Physical geography, or The terraqueous globe and its phenomena

Front matter

SMI

Q

**

CHAPTER I. Page

Universal Character of Terrestrial Physics. — The Sky. — Apparent Motion of the Heavens. — The Planets. — The Sim controls the System. — Universal Gfravitation. — 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. — ^Nebulse. — 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. — Motions 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 HI.

Ancient observation of the 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

X CONTENTS.

Page Effects of Rotation on the Earth's figure.— Irregularities of Gravitation 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.— SchehaUien.— 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 Glohe.—Maps— modes of proj ecting. — Orthographic Proj ection — Stereographic. — Development of a Gone. — 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. — Temperatm-e. — 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 — Minas. — 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 VIL

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 Eai'th — and on the Moon

CONTENTS.

CHAPTER VIII. Page

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 Transforrdations. — 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. — Lament'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 Temperatm-e.— Effect of Clouds 186

Chapter XI

Lines of equal Temperature — Hypsometrical. — Isothermals at the Level of the 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. — Land- and Sea-breezes — how caused— ^aspirated or propelled. — The Trade- Winds — their limits in the Atlantic — in the Pacific Ocean. — Deflection of the Winds. — Effects of the 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 163

Xll CONTENTS.

CHAPTER Xin.

Page The Monsoona— their Limits— regvilar and alternating.— Winds of the Polar Circle.— The East Winds the prime movers.- Tahle of Comparative Frequency. — ^Effects of Heat and Deflection controverted. — Table of the Strength of Winds.— West Winds suhordinate.— East Winds the prime movers in Aerial Circulation. — Objections. — The Barometer, what it Measures 180

Chapter XIV

Cyclones. — ^Whirlwinds variously generated. — Conflagration. — SandpiUars. — 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 . . 194

Chapter XV

Clouds. — Lightness of Vapour — ^Tendency to regular Structure. — Howard's Nomenclature — Cirrus — .Cumulus — Stratus. — Vain attempts to multiply Distinctions. — The support of the Clouds.— -The Vesicular Theory. — ^Viscosity. — Clouds merely transient. — Proofs of some Stability. — Cumuli in dry weather. — Clouds travel fax. — 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. — Unequal Distribution. — Sudden Rains. — Constant Rains. — Region of two Rainy Seasons. — Winter Rains. — Summer Rains. — Rains at all Seasons. — Rains of Eastern Africa. — Rainless Tracts. — Rains of Europe — of the United States 227

CHAPTER XVn.

Moisture on the Eaxth — 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 sufiiciency of Rain. — Underground Reservoirs. — Artesian Wells. — In-

CONTENTS. Xni

Pago termittent Springs. — Steam-Wells from Volcanoes. — Tlie Geysers. — Brine-Springs. — Connexion of the Sea with Land-Springs. — Bivulets collected into Rivers. — Basins and Watersheds. — Great elevation of River-sources not essential. — Velocity and Force of Rivers. — Deltas. — Erosion of Channels. — Cafions. — Deflection of Rivers by the Earth's Rotation. — Improvement of Rivers. — The Kalihari Desert and the River Orange 244

CHAPTER XVm.

The Ocean — ^its Oifices — moderates Temperatm-e — 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 — Circidation by Heat — Objections answered — Temperature of its Depths 262

CHAPTER XIX.

Waves, how produced — oscillating Motion — ^the onward Movement of the Water onlyapparent — 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 — Spring- and Neap-tides. — Establishment of a Port. — Dr. WheweU's Cotidal Lines.^^Tides round the British Islands 282

CHAPTER XX.

Currents, their Origin. — Equatorial Current in the Atlantic. — Gulf-stream. — Strait of Bemini. — Warm 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 Cmrent.— Humboldt's Current — Mosambique — Pacific. — Deflection of Currents. — The Cold Currents famed for the best Fish. — Currents of the Mediterranean, &c 303

CHAPTER XXL

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

XIV 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 XXn.

Mountains — their Importance — ^Impressions made by them. — View from the Summit of the Alps. — High Snow. — The Chasm. — The Fim or N^v^. — 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 CEtzthal group of Mountains > . . c > . 331

CHAPTER XXm.

Mountains — their Physical Influence — Steepness — interfere with the lower Atmosphere. — Temperature on Mountains. — Limits of Vegetation on the Alps. — Temperature of the Ice — ^Winds — Rains. — Decreased Amplitude of Variation. — Union of Heat and Cold. — The Andes —their Rauis. — Himalaya, its Rains. — Passes of the Alps, Andes, and Himalaya. — Springs. — Lakes. — Mountaia-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, Frankliu, Richman, &c. — Clouds as electrical Conductors. — Thunderstorms. — Fogs.— The Path of Lightning.— Fulgurites.— Height of Thunder-clouds. — Periodicity of Electricity — Illustrated.- Fireballs or Thunderbolts.— Singular Effects imputed to Lightning.— Lightning-rods 3gj

CHAPTER XXV.

Hail, unlike Snow. — Hailstones, Peculiarities of — unequaUy Distributed — Destructive— singular remark of Von Buch. — Great Damage done

CONTENTS. XV

Page to the Vineyards of France. — Influence ascribed to Forests. — At^ tempts to explain the Origin of Hail — ^Volta — M. de la Rive — M. Dufour. ^Intense Cold the first Condition — this produced hy 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 eyinced 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 .889

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 Sim'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 — ^Efiects of its VaidabUity. — 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 Com-se of Nature. — Change in the Atmosphere — its Effect on Animal Forms 407

VI CONTENTS.

CHAPTEE XXX,

Page

Tlie 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 420

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 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. — NebulsB. — Parallax of the Stars. — The Nehular Hypothesis of Creation. — Opinions of Kant, Herschel, and Laplace — Objections.

Since Physical Geography has for its object to establish the connexion of Physics with Greography, or to explain hovsr 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 vrith 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 yiews 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 primaeval 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 o-Kid (a shade).

B 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 ahnost 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 all directions, seems to have the form of a vault or concave sphere.

The splendoin- 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 innmnerable 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, participating 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 inorder to come up with the sun and complete the mean solar day of 24 hours.

In the zone of the heavens immediately about the ecliptid or sun's path, and which is called the Zodiac, from feoStov, a figure

THE PLAi^ETS. 6

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 wanderers ; 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 Fig- !■

to b and back again, the path of P would be, '^

a straight line back and forward, supposing i* .^

P's orbit and that of the earth to be 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 wiU appear unequal, and the ellipse described by -^ it will assume a new and convoluted form ^ — Q_J>-^ (fig. 2).

The sun, by far the largest body in the solar system, occupies the centre, and holds aU the planets in their orbits by its attraction ; for it can be perfectly demonstrated that the rectilineal course in which they would hurry along if left to themselves is converted by the central attraction into an elliptical orbit round the central body. The attraction exercised by the sun in con-

b2

4 PHYSICAL GBOGBAPHY.

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 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 contrivance, 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 p^g. 4. (a, fig. 4) with another more distant, 6, 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 h, 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 vsdll be its angular ~*~ motion. If the more distant object be a fixed star, then fines 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.

Pig. 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 between 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 6, is the 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 observation possible), and is the recorded lunar parallax of astronomers.

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. 6.

Fig. 7.

bQz 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 p,._^_ g

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

magnitudes 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 31-7 to 114, in volume as 1 to 49.

The sun's parallax, owing to the distance of the luminary, is so

PHYSICAL GEOGRAPHY.

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 halfmoon, the sun's distance could be exactly derived from it for at that moment the lines S M and B M (fig. 9), drawn from the sun and the earth to the moon, form a right angle, SMB. The angular distance 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, ME, already found, may be deduced the sun's distance. But in this mode of proceeding the fundamental observation can never be satisfactory.

A far better means of finding the parallax and distance of the sun is afforded by the transit of one of the inferior planets over the sun's disk. That of Venus, as being nearer to the earth, offers most advantage. Suppose the transit to be observed from two points on the same meridian, then the two tracks seen of the planet's passage over the face of the luminary (fig. 10) will be at some distance asunder and of different lengths. At each place of observation the contacts, internal and external, of the planet with the sun's edge must be carefully noted at ingress and egress. This noting of time may be done with extreme precision, as the transit is

comparatively slow. The length of each track is proportional to the time taken to perform it, which is easily observed. The size of the solar disk being well known, there can be no difficulty in placing the two observed chords accurately within it, and thus ascertaining the distance, m n, between them. But as the distance of Venus from the sun, V S, is to its distance from the earth, VB, so is the distance, mn, between the observed chords to the

PARALLAX OF THE SUN. 7

true solar parallax, which, is thus found. The differing observations of transit made at different places, and not on the same meridian, are all conducive to accurate results, though by means of intricate calculations. But here it is enough to point out the general principle and method of proceeding.

Astronomers have been long agreed in adopting 8""6, or more strictly 8""5766, as the best expression of the sun's parallax ; but observations with improved instruments, together with theoretical considerations of great weight, recommend the adoption of 8"'89 as a better estimate. This makes the sun's distance from the earth to be about 91,600,000 miles. This increase of the solar parallax, it may be observed, is less than the hundred-and-eightieth part of a minute of space, or of the smallest point clearly visible to the naked eye ; yet it has the effect of reducing by one twentyeighth part the scale of the whole solar system, bringing the earth nearly four millions of miles and the polar planet Neptune 100 millions of miles nearer to the sun *. Having determined the distance of the central body and observed the movements of the planets about it, or the periodic times and variations of speed with which the orbits are described, we thence deduce their real dimensions ; whUe the measurable disks of the planets, varying in size as they are more or less distant, afford additional means of arriving by repeated observation at accurate results.

The bodies composing the solar system, with their relative magnitude, distances from the centre, and densities, the unit of measure under each head being furnished by the earth, are as follows : —

Diameter. Distance. Density.

The Earth Ill

The Asteroids

* A minute of space is about the smallest point plainly discernible by the human eye.

The equatorial diameter of the earth is 7924 statute miles ; that of the sun, therefore, being 107 times as much, will be 847,868 miles. The distance of the earth from the sun, the unit of distance of the preceding Table, is about 91,600,000 miles ; the distance of Neptune therefore from the centre of heat, light, and gravitation is 2750 millions of miles. The density of the earth seems to be about 5-53, that of water being taken as unit ; and the mean density of the minerals on its surface being not more than 2-66, it follows that its density must increase downwards tiU at the centre it is more than double of that at the surface. The density of Saturn, 0"14, being considerably less than that of water (O'lSl when the density of the earth is taken as unit), that planet would appear to be no heavier than cork.

The earth, the largest of the first four or inferior planets, is also distinguished from them by having a satellite. Jupiter, the largest of all the planets, its bulk being 1330 times that of the earth, has four satellites. To eight satelUtes, which are most of them discoverable only by very powerful telescopes, Saturn adds a peculiar appendage in the form of a flat hoop ring, or two or more rings lying close together. The satellites of Uranus, six in number, were discovered by Sir Wm. Herschel ; but as two of them have never been rediscovered by succeeding astronomers, their existence is now matter of doubt. These satelHtes present a remarkable singularity, their orbits being nearly perpendicular to the plane of the ecliptic, while their motion in these orbits is said to be retrograde. The great distance of Neptune secures his attendants from observation, and as yet the existence of only two satellites has been perfectly ascertained.