Chapter IV: Preface: To First Edition (4)

Our earth at the present day shows many indications of being much hotter within than it is on the surface. The volcanoes themselves are mere outbreaks of incandescent material from inside. Then there are hot springs of water at Bath, which gush out from the earth. There are geysers of hot water in Iceland and in the Yellowstone Park in America, and in other places. And there are other indications also, with which every miner is familiar. Wherever a deep pit is sunk into the earth, the rocks below are always found to be warmer than those on the surface, and the deeper the pit the greater is the heat that is encountered. Thus, from all over the world we obtain proofs of the present existence of internal heat. Great as is the earth, we must still apply the simple common-sense principles that we use in our everyday life here. Let me give an illustration. Suppose that a servant came into the room and placed a jug of water on the table, and that an hour afterwards you went to the jug of water and found it to be cold, you would not from that fact alone be able to infer anything with certainty, as to whether the water had been warm or cold when it was brought in. It might have been perfectly cold, as it is at present, though on the other hand the water might have been warm at first, and have since cooled down to the temperature of the room during the hour.

Suppose, however, that when you went to the jug of water, which had stood on the table for an hour, you found it tepid, no matter how slightly its temperature might be above that of the room, do you not see the inference you would be able to draw? You would argue in this way: that water has still some heat; it must, of course, be gradually cooling, and therefore it was hotter a minute ago than it is now; it was hotter still two minutes ago, or ten minutes; and must have been very hot and perhaps boiling when it was brought in an hour ago.

I want you to apply exactly the same reasoning to our earth. It is, as I have shown you, still hot and warm inside. Of course, that heat is gradually becoming lost; so that the earth will from year to year gradually cool down, though at an extremely slow rate. But we must look back into what has happened during past ages. Just as we inferred that the jug must have contained very hot water an hour before from the mere fact that the water was still warm, so we are entitled to infer, from the fact that the earth still retains some heat, that it must in ages gone by have been exceedingly hot. In fact, the further we look back, the hotter do we see the earth growing, until at last we are constrained to think of a period, in the excessively remote past, long ere life began to dawn on this earth, when even the surface of the earth was hot. Back further still we see the earth no longer covered with the hard, the dark, and the cold surface we now find; we are to think of it in these primitive times as a huge glowing mass, in which all the substances that now form the rocks were then incandescent, and even molten material.

There is good reason for knowing that in those early times the moon also was molten with heat; and thus our reasoning has led us to think of a period when there were two great red-hot globes--one of which had about four times the diameter of the other--starting on their career of gradually cooling down. Recall our little experiment with the two cooling globes of iron; imagine these globes to preserve their relative proportions, but that one of them was 8000 miles and the other 2000 miles across. Ages will, no doubt, elapse ere they part with their heat sufficiently to allow the surfaces to cool and to consolidate. We may, however, be sure that the small globe will cool the faster, that its outside will become hard sooner than will the surface of the large one, and long after the small globe has become cold to the centre, the large one may continue to retain some of its primeval heat. We can thus readily understand why all the volcanoes on the moon have ceased--their day is over. It is over because the moon, being so small, has grown so cold that it no longer sustains the internal fires which are necessary for volcanic outbreaks. Our earth, in consequence of its much greater size, has grown cold more slowly. It has no doubt lost the high temperature on the exterior, and its volcanic energy has probably abated from what it once was. But there is still sufficient power in the subterranean fires to awaken us occasionally by a Krakatoa, or to supply Vesuvius with sufficient materials and vigor for its more frequent outbursts. The argument shows us that the time will at last come when this earth shall have parted with so large a proportion of its heat that it will be no longer able to provide volcanic phenomena, and then we shall pass into the exhausted stage which the moon attained ages ago.

THE MOVEMENTS OF THE MOON.

Though the moon is going round and round the earth incessantly, yet it always manages to avoid affording us a view of what is on the other side. Our satellite always directs the same face towards the earth, and we may reasonably conjecture that the other side is covered, like the side we know, with rings and other traces of former volcanoes. In this respect the moon is quite a peculiar object. The other great celestial bodies, such as the sun or Jupiter, turn round on their axes, and show us now one side and then the other, with complete impartiality. The way in which the moon revolves may be illustrated by taking your watch and chain, and as you hold the chain at the centre making the watch revolve in a circular path. At every point of its path the ring of the watch is, of course, pointed to the centre where the chain is held. If you imagine your eye placed at the centre, to represent the earth, the movements of the watch would exemplify the way the moon turns round it.

One more point I must explain about the moon before we close this lecture. There is nothing more familiar than the fact that a heavy body will fall to the ground. Indeed, it hardly matters what the material of the body may be, for you see I have a small iron ball in one hand and I hold a cork in the other. I drop them at the same moment, and they reach the ground together. Perhaps you would have expected that the cork would have lagged behind the iron. I try the experiment again and again, and you can see no difference in the times of their falling, though I do not say this would be true if they were dropped from the top of the Monument. In general we may say that bodies let drop will fall sixteen feet in the first second. Even a bit of paper and a penny piece will fall through the same height in the same time if you can get over the difficulty of the resistance of the air. This is easily managed. Cut a small piece of tissue paper which will lie flat on the top of the penny, and hold the penny horizontal with the paper uppermost. Though there is nothing to fasten the paper to the penny, you will find that they fall together. If we could conduct the experiment of dropping the penny and the bit of paper in a vacuum, then, whether the paper was laid on the penny or placed in any other way, the two objects would reach the table at the same moment if released at the same moment at equal heights.

Wherever we go we find that bodies will always tend to fall in towards the centre of the earth; thus in New Zealand, at the opposite side of our globe from where we are now standing, bodies will fall up towards us, and this law of falling is obeyed at the top of a mountain as it is down here. No matter how high may be the ascent made in a balloon, a body released will fall towards the earth’s centre. Of course, we can only ascend some five or six miles high, even in the most buoyant of balloons; but we know that the attraction by which bodies are pulled downwards towards the earth extends far beyond this limit. If we could go ten, twenty, or fifty miles up, we should still find that the earth tried to pull us down. Nor, even if you could imagine an ascent made to the height of 1000 miles, would gravitation have ceased. A cork or an iron ball, or any other object dropped from the height of 1000 miles, would assuredly tumble down on the ground below.

Suppose that by some device we were able to soar aloft to a height of 4000 miles. I name that elevation because we should then be as high above the earth as the centre of the earth is below our feet. We should have doubled our distance from the centre of the earth, and the intensity of the gravitation would have decreased to one-quarter of what it is at the surface. A body which at the earth’s surface falls sixteen feet in a second would there fall only four feet in a second, and the apparent weight of any body would be so much reduced that it would seem to weigh only a quarter of what it weighs down here. Thus, the higher and higher we go, the less and less does gravity become; but it does not cease, even at a distance of millions of miles. Therefore you might say that as gravity tries to pull everything down, wherever it may be, why does it not pull down the moon? This is a difficulty which we must carefully consider. Supposing that the earth and the moon were simply held apart, both being at rest, and that then the moon were to be let go, it would no doubt drop down directly on the earth. The movement of the moon would, however, be very different if, instead of being merely let fall, it was thrown sideways. The effect of the earth’s pull upon the moon would then be shown in keeping the moon revolving around us instead of allowing it to fly away altogether, as it would have done had the earth not been there to attract it.

We can explain this by an illustration. On the top of a mountain I have placed a big cannon (Fig. 44). We fire off the cannon, and the bullet flies away in a curved path, with a gradual descent until it falls to the ground. I have made the mountain look hundreds of times larger than any mountain could possibly be; and now I want you to imagine a cannon far stronger and gunpowder more potent than any powder or cannon that has ever yet been manufactured. Fire off a bullet with a still greater charge than the last time, and now the path is a much longer one, but still the bullet curves down so as ultimately to fall on the earth. But make now one final shot with a charge sufficiently powerful, and away flies the bullet, following this time the curvature of the earth, for the earth’s attraction has the effect of bending the path of the bullet from a straight line into this circular form. By the time the bullet has travelled a quarter of the way round, it is no nearer to the earth than it was at first, nor has it parted with any of its original speed. Thus, notwithstanding its long journey, the bullet has practically just as much energy as when it first left the muzzle of the cannon. Away it will fly round another quarter of the earth, and still in the same condition it will accomplish the third and the fourth quarters, thus returning to the point from which it started. If we have cleared the cannon out of the way, the bullet will fly again over the mountain top without having lost any of its speed by its voyage round the earth. Therefore it will be in a condition to start again, and thus to revolve around the earth permanently. If, then, from the top of a mountain 240,000 miles high a great bullet 2000 miles in diameter had once been projected with the proper velocity, that bullet would continue forever to circle round and round the earth, and even though the mountain and the cannon disappeared, the motion would be preserved indefinitely. This illustration will, at all events, show how a continuous revolution of the moon round the earth can exist, notwithstanding that the earth is constantly pulling our satellite down towards its surface.

ON THE POSSIBILITY OF LIFE IN THE MOON.

Astronomers are often asked whether any animals can be living on the moon. No observations we can make with the telescope can answer that question directly. There are great plains to be seen on the moon, of course, but even if there were elephants tramping over those plains, our telescopes could not show them. Nor will our instruments pronounce at once whether plants or trees flourish on the moon. The mammoth trees of California are so big that a tunnel has been cut through the trunk of one large enough to give passage for a carriage and pair. Even if there were trees as big as this on the moon, they would not be visible from the most famous observatories.

Let us think what we should ourselves experience if we could in some marvellous manner be transferred from the earth to its satellite, and tried to explore that new and wonderful country. Alas, we should find it utterly impossible to live there for an hour, or even for a minute! Troops of difficulties would immediately beset us. The very first would be the want of air. Ponder for a moment on the invariable presence of air around our own globe. Even if you climb to the top of a high mountain, or if you take a lofty voyage in a balloon, you are all the time bathed in air. It is air which supports the balloon, just as a cork is buoyed up by water. In all circumstances, we must have air to breathe. In that air is oxygen gas, and we must have oxygen incessantly supplied to our lungs to reinvigorate our blood. We require, too, that this oxygen shall be diluted with a much larger amount of nitrogen gas, for our lungs and system of circulation are adapted for abode in that particular mixture of gases which we find here. The atmosphere becomes more and more rarefied the higher we ascend, and apparently terminates altogether some two or three hundred miles over our heads. Beyond the limits of the atmosphere it seems as if empty space would be met with all the way from the earth to the moon. We could not procure a single breath of air, and life would be, of course, impossible. Even at a height of three or four miles, respiration becomes difficult, and doubtless life could not possibly be sustained at a height of ten miles.

It is therefore plain that for a voyage to the moon we should require an ample supply of air, or, at least, of life-giving oxygen, which in some way or other was to be inhaled during the progress of the journey. When at length 240,000 miles had been traversed, and we were about to land on the moon, we would first of all ascertain whether it was surrounded with a coating of air. Most of the globes through space are, so far as we can learn, covered and warmed with an enveloping atmosphere of some kind; but, unhappily, the poor moon has been left entirely, or almost entirely, without any such clothing. She is quite bare of atmosphere at all comparable in density or in volume to that which surrounds us, though possibly we do now and then perceive some traces of air, or of some kind of gas, in small quantities in the lunar valleys.

I am sure each intelligent boy or girl will want to know how we are able to tell all this. We have never been at the moon, and how then can we say that it is nearly destitute of air? Nor can our telescope answer this question immediately, for you could hardly expect to see air, even if it were there. How then are we able to make such assertions? There are many different ways in which we have learned the absence of air from the moon. I will tell you one of the easiest and the most certain of these methods. First let me say that air is not perfectly transparent. No doubt I can see you, and you can see me, though a good many feet of air may lie between us; but when we deal with distances much greater, there is a very simple way in which we can show that air is not quite transparent. In the evening, when the sun is setting and the sky is clear, you can look at him without discomfort; but in the middle of the day you know that it is impossible to look at the sun without shading your eyes with smoked glass or protecting them by some similar contrivance. The reason is, that when the sun is either setting or rising we look at it through an immense thickness of air, which not being perfectly transparent stops some of the light. Thus it is that the sun in these circumstances loses its dazzling brilliancy, and we can view it without discomfort.

At the seaside you can notice the same effect in a different manner. Go out on a fine and clear night, when the stars in their thousands are glittering overhead, and then look down gradually towards the horizon, and you will find the stars becoming fainter and fainter. Indeed, even the brightest star cannot be seen when it is at the horizon, because an immense thickness of the atmosphere is not transparent.

We can now state the argument by which we may prove that there is little or no air on our satellite. The moon will frequently pass between the earth and a star, and when the star is a really bright one the observations that can be made are of great interest. Let me first describe what we actually see. The star is shining brightly until the moment when the moon eclipses it. Generally speaking, its disappearance is instantaneous. But this would not be the case if the moon were encircled with an atmosphere. If the moon were coated with air, the light from the star would not be extinguished _instantly_; it would gradually decline, according as it had to pass through more and more of the moon’s atmosphere. Thus you would find that the star dwindled down in brightness before the solid body of the moon had advanced far enough to shut it out. The sudden extinction of the stars demonstrates the airless state of our satellite.

There would be another insuperable difficulty in adopting the moon as a residence, even supposing that you could get there. Water is absent from its surface. We have examined every part of it, and we find no evidences of seas or of oceans, of lakes or rivers; we never see anything like clouds or mists, which are, of course, only water in the vaporous form. We are, therefore, assured that, so far as water is concerned, the moon is an absolute desert. This is, perhaps, the most striking contrast between the aspect of the earth and the aspect of the moon. Were an astronomer on the moon to look at our earth he would find most of its surface concealed beneath clouds, and through the openings in these clouds he would see that by far the greater part of this globe was covered by the expanse of ocean; in fact, when the lunar astronomer had realized the prevalence of water upon this earth, either in the form of ocean or cloud, I feel sure he would come to the conclusion that nothing could live here except seals or other amphibious animals.

Owing to the absence of air and water, the moon would be totally disqualified for the support of life of those types in which we know it. For air and water are necessary to every animal, from the humblest animalcule up to whales or elephants. Air and water are necessary for every form of vegetable life, from the lichen which grows on a stone up to the noble old oak of the forest. But even supposing that we could land on the moon, bearing with us an ample supply of oxygen to breathe, and of water to drink, we should find ourselves perplexed and embarrassed, to say the very least of it, by an extraordinary difference that would immediately attract our notice. That familiar experience of gravity, or the weights of things, which we have acquired in our residence on a great globe like the earth, would seem ludicrously altered when we began to walk about on a little globe like the moon. We should be astonished at the transformation by which the weight of everything was much lessened; when you pulled out your watch you would hardly feel it at the end of the chain; it would seem like a mere shell; but yet the watch is all right, it is going as well as ever. Nothing has altered about it except its weight. A big stone attracts your notice, and, to your amazement, you find that it does not weigh so much as a piece of wood of the same size would weigh down here. A stone that you could hardly stir on the earth, you can carry about on the moon. Nor is this to be explained by any peculiarity in the constitution of the lunar stone. Most probably it will be not very dissimilar to some of the rocks on the earth. The relative lightness of a lunar stone is not due to its being formed of some very special material; we must seek for some other explanation. Every object on the moon would be found only one-sixth as heavy as the same object on the earth. A sturdy laborer at one of the docks can carry one sack of corn on his back here, and he finds that this load is as much as is convenient. He would, however, discover, were he placed on the moon, that his load had suddenly become lightened to one-sixth part (Fig. 45). The laborer would find that he could carry six sacks of corn on the moon without making a greater effort than the support of a single sack on the earth cost him. To explain how such a change as this has occurred, look at these two pictures: one shows the laborer on a small body like the moon, the other shows him on a great globe like the earth. What the laborer actually does feel is not quite so simple a thing as he imagines. He imagines that it is the weight of the corn, and the corn alone, which produces that pressure on his shoulders which he knows so well. But that is not exactly the manner in which the philosopher will look at the same question. What the laborer does actually feel is the attraction between the earth beneath his feet and the corn on his back. It is this force which produces the pressure on his shoulders. Its magnitude no doubt depends upon the quantity of corn in the sack, but it also depends on the quantity of matter on the earth beneath his feet. In fact, the force between two attracting bodies depends upon the masses of both the attracting bodies. When the laborer is transferred to the moon, of which the mass is so much less than that of the earth, the attraction is less there than it is here, even though the corn is the same in the two cases.

Many odd instances could be given of the extraordinary consequences of life on a world where all weights are reduced to a sixth part. One occurred to me the other day when I saw a postman going his rounds with an amazing load of Christmas presents and parcels. I thought, how much happier must be the lot of a postman on the moon, if such functionaries are wanted there! All the presents of toys or more substantial donations might be the same as before, the only alteration would be that they would not feel nearly so heavy. A box which contains a pound of chocolate bonbons might still contain exactly the same quantity of sweetmeat on the moon, but the exertion of carrying it would be reduced to one-sixth. It would only weigh as much as two or three ounces do on the earth. Our streets provide another admirable illustration of the drawbacks of our life here as compared with the facilities offered by life on the moon. I feel quite confident that no perambulators can be necessary there. I cannot indeed say that there are babies to be found on the moon, but of this I am certain, that even if the lunar babies were as plump and as sturdy as ours, they must still only weigh about a sixth as much as ours do. A lunar nurse would scorn to use a perambulator, even for a pair of twins; she might take them both out on her arm for an airing, and even then only bear one-third of the load that her terrestrial sister must sustain if she is carrying but a single child.

The lightness of bodies in the moon would entirely transform many of our most familiar games. In cricket, for instance, I don’t think the bowling would be so much affected, but the hits on the moon would be truly terrific. I believe an exceptionally good throw of the cricket-ball here is about a hundred yards, but the same man, using the same ball and applying the same force to it, would send the ball six hundred yards on the moon. So, too, every hit would in the lunar game carry the ball to six times the distance it does here. Football would show a striking development in lunar play; a good kick would not only send the ball over the cross-bar, but it would go soaring over the houses, and perhaps drop in the next parish.

Our own bodies would, of course, participate in the general buoyancy, so that, while muscular power remained unabated, we should be almost able to run and jump as if we had on the famous seven-league boots. I have seen an athlete in a circus jump over ten horses placed side by side. The same athlete, making the same effort, would jump over sixty horses on the moon.

A run with a pack of lunar foxhounds would indeed be a marvellous spectacle. There need be no looking round by timid horsemen to find open roads or easy gaps. The five-barred gate itself would be utterly despised by a huntsman who could easily clear a hay-rick. It would hardly be worth taking a serious jump to clear a canal unless there was a road and a railway or so, which could be disposed of at the same time.

To illustrate this subject of gravitation in another way, suppose that we were to be transferred from this earth to some globe much greater than the earth--to a globe, for instance, as large and massive as the sun. We can then show that the weight of every object would be increased. Indeed, everything would weigh about twenty-seven times as much as we find it does here. To pull out your watch would be to hoist a weight of about five or six pounds out of your pocket. Indeed, I do not see how you could draw out your watch, for even to raise your arm would be impossible; it would feel heavier by far than if it were made of solid lead. It is, perhaps, conceivable that you might stand upright for a moment, particularly if you had a wall to lean up against; but of this I feel certain, that if you once got down on the ground, it would be utterly out of your power to rise again.

These illustrations will at least answer one purpose: they will show how difficult it is for us to form any opinion as to the presence or the absence of life on the other globes in space. We are just adapted in every way for a residence on this particular earth of a particular size and climate, and with atmosphere of a particular composition. Within certain slender limits our vital powers can become accommodated to change, but the conditions of other worlds seem to be so utterly different from those we find here, that it would probably be quite impossible for beings constituted as we are to remain alive for five minutes on any other globe in space.

It is, however, quite another question as to whether there may not be inhabitants of some kind on many of the other splendid globes. We have through the wide extent of space inconceivable myriads of worlds, presenting, no doubt, every variety of size and climate, of atmosphere and soil. It seems quite preposterous to imagine that among all these globes ours alone should be the abode of life. The most reasonable conclusion for us to come to is that these bodies may be endowed with life of types which are just as appropriate to the physical conditions around them as is the life, both animal and vegetable, on this globe to the special circumstances in which it is placed.

LECTURE III.

THE INNER PLANETS.

Mercury, Venus, and Mars--How to make a Drawing of our System--The
Planet Mercury--The Planet Venus--The Transit of Venus--Venus
as a World--The Planet Mars and his Movements--The Ellipse--The
Discoveries made by Tycho and Kepler--The Discoveries made by
Newton--The Geography of Mars--The Satellites of Mars--How the
Telescope aids in Viewing Faint Objects--The Asteroids, or Small
Planets.

MERCURY, VENUS, AND MARS.

We can hardly think of either the sun or the moon as a world in the sense in which our earth is a world, but there are some bodies called planets which seem more like worlds, and it is about them that we are now going to talk. Besides our Earth there are seven planets of considerable size, and a whole host of insignificant little ones. These planets are like ours in a good many respects. One of them, Venus, is about the same size as this earth; but the two others, Mercury and Mars, are very much smaller. There are also some planets very much larger than any of these, namely, Jupiter, Saturn, Uranus, and Neptune. We shall in this lecture chiefly discuss three bodies, namely, Mercury, Venus, and Mars, which, with the earth, form the group of “inner” planets.

The planets are all members of the great family dependent on the sun. Venus and the earth may be considered the pair of twins, alike in size and weight. Mercury and Mars are the babies of the system. The big brothers are Jupiter and Saturn. All the planets revolve round the sun, and derive their light and their heat from his beams. We should like to get a little closer to some of our fellow-planets and learn their actual geography. Unfortunately, even under the most favorable circumstances, they are a very long way off. They are many millions of miles distant, and are always at least a hundred times as far as the moon. But far as the planets may be, astronomers have been familiar with their existence for ages past. I can give you a curious proof of this. You remember how we said the first and the second days of the week were called after the sun and the moon, Sun-day and Moon-day, or Monday, respectively. Let us see about the other days. Tuesday is not quite so obvious, but translate it into French and we have at once _Mardi_; this word means nothing but Mars’ day, and our Tuesday means exactly the same. Wednesday is also readily interpreted by the French word _Mercredi_, or Mercury’s day, while Venus corresponds to Friday. Jupiter’s day is Thursday, while Saturn’s day is naturally Saturday. The familiar names of the days of the week are thus associated with the seven moving celestial bodies which have been known for uncounted ages.

HOW TO MAKE A DRAWING OF OUR SYSTEM.

I want every one who reads this book to make a little drawing of the sun and the planets. The apparatus that you will need is a pair of compasses; any sort of compasses that will carry a bit of pencil will do. You must also get a little scale that has inches and parts of inches divided upon it; any carpenter’s rule will answer. The drawing is intended to give a notion of the true sizes and positions of the fine family of which the earth is one member. The figure I have given (Fig. 46) is not on so large a scale as that which I ask you to use, and which I shall here mention. Try and do the work neatly, and then pin up your little drawings where you will be able to see them every day until you are quite familiar with the notion of what we mean by our solar system.

First open the compasses one inch, and then describe a circle, and mark a dot on this as “MERCURY,” in neat letters, and also write on the circle “88 days.” At the centre you are to show the “SUN.” This circle gives the track followed by Mercury in its journey round the sun in the period of 88 days. Next open your compasses to 1¾ in., which you must do accurately by the scale. The circle drawn with this radius shows the relative size of the path of Venus, and to indicate the periodic time, you should mark it, “225 days.” The next circle you have to draw is a very interesting one. The compass is to be opened 2½ in. this time, and the path that it makes is to be marked “365 days.” This shows the high road along which we ourselves journey every year, along which we are, indeed, journeying at this moment. If you wanted to obtain from your figure any notions of the true dimensions of the system, the path of the earth will be the most convenient means of doing so. The earth is 93,000,000 miles from the sun, and our drawing shows its orbit as a circle of 2½ in. radius. It follows that each inch on our little scale will correspond to about 37,000,000 miles. As, therefore, the radius of the orbit of Mercury has been taken to be one inch, it follows that the distance of Mercury from the sun is about 37,000,000 miles.

We have, however, still one more circle to draw before we complete this little sketch. The compass must now open to four inches, and a circle which represents the orbit of Mars is then to be drawn. We mark on this “687 days,” and the inner part of the solar system is then fully represented. You see, this diagram shows how our earth is in every sense a planet. It happens that one of the four planets revolves outside the earth’s path, while there are two inside. By marking the days on the circles which show the periods of the planets, you perceive that the further a planet is from the sun, the longer is the time that it takes to go round. Perhaps you will not be surprised at this, for the length of the journey is, of course, greater in the greater orbits; but this consideration will not entirely explain the augmentation of the time of revolution. The further a planet is from the sun, the more slowly does it actually move, and therefore, for a double reason, the larger orbit will take a longer time. From London to Brighton is a much longer journey than from London to Greenwich, and, therefore, the journey by rail to Brighton will, of course, be a longer one than by rail to Greenwich. But suppose that you compared the railway journey to Greenwich with the journey, not by rail, but by coach, to Brighton, here the comparative slowness of the coach would form another reason besides the greater length of the journey for making the Brighton trip a much more tedious one than that to Greenwich. Mars may be likened to the coach which has to go all the way to Brighton, while Mercury may be likened to the train which flies along over the very short journey to Greenwich.

We can easily show from our little sketch that Mercury must be moving more quickly than Mars, for the radii of the two circles are respectively one inch and four inches, and therefore the path of Mars must be four times as long as the orbit of Mercury. If Mars moved as fast as Mercury, he would, of course, require only four times as many days to complete his large path as Mercury takes for his small path; but four times 88 is 352, and, consequently, Mars ought to get round in 352 days if he moved as fast as Mercury does. As a matter of fact, Mars requires nearly twice that number of days; indeed, no less than 687, and hence we infer that the average speed of Mars cannot be much more than half that of Mercury.

To appreciate duly the position of the earth with regard to its brothers and sisters in the sun’s family it will be necessary to use your compasses in drawing another little sketch, by which the sizes of the four bodies themselves shall be fairly represented. Remember that the last drawing showed nothing whatever about the sizes of the bodies; it merely exhibited the dimensions of the paths in which they moved. As Mercury is the smallest globe of the four, we shall open the compasses half an inch and describe a circle to represent it. The earth and Venus are so nearly the same size (though the earth is a trifle the larger) that it is not necessary to attempt to exhibit the difference between them, so we shall represent both bodies by circles, each 1¼ inches in radius. Mars, like Mercury, is one of the globes smaller than the earth, and the circle that represents it will have a radius of ¾ of an inch. You should draw these figures neatly, and by a little shading make them look like globes. It would be better still if you were to make actual models, taking care, of course, to give each of them the exact size. A comparative view of the principal planets is shown in Fig. 47.

THE PLANET MERCURY.

Quicksilver is a bright and pretty metal, and, unlike every other metal, it is a liquid under ordinary circumstances. If you spill quicksilver, it is a difficult task to gather the liquid up again. It breaks into little drops, and you cannot easily lift them with your fingers; they slip away and escape your grasp. Quicksilver will run easily through a hole so small that water would hardly pass, and it is so heavy that an iron nail or a bunch of keys will float upon it. Now, this heavy, bright, nimble metal is known by another name besides quicksilver; a chemist would call it mercury, and the astronomers use exactly the same word to denote a pretty, bright, nimble, and heavy planet which seems to try to elude our vision. Though Mercury is so hard to see, yet it was discovered so long ago, that all record is lost of who the discoverer was.

You must take special pains if you want to see the planet Mercury, for during the greater part of the year it is not to be seen at all. Every now and then a glimpse is to be had, but you must be on the alert to look out just after sunset, or you must be up very early in the morning so as to see it just before sunrise. Mercury is always found to be in attendance on the sun, so that you must search for him near the sun; that is, low down in the west in the evenings, or low down in the east in the mornings. To ascertain the proper time of the year at which to look for him you must refer to the almanac.

We have seen how Mercury revolves in a path inside that of Venus, and it is therefore nearer to the sun. Indeed, Mercury is so close to the sun that it is generally overpowered by his brilliance and cannot be seen at all. Like every other planet, Mercury is lighted by the sun’s rays, and shows phases in the telescope just as the moon does (Fig. 48). In this figure the different apparent sizes of the planet at different parts of its path are shown. Of course the nearer Mercury is to the earth the larger does it seem.

If we can only see Mercury so rarely, and if even then it is a very long way off, does it not seem strange that we can tell how heavy it is? Even if we had a pair of scales big enough to hold a planet, what, it may be asked, would be the use of the scales when the body to be weighed was about a hundred millions of miles away? Of course the weighing of a planet must be conducted in some manner totally different from the kind of weighing that we ordinarily use. Astronomers have, however, various methods for weighing these big globes, even though they can never touch them. We do not, of course, want to know how many pounds, or how many millions of tons they contain; there is but little use in trying to express the weight in that way. It gives no conception of a planet’s true importance. One world must be compared with another world, and we therefore estimate the weights of the other worlds by comparing them with that of our own. We accordingly have to consider Mercury placed beside the earth, and to see which of the two bodies is the bigger and the heavier, or what is the proportion between them. It so happens that Mercury, viewed as a world, is a very small body. It is a good deal less in size than our earth, and it is not nearly so massive. To show you how we found out the mass of Mercury I shall venture on a little story. It will explain one of the strange devices that astronomers have to use when they want to weigh a distant body in space.

There was once, and there is still, a little comet which flits about the sky; we shall call it after the name of its discoverer, Encke. There are sometimes splendid comets which everybody can see--we will talk about these afterwards--but Encke is not such a one. It is very faint and delicate, but astronomers are interested in it, and they always look out for it with their telescopes; indeed, they could not see the poor little thing without them. Encke goes for long journeys through space--so far that it becomes quite invisible, and remains out of sight for two or three years. All this time it is tearing along at a tremendous speed. If you were to take a ride on the comet, it would whirl you along far more swiftly than if you were sitting on a cannon-ball. When the comet has reached the end of its journey, then it turns round and returns by a different road, until at last it comes near enough to show itself. Astronomers give it all the welcome they can, but it won’t remain; sometimes it will hardly stay long enough for us to observe that it has come at all, and sometimes it is so thin and worn after all its wanderings that we are hardly able to see it. The comet never takes any rest; even during its brief visit to us it is scampering along all the time, and then again it darts off, gradually to sink into the depths of space, whither even our best telescopes cannot follow it. No more is there to be seen of Encke for another three years, when again it will come back for a while. Encke is like the cuckoo, which only comes for a brief visit every spring, and even then is often not heard by many who dearly love his welcome note; but Encke is a greater stranger than the cuckoo, for the comet never repeats his visit of a few weeks more than once in three years; and he is then so shy that usually very few catch a glimpse of him.

An astronomer and a mathematician were great friends, and they used to help each other in their work. The astronomer watched Encke’s comet, noted exactly where it was, on each night it was visible, and then told the mathematician all he had seen. Provided with this information the mathematician sharpens his pencil, sits down at his desk, and begins to work long columns of figures, until at length he discovers how to make a time table which shall set forth the wanderings of Encke. He is able to verify the accuracy of his table in a very unmistakable way by venturing upon prophecies. The mathematician predicts to the astronomer the very day and the very hour at which the comet will reappear. He even indicates the very part of the heavens to which the telescope must be directed, in order to greet the wanderer on his return. When the time comes the astronomer finds that his friend has been a true prophet; there is the comet on the expected day, and in the expected constellation.

This happens again and again, so that the mathematician, with his pencil and his figures, marks stage by stage the progress of Encke through the years of his invisible voyage. At each moment he knows where the comet is situated, though utterly unable to see it.

The joint labors of the two friends having thus discovered law and order in the movements of the comet, you may judge of their dismay when on one occasion Encke disappointed them. He appeared, it is true, but then he was a little late, and he was also not in the spot where he was expected. There was nearly being a serious difference between the two friends. The astronomer accused the mathematician of having made mistakes in his figures, the mathematician retorted that the astronomer must have made some blunder in his observations. A quarrel was imminent, when finally it was suggested to interrogate Encke himself, and see whether he could offer any explanation. The mathematician employed peculiar methods that I could not explain, so I shall transform his processes into a dialogue between himself and the offending comet.

“You are late,” said he to the comet. “You have not turned up at the time I expected you, nor are you exactly in the right place; nor, indeed, for that matter, are you now moving exactly as you ought to do. In fact, you are entirely out of order, and what explanation have you to give of this irregularity?”

You see the questioner felt quite confident that there must have been some cause at work that he did not know of. Mathematicians have one great privilege; they are the only people in the world who never make any mistakes. If they knew accurately all the various influences that were at work on the comet, they could, by working out the figures, have found exactly where the comet would be placed. If the comet was not there, it is inevitable that there must have been something or other acting upon the comet, of which the mathematician was in ignorance.

The comet, like every other transgressor, immediately began to make excuses, and to shuffle off the blame on somebody else. “I was,” said Encke, “going quietly on my rounds as usual. I was following out stage by stage the track that you know so well, and I would certainly have completed my journey and have arrived here in good time and in the spot where you expected me had I been let alone, but unfortunately I was not let alone. In the course of my long travels--but at a time when you could not have seen me--I had the misfortune to come very close to a planet, of which I dare say you have heard--it is called Mercury. I did not want to interfere with Mercury; I was only anxious to hurry past and keep on my journey, but he was meddlesome, and began to pull me about, and I had a great deal of trouble to get free from him, but at last I did shake him off. I kept my pace as well as I could afterwards, but I could not make up the lost time, and consequently I am here a little late. I know I am not just where I ought to be, nor am I now moving quite as you expect me to do; the fact is, I have not yet quite recovered from the bad treatment I have experienced.”

The astronomer and the mathematician proceeded to test this story. They found out what Mercury was doing; they knew where he was at the time, and they ascertained that what the comet had said was true, and that it had come very close indeed to the planet. The astronomer was quite satisfied, and was proposing to turn to some other matter, when the mathematician said:--

“Tarry a moment, my friend. It is the part of a wise man to extract special benefit from mishaps and disasters. Let us see whether the tribulations of poor Encke cannot be made to afford some very valuable information. We expected to find Encke here. Well, he is not here--he is there, a little way off. Let us measure the distance between the place where Encke is, and the place where he ought to have been.”

This the astronomer did. “Well,” he said, “what will this tell you? It merely expresses the amount of delinquency on the part of Encke.”

“No doubt,” said the mathematician, “that is so; but we must remember that the delinquency, as you call it, was caused by Mercury. The bigger and the heavier Mercury was, the greater would be his power of doing mischief, the more would he have troubled poor Encke, and the larger would be the derangement of the comet in consequence of the unfortunate incident. We have measured how much Encke has actually been led astray. Had Mercury been heavier than he is, that distance would have been larger; and if Mercury had been lighter than he is, you would not, of course, have found so large an error in the comet.”

We may illustrate what is meant in this way. A steamer sails from Liverpool to New York, and in favorable circumstances the voyage across the Atlantic should be accomplished within a week. But supposing that in the middle of the ocean a storm is encountered, by which the ship is driven from her course. She will, of course, be delayed, and her voyage will be lengthened. A trifling storm, perhaps, she will not mind, but a heavy storm might delay her six hours; a still greater storm might keep her back half a day; while cases are not infrequent in which the delay has amounted to one day, or two days, or even more.

The delay which the ship has experienced may be taken as a measure of the vehemence of the storm. I am not supposing that her machinery has broken down; of course, that sometimes happens at sea, as do calamities of a far more tragic nature. I am merely supposing the ship to be exposed to very heavy weather, from which she emerges just as sound as she was when the storm began. In such cases as this we may reasonably measure the intensity of the storm by the number of hours’ delay to which the passengers were subjected. “The weather we had was much worse than the weather you had,” one traveller may say to another. “Our ship was two days late, while you escaped with a loss of one day.”