The Planet Mars, as drawn by N. E. Green.
(See Notes to Illustrations)
LECTURE IV
VISITS TO THE MOON AND PLANETS
You may perhaps think that we take a long time getting started, seeing that we have spent three lectures out of six in discussing where we are to start from, in what we are to travel, how far we are to go, and so on. But I find that this is the method approved by my great predecessors as celestial guides. Jules Verne, you find, takes a whole book talking about the start; how the big gun is to be built capable of shooting people to the Moon; how, moreover, a big telescope is provided to watch them when they go; and it is only at the end of the first book that they really get started.
However, here we are now started; and if we are not exactly arrived on Mars, we are as near Mars as the telescope will take us; let us say about a thousand times nearer than our eyes can take us. I told you that the telescope would magnify as much as you like, but that you might not be satisfied with the result if you magnified too much. Roughly speaking, the best telescopes we have will magnify about a thousand times, so that we reduce the distance of Mars to about one thousandth of the actual distance, let us say 50,000 or 100,000 miles; and that is as near as the telescope will take us. One peculiarity of a telescope is that it usually turns things upside down, so that the South appears at the top. The white patch at the top of the picture is therefore not the North polar cap but the South polar cap. I wonder whether any one is at present making a South Polar Expedition in Mars? It is claimed that the white patch represents ice and snow, such as we have' at the poles of our Earth; others, however, question whether it can be ice and snow, because certain observations made with the spectroscope render it a little doubtful whether there is sufficient water in the air of Mars to make ice and snow at the Poles. One thing we know with certainty; that in the Martian Summer the cap diminishes in size as though it were ice and snow melting, and in the Winter it increases in size as though it were ice and snow forming. But, after all, other things besides water may give this appearance. At the end of the lecture, when we come to an experiment or two, we shall see that liquid air may give a beautiful snowy appearance, and it is possible that the Martian polar caps are not made of ice and snow but of liquid air or frozen carbonic acid gas, or some other gas which on the very cold surface of Mars becomes liquid or even solid.
There are other markings on the picture, but what you will probably look for are the famous canals of Mars. I am afraid there are none shown on this picture because Mr. N. E. Green, who made it, did not see any canals. Some people do not see them, and claim that therefore they cannot be there. One scarcely knows what to say about this claim: it is rather like calling witnesses, after others have testified that they saw a theft committed, to declare that they did not see the theft. We should probably pay more attention to the first lot who saw something and agreed among themselves about it. Nevertheless, we must be careful to listen to the others also, in case there is more in what they say than might appear at first. A jury must be very careful before they find a man guilty, and one thing they must be specially careful about is not to let their feelings of affection or dislike influence them. So well known is it that such feelings may interfere with true justice that every effort is made to have the jury composed of people who do not know the prisoner being tried, because a friend might interfere with justice in one way or an enemy in the other. Now in this case of the canals of Mars it is rather harder than usual to keep the friends and enemies off the jury. Some people are very anxious to believe that the planets are inhabited and they are inclined to jump at thing which seems like an indication of life on Mars. Others are equally anxious that our little Earth should be the only place in the whole universe with life upon it; does not this seem rather selfish? Yet a very great and unselfish man, who helped to make one of the greatest of scientific discoveries, has written a big book to try and establish this fact—that this little Earth of ours is the only place in the whole universe on which there is life. All the thousands of millions of stars in the sky may be suns like ours, and each of them may have many planets circling round it as we circle round the Sun, and yet Mr. Alfred Russel Wallace sincerely believes that there is not a sign of life on any of those thousands of millions!
You see there are very different views about Mars and whether it is inhabited; and if we wish to be a good jury we must listen to both sides carefully. But we have not time to do so in this brief hour; all we can do is to notice that there is evidence on both sides, so that we may be careful not to make up our minds too quickly. If you wish to make up your mind at all you ought to read a good many books but it is easier not to make up one's mind at all, and sometimes that is the best plan. Another plan is to get some one to make up your mind for you, and perhaps you hope that I will do it by telling you what I think. I don't feel sure that that would be a good plan in any case, but one thing makes it impossible to act upon, I have not made up my mind myself. At least I have not made it up about Mars. On the big question raised by Mr. Wallace, whether our little Earth is the only place for life, I have made it up; I don't think for a moment that it is. I have read his book carefully, but cannot see that he makes out a case for so strange an idea. That there is plenty of life elsewhere seems to me practically certain; but whether Mr. Lowell has really detected signs of it on Mars is more doubtful. Of one thing, however, there is no doubt; he has certainly written some most fascinating books, which will teach you much and tell you a story at the same time; I heartily advise you to read some or all of them. The kind of story he has to tell is this: Mars has become dried up, so that it has no great oceans as we have, making rains which fertilize the ground so that plants may grow; there is very little water indeed on Mars. Every one agrees to that. Mr. Lowell, however, says that there is some water, mostly frozen up in the polar caps; when these melt at the edges with the coming of Spring and Summer, the inhabitants draw this water off quickly to the thirsty land in long sluices; vegetation immediately springs up on the banks of these sluices, making a wide belt of verdure. Something of the kind may be seen in Egypt when the Nile floods: the surrounding country, previously brown and bare, becomes covered with crops. An observer in the Moon or in Mars might not see the thin river Nile in the dry season, but after the flood, when the whole width of the country is green, he would scarcely fail to notice it. So with Mars: the "canals" disappear at times; then, Mr. Lowell says, there is little or no water in them and the surrounding country is bare and dry. The polar cap melts a little, the water is drawn into the sluices, the crops spring up on the banks, bordering the narrow sluice with a margin wide enough for our telescopes on Earth to see, and they show us a "canal." From the straightness and arrangement of the sluices thus indicated Mr. Lowell infers that the 7 were made by intelligent beings in desperate need of water; so that he considers the "canals" a proof of life in Mars.
It may surprise you that, if there is really any chance of there being wide tracts of cultivated land, there should be any difference of opinion on the subject. Why can some people see them and others not? I can easily show you one reason. You see here a clear picture on the screen, but you seldom see a clear picture in a telescope because of air currents. By a little trick the lanternist will imitate some air currents for us; and now the picture instead of being clear and definite is all fuzzy, with the details blurred; then perhaps there comes over it now and then a clear moment, when you glimpse it clearly; but almost at once it blurs again, and you wonder whether perhaps after all you really did get that glimpse or whether it was a mistake. You wait for another clear moment, and get the same glimpse; that reassures you that you did see it; but the best way of removing doubt is for some one else to see the same thing. That is the way in which telescope observations are rendered difficult in a bad climate; and that is why people go up high mountains, in order to see whether they cannot reduce the air currents. Even when, like Mr. Percival Lowell, they build a big telescope in a specially good climate to study Mars, they can only grasp a few details at a time; and two pictures made at the same time by different people may be very different. Fig. 37 shows two drawings made by Mr. Percival Lowell and his assistant on August 14, showing about what they could see at that time; and you see that although they have tried as faithfully as they could to put down what they saw, Mr. Lowell saw a straight canal nearly due North and South, while his assistant saw it slope to the East. When there is difference of opinion of this kind the only way is to make vast numbers of drawings, throwing away what only one man sees, and keeping what everybody sees;
Fig. 37.
in this way only can they really make a trustworthy chart of Mars. You may say: Why not photograph the planet? But photographing is even less satisfactory because the camera cannot always seize good moments as the eye can; it often photographs at the wrong moment. By taking many photographs, however, one after another, we can pick out the good ones; this plan has only recently been introduced, but it has given us already some good photographs of Mars. Fig. 38 shows three taken by Professor Barnard on which you see the polar caps clearly. But, after all, we shall much better understand the difficulty of coming to conclusions about the planets if now we have got to Mars we take the big telescope that has carried us there and turn it on to the Earth;
Fig. 38.—Mars Photographed with the 40-in. Lens of the Yerkes Observatory, 1909.
and let us try what we can see of our Earth from that distance. I propose to arm you with the best telescope in our possession,
Fig. 39.
and to ask you to look with it at the Earth. Of course I must provide you artificially with the view you will get; and Fig. 39 provides two views of our Earth, differing in some respects, but both of them better views than you would have any right to expect if you were really looking from Mars, even with our very best telescope. You see the outline of Africa and other well-known outlines; these need not be regarded very closely; they are the same in both pictures. The only difference is in the British Isles. To make these two pictures, a globe was photographed twice, in as nearly the same conditions as possible, with one exception: the British Isles were cut out in paper, in the first case in their usual shape, in the second case
in very much altered shape.
Fig. 40.
You will see the alterations in a moment when I put both pictures on the screen; but first I want you to try whether you can see them from Mars. If such alterations were made in England, could the Martians armed with our best telescope detect them at all? Well, you are practically in that position; what do you say? I fancy you cannot detect much difference. Now let us look at Fig. 40 on a scale which will show us their real differences; you see that our well-known wavy coastline has been made straight, various islands, such as Anglesea and the Hebrides have been swept away, the Thames and Severn have given place to two straight canals, and there is Home Rule in Ireland! Of all this you as Martians could detect nothing, even through the best telescope. How, then, are we to find signs of life in Mars? You will perhaps agree with me that it is a very difficult quest.
Fig. 41.—Jupiter, drawn by Mr. Scriven Bolton.
(See Notes to Illustrations.)
With other planets we can see less still, either because they are too near the Sun, like Mercury and Venus, or because they are even further away than Mars, like Jupiter and others. Here are two pictures of Jupiter showing the "red spot" (see Fig. 41). We learn from it that Jupiter rotates on its axis as the Earth does, because the red spot goes round: it crosses the disc, disappears on the other side, and then reappears again. It comes back to the same place pretty regularly after about 10 hours, and other spots on Jupiter, not so large as the red spot but quite noticeable, also return to their places in about 10 hours; so that this must be about the length of time in which Jupiter rotates. But now comes a curious thing, quite different from what we should expect. If the Martians are looking at the Earth, they see the markings on the surface go round to the other side and then reappear again in exactly 24 hours. Everything comes round in precisely the same time, England and Ireland and China and Australia, the mountains, and the seas, all keep their places on the Earth, just as they do on a globe of the Earth which I can turn round. If they did not keep their places we could not learn geography, which would be a terrible distress to us all. But when we look at Jupiter the spots do not keep their places: some of them come round faster than others, so that we cannot make a map which will do for more than a very little time. It is as though the Martians noticed the positions, not of England or China or Australia, but of the ships crossing our oceans. They are scarcely likely to do this unless they have very much more powerful telescopes than ours, because you have seen how very small even a whole country like England would appear to them, and a ship would be far too small. But we keep building ships bigger and bigger, and perhaps the Martians might see them at last; and if they watched carefully they would find that after 24 hours exactly the ship was not quite in the same place on the Earth's surface. Shall we, then, say that the spots we see on Jupiter are ships moving on its oceans? It seems more probable that they are cloud effects of some kind; all we know is that we are not looking at solidly arranged countries like our Earth; and if we want to know what we are looking at, we have some choice of opinion. Most astronomers would say the spots are cloud effects high above the real surface of Jupiter, which is hidden from view.
Another thing I would like you to realize about Jupiter is his vast size. Here is a model of Jupiter on the scale of 10,000 miles to the inch, and the model is about 9 inches across, so that Jupiter himself must be 90,000 miles from side to side; and now look at our Earth less than an inch across, and our little Moon less than a quarter of an inch. How small they are compared with Jupiter! Why, to go round Jupiter is a longer voyage than from the Earth to the Moon!
Another big planet is Saturn, which I hope you recognize by his ring. In this model we have had to imitate the ring in solid cardboard, but if the real ring were solid like this, it would soon fall on to Saturn, as a great mathematician (Clerk Maxwell) showed by calculation. He went on to show that the ring must be made up of millions of little tiny satellites all separate from one another and revolving round Saturn at different rates. They are, however, so close together that we cannot see them separate even with our best telescopes; consequently we do not see that one travels faster than another, as we can see one of Jupiter's spots catch up and pass another; at any rate we cannot see this in the ordinary way; but by a wonderful property of light, the spectroscope enables us to see it in another way of which we will say more in another lecture. We are able to measure the speed with which a shining body is coming towards us or going away from us; and we find that though we cannot see the satellites separate, their different speeds declare themselves in the spectroscope, just as Clerk Maxwell said (see p. 275). One more point about the ring: it does not always appear to us in the same aspect; sometimes it is open and sometimes it closes up until we see it edgeways, and then it disappears altogether. This shows how very thin it must be, and we can infer that the little satellites composing it must be very small indeed. You might say they need only be very flat; but it is hardly likely that they would keep their flatness always in the plane of the ring when they are jostling round at different rates. It seems most reasonable to think that they are small in all ways.
Now please look at this diagram showing the distances of the planets from the Sun. We can only imitate them on a very small scale, of course. You remember that our Earth is about 93 million miles from the Sun; let us call this 10 inches or 10 millimetres or whatever we like. Then the other distances can be written down as follows
| Bode's Law | Distance | ||||
| Mercury | 4 | + | 0 | = | 4 |
| Venus | 4 | + | 3 | = | 7 |
| Earth | 4 | + | 6 | = | 10 |
| Mars | 4 | + | 12 | = | 16 |
| (Minor Planets) | 4 | + | 24 | = | (28) |
| Jupiter | 4 | + | 48 | = | 52 |
| Saturn | 4 | + | 96 | = | 100 |
| Uranus | 4 | + | 192 | = | 196 |
| Neptune | 4 | + | 384 | = | 388 |
These are not the actual distances, but they are so close to them that the extra convenience of being able to remember them or to write them down out-weighs the disadvantage of inaccuracy for many purposes.
The law which helps us to remember them was first stated by a man called Titius, and we ought to call it by his name if every one had his rights; but Bode made a sensational use of the law, and so it is generally known as Bode's Law. You can easily see what it amounts to: write down a set of 4's,
Saturn: photographed by Barnard in 1911, with the 5-foot Reflector on Mount Wilson.
and add to them other numbers which are doubled every time, beginning with 3 for Venus. The easiest way to remember it is to try and remember that the Earth's distance comes out 10; and perhaps also that Saturn is 100; from these two facts you could recover the law if you had forgotten it.
Let us spend a moment or two on the use made of the law by Bode, about the end of the eighteenth century. At that time there was a break in the series, for none of the minor planets had been found. Nowadays we know nearly 1000 of these little bodies; many of them are very tiny, only a few miles in diameter. They are probably made of the same kind of stuff as our Earth, and reflect the Sun's light as the Earth and the other planets do; but being so tiny they cannot reflect much of it and are consequently very hard to see. Most of the brightest of them have probably been found by this time, but every year more and more faint ones are found, so that we scarcely know whether there is any limit to their number. As regards distance from the Sun, they differ considerably among themselves, but with one or two rare exceptions they agree to keep within the gap between Mars and Jupiter. Now in Bode's time this was a real gap since none of the minor planets were known, and Bode thought that there must be a planet to fill the gap. The idea of a number of little planets doing duty for a big one had not at that time occurred to any one: it was merely thought that there was one missing planet, and Bode set the police on the track of the culprit. This may sound a strange statement; what he actually did was to get a number of astronomers to agree among themselves to watch different parts of the sky for the missing planet; but the little band of searchers were jokingly called the "astronomical police," so that my statement is not far wrong. It must have been mortifying to them when the culprit was first seen by an outsider (sometimes that happens to our earthly police in spite of their vigilance). Another astronomer called Piazzi, who was not looking for the planet at all, happened to find it; he watched it for a few nights to be sure of it, and sent a note of its position to a brother astronomer—one of the "police," but the post travelled very slowly in those days when there were no railways, so that it was a long time before the other astronomer could look for the planet; Piazzi himself had fallen ill, and the planet, seizing the opportunity (almost like an actual thief with the crowd after him), dodged into the Sun and was lost to view. Perhaps you will not understand rightly what I mean by dodging into the Sun. You know how the Sun goes the round of the Zodiac: The Ram, the Bull, the Heavenly Twins, and so on (have you learnt that verse thoroughly?). As it visits them, each in turn disappears from view in the glare of daylight so that you cannot see the Ram in April, or the Bull in May. Indeed, not only one, but two or three of them are invisible at any given time of year. Now a planet moves about among the constellations: sometimes it is in one that is visible and sometimes in one that is lost in the Sun's glare; and Ceres, as the first-discovered minor planet was called, passed from one to the other very soon after the discovery. It seemed as though she had been found only to be lost again. You may say, could they, not wait till she came out and catch her again? Yes, but sometimes it is very hard to tell when and where a burglar will come out if once you let him disappear. He may get on to the roof and crawl along to other houses, down through them and out at some back door, when the police are watching all the time in the wrong place. A planet is not quite so clever and erratic as a burglar; indeed, if we have watched his or her movements long enough, we can calculate almost exactly where he or she will be at a future time. That is what Kepler and Newton have done for us by finding out the great Law of Gravity which controls the movements. But everything depends on that if; if we have watched her long enough. Ceres had only been watched for a very little time before Piazzi fell ill, and those who were accustomed to such calculations said it was not nearly long enough to enable the orbit to be calculated. You will understand the difficulty perhaps from Figure 42: if you have a large portion ABC of a circle given, it is pretty easy to find the centre O and draw the rest of the curve (dotted) from A to C. If the given part is much smaller, like DE, it is harder to find the centre P and draw the rest,
Fig. 42.
though it can still be done. But when there is only a tiny bit like GH given you, you scarcely know whether the circle should come out like GHK or like GHL: you may be watching for the burglar to come out at K when he is really at L. That was the difficulty about Ceres; she had only shown a very small bit of her circle before disappearing. Look at the minute spaces on your watch—there are 60 of them in the complete circle. If you had one of them drawn on a piece of paper, and one only, and tried from that to draw the whole watch dial, you would find it pretty hard; and Ceres did not even give the astronomers a whole space—she gave them less than half a space! You will agree with me, I feel sure, that the man who showed them how to draw the whole dial in such circumstances was a remarkably clever man—a wonderful mathematician. His great book, in which he explained the method, his Theoria Motus, is one of those books which are likely to last as long as the world does. The little planet was found again, and to the astonishment of the world, another was also found in the search: and then two more—four burglars when the police had never suspected more than one! However, they were all securely handcuffed and tethered—that is to say, the mathematicians calculated their orbits very carefully, so as to know exactly where to find them when wanted—and the world settled down again after the excitement of the chase. I forgot to tell you that Ceres was first seen on the very first day of the nineteenth century, January 1, 1801; and the other three, Vesta, Pallas and Juno, were all found by March 1807. After that no more were found for forty years, and Hencke, who found the next, had been looking for fifteen years before he found it. Is that not strange when, as we know now, there are hundreds of these little planets in the sky? It shows the difficulties of finding them in the days before photography; since we have been able to take photographs, discovery is much easier; astronomers merely take a photograph of part of the sky, showing all the stars as fixed points, but if one of them seems to have moved or "trailed" during the exposure it is probably a planet.
But we must return to the more important planets, as I feel sure you cannot afford much time to visit these tiny bits of rock. It is quite possible that they have their inhabitants on a small scale; they may be like dolls' houses, which are always most attractive; but we have to make calls at more important dwellings. The main point of our visit was to notice how the gap in Bode's Law was filled, and how it came in consequence to be called Bode's Law; and the next two big planets in the list have also something to say about filling vacant places. There were no more gaps to fill in the middle of the series, but we can have any number of places at the end. Before Uranus and Neptune were discovered the series stopped short at Saturn, with 4 + 96 = 100; and it naturally seemed to confirm the law when Uranus was found by Herschel, and its distance was seen to agree closely with the next term, 4 + 192 = 196. This had a good deal to do with Bode's formation of "the Astronomical Police."
I have already told you of Herschel' s discovery of Uranus, when we were talking about his telescope. He thought at first he had found a comet, and it was not until after some time that the true nature of the discovery became clear. Then it created an immense sensation, for never before in the memory of man had any one found a new planet. All those previously known had been known for ages; they give their names to the days of the week, Satur(n)-day, Sun-day, Mo(o)n-day are called after Saturn, the Sun, the Moon. Our English names for the other days do not remind us of the planets, but the French names (Mar-di for Mars, Mercre-di for Mercury, Jeudi for Jupiter, Vendredi for Venus) are probably known to you. We have for some reason put instead of Mars, who was the Roman god of war, the corresponding Scandinavian god of war Tuya, leading to Tuesday; and for Venus, the Roman goddess of love and beauty, we have substituted the Scandinavian Freya, leading to Friday. Perhaps as I have mentioned these days of the week I may as well explain the curious order in which they come. The ancients did not know the distances of the planets away from us or from the Sun,
Fig. 43.
but they could easily observe which of them moved most slowly and which most quickly. They put them in the order of speed, beginning with Saturn, the slowest, then Jupiter, Mars, the Sun, Venus, Mercury, and the Moon, which moves quickest of all (see Fig. 43). The old astrologers then assigned each planet influence for an hour; Saturn started with the first hour of his day, followed by Jupiter for the second hour, Mars for the third, and so on. When all seven planets had been used up, Saturn's turn came round again, and in the first 21 hours of the 24 all the planets would get 3 turns each. Then Saturn, Jupiter, Mars would finish up the day, and the Sun's turn would come in the first hour of the next day. In Fig. 43 let us show that the Sun follows Saturn by joining up Saturn to the Sun by a line, missing two planets (Jupiter and Mars). Then to find what planet will rule the next day we miss Venus and Mercury and join to the Moon: miss two and join to Mars, and so on. If you follow round the star in the direction of the arrows you will find the days of the week in the right order, Saturnday, Sunday, Moonday, Mardi, Mercredi, Jeudi, Vendredi. I have begun with Saturday to make the explanation simpler; but you can see that one may begin anywhere; and the Sun is so important that they began the week with him.
Of course, when Uranus was found they could not upset so well-established an order as the days of the week to make room for him in it; and a very good thing it is they did not try, because there would have been another upset half-a-century later when Neptune was found.
That was a discovery even more remarkable than the discovery of Uranus, because it was made by calculation. We have already said that when a planet has been watched long enough, we can calculate just where it will be in the future; we must take into account, not only the attraction of the Sun, but that of other planets as well, since they are all pulling at one another. Though this is hard and toilsome work it can be done, and was done for the planet Uranus when it had been watched long enough. But the curious thing was that Uranus did not follow the track calculated for it, and ultimately two very clever men, Adams, an Englishman, and Leverrier, a Frenchman, calculated that there must be a yet undiscovered planet pulling it out of the place; they independently calculated whereabouts this unknown planet must be, and in that position it was actually found.
Fig. 44.
One thing I would like you to realize is the very small indication they had to go upon. You know stories of scouts who have been able to track their prey by small indications, a bent twig here, a footprint there—indications which our untrained eyes would pass unnoticed? Well, the indications left by Uranus in his journey were small and faint like those, so that clever scouts were required to read them properly. Suppose Uranus travelling in the circular path at the top of Fig. 44; let us put the real Uranus and the Uranus-as-it-might-be (undisturbed) both in their places; then you could not see the difference between them at all. More than that: suppose you magnify that circle on the screen 50 times, so that it becomes a big circle nearly the size of the room. We shall only have space on the screen for a little bit of it, which looks almost straight, owing to the slow curvature. Even now you could not distinguish the real Uranus from the theoretical one in the calculated place. The disturbance can only be seen when that again is magnified 50 times; then you can just see the difference between the real and expected places. That small difference in the picture, diminished 2500 times, was all that Adams and Leverrier had to go upon, but from that tiny discrepancy they were able to infer the existence of the mighty planet Neptune.
Another point worthy of notice is that Bode's Law had by this time taken so firm a place in men's minds that both Adams and Leverrier used it to help them in their calculations. This is scarcely surprising when we remember that, first of all, Uranus had been found to fit in with the law; and that, secondly, the gap had been filled by the minor planets. But unfortunately this law, which they thought would be a help, was only a hindrance, for it no longer held. You may at some time or other have been going down a dark staircase, perhaps in some old tower, and the steps have been so even for a long time that you think you know just how far down to put your foot for the next, when suddenly there comes a short step or an extra long step which gives you quite a shock. It is often like that in scientific work: you think you have found out some law which will enable you to set your foot confidently on the next step, but owing to some unknown cause the next step is of a different length and you get a shock. Sometimes you can find out the reason for the exception, which soon leads to a discovery; sometimes the reason is not found for a long time. We do not yet know why the steps in Bode's Law are so nearly regular up to Uranus and why there is then a short step to Neptune, but so it is; and both Adams and Leverrier, who confidently put out their feet for a step of the usual length, got a jar in consequence. It was not enough to make them miss the step altogether; in other words, they found Neptune all right; but the stumble was so obvious that it excited many remarks. Some people even went so far as to say that they had not found Neptune at all, but that the discovery was made by accident! It would take too long to explain the full meaning of this criticism; you may like to read all about it some day, especially if you like mathematics. But before leaving the story of this great discovery I should like just to tell you how it came to be connected with the names of two different people.
J. C. Adams was quite a young man who had just taken his B.A. degree at Cambridge when he carried out his resolution of calculating where the planet must be that was disturbing Uranus. He finished the calculations, and took them to the Professor of Astronomy in Cambridge. Unfortunately that particular professor did not happen to be very clever; sometimes there are professors who are not very clever, or are lazy, though you might not believe it, and it is hard luck on the students. However, this professor had enough sense to suggest that Adams should get help from some one else, and he recommended him to go to the Astronomer Royal (Airy) at Greenwich, and then he thought that he had done everything that could be reasonably expected of him and went peacefully to sleep. At any rate he seems never to have enquired whether Adams went to Greenwich, or what happened to him there. He did go to Greenwich, but the Astronomer Royal was away in London on Government business; Adams called again later, but he was at dinner, and his faithful servants would not disturb him. The poor young man was getting a little discouraged, but he left a note of his results for the Astronomer Royal to look at after dinner. "According to my calculations," it read, "the observed irregularities in the movements of the planet Uranus may be accounted for by supposing the existence of an exterior body, the orbit of which is as follows." This note has been preserved and bears the date "October 1845" in the hand-writing of the Astronomer Royal; for Adams himself put no date at all on this most important document, and Airy must have supplied it later on when he had already forgotten the exact day; so that we may judge he did not pay it very much attention at the time. But he did reply to Adams, asking him what he regarded as a test question. Adams got the impression that his careful calculations were mistrusted, and was so disappointed and heartbroken after all his work that he did not reply; and the whole incident dropped. It was the very greatest pity; a little more persistence on the part of any of these three men would have almost certainly led to the discovery of Neptune at once. Some one like Halley was wanted to keep them up to the mark; but no successor to Halley appeared, and the great chance was lost.
Meanwhile Leverrier had begun work on the same problem. He was already a famous astronomer and published his results in a series of famous papers, ending up by pointing to the place near which Neptune must be.
The statue of Leverrier at the Paris Observatory.
Now he pointed to very nearly the same place in the sky as Adams; as Airy found on comparing Leverrier's paper with the note he had received from Adams nearly a year before; and this made Airy think there must be enough chance of finding the planet to be worth trying. So he woke up the professor at Cambridge and told him to begin searching. The professor set to work, but was still half asleep, so that though he looked straight at Neptune more than once, he did not recognize it. Meanwhile Leverrier had set Galle, a German astronomer, to work (it is very curious how all these people set other folk to work instead of looking themselves, as they might easily have done), and Galle found the planet on the first night! You can well imagine that there was then a great fuss as to whose planet it really was; everybody blamed somebody else, except Adams, who never blamed any one but himself for being too shy and easily discouraged. We need not follow the story further now than to say that it was at last honourably agreed to share the merit of the discovery equally between Adams and Leverrier.
But I scarcely think we English have done enough public honour to the part played by Adams; there is indeed a plaque of his head in Westminster Abbey; but in the centre of the courtyard of the Paris Observatory there is a fine big statue of Leverrier, with head erect, pointing with his finger at a globe representing Neptune. "They order this matter better in France."
We must now turn from the planets to their satellites. In one of the books which I mentioned at the outset, Mr. Griffith's Honeymoon in Space, it is suggested that instead of landing on Jupiter, which may still be so hot as to burn our feet, we should land on one of his satellites. We are not yet sure how many he has, but we know that he has at least eight.[1] Four of them can easily be seen with quite a small telescope, and were seen by Galileo with the first telescope ever turned to the skies. The other four are very faint and were not discovered until recently. Just as Jupiter is so much bigger than our Earth, so is his system of satellites on a much grander scale than ours. Our model of the little Earth has our single Moon at a distance of 2 feet; but though one or two of Jupiter's satellites are about as near as this, others would be outside this lecture hall on the same scale, and one of them half-way down Albemarle Street!
Saturn has also a number of separate satellites besides the great crowd of little tiny ones which make up the ring. We know of nine or ten already (the tenth has been announced, but has not yet been satisfactorily identified), and they also are scattered to great distances from the planet himself.
If we chose to land on one of these satellites we should be liable to an experience which is quite unfamiliar on our Earth—that of a long eclipse of the Sun, when day would be turned into night. It is just possible to have a total eclipse of the Sun even on our Earth, but only for a few minutes. The Moon is not large enough to cover up the Sun for very long. Perhaps you have read an exciting book called King Solomon's Mines? When Sir Rider Haggard first wrote it, he introduced a total eclipse of the Sun which was quite impossibly long; it lasted (if I reremember rightly) several hours, and the darkness was so great that the people could only grope their way about, being quite unable to see "their hands before their faces." Somebody must have told him this was all quite a mistake, because in later editions of the book it was cut out. It has been my duty to observe a number of total eclipses of the Sun, but they never last more than a few minutes, and the darkness is not so great but that one can read a watch face. However, if we landed on one of Jupiter's satellites, we could see such an eclipse as Sir Rider Haggard describes; for instead of having only our little Moon to act as a screen between us and the Sun (see diagram on p. 232), we should have great big Jupiter, and he would cut off the light for a long time. We can imitate what would happen by using a model. The Sun's light is represented by a beam from the electric lantern, which lights up one side of the model and leaves the other dark. On the bright side it is day on Jupiter, on the dark side it is night. But the darkness does not merely affect the surface; it streams away in a cylinder or cone of shadow. Now I will take a billiard ball to represent a satellite, holding it by a string. While it is anywhere outside this cone of shadow it is illuminated by the beam much as Jupiter itself is; one side of it is bright and has daytime, the other side is dark and is having night time. But as I circulate it round Jupiter, it comes into this cone of shadow; the light of the beam is cut off, and there is a total eclipse, which lasts all the time the satellite is passing through the shadow until it emerges on the other side. Some of you may not be able to see this emergence because Jupiter blocks your view; but those in a different part of the room can see it quite well, and so could the others if they changed their position. In looking at the real Jupiter, we on Earth are constantly changing our position as the Earth travels round the Sun; hence we can sometimes see the emergence and sometimes Jupiter blocks the way. You see the two kinds of blocking: he blocks the sunlight in one case, to make the eclipse; but he may also block the view from the Earth, if the Earth lies nearly in the same direction as the Sun.
There are two different cases in which the Earth may lie nearly in this direction; it may be on the near side of the Sun or on the further side; and because of the difference between the two cases a very important discovery was made, viz. that light takes time to travel. We know that sound takes time to travel because of echoes; we shout "Jack!" and after 'a perceptible interval a faint shout "Jack!" comes back to us from a distant wall or hill. If we measure the interval between shout and echo, and also measure the distance of the wall from us, we can find how quickly sound travels—about 1100 feet per second. Perhaps you have recognized this fact in another way; when there is a thunderstorm, we first see the lightning and we hear the thunder later. If we count the number of seconds between the flash and the first clap of thunder, and multiply by 1100 we can find how many feet away the flash took place. (As regards the rest of the thunder I suppose it is made up of echoes, partly at any rate.)
Now, has it ever occurred to you that the strike of a church clock never gives you the exact time unless you are close to it? If you are 1100 feet away you will not hear the strike until a whole second after it has occurred; if you are a mile away, you will hear the clock nearly 5 seconds wrong! Many people set their watches by a big clock striking without thinking of this: perhaps 5 seconds is not very important to them. But now suppose a man had two houses, his dwelling and his office, let us say, one three miles away from the city clock, and the other only two miles away; and suppose his watch and the city clock were both keeping perfect time. Nevertheless when he was at home he would always think his watch was 15 seconds too fast, and when at his office he would think it was only 10 seconds too fast. The first time he noticed the difference, he might think his watch had gone wrong; but if he went on noticing he would soon find out that the watch was all right and that the discrepancy must be due to something else; and he would perhaps find out the real reason, namely, that the extra mile between house and office made the difference of 5 seconds because sound took that time to travel one mile.
In this kind of way it was found that light takes time to travel. For the striking clock we substitute an eclipse of one of Jupiter's satellites as seen from our Earth. For the dwelling and the office we substitute the two positions of the Earth, on opposite sides of the Sun, one far from the eclipse, the other nearer. The great Danish astronomer Roemer calculated times for the eclipses (which we may regard as his watch); and when he compared them with the observed times (which we may regard as the clock) he got a regular difference according as he was on the hither side (office) or the further side (dwelling) of the Sun, and he reasoned that light must take time to travel. Compared with sound, it travels fearfully quick, no less than 188,000 miles a second, so that you might think the difference would be very small. But you must remember that instead of the single mile between dwelling and office, we have now the enormous distance between one side of the Earth's orbit and the other, about 186,000,000 miles, or nearly one thousand times the distance travelled by light in one second. Hence Roemer found a difference of nearly 1000 seconds or about 16 minutes, and in this way the velocity of light was found for the first time.
Now, you may say, that is very interesting as a story, but you do not see any particular good can come of knowing that light takes time to travel; such a piece of knowledge is quite remote from our practical everyday life. But very often scientific discoveries of this kind lead to practical results of immense importance. The great man whose statue is in the entrance hall, Michael Faraday, made discoveries as to the behaviour of little bits of wire and glass which seemed equally unpractical; and yet they led to our electric railways, and motor-cars, and aeroplanes, which could never have existed but for Faraday's discoveries made with little bits of wire in this Royal Institution. Another great man following Faraday, James Clerk Maxwell, noticed that the velocity of light, first detected by Roemer, was the same as a measurement made with the electro-magnetic apparatus due to Faraday; and he concluded that light was electro-magnetic in its action. And then another great man, Hertz, said, "If light consists of waves, and is electro-magnetic, then we ought to be able to get electric waves," and he found out that he could; and his discoveries made wireless telegraphy possible. And so you see a practical discovery which you probably regard as one of the most wonderful of modern times can be traced back step by step to this discovery of Roemer's which seems entirely unpractical!
But we are getting rather away from the satellites themselves, in this talk about their eclipses, and I want us to think for a few minutes what a satellite really is. Why should there be these moons, these smaller bodies revolving round the planets? And indeed why should there be planets revolving round the Sun? For the same sort of answer will do for both these questions. The planets are satellites of the Sun just as our Moon is a satellite of the Earth and Jupiter's eight moons are his satellites; in all these cases we have reason to think that parts of the central body have become detached from it to form satellites. First of all remember that all these bodies are rotating—turning round on their axes. We proved that the Earth was rotating by the pendulum experiment in the first lecture; we can see Jupiter rotating by noticing his red spot; we can see the Sun rotating by means of sunspots, as we shall remark in the next lecture. Now, when a body is rotating, the outermost parts tend to fly off, and if they can be detached they will fly off. When a boatman twirls a wet mop, drops of water fly out in all directions. There are several pretty pieces of apparatus in the stores of this Institution which illustrate this principle. In Fig. 45 a weight A slides on a wire BC. A string DEFG attaches it to a hanging weight G. When everything is at rest the hanging weight G drops to its lowest point, pulling the slider A towards the centre E. But now let us spin the apparatus about the vertical axis FK. The slider will tend to fly away from the axis and will pull up the weight G, unless we make G very heavy. If G is heavy enough it will hold A near the centre; but if it is not heavy enough A will fly out.
Fig. 45.
Whether G is heavy enough depends on how fast we spin the apparatus. If we spin it slowly, G will be strong enough to hold A in close; but as we spin faster and faster, A tries harder and harder to fly out from the centre and is at last too much for G. This is very much what happens in the formation of a satellite. If the parent body is spinning slowly, its gravity or attraction (which we have represented by the weight G and the string) will hold all its parts in close together; but if for any reason the spin is made faster and faster, there will come a point at which some of the outermost portions will have a tendency to fly out too great for the gravity or attraction, and a satellite will be detached. We have still to explain why the spin should get faster and faster, and we will come back to that in a moment; but as we have the turn-table here,
Fig. 46.Fig. 47.
let us first do one or two more experiments. In Fig. 46 is a hoop, or rather a pair of hoops, which when at rest are circular, but flatten down as in Fig. 47 when they are spun, because the outer portions AB tend to fly away from the axis; you can easily see that in the second figure they are farther from the axis in the positions ab than in the first figure. The hoops are strong enough to hold together however fast we spin them in this case; but you must remember that I am not very strong and cannot spin very fast. If we set machinery to work and buzzed the hoops round at a terrific pace we could make the portions ab break away and fly off. It is only a question of speed to break even the strongest flywheel.
Fig. 48.
Here is another experiment, which shows us something else. We generally say that the satellites go round the planets, as though the planets themselves did not move. But this is not strictly true. The Earth and Moon are like a pair of partners waltzing, one partner being very much heavier than the other. The little partner almost flies round the big one, but the big one has to move a little.
Fig. 49.
Here is a big ball A and a little one B tied together (Fig. 48), and both are sliding on the same wire. If we put them at equal distances from the centre C and then spin the apparatus, the big ball immediately flies to the end a, pulling the little one with it. But if we put A at the centre (Fig. 49) and spin again, the little one flies to its end b and pulls the big one. If we are to get a proper balance we must put the big one, not at the centre, but certainly nearer than the other; and if we try once or twice we can find a position when they are just balanced, so that neither pulls the other. It is quite a nice game to balance them. Now I think we have them. The little one is doing most of the revolving, but the big one is by no means at rest, you see.
Fig. 50.
So you must remember that our big Earth is itself waltzing a little with the Moon, though the Moon does most of the dancing.
Our next spinning experiment shall be a rough imitation of the detaching of a satellite. It requires some delicate adjustment, and you owe your thanks to Mr. Green for making us this mixture of alcohol and water, so carefully made that a little oil will collect on the little table in the middle without wanting to go either up or down. Now Mr. Heath turns the table so that the oil is made to spin; and at once you see it flatten out as the hoops did. Now he turns it a little faster and—yes! there you see a drop of the oil flies off to form a satellite!
And so all we have now to consider is—why a planet should spin quicker. It was easy for us to make the oil spin quicker; all we had to do was to ask Mr. Heath to turn the handle a little quicker. But what turns the handle for a planet? Well, the simple reason is that the planet is continually shrinking with the cold, and as it shrinks it automatically spins quicker. We will illustrate that by experiment. In Fig. 50 two wooden bars, CA, CB, are loosely hinged at C. A string APB, passing between the jaws of a spring clothes-peg at P, holds the bars horizontal; and the whole is hung by a string and can be set spinning round it. To make the experiment more striking, there are two leaden weights at A and B. Now if the spring of the clothes-peg be nipped, the string is released and the ends A and B drop into the vertical position (Fig. 51). This is our way of representing the shrinking of the planet. We might make the bars CA and CB actually shorten themselves, but this is not easy to do, and so we make the weights A and B come near the axis in a different way, by dropping them downwards; the main point is that in the first position they are far out from the axis and in the second they are close to it. In the case of a planet, the shrinking may take millions of years; but to save time, we do it in a fraction of a second. Now I will spin the apparatus slowly with the bars extended. Without interfering with the spin, I now nip the clothes-peg and the weights drop, when you see that the spin immediately becomes much more rapid. Shrinking towards the axis quickens up the spin just as well as telling Mr. Heath to turn the handle quicker;
Fig. 51.
and so we get our satellites formed because the planets shrink as they cool. There is another and perhaps a better way of making this experiment, using human arms instead of wooden ones. We place the human being on a turn-table (made to turn very easily by ball-bearings) with arms extended and holding a pair of dumb-bells (Fig. 52), and then give her a gentle rotation. If she drops her arms (Fig. 53) she will at once spin much quicker.
I must now tell you about a very extraordinary discovery of quite recent times. I don't know whether you noticed that when we made a satellite by spinning the oil,
Fig. 52.Fig. 53.-->
it went sailing round its planet in the same direction as the spin; and this seems natural. We should expect a planet spinning with the arrows (Fig. 54) to have a satellite like A, revolving in the same direction and not like B, revolving in the opposite direction; and yet, to our great surprise, such satellites have been discovered. The story begins with the finding of Phœbe, the ninth satellite of Saturn, by Professor W. H. Pickering, of Harvard Observatory. He found it by examining photographs of Saturn, with the stars all round it; and as he was able to identify it on a number of such photographs taken on different days, he calculated an orbit for it so as to be able to find it again when wanted. You remember the case of the little planet Ceres, which had been observed for so short a time that astronomers despaired of finding it again, until Gauss showed them how to calculate the orbit—even from a very few days' observations? Professor Pickering was luckier than Piazzi; he had quite enough observations to calculate the orbit so he thought; but the trouble was that when he looked for the little satellite in its expected place (after the Sun had hidden it, as it hid Ceres) he could not find it anywhere! This was very puzzling indeed. Of course the little satellite might have been destroyed in the meantime by some unknown agency, but this did not seem likely; it seemed much more likely that he had made a mistake, and so indeed he had—the mistake of thinking that the satellite was travelling round Saturn in the usual way, like satellite A in Fig. 54. It is really travelling like satellite B, and when Professor Pickering persuaded himself, very reluctantly, to try this other supposition, he, to his great delight, found the little satellite in its calculated place. You may wonder why he made any supposition at all when he had actual observations to go upon; it would take too long to explain it here, but I may remind you that both Adams and Leverrier, in their calculations to find Neptune, made the supposition that Bode's Law would go on in regular steps, and they too got led astray by this supposition. Suppositions are often made in such work, in order to simplify the calculations; but it would appear from these two cases far better to do without them if we can.
Fig. 54.
This discovery of Phœbe ultimately brought about an interesting situation. All the discoveries of satellites were at first made by other nations than England or America. Galileo started by finding four satellites of Jupiter, and it was not until nine in all had been discovered that anything was done in England. Once having started, however, England had the credit of seven out of the next eight, the other one falling to America. Then America continued with two satellites of Mars in 1877, a satellite of Jupiter in 1892, and Professor Pickering's satellite of Saturn in 1899. Stimulated by this last, Professor Perrine, of the Lick Observatory, gave America the credit of two more satellites to Jupiter in 1904, making the score England 7, America 7, as you will see by the following table.
| Date. | Mars. | Jupiter. | Saturn. | Uranus. | Neptune. | ||
| To 1685 | 4 | 5 | Foreign | ||||
| 1787 | — | — | 2 | 2 | — | England | |
| 1789 | |||||||
| 1848 | — | — | 1 | — | — | America | |
| 1846 | — | — | — | 2 | 1 | England | |
| 1851 | |||||||
| 1877 | 2 | — | — | — | — | America | |
| 1892 | — | 1 | — | — | — | America | |
| 1899 | — | — | 1 | — | — | America | |
| 1904 | — | 2 | — | — | — | America | |
| 1908 | — | 1 | — | — | — | England | |
| 1914[2] | — | 1 | — | — | — | America | |
| 2 | 9 | 9 | 4 | 1 | |||
Score: Foreigners 9; England 8; America 8.
Now, who was to kick the next goal? I am glad to say that Mr. Melotte, of the Royal Observatory at Greenwich, scored for England by finding an eighth satellite to Jupiter in 1908. The Astronomer Royal has kindly lent for your inspection this beautiful model of Jupiter and his eight satellite orbits. The four found by Galileo are comparatively close to Jupiter, but the sharp eyes of Professor Barnard found one inside them—a very faint one—in 1892. The other three are well away from these; Mr. Perrine's sixth and seventh are at nearly the same distance, and may be regarded as a twin pair. Then comes this tangle of wires far outside everything else, looking like a whole lot of orbits; but it is really only one orbit—that of Mr. Melotte's eighth satellite. The fact is that this satellite is so far from Jupiter that the Sun gets quite an undue pull upon it; it is like the man in the Bible who was trying to serve two masters, the result being unsatisfactory, as you remember. But it is even more important to notice that, like Phcebe, it is going round its planet in the wrong direction. We now know of three[3] such satellites; that of Neptune has been known to go round "retrograde" for a number of years, but as it was the single exception to the usual rule no one paid much attention to it. When, however, Professor Pickering found Phœbe, and especially when this going round in the wrong direction had been such a trouble to him, he looked for some explanation; and the one he suggested he illustrated by an experiment with a gyroscope, which we will repeat here. I have a gyroscope mounted in a wooden frame which I can grip firmly with both hands, or one of you can perhaps hold it after it has been set spinning. Now you see when I turn slowly in this direction (which is the same as that in which the gyroscope is spinning) nothing much happens; but if I turn round in the opposite direction, the gyroscope turns upside down. Try it yourself: you must grip the frame firmly, because when the gyroscope turns over it gives you quite a shock and you must be careful not to drop it. The reason why it turns over may be put in this way: that it insists on spinning in the same direction in which the holder is turning. If he turns in the opposite direction, then the somersault of the gyroscope will practically make it spin in the other direction. You can verify this for yourself by turning your watch face downwards and imagining that you can see the hands through the back; you would see them moving contrary to their usual direction. People who go to the Southern hemisphere find the Sun going round the wrong way; that is because they have themselves turned upside down, pointing their feet in the direction of the heads of northern folk; and it would be the same if they remained at home, but the Earth turned a somersault.
Suppose now that the Earth did turn a somersault; the Moon would be going round us in the direction opposite to our rotation, like Phœbe does round Saturn. If, now, the Earth flung off a new moon, we should have two moons, going round in opposite directions. We explain the existence of Phœbe in this way: she is Saturn's eldest child, born at a time when he was turning on his axis in the direction which Phœbe now indicates. Subsequently he turned a somersault, and the other eight or nine children, all born after the somersault, naturally adopt the new direction. The only thing still to be explained is what made him turn the somersault, and Mr. Stratton has found a sufficient explanation of this in the action of the tides. We must therefore suppose that our tides are tending to turn our Earth over again, though the action is so slight and so slow that we cannot detect it by direct observation.
The mention of our own tides reminds us that we have shamefully neglected our own Moon. We have been visiting other planets' satellites and neglecting our own child all the time. Jules Verne and H. G. Wells were very different: they spent all their attention on the Moon. Let us look first at some pictures of the Moon's surface, which is very mountainous. Now-a-days we can get such pictures by photography very easily, but before we had this great help it took a long time to make an accurate picture. A little more than a century ago a great artist, John Russell, R.A., spent about twenty years making a careful drawing which is still preserved at the Radcliffe Observatory at Oxford; we can now get as good a picture by photography in a second or less. Perhaps some of the fine detail would not be as good, for the human eye still beats the photograph in drawing fine detail; but the more conspicuous features would be more faithfully in their exact places. A photograph is wonderfully accurate and faithful, as can be proved by taking two photographs with different telescopes—say one in Paris and one in Chicago—and comparing the results. A great English astronomer whom we have recently lost, Mr. Saunder, had the most careful measures made on two such pictures and showed that they agreed extraordinarily well. Not only that, but he mapped out the surface of the Moon so accurately that in some ways we know it better than that of our own Earth. He gave one very striking proof of the accuracy of his knowledge. The photographs sent to him all had marked on them the date and time when they were taken, but Mr. Saunder was able to say that in one case the wrong month had been given and in another case the wrong day! You can imagine that the astronomer who had taken the photographs did not like being told that he had made mistakes of that kind, and at first he was inclined to dispute it; but Mr. Saunder's case was too strong for him. Now, I want you to realize what this means: it is not as though anything were in a different place on the Moon itself from one day to another; so far as we can judge everything that we can see remains perfectly steady; otherwise Mr. Saunder could not make two different photographs fit. But on different days the Earth looks at the Moon from a slightly different angle, which can be allowed for if you know the correct date. If the wrong date is given, the wrong allowance is made and the measures will not fit those of other photographs. This was what Mr. Saunder found; but I need scarcely say that if his work had not been wonderfully accurate, he could not have found it out; and even as it was, it took him several days' hard work at his calculations, for all such work involves a great deal of arithmetic.
When we make a map of the Earth we may put in the places of the mountains without saying how high they are; but the best maps tell you the heights of all the hills. It is wonderful to think that we can also find the heights of the mountains and hills of the Moon, so that we can make not only a map, but a relief model. The Royal Astronomical Society has kindly lent us a beautiful relief model of a part of the Moon's surface, with a model of the neighbourhood of Vesuvius alongside it, so that we can compare the two. You see that on the whole the Earth's mountains are distinctly smaller than those on the Moon. One reason for this is that our mountains are being continually washed away by the rain which falls on them, so that they are smaller now-a-days than they used to be. On the Moon there is no rain, and so the mountains remain unwashed.
Part of Moon.Vesuvius and Bay of Naples.
From Plate VI of "The Moon" (Nasmyth & Carpenter)
One way of measuring the heights of a mountain in the Moon is to measure the length of the shadow it casts. Of course a shadow does not always remain the same length; as you walk past a lamp-post you can see your own shadow grow shorter and longer; but if you stay in the same spot near the lamp it will remain of the same length; or if you go back to that spot it will come back to the same length; or if any one were told where the lamp was, he could calculate the length. The lamp that casts the shadows of the lunar mountains is of course the Sun; and we can find his position at any date from the Nautical Almanac or a similar book, and then we can calculate the length of shadow if we know how high the mountain is; or, if we measure the length of shadow, we can calculate the height of the mountain. But there is one thing to be very careful about: if the plain on which the shadow falls is not quite flat we may be misled.
Fig. 55.
We can see that by a little experiment. Here is a rough model of a lunar mountain attached to a flat board, and from the lantern, which represents the Sun, we will cast a shadow on the board. But now the board is made up of two pieces hinged together, and if I slope the outer piece a little, you will see how the shadow alters in length; I can lengthen it or shorten it by a very slight inclination of the hinged piece (Figs. 55 and 56). I can, of course, see that I am sloping the board, and make due allowance; but on the Moon we do not know whether the plain is flat or not, so that when we measure heights in this way, we are liable to some uncertainties. The case is not really so bad as I have made it appear to you; I have exaggerated things for the sake of plainness. One thing which helps us is that the Sun moves about, throwing shadows in different directions and of different lengths from the same mountain; so that if the supposed plain round the base slopes upward in one place and downwards in another, the lengths of the shadows will show it.
Fig. 56.
If there are mountains in the Moon like our own mountains, are there also inhabitants like those on the Earth? At the beginning of this lecture we considered the case of Mars, and I showed you how hard it would be to see any signs of life on Mars by turning things round and supposing that we were looking at the Earth from Mars. You remember that we could not see violent changes in the British Isles even with the best telescope from that enormous distance. But the Moon is much nearer to us than Mars is: only a quarter of a million miles instead of (say) 50 million. If we use a telescope magnifying 1000 times, it is as though we were looking at things with the naked eye from 240 miles away. You see that this is still a considerable distance; you would not expect to recognize much at such a distance. Some of you have been at sea and seen a ship on the horizon; you know how small it looks when it is five or six miles away, and how close it must come before you can see the people on its decks. You can well imagine that at 240 miles you might not see the ship at all with the naked eye—even a very big ship. We have as yet been unable to see any signs of life on the Moon, but that does not mean that there is no life; still less that there has never been any life. Perhaps life may by this time have disappeared from the Moon, as there is apparently very little air and water, if any. The Moon has dried up. But that there was life on it once I firmly believe. It seems reasonably certain that bodies which have so much in common with our own Earth, such as the planets and satellites of our system (and probably of other systems, for the stars are Suns like our own and probably have planets and satellites as our Sun has), have also life on them as our Earth has. It would be strange indeed if this peculiarity were confined to a single, rather insignificant body.
But whether there are men and women like ourselves on the Moon, or ever were, is quite a different question. Let me remind you what a vast number of forms of life there are even on our own Earth. There are fishes which live in the water; there are birds which fly in the air; there are insects which crawl and there are rabbits which live underground. Mr. H. G. Wells in his fascinating book has supposed that the inhabitants of the Moon are a combination of the last two classes; he makes them large insects which live underground. Let me recommend you again to read his book, which will teach you a great deal.
Liquid Air Experiment.
(See Notes to Illustrations.)
His reason for making the inhabitants live underground is that there is certainly very little air on the Moon—some people say none at all, but Mr. Wells gives it the benefit of the doubt; he considers that there may be none on the outside for us to detect, but there may be some in the interior; and accordingly he puts the dwellings down inside the Moon, so that the people can breathe. But he allows some air for the outside, and he brings it into the story in a very interesting way, all frozen. The "first men in the Moon " land in a regular snow-drift of frozen air, due to the fact that they land on a part of the surface on which the Sun is not shining, so that it is bitterly cold—so cold as to freeze the air. Not very long ago no one had ever seen air frozen or even liquid; but you fortunate young people of to-day can have it shown to you quite easily, and we will conclude this lecture with one or two experiments showing liquid air.
Here is a regular fountain of it at the back of the room. It is frightfully cold, and though it will not hurt you to put your hand in it for a moment, you could easily burn the skin off your hands by exposing them to it for too long. If we put an india-rubber ball into liquid air it is frozen so hard that it breaks when we throw it down. Even a flower dipped into liquid air becomes quite brittle. But perhaps the prettiest effect of liquid air can be seen by pouring some into a bath of warm water (see illustration). It makes a beautiful snowy cloud over the surface of the water. Now if some of the audience will kindly take these fans and fan away that cloud, they will find underneath little frozen cakes of air, bubbling furiously. One can show that they are cakes of air by the effect on a glowing wooden pipelight; we will light such a pipelight or a big wooden match: then blow out the flame, but leave the end glowing. Now put the glowing end in the bubbles of one of the ice-floes and you see the flame burst out again. If the bubbles were water vapour this would not happen; they are really air, and the rush of air made by the rapid evaporation lights up the match again.
- ↑ A ninth satellite to Jupiter was discovered by Mr. Seth B. Nicholson in July 1914.
- ↑ The discovery of Mr. Nicholson in 1914 again brings the score level.
- ↑ Since these lectures were given, a ninth satellite to Jupiter, also going round the wrong way, has been found in America. This makes four retrograde satellites.