We have talked of the primary great difficulty of getting away from the Earth at all. But it is not quite fatal, for balloons and aeroplanes do allow us to leave the solid Earth in some measure. We have just read in the newspapers that an aeroplane has ascended to a height of nearly four miles, and the question arises whether this is as far away as we wish to go. If we have any idea of visiting the Moon, then it is not nearly far enough, for the Moon is not four, but 240,000 miles away; an aeroplane must beat the present record by a considerable margin if it is to take us to the Moon. It is not altogether easy to realize what such a distance means, even when we remember that it is about ten times round the Earth. A little time ago there was a story in Pearson's Magazine of an American who used a huge searchlight apparatus to throw an advertisement on to the Moon, "USE MOON SOAP." The picture does not suggest any great difficulty in this achievement until we remember that owing to the Moon being 240,000 miles away, each of the letters would be hundreds of miles high; and then we begin to wonder whether even the most powerful searchlight possible would send light enough to illuminate brightly such an enormous area. It seems probable

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The Wreck of the imaginary Searchlight Apparatus for advertising on the Moon
(See Notes to Illustrations.)

that it would not. But you can never tell what they may do next in America.

But now I do not want you to take this statement about the distance of the Moon on my simple word; I want you to know for yourselves how it is measured. We saw what an advantage it was to Galileo to try things for himself rather than take them on trust; and though we cannot exactly measure the Moon's distance in this room, we can imitate the method with something else. We cannot perhaps make so good an imitation as we did of Galileo's experiment, by dropping balls from the roof, but we will make as good an imitation as we can. When astronomers measure the distance of the Moon or of any other object in the sky, they use precisely the same method as surveyors use on Earth, and indeed the method which every one of us uses nearly every moment of our wakeful lives. With our two eyes we are continually trying to estimate how far away men and things are from us. At this moment, for instance, I am trying to see whether the important people are in the front rows[1] and the unimportant people at the back; and I do it really by squinting. I turn inwards my two eyes to converge on some one in the front row, and I can feel how much squint it takes: then I converge them similarly on some one in the back row and I feel that it does not take so much. This process is quite unconscious because we have all done it so many millions of times that we do it without thinking, but our muscles and nerves tell us the result just as well. From what they tell me I conclude that the people are pretty much in their right places.

Not only is this measurement of distance with our eyes generally unconscious, but it is also vague: that is to say, we seldom express the distances in feet and inches: we usually judge whether one thing is nearer than another, or farther away, or at the same distance. Thus, when I try to light a cigar, I have to use my eyes to tell me when the match is at the same distance as the end of the cigar: for if I put the match nearer I shall spoil the cigar by lighting it in the middle; and if I put the match farther away than the end I shall not get a light at all. This equality of distance is gauged by squinting with the eyes; and if you look at your father's eyes when he is lighting a short cigar, or a short stump of one, you will see him squint horribly. But if he has a very long cigar like this,[2] he will squint far less. For that reason it is not quite so easy to light a big cigar. If I could get one as long as this room, even if my arms were long enough to put the match near the end, I might not hit the end without some trouble: for the amount of squint would be so small that I could scarcely feel it, and it would be very nearly the same even if I put the match a foot away from the cigar end.

Now astronomers use the same kind of method, and meet with the same difficulties, when they measure the distance to the Moon. Instead of two eyes, they use two telescopes, one at a place we will call A, the other at a place B; and they measure the distance AB, which is called the "base," and is of great importance (we do not need to measure the distance between our two eyes, which is the "base" for ordinary life, because we are thoroughly familiar with it, which does nearly as well). The base may be a few feet long, or some miles, or many millions of miles, according to the distance to be measured: but we must know its length before we can measure the distance required. Then the two telescopes at the ends are pointed at the Moon (or other object at the distance to be measured), and we see how much they "squint." Some

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Fig. 9.

people may tell you that the proper word for this is the Greek word "parallax": but we will be content to use the good old English word "squint."

Let us try an actual experiment to see how a distance is determined by this squint. To save time we will put all the squint in one eye; that is to say, we will have one telescope (Fig. 9) not squinting at all, but pointing straightforwards from the base AB to the point C. We must measure the length of the base AB with this tape measure, and we will make it just 12 feet, putting the other telescope at B just 12 feet from A. Instead of telescopes, however, through which only one person can look at a time, we will have small searchlights which will make the direction visible to every one. You can all see when the searchlight B is pointed so as to throw its light on to C. To save us time in measuring the different distances I have had various positions of C marked off at 6 feet, 12 feet, 18 feet, and so on, from A: and you will be able to see on which of these the light falls. But I want one of my young friends to turn his back on the screen so that he cannot see where the light falls: all he can now see is the amount of "squint" marked on this card, and yet you will find that by reading the "squint" he can tell you where the light is falling. Thus I point the telescope and from the "squint" he tells you that the squint is marked 1, meaning that the distance is 12 feet: we change it and he tells you that the "squint" is 1½, that means 1½ times the base, or 18 feet; change it again and he says it is between ½ and 1; or between 6 feet and 12 feet, as we see it is. He can tell just as well what the distance is from the "squint," as we can by seeing it on the screen. And this is no conjuring trick: it is the simplest and most straightforward process: and it is the same process which the astronomer uses to find the distance of the Moon.

But before going on to the Moon, let us consider this card of "squint" a little (Fig. 10). We put 1 for a "squint" on an object just as far away as the base. In our case the base was 12 feet; but the same card would serve if the base were a mile long: when the "squint" was 1, the distance would then be a mile. The mark 2 tells us the "squint" corresponding to twice the base: 3 to 3 times the base, and so on. We can also mark ½ for half the base; or other fractions. But what I want you to note is that all the marks for distances greater than 1 fall in one half of the arc: they all fall between E and Z: in the part AE the marks are all for distances less than 1. Moreover, the distance 2 uses up a considerable part of EZ, leaving only FZ for all distances greater than 2; 3 uses up still more, leaving only GZ: and yet we have a great many still to come, 4, 5, 6, up to hundreds and millions. All these have to be crowded into an arc which gets continually smaller and smaller, so that we find it more and more difficult to make the marks or to distinguish between them. When you get home, take a card and mark it for

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Fig. 10.

yourself and see how many "squints" you can put accurately upon it. You may get up to 10, or 20 even; but you will soon find how difficult it becomes. Probably you have a watch with the 60 minute spaces marked. Well, one minute space is the "squint" mark for 10: divide it into halves and you get the "squint" mark for 20. One half must have all the marks between 10 and 20, the other must have everything above 20! When we use our eyes we have no card, but we attend to the feelings of our nerves and muscles, which can scarcely distinguish between these various "squints" at long distances. That is why it is harder to put the match to the end of a very long cigar than of a short one.

And when the astronomer tries to determine distances of objects in the sky, such as the Moon, he comes across this same difficulty, since the "squint" is always small, owing to the object being very far away. Now there is one method of getting over this difficulty which I have not yet mentioned: if we could put our eyes at the end of long horns, not like those of a snail, which project forward, but horns projecting sideways a foot or two, then for both eyes to look at the same object they would have to "squint" much more than at present (Fig. 11). This is because we have made the base much bigger:

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Fig. 11.

instead of being about 2 inches we have made it several feet. We have not heard of any actual men with eyes set on horns in this way: Sir John Maundevile related some wonderful "traveller's tales" in old days, when there was less chance of finding him out than there is now: he wrote

" of anthropophagi and men whose heads
Do grow beneath their shoulders,"

but I don't think he ever mentioned any race whose eyes grew on horns. Yet by means of an apparatus called a "range-finder" which soldiers and sailors use to find the distance of the enemy, they manage to use their eyes just as though they were on long horns.

In Fig. 12 A and B represent the real eyes of

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Fig. 12.

the observer, close together; C and D his artificial eyes wide apart. They are two lenses C and D, with mirrors behind them G and H: and these artificial eyes can be turned on the object by screws, just as our eyes can be turned by our muscles. Rays of light EC from the distant object falling on C are reflected by the little mirror behind it so as to travel along GJ: and another little mirror at J sends them into the eye A. Rays which start along FD are similarly reflected along HKB into the eye B. Hence without moving his own eyes at all, the observer measures the "squint" of the wide artificial eyes C and D by turning the screw (or both screws, if there are two). He then knows the distance of the enemy and tells his comrades for what "range" to sight their rifles. If they did not find out the "range" in this way, the bullets would either go over the heads of the enemy, or hit the ground in front of them.[3]

Now the astronomer is fully alive to this method of getting over the great difficulty. What it comes to is that we make the base as long as possible. Nature has put our eyes so close together that the base is very short: and though it suffices for estimating the distance of a cigar tip, or of people in the same room, it is too short for telling us the distance of the enemy half-a-mile away. So the range-finder is made with a much longer base and the difficulty is largely reduced. But for greater distances still, such as that of the Moon, the range-finder becomes as useless as our eyes: the base is not nearly large enough. Instead of having our artificial eyes a few feet apart, we must put them miles apart; and we may as well put them as far apart as we can, let us say, on opposite sides of the Earth (Fig. 13); and then we find that the "squint" required to set both eyes (that is to say, two telescopes at the end of the base) on the Moon is quite considerable, and by measuring it we find that the distance of the Moon is 240,000 miles, as I told you. And it was to find out this fact that our Government established the Royal observatories at Greenwich and at the Cape of Good Hope, one in the northern hemisphere and one in the southern. They are not quite as far apart as possible, because one might have been at the North Pole and one at the South Pole: but you will probably agree with

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Fig. 13.

me that the astronomers would not have been quite so comfortable in that case, and we have to sacrifice something for comfort.

This old difficulty, however, is not yet done with: it crops up again in a most tiresome way when we want to find the distance of the Sun instead of that of the Moon, because the Sun is still further away—nearly a hundred million miles: and even when our base is the biggest we can get on our Earth, the "squint" required for the Sun is very small, and the difficulty of measuring it very great. The only thing then left for the astronomer is to take advantage of special occasions when the difficulty happens to be less than usual. Such an occasion was the Transit of Venus. Perhaps my present audience has not heard so much about the Transit of Venus as their parents and grandparents did, because the last Transits occurred in 1874 and 1882, long before you were born: and the next will happen in 2004 and 2012, when all of you will be a good deal older. But the echo of the great sensations may not yet have died down, and so you may have heard of a Transit of Venus and wondered what it was.

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Fig. 14.

What concerns us is that it is simply an occasion when the difficulties of measuring the Sun's distance are less than usual. The base remains the same, because we cannot get away from the Earth: the best we can do is to put our eyes or telescopes on opposite sides of it, and indeed in all parts of it. Astronomers were scattered for the Transits of Venus to such places as the Sandwich Islands, New Zealand, and Australia, Mexico, Kerguelen Island (a very desolate spot), Egypt, and so on: and they watched Venus cross (or "transit") the Sun, and noticed the exact moment when the transit began and ended. You will scarcely want me to explain fully how this told them the Sun's distance; but I think you can see in a general way how they worked it out if you look at Fig. 14. You can see that for a telescope at A on one side of the Earth, Venus would cross the Sun's disc along the path CD; while a telescope B on the other side would see Venus travel in a different path EF; and you can see also that if the Sun were brought nearer, the two paths would not be so different: if

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Fig. 15.—An Ideal Transit of Venus.

the Sun were further away the paths would be more different. In other words, the amount of difference tells us just how far off the Sun is, if we can measure the difference accurately.

Unfortunately astronomers found, when the great events had taken place, that they could not measure the difference as accurately as they had hoped. They hoped to determine the exact second when the transit began or ended. It would have been best for them if there could have been a moment such as that labelled II in Fig. 15, when the black disc of Venus just touched the bright edge of the Sun. Before that moment, as in I, it would have been only partly on the Sun, and after it, as in III, the black disc is completely within the Sun. It would have been nice for the astronomers if between I and III there had been just a single instant like II, which they could note accurately. Then they would have found out the Sun's distance very exactly. Unfortunately it was not so, and they knew from previous Transits that it would not be so. Instead of

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Fig. 16.—The "Black Drop."

an appearance like II, they knew they would see something like Fig. 16, the black disc being not at all round, but pear-shaped. This does not mean that Venus is really of that shape, for when it is fully on the Sun we can see that it is properly round, as in Fig. 17; and we also see that it has an atmosphere like ours, through which the Sun's rays are bent. The pear-shape is, however, not caused by this atmosphere being illuminated (though this illumination causes trouble of another kind), but by the peculiar behaviour of light when a bright light is screened off. Near the edge of the screen we get peculiar appearances which go by the name of diffraction. You can see them if you look (with one eye only) at the sky through two finger-tips nearly closed together: when

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Fig. 17.—Venus and its Atmosphere.

they are very close, one seems to go out to meet the other, though you can feel that they are not yet touching. Astronomers knew that there would be these "diffraction" appearances, but they hoped to make good allowance for them by practising beforehand with models of the expected Transit which were ingeniously constructed. But they were disappointed. They knew they had been disappointed long before they collected and compared the observations made at different stations; because two people side by side using similar telescopes gave quite different times instead of the same time. Hence they knew that one of them, and perhaps both, must be wrong: and you cannot get a right result from wrong observations.

Transits of Venus only rarely occur. We have said that the next pair (they always occur in pairs like twins) will be in 2004 and 2012, and the last pair were in 1874 and 1882. The pair before that were in 1761 and 1769, and one of the people who observed the Transit of 1769 was the famous Captain Cook, who was killed by the natives of the island of Hawaii ten years afterwards. A more famous Transit still was that of 1639, because it was the first that any one ever observed. It occurred to a young clergyman named Horrox that there might be such a thing, though no one had thought of it before; and he calculated the day on which it would fall, which turned out to be Sunday. He got out his telescope at sunrise, and set it to look at the sun; for he did not quite know at what time the Transit would come. The hours passed on, and it began to get near church time. Then came rather a struggle between duty and inclination. What was he to do? Transits of Venus do not come every Sunday. Was he to go to church and take the service, or to watch Venus? Well, really he had no doubt what he should do. He lived long before Nelson, but he knew what "England expects of every man." So he went to his duty first, and took the service. Should you think that he thought of Venus sometimes? At any rate, when it was over, he flung off his surplice, and hurried back to his study, and, to his great delight, he saw Venus just coming on to the Sun. So Horrox and his friend, Crabtree, a weaver, whom he had told of the great event, were the first to see a Transit of Venus. Horrox was quite a young fellow, and I am sorry to say that he died at the age of only 24. He had done so much before 24 that some people think that if he had lived he might, perhaps, have been an even greater man than Newton.

We see that our Voyage in Space will take us over big distances; 240,000 miles to the Moon, and 93 million miles to the Sun. It hardly seems worth while in starting to bother about the first few miles. But perhaps you have noticed, when you have taken

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Horrox observing a Transit of Venus for the first time.
(See Notes to Illustrations.)

a railway journey, what a long time it takes to get out of the station; or if you take a voyage on a ship, what a business it is getting out of dock. Afterwards when the ship has started the voyage becomes rather uneventful; but at any rate the first bit is not uneventful. So I think for a few moments we might think about the air through which we must first ascend, and which corresponds to the dock from which we start for a sea voyage.

There is one property of the air that seems very remarkable to those who have never thought about it before. Here it is all around us. It does not seem to be exerting any pressure and yet it is pressing on us all with tremendous force—15 pounds to every square inch. We are all supporting that great pressure, both inside and outside. If we had not the pressure outside, I suppose we should burst; and if we had not the pressure inside we should go flat. We can easily prove there is that kind of pressure by one of those old experiments which I think every generation ought to see, though it has been performed many times, and was first done many years ago: viz. the experiment of the "Magdeburg Hemispheres." We take two cups or hemispheres that are easily pulled apart when there is air both inside and outside. But if we fit them together, and if Mr. Heath kindly connects up the exhaust and takes all the air from inside, they do not, indeed, go flat, because they are made of good solid brass, but we shall find it very difficult to pull them apart. Now the air is sufficiently exhausted, and I want two very strong men from the front row. Mr. Heath and I will stand behind them in case by some accident the air is not all out and they come apart too easily. But you see it is all out, and though these strong giants are pulling as hard as they can, they cannot separate them. Even when Mr. Heath and I help them we cannot do it. We will now let the air in; and you will see it is as easy to pull them apart as it was at first.

We need not always use an air-pump to get rid of air. Sometimes one can get the air out from between one's hands by squeezing them tightly together, and there are people who can make a fascinating noise in that way. I have here two beautiful planes, that Mr. Whitworth told us how to make; they fit one another so closely that we can exclude the air without a pump. We must not merely put them together in the ordinary way; we must slide one on to the other in order to squeeze out the air: and then you find them stick so that they are as hard to pull apart as the Magdeburg hemispheres.

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These two experiments show the enormous pressure air has. But that is only near the Earth; as we go up it gets less and less; or, perhaps, a better way to look at it is that as we come down in the air the pressure gets greater, because every layer has to support all the layers above it. Fig. 18 is a picture of a totem-post in the Oxford Museum, kindly made for me by a member of the audience and her mother. It used to be outside the tent of an Indian chief. There is a raven at the bottom, and on that a bear holding a hunter; then another bird; above that another bear hugging

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Fig. 18.Fig. 19.

a hunter. I believe the idea is that when a tribe conquers another tribe they take their totem and put their own on the top: and then again, if that tribe is conquered, the totem of the conquering tribe is again put on top. Now obviously

these animals are strong and solid, so they can bear all that weight upon them without getting squashed. But the air is not like that. The air beneath gets squashed down by the air on top. So it would be like the second picture (Fig. 19), when the raven gets squashed flat by all the weight it supports: and even the bear above it gets very much flattened. That is how our air gets treated near the Earth. But high up it gets thin like the chief on the top of the second picture, because there is nothing to squash it. The air is not of course divided into separate layers, as the totem-post is divided into separate animals, but it will do no harm to treat it as though it were, in thinking out what happens.

Let us make believe that there are different layers, as in Fig. 20, copied from a beautiful picture at the Meteorological Office at South Kensington

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Fig. 20.

(which you ought to visit some time, because you will find all sorts of interesting things there). The first layer takes us about six miles up, and may be called the layer of mountains. The highest mountain in the world (Mount Everest) reaches just about to the top, and you see how small the Eiffel Tower looks beside it. The highest aeroplane record is about four miles, so that the first layer is also that of aeroplanes.

A kite has been up a little higher than that. And a man in a balloon has actually been just into the second layer, but only just. So that our human experience of the air is practically confined to the first layer, and we knew nothing about the layers above it till quite recently. All our knowledge of them is due to the use of balloons, rather like toy balloons, though larger. There is one hung up near the roof; perhaps we can get it down at the end of the lecture. These balloons do not carry men, they only carry apparatus; but this apparatus is so skilfully made that it brings down for us information about these upper layers. We learn that the pressure gets less and less, as we expected; but we also learn something that we did not at all expect. We thought that it would get colder and colder as we went higher, but these "sounding-balloons" (ballons sondes) tell us that soon after we leave the first layer it ceases to get any colder. The figures for the temperature in Fig. 20 show that it does not get very much colder. They are measured from what we call absolute zero. You know well the ordinary thermometer which your mother puts into your mouth when she thinks you "have a temperature." If we used that we should have to alter these figures a good deal: and if your mother used the one the Meteorological Office uses, I fear she would get a great fright, for she always hopes to find your temperature below 100°, doesn't she? Yet with this thermometer she would find it over 300°, even if there was nothing the matter with you at all. But, of course, if she were warned beforehand that that was the proper temperature for you to have with this kind of thermometer, she would not be alarmed: there are such various kinds of thermometers, that before using any of them it is well to know beforehand what to expect.

Well, the great fact we have recently learnt about these upper layers of the air is that the temperature ceases to fall in a way that has been a great surprise to us. The sounding balloons have gone up higher and higher until they burst, and then the apparatus carried by them falls down to the ground. You might think it would get hopelessly smashed by such a fall (from a height of 20 miles, say), but the burst balloon case acts as a kind of parachute, or drag, so that the blow on the ground is not really so very hard, and also the apparatus is attached to an ingenious light framework called a "spider," which eases the blow still further: so that usually it is not damaged, and not only does it tell us what happened to it in the upper layers, where man has never reached, but it can be used again and again to get more news. The news has been of intense interest to scientific men, but would scarcely interest us enough to justify dwelling longer on it. Rather, let us return to the bottom layer and recall a few things which have happened in it.

Let us begin with the mountains. Many people have ascended mountains and got above the clouds. In order to get above the clouds they have put a magnificent observatory on Mt. Wilson in California, 6000 feet above the dust, which can also be seen lying below them. Astronomers have begun to use mountains not only to get above dust and clouds, but because they also get above a considerable amount of air. Professor Campbell went up Mount Whitney, sometimes called the "top of the United States," to make observations upon Mars, especially to see whether it has an atmosphere containing water vapour. He made his arduous expedition in order that he might not mistake for water vapour in Mars what really belonged to our own Earth. Having much less of our own air between him and Mars, he was less likely to make that mistake. He concluded that there could not be very much, perhaps not more than on the Moon, which we know to be very dry indeed. He lived in a desolate little hut on the top of Mount Whitney for a considerable time to make

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these observations, but usually he lives on the top of a more comfortable mountain (4000 feet high) at the Lick Observatory. I got a pretty Christmas card from him, showing the Lick Observatory in winter with the snow on it, which I am sure he would like me to share with you. The snow is, of course, only there in winter, and rarely even then; generally the Lick Observatory has a beautiful summer climate. But there are mountains, as you all know, on which there is perpetual snow, and yet climbers of mountains have gone up even higher than aeroplanes have gone. But I am sure you are much more interested in aeroplanes, because to go in one seems more like leaving the Earth behind; whilst we are on a mountain we are still touching the Earth. Aeroplanes are becoming so common that we have nearly forgotten how hard it was to invent them, and who did the early pioneer work which led to the invention. Who first thought of the name "aeroplane"? When you want to find out a thing like that, one good way is to look in the dictionary: and I hope you all look in the dictionary when you want to find out things, instead of bothering other people with questions which they cannot answer. But if one wants to be sure of getting the right answer, an even better way is to bother the man who makes the dictionary: and we are fortunate enough to have in Oxford Sir James Murray,[4] who is making a very big dictionary, and who was able to tell me a curious thing about aeroplanes. First of all, the word means a plane used for experiments on air. That is the way he has defined it in his dictionary, of which the letter A was published in 1888 (though they have not yet got to Z!). He says the word was then used in England to mean "a plane placed in the air for aerostatistical experiments." But he also says that "aeroplane" was used in France in quite a different way; plane meaning there not a plane surface at all, but a thing which soars. Those who know French will know that they use the word "planer" to mean soaring like birds. I am sure you have all heard of a "vol plané," which means a soaring flight. And these two different ways of using the word may be illustrated in this way. A piece of paper like this is a plane: and we might put it at the end of a mechanical arm, and find the amount of pressure on it when it is moved about. Several such arms or sails put together would make a toy windmill: and, although that is scarcely a piece of scientific apparatus as it is usually made, a very little alteration would make it into one. In the toy, all the sails are set at the same angle, and they blow round merrily. But suppose we set some one way and some another, so that they want to go round in opposite directions, we could balance them so that the machine would turn neither way, however strong the wind was, and now we have a piece of scientific apparatus. Lord Rayleigh made many experiments with a simple machine of this kind, like a toy windmill, of which he could put the vanes at different angles. I think there were six vanes, and he set two of them in one direction and four in the other, but at a different angle, which he chose, so that the two would balance the four, and then the machine would not go round at all. By varying these angles he learnt a great deal: and the plane sails of his apparatus were aeroplanes. But I can take the same piece of paper and make it into another kind of aeroplane—a thing which soars; you probably know how to make this kind of dart, which soars about. That is a different kind of aeroplane, like those in which people fly. Here is a beautiful little model of an aeroplane that has been kindly lent from the Grahame-White Company. It was the prize at one of the Hendon Race Meetings, I believe. And the same company have kindly lent a few slides, which I am sure you will like to see. You are regularly spoilt children, because every one seems so ready to lend things to be shown to you.

When we went over to America in 1910 to have a meeting of astronomers who study the Sun—the modern Sun worshippers—I was lucky enough to see a great aeroplane race meeting: and it was specially interesting to see Mr. Grahame White dropping chalk balls to try and hit the funnels of an imaginary ship marked out on the ground. It was quite difficult to hit the funnel from 100 feet up, when travelling at full speed, and of course an aeroplane would have to fly much higher up in time of war to avoid being shot at. So at the end of the meeting Mr. Grahame White kindly volunteered to try from a much greater height—1700 feet, I think. A hard chalk ball falling from this height might by accident hurt some one badly, so in this instance he dropped eggs, and presently a man came out with a megaphone to say what the result was; and he said something like this—


You can well believe that the crowd laughed at this odd result, and it was suggested that the eggs hatched out on the way down and flew away!

Here you see that wonderful modern achievement called looping the loop. But, with all our admiration for aeroplanes, we must not forget that in the old days balloonist s did some extraordinarily plucky things. There is a fascinating book about ballooning called Travels in the Air, by James Glaisher,[5] but I fear it is now out of print, so that your only chance is to find it in some library. It has beautiful pictures of the

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balloon travelling above the clouds, with wonderful sunrise effects. Sometimes the balloonists saw their own shadow thrown on the clouds: and on one occasion, in 1866, they saw a great shower of meteors. But the most exciting occasion was when they got nearly seven miles high and nearly died from the cold. Mr. Glaisher himself fainted: his companion, Mr. Coxwell, wished to open the valve so that the balloon might descend, but found that his hands were frostbitten: however, he managed to climb up with his elbows and knees and open it with his teeth, and then they dropped quickly to a warmer region and revived. Mr. Glaisher's object in making these ascents was to find out the temperature and pressure of the upper air, for which purpose he carried up a whole trayful of instruments: and he did find out a very great deal—practically all we knew until recent times. But since seven miles was the highest he ever went, we knew nothing at all about the air above that point. We could only guess, and we guessed quite wrong.

And now we come to the balloons that have really taught us about the upper air, about which the people who study the weather have been trying to learn for years, and have at last succeeded. First of all they tried by sending up kites of a peculiar shape: you see a fine specimen up there. They are not like ordinary kites, but are in the shape of a box almost; but they have been copied in the form of toys, and perhaps this one does not seem so strange to you as it would have to us in our childhood. These kites can be got up to great heights by attaching one to another in a series. The top kite has been up above the aeroplane record, but not so high as the man-balloon record. They do not, of course, take up men with them, but they carry a recording apparatus, including a barometer and thermometer. The thermometer is that curious spiral spring at the back. All these things write their own story on this piece of paper in front. I wonder if you can guess what is the use of that ping-pong ball tied to a thread? It tells the force of the wind, because the wind blows the ball away from the kite, so that it pulls the string, and the more it pulls the more it writes on this diagram. Then presently the kite is hauled down, and this diagram is read; and from this we find out what has been going on in the upper air. When these kites are up miles high, the pull on the wire (they use wire and not string) is tremendous, and special machines are necessary for winding the wire.

But lately it has been found better to use balloons instead of kites; balloons filled with hydrogen, which is so light that it takes them up for miles and miles before they burst. They always burst at last, because the pressure of the air outside keeps getting less and less as we go up (you remember how we illustrated that with the totem-post pictures), but the hydrogen inside does not get less, and so swells the balloon out more and more. I told you that if there were no air outside us, we might burst; and that is actually what happens to the balloon when it gets so high that the air outside it scarcely presses it at all. And then down it comes like a parachute, bringing the precious records, like those attached to the kite. But of course the kite is pulled down gently and smoothly by means of its wire, whereas the balloon-case drops from a great height, and it is important to prevent a jar on reaching the ground which might damage the instruments which make the records. They are costly, and must be used again and again if possible. Accordingly the instruments are made of very light materials; and, as already mentioned, they are attached to a light framework called a "spider," consisting of three bamboo rods tied together, at their middle points, and set each one at right angles to the other two. To keep them in this position their ends are joined up by thin strings, the whole forming a framework which is very strong for its weight, and which may fall on any part of itself without much damage. The precious apparatus is securely fixed to the very centre of this framework, so that when the balloon drops, it falls always on its feet so to speak. You know how a cat prevents its body being hurt when it falls by twisting in a remarkable way so as to fall on its springy feet? Well, we cannot provide the apparatus with the agility of a cat, so as to make its feet twist under it; but the "spider" has feet in all directions, which acts just as well. Make a "spider" for yourselves when you get home, and see how strong and light it is. Here is a miniature "spider" which Mr. Bellamy has kindly made out of three bits of straw and some thread; and it is so light that even a toy balloon will carry it up if properly filled with hydrogen. We will make the experiment. There! you see; up it goes! It cannot rise high enough to burst in the proper way because the roof stops it; but fortunately Galileo is still up there in the roof, or has gone back again, and he can arrange the bursting for us; and down come the case and spider and all! One very important thing I had nearly forgotten. The spider may fall anywhere—perhaps far away from any town or people. Who is to find it, and send it home? It stands well above the ground and can be seen from some distance, so that it is not long before some wanderer finds it; and he also finds a label attached to it, saying that if he will send the apparatus and records to the Meteorological Office he will get five shillings reward. I am sure five shillings would be very useful to many of you, so I advise you to keep a sharp look-out whenever you are in the country to see whether you can find a "spider," and get the five shillings reward.

There is one thing about the upper atmosphere which these sounding-balloons do not tell us, though if they could bring down some of it they might; and perhaps it may be arranged at some future time that they shall bring samples down with them, and then we should be able to verify what we believe to be true, viz., that certain gases, which are present in our lowest layer in very minute quantities, become much more common up there. When I was a boy, we were taught that the air consisted of oxygen, nitrogen, carbon dioxide (or carbonic acid gas, if you like that name better), and water vapour; nothing else but these four. But in the year 1894 Lord Rayleigh announced to the British Association, which was then meeting at Oxford, that he had found something else in the air—a gas so like nitrogen that he had had the utmost difficulty in separating the two, but clearly different from nitrogen when sufficient care was taken to divide them. It was a most exciting announcement, because we all thought that we knew practically all there was to be known about the nature of air, and it was a great shock to find that we were all wrong. The new gas was called argon; and soon other new gases were found, especially by Sir William Ramsay—helium, and neon, and xenon, and a heap of others; so that unfortunate school-boys, instead of having to learn only four things as making up air, as we did, have now to learn a large number of names. Life has a terrible way of getting more and more difficult!

Well, now, one of the reasons why these gases were not found before is that down here in the lowest layers they are very scarce, like rare butterflies, such as the Camberwell Beauty, which is very hard indeed to find in England. But if you go abroad, you may find Camberwell Beauties by no means uncommon. So also, if we could go "abroad" into the upper air, we might find these rare gases much more easily. And there is one thing about them that will interest you, I hope, though their names may be tiresome to learn; they give very pretty colours when we electrify them. We have a series of tubes here filled with specimens of various gases, and when we pass a current through them you will see their different beautiful colours.

There is one other way in which we learn about the upper air, and that is through those meteors, which you saw on one of the balloon slides. A meteor, or, as it is often called, a shooting star, is a bright light like a star that darts across the sky; but it has nothing to do with stars; it is only a piece of stone or metal. Here are some meteors and fragments of meteors; many of the shooting stars you see would be much less than any of these; tiny specks so small that they get burnt up entirely. What makes them shine and burn is the friction of the air as they rush into it. You have heard of producing fire by rubbing two pieces of wood together? The faster you rub the sooner you get the fire. It is not at all easy to make the fire because it is not easy to rub fast enough; but a meteor rushes into the air at a great speed, fifty miles a second at least. It seems to us a great speed when our motor-car goes at 50 miles an hour; think what it must be to go at 50 miles a second, nearly 4000 times as fast as a motor-car! I am afraid the police would not allow us to try it; but if we could increase the speed more and

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Fig. 21.

more, we should find that the air which at first gently blows us cool, was becoming a fierce wind; and presently, instead of being at all cool, it would seem hot; and long before we reached the speed of a meteor t would be unbearable.

It may seem surprising that anything so soft and thin as air can produce the same effect by friction as we get from hard wood, but we can actually prove it by experiment. Sir James Dewar devised this ingenious apparatus to do so (Fig. 21). A wooden arm AB can be spun about the end A. The end B thus rushes at a considerable speed through the air not the speed of a meteor, or anything like it, but still a sufficient speed to be heated by friction. The heating will not be very great, but I think we can detect it and even show it to you on the screen; and the method is this: we join two wires of different metals at both ends, and put one junction at the end A of the spinning bar and the other at the end B. Now it is a well-known fact that if one of these junctions be heated more than the other, an electric current will flow in the wires, which we can detect by using this galvanometer. You see that spot of light on the screen? Well, when an electric current flows it will move to right or left; and we want to know which way it moves when the end B is heated. Perhaps one of the ladies in the front row will kindly put her cheek against the end B for a moment. There goes the spot to the right, showing quite a warm cheek! (Perhaps that is better than a cool cheek?) Accordingly we know that if the end B gets warm, the spot will move to the right. Now we will buzz the arm round as quickly as we can, and there goes the spot to the right again, showing that the junction B is heated by its rushing through the air. The end A, you see, is so near the centre that it remains practically stationary.

If we could whirl the arm round ever so much more quickly we might heat the wire so much that it would shine like a meteor, and perhaps be burnt up; but you will easily understand why I cannot do this.

Some meteors, however, are too big to get burnt up. They come right through the air and bury themselves in the ground, as you remember the leaden ball buried itself when Galileo dropped it from the roof. I hope you will never have the ill-luck to be hit by a meteor, for it would hurt much worse than any bullet. Once a monk was sleeping in a tent, and a meteor went right through tent and monk and bed below him, and buried itself deep in the ground. But that is the only case I ever heard of where any one has been struck in this way, because meteors are very rare. Mr. Gregory has been kind enough to lend us his wonderful collection for you to look at, and I hope you will look carefully at them after the lecture.

Before a meteor enters our air it is travelling

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The Gross Divina Meteorite.

through the terribly cold regions of space, and is chilled to its marrow. When it rushes into our air it becomes heated as we have said; but it travels so quickly that it comes right through the air in a few seconds, and though the outside gets very hot, there is not time for the heat to get inside. The inside remains just as cold as it was before, and only a thin coating near the surface is heated. When the meteor comes to rest in the ground, the heat of the crust is soon overcome by the bitter cold of the inside; and in a very few minutes the crust which was so hot as to be shining brightly is so cold that hoar frost settles on it; and in this state meteors have often been found by those who have seen them flash down, have marked the spot where they fell, and hastened towards them as quickly as possible. Mr. Gregory has kindly cut some of his meteors so that you can see where this thin outer crust stops; you see how very little had time to get heated; all the inside remained cool.

Here is a specimen which should be of particular interest to the ladies present, because it has diamonds in it: not very large ones, but still unmistakable diamonds. Mr. Parsons (whom you all know, I hope, as the inventor of the turbine) tells me that he thinks these diamonds are made when two meteors strike one another. I have told you at what fearful speeds they travel, 50 or 100 miles a second, say; and if two of them hit, the pressure developed must be enormous, far greater than anything we can imitate on this Earth. And Mr. Parsons further tells me that he believes all diamonds are made in this way. He has himself tried many times to make diamonds without success, and he thinks it is because he cannot get pressures nearly big enough; and so he inclines to the view that all the diamonds we find have been brought to the Earth by meteors which fell on it in ages past. This makes us look with increased respect at Mr. Gregory's collection, doesn't it?

But diamonds are not the only substance meteors have brought us; shut up inside them various gases have been found. Where were these gases collected? Probably in the upper layers of our own atmosphere. You remember how we regretted that the sounding-balloons were not able to bring us down samples of the air up aloft? Well, fortunately, meteors are able to do this very thing to a limited extent. In their fierce rush they catch the gases and shut them up

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The Appley Bridge Aerolite, Oct. 13, 1914. Scale 1/3
(See Notes to Illustrations.)

inside themselves so that chemists have been able to get them out afterwards and find their nature. They have often found hydrogen in this way inside meteors, and so we learn, what can be shown to be probable in other ways, that there is a good deal of hydrogen in the uppermost layers. Unfortunately these gases often escape before the chemists can get at them; they burst through the outer covering. In some of Mr. Gregory's specimens you see the holes through which the gases have burst out before we could examine them.

In one way and another, therefore, we have learnt a good deal about the upper air lately, so that we know what we should have to pass through in leaving port when we start on our voyage. The air will get thinner and colder and change ultimately into hydrogen, and then—what after that? We shall get out into the cold and silence of space—the great Silence. Perhaps it had not occurred to you that we should lose all sensation of sound? Our ancestors thought that the heavenly bodies were making beautiful music—the "music of the spheres"; but there is nothing to carry music in space. Most sounds we hear are carried by air; though they can be carried by other things as well. Here is a wooden rod which goes down through the floor into the basement where there is a musical-box. The sound of the music is coming up this rod into this room, though only those close to the rod can hear it, even faintly. But if two of my audience will kindly lift this wooden tray on to the top of the rod, the tray will act as a resonator and the whole room will hear the musical-box through the instrumentality of the rod. (Experiment as indicated.) So that materials other than air can carry sound. But in outer space there is no material at all, not even air; so that no sounds can travel. We can realize this by another experiment with this bell, which is ringing plainly enough even when I cover it with a glass jar. But now, if we exhaust the air from the jar the sound will die away until you cannot hear it at all, though you can see that the hammer is still striking the bell as vigorously as ever. (Experiment.) And if we gradually let the air in again the sound comes back. All the apparatus for making the sound was the same throughout; only the air which carries it was removed, and the other materials round (such as the table) did not carry it sufficiently for you to hear.

Hence we must be prepared for a great silence on our voyage. When Jules Verne's travellers were shot out of the enormous cannon to the Moon, they were puzzled because they did not hear even the sound of the explosion which started them; and they reasoned it out that they travelled more quickly than sound, so that the sound was unable to catch them up. Of course, they talked to one another inside the projectile because they took air with them; but they were cut off from all outside sounds. Other projectiles might have blown foghorns louder than any steamer that ever passed us on the sea, but the warning would not be of the slightest use because it could not be heard.

It seems almost better for us not to venture into this great silence, doesn't it? Especially if I am to continue these lectures. You will not be able to hear me if we have no air, for several reasons. Consequently I shall next time propose a method of making our voyage which will avoid this difficulty, and allow us to stay comfortably in this room all the time.

  1. These lectures being "adapted to a juvenile audience," the front rows were "reserved for juveniles."
  2. A curiosity called a "Giant," purchased at Winchester.
  3. Since these lectures were delivered, the use of the periscope has become familiar to most of us. The eyes on horns could be represented by using a periscope for each eye.
  4. Sir James Murray died while this book was in the press.
  5. Bentley, London, 1871. New edition, 1880. Price 255.