As the most valuable impression which the student can receive from his study of Nature is that sense of the order which has made possible all life, including his own, it will be well for him to imagine, as he may readily do, what would be the effect arising from changes in relations of earth and sun. Bringing the earth's axis in imagination into a position at right angles to the plane of the orbit, he will see that the effect would be to intensify the equatorial 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 equatorial point at the pole nearest the sun; thence the temperatures would grade away to the present equator, beyond which half the earth would be in more refrigerating condition than are the poles at the present day. In considering the movements of our planet, we shall see that no great changes in the position of the polar axis can have taken place. On this account the suggested alterations of the axis should not be taken as other than imaginary changes.

It is easy to see that with a circular orbit and with an inclined axis winter and summer would normally come always at the same point in the orbit, and that these seasons would be of perfectly even length. But, as we have before noted, the earth's path around the sun is in its form greatly affected by the attractions which are exercised by the neighbouring planets, principally by those great spheres which lie in the realm without its orbit, Jupiter 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 eccentricity which can be brought about by these alterations is very great. When the path is most nearly circular, the difference in the major and minor axis may amount to as little as about five hundred thousand miles, or about one one hundred and eighty-sixth of its average diameter. When the variation is greatest the difference in these measurements may be as much as near thirteen million miles, or about one seventh of the mean width of the orbit.

The first and most evident effect arising from these changes of the orbit comes from the difference in the amount of heat which the earth may receive according as it is nearer or farther from the sun. As in the case of other fires, the nearer a body is to it the larger the share of light and heat which it will receive. In an orbit made elliptical by the planetary attraction 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 clearly show that the total amount of heat and the attendant light which the earth receives in a year is not affected by these changes in the form of its path. While it is true that it receives heat more rapidly in the half of the ellipse which is nearest the source of the inundation, it obtains less while it is farther away, and these two variations just balance each other.

Although the alterations in the eccentricity of its orbit do not vary the annual supply of heat which the earth receives, they are capable of changing the character of the seasons, and this in the way which we will now endeavour to set forth, though we must do it at the cost of considerable attention on the part of the reader, for the facts are somewhat complicated. In the first place, we must note that the ellipticity of the earth's orbit is not developed on fixed lines, but is endlessly varied, as we can readily imagine it would be for the reason that its 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, and sometimes farther away. It may thus well come about that at one time the summer season of a hemisphere arrives when it is nearest the sun, so that the season, though hot, will be very short, while at another time the same season will arrive when the earth is farthest from the sun, and receives much less heat, which would tend to make a long and relatively cool summer. The reason for the difference in length of the seasons is to be found in the relative swiftness of the earth's revolution when it is nearest the sun, and the slowness when it is farther away.

There is a further complication arising from that curious phenomenon called the precession of the equinoxes, which has to be taken into account before we can sufficiently comprehend the effect of the varying eccentricity of the orbit on the earth's seasons. To understand this feature of precession we should first note that it means that each year the change from the winter to the summer—or, as we phrase it, the passage of the equinoctial line—occurs a little sooner than the year before. The cause of this is to be found in the attraction which the heavenly bodies, 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 plane of the equator will constantly be altering its position. Now, as the equinoctial points in the orbit depend for their position upon the attitude of the equatorial plane, we can conceive that the effect is a change in position of the place in that orbit where summer and winter begin. The actual result is to bring the seasonal points backward, step by step, through the orbit in a regular measure until in twenty-two thousand five hundred years they return to the place where they were before. This cycle of change was of old called the Annus Magnus, or great year.

If the earth's orbit were an ellipse, the major axis of which remained in the same position, we could readily reckon all the effects which arise from the variations of the great year. But this ellipse is ever changing in form, and in the measure of its departure from a circle the effects on the seasons distributed over a great period of time are exceedingly irregular. Now and then, at intervals of hundreds of thousands or millions of years, the orbit becomes 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 the sun that part of the year would have the temperature mitigated by its removal to a greater distance from the source of heat. A corresponding effect would be produced in the winter season. As long as the orbit remained eccentric the tendency would be to give alternately intense seasons to each hemisphere through periods of about twelve thousand years, the other hemisphere having at the same time a relatively slight variation in the summer and winter.

At first sight it may seem to the reader that these studies we have just been making in matters concerning the shape of the orbit and the attendant circumstances which regulate 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 climatal changes on the earth, including the Glacial periods, accidents which have had the utmost importance in the history of man, as well as of all the other life of the planet.

It is now time to give some account as to what is known concerning the general conditions of the solar bodies—the planets and satellites of our own celestial group. For our purpose we need attend only to the general physical state of these orbs so far as it is known to us by the studies of astronomers. The nearest planet to the sun is Mercury. This little sphere, less than half the diameter of our earth, is so close to 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 this, as most else that has been stated concerning this planet, is doubtful. The sphere is so near to the sun that if it were possessed of water it would inevitably bear an atmosphere full of vapour. Under any conceivable conditions of a planet placed as Mercury is, provided it had an atmosphere to retain the heat, its temperature would necessarily be very high. Life as we know it could not well exist upon such a sphere.

Next beyond Mercury is Venus, a sphere only a little less in diameter than the earth. Of this sphere we know more than we do of Mercury, for the reason that it is farther from the sun and so appears in the darkened sky. Most astronomers hold that the surface 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; it is, indeed, by observing them that the rotation period of the planet has, according to some observers, been determined. It therefore seems likely that these spots are the summits of mountains, which, like many of our own earth, rise above the cloud level.

Recent observations on Venus made by Mr. Percival Lowell appear to show that the previous determinations of the rotation of that planet, as well as regards its cloud wrap, are in error. According to these observations, the sphere moves about the sun, always keeping the same side turned toward 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 made with an excellent telescope and in the very translucent and steady air of the Flagstaff Station, they are more likely to be correct than those obtained by other students. If it be true that Venus does not turn upon its axis, such is likely to be the case also with the planet Mercury.

Next in the series of the planets is our own earth. As the details of this planet are to occupy us during nearly all the remainder of this work, we shall for the present pass it by.

Beyond the earth we pass first to the planet Mars, a sphere which has already revealed to us much concerning its peculiarities of form and physical state, and which is likely in the future to give more information than we shall obtain from any other of our companions in space, except perhaps the moon. Mars is not only nearer to us than any other planet, but it is so placed that it receives the light of the sun under favourable conditions for our vision. Moreover, its sky appears to be generally almost cloudless, so that when in its orbital course the sphere is nearest our earth it 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 snow. This covering extends rather less far toward the planet's equator than does the snow sheet on our continents. Taking into account the colour of the coating, and the fact that it disappears when the summer season comes to the hemisphere in which it was formed, we are, in fact, forced to believe that the deposit is frozen water, though it has been suggested that it may be frozen carbonic acid. Taken in connection with what we have shortly to note concerning the apparent seas of this sphere, the presumption is overwhelmingly to the effect that Mars has seasons not unlike our own.

The existence of snow on any sphere may safely be taken as evidence that there is an atmosphere. In the case of Mars, this supposition is borne out by the appearance of its surface. The ruddy light which it sends back to us, and the appearance on the margin of the sphere, which is somewhat dim, appears to indicate that its atmosphere is dense. In fact, the existence of an atmosphere much denser than that of 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 seasonal changes as we have observed could not take place. The planet receives one third less heat than an equal area of the earth, and its likeness to our own temperature, if such exists, is doubtless brought about by the greater density of its atmosphere, that serves to retain the heat which comes upon its surface. The manner in which this is effected will be set forth in the study of the earth's atmosphere.

As is shown by the maps of Mars, the surface is occupied by shadings which seem to indicate the existence of water and lands. Those portions of the area which are taken to be land are very much divided by what appear to be narrow seas. The general geographic conditions differ much from those of our own sphere in that 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 the Martial lands and seas as regards their form, at least, is more like that of Europe than that of any other part of the earth's surface. Europe in the early Tertiary times had a configuration even more like that of Mars than it exhibits at present, for in that period the land was very much more divided than it now is.

If the lands of Mars are framed as are those of our own earth, there should be ridges of mountains constituting what we may term the backbones of the continent. As yet such have not been discerned, which may be due to the fact that they have not been carefully looked for. The only peculiar 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 observed by but few students, so that their features are not yet well recorded; as far as we know them, these fissures have no likeness in the existing conditions of our earth. It is difficult to understand how they are formed or preserved on a surface which is evidently subjected to rainfalls.

It will require much more efficient telescopes than we now have before it will be possible to begin any satisfactory study on the geography of this marvellous planet. We can not hope as yet to obtain any indications as to the details of its 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 research during the coming century, if made with the same speed as during the last, will perhaps enable us to interpret the nature of this neighbour, and thereby to extend the conception of planetary histories which we derive from our own earth.

Beyond Mars we find one of the most singular features of our solar system in a group of small planetary bodies, the number of which now known amounts to some two hundred, and the total may be far greater. These bodies are evidently all small; it is doubtful if the largest is three hundred and the smaller more than twenty miles in diameter. So far as it has been determined by the effect of their aggregate mass in attracting the other spheres, they would, if put together, make a sphere far 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 of the sphere was accomplished, and the matter solidified, the mass was broken into bits in some way which we can not yet conceive. It has been conjectured that such a solid sphere might have been driven asunder by a collision with some wandering celestial body; but all we can conceive of such actions leads us to suppose that a blow of this nature would tend to melt or convert materials subjected to it into the state of vapour, rather than to drive them asunder in the manner of an explosion.

The four planets which lie beyond the asteroids give us relatively little information concerning their physical condition, though they afford a 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 has brought to the task of interpreting the planetary conditions the skill of a well-trained astronomer and a remarkable constructive imagination.

The planet Jupiter, by far the largest of the children of the sun, appears to be still in a state where its internal heat has not so far escaped that the surface has cooled down in the manner of our earth. What appear to be good observations show that the equatorial part of its area, 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 Jovian sphere were no larger than the earth, it would, on account of its greater age, long ago have parted with its heat; but on account of its great size it has been able, notwithstanding its antiquity, to retain a measure of temperature which has long since passed away from our earth.

In the case of Saturn, the cloud bands are somewhat less visible than on Jupiter, but there is reason to suppose in this, as in the last-named planet, that we do not behold the more solid surface of the sphere, but see only a cloud wrap, which is probably due rather 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 conceive that any density of the atmosphere whatever would suffice to hold in enough temperature to produce ordinary clouds. Moreover, from time to time bright spots appear on the surface of the planet, which must be due to some form of eruptions from its interior.

Beyond Saturn the two planets Uranus and Neptune, which occupy the outer part of the solar system, are so remote that even our best telescopes discern little more than their presence, and the fact that they have attendant moons.

From the point of view of astronomical science, the outermost planet Neptune, of peculiar interest for the reason that it was, as we may say, discovered by computation. Astronomers had for many years remarked the fact that the next inner planetary sphere exhibited peculiarities in its 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 the telescope was turned to the given field a brief study revealed the planet. Nothing else in the history of the science of astronomy, unless it be the computation of eclipses, so clearly and popularly shows the accuracy of the methods by which the work of that science may be done.

As we shall see hereafter, in the chapters which are devoted to terrestrial phenomena, the physical condition of the sun determines the course of all the more 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 especially designed to make the earth more interpretable to us, we should give a somewhat special attention to what is known under the title of "Solar Physics."

The reader has already been told that the sun is one of many million similar bodies which exist in space, and, furthermore, that these aggregations of matter have been developed from an original nebulous condition. The facts indicate that the natural history of the sun, as well as that of its attendant spheres, exhibits three momentous stages: First, that of vapour; second, that of igneous fluidity; third, that 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 nebulous state; the last of the successively developed rings has broken, and has formed itself into the smallest of the planets, which by its distance from the sun seems to indicate that the process of division by rings long ago attained in our solar system its end, the remainder of its nebulous material concentrating on its centre without sign of any remaining tendency to produce these planet-making circles.

The Constitution of the Sun.

Before the use of the telescope in astronomical work, which was begun by the illustrious Galileo in 1608, astronomers were unable to approach the problem of the structure of the sun. They could discern no more than can be seen by any one who looks at the great sphere through a bit of smoked glass, as we know this 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 an aspect like a very thin cloud or fog, which rises in streamer like projections at points to a quarter of a million miles or more above the surface of the sphere. The appearance of this shining field, which is called the corona, reminds one of the aurora which glows in the region about either pole of the earth.

One of the first results of the invention of the telescope was the revelation of the curious dark objects on the sun's disk, known by the name of spots from the time of their discovery, or, at least, from the time when it was clearly perceived that they were not planets, but really on the solar body. The interest 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 astronomers as to the interpretation of their observations, but certain points of great general interest have been tolerably well determined. These may be briefly set forth by an account of what would meet the eye if an observer were able to pass from the surface of the earth to the central part of the sun.

In passing from the earth to a point about a quarter of a million miles from the sun's surface—a distance about that of the moon from our sphere—the observer would traverse the uniformly empty spaces of the heavens, where, but for the rare chance of a passing meteorite or comet, there would be 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 mile; yet, as they would glow intensely in the central light, they would be sufficient to give the illumination which is visible in an eclipse. These particles are most likely driven up from the sun by some electrical action, and are constantly in motion, much as are the streamers of the aurora.

Below the corona and sharply separated from it the observer finds another body of very dense vapour, which is termed the chromosphere, and which has been regarded as the atmosphere of the sun. This layer is probably several thousand miles thick. From the manner in which it moves, in the way the air of our own planet does in great storms, it is not easy to believe that it is a fluid, yet its sharply defined upper surface leads us to suppose that it can not well be a mere mass of vapour. The spectroscope shows us that this chromosphere 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 matter have the aspect of flames, but they would not be such in fact, for the materials are not burning, but merely kept at a high temperature by the heat of the great sphere beneath. They spring up with such energy that they at times move with a speed of one hundred and fifty miles a second, or at a rate which is attained by no other matter in the visible universe, except that strange, wandering star known to astronomers as "Grombridge, 1830," which is traversing the firmament with a speed of not less than two hundred miles a second.

Below the chromosphere is the photosphere, the lower envelope of the sun, if it be not indeed the body of the sphere itself; from this comes the light and heat of the mass. This, too, can not well be a firm-set mass, for the reason that the spots appear to form in and move over it. It may be regarded as an extremely dense mass of gas, so weighed down by the 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 the full blaze of the sun is cut off in a total eclipse. The fact that the photosphere, except when broken by the so-called spots, lies like a great smooth sea, with no parts which lie above the general line, shows that it has a very different structure from the envelope which lies upon it. If they were both vaporous, there would be a gradation between them.

On the surface of the photosphere, almost altogether within thirty degrees of the equator of the sun, a field corresponding approximately to the tropical belt of the earth, there appear from time to time the curious disturbances which are termed spots. These appear to be uprushes of matter in the gaseous state, the upward movement being upon the margins of the 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 that they are most abundant at intervals of about eleven years, the last period of abundance being in 1893. The next to come may thus be expected in 1904. In the times of least spotting more than half the days of a year may pass without the surface of the photosphere being broken, while in periods of plenty no day in the year is likely to fail to show them.

It is doubtful if the closest seeing would reveal the cause of the solar spots. The studies of the physicists who have devoted the most skill to the matter show little more than that they are tumults in the photosphere, attended 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 uncertain. It is also uncertain what effect these convulsions of the sun have on the amount of the heat and light which is poured forth from the orb. The common opinion that the sun-spot years are the hottest is not yet fully verified.

Below the photosphere lies the vast unknown mass of the unseen solar realm. It was at one time supposed that the dark colour of the spots was due to the fact that the photosphere was broken through in those spaces, and that we looked down through them upon the surface of the slightly illuminated central part of the sphere. This view is untenable, and 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 compressed gases, which are kept from becoming solid or even fluid by the very high temperature which exists in them. This view is apparently supported by the fact that, while the pressure upon its matter is twenty-seven times greater in the sun than it is in the earth, the weight of the whole mass is less than we should expect under these conditions.

As for the temperature of the sun, we only know that it is hot enough to turn the metals into gases in the manner in which this is done in a strong electric arc, but no satisfactory method of reckoning the scale of this heat has been devised. The probabilities are to 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 the combustion of substances, as in a fire. It must be produced by the gradual falling in of the materials, due to the gravitation of the mass toward its centre, each particle converting its energy of position into heat, as does the meteorite when it comes into the air.

It is well to close this very imperfect account of the learning which relates to the sun with a brief tabular statement showing the relative masses of the several bodies 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 gravitative attraction which it exercises on other celestial bodies. In this test the sun is taken as the measure, and its mass is for convenience reckoned at 1,000,000,000.

It thus appears that the mass of all the planets is about one seven hundredth that of the sun.

Those who wish to make a close study of celestial geography will do well to procure the interesting set of diagrams prepared by the late James Freeman Clarke, in which transparencies placed in a convenient 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 manner. After the student has learned the position of a dozen of the constellations visible in the northern hemisphere, he can rapidly advance his knowledge in the admirable method invented by Dr. Clarke.

Having learned the constellations, the student may well proceed to find the several planets, and to trace them in their apparent path across the fixed stars. It will be well for him here to gain if he can the conception that their apparent movement is compounded of their motion 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 planets round the earth. This discovery was one of the great feats of the human mind; it baffled the wits of the best men for thousands of years. Therefore the inquirer who works over the evidence is treading one of the famous paths by which his race climbed the steeps of science.

The student must not expect to find the evidence that the sun is the centre of the solar system very easy to interpret; and yet any youth of moderate curiosity, and that interest in the world about him which is the foundation of scientific insight, can see through the matter. He will best begin his inquiries by getting a clear notion of the fact that the moon goes round the earth. This is the simplest case of movements of this nature which he can see in the solar system. Noting that the moon occupies a different place at a given hour in the twenty-four, but is evidently at all times at about the same distance from the earth, he readily perceives that it circles about our sphere. This the people knew of old, but they made of it an evidence that the sun also went around our sphere. Here, then, is the critical point. Why does the sun not behave in the same manner as the moon? At this stage of his inquiry the student best notes what takes place in the motions of the planets 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 traversed by the inner planets. The wonder is that with these simple facts before them, and with ample time to think the matter over, the early astronomers did not learn the great truth about the solar system—namely, that the sun is the centre about which the planets circled. Their difficulty lay mainly in the fact that they did not conceive the earth as a sphere, and even after they attained that conception they believed that our globe was vastly larger than the planets, or even than the sun. This misconception kept even the thoughtful Greeks, who knew that the earth was spherical in form, from a clear notion as to the structure of our system. It was not, indeed, until mathematical astronomy attained a considerable advance, and men began to measure the distances in the solar system, and until the Newtonian theory of gravitation was developed, that the planetary orbits and the relation of the various bodies in the solar system to each other could be perfectly discerned.

Care has been taken in the above statements to give the student indices which may assist him in working out for himself the evidence which may properly lead a person, even without mathematical considerations of a formal kind, to construct a theory as to the relation of the planets to the sun. It is not likely that he can go through all the steps of this argument at once, but it will be most useful 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 form. He should endeavour to see if the facts are reconcilable with any other supposition than that the earth and the other planets move around the sun. When he has done his task, he will have passed over one of the most difficult roads which his predecessors had to traverse on their way to an understanding of the heavens. Even if he fail he will have helped himself to some large understandings.

The student will find it useful to make a map of the heavens, or rather make several representing their condition at different times in the year. 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 different times. In this way, though at first his efforts will be very awkward, he will soon come to know the general geography of the heavens.

Although the possession or at least the use of a small astronomical telescope is a great advantage to a student after he has made a certain advance in his work, such an instrument 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 identifying the planets, and in getting a better idea as to the form of the moon's surface—a matter which will be treated in this work in connection with the structure of the earth.

CHAPTER   IV.
the earth.

With the terrestrial globe in hand, the student can readily construct an image which will represent, at least in outline, the appearance which the sphere he inhabits would present when seen from a distance of about a quarter of a million miles away. The continent of Europe-Asia would of itself appear larger than all the lunar surface which is visible to us. Every continent and all the greater islands would be clearly indicated. The snow covering which in the winter of the northern hemisphere wraps so much of the land would be seen to come and go in the changes of the seasons; even the permanent ice about either pole, and the greater regions of glaciers, such as those of the Alps and the Himalayas, would appear as brilliant patches of white amid fields of darker hue. Even the changes in the aspect of the vegetation which at one season clothes the wide land with a green mantle, and at another assumes the dun hue of winter, would be, to the unaided eye, very distinct. It is probable that all the greater rivers would be traceable as lines of light across the relatively dark surface of the continents. By such exercises of the constructive imagination—indeed, in no other way—the student can acquire the habit of considering the earth as a vast whole. From time to time as he studies the earth from near by he should endeavour to assemble the phenomena in the general way which we have indicated.

The reader has doubtless already learned that the earth is a slightly flattened sphere, having an average diameter of about eight thousand miles, the average section at the equator being about twenty-six miles greater than that from pole to pole. In a body of such large proportions this difference in measurement appears not important; it is, however, most significant, for it throws light upon the history of the earth's mass. Computation shows that the measure of flattening at the poles is just what would occur if the earth were or had been at the time when it assumed its present form in a fluid condition. We readily conceive that a soft body revolving in space, while all its particles by gravitation tended to the centre, would in turning around, as our earth does upon its axis, tend to bulge out in those parts which were remote from the line upon which the turning took place. Thus the flattening of our sphere at the poles corroborates the opinion that its mass was once molten—in a word, that its ancient history was such as the nebular theory suggests.

Although we have for convenience termed the earth a flattened spheroid, it is only such in a very general sense. It has an infinite number of minor irregularities which it is the province of the geographer to trace and that of the geologist to account for. In the first place, its surface is occupied by a great array of ridges and hollows. The larger of these, the oceans and continents, first deserve our attention. The difference in altitude of the earth's surface from the height of the continents to the deepest part of the sea is probably between ten and eleven miles, thus amounting to about two fifths of the polar flattening before noted. The average difference between the ocean floor and the summits of the neighbouring continents is probably rather less than four miles. It happens, most fortunately for the history of the earth, that the water upon its surface fills its great concavities on the average to about four fifths of their total depth, leaving only about one fifth of the relief projecting above the ocean level. We have termed this arrangement fortunate, for it insures that rainfall visits almost all the land areas, and thereby makes those realms fit for the uses of life. If the ocean had only half its existing area, the lands would be so wide that only their fringes would be fertile. If it were one fifth greater than it is, the dry areas would be reduced to a few scattered islands.

From all points of view the most important feature of the earth's surface arises from its division into land and water areas, and this for the reason that the physical and vital work of our sphere is inevitably determined by this distribution. The shape of the seas and lands is fixed by the positions at which the upper level of the great water comes against the ridges which fret the earth's surface. These elevations are so disposed that about two thirds of the hard mass is at the present time covered with water, and only one third exposed to the atmosphere. This proportion is inconstant. Owing to the endless up-and-down goings of the earth's surface, the place of the shore lines varies from year to year, and in the geological ages great revolutions in the forms and relative area of water and land are brought about.

Noting the greater divisions of land and water as they are shown on a globe, we readily perceive that those parts of the continental ridges which rise above the sea level are mainly accumulated in the northern hemisphere—in fact, far more than half the dry realm is in that part of the world. We furthermore perceive that all the continents more or less distinctly point to the southward; they are, in a word, triangles, with their bases to the northward, and their apices, usually rather acute, directed to the southward. This form is very well indicated in three of the great lands, North and South America and Africa; it is more indistinctly shown in Asia and in Australia. As yet we do not clearly understand the reason why the continents are triangular, why they point toward the south pole, or why they are mainly accumulated in the northern hemisphere. As stated in the chapter on astronomy, some trace of the triangular form appears in the land masses of the planet Mars. There, too, these triangles appear to point toward one pole.

Besides the greater lands, the seas are fretted by a host of smaller dry areas, termed islands. These, as inquiry has shown, are of two very diverse natures. Near the continents, practically never more than a thousand miles from their shores, we find isles, often of great size, such as Madagascar, which in their structure are essentially like the continents—that is, they are built in part or in whole of non-volcanic rocks, sandstones, limestones, etc. In most cases these islands, to which we may apply the term continental, have at some time been connected with the neighbouring mainland, and afterward separated from it by a depression of the surface which permitted the sea to flow over the lowlands. Geologists have traced many cases where in the past elevations which are now parts of a continent were once islands next its shore. In the deeper seas far removed from the margins of the continents the islands are made up of volcanic ejections of lava, pumice, and dust, which has been thrown up from craters and fallen around their margin or are formed of coral and other organic remains.

Next after this general statement as to the division of sea and land we should note the peculiarities which the earth's surface exhibits where it is bathed by the air, and where it is covered by the water. Beginning with the best-known region, that of the dry land, we observe that the surface is normally made up of continuous slopes of varying declivity, which lead down from the high points to the sea. Here and there, though rarely, these slopes centre in a basin which is occupied by a lake or a dead sea. On the deeper ocean floors, so far as we may judge with the defective information which the plumb line gives us, there is no such continuity in the downward sloping of the surface, the area being cast into numerous basins, each of great extent.

When we examine in some detail the shape of the land surface, we readily perceive that the continuous down slopes are due to the cutting action of rivers. In the basin of a stream the waters act to wear away the original heights, filling them into the hollows, until the whole area has a continuous down grade to the point where the waters discharge into the ocean or perhaps into a lake. On the bottom of the sea, except near the margin of the continent, where the floor may in recent geological times have been elevated into the air, and thus exposed to river action, there is no such agent working to produce continuous down grades.

Looking upon a map of a continent which shows the differences in altitude of the land, we readily perceive that the area is rather clearly divided into two kinds of surface, mountains and plains, each kind being sharply distinguished from the other by many important peculiarities. Mountains are characteristically made up of distinct, more or less parallel ridges and valleys, which are grouped in very elongated belts, which, in the case of the American Cordilleras, extend from the Arctic to the Antarctic Circle. Only in rare instances do we find mountains occupying an area which is not very distinctly elongated, and in such cases the elevations are usually of no great height. Plains, on the other hand, commonly occupy the larger part of the continent, and are distributed around the flanks of the mountain systems. There is no rule as to their shape; they normally grade away from the bases of the mountains toward the sea, and are often prolonged below the level of the water for a considerable distance beyond the shore, forming what is commonly known as the continental shelf or belt of shallows along the coast line. We will now consider some details concerning the form and structure of mountains.

In almost any mountain region a glance over the surface of the country will give the reader a clew to the principal factor which has determined the existence of these elevations. Wherever the bed rocks are revealed he will recognise the fact that they have been much disturbed. Almost everywhere the strata are turned at high angles; often their slopes are steeper than those of house roofs, and not infrequently they stand in attitudes where they appear vertical. Under the surface of plains bedded rocks generally retain the nearly horizontal position in which all such deposits are most likely to be found. If the observer will attentively study the details of position of these tilted rocks of mountainous districts, he will in most cases be able to perceive that the beds have been flexed or folded in the manner indicated by the diagram. Sometimes, though rarely, the tops of these foldings or arches have been preserved, so that the nature of the movement can be clearly discerned. More commonly the upper parts of the upward-arching strata have been cut off by the action of the decay-bringing forces—frost, flowing water, or creeping ice in glaciers—so that only the downward pointing folds which were formed in the mountain-making are well preserved, and these are almost invariably hidden within the earth.