The Earth: a descriptive history of the phenomena of the life of the Globe ... (The Ocean, Atmosphere and Life; being the second series of A descriptive history of the phenomena of the life of the Globe, etc.) Translated by ... B. B. Woodward and edited by H. Woodward ... Illustrated, etc

Front matter

THE EARTH CONTINENTS Section I. The Earth. Vol. 1. MAP ©F Tl I The regions tintl hydTvjlegifral bar -letto the sea). Vincent BroGkB.Da.yJI CHAPMAN & : Kng4 by EAu-d Draim try A .Vmllemwi 1 ALL, L03ND0N THE EARTH A DESCRIPTIVE HISTORY OF THE PHENOMENA OF THE LIFE OF THE GLOBE Bv EEISEE RECLUS Translated by the late B. B. WOODWARD, m.a. AND Edited by HENRY WOODWARD, British Museum CONTINENTS __n_.sa.__M> bv TWO HUNDRED AND THIRTY MAPS INSERTED IN THE TEXT AND TWENTY-FOUR PAGE MAPS PRINTED IN COLOURS

Section I. London- Chapman And Hall

193, PICCADILLY 18-1 LONDON PHINTl-P BY VIRTUE >ND CO,, CITY KOAD. LIST OF COLOUEED ILLTJSTKATIONS. ' PLATE TO FACE PAGE I. Map of the "World Frontispiece. II. Geological Chart of the World 1 III. Geological Map of England 30 IV. North America 48 V. South America 54 VT. Europe 60 VII. Africa • .... 64 VIII. Asia 68 IX. Australia and the Adjacent Archipelago 70 X. The Alps 164 XI. The Mer de Glace and its Affluents 200 XII. Glaciers of Geiseerg and Eothmoos 220 XIII. Glaciers of Langthal and Gurgl 228 XIV. Glacier of Vernagt 232 XV. Former Glaciers of the Valley of the Adige .... 240 XVI. Middle Course of the Mississippi 360 XVII. Delta of the Ganges 410 XVIII. Lake Garda 450 XIX. The Bugors of the Caspian Sea 474 XX. Eruptions of Etna . . _, 496 XXI. Volcanoes . 504 XXII. Crater of Mauna-Loa 536 XXIII. Upheavals and Depressions . 616 XXIV. Atoll Ari 658 C

Preface

TO THE ENGLISH EDITION. This work— translated hy the late B. B. Woodward, Esq., B.A. Lond., F.S.A., the Queen's Librarian at Windsor Castle, in his leisure hours, and edited, since his death, hy his brother — is the result of more than fifteen years' careful study, travel, and research, by one of the most able living French authors, Elisee Reclus. It has already passed through two French editions, and the pages of the present volume have, from time to time, received the benefit of the Author's attention. We know of no work, either French or English, in which so great a task has been attempted, and achieved with such wonderful success ; and we cannot doubt that the English edition will obtain as favourable a reception, and arrive as rapidly at a second edition, as the original French work. The illustrations, both in number and excellence, surpass those of any similar work upon Physical Geography with which we are acquainted.

The careful and laborious research displayed in the elaboration and completion of each portion of the work, and the harmonious manner in which the several parts are united, will insure for M. Reclus an honourable place in English popular scientific literature. As to the English translation, it is only due to the Author to state that such a work must necessarily be somewhat literal, and cannot but fail, to a certain extent, in doing justice to the elegance of diction and beauty of composition of the original French. b PAGE CONTENTS.

Part I. The Earth As

A PLANET. Chapter I. — Smallness of the Earth as compared with the Sun and Fixed Stars ; Grandeur of its Phenomena. — Form of the Terrestrial Glohe ; its Dimensions 1 Chapter II. — Motion of the Planet. — Diurnal Rotation and Annual Revolution. — Sidereal and Solar Day. — Succession of Days and Seasons. — Difference of Duration ofthe Seasons in the Two Hemispheres. — Precession of the Equinoxes. — Nutation. — Planetary Perturbations. — Movement of the Eaith towards the Constellation Hercules ........ 5 Chapter III. — Various Opinions as to the Formation of the Earth. — Laplace's Hypothesis ; grave Objections raised to it. — Theory of a Central Fire ; Objections to it 14 Chapter FV. — Geological Strata. — Conglomerates. — Sandstones. — Clays. — Limestones. — Fossiliferous Beds. — Sequence of Organic Beings. — General Classification of Strata. — Duration of Geological Periods 24 Chapter V. — Incessant Modification in the Shape of Continents. — Attempts made to learn the former Distribution of Soils and Climates. — Object of Geology. ■ — Province of Physical Geography ....... .32 PART n. THE LAND.

Chapter VI. — Regular Distribution of Continents. — Ideas of Ancient Nations on this Point. — Hindoo Legends. — Atlas and the Giant Chibchacum. — Homer's Shield. — Strabo 40 Chapter VII. — Inequality of Land and Water. — The Oceanic Hemisphere- — The Semicircle of Land. — Distribution of the highest Plateaux and loltiest Mountain Chains round the Indian and Southern Oceans. — Polar Circle. — Circle of Lakes and Deserts. — Coasts arranged in Arcs of a Circle . . 45 Chapter VIII. — Division of the Land into the Old and New Worlds. — Double American Continent. — Double Continent of Europe and Africa. — Double Continent of Asia and Australia CONTENTS.

PAGE Chapter IX. — Principal Analogies between Continents. — Pyramid Form of Portions of the World. — Slopes and Declivities. — Closed Basins of each Continent. — Southern Peninsulas in each Group of Continents. — Hypothesis of Periodical Deluges. — Rhythmical Arrangements of Peninsulas . . 61 Chapter X. — Numerous Indentations of the Northern Continent. — Heaviness of Form in the Southern Continents. — Inequality of Size in the Continents of the Old World.— Extent of Coast-line in Inverse Ratio to the Area of Land. —Contrasts between the Old World and the New.— The Transverse Position of the Axes of America and the Old World. — Contrasts of Climate in the various Continents ; North and South, East and West 67 Chapter XL— Harmony of Shape in Oceans. — The two Basins of the Pacific— The two Basins of the Atlantic. — The Arctic Frozen Ocean and the Antarctic Continent. — Contrasts, an Essential Condition of Planetary Vitality . . 76 Chapter XII.— General Aspect of Plains.— Alluvial Plains.— Cultivated Plains. —Uniformity in Uncultivated Plains. — Varieties in Appearance produced by Climates and Different Physical Conditions ....... 79 Chapter XIII. — The French Landes. — The Brandes and the Alios. — The Campine. — The Heaths of Holland and Northern Germany. — The Puszta of Hungary. — The Grassy Steppes of Russia. — The Salt Steppes of the Caspian and the Aral.— The Tundras 82 Chapter XIV. — Semicircle of Deserts parallel to the Semicircle of Landes and Steppes. — The Sahara, — Sands, Rocks, Oases. — The Deserts of Arabia. — The Nefoud. — Deserts of Iran and the Indus. — The Desert of Cobi ... 92 Chapter XV. — Plains and Deserts of the New World. — Humidity of the Amerit can Continents. — Distribution of Savannahs and sterile Tracts. — The Prairies of North America. — The Llanos and Pampas ...... 102 Chapter XVI. — American Deserts. — The Great Basin of Utah. — The Desert of Colorado. — The A tacama and the Pampa of Tamarugal. — Deposits of Salt, Saltpetre, and Guano ........... 107 Chapter XVII. — Difference between Plateaux and Plains. — Material Importance of Plateaux in the Economy of the Globe. — Distribution of elevated Regions on the Surface of Continents .......... 113 Chapter XVIII. — The Great Plateaux of Central Asia and the Gate of the Hindoo Kutch. — Plateaux of Europe. — Their Symmetrical Arrangement. — Plateaux of the Two Americas. — Similarity between the closed Basin of Bolivia aud the District of Utah. — Plateaux "of Africa , . , . .118 Chapter XIX. — Isolated Mountains. — Mountains in Groups. — Chains and Systems of Mountains. — The Beauty of Mountain Peaks. — Sacred Mountains. — Pleasures of Mountain Climbers . . . . . . , .125 Chapter XX. — Various Forms of Mountains. — Poverty of polished Languages in describing their Appearance. — Richness in this Respect of the Spanish Language and the Alpine and Pyrenean Patois. — The numerous Provincial Terms employed for various Shapes of Hills and Mountains . . . .132 Chapter XXI. —

Inequalities and Depressions in the Vertical Outline of Mountains. — Origin of Valleys, Gorges, and other Depressions. — Longitudinal Valleys. — 'transverse Valleys. — Winding Valleys with Parallel Sides.

Valleys with Defiles and Gradations of Levels. — Cluses and Canons. General Arrangement of Valleys. — Amphitheatres. — The Oules of the Pyrenees u0 Chapter XXII. — Depressions in Mountain Ridges. — Diversity in the Form of Passes (Cols). — Relation between the respective Altitudes of Summits and Passes. — Law of Debouchments. — Real and Ideal Slopes of II untains. Estimated Solid Contents of Mountain Groups ...... CONTENTS. IX PAGE Chapter XXIII.— Hypotheses as to the General Laws of Mountain Chains. — M. Elie de Beaumont's Theory of Parallel Upheavals.— Chain of the Pyrenees taken as a Type of the Cordilleras or Longitudinal Chain. — Various Irregularities in the Chain. — The Pyrenees as an Ethnological Barrier 153 Chapter XXIV. — Mountains of Central Europe. — Contrast between the Alps and the Jura.— The Jura as a Type of a System of Mountains with Parallel Chains.— Apparent Chaos of the Alps. — Central Group of St. Gothard.— Groups of Monte Rosa and Mont Blanc. — The Alps considered as a Frontier 159 Chapter XXV. — Mountain Chains of Central Asia. — The Kouen-Lun, the Earakorum, the Himalaya. — The South American Andes, a Type of the Bifurcated Chain 167 Chapter XXVI.— Gradual Cooling of the Air on Mountain Sides.— Difficulty of Ascents. — Limits of Man's Habitation. — Illness felt by Mountain Travellers 173 Chapter XXVII. — Gradual Subsidence of Mountains during the Lapse of Ages. — Sudden Downfalls and Chao3. — The Fall at Felsberg. — Slow Action of Meteoric Agencies ........... 177

Part III. The Circulation Of Water

Chapter XXVIII. — Snow-fall on Mountains. — Lower Limit of Snow. — Zone of Perpetual or Permanent Snow 181 Chapter XXIX. — Influence of the Sun and Meteoric Agents on the Snow. — Avalanches. — Protecting Forests. — Defensive Works against Downfalls of Avalanches 188 Chapter XXX. — Gradual Transformation of Snow into Ice. — Neves, or Glacier-reservoirs. — Phenomenon of Regelation. — Crystals of Ice. — Glaciers of the First and Second Order .......... 194 Chapter XXXI. — Movement of Glaciers. — Experiments and Theories. — Convexity ofthe Central Part ofa Cllaeier. — Its successive Windings. — Friction of the Ice against the Bottom and Sides of the Bed. — The Glacier Gauge. — Inclination of the Glacier Bed 199 Chapter XXXII. — Marginal, Transversal, and Longitudinal Crevasses. — Seracs. — Moulins. — Bridges of Snow. — Veins of Fresh Ice. — Surface -streams on Glaciers. — Gouilles. — Lakes and Inundations. — Discharging Channels . 206 Chapter XXXTII. — Debris lying on the Surface of the Glacier. — Holes in the Surface. — Glacial Tables. — Moraines ; Lateral, Medial, and Frontal. — Ribbons of Mud. — Measurement of the Speed of Glaciers. — Ablation. — Subglaciary Streams. — Terminal Arches. — Contrast between the Glacier Ice and the surrounding Vegetation 217 Chapter XXXTV. — Progress and Retirement of Glaciers. — Appearance of the Bed when abandoned by the Ice. — Roches Moutonnees. — Parallel Furrows . 228 Chapter XXXV. — Distribution of Glaciers over the Surface of the Earth . . 234 Chapter XXXVI. — The Glacial Period. — Ancient Glaciers of Europe — Dispersion of Rocks and Boulders from Scandinavia and in North America. — Ancient Glaciers in Tropical Regions 246 Chapter XXXVII. — Secondary Part taken by Glaciers in the Circulation of Water. — Mountain Flood-waters. — Absorption of Rain and Melted Snow by the Earth, Peat-mosses, and Rocks. — Springs and their Nymphs . .

Contents

X PAGE Chapter XXXVIII. —Variation in the Discharge of Springs.— Estavelles.— Equalisation of the Supply in Springs with Deep Sources. — Intermittent Springs 260 Chapter XXXIX.— Ascending Springs.— Artesian Wells.— Temperature of Jetting Springs ........••■•• "oo Chapter XL. — Cold and Thermal Springs 269 Chapter XLI. — Mineral Springs. — Incrusting Springs. — Metallic Veins. — Salt Springs .........•••• 273 Chapter XLII. — Subterranean Rivers. — The Spring of Vaucluse, the Touvre. — Submarine Affluents. — The Rios of Yucatan. — The " Mud-lumps" of the Mississippi ............. 282 Chapter XLIII. — System of Subterranean Streams. — Joints and Fissures of Rocks.— Stalactites.— The Inhabitants of Caves. — The Mammoth Cave. — Caverns of Carniola and Istria ....••••• 289 Chapter XLIV. — Rivers. — Various Denominations of Water-courses. — Determination of the principal Branch among the Affluents of a River. — River Basins and Watersheds. — Forks of certain Rivers ..... 300 Chapter XLV. — The Hydrographical Systems of various Parts of the World . 312 Chapter XLVT. — The Rivejof the Amazons. — Diversity in the Character of Water-courses. — Unity of the Law which governs them. — Equalisation of their Slopes. — Upper, Middle, and Lower Courses of Rivers . . . 322 Chapter XLVII. — Mountain Torrents. — Inequalities oftheir Beds and of their Discharge of Water. — Temporary Streams. — Filling up of Lakes. — Erosions, Gorges, and Slopes. — Torrents of the French Alps 328 Chapter XLVIIL— Erosion of Lacustrine Dikes. — Cataracts and Rapids . . 342 Chapter XLIX. — Formation of Islands. — Reciprocity of Curves. — Windings and Cuttings. — Shifting of the Courses of Affluents 354 Chapter L. — Periodical Rising of Streams. — "Embarras" of Floating Trees. — Ice-floods in the Northern Rivers. — Inundations 367 Chapter LI. — Means of Preventing Floods. — Natural and Artificial Reservoirs. — Irrigation Channels. — Embankments, and Cracks in them . . . 377 Chapter LH. — The Mouths of Rivers. — Estuaries. — Long Banks of Sand. — Deltas. — Network of Branches of Rivers in Alluvial Plains . . . 394 Chapter LIII. — The Channels ofthe Mississippi. — "Working Rivers." — Shifting of the Point of Bifurcation. — Raising of the River-bed above the Delta. — Alteration in the Situation of the Mouths of Rivers ..... 405 Chapter LIV. — Bars of Rivers. — Operations undertaken for Deepening the Mouths of Rivers 422 Chapter LV. — Alteration in the Position of Water-courses in Consequence of the Rotation of the Earth. — Masses of Water brought down to the Sea by Rivers. — General Considerations ......... 433 Chapter LVI. — Lakes. — Formation of Lakes. — Their Increase and Diminution. — Their Form and their Depth. — Lakes lying in Successive Gradation of Elevation ............. 446 Chapter LVII. — Various Phenomena in Lakes. — Colour of their Waters. — Seiches. — Currents and Tides. — Formation of Ice in Lakes .... 459 Chapter LVIII. — Lakes acting as Regulators of the Rivers which pass

through them. — Fresh-water and Salt-water Lakes. — The Caspian Sea . . .

CONTENTS. XI PAGE Chapter LIX.— The Dead Sea.— The Salt Lakes of Asia Minor and the Russian Steppes.— The Great Salt Lake. — The Melr'ir 474 Chapter LX. — Marshes. — Swamps of North America. — Peat-bogs. — Unhealthiness of Marshes 482 PAKT IV. SUBTERRANEAN FORCES.

Chapter I. Smallness Of The Eaeth As Compared With The Sun And

FIXED STARS J GRANDEUR OF ITS PHENOMENA. FORM OF THE TERRESTRIAL GLOBE J ITS DIMENSIONS. The earth on which we dwell is one of the lowest in rank among the heavenly bodies. If an astronomer in some other planet were exploring the immensity of space, our earth, owing to its small size, might readily elude his intelligent view. A mere satellite of the sun, the volume of wrhich is 1,255,000 times greater, the earth is but a point as compared with the immense tract of ether traversed by the planets in their courses round their central globe. The sun itself is only a spark, which seems lost amid the eighteen millions of stars which Herschel's telescope discerned in the Milky Way ; the latter, an immense agglomeration of suns and planets, which looks to us like a broad streak of light round the whole universe, is in reality nothing but a nebula ; that is, a cloud of stars resembling a mist, which would be as nothing in infinite space.

Beyond our own sky, other skies stretchfar away into infinity, and others beyond these, so that light, notwithstanding its prodigious rapidity, takes eternities to cross them. How small the earth seems in this fathomless abyss of stars ! Individually, it may seem immense to us ; all too vast for our littleness, we have not yet succeeded in investigating the whole of its surface ; but, as compared with the whole sidereal cosmos, it is less than a grain of sand by the side B of a mass of mountains, or an atmospheric particle compared with aerial space. True enough that the earth is nothing but an almost impalpable grain of dust to the vision of the astronomer scanning the nebulæ in the field of his telescope, but it is, nevertheless, quite as much worthy of study as any other of the heavenly bodies.

If it does not possess magnitude of dimensions, it presents an infinite variety in all its details. "Whole generations, living one after the other upon its face, might pass their lives in studying its phenomena without comprehending all their full beauty. There is not even any special science, having for its aim some portion of the terrestrial surface or some particular series of its products, which does not present to our savants an inexhaustible field of inquiry. Moreover, is not our httle globe, as well as the sky, a real cosmos, both by the admirable arrangement of its parts, and by its supreme harmony as a whole ? In a certain point of view, is not our almost imperceptible planet as great as the universe, in that it is the expression of the same laws ? In the form of its orbit, in its movements round the sun and on its own axis, in the succession of days and seasons, and in all the phenomena governed by the great law of attraction, the earth becomes the representative of all the other planets ; in studying it, We study all the heavenly bodies.

Om' planet is a spheroid; that is, a sphere flattened at the two poles and enlarged at the equator, so that all the circles passing through the extremity of the polar axis form elhpses. The presumed depression of each pole is about thirteen miles, nearly a three hundredth part of the radius of the earth;* but it is not altogether certain that the two poles are equaUy flattened. Perhaps a contrast exists between the two hemispheres, not only in the features of their continents and the distribution of seas, but also in their geometrical shape. Be this as it may, it appears to be proved that the curvature is not exactly the same at all points of the earth at an equal distance from the poles ; the meridians appear without exception to be irre gular ellipses. The recent measurement of degrees carried out by astronomers, and especiaUy the great trigonometrical survey made between 1816 and 1852, under the direction of Struve, from the coasts of the Frozen Ocean to the banks of the Danube, have dis closed some singular deviations in the form of the earth, caused either by the geological nature of the crust or by the vicinity of * According to Bessel, the astronomer, 299T528.

All possible eirors are embraced between 302'301 and 296-005. AS A PLANET. 3 considerable mountain chains. Thus, among the countries of Europe, the surfaces of England and Italy are sensibly depressed in com parison with adjacent countries. These inequalities of curvation, which are doubtless variable, and correspond to the changes in the position of the earth's centre of gravity, are cognizable only by the astronomer, and nowhere interrupt the apparent horizontal character of the surface of plains and seas. As far as man is concerned, the roughness and hoUows forming our plains, mountains, and vaUeys, are more important than any inequalities in the roundness of the globe. According to Von Schubert, the Academician, an enlargement, perpendicular to the equator, and therefore paraUel to the meridian, bulges out aU round the globe, passing through Em-ope and Africa ; this hypothesis is not, however, made good by the measurements of an arc of the meridian recently made in India.

The dimensions of the earth, as we have already seen, are almost as nothing compared with the larger celestial bodies, and especiaUy with the extent of space which can be explored by the telescope. If light, the speed of which has been adopted in astronomy as a term of comparison, could be diffused in a curved line, it would travel seven times round tho globe in a second of time ; this standard of measure ment, therefore, the only one suited to the steUary field, is completely inapplicable to the surface of omglobe. Man, smaU as he is in com parison with the planet on which he lives, in the first instance chose out for the measurement of his domain, either parts of his own body, such as the foot, cubit, or fathom ; or the distance traveUed during a certain period of time, as the parasang, stadium, mile, or league.

It was not untU the end of the last century that the savants who then adorned France, conceived the idea of dividing the circumference of the earth into equal parts, which for the future should serve as a standard of measure for aU terrestrial distances. This measure, or metre, which, with the aid of its multiples and divisions, enables us to estimate, with equal ease, the circumference of the globe or that of an almost invisible molecule, is the ten-millionth part of the arc described from the equator to one of the poles. Owing to errors which the difficulties of actual measurements rendered inevitable, the ideal metre exceeds the customary one by nearly the eleventh part of a millimetre; but this very trifling difference, which is imperceptible to .the naked eye, may be disregarded in practice without any incon venience.

A line, therefore, going round the earth, and passing through the two poles, woidd be of the length of about 40 millions of b metres, or 40,000 kilometres. Thus, as Schubert* remarks, it is about the distance which the usual pace of a man would travel over in a year — that is, if he did not stop for a single instant. The superficies of the globe, as calculated by Wolfers, according to the most recent measurements which astronomers have made of the arcs of the longitude and latitude in various countries, is 197,124,000 square miles. According to Encke, the astronomer, it amounts to 197,108,580 square miles, and the planetary mass would attain to a bidk of 256,000 nullions of cubic miles. * Geschichtc der See/e. MOTION OF THE PLANET.

Chapter II. Motion Of The Planet

DIURNAL ROTATION AND ANNUAL EEVOLUTION. SIDEREAL AND SOLAR DAY. SUCCESSION OF DAYS AND SEASONS. — DIFFER- ENCE OF DURATION OF THE SEASONS IN THE TWO HEMISPHERES. PRE- CESSION OF THE EQUINOXES. NUTATION. PLANETARY PEETUEBATIONS. MOVEMENT OF THE EAETH TOWAEDS THE CONSTELLATION HEECULES. The isolated globule in the immensity of space, which we caU the earth, is not motionless, as the ancients necessarily supposed, looking upon it, as they did, as the immovable base of the firmament of heaven. Hurried on in the vortex of universal vitality, our globe is ever actuated by ceaseless motion, describing in ether a series of elliptic spirals so complicated that astronomers have not yet been able to calculate their various curves. Besides rotating on its own axis, the earth describes an ellipse round the sun, and, under the influence of this body, is drawn along from one heaven to another towards distant consteUations.

It also oscillates and rocks on its axis, and deviates more or less from its path, to salute, as it were, every heavenly body which meets it. It is probable that it never passes a second time through the same regions of the air ; yet, if it has again to traverse the spiral line of ellipses it has already described, it would be after a cycle of so many thousands of millions of years, that the earth itself, completely transformed, would be no longer the same planet. Nature, immutable in its laws, but for ever variable in its phenomena, never repeats itself. The motion of the earth, the immediate effects of which are the most obvious to the notice of men, is the daUy rotation which takes place round an ideal axis passing through the two poles. The globe turns from right to left, or from west to east — that is, in a contrary direction to the apparent motion of the sun and stars, which seem to rise in the east and to set in the west.

As the earth's axis terminates at each pole, there is least surface-motion at those points, and the motion is the more rapid in any part of the surface of the globe the farther it is from the central axis. At St. Petersburg, in 60° latitude, the speed of rotation is about nine mUes a minute ; in Paris, it exceeds eleven and a half nhles during the same brief time ; on the equatorial line, which may be looked upon as the ring of an immense wheel, the speed of the earth is twice as great as it is at 60° of latitude — that is, about eighteen miles a minute, or 528 yards a second — a rapidity equal to the flight of a 26-pound cannon-baU impeUed by thirteen pounds of powder. By means of this rotatory motion, the earth presents towards the sun each of its faces alternately, and each also in turn towards the comparatively darker regions of space ; the succession of day and night is thus constituted.

In addition to this, the rotation of the earth is an important fact which must always be taken into account in determining the direction of fluids in motion on the surface of the globe, such as streams and rivers, also marine and atmospheric currents.* The annual revolution which the earth performs round the sun foUows the line of an eUipse ; one of the foci of which is occupied by the central star ; the eccentricity of the eUipse is nearly equal to -\\-^ of the great axis. The distance between the sun and the earth always varies according to the particular point of its orbit which the latter is travelling over. At its aphelion, that is, its greatest remoteness, this distance is about 93f millions of miles ; at the period of its perihelion, when the two heavenly bodies are nearest to each other, it is approximately 90,259,000 nhles.

The mean distance, as estimated by astronomers since the corrections of Encke, Hansen, Foucault, and Hind, is 91,839,000 nhles. This extent of space is traversed by the solar rays in 8 minutes, 16 seconds ; sound woidd take fifteen years in passing through the same distance. As Kepler has laid down in his celebrated laws, our planet moves with an increased rapidity as it approaches nearer to the sun, and travels more slowly in proportion to its distance from that luminary ; but its mean speed may be -estimated at nearly 19 miles a second, or sixty times the rapidity of a baU from the cannon's mouth. This speed, which makes one dizzy to think of, is to be added, as regards each point in the surface of the earth, to the rotatory motion which impels it round the polar axis. Modified by this latter motion, the line described by any one point on the terrestrial superficies becomes a spiral.

After having turned round 366 times on its axis, our planet has terminated its orbicular course, and is in the same position relatively to the sun as at its starting-point ; it has then accomplished its year. During this period of time, composed of 366 terrestrial rotations, the sun has only iUumined each hemisphere in turn 365 times. How Vide the chapters as to "Rivers," "Currents," "The Atmosphere, and Winds." MOTION OF THE PLANET. 7 does this apparent anomaly arise ? How does it happen that a complete movement of rotation performed by the globe round its own axis does not exactly coincide with the solar day ? The cause is this — that the earth in its rotation, carried on as it is in its immense orbit, is constantly changing its position in respect to the sun. As regards the fixed stars, situate at an almost infinite distance from our planet, the earth remains, so to speak, always in the same position ; consequently, the sidereal day, that is the interval which separates two transits of the same star over the same terrestrial meridian, has the precise duration of one rotation of our globe.

After each of its diurnal rotations, our earth presents to these far-remote stars the Fig. 1.— Inequality of the Solar and SiderealDi same part of its surface, and if the light of the sun became suddenly extinct, and if a star, such as Sirius or Aldebaran, became our great focus of iUumination, our days would have the exact duration of a terrestrial rotation, that is, about 23 hours, 56 minutes. But the sun, although a fixed star, is comparatively near to the earth. 'WhUst the latter is performing a movement of rotation, it alters its position 1,604,300 nhles along the course of its orbit ; consequently, the sun, in its apparent progress, seems to retrograde this distance, and in order that the earth shoidd present to it exactly the same portion of its surface as at the commencement of its rotation, it woidd be necessary for it to turn round four minutes more.

The next day, a fresh change in the position of the earth again adds four minutes to the duration of the day, and so on tiU the end of the year. These dady additions of four minutes to the length of the day form during a whole year a period equal to the duration of one of the diurnal rotations ; the result is that the sidereal days in the year exceed the solar by one.* Thus the dady rotation of the earth round its axis produces the succession of days and nights, and in the same way, its annual revolution round the sun causes the alternations of the seasons. If the axis of the earth, that is, the ideal line which passes through its two poles, were perpendicular to the plane of its annual orbit, it is evident that the portion of the globe lighted by the sun would invariably extend from one pole to the other, and that in both hemispheres the days and nights would always consist of twelve hours each.