Outlines of the Earth's History: A Popular Study in Physiography

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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 plane

ct 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 wo

ir 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

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

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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

mer 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

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 d

r 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 qu

s, 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 sid

fetime 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 inconce

ul?, 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 lea

, Jan. 26, 188

sappeared, 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 wou

ns 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

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° below 0° 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 actual

re 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

ers 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, ab

hich 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

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, an

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 an

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; wi

cause 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 wh

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 trave

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 case

hysical universe, masses of matter which for some reason as yet unknown have not progressed in their con

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 a

his 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 t

our solar system, and perhaps of other similar systems, and in part wanderers in the vast realm which intervenes between the solar systems.

sphere 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 (s

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

masses. As they move in toward the sun long trails of vapour stream back from the somewhat consolidated head. Swinging around that centre, they journey again into the outer realm. As they retreat, their tail-like streamers appear to gather again upon their centres, and when they fa

1811, one of the many var

kings. Again, in more modern times, when they were not regarded as portents of calamity, it was feared that these wanderers moving vagariously through our solar system might by chance come in contact with the earth with disastrous results. Such

of the

cond. Our whole solar system is journeying away from certain stars, and in the direction of others which are situated in the opposite part of the heavens. The proof of this fact is found in the observations which show that on one side of us the stars are apparently coming closer together, while on the other side they are going farther apart. The phenomenon, in a word, is one of perspective, and may be made real

d from the study of the lines in the light which the spectrum reveals to us when critically examined. The position of these cross lines is, as we know, affected by the motion of the body whence the light comes, and by close analysis of the facts i

how that the matter in the stars is akin to that of our own earth, there is reas

ich it deals is of a nature to be apprehended by the student who does not approach it in a somewhat professional way. We shall therefore now turn to a description of the portion of the starry world which is found in the limi

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h makes our own solar system-are the bodies sufficiently near to our eyes for us, even with the resources of our telescopes and other instruments, to divine something of the details which they exhibit. In studying what we may concerning the family of the

ivided, cloudy state, was separated from the similar material which went to make the neighbouring fixed stars. The period when our solar system began its individual life was remote beyond the possibility of conception. Naturali

isklike mass, each breaking up and consolidating into a body of nebulous matter which followed in the same path, generally forming rings which became by the same process the moons or satellites of the sphere. In this way the sun produced eight planets which are known, and possibly others of small size on the outer verge of the system which have eluded discovery. According t

e two bodies would cause them to circle around a common centre of gravity, which, if the planet were not larger or the sun smaller than is the case in our solar system, would lie within the mass of the sun. In proportion as the two bodies might approach each other in size, the centre of gravity would come the nearer to the middle point in a line connecting the two spheres. In this condition of a sun with a single planet, whatever were the relative size of sun and planet, the orbits which they traverse would be circular. In this state of affairs it should be noted that each of the two bodies would have its plane of rotation permanently in the same position. Even if

ersely as the square of the distance; each sphere, pulling on the equatorial bulging of the other, pulls most effectively on the part of it which is nearest, and tends to draw it down toward its centre.

tre would be to draw the first-mentioned planet a little farther away from the centre of the sun; but in our own solar system, and probably in all others, this supposition is inadmissible, because the planets have longer journeys to go and also move slower, the farther they are from the sun. Thus Mercury completes the circle of its year in eighty-eight of our days, while the outermost planet requires sixty thousand days (more than one hundred and sixty-four years) for the same task. The result is not only that the centre of gravity of the system is somewhat displaced-itself a matter of no great account-but also that the orbit of the original planet ceases to be circled and becomes elliptical, and this for the evident reason that the sph

ication that if we should essay a diagram of it on the scale of this page it would not be possible to represent any considerable part of its deviations. These, in fact, would elude depiction, even if the draughtsman had a sheet for his drawing as large as the orbit itself, for every particle of matter in space, even if it be lodged beyond the limits of the farthest stars revealed to us by the tel

f some million miles; we readily perceive that the particles of matter in the outer part of the belt would have a swifter movement around the sun than those on the inside. When by some disturbance, as possibly by the passage of a great meteoric body of a considerable gravitative power, this ring was broken in two, the particles composing it on either side would, because of their mutual attraction, tend to draw away from the breach, widening that gap until the matter of the broken ring

and concentrated would constitute secondary planets or satellites, such as our moon. For some reason as yet unknown the outer planets-in fact, all those in the solar system except the two inner, Venus and Mercury and the as

of light and heat which would fall upon any one point on the sphere would be perfectly uniform. There would be no variations in the length of day or night; no changes in the seasons; the winds everywhere would blow with exceeding steadiness-in fact, the present atmospheric confusion would be reduced to something like order. From age to age, except so far as the sun itself might vary in the amount of energy which it radiated, or lands rose up into the air or sunk down toward the sea level, the climate of each region would be perfectly stable. In the existing conditions the influences bring about unending variety. First of all, the inclined position of the polar axis causes the sun apparently to move across the heavens, so that it comes in an overhead position once or twice in the year in quite half the area of the lands and seas. This apparent swayin

n of Planetary Orb

uatorial heat, and to rob the high latitudes of the share which they now have. On moving the axis gradually to positions where it approaches the plane of the orbit, he will note that each stage of the change widens the tropic belt. Bringing the polar axis down to the plane of the orbit, one hemisphere would receive unbroken sunshine, the other remaining in perpetual darkness and cold. In this condition, in place of an equatorial line we should have an eq

er and Saturn. When these attracting bodies, as is the case from time to time, though at long intervals, are brought together somewhere near to that part of the solar system in which the earth is moving around the sun, they draw our planet toward them, and so make its path very elliptical. When, however, they are so distributed that their pulling actions neutralize each other, the orbit returns more nearly to a circular form. The range in its ecc

the sun necessarily occupies one of the foci of the ellipse. The result is, of course, that the side of the earth which is toward the sun, while it is thus brought the nearer to the luminary, receives more energy in the form of light and heat than come to any part which is exposed when the spheres are farther away from each other in the other part of the orbit. Computations

form depends upon the wandering of the outer planetary spheres which pull the earth about. The longer axis of the ellipse is itself in constant motion in the direction in which the earth travels. This movement is slow, and at an irregular rate. It is easy to see that the effect of this action, which is called the revolution of the apsides, or, as the word means, the movement of the poles of the ellipse, is to bring the earth, when a given hemisphere is turned toward the sun, sometimes in the part of the orbit which is nearest the source of light and heat, an

, practically altogether the moon, exercises on the equatorial protuberance of the earth. We know that the diameter of our sphere at the equator is, on the average, something more than twenty-six miles greater than it is through the poles. We know, furthermore, that the position of the moon in relation to the earth is such that it causes the attraction on one half of this protuberance to be greater than it is upon the other. We readily perceive that this action will cause the polar axis to make a certain revolution, or, what comes to the same thing, that the pla

comes very elliptical; then again for long periods it may in form approach a circle. When in the state of extreme ellipticity, the precession of the equinoxes will cause the hemispheres in turn each to have their winter and summer alternately near and far from the sun. It is easily seen that when the summer season comes to a hemisphere in the part of the orbit which is then nearest the sun the period will be very hot. When the summer came farthest from

late the seasons were of no very great consequence; but, in the opinion of some students of climate, we are to look to these processes for an explanation of certain climata

o the sun that even when most favourably placed for observation it is visible for but a few minutes before sunrise and after sunset. Although it may without much difficulty be found by the ordinary eye, very few people have ever seen it. To the telescope when it is in the full moon state it appears as a brilliant disk; it is held by most astronomers that the surface which we see is made up altogether of clouds, but th

e of this planet apparently is almost completely and continually hidden from us by what appears to be a dense cloud envelope, through which from time to time certain spots appear of a dark colour. These, it is claimed, retain their place in a permanent way;

ard the solar centre, just as the moon does in its motion around the earth. Moreover, Mr. Lowell has failed to discover any traces of clouds upon the surface of the planet. As yet these results have not been verified by the work of other astronomers; resting, however, as they do on studies

etails of this planet are to occupy us during nearly all the

is under favourable conditions for telescopic observation. At such times there is revealed to the astronomer a surface which is covered with an amazing number of shadings and markings which as yet have been incompletely interpreted. The faint nature of these indications has led to very contradictory statements as to their form; no two maps which have been drawn agree except in their generalities. There is reason to believe that Mars has an atmosphere; this is shown by the fact that in the appropriate season the region about either pole is covered by a white coating, presumably sn

f our own earth appears to be demanded by the fact that the temperatures are such as to permit the coming and going of snow. It is well known that the temperature of any point on the earth, other things being equal, is proportionate to the depth of atmosphere above its surface. If Mars had no more air over its surface than has an equal area of the earth, it would remain at a temperature so low that such se

2 (Guiot), the white patch i

the parts of the planet about the water level are not grouped in great continents, and there are no large oceans. The only likeness to the conditions of our earth which we can perceive is in a general pointing of the somewhat triangular masses of what appears to be land toward one pole. As a whole, the conditions of t

physical features which have as yet been discerned on the lands of Mars are certain long, straight, rather narrow crevicelike openings, which have received the name of "canals." These features are very indistinct, and are just on the limit of visibility. As yet they have been carefully obse

s structure; we can not see closely enough to determine whether rivers exist, or whether there is a coating which we may interpret as vegetation, changing its hues in the different seasons of the year. An advance in our instruments of rese

ve Sizes of the P

less in diameter than our earth, perhaps not more than five hundred miles through. The forms of these asteroids is as yet unknown; we therefore can not determine whether their shapes are spheroidal, as are those of the other planets, or whether they are angular bits like the meteorites. We are thus not in a position to conjecture whether their independence began when the nebulous matter of the ring to which they belonged was in process of consolidation, or whether, after the aggregation o

wide field for the philosophic imagination. From this point of view the reader is advised to consult the writings of the late R.A. Proctor, who

at least, still glows from its own heat. The sphere is cloud-wrapped, but it is doubtful whether the envelope be of watery vapour; it is, indeed, quite possible that besides such vapour it may contain some part of the many substances which occupy the atmosphere of the sun. If the Jo

r to the heat of the sphere itself than to that which comes to it from the sun. At the distance of Saturn from the centre of the solar system a given area of surface receives less than one ninetieth of the sun's heat as compared with the earth; therefore we can not

t of the solar system, are so remote that even our best telescopes discern l

ts orbit which could only be accounted for on the supposition that it was subjected to the attraction of another wandering body which had escaped observation. By skilful computation the place in the heavens in which this disturbing element lay was so accurately determined that when

ore important events which take place on the surface of the earth. It is therefore fit that in this preliminary study of the celestial bodies, which is esp

hat in which the sphere is so far congealed that it becomes dark. Neither of these states is sharply separated from the other; a mass may be partly nebulous and partly fluid; even when it has been converted into fluid, or possibly into the solid state, it may still retain on the exterior some share of its original vaporous condition. In our sun the concentration has long since passed beyond the limits of the

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s reveals a disklike body of very uniform appearance. The only variation in this simple aspect occurs at the time of a total eclipse, when for a minute or two the moon hides the whole body of the sun. On such occasions even the unaided eye can see that there is about the sphere a broad, rather bright field, of a

terest in the constitution of the sphere has increased during the last fifty years. This interest has rapidly grown until at the present time a vast body of learning has been gathered for the solution of the many problems concerning the centre of our system. As yet there is great divergence in the views of

sea. Note the "ropy" character of the half-frozen r

nothing that we term matter. Arriving at a point some two or three hundred thousand miles from the body of the sun, he would enter the realm of the corona; here he would find scattered particles of matter, the bits so far apart that there would perhaps be not more than one or two in the cubic m

e contains in the state of vapour a number of metallic substances, such as iron and magnesium. To an observer who could behold this envelope of the sun from the distance at which we see the moon, the spectacle would be more magnificent than the imagination, guided by the sight of all the relatively trifling fractures of our earth, can possibly conceive. From the surface of the fiery sea vast uprushes of heated matter rise to the height of two or three hundred thousand miles, and then fall back upon its surface. These jets of heated

vast attraction of the great sphere below it that it is in effect a fluid. The near-at-hand observer would doubtless find this photosphere, as it appears in the telescope, to be sharply separated from the thinner and more vaporous envelopes-the chromosphere and the corona-which are, indeed, so thin that they are invisible even with the telescope, except when

he field and a downward motion taking place in the middle of the irregular opening, which is darkened in its central part, thus giving it, when seen by an ordinary telescope, the aspect of a black patch on the glowing surface. These spots, which are from some hundred to some thousand miles in diameter, may endure for months before they fade away. It is clear

ary Sun-spot,

d by an uprush of vapours, in which iron and other metals exist; but whether these movements are due to outbreaks from the deeper parts of the sun or to some action like the whirling storms of the earth's atmosphere is uncertai

in its place we have to assume that for the eight hundred and sixty thousand miles of its diameter the sun is composed of matter such as is found in our earth, but throughout in a state of heat which vastly exceeds that known on or in our planet. Owing to its heat, this matter is possibly not in either the solid or the fluid state, but in that of very

the effect that the heat is to be counted by the tens of thousands of degrees Fahrenheit, and it may amount to hundreds of thousands; it has, indeed, been reckoned as high as a million degrees. This vast discharge is not due to any kind of burning action-i.e., to th

dies of the solar system. It should be understood that by mass is meant not the bulk of the object, but the actual amount of matter in it as determined by the gravit

e Masses of Sun

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ent lantern show the grouping of the important stars in each constellation. The advantage of this arrangement is that the little maps can be consulted at night and in the open air in a very convenient m

around the sun and that of our own sphere; that it would be very different if our earth stood still in the heavens. At this stage he may well begin to take in mind the evidence which the planetary motion supplies that the earth really moves round the sun, and not the sun and p

ts between the earth and the sun. He observes that those so-called inferior planets Mercury and Venus are never very far away from the central body; that they appear to rise up from it, and then to go back to it, and that they have phases like the moon. Now and then Venus may be observed as a black spot crossing the disk of the sun. A little consideration will show that on the theory that bodies revolve round each other in the solar system these movements of the inner planets can only be explained on the supposition that they at least travel around the great central fire. Now, taking up the outer planets, we observe that they occasionally appear very bright, and that they are then at a place in the heavens where we see that they are far from the solar centre. Gradually they move down toward the sunset and disappear from view. Here, too, the movement, though less clearly so, is best reconcilable with the idea that these bodies travel in orbits, such as those which are

l to him to ponder upon the problem, and gradually win his way to a full understanding of it. With that purpose in mind, he should avoid reading what astronomers have to say on the matter until he is satisfied that he has done as much as he can with the matter on his own account. He should, however, state his observations, and as far as possible draw the results in his note-book in a diagrammatic

ear. On this plot he should put down only the stars whose places and names he has learned, but he should plot the position of the planets at

is not at all necessary, or, indeed, desirable at the outset of his studies. An ordinary opera-glass, however, will help him in picking out the stars in the constellations, in id