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If you are not located in the United States, you'll have -to check the laws of the country where you are located before using this ebook. - -Title: Star-land - -Author: Robert Stawell Ball - -Release Date: September 18, 2019 [EBook #60318] - -Language: English - -Character set encoding: UTF-8 - -*** START OF THIS PROJECT GUTENBERG EBOOK STAR-LAND *** - - - - -Produced by deaurider, Charlie Howard, and the Online -Distributed Proofreading Team at http://www.pgdp.net (This -file was produced from images generously made available -by The Internet Archive) - - - - - - - - - -[Illustration] - - - - - STAR-LAND - - _BEING TALKS WITH YOUNG PEOPLE ABOUT THE - WONDERS OF THE HEAVENS_ - - - BY - - SIR ROBERT STAWELL BALL, F.R.S. - - LOWNDEAN PROFESSOR OF ASTRONOMY IN - THE UNIVERSITY OF CAMBRIDGE - AUTHOR OF “THE STORY OF THE HEAVENS,” ETC. - - - Illustrated - - - NEW AND REVISED EDITION - - - BOSTON, U.S.A., AND LONDON - GINN & COMPANY, PUBLISHERS - The Athenæum Press - 1899 - - - - - ENTERED AT STATIONERS’ HALL - - COPYRIGHT, 1899, BY - CASSELL & COMPANY, LIMITED - - ALL RIGHTS RESERVED - - - - - To - THOSE YOUNG FRIENDS - WHO HAVE ATTENDED MY CHRISTMAS LECTURES - THIS LITTLE BOOK - IS DEDICATED - - - - -PREFACE TO FIRST EDITION. - - -It has long been the custom at the Royal Institution of Great Britain -to provide each Christmastide a course of Lectures specially addressed -to a juvenile audience. - -On two occasions, namely, in 1881 and in 1887, the Managers entrusted -this honorable duty to me. The second course was in the main a -repetition of the first; and on my notes and recollections of both the -present little volume has been founded. - -I am indebted to my friends Rev. MAXWELL CLOSE, Mr. ARTHUR RAMBAUT, and -Dr. JOHN TODHUNTER for their kindness in reading the proofs. - - ROBERT S. BALL. - - OBSERVATORY, - CO. DUBLIN, - _Oct. 22, 1889_. - - - - -CONTENTS. - - - LECTURE I. - - THE SUN. - PAGE - - The Heat and Brightness of the Sun--Further Benefits that we - receive from the Sun--The Distance of the Sun--How Astronomers - measure the Distances of the Heavenly Bodies--The Apparent - Smallness of Distant Objects--The Shape and Size of the Sun-- - The Spots on the Sun--Appearances seen during a Total Eclipse - of the Sun--Night and Day--The Daily Rotation of the Earth-- - The Annual Motion of the Earth round the Sun--The Changes of - the Seasons--Sunshine at the North Pole 1 - - - LECTURE II. - - THE MOON. - - The Phases of our Attendant the Moon--The Size of the Moon--How - Eclipses are produced--Effect of the Moon’s Distance on its - Appearance--A Talk about Telescopes--How the Telescope aids us - in Viewing the Moon--Telescopic Views of the Lunar Scenery--On - the Origin of the Lunar Craters--The Movements of the Moon--On - the Possibility of Life in the Moon 74 - - - LECTURE III. - - THE INNER PLANETS. - - Mercury, Venus, and Mars--How to make a Drawing of our System-- - The Planet Mercury--The Planet Venus--The Transit of Venus-- - Venus as a World--The Planet Mars and his Movements--The - Ellipse--The Discoveries made by Tycho and Kepler--The - Discoveries made by Newton--The Geography of Mars--The - Satellites of Mars--How the Telescope aids in Viewing Faint - Objects--The Asteroids, or Small Planets 134 - - - LECTURE IV. - - THE GIANT PLANETS. - - Jupiter, Saturn, Uranus, Neptune--Jupiter--The Satellites of - Jupiter--Saturn--The Nature of the Rings--William Herschel-- - The Discovery of Uranus--The Satellites of Uranus--The - Discovery of Neptune 212 - - - LECTURE V. - - COMETS AND SHOOTING STARS. - - The Movements of a Comet--Encke’s Comet--The Great Comet of - Halley--How the Telegraph is used for Comets--The Parabola-- - The Materials of a Comet--Meteors--What becomes of the Shooting - Stars--Grand Meteors--The Great November Showers--Other Great - Showers--Meteorites 255 - - - LECTURE VI. - - STARS. - - We try to make a Map--The Stars are Suns--The Numbers of the - Stars--The Clusters of Stars--The Rank of the Earth as a Globe - in Space--The Distances of the Stars--The Brightness and Color - of Stars--Double Stars--How we find what the Stars are made - of--The Nebulæ--What the Nebulæ are made of--Photographing the - Nebulæ--Conclusion 318 - - - CONCLUDING CHAPTER. - - HOW TO NAME THE STARS. 381 - - - - -STAR-LAND. - - - - -LECTURE I. - -THE SUN. - - The Heat and Brightness of the Sun--Further Benefits that we receive - from the Sun--The Distance of the Sun--How Astronomers measure - the Distances of the Heavenly Bodies--The Apparent Smallness of - Distant Objects--The Shape and Size of the Sun--The Spots on the - Sun--Appearances seen during a Total Eclipse of the Sun--Night - and Day--The Daily Rotation of the Earth--The Annual Motion of - the Earth round the Sun--The Changes of the Seasons--Sunshine at - the North Pole. - - -THE HEAT AND BRIGHTNESS OF THE SUN. - -We can all feel that the sun is very hot, and we know that it is very -big and a long way off. Let us first talk about the heat from the sun. -On a cold day it is pleasant to go into a room with a good fire, and -everybody knows that the nearer we go to the fire, the more strongly -we feel the heat. The boy who is at the far end of the room may be -shivering with cold, while those close to the fire are as hot as they -find to be pleasant. If we could draw much nearer to the sun than we -actually are, we should find the heat greatly increased. Indeed, if we -went close enough, the temperature would rise so much that we could -not endure it; we should be roasted. On the other hand, we should -certainly be frozen to death if we were transported much further away -from the sun than we are now. We are able to live comfortably, because -our bodies are just arranged to suit the warmth which the sun sends to -that distance from it at which the earth is actually placed. - -Suppose you were able to endure any degree of heat, and that you had -some way of setting out on a voyage to the sun. Take with you a wax -candle, a leaden bullet, a penny, a poker, and a flint. Soon after -you have started you find the warmth from the sun increasing, and the -candle begins to get soft and melt away. Still, on you go, and you -notice that the leaden bullet gets hotter and hotter, until it becomes -too hot to touch, until at last the lead has melted, as the wax had -previously done. However, you are still a very long way from the sun, -and you have the penny, the poker, and the flint remaining. As you -approach closer to the luminary the heat is ever increasing, and at -last you notice that the penny is beginning to get red-hot; go still -nearer, and it melts away, and follows the example of the bullet and -the candle. If you still press onwards, you find that the iron poker, -which was red-hot when the penny melted, begins to get brighter and -brighter, till at last it is brilliantly white, and becomes so dazzling -that you can hardly bear to look at it; then melting commences, and the -poker is changed into liquid like the penny, the lead, and the wax. -Yet a little nearer you may carry the flint, which is now glowing with -the same fervor which fused the poker, but even the flint itself will -have to yield at last and become, not merely a liquid like water, but -a vapor like steam. - -You will ask, how do we learn all this? As nobody could ever make such -a journey, how can we feel certain that the sun is so excessively -hot? I know that what I say is true for various reasons, but I will -only mention one, which is derived from an experiment with the -burning-glass, that most boys have often tried. - -[Illustration: FIG. 1.--How to use the Burning-glass.] - -[Illustration: FIG. 2.--The Noonday Gun.] - -We may use one of those large lenses that are intended for magnifying -photographs. But almost any kind of lens will do, except it be too -flat, as those in spectacles generally are. On a fine sunny day in -summer, you turn the burning-glass to the sun, and by holding a -piece of paper at the proper distance a bright spot will be obtained -(Fig. 1). At that spot there is intense heat, by which a match can be -lighted, gunpowder exploded, or the paper itself kindled. The broad -lens collects together the rays from the sun that fall upon it, and -concentrates them in one spot, which consequently becomes hot and -bright. If we merely used a flat piece of glass the sunbeams would go -straight through; they would not be gathered together, and they would -not be strong enough to burn the paper. The lens, you see, is not -flat; its faces are curved, and they thus acquire the power of bending -in rays of light or heat, so as to unite their effect on that one -point which we call the _focus_. When a great number of rays are thus -collected on the same spot, each of them contributes a little warmth. - -[Illustration: FIG. 3.--A Tell-tale for the Sun.] - -Some ingenious person has turned this principle to an odd use, by -arranging a burning-glass over a cannon in such a way that just when -noon arrived the spot of light should reach the touch-hole of the -cannon and fire it off. Thus the sun itself is made to announce the -middle of the day (Fig. 2). - -Another application of the burning-glass is to obtain a record of the -number of hours of sunshine in each day. You will understand the -apparatus from Fig. 3; the lens is here replaced by a glass globe, -which acts as a burning-glass. As the sun moves over the sky the bright -spot of light also moves, and therefore burns its track on a sheet of -paper marked with lines corresponding to the hours. When the sun is -hidden by clouds the burning ceases, so by preserving each day the -piece of paper, we have an unerring tell-tale, which shows us during -what hours the sun was shining brightly, and the hours during which he -was hidden. You see, the burning-glass is not merely a toy, it can be -made useful in helping us to learn something about the weather. - -Another experiment with the burning-glass will also teach us something. -Take a candle, and from its flame you can get a bright point at the -focus. It may fall upon your hand, but you can hardly feel it, and -you will readily believe that the focus is not nearly so hot as the -candle. Even when a burning-glass is held in front of a bright fire -there is comparatively little heat in the focus. By using a lens to -condense the beams from an electric lamp, Professor Tyndall has shown -how to light a piece of paper, and to produce many other effects. -But, nevertheless, the focus is not nearly so hot as the arc between -the two glowing carbons. You might move your finger through the focus -without much inconvenience, but I would not recommend you to trust your -finger between the poles of the electric light itself. The temperature -obtained at the focus of a burning-glass seems thus to be always less -than that prevailing at the source of heat itself. This principle -will be equally true when we turn a burning-glass to the sun, and -hence we know that the sun must be hotter than any heat which can be -obtained by the biggest burning-glass on the brightest of summer days. -But burning-glasses a yard wide have been made, and astonishing heat -effects have been produced. Steel has thus been melted by the sunbeams, -and so have other substances which even our greatest furnaces cannot -fuse. Therefore the sun must have a higher temperature than that of -molten steel; higher, indeed, than any temperature we can produce on -the earth. - -I have tried to prove to you that the sun is very hot; but it would be -well to see what arguments might be used on the other side. Indeed, it -is by considering objections that we often learn. So I shall tell you -of a difficulty that was once raised when I was endeavoring to explain -the heat of the sun to an intelligent man. “I am sure,” said my friend, -“that you must be quite wrong. You said that the nearer you got to -the sun the hotter it would be; but I know this to be a mistake. When -tourists go to Switzerland, they sometimes climb very high mountains. -But the top of a mountain, of course, is nearer the sun than below; and -so, if the sun were really hot, the climber should have found it much -warmer on the top of the mountain than at its base. But every one knows -that there is abundant ice and snow on lofty Alpine summits, while down -below in the valleys there may be at the same time excessively warm -weather. Does it not therefore seem that the nearer we go to the sun -the colder it is, and the further we are from the sun the warmer it -is?” - -But my friend was quite wrong in his argument. The coldness of the -mountain tops depends upon something which he had not taken into -account. There is something else besides the sun which helps to make -us so warm and comfortable. This other essential thing is more or less -deficient at great heights. You know that we live by breathing air, and -we find air wherever we go, over land and sea, all round the earth. -Those who ascend in balloons are borne upwards by the air, and thus we -can show that air extends for miles and miles over our heads, though it -becomes lighter and thinner the loftier the elevation. - -We not only utilize the air for breathing, but it is also of -indispensable service to us in another way. It acts as a blanket to -keep the earth warm; indeed, we ought rather to describe the air as -a pile of blankets one over the other. These air blankets enable the -earth to preserve the heat received from the sunbeams by preventing it -from escaping back again into space. Thus warmth is maintained, and -our globe is rendered habitable. You see then, that for our comfort -we require not only the sun to give us the heat, but also the set of -blankets to keep it when we have got it. If we threw off the blankets -we should be uncomfortable, though the sun were as bright as before. -A man who goes to the top of a mountain at mid-day does approach the -sun to some extent, and, so far as this goes, he ought no doubt to -feel warmer, but the gain is far too small to be thought of. Even at -the top of Mont Blanc the increase in heat due to the approach to the -sun would be only one ten-millionth part of the whole. This would be -utterly inappreciable; even a thermometer would not be delicate enough -to show it. On the other hand, by ascending to the top of the mountain, -the climber has got above the lower regions of the air; he has not, it -is true, reached even halfway to the upper surface--that is still very -far over his head--but the higher layers of the atmosphere are so very -thin that they form most indifferent blankets. The Alpine climber on -the top of the mountain has thus thrown off the best portion of his -blankets, and receives a chill; while the gain of heat arising from his -closer approach to the sun is imperceptible. Perhaps you will now be -able to understand why eternal snow rests on the summits of the great -mountains. They are chilled because they have not so many air blankets -as the snug valleys beneath. - -The brightness of the sun is among the most wonderful things in nature, -and there are three points that I ask you to remember, and then indeed -you will agree with Milton, that the sun is “with surpassing glory -crowned.” First think of the beauty and brilliancy of a lovely day in -June. Then remember that all this flood of light comes from a single -lamp at a most tremendous distance; and thirdly, recollect that the -sun is not like a bull’s-eye lantern, concentrating all his light -_specially_ for our benefit, but that he diffuses it equally around, -and that we do not get on this earth the two-thousand-millionth part -of what he gives out so plenteously! When we think of the brightness -of day, of the distance from which the light has come, though Nature -has not adjusted any vast lenses to direct the light specially in our -direction, we begin to comprehend the sun’s true magnificence. - - -FURTHER BENEFITS THAT WE RECEIVE FROM THE SUN. - -I want to show you how great should be the extent of our gratitude to -the sun. Of course, on a bright summer’s day, when we are revelling in -the genial warmth and enjoying the gladness of sunshine, it needs no -words to convince us of the utility and of the beneficence of sunbeams. -So we will not take midsummer. Let us take midwinter. Take this very -Christmas season when the days are short and cheerless, the nights -are long and dark and cold. We might be tempted to think that the sun -had well-nigh forgotten us. It is true he only seems to pay us very -occasional visits, and between fogs and clouds we in England see but -little of him; but, visible or invisible, the sun incessantly tends -us, and provides for our welfare in ways that perhaps we do not always -remember. - -Let me give an illustration of what I mean. You will go back this dull -and cold afternoon to the happy home where your Christmas holidays are -being enjoyed. It will be quite dark ere you get there, for the sun in -these wintry days sets so very early. You will gather around a cheerful -fire. The curtains will be drawn, the lamps will be lighted, and the -disagreeable weather outside will be forgotten in the pleasant warmth -and light within. Five o’clock has arrived, the pretty wicker table has -been placed near mamma’s chair; on it are the cups and saucers and the -fancy teapot. Under the table is a little shelf, with some tempting -cakes and a tender muffin. Two or three welcome friends have joined the -little group, and a delightful half-hour is sure to follow. - -But you may say, “What have tea and muffins, lamps and fireplaces to do -with the sun? Are they not all mere artificial devices, as far removed -as possible from the sunbeams or the natural beauties which sunbeams -create?” Well, not so far, perhaps, as you may think. Let us see. - -Poke up the fire, and while it is throwing forth that delicious warmth, -and charming but flickering light, we will try to discover where that -light and heat have come from. No doubt they have come from the coal, -but then, whence came the coal? It came from the mine, where brave -colliers hewed it out deep under the ground, and then it was hoisted -to the surface by steam engines. Our inquiry must not stop here, for -another question immediately arises, as to how this wonderful fuel -came into the earth? When we examine coal carefully, by using the -microscope to see its structure, we find that it is not like a stone; -it is composed of trees and other plants, the leaves and stems of which -can be sometimes recognized. Indeed, the fossil trunks and roots of -the great trees are occasionally conspicuous in the coal-pit. It is -quite plain that these are only the remains of a vegetation which was -formerly growing and flourishing, and on further inquiry we learn that -coal must have been produced in the following manner:-- - -Once upon a time a great forest flourished. The sun shone down on this -forest, and it was watered by genial showers, while insects and other -creatures sported in its shades. It is true that the trees and plants -were not like those we now see about us. They were more like ferns and -mare’s-tails and gigantic club-mosses. In the fulness of time they -died, and fell, and decayed, and others sprang up to meet the like end. -Thus it happened that, in course of ages, the remains of leaves, and -fruits, and trunks accumulated over the soil. The forest was situated -near the seashore, and then a remarkable change took place--the land -began slowly to sink. You need not think that this is impossible. Land -has often been known to change its level gradually. In fact, a sinking -process is slowly going on now in many places on the earth, while -the land is rising in other localities. As the forest gradually sank -lower and lower, the sea-water began to inundate it, and all the trees -perished until, at last, deep water submerged the surface which had -once been covered by a fine forest. At the bottom of this sea lay the -decaying vegetation. - -That which was the destruction of the growing forest, proved to be the -means of preserving its remains, for, then as now, the rivers flowed -into the sea, and the waters of the rivers, especially in times of -flood, carried down with them clay or mud, held in suspension. Upon -the floor of the ocean this material was slowly deposited; and thus -a coating of mud overlay the remains of the forest. In the course of -ages, these layers grew thick and heavy, and hardened into a great flat -rock, while the trunks and leaves underneath were squeezed together by -the weight, and packed into a solid mass which became black, and in -the course of time was transformed into coal. - -After ages and ages had passed by, the bed of the sea ceased to sink, -and began slowly to rise. The water over the newly made layers of stone -became shallower, and at last the floor was raised until it emerged -from the sea. But, of course, it would not be the original ground -which formed the surface of the newly uncovered land. The sheets of -consolidated clay lay on the top; over the fresh surface life gradually -spread, until man himself came to dwell there, while far beneath his -feet the remains of the ancient vegetation were buried. - -When we now dig down through the rocks we come upon the portions of -trees and other plants which the lapse of time, and the influence of -pressure, have turned from leaves and wood into our familiar coal. - -That ancient forest grew because sunbeams abounded in those early -times, and nourished a luxuriant vegetation. The heat and the light -then expended so liberally by the sun were seized by the leaves of -flourishing plants, and were stored away in their stems and foliage. -Thus it is that the ancient sunbeams have been preserved in our -coal-beds for uncounted thousands of years. When we put a lump of coal -on our fire this evening, and when it sends forth a grateful warmth and -cheerful light, it but reproduces for our benefit some of that store of -preserved sunbeams of which our earth holds so large a treasure. Thus, -the sun has contributed very materially to our comfort, for it has -provided the fire to keep us warm. - -The orb of day has, however, ministered further to our tea party, -for has it not produced the tea itself? The tea grew a long way off, -most likely in China, where the plant was matured by the warmth of the -sunbeams. From China the tea-chests were brought by a sailing vessel to -London; the ship performed this long voyage by the use of sails, blown -by what we call wind, which is merely the passage of great volumes of -air as they hurry from one part of the earth to another. - -We may ask what makes the air move, for it will not rush about in -this way unless there be considerable force to drive it. Here again -we perceive the influence of the sun. Tracts of land are warmed by -the genial sunbeams. The air receives the heat from the land, and the -warm air is buoyant and ascends, while cooler air continually flows in -to supply its place. To do this it has, of course, to rush across the -country, and thus wind is caused. All the air currents on our earth are -consequently due to the sun. You see, therefore, how greatly we are -indebted to our brilliant luminary for the enjoyment of our tea-table. -Not only has the sun given us the coal and the tea, but it has actually -provided the means by which the tea was carried all the way from China -to our own shores. - -We can also trace the connection between the hot water and the sun. Of -course, the water has come immediately from the kettle, and that has -been taken from the fire, and the fire was produced by sunbeams. Thus -we learn that it is the warmth of the sun that has made the water boil. -If you visit the water-works you will see great reservoirs. In some -cases they have been filled by a river, sometimes the water is pumped -from a deep well in the ground, sometimes it is the surface-water -caught on a mountain side. Whatever be the immediate source of our -water supply, the real origin is to be sought, not in the earth -beneath, but in the heavens above. All the water we use day by day has -come from the clouds. It is the clouds which sent down the rain, or -sometimes the snow, or the hail, and it is this water from the clouds -which fills our rivers. It is this water also which sinks deep into the -earth and supplies our wells, so that from whatever apparent source the -water seems to have come, it is indeed the clouds which have been the -real benefactors. The water in your teacup to-night was, a little while -ago, in a cloud, floating far overhead in the sky. - -We may look a little further and find whence the clouds have come. It -is certain that clouds are merely a form of steam or vapor of water, -and as they are so continually sending down rain on the earth, there -must be some means by which their supply will be replenished. Here -again our excellent friend the sun is to be found ever helping us -secretly, if not helping us openly. He pours down his rich and warm -beams on the great oceans, and the heat turns some of the water into -vapor, which, being lighter than the air, ascends upwards for miles. -There the vapor often passes into the form of clouds, and the winds -waft these clouds to refresh the thirsty lands of the earth. Thus, you -see, it is the sun which procures for us water from the great oceans -which cover so much of our globe, and sends it on by the winds to -supply our water-works, and fill our teapots. Notice another little -kindliness of our great benefactor. The water of the oceans is quite -salt. But we could not make tea with salt water, so the sun, when -lifting the vapor from the sea, most thoughtfully leaves all the salt -behind, and thus provides us with the purest of sweet water. - -That nice muffin was baked by the sun, toasted by the sun, and made -from wheat grown by the sun. If the wheat was ground in a wind-mill, -then the sun raised the wind which turned the mill. Perhaps the -flour-mill was driven by steam, in which case the sun, long ago, -provided the coal for the boiler. The miller might have lived on a -river and used a water-mill, but if he did, then here again the sun -actually did the work. The sun raised the water to the clouds, and -after it had fallen in rain, and was on its way back to the sea, its -descent was utilized to turn the water-wheel. The water derives its -power to turn the mill from the fact that it is running downhill, -but it could not run down unless it had first been raised up; and -thus it is indeed the sun which drives the water-wheel. Nor can the -baker dispense with the sun’s aid even if he rejected wind-mills, or -steam-mills, or water-mills, and determined to grind the corn himself -with a pestle and mortar. Here, at least, it might be thought that it -is a man’s sinews and muscles that are doing the work, and so no doubt -they are. But you are mistaken if you think the sun has not rendered -indispensable aid. The sun has just as surely provided the power -which moves the baker’s arms as it has raised the wind which turned -the wind-mill. The force exerted in grinding with the pestle has been -derived from the food that the man has eaten; that food was grown by -the sun, and the man received from the food the energy it had derived -from the sun’s heat. So that, look at it any way you please, even for -the grinding of the wheat to make the muffin for your tea party, you -are wholly indebted to the sun. - -It is the sun which has bleached the tablecloth to that snowy -whiteness. The sun has given those bright colors which look so pretty -in the girls’ dresses. With how much significance can we say and feel -that light is pleasant to the eye, and what prettier name than Little -Sunbeam can we have for the darling child who makes our home so bright? - - -THE DISTANCE OF THE SUN. - -The sun is a very long way off. It is not easy for you to imagine a -distance so great, but if you want to learn astronomy you must make -the attempt. This is the first measurement that we shall have to make -on our way to that far-off country called Star-Land; but long as we -shall find it to be, we shall afterwards have to consider distances -very much longer. When you are out in the street, or taking a walk in -the country, you can see at once that this man is near, or that house -is far, or that mountain is many miles away. This is because you have -other objects between to help you to judge of the distances of these -different objects. You will see, for example, that there are many -houses or farmyards, and you will notice hedges dividing different -fields between you and the mountain. You also see that there are -woods and parks, and perhaps stretches of moorland extending up the -slopes. You have an impression that the farmyards and fields are of -considerable size, and that the woods or moors are wide and extensive; -and putting these things together, you realize that the mountain must -be miles away. - -But when we look at the sun we have no aids conveniently placed to -help us in judging his distance. There are no intervening objects, and -merely gazing at the sun helps us but little in obtaining any accurate -knowledge. We must go to the astronomer and ask him to tell us how far -he has found the sun to be, and then we must also beg from him some -explanation of the method he has used in making his measurements. - -It has been found that the sun is, on the average, about ninety-three -millions of miles from the earth; but sometimes it is a little further -and sometimes it is a little nearer. Let us first try to count -93,000,000. The easiest way will be to get the clock to do this for -us; and here is a sum that I would suggest for you to work out. How -long will the clock have to tick before it has made as many ticks as -there are miles between the earth and the sun? Every minute the clock, -of course, makes 60 ticks, and in 24 hours the total number will reach -86,400. By dividing this into 93,000,000 you will find that more than -1076 days, or nearly three years, will be required for the clock to -perform the task. - -We may consider the subject in another way, and find how long an -express train would take to go all the way from the earth to the sun. -We shall suppose the speed of the train to be 40 miles an hour; and -if the train ran for a whole day and a whole night without stopping, -it would then accomplish 960 miles. In a year the distance travelled -would reach 350,400 miles, and by dividing this into 93,000,000 we -arrive at the conclusion that a train would have to travel at a pace -of 40 miles an hour, not alone for days and for weeks and for years, -but even for centuries. Indeed, not until 265 years had elapsed would -the mighty journey have been ended. Even though King Charles I. had -been present when the train began to move, the destination would not -yet have been reached. No one who started in the train could expect to -reach the end of the trip. That would not occur till the time of his -great-great-grandchildren. - - -HOW ASTRONOMERS MEASURE THE DISTANCES OF THE HEAVENLY BODIES. - -I shall so often have to speak of the distances of the celestial bodies -that I may once for all explain how it is that we have been able to -discover what these distances are. This would be a very puzzling matter -if we were to try and describe it fully, but the principle of the -method is not at all difficult. Do you know why you have been provided -with two eyes? It is undoubted that one of the reasons is to aid you in -estimating distances. You see this boy (Fig. 4) judges of the distance -of his finger by the inclination of his two eyes when directed at it. -In a similar way we judge of the distance of a heavenly body by making -observations on it from two different stations. - -[Illustration: FIG. 4.--Two Eyes are better than One.] - -[Illustration: FIG. 5.--How we measured the Height of the Ball.] - -I shall illustrate our method of measuring the actual distance of a -body in the heavens by showing you how we can find the height of that -large india-rubber ball which is hanging from the ceiling. Of course, -I do not intend to have a measuring tape from the ball itself, because -I want to solve the problem on the same principle as that by which we -measure the distance of the sun or of any other celestial body which we -cannot reach. I will ask the aid of a boy and a girl, who will please -stand one at each end of the lecture table. The apparatus we shall want -is very simple; it consists of two cards and a pair of scissors. The -boy will kindly shape his card to such an angle that when he holds it -to his eye one side of the angle shall point straight at the little -girl, and the other side shall point straight at the ball, just as you -see in the picture (Fig. 5). The girl will also please do the same -with her card, so that along one side she just sees the little boy’s -face, while the other side points up to the ball. It will be necessary -to cut these angles properly. If the angle be too big, then when one -side points to the boy’s face, the other will be directed above the -ball. If the angle on the card be too small, then one side will be -directed below the ball, while the other is pointed to the boy. The -whole accuracy of our little observations depends upon cutting the -card angles properly. When they have been truly shaped it will be easy -to find the distance of the ball. We first take a foot rule and measure -the length of our table from one of our young friends to the other. -That length is twelve feet, and to discover the distance of the ball -we must make a drawing. We get a sheet of paper, and first rule a line -twelve inches long. That will represent the length of the table, it -being understood that each inch of the drawing is to correspond to a -foot of the actual table. Let the end where the girl stood be marked -B, and that of the boy, A, and now bring the cards and place them on -the line just as shown in the figure. The card the girl has shaped is -to be put so that the corner of it lies at B, and one edge along B A. -Then the boy’s card is to be so put that its corner is at A and one -edge along A B. Next with a pencil we rule lines on the other edges of -the cards, taking care that they are kept all the time in their proper -positions. These two lines carried on will meet at C; and this must be -the position of the ball on the scale of our little sketch. It only now -remains to take the foot rule and measure on the drawing the length -from A to C. I find it to be twenty inches, and I have so arranged it -that the distance from B to C is the same. - -[Illustration: FIG. 6.--This is what we wanted the Cards for.] - -I do not intend to trouble you much with Euclid in these lectures, but -as many of my young friends have learned the sixth book, I will just -refer to the well-known proposition, which tells us that the lengths -of the corresponding sides of two similar triangles are proportional. -We have here two similar triangles. There is the big one with the boy -at one corner, the girl at the other, and the ball overhead. Here -is the small triangle which we have just drawn. These triangles are -similar because they have got the same angles, and it was to insure -that they should have the same angles that we were so careful in -shaping the cards. As these two triangles are similar, their sides must -be proportional. We have agreed that the line A B, which is twelve -inches long, is to represent the length of the table between the little -boy and girl. Hence the distance, A C, must, on the same scale, be the -interval between the ball and the boy at the end. This is twenty inches -on the drawing, and therefore the actual distance from the end of the -table to the ball is twenty feet. - -Hence you see that without going up to the ball or having a string from -it, or in any other way making direct communication with it, we have -been able to ascertain how far up in the air the ball is actually hung. -This simple illustration explains the principle of the method by which -astronomers are able to learn the distances of the different celestial -bodies from the earth. You must think of the sun, the moon, and the -stars as globes supported in some manner over our heads, and we seek to -discover their distances from measurements of angles made at the ends -of a base-line. - -[Illustration: FIG. 7.--This would be our Base-line when finding the -Sun’s Distance.] - -Of course, astronomers must choose two stations which are far more -widely separated than are those in our little experiment. In fact, the -greater the interval between the two stations, the better. Astronomers -require a much longer distance than from one side of this room to -the other, or from one side of London to the other side. If it were -merely a balloon at which we were looking, then, when one observer -at one side of London and another at the opposite side shaped their -cards carefully, we should be able to tell the height of the balloon -very easily. But as the sun is so much further off than any balloon -could ever be, we must separate the observers much more widely. -Even the breadth of England would not be enough, so we have to make -them separate more and more until they are as widely divided as it -is possible for any two people on this earth to be. One astronomer -takes up his position at A (Fig. 7), and the other at the opposite -side at B, so that they can both see the sun. They are obliged to -use a much more accurate way of measuring the angles than by cutting -out cards with pairs of scissors; and as the astronomer at A is not -able to see his friend at B, it becomes no easy matter to measure the -angles accurately. However, we shall not now trouble ourselves about -such difficulties. It may suffice for the present to know that the -angles are measured by delicate and very accurate instruments used -by astronomers. They will not, indeed, make a little sketch such as -sufficed for our purpose. They make a calculation which is a much more -accurate way of effecting true measurement. The astronomers know the -size of the earth, and thus they know how many thousands of miles lie -between the two stations where the observations are made. This distance -means in their calculation just what the length of the table did in our -sketch. From each end of the line they set off an angle just as we did, -and the astronomer must use the principle of similar triangles which he -finds in Euclid, just we had to do. At last, when they have calculated -the sides of their triangle, they obtain the distance of the sun. - - -THE APPARENT SMALLNESS OF DISTANT OBJECTS. - -[Illustration: FIG. 8.--The nearer you are, the bigger the Globe looks.] - -[Illustration: FIG. 9.--The Globe is so far off that it lies beyond the -Picture. The dotted lines show how small it seems.] - -I ought here to explain a principle which those who are learning -about the stars must always bear in mind. The principle asserts that -the further a body is, the smaller it looks. Perhaps this will be -understood from the adjoining little sketch (Fig. 8). It represents a -great globe, on which oceans and continents are shown, and you see -a little boy and a little girl are looking at the globe. The girl -stands quite close to it, and I have drawn two dotted lines from her -eye, one to the top of the globe, and the other to the under surface. -If she wants to examine the entire side of the globe which is visible -to her, she must first look along the upper dotted line, and then she -must turn her glance downwards until she comes to the lower line, and -having to turn her eyes thus up and down she will think the globe is -very big, and she will be quite right. The boy is, as you see, on the -other side of the globe, but I have put him much further off than the -girl. I have also drawn two dotted lines from his eye to the globe, and -it is plain that he will not have to turn his head much up and down to -see the whole globe. He can take it all in at a glance, and to him, -therefore, the globe will appear to be comparatively small, because he -is sufficiently far from it. The more distant he is, the smaller it -will appear. You can easily imagine that, if the globe were far enough, -the two lines that would include the whole would be like those shown -(Fig. 9), in which the globe is so distant that it cannot be seen in -the picture. The apparent size of the globe, which is really measured -by the angle between these two lines, would always be smaller and -smaller according as the distance was greater. Now you can understand -why an object seems smaller the further away it is; indeed, when -sufficiently far, the object ceases to be visible at all. - -I could give many illustrations of the diminution of size by distance, -and so, doubtless, could you. Every boy knows that his kite looks -smaller and smaller the greater the length of string that he lets -out. I have seen in the West of Ireland a bird that seemed like a -little speck high up near the clouds, but from its flight and other -circumstances I knew that the speck was not a little bird. It was, -indeed, a great eagle, which was dwarfed by the elevation to which it -had soared. - -It is in astronomy that we have the best illustrations of this -principle. Enormous objects seem to be small because they are so very -far off. You must therefore always remember that although an object may -appear to be small, this appearance may be only a delusion. It may be -that the object is very big, but very distant. In astronomy, this is -almost always the case, there is so much room above us, around us, on -all sides in space. Look up at the ceiling. It certainly does not bound -space, for there is another side to it; and then there is the roof of -the house. But the roof is not a boundary, for, of course, there is the -air above it, and then, higher up still, there are the clouds, and so -we can carry our imagination on and on through and beyond the air up -to where the stars are, and still on and on. And as there is unlimited -room, the celestial bodies take advantage of it, and are, generally -speaking, at distances so gigantic that, no matter how small they may -appear, their smallness is merely deceptive. - -Let us try to illustrate in another way the exceeding remoteness of the -sun. So please imagine that you were on the sun, and that you took a -view of our earth from that distance. To find out what we must expect -to see, let us think of a balloon voyage. If you were to go up in a -balloon, you would at first see only the houses, or objects immediately -about you, but as you rose the view would become wider and wider. You -would see that London was surrounded by the country, and then, as you -still soared up and up, the sea would become visible, and you would -be able to trace out the coasts, east and west and south. If, in some -way, you could soar higher than any balloon could carry you, the whole -of the British Islands would presently lie spread like a map beneath. -Still on and on, and then the continent of Europe would be gradually -opened out, until the great oceans, and even other continents, would -at last be caught sight of, and then you would perceive that our whole -earth was indeed a globe. The higher you went, the less distinctly -would you be able to see the details on the surface. At last the -outlines of the continents and oceans would fade, and you would begin -to lose any perception of the shape of the earth itself. Long ere you -had reached the distance of the sun, the earth would look merely as the -planet Venus now does to us. It is instructive to consider how small -our earth would seem if it were possible to view it from the sun. Think -of that very familiar little globe, a lawn-tennis ball, which is two -and three-quarter inches in diameter. But suppose a tennis ball were -at the opposite side of the street, or still further away; suppose, -for example, that it were half a mile away, what could you expect to -see of it? And yet the earth, as seen from the sun, would appear to be -no larger than a tennis ball would look when viewed from a distance of -half a mile. - - -THE SHAPE AND SIZE OF THE SUN. - -We have spoken of the heat of the sun, how hot he is; of the distance -of the sun, how far he is; and now we must say a little about the size -of the sun; and also about his shape. It is plain that the sun is -round, that it has the shape of a ball. We are sure of this because, -though a plate is circular, yet, if it were placed so that we only -saw it edgeways from a distance, it would not appear to be round. The -sun is always rotating, and as it always seems to be a circle, we are -therefore certain that the true shape of the sun must be globular, and -not merely circular like a flat plate. - -In the middle of the day, when the sun is high in the heavens, it is -impossible for us to form a notion of the size of the sun. People -will form very different estimates as to his apparent bigness. Some -will say he looks as large as a dinner plate, but such statements are -meaningless, unless we say where the plate is to be held. If it be near -the eye, of course the plate may hide the sun, and, for that matter, -everything else also. If the plate were about a hundred feet away, then -it would often hide the sun. If the plate were more than a hundred feet -distant, then it could not hide the sun entirely, and the further the -plate, the smaller it would seem. - -No means of estimating the sun’s size are available when his orb stands -high in the heavens. But when he is rising or setting, we see that he -passes behind trees or mountains, so that there are intervening objects -with which we can compare him; then we have actual proof that the sun -must be a very large body indeed. - -[Illustration: FIG. 10.--A Sunset viewed from Marseilles (_Marcus -Codde_).] - -I give here a picture, by Marcus Codde, taken from a French journal, -_l’Astronomie_, which gives a charming illustration of a sunset at -Marseilles (Fig. 10). If you wish to see that the sun is bigger than -a mountain, you may go to the top of Notre Dame de la Garde, but you -must choose either the 10th of February or the 31st of October for your -visit, because it is only on the evenings of those days that the sun -sets in the right position. - -On both these evenings the sun sinks directly behind Mount Carigou in -the Pyrenees; this mountain is a long way from Marseilles--no less, -indeed, than one hundred and fifty-eight miles. But the mountain is so -lofty, that when the sky is clear, the summit can be distinctly seen -upon the sun as a background, in the way shown in the picture. This -must be a very pretty sight, and it teaches us an important lesson. -The sun is further away than the mountain, and yet you see the sun on -both sides of the mountain, and above it. Here, then, we learn without -any calculations, that the sun must be bigger than the upper part of a -great mountain in the Pyrenees. - -When we calculate the size of the sun from the measurements made by -astronomers, we discover that it is much bigger than Mount Carigou; we -see that even the entire range of the Pyrenees, the whole of Europe, -and even our whole globe, are insignificant by comparison. - -[Illustration: FIG. 11.--How we compare the Earth and the Sun.] - -There is a football on the table, shown in Fig. 11. We shall suppose it -to represent the sun; we shall now choose something else to represent -the earth. We must, however, exhibit the proportions accurately. A -tennis ball will not do; it is far too large. The fact is, the width -of the earth is less than the one-hundredth part of the width of the -sun. The tennis ball is, however, only a quarter the width of the -football, so we must choose something a good deal smaller. I try with -a marble, even with the smallest marble I can find, but when I measure -it, I find that one hundred such marbles, placed side by side, would -be far longer than the width of the football; I must therefore look -for something still smaller. A grain of small-sized shot will give -the right size for the model of our earth. About one hundred of these -grains placed side by side will extend to a length equal to the width -of the football. Now you will be able to form some conception of how -enormous the sun really is. Think of this earth, how big we find it -when we begin to travel. What a tremendous voyage we have to take to -get to New Zealand, and even then we have only got halfway round the -globe. Then think that the sun is in the same proportion bigger than -the earth as that football is bigger than that grain of shot. If a -million of such grains of shot were melted and cast into one globe, -it would not be so large as that football. If a million globes, as -large as our earth, could be united together, no doubt a vast globe -would be produced, but it would not be so large as the sun. Think of a -single house, with three or four people living in it, and then think of -this mighty London, with its millions of inhabitants. The house will -represent our earth, while great London represents the sun! - - -THE SPOTS ON THE SUN. - -I have shown you that the sun is intensely hot, and a very long way -off, and enormously big. And now we have to describe the appearance of -the surface of the sun when we examine it closely. - -[Illustration: FIG. 12.--Looking at the Sun.] - -If you get a piece of very dark glass, or if you smoke a piece of glass -over a candle, then you can look directly at the sun with comfort. A -nicer plan is to prick a pinhole in a card, through which you can look -at the sun without any inconvenience. Generally speaking, a view of -the sun in this way will show you only a uniformly bright surface. To -study the face of our great luminary carefully, you must use the aid -which the telescope gives to the astronomer. A very good way of doing -this is shown in Fig. 12. A small telescope, fixed on a stand, is -pointed to the sun, and, the eyepiece being drawn out somewhat further -than when direct observations are being made, the sun draws its own -picture on a screen. This may be examined without any inconvenience, or -without the necessity for any protection to the eye, and a number of -young astronomers can all view the sun at the same moment. On such a -picture you will generally see the brilliant surface marked with dark -spots, which are sometimes as numerous as in the case represented in -Fig. 13. These spots present very different appearances according to -circumstances. One such spot when seen with a very powerful telescope -showed the wonderful structure which is represented in Fig. 14. - -[Illustration: FIG. 13.--This is what the Sun sometimes looks like.] - -The visible surface of the sun is entirely formed of intensely heated -vapors. We might almost say that the spots are holes, by which we can -look through the brilliant surface to the interior and darker parts. -Sometimes the spots close up, and fresh ones will open elsewhere. Now -and then the whole surface is mottled over in a remarkable way. I give -here a picture which was taken from Mr. Nasmyth’s beautiful drawing, -in which he shows how the sun sometimes assumes the appearance which -has been likened to willow leaves (Fig. 15). This appearance was very -noticeable in the great spot of September, 1898. - -[Illustration: FIG. 14.--A Sun-spot (_after Janssen_).] - -[Illustration: FIG. 15.--Nasmyth’s Drawing of the Willow-leaved -Structure of the Sun.] - -[Illustration: FIG. 16.--Spot nearing the Sun’s Edge.] - -The spots often last long enough to demonstrate a remarkable fact. We -must remember that the sun is a great globe, and that it is poised -freely in space. There is nothing to hold it up, and there is nothing -to prevent it from turning round. That it does turn round, we can prove -by careful observation of the spots. I can best illustrate what I want -by Fig. 17, which shows six imaginary pictures. The first represents -the sun on the 1st day of the month; the next shows it five days later, -on the 6th; another view is five days later still, on the 11th; and -so on until the last picture, which corresponds to the 26th. You see, -on the first day there is a spot near the left edge; by the 6th, -this spot is near the middle; by the 11th, it is near the right edge; -then you do not see it at all on the 16th, or on the 21st; but on the -26th it is back in the same place from which it started. We find -other spots to have a similar history. They appear to move across the -face, and then to return in a little less than four weeks to the same -place where they were originally noticed. These appearances can be -illustrated very simply by cutting a small hole through the rind of an -orange down to the white interior skin, which may be darkened with ink. -Put a knitting needle through the axis of the orange, and then turn it -slowly round. The spot will be found to go through the changes that we -have seen. We start with the spot near the left, it moves across the -face, and then passes to invisibility by moving behind the globe until -it reappears again, after having moved round the back. As the same may -be observed with every spot which lasts long enough, we learn that the -changes in the places must be produced by the turning round of the sun. -Here you see is the way in which an astronomical discovery is made. We -first observe the fact that the spots do always appear to move. Then -we try to account for this, and we find a very simple explanation, by -supposing that the whole sun, spots and all, turns steadily round and -round. It can also be proved in a very conclusive manner that no other -explanation is possible. This rotation of the sun is always going on -uniformly, and some curious consequences follow from it. The view of -the sun which is turned towards us to-day is quite different from that -which was towards us a fortnight ago, or from that which we shall see -in a fortnight hence. There is no actual or visible axis about which -the sun rotates. In this the sun is like the earth and other celestial -bodies. - -[Illustration: FIG. 17.--How the Sun turns round.] - - -APPEARANCES SEEN DURING A TOTAL ECLIPSE OF THE SUN. - -For a great deal of our knowledge about the sun we are indebted to -the moon. It will sometimes happen that the moon comes in between -us and the sun, and produces an eclipse. At first you might think -that an eclipse would only have the effect of preventing us from -seeing anything of the sun, but it really reveals most beautiful -and interesting objects, of whose existence we should otherwise be -ignorant. The great luminary has curious appendages which are quite -hidden under ordinary circumstances. In the full glare of day the -dazzling splendor of the sun obliterates and renders invisible these -appendages, which only shine with comparatively feeble light. It -fortunately happens that the moon is just large enough to intercept -the whole of the direct light from the sun, or rather, I should -say, from the central parts of the sun. Surrounding that central -and more familiar part from which the brilliancy is chiefly derived -is a remarkable fringe of delicate and beautiful objects which are -self-luminous no doubt, but with a light so feeble that when presented -to us amid the full blaze of sunlight they are invisible. When, -however, the moon so kindly stops all the stronger beams, then these -faint objects spring into visibility, and we have the exquisite -spectacle of a total eclipse. The objects that I desire to mention -particularly are the corona and the prominences. - -[Illustration: FIG. 18.--Total Eclipse of the Sun, May 6, 1883 (_drawn -by Trouvelot_).] - -A pretty picture of the total eclipse of the sun which occurred on May -6, 1883, is here shown (Fig. 18). It is taken from a drawing made by -M. Trouvelot, who was sent out with a French observing party. They -went a very long way to see an eclipse, but what they saw recompensed -them for all their trouble. The track along which the phenomenon could -be best seen lay in the Pacific Ocean, and a place had to be selected -which was so situated that the sun should be high in the heavens at -the important moment, and also that the duration while the total -eclipse lasted should be as long as possible. They accordingly went to -Caroline Island, and all this journey to the other side of the earth -was taken to witness a phenomenon that only lasted five minutes and -twenty-three seconds. Short though these precious minutes were, they -were long enough to enable good work to be done. Careful preparations -had been made so that not a moment should be thrown away. Each member -of the party had his special duty allotted to him, and this had been -rehearsed so carefully beforehand that when the long-expected moment -of “totality” arrived there was neither haste nor confusion; every one -carefully went through his part of the programme. M. Trouvelot, for -instance, occupied himself for two minutes and a few seconds in making -the sketch that we now show. No doubt an accomplished astronomical -artist like M. Trouvelot would gladly have taken longer time for his -sketch of so unique a sight, but brevity was imperative. He had already -had experience of similar eclipses, so that he was prepared at once -to note what ought to be noted, and the picture we have shown is the -result. This was completed within less than half of the duration of -totality, and the artist had still three minutes left to devote to -another and quite different part of the work, which does not concern us -at present. - -[Illustration: FIG. 19.--The Corona of the Sun, 1882 (_by Schuster_).] - -I want you particularly to look at these long branches or projections -which we see surrounding the sun when totally eclipsed. They shine -with a pearly light, and, in fact, it is stated that even during the -gloomiest portion of the time there was still as much illumination as -on a bright moonlight night. All that light came from this glorious -halo round the sun which astronomers call the “corona.” We do not -under ordinary circumstances obtain even the slightest glimpse of this -object. Even during a partial eclipse of the sun it is not visible, but -directly the moon quite covers the sun, so as to cut off all the direct -light, then the corona springs into visibility. It is always there, no -doubt, though we cannot see it. - -One of the most interesting photographs of the eclipsed sun which has -ever been taken was that by Professor Schuster in 1882 (Fig. 19). The -corona is well shown, and also a comet. - -The other appendages to the sun which can be seen during an eclipse are -the objects which we call “prominences.” They are of a ruddy color, and -seem to be great flames, which leap upwards from the glowing surface -of the sun below. Though the existence of the prominences was first -discovered by their presence during eclipses, it fortunately happens -that we are no longer wholly dependent on eclipses for the purpose -of making our observations of these remarkable objects. It is true -that we may look at the sun with even the biggest and most powerful -telescope in the world, and still not be able to perceive anything of -the prominences. We require the aid of a special appliance called the -spectroscope to render them visible. But I am not now going to describe -this ingenious contrivance. I am only going to speak of the results -which have been obtained by its means. We shall here again avail -ourselves of the experience of M. Trouvelot for a picture of two of -these wonderful appendages. - -[Illustration: FIG. 20.--Solar Prominences (_drawn by Trouvelot_).] - -The view (Fig. 20) shows the ordinary aspect of the sun diversified -with groups of dark spots. The fringe around the margin of the globe -is of some ruddy material, forming the base of the flames which rise -from the glowing surface. No doubt these flames are also often present -on the face of the sun, but we cannot see them against the brilliant -background. They are only perceptible when shown against the sky -behind. At two points of this ruddy fringe, which happen curiously -enough to be nearly opposite to each other, two colossal flames have -burst forth. They extend to a vast distance, which is quite one-third -of the width of the sun. The vigor of these outbreaks may be estimated -by the remarkable changes which are incessantly going on. These great -flames may indeed be said to flicker; only, considering their size, we -must allow them a little more time than is demanded for the movements -of flames of ordinary dimensions. The great flame on the left was -obviously declining in brilliancy when first seen. In a quarter of an -hour it had broken up into fragments, some of which were still to be -seen floating in the sun’s atmosphere. In ten minutes more the light of -this flame had almost entirely vanished. Surely these are changes of -extraordinary rapidity when we remember the size of this prominence. It -was nearly 300,000 miles in height--that is to say, about thirty-seven -times the width of our earth. - -Great as are these prominences, others have been recorded which are -even larger. One of them has been seen to rush up with a speed of -200,000 miles an hour--that is, with more than two hundred times the -pace of the swiftest of rifle-bullets. - - -NIGHT AND DAY. - -The sun is bright, and the earth is dark. The sun gives light and heat, -and the earth receives light and heat. We should be in utter darkness -were it not for the sun; at least, all the light we should have, beyond -our trivial artificial light, would come from the feeble twinkle of -the stars. The moon would be no use, for the brightness of the moon is -merely the reflection of the sunbeams. Were the sun’s light completely -extinguished we could never again see the moon, and we should also miss -from the sky a few other bodies, which we call planets, such as Jupiter -and Venus, Mars and Saturn. But the stars would be the same as before, -for they do not depend upon the sun for their light. We shall, indeed, -afterwards see that each star is itself a sun. - -Picture to yourself the earth as receiving a stream of sunbeams. These -beams fall on one half of our globe, and give to it the brilliance -of day. The other half of the earth of course receives no sunlight. -It is in the shadow, and consequently the darkness of night there -prevails. The boundary between light and darkness is not quite sharply -defined, for the pleasant twilight softens it a little, so that we -pass gradually from day to night. Looking at the progress of the sun -in the course of the day, we see that he rises far away in the east, -then he gradually moves across the heavens past the south, and in the -evening declines to the west, sets, and disappears. All through the -night the sun is gradually moving round the opposite side of the earth, -illuminating New Zealand and Japan and other remote countries, and then -gradually working round to the east, where he starts afresh to give us -a new day here. - -Our ancestors many ages ago did not know that the earth was round. They -thought it was a great flat plain, and that it extended endlessly in -every direction. They were, however, much puzzled about the sun. They -could see from the coasts of France and Spain or Britain that the sun -gradually disappeared in the ocean; they thought that it actually took -a plunge into the sea. This would certainly quench the glowing sun; -and some of the ancients used to think they heard the dreadful hissing -noise when the great red-hot body dropped into the Atlantic. But there -was here a difficulty. If the sun were to be chilled down every evening -by dropping into the water hundreds of miles away to the west, how did -it happen that early the next morning he came up as fresh and as hot as -ever, hundreds of miles away to the east? For this, indeed, it seemed -hard to account. Some said that we had an entirely new sun every day. -The gods started the sun far off in the east, and after having run -its course it perished in the west. All the night the gods were busy -preparing a new sun to be used on the succeeding day. But this was -thought to be such a waste of good suns that a more economical theory -was afterwards proposed. The ancients believed that the continents of -the earth, so far as they knew them, were surrounded by a limitless -ocean. At the north, there were high mountains and ice and snow, which -they thought prevented access to this ocean from civilized regions. -Vulcan was the presiding deity who navigated those wastes of waters, -and to him was intrusted the responsible duty of saving the sun from -extinction. He had a great boat ready, so that when the sun was just -dropping into the ocean at sunset he caught it, and during all the -night he paddled with his glorious cargo round by the north. The glow -of the sun during the voyage could even be sometimes traced in summer -over the great highlands to the north. This, at all events, was their -way of accounting for the long midsummer twilight. After a tedious -night’s voyage Vulcan got round to the east in good time for sunrise. -Then he shot the sun up with such terrific force that it would go -across the whole sky, and then the industrious deity paddled back with -all his might by the way he had come, so as to be ready to catch the -sun in the evening and thus repeat his never-ending task. - - -THE DAILY ROTATION OF THE EARTH. - -Vulcan and his boat seemed a pretty way of accounting for the sun’s -apparent motion. The chief drawback was that it was all work and -no play for poor Vulcan. There were also a few other difficulties. -Captains of ships told us that they had sailed out on the great sea, -and that so far from finding that the ocean extended on and on in one -flat plain forever, the water seemed to bend round, so that, in fact, -after sailing far enough in the same direction, they found that they -would be brought back again to the place from which they started. They -also knew a little about the north. They told us that there could be no -such ocean as that which Vulcan in this fable was supposed to navigate. -It also appeared that ships had been voyaging all over the globe -night and day in every direction, and that no captain had ever seen -the sun coming down to the sea, and still less had he ever met with -Vulcan in the course of his incessant voyages. Thus it was discovered -that the earth could not be a never-ending flat, but that it must be -a globe, poised freely in space without any attachment to hold it up. -It was thought that the change from day to night might be accounted -for by supposing that the sun actually went round the earth through -the space underneath our feet. This is, indeed, what it seems to do. -But there was a great difficulty about this explanation, which began -to be perceived when the size and distance of the sun were considered. -It required the sun to possess an alarming activity. He would actually -have to rush round a circle one hundred and eighty million miles in -diameter and complete this astonishing voyage once every day. - -[Illustration: FIG. 21.--How we illustrate the Changes between Day and -Night.] - -A little reflection will show that a very much simpler explanation was -available. It was shown that the sun need not revolve round the earth -once every day, but that everything would be explained if the earth -itself turned round in such a way as to produce the changes from day -to night. We may illustrate the case by this figure (Fig. 21). The -small globe is the earth, which I can turn by the handle. The lamp will -represent the sun, and, as at present shown, the side of the earth, -on which England lies, is towards the lamp and in full day. On the -opposite side of the globe are other countries such as New Zealand, and -there it is dark. You see that by simply turning the handle I can move -England gradually round so that it passes into the dark side, and then -night falls over the country. At the same time New Zealand is turned -round to enjoy the smiles of day. This is a very simple method of -accounting for the succession of day and night, and it is also the true -method. We have already seen that the sun turns round, and now we find -that the earth also turns, but the little body, the earth, goes much -the faster, for it makes twenty-five turns while the sun goes round -once. - -Our earth is at this moment spinning round at a speed so great that -London moves many hundreds of miles every hour. A town near the -equator would gallop round at a pace of more than a thousand miles an -hour--quicker, in fact, than a rifle-bullet. Don’t you think that we -ought to perceive that we are being whirled about in this terrific -fashion? We know that when we are flying along in a railway train, we -feel the jolting and we hear the noise, and we feel the blast of air -if we put our heads out of window, and we see the trees as they appear -to rush past. All these things tell us that we are in rapid motion. -But suppose these sensations were absent. Imagine a line so perfectly -laid that no jolts are perceptible, and that no racket is heard; draw -down the blinds so that nothing can be seen, how then are we to know -that we are moving? Indeed, your grandfathers used to be able to enjoy -such a tranquil locomotion. I remember seeing in my childhood the -fly-boats, as they were called, on the Royal Canal, wherein passengers -were conveyed from Dublin to the West of Ireland, before the railway -was made. The fly-boat was a sort of Noah’s ark in appearance, drawn -by a horse cantering along the towing-path. In the cabin of such -a vessel, where there was not the slightest motion of rolling or -pitching--nothing but noiseless gliding along the canal--no one would -be conscious of motion, so long as he did not look through the cabin -windows. No one was ever seasick in a fly-boat; it was the perfection -of travelling for those who loved ease and quiet. - -The motion of the earth round its axis is, so far, like that of the -fly-boat. It is so absolutely smooth that we do not feel anything, and -we only become conscious of it by looking at outside objects. These -are the sun, or the moon, or the stars. We see these bodies apparently -going through their unvarying rising and setting, just as, in looking -out from the fly-boat, the passengers in that quaint old conveyance -could see the houses and trees as they passed. - -Seeing is believing; and I should like here, in this very theatre, to -show you that we are actually turning round; and this I am enabled to -do by the kindness of my distinguished friend, Professor Dewar. - -[Illustration: FIG. 22.--A Pendulum.] - -I am tempted to wish that I had Aladdin’s lamp for the moment, for I -would rub it, and when the great genie appeared, I would bid him take -the Royal Institution, and all of us here, to a place which everybody -has heard of, and nobody has seen--I mean the North Pole. It would be -so easy to describe the experiment I am about to show you, there. It is -not so easy here. But it will be sufficiently accurate for our purpose -to suppose that we actually have made the voyage, and that this is the -Pole at the centre of the lecture-table. The direction of the axis -round which the earth is turning is a line pointing up straight to the -ceiling. This lecture-table and all the rest of the theatre is going -round. In about six hours it will have moved a quarter of the way, and -in twenty-four hours it will have gone completely round. That is, at -least, what would happen if we were actually at the Pole. As we are -not there, for the Pole is many miles away from the Royal Institution, -I must slightly modify this statement, and say that the table here -takes more than twenty-four hours to go round. And now I want some -way of proving that such is actually the case. There is no use in our -merely looking at it, because we ourselves, and this whole building, -and the whole of London, are all turning together. What we want is -something which does not partake of the motion. Here is a heavy leaden -ball (Fig. 22). It is fastened to the roof by a fine steel wire, and -you see it swings to and fro with a deliberate and graceful motion. I -want it to oscillate very steadily, so I draw it to one side and tie -it by a piece of thread to a support, and then I burn the thread, and -the great ball begins to swing to and fro. It would continue to do so -for an hour, or indeed for several hours, and it is a peculiarity of -this motion that the vibration always remains in the same direction -in space. Even the rotation of the earth will not affect the plane of -this great pendulum, so far at least as our experiment is concerned. -Here, then, we have a method of testing my assertion about the turning -round of this theatre. I mark a line on the table, directly underneath -the motion of the ball to and fro. If we could wait for an hour or so, -we should see that the motion of the ball seemed to have altered to -a direction inclined to its original position, but it is really the -table that has moved, for the direction of the motion of the ball is -unaltered. We cannot, however, wait so long, therefore I show you the -ingenious method which Professor Dewar has devised. By a beam from -the electric light, he has succeeded in so magnifying the effect that -even in a single minute it is quite obvious that the whole of this -room is distinctly turning round, with respect to the oscillations of -the pendulum. This celebrated experiment proves by actual inspection -that the earth must be rotating. By measuring the motion we might even -calculate the length of the day, though I do not say it would be an -accurate method of doing so. - -The proper way of finding how long the earth takes to turn round is -by observing the stars. Fix on any star you please, and note it in a -certain position to-night; if you then observe the moment when the star -is in the same place to-morrow, the interval of time that has elapsed -is the true duration of one complete rotation. When accurately measured -its length is found to be 23 hours 56 minutes 4 seconds, or about four -minutes shorter than the ordinary day, measured from one noon to the -next. - - -ANNUAL MOTION OF THE EARTH ROUND THE SUN. - -[Illustration: FIG. 23.--The Changes of the Sun with the Seasons.] - -I have as yet only been speaking of the _daily_ movements by which the -sun appears to go across the heavens between morning and evening. We -next consider the annual movements which give rise to the changes of -the seasons. It is now Christmastide, when the days are short and dark, -while six months ago the days were long and glorious in the warmth -and brightness of summer. A similar recurrence of the seasons takes -place every year, and thus we learn that some great changes alter the -relation between the earth and the sun year after year. We must try and -explain this. Why is it that we enjoy warmth at one season, and suffer -from frost and snow at another? - -Note first a great difference between the sun in summer and the sun in -winter. I will ask you to look out at noon any day when the clouds are -absent, and you will then find the sun at the highest point it reaches -during the day. All the morning the sun has been gradually climbing -from the east; all the afternoon it will be gradually sinking down to -the west. Let us make the same observation at different parts of the -year. Suppose we take the shortest day in December. You will look out -about twelve o’clock from some situation which affords a view towards -the south, and there, as shown in the adjoining sketch (Fig. 23), is -the midwinter sun. - -But now the spring approaches, and the days begin to lengthen. If you -watch the sun you will see it pass higher and higher every noon until -Midsummer Day is reached, and then the sun at noon is found quite high -up in the sky. As autumn draws near, the sun at noon creeps downwards -again until, when the next shortest day has come round, we find that -it passes just where it did at the previous midwinter. With unceasing -regularity year after year the sun goes through these changes. When he -is high at noon we have days both long and warm; when he is low at noon -we have days both short and cold. - -Vulcan with his golden boat was naturally expected to give an -explanation of this. As the summer drew on, each day Vulcan shot out -the sun with a stronger impulse, so that it should ascend higher and -higher. His greatest effort was made on Midsummer Day, when, after -rowing but a little way round from the north towards the east, he drove -off the sun with a terrific effort. The sun soared aloft to the utmost -height it could reach, and in the meantime Vulcan returned to the west -to be ready to catch the sun as it descended. On the other hand, in -midwinter, he came round much further through the east to the south, -and then shot up the sun with his feeblest effort, and had to paddle as -hard as ever he could so as to complete his long return voyage during -the brief day. - -[Illustration: FIG. 24.--How the Stars are to be seen in broad -Daylight.] - -It is evident that there are two quite distinct kinds of motion of the -sun. There is first the daily rising and setting, for which we have -accounted by showing that it is merely an appearance produced by the -fact that the earth is turning round. But now we have been considering -quite a different motion by which the sun seems to creep up and down in -the heavens, and this takes a whole year to go through its changes. - -There is still another point which we must consider before we can -understand all these puzzling movements of the sun. We shall ask the -stars to help us by their familiar constellations. You know, perhaps, -the Great Bear, or the Plough as it is often called, and Orion. There -are also Aries the Ram, Taurus the Bull, and other fancifully named -systems. These constellations have been known for countless ages, and -for our present purposes we may think of them as permanent groups -in the heavens, which do not alter either their own shapes or their -positions relatively to each other. These groups of stars extend all -around the sky. They are not only over our heads and on all sides down -to the horizon, but if we could dig a deep hole through the earth, -coming out somewhere near New Zealand, and if we then looked through, -we should see that there was another vault of stars beneath us. We -stand on our comparatively little earth in what seems the centre of -this great universe of stars all around. It is true we do not often -see the stars in broad daylight, but they are there nevertheless. The -blaze of sunlight makes them invisible. A good telescope will always -show the stars, and even without a telescope they can sometimes be -seen in daylight in rather an odd way. If you can obtain a glimpse of -the blue sky on a fine day from the bottom of a coal pit, stars are -often visible. The top of the shaft is, however, generally obstructed -by the machinery for hoisting up the coal, but the stars may be seen -occasionally through the tall chimney attached to a manufactory when an -opportune disuse of the chimney permits of the observation being made -(Fig. 24). The fact is that the long tube has the effect of completely -screening from the eye the direct light of the sun. The eye thus -becomes more sensitive, and the feeble light from the stars can make -its impression, and produce vision. From all these various lines of -reasoning we see that there can be no doubt of the continuous presence -of stars above and around us, and below us, on every side, and at all -times. - -[Illustration: FIG. 25.--The Sun seems to revolve around the Earth.] - -If you look out at Christmas time, towards the south, you will see the -Belt of Orion and the Dog Star in a splendid portion of the heavens. -These stars you will see every winter in the same place. But you may -look in vain for them in summer. No doubt you can see stars in the -summer evenings, but they will be totally different from those that -adorned the skies in winter. Each season has its own constellations. -This simple fact was known to the ancients, and we shall find -its explanation full of meaning. Let us select four well-known -constellations which will best answer our purpose. They lie in a circle -round the heavens. They are Orion, Virgo, Scorpio, and Pisces. I am -supposing that you are looking out at midnight towards the south. In -December you will see Orion; in March, Virgo; in June, Scorpio; and in -September, Pisces; and then next December you will be looking at Orion -again. See what this proves. At midnight, of course, the sun is at the -other side of the earth, so that if I am looking at Orion in midwinter -the sun must be behind my back. Look at our little picture (Fig. 25). -The earth is in the middle, and the sun must be on the opposite side -to Orion. That is, the sun must be somewhere about the position I have -marked at A. In March we see Virgo in the south at midnight, when, of -course, the sun is at the other side of the earth; so that the sun must -be somewhere at B. In June Scorpio is seen, so that the sun must be at -the other side, at C. That is to say, in midsummer the sun is in that -part of the sky where Orion is situated. If, therefore, on a bright -June day we could see the stars, we should find Orion in the south. -But, of course, the light of the sun makes Orion invisible. We can, -however, see the stars by our telescopes, and on rare occasions an -eclipse of the sun will occur, by which he is temporarily extinguished, -and then we can see the stars without the help of a telescope, even -though it is daytime. - -[Illustration: FIG. 26.--The Earth, however, really revolves around the -Sun.] - -Thus it would seem as if the sun were first at A and then at B, C, and -D, and then began to go round again. I say it would _seem_ as if the -sun had these movements, and the ancients thought there was no doubt -about the matter. Even after it was plain that the earth turned round -on its axis so as to give the changes of day and night, it was still -thought necessary to suppose that the sun went round the earth once in -the year, in order to explain how the changes in the stars during the -different seasons were produced. - -Here is another case in which we must be careful to distinguish between -what appears to be true and what is actually the case. Everything that -we undoubtedly see would be just as well explained by supposing that -the sun remained at rest, and that the earth revolved around it, as -in Fig. 26. If, for instance, the earth were at A in midwinter, then -the sun is on the opposite side to Orion, and of course at midnight we -shall be able to see Orion. So in spring the earth is at B, and we see -Virgo, and similarly in summer we have Scorpio, and in autumn Pisces. -Thus all that is actually visible could be fully accounted for by -regarding the sun as fixed in the centre, and the earth as travelling -round it from A to B, to C and to D respectively, and completing the -journey in a twelvemonth. Which idea are we to adopt? Shall we say that -the earth goes round the sun, or the sun goes round the earth? - -I remember an old college story, which I cannot help giving you at this -place. It may serve to lighten what I fear you must otherwise have -thought rather a tedious part of our subject. There were three students -brought up for examination in astronomy, and they showed a lamentable -ignorance of the subject, but the examiner, being a kind-hearted man, -wished, if possible, to pass them; and so he proposed to the three -youths the very simplest question that he could think of. Accordingly, -addressing the first student, he said: “Now tell me, does the earth go -round the sun, or the sun go round the earth?” “It is--the earth--goes -round the sun.” “What do you say?” he inquired, turning rather suddenly -on the next, who gasped out: “Oh, sir--of course--it is the sun goes -round the earth.” “What do you say?” he shouted at the third unhappy -victim. “Oh, sir, it is--sometimes one way, sir, and sometimes the -other!” - -But which is it? Well, we must remember that the earth is comparatively -a very little body and the sun a very big one, so it is not at all -surprising to learn that the earth goes round the sun, which remains, -practically speaking, at rest in the centre. Thus our great earth and -all it contains are continually bound in what is very nearly a circular -course round the great luminary. You will find it instructive to work -out this little sum. How fast is the earth moving, or how far do we go -in a second? We are about 93,000,000 miles from the sun, and the great -circle that we go round has a diameter twice as great as this--that is, -about 186,000,000 miles. The circumference of a circle is nearly three -and one-seventh times its diameter, and accordingly the whole length -of the voyage in the year is about 585,000,000 miles. This has to be -accomplished in 365 days, so that the daily run must be about 1,600,000 -miles. We divide this by 24, to find the distance journeyed each hour, -which we find to be about 67,000 miles; and we must divide this again -by 60 to find the length covered in a minute, and by 60 again for the -progress made each second. It is truly startling to find that, night -and day, this great earth has to travel more than eighteen miles every -second in order to get round its mighty path in the allotted time. - -I began this lecture about forty minutes ago, and I think from what I -have said you will be able to calculate a result that will, I dare say, -astonish you. In these forty minutes we have moved about 45,000 miles. -No doubt my lecture commenced in this hall, and in your presence; but -can I truly say I began it _here_? Well, no; I began it not here, but -at a place 45,000 miles away; but we have all been travelling together, -and the journey has been so very smooth and free from all jolts, that -we never thought anything about the motion. - -I am sure many of those to whom I am now speaking have read accounts -of voyages in the Arctic regions. You have been told of the sufferings -of the crews during the long winters, amid the ice and snow; and you -have heard how, during that dismal period, there is total darkness, -for the sun never rises for weeks and months together. On the other -hand, these northern regions often present a more cheerful picture. -During midsummer, the long darkness of winter is atoned for by -perpetual sunshine. At midnight there is still the full brilliance -of day, and the sun, though low, no doubt, has not passed below the -horizon. Even in the northerly parts of Europe we can see the midnight -sun. Lord Dufferin, in his delightful narrative of a cruise, entitled -“Letters from High Latitudes,” gives an interesting illustration -of the perplexities arising from endless daylight. It appears that -everything went on happily until the fatal moment when the yacht -crossed the Arctic circle. Then it was that dire tribulation arose -among the poultry. A fine cock was the cause of the trouble. Knowing -his duty, he always liked to be particular about performing the -important task of crowing at sunrise. This he could do regularly, so -long as the yacht remained in reasonable latitudes, where the sun -behaved properly. But when they crossed the Arctic circle, the cock -was confronted with a wholly new experience. The sun never set in the -evening, and consequently never had to rise in the morning. What was -the distracted bird to do? He did everything. He burst into occasional -fits of terrific crowing at all sorts of hours, then he gave up crowing -altogether, but finding that did not mend matters, he took to crowing -incessantly. Exhaustion was succeeded by delirium, and rather than -live any longer in a universe where the sun was capable of pranks -so heartless, the indignant fowl flung himself from the vessel and -perished in the Arctic Ocean. - - -THE CHANGES OF THE SEASONS. - -[Illustration: FIG. 27.--The Changes of the Seasons.] - -In the adjoining figure, I show a little sketch (Fig. 27), by which I -try to explain the changes of the seasons. It exhibits four positions -of the earth, one on each side of the sun. The left. A, represents the -earth when summer gladdens the northern hemisphere; while the right, -C, shows winter in the same region. You will see the two central lines -which represent the axis about which the earth rotates. Of course, -the earth has no visible axis. The line which runs through the globe -from the North to the South Pole is imaginary. It remains fixed in the -earth, for we can prove in our observatories that the Pole does not -shift its position to any considerable extent in the earth itself. -In fact, if we could reach the North Pole and drive a peg into the -ground year after year to mark the exact spot, we should find that -the position of the Pole was sensibly the same. Does it not seem -strange that we should be able to know so much about the Pole, though -we have never been able to get there; have never, in fact, been able -to get within less than 400 miles of it? I think you will be able to -understand the point quite easily. The latitude of a place, as you know -from your geography, is the number of degrees, and parts of a degree, -between that place and the equator. In our observatories, we can -determine this so accurately that the difference between the latitude -of one side of a room and of the other side of the same room is quite -perceptible. As we find that the latitudes of our observatories remain -sensibly unchanged from year to year, we are certain that the Pole -must remain in the same place. Indeed, if the Pole were to alter its -position by the distance of a stone’s throw, the careful watchers in -many observatories would speedily detect the occurrence. - -And now I must direct your attention to something apparently quite -different. When the battle of Waterloo was fought, the great victory -was won with the aid of the old-fashioned musket, a smooth-bore gun -which was loaded at the muzzle with a good charge of powder, and then a -round bullet was rammed down. “Brown Bess,” as the musket was called, -was a most efficient weapon at close quarters, and indeed at any -distance _when the bullet hit_; but there was the difficulty. The round -bullets, rushing up the tube and out into the air in a somewhat vague -manner, had a habit of roaming about, which was quite incompatible with -the accurate shooting of our modern rifles. - -One great improvement in small arms consisted in giving to the bullet a -rapid rotation about an axis which is in the line of fire. This is what -the _rifle_ accomplishes. The grooves in the barrel of the rifle twist -round, and though they only give half a complete turn in the length of -the barrel, yet the speed of the bullet is so great that when it flies -off it is actually spinning with the tremendous velocity of about one -hundred and fifty revolutions a second. Even with the old-fashioned -round bullet, the rifling of the barrel effected great improvement in -the accuracy of the shooting. The introduction of the elongated bullets -was another great improvement, while the adaptation of breech-loading -enabled a bullet to be used rather larger than that which could have -been forced down the barrel, and thus it was insured that the grooves -should bite into the bullet as it hurries past and impart the necessary -spin. - -A body rapidly rotating about an axis has a tendency to preserve the -direction of that axis, and powerfully resists any attempt to change -it. Our earth is spinning in this fashion. It is true that the rotation -is, in one sense, a slow one, for it requires almost an entire day for -each rotation. But when we remember the dimensions of our earth, we -shall modify this notion. We have already stated that any place on the -equator has to travel more than one thousand miles each hour in order -to accomplish the journey within the required time. So far, therefore, -the earth moves like a rifle-bullet, and the direction of its axis -remains constant. - -In the course of the great voyage between summer and winter, the earth -travels from one side of the sun to the opposite side, and in doing -so it still continues to spin about an axis parallel to the original -direction. See the consequences which follow. The sun illuminates half -the earth, and in the left position in Fig. 27, representing summer, -the North Pole is turned over towards the sun, and lies in the bright -half of the earth. There is continual day at the North Pole, and night -is unknown there at this time of year, because the turning of the earth -about its axis will not bring the Pole nor the regions near the Pole -into the dark hemisphere. Thus it is that the Arctic regions enjoy -perpetual day at this season. Look now at the position of England when -the northern hemisphere is tilted towards the sun, and is consequently -enjoying the full splendor of midsummer. As the earth turns round, -England will gradually cross the boundary between light and shade, and -will enter upon the darkened hemisphere. Then there will be night in -England, but you will see from the figure that the day is much longer -than the night, and hence it is that we enjoy the fine long days in -summer. - -We next look at a different scene six months later. The earth has -reached the other side of the sun, but the axis has remained parallel -to itself, consequently the North Pole is now inclined entirely away -from the sun. The earth continues to turn round as before, but its -movements do not bring the North Pole or the surrounding Arctic regions -out of the dark hemisphere, and consequently the night must be unbroken -in these dismal circumstances. The long continuous day which forms the -Polar midsummer is dearly purchased by the gloom and cold of a winter -in which there is no sun for many weeks in succession. Observe also the -changed circumstances of England. In the course of each twenty-four -hours it lies much longer in the dark half of the earth than in the -bright, and consequently there is only a short day succeeded by a long -night. - - -SUNSHINE AT THE NORTH POLE. - -It is a privilege of astronomers to be able to predict events that will -happen in thousands of years to come, and to describe things accurately -though they never saw them, and though nobody else has ever seen them -either. No one has ever yet got to the North Pole, but whenever they -do, we are able to tell them much of what they will see there. We may -leave it to Jules Verne to describe how the journey is to be made, and -how the party are to be kept alive at the North Pole. I shall give a -picture of the changes of the seasons, and of the appearance in the -stars, as seen from thence. - -We shall, therefore, prepare to make observations from that very -particular spot on this earth--the North Pole. I suppose that eternal -ice and snow abide there. I don’t think it would be a pleasant -residence. However, we shall arrange to arrive on Midsummer Day, -prepared to make a year’s sojourn. The first question to be settled is -the erection of the hut. In a cold country it is important to give the -right aspect, and we are in the habit of saying that a southerly aspect -is the best and warmest, while the north and the east are suggestive -only of chills and discomfort. But what is a southerly aspect at the -North Pole, or, rather, what is not a southerly aspect? Whatever way -we look from the North Pole we are facing due south. There is no such -thing as east or west; every way is the southward way. This is truly -an odd part of the earth. The only other locality at all resembling it -would be the South Pole, from which all directions would be north. - -The sun would be moving all through the day in a fashion utterly unlike -its behavior in our latitudes. There would, of course, be no such thing -as rising and setting. The sun would, indeed, at first seem neither -to go any nearer to the horizon nor to rise any higher above it, but -would simply go round and round the sky. Then it would gradually get -lower and lower, moving round day after day in a sort of spiral, until -at last it would get down so low that it would just graze the horizon, -right round which it would circulate till half the sun was below, and -then until the whole disk had disappeared. Even though the sun had now -vanished, a twilight glow would for some time be continuous. It would -seem to come from a source moving round and round below the horizon, -then gradually the light would become fainter and fainter until at last -the winter of utter and continuous blackness had set in. The first -indications of the return of spring would be detected by a feeble -glow near the horizon, which would seem to move round and round day -after day. Then this glow would pass into a continuous dawn, gradually -increasing until the sun’s edge crept into visibility, and the great -globe would at last begin to climb the heavens by its continual spiral -until midsummer was reached, when the change would go on again as -before. - -Our first excursion to the country of Star-land has now been taken, and -we have naturally commenced by studying that sun to which we owe so -much. But we shall have to learn that though our sun is of such vital -importance to us, yet, in magnificence and size, he has many rivals -among the host of stars. - - - - -LECTURE II. - -THE MOON. - - The Phases of our Attendant, the Moon--The Size of the Moon--How - Eclipses are produced--Effect of the Moon’s Distance on its - Appearance--A Talk about Telescopes--How the Telescope aids us in - Viewing the Moon--Telescopic Views of the Lunar Scenery--On the - Origin of the Lunar Craters--The Movements of the Moon--On the - Possibility of Life in the Moon. - - -THE PHASES OF OUR ATTENDANT, THE MOON. - -The first day of the week is related to the greatest body in the -heavens--the sun--and accordingly we call that day Sun-day. The second -day of the week is similarly called after the next most important -celestial body--the moon--and though we do not actually say Moon-day, -we do say Monday, which is very nearly the same. In French, too, we -have _lune_ for moon, and _Lundi_ signifies our Monday. The other days -of the week also have names derived from the heavens, but of these we -shall speak hereafter. We are now going to talk about the moon. - -We can divide the objects in this room into two classes. There are the -bright faces in front of me, and there are the bright electric lights -above. The electric lights give light, and the faces receive it. I can -see both lights and faces; but I see the electric lamps by the light -which they themselves give. I see the faces by the illumination which -they have received from the electric lights. This is a very simple -distinction, but it is a very important one in Star-land. Among all -these bodies which glitter in the heavens there are some which shine -by their own light, like the lamps. There are others only brilliant by -reflected light, like the faces. It seems impossible for us to confuse -the brightness of a pleasant face with the beam from a pretty lamp, but -it is often not very easy to distinguish in the heavens between a body -which shines by its own light and a body which merely shines by some -other light reflected from it. I think many people would make great -mistakes if asked to point out which objects on the sky were really -self-luminous and which objects were merely lighted up by other bodies. -Astronomers themselves have been sometimes deceived in this way. - -The easiest example we can give of bodies so contrasted is found in the -case of the sun and the moon. Of course, as we have already seen, the -sun is the splendid source of light which it scatters all around. Some -of that light falls on our earth to give us the glories of the day; -some of the sunbeams fall on the moon, and though the moon has itself -no more light than earth or stones, yet when exposed to a torrent of -sunbeams, she enjoys a day as we do. One side of her is brilliantly -lighted; and this it is which renders our satellite visible. - -Hence we explain the marked contrast between the sun and the moon. The -whole of the sun is always bright; while half of the moon is always in -darkness. When the bright side of the moon is turned directly towards -us, then, no doubt, we see a complete circle, and we say the moon -is full. On other occasions a portion only of the bright surface is -directed to us, and thus are produced the beautiful crescents and -semicircles and other phases of the moon. - -[Illustration: FIG. 28.--To show that the Moon is lighted by Sunbeams.] - -A simple apparatus (illustrated in Fig. 28) will explain their various -appearances. The large india-rubber ball there shown represents the -moon, which I shall illuminate by a beam from the electric light. The -side of the ball turned towards the light is glowing brilliantly, and -from the right side of the room you see nearly the whole of the bright -side. To you the moon is nearly full. From the centre of the room you -see the moon like a semicircle, and from the left it appears a thin -crescent of light. I alter the position of the ball with respect to the -lamp, and now you see the phases are quite changed. To those on my left -our mimic moon is now full; to those on my right the moon is almost -new, or is visible with only a slender crescent. From the centre of the -room the quarter is visible as before. We can also show the same series -of changes by a little contrivance of Figs. 29 and 30. - -Thus every phase of the moon (Fig. 31), from the thinnest beautiful -crescent of light that you can just see low in the west after sunset -up to the splendor of the full moon, can be completely accounted for -by the different aspects of a globe, of which one-half is brilliantly -illuminated. - -[Illustration: - - FIG. 29. FIG. 30. - -The Phases of the Moon.] - -We can now explain a beautiful phenomenon that you will see when the -moon is still quite young. We fancifully describe the old moon as lying -in the new moon’s arms when we observe the faintly illuminated portion -of the rest of that circle, of which a part is the brilliant crescent. -This can only be explained by showing how some light has fallen on the -shadowed side; for nothing which is not itself a source of light can -ever become visible unless illuminated by light from some other body. - -[Illustration: FIG. 31.--The Changes in the Moon.] - -Let us suppose that there is a man on the moon who is looking at the -earth. To him the earth will appear in the same way as the moon appears -to us, only very much larger. At the time of new moon the bright side -of the earth will be turned directly towards him, so that the man in -the moon will see an earth nearly full, and consequently pouring -forth a large flood of light. Think of the brightest of all the bright -moonlight nights you have ever seen on earth, and then think of a light -which would be produced if you had thirteen moons, all as big and as -bright as our full moon, shining together. How splendid the night -would then be! You would be able to read a book quite easily! Well, -that is the sort of illumination which the lunar man will enjoy under -these circumstances; all the features of his country will be brightly -lighted up by the full earth. Of course, this earth-lighted side of -the moon cannot be compared in brilliancy with the sun-lighted side, -but the brightness will still be perceptible, so that when from the -earth we look at the moon, we see this glow distributed all over the -dark portion; that is, we observe the feebly lighted globe clasped in -the brilliant arms of the crescent. At a later phase the dark part of -the moon entirely ceases to be visible, and this for a double reason: -firstly, the bright side of the earth is then not so fully turned to -the moon, and therefore the illumination it receives from earth-shine -is not so great; and, secondly, the increasing size of the sun-lighted -part of the moon has such an augmented glow that the fainter light is -overpowered by contrast. You must remember that more light does not -always increase the number of things that can be seen. It has sometimes -the opposite effect. Have we not already mentioned how the brightness -of day makes the stars invisible? The moon herself, seen in full -daylight, seems no brighter than a small particle of white cloud. - - -THE SIZE OF THE MOON. - -It is not easy to answer the question which I am sometimes asked, “Is -the moon very big?” I would meet that question by another, “Is a cat a -big animal?” The fact is, there is no such thing as absolute bigness or -smallness. The cat is no doubt a small animal when compared with the -tiger, but I think a mouse would probably tell you that the cat was -quite a big animal--rather too big, indeed, in the mouse’s opinion. And -the tiger himself is small compared with an elephant, while the mouse -is large as compared with a fly. - -When we talk of the bigness or the smallness of a body, we must always -consider what we are going to compare it with. It is natural in -speaking of the moon to compare it with our own globe, and then we can -say that the moon is a small body. - -The relative sizes of the earth and the moon may be illustrated by -objects of very much smaller dimensions. Both a tennis ball and a -football are no doubt familiar objects to everybody. If the earth be -represented by the football, then the moon would be about as large as -the lawn-tennis ball. But this proportion is not quite accurate, so -I will suggest to you an instructive way of making a better pair of -models of the earth and the moon. In fact, experiments somewhat similar -to those I describe have been actually going on in every kitchen in the -land during this festive season. For have not globes and balls of all -sorts and sizes been made of plum-pudding, and it will only require a -little care on the part of the cook to make a pair of luscious spheres -that shall fairly set forth the sizes of the earth and the moon. There -is first to be a nice little round plum-pudding, three inches in -diameter. It is just a little bigger than a cricket ball. It should, -however, only make its appearance at a bachelor’s table. Were it set -down before a hearty circle on Christmas Day, dire disappointment would -result. One boy of sound constitution could eat it all. Perhaps it -would weigh about three-quarters of a pound. This little globe is to -represent the moon. - -[Illustration: FIG. 32.--Relative Sizes of the Earth and Moon.] - -Another plum-pudding is to be constructed which shall represent -the earth (Fig. 32). We must, however, beg the cook to observe the -proportions. The width of the earth, or the diameter, to use the proper -word, is about four times the diameter of the moon. Hence, as the -small plum-pudding was three inches across, the large one must have -a diameter of twelve inches. This will be a family pudding of truly -satisfactory dimensions; perhaps the cook will be a little surprised -to find the alarming quantity of materials that will be required to -complete a sphere of plum-pudding a foot in diameter. - -These models having been duly made, and boiled, and placed on the -table, we are now to propose the following problem:-- - -“If one schoolboy could eat the small plum-pudding, how many boys would -be required to dispose of the large one?” - -The hasty person, who does not reflect, will at once dash out the -answer, “Four!” He will say, “It is quite plain that, since one of -the puddings has four times the diameter of the other, it must be -four times as big; and therefore, as one boy is able to eat the small -pudding, four boys will be adequate for the large one.” But the hasty -person will, as usual, be quite wrong. His argument would be sound -if it were merely two pieces of sugar-stick that he was comparing; -no doubt there is only four times as much material in a piece twelve -inches long as there is in a piece three inches long. But the -plum-puddings have breadth and depth, which are in the same proportions -as the length, and the consequence is that the large plum-pudding is -far more than four times as big as the small one. No four boys, however -admirable their capacities, would be equal to the task of consuming it. -Nor even if four more boys were called in to help would the dish be -cleared. Twenty boys, forty boys, fifty boys would not be enough. It -would take sixty-four boys to demolish the magnificent plum-pudding one -foot in diameter. - -If the cook will try the experiment, she will find that by taking the -materials sufficient for sixty-four small plum-puddings all of the -same size, and mixing them together, she will, no doubt, make a large -plum-pudding, but its diameter will only be four times that of the -small puddings. - -As a matter of fact, the moon is 2160 miles in diameter, and the earth -is 7918 miles. These numbers are so nearly 2000 and 8000 respectively, -that for simplicity I have spoken of the earth as having a diameter -four times as great as the moon. If we want to be very accurate, we -ought to determine the ratio of the two quantities from the figures -just given. Our illustration of the plum-puddings must, therefore, be -a little modified. The earth is not quite so much as sixty-four times -as big as the moon; but this figure is sufficiently accurate for our -present purpose. - -Another interesting question may be proposed, namely: How much land -is there on the moon? We might state the answer in acres or in square -miles; but it will, perhaps, be more instructive to make a comparison -between the moon and the earth. - -Here also I shall use an illustration; and we shall again consider -two globes which are respectively three inches and twelve inches in -diameter. The globes I use this time are hollow balls of india-rubber. -These will represent the earth and the moon with sufficient accuracy, -and the relative surfaces of these two globes is what I want to find. -There are different ways in which the comparison might be made. I -might, for instance, paint the two globes and see the quantity of -paint that each requires. If I did this, I should find that the great -globe took just sixteen times as much paint as the small one. We can -adopt a simpler plan. The india-rubber in one of these balls has the -same thickness as in the other, as they are each hollow, so that the -quantity which is required for each ball may be taken to represent its -surface. By simply weighing the two balls, I perceive that the large -one is sixteen times as heavy as the small one. You notice here the -difference between the comparative weights of two hollow balls and -two solid ones of the same material. Had these globes been of solid -india-rubber, the large one would have weighed sixty-four times as much -as the small one, just as in the case of the plum-puddings; but being -hollow, the ratio of their weights is only the square of the ratio of -their diameters--that is to say, four times four, or sixteen. - -We are thus taught that if the moon were exactly one-fourth of the -diameter of the earth, its surface would be one-sixteenth part of -that of the earth. It would, no doubt, have made our subject a little -easier and simpler if the moon had been created somewhat smaller than -it is. As, however, the universe has not been solely constructed for -the purpose of these talks about Star-land, we must take things as we -find them. This proportion is not four; it is more nearly 3⅔, and the -relative surfaces of the two bodies is the square of 11/3, or about -13½. In other words, the entire extent of the surface of our globe is -about thirteen and a half times that of the moon. - -The face of the full moon, being half the entire extent of the -surface, is, therefore, about one-twenty-seventh part of the earth’s -surface--continents, oceans, seas, and islands all taken together. The -British Empire and the Russian Empire are each of them as large as the -face of the full moon. - - -HOW ECLIPSES ARE PRODUCED. - -The moon is the attendant, or the satellite of the earth, ministering -to the wants of the earth by mitigating the darkness of our nights. The -earth goes around the sun in its annual journey of 365 days. The moon -revolves around the earth once every twenty-seven days. The motion of -the moon is thus a very complicated one, for it is, in fact, moving -round a body which is itself in constant motion (Fig. 33). - -You will see by your almanacs every year that certain eclipses are to -take place; and after what we have said about the sun and the moon, it -will be easy to understand how eclipses arise. There are two different -kinds. You will sometimes see an eclipse of the moon, and sometimes -those eclipses of the sun of which we have spoken in the last Lecture. -You may be surprised to find with what accuracy the eclipses can be -predicted. We can tell not only those that will occur this year and -next year, but we could also foretell the eclipses that will appear -in a hundred or a thousand years to come; or we can, with equal ease, -calculate backwards, so as to find the circumstances of eclipses that -happened thousands of years ago. This shows how well we have learned -the way the moon moves. - -[Illustration: FIG. 33.--To show how the Earth goes round the Sun and -the Moon round the Earth.] - -[Illustration: FIG. 34.--A Total Eclipse to the Girl and a Partial -Eclipse to the Boy.] - -[Illustration: - - Partial. Annular. - -FIG. 35.--Different Kinds of Solar Eclipse.] - -An eclipse of the sun is the simpler occurrence, so we shall describe -it first. It happens when the moon comes between the earth and the sun. -Look at our little astronomers shown in Fig. 34. A boy and a girl are -both gazing at the sun, when the moon comes between. To the boy the -moon appears to take a great bite out of the sun, so that it looks like -the left-hand picture in Fig. 35. (I have drawn a line from the end -of the telescope in Fig. 34, which shows how much of the sun is cut -off.) This would be called a partial eclipse of the sun. The almanac -will sometimes describe the eclipses as visible in London, or visible -at Greenwich; but that need not be taken so literally as was supposed -by a Kensington gentleman, who, on noticing that the almanac said an -eclipse was to be visible in London, called a cab and drove into the -city to look for it. His almanac had not mentioned that it would be -visible from his own house. You may usually take for granted that when -an eclipse is said to be visible from London or Greenwich, it will be -more or less visible all over England. Most of these eclipses are only -partial, and though they are interesting to watch they do not teach us -much. By far the most wonderful kind of eclipse is that in which the -whole of the bright part of the sun is blotted out. Then, indeed, we do -see wonders. But such eclipses are rare, and even when they do occur -they only last a very few minutes. The sights that are displayed are so -interesting that astronomers often travel thousands of miles to reach -a suitable locality for making observations. - -The girl in Fig. 34 is placed in the best possible position for seeing -the eclipse. There you find her right in the line of the sun and moon; -and I think you will agree that she cannot see any part of the sun, for -the moon is altogether in the way. I have drawn two dotted lines, one -at each side. All that she can see beyond the moon must lie outside -these dotted lines, and she will be in the dark as long as the moon -stays in the way. When the eclipse is complete, comparative darkness -steals over the land. The birds are deceived, and fly home to the trees -to roost. The owls and the bats, thinking their time has arrived, -venture forth on their nocturnal business. Even flowers close their -petals, only to open a few minutes later when the sun again bursts -forth. Other flowers that give forth their fragrance at night are also -sweetly perceptible so long as the sun remains obscured. An unruly -cow, accustomed to break into a meadow at night, was found there after -an eclipse was over; while I learn from the same authority that a man -rushed over in great excitement to see what his chickens were doing, -but came back much disappointed on finding them pecking away as if -nothing had happened. - -It will sometimes happen that the moon is so placed that the edge of -the sun can be seen all round it. A case of the kind is shown in the -right-hand picture of Fig. 35. It is called an annular, or ring-shaped -eclipse. - -The eclipses of which we have been speaking are, of course, only to be -seen during the day when the sun must be up. The lunar eclipses, which -are visible at night, are due to the interposition of the earth between -the sun and the moon. The sun is at night-time under our feet at the -other side of the earth, and the earth throws a long shadow upwards. -If the moon enter into this shadow, it is plain that the sunlight is -partly or wholly cut off, and since the moon shines by no light of her -own, but only by light borrowed from the sun, it follows that when -she is buried in the shadow all the direct light is intercepted, and -she must lose her brilliancy. Thus we obtain what is called a lunar -eclipse. It is total if the moon be entirely in the shadow. The eclipse -is partial if the moon be only partly in the shadow. The lunar eclipse -is visible to everybody on the dark hemisphere of the earth if the -clouds will keep out of the way, so that usually a great many more -people can see a lunar eclipse than a solar eclipse, which is only -visible from a limited part of the earth. It thus happens that the -lunar eclipse is the more familiar spectacle of the two. - -When the moon is entirely in the shadow, one might naturally think -that it would become totally invisible. This is not always the case. -It is a curious fact that in the depth of a total eclipse the moon is -often still visible, for she glows with a copper-colored light, which -is bright enough to render some of the chief marks on her surface -distinctly discernible. - - -EFFECT OF THE MOON’S DISTANCE ON ITS APPEARANCE. - -We are now about to take a good look at the moon and examine the -different objects which are marked upon it. There is a peculiar -interest attached to this particular orb, because it is much the -nearest of all the heavenly bodies to our globe, and therefore the one -that we can see the best. Every other object--sun, star, or planet--is -hundreds, or perhaps thousands, of times as far off as the moon. It is -right that we should desire to learn all we can about the bodies in -space. We know that the earth is a great ball, and we see that there -are many other such bodies. Some of them are much larger, and some of -them are smaller than the globe on which we dwell; some of them are -dark bodies like the earth, and among them the moon is one. Is it not -reasonable that we should make special efforts to find out all we can -about this interesting neighbor? - -Though the moon is so close to us relatively to other objects in space, -yet when we express its distance in the ordinary methods of measurement -it is a very long way off--about 240,000 miles--a length nearly as -great as that of all the railways in the world put together. An express -train which runs forty miles an hour would travel 240 miles in six -hours, and the whole distance to the moon would be accomplished in -6000 hours, so that travelling by night and day incessantly you would -accomplish the journey in 250 days. To take another illustration, if -you wrapped a thread ten times round the equator of the earth, it would -be long enough to stretch from the earth to the moon. Or suppose a -cannon could be made sufficiently strong to be fired with a report loud -enough to be audible 240,000 miles away. The sound would only be heard -at that distance a fortnight after the discharge had taken place. - -The moon is too far for us to examine the particular features on its -surface by the unaided eye. Suppose that there was a mighty city -like London on the moon, with great buildings and teeming millions -of people, and you went out on a fine night to take a look at our -neighbor. What do you think you would be able to see of the great lunar -metropolis? Would you be able to see its streets full of omnibuses, or -even its great buildings? Would you see St. Paul’s and Westminster--the -great parks and the river? Of all these things your unaided eye would -show you almost nothing. I can give you a little illustration. Suppose -that you made a tiny model of London; imagine this little structure -all complete, so that the streets, the buildings, the bridges, -the railways, the parks, and the Thames were placed in their true -proportions; suppose that the miniature city was so small that it could -stand on a penny postage stamp, surely everything would look very -insignificant, even if you had the model in your hand and looked at -it with the aid of a magnifying glass. But suppose it were put on the -other side of the table or on the other side of the room, or the other -side of the street. Even St. Paul’s Cathedral itself would have ceased -to be distinguishable; but yet the distance is not nearly great enough. -You would have to put the little model a quarter of a mile away before -it would be in the right position to illustrate the appearance of a -lunar London to the unaided eye. - - -A TALK ABOUT TELESCOPES. - -The astronomer will not be contented with a mere naked-eye inspection -of a world so interesting as the moon. He will get a telescope to help -his vision. The word “telescope” means a contrivance for looking at -objects which are a long way off. We have explained that the further -an object is, the smaller it appears to be. The telescope enables us -to largely overcome this inconvenience. It has the effect of making a -distant object look larger. - -There are great differences in the forms of telescopes; and some -instruments are large and some small, according to the purposes for -which they are required. Perhaps the most useful practical application -of the telescope is by the officer on duty on board a ship. He is -generally provided with a pair of these instruments bound together to -form the “binocular.” - -You are all acquainted with this useful contrivance, or at all -events with the opera-glass, that is used for purposes with which -landsmen are more familiar. The ship’s telescope, or the binocular, -or the opera-glass, is feeble in power when compared with the great -instruments of the Observatory. The officer on the ship will generally -be satisfied with a telescope which shall show the objects with which -he is concerned at about one-third of their actual distance. Thus, -suppose his attention is directed to a great steamer three miles -away, he wishes to see her more clearly, and accordingly he takes a -view through his binocular. Immediately the vessel is so transformed -that it seems to be only one mile away. The apparent dimensions of -the object are increased threefold. The hull is three times as long, -the masts and the funnel are three times as high, the sailors are -three times as tall; various objects on the ship too small to be seen -at three miles would be visible from one mile, and to that apparent -distance the ship has now been brought. - -If the sailor desires to reduce the apparent distance of objects, how -much more keenly does the astronomer feel the same want? At best, the -sailor only has to scan a range of a few miles with his glass, but what -are a few miles to the astronomer? It is true that he can count the -distance of the moon by thousands of miles, a good many thousands, no -doubt, but for all other objects he must use millions, while for most -bodies in space, millions of millions of miles are the figures we are -constrained to employ. Need it be said that the astronomer must resort -to every device he can to make the body appear closer. He does not -despise the modest binocular. It is often a useful instrument in the -Observatory. It gives most beautiful pictures of the celestial scenery, -and you would be amazed to find how many thousands of stars you can -see with its help which your unaided eye would not show you at all. -The binocular will also greatly improve the appearance of the moon, -but still its powers fall far short of what we require for the study -of lunar landscapes. Even though we can reduce the moon’s apparent -distance to one-third its actual amount, yet still that third is a -very considerable distance. One-third of 240,000 is 80,000, so that we -can see the moon no better with a binocular than we should see it were -it 80,000 miles away, and were we viewing it with the unaided eye. - -[Illustration: FIG. 36.--The Dome at Dunsink Observatory.] - -[Illustration: FIG. 37.--The Equatorial at Dunsink Observatory.] - -I am not going to enter here upon any detailed account of the -telescope, because I shall say a little more on the subject in a later -lecture; at present I only describe that form of instrument which is -most convenient for studying the moon. I take as an illustration the -South Equatorial at Dunsink Observatory, which belongs to Trinity -College, Dublin. - -This telescope has a building to itself, which stands on the lawn in -front of the house. The site is open and elevated, so as to command an -extensive prospect of the heavens. You will see in Fig. 36 a picture -of the structure. It is circular in form and is entered by the little -porch. The most peculiar feature of an edifice intended to contain -this kind of telescope is its roof, or _Dome_, as we call it. It is of -a hemispherical shape with a projecting rim at the bottom. But no one -would go to the trouble and expense of making a round dome like that -over the Observatory if it were not necessary for a particular purpose. -The dome is very unlike ordinary roofs, not only in appearance, but -also because it can turn round. In the next figure you will see a -section through the building, and the wheels are exposed by which the -dome is carried. These wheels run easily on rails, so that when the -attendant pulls the rope which you see in his hands, he turns round a -large pulley, and that operates a little cogwheel which works into a -rack, and thus makes the dome revolve. The roof is built of timber, -covered with copper; it weighs more than six tons, but the machinery -is so nicely adjusted, that a child four years old can easily set -the whole in motion. The object of all this machinery is seen when -we learn that there is only one opening in the dome. It is covered -by the shutter shown over the doorway in Fig. 36. When opened to the -top, it gives a long and wide aperture, through which the astronomer -can look out at the heavens. Of course the dome has to be turned until -the opening has been brought to face the required aspect. The big -telescope can thus be directed to any object above the horizon. You -see a gentleman using the telescope (Fig. 37), and this shows that -the great instrument is nearly three times as long as the astronomer -himself! No doubt the telescope seems to be composed of a good many -different parts, but the essential portions of the instrument are -comparatively few and simple. At the upper end is the object glass, -which consists of two lenses, one of flint glass and the other of crown -glass. Both of these must be of exceptional purity, and the shape to -be given to the lenses is a matter of the utmost importance. It is in -the making of this pair of glasses that the skill of the optician has -to be specially put forth. So valuable indeed is an object glass which -fulfils all the requirements, that it is by far the most costly part -of the instrument. There are no glasses in the interior of the tube -until you come to the end where the observer is looking in. This is -closed by an eyepiece consisting of a lens, or a pair of lenses. There -are usually many different eyepieces for a telescope, and they contain -lenses of varied powers, to be used according to the state of the -atmosphere, or to the particular kinds of observation in progress. - -If you point a big telescope to the sky, and see therein the sun or -the moon or any of the stars, you will speedily find that the objects -pass away out of view. Remember our earth is constantly turning -round, and bears, of course, the Observatory with it, so that though -the telescope be rightly pointed to the heavens at one moment, by -the next it will have been turned aside. To you who are using the -telescope, the appearance produced is as if the heavenly bodies were -themselves moving. We can counteract this inconvenience. The telescope -is supported on a pedestal, which is built on masonry, that goes down -through the floor to its foundation on the solid rock beneath. In the -iron casing at the top of the pedestal you will see a little window, -and inside is clockwork driven by a heavy weight. This clockwork turns -the whole telescope round in the opposite direction to that in which -the earth is moving. The consequence is that the telescope remains -constantly pointed to the same part of the heavens. - -[Illustration: FIG. 38.--The Yerkes Telescope, University of Chicago.] - -This instrument is no doubt a large one, but of late years many much -greater have been built. The most powerful telescope that has ever been -erected is the great Yerkes instrument belonging to the University of -Chicago, of which a picture is shown in Fig. 38. The object glass is 40 -inches across. - - -HOW THE TELESCOPE AIDS US IN VIEWING THE MOON. - -Those who are in charge of an observatory are often visited by persons -who, coming to see the wonders of the heavens, and finding instruments -of such great proportions, not unnaturally expect the views they are to -obtain of the celestial bodies shall be of corresponding magnificence. -So they are, no doubt, but then it frequently happens that the pictures -which even the greatest telescope can display will fall far short of -the ideal pictures which the visitors have conjured up in their own -imaginations, so that they are often sadly disappointed. Especially is -this true with regard to the moon. I have seen people who, when they -had a view of the moon through a great telescope, were surprised not -to find vast ranges of mountains which looked to them as big as the -Alps, or mighty deserts, over which the eye could roam for thousands of -miles. They have sometimes expected to behold stupendous volcanoes that -not only were, but that looked to be as big as Vesuvius. Others seem -to have thought they ought to see the moon with such clearness that -the fields were to be quite visible, and some would not have been much -astonished if they had observed houses and farmyards, and, perhaps, -even cocks and hens. - -There are different ways of estimating the apparent dimensions of -an object, but the size the moon appears to me to have in a great -telescope may be illustrated by taking an orange in your hand and -looking at the innumerable little marks and spots on its surface. The -amount of detail that the eye will show on the orange is about equal -to the amount of detail that a good telescope will show on the moon. A -desert on the moon, which really is a hundred miles across, will then -correspond to a mark about an eighth of an inch in diameter on the -orange. Some of you may ask what is gained by the use of a telescope, -for the moon looks to us as large as a plate with the unaided eye, and -now we hear it only looks as big as an orange in the telescope. But -where is the plate with which you compare your moon supposed to be -held? It is surely not in your hand. It is imagined to be up in the -sky, a very long way off. Though an orange is much smaller than a -plate, yet you will be able to see many more details in the orange by -taking it in your hand than you could see on a plate which was at the -other side of the street. - -[Illustration: FIG. 39.--The Advantage of using a Telescope.] - -I sometimes find that people will not believe how much the telescope -that they are using is magnifying the moon until they use both eyes -together, of which one is applied to the telescope, while the other -is directed to the moon. It will then be seen, even with a very small -instrument, that the telescopic moon is as big as the larger of the two -crescents in the adjoining figure (Fig. 39), while the naked-eye moon -is like the smaller. - -The greatest telescopes are capable of reducing the apparent distance -of an object to about one-thousandth part of its actual amount. If, -therefore, a body were a thousand miles away, it would, when viewed by -one of these mighty instruments, be seen as large as our unaided vision -would show it, were the body only a single mile distant. No doubt this -is a large accession to our power, but it often falls far short of what -the astronomer would desire. The distances of the stars are all so -great that even when divided by one thousand, they are still enormous. -If you have a number expressed by 100,000,000,000,000, then dividing it -by a thousand merely means taking off three of the ciphers, and there -is still a large number left. We are, however, at present concerned -with the moon, and, as its distance is about 240,000 miles, the effect -of the best telescope is to reduce this distance apparently to 240 -miles. Here, then, we find a limit to what the best of all telescopes -can do. It can never show us the moon better than, hardly indeed so -well as, we could see it with our unaided eye were it only 240 miles -over our heads. We cannot expect the most powerful instruments to -reveal any object on the moon unless that object were big enough to be -seen by the unaided eye when 240 miles away. What could we expect to -see at a distance of 240 miles? - -Here is a little experiment which I made to study this point. I marked -a round black dot on a sheet of white paper. The dot was a quarter of -an inch in diameter, and then I fastened this on a door in the garden, -and walked backwards until the dot ceased to be visible. I found this -distance to be about thirty-six yards. I tried a little boy of eight -years old, and it appeared that the dot became invisible to him about -the same time as it did to me. “What has this to do with the moon?” you -will say. Well, we shall soon see. In thirty-six yards there are 5184 -quarters of an inch, and as it is unnecessary to be very particular -about the figures, we may say, in round numbers, that the distance when -we ceased to be able to distinguish the dot was about five thousand -times as great as the width of the dot itself. You need not, therefore, -expect to see anything on the moon or on anything else which is not at -least as wide as the five-thousandth part of the distance from which -we are viewing it. The great telescope practically places the moon at -a distance of 240 miles, and the five-thousandth part of that is about -eighty yards; consequently a round object on the moon about eighty -yards in diameter would be just glimpsed as the merest dot in the most -powerful telescope. To attract attention, a lunar object should be -much larger than this. If St. Paul’s Cathedral stood on a lunar plain, -it would be visible in our great telescopes. It is true that we could -not see any details. We should not be able to distinguish between a -Cathedral and a Town-hall. There would just be something visible, so -that the artist who was making a sketch of that part would put down a -mark with his pencil to show that something was there. This will show -us that we need not expect to see objects on the moon, even with the -mightiest of telescopes, unless they are of great size. - - -TELESCOPIC VIEWS OF LUNAR SCENERY. - -I have already warned you not to expect too much, even with the biggest -of telescopes; and just as a caution, I may, perhaps, tell you a story -I once heard of an astronomer who had a great telescope. It was a very -famous instrument, and people often came to the Observatory at night -to enjoy a look at the heavens. Sometimes these visitors were grave -philosophers, but frequently they were not very accomplished men of -science. One evening such a visitor came to the Observatory, and sent -in his name and an introduction to the astronomer, with a request -that he might enter the temple of mystery. The astronomer courteously -welcomed the stranger, and asked him what he specially desired to see. - -“Oh!” said the visitor, “I have specially come to see the moon--that is -the object I am particularly interested about.” - -“But,” said the astronomer, “my dear sir, I would show you the moon -with pleasure, if you were here at the proper time; but what brings you -here now? Look up; the evening is fine. There are the stars shining -brightly, but where is the moon? You see it is not up at present. In -fact, it won’t rise till about half-past two to-morrow morning, and it -is only nine o’clock now. Come back again in five or six hours, and you -shall observe the moon with the great telescope.” - -But the visitor evidently thought the astronomer was merely trying to -get rid of him by a pretext. And he was equal to the occasion--he was -not going to be put off in that way. - -“Of course, the moon is not up,” he replied; “any one can see that, and -that is the reason why I have come, for _if the moon had been up, I -could have seen it without your telescope at all_!” - -Although no explorer can ever reach our satellite, yet it is hardly -an exaggeration to say that in some respects we know the geography of -the moon a good deal better than we know the geography of the earth. -Think of the continent of Africa. In that great country there are -mighty tracts, there are vast lakes and ranges of mountains, of which -we know but little. We could make a better map of Africa, so far at -least as its broad outlines are concerned, if it were fastened up on -our side of the moon than we actually possess at this moment. There -is no spot on the nearer side of the moon as large as an ordinary -parish in this country which has not been surveyed. There are maps -and charts of the moon showing every part of it, which is as big as a -good-sized field. Indeed, as there are no lunar clouds, the features -of its surface are never obscured whenever our own atmosphere will -permit us to make our observation. Artists have frequently sketched -the lunar features, and there is plenty of material for them to work -on. We have also had photographs taken of the moon, but there is a -special difficulty to be encountered in taking photographs of celestial -bodies which photographers of familiar objects on this earth do not -experience. For a photograph to be successful, everybody knows that -the first requisite is for the sitter to stay quiet while the plate -is being exposed. This is, unhappily, just what the moon cannot do. We -endeavor to obviate the difficulty by moving the telescope round so as -to follow the moon in its progress. This can be done with considerable -accuracy, but, unfortunately, there is another difficulty which lies -entirely beyond our control. As the rays of light from the moon perform -their journey through hundreds of miles of unsteady air, the rays are -bent hither and thither, so that the picture is more affected by the -atmosphere than in the case of a photographer’s portrait taken in the -studio. If we are merely _viewing_ the moon through the telescope, the -quivering effect on the rays of this long atmospheric voyage, though -rather inconvenient, does not prevent us from seeing the object, and we -can readily detect the true shape of each feature in spite of incessant -fluctuations. When, however, these rays fall not on the eye, but on -the photographic plate, they produce by their motion a picture which -cannot be much magnified without becoming very confused and wanting in -sharpness. Nevertheless, for the general outlines of our satellite’s -appearance and for the portraiture of its splendid features we have -derived the greatest assistance from photography. - -[Illustration: FIG. 40.--The Full Moon.] - -The adjoining picture (Fig. 40) gives a fair idea of what the full moon -looks like when viewed through a small telescope. I do not, however, -say that the lunar objects can then be observed under favorable -conditions; for when the moon is full is the very worst time for making -observations of our satellite. In fact, at this phase you can hardly -see anything except slight differences between the colors of different -parts. The best time for observing the moon is at the first quarter; -but even then you can only observe satisfactorily those objects which -happen to lie along the border between light and shade. To study the -moon properly you must, therefore, watch it during several different -phases, from the time when it presents a thin and delicate crescent -(just after new moon) until it has again waned to a thin and delicate -crescent (just before the next new moon). We want the relief given by -shadows to bring out the full beauty of lunar scenery. - -On the map you will first notice the large dark-colored patches which -are so conspicuous on the moon’s face. They are, apparently, the empty -basins which great seas once filled. But if water was ever there it has -at all events now quite disappeared. These dark parts are, no doubt, a -good deal smoother than the rest of the surface; but we can see many -little irregularities which tell us that we are not looking at oceans. -The chief features I want you to observe are the curious rings which -you see in the figure; there is a very well-marked one a little below -the centre, and in the upper part many rings--large and small--are -crowded together. We call them lunar craters. You will see what they -are like from the model, of which a picture is shown in Fig. 42. But to -realize from this picture the proper scale of the object, you should -imagine it to be some miles in width. The cliffs which rise all round -to form the wall, as well as the mountain which adorns the centre, are -quite as high as any of the mountains in Great Britain. - -[Illustration: FIG. 41.--View on the Moon. - -(_By Lœwy and Puiseux, Paris Observatory._) - -The large central crater is Hipparchus and above it is Albategnius.] - -[Illustration: FIG. 42.--Our Model of a Lunar Crater.] - -You may desire to know how we are able to measure the heights of -mountains on the moon. That is what I am now going to show you; and for -this purpose we shall look at our imitation lunar crater. Here is the -great ring, or circular enclosure, surrounded by cliffs, and here is a -sharp mountain peak rising in the centre. I shall ask to have the beam -from the electric lamp turned on our model. You see how prettily it -is lighted up. I have placed the lamp so that the beams are sloping; -and I have done this with the express object of making the shadows -long. In fact, as we look at a lunar crater, which lies on the border -between light and shade, the sun illuminates the object under the same -conditions as those shown in the figure. I dare say you have often -noticed what long shadows are cast at sunset. Similar shadows are made -to teach the astronomer the altitudes of the lunar mountains; for he -measures the length of the shadow, and then by a little calculation he -can find the height of the object by which that shadow has been cast. -I shall suppose that we want to measure the height of a flagstaff -(Fig. 43). It is quite possible to do this by merely measuring the -length of the shadow which that flagstaff casts at noon. It would not -be correct to say that the height of the flagstaff is the length of -its shadow. This will, indeed, be the case if you are fortunate enough -to make your measurement at or near London on either the 6th of -April or the 5th of September. On all other days in the year a little -calculation must be made, which I need not now mention, but which the -astronomer, with the aid of his Nautical Almanac, can do in a very few -minutes. In a similar manner, by measuring the lengths of the shadows -on the moon, and by finding the number of miles in the shadow, we are -able to calculate the altitudes of the lunar mountains and of the -ranges of cliffs by which the walled plains are surrounded. - -[Illustration: FIG. 43.--How we found the Height of the Flagstaff.] - - -ON THE ORIGIN OF THE LUNAR CRATERS. - -We have now to offer an explanation of the curious rings which are -the most characteristic features on the moon. To account for them we -must look for a moment at some objects on the earth. You have all -heard of volcanoes or burning mountains, such as Vesuvius or Etna, -which occasionally break out into violent eruptions, and send forth -great showers of ashes and torrents of molten lava. In the Sandwich -Islands there is a celebrated volcano called Kilauea. It is like a -vast lake of lava, so hot that it is actually molten, and glows with -heat like red-hot iron. The adventurous tourist who visits this crater -can climb to the brink of a lofty range of cliffs which surround it, -and gaze down upon the fervid sea beneath. Suppose that by some great -change the internal heat which keeps this mighty basin glowing were -to decline and go out, the sea of lava would cease to be liquid, and -would ultimately grow hard and cold, and we should then have an immense -flat plain, surrounded by a range of cliffs. Elsewhere in the Sandwich -Islands examples of extinct craters may be found at the present day. -Those who have studied these interesting localities point out how such -terrestrial craters explain the ringed plains in the moon. It seems -certain that in ancient days great volcanoes abounded on our satellite, -and the rings were often much larger than those on the Sandwich -Islands, some of them being one hundred miles or more in diameter. -The volcanoes must long ago have been raging on the moon with a fury -altogether unknown in any active volcanoes which this earth can now -show. We can also surmise how the lofty mountain peak, which so often -rises in the centre of a lunar ring, has been upheaved. When the fires -had almost subsided, and the floor had grown nearly cold, one last -and expiring effort is made by which the congealing surface is burst -through at the centre, and materials are shot forth which remain as the -central mountain to the present day. - -I must, however, impress upon you that even our greatest telescopes -never exhibit to us any volcanic eruptions at present going on in the -moon; in fact, it is most doubtful if any change has been noticed in -the features on its surface since the date of the invention of the -telescope. The volcanoes sculptured the crust of the moon into the form -in which we see it, and that form our satellite has preserved for ages, -of which we cannot estimate the duration. All the craters and all the -volcanoes in the moon can only be described as extinct. - -It would be interesting for us to compare the present condition of -the volcanoes in the earth with that of the ringed craters in the -moon. The noisy volcanoes on our globe are those most talked about; we -often hear of Vesuvius being in eruption, and in August, 1883, there -was a terrific eruption at Krakatoa, during which a large quantity -of dust was shot up into the air, to such a height that it was borne -right round the earth, and produced beautiful sunsets and unwonted sky -hues in almost every country in the world. The explosion at Krakatoa -made the loudest noise that history has recorded. Fortunately such -convulsions of the earth do not often happen, for, on that occasion, -the sea rushed in on the land, and thousands of lives were lost. There -are said to be one hundred and fifty volcanoes on different parts of -the earth, which are more or less active, but there are many others in -which the fire has gone out, and which seem to be just as cold and -just as extinct as any volcanoes in the moon. Even in our own islands -there are abundant remains of ancient volcanoes. Masses of lava are -found in many places where now there is no trace of an active volcano. -Perhaps there is no more remarkable sight in the British Isles than -that lofty rock which is crowned by Edinburgh Castle; it is the remnant -of a former volcano, while Arthur’s Seat, close by, is another. In -the centre of France is the beautiful district of Auvergne, in which -ancient volcanoes abound; and the lava streams can be traced for miles -across the country. These volcanoes have been extinct for thousands of -years, during which time the lava has become largely covered with soil -and vegetation, and in some places vineyards are cultivated upon it. - -We are now able to contrast the earth with the moon, in so far as -volcanoes are concerned. On the earth we have some that are active, -and a much greater number that are extinct. On the moon we find no -active volcanoes, for there all are extinct. I can explain how this -difference has arisen, but first let me show you a simple experiment. -My assistant will kindly bring to me from that furnace two iron balls, -which we placed there before the commencement of this lecture; there -they are, you see, both glowing with a bright red heat, for at present -they are equally hot. We will place them on these stands, and allow -them to grow cold. One of these balls is a small cannon-ball, four -inches in diameter, while the other is only one inch. They are in the -same proportion as the earth is to the moon; but look, even while I am -speaking the balls have ceased to preserve the same temperature, for -the little one has become almost black from loss of its heat, while the -large one still looks nearly as red as it did at the beginning; this -simple experiment will illustrate the principle that two heated bodies -will cool at very different rates, if their sizes be different, while -the other conditions are the same. The small body will always cool -faster than the large one. They need not be globes for this experiment; -if you put a poker and a knitting needle into the fire, and leave both -there until they are red-hot, and then put them out into the fender, -you will speedily find that though they were at the same temperature -when drawn from the fire, they do not long remain so; indeed, the -knitting needle has become cold enough to handle before the poker has -ceased to glow. Our experiments have been made with, no doubt, small -objects only, but the law about which they inform us will remain true, -even for the greatest objects. - -Our earth at the present day shows many indications of being much -hotter within than it is on the surface. The volcanoes themselves are -mere outbreaks of incandescent material from inside. Then there are -hot springs of water at Bath, which gush out from the earth. There -are geysers of hot water in Iceland and in the Yellowstone Park in -America, and in other places. And there are other indications also, -with which every miner is familiar. Wherever a deep pit is sunk into -the earth, the rocks below are always found to be warmer than those -on the surface, and the deeper the pit the greater is the heat that -is encountered. Thus, from all over the world we obtain proofs of -the present existence of internal heat. Great as is the earth, we -must still apply the simple common-sense principles that we use in -our everyday life here. Let me give an illustration. Suppose that a -servant came into the room and placed a jug of water on the table, and -that an hour afterwards you went to the jug of water and found it to -be cold, you would not from that fact alone be able to infer anything -with certainty, as to whether the water had been warm or cold when it -was brought in. It might have been perfectly cold, as it is at present, -though on the other hand the water might have been warm at first, and -have since cooled down to the temperature of the room during the hour. - -Suppose, however, that when you went to the jug of water, which had -stood on the table for an hour, you found it tepid, no matter how -slightly its temperature might be above that of the room, do you not -see the inference you would be able to draw? You would argue in this -way: that water has still some heat; it must, of course, be gradually -cooling, and therefore it was hotter a minute ago than it is now; it -was hotter still two minutes ago, or ten minutes; and must have been -very hot and perhaps boiling when it was brought in an hour ago. - -I want you to apply exactly the same reasoning to our earth. It is, -as I have shown you, still hot and warm inside. Of course, that heat -is gradually becoming lost; so that the earth will from year to year -gradually cool down, though at an extremely slow rate. But we must -look back into what has happened during past ages. Just as we inferred -that the jug must have contained very hot water an hour before from -the mere fact that the water was still warm, so we are entitled to -infer, from the fact that the earth still retains some heat, that it -must in ages gone by have been exceedingly hot. In fact, the further -we look back, the hotter do we see the earth growing, until at last we -are constrained to think of a period, in the excessively remote past, -long ere life began to dawn on this earth, when even the surface of the -earth was hot. Back further still we see the earth no longer covered -with the hard, the dark, and the cold surface we now find; we are to -think of it in these primitive times as a huge glowing mass, in which -all the substances that now form the rocks were then incandescent, and -even molten material. - -There is good reason for knowing that in those early times the moon -also was molten with heat; and thus our reasoning has led us to think -of a period when there were two great red-hot globes--one of which had -about four times the diameter of the other--starting on their career -of gradually cooling down. Recall our little experiment with the two -cooling globes of iron; imagine these globes to preserve their relative -proportions, but that one of them was 8000 miles and the other 2000 -miles across. Ages will, no doubt, elapse ere they part with their -heat sufficiently to allow the surfaces to cool and to consolidate. We -may, however, be sure that the small globe will cool the faster, that -its outside will become hard sooner than will the surface of the large -one, and long after the small globe has become cold to the centre, -the large one may continue to retain some of its primeval heat. We -can thus readily understand why all the volcanoes on the moon have -ceased--their day is over. It is over because the moon, being so small, -has grown so cold that it no longer sustains the internal fires which -are necessary for volcanic outbreaks. Our earth, in consequence of its -much greater size, has grown cold more slowly. It has no doubt lost the -high temperature on the exterior, and its volcanic energy has probably -abated from what it once was. But there is still sufficient power in -the subterranean fires to awaken us occasionally by a Krakatoa, or -to supply Vesuvius with sufficient materials and vigor for its more -frequent outbursts. The argument shows us that the time will at last -come when this earth shall have parted with so large a proportion of -its heat that it will be no longer able to provide volcanic phenomena, -and then we shall pass into the exhausted stage which the moon attained -ages ago. - - -THE MOVEMENTS OF THE MOON. - -Though the moon is going round and round the earth incessantly, yet it -always manages to avoid affording us a view of what is on the other -side. Our satellite always directs the same face towards the earth, and -we may reasonably conjecture that the other side is covered, like the -side we know, with rings and other traces of former volcanoes. In this -respect the moon is quite a peculiar object. The other great celestial -bodies, such as the sun or Jupiter, turn round on their axes, and show -us now one side and then the other, with complete impartiality. The way -in which the moon revolves may be illustrated by taking your watch -and chain, and as you hold the chain at the centre making the watch -revolve in a circular path. At every point of its path the ring of the -watch is, of course, pointed to the centre where the chain is held. If -you imagine your eye placed at the centre, to represent the earth, the -movements of the watch would exemplify the way the moon turns round it. - -One more point I must explain about the moon before we close this -lecture. There is nothing more familiar than the fact that a heavy body -will fall to the ground. Indeed, it hardly matters what the material -of the body may be, for you see I have a small iron ball in one hand -and I hold a cork in the other. I drop them at the same moment, and -they reach the ground together. Perhaps you would have expected that -the cork would have lagged behind the iron. I try the experiment -again and again, and you can see no difference in the times of their -falling, though I do not say this would be true if they were dropped -from the top of the Monument. In general we may say that bodies let -drop will fall sixteen feet in the first second. Even a bit of paper -and a penny piece will fall through the same height in the same time -if you can get over the difficulty of the resistance of the air. This -is easily managed. Cut a small piece of tissue paper which will lie -flat on the top of the penny, and hold the penny horizontal with the -paper uppermost. Though there is nothing to fasten the paper to the -penny, you will find that they fall together. If we could conduct the -experiment of dropping the penny and the bit of paper in a vacuum, -then, whether the paper was laid on the penny or placed in any other -way, the two objects would reach the table at the same moment if -released at the same moment at equal heights. - -Wherever we go we find that bodies will always tend to fall in towards -the centre of the earth; thus in New Zealand, at the opposite side of -our globe from where we are now standing, bodies will fall up towards -us, and this law of falling is obeyed at the top of a mountain as it is -down here. No matter how high may be the ascent made in a balloon, a -body released will fall towards the earth’s centre. Of course, we can -only ascend some five or six miles high, even in the most buoyant of -balloons; but we know that the attraction by which bodies are pulled -downwards towards the earth extends far beyond this limit. If we could -go ten, twenty, or fifty miles up, we should still find that the earth -tried to pull us down. Nor, even if you could imagine an ascent made to -the height of 1000 miles, would gravitation have ceased. A cork or an -iron ball, or any other object dropped from the height of 1000 miles, -would assuredly tumble down on the ground below. - -Suppose that by some device we were able to soar aloft to a height -of 4000 miles. I name that elevation because we should then be as -high above the earth as the centre of the earth is below our feet. We -should have doubled our distance from the centre of the earth, and the -intensity of the gravitation would have decreased to one-quarter of -what it is at the surface. A body which at the earth’s surface falls -sixteen feet in a second would there fall only four feet in a second, -and the apparent weight of any body would be so much reduced that it -would seem to weigh only a quarter of what it weighs down here. Thus, -the higher and higher we go, the less and less does gravity become; but -it does not cease, even at a distance of millions of miles. Therefore -you might say that as gravity tries to pull everything down, wherever -it may be, why does it not pull down the moon? This is a difficulty -which we must carefully consider. Supposing that the earth and the moon -were simply held apart, both being at rest, and that then the moon -were to be let go, it would no doubt drop down directly on the earth. -The movement of the moon would, however, be very different if, instead -of being merely let fall, it was thrown sideways. The effect of the -earth’s pull upon the moon would then be shown in keeping the moon -revolving around us instead of allowing it to fly away altogether, as -it would have done had the earth not been there to attract it. - -[Illustration: FIG. 44.--An Illustration to explain the Movement of the -Moon] - -We can explain this by an illustration. On the top of a mountain I have -placed a big cannon (Fig. 44). We fire off the cannon, and the bullet -flies away in a curved path, with a gradual descent until it falls to -the ground. I have made the mountain look hundreds of times larger than -any mountain could possibly be; and now I want you to imagine a cannon -far stronger and gunpowder more potent than any powder or cannon that -has ever yet been manufactured. Fire off a bullet with a still greater -charge than the last time, and now the path is a much longer one, but -still the bullet curves down so as ultimately to fall on the earth. But -make now one final shot with a charge sufficiently powerful, and away -flies the bullet, following this time the curvature of the earth, for -the earth’s attraction has the effect of bending the path of the bullet -from a straight line into this circular form. By the time the bullet -has travelled a quarter of the way round, it is no nearer to the earth -than it was at first, nor has it parted with any of its original speed. -Thus, notwithstanding its long journey, the bullet has practically just -as much energy as when it first left the muzzle of the cannon. Away it -will fly round another quarter of the earth, and still in the same -condition it will accomplish the third and the fourth quarters, thus -returning to the point from which it started. If we have cleared the -cannon out of the way, the bullet will fly again over the mountain top -without having lost any of its speed by its voyage round the earth. -Therefore it will be in a condition to start again, and thus to revolve -around the earth permanently. If, then, from the top of a mountain -240,000 miles high a great bullet 2000 miles in diameter had once been -projected with the proper velocity, that bullet would continue forever -to circle round and round the earth, and even though the mountain and -the cannon disappeared, the motion would be preserved indefinitely. -This illustration will, at all events, show how a continuous revolution -of the moon round the earth can exist, notwithstanding that the earth -is constantly pulling our satellite down towards its surface. - - -ON THE POSSIBILITY OF LIFE IN THE MOON. - -Astronomers are often asked whether any animals can be living on the -moon. No observations we can make with the telescope can answer that -question directly. There are great plains to be seen on the moon, of -course, but even if there were elephants tramping over those plains, -our telescopes could not show them. Nor will our instruments pronounce -at once whether plants or trees flourish on the moon. The mammoth trees -of California are so big that a tunnel has been cut through the trunk -of one large enough to give passage for a carriage and pair. Even if -there were trees as big as this on the moon, they would not be visible -from the most famous observatories. - -Let us think what we should ourselves experience if we could in some -marvellous manner be transferred from the earth to its satellite, and -tried to explore that new and wonderful country. Alas, we should find -it utterly impossible to live there for an hour, or even for a minute! -Troops of difficulties would immediately beset us. The very first would -be the want of air. Ponder for a moment on the invariable presence -of air around our own globe. Even if you climb to the top of a high -mountain, or if you take a lofty voyage in a balloon, you are all the -time bathed in air. It is air which supports the balloon, just as a -cork is buoyed up by water. In all circumstances, we must have air to -breathe. In that air is oxygen gas, and we must have oxygen incessantly -supplied to our lungs to reinvigorate our blood. We require, too, that -this oxygen shall be diluted with a much larger amount of nitrogen -gas, for our lungs and system of circulation are adapted for abode in -that particular mixture of gases which we find here. The atmosphere -becomes more and more rarefied the higher we ascend, and apparently -terminates altogether some two or three hundred miles over our heads. -Beyond the limits of the atmosphere it seems as if empty space would be -met with all the way from the earth to the moon. We could not procure -a single breath of air, and life would be, of course, impossible. Even -at a height of three or four miles, respiration becomes difficult, -and doubtless life could not possibly be sustained at a height of ten -miles. - -It is therefore plain that for a voyage to the moon we should require -an ample supply of air, or, at least, of life-giving oxygen, which in -some way or other was to be inhaled during the progress of the journey. -When at length 240,000 miles had been traversed, and we were about -to land on the moon, we would first of all ascertain whether it was -surrounded with a coating of air. Most of the globes through space -are, so far as we can learn, covered and warmed with an enveloping -atmosphere of some kind; but, unhappily, the poor moon has been left -entirely, or almost entirely, without any such clothing. She is quite -bare of atmosphere at all comparable in density or in volume to that -which surrounds us, though possibly we do now and then perceive some -traces of air, or of some kind of gas, in small quantities in the lunar -valleys. - -I am sure each intelligent boy or girl will want to know how we are -able to tell all this. We have never been at the moon, and how then can -we say that it is nearly destitute of air? Nor can our telescope answer -this question immediately, for you could hardly expect to see air, -even if it were there. How then are we able to make such assertions? -There are many different ways in which we have learned the absence of -air from the moon. I will tell you one of the easiest and the most -certain of these methods. First let me say that air is not perfectly -transparent. No doubt I can see you, and you can see me, though a good -many feet of air may lie between us; but when we deal with distances -much greater, there is a very simple way in which we can show that air -is not quite transparent. In the evening, when the sun is setting and -the sky is clear, you can look at him without discomfort; but in the -middle of the day you know that it is impossible to look at the sun -without shading your eyes with smoked glass or protecting them by some -similar contrivance. The reason is, that when the sun is either setting -or rising we look at it through an immense thickness of air, which not -being perfectly transparent stops some of the light. Thus it is that -the sun in these circumstances loses its dazzling brilliancy, and we -can view it without discomfort. - -At the seaside you can notice the same effect in a different manner. Go -out on a fine and clear night, when the stars in their thousands are -glittering overhead, and then look down gradually towards the horizon, -and you will find the stars becoming fainter and fainter. Indeed, even -the brightest star cannot be seen when it is at the horizon, because an -immense thickness of the atmosphere is not transparent. - -We can now state the argument by which we may prove that there is -little or no air on our satellite. The moon will frequently pass -between the earth and a star, and when the star is a really bright one -the observations that can be made are of great interest. Let me first -describe what we actually see. The star is shining brightly until the -moment when the moon eclipses it. Generally speaking, its disappearance -is instantaneous. But this would not be the case if the moon were -encircled with an atmosphere. If the moon were coated with air, the -light from the star would not be extinguished _instantly_; it would -gradually decline, according as it had to pass through more and more -of the moon’s atmosphere. Thus you would find that the star dwindled -down in brightness before the solid body of the moon had advanced far -enough to shut it out. The sudden extinction of the stars demonstrates -the airless state of our satellite. - -There would be another insuperable difficulty in adopting the moon -as a residence, even supposing that you could get there. Water is -absent from its surface. We have examined every part of it, and we -find no evidences of seas or of oceans, of lakes or rivers; we never -see anything like clouds or mists, which are, of course, only water in -the vaporous form. We are, therefore, assured that, so far as water is -concerned, the moon is an absolute desert. This is, perhaps, the most -striking contrast between the aspect of the earth and the aspect of the -moon. Were an astronomer on the moon to look at our earth he would find -most of its surface concealed beneath clouds, and through the openings -in these clouds he would see that by far the greater part of this globe -was covered by the expanse of ocean; in fact, when the lunar astronomer -had realized the prevalence of water upon this earth, either in the -form of ocean or cloud, I feel sure he would come to the conclusion -that nothing could live here except seals or other amphibious animals. - -Owing to the absence of air and water, the moon would be totally -disqualified for the support of life of those types in which we know -it. For air and water are necessary to every animal, from the humblest -animalcule up to whales or elephants. Air and water are necessary -for every form of vegetable life, from the lichen which grows on a -stone up to the noble old oak of the forest. But even supposing that -we could land on the moon, bearing with us an ample supply of oxygen -to breathe, and of water to drink, we should find ourselves perplexed -and embarrassed, to say the very least of it, by an extraordinary -difference that would immediately attract our notice. That familiar -experience of gravity, or the weights of things, which we have -acquired in our residence on a great globe like the earth, would seem -ludicrously altered when we began to walk about on a little globe -like the moon. We should be astonished at the transformation by which -the weight of everything was much lessened; when you pulled out your -watch you would hardly feel it at the end of the chain; it would seem -like a mere shell; but yet the watch is all right, it is going as well -as ever. Nothing has altered about it except its weight. A big stone -attracts your notice, and, to your amazement, you find that it does -not weigh so much as a piece of wood of the same size would weigh down -here. A stone that you could hardly stir on the earth, you can carry -about on the moon. Nor is this to be explained by any peculiarity in -the constitution of the lunar stone. Most probably it will be not very -dissimilar to some of the rocks on the earth. The relative lightness -of a lunar stone is not due to its being formed of some very special -material; we must seek for some other explanation. Every object on -the moon would be found only one-sixth as heavy as the same object on -the earth. A sturdy laborer at one of the docks can carry one sack -of corn on his back here, and he finds that this load is as much as -is convenient. He would, however, discover, were he placed on the -moon, that his load had suddenly become lightened to one-sixth part -(Fig. 45). The laborer would find that he could carry six sacks of -corn on the moon without making a greater effort than the support of -a single sack on the earth cost him. To explain how such a change as -this has occurred, look at these two pictures: one shows the laborer -on a small body like the moon, the other shows him on a great globe -like the earth. What the laborer actually does feel is not quite so -simple a thing as he imagines. He imagines that it is the weight of the -corn, and the corn alone, which produces that pressure on his shoulders -which he knows so well. But that is not exactly the manner in which -the philosopher will look at the same question. What the laborer does -actually feel is the attraction between the earth beneath his feet and -the corn on his back. It is this force which produces the pressure on -his shoulders. Its magnitude no doubt depends upon the quantity of corn -in the sack, but it also depends on the quantity of matter on the earth -beneath his feet. In fact, the force between two attracting bodies -depends upon the masses of both the attracting bodies. When the laborer -is transferred to the moon, of which the mass is so much less than that -of the earth, the attraction is less there than it is here, even though -the corn is the same in the two cases. - -[Illustration: FIG. 45.--The Lessened Gravitation on the Moon.] - -Many odd instances could be given of the extraordinary consequences -of life on a world where all weights are reduced to a sixth part. One -occurred to me the other day when I saw a postman going his rounds with -an amazing load of Christmas presents and parcels. I thought, how much -happier must be the lot of a postman on the moon, if such functionaries -are wanted there! All the presents of toys or more substantial -donations might be the same as before, the only alteration would be -that they would not feel nearly so heavy. A box which contains a pound -of chocolate bonbons might still contain exactly the same quantity of -sweetmeat on the moon, but the exertion of carrying it would be reduced -to one-sixth. It would only weigh as much as two or three ounces do -on the earth. Our streets provide another admirable illustration of -the drawbacks of our life here as compared with the facilities offered -by life on the moon. I feel quite confident that no perambulators can -be necessary there. I cannot indeed say that there are babies to be -found on the moon, but of this I am certain, that even if the lunar -babies were as plump and as sturdy as ours, they must still only weigh -about a sixth as much as ours do. A lunar nurse would scorn to use a -perambulator, even for a pair of twins; she might take them both out -on her arm for an airing, and even then only bear one-third of the -load that her terrestrial sister must sustain if she is carrying but a -single child. - -The lightness of bodies in the moon would entirely transform many of -our most familiar games. In cricket, for instance, I don’t think the -bowling would be so much affected, but the hits on the moon would -be truly terrific. I believe an exceptionally good throw of the -cricket-ball here is about a hundred yards, but the same man, using -the same ball and applying the same force to it, would send the ball -six hundred yards on the moon. So, too, every hit would in the lunar -game carry the ball to six times the distance it does here. Football -would show a striking development in lunar play; a good kick would not -only send the ball over the cross-bar, but it would go soaring over the -houses, and perhaps drop in the next parish. - -Our own bodies would, of course, participate in the general buoyancy, -so that, while muscular power remained unabated, we should be almost -able to run and jump as if we had on the famous seven-league boots. I -have seen an athlete in a circus jump over ten horses placed side by -side. The same athlete, making the same effort, would jump over sixty -horses on the moon. - -A run with a pack of lunar foxhounds would indeed be a marvellous -spectacle. There need be no looking round by timid horsemen to find -open roads or easy gaps. The five-barred gate itself would be utterly -despised by a huntsman who could easily clear a hay-rick. It would -hardly be worth taking a serious jump to clear a canal unless there was -a road and a railway or so, which could be disposed of at the same time. - -To illustrate this subject of gravitation in another way, suppose that -we were to be transferred from this earth to some globe much greater -than the earth--to a globe, for instance, as large and massive as -the sun. We can then show that the weight of every object would be -increased. Indeed, everything would weigh about twenty-seven times as -much as we find it does here. To pull out your watch would be to hoist -a weight of about five or six pounds out of your pocket. Indeed, I do -not see how you could draw out your watch, for even to raise your arm -would be impossible; it would feel heavier by far than if it were made -of solid lead. It is, perhaps, conceivable that you might stand upright -for a moment, particularly if you had a wall to lean up against; but of -this I feel certain, that if you once got down on the ground, it would -be utterly out of your power to rise again. - -These illustrations will at least answer one purpose: they will show -how difficult it is for us to form any opinion as to the presence or -the absence of life on the other globes in space. We are just adapted -in every way for a residence on this particular earth of a particular -size and climate, and with atmosphere of a particular composition. -Within certain slender limits our vital powers can become accommodated -to change, but the conditions of other worlds seem to be so utterly -different from those we find here, that it would probably be quite -impossible for beings constituted as we are to remain alive for five -minutes on any other globe in space. - -It is, however, quite another question as to whether there may not -be inhabitants of some kind on many of the other splendid globes. We -have through the wide extent of space inconceivable myriads of worlds, -presenting, no doubt, every variety of size and climate, of atmosphere -and soil. It seems quite preposterous to imagine that among all these -globes ours alone should be the abode of life. The most reasonable -conclusion for us to come to is that these bodies may be endowed with -life of types which are just as appropriate to the physical conditions -around them as is the life, both animal and vegetable, on this globe to -the special circumstances in which it is placed. - - - - -LECTURE III. - -THE INNER PLANETS. - - Mercury, Venus, and Mars--How to make a Drawing of our System--The - Planet Mercury--The Planet Venus--The Transit of Venus--Venus - as a World--The Planet Mars and his Movements--The Ellipse--The - Discoveries made by Tycho and Kepler--The Discoveries made by - Newton--The Geography of Mars--The Satellites of Mars--How the - Telescope aids in Viewing Faint Objects--The Asteroids, or Small - Planets. - - -MERCURY, VENUS, AND MARS. - -We can hardly think of either the sun or the moon as a world in the -sense in which our earth is a world, but there are some bodies called -planets which seem more like worlds, and it is about them that we -are now going to talk. Besides our Earth there are seven planets of -considerable size, and a whole host of insignificant little ones. These -planets are like ours in a good many respects. One of them, Venus, is -about the same size as this earth; but the two others, Mercury and -Mars, are very much smaller. There are also some planets very much -larger than any of these, namely, Jupiter, Saturn, Uranus, and Neptune. -We shall in this lecture chiefly discuss three bodies, namely, Mercury, -Venus, and Mars, which, with the earth, form the group of “inner” -planets. - -The planets are all members of the great family dependent on the sun. -Venus and the earth may be considered the pair of twins, alike in size -and weight. Mercury and Mars are the babies of the system. The big -brothers are Jupiter and Saturn. All the planets revolve round the -sun, and derive their light and their heat from his beams. We should -like to get a little closer to some of our fellow-planets and learn -their actual geography. Unfortunately, even under the most favorable -circumstances, they are a very long way off. They are many millions of -miles distant, and are always at least a hundred times as far as the -moon. But far as the planets may be, astronomers have been familiar -with their existence for ages past. I can give you a curious proof of -this. You remember how we said the first and the second days of the -week were called after the sun and the moon, Sun-day and Moon-day, or -Monday, respectively. Let us see about the other days. Tuesday is not -quite so obvious, but translate it into French and we have at once -_Mardi_; this word means nothing but Mars’ day, and our Tuesday means -exactly the same. Wednesday is also readily interpreted by the French -word _Mercredi_, or Mercury’s day, while Venus corresponds to Friday. -Jupiter’s day is Thursday, while Saturn’s day is naturally Saturday. -The familiar names of the days of the week are thus associated with the -seven moving celestial bodies which have been known for uncounted ages. - - -HOW TO MAKE A DRAWING OF OUR SYSTEM. - -I want every one who reads this book to make a little drawing of the -sun and the planets. The apparatus that you will need is a pair of -compasses; any sort of compasses that will carry a bit of pencil will -do. You must also get a little scale that has inches and parts of -inches divided upon it; any carpenter’s rule will answer. The drawing -is intended to give a notion of the true sizes and positions of the -fine family of which the earth is one member. The figure I have given -(Fig. 46) is not on so large a scale as that which I ask you to use, -and which I shall here mention. Try and do the work neatly, and then -pin up your little drawings where you will be able to see them every -day until you are quite familiar with the notion of what we mean by our -solar system. - -[Illustration: FIG. 46.--The Orbits of the Four Inner Planets.] - -First open the compasses one inch, and then describe a circle, and mark -a dot on this as “MERCURY,” in neat letters, and also write on the -circle “88 days.” At the centre you are to show the “SUN.” This circle -gives the track followed by Mercury in its journey round the sun in the -period of 88 days. Next open your compasses to 1¾ in., which you must -do accurately by the scale. The circle drawn with this radius shows the -relative size of the path of Venus, and to indicate the periodic time, -you should mark it, “225 days.” The next circle you have to draw is a -very interesting one. The compass is to be opened 2½ in. this time, -and the path that it makes is to be marked “365 days.” This shows the -high road along which we ourselves journey every year, along which we -are, indeed, journeying at this moment. If you wanted to obtain from -your figure any notions of the true dimensions of the system, the path -of the earth will be the most convenient means of doing so. The earth -is 93,000,000 miles from the sun, and our drawing shows its orbit as a -circle of 2½ in. radius. It follows that each inch on our little scale -will correspond to about 37,000,000 miles. As, therefore, the radius of -the orbit of Mercury has been taken to be one inch, it follows that the -distance of Mercury from the sun is about 37,000,000 miles. - -We have, however, still one more circle to draw before we complete -this little sketch. The compass must now open to four inches, and a -circle which represents the orbit of Mars is then to be drawn. We mark -on this “687 days,” and the inner part of the solar system is then -fully represented. You see, this diagram shows how our earth is in -every sense a planet. It happens that one of the four planets revolves -outside the earth’s path, while there are two inside. By marking the -days on the circles which show the periods of the planets, you perceive -that the further a planet is from the sun, the longer is the time that -it takes to go round. Perhaps you will not be surprised at this, for -the length of the journey is, of course, greater in the greater orbits; -but this consideration will not entirely explain the augmentation of -the time of revolution. The further a planet is from the sun, the more -slowly does it actually move, and therefore, for a double reason, the -larger orbit will take a longer time. From London to Brighton is a -much longer journey than from London to Greenwich, and, therefore, the -journey by rail to Brighton will, of course, be a longer one than by -rail to Greenwich. But suppose that you compared the railway journey -to Greenwich with the journey, not by rail, but by coach, to Brighton, -here the comparative slowness of the coach would form another reason -besides the greater length of the journey for making the Brighton trip -a much more tedious one than that to Greenwich. Mars may be likened to -the coach which has to go all the way to Brighton, while Mercury may be -likened to the train which flies along over the very short journey to -Greenwich. - -[Illustration: FIG. 47.--Comparative Sizes of the Planets.] - -We can easily show from our little sketch that Mercury must be -moving more quickly than Mars, for the radii of the two circles are -respectively one inch and four inches, and therefore the path of Mars -must be four times as long as the orbit of Mercury. If Mars moved as -fast as Mercury, he would, of course, require only four times as many -days to complete his large path as Mercury takes for his small path; -but four times 88 is 352, and, consequently, Mars ought to get round -in 352 days if he moved as fast as Mercury does. As a matter of fact, -Mars requires nearly twice that number of days; indeed, no less than -687, and hence we infer that the average speed of Mars cannot be much -more than half that of Mercury. - -[Illustration: FIG. 48.--Phases of an Inferior Planet.] - -To appreciate duly the position of the earth with regard to its -brothers and sisters in the sun’s family it will be necessary to use -your compasses in drawing another little sketch, by which the sizes of -the four bodies themselves shall be fairly represented. Remember that -the last drawing showed nothing whatever about the sizes of the bodies; -it merely exhibited the dimensions of the paths in which they moved. As -Mercury is the smallest globe of the four, we shall open the compasses -half an inch and describe a circle to represent it. The earth and Venus -are so nearly the same size (though the earth is a trifle the larger) -that it is not necessary to attempt to exhibit the difference between -them, so we shall represent both bodies by circles, each 1¼ inches -in radius. Mars, like Mercury, is one of the globes smaller than the -earth, and the circle that represents it will have a radius of ¾ of an -inch. You should draw these figures neatly, and by a little shading -make them look like globes. It would be better still if you were to -make actual models, taking care, of course, to give each of them the -exact size. A comparative view of the principal planets is shown in -Fig. 47. - - -THE PLANET MERCURY. - -Quicksilver is a bright and pretty metal, and, unlike every other -metal, it is a liquid under ordinary circumstances. If you spill -quicksilver, it is a difficult task to gather the liquid up again. It -breaks into little drops, and you cannot easily lift them with your -fingers; they slip away and escape your grasp. Quicksilver will run -easily through a hole so small that water would hardly pass, and it -is so heavy that an iron nail or a bunch of keys will float upon it. -Now, this heavy, bright, nimble metal is known by another name besides -quicksilver; a chemist would call it mercury, and the astronomers use -exactly the same word to denote a pretty, bright, nimble, and heavy -planet which seems to try to elude our vision. Though Mercury is so -hard to see, yet it was discovered so long ago, that all record is lost -of who the discoverer was. - -You must take special pains if you want to see the planet Mercury, for -during the greater part of the year it is not to be seen at all. Every -now and then a glimpse is to be had, but you must be on the alert to -look out just after sunset, or you must be up very early in the morning -so as to see it just before sunrise. Mercury is always found to be in -attendance on the sun, so that you must search for him near the sun; -that is, low down in the west in the evenings, or low down in the east -in the mornings. To ascertain the proper time of the year at which to -look for him you must refer to the almanac. - -We have seen how Mercury revolves in a path inside that of Venus, and -it is therefore nearer to the sun. Indeed, Mercury is so close to the -sun that it is generally overpowered by his brilliance and cannot -be seen at all. Like every other planet, Mercury is lighted by the -sun’s rays, and shows phases in the telescope just as the moon does -(Fig. 48). In this figure the different apparent sizes of the planet at -different parts of its path are shown. Of course the nearer Mercury is -to the earth the larger does it seem. - -If we can only see Mercury so rarely, and if even then it is a -very long way off, does it not seem strange that we can tell how -heavy it is? Even if we had a pair of scales big enough to hold a -planet, what, it may be asked, would be the use of the scales when -the body to be weighed was about a hundred millions of miles away? -Of course the weighing of a planet must be conducted in some manner -totally different from the kind of weighing that we ordinarily use. -Astronomers have, however, various methods for weighing these big -globes, even though they can never touch them. We do not, of course, -want to know how many pounds, or how many millions of tons they -contain; there is but little use in trying to express the weight in -that way. It gives no conception of a planet’s true importance. One -world must be compared with another world, and we therefore estimate -the weights of the other worlds by comparing them with that of our own. -We accordingly have to consider Mercury placed beside the earth, and -to see which of the two bodies is the bigger and the heavier, or what -is the proportion between them. It so happens that Mercury, viewed as -a world, is a very small body. It is a good deal less in size than our -earth, and it is not nearly so massive. To show you how we found out -the mass of Mercury I shall venture on a little story. It will explain -one of the strange devices that astronomers have to use when they want -to weigh a distant body in space. - -There was once, and there is still, a little comet which flits about -the sky; we shall call it after the name of its discoverer, Encke. -There are sometimes splendid comets which everybody can see--we will -talk about these afterwards--but Encke is not such a one. It is very -faint and delicate, but astronomers are interested in it, and they -always look out for it with their telescopes; indeed, they could not -see the poor little thing without them. Encke goes for long journeys -through space--so far that it becomes quite invisible, and remains -out of sight for two or three years. All this time it is tearing along -at a tremendous speed. If you were to take a ride on the comet, it -would whirl you along far more swiftly than if you were sitting on a -cannon-ball. When the comet has reached the end of its journey, then -it turns round and returns by a different road, until at last it comes -near enough to show itself. Astronomers give it all the welcome they -can, but it won’t remain; sometimes it will hardly stay long enough for -us to observe that it has come at all, and sometimes it is so thin and -worn after all its wanderings that we are hardly able to see it. The -comet never takes any rest; even during its brief visit to us it is -scampering along all the time, and then again it darts off, gradually -to sink into the depths of space, whither even our best telescopes -cannot follow it. No more is there to be seen of Encke for another -three years, when again it will come back for a while. Encke is like -the cuckoo, which only comes for a brief visit every spring, and even -then is often not heard by many who dearly love his welcome note; -but Encke is a greater stranger than the cuckoo, for the comet never -repeats his visit of a few weeks more than once in three years; and he -is then so shy that usually very few catch a glimpse of him. - -An astronomer and a mathematician were great friends, and they used to -help each other in their work. The astronomer watched Encke’s comet, -noted exactly where it was, on each night it was visible, and then told -the mathematician all he had seen. Provided with this information the -mathematician sharpens his pencil, sits down at his desk, and begins -to work long columns of figures, until at length he discovers how to -make a time table which shall set forth the wanderings of Encke. He is -able to verify the accuracy of his table in a very unmistakable way by -venturing upon prophecies. The mathematician predicts to the astronomer -the very day and the very hour at which the comet will reappear. He -even indicates the very part of the heavens to which the telescope -must be directed, in order to greet the wanderer on his return. When -the time comes the astronomer finds that his friend has been a true -prophet; there is the comet on the expected day, and in the expected -constellation. - -This happens again and again, so that the mathematician, with his -pencil and his figures, marks stage by stage the progress of Encke -through the years of his invisible voyage. At each moment he knows -where the comet is situated, though utterly unable to see it. - -The joint labors of the two friends having thus discovered law and -order in the movements of the comet, you may judge of their dismay -when on one occasion Encke disappointed them. He appeared, it is -true, but then he was a little late, and he was also not in the spot -where he was expected. There was nearly being a serious difference -between the two friends. The astronomer accused the mathematician of -having made mistakes in his figures, the mathematician retorted that -the astronomer must have made some blunder in his observations. A -quarrel was imminent, when finally it was suggested to interrogate -Encke himself, and see whether he could offer any explanation. The -mathematician employed peculiar methods that I could not explain, so I -shall transform his processes into a dialogue between himself and the -offending comet. - -“You are late,” said he to the comet. “You have not turned up at the -time I expected you, nor are you exactly in the right place; nor, -indeed, for that matter, are you now moving exactly as you ought to do. -In fact, you are entirely out of order, and what explanation have you -to give of this irregularity?” - -You see the questioner felt quite confident that there must have been -some cause at work that he did not know of. Mathematicians have one -great privilege; they are the only people in the world who never make -any mistakes. If they knew accurately all the various influences that -were at work on the comet, they could, by working out the figures, have -found exactly where the comet would be placed. If the comet was not -there, it is inevitable that there must have been something or other -acting upon the comet, of which the mathematician was in ignorance. - -The comet, like every other transgressor, immediately began to make -excuses, and to shuffle off the blame on somebody else. “I was,” said -Encke, “going quietly on my rounds as usual. I was following out stage -by stage the track that you know so well, and I would certainly have -completed my journey and have arrived here in good time and in the -spot where you expected me had I been let alone, but unfortunately I -was not let alone. In the course of my long travels--but at a time -when you could not have seen me--I had the misfortune to come very -close to a planet, of which I dare say you have heard--it is called -Mercury. I did not want to interfere with Mercury; I was only anxious -to hurry past and keep on my journey, but he was meddlesome, and began -to pull me about, and I had a great deal of trouble to get free from -him, but at last I did shake him off. I kept my pace as well as I could -afterwards, but I could not make up the lost time, and consequently I -am here a little late. I know I am not just where I ought to be, nor am -I now moving quite as you expect me to do; the fact is, I have not yet -quite recovered from the bad treatment I have experienced.” - -The astronomer and the mathematician proceeded to test this story. They -found out what Mercury was doing; they knew where he was at the time, -and they ascertained that what the comet had said was true, and that -it had come very close indeed to the planet. The astronomer was quite -satisfied, and was proposing to turn to some other matter, when the -mathematician said:-- - -“Tarry a moment, my friend. It is the part of a wise man to extract -special benefit from mishaps and disasters. Let us see whether the -tribulations of poor Encke cannot be made to afford some very valuable -information. We expected to find Encke here. Well, he is not here--he -is there, a little way off. Let us measure the distance between the -place where Encke is, and the place where he ought to have been.” - -This the astronomer did. “Well,” he said, “what will this tell you? It -merely expresses the amount of delinquency on the part of Encke.” - -“No doubt,” said the mathematician, “that is so; but we must remember -that the delinquency, as you call it, was caused by Mercury. The bigger -and the heavier Mercury was, the greater would be his power of doing -mischief, the more would he have troubled poor Encke, and the larger -would be the derangement of the comet in consequence of the unfortunate -incident. We have measured how much Encke has actually been led astray. -Had Mercury been heavier than he is, that distance would have been -larger; and if Mercury had been lighter than he is, you would not, of -course, have found so large an error in the comet.” - -We may illustrate what is meant in this way. A steamer sails from -Liverpool to New York, and in favorable circumstances the voyage across -the Atlantic should be accomplished within a week. But supposing that -in the middle of the ocean a storm is encountered, by which the ship is -driven from her course. She will, of course, be delayed, and her voyage -will be lengthened. A trifling storm, perhaps, she will not mind, but a -heavy storm might delay her six hours; a still greater storm might keep -her back half a day; while cases are not infrequent in which the delay -has amounted to one day, or two days, or even more. - -The delay which the ship has experienced may be taken as a measure of -the vehemence of the storm. I am not supposing that her machinery has -broken down; of course, that sometimes happens at sea, as do calamities -of a far more tragic nature. I am merely supposing the ship to be -exposed to very heavy weather, from which she emerges just as sound as -she was when the storm began. In such cases as this we may reasonably -measure the intensity of the storm by the number of hours’ delay to -which the passengers were subjected. “The weather we had was much worse -than the weather you had,” one traveller may say to another. “Our ship -was two days late, while you escaped with a loss of one day.” - -When the comet at last returned to the earth after a cruise of three -years through space, the number of hours by which it was late expressed -the vehemence of the storm it experienced. The only storm that the -comet would have met with, at least in so far as our present object -is concerned, was the trouble that it had with Mercury. The mass of -Mercury was, therefore, involved in the delay of the comet. In fact, -the delay was a measure of the mass of the planet. I do not attempt to -describe to you all the long work through which the mathematician had -to plod before he could ascertain the mass of Mercury. It was a very -tedious and a very hard sum, but at last his calculations arrived at -the answer, and showed that Mercury must be a light globe compared to -the earth. In fact, it would take twenty-five globes, each equal to -Mercury, to weigh as much as the earth. - -I dare say you will think that this was a very long and roundabout way -of weighing. Supposing, however, we had to weigh a mountain, or rather -a body which was bigger than fifty thousand mountains, and which was -also many millions of miles away, all sorts of expedients would have -to be resorted to. I have told you one of them. If you feel any doubts -as to the accuracy with which such weighings can be made, then I must -tell you that there are many other methods, and that these all agree in -giving concordant results. - -[Illustration: FIG. 49.--Relative Weights of Mercury and the Earth.] - -We hardly know anything as to what the globe of Mercury may be like. -We can see little or nothing of the nature of its surface. We only -perceive the planet to be a ball, brightly lighted by the sun, and we -cannot satisfactorily discern permanent features thereon, as we are -able to do on some of the other planets. - - -THE PLANET VENUS. - -You will have no difficulty in recognizing Venus, but you must choose -the right time to look out for her. In the first place, you need never -expect to see Venus very late at night. You should look for the planet -in the evening, as soon as it is dark, towards the west, or in the -morning, a little before sunrise, towards the east. I do not, however, -say that you can always see Venus, either before sunrise or after -sunset. In fact, for a large part of the year, this planet is not to be -seen at all. You should therefore consult the almanac, and unless you -find that Venus is stated to be an evening star or a morning star, you -need not trouble to search for it. I may, however, tell you that Venus -can never be an evening star and a morning star at the same time. If -you can see it this evening after sundown, there is no use in getting -up early in the morning to look out for it again. The planet will -remain for several weeks a splendid object after sunset, and then will -gradually disappear from the west, and in a couple of months later will -be the morning star in the east. Venus requires a year and seven months -to run through her changes, so that if you find her a bright evening -star to-night, you may feel sure that she was a bright evening star a -year and seven months ago, and that she will be a bright evening star -in a year and seven months to come. Nor must you ever expect to see her -right overhead; she is always to the west or to the east. - -The splendor of Venus, when at her best, will prevent you at such times -from mistaking this planet for an ordinary star. She is then more than -twenty times as bright as any star in the heavens. The most conclusive -proof of the unrivalled brightness of Venus is found in the fact that -she can be recognized in broad daylight without a telescope. Even on -the brightest June afternoons the lovely planet is sometimes to be -discerned like a morsel of white cloud on the perfect azure of the sky. - -Venus is so brilliant that perhaps you will hardly credit me when I -tell you that she has no more light of her own than has a stone or a -handful of earth, or a button. Is it possible that this is the case, -you will say, for as we see the planet so exquisitely beautiful, how -can she be merely a huge stone high up in the heavens? The fact is that -Venus shines by light not her own, but by light which falls upon her -from the sun. She is lighted up just as the moon, or just as our own -earth is lighted. Her radiance merely arises from the sunbeams which -fall upon her. It seems at first surprising that mere sunbeams on the -planet can give her the brilliancy that is sometimes so attractive. -Let me show you an illustration which will, I trust, convince you that -sunbeams will be adequate even for the glory of Venus. - -Here is a button. I hang it by a piece of fine thread, and when I -dip it into the beam from the electric lamp, look at the brilliancy -with which the mimic planet glitters. You cannot see the shape of the -button; it is too small for that; you merely see it as a brilliant gem, -radiating light all around. Therefore, we need not be surprised to -learn that the brilliancy of the evening star is borrowed from the sun, -and that if, while we are looking at the planet in the evening, the sun -were to be suddenly extinguished, the planet would also vanish from -view, though the stars would shine as before. - -Thus we explain the appearance of Venus. The evening star is a -beautiful, luminous point, but it has no shape which can be discerned -with the unaided eye. When, however, the telescope is turned towards -Venus we have the delightful spectacle of a tiny moon, which goes -through its phases just as does our own satellite. When first seen as -an evening star Venus will often be like the moon at the quarter, and -then it will pass to the crescent shape. Then the crescent becomes -gradually thinner, and next will follow a brief period of invisibility -before the appearance of Venus as the morning star. It seems at first -a little strange that Venus when brightest should not be full like the -moon, which in similar circumstances is, of course, a complete circle -of light. The planet, however, has a very marked crescent-shaped form -in these circumstances. But at this time the planet is so near us -that the gain of brilliancy from the diminution of distance more than -compensates for the small part of the illuminated side which is turned -towards us. - -You ought all to try to get some one to show you Venus through a -telescope. A very large instrument is not necessary, and I feel sure -you will be delighted to see the beautiful moon-shaped planet. You will -then have no difficulty in understanding how the brightness of the -planet has come from the sun. The changes in the crescent merely depend -upon the proportion of the illuminated side which is turned towards us. -Were Venus itself a sunlike body we should, of course, see no crescent, -but only a bright circle of light. - -In Fig. 50 you will notice an imaginary picture of a young astronomer -surveying Venus with a telescope. I have not, as is obvious, attempted -to show the different objects in their proper proportions. The sun is -supposed to have set, so that his beams do not reach the astronomer. -Night has begun at his observatory; but the sunbeams fall on Venus, -and light her up on that side turned towards the sun. A part of this -lighted side is, of course, seen by the telescope which the astronomer -is using, and thus the planet seems to him like a crescent of light. - -[Illustration: FIG. 50.--To show that Venus shines by Sunlight.] - - -THE TRANSIT OF VENUS. - -We might naturally think from Fig. 46 that Venus must pass at every -revolution directly between the earth and the sun; and therefore it -might appear that what is called the transit of Venus across the sun -ought to occur every time between the appearance of the planet as the -evening star and the next following appearance as the morning star. -No doubt on each of these occasions Venus seems to approach the sun -closely; but the orbits of Venus and the Earth do not lie quite in the -same plane, and hence the planet usually passes just over or just under -the sun, so that it is a very rare event indeed for her to come right -in front of the sun. But this does sometimes happen. It happened, for -instance, in the year 1874, and again in the year 1882; but, alas! -I cannot hold out to you the prospect of ever seeing another such -spectacle. There will be no further occurrence of the transit of Venus -until the year 2004, though there will be another eight years later, in -2012. - -It seems rather odd that one transit of Venus should be followed by -another after an interval of eight years, and that then a period of -much more than a century should have to elapse before there will be -a repetition of a similar pair. This is in consequence of a curious -relation between the motion of Venus and the motion of the Earth, which -I must endeavor to explain with the help of a little illustration. - -Let us suppose a clock with ordinary numbers round the dial, but so -arranged that the slowly moving short hand requires 365.26 days to -complete one revolution round the dial, while the more rapidly moving -long hand revolves in 224.70 days. The short hand will then go round -once in a year, and the long hand once during the revolution of Venus. -Let us suppose that both hands start together from XII, then in 224.70 -days the long hand is round to XII again, but the short hand will have -only advanced to about VII, and by the time it reaches XII the long -hand will have completed a large part of a second circuit. It happens -that the two numbers 224.70 and 365.26 are very nearly in the ratio -of 8 to 13. In fact, if the numbers had only been 224.8 and 365.3 -respectively, they would be exactly in the proportion of 8 to 13. It, -therefore, follows that eight revolutions of the short hand must occupy -very nearly the same time as thirteen revolutions of the long hand. -After eight years the short hand will of course be found again at XII; -and at the same moment the long hand will also be back at XII, after -completing thirteen revolutions. - -We can now understand why the transits, when they do occur, generally -arrive in pairs at an interval of eight years. Suppose that at a -certain time Venus happens to interpose itself directly between the -earth and the sun, then, when eight years have elapsed, the earth is, -of course, restored for the eighth time since the first transit to the -same place, and Venus has returned to almost the same spot for the -thirteenth time. The two bodies are practically in the same condition -as they were at first, and, therefore, Venus again intervenes, and the -planet is beheld as a black spot on the sun’s surface. We must not -push this argument too far; the relation between the two periods of -revolution, though nearly, is not exactly 8 to 13. The consequence is -that when another eight years have elapsed, the planet passes a little -above the sun or a little below the sun, and thus a third occurrence of -the transit is avoided for more than a century. The next transit will -take place at the opposite side of the path. - -We were fortunate enough to be able to see the transit of Venus in 1882 -from Great Britain. Perhaps I should say a part of the transit, for the -sun had set long before the planet had finished its journey across the -disk. Venus looked like a small round black spot, stealing in on the -bright surface of the sun and gradually advancing along the short chord -that formed its track. - -[Illustration: FIG. 51.--Venus in Transit across the Sun.] - -An immense deal of trouble was taken in 1882, as well as in 1874, to -observe this rare occurrence. Expeditions were sent to various places -over the earth where the circumstances were favorable. Indeed, I do not -suppose that there was ever any other celestial event about which so -much interest was created. The reason why the event attracted so much -attention was not solely on account of its beauty or its singularity; -it was because the transit of Venus affords us a valuable means of -learning the distance of the sun. When observations of the transit of -Venus made at opposite sides of the earth are brought together, we are -enabled to calculate from them the distance of Venus, and knowing that, -we can find the distance of the sun and the distances and the sizes of -the planets. This is very valuable information; but you would have to -read some rather hard books on astronomy if you wanted to understand -clearly how it is that the transit of Venus tells us all these -wonderful things. I may, however, say that the principle of the method -is really the same as that mentioned on pp. 19–25. When you remember -that not we ourselves, nor our children, and hardly our grandchildren, -will ever be able to see another transit of Venus, you will, perhaps, -not be surprised that we tried to make the most of such transits as -have occurred in our time. - - -VENUS AS A WORLD. - -Though Venus exhibits such pretty crescents in the telescope, yet -I must say that in other respects a view of the planet is rather -disappointing. Venus is adorned by such a very bright dress of sunbeams -that we can see but little more than those sunbeams, and we can hardly -make out anything of the actual nature of the planet itself. We can -sometimes discern faint marks upon the globe, but it is impossible -even to make a conjecture of what the Venus country is like. This is -greatly to be regretted, for Venus approaches comparatively close -to the earth, and is a world so like our own in size and other -circumstances that we feel a legitimate curiosity to learn something -more about her. - -But the marks on the planet, though very faint, are still sufficiently -definite to have enabled some sharp-sighted astronomers to answer a -question of much interest. They have made it plain that in one most -important respect Venus is very unlike our Earth. Our globe, of course, -rotates on its axis once each day, but Venus requires no less than 225 -days to complete each rotation. In fact, this planet rotates in such a -fashion that she always keeps the same face to the sun. The inhabitants -of Venus will therefore find that it is perennial day on one side of -this globe and everlasting night on the other. - -Venus is one of the few globes which might conceivably be the abode -of beings not very widely different from ourselves. In one condition -especially--namely, that of weight--she resembles the earth so closely -that those bodies which we actually possess would probably be adapted, -so far as strength is concerned, for a residence on the sister planet. -Our present muscles would not be unnecessarily strong, as they would be -on the moon, nor should we find them too weak, as they would certainly -prove to be were we placed on one of the very heavy bodies of our -system. Nor need the temperature of Venus be regarded as presenting any -insuperable difficulties. She is, of course, nearer to the sun than -we are, but then climate depends on other conditions besides nearness -to the sun, so that the question as to whether Venus would be too hot -for our abode could not be readily decided. The composition of the -atmosphere surrounding the planet would be the most material point in -deciding whether terrestrial beings could live there. I think it to -be in the highest degree unlikely that the atmosphere of Venus should -chance to suit us in the requisite particulars, and therefore I think -there is not much likelihood that Venus is inhabited by any men, women, -or children resembling those on this earth. - - -THE PLANET MARS AND HIS MOVEMENTS. - -The path of the earth lies between the orbits of the planets Venus and -Mars. It is natural for us to endeavor to learn what we can about our -neighbors. We ought to know something, at all events, as to the people -who live next door to us on each side. I have, however, already said -that we cannot observe very much upon Venus. The case is very different -with respect to Mars. He is a planet which we are fortunately enabled -to study minutely, and he is full of interest when we examine him -through a good telescope. - -The right season for observing Mars must, of course, be awaited, as he -is not always visible. Such seasons recur about every two years, and -then for months together Mars will be a brilliant object in the skies -every night. Nor has Mars necessarily to be sought in the early morn or -immediately after sunset, in the manner we have already described for -Venus and Mercury. At the time Mars is at his best he comes into the -highest position at midnight, and he can generally be seen for hours -before, and be followed for hours subsequently. You may, however, find -some difficulty in recognizing him. You probably would not at first be -able to distinguish Mars from a fixed star. No doubt this planet is a -ruddy object, but some stars are also ruddy, and this is at the best a -very insecure characteristic for identification. I cannot give you any -more general directions, except that you should get your papa to point -out Mars to you the next time it is visible. It is just conceivable -that papa himself might not know how to find Mars. If so, the sooner he -gets a set of star maps and begins to teach himself and to teach you, -the better it will be for you both. - -Mars, though apparently so like a star, differs in some essential -points from any star in the sky. The stars proper are all fixed in the -constellations, and they never change their relative positions. The -groups which form the Great Bear or the Belt of Orion do not alter, -they are just the same now as they were centuries ago. But the case is -very different with a planet such as Mars. The very word planet means a -_wanderer_, and it is justly applied, because Mars, instead of staying -permanently in any one constellation, goes constantly roaming from one -group to the other. He is a very restless body; sometimes he pays his -respects to the heavenly Twins, and is found near Castor and Pollux in -Gemini, then he goes off and has a brief sojourn with the Bull, but it -looks as if that fierce animal got tired of his company and hunted -him off to the Lion. His quarters then become still more critical. -Sometimes it looks as if he desired to seek for peace beneath the -waters, and so he visits Aquarius, while at other times he is found in -dangerous proximity to the claws of the Crab. - -Mars cannot even make up his mind to run steadily round the heavens in -one direction; sometimes he will bolt off rapidly, then pause for a -while, and turn back again; then the original impulse will return, and -he will resume his journey in the direction he at first intended. It -is no wonder that I am not able to give you very explicit directions -as to how you may secure a sight of a truant whose wanderings are -apparently so uncertain. Yet there is a definite order underlying all -his movements. Astronomers, who make it their business to study the -movements of Mars, can follow him on his way; they know exactly where -he is now, and where he will be every night for years and years to -come. The people who make the almanacs come to the astronomers and get -hints from them as to what Mars intends to do, so that the almanacs -announce the positions in which the planet will be found with as much -regularity as if he was in the habit of behaving with the orderly -propriety of the sun or the moon. - -We must not lay all the blame on Mars for the eccentricities of his -movements. Our earth is to a very large extent responsible. What we -think to be Mars’ vagaries are often to be explained by the fact that -we ourselves on the earth are rapidly shifting about and altering our -point of view. - -[Illustration: FIG. 52.--How the Tree seems to move about.] - -I was driving down a pretty country road with a little girl three years -old beside me, when I was addressed with the little remark, “Look at -the tree going about in the field.” Now, you or I, with our longer -experience of the world around us, know that it is not the custom of -trees to take themselves up and walk about the fields. But this was -what this little girl saw, or rather what she thought she saw; and very -often what we do see is something very different from what we think we -see. We think we see Mars performing all these extraordinary movements, -as the little girl thought she saw the tree moving about. But just as -that little girl, when she grew to be a big girl, found that what she -thought was a tree walking across the field must really have some quite -different explanation, so we, too, find that what Mars seems to do is -one thing, and what Mars actually does is quite another thing. - -Let us see what the little girl noticed. She was looking at the tree, -and first she saw it on one side of the house, and then she saw it on -the opposite side (Fig. 52). If it had been a cow instead of a tree, of -course the natural supposition would have been that the cow had walked. -Our little friend may, perhaps, have thought it unusual for a tree to -walk, but still she saw the undoubted fact that the tree had shifted to -the other side of the house, and therefore, perhaps, remembering what -the cow could do, she said the tree had moved. - -[Illustration: FIG. 53.--A Specimen of the Track of Mars.] - -The little girl did not stop to reflect that she herself had entirely -changed her position, and hence arose the surprising phenomenon of a -tree that could move about. You will understand this, at once, from the -two positions of the car here shown. In the first position, as the girl -looks at the tree, the dotted line shows the direction of her glance, -and the other dotted line shows how the apparent places of the tree and -the house have altered. It is her change of place that has accomplished -the transformation. Observe also that the tree appeared to her to move -in the direction opposite to that in which she is going. - -Mars generally appears to move round among the stars from west to east. -In fact, if we were viewing him from the sun he would always seem to -move in this manner. But at certain seasons our earth is moving very -fast past Mars, and this will make him appear to move in the opposite -direction. This apparent motion is sometimes so much in excess of his -real motion, that it may give us an entirely incorrect idea of what the -planet is actually doing. - -Thus, notwithstanding that Mars is moving one way, he may appear to us -who dwell on the earth to be going in the opposite way. This illusion -only happens for a short time, just when we are passing Mars, as we -do every two years. The effect on the planet is to make the path he -pursues at this time something like that shown in Fig. 53. The planet -is nearest to us at the time he is moving in this loop. He is then -to be seen at his best in the telescope, so that it is especially -interesting to watch Mars through this critical part of his career. - -I want to show you how to make a little calculation which will explain -the law by which the seasons when we can see Mars best will follow -each other. The period he requires for a voyage round the sun is not -quite two years, for that would be 730 days, and Mars only takes 687 -days for his journey. It is, however, true that 1-15/17 years is very -nearly the period of Mars. Hence, every 32 years Mars will complete 17 -rounds. From this we shall be able to see how long it will take after -the earth once passes Mars before they pass again. I shall suppose -there is a circular course, around which two boys start together to -run a race. One of these boys is such a good runner that he will get -quite round in 17 minutes; but the other boy can hardly run more than -half as quickly, for he will require 32 minutes to complete one circle. -Here then is the question. Suppose the two boys to start together: how -long will it be before the faster runner gains one complete circuit on -the other? By the time the good runner (A) has completed one circuit, -the bad runner (B) has only got a little more than halfway. When A -has completed his second circuit, he has, of course, run for twice -17 minutes--that is, for 34 minutes. This is two minutes longer than -the time B requires to get round once; therefore B is only ahead by -a distance which A could cover in about one minute; but B will have -advanced during this minute a distance for which A will require another -half-minute, during which B covers a distance for which A will need a -further quarter, and so on. But all these intervals--one minute, half -a minute, a quarter of a minute, one-eighth, one-sixteenth, and so -on--added together amount to two minutes, and hence it follows that B -will not be overtaken until about two minutes after A has completed his -second round--that is, in 36 minutes altogether. - -We can pass from this illustration to the case of the planet Mars -and the earth. The orbit of the earth is traversed in a year, and -therefore, after the earth has once passed Mars, which is then, as -astronomers would say, in _opposition_, about two years and the eighth -of a year--that is, two years and six or seven weeks--will elapse -before Mars is again favorably placed. You will thus see that we need -not expect to observe Mars under the best conditions every year. -Besides, the distance of the planet from the earth at opposition -varies so greatly that some oppositions are more favorable than others. - -The time has come when I must tell you something about the shapes -of the paths in which the earth and the other planets perform their -great journeys round the sun. Perhaps you will think that I am going -to contradict some of the things that I have told you before. I have -often represented the orbits of the planets as circles, and now I am -going to tell you that this is not correct. The fact is that the paths -are nearly circles; but, still, there is some departure from the exact -circular shape. Mars, in particular, moves in a path which is more -different from a circle than the path of the earth, and consequently it -is appropriate to introduce this subject when we are engaged about Mars. - -[Illustration: FIG. 54.--How to draw an Ellipse.] - -We must first take another lesson in drawing, and the appliances I -want you to use for the purpose are very simple. You must have a smooth -board and some tacks or drawing-pins, besides paper, pencil, and twine. - -[Illustration: FIG. 55.--Specimens of Ellipses.] - -We first lay a sheet of paper on the board, and then put in two tacks -through the paper and into the board. It does not much matter where -we put them in. Next we take a piece of twine and tie the two ends -together so as to form a loop, which we pass round the two tacks -(Fig. 54). In the loop I place the pencil, and then you see I move it -round, taking care to keep the twine stretched. Thus I produce a pretty -curve, which we call the ellipse. I must ask all of you to practise -this experiment. Try with different lengths of string, and try using -different distances between the tacks. Here are some sketches of two -shapes of ellipse and a parabola (Fig. 55). Elliptic curves can be made -almost circles by putting the two tacks close together, or they can be -made very long in comparison with their width. They are all pretty and -graceful figures, and are often useful for ornamental work. The ellipse -is a pretty shape for beds of flowers in a grass-plot. - -The importance of the ellipse to astronomers is greater than that of -any other geometrical figure. In fact, all the planets, as they perform -their long and unceasing journeys round the sun, move in ellipses; and -though it is true that these ellipses are very nearly circles, yet the -difference is quite appreciable. - -It is also important to observe that the sun is not in the centre -of the ellipse which the planet describes. The sun is nearer to one -end than to the other. And the actual position of the sun must be -particularly noted. Suppose that some mighty giant were preparing to -draw an exact path for the earth, or for Mars, of course he would -want to have millions of miles of string for producing a big enough -curve, and one of the nails that he used would have to be driven right -into the sun. The following is the astronomer’s more accurate method -of stating the facts. He calls each of the points represented by the -tacks around which the string is looped a _focus_ of the ellipse; the -two points together are said to be the _foci_; and as the planet is -describing its orbit, the position of the sun will lie exactly at one -of the foci. - -The ellipse is a curve that nature is very fond of reproducing. From -an electric light, a brilliant beam will diverge. If you hold a globe -in the beam, and let the shadow fall on a sheet of paper, it forms an -ellipse. If you hold the sheet squarely, the shadow is a circle; but as -you incline it, you obtain a beautiful oval, and by gradually altering -the position, you can get a greatly elongated curve. Indeed, you can -thus produce an ellipse of almost any form. The electric light is not -indispensable for this purpose; any ordinary bright lamp with a small -flame will answer, and by taking different sized balls and putting them -in various positions, you can make many ellipses, great and small. - - -THE DISCOVERIES MADE BY TYCHO AND KEPLER. - -It was by the observations of a celebrated old astronomer, named Tycho -Brahe, that the true shape of a planet’s path came to be afterwards -determined. Tycho lived in days before telescopes were invented. He had -few of the excellent contrivances for measuring which we have in our -observatories. We shall take a look at this fine old astronomer, as he -sits amid his curious astronomical machines. - -[Illustration: FIG. 56.--Tycho Brahe in his Observatory.] - -He lived on an island near Copenhagen, and he has given us a picture -of himself (Fig. 56), as he is seated with his quaint apparatus, and -his assistants around him, busily engaged in observing the heavens. -You see the walls of his observatory are decorated with pictures; and -one of the great Danish hounds which the King of Denmark had presented -to him lies asleep at his feet. I do not think we should now encourage -big dogs in the observatory at night. Nor do modern astronomers put on -their velvet robes of state, as Tycho was said to have done when he -entered into the presence of the stars, as, by so doing, he showed his -respect for the heavens. Astronomers, nowadays, rather prefer to wear -some comfortable coat which shall keep out the cold, no matter what may -be its appearance from the picturesque point of view. In this wonderful -contrivance, you see Tycho Brahe did not use any actual telescope. He -observed through a small opening in the wall, and lest there should be -any mistake as to what is going on, you see he is pointing towards it, -and giving his three assistants their instructions. The most important -work is being done by the man on the right. He is engaged in making the -actual observation. But he has no aid from magnifying lenses. All he -can do is to slide a pointer up or down till it is just in line with -the planet or star as he sees it through the hole opposite. - -On the circle a number of marks have been engraved, and there are -numbers placed opposite to the marks; it is by these that the position -of the object is to be ascertained. If the object is high, then the -pointer will be low; and if the object is low, then the pointer will -be high. The observer calls out the position when he has found it, and -there, you see, is a man ready with writing materials to take down -the observation. Notice also the other astronomer who is looking at -the clock. He gives the time, which must also be recorded accurately. -In fact, the entire process of finding the place of a heavenly body -consists in two observations--one from the circle and the other from -the clock; so that though Tycho had no telescope to aid his vision, yet -the principle on which his work was done was the same as that which we -use in our observatories at this moment. - -You may think that such a concern would hardly be capable of producing -much reliable work. However, Tycho compensated in a great degree for -the imperfection of his instrument by the skill with which he used it. -He had a noble determination to do his very best. Perseverance will -accomplish wonders even with very imperfect means. A great astronomer -has said that a skilful observer ought to be able to make valuable -measurements with a common cart-wheel! - -It was with instruments on the principle of that which I have here -shown that Tycho made his celebrated observations of Mars. Week -after week, month after month, year after year, did the patient old -astronomer track the planet through his capricious wanderings. - -Before we try to explain anything, it is of course necessary to -ascertain, with all available accuracy, what the thing actually is. -Therefore, when we seek to explain the irregular movements of a planet, -the first thing to be done is to make a careful examination of the -nature of those irregularities. And this was what Tycho strove to do -with the best means at his disposal. - -The full benefit of Tycho’s work was realized by Kepler when he -commenced to search out the kind of figure in which Mars was moving. -First he tried various circles, and then he sought, by placing the -centre in different positions, to see whether it would not be possible -to account thus for the irregularities of the wayward planet. It would -not do; the movement was not circular. This was thought very strange -in those days, for the circle was regarded as the only perfect curve, -and it was considered quite impossible for a planet to have any motion -except it were the most perfect. There was, however, no help for -it; so Kepler sagaciously tried the ellipse, which he considered to -be the most perfect curve next to the circle. He continued his long -calculations, until at last he succeeded in finding one particular -ellipse, placed in one particular position, which would just explain -the strange wanderings of our erratic neighbor. It was not alone -that the motion of the planet traced out an ellipse; it was further -discovered that the sun lies at one of the foci of the curve. If the -sun were anywhere else, the motion of the planet would have been -different from that which Tycho had found it to be. - -You must know that this discovery is one of the very greatest that -have ever been made in the whole extent of human knowledge. After it -had been proved that the orbit of Mars was elliptic, it became plain -that the same path must be traced by every planet. There are very big -planets, and there are small ones; there are planets which move in -very large orbits, and there are planets whose paths are comparatively -small. In all cases the high road which the planet follows is -invariably an ellipse, and the sun is invariably to be found situated -at the focus. It is surely interesting to find that these beautiful -ellipses which we can draw so simply with a piece of twine and a pencil -should be also the very same figures which our great earth and all the -other bodies which revolve around the sun are ever compelled to follow. - -Kepler also made another great discovery in connection with the same -subject. If the planet moved in a circle with the sun in the centre, -then there would be very good reason to expect that it would always -move at the same speed, for there would be no reason why it should go -faster at one place than at another. In fact, the planet would then be -revolving always at the same distance from the sun, and every part of -its path would be exactly like every other part. But when we consider -that the motion is performed in an ellipse, so that the planet is -curving round more rapidly at the extremities of its path than in the -other parts where the curvature is less perceptible, we have no reason -to expect that the speed shall remain the same all round. - -We know that the engine-driver of a railway train always has to slacken -speed when he is going round a sharp curve. If he did not do so, his -train would be very likely to run off the line, and a dreadful accident -would follow. The engine-driver is well aware that the conditions of -pace are dependent on the curvature of his line. The planet finds that -it, too, must pay attention to the curves; but the extraordinary -point is that the planet acts exactly in the opposite way to the -engine-driver. The planet puts on its highest pace at one of the most -critical curves in the whole journey. There are two specially sharp -curves in the planet’s path. These are, of course, the two extremities -of the ellipse which it follows. The cautious engine-driver would, of -course, creep round these with equal care, and no doubt the planet -goes slowly enough about that end of the ellipse which is farthest -from the sun. There its pace is slower than anywhere else; but from -that moment onwards the planet steadily applies itself to getting -up more and more speed. As it traverses the comparatively straight -portion of the celestial road, the pace is ever accelerating until the -sharp curve near the sun is being approached; then the velocity gets -more and more alarming, until at last, in utter defiance of all rules -of engine-driving, the planet rushes round one of the worst parts of -the orbit at the highest possible speed. And yet no accident happens, -though the planet has no nicely laid lines to keep it on the track. - -If lines are necessary to save a railway train from destruction, how -can we possibly escape when we have no similar assistance to keep us -from flying away from the sun and off into infinite space? Kepler -has taught us to measure the changes in the speed of the body with -precision. He has shown that the planet must, at every point of its -long journey, possess exactly the right speed; otherwise everything -would go wrong. I dare say you have seen, at different points along -a line of railway, boards put up here and there, with notices like, -“Ten miles an hour.” These words are, of course, an intimation to the -engine-driver that he is not to vary from the speed thus stated. Kepler -has given us a law which is equivalent to a large number of caution -boards, fixed all round the planet’s path, indicating the safe speed -for the journey at every stage. It is fortunate for us that the planet -is careful to observe these regulations. If the earth were to leave -her track, the consequences would be far worse than those of the most -frightful railway accident that ever happened. Whichever side we took -would be almost equally disastrous. If we went inwards we should plunge -into the sun, and if we went outwards we should be frozen by cold. - -We owe our safety to the care with which the speed of the earth is -prescribed. When near the sun, the earth is pulled inwards with -exceptionally strong attraction. We are often told that when a strong -temptation seizes us, the wisest thing that we can do is to run away -as hard as possible. This is just what the laws of dynamics cause the -earth to do at this critical time. She puts on her very best pace, and -only slackens when she has got well away from the danger. - -The peril that we are exposed to when the earth is at the other end -of the orbit is of an opposite character. We are then a long way -from the sun, and the pull which it can exercise upon the earth is -correspondingly lessened. Care is then required lest we should escape -altogether from the sun’s warmth and his guidance. We must therefore -give time to the sun to exercise his power, so as to enable the earth -to be recalled; accordingly we move as slowly as possible until the -sun conquers the earth’s disposition to fly off, and we begin to return. - -You may remember that when we were speaking about the moon, I showed -you how a body might revolve around the earth in a circle under the -influence of an attraction towards the earth’s centre. So long as the -path is really a circle, then the power with which the earth is drawing -the body remains the same. In a precisely similar way, a body could -revolve around the sun in a circle, in which case also the attraction -of the sun will remain the same all round. But now we have a very much -more difficult case to consider. If the body does not always remain at -the same distance, the power of the sun will not be the same at the -different places. Whenever the object is near the sun, the attraction -will be greater than when it is farther off. For example, when the -distance between the two bodies is doubled, then the pull is reduced to -the fourth part of what it was before. - - -THE DISCOVERIES MADE BY NEWTON. - -I have now some great discoveries to talk to you about, which were made -by Sir Isaac Newton. He was not an astronomer who looked much through -a telescope, though he made many remarkable experiments. He used to -sit in his study and think, and then he used to draw figures with his -pencil, and make long calculations. At last he was able to give answers -to the questions: What is the reason why the planet moves in an -ellipse? Why should it move in this curve rather than in any other? Why -should this ellipse be so placed that the sun lies at one of the foci? - -If the planet had run uniformly round its course, Newton would have -found his task an impossible one. But I have already explained that -the motion is not uniform. I described how the planet hurried along -with extra speed at certain parts of its path; how it lingered at -other parts; how, in fact, it never preserved the same rate for even a -single minute during the whole journey. Kepler had shown how to make a -time-table for the whole journey. In fact, just as a captain on a long -voyage keeps a record of each day’s run, and shows how to-day he makes -170 miles, and to-morrow perhaps 200, and the next day 210, while the -day after he may fall back to 120, so Kepler gave rules by which the -log of a planet in its voyage round the sun might be so faithfully kept -that every day’s run would be accurately recorded. - -When Newton commenced his work, one of the first questions he had to -consider was the following: Suppose that a great globe like a planet, -or a small globe like a marble, or an irregular body like an ordinary -stone, were to be thrown into space, and were then to be left to follow -its course without any force whatever acting upon it, where would it go -to? - -You may say, at once, that a body under such circumstances will -presently fall down to the ground; and so, of course, it will, if it be -near the earth. I am not, however, talking of anything near the earth; -I want you to imagine a body far off in the depths of space, among the -stars. Such a body need not necessarily fall down here, for you see the -moon does not fall, and the sun does not. - -If you were at a great distance from our globe and from all other large -globes--so far, indeed, that their attractions were imperceptible--you -could try the experiment that I wish now to describe. Throw a stone -as hard as ever you can, and what will happen? Of course, when you do -it down here, it moves in a pretty curve through the air, and tumbles -to the ground; but away in open space, what will the stone do? There -will be no such motion as up or down, as we ordinarily understand it; -for though the earth, no doubt, will lie in one particular direction -at a great distance, yet there will be other bodies just as large in -other directions; and there is no reason why the stone should move -towards one of these rather than to another; in fact, if they are all -far enough, as the stars are from us, their attractions will be quite -inappreciable. There is, therefore, not the slightest reason why the -stone should swerve to one side more than to another. There is no more -reason why it should turn to the right than why it should turn to the -left. Nor could you throw the stone so as to make it follow a curved -path. You can, of course, make it describe a curve while it remains in -your hand, but the moment the stone has left your hand, it proceeds on -its journey by a law over which you have no control. As the direction -cannot be changed towards one side more than towards the other, the -stone must simply follow a straight line from the very moment when it -is released from your hand. - -The speed with which the stone is started will also not change. You -might at first think that it would gradually abate, and ultimately -cease. No doubt a stone thrown along the road will behave in this way, -but that is because the stone rubs against the ground. If you throw -a stone across a sheet of ice, then it will run a very long distance -before it stops, and all the time it will be moving in a straight line. -In this case there is but little loss by rubbing against the ice, -because it is so smooth. Thus we see that if the path be exceedingly -smooth, the body will run a long way before it stops. Think of the -distance a railway train will run if, while travelling at full speed -along a level line, the steam is turned off. - -These illustrations all show that if you let a body alone, after having -once started it, and do not try to pull it this way or that way, and -do not make it rub against things, that body will move on continually -in a straight line, and will keep up a uniform speed. We can apply -this reasoning to a stone out in space. It would certainly move in a -straight line, and would go on and on forever, without losing any of -its pace. - -I need hardly tell you that no one has ever been able to try this -experiment. In the first place, we reside upon the surface of the -earth, and we have no means of ascending into those elevated regions -where the stone is supposed to be projected. There is also another -difficulty which we cannot entirely avoid, and that arises from the -resistance of the air. All movements down here are impeded because -the body has to force its way through the air; and in doing so it -invariably loses some of its speed. Out in open space there is, of -course, no air, and no loss of speed can therefore arise from this -cause. - -[Illustration: FIG. 57.--The Humming-top.] - -There are, however, several actual experiments by which we can assure -ourselves of the general truth. Set a humming-top spinning (Fig. 57); -it gradually comes to rest, partly because of the rubbing of its point -on the table, and partly because it has to force its way through the -air. In fact, the hum of the top that you hear is only produced at the -expense of its motion. Supposing I use a much heavier top; if I set it -spinning it will keep up for many minutes, because its weight gives it -a better store of power wherewith to overcome the resistance of the -air. I remember hearing a story about Professor Clerk-Maxwell. He had, -when at Cambridge, invented one of these large and heavy tops, which -would spin for a long time. One evening the top was left spinning on a -plate in his room when his friends took their departure, and no doubt -it came to rest in due time. Early the next morning, Professor Maxwell, -hearing the same friends coming up to his rooms again, jumped out of -bed, set the top spinning, and then got back to bed, and pretended to -be asleep. He thus astounded his friends, who, of course, imagined that -the top must have been spinning all the night long! - -If we spin a top under the receiver of an air pump (Fig. 58), it will -keep up its motion for a very much longer time after the air has been -exhausted than it would in ordinary circumstances. Such experiments -prove that the motion of a body will not of itself naturally die out, -and that if we could only keep away the interfering forces altogether, -the motion would continue indefinitely with unabated speed. What I have -been endeavoring to illustrate is called the first law of motion. It is -written thus:-- - -“_Every body continues in its state of rest or of uniform motion in a -straight line, except in so far as it may be compelled by impressed -forces to change that state._” - -[Illustration: FIG. 58.--To illustrate the First Law of Motion.] - -I would recommend you to learn this by heart. I can assure you it is -quite as well worth knowing as those rules in the Latin Grammar with -which many of you, I have no doubt, are acquainted. The best proof -of the first law of motion is derived, not from any experiments, but -from astronomy. We make many calculations about the movements of the -sun, the moon, the stars, and then we venture on predictions, and -we find those predictions verified. Thus we had a transit of Venus -across the sun in 1882, and every astronomer knew that this was going -to occur, and many went to the ends of the earth so that they might -see it favorably. Their anticipations were realized; they always are. -Astronomers make no mistakes in these matters. They know that there -will be another transit of Venus in the year 2004, but not sooner. The -calculations by which these accurate prophecies are made involve this -first law of motion; and as we find that such prophecies are always -fulfilled, we know that the first law of motion must be true also. - -Newton knew that if a planet were merely left alone in space, it would -continue to move on forever in a straight line. But Kepler had shown -that the planet did not move in a straight line, but that it described -an ellipse. One conclusion was obvious. There must be some force acting -upon the planet which pulls it away from the straight line it would -otherwise pursue. We may, for the sake of illustration, imagine this -force to be applied by a rope attached to the planet so that at every -moment it is dragged by some unseen hand. To find the direction this -rope must have, we take the law of Kepler, which explains the rules -according to which the planet varies its speed. I cannot enter into -the question fully, as it would be too difficult for us to discuss -now. I should have to talk a great deal more about mathematics than -would be convenient just at present; but I think you can all understand -the result to which Newton was led. He showed that the rope must always -be directed towards the sun. In other words, suppose that there was -no sun, but that in the place which it occupied there was a strong -enough giant constantly pulling away at the planet, then we should find -that the speed of the planet would alter just in the way it actually -does. Thus we learn that some force must reside in the sun by which -the planet is drawn, and this force is exerted, although there is no -visible bond between the sun and the planet. - -There is another fact to be learned about the sun’s attraction, and -this time we obtain it by knowing the shape of the curve followed by -the planet. The laws by which the planet’s speed is regulated prove -that the force emanates from the sun. We shall now learn much more when -we take into account that the path of the planet is an ellipse, of -which the sun lies at the focus. Nothing has been said as yet regarding -the magnitude of the pull which is being exerted by the sun. Is that -pull to be always the same, or is it to be greater at some times than -at other times? Newton showed that no ellipse other than a circle -could be described, if the pull from the sun were always the same. Its -magnitude must be continually changed, and the nearer the planet lies -to the sun, the more vehement is the pull it receives. Newton laid down -the exact law by which the force on the planet at any one place in -its path could be compared with the force at any other position. Let -us suppose that the planet is in a certain position, and that it then -passes into a second position, which is twice as far from the sun. The -pull upon the planet at the shorter distance is not only greater than -the pull at the longer distance, but it is actually four times as much. -Stating this result a little more generally, we assert, in the language -of astronomers, that the _attraction varies inversely as the square of -the distance_. If this law were departed from, then I do not say that -it would be impossible for the planet to revolve around the sun in some -fashion, but the motion would not be performed in an ellipse described -around the sun in the focus. - -You see how very instructive are the laws which Kepler discovered. -From the first of them we were able to infer that the sun attracts the -planets; from the second, we have learned how the magnitude of the -attracting force varies. - -The true importance of these great discoveries will be manifest when -we compare them with what we have already learned with regard to the -movements of the moon. As the moon revolves around the earth it is held -by the earth’s attraction, and the moon follows a path which, though -nearly a circle, is really an ellipse. This orbit is described around -the earth just as the earth describes its path around the sun. That -law by which a stone falls to the ground in consequence of the earth’s -attraction is merely an illustration of a great general principle. -Every body in the whole universe attracts every other body. - -Think of two weights lying on the table. They no doubt attract each -other, but the force is an extremely small one--so small, indeed, that -you could not measure it by any ordinary appliance. One or both of the -attracting masses must be enormously big if their mutual gravitation is -to be readily appreciable. The attraction of the earth on a stone is -a considerable force, because the earth is so large, even though the -stone may be small. Imagine a pair of colossal solid iron cannon-balls, -each 53 yards in diameter, and weighing about 417,000 tons. Suppose -these two globes were placed a mile apart, the pull of one of them on -the other by gravitation would be just a pound weight. Notwithstanding -the size of these masses, the hand of a child could prevent any motion -of one ball by the attraction of the other. If, however, they were -quite free to move, and there was absolutely no friction, the balls -would begin to draw together; at first they would creep so slowly that -the motion would hardly be noticed. The pace would no doubt continue to -improve slowly, but still not less than three or four days must elapse -before they will have come together. - -By the kindness of Professor Dewar, I am enabled to exhibit a -contrivance with which we can illustrate the motion of a planet around -the sun. Here is a long wire suspended from the roof of this theatre, -and attached to its lower end is an iron ball, made hollow for the sake -of lightness. When I draw the ball aside, it swings to and fro with -the regularity of a great pendulum. But when I place a powerful magnet -in its neighborhood (Fig. 59), you see that as soon as the ball gets -near the magnet it is violently drawn to one side, and follows a curved -path. This magnet may be taken to represent the sun, while the ball is -like our earth, or any other planet, which would move in a straight -line were it not for the attraction of the sun which draws the body -aside. - -[Illustration: FIG. 59.--The Effect of Attraction.] - - -THE GEOGRAPHY OF MARS. - -We will now say something with respect to the geography of our -fellow-planet, a subject which seems all the more interesting because -Mars is so like the earth in many respects. We require a fairly -good telescope for the purpose of seeing him well, but when such an -instrument is directed to the planet, a beautiful picture of another -world is unfolded (Fig. 60). There are many things visible on his -surface, but we must always remember that even with our most powerful -telescopes the planet still appears a long way off. - -[Illustration: FIG. 60.--Views of Mars.] - -[Illustration: FIG. 61.--Mars. - -(_By Douglass, Lowell Observatory._)] - -In the most favorable circumstances, Mars is at least one hundred times -as far from us as the moon. But we know that an object on the moon must -be as large as St. Paul’s Cathedral if it is to be visible in our -telescopes. An object on Mars must be, therefore, at least one hundred -times as broad and one hundred times as long as St. Paul’s Cathedral if -it is to be discernible by astronomers on our earth. We can, therefore, -only expect to see the general features of our fellow-planet. Were we -looking at our earth from a similar distance, and with equally good -telescopes, the continents and oceans, and the larger seas and islands, -would all be large enough to be conspicuous. It is, however, doubtful -whether they could ever be properly revealed through the serious -impediment to vision which our atmosphere would offer. - -It fortunately happens that the surface of Mars is only obscured -by clouds to a very trifling extent, and we are thus able to see a -panorama of our neighboring globe laid before us. Mars is not nearly -so large as our earth, the diameters of the two bodies being nearly -as two to one. It follows that the number of acres on the planet is -only a quarter of the number of acres on the earth. Careful telescopic -scrutiny shows that the chief features which we see on Mars are of a -permanent character. In this respect Mars is much more like the moon -than the sun. The latter presents to us merely glowing vapors, with -hardly more permanence than is possessed by the clouds in our own sky. -On the other hand, the entire absence of clouds from the moon enables -us to see the permanent features on its surface. Most of the visible -features on Mars are also invariable; though occasionally it would seem -that the climate produces some changes in its appearance. - -[Illustration: FIG. 62.--The South Pole of Mars, September, 1877 -(_Green_).] - -We first notice that there are differently colored parts on Mars. The -darkish or bluish regions are usually spoken of as seas or oceans; -though we should be going beyond our strict knowledge were we to assert -that water is actually found there. Look at the horn-shaped object in -the centre of the lower picture in Fig. 60. We call it the Kaiser Sea, -and it is so strongly marked that even in a small telescope it can be -often seen. You must not, however, always expect to notice this feature -when you look at the planet through a telescope, for it turns round -and round. We can make a globe representing Mars. On this are to be -depicted this great sea and the other characteristic objects. But as -we turn the globe around, the opposite side of the planet is brought -into view, and other features are revealed like those represented in -the upper figure. Mars requires 24 hours 37 minutes 22.7 seconds to -complete a single rotation. It is somewhat remarkable that this only -differs from the earth’s period of rotation by a little more than half -an hour. - -Mars contains what we call continents as well as oceans, and we also -find there lakes and seas and straits. These objects are indicated in -the drawings that are here represented. But the most striking features -which the planet displays are the marvellous white regions, which are -seen both at its North Pole and at its South Pole (Fig. 62). If we -were able to soar aloft above our earth and take a bird’s-eye view of -our own polar regions, we should see a white cap at the middle of the -arctic circle. This appearance would be produced by the eternal ice and -snow. It would increase during the long, dark winter, and be somewhat -reduced by melting during the continuously bright summer. Though we -cannot thus see our earth, yet we can sometimes observe one Pole of -Mars and sometimes the other, and we find each of these Poles crowned -with a dense white cap, which increases during the severity of its -winter, and which declines again with the warmth of the ensuing summer. - -Sketches of Mars have been made by many astronomers; among them we may -mention Mr. Green, who made a beautiful series of pictures at Madeira -in 1877. These may be supplemented by the drawings of Mr. Knobel in -1884, when the opposite Pole of the planet was turned to view. The -drawings show the polar snows, and there seem to be some elevated -districts in his arctic regions which retain a little patch of snow -after the main body of the ice cap has shrunk within its summer limits. -An interesting case of this kind is shown in Fig. 62, which has been -copied from one of Mr. Green’s drawings. - -It has lately been surmised that the continents on Mars are -occasionally inundated by floods of water. There are also indications -of clouds hanging over the Martian lands, but the inhabitants of -that planet, in this respect, escape much better than we do. A -certain amount of atmosphere always surrounds Mars, though it is much -less copious than that we have here. As to the composition of this -atmosphere we know nothing. For anything we can tell, it might be a -gas so poisonous that a single inspiration would be fatal to us; or if -it contained oxygen in much larger proportion than our air does, it -might be fatal from the mere excitement to our circulation which an -over-supply of stimulant would produce. I do not think it the least -likely that our existence could be supported on Mars, even if we could -get there. We also require certain conditions of climate, which would -probably be totally different from those we should find on Mars. - -Many remarkable observations of Mars have been lately made by Mr. -Percival Lowell. It seems very doubtful how far our former division -of continents and oceans on Mars can be maintained. Mr. Lowell has -paid special attention to a wonderful system of lines on the planet’s -surface to which the name of “canals” has been given, which often show -such a degree of regularity as would almost suggest the idea that they -had been laid down by intelligent guidance. - - -THE SATELLITES OF MARS. - -When Mars appeared in his full splendor in 1877, he was for the first -time honored with the notice of instruments capable of doing him -justice. I do not, however, mean that in former apparitions he was -not also carefully observed, but a great improvement had recently -taken place in telescopes, and it was thus under specially favorable -auspices that his return was welcomed in 1877. This year will be always -celebrated in astronomical history for a beautiful discovery made by -Professor Asaph Hall, the illustrious astronomer at Washington. - -Before I can explain what this discovery was, I must have a little talk -about moons, or satellites as they are often called. You know that -we have one moon, which is constantly revolving round the earth, and -accompanies the earth in its long voyage round the sun. But the earth -is only a planet, and there are many other planets which are worlds -like ours. It is natural to compare these worlds, and as we have one -moon, why should not the other planets also have moons? If there are -children in one house in a square, why should there not be children -in the other houses? We find that some of the other planets have -satellites, but they do not seem to be distributed very regularly. In -fact, they are almost as capriciously allotted as the children would be -in eight houses that you might take at random. - -[Illustration: FIG. 63.--Mars and his Two Satellites.] - -In Number One there lives an old bachelor, and in Number Two a single -lady. These are Mercury and Venus, and of course there are no children -in either of these houses. Number Three is inhabited by old mother -Earth, and she has got a fine big son, called the Moon. Number Four is -a nice little house inhabited by Mars. There are to be found a pair -of little twins, and nimble creatures they are too. Number Five is a -great mansion. A very big man lives here, called Jupiter, with four -robust sons and daughters that everybody knows. I fancy they must go -to many dancing parties, for every night they may be seen whirling -round and round. For three hundred years these four moons have been -known to astronomers, but in 1892 there was an addition to the family -in the shape of a tiny moon which had never been seen up to that time. -Number Six is also a fine big house, though not quite so big as Number -Five, but larger than any of the others. It is inhabited by Saturn, -and contains the biggest family of all. Up till the other day eight -sons and daughters were known to live here, but they are not nearly so -sturdy as Jupiter’s children; in fact, the young Saturns do not make -much display, and some of them are so delicate that they are hardly -ever seen. In this household, too, a new member has recently appeared. -For fifty years the family was known to consist of these eight sons -and daughters, but in August, 1898, when they were being photographed -in a group, it was discovered that a ninth moon had been added. Number -Seven is also a fine large house; but Uranus, who lives there, is such -a recluse that unless you carefully keep your eye on his house, you -will hardly ever catch a glimpse of him. There are four children in -that house, I believe, but we hardly know them. They move in circles -of their own, and apparently have seen a good deal of trouble. Only -one more house is to be mentioned, and that is Number Eight, inhabited -by Neptune. It contains one child, but we are hardly on visiting terms -with this household, and we know next to nothing about it. - -Before 1877, Mars appeared to be in the same condition as Venus or -Mercury--that is, devoid of the dignity of attendants. There was, -however, good reason for thinking that there might be some satellites -to Mars, only that we had not seen them. You see that, as Number Three -had one child, and Numbers Five, Six, Seven, and Eight had each one, or -more than one, it seemed hard that poor Number Four should have none -at all. It was, however, certain that if there were any satellites to -Mars, they must be comparatively small things; for if Mars had even -one considerable moon, it must have been discovered long ago. - -On the memorable occasion in 1877, Professor Hall discovered that the -ruddy planet Mars was attended, not alone by one moon, but by two. -Their behavior was most extraordinary. It appeared to him at first -almost as if one of these little moons was playing at hide-and-seek. -Sometimes it would peep out at one side of the planet, and sometimes -at the other side. I have here a picture (Fig. 63) which shows how -these moons of Mars revolve. That is the globe of the planet himself -in the middle, and he is turning round steadily in a period which is -nearly the same as our day. But the remarkable point is that the inner -of the moons of Mars runs round the planet in 7 hours 39 minutes. It -would seem very strange in our sky if we had a little moon which rose -in the west instead of in the east, and which galloped right across -the heavens three times every day--and this is what Mars has. The -outer moon takes a more leisurely journey, for he requires 30 hours 18 -minutes to complete a circuit. If for no other reason than to see these -wonderful moons, it would be very interesting to visit Mars. - -The satellites of this planet are in contrast to our moon. In the -first place, our moon takes 27 days to go round the earth, and is -comparatively a long way off. The moons of Mars are much nearer to -their planet, and they go round much more quickly. There is also -another difference. The moons of Mars are much smaller bodies than -our moon. If we represent Mars by a good-sized football, his moons, -on the same scale, would be hardly so big as the smallest-sized -grains of shot. Does it not speak well for the power of telescopes -in these modern days that objects so small as the satellites of Mars -should be seen at all? You remember, of course, that neither Mars -himself nor his moons have any light of their own. They shine solely -in consequence of the sunlight which falls upon them. They are merely -lighted like the earth itself, or like the moon. The difficulty about -observing the satellites is all the greater because they are seen in -the telescope close to such a brilliant body as Mars. The glare from -the bright planet is such that when we want to see faint objects like -the satellites we have to hide Mars, so as to get a comparatively dark -space in which to search. - -Now that they know exactly what to look for, a good many astronomers -have observed the satellites of Mars. A superb telescope is -nevertheless required. And, in fact, you could not find a better -test for the excellence of an instrument than to try if it will show -these delicate objects. But do not imagine that merely having a -good telescope and a clear sky is all that is requisite for making -astronomical discoveries. You might just as well say that by putting -a first-rate cricket-bat in any man’s hands will ensure his making a -grand score. Every boy knows that the bat does not make the cricketer, -and I can assure him that neither will the telescope make the -astronomer. In both cases, no doubt, there is some element of luck. But -of this you may be certain: that as it is the man that makes the score, -and not the bat, so it is the astronomer that makes the discovery, and -not his telescope. - -Deimos and Phobos were the names of the two personages, according to -Homer, whose duty it was to attend on the god Mars, and to yoke his -steeds. A conclave of classical scholars and astronomers appropriately -decided that Deimos and Phobos must be the names of the two satellites -to the planet which bears the name of Mars. - - -HOW THE TELESCOPE AIDS IN VIEWING FAINT OBJECTS. - -We have been hitherto talking about large planets, which, if not as big -as our earth, are at least as big as our moon. But now we have to say a -few words about a number of little planets, many of them being so very -small that a million rolled together would not form a globe so big as -this earth. These little objects you cannot see with your unaided eye, -and even with a telescope they only look like very small stars. - -I have often been asked why it is that a telescope enables us to -see objects, both faint and small, which our unaided eyes fail to -show. Perhaps this will be a good opportunity to say a few words on -the subject. I think we can explain the utility of the telescope by -examining our own eyes. The eye undergoes a remarkable transformation -when its owner passes from darkness into a brilliantly lighted room -(Fig. 64). Here you see two views of an eye, and you notice the great -difference between them. They are not intended to be the eyes of two -different people, or the two eyes of the same person; they are merely -two conditions of the same eye. They are intended to illustrate two -different states of the eye of a collier. The right shows his eye when -he is above ground in bright daylight; the left is his eye when he has -gone down the coal-pit to his useful work in the dark regions below. I -remember when I went down a coal-pit I was lowered down a long shaft, -and when the bottom was reached a safety lamp was handed to me. The -gloom was such, that at first I found some little difficulty in guiding -my steps, but the capable guide beside me said in an encouraging voice, -“You will be all right, sir, in a few moments, for you will get your -_pit-eyes_.” I did get my “pit-eyes,” as he promised, and was able to -see my way along sufficiently to enjoy the wonderful sights that are -met with in the depths below. - -[Illustration: This shows the Eye in the Dark. - -This shows the Eye in the Daylight. - -FIG. 64.] - -The change that came over my eyes is that which these two pictures -illustrate: the black, round spot in the centre is an opening covered -with a transparent window, by which light enters the eye; the black -spot is called the pupil, and nature has provided a beautiful -contrivance by which the pupil can get larger or smaller, so as to make -vision agreeable. When there is a great deal of light we limit the -amount that enters by contracting the pupil so as to make the opening -smaller. Thus the picture with the small pupil represents the state of -the collier’s eye when he is above ground in bright sunlight. When he -descends into the pit, where the light is very scanty, then he wants to -grasp as much of it as ever he can, and consequently his pupil enlarges -so as to make a wider opening, and this is what he calls getting his -“pit-eyes.” - -But you need not go down a coal-mine to see the use of the iris--for so -that pretty membrane is called which surrounds the pupil. Every time -you pass from light into darkness the same thing can be perceived. -When we turn down the lights in a room, so that we are in comparative -darkness, our pupils gradually expand. As soon as the lights are -turned up again, then our pupils begin to contract. Other animals have -the same contrivance in their eyes. You may notice in the Zoölogical -Gardens how quickly the pupil of the lion contracts when he raises his -eyes to the light. The power of rapidly changing the pupil might be -of service to a beast of prey. Imagine him crouching in a dense shade -to wait for his dinner; then of course the pupil will be large from -deficiency of light; but when he springs out suddenly on his victim, in -bright light, it would surely be of advantage to him to be able at once -to see clearly. Accordingly his pupil adjusts itself to the altered -conditions with a rapidity that might not be necessary for creatures of -less predaceous habits. - -These changes of the pupil explain how the telescope aids our eyes -when we want to discern any faint objects, like the little planets. -Such bodies are not visible to the unaided eyes, because our pupils -are not large enough to grasp sufficient light for the purpose. Even -when they are opened to the utmost, we want something that shall enable -them to open wider still. We must therefore borrow assistance from -some device which shall have an effect equivalent to an enlargement -of the pupil beyond the limits that nature has actually assigned to -it. What we want is something like a funnel which shall transform a -large beam of rays into a small one. I may explain what I mean by the -following illustration: Suppose that it is raining heavily, and that -you want to fill a bucket with water. If you merely put the bucket out -in the middle of a field, it will never be filled; but bring it to -where the rain-shoot from a house-top is running down, and then your -bucket will be running over in a few moments. The reason, of course, -is that the broad top of the house has caught a vast number of drops -and brought them together in the narrow shoot, and so the bucket is -filled. In the same way the telescope gathers the rays of light that -fall on the object glass, and condenses them into a small beam which -can enter the eye. We thus have what is nearly equivalent to an eye -with a pupil as big as the object glass. Thus the effect of a grand -telescope amounts to a practical increase of the pupil from the size of -a threepenny-piece up to that of a dinner-plate, or even much larger -still. - - -THE ASTEROIDS OR SMALL PLANETS. - -An asteroid is like a tiny star, and in fact the two bodies are very -often mistaken. If we could get close to the objects, we should see a -wide difference between them. We should find the asteroid to be a dark -planet like our earth, lighted only by the rays from the sun. The star, -small and faint though it may seem, is itself a bright sun, at such a -vast distance that it is only visible as a small point. The star is -millions of times as far from us as the planet, and utterly different -in every respect. - -It is a curious fact that the planets should happen to resemble the -stars so closely. We can find an analogous fact in quite another part -of nature. In visiting a good entomological collection, you will be -shown some of the wonderful leaf-like insects. These creatures have -wings, exactly formed to imitate leaves of trees, with the stalks and -veins completely represented. When one of these insects lies at rest, -with its wings folded, among a number of leaves, it would be almost -impossible to penetrate the disguise. This mimicry is no doubt an -ingenious artifice to deceive the birds or other enemies that want to -eat the insect. There is, however, one test which the cunning bird -could apply: the leaves do not move about of their own accord, but the -leaf-insects do. If therefore the bird will only have the patience -to wait, he will see a pair of the seeming leaves move, and then the -deception will be to him a deception no longer, and he will gobble up -the poor insect. - -In our attempts to discover the planets we experience just the same -difficulties as the insect-eating bird. Wide as is the true difference -between a planet and a star, there is yet such a seeming resemblance -between them that we are often puzzled to know which is which. The -planets imitate the stars so successfully, that when one of them -is presented to us among myriads of stars it is impossible for us -to detect the planet by its appearance. But we can be cunning--we -can steadily watch, and the moment we find one of these star-like -points beginning to creep about we can pounce upon it. We know by its -movements that it is only disguised as a star, but that it is really -one of the planets. - -It is not always easy to discover the asteroids even by this principle, -for unfortunately these bodies move very slowly. If you have a planet -in the field of view, it will creep along so gradually that an hour or -more must have elapsed before it has shifted its position with respect -to the neighboring stars to any appreciable extent. The search for such -little planets is therefore a tedious one, but there are two methods of -conducting it: the new one, which has only recently come into use, and -the old one. I shall speak of the old one first. - -Although the body’s motion is so slow, yet when sufficient time is -allowed, the planet will not only move away from the stars close by, -but will even journey round the entire heavens. The surest way of -making the discovery is to study a small part of the heavens now and -to examine the same locality again months or years afterwards. Memory -will not suffice for this purpose. No one could recollect all the stars -he saw with sufficient distinctness to be confident that the field of -the telescope on the second occasion contained either more or fewer -stars than it did on the first. The only way of doing this work is to -draw a map of the stars very carefully. This is a tedious business, -for the stars are so numerous that even in a small part of the heavens -there will be many thousands of stars visible in the telescope. All of -these will have to be entered faithfully in their true places on the -map. When this has been done the map must be laid aside for a season, -and then it is brought out again and compared with the sky. No doubt -the great majority of the objects will be found just as they were -before. These are the stars, the distant suns, and our concern is not -at present with them. Sometimes it will happen that an object marked -on the first map has left a vacant place on the second. This, however, -does not help us much, for, whatever the object was, it has vanished -into obscurity, and a new planet could hardly be discovered in this -way. But sometimes it will happen that there is a small point of light -seen in the second map which has no corresponding point in the first. -Then, indeed, the expectation of the astronomer is aroused; he may be -on the brink of a discovery. Of course he watches accurately the little -stranger. It might be some star that had been accidentally overlooked -when forming the map, or it might possibly be a star that has become -bright in the interval. But here is a ready test: is the body moving? -He looks at it very carefully, and notes its position with respect to -the adjacent stars. In an hour or two his suspicions may be confirmed; -if the object be in motion, then it is really a planet. A few further -observations, made on subsequent days, will show the path of the body. -And the astronomer has only to assure himself that the object is not -one of the planets that have been already found before he announces his -discovery. - -The new method of searching for small planets, which has only come into -use in recent years, is a very beautiful one, and renders the process -of making such discoveries much more easy than the older method which I -have just described. - -We can take photographs of the heavenly bodies by adjusting a sensitive -plate in the telescope so that the images of the objects we desire to -see shall fall upon it. The method will apply to very small stars, if -by excellent clockwork and careful guiding we can keep the telescope -constantly pointing to the same spot until the stars have had time to -imprint their little images. Thus we obtain a map of the heavens, made -in a thoroughly accurate manner. Indeed, the delicacy of photography -for this purpose is so great, that the plates show many stars which -cannot be seen with even the greatest of telescopes. Suppose that -a little planet happened to lie among the stars which are being -photographed. All the time that the plate is being exposed the wanderer -is, of course, creeping along, and after an hour (exposures even longer -are often used), it may have moved through a distance sufficient to -ensure its detection. The plate will, therefore, show the stars as -points, but the planet will betray its presence by producing a streak. - -The asteroids now known number between 400 and 500. Out of this host -a few afford some information to the astronomer, but the majority of -them are objects possessing individually only the slightest interest. -No small planet is worth looking at as a telescopic picture. We should -consider that asteroid to be a large one which possessed a surface -altogether as great as England or France. Many of these planets have a -superficial extent not so large as some of our great counties. A globe -which was just big enough to be covered by Yorkshire--if you could -imagine that large county neatly folded round it--would make a very -respectable minor planet. - -We know hardly anything of the nature of these small worlds, but it is -certain that any living beings they could support must have a totally -different nature from the creatures that we know on this earth. We can -easily prove this by making a calculation. I shall suppose a small -planet one hundred miles wide, its diameter being, therefore, the -one-eightieth part of the diameter of the earth. If we were landed -on such a globe, we should be far more puzzled by the extraordinary -lightness of everything than we should be in the similar case of the -moon to which I referred (p. 124). If we suppose the planet to be -constructed of materials which had the same density as those of which -the earth is made, then every weight would be reduced to the eightieth -part of what it is here. - -There would be one curious consequence of residence on such a globe. -We have heard of attempts to make flying machines, or to provide a -man with wings by which he shall soar aloft like the birds. All such -contrivances have hitherto failed. It may be possible to make a pair of -wings by which a man can fly down, but it is quite another matter when -he tries to fly up again. Suppose, however, we were living on a small -planet, it would be perfectly easy to fly, for as our bodies would only -seem to weigh a couple of pounds, we ought to be able to flap a pair of -wings strong enough to overcome so trivial a force. I should, however, -add that this is on the supposition that the atmosphere has the same -density as our own. - -Life on these small planets would indeed be extraordinary. Let us take, -for example, Flora, and see how a game of lawn tennis on that body -would be managed. The very slightest blow of the racket would drive the -ball a prodigious distance before it could touch the ground; indeed, -unless the courts were about half a mile long, it would be impossible -to serve any ball that was not a fault. Nor is there any great exertion -necessary for playing lawn tennis on Flora, even though the courts are -several hundred acres in extent. As a young lady ran to meet the ball -and return it, each of her steps might cover a hundred yards or so -without extra effort; and should she have the misfortune to get a fall, -her descent to the ground would be as gentle as if she was seeking -repose on a bed of the softest swan’s-down. - -These little planets cluster together in a certain part of our -system. Inside are the four inner planets, of which we have already -spoken; outside are the four outer planets, of which we have soon to -speak. Between these two groups there was a vacant space. It seemed -unreasonable that where there was room for planets, planets should -not be found. Accordingly the search was made, and these objects -were discovered. Even at the present day, more and more are being -constantly added to the list. - -Up till quite recently all the small planets which had been discovered -confined themselves to the space lying between the paths of the -major planets Mars and Jupiter. This invariable rule was, however, -departed from in the case of one of these bodies which was discovered -in August, 1898. This little body, which was known for some time by -the provisional appellation of D Q, and which has now been definitely -christened Eros, is an exception to this rule. It travels at an average -distance from the sun actually less than that of Mars, and at the -nearest point can come within 15,000,000 miles of the earth. - -We occasionally get information from these little bodies; for in their -revolutions through the solar system, they sometimes pick up scraps of -useful knowledge, which we can elicit from them by careful examination. -For example, one of the most important problems in the whole of -astronomy is to determine the sun’s distance. I have already mentioned -one of the ways of doing this, which is given by the transit of Venus. -Astronomers never like to rely on a single method; we are therefore -glad to discover any other means of solving the same problem. This -it is which the little planets will sometimes do for us. Juno on one -occasion approached very close to the earth, and astronomers in various -parts of the globe observed her at the same time. When they compared -their observations they measured the sun’s distance. But I am not going -to trouble you now with a matter so difficult. Suffice it to say, -that for this, as for all similar investigations, the observers were -constrained to use the very same principle as that which we illustrated -in Fig. 5. - -Let me rather close this lecture with the remark that we have here been -considering only the lesser members of the great family which circulate -round the sun, and that we shall speak in our next lecture of the giant -members of our system. - - - - -LECTURE IV. - -JUPITER, SATURN, URANUS, NEPTUNE. - - Jupiter, Saturn, Uranus, Neptune--Jupiter--The Satellites of - Jupiter--Saturn--The Nature of the Rings--William Herschel--The - Discovery of Uranus--The Satellites of Uranus--The Discovery of - Neptune. - - -Our lecture to-day ought to make us take a very humble view of the size -of our earth. Mercury, Venus, and Mars may be regarded as the earth’s -peers, though we are slightly larger than Venus, and a good deal larger -than Mercury or Mars; but all these four globes are insignificant in -comparison with the gigantic planets which lie in the outer parts of -our system. These great bodies do not enjoy the benefits of the sun to -the same extent that we are permitted to do; they are so far off that -the sun’s rays become greatly enfeebled before they can traverse the -distance; but the gloom of their situation seems to matter but little, -for it is highly improbable that any of these bodies could be inhabited. - -A view of parts of the paths of these four great planets is shown -in Fig. 65. The innermost is Jupiter, which completes a circuit in -about twelve years; then comes Saturn, revolving in an orbit so great -that twenty-nine years and a half are required before the complete -journey is finished. Still further outside is Uranus, which has a -longer journey than Saturn, and moves so much more slowly that a man -would have to live to the ripe old age of eighty-four if a complete -revolution of Uranus was to be accomplished during his lifetime. At the -boundary of our system revolves the planet Neptune, and though it is a -mighty globe, yet we cannot see it without a telescope. It is invisible -to the naked eye for two reasons: first of all, because it is so far -from the sun that the light which illuminates it is excessively feeble; -and, secondly, because it is so far from us that whatever brilliancy it -has is largely reduced. - -[Illustration: FIG. 65.--The Orbits of the Four Giant Planets.] - - -JUPITER. - -Of all these bodies Jupiter is by far the greatest; he is, indeed, -greater than all the other planets rolled into one. The relative -insignificance of the earth when compared with Jupiter is well -illustrated by the fact that if we took 1200 globes each as big as our -earth, and made them into a single globe, it would only be as large as -the greatest of the planets. A view of the comparative sizes of the -earth and Jupiter is shown in Fig. 66. - -[Illustration: FIG. 66.--Jupiter and the Earth compared.] - -Fig. 67 shows a picture of Jupiter as seen through the telescope. -First, you will notice that the outline of the planet’s shape is not -circular, for it is plain that the vertical diameter in this picture -is shorter than the horizontal one; in fact, Jupiter is flattened at -the Poles and bulges out at the equator, so that a section through the -Poles is an ellipse. Jupiter is turning round rapidly on his axis, and -this will account for the protuberance. We find that the planet has -assumed almost the same form as if it were actually a liquid. This -we can illustrate by a globe of oil which is poised in a mixture of -spirits of wine and water so carefully adjusted that the oil has no -tendency to rise or fall. As we make the globe of oil rotate, which we -can easily do by passing a spindle through it, we see that it bulges -out in the form that Jupiter as well as other planets have taken. - -[Illustration: FIG. 67.--The Clouds of Jupiter.] - -On the picture of the planets you will see shaded bands. These are -constantly changing their aspect, and for a double reason. In the first -place, they change because Jupiter is rotating so quickly that in -five hours the whole side of the planet which is towards us has been -carried out of sight. In another five hours the original side of the -globe will be back again, for the entire rotation occupies about 10 -hours, or, more precisely, 9 hours 55 minutes 21 seconds. - -But these bands are themselves not permanent objects. They have no more -permanence than the clouds over our own sky. Sometimes Jupiter’s clouds -are more strongly marked than on other occasions. Sometimes, indeed, -they are hardly to be seen at all. It is from this we learn that those -markings which we see when we look at the great planet are merely the -masses of cloud which surround and obscure whatever may constitute his -interior. - -There is a circumstance which demonstrates that Jupiter must be an -object exceedingly different from the earth, though both bodies agree -in so far as having clouds are concerned. What would you think when I -tell you that we were able to weigh Jupiter by the aid of his little -moons, of which I shall afterwards speak? These little bodies inform us -that Jupiter is about 300 times as heavy as our earth, and we have no -doubt about this, for it has been confirmed in other ways. But we have -found by actual measurement that Jupiter is 1200 times as big as the -earth, and therefore, if he were constituted like the earth, he ought -to be 1200 times as heavy. This is, I think, quite plain; for if two -cakes were made of the same material, and one contained twice the bulk -of the other, then it would certainly be twice as heavy. If there be -two balls of iron, one twice the bulk of the other, then, of course, -one has twice the weight of the other. But if a ball of lead have -twice the bulk of a ball of iron, then the leaden ball would be more -than twice as heavy as the iron, because lead is the heavier material. -In the same way, the weights of the earth and Jupiter are not what we -might expect from their relative sizes. If the two bodies were made -of the same materials and in the same state, then Jupiter would be -certainly four times as heavy as we find him to be. We are, therefore, -led to the belief that Jupiter is not a solid body, at least in its -outer portions. The masses of cloud which surround the planet seem -to be immensely thick, and as clouds are, of course, light bodies in -comparison with their bulk, they have the effect of largely increasing -the apparent size of Jupiter, while adding very little to his weight. -There is thus a great deal of mere inflation about this planet, by -which he looks much bigger than his actual materials would warrant if -he were constituted like the earth. - -These facts suggest an interesting question. Why has Jupiter such an -immense atmosphere, if we may so call it? The clouds we are so familiar -with down here on the earth are produced by the heat of the sun, which -beats down upon the wide surface of the ocean, evaporates the water, -and raises the vapor up to where it forms the clouds. Heat, therefore, -is necessary for the formation of cloud; and with clouds so dense and -so massive as those on Jupiter, more heat would apparently be necessary -than is required for the moderate clouds on this earth. Whence is -Jupiter to get this heat? Have we not seen that the great planet is -far more distant from the sun than we are? In fact, the intensity of -the sun’s heat on Jupiter is not more than the twenty-fifth part of -what we derive from the same source. We can hardly believe that the -sun supplied the heat to make those big clouds on the great planet; so -we must cast about for an additional source, which can only be inside -the planet itself. So far as his internal heat is concerned, Jupiter -seems to be in much the same condition now as our earth was once, ages -ago, before its surface had cooled down to the present temperature. -As Jupiter is so much larger than the earth, he has been slower in -parting with his heat. The planet seems not yet to have had time to -cool sufficiently to enable water to remain on his surface. Thus the -internal heat of the planet supplies an explanation of his clouds. We -may also remark that as the present condition of Jupiter illustrates -the early condition of our earth, so the present condition of the earth -foreshadows the future reserved for Jupiter when he shall have had time -to cool down, and when the waters that now exist in the form of vapor -shall be condensed into oceans on his surface. - - -THE SATELLITES OF JUPITER. - -Every owner of a telescope delights to turn it on the planet Jupiter, -both for the spectacle the globe itself affords him, and for a view of -the wonderful system of moons by which the giant planet is attended. -Fortunately the four satellites of Jupiter lie within reach of even -the most modest telescope, and their incessant changes relative to -Jupiter and each other give them a never-ending interest for the -astronomer. Compared with the torpid performance of our moon, which -requires a month to complete a circuit around the earth, Jupiter’s -moons are wonderfully brisk and lively. Nor are they small bodies like -the satellites of Mars, for the second of Jupiter’s satellites is quite -as big as our moon, and the other three are very much larger. It is, -however, true that his satellites appear insignificant when compared -with Jupiter’s own enormous bulk. - -The innermost of these little bodies flies right round in a period of -one day and eighteen or nineteen hours, while the outermost of them -takes a little more than a fortnight--that is, rather more than half -the time that our moon demands for a complete revolution. Jupiter’s -satellites are too far off for us to see much with respect to their -structure or appearance even with mighty telescopes. It is, of course, -their great distance from us that makes them look insignificant. They -would, however, be bright enough to be seen like small stars were -it not that, being so close to Jupiter, his overpowering brightness -renders such faint objects in his vicinity invisible. - -It was by means of the satellites of Jupiter that one of the most -beautiful scientific discoveries was made. As a satellite revolves -round the giant planet it often happens that the little body enters -into the shadow of the great planet. No sunlight will then fall upon -the satellite, and as it has no light of its own, it disappears from -sight until it has passed through the shadow and again receives -sunlight on the other side. We can watch these eclipses with our -telescopes, and there can be no more interesting employment for -a small telescope. The movements of these bodies are now known so -thoroughly that the occurrence of the eclipses can be predicted. The -almanacs will tell when the satellite is calculated to disappear, and -when it ought again to return to visibility. When astronomers first -began to make these computations a couple of hundred years ago, the -little satellites gave a great deal of trouble. They would not keep -their time. Sometimes they were a quarter of an hour too soon, and -sometimes a quarter of an hour too late. At last, however, the reason -for these irregularities was discovered, and a wonderful reason it was. - -Suppose there were a number of cannons all over Hyde Park, and that -these cannons were fired at the same moment by electricity. Though the -sounds would all be produced simultaneously, yet, no matter where you -stood, you would not hear them altogether; the noise from the cannons -close at hand would reach your ears first, and the more distant reports -would come in subsequently. You can calculate the distance of a flash -of lightning if you allow a mile for every five seconds that elapse -between the time you saw the flash and the time you heard the peal -of thunder which followed it. The light and the noise were produced -simultaneously, but the sound takes five seconds to pass over every -mile, while the light, in comparison to sound, may be said to move -instantaneously. That sound travelled with a limited velocity was -always obvious, but never until the discrepancies arose about Jupiter’s -satellites was it learned that light also takes time to travel. It is -true that light travels much more quickly than sound--indeed, about -a million times as fast. Light goes so quickly, that it would rush -more than seven times around the earth in a single second. So far as -terrestrial distances are concerned, the velocity of light is such -that the time required for a journey is inappreciable. The distances, -however, between one celestial body and another are so enormous, that -even a ray of light, moving as quickly as it alone can move, will -occupy a measurable time on the way. Our moon is comparatively so near -us, that light takes little more than a second to cover that short -distance. Eight minutes are, however, required for light to travel from -the sun to the earth; in fact, the sunbeams that now come into our -eyes left the sun eight minutes ago. If the sun were to be suddenly -extinguished, it would still seem to shine as brightly as ever in the -eyes of the inhabitants of this earth for eight minutes longer. As -Jupiter is five times as far from the sun as we are, it follows that -the light from the sun to Jupiter will spend forty minutes on the -journey, and the light from Jupiter to the earth will take a somewhat -similar time. When we look at Jupiter and his moons, we do not see him -as he is now, we see him as he was more than half an hour ago, but -the interval will vary somewhat according to our different distances -from the planet. Sometimes the light from Jupiter will reach us in as -little as thirty-two minutes, while sometimes it will take as much as -forty-eight--that is, the light sometimes requires for its journey a -quarter of an hour more than is sufficient at other times. - -We can therefore understand that irregularity of Jupiter’s satellites -which puzzled the early astronomers. An eclipse sometimes appeared a -quarter of an hour before it was expected; because the earth was then -as near as it could be to Jupiter, while the calculations had been -made from observations when Jupiter was at his greatest distance. -It was these eclipses of the satellites which first suggested the -possibility that light must have a measurable speed. When this was -taken into account, then the occasional delay of the eclipses was found -to be satisfactorily explained. Confirmation flowed in from other -sources, and thus the discovery of the velocity of light was completely -established. - -Professor Barnard, when studying Jupiter in 1892 with the splendid -refractor at the Lick Observatory, saw a very small point of light -nearer to the planet than the nearest of the four satellites already -known. Further examination showed that this little object was indeed -another satellite. Thus Jupiter has a fifth moon in addition to the -four which have been known so long. This little body is so small and -faint that it can only be discerned under the most favorable conditions -by the most powerful telescopes. - - -SATURN. - -Next outside Jupiter, on the confines of the ancient planetary system, -revolves another grand planet, called Saturn. His distance from us is -sometimes nearly a thousand millions of miles, and he requires more -than a quarter of a century for the completion of each revolution. -Sometimes people do not pronounce the names of the planets quite -correctly. I have heard of a gardener who has a taste for astronomy, -and sometimes begins to talk about the planets Juniper and Citron. -Probably you will know what he meant to say. The ancients had -discovered Saturn to be a planet, for though he looked like a star, yet -his movement through the constellations could not escape their notice -when attention was paid to the heavens. - -[Illustration: FIG. 68.--Saturn and the Earth compared.] - -In the matter of size Saturn is only surpassed by Jupiter among the -planets. He is about 600 times as large as the earth; the small object, -E, shown in Fig. 68, represents our earth in its true comparative size -to the ringed planet; but Saturn is so far off, that even at his best -he is never so bright as Venus, or Mars, or Jupiter become when they -are favorably situated. On the globe of Saturn we can sometimes see a -few bands, but they are faint compared with those on Jupiter. There -is, however, no doubt that what we see upon Saturn is a dense mass of -cloud. Indeed, he can have comparatively little solid matter inside, -for this planet does not weigh so much as a ball of water the same size -would do. Saturn, like Jupiter, must be highly heated in his interior. - -The ring, or rather series of rings, by which the planet is surrounded -are also shown in Fig. 68: these appendages are not fastened to the -globe of Saturn by any material bonds; they are poised in space, -without any support, while the globe or planet proper is placed -symmetrically in the interior. - -I have made a model which shows Saturn with his rings, but it is -necessary for me to fasten the rings by little pieces of wire to the -globe, for there is no mechanical means by which the rings of the model -could be poised without support, as they are around the planet. If -we throw the beam of the electric lamp on the little planet, we see -the shadow which the planet casts on its ring. Similar shadows can be -observed in the actual Saturn of the sky, and this is a proof that -the planet does not shine by its own light, but by the light of the -sun which falls upon it. Here again we illustrate the wide difference -between a planet and a star, for were our sun to be put out, Saturn -and all the other planets in the sky would vanish from sight, while -the stars would, of course, twinkle on as before. There are three -rings round Saturn; they all lie in the same plane, and they are so -thin, that when turned edgewise towards us the whole system almost -disappears, except in very powerful telescopes. The outer and the -inner bright rings are divided by a dark line, which can be traced -entirely round. At the inner edge of the inner ring begins that strange -structure called the _crape_ ring, which extends halfway towards the -globe of the planet. The most remarkable point about the crape ring is -its semi-transparency, for we can sometimes see the globe of the planet -through this strange curtain. The crape ring can only be observed with -a powerful telescope. The other two rings are within the power of very -moderate instruments. - - -THE NATURE OF THE RINGS. - -For the explanation of the nature of Saturn’s rings we are indebted -to the calculations of mathematicians. You might have thought, -perhaps, that nothing would be simpler than to suppose the rings -were stiff plates made from solid material. But the question cannot -be thus settled. We know that the ring could not bear the strain -of the planet’s attraction upon it if it were a solid body. I may -illustrate the argument by familiar facts about bridges. Where the -span is but a small one, as, for instance, when a road has to cross -a railway, a canal, or a river, the arch is, of course, the proper -kind of structure. There is, for example, a specially beautiful arch -over the river Dee at Chester. But if the bridge be longer than this, -masonry arches are not suitable. Where a considerable span has to be -crossed, as at the Menai Straits, or a gigantic one, as at the Firth -of Forth, then arches have to be abandoned, and iron bridges of a -totally different construction have to be employed. Arches cannot be -used beyond a limited span, because the strain upon the materials -becomes too great for their powers of resistance to withstand. Each of -the stones in an arch is squeezed by intense pressure, and there is a -limit beyond which even the stoutest stones cannot be relied upon. As -soon, therefore, as the span of the arch is so great that the stones -it contains are squeezed as far as is compatible with safety, then the -limit of size for that form of arch has been reached. - -Suppose that you stood on Saturn at his equator, and looked up at the -mighty ring which would stretch edgewise across your sky. It would rise -up from the horizon on one side, and, passing over your head, would -slope down to the horizon on the other. You would, in fact, be under an -arch of which the span was about 100,000 miles. Owing to the attraction -of Saturn, every part of that structure would be pulled forcibly -towards his surface, and thus the materials of the arch, if it were a -solid body, would be compressed with terrific force. - -It does not really signify that the arch I am now speaking of is half -of a ring the other half of which is below the globe of the planet. -That is only a difference with respect to the support of two ends of -the arch, and does not affect the question as to the pressure upon -its materials; nor does the fact that the ring is revolving remove -the difficulty, though it undoubtedly lessens it. We know no solid -substance which could endure the pressure. Even the toughest steel -that ever was made would bend up like dough under such conditions. We -cannot, therefore, account for Saturn’s ring by supposing it to be a -solid, for no solid would be strong enough. - -Do you not remember the old fable of the oak tree and the pliant -reed--how when the storm was about to arise the oak laughed at the -poor reed, and said it would never be able to withstand the blasts? -But matters did not so turn out. The mighty oak, which would not -yield to the storm, was blown down, while the slender reed bent to -the wind and suffered no injury. This gives us a hint as to the true -constitution of Saturn’s ring; it is not a solid body, trying to resist -by mere strength; it is rather to be explained as an excessively pliant -structure. Indeed, I ought not to call it a structure at all; it is -rather a multitude of small bodies not in the least attached together. -I do not know what the size of these bodies may be. For anything we can -tell, they may be no larger than the pebbles you find on a gravel walk. - -Let us see how we could encircle our earth with rings like those which -surround Saturn. I shall ask you to be provided with a sufficiently -large number of pebbles, and you must also imagine that I have the -means of ascending high up into space, halfway from here to the moon. -Suppose I went up there and simply dropped the pebble, of course, it -would tumble straight down to the earth again. If, however, I threw it -out with proper speed and in the proper direction, I could start it -off like a little moon, and it would go on round and round our earth -in a circle. I mention a pebble, but really it is little matter what -the size of the object may be--it may be as small as a grain of shot -or as big as a cannon-ball. Now take another pebble. Cast it also in a -somewhat similar path, taking care, however, that the planes of the two -orbits shall be the same. Each of these little bodies shall pursue its -journey without interference from the other. Then proceed in the same -way with a third, a fourth, with thousands and millions and billions of -pebbles, until at last the small bodies will become so numerous that -they almost fill a large part of the plane with a continuous shoal. -Each little object, guided entirely by the earth’s attraction, will -pursue its path with undeviating regularity. Its neighbors will not -interfere with it, nor will it interfere with them. Let us circumscribe -the limits of our flat shoal of moonlets. We first take away all those -that lie outside a certain large circle; then we shall clear away -sufficient to make a vacant space between the outer ring and the inner -ring, and thus the two conspicuous rings have been made; at the inside -of the inner ring we shall take out numbers of pebbles here and there, -so as to make this part much less dense than the outer portions, and -thus produce a semi-transparent crape ring; then we shall clear away -those that come too close to the planet, and form a neat inner boundary. - -Could we then view our handiwork from the standpoint of another planet, -what appearance would our earth present? The several pebbles, though -individually so small, would yet, by their countless numbers, reflect -the sun’s light so as to produce the appearance of a continuous sheet. -Thus we should find a large bright outer ring surrounding the earth, -separated by a dark interval from the inner ring, and at the margin of -the inner ring the pebbles would be so much more sparsely distributed -that we should be able to see through them to some extent. That -beautiful system of rings which Saturn displays is undoubtedly of a -similar character to the hypothetical system which I have endeavored to -describe. No other explanation will account for the facts, especially -for the semi-transparency of the crape ring. The separate bodies from -which Saturn’s rings are constituted seem, however, so small that we -are not able to see them individually. There are some other fine lines -running round the rings beside the great division, and these can also -be explained by the theory I have stated. - -Saturn has other claims on our attention besides those of its rings. -It has an elaborate retinue of satellites--no fewer, indeed, than -nine; but some of them are very faint objects, and not by any means -so interesting as the system by which Jupiter is attended. The ninth -of these was discovered quite recently by Professor W. H. Pickering, -of Harvard College Observatory. This little moon, for which the name -“Phoebe” has been suggested, is further from the planet than any of the -others. It is a minute object shining as a star of the 15th or 16th -magnitude, and moves around the planet in a period of about sixteen -months. - -Saturn was the last and outermost of the planets with which the -ancients were acquainted. Its path lay on the frontiers of the then -known solar system, and the magnificence of the planet itself, with -its attendant luminaries and its marvellous rings, rendered it worthy -indeed of a position so dignified. These five planets--namely, Mercury, -Venus, Mars, Jupiter, and Saturn--made up with the sun and the moon -the seven “planets” of the ancients. They were supposed to complete -the solar system, and, furthermore, the existence of other members was -thought to be impossible. In modern times it has been discovered that -there are yet two more planets. I do not now refer to those little -bodies which run about in scores between Mars and Jupiter. I mean two -grand first-class planets, far bigger than our earth. One of them is -Uranus, which revolves far outside Saturn, and the other is Neptune, -which is much further still, and whose mighty orbit includes the whole -planetary system in its circuit. To complete its journey round the sun -not less than 165 years is required. - - -WILLIAM HERSCHEL. - -I have to begin the account of this discovery by telling you a little -story. In the middle of the last century there lived at Hanover a -teacher of music whose name was Isaac Herschel. He had a family of ten -children, and he did the best for them that his scanty means would -permit. Of his children William was the fourth, and he inherited his -father’s talents for music, as did most of his brothers and sisters. -He was a bright, clever boy at school, and he made such good progress -in his music that by the time he was fourteen years old he was able -to play in the military band of the Hanoverian Guards. War broke out -between France and England, and as Hanover was then under the English -crown, the French invaded it, and a battle was fought in which the poor -Hanoverian Guards suffered very terribly. Young Herschel spent the -night after the battle in a ditch, and he came to the conclusion that -he did not like fighting, though he was only a member of the band, and -he resolved to change his profession. That was not so easy to do just -then, for even a bandsman cannot leave the service in war time at his -own free will. William Herschel, however, showed all through his life -that he was not the man to be baffled by difficulties. I do not know -whether he asked for leave, but at all events he took it. He deserted, -in fact, and his friends succeeded in sending him away to England. - -He was nineteen years old when he commenced to look for a career over -here, and certainly he found his prospects in the musical profession -very discouraging. Herschel was, however, very industrious; and at last -he succeeded in getting appointed as organist of the Octagon Chapel at -Bath. He gradually became famous for his musical skill, and had numbers -of pupils. He used also to conduct concerts and oratorios, and was -well known in this way over the West of England. Busy as Herschel was -with his profession, he still retained his love of reading and study. -Every moment he could spare from his duties he devoted to his books. -It was natural that a musician should specially desire to study the -theory of music, and to understand it properly you should know Euclid -and algebra, and, indeed, higher branches of mathematics as well. -Herschel did not know these things at first; he had not the means of -learning them when he was a boy, so he worked very hard after he became -a man. And he studied with such success that he made fair progress in -mathematics, and then it appeared to him that it would be interesting -to learn something about astronomy. After he had begun to read about -the stars, he thought he would like to see them, and so he borrowed a -telescope. It was only a little instrument, but it delighted him so -much that he said he must have one for himself. So he wrote to London -to make inquiries. - -Telescopes were much dearer in those days than they are now, and -Herschel could not give the price that the opticians demanded. Here -again his invincible determination came to his aid. What was there to -prevent him from making a telescope? he asked himself; and forthwith -he began the attempt. You will think it strange, perhaps, that a -music-teacher who had no special training as a mechanic should at once -commence so delicate and difficult a task; but it is not really so hard -to make a telescope as might be imagined. The amateur cannot make such -a pretty-looking instrument as he is able to buy at the shops--the -tubes will not be so beautifully polished and the finish will be such -as a trained workman would be ashamed of--but the essential part of a -telescope is comparatively easy to make; at least, I should say of a -reflecting telescope, which is the kind Herschel attempted to make, -and succeeded in making. You must know that there are two kinds of -telescopes. The commoner one with which you are more familiar is called -the refracting telescope, and it has glass lenses. It was an instrument -constructed on this principle that we spoke of in a former lecture -(p. 97). The reflecting telescope depends for its power upon a bright -mirror at the lower end, and when using this instrument you look at -the reflection of the stars in the mirror. It was a reflector like -this that Herschel began to construct, and he engaged in the task with -enthusiasm. His sister Caroline had come to live with him, and she used -to help him at his work. So much in earnest was he that he used to rush -into his workshop directly he came home from a concert, and without -taking off his best clothes he would plunge into the grinding and -polishing of his mirrors. His sister tried to keep the house as tidy as -possible, but Herschel put up a carpenter’s shop in the drawing-room, -and turning-lathes in the best bedroom. At last he succeeded. He made -a mirror of the right shape, and found that it exhibited the stars -properly. It was not a looking-glass in the ordinary sense, with glass -on one side and quicksilver on the other. The mirror that Herschel -constructed was entirely of metal. It consisted of a mixture of two -parts of copper with one of tin. - -[Illustration: FIG. 69.--The Mirror.] - -The copper has first to be melted in a furnace, for the metal must be -above a red heat before it will begin to run. Then the tin has to be -carefully added, and the casting of the mirror is effected by pouring -the molten metal into a flat mould. Thus the rough mirror is obtained, -which in Herschel’s earlier telescopes seems to have been about six -or seven inches in diameter, and nearly an inch thick. Though copper -is such a tough substance, and though tin is also tough, yet when -melted together to make speculum metal, as this alloy is called, they -produce an exceedingly hard and brittle material. When we remember -that we could never break a copper penny piece by throwing it down on -the flags, it may seem strange that the “speculum metal” should be so -exceedingly brittle. A piece the size of a penny would be more brittle -than a bit of glass of the same dimensions, and when the speculum is -cast, unless it is cooled very carefully, it will fly into pieces. -Herein lay one of the difficulties that Herschel encountered. Speculum -metal must be put into an oven as soon as the casting has become solid, -and then the heat is gradually allowed to abate. When the speculum has -been at last obtained, next follows the labor of giving it the true -figure and polish. It is not only more fragile than glass, but it is -also quite as hard, and therefore the grinding is a tedious operation. -First the surface has to be ground with coarse sand, and then with -emery, which is gradually made finer and finer until the true figure -has been given (Fig. 69). The mirror is then somewhat basin-shaped, -but the depression is very slight. For example, in a mirror six inches -across the depression at the centre would perhaps be not more than the -twentieth of an inch. Small though this depression is, yet it has to be -made with exactness. In fact, if it were wrong at any point by so much -as the tenth of the thickness of this sheet of paper, the telescope -would not perform accurately. The tool that is used in grinding is -made of cast iron, and has been turned in a lathe to the right shape. -It is divided into squares in the manner shown in Fig. 70. After the -grinding comes the polishing, and this is effected with a tool like the -grinder in shape. This has to be covered over with little squares of -pitch, so that when warmed and put down on the mirror it is soft enough -to receive the right shape. Some rouge and water is spread over the -mirror, and the polisher is worked backward and forward with the hand -until a brilliant surface is obtained. - -[Illustration: FIG. 70.--The Grinding Tool.] - -When the amateur astronomer has completed this part of the task, all -the great difficulties about his telescope are conquered. The tube may -be made of wood, and, indeed, a square tube will do just as well as a -round one. He must also provide for the top of the tube a small mirror, -which has to be perfectly flat. The preparation of this requires much -care, because it is not so easy as one might suppose to obtain an -accurately flat surface. One way of doing this is to get three pieces, -and grind each two of them together until every pair will touch all -over; then they will certainly all be flat. One more part you want, -and that is an eyepiece. This presents no difficulty. A single glass -lens can be made to answer and your telescope is complete. - - -THE DISCOVERY OF URANUS. - -It was in the year 1774 that Herschel first had a view of the heavens -through the telescope he had himself constructed. During the early part -of his career he does not seem to have made any important discoveries. -He was gradually preparing himself for the great achievement by which -his name became famous. - -It was on the 13th of March, 1781, that the organist of the Octagon -Chapel at Bath turned his telescope on the constellation of the Twins, -and began to look at one star after another. You must know that a star -merely looks like a little point in a telescope; even the greatest -instrument will only make a star look brighter, and will never show -it with a perceptible disk. In looking over the stars that night, -Herschel’s attention was arrested by one object that did look larger -when magnified, and therefore was not a star. The only other objects -which would behave in this way were the planets, or possibly a comet. -Indeed, at first Herschel imagined that what he saw must be a comet. It -could hardly have occurred to him that he was to have such good fortune -as to discover a new planet. The five great planets had been known from -all antiquity. Was it reasonable to suppose that there could be yet -another that had never been perceived? Fortunately, there was a test -available. A star remains in the same place from night to night and -from year to year; while a planet, as we have already had occasion to -mention, is a body which is wandering about. The movements of a planet -are, however, not at all like those of a comet. To decide on the nature -of Herschel’s newly discovered body, it was sufficient to observe the -character of its motion. A few nights sufficed to do this. The position -of the body was carefully marked relatively to the neighboring stars, -and it was soon shown that it was a planet. - -Here, then, a great discovery was made. A new planet, now called -Uranus, was added to our system. It would be nothing to discover a -new star. You might as well talk of discovering a new grain of sand -on the seashore. The stars are in untold myriads. They are so far off -that they have no relation whatever to our system, which is presided -over by the sun. But by the detection of a new planet, revolving far -outside Saturn, Herschel showed that a new and most interesting member -had to be added to the five old planets which have been known from the -earliest records of history. - -It may well be imagined that a discovery so startling as this excited -astonishment throughout the scientific world. “Who is this Bath -organist?” everybody asked. Accounts of him and his discoveries -appeared in the papers. His fellow-citizens were not so familiar with -the name as we are, happily, now; and the spelling of the unusual name -showed many varieties. When George III. heard of Herschel’s great -achievement, he directed the astronomer to be summoned to Windsor, -that the King might receive an account of the wonderful discovery from -the lips of the discoverer himself. Herschel of course obeyed, and he -brought with him his famous telescope, and also a map of the whole -solar system, to show to the King. No doubt he thought that his Majesty -had probably not paid much attention to astronomy. Herschel was, -therefore, prepared to explain to the King what it would be necessary -for him to know before he could fully appreciate the magnitude of the -discovery. - -You will remember that Herschel while still a boy had deserted from the -army, many years previously. It appears that the King had learned this -fact in some way, so that when Herschel was ushered into his presence -his Majesty said that before the great astronomer could discuss science -there was a little matter of business that must be disposed of. The -King accordingly handed Herschel a paper, in which he was, I dare say, -greatly surprised to find a pardon to the deserter written out by the -King himself. - -Then Herschel unfolded his wonderful discovery, which the King -thoroughly appreciated, and in the evening the telescope was set up in -the gardens, and the glories of the heavens were displayed. Herschel -made a most favorable impression on his Majesty, and when the King -told the ladies of the Castle next day of all that Herschel had shown -him, their astronomical ardor was also aroused, and they asked to see -through the marvellous tube. Of course Herschel was ready to comply, -and the telescope was accordingly carried to the windows of the Queen’s -apartments at Windsor, which would have commanded a fine view if the -clouds had not been in the way, which they unfortunately were. Even -for royalty the clouds would not disperse, so what was to be done? -Herschel was equal to the occasion. He specially wanted to exhibit -Saturn, for it is one of the most beautiful objects in the sky, and -will fascinate any intelligent beholder. No astronomers would have been -able to see Saturn through the clouds, but Herschel did not disappoint -his visitors; he directed the instrument, not to the sky (nothing was -there to be seen); he turned it towards a distant garden wall. Now what -would you expect to see by looking through a telescope at a garden -wall--bricks, perhaps, or ivy? What these ladies saw was a beautiful -image of Saturn, his globe in the centre and his rings all complete, -forming so true a resemblance to the planet that even an experienced -astronomer might have been deceived. In the afternoon Herschel had seen -that the clouds were thick, and that there would be little probability -of using the telescope properly. Accordingly he cut out a little -image of Saturn, illuminated it by lamps, and set it up at a suitable -distance on a garden wall. - -Herschel’s visit to Windsor was productive of important consequences. -The King said it was a pity that so great an astronomer should devote -himself to music, and that it would be far better for him to give up -that profession and come and live at Windsor. His Majesty promised that -he would pay him a salary, and he also undertook to provide the cost of -erecting great telescopes. His faithful sister Caroline came with him -as his assistant, and also received some bounty from the King. From -that moment Herschel renounced all his musical business, and devoted -himself to his great life-task of observing the heavens. - -He built telescopes of proportions far exceeding those that had ever -been then thought of. He used to stand at night in the open air from -dusk to dawn gazing down the tube of his mighty reflector, watching the -stars and other objects in the heavens as they moved past. He would -dictate what he saw to Caroline, who sat near him. It was her business -to write down his notes and to record the position of the objects which -he was describing. Sometimes, she tells us, the cold was so great that -the ink used to freeze in her pen when she was at this work. Until he -became a very old man, Herschel devoted himself to his astronomical -labors. His discoveries are to be counted by thousands, though not one -of them was so striking or so important as the detection of the new -planet which first brought him fame. - -The question of a name for the addition to the sun’s family had, of -course, to be settled. Herschel had surely a right to be heard at the -christening, and as a compliment to his Majesty he named the stranger -the Georgium Sidus. So, indeed, for a brief while, the planet was -actually styled. The Continental astronomers, however, would not accept -this designation; all the other planets were named after ancient -divinities, and it was thought that the King of England would seem -oddly associated with Jupiter and Saturn; perhaps also they considered -that the British dominions, on which the sun never sets, were already -quite large enough, without further extension to the celestial -regions. Accordingly a consultation was held, the result of which was -that George III. was deprived of his planetary honors, and the body was -given the name of Uranus, which, by universal consent, it now bears. - -The planet Uranus lies just on the verge of visibility with the -unaided eye. It can sometimes be glimpsed like a faint star, and, of -course, with a telescope it is readily perceived. Many generations of -astronomers before Herschel’s time had been observing the heavens, -making maps of the stars, and compiling great catalogues in which the -places of the stars were accurately put down. It often happened that -Uranus came under their notice, but it never occurred to them that -what seemed so like a star was really a planet. I have, no doubt, said -that Uranus looked unlike a star when Herschel examined it; but then -that was because Herschel was a particularly skilful astronomer. To -an observer of a more ordinary type Uranus would not present any very -remarkable appearance, and would be passed over merely as a small star. -In fact, the planet was thus observed not once or twice, but no fewer -than seventeen times, before the acute eye of Herschel perceived its -true character. On many previous occasions the planet had been noted -as a star by astronomers who are in every way entitled to our respect. -It required a Herschel, determined to see everything in the very best -manner, to grasp the discovery which eluded so many others. - -When Uranus was observed on these former occasions and mistaken for -a star, its place had been carefully put down. These records are at -present of the utmost use, because they show the past history of the -planet; and they appear all the more valuable when we remember that -Uranus requires no less than eighty-four years to accomplish a single -revolution around the sun. Thus, since the planet was discovered in -1781, it had completed one revolution by 1865, and is now (1899) about -one-third of the way around another. The earlier observations extend -backwards almost 200 years, so that altogether we have more or less -information about the movements of the planet during the completion of -two circuits and a half. - -Uranus is a great deal bigger than the earth, as you will see in the -view of the comparative sizes of the planets (Fig. 47). It appears to -be of a bluish hue, but we cannot tell whether it turns round on its -axis, or rather, I should say, we are not able to _see_ whether it -turns round on its axis; for we can hardly doubt that it does so. - -Notwithstanding that Uranus is at so great a distance from the earth, -we have been able to put this planet, no less than the nearer ones, -in the weighing scales, and we assert with confidence that Uranus is -fifteen times as heavy as our earth. We are indebted to the satellites -for this information. - - -THE SATELLITES OF URANUS. - -You must use a very good telescope to see the satellites of Uranus. -They are four in number, bearing the names of Ariel, Umbriel, Titania, -and Oberon. The innermost of these, Ariel, completes a journey round -the planet in two days and a half; Oberon, the most distant, requires -thirteen days and a half. A planet is always tending to pull its -satellite down, and the satellite is kept from falling by the speed -with which it revolves. The heavier the planet, the faster must its -satellites go round. Thus, to take an illustration from our own moon, -we know that, if the earth were to be made four times heavier than it -is, the moon would have to spin round twice as fast as it does, in -order to remain in the same orbit. The speed with which the satellites -of Uranus revolve accordingly affords a measure of the mass of the -planet. Were Uranus heavier than he is, his satellites would revolve -more quickly than they do; were he lighter, the satellites would take a -longer period to go round. - -Uranus also seems to be greatly swollen by clouds, in the same manner -as are both Jupiter and Saturn; in fact, if our earth was as big as -Uranus, it would weigh four or five times as much as Uranus does. Hence -we are certain that Uranus must consist of materials less dense on the -whole than are those of which our earth is made. - -There is another singular circumstance connected with the moons of -Uranus. I have told you how every body revolving round another by -gravitation will describe an ellipse; but, of course, there are many -different kinds of this curve, and some may be nearly circles. There -is nothing whatever to prevent a satellite from revolving around its -primary in an exact circle if it be started properly; that is, in the -right direction and with the right speed. All the four satellites of -this planet seem to revolve in circles so perfect that we can make an -accurate picture of this system with a pair of compasses. It is further -to be noticed that the four circles seem to lie exactly in the same -plane. The orbits of the other great planets and of their satellites -lie in planes inclined at angles of less than 35° to the ecliptic, the -plane in which the earth moves. Here again the satellites of Uranus are -exceptional. The plane in which they are contained stands up almost -squarely to the plane in which the motion of the planet is performed. -The moons of Uranus seem to have got a twist, from some accidental -circumstance for which we are not able to account. - - -THE DISCOVERY OF NEPTUNE. - -The boundaries of the solar system had been much extended by the -discovery of Uranus, but they were destined to receive still further -enlargement by the detection of another vast planet, revolving far -outside Uranus, the orbit of which forms, according to our present -knowledge, the outline of the planetary system. - -I have here to describe one of the greatest discoveries that have ever -been made. It is not the magnificence of the outermost planet itself -that I refer to, though, indeed, it is bigger than Uranus. I am rather -thinking of the _way_ in which the discovery was made. I do not mean -any disrespect to Herschel when I say that the discovery of Uranus was -chiefly a stroke of good fortune; but I may be permitted to describe -it in this manner by way of emphasizing as strongly as I can how -utterly different was the train of ideas which led to the discovery of -Neptune. Herschel merely looked at one star after another till suddenly -he dropped on the planet, having beforehand not the slightest notion -that any such planet was likely to exist. But Neptune was shown to -exist before it was ever seen, and, in fact, the man that first saw -the planet, and knew it to be a planet, was not the discoverer. This -is rather a difficult subject; and it would take you years of hard -study to be able to understand the train of reasoning by which Neptune -was found. I shall, however, make an attempt to explain this matter -sufficiently to give at least some idea of the kind of problem that had -to be solved. - -You will remember that law of Kepler which tells us that every planet -moves round the sun in an ellipse. If the planet be uninterfered -with in any way and guided only by the attraction of the sun, it -will forever continue to describe precisely the same ellipse without -the slightest alteration. It was ascertained that the path which -Uranus followed was not always regular. The early observations of the -planet, when it was mistaken for a star, have here been of the utmost -service. They have indicated the ellipse which Uranus described the -last time it went round, and our modern observations have taught us -the path which the planet is at present describing. These two ellipses -are slightly different, and the consequence is that, supposing we -take the observations of Uranus made 100 years ago, and calculate -from them where Uranus ought to be now, we find that the planet is -a little astray. Astronomers are not accustomed to be wrong in such -calculations, and when discrepancies arise, the first thing to be -done is to see what has caused them. It is certain that the position -in which Uranus is found this very night, for example, is not what it -would have been had the sun alone been guiding the planet. Perhaps you -will think that it is impossible for reliable computations to be made -about such matters; but I assure you they can, and the very fact that -the motion of Uranus appeared to be irregular made it interesting to -try and find out the cause of the disturbance. - -I have already explained, when speaking about Mars (p. 187), that -there is an attraction between every two bodies, but in the group of -planets to which the earth belongs the sun’s attraction is so much -stronger than any other force that all the movements are guided mainly -by it. Nevertheless it is true that not only does the sun pull our -earth and all the other planets as well, but all the planets, including -the earth, are pulling one another. In fact, there is an incessant -struggle going on in the family party. Fortunately the sun is so much -more powerful than any other member, that he keeps them all pretty -well in order; and unless you look very carefully you will not see the -effects of the little struggles that are going on between every pair -of the system. Our earth itself is pulled and swayed to and fro by the -actions of its brothers and sisters. It is dragged perhaps a thousand -or two thousand miles this way by Jupiter, or it gets a good tug in the -other direction by Venus. Mars and Saturn also do their little best -to force the earth away from its strict path. However, our earth does -not suffer much from these irregularities. It pursues its route fairly -enough, just as a coach from London to Brighton will get safely to its -destination notwithstanding the fact that it has to swerve a little -from its path whenever it meets other vehicles on the way, or when the -coachman wishes to avoid a piece of the road on which stones have been -freshly laid down. - -The track followed by Uranus was found to be somewhat irregular, -like that of every other planet. Jupiter gave it a pull, and so did -Saturn, and at first it was thought that the irregularities which were -perceived could be explained by the action of these planets, so big and -so well known. Here is a question for calculation; it involves a very -long and a very hard piece of work, but it is possible to estimate how -far each of the other planets is capable of dragging Uranus from its -path. Is it not remarkable that by working out long calculations we -should be able to find what one planet hundreds of millions of miles -away was able to do to another planet still further off, and not only -for to-day or yesterday, but for past time extending over more than a -century? If, however, you will listen to me a little longer, I think I -shall give you a proof that these sums could be worked out correctly. - -When the calculations had been made which showed how much the known -planets could disturb Uranus, it was found that there were still some -deviations of the planet that remained unexplained. They were not -large; they only amounted to showing that the body was just a little -astray from the spot where the calculations indicated it should be. The -rest of astronomy was so perfect, and the law of attraction prevailed -so universally, that it was thought the law of attraction must provide -some way of explaining the behavior of Uranus. He could not have left -his track of his own accord; therefore there must be some agency at -work upon him of which we did not know. What could this unknown source -of disturbance be? Every such trouble had hitherto been found to be a -consequence of the attraction of gravitation; therefore there must be -some unknown body pulling at Uranus which no one had ever seen. Where -could it be? How was it to be discovered? Such were the questions that -were asked, and they were answered in a most satisfactory manner. - -First of all, what sort of body could it be that was pulling Uranus? -It is obvious that none of the stars would be competent to produce -so great an effect; they are all so far off that they have nothing -whatever to say to any of the domestic matters in our little solar -system, which is simply a group by itself. It would be more reasonable -to suppose that there must be yet another planet which nobody had ever -recognized, but which affected Uranus so as to account for his truant -behavior. To begin to search for this planet with telescopes without -some guidance would be futile; in fact, astronomers had been scanning -the heavens for planets for nearly fifty years, and though several -had been discovered, they all belonged to the zone of little planets, -and none of them were big enough to pull Uranus about appreciably. Of -course, if all the stars could be blotted out of the sky, so that -nothing but planets were left, then, by sweeping the telescope over -the heavens, every planet that exists might be speedily found. The -difficulty is that the planets, which are either small or very distant, -look so like the stars that it is impossible to recognize them among -the millions of glittering points in the sky. It was, however, hoped -that the unknown planet would be large enough to be visible in the -telescope, if only we knew exactly where to point it. - -Two illustrious astronomers, Adams of Cambridge, and Leverrier of -Paris, both separately undertook an astonishing piece of calculation. -They tried to find out the position of the unknown planet from the -mere fact that it deranged Uranus in a particular way. I dare say many -of those who are reading this book have learned simple equations in -algebra, and they have worked such questions as to find the length -of a pole, half of which is in mud, a quarter in water, and ten -feet above the water. Those who know this much can perhaps realize -the problem that had to be solved in trying to discover the unknown -planet. So difficult a question as this had to be solved in a way that -your masters would hardly allow you to use when working your sums in -algebra. I do not think they would let you make a series of guesses. -Let us try 20 feet, for instance, as the length of the pole; that will -make 10 feet in the mud, 5 feet in the water, and 5 feet outside. This -will not do; it is not enough; we must try again; and after another -guess or two, we see that a pole 40 feet long will exactly answer. We -do not use this method of guessing in algebra, because solving the -simple equation is a much better method. Adams and Leverrier found -that to discover the unknown planet was a question so very difficult, -that they were obliged to use a sort of guessing, but very intelligent -guessing, I need hardly assure you. They proceeded in this way -(Fig. 71). They would draw a circle outside the path of Uranus, and -then suppose that a planet was revolving in that circle. Its effect -upon Uranus would then be calculated, and it would be found whether -the observed irregularities could be in this manner accounted for. The -first planet they tried was not the right one; then they began again -with another, until at last, after many trials and much very hard work, -they saw that there might be a planet in a particular path far outside -Uranus, such that if this planet were of the right weight and moving -with the right speed, then it would pull Uranus exactly in the way -that astronomers had observed it to be pulled. They found at last that -there could be little doubt about the matter; for this unknown body -would account for all the facts. Then, indeed, they had solved their -equation; they had found the unknown. - -[Illustration: FIG. 71.--Orbits of Uranus and Neptune.] - -The two great astronomers had thus discovered a planet, but as yet it -was only a planet on paper. Those who could judge of the subject had -no doubt that the planet was really in the sky; but just as you like -to prove that you have found the correct answer to your sum, so people -were naturally anxious to prove the truth of this wonderful sum that -Adams and Leverrier had worked out. This was to be done by actually -seeing the planet of which the astronomers had asserted the existence. -Leverrier calculated that the new planet in a certain night would be in -a particular position on the sky. Accordingly he wrote to Dr. Galle, of -the observatory at Berlin, requesting him on the evening in question -to point his telescope to the very spot indicated, and there he would -see a planet which human eyes had never before beheld. Of course, Dr. -Galle was only too delighted to undertake so marvellous a commission. -The evening was fine; the telescope was opened; it was directed towards -the heavens; and there, in the very spot which the calculations -of Leverrier had indicated, shone the beautiful little planet. At -Cambridge arrangements had also been made to search for the new member -of the solar system, in accordance with Professor Adams’ calculations. -There also the planet that had given all this trouble to Uranus was -brought to light. At first it looked like a star, as all such planets -do; but that it was not a star was speedily proved, by the two tests -which are sure indications of a planet. First the body was so moving -that its position with respect to the adjacent stars was constantly -changing. Then, when a strong magnifying power was placed on the -telescope, the little object was seen, not to be a mere starlike point, -but to expand into the little disk which shows us we are not looking at -a distant sun, but at a world like our own. - -Was not this truly a great discovery? Have we not shown you how -entitled the calculations of astronomers are to our respect, when we -find that they actually discovered the existence of a majestic planet -before the telescope had revealed it? See also the greatly increased -interest that belongs to Herschel’s discovery of Uranus. We can hardly -imagine anything that would have given more gratification to this old -astronomer than to think that his Uranus should have given rise to a -discovery even more splendid than his own. He died, however, more than -twenty years before this achievement. - -The authorities who decide on such matters christened the new planet -Neptune; and this body wanders round on the outskirts of our solar -system, requiring for each journey a period of no less than 165 years. -The circle thus described has a radius thirty times as great as that of -the earth’s track. - -Neptune is altogether invisible to the unaided eye, but it is -sufficiently bright to have been occasionally recorded as a star. -Indeed, nearly fifty years before it was actually discovered to be a -planet it had been included by the astronomer Lalande in a list of -stars he was observing. A curious circumstance was afterwards brought -to light. When reference was made to the books in which Lalande’s -observations were written, it was found that he had observed this -object twice, namely, on May 8 and May 10, 1785. Of course, if the -object had indeed been a star its position on the two days would have -been the same, but being a planet it had moved. When Lalande, on -looking over his papers, saw that the places of this supposed star -were different on the two nights, he concluded that he must have made -a mistake on the first night, and accordingly treated the object as -if the place on the 10th was the right one. Just think how narrowly -Lalande missed making a discovery! Unhappily for his renown, he took it -for granted that one or both of his observations were erroneous, and so -they must have been if the object had been a star. But they were both -right; it was the planet which had moved in the interval. - -As Neptune is half as far again from the earth as Uranus, we can hardly -expect to learn much about the actual nature of the planet. We do know -that it has four times the diameter of the earth, so that it exceeds -the earth in the same proportion that the earth is larger than the moon. - -Like the other great planets, Neptune is also enveloped with copious -clouds; in fact, it only weighs one-fifth part as much as it would do -if it were made of materials as substantial as are those of the earth. -Like our earth, Neptune is attended by one moon, which revolves round -the planet in a little more than six days. - -The orbit of this great planet marks the boundary of our known system -of planets. We have seen how the five great planets of antiquity have -been increased in these modern days by the addition of two more, Uranus -and Neptune, while the discovery of a multitude of small planets has -given a further increase to the number of the sun’s family. We have -still some other objects in our solar system to describe; some of them -are excessively big; these are the comets. Some of them are exceedingly -small; they are the shooting stars. We shall talk about comets and -shooting stars in our next lecture. - - - - -LECTURE V. - -COMETS AND SHOOTING STARS. - - The Movements of a Comet--Encke’s Comet--The Great Comet of - Halley--How the Telegraph is used for Comets--The Parabola--The - Materials of a Comet--Meteors--What becomes of the Shooting - Stars--Grand Meteors--The Great November Showers--Other Great - Showers--Meteorites. - - -THE MOVEMENTS OF A COMET. - -The planets are all massive globes, more or less flattened at the -Poles; but now we have to talk about a multitude of objects of the -most irregular shapes, and of the most flimsy description. We call -them _comets_, and they exist in such numbers that an old astronomer -has said “there were more comets in the sky than fishes in the sea,” -though I think we cannot quite believe him. There is also another -wide difference between planets and comets: planets move round in -nearly circular ellipses, and not only do we know where a planet is -to-night, but we know where it was a month ago, or a hundred years -ago, or where it will be in a hundred years or a thousand years to -come. All such movements are conducted with conspicuous regularity -and order; but now we are to speak of bodies which generally come in -upon us in the most uncertain and irregular fashion. They visit us we -hardly know whence, except that it is from outer space, and they are -adorned in a glittering raiment, almost spiritual in its texture. -They are always changing their appearance in a baffling, but still -very fascinating manner. If an artist tries to draw a comet, he will -have hardly finished his picture of it in one charming robe before he -finds it arrayed in another. The astronomer has also his complaints to -make against the comets. I have told you how thoroughly we can rely on -the movements of the planets, but comets often play sad pranks with -our calculations. They sometimes take the astronomers by surprise, and -blaze out with their long tails just when we do not expect them. Then -by way of compensation they frequently disappoint us by not appearing -when they have been most anxiously looked for. - -After a voyage through space the comet at length begins to draw in -towards the central parts of our system, and as it approaches the sun, -its pace becomes gradually greater and greater; in fact, as the body -sweeps round the sun the speed is sometimes 20,000 times faster than -that of an express train. It is sometimes more than 1000 times as fast -as the swiftest of rifle bullets, occasionally attaining the rate of -200 miles a second. The closer the comet goes to the sun, the faster -it moves; and a case has been known in which a comet, after coming in -for an incalculable duration of time towards the sun, has acquired a -speed so tremendous, that in two hours it has whirled round the sun and -has commenced to return to the depths of outer space. This terrific -outburst of speed does not last long. A pace which near the sun is -20,000 times that of our express trains diminishes to 10,000 times, to -fifty times, to ten times that pace; while in the outermost part of -its path the comet seems to creep along so slowly that we might think -it had been fatigued by its previous exertions. - -[Illustration: FIG. 72.--How the Comet’s Tail is disposed.] - -We have so often seen a stream of sparks stretching out along the track -of a sky-rocket, that we might naturally suppose the tail of a comet -streamed out along its path in a somewhat similar manner. This would -be quite wrong. You see from Fig. 72 that the tail does not lie along -the comet’s path, but is always directed outwards from the sun. If you -will draw a line from the sun to the head of the comet and follow the -direction of the line, it shows the way in which the tail is arranged. -You will also notice how the tail of the comet seems to grow in length -as it approaches the sun. When the comet is first seen, the tail is -often a very insignificant affair, but it shoots out with enormous -rapidity until it becomes many millions of miles long by the time the -comet is whirling round the sun. Those glories soon begin to wane as -the comet flies outward; the tail gradually vanishes, and the wanderer -retreats again to the depths of space in the same undecorated condition -as that in which it first approached. - -When a comet appears, it is always a matter of interest to see whether -it is an entirely new object, or whether it may not be only another -return of a comet which has paid us one or more previous visits. The -question then arises as to how they are to be identified. Here we see a -wide contrast between unsubstantial bodies like comets and the weighty -and stately planets. Sketches of the various planets or of the face of -the sun, though they might show slight differences from time to time, -are still always sufficiently characteristic, just as a photographic -portrait will identify the individual, even though the lapse of years -will bring some changes in his appearance. But the drawing of a comet -is almost useless for identification. You might as well try to identify -a cloud or a puff of smoke by making a picture of it. Make a drawing of -a comet at one appearance, and sketch particularly the ample tail with -which it is provided. The next time the comet comes round it may very -possibly have two tails, or possibly no tail at all. We are therefore -unable to place any reliance on the comet’s personal appearance in our -efforts to identify it. The highway which it follows through the sky -affords the only means of recognition; for the comet, if undisturbed -by other objects, will never change its actual orbit. But even this -method of identification often fails, for it not unfrequently happens -that during its erratic movements the comet gets into fearful trouble -with other heavenly bodies. In such cases the poor comet is sometimes -driven so completely out of its road that it has to make for itself -an entirely new path, and our efforts to identify it are plunged in -confusion. It has happened that a second comet or even a third will be -found in nearly the same track, but whether these are wholly different, -or whether they are merely parts of the same original object, it is -often impossible to determine. - -The great majority of comets are only to be seen with a telescope, and -hardly a year passes without the detection of at least a few of these -faint objects. The number of really brilliant comets that can be seen -in a lifetime could, however, be counted on the fingers. - - -ENCKE’S COMET. - -We have already alluded to a little body called Encke’s comet, which -was discovered by an astronomer at Marseilles. It was in the year 1818 -that he was scanning the heavens with a small telescope, when an object -attracted his attention. It was not one of those grand long-tailed -comets which every one notices; this body was so faint that it merely -appeared as a very small cloud of light, and was recognized as a comet -by the fact that it was moving. It happens that there are other bodies -in the sky very like comets; we call them nebulæ, and we shall have -something to say about them afterwards. But it is remarkable that -just as a planet is liable to be mistaken for a star, so a comet is -liable to be mistaken for a nebula. However, in each case the fact of -its movement is the test by which the planet or the comet is at once -detected. A nebula stays always in the same spot, like a star, while a -comet is incessantly moving. In fact, with a telescope you can actually -watch a comet stealing past the stars that lie near it. You know that -an object a very long way off may appear to move slowly, though in -reality it is moving very rapidly. Look at a steamer near the horizon -at sea. In the course of a minute or two it will not appear to have -shifted its position to any appreciable extent, but that is because -it is far off. If you were near the ship, you would see that it was -dashing along at the rate of perhaps fifteen or twenty miles an hour. -In a similar manner the comet seems to move slowly, because it is at -such a great distance. As a matter of fact it is moving faster at the -time we see it than any steamer, faster than any express train, faster -than any cannon-ball. There were special reasons why the movements -of Encke’s comet should be watched with peculiar care, and the track -which it pursued be ascertained. If you can observe a comet three -times and measure its position in the sky, the movement of that comet -is completely determined. Perhaps I should say would be determined if -the comet were let alone, which, unfortunately, is not often the case. -Indeed, you may remember how I told you some of the misadventures of -this very comet when we were speaking about the planet Mercury. Encke’s -comet comes round in a period of a little more than three years, and it -gives us some curious information that has been ascertained during its -journeys. One of the facts we have thus learned is so important that we -cannot omit to notice it (Fig. 73). - -[Illustration: FIG. 73.--The Orbit of Encke’s Comet.] - -At increasing heights above the earth’s surface there is gradually -less and less air; until at last, at about 200 or 300 miles above the -surface on which we dwell, there would be none. You might as well try -to quench your thirst by drinking out of an empty cup as attempt to -breathe in the open space which begins a few hundred miles aloft. -In open space motion could take place quite freely. Down here the -resistance of the air is a great impediment to movement, especially -when very rapid. A heavy cannon-ball is checked and robbed of its pace -by having to plough its way through our dense atmosphere. The motion -is arrested in the same way, though not of course to the same degree, -as if the cannon-ball had been fired into water. Unsubstantial objects -are, of course, impeded by the air to a far greater extent than such -heavy bodies as cannon-balls. You know that you cannot throw a handful -of feathers across the road in the same way that you could throw a -handful of gravel. The light feathers cannot force their way through -the air so well as the pebbles. A body so flimsy as a comet would -never be able to push its way through an atmosphere like ours; but out -in empty space the comet meets with no resistance during the greater -part of its path. Accordingly, though it has little more substance -than a will-o’-the-wisp, the comet pursues its journey with as much -resolute dignity as if it were made of cast iron. If in any part of -its track the body should have to pierce its way through any material -like even the thinnest possible air, then the unsubstantial nature of -the cometary materials would be at once shown. The motion would be -impeded, and the body’s path would be changed. In this way a comet may -be made very instructive, for it will show whether space is really -so empty as we sometimes suppose it to be. During the greater part -of its course the flimsy little Encke tears along with such ease and -speed that there seems to be nothing to impede it, and thus we learn -that space is generally empty. However, when the comet begins to wheel -around the sun, the freedom of its movements seems to receive a check. -The unsubstantial object has to force its way with a difficulty that -it did not experience so long as it was moving round the greater part -of its orbit. We thus learn that there is a thin diffused atmosphere -surrounding the sun. We cannot, indeed, say that it is like our air. -Its composition is quite different, and almost the only way we know -of its existence is by the evidence which this comet affords. In a -former lecture I showed how Encke’s comet told us the mass of the -planet Mercury. Now we see how the travels of the same body give us -information about the sun himself. I ought, however, to add that some -more recent observations seem not to have confirmed the belief that -there is the resistance of the kind we have just been considering. - - -THE GREAT COMET OF HALLEY. - -I dare say you would think it more interesting to talk about some big -and bright comets rather than about objects so faint as that of Encke. -It unfortunately happens that most of the fine comets pay our system -only a single visit. There is only one of the really splendid objects -of this kind that comes back to us with anything like regularity. - -It was last seen in the year 1835, and I am glad to tell you that it -is coming again; it is expected about the year 1910. You may ask, How -can we feel sure that such a prediction as I have mentioned will turn -out correctly? The fact is that this comet has been watched for a great -many centuries. We find ancient records, some of them nearly 2000 years -old, of the appearance of grand comets, and several of these are found -to fit in with the supposition that there is a body which accomplishes -its journey in a period of about seventy-five or seventy-six years. Of -course there are thousands of other comets recorded in these old books -as well; but what I mean is that among the records many are found which -clearly indicate some successive returns of this particular body. - -I will explain how the movements of this comet were discovered. There -was a great astronomer called Halley, who lived two hundred years -ago, and in the year 1682 he, like every one else, was looking with -admiration at a splendid comet with a magnificent tail which adorned -the sky in that year. At the observatories, of course, they diligently -set down the positions of the comet, which they ascertained by -carefully measuring it with telescopes. Halley first calculated the -highway which this comet followed through the heavens, and then he -looked at the list of old comets that had been seen before. He thus -found that in 1607--that was, seventy-five years earlier--a great -comet had also appeared, the path of which seemed much the same as -that which he found for the body that he had himself observed. This -was a remarkable fact, and it became still more significant when he -discovered that seventy-six years earlier--namely, in 1531--another -great comet had been recorded, which moved in a path also agreeing -with those of 1607 and 1682. It then occurred to Halley that possibly -these were not three different objects, but only different exhibitions -of one and the same, which moved round in the period of seventy-five or -seventy-six years. - -There is a test which an astronomer can often apply in the proof -of his theory, and it is a very severe test. He will not only show -himself to be wrong if it fails, but he will also make himself somewhat -ridiculous. Halley ventured to submit his reputation to this ordeal. He -prophesied that the comet would appear again in another seventy-five -or seventy-six years. He knew that he would, of course, be dead long -before 1758 should arrive; but when he ventured to make the prediction, -he said that he hoped posterity would not refuse to admit that this -discovery had been made by an Englishman. - -You can easily imagine that as 1758 drew near, great interest was -excited among astronomers to see if the prediction of Halley would be -fulfilled. We are accustomed in these days to find many astronomical -events foretold with the same sort of punctuality as we expect in -railway time-tables. The Nautical Almanac is full of such prophecies, -and we find them universally fulfilled. Even now, however, we are not -able to set forth our time-tables for comets with the same confidence -that we show when issuing them for the sun, the moon, or the stars. -How astonishing, then, must Halley’s prediction have seemed! Here was -a vast comet which had to make a voyage through space to the extent of -many hundreds of millions of miles. For three-quarters of a century -it would be utterly invisible in the greatest telescopes, and the only -way in which it could be perceived was by figures and calculations -which enabled the mind’s eye to follow the hidden body all around its -mysterious track. For fifty, or sixty, or seventy years nothing had -been seen of the comet, nor, indeed, was anything expected to be seen -of it; but as seventy-one, and seventy-two, and seventy-three years -had passed, it was felt that the wanderer, though still unseen, must -be rapidly drawing near. The problem was made more difficult for those -skilful mathematicians who essayed to calculate it by the fact that the -comet approached the thoroughfares where the planets circulate; and, of -course, the flimsy object would be pulled hither and thither out of its -path by the attractions of the weighty bodies. It was computed that the -influence of Saturn alone was sufficient to delay the comet for more -than three months, while it appeared that the attraction of Jupiter was -potent enough to retard the expected event for a year and a half more. -Was it not wonderful that mathematicians should be able to find out all -these facts from merely knowing the track which the comet was expected -to follow? Clairaut, who devoted himself to this problem, suggested -that there might also be some disturbances from other causes of which -he did not know, and that consequently the expected return of the comet -might be a month wrong either way. Great indeed was the admiration in -astronomical circles when, true to prediction, the comet blazed upon -the world within the limits of time Clairaut had specified. - -The remarkable fulfilment of this prophecy entitles us to speak with -confidence about the past performances of this comet. Among all the -apparitions of Halley’s comet for the last two thousand years, perhaps -the most remarkable is that which took place in the year 1066. I am -sure you will all remember this date in your English history; it -was the year of the Conquest. In those days they did not understand -astronomy as we understand it now; they used to think of a comet as -a fearful portent of evil, sent to threaten some frightful calamity; -such as a pestilence, a war, a famine, or something else equally -disagreeable. Hence in the year of the Conquest the appearance of so -terrific an object in the sky was a very significant omen. Attention -was concentrated upon the spectacle, and a picture of Halley’s comet -as it appeared to the somewhat terrified imaginations of the people of -those days has been preserved. There is a celebrated tapestry at Bayeux -on which historical incidents are represented by beautifully worked -pictures. On this fabric we have a view of Halley’s comet in a quaint -and rather ludicrous aspect. You will read of this comet also in the -early pages of Tennyson’s “Harold.” - - -HOW THE TELEGRAPH IS USED FOR COMETS. - -In these days the study of comets is prosecuted with energy. Over the -world observatories are situated, and whenever a comet is discovered, -tidings of the event are diffused among those likely to be interested. -Suppose that one is discovered in the southern hemisphere, the -astronomers then write to warn the northern observatories of the -event. But comets often move faster than her Majesty’s mails, so that -the telegraph has to be put into requisition. The kind of message is -one which shall show the position and the movements of the body. It -necessarily involves a good many figures and words, and consequently -it is desirable to abbreviate as much as possible for the sake of -economy. There is a further difficulty in using the telegraph, because -the messages are not of an intelligible description to those not -specially versed in astronomy. Skilful as the telegraph clerks are, -they can hardly be expected to be familiar with the technicalities of -astronomers. The clerk at the receiving end is handed a message which -he does not understand very clearly. The clerk at the other end does -not understand the message which is delivered to him, and between them -it has happened that they have transformed the message into something -which not only they do not understand, but which, unfortunately, nobody -else can understand either. These difficulties have been surmounted by -an agreement between astronomers, which is so simple and interesting -that I must mention it. - -The kind of message that expresses the place of a comet will consist of -sentences something of this kind: “One hundred and twenty-three degrees -and forty-five minutes.” Surely it would be an advantage to be able to -replace all these words by a single word, particularly if by doing so -the risk of error would be diminished. This is what the astronomers’ -telegraphic arrangement enables them to accomplish. There is a certain -excellent Dictionary known as Worcester’s. I am sure when Mr. Worcester -arranged this work, he had not the slightest anticipation of an odd -use to which it would occasionally be put. Every astronomer who is -co-operating in the comet scheme must have a copy of the book. To send -the message I have just referred to, he has to take up his Dictionary -and look out page 123. Then he will count down the column until he -comes to the forty-fifth word on that page, which he finds to be -“constituent,” and according to this plan the message, or at least this -part of it, is merely that one word, “constituent.” The astronomer who -receives this message and wishes to interpret it takes up his copy of -Worcester’s Dictionary and looks out for “constituent.” He sees that it -is on page 123, and that it is the forty-fifth word down on that page; -and therefore he knows that the interpretation of the message is to be -one hundred and twenty-three degrees and forty-five minutes. - - -THE PARABOLA. - -Generally speaking, great comets come to us once and are then never -seen again. Such bodies do not move in closed ovals or ellipses, they -follow another kind of curve, like that represented in Fig. 74. It -is one that every boy ought to know. In fact, in one of his earliest -accomplishments he learned how to make a parabola. It is true he did -not call it by any name so fine as this, but every time a ball is -thrown into the air it describes a part of the beautiful curve which -geometers know by this word (Fig. 74). In fact, you could not throw -a ball so that it should describe any other curve except a parabola. -No boy could throw a stone in a truly horizontal line. It will always -curve down a little, will always, in fact, be a portion of a parabola. - -[Illustration: FIG. 74.--The Path of a Projectile is a Parabola.] - -There are big parabolas and there are small ones. One of the shells -which are thrown into a town when bombarded from a distance describes, -as it rises and then slopes down again, part of a mighty parabola. So -does a tennis ball thrown by the hand or struck by the racket; though -here, indeed, I admit that a spin may be given to the ball which will -somewhat detract from the simplicity of its movement. In playing -baseball, a large part of the skill of the pitcher consists in throwing -the ball in such a way that it shall not move in a parabola, but in -some twisting curve by which he hopes to baffle his adversary. Setting -aside these exceptions, and such another as the case of a body tossed -straight up or dropped straight down, we may assert that the path of a -projectile is a parabola. - -[Illustration: FIG. 75.--The Lighthouse Reflector.] - -There are some remarkable applications of the same curve for practical -purposes. From our lighthouses we want to send beams off to sea, so -as to guide ships into port. If we merely employed a lamp without -concentrating its rays, we should have a very imperfect lighthouse, -for the lamp scatters light about in all directions. Much of it goes -straight up into the air, much of it would be directed inland; in fact, -there is only an extremely small part of the entire number of rays -that will naturally take the useful direction. We therefore require -something round the lamp which shall catch the truant rays that are -running away to idleness and loss, and shall concentrate them into the -direction in which they will be useful to the mariner. An effective -way of doing this is to furnish the lamp with a reflector. On its -bright surface (Fig. 75) all the rays fall which would otherwise -have gone astray, and each of them is properly redirected, where the -sailors can see it. It is essential that the mirror shall do this work -accurately, and this it will only do when it has been truly shaped so -as to be a parabola. - -You will remember, also, how I described to you the reflector which -Herschel made for his great telescope. The shape of the mirror must -be most accurately worked, and it, too, must have a parabola for its -section; so that you see this curve is one of importance in a variety -of ways. - -But the grandest of all parabolas are those which the comets pursue. -Unlike the ellipse, the parabola is an open curve; it has two branches -stretching away and away forever, and always getting further apart. Of -course, in the examples of this curve that I have given it is only a -small part of the figure that is concerned. When you throw a stone it -only describes that part of the parabola that lies between your hand -and the spot where the stone hits the ground. It is just a part of the -curve in the same way that a crescent may be a bit of a circle. It is -to comets that we must look for the most complete illustration of the -ample extent of a parabola. - -The shape of this grand curve will explain why so many comets only -appear to us once. It is quite clear that if you begin to run round a -closed racecourse, you may, if you continue your career long enough, -pass and repass the starting-post thousands of times. Thus, comets -which move in ellipses, and are consequently tracing closed curves, -will pass the earth times without number. For this reason we may see -them over and over again, as we do Encke’s comet or Halley’s comet. But -suppose you were travelling along a road which, no matter how it may -turn, never leads again into itself, then it is quite plain that, even -if you were to continue your journey forever, you can never twice pass -the same house on the roadside. That is exactly the condition in which -most of the comets are moving. Their orbits are parabolas which bend -round the sun; and, generally speaking, the sun is very close to the -turning-point. The earth is also, comparatively speaking, close to the -sun; so that while the comet is in that neighborhood we can sometimes -see it. We do not see the comet for a long time before it approaches -the sun, or for a long time after it has passed the sun. All we know, -therefore, of its journey is that the two ends of the parabola stretch -on and on forever into space. The comet is first perceived coming in -along one of these branches to whirl round the sun; and after doing so, -it retreats along the other branch, and gradually sinks into the depths -of space. - -Why one of these mysterious wanderers should approach in such a hurry, -and then why it should fly back again, can be partially explained -without the aid of mathematics. - -Let us suppose that, at a distance of thousands of millions of miles, -there floated a mass of flimsy material resembling that from which -comets are made. Notwithstanding its vast distance from the sun, the -attraction of that great body will extend thither. It is true the -pull of the sun on the comet will be of the feeblest and slightest -description, on account of the enormously great distance. Still, the -comet will respond in some degree, and will commence gradually to move -in the direction in which the sun invites it. Perhaps centuries, or -perhaps thousands, or even tens of thousands, of years will elapse -before the object has gained the solar system. By that time its speed -will be augmented to such a degree, that after a terrific whirl around -the sun, it will at once fly off again along the other branch of the -parabola. Perhaps you will wonder why it does not tumble straight -into the sun. It would do so, no doubt, if it started at first from a -position of rest; generally, however, the comet has a motion to begin -with which would not be directed exactly to the luminary. This it is -which causes the comet to miss actually hitting the sun. - -It may also be difficult to understand why the sun does not keep the -comet when at last it has arrived. Why should the wandering body be in -such a hurry to recede? Surely it might be expected that the attraction -of the sun ought to hold it. If something were to check the pace of -the comet in its terrific dash round the sun, then, no doubt, the -object would simply tumble down into the sun and be lost. The sun has, -however, not time to pull in the comet when it comes up with a speed -20,000 times that of an express train. But the sun does succeed in -altering the _direction_ of the motion of the comet, and the attraction -has shown itself in that way. - -I can illustrate what happens in this manner. Here is a heavy weight -suspended from the ceiling by a wire; it hangs straight down, of -course, and there it is kept by the pull of the earth. Supposing I draw -the weight aside and allow it to swing to and fro, then the motion -continues like the beat of a pendulum. The weight is always pulled -down as near to the earth as possible, but when it gets to the lowest -point, it does not stay there, it goes through that point, and rises -up at the other side. The reason is that the weight has acquired speed -by the time it reaches the lowest point; and that, in virtue of its -speed, it passes through the position in which it would naturally rest, -and actually ascends the other side in opposition to the earth’s pull, -which is dragging it back all the time. This will illustrate how the -comet can pass by and even recede from the body which is continually -attracting it. - -Just a few words of caution must be added. Suppose you had an ellipse -so long that the comet would take thousands and thousands of years to -complete a circuit, then the part of the ellipse in which the comet -moves during the time when we can see it is so like a parabola, that -we might possibly be mistaken in the matter. In fact, a geometer will -tell us that if one end of an ellipse was to go further and further -away, the end that stayed with us would gradually become more and more -like this curve. Therefore, some of those comets which seem to move in -parabolas may really be moving in extremely elongated ellipses, and -thus, after excessively long periods of time, may come back to revisit -us. - - -THE MATERIALS OF A COMET. - -A comet is made of very unsubstantial material. This we can show in a -very interesting manner, when we see it moving over the sky between -the earth and the stars. Sometimes a comet will pass over a cluster of -very small stars, so faint that the very lightest cloud that is ever -in the sky would be quite sufficient to hide them. Yet the stars are -distinctly visible right through the comet, notwithstanding that it may -be hundreds of thousands of miles thick. This shows us how excessively -flimsy is the substance of a comet, for while a few feet of haze or -mist suffice to extinguish the brightest of stars, this immense curtain -of comet stuff, whatever it may be made of, is practically transparent. - -I have often told you that we are able to weigh the heavenly bodies, -but a comet gives us a great deal of trouble. You see that the weighing -machine must be of a very delicate kind if you are going to weigh a -very light object. Take, for example, a little lock of golden hair, -which no doubt has generally a value quite independent of the number of -grains that it contains. Suppose, however, that we are so curious as to -desire to know its weight, then one of those beautiful balances in our -laboratories will tell us. In fact, if you snipped a little fragment -from a single hair, the balance would be sensitive enough to weigh it. -If, however, you were only provided with a common pair of scales like -those which are suited for the parcel post, then you could never weigh -anything so light as a lock of hair. You have not small enough weights -to begin with, and even if you had they would be of no use, for the -scale is too coarse to estimate such a trifle. This is precisely the -sort of difficulty we experience when we try to weigh a comet. The -body, though so big, is very light, and our scales are so cumbersome -that we are in a position of one who would try to weigh a lock of hair -with a parcel-post balance. We cannot always find suitable scales for -weighing celestial bodies. We have to use for the purpose whatever -methods of discovering the weights happen to be available. So far, the -methods I have mentioned are of the rudest description; they serve well -enough for weighing heavy masses like planets, but they will not do for -such unsubstantial bodies as comets. - -But, though we fail in this endeavor, _i.e._ to weigh comets, yet -skilful astronomers have succeeded in something which at first you -might think to be almost impossible. They have actually been able to -discover some of the ingredients of which a comet is made. This is so -important a subject that I must explain it fully. - -The most instructive comet which we have seen in modern days is that -which appeared in the year 1882. It was an object so great that its -tail alone was double as long as from the earth to the sun. It was -discovered at the observatories in the southern hemisphere early in -September of that year. A little later it was observed in the northern -hemisphere in extraordinary circumstances. It must be remembered that -a comet is generally a faint object, and that even those comets which -are large enough and bright enough to form glorious spectacles in the -sky at night are usually invisible during the brightness of day. For a -comet to be seen in daylight was indeed an unusual occurrence; but on -the forenoon of Sunday, September 17, Mr. Common at Ealing saw a great -comet close to the sun. Unfortunately clouds intervened, and he was -prevented from observing the critical occurrence just approaching. An -astronomer at the Cape of Good Hope--Mr. Finlay--who had also been one -of the earliest discoverers of the comet, was watching the body on the -same day. He followed it as it advanced close up to the sun; bright -indeed must that comet have been which permitted such a wonderful -observation. At the sun’s edge the comet disappeared instantly; in -fact, the observers thought that it must have gone behind the sun. They -could not otherwise account for the suddenness with which it vanished. -This was not what really happened. It was afterwards ascertained that -the comet had not passed behind the sun; it had, indeed, come between -us and our luminary. In its further progress this body showed in a -striking degree the incoherent nature of the materials of which a comet -is composed. It seemed to throw off portions of its mass along its -track, each of which continued an independent journey. Even the central -part in the head of the comet--the nucleus, as it is called--showed -itself to be of a widely different nature from a substantial planetary -body. The nucleus divided into two, three, four, or even five distinct -parts, which seemed, in the words of one observer, to be connected -together like pearls on a string. - -The comet of 1882 was also very instructive with regard to the actual -materials from which such bodies are made. Astronomers have a beautiful -method by which they find out the substances present in a heavenly -body, even though they never can get a specimen of the body into their -hands. We know at least three materials which were present in this -comet. The first of them is an ingredient which is very commonly found -in comets--a chemist calls it carbon. It is an extremely familiar -material on the earth; for instance, coal is chiefly composed of -carbon. Charcoal when burned leaves only a few ashes. All the substance -that has vanished during combustion is carbon; in fact, it is not too -much to say that carbon is found abundantly not only in wood, but -in almost every form of vegetable matter. The food we eat contains -abundant carbon, and it is an important constituent in the building up -of our own bodies. Generally speaking, carbon is not found in a pure -state--it is associated with other substances. Soot and lampblack are -largely composed of it; but the purest form of this element carbon that -we know is the diamond. - -It is interesting to note that carbon is certainly found as a frequent -constituent of comets. The great comet of 1882 undoubtedly contained -it, as well as certain other substances. Of these we know two: the -first is the element sodium, an extremely abundant material on earth, -inasmuch as the salt in the sea is mainly composed of it. It was also -discovered that the same great comet contained another substance very -common here and extremely useful to mankind. Dr. Copeland and Dr. Lohse -at Dunecht showed that iron was present in this body which had come in -to visit us from the depths of space. - -These discoveries are especially interesting because they seem to show -the uniformity of material composing our system. We already knew that -sodium and iron abounded in the sun, and now we have learned that -these bodies and carbon as well are present in the comets. In the -next chapter we shall learn that the very same materials--sodium and -iron--are met with in bodies far more remote from us than any bodies of -our own system. - -Comets have such a capricious habit of dashing into the solar system -at any time and from any direction, that it is worth while asking -whether a comet might not sometimes happen to come into collision with -the earth. There is nothing impossible in such an occurrence. There -is, however, no reason to apprehend that any disastrous consequences -would ensue to the earth. Man has lived on this globe for many, many -thousands of years, and the rocks are full of the remains of fossil -animals which have flourished during past ages; indeed, we cannot -possibly estimate the number of millions of years that have elapsed -since living things first crawled about this globe. There has never -been any complete break in the succession of life, consequently during -all those millions of years we are certain that no such dire calamity -has happened to the earth as a frightful collision would have produced, -and we need not apprehend any such catastrophe in the future. - -I do not mean, however, that harmless collisions with comets may not -have occasionally happened; in fact, there is good reason for knowing -that they have actually taken place. In the year 1861 a fine comet -appeared; and it is not so well remembered as its merits deserve, -because it happened, unfortunately for its own renown, to appear -just three years after the comet of 1858, which was one of the most -gorgeous objects of this kind in modern times. But in 1861 we had a -novel experience. On a Sunday evening in midsummer of that year, we -dashed into the comet, or it dashed into us. We were not, it is true, -in collision with its densest part; it was rather the end of the tail -which we encountered. There were, fortunately, no very serious results. -Indeed, most of us never knew that anything had happened at all, and -the rest only learned of the accident long after it was all over. -For a couple of hours that night it would seem that we were actually -in the tail of the comet, but so far as I know no one was injured or -experienced any alarming inconvenience. Indeed, I have only heard of -one calamity arising from the collision. A clergyman tells us that at -midsummer he was always able in ordinary years to read his sermon at -evening service without artificial light. On this particular occasion, -however, the sky was overcast with a peculiar glow, while the ordinary -light was so much interfered with that the sexton had to provide a pair -of candles to enable him to get through the sermon. The expense of -those candles was, I believe, the only loss to the earth in consequence -of its collision with the comet of 1861. - -[Illustration: FIG. 76.--How the Tail of a Comet arises.] - -The tail of a comet appears to develop under the influence of the sun. -As the wandering body approaches the source of central heat it grows -warm, and as it gets closer and closer to the sun, the fervor becomes -greater and greater, until sometimes the comet experiences a heat more -violent than any we could produce in our furnaces. The most infusible -substances, such as stones or earth, would be heated white-hot and -melted, and even driven off into vapor, under the intense heat to which -a comet is sometimes exposed. Comets, indeed, have been known to sweep -round the sun so closely as to pass within a seventh part of its radius -from the surface. It seems that certain materials present in the comet, -when heated to this extraordinary temperature, are driven away from -the head, and thus form the tail (Fig. 76). Hence we see that the tail -consists of a stream of vaporous particles, dashing away from the sun -as if the heat which had called them into being was a torment from -which they were endeavoring to escape. - -The tail of a comet is, therefore, not a permanent part of the body. -It is more like the smoke from a great chimney. The smoke is being -incessantly renewed at one end as the column gets dispersed into the -air at the other. As the comet retreats, the sun’s heat loses its -power. In the chills of space there is, therefore, no tail-making -in progress, while the small mass of the comet renders it unable to -gather back again by its attraction the materials which have been -expelled. Should it happen that the comet moves in an elliptic orbit, -and thus comes back time after time to be invigorated by a good -roasting from the sun, it will, of course, endeavor to manufacture a -tail each time that it approaches the source of heat. The quantity -of material available for the formation of tails is limited to the -amount with which the comet originally started; no fresh supply can be -added. If, therefore, the comet expends a portion of this every time -it comes round, an inevitable consequence seems to follow. Suppose a -boy receives a sovereign when he goes back to school, and that every -time he passes the pastry-cook’s shop some of his money disappears in -a manner that I dare say you can conjecture, I need not tell you that -before long the sovereign will have totally vanished. In a similar way -comets cannot escape the natural consequences of their extravagance; -their store of tail-making substance must, therefore, gradually -diminish. At each successive return the tails produced must generally -decline in size and magnificence, until at last the necessary -materials have been all squandered, and we have the pitiful spectacle -of a comet without any tail at all. - -The gigantic size of comets must excite our astonishment. A pebble -tossed into a river would not be more completely engulfed than is our -whole earth when it enters the tail of one of these bodies. But we now -pass by a sudden transition to speak of the very smallest bodies, of -little objects so minute that you could carry them in your waistcoat -pocket. You will perhaps be surprised that such things can play an -important part in our system and have a momentous connection with -mighty comets. - - -METEORS. - -If you look out from your window at the midnight sky, or take a walk -on a fine clear night, you will occasionally see a streak of light -dash over the heavens, thus forming what is called a falling, or a -shooting, star (Fig. 77). It is not really one of the regular stars -that has darted from its place. The objects we are now talking of are -quite different from stars proper. To begin with, the shooting stars -are comparatively close to us when we see them, and they are very -small, whereas the stars themselves are enormous globes, far bigger -than our earth, or often even bigger than the sun. Sometimes a great -shooting star is seen which makes a tremendous blaze of light as bright -as the moon, or even brighter still. These objects we call meteors, -and you will be very fortunate if you can ever see a really fine one. -Astronomers cannot predict these things as they predict the appearance -of the planets. Bright meteors consequently appear quite unexpectedly, -and it is a matter of chance as to who shall enjoy the privilege of -beholding them. But it is not about the great meteors that we are now -going to speak particularly; they are often not so interesting as the -small ones. - -[Illustration: FIG. 77.--A Brilliant Meteor.] - -These little meteoroids, as we shall call them, have a curious history. -They become visible to us only at the very last moment of their -existence--in fact, the streak of light which forms a shooting star is -merely the destruction of a meteoroid. You must always remember that -we are here living at the bottom of a great ocean of air, and above -the air extends the empty space. Air is a great impediment to motion; -a large part of the power of a locomotive engine has to be expended -solely in pushing the air out of the way so as to allow the train to -get through. The faster the speed, the greater is the tax which the -air imposes on the moving body. A cannon-ball, for instance, loses -an immensity of its speed, and consequently of its power, by having -to bore its way through the air. In outer space beyond the limits of -this atmosphere, a freedom of movement can be enjoyed of which we -know nothing down here. I spoke of this when discussing the movements -of Encke’s comet. Even this very unsubstantial body could dash along -without appreciable resistance until it traversed the atmosphere -surrounding the sun. But now we have to speak of the motion of a little -object both small and dense, resembling perhaps a pebble or a fragment -of iron, or some substance of that description. It is a little body -such as this which produces a shooting star. - -For ages and ages the meteoroid has been moving freely through space. -The speed with which it dashes along greatly exceeds that of any of the -motions with which we are familiar. It is about 100 times as swift as -the pace of a rifle-bullet. About twenty miles would be covered in a -second. You can hardly imagine what that speed is capable of. Suppose -that you put one of these flying meteoroids beside an express train to -race from London to Edinburgh, the meteoroid would have won the race -before the train had got out of the station. Or suppose that a shooting -star determined to make the circuit of the earth, it might, so far as -pace is concerned, go entirely around the globe and back to the point -from which it started in a little more than twenty minutes. But the -fact is, you could not make any object down here move as fast as a -shooting star. No gunpowder that could be made would be strong enough, -in the first place, and even if the body could once receive the speed, -it would never be able to force its way through the air uninjured. - -So long as a little shooting star is tearing away through open space -we are not able to see it. The largest telescope in the world would -not reveal a glimpse of anything so small. The meteoroid has no light -of its own, and it is not big enough to exhibit the light reflected -from the sun in the same manner as a little planet would do. It is -only at the moment when it begins to be destroyed that its visibility -commences. If the little object can succeed in dashing past our earth -without becoming entangled in the atmosphere, then it will pursue its -track with perhaps only a slight alteration in its path, due to the -pull exercised by the earth. The air which surrounds our globe may be -likened to a vast net, in which if any little meteor becomes caught -its career is over. For when the little body, after rejoicing in the -freedom of open space, dashes into air, immediately it experiences a -terrific resistance; it has to force the particles of air out of the -way so as to make room for itself, and in doing so it rubs against them -with such vehemence that heat is produced. - -I am sure every boy knows that if he rubs a button upon a board it -becomes very hot, in consequence of the friction. There are many other -ways in which we can illustrate the production of heat in the same -manner. One is a contrivance by which we spin round rapidly a piece of -stick pressed against a board. Quantities of heat are thus produced -by the friction, and volumes of smoke rise up. We have read how some -savages are able to produce fire by means of friction in a somewhat -similar manner, but to do so requires a rare amount of skill and -patience. There is another illustration by which to show how heat can -be produced by friction. A brass tube full of water is so arranged that -it can be turned around very rapidly by the whirling table. We apply -pressure to the tube, and after a minute or two the water begins to get -hot, and then at last to boil, until ultimately the cork is driven out -and a diminutive and, fortunately, harmless explosion of the friction -boiler takes place. Engineers are aware how frequently heat is produced -by friction, when it is very inconvenient or dangerous. Indeed, unless -the wheels of railway carriages are kept well greased, the rubbing -of the axle may generate so much heat that conflagrations in the -carriage will ensue. Nature, in the little shooting star, gives us a -striking illustration of the same fact. Perhaps you may be surprised to -hear that the whole brilliancy of the shooting star is simply due to -friction. As the little body dashes through the air it becomes first -red-hot, then white-hot, until at last it is melted and turned into -vapor. Thus is formed that glowing streak which we, standing very many -miles below, see as a shooting star. - -A bullet when fired from a rifle will fly into pieces after it has -struck against the target, and if you quickly pick up one of these -pieces you will generally find it quite hot. Whence comes this heat? -The bullet, of course, was cold before the rifleman pulled the -trigger. No doubt there was a considerable amount of heat developed -by the burning of the gunpowder, but the bullet was so short a time in -contact with the wad, through which so little heat would pass, that we -must look to some other source for the warmth that has been acquired. -Friction against the barrel as the bullet passed to the mouth must have -warmed the missile a good deal, and when rubbing against the air the -same influence must have added still further to its temperature, while -the blow against the target would finally warm it yet more. - -In comparing the shooting star with the rifle-bullet we must remember -that the celestial object is travelling with a pace 100 times as swift -as the utmost velocity that the rifle can produce, and the heat which -is generated by friction is increased in still greater proportion. -If we double the speed, we shall increase the quantity of heat by -friction fourfold; if we increase the speed three times, then friction -will be capable of producing nine times as much heat. In fact, we must -multiply the number expressing the relative speed by itself--that is, -we must form its square--if we want to find an accurate measure for -the quantity of heat which friction is able to produce when a rapidly -moving body is being stopped by its aid. The shooting star may have -a pace 100 times that of the rifle-bullet, and if we multiply 100 by -100 we get 10,000; consequently we see that the heat produced by the -shooting star before its motion was arrested in dashing through the air -would be 10,000 times that gained by the rifle-bullet in its flight. -If the temperature of the rifle-bullet only rose a single degree by -friction, it would thus be possible for the shooting star to gain -10,000 degrees, and this would be enough to melt and boil away any -object which ever existed. Thus we need not be surprised that friction -through the air, and friction alone, has proved an adequate cause -for the production of all the heat necessary to account for the most -brilliant of meteors. - -It is rather fortunate for us that the meteors do dash in with this -frightful speed; had the little bodies only moved as quickly as a -rifle-bullet, or even only four or five times as fast, they would have -pelted down on the earth in solid form. Indeed, on rare occasions it -does happen that bodies from the heavens strike down on the ground. -The great majority of those that fall on the ground, however, become -entirely transformed into harmless vapor. The earth would, indeed, -be almost uninhabitable from this cause alone were it not for the -protection that the air affords us. All day and all night innumerable -missiles would be whizzing about us, and though many of them are -undoubtedly very small, yet as their speed is 100 times that of a -rifle-bullet, the fusillade would be very unpleasant. It is, indeed, -the intense hurry of these celestial bullets to get at us which is the -very source of our safety. It dissipates the meteors into streaks of -harmless vapor. - - -WHAT BECOMES OF THE SHOOTING STARS. - -When we throw a lump of coal on the fire, all that is soon left is a -little pinch of ashes, and the rest of the coal has vanished. You might -think it had been altogether annihilated, but that is not nature’s -way. Nothing is ever completely destroyed; it is merely transformed or -changed into something else. The greater part of the coal has united -with the oxygen which it has obtained from the air, and has formed a -new gas, which has ascended the chimney. Every particle that was in the -coal can be thus accounted for, and in the act of transformation heat -is given out. - -A meteor also becomes transformed, but the substance it contains is not -lost, though it is changed into glowing vapors. It is known that with -heat enough any substance can be turned into vapor, just as water can -be boiled into steam. Look at an electric light flashing between two -pieces of carbon. Though carbon is one of the most difficult substances -to melt, yet such is the intense heat generated by the electric -current that the carbon is not only melted, but is actually turned -into a vapor, and it is this vapor glowing with heat that gives us the -brilliant light. In a similar manner iron can be rendered red-hot, -white-hot, and then boiled and transformed into an iron vapor, if we -may so call it. There is, indeed, plenty of such iron vapor in the -universe. Quantities of it surround the sun and some of the stars. - -When ordinary steam is chilled it condenses into little drops of water. -So, too, if iron be heated until it is transformed into vapor, and if -that vapor be allowed to condense, it will ultimately form a dust, -consisting of bits of iron so small that you would require a microscope -to examine them. There is iron present in the small shooting stars. -Other substances are also contained therein, and all these materials, -after being boiled by the intense heat, are transformed into vapor. -When the heat subsides, the vapor condenses again into fine dust, -so that the ultimate effect of the atmosphere on a shooting star is -to grind the little object into excessively fine powder, which is -scattered along the track which the object has pursued. Sometimes this -powder will continue to glow for minutes after the meteor has vanished, -and in the case of some great meteors this stream of luminous dust in -the air forms a very striking spectacle. A great meteor, or fire-ball -as it is often called, appeared on the 6th of November, 1869. It flew -over Devonshire and Cornwall, and left a track fifty miles long and -four miles wide. The dust remained visible all along the great highway -for nearly an hour; it formed a glowing cloud hanging in the sky, and -though originally nearly straight, it became bent and twisted by the -winds before it finally disappeared from view. - -We have now to see what becomes of this meteoric dust which is being -incessantly poured into the air from external space. None of it ever -gets away again; for whenever an unfortunate meteor just touches the -air it is inevitably captured and pulverized. That dust subsides -slowly, but we do not find it easy to distinguish the particles which -have come from the shooting stars, because there is so much floating -dust which has come from other sources. - -A sunbeam is the prettiest way of revealing the existence of the motes -with which the air is charged. The sunbeam renders these motes visible -exactly in the same way as planets become visible when sunbeams fall -on them in far-distant space. But if we have not the sunbeams here, -we can throw across the room a beam of electric light, and it is seen -glowing all along its track, simply because the air of the room, like -air everywhere, is charged with myriads of small floating particles. -If you hold the flame of a spirit-lamp beneath this beam, you will see -what seems like columns of black smoke ascending through it. But these -columns are not smoke, they are pure air, or rather air in which the -solid particles have been transformed into vapor by the heat from the -spirit-flame. - -The motes abound everywhere in the air. We take thousands of them -into our lungs every time we breathe. They are on the whole gradually -sinking and subsiding downwards, but they yield to every slightest -current, so that when looking at a sunbeam you will find them moving in -all directions. It is sometimes hard to believe that the little objects -are tending downwards, but if you look on the top of a book that has -lain for a time on a book-shelf, you find there a quantity of dust, -produced by the motes which have gradually subsided where they found a -quiet spot and were allowed sufficient time to do so. - -The great majority of these particles consist, no doubt, of fragments -of terrestrial objects. The dust from the roads, the smoke from the -factories, and numerous other sources, are incessantly adding their -objectionable particles to the air. There can be no doubt that the -shooting stars also contribute their mites to the dust with which the -atmosphere is ever charged. The motes in the murky air of our towns -have no doubt chiefly originated from sources on this earth. Many -of these sources it would be impossible to regard as of a romantic -description. We may, however, feel confident that among those teeming -myriads of small floating objects are many little particles which, -having had their origin from shooting stars, are now gradually sinking -to the earth. - -This is not a mere surmise, for dust has been collected from lofty -Alpine snows, from the depths of the sea, and from other localities far -removed from the haunts of men. From such collections, tiny particles -of iron have been obtained, which have evidently been once in a molten -condition. There is no conceivable explanation for the origin of iron -fragments in such situations, except that they have been dropped from -shooting stars. - -I am sure you have often helped in the making of a gigantic snowball. -You begin with a small quantity of snow that can be worked with your -hands. Then you have rolled it along the ground until it has become -so big and so heavy that you must get a few playmates to help you, -until at last it has grown so unwieldy that you can move it no longer, -and then you apply your artistic powers to carving out a statue. The -snowball has grown by the addition of material to it from without, -and as it became heavier and heavier, it lapped up more and more of -the snow as it rolled along; so that with each increase of size, its -capacity for becoming still larger has also increased. I want to liken -our earth to a snowball which goes rolling on through space, and -every day, every hour, every minute, is gathering up and taking into -it the little shooting stars that it meets with on its way. No doubt -the annual accumulation is a very small quantity when compared with the -whole size of the earth; but the earth is always drawing in, and now, -at all events, never giving back again; so that when this process is -carried on long enough, astonishing results may be obtained. - -You have all heard many maxims on this subject--how every little saving -will at length reach a respectable or a gigantic total. Nature abounds -with illustrations of the principle. All the water that thunders over -Niagara is merely a sufficient number of little drops of rain collected -together. Our earth has been gradually hoarding up, during countless -ages, all the meteor dust that has rained upon it; and the larger the -earth grows, the bigger is the net which it spreads, and the greater is -the power it has to capture the wandering bodies. Thus, our earth, ages -and ages ago, may have been considerably smaller than it is at present; -in fact, a large proportion of this globe on which we dwell may have -been derived from the little shooting stars which incessantly rain in -upon its surface. - - -GRAND METEORS. - -I dare say that many of those present will, in the course of their -lives, have opportunities of seeing some of the great meteors, or -fire-balls, which are occasionally displayed. Generally speaking, about -one hundred or so of these splendid objects are recorded every year. -We are never apprised that they are coming; they take us unawares, and -therefore we have no opportunity to make proper arrangements for seeing -them. There is only the chance that such persons as have been fortunate -enough to see them will have noted the circumstances with sufficient -accuracy to enable us to make use of their observations. - -[Illustration: FIG. 78.--How to find the Height of a Meteor.] - -The chief point to determine is the height of the meteor above the -earth. For this we must have two observations at least, made in places -as far asunder as possible. Suppose an observer at London and an -observer at York were both witnesses of a splendid meteor; if they -find, on subsequent comparison, that their observations were made at -the same moment, there is no reasonable doubt that it was the same -object they both saw. The observer at York describes the meteor as -lying to the south, halfway down from the point directly over his head -towards the horizon. The London observer speaks of the meteor as being -to the north; and to him also it appeared that the object was halfway -down towards his horizon from the point directly over his head. If you -know a little Euclid, you can easily show from these facts that the -height of the meteor must have been half the distance between London -and York, that is, 85 miles (Fig. 78). - -I do not mean to say that the mode of discovering the meteor’s height -will be always quite such a simple process as it has been in the -case of the London and York observations. The principle is, however, -the same--that whenever from two sufficiently distant positions the -direction of the meteor has been observed, its path is known--just as -on p. 21 we showed how the height of the suspended ball was obtained -from observations at each end of the table. Generally speaking, bright -meteors begin at an elevation of between fifty and one hundred miles, -and they become extinguished before they are within twenty miles of the -ground. - -Sometimes a tremendous explosion will take place during the passage of -a meteor through the air. There was a celebrated instance in America -on the 21st of December, 1876, which will give an idea of one of these -objects possessing exceptional magnificence. It began in Kansas about -seventy-five miles high, and thence it flew for a thousand miles at a -speed of ten or fifteen miles a second, until it disappeared somewhere -near Lake Ontario. Over a certain region between Chicago and St. -Louis, the great ball of fire burst into a number of pieces, and -formed a cluster of glowing stars that seemed to chase each other over -the sky. This cluster must have been about forty miles long and five -miles wide, and when the explosion occurred a most terrific noise was -produced, so loud that many thought it was an earthquake. A remarkable -circumstance illustrates the tremendous height at which this explosion -occurred. The meteor had burst into pieces, the display was all over, -and was beginning to be forgotten, and yet nothing had been _heard_. It -was not until a quarter of an hour after the explosion had been _seen_ -that a fearful crash was heard at Bloomington. The explosion actually -occurred 180 miles from the spot, and as sound takes five seconds to -travel a mile, you can easily calculate that the noise required a -quarter of an hour for its journey. What a tremendous noise it must -have been! - -Shooting stars are of every grade of brightness. Beginning with the -more gorgeous objects which have been compared with the moon or -even with the sun himself, we descend to others as bright as Venus -or as Jupiter; others are as bright as stars of various degrees of -brilliancy. Fainter shooting stars are much more numerous than the -conspicuous ones; in fact, there are multitudes of these objects so -extremely feeble that the unaided eye would not show them. They only -become perceptible in a telescope. It is not uncommon while watching -the heavens at night to notice a faint streak of light dashing across -the field of the instrument. This is a shooting star which is invisible -except through the telescope. - - -THE GREAT NOVEMBER SHOWERS. - -[Illustration: FIG. 79.--A Great Shower of Shooting Stars.] - -Occasionally we have the superb spectacle of a shower of shooting -stars. None of you, my young friends, can as yet have had the good -fortune to witness one of the specially grand displays, but you may -live in hope; there are still showers to come. Astronomers have -ventured on the prophecy that in or about the year 1899 you will have -the opportunity of seeing a magnificent exhibition of this kind. There -is only one ground for anxiety, and that is as to whether the clouds -will keep out of the way for the occasion. I think I cannot explain -my subject better than by taking you into my confidence and showing -you the reasons on which we base this prediction. The last great -shooting-star shower took place in the year 1866, or, perhaps, I should -rather say that this was the last display from the same shooting-star -system as that about which we are now going to speak. On the night of -the 13th of November, 1866, astronomers were everywhere delighted by -a superb spectacle. Enjoyment of the wondrous sight was not only for -astronomers. Every one who loves to see the great sights of nature -will have good reason for remembering that night. I certainly shall -never forget it. It was about ten o’clock when a brilliant meteor or -two first flashed across the sky, then presently they came in twos and -threes, and later on in dozens, in scores, in hundreds. These meteors -were brilliant objects, any one of which would have extorted admiration -on an ordinary night. What, then, was the splendor of the display when -they came on in multitudes? For two or three hours the great shower -lasted, and then gradually subsided. - -We were not taken unawares on this occasion, for the shower was -expected, and had been, in fact, awaited with eager anticipation. It -should first be noticed that each year some shooting stars may always -be looked for on or about the 13th of November. Every thirty-three -years, or thereabouts, the ordinary spectacle breaks out into a -magnificent display. It has also been found that for nearly 1000 years -there have been occasional grand showers of meteors at the time of year -mentioned, and all these incidents agree with the supposition that -they are merely repetitions of the regular thirty-three-year shower. -The first was in the year A.D. 902, which an old chronicle speaks -of as the “year of the stars,” from the extraordinary display which -then took place. I do not think the good people 1000 years ago fully -appreciated the astronomical interest of such spectacles; in fact, -they were often frightened out of their wits, and thought the end of -the world had come. Doubtless many ancient showers have taken place -of which we have no record whatever. In more modern days we have had -somewhat fuller information; for example, on the night between the 12th -and 13th of November, 1833, a shower was magnificently seen in America. -Mr. Kirkwood tells us that a gentleman of South Carolina described the -effect on the negroes of his plantation as follows: “I was suddenly -awakened by the most distressing cries that ever fell on my ears. -Shrieks of horror and cries for mercy I could hear from most of the -negroes of the three plantations, amounting in all to about 600 or 800. -While earnestly listening for the cause, I heard a faint voice near the -door calling my name. I arose, and taking my sword, stood at the door. -At this moment I heard the same voice still beseeching me to arise, -and crying out that the world was on fire. I then opened the door, and -it is difficult to say which excited me the most--the awfulness of the -scene or the distressed cries of the negroes. Upwards of a hundred lay -prostrate on the ground, some speechless, and some with the bitterest -cries, but with their hands raised praying for mercy. The scene was -truly awful, for never did rain fall much thicker than the meteors fell -towards the earth.” - -By the study of many records of great showers it was learned that the -interval at which these grand displays succeeded one another was about -thirty-three years; and when it was remembered that the last great -shower was in 1833 it was confidently expected that another similar -display would take place in 1866. This was fully confirmed. Yet another -thirty-three years brings us to 1899, when we have good reason for -looking forward to a grand shower of these bodies. It may be expected -to occur on November 14 or November 15. It may, however, possibly be -that a shower will occur on the same days of the succeeding year. - -[Illustration: FIG. 80.--The Earth crossing the Track of Meteors.] - -We know a good deal now as regards the movements of these little -objects. I want you to think of a vast swarm, something like a flock -of birds, which I dare say you have often seen flying high in the air; -the difference, however, is that the flock of meteors is enormously -greater than any flock of birds ever was; and the meteors, too, are -scattered so widely apart, that each one may be miles away from its -next neighbors. Usually the meteoric shoal is many millions of miles -long, and perhaps a hundred thousand miles in width. The great flock -of meteors travels through space in a certain definite track. We have -learned how the sun guides a planet, and forces the planet to move -around him in an ellipse. But our sun will also condescend to guide -an object no bigger than a shooting star. A bullet, a pea, or even -a grain of sand will be held to an elliptic course around the sun as -carefully as the great Jupiter himself. The entire shoal of meteors may -therefore pursue their common journey around the sun as if inspired by -a common purpose, each individual member of the host being, however, -guided by the sun, and performing its path in real independence of its -neighbors. The orbit followed by this shoal of meteors is enormously -large and wide. Here is a sketch of the path (Fig. 80), and I have -laid down the position of the orbit of the earth, but not on the same -scale. The ellipse is elongated, so that while the shoal approaches -comparatively close to the sun at one end of its journey, at the other -end it goes out to an enormous distance, far beyond the orbit of the -earth--beyond, indeed, the orbit of Jupiter or Saturn; in fact, it -reaches to the path of Uranus. To accomplish so vast a journey as this -thirty-three years and a quarter are required, and now you will be -easily able to see why we get periodical visits from the shoal. - -It is, however, a mere piece of good fortune that we ever encounter the -November meteors. Probably there are numerous other shoals of meteors -quite as important which we never see, just in the same way as there -are many shoals of fish in the sea that never come into our net. The -earth moves round the sun in a path which is very nearly a circle, and -the shoal moves round in this long oval. We cannot easily represent -the true state of things by mere diagrams which show all these objects -on the same plane. This does not give an accurate representation of -the orbits. I think you will better understand what I mean by means of -some wire rings. Make a round one to represent the path of the earth, -and a long oval one to represent the path of the meteors. There is to -be a small opening in the circular ring so that we can slip one of the -orbits inside the other. If we are to see the meteors, it is of course -necessary that they should strike the earth’s atmosphere, for they are -not visible to us when they lie at a distance like the moon or like the -planets. It is necessary that there be a collision between the earth -and the shoal of meteors. But there never could be a collision between -two trains unless the lines on which these trains run meet each other; -therefore, it is necessary that this long ellipse shall actually cross -the earth’s track; it will not do to have it pass inside like the two -links of a chain; our earth would then miss the meteors altogether, -and we should never see them. There are very likely many of such shoals -of meteors revolving in this way, and thus escaping our notice entirely. - -You will also understand why there is no use in looking for these -showers except on a particular day of November. On that day, and on -that day alone, the earth appears at that particular point of its -route where the latter crosses the track of the shoal. On the 1st of -November, for instance, the earth has not yet reached the point where -it could meet with these bodies. By the end of November it has passed -too far. But even supposing that the earth is crossing the track of -the meteors on the 13th of November, it is still possible that only -a few, or none at all, shall be seen. The shoal may not happen to be -at that spot at the right time. For a display of meteors to occur, it -is therefore necessary that the shoal shall happen to be passing this -particular stage of the journey on the 13th of November. In 1866 the -earth dipped through the shoal and caught a great many of these meteors -in its net. For a few hours the earth was engaged in the capture, until -it emerged on the other side of the shoal, and the display was at an -end. - -Sometimes it happens that in two years following each other, grand -showers of meteors are seen. The reason of this is that the shoal is -very long and thin, and consequently if the earth passes through the -beginning of the shoal one year, it may have returned to the same point -next year before the whole length of the shoal has completely passed. -In this case we shall have two great showers in consecutive years. -Thus a very fine display was seen in America on the proper day in 1867, -while many stragglers were also observed during the three subsequent -recurrences of the same date. - -Whenever the 13th of November comes round we generally meet with at -least five shooting stars belonging to this same system, and we must -explain how this occurs. Suppose there is a small racecourse so that -the competitors will have to run a great many times round before the -race is over. Let there be a very large number of entries, and let -the majority of the athletes be fairly good runners, while a few are -exceptionally good with varying degrees of excellence, and a few are -very bad, some being worse than others. The whole group starts together -in a cluster at the signal, and perhaps for the first round or two they -may keep tolerably well together. It will be noticed the cluster begins -to elongate as one circuit after another is made; the better runners -draw out to the front, and the slower runners lag further and further -behind; at last it may happen that those at the head will have gained -a whole round on those at the tail, while the other runners of varying -degrees of speed will be scattered all round the course. The majority -of the runners, if of nearly equal speed, may continue in a pretty -dense group. - -Precisely similar has been the great celestial race which these meteors -are running. They started on their grand career centuries ago, and ever -since then they have been flying round and round their mighty course. -The greater proportion of the meteors still stay close together, and -their pace is nearly uniform. The exceptionally smart ones have shot -ahead, the exceptionally slow ones have lagged behind, and thus it -happens that, after fifty or more revolutions have been completed, -the shape of the original swarm has become considerably modified. -Its length has been drawn out, while the stragglers and the fastest -runners have been scattered all around the path. Across this course -our earth carries us every November; there we usually encounter some -of the members of this swarm which have strayed from the great host; -they flash into the air, and thus it is that some of these bodies are -generally seen every November. - -[Illustration: FIG. 81.--The Radiant.] - -During a shooting-star shower it is interesting to notice that all the -meteors seem to diverge from a single point. In the adjoining figure -(Fig. 81), which shows the directions of a number of meteors’ tracks, -you will notice that every one seems to radiate from a certain point of -the sky. In the case of the shower of the 13th–15th of November this -point lies in the constellation Leo. I must refer you to the Appendix -for a description of the way to find Leo or the Lion. The radiant -point, as we term it, of this system of meteors is there situated. It -is true that the meteors themselves do not generally seem to come all -the way from this place. It is the direction of their luminous trails -produced backward that carries the eye to the radiant (Fig. 81). If a -meteor were actually seen there, it would be certainly coming straight -towards us; it would not then appear as a streak of light at all; it -would merely seem like a star which suddenly blazed into splendor and -then again sank down into invisibility. Every meteor which appeared -near this point would be directed very nearly at the observer, and its -path would therefore seem very much foreshortened. I can illustrate -this with a long straight rod. If I point it directly at you, you can -only see the end. If I point it nearly at you, it will seem very much -shortened. During the great shower in 1866 many of the meteors could be -observed so close to the radiant in Leo that they seemed merely like -very short marks in the sky; some of them, indeed, seemed to be merely -starlike points swelling up into brilliance and then going out. Hence -it is that we call this system of shooting stars the “Leonids.” They -bear this name because their radiant lies in the constellation Leo, -and unless the direction of a shooting star emanates from this point it -does not belong to the Leonids. Even if it did so, the meteor would not -be a Leonid unless the date was right, namely, on the 13th of November, -or within a day thereof. We thus have two characteristics which belong -to a system of shooting stars; there is the date on which they occur -and the point from which they radiate. - - -OTHER GREAT SHOWERS. - -To illustrate what I have said, we will speak about another system of -shooting stars; they are due every August, from the 9th to the 11th, -and their directions diverge from a point in the constellation of -Perseus. I may remind you of the dates of the recurrence of this shower -as well as of the November meteors of which we have just spoken, by -quoting the following production:-- - - If you November’s stars would see, - About the fourteenth watching be. - In August, too, stars shine through heaven, - On nights between nine and eleven. - -It may be worth your while to remember these lines, and always to keep -a look-out on the days named. The August meteors, the Perseids we -often call them, do not make gorgeous displays, in particular years, -with the regularity of the Leonids. There have been, no doubt, some -exceptionally grand showers between the 9th and the 11th of August, but -we cannot predict when the next splendid one is due. There are vast -numbers of stragglers all round the track of the Perseids. In fact, -it would seem as if the great race had gone on for such a long period -that the cluster had to a great extent broken up, and that a large -proportion of the meteors were now scattered the whole way around the -course with tolerable uniformity. This being so, it follows that every -time we cross the track we are nearly certain to fall in with a few of -the stragglers, though we may never enjoy the tremendous spectacle of a -plunge through a dense host of meteoroids. - -There are many other showers besides the two I have mentioned. Some -shooting stars are to be seen almost every fine night, and those -astronomers who pay particular attention to this subject are able -to make out scores of small showers which might not interest you. -Each of these is fully defined by the night of the year on which it -occurs and the position of the point in the heavens from which the -meteors radiate. Of these I must mention one. It is not usually very -attractive, but it has a particular interest, as I shall now explain. - -On the 27th of November, 1872, a beautiful meteoric shower took place. -You will notice that though the month is the same, the day is entirely -different from that on which the Leonids appear. This shower of the -27th is called the Andromedes, because the lines of direction of the -shooting stars of which it is composed seem to diverge from a point in -the constellation Andromeda. Ordinarily speaking, there is no special -display of meteors connected with the annual return of this day; but in -1872 astronomers were astonished by an exhibition of shooting stars -belonging to this system. They were not at all bright when compared -with the Leonid meteors. They were, however, sufficiently numerous to -arrest the attention of very many, even among those who do not usually -pay much attention to the heavens. - -The chief interest of the shower of Andromedes centers in a remarkable -discovery connecting meteors and comets. There is a comet which was -discovered by the astronomer Biela. It is a small object, requiring a -telescope to show it. This comet completes each revolution in a period -of about seven years; or rather, I should say, that was the time which -the comet used to spend on its journey, for a life of trouble and -disaster seems of late to have nearly extinguished the unfortunate -object. In 1872 the comet was due in our neighborhood, and on the -night of the 27th of November, in the same year, the earth crossed the -track, and, in doing so, the shower of shooting stars was seen. This -was a remarkable coincidence. We crossed the path of the comet at the -time when we knew the comet ought to be there; and though we did not -then see the comet, we saw a shower of shooting stars, and a wonderful -shower too. A circumstance so peculiar suggested at once that the comet -and the shooting stars must in some way or other be connected together. -This is a suggestion we can test in another manner. We know the history -of the comet, and we are aware that at the very time of the shower, -the comet was approaching from the direction of the constellation of -Andromeda. It was coming, in fact, from the very quarter whence the -shooting stars have themselves travelled. Taking all these things -together, it seems impossible to doubt that the shoal of shooting stars -was, if not actually the comet itself, something closely connected with -that famous body. - - -METEORITES. - -Some years ago, a farmer living near Rowton, in Shropshire, noticed -on a path in a field a hole which had been suddenly made by some -mysterious and unknown agent. The laborers who were near told him they -had just heard a remarkable noise; and when the farmer put his hand -down into the hole, he felt something hot at the bottom of it. He took -a spade and dug up the strange body, and found it to be a piece of -iron, weighing about seven pounds. He was naturally amazed at such an -occurrence, and brought the body home with him. - -Where did that piece of iron come from? It is plain that it could not -have been always in the ground. The noise and the recently made hole -showed that was not the case, and that is confirmed by the fact that -the iron was hot. A piece of iron within a few feet of the earth’s -surface cannot have remained warm for any length of time. It is -therefore clear that the iron must have tumbled from the sky. This is -a marvellous notion; in fact, it seems so incredible that at first -people refused to believe that such things as stones or solid lumps of -iron could have fallen from the heavens to the earth. But they had to -believe it; the evidence was too conclusive. Fortunately, however, -the occurrence is a comparatively rare one; indeed, our life on this -globe would have an intolerable anxiety added to it if showers of iron -hailstones like that at Rowton were at all of frequent occurrence. We -should want umbrellas of a more substantial description than those -which suffice for the rains we actually experience. There are, indeed, -instances on record of persons having been killed by the fearful blows -given by these bodies in falling. - -The Rowton siderite is a comparatively small one; pieces weighing -hundredweights, and even tons, have been collected together in our -museums. I would recommend you to pay a visit to that interesting room -in our great British Museum in which these meteorites are exhibited. -There we see actual specimens of celestial bodies which we can feel -or weigh, and which our chemists can analyze. It may be noticed that -they only contain substances that we already know on this earth. This -celestial iron has often been made use of in primitive times before -man understood how to smelt iron from its ore and how to transform it -from cast iron to wrought iron. Nature seems to have taken heed of -their wants, and occasionally to have thrown down a lump or two for the -benefit of those who were so fortunate as to secure them. - -That these stones or irons drop from the sky is absolutely certain, -but when we try to find out their earlier history we become involved -in not a few difficulties. Nobody really knows the true history of -these objects, but the view of their origin which seems to me to -possess fewer difficulties than any other view is that which we may -call the Columbiad Theory. I use this expression because every boy or -girl listening to me ought to have read Jules Verne’s wonderful book, -“From the Earth to the Moon,” and if any of you have not read it, the -sooner you do so the better. It is there narrated how the gun club of -Baltimore designed a magnificent cannon which was sunk deep into the -ground, and then received a terrific charge of guncotton, on which a -great hollow projectile was carefully lowered, containing inside the -three adventurous explorers who desired to visit the moon. Calculations -were produced with a view of showing that by firing on a particular -day the explosion would drive the projectile up to the moon. There -was, however, the necessary condition that the speed of projection -should be great enough. The gun club were accurate in saying that if -the cannon were able to discharge the projectile with a speed twenty -or thirty times as great as that which had ever been obtained with -any other cannon, then the missile would ascend up and up forever if -no further influence were exerted on it. No doubt we have to overlook -the resistance on the air and a few other little difficulties, but to -this extent, at all events, the gun club were right: that a velocity of -about six or seven miles a second would suffice to carry a body away -from the gravitation of the earth. - -No one supposes that there were ever Columbiad cannons on our globe by -which projectiles were shot up into space; but it seems possible that -there may have been in very ancient days volcanoes on the earth with a -shooting power as great as that which President Barbicane designed for -the big cannon. - -Even now we have some active volcanoes of great energy on our earth, -and we know that in former days the volcanoes must have been still more -powerful; that, in fact, the Vesuvius of the present must be merely -a popgun in comparison with volcanoes which have shaken the earth in -those primitive days when it had just cooled down from its original -fiery condition. Some of these early volcanoes, in the throes of their -mighty eruptions, appear to have shot forth pieces of iron and volcanic -substances with a violence great enough to carry them off into space. - -Suppose that a missile were projected upwards, it would ascend higher -and higher, and gravity would, of course, tend to drag it back again -down to earth. It can be shown that with an initial speed of six or -seven miles a second the missiles would never return to the earth if -only influenced by its attraction. The subsequent history of such a -projectile would be guided by the laws according to which a planet -moves. The body is understood to escape the destination which was aimed -at by the Columbiad. I mean, of course, that it is not supposed to hit -the moon. Of course, this might conceivably happen; but most of the -projectiles would go quite wide of the mark, and would travel off into -space. - -Though the earth would be unable to recall the projectile, the -attraction of the sun would still guide it, whether it was as big as -a paving-stone or ever so much larger or smaller. The body would be -constrained to follow a path like a little planet around the sun. This -track it would steadily pursue for ages. The wanderer would, however, -cross the earth’s track once during each of its revolutions at the -point from which it was projected. Of course, it will generally happen -that the earth will not be there at the time the meteorite is crossing, -and the meteorite will not be there at the time the earth is crossing. -Nothing will therefore happen, and the object goes again on its long -rounds. But sometimes it must occur that a meteor does not get past -the junction without coming so close to the earth that it plunges into -the air, often producing a noise and generating a streak of light like -a shooting star. Then it tumbles down, and is restored to that earth -whence it originally came. - -If this be the true view--and I think there are less weighty objections -to it than to any other I know of--then the history of the piece -of iron that was found in Shropshire would be somewhat as follows. -Many millions of years ago, when the fires of our earth were much -more vigorous than they are in these dull times, a terrific volcanic -outbreak took place, and vast quantities of material were shot into -space, of which this is one of the fragments. During all the ages that -have since elapsed this piece of iron has followed its lonely track. In -a thousandth part of the time rust and decay would have destroyed it -had it lain on the earth, but in the solitudes of space there was found -no air or damp to produce corrosion. At last, after the completion of -its long travels, it again crashed down on the earth. - -We have now briefly surveyed the extent of the solar system. We began -with the sun, which presides over all, and then we discussed the -various planets with their satellites, next we considered the eccentric -comets, and finally the minute bodies which, as shooting stars or -meteorites, must be regarded as forming part of the Sun’s system. In -our closing lecture we shall have to deal with objects of a far more -magnificent character. - - - - -LECTURE VI. - -STARS. - - We try to make a Map--The Stars are Suns--The Numbers of the - Stars--The Clusters of Stars--The Rank of the Earth as a Globe - in Space--The Distances of the Stars--The Brightness and Color - of Stars--Double Stars--How we find what the Stars are made - of--The Nebulæ--What the Nebulæ are made of--Photographing the - Nebulæ--Conclusion. - - -WE TRY TO MAKE A MAP. - -The group of bodies which cluster around our sun forms a little island, -so to speak, in the extent of infinite space. We may illustrate this -by a map in which we shall endeavor to show the stars placed at their -proper relative distances. We first open the compasses one inch, and -thus draw a little circle to represent the path of the earth. We are -not going to put in all the planets. We take Neptune, the outermost, at -once. To draw its path I open the compasses to thirty inches, and draw -a circle with that radius. That will do for our solar system, though -the comets no doubt will roam beyond these limits. To complete our map -we ought of course to put in some stars. There are a hundred million to -choose from, and we shall begin with the brightest. It is often called -the Dog Star, but astronomers know it better as Sirius. Let us see -where it is to be placed on our map. Sirius is beyond Neptune, so it -must be outside somewhere. Indeed, it is a good deal further off than -Neptune; so I try at the edge of the drawing-board; I have got a method -of making a little calculation that I do not intend to trouble you -with, but I can assure you that the results it leads me to are quite -correct; they show me that this board is not big enough. But could a -board which was big enough fit into this lecture theatre? Here, again, -I make my little calculations, and I find that there would not be room -for a board sufficiently great; in fact, if I put the sun here at one -end, with its planets around it, Sirius would be too near on the same -scale if it were at the further corner. The board would have to go out -through the wall of the theatre, out through London. Indeed, big as -London is, it would not be large enough to contain the drawing-board -that I should require. It would have to stretch about twenty miles from -where we are now assembled. We may therefore dismiss any hope of making -a practical map of our system on this scale if Sirius is to have its -proper place. Let us, then, take some other star. We shall naturally -try with the nearest of all. It is one that we do not know in this -part of the world, but those that live in the southern hemisphere are -well acquainted with it. The name of this star is Alpha Centauri. Even -for this star, we should require a drawing three or four miles long -if the distance from the earth to the sun is to be taken as one inch. -You see what an isolated position our sun and his planets occupy. The -members of the family are all close together, and the nearest neighbors -are situated at enormous distances. There is a good reason for this -separation. The stars are very pretty and perfectly harmless to us -where they are at present situated. They might be very troublesome -neighbors if they were very much closer to our system. It is therefore -well they are so far off; they would be constantly making disturbance -in the sun’s family if they were near at hand. Sometimes they would be -dragging us into unpleasantly great heat by bringing us too close to -the sun, or producing a coolness by pulling us away from the sun, which -would be quite as disagreeable. - - -THE STARS ARE SUNS. - -We are about to discuss one of the grandest truths in the whole -of nature. We have had occasion to see that this sun of ours is a -magnificent globe immensely larger than the greatest of his planets, -while the greatest of these planets is immensely larger than this -earth; but now we are to learn that our sun is, indeed, only a star -not nearly so bright as many of those which shine over our heads every -night. We are comparatively close to the sun, so that we are able -to enjoy his beautiful light and cheering heat. Each of those other -myriads of stars is a sun, and the splendor of those distant suns is -often far greater than that of our own. We are, however, so enormously -far from them that they appear dwindled down to insignificance. To -judge impartially between our sun or star and such a sun or star as -Sirius we should stand halfway between the two; it is impossible to -make a fair estimate when we find ourselves situated close to one star -and a million times as far from the other. After allowance is made for -the imperfections of our point of view, we are enabled to realize the -majestic truth that the sun is no more than a star, and that the other -stars are no less than suns. This gives us an imposing idea of the -extent and the magnificence of the universe in which we are situated. -Look up at the sky at night--you will see a host of stars; try to think -that every one of them is itself a sun. It may probably be that those -suns have planets circulating round them, but it is hopeless for us to -expect to see such planets. Were you standing on one of those stars -and looking towards our system, you would not perceive the sun to be -the brilliant and gorgeous object that we knew so well. If you could -see him at all, he would merely seem like a star, not nearly so bright -as many of those you can see at night. Even if you had the biggest -of telescopes to aid your vision, you could never discern from one -of these bodies the planets which surround the sun. No astronomer in -the stars could see Jupiter even if his sight were a thousand times -as good or his telescopes a thousand times as powerful as any sight -or telescope that we know. So minute an object as our earth would, of -course, be still more hopelessly beyond the possibility of vision. - - -THE NUMBERS OF THE STARS. - -To count the stars involves a task which lies beyond the power of man -to accomplish. Even without the aid of any telescope, we can see a -great multitude of stars from this part of the world. There are also -many constellations in the southern hemisphere which never appear -above our horizon. If, however, we were to go to the equator, then, -by waiting there for a twelve-month, all the stars in the heavens -would have been successively exposed to view. An astronomer, Houzeau, -with the patience to count them, enumerated about 6000. This is the -naked-eye estimate of the star-population of the heavens; but if, -instead of relying on unaided vision, you get the assistance of a -little telescope, you will be astounded at the enormous multitude of -stars which are disclosed. - -[Illustration: FIG. 82.--The Great Bear and the Pole.] - -An ordinary opera-glass or binocular is a very useful instrument for -looking at the stars in the heavens. If you employ an instrument of -this sort, you will be amazed to find that the heavens teem with -additional hosts of stars that your unaided vision would never have -given you knowledge of. Any part of the sky may be observed; but, just -to give an illustration, I shall take one special region, namely, that -of the Great Bear (Fig. 82). The seven well-known stars are here shown, -four of which form a sort of oblong, while the other three represent -the tail. I would like you to make this little experiment. On a fine -clear night, count how many stars there are within this oblong; they -are all very faint, but you will be able to see a few, and, with good -sight, and on a clear night, you may see perhaps ten. Next take your -opera-glass and sweep it over the same region; if you will carefully -count the stars it shows, you will find fully 200; so that the -opera-glass has, in this part of the sky, revealed nearly twenty times -as many stars as could be seen without its aid. As 6000 stars can be -seen by the eye all over the heavens, we may fairly expect that twenty -times that number--that is to say, 120,000 stars--could be shown by the -opera-glass over the entire sky. Let us go a step further, and employ -a telescope, the object-glass of which is three inches across. This is -a useful telescope to have, and, if a good one, will show multitudes -of pleasing objects, though an astronomer would not consider it very -powerful. An instrument like this, small enough to be carried in the -hand, has been applied to the task of enumerating the stars in the -northern half of the sky, and 320,000 stars were counted. Indeed, -the actual number that might have been seen with it is considerably -greater, for when the astronomer Argelander made this memorable -investigation he was unable to reckon many of the stars in localities -where they lay very close together. This grand count only extended to -half the sky, and, assuming that the other half is as richly inlaid -with stars, we see that a little telescope like that we have supposed -will, when swept over the heavens, reveal a number of stars which -exceeds that of the population of any city in England except London. It -exhibits more than one hundred times as many stars as our eyes could -possibly reveal. Still, we are only at the beginning of the count; the -very great telescopes add largely to the number. There are multitudes -of stars which in small instruments we cannot see, but which are -distinctly visible from our great observatories. That telescope would -be still but a comparatively small one which would show as many stars -in the sky as there are people living in this mighty city of London; -and with the greatest instruments, the tale of stars has risen to a -number far greater than that of the entire population of Great Britain. - -In addition to those stars which the largest telescopes show us, there -are myriads which make their presence evident in a wholly different -way. It is only in quite recent times that an attempt has been made to -develop fully the powers of photography in representing the celestial -objects. On a photographic plate which has been exposed to the sky in -a great telescope the stars are recorded by thousands. Many of these -may, of course, be observed with a good telescope, but there are not a -few others which no one ever saw in a telescope, which apparently no -one ever could see, though the photograph is able to show them. We do -not, however, employ a camera like that which the photographer uses who -is going to take your portrait. The astronomer’s plate is put into his -telescope, and then the telescope is turned towards the sky. On that -plate the stars produce their images, each by its own light. Some of -these images are excessively faint, but we give a very long exposure -of an hour or two hours; sometimes as much as four hours’ exposure -is given to a plate so sensitive that a mere fraction of a second -would sufficiently expose it during the ordinary practice of taking a -photograph in daylight. We thus afford sufficient time to enable the -fainter objects to indicate their presence upon the sensitive film. -Even with an exposure of a single hour a picture exhibiting 16,000 -stars has been taken by Mr. Isaac Roberts, of Liverpool. Yet the -portion of the sky which it represents is only one ten-thousandth part -of the entire heavens. It should be added that the region which Mr. -Roberts has photographed is furnished with stars in rather exceptional -profusion. - -Here, at last, we have obtained some conception of the sublime scale on -which the stellar universe is constructed. Yet even these plates cannot -represent all the stars that the heavens contain. We have every reason -for knowing that with larger telescopes, with more sensitive plates, -with more prolonged exposures, ever fresh myriads of stars will be -brought within our view. - -You must remember that every one of these stars is truly a sun, a -lamp, as it were, which doubtless gives light to other objects in its -neighborhood as our sun sheds light upon this earth and the other -planets. In fact, to realize the glories of the heavens you should try -to think that the brilliant points you see are merely the luminous -points of the otherwise invisible universe. - -Standing one fine night on the deck of a Cunarder we passed in open -ocean another great Atlantic steamer. The vessel was near enough for -us to see not only the light from the mast-head but also the little -beams from the several cabin ports; and we could see nothing of the -ship herself. Her very existence was only known to us by the twinkle -of these lights. Doubtless her passengers could see, and did see, the -similar lights from our own vessel, and they probably drew the correct -inference that these lights indicated a great ship. - -Consider the multiplicity of beings and objects in a ship: the captain -and the crew, the passengers, the cabins, the engines, the boats, -the rigging, and the stores. Think of all the varied interests there -collected and then reflect that out on the ocean, at night, the sole -indication of the existence of this elaborate structure was given by -the few beams of light that happened to radiate from it. Now raise your -eyes to the stars; there are the twinkling lights. We cannot see what -those lights illuminate, we can only conjecture what untold wealth of -non-luminous bodies may also lie in their vicinity; we may, however, -feel certain that just as the few gleaming lights from a ship are -utterly inadequate to give a notion of the nature and the contents of -an Atlantic steamer, so are the twinkling stars utterly inadequate to -give even the faintest conception of the extent and the interest of -the universe. We merely see self-luminous bodies, but of the multitudes -of objects and the elaborate systems of which these bodies are only -the conspicuous points we see nothing and we know very little. We are, -however, entitled to infer from an examination of our own star--the -sun--and of the beautiful system by which it is surrounded, that these -other suns may be also splendidly attended. This is quite as reasonable -a supposition as that a set of lights seen at night on the Atlantic -Ocean indicates the existence of a fine ship. - - -THE CLUSTERS OF STARS. - -On a clear night you can often see, stretching across the sky, a track -of faint light, which is known to astronomers as the “Milky Way.” It -extends below the horizon and then round the earth to form a girdle -about the heavens. When we examine the Milky Way with a telescope we -find, to our amazement, that it consists of myriads of stars, so small -and so faint that we are not able to distinguish them individually; we -merely see the glow produced from their collective rays. Remembering -that our sun is a star, and that the Milky Way surrounds us, it would -almost seem as if our sun were but one of the host of stars which form -this cluster. - -[Illustration: FIG. 83.--Globular Cluster in the Centaur.] - -There are also other clusters of stars, some of which are exquisitely -beautiful telescopic spectacles. I may mention a celebrated pair of -these objects which lies in the constellation of Perseus. The sight of -them in a great telescope is so imposing that no one who is fit to -look through a telescope could resist a shout of wonder and admiration -when first they burst on his view. But there are other clusters. Here -is a picture of one which is known as the “Globular Cluster in the -Centaur” (Fig. 83). It consists of a ball of stars, so far off that, -however large these several suns may actually be, they have dwindled -down to extremely small points of light. A homely illustration may -serve to show the appearance which a globular cluster presents in a -good telescope. I take a pepper-castor and on a sheet of white paper -I begin to shake out the pepper until there is a little heap at the -centre and other grains are scattered loosely about. Imagine that -every one of those grains of pepper was to be transformed into a tiny -electric light, and then you have some idea of what a cluster of stars -would look like when viewed through a telescope of sufficient power. -There are multitudes of such groups scattered through the depths of -space. They require our biggest telescopes to show them adequately. -We have seen that our sun is a star, being only one of a magnificent -cluster that form the Milky Way. We have also seen that there are other -groups scattered through the length and depth of space. It is thus we -obtain a notion of the rank which our earth holds in the scheme of -things celestial. - - -THE RANK OF THE EARTH AS A GLOBE IN SPACE. - -Let me give an illustration with the view of explaining more fully -the nature of the relation which the earth bears to the other globes -which abound through space, and you must allow me to draw a little -upon my imagination. I shall suppose that Her Majesty’s mails extend -not only over this globe, but that they also communicate with other -worlds; that postal arrangements exist between Mars and the earth, -between the sun and Orion--in fact, everywhere throughout the whole -extent of the universe. We shall consider how our letters are to be -addressed. Let us take the case of Mr. John Smith, merchant, who lives -at 1001, Piccadilly; and let us suppose that Mr. John Smith’s business -transactions are of such an extensive nature that they reach not only -all over this globe, but away throughout space. I shall suppose that -the firm has a correspondent residing--let us say in the constellation -of the Great Bear; and when this man of business wants to write to Mr. -Smith from these remote regions, what address must he put upon the -letter, so that the Postmaster-General of the universe shall make no -mistake about its delivery? He will write as follows:-- - - MR. JOHN SMITH, - 1001 Piccadilly, - London, - England, - Europe, - Earth, - Near the Sun, - Milky Way, - The Universe. - -Let us now see what the several lines of this address mean. Of course -we put down the name of Mr. John Smith in the first line, and then -we will add “1001 Piccadilly” for the second; but as the people in -the Great Bear are not likely to know where Piccadilly is, we shall -add “London” underneath. As even London itself cannot be well known -everywhere, it is better to write “England.” This would surely find -Mr. John Smith from any post-office on this globe. From other globes, -however, the supreme importance of England may not be so immediately -recognized, and therefore it is as well to add another line, “Europe.” -This ought to be sufficient, I think, for any post-office in the solar -system. Europe is big enough to be visible from Mars or Venus, and -should be known to the post-office people there, just as we know and -have names for the continents on Mars. But further away there might -be a little difficulty; from Uranus and Neptune the different regions -on our earth can never have been distinguished, and therefore we must -add another line to indicate the particular globe of the solar system -which contains Europe. Mark Twain tells us that there was always -one thing in astronomy which specially puzzled him, and that was to -know how we found out the names of the stars. We are, of course, in -hopeless ignorance of the name by which this earth is called among -other intelligent beings elsewhere who can see it. I can only adopt -the title of “Earth,” and therefore I add this line. Now our address -is so complete that from anywhere in the solar system--from Mercury, -from Jupiter, or Neptune--there ought to be no mistake about the letter -finding its way to Mr. John Smith. But from his correspondent in the -Great Bear this address would be still incomplete; they cannot see our -earth from there, and even the sun himself only looks like a small -star--like one, in fact, of thousands of stars elsewhere. However, each -star can be distinguished, and our sun may, for instance, be recognized -from the Great Bear by some designation. We shall add the line “Near -the Sun,” and then I think that from this constellation, or from any -of the other stars around us, the address of Mr. John Smith may be -regarded as complete. But Mr. Smith’s correspondence may be still -wider. He may have an agent living in the cluster of Perseus or on some -other objects still fainter and more distant; then “Near the Sun” is -utterly inadequate as a concluding line to the address, for the sun, -if it can be seen at all from thence, will be only of the significance -of an excessively minute star, no more to be designated by a special -name than are each of the several leaves on the trees of a forest. What -this distant correspondent will be acquainted with is not the earth or -the sun, but only the cluster of stars among which the sun is but a -unit. Again we use our own name to denote the cluster, and we call it -the “Milky Way.” When we add this line, we have made the address of Mr. -John Smith as complete as circumstances will permit. I think a letter -posted to him anywhere ought to reach its destination. To perfect it, -however, we will finish up with one line more--“_The Universe_.” - - -THE DISTANCES OF THE STARS. - -I must now tell you something about the distances of the stars. I -shall not make the attempt to explain fully how astronomers make such -measurements, but I will give you some notion of how it is done. You -may remember I showed you how we found the distance of a globe that -was hung from the ceiling. The principle of the method for finding the -distance of a star is somewhat similar, except that we make the two -observations not from the two ends of a table, not even from opposite -sides of the earth, but from two opposite points on the earth’s orbit, -which are therefore at a distance of 186,000,000 miles. Imagine that on -Midsummer Day, when standing on the earth here, I measure with a piece -of card the angle between the star and the sun. Six months later, on -Midwinter Day, when the earth is at the opposite point of its orbit, -I again measure the angle between the same star and the sun, and we -can now determine the star’s distance by making a triangle. I draw a -line a foot long, and we will take this foot to represent 186,000,000 -miles, the distance between the two stations; then placing the cards -at the corners, I rule the two sides and complete the triangle, and the -star must be at the remaining corner; then I measure the sides of the -triangle, and find how many feet they contain, and recollecting that -each foot corresponds to 186,000,000 miles, we discover the distance of -the star. If the stars were comparatively near us, the process would be -a very simple one; but, unfortunately, the stars are so extremely far -off that this triangle, even with a base of only one foot, must have -its sides many miles long. Indeed, astronomers will tell you that there -is no more delicate or troublesome work in the whole of their science -than that of discovering the distance of a star. - -In all such measurements we take the distance from the earth to the -sun as a conveniently long measuring-rod, whereby to express the -results. The nearest stars are still hundreds of thousands of times -as far off as the sun. Let us ponder for a little on the vastness of -these distances. We shall first express them in miles. Taking the sun’s -distance to be 93,000,000 miles, then the distance of the nearest fixed -star is about twenty millions of millions of miles--that is to say, -we express this by putting down a 2 first, and then writing thirteen -ciphers after it. It is, no doubt, easy to speak of such figures, but -it is a very different matter when we endeavor to imagine the awful -magnitude which such a number indicates. I must try to give some -illustrations which will enable you to form a notion of it. At first I -was going to ask you to try and count this number, but when I found it -would require at least 300,000 years, counting day and night without -stopping, before the task was over, it became necessary to adopt some -other method. - -When on a visit in Lancashire I was once kindly permitted to visit a -cotton mill, and I learned that the cotton yarn there produced in a -single day would be long enough to wind round this earth twenty-seven -times at the equator. It appears that the total production of cotton -yarn each day in all the mills together would be on the average about -155,000,000 miles. In fact, if they would only spin about one-fifth -more, we could assert that Great Britain produced enough cotton yarn -every day to stretch from the earth to the sun and back again! It is -not hard to find from these figures how long it would take for all the -mills in Lancashire to produce a piece of yarn long enough to reach -from our earth to the nearest of the stars. If the spinners worked -as hard as ever they could for a year, and if all the pieces were -then tied together, they would extend to only a small fraction of the -distance; nor if they worked for ten years, or for twenty years, would -the task be fully accomplished. Indeed, upwards of 400 years would be -necessary before enough cotton could be grown in America and spun in -this country to stretch over a distance so enormous. All the spinning -that has ever yet been done in the world has not formed a long enough -thread! - -There is another way in which we can form some notion of the immensity -of these sidereal distances. You will recollect that, when we were -speaking of Jupiter’s moons (p. 219), I told you of the beautiful -discovery which their eclipses enabled astronomers to make. It was -thus found that light travels at the enormous speed of about 185,000 -miles per second. It moves so quickly that within a single second a ray -would flash two hundred times from London to Edinburgh and back again. - -We said that a meteor travels one hundred times as swiftly as a -rifle-bullet; but even this great speed seems almost nothing when -compared with the speed of light, which is 10,000 times as great. -Suppose some brilliant outbreak of light were to take place in a -distant star--an outbreak which would be of such intensity that the -flash from it would extend far and wide throughout the universe. -The light would start forth on its voyage with terrific speed. Any -neighboring star which was at a distance of less than 185,000 miles -would, of course, see the flash within a second after it had been -produced. More distant bodies would receive the intimation after -intervals of time proportional to their distances. Thus, if a body -were 1,000,000 miles away the light would reach it in from five to six -seconds, while over a distance as great as that which separates the -earth from the sun the news would be carried in about eight minutes. We -can calculate how long a time must elapse ere the light shall travel -over a distance so great as that between the star and our earth. You -will find that from the nearest of the stars the time required for the -journey will be over three years. Ponder on all that this involves. -That outbreak in the star might be great enough to be visible here, -but we could never become aware of it till three years after it had -happened. When we are looking at such a star to-night we do not see it -as it is at present, for the light that is at this moment entering -our eyes has travelled so far that it has been three years on the way. -Therefore, when we look at the star now we see it as it was three years -previously. In fact, if the star were to go out altogether, we might -still continue to see it twinkling for a period of three years longer, -because a certain amount of light was on its way to us at the moment -of extinction, and so long as that light keeps arriving here, so long -shall we see the star showing as brightly as ever. When, therefore, you -look at the thousands of stars in the sky to-night, there is not one -that you see as it is now, but as it was years ago. - -I have been speaking of the stars that are nearest to us, but there -are others much farther off. It is true we cannot find the distance -of these more remote objects with any degree of accuracy, but we -can convince ourselves how great that distance is by the following -reasoning. Look at one of the brightest stars. Try to conceive that the -object was carried away further into the depths of space, until it was -ten times as far from us as it is at present, it would still remain -bright enough to be recognized in quite a small telescope; even if it -were taken to one hundred times its original distance it would not have -withdrawn from the view of a good telescope; while if it retreated -one thousand times as far as it was at first it would still be a -recognizable point in our mightiest instruments. Among the stars which -we can see with our telescopes, we feel confident there must be many -from which the light has expended hundreds of years, or even thousands -of years, on the journey. When, therefore, we look at such objects, we -see them, not as they are now, but as they were ages ago; in fact, a -star might have ceased to exist for thousands of years, and still be -seen by us every night as a twinkling point in our great telescopes. - -Remembering these facts, you will, I think, look at the heavens with a -new interest. There is a bright star, Vega or Alpha Lyræ, a beautiful -gem, so far off that the light from it which now reaches our eyes -started before many of my audience were born. Suppose that there are -astronomers residing on worlds amid the stars, and that they have -sufficiently powerful telescopes to view this globe, what do you think -they would observe? They will not see our earth as it is at present, -they will see it as it was years, and sometimes many years, ago. There -are stars from which, if England could now be seen, the whole of the -country would be observed at this present moment to be in a great -state of excitement at a very auspicious event. Distant astronomers -might notice a great procession in London, and they could watch the -coronation of a youthful queen amid the enthusiasm of a nation. There -are other stars still further, from which, if the inhabitants had good -enough telescopes, they would now see a mighty battle in progress not -far from Brussels. One splendid army could be beheld hurling itself -time after time against the immovable ranks of the other. They would -not, indeed, be able to hear the ever-memorable, “Up, Guards, and at -them!” but there can be no doubt that there are stars so far away that -the rays of light which started from the earth on the day of the -battle of Waterloo are only just arriving there. Further off still, -there are stars from which a bird’s-eye view could be taken at this -very moment of the signing of Magna Charta. There are even stars from -which England, if it could be seen at all, would now appear, not as the -great England we know, but as a country covered by dense forests, and -inhabited by painted savages, who waged incessant war with wild beasts -that roamed through the island. The geological problems that now puzzle -us would be quickly solved could we only go far enough into space and -had we only powerful enough telescopes. We should then be able to view -our earth through the successive epochs of past geological time; we -should be actually able to see those great animals whose fossil remains -are treasured in our museums tramping about over the earth’s surface, -splashing across its swamps, or swimming with broad flippers through -its oceans. Indeed, if we could view our own earth reflected from -mirrors in the stars, we might still see Moses crossing the Red Sea, or -Adam and Eve being expelled from Eden. - -So important is the subject of star distance that I am tempted to give -one more illustration in order to bring before you some conception of -how vast such distances are. I shall take, as before, the nearest of -the stars so far as known to us, and I hope to be forgiven for taking -an illustration of a practical and a commercial kind instead of one -more purely scientific. I shall suppose that a railway is about to -be made from London to Alpha Centauri. The length of that railway, -of course, we have already stated: it is twenty billions of miles. -So I am now going to ask your attention to the simple question as to -the fare which it would be reasonable to charge for the journey. We -shall choose a very cheap scale on which to compute the price of a -ticket. The parliamentary rate here is, I believe, a penny for every -mile. We will make our interstellar railway fares much less even than -this; we shall arrange to travel at the rate of one hundred miles for -every penny. That, surely, is moderate enough. If the charges were so -low that the journey from London to Edinburgh only cost fourpence, -then even the most unreasonable passenger would be surely contented. -On these terms how much do you think the fare from London to this -star ought to be? I know of one way in which to make our answer -intelligible. There is a National Debt with which your fathers are, -unhappily, only too well acquainted; you will know quite enough about -it yourselves in those days when you have to pay income tax. This debt -is so vast that the interest upon it is about sixty thousand pounds -a day, the whole amount of the National Debt being six hundred and -thirty-eight millions of pounds (April, 1898). - -If you went to the booking office with the whole of this mighty sum -in your pocket--but stop a moment; could you carry it in your pocket? -Certainly not, if it were in sovereigns. You would find that after you -had as many sovereigns as you could conveniently carry there would -still be some left--so many, indeed, that it would be necessary to get -a cart to help you on with the rest. When the cart had as great a load -of sovereigns as the horse could draw there would be still some more, -and you would have to get another cart; but ten carts, twenty carts, -fifty carts, would not be enough. You would want five thousand of these -before you would be able to move off towards the station with your -money. When you did get there and asked for a ticket at the rate of one -hundred miles for a penny, do you think you would get any change? No -doubt some little time would be required to count the money, but when -it was counted the clerk would tell you that there was not enough, that -he must have nearly two hundred millions of pounds more. - -That will give some notion of the distance of the nearest star, and we -may multiply it by ten, by one hundred, and even by one thousand, and -still not attain to the distance of some of the more remote stars that -the telescope shows us. - -On account of the immense distances of the stars we can only perceive -them to be mere points of light. We can never see a star to be a globe -with marks on it like the moon, or like one of the planets--in fact, -the better the telescope the smaller does the star seem, though, -of course, its brightness is increased with every addition to the -light-grasping power of the instrument. - - -THE BRIGHTNESS AND COLOR OF STARS. - -Another point to be noticed is the arrangement of stars in classes, -according to their lustre. The brightest stars, of which there are -about twenty, are said to be of the first magnitude. Those just -inferior to the first magnitude are ranked as the second; and those -just lower than the second are estimated as the third; and so on. The -smallest points that your unaided eyes will show you are of about the -sixth magnitude. Then the telescope will reveal stars still fainter and -fainter, down to what we term the seventeenth or eighteenth magnitudes, -or even lower still. The number of stars of each magnitude increases -very much in the classes of small ones. - -Most of the stars are white, but many are of a somewhat ruddy hue. -There are a few telescopic points which are intensely red, some exhibit -beautiful golden tints, while others are blue or green. - -There are some curious stars which regularly change their brilliancy. -Let me try to illustrate the nature of these variables. Suppose that -you were looking at a street gas-lamp from a very long distance, so -that it seemed a little twinkling light; and suppose that some one was -preparing to turn the gas-cock up and down. Or, better still, imagine -a little machine which would act regularly so as to keep the light -first of all at its full brightness for two days and a half, and then -gradually turn it down until in three or four hours it declines to a -feeble glimmer. In this low state the light remains for twenty minutes; -then during three or four hours the gas is to be slowly turned on again -until it is full. In this condition the light will remain for two days -and a half, and then the same series of changes is to recommence. This -would be a very odd form of gas-lamp. There would be periods of two -days and a half during which it would remain at its full; these would -be separated by intervals of about seven hours, when the gradual -turning down and turning up again would be in progress. - -[Illustration: FIG. 84.--Perseus and its Neighboring Stars, including -Algol.] - -The imaginary gas-lamp is exactly paralleled by a star Algol, in the -constellation of Perseus (Fig. 84), which goes through the series of -changes I have indicated. Ordinarily speaking, it is a bright star of -the second magnitude, and, whatever be the cause, the star performs -its variations with marvellous uniformity. In fact, Algol has always -arrested the attention of those who observed the heavens, and in early -times was looked on as the eye of a Demon. There are many other stars -which also change their brilliancy. Most of them require much longer -periods than Algol, and sometimes a new star which nobody has ever -seen before will suddenly kindle into brilliancy. It is now known that -the bright star Algol is attended by a dark companion. This dark star -sometimes comes between Algol and the observer and cuts off the light. -Thus it is that the diminution of brightness is produced. - - -DOUBLE STARS. - -Whenever you have a chance of looking at the heavens through a -telescope, you should ask to be shown what is called _a double star_. -There are many stars in the heavens which present no remarkable -appearance to the unaided eye, but which a good telescope at once -shows to be of quite a complex nature. These are what we call double -stars, in which two quite distinct stars are placed so close together -that the unaided eye is unable to separate them. Under the magnifying -power of the telescope, however, they are seen to be distinct. In -order to give some notion of what these objects are like, I shall -briefly describe three of them. The first lies in that best known of -constellations, the Great Bear. If you look at his tail, which consists -of three stars, you will see that near the middle one of the three a -small star is situated; we call this little star Alcor, but it is the -brighter one near Alcor to which I specially call your attention. The -sharpest eye would never suspect that it was composed of two stars -placed close together. Even a small telescope will, however, show this -to be the case, and this is the easiest and the first observation that -a young astronomer should make when beginning to turn a telescope to -the heavens. Of course, you will not imagine that I mean Alcor to be -the second component of the double star; it is the bright star near -Alcor which is the double. Here are two marbles, and these marbles are -fastened an inch apart. You can see them, of course, to be separate; -but if the pair were moved further and further away, then you would -soon not be able to distinguish between them, though the actual -distance between the marbles had not altered. Look at these two wax -tapers which are now lighted; the little flames are an inch apart. You -would have to view them from a station a third of a mile away if the -distance between the two flames were to appear the same as that between -the two components of this double star. Your eye would never be able -to discriminate between two lights only an inch apart at so great a -distance; a telescope would, however, enable you to do so, and this is -the reason why we have to use telescopes to show us double stars. - -You might look at that double star year after year throughout the -course of a long life without finding any appreciable change in the -relative positions of its components. But we know that there is no -such thing as rest in the universe; even if you could balance a body -so as to leave it for a moment at rest, it would not stay there, for -the simple reason that all the bodies round it in every direction are -pulling at it, and it is certain that the pull in one direction will -preponderate, so that move it must. Especially is this true in the case -of two suns like those forming a double star. Placed comparatively near -each other they could not remain permanently in that position; they -must gradually draw together and come into collision with an awful -crash. There is only one way by which such a disaster could be averted. -That is by making one of these stars revolve around the other just as -the earth revolves around the sun, or the moon revolves around the -earth. Some motion must, therefore, be going on in every genuine double -star, whether we have been able to see that motion or not. - -Let us now look at another double star of a different kind. This time -it is in the constellation of Gemini. The heavenly twins are called -Castor and Pollux. Of these, Castor is a very beautiful double star, -consisting of two bright points, a great deal closer together than were -those in the Great Bear; consequently a better telescope is required -for the purpose of showing them separately. Castor has been watched -for many years, and it can be seen that one of these stars is slowly -revolving around the other; but it takes a very long time, amounting -to hundreds of years, for a complete circuit to be accomplished. This -seems very astonishing, but when you remember how exceedingly far -Castor is, you will perceive that that pair of stars which appear so -close together that it requires a telescope to show them apart must -indeed be separated by hundreds of millions of miles. Let us try to -conceive our own system transformed into a double star. If we took -our outermost planet--Neptune--and enlarged him a good deal, and then -heated him sufficiently to make him glow like a sun, he would still -continue to revolve round our sun at the same distance, and thus a -double star would be produced. An inhabitant of Castor who turned his -telescope towards us would be able to see the sun as a star. He would -not, of course, be able to see the earth, but he might see Neptune like -another small star close to the sun. If generations of astronomers in -Castor continued their observations of our system, they would find a -binary star, of which one component took a century and a half to go -round the other. Need we then be surprised that when we look at Castor -we observe movements that seem very slow? - -There is often so much diffused light about the bright stars seen in -a telescope, and so much twinkling in some states of the atmosphere, -that stars appear to dance about in rather a puzzling fashion, -especially to one who is not accustomed to astronomical observations. -I remember hearing how a gentleman once came to visit an observatory. -The astronomer showed him Castor through a powerful telescope as a fine -specimen of a double star, and then, by way of improving his little -lesson, the astronomer mentioned that one of these stars was revolving -around the other. “Oh, yes,” said the visitor, “I saw them going round -and round in the telescope.” He would, however, have had to wait for a -few centuries with his eye to the instrument before he would have been -entitled to make this assertion. - -Double stars also frequently delight us by giving beautifully -contrasted colors. I dare say you have often noticed the red and the -green lights that are used on railways in the signal lamps. Imagine one -of those red and one of those green lights away far up in the sky and -placed close together, then you would have some idea of the appearance -that a colored double star presents, though, perhaps, I should add that -the hues in the heavenly bodies are not so vividly different as are -those which our railway people find necessary. There is a particularly -beautiful double star of this kind in the constellation of the Swan. -You could make an imitation of it by boring two holes, with a red-hot -needle, in a piece of card, and then covering one of these holes with a -small bit of the topaz-colored gelatine with which Christmas crackers -are made. The other star is to be similarly colored with blue gelatine. -A slide made on this principle placed in the lantern gives a very good -representation of these two stars on the screen. There are many other -colored doubles besides this one; and, indeed, it is noteworthy that -we hardly ever find a blue or a green star by itself in the sky; it is -always as a member of one of these pairs. - - -HOW WE FIND WHAT THE STARS ARE MADE OF. - -Here is a piece of stone. If I wanted to know what it was composed -of, I should ask a chemist to tell me. He would take it into his -laboratory, and first crush it into powder, and then, with his test -tubes, and with the liquids which his bottles contain, and his -weighing scales, and other apparatus, he will tell all about it; -there is so much of this, and so much of that, and plenty of this, and -none at all of that. But now, suppose you ask this chemist to tell -you what the sun is made of, or one of the stars. Of course, you have -not a sample of it to give him; how, then, can he possibly find out -anything about it? Well, he can tell you something, and this is the -wonderful discovery that I want to explain to you. We now put down the -gas, and I kindle a brilliant red light. Perhaps some of those whom -I see before me have occasionally ventured on the somewhat dangerous -practice of making fireworks. If there is any boy here who has ever -constructed sky-rockets, and put the little balls into the top which -are to burn with such vivid colors when the explosion takes place, he -will know that the substance which tinged that red fire must have been -_strontium_. He will recognize it by the color; because _strontium_ -gives a red light which nothing else will give. Here are some of these -lightning papers, as they are called; they are very pretty and very -harmless; and these, too, give brilliant red flashes as I throw them. -The red tint has, no doubt, been produced by _strontium_ also. You -see we recognized the substance simply by the color of the light it -produced when burning. - -Perhaps some of you have tried to make a ghost at Christmas by dressing -up in a sheet, and bearing in your hand a ladle blazing with a mixture -of common salt and spirits of wine, the effect produced being a most -ghastly one. Some mammas will hardly thank me for this suggestion, -unless I add that the ghost must walk about cautiously, for otherwise -the blazing spirit would be very apt to produce conflagrations of a -kind more extensive than those intended. However, by the kindness of -Professor Dewar, I am enabled to show the phenomenon on a splendid -scale, and also free from all danger. I kindle a vivid flame of an -intensely yellow color, which I think the ladies will unanimously -agree is not at all becoming to their complexions, while the pretty -dresses have lost their variety of colors. Here is a nice bouquet, -and yet you can hardly distinguish the green of the leaves from the -brilliant colors of the flowers, except by trifling differences of -shade. Expose to this light a number of pieces of variously colored -ribbon, pink and red and green and blue, and their beauty is gone; and -yet we are told that this yellow is a perfectly pure color; in fact, -the purest color that can be produced. I think we have to be thankful -that the light which our good sun sends us does not possess purity -of that description. There is one substance which will produce that -yellow light; it is a curious metal called sodium--a metal so soft -that you can cut it with a knife, and so light that it will float on -water; while, still more strange, it actually takes fire the moment it -is dropped on the water. It is only in a chemical laboratory that you -will be likely to meet with the actual metallic sodium, yet in other -forms the substance is one of the most abundant in nature. Indeed, -common salt is nothing but sodium closely united with a most poisonous -gas, a few respirations of which would kill you. But this strange metal -and this noxious gas, when united, become simply the salt for our eggs -at breakfast. This pure yellow light, wherever it is seen, either in -the flame of spirits of wine mixed with salt or in that great blaze at -which we have been looking, is characteristic of sodium. Wherever you -see that particular kind of light, you know that sodium must have been -present in the body from which it came. - -We have accordingly learned to recognize two substances, namely, -_strontium_ and _sodium_, by the different lights which they give out -when burning. To these two metals we may add a third. Here is a strip -of white metallic ribbon. It is called magnesium. It seems like a bit -of tin at the first glance, but indeed it is a very different substance -from tin; for, look, when I hold it in the spirit-lamp, the strip of -metal immediately takes fire, and burns with a white light so dazzling -that it pales the gas-flames to insignificance. There is no other -substance which will, when kindled, give that particular kind of light -which we see from magnesium. I can recommend this little experiment -as quite suitable for trying at home; you can buy a bit of magnesium -ribbon for a trifle at the optician’s; it cannot explode or do any -harm, nor will you get into any trouble with the authorities provided -you hold it when burning over a tray or a newspaper, so as to prevent -the white ashes from falling on the carpet. - -There are, in nature, a number of simple bodies called elements. Every -one of these, when ignited under suitable conditions, emits a light -which belongs to it alone, and by which it can be distinguished from -every other substance. I do not say that we can try the experiments in -the simple way I have here indicated. Many of the materials will yield -light which will require to be studied by much more elaborate artifices -than those which have sufficed for us. But you see that the method -affords a means of finding out the actual substances present in the -sun or in the stars. There is a practical difficulty in the fact that -each of the heavenly bodies contains a number of different elements; so -that in the light it sends us the hues arising from distinct substances -are blended into one beam. The first thing to be done is to get some -way of splitting up a beam of light, so as to discover the components -of which it is made. You might have a skein of silks of different hues -tangled together, and this would be like the sunbeam as we receive it -in its unsorted condition. How shall we untangle the light from the -sun or a star? I will show you by a simple experiment. Here is a beam -from the electric light; beautifully white and bright, is it not? It -looks so pure and simple, but yet that beam is composed of all sorts of -colors mingled together, in such proportions as to form white light. I -take a wedge-shaped piece of glass called a prism, and when I introduce -it into the course of the beam, you see the transformation that has -taken place (Fig. 85). Instead of the white light you have now all -the colors of the rainbow--red, orange, yellow, green, blue, indigo, -violet, marked by their initial letters in the figure. These colors are -very beautiful, but they are transient, for the moment we take away -the prism they all unite again to form white light. You see what the -prism has done; it has bent all the light in passing through it; but it -is more effective in bending the blue than the red, and consequently -the blue is carried away much further than the red. Such is the way in -which we study the composition of a heavenly body. We take a beam of -its light, we pass it through a prism, and immediately it is separated -into its components; then we compare what we find with the lights -given by the different elements, and thus we are enabled to discover -the substances which exist in the distant object whose light we have -examined. I do not mean to say that the method is a simple one; all -I am endeavoring to show is a general outline of the way in which we -have discovered the materials present in the stars. The instrument that -is employed for this purpose is called the spectroscope. And perhaps -you may remember that name by these lines, which I have heard from an -astronomical friend:-- - -[Illustration: FIG. 85.--How to split up a Ray of Light.] - - “Twinkle, twinkle, little star, - Now we find out what you are, - When unto the midnight sky, - We the spectroscope apply.” - -I am sure it will interest everybody to know that the elements which -the stars contain are not altogether different from those of which the -earth is made. It is true there may be substances in the stars of which -we know nothing here; but it is certain that many of the most common -elements on the earth are present in the most distant bodies. I shall -only mention one, the metal iron. That useful substance has been found -in some of the stars which lie at almost incalculable distances from -the earth. - - -THE NEBULÆ. - -In drawing towards the close of these lectures I must say a few words -about some dim and mysterious objects to which we have not yet alluded. -They are what are called nebulæ, or little clouds; and in one sense -they are justly called little, for each of them occupies but a very -small spot in the sky as compared with that which would be filled by -an ordinary cloud in our air. The nebulæ are, however, objects of the -most stupendous proportions. Were our earth and thousands of millions -of bodies quite as big all put together, they would not be nearly so -great as one of these nebulæ. Astronomers reckon up the various nebulæ -by thousands, but I must add that most of them are apparently faint and -uninteresting. A nebula is sometimes liable to be mistaken for a comet. -The comet is, as I have already explained, at once distinguished by the -fact that it is moving and changing its appearance from hour to hour, -while scores of years elapse without changes in the aspect or position -of a nebula. The most powerful telescopes are employed in observing -these faint objects. I take this opportunity of showing a picture of an -instrument suitable for such observations. It is the great reflector of -the Paris Observatory (Fig. 87). - -[Illustration: FIG. 86.--The Ring Nebula in Lyra, under Different -Telescopic Powers.] - -There are such multitudes of nebulæ that I can only show a few of the -more remarkable kinds. In Fig. 86 will be seen pictures of a curious -object in the constellation of Lyra seen under different telescopic -powers. This is a gigantic ring of luminous gas. To judge of the size -of this ring let us suppose that a railway were laid across it, and -the train you entered at one side was not to stop until it reached -the other side, how long do you think this journey would require? -I recollect some time ago a picture in _Punch_ which showed a train -about to start from London to Brighton, and the guard walking up and -down announcing to the passengers the alarming fact that “this train -stops nowhere.” An old gentleman was seen vainly gesticulating out of -the window and imploring to be let out ere the frightful journey was -commenced. In the nebular railway the passengers would almost require -such a warning. - -[Illustration: FIG. 87.--A Great Reflecting Telescope.] - -Let the train start at a speed of a mile a minute, you would think, -surely, that it must soon cross the ring. But the minutes pass, an -hour has elapsed; so the distance must be sixty miles, at all events. -The hours creep on into days, the days advance into years, and still -the train goes on. The years would lengthen out into centuries, and -even when the train had been rushing on for a thousand years with an -unabated speed of a mile a minute, the journey would certainly not have -been completed. Nor do I venture to say what ages must elapse ere the -terminus at the other side of the ring nebula would be reached. - -A cluster of stars viewed in a small telescope will often seem like a -nebula, for the rays of the stars become blended. A powerful telescope -will, however, dispel the illusion and reveal the separate stars. It -was, therefore, thought that all the nebulæ might be merely clusters -so exceedingly remote that our mightiest instruments failed to resolve -them into stars. But this is now known not to be the case. Many of -these objects are really masses of glowing gas; such are, for instance, -the ring nebulæ, of which I have just spoken, and the form of which I -can simulate by a pretty experiment. - -[Illustration: FIG. 88.--How to make the Smoke-rings.] - -We take a large box with a round hole cut in one face, and a canvas -back at the opposite side. I first fill this box with smoke, and there -are different ways of doing so. Burning brown paper does not answer -well, because the supply of smoke is too irregular and the paper -itself is apt to blaze. A little bit of phosphorus set on fire yields -copious smoke, but it would be apt to make people cough, and, besides, -phosphorus is a dangerous thing to handle incautiously, and I do not -want to suggest anything which might be productive of disaster if the -experiment was repeated at home. A little wisp of hay, slightly damped -and lighted, will safely yield a sufficient supply, and you need not -have an elaborate box like this; any kind of old packing-case, or even -a band-box with a duster stretched across its open top and a round hole -cut in the bottom, will answer capitally. While I have been speaking, -my assistant has kindly filled this box with smoke, and in order to -have a sufficient supply, and one which shall be as little disagreeable -as possible, he has mixed together the fumes of hydrochloric acid and -ammonia from two retorts shown in Fig. 88. A still simpler way of doing -the same thing is to put a little common salt in a saucer and pour -over it a little oil of vitriol; this is put into the box, and over -the floor of the box common smelling-salts is to be scattered. You see -there are dense volumes of white smoke escaping from every corner of -the box. I uncover the opening and give a push to the canvas, and you -see a beautiful ring flying across the room; another ring and another -follow. If you were near enough to feel the ring, you would experience -a little puff of wind; I can show this by blowing out a candle which -is at the other end of the table. These rings are made by the air -which goes into a sort of eddy as it passes through the hole. All the -smoke does is to render the air visible. The smoke-ring is indeed -quite elastic. If we send a second ring hurriedly after the first, we -can produce a collision, and you see each of the two rings remains -unbroken, though both are quivering from the effects of the blow. They -are beautifully shown along the beam of the electric lamp, or, better -still, along a sunbeam. - -We can make many experiments with smoke-rings. Here, for instance, I -take an empty box, so far as smoke is concerned, but air-rings can be -driven forth from it, though you cannot see them, but you can feel them -even at the other side of the room, and they will, as you see, blow out -a candle. I can also shoot invisible air-rings at a column of smoke, -and when the missile strikes the smoke it produces a little commotion -and emerges on the other side, carrying with it enough of the smoke to -render itself visible, while the solid black looking ring of air is -seen in the interior. Still more striking is another way of producing -these rings, for I charge this box with ammonia, and the rings from -it you cannot see. There is a column of the vapor of hydrochloric -acid that also you cannot see; but when the invisible ring enters the -invisible column, then a sudden union takes place between the vapor -of the ammonia and the vapor of the hydrochloric acid; the result is -a solid white substance in extremely fine dust which renders the ring -instantly visible. - - -WHAT THE NEBULÆ ARE MADE OF. - -There is a fundamental difference between the illumination of these -little rings that I have shown you and the great rings in the heavens. -I had to illuminate our smoke with the help of the electric light, -for, unless I had done so, you would not have been able to see them. -This white substance formed by the union of ammonia and hydrochloric -acid has, of course, no more light of its own than a piece of chalk; -it requires other light falling upon it to make it visible. Were the -ring nebula in Lyra composed of this material, we could not see it. -The sunlight which illuminates the planets might, of course, light up -such an object as the ring, if it were comparatively near us; but Lyra -is at such a stupendous distance that any light which the sun could -send out there would be just as feeble as the light we receive from a -fixed star. Should we be able to show our smoke-rings, for instance, -if, instead of having the electric light, I merely cut a hole in the -ceiling and allowed the feeble twinkle of a star in the Great Bear to -shine through? In a similar way the sunbeams would be utterly powerless -to effect any illumination of objects in these stellar distances. If -the sun were to be extinguished altogether, the calamity would no doubt -be a very dire one so far as we are concerned, but the effect on the -other celestial bodies (moon and planets excepted) would be of the -slightest possible description. All the stars of heaven would continue -to shine as before. Not a point in one of the constellations would be -altered, not a variation in the brightness, not a change in the hue of -any star could be noticed. The thousands of nebulæ and clusters would -be absolutely unaltered; in fact, the total extinction of the sun would -be hardly remarked in the newspapers published in the Pleiades or in -Orion. There might possibly be a little line somewhere in an odd corner -to the effect “Mr. So-and-So, our well-known astronomer, has noticed -that a tiny star, inconspicuous to the eye, and absolutely of no -importance whatever, has now become invisible.” - -If, therefore, it be not the sun which lights up this nebula, where -else can be the source of its illumination? There can be no other star -in the neighborhood adequate to the purpose, for, of course, such an -object would be brilliant to us if it were large enough and bright -enough to impart sufficient illumination to the nebula. It would be -absurd to say that you could see a man’s face by the light of a candle -while the candle itself was too faint or too distant to be visible. -The actual facts are, of course, the other way; the candle might be -visible, when it was impossible to discern the face which it lighted. - -Hence we learn that the ring nebula must shine by some light of its -own, and now we have to consider how it can be possible for such -material to be self-luminous. The light of a nebula does not seem to -be like flame; it can, perhaps, be better represented by the pretty -electrical experiment with Geissler’s tubes. These are glass vessels -of various shapes, and they are all very nearly empty, as you will -understand when I tell you the way in which they have been prepared. A -little gas was allowed into each tube, and then almost all the gas was -taken out again, so that only a mere trace was left. I pass a current -of electricity through these tubes, and now you see they are glowing -with beautiful colors. The different gases give out lights of different -hues, and the optician has exerted his skill so as to make the effect -as beautiful as possible. The electricity, in passing through these -tubes, heats the gas which they contain, and makes it glow; and just -as this gas can, when heated sufficiently, give out light, so does the -great nebula, which is a mass of gas poised in space, become visible in -virtue of the heat which it contains. - -We are not left quite in doubt as to the constitution of these gaseous -nebulæ, for we can submit their light to the prism in the way I -explained when we were speaking of the stars. Distant though that ring -in Lyra may be, it is interesting to learn that the ingredients from -which it is made are not entirely different from substances we know -on our earth. The water in this glass, and every drop of water, is -formed by the union of two gases, of which one is hydrogen. This is an -extremely light material, as you see by a little balloon which ascends -so prettily when filled with it. Hydrogen also burns very readily, -though the flame is almost invisible. When I blow a jet of oxygen -through the hydrogen, I produce a little flame with a very intense -heat. For instance, I hold a steel pen in the flame, and it glows and -sputters, and falls down in white-hot drops. It is needless to say -that, as a constituent of water, hydrogen is one of the most important -elements on this earth. It is, therefore, of interest to learn that -hydrogen in some form or other is a constituent of the most distant -objects in space that the telescope has revealed. - - -PHOTOGRAPHING THE NEBULÆ. - -Of late years we have learned a great deal about nebulæ, by the help -which photography has given to us. Look at this group of stars which -constitutes that beautiful little configuration known as the Pleiades -(Fig. 89). It looks like a miniature representation of the Great -Bear; in fact, it would be far more appropriate to call the Pleiades -the Little Bear than to apply that title to another quite different -constellation, as has unfortunately been done. The Pleiades form a -group containing six or seven stars visible to the ordinary eye, though -persons endowed with exceptionally good vision can usually see a few -more. In an opera-glass the Pleiades becomes a beautiful spectacle, -though in a large telescope the stars appear too far apart to make a -really effective cluster. When Mr. Roberts took a photograph of the -Pleiades he placed a highly sensitive plate in his telescope, and on -that plate the Pleiades engraved their picture with their own light. -He left the plate exposed for hours, and on developing it not only -were the stars seen, but there were also patches of faint light due to -the presence of nebula. It could not be said that the objects on the -plate were fallacious, for another photograph was taken, when the same -appearances were reproduced. - -[Illustration: FIG. 89.--The Pleiades.] - -When we look at that pretty group of stars which has attracted -admiration during all time, we are to think that some of those stars -are merely the bright points in a vast nebula, invisible to our unaided -eyes or even to our mighty telescopes, though capable of recording -its trace on the photographic plate. Does not this give us a greatly -increased notion of the extent of the universe, when we reflect that -by photography we are enabled to see much which the mightiest of -telescopes had previously failed to disclose? - -Of all the nebulæ, now numbering some thousands, there is but a single -one which can be seen without a telescope. It is in the constellation -of Andromeda, and on a clear dark night can just be seen with the -unaided eye as a faint stain of light on the sky. It has happened -before now that persons noticing this nebula for the first time have -thought they had discovered a comet. I would like you to try and find -out this object for yourselves. - -If you look at it with an opera-glass it appears to be distinctly -elongated. You can see more of its structure when you view it in larger -instruments, but its nature was never made clear until some beautiful -photographs were taken by Mr. Roberts (Fig. 90). Unfortunately, the -nebula in Andromeda has not been placed in the best position for its -portrait from our point of view. It seems as if it were a rather -flat-shaped object, turned nearly edgewise towards us. To look at the -pattern on a plate, you would naturally hold the plate so as to be -able to look at it squarely. The pattern would not be seen well if -the plate were so tilted that its edge was turned towards you. That -seems to be nearly the way in which we are forced to view the nebula -in Andromeda. We can trace in the photograph some divisions extending -entirely round the nebula, showing that it seems to be formed of a -series of rings; and there are some outlying portions which form part -of the same system. Truly this is a marvellous object. It is impossible -for us to form any conception of the true dimensions of this gigantic -nebula; it is so far off that we have never yet been able to determine -its distance. Indeed, I may take this opportunity of remarking that -no astronomer has yet succeeded in ascertaining the distance of any -nebula. Everything, however, points to the conclusion that they are at -least as far as the stars. - -[Illustration: FIG. 90.--The Great Nebula in Andromeda. - -(_From Mr. Roberts’ Photograph._)] - -It is almost impossible to apply the methods which we use in finding -the distance of a star to the discovery of the distance of the -nebulæ. These flimsy bodies are usually too ill-defined to admit of -being measured with the precision and the delicacy required for the -determination of distance. The measurements necessary for this purpose -can only be made from one star-like point to another similar point. If -we could choose a star in the nebula and determine its distance, then, -of course, we should have the distance of the nebula itself; but the -difficulty is that we have, in general, no means of knowing whether the -star does actually lie in the object. It may, for anything we can tell, -lie billions of miles nearer to us, or billions of miles further off, -and, by merely happening to lie in the line of sight, appear to glimmer -in the nebula itself. - -[Illustration: FIG. 91.--To show how Small the Solar System is compared -with a Great Nebula.] - -If we have any assurance that the star is surrounded by a mass of -this glowing vapor, then it may be possible to measure that nebula’s -distance. It will occasionally happen that grounds can be found for -believing that a star which appears to be in the glowing gas does -veritably lie therein, and is not merely seen in the same direction. -There are hundreds of stars visible on a good drawing or a good -photograph of the famous object in Andromeda, and doubtless large -numbers of these are merely stars which happen to lie in the same line -of sight. The peculiar circumstances attending the history of one star -seem, however, to warrant us in making the assumption that it was -certainly in the nebula. The history of this star is a remarkable one. -It suddenly kindled from invisibility into brilliancy. How is a change -so rapid in the lustre of a star to be accounted for? In a few days -its brightness had undergone an extraordinary increase. Of course, -this does not tell us for certain that the star lay in the glowing -gas; but the most rational explanation that I have heard offered of -this occurrence is that due, I believe, to my friend Mr. Monck. He has -suggested that the sudden outbreak in brilliancy might be accounted for -on the same principles as those by which we explain the ignition of -meteors in our atmosphere. If a dark star, moving along with terrific -speed through space, were suddenly to plunge into a dense region of -the nebula, heat and light must be evolved in sufficient abundance to -transform the star into a brilliant object. If, therefore, we knew the -distance of this star at the time it was in Andromeda, we should, of -course, learn the distance of that interesting object. This has been -attempted, and it has thus been proved that the Great Nebula must -be very much further from us than is that star of whose distance I -attempted some time ago to give you a notion. - -We thus realize the enormous size of the Great Nebula. It appears that -if, on a map of this object, we were to lay down, accurately to scale, -a map of the solar system, putting the sun in the centre and all the -planets around in their true proportions out to the boundary traced by -Neptune, this area, vast though it is, would be a mere speck on the -drawing of the object. Our system would have to be enormously bigger -before it sufficed to cover anything like the area of the sky included -in one of these great objects. Here is a sketch of a nebula (Fig. 91), -and near it I have marked a dot which is to indicate our solar system. -We may feel confident that the Great Nebula is at the very least as -mighty as this proportion would indicate. - - -CONCLUSION. - -And now, my young friends, I am drawing near the close of that -course of lectures which has occupied us, I hope you will think -not unprofitably, for a portion of our Christmas holidays. We have -spoken of the sun and of the moon, of comets and of stars, and I have -frequently had occasion to allude to the relative position of our earth -in the universe. No doubt it is a noble globe which we inhabit, but I -have failed in my purpose if I have not shown you how insignificant -is this earth when compared with the vast extent of some of the other -bodies that abound in space. We have, however, been endowed with a -feeling of curiosity which makes us long to know of things beyond -the confines of our own earth. Astronomers can tell us a little, but -too often only a little. They will say--That is a star, and That is -a planet, and That is so big, and That so far; such is the meagre -style of information with which we often have to be content. The -astronomers who live on other worlds, if their faculties be in any -degree comparable with ours, must be similarly ignorant with regard to -this earth. Inhabitants of our fellow-planets can know hardly anything -more than that the earth on which we dwell is a globe 8000 miles -across, with many clouds around us. Some of the planets would not even -pay us the compliment of recognizing our existence; while from the -other systems--the countless other systems--of space we are absolutely -imperceptible and unknown. - -Out of all the millions of bodies which we can see, you could very -nearly count on your fingers those from which our earth would be -visible. This reflection is calculated to show us how vast must be -the real extent of that universe around us. Here is our globe, with -its inhabitants, with its great continents, with its oceans, with its -empires, its kingdoms, with its arts, its commerce, its literature, its -sciences, and yet it would seem that all these things are absolutely -unknown to any inhabitants that may exist elsewhere. I do not think -that any reasonable person will doubt that there must be inhabitants -elsewhere. There are millions of globes, many of them more splendid -than ours. Surely it would be presumptuous to say that this is the -only one of all the bodies in the universe on the surface of which -life, with all that life involves, is manifested. You will rather think -that our globe is but one in the mighty fabric, and that other globes -may teem with interest just as ours does. We can, of course, make no -conjecture as to what the nature of the life may be elsewhere. Could -a traveller visit some other globes and bring back specimens of the -natural objects that he found there, no collections that the world has -ever seen could rival them in interest. When I go into the British -Natural History Museum and look around that marvellous collection, -it awakens in me a feeling of solemnity. I see there the remains of -mighty extinct animals which once roamed over this earth; also objects -which have been dredged from the bottom of the sea at a depth of some -miles; there I can examine crystals which have required incalculable -ages for their formation; and there I look at meteorites which have -travelled from the heavens above down on to the earth beneath. Such -sights, and the reflections they awaken, bring before us in an imposing -manner the phenomena of our earth, and the extent and interest of its -past history. Oliver Wendell Holmes said that the only way to see the -British Museum was to take lodgings close by when you were a boy, and -to stay in the Museum from nine to five every day until you were an old -man; then you would begin to have some notion of what this Institution -contains. Think what millions of British Museums would be required were -the universe to be adequately illustrated: one museum for the earth, -another for Mars, another for Venus--but it would be useless attempting -to enumerate them! - -Most of us must be content with acquiring the merest shred of -information with regard even to our own earth. Perhaps a schoolboy -will think it fortunate that we are so ignorant with respect to the -celestial bodies. What an awful vista of lessons to be learned would -open before his view, if only we had a competent knowledge of the other -globes which surround us in space! I should like to illustrate the -extent of the universe by following this reflection a little further. I -shall just ask you to join with me in making a little calculation as to -the extent of the lessons you would have to learn if astronomers should -succeed in discovering some of the things they want to know. - -Of course, all of us learn geography and history. We must know the -geography of the leading countries of the globe, and we must have some -knowledge of their inhabitants and of their government, their resources -and their civilization. It would seem shockingly ignorant not to know -something about China, or not to have some ideas on the subject of -India or Egypt. The discovery of the New World also involves matters -on which every boy and girl has to be instructed. Supposing we were -so far acquainted with the other globes scattered through space that -we were able to gain some adequate knowledge of their geography and -natural history, of the creatures that inhabit them, of their different -products and climates, then everybody would be anxious to learn those -particulars; and even when the novelty had worn off, it would still be -right for us to know something about countries perhaps more populous -than China, about nations more opulent than our own, about battles -mightier than Waterloo, about animals and plants far stranger than -any we have ever dreamt of. An outline of all such matters should, of -course, be learned, and as the amount of information would be rather -extensive, we will try to condense it as much as possible. - -To aid us in realizing the full magnificence of that scheme in -the heavens of which we form a part, I shall venture to give an -illustration. Let us attempt to form some slight conception of the -number and of the bulk of the books which would be necessary for -conveying an adequate description of that marvellous universe of stars -which surround us. These stars being suns, and many of them being -brighter and larger than our own sun, it is but reasonable to presume -that they may be attended by planetary systems. I do not say that we -have any right to infer that such systems are like ours. It is not -improbable that many of the suns around us have a much poorer retinue -than that which dignifies our sun. On the other hand, it is just as -likely that many of these other suns may be the centres of systems -far more brilliant and interesting, with far greater diversity of -structure, with far more intensity and variety of life and intelligence -than are found in the system of which we form a part. It is only -reasonable for us to suppose that, as our earth is an average planet, -so our sun is an average star both in size and in the importance of its -attendants. We may take the number of stars in the sky at about one -hundred millions; and thus we see that the books which are to contain -a description of the entire universe--or rather, I should say, of the -entire universe that we see--must describe 100,000,000 times as much as -is contained in our single system. Of course, we know next to nothing -of what the books should contain; but we can form some conjecture of -the number of those books, and this is the notion to which I now ask -your attention. - -So vast is the field of knowledge that has to be traversed, that we -should be obliged to compress our descriptions into the narrowest -compass. We begin with a description of our earth, for nearly all -the books in the libraries that exist at this moment are devoted to -subjects connected with this earth. They include various branches -of history, innumerable languages and literatures and religions, -everything relating to life on this globe, to its history in past -geological times, to its geography, to its politics, to every variety -of manufacture and agriculture, and all the innumerable matters which -concern our earth’s inhabitants, past and present. But this tremendous -body of knowledge must be much condensed before it would be small -enough to retire to its just position in the great celestial library. I -can only allow to the earth one volume of about 500 pages. Everything -that has to be said about our earth must be packed within this compass. -All terrestrial languages, histories, and sciences that cannot be -included between its covers can find no other place on our shelves. I -cannot spare any more room. Our celestial library will be big enough, -as you shall presently see. I am claiming a good deal for our earth -when I regard it as one of the most important bodies in the solar -system. Of course it is not the biggest--very far from it; but it seems -as if the big planets and the sun were not likely to be inhabited, so -that if we allow one other volume to the rest of the solar system, it -will perhaps be sufficient, though it must be admitted that Venus, of -which we know next to nothing, except that it is as large as the earth, -may also be quite as full of life and interest. Mars and Mercury are -also among the planets with possible inhabitants. We are, therefore, -restricting the importance of the solar system as much as possible, -perhaps even too much, by allowing it two. Within those two volumes -every conceivable thing about the entire solar system--sun, planets -(great and small), moons, comets, and meteors--must be included, or -else it would not be represented at all in the great celestial library. - -We shall deal on similar principles with the other systems through -space. Each of the 100,000,000 stars will have two volumes allotted -to it. Within the two volumes devoted to each star we must compress -our description of the body itself and of the system which surrounds -it; the planets, their inhabitants, histories, arts, sciences, and all -other information. I am not, remember, discussing the contents, but -only the number of books we should have to read ere we could obtain -even the merest outline of the true magnificence of the heavens. Let -us try to form some estimate as to the kind of library that would be -required to accommodate 200,000,000 volumes. I suppose a long straight -hall, so lofty that there could be fifty shelves of books on each side. -As you enter you look on the right hand and on the left, and you see -it packed from floor to ceiling with volumes. We have arranged them -according to the constellations. All the shelves in one part contain -the volumes relating to the worlds in the Great Bear, while upon the -other side may repose ranks upon ranks of volumes relating to the -constellation of Orion. - -I shall suppose that the volumes are each about an inch and a half -thick, and as there are fifty shelves on each side, you will easily -see that for each foot of its length the hall will accommodate 800 -books. We can make a little calculation as to the length of this -library, which, as we walk down through it, stretches out before us -in a majestic corridor, with books, books everywhere. Let us continue -our stroll, and as we pass by we find the shelves on both sides packed -with their thousands of volumes; and we walk on and on, and still see -no end to the vista that ever opens before us. In fact, no building -that was ever yet constructed would hold this stupendous library. Let -the hall begin on the furthest outskirts of the west of London, carry -it through the heart of the city, and away to the utmost limits of -the east--not a half of the entire books could be accommodated. The -mighty corridor would have to be fifty miles long, and to be packed -from floor to ceiling with fifty shelves of books on each side, if it -is to contain even this very inadequate description of the contents -of the visible universe. Imagine the solemn feelings with which we -should enter such a library, could it be created by some miracle! As -we took down one of the volumes, with what mysterious awe should we -open it, and read therein of some vast world which eye had never -seen! There we might learn strange problems in philosophy, astonishing -developments in natural history; with what breathless interest we -should read of inhabitants of an organization utterly unknown to our -merely terrestrial experience! Notwithstanding the vast size of the -library, the description of each globe would have to be very scanty. -Thus, for instance, in the single book which referred to the earth I -suppose a little chapter might be spared to an island called England, -and possibly a page or so to its capital, London. Similarly meagre -would have to be the accounts of the other bodies in the universe; and -yet, for this most inadequate of abstracts, a library fifty miles long, -and lined closely with fifty shelves of books on each side, would be -required! - -Methuselah lived, we are told, nine hundred and sixty-nine years; but -even if he had attained his thousandth birthday he would have had to -read about 300 of these books through every day of his life before he -accomplished the task of learning even the merest outline about the -contents of space. - -If, indeed, we were to have a competent knowledge of all these other -globes, of all their countries, their geographies, their nations, their -climates, their plants, their animals, their sciences, languages, arts -and literatures, it is not a volume, or a score of volumes, that would -be required, but thousands of books would have to be devoted to the -description of each world alone, just as thousands of volumes have been -devoted to the affairs of this earth without exhausting the subjects -of interest it presents. Hundreds of thousands of libraries, each -as large as the British Museum, would not contain all that should be -written, were we to have anything like a detailed description of the -universe _which we see_. I specially emphasize the words just written, -and I do so because the grandest thought of all, and that thought -with which I conclude, brings before us the overwhelming extent of -the unseen universe. Our telescopes can, no doubt, carry our vision -to an immeasurable distance into the depths of space. But there are, -doubtless, stars beyond the reach of our mightiest telescopes. There -are stars so remote that they cannot record themselves on the most -sensitive of photographic plates. - -On the blackboard I draw a little circle with a piece of chalk. I think -of our earth as the centre, and this circle will mark for us the limit -to which our greatest telescopes can sound. Every star which we see, -or which the photographic plate sees, lies within this circle; but, -are there no stars outside? It is true that we can never see them, -but it is impossible to believe that space is utterly void and empty -where it lies beyond the view of our telescopes. Are we to say that -inside this circle stars, worlds, nebulæ, and clusters are crowded, and -that outside there is nothing? Everything teaches us that this is not -so. We occasionally gain accession to our power by adding perhaps an -inch to the diameter of our object-glass, or by erecting a telescope -in an improved situation on a lofty mountain peak, or by procuring a -photographic plate of increased sensibility. It thus happens that we -are enabled to extend our vision a little further and to make this -circle a little larger, and thus to add a little more to the known -inside which has been won from the unknown outside. Whenever this is -done we invariably find that the new region thus conquered is also -densely filled with stars, with clusters, and with nebulæ; it is thus -unreasonable to doubt that the rest of space also contains untold -myriads of objects, even though they may never, by any conceivable -improvement in our instruments, be brought within the range of our -observation. Reflect that this circle is comparatively small with -respect to the space outside. It occupied but a small spot on this -blackboard, the blackboard itself occupies only a small part of the -end of the theatre, while the end of the theatre is an area very small -compared with that of London, of England, of the world, of the solar -system, of the actual distance of the stars. In a similar way the -region of space which is open to our inspection is an inconceivably -small portion of the entire extent of space. The unknown outside is -so much larger than the known inside, it is impossible to express the -proportion. I write down unity in this corner and a cipher after it -to make ten, and six ciphers again to make ten millions, and again, -six ciphers more to make ten billions; but I might write six more, -ay, I might cover the whole of this blackboard with ciphers, and even -then I should not have got a number big enough to express how greatly -the extent of the space we cannot see exceeds that of the space we -see. If, therefore, we admit the fact, which no reasonable person can -doubt, that this outside, this unknown, this unreachable and, to us, -invisible space does really contain worlds and systems as does this -small portion of space in which we happen to be placed--then, indeed, -we shall begin truly to comprehend the majesty of the universe. What -figures are to express the myriads of stars that should form a suitable -population for a space inconceivably greater than that which contains -100,000,000 stars? But our imagination will extend still further. It -brings before us these myriads of unseen stars with their associated -worlds, it leads us to think that these worlds may be full to the brim -with interests as great as those which exist on our world. When we -remember that, for an adequate description of the worlds which we can -see, one hundred thousand libraries, each greater than any library on -earth, would be utterly insufficient, what conception are we to form -when we now learn that even this would only amount to a description of -an inconceivably small fragment of the entire universe? - -Let us conceive that omniscience granted to us an adequate revelation -of the ample glories of the heavens, both in that universe which we -do see and in that infinitely greater universe which we do not see. -Let a full inventory be made of all those innumerable worlds, with -descriptions of their features and accounts of their inhabitants and -their civilizations, their geology and their natural history, and all -the boundless points of interest of every kind which a world in the -sense in which we understand it does most naturally possess. Let those -things be written every one, then may we say that were this whole earth -of ours covered with vast buildings, lined from floor to ceiling with -book-shelves--were every one of these shelves stored full with volumes, -yet, even then this library would be inadequate to receive the books -that would be necessary to contain a description of the glories of the -sidereal heavens. - - - - -CONCLUDING CHAPTER. - -HOW TO NAME THE STARS. - - -Every one who wishes to learn something about astronomy should -make a determined effort to become acquainted with the principal -constellations, and to find out the names of the brighter and more -interesting stars. I have therefore added to STAR-LAND this little -chapter, in which I have tried to make the study of the stars so simple -that, by taking advantage of a few clear nights, there ought to be no -difficulty in obtaining a knowledge of a few constellations. - -The first step is to become familiar with the Great Bear, or Ursa -Major, as astronomers more generally call the group. We begin with -this, because after it has been once recognized, then you will find -it quite easy to make out the other constellations and stars. It may -save you some trouble if you can get some one to point out to you -the Great Bear; but even without such aid, I think you will be able -to make out the seven bright stars which form this remarkable group, -from the figure here given (Fig. 92). Of course, the position of this -constellation, as of every other in the heavens, changes with the hour -of the night, and changes with the seasons of the year. About April -the constellation at 11 o’clock at night is high over your head. In -September at the same hour, the Great Bear is low down in the north. -It is to be seen in the west in July, and at Christmas it lies in -the east at convenient hours in the evening for observation. One of -the advantages of using the Great Bear as the foundation of our study -of the stars arises from the fact that to an observer in the British -Islands or in similar latitudes this group never sets. Whenever the sky -is clear after nightfall, the Great Bear is to be seen somewhere, while -the brightness of its component stars makes it a conspicuous object. -Indeed, there is only one constellation in the sky, namely, that of -Orion, which is a more brilliant group than the Great Bear. We shall -tell you about Orion presently, but it does not suit to begin with, -because it can only be seen in winter, and is then placed very low down -in the heavens. - -[Illustration: FIG. 92.--The Great Bear and the Pole Star.] - -Your next lesson will be to utilize the Great Bear for the purpose of -pointing out the Pole Star. Look at the two stars marked α and β. They -are called the “Pointers,” because if you follow the direction they -indicate along the dotted line in the figure, they will conduct your -glance to the Pole. This is the most important star in the heavens to -astronomers, because it happens to mark very nearly the position of -the Pole on the sky. You will easily note the peculiarity of the Pole -Star if you will look at it two or three times in the course of the -night. It will appear to remain in the same place in the sky, while the -other stars change their places from hour to hour. It is very fortunate -that we have a star like this in the northern heavens; the astronomers -in Australia or New Zealand can see no bright star lying near the -Southern Pole which will answer the purposes that the Pole Star does so -conveniently for us in the north. - -The Pole Star belongs to a constellation which we call the Little Bear; -two other conspicuous members of this group are the two “Guards”; you -will see how they are situated from Fig. 82, p. 322. They lie nearly -midway between the Pole Star and the last of the three stars which -form the Great Bear’s Tail. The same figure will also introduce us to -another beautiful constellation, namely, Cassiopeia. You will never -find any difficulty in identifying the figure that marks this group if -you will notice that the Pole lies midway between it and the Great Bear. - -[Illustration: FIG. 93.--The Great Square of Pegasus.] - -Cassiopeia is also one of the constellations that never set to British -observers; but now we have to speak of groups which do set, and which, -therefore, can only be observed when the proper season comes round. The -first of these is “the Great Square of Pegasus”; you cannot see this -group conveniently in the spring or summer, but during the autumn and -winter it is well placed after nightfall. There are four conspicuous -stars forming the corners of the square, and then three others marked -α, γ, and β (Fig. 93), which form a sort of handle to the square. In -fact, if you once recognize this group, you will perhaps see in it a -resemblance to a great saucepan with a somewhat bent handle, and then -you will be acquainted with a large tract of Star-land near the Square -of Pegasus. From the figure you will see that a line imagined to be -drawn from the Pole Star over the end of Cassiopeia, and then produced -as far again, will just lead to the Great Square. I have also marked on -this figure two objects that are of great telescopic interest; one of -them is the Nebula in Andromeda, of which we had an account in the last -lecture. You see it lies halfway between the corner α of the square and -the group of Cassiopeia. Another interesting object is the star marked -γ Andromedæ. The telescope shows it to consist of a pair of stars, the -colors of which are beautifully contrasted. - -At the end of this handle to the Great Square of Pegasus is the star -α, in the constellation of Perseus. It lies between two other stars γ -and δ. We refer to Fig. 84, in which these stars are shown. We there -employed the figure to indicate the position of Algol, the remarkable -variable star. Your map will also point out some other important -stellar features. If we curve round the three marked γ, α, and δ of -Perseus, the eye is conducted to Capella, a gem of the first magnitude -in the constellation of Auriga. Close to Capella is a long triangle, -the corners of which are the “Hœdi,” the three kids--which Capella is -supposed to nurture. - -If we carry a curve through γ, α, δ, of Perseus, and now bend it in the -opposite way, the eye is led through ε and ζ in the same constellation, -and then on to the Pleiades, of which we have already spoken. - -[Illustration: FIG. 94.--Orion and Sirius.] - -Perseus lies in one of the richest parts of the heavens. The Milky -Way stretches across the group, and the sky is strewn with stars beyond -number. Even an opera-glass directed to this teeming constellation -cannot fail to afford the observer a delightful glimpse of celestial -scenery. - -We may, however, specially remind the beginner that the objects on -this map are not always to be seen, and as an illustration of the way -in which the situation and the visibility of the constellations are -affected by the time of year, I shall take the case of the Pleiades and -follow them through a season. Let us suppose that we make a search for -this group at 11 P.M. every night. On the 1st of January, the Pleiades -will be found high up in the sky in the southwest. On the 1st of March, -they will be setting in the west at the same hour. On the 1st of May, -the Pleiades are not visible, neither are they on the 1st of July. On -the 1st of September, they will be seen low down in the east. On the -1st of November, they will be high in the heavens in the southeast. On -the ensuing 1st of January, the Pleiades will be found back in the same -place which they occupied on the same date in the preceding year, and -so on throughout the cycle. Of course, you will not suppose that their -changes are due to actual motions in the group of stars themselves. -They are merely apparent, and are to be explained by the motion of the -earth round its axis, and the revolution around the sun. - -Next we are to become acquainted with the glory of our winter skies, -the constellation of Orion, Fig. 94. I dare say many of my readers are -already familiar with the well-known twin stars which form the belt of -Orion, but if not, they will be able to recognize it by the help of the -groups already learned. Imagine a line drawn from the Pole Star through -Capella, and then produced as much further again, and we shall be -conducted into the precincts of Orion. This group lies on the equator, -and, consequently, it is equally familiar to southern astronomers and -to those of the north. It can be best seen by those who observe it from -or near the equator. - -The brightest star in Orion is known either as α Orionis or as -Betelgeuze, by which name it is represented in the figure. This star -is of the first magnitude, and so is Rigel on the opposite side of the -belt. The three stars of the belt and the two others, γ and κ, at which -they point above and below, are of second magnitude. - -The owner of a telescope finds especial attractions in this -constellation. Notably among the subjects which will interest him is -the Great Nebula, the position of which is indicated in our figure. -Under the middle of the belt are a few stars, around which is a hazy -light that is perceptible with the smallest telescopic aid. Viewed -by instruments of adequate proportions, these have developed into a -marvellous nebula of glowing gas, attaining to dimensions so vast that -no one has yet ever attempted to estimate them. - -The vicinity of Orion is also enriched with some of the most -interesting stellar objects. Follow the line of the belt upwards to -the right, and your eye is conducted to a ruddy first magnitude star -named Aldebaran, in the constellation of the Bull. This is a pleasing -object, which the beginner will sometimes be apt to confuse with the -planet Mars, to which, under certain circumstances, it certainly bears -a resemblance. Another very pleasing little group, known as the Hyades, -will be found near Aldebaran. If the line of the belt of Orion be -carried down to the left, it will be found to point to Sirius, or the -Dog Star. - -You will find it an interesting occupation to make for yourself maps -of small parts of the heavens. First copy out the chief stars in their -proper places from the star atlas, and then fill in the smaller stars -with your own observations. Try first on some limited region of the -heavens; take the figure of Cassiopeia, for instance, or the Square -of Pegasus, and see if you can produce a fair representation of those -groups by marking in the stars that your instrument will show you; -or take the Pleiades and make a tracing of the principal stars of -the group from the sketch that we have given (Fig. 89), then take an -opera-glass and fill in as carefully as you can all that it will show. -I can assure you that you will find a little definite work of this kind -full of interest and instruction. - -But I hope you will desire to advance further in the study of the -heavens. It is to be remembered that with even the most moderate -instruments there is much to be done. Many comets have been detected, -and many planets have been discovered, by the use of telescopes so -small that they could be easily carried out from the house for the -evening’s work and brought back again after the observations were over. - -It remains for me to add a few words which will help you in finding -the planets. It is, of course, impossible to represent such objects -as Jupiter, Saturn, Venus, Mars, and Mercury on maps of the heavens, -because these bodies are constantly moving about, and if their places -were right to-day they would be wrong to-morrow. The almanac will be -necessary for you here. You must find out by its help what planets are -visible and in what part of the sky they are placed. Then you will -have to compare your maps with the heavens, and when you find a bright -star-like body that is not shown on your maps you may conclude at once -that it is the planet. Although these objects are so star-like to the -unaided eye, yet the resemblance is at once dispelled when we use a -telescope. The star is only a bright point of light and white, the -planet shows a visible shape. This is, at least, the case with the five -planets I have named; for there are others, such as Uranus and Neptune, -which are too far to be much more than star-like points in ordinary -telescopes. The minor planets would not interest you. - -I hope that the reading of STAR-LAND will, at all events, induce you to -make a beginning of the study of the heavens, if you have not already -done so. If you have the advantage of a telescope, so much the better; -but, if this is impossible, make the best use of your own eyes. Do not -put it off or wait till you get some one to teach you. If it be clear -this very night, go out and find the Great Bear and the Pole Star, and -as many of the other constellations as you can, and at once commence -your career as an astronomer. - - - - -TABLE OF USEFUL ASTRONOMICAL FACTS. - - -The sun’s mean distance from the earth is 92,700,000 miles; his -diameter is 865,000 miles, and he rotates in a period between 25 and 26 -days. - -The moon’s mean distance from the earth is 238,000 miles; the diameter -of the moon is 2160 miles, and the time of revolution round the earth -is 27.322 days. - - -THE PLANETS. - - --------+---------------+---------------+-----------+----------------- - | Mean Distance | Periodic Time | Diameter | - | from the Sun | of Revolution | of Planet | Axial Rotation. - | in Millions | in Days. | in Miles. | - | of Miles. | | | - --------+---------------+---------------+-----------+----------------- - | | | | Hrs. Mins. Secs. - Mercury | 35.9 | 87.969 | 2,992 | Uncertain. - Venus | 67.0 | 224.70 | 7,660 | Uncertain. - Earth | 92.7 | 365.26 | 7,918 | 23 56 4.09 - Mars | 141 | 686.98 | 4,200 | 24 37 22.7 - Jupiter | 482 | 4,332.6 | 85,000 | 9 55 -- - Saturn | 884 | 10,759 | 71,000 | 10 14 23.8 - Uranus | 1,780 | 30,687 | 31,700 | Unknown. - Neptune | 2,780 | 60,127 | 34,500 | Unknown. - --------+---------------+---------------+-----------+----------------- - - -THE SATELLITES OF MARS. - - ----------+--------------------+------------------- - Name. | Mean Distance from | Periodic Time. - | Centre of Mars. | - ----------+--------------------+------------------- - | | Hrs. Mins. Secs. - Phobos | 5,800 miles. | 7 39 14 - Deimos | 14,500 ” | 30 17 54 - ----------+--------------------+------------------- - - -THE SATELLITES OF JUPITER. - - ----------+--------------------+------------------------- - Name. | Mean Distance from | Periodic Time. - | Centre of Jupiter. | - ----------+--------------------+------------------------- - | | Days Hrs. Mins. Secs. - I | 262,000 miles. | 1 18 27 34 - II | 417,000 ” | 3 13 13 42 - III | 664,000 ” | 7 3 42 33 - IV | 1,170,000 ” | 16 16 32 11 - V | 112,400 ” | -- 11 57 (?) - ----------+--------------------+-------------------------- - - -THE SATELLITES OF SATURN. - - ----------+--------------------+------------------------- - Name. | Mean Distance from | Periodic Time. - | Centre of Saturn. | - ----------+--------------------+------------------------- - | | Days Hrs. Mins. Secs. - Mimas | 118,000 miles. | 0 22 37 27.9 - Enceladus | 152,000 ” | 1 8 53 6.7 - Tethys | 188,000 ” | 1 21 18 25.7 - Dione | 241,000 ” | 2 17 41 8.9 - Rhea | 337,000 ” | 4 12 25 10.8 - Titan | 781,000 ” | 15 22 41 25.2 - Hyperion | 946,000 ” | 21 7 7 40.8 - Iapetus | 2,280,000 ” | 79 7 54 40.4 - ----------+--------------------+------------------------- - -A ninth satellite was discovered in August, 1898, by Prof. W. H. -Pickering, but its mean distance and periodic time have not yet been -determined with precision. - - -THE SATELLITES OF URANUS. - - ----------+--------------------+--------------- - Name. | Mean Distance from | Periodic Time. - | Centre of Uranus. | Days. - ----------+--------------------+--------------- - Ariel | 119,000 miles. | 2.520383 - Umbriel | 166,000 ” | 4.144121 - Titania | 272,000 ” | 8.705897 - Oberon | 363,000 ” | 13.463269 - ----------+--------------------+--------------- - - -THE SATELLITE OF NEPTUNE. - - ----------+--------------------+--------------- - Name. | Mean Distance from | Periodic Time. - | Centre of Neptune. | Days. - ----------+--------------------+--------------- - Anonymous | 220,000 miles. | 5.87690 - ----------+--------------------+--------------- - - - - -INDEX. - - - A. - - Active Volcanoes, Number of, 112. - - Adams, of Cambridge, and Leverrier, of Paris, 249. - - Address of Mr. John Smith, 330. - - Africa would be better known if on Moon, 105. - - Air as Blanket to keep Earth Warm, 8. - - Air not Transparent, 125. - - ” Pump, 183. - - ” Resistance of, 182. - - Alcor, 343. - - Aldebaran, 389. - - Algol, 342, 385. - - Alpha Centauri, 319; - Railway to, 338. - - Ancient Theory to account for Rising and Setting of Sun, 48. - - Andromeda, 385. - - Andromeda, Nebula in, photographed by Mr. Roberts, 364. - - Andromedes, 310. - - Annual Motion of Earth round Sun, 56. - - Annular Eclipses, 88. - - Apparent Smallness of Distant Objects, 25. - - Appearance of the Sun, 35. - - Appearance of the Sun during a Total Eclipse, 40. - - Arctic Sun, 66. - - Area of Moon’s Surface, 82. - - Ariel, 243. - - ” Distance and Period of, 392. - - Arthur’s Seat, Volcano, 114. - - Asaph Hall, Professor, 195. - - Asteroids or Small Planets, 203. - - Astronomer and Mathematician, 144. - - Astronomers, How they measure the Distances of the Heavenly Bodies, 19. - - Astronomical Facts, Table of, 391. - - _Astronomie, l’_, A French Journal, 30. - - Athlete on Moon, 131. - - Atlantic, Sun dropped into, 48. - - Atmosphere of Moon, 125. - - Attraction of Gravitation, 119, 186. - - August Meteors, 309. - - Auriga, 385. - - Auvergne, Ancient Volcanoes in, 114. - - Awful Vista of Lessons, 371. - - Axis of Earth Constant in Direction, 69. - - - B. - - Babies on the Moon, 130. - - Balloon, How supported, 124. - - Bands on Saturn, 224. - - Bear, Great, 381; - Little, 383. - - Belts on Jupiter, 215. - - Benefits that we receive from the Sun, 10. - - Betelgeuze, 388. - - Biela’s Comet, 311. - - Blanket to keep Earth Warm, 8. - - Books, Number of, Necessary to describe Universe, 372. - - Brahe, Tycho, 170. - - Brightness of Saturn, 223. - - ” ” the Stars, 340. - - ” ” ” Sun, 1. - - Brighton Coach, 138. - - British Museum Collection of Meteorites, 313. - - British Natural History Museum, 370. - - “Brown Bess,” 68. - - Brussels, 328. - - Bull, Constellation of the, 388. - - Burning-glass, Experiment with, 3. - - Button illuminated, 152. - - - C. - - Cambridge Observations of Neptune, 251. - - Capella, 385. - - Carigou, Mount, in Pyrenees, 31. - - Caroline Island, 42. - - Cart-wheel, Measurements with, 173. - - Cassiopeia, 384. - - Castor and Pollux, 345. - - Celestial Library, 372. - - ” ” Size of, 373. - - Changes of the Seasons, 66. - - Changes of the Sun with the Seasons, 57. - - Charles I., 19. - - Christmas Time, What the Sun does for us at, 10. - - Clerk-Maxwell’s Top, 182. - - Clock to count Sun’s Distance, 18. - - Clouds a Form of Steam, 15. - - ” fill our Rivers, etc., 15. - - ” on Jupiter, 216. - - ” ” Mars, 191. - - ” ” Neptune, 253. - - ” ” Saturn, 224. - - ” ” Uranus, 243. - - Cluster in the Centaur, 328. - - Clusters of Stars, 327. - - Coal, Mode of Production of, 11. - - ” Whence came it?, 11. - - Coal-pit, 201. - - Cock and the Sun, 66. - - Codde, Marcus, Picture of Sunset, 30. - - Coldness of Mountain Tops, 8. - - Collier, Eye of a, 201. - - Color of Stars, 340. - - Columbiad Theory of Meteorites, 314. - - Comet attracted by Sun, 274. - - ” colliding with Earth, 281. - - ” Encke’s, 143, 259. - - Comet, Great, of September, 1882, 277. - - Comet, Halley’s, 263. - - ” Identification of a, 258. - - ” Materials of a, 276. - - ” Movements of a, 255. - - ” of Biela, 311. - - ” of 1861, 281. - - ” seen at Sun’s Edge, 278. - - ” Speed of a, 256. - - ” Weighing Scales for a, 276. - - Comets and Shooting Stars, 255. - - ” Disposition of Tails of, 257. - - ” Extravagance of, 283. - - ” Tails of, 281. - - Common, Mr., 278. - - Comparative Sizes of Planets, 139. - - Comparison of Solar System and Nebula, 367. - - Cooling of Earth and Moon, Illustration of, 116. - - Copenhagen, Residence of Tycho, 170. - - Corona of the Sun, 44. - - Cotton Yarn, 334. - - Crape Ring of Saturn, 225. - - Craters on the Moon, 108. - - Craters, Terrestrial and Lunar, compared, 113. - - Cricket on the Moon, 131. - - Cunarder’s Lights at Sea, 326. - - - D. - - Danish Hounds of Tycho Brahe, 172. - - Day and Night, 46, 51. - - Daylight, Stars seen in, 59. - - Deimos and Phobos, Satellites of Mars, 200. - - Deimos, Distance and Period of, 391. - - Desertion, Herschel’s, 231. - - Dewar, Professor, 53, 55, 187. - - Dictionary, Worcester’s, Use of, 269. - - Dione, Distance and Period of, 392. - - Direction of Earth’s Axis Constant, 69. - - Discoveries of Kepler, 174. - - ” ” Newton, 178. - - Disguised Stars, 205. - - Disposition of Comets’ Tails, 257. - - Distance of the Sun, 17, 210, 391. - - Distances of Heavenly Bodies, How measured, 19. - - Distances of Nebulæ, 365. - - ” ” the Stars, 332. - - Distant Objects, Apparent Smallness of, 25. - - Double Stars, 342; - Motion of, 344. - - D. Q., 210. - - Drawing of the Solar System, 135. - - Dufferin, Lord, 65. - - Dunsink Observatory, Telescope of, 95. - - Dust from Meteors, 292. - - - E. - - Eagle in West of Ireland, 27. - - Earth, Annual Motion of, 56. - - ” colliding with Comet, 281. - - ” Diameter of, 391. - - ” Distance of, from Sun, 391. - - Earth, History of, as seen from Stars, 337. - - Earth, Internal Heat of, 115. - - ” Moon-view of, 78. - - Earth, Period of, 391. - - ” Rank of, in Space, 329. - - ” Rotation of, 49, 391. - - ” viewed from Sun, 28. - - ” Visibility of, 369. - - Eclipse, Total, Appearances seen during, 40. - - Eclipses, How produced, 84. - - ” Annular, 88. - - ” of Jupiter’s Satellites, 219. - - ” ” Moon, 89. - - Edinburgh Castle, 114. - - Effect of Moon’s Distance on its Appearance, 89. - - Electric Lamp, Heating Effect of, 6. - - Ellipse, 167; - Importance of, 169. - - ” and Parabola, 169, 275. - - Elliptic Paths of Planets, 169. - - Enceladus, Distance and Period of, 392. - - Encke’s Comet, 143, 259; - Periodicity of, 260. - - Eros, 210. - - Eternal Snow on Mountain Summits, 9. - - Euclid, 22. - - Evening Star, 151. - - Examining Moon, Quarter the Best Time for, 107. - - Experiment with Burning-glass, 3. - - Explosion of Krakatoa, 113. - - Express Train to the Sun, 19. - - Extinct Craters on the Earth, 114; - on the Moon, 114. - - Eye of a Collier, 201. - - Eyes, Use of Two, 19. - - - F. - - Facts, Table of Useful Astronomical, 391. - - Five o’Clock Tea, Sun’s Share in, 10. - - Finlay, Mr., 278. - - Flagstaff, Height of, 110. - - Flora, Lawn-tennis on, 209. - - Fly-boats on Royal Canal, Smooth Motion of, 52. - - Flying Machines, 208. - - Focus, 5; - of Ellipse, 170. - - Football on Moon, 131. - - Fossil Trees, 11. - - France, Extinct Craters in, 114. - - Friday, 135. - - - G. - - Galle of Berlin finds Neptune, 251. - - Geissler’s Tubes, 361. - - Geography of Mars, 188. - - ” ” the Moon, 105. - - George III. and Herschel, 238. - - Georgium Sidus, 240. - - Giant Planets, 212; - Orbits of, 213. - - Globe, Shadow of, 170. - - Grand Meteors, 295. - - Gravitation, 120, 186. - - ” on Moon, 128. - - ” ” Small Planets, 209. - - ” ” the Sun, 132. - - Great Bear, 161; - Number of Stars in, 323; - How to find, 381. - - Green, Mr., 193. - - Greenwich, 138. - - Grinding Specula, 235. - - “Guards,” 383. - - - H. - - Habitability of Moon, 123. - - ” ” Other Worlds, 133. - - Hall, Professor Asaph, 195. - - Halley’s Comet, 263; - Reappearance of, 266. - - Heat, Internal, of Earth, 115. - - ” of the Sun, 1. - - Heating Effect of Electric Lamp, 6. - - Height of Flagstaff, 110. - - ” ” India-rubber Ball, 19. - - ” ” Meteor, 296. - - ” ” the Sun, 57. - - Herschel, Caroline, 233, 239. - - ” William, 230. - - ” ” at Windsor, 238. - - Herschel’s Saturn, 239. - - “Hœdi,” or Kids, 385. - - Holmes, Oliver Wendell, 370. - - Hot Water and the Sun, 14. - - Houzeau, 322. - - How Astronomers measure the Distances of the Heavenly Bodies, 19. - - How Planets are weighed, 142. - - ” to find the Planets, 390. - - ” ” name the Stars, 381. - - ” ” split up a Ray of Light, 351. - - Humming-top, 182. - - Hundreds of Thousands of Libraries required, 376. - - Hyperion, Distance and Period of, 392. - - - I. - - Iapetus, Distance and Period of, 392. - - Identification of Comets, 258. - - India-rubber Ball, Height of, 19. - - Inner Planets, The, 134. - - Insects, Leaf-like, 204. - - Institution, Royal, 53. - - Internal Heat of Earth, 115. - - Inventory of Worlds, 379. - - Iris of the Eye, 202. - - Iron Vapor, 291. - - Island, Caroline, 42. - - - J. - - Janssen’s Picture of Sun-spot, 36. - - Jupiter, 214. - - ” compared with Earth, 214. - - Jupiter’s Belts, 215. - - ” Clouds, 216. - - ” Diameter, 391. - - ” Distance, 391. - - ” Internal Heat, 218. - - Jupiter’s Period of Revolution, 212, 391. - - Jupiter’s Rotation, 215, 391. - - ” Satellites, 218. - - ” ” Eclipses of, 220. - - Jupiter’s Satellites, Distances and Periods of, 392. - - Jupiter’s Weight, 214. - - - K. - - Kaiser Sea, 192. - - Kepler, 174; - His Laws, 177, 186. - - Kilauea, Volcano, 112. - - Kirkwood’s Description of Meteor Shower, 1833, 301. - - Krakatoa Eruption, 113. - - - L. - - Lalande’s Observations of Neptune, 253. - - Latitude defined, 68. - - Lawn-tennis on Flora, 209. - - Law of Motion, First, 183. - - Laws of Kepler, 177. - - Leaf-like Insects, 204. - - Leonids, 308. - - ” Orbit of, 302. - - “Letters from High Latitudes,” 65. - - Leverrier, of Paris, and Adams, of Cambridge, 249. - - Life on the Moon, 123. - - ” ” Other Worlds, 369. - - ” ” Small Planets, 209. - - Light, Velocity of, 219. - - Limit of Visibility, 102. - - Lion, Eyes of, 202. - - Little Bear, 383. - - Little Sunbeam, 17. - - London, Model of, 91. - - Lowell, Mr., 194. - - Lunar Athlete, 131. - - ” Babies, 130. - - ” Craters, 108. - - ” ” Origin of, 111. - - ” Cricket and Football, 131. - - ” Eclipses, 89. - - ” Foxhounds, 131. - - ” Geography, 105. - - ” Postman, 130. - - ” Seas, 108. - - - M. - - Magnesium, 350. - - Magnet attracting Ball, 187. - - Man on the Moon, 77. - - Maps of the Stars, 206, 318; - How to make, 389. - - Mars, 134, 160. - - ” and his Movements, 160. - - ” Atmosphere of, 194. - - ” Color of, 161. - - Mars, Diameter and Distance of, 391. - - Mars, General Direction of Motion of, 165. - - Mars, Geography of, 188. - - ” Period of, 391. - - ” Polar Snows on, 193. - - ” Retrograde Motion of, 162. - - ” Rotation of, 192, 391. - - ” Satellites of, 194. - - ” Seas on, 193. - - ” Views of, 189, 190. - - ” When to observe, 160. - - Materials of a Comet, 276. - - Mathematician and Astronomer, 144. - - Measurements with a Cart-wheel, 173. - - Measuring-rod used by Astronomers, 333. - - Mercury, 134, 141. - - Mercury, Diameter, Distance, Period, and Rotation of, 391. - - Mercury, Transit of, 142. - - ” Weight of, 142. - - ” Where to find, 142. - - Meteor, Height of a, 296. - - ” of Dec. 21, 1876, 297. - - ” ” Nov. 6, 1869, 292. - - Meteoric Dust, 292. - - Meteorites, 312. - - Meteorites, Columbiad Theory of, 314. - - Meteorites in British Museum, 313. - - Meteoroids, 285. - - Meteoroids heated by Friction of Air, 288. - - Meteoroids, Velocity of, 286. - - Meteors, 284. - - ” August, 309. - - Methuselah, 376. - - Milky Way, 327. - - Mimas, Distance and Period of, 392. - - Minor Planets, Size and Number of, 207. - - Mirror for Reflecting Telescope, 234. - - Mode of Production of Coal, 11. - - Model of Lunar Crater, 110. - - Monday, Why so called, 74, 135. - - Mont Blanc, 8. - - Moon always shows Same Face, 118. - - Moon, Imaginary Voyage to, 124. - - ” Life on, 123. - - ” rising in West, 198. - - ” Size of, 79, 101. - - Moon’s Appearance, Effect of Distance on, 89. - - Moon’s Area, 82. - - ” Atmosphere, 125. - - ” Diameter, 391. - - ” Distance, 391. - - ” Movements, 84, 118. - - ” Phases, 74. - - ” Time of Revolution, 391. - - Moon-view of Earth, 78. - - Morning Star, 151. - - Motes in Sunbeam, 292. - - Motion, Annual, of Earth, 56. - - Motion of Planet round Sun illustrated, 188. - - Mount Carigou in the Pyrenees, 31. - - Mountains on the Moon, How measured, 108. - - Mountain Tops, Coldness of, 8. - - - N. - - Naming the Stars, 381. - - Nasmyth, 35. - - National Debt, 339. - - Nature of Saturn’s Rings, 225. - - Nebula in Orion, 388. - - ” Ring, in Lyra, 354. - - Nebulæ, 353. - - Nebulæ and Solar System compared, 367. - - Nebulæ, Distances of, 366. - - ” Photographs of, 362. - - Nebulæ, Stars in, 366. - - ” What made of, 359. - - Neptune, Discovery of, 244. - - Neptune, Former Observations of, 252. - - Neptune’s Brightness, 252. - - ” Clouds, 253. - - Neptune’s Diameter, Distance, Period, and Rotation, 391. - - Neptune’s Size, 253. - - ” Satellite, 253. - - Neptune’s Satellite, Distance and Period of, 393. - - Neptune’s Time of Revolution, 213. - - Newton’s Discoveries, 178. - - Night and Day, 46, 51. - - Noonday Gun, 5. - - North Pole, 53; - Continual Day at, 70; - Sunshine at, 71. - - November Showers of Meteors, 299. - - Number of Books Necessary to describe Universe, 372. - - Number of Minor Planets, 207. - - ” ” Stars, 321. - - - O. - - Oberon, 243; - Distance and Period of, 392. - - Objects, Distant, Apparent Smallness of, 25. - - Observing Robes, 172. - - Occultation of Star by Moon, 126. - - Octagon Chapel, Bath, Organist of, 231. - - Old Moon in New Moon’s Arms, 77. - - Orbit of Leonids, 302. - - Orbits of Giant Planets, 213. - - ” ” Uranus and Neptune, 250. - - Orion, 387. - - Oxygen Necessary to Life, 124. - - - P. - - Pacific Ocean, Track of Eclipse across, 42. - - Parabola, 169, 269. - - ” and Ellipse, 169, 275. - - Parabolic Reflectors, 272. - - Pegasus, Square of, 384. - - Pendulum, 54. - - Periodicity of Comets, 263. - - Perseids, 309. - - Perseus, 327, 385. - - Perturbation of Encke’s Comet, 146. - - Phases of Mercury, 142. - - ” ” the Moon, 74. - - Phases of Venus, 152. - - Phobos and Deimos, Satellites of Mars, 200. - - Phobos, Distance and Period of, 391. - - Phœbe, 229. - - Photographic Search for Planets, 207. - - Photographing the Nebulæ, 362. - - Photographs of the Heavenly Bodies, 207. - - Photographs of the Moon, 107, 109. - - Pit-eyes, 201. - - Planetary Time Table, 179. - - Planets, How to find the, 390. - - ” Small, or Asteroids, 203. - - ” Small, Search for, 205. - - Planets, Small, Size and Number of, 207. - - Pleiades, 363, 385; - Apparent Change in Position of, 387. - - “Pointers,” 383. - - Polar Snows on Mars, 193. - - Pole, North, 53; - Continual Day at, 70; - Sunshine at, 71. - - Pole Star, 383. - - Postman on the Moon, 130. - - Prediction of Halley, 264. - - Preserved Sunbeams, 13. - - Prism, Refraction of Light through, 351. - - Prominences on Sun, 44. - - Proportion of Sunlight received by Earth, 9. - - Pupil of the Eye, 201. - - Pupil of the Eye as Large as a Dinner Plate, 203. - - - Q. - - Quarter the Best Time for examining the Moon, 107. - - Quicksilver, 141. - - - R. - - Radiant Point of Meteor Shower, 308. - - Railway to Alpha Centauri, 338. - - Rank of Earth in Space, 329. - - Reappearance of Halley’s Comet, 267. - - Recorder, Sunshine, 5. - - Reflectors for Lighthouses, 271. - - Refraction of Light through Prism, 351. - - Relative Sizes of Earth and Sun, 31. - - Requisites for Astronomical Discoveries, 199. - - Resistance of Air, 182. - - Retrograde Motion of Mars, 162. - - Rhea, Distance and Period of, 392. - - Rifle, 68. - - Rigel, 388. - - Ring Nebula in Lyra, 354. - - Rings of Saturn, 224. - - Rings of Smoke, 357. - - Rising and Setting of Sun, Ancient Theories of, 48. - - Roberts, Mr. Isaac, 325; - His Photograph of Great Andromeda Nebula 364. - - Rotating Globe of Oil, 215. - - Rotation of Earth, 49. - - Rotation of Earth, Illustration of, 53. - - Rotation of Jupiter, 215. - - ” ” Mars, 192. - - ” ” Sun, 39. - - Rowton Meteorite, 313. - - Royal Canal Fly-boats, 52. - - Royal Institution, 53. - - - S. - - St. Paul’s Cathedral on the Moon, 103. - - Sandwich Isles, Crater in the, 112. - - Satellites, 195. - - ” of Jupiter, 218. - - ” ” Mars, 194. - - ” ” Saturn, 229. - - Satellites of Uranus, Revolution of, 243. - - Saturday, 135. - - Saturn, 222. - - ” and Earth, 223. - - Saturn’s Bands and Clouds, 224. - - ” Brightness, 223. - - Saturn’s Diameter and Distance, 391. - - Saturn’s Internal Heat, 224. - - ” Period, 391. - - ” Rings, 224. - - ” Rotation, 391. - - ” Satellites, 229. - - Saturn’s Satellites, Distances and Periods of, 392. - - Saturn’s Time of Revolution, 212. - - Scale of Universe, 319. - - Search for Small Planets, 205. - - Seas on Mars, 193. - - ” ” the Moon, 108. - - Seasons, Changes of the, 66. - - Seasons, Changes of the Sun with the, 57. - - Seen and Unseen Universe, 377. - - Shadow of Globe, 170. - - Shape and Size of the Sun, 29. - - Shooting Stars and Comets, 255. - - Shooting Stars, What becomes of them, 290. - - Sidus, Georgium, 240. - - Sirius, 318. - - ” How to find, 389. - - Sirius, Position of, on Map of Universe, 319. - - Size of Celestial Library, 373. - - ” Minor Planets, 207. - - ” Moon, 79, 101. - - ” the Sun, 29. - - Sizes, Comparative, of Planets, 139. - - Sizes, Comparative, of Earth and Sun, 31. - - Smith, Mr. John, his Address, 330. - - Smoke Rings, 357. - - Snow on Mountain Tops, 9. - - Snowball, 294. - - Sodium, 350. - - Solar Gravitation, 132. - - ” Prominences, 44. - - Solar System and Nebula compared, 367. - - Solar System, Drawing of the, 135. - - Spectroscope, 352. - - Speculum Grinding, 235. - - ” Metal, 235. - - Speed of Planets, 138. - - Splendor of Venus, 151. - - Spots on the Sun, 33. - - Square of Pegasus, 384. - - Star eclipsed or occulted by Moon, 126. - - Star Maps, 206, 318; - How to make, 389. - - Stars, 318; - Number of, 321. - - ” are Suns, 320. - - ” Clusters of, 327. - - ” Disguised, 205. - - ” Distances of, 332. - - ” Double, 342; - Motion of, 344. - - ” How to name the, 381. - - ” seen in Daylight, 59. - - ” Variable, 341. - - ” What made of, 347. - - Steam, Clouds a Form of, 15. - - Steel melted by Sunbeams, 7. - - Strontium, 348. - - Sun and Sirius compared in Brightness, 320. - - Sun, Benefits that we receive from the, 10. - - Sun, Express Train to, 18. - - Sun must be Hotter than Molten Steel, 7. - - Sun’s Appearance, 35. - - ” Attraction on a Comet, 274. - - ” Corona, 44. - - ” Diameter, 391. - - ” Distance, 17, 158, 210, 391. - - ” Heat and Brightness, 1. - - ” Rotation, 39, 391. - - ” Shape and Size, 29. - - ” Share in Five o’Clock Tea, 10. - - ” Spots, 33. - - ” Sunbeams, Preserved, 13. - - Sunday, 74, 135. - - Sunlight, Proportion of, received by Earth, 9. - - Sunset at Marseilles, 30. - - Sunshine at North Pole, 71. - - ” Recorder, 5. - - - T. - - Tails of Comets, 281. - - Telegraph used for Comets, 267. - - Telescope of Dunsink Observatory, 96. - - Telescope of Yerkes Observatory, 98. - - Telescopes, 92; - Reflecting, 232; - Herschel’s, 233. - - Telescopic Aid, 200. - - ” View of Moon, 101, 104. - - Tethys, Distance and Period of, 392. - - Theory to account for the Sun’s Rising and Setting, 48. - - Thursday, 135. - - Ticket to Alpha Centauri, 339. - - Time occupied by Light and Stars, 335. - - Titan, Distance and Period of, 392. - - Titania, 243. - - Titania, Distance and Period of, 392. - - Top-spinning under Air Pump, 183. - - Total Eclipse of the Sun, Appearances during, 40. - - Toy, Burning-glass not merely a, 6. - - Train to the Sun, 18. - - Transit of Venus, 154. - - Trouvelot’s Drawing of Eclipse, 41. - - Trouvelot’s Drawing of Solar Prominences, 45. - - Tuesday, 135. - - Twilight, 47. - - Two Eyes Better than One, 19. - - Tycho Brahe, 170. - - Tycho’s Method of Observing, 172. - - Tyndall’s, Professor, Heating Effect of Electric Lamp, 6. - - - U. - - Umbriel, 243. - - Umbriel, Distance and Period of, 392. - - Universe, Seen and Unseen, 377. - - “Up, Guards, and at them,” 337. - - Uranus and Earth compared, 243. - - Uranus, Clouds on, 243. - - ” Color of, 242. - - ” Diameter of, 391. - - ” Discovery of, 237. - - ” Distance of, 391. - - Uranus, Former Observations of, 241. - - Uranus, Period of, 391. - - ” Rotation of, 391. - - ” Satellites of, 242. - - Uranus, Satellites of, Distances and Periods of, 392. - - Uranus, Time of Revolution, 213. - - ” Weight of, 243. - - Ursa Major, or Great Bear, 381. - - Use of Two Eyes, 19. - - Utility of Telescope, 200. - - - V. - - Vapor of Iron, 291. - - Variable Stars, 341. - - Vega, 337. - - Velocity of a Planet, 138. - - ” ” Light, 219. - - Venus, 134, 150. - - Venus, Appearance of, during Transit, 157. - - Venus as a World, 158. - - ” Composition of Air on, 160. - - Venus, Diameter and Distance of, 391. - - Venus, Gravitation on, 159. - - ” Greatest Brilliance of, 152. - - Venus, in Telescope, 151. - - ” Period and Rotation of, 391. - - Venus, Sun’s Distance deduced from Transit of, 158. - - Venus, Temperature of, 159. - - ” Transit of, 154. - - Venus, When and where to seek, 150. - - View of Earth from the Sun, 28. - - Views of Lunar Scenery, 104, 109. - - ” ” Mars, 189, 190. - - Visibility, Limit of, 102. - - ” of Earth, 369. - - Volcanoes, Number of, 112. - - Voyage to the Moon, 124. - - ” ” Sun, 2. - - Vulcan, 48, 49, 57. - - - W. - - Waste of the Tail-making Material of Comets, 283. - - Water, Hot, and the Sun, 14. - - Water-mill turned by Sun, 16. - - Water on Moon, 127. - - Waterloo, 68, 337. - - Wednesday, 135. - - Weighed, How Planets are, 142. - - Weighing Scales for Comets, 276. - - Weight of Jupiter, 214. - - ” ” Mercury, 149. - - ” ” Uranus, 243. - - Weights on the Moon, 127. - - ” ” ” Sun, 132. - - What Nebulæ are made of, 359. - - ” Stars are made of, 347. - - Wind, Cause of, 14. - - Wind-mill turned by Sun, 15. - - Worcester’s Dictionary, Use of, 269. - - - Y. - - Yellowstone Park, 115. - - Yerkes Observatory, Chicago, Telescope at, 98. - - - - -Transcriber’s Notes - - -The Table of Contents does not mention the “Table of Useful -Astronomical Facts”, which begins on page 391, immediately preceding -the Index. - -Punctuation, hyphenation, and spelling were made consistent when a -predominant preference was found in the original book; 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