OUTLINES OF THE EARTH'S HISTORY

A POPULAR STUDY IN PHYSIOGRAPHY

BY

NATHANIEL SOUTHGATE SHALER

PROFESSOR OF GEOLOGY IN HARVARD UNIVERSITY DEAN OF LAWRENCE SCIENTIFIC SCHOOL

ILLUSTRATED WITH INDEX

NEW YORK AND LONDON D. APPLETON AND COMPANY

1898, 1910



PREFACE.

The object of this book is to provide the beginner in the study of the earth's history with a general account of those actions which can be readily understood and which will afford him clear understandings as to the nature of the processes which have made this and other celestial spheres. It has been the writer's purpose to select those series of facts which serve to show the continuous operations of energy, so that the reader might be helped to a truer conception of the nature of this sphere than he can obtain from ordinary text-books.

In the usual method of presenting the elements of the earth's history the facts are set forth in a manner which leads the student to conceive that history as in a way completed. The natural prepossession to the effect that the visible universe represents something done, rather than something endlessly doing, is thus re-enforced, with the result that one may fail to gain the largest and most educative impression which physical science can afford him in the sense of the swift and unending procession of events.

It is well known to all who are acquainted with the history of geology that the static conception of the earth—the idea that its existing condition is the finished product of forces no longer in action—led to prejudices which have long retarded, and indeed still retard, the progress of that science. This fact indicates that at the outset of a student's work in this field he should be guarded against such misconceptions. The only way to attain the end is by bringing to the understanding of the beginner a clear idea of successions of events which are caused by the forces operating in and on this sphere. Of all the chapters of this great story, that which relates to the history of the work done by the heat of the sun is the most interesting and awakening. Therefore an effort has been made to present the great successive steps by which the solar energy acts in the processes of the air and the waters.

The interest of the beginner in geology is sure to be aroused when he comes to see how very far the history of the earth has influenced the fate of men. Therefore the aim has been, where possible, to show the ways in which geological processes and results are related to ourselves; how, in a word, this earth has been the well-appointed nursery of our kind.

All those who are engaged in teaching elementary science learn the need of limiting the story they have to tell to those truths which can be easily understood by beginners. It is sometimes best, as in stating such difficult matters as those concerning the tides, to give explanations which are far from complete, and which, as to their mode of presentation, would be open to criticism were it not for the fact that any more elaborate statements would most likely be incomprehensible to the novice, thus defeating the teacher's aim.

It will be observed that no account is here given of the geological ages or of the successions of organic life. Chapters on these subjects were prepared, but were omitted for the reason that they made the story too long, and also because they carried the reader into a field of much greater difficulty than that which is found in the physical history of the earth.

N.S.S. March, 1898.



CONTENTS.

CHAPTER PAGE

I.—INTRODUCTION TO THE STUDY OF NATURE 1 II.—WAYS AND MEANS OF STUDYING NATURE 9 III.—THE STELLAR REALM 31 IV.—THE EARTH 81 V.—THE ATMOSPHERE 97 VI.—GLACIERS 207 VII.—THE WORK OF UNDERGROUND WATER 250 VIII.—THE SOIL 313 IX.—THE ROCKS AND THEIR ORDER 349



LIST OF FULL-PAGE ILLUSTRATIONS.

FACING PAGE

Dunes at Ipswich Light, Massachusetts Frontispiece Seal Rocks near San Francisco, California 33 Lava stream, in Hawaiian Islands, flowing into the sea 72 Waterfall near Gadsden, Alabama 90 South shore, Martha's Vineyard, Massachusetts 121 Pocket Creek, Cape Ann, Massachusetts 163 Muir Glacier, Alaska 207 Front of Muir Glacier 240 Mount AEtna, seen from near Catania 201 Mountain gorge, Himalayas, India 330



OUTLINES OF THE EARTH'S HISTORY.

CHAPTER I.

AN INTRODUCTION TO THE STUDY OF NATURE.

The object of this book is to give the student who is about to enter on the study of natural science some general idea as to the conditions of the natural realm. As this field of inquiry is vast, it will be possible only to give the merest outline of its subject-matter, noting those features alone which are of surpassing interest, which are demanded for a large understanding of man's place in this world, or which pertain to his duties in life.

In entering on any field of inquiry, it is most desirable that the student should obtain some idea as to the ways in which men have been led to the knowledge which they possess concerning the world about them. Therefore it will be well briefly to sketch the steps by which natural science has come to be what it is. By so doing we shall perceive how much we owe to the students of other generations; and by noting the difficulties which they encountered, and how they avoided them, we shall more easily find our own way to knowledge.

The primitive savages, who were the ancestors of all men, however civilized they may be, were students of Nature. The remnants of these lowly people who were left in different parts of the world show us that man was not long in existence before he began to devise some explanation concerning the course of events in the outer world. Seeing the sun rise and set, the changes of the moon, the alternation of the seasons, the incessant movement of the streams and sea, and the other more or less orderly successions of events, our primitive forefathers were driven to invent some explanation of them. This, independently, and in many different times and places, they did in a simple and natural way by supposing that the world was controlled by a host of intelligent beings, each of which had some part in ordering material things. Sometimes these invisible powers were believed to be the spirits of great chieftains, who were active when on earth, and who after death continued to exercise their power in the larger realms of Nature. Again, and perhaps more commonly, these movements of Nature were supposed to be due to the action of great though invisible beasts, much like those which the savage found about him. Thus among our North American Indians the winds are explained by the supposition that the air is fanned by the wings of a great unseen bird, whose duty it is to set the atmosphere into motion. That no one has ever seen the bird doing the work, or that the task is too great for any conceivable bird, is to the simple, uncultivated man no objection to this view. It is long, indeed, before education brings men to the point where they can criticise their first explanations of Nature.

As men in their advance come to see how much nobler are their own natures than those of the lower animals, they gradually put aside the explanation of events by the actions of beasts, and account for the order of the world by the supposition that each and every important detail is controlled by some immortal creature essentially like a man, though much more powerful than those of their own kind. This stage of understanding is perhaps best shown by the mythology of the Greeks, where there was a great god over all, very powerful but not omnipotent; and beneath him, in endless successions of command, subordinate powers, each with a less range of duties and capacities than those of higher estate, until at the bottom of the system there were minor deities and demigods charged with the management of the trees, the flowers, and the springs—creatures differing little from man, except that they were immortal, and generally invisible, though they, like all the other deities, might at their will display themselves to the human beings over whom they watched, and whose path in life they guided.

Among only one people do we find that the process of advance led beyond this early and simple method of accounting for the processes of Nature, bringing men to an understanding such as we now possess. This great task was accomplished by the Greeks alone. About twenty-five hundred years ago the philosophers of Greece began to perceive that the early notion as to the guidance of the world by creatures essentially like men could not be accepted, and must be replaced by some other view which would more effectively account for the facts. This end they attained by steps which can not well be related here, but which led them to suppose separate powers behind each of the natural series—powers having no relation to the qualities of mankind, but ever acting to a definite end. Thus Plato, who represents most clearly this advance in the interpretation of facts, imagined that each particular kind of plant or animal had its shape inevitably determined by something which he termed an idea, a shape-giving power which existed before the object was created, and which would remain after it had been destroyed, ever ready again to bring matter to the particular form. From this stage of understanding it was but a short step to the modern view of natural law. This last important advance was made by the great philosopher Aristotle, who, though he died about twenty-two hundred years ago, deserves to be accounted the first and in many ways the greatest of the ancient men of science who were informed with the modern spirit.

With Aristotle, as with all his intellectual successors, the operations of Nature were conceived as to be accounted for by the action of forces which we commonly designate as natural laws, of which perhaps the most familiar and universal is that of gravitation, which impels all bodies to move toward each other with a degree of intensity which is measured by their weight and the distance by which they are separated.

For many centuries students used the term law in somewhat the same way as the more philosophical believers in polytheism spoke of their gods, or as Plato of the ideas which he conceived to control Nature. We see by this instance how hard it is to get rid of old ways of thinking. Even when the new have been adopted we very often find that something of the ancient and discarded notions cling in our phrases. The more advanced of our modern philosophers are clear in their mind that all we know as to the order of Nature is that, given certain conditions, certain consequences inevitably follow.

Although the limitations which modern men of science perceive to be put upon their labours may seem at first sight calculated to confine our understanding within a narrow field of things which can be seen, or in some way distinctly proved to exist, the effect of this limitation has been to make science what it is—a realm of things known as distinct from things which may be imagined. All the difference between ancient science and modern consists in the fact that in modern science inquirers demand a businesslike method in the interpretation of Nature. Among the Greeks the philosopher who taught explanations of any feature in the material world which interested him was content if he could imagine some way which would account for the facts. It is the modern custom now to term the supposition of an explanation a working hypothesis, and only to give it the name of theory after a very careful search has shown that all the facts which can be gathered are in accordance with the view. Thus when Newton made his great suggestion concerning the law of gravitation, which was to the effect that all bodies attracted each other in proportion to their masses, and inversely as the square of their distance from each other, he did not rest content, as the old Greeks would have done, with the probable truth of the explanation, but carefully explored the movements of the planets and satellites of the solar system to see if the facts accorded with the hypothesis. Even the perfect correspondence which he found did not entirely content inquirers, and in this century very important experiments have been made which have served to show that a ball suspended in front of a precipice will be attracted toward the steep, and that even a mass of lead some tons in weight will attract toward itself a small body suspended in the manner of a pendulum.

It is this incessant revision of the facts, in order to see if they accord with the assumed rule or law, which has given modern science the sound footing that it lacked in earlier days, and which has permitted our learning to go on step by step in a safe way up the heights to which it has climbed. All explanations of Nature begin with the work of the imagination. In common phrase, they all are guesses which have at first but little value, and only attain importance in proportion as they are verified by long-continued criticism, which has for its object to see whether the facts accord with the theory. It is in this effort to secure proof that modern science has gathered the enormous store of well-ascertained facts which constitutes its true wealth, and which distinguishes it from the earlier imaginative and to a great extent unproved views.

In the original state of learning, natural science was confounded with political and social tradition, with the precepts of duty which constitute the law of the people, as well as with their religion, the whole being in the possession of the priests or wise men. So long as natural action was supposed to be in the immediate control of numerous gods and demigods, so long, in a word, as the explanation of Nature was what we term polytheistic, this association of science with other forms of learning was not only natural but inevitable. Gradually, however, as the conception of natural law replaced the earlier idea as to the intervention of a spirit, science departed from other forms of lore and came to possess a field to itself. At first it was one body of learning. The naturalists of Aristotle's time, and from his day down to near our own, generally concerned themselves with the whole field of Nature. For a time it was possible for any one able and laborious man to know all which had been ascertained concerning astronomy, chemistry, geology, as well as the facts relating to living beings. The more, however, as observation accumulated, and the store of facts increased, it became difficult for any one man to know the whole. Hence it has come about that in our own time natural learning is divided into many distinct provinces, each of which demands a lifetime of labour from those who would know what has already been done in the field, and what it is now important to do in the way of new inquiries.

The large divisions which naturalists have usually made of their tasks rest in the main on the natural partitions which we may readily observe in the phenomenal world. First of all comes astronomy, including the phenomena exhibited in the heavens, beyond the limits of the earth's atmosphere. Second, geology, which takes account of all those actions which in process of time have been developed in our own sphere. Third, physics, which is concerned with the laws of energy, or those conditions which affect the motion of bodies, and the changes which are impressed upon them by the different natural forces. Fourth, chemistry, which seeks to interpret the principles which determine the combination of atoms and the molecules which are built of them under the influence of the chemical affinities. Fifth, biology, or the laws of life, a study which pertains to the forms and structures of animals and plants, and their wonderful successions in the history of the world. Sixth, mathematics, or the science of space and number, that deals with the principles which underlie the order of Nature as expressed at once in the human understanding and in the material universe. By its use men were made able to calculate, as in arithmetic, the problems which concern their ordinary business, as well as to compute the movements of the celestial bodies, and a host of actions which take place on the earth that would be inexplicable except by the aid of this science. Last of all among the primary sciences we may name that of psychology, which takes account of mental operations among man and his lower kindred, the animals.

In addition to the seven sciences above mentioned, which rest in a great measure on the natural divisions of phenomena, there are many, indeed, indefinitely numerous, subdivisions which have been made to suit the convenience of students. Thus astronomy is often separated into physical and mathematical divisions, which take account either of the physical phenomena exhibited by the heavenly bodies or of their motions. In geology there are half a dozen divisions relating to particular branches of that subject. In the realm of organic life, in chemistry, and in physics there are many parts of these sciences which have received particular names.

It must not be supposed that these sciences have the independence of each other which their separate names would imply. In fact, the student of each, however, far he may succeed in separating his field from that of the other naturalists, as we may fitly term all students of Nature, is compelled from time to time to call in the aid of his brethren who cultivate other branches of learning. The modern astronomer needs to know much of chemistry, or else he can not understand many of his observations on the sun. The geologists have to share their work with the student of animal and vegetable life, with the physicists; they must, moreover, know something of the celestial spheres in order to interpret the history of the earth. In fact, day by day, with the advance of learning, we come more clearly to perceive that all the processes of Nature are in a way related to each other, and that in proportion as we understand any part of the great mechanism, we are forced in a manner to comprehend the whole. In other words, we are coming to understand that these divisions of the field of science depend upon the limitations of our knowledge, and not upon the order of Nature itself. For the purposes of education it is important that every one should know something of the great truths which each science has disclosed. No mortal man can compass the whole realm of this knowledge, but every one can gain some idea of the larger truths which may help him to understand the beauty and grandeur of the sphere in which he dwells, which will enable him the better to meet the ordinary duties of life, that in almost all cases are related to the facts of the world about us. It has been of late the custom to term this body of general knowledge which takes account of the more evident facts and important series of terrestrial actions physiography, or, as the term implies, a description of Nature, with the understanding that the knowledge chosen for the account is that which most intimately concerns the student who seeks information that is at once general and important. Therefore, in this book the effort is made first to give an account as to the ways and means which have led to our understanding of scientific problems, the methods by which each person may make himself an inquirer, and the outline of the knowledge that has been gathered since men first began to observe and criticise the revelations the universe may afford them.



CHAPTER II.

WAYS AND MEANS OF STUDYING NATURE.

It is desirable that the student of Nature keep well in mind the means whereby he is able to perceive what goes on in the world about him. He should understand something as to the nature of his senses, and the extent to which these capacities enable him to discern the operations of Nature. Man, in common with his lower kindred, is, by the mechanism of the body, provided with five somewhat different ways by which he may learn something of the things about him. The simplest of these capacities is that of touch, a faculty that is common to the general surface of the body, and which informs us when the surface is affected by contact with some external object. It also enables us to discern differences of temperature. Next is the sense of taste, which is limited to the mouth and the parts about it. This sense is in a way related to that of touch, for the reason that it depends on the contact of our body with material things. Third is the sense of smell, so closely related to that of taste that it is difficult to draw the line between the two. Yet through the apparatus of the nose we can perceive the microscopically small parts of matter borne to us through the air, which could not be appreciated by the nerves of the mouth. Fourth in order of scope comes the hearing, which gives us an account of those waves of matter that we understand as sound. This power is much more far ranging than those before noted; in some cases, as in that of the volcanic explosions from the island of Krakatoa, in the eruption of 1883, the convulsions were audible at the distance of more than a thousand miles away. The greater cannon of modern days may be heard at the distance of more than a hundred miles, so that while the sense of touch, taste, and smell demand contact with the bodies which we appreciate, hearing gives us information concerning objects at a considerable distance. Last and highest of the senses, vastly the most important in all that relates to our understanding of Nature, is sight, or the capacity which enables us to appreciate the movement of those very small waves of ether which constitute light. The eminent peculiarity of sight is that it may give us information concerning things which are inconceivably far away; it enables us to discern the light of suns probably millions of times as remote from us as is the centre of our own solar system.

Although much of the pleasure which the world affords us comes through the other senses, the basis of almost all our accurate knowledge is reported by sight. It is true that what we have observed with our eyes may be set forth in words, and thus find its way to the understanding through the ears; also that in many instances the sense of touch conveys information which extends our perceptions in many important ways; but science rests practically on sight, and on the insight that comes from the training of the mind which the eyes make possible.

The early inquirers had no resources except those their bodies afforded; but man is a tool-making creature, and in very early days he began to invent instruments which helped him in inquiry. The earliest deliberate study was of the stars. Science began with astronomy, and the first instruments which men contrived for the purpose of investigation were astronomical. In the beginning of this search the stars were studied in order to measure the length of the year, and also for the reason that they were supposed in some way to control the fate of men. So far as we know, the first pieces of apparatus for this purpose were invented in Egypt, perhaps about four thousand years before the Christian era. These instruments were of a simple nature, for the magnifying glass was not yet contrived, and so the telescope was impossible. They consisted of arrangements of straight edges and divided circles, so that the observers, by sighting along the instruments, could in a rough way determine the changes in distance between certain stars, or the height of the sun above the horizon at the various seasons of the year. It is likely that each of the great pyramids of Egypt was at first used as an observatory, where the priests, who had some knowledge of astronomy, found a station for the apparatus by which they made the observations that served as a basis for casting the horoscope of the king.

In the progress of science and of the mechanical invention attending its growth, a great number of inventions have been contrived which vastly increase our vision and add inconceivably to the precision it may attain. In fact, something like as much skill and labour has been given to the development of those inventions which add to our learning as to those which serve an immediate economic end. By far the greatest of these scientific inventions are those which depend upon the lens. By combining shaped bits of glass so as to control the direction in which the light waves move through them, naturalists have been able to create the telescope, which in effect may bring distant objects some thousand times nearer to view than they are to the naked eye; and the microscope, which so enlarges minute objects as to make them visible, as they were not before. The result has been enormously to increase our power of vision when applied to distant or to small objects. In fact, for purposes of learning, it is safe to say that those tools have altogether changed man's relation to the visible universe. The naked eye can see at best in the part of the heavens visible from any one point not more than thirty thousand stars. With the telescope somewhere near a hundred million are brought within the limits of vision. Without the help of the microscope an object a thousandth of an inch in diameter appears as a mere point, the existence of which we can determine only under favourable circumstances. With that instrument the object may reveal an extended and complicated structure which it may require a vast labour for the observer fully to explore.

Next in importance to the aid of vision above noted come the scientific tools which are used in weighing and measuring. These balances and gauges have attained such precision that intervals so small as to be quite invisible, and weights as slight as a ten-thousandth of a grain, can be accurately measured. From these instruments have come all those precise examinations on which the accuracy of modern science intimately depends. All these instruments of precision are the inventions of modern days. The simplest telescopes were made only about two hundred and fifty years ago, and the earlier compound microscopes at a yet later date. Accurate balances and other forms of gauges of space, as well as good means of dividing time, such as our accurate astronomical clocks and chronometers, are only about a century old. The instruments have made science accurate, and have immensely extended its powers in nearly all the fields of inquiry.

Although the most striking modern discoveries are in the field which was opened to us by the lens in its manifold applications, it is in the chemist's laboratory that we find that branch of science, long cultivated, but rapidly advanced only within the last two centuries, which has done the most for the needs of man. The ancients guessed that the substances which make up the visible world were more complicated in their organization than they appear to our vision. They even suggested the great truth that matter of all kinds is made up of inconceivably small indivisible bits which they and we term atoms. It is likely that in the classic days of Greece men began to make simple experiments of a chemical nature. A century or two after the time of Mohammed, the Arabians of his faith, a people who had acquired Greek science from the libraries which their conquests gave them, conducted extensive experiments, and named a good many familiar chemical products, such as alcohol, which still bears its Arabic name.

These chemical studies were continued in Europe by the alchemists, a name also of Arabic origin, a set of inquirers who were to a great extent drawn away from scientific studies by vain though unending efforts to change the baser metals into gold and silver, as well as to find a compound which would make men immortal in the body. By the invention of the accurate balance, and by patient weighing of the matters which they submitted to experiment, by the invention of hypotheses or guesses at truth, which were carefully tested by experiment, the majestic science of modern chemistry has come forth from the confused and mystical studies of the alchemists. We have learned to know that there are seventy or more primitive or apparently unchangeable elements which make up the mass of this world, and probably constitute all the celestial spheres, and that these elements in the form of their separate atoms may group themselves in almost inconceivably varied combinations. In the inanimate realm these associations, composed of the atoms of the different substances, forming what are termed molecules, are generally composed of but few units. Thus carbonic-acid gas, as it is commonly called, is made up of an aggregation of molecules, each composed of one atom of carbon and two of oxygen; water, of two atoms of hydrogen and one of oxygen; ordinary iron oxide, of two atoms of iron and three of oxygen. In the realm of organic life, however, these combinations become vastly more complicated, and with each of them the properties of the substance thus produced differ from all others. A distinguished chemist has estimated that in one group of chemical compounds, that of carbon, it would be possible to make such an array of substances that it would require a library of many thousand ordinary volumes to contain their names alone.

It is characteristic of chemical science that it takes account of actions which are almost entirely invisible. No contrivances have been or are likely to be invented which will show the observer what takes place when the atoms of any substance depart from their previous combination and enter on new arrangements. We only know that under certain conditions the old atomic associations break up, and new ones are formed. But though the processes are hidden, the results are manifest in the changes which are brought about upon the masses of material which are subjected to the altering conditions. Gradually the chemists of our day are learning to build up in their laboratories more and more complicated compounds; already they have succeeded in producing many of the materials which of old could only be obtained by extracting them from plants. Thus a number of the perfumes of flowers, and many of the dye-stuffs which a century ago were extracted from vegetables, and were then supposed to be only obtainable in that way, are now readily manufactured. In time it seems likely that important articles of food, for which we now depend upon the seeds of plants, may be directly built up from the mineral kingdom. Thus the result of chemical inquiry has been not only to show us much of the vast realm of actions which go on in the earth, but to give us control of many of these movements so that we may turn them to the needs of man.

Animals and plants were at an early day very naturally the subjects of inquiry. The ancients perceived that there were differences of kind among these creatures, and even in Aristotle's time the sciences of zooelogy and botany had attained the point where there were considerable treatises on those subjects. It was not, however, until a little more than a century ago that men began accurately to describe and classify these species of the organic world. Since the time of Linnaeus the growth of our knowledge has gone forward with amazing swiftness. Within a century we have come to know perhaps a hundred times as much concerning these creatures as was learned in all the earlier ages. This knowledge is divisible into two main branches: in one the inquirers have taken account of the different species, genera, families, orders, and classes of living forms with such effect that they have shown the existence at the present time of many hundred thousand distinct species, the vast assemblage being arranged in a classification which shows something as to the relationship which the forms bear to each other, and furthermore that the kinds now living have not been long in existence, but that at each stage in the history of the earth another assemblage of species peopled the waters and the lands.

At first naturalists concerned themselves only with the external forms of living creatures; but they soon came to perceive that the way in which these organisms worked, their physiology, in a word, afforded matters for extended inquiry. These researches have developed the science of physiology, or the laws of bodily action, on many accounts the most modern and extensive of our new acquisitions of natural learning. Through these studies we have come to know something of the laws or principles by which life is handed on from generation to generation, and by which the gradations of structure have been advanced from the simple creatures which appear like bits of animated jelly to the body and mind of man.

The greatest contribution which modern naturalists have made to knowledge concerns the origin of organic species. The students of a century ago believed that all these different kinds had been suddenly created either through natural law or by the immediate will of God. We now know that from the beginning of organic life in the remote past to the present day one kind of animal or plant has been in a natural and essentially gradual way converted into the species which was to be its successor, so that all the vast and complicated assemblage of kinds which now exists has been derived by a process of change from the forms which in earlier ages dwelt upon this planet. The exact manner in which these alterations were produced is not yet determined, but in large part it has evidently been brought about by the method indicated by Mr. Darwin, through the survival of the fittest individuals in the struggle for existence.

Until men came to have a clear conception as to the spherical form of the earth, it was impossible for them to begin any intelligent inquiries concerning its structure or history. The Greeks knew the earth to be a sphere, but this knowledge was lost among the early Christian people, and it was not until about four hundred years ago that men again came to see that they dwelt upon a globe. On the basis of this understanding the science of geology, which had in a way been founded by the Greeks, was revived. As this science depends upon the knowledge which we have gained of astronomy, physics, chemistry, and biology, all of which branches of learning have to be used in explaining the history of the earth, the advance which has been made has been relatively slow. Geology as a whole is the least perfectly organized of all the divisions of learning. A special difficulty peculiar to this science has also served to hinder its development. All the other branches of learning deal mainly, if not altogether, with the conditions of Nature as they now exist. In this alone is it necessary at every step to take account of actions which have been performed in the remote past.

It is an easy matter for the students of to-day to imagine that the earth has long endured; but to our forefathers, who were educated in the view that it had been brought from nothingness into existence about seven thousand years ago, it was most difficult and for a time impossible to believe in its real antiquity. Endeavouring, as they naturally did, to account for all the wonderful revolutions, the history of which is written in the pages of the great stone book, the early geologists supposed this planet to have been the seat of frequent and violent changes, each of which revolutionized its shape and destroyed its living tenants. It was only very gradually that they became convinced that a hundred million years or more have elapsed since the dawn of life on the earth, and that in this vast period the march of events has been steadfast, the changes taking place at about the same rate in which they are now going on. As yet this conception as to the history of our sphere has not become the general property of the people, but the fact of it is recognised by all those who have attentively studied the matter. It is now as well ascertained as any of the other truths which science has disclosed to us.

It is instructive to note the historic outlines of scientific development. The most conspicuous truth which this history discloses is that all science has had its origin and almost all its development among the peoples belonging to the Aryan race. This body of folk appears to have taken on its race characteristics, acquired its original language, its modes of action, and the foundations of its religion in that part of northern Europe which is about the Baltic Sea. Thence the body of this people appear to have wandered toward central Asia, where after ages of pastoral life in the high table lands and mountains of their country it sent forth branches to India, Asia Minor and Greece, to Persia, and to western Europe. It seems ever to have been a characteristic of these Aryan peoples that they had an extreme love for Nature; moreover, they clearly perceived the need of accounting for the things that happened in the world about them. In general they inclined to what is called the pantheistic explanation of the universe. They believed a supreme God in many different forms to be embodied in all the things they saw. Even their own minds and bodies they conceived as manifestations of this supreme power. Among the Aryans who came to dwell in Europe and along the eastern Mediterranean this method of explaining Nature was in time changed to one in which humanlike gods were supposed to control the visible and invisible worlds. In that marvellous centre of culture which was developed among the Greeks this conception of humanlike deities was in time replaced by that of natural law, and in their best days the Greeks were men of science essentially like those of to-day, except that they had not learned by experience how important it was to criticise their theories by patiently comparing them with the facts which they sought to explain. The last of the important Greek men of science, Strabo, who was alive when Christ was born, has left us writings which in quality are essentially like many of the able works of to-day. But for the interruption in the development of Greek learning, natural science would probably have been fifteen hundred years ahead of its present stage. This interruption came in two ways. In one, through the conquest of Greece and the destruction of its intellectual life by the Romans, a people who were singularly incapable of appreciating natural science, and who had no other interest in it except now and then a vacant and unprofitable curiosity as to the processes of the natural world. A second destructive influence came through the fact that Christianity, in its energetic protest against the sins of the pagan civilization, absolutely neglected and in a way despised all forms of science.

The early indifference of Christians to natural learning is partly to be explained by the fact that their religion was developed among the Hebrews, a people remarkable for their lack of interest in the scientific aspects of Nature. To them it was a sufficient explanation that one omnipotent God ruled all things at his will, the heavens and the earth alike being held in the hollow of his hand.

Finding the centre of its development among the Romans, Christianity came mainly into the control of a people who, as we have before remarked, had no scientific interest in the natural world. This condition prolonged the separation of our faith from science for fifteen hundred years after its beginning. In this time the records of Greek scientific learning mostly disappeared. The writings of Aristotle were preserved in part for the reason that the Church adopted many of his views concerning questions in moral philosophy and in politics. The rest of Greek learning was, so far as Europe was concerned, quite neglected.

A large part of Greek science which has come down to us owes its preservation to a very singular incident in the history of learning. In the ninth century, after the Arabs had been converted to Mohammedanism, and on the basis of that faith had swiftly organized a great and cultivated empire, the scholars of that folk became deeply interested in the remnants of Greek learning which had survived in the monastic and other libraries about the eastern Mediterranean. So greatly did they prize these records, which were contemned by the Christians, that it was their frequent custom to weigh the old manuscripts in payment against the coin of their realm. In astronomy, mathematics, chemistry, and geology the Arabian students, building on the ancient foundations, made notable and for a time most important advances. In the tenth century of our era they seemed fairly in the way to do for science what western Europe began five centuries later to accomplish. In the fourteenth century the centre of Mohammedan strength was transferred from the Arabians to the Turks, from a people naturally given to learning to a folk of another race, who despised all such culture. Thenceforth in place of the men who had treasured and deciphered with infinite pains all the records of earlier learning, the followers of Mohammed zealously destroyed all the records of the olden days. Some of these records, however, survived among the Arabs of Spain, and others were preserved by the Christian scholars who dwelt in Byzantium, or Constantinople, and were brought into western Europe when that city was captured by the Turks in the fifteenth century.

Already the advance of the fine arts in Italy and the general tendency toward the study of Nature, such as painting and sculpture indicate, had made a beginning, or rather a proper field for a beginning, of scientific inquiry. The result was a new interest in Greek learning in all its branches, and a very rapid awakening of the scientific spirit. At first the Roman Church made no opposition to this new interest which developed among its followers, but in the course of a few years, animated with the fear that science would lead men to doubt many of the dogmas of the Church, it undertook sternly to repress the work of all inquirers.

The conflict between those of the Roman faith and the men of science continued for above two hundred years. In general, the part which the Church took was one of remonstrance, but in a few cases the spirit of fanaticism led to the persecution of the men who did not obey its mandates and disavow all belief in the new opinions which were deemed contrary to the teachings of Scripture. The last instance of such oppression occurred in France in the year 1756, when the great Buffon was required to recant certain opinions concerning the antiquity of the earth which he had published in his work on Natural History. This he promptly did, and in almost servile language withdrew all the opinions to which the fathers had objected. A like conflict between the followers of science and the clerical authorities occurred in Protestant countries. Although in no case were the men of science physically tortured or executed for their opinions, they were nevertheless subjected to great religious and social pressure: they were almost as effectively disciplined as were those who fell under the ban of the Roman Church.

Some historians have criticised the action of the clerical authorities toward science as if the evil which was done had been performed in our own day. It should be remembered, however, that in the earlier centuries the churches regarded themselves as bound to protect all men from the dangers of heresy. For centuries in the early history of Christianity the defenders of the faith had been engaged in a life-and-death struggle with paganism, the followers of which held all that was known of Nature. Quite naturally the priestly class feared that the revival of scientific inquiry would bring with it the evils from which the world had suffered in pagan times. There is no doubt that these persecutions of science were done under what seemed the obligations of duty. They may properly be explained particularly by men of science as one of the symptoms of development in the day in which they were done. It is well for those who harshly criticise the relations of the Church to science to remember that in our own country, about two centuries ago, among the most enlightened and religious people of the time, Quakers were grievously persecuted, and witches hanged, all in the most dutiful and God-fearing way. In considering these relations of science to our faith, the matter should be dealt with in a philosophical way, and with a sense of the differences between our own and earlier ages.

To the student of the relations between Christianity and science it must appear doubtful whether the criticism or the other consequences which the men of science had to meet from the Church was harmful to their work. The early naturalists, like the Greeks whom they followed, were greatly given to speculations concerning the processes of Nature, which, though interesting, were unprofitable. They also showed a curious tendency to mingle their scientific speculations with ancient and base superstitions. They were often given to the absurdity commonly known as the "black art," or witchcraft, and held to the preposterous notions of the astrologists. Even the immortal astronomer Kepler, who lived in the sixteenth century, was a professional astrologer, and still held to the notion that the stars determined the destiny of men. Many other of the famous inquirers in those years which ushered in modern science believed in witchcraft. Thus for a time natural learning was in a way associated with ancient and pernicious beliefs which the Church was seeking to overthrow. One result of the clerical opposition to the advancement of science was that its votaries were driven to prove every step which led to their conclusions. They were forced to abandon the loose speculation of their intellectual guides, the Greeks, and to betake themselves to observation. Thus a part of the laborious fact-gathering habit on which the modern advance of science has absolutely depended was due to the care which men had to exercise in face of the religious authorities.

In our own time, in the latter part of the nineteenth century, the conflict between the religious authority and the men of science has practically ceased. Even the Roman Church permits almost everywhere an untrammelled teaching of the established learning to which it was at one time opposed. Men have come to see that all truth is accordant, and that religion has nothing to fear from the faithful and devoted study of Nature.

The advance of science in general in modern times has been greatly due to the development of mechanical inventions. Among the ancients, the tools which served in the arts were few in number, and these of exceeding simplicity. So far as we can ascertain, in the five hundred years during which the Greeks were in their intellectual vigour, not more than half a dozen new machines were invented, and these were exceedingly simple. The fact seems to be that a talent for mechanical invention is mainly limited to the peoples of France, Germany, and of the English-speaking folk. The first advances in these contrivances were made in those countries, and all our considerable gains have come from their people. Thus, while the spirit of science in general is clearly limited to the Aryan folk, that particular part of the motive which leads to the invention of tools is restricted to western and northern Europe, to the people to whom we give the name of Teutonic.

Mechanical inventions have aided the development of our sciences in several ways. They have furnished inquirers with instruments of precision; they have helped to develop accuracy of observation; best of all, they have served ever to bring before the attention of men a spectacle of the conditions in Nature which we term cause and effect. The influence of these inventions on the development of learning has been particularly great where the machines, such as our wind and water mills, and our steam engine, make use of the forces of Nature, subjugating them to the needs of man. Such instruments give an unending illustration as to the presence in Nature of energy. They have helped men to understand that the machinery of the universe is propelled by the unending application of power. It was, in fact, through such machines that men of science first came to understand that energy, manifested in the natural forces, is something that eternally endures; that we may change its form in our arts as its form is changed in the operations of Nature, but the power endures forever.

It is interesting to note that the first observation which led to this most important scientific conclusion that energy is indestructible however much it may change its form, was made by an American, Benjamin Thompson, who left this country at the time of the Revolution, and after a curious life became the executive officer, and in effect king, of Bavaria. While engaged in superintending the manufacture of cannon, he observed that in boring out the barrel of the gun an amount of heat was produced which evaporated a certain amount of water. He therefore concluded that the energy required to do the boring of the metal passed into the state of heat, and thus only changed its state, in no wise disappearing from the earth. Other students pursuing the same line of inquiry have clearly demonstrated what is called the law of the conservation of energy, which more than anything has helped us to understand the large operations of Nature. Through these studies we have come to see that, while the universe is a place of ceaseless change, the quantities of energy and of matter remain unaltered.

The foregoing brief sketch, which sets forth some of the important conditions which have affected the development of science, may in a way serve to show the student how he can himself become an interpreter of Nature. The evidence indicates that the people of our race have been in a way chosen among all the varieties of mankind to lead in this great task of comprehending the visible universe. The facts, moreover, show that discovery usually begins with the interest which men feel in the world immediately about them, or which is presented to their senses in a daily spectacle. Thus Benjamin Franklin, in the midst of a busy life, became deeply interested in the phenomena of lightning, and by a very simple experiment proved that this wonder of the air was due to electrical action such as we may arouse by rubbing a stick of sealing-wax or a piece of amber with a cloth. All discoveries, in a word, have had their necessary beginnings in an interest in the facts which daily experience discloses. This desire to know something more than the first sight exhibits concerning the actions in the world about us is native in every human soul—at least, in all those who are born with the heritage of our race. It is commonly strong in childhood; if cultivated by use, it will grow throughout a lifetime, and, like other faculties, becomes the stronger and more effective by the exertions which it inspires. It is therefore most important that every one should obey this instinctive command to inquiry, and organize his life and work so that he may not lose but gain more and more as time goes on of this noble capacity to interrogate and understand the world about him.

It is best that all study of Nature should begin not in laboratories, nor with the things which are remote from us, but in the field of Nature which is immediately about us. The student, even if he dwell in the unfavourable conditions of a great city, is surrounded by the world which has yielded immeasurable riches in the way of learning, which he can appropriate by a little study. He can readily come to know something of the movements of the air; the buildings will give him access to a great many different kinds of stone; the smallest park, a little garden, or even a few potted plants and captive animals, may tell him much concerning the forms and actions of living beings. By studying in this way he can come to know something of the differences between things and their relations to each other. He will thus have a standard by which he can measure and make familiar the body of learning concerning Nature which he may find in books. From printed pages alone, however well they be written, he can never hope to catch the spirit that animates the real inquirer, the true lover of Nature.

On many accounts the most attractive way of beginning to form the habit of the naturalist is by the study of living animals and plants. To all of us life adds interest, and growth has a charm. Therefore it is well for the student to start on the way of inquiry by watching the actions of birds and insects or by rearing plants. It is fortunate if he can do both these agreeable things. When the habit of taking an account of that most important part of the world which is immediately about him has been developed in the student, he may profitably proceed to acquire the knowledge of the invisible universe which has been gathered by the host of inquirers of his race. However far he journeys, he should return to the home world that lies immediately and familiarly about him, for there alone can he acquire and preserve that personal acquaintance with things which is at once the inspiration and the test of all knowledge.

Along with this study of the familiar objects about us the student may well combine some reading which may serve to show him how others have been successful in thus dealing with Nature at first hand. For this purpose there are, unfortunately, but few works which are well calculated to serve the needs of the beginner. Perhaps the best naturalist book, though its form is somewhat ancient, is White's Natural History of Selborne. Hugh Miller's works, particularly his Old Red Sandstone and My Schools and Schoolmasters, show well how a man may become a naturalist under difficulties. Sir John Lubbock's studies on Wasps, and Darwin's work on Animals and Plants under Domestication are also admirable to show how observation should be made. Dr. Asa Gray's little treatise on How Plants Grow will also be useful to the beginner who wishes to approach botany from its most attractive side—that of the development of the creature from the seed to seed.

There is another kind of training which every beginner in the art of observing Nature should obtain, and which many naturalists of repute would do well to give themselves—namely, an education in what we may call the art of distance and geographical forms. With the primitive savage the capacity to remember and to picture to the eye the shape of a country which he knows is native and instinctive. Accustomed to range the woods, and to trust to his recollection to guide him through the wilderness to his home, the primitive man develops an important art which among civilized people is generally dormant. In fact, in our well-trodden ways people may go for many generations without ever being called upon to use this natural sense of geography. The easiest way to cultivate the geographic sense is by practising the art of making sketch maps. This the student, however untrained, can readily do by taking first his own dwelling house, on which he should practise until he can readily from memory make a tolerably correct and proportional plan of all its rooms. Then on a smaller scale he should begin to make also from recollection a map showing the distribution of the roads, streams, and hills with which his daily life makes him familiar. From time to time this work from memory should be compared with the facts. At first the record will be found to be very poor, but with a few months of occasional endeavour the observer will find that his mind takes account of geographic features in a way it did not before, and, moreover, that his mind becomes enriched with impressions of the country which are clear and distinct, in place of the shadowy recollections which he at first possessed.

When the student has attained the point where, after walking or riding over a country, he can readily recall its physical features of the simpler sort, he will find it profitable to undertake the method of mapping with contour lines—that is, by pencilling in indications to show the exact shape of the elevations and depressions. The principle of contour lines is that each of them represents where water would come against the slope if the area were sunk step by step below the sea level—in other words, each contour line marks the intersection of a horizontal plane with the elevation of the country. Practice on this somewhat difficult task will soon give the student some idea as to the complication of the surface of a region, and afford him the basis for a better understanding of what geography means than all the reading he can do will effect. It is most desirable that training such as has been described should be a part of our ordinary school education.

Very few people have clear ideas of distances. Even the men whose trade requires some such knowledge are often without that which a little training could give them. Without some capacity in this direction, the student is always at a disadvantage in his contact with Nature. He can not make a record of what he sees as long as the element of horizontal and vertical distance is not clearly in mind. To attain this end the student should begin by pacing some length of road where the distances are well known. In this way he will learn the length of his step, which with a grown man generally ranges between two and a half and three feet. Learning the average length of his stride by frequent counting, it is easy to repeat the trial until one can almost unconsciously keep the count as he walks. Properly to secure the training of this sort the observer should first attentively look across the distance which is to be determined. He should notice how houses, fences, people, and trees appear at that distance. He will quickly perceive that each hundred feet of additional interval somewhat changes their aspect. In training soldiers to measure with the eye the distances which they have to know in order effectively to use the modern weapons of war, a common device is to take a squad of men, or sometimes a company, under the command of an officer, who halts one man at each hundred yards until the detachment is strung out with that interval as far as the eye can see them. The men then walk to and fro so that the troops who are watching them may note the effects of increased distance on their appearance, whether standing or in motion. At three thousand yards a man appears as a mere dot, which is not readily distinguishable. Schoolboys may find this experiment amusing and instructive.

After the student has gained, as he readily may, some sense of the divisions of distance within the range of ordinary vision, he should try to form some notion of greater intervals, as of ten, a hundred, and perhaps a thousand miles. The task becomes more difficult as the length of the line increases, but most persons can with a little address manage to bring before their eyes a tolerably clear image of a hundred miles of distance by looking from some elevation which commands a great landscape. It is doubtful, however, whether the best-trained man can get any clear notion of a thousand miles—that is, can present it to himself in imagination as he may readily do with shorter intervals.

The most difficult part of the general education which the student has to give himself is begun when he undertakes to picture long intervals of time. Space we have opportunities to measure, and we come in a way to appreciate it, but the longest lived of men experiences at most a century of life, and this is too small a measure to give any notion as to the duration of such great events as are involved in the history of the earth, where the periods are to be reckoned by the millions of years. The only way in which we can get any aid in picturing to ourselves great lapses of time is by expressing them in units of distance. Let a student walk away on a straight road for the distance of a mile; let him call each step a year; when he has won the first milestone, he may consider that he has gone backward in time to the period of Christ's birth. Two miles more will take him to the station which will represent the age when the oldest pyramids were built. He is still, however, in the later days of man's history on this planet. To attain on the scale the time when man began, he might well have to walk fifty miles away, while a journey which would thus by successive steps describe the years of the earth's history since life appeared upon its surface would probably require him to circle the earth at least four times. We may accept it as impossible for any one to deal with such vast durations save with figures which are never really comprehended. It is well, however, to enlarge our view as to the age of the earth by such efforts as have just been indicated.

When we go beyond the earth into the realm of the stars all efforts toward understanding the ranges of space or the durations of time are quite beyond the efforts of man. Even the distance of about two hundred and forty thousand miles which separates us from the moon can not be grasped by even the greater minds. No human intelligence, however cultivated, can conceive the distance of about ninety-five million miles which separates us from the sun. In the celestial realm we can only deal with relations of space and time in a general and comparative way. We can state the distances if we please in millions of miles, or we can reckon the ampler spaces by using the interval which separates the earth from the sun as we do a foot rule in our ordinary work, but the depths of the starry spaces can only be sounded by the winged imagination.

Although the student has been advised to begin his studies of Nature on the field whereon he dwells, making that study the basis of his most valuable communications with Nature, it is desirable that he should at the same time gain some idea as to the range and scope of our knowledge concerning the visible universe. As an aid toward this end the following chapters of this book will give a very brief survey of some of the most important truths concerning the heavens and the earth which have rewarded the studies of scientific men. Of remoter things, such as the bodies in the stellar spaces, the account will be brief, for that which is known and important to the general student can be briefly told. So, too, of the earlier ages of the earth's history, although a vast deal is known, the greater part of the knowledge is of interest and value mainly to geologists who cultivate that field. That which is most striking and most important to the mass of mankind is to be found in the existing state of our earth, the conditions which make it a fit abode for our kind, and replete with lessons which he may study with his own eyes without having to travel the difficult paths of the higher sciences.

Although physiography necessarily takes some account of the things which have been, even in the remote past, and this for the reason that everything in this day of the world depends on the events of earlier days, the accent of its teaching is on the immediate, visible, as we may say, living world, which is a part of the life of all its inhabitants.



CHAPTER III.

THE STELLAR REALM.

Even before men came to take any careful account of the Nature immediately about them they began to conjecture and in a way to inquire concerning the stars and the other heavenly bodies. It is difficult for us to imagine how hard it was for students to gain any adequate idea of what those lights in the sky really are. At first men imagined the celestial bodies to be, as they seemed, small objects not very far away. Among the Greeks the view grew up that the heavens were formed of crystal spheres in which the lights were placed, much as lanterns may be hung upon a ceiling. These spheres were conceived to be one above the other; the planets were on the lower of them, and the fixed stars on the higher, the several crystal roofs revolving about the earth. So long as the earth was supposed to be a flat and limitless expanse, forming the centre of the universe, it was impossible for the students of the heavens to attain any more rational view as to their plan.

The fact that the earth was globular in form was understood by the Greek men of science. They may, indeed, have derived the opinion from the Egyptian philosophers. The discovery rested upon the readily observed fact that on a given day the shadow of objects of a certain height was longer in high latitude than in low. Within the tropics, when the sun was vertical, there would be no shadow, while as far north as Athens it would be of considerable length. The conclusion that the earth was a sphere appears to have been the first large discovery made by our race. It was, indeed, one of the most important intellectual acquisitions of man.

Understanding the globular form of the earth, the next and most natural step was to learn that the earth was not the centre of the planetary system, much less of the universe, but that that centre was the sun, around which the earth and the other planets revolved. The Greeks appear to have had some idea that this was the case, and their spirit of inquiry would probably have led them to the whole truth but for the overthrow of their thought by the Roman conquest and the spread of Christianity. It was therefore not until after the revival of learning that astronomers won their way to our modern understanding concerning the relation of the planets to the sun. With Galileo this opinion was affirmed. Although for a time the Church, resting its opposition on the interpretation of certain passages of Scripture, resisted this view, and even punished the men who held it, it steadfastly made its way, and for more than two centuries has been the foundation of all the great discoveries in the stellar realm. Yet long after the fact that the sun was the centre of the solar system was well established no one understood why the planets should move in their ceaseless, orderly procession around the central mass. To Newton we owe the studies on the law of gravitation which brought us to our present large conception as to the origin of this order. Starting with the view that bodies attracted each other in proportion to their weight, and in diminishing proportion as they are removed from each other, Newton proceeded by most laborious studies to criticise this view, and in the end definitely proved it by finding that the motions of the moon about the earth, as well as the paths of the planets, exactly agreed with the supposition.

The last great path-breaking discovery which has helped us in our understanding of the stars was made by Fraunhofer and other physicists, who showed us that substances when in a heated, gaseous, or vaporous state produced, in a way which it is not easy to explain in a work such as this, certain dark lines in the spectrum, or streak of divided light which we may make by means of a glass prism, or, as in the rainbow, by drops of water. Carefully studying these very numerous lines, those naturalists found that they could with singular accuracy determine what substances there were in the flame which gave the light. So accurate is this determination that it has been made to serve in certain arts where there is no better means of ascertaining the conditions of a flaming substance except by the lines which its light exhibits under this kind of analysis. Thus, in the manufacture of iron by what is called the Bessemer process, it has been found very convenient to judge as to the state of the molten metal by such an analysis of the flame which comes forth from it.



No sooner was the spectroscope invented than astronomers hastened by its aid to explore the chemical constitution of the sun. These studies have made it plain that the light of our solar centre comes forth from an atmosphere composed of highly heated substances, all of which are known among the materials forming the earth. Although for various reasons we have not been able to recognise in the sun all the elements which are found in our sphere, it is certain that in general the two bodies are alike in composition. An extension of the same method of inquiry to the fixed stars was gradually though with difficulty attained, and we now know that many of the elements common to the sun and earth exist in those distant spheres. Still further, this method of inquiry has shown us, in a way which it is not worth while here to describe, that among these remoter suns there are many aggregations of matter which are not consolidated as are the spheres of our own solar system, but remain in the gaseous state, receiving the name of nebulae.

Along with the growth of observational astronomy which has taken place since the discoveries of Galileo, there has been developed a view concerning the physical history of the stellar world, known as the nebular hypothesis, which, though not yet fully proved, is believed by most astronomers and physicists to give us a tolerably correct notion as to the way in which the heavenly spheres were formed from an earlier condition of matter. This majestic conception was first advanced, in modern times at least, by the German philosopher Immanuel Kant. It was developed by the French astronomer Laplace, and is often known by his name. The essence of this view rests upon the fact previously noted that in the realm of the fixed stars there are many faintly shining aggregations of matter which are evidently not solid after the manner of the bodies in our solar system, but are in the state where their substances are in the condition of dustlike particles, as are the bits of carbon in flame or the elements which compose the atmosphere. The view held by Laplace was to the effect that not only our own solar system, but the centres of all the other similar systems, the fixed stars, were originally in this gaseous state, the material being disseminated throughout all parts of the heavenly realm, or at least in that portion of the universe of which we are permitted to know something. In this ancient state of matter we have to suppose that the particles of it were more separated from each other than are the atoms of the atmospheric gases in the most perfect vacuum which we can produce with the air-pump. Still we have to suppose that each of these particles attract the other in the gravitative way, as in the present state of the universe they inevitably do.

Under the influence of the gravitative attraction the materials of this realm of vapour inevitably tended to fall in toward the centre. If the process had been perfectly simple, the result would have been the formation of one vast mass, including all the matter which was in the original body. In some way, no one has yet been able to make a reasonable suggestion of just how, there were developed in the process of concentration a great many separate centres of aggregation, each of which became the beginning of a solar system. The student may form some idea of how readily local centres may be produced in materials disseminated in the vaporous state by watching how fog or the thin, even misty clouds of the sunrise often gather into the separate shapes which make what we term a "mackerel" sky. It is difficult to imagine what makes centres of attraction, but we readily perceive by this instance how they might have occurred.

When the materials of each solar system were thus set apart from the original mass of star dust or vapour, they began an independent development which led step by step, in the case of our own solar system at least, and presumably also in the case of the other suns, the fixed stars, to the formation of planets and their moons or satellites, all moving around the central sun. At this stage of the explanation the nebular hypothesis is more difficult to conceive than in the parts of it which have already been described, for we have now to understand how the planets and satellites had their matter separated from each other and from the solar centre, and why they came to revolve around that central body. These problems are best understood by noting some familiar instances connected with the movement of fluids and gases toward a centre. First let us take the case of a basin in which the water is allowed to flow out through a hole in its centre. When we lift the stopper the fluid for a moment falls straight down through the opening. Very quickly, however, all the particles of the water start to move toward the centre, and almost at once the mass begins to whirl round with such speed that, although it is working toward the middle, it is by its movement pushed away from the centre and forms a conical depression. As often as we try the experiment, the effect is always the same. We thus see that there is some principle which makes particles of fluid that tend toward a centre fail directly to attain it, but win their way thereto in a devious, spinning movement.

Although the fact is not so readily made visible to the eye, the same principle is illustrated in whirling storms, in which, as we shall hereafter note with more detail, the air next the surface of the earth is moving in toward a kind of chimney by which it escapes to the upper regions of the atmosphere. A study of cyclones and tornadoes, or even of the little air-whirls which in hot weather lift the dust of our streets, shows that the particles of the atmosphere in rushing in toward the centre of upward movement take on the same whirling motion as do the molecules of water in the basin—in fact, the two actions are perfectly comparable in all essential regards, except that the fluid is moving downward, while the air flows upward. Briefly stated, the reason for the movement of fluid and gas in the whirling way is as follows: If every particle on its way to the centre moved on a perfectly straight line toward the point of escape, the flow would be directly converging, and the paths followed would resemble the spokes of a wheel. But when by chance one of the particles sways ever so little to one side of the direct way, a slight lateral motion would necessarily be established. This movement would be due to the fact that the particle which pursued the curved line would press against the particles on the out-curved side of its path—or, in other words, shove them a little in that direction—to the extent that they departed from the direct line they would in turn communicate the shoving to the next beyond. When two particles are thus shoving on one side of their paths, the action which makes for revolution is doubled, and, as we readily see, the whole mass may in this way become quickly affected, the particles driven out of their path, moving in a curve toward the centre. We also see that the action is accumulative: the more curved the path of each particle, the more effectively it shoves; and so, in the case of the basin, we see the whirling rapidly developed before our eyes.

In falling in toward the centre the particles of star dust or vapour would no more have been able one and all to pursue a perfectly straight line than the particles of water in the basin. If a man should spend his lifetime in filling and emptying such a vessel, it is safe to say that he would never fail to observe the whirling movement. As the particles of matter in the nebular mass which was to become a solar system are inconceivably greater than those of water in the basin, or those of air in the atmospheric whirl, the chance of the whirling taking place in the heavenly bodies is so great that we may assume that it would inevitably occur.

As the vapours in the olden day tended in toward the centre of our solar system, and the mass revolved, there is reason to believe that ringlike separations took place in it. Whirling in the manner indicated, the mass of vapour or dust would flatten into a disk or a body of circular shape, with much the greater diameter in the plane of its whirling. As the process of concentration went on, this disk is supposed to have divided into ringlike masses, some approach to which we can discern in the existing nebulae, which here and there among the farther fixed stars appear to be undergoing such stages of development toward solar systems. It is reasonably supposed that after these rings had been developed they would break to pieces, the matter in them gathering into a sphere, which in time was to become a planet. The outermost of these rings led to the formation of the planet farthest from the sun, and was probably the first to separate from the parent mass. Then in succession rings were formed inwardly, each leading in turn to the creation of another planet, the sun itself being the remnant, by far the greater part of the whole mass of matter, which did not separate in the manner described, but concentrated on its centre. Each of these planetary aggregations of vapour tended to develop, as it whirled upon its centre, rings of its own, which in turn formed, by breaking and concentrating, the satellites or moons which attend the earth, as they do all the planets which lie farther away from the sun than our sphere.



As if to prove that the planets and moons of the solar system were formed somewhat in the manner in which we have described it, one of these spheres, Saturn, retains a ring, or rather a band which appears to be divided obscurely into several rings which lie between its group of satellites and the main sphere. How this ring has been preserved when all the others have disappeared, and what is the exact constitution of the mass, is not yet well ascertained. It seems clear, however, that it can not be composed of solid matter. It is either in the form of dust or of small spheres, which are free to move on each other; otherwise, as computation shows, the strains due to the attraction which Saturn itself and its moons exercise upon it would serve to break it in pieces. Although this ring theory of the formation of the planets and satellites is not completely proved, the occurrence of such a structure as that which girdles Saturn affords presumptive evidence that it is true. Taken in connection with what we know of the nebulae, the proof of Laplace's nebular hypothesis may fairly be regarded as complete.

It should be said that some of the fixed stars are not isolated suns like our own, but are composed of two great spheres revolving about one another; hence they are termed double stars. The motions of these bodies are very peculiar, and their conditions show us that it is not well to suppose that the solar system in which we dwell is the only type of order which prevails in the celestial families; there may, indeed, be other variations as yet undetected. Still, these differences throw no doubt on the essential truth of the theory as to the process of development of the celestial systems. Though there is much room for debate as to the details of the work there, the general truth of the theory is accepted by nearly all the students of the problem.

A peculiar advantage of the nebular hypothesis is that it serves to account for the energy which appears as light and heat in the sun and the fixed stars, as well as that which still abides in the mass of our earth, and doubtless also in the other large planets. When the matter of which these spheres were composed was disseminated through the realms of space, it is supposed to have had no positive temperature, and to have been dark, realizing the conception which appears in the first chapter of Genesis, "without form, and void." With each stage of the falling in toward the solar centres what is called the "energy of position" of this original matter became converted into light and heat. To understand how this took place, the reader should consider certain simple yet noble generalizations of physics. We readily recognise the fact that when a hammer falls often on an anvil it heats itself and the metal on which it strikes. Those who have been able to observe the descent of meteoric stones from the heavens have remarked that when they came to the earth they were, on their surfaces at least, exceedingly hot. Any one may observe shining meteors now and then flashing in the sky. These are known commonly to be very small bits of matter, probably not larger than grains of sand, which, rushing into our atmosphere, are so heated by the friction which they encounter that they burn to a gas or vapour before they attain the earth. As we know that these particles come from the starry spaces, where the temperature is somewhere near 500 deg. below 0 deg. Fahr., it is evident that the light and heat are not brought with them into the atmosphere; it can only be explained by the fact that when they enter the air they are moving at an average speed of about twenty miles a second, and that the energy which this motion represents is by the resistance which the body encounters converted into heat. This fact will help us to understand how, as the original star dust fell in toward the centre of attraction, it was able to convert what we have termed the energy of position into temperature. We see clearly that every such particle of dust or larger bit of matter which falls upon the earth brings about the development of heat, even though it does not actually strike upon the solid mass of our sphere. The conception of what took place in the consolidation of the originally disseminated materials of the sun and planets can be somewhat helped by a simple experiment. If we fit a piston closely into a cylinder, and then suddenly drive it down with a heavy blow, the compressed air is so heated that it may be made to communicate fire. If the piston should be slowly moved, the same amount of heat would be generated, or, as we may better say, liberated by the compression, though the effect would not be so striking. A host of experiments show that when a given mass of matter is brought to occupy a less space the effect is in practically all cases to increase the temperature. The energy which kept the particles apart is, when they are driven together, converted into heat. These two classes of actions are somewhat different in their nature; in the case of the meteors, or the equivalent star dust, the coming together of the particles is due to gravitation. In the experiment with the cylinder above described, the compression is due to mechanical energy, a force of another nature.

There is reason for believing that all our planets, as well as the sun itself, and also the myriad other orbs of space, have all passed through the stages of a transition in which a continually concentrating vapour, drawn together by gravitation, became progressively hotter and more dense until it assumed the condition of a fluid. This fluid gradually parted with its heat to the cold spaces of the heavens, and became more and more concentrated and of a lower temperature until in the end, as in the case of our earth and of other planets, it ceased to glow on the outside, though it remained intensely heated in the inner parts. It is easy to see that the rate of this cooling would be in some proportion to the size of the sphere. Thus the earth, which is relatively small, has become relatively cold, while the sun itself, because of its vastly greater mass, still retains an exceedingly high temperature. The reason for this can readily be conceived by making a comparison of the rate of cooling which occurs in many of our ordinary experiences. Thus a vial of hot water will quickly come down to the temperature of the air, while a large jug filled with the fluid at the same temperature will retain its heat many times as long. The reason for this rests upon the simple principle that the contents of a sphere increase with its enlargement more rapidly than the surface through which the cooling takes place.

The modern studies on the physical history of the sun and other celestial bodies show that their original store of heat is constantly flowing away into the empty realms of space. The rate at which this form of energy goes away from the sun is vast beyond the powers of the imagination to conceive; thus, in the case of our earth, which viewed from the sun would appear no more than a small star, the amount of heat which falls upon it from the great centre is enough each day to melt, if it all could be put to such work, about eight thousand cubic miles of ice. Yet the earth receives only 1/2,170,000,000 part of the solar radiation. The greater part of this solar heat—in fact, we may say nearly all of it—slips by the few and relatively small planets and disappears in the great void.

The destiny of all the celestial spheres seems in time to be that they shall become cooled down to a temperature far below anything which is now experienced on this earth. Even the sun, though its heat will doubtless endure for millions of years to come, must in time, so far as we can see, become dark and cold. So far as we know, we can perceive no certain method by which the life of the slowly decaying suns can be restored. It has, however, been suggested that in many cases a planetary system which has attained the lifeless and lightless stage may by collision with some other association of spheres be by the blow restored to its previous state of vapour, the joint mass of the colliding systems once again to resume the process of concentration through which it had gone before. Now and then stars have been seen to flash suddenly into great brilliancy in a way which suggests that possibly their heat had been refreshed by a collision with some great mass which had fallen into them from the celestial spaces. There is room for much speculation in this field, but no certainty appears to be attainable.

The ancients believed that light and heat were emanations which were given off from the bodies that yielded them substantially as odours are given forth by many substances. Since the days of Newton inquiry has forced us to the conviction that these effects of temperature are produced by vibrations having the general character of waves, which are sent through the spaces with great celerity. When a ray of light departs from the sun or other luminous body, it does not convey any part of the mass; it transmits only motion. A conception of the action can perhaps best be formed by suspending a number of balls of ivory, stone, or other hard substance each by a cord, the series so arranged that they touch each other. Then striking a blow against one end of the line, we observe that the ball at the farther end of the line is set in motion, swinging a little away from the place it occupied before. The movement of the intermediate balls may be so slight as to escape attention. We thus perceive that energy can be transmitted from one to another of these little spheres. Close observation shows us that under the impulse which the blow gives each separate body is made to sway within itself much in the manner of a bell when it is rung, and that the movement is transmitted to the object with which it is in contact. In passing from the sun to the earth, the light and heat traverse a space which we know to be substantially destitute of any such materials as make up the mass of the earth or the sun. Judged by the standards which we can apply, this space must be essentially empty. Yet because motions go through it, we have to believe that it is occupied by something which has certain of the properties of matter. It has, indeed, one of the most important properties of all substances, in that it can vibrate. This practically unknown thing is called ether.

The first important observational work done by the ancients led them to perceive that there was a very characteristic difference between the planets and the fixed stars. They noted the fact that the planets wandered in a ceaseless way across the heavens, while the fixed stars showed little trace of changing position in relation to one another. For a long time it was believed that these, as well as the remoter fixed stars, revolved about the earth. This error, though great, is perfectly comprehensible, for the evident appearance of the movement is substantially what would be brought about if they really coursed around our sphere. It was only when the true nature of the earth and its relations to the sun were understood that men could correct this first view. It was not, indeed, until relatively modern times that the solar system came to be perceived as something independent and widely detached from the fixed stars system; that the spaces which separate the members of our own solar family, inconceivably great as they are, are but trifling as compared with the intervals which part us from the nearer fixed stars. At this stage of our knowledge men came to the noble suggestion that each of the fixed stars was itself a sun, each of the myriad probably attended by planetary bodies such as exist about our own luminary.

It will be well for the student to take an imaginary journey from the sun forth into space, along the plane in which extends that vast aggregation of stars which we term the Milky Way. Let him suppose that his journey could be made with something like the speed of light, or, say, at the rate of about two hundred thousand miles a second. It is fit that the imagination, which is free to go through all things, should essay such excursions. On the fancied outgoing, the observer would pass the interval between the sun and the earth in about eight minutes. It would require some hours before he attained to the outer limit of the solar system. On his direct way he would pass the orbits of the several planets. Some would have their courses on one side or the other of his path; we should say above or below, but for the fact that we leave these terms behind in the celestial realm. On the margin of the solar system the sun would appear shrunken to the state where it was hardly greater than the more brilliant of the other fixed stars. The onward path would then lead through a void which it would require years to traverse. Gradually the sun which happened to lie most directly in his path would grow larger; with nearer approach, it would disclose its planets. Supposing that the way led through this solar system, there would doubtless be revealed planets and satellites in their order somewhat resembling those of our own solar family, yet there would doubtless be many surprises in the view. Arriving near the first sun to be visited, though the heavens would have changed their shape, all the existing constellations having altered with the change in the point of view, there would still be one familiar element in that the new-found planets would be near by, and the nearest fixed stars far away in the firmament.

With the speed of light a stellar voyage could be taken along the path of the Milky Way, which would endure for thousands of years. Through all the course the journeyer would perceive the same vast girdle of stars, faint because they were far away, which gives the dim light of our galaxy. At no point is it probable that he would find the separate suns much more aggregated or greatly farther apart than they are in that part of the Milky Way which our sun now occupies. Looking forth on either side of the "galactic plane," there would be the same scattering of stars which we now behold when we gaze at right angles to the way we are supposing the spirit to traverse.

As the form of the Milky Way is irregular, the mass, indeed, having certain curious divisions and branches, it well might be that the supposed path would occasionally pass on one or the other side of the vast star layer. In such positions the eye would look forth into an empty firmament, except that there might be in the far away, tens of thousands of years perhaps at the rate that light travels away from the observer, other galaxies or Milky Ways essentially like that which he was traversing. At some point the journeyer would attain the margin of our star stratum, whence again he would look forth into the unpeopled heavens, though even there he might discern other remote star groups separated from his own by great void intervals.

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The revelations of the telescope show us certain features in the constitution and movements of the fixed stars which now demand our attention. In the first place, it is plain that not all of these bodies are in the same physical condition. Though the greater part of these distant luminous masses are evidently in the state of aggregation displayed by our own sun, many of them retain more or less of that vaporous, it may be dustlike, character which we suppose to have been the ancient state of all the matter in the universe. Some of these masses appear as faint, almost indistinguishable clouds, which even to the greatest telescope and the best-trained vision show no distinct features of structure. In other cases the nebulous appearance is hardly more than a mist about a tolerably distinct central star. Yet again, and most beautifully in the great nebula of the constellation of Orion, the cloudy mass, though hardly visible to the naked eye, shows a division into many separate parts, the whole appearing as if in process of concentration about many distinct centres.

The nebulas are reasonably believed by many astronomers to be examples of the ancient condition of the physical universe, masses of matter which for some reason as yet unknown have not progressed in their consolidation to the point where they have taken on the characteristics of suns and their attendant planets.

Many of the fixed stars, the incomplete list of which now amounts to several hundred, are curiously variable in the amount of light which they send out to the earth. Sometimes these variations are apparently irregular, but in the greater number of cases they have fixed periods, the star waxing and waning at intervals varying from a few months to a few years. Although some of the sudden flashings forth of stars from apparent small size to near the greatest brilliancy may be due to catastrophes such as might be brought about by the sudden falling in of masses of matter upon the luminous spheres, it is more likely that the changes which we observe are due to the fact that two suns revolving around a common centre are in different stages of extinction. It may well be that one of these orbs, presumably the smaller, has so far lost temperature that it has ceased to glow. If in its revolution it regularly comes between the earth and its luminous companion, the effect would be to give about such a change in the amount of light as we observe.

The supposition that a bright sun and a relatively dark sun might revolve around a common centre of gravity may at first sight seem improbable. The fact is, however, that imperfect as our observations on the stars really are, we know many instances in which this kind of revolution of one star about another takes place. In some cases these stars are of the same brilliancy, but in others one of the lights is much brighter than the other. From this condition to the state where one of the stars is so nearly dark as to be invisible, the transition is but slight. In a word, the evidence goes to show that while we see only the luminous orbs of space, the dark bodies which people the heavens are perhaps as numerous as those which send us light, and therefore appear as stars.

Besides the greater spheres of space, there is a vast host of lesser bodies, the meteorites and comets, which appear to be in part members of our solar system, and perhaps of other similar systems, and in part wanderers in the vast realm which intervenes between the solar systems. Of these we will first consider the meteors, of which we know by far the most; though even of them, as we shall see, our knowledge is limited.

From time to time on any starry night, and particularly in certain periods of the year, we may behold, at the distance of fifty or more miles above the surface of the earth, what are commonly called "shooting stars." The most of these flashing meteors are evidently very small, probably not larger than tiny sand grains, possibly no greater than the fragments which would be termed dust. They enter the air at a speed of about thirty miles a second. They are so small that they burn to vapour in the very great heat arising from their friction on the air, and do not attain the surface of the earth. These are so numerous that, on the average, some hundreds of thousands probably strike the earth's atmosphere each day. From time to time larger bodies fall—bodies which are of sufficient bulk not to be burned up in the air, but which descend to the ground. These may be from the smallest size which may be observed to masses of many hundred pounds in weight. These are far less numerous than the dust meteorites; it is probable, however, that several hundred fragments each year attain the earth's surface. They come from various directions of space, and there is as yet no means of determining whether they were formed in some manner within our planetary system or whether they wander to us from remoter realms. We know that they are in part composed of metallic iron commingled with nickel and carbon (sometimes as very small diamonds) in a way rarely if ever found on the surface of our sphere, and having a structure substantially unknown in its deposits. In part they are composed of materials which somewhat resemble certain lavas. It is possible that these fragments of iron and stone which constitute the meteorites have been thrown into the planetary spaces by the volcanic eruption of our own and other planets. If hurled forth with a sufficient energy, the fragments would escape from the control of the attraction of the sphere whence they came, and would become independent wanderers in space, moving around the sun in varied orbits until they were again drawn in by some of the greater planets.

As they come to us these meteorites often break up in the atmosphere, the bits being scattered sometimes over a wide area of country. Thus, in the case of the Cocke County meteorite of Tennessee, one of the iron species, the fragments, perhaps thousands in number, which came from the explosion of the body were scattered over an area of some thousand square miles. When they reach the surface in their natural form, these meteors always have a curious wasted and indented appearance, which makes it seem likely that they have been subject to frequent collisions in their journeys after they were formed by some violent rending action.