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information may be looked for. In the first three parts of the article GEOLOGY, a large section of what is usually included under physical geography will be found.

1. The Earth in its Cosmical Relations. From astronomy we learn the shape and size of the earth, its motions of rotation round its axis and of revolution in an elliptical orbit round the sun, the origin of day and night, and of the seasons. Speculating on the original condition of the whole solar system, we may regard it as having been in the condition of a nebula, gradually contracting, condensing, and leaving behind successive rings, which on disruption and reaggregation formed planets. Hence the primitive condition of our globe as a separate mass must have been gaseous or fluid. Since that time the earth has been cooling and contracting, but still retains a high residual temperature in its interior. This original condition, and the internal heat of the earth, must be constantly kept in view as an explanation of many of the features of its outer surface. See GEOLOGY, part i.; ASTRONOMY, chapter i.; | GEOGRAPHY (MATHEMATICAL); GEODESY.

2. The Atmosphere or Gaseous Envelope of the Earth.The solid planet is covered by two envelopes, one of gas which completely surrounds it, and one of water, which occupies about three-fourths of its surface. In studying the atmosphere we have to consider its height, its composition, its temperature, its moisture, and its pressure (see ATMOSPHERE, METEOROLOGY), Its height must be at least 40 or 50 miles. This deep gaseous ocean consists of a mixture of the two gases, oxygen (21 parts by weight) and nitrogen (79 parts), with a minute proportion of carbonic acid (004) and of aqueous vapour. The physical geographer takes note of the manifold importance of oxygen, not only in supporting animal life, but in the general oxidation of the earth's outer crust. He recognizes the atmospheric carbonic acid as the source of the carbon built up into the structure of plants. He cannot contemplate without everincreasing wonder and delight the coming and going of the water-vapour in the air, as it rises incessantly from every sea and land, and after condensation into visible form courses over the land as rain, brooks, rivers, and glaciers (see GEOLOGY, part iii.). The consideration of the temperature of the atmosphere elicits the facts that temperature falls as we rise above the sea-level, and as we recede from the equator to the poles, and that it is profoundly affected by the relative positions of sea and land. The want of strict dependence upon latitude in this distribution of temperature is strikingly brought out by the contrast between the mean temperature of Labrador and Ireland on the same parallels (see CLIMATE, ISOTHERMS). In dealing with the moisture of the air we have to consider the phenomena of evaporation and condensation, the formation of dew, clouds, rain, snow, and hail, the distribution of rain, the position of the snow-line, the occurrence of deserts, &c. (see METEOROLOGY). The study of the pressure of the atmosphere, which appears to vary with variations in temperature and amount of vapour, brings before us the cause of the constant aerial movements. The law has now been well established that air always flows out from tracts where the barometric pressure is high into those where it is low. A knowledge of the distribution of pressure over the globe furnishes the key to the great movements of the atmospheric circulation. The trade winds, for example, blow constantly from a belt of high pressure towards the equator, where the pressure remains low. Periodic winds, like the monsoons and land and sea breezes, shift with the changes in atmospheric pressure. Thus Asia during winter is a vast region of high pressure; the winds round its margin therefore flow out towards the sea. In summer, on the other hand, it becomes a region of low pressure, and the winds consequently blow inland from the sea. Sudden and

violent atmospheric movements, such as tempests and hurricanes, are illustrations of the same law, the force of the wind being always proportional to the shortness of the space between great extremes of pressure (see ATMOSPHERE). 3. The Ocean or Water-Envelope of the Earth, from the point of view of physical geography, presents for consideration the form of the basins in which it is contained, the shape and nature of their bottom, their submarine ridges and islands, the density and composition of the water, the distribution of marine temperature, the ice of the sea, and the movements of the ocean due to cosmical causes as in the tides, to the effects of winds as in surface drifts, currents, and waves, and to differences of temperature. The largest additions in recent years to our knowledge of the earth have been made in the ocean, notably by the different expeditions and cruises equipped for the purpose by the British Government. The climates of the sea have been systematically determined, and the extraordinary fact has been brought to light that the great mass of the ocean water is cold, or below 40° Fahr. Even in the equatorial parts of the ATLANTIC and PACIFIC OCEANS (q.v.), though the upper layers of water partake in the heat of the intertropical latitudes, a temperature of 40° is found within 300 fathoms of the surface, while at the bottom, at depths of 2500 or 3000 fathoms, the temperature (32°-4 to 33° Fahr.) is very little above that of the freezing-point of fresh water. It has been proved that the bottom temperature of every ocean in free communication with the poles has a temperature little different from that of the water in polar latitudes. Between Scotland and the Faroe Islands a sounding was obtained giving even a temperature of 29°-6, or 2.4 degrees below the freezing-point of fresh water, and very little above that of salt water. These observations warrant the conclusion that a vast system of circulation takes place in the ocean. The cold heavy polar water creeps slowly towards the equator under the upper lighter water, which moves away towards the poles.

4. The Land.-We have to consider the distribution of the land over the face of the globe, the grouping of the continents, the forms and trend of the great terrestrial ridges, the relation of coast-line to superficial area, the contours of the land, as mountains, table-lands, valleys, and plains, the relation of the continents to each other as regards general mass (see GEOLOGY, part ii.; AFRICA, AMERICA, ASIA, EUROPE). Over this framework of land there is a ceaseless circulation of water. The vapour raised by the sun's heat from every ocean and surface of water on the land, after being condensed into clouds and rain, falls in large measure upon the land, and courses over its surface from mountain to shore in brooks and rivers, which again have their own distinguishing phenomena, such as the formation of terraces, deltas, &c. Part of the water performs an underground circulation and returns to the surface in springs. Another portion falls as snow upon the mountains and descends into valleys in the form of glaciers. In this ceaseless flow of water from the summits to the sea we must recognize one of the great agencies by which the present contour of the land has been moulded (see GEOLOGY, part iii., section ii.).

The physical geographer collects, moreover, data which show the reaction of the earth's interior upon its surface,proofs from bores and mines of a progressive increase of temperature downwards, the evidence of hot springs, and of earthquakes and volcanoes. He finds proofs of oscillations in the level of the land, some regions having been raised and others depressed within the times of human history. From the geologist he learns that such instability has characterized the outer crust of the planet from very ancient times, and that indeed it is to the results of terrestrial movements that we owe the existence of mountain ranges and even the dry land itself (see GEOLOGY, part iii.,

section i., and part vii.). He perceives that the present area of land on the earth's surface is the result of the balance of two antagonistic processes-the destruction caused by superficial agents on every portion of land exposed to their influence, and the periodic elevation, by subterranean action, of the land so wasted, or of new land from beneath the sea.

5. Distribution of Animal and Vegetable Life.—It is usual to include in treatises on physical geography an outline of the distribution of plants and animals, with an account of the great regions or provinces into which zoologists and botanists have divided the continents. The question naturally arises why the distribution should be as it is. Two answers obviously suggest themselves-1st, climate, and 2d, the power possessed by plants and animals of diffusing themselves. Yet climate only explains a part of this problem, and it is evident that migration cannot

| possibly account for the diffusion of innumerable organisms. There is a large residuum of unexplained phenomena on which much light is thrown by geological inquiry. Thus, for example, the presence of living Arctic forms of vegetation on the mountains of central Europe can be connected with the occurrence of the remains of Arctic animals in the superficial deposits of that region, and with other facts which make it clear that at no very distant date an Arctic climate prevailed over most of Europe, that at that time a northern vegetation spread southwards and covered the plains and heights of Europe even as far south as the Alps and Pyrenees, and that as the climate gradually ameliorated the northern vegetation was extirpated from the low grounds by the advance of plants better suited to the milder temperature, but continued to maintain its ground amid the congenial frosts and snows of the mountains, where to this day it still flourishes (see DISTRIBUTION). (A. GE.)


GEOLOGY is the science, which investigates the history


EOLOGY is the science which investigates the history | the task of the geologist to group these elements in such a of the earth. Its object is to trace the progress of way that they may be made to yield up their evidence as our planet from the earliest beginnings of its separate ex- to the march of events in the evolution of the planet. istence, through its various stages of growth, down to the finds that they have in large measure arranged them.selves present condition of things. It seeks to determine the in chronological sequence, the oldest lying at the bottom manner in which the evolution of the earth's great surface and the newest at the top. Relics of an ancient sea-floor features has been effected. It unravels the complicated are overlaid by traces of a vanished land-surface; these are processes by which each continent has been built up. It in turn covered by the deposits of a former lake, above follows, even into detail, the varied sculpture of mountain which once more appear proofs of the return of the sea. and valley, crag and ravine. Nor does it confine itself Among these rocky records lie the lavas and ashes of longmerely to changes in the inorganic world. Geology shows extinct volcanoes. The ripple left upon the shore, the that the present races of plants and animals are the descend- cracks formed by the sun's heat upon the muddy bottom ants of other and very different races which once peopled of a dried-up pool, the very imprint of the drops of a passthe earth. It teaches that there has been a progress of the ing rain-shower, have all been accurately preserved, and inhabitants, as well as one of the globe on which they yield their evidence as to geographical conditions widely dwelt; that each successive period in the earth's history, different from those which exist where such markings are since the introduction of living things, has been marked by now found. characteristic types of the animal and vegetable kingdoms; and that, however imperfectly they have been preserved or may be deciphered, materials exist for a history of life upon the planet. The geographical distribution of existing faunas and floras is often made clear and intelligible by geological evidence; and in the same way light is thrown upon some of the remoter phases in the history of man himself. A subject so comprehensive as this must require a wide and varied basis of evidence. It is one of the characteristics of geology to gather evidence from sources which at first sight seem far removed from its scope, and to seek aid from almost every other leading branch of science. Thus, in dealing with the earliest conditions of the planet, the geologist must fully avail himself of the labours of the astronomer. Whatever is ascertainable by telescope, spectroscope, or chemical analysis, regarding the constitution of other heavenly bodies, has a geological bearing. The experiments of the physicist, undertaken to determine conditions of matter and of energy, may sometimes be taken as the starting-points of geological investigation. The work of the chemical laboratory forms the foundation of a vast and increasing mass of geological inquiry. To the botanist, the zoologist, even to the unscientific, if observant, traveller by land or sea, the geologist turns for information and assistance.

But while thus culling freely from the dominions of other sciences, geology claims as its peculiar territory the rocky framework of the globe. In the materials composing that framework, their composition and arrangement, the processes of their formation, the changes which they have undergone, and the terrestrial revolutions to which they bear witness, lie the main data of geological history. It is

But it is mainly by the remains of plants and animals imbedded in the rocks that the geologist is guided in un ravelling the chronological succession of geological changes. He has found that a certain order of appearance characterizes these organic remains, that each great group of rocks is marked by its own special types of life, and that these types can be recognized, and the rocks in which they occur can be correlated even in distant countries, and where no other means of comparison would be possible. At one moment he has to deal with the bones of some large mammal scattered through a deposit of superficial gravel, at another time with the minute foraminifers and ostracods of an upraised sea-bottom. Corals and crinoids crowded and crushed into a massive limestone where they lived and died, ferns and terrestrial plants matted together into a bed of coal where they originally grew, the scattered shells of a submarine sand-bank, the snails and lizards which lived and died within a hollow tree, the insects which have been imprisoned within the exuding resin of old forests, the footprints of birds and quadrupeds, the trails of worms left upon former shores-these, and innumerable other pieces of evidence, enable the geologist to realize in some measure what the faunas and floras of successive periods have been, and what geographical changes the site of every land has undergone.

It is evident that to deal successfully with these varied materials, a considerable acquaintance with different branches of science is needful. Especially necessary is a tolerably wide knowledge of the processes now at work in changing the surface of the earth, and of at least those forms of plant and animal life whose remains are apt to be preserved in geological deposits, or which in their structure

and habitat enable us to realize what their forerunners were. It has often been insisted upon that the present is the key to the past; and in a wide sense this assertion is eminently true. Only in proportion as we understand the present, where everything is open on all sides to the fullest investigation, can we expect to decipher the past, where so much is obscure, imperfectly preserved, or not preserved at all. A study of the existing economy of nature ought thus to be the foundation of the geologist's training.

While, however, the present condition of things is thus employed, we must obviously be on our guard against the danger of unconsciously assuming that the phase of nature's operations which we now witness has been the same in all past time, that geological changes have taken place in former ages in the manner and on the scale which we behold to-day, and that at the present time all the great geological processes, which have produced changes in the past eras of the earth's history, are still existent and active. Of course we may assume this uniformity of action, and use the assumption as a working hypothesis. But it ought not to be allowed any firmer footing, nor on any account be suffered to blind us to the obvious truth that the few centuries wherein man has been observing nature form much too brief an interval, by which to measure the intensity of geological action in all past time. For aught we can tell the present is an era of quietude and slow change, compared with some of the eras which have preceded it. Nor can we be sure that, when we have explored every geological process now in progress, we have exhausted all the causes of change which, even iu comparatively recent times, have been at work.

In dealing with the Geological Record, as the accessible solid part of the globe is called, we cannot too vividly realize that at the best it forms but an imperfect chronicle. Geological history cannot be compiled from a full and continuous series of documents. From the very nature of its origin the record is necessarily fragmentary, and it has been further mutilated and obscured by the revolutions of successive ages. And even where the chronicle of events is continuous, it is of very unequal value in different places. In one case, for example, it may present us with an unbroken succession of deposits many thousands of feet in thickness, from which, however, only a few meagre facts as to geological history can be gleaned. In another instance it brings before us, within the compass of a few yards, the evidence of a most varied and complicated series of changes in physical geography, as well as an abundant and interesting suite of organic remains. These and other characteristics of the geological record will become more apparent and intelligible as we proceed in the study of the science.

In the systematic treatment of the subject the following arrangement will here be followed :

1. The Cosmical Aspects of Geology.-Under this head we may consider the evidence supplied by astronomy and physics regarding the form and motions of the earth, the composition of the sun and planets, and the probable history of the solar system.

2. Geognosy, an Inquiry into the Materials of the Earth's Substance. In this division we deal with the parts of the earth, its envelopes of air and water, its solid crust, and the *probable condition of its interior. Especially, we have to study the more important minerals of the crust, and the chief rocks of which that crust is built up. In this way we lay a foundation of knowledge regarding the nature of the materials constituting the mass of the globe, and may next proceed to investigate the processes by which these materials are produced and altered.

3. Dynamical Geology embraces an investigation of the various agencies whereby the rocks of the earth's crust are formed and metamorphosed, and by which changes are

effected upon the distribution of sea and land, and upon the forms of terrestrial surfaces. Such an inquiry necessitates a careful study of the existing geological economy of nature, and forms a fitting introduction to the investigation of the geological changes of former periods. This and the previous section include most of what is embraced under Physical Geography; and for the reason stated under that heading the subject will here be treated more in detail than is usual in geological treatises.

4. Structural Geology, or the Architecture of the Earth. We now advance to consider how the various materials composing the crust of the earth have been arranged. We learn that some have been formed in beds or strata on the floor of the sea, that others have been built up by the slow aggregation of organic forms, that others have been poured out in a molten condition or in showers of loose dust from subterranean sources. We further find that, though originally laid down in almost horizontal beds, the rocks have subsequently been crumpled, contorted, and dislocated, that they have been incessantly worn down, and have often been depressed and buried beneath later accumulations. 5. Palæontological Geology.—This branch of the subject deals with the organic forms which are found preserved in the crust of the earth. It includes such questions as the relations between extinct and living types, the laws which appear to have governed the distribution of life in time and in space, the relative importance of different genera of animals in geological inquiry, the nature and use of the evidence from organic remains regarding former conditions of physical geography. This subject will be more properly discussed in the article PALEONTOLOGY, and will therefore be only cursorily treated in the following pages. 6. Stratigraphical Geology. -This section might be called geological history. It works out the chronological succession of the great formations of the earth's crust, and endeavours to trace the sequence of events of which they contain the record. More particularly it determines the order of succession of the various plants and animals which in past time have peopled the earth, and thus ascertains what has been the grand march of life upon the planet.

7. Physiographical Geology, starting from the basis of fact laid down by stratigraphical geology regarding former geographical changes, embraces an inquiry into the origin and history of the features of the earth's surface-continental ridges and ocean basins, plains, valleys, and mountains. It explains the causes on which local differences of scenery depend, and shows under what very different circumstances, and at what widely separated intervals, the hills and mountains, even of a single country, have been produced.


Before geology had attained to the position of an induc. tive science, it was customary to begin all investigations into the history of the earth by propounding or adopting some more or less fanciful hypothesis in explanation of the origin of our planet, or even of the universe. Such preliminary notions were looked upon as essential to a right understanding of the manner in which the materials of the globe had been put together. To the illustrious James Hutton (1785) geologists are indebted for strenuously upholding the doctrine that it is no part of the province of geology to discuss the origin of things. He taught them that in the materials from which geological evidence is to be compiled there can be found "no traces of a beginning, no prospect of an end." In England, mainly to the influence of the school which he founded, and to the subsequent rise of the Geological Society (1807), which resolved to collect facts instead of fighting over hypotheses, is due

the disappearance of the crude and unscientific cosmologies by which the writings of the earlier geologists were distinguished.

But there can now be little doubt that in the reaction against those visionary and often grotesque speculations, geologists were carried too far in an opposite direction. In allowing themselves to believe that geology had nothing to do with questions of cosmogony, they gradually grew up in the conviction that such questions could never be other than mere speculation, interesting or amusing as a theme for the employment of the fancy, but hardly coming within the domain of sober and inductive science. Nor would they soon have been awakened out of this belief by anything in their own science. It is still true that in the data with which they are accustomed to deal, as comprising the sum of geological evidence, there can be found no trace of a The oldest rocks which have been discovered beginning. on any part of the globe have probably been derived from other rocks older than themselves. Geology by itself has not yet revealed, and is little likely ever to reveal, a trace of the first solid crust of our globe. If then geological history is to be compiled from direct evidence furnished by the rocks of the earth, it cannot begin at the beginning of things, but must be content to date its first chapter from the earliest period of which any record has been preserved among the rocks.

Nevertheless, though geology in its usual restricted sense has been, and must ever be, unable to reveal the earliest history of our planet, it no longer ignores, as mere speculation, what is attempted in this subject by its sister sciences. Astronomy, physics, and chemistry have in late years all contributed to cast much light on the earlier stages of the earth's existence, previous to the beginning of what is commonly regarded as geological history. But whatever extends our knowledge of the former conditions of our globe may be legitimately claimed as part of the domain of geology. If this branch of inquiry therefore is to continue worthy of its name as the science of the earth, it must take cognizance of these recent contributions from other sciences. It must no longer be content to begin its annals with the records of the oldest rocks, but must endeavour to grope its way through the ages which preceded the formation of any rocks. Thanks to the results achieved with the telescope, the spectroscope, and the chemical laboratory, the story of these earliest ages of our earth is every year becoming more definite and intelligible.

RELATIONS OF THE EARTH IN THE SOLAR SYSTEM. Before entering upon the study of the structure and history of the earth, we may with advantage consider the general relations of our planet to the solar system, especially in view of its origin and history. It is now regarded as in the highest degree probable that all the members of that system have had a common origin. The investigations of recent years have revived and given a new form and meaning to the well-known nebular hypothesis, in which Laplace sketched the progress of the system from the state of an original nebula to its existing condition of a central incandescent sun with surrounding cool planetary bodies. He supposed that the nebula, originally diffused at least as far as the furthest member of the system, began to condense towards the centre, and that in so doing it threw off or left behind successive rings which on disruption and further condensation assumed the form of planets, sometimes with a further formation, of rings, which in the case of Saturn remain, though in other planets they have broken up and united into satellites.

According to this view we should expect that the matter composing the various members of the solar system should

be everywhere nearly the same. The fact of condensation round centres, however, indicates at least differences of density throughout the nebula. Mr Lockyer has, indeed, suggested that the materials composing the nebula arranged themselves according to their respective densities, the lightest occupying the exterior and the heaviest the interior of the mass. And if we compare the densities of the various planets, they certainly seem to support this suggestion. These densities are shown in the following table, that of the earth being taken as the unit :--Density of the Sun. Mercury

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There is not indeed a strict progression in the diminution of density, but the fact remains that, while the planets near the sun are about twice as heavy as they would be if they consisted of such a substance as granite, towards the outer limits of the system they are composed of matter as light as cork. Again, in some cases, a similar relation has been observed between the densities of the satellites and their primaries. The moon, for example, has a density little more than half that of the earth. The first satellite of Jupiter is less dense, though the other three are found to be more dense than the planet. Further, in the condition of the earth itself, a very light gaseous atmosphere forms the outer portion, beneath which lies a heavier layer of water, while within these two envelopes the materials forming the solid substance of the planet are so arranged that the outer layer or crust has only about half the density of the whole globe. Mr Lockyer finds in the sun itself evidence of the same tendency towards a stratified arrangement in accordance with relative densities, as will be immediately further alluded to.

There seems therefore to be much probability in the hypothesis that, in the gradual condensation of the original nebula, each successive mass left behind represented the density of its parent layer, and consisted of progressively heavier matter. The remoter planets, with their low density and vast absorbing atmospheres, may be supposed to consist of metalloids like the outer parts of the sun's atmosphere, while the interior planets are no doubt mainly metallic. The rupture of each planetary ring would, it is conceived, raise the temperature of the resultant nebulous planet to such a height as to allow the vapours to rearrange themselves by degrees in successive layers, or rather shells, according to density. And when the planet gave off a satellite, that body would, it might be expected, have the composition and density of the outer layers of its primary.1 For many years the only evidence available as to the actual composition of other heavenly bodies than our own earth was furnished by the aerolites, meteorites, or falling stars, which from time to time have entered our atmo sphere from planetary space, and have descended upon the surface of the globe. Subjected to chemical analysis these foreign bodies show considerable diversities of composition; but in no case have they yet yielded a trace of any element' not already recognized among terrestrial materials. Upwards of twenty of our elements have been detected in aerolites, sometimes in the free state, sometimes combined with each other. More than half of them are metals, including iron, nickel, manganese, calcium, sodium, and potas

1 Mr Lockyer communicated some of his views to Professor Prestwich, who gave them in his interesting Inaugural Lecture at Oxford, in 1875. He has further stated them in his Manchester Lectures, Why the Earth's Chemistry is as it is.




sium. There occur also carbon, silicon, phosphorus, sulphur, | been recognized, pointing to a structure resembling that of oxygen, nitrogen, and hydrogen. In some of their combinations these elements, as found in the meteoric stones, differ from their mode of occurrence in the accessible parts of the earth. Iron, for example, occurs as native metal, alloyed with a variable proportion (6 to 10 per cent.) of metallic nickel. But in other respects they closely resemble some of the familiar materials of the earth's rocky crust. Thus we have such minerals as pyrite, apatite, olivine, angite, hornblende, and labradorite. No more reliable proof could be desired that some at least of the other members of the solar system are formed of the same materials as compose the earth.

But in recent years a far more precise and generally applicable method of research into the composition of the heavenly bodies has been found in the spectroscope. By means of this instrument, the light emitted from selfluminous bodies can be analysed in such a way as to show what elements are present in their intensely hot luminous vapour. When the light of a burning metal is examined with a properly-arranged prism, it is seen to give a dark band or spectrum which is traversed by certain vertical bright lines. This is termed a radiation-spectrum. Each element appears to have its own characteristic arrange ment of lines, which retain the same relative position, intensity, and colours. Moreover, gases and the vapours of solid bodies are found to intercept those rays of light which they themselves emit. The spectrum of burning sodium, for example, shows two bright yellow lines. If therefore white light from some other source passes through the vapour of sodium, these two bright lines become dark lines, that portion of the light being exactly cut off which would have been given out by the sodium itself. This is called an absorption-spectrum.

By this method of examination it has been ascertained that many of the elements of which our earth is composed exist in the state of incandescent vapour in the atmosphere of the sun. Among these are some of our most familiar metals-iron, zinc, copper, nickel, with sodium, magnesium, barium, calcium, and vast quantities of free hydrogen. Moreover, as Mr Lockyer has pointed out, these elements. appear to succeed each other in relation to their respective densities. Thus the coronal atmosphere which, as seen ir total eclipses, extends to so prodigious a distance beyond the orb of the sun, consists mainly of sub-incandescent hydrogen and another element which may be new. Beneath this external vaporous envelope lies the chromosphere where the vapours of incandescent hydrogen, calcium, and magnesium can be detected. Further inward the spot-zone shows the presence of sodium, titanium, &c.; while still lower, a layer (the reversing layer) of intensely hot vapours, lying probably next to the inner brilliant photosphere gives spectroscopic evidence of the existence of incandescent iron, manganese, cobalt, nickel, copper, and other well-known terrestrial metals.1

The spectrosope has likewise been successfully applied by Mr Huggins and others to the observation of the fixed stars and nebulæ, with the result of establishing a similarity of elements between our own system and other bodies in sidereal space. In the radiation spectra of nebula Mr Huggins finds the hydrogen lines very prominent; and he conceives that they may be glowing masses of that element. Sir William Thomson and Professor Tait have suggested, on the other hand, that they are more probably clouds of stones in rapid motion, perhaps in an atmosphere of hydrogen. Among the fixed stars absorption spectra lave

On the constitution of the sun see Roscoe's Spectrum Analysis; Lockyer's Solar Physics, 1873; and memoirs in Proc. of Roy. Soc., by B. Stewart, Loewy, and De la Rue.

our sun, viz., a solid or liquid incandescent nucleus, surrounded with an atmosphere of glowing vapour.2 According to Mr Lockyer, those stars or nebula which have the highest temperature have the simplest spectra, and in proportion as they cool their materials become more and more differentiated into what we call elements. He remarks that the most brilliant or hottest stars show in their spectra only the lines of gases, as hydrogen. Cooler stars, like our sun, give indications of the presence, in addition, of the more stable metals-magnesium, sodium, calcium, iron. A still lower temperature he regards as marked by the appearance of the other metals, metalloids, and compounds, so that the older a star or planet is the more will it lose free hydrogen, till, when it comes to the condition of our earth, all its free hydrogen will have disappeared. According to this view the atoms of all the elements existed originally in the nebula dissociated from each other by reason of the intense heat. As the nebula gravitated towards its nucleus and cooled, the atoms came together, and the elements appeared in a certain order, beginning with hydrogen, and passing on through the metals and metalloids into compounds such as we find on our globe. The sun would thus be a star considerably advanced in the process of differentiation or association of its atoms. It contains, so far as we know, no metalloids or compounds, while stars like Sirius show the presence only of hydrogen, with but a feeble proportion of metallic vapours; and on the other hand, the red stars indicate by their spectra that their metallic vapours have entered into combination, whence it is inferred that their temperature is lower than that of our sun.

Further confirmation of these views as to the order of planetary evolution is furnished by the form and structure of the earth. Reference has already been made to the fact that the outer crust of our planet possesses only about half the density of the whole mass. It consists largely of metalloids-oxygen, silicon, carbon, sulphur, chlorine. On the other hand, lavas and mineral veins, which are believed to have been supplied from some considerable depth, contain abundance of metallic ingredients.

The form of the globe likewise points to a former fluid condition. As the result of computations from ten measured arcs of the meridian made by different observers between the latitudes of Sweden and the Cape of Good Hope, Bessel obtained the following data for the dimensions of the earth :—

Equatorial diameter. Polar diameter..

.41,847,192 feet, or 7925 604 miles. .41,707,314 7899-114 26.471

Amount of polar flattening, 139,768

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The equatorial circumference is thus a little less than 25,000 miles, and the difference between the polar and equatorial diameters (nearly 26 miles) amounts to about 3th of the equatorial diameter. More recently, however, it has been shown that the oblate spheroid indicated by these measurements is not a symmetrical body, the equatorial circumference being an ellipse instead of a circle. The diameter of which the vertices touch the surface of the globe in longitudes 14° 23′ E. and 194° 23' E. of Greenwich is nearly two miles longer than that at right angles to it.5 In obedience to the influence of rotation on its axis, our planet would tend to assume exactly such a flattening at This was disthe poles as it has been proved to possess. covered and demonstrated by Newton, and the amount of

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