticed on 
 icebergs on the ocean. The debris of the mountains is 
 thus actually borne by the ice out to sea, and may travel 
 for many hundreds of miles before, as the bergs melt, it 
 sinks at last to the bottom of the ocean. 
 
 28. Erosion. In the second place, a little further 
 observation will suffice to show that the carrying of 
 materials down its valley is not all the work which the 
 glacier performs. Let us consider the river of muddy 
 water that escapes from the end of the ice. Muddy at 
 every season of the year, it is most copious and dis- 
 coloured during the warm dry weather of summer and 
 autumn. The mud cannot come from the melting of 
 the glacier itself, for the ice is clear and pure. Very 
 little of it can be derived from the bright little brooks 
 and torrents which rush down from the melting snows 
 and the springs on either side of the valley. Yet it un- 
 doubtedly comes out from beneath the glacier. Mud 
 consists merely of the finer particles worn from rocks. 
 There must, therefore, be a great deal of waste some- 
 where to account for this constant and plentiful supply 
 of mud.
 
 312 PHYSICAL GEOGRAPHY. [LESS. 
 
 29. Following this inquiry further, we observe that 
 the rocks which rise on either side of the valley from 
 under the glacier are remarkably smoothed. Under- 
 neath the ice, as we may sometimes detect, the floor of 
 the valley is similarly smoothed and polished. The 
 contrast between the rounded and smoothed outlines of 
 the knobs and hummocks of rock near the ice, and the 
 sharp, rugged forms of the crags above, is often singu- 
 larly well-defined. Besides this smoothing and polishing, 
 the surface of the rocks may be observed to be covered 
 with many parallel or intersecting scratches and groov- 
 ings, varying from such lines as might have been graven 
 by a hard grain of sand to such deep ruts as would re- 
 quire the forcible pressure of some sharp edge or blunt 
 corner of stone. These markings are seen to run in a 
 general direction down the valley. They have been 
 evidently produced by some agent which has descended 
 with sufficient force and steadiness to grind the rocks 
 along the bottom and the lower slopes of the valley. 
 
 SO. It is now and then possible to creep in under the 
 ice at the end of the glacier, and to see where it rests 
 upon its rocky bed. At such times we may, as it were, 
 catch the glacier in the very act of grinding down and 
 striating the rocks below it. Pieces of stone and grains 
 of sand are jammed between the ice and the rock over 
 which it moves. Held there, and pressed against the 
 rock, they groove and scratch it. As this goes on year 
 after year, the surface of the rock necessarily undergoes 
 continual waste, and acquires that smoothed, polished, 
 and striated appearance so characteristic of the bottom 
 and sides of glacier-valleys. 
 
 31. Here, then, is the main cause of the unfailing 
 muddiness of the water that issues from the end of a 
 glacier. The glacier is busily engaged in wearing down 
 its channel with the same kind of grinding powder which 
 a river uses ; but employs these materials in its own way, 
 and forms with them a peculiar smoothed and grooved
 
 xxviii.] GLACIERS. 313 
 
 surface, such as no other agent in nature can produce. 
 The earth, sand, and stones that fall from the moraine 
 heaps through the crevasses, or between the glacier 
 and its rocky sides, form the hard materials by which 
 the ice erodes its channel. While employed to wear 
 down the solid rocks, they themselves are undergoing 
 constant wear and tear. They become smoothed, pol- 
 ished, and striated like the solid rocks, over which they 
 are ground along. One of these ice-smoothed and stri- 
 ated stones is represented in Fig. 71. 
 
 FIG. 71. Stone polished and striated under glacier-ice. 
 
 32. We conclude, then, that another great task in 
 which every glacier is ceaselessly engaged is the erosion 
 of the sides and bottom of its valley. That this is 
 an important work in regard to the general scenery of 
 a country may be shown by the great height and the 
 long distances to which the peculiar forms of ice-worn 
 rocks may be traced. The former greater thickness and 
 wider extent of the Alpine glaciers are not more de- 
 cisively shown by the dispersion of the erratic blocks 
 than by the range of the polished and striated rock-sur- 
 faces. By this test it can be proved that a great part 
 of Northern Europe and America has been under mov- 
 ing sheets of land-ice, which, passing over the land from 
 mountain-ridge to seashore, have left behind them their
 
 314 PHYSICAL GEOGRAPHY. [LESS. 
 
 memorial in the almost indelible markings engraved 
 upon the rocks. 
 
 33. Again, no one can see the tumultuous body of 
 discoloured water escape from a glacier without appre- 
 ciating that in time a sensible deepening of the valley 
 must take place in consequence of this ceaseless erosion 
 and removal of materials. The deepening cannot be 
 uniformly spread over the whole of the valley. There 
 are places where the ice exerts more grinding power than 
 at others, as a river at its falls and rapids effects more 
 destruction than among meadows and plains. The 
 rocks, too, of the glacier's bed vary much in hardness 
 and power of resistance. Some parts must be more 
 readily scooped out than others. So that, should the 
 glacier retreat up the valley, these more deeply excavated 
 portions, unless concealed under moraine rubbish, would 
 become basins filled with water. Lake-basins of this 
 kind in the midst of ice-worn rocks are a marked feature 
 of glacier districts, and of all those regions of Northern 
 Europe and Northern America which have just been re- 
 ferred to as having been at one time buried under ice 
 (Lesson XXVI. Art. 4). 
 
 LESSON XXIX. The Sculpture of the Land. 
 
 1. In Lesson XX. the leading features of the general 
 external form of the land were described its mountains, 
 table -lands, valleys, and plains. At the end of that 
 Lesson the question naturally arose whether any ex- 
 planation could be given of the origin and history of 
 these various features, but the answer to this question 
 was postponed until after some consideration had been 
 given to the nature of the materials and the internal 
 constitution of the globe, and to the operations of water 
 upon the surface of the land. We are now, therefore, 
 in a position to return to the subject, and to apply to
 
 xxix.] THE SCULPTURE OF THE LAND. 315 
 
 the investigation of it the facts and deductions about the 
 interior and exterior of the land which have come before 
 us in the last eight Lessons. 
 
 2. The existing land consists of the higher parts, pro- 
 jecting above sea-level, of those ridges into which the 
 exterior of our planet has been wrinkled during its 
 gradual consolidation and contraction from an original 
 fluid or viscous condition (Lesson XXII. Art. 20). We 
 must not hastily conclude, however, that the land, such 
 as we now see it, is the original surface of the solid 
 globe. That it cannot be so must be evident from two 
 considerations, to which reference has already been made, 
 rst. All the land, as far as we know, has been under 
 the sea, and, even up to its mountain tops, consists in 
 great part of hardened and altered sand, mud, and other 
 materials, which were originally laid down upon the 
 floor of the sea (Lesson XXI. Art. 13). 2d. The whole 
 surface of the land is subject to a constant and enormous, 
 though unequally distributed decay and removal. A 
 comparatively short period would suffice for the entire 
 destruction of the continents, were their surfaces to be 
 continuously wasted even at the rate of the Mississippi's 
 operations and other rivers work considerably faster. 
 At any probable rate of degradation, the land surface 
 that first appeared above the earliest ocean must have 
 been long since destroyed, and we can hardly hope to 
 find any trace of it, even buried under the later accumula- 
 tions of which the continents consist (Lesson XXVII. 
 Art. 22). 
 
 3. But though no portion of the present land can be 
 looked upon as part of the original or earliest solid 
 surface of the planet, there can be no doubt that the 
 existing continents must be very old. Not improbably 
 they occupy the sites of the first ridges that appeared 
 upon the cooling and shrinking mass of the globe. In 
 the course of ages, these primeval ridges would be worn 
 down by the action of water and air. But from time to
 
 PHYSICAL GEOGRAPHY. 
 
 [LESS. 
 
 time, if renewed uprisings took place along the same 
 original lines, the land would be formed and destroyed, 
 and then formed and destroyed again. That this view 
 is not mere theory, but rests on a strong basis of pro- 
 bability, may be shown by a consideration of the way 
 in which the materials forming the land have actually 
 been put together. If the existing ridges can be proved 
 to have been upraised again and again during past ages, 
 they may at least be plausibly conjectured to mark 
 generally the primeval lines of elevation on the surface 
 of the globe. 
 
 FIG. 72. Quarry in flat stratified rocks. 
 
 4. Let us, then, return to the composition of the earth 
 described in Lesson XXI. All over the globe, it is 
 found that by far the largest mass of the land is built up 
 of materials that have been slowly accumulated as sedi- 
 ment on the floor of the sea. These materials are 
 arranged in layers or strata which have been laid down 
 upon each other, until a depth of many thousands of feet 
 has been formed. It is evident that the original posi- 
 tion of these strata must have been nearly or quite hori- 
 zontal, seeing that they were piled one upon another, as 
 sand and mud are laid down on the level or gently slop-
 
 xxix.] THE SCULPTURE OF THE LAND. 317 
 
 ing bed of the sea at the present time. The subterranean 
 movements whereby they have been raised above the 
 sea into dry land, have taken place over such wide re- 
 gions that this original level or gently inclined position 
 may remain. The flat bedding of the rocks shown in 
 Fig. 26 is of common occurrence, and most people will 
 recognise the familiar look of such sections as that of 
 the quarry represented in Fig. 72. In such cases of 
 horizontality, we may conceive a large tract of the sea- 
 floor to have been raised up into land so uniformly and 
 equably, that the sheets of hardened sand and mud re- 
 mained nearly, or quite in their original level condition. 
 In Central and Northern Russia, in China, and in the 
 Western Territories of the United States, this gentle 
 and equable upheaval has taken place over regions many 
 thousands of square miles in extent. 
 
 5. While the earth has been contracting as a result 
 of its cooling, the effects of this contraction have not 
 been uniformly distributed over the surface. The vast 
 basins of the oceans, no doubt, mark the regions where 
 the subsidence has been greatest. Probably they have 
 been depressions from the beginning, though portions of 
 them, particularly along their margins, have from time 
 to time alternately risen and sunk. Every tract which 
 sinks requires to accommodate itself to a diminished 
 superficial area, and therefore exerts a strong lateral 
 thrust upon the adjoining more stable parts. Under the 
 influence of this force, long ridges have been raised into 
 land between the ocean basins. Every successive sub- 
 sidence may thus have carried with it a corresponding 
 upheaval. So that, as regards the whole globe, although 
 subsidence was the rule, and although the land was being 
 continually wasted by air, rain, rivers, and the sea, never- 
 theless these periodic uplifts along the same general 
 lines of movement, that is, along the axes of the conti- 
 nents, have compensated for the loss, and seem to have 
 maintained, on the whole, the balance of dry land.
 
 PHYSICAL GEOGRAPHY. 
 
 [LESS. 
 
 Ja 
 
 41* 
 
 6. But it could hardly happen, in the midst of these 
 movements, that the upheaval should be 
 always so gentle and uniform as not to 
 disturb the original level, or nearly level, 
 position of the strata. On the contrary, 
 over wide tracts of land, and more parti- 
 cularly along vast extended lines, the rocks 
 have not only been upraised, but have 
 been crumpled up and broken. Instead 
 of remaining in horizontal or slightly in- 
 clined sheets, they may be found tilted up 
 in all directions, and often placed on end 
 like books on a library-shelf. Every great 
 mountain-chain furnishes examples of these 
 more complex arrangements. On the plains 
 and lowlands, the rocks may stretch for 
 hundreds of miles as level as before they 
 rose out of the sea. But towards the 
 interior they begin to bend in wave-like 
 undulations, which increase in magnitude 
 until, along the flanks of the mountains, 
 the rocks are sometimes found actually so 
 thrown over that the lowest lie uppermost. 
 This structure is explained in the accom- 
 panying diagram (Fig. 73). 
 
 7. In the form of mountain-structure, 
 illustrated in Fig. 73, there is evidence of 
 only one general upheaval which took 
 place after the formation of the various 
 rocks of the region, for these have partici- 
 pated in the change of position. In the 
 example represented in Fig. 74, there is 
 proof of two upheavals. First, the older 
 series of rocks, A, was contorted and 
 raised ; then, against its sides and upon 
 its broken and worn edges, the series B 
 was formed, and thereafter raised into land. A still 
 
 
 '3 6
 
 xxix.] THE SCULPTURE OF TIJE LAND. 319 
 
 longer succession of movements is shown in Fig. 75. 
 We there see that after the two movements which dis- 
 turbed the rocks A and B, a third uplift took place, 
 whereby a still newer series, C, which had been deposited 
 upon the side of B, came to be raised into land. 
 
 8. Two facts about mountains are presented to the 
 mind by such sections as these. First, that a great axis 
 of elevation on the earth's surface, in other words, a 
 mountain chain, may have again and again served as a 
 line of relief from the strain of terrestrial contraction, 
 and may have consequently been successively pushed 
 up between subsiding areas on either side. And secondly, 
 
 A 
 
 FIG. 74. Section across a mountain-chain, showing two successive 
 periods of uplift. 
 
 that between each period of uplift there has been great 
 waste of the upraised land air, rain, frosts, brooks, rivers, 
 glaciers, and the sea, all wearing down the surface and 
 producing the materials out of which the next series of 
 rocks was formed. 
 
 9. It is evident also that by means of such sections 
 we may compare the relative dates of different mountain- 
 chains. If the rocks represented in Fig. 73 be the same 
 as those marked C in Fig. 75, then the mountain shown 
 in the former section must be much younger than that 
 shown in the latter. We see, indeed, that the mountain 
 in Fig. 7 5 is not only older, but older by two earlier up- 
 heavals. It is the province of the science of Geology to 
 investigate these questions regarding the structure and 
 age of mountain -chains. Geologists have discovered 
 that among the rocks of the earth's crust a chronological 
 order can be made out, and they can thus assign to each
 
 320 PHYSICAL GEOGRAPHY. [LESS. 
 
 great elevation on the surface of the globe its relative 
 date in the history of mountains. 
 
 1O. But while the existence of dry land and of the 
 mountain-chains which traverse it must be ascribed to 
 movements of the solid crust of the earth, the present 
 aspect of the surface of the land cannot be its original 
 aspect, but must have been largely determined by the 
 action of those various agents which wear down its sur- 
 face (Art. 2). The vast amount of mud annually trans- 
 ported into the sea by rivers proves how much material 
 is continually removed from the land, and therefore, how 
 
 FIG. 75. Section across a mountain-chain, showing three successive 
 periods of uplift. 
 
 greatly, though it may be insensibly, the height and 
 appearance of the mountains and valleys must, in course 
 of time, be changed. This progress of disintegration 
 and removal is going on all over the globe, here more 
 rapidly, there more slowly, but always advancing, and 
 always involving changes in the aspect of the land. In 
 the lapse of the long periods of time during which it 
 has been in progress, how vast must have been the 
 mutations on the surface of the globe, how many suc- 
 cessive mountain ranges may have been upraised and 
 worn away ! 
 
 11. The working of the chief agents that erode the 
 surface of the land has been described in previous lessons 
 the air, by its gases and vapours, its winds and changes 
 of temperature ; frost by its oft-renewed wedges of ice ; 
 rain, brooks, and rivers, by their movement down the 
 land, and their power to sweep along the loosened debris ; 
 avalanches and glaciers, by the fragments of stone with
 
 xxix.] THE SCULPTURE OF THE LAND. 321 
 
 which they grind and polish the rocks of the valleys ; the 
 sea, by the waves thrown incessantly against its shores. 
 We may compare the general results of the co-operation 
 of all these forces to the work of a sculptor. They are, 
 so to speak, the different tools with which the framework 
 of the land is carved. But the sculpture they achieve 
 is never completed. It goes on continuously so long as 
 the land remains above the sea. 
 
 12. At first it may seem almost incredible that the 
 whole surface of the land, even the loftiest and stateliest 
 mountains, should thus be crumbling down. But the 
 more we search for proofs of the assertion, the more clear 
 and abundant do they become. We learn that, whatever 
 may have been the aspect of the land when first pushed 
 out of the sea, it has been, and is now being, chiselled from 
 its highest peaks down to'beiow the tide-marks. Its cliffs 
 and pinnacles are split upland grow more shattered and 
 sharp every year. Its ravines are widened and deepened. 
 Its hilly surfaces become more roughened and more 
 deeply seamed by the lines which running water traces 
 over them. Its valleys and plains are levelled and 
 strewn with debris washed down from higher grounds. 
 
 13. In travelling from place to place we cannot fail 
 to notice the evidence of this universal decay, and, on 
 further observation, to remark that, though the wearing 
 down of the land may be traced more or less clearly 
 everywhere, its rate and the changes of scenery which it 
 brings with it depend very much upon the nature of the 
 rocks of each region. Here again we may have recourse 
 to the simile of the sculptor's work. The character of a 
 statue depends not only on the design and manipulation 
 of the artist, but also on the material employed. Out of 
 a piece of granite or of pudding-stone, no matter what 
 amount of genius and skill were bestowed on it, the same 
 effect could never be produced as from a block of white 
 marble. So we find that the hills, valleys, and mountains 
 differ from each other, in great measure, according to 
 
 Y
 
 322 
 
 PHYSICAL GEOGRAPHY. 
 
 [LESS. 
 
 the nature of the rocks of which they consist. In our 
 journeys, whether in our own district or in other regions, 
 we shall find it not uninteresting nor uninstructive to take 
 note of the changes in the aspect of the scenery through 
 which we pass, and to connect them with variations in 
 the character of the rocks. 
 
 FIG. 76. Scene on the Coast of Caithness. Influence of joints among 
 stratified rocks in the formation of vertical cliffs and outstanding square 
 blocks of rock. 
 
 14. Apart from the varying nature of the materials, 
 nothing contributes more to the character of scenery 
 than those lines of division or " joints " which have 
 already been referred to as traversing all rocks. They 
 serve as channels for the descent and reascent of sub-
 
 xxix.] THE SCULPTURE OF THE LAND. 323 
 
 terranean water (Lesson XXIV. Art. 17). They are 
 
 FIG. 77. 'Portion of the west front of Salisbury Crag, Edinburgh, showing 
 the influence of joints in promoting the splitting up of the igneous rock, 
 and the preservation of a vertical face to the cliff. 
 
 made use of by frost as the lines along which the wedges
 
 324 PHYSICAL GEOGRAPHY. [LESS. 
 
 of ice are driven most effectively into exposed faces of 
 rock (Lesson XXVIII. Art. 10). Every mountain-peak 
 and cliff, indeed, every large naked mass of rock which 
 projects into the air, owes more or less of its character- 
 istic outlines to the way in which its joints have been 
 split open. Among the stratified rocks the joints allow 
 vertical cliffs to be formed and large square buttress-like 
 masses to project from the cliff or even to be isolated 
 from them. The foregoing drawing (Fig. 76), for ex- 
 ample, shows how the forms of coast -cliffs are deter- 
 mined by the position of the intersecting lines of joint, 
 each vertical face of rock corresponding with the direc- 
 tion of one of these lines. Where such rocks form lofty 
 mountainous ground, they are often carved into the most 
 picturesque forms of pinnacle and buttress. Again, 
 among the unstratified rocks, such as granite and basalt, 
 the influence of the joints is no less marked, as, for 
 instance, where it defines the ledges and rifts in a preci- 
 pice, or where it has allowed the most solid rock to be 
 so completely shattered as to look like a huge mass of 
 ruin. In Fig. 77 a representation is given of part of 
 the face of a basalt cliff, where abundant joints traverse 
 the rock in such a way as to divide it into rude prisms, 
 which are gradually wedged off from each other by frost, 
 until, detached at last, they fall to the base of the preci- 
 pice. The ruined masses below are further broken up 
 and carried away piecemeal by frost, rain, and in the 
 general process of " weathering," or in other cases, by 
 waves along a sea-shore or by the flow of a brook or 
 river. Their removal permits the continuous decay of 
 the cliff and preserves the steep face of the precipice, 
 which thus slowly recedes as slice after slice is cut away 
 from its front. But where the detached blocks gather 
 at the base, they form in the end a protecting bulwark, 
 and either retard or prevent the farther recession of the 
 line of cliff. 
 
 15. Among the higher parts of the mountains this
 
 xxix.] THE SCULPTURE OF THE LAND. 325 
 
 kind of rude chiselling of the rocks is most conspicuous. 
 Not only are steep crags and lofty precipices formed, 
 but the very mountain ridges are cut away into sharp 
 crests. These, again, still further split and splintered 
 by the severe frosts and furious storms of the mountain 
 climate, are cut into slender pinnacles and spires, some- 
 times at a distance seeming so needle-like in their slimness 
 and sharpness, as to have received, among the Alps, the 
 name of aigralles, or needles. The blocks loosened from 
 these high crags and crests furnish abundant stones for 
 glacier moraines (Lesson XXVIII. Art. 22). 
 
 16. From the top of a high hill, or of a mountain 
 beneath the snow-line, one may sometimes look down 
 upon a wide region and mark, as in a vast map or model, 
 how the little gullies on the sides of the slopes widen out 
 into larger channels, how these run together into valleys, 
 and how the whole landscape seems thus to be trenched 
 with water-courses. One who has the good fortune to 
 see such a scene as this, after having learned to appre- 
 ciate how ceaselessly and potently every brook and river 
 is cutting out its channel (Lesson XXVI.), will realise 
 more vividly than from any map or description how the 
 valleys are carved out by the power of running water. 
 Every little gully and ravine down the steeper declivities 
 is a sample of the way in which the forms of the solid 
 land are changed ; every gradation of size and shape 
 may be traced, from the trench that was opened by some 
 storm last winter, up to the deep and wide gorge through 
 which a foaming river rushes, or the ample valley down 
 which the collected waters from a whole range of 
 mountains sweep onward to the sea. One may not be 
 able to tell how far the line of any particular valley may 
 have been originally determined by the shape which the 
 ground had when it first rose out of the sea. But, as 
 a heavy shower of rain produces runnels which soon cut 
 out a miniature drainage system on a roadway, so, in 
 the course of time, the flow of brooks and rivers over
 
 326 PHYSICAL GEOGRAPHY. [LESS. 
 
 the surface of the land must necessarily erode systems 
 of valleys. Whatever might have been the original 
 shape of a country, rain and frost, brooks and rivers, 
 snow-fields and glaciers could not have been at work 
 upon it for even a comparatively short time, without 
 carving out valleys for themselves, and sculpturing the 
 mountains into such sharp and rugged forms as they now 
 wear. Everywhere there lie proofs of excavation ; hill- 
 sides are furrowed with gulleys, mountain slopes are 
 trenched with ravines, tablelands are cut down until 
 they become only chains of ridges separating the valleys 
 which have been carved out of them (Lesson XX. Art. 
 
 19). 
 
 17. While the effects of disintegration in roughening 
 the surface of the land are most marked among the high 
 grounds, the results of this process show themselves 
 among the lower regions in the strewing of the crumbled 
 fragments of the hills over the valleys and plains. Every 
 tract of fertile meadow or level field bears witness to the 
 way in which the lowlands have been smoothed and 
 raised in level by the sand and earth spread over them 
 by rain and streams. And yet this increase of height 
 in the plains does not really compensate for the waste 
 of the high-grounds. A little consideration of the matter 
 shows, in the first place, that though the plains do 
 obtain considerable additions to their surface from the 
 materials swept down upon them by rivers, they receive 
 only a part of these materials, the remainder being 
 carried out to sea ; and in the second place, that even 
 the plains themselves are wasted ; floods tear up their 
 soil and sweep away their river-banks. 
 
 18. It appears, then, that the tendency of the process 
 of sculpture, which gives to landscapes their character- 
 istic details of outline, is in the end to reduce the dry 
 land to the level of the sea. But this is not all. While 
 the general surface of the land undergoes attacks from 
 the atmospheric influences its margin is continually
 
 xxix.] THE SCULPTURE OF THE LAND. 327 
 
 suffering from the assaults of the waves. Only the 
 parts of the earth's surface lying under a considerable 
 depth of ocean are protected from decay (Lesson XVIII. 
 Art. 19). Along the margin of the land the waves are 
 gnawing away the coast-line, or are only kept from doing 
 so by the bar of detritus which has been thrown up 
 against them. 
 
 19. Were no other operation to come into play, the 
 natural and inevitable result of this ceaseless destruction 
 would be the final disappearance of dry land. But here 
 we see the meaning and importance of the underground 
 movements already referred to as the results of terrestrial 
 contraction. The great ocean basins have from time to 
 time sunk down, and in so doing have pushed up ridges 
 of land between them. These ridges, on each successive 
 uplift from beneath sea- level, consisted mainly of the 
 more or less consolidated debris worn away from their 
 predecessors. The same materials have thus served, 
 over and over again, to form the framework of the 
 upraised land, and thus, while looking at the subject 
 from one side, we see only ceaseless destruction, moun- 
 tain and valley continually crumbling down before us ; 
 yet, taking a wider view, we perceive that decay of the 
 surface is needed to furnish soil for the support of living 
 plants and animals which people the earth, and that the 
 materials so removed from sight are not lost, but are 
 carefully stored away, to be, in some future time, raised 
 into new land, thereafter to go again through a similar 
 cycle of change.
 
 .328 PHYSICAL GEOGRAPHY. [LESS. 
 
 CHAPTER V. 
 
 LIFE. 
 
 LESSON XXX. The Geographical Distribution of 
 Plants and Animals. 
 
 1. The foregoing Lessons have treated of the parts of 
 the earth, their relation to each other, and the constant 
 changes and reactions between them which constitute the 
 Life of the globe. But above and apart from all these 
 movements within or upon the surface of the earth, 
 another kind of activity and progress now claims our 
 attention, where the forces concerned are not air, sea, 
 and land, but the living energy of plants and animals. 
 Our planet is not merely a theatre for the evolution of 
 physical phenomena. It has been appointed as the 
 dwelling-place of a vast and varied series of living things, 
 which move through the air and people both land and 
 water. The study of these living organisms is comprised 
 under the general name of Biology, or the science which 
 deals with vegetable and animal life. 
 
 2. So vast a study, opening up wide fields of inquiry 
 far beyond those over which we have been travelling in 
 these Lessons, must needs be subdivided into different 
 departments. Thus one branch inquires into the struc- 
 ture and growth of plants, another deals with the way in 
 which plants are distributed over the globe, a third treats 
 of the structure of the various tribes of animals, a fourth 
 follows the action of the different parts of an animal's 
 body and the part which each of these plays in the life
 
 into which the Surface of the 
 
 Earth is divided. 
 (l>r Srioter A A/r A R. Wallace.)
 
 PLATE. X. 
 
 20 40 60L- t 80.rck~.IOO 120 I4O 160
 
 xxx.] BIOLOGICAL REGIONS. 329 
 
 of that body, a fifth arranges the enormous numbers of 
 animal forms in due order, to show their grade in the 
 scale of being, and to allow the general assemblage of 
 living forms in one region to be compared with that in 
 another. These and the other branches of biology deal 
 chiefly with the plants and animals as they are in them- 
 selves, or as they stand in relation to each other. 
 
 3. But it is evident that, just as we ourselves are en- 
 compassed by external conditions of geography, climate, 
 and vegetation which, it may be unconsciously, govern 
 our everyday life, so each plant and animal on the globe 
 comes under the control of similar surrounding influences. 
 Apart, therefore, from the structure, functions, or classi- 
 fication of vegetable and animal life, we may study it 
 with reference to its relation to these external and domi- 
 nant conditions. From this point of view, physical 
 geography and biology are seen to be closely linked to 
 each other. It is impossible to gain any intelligent con- 
 ception of the present distribution of plants and animals 
 over the globe without entering upon inquiries which 
 form part of the scope of physical geography. 
 
 4. We all know how greatly the plants and animals 
 of different quarters of the globe differ from each other. 
 The equatorial regions nourish a rank and luxuriant 
 vegetation, including palms, bananas, tall tree -grasses 
 with rope -like lianas twisting round their stems, and 
 bright-hued, strangely-shaped orchids hanging from their 
 branches. The animals are equally characteristic, for 
 they include lions, tigers, elephants, rhinoceroses, camels, 
 giraffes, crocodiles, large serpents, with crowds of gor- 
 geously-plumed birds and brilliant butterflies. 
 
 5. The temperate zone is distinguished by a very 
 different assemblage of plants and animals. The forests 
 and woods show such trees as the oak, ash, elm, syca- 
 more, beech, poplar, birch, hazel, and pine. The dells 
 are bright in spring with snowdrops, anemones, and 
 primroses ; and in summer with speedwells, geraniums,
 
 330 PHYSICAL GEOGRAPHY. [LESS. 
 
 and wild roses. But both the plants and the animals 
 are more sober in colouring than in the hotter parts of 
 the earth. The birds include thrushes, larks, and other 
 songsters. Among the wild animals of the low-ground 
 we find mice, rats, weasels, hedgehogs, badgers, otters, 
 foxes ; and in hilly districts wild-cats, wolves, and bears. 
 
 6. Within the Arctic and Polar regions life becomes 
 much less abundant and varied. As we advance, trees 
 disappear, though stunted forms of birch, fir, and willow 
 extend a long way northwards. By degrees these too 
 die out, and the scanty vegetation consists, at last, 
 mainly of mosses and lichens, with saxifrages, gentians, 
 and a few more flowering plants. These snow-covered 
 lands are wandered over by polar bears, white foxes, 
 reindeer, musk-oxen, and ermines ; the seas are fre- 
 quented by seals, walruses, and whales ; while the coasts 
 are sought by large flocks of northern sea-fowl and by 
 snowy falcons, buntings, ptarmigans, owls, and other 
 white-feathered birds. At the extreme northern limits 
 reached by explorers, life of any kind is hardly to be met 
 with among the deep snow-fields and piled-up heaps of 
 ice which cover land and sea. 
 
 7. From the extreme exuberance and variety of plant 
 and animal life in the equatorial and tropical lands there 
 is thus a gradual diminution polewards, until in the far 
 polar regions almost the zero of vegetation and of animal 
 existence is reached. There can be no doubt, therefore, 
 that one grand influence in the distribution of plants and 
 animals is temperature ; warmth being favourable, cold 
 unfavourable, to the growth of living things. 
 
 8. But if temperature were the sole cause that deter- 
 mined the character of the plants and animals of any 
 country, then every zone of latitude should be marked 
 by the same kind of vegetation and by the same groups 
 of animals. It would then be enough to know the geo- 
 graphical position of any place to be able to tell what 
 must be its flora^ that is, its assemblage of plants, and
 
 xxx.] BIOLOGICAL REGIONS. 331 
 
 its fauna, or population of animals. But a very little 
 inquiry suffices to show that no such strict coincidence 
 between latitude and the distribution of plants and animals 
 really exists. The Old and the New Worlds are traversed 
 by the same isothermal bands (Lesson IX. Art. 2), and 
 have similar kinds of climate, soil, and exposure. Yet, 
 in regions closely resembling each other as to conditions 
 of physical geography, the plants and animals, though 
 sometimes presenting such general resemblance as to 
 show them to be mutually representative of each other, 
 are often widely different in the two hemispheres. The 
 lion of the Old World gives way to the puma in the 
 New ; the tiger is replaced by the jaguar ; the elephant, 
 rhinoceros, and hippopotamus, by tapirs and peccaries ; 
 camels by vicunas ; apes and baboons by flat-nosed 
 monkeys and marmosets. The birds are not less dis- 
 tinct ; the Old- World vulture being represented by the 
 New-World condor ; sun-birds by humming-birds. 
 
 9. The present arrangement oi plants and animals 
 over the earth's surface is one of the most difficult ques- 
 tions with which science has to deal. The first step 
 towards its investigation must necessarily be a careful 
 inquiry into the actual facts regarding the distribution of 
 the various kinds of plant and animal life. This inquiry 
 has been carried on by naturalists in all parts of the 
 world with the result of enabling them to parcel out 
 the earth's surface into distinct regions and sub-regions, 
 each characterised by a peculiar flora and fauna, though 
 sometimes containing a number of the same plants and 
 animals in common. These regions, to which the fol- 
 lowing names and limits have been assigned, are shown 
 in Plate X. 1 
 
 O. (i.) Palsearctic Region, comprising all Europe 
 with the temperate parts of Asia, and the tracts of Africa 
 
 1 The arrangement here followed is that of Mr. Sclater, as modified by 
 Mr. Wallace in his admirable work on The Geographical Distribution oj 
 Animals.
 
 332 PHYSICAL GEOGRAPHY. [LESS. 
 
 lying to the north of the Sahara desert ; or the northern 
 parts of the Old World, from Iceland to Behring Strait, 
 and from the Azores to Japan. In the Arctic portions 
 of this vast region the vegetation is comparatively meagre, 
 showing an abundance of mosses and lichens, which, in 
 the tundras of Siberia, cover thousands of square miles 
 of barren waste. In favourable places, flowering plants, 
 such as saxifrages and gentians, peep out from beneath 
 the snows during the short but warm summer ; stunted 
 forms of willow, azalea, and rhododendron form here 
 and there a kind of scrub upon the slopes, while south- 
 wards from the mean annual isotherm of 32, pine-trees 
 make their appearance and increase in number, till they 
 form wide ranges of dark forest, as in Norway, and on 
 the higher mountain groups farther south. Between the 
 isotherms of 40 and 60 the sombre pine-forests, retain- 
 ing their leaves throughout the year, give place to a 
 more varied and luxuriant deciduous vegetation, which 
 sheds its leaves in autumn and renews them again in 
 spring. The trees include many noble kinds, birch, 
 alder, beech, ash, oak, elm, sycamore, walnut, chestnut, 
 and maple. Northern fruits, like the cranberry, cloud- 
 berry, bilberry, strawberry, currant, and raspberry, are 
 succeeded farther south by luscious pears and apples, 
 almonds, olives, figs, grapes, and oranges. The cereals 
 wheat, barley, oats, etc. are abundantly cultivated 
 throughout most of the region. In the more southern 
 countries, with a mean annual temperature, between 60 
 and 70, the trees do not lose their leaves in winter. 
 Here we find the evergreen oak, myrtle, and laurel, with 
 some plants, such as palms, which more properly belong 
 to the tracts lying nearer the equator. 
 
 11. The animals of the region show a development 
 similarly connected with the distribution of temperature. 
 In the extreme north, white bears and foxes, reindeer, 
 whales, walruses and seals, many peculiar sea-birds, 
 together with white owls and ptarmigan, form a distinct
 
 xxx.] BIOLOGICAL REGIONS. 333 
 
 assemblage. As most of these species disappear south- 
 wards, their places are taken by brown bears, badgers, 
 otters, horses, buffaloes, fallow and roe-deer, chamois, 
 wild goats, wild sheep, hares, rabbits, moles, hedgehogs, 
 and dormice ; golden eagles, hawks, grouse, pheasants, 
 house-sparrows, magpies, jays, and thrushes. Farther 
 south the camel is the most characteristic animal. 
 
 12. (ii.) The Ethiopian Region embraces Central 
 and Southern Africa, with the tropical part of Arabia, 
 Madagascar, and the neighbouring islands. To the south 
 of the great desert of Sahara the western portion of the 
 African continent is largely covered with dense forests, 
 where the oil-palm, the huge baobab, euphorbias, big- 
 nonias, tamarinds, and many other tropical plants, form 
 a dense luxuriant vegetation in the hot moist air of that 
 climate. Farther to the east lie vast elevated lands, 
 covered with tall grasses and sedges, and dotted with 
 patches of forest. The flora of the southern part of 
 Africa is distinguished by its great variety of heaths, its 
 fig-marigolds, carrion-scented stapelias, aloes, and pelar- 
 goniums. 
 
 13. The fauna of the Ethiopian region is marked on 
 the one hand by the absence of such wide-spread animals 
 as camels, deer, goats, sheep, wild oxen, wild boars, and 
 bears ; and, on the other hand, by the presence of many 
 remarkable forms of life, including the gorilla, chim- 
 panzee, baboon, lemurs, aye -aye, lion, leopard, civet, 
 hyaena, zebra, rhinoceros, hippopotamus, giraffe, antelope, 
 elephant, ostrich, ibis, flamingo, chameleon, and crocodile. 
 
 14. (iii.) The Oriental Region includes Southern 
 Asia, from the mouth of the Indus along the southward 
 slopes of the Himalaya mountains and the Chinese up- 
 lands to Ningpo, with Formosa, the Philippines, Borneo, 
 and the Malay Islands as far as the south-east end of 
 Java. Much of the surface of this region is covered 
 with dense forests of tropical vegetation. Among the 
 better-known plants occur the ginger, arrow-root, banana,
 
 334 PHYSICAL GEOGRAPHY. [LESS. 
 
 cocoa-nut, screw-pine, yam, bamboo, rice, gourd, custard- 
 apple, mango, coffee -tree, mangrove, ebony -tree, big- 
 nonia, hemp, and sandalvvood. 
 
 15. The fauna contains some characteristic animals 
 the ourang-utan, long-armed monkeys, flying lemur, 
 many civets, the tiger, hyaena, jackal, wild cattle, ele- 
 phant, rhinoceros ; many bright-feathered birds, as tro- 
 gons, hornbills, goat-suckers, sunbirds, long-tailed parrots, 
 and peacocks ; numerous reptiles, including ground and 
 tree-snakes, cobras, and crocodiles ; and a vast assem- 
 blage of insects, among which the size and brilliancy of 
 the butterflies and many of the beetles are remarkable. 
 
 16. (iv.) The Australian Region, embracing 
 Australia, New Zealand, and the numerous islands to 
 the east of Java, Borneo, and the Philippine group, con- 
 sists wholly of islands, which, lying apart from all the 
 great continental masses of land, show a peculiar assem- 
 blage of plants and animals. The vast insular expanse 
 of Australia, situated partly within and partly without 
 the tropics, and exposing a wide desert interior to the 
 hot rays of the sun, while its coast-line is washed by the 
 open sea, presents contrasts of climate not met with in 
 the smaller islands of the region. From its she, also, 
 and its proximity to the south-eastern limits of the Ori- 
 ental region, it contains a greater diversity in its flora 
 and fauna. Over the dry and warm tracts of Australia, 
 the general heath-like vegetation is marked by a per- 
 vading dead blue-gi;een colour, with dull leaves so ar- 
 ranged upon the plants as to afford but little shade. 
 The eucalyptus, or gum-tree, and other trees and shrubs 
 bearing bright honeyed flowers, together with thickets of 
 acacia and scattered marsh-oaks, give a peculiar charac- 
 ter to the forest-lands. Vast regions are covered with 
 grasses, and furnish good pasture. Along the northern 
 limits, where this region borders the Oriental islands, 
 the flora contains some of the forms of vegetation found 
 more plentifully towards the north and north-west, such
 
 xxx.] BIOLOGICAL REGIONS. 335 
 
 as pandanus, cabbage-palm, fig, nutmeg, and sandalwood. 
 On its southern margin, where the climate assumes a 
 moister and more temperate character, ferns, cycads, 
 and pines abound, while the heath -like epacris, and 
 numerous proteas enliven the surface with their bright 
 blossoms. New Zealand is distinguished by the verdure 
 of its flora, which consists largely of ferns, often growing 
 as trees, and many kinds of pines. The scattered 
 islands of the Pacific have their cocoa-nut palm, bread- 
 fruit, tacca, grasses, and sedges. 
 
 17. The fauna of the Australian region is the most 
 peculiar on the face of the globe, both for the types 
 which it contains, and for the almost universally diffused 
 forms which it has not. No apes or monkeys chatter 
 in its woods, no wild horses, cattle, or sheep browse 
 over its pastures, no wolves, foxes, tigers, or other simi- 
 lar beasts of prey prowl across its hills and valleys. 
 The place of these various and wide-spread species is 
 taken in Australia by a totally distinct and less highly- 
 organised class of animals called marsupials, of which 
 the kangaroo may serve as the type. Of these there 
 are many varieties, some living on fruits, others on roots, 
 others on smaller animals of their own kind, others on 
 insects ; some keeping to the ground, others taking to 
 trees. The birds likewise are peculiar, for they include 
 the bird-of-paradise, lyre-bird, cassowary, paroquets, and 
 honey-suckers. 
 
 18. (v.) Neotropical Region. Under this term 
 are included the whole of South America, the islands of 
 the Antilles group, and the tropical part of North America. 
 Ranging across the whole zone of the tropics, and as far 
 south as the 5 6th parallel of south latitude, and rising up 
 to the snow-line in the Andes, this region presents many 
 varieties of climate, which are well shown by differences 
 of vegetation. The lower grounds within the tropics 
 present the most luxuriant flora in the world. It abounds 
 in mangroves, palms (cabbage -palm, ivory-palm, and
 
 336 PHYSICAL GEOGRAPHY. [LESS. 
 
 other kinds), bananas, tree-ferns, and mimosas, growing 
 in dense jungles, and having their stems and branches 
 clustered round with many smaller plants, such as lianas 
 and ferns, or gorgeously-blossomed cactuses, orchids, and 
 passion-flowers. The vast plains or llanos of the Orinoco, 
 with their tall grasses and occasional clumps of pines 
 and mimosas, represent the pasture lands of the Old 
 World, but with their bright lilies show a brilliancy of 
 colour peculiar to themselves. Farther south the basin 
 of the La Plata presents similar plains or pampas, which, 
 getting less and less luxuriant in their vegetation, are at 
 length succeeded by the barren moors of Patagonia and 
 Tierra del Fuego. On the lower mountain slopes, 
 characteristic trees are cinchonas, from the bark of 
 which quinine is prepared. Mahogany, rosewood, the 
 indiarubber tree, with many plants yielding spices, bal- 
 sams, and perfumes, give a distinctive character to the 
 South American flora. On the more elevated tracts, 
 calceolarias, gentians, and low -growing plants occur, 
 that remind the traveller of some features of mountain 
 vegetation near the snow-line in the Old World. 
 
 19. The fauna of this region is more varied than that 
 of any other. It contains the peculiar jaguar, flat-nosed 
 monkeys and marmosets, blood-sucking bats, chinchillas, 
 sloths, armadilloes, ant-eaters, racoons, opossums, deer, 
 llamas, alpacas, vicunas, tapirs, and peccaries ; but no 
 native sheep or oxen. Among the birds occur condors, 
 curassows, rheas, or American ostriches, toucans, jaca- 
 mars, mot-mots, macaws, and numerous forms of hum- 
 ming-bird. The reptiles include the boa-constrictor and 
 many other serpents, the alligator, crocodiles, tortoises, 
 and turtles. The insect life is immensely abundant and 
 varied. 
 
 20. (vi.) The Nearctic Region embraces all North 
 America lying to the north of the tropic of Cancer. Its 
 greatest breadth lies towards the cold northern regions ; 
 whence it rapidly narrows southward, so as to be con-
 
 xxxi.] PLANTS AND ANIMALS. 337 
 
 nected with the Neotropical region merely by a narrow 
 strip of land. This isolation is accompanied by a some- 
 what less varied flora and fauna than in the correspond- 
 ing region of the Old World. The plants and animals, 
 taken as a whole, present much less contrast to those of 
 the Palasarctic region than those of the Neotropical and 
 Ethiopian regions do to each other. The northern tracts 
 of North America extend far within the Arctic Circle, 
 into the snow-covered lands where vegetation reaches its 
 lowest point of development. The southern limits of 
 the province, on the other hand, lie towards the tropical 
 zone, where the sugar-cane, yucca, cotton, maize, and 
 tobacco are characteristic plants. In California and 
 Oregon many large and distinct kinds of pine occur in 
 the forests, such as the gigantic Sequoia (Wellingtonia) 
 and the Douglas pine. Eastward of the Rocky Moun- 
 tains vast undulating pasture lands or prairies stretch 
 over the basin of the Mississippi and its tributaries. 
 The British Possessions are covered with extensive 
 forests, which, as the country becomes peopled, are 
 gradually giving way to pasture and cultivation. 
 
 21. The fauna varies with the latitude. In the north 
 are found musk-sheep, moose-elks, reindeer, gluttons, 
 skunks, racoons, beavers, lemmings, jumping-mice, and 
 tree -porcupines. Farther south vast herds of bisons 
 roam over the prairies. Other typical animals are the 
 grizzly bear, black bear, puma, lynx, prong-horned ante- 
 lope, prairie-dog, flying-squirrel, pouched-rat, opossum, 
 humming-bird, blue-crow, and rattlesnake. 
 
 LESSON XXXI. The Diffusion of Plants and Animals. 
 
 Climate. Migration and Transport. Changes 
 of Land and Sea. 
 
 1. It is not enough to know how the various tribes of 
 Z
 
 338 PHYSICAL GEOGRAPHY. [LESS. 
 
 plants and animals are distributed over the surface of the 
 globe. We are irresistibly led to ask ourselves why and 
 how their distribution has come to be as we have traced 
 it in the preceding lesson. Not very long ago, men were 
 content with supposing that the present arrangement had 
 always existed, ever since the different continents and 
 islands rose out of the sea and received their earliest 
 inhabitants. 
 
 2. But in the gravels and clays beneath the soil, or 
 in the limestones, sandstones, and other rocks lying 
 below, traces have been found all over the world of 
 older and different plants and animals, which occupied 
 the land before the present denizens had appeared. 
 Our modern horses and cattle, for example, were pre- 
 ceded by other kinds which are no longer living. The 
 bears, wolves, and hyaenas of to-day are not quite the 
 same as those of which the teeth and bones are found 
 in caves and ancient alluvial deposits. 
 
 3. A thoughtful study of this subject suggests three 
 main influences which may have guided the distribu- 
 tion of the present faunas and floras of the earth's sur- 
 face, ist. Climate. 2d. Migration and transport. 3d. 
 Changes in the form and height of the land and in the 
 depth and extent of the seas. A knowledge of the nature 
 and effect of these influences helps us to understand 
 much that would otherwise be inexplicable, but there 
 still remain, and perhaps must ever remain, many diffi- 
 culties which no amount of research may be able to 
 remove. 
 
 I. Climate. 
 
 4. This term includes the general temperature, mois- 
 ture, winds, and other atmospheric conditions which 
 prevail in any district, and which directly affect the 
 growth and vigour of plants and animals. From the 
 statements made in Lesson IX. it appeared that the
 
 xxxi.] PLANTS AND ANIMALS. 339 
 
 climates of different portions of the globe greatly differ, 
 and some of the causes of such differences were there 
 traced. But a knowledge of the annual distribution of 
 heat at any place, though it gives us one main element 
 in determining the climate of that place, requires to be 
 enlarged by further knowledge respecting the rainfall, 
 the direction of the prevalent winds, the shape, height, 
 and position of the ground, the character of the soil, 
 vegetation, and other more local details. Since Lesson 
 IX. we have been led over most of these subjects, so 
 that we may now return to the question of climate in 
 reference to the arrangement of plants and animals. 
 
 5. On due consideration of this subject, five distinct 
 influences by which the climate of any place is deter- 
 mined may be recognised. 1st. Distance from the 
 equator. 2d. Distance from the sea. 3d. Height 
 above the sea, 4th. Prevailing winds ; and 5th. Local 
 influences, such as soil, vegetation, and proximity to 
 lakes or to mountains. Each of these causes directly, 
 and often powerfully, controls the spread of vegetable 
 and animal life. 
 
 6. (i.). Distance from the Equator. Climate, 
 having temperature for its main element, must follow 
 generally the course of the isothermal bands over the 
 earth's surface. The warmest climates are necessarily 
 those of the inter-tropical regions, where the sun's rays 
 are vertical, or not much inclined from vertical. In 
 proportion as we recede from the equator, the rays fall 
 more and more obliquely, and the same amount of heat- 
 rays is therefore spread over an increasing breadth of 
 surface, while, moreover, they have to pierce a greater 
 mass of air. Round the poles, the least amount of heat 
 is received, and the climates are coldest. 
 
 7. Were the earth's whole surface either land or water, 
 the climates would be arranged in parallel and regular 
 bands from the equator to the poles. But owing to the 
 way in which land and water are grouped, such an
 
 340 PHYSICAL GEOGRAPHY. [LESS. 
 
 arrangement has been prevented (Lesson IX.). Two 
 places on the same latitude may have very different 
 average temperatures, and therefore very dissimilar 
 climates. Nevertheless, the great predominating influ- 
 ence of position with regard to the equator is, on the 
 whole, maintained in the arrangement of climates. It 
 influences, in a marked way, vegetable growth, as is 
 shown by the time of flowering or of ripening among 
 widely-distributed plants. On the continents, this time 
 becomes later in proportion as the country is distant 
 from the equator. Thus the elm comes into leaf, at 
 Naples about the beginning of February ; at Paris, not 
 until late in March ; and in the centre of England, not 
 until the middle of April. Ripe cherries may be gathered 
 in the south of Italy about the beginning of May ; they 
 are ready in Northern France and Central Germany at 
 the end of June ; but not generally in England for three 
 or four weeks later. Nothing could show more strikingly 
 the difference of climates between the different latitudes 
 of a continent. 
 
 8. But these differences are not merely marked by 
 the variations in the growth of the same plants. As 
 shown in last lesson, when we pass from one climate to 
 another we encounter different plants and different 
 animals. One by one characteristic forms of life drop 
 away, and their places are taken by others. So constant 
 and marked are these changes that such expressions as 
 " an arctic vegetation," " a temperate flora," " a tropical 
 fauna," have passed into general use, and convey a dis- 
 tinct picture to the mind. 
 
 9. We have found, however, that if such a distribu- 
 tion of plants and animals were due to differences of 
 climate alone, wherever the same climate recurs it 
 should be accompanied by the same kind of vegetation 
 and of animal life, but that no general coincidence of 
 this kind exists, when regions remote from each other 
 are compared. The climate of Central Europe closely
 
 xxxi.] PLANTS AND ANIMALS. 341 
 
 resembles that of parts of the United States. Yet the 
 wild animals and birds are strikingly different ; mice, 
 hedgehogs, buffaloes, chamois, and jays in the Old 
 World are replaced by jumping- mice, racoons, opos- 
 sums, bisons, llamas, and humming-birds in the New. 
 In Central South America the forests are tenanted by 
 jaguars, sloths, armadilloes, tapirs, curassows, and tou- 
 cans. On corresponding latitudes in equatorial Africa 
 these animals are represented by lions, leopards, hyaenas, 
 hippopotamuses, elephants, guinea-fowl, and touracoes. 
 In Australia these forms are again replaced by a strange 
 and peculiar assemblage of animals, including kangaroos, 
 wombats, flying opossums, emus, lyre-birds, and crested 
 pigeons. While, therefore, difference of latitude usually 
 means difference of climate and of plant and animal life, 
 identity of latitude with similarity of climate does not 
 necessarily imply agreement in the character of the flora 
 and fauna. 
 
 10. (ii.) Distance from the Sea. The influence of 
 the sea upon the distribution of temperature and moisture 
 has been already described (Lessons IX., X., and XVIII.) 
 As water is more slowly heated and cooled than land, 
 the climates of the sea and of the coasts of the land are 
 much more moist and equable than those of the interior 
 of the land. In proportion, therefore, as places recede 
 from the sea, their climates become more extreme. An 
 insular or oceanic climate is one where the difference 
 between summer and winter temperature is reduced to a 
 minimum, and where there is a copious supply of mois- 
 ture from the large water-surface. A continental climate 
 is one where the summer is hot, the winter cold, and 
 where the rainfall is comparatively slight. 
 
 11. These variations cannot but make themselves 
 visible in the distribution of plant and animal life. They 
 are well shown by contrasting the times of flowering and 
 ripening of the same plants along the Atlantic border and 
 in the central countries of Europe. It will be remem-
 
 342 PHYSICAL GEOGRAPHY. [LESS. xxxi. 
 
 bered that owing to the influence of the warm Atlantic 
 water the temperature of the whole of the north-west of 
 that continent is raised considerably higher than it would 
 otherwise be. (Lesson XVIII. Arts. 7, 8.) Conse- 
 quently vegetation is much earlier in the south of Sweden 
 than in the same latitudes to the east. The lilac and 
 elm begin to show their leaves sooner at Upsala than at 
 Paris, and while winter still reigns to the east of the 
 Baltic, spring blossoms have already spread far up into 
 Scandinavia. 
 
 12. The difference between an insular or oceanic and 
 a continental climate is likewise well brought out by the 
 fact that such evergreens as the Portugal laurel, aucuba, 
 and laurustinus grow luxuriantly even in the north of 
 Scotland, while they cannot withstand the severe cold of 
 the winter at Lyons. 
 
 13. (iii.) Height above the Sea. In Lesson IX. 
 Art. 20 reference was made to the gradual diminution of 
 temperature with increase of elevation above the sea. 
 This cause of variation in climate is of a more local 
 character than the two already illustrated. But its effects 
 upon the spread of plants and animals is singularly well 
 marked. If the fall of the thermometer be taken at i 
 Fahr. for every 300 feet of ascent, we can readily per- 
 ceive that the times of flowering and ripening of the same 
 plants must become later, in proportion to the height of 
 their place of growth, until at last the ground is too high 
 and bleak to let them ripen at all before the winter sets 
 in. They have thus an upper limit beyond which they 
 cannot extend. But while they disappear, other plants, 
 better able to withstand the rigorous climate of the up- 
 lands, take their place. As we climb to higher elevations 
 the familiar vegetation of the plains is gradually succeeded 
 by a vegetation peculiar to the mountains, until at last 
 we reach the edge of the snow line. This influence of 
 height on vegetation is illustrated in a graphical form by 
 Fig. 78.
 
 o o o o o 
 o o o o o 
 o o o o o
 
 344 PHYSICAL GEOGRAPHY. [LESS. 
 
 On the animal world, too, the influence of elevation 
 may be distinctly seen. In Europe rabbits, moles, 
 hedgehogs, otters, foxes, larks, thrushes, lapwings, and 
 many other common forms occur among the lower 
 grounds, while in the mountains such animals as the 
 marmot, goat, ibex, chamois, brown bear, and eagle find 
 their congenial home. 
 
 14. (iv.) Prevailing Winds. Air lying upon the 
 surface of any part of the globe tends to acquire the 
 temperature of that surface. Consequently winds which 
 come from a cold region are cold, those from a warm 
 region are warm. Winds from the sea are usually moist, 
 those from the land are generally dry. Sea-breezes are 
 not liable to the same extremes of temperature as those 
 from the land. The vapour which they carry with them 
 cools the heat of summer and lessens the cold of winter. 
 On the other hand, winds blowing from the interior of a 
 continent are apt to be hot and suffocating in summer, 
 piercingly cold and dry in winter. Winds which come 
 from lower into higher latitudes, or from warmer to cooler 
 climates, have their moisture condensed, and are there- 
 fore rainy, while those which blow from higher to lower 
 latitudes, or from cold to warm regions, are dry. 
 
 15. Much, therefore, in the climate of any place, must 
 be due to the prevailing winds. This is more particularly 
 noticeable on the coasts of the continents, where the 
 winds blow alternately from and to the sea. The striking 
 contrasts between the extremely rainy and almost rainless 
 districts in certain parts of India have already been re- 
 ferred to as showing the great influence of the winds in 
 determining the moisture of a climate. (Lesson X. Arts. 
 32, 34-) 
 
 16. It is evident that, as regards plant-growth, mois- 
 ,ture is hardly less important an element than temperature 
 
 in the climate. Those tracts are most plentifully covered 
 with vegetation which are most copiously watered. Both 
 the abundance and the character of the vegetation de-
 
 xxxi.] PLANTS AND ANIMALS. 345 
 
 pend greatly upon the amount of rain-fall. The west 
 side of the British Islands, for example, which receives 
 the first and largest precipitation of the moisture from 
 the Atlantic, is much greener and more luxuriantly clothed 
 with vegetation than the east side. 
 
 17. Whatever regulates the growth and distribution of 
 plants must tell effectually upon the spread of animals. 
 The herbivorous species naturally haunt those regions 
 where their supplies of vegetable food are most abundant. 
 In their train come the predatory kinds which prey upon 
 them. Any change of climate, therefore, unfavourable to 
 the vitality of the pasture will drive away or even locally 
 exterminate the herds of plant-eating animals, and when 
 they disappear the beasts of prey must vanish also. 
 
 18. (v.) Local Influences. Various minor causes 
 of a more local kind help to modify the climates of differ- 
 ent places, and thereby to affect the flora and fauna. 
 The nature of the soil is one of the most important of 
 these. Wet, marshy ground lowers the mean tempera- 
 ture, seeing that its water absorbs and conveys down- 
 ward the heat which would otherwise warm the soil. 
 Consequently the effect of drainage is to raise the mean 
 annual temperature. In Britain increase from this cause 
 amounts sometimes to as much as l 3 Fahr., which 
 is as great a change as if the drained ground had actually 
 been transported 100 or 150 miles farther south. A 
 waste of sand presents the greatest extremes of climate, 
 for while the dry surface readily absorbs the sun's heat, 
 so as to rise even to 200 Fahr. during the day, it cools 
 rapidly by radiation, and during a clear night may grow 
 ice-cold. 
 
 19. A surface of vegetation prevents the soil from 
 being as much warmed and cooled as it would be if bare, 
 and since leaves never become so hot as soil, they equalise 
 the temperature. A large mass of forest thus exercises 
 a well-marked influence on the climate of the region, 
 tempering alike the heat of the day and the cold of night.
 
 346 PHYSICAL GEOGRAPHY. [LESS. 
 
 20. Similar effects are produced by lakes. The sur- 
 face water, chilled by the cold of winter, descends to the 
 bottom, leaving a warmer layer at the top, which, cooled 
 in its turn, sinks down and allows another warmer portion 
 to lie at the surface. By this means the temperature 
 of the air overlying the water is kept above that of the 
 air overlying the adjoining land, while the colder air 
 from all sides flows down to the lake and is there 
 warmed. (Lesson XXVI. Art. 17.) Many deep lakes 
 do not freeze in winter, and then serve as reservoirs 
 of warmth to keep the temperature of the surrounding 
 ground higher than that of places only a short distance 
 away. On the other hand, during summer the water 
 cools the air lying upon it, and thereby lessens the heat 
 of the locality. 
 
 21. One other local cause affecting climate may be re- 
 ferred to, viz. proximity to hills or mountains. The 
 influence of high ground shows itself in augmenting rain- 
 fall (Lesson X. Art. 29), and in producing currents of 
 air, which, moving alternately up and down the valleys 
 (Lesson XL Art. 8), give rise to gusts and blasts of cold 
 wind that rush down to the plains. 
 
 22. It was formerly imagined that each climate had 
 its own characteristic forms of life, and that the bound- 
 aries between the different botanical and zoological 
 regions were as ancient and as well defined as between 
 the various climates. But while similarity of climate 
 does not always bring similarity of vegetation and of ani- 
 mals, the want of resemblance between the plants and 
 animals of two distant countries having similar climates 
 does not arise from any unfitness in the one country for 
 the organisms of the other. Cattle and horses introduced 
 by the Spaniards into South America now run wild there 
 in vast herds. The rat, originally not a native of America, 
 may now be found in all parts of the continent. Hogs, 
 goats, cats, and dogs, first brought into the New World 
 by Columbus and his successors, are to-day found running
 
 xxxi.] PLANTS AND ANIMALS. 347 
 
 wild in great numbers. In Australia, too, the domestic 
 animals introduced by the colonists are rapidly supplant- 
 ing the kangaroos and other aboriginal forms. A fresh- 
 water plant accidentally imported from America has 
 spread rapidly over England, and is choking up canals 
 and the channels of rivers. 
 
 23. From the foregoing statements it may be con- 
 cluded that under similar climates remarkably dissimilar 
 assemblages of plants and animals may exist if they 
 are sufficiently isolated from each other ; that such 
 botanical and zoological distribution is not referable to 
 the influence of climate, for plants and animals when 
 artificially removed from the areas within which they are 
 naturally restricted have been found to increase rapidly 
 when transported to a distant but similar climate ; and 
 that while climate has evidently an important influence 
 in the distribution of life over the globe, it is not sufficient 
 to account for all that we see. 
 
 II. Migration and Transport. 
 
 24. It might be supposed that the present plants and 
 animals first appeared in one region or continent, whence 
 they gradually spread over the whole of the globe. No 
 doubt, many species are endowed with remarkable 
 powers of diffusing themselves, and of living even vigor- 
 ously under the greatest extremes of climate. But further 
 consideration suffices to convince us that this explanation 
 is wholly incapable of accounting for the existing arrange- 
 ment of the faunas and floras of the earth. 
 
 25. Plants have many facilities for spreading them- 
 selves. Their seeds are often swept up into the air by 
 whirlwinds, and may be carried along for hundreds of 
 miles before being dropped again to the ground. Should 
 the soil, climate, and other conditions be favourable, these 
 transported seeds may take root and spread over their
 
 348 PHYSICAL GEOGRAPHY. [LESS. 
 
 new abode. In other cases, seeds may be borne for long 
 distances over the sea, either floating by themselves or 
 enclosed among earth and leaves in masses of drift-wood. 
 Cast up at last on some remote shore, they sometimes 
 find a fitting home and take root there. To the feathers 
 of birds and the fur of animals seeds must often adhere, 
 and may thus be carried far away from their original 
 source. Seeds which have been for many hours in the 
 crops of birds have been found to be still alive. It may 
 now and then happen, therefore, that when, after flying 
 across hundreds of miles of sea, the birds fall a prey to 
 other predatory members of the feathered race, seeds, fall- 
 ing from their torn crops, find a lodgment in the earth, 
 where eventually they spring into leaf. In these and 
 other ways, many kinds of plants may have spread far 
 beyond their original bounds. Yet, at the best, these are 
 but limited means of transport. Differences of climate 
 and soil, lofty mountain-chains, and intervening seas, have 
 placed barriers in the way of such diffusion, which com- 
 paratively few species of plants can ever surmount. 
 
 26. Animals enjoy greater facilities for dispersal, since 
 their movements are voluntary as well as involuntary. 
 On some of the large tropical rivers rafts of drift-wood 
 are now and then to be seen, with monkeys and other 
 wild animals upon them, all sailing down the current on 
 their way to the ocean. In the great majority of cases 
 rafts of this kind are broken up at sea, and their un- 
 fortunate denizens are drowned. But cases have been 
 known where the animals have actually found their way 
 to land. We may suppose, therefore, that islands in mid- 
 ocean may sometimes have had both plants and animals 
 introduced into them by these means. Again, in the 
 Arctic seas, polar bears have been noticed upon icebergs 
 at a great distance from land ; so that by drifting ice as 
 well as by floating vegetation, animals may be diffused 
 from one country to another. But here again the means 
 of transport are so scanty, and the chances of the animals
 
 xxxi.] PLANTS AND ANIMALS. 349 
 
 being able to live and multiply in their new home are so 
 small, that we may be sure it is not in this way that the 
 continents have been peopled. 
 
 27. While most animals live within tolerably well- 
 defined limits, marked out by the climate and the kind of 
 vegetation which the climate supports, some species have 
 great powers of migration, and when impelled by their 
 migratory instinct, whether from stress of hunger or from 
 change of season, will travel for hundreds or thousands 
 of miles. In North America, many remarkable instances 
 are on record of the hordes of bisons, beavers, and 
 squirrels, which from time to time have quitted their 
 previous haunts in search of a new home. Birds show 
 this instinct strongly. Many of the most familiar birds 
 of the temperate region, both in the Old and the New 
 World, are migratory. They go north in summer to 
 breed, and after spending some months in a cooler 
 climate, and seeing their young brood able to fly, they 
 again take wing and return to their winter quarters in 
 the south. In Europe the swift, swallow, and cuckoo 
 wing their way in summer even far up within the Arctic 
 Circle, but before winter has set in, they have crossed 
 the Mediterranean to the milder air of Northern 
 Africa. 
 
 28. But while a limited number of animals are fitted 
 to spread over wide regions and to endure great diversities 
 of climate, the vast majority are confined within their 
 own district, beyond which they cannot stray, not because 
 they are in all cases unfitted for distant journeys, but 
 because of the insuperable barriers to their advance. Of 
 these obstacles, the most potent is no doubt climate, 
 which acts not only on the animals themselves, but 
 on the vegetation that directly or indirectly furnishes 
 their food. Some species of animals can live only in 
 woods, others cannot stray far from the marshes and 
 jungles, where alone they find their proper support ; 
 some are adapted for life solely in the moist hot air and
 
 350 PHYSICAL GEOGRAPHY. {LESS. 
 
 luxuriant vegetation of the tropics ; others find their 
 congenial home among arctic snows. 
 
 29. Even, however, where an animal is endowed, like 
 the tiger, with extraordinary powers of accommodating 
 itself to wide extremes of temperature and great variety 
 of food, it is surrounded by many obstacles to its dif- 
 fusion. A lofty snow -covered mountain -chain may 
 effectually prevent it from crossing into districts where, 
 if it could once reach them, it would find abundance of 
 food and shelter. A strip of barren arid desert is another 
 efficacious barrier, preventing the animals of one province 
 from passing over into another. An arm of the sea or 
 strait only a few miles in breadth suffices to keep the 
 plants and animals of the opposite shores distinct ; 
 while of course a wide and deep ocean is an insuperable 
 barrier. 
 
 30. Let us suppose, however, that through some excep- 
 tionally favourable circumstances, a few animals of one 
 or more species have succeeded in crossing one of these 
 natural barriers, what are the chances that they will be 
 able to establish themselves on the farther side ? The 
 climate must be one in which they can live and increase. 
 They must be able to find enough of their proper food. 
 If herbivorous they must needs find vegetation fitted to 
 support them ; and if carnivorous they will require to 
 meet with animals less powerful than themselves, in num- 
 bers sufficient to yield them subsistence. It would seldom 
 happen that the invaders would have all these chances 
 in their favour. But if they were able to maintain them- 
 selves at first, they would soon find their advent op- 
 posed by some rival species already long established 
 and numerous. In the struggle that would follow the 
 new comers could seldom make good their hold in the 
 country, and unless able to return to their original haunts 
 would in most cases perish. 
 
 31. The vast majority of animals are thus hemmed in 
 by barriers climate, food, mountains, deserts, wide
 
 xxxi.] PLANTS AND ANIMALS. 351 
 
 rivers, seas, or rival species barriers which, whether 
 seen or unseen, effectually restrain them from spreading 
 beyond the limits of their own district or region. We 
 are therefore forced to conclude that most of the present 
 species of animals (and the same holds true for the great 
 majority of plants) cannot have spread from any one 
 common centre, but from their very nature and require- 
 ments must always have been restricted in their distribu- 
 tion, generally to the same tracts in which they now 
 live. While certain forms of plant and animal have 
 been able to diffuse themselves over almost the whole 
 globe, the flora and fauna of each of the great biological 
 regions remain distinctly marked out by more or less 
 definite boundaries. That these regions have in every 
 case had a long history, and that their existing species 
 of plants and animals have been preceded by other and 
 different species, is shown by the rocks which form the 
 land, and the traces both of former vegetable and animal 
 life found in these rocks. In trying to discover how 
 and whence the continents have received their mantle of 
 vegetation and their hosts of animals, science needs to 
 grope backwards among the records contained in the 
 rocks, which form the subject of Geology. To some 
 aspects of this question, which show how closely Physical 
 Geography is linked with Biology, and how the plants 
 and the animals of a continent may be made to tell a 
 part of the ancient history of the land on which they 
 live, the concluding pages of these Lessons may fittingly 
 be given. 
 
 III. Changes of Land and Sea and of Climate. 
 
 32. From the facts recorded in previous Lessons it 
 is manifest that the present heights and hollows of the 
 land have not always existed, that the continents have 
 been uplifted at different times from the bed of the sea,
 
 352 PHYSICAL GEOGRAPHY. [LESS. 
 
 that each mountain-chain has been ridged up and altered 
 at various periods, and that the valleys have been slowly 
 deepened and widened by the rivers flowing in them. 
 We cannot suppose that such important changes could 
 take place without affecting more or less potently the 
 various forms of plants and animals. Investigation 
 shows that the present distribution of life sometimes 
 bears striking and independent witness to these changes. 
 How this is made evident will be clear from one of the 
 simpler illustrations. 
 
 33. The plains of Central Europe up to the shores of 
 the Baltic are clothed with a vegetation which has one 
 common character throughout. Many of the plants are 
 of course local, but a vast number range far and wide 
 over the region. Crossing from the continent into 
 Britain we meet with the same general assemblage of 
 plants, and as the greater number of these could not 
 have been drifted across the intervening sea, but must 
 have travelled by land, they show that originally Britain 
 formed a part of the European continent, and that its 
 separation into islands has taken place since the present 
 species of plants spread over its surface. 
 
 34. Any one who ascends the higher hills in Britain, 
 or a part of the mountain - chain of the Alps or the 
 Pyrenees, finds that as he reaches higher elevations he 
 loses the common and characteristic plants of the plains. 
 The vegetation, as it becomes less luxuriant, assumes a 
 more and more distinct type, not only by the disappear- 
 ance of lowland species, but by the occurrence of others, 
 such as peculiar gentians and saxifrages never seen below, 
 but plentiful on the higher hill-tops and mountain slopes. 
 So general is this change, that every hill or mountain in 
 Central and Northern Europe, rising high enough to 
 reach the fitting climate, may be expected to contain 
 more or less fully this " alpine " flora. And this is 
 found to be the case even when the groups of mountains 
 are separated by wide intervals of low country. The
 
 xxxi.] PLANTS AND ANIMALS. 353 
 
 Scottish Highlands furnish on their higher slopes an 
 abundant growth of alpine forms of vegetation. Cross- 
 ing the Lowlands, where none of these plants occur, we 
 again meet with them on some of the more elevated 
 summits in the Cheviot Hills. After another interval 
 they reappear on the hill-tops in the Cumberland lake- 
 district, and again on the higher mountains of Wales. 
 Across the whole breadth of England they are absent 
 from the low grounds. They are not to be found on the 
 opposite si ores of the Continent. But far to the south 
 they reappear abundantly on the tops of the Pyrenees, 
 and below the snow-line along the whole chain of the 
 Alps. 
 
 35. We must not suppose these plants to be merely 
 species peculiar to lofty elevations and always found 
 there. They do not occur on mountains lying to the 
 south of the Palsearctic region. On the Peak of Teneriffe, 
 for instance, they are absent, though the climate and soil 
 would have been well fitted for them. On comparing 
 the heights at which they are met with, we perceive that 
 they approach nearer the sea-level the farther north we 
 trace them. In the Alps they grow in the zone between 
 the upper limit of trees and the snow-line, or at a height 
 of from 6000 to 10,000 feet above the sea. In the 
 Scottish Highlands they descend to 2000 feet, or even 
 lower. In Scandinavia they come down to the sea- 
 level, and grow in such vigour and abundance there as 
 to show that they are really northern or arctic plants. 
 
 36. How then did an arctic flora overspread the 
 mountains so far south as the Alps ? It could not do so 
 at present. The intervening low grounds are clothed 
 with an abundant vegetation of another type, across which 
 the northern plants could not force their way. To enable 
 them to advance southwards this lowland vegetation 
 would need to be removed. If the climate of Central 
 Europe were to grow as severe as that of Norway or of 
 the higher Alps, the effect of the change would be to kill 
 
 2 A
 
 354 PHYSICAL GEOGRAPHY. [LESS. 
 
 the vegetation of the plains, or as much of it as could 
 not endure the greater cold. At the same time the 
 arctic plants, finding their congenial temperature pro- 
 longed across the European plain, would gradually spread 
 southwards, descend from the mountains, and finally 
 become the dominant vegetation over the whole region 
 where the arctic climate prevailed. This seems to have 
 been the condition of things when the northern plants 
 found their way to the Alps and Pyrenees. 
 
 37. Is there, then, any corroborative evidence that 
 such a wonderful change in the aspect of Europe did 
 really take place ? Undoubtedly there is. Below the soil 
 in different parts of the lowlands of France and England, 
 as well as in the deposits covering the floors of caves, 
 bones of reindeer have been found in considerable abun- 
 dance. This we know is an arctic animal. Remains also 
 of the musk- sheep, glutton, arctic fox, lemming and 
 others, which are all northern forms, have been exhumed 
 from similar situations. So that there can be no doubt 
 that at one time characteristic animals of the arctic 
 regions roamed over Europe as far at least as the south 
 of France. We cannot doubt that this could have hap- 
 pened only through some change of climate which, 
 driving out the usual denizens of the plains and forests, 
 allowed the northern animals to occupy their place. 
 
 38. But further and abundant proof of an extremely 
 rigorous climate having overspread Europe is supplied 
 by the polished rocks and heaps of earth described 
 already (Lesson XXVIII.), as part of the work of 
 glacier-ice. Traces of former glaciers are found through- 
 out the more hilly districts of Britain. Similar traces 
 occur both in Norway and among the Alps and Pyrenees, 
 far beyond the limits of the present glaciers, showing 
 that the ice formerly extended in vast sheets into the 
 plains. 
 
 39. It was during some part of this cold period that 
 the arctic plants and animals overspread Europe. Since
 
 xxxi.] PLANTS AND ANIMALS. 355 
 
 that time the climate has gradually ameliorated. Step 
 by step, in the struggle for life, the northern plants have 
 been driven out of the plains and up into the mountains, 
 where, among the congenial frosts and snows, to which 
 their competitors on the lower grounds do not follow 
 them, they are able to maintain themselves in scattered 
 colonies. The animals have long since been pushed 
 back into the icy north. 
 
 40. In North America a similar record has been pre- 
 served. As far south as the White Mountains of New 
 Hampshire in the United States (lat. 45) the summits 
 are peopled with Labrador plants, which once no doubt 
 extended over the low grounds up into the northern lands, 
 where the same species are now found abundantly down 
 to the sea-level. 
 
 41. This illustration by showing how far the present 
 distribution of plants and animals may be from that which 
 once existed, and also how distinctly groups of plants 
 or of animals may sometimes tell of former changes in 
 physical geography may serve to indicate why the prob- 
 lem of accounting for the existence and boundaries of 
 the biological regions should be so difficult. So many 
 causes have to be considered and so much knowledge is 
 needed regarding the events which preceded the present 
 state of things. Much has been done by naturalists in 
 this department of research during recent years, but they 
 have as yet only entered upon the beginning of the in- 
 quiry. The story of the peopling of each of the great 
 regions with plants and animals may never be fully told. 
 But that it will be made far fuller and clearer than it is, 
 and that it will help to illustrate the history of the con- 
 tinents themselves, cannot be doubted. 
 
 42. One grand object of science is to link the present 
 with the past, to show how the condition of the globe to- 
 day is the result of former changes, to trace the progress 
 of the continents back through long ages to their earliest 
 beginnings, to connect the abundant life now teeming in
 
 356 PHYSICAL GEOGRAPHY. [LESS. xxxi. 
 
 air, on land, and in the sea, with earlier forms long since 
 extinct, but which all bore their part in the grand on- 
 ward march of life, now headed by man ; and thus, 
 learning ever more and more of that marvellous plan after 
 which this vast world has been framed, to gain a deeper 
 insight into the harmony and beauty of creation, with a 
 yet profounder reverence for Him who made and who 
 upholds it all.
 
 INDEX. 
 
 ADELSBERG GROTTO, 240 
 
 Adriatic Sea, 292 
 
 Africa, winds of, 93, 95 ; coast-line of, 
 168 ; average height of, 170 ; 
 deserts of, 179; plateau character 
 of, 181 ; coral reefs of, 219 ; great 
 lakes of, 262, 266 ; fauna and flora 
 
 f> 333 
 
 Air, composition of, 38 ; capacity of, 
 for vapour, 43. 65, 80 ; height of, 
 45 ; pressure of, 47 ; temperature 
 of, 54 ; moisture of, 64 ; movements 
 of, 70, 83, 143 
 
 Algeria, wells of, 226 
 
 Alluvium, 285 
 
 Alps, comparative mass of, 170; de- 
 scription of, 174 ; snow-line of, 177 ; 
 valleys of, 177 ; glaciers of, 301, 
 309, 313 ; vegetation of, 176, 352 
 
 Amazon, River, 290 
 
 America, coast -line of, 168; axis of, 
 171 ; plains of, 172, 178, 200; salt- 
 lakes of, 173 ; mountain-chains of, 
 177 ; table-lands of, 180; drainage 
 of, 250, 253, 290 ; water-sheds of, 
 251 ; lagoons of, 264 ; great lakes 
 of, 266 
 
 Anchor-ice, 137 
 
 Animals, geographical distribution of, 
 328 ; migration and transport of, 
 
 347. 
 
 Antarctic Ocean, 109, 114, 129 
 Anticyclone, 84 
 Antiparos Grotto, 241 
 Antipodes, 16 
 
 Arabia, desert climate of, 77, 226 
 Aral, Sea of, 112, 271 
 Archipelago, 165 
 Arctic Circle, 15 
 Arctic Ocean, 109 
 Arctic regions, ice of, 131, 135 ; plants 
 
 and animals of, 330, 332, 336 
 
 Artesian wells, 230 
 
 Ascension Island, no 
 
 Asia, winds of, 93 ; position of axis of, 
 171 ; salt-lakes of, 173 ; mountain 
 system of, 178 ; plateau of, 180 
 
 Atlantic Ocean, form and depth of, 
 109; density of water of, 114; 
 temperature of, 126 ; height of 
 waves in, 142 ; currents of, 144 ; 
 form of, how determined, 165 
 
 Atmosphere, 38 
 
 Atmospheric pressure, 47, 83, 96 
 
 Attraction, force of, 12 
 
 Australia, proportion of coast to area 
 of, 1 68 ; barrier-reef of, 220; fauna 
 and flora of, 334 ; animals intro- 
 duced by man into, 347 
 
 Australian region, 334 
 
 Avalanches, 299 
 
 Azores, no 
 
 1! 
 
 BALTIC SEA, ground-ice of, 137 
 
 Barometer, 48 
 
 Bays, 166 
 
 Bay of Biscay, sand-dunes of, 101 
 
 Beach, origin of a, 152 
 
 Beaches, raised, 217 
 
 Black Lands of Russia, 179 
 
 Black Sea, 113, 295 
 
 Blood-rain, 92 
 
 Bore of the tidal wave, 150 
 
 Borings, internal heat of earth shown 
 
 , b , y > I? 
 Brazil-current, 144 
 
 Breakers, 142 
 
 Britain, rainfall of, 76; winds of, 90, 
 
 96 ; fall of volcanic dust in, 91 ; 
 
 sand-dunes of, 101 ; position of 
 
 axis of, 171 ; table-land of, 181 ; 
 
 warm springs of, 191, 235
 
 358 
 
 INDEX. 
 
 CANADA, temperature in, 61 ; winter 
 in, 137, 295 ; forests of, 336 
 
 Canary Islands, dust showers in, 92 
 
 Cancer, Tropic of, 16 ; Calms of, 90 
 
 Cape Horn current, 145 
 
 Capricorn, Tropic of, 16 ; Calms of, 90 
 
 Carbon, 41 
 
 Carbonic acid, or carbon dioxide, in 
 the air, 41 ; given off in volcanic 
 districts, 206, 237 ; solvent power 
 of in water, 237 
 
 Caribbean Sea, no 
 
 Carlsbad, hot springs of, 191, 235 
 
 Caspian Sea, 112, 173, 179, 270 
 
 Catchment-basin, 250 
 
 Caverns, formation of by acidulous 
 water, 240 ; tunnelled, cut by the 
 sea, 158 
 
 Chalk, origin of, 186 
 
 Challenger expedition, 108-112, 123 
 
 Climate, origin of differences of, 60, 
 156; continental and insular, 156, 
 342 ; how determined, 338 ; affected 
 by distance from Equator, 339 ; af- 
 fected by distance from the sea, 
 341 ; affected by height above the 
 sea, 342 ; affected by prevailing 
 winds, 344 ; affected by local in- 
 fluences, 345 ; secular changes of, 
 
 35 1 
 
 Clouds, formation of, 69 ; check forma- 
 tion of dew, 68 ; as indications of 
 aerial currents, 91 
 
 Coal, origin of, 186 
 
 Coast-lines, 166 
 
 Condensation, 66 
 
 Conduction, 56 
 
 Continents of the globe, 35 ; distribu- 
 tion of, 164; coast -lines of, 166; 
 general relief of, 169 ; average 
 height of, 170 ; axes of, 170 ; an- 
 tiquity of, 315 
 
 Convection, 56 
 
 Coral-reefs, 219 
 
 Cotopaxi, 197 
 
 Craters, volcanic, 197 
 
 Crust of the earth, corrugation of, 209, 
 3i5, 3i7 
 
 Currents of the sea, 143 
 
 Cyclone, 84 
 
 DANUBE, annual discharge of, 258 ; 
 mineral matter transported by, 
 283, 284 
 
 Day, causes in difference of length ol 
 
 the, 13 
 
 Dead Sea, 173, 271 
 Deltas, 247, 291 
 Deposit by running water, 275 
 Deserts, climate of, 55, 77, 345 ; sand 
 
 wastes of, 101 ; aspects of, 179 
 Dew, 68 
 Dew-point, 68 
 Diatoms, deposits of, on sea -bottom, 
 
 124 
 
 Diliquescence, 116 
 Disease-germs in the air, 39 
 Distribution of plants and animals, 
 
 328 
 
 Dolphin Rise, no 
 Drainage, effects of, 232, 345 
 Drainage-basin, 250 
 Dredge, use of, 108 
 Dust, importance of, in dry climates, 
 
 102 
 
 E 
 
 EARTH as a planet, 8 ; proved to be a 
 globe, 8 ; axis of, n ; movements 
 of, n ; orbit of, 12 ; hot interior 
 of, 18, 21, 190, 195, 209; history 
 of, 21, 107 ; measurement and map- 
 ping of, 24 ; diameter of, 29 ; gene- 
 ral view of, 32, 196 ; size of, 32 ; 
 density of, 100 ; possible metallic 
 interior of, 196 ; contraction of, 
 107, 209, 313, 3'7 
 
 Earthquakes, 211 
 
 Eclipse, 9 
 
 Elbe, bore of, 151 
 
 Elevation of land. See Land 
 
 England. See Britain 
 
 Equator, n 
 
 Equatorial calms, 88 ; current, 143 ; 
 regions, plants and animals of, 329, 
 
 333. 335 
 
 Equinoxes, 13 
 
 Erosion by running water, 274, 275 
 
 Erratics, 310 
 
 Ethiopian region, 333 
 
 Europe, winds of, 90, 93, 94, 95, 96 ; 
 climate of, 155, 342 ; coast-line of, 
 contrasted with that of Africa, 168 : 
 average height of, 170 ; axis of, 171 ; 
 plains of, 172, 179; salt -lakes of, 
 173; table -lands of, 181 ; water- 
 shed of, 250 ; lagoons of, 264 : 
 glaciation of, 313, 354 ; changes of 
 climate in, 352 
 
 Evaporation, 43, 64, 75, 153
 
 INDEX. 
 
 359 
 
 F 
 
 FAROE ISLANDS, no 
 
 Fauna, the assemblage of animals of a 
 
 district, 331 
 Field-ice, 133 
 
 Firn of Alpine snow-fields, 300 
 Floe-ice, 133 
 Flood-plain, 286 
 Flora, the assemblage of plants of a 
 
 district, 330 ; Alpine, of Europe, 
 
 352 
 
 Florida, peninsula of, no 
 Fog, formation of, 69 
 Fohn, of the Alps, 256 
 France, dunes of, 101 ; tidal wave on 
 
 coast of, 151 ; hot springs of, 192 ; 
 
 extinct volcanoes of, 192, 200 ; 
 
 vegetation in, affected by climate, 
 
 342 
 
 1'rost, 79, 294, 323 
 Fundy Bay, tides of, 150 
 
 GANGES, drainage area of, 250 ; mineral 
 matter transported by, 283 ; delta 
 of, 292, 293 
 
 Geneva. See Lake of Geneva 
 
 Geography, physical, defined, 3 
 
 Geological changes influencing climate 
 and the distribution of plants and 
 animals, 351 
 
 Geysers, 192 
 
 Glaciers, formation of, 131^ 290, 300; 
 transport by, 306 ; erosion by, 311 
 
 Globigerina ooze of sea-bottom, 123 
 
 Gorges, excavation of, by rivers, 277 
 
 Gravity, action of, 12 
 
 Great basin of North America, 181 
 
 Great Salt Lake of Utah, 269 
 
 Greenland, glaciers of, 131, 311 ; ice- 
 foot of, 135 
 
 Grottos, formation of, 240 
 
 Ground-ice, 137 
 
 Ground-swell, 141 
 
 Gulf Stream, 60, 144-154 
 
 H 
 
 HAIL, 82 
 
 " Hard " water, 236 
 Harmattan, 95 
 Hawai, in, 207 
 Headlands, 166 
 
 Heights,' measurement of, by baro- 
 meter, 49 
 
 Hemispheres of the Globe, 15, 33 ; 
 excess of density in Southern, 107 ; 
 excess of land in Northern, 164 
 
 High-water, 148, 152 
 
 Himalaya Mountains, height of, 169; 
 height of snow- line on, 80, 343; 
 formation of, 187 ; glaciers of, 253 ; 
 vegetation of, 177 
 
 Hindostan. See India 
 
 Hoar-frost, 69 
 
 Horizon, extent of the visible, 103 
 
 ICE, formation of, 78 
 
 Icebergs, 129 
 
 Ice-foot, 135 
 
 Iceland, climate of, 61 ; volcanic erup- 
 tions of, 91 ; position of, on a sub- 
 marine plateau, no; hot springs 
 of, 192 
 
 India, rainfall, 76, 99 ; water-shed of, 
 251 ; lagoons of, 264 ; alluvial plains 
 of, 200 
 
 Indian Ocean, temperature of, 127 ; 
 moisture supplied by, 154 
 
 Indus River, 290 
 
 Inland seas, 112, 269 
 
 Irrawaddy, transport of mineral matter 
 by, 283 
 
 Islands, oceanic, of volcanic origin, 
 in 
 
 Isobars, 53 
 
 Isotherms, 55 
 
 Isthmus, 164 
 
 !AN MAVEN ISLAND, 207 
 apan, winds of, 93 ; volcanoes of, 
 207 ; current, 145 
 
 {upiter, size of the planet, 30 
 ura Mountains, 174, 310 
 
 K 
 
 KARST, honeycombed structure of 
 
 the, 240 
 
 Kentucky, Mammoth Cave of, 241 
 Khasi Hills, rainfall of, 76, 99 
 Krakatau, volcanic explosion of, zca, 
 
 214
 
 360 
 
 INDEX. 
 
 LABRADOR, coast ice of, 133 ; cold cur- 
 rents and climate of, 60, 156 
 
 Lagoons, 264 
 
 Lake Baikal, 266 ; Brienz, 288 ; Como, 
 267 ; Geneva, 268, 287 ; Maggiore, 
 267 ; Sabatino, 268 ; Superior, 266 ; 
 Thun, 288 
 
 Lakes, formation of, 172 ; without out- 
 let (salt lakes), 172, 181, 262 ; de- 
 fined, 258 ; abundant in Northern 
 latitudes, 259 ; connected with ice- 
 action, 260, 314 ; disappearance of, 
 265, 288 ; sources of, 265 ; storms 
 on, 266 ; depth of, 266 ; survival 
 of marine forms in, 267 ; distribu- 
 tion of temperature in, 267 ; offices 
 of, 268, 287 ; filter rivers, 287 ; 
 freezing of, 295, 346 ; influence of, 
 on climate, 346 ; revolutions in dis- 
 tribution of, 351 
 
 Land, general aspects of the, 162 ; 
 distribution of, 33, 164 ; average 
 height of, 170 ; relief of, 173 ; nature 
 of materials forming, 182-185; for- 
 merly under water,i88 ; movements 
 of, 210; upheaval of, 212, 216, 316 ; 
 subsidence of, 213, 218; lowering 
 of level of, by erosion, 284, 320 ; 
 sculpture of, 314 ; not original part 
 of the surface of globe, 315 
 
 Land-breeze, origin of, 86 
 
 Landslips, 242 
 
 Latitude, finding the, 27 ; influence of, 
 on temperature, 58 
 
 Lava, nature of, 197 ; outpouring of, 
 203 ; absorbed vapours of, 210 
 
 Left bank of a river, meaning of, 240 
 
 Life, plant and animal, on the globe, 
 328 
 
 Lime, carbonate of, in sea-water, 118 ; 
 in springs, 235, 237 
 
 Limestone, origin of, 187 
 
 Loch Ness, 267 ; Lomond, 267 
 
 Longitude, finding the, 25 
 
 Low-water, 148, 152 
 
 M 
 
 MAELSTROM WHIRLPOOL, 152 
 Mahanadi River, 226, 254 
 Malay Archipelago, 207, 333 
 Map, construction of a, 30 
 Medicinal springs, 235 
 Mediterranean Seas, 112 
 
 Mercury, distance of planet, from the 
 Sun, 30 
 
 Meridians, of longitude, 26 ; measure- 
 ment of a degree of, 29 
 
 Meteors, 45 
 
 Migration of plants and animals, 347 
 
 Mines, evidence furnished by, as to 
 internal temperature of the earth, 
 19. 
 
 Mississippi, windings of, 249 ; breadth 
 of, 257 ; volume of annual dis- 
 charge of, 257 ; mineral matter 
 transported by, 283, 284 ; alluvial 
 plains of, 290 ; delta of, 291, 292 
 
 Missouri, mean slope of, 256 
 
 Mist, formation of, 69 
 
 Mistral, 94 
 
 Moisture, distribution of by winds, 
 
 7S ' 98 
 Monsoons, 77, 93 
 
 Moon, craters of the, 19 
 Moraines, 307 
 
 Mountains, varieties of, 174 ; formation 
 of, 318 ; influence of, on climate, 
 , 7, 346 . 
 Mountain-chains, 173 
 
 N 
 
 NEAP-TIDES, 148 
 
 Nearctic Region, 336 
 
 Nebulae, 23 
 
 Nebular theory, 23 
 
 Neotropical region, 335 
 
 Neptune, planet, distance of, from 
 Sun, 30 
 
 Neve of Alpine snow-fields, 300 
 
 Newfoundland, banks of, no 
 
 New Zealand, on a submarine ridge ; 
 in ; hot springs of, 194 
 
 Niagara, River and Falls, 280 
 
 Nile, delta of, 246 ; annual rise of, 
 252, 254 ; alluvium of, 289 
 
 Norway, coast-line of, 152, 155 ; snow- 
 fields and glaciers of, 301, 311 
 
 Nova Scotia, 146, 150, 156 
 
 O 
 
 OASES of deserts, 179, 226 
 
 Oceans, 34. See Sea 
 
 Ooze of sea-bottom, 124 
 
 Organic matter in sea-water, 119; in 
 
 soil, 183 
 
 Oriental region, 333 
 Ozone, 40
 
 INDEX. 
 
 361 
 
 PACIFIC OCEAN, basin of, in ; depth 
 of, 112 ; density of water of, 114 : 
 deep-sea deposits of, 124 ; tem- 
 perature of, 127 ; tidal wave in, 
 150 ; influence of, on climate of 
 America, 154 ; volcanic girdle of, 
 206 ; coral-reefs of, 219 
 
 Pa!a;arctic region, 331 
 
 Parallels of latitude, 28 
 
 Perched blocks, 309 
 
 Persian Gulf, filling up of, 290 
 
 Peru, rainless climate of, 77 ; elevation 
 of, 217 
 
 Plains of the globe, 178 ; formed by 
 alluvial deposits, 326 
 
 Planets, 22 
 
 Plants, distribution of, according to 
 height, 176 ; geographical distri- 
 bution of, 328 ; migration and 
 transport of, 347 ; remains of in 
 rocks, 1 86 
 
 Plateaux, 180 
 
 Po, delta of, 291 
 
 Polar regions, plants and animals of, 
 
 33o, 33^1 337 
 Pole the Celestial, 27 
 Poles, North and South, n 
 Pompeii, buried under the ashes of 
 
 Vesuvius, 199, 202 
 Pot-holes, 277 
 Prairies, 178 
 Precipitation, 2one of Constant, 76, 
 
 88 
 
 Protoplasm, in sea-water, 119 
 Pumice of sea-bottom, 123 
 Pyrenees, arctic vegetation of, 352 
 
 RACE of the tides, 151 
 
 Radiation, 56 ; checked by water- 
 vapour in the air, 67 
 
 Rndiolaria, deposits of, 124 
 
 Rain, washes the air, 40, 78; forma- 
 tion of, 74 ; composition of, 78, 
 114, 233 ; floats on sea-water, 115; 
 supplies spring--, 224 ; work done 
 by, 272 
 
 Rainfall, distribution of, 76, 153, 282 ; 
 
 proportion of, carried to sea by 
 
 rivers, 253 ; of Britain, 76 : Caspian 
 
 basin, 113 ; Europe, 77 ; India, 76, 
 
 _ 77 ; South America, 77 
 
 Rainless climates, 77, 154, 179, 235 
 
 Rain-prints, 273 
 
 Rainy seasons, 16, 254 
 
 Raised beaches, 217 
 
 Rapids, 279 
 
 Revolution of the earth, 12 
 
 Rhine, drainage area of, 250 ; annual 
 rise of, 255 ; mineral matter in 
 solution in water of, 281 
 
 Rhone, mineral matter transported in 
 water of, 281, 284 
 
 Right bank of a river, meaning of, 249 
 
 Rivers, underground, 232, 240 ; course 
 of typical, 245 ; sources of, 252 ; 
 flow, 256 ; volume of water dis 
 charged by, 257 ; erosion by, 275 ; 
 transporting power of, 280 ; de- 
 posits from, 285 ; freezing of, 295 
 
 River-bars, 290 
 
 River-basins, 244 
 
 Rocks, bedded, 185, 316 ; containing 
 plant remains, 186 ; containing 
 animal remains, 186 ; crystalline, 
 188 ; specific gravity of, 190 
 
 Rocky Mountains, 177, 178, 187, 337 
 
 Rotation of the earth, 1 1 
 
 SAHARA, Desert of, 77, 101, 179, 218, 
 226, 333 
 
 St. Helena, height of tide at, 150 
 
 St. Paul's Rock, Atlantic Ocean, no 
 
 Salt, incrustations of, 263 
 
 Salt-lakes 262,' 269 
 
 Salts, of sea-water, 115 
 
 Sand-dunes, 100 
 
 Sandstone, origin of, 185 
 
 Sandwich Islands, in, 203,207 
 
 Sargasso Sea, 145 
 
 Scandinavia, oscillation of level in, 
 218 ; position of axis of, 171 ; table- 
 land of, 181 
 
 Scenery, how influenced by sculpture 
 of the land, 320 
 
 Sea, general aspects of the, 104 ; depth 
 of, 106, no, in, 112 ; deepest 
 abysses of, 112 ; saltness of, 113 ; 
 density of water of, 114 ; composi- 
 tion of water of, 117-119; nature 
 of floor of, 120, 165 ; plants and 
 animals on bottom of, 120; tem- 
 perature of, 125 ; ice of, 129 ; 
 movements, 137 ; offices of, 152 ; 
 supplies the atmosphere with 
 moisture, 153 ; regulates the dis- 
 tribution of temperature, 60, 154 ; 
 wears away its shores, and thus
 
 362 
 
 INDEX. 
 
 tends to reduce the area of the 
 dryland, 156; receives and pre- 
 serves the materials out of which 
 new land will be formed, 160 ; 
 action of, on the whole conserva- 
 tive, 161 ; submarine ridges and 
 oceanic islands of, 165 
 
 Sea-basins of the globe, 103 
 
 Sea-breeze, origin of, 86 
 
 Sea-dust, 92 
 
 Sea-level, 35 
 
 Sea-shore, forms of, 166 ; waste of, 
 156 ; gain of land at, 160 
 
 Seasons, cause of the, 15 
 
 Sebka-el-Faroon, 266 
 
 Seine, bore of the, 151 
 
 Shannon, drainage area of, 250 
 
 Siberian tundras, 179, 332 
 
 Silica in sea- water, 117 
 
 Silvas of South America, 290 
 
 Simoom, 95 
 
 Sinter of hot springs, 192 
 
 Sirocco, 94 
 
 Sleet, 82 
 
 Snow, 78, 298 
 
 Snow-fields, 298 
 
 Snow-line, 81, 177, 343 
 
 Sodium-chloride in sea-water, 116 
 
 " Soft " water, 236 
 
 Soil, formation of, 182 ; influence of, 
 on climate, 345 
 
 Solano, 95 
 
 Solar System, 23 
 
 Sounding-line, use of, 108 
 
 Space, vastness of, i 
 
 Spain, table-land of, 181 ; hot springs 
 of, 191 
 
 Spitzbergen, 144 
 
 Spring-tides, 148 
 
 Springs, 222 ; dependent on rainfall, 
 224 ; surface, 227 ; deep-seated, 
 228 ; composition of water of, 233 ; 
 mineral, 235 ; temperature of, 235 ; 
 thermal, 191, 236 ; quantity of 
 mineral matter abstracted by, 237 ; 
 origin of substances dissolved by, 
 
 237 
 
 Stalactites, origin of, 238 
 Steppes, 179 
 Storms, 95 ; how caused, 53 ; destruc- 
 
 tiveness of, 100 
 Stratification of rocks, 185 
 Striation of rocks by ice, 312 
 Subsidence of land, 173, 218 
 Subsoil, formation of, 184 
 Sun, distance of, 10, 12, 30; heat of, 
 
 21, 56, 64 ; composition of the, 22 ; 
 
 rotation of, 22 : size of, 30 
 Sun-spots and weather, 63 
 
 TABLE-LANDS, 180 
 
 Tahiti, in 
 
 Tay, drainage area of, 250 ; annual 
 
 discharge of, 258 
 Temperate regions, plants and animals 
 
 of, 329, 332, 337, 340 
 Temperature, how determined, 54 ; 
 
 how interchanged, 56; regulated 
 
 by latitude, 58 ; regulated by form I 
 
 of land and height above sea, 63 ; 
 
 daily range of, 63 ; distribution of 
 
 by winds, 97 
 
 TenerifFe, Peak of, 198, 353 
 Thames, drainage area of, 250 ; mineral 
 
 matter in water of, 281 ; frozen 
 
 over, 295 
 
 Thermometer, 54, 108 
 Tiber, delta of, 291 
 Tides, 139, 147 
 Tigris River, 290 
 Trade-winds, 90 
 Transport by running water, 275, 280 ; 
 
 of plants and animals, 347 
 Travelled blocks, 310 
 Triangulation, 31 
 Tristan d'Acunha, no 
 Tropical regions, rainfall of, 76, 153 ; 
 
 fauna and flora of, 177, 329, 333, 
 
 336 
 
 Tropics, 16 
 Tundras, 179, 270 
 Titscarora expedition, in, 112 
 
 U 
 
 UNDERGROUND circulation of water, 
 
 222 
 
 United States, winds of, 89, 94 ; mount- 
 ains of, 177, 180; plains of, 178; 
 salt-lakes of, 173 ; table-lands of, 
 180; geysers of, 194; volcanic 
 action in, 206 ; earthquakes in. 
 213 ; fauna and flora of, 336, 341, 
 349, 055 
 
 Upheaval of land, 317 
 Utah, Great Salt Lake of, 269 
 
 VALLEYS, longitudinal and transverse, 
 177 ; hollowed out by running 
 water, 325 ; deepened by glaciers, 
 312
 
 INDEX. 
 
 363 
 
 Vegetation, influence of, upon climate, 
 
 345 ; distribution of, regulated by 
 
 climate, 338 
 Vesuvius, eruptions of, 108, 201, 203, 
 
 204, 205 
 Volcanic dust, transported by aerial 
 
 currents, 91 ; eruption, 198 
 Volcanoes, nature of, 194, 196 
 Volga, mean slope of, 256, 270 
 
 W 
 
 WADYS OF SYRIA, 255 
 
 Water, circulation of, on the globe, 
 44, 114, 222 ; its three conditions, 
 68 ; underground circulation of, 
 224 ; possible ultimate abstraction 
 of from the surface, 227 ; hard and 
 soft, 234 ; work of running, 272, 
 325 ; point of maximum density 
 of, 294 
 
 Water-courses, 244 
 
 Waterfalls, 277 
 
 Water-shed, 250 
 
 Water-vapour in the air, 43, 56, 64, 86 
 
 Water-worn character of detritus, 275 
 
 Waves, formed by wind, 140 ; height 
 of, 142 ; force of, 142, 157 ; caused 
 by earthquakes, 213 
 
 Weather forecasts, 95 
 
 Wells, 225, 230 
 
 West Indian Islands, no; plants 
 drifted from, to shores of Europe, 
 139 
 
 Westward growth of European towns, 
 cause of, 90 
 
 Whirlpools, 152 
 
 Wiesbaden, hot springs of, IQI, 235 
 
 Winds, cause of, 85 ; periodical (seas- 
 onal), 77, 93 ; constant, 88 ; local, 
 94 ; rate of, in storms, 96 ; office 
 of, 97, 1 1 8, 256 ; wet and dry, 99 
 
 YEAR, how determined, 12 
 Yellowstone, geysers of the, 194 
 
 ZENITH, 28 
 
 Zirknitz, Lake of, 241, 265 
 
 Printed by R & R. CLARK, Edinburgh.
 
 MESSRS. MACMILLAN AND CO.'S PUBLICATIONS. 
 
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