Physical Geography, or the Terraqueous globe and its phenomena. Illustrated with 125 wood-engravings, frontispiece, and 12 maps

Preface

. Many, perhaps, on looking at this volume will be dis posed to ask, " What is Physical Geography ? " To this question (which, under existing circumstances, is not inex cusable) I would reply that it is the department of science which embraces the course of Physics reigning on the earth's surface, over land, sea, and air, and of which, as it depends to some extent on the feature of that surface, Geography is a function. Physics (from the Greek (pvaig, nature), or the various branches of the science of nature ordinarily combined under the name of Natural Philosophy, have for subject not inert matter merely as such, but the properties, motions, and other incidents of bodies, their mutual relations, their causes and effects. Nature is regarded by the philosopher as an organic whole, connected throughout and regulated by laws.

In his language the word Physical always refers to nature thus conceived, to its rational principles and its laws. Thus, while Descriptive Astronomy depicts the heavens and enumerates the various bodies that revolve in them, Physical Astronomy explains how they are governed in all their movements by gravitation. Immanuel Kant, the great philosopher who first drew attention to Physical Geography, was disposed to allow it FREFACE. VI a very wide range. Iu discussing it he roved through all nature. Dr. J. C. E. Schmidt, whose work on the subject is in the mathematical treatment most complete, and Dr. Maedler, the Astronomer, have both viewed Physical Geography as a part of the Philosophy of the universe • and it occupies no small part of A. von Humboldt's ' Kosmos.' Sir J. Herschel describes it in the following words : — " We find ourselves thus introduced to the domain of Physical Geography, or the description of the actual state of the earth's surface in its three great divisions — land, sea, and air, as prepared for the habitation of organic beings, and as exhibiting the play of all those complex agencies on which depend the distribution of temperature and moisture, aerial and oceanic currents, and those conditions which, under the general title of climate, determine the abundance and limits of vegetable and animal forms."

Thus as to the objects and scope of Physical Geography we are fully instructed by the eminent men who brought it into vogue. But about half a century ago, when physical science was little cultivated in England, some ardent students of Geology were pleased to collect those rudiments of Geography which are more immediately connected with their own pursuits, and converting them into superficial treatises of Geology, presented them to the public ennobled with the title of Physical Geography, the word physical being here used in its vulgar sense, to exclude moral, political, and other such considerations. In this sense it is evident that every book on Geography includes Physical Geography. In the writings of Sir Charles Lyell and of Sir R. I. Murchison, the expression Physical Geography always

PREFACE.

. VII signifies merely the conformation of the earth's surface. Mr. B. Jukes defended this as the correct application of the name. When these authors tell us of frequent changes in the physical geography of the earth, we must under stand them to speak of an altered conformation or change in the distribution of sea and land. It is obvious that in this sense Physical Geography cannot be a science ; yet it is often appealed to as such. The same confusion of ideas has descended to the disciples of the above-named founders of Geology ; and we have at present numerous treatises on Physical Geography which are in reality merely outlines of Geology, without a trace of Physical Science. Under the patronage of the Royal Geographical Society, the study of Geography has been divided into two branches, viz. Physical and Political ; but the purpose or advantage of this division has never been explained.

Indeed it would appear, from the published specimens of the examination papers, that no thought has been expended on the subject. Synclinal and anticlinal lines, escarpments, and all details pertaining to Geology belong to Physical Geography • but mountains, rivers, and every thing else lie equally within the province of Political Geography. In short, a feeble and useless attempt is made to distinguish between a description of the Earth and a description of the countries and kingdoms of the Earth, the natural philosophy involved with geographical considerations being in the mean time forgotten . Some writers have included in Physical Geography the distribution of plants and animals over the earth. There is, however, a broad distinction between Physical Science and Natural History, and the attempt to blend them can never succeed ; for Natural History is made up of multi tudinous details. Species of plants and animals are

PREFACE.

. VIII reckoned by thousands. But the generalizations of science ought to follow the particulars, and can be fully under stood only by those well acquainted with the latter. An attempt to explain the geographical distribution of plants, for example, if it does not give a full account of them, but only their names, must be dry and uninstructive ; but if it does give such an account, it is no longer Geography, but Natural History. Since the first sheet of this volume was sent to press, the number of asteroids discovered has been increased by 8. They now amount to 176. W.D.C. November 26th, 1875. CONTENTS.

Chapter I

Page Universal Character of Terrestrial Physics. — The Sky. — Apparent Motion of the Heavens. — The Planets. — The Sun controls the System. — Universal Gravitation. — The Cosmogony of the Ancients. — Measure of Gravitation. — Estimation of Distance in the Heavens. — Parallax. — Amount of the Lunar Parallax. — The Sun's Parallax — how found from the Transit of an Inferior Planet. — The Solar System — its magnitude. — Superior and Inferior Planets. — Asteroids. — Comets. — Aspects of Sun and Moon. — Fixed Stars — their classes and numbers. — Binary and Multiple Stars. — Nebulae. — Parallax of the Stars. — The Nebular Hypothesis of Creation. — Opinions of Kant, Herschel, and Laplace — Objections 1

Chapter II

The Earth. — Early Opinions as to its Figure. — Found to be a Globe. — Just Views of Aristarchus — why not adopted. — Morions of the Earth reflected from the Heavens. — Epicycles. — Copernicus. — Kepler and his Laws. — Newton — his Discovery of Universal Gravitation. — The Movement in Elliptic Orbits explained. — The Earth's Orbit, Dimensions, &c. — The Plane of the Ecliptic. — Constancy of the Earth's Axis. — Revolution in the Orbit. — Time and Seasons.— Equality of Rotation. — Sidereal and Solar time. — Perturbations. — Division of Time. — Origin of the Week 19

Chapter III

Ancient observation ofthe Heavens. — Description of the Celestial Sphere its Divisions transferred to the Earth. — The Sphericity of the Earth recognized — its Magnitude conjectured. — Eratosthenes, his measurement of a degree — his method revived by Fresnel in the 17th century. — Snell's attempted triangulation. — Geodetic Surveys, France, Great Britain, India, Russia, &c. — Newton's Work. — b CONTENTS. X Page Effects of Rotation on the Earth's figure.— Irregularities of Gravi tation at the Surface.— Increase of Gravity from the Equator to the Pole— and of the length of Degrees.— Figure and size of the Earth determined.— The Pendulum as a means of measuring Gravitation. —The Attraction of Mountains.— Schehallien — The Himalaya.— The Torsion Balance.— The Specific Density of the Earth 32

Chapter IV

The Earth assumed to be a Perfect Sphere.— Positions on it how determined.— The Pole and Equator.— Latitude and Longitude.— The Zenith and Horizon.— Altitude and Azimuth.— The Representation of the Earth.— Insufficiency of the Artificial Globe.— Maps— modes of projecting. — Orthographic Projection — Stereographic. — Development of a Cone. — Flamsteed's Projection. — Mercator's Chart — its Peculiarities 49

Chapter V

The Globe, its Constituents — Solid, Fluid, and Gaseous. — Heat the Prime Mover of Circulation. — Dilatation by Heat. — Specific Weight thus changed. — Dilatation, a Measure of Heat.— The Thermometer — Leslie's Differential. — Temperature. — Quantity of Heat. — Specific Heat. — Effects of Contraction. — Convection of Heat. — Conduction. — Reflection.— Diathermancy. — Radiation between Bodies 61

Chapter VI

Sources of Heat — internal — proved in Wells — Mines. — Shergin's Well. — Heat of Earth now constant. — Star Heat. — The Sun, the chief source. — Effect of Solar Spots. — Conditions of Insolation. — Angle of Incidence. — Loss by Obliquity. — Effect of Sun's Declination. — Distribution of Solar Heat. — Length of the Day. — Penetration of Heat into the Ground. — Line of Constant Temperature. — Effects of Sea and Land. — Ptolemy's Climates. — Heating Power of the Sun .... 78

Chapter VII

Terrestrial Radiation — depends on the Character of the radiating Surface — varies with Season and state of the Sky — proved by Experiment — lowers Temperature — Dew and Hoarfrost. — Radiation varies as Insolation.— Making of Ice, in India and China.— Attempts to measure Radiation — its Effects on the Earth — and on the Moon . . CONTENTS. Page 163

Chapter VIII

The Atmosphere — its Composition — Constancy — Incidental Gases. — Carbonic Acid Gas — Vapour — Law of Mariotte. — Atmospheric Pressure. — The Barometer — Weight of the Atmosphere — its Height — affected by Temperature — and by the Earth's Rotation — Periodical Fluctuations. — Table of Atmospheric Pressures 108

Chapter IX

Water — its Composition — its Metamorphoses and change of State. — Dilatation and Contraction. — Specific Gravity. — Wonderful Transformations. — Aqueous Vapour. — Table of Evaporation — its Conditions. — Tension of Vapour. — Saturation — how Measured. — Humidity unequally distributed over the Globe 124

Chapter X

Temperature — of the Atmosphere. — Mean Temperature — how found. — Lamont's Table. — Meech's Table — its defects. — Radical Imperfection of Tables of Temperature. — Insolation regular, its effects irregular. — Dry Air diathermanous. — Glaisher's Table of Atmospheric Temperature. — Effect of Clouds 136

Chapter XI

Lines of equal Temperature — Hypsometrical. — Isothermals at the Level ofthe Sea. — Thermal Equator — its Position and Oscillation. — Line of Freezing Temperature traced in October — in January — its Position in July. — Isotherms in the Southern Hemisphere. — Influences of Sea and Land. — Abnormal Temperature of Western Europe — not due to predominance of Land — its true Cause. — Unequal Polar Seas. — Isobars, their relation to Isotherms 150

Chapter XII

Winds. — Landand Sea-breezes — how caused— aspirated or propelled. — The Trade- Winds — their limits in the Atlantic — in the Pacific Ocean. — Deflection of the Winds. — Effects ofthe Earth's rotation. — Revolution of the Plane of Oscillation. — Belt of Calms. — The returning Winds or Up Trades — winds neither polar nor equatorial. — Zone of Tropical Calms. — Maury's Theories CONTENTS. Page 180 194 CHAPTER Xin. The Monsoons— their Limits— regular and alternating.— Winds of the Polar Circle. — The East Winds the prime movers. — Table of Comparative Frequency. — Effects of Heat and Deflection controverted. — Table of the Strength of Winds. — West Winds subordinate. — East Winds the prime movers in Aerial Circulation. — Objections. — The Barometer, what it Measures

Chapter XIV

Cyclones. — Whirlwinds variously generated. — Conflagration. — Sand-pillars. — Waterspouts. — Trombes. — Tornadoes. — Hurricanes — their Course in the West Indies — near Mauritius — Bay of Bengal — Chinese seas — their Diameter, Speed, &c. — not strictly circular — Violence — Advantages to be derived from Knowledge of their Nature — Explanations of Peltier, Dove, &c. — their probable Origin . .

Chapter XV

Clouds. — Lightness of Vapour — Tendency to regular Structure. — Howard's Nomenclature — Cirrus — Cumulus — Stratus. — Vain attempts to multiply Distinctions. — The support ofthe Clouds. — The Vesicular Theory. — Viscosity. — Clouds merely transient— Proofs of some Stability. — Cumuli in dry weather. — Clouds travel far. — The Globular Theory justified. — Why Clouds do not exhibit the prismatic colours 210

Chapter XVI

Rain. — Causes of Precipitation. — Cold, Pressure, Mixture. — Rate of Rainfall — how measured. — Growth of Rain-drops. — Lnequal Distribution. — Sudden Rains. — Constant Rains. — Region of two Rainy Seasons. — "Winter Rains. — Summer Rains. — Rains at a"ll Seasons. — Rains of Eastern Africa. — Rainless Tracts. — Rains of Europe — of the United States 227

Chapter XVII

Moisture on the Earth— permeates the ground.— Formation of Springs. —Opinions of the Ancients as to the Source of Springs— all from the Clouds.— Halley's miscalculation of Rain and Rivers.— The sufficiency of Rain.— Underground Reservoirs.— Artesian Wells.— In- CONTENTS. XIII Page termittent Springs.— Steam-Wells from Volcanoes. — The Geysers.— Brine-Springs. — Connexion ofthe Sea with Land-Springs.— Rivulets collected into Rivers. — Basins and Watersheds. — Great elevation of River-sources not essential. — Velocity and Force of Rivers. — Deltas. — Erosion of Channels. — Canons. — Deflection of Rivers by the Earth's Rotation. — Improvement of Rivers. — The Kalihari Desert and the River Orange 244 CHAPTER XVHI. The Ocean — its Offices — moderates Temperature — maintains Humidity — General Depth — Deep-sea Soundings — The Atlantic — The Pacific Ocean. — The Permanence of the Sea — its Saltness — Density — Miscellaneous Contents — Transparency — Colour — Temperature at the Surface — Circulation by Heat — Objections answered — Temperature of its Depths 262

Chapter XIX

Waves, how produced — oscillating Motion — the onward Movement of the Water only apparent — Form of Waves — Velocity. — Earthquake- Waves — effect of Wind — increase to leeward — estimated height — Direction changed by Friction — Depth affected by them — Feelings connected with them. — Tides, how caused — Variations in a Lunar Month— Lagging and Priming — Springand Neap-tides. —Establishment of a Port. — Dr. Whewell's Cotidal Lines. — Tides round the British Islands 282

Chapter XX

Currents, their Origin. — Equatorial Current in the Atlantic. — Gulf-stream. — Strait of Bernini. — Warrn and Cold Bands. — Maury 's Description. — Magnitude of the Warm Stream, its Course and Limits. — Atlantic Current to the Polar Sea. — Currents the consequences of Disturbance, not Compensations. — Japan Current. — Humboldt's Current — Mosambique — Pacific. — Deflection of Currents. — The Cold Currents famed for the best Fish. — Currents of the Mediterranean, &c 303

Chapter XXI

Congelation in Fresh Water — in the Sea. — Thickness attainable by Ice. — Varieties of Sea-Ice. — Source of Icebergs. — Limits of Polar Ice. — Grounding of Icebergs — Distance reached by them towards the CONTENTS. Page Equator. — Snow, its Forms— Density— Limits at Sea-level.— Snow in the Sahra.— Good Effects of Snow.— Perpetual Snow- Conditions determining its Height.— Table of Altitude of the Snow line _The Line of Congelation different. — Disadvantage of Ice and Snow. — Perpetually Frozen Ground. — Ground-Ice. — Iceless Cold Lakes.— Congelation fatal to Fish— and to Animal Life in general . . 319

Chapter XXII

Mountains — their Importance — Impressions made by them. — View from the Summit of the Alps. — High Snow. — The Chasm. — The Firn or Ne*ve. — Depth of Snow. — Glaciers — Conditions of their Formation — Primary and Secondary. — Moraines. — Guffer-lines. — Seracs. — Tables — Veined Structure. — Dirt-bands. — Number and Size of Glaciers — their Movements — Variation. — The Œtzthal group of Mountains t, . . . <. . . . 331

Chapter XXIII

Mountains — their Physical Influence — Steepness — interfere with the lower Atmosphere. — Temperature on Mountains. — Limits of Vegetation on the Alps. — Temperature ofthe Ice — Winds — Rains. — Decreased Amplitude of Variation. — Union of Heat and Cold. — The Andes — their Rains. — Himalaya, its Rains. — Passes of the Alps, Andes, and Himalaya. — Springs. — Lakes. — Mountain-dwellings. — Villages in Tibet. — Gold-Mines of Thok Jalung. — Table of highest Mountains 348

Chapter XXIV

Electricity — how developed — its Nature and Power. — The Electroscope. — Atmospheric Electricity — its Daily Variation — most abundant in Winter — identified with Lightning. — Dr. Wall, Franklin, Richman, &c. — Clouds as electrical Conductors. — Thunderstorms. — Fogs. — The Path of Lightning. — Fulgurites. — Height of Thunder-clouds. — Periodicity of Electricity — Elustrated. — Fireballs or Thunderbolts. — Singular Effects imputed to Lightning.— Lightning-rods 361

Chapter XXV

Hail, unlike Snow. — Hailstones, Peculiarities of— unequally Distributed —Destructive— singular remark of Von Buch.— Great Damage done CONTENTS. XV Page to the Vineyards of France.— Influence ascribed to Forests.— Attempts to explain the Origin of Hail— Volta— M. de la Rive— M. Dufour.— Intense Cold the first Condition— this produced by the rapid Expansion produced by an Electric Discharge 371

Chapter XXVI

Magnetism. — Properties of the Magnet. — Terrestrial Magnetism. — Declination of the Needle — its History — Variation, Diurnal and Annual. — Inclination or Dip — its Changes — Intensity. — Isogonal, Isoclinal, and Isodynamic Lines. — Magnetic Equator. — Table of Magnetic Elements. — Magnetic Storms. — Theory of Terrestrial Magnetism — Hansteen — Gauss — Ampere. — Power of Magnetism evinced in Basaltic Columns ,378

Chapter XXVII

Aurora Polaris. — Interchange of Electricity between the Pole and Equator. — M. de la Rive's Theory. —The Electric Brush. — Phenomena of Aurora described — rarely complete. — Cosmical Origin of Aurora disproved. — Prof. Olmsted's views. — Phenomena widely visible not therefore Cosmical. — Aurora not equally visible from all sides — its great Velocity — an Optical Phenomenon — Zone of its frequent Appearance 389

Chapter XXVIII

Light, its Refraction — Dispersion. — The operation of these in a drop of Rain — with one Reflection or two.— The Rainbow — why circular — disposal of the Colours — Angular Limits. — The Rainbow a mere vision only optically existing — varies in Magnitude inversely as the Sun's Altitude. — Refraction in the Atmosphere. — Incurvation of Rays of Light. — Objects raised by Refraction. — Incurvation inverted by great Heat. — Mirage. — Travellers' Descriptions 397

Chapter XXIX

Past Changes of Climate on the Earth. — The Eccentricity of the Earth's Orbit — Effects of its Variability. — Precession of the Equinoxes. — Table of Successive Eccentricities.— The Glacial Period.— The Polar Snow-cap. — Oscillation of the Ocean — Hypotheses weighed — Disordered Calculations. — Uniformity in the Course of Nature. — Change in the Atmosphere — its Effect on Animal Forms CONTENTS.

Chapter XXX

Page The received views of Creation. — The Earth not at first liquid. — The origin of the Ocean and of Mountains. — Denudation. — Volcanoes incidental and not permanent. — Subsidence and Rise of Land. — The Natural Hypothesis of Creation. — The gathering of the Ocean. — Fracture of the Earth's Crust — its consequences. — Change of Level attributable to the Ocean PHYSICAL GEOGRAPHY.

Chapter I

Universal Character of Terrestrial Physics. — The Sky. — Apparent Motion of the Heavens. — The Planets. — The Sun controls the System. — Universal Gravitation. — The Cosmogony ofthe Ancients. — Measure of Gravitation. — Estimation of Distance in the Heavens. — Parallax. — Amount of the Lunar Parallax. — The Sun's Parallax — how found from the Transit of an Inferior Planet. — The Solar System — its magnitude. — Superior and Inferior Planets. — Asteroids. — Comets. — Aspects of Sun and Moon. — Fixed Stars — their classes and numbers. — Binary and Multiple Stars. — Nebulæ. — Parallax of the Stars. — The Nebular Hypothesis of Creation. — Opinions of Kant, Herschel, and Laplace — Objections. Since Physical Geography has for its object to establish the connexion of Physics with Geography, or to explain how physical laws and phenomena are modified by position on the earth's surface, we cannot commence the study of it more fitly than by making ourselves acquainted with the chief physical laws of the universe and with the system visibly upheld and controlled by them.

The science of nature compels us to carry our views beyond the earth. It is from observation of the heavens that we can best learn the nature of gravitation. For the cause of the alternation of night and day, and of the change of seasons, we must look to the earth rotating on its axis, while revolving at the same time in its orbit round the sun. As to the primæval condition of the earth when first called into existence, no attempt can be made to investigate it without considering at the same time the origin of the system of which the earth is a member. The sky appears by day to be an azure vault *. Its colour, * The word " skye " in Danish signifies " a cloud," and may possibly be connected in origin with the Greek mcta (a shade). B PHYSICAL GEOGKAPHY. however, is not due to the distant heavens, but to the terrestrial atmosphere, which, by absorbing a certain portion of the light, colours that which it transmits.

Consequently when viewed from a considerable elevation, where the air is much rarefied, the sky appears almost black. As soon as the more dense atmosphere, retentive of illumination, is left behind, the eye discovers a dark unfathomable abyss, which, being penetrated by it equally in aU directions, seems to have the form of a vault or concave sphere. The splendour of the sun above the horizon extinguishes all other objects in the visible heavens. But after sunset, and when twilight is at an end, the sky resumes its natural darkness, with innumerable stars, minute fountains of light, irregularly scattered over it. These are to ordinary perception immovable with respect to each other, and keep the same relative positions from age to age. The whole spangled canopy moves with perfect constancy from east to west, or from the left to the right hand of one looking southwards (in the northern hemisphere), and makes the circuit of the heavens in the sidereal day, which is the true or perfectly constant unit of time, being equal to 23 hours 56 minutes, and 4*09 seconds of mean solar time.

The sun, par ticipating in the general diurnal movement of the heavens from east to west, moves also from west to east among the stars at the mean rate of 59 minutes, 8 seconds of space, or nearly twice its own diameter, in a day, so as to complete the circuit of the heavens in the tropical year, or 365 days 6 hours 9 minutes and 9*6 seconds. The apparent diurnal revolution of the collective heavens is obviously due to the earth's rotation on its axis, the real motion of the spectator causing the apparent motion of the scene before his eyes. The sun's apparent course among the stars is, in like manner, the effect of the earth's advance in its orbit. The earth, while rotating on its axis, also goes forward in its path ; and the sun therefore seems to go in the opposite direction. When the earth's diurnal rotation is complete with respect to the stars, it must still continue for 3 minutes 56 seconds in order to come up with the sun and complete the mean solar day of 24 hours.

In the zone of the heavens immediately about the ecliptic or sun's path, and which is called the Zodiac, from &Siov, a figure THE PLANETS. of an animal, because the stars within it were, for convenient reference, anciently grouped into such imaginary figures, may be seen a few small bodies, hardly distinguishable by the naked eye from stars, though differing from them in having measurable disks as well as motions generally direct or from west to east, yet also, in some instances and at certain periods, from east to west, or retrograde. Some of them make in the course of years, by what appears to be a very complicated path, the circuit of the heavens. Others never extend their excursions beyond a limited distance from the sun. These bodies are called Planets, or wan derers ; and close observation discovers that they all revolve round the sun, nearly in the same plane, which is called the plane of the ecliptic, because in it alone take place the eclipses of the sun and moon ; that the earth is one of the same class, being itself a planet ; and that the apparent complexity of the planetary motions is due to the circumstance that the proper motion of the earth round the sun is combined with those of the planets.

If the earth, E (fig. 1), stood still while the planet P described its orbit round the sun, from a to b and back again, the path of P would be a straight line back and forward, supposing P's orbit and that of the earth to he in the same plane — or a compressed ellipse, if the orbits be inclined to each other at a small angle. But if the earth, instead of standing still, constantly moves on in its orbit, the line marking P's course will continually change its position ; the alternate movements of the planet back and forward will appear unequal, and the ellipse described by it will assume a new and convoluted form (fig. 2). Fig. 1. Fig. 2. The sun, by far the largest body in the solar system, occupies the centre, and holds all the planets in their orbits by its attraction ; for it can be perfectly demonstrated that the rectilineal course in wbich they would hurry along if left to themselves is converted by the central attraction into an elliptical orbit round the central body.

Tbe attraction exercised by tbe sun in conb2 sequence of its surpassing magnitude, presents to the earth the most striking example of universal gravitation. The ancients did not regard the sun as the central body. They believed the earth to be the fixed centre round which the sun and all the bodies in the heavens moved in circles ; for they deemed the circle the most perfect figure. When movements decidedly not circular were subsequently detected Fig. 3. p in the heavens, they were referred to epicycles, or circles the centres of which revolve on other circles (fig. 3). It is evident that by this contri vance, with careful adaptation of the circle and its epicycle in respect of size and velocity of revolution, an ellipse or any other curve may be approximately represented. The attraction of gravitation is manifest in its action ; but to calculate its force it is necessary to know the distance between the attracting body and that attracted.

Observation of the heavens generally reveals only proportional or relative, but no absolute, distances. Thus the distance of the sun from the earth is easily found to be about 400 times that of the moon ; but the absolute and exact distances of those bodies are not so discoverable. The distance of an unapproachable object can be found only by observation of its parallax, or that change in its apparent place which is caused by and corresponds with the spectator's change of place, the distance between his successive positions being known. The optical coincidence of one object (a, fig. 4) with another more distant, b, takes place only when the observer stands in the continuation of the line ab, which joins them both. When he moves away from that line, or from g to /., the nearer object moves, in respect of the more distant, in the opposite direction, to c ; and the nearer it is to the observer, the greater will be its angular Fig. 4. motion.

If the more distant object be a fixed star, then lines drawn from the observer to the star, whatever be his change of place, may be considered as parallel to each other, because, the earth being a mere point as seen from the star, any angle subtended by a portion of its surface must be wholly in- THE LUNAR PARALLAX. 5 appreciable. This being understood, let us suppose that at two observatories (a and b, fig. 5) widely asunder and, to make the case more simple, on the same meridian, the moon's position be noted at one and the same instant, the effect of the distance be tween the observers will be evident in the circumstance that the moon will appear at the northern station to be further south, at the southern further north, among the stars. The difference between these two observations, or the angle a M b, is the Fig. 5. parallax corresponding to the measured arc between the two observatories ; and from this again may be accurately deduced the angle PMC (fig. 6), the horizontal or maximum parallax (for 90°).

This, called also the geocentric parallax, is that which would be found by observations made at the pole and the centre of the earth (were such Fig. 6. observation possible), and is the recorded lunar parallax of astro nomers. The horizontal parallax of the moon, or the angle subtended by the earth's semidiameter as seen from the moon (fig. 7), is found Fig. 7. to be 57' 2"-325. Consequently, as the sine (0-165779) of that angle is to radius (1), so is 3912, the earth's polar semidiameter, to the moon's distance, or 238,793 miles, about 60 times that semidiameter. Again, it is obvious that when bodies are compared together at a fixed distance, their real and their apparent mao-nitudes are in the same proportion. Now the earth seen from the moon (E, fig. 8) has a diameter of 114' (double of the horizontal parallax), while that of the moon seen from the earth is but 31'-7.

It follows, therefore, that the moon is to the earth in diameter as 3P7 to 114, in volume as 1 to 49. Fig. 8. The sun's parallax, owing to the distance of the luminary, is so small an angle that it is hard to determine it with the precision required by its importance ; for the smaller the angle the greater proportion do the inevitable errors of observation bear to the whole result. Were it possible to find exactly the instant of lunar quadrature, or half moon, the sun's distance could be exactly derived from it for at that moment the lines S M and E M (fig. 9), drawn from the sun and the earth to the moon, form a right angle, SME. The angular dis tance of the sun from the moon, S E M, may be learned from the clock. The three angles of the triangle being thus known, the mutual proportions of its sides are also known ; and therefore from the moon's distance, M E, already found, may be deduced the sun's distance.