,illustration ,,star-,,land ,,being ,,talks ,,with ,,young ,,people ,,about ,,the ,,wonders ,,of ,,the ,,heavens ,,by ,,sir ,,robert ,,stawell ,,ball1 ,f4,r4,s4 ,,lowndean ,,professor ,,of ,,astronomy ,,in ,,the ,,university ,,of ,,cambridge ,,author ,,of 8,,the ,,story ,,of ,,the ,,heavens10 ,,etc4 ,illustrated ,,new ,,and ,,revised ,,edition ,,boston1 ,u4,s4,a41 ,,and ,,london ,,ginn ,,company1 ,,publishers ,the ,athenum ,press #ahii ,,entered ,,at ,,stationers' ,,hall ,,copyright1 #ahii1 ,,by ,,cassell ,,company1 ,,limited ,,all ,,rights ,,reserved ,to ,,those ,,young ,,friends ,,who ,,have ,,attended ,,my ,,christmas ,,lectures ,,this ,,little ,,book ,,is ,,dedicated ,,preface ,,to ,,first ,,edition4 ,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 audience4 ,on two occasions1 namely1 in #ahha and in #ahhg1 the ,managers entrusted this honorable duty to me4 ,the second course was in the main a repetition of the first2 and on my notes and recollections of both the present little volume has been founded4 ,i am indebted to my friends ,rev4 ,,maxwell ,,close1 ,mr4 ,,arthur ,,rambaut1 and ,dr4 ,,john ,,todhunter for their kindness in reading the proofs4 ,,robert ,s4 ,,ball4 ,,observatory1 ,,co4 ,,dublin1 ,oct4 #bb1 #ahhi4 ,,contents4 ,,lecture ,i4 ,,the ,,sun4 ,,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 #a ,,lecture ,,ii4 ,,the ,,moon4 ,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 #gd ,,lecture ,,iii4 ,,the ,,inner ,,planets4 ,mercury1 ,venus1 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 ,asteroids1 or ,small ,planets #acd ,,lecture ,,iv4 ,,the ,,giant ,,planets4 ,jupiter1 ,saturn1 ,uranus1 ,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 #bab ,,lecture ,v4 ,,comets ,,and ,,shooting ,,stars4 ,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 #bee ,,lecture ,,vi4 ,,stars4 ,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 ,nebul--,what the ,nebul are made of--,photographing the ,nebul--,conclusion #cah ,,concluding ,,chapter4 ,,how ,,to ,,name ,,the ,,stars4 #cha ,,star-,,land4 ,,lecture ,i4 ,,the ,,sun4 ,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 ,pole4 ,,the ,,heat ,,and ,,brightness ,,of ,,the ,,sun4 ,we can all feel that the sun is very hot1 and we know that it is very big and a long way off4 ,let us first talk about the heat from the sun4 ,on a cold day it is pleasant to go into a room with a good fire1 and everybody knows that the nearer we go to the fire1 the more strongly we feel the heat4 ,the boy who is at the far end of the room may be shivering with cold1 while those close to the fire are as hot as they find to be pleasant4 ,if we could draw much nearer to the sun than we actually are1 we should find the heat greatly increased4 ,indeed1 if we went close enough1 the temperature would rise so much that we could not endure it2 we should be roasted4 ,on the other hand1 we should certainly be frozen to death if we were transported much further away from the sun than we are now4 ,we are able to live comfortably1 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 placed4 ,suppose you were able to endure any degree of heat1 and that you had some way of setting out on a voyage to the sun4 ,take with you a wax candle1 a leaden bullet1 a penny1 a poker1 and a flint4 ,soon after you have started you find the warmth from the sun increasing1 and the candle begins to get soft and melt away4 ,still1 on you go1 and you notice that the leaden bullet gets hotter and hotter1 until it becomes too hot to touch1 until at last the lead has melted1 as the wax had previously done4 ,however1 you are still a very long way from the sun1 and you have the penny1 the poker1 and the flint remaining4 ,as you approach closer to the luminary the heat is ever increasing1 and at last you notice that the penny is beginning to get red-hot2 go still nearer1 and it melts away1 and follows the example of the bullet and the candle4 ,if you still press onwards1 you find that the iron poker1 which was red-hot when the penny melted1 begins to get brighter and brighter1 till at last it is brilliantly white1 and becomes so dazzling that you can hardly bear to look at it2 then melting commences1 and the poker is changed into liquid like the penny1 the lead1 and the wax4 ,yet a little nearer you may carry the flint1 which is now glowing with the same fervor which fused the poker1 but even the flint itself will have to yield at last and become1 not merely a liquid like water1 but a vapor like steam4 ,you will ask1 how do we learn all this8 ,as nobody could ever make such a journey1 how can we feel certain that the sun is so excessively hot8 ,i know that what ,i say is true for various reasons1 but ,i will only mention one1 which is derived from an experiment with the burning-glass1 that most boys have often tried4 ,illustration3 ,,fig4 #a4--;,how to use the ,burning-glass4 ,illustration3 ,,fig4 #b4--,the ,noonday ,gun4 ,we may use one of those large lenses that are intended for magnifying photographs4 ,but almost any kind of lens will do1 except it be too flat1 as those in spectacles generally are4 ,on a fine sunny day in summer1 you turn the burning-glass to the sun1 and by holding a piece of paper at the proper distance a bright spot will be obtained 7,fig4 #a74 ,at that spot there is intense heat1 by which a match can be lighted1 gunpowder exploded1 or the paper itself kindled4 ,the broad lens collects together the rays from the sun that fall upon it1 and concentrates them in one spot1 which consequently becomes hot and bright4 ,if we merely used a flat piece of glass the sunbeams would go straight through2 they would not be gathered together1 and they would not be strong enough to burn the paper4 ,the lens1 you see1 is not flat2 its faces are curved1 and they thus acquire the power of bending in rays of light or heat1 so as to unite their effect on that one point which we call the focus4 ,when a great number of rays are thus collected on the same spot1 each of them contributes a little warmth4 ,illustration3 ,,fig4 #c4--;,a ,tell-tale for the ,sun4 ,some ingenious person has turned this principle to an odd use1 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 off4 ,thus the sun itself is made to announce the middle of the day 7,fig4 #b74 ,another application of the burning-glass is to obtain a record of the number of hours of sunshine in each day4 ,you will understand the apparatus from ,fig4 #c2 the lens is here replaced by a glass globe1 which acts as a burning-glass4 ,as the sun moves over the sky the bright spot of light also moves1 and therefore burns its track on a sheet of paper marked with lines corresponding to the hours4 ,when the sun is hidden by clouds the burning ceases1 so by preserving each day the piece of paper1 we have an unerring tell-tale1 which shows us during what hours the sun was shining brightly1 and the hours during which he was hidden4 ,you see1 the burning-glass is not merely a toy1 it can be made useful in helping us to learn something about the weather4 ,another experiment with the burning-glass will also teach us something4 ,take a candle1 and from its flame you can get a bright point at the focus4 ,it may fall upon your hand1 but you can hardly feel it1 and you will readily believe that the focus is not nearly so hot as the candle4 ,even when a burning-glass is held in front of a bright fire there is comparatively little heat in the focus4 ,by using a lens to condense the beams from an electric lamp1 ,professor ,tyndall has shown how to light a piece of paper1 and to produce many other effects4 ,but1 nevertheless1 the focus is not nearly so hot as the arc between the two glowing carbons4 ,you might move your finger through the focus without much inconvenience1 but ,i would not recommend you to trust your finger between the poles of the electric light itself4 ,the temperature obtained at the focus of a burning-glass seems thus to be always less than that prevailing at the source of heat itself4 ,this principle will be equally true when we turn a burning-glass to the sun1 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 days4 ,but burning-glasses a yard wide have been made1 and astonishing heat effects have been produced4 ,steel has thus been melted by the sunbeams1 and so have other substances which even our greatest furnaces cannot fuse4 ,therefore the sun must have a higher temperature than that of molten steel2 higher1 indeed1 than any temperature we can produce on the earth4 ,i have tried to prove to you that the sun is very hot2 but it would be well to see what arguments might be used on the other side4 ,indeed1 it is by considering objections that we often learn4 ,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 man4 8,i am sure10 said my friend1 8that you must be quite wrong4 ,you said that the nearer you got to the sun the hotter it would be2 but ,i know this to be a mistake4 ,when tourists go to ,switzerland1 they sometimes climb very high mountains4 ,but the top of a mountain1 of course1 is nearer the sun than below2 and so1 if the sun were really hot1 the climber should have found it much warmer on the top of the mountain than at its base4 ,but every one knows that there is abundant ice and snow on lofty ,alpine summits1 while down below in the valleys there may be at the same time excessively warm weather4 ,does it not therefore seem that the nearer we go to the sun the colder it is1 and the further we are from the sun the warmer it is80 ,but my friend was quite wrong in his argument4 ,the coldness of the mountain tops depends upon something which he had not taken into account4 ,there is something else besides the sun which helps to make us so warm and comfortable4 ,this other essential thing is more or less deficient at great heights4 ,you know that we live by breathing air1 and we find air wherever we go1 over land and sea1 all round the earth4 ,those who ascend in balloons are borne upwards by the air1 and thus we can show that air extends for miles and miles over our heads1 though it becomes lighter and thinner the loftier the elevation4 ,we not only utilize the air for breathing1 but it is also of indispensable service to us in another way4 ,it acts as a blanket to keep the earth warm2 indeed1 we ought rather to describe the air as a pile of blankets one over the other4 ,these air blankets enable the earth to preserve the heat received from the sunbeams by preventing it from escaping back again into space4 ,thus warmth is maintained1 and our globe is rendered habitable4 ,you see then1 that for our comfort we require not only the sun to give us the heat1 but also the set of blankets to keep it when we have got it4 ,if we threw off the blankets we should be uncomfortable1 though the sun were as bright as before4 ,a man who goes to the top of a mountain at mid-day does approach the sun to some extent1 and1 so far as this goes1 he ought no doubt to feel warmer1 but the gain is far too small to be thought of4 ,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 whole4 ,this would be utterly inappreciable2 even a thermometer would not be delicate enough to show it4 ,on the other hand1 by ascending to the top of the mountain1 the climber has got above the lower regions of the air2 he has not1 it is true1 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 blankets4 ,the ,alpine climber on the top of the mountain has thus thrown off the best portion of his blankets1 and receives a chill2 while the gain of heat arising from his closer approach to the sun is imperceptible4 ,perhaps you will now be able to understand why eternal snow rests on the summits of the great mountains4 ,they are chilled because they have not so many air blankets as the snug valleys beneath4 ,the brightness of the sun is among the most wonderful things in nature1 and there are three points that ,i ask you to remember1 and then indeed you will agree with ,milton1 that the sun is 8with surpassing glory crowned40 ,first think of the beauty and brilliancy of a lovely day in ,june4 ,then remember that all this flood of light comes from a single lamp at a most tremendous distance2 and thirdly1 recollect that the sun is not like a bull's-eye lantern1 concentrating all his light specially for our benefit1 but that he diffuses it equally around1 and that we do not get on this earth the two-thousand-millionth part of what he gives out so plenteously6 ,when we think of the brightness of day1 of the distance from which the light has come1 though ,nature has not adjusted any vast lenses to direct the light specially in our direction1 we begin to comprehend the sun's true magnificence4 ,,further ,,benefits ,,that ,,we ,,receive ,,from ,,the ,,sun4 ,i want to show you how great should be the extent of our gratitude to the sun4 ,of course1 on a bright summer's day1 when we are revelling in the genial warmth and enjoying the gladness of sunshine1 it needs no words to convince us of the utility and of the beneficence of sunbeams4 ,so we will not take midsummer4 ,let us take midwinter4 ,take this very ,christmas season when the days are short and cheerless1 the nights are long and dark and cold4 ,we might be tempted to think that the sun had well-nigh forgotten us4 ,it is true he only seems to pay us very occasional visits1 and between fogs and clouds we in ,england see but little of him2 but1 visible or invisible1 the sun incessantly tends us1 and provides for our welfare in ways that perhaps we do not always remember4 ,let me give an illustration of what ,i mean4 ,you will go back this dull and cold afternoon to the happy home where your ,christmas holidays are being enjoyed4 ,it will be quite dark ere you get there1 for the sun in these wintry days sets so very early4 ,you will gather around a cheerful fire4 ,the curtains will be drawn1 the lamps will be lighted1 and the disagreeable weather outside will be forgotten in the pleasant warmth and light within4 ,five o'clock has arrived1 the pretty wicker table has been placed near mamma's chair2 on it are the cups and saucers and the fancy teapot4 ,under the table is a little shelf1 with some tempting cakes and a tender muffin4 ,two or three welcome friends have joined the little group1 and a delightful half-hour is sure to follow4 ,but you may say1 8,what have tea and muffins1 lamps and fireplaces to do with the sun8 ,are they not all mere artificial devices1 as far removed as possible from the sunbeams or the natural beauties which sunbeams create80 ,well1 not so far1 perhaps1 as you may think4 ,let us see4 ,poke up the fire1 and while it is throwing forth that delicious warmth1 and charming but flickering light1 we will try to discover where that light and heat have come from4 ,no doubt they have come from the coal1 but then1 whence came the coal8 ,it came from the mine1 where brave colliers hewed it out deep under the ground1 and then it was hoisted to the surface by steam engines4 ,our inquiry must not stop here1 for another question immediately arises1 as to how this wonderful fuel came into the earth8 ,when we examine coal carefully1 by using the microscope to see its structure1 we find that it is not like a stone2 it is composed of trees and other plants1 the leaves and stems of which can be sometimes recognized4 ,indeed1 the fossil trunks and roots of the great trees are occasionally conspicuous in the coal-pit4 ,it is quite plain that these are only the remains of a vegetation which was formerly growing and flourishing1 and on further inquiry we learn that coal must have been produced in the following manner3-- ,once upon a time a great forest flourished4 ,the sun shone down on this forest1 and it was watered by genial showers1 while insects and other creatures sported in its shades4 ,it is true that the trees and plants were not like those we now see about us4 ,they were more like ferns and mare's-tails and gigantic club-mosses4 ,in the fulness of time they died1 and fell1 and decayed1 and others sprang up to meet the like end4 ,thus it happened that1 in course of ages1 the remains of leaves1 and fruits1 and trunks accumulated over the soil4 ,the forest was situated near the seashore1 and then a remarkable change took place--the land began slowly to sink4 ,you need not think that this is impossible4 ,land has often been known to change its level gradually4 ,in fact1 a sinking process is slowly going on now in many places on the earth1 while the land is rising in other localities4 ,as the forest gradually sank lower and lower1 the sea-water began to inundate it1 and all the trees perished until1 at last1 deep water submerged the surface which had once been covered by a fine forest4 ,at the bottom of this sea lay the decaying vegetation4 ,that which was the destruction of the growing forest1 proved to be the means of preserving its remains1 for1 then as now1 the rivers flowed into the sea1 and the waters of the rivers1 especially in times of flood1 carried down with them clay or mud1 held in suspension4 ,upon the floor of the ocean this material was slowly deposited2 and thus a coating of mud overlay the remains of the forest4 ,in the course of ages1 these layers grew thick and heavy1 and hardened into a great flat rock1 while the trunks and leaves underneath were squeezed together by the weight1 and packed into a solid mass which became black1 and in the course of time was transformed into coal4 ,after ages and ages had passed by1 the bed of the sea ceased to sink1 and began slowly to rise4 ,the water over the newly made layers of stone became shallower1 and at last the floor was raised until it emerged from the sea4 ,but1 of course1 it would not be the original ground which formed the surface of the newly uncovered land4 ,the sheets of consolidated clay lay on the top2 over the fresh surface life gradually spread1 until man himself came to dwell there1 while far beneath his feet the remains of the ancient vegetation were buried4 ,when we now dig down through the rocks we come upon the portions of trees and other plants which the lapse of time1 and the influence of pressure1 have turned from leaves and wood into our familiar coal4 ,that ancient forest grew because sunbeams abounded in those early times1 and nourished a luxuriant vegetation4 ,the heat and the light then expended so liberally by the sun were seized by the leaves of flourishing plants1 and were stored away in their stems and foliage4 ,thus it is that the ancient sunbeams have been preserved in our coal-beds for uncounted thousands of years4 ,when we put a lump of coal on our fire this evening1 and when it sends forth a grateful warmth and cheerful light1 it but reproduces for our benefit some of that store of preserved sunbeams of which our earth holds so large a treasure4 ,thus1 the sun has contributed very materially to our comfort1 for it has provided the fire to keep us warm4 ,the orb of day has1 however1 ministered further to our tea party1 for has it not produced the tea itself8 ,the tea grew a long way off1 most likely in ,china1 where the plant was matured by the warmth of the sunbeams4 ,from ,china the tea-chests were brought by a sailing vessel to ,london2 the ship performed this long voyage by the use of sails1 blown by what we call wind1 which is merely the passage of great volumes of air as they hurry from one part of the earth to another4 ,we may ask what makes the air move1 for it will not rush about in this way unless there be considerable force to drive it4 ,here again we perceive the influence of the sun4 ,tracts of land are warmed by the genial sunbeams4 ,the air receives the heat from the land1 and the warm air is buoyant and ascends1 while cooler air continually flows in to supply its place4 ,to do this it has1 of course1 to rush across the country1 and thus wind is caused4 ,all the air currents on our earth are consequently due to the sun4 ,you see1 therefore1 how greatly we are indebted to our brilliant luminary for the enjoyment of our tea-table4 ,not only has the sun given us the coal and the tea1 but it has actually provided the means by which the tea was carried all the way from ,china to our own shores4 ,we can also trace the connection between the hot water and the sun4 ,of course1 the water has come immediately from the kettle1 and that has been taken from the fire1 and the fire was produced by sunbeams4 ,thus we learn that it is the warmth of the sun that has made the water boil4 ,if you visit the water-works you will see great reservoirs4 ,in some cases they have been filled by a river1 sometimes the water is pumped from a deep well in the ground1 sometimes it is the surface-water caught on a mountain side4 ,whatever be the immediate source of our water supply1 the real origin is to be sought1 not in the earth beneath1 but in the heavens above4 ,all the water we use day by day has come from the clouds4 ,it is the clouds which sent down the rain1 or sometimes the snow1 or the hail1 and it is this water from the clouds which fills our rivers4 ,it is this water also which sinks deep into the earth and supplies our wells1 so that from whatever apparent source the water seems to have come1 it is indeed the clouds which have been the real benefactors4 ,the water in your teacup to-night was1 a little while ago1 in a cloud1 floating far overhead in the sky4 ,we may look a little further and find whence the clouds have come4 ,it is certain that clouds are merely a form of steam or vapor of water1 and as they are so continually sending down rain on the earth1 there must be some means by which their supply will be replenished4 ,here again our excellent friend the sun is to be found ever helping us secretly1 if not helping us openly4 ,he pours down his rich and warm beams on the great oceans1 and the heat turns some of the water into vapor1 which1 being lighter than the air1 ascends upwards for miles4 ,there the vapor often passes into the form of clouds1 and the winds waft these clouds to refresh the thirsty lands of the earth4 ,thus1 you see1 it is the sun which procures for us water from the great oceans which cover so much of our globe1 and sends it on by the winds to supply our water-works1 and fill our teapots4 ,notice another little kindliness of our great benefactor4 ,the water of the oceans is quite salt4 ,but we could not make tea with salt water1 so the sun1 when lifting the vapor from the sea1 most thoughtfully leaves all the salt behind1 and thus provides us with the purest of sweet water4 ,that nice muffin was baked by the sun1 toasted by the sun1 and made from wheat grown by the sun4 ,if the wheat was ground in a wind-mill1 then the sun raised the wind which turned the mill4 ,perhaps the flour-mill was driven by steam1 in which case the sun1 long ago1 provided the coal for the boiler4 ,the miller might have lived on a river and used a water-mill1 but if he did1 then here again the sun actually did the work4 ,the sun raised the water to the clouds1 and after it had fallen in rain1 and was on its way back to the sea1 its descent was utilized to turn the water-wheel4 ,the water derives its power to turn the mill from the fact that it is running downhill1 but it could not run down unless it had first been raised up2 and thus it is indeed the sun which drives the water-wheel4 ,nor can the baker dispense with the sun's aid even if he rejected wind-mills1 or steam-mills1 or water-mills1 and determined to grind the corn himself with a pestle and mortar4 ,here1 at least1 it might be thought that it is a man's sinews and muscles that are doing the work1 and so no doubt they are4 ,but you are mistaken if you think the sun has not rendered indispensable aid4 ,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-mill4 ,the force exerted in grinding with the pestle has been derived from the food that the man has eaten2 that food was grown by the sun1 and the man received from the food the energy it had derived from the sun's heat4 ,so that1 look at it any way you please1 even for the grinding of the wheat to make the muffin for your tea party1 you are wholly indebted to the sun4 ,it is the sun which has bleached the tablecloth to that snowy whiteness4 ,the sun has given those bright colors which look so pretty in the girls' dresses4 ,with how much significance can we say and feel that light is pleasant to the eye1 and what prettier name than ,little ,sunbeam can we have for the darling child who makes our home so bright8 ,,the ,,distance ,,of ,,the ,,sun4 ,the sun is a very long way off4 ,it is not easy for you to imagine a distance so great1 but if you want to learn astronomy you must make the attempt4 ,this is the first measurement that we shall have to make on our way to that far-off country called ,star-,land2 but long as we shall find it to be1 we shall afterwards have to consider distances very much longer4 ,when you are out in the street1 or taking a walk in the country1 you can see at once that this man is near1 or that house is far1 or that mountain is many miles away4 ,this is because you have other objects between to help you to judge of the distances of these different objects4 ,you will see1 for example1 that there are many houses or farmyards1 and you will notice hedges dividing different fields between you and the mountain4 ,you also see that there are woods and parks1 and perhaps stretches of moorland extending up the slopes4 ,you have an impression that the farmyards and fields are of considerable size1 and that the woods or moors are wide and extensive2 and putting these things together1 you realize that the mountain must be miles away4 ,but when we look at the sun we have no aids conveniently placed to help us in judging his distance4 ,there are no intervening objects1 and merely gazing at the sun helps us but little in obtaining any accurate knowledge4 ,we must go to the astronomer and ask him to tell us how far he has found the sun to be1 and then we must also beg from him some explanation of the method he has used in making his measurements4 ,it has been found that the sun is1 on the average1 about ninety-three millions of miles from the earth2 but sometimes it is a little further and sometimes it is a little nearer4 ,let us first try to count #ic1jjj1jjj4 ,the easiest way will be to get the clock to do this for us2 and here is a sum that ,i would suggest for you to work out4 ,how long will the clock have to tick before it has made as many ticks as there are miles between the earth and the sun8 ,every minute the clock1 of course1 makes #fj ticks1 and in #bd hours the total number will reach #hf1djj4 ,by dividing this into #ic1jjj1jjj you will find that more than #ajgf days1 or nearly three years1 will be required for the clock to perform the task4 ,we may consider the subject in another way1 and find how long an express train would take to go all the way from the earth to the sun4 ,we shall suppose the speed of the train to be #dj miles an hour2 and if the train ran for a whole day and a whole night without stopping1 it would then accomplish #ifj miles4 ,in a year the distance travelled would reach #cej1djj miles1 and by dividing this into #ic1jjj1jjj we arrive at the conclusion that a train would have to travel at a pace of #dj miles an hour1 not alone for days and for weeks and for years1 but even for centuries4 ,indeed1 not until #bfe years had elapsed would the mighty journey have been ended4 ,even though ,king ,charles ,i4 had been present when the train began to move1 the destination would not yet have been reached4 ,no one who started in the train could expect to reach the end of the trip4 ,that would not occur till the time of his great-great-grandchildren4 ,,how ,,astronomers ,,measure ,,the ,,distances ,,of ,,the ,,heavenly ,,bodies4 ,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 are4 ,this would be a very puzzling matter if we were to try and describe it fully1 but the principle of the method is not at all difficult4 ,do you know why you have been provided with two eyes8 ,it is undoubted that one of the reasons is to aid you in estimating distances4 ,you see this boy 7,fig4 #d7 judges of the distance of his finger by the inclination of his two eyes when directed at it4 ,in a similar way we judge of the distance of a heavenly body by making observations on it from two different stations4 ,illustration3 ,,fig4 #d4--,two ,eyes are better than ,one4 ,illustration3 ,,fig4 #e4--;,how we measured the ,height of the ,ball4 ,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 ceiling4 ,of course1 ,i do not intend to have a measuring tape from the ball itself1 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 reach4 ,i will ask the aid of a boy and a girl1 who will please stand one at each end of the lecture table4 ,the apparatus we shall want is very simple2 it consists of two cards and a pair of scissors4 ,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 girl1 and the other side shall point straight at the ball1 just as you see in the picture 7,fig4 #e74 ,the girl will also please do the same with her card1 so that along one side she just sees the little boy's face1 while the other side points up to the ball4 ,it will be necessary to cut these angles properly4 ,if the angle be too big1 then when one side points to the boy's face1 the other will be directed above the ball4 ,if the angle on the card be too small1 then one side will be directed below the ball1 while the other is pointed to the boy4 ,the whole accuracy of our little observations depends upon cutting the card angles properly4 ,when they have been truly shaped it will be easy to find the distance of the ball4 ,we first take a foot rule and measure the length of our table from one of our young friends to the other4 ,that length is twelve feet1 and to discover the distance of the ball we must make a drawing4 ,we get a sheet of paper1 and first rule a line twelve inches long4 ,that will represent the length of the table1 it being understood that each inch of the drawing is to correspond to a foot of the actual table4 ,let the end where the girl stood be marked ,b1 and that of the boy1 ,a1 and now bring the cards and place them on the line just as shown in the figure4 ,the card the girl has shaped is to be put so that the corner of it lies at ,b1 and one edge along ,b ,a4 ,then the boy's card is to be so put that its corner is at ,a and one edge along ,a ,b4 ,next with a pencil we rule lines on the other edges of the cards1 taking care that they are kept all the time in their proper positions4 ,these two lines carried on will meet at ,c2 and this must be the position of the ball on the scale of our little sketch4 ,it only now remains to take the foot rule and measure on the drawing the length from ,a to ,c4 ,i find it to be twenty inches1 and ,i have so arranged it that the distance from ,b to ,c is the same4 ,illustration3 ,,fig4 #f4--,this is what we wanted the ,cards for4 ,i do not intend to trouble you much with ,euclid in these lectures1 but as many of my young friends have learned the sixth book1 ,i will just refer to the well-known proposition1 which tells us that the lengths of the corresponding sides of two similar triangles are proportional4 ,we have here two similar triangles4 ,there is the big one with the boy at one corner1 the girl at the other1 and the ball overhead4 ,here is the small triangle which we have just drawn4 ,these triangles are similar because they have got the same angles1 and it was to insure that they should have the same angles that we were so careful in shaping the cards4 ,as these two triangles are similar1 their sides must be proportional4 ,we have agreed that the line ,a ,b1 which is twelve inches long1 is to represent the length of the table between the little boy and girl4 ,hence the distance1 ,a ,c1 must1 on the same scale1 be the interval between the ball and the boy at the end4 ,this is twenty inches on the drawing1 and therefore the actual distance from the end of the table to the ball is twenty feet4 ,hence you see that without going up to the ball or having a string from it1 or in any other way making direct communication with it1 we have been able to ascertain how far up in the air the ball is actually hung4 ,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 earth4 ,you must think of the sun1 the moon1 and the stars as globes supported in some manner over our heads1 and we seek to discover their distances from measurements of angles made at the ends of a base-line4 ,illustration3 ,,fig4 #g4--,this would be our ,base-line when finding the ,sun's ,distance4 ,of course1 astronomers must choose two stations which are far more widely separated than are those in our little experiment4 ,in fact1 the greater the interval between the two stations1 the better4 ,astronomers require a much longer distance than from one side of this room to the other1 or from one side of ,london to the other side4 ,if it were merely a balloon at which we were looking1 then1 when one observer at one side of ,london and another at the opposite side shaped their cards carefully1 we should be able to tell the height of the balloon very easily4 ,but as the sun is so much further off than any balloon could ever be1 we must separate the observers much more widely4 ,even the breadth of ,england would not be enough1 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 be4 ,one astronomer takes up his position at ,a 7,fig4 #g71 and the other at the opposite side at ,b1 so that they can both see the sun4 ,they are obliged to use a much more accurate way of measuring the angles than by cutting out cards with pairs of scissors2 and as the astronomer at ,a is not able to see his friend at ,b1 it becomes no easy matter to measure the angles accurately4 ,however1 we shall not now trouble ourselves about such difficulties4 ,it may suffice for the present to know that the angles are measured by delicate and very accurate instruments used by astronomers4 ,they will not1 indeed1 make a little sketch such as sufficed for our purpose4 ,they make a calculation which is a much more accurate way of effecting true measurement4 ,the astronomers know the size of the earth1 and thus they know how many thousands of miles lie between the two stations where the observations are made4 ,this distance means in their calculation just what the length of the table did in our sketch4 ,from each end of the line they set off an angle just as we did1 and the astronomer must use the principle of similar triangles which he finds in ,euclid1 just we had to do4 ,at last1 when they have calculated the sides of their triangle1 they obtain the distance of the sun4 ,,the ,,apparent ,,smallness ,,of ,,distant ,,objects4 ,illustration3 ,,fig4 #h4--,the nearer you are1 the bigger the ,globe looks4 ,illustration3 ,,fig4 #i4--,the ,globe is so far off that it lies beyond the ,picture4 ,the dotted lines show how small it seems4 ,i ought here to explain a principle which those who are learning about the stars must always bear in mind4 ,the principle asserts that the further a body is1 the smaller it looks4 ,perhaps this will be understood from the adjoining little sketch 7,fig4 #h74 ,it represents a great globe1 on which oceans and continents are shown1 and you see a little boy and a little girl are looking at the globe4 ,the girl stands quite close to it1 and ,i have drawn two dotted lines from her eye1 one to the top of the globe1 and the other to the under surface4 ,if she wants to examine the entire side of the globe which is visible to her1 she must first look along the upper dotted line1 and then she must turn her glance downwards until she comes to the lower line1 and having to turn her eyes thus up and down she will think the globe is very big1 and she will be quite right4 ,the boy is1 as you see1 on the other side of the globe1 but ,i have put him much further off than the girl4 ,i have also drawn two dotted lines from his eye to the globe1 and it is plain that he will not have to turn his head much up and down to see the whole globe4 ,he can take it all in at a glance1 and to him1 therefore1 the globe will appear to be comparatively small1 because he is sufficiently far from it4 ,the more distant he is1 the smaller it will appear4 ,you can easily imagine that1 if the globe were far enough1 the two lines that would include the whole would be like those shown 7,fig4 #i71 in which the globe is so distant that it cannot be seen in the picture4 ,the apparent size of the globe1 which is really measured by the angle between these two lines1 would always be smaller and smaller according as the distance was greater4 ,now you can understand why an object seems smaller the further away it is2 indeed1 when sufficiently far1 the object ceases to be visible at all4 ,i could give many illustrations of the diminution of size by distance1 and so1 doubtless1 could you4 ,every boy knows that his kite looks smaller and smaller the greater the length of string that he lets out4 ,i have seen in the ,west of ,ireland a bird that seemed like a little speck high up near the clouds1 but from its flight and other circumstances ,i knew that the speck was not a little bird4 ,it was1 indeed1 a great eagle1 which was dwarfed by the elevation to which it had soared4 ,it is in astronomy that we have the best illustrations of this principle4 ,enormous objects seem to be small because they are so very far off4 ,you must therefore always remember that although an object may appear to be small1 this appearance may be only a delusion4 ,it may be that the object is very big1 but very distant4 ,in astronomy1 this is almost always the case1 there is so much room above us1 around us1 on all sides in space4 ,look up at the ceiling4 ,it certainly does not bound space1 for there is another side to it2 and then there is the roof of the house4 ,but the roof is not a boundary1 for1 of course1 there is the air above it1 and then1 higher up still1 there are the clouds1 and so we can carry our imagination on and on through and beyond the air up to where the stars are1 and still on and on4 ,and as there is unlimited room1 the celestial bodies take advantage of it1 and are1 generally speaking1 at distances so gigantic that1 no matter how small they may appear1 their smallness is merely deceptive4 ,let us try to illustrate in another way the exceeding remoteness of the sun4 ,so please imagine that you were on the sun1 and that you took a view of our earth from that distance4 ,to find out what we must expect to see1 let us think of a balloon voyage4 ,if you were to go up in a balloon1 you would at first see only the houses1 or objects immediately about you1 but as you rose the view would become wider and wider4 ,you would see that ,london was surrounded by the country1 and then1 as you still soared up and up1 the sea would become visible1 and you would be able to trace out the coasts1 east and west and south4 ,if1 in some way1 you could soar higher than any balloon could carry you1 the whole of the ,british ,islands would presently lie spread like a map beneath4 ,still on and on1 and then the continent of ,europe would be gradually opened out1 until the great oceans1 and even other continents1 would at last be caught sight of1 and then you would perceive that our whole earth was indeed a globe4 ,the higher you went1 the less distinctly would you be able to see the details on the surface4 ,at last the outlines of the continents and oceans would fade1 and you would begin to lose any perception of the shape of the earth itself4 ,long ere you had reached the distance of the sun1 the earth would look merely as the planet ,venus now does to us4 ,it is instructive to consider how small our earth would seem if it were possible to view it from the sun4 ,think of that very familiar little globe1 a lawn-tennis ball1 which is two and three-quarter inches in diameter4 ,but suppose a tennis ball were at the opposite side of the street1 or still further away2 suppose1 for example1 that it were half a mile away1 what could you expect to see of it8 ,and yet the earth1 as seen from the sun1 would appear to be no larger than a tennis ball would look when viewed from a distance of half a mile4 ,,the ,,shape ,,and ,,size ,,of ,,the ,,sun4 ,we have spoken of the heat of the sun1 how hot he is2 of the distance of the sun1 how far he is2 and now we must say a little about the size of the sun2 and also about his shape4 ,it is plain that the sun is round1 that it has the shape of a ball4 ,we are sure of this because1 though a plate is circular1 yet1 if it were placed so that we only saw it edgeways from a distance1 it would not appear to be round4 ,the sun is always rotating1 and as it always seems to be a circle1 we are therefore certain that the true shape of the sun must be globular1 and not merely circular like a flat plate4 ,in the middle of the day1 when the sun is high in the heavens1 it is impossible for us to form a notion of the size of the sun4 ,people will form very different estimates as to his apparent bigness4 ,some will say he looks as large as a dinner plate1 but such statements are meaningless1 unless we say where the plate is to be held4 ,if it be near the eye1 of course the plate may hide the sun1 and1 for that matter1 everything else also4 ,if the plate were about a hundred feet away1 then it would often hide the sun4 ,if the plate were more than a hundred feet distant1 then it could not hide the sun entirely1 and the further the plate1 the smaller it would seem4 ,no means of estimating the sun's size are available when his orb stands high in the heavens4 ,but when he is rising or setting1 we see that he passes behind trees or mountains1 so that there are intervening objects with which we can compare him2 then we have actual proof that the sun must be a very large body indeed4 ,illustration3 ,,fig4 #aj4--;,a ,sunset viewed from ,marseilles 7,marcus ,codde74 ,i give here a picture1 by ,marcus ,codde1 taken from a ,french journal1 l',astronomie1 which gives a charming illustration of a sunset at ,marseilles 7,fig4 #aj74 ,if you wish to see that the sun is bigger than a mountain1 you may go to the top of ,notre ,dame de la ,garde1 but you must choose either the #ajth of ,february or the #cast of ,october for your visit1 because it is only on the evenings of those days that the sun sets in the right position4 ,on both these evenings the sun sinks directly behind ,mount ,carigou in the ,pyrenees2 this mountain is a long way from ,marseilles--no less1 indeed1 than one hundred and fifty-eight miles4 ,but the mountain is so lofty1 that when the sky is clear1 the summit can be distinctly seen upon the sun as a background1 in the way shown in the picture4 ,this must be a very pretty sight1 and it teaches us an important lesson4 ,the sun is further away than the mountain1 and yet you see the sun on both sides of the mountain1 and above it4 ,here1 then1 we learn without any calculations1 that the sun must be bigger than the upper part of a great mountain in the ,pyrenees4 ,when we calculate the size of the sun from the measurements made by astronomers1 we discover that it is much bigger than ,mount ,carigou2 we see that even the entire range of the ,pyrenees1 the whole of ,europe1 and even our whole globe1 are insignificant by comparison4 ,illustration3 ,,fig4 #aa4--;,how we compare the ,earth and the ,sun4 ,there is a football on the table1 shown in ,fig4 #aa4 ,we shall suppose it to represent the sun2 we shall now choose something else to represent the earth4 ,we must1 however1 exhibit the proportions accurately4 ,a tennis ball will not do2 it is far too large4 ,the fact is1 the width of the earth is less than the one-hundredth part of the width of the sun4 ,the tennis ball is1 however1 only a quarter the width of the football1 so we must choose something a good deal smaller4 ,i try with a marble1 even with the smallest marble ,i can find1 but when ,i measure it1 ,i find that one hundred such marbles1 placed side by side1 would be far longer than the width of the football2 ,i must therefore look for something still smaller4 ,a grain of small-sized shot will give the right size for the model of our earth4 ,about one hundred of these grains placed side by side will extend to a length equal to the width of the football4 ,now you will be able to form some conception of how enormous the sun really is4 ,think of this earth1 how big we find it when we begin to travel4 ,what a tremendous voyage we have to take to get to ,new ,zealand1 and even then we have only got halfway round the globe4 ,then think that the sun is in the same proportion bigger than the earth as that football is bigger than that grain of shot4 ,if a million of such grains of shot were melted and cast into one globe1 it would not be so large as that football4 ,if a million globes1 as large as our earth1 could be united together1 no doubt a vast globe would be produced1 but it would not be so large as the sun4 ,think of a single house1 with three or four people living in it1 and then think of this mighty ,london1 with its millions of inhabitants4 ,the house will represent our earth1 while great ,london represents the sun6 ,,the ,,spots ,,on ,,the ,,sun4 ,i have shown you that the sun is intensely hot1 and a very long way off1 and enormously big4 ,and now we have to describe the appearance of the surface of the sun when we examine it closely4 ,illustration3 ,,fig4 #ab4--,looking at the ,sun4 ,if you get a piece of very dark glass1 or if you smoke a piece of glass over a candle1 then you can look directly at the sun with comfort4 ,a nicer plan is to prick a pinhole in a card1 through which you can look at the sun without any inconvenience4 ,generally speaking1 a view of the sun in this way will show you only a uniformly bright surface4 ,to study the face of our great luminary carefully1 you must use the aid which the telescope gives to the astronomer4 ,a very good way of doing this is shown in ,fig4 #ab4 ,a small telescope1 fixed on a stand1 is pointed to the sun1 and1 the eyepiece being drawn out somewhat further than when direct observations are being made1 the sun draws its own picture on a screen4 ,this may be examined without any inconvenience1 or without the necessity for any protection to the eye1 and a number of young astronomers can all view the sun at the same moment4 ,on such a picture you will generally see the brilliant surface marked with dark spots1 which are sometimes as numerous as in the case represented in ,fig4 #ac4 ,these spots present very different appearances according to circumstances4 ,one such spot when seen with a very powerful telescope showed the wonderful structure which is represented in ,fig4 #ad4 ,illustration3 ,,fig4 #ac4--,this is what the ,sun sometimes looks like4 ,the visible surface of the sun is entirely formed of intensely heated vapors4 ,we might almost say that the spots are holes1 by which we can look through the brilliant surface to the interior and darker parts4 ,sometimes the spots close up1 and fresh ones will open elsewhere4 ,now and then the whole surface is mottled over in a remarkable way4 ,i give here a picture which was taken from ,mr4 ,nasmyth's beautiful drawing1 in which he shows how the sun sometimes assumes the appearance which has been likened to willow leaves 7,fig4 #ae74 ,this appearance was very noticeable in the great spot of ,september1 #ahih4 ,illustration3 ,,fig4 #ad4--;,a ,sun-spot 7after ,janssen74 ,illustration3 ,,fig4 #ae4--,nasmyth's ,drawing of the ,willow-leaved ,structure of the ,sun4 ,illustration3 ,,fig4 #af4--,spot nearing the ,sun's ,edge4 ,the spots often last long enough to demonstrate a remarkable fact4 ,we must remember that the sun is a great globe1 and that it is poised freely in space4 ,there is nothing to hold it up1 and there is nothing to prevent it from turning round4 ,that it does turn round1 we can prove by careful observation of the spots4 ,i can best illustrate what ,i want by ,fig4 #ag1 which shows six imaginary pictures4 ,the first represents the sun on the #ast day of the month2 the next shows it five days later1 on the #fth2 another view is five days later still1 on the #aath2 and so on until the last picture1 which corresponds to the #bfth4 ,you see1 on the first day there is a spot near the left edge2 by the #fth1 this spot is near the middle2 by the #aath1 it is near the right edge2 then you do not see it at all on the #afth1 or on the #bast2 but on the #bfth it is back in the same place from which it started4 ,we find other spots to have a similar history4 ,they appear to move across the face1 and then to return in a little less than four weeks to the same place where they were originally noticed4 ,these appearances can be illustrated very simply by cutting a small hole through the rind of an orange down to the white interior skin1 which may be darkened with ink4 ,put a knitting needle through the axis of the orange1 and then turn it slowly round4 ,the spot will be found to go through the changes that we have seen4 ,we start with the spot near the left1 it moves across the face1 and then passes to invisibility by moving behind the globe until it reappears again1 after having moved round the back4 ,as the same may be observed with every spot which lasts long enough1 we learn that the changes in the places must be produced by the turning round of the sun4 ,here you see is the way in which an astronomical discovery is made4 ,we first observe the fact that the spots do always appear to move4 ,then we try to account for this1 and we find a very simple explanation1 by supposing that the whole sun1 spots and all1 turns steadily round and round4 ,it can also be proved in a very conclusive manner that no other explanation is possible4 ,this rotation of the sun is always going on uniformly1 and some curious consequences follow from it4 ,the view of the sun which is turned towards us to-day is quite different from that which was towards us a fortnight ago1 or from that which we shall see in a fortnight hence4 ,there is no actual or visible axis about which the sun rotates4 ,in this the sun is like the earth and other celestial bodies4 ,illustration3 ,,fig4 #ag4--;,how the ,sun turns round4 ,,appearances ,,seen ,,during ,a ,,total ,,eclipse ,,of ,,the ,,sun4 ,for a great deal of our knowledge about the sun we are indebted to the moon4 ,it will sometimes happen that the moon comes in between us and the sun1 and produces an eclipse4 ,at first you might think that an eclipse would only have the effect of preventing us from seeing anything of the sun1 but it really reveals most beautiful and interesting objects1 of whose existence we should otherwise be ignorant4 ,the great luminary has curious appendages which are quite hidden under ordinary circumstances4 ,in the full glare of day the dazzling splendor of the sun obliterates and renders invisible these appendages1 which only shine with comparatively feeble light4 ,it fortunately happens that the moon is just large enough to intercept the whole of the direct light from the sun1 or rather1 ,i should say1 from the central parts of the sun4 ,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 doubt1 but with a light so feeble that when presented to us amid the full blaze of sunlight they are invisible4 ,when1 however1 the moon so kindly stops all the stronger beams1 then these faint objects spring into visibility1 and we have the exquisite spectacle of a total eclipse4 ,the objects that ,i desire to mention particularly are the corona and the prominences4 ,illustration3 ,,fig4 #ah4--,total ,eclipse of the ,sun1 ,may #f1 #ahhc 7drawn by ,trouvelot74 ,a pretty picture of the total eclipse of the sun which occurred on ,may #f1 #ahhc1 is here shown 7,fig4 #ah74 ,it is taken from a drawing made by ,m4 ,trouvelot1 who was sent out with a ,french observing party4 ,they went a very long way to see an eclipse1 but what they saw recompensed them for all their trouble4 ,the track along which the phenomenon could be best seen lay in the ,pacific ,ocean1 and a place had to be selected which was so situated that the sun should be high in the heavens at the important moment1 and also that the duration while the total eclipse lasted should be as long as possible4 ,they accordingly went to ,caroline ,island1 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 seconds4 ,short though these precious minutes were1 they were long enough to enable good work to be done4 ,careful preparations had been made so that not a moment should be thrown away4 ,each member of the party had his special duty allotted to him1 and this had been rehearsed so carefully beforehand that when the long-expected moment of 8totality0 arrived there was neither haste nor confusion2 every one carefully went through his part of the programme4 ,m4 ,trouvelot1 for instance1 occupied himself for two minutes and a few seconds in making the sketch that we now show4 ,no doubt an accomplished astronomical artist like ,m4 ,trouvelot would gladly have taken longer time for his sketch of so unique a sight1 but brevity was imperative4 ,he had already had experience of similar eclipses1 so that he was prepared at once to note what ought to be noted1 and the picture we have shown is the result4 ,this was completed within less than half of the duration of totality1 and the artist had still three minutes left to devote to another and quite different part of the work1 which does not concern us at present4 ,illustration3 ,,fig4 #ai4--,the ,corona of the ,sun1 #ahhb 7by ,schuster74 ,i want you particularly to look at these long branches or projections which we see surrounding the sun when totally eclipsed4 ,they shine with a pearly light1 and1 in fact1 it is stated that even during the gloomiest portion of the time there was still as much illumination as on a bright moonlight night4 ,all that light came from this glorious halo round the sun which astronomers call the 8corona40 ,we do not under ordinary circumstances obtain even the slightest glimpse of this object4 ,even during a partial eclipse of the sun it is not visible1 but directly the moon quite covers the sun1 so as to cut off all the direct light1 then the corona springs into visibility4 ,it is always there1 no doubt1 though we cannot see it4 ,one of the most interesting photographs of the eclipsed sun which has ever been taken was that by ,professor ,schuster in #ahhb 7,fig4 #ai74 ,the corona is well shown1 and also a comet4 ,the other appendages to the sun which can be seen during an eclipse are the objects which we call 8prominences40 ,they are of a ruddy color1 and seem to be great flames1 which leap upwards from the glowing surface of the sun below4 ,though the existence of the prominences was first discovered by their presence during eclipses1 it fortunately happens that we are no longer wholly dependent on eclipses for the purpose of making our observations of these remarkable objects4 ,it is true that we may look at the sun with even the biggest and most powerful telescope in the world1 and still not be able to perceive anything of the prominences4 ,we require the aid of a special appliance called the spectroscope to render them visible4 ,but ,i am not now going to describe this ingenious contrivance4 ,i am only going to speak of the results which have been obtained by its means4 ,we shall here again avail ourselves of the experience of ,m4 ,trouvelot for a picture of two of these wonderful appendages4 ,illustration3 ,,fig4 #bj4--,solar ,prominences 7drawn by ,trouvelot74 ,the view 7,fig4 #bj7 shows the ordinary aspect of the sun diversified with groups of dark spots4 ,the fringe around the margin of the globe is of some ruddy material1 forming the base of the flames which rise from the glowing surface4 ,no doubt these flames are also often present on the face of the sun1 but we cannot see them against the brilliant background4 ,they are only perceptible when shown against the sky behind4 ,at two points of this ruddy fringe1 which happen curiously enough to be nearly opposite to each other1 two colossal flames have burst forth4 ,they extend to a vast distance1 which is quite one-third of the width of the sun4 ,the vigor of these outbreaks may be estimated by the remarkable changes which are incessantly going on4 ,these great flames may indeed be said to flicker2 only1 considering their size1 we must allow them a little more time than is demanded for the movements of flames of ordinary dimensions4 ,the great flame on the left was obviously declining in brilliancy when first seen4 ,in a quarter of an hour it had broken up into fragments1 some of which were still to be seen floating in the sun's atmosphere4 ,in ten minutes more the light of this flame had almost entirely vanished4 ,surely these are changes of extraordinary rapidity when we remember the size of this prominence4 ,it was nearly #cjj1jjj miles in height--that is to say1 about thirty-seven times the width of our earth4 ,great as are these prominences1 others have been recorded which are even larger4 ,one of them has been seen to rush up with a speed of #bjj1jjj miles an hour--that is1 with more than two hundred times the pace of the swiftest of rifle-bullets4 ,,night ,,and ,,day4 ,the sun is bright1 and the earth is dark4 ,the sun gives light and heat1 and the earth receives light and heat4 ,we should be in utter darkness were it not for the sun2 at least1 all the light we should have1 beyond our trivial artificial light1 would come from the feeble twinkle of the stars4 ,the moon would be no use1 for the brightness of the moon is merely the reflection of the sunbeams4 ,were the sun's light completely extinguished we could never again see the moon1 and we should also miss from the sky a few other bodies1 which we call planets1 such as ,jupiter and ,venus1 ,mars and ,saturn4 ,but the stars would be the same as before1 for they do not depend upon the sun for their light4 ,we shall1 indeed1 afterwards see that each star is itself a sun4 ,picture to yourself the earth as receiving a stream of sunbeams4 ,these beams fall on one half of our globe1 and give to it the brilliance of day4 ,the other half of the earth of course receives no sunlight4 ,it is in the shadow1 and consequently the darkness of night there prevails4 ,the boundary between light and darkness is not quite sharply defined1 for the pleasant twilight softens it a little1 so that we pass gradually from day to night4 ,looking at the progress of the sun in the course of the day1 we see that he rises far away in the east1 then he gradually moves across the heavens past the south1 and in the evening declines to the west1 sets1 and disappears4 ,all through the night the sun is gradually moving round the opposite side of the earth1 illuminating ,new ,zealand and ,japan and other remote countries1 and then gradually working round to the east1 where he starts afresh to give us a new day here4 ,our ancestors many ages ago did not know that the earth was round4 ,they thought it was a great flat plain1 and that it extended endlessly in every direction4 ,they were1 however1 much puzzled about the sun4 ,they could see from the coasts of ,france and ,spain or ,britain that the sun gradually disappeared in the ocean2 they thought that it actually took a plunge into the sea4 ,this would certainly quench the glowing sun2 and some of the ancients used to think they heard the dreadful hissing noise when the great red-hot body dropped into the ,atlantic4 ,but there was here a difficulty4 ,if the sun were to be chilled down every evening by dropping into the water hundreds of miles away to the west1 how did it happen that early the next morning he came up as fresh and as hot as ever1 hundreds of miles away to the east8 ,for this1 indeed1 it seemed hard to account4 ,some said that we had an entirely new sun every day4 ,the gods started the sun far off in the east1 and after having run its course it perished in the west4 ,all the night the gods were busy preparing a new sun to be used on the succeeding day4 ,but this was thought to be such a waste of good suns that a more economical theory was afterwards proposed4 ,the ancients believed that the continents of the earth1 so far as they knew them1 were surrounded by a limitless ocean4 ,at the north1 there were high mountains and ice and snow1 which they thought prevented access to this ocean from civilized regions4 ,vulcan was the presiding deity who navigated those wastes of waters1 and to him was intrusted the responsible duty of saving the sun from extinction4 ,he had a great boat ready1 so that when the sun was just dropping into the ocean at sunset he caught it1 and during all the night he paddled with his glorious cargo round by the north4 ,the glow of the sun during the voyage could even be sometimes traced in summer over the great highlands to the north4 ,this1 at all events1 was their way of accounting for the long midsummer twilight4 ,after a tedious night's voyage ,vulcan got round to the east in good time for sunrise4 ,then he shot the sun up with such terrific force that it would go across the whole sky1 and then the industrious deity paddled back with all his might by the way he had come1 so as to be ready to catch the sun in the evening and thus repeat his never-ending task4 ,,the ,,daily ,,rotation ,,of ,,the ,,earth4 ,vulcan and his boat seemed a pretty way of accounting for the sun's apparent motion4 ,the chief drawback was that it was all work and no play for poor ,vulcan4 ,there were also a few other difficulties4 ,captains of ships told us that they had sailed out on the great sea1 and that so far from finding that the ocean extended on and on in one flat plain forever1 the water seemed to bend round1 so that1 in fact1 after sailing far enough in the same direction1 they found that they would be brought back again to the place from which they started4 ,they also knew a little about the north4 ,they told us that there could be no such ocean as that which ,vulcan in this fable was supposed to navigate4 ,it also appeared that ships had been voyaging all over the globe night and day in every direction1 and that no captain had ever seen the sun coming down to the sea1 and still less had he ever met with ,vulcan in the course of his incessant voyages4 ,thus it was discovered that the earth could not be a never-ending flat1 but that it must be a globe1 poised freely in space without any attachment to hold it up4 ,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 feet4 ,this is1 indeed1 what it seems to do4 ,but there was a great difficulty about this explanation1 which began to be perceived when the size and distance of the sun were considered4 ,it required the sun to possess an alarming activity4 ,he would actually have to rush round a circle one hundred and eighty million miles in diameter and complete this astonishing voyage once every day4 ,illustration3 ,,fig4 #ba4--;,how we illustrate the ,changes between ,day and ,night4 ,a little reflection will show that a very much simpler explanation was available4 ,it was shown that the sun need not revolve round the earth once every day1 but that everything would be explained if the earth itself turned round in such a way as to produce the changes from day to night4 ,we may illustrate the case by this figure 7,fig4 #ba74 ,the small globe is the earth1 which ,i can turn by the handle4 ,the lamp will represent the sun1 and1 as at present shown1 the side of the earth1 on which ,england lies1 is towards the lamp and in full day4 ,on the opposite side of the globe are other countries such as ,new ,zealand1 and there it is dark4 ,you see that by simply turning the handle ,i can move ,england gradually round so that it passes into the dark side1 and then night falls over the country4 ,at the same time ,new ,zealand is turned round to enjoy the smiles of day4 ,this is a very simple method of accounting for the succession of day and night1 and it is also the true method4 ,we have already seen that the sun turns round1 and now we find that the earth also turns1 but the little body1 the earth1 goes much the faster1 for it makes twenty-five turns while the sun goes round once4 ,our earth is at this moment spinning round at a speed so great that ,london moves many hundreds of miles every hour4 ,a town near the equator would gallop round at a pace of more than a thousand miles an hour--quicker1 in fact1 than a rifle-bullet4 ,don't you think that we ought to perceive that we are being whirled about in this terrific fashion8 ,we know that when we are flying along in a railway train1 we feel the jolting and we hear the noise1 and we feel the blast of air if we put our heads out of window1 and we see the trees as they appear to rush past4 ,all these things tell us that we are in rapid motion4 ,but suppose these sensations were absent4 ,imagine a line so perfectly laid that no jolts are perceptible1 and that no racket is heard2 draw down the blinds so that nothing can be seen1 how then are we to know that we are moving8 ,indeed1 your grandfathers used to be able to enjoy such a tranquil locomotion4 ,i remember seeing in my childhood the fly-boats1 as they were called1 on the ,royal ,canal1 wherein passengers were conveyed from ,dublin to the ,west of ,ireland1 before the railway was made4 ,the fly-boat was a sort of ,noah's ark in appearance1 drawn by a horse cantering along the towing-path4 ,in the cabin of such a vessel1 where there was not the slightest motion of rolling or pitching--nothing but noiseless gliding along the canal--no one would be conscious of motion1 so long as he did not look through the cabin windows4 ,no one was ever seasick in a fly-boat2 it was the perfection of travelling for those who loved ease and quiet4 ,the motion of the earth round its axis is1 so far1 like that of the fly-boat4 ,it is so absolutely smooth that we do not feel anything1 and we only become conscious of it by looking at outside objects4 ,these are the sun1 or the moon1 or the stars4 ,we see these bodies apparently going through their unvarying rising and setting1 just as1 in looking out from the fly-boat1 the passengers in that quaint old conveyance could see the houses and trees as they passed4 ,seeing is believing2 and ,i should like here1 in this very theatre1 to show you that we are actually turning round2 and this ,i am enabled to do by the kindness of my distinguished friend1 ,professor ,dewar4 ,illustration3 ,,fig4 #bb4--;,a ,pendulum4 ,i am tempted to wish that ,i had ,aladdin's lamp for the moment1 for ,i would rub it1 and when the great genie appeared1 ,i would bid him take the ,royal ,institution1 and all of us here1 to a place which everybody has heard of1 and nobody has seen--,i mean the ,north ,pole4 ,it would be so easy to describe the experiment ,i am about to show you1 there4 ,it is not so easy here4 ,but it will be sufficiently accurate for our purpose to suppose that we actually have made the voyage1 and that this is the ,pole at the centre of the lecture-table4 ,the direction of the axis round which the earth is turning is a line pointing up straight to the ceiling4 ,this lecture-table and all the rest of the theatre is going round4 ,in about six hours it will have moved a quarter of the way1 and in twenty-four hours it will have gone completely round4 ,that is1 at least1 what would happen if we were actually at the ,pole4 ,as we are not there1 for the ,pole is many miles away from the ,royal ,institution1 ,i must slightly modify this statement1 and say that the table here takes more than twenty-four hours to go round4 ,and now ,i want some way of proving that such is actually the case4 ,there is no use in our merely looking at it1 because we ourselves1 and this whole building1 and the whole of ,london1 are all turning together4 ,what we want is something which does not partake of the motion4 ,here is a heavy leaden ball 7,fig4 #bb74 ,it is fastened to the roof by a fine steel wire1 and you see it swings to and fro with a deliberate and graceful motion4 ,i want it to oscillate very steadily1 so ,i draw it to one side and tie it by a piece of thread to a support1 and then ,i burn the thread1 and the great ball begins to swing to and fro4 ,it would continue to do so for an hour1 or indeed for several hours1 and it is a peculiarity of this motion that the vibration always remains in the same direction in space4 ,even the rotation of the earth will not affect the plane of this great pendulum1 so far at least as our experiment is concerned4 ,here1 then1 we have a method of testing my assertion about the turning round of this theatre4 ,i mark a line on the table1 directly underneath the motion of the ball to and fro4 ,if we could wait for an hour or so1 we should see that the motion of the ball seemed to have altered to a direction inclined to its original position1 but it is really the table that has moved1 for the direction of the motion of the ball is unaltered4 ,we cannot1 however1 wait so long1 therefore ,i show you the ingenious method which ,professor ,dewar has devised4 ,by a beam from the electric light1 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 round1 with respect to the oscillations of the pendulum4 ,this celebrated experiment proves by actual inspection that the earth must be rotating4 ,by measuring the motion we might even calculate the length of the day1 though ,i do not say it would be an accurate method of doing so4 ,the proper way of finding how long the earth takes to turn round is by observing the stars4 ,fix on any star you please1 and note it in a certain position to-night2 if you then observe the moment when the star is in the same place to-morrow1 the interval of time that has elapsed is the true duration of one complete rotation4 ,when accurately measured its length is found to be #bc hours #ef minutes #d seconds1 or about four minutes shorter than the ordinary day1 measured from one noon to the next4 ,,annual ,,motion ,,of ,,the ,,earth ,,round ,,the ,,sun4 ,illustration3 ,,fig4 #bc4--,the ,changes of the ,sun with the ,seasons4 ,i have as yet only been speaking of the daily movements by which the sun appears to go across the heavens between morning and evening4 ,we next consider the annual movements which give rise to the changes of the seasons4 ,it is now ,christmastide1 when the days are short and dark1 while six months ago the days were long and glorious in the warmth and brightness of summer4 ,a similar recurrence of the seasons takes place every year1 and thus we learn that some great changes alter the relation between the earth and the sun year after year4 ,we must try and explain this4 ,why is it that we enjoy warmth at one season1 and suffer from frost and snow at another8 ,note first a great difference between the sun in summer and the sun in winter4 ,i will ask you to look out at noon any day when the clouds are absent1 and you will then find the sun at the highest point it reaches during the day4 ,all the morning the sun has been gradually climbing from the east2 all the afternoon it will be gradually sinking down to the west4 ,let us make the same observation at different parts of the year4 ,suppose we take the shortest day in ,december4 ,you will look out about twelve o'clock from some situation which affords a view towards the south1 and there1 as shown in the adjoining sketch 7,fig4 #bc71 is the midwinter sun4 ,but now the spring approaches1 and the days begin to lengthen4 ,if you watch the sun you will see it pass higher and higher every noon until ,midsummer ,day is reached1 and then the sun at noon is found quite high up in the sky4 ,as autumn draws near1 the sun at noon creeps downwards again until1 when the next shortest day has come round1 we find that it passes just where it did at the previous midwinter4 ,with unceasing regularity year after year the sun goes through these changes4 ,when he is high at noon we have days both long and warm2 when he is low at noon we have days both short and cold4 ,vulcan with his golden boat was naturally expected to give an explanation of this4 ,as the summer drew on1 each day ,vulcan shot out the sun with a stronger impulse1 so that it should ascend higher and higher4 ,his greatest effort was made on ,midsummer ,day1 when1 after rowing but a little way round from the north towards the east1 he drove off the sun with a terrific effort4 ,the sun soared aloft to the utmost height it could reach1 and in the meantime ,vulcan returned to the west to be ready to catch the sun as it descended4 ,on the other hand1 in midwinter1 he came round much further through the east to the south1 and then shot up the sun with his feeblest effort1 and had to paddle as hard as ever he could so as to complete his long return voyage during the brief day4 ,illustration3 ,,fig4 #bd4--;,how the ,stars are to be seen in broad ,daylight4 ,it is evident that there are two quite distinct kinds of motion of the sun4 ,there is first the daily rising and setting1 for which we have accounted by showing that it is merely an appearance produced by the fact that the earth is turning round4 ,but now we have been considering quite a different motion by which the sun seems to creep up and down in the heavens1 and this takes a whole year to go through its changes4 ,there is still another point which we must consider before we can understand all these puzzling movements of the sun4 ,we shall ask the stars to help us by their familiar constellations4 ,you know1 perhaps1 the ,great ,bear1 or the ,plough as it is often called1 and ,orion4 ,there are also ,aries the ,ram1 ,taurus the ,bull1 and other fancifully named systems4 ,these constellations have been known for countless ages1 and for our present purposes we may think of them as permanent groups in the heavens1 which do not alter either their own shapes or their positions relatively to each other4 ,these groups of stars extend all around the sky4 ,they are not only over our heads and on all sides down to the horizon1 but if we could dig a deep hole through the earth1 coming out somewhere near ,new ,zealand1 and if we then looked through1 we should see that there was another vault of stars beneath us4 ,we stand on our comparatively little earth in what seems the centre of this great universe of stars all around4 ,it is true we do not often see the stars in broad daylight1 but they are there nevertheless4 ,the blaze of sunlight makes them invisible4 ,a good telescope will always show the stars1 and even without a telescope they can sometimes be seen in daylight in rather an odd way4 ,if you can obtain a glimpse of the blue sky on a fine day from the bottom of a coal pit1 stars are often visible4 ,the top of the shaft is1 however1 generally obstructed by the machinery for hoisting up the coal1 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 7,fig4 #bd74 ,the fact is that the long tube has the effect of completely screening from the eye the direct light of the sun4 ,the eye thus becomes more sensitive1 and the feeble light from the stars can make its impression1 and produce vision4 ,from all these various lines of reasoning we see that there can be no doubt of the continuous presence of stars above and around us1 and below us1 on every side1 and at all times4 ,illustration3 ,,fig4 #be4--,the ,sun seems to revolve around the ,earth4 ,if you look out at ,christmas time1 towards the south1 you will see the ,belt of ,orion and the ,dog ,star in a splendid portion of the heavens4 ,these stars you will see every winter in the same place4 ,but you may look in vain for them in summer4 ,no doubt you can see stars in the summer evenings1 but they will be totally different from those that adorned the skies in winter4 ,each season has its own constellations4 ,this simple fact was known to the ancients1 and we shall find its explanation full of meaning4 ,let us select four well-known constellations which will best answer our purpose4 ,they lie in a circle round the heavens4 ,they are ,orion1 ,virgo1 ,scorpio1 and ,pisces4 ,i am supposing that you are looking out at midnight towards the south4 ,in ,december you will see ,orion2 in ,march1 ,virgo2 in ,june1 ,scorpio2 and in ,september1 ,pisces2 and then next ,december you will be looking at ,orion again4 ,see what this proves4 ,at midnight1 of course1 the sun is at the other side of the earth1 so that if ,i am looking at ,orion in midwinter the sun must be behind my back4 ,look at our little picture 7,fig4 #be74 ,the earth is in the middle1 and the sun must be on the opposite side to ,orion4 ,that is1 the sun must be somewhere about the position ,i have marked at ,a4 ,in ,march we see ,virgo in the south at midnight1 when1 of course1 the sun is at the other side of the earth2 so that the sun must be somewhere at ,b4 ,in ,june ,scorpio is seen1 so that the sun must be at the other side1 at ,c4 ,that is to say1 in midsummer the sun is in that part of the sky where ,orion is situated4 ,if1 therefore1 on a bright ,june day we could see the stars1 we should find ,orion in the south4 ,but1 of course1 the light of the sun makes ,orion invisible4 ,we can1 however1 see the stars by our telescopes1 and on rare occasions an eclipse of the sun will occur1 by which he is temporarily extinguished1 and then we can see the stars without the help of a telescope1 even though it is daytime4 ,illustration3 ,,fig4 #bf4--,the ,earth1 however1 really revolves around the ,sun4 ,thus it would seem as if the sun were first at ,a and then at ,b1 ,c1 and ,d1 and then began to go round again4 ,i say it would seem as if the sun had these movements1 and the ancients thought there was no doubt about the matter4 ,even after it was plain that the earth turned round on its axis so as to give the changes of day and night1 it was still thought necessary to suppose that the sun went round the earth once in the year1 in order to explain how the changes in the stars during the different seasons were produced4 ,here is another case in which we must be careful to distinguish between what appears to be true and what is actually the case4 ,everything that we undoubtedly see would be just as well explained by supposing that the sun remained at rest1 and that the earth revolved around it1 as in ,fig4 #bf4 ,if1 for instance1 the earth were at ,a in midwinter1 then the sun is on the opposite side to ,orion1 and of course at midnight we shall be able to see ,orion4 ,so in spring the earth is at ,b1 and we see ,virgo1 and similarly in summer we have ,scorpio1 and in autumn ,pisces4 ,thus all that is actually visible could be fully accounted for by regarding the sun as fixed in the centre1 and the earth as travelling round it from ,a to ,b1 to ,c and to ,d respectively1 and completing the journey in a twelvemonth4 ,which idea are we to adopt8 ,shall we say that the earth goes round the sun1 or the sun goes round the earth8 ,i remember an old college story1 which ,i cannot help giving you at this place4 ,it may serve to lighten what ,i fear you must otherwise have thought rather a tedious part of our subject4 ,there were three students brought up for examination in astronomy1 and they showed a lamentable ignorance of the subject1 but the examiner1 being a kind-hearted man1 wished1 if possible1 to pass them2 and so he proposed to the three youths the very simplest question that he could think of4 ,accordingly1 addressing the first student1 he said3 8,now tell me1 does the earth go round the sun1 or the sun go round the earth80 8,it is--the earth--goes round the sun40 8,what do you say80 he inquired1 turning rather suddenly on the next1 who gasped out3 8,oh1 sir--of course--it is the sun goes round the earth40 8,what do you say80 he shouted at the third unhappy victim4 8,oh1 sir1 it is--sometimes one way1 sir1 and sometimes the other60 ,but which is it8 ,well1 we must remember that the earth is comparatively a very little body and the sun a very big one1 so it is not at all surprising to learn that the earth goes round the sun1 which remains1 practically speaking1 at rest in the centre4 ,thus our great earth and all it contains are continually bound in what is very nearly a circular course round the great luminary4 ,you will find it instructive to work out this little sum4 ,how fast is the earth moving1 or how far do we go in a second8 ,we are about #ic1jjj1jjj miles from the sun1 and the great circle that we go round has a diameter twice as great as this--that is1 about #ahf1jjj1jjj miles4 ,the circumference of a circle is nearly three and one-seventh times its diameter1 and accordingly the whole length of the voyage in the year is about #ehe1jjj1jjj miles4 ,this has to be accomplished in #cfe days1 so that the daily run must be about #a1fjj1jjj miles4 ,we divide this by #bd1 to find the distance journeyed each hour1 which we find to be about #fg1jjj miles2 and we must divide this again by #fj to find the length covered in a minute1 and by #fj again for the progress made each second4 ,it is truly startling to find that1 night and day1 this great earth has to travel more than eighteen miles every second in order to get round its mighty path in the allotted time4 ,i began this lecture about forty minutes ago1 and ,i think from what ,i have said you will be able to calculate a result that will1 ,i dare say1 astonish you4 ,in these forty minutes we have moved about #de1jjj miles4 ,no doubt my lecture commenced in this hall1 and in your presence2 but can ,i truly say ,i began it here8 ,well1 no2 ,i began it not here1 but at a place #de1jjj miles away2 but we have all been travelling together1 and the journey has been so very smooth and free from all jolts1 that we never thought anything about the motion4 ,i am sure many of those to whom ,i am now speaking have read accounts of voyages in the ,arctic regions4 ,you have been told of the sufferings of the crews during the long winters1 amid the ice and snow2 and you have heard how1 during that dismal period1 there is total darkness1 for the sun never rises for weeks and months together4 ,on the other hand1 these northern regions often present a more cheerful picture4 ,during midsummer1 the long darkness of winter is atoned for by perpetual sunshine4 ,at midnight there is still the full brilliance of day1 and the sun1 though low1 no doubt1 has not passed below the horizon4 ,even in the northerly parts of ,europe we can see the midnight sun4 ,lord ,dufferin1 in his delightful narrative of a cruise1 entitled 8,letters from ,high ,latitudes10 gives an interesting illustration of the perplexities arising from endless daylight4 ,it appears that everything went on happily until the fatal moment when the yacht crossed the ,arctic circle4 ,then it was that dire tribulation arose among the poultry4 ,a fine cock was the cause of the trouble4 ,knowing his duty1 he always liked to be particular about performing the important task of crowing at sunrise4 ,this he could do regularly1 so long as the yacht remained in reasonable latitudes1 where the sun behaved properly4 ,but when they crossed the ,arctic circle1 the cock was confronted with a wholly new experience4 ,the sun never set in the evening1 and consequently never had to rise in the morning4 ,what was the distracted bird to do8 ,he did everything4 ,he burst into occasional fits of terrific crowing at all sorts of hours1 then he gave up crowing altogether1 but finding that did not mend matters1 he took to crowing incessantly4 ,exhaustion was succeeded by delirium1 and rather than live any longer in a universe where the sun was capable of pranks so heartless1 the indignant fowl flung himself from the vessel and perished in the ,arctic ,ocean4 ,,the ,,changes ,,of ,,the ,,seasons4 ,illustration3 ,,fig4 #bg4--,the ,changes of the ,seasons4 ,in the adjoining figure1 ,i show a little sketch 7,fig4 #bg71 by which ,i try to explain the changes of the seasons4 ,it exhibits four positions of the earth1 one on each side of the sun4 ,the left4 ,a1 represents the earth when summer gladdens the northern hemisphere2 while the right1 ,c1 shows winter in the same region4 ,you will see the two central lines which represent the axis about which the earth rotates4 ,of course1 the earth has no visible axis4 ,the line which runs through the globe from the ,north to the ,south ,pole is imaginary4 ,it remains fixed in the earth1 for we can prove in our observatories that the ,pole does not shift its position to any considerable extent in the earth itself4 ,in fact1 if we could reach the ,north ,pole and drive a peg into the ground year after year to mark the exact spot1 we should find that the position of the ,pole was sensibly the same4 ,does it not seem strange that we should be able to know so much about the ,pole1 though we have never been able to get there2 have never1 in fact1 been able to get within less than #djj miles of it8 ,i think you will be able to understand the point quite easily4 ,the latitude of a place1 as you know from your geography1 is the number of degrees1 and parts of a degree1 between that place and the equator4 ,in our observatories1 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 perceptible4 ,as we find that the latitudes of our observatories remain sensibly unchanged from year to year1 we are certain that the ,pole must remain in the same place4 ,indeed1 if the ,pole were to alter its position by the distance of a stone's throw1 the careful watchers in many observatories would speedily detect the occurrence4 ,and now ,i must direct your attention to something apparently quite different4 ,when the battle of ,waterloo was fought1 the great victory was won with the aid of the old-fashioned musket1 a smooth-bore gun which was loaded at the muzzle with a good charge of powder1 and then a round bullet was rammed down4 8,brown ,bess10 as the musket was called1 was a most efficient weapon at close quarters1 and indeed at any distance when the bullet hit2 but there was the difficulty4 ,the round bullets1 rushing up the tube and out into the air in a somewhat vague manner1 had a habit of roaming about1 which was quite incompatible with the accurate shooting of our modern rifles4 ,one great improvement in small arms consisted in giving to the bullet a rapid rotation about an axis which is in the line of fire4 ,this is what the rifle accomplishes4 ,the grooves in the barrel of the rifle twist round1 and though they only give half a complete turn in the length of the barrel1 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 second4 ,even with the old-fashioned round bullet1 the rifling of the barrel effected great improvement in the accuracy of the shooting4 ,the introduction of the elongated bullets was another great improvement1 while the adaptation of breech-loading enabled a bullet to be used rather larger than that which could have been forced down the barrel1 and thus it was insured that the grooves should bite into the bullet as it hurries past and impart the necessary spin4 ,a body rapidly rotating about an axis has a tendency to preserve the direction of that axis1 and powerfully resists any attempt to change it4 ,our earth is spinning in this fashion4 ,it is true that the rotation is1 in one sense1 a slow one1 for it requires almost an entire day for each rotation4 ,but when we remember the dimensions of our earth1 we shall modify this notion4 ,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 time4 ,so far1 therefore1 the earth moves like a rifle-bullet1 and the direction of its axis remains constant4 ,in the course of the great voyage between summer and winter1 the earth travels from one side of the sun to the opposite side1 and in doing so it still continues to spin about an axis parallel to the original direction4 ,see the consequences which follow4 ,the sun illuminates half the earth1 and in the left position in ,fig4 #bg1 representing summer1 the ,north ,pole is turned over towards the sun1 and lies in the bright half of the earth4 ,there is continual day at the ,north ,pole1 and night is unknown there at this time of year1 because the turning of the earth about its axis will not bring the ,pole nor the regions near the ,pole into the dark hemisphere4 ,thus it is that the ,arctic regions enjoy perpetual day at this season4 ,look now at the position of ,england when the northern hemisphere is tilted towards the sun1 and is consequently enjoying the full splendor of midsummer4 ,as the earth turns round1 ,england will gradually cross the boundary between light and shade1 and will enter upon the darkened hemisphere4 ,then there will be night in ,england1 but you will see from the figure that the day is much longer than the night1 and hence it is that we enjoy the fine long days in summer4 ,we next look at a different scene six months later4 ,the earth has reached the other side of the sun1 but the axis has remained parallel to itself1 consequently the ,north ,pole is now inclined entirely away from the sun4 ,the earth continues to turn round as before1 but its movements do not bring the ,north ,pole or the surrounding ,arctic regions out of the dark hemisphere1 and consequently the night must be unbroken in these dismal circumstances4 ,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 succession4 ,observe also the changed circumstances of ,england4 ,in the course of each twenty-four hours it lies much longer in the dark half of the earth than in the bright1 and consequently there is only a short day succeeded by a long night4 ,,sunshine ,,at ,,the ,,north ,,pole4 ,it is a privilege of astronomers to be able to predict events that will happen in thousands of years to come1 and to describe things accurately though they never saw them1 and though nobody else has ever seen them either4 ,no one has ever yet got to the ,north ,pole1 but whenever they do1 we are able to tell them much of what they will see there4 ,we may leave it to ,jules ,verne to describe how the journey is to be made1 and how the party are to be kept alive at the ,north ,pole4 ,i shall give a picture of the changes of the seasons1 and of the appearance in the stars1 as seen from thence4 ,we shall1 therefore1 prepare to make observations from that very particular spot on this earth--the ,north ,pole4 ,i suppose that eternal ice and snow abide there4 ,i don't think it would be a pleasant residence4 ,however1 we shall arrange to arrive on ,midsummer ,day1 prepared to make a year's sojourn4 ,the first question to be settled is the erection of the hut4 ,in a cold country it is important to give the right aspect1 and we are in the habit of saying that a southerly aspect is the best and warmest1 while the north and the east are suggestive only of chills and discomfort4 ,but what is a southerly aspect at the ,north ,pole1 or1 rather1 what is not a southerly aspect8 ,whatever way we look from the ,north ,pole we are facing due south4 ,there is no such thing as east or west2 every way is the southward way4 ,this is truly an odd part of the earth4 ,the only other locality at all resembling it would be the ,south ,pole1 from which all directions would be north4 ,the sun would be moving all through the day in a fashion utterly unlike its behavior in our latitudes4 ,there would1 of course1 be no such thing as rising and setting4 ,the sun would1 indeed1 at first seem neither to go any nearer to the horizon nor to rise any higher above it1 but would simply go round and round the sky4 ,then it would gradually get lower and lower1 moving round day after day in a sort of spiral1 until at last it would get down so low that it would just graze the horizon1 right round which it would circulate till half the sun was below1 and then until the whole disk had disappeared4 ,even though the sun had now vanished1 a twilight glow would for some time be continuous4 ,it would seem to come from a source moving round and round below the horizon1 then gradually the light would become fainter and fainter until at last the winter of utter and continuous blackness had set in4 ,the first indications of the return of spring would be detected by a feeble glow near the horizon1 which would seem to move round and round day after day4 ,then this glow would pass into a continuous dawn1 gradually increasing until the sun's edge crept into visibility1 and the great globe would at last begin to climb the heavens by its continual spiral until midsummer was reached1 when the change would go on again as before4 ,our first excursion to the country of ,star-land has now been taken1 and we have naturally commenced by studying that sun to which we owe so much4 ,but we shall have to learn that though our sun is of such vital importance to us1 yet1 in magnificence and size1 he has many rivals among the host of stars4 ,,lecture ,,ii4 ,,the ,,moon4 ,the ,phases of our ,attendant1 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 ,moon4 ,,the ,,phases ,,of ,,our ,,attendant1 ,,the ,,moon4 ,the first day of the week is related to the greatest body in the heavens--the sun--and accordingly we call that day ,sun-day4 ,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-day1 we do say ,monday1 which is very nearly the same4 ,in ,french1 too1 we have lune for moon1 and ,lundi signifies our ,monday4 ,the other days of the week also have names derived from the heavens1 but of these we shall speak hereafter4 ,we are now going to talk about the moon4 ,we can divide the objects in this room into two classes4 ,there are the bright faces in front of me1 and there are the bright electric lights above4 ,the electric lights give light1 and the faces receive it4 ,i can see both lights and faces2 but ,i see the electric lamps by the light which they themselves give4 ,i see the faces by the illumination which they have received from the electric lights4 ,this is a very simple distinction1 but it is a very important one in ,star-land4 ,among all these bodies which glitter in the heavens there are some which shine by their own light1 like the lamps4 ,there are others only brilliant by reflected light1 like the faces4 ,it seems impossible for us to confuse the brightness of a pleasant face with the beam from a pretty lamp1 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 it4 ,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 bodies4 ,astronomers themselves have been sometimes deceived in this way4 ,the easiest example we can give of bodies so contrasted is found in the case of the sun and the moon4 ,of course1 as we have already seen1 the sun is the splendid source of light which it scatters all around4 ,some of that light falls on our earth to give us the glories of the day2 some of the sunbeams fall on the moon1 and though the moon has itself no more light than earth or stones1 yet when exposed to a torrent of sunbeams1 she enjoys a day as we do4 ,one side of her is brilliantly lighted2 and this it is which renders our satellite visible4 ,hence we explain the marked contrast between the sun and the moon4 ,the whole of the sun is always bright2 while half of the moon is always in darkness4 ,when the bright side of the moon is turned directly towards us1 then1 no doubt1 we see a complete circle1 and we say the moon is full4 ,on other occasions a portion only of the bright surface is directed to us1 and thus are produced the beautiful crescents and semicircles and other phases of the moon4 ,illustration3 ,,fig4 #bh4--,to show that the ,moon is lighted by ,sunbeams4 ,a simple apparatus 7illustrated in ,fig4 #bh7 will explain their various appearances4 ,the large india-rubber ball there shown represents the moon1 which ,i shall illuminate by a beam from the electric light4 ,the side of the ball turned towards the light is glowing brilliantly1 and from the right side of the room you see nearly the whole of the bright side4 ,to you the moon is nearly full4 ,from the centre of the room you see the moon like a semicircle1 and from the left it appears a thin crescent of light4 ,i alter the position of the ball with respect to the lamp1 and now you see the phases are quite changed4 ,to those on my left our mimic moon is now full2 to those on my right the moon is almost new1 or is visible with only a slender crescent4 ,from the centre of the room the quarter is visible as before4 ,we can also show the same series of changes by a little contrivance of ,figs4 #bi and #cj4 ,thus every phase of the moon 7,fig4 #ca71 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 moon1 can be completely accounted for by the different aspects of a globe1 of which one-half is brilliantly illuminated4 ,illustration3 ,,fig4 #bi4 ,,fig4 #cj4 ,the ,phases of the ,moon4 ,we can now explain a beautiful phenomenon that you will see when the moon is still quite young4 ,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 circle1 of which a part is the brilliant crescent4 ,this can only be explained by showing how some light has fallen on the shadowed side2 for nothing which is not itself a source of light can ever become visible unless illuminated by light from some other body4 ,illustration3 ,,fig4 #ca4--,the ,changes in the ,moon4 ,let us suppose that there is a man on the moon who is looking at the earth4 ,to him the earth will appear in the same way as the moon appears to us1 only very much larger4 ,at the time of new moon the bright side of the earth will be turned directly towards him1 so that the man in the moon will see an earth nearly full1 and consequently pouring forth a large flood of light4 ,think of the brightest of all the bright moonlight nights you have ever seen on earth1 and then think of a light which would be produced if you had thirteen moons1 all as big and as bright as our full moon1 shining together4 ,how splendid the night would then be6 ,you would be able to read a book quite easily6 ,well1 that is the sort of illumination which the lunar man will enjoy under these circumstances2 all the features of his country will be brightly lighted up by the full earth4 ,of course1 this earth-lighted side of the moon cannot be compared in brilliancy with the sun-lighted side1 but the brightness will still be perceptible1 so that when from the earth we look at the moon1 we see this glow distributed all over the dark portion2 that is1 we observe the feebly lighted globe clasped in the brilliant arms of the crescent4 ,at a later phase the dark part of the moon entirely ceases to be visible1 and this for a double reason3 firstly1 the bright side of the earth is then not so fully turned to the moon1 and therefore the illumination it receives from earth-shine is not so great2 and1 secondly1 the increasing size of the sun-lighted part of the moon has such an augmented glow that the fainter light is overpowered by contrast4 ,you must remember that more light does not always increase the number of things that can be seen4 ,it has sometimes the opposite effect4 ,have we not already mentioned how the brightness of day makes the stars invisible8 ,the moon herself1 seen in full daylight1 seems no brighter than a small particle of white cloud4 ,,the ,,size ,,of ,,the ,,moon4 ,it is not easy to answer the question which ,i am sometimes asked1 8,is the moon very big80 ,i would meet that question by another1 8,is a cat a big animal80 ,the fact is1 there is no such thing as absolute bigness or smallness4 ,the cat is no doubt a small animal when compared with the tiger1 but ,i think a mouse would probably tell you that the cat was quite a big animal--rather too big1 indeed1 in the mouse's opinion4 ,and the tiger himself is small compared with an elephant1 while the mouse is large as compared with a fly4 ,when we talk of the bigness or the smallness of a body1 we must always consider what we are going to compare it with4 ,it is natural in speaking of the moon to compare it with our own globe1 and then we can say that the moon is a small body4 ,the relative sizes of the earth and the moon may be illustrated by objects of very much smaller dimensions4 ,both a tennis ball and a football are no doubt familiar objects to everybody4 ,if the earth be represented by the football1 then the moon would be about as large as the lawn-tennis ball4 ,but this proportion is not quite accurate1 so ,i will suggest to you an instructive way of making a better pair of models of the earth and the moon4 ,in fact1 experiments somewhat similar to those ,i describe have been actually going on in every kitchen in the land during this festive season4 ,for have not globes and balls of all sorts and sizes been made of plum-pudding1 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 moon4 ,there is first to be a nice little round plum-pudding1 three inches in diameter4 ,it is just a little bigger than a cricket ball4 ,it should1 however1 only make its appearance at a bachelor's table4 ,were it set down before a hearty circle on ,christmas ,day1 dire disappointment would result4 ,one boy of sound constitution could eat it all4 ,perhaps it would weigh about three-quarters of a pound4 ,this little globe is to represent the moon4 ,illustration3 ,,fig4 #cb4--,relative ,sizes of the ,earth and ,moon4 ,another plum-pudding is to be constructed which shall represent the earth 7,fig4 #cb74 ,we must1 however1 beg the cook to observe the proportions4 ,the width of the earth1 or the diameter1 to use the proper word1 is about four times the diameter of the moon4 ,hence1 as the small plum-pudding was three inches across1 the large one must have a diameter of twelve inches4 ,this will be a family pudding of truly satisfactory dimensions2 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 diameter4 ,these models having been duly made1 and boiled1 and placed on the table1 we are now to propose the following problem3-- 8,if one schoolboy could eat the small plum-pudding1 how many boys would be required to dispose of the large one80 ,the hasty person1 who does not reflect1 will at once dash out the answer1 8,four60 ,he will say1 8,it is quite plain that1 since one of the puddings has four times the diameter of the other1 it must be four times as big2 and therefore1 as one boy is able to eat the small pudding1 four boys will be adequate for the large one40 ,but the hasty person will1 as usual1 be quite wrong4 ,his argument would be sound if it were merely two pieces of sugar-stick that he was comparing2 no doubt there is only four times as much material in a piece twelve inches long as there is in a piece three inches long4 ,but the plum-puddings have breadth and depth1 which are in the same proportions as the length1 and the consequence is that the large plum-pudding is far more than four times as big as the small one4 ,no four boys1 however admirable their capacities1 would be equal to the task of consuming it4 ,nor even if four more boys were called in to help would the dish be cleared4 ,twenty boys1 forty boys1 fifty boys would not be enough4 ,it would take sixty-four boys to demolish the magnificent plum-pudding one foot in diameter4 ,if the cook will try the experiment1 she will find that by taking the materials sufficient for sixty-four small plum-puddings all of the same size1 and mixing them together1 she will1 no doubt1 make a large plum-pudding1 but its diameter will only be four times that of the small puddings4 ,as a matter of fact1 the moon is #bafj miles in diameter1 and the earth is #giah miles4 ,these numbers are so nearly #bjjj and #hjjj respectively1 that for simplicity ,i have spoken of the earth as having a diameter four times as great as the moon4 ,if we want to be very accurate1 we ought to determine the ratio of the two quantities from the figures just given4 ,our illustration of the plum-puddings must1 therefore1 be a little modified4 ,the earth is not quite so much as sixty-four times as big as the moon2 but this figure is sufficiently accurate for our present purpose4 ,another interesting question may be proposed1 namely3 ,how much land is there on the moon8 ,we might state the answer in acres or in square miles2 but it will1 perhaps1 be more instructive to make a comparison between the moon and the earth4 ,here also ,i shall use an illustration2 and we shall again consider two globes which are respectively three inches and twelve inches in diameter4 ,the globes ,i use this time are hollow balls of india-rubber4 ,these will represent the earth and the moon with sufficient accuracy1 and the relative surfaces of these two globes is what ,i want to find4 ,there are different ways in which the comparison might be made4 ,i might1 for instance1 paint the two globes and see the quantity of paint that each requires4 ,if ,i did this1 ,i should find that the great globe took just sixteen times as much paint as the small one4 ,we can adopt a simpler plan4 ,the india-rubber in one of these balls has the same thickness as in the other1 as they are each hollow1 so that the quantity which is required for each ball may be taken to represent its surface4 ,by simply weighing the two balls1 ,i perceive that the large one is sixteen times as heavy as the small one4 ,you notice here the difference between the comparative weights of two hollow balls and two solid ones of the same material4 ,had these globes been of solid india-rubber1 the large one would have weighed sixty-four times as much as the small one1 just as in the case of the plum-puddings2 but being hollow1 the ratio of their weights is only the square of the ratio of their diameters--that is to say1 four times four1 or sixteen4 ,we are thus taught that if the moon were exactly one-fourth of the diameter of the earth1 its surface would be one-sixteenth part of that of the earth4 ,it would1 no doubt1 have made our subject a little easier and simpler if the moon had been created somewhat smaller than it is4 ,as1 however1 the universe has not been solely constructed for the purpose of these talks about ,star-land1 we must take things as we find them4 ,this proportion is not four2 it is more nearly #c1 and the relative surfaces of the two bodies is the square of #aa_/c1 or about #ac4 ,in other words1 the entire extent of the surface of our globe is about thirteen and a half times that of the moon4 ,the face of the full moon1 being half the entire extent of the surface1 is1 therefore1 about one-twenty-seventh part of the earth's surface--continents1 oceans1 seas1 and islands all taken together4 ,the ,british ,empire and the ,russian ,empire are each of them as large as the face of the full moon4 ,,how ,,eclipses ,,are ,,produced4 ,the moon is the attendant1 or the satellite of the earth1 ministering to the wants of the earth by mitigating the darkness of our nights4 ,the earth goes around the sun in its annual journey of #cfe days4 ,the moon revolves around the earth once every twenty-seven days4 ,the motion of the moon is thus a very complicated one1 for it is1 in fact1 moving round a body which is itself in constant motion 7,fig4 #cc74 ,you will see by your almanacs every year that certain eclipses are to take place2 and after what we have said about the sun and the moon1 it will be easy to understand how eclipses arise4 ,there are two different kinds4 ,you will sometimes see an eclipse of the moon1 and sometimes those eclipses of the sun of which we have spoken in the last ,lecture4 ,you may be surprised to find with what accuracy the eclipses can be predicted4 ,we can tell not only those that will occur this year and next year1 but we could also foretell the eclipses that will appear in a hundred or a thousand years to come2 or we can1 with equal ease1 calculate backwards1 so as to find the circumstances of eclipses that happened thousands of years ago4 ,this shows how well we have learned the way the moon moves4 ,illustration3 ,,fig4 #cc4--,to show how the ,earth goes round the ,sun and the ,moon round the ,earth4 ,illustration3 ,,fig4 #cd4--;,a ,total ,eclipse to the ,girl and a ,partial ,eclipse to the ,boy4 ,illustration3 ,partial4 ,annular4 ,,fig4 #ce4--;,different ,kinds of ,solar ,eclipse4 ,an eclipse of the sun is the simpler occurrence1 so we shall describe it first4 ,it happens when the moon comes between the earth and the sun4 ,look at our little astronomers shown in ,fig4 #cd4 ,a boy and a girl are both gazing at the sun1 when the moon comes between4 ,to the boy the moon appears to take a great bite out of the sun1 so that it looks like the left-hand picture in ,fig4 #ce4 7,i have drawn a line from the end of the telescope in ,fig4 #cd1 which shows how much of the sun is cut off47 ,this would be called a partial eclipse of the sun4 ,the almanac will sometimes describe the eclipses as visible in ,london1 or visible at ,greenwich2 but that need not be taken so literally as was supposed by a ,kensington gentleman1 who1 on noticing that the almanac said an eclipse was to be visible in ,london1 called a cab and drove into the city to look for it4 ,his almanac had not mentioned that it would be visible from his own house4 ,you may usually take for granted that when an eclipse is said to be visible from ,london or ,greenwich1 it will be more or less visible all over ,england4 ,most of these eclipses are only partial1 and though they are interesting to watch they do not teach us much4 ,by far the most wonderful kind of eclipse is that in which the whole of the bright part of the sun is blotted out4 ,then1 indeed1 we do see wonders4 ,but such eclipses are rare1 and even when they do occur they only last a very few minutes4 ,the sights that are displayed are so interesting that astronomers often travel thousands of miles to reach a suitable locality for making observations4 ,the girl in ,fig4 #cd is placed in the best possible position for seeing the eclipse4 ,there you find her right in the line of the sun and moon2 and ,i think you will agree that she cannot see any part of the sun1 for the moon is altogether in the way4 ,i have drawn two dotted lines1 one at each side4 ,all that she can see beyond the moon must lie outside these dotted lines1 and she will be in the dark as long as the moon stays in the way4 ,when the eclipse is complete1 comparative darkness steals over the land4 ,the birds are deceived1 and fly home to the trees to roost4 ,the owls and the bats1 thinking their time has arrived1 venture forth on their nocturnal business4 ,even flowers close their petals1 only to open a few minutes later when the sun again bursts forth4 ,other flowers that give forth their fragrance at night are also sweetly perceptible so long as the sun remains obscured4 ,an unruly cow1 accustomed to break into a meadow at night1 was found there after an eclipse was over2 while ,i learn from the same authority that a man rushed over in great excitement to see what his chickens were doing1 but came back much disappointed on finding them pecking away as if nothing had happened4 ,it will sometimes happen that the moon is so placed that the edge of the sun can be seen all round it4 ,a case of the kind is shown in the right-hand picture of ,fig4 #ce4 ,it is called an annular1 or ring-shaped eclipse4 ,the eclipses of which we have been speaking are1 of course1 only to be seen during the day when the sun must be up4 ,the lunar eclipses1 which are visible at night1 are due to the interposition of the earth between the sun and the moon4 ,the sun is at night-time under our feet at the other side of the earth1 and the earth throws a long shadow upwards4 ,if the moon enter into this shadow1 it is plain that the sunlight is partly or wholly cut off1 and since the moon shines by no light of her own1 but only by light borrowed from the sun1 it follows that when she is buried in the shadow all the direct light is intercepted1 and she must lose her brilliancy4 ,thus we obtain what is called a lunar eclipse4 ,it is total if the moon be entirely in the shadow4 ,the eclipse is partial if the moon be only partly in the shadow4 ,the lunar eclipse is visible to everybody on the dark hemisphere of the earth if the clouds will keep out of the way1 so that usually a great many more people can see a lunar eclipse than a solar eclipse1 which is only visible from a limited part of the earth4 ,it thus happens that the lunar eclipse is the more familiar spectacle of the two4 ,when the moon is entirely in the shadow1 one might naturally think that it would become totally invisible4 ,this is not always the case4 ,it is a curious fact that in the depth of a total eclipse the moon is often still visible1 for she glows with a copper-colored light1 which is bright enough to render some of the chief marks on her surface distinctly discernible4 ,,effect ,,of ,,the ,,moon',s ,,distance ,,on ,,its ,,appearance4 ,we are now about to take a good look at the moon and examine the different objects which are marked upon it4 ,there is a peculiar interest attached to this particular orb1 because it is much the nearest of all the heavenly bodies to our globe1 and therefore the one that we can see the best4 ,every other object--sun1 star1 or planet--is hundreds1 or perhaps thousands1 of times as far off as the moon4 ,it is right that we should desire to learn all we can about the bodies in space4 ,we know that the earth is a great ball1 and we see that there are many other such bodies4 ,some of them are much larger1 and some of them are smaller than the globe on which we dwell2 some of them are dark bodies like the earth1 and among them the moon is one4 ,is it not reasonable that we should make special efforts to find out all we can about this interesting neighbor8 ,though the moon is so close to us relatively to other objects in space1 yet when we express its distance in the ordinary methods of measurement it is a very long way off--about #bdj1jjj miles--a length nearly as great as that of all the railways in the world put together4 ,an express train which runs forty miles an hour would travel #bdj miles in six hours1 and the whole distance to the moon would be accomplished in #fjjj hours1 so that travelling by night and day incessantly you would accomplish the journey in #bej days4 ,to take another illustration1 if you wrapped a thread ten times round the equator of the earth1 it would be long enough to stretch from the earth to the moon4 ,or suppose a cannon could be made sufficiently strong to be fired with a report loud enough to be audible #bdj1jjj miles away4 ,the sound would only be heard at that distance a fortnight after the discharge had taken place4 ,the moon is too far for us to examine the particular features on its surface by the unaided eye4 ,suppose that there was a mighty city like ,london on the moon1 with great buildings and teeming millions of people1 and you went out on a fine night to take a look at our neighbor4 ,what do you think you would be able to see of the great lunar metropolis8 ,would you be able to see its streets full of omnibuses1 or even its great buildings8 ,would you see ,st4 ,paul's and ,westminster--the great parks and the river8 ,of all these things your unaided eye would show you almost nothing4 ,i can give you a little illustration4 ,suppose that you made a tiny model of ,london2 imagine this little structure all complete1 so that the streets1 the buildings1 the bridges1 the railways1 the parks1 and the ,thames were placed in their true proportions2 suppose that the miniature city was so small that it could stand on a penny postage stamp1 surely everything would look very insignificant1 even if you had the model in your hand and looked at it with the aid of a magnifying glass4 ,but suppose it were put on the other side of the table or on the other side of the room1 or the other side of the street4 ,even ,st4 ,paul's ,cathedral itself would have ceased to be distinguishable2 but yet the distance is not nearly great enough4 ,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 eye4 ,a ,,talk ,,about ,,telescopes4 ,the astronomer will not be contented with a mere naked-eye inspection of a world so interesting as the moon4 ,he will get a telescope to help his vision4 ,the word 8telescope0 means a contrivance for looking at objects which are a long way off4 ,we have explained that the further an object is1 the smaller it appears to be4 ,the telescope enables us to largely overcome this inconvenience4 ,it has the effect of making a distant object look larger4 ,there are great differences in the forms of telescopes2 and some instruments are large and some small1 according to the purposes for which they are required4 ,perhaps the most useful practical application of the telescope is by the officer on duty on board a ship4 ,he is generally provided with a pair of these instruments bound together to form the 8binocular40 ,you are all acquainted with this useful contrivance1 or at all events with the opera-glass1 that is used for purposes with which landsmen are more familiar4 ,the ship's telescope1 or the binocular1 or the opera-glass1 is feeble in power when compared with the great instruments of the ,observatory4 ,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 distance4 ,thus1 suppose his attention is directed to a great steamer three miles away1 he wishes to see her more clearly1 and accordingly he takes a view through his binocular4 ,immediately the vessel is so transformed that it seems to be only one mile away4 ,the apparent dimensions of the object are increased threefold4 ,the hull is three times as long1 the masts and the funnel are three times as high1 the sailors are three times as tall2 various objects on the ship too small to be seen at three miles would be visible from one mile1 and to that apparent distance the ship has now been brought4 ,if the sailor desires to reduce the apparent distance of objects1 how much more keenly does the astronomer feel the same want8 ,at best1 the sailor only has to scan a range of a few miles with his glass1 but what are a few miles to the astronomer8 ,it is true that he can count the distance of the moon by thousands of miles1 a good many thousands1 no doubt1 but for all other objects he must use millions1 while for most bodies in space1 millions of millions of miles are the figures we are constrained to employ4 ,need it be said that the astronomer must resort to every device he can to make the body appear closer4 ,he does not despise the modest binocular4 ,it is often a useful instrument in the ,observatory4 ,it gives most beautiful pictures of the celestial scenery1 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 all4 ,the binocular will also greatly improve the appearance of the moon1 but still its powers fall far short of what we require for the study of lunar landscapes4 ,even though we can reduce the moon's apparent distance to one-third its actual amount1 yet still that third is a very considerable distance4 ,one-third of #bdj1jjj is #hj1jjj1 so that we can see the moon no better with a binocular than we should see it were it #hj1jjj miles away1 and were we viewing it with the unaided eye4 ,illustration3 ,,fig4 #cf4--,the ,dome at ,dunsink ,observatory4 ,illustration3 ,,fig4 #cg4--,the ,equatorial at ,dunsink ,observatory4 ,i am not going to enter here upon any detailed account of the telescope1 because ,i shall say a little more on the subject in a later lecture2 at present ,i only describe that form of instrument which is most convenient for studying the moon4 ,i take as an illustration the ,south ,equatorial at ,dunsink ,observatory1 which belongs to ,trinity ,college1 ,dublin4 ,this telescope has a building to itself1 which stands on the lawn in front of the house4 ,the site is open and elevated1 so as to command an extensive prospect of the heavens4 ,you will see in ,fig4 #cf a picture of the structure4 ,it is circular in form and is entered by the little porch4 ,the most peculiar feature of an edifice intended to contain this kind of telescope is its roof1 or ,dome1 as we call it4 ,it is of a hemispherical shape with a projecting rim at the bottom4 ,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 purpose4 ,the dome is very unlike ordinary roofs1 not only in appearance1 but also because it can turn round4 ,in the next figure you will see a section through the building1 and the wheels are exposed by which the dome is carried4 ,these wheels run easily on rails1 so that when the attendant pulls the rope which you see in his hands1 he turns round a large pulley1 and that operates a little cogwheel which works into a rack1 and thus makes the dome revolve4 ,the roof is built of timber1 covered with copper2 it weighs more than six tons1 but the machinery is so nicely adjusted1 that a child four years old can easily set the whole in motion4 ,the object of all this machinery is seen when we learn that there is only one opening in the dome4 ,it is covered by the shutter shown over the doorway in ,fig4 #cf4 ,when opened to the top1 it gives a long and wide aperture1 through which the astronomer can look out at the heavens4 ,of course the dome has to be turned until the opening has been brought to face the required aspect4 ,the big telescope can thus be directed to any object above the horizon4 ,you see a gentleman using the telescope 7,fig4 #cg71 and this shows that the great instrument is nearly three times as long as the astronomer himself6 ,no doubt the telescope seems to be composed of a good many different parts1 but the essential portions of the instrument are comparatively few and simple4 ,at the upper end is the object glass1 which consists of two lenses1 one of flint glass and the other of crown glass4 ,both of these must be of exceptional purity1 and the shape to be given to the lenses is a matter of the utmost importance4 ,it is in the making of this pair of glasses that the skill of the optician has to be specially put forth4 ,so valuable indeed is an object glass which fulfils all the requirements1 that it is by far the most costly part of the instrument4 ,there are no glasses in the interior of the tube until you come to the end where the observer is looking in4 ,this is closed by an eyepiece consisting of a lens1 or a pair of lenses4 ,there are usually many different eyepieces for a telescope1 and they contain lenses of varied powers1 to be used according to the state of the atmosphere1 or to the particular kinds of observation in progress4 ,if you point a big telescope to the sky1 and see therein the sun or the moon or any of the stars1 you will speedily find that the objects pass away out of view4 ,remember our earth is constantly turning round1 and bears1 of course1 the ,observatory with it1 so that though the telescope be rightly pointed to the heavens at one moment1 by the next it will have been turned aside4 ,to you who are using the telescope1 the appearance produced is as if the heavenly bodies were themselves moving4 ,we can counteract this inconvenience4 ,the telescope is supported on a pedestal1 which is built on masonry1 that goes down through the floor to its foundation on the solid rock beneath4 ,in the iron casing at the top of the pedestal you will see a little window1 and inside is clockwork driven by a heavy weight4 ,this clockwork turns the whole telescope round in the opposite direction to that in which the earth is moving4 ,the consequence is that the telescope remains constantly pointed to the same part of the heavens4 ,illustration3 ,,fig4 #ch4--,the ,yerkes ,telescope1 ,university of ,chicago4 ,this instrument is no doubt a large one1 but of late years many much greater have been built4 ,the most powerful telescope that has ever been erected is the great ,yerkes instrument belonging to the ,university of ,chicago1 of which a picture is shown in ,fig4 #ch4 ,the object glass is #dj inches across4 ,,how ,,the ,,telescope ,,aids ,,us ,,in ,,viewing ,,the ,,moon4 ,those who are in charge of an observatory are often visited by persons who1 coming to see the wonders of the heavens1 and finding instruments of such great proportions1 not unnaturally expect the views they are to obtain of the celestial bodies shall be of corresponding magnificence4 ,so they are1 no doubt1 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 imaginations1 so that they are often sadly disappointed4 ,especially is this true with regard to the moon4 ,i have seen people who1 when they had a view of the moon through a great telescope1 were surprised not to find vast ranges of mountains which looked to them as big as the ,alps1 or mighty deserts1 over which the eye could roam for thousands of miles4 ,they have sometimes expected to behold stupendous volcanoes that not only were1 but that looked to be as big as ,vesuvius4 ,others seem to have thought they ought to see the moon with such clearness that the fields were to be quite visible1 and some would not have been much astonished if they had observed houses and farmyards1 and1 perhaps1 even cocks and hens4 ,there are different ways of estimating the apparent dimensions of an object1 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 surface4 ,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 moon4 ,a desert on the moon1 which really is a hundred miles across1 will then correspond to a mark about an eighth of an inch in diameter on the orange4 ,some of you may ask what is gained by the use of a telescope1 for the moon looks to us as large as a plate with the unaided eye1 and now we hear it only looks as big as an orange in the telescope4 ,but where is the plate with which you compare your moon supposed to be held8 ,it is surely not in your hand4 ,it is imagined to be up in the sky1 a very long way off4 ,though an orange is much smaller than a plate1 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 street4 ,illustration3 ,,fig4 #ci4--,the ,advantage of using a ,telescope4 ,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 together1 of which one is applied to the telescope1 while the other is directed to the moon4 ,it will then be seen1 even with a very small instrument1 that the telescopic moon is as big as the larger of the two crescents in the adjoining figure 7,fig4 #ci71 while the naked-eye moon is like the smaller4 ,the greatest telescopes are capable of reducing the apparent distance of an object to about one-thousandth part of its actual amount4 ,if1 therefore1 a body were a thousand miles away1 it would1 when viewed by one of these mighty instruments1 be seen as large as our unaided vision would show it1 were the body only a single mile distant4 ,no doubt this is a large accession to our power1 but it often falls far short of what the astronomer would desire4 ,the distances of the stars are all so great that even when divided by one thousand1 they are still enormous4 ,if you have a number expressed by #ajj1jjj1jjj1jjj1jjj1 then dividing it by a thousand merely means taking off three of the ciphers1 and there is still a large number left4 ,we are1 however1 at present concerned with the moon1 and1 as its distance is about #bdj1jjj miles1 the effect of the best telescope is to reduce this distance apparently to #bdj miles4 ,here1 then1 we find a limit to what the best of all telescopes can do4 ,it can never show us the moon better than1 hardly indeed so well as1 we could see it with our unaided eye were it only #bdj miles over our heads4 ,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 #bdj miles away4 ,what could we expect to see at a distance of #bdj miles8 ,here is a little experiment which ,i made to study this point4 ,i marked a round black dot on a sheet of white paper4 ,the dot was a quarter of an inch in diameter1 and then ,i fastened this on a door in the garden1 and walked backwards until the dot ceased to be visible4 ,i found this distance to be about thirty-six yards4 ,i tried a little boy of eight years old1 and it appeared that the dot became invisible to him about the same time as it did to me4 8,what has this to do with the moon80 you will say4 ,well1 we shall soon see4 ,in thirty-six yards there are #eahd quarters of an inch1 and as it is unnecessary to be very particular about the figures1 we may say1 in round numbers1 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 itself4 ,you need not1 therefore1 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 it4 ,the great telescope practically places the moon at a distance of #bdj miles1 and the five-thousandth part of that is about eighty yards2 consequently a round object on the moon about eighty yards in diameter would be just glimpsed as the merest dot in the most powerful telescope4 ,to attract attention1 a lunar object should be much larger than this4 ,if ,st4 ,paul's ,cathedral stood on a lunar plain1 it would be visible in our great telescopes4 ,it is true that we could not see any details4 ,we should not be able to distinguish between a ,cathedral and a ,town-hall4 ,there would just be something visible1 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 there4 ,this will show us that we need not expect to see objects on the moon1 even with the mightiest of telescopes1 unless they are of great size4 ,,telescopic ,,views ,,of ,,lunar ,,scenery4 ,i have already warned you not to expect too much1 even with the biggest of telescopes2 and just as a caution1 ,i may1 perhaps1 tell you a story ,i once heard of an astronomer who had a great telescope4 ,it was a very famous instrument1 and people often came to the ,observatory at night to enjoy a look at the heavens4 ,sometimes these visitors were grave philosophers1 but frequently they were not very accomplished men of science4 ,one evening such a visitor came to the ,observatory1 and sent in his name and an introduction to the astronomer1 with a request that he might enter the temple of mystery4 ,the astronomer courteously welcomed the stranger1 and asked him what he specially desired to see4 8,oh60 said the visitor1 8,i have specially come to see the moon--that is the object ,i am particularly interested about40 8,but10 said the astronomer1 8my dear sir1 ,i would show you the moon with pleasure1 if you were here at the proper time2 but what brings you here now8 ,look up2 the evening is fine4 ,there are the stars shining brightly1 but where is the moon8 ,you see it is not up at present4 ,in fact1 it won't rise till about half-past two to-morrow morning1 and it is only nine o'clock now4 ,come back again in five or six hours1 and you shall observe the moon with the great telescope40 ,but the visitor evidently thought the astronomer was merely trying to get rid of him by a pretext4 ,and he was equal to the occasion--he was not going to be put off in that way4 8,of course1 the moon is not up10 he replied2 8any one can see that1 and that is the reason why ,i have come1 for if the moon had been up1 ,i could have seen it without your telescope at all60 ,although no explorer can ever reach our satellite1 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 earth4 ,think of the continent of ,africa4 ,in that great country there are mighty tracts1 there are vast lakes and ranges of mountains1 of which we know but little4 ,we could make a better map of ,africa1 so far at least as its broad outlines are concerned1 if it were fastened up on our side of the moon than we actually possess at this moment4 ,there is no spot on the nearer side of the moon as large as an ordinary parish in this country which has not been surveyed4 ,there are maps and charts of the moon showing every part of it1 which is as big as a good-sized field4 ,indeed1 as there are no lunar clouds1 the features of its surface are never obscured whenever our own atmosphere will permit us to make our observation4 ,artists have frequently sketched the lunar features1 and there is plenty of material for them to work on4 ,we have also had photographs taken of the moon1 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 experience4 ,for a photograph to be successful1 everybody knows that the first requisite is for the sitter to stay quiet while the plate is being exposed4 ,this is1 unhappily1 just what the moon cannot do4 ,we endeavor to obviate the difficulty by moving the telescope round so as to follow the moon in its progress4 ,this can be done with considerable accuracy1 but1 unfortunately1 there is another difficulty which lies entirely beyond our control4 ,as the rays of light from the moon perform their journey through hundreds of miles of unsteady air1 the rays are bent hither and thither1 so that the picture is more affected by the atmosphere than in the case of a photographer's portrait taken in the studio4 ,if we are merely viewing the moon through the telescope1 the quivering effect on the rays of this long atmospheric voyage1 though rather inconvenient1 does not prevent us from seeing the object1 and we can readily detect the true shape of each feature in spite of incessant fluctuations4 ,when1 however1 these rays fall not on the eye1 but on the photographic plate1 they produce by their motion a picture which cannot be much magnified without becoming very confused and wanting in sharpness4 ,nevertheless1 for the general outlines of our satellite's appearance and for the portraiture of its splendid features we have derived the greatest assistance from photography4 ,illustration3 ,,fig4 #dj4--,the ,full ,moon4 ,the adjoining picture 7,fig4 #dj7 gives a fair idea of what the full moon looks like when viewed through a small telescope4 ,i do not1 however1 say that the lunar objects can then be observed under favorable conditions2 for when the moon is full is the very worst time for making observations of our satellite4 ,in fact1 at this phase you can hardly see anything except slight differences between the colors of different parts4 ,the best time for observing the moon is at the first quarter2 but even then you can only observe satisfactorily those objects which happen to lie along the border between light and shade4 ,to study the moon properly you must1 therefore1 watch it during several different phases1 from the time when it presents a thin and delicate crescent 7just after new moon7 until it has again waned to a thin and delicate crescent 7just before the next new moon74 ,we want the relief given by shadows to bring out the full beauty of lunar scenery4 ,on the map you will first notice the large dark-colored patches which are so conspicuous on the moon's face4 ,they are1 apparently1 the empty basins which great seas once filled4 ,but if water was ever there it has at all events now quite disappeared4 ,these dark parts are1 no doubt1 a good deal smoother than the rest of the surface2 but we can see many little irregularities which tell us that we are not looking at oceans4 ,the chief features ,i want you to observe are the curious rings which you see in the figure2 there is a very well-marked one a little below the centre1 and in the upper part many rings--large and small--are crowded together4 ,we call them lunar craters4 ,you will see what they are like from the model1 of which a picture is shown in ,fig4 #db4 ,but to realize from this picture the proper scale of the object1 you should imagine it to be some miles in width4 ,the cliffs which rise all round to form the wall1 as well as the mountain which adorns the centre1 are quite as high as any of the mountains in ,great ,britain4 ,illustration3 ,,fig4 #da4--,view on the ,moon4 7,by ,lwy and ,puiseux1 ,paris ,observatory47 ,the large central crater is ,hipparchus and above it is ,albategnius4 ,illustration3 ,,fig4 #db4--,our ,model of a ,lunar ,crater4 ,you may desire to know how we are able to measure the heights of mountains on the moon4 ,that is what ,i am now going to show you2 and for this purpose we shall look at our imitation lunar crater4 ,here is the great ring1 or circular enclosure1 surrounded by cliffs1 and here is a sharp mountain peak rising in the centre4 ,i shall ask to have the beam from the electric lamp turned on our model4 ,you see how prettily it is lighted up4 ,i have placed the lamp so that the beams are sloping2 and ,i have done this with the express object of making the shadows long4 ,in fact1 as we look at a lunar crater1 which lies on the border between light and shade1 the sun illuminates the object under the same conditions as those shown in the figure4 ,i dare say you have often noticed what long shadows are cast at sunset4 ,similar shadows are made to teach the astronomer the altitudes of the lunar mountains2 for he measures the length of the shadow1 and then by a little calculation he can find the height of the object by which that shadow has been cast4 ,i shall suppose that we want to measure the height of a flagstaff 7,fig4 #dc74 ,it is quite possible to do this by merely measuring the length of the shadow which that flagstaff casts at noon4 ,it would not be correct to say that the height of the flagstaff is the length of its shadow4 ,this will1 indeed1 be the case if you are fortunate enough to make your measurement at or near ,london on either the #fth of ,april or the #eth of ,september4 ,on all other days in the year a little calculation must be made1 which ,i need not now mention1 but which the astronomer1 with the aid of his ,nautical ,almanac1 can do in a very few minutes4 ,in a similar manner1 by measuring the lengths of the shadows on the moon1 and by finding the number of miles in the shadow1 we are able to calculate the altitudes of the lunar mountains and of the ranges of cliffs by which the walled plains are surrounded4 ,illustration3 ,,fig4 #dc4--;,how we found the ,height of the ,flagstaff4 ,,on ,,the ,,origin ,,of ,,the ,,lunar ,,craters4 ,we have now to offer an explanation of the curious rings which are the most characteristic features on the moon4 ,to account for them we must look for a moment at some objects on the earth4 ,you have all heard of volcanoes or burning mountains1 such as ,vesuvius or ,etna1 which occasionally break out into violent eruptions1 and send forth great showers of ashes and torrents of molten lava4 ,in the ,sandwich ,islands there is a celebrated volcano called ,kilauea4 ,it is like a vast lake of lava1 so hot that it is actually molten1 and glows with heat like red-hot iron4 ,the adventurous tourist who visits this crater can climb to the brink of a lofty range of cliffs which surround it1 and gaze down upon the fervid sea beneath4 ,suppose that by some great change the internal heat which keeps this mighty basin glowing were to decline and go out1 the sea of lava would cease to be liquid1 and would ultimately grow hard and cold1 and we should then have an immense flat plain1 surrounded by a range of cliffs4 ,elsewhere in the ,sandwich ,islands examples of extinct craters may be found at the present day4 ,those who have studied these interesting localities point out how such terrestrial craters explain the ringed plains in the moon4 ,it seems certain that in ancient days great volcanoes abounded on our satellite1 and the rings were often much larger than those on the ,sandwich ,islands1 some of them being one hundred miles or more in diameter4 ,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 show4 ,we can also surmise how the lofty mountain peak1 which so often rises in the centre of a lunar ring1 has been upheaved4 ,when the fires had almost subsided1 and the floor had grown nearly cold1 one last and expiring effort is made by which the congealing surface is burst through at the centre1 and materials are shot forth which remain as the central mountain to the present day4 ,i must1 however1 impress upon you that even our greatest telescopes never exhibit to us any volcanic eruptions at present going on in the moon2 in fact1 it is most doubtful if any change has been noticed in the features on its surface since the date of the invention of the telescope4 ,the volcanoes sculptured the crust of the moon into the form in which we see it1 and that form our satellite has preserved for ages1 of which we cannot estimate the duration4 ,all the craters and all the volcanoes in the moon can only be described as extinct4 ,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 moon4 ,the noisy volcanoes on our globe are those most talked about2 we often hear of ,vesuvius being in eruption1 and in ,august1 #ahhc1 there was a terrific eruption at ,krakatoa1 during which a large quantity of dust was shot up into the air1 to such a height that it was borne right round the earth1 and produced beautiful sunsets and unwonted sky hues in almost every country in the world4 ,the explosion at ,krakatoa made the loudest noise that history has recorded4 ,fortunately such convulsions of the earth do not often happen1 for1 on that occasion1 the sea rushed in on the land1 and thousands of lives were lost4 ,there are said to be one hundred and fifty volcanoes on different parts of the earth1 which are more or less active1 but there are many others in which the fire has gone out1 and which seem to be just as cold and just as extinct as any volcanoes in the moon4 ,even in our own islands there are abundant remains of ancient volcanoes4 ,masses of lava are found in many places where now there is no trace of an active volcano4 ,perhaps there is no more remarkable sight in the ,british ,isles than that lofty rock which is crowned by ,edinburgh ,castle2 it is the remnant of a former volcano1 while ,arthur's ,seat1 close by1 is another4 ,in the centre of ,france is the beautiful district of ,auvergne1 in which ancient volcanoes abound2 and the lava streams can be traced for miles across the country4 ,these volcanoes have been extinct for thousands of years1 during which time the lava has become largely covered with soil and vegetation1 and in some places vineyards are cultivated upon it4 ,we are now able to contrast the earth with the moon1 in so far as volcanoes are concerned4 ,on the earth we have some that are active1 and a much greater number that are extinct4 ,on the moon we find no active volcanoes1 for there all are extinct4 ,i can explain how this difference has arisen1 but first let me show you a simple experiment4 ,my assistant will kindly bring to me from that furnace two iron balls1 which we placed there before the commencement of this lecture2 there they are1 you see1 both glowing with a bright red heat1 for at present they are equally hot4 ,we will place them on these stands1 and allow them to grow cold4 ,one of these balls is a small cannon-ball1 four inches in diameter1 while the other is only one inch4 ,they are in the same proportion as the earth is to the moon2 but look1 even while ,i am speaking the balls have ceased to preserve the same temperature1 for the little one has become almost black from loss of its heat1 while the large one still looks nearly as red as it did at the beginning2 this simple experiment will illustrate the principle that two heated bodies will cool at very different rates1 if their sizes be different1 while the other conditions are the same4 ,the small body will always cool faster than the large one4 ,they need not be globes for this experiment2 if you put a poker and a knitting needle into the fire1 and leave both there until they are red-hot1 and then put them out into the fender1 you will speedily find that though they were at the same temperature when drawn from the fire1 they do not long remain so2 indeed1 the knitting needle has become cold enough to handle before the poker has ceased to glow4 ,our experiments have been made with1 no doubt1 small objects only1 but the law about which they inform us will remain true1 even for the greatest objects4 ,our earth at the present day shows many indications of being much hotter within than it is on the surface4 ,the volcanoes themselves are mere outbreaks of incandescent material from inside4 ,then there are hot springs of water at ,bath1 which gush out from the earth4 ,there are geysers of hot water in ,iceland and in the ,yellowstone ,park in ,america1 and in other places4 ,and there are other indications also1 with which every miner is familiar4 ,wherever a deep pit is sunk into the earth1 the rocks below are always found to be warmer than those on the surface1 and the deeper the pit the greater is the heat that is encountered4 ,thus1 from all over the world we obtain proofs of the present existence of internal heat4 ,great as is the earth1 we must still apply the simple common-sense principles that we use in our everyday life here4 ,let me give an illustration4 ,suppose that a servant came into the room and placed a jug of water on the table1 and that an hour afterwards you went to the jug of water and found it to be cold1 you would not from that fact alone be able to infer anything with certainty1 as to whether the water had been warm or cold when it was brought in4 ,it might have been perfectly cold1 as it is at present1 though on the other hand the water might have been warm at first1 and have since cooled down to the temperature of the room during the hour4 ,suppose1 however1 that when you went to the jug of water1 which had stood on the table for an hour1 you found it tepid1 no matter how slightly its temperature might be above that of the room1 do you not see the inference you would be able to draw8 ,you would argue in this way3 that water has still some heat2 it must1 of course1 be gradually cooling1 and therefore it was hotter a minute ago than it is now2 it was hotter still two minutes ago1 or ten minutes2 and must have been very hot and perhaps boiling when it was brought in an hour ago4 ,i want you to apply exactly the same reasoning to our earth4 ,it is1 as ,i have shown you1 still hot and warm inside4 ,of course1 that heat is gradually becoming lost2 so that the earth will from year to year gradually cool down1 though at an extremely slow rate4 ,but we must look back into what has happened during past ages4 ,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 warm1 so we are entitled to infer1 from the fact that the earth still retains some heat1 that it must in ages gone by have been exceedingly hot4 ,in fact1 the further we look back1 the hotter do we see the earth growing1 until at last we are constrained to think of a period1 in the excessively remote past1 long ere life began to dawn on this earth1 when even the surface of the earth was hot4 ,back further still we see the earth no longer covered with the hard1 the dark1 and the cold surface we now find2 we are to think of it in these primitive times as a huge glowing mass1 in which all the substances that now form the rocks were then incandescent1 and even molten material4 ,there is good reason for knowing that in those early times the moon also was molten with heat2 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 down4 ,recall our little experiment with the two cooling globes of iron2 imagine these globes to preserve their relative proportions1 but that one of them was #hjjj miles and the other #bjjj miles across4 ,ages will1 no doubt1 elapse ere they part with their heat sufficiently to allow the surfaces to cool and to consolidate4 ,we may1 however1 be sure that the small globe will cool the faster1 that its outside will become hard sooner than will the surface of the large one1 and long after the small globe has become cold to the centre1 the large one may continue to retain some of its primeval heat4 ,we can thus readily understand why all the volcanoes on the moon have ceased--their day is over4 ,it is over because the moon1 being so small1 has grown so cold that it no longer sustains the internal fires which are necessary for volcanic outbreaks4 ,our earth1 in consequence of its much greater size1 has grown cold more slowly4 ,it has no doubt lost the high temperature on the exterior1 and its volcanic energy has probably abated from what it once was4 ,but there is still sufficient power in the subterranean fires to awaken us occasionally by a ,krakatoa1 or to supply ,vesuvius with sufficient materials and vigor for its more frequent outbursts4 ,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 phenomena1 and then we shall pass into the exhausted stage which the moon attained ages ago4 ,,the ,,movements ,,of ,,the ,,moon4 ,though the moon is going round and round the earth incessantly1 yet it always manages to avoid affording us a view of what is on the other side4 ,our satellite always directs the same face towards the earth1 and we may reasonably conjecture that the other side is covered1 like the side we know1 with rings and other traces of former volcanoes4 ,in this respect the moon is quite a peculiar object4 ,the other great celestial bodies1 such as the sun or ,jupiter1 turn round on their axes1 and show us now one side and then the other1 with complete impartiality4 ,the way in which the moon revolves may be illustrated by taking your watch and chain1 and as you hold the chain at the centre making the watch revolve in a circular path4 ,at every point of its path the ring of the watch is1 of course1 pointed to the centre where the chain is held4 ,if you imagine your eye placed at the centre1 to represent the earth1 the movements of the watch would exemplify the way the moon turns round it4 ,one more point ,i must explain about the moon before we close this lecture4 ,there is nothing more familiar than the fact that a heavy body will fall to the ground4 ,indeed1 it hardly matters what the material of the body may be1 for you see ,i have a small iron ball in one hand and ,i hold a cork in the other4 ,i drop them at the same moment1 and they reach the ground together4 ,perhaps you would have expected that the cork would have lagged behind the iron4 ,i try the experiment again and again1 and you can see no difference in the times of their falling1 though ,i do not say this would be true if they were dropped from the top of the ,monument4 ,in general we may say that bodies let drop will fall sixteen feet in the first second4 ,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 air4 ,this is easily managed4 ,cut a small piece of tissue paper which will lie flat on the top of the penny1 and hold the penny horizontal with the paper uppermost4 ,though there is nothing to fasten the paper to the penny1 you will find that they fall together4 ,if we could conduct the experiment of dropping the penny and the bit of paper in a vacuum1 then1 whether the paper was laid on the penny or placed in any other way1 the two objects would reach the table at the same moment if released at the same moment at equal heights4 ,wherever we go we find that bodies will always tend to fall in towards the centre of the earth2 thus in ,new ,zealand1 at the opposite side of our globe from where we are now standing1 bodies will fall up towards us1 and this law of falling is obeyed at the top of a mountain as it is down here4 ,no matter how high may be the ascent made in a balloon1 a body released will fall towards the earth's centre4 ,of course1 we can only ascend some five or six miles high1 even in the most buoyant of balloons2 but we know that the attraction by which bodies are pulled downwards towards the earth extends far beyond this limit4 ,if we could go ten1 twenty1 or fifty miles up1 we should still find that the earth tried to pull us down4 ,nor1 even if you could imagine an ascent made to the height of #ajjj miles1 would gravitation have ceased4 ,a cork or an iron ball1 or any other object dropped from the height of #ajjj miles1 would assuredly tumble down on the ground below4 ,suppose that by some device we were able to soar aloft to a height of #djjj miles4 ,i name that elevation because we should then be as high above the earth as the centre of the earth is below our feet4 ,we should have doubled our distance from the centre of the earth1 and the intensity of the gravitation would have decreased to one-quarter of what it is at the surface4 ,a body which at the earth's surface falls sixteen feet in a second would there fall only four feet in a second1 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 here4 ,thus1 the higher and higher we go1 the less and less does gravity become2 but it does not cease1 even at a distance of millions of miles4 ,therefore you might say that as gravity tries to pull everything down1 wherever it may be1 why does it not pull down the moon8 ,this is a difficulty which we must carefully consider4 ,supposing that the earth and the moon were simply held apart1 both being at rest1 and that then the moon were to be let go1 it would no doubt drop down directly on the earth4 ,the movement of the moon would1 however1 be very different if1 instead of being merely let fall1 it was thrown sideways4 ,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 altogether1 as it would have done had the earth not been there to attract it4 ,illustration3 ,,fig4 #dd4--;,an ,illustration to explain the ,movement of the ,moon ,we can explain this by an illustration4 ,on the top of a mountain ,i have placed a big cannon 7,fig4 #dd74 ,we fire off the cannon1 and the bullet flies away in a curved path1 with a gradual descent until it falls to the ground4 ,i have made the mountain look hundreds of times larger than any mountain could possibly be2 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 manufactured4 ,fire off a bullet with a still greater charge than the last time1 and now the path is a much longer one1 but still the bullet curves down so as ultimately to fall on the earth4 ,but make now one final shot with a charge sufficiently powerful1 and away flies the bullet1 following this time the curvature of the earth1 for the earth's attraction has the effect of bending the path of the bullet from a straight line into this circular form4 ,by the time the bullet has travelled a quarter of the way round1 it is no nearer to the earth than it was at first1 nor has it parted with any of its original speed4 ,thus1 notwithstanding its long journey1 the bullet has practically just as much energy as when it first left the muzzle of the cannon4 ,away it will fly round another quarter of the earth1 and still in the same condition it will accomplish the third and the fourth quarters1 thus returning to the point from which it started4 ,if we have cleared the cannon out of the way1 the bullet will fly again over the mountain top without having lost any of its speed by its voyage round the earth4 ,therefore it will be in a condition to start again1 and thus to revolve around the earth permanently4 ,if1 then1 from the top of a mountain #bdj1jjj miles high a great bullet #bjjj miles in diameter had once been projected with the proper velocity1 that bullet would continue forever to circle round and round the earth1 and even though the mountain and the cannon disappeared1 the motion would be preserved indefinitely4 ,this illustration will1 at all events1 show how a continuous revolution of the moon round the earth can exist1 notwithstanding that the earth is constantly pulling our satellite down towards its surface4 ,,on ,,the ,,possibility ,,of ,,life ,,in ,,the ,,moon4 ,astronomers are often asked whether any animals can be living on the moon4 ,no observations we can make with the telescope can answer that question directly4 ,there are great plains to be seen on the moon1 of course1 but even if there were elephants tramping over those plains1 our telescopes could not show them4 ,nor will our instruments pronounce at once whether plants or trees flourish on the moon4 ,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 pair4 ,even if there were trees as big as this on the moon1 they would not be visible from the most famous observatories4 ,let us think what we should ourselves experience if we could in some marvellous manner be transferred from the earth to its satellite1 and tried to explore that new and wonderful country4 ,alas1 we should find it utterly impossible to live there for an hour1 or even for a minute6 ,troops of difficulties would immediately beset us4 ,the very first would be the want of air4 ,ponder for a moment on the invariable presence of air around our own globe4 ,even if you climb to the top of a high mountain1 or if you take a lofty voyage in a balloon1 you are all the time bathed in air4 ,it is air which supports the balloon1 just as a cork is buoyed up by water4 ,in all circumstances1 we must have air to breathe4 ,in that air is oxygen gas1 and we must have oxygen incessantly supplied to our lungs to reinvigorate our blood4 ,we require1 too1 that this oxygen shall be diluted with a much larger amount of nitrogen gas1 for our lungs and system of circulation are adapted for abode in that particular mixture of gases which we find here4 ,the atmosphere becomes more and more rarefied the higher we ascend1 and apparently terminates altogether some two or three hundred miles over our heads4 ,beyond the limits of the atmosphere it seems as if empty space would be met with all the way from the earth to the moon4 ,we could not procure a single breath of air1 and life would be1 of course1 impossible4 ,even at a height of three or four miles1 respiration becomes difficult1 and doubtless life could not possibly be sustained at a height of ten miles4 ,it is therefore plain that for a voyage to the moon we should require an ample supply of air1 or1 at least1 of life-giving oxygen1 which in some way or other was to be inhaled during the progress of the journey4 ,when at length #bdj1jjj miles had been traversed1 and we were about to land on the moon1 we would first of all ascertain whether it was surrounded with a coating of air4 ,most of the globes through space are1 so far as we can learn1 covered and warmed with an enveloping atmosphere of some kind2 but1 unhappily1 the poor moon has been left entirely1 or almost entirely1 without any such clothing4 ,she is quite bare of atmosphere at all comparable in density or in volume to that which surrounds us1 though possibly we do now and then perceive some traces of air1 or of some kind of gas1 in small quantities in the lunar valleys4 ,i am sure each intelligent boy or girl will want to know how we are able to tell all this4 ,we have never been at the moon1 and how then can we say that it is nearly destitute of air8 ,nor can our telescope answer this question immediately1 for you could hardly expect to see air1 even if it were there4 ,how then are we able to make such assertions8 ,there are many different ways in which we have learned the absence of air from the moon4 ,i will tell you one of the easiest and the most certain of these methods4 ,first let me say that air is not perfectly transparent4 ,no doubt ,i can see you1 and you can see me1 though a good many feet of air may lie between us2 but when we deal with distances much greater1 there is a very simple way in which we can show that air is not quite transparent4 ,in the evening1 when the sun is setting and the sky is clear1 you can look at him without discomfort2 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 contrivance4 ,the reason is1 that when the sun is either setting or rising we look at it through an immense thickness of air1 which not being perfectly transparent stops some of the light4 ,thus it is that the sun in these circumstances loses its dazzling brilliancy1 and we can view it without discomfort4 ,at the seaside you can notice the same effect in a different manner4 ,go out on a fine and clear night1 when the stars in their thousands are glittering overhead1 and then look down gradually towards the horizon1 and you will find the stars becoming fainter and fainter4 ,indeed1 even the brightest star cannot be seen when it is at the horizon1 because an immense thickness of the atmosphere is not transparent4 ,we can now state the argument by which we may prove that there is little or no air on our satellite4 ,the moon will frequently pass between the earth and a star1 and when the star is a really bright one the observations that can be made are of great interest4 ,let me first describe what we actually see4 ,the star is shining brightly until the moment when the moon eclipses it4 ,generally speaking1 its disappearance is instantaneous4 ,but this would not be the case if the moon were encircled with an atmosphere4 ,if the moon were coated with air1 the light from the star would not be extinguished instantly2 it would gradually decline1 according as it had to pass through more and more of the moon's atmosphere4 ,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 out4 ,the sudden extinction of the stars demonstrates the airless state of our satellite4 ,there would be another insuperable difficulty in adopting the moon as a residence1 even supposing that you could get there4 ,water is absent from its surface4 ,we have examined every part of it1 and we find no evidences of seas or of oceans1 of lakes or rivers2 we never see anything like clouds or mists1 which are1 of course1 only water in the vaporous form4 ,we are1 therefore1 assured that1 so far as water is concerned1 the moon is an absolute desert4 ,this is1 perhaps1 the most striking contrast between the aspect of the earth and the aspect of the moon4 ,were an astronomer on the moon to look at our earth he would find most of its surface concealed beneath clouds1 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 ocean2 in fact1 when the lunar astronomer had realized the prevalence of water upon this earth1 either in the form of ocean or cloud1 ,i feel sure he would come to the conclusion that nothing could live here except seals or other amphibious animals4 ,owing to the absence of air and water1 the moon would be totally disqualified for the support of life of those types in which we know it4 ,for air and water are necessary to every animal1 from the humblest animalcule up to whales or elephants4 ,air and water are necessary for every form of vegetable life1 from the lichen which grows on a stone up to the noble old oak of the forest4 ,but even supposing that we could land on the moon1 bearing with us an ample supply of oxygen to breathe1 and of water to drink1 we should find ourselves perplexed and embarrassed1 to say the very least of it1 by an extraordinary difference that would immediately attract our notice4 ,that familiar experience of gravity1 or the weights of things1 which we have acquired in our residence on a great globe like the earth1 would seem ludicrously altered when we began to walk about on a little globe like the moon4 ,we should be astonished at the transformation by which the weight of everything was much lessened2 when you pulled out your watch you would hardly feel it at the end of the chain2 it would seem like a mere shell2 but yet the watch is all right1 it is going as well as ever4 ,nothing has altered about it except its weight4 ,a big stone attracts your notice1 and1 to your amazement1 you find that it does not weigh so much as a piece of wood of the same size would weigh down here4 ,a stone that you could hardly stir on the earth1 you can carry about on the moon4 ,nor is this to be explained by any peculiarity in the constitution of the lunar stone4 ,most probably it will be not very dissimilar to some of the rocks on the earth4 ,the relative lightness of a lunar stone is not due to its being formed of some very special material2 we must seek for some other explanation4 ,every object on the moon would be found only one-sixth as heavy as the same object on the earth4 ,a sturdy laborer at one of the docks can carry one sack of corn on his back here1 and he finds that this load is as much as is convenient4 ,he would1 however1 discover1 were he placed on the moon1 that his load had suddenly become lightened to one-sixth part 7,fig4 #de74 ,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 him4 ,to explain how such a change as this has occurred1 look at these two pictures3 one shows the laborer on a small body like the moon1 the other shows him on a great globe like the earth4 ,what the laborer actually does feel is not quite so simple a thing as he imagines4 ,he imagines that it is the weight of the corn1 and the corn alone1 which produces that pressure on his shoulders which he knows so well4 ,but that is not exactly the manner in which the philosopher will look at the same question4 ,what the laborer does actually feel is the attraction between the earth beneath his feet and the corn on his back4 ,it is this force which produces the pressure on his shoulders4 ,its magnitude no doubt depends upon the quantity of corn in the sack1 but it also depends on the quantity of matter on the earth beneath his feet4 ,in fact1 the force between two attracting bodies depends upon the masses of both the attracting bodies4 ,when the laborer is transferred to the moon1 of which the mass is so much less than that of the earth1 the attraction is less there than it is here1 even though the corn is the same in the two cases4 ,illustration3 ,,fig4 #de4--,the ,lessened ,gravitation on the ,moon4 ,many odd instances could be given of the extraordinary consequences of life on a world where all weights are reduced to a sixth part4 ,one occurred to me the other day when ,i saw a postman going his rounds with an amazing load of ,christmas presents and parcels4 ,i thought1 how much happier must be the lot of a postman on the moon1 if such functionaries are wanted there6 ,all the presents of toys or more substantial donations might be the same as before1 the only alteration would be that they would not feel nearly so heavy4 ,a box which contains a pound of chocolate bonbons might still contain exactly the same quantity of sweetmeat on the moon1 but the exertion of carrying it would be reduced to one-sixth4 ,it would only weigh as much as two or three ounces do on the earth4 ,our streets provide another admirable illustration of the drawbacks of our life here as compared with the facilities offered by life on the moon4 ,i feel quite confident that no perambulators can be necessary there4 ,i cannot indeed say that there are babies to be found on the moon1 but of this ,i am certain1 that even if the lunar babies were as plump and as sturdy as ours1 they must still only weigh about a sixth as much as ours do4 ,a lunar nurse would scorn to use a perambulator1 even for a pair of twins2 she might take them both out on her arm for an airing1 and even then only bear one-third of the load that her terrestrial sister must sustain if she is carrying but a single child4 ,the lightness of bodies in the moon would entirely transform many of our most familiar games4 ,in cricket1 for instance1 ,i don't think the bowling would be so much affected1 but the hits on the moon would be truly terrific4 ,i believe an exceptionally good throw of the cricket-ball here is about a hundred yards1 but the same man1 using the same ball and applying the same force to it1 would send the ball six hundred yards on the moon4 ,so1 too1 every hit would in the lunar game carry the ball to six times the distance it does here4 ,football would show a striking development in lunar play2 a good kick would not only send the ball over the cross-bar1 but it would go soaring over the houses1 and perhaps drop in the next parish4 ,our own bodies would1 of course1 participate in the general buoyancy1 so that1 while muscular power remained unabated1 we should be almost able to run and jump as if we had on the famous seven-league boots4 ,i have seen an athlete in a circus jump over ten horses placed side by side4 ,the same athlete1 making the same effort1 would jump over sixty horses on the moon4 ,a run with a pack of lunar foxhounds would indeed be a marvellous spectacle4 ,there need be no looking round by timid horsemen to find open roads or easy gaps4 ,the five-barred gate itself would be utterly despised by a huntsman who could easily clear a hay-rick4 ,it would hardly be worth taking a serious jump to clear a canal unless there was a road and a railway or so1 which could be disposed of at the same time4 ,to illustrate this subject of gravitation in another way1 suppose that we were to be transferred from this earth to some globe much greater than the earth--to a globe1 for instance1 as large and massive as the sun4 ,we can then show that the weight of every object would be increased4 ,indeed1 everything would weigh about twenty-seven times as much as we find it does here4 ,to pull out your watch would be to hoist a weight of about five or six pounds out of your pocket4 ,indeed1 ,i do not see how you could draw out your watch1 for even to raise your arm would be impossible2 it would feel heavier by far than if it were made of solid lead4 ,it is1 perhaps1 conceivable that you might stand upright for a moment1 particularly if you had a wall to lean up against2 but of this ,i feel certain1 that if you once got down on the ground1 it would be utterly out of your power to rise again4 ,these illustrations will at least answer one purpose3 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 space4 ,we are just adapted in every way for a residence on this particular earth of a particular size and climate1 and with atmosphere of a particular composition4 ,within certain slender limits our vital powers can become accommodated to change1 but the conditions of other worlds seem to be so utterly different from those we find here1 that it would probably be quite impossible for beings constituted as we are to remain alive for five minutes on any other globe in space4 ,it is1 however1 quite another question as to whether there may not be inhabitants of some kind on many of the other splendid globes4 ,we have through the wide extent of space inconceivable myriads of worlds1 presenting1 no doubt1 every variety of size and climate1 of atmosphere and soil4 ,it seems quite preposterous to imagine that among all these globes ours alone should be the abode of life4 ,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 life1 both animal and vegetable1 on this globe to the special circumstances in which it is placed4 ,,lecture ,,iii4 ,,the ,,inner ,,planets4 ,mercury1 ,venus1 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 ,asteroids1 or ,small ,planets4 ,,mercury1 ,,venus1 ,,and ,,mars4 ,we can hardly think of either the sun or the moon as a world in the sense in which our earth is a world1 but there are some bodies called planets which seem more like worlds1 and it is about them that we are now going to talk4 ,besides our ,earth there are seven planets of considerable size1 and a whole host of insignificant little ones4 ,these planets are like ours in a good many respects4 ,one of them1 ,venus1 is about the same size as this earth2 but the two others1 ,mercury and ,mars1 are very much smaller4 ,there are also some planets very much larger than any of these1 namely1 ,jupiter1 ,saturn1 ,uranus1 and ,neptune4 ,we shall in this lecture chiefly discuss three bodies1 namely1 ,mercury1 ,venus1 and ,mars1 which1 with the earth1 form the group of 8inner0 planets4 ,the planets are all members of the great family dependent on the sun4 ,venus and the earth may be considered the pair of twins1 alike in size and weight4 ,mercury and ,mars are the babies of the system4 ,the big brothers are ,jupiter and ,saturn4 ,all the planets revolve round the sun1 and derive their light and their heat from his beams4 ,we should like to get a little closer to some of our fellow-planets and learn their actual geography4 ,unfortunately1 even under the most favorable circumstances1 they are a very long way off4 ,they are many millions of miles distant1 and are always at least a hundred times as far as the moon4 ,but far as the planets may be1 astronomers have been familiar with their existence for ages past4 ,i can give you a curious proof of this4 ,you remember how we said the first and the second days of the week were called after the sun and the moon1 ,sun-day and ,moon-day1 or ,monday1 respectively4 ,let us see about the other days4 ,tuesday is not quite so obvious1 but translate it into ,french and we have at once ,mardi2 this word means nothing but ,mars' day1 and our ,tuesday means exactly the same4 ,wednesday is also readily interpreted by the ,french word ,mercredi1 or ,mercury's day1 while ,venus corresponds to ,friday4 ,jupiter's day is ,thursday1 while ,saturn's day is naturally ,saturday4 ,the familiar names of the days of the week are thus associated with the seven moving celestial bodies which have been known for uncounted ages4 ,,how ,,to ,,make ,a ,,drawing ,,of ,,our ,,system4 ,i want every one who reads this book to make a little drawing of the sun and the planets4 ,the apparatus that you will need is a pair of compasses2 any sort of compasses that will carry a bit of pencil will do4 ,you must also get a little scale that has inches and parts of inches divided upon it2 any carpenter's rule will answer4 ,the drawing is intended to give a notion of the true sizes and positions of the fine family of which the earth is one member4 ,the figure ,i have given 7,fig4 #df7 is not on so large a scale as that which ,i ask you to use1 and which ,i shall here mention4 ,try and do the work neatly1 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 system4 ,illustration3 ,,fig4 #df4--,the ,orbits of the ,four ,inner ,planets4 ,first open the compasses one inch1 and then describe a circle1 and mark a dot on this as 8,,mercury10 in neat letters1 and also write on the circle 8#hh days40 ,at the centre you are to show the 8,,sun40 ,this circle gives the track followed by ,mercury in its journey round the sun in the period of #hh days4 ,next open your compasses to #a in41 which you must do accurately by the scale4 ,the circle drawn with this radius shows the relative size of the path of ,venus1 and to indicate the periodic time1 you should mark it1 8#bbe days40 ,the next circle you have to draw is a very interesting one4 ,the compass is to be opened #b in4 this time1 and the path that it makes is to be marked 8#cfe days40 ,this shows the high road along which we ourselves journey every year1 along which we are1 indeed1 journeying at this moment4 ,if you wanted to obtain from your figure any notions of the true dimensions of the system1 the path of the earth will be the most convenient means of doing so4 ,the earth is #ic1jjj1jjj miles from the sun1 and our drawing shows its orbit as a circle of #b in4 radius4 ,it follows that each inch on our little scale will correspond to about #cg1jjj1jjj miles4 ,as1 therefore1 the radius of the orbit of ,mercury has been taken to be one inch1 it follows that the distance of ,mercury from the sun is about #cg1jjj1jjj miles4 ,we have1 however1 still one more circle to draw before we complete this little sketch4 ,the compass must now open to four inches1 and a circle which represents the orbit of ,mars is then to be drawn4 ,we mark on this 8#fhg days10 and the inner part of the solar system is then fully represented4 ,you see1 this diagram shows how our earth is in every sense a planet4 ,it happens that one of the four planets revolves outside the earth's path1 while there are two inside4 ,by marking the days on the circles which show the periods of the planets1 you perceive that the further a planet is from the sun1 the longer is the time that it takes to go round4 ,perhaps you will not be surprised at this1 for the length of the journey is1 of course1 greater in the greater orbits2 but this consideration will not entirely explain the augmentation of the time of revolution4 ,the further a planet is from the sun1 the more slowly does it actually move1 and therefore1 for a double reason1 the larger orbit will take a longer time4 ,from ,london to ,brighton is a much longer journey than from ,london to ,greenwich1 and1 therefore1 the journey by rail to ,brighton will1 of course1 be a longer one than by rail to ,greenwich4 ,but suppose that you compared the railway journey to ,greenwich with the journey1 not by rail1 but by coach1 to ,brighton1 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 ,greenwich4 ,mars may be likened to the coach which has to go all the way to ,brighton1 while ,mercury may be likened to the train which flies along over the very short journey to ,greenwich4 ,illustration3 ,,fig4 #dg4--;,comparative ,sizes of the ,planets4 ,we can easily show from our little sketch that ,mercury must be moving more quickly than ,mars1 for the radii of the two circles are respectively one inch and four inches1 and therefore the path of ,mars must be four times as long as the orbit of ,mercury4 ,if ,mars moved as fast as ,mercury1 he would1 of course1 require only four times as many days to complete his large path as ,mercury takes for his small path2 but four times #hh is #ceb1 and1 consequently1 ,mars ought to get round in #ceb days if he moved as fast as ,mercury does4 ,as a matter of fact1 ,mars requires nearly twice that number of days2 indeed1 no less than #fhg1 and hence we infer that the average speed of ,mars cannot be much more than half that of ,mercury4 ,illustration3 ,,fig4 #dh4--,phases of an ,inferior ,planet4 ,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 sketch1 by which the sizes of the four bodies themselves shall be fairly represented4 ,remember that the last drawing showed nothing whatever about the sizes of the bodies2 it merely exhibited the dimensions of the paths in which they moved4 ,as ,mercury is the smallest globe of the four1 we shall open the compasses half an inch and describe a circle to represent it4 ,the earth and ,venus are so nearly the same size 7though the earth is a trifle the larger7 that it is not necessary to attempt to exhibit the difference between them1 so we shall represent both bodies by circles1 each #a inches in radius4 ,mars1 like ,mercury1 is one of the globes smaller than the earth1 and the circle that represents it will have a radius of of an inch4 ,you should draw these figures neatly1 and by a little shading make them look like globes4 ,it would be better still if you were to make actual models1 taking care1 of course1 to give each of them the exact size4 ,a comparative view of the principal planets is shown in ,fig4 #dg4 ,,the ,,planet ,,mercury4 ,quicksilver is a bright and pretty metal1 and1 unlike every other metal1 it is a liquid under ordinary circumstances4 ,if you spill quicksilver1 it is a difficult task to gather the liquid up again4 ,it breaks into little drops1 and you cannot easily lift them with your fingers2 they slip away and escape your grasp4 ,quicksilver will run easily through a hole so small that water would hardly pass1 and it is so heavy that an iron nail or a bunch of keys will float upon it4 ,now1 this heavy1 bright1 nimble metal is known by another name besides quicksilver2 a chemist would call it mercury1 and the astronomers use exactly the same word to denote a pretty1 bright1 nimble1 and heavy planet which seems to try to elude our vision4 ,though ,mercury is so hard to see1 yet it was discovered so long ago1 that all record is lost of who the discoverer was4 ,you must take special pains if you want to see the planet ,mercury1 for during the greater part of the year it is not to be seen at all4 ,every now and then a glimpse is to be had1 but you must be on the alert to look out just after sunset1 or you must be up very early in the morning so as to see it just before sunrise4 ,mercury is always found to be in attendance on the sun1 so that you must search for him near the sun2 that is1 low down in the west in the evenings1 or low down in the east in the mornings4 ,to ascertain the proper time of the year at which to look for him you must refer to the almanac4 ,we have seen how ,mercury revolves in a path inside that of ,venus1 and it is therefore nearer to the sun4 ,indeed1 ,mercury is so close to the sun that it is generally overpowered by his brilliance and cannot be seen at all4 ,like every other planet1 ,mercury is lighted by the sun's rays1 and shows phases in the telescope just as the moon does 7,fig4 #dh74 ,in this figure the different apparent sizes of the planet at different parts of its path are shown4 ,of course the nearer ,mercury is to the earth the larger does it seem4 ,if we can only see ,mercury so rarely1 and if even then it is a very long way off1 does it not seem strange that we can tell how heavy it is8 ,even if we had a pair of scales big enough to hold a planet1 what1 it may be asked1 would be the use of the scales when the body to be weighed was about a hundred millions of miles away8 ,of course the weighing of a planet must be conducted in some manner totally different from the kind of weighing that we ordinarily use4 ,astronomers have1 however1 various methods for weighing these big globes1 even though they can never touch them4 ,we do not1 of course1 want to know how many pounds1 or how many millions of tons they contain2 there is but little use in trying to express the weight in that way4 ,it gives no conception of a planet's true importance4 ,one world must be compared with another world1 and we therefore estimate the weights of the other worlds by comparing them with that of our own4 ,we accordingly have to consider ,mercury placed beside the earth1 and to see which of the two bodies is the bigger and the heavier1 or what is the proportion between them4 ,it so happens that ,mercury1 viewed as a world1 is a very small body4 ,it is a good deal less in size than our earth1 and it is not nearly so massive4 ,to show you how we found out the mass of ,mercury ,i shall venture on a little story4 ,it will explain one of the strange devices that astronomers have to use when they want to weigh a distant body in space4 ,there was once1 and there is still1 a little comet which flits about the sky2 we shall call it after the name of its discoverer1 ,encke4 ,there are sometimes splendid comets which everybody can see--we will talk about these afterwards--but ,encke is not such a one4 ,it is very faint and delicate1 but astronomers are interested in it1 and they always look out for it with their telescopes2 indeed1 they could not see the poor little thing without them4 ,encke goes for long journeys through space--so far that it becomes quite invisible1 and remains out of sight for two or three years4 ,all this time it is tearing along at a tremendous speed4 ,if you were to take a ride on the comet1 it would whirl you along far more swiftly than if you were sitting on a cannon-ball4 ,when the comet has reached the end of its journey1 then it turns round and returns by a different road1 until at last it comes near enough to show itself4 ,astronomers give it all the welcome they can1 but it won't remain2 sometimes it will hardly stay long enough for us to observe that it has come at all1 and sometimes it is so thin and worn after all its wanderings that we are hardly able to see it4 ,the comet never takes any rest2 even during its brief visit to us it is scampering along all the time1 and then again it darts off1 gradually to sink into the depths of space1 whither even our best telescopes cannot follow it4 ,no more is there to be seen of ,encke for another three years1 when again it will come back for a while4 ,encke is like the cuckoo1 which only comes for a brief visit every spring1 and even then is often not heard by many who dearly love his welcome note2 but ,encke is a greater stranger than the cuckoo1 for the comet never repeats his visit of a few weeks more than once in three years2 and he is then so shy that usually very few catch a glimpse of him4 ,an astronomer and a mathematician were great friends1 and they used to help each other in their work4 ,the astronomer watched ,encke's comet1 noted exactly where it was1 on each night it was visible1 and then told the mathematician all he had seen4 ,provided with this information the mathematician sharpens his pencil1 sits down at his desk1 and begins to work long columns of figures1 until at length he discovers how to make a time table which shall set forth the wanderings of ,encke4 ,he is able to verify the accuracy of his table in a very unmistakable way by venturing upon prophecies4 ,the mathematician predicts to the astronomer the very day and the very hour at which the comet will reappear4 ,he even indicates the very part of the heavens to which the telescope must be directed1 in order to greet the wanderer on his return4 ,when the time comes the astronomer finds that his friend has been a true prophet2 there is the comet on the expected day1 and in the expected constellation4 ,this happens again and again1 so that the mathematician1 with his pencil and his figures1 marks stage by stage the progress of ,encke through the years of his invisible voyage4 ,at each moment he knows where the comet is situated1 though utterly unable to see it4 ,the joint labors of the two friends having thus discovered law and order in the movements of the comet1 you may judge of their dismay when on one occasion ,encke disappointed them4 ,he appeared1 it is true1 but then he was a little late1 and he was also not in the spot where he was expected4 ,there was nearly being a serious difference between the two friends4 ,the astronomer accused the mathematician of having made mistakes in his figures1 the mathematician retorted that the astronomer must have made some blunder in his observations4 ,a quarrel was imminent1 when finally it was suggested to interrogate ,encke himself1 and see whether he could offer any explanation4 ,the mathematician employed peculiar methods that ,i could not explain1 so ,i shall transform his processes into a dialogue between himself and the offending comet4 8,you are late10 said he to the comet4 8,you have not turned up at the time ,i expected you1 nor are you exactly in the right place2 nor1 indeed1 for that matter1 are you now moving exactly as you ought to do4 ,in fact1 you are entirely out of order1 and what explanation have you to give of this irregularity80 ,you see the questioner felt quite confident that there must have been some cause at work that he did not know of4 ,mathematicians have one great privilege2 they are the only people in the world who never make any mistakes4 ,if they knew accurately all the various influences that were at work on the comet1 they could1 by working out the figures1 have found exactly where the comet would be placed4 ,if the comet was not there1 it is inevitable that there must have been something or other acting upon the comet1 of which the mathematician was in ignorance4 ,the comet1 like every other transgressor1 immediately began to make excuses1 and to shuffle off the blame on somebody else4 8,i was10 said ,encke1 8going quietly on my rounds as usual4 ,i was following out stage by stage the track that you know so well1 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 alone1 but unfortunately ,i was not let alone4 ,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 planet1 of which ,i dare say you have heard--it is called ,mercury4 ,i did not want to interfere with ,mercury2 ,i was only anxious to hurry past and keep on my journey1 but he was meddlesome1 and began to pull me about1 and ,i had a great deal of trouble to get free from him1 but at last ,i did shake him off4 ,i kept my pace as well as ,i could afterwards1 but ,i could not make up the lost time1 and consequently ,i am here a little late4 ,i know ,i am not just where ,i ought to be1 nor am ,i now moving quite as you expect me to do2 the fact is1 ,i have not yet quite recovered from the bad treatment ,i have experienced40 ,the astronomer and the mathematician proceeded to test this story4 ,they found out what ,mercury was doing2 they knew where he was at the time1 and they ascertained that what the comet had said was true1 and that it had come very close indeed to the planet4 ,the astronomer was quite satisfied1 and was proposing to turn to some other matter1 when the mathematician said3-- 8,tarry a moment1 my friend4 ,it is the part of a wise man to extract special benefit from mishaps and disasters4 ,let us see whether the tribulations of poor ,encke cannot be made to afford some very valuable information4 ,we expected to find ,encke here4 ,well1 he is not here--he is there1 a little way off4 ,let us measure the distance between the place where ,encke is1 and the place where he ought to have been40 ,this the astronomer did4 8,well10 he said1 8what will this tell you8 ,it merely expresses the amount of delinquency on the part of ,encke40 8,no doubt10 said the mathematician1 8that is so2 but we must remember that the delinquency1 as you call it1 was caused by ,mercury4 ,the bigger and the heavier ,mercury was1 the greater would be his power of doing mischief1 the more would he have troubled poor ,encke1 and the larger would be the derangement of the comet in consequence of the unfortunate incident4 ,we have measured how much ,encke has actually been led astray4 ,had ,mercury been heavier than he is1 that distance would have been larger2 and if ,mercury had been lighter than he is1 you would not1 of course1 have found so large an error in the comet40 ,we may illustrate what is meant in this way4 ,a steamer sails from ,liverpool to ,new ,york1 and in favorable circumstances the voyage across the ,atlantic should be accomplished within a week4 ,but supposing that in the middle of the ocean a storm is encountered1 by which the ship is driven from her course4 ,she will1 of course1 be delayed1 and her voyage will be lengthened4 ,a trifling storm1 perhaps1 she will not mind1 but a heavy storm might delay her six hours2 a still greater storm might keep her back half a day2 while cases are not infrequent in which the delay has amounted to one day1 or two days1 or even more4 ,the delay which the ship has experienced may be taken as a measure of the vehemence of the storm4 ,i am not supposing that her machinery has broken down2 of course1 that sometimes happens at sea1 as do calamities of a far more tragic nature4 ,i am merely supposing the ship to be exposed to very heavy weather1 from which she emerges just as sound as she was when the storm began4 ,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 subjected4 8,the weather we had was much worse than the weather you had10 one traveller may say to another4 8,our ship was two days late1 while you escaped with a loss of one day40 ,when the comet at last returned to the earth after a cruise of three years through space1 the number of hours by which it was late expressed the vehemence of the storm it experienced4 ,the only storm that the comet would have met with1 at least in so far as our present object is concerned1 was the trouble that it had with ,mercury4 ,the mass of ,mercury was1 therefore1 involved in the delay of the comet4 ,in fact1 the delay was a measure of the mass of the planet4 ,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 ,mercury4 ,it was a very tedious and a very hard sum1 but at last his calculations arrived at the answer1 and showed that ,mercury must be a light globe compared to the earth4 ,in fact1 it would take twenty-five globes1 each equal to ,mercury1 to weigh as much as the earth4 ,i dare say you will think that this was a very long and roundabout way of weighing4 ,supposing1 however1 we had to weigh a mountain1 or rather a body which was bigger than fifty thousand mountains1 and which was also many millions of miles away1 all sorts of expedients would have to be resorted to4 ,i have told you one of them4 ,if you feel any doubts as to the accuracy with which such weighings can be made1 then ,i must tell you that there are many other methods1 and that these all agree in giving concordant results4 ,illustration3 ,,fig4 #di4--,relative ,weights of ,mercury and the ,earth4 ,we hardly know anything as to what the globe of ,mercury may be like4 ,we can see little or nothing of the nature of its surface4 ,we only perceive the planet to be a ball1 brightly lighted by the sun1 and we cannot satisfactorily discern permanent features thereon1 as we are able to do on some of the other planets4 ,,the ,,planet ,,venus4 ,you will have no difficulty in recognizing ,venus1 but you must choose the right time to look out for her4 ,in the first place1 you need never expect to see ,venus very late at night4 ,you should look for the planet in the evening1 as soon as it is dark1 towards the west1 or in the morning1 a little before sunrise1 towards the east4 ,i do not1 however1 say that you can always see ,venus1 either before sunrise or after sunset4 ,in fact1 for a large part of the year1 this planet is not to be seen at all4 ,you should therefore consult the almanac1 and unless you find that ,venus is stated to be an evening star or a morning star1 you need not trouble to search for it4 ,i may1 however1 tell you that ,venus can never be an evening star and a morning star at the same time4 ,if you can see it this evening after sundown1 there is no use in getting up early in the morning to look out for it again4 ,the planet will remain for several weeks a splendid object after sunset1 and then will gradually disappear from the west1 and in a couple of months later will be the morning star in the east4 ,venus requires a year and seven months to run through her changes1 so that if you find her a bright evening star to-night1 you may feel sure that she was a bright evening star a year and seven months ago1 and that she will be a bright evening star in a year and seven months to come4 ,nor must you ever expect to see her right overhead2 she is always to the west or to the east4 ,the splendor of ,venus1 when at her best1 will prevent you at such times from mistaking this planet for an ordinary star4 ,she is then more than twenty times as bright as any star in the heavens4 ,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 telescope4 ,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 sky4 ,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 earth1 or a button4 ,is it possible that this is the case1 you will say1 for as we see the planet so exquisitely beautiful1 how can she be merely a huge stone high up in the heavens8 ,the fact is that ,venus shines by light not her own1 but by light which falls upon her from the sun4 ,she is lighted up just as the moon1 or just as our own earth is lighted4 ,her radiance merely arises from the sunbeams which fall upon her4 ,it seems at first surprising that mere sunbeams on the planet can give her the brilliancy that is sometimes so attractive4 ,let me show you an illustration which will1 ,i trust1 convince you that sunbeams will be adequate even for the glory of ,venus4 ,here is a button4 ,i hang it by a piece of fine thread1 and when ,i dip it into the beam from the electric lamp1 look at the brilliancy with which the mimic planet glitters4 ,you cannot see the shape of the button2 it is too small for that2 you merely see it as a brilliant gem1 radiating light all around4 ,therefore1 we need not be surprised to learn that the brilliancy of the evening star is borrowed from the sun1 and that if1 while we are looking at the planet in the evening1 the sun were to be suddenly extinguished1 the planet would also vanish from view1 though the stars would shine as before4 ,thus we explain the appearance of ,venus4 ,the evening star is a beautiful1 luminous point1 but it has no shape which can be discerned with the unaided eye4 ,when1 however1 the telescope is turned towards ,venus we have the delightful spectacle of a tiny moon1 which goes through its phases just as does our own satellite4 ,when first seen as an evening star ,venus will often be like the moon at the quarter1 and then it will pass to the crescent shape4 ,then the crescent becomes gradually thinner1 and next will follow a brief period of invisibility before the appearance of ,venus as the morning star4 ,it seems at first a little strange that ,venus when brightest should not be full like the moon1 which in similar circumstances is1 of course1 a complete circle of light4 ,the planet1 however1 has a very marked crescent-shaped form in these circumstances4 ,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 us4 ,you ought all to try to get some one to show you ,venus through a telescope4 ,a very large instrument is not necessary1 and ,i feel sure you will be delighted to see the beautiful moon-shaped planet4 ,you will then have no difficulty in understanding how the brightness of the planet has come from the sun4 ,the changes in the crescent merely depend upon the proportion of the illuminated side which is turned towards us4 ,were ,venus itself a sunlike body we should1 of course1 see no crescent1 but only a bright circle of light4 ,in ,fig4 #ej you will notice an imaginary picture of a young astronomer surveying ,venus with a telescope4 ,i have not1 as is obvious1 attempted to show the different objects in their proper proportions4 ,the sun is supposed to have set1 so that his beams do not reach the astronomer4 ,night has begun at his observatory2 but the sunbeams fall on ,venus1 and light her up on that side turned towards the sun4 ,a part of this lighted side is1 of course1 seen by the telescope which the astronomer is using1 and thus the planet seems to him like a crescent of light4 ,illustration3 ,,fig4 #ej4--,to show that ,venus shines by ,sunlight4 ,,the ,,transit ,,of ,,venus4 ,we might naturally think from ,fig4 #df that ,venus must pass at every revolution directly between the earth and the sun2 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 star4 ,no doubt on each of these occasions ,venus seems to approach the sun closely2 but the orbits of ,venus and the ,earth do not lie quite in the same plane1 and hence the planet usually passes just over or just under the sun1 so that it is a very rare event indeed for her to come right in front of the sun4 ,but this does sometimes happen4 ,it happened1 for instance1 in the year #ahgd1 and again in the year #ahhb2 but1 alas6 ,i cannot hold out to you the prospect of ever seeing another such spectacle4 ,there will be no further occurrence of the transit of ,venus until the year #bjjd1 though there will be another eight years later1 in #bjab4 ,it seems rather odd that one transit of ,venus should be followed by another after an interval of eight years1 and that then a period of much more than a century should have to elapse before there will be a repetition of a similar pair4 ,this is in consequence of a curious relation between the motion of ,venus and the motion of the ,earth1 which ,i must endeavor to explain with the help of a little illustration4 ,let us suppose a clock with ordinary numbers round the dial1 but so arranged that the slowly moving short hand requires #cfe4bf days to complete one revolution round the dial1 while the more rapidly moving long hand revolves in #bbd4gj days4 ,the short hand will then go round once in a year1 and the long hand once during the revolution of ,venus4 ,let us suppose that both hands start together from ,,xii1 then in #bbd4gj days the long hand is round to ,,xii again1 but the short hand will have only advanced to about ,,vii1 and by the time it reaches ,,xii the long hand will have completed a large part of a second circuit4 ,it happens that the two numbers #bbd4gj and #cfe4bf are very nearly in the ratio of #h to #ac4 ,in fact1 if the numbers had only been #bbd4h and #cfe4c respectively1 they would be exactly in the proportion of #h to #ac4 ,it1 therefore1 follows that eight revolutions of the short hand must occupy very nearly the same time as thirteen revolutions of the long hand4 ,after eight years the short hand will of course be found again at ,,xii2 and at the same moment the long hand will also be back at ,,xii1 after completing thirteen revolutions4 ,we can now understand why the transits1 when they do occur1 generally arrive in pairs at an interval of eight years4 ,suppose that at a certain time ,venus happens to interpose itself directly between the earth and the sun1 then1 when eight years have elapsed1 the earth is1 of course1 restored for the eighth time since the first transit to the same place1 and ,venus has returned to almost the same spot for the thirteenth time4 ,the two bodies are practically in the same condition as they were at first1 and1 therefore1 ,venus again intervenes1 and the planet is beheld as a black spot on the sun's surface4 ,we must not push this argument too far2 the relation between the two periods of revolution1 though nearly1 is not exactly #h to #ac4 ,the consequence is that when another eight years have elapsed1 the planet passes a little above the sun or a little below the sun1 and thus a third occurrence of the transit is avoided for more than a century4 ,the next transit will take place at the opposite side of the path4 ,we were fortunate enough to be able to see the transit of ,venus in #ahhb from ,great ,britain4 ,perhaps ,i should say a part of the transit1 for the sun had set long before the planet had finished its journey across the disk4 ,venus looked like a small round black spot1 stealing in on the bright surface of the sun and gradually advancing along the short chord that formed its track4 ,illustration3 ,,fig4 #ea4--,venus in ,transit across the ,sun4 ,an immense deal of trouble was taken in #ahhb1 as well as in #ahgd1 to observe this rare occurrence4 ,expeditions were sent to various places over the earth where the circumstances were favorable4 ,indeed1 ,i do not suppose that there was ever any other celestial event about which so much interest was created4 ,the reason why the event attracted so much attention was not solely on account of its beauty or its singularity2 it was because the transit of ,venus affords us a valuable means of learning the distance of the sun4 ,when observations of the transit of ,venus made at opposite sides of the earth are brought together1 we are enabled to calculate from them the distance of ,venus1 and knowing that1 we can find the distance of the sun and the distances and the sizes of the planets4 ,this is very valuable information2 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 things4 ,i may1 however1 say that the principle of the method is really the same as that mentioned on pp4 #ai-be4 ,when you remember that not we ourselves1 nor our children1 and hardly our grandchildren1 will ever be able to see another transit of ,venus1 you will1 perhaps1 not be surprised that we tried to make the most of such transits as have occurred in our time4 ,,venus ,,as ,a ,,world4 ,though ,venus exhibits such pretty crescents in the telescope1 yet ,i must say that in other respects a view of the planet is rather disappointing4 ,venus is adorned by such a very bright dress of sunbeams that we can see but little more than those sunbeams1 and we can hardly make out anything of the actual nature of the planet itself4 ,we can sometimes discern faint marks upon the globe1 but it is impossible even to make a conjecture of what the ,venus country is like4 ,this is greatly to be regretted1 for ,venus approaches comparatively close to the earth1 and is a world so like our own in size and other circumstances that we feel a legitimate curiosity to learn something more about her4 ,but the marks on the planet1 though very faint1 are still sufficiently definite to have enabled some sharp-sighted astronomers to answer a question of much interest4 ,they have made it plain that in one most important respect ,venus is very unlike our ,earth4 ,our globe1 of course1 rotates on its axis once each day1 but ,venus requires no less than #bbe days to complete each rotation4 ,in fact1 this planet rotates in such a fashion that she always keeps the same face to the sun4 ,the inhabitants of ,venus will therefore find that it is perennial day on one side of this globe and everlasting night on the other4 ,venus is one of the few globes which might conceivably be the abode of beings not very widely different from ourselves4 ,in one condition especially--namely1 that of weight--she resembles the earth so closely that those bodies which we actually possess would probably be adapted1 so far as strength is concerned1 for a residence on the sister planet4 ,our present muscles would not be unnecessarily strong1 as they would be on the moon1 nor should we find them too weak1 as they would certainly prove to be were we placed on one of the very heavy bodies of our system4 ,nor need the temperature of ,venus be regarded as presenting any insuperable difficulties4 ,she is1 of course1 nearer to the sun than we are1 but then climate depends on other conditions besides nearness to the sun1 so that the question as to whether ,venus would be too hot for our abode could not be readily decided4 ,the composition of the atmosphere surrounding the planet would be the most material point in deciding whether terrestrial beings could live there4 ,i think it to be in the highest degree unlikely that the atmosphere of ,venus should chance to suit us in the requisite particulars1 and therefore ,i think there is not much likelihood that ,venus is inhabited by any men1 women1 or children resembling those on this earth4 ,,the ,,planet ,,mars ,,and ,,his ,,movements4 ,the path of the earth lies between the orbits of the planets ,venus and ,mars4 ,it is natural for us to endeavor to learn what we can about our neighbors4 ,we ought to know something1 at all events1 as to the people who live next door to us on each side4 ,i have1 however1 already said that we cannot observe very much upon ,venus4 ,the case is very different with respect to ,mars4 ,he is a planet which we are fortunately enabled to study minutely1 and he is full of interest when we examine him through a good telescope4 ,the right season for observing ,mars must1 of course1 be awaited1 as he is not always visible4 ,such seasons recur about every two years1 and then for months together ,mars will be a brilliant object in the skies every night4 ,nor has ,mars necessarily to be sought in the early morn or immediately after sunset1 in the manner we have already described for ,venus and ,mercury4 ,at the time ,mars is at his best he comes into the highest position at midnight1 and he can generally be seen for hours before1 and be followed for hours subsequently4 ,you may1 however1 find some difficulty in recognizing him4 ,you probably would not at first be able to distinguish ,mars from a fixed star4 ,no doubt this planet is a ruddy object1 but some stars are also ruddy1 and this is at the best a very insecure characteristic for identification4 ,i cannot give you any more general directions1 except that you should get your papa to point out ,mars to you the next time it is visible4 ,it is just conceivable that papa himself might not know how to find ,mars4 ,if so1 the sooner he gets a set of star maps and begins to teach himself and to teach you1 the better it will be for you both4 ,mars1 though apparently so like a star1 differs in some essential points from any star in the sky4 ,the stars proper are all fixed in the constellations1 and they never change their relative positions4 ,the groups which form the ,great ,bear or the ,belt of ,orion do not alter1 they are just the same now as they were centuries ago4 ,but the case is very different with a planet such as ,mars4 ,the very word planet means a wanderer1 and it is justly applied1 because ,mars1 instead of staying permanently in any one constellation1 goes constantly roaming from one group to the other4 ,he is a very restless body2 sometimes he pays his respects to the heavenly ,twins1 and is found near ,castor and ,pollux in ,gemini1 then he goes off and has a brief sojourn with the ,bull1 but it looks as if that fierce animal got tired of his company and hunted him off to the ,lion4 ,his quarters then become still more critical4 ,sometimes it looks as if he desired to seek for peace beneath the waters1 and so he visits ,aquarius1 while at other times he is found in dangerous proximity to the claws of the ,crab4 ,mars cannot even make up his mind to run steadily round the heavens in one direction2 sometimes he will bolt off rapidly1 then pause for a while1 and turn back again2 then the original impulse will return1 and he will resume his journey in the direction he at first intended4 ,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 uncertain4 ,yet there is a definite order underlying all his movements4 ,astronomers1 who make it their business to study the movements of ,mars1 can follow him on his way2 they know exactly where he is now1 and where he will be every night for years and years to come4 ,the people who make the almanacs come to the astronomers and get hints from them as to what ,mars intends to do1 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 moon4 ,we must not lay all the blame on ,mars for the eccentricities of his movements4 ,our earth is to a very large extent responsible4 ,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 view4 ,illustration3 ,,fig4 #eb4--;,how the ,tree seems to move about4 ,i was driving down a pretty country road with a little girl three years old beside me1 when ,i was addressed with the little remark1 8,look at the tree going about in the field40 ,now1 you or ,i1 with our longer experience of the world around us1 know that it is not the custom of trees to take themselves up and walk about the fields4 ,but this was what this little girl saw1 or rather what she thought she saw2 and very often what we do see is something very different from what we think we see4 ,we think we see ,mars performing all these extraordinary movements1 as the little girl thought she saw the tree moving about4 ,but just as that little girl1 when she grew to be a big girl1 found that what she thought was a tree walking across the field must really have some quite different explanation1 so we1 too1 find that what ,mars seems to do is one thing1 and what ,mars actually does is quite another thing4 ,let us see what the little girl noticed4 ,she was looking at the tree1 and first she saw it on one side of the house1 and then she saw it on the opposite side 7,fig4 #eb74 ,if it had been a cow instead of a tree1 of course the natural supposition would have been that the cow had walked4 ,our little friend may1 perhaps1 have thought it unusual for a tree to walk1 but still she saw the undoubted fact that the tree had shifted to the other side of the house1 and therefore1 perhaps1 remembering what the cow could do1 she said the tree had moved4 ,illustration3 ,,fig4 #ec4--;,a ,specimen of the ,track of ,mars4 ,the little girl did not stop to reflect that she herself had entirely changed her position1 and hence arose the surprising phenomenon of a tree that could move about4 ,you will understand this1 at once1 from the two positions of the car here shown4 ,in the first position1 as the girl looks at the tree1 the dotted line shows the direction of her glance1 and the other dotted line shows how the apparent places of the tree and the house have altered4 ,it is her change of place that has accomplished the transformation4 ,observe also that the tree appeared to her to move in the direction opposite to that in which she is going4 ,mars generally appears to move round among the stars from west to east4 ,in fact1 if we were viewing him from the sun he would always seem to move in this manner4 ,but at certain seasons our earth is moving very fast past ,mars1 and this will make him appear to move in the opposite direction4 ,this apparent motion is sometimes so much in excess of his real motion1 that it may give us an entirely incorrect idea of what the planet is actually doing4 ,thus1 notwithstanding that ,mars is moving one way1 he may appear to us who dwell on the earth to be going in the opposite way4 ,this illusion only happens for a short time1 just when we are passing ,mars1 as we do every two years4 ,the effect on the planet is to make the path he pursues at this time something like that shown in ,fig4 #ec4 ,the planet is nearest to us at the time he is moving in this loop4 ,he is then to be seen at his best in the telescope1 so that it is especially interesting to watch ,mars through this critical part of his career4 ,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 other4 ,the period he requires for a voyage round the sun is not quite two years1 for that would be #gcj days1 and ,mars only takes #fhg days for his journey4 ,it is1 however1 true that #a-ae_/ag years is very nearly the period of ,mars4 ,hence1 every #cb years ,mars will complete #ag rounds4 ,from this we shall be able to see how long it will take after the earth once passes ,mars before they pass again4 ,i shall suppose there is a circular course1 around which two boys start together to run a race4 ,one of these boys is such a good runner that he will get quite round in #ag minutes2 but the other boy can hardly run more than half as quickly1 for he will require #cb minutes to complete one circle4 ,here then is the question4 ,suppose the two boys to start together3 how long will it be before the faster runner gains one complete circuit on the other8 ,by the time the good runner 7,a7 has completed one circuit1 the bad runner 7,b7 has only got a little more than halfway4 ,when ,a has completed his second circuit1 he has1 of course1 run for twice #ag minutes--that is1 for #cd minutes4 ,this is two minutes longer than the time ,b requires to get round once2 therefore ,b is only ahead by a distance which ,a could cover in about one minute2 but ,b will have advanced during this minute a distance for which ,a will require another half-minute1 during which ,b covers a distance for which ,a will need a further quarter1 and so on4 ,but all these intervals--one minute1 half a minute1 a quarter of a minute1 one-eighth1 one-sixteenth1 and so on--added together amount to two minutes1 and hence it follows that ,b will not be overtaken until about two minutes after ,a has completed his second round--that is1 in #cf minutes altogether4 ,we can pass from this illustration to the case of the planet ,mars and the earth4 ,the orbit of the earth is traversed in a year1 and therefore1 after the earth has once passed ,mars1 which is then1 as astronomers would say1 in opposition1 about two years and the eighth of a year--that is1 two years and six or seven weeks--will elapse before ,mars is again favorably placed4 ,you will thus see that we need not expect to observe ,mars under the best conditions every year4 ,besides1 the distance of the planet from the earth at opposition varies so greatly that some oppositions are more favorable than others4 ,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 sun4 ,perhaps you will think that ,i am going to contradict some of the things that ,i have told you before4 ,i have often represented the orbits of the planets as circles1 and now ,i am going to tell you that this is not correct4 ,the fact is that the paths are nearly circles2 but1 still1 there is some departure from the exact circular shape4 ,mars1 in particular1 moves in a path which is more different from a circle than the path of the earth1 and consequently it is appropriate to introduce this subject when we are engaged about ,mars4 ,illustration3 ,,fig4 #ed4--;,how to draw an ,ellipse4 ,we must first take another lesson in drawing1 and the appliances ,i want you to use for the purpose are very simple4 ,you must have a smooth board and some tacks or drawing-pins1 besides paper1 pencil1 and twine4 ,illustration3 ,,fig4 #ee4--,specimens of ,ellipses4 ,we first lay a sheet of paper on the board1 and then put in two tacks through the paper and into the board4 ,it does not much matter where we put them in4 ,next we take a piece of twine and tie the two ends together so as to form a loop1 which we pass round the two tacks 7,fig4 #ed74 ,in the loop ,i place the pencil1 and then you see ,i move it round1 taking care to keep the twine stretched4 ,thus ,i produce a pretty curve1 which we call the ellipse4 ,i must ask all of you to practise this experiment4 ,try with different lengths of string1 and try using different distances between the tacks4 ,here are some sketches of two shapes of ellipse and a parabola 7,fig4 #ee74 ,elliptic curves can be made almost circles by putting the two tacks close together1 or they can be made very long in comparison with their width4 ,they are all pretty and graceful figures1 and are often useful for ornamental work4 ,the ellipse is a pretty shape for beds of flowers in a grass-plot4 ,the importance of the ellipse to astronomers is greater than that of any other geometrical figure4 ,in fact1 all the planets1 as they perform their long and unceasing journeys round the sun1 move in ellipses2 and though it is true that these ellipses are very nearly circles1 yet the difference is quite appreciable4 ,it is also important to observe that the sun is not in the centre of the ellipse which the planet describes4 ,the sun is nearer to one end than to the other4 ,and the actual position of the sun must be particularly noted4 ,suppose that some mighty giant were preparing to draw an exact path for the earth1 or for ,mars1 of course he would want to have millions of miles of string for producing a big enough curve1 and one of the nails that he used would have to be driven right into the sun4 ,the following is the astronomer's more accurate method of stating the facts4 ,he calls each of the points represented by the tacks around which the string is looped a focus of the ellipse2 the two points together are said to be the foci2 and as the planet is describing its orbit1 the position of the sun will lie exactly at one of the foci4 ,the ellipse is a curve that nature is very fond of reproducing4 ,from an electric light1 a brilliant beam will diverge4 ,if you hold a globe in the beam1 and let the shadow fall on a sheet of paper1 it forms an ellipse4 ,if you hold the sheet squarely1 the shadow is a circle2 but as you incline it1 you obtain a beautiful oval1 and by gradually altering the position1 you can get a greatly elongated curve4 ,indeed1 you can thus produce an ellipse of almost any form4 ,the electric light is not indispensable for this purpose2 any ordinary bright lamp with a small flame will answer1 and by taking different sized balls and putting them in various positions1 you can make many ellipses1 great and small4 ,,the ,,discoveries ,,made ,,by ,,tycho ,,and ,,kepler4 ,it was by the observations of a celebrated old astronomer1 named ,tycho ,brahe1 that the true shape of a planet's path came to be afterwards determined4 ,tycho lived in days before telescopes were invented4 ,he had few of the excellent contrivances for measuring which we have in our observatories4 ,we shall take a look at this fine old astronomer1 as he sits amid his curious astronomical machines4 ,illustration3 ,,fig4 #ef4--,tycho ,brahe in his ,observatory4 ,he lived on an island near ,copenhagen1 and he has given us a picture of himself 7,fig4 #ef71 as he is seated with his quaint apparatus1 and his assistants around him1 busily engaged in observing the heavens4 ,you see the walls of his observatory are decorated with pictures2 and one of the great ,danish hounds which the ,king of ,denmark had presented to him lies asleep at his feet4 ,i do not think we should now encourage big dogs in the observatory at night4 ,nor do modern astronomers put on their velvet robes of state1 as ,tycho was said to have done when he entered into the presence of the stars1 as1 by so doing1 he showed his respect for the heavens4 ,astronomers1 nowadays1 rather prefer to wear some comfortable coat which shall keep out the cold1 no matter what may be its appearance from the picturesque point of view4 ,in this wonderful contrivance1 you see ,tycho ,brahe did not use any actual telescope4 ,he observed through a small opening in the wall1 and lest there should be any mistake as to what is going on1 you see he is pointing towards it1 and giving his three assistants their instructions4 ,the most important work is being done by the man on the right4 ,he is engaged in making the actual observation4 ,but he has no aid from magnifying lenses4 ,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 opposite4 ,on the circle a number of marks have been engraved1 and there are numbers placed opposite to the marks2 it is by these that the position of the object is to be ascertained4 ,if the object is high1 then the pointer will be low2 and if the object is low1 then the pointer will be high4 ,the observer calls out the position when he has found it1 and there1 you see1 is a man ready with writing materials to take down the observation4 ,notice also the other astronomer who is looking at the clock4 ,he gives the time1 which must also be recorded accurately4 ,in fact1 the entire process of finding the place of a heavenly body consists in two observations--one from the circle and the other from the clock2 so that though ,tycho had no telescope to aid his vision1 yet the principle on which his work was done was the same as that which we use in our observatories at this moment4 ,you may think that such a concern would hardly be capable of producing much reliable work4 ,however1 ,tycho compensated in a great degree for the imperfection of his instrument by the skill with which he used it4 ,he had a noble determination to do his very best4 ,perseverance will accomplish wonders even with very imperfect means4 ,a great astronomer has said that a skilful observer ought to be able to make valuable measurements with a common cart-wheel6 ,it was with instruments on the principle of that which ,i have here shown that ,tycho made his celebrated observations of ,mars4 ,week after week1 month after month1 year after year1 did the patient old astronomer track the planet through his capricious wanderings4 ,before we try to explain anything1 it is of course necessary to ascertain1 with all available accuracy1 what the thing actually is4 ,therefore1 when we seek to explain the irregular movements of a planet1 the first thing to be done is to make a careful examination of the nature of those irregularities4 ,and this was what ,tycho strove to do with the best means at his disposal4 ,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 moving4 ,first he tried various circles1 and then he sought1 by placing the centre in different positions1 to see whether it would not be possible to account thus for the irregularities of the wayward planet4 ,it would not do2 the movement was not circular4 ,this was thought very strange in those days1 for the circle was regarded as the only perfect curve1 and it was considered quite impossible for a planet to have any motion except it were the most perfect4 ,there was1 however1 no help for it2 so ,kepler sagaciously tried the ellipse1 which he considered to be the most perfect curve next to the circle4 ,he continued his long calculations1 until at last he succeeded in finding one particular ellipse1 placed in one particular position1 which would just explain the strange wanderings of our erratic neighbor4 ,it was not alone that the motion of the planet traced out an ellipse2 it was further discovered that the sun lies at one of the foci of the curve4 ,if the sun were anywhere else1 the motion of the planet would have been different from that which ,tycho had found it to be4 ,you must know that this discovery is one of the very greatest that have ever been made in the whole extent of human knowledge4 ,after it had been proved that the orbit of ,mars was elliptic1 it became plain that the same path must be traced by every planet4 ,there are very big planets1 and there are small ones2 there are planets which move in very large orbits1 and there are planets whose paths are comparatively small4 ,in all cases the high road which the planet follows is invariably an ellipse1 and the sun is invariably to be found situated at the focus4 ,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 follow4 ,kepler also made another great discovery in connection with the same subject4 ,if the planet moved in a circle with the sun in the centre1 then there would be very good reason to expect that it would always move at the same speed1 for there would be no reason why it should go faster at one place than at another4 ,in fact1 the planet would then be revolving always at the same distance from the sun1 and every part of its path would be exactly like every other part4 ,but when we consider that the motion is performed in an ellipse1 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 perceptible1 we have no reason to expect that the speed shall remain the same all round4 ,we know that the engine-driver of a railway train always has to slacken speed when he is going round a sharp curve4 ,if he did not do so1 his train would be very likely to run off the line1 and a dreadful accident would follow4 ,the engine-driver is well aware that the conditions of pace are dependent on the curvature of his line4 ,the planet finds that it1 too1 must pay attention to the curves2 but the extraordinary point is that the planet acts exactly in the opposite way to the engine-driver4 ,the planet puts on its highest pace at one of the most critical curves in the whole journey4 ,there are two specially sharp curves in the planet's path4 ,these are1 of course1 the two extremities of the ellipse which it follows4 ,the cautious engine-driver would1 of course1 creep round these with equal care1 and no doubt the planet goes slowly enough about that end of the ellipse which is farthest from the sun4 ,there its pace is slower than anywhere else2 but from that moment onwards the planet steadily applies itself to getting up more and more speed4 ,as it traverses the comparatively straight portion of the celestial road1 the pace is ever accelerating until the sharp curve near the sun is being approached2 then the velocity gets more and more alarming1 until at last1 in utter defiance of all rules of engine-driving1 the planet rushes round one of the worst parts of the orbit at the highest possible speed4 ,and yet no accident happens1 though the planet has no nicely laid lines to keep it on the track4 ,if lines are necessary to save a railway train from destruction1 how can we possibly escape when we have no similar assistance to keep us from flying away from the sun and off into infinite space8 ,kepler has taught us to measure the changes in the speed of the body with precision4 ,he has shown that the planet must1 at every point of its long journey1 possess exactly the right speed2 otherwise everything would go wrong4 ,i dare say you have seen1 at different points along a line of railway1 boards put up here and there1 with notices like1 8,ten miles an hour40 ,these words are1 of course1 an intimation to the engine-driver that he is not to vary from the speed thus stated4 ,kepler has given us a law which is equivalent to a large number of caution boards1 fixed all round the planet's path1 indicating the safe speed for the journey at every stage4 ,it is fortunate for us that the planet is careful to observe these regulations4 ,if the earth were to leave her track1 the consequences would be far worse than those of the most frightful railway accident that ever happened4 ,whichever side we took would be almost equally disastrous4 ,if we went inwards we should plunge into the sun1 and if we went outwards we should be frozen by cold4 ,we owe our safety to the care with which the speed of the earth is prescribed4 ,when near the sun1 the earth is pulled inwards with exceptionally strong attraction4 ,we are often told that when a strong temptation seizes us1 the wisest thing that we can do is to run away as hard as possible4 ,this is just what the laws of dynamics cause the earth to do at this critical time4 ,she puts on her very best pace1 and only slackens when she has got well away from the danger4 ,the peril that we are exposed to when the earth is at the other end of the orbit is of an opposite character4 ,we are then a long way from the sun1 and the pull which it can exercise upon the earth is correspondingly lessened4 ,care is then required lest we should escape altogether from the sun's warmth and his guidance4 ,we must therefore give time to the sun to exercise his power1 so as to enable the earth to be recalled2 accordingly we move as slowly as possible until the sun conquers the earth's disposition to fly off1 and we begin to return4 ,you may remember that when we were speaking about the moon1 ,i showed you how a body might revolve around the earth in a circle under the influence of an attraction towards the earth's centre4 ,so long as the path is really a circle1 then the power with which the earth is drawing the body remains the same4 ,in a precisely similar way1 a body could revolve around the sun in a circle1 in which case also the attraction of the sun will remain the same all round4 ,but now we have a very much more difficult case to consider4 ,if the body does not always remain at the same distance1 the power of the sun will not be the same at the different places4 ,whenever the object is near the sun1 the attraction will be greater than when it is farther off4 ,for example1 when the distance between the two bodies is doubled1 then the pull is reduced to the fourth part of what it was before4 ,,the ,,discoveries ,,made ,,by ,,newton4 ,i have now some great discoveries to talk to you about1 which were made by ,sir ,isaac ,newton4 ,he was not an astronomer who looked much through a telescope1 though he made many remarkable experiments4 ,he used to sit in his study and think1 and then he used to draw figures with his pencil1 and make long calculations4 ,at last he was able to give answers to the questions3 ,what is the reason why the planet moves in an ellipse8 ,why should it move in this curve rather than in any other8 ,why should this ellipse be so placed that the sun lies at one of the foci8 ,if the planet had run uniformly round its course1 ,newton would have found his task an impossible one4 ,but ,i have already explained that the motion is not uniform4 ,i described how the planet hurried along with extra speed at certain parts of its path2 how it lingered at other parts2 how1 in fact1 it never preserved the same rate for even a single minute during the whole journey4 ,kepler had shown how to make a time-table for the whole journey4 ,in fact1 just as a captain on a long voyage keeps a record of each day's run1 and shows how to-day he makes #agj miles1 and to-morrow perhaps #bjj1 and the next day #baj1 while the day after he may fall back to #abj1 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 recorded4 ,when ,newton commenced his work1 one of the first questions he had to consider was the following3 ,suppose that a great globe like a planet1 or a small globe like a marble1 or an irregular body like an ordinary stone1 were to be thrown into space1 and were then to be left to follow its course without any force whatever acting upon it1 where would it go to8 ,you may say1 at once1 that a body under such circumstances will presently fall down to the ground2 and so1 of course1 it will1 if it be near the earth4 ,i am not1 however1 talking of anything near the earth2 ,i want you to imagine a body far off in the depths of space1 among the stars4 ,such a body need not necessarily fall down here1 for you see the moon does not fall1 and the sun does not4 ,if you were at a great distance from our globe and from all other large globes--so far1 indeed1 that their attractions were imperceptible--you could try the experiment that ,i wish now to describe4 ,throw a stone as hard as ever you can1 and what will happen8 ,of course1 when you do it down here1 it moves in a pretty curve through the air1 and tumbles to the ground2 but away in open space1 what will the stone do8 ,there will be no such motion as up or down1 as we ordinarily understand it2 for though the earth1 no doubt1 will lie in one particular direction at a great distance1 yet there will be other bodies just as large in other directions2 and there is no reason why the stone should move towards one of these rather than to another2 in fact1 if they are all far enough1 as the stars are from us1 their attractions will be quite inappreciable4 ,there is1 therefore1 not the slightest reason why the stone should swerve to one side more than to another4 ,there is no more reason why it should turn to the right than why it should turn to the left4 ,nor could you throw the stone so as to make it follow a curved path4 ,you can1 of course1 make it describe a curve while it remains in your hand1 but the moment the stone has left your hand1 it proceeds on its journey by a law over which you have no control4 ,as the direction cannot be changed towards one side more than towards the other1 the stone must simply follow a straight line from the very moment when it is released from your hand4 ,the speed with which the stone is started will also not change4 ,you might at first think that it would gradually abate1 and ultimately cease4 ,no doubt a stone thrown along the road will behave in this way1 but that is because the stone rubs against the ground4 ,if you throw a stone across a sheet of ice1 then it will run a very long distance before it stops1 and all the time it will be moving in a straight line4 ,in this case there is but little loss by rubbing against the ice1 because it is so smooth4 ,thus we see that if the path be exceedingly smooth1 the body will run a long way before it stops4 ,think of the distance a railway train will run if1 while travelling at full speed along a level line1 the steam is turned off4 ,these illustrations all show that if you let a body alone1 after having once started it1 and do not try to pull it this way or that way1 and do not make it rub against things1 that body will move on continually in a straight line1 and will keep up a uniform speed4 ,we can apply this reasoning to a stone out in space4 ,it would certainly move in a straight line1 and would go on and on forever1 without losing any of its pace4 ,i need hardly tell you that no one has ever been able to try this experiment4 ,in the first place1 we reside upon the surface of the earth1 and we have no means of ascending into those elevated regions where the stone is supposed to be projected4 ,there is also another difficulty which we cannot entirely avoid1 and that arises from the resistance of the air4 ,all movements down here are impeded because the body has to force its way through the air2 and in doing so it invariably loses some of its speed4 ,out in open space there is1 of course1 no air1 and no loss of speed can therefore arise from this cause4 ,illustration3 ,,fig4 #eg4--,the ,humming-top4 ,there are1 however1 several actual experiments by which we can assure ourselves of the general truth4 ,set a humming-top spinning 7,fig4 #eg72 it gradually comes to rest1 partly because of the rubbing of its point on the table1 and partly because it has to force its way through the air4 ,in fact1 the hum of the top that you hear is only produced at the expense of its motion4 ,supposing ,i use a much heavier top2 if ,i set it spinning it will keep up for many minutes1 because its weight gives it a better store of power wherewith to overcome the resistance of the air4 ,i remember hearing a story about ,professor ,clerk-,maxwell4 ,he had1 when at ,cambridge1 invented one of these large and heavy tops1 which would spin for a long time4 ,one evening the top was left spinning on a plate in his room when his friends took their departure1 and no doubt it came to rest in due time4 ,early the next morning1 ,professor ,maxwell1 hearing the same friends coming up to his rooms again1 jumped out of bed1 set the top spinning1 and then got back to bed1 and pretended to be asleep4 ,he thus astounded his friends1 who1 of course1 imagined that the top must have been spinning all the night long6 ,if we spin a top under the receiver of an air pump 7,fig4 #eh71 it will keep up its motion for a very much longer time after the air has been exhausted than it would in ordinary circumstances4 ,such experiments prove that the motion of a body will not of itself naturally die out1 and that if we could only keep away the interfering forces altogether1 the motion would continue indefinitely with unabated speed4 ,what ,i have been endeavoring to illustrate is called the first law of motion4 ,it is written thus3-- 8,every body continues in its state of rest or of uniform motion in a straight line1 except in so far as it may be compelled by impressed forces to change that state40 ,illustration3 ,,fig4 #eh4--,to illustrate the ,first ,law of ,motion4 ,i would recommend you to learn this by heart4 ,i can assure you it is quite as well worth knowing as those rules in the ,latin ,grammar with which many of you1 ,i have no doubt1 are acquainted4 ,the best proof of the first law of motion is derived1 not from any experiments1 but from astronomy4 ,we make many calculations about the movements of the sun1 the moon1 the stars1 and then we venture on predictions1 and we find those predictions verified4 ,thus we had a transit of ,venus across the sun in #ahhb1 and every astronomer knew that this was going to occur1 and many went to the ends of the earth so that they might see it favorably4 ,their anticipations were realized2 they always are4 ,astronomers make no mistakes in these matters4 ,they know that there will be another transit of ,venus in the year #bjjd1 but not sooner4 ,the calculations by which these accurate prophecies are made involve this first law of motion2 and as we find that such prophecies are always fulfilled1 we know that the first law of motion must be true also4 ,newton knew that if a planet were merely left alone in space1 it would continue to move on forever in a straight line4 ,but ,kepler had shown that the planet did not move in a straight line1 but that it described an ellipse4 ,one conclusion was obvious4 ,there must be some force acting upon the planet which pulls it away from the straight line it would otherwise pursue4 ,we may1 for the sake of illustration1 imagine this force to be applied by a rope attached to the planet so that at every moment it is dragged by some unseen hand4 ,to find the direction this rope must have1 we take the law of ,kepler1 which explains the rules according to which the planet varies its speed4 ,i cannot enter into the question fully1 as it would be too difficult for us to discuss now4 ,i should have to talk a great deal more about mathematics than would be convenient just at present2 but ,i think you can all understand the result to which ,newton was led4 ,he showed that the rope must always be directed towards the sun4 ,in other words1 suppose that there was no sun1 but that in the place which it occupied there was a strong enough giant constantly pulling away at the planet1 then we should find that the speed of the planet would alter just in the way it actually does4 ,thus we learn that some force must reside in the sun by which the planet is drawn1 and this force is exerted1 although there is no visible bond between the sun and the planet4 ,there is another fact to be learned about the sun's attraction1 and this time we obtain it by knowing the shape of the curve followed by the planet4 ,the laws by which the planet's speed is regulated prove that the force emanates from the sun4 ,we shall now learn much more when we take into account that the path of the planet is an ellipse1 of which the sun lies at the focus4 ,nothing has been said as yet regarding the magnitude of the pull which is being exerted by the sun4 ,is that pull to be always the same1 or is it to be greater at some times than at other times8 ,newton showed that no ellipse other than a circle could be described1 if the pull from the sun were always the same4 ,its magnitude must be continually changed1 and the nearer the planet lies to the sun1 the more vehement is the pull it receives4 ,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 position4 ,let us suppose that the planet is in a certain position1 and that it then passes into a second position1 which is twice as far from the sun4 ,the pull upon the planet at the shorter distance is not only greater than the pull at the longer distance1 but it is actually four times as much4 ,stating this result a little more generally1 we assert1 in the language of astronomers1 that the attraction varies inversely as the square of the distance4 ,if this law were departed from1 then ,i do not say that it would be impossible for the planet to revolve around the sun in some fashion1 but the motion would not be performed in an ellipse described around the sun in the focus4 ,you see how very instructive are the laws which ,kepler discovered4 ,from the first of them we were able to infer that the sun attracts the planets2 from the second1 we have learned how the magnitude of the attracting force varies4 ,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 moon4 ,as the moon revolves around the earth it is held by the earth's attraction1 and the moon follows a path which1 though nearly a circle1 is really an ellipse4 ,this orbit is described around the earth just as the earth describes its path around the sun4 ,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 principle4 ,every body in the whole universe attracts every other body4 ,think of two weights lying on the table4 ,they no doubt attract each other1 but the force is an extremely small one--so small1 indeed1 that you could not measure it by any ordinary appliance4 ,one or both of the attracting masses must be enormously big if their mutual gravitation is to be readily appreciable4 ,the attraction of the earth on a stone is a considerable force1 because the earth is so large1 even though the stone may be small4 ,imagine a pair of colossal solid iron cannon-balls1 each #ec yards in diameter1 and weighing about #dag1jjj tons4 ,suppose these two globes were placed a mile apart1 the pull of one of them on the other by gravitation would be just a pound weight4 ,notwithstanding the size of these masses1 the hand of a child could prevent any motion of one ball by the attraction of the other4 ,if1 however1 they were quite free to move1 and there was absolutely no friction1 the balls would begin to draw together2 at first they would creep so slowly that the motion would hardly be noticed4 ,the pace would no doubt continue to improve slowly1 but still not less than three or four days must elapse before they will have come together4 ,by the kindness of ,professor ,dewar1 ,i am enabled to exhibit a contrivance with which we can illustrate the motion of a planet around the sun4 ,here is a long wire suspended from the roof of this theatre1 and attached to its lower end is an iron ball1 made hollow for the sake of lightness4 ,when ,i draw the ball aside1 it swings to and fro with the regularity of a great pendulum4 ,but when ,i place a powerful magnet in its neighborhood 7,fig4 #ei71 you see that as soon as the ball gets near the magnet it is violently drawn to one side1 and follows a curved path4 ,this magnet may be taken to represent the sun1 while the ball is like our earth1 or any other planet1 which would move in a straight line were it not for the attraction of the sun which draws the body aside4 ,illustration3 ,,fig4 #ei4--,the ,effect of ,attraction4 ,,the ,,geography ,,of ,,mars4 ,we will now say something with respect to the geography of our fellow-planet1 a subject which seems all the more interesting because ,mars is so like the earth in many respects4 ,we require a fairly good telescope for the purpose of seeing him well1 but when such an instrument is directed to the planet1 a beautiful picture of another world is unfolded 7,fig4 #fj74 ,there are many things visible on his surface1 but we must always remember that even with our most powerful telescopes the planet still appears a long way off4 ,illustration3 ,,fig4 #fj4--,views of ,mars4 ,illustration3 ,,fig4 #fa4--,mars4 7,by ,douglass1 ,lowell ,observatory47 ,in the most favorable circumstances1 ,mars is at least one hundred times as far from us as the moon4 ,but we know that an object on the moon must be as large as ,st4 ,paul's ,cathedral if it is to be visible in our telescopes4 ,an object on ,mars must be1 therefore1 at least one hundred times as broad and one hundred times as long as ,st4 ,paul's ,cathedral if it is to be discernible by astronomers on our earth4 ,we can1 therefore1 only expect to see the general features of our fellow-planet4 ,were we looking at our earth from a similar distance1 and with equally good telescopes1 the continents and oceans1 and the larger seas and islands1 would all be large enough to be conspicuous4 ,it is1 however1 doubtful whether they could ever be properly revealed through the serious impediment to vision which our atmosphere would offer4 ,it fortunately happens that the surface of ,mars is only obscured by clouds to a very trifling extent1 and we are thus able to see a panorama of our neighboring globe laid before us4 ,mars is not nearly so large as our earth1 the diameters of the two bodies being nearly as two to one4 ,it follows that the number of acres on the planet is only a quarter of the number of acres on the earth4 ,careful telescopic scrutiny shows that the chief features which we see on ,mars are of a permanent character4 ,in this respect ,mars is much more like the moon than the sun4 ,the latter presents to us merely glowing vapors1 with hardly more permanence than is possessed by the clouds in our own sky4 ,on the other hand1 the entire absence of clouds from the moon enables us to see the permanent features on its surface4 ,most of the visible features on ,mars are also invariable2 though occasionally it would seem that the climate produces some changes in its appearance4 ,illustration3 ,,fig4 #fb4--,the ,south ,pole of ,mars1 ,september1 #ahgg 7,green74 ,we first notice that there are differently colored parts on ,mars4 ,the darkish or bluish regions are usually spoken of as seas or oceans2 though we should be going beyond our strict knowledge were we to assert that water is actually found there4 ,look at the horn-shaped object in the centre of the lower picture in ,fig4 #fj4 ,we call it the ,kaiser ,sea1 and it is so strongly marked that even in a small telescope it can be often seen4 ,you must not1 however1 always expect to notice this feature when you look at the planet through a telescope1 for it turns round and round4 ,we can make a globe representing ,mars4 ,on this are to be depicted this great sea and the other characteristic objects4 ,but as we turn the globe around1 the opposite side of the planet is brought into view1 and other features are revealed like those represented in the upper figure4 ,mars requires #bd hours #cg minutes #bb4g seconds to complete a single rotation4 ,it is somewhat remarkable that this only differs from the earth's period of rotation by a little more than half an hour4 ,mars contains what we call continents as well as oceans1 and we also find there lakes and seas and straits4 ,these objects are indicated in the drawings that are here represented4 ,but the most striking features which the planet displays are the marvellous white regions1 which are seen both at its ,north ,pole and at its ,south ,pole 7,fig4 #fb74 ,if we were able to soar aloft above our earth and take a bird's-eye view of our own polar regions1 we should see a white cap at the middle of the arctic circle4 ,this appearance would be produced by the eternal ice and snow4 ,it would increase during the long1 dark winter1 and be somewhat reduced by melting during the continuously bright summer4 ,though we cannot thus see our earth1 yet we can sometimes observe one ,pole of ,mars and sometimes the other1 and we find each of these ,poles crowned with a dense white cap1 which increases during the severity of its winter1 and which declines again with the warmth of the ensuing summer4 ,sketches of ,mars have been made by many astronomers2 among them we may mention ,mr4 ,green1 who made a beautiful series of pictures at ,madeira in #ahgg4 ,these may be supplemented by the drawings of ,mr4 ,knobel in #ahhd1 when the opposite ,pole of the planet was turned to view4 ,the drawings show the polar snows1 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 limits4 ,an interesting case of this kind is shown in ,fig4 #fb1 which has been copied from one of ,mr4 ,green's drawings4 ,it has lately been surmised that the continents on ,mars are occasionally inundated by floods of water4 ,there are also indications of clouds hanging over the ,martian lands1 but the inhabitants of that planet1 in this respect1 escape much better than we do4 ,a certain amount of atmosphere always surrounds ,mars1 though it is much less copious than that we have here4 ,as to the composition of this atmosphere we know nothing4 ,for anything we can tell1 it might be a gas so poisonous that a single inspiration would be fatal to us2 or if it contained oxygen in much larger proportion than our air does1 it might be fatal from the mere excitement to our circulation which an over-supply of stimulant would produce4 ,i do not think it the least likely that our existence could be supported on ,mars1 even if we could get there4 ,we also require certain conditions of climate1 which would probably be totally different from those we should find on ,mars4 ,many remarkable observations of ,mars have been lately made by ,mr4 ,percival ,lowell4 ,it seems very doubtful how far our former division of continents and oceans on ,mars can be maintained4 ,mr4 ,lowell has paid special attention to a wonderful system of lines on the planet's surface to which the name of 8canals0 has been given1 which often show such a degree of regularity as would almost suggest the idea that they had been laid down by intelligent guidance4 ,,the ,,satellites ,,of ,,mars4 ,when ,mars appeared in his full splendor in #ahgg1 he was for the first time honored with the notice of instruments capable of doing him justice4 ,i do not1 however1 mean that in former apparitions he was not also carefully observed1 but a great improvement had recently taken place in telescopes1 and it was thus under specially favorable auspices that his return was welcomed in #ahgg4 ,this year will be always celebrated in astronomical history for a beautiful discovery made by ,professor ,asaph ,hall1 the illustrious astronomer at ,washington4 ,before ,i can explain what this discovery was1 ,i must have a little talk about moons1 or satellites as they are often called4 ,you know that we have one moon1 which is constantly revolving round the earth1 and accompanies the earth in its long voyage round the sun4 ,but the earth is only a planet1 and there are many other planets which are worlds like ours4 ,it is natural to compare these worlds1 and as we have one moon1 why should not the other planets also have moons8 ,if there are children in one house in a square1 why should there not be children in the other houses8 ,we find that some of the other planets have satellites1 but they do not seem to be distributed very regularly4 ,in fact1 they are almost as capriciously allotted as the children would be in eight houses that you might take at random4 ,illustration3 ,,fig4 #fc4--,mars and his ,two ,satellites4 ,in ,number ,one there lives an old bachelor1 and in ,number ,two a single lady4 ,these are ,mercury and ,venus1 and of course there are no children in either of these houses4 ,number ,three is inhabited by old mother ,earth1 and she has got a fine big son1 called the ,moon4 ,number ,four is a nice little house inhabited by ,mars4 ,there are to be found a pair of little twins1 and nimble creatures they are too4 ,number ,five is a great mansion4 ,a very big man lives here1 called ,jupiter1 with four robust sons and daughters that everybody knows4 ,i fancy they must go to many dancing parties1 for every night they may be seen whirling round and round4 ,for three hundred years these four moons have been known to astronomers1 but in #ahib there was an addition to the family in the shape of a tiny moon which had never been seen up to that time4 ,number ,six is also a fine big house1 though not quite so big as ,number ,five1 but larger than any of the others4 ,it is inhabited by ,saturn1 and contains the biggest family of all4 ,up till the other day eight sons and daughters were known to live here1 but they are not nearly so sturdy as ,jupiter's children2 in fact1 the young ,saturns do not make much display1 and some of them are so delicate that they are hardly ever seen4 ,in this household1 too1 a new member has recently appeared4 ,for fifty years the family was known to consist of these eight sons and daughters1 but in ,august1 #ahih1 when they were being photographed in a group1 it was discovered that a ninth moon had been added4 ,number ,seven is also a fine large house2 but ,uranus1 who lives there1 is such a recluse that unless you carefully keep your eye on his house1 you will hardly ever catch a glimpse of him4 ,there are four children in that house1 ,i believe1 but we hardly know them4 ,they move in circles of their own1 and apparently have seen a good deal of trouble4 ,only one more house is to be mentioned1 and that is ,number ,eight1 inhabited by ,neptune4 ,it contains one child1 but we are hardly on visiting terms with this household1 and we know next to nothing about it4 ,before #ahgg1 ,mars appeared to be in the same condition as ,venus or ,mercury--that is1 devoid of the dignity of attendants4 ,there was1 however1 good reason for thinking that there might be some satellites to ,mars1 only that we had not seen them4 ,you see that1 as ,number ,three had one child1 and ,numbers ,five1 ,six1 ,seven1 and ,eight had each one1 or more than one1 it seemed hard that poor ,number ,four should have none at all4 ,it was1 however1 certain that if there were any satellites to ,mars1 they must be comparatively small things2 for if ,mars had even one considerable moon1 it must have been discovered long ago4 ,on the memorable occasion in #ahgg1 ,professor ,hall discovered that the ruddy planet ,mars was attended1 not alone by one moon1 but by two4 ,their behavior was most extraordinary4 ,it appeared to him at first almost as if one of these little moons was playing at hide-and-seek4 ,sometimes it would peep out at one side of the planet1 and sometimes at the other side4 ,i have here a picture 7,fig4 #fc7 which shows how these moons of ,mars revolve4 ,that is the globe of the planet himself in the middle1 and he is turning round steadily in a period which is nearly the same as our day4 ,but the remarkable point is that the inner of the moons of ,mars runs round the planet in #g hours #ci minutes4 ,it would seem very strange in our sky if we had a little moon which rose in the west instead of in the east1 and which galloped right across the heavens three times every day--and this is what ,mars has4 ,the outer moon takes a more leisurely journey1 for he requires #cj hours #ah minutes to complete a circuit4 ,if for no other reason than to see these wonderful moons1 it would be very interesting to visit ,mars4 ,the satellites of this planet are in contrast to our moon4 ,in the first place1 our moon takes #bg days to go round the earth1 and is comparatively a long way off4 ,the moons of ,mars are much nearer to their planet1 and they go round much more quickly4 ,there is also another difference4 ,the moons of ,mars are much smaller bodies than our moon4 ,if we represent ,mars by a good-sized football1 his moons1 on the same scale1 would be hardly so big as the smallest-sized grains of shot4 ,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 all8 ,you remember1 of course1 that neither ,mars himself nor his moons have any light of their own4 ,they shine solely in consequence of the sunlight which falls upon them4 ,they are merely lighted like the earth itself1 or like the moon4 ,the difficulty about observing the satellites is all the greater because they are seen in the telescope close to such a brilliant body as ,mars4 ,the glare from the bright planet is such that when we want to see faint objects like the satellites we have to hide ,mars1 so as to get a comparatively dark space in which to search4 ,now that they know exactly what to look for1 a good many astronomers have observed the satellites of ,mars4 ,a superb telescope is nevertheless required4 ,and1 in fact1 you could not find a better test for the excellence of an instrument than to try if it will show these delicate objects4 ,but do not imagine that merely having a good telescope and a clear sky is all that is requisite for making astronomical discoveries4 ,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 score4 ,every boy knows that the bat does not make the cricketer1 and ,i can assure him that neither will the telescope make the astronomer4 ,in both cases1 no doubt1 there is some element of luck4 ,but of this you may be certain3 that as it is the man that makes the score1 and not the bat1 so it is the astronomer that makes the discovery1 and not his telescope4 ,deimos and ,phobos were the names of the two personages1 according to ,homer1 whose duty it was to attend on the god ,mars1 and to yoke his steeds4 ,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 ,mars4 ,,how ,,the ,,telescope ,,aids ,,in ,,viewing ,,faint ,,objects4 ,we have been hitherto talking about large planets1 which1 if not as big as our earth1 are at least as big as our moon4 ,but now we have to say a few words about a number of little planets1 many of them being so very small that a million rolled together would not form a globe so big as this earth4 ,these little objects you cannot see with your unaided eye1 and even with a telescope they only look like very small stars4 ,i have often been asked why it is that a telescope enables us to see objects1 both faint and small1 which our unaided eyes fail to show4 ,perhaps this will be a good opportunity to say a few words on the subject4 ,i think we can explain the utility of the telescope by examining our own eyes4 ,the eye undergoes a remarkable transformation when its owner passes from darkness into a brilliantly lighted room 7,fig4 #fd74 ,here you see two views of an eye1 and you notice the great difference between them4 ,they are not intended to be the eyes of two different people1 or the two eyes of the same person2 they are merely two conditions of the same eye4 ,they are intended to illustrate two different states of the eye of a collier4 ,the right shows his eye when he is above ground in bright daylight2 the left is his eye when he has gone down the coal-pit to his useful work in the dark regions below4 ,i remember when ,i went down a coal-pit ,i was lowered down a long shaft1 and when the bottom was reached a safety lamp was handed to me4 ,the gloom was such1 that at first ,i found some little difficulty in guiding my steps1 but the capable guide beside me said in an encouraging voice1 8,you will be all right1 sir1 in a few moments1 for you will get your pit-eyes40 ,i did get my 8pit-eyes10 as he promised1 and was able to see my way along sufficiently to enjoy the wonderful sights that are met with in the depths below4 ,illustration3 ,this shows the ,eye in the ,dark4 ,this shows the ,eye in the ,daylight4 ,,fig4 #fd4 ,the change that came over my eyes is that which these two pictures illustrate3 the black1 round spot in the centre is an opening covered with a transparent window1 by which light enters the eye2 the black spot is called the pupil1 and nature has provided a beautiful contrivance by which the pupil can get larger or smaller1 so as to make vision agreeable4 ,when there is a great deal of light we limit the amount that enters by contracting the pupil so as to make the opening smaller4 ,thus the picture with the small pupil represents the state of the collier's eye when he is above ground in bright sunlight4 ,when he descends into the pit1 where the light is very scanty1 then he wants to grasp as much of it as ever he can1 and consequently his pupil enlarges so as to make a wider opening1 and this is what he calls getting his 8pit-eyes40 ,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 pupil4 ,every time you pass from light into darkness the same thing can be perceived4 ,when we turn down the lights in a room1 so that we are in comparative darkness1 our pupils gradually expand4 ,as soon as the lights are turned up again1 then our pupils begin to contract4 ,other animals have the same contrivance in their eyes4 ,you may notice in the ,zological ,gardens how quickly the pupil of the lion contracts when he raises his eyes to the light4 ,the power of rapidly changing the pupil might be of service to a beast of prey4 ,imagine him crouching in a dense shade to wait for his dinner2 then of course the pupil will be large from deficiency of light2 but when he springs out suddenly on his victim1 in bright light1 it would surely be of advantage to him to be able at once to see clearly4 ,accordingly his pupil adjusts itself to the altered conditions with a rapidity that might not be necessary for creatures of less predaceous habits4 ,these changes of the pupil explain how the telescope aids our eyes when we want to discern any faint objects1 like the little planets4 ,such bodies are not visible to the unaided eyes1 because our pupils are not large enough to grasp sufficient light for the purpose4 ,even when they are opened to the utmost1 we want something that shall enable them to open wider still4 ,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 it4 ,what we want is something like a funnel which shall transform a large beam of rays into a small one4 ,i may explain what ,i mean by the following illustration3 ,suppose that it is raining heavily1 and that you want to fill a bucket with water4 ,if you merely put the bucket out in the middle of a field1 it will never be filled2 but bring it to where the rain-shoot from a house-top is running down1 and then your bucket will be running over in a few moments4 ,the reason1 of course1 is that the broad top of the house has caught a vast number of drops and brought them together in the narrow shoot1 and so the bucket is filled4 ,in the same way the telescope gathers the rays of light that fall on the object glass1 and condenses them into a small beam which can enter the eye4 ,we thus have what is nearly equivalent to an eye with a pupil as big as the object glass4 ,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-plate1 or even much larger still4 ,,the ,,asteroids ,,or ,,small ,,planets4 ,an asteroid is like a tiny star1 and in fact the two bodies are very often mistaken4 ,if we could get close to the objects1 we should see a wide difference between them4 ,we should find the asteroid to be a dark planet like our earth1 lighted only by the rays from the sun4 ,the star1 small and faint though it may seem1 is itself a bright sun1 at such a vast distance that it is only visible as a small point4 ,the star is millions of times as far from us as the planet1 and utterly different in every respect4 ,it is a curious fact that the planets should happen to resemble the stars so closely4 ,we can find an analogous fact in quite another part of nature4 ,in visiting a good entomological collection1 you will be shown some of the wonderful leaf-like insects4 ,these creatures have wings1 exactly formed to imitate leaves of trees1 with the stalks and veins completely represented4 ,when one of these insects lies at rest1 with its wings folded1 among a number of leaves1 it would be almost impossible to penetrate the disguise4 ,this mimicry is no doubt an ingenious artifice to deceive the birds or other enemies that want to eat the insect4 ,there is1 however1 one test which the cunning bird could apply3 the leaves do not move about of their own accord1 but the leaf-insects do4 ,if therefore the bird will only have the patience to wait1 he will see a pair of the seeming leaves move1 and then the deception will be to him a deception no longer1 and he will gobble up the poor insect4 ,in our attempts to discover the planets we experience just the same difficulties as the insect-eating bird4 ,wide as is the true difference between a planet and a star1 there is yet such a seeming resemblance between them that we are often puzzled to know which is which4 ,the planets imitate the stars so successfully1 that when one of them is presented to us among myriads of stars it is impossible for us to detect the planet by its appearance4 ,but we can be cunning--we can steadily watch1 and the moment we find one of these star-like points beginning to creep about we can pounce upon it4 ,we know by its movements that it is only disguised as a star1 but that it is really one of the planets4 ,it is not always easy to discover the asteroids even by this principle1 for unfortunately these bodies move very slowly4 ,if you have a planet in the field of view1 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 extent4 ,the search for such little planets is therefore a tedious one1 but there are two methods of conducting it3 the new one1 which has only recently come into use1 and the old one4 ,i shall speak of the old one first4 ,although the body's motion is so slow1 yet when sufficient time is allowed1 the planet will not only move away from the stars close by1 but will even journey round the entire heavens4 ,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 afterwards4 ,memory will not suffice for this purpose4 ,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 first4 ,the only way of doing this work is to draw a map of the stars very carefully4 ,this is a tedious business1 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 telescope4 ,all of these will have to be entered faithfully in their true places on the map4 ,when this has been done the map must be laid aside for a season1 and then it is brought out again and compared with the sky4 ,no doubt the great majority of the objects will be found just as they were before4 ,these are the stars1 the distant suns1 and our concern is not at present with them4 ,sometimes it will happen that an object marked on the first map has left a vacant place on the second4 ,this1 however1 does not help us much1 for1 whatever the object was1 it has vanished into obscurity1 and a new planet could hardly be discovered in this way4 ,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 first4 ,then1 indeed1 the expectation of the astronomer is aroused2 he may be on the brink of a discovery4 ,of course he watches accurately the little stranger4 ,it might be some star that had been accidentally overlooked when forming the map1 or it might possibly be a star that has become bright in the interval4 ,but here is a ready test3 is the body moving8 ,he looks at it very carefully1 and notes its position with respect to the adjacent stars4 ,in an hour or two his suspicions may be confirmed2 if the object be in motion1 then it is really a planet4 ,a few further observations1 made on subsequent days1 will show the path of the body4 ,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 discovery4 ,the new method of searching for small planets1 which has only come into use in recent years1 is a very beautiful one1 and renders the process of making such discoveries much more easy than the older method which ,i have just described4 ,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 it4 ,the method will apply to very small stars1 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 images4 ,thus we obtain a map of the heavens1 made in a thoroughly accurate manner4 ,indeed1 the delicacy of photography for this purpose is so great1 that the plates show many stars which cannot be seen with even the greatest of telescopes4 ,suppose that a little planet happened to lie among the stars which are being photographed4 ,all the time that the plate is being exposed the wanderer is1 of course1 creeping along1 and after an hour 7exposures even longer are often used71 it may have moved through a distance sufficient to ensure its detection4 ,the plate will1 therefore1 show the stars as points1 but the planet will betray its presence by producing a streak4 ,the asteroids now known number between #djj and #ejj4 ,out of this host a few afford some information to the astronomer1 but the majority of them are objects possessing individually only the slightest interest4 ,no small planet is worth looking at as a telescopic picture4 ,we should consider that asteroid to be a large one which possessed a surface altogether as great as ,england or ,france4 ,many of these planets have a superficial extent not so large as some of our great counties4 ,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 planet4 ,we know hardly anything of the nature of these small worlds1 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 earth4 ,we can easily prove this by making a calculation4 ,i shall suppose a small planet one hundred miles wide1 its diameter being1 therefore1 the one-eightieth part of the diameter of the earth4 ,if we were landed on such a globe1 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 7p4 #abd74 ,if we suppose the planet to be constructed of materials which had the same density as those of which the earth is made1 then every weight would be reduced to the eightieth part of what it is here4 ,there would be one curious consequence of residence on such a globe4 ,we have heard of attempts to make flying machines1 or to provide a man with wings by which he shall soar aloft like the birds4 ,all such contrivances have hitherto failed4 ,it may be possible to make a pair of wings by which a man can fly down1 but it is quite another matter when he tries to fly up again4 ,suppose1 however1 we were living on a small planet1 it would be perfectly easy to fly1 for as our bodies would only seem to weigh a couple of pounds1 we ought to be able to flap a pair of wings strong enough to overcome so trivial a force4 ,i should1 however1 add that this is on the supposition that the atmosphere has the same density as our own4 ,life on these small planets would indeed be extraordinary4 ,let us take1 for example1 ,flora1 and see how a game of lawn tennis on that body would be managed4 ,the very slightest blow of the racket would drive the ball a prodigious distance before it could touch the ground2 indeed1 unless the courts were about half a mile long1 it would be impossible to serve any ball that was not a fault4 ,nor is there any great exertion necessary for playing lawn tennis on ,flora1 even though the courts are several hundred acres in extent4 ,as a young lady ran to meet the ball and return it1 each of her steps might cover a hundred yards or so without extra effort2 and should she have the misfortune to get a fall1 her descent to the ground would be as gentle as if she was seeking repose on a bed of the softest swan's-down4 ,these little planets cluster together in a certain part of our system4 ,inside are the four inner planets1 of which we have already spoken2 outside are the four outer planets1 of which we have soon to speak4 ,between these two groups there was a vacant space4 ,it seemed unreasonable that where there was room for planets1 planets should not be found4 ,accordingly the search was made1 and these objects were discovered4 ,even at the present day1 more and more are being constantly added to the list4 ,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 ,jupiter4 ,this invariable rule was1 however1 departed from in the case of one of these bodies which was discovered in ,august1 #ahih4 ,this little body1 which was known for some time by the provisional appellation of ,d ,q1 and which has now been definitely christened ,eros1 is an exception to this rule4 ,it travels at an average distance from the sun actually less than that of ,mars1 and at the nearest point can come within #ae1jjj1jjj miles of the earth4 ,we occasionally get information from these little bodies2 for in their revolutions through the solar system1 they sometimes pick up scraps of useful knowledge1 which we can elicit from them by careful examination4 ,for example1 one of the most important problems in the whole of astronomy is to determine the sun's distance4 ,i have already mentioned one of the ways of doing this1 which is given by the transit of ,venus4 ,astronomers never like to rely on a single method2 we are therefore glad to discover any other means of solving the same problem4 ,this it is which the little planets will sometimes do for us4 ,juno on one occasion approached very close to the earth1 and astronomers in various parts of the globe observed her at the same time4 ,when they compared their observations they measured the sun's distance4 ,but ,i am not going to trouble you now with a matter so difficult4 ,suffice it to say1 that for this1 as for all similar investigations1 the observers were constrained to use the very same principle as that which we illustrated in ,fig4 #e4 ,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 sun1 and that we shall speak in our next lecture of the giant members of our system4 ,,lecture ,,iv4 ,,jupiter1 ,,saturn1 ,,uranus1 ,,neptune4 ,jupiter1 ,saturn1 ,uranus1 ,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 ,neptune4 ,our lecture to-day ought to make us take a very humble view of the size of our earth4 ,mercury1 ,venus1 and ,mars may be regarded as the earth's peers1 though we are slightly larger than ,venus1 and a good deal larger than ,mercury or ,mars2 but all these four globes are insignificant in comparison with the gigantic planets which lie in the outer parts of our system4 ,these great bodies do not enjoy the benefits of the sun to the same extent that we are permitted to do2 they are so far off that the sun's rays become greatly enfeebled before they can traverse the distance2 but the gloom of their situation seems to matter but little1 for it is highly improbable that any of these bodies could be inhabited4 ,a view of parts of the paths of these four great planets is shown in ,fig4 #fe4 ,the innermost is ,jupiter1 which completes a circuit in about twelve years2 then comes ,saturn1 revolving in an orbit so great that twenty-nine years and a half are required before the complete journey is finished4 ,still further outside is ,uranus1 which has a longer journey than ,saturn1 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 lifetime4 ,at the boundary of our system revolves the planet ,neptune1 and though it is a mighty globe1 yet we cannot see it without a telescope4 ,it is invisible to the naked eye for two reasons3 first of all1 because it is so far from the sun that the light which illuminates it is excessively feeble2 and1 secondly1 because it is so far from us that whatever brilliancy it has is largely reduced4 ,illustration3 ,,fig4 #fe4--,the ,orbits of the ,four ,giant ,planets4 ,,jupiter4 ,of all these bodies ,jupiter is by far the greatest2 he is1 indeed1 greater than all the other planets rolled into one4 ,the relative insignificance of the earth when compared with ,jupiter is well illustrated by the fact that if we took #abjj globes each as big as our earth1 and made them into a single globe1 it would only be as large as the greatest of the planets4 ,a view of the comparative sizes of the earth and ,jupiter is shown in ,fig4 #ff4 ,illustration3 ,,fig4 #ff4--;,jupiter and the ,earth compared4 ,fig4 #fg shows a picture of ,jupiter as seen through the telescope4 ,first1 you will notice that the outline of the planet's shape is not circular1 for it is plain that the vertical diameter in this picture is shorter than the horizontal one2 in fact1 ,jupiter is flattened at the ,poles and bulges out at the equator1 so that a section through the ,poles is an ellipse4 ,jupiter is turning round rapidly on his axis1 and this will account for the protuberance4 ,we find that the planet has assumed almost the same form as if it were actually a liquid4 ,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 fall4 ,as we make the globe of oil rotate1 which we can easily do by passing a spindle through it1 we see that it bulges out in the form that ,jupiter as well as other planets have taken4 ,illustration3 ,,fig4 #fg4--,the ,clouds of ,jupiter4 ,on the picture of the planets you will see shaded bands4 ,these are constantly changing their aspect1 and for a double reason4 ,in the first place1 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 sight4 ,in another five hours the original side of the globe will be back again1 for the entire rotation occupies about #aj hours1 or1 more precisely1 #i hours #ee minutes #ba seconds4 ,but these bands are themselves not permanent objects4 ,they have no more permanence than the clouds over our own sky4 ,sometimes ,jupiter's clouds are more strongly marked than on other occasions4 ,sometimes1 indeed1 they are hardly to be seen at all4 ,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 interior4 ,there is a circumstance which demonstrates that ,jupiter must be an object exceedingly different from the earth1 though both bodies agree in so far as having clouds are concerned4 ,what would you think when ,i tell you that we were able to weigh ,jupiter by the aid of his little moons1 of which ,i shall afterwards speak8 ,these little bodies inform us that ,jupiter is about #cjj times as heavy as our earth1 and we have no doubt about this1 for it has been confirmed in other ways4 ,but we have found by actual measurement that ,jupiter is #abjj times as big as the earth1 and therefore1 if he were constituted like the earth1 he ought to be #abjj times as heavy4 ,this is1 ,i think1 quite plain2 for if two cakes were made of the same material1 and one contained twice the bulk of the other1 then it would certainly be twice as heavy4 ,if there be two balls of iron1 one twice the bulk of the other1 then1 of course1 one has twice the weight of the other4 ,but if a ball of lead have twice the bulk of a ball of iron1 then the leaden ball would be more than twice as heavy as the iron1 because lead is the heavier material4 ,in the same way1 the weights of the earth and ,jupiter are not what we might expect from their relative sizes4 ,if the two bodies were made of the same materials and in the same state1 then ,jupiter would be certainly four times as heavy as we find him to be4 ,we are1 therefore1 led to the belief that ,jupiter is not a solid body1 at least in its outer portions4 ,the masses of cloud which surround the planet seem to be immensely thick1 and as clouds are1 of course1 light bodies in comparison with their bulk1 they have the effect of largely increasing the apparent size of ,jupiter1 while adding very little to his weight4 ,there is thus a great deal of mere inflation about this planet1 by which he looks much bigger than his actual materials would warrant if he were constituted like the earth4 ,these facts suggest an interesting question4 ,why has ,jupiter such an immense atmosphere1 if we may so call it8 ,the clouds we are so familiar with down here on the earth are produced by the heat of the sun1 which beats down upon the wide surface of the ocean1 evaporates the water1 and raises the vapor up to where it forms the clouds4 ,heat1 therefore1 is necessary for the formation of cloud2 and with clouds so dense and so massive as those on ,jupiter1 more heat would apparently be necessary than is required for the moderate clouds on this earth4 ,whence is ,jupiter to get this heat8 ,have we not seen that the great planet is far more distant from the sun than we are8 ,in fact1 the intensity of the sun's heat on ,jupiter is not more than the twenty-fifth part of what we derive from the same source4 ,we can hardly believe that the sun supplied the heat to make those big clouds on the great planet2 so we must cast about for an additional source1 which can only be inside the planet itself4 ,so far as his internal heat is concerned1 ,jupiter seems to be in much the same condition now as our earth was once1 ages ago1 before its surface had cooled down to the present temperature4 ,as ,jupiter is so much larger than the earth1 he has been slower in parting with his heat4 ,the planet seems not yet to have had time to cool sufficiently to enable water to remain on his surface4 ,thus the internal heat of the planet supplies an explanation of his clouds4 ,we may also remark that as the present condition of ,jupiter illustrates the early condition of our earth1 so the present condition of the earth foreshadows the future reserved for ,jupiter when he shall have had time to cool down1 and when the waters that now exist in the form of vapor shall be condensed into oceans on his surface4 ,,the ,,satellites ,,of ,,jupiter4 ,every owner of a telescope delights to turn it on the planet ,jupiter1 both for the spectacle the globe itself affords him1 and for a view of the wonderful system of moons by which the giant planet is attended4 ,fortunately the four satellites of ,jupiter lie within reach of even the most modest telescope1 and their incessant changes relative to ,jupiter and each other give them a never-ending interest for the astronomer4 ,compared with the torpid performance of our moon1 which requires a month to complete a circuit around the earth1 ,jupiter's moons are wonderfully brisk and lively4 ,nor are they small bodies like the satellites of ,mars1 for the second of ,jupiter's satellites is quite as big as our moon1 and the other three are very much larger4 ,it is1 however1 true that his satellites appear insignificant when compared with ,jupiter's own enormous bulk4 ,the innermost of these little bodies flies right round in a period of one day and eighteen or nineteen hours1 while the outermost of them takes a little more than a fortnight--that is1 rather more than half the time that our moon demands for a complete revolution4 ,jupiter's satellites are too far off for us to see much with respect to their structure or appearance even with mighty telescopes4 ,it is1 of course1 their great distance from us that makes them look insignificant4 ,they would1 however1 be bright enough to be seen like small stars were it not that1 being so close to ,jupiter1 his overpowering brightness renders such faint objects in his vicinity invisible4 ,it was by means of the satellites of ,jupiter that one of the most beautiful scientific discoveries was made4 ,as a satellite revolves round the giant planet it often happens that the little body enters into the shadow of the great planet4 ,no sunlight will then fall upon the satellite1 and as it has no light of its own1 it disappears from sight until it has passed through the shadow and again receives sunlight on the other side4 ,we can watch these eclipses with our telescopes1 and there can be no more interesting employment for a small telescope4 ,the movements of these bodies are now known so thoroughly that the occurrence of the eclipses can be predicted4 ,the almanacs will tell when the satellite is calculated to disappear1 and when it ought again to return to visibility4 ,when astronomers first began to make these computations a couple of hundred years ago1 the little satellites gave a great deal of trouble4 ,they would not keep their time4 ,sometimes they were a quarter of an hour too soon1 and sometimes a quarter of an hour too late4 ,at last1 however1 the reason for these irregularities was discovered1 and a wonderful reason it was4 ,suppose there were a number of cannons all over ,hyde ,park1 and that these cannons were fired at the same moment by electricity4 ,though the sounds would all be produced simultaneously1 yet1 no matter where you stood1 you would not hear them altogether2 the noise from the cannons close at hand would reach your ears first1 and the more distant reports would come in subsequently4 ,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 it4 ,the light and the noise were produced simultaneously1 but the sound takes five seconds to pass over every mile1 while the light1 in comparison to sound1 may be said to move instantaneously4 ,that sound travelled with a limited velocity was always obvious1 but never until the discrepancies arose about ,jupiter's satellites was it learned that light also takes time to travel4 ,it is true that light travels much more quickly than sound--indeed1 about a million times as fast4 ,light goes so quickly1 that it would rush more than seven times around the earth in a single second4 ,so far as terrestrial distances are concerned1 the velocity of light is such that the time required for a journey is inappreciable4 ,the distances1 however1 between one celestial body and another are so enormous1 that even a ray of light1 moving as quickly as it alone can move1 will occupy a measurable time on the way4 ,our moon is comparatively so near us1 that light takes little more than a second to cover that short distance4 ,eight minutes are1 however1 required for light to travel from the sun to the earth2 in fact1 the sunbeams that now come into our eyes left the sun eight minutes ago4 ,if the sun were to be suddenly extinguished1 it would still seem to shine as brightly as ever in the eyes of the inhabitants of this earth for eight minutes longer4 ,as ,jupiter is five times as far from the sun as we are1 it follows that the light from the sun to ,jupiter will spend forty minutes on the journey1 and the light from ,jupiter to the earth will take a somewhat similar time4 ,when we look at ,jupiter and his moons1 we do not see him as he is now1 we see him as he was more than half an hour ago1 but the interval will vary somewhat according to our different distances from the planet4 ,sometimes the light from ,jupiter will reach us in as little as thirty-two minutes1 while sometimes it will take as much as forty-eight--that is1 the light sometimes requires for its journey a quarter of an hour more than is sufficient at other times4 ,we can therefore understand that irregularity of ,jupiter's satellites which puzzled the early astronomers4 ,an eclipse sometimes appeared a quarter of an hour before it was expected2 because the earth was then as near as it could be to ,jupiter1 while the calculations had been made from observations when ,jupiter was at his greatest distance4 ,it was these eclipses of the satellites which first suggested the possibility that light must have a measurable speed4 ,when this was taken into account1 then the occasional delay of the eclipses was found to be satisfactorily explained4 ,confirmation flowed in from other sources1 and thus the discovery of the velocity of light was completely established4 ,professor ,barnard1 when studying ,jupiter in #ahib with the splendid refractor at the ,lick ,observatory1 saw a very small point of light nearer to the planet than the nearest of the four satellites already known4 ,further examination showed that this little object was indeed another satellite4 ,thus ,jupiter has a fifth moon in addition to the four which have been known so long4 ,this little body is so small and faint that it can only be discerned under the most favorable conditions by the most powerful telescopes4 ,,saturn4 ,next outside ,jupiter1 on the confines of the ancient planetary system1 revolves another grand planet1 called ,saturn4 ,his distance from us is sometimes nearly a thousand millions of miles1 and he requires more than a quarter of a century for the completion of each revolution4 ,sometimes people do not pronounce the names of the planets quite correctly4 ,i have heard of a gardener who has a taste for astronomy1 and sometimes begins to talk about the planets ,juniper and ,citron4 ,probably you will know what he meant to say4 ,the ancients had discovered ,saturn to be a planet1 for though he looked like a star1 yet his movement through the constellations could not escape their notice when attention was paid to the heavens4 ,illustration3 ,,fig4 #fh4--,saturn and the ,earth compared4 ,in the matter of size ,saturn is only surpassed by ,jupiter among the planets4 ,he is about #fjj times as large as the earth2 the small object1 ,e1 shown in ,fig4 #fh1 represents our earth in its true comparative size to the ringed planet2 but ,saturn is so far off1 that even at his best he is never so bright as ,venus1 or ,mars1 or ,jupiter become when they are favorably situated4 ,on the globe of ,saturn we can sometimes see a few bands1 but they are faint compared with those on ,jupiter4 ,there is1 however1 no doubt that what we see upon ,saturn is a dense mass of cloud4 ,indeed1 he can have comparatively little solid matter inside1 for this planet does not weigh so much as a ball of water the same size would do4 ,saturn1 like ,jupiter1 must be highly heated in his interior4 ,the ring1 or rather series of rings1 by which the planet is surrounded are also shown in ,fig4 #fh3 these appendages are not fastened to the globe of ,saturn by any material bonds2 they are poised in space1 without any support1 while the globe or planet proper is placed symmetrically in the interior4 ,i have made a model which shows ,saturn with his rings1 but it is necessary for me to fasten the rings by little pieces of wire to the globe1 for there is no mechanical means by which the rings of the model could be poised without support1 as they are around the planet4 ,if we throw the beam of the electric lamp on the little planet1 we see the shadow which the planet casts on its ring4 ,similar shadows can be observed in the actual ,saturn of the sky1 and this is a proof that the planet does not shine by its own light1 but by the light of the sun which falls upon it4 ,here again we illustrate the wide difference between a planet and a star1 for were our sun to be put out1 ,saturn and all the other planets in the sky would vanish from sight1 while the stars would1 of course1 twinkle on as before4 ,there are three rings round ,saturn2 they all lie in the same plane1 and they are so thin1 that when turned edgewise towards us the whole system almost disappears1 except in very powerful telescopes4 ,the outer and the inner bright rings are divided by a dark line1 which can be traced entirely round4 ,at the inner edge of the inner ring begins that strange structure called the crape ring1 which extends halfway towards the globe of the planet4 ,the most remarkable point about the crape ring is its semi-transparency1 for we can sometimes see the globe of the planet through this strange curtain4 ,the crape ring can only be observed with a powerful telescope4 ,the other two rings are within the power of very moderate instruments4 ,,the ,,nature ,,of ,,the ,,rings4 ,for the explanation of the nature of ,saturn's rings we are indebted to the calculations of mathematicians4 ,you might have thought1 perhaps1 that nothing would be simpler than to suppose the rings were stiff plates made from solid material4 ,but the question cannot be thus settled4 ,we know that the ring could not bear the strain of the planet's attraction upon it if it were a solid body4 ,i may illustrate the argument by familiar facts about bridges4 ,where the span is but a small one1 as1 for instance1 when a road has to cross a railway1 a canal1 or a river1 the arch is1 of course1 the proper kind of structure4 ,there is1 for example1 a specially beautiful arch over the river ,dee at ,chester4 ,but if the bridge be longer than this1 masonry arches are not suitable4 ,where a considerable span has to be crossed1 as at the ,menai ,straits1 or a gigantic one1 as at the ,firth of ,forth1 then arches have to be abandoned1 and iron bridges of a totally different construction have to be employed4 ,arches cannot be used beyond a limited span1 because the strain upon the materials becomes too great for their powers of resistance to withstand4 ,each of the stones in an arch is squeezed by intense pressure1 and there is a limit beyond which even the stoutest stones cannot be relied upon4 ,as soon1 therefore1 as the span of the arch is so great that the stones it contains are squeezed as far as is compatible with safety1 then the limit of size for that form of arch has been reached4 ,suppose that you stood on ,saturn at his equator1 and looked up at the mighty ring which would stretch edgewise across your sky4 ,it would rise up from the horizon on one side1 and1 passing over your head1 would slope down to the horizon on the other4 ,you would1 in fact1 be under an arch of which the span was about #ajj1jjj miles4 ,owing to the attraction of ,saturn1 every part of that structure would be pulled forcibly towards his surface1 and thus the materials of the arch1 if it were a solid body1 would be compressed with terrific force4 ,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 planet4 ,that is only a difference with respect to the support of two ends of the arch1 and does not affect the question as to the pressure upon its materials2 nor does the fact that the ring is revolving remove the difficulty1 though it undoubtedly lessens it4 ,we know no solid substance which could endure the pressure4 ,even the toughest steel that ever was made would bend up like dough under such conditions4 ,we cannot1 therefore1 account for ,saturn's ring by supposing it to be a solid1 for no solid would be strong enough4 ,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 reed1 and said it would never be able to withstand the blasts8 ,but matters did not so turn out4 ,the mighty oak1 which would not yield to the storm1 was blown down1 while the slender reed bent to the wind and suffered no injury4 ,this gives us a hint as to the true constitution of ,saturn's ring2 it is not a solid body1 trying to resist by mere strength2 it is rather to be explained as an excessively pliant structure4 ,indeed1 ,i ought not to call it a structure at all2 it is rather a multitude of small bodies not in the least attached together4 ,i do not know what the size of these bodies may be4 ,for anything we can tell1 they may be no larger than the pebbles you find on a gravel walk4 ,let us see how we could encircle our earth with rings like those which surround ,saturn4 ,i shall ask you to be provided with a sufficiently large number of pebbles1 and you must also imagine that ,i have the means of ascending high up into space1 halfway from here to the moon4 ,suppose ,i went up there and simply dropped the pebble1 of course1 it would tumble straight down to the earth again4 ,if1 however1 ,i threw it out with proper speed and in the proper direction1 ,i could start it off like a little moon1 and it would go on round and round our earth in a circle4 ,i mention a pebble1 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-ball4 ,now take another pebble4 ,cast it also in a somewhat similar path1 taking care1 however1 that the planes of the two orbits shall be the same4 ,each of these little bodies shall pursue its journey without interference from the other4 ,then proceed in the same way with a third1 a fourth1 with thousands and millions and billions of pebbles1 until at last the small bodies will become so numerous that they almost fill a large part of the plane with a continuous shoal4 ,each little object1 guided entirely by the earth's attraction1 will pursue its path with undeviating regularity4 ,its neighbors will not interfere with it1 nor will it interfere with them4 ,let us circumscribe the limits of our flat shoal of moonlets4 ,we first take away all those that lie outside a certain large circle2 then we shall clear away sufficient to make a vacant space between the outer ring and the inner ring1 and thus the two conspicuous rings have been made2 at the inside of the inner ring we shall take out numbers of pebbles here and there1 so as to make this part much less dense than the outer portions1 and thus produce a semi-transparent crape ring2 then we shall clear away those that come too close to the planet1 and form a neat inner boundary4 ,could we then view our handiwork from the standpoint of another planet1 what appearance would our earth present8 ,the several pebbles1 though individually so small1 would yet1 by their countless numbers1 reflect the sun's light so as to produce the appearance of a continuous sheet4 ,thus we should find a large bright outer ring surrounding the earth1 separated by a dark interval from the inner ring1 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 extent4 ,that beautiful system of rings which ,saturn displays is undoubtedly of a similar character to the hypothetical system which ,i have endeavored to describe4 ,no other explanation will account for the facts1 especially for the semi-transparency of the crape ring4 ,the separate bodies from which ,saturn's rings are constituted seem1 however1 so small that we are not able to see them individually4 ,there are some other fine lines running round the rings beside the great division1 and these can also be explained by the theory ,i have stated4 ,saturn has other claims on our attention besides those of its rings4 ,it has an elaborate retinue of satellites--no fewer1 indeed1 than nine2 but some of them are very faint objects1 and not by any means so interesting as the system by which ,jupiter is attended4 ,the ninth of these was discovered quite recently by ,professor ,w4 ,h4 ,pickering1 of ,harvard ,college ,observatory4 ,this little moon1 for which the name 8,phoebe0 has been suggested1 is further from the planet than any of the others4 ,it is a minute object shining as a star of the #aeth or #afth magnitude1 and moves around the planet in a period of about sixteen months4 ,saturn was the last and outermost of the planets with which the ancients were acquainted4 ,its path lay on the frontiers of the then known solar system1 and the magnificence of the planet itself1 with its attendant luminaries and its marvellous rings1 rendered it worthy indeed of a position so dignified4 ,these five planets--namely1 ,mercury1 ,venus1 ,mars1 ,jupiter1 and ,saturn--made up with the sun and the moon the seven 8planets0 of the ancients4 ,they were supposed to complete the solar system1 and1 furthermore1 the existence of other members was thought to be impossible4 ,in modern times it has been discovered that there are yet two more planets4 ,i do not now refer to those little bodies which run about in scores between ,mars and ,jupiter4 ,i mean two grand first-class planets1 far bigger than our earth4 ,one of them is ,uranus1 which revolves far outside ,saturn1 and the other is ,neptune1 which is much further still1 and whose mighty orbit includes the whole planetary system in its circuit4 ,to complete its journey round the sun not less than #afe years is required4 ,,william ,,herschel4 ,i have to begin the account of this discovery by telling you a little story4 ,in the middle of the last century there lived at ,hanover a teacher of music whose name was ,isaac ,herschel4 ,he had a family of ten children1 and he did the best for them that his scanty means would permit4 ,of his children ,william was the fourth1 and he inherited his father's talents for music1 as did most of his brothers and sisters4 ,he was a bright1 clever boy at school1 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 ,guards4 ,war broke out between ,france and ,england1 and as ,hanover was then under the ,english crown1 the ,french invaded it1 and a battle was fought in which the poor ,hanoverian ,guards suffered very terribly4 ,young ,herschel spent the night after the battle in a ditch1 and he came to the conclusion that he did not like fighting1 though he was only a member of the band1 and he resolved to change his profession4 ,that was not so easy to do just then1 for even a bandsman cannot leave the service in war time at his own free will4 ,william ,herschel1 however1 showed all through his life that he was not the man to be baffled by difficulties4 ,i do not know whether he asked for leave1 but at all events he took it4 ,he deserted1 in fact1 and his friends succeeded in sending him away to ,england4 ,he was nineteen years old when he commenced to look for a career over here1 and certainly he found his prospects in the musical profession very discouraging4 ,herschel was1 however1 very industrious2 and at last he succeeded in getting appointed as organist of the ,octagon ,chapel at ,bath4 ,he gradually became famous for his musical skill1 and had numbers of pupils4 ,he used also to conduct concerts and oratorios1 and was well known in this way over the ,west of ,england4 ,busy as ,herschel was with his profession1 he still retained his love of reading and study4 ,every moment he could spare from his duties he devoted to his books4 ,it was natural that a musician should specially desire to study the theory of music1 and to understand it properly you should know ,euclid and algebra1 and1 indeed1 higher branches of mathematics as well4 ,herschel did not know these things at first2 he had not the means of learning them when he was a boy1 so he worked very hard after he became a man4 ,and he studied with such success that he made fair progress in mathematics1 and then it appeared to him that it would be interesting to learn something about astronomy4 ,after he had begun to read about the stars1 he thought he would like to see them1 and so he borrowed a telescope4 ,it was only a little instrument1 but it delighted him so much that he said he must have one for himself4 ,so he wrote to ,london to make inquiries4 ,telescopes were much dearer in those days than they are now1 and ,herschel could not give the price that the opticians demanded4 ,here again his invincible determination came to his aid4 ,what was there to prevent him from making a telescope8 he asked himself2 and forthwith he began the attempt4 ,you will think it strange1 perhaps1 that a music-teacher who had no special training as a mechanic should at once commence so delicate and difficult a task2 but it is not really so hard to make a telescope as might be imagined4 ,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 make2 at least1 ,i should say of a reflecting telescope1 which is the kind ,herschel attempted to make1 and succeeded in making4 ,you must know that there are two kinds of telescopes4 ,the commoner one with which you are more familiar is called the refracting telescope1 and it has glass lenses4 ,it was an instrument constructed on this principle that we spoke of in a former lecture 7p4 #ig74 ,the reflecting telescope depends for its power upon a bright mirror at the lower end1 and when using this instrument you look at the reflection of the stars in the mirror4 ,it was a reflector like this that ,herschel began to construct1 and he engaged in the task with enthusiasm4 ,his sister ,caroline had come to live with him1 and she used to help him at his work4 ,so much in earnest was he that he used to rush into his workshop directly he came home from a concert1 and without taking off his best clothes he would plunge into the grinding and polishing of his mirrors4 ,his sister tried to keep the house as tidy as possible1 but ,herschel put up a carpenter's shop in the drawing-room1 and turning-lathes in the best bedroom4 ,at last he succeeded4 ,he made a mirror of the right shape1 and found that it exhibited the stars properly4 ,it was not a looking-glass in the ordinary sense1 with glass on one side and quicksilver on the other4 ,the mirror that ,herschel constructed was entirely of metal4 ,it consisted of a mixture of two parts of copper with one of tin4 ,illustration3 ,,fig4 #fi4--,the ,mirror4 ,the copper has first to be melted in a furnace1 for the metal must be above a red heat before it will begin to run4 ,then the tin has to be carefully added1 and the casting of the mirror is effected by pouring the molten metal into a flat mould4 ,thus the rough mirror is obtained1 which in ,herschel's earlier telescopes seems to have been about six or seven inches in diameter1 and nearly an inch thick4 ,though copper is such a tough substance1 and though tin is also tough1 yet when melted together to make speculum metal1 as this alloy is called1 they produce an exceedingly hard and brittle material4 ,when we remember that we could never break a copper penny piece by throwing it down on the flags1 it may seem strange that the 8speculum metal0 should be so exceedingly brittle4 ,a piece the size of a penny would be more brittle than a bit of glass of the same dimensions1 and when the speculum is cast1 unless it is cooled very carefully1 it will fly into pieces4 ,herein lay one of the difficulties that ,herschel encountered4 ,speculum metal must be put into an oven as soon as the casting has become solid1 and then the heat is gradually allowed to abate4 ,when the speculum has been at last obtained1 next follows the labor of giving it the true figure and polish4 ,it is not only more fragile than glass1 but it is also quite as hard1 and therefore the grinding is a tedious operation4 ,first the surface has to be ground with coarse sand1 and then with emery1 which is gradually made finer and finer until the true figure has been given 7,fig4 #fi74 ,the mirror is then somewhat basin-shaped1 but the depression is very slight4 ,for example1 in a mirror six inches across the depression at the centre would perhaps be not more than the twentieth of an inch4 ,small though this depression is1 yet it has to be made with exactness4 ,in fact1 if it were wrong at any point by so much as the tenth of the thickness of this sheet of paper1 the telescope would not perform accurately4 ,the tool that is used in grinding is made of cast iron1 and has been turned in a lathe to the right shape4 ,it is divided into squares in the manner shown in ,fig4 #gj4 ,after the grinding comes the polishing1 and this is effected with a tool like the grinder in shape4 ,this has to be covered over with little squares of pitch1 so that when warmed and put down on the mirror it is soft enough to receive the right shape4 ,some rouge and water is spread over the mirror1 and the polisher is worked backward and forward with the hand until a brilliant surface is obtained4 ,illustration3 ,,fig4 #gj4--,the ,grinding ,tool4 ,when the amateur astronomer has completed this part of the task1 all the great difficulties about his telescope are conquered4 ,the tube may be made of wood1 and1 indeed1 a square tube will do just as well as a round one4 ,he must also provide for the top of the tube a small mirror1 which has to be perfectly flat4 ,the preparation of this requires much care1 because it is not so easy as one might suppose to obtain an accurately flat surface4 ,one way of doing this is to get three pieces1 and grind each two of them together until every pair will touch all over2 then they will certainly all be flat4 ,one more part you want1 and that is an eyepiece4 ,this presents no difficulty4 ,a single glass lens can be made to answer and your telescope is complete4 ,,the ,,discovery ,,of ,,uranus4 ,it was in the year #aggd that ,herschel first had a view of the heavens through the telescope he had himself constructed4 ,during the early part of his career he does not seem to have made any important discoveries4 ,he was gradually preparing himself for the great achievement by which his name became famous4 ,it was on the #acth of ,march1 #agha1 that the organist of the ,octagon ,chapel at ,bath turned his telescope on the constellation of the ,twins1 and began to look at one star after another4 ,you must know that a star merely looks like a little point in a telescope2 even the greatest instrument will only make a star look brighter1 and will never show it with a perceptible disk4 ,in looking over the stars that night1 ,herschel's attention was arrested by one object that did look larger when magnified1 and therefore was not a star4 ,the only other objects which would behave in this way were the planets1 or possibly a comet4 ,indeed1 at first ,herschel imagined that what he saw must be a comet4 ,it could hardly have occurred to him that he was to have such good fortune as to discover a new planet4 ,the five great planets had been known from all antiquity4 ,was it reasonable to suppose that there could be yet another that had never been perceived8 ,fortunately1 there was a test available4 ,a star remains in the same place from night to night and from year to year2 while a planet1 as we have already had occasion to mention1 is a body which is wandering about4 ,the movements of a planet are1 however1 not at all like those of a comet4 ,to decide on the nature of ,herschel's newly discovered body1 it was sufficient to observe the character of its motion4 ,a few nights sufficed to do this4 ,the position of the body was carefully marked relatively to the neighboring stars1 and it was soon shown that it was a planet4 ,here1 then1 a great discovery was made4 ,a new planet1 now called ,uranus1 was added to our system4 ,it would be nothing to discover a new star4 ,you might as well talk of discovering a new grain of sand on the seashore4 ,the stars are in untold myriads4 ,they are so far off that they have no relation whatever to our system1 which is presided over by the sun4 ,but by the detection of a new planet1 revolving far outside ,saturn1 ,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 history4 ,it may well be imagined that a discovery so startling as this excited astonishment throughout the scientific world4 8,who is this ,bath organist80 everybody asked4 ,accounts of him and his discoveries appeared in the papers4 ,his fellow-citizens were not so familiar with the name as we are1 happily1 now2 and the spelling of the unusual name showed many varieties4 ,when ,george ,,iii4 heard of ,herschel's great achievement1 he directed the astronomer to be summoned to ,windsor1 that the ,king might receive an account of the wonderful discovery from the lips of the discoverer himself4 ,herschel of course obeyed1 and he brought with him his famous telescope1 and also a map of the whole solar system1 to show to the ,king4 ,no doubt he thought that his ,majesty had probably not paid much attention to astronomy4 ,herschel was1 therefore1 prepared to explain to the ,king what it would be necessary for him to know before he could fully appreciate the magnitude of the discovery4 ,you will remember that ,herschel while still a boy had deserted from the army1 many years previously4 ,it appears that the ,king had learned this fact in some way1 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 of4 ,the ,king accordingly handed ,herschel a paper1 in which he was1 ,i dare say1 greatly surprised to find a pardon to the deserter written out by the ,king himself4 ,then ,herschel unfolded his wonderful discovery1 which the ,king thoroughly appreciated1 and in the evening the telescope was set up in the gardens1 and the glories of the heavens were displayed4 ,herschel made a most favorable impression on his ,majesty1 and when the ,king told the ladies of the ,castle next day of all that ,herschel had shown him1 their astronomical ardor was also aroused1 and they asked to see through the marvellous tube4 ,of course ,herschel was ready to comply1 and the telescope was accordingly carried to the windows of the ,queen's apartments at ,windsor1 which would have commanded a fine view if the clouds had not been in the way1 which they unfortunately were4 ,even for royalty the clouds would not disperse1 so what was to be done8 ,herschel was equal to the occasion4 ,he specially wanted to exhibit ,saturn1 for it is one of the most beautiful objects in the sky1 and will fascinate any intelligent beholder4 ,no astronomers would have been able to see ,saturn through the clouds1 but ,herschel did not disappoint his visitors2 he directed the instrument1 not to the sky 7nothing was there to be seen72 he turned it towards a distant garden wall4 ,now what would you expect to see by looking through a telescope at a garden wall--bricks1 perhaps1 or ivy8 ,what these ladies saw was a beautiful image of ,saturn1 his globe in the centre and his rings all complete1 forming so true a resemblance to the planet that even an experienced astronomer might have been deceived4 ,in the afternoon ,herschel had seen that the clouds were thick1 and that there would be little probability of using the telescope properly4 ,accordingly he cut out a little image of ,saturn1 illuminated it by lamps1 and set it up at a suitable distance on a garden wall4 ,herschel's visit to ,windsor was productive of important consequences4 ,the ,king said it was a pity that so great an astronomer should devote himself to music1 and that it would be far better for him to give up that profession and come and live at ,windsor4 ,his ,majesty promised that he would pay him a salary1 and he also undertook to provide the cost of erecting great telescopes4 ,his faithful sister ,caroline came with him as his assistant1 and also received some bounty from the ,king4 ,from that moment ,herschel renounced all his musical business1 and devoted himself to his great life-task of observing the heavens4 ,he built telescopes of proportions far exceeding those that had ever been then thought of4 ,he used to stand at night in the open air from dusk to dawn gazing down the tube of his mighty reflector1 watching the stars and other objects in the heavens as they moved past4 ,he would dictate what he saw to ,caroline1 who sat near him4 ,it was her business to write down his notes and to record the position of the objects which he was describing4 ,sometimes1 she tells us1 the cold was so great that the ink used to freeze in her pen when she was at this work4 ,until he became a very old man1 ,herschel devoted himself to his astronomical labors4 ,his discoveries are to be counted by thousands1 though not one of them was so striking or so important as the detection of the new planet which first brought him fame4 ,the question of a name for the addition to the sun's family had1 of course1 to be settled4 ,herschel had surely a right to be heard at the christening1 and as a compliment to his ,majesty he named the stranger the ,georgium ,sidus4 ,so1 indeed1 for a brief while1 the planet was actually styled4 ,the ,continental astronomers1 however1 would not accept this designation2 all the other planets were named after ancient divinities1 and it was thought that the ,king of ,england would seem oddly associated with ,jupiter and ,saturn2 perhaps also they considered that the ,british dominions1 on which the sun never sets1 were already quite large enough1 without further extension to the celestial regions4 ,accordingly a consultation was held1 the result of which was that ,george ,,iii4 was deprived of his planetary honors1 and the body was given the name of ,uranus1 which1 by universal consent1 it now bears4 ,the planet ,uranus lies just on the verge of visibility with the unaided eye4 ,it can sometimes be glimpsed like a faint star1 and1 of course1 with a telescope it is readily perceived4 ,many generations of astronomers before ,herschel's time had been observing the heavens1 making maps of the stars1 and compiling great catalogues in which the places of the stars were accurately put down4 ,it often happened that ,uranus came under their notice1 but it never occurred to them that what seemed so like a star was really a planet4 ,i have1 no doubt1 said that ,uranus looked unlike a star when ,herschel examined it2 but then that was because ,herschel was a particularly skilful astronomer4 ,to an observer of a more ordinary type ,uranus would not present any very remarkable appearance1 and would be passed over merely as a small star4 ,in fact1 the planet was thus observed not once or twice1 but no fewer than seventeen times1 before the acute eye of ,herschel perceived its true character4 ,on many previous occasions the planet had been noted as a star by astronomers who are in every way entitled to our respect4 ,it required a ,herschel1 determined to see everything in the very best manner1 to grasp the discovery which eluded so many others4 ,when ,uranus was observed on these former occasions and mistaken for a star1 its place had been carefully put down4 ,these records are at present of the utmost use1 because they show the past history of the planet2 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 sun4 ,thus1 since the planet was discovered in #agha1 it had completed one revolution by #ahfe1 and is now 7#ahii7 about one-third of the way around another4 ,the earlier observations extend backwards almost #bjj years1 so that altogether we have more or less information about the movements of the planet during the completion of two circuits and a half4 ,uranus is a great deal bigger than the earth1 as you will see in the view of the comparative sizes of the planets 7,fig4 #dg74 ,it appears to be of a bluish hue1 but we cannot tell whether it turns round on its axis1 or rather1 ,i should say1 we are not able to see whether it turns round on its axis2 for we can hardly doubt that it does so4 ,notwithstanding that ,uranus is at so great a distance from the earth1 we have been able to put this planet1 no less than the nearer ones1 in the weighing scales1 and we assert with confidence that ,uranus is fifteen times as heavy as our earth4 ,we are indebted to the satellites for this information4 ,,the ,,satellites ,,of ,,uranus4 ,you must use a very good telescope to see the satellites of ,uranus4 ,they are four in number1 bearing the names of ,ariel1 ,umbriel1 ,titania1 and ,oberon4 ,the innermost of these1 ,ariel1 completes a journey round the planet in two days and a half2 ,oberon1 the most distant1 requires thirteen days and a half4 ,a planet is always tending to pull its satellite down1 and the satellite is kept from falling by the speed with which it revolves4 ,the heavier the planet1 the faster must its satellites go round4 ,thus1 to take an illustration from our own moon1 we know that1 if the earth were to be made four times heavier than it is1 the moon would have to spin round twice as fast as it does1 in order to remain in the same orbit4 ,the speed with which the satellites of ,uranus revolve accordingly affords a measure of the mass of the planet4 ,were ,uranus heavier than he is1 his satellites would revolve more quickly than they do2 were he lighter1 the satellites would take a longer period to go round4 ,uranus also seems to be greatly swollen by clouds1 in the same manner as are both ,jupiter and ,saturn2 in fact1 if our earth was as big as ,uranus1 it would weigh four or five times as much as ,uranus does4 ,hence we are certain that ,uranus must consist of materials less dense on the whole than are those of which our earth is made4 ,there is another singular circumstance connected with the moons of ,uranus4 ,i have told you how every body revolving round another by gravitation will describe an ellipse2 but1 of course1 there are many different kinds of this curve1 and some may be nearly circles4 ,there is nothing whatever to prevent a satellite from revolving around its primary in an exact circle if it be started properly2 that is1 in the right direction and with the right speed4 ,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 compasses4 ,it is further to be noticed that the four circles seem to lie exactly in the same plane4 ,the orbits of the other great planets and of their satellites lie in planes inclined at angles of less than #ce to the ecliptic1 the plane in which the earth moves4 ,here again the satellites of ,uranus are exceptional4 ,the plane in which they are contained stands up almost squarely to the plane in which the motion of the planet is performed4 ,the moons of ,uranus seem to have got a twist1 from some accidental circumstance for which we are not able to account4 ,,the ,,discovery ,,of ,,neptune4 ,the boundaries of the solar system had been much extended by the discovery of ,uranus1 but they were destined to receive still further enlargement by the detection of another vast planet1 revolving far outside ,uranus1 the orbit of which forms1 according to our present knowledge1 the outline of the planetary system4 ,i have here to describe one of the greatest discoveries that have ever been made4 ,it is not the magnificence of the outermost planet itself that ,i refer to1 though1 indeed1 it is bigger than ,uranus4 ,i am rather thinking of the way in which the discovery was made4 ,i do not mean any disrespect to ,herschel when ,i say that the discovery of ,uranus was chiefly a stroke of good fortune2 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 ,neptune4 ,herschel merely looked at one star after another till suddenly he dropped on the planet1 having beforehand not the slightest notion that any such planet was likely to exist4 ,but ,neptune was shown to exist before it was ever seen1 and1 in fact1 the man that first saw the planet1 and knew it to be a planet1 was not the discoverer4 ,this is rather a difficult subject2 and it would take you years of hard study to be able to understand the train of reasoning by which ,neptune was found4 ,i shall1 however1 make an attempt to explain this matter sufficiently to give at least some idea of the kind of problem that had to be solved4 ,you will remember that law of ,kepler which tells us that every planet moves round the sun in an ellipse4 ,if the planet be uninterfered with in any way and guided only by the attraction of the sun1 it will forever continue to describe precisely the same ellipse without the slightest alteration4 ,it was ascertained that the path which ,uranus followed was not always regular4 ,the early observations of the planet1 when it was mistaken for a star1 have here been of the utmost service4 ,they have indicated the ellipse which ,uranus described the last time it went round1 and our modern observations have taught us the path which the planet is at present describing4 ,these two ellipses are slightly different1 and the consequence is that1 supposing we take the observations of ,uranus made #ajj years ago1 and calculate from them where ,uranus ought to be now1 we find that the planet is a little astray4 ,astronomers are not accustomed to be wrong in such calculations1 and when discrepancies arise1 the first thing to be done is to see what has caused them4 ,it is certain that the position in which ,uranus is found this very night1 for example1 is not what it would have been had the sun alone been guiding the planet4 ,perhaps you will think that it is impossible for reliable computations to be made about such matters2 but ,i assure you they can1 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 disturbance4 ,i have already explained1 when speaking about ,mars 7p4 #ahg71 that there is an attraction between every two bodies1 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 it4 ,nevertheless it is true that not only does the sun pull our earth and all the other planets as well1 but all the planets1 including the earth1 are pulling one another4 ,in fact1 there is an incessant struggle going on in the family party4 ,fortunately the sun is so much more powerful than any other member1 that he keeps them all pretty well in order2 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 system4 ,our earth itself is pulled and swayed to and fro by the actions of its brothers and sisters4 ,it is dragged perhaps a thousand or two thousand miles this way by ,jupiter1 or it gets a good tug in the other direction by ,venus4 ,mars and ,saturn also do their little best to force the earth away from its strict path4 ,however1 our earth does not suffer much from these irregularities4 ,it pursues its route fairly enough1 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 way1 or when the coachman wishes to avoid a piece of the road on which stones have been freshly laid down4 ,the track followed by ,uranus was found to be somewhat irregular1 like that of every other planet4 ,jupiter gave it a pull1 and so did ,saturn1 and at first it was thought that the irregularities which were perceived could be explained by the action of these planets1 so big and so well known4 ,here is a question for calculation2 it involves a very long and a very hard piece of work1 but it is possible to estimate how far each of the other planets is capable of dragging ,uranus from its path4 ,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 off1 and not only for to-day or yesterday1 but for past time extending over more than a century8 ,if1 however1 you will listen to me a little longer1 ,i think ,i shall give you a proof that these sums could be worked out correctly4 ,when the calculations had been made which showed how much the known planets could disturb ,uranus1 it was found that there were still some deviations of the planet that remained unexplained4 ,they were not large2 they only amounted to showing that the body was just a little astray from the spot where the calculations indicated it should be4 ,the rest of astronomy was so perfect1 and the law of attraction prevailed so universally1 that it was thought the law of attraction must provide some way of explaining the behavior of ,uranus4 ,he could not have left his track of his own accord2 therefore there must be some agency at work upon him of which we did not know4 ,what could this unknown source of disturbance be8 ,every such trouble had hitherto been found to be a consequence of the attraction of gravitation2 therefore there must be some unknown body pulling at ,uranus which no one had ever seen4 ,where could it be8 ,how was it to be discovered8 ,such were the questions that were asked1 and they were answered in a most satisfactory manner4 ,first of all1 what sort of body could it be that was pulling ,uranus8 ,it is obvious that none of the stars would be competent to produce so great an effect2 they are all so far off that they have nothing whatever to say to any of the domestic matters in our little solar system1 which is simply a group by itself4 ,it would be more reasonable to suppose that there must be yet another planet which nobody had ever recognized1 but which affected ,uranus so as to account for his truant behavior4 ,to begin to search for this planet with telescopes without some guidance would be futile2 in fact1 astronomers had been scanning the heavens for planets for nearly fifty years1 and though several had been discovered1 they all belonged to the zone of little planets1 and none of them were big enough to pull ,uranus about appreciably4 ,of course1 if all the stars could be blotted out of the sky1 so that nothing but planets were left1 then1 by sweeping the telescope over the heavens1 every planet that exists might be speedily found4 ,the difficulty is that the planets1 which are either small or very distant1 look so like the stars that it is impossible to recognize them among the millions of glittering points in the sky4 ,it was1 however1 hoped that the unknown planet would be large enough to be visible in the telescope1 if only we knew exactly where to point it4 ,two illustrious astronomers1 ,adams of ,cambridge1 and ,leverrier of ,paris1 both separately undertook an astonishing piece of calculation4 ,they tried to find out the position of the unknown planet from the mere fact that it deranged ,uranus in a particular way4 ,i dare say many of those who are reading this book have learned simple equations in algebra1 and they have worked such questions as to find the length of a pole1 half of which is in mud1 a quarter in water1 and ten feet above the water4 ,those who know this much can perhaps realize the problem that had to be solved in trying to discover the unknown planet4 ,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 algebra4 ,i do not think they would let you make a series of guesses4 ,let us try #bj feet1 for instance1 as the length of the pole2 that will make #aj feet in the mud1 #e feet in the water1 and #e feet outside4 ,this will not do2 it is not enough2 we must try again2 and after another guess or two1 we see that a pole #dj feet long will exactly answer4 ,we do not use this method of guessing in algebra1 because solving the simple equation is a much better method4 ,adams and ,leverrier found that to discover the unknown planet was a question so very difficult1 that they were obliged to use a sort of guessing1 but very intelligent guessing1 ,i need hardly assure you4 ,they proceeded in this way 7,fig4 #ga74 ,they would draw a circle outside the path of ,uranus1 and then suppose that a planet was revolving in that circle4 ,its effect upon ,uranus would then be calculated1 and it would be found whether the observed irregularities could be in this manner accounted for4 ,the first planet they tried was not the right one2 then they began again with another1 until at last1 after many trials and much very hard work1 they saw that there might be a planet in a particular path far outside ,uranus1 such that if this planet were of the right weight and moving with the right speed1 then it would pull ,uranus exactly in the way that astronomers had observed it to be pulled4 ,they found at last that there could be little doubt about the matter2 for this unknown body would account for all the facts4 ,then1 indeed1 they had solved their equation2 they had found the unknown4 ,illustration3 ,,fig4 #ga4--,orbits of ,uranus and ,neptune4 ,the two great astronomers had thus discovered a planet1 but as yet it was only a planet on paper4 ,those who could judge of the subject had no doubt that the planet was really in the sky2 but just as you like to prove that you have found the correct answer to your sum1 so people were naturally anxious to prove the truth of this wonderful sum that ,adams and ,leverrier had worked out4 ,this was to be done by actually seeing the planet of which the astronomers had asserted the existence4 ,leverrier calculated that the new planet in a certain night would be in a particular position on the sky4 ,accordingly he wrote to ,dr4 ,galle1 of the observatory at ,berlin1 requesting him on the evening in question to point his telescope to the very spot indicated1 and there he would see a planet which human eyes had never before beheld4 ,of course1 ,dr4 ,galle was only too delighted to undertake so marvellous a commission4 ,the evening was fine2 the telescope was opened2 it was directed towards the heavens2 and there1 in the very spot which the calculations of ,leverrier had indicated1 shone the beautiful little planet4 ,at ,cambridge arrangements had also been made to search for the new member of the solar system1 in accordance with ,professor ,adams' calculations4 ,there also the planet that had given all this trouble to ,uranus was brought to light4 ,at first it looked like a star1 as all such planets do2 but that it was not a star was speedily proved1 by the two tests which are sure indications of a planet4 ,first the body was so moving that its position with respect to the adjacent stars was constantly changing4 ,then1 when a strong magnifying power was placed on the telescope1 the little object was seen1 not to be a mere starlike point1 but to expand into the little disk which shows us we are not looking at a distant sun1 but at a world like our own4 ,was not this truly a great discovery8 ,have we not shown you how entitled the calculations of astronomers are to our respect1 when we find that they actually discovered the existence of a majestic planet before the telescope had revealed it8 ,see also the greatly increased interest that belongs to ,herschel's discovery of ,uranus4 ,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 own4 ,he died1 however1 more than twenty years before this achievement4 ,the authorities who decide on such matters christened the new planet ,neptune2 and this body wanders round on the outskirts of our solar system1 requiring for each journey a period of no less than #afe years4 ,the circle thus described has a radius thirty times as great as that of the earth's track4 ,neptune is altogether invisible to the unaided eye1 but it is sufficiently bright to have been occasionally recorded as a star4 ,indeed1 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 observing4 ,a curious circumstance was afterwards brought to light4 ,when reference was made to the books in which ,lalande's observations were written1 it was found that he had observed this object twice1 namely1 on ,may #h and ,may #aj1 #aghe4 ,of course1 if the object had indeed been a star its position on the two days would have been the same1 but being a planet it had moved4 ,when ,lalande1 on looking over his papers1 saw that the places of this supposed star were different on the two nights1 he concluded that he must have made a mistake on the first night1 and accordingly treated the object as if the place on the #ajth was the right one4 ,just think how narrowly ,lalande missed making a discovery6 ,unhappily for his renown1 he took it for granted that one or both of his observations were erroneous1 and so they must have been if the object had been a star4 ,but they were both right2 it was the planet which had moved in the interval4 ,as ,neptune is half as far again from the earth as ,uranus1 we can hardly expect to learn much about the actual nature of the planet4 ,we do know that it has four times the diameter of the earth1 so that it exceeds the earth in the same proportion that the earth is larger than the moon4 ,like the other great planets1 ,neptune is also enveloped with copious clouds2 in fact1 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 earth4 ,like our earth1 ,neptune is attended by one moon1 which revolves round the planet in a little more than six days4 ,the orbit of this great planet marks the boundary of our known system of planets4 ,we have seen how the five great planets of antiquity have been increased in these modern days by the addition of two more1 ,uranus and ,neptune1 while the discovery of a multitude of small planets has given a further increase to the number of the sun's family4 ,we have still some other objects in our solar system to describe2 some of them are excessively big2 these are the comets4 ,some of them are exceedingly small2 they are the shooting stars4 ,we shall talk about comets and shooting stars in our next lecture4 ,,lecture ,v4 ,,comets ,,and ,,shooting ,,stars4 ,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--,meteorites4 ,,the ,,movements ,,of ,a ,,comet4 ,the planets are all massive globes1 more or less flattened at the ,poles2 but now we have to talk about a multitude of objects of the most irregular shapes1 and of the most flimsy description4 ,we call them comets1 and they exist in such numbers that an old astronomer has said 8there were more comets in the sky than fishes in the sea10 though ,i think we cannot quite believe him4 ,there is also another wide difference between planets and comets3 planets move round in nearly circular ellipses1 and not only do we know where a planet is to-night1 but we know where it was a month ago1 or a hundred years ago1 or where it will be in a hundred years or a thousand years to come4 ,all such movements are conducted with conspicuous regularity and order2 but now we are to speak of bodies which generally come in upon us in the most uncertain and irregular fashion4 ,they visit us we hardly know whence1 except that it is from outer space1 and they are adorned in a glittering raiment1 almost spiritual in its texture4 ,they are always changing their appearance in a baffling1 but still very fascinating manner4 ,if an artist tries to draw a comet1 he will have hardly finished his picture of it in one charming robe before he finds it arrayed in another4 ,the astronomer has also his complaints to make against the comets4 ,i have told you how thoroughly we can rely on the movements of the planets1 but comets often play sad pranks with our calculations4 ,they sometimes take the astronomers by surprise1 and blaze out with their long tails just when we do not expect them4 ,then by way of compensation they frequently disappoint us by not appearing when they have been most anxiously looked for4 ,after a voyage through space the comet at length begins to draw in towards the central parts of our system1 and as it approaches the sun1 its pace becomes gradually greater and greater2 in fact1 as the body sweeps round the sun the speed is sometimes #bj1jjj times faster than that of an express train4 ,it is sometimes more than #ajjj times as fast as the swiftest of rifle bullets1 occasionally attaining the rate of #bjj miles a second4 ,the closer the comet goes to the sun1 the faster it moves2 and a case has been known in which a comet1 after coming in for an incalculable duration of time towards the sun1 has acquired a speed so tremendous1 that in two hours it has whirled round the sun and has commenced to return to the depths of outer space4 ,this terrific outburst of speed does not last long4 ,a pace which near the sun is #bj1jjj times that of our express trains diminishes to #aj1jjj times1 to fifty times1 to ten times that pace2 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 exertions4 ,illustration3 ,,fig4 #gb4--;,how the ,comet's ,tail is disposed4 ,we have so often seen a stream of sparks stretching out along the track of a sky-rocket1 that we might naturally suppose the tail of a comet streamed out along its path in a somewhat similar manner4 ,this would be quite wrong4 ,you see from ,fig4 #gb that the tail does not lie along the comet's path1 but is always directed outwards from the sun4 ,if you will draw a line from the sun to the head of the comet and follow the direction of the line1 it shows the way in which the tail is arranged4 ,you will also notice how the tail of the comet seems to grow in length as it approaches the sun4 ,when the comet is first seen1 the tail is often a very insignificant affair1 but it shoots out with enormous rapidity until it becomes many millions of miles long by the time the comet is whirling round the sun4 ,those glories soon begin to wane as the comet flies outward2 the tail gradually vanishes1 and the wanderer retreats again to the depths of space in the same undecorated condition as that in which it first approached4 ,when a comet appears1 it is always a matter of interest to see whether it is an entirely new object1 or whether it may not be only another return of a comet which has paid us one or more previous visits4 ,the question then arises as to how they are to be identified4 ,here we see a wide contrast between unsubstantial bodies like comets and the weighty and stately planets4 ,sketches of the various planets or of the face of the sun1 though they might show slight differences from time to time1 are still always sufficiently characteristic1 just as a photographic portrait will identify the individual1 even though the lapse of years will bring some changes in his appearance4 ,but the drawing of a comet is almost useless for identification4 ,you might as well try to identify a cloud or a puff of smoke by making a picture of it4 ,make a drawing of a comet at one appearance1 and sketch particularly the ample tail with which it is provided4 ,the next time the comet comes round it may very possibly have two tails1 or possibly no tail at all4 ,we are therefore unable to place any reliance on the comet's personal appearance in our efforts to identify it4 ,the highway which it follows through the sky affords the only means of recognition2 for the comet1 if undisturbed by other objects1 will never change its actual orbit4 ,but even this method of identification often fails1 for it not unfrequently happens that during its erratic movements the comet gets into fearful trouble with other heavenly bodies4 ,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 path1 and our efforts to identify it are plunged in confusion4 ,it has happened that a second comet or even a third will be found in nearly the same track1 but whether these are wholly different1 or whether they are merely parts of the same original object1 it is often impossible to determine4 ,the great majority of comets are only to be seen with a telescope1 and hardly a year passes without the detection of at least a few of these faint objects4 ,the number of really brilliant comets that can be seen in a lifetime could1 however1 be counted on the fingers4 ,,encke',s ,,comet4 ,we have already alluded to a little body called ,encke's comet1 which was discovered by an astronomer at ,marseilles4 ,it was in the year #ahah that he was scanning the heavens with a small telescope1 when an object attracted his attention4 ,it was not one of those grand long-tailed comets which every one notices2 this body was so faint that it merely appeared as a very small cloud of light1 and was recognized as a comet by the fact that it was moving4 ,it happens that there are other bodies in the sky very like comets2 we call them nebul1 and we shall have something to say about them afterwards4 ,but it is remarkable that just as a planet is liable to be mistaken for a star1 so a comet is liable to be mistaken for a nebula4 ,however1 in each case the fact of its movement is the test by which the planet or the comet is at once detected4 ,a nebula stays always in the same spot1 like a star1 while a comet is incessantly moving4 ,in fact1 with a telescope you can actually watch a comet stealing past the stars that lie near it4 ,you know that an object a very long way off may appear to move slowly1 though in reality it is moving very rapidly4 ,look at a steamer near the horizon at sea4 ,in the course of a minute or two it will not appear to have shifted its position to any appreciable extent1 but that is because it is far off4 ,if you were near the ship1 you would see that it was dashing along at the rate of perhaps fifteen or twenty miles an hour4 ,in a similar manner the comet seems to move slowly1 because it is at such a great distance4 ,as a matter of fact it is moving faster at the time we see it than any steamer1 faster than any express train1 faster than any cannon-ball4 ,there were special reasons why the movements of ,encke's comet should be watched with peculiar care1 and the track which it pursued be ascertained4 ,if you can observe a comet three times and measure its position in the sky1 the movement of that comet is completely determined4 ,perhaps ,i should say would be determined if the comet were let alone1 which1 unfortunately1 is not often the case4 ,indeed1 you may remember how ,i told you some of the misadventures of this very comet when we were speaking about the planet ,mercury4 ,encke's comet comes round in a period of a little more than three years1 and it gives us some curious information that has been ascertained during its journeys4 ,one of the facts we have thus learned is so important that we cannot omit to notice it 7,fig4 #gc74 ,illustration3 ,,fig4 #gc4--,the ,orbit of ,encke's ,comet4 ,at increasing heights above the earth's surface there is gradually less and less air2 until at last1 at about #bjj or #cjj miles above the surface on which we dwell1 there would be none4 ,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 aloft4 ,in open space motion could take place quite freely4 ,down here the resistance of the air is a great impediment to movement1 especially when very rapid4 ,a heavy cannon-ball is checked and robbed of its pace by having to plough its way through our dense atmosphere4 ,the motion is arrested in the same way1 though not of course to the same degree1 as if the cannon-ball had been fired into water4 ,unsubstantial objects are1 of course1 impeded by the air to a far greater extent than such heavy bodies as cannon-balls4 ,you know that you cannot throw a handful of feathers across the road in the same way that you could throw a handful of gravel4 ,the light feathers cannot force their way through the air so well as the pebbles4 ,a body so flimsy as a comet would never be able to push its way through an atmosphere like ours2 but out in empty space the comet meets with no resistance during the greater part of its path4 ,accordingly1 though it has little more substance than a will-o'-the-wisp1 the comet pursues its journey with as much resolute dignity as if it were made of cast iron4 ,if in any part of its track the body should have to pierce its way through any material like even the thinnest possible air1 then the unsubstantial nature of the cometary materials would be at once shown4 ,the motion would be impeded1 and the body's path would be changed4 ,in this way a comet may be made very instructive1 for it will show whether space is really so empty as we sometimes suppose it to be4 ,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 it1 and thus we learn that space is generally empty4 ,however1 when the comet begins to wheel around the sun1 the freedom of its movements seems to receive a check4 ,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 orbit4 ,we thus learn that there is a thin diffused atmosphere surrounding the sun4 ,we cannot1 indeed1 say that it is like our air4 ,its composition is quite different1 and almost the only way we know of its existence is by the evidence which this comet affords4 ,in a former lecture ,i showed how ,encke's comet told us the mass of the planet ,mercury4 ,now we see how the travels of the same body give us information about the sun himself4 ,i ought1 however1 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 considering4 ,,the ,,great ,,comet ,,of ,,halley4 ,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 ,encke4 ,it unfortunately happens that most of the fine comets pay our system only a single visit4 ,there is only one of the really splendid objects of this kind that comes back to us with anything like regularity4 ,it was last seen in the year #ahce1 and ,i am glad to tell you that it is coming again2 it is expected about the year #aiaj4 ,you may ask1 ,how can we feel sure that such a prediction as ,i have mentioned will turn out correctly8 ,the fact is that this comet has been watched for a great many centuries4 ,we find ancient records1 some of them nearly #bjjj years old1 of the appearance of grand comets1 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 years4 ,of course there are thousands of other comets recorded in these old books as well2 but what ,i mean is that among the records many are found which clearly indicate some successive returns of this particular body4 ,i will explain how the movements of this comet were discovered4 ,there was a great astronomer called ,halley1 who lived two hundred years ago1 and in the year #afhb he1 like every one else1 was looking with admiration at a splendid comet with a magnificent tail which adorned the sky in that year4 ,at the observatories1 of course1 they diligently set down the positions of the comet1 which they ascertained by carefully measuring it with telescopes4 ,halley first calculated the highway which this comet followed through the heavens1 and then he looked at the list of old comets that had been seen before4 ,he thus found that in #afjg--that was1 seventy-five years earlier--a great comet had also appeared1 the path of which seemed much the same as that which he found for the body that he had himself observed4 ,this was a remarkable fact1 and it became still more significant when he discovered that seventy-six years earlier--namely1 in #aeca--;another great comet had been recorded1 which moved in a path also agreeing with those of #afjg and #afhb4 ,it then occurred to ,halley that possibly these were not three different objects1 but only different exhibitions of one and the same1 which moved round in the period of seventy-five or seventy-six years4 ,there is a test which an astronomer can often apply in the proof of his theory1 and it is a very severe test4 ,he will not only show himself to be wrong if it fails1 but he will also make himself somewhat ridiculous4 ,halley ventured to submit his reputation to this ordeal4 ,he prophesied that the comet would appear again in another seventy-five or seventy-six years4 ,he knew that he would1 of course1 be dead long before #ageh should arrive2 but when he ventured to make the prediction1 he said that he hoped posterity would not refuse to admit that this discovery had been made by an ,englishman4 ,you can easily imagine that as #ageh drew near1 great interest was excited among astronomers to see if the prediction of ,halley would be fulfilled4 ,we are accustomed in these days to find many astronomical events foretold with the same sort of punctuality as we expect in railway time-tables4 ,the ,nautical ,almanac is full of such prophecies1 and we find them universally fulfilled4 ,even now1 however1 we are not able to set forth our time-tables for comets with the same confidence that we show when issuing them for the sun1 the moon1 or the stars4 ,how astonishing1 then1 must ,halley's prediction have seemed6 ,here was a vast comet which had to make a voyage through space to the extent of many hundreds of millions of miles4 ,for three-quarters of a century it would be utterly invisible in the greatest telescopes1 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 track4 ,for fifty1 or sixty1 or seventy years nothing had been seen of the comet1 nor1 indeed1 was anything expected to be seen of it2 but as seventy-one1 and seventy-two1 and seventy-three years had passed1 it was felt that the wanderer1 though still unseen1 must be rapidly drawing near4 ,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 circulate2 and1 of course1 the flimsy object would be pulled hither and thither out of its path by the attractions of the weighty bodies4 ,it was computed that the influence of ,saturn alone was sufficient to delay the comet for more than three months1 while it appeared that the attraction of ,jupiter was potent enough to retard the expected event for a year and a half more4 ,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 follow8 ,clairaut1 who devoted himself to this problem1 suggested that there might also be some disturbances from other causes of which he did not know1 and that consequently the expected return of the comet might be a month wrong either way4 ,great indeed was the admiration in astronomical circles when1 true to prediction1 the comet blazed upon the world within the limits of time ,clairaut had specified4 ,the remarkable fulfilment of this prophecy entitles us to speak with confidence about the past performances of this comet4 ,among all the apparitions of ,halley's comet for the last two thousand years1 perhaps the most remarkable is that which took place in the year #ajff4 ,i am sure you will all remember this date in your ,english history2 it was the year of the ,conquest4 ,in those days they did not understand astronomy as we understand it now2 they used to think of a comet as a fearful portent of evil1 sent to threaten some frightful calamity2 such as a pestilence1 a war1 a famine1 or something else equally disagreeable4 ,hence in the year of the ,conquest the appearance of so terrific an object in the sky was a very significant omen4 ,attention was concentrated upon the spectacle1 and a picture of ,halley's comet as it appeared to the somewhat terrified imaginations of the people of those days has been preserved4 ,there is a celebrated tapestry at ,bayeux on which historical incidents are represented by beautifully worked pictures4 ,on this fabric we have a view of ,halley's comet in a quaint and rather ludicrous aspect4 ,you will read of this comet also in the early pages of ,tennyson's 8,harold40 ,,how ,,the ,,telegraph ,,is ,,used ,,for ,,comets4 ,in these days the study of comets is prosecuted with energy4 ,over the world observatories are situated1 and whenever a comet is discovered1 tidings of the event are diffused among those likely to be interested4 ,suppose that one is discovered in the southern hemisphere1 the astronomers then write to warn the northern observatories of the event4 ,but comets often move faster than her ,majesty's mails1 so that the telegraph has to be put into requisition4 ,the kind of message is one which shall show the position and the movements of the body4 ,it necessarily involves a good many figures and words1 and consequently it is desirable to abbreviate as much as possible for the sake of economy4 ,there is a further difficulty in using the telegraph1 because the messages are not of an intelligible description to those not specially versed in astronomy4 ,skilful as the telegraph clerks are1 they can hardly be expected to be familiar with the technicalities of astronomers4 ,the clerk at the receiving end is handed a message which he does not understand very clearly4 ,the clerk at the other end does not understand the message which is delivered to him1 and between them it has happened that they have transformed the message into something which not only they do not understand1 but which1 unfortunately1 nobody else can understand either4 ,these difficulties have been surmounted by an agreement between astronomers1 which is so simple and interesting that ,i must mention it4 ,the kind of message that expresses the place of a comet will consist of sentences something of this kind3 8,one hundred and twenty-three degrees and forty-five minutes40 ,surely it would be an advantage to be able to replace all these words by a single word1 particularly if by doing so the risk of error would be diminished4 ,this is what the astronomers' telegraphic arrangement enables them to accomplish4 ,there is a certain excellent ,dictionary known as ,worcester's4 ,i am sure when ,mr4 ,worcester arranged this work1 he had not the slightest anticipation of an odd use to which it would occasionally be put4 ,every astronomer who is co-operating in the comet scheme must have a copy of the book4 ,to send the message ,i have just referred to1 he has to take up his ,dictionary and look out page #abc4 ,then he will count down the column until he comes to the forty-fifth word on that page1 which he finds to be 8constituent10 and according to this plan the message1 or at least this part of it1 is merely that one word1 8constituent40 ,the astronomer who receives this message and wishes to interpret it takes up his copy of ,worcester's ,dictionary and looks out for 8constituent40 ,he sees that it is on page #abc1 and that it is the forty-fifth word down on that page2 and therefore he knows that the interpretation of the message is to be one hundred and twenty-three degrees and forty-five minutes4 ,,the ,,parabola4 ,generally speaking1 great comets come to us once and are then never seen again4 ,such bodies do not move in closed ovals or ellipses1 they follow another kind of curve1 like that represented in ,fig4 #gd4 ,it is one that every boy ought to know4 ,in fact1 in one of his earliest accomplishments he learned how to make a parabola4 ,it is true he did not call it by any name so fine as this1 but every time a ball is thrown into the air it describes a part of the beautiful curve which geometers know by this word 7,fig4 #gd74 ,in fact1 you could not throw a ball so that it should describe any other curve except a parabola4 ,no boy could throw a stone in a truly horizontal line4 ,it will always curve down a little1 will always1 in fact1 be a portion of a parabola4 ,illustration3 ,,fig4 #gd4--,the ,path of a ,projectile is a ,parabola4 ,there are big parabolas and there are small ones4 ,one of the shells which are thrown into a town when bombarded from a distance describes1 as it rises and then slopes down again1 part of a mighty parabola4 ,so does a tennis ball thrown by the hand or struck by the racket2 though here1 indeed1 ,i admit that a spin may be given to the ball which will somewhat detract from the simplicity of its movement4 ,in playing baseball1 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 parabola1 but in some twisting curve by which he hopes to baffle his adversary4 ,setting aside these exceptions1 and such another as the case of a body tossed straight up or dropped straight down1 we may assert that the path of a projectile is a parabola4 ,illustration3 ,,fig4 #ge4--,the ,lighthouse ,reflector4 ,there are some remarkable applications of the same curve for practical purposes4 ,from our lighthouses we want to send beams off to sea1 so as to guide ships into port4 ,if we merely employed a lamp without concentrating its rays1 we should have a very imperfect lighthouse1 for the lamp scatters light about in all directions4 ,much of it goes straight up into the air1 much of it would be directed inland2 in fact1 there is only an extremely small part of the entire number of rays that will naturally take the useful direction4 ,we therefore require something round the lamp which shall catch the truant rays that are running away to idleness and loss1 and shall concentrate them into the direction in which they will be useful to the mariner4 ,an effective way of doing this is to furnish the lamp with a reflector4 ,on its bright surface 7,fig4 #ge7 all the rays fall which would otherwise have gone astray1 and each of them is properly redirected1 where the sailors can see it4 ,it is essential that the mirror shall do this work accurately1 and this it will only do when it has been truly shaped so as to be a parabola4 ,you will remember1 also1 how ,i described to you the reflector which ,herschel made for his great telescope4 ,the shape of the mirror must be most accurately worked1 and it1 too1 must have a parabola for its section2 so that you see this curve is one of importance in a variety of ways4 ,but the grandest of all parabolas are those which the comets pursue4 ,unlike the ellipse1 the parabola is an open curve2 it has two branches stretching away and away forever1 and always getting further apart4 ,of course1 in the examples of this curve that ,i have given it is only a small part of the figure that is concerned4 ,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 ground4 ,it is just a part of the curve in the same way that a crescent may be a bit of a circle4 ,it is to comets that we must look for the most complete illustration of the ample extent of a parabola4 ,the shape of this grand curve will explain why so many comets only appear to us once4 ,it is quite clear that if you begin to run round a closed racecourse1 you may1 if you continue your career long enough1 pass and repass the starting-post thousands of times4 ,thus1 comets which move in ellipses1 and are consequently tracing closed curves1 will pass the earth times without number4 ,for this reason we may see them over and over again1 as we do ,encke's comet or ,halley's comet4 ,but suppose you were travelling along a road which1 no matter how it may turn1 never leads again into itself1 then it is quite plain that1 even if you were to continue your journey forever1 you can never twice pass the same house on the roadside4 ,that is exactly the condition in which most of the comets are moving4 ,their orbits are parabolas which bend round the sun2 and1 generally speaking1 the sun is very close to the turning-point4 ,the earth is also1 comparatively speaking1 close to the sun2 so that while the comet is in that neighborhood we can sometimes see it4 ,we do not see the comet for a long time before it approaches the sun1 or for a long time after it has passed the sun4 ,all we know1 therefore1 of its journey is that the two ends of the parabola stretch on and on forever into space4 ,the comet is first perceived coming in along one of these branches to whirl round the sun2 and after doing so1 it retreats along the other branch1 and gradually sinks into the depths of space4 ,why one of these mysterious wanderers should approach in such a hurry1 and then why it should fly back again1 can be partially explained without the aid of mathematics4 ,let us suppose that1 at a distance of thousands of millions of miles1 there floated a mass of flimsy material resembling that from which comets are made4 ,notwithstanding its vast distance from the sun1 the attraction of that great body will extend thither4 ,it is true the pull of the sun on the comet will be of the feeblest and slightest description1 on account of the enormously great distance4 ,still1 the comet will respond in some degree1 and will commence gradually to move in the direction in which the sun invites it4 ,perhaps centuries1 or perhaps thousands1 or even tens of thousands1 of years will elapse before the object has gained the solar system4 ,by that time its speed will be augmented to such a degree1 that after a terrific whirl around the sun1 it will at once fly off again along the other branch of the parabola4 ,perhaps you will wonder why it does not tumble straight into the sun4 ,it would do so1 no doubt1 if it started at first from a position of rest2 generally1 however1 the comet has a motion to begin with which would not be directed exactly to the luminary4 ,this it is which causes the comet to miss actually hitting the sun4 ,it may also be difficult to understand why the sun does not keep the comet when at last it has arrived4 ,why should the wandering body be in such a hurry to recede8 ,surely it might be expected that the attraction of the sun ought to hold it4 ,if something were to check the pace of the comet in its terrific dash round the sun1 then1 no doubt1 the object would simply tumble down into the sun and be lost4 ,the sun has1 however1 not time to pull in the comet when it comes up with a speed #bj1jjj times that of an express train4 ,but the sun does succeed in altering the direction of the motion of the comet1 and the attraction has shown itself in that way4 ,i can illustrate what happens in this manner4 ,here is a heavy weight suspended from the ceiling by a wire2 it hangs straight down1 of course1 and there it is kept by the pull of the earth4 ,supposing ,i draw the weight aside and allow it to swing to and fro1 then the motion continues like the beat of a pendulum4 ,the weight is always pulled down as near to the earth as possible1 but when it gets to the lowest point1 it does not stay there1 it goes through that point1 and rises up at the other side4 ,the reason is that the weight has acquired speed by the time it reaches the lowest point2 and that1 in virtue of its speed1 it passes through the position in which it would naturally rest1 and actually ascends the other side in opposition to the earth's pull1 which is dragging it back all the time4 ,this will illustrate how the comet can pass by and even recede from the body which is continually attracting it4 ,just a few words of caution must be added4 ,suppose you had an ellipse so long that the comet would take thousands and thousands of years to complete a circuit1 then the part of the ellipse in which the comet moves during the time when we can see it is so like a parabola1 that we might possibly be mistaken in the matter4 ,in fact1 a geometer will tell us that if one end of an ellipse was to go further and further away1 the end that stayed with us would gradually become more and more like this curve4 ,therefore1 some of those comets which seem to move in parabolas may really be moving in extremely elongated ellipses1 and thus1 after excessively long periods of time1 may come back to revisit us4 ,,the ,,materials ,,of ,a ,,comet4 ,a comet is made of very unsubstantial material4 ,this we can show in a very interesting manner1 when we see it moving over the sky between the earth and the stars4 ,sometimes a comet will pass over a cluster of very small stars1 so faint that the very lightest cloud that is ever in the sky would be quite sufficient to hide them4 ,yet the stars are distinctly visible right through the comet1 notwithstanding that it may be hundreds of thousands of miles thick4 ,this shows us how excessively flimsy is the substance of a comet1 for while a few feet of haze or mist suffice to extinguish the brightest of stars1 this immense curtain of comet stuff1 whatever it may be made of1 is practically transparent4 ,i have often told you that we are able to weigh the heavenly bodies1 but a comet gives us a great deal of trouble4 ,you see that the weighing machine must be of a very delicate kind if you are going to weigh a very light object4 ,take1 for example1 a little lock of golden hair1 which no doubt has generally a value quite independent of the number of grains that it contains4 ,suppose1 however1 that we are so curious as to desire to know its weight1 then one of those beautiful balances in our laboratories will tell us4 ,in fact1 if you snipped a little fragment from a single hair1 the balance would be sensitive enough to weigh it4 ,if1 however1 you were only provided with a common pair of scales like those which are suited for the parcel post1 then you could never weigh anything so light as a lock of hair4 ,you have not small enough weights to begin with1 and even if you had they would be of no use1 for the scale is too coarse to estimate such a trifle4 ,this is precisely the sort of difficulty we experience when we try to weigh a comet4 ,the body1 though so big1 is very light1 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 balance4 ,we cannot always find suitable scales for weighing celestial bodies4 ,we have to use for the purpose whatever methods of discovering the weights happen to be available4 ,so far1 the methods ,i have mentioned are of the rudest description2 they serve well enough for weighing heavy masses like planets1 but they will not do for such unsubstantial bodies as comets4 ,but1 though we fail in this endeavor1 i4e4 to weigh comets1 yet skilful astronomers have succeeded in something which at first you might think to be almost impossible4 ,they have actually been able to discover some of the ingredients of which a comet is made4 ,this is so important a subject that ,i must explain it fully4 ,the most instructive comet which we have seen in modern days is that which appeared in the year #ahhb4 ,it was an object so great that its tail alone was double as long as from the earth to the sun4 ,it was discovered at the observatories in the southern hemisphere early in ,september of that year4 ,a little later it was observed in the northern hemisphere in extraordinary circumstances4 ,it must be remembered that a comet is generally a faint object1 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 day4 ,for a comet to be seen in daylight was indeed an unusual occurrence2 but on the forenoon of ,sunday1 ,september #ag1 ,mr4 ,common at ,ealing saw a great comet close to the sun4 ,unfortunately clouds intervened1 and he was prevented from observing the critical occurrence just approaching4 ,an astronomer at the ,cape of ,good ,hope--,mr4 ,finlay--who had also been one of the earliest discoverers of the comet1 was watching the body on the same day4 ,he followed it as it advanced close up to the sun2 bright indeed must that comet have been which permitted such a wonderful observation4 ,at the sun's edge the comet disappeared instantly2 in fact1 the observers thought that it must have gone behind the sun4 ,they could not otherwise account for the suddenness with which it vanished4 ,this was not what really happened4 ,it was afterwards ascertained that the comet had not passed behind the sun2 it had1 indeed1 come between us and our luminary4 ,in its further progress this body showed in a striking degree the incoherent nature of the materials of which a comet is composed4 ,it seemed to throw off portions of its mass along its track1 each of which continued an independent journey4 ,even the central part in the head of the comet--the nucleus1 as it is called--showed itself to be of a widely different nature from a substantial planetary body4 ,the nucleus divided into two1 three1 four1 or even five distinct parts1 which seemed1 in the words of one observer1 to be connected together like pearls on a string4 ,the comet of #ahhb was also very instructive with regard to the actual materials from which such bodies are made4 ,astronomers have a beautiful method by which they find out the substances present in a heavenly body1 even though they never can get a specimen of the body into their hands4 ,we know at least three materials which were present in this comet4 ,the first of them is an ingredient which is very commonly found in comets--a chemist calls it carbon4 ,it is an extremely familiar material on the earth2 for instance1 coal is chiefly composed of carbon4 ,charcoal when burned leaves only a few ashes4 ,all the substance that has vanished during combustion is carbon2 in fact1 it is not too much to say that carbon is found abundantly not only in wood1 but in almost every form of vegetable matter4 ,the food we eat contains abundant carbon1 and it is an important constituent in the building up of our own bodies4 ,generally speaking1 carbon is not found in a pure state--it is associated with other substances4 ,soot and lampblack are largely composed of it2 but the purest form of this element carbon that we know is the diamond4 ,it is interesting to note that carbon is certainly found as a frequent constituent of comets4 ,the great comet of #ahhb undoubtedly contained it1 as well as certain other substances4 ,of these we know two3 the first is the element sodium1 an extremely abundant material on earth1 inasmuch as the salt in the sea is mainly composed of it4 ,it was also discovered that the same great comet contained another substance very common here and extremely useful to mankind4 ,dr4 ,copeland and ,dr4 ,lohse at ,dunecht showed that iron was present in this body which had come in to visit us from the depths of space4 ,these discoveries are especially interesting because they seem to show the uniformity of material composing our system4 ,we already knew that sodium and iron abounded in the sun1 and now we have learned that these bodies and carbon as well are present in the comets4 ,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 system4 ,comets have such a capricious habit of dashing into the solar system at any time and from any direction1 that it is worth while asking whether a comet might not sometimes happen to come into collision with the earth4 ,there is nothing impossible in such an occurrence4 ,there is1 however1 no reason to apprehend that any disastrous consequences would ensue to the earth4 ,man has lived on this globe for many1 many thousands of years1 and the rocks are full of the remains of fossil animals which have flourished during past ages2 indeed1 we cannot possibly estimate the number of millions of years that have elapsed since living things first crawled about this globe4 ,there has never been any complete break in the succession of life1 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 produced1 and we need not apprehend any such catastrophe in the future4 ,i do not mean1 however1 that harmless collisions with comets may not have occasionally happened2 in fact1 there is good reason for knowing that they have actually taken place4 ,in the year #ahfa a fine comet appeared2 and it is not so well remembered as its merits deserve1 because it happened1 unfortunately for its own renown1 to appear just three years after the comet of #aheh1 which was one of the most gorgeous objects of this kind in modern times4 ,but in #ahfa we had a novel experience4 ,on a ,sunday evening in midsummer of that year1 we dashed into the comet1 or it dashed into us4 ,we were not1 it is true1 in collision with its densest part2 it was rather the end of the tail which we encountered4 ,there were1 fortunately1 no very serious results4 ,indeed1 most of us never knew that anything had happened at all1 and the rest only learned of the accident long after it was all over4 ,for a couple of hours that night it would seem that we were actually in the tail of the comet1 but so far as ,i know no one was injured or experienced any alarming inconvenience4 ,indeed1 ,i have only heard of one calamity arising from the collision4 ,a clergyman tells us that at midsummer he was always able in ordinary years to read his sermon at evening service without artificial light4 ,on this particular occasion1 however1 the sky was overcast with a peculiar glow1 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 sermon4 ,the expense of those candles was1 ,i believe1 the only loss to the earth in consequence of its collision with the comet of #ahfa4 ,illustration3 ,,fig4 #gf4--;,how the ,tail of a ,comet arises4 ,the tail of a comet appears to develop under the influence of the sun4 ,as the wandering body approaches the source of central heat it grows warm1 and as it gets closer and closer to the sun1 the fervor becomes greater and greater1 until sometimes the comet experiences a heat more violent than any we could produce in our furnaces4 ,the most infusible substances1 such as stones or earth1 would be heated white-hot and melted1 and even driven off into vapor1 under the intense heat to which a comet is sometimes exposed4 ,comets1 indeed1 have been known to sweep round the sun so closely as to pass within a seventh part of its radius from the surface4 ,it seems that certain materials present in the comet1 when heated to this extraordinary temperature1 are driven away from the head1 and thus form the tail 7,fig4 #gf74 ,hence we see that the tail consists of a stream of vaporous particles1 dashing away from the sun as if the heat which had called them into being was a torment from which they were endeavoring to escape4 ,the tail of a comet is1 therefore1 not a permanent part of the body4 ,it is more like the smoke from a great chimney4 ,the smoke is being incessantly renewed at one end as the column gets dispersed into the air at the other4 ,as the comet retreats1 the sun's heat loses its power4 ,in the chills of space there is1 therefore1 no tail-making in progress1 while the small mass of the comet renders it unable to gather back again by its attraction the materials which have been expelled4 ,should it happen that the comet moves in an elliptic orbit1 and thus comes back time after time to be invigorated by a good roasting from the sun1 it will1 of course1 endeavor to manufacture a tail each time that it approaches the source of heat4 ,the quantity of material available for the formation of tails is limited to the amount with which the comet originally started2 no fresh supply can be added4 ,if1 therefore1 the comet expends a portion of this every time it comes round1 an inevitable consequence seems to follow4 ,suppose a boy receives a sovereign when he goes back to school1 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 conjecture1 ,i need not tell you that before long the sovereign will have totally vanished4 ,in a similar way comets cannot escape the natural consequences of their extravagance2 their store of tail-making substance must1 therefore1 gradually diminish4 ,at each successive return the tails produced must generally decline in size and magnificence1 until at last the necessary materials have been all squandered1 and we have the pitiful spectacle of a comet without any tail at all4 ,the gigantic size of comets must excite our astonishment4 ,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 bodies4 ,but we now pass by a sudden transition to speak of the very smallest bodies1 of little objects so minute that you could carry them in your waistcoat pocket4 ,you will perhaps be surprised that such things can play an important part in our system and have a momentous connection with mighty comets4 ,,meteors4 ,if you look out from your window at the midnight sky1 or take a walk on a fine clear night1 you will occasionally see a streak of light dash over the heavens1 thus forming what is called a falling1 or a shooting1 star 7,fig4 #gg74 ,it is not really one of the regular stars that has darted from its place4 ,the objects we are now talking of are quite different from stars proper4 ,to begin with1 the shooting stars are comparatively close to us when we see them1 and they are very small1 whereas the stars themselves are enormous globes1 far bigger than our earth1 or often even bigger than the sun4 ,sometimes a great shooting star is seen which makes a tremendous blaze of light as bright as the moon1 or even brighter still4 ,these objects we call meteors1 and you will be very fortunate if you can ever see a really fine one4 ,astronomers cannot predict these things as they predict the appearance of the planets4 ,bright meteors consequently appear quite unexpectedly1 and it is a matter of chance as to who shall enjoy the privilege of beholding them4 ,but it is not about the great meteors that we are now going to speak particularly2 they are often not so interesting as the small ones4 ,illustration3 ,,fig4 #gg4--;,a ,brilliant ,meteor4 ,these little meteoroids1 as we shall call them1 have a curious history4 ,they become visible to us only at the very last moment of their existence--in fact1 the streak of light which forms a shooting star is merely the destruction of a meteoroid4 ,you must always remember that we are here living at the bottom of a great ocean of air1 and above the air extends the empty space4 ,air is a great impediment to motion2 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 through4 ,the faster the speed1 the greater is the tax which the air imposes on the moving body4 ,a cannon-ball1 for instance1 loses an immensity of its speed1 and consequently of its power1 by having to bore its way through the air4 ,in outer space beyond the limits of this atmosphere1 a freedom of movement can be enjoyed of which we know nothing down here4 ,i spoke of this when discussing the movements of ,encke's comet4 ,even this very unsubstantial body could dash along without appreciable resistance until it traversed the atmosphere surrounding the sun4 ,but now we have to speak of the motion of a little object both small and dense1 resembling perhaps a pebble or a fragment of iron1 or some substance of that description4 ,it is a little body such as this which produces a shooting star4 ,for ages and ages the meteoroid has been moving freely through space4 ,the speed with which it dashes along greatly exceeds that of any of the motions with which we are familiar4 ,it is about #ajj times as swift as the pace of a rifle-bullet4 ,about twenty miles would be covered in a second4 ,you can hardly imagine what that speed is capable of4 ,suppose that you put one of these flying meteoroids beside an express train to race from ,london to ,edinburgh1 the meteoroid would have won the race before the train had got out of the station4 ,or suppose that a shooting star determined to make the circuit of the earth1 it might1 so far as pace is concerned1 go entirely around the globe and back to the point from which it started in a little more than twenty minutes4 ,but the fact is1 you could not make any object down here move as fast as a shooting star4 ,no gunpowder that could be made would be strong enough1 in the first place1 and even if the body could once receive the speed1 it would never be able to force its way through the air uninjured4 ,so long as a little shooting star is tearing away through open space we are not able to see it4 ,the largest telescope in the world would not reveal a glimpse of anything so small4 ,the meteoroid has no light of its own1 and it is not big enough to exhibit the light reflected from the sun in the same manner as a little planet would do4 ,it is only at the moment when it begins to be destroyed that its visibility commences4 ,if the little object can succeed in dashing past our earth without becoming entangled in the atmosphere1 then it will pursue its track with perhaps only a slight alteration in its path1 due to the pull exercised by the earth4 ,the air which surrounds our globe may be likened to a vast net1 in which if any little meteor becomes caught its career is over4 ,for when the little body1 after rejoicing in the freedom of open space1 dashes into air1 immediately it experiences a terrific resistance2 it has to force the particles of air out of the way so as to make room for itself1 and in doing so it rubs against them with such vehemence that heat is produced4 ,i am sure every boy knows that if he rubs a button upon a board it becomes very hot1 in consequence of the friction4 ,there are many other ways in which we can illustrate the production of heat in the same manner4 ,one is a contrivance by which we spin round rapidly a piece of stick pressed against a board4 ,quantities of heat are thus produced by the friction1 and volumes of smoke rise up4 ,we have read how some savages are able to produce fire by means of friction in a somewhat similar manner1 but to do so requires a rare amount of skill and patience4 ,there is another illustration by which to show how heat can be produced by friction4 ,a brass tube full of water is so arranged that it can be turned around very rapidly by the whirling table4 ,we apply pressure to the tube1 and after a minute or two the water begins to get hot1 and then at last to boil1 until ultimately the cork is driven out and a diminutive and1 fortunately1 harmless explosion of the friction boiler takes place4 ,engineers are aware how frequently heat is produced by friction1 when it is very inconvenient or dangerous4 ,indeed1 unless the wheels of railway carriages are kept well greased1 the rubbing of the axle may generate so much heat that conflagrations in the carriage will ensue4 ,nature1 in the little shooting star1 gives us a striking illustration of the same fact4 ,perhaps you may be surprised to hear that the whole brilliancy of the shooting star is simply due to friction4 ,as the little body dashes through the air it becomes first red-hot1 then white-hot1 until at last it is melted and turned into vapor4 ,thus is formed that glowing streak which we1 standing very many miles below1 see as a shooting star4 ,a bullet when fired from a rifle will fly into pieces after it has struck against the target1 and if you quickly pick up one of these pieces you will generally find it quite hot4 ,whence comes this heat8 ,the bullet1 of course1 was cold before the rifleman pulled the trigger4 ,no doubt there was a considerable amount of heat developed by the burning of the gunpowder1 but the bullet was so short a time in contact with the wad1 through which so little heat would pass1 that we must look to some other source for the warmth that has been acquired4 ,friction against the barrel as the bullet passed to the mouth must have warmed the missile a good deal1 and when rubbing against the air the same influence must have added still further to its temperature1 while the blow against the target would finally warm it yet more4 ,in comparing the shooting star with the rifle-bullet we must remember that the celestial object is travelling with a pace #ajj times as swift as the utmost velocity that the rifle can produce1 and the heat which is generated by friction is increased in still greater proportion4 ,if we double the speed1 we shall increase the quantity of heat by friction fourfold2 if we increase the speed three times1 then friction will be capable of producing nine times as much heat4 ,in fact1 we must multiply the number expressing the relative speed by itself--that is1 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 aid4 ,the shooting star may have a pace #ajj times that of the rifle-bullet1 and if we multiply #ajj by #ajj we get #aj1jjj2 consequently we see that the heat produced by the shooting star before its motion was arrested in dashing through the air would be #aj1jjj times that gained by the rifle-bullet in its flight4 ,if the temperature of the rifle-bullet only rose a single degree by friction1 it would thus be possible for the shooting star to gain #aj1jjj degrees1 and this would be enough to melt and boil away any object which ever existed4 ,thus we need not be surprised that friction through the air1 and friction alone1 has proved an adequate cause for the production of all the heat necessary to account for the most brilliant of meteors4 ,it is rather fortunate for us that the meteors do dash in with this frightful speed2 had the little bodies only moved as quickly as a rifle-bullet1 or even only four or five times as fast1 they would have pelted down on the earth in solid form4 ,indeed1 on rare occasions it does happen that bodies from the heavens strike down on the ground4 ,the great majority of those that fall on the ground1 however1 become entirely transformed into harmless vapor4 ,the earth would1 indeed1 be almost uninhabitable from this cause alone were it not for the protection that the air affords us4 ,all day and all night innumerable missiles would be whizzing about us1 and though many of them are undoubtedly very small1 yet as their speed is #ajj times that of a rifle-bullet1 the fusillade would be very unpleasant4 ,it is1 indeed1 the intense hurry of these celestial bullets to get at us which is the very source of our safety4 ,it dissipates the meteors into streaks of harmless vapor4 ,,what ,,becomes ,,of ,,the ,,shooting ,,stars4 ,when we throw a lump of coal on the fire1 all that is soon left is a little pinch of ashes1 and the rest of the coal has vanished4 ,you might think it had been altogether annihilated1 but that is not nature's way4 ,nothing is ever completely destroyed2 it is merely transformed or changed into something else4 ,the greater part of the coal has united with the oxygen which it has obtained from the air1 and has formed a new gas1 which has ascended the chimney4 ,every particle that was in the coal can be thus accounted for1 and in the act of transformation heat is given out4 ,a meteor also becomes transformed1 but the substance it contains is not lost1 though it is changed into glowing vapors4 ,it is known that with heat enough any substance can be turned into vapor1 just as water can be boiled into steam4 ,look at an electric light flashing between two pieces of carbon4 ,though carbon is one of the most difficult substances to melt1 yet such is the intense heat generated by the electric current that the carbon is not only melted1 but is actually turned into a vapor1 and it is this vapor glowing with heat that gives us the brilliant light4 ,in a similar manner iron can be rendered red-hot1 white-hot1 and then boiled and transformed into an iron vapor1 if we may so call it4 ,there is1 indeed1 plenty of such iron vapor in the universe4 ,quantities of it surround the sun and some of the stars4 ,when ordinary steam is chilled it condenses into little drops of water4 ,so1 too1 if iron be heated until it is transformed into vapor1 and if that vapor be allowed to condense1 it will ultimately form a dust1 consisting of bits of iron so small that you would require a microscope to examine them4 ,there is iron present in the small shooting stars4 ,other substances are also contained therein1 and all these materials1 after being boiled by the intense heat1 are transformed into vapor4 ,when the heat subsides1 the vapor condenses again into fine dust1 so that the ultimate effect of the atmosphere on a shooting star is to grind the little object into excessively fine powder1 which is scattered along the track which the object has pursued4 ,sometimes this powder will continue to glow for minutes after the meteor has vanished1 and in the case of some great meteors this stream of luminous dust in the air forms a very striking spectacle4 ,a great meteor1 or fire-ball as it is often called1 appeared on the #fth of ,november1 #ahfi4 ,it flew over ,devonshire and ,cornwall1 and left a track fifty miles long and four miles wide4 ,the dust remained visible all along the great highway for nearly an hour2 it formed a glowing cloud hanging in the sky1 and though originally nearly straight1 it became bent and twisted by the winds before it finally disappeared from view4 ,we have now to see what becomes of this meteoric dust which is being incessantly poured into the air from external space4 ,none of it ever gets away again2 for whenever an unfortunate meteor just touches the air it is inevitably captured and pulverized4 ,that dust subsides slowly1 but we do not find it easy to distinguish the particles which have come from the shooting stars1 because there is so much floating dust which has come from other sources4 ,a sunbeam is the prettiest way of revealing the existence of the motes with which the air is charged4 ,the sunbeam renders these motes visible exactly in the same way as planets become visible when sunbeams fall on them in far-distant space4 ,but if we have not the sunbeams here1 we can throw across the room a beam of electric light1 and it is seen glowing all along its track1 simply because the air of the room1 like air everywhere1 is charged with myriads of small floating particles4 ,if you hold the flame of a spirit-lamp beneath this beam1 you will see what seems like columns of black smoke ascending through it4 ,but these columns are not smoke1 they are pure air1 or rather air in which the solid particles have been transformed into vapor by the heat from the spirit-flame4 ,the motes abound everywhere in the air4 ,we take thousands of them into our lungs every time we breathe4 ,they are on the whole gradually sinking and subsiding downwards1 but they yield to every slightest current1 so that when looking at a sunbeam you will find them moving in all directions4 ,it is sometimes hard to believe that the little objects are tending downwards1 but if you look on the top of a book that has lain for a time on a book-shelf1 you find there a quantity of dust1 produced by the motes which have gradually subsided where they found a quiet spot and were allowed sufficient time to do so4 ,the great majority of these particles consist1 no doubt1 of fragments of terrestrial objects4 ,the dust from the roads1 the smoke from the factories1 and numerous other sources1 are incessantly adding their objectionable particles to the air4 ,there can be no doubt that the shooting stars also contribute their mites to the dust with which the atmosphere is ever charged4 ,the motes in the murky air of our towns have no doubt chiefly originated from sources on this earth4 ,many of these sources it would be impossible to regard as of a romantic description4 ,we may1 however1 feel confident that among those teeming myriads of small floating objects are many little particles which1 having had their origin from shooting stars1 are now gradually sinking to the earth4 ,this is not a mere surmise1 for dust has been collected from lofty ,alpine snows1 from the depths of the sea1 and from other localities far removed from the haunts of men4 ,from such collections1 tiny particles of iron have been obtained1 which have evidently been once in a molten condition4 ,there is no conceivable explanation for the origin of iron fragments in such situations1 except that they have been dropped from shooting stars4 ,i am sure you have often helped in the making of a gigantic snowball4 ,you begin with a small quantity of snow that can be worked with your hands4 ,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 you1 until at last it has grown so unwieldy that you can move it no longer1 and then you apply your artistic powers to carving out a statue4 ,the snowball has grown by the addition of material to it from without1 and as it became heavier and heavier1 it lapped up more and more of the snow as it rolled along2 so that with each increase of size1 its capacity for becoming still larger has also increased4 ,i want to liken our earth to a snowball which goes rolling on through space1 and every day1 every hour1 every minute1 is gathering up and taking into it the little shooting stars that it meets with on its way4 ,no doubt the annual accumulation is a very small quantity when compared with the whole size of the earth2 but the earth is always drawing in1 and now1 at all events1 never giving back again2 so that when this process is carried on long enough1 astonishing results may be obtained4 ,you have all heard many maxims on this subject--how every little saving will at length reach a respectable or a gigantic total4 ,nature abounds with illustrations of the principle4 ,all the water that thunders over ,niagara is merely a sufficient number of little drops of rain collected together4 ,our earth has been gradually hoarding up1 during countless ages1 all the meteor dust that has rained upon it2 and the larger the earth grows1 the bigger is the net which it spreads1 and the greater is the power it has to capture the wandering bodies4 ,thus1 our earth1 ages and ages ago1 may have been considerably smaller than it is at present2 in fact1 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 surface4 ,,grand ,,meteors4 ,i dare say that many of those present will1 in the course of their lives1 have opportunities of seeing some of the great meteors1 or fire-balls1 which are occasionally displayed4 ,generally speaking1 about one hundred or so of these splendid objects are recorded every year4 ,we are never apprised that they are coming2 they take us unawares1 and therefore we have no opportunity to make proper arrangements for seeing them4 ,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 observations4 ,illustration3 ,,fig4 #gh4--;,how to find the ,height of a ,meteor4 ,the chief point to determine is the height of the meteor above the earth4 ,for this we must have two observations at least1 made in places as far asunder as possible4 ,suppose an observer at ,london and an observer at ,york were both witnesses of a splendid meteor2 if they find1 on subsequent comparison1 that their observations were made at the same moment1 there is no reasonable doubt that it was the same object they both saw4 ,the observer at ,york describes the meteor as lying to the south1 halfway down from the point directly over his head towards the horizon4 ,the ,london observer speaks of the meteor as being to the north2 and to him also it appeared that the object was halfway down towards his horizon from the point directly over his head4 ,if you know a little ,euclid1 you can easily show from these facts that the height of the meteor must have been half the distance between ,london and ,york1 that is1 #he miles 7,fig4 #gh74 ,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 observations4 ,the principle is1 however1 the same--that whenever from two sufficiently distant positions the direction of the meteor has been observed1 its path is known--just as on p4 #ba we showed how the height of the suspended ball was obtained from observations at each end of the table4 ,generally speaking1 bright meteors begin at an elevation of between fifty and one hundred miles1 and they become extinguished before they are within twenty miles of the ground4 ,sometimes a tremendous explosion will take place during the passage of a meteor through the air4 ,there was a celebrated instance in ,america on the #bast of ,december1 #ahgf1 which will give an idea of one of these objects possessing exceptional magnificence4 ,it began in ,kansas about seventy-five miles high1 and thence it flew for a thousand miles at a speed of ten or fifteen miles a second1 until it disappeared somewhere near ,lake ,ontario4 ,over a certain region between ,chicago and ,st4 ,louis1 the great ball of fire burst into a number of pieces1 and formed a cluster of glowing stars that seemed to chase each other over the sky4 ,this cluster must have been about forty miles long and five miles wide1 and when the explosion occurred a most terrific noise was produced1 so loud that many thought it was an earthquake4 ,a remarkable circumstance illustrates the tremendous height at which this explosion occurred4 ,the meteor had burst into pieces1 the display was all over1 and was beginning to be forgotten1 and yet nothing had been heard4 ,it was not until a quarter of an hour after the explosion had been seen that a fearful crash was heard at ,bloomington4 ,the explosion actually occurred #ahj miles from the spot1 and as sound takes five seconds to travel a mile1 you can easily calculate that the noise required a quarter of an hour for its journey4 ,what a tremendous noise it must have been6 ,shooting stars are of every grade of brightness4 ,beginning with the more gorgeous objects which have been compared with the moon or even with the sun himself1 we descend to others as bright as ,venus or as ,jupiter2 others are as bright as stars of various degrees of brilliancy4 ,fainter shooting stars are much more numerous than the conspicuous ones2 in fact1 there are multitudes of these objects so extremely feeble that the unaided eye would not show them4 ,they only become perceptible in a telescope4 ,it is not uncommon while watching the heavens at night to notice a faint streak of light dashing across the field of the instrument4 ,this is a shooting star which is invisible except through the telescope4 ,,the ,,great ,,november ,,showers4 ,illustration3 ,,fig4 #gi4--;,a ,great ,shower of ,shooting ,stars4 ,occasionally we have the superb spectacle of a shower of shooting stars4 ,none of you1 my young friends1 can as yet have had the good fortune to witness one of the specially grand displays1 but you may live in hope2 there are still showers to come4 ,astronomers have ventured on the prophecy that in or about the year #ahii you will have the opportunity of seeing a magnificent exhibition of this kind4 ,there is only one ground for anxiety1 and that is as to whether the clouds will keep out of the way for the occasion4 ,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 prediction4 ,the last great shooting-star shower took place in the year #ahff1 or1 perhaps1 ,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 speak4 ,on the night of the #acth of ,november1 #ahff1 astronomers were everywhere delighted by a superb spectacle4 ,enjoyment of the wondrous sight was not only for astronomers4 ,every one who loves to see the great sights of nature will have good reason for remembering that night4 ,i certainly shall never forget it4 ,it was about ten o'clock when a brilliant meteor or two first flashed across the sky1 then presently they came in twos and threes1 and later on in dozens1 in scores1 in hundreds4 ,these meteors were brilliant objects1 any one of which would have extorted admiration on an ordinary night4 ,what1 then1 was the splendor of the display when they came on in multitudes8 ,for two or three hours the great shower lasted1 and then gradually subsided4 ,we were not taken unawares on this occasion1 for the shower was expected1 and had been1 in fact1 awaited with eager anticipation4 ,it should first be noticed that each year some shooting stars may always be looked for on or about the #acth of ,november4 ,every thirty-three years1 or thereabouts1 the ordinary spectacle breaks out into a magnificent display4 ,it has also been found that for nearly #ajjj years there have been occasional grand showers of meteors at the time of year mentioned1 and all these incidents agree with the supposition that they are merely repetitions of the regular thirty-three-year shower4 ,the first was in the year ,a4,d4 #ijb1 which an old chronicle speaks of as the 8year of the stars10 from the extraordinary display which then took place4 ,i do not think the good people #ajjj years ago fully appreciated the astronomical interest of such spectacles2 in fact1 they were often frightened out of their wits1 and thought the end of the world had come4 ,doubtless many ancient showers have taken place of which we have no record whatever4 ,in more modern days we have had somewhat fuller information2 for example1 on the night between the #abth and #acth of ,november1 #ahcc1 a shower was magnificently seen in ,america4 ,mr4 ,kirkwood tells us that a gentleman of ,south ,carolina described the effect on the negroes of his plantation as follows3 8,i was suddenly awakened by the most distressing cries that ever fell on my ears4 ,shrieks of horror and cries for mercy ,i could hear from most of the negroes of the three plantations1 amounting in all to about #fjj or #hjj4 ,while earnestly listening for the cause1 ,i heard a faint voice near the door calling my name4 ,i arose1 and taking my sword1 stood at the door4 ,at this moment ,i heard the same voice still beseeching me to arise1 and crying out that the world was on fire4 ,i then opened the door1 and it is difficult to say which excited me the most--the awfulness of the scene or the distressed cries of the negroes4 ,upwards of a hundred lay prostrate on the ground1 some speechless1 and some with the bitterest cries1 but with their hands raised praying for mercy4 ,the scene was truly awful1 for never did rain fall much thicker than the meteors fell towards the earth40 ,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 years2 and when it was remembered that the last great shower was in #ahcc it was confidently expected that another similar display would take place in #ahff4 ,this was fully confirmed4 ,yet another thirty-three years brings us to #ahii1 when we have good reason for looking forward to a grand shower of these bodies4 ,it may be expected to occur on ,november #ad or ,november #ae4 ,it may1 however1 possibly be that a shower will occur on the same days of the succeeding year4 ,illustration3 ,,fig4 #hj4--,the ,earth crossing the ,track of ,meteors4 ,we know a good deal now as regards the movements of these little objects4 ,i want you to think of a vast swarm1 something like a flock of birds1 which ,i dare say you have often seen flying high in the air2 the difference1 however1 is that the flock of meteors is enormously greater than any flock of birds ever was2 and the meteors1 too1 are scattered so widely apart1 that each one may be miles away from its next neighbors4 ,usually the meteoric shoal is many millions of miles long1 and perhaps a hundred thousand miles in width4 ,the great flock of meteors travels through space in a certain definite track4 ,we have learned how the sun guides a planet1 and forces the planet to move around him in an ellipse4 ,but our sun will also condescend to guide an object no bigger than a shooting star4 ,a bullet1 a pea1 or even a grain of sand will be held to an elliptic course around the sun as carefully as the great ,jupiter himself4 ,the entire shoal of meteors may therefore pursue their common journey around the sun as if inspired by a common purpose1 each individual member of the host being1 however1 guided by the sun1 and performing its path in real independence of its neighbors4 ,the orbit followed by this shoal of meteors is enormously large and wide4 ,here is a sketch of the path 7,fig4 #hj71 and ,i have laid down the position of the orbit of the earth1 but not on the same scale4 ,the ellipse is elongated1 so that while the shoal approaches comparatively close to the sun at one end of its journey1 at the other end it goes out to an enormous distance1 far beyond the orbit of the earth--beyond1 indeed1 the orbit of ,jupiter or ,saturn2 in fact1 it reaches to the path of ,uranus4 ,to accomplish so vast a journey as this thirty-three years and a quarter are required1 and now you will be easily able to see why we get periodical visits from the shoal4 ,it is1 however1 a mere piece of good fortune that we ever encounter the ,november meteors4 ,probably there are numerous other shoals of meteors quite as important which we never see1 just in the same way as there are many shoals of fish in the sea that never come into our net4 ,the earth moves round the sun in a path which is very nearly a circle1 and the shoal moves round in this long oval4 ,we cannot easily represent the true state of things by mere diagrams which show all these objects on the same plane4 ,this does not give an accurate representation of the orbits4 ,i think you will better understand what ,i mean by means of some wire rings4 ,make a round one to represent the path of the earth1 and a long oval one to represent the path of the meteors4 ,there is to be a small opening in the circular ring so that we can slip one of the orbits inside the other4 ,if we are to see the meteors1 it is of course necessary that they should strike the earth's atmosphere1 for they are not visible to us when they lie at a distance like the moon or like the planets4 ,it is necessary that there be a collision between the earth and the shoal of meteors4 ,but there never could be a collision between two trains unless the lines on which these trains run meet each other2 therefore1 it is necessary that this long ellipse shall actually cross the earth's track2 it will not do to have it pass inside like the two links of a chain2 our earth would then miss the meteors altogether1 and we should never see them4 ,there are very likely many of such shoals of meteors revolving in this way1 and thus escaping our notice entirely4 ,you will also understand why there is no use in looking for these showers except on a particular day of ,november4 ,on that day1 and on that day alone1 the earth appears at that particular point of its route where the latter crosses the track of the shoal4 ,on the #ast of ,november1 for instance1 the earth has not yet reached the point where it could meet with these bodies4 ,by the end of ,november it has passed too far4 ,but even supposing that the earth is crossing the track of the meteors on the #acth of ,november1 it is still possible that only a few1 or none at all1 shall be seen4 ,the shoal may not happen to be at that spot at the right time4 ,for a display of meteors to occur1 it is therefore necessary that the shoal shall happen to be passing this particular stage of the journey on the #acth of ,november4 ,in #ahff the earth dipped through the shoal and caught a great many of these meteors in its net4 ,for a few hours the earth was engaged in the capture1 until it emerged on the other side of the shoal1 and the display was at an end4 ,sometimes it happens that in two years following each other1 grand showers of meteors are seen4 ,the reason of this is that the shoal is very long and thin1 and consequently if the earth passes through the beginning of the shoal one year1 it may have returned to the same point next year before the whole length of the shoal has completely passed4 ,in this case we shall have two great showers in consecutive years4 ,thus a very fine display was seen in ,america on the proper day in #ahfg1 while many stragglers were also observed during the three subsequent recurrences of the same date4 ,whenever the #acth of ,november comes round we generally meet with at least five shooting stars belonging to this same system1 and we must explain how this occurs4 ,suppose there is a small racecourse so that the competitors will have to run a great many times round before the race is over4 ,let there be a very large number of entries1 and let the majority of the athletes be fairly good runners1 while a few are exceptionally good with varying degrees of excellence1 and a few are very bad1 some being worse than others4 ,the whole group starts together in a cluster at the signal1 and perhaps for the first round or two they may keep tolerably well together4 ,it will be noticed the cluster begins to elongate as one circuit after another is made2 the better runners draw out to the front1 and the slower runners lag further and further behind2 at last it may happen that those at the head will have gained a whole round on those at the tail1 while the other runners of varying degrees of speed will be scattered all round the course4 ,the majority of the runners1 if of nearly equal speed1 may continue in a pretty dense group4 ,precisely similar has been the great celestial race which these meteors are running4 ,they started on their grand career centuries ago1 and ever since then they have been flying round and round their mighty course4 ,the greater proportion of the meteors still stay close together1 and their pace is nearly uniform4 ,the exceptionally smart ones have shot ahead1 the exceptionally slow ones have lagged behind1 and thus it happens that1 after fifty or more revolutions have been completed1 the shape of the original swarm has become considerably modified4 ,its length has been drawn out1 while the stragglers and the fastest runners have been scattered all around the path4 ,across this course our earth carries us every ,november2 there we usually encounter some of the members of this swarm which have strayed from the great host2 they flash into the air1 and thus it is that some of these bodies are generally seen every ,november4 ,illustration3 ,,fig4 #ha4--,the ,radiant4 ,during a shooting-star shower it is interesting to notice that all the meteors seem to diverge from a single point4 ,in the adjoining figure 7,fig4 #ha71 which shows the directions of a number of meteors' tracks1 you will notice that every one seems to radiate from a certain point of the sky4 ,in the case of the shower of the #acth-#aeth of ,november this point lies in the constellation ,leo4 ,i must refer you to the ,appendix for a description of the way to find ,leo or the ,lion4 ,the radiant point1 as we term it1 of this system of meteors is there situated4 ,it is true that the meteors themselves do not generally seem to come all the way from this place4 ,it is the direction of their luminous trails produced backward that carries the eye to the radiant 7,fig4 #ha74 ,if a meteor were actually seen there1 it would be certainly coming straight towards us2 it would not then appear as a streak of light at all2 it would merely seem like a star which suddenly blazed into splendor and then again sank down into invisibility4 ,every meteor which appeared near this point would be directed very nearly at the observer1 and its path would therefore seem very much foreshortened4 ,i can illustrate this with a long straight rod4 ,if ,i point it directly at you1 you can only see the end4 ,if ,i point it nearly at you1 it will seem very much shortened4 ,during the great shower in #ahff many of the meteors could be observed so close to the radiant in ,leo that they seemed merely like very short marks in the sky2 some of them1 indeed1 seemed to be merely starlike points swelling up into brilliance and then going out4 ,hence it is that we call this system of shooting stars the 8,leonids40 ,they bear this name because their radiant lies in the constellation ,leo1 and unless the direction of a shooting star emanates from this point it does not belong to the ,leonids4 ,even if it did so1 the meteor would not be a ,leonid unless the date was right1 namely1 on the #acth of ,november1 or within a day thereof4 ,we thus have two characteristics which belong to a system of shooting stars2 there is the date on which they occur and the point from which they radiate4 ,,other ,,great ,,showers4 ,to illustrate what ,i have said1 we will speak about another system of shooting stars2 they are due every ,august1 from the #ith to the #aath1 and their directions diverge from a point in the constellation of ,perseus4 ,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 spoken1 by quoting the following production3-- ,if you ,november's stars would see1 ,about the fourteenth watching be4 ,in ,august1 too1 stars shine through heaven1 ,on nights between nine and eleven4 ,it may be worth your while to remember these lines1 and always to keep a look-out on the days named4 ,the ,august meteors1 the ,perseids we often call them1 do not make gorgeous displays1 in particular years1 with the regularity of the ,leonids4 ,there have been1 no doubt1 some exceptionally grand showers between the #ith and the #aath of ,august1 but we cannot predict when the next splendid one is due4 ,there are vast numbers of stragglers all round the track of the ,perseids4 ,in fact1 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 up1 and that a large proportion of the meteors were now scattered the whole way around the course with tolerable uniformity4 ,this being so1 it follows that every time we cross the track we are nearly certain to fall in with a few of the stragglers1 though we may never enjoy the tremendous spectacle of a plunge through a dense host of meteoroids4 ,there are many other showers besides the two ,i have mentioned4 ,some shooting stars are to be seen almost every fine night1 and those astronomers who pay particular attention to this subject are able to make out scores of small showers which might not interest you4 ,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 radiate4 ,of these ,i must mention one4 ,it is not usually very attractive1 but it has a particular interest1 as ,i shall now explain4 ,on the #bgth of ,november1 #ahgb1 a beautiful meteoric shower took place4 ,you will notice that though the month is the same1 the day is entirely different from that on which the ,leonids appear4 ,this shower of the #bgth is called the ,andromedes1 because the lines of direction of the shooting stars of which it is composed seem to diverge from a point in the constellation ,andromeda4 ,ordinarily speaking1 there is no special display of meteors connected with the annual return of this day2 but in #ahgb astronomers were astonished by an exhibition of shooting stars belonging to this system4 ,they were not at all bright when compared with the ,leonid meteors4 ,they were1 however1 sufficiently numerous to arrest the attention of very many1 even among those who do not usually pay much attention to the heavens4 ,the chief interest of the shower of ,andromedes centers in a remarkable discovery connecting meteors and comets4 ,there is a comet which was discovered by the astronomer ,biela4 ,it is a small object1 requiring a telescope to show it4 ,this comet completes each revolution in a period of about seven years2 or rather1 ,i should say1 that was the time which the comet used to spend on its journey1 for a life of trouble and disaster seems of late to have nearly extinguished the unfortunate object4 ,in #ahgb the comet was due in our neighborhood1 and on the night of the #bgth of ,november1 in the same year1 the earth crossed the track1 and1 in doing so1 the shower of shooting stars was seen4 ,this was a remarkable coincidence4 ,we crossed the path of the comet at the time when we knew the comet ought to be there2 and though we did not then see the comet1 we saw a shower of shooting stars1 and a wonderful shower too4 ,a circumstance so peculiar suggested at once that the comet and the shooting stars must in some way or other be connected together4 ,this is a suggestion we can test in another manner4 ,we know the history of the comet1 and we are aware that at the very time of the shower1 the comet was approaching from the direction of the constellation of ,andromeda4 ,it was coming1 in fact1 from the very quarter whence the shooting stars have themselves travelled4 ,taking all these things together1 it seems impossible to doubt that the shoal of shooting stars was1 if not actually the comet itself1 something closely connected with that famous body4 ,,meteorites4 ,some years ago1 a farmer living near ,rowton1 in ,shropshire1 noticed on a path in a field a hole which had been suddenly made by some mysterious and unknown agent4 ,the laborers who were near told him they had just heard a remarkable noise2 and when the farmer put his hand down into the hole1 he felt something hot at the bottom of it4 ,he took a spade and dug up the strange body1 and found it to be a piece of iron1 weighing about seven pounds4 ,he was naturally amazed at such an occurrence1 and brought the body home with him4 ,where did that piece of iron come from8 ,it is plain that it could not have been always in the ground4 ,the noise and the recently made hole showed that was not the case1 and that is confirmed by the fact that the iron was hot4 ,a piece of iron within a few feet of the earth's surface cannot have remained warm for any length of time4 ,it is therefore clear that the iron must have tumbled from the sky4 ,this is a marvellous notion2 in fact1 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 earth4 ,but they had to believe it2 the evidence was too conclusive4 ,fortunately1 however1 the occurrence is a comparatively rare one2 indeed1 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 occurrence4 ,we should want umbrellas of a more substantial description than those which suffice for the rains we actually experience4 ,there are1 indeed1 instances on record of persons having been killed by the fearful blows given by these bodies in falling4 ,the ,rowton siderite is a comparatively small one2 pieces weighing hundredweights1 and even tons1 have been collected together in our museums4 ,i would recommend you to pay a visit to that interesting room in our great ,british ,museum in which these meteorites are exhibited4 ,there we see actual specimens of celestial bodies which we can feel or weigh1 and which our chemists can analyze4 ,it may be noticed that they only contain substances that we already know on this earth4 ,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 iron4 ,nature seems to have taken heed of their wants1 and occasionally to have thrown down a lump or two for the benefit of those who were so fortunate as to secure them4 ,that these stones or irons drop from the sky is absolutely certain1 but when we try to find out their earlier history we become involved in not a few difficulties4 ,nobody really knows the true history of these objects1 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 ,theory4 ,i use this expression because every boy or girl listening to me ought to have read ,jules ,verne's wonderful book1 8,from the ,earth to the ,moon10 and if any of you have not read it1 the sooner you do so the better4 ,it is there narrated how the gun club of ,baltimore designed a magnificent cannon which was sunk deep into the ground1 and then received a terrific charge of guncotton1 on which a great hollow projectile was carefully lowered1 containing inside the three adventurous explorers who desired to visit the moon4 ,calculations were produced with a view of showing that by firing on a particular day the explosion would drive the projectile up to the moon4 ,there was1 however1 the necessary condition that the speed of projection should be great enough4 ,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 cannon1 then the missile would ascend up and up forever if no further influence were exerted on it4 ,no doubt we have to overlook the resistance on the air and a few other little difficulties1 but to this extent1 at all events1 the gun club were right3 that a velocity of about six or seven miles a second would suffice to carry a body away from the gravitation of the earth4 ,no one supposes that there were ever ,columbiad cannons on our globe by which projectiles were shot up into space2 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 cannon4 ,even now we have some active volcanoes of great energy on our earth1 and we know that in former days the volcanoes must have been still more powerful2 that1 in fact1 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 condition4 ,some of these early volcanoes1 in the throes of their mighty eruptions1 appear to have shot forth pieces of iron and volcanic substances with a violence great enough to carry them off into space4 ,suppose that a missile were projected upwards1 it would ascend higher and higher1 and gravity would1 of course1 tend to drag it back again down to earth4 ,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 attraction4 ,the subsequent history of such a projectile would be guided by the laws according to which a planet moves4 ,the body is understood to escape the destination which was aimed at by the ,columbiad4 ,i mean1 of course1 that it is not supposed to hit the moon4 ,of course1 this might conceivably happen2 but most of the projectiles would go quite wide of the mark1 and would travel off into space4 ,though the earth would be unable to recall the projectile1 the attraction of the sun would still guide it1 whether it was as big as a paving-stone or ever so much larger or smaller4 ,the body would be constrained to follow a path like a little planet around the sun4 ,this track it would steadily pursue for ages4 ,the wanderer would1 however1 cross the earth's track once during each of its revolutions at the point from which it was projected4 ,of course1 it will generally happen that the earth will not be there at the time the meteorite is crossing1 and the meteorite will not be there at the time the earth is crossing4 ,nothing will therefore happen1 and the object goes again on its long rounds4 ,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 air1 often producing a noise and generating a streak of light like a shooting star4 ,then it tumbles down1 and is restored to that earth whence it originally came4 ,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 follows4 ,many millions of years ago1 when the fires of our earth were much more vigorous than they are in these dull times1 a terrific volcanic outbreak took place1 and vast quantities of material were shot into space1 of which this is one of the fragments4 ,during all the ages that have since elapsed this piece of iron has followed its lonely track4 ,in a thousandth part of the time rust and decay would have destroyed it had it lain on the earth1 but in the solitudes of space there was found no air or damp to produce corrosion4 ,at last1 after the completion of its long travels1 it again crashed down on the earth4 ,we have now briefly surveyed the extent of the solar system4 ,we began with the sun1 which presides over all1 and then we discussed the various planets with their satellites1 next we considered the eccentric comets1 and finally the minute bodies which1 as shooting stars or meteorites1 must be regarded as forming part of the ,sun's system4 ,in our closing lecture we shall have to deal with objects of a far more magnificent character4 ,,lecture ,,vi4 ,,stars4 ,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 ,nebul--,what the ,nebul are made of--,photographing the ,nebul--,conclusion4 ,,we ,,try ,,to ,,make ,a ,,map4 ,the group of bodies which cluster around our sun forms a little island1 so to speak1 in the extent of infinite space4 ,we may illustrate this by a map in which we shall endeavor to show the stars placed at their proper relative distances4 ,we first open the compasses one inch1 and thus draw a little circle to represent the path of the earth4 ,we are not going to put in all the planets4 ,we take ,neptune1 the outermost1 at once4 ,to draw its path ,i open the compasses to thirty inches1 and draw a circle with that radius4 ,that will do for our solar system1 though the comets no doubt will roam beyond these limits4 ,to complete our map we ought of course to put in some stars4 ,there are a hundred million to choose from1 and we shall begin with the brightest4 ,it is often called the ,dog ,star1 but astronomers know it better as ,sirius4 ,let us see where it is to be placed on our map4 ,sirius is beyond ,neptune1 so it must be outside somewhere4 ,indeed1 it is a good deal further off than ,neptune2 so ,i try at the edge of the drawing-board2 ,i have got a method of making a little calculation that ,i do not intend to trouble you with1 but ,i can assure you that the results it leads me to are quite correct2 they show me that this board is not big enough4 ,but could a board which was big enough fit into this lecture theatre8 ,here1 again1 ,i make my little calculations1 and ,i find that there would not be room for a board sufficiently great2 in fact1 if ,i put the sun here at one end1 with its planets around it1 ,sirius would be too near on the same scale if it were at the further corner4 ,the board would have to go out through the wall of the theatre1 out through ,london4 ,indeed1 big as ,london is1 it would not be large enough to contain the drawing-board that ,i should require4 ,it would have to stretch about twenty miles from where we are now assembled4 ,we may therefore dismiss any hope of making a practical map of our system on this scale if ,sirius is to have its proper place4 ,let us1 then1 take some other star4 ,we shall naturally try with the nearest of all4 ,it is one that we do not know in this part of the world1 but those that live in the southern hemisphere are well acquainted with it4 ,the name of this star is ,alpha ,centauri4 ,even for this star1 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 inch4 ,you see what an isolated position our sun and his planets occupy4 ,the members of the family are all close together1 and the nearest neighbors are situated at enormous distances4 ,there is a good reason for this separation4 ,the stars are very pretty and perfectly harmless to us where they are at present situated4 ,they might be very troublesome neighbors if they were very much closer to our system4 ,it is therefore well they are so far off2 they would be constantly making disturbance in the sun's family if they were near at hand4 ,sometimes they would be dragging us into unpleasantly great heat by bringing us too close to the sun1 or producing a coolness by pulling us away from the sun1 which would be quite as disagreeable4 ,,the ,,stars ,,are ,,suns4 ,we are about to discuss one of the grandest truths in the whole of nature4 ,we have had occasion to see that this sun of ours is a magnificent globe immensely larger than the greatest of his planets1 while the greatest of these planets is immensely larger than this earth2 but now we are to learn that our sun is1 indeed1 only a star not nearly so bright as many of those which shine over our heads every night4 ,we are comparatively close to the sun1 so that we are able to enjoy his beautiful light and cheering heat4 ,each of those other myriads of stars is a sun1 and the splendor of those distant suns is often far greater than that of our own4 ,we are1 however1 so enormously far from them that they appear dwindled down to insignificance4 ,to judge impartially between our sun or star and such a sun or star as ,sirius we should stand halfway between the two2 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 other4 ,after allowance is made for the imperfections of our point of view1 we are enabled to realize the majestic truth that the sun is no more than a star1 and that the other stars are no less than suns4 ,this gives us an imposing idea of the extent and the magnificence of the universe in which we are situated4 ,look up at the sky at night--you will see a host of stars2 try to think that every one of them is itself a sun4 ,it may probably be that those suns have planets circulating round them1 but it is hopeless for us to expect to see such planets4 ,were you standing on one of those stars and looking towards our system1 you would not perceive the sun to be the brilliant and gorgeous object that we knew so well4 ,if you could see him at all1 he would merely seem like a star1 not nearly so bright as many of those you can see at night4 ,even if you had the biggest of telescopes to aid your vision1 you could never discern from one of these bodies the planets which surround the sun4 ,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 know4 ,so minute an object as our earth would1 of course1 be still more hopelessly beyond the possibility of vision4 ,,the ,,numbers ,,of ,,the ,,stars4 ,to count the stars involves a task which lies beyond the power of man to accomplish4 ,even without the aid of any telescope1 we can see a great multitude of stars from this part of the world4 ,there are also many constellations in the southern hemisphere which never appear above our horizon4 ,if1 however1 we were to go to the equator1 then1 by waiting there for a twelve-month1 all the stars in the heavens would have been successively exposed to view4 ,an astronomer1 ,houzeau1 with the patience to count them1 enumerated about #fjjj4 ,this is the naked-eye estimate of the star-population of the heavens2 but if1 instead of relying on unaided vision1 you get the assistance of a little telescope1 you will be astounded at the enormous multitude of stars which are disclosed4 ,illustration3 ,,fig4 #hb4--,the ,great ,bear and the ,pole4 ,an ordinary opera-glass or binocular is a very useful instrument for looking at the stars in the heavens4 ,if you employ an instrument of this sort1 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 of4 ,any part of the sky may be observed2 but1 just to give an illustration1 ,i shall take one special region1 namely1 that of the ,great ,bear 7,fig4 #hb74 ,the seven well-known stars are here shown1 four of which form a sort of oblong1 while the other three represent the tail4 ,i would like you to make this little experiment4 ,on a fine clear night1 count how many stars there are within this oblong2 they are all very faint1 but you will be able to see a few1 and1 with good sight1 and on a clear night1 you may see perhaps ten4 ,next take your opera-glass and sweep it over the same region2 if you will carefully count the stars it shows1 you will find fully #bjj2 so that the opera-glass has1 in this part of the sky1 revealed nearly twenty times as many stars as could be seen without its aid4 ,as #fjjj stars can be seen by the eye all over the heavens1 we may fairly expect that twenty times that number--that is to say1 #abj1jjj stars--could be shown by the opera-glass over the entire sky4 ,let us go a step further1 and employ a telescope1 the object-glass of which is three inches across4 ,this is a useful telescope to have1 and1 if a good one1 will show multitudes of pleasing objects1 though an astronomer would not consider it very powerful4 ,an instrument like this1 small enough to be carried in the hand1 has been applied to the task of enumerating the stars in the northern half of the sky1 and #cbj1jjj stars were counted4 ,indeed1 the actual number that might have been seen with it is considerably greater1 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 together4 ,this grand count only extended to half the sky1 and1 assuming that the other half is as richly inlaid with stars1 we see that a little telescope like that we have supposed will1 when swept over the heavens1 reveal a number of stars which exceeds that of the population of any city in ,england except ,london4 ,it exhibits more than one hundred times as many stars as our eyes could possibly reveal4 ,still1 we are only at the beginning of the count2 the very great telescopes add largely to the number4 ,there are multitudes of stars which in small instruments we cannot see1 but which are distinctly visible from our great observatories4 ,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 ,london2 and with the greatest instruments1 the tale of stars has risen to a number far greater than that of the entire population of ,great ,britain4 ,in addition to those stars which the largest telescopes show us1 there are myriads which make their presence evident in a wholly different way4 ,it is only in quite recent times that an attempt has been made to develop fully the powers of photography in representing the celestial objects4 ,on a photographic plate which has been exposed to the sky in a great telescope the stars are recorded by thousands4 ,many of these may1 of course1 be observed with a good telescope1 but there are not a few others which no one ever saw in a telescope1 which apparently no one ever could see1 though the photograph is able to show them4 ,we do not1 however1 employ a camera like that which the photographer uses who is going to take your portrait4 ,the astronomer's plate is put into his telescope1 and then the telescope is turned towards the sky4 ,on that plate the stars produce their images1 each by its own light4 ,some of these images are excessively faint1 but we give a very long exposure of an hour or two hours2 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 daylight4 ,we thus afford sufficient time to enable the fainter objects to indicate their presence upon the sensitive film4 ,even with an exposure of a single hour a picture exhibiting #af1jjj stars has been taken by ,mr4 ,isaac ,roberts1 of ,liverpool4 ,yet the portion of the sky which it represents is only one ten-thousandth part of the entire heavens4 ,it should be added that the region which ,mr4 ,roberts has photographed is furnished with stars in rather exceptional profusion4 ,here1 at last1 we have obtained some conception of the sublime scale on which the stellar universe is constructed4 ,yet even these plates cannot represent all the stars that the heavens contain4 ,we have every reason for knowing that with larger telescopes1 with more sensitive plates1 with more prolonged exposures1 ever fresh myriads of stars will be brought within our view4 ,you must remember that every one of these stars is truly a sun1 a lamp1 as it were1 which doubtless gives light to other objects in its neighborhood as our sun sheds light upon this earth and the other planets4 ,in fact1 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 universe4 ,standing one fine night on the deck of a ,cunarder we passed in open ocean another great ,atlantic steamer4 ,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 ports2 and we could see nothing of the ship herself4 ,her very existence was only known to us by the twinkle of these lights4 ,doubtless her passengers could see1 and did see1 the similar lights from our own vessel1 and they probably drew the correct inference that these lights indicated a great ship4 ,consider the multiplicity of beings and objects in a ship3 the captain and the crew1 the passengers1 the cabins1 the engines1 the boats1 the rigging1 and the stores4 ,think of all the varied interests there collected and then reflect that out on the ocean1 at night1 the sole indication of the existence of this elaborate structure was given by the few beams of light that happened to radiate from it4 ,now raise your eyes to the stars2 there are the twinkling lights4 ,we cannot see what those lights illuminate1 we can only conjecture what untold wealth of non-luminous bodies may also lie in their vicinity2 we may1 however1 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 steamer1 so are the twinkling stars utterly inadequate to give even the faintest conception of the extent and the interest of the universe4 ,we merely see self-luminous bodies1 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 little4 ,we are1 however1 entitled to infer from an examination of our own star--the sun--and of the beautiful system by which it is surrounded1 that these other suns may be also splendidly attended4 ,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 ship4 ,,the ,,clusters ,,of ,,stars4 ,on a clear night you can often see1 stretching across the sky1 a track of faint light1 which is known to astronomers as the 8,milky ,way40 ,it extends below the horizon and then round the earth to form a girdle about the heavens4 ,when we examine the ,milky ,way with a telescope we find1 to our amazement1 that it consists of myriads of stars1 so small and so faint that we are not able to distinguish them individually2 we merely see the glow produced from their collective rays4 ,remembering that our sun is a star1 and that the ,milky ,way surrounds us1 it would almost seem as if our sun were but one of the host of stars which form this cluster4 ,illustration3 ,,fig4 #hc4--;,globular ,cluster in the ,centaur4 ,there are also other clusters of stars1 some of which are exquisitely beautiful telescopic spectacles4 ,i may mention a celebrated pair of these objects which lies in the constellation of ,perseus4 ,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 view4 ,but there are other clusters4 ,here is a picture of one which is known as the 8,globular ,cluster in the ,centaur0 7,fig4 #hc74 ,it consists of a ball of stars1 so far off that1 however large these several suns may actually be1 they have dwindled down to extremely small points of light4 ,a homely illustration may serve to show the appearance which a globular cluster presents in a good telescope4 ,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 about4 ,imagine that every one of those grains of pepper was to be transformed into a tiny electric light1 and then you have some idea of what a cluster of stars would look like when viewed through a telescope of sufficient power4 ,there are multitudes of such groups scattered through the depths of space4 ,they require our biggest telescopes to show them adequately4 ,we have seen that our sun is a star1 being only one of a magnificent cluster that form the ,milky ,way4 ,we have also seen that there are other groups scattered through the length and depth of space4 ,it is thus we obtain a notion of the rank which our earth holds in the scheme of things celestial4 ,,the ,,rank ,,of ,,the ,,earth ,,as ,a ,,globe ,,in ,,space4 ,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 space1 and you must allow me to draw a little upon my imagination4 ,i shall suppose that ,her ,majesty's mails extend not only over this globe1 but that they also communicate with other worlds2 that postal arrangements exist between ,mars and the earth1 between the sun and ,orion--in fact1 everywhere throughout the whole extent of the universe4 ,we shall consider how our letters are to be addressed4 ,let us take the case of ,mr4 ,john ,smith1 merchant1 who lives at #ajja1 ,piccadilly2 and let us suppose that ,mr4 ,john ,smith's business transactions are of such an extensive nature that they reach not only all over this globe1 but away throughout space4 ,i shall suppose that the firm has a correspondent residing--let us say in the constellation of the ,great ,bear2 and when this man of business wants to write to ,mr4 ,smith from these remote regions1 what address must he put upon the letter1 so that the ,postmaster-,general of the universe shall make no mistake about its delivery8 ,he will write as follows3-- ,,mr4 ,,john ,,smith1 #ajja ,piccadilly1 ,london1 ,england1 ,europe1 ,earth1 ,near the ,sun1 ,milky ,way1 ,the ,universe4 ,let us now see what the several lines of this address mean4 ,of course we put down the name of ,mr4 ,john ,smith in the first line1 and then we will add 8#ajja ,piccadilly0 for the second2 but as the people in the ,great ,bear are not likely to know where ,piccadilly is1 we shall add 8,london0 underneath4 ,as even ,london itself cannot be well known everywhere1 it is better to write 8,england40 ,this would surely find ,mr4 ,john ,smith from any post-office on this globe4 ,from other globes1 however1 the supreme importance of ,england may not be so immediately recognized1 and therefore it is as well to add another line1 8,europe40 ,this ought to be sufficient1 ,i think1 for any post-office in the solar system4 ,europe is big enough to be visible from ,mars or ,venus1 and should be known to the post-office people there1 just as we know and have names for the continents on ,mars4 ,but further away there might be a little difficulty2 from ,uranus and ,neptune the different regions on our earth can never have been distinguished1 and therefore we must add another line to indicate the particular globe of the solar system which contains ,europe4 ,mark ,twain tells us that there was always one thing in astronomy which specially puzzled him1 and that was to know how we found out the names of the stars4 ,we are1 of course1 in hopeless ignorance of the name by which this earth is called among other intelligent beings elsewhere who can see it4 ,i can only adopt the title of 8,earth10 and therefore ,i add this line4 ,now our address is so complete that from anywhere in the solar system--from ,mercury1 from ,jupiter1 or ,neptune--there ought to be no mistake about the letter finding its way to ,mr4 ,john ,smith4 ,but from his correspondent in the ,great ,bear this address would be still incomplete2 they cannot see our earth from there1 and even the sun himself only looks like a small star--like one1 in fact1 of thousands of stars elsewhere4 ,however1 each star can be distinguished1 and our sun may1 for instance1 be recognized from the ,great ,bear by some designation4 ,we shall add the line 8,near the ,sun10 and then ,i think that from this constellation1 or from any of the other stars around us1 the address of ,mr4 ,john ,smith may be regarded as complete4 ,but ,mr4 ,smith's correspondence may be still wider4 ,he may have an agent living in the cluster of ,perseus or on some other objects still fainter and more distant2 then 8,near the ,sun0 is utterly inadequate as a concluding line to the address1 for the sun1 if it can be seen at all from thence1 will be only of the significance of an excessively minute star1 no more to be designated by a special name than are each of the several leaves on the trees of a forest4 ,what this distant correspondent will be acquainted with is not the earth or the sun1 but only the cluster of stars among which the sun is but a unit4 ,again we use our own name to denote the cluster1 and we call it the 8,milky ,way40 ,when we add this line1 we have made the address of ,mr4 ,john ,smith as complete as circumstances will permit4 ,i think a letter posted to him anywhere ought to reach its destination4 ,to perfect it1 however1 we will finish up with one line more--8,the ,universe40 ,,the ,,distances ,,of ,,the ,,stars4 ,i must now tell you something about the distances of the stars4 ,i shall not make the attempt to explain fully how astronomers make such measurements1 but ,i will give you some notion of how it is done4 ,you may remember ,i showed you how we found the distance of a globe that was hung from the ceiling4 ,the principle of the method for finding the distance of a star is somewhat similar1 except that we make the two observations not from the two ends of a table1 not even from opposite sides of the earth1 but from two opposite points on the earth's orbit1 which are therefore at a distance of #ahf1jjj1jjj miles4 ,imagine that on ,midsummer ,day1 when standing on the earth here1 ,i measure with a piece of card the angle between the star and the sun4 ,six months later1 on ,midwinter ,day1 when the earth is at the opposite point of its orbit1 ,i again measure the angle between the same star and the sun1 and we can now determine the star's distance by making a triangle4 ,i draw a line a foot long1 and we will take this foot to represent #ahf1jjj1jjj miles1 the distance between the two stations2 then placing the cards at the corners1 ,i rule the two sides and complete the triangle1 and the star must be at the remaining corner2 then ,i measure the sides of the triangle1 and find how many feet they contain1 and recollecting that each foot corresponds to #ahf1jjj1jjj miles1 we discover the distance of the star4 ,if the stars were comparatively near us1 the process would be a very simple one2 but1 unfortunately1 the stars are so extremely far off that this triangle1 even with a base of only one foot1 must have its sides many miles long4 ,indeed1 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 star4 ,in all such measurements we take the distance from the earth to the sun as a conveniently long measuring-rod1 whereby to express the results4 ,the nearest stars are still hundreds of thousands of times as far off as the sun4 ,let us ponder for a little on the vastness of these distances4 ,we shall first express them in miles4 ,taking the sun's distance to be #ic1jjj1jjj miles1 then the distance of the nearest fixed star is about twenty millions of millions of miles--that is to say1 we express this by putting down a #b first1 and then writing thirteen ciphers after it4 ,it is1 no doubt1 easy to speak of such figures1 but it is a very different matter when we endeavor to imagine the awful magnitude which such a number indicates4 ,i must try to give some illustrations which will enable you to form a notion of it4 ,at first ,i was going to ask you to try and count this number1 but when ,i found it would require at least #cjj1jjj years1 counting day and night without stopping1 before the task was over1 it became necessary to adopt some other method4 ,when on a visit in ,lancashire ,i was once kindly permitted to visit a cotton mill1 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 equator4 ,it appears that the total production of cotton yarn each day in all the mills together would be on the average about #aee1jjj1jjj miles4 ,in fact1 if they would only spin about one-fifth more1 we could assert that ,great ,britain produced enough cotton yarn every day to stretch from the earth to the sun and back again6 ,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 stars4 ,if the spinners worked as hard as ever they could for a year1 and if all the pieces were then tied together1 they would extend to only a small fraction of the distance2 nor if they worked for ten years1 or for twenty years1 would the task be fully accomplished4 ,indeed1 upwards of #djj years would be necessary before enough cotton could be grown in ,america and spun in this country to stretch over a distance so enormous4 ,all the spinning that has ever yet been done in the world has not formed a long enough thread6 ,there is another way in which we can form some notion of the immensity of these sidereal distances4 ,you will recollect that1 when we were speaking of ,jupiter's moons 7p4 #bai71 ,i told you of the beautiful discovery which their eclipses enabled astronomers to make4 ,it was thus found that light travels at the enormous speed of about #ahe1jjj miles per second4 ,it moves so quickly that within a single second a ray would flash two hundred times from ,london to ,edinburgh and back again4 ,we said that a meteor travels one hundred times as swiftly as a rifle-bullet2 but even this great speed seems almost nothing when compared with the speed of light1 which is #aj1jjj times as great4 ,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 universe4 ,the light would start forth on its voyage with terrific speed4 ,any neighboring star which was at a distance of less than #ahe1jjj miles would1 of course1 see the flash within a second after it had been produced4 ,more distant bodies would receive the intimation after intervals of time proportional to their distances4 ,thus1 if a body were #a1jjj1jjj miles away the light would reach it in from five to six seconds1 while over a distance as great as that which separates the earth from the sun the news would be carried in about eight minutes4 ,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 earth4 ,you will find that from the nearest of the stars the time required for the journey will be over three years4 ,ponder on all that this involves4 ,that outbreak in the star might be great enough to be visible here1 but we could never become aware of it till three years after it had happened4 ,when we are looking at such a star to-night we do not see it as it is at present1 for the light that is at this moment entering our eyes has travelled so far that it has been three years on the way4 ,therefore1 when we look at the star now we see it as it was three years previously4 ,in fact1 if the star were to go out altogether1 we might still continue to see it twinkling for a period of three years longer1 because a certain amount of light was on its way to us at the moment of extinction1 and so long as that light keeps arriving here1 so long shall we see the star showing as brightly as ever4 ,when1 therefore1 you look at the thousands of stars in the sky to-night1 there is not one that you see as it is now1 but as it was years ago4 ,i have been speaking of the stars that are nearest to us1 but there are others much farther off4 ,it is true we cannot find the distance of these more remote objects with any degree of accuracy1 but we can convince ourselves how great that distance is by the following reasoning4 ,look at one of the brightest stars4 ,try to conceive that the object was carried away further into the depths of space1 until it was ten times as far from us as it is at present1 it would still remain bright enough to be recognized in quite a small telescope2 even if it were taken to one hundred times its original distance it would not have withdrawn from the view of a good telescope2 while if it retreated one thousand times as far as it was at first it would still be a recognizable point in our mightiest instruments4 ,among the stars which we can see with our telescopes1 we feel confident there must be many from which the light has expended hundreds of years1 or even thousands of years1 on the journey4 ,when1 therefore1 we look at such objects1 we see them1 not as they are now1 but as they were ages ago2 in fact1 a star might have ceased to exist for thousands of years1 and still be seen by us every night as a twinkling point in our great telescopes4 ,remembering these facts1 you will1 ,i think1 look at the heavens with a new interest4 ,there is a bright star1 ,vega or ,alpha ,lyr1 a beautiful gem1 so far off that the light from it which now reaches our eyes started before many of my audience were born4 ,suppose that there are astronomers residing on worlds amid the stars1 and that they have sufficiently powerful telescopes to view this globe1 what do you think they would observe8 ,they will not see our earth as it is at present1 they will see it as it was years1 and sometimes many years1 ago4 ,there are stars from which1 if ,england could now be seen1 the whole of the country would be observed at this present moment to be in a great state of excitement at a very auspicious event4 ,distant astronomers might notice a great procession in ,london1 and they could watch the coronation of a youthful queen amid the enthusiasm of a nation4 ,there are other stars still further1 from which1 if the inhabitants had good enough telescopes1 they would now see a mighty battle in progress not far from ,brussels4 ,one splendid army could be beheld hurling itself time after time against the immovable ranks of the other4 ,they would not1 indeed1 be able to hear the ever-memorable1 8,up1 ,guards1 and at them60 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 there4 ,further off still1 there are stars from which a bird's-eye view could be taken at this very moment of the signing of ,magna ,charta4 ,there are even stars from which ,england1 if it could be seen at all1 would now appear1 not as the great ,england we know1 but as a country covered by dense forests1 and inhabited by painted savages1 who waged incessant war with wild beasts that roamed through the island4 ,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 telescopes4 ,we should then be able to view our earth through the successive epochs of past geological time2 we should be actually able to see those great animals whose fossil remains are treasured in our museums tramping about over the earth's surface1 splashing across its swamps1 or swimming with broad flippers through its oceans4 ,indeed1 if we could view our own earth reflected from mirrors in the stars1 we might still see ,moses crossing the ,red ,sea1 or ,adam and ,eve being expelled from ,eden4 ,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 are4 ,i shall take1 as before1 the nearest of the stars so far as known to us1 and ,i hope to be forgiven for taking an illustration of a practical and a commercial kind instead of one more purely scientific4 ,i shall suppose that a railway is about to be made from ,london to ,alpha ,centauri4 ,the length of that railway1 of course1 we have already stated3 it is twenty billions of miles4 ,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 journey4 ,we shall choose a very cheap scale on which to compute the price of a ticket4 ,the parliamentary rate here is1 ,i believe1 a penny for every mile4 ,we will make our interstellar railway fares much less even than this2 we shall arrange to travel at the rate of one hundred miles for every penny4 ,that1 surely1 is moderate enough4 ,if the charges were so low that the journey from ,london to ,edinburgh only cost fourpence1 then even the most unreasonable passenger would be surely contented4 ,on these terms how much do you think the fare from ,london to this star ought to be8 ,i know of one way in which to make our answer intelligible4 ,there is a ,national ,debt with which your fathers are1 unhappily1 only too well acquainted2 you will know quite enough about it yourselves in those days when you have to pay income tax4 ,this debt is so vast that the interest upon it is about sixty thousand pounds a day1 the whole amount of the ,national ,debt being six hundred and thirty-eight millions of pounds 7,april1 #ahih74 ,if you went to the booking office with the whole of this mighty sum in your pocket--but stop a moment2 could you carry it in your pocket8 ,certainly not1 if it were in sovereigns4 ,you would find that after you had as many sovereigns as you could conveniently carry there would still be some left--so many1 indeed1 that it would be necessary to get a cart to help you on with the rest4 ,when the cart had as great a load of sovereigns as the horse could draw there would be still some more1 and you would have to get another cart2 but ten carts1 twenty carts1 fifty carts1 would not be enough4 ,you would want five thousand of these before you would be able to move off towards the station with your money4 ,when you did get there and asked for a ticket at the rate of one hundred miles for a penny1 do you think you would get any change8 ,no doubt some little time would be required to count the money1 but when it was counted the clerk would tell you that there was not enough1 that he must have nearly two hundred millions of pounds more4 ,that will give some notion of the distance of the nearest star1 and we may multiply it by ten1 by one hundred1 and even by one thousand1 and still not attain to the distance of some of the more remote stars that the telescope shows us4 ,on account of the immense distances of the stars we can only perceive them to be mere points of light4 ,we can never see a star to be a globe with marks on it like the moon1 or like one of the planets--in fact1 the better the telescope the smaller does the star seem1 though1 of course1 its brightness is increased with every addition to the light-grasping power of the instrument4 ,,the ,,brightness ,,and ,,color ,,of ,,stars4 ,another point to be noticed is the arrangement of stars in classes1 according to their lustre4 ,the brightest stars1 of which there are about twenty1 are said to be of the first magnitude4 ,those just inferior to the first magnitude are ranked as the second2 and those just lower than the second are estimated as the third2 and so on4 ,the smallest points that your unaided eyes will show you are of about the sixth magnitude4 ,then the telescope will reveal stars still fainter and fainter1 down to what we term the seventeenth or eighteenth magnitudes1 or even lower still4 ,the number of stars of each magnitude increases very much in the classes of small ones4 ,most of the stars are white1 but many are of a somewhat ruddy hue4 ,there are a few telescopic points which are intensely red1 some exhibit beautiful golden tints1 while others are blue or green4 ,there are some curious stars which regularly change their brilliancy4 ,let me try to illustrate the nature of these variables4 ,suppose that you were looking at a street gas-lamp from a very long distance1 so that it seemed a little twinkling light2 and suppose that some one was preparing to turn the gas-cock up and down4 ,or1 better still1 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 half1 and then gradually turn it down until in three or four hours it declines to a feeble glimmer4 ,in this low state the light remains for twenty minutes2 then during three or four hours the gas is to be slowly turned on again until it is full4 ,in this condition the light will remain for two days and a half1 and then the same series of changes is to recommence4 ,this would be a very odd form of gas-lamp4 ,there would be periods of two days and a half during which it would remain at its full2 these would be separated by intervals of about seven hours1 when the gradual turning down and turning up again would be in progress4 ,illustration3 ,,fig4 #hd4--,perseus and its ,neighboring ,stars1 including ,algol4 ,the imaginary gas-lamp is exactly paralleled by a star ,algol1 in the constellation of ,perseus 7,fig4 #hd71 which goes through the series of changes ,i have indicated4 ,ordinarily speaking1 it is a bright star of the second magnitude1 and1 whatever be the cause1 the star performs its variations with marvellous uniformity4 ,in fact1 ,algol has always arrested the attention of those who observed the heavens1 and in early times was looked on as the eye of a ,demon4 ,there are many other stars which also change their brilliancy4 ,most of them require much longer periods than ,algol1 and sometimes a new star which nobody has ever seen before will suddenly kindle into brilliancy4 ,it is now known that the bright star ,algol is attended by a dark companion4 ,this dark star sometimes comes between ,algol and the observer and cuts off the light4 ,thus it is that the diminution of brightness is produced4 ,,double ,,stars4 ,whenever you have a chance of looking at the heavens through a telescope1 you should ask to be shown what is called a double star4 ,there are many stars in the heavens which present no remarkable appearance to the unaided eye1 but which a good telescope at once shows to be of quite a complex nature4 ,these are what we call double stars1 in which two quite distinct stars are placed so close together that the unaided eye is unable to separate them4 ,under the magnifying power of the telescope1 however1 they are seen to be distinct4 ,in order to give some notion of what these objects are like1 ,i shall briefly describe three of them4 ,the first lies in that best known of constellations1 the ,great ,bear4 ,if you look at his tail1 which consists of three stars1 you will see that near the middle one of the three a small star is situated2 we call this little star ,alcor1 but it is the brighter one near ,alcor to which ,i specially call your attention4 ,the sharpest eye would never suspect that it was composed of two stars placed close together4 ,even a small telescope will1 however1 show this to be the case1 and this is the easiest and the first observation that a young astronomer should make when beginning to turn a telescope to the heavens4 ,of course1 you will not imagine that ,i mean ,alcor to be the second component of the double star2 it is the bright star near ,alcor which is the double4 ,here are two marbles1 and these marbles are fastened an inch apart4 ,you can see them1 of course1 to be separate2 but if the pair were moved further and further away1 then you would soon not be able to distinguish between them1 though the actual distance between the marbles had not altered4 ,look at these two wax tapers which are now lighted2 the little flames are an inch apart4 ,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 star4 ,your eye would never be able to discriminate between two lights only an inch apart at so great a distance2 a telescope would1 however1 enable you to do so1 and this is the reason why we have to use telescopes to show us double stars4 ,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 components4 ,but we know that there is no such thing as rest in the universe2 even if you could balance a body so as to leave it for a moment at rest1 it would not stay there1 for the simple reason that all the bodies round it in every direction are pulling at it1 and it is certain that the pull in one direction will preponderate1 so that move it must4 ,especially is this true in the case of two suns like those forming a double star4 ,placed comparatively near each other they could not remain permanently in that position2 they must gradually draw together and come into collision with an awful crash4 ,there is only one way by which such a disaster could be averted4 ,that is by making one of these stars revolve around the other just as the earth revolves around the sun1 or the moon revolves around the earth4 ,some motion must1 therefore1 be going on in every genuine double star1 whether we have been able to see that motion or not4 ,let us now look at another double star of a different kind4 ,this time it is in the constellation of ,gemini4 ,the heavenly twins are called ,castor and ,pollux4 ,of these1 ,castor is a very beautiful double star1 consisting of two bright points1 a great deal closer together than were those in the ,great ,bear2 consequently a better telescope is required for the purpose of showing them separately4 ,castor has been watched for many years1 and it can be seen that one of these stars is slowly revolving around the other2 but it takes a very long time1 amounting to hundreds of years1 for a complete circuit to be accomplished4 ,this seems very astonishing1 but when you remember how exceedingly far ,castor is1 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 miles4 ,let us try to conceive our own system transformed into a double star4 ,if we took our outermost planet--,neptune--and enlarged him a good deal1 and then heated him sufficiently to make him glow like a sun1 he would still continue to revolve round our sun at the same distance1 and thus a double star would be produced4 ,an inhabitant of ,castor who turned his telescope towards us would be able to see the sun as a star4 ,he would not1 of course1 be able to see the earth1 but he might see ,neptune like another small star close to the sun4 ,if generations of astronomers in ,castor continued their observations of our system1 they would find a binary star1 of which one component took a century and a half to go round the other4 ,need we then be surprised that when we look at ,castor we observe movements that seem very slow8 ,there is often so much diffused light about the bright stars seen in a telescope1 and so much twinkling in some states of the atmosphere1 that stars appear to dance about in rather a puzzling fashion1 especially to one who is not accustomed to astronomical observations4 ,i remember hearing how a gentleman once came to visit an observatory4 ,the astronomer showed him ,castor through a powerful telescope as a fine specimen of a double star1 and then1 by way of improving his little lesson1 the astronomer mentioned that one of these stars was revolving around the other4 8,oh1 yes10 said the visitor1 8,i saw them going round and round in the telescope40 ,he would1 however1 have had to wait for a few centuries with his eye to the instrument before he would have been entitled to make this assertion4 ,double stars also frequently delight us by giving beautifully contrasted colors4 ,i dare say you have often noticed the red and the green lights that are used on railways in the signal lamps4 ,imagine one of those red and one of those green lights away far up in the sky and placed close together1 then you would have some idea of the appearance that a colored double star presents1 though1 perhaps1 ,i should add that the hues in the heavenly bodies are not so vividly different as are those which our railway people find necessary4 ,there is a particularly beautiful double star of this kind in the constellation of the ,swan4 ,you could make an imitation of it by boring two holes1 with a red-hot needle1 in a piece of card1 and then covering one of these holes with a small bit of the topaz-colored gelatine with which ,christmas crackers are made4 ,the other star is to be similarly colored with blue gelatine4 ,a slide made on this principle placed in the lantern gives a very good representation of these two stars on the screen4 ,there are many other colored doubles besides this one2 and1 indeed1 it is noteworthy that we hardly ever find a blue or a green star by itself in the sky2 it is always as a member of one of these pairs4 ,,how ,,we ,,find ,,what ,,the ,,stars ,,are ,,made ,,of4 ,here is a piece of stone4 ,if ,i wanted to know what it was composed of1 ,i should ask a chemist to tell me4 ,he would take it into his laboratory1 and first crush it into powder1 and then1 with his test tubes1 and with the liquids which his bottles contain1 and his weighing scales1 and other apparatus1 he will tell all about it2 there is so much of this1 and so much of that1 and plenty of this1 and none at all of that4 ,but now1 suppose you ask this chemist to tell you what the sun is made of1 or one of the stars4 ,of course1 you have not a sample of it to give him2 how1 then1 can he possibly find out anything about it8 ,well1 he can tell you something1 and this is the wonderful discovery that ,i want to explain to you4 ,we now put down the gas1 and ,i kindle a brilliant red light4 ,perhaps some of those whom ,i see before me have occasionally ventured on the somewhat dangerous practice of making fireworks4 ,if there is any boy here who has ever constructed sky-rockets1 and put the little balls into the top which are to burn with such vivid colors when the explosion takes place1 he will know that the substance which tinged that red fire must have been strontium4 ,he will recognize it by the color2 because strontium gives a red light which nothing else will give4 ,here are some of these lightning papers1 as they are called2 they are very pretty and very harmless2 and these1 too1 give brilliant red flashes as ,i throw them4 ,the red tint has1 no doubt1 been produced by strontium also4 ,you see we recognized the substance simply by the color of the light it produced when burning4 ,perhaps some of you have tried to make a ghost at ,christmas by dressing up in a sheet1 and bearing in your hand a ladle blazing with a mixture of common salt and spirits of wine1 the effect produced being a most ghastly one4 ,some mammas will hardly thank me for this suggestion1 unless ,i add that the ghost must walk about cautiously1 for otherwise the blazing spirit would be very apt to produce conflagrations of a kind more extensive than those intended4 ,however1 by the kindness of ,professor ,dewar1 ,i am enabled to show the phenomenon on a splendid scale1 and also free from all danger4 ,i kindle a vivid flame of an intensely yellow color1 which ,i think the ladies will unanimously agree is not at all becoming to their complexions1 while the pretty dresses have lost their variety of colors4 ,here is a nice bouquet1 and yet you can hardly distinguish the green of the leaves from the brilliant colors of the flowers1 except by trifling differences of shade4 ,expose to this light a number of pieces of variously colored ribbon1 pink and red and green and blue1 and their beauty is gone2 and yet we are told that this yellow is a perfectly pure color2 in fact1 the purest color that can be produced4 ,i think we have to be thankful that the light which our good sun sends us does not possess purity of that description4 ,there is one substance which will produce that yellow light2 it is a curious metal called sodium--a metal so soft that you can cut it with a knife1 and so light that it will float on water2 while1 still more strange1 it actually takes fire the moment it is dropped on the water4 ,it is only in a chemical laboratory that you will be likely to meet with the actual metallic sodium1 yet in other forms the substance is one of the most abundant in nature4 ,indeed1 common salt is nothing but sodium closely united with a most poisonous gas1 a few respirations of which would kill you4 ,but this strange metal and this noxious gas1 when united1 become simply the salt for our eggs at breakfast4 ,this pure yellow light1 wherever it is seen1 either in the flame of spirits of wine mixed with salt or in that great blaze at which we have been looking1 is characteristic of sodium4 ,wherever you see that particular kind of light1 you know that sodium must have been present in the body from which it came4 ,we have accordingly learned to recognize two substances1 namely1 strontium and sodium1 by the different lights which they give out when burning4 ,to these two metals we may add a third4 ,here is a strip of white metallic ribbon4 ,it is called magnesium4 ,it seems like a bit of tin at the first glance1 but indeed it is a very different substance from tin2 for1 look1 when ,i hold it in the spirit-lamp1 the strip of metal immediately takes fire1 and burns with a white light so dazzling that it pales the gas-flames to insignificance4 ,there is no other substance which will1 when kindled1 give that particular kind of light which we see from magnesium4 ,i can recommend this little experiment as quite suitable for trying at home2 you can buy a bit of magnesium ribbon for a trifle at the optician's2 it cannot explode or do any harm1 nor will you get into any trouble with the authorities provided you hold it when burning over a tray or a newspaper1 so as to prevent the white ashes from falling on the carpet4 ,there are1 in nature1 a number of simple bodies called elements4 ,every one of these1 when ignited under suitable conditions1 emits a light which belongs to it alone1 and by which it can be distinguished from every other substance4 ,i do not say that we can try the experiments in the simple way ,i have here indicated4 ,many of the materials will yield light which will require to be studied by much more elaborate artifices than those which have sufficed for us4 ,but you see that the method affords a means of finding out the actual substances present in the sun or in the stars4 ,there is a practical difficulty in the fact that each of the heavenly bodies contains a number of different elements2 so that in the light it sends us the hues arising from distinct substances are blended into one beam4 ,the first thing to be done is to get some way of splitting up a beam of light1 so as to discover the components of which it is made4 ,you might have a skein of silks of different hues tangled together1 and this would be like the sunbeam as we receive it in its unsorted condition4 ,how shall we untangle the light from the sun or a star8 ,i will show you by a simple experiment4 ,here is a beam from the electric light2 beautifully white and bright1 is it not8 ,it looks so pure and simple1 but yet that beam is composed of all sorts of colors mingled together1 in such proportions as to form white light4 ,i take a wedge-shaped piece of glass called a prism1 and when ,i introduce it into the course of the beam1 you see the transformation that has taken place 7,fig4 #he74 ,instead of the white light you have now all the colors of the rainbow--red1 orange1 yellow1 green1 blue1 indigo1 violet1 marked by their initial letters in the figure4 ,these colors are very beautiful1 but they are transient1 for the moment we take away the prism they all unite again to form white light4 ,you see what the prism has done2 it has bent all the light in passing through it2 but it is more effective in bending the blue than the red1 and consequently the blue is carried away much further than the red4 ,such is the way in which we study the composition of a heavenly body4 ,we take a beam of its light1 we pass it through a prism1 and immediately it is separated into its components2 then we compare what we find with the lights given by the different elements1 and thus we are enabled to discover the substances which exist in the distant object whose light we have examined4 ,i do not mean to say that the method is a simple one2 all ,i am endeavoring to show is a general outline of the way in which we have discovered the materials present in the stars4 ,the instrument that is employed for this purpose is called the spectroscope4 ,and perhaps you may remember that name by these lines1 which ,i have heard from an astronomical friend3-- ,illustration3 ,,fig4 #he4--;,how to split up a ,ray of ,light4 8,twinkle1 twinkle1 little star1 ,now we find out what you are1 ,when unto the midnight sky1 ,we the spectroscope apply40 ,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 made4 ,it is true there may be substances in the stars of which we know nothing here2 but it is certain that many of the most common elements on the earth are present in the most distant bodies4 ,i shall only mention one1 the metal iron4 ,that useful substance has been found in some of the stars which lie at almost incalculable distances from the earth4 ,,the ,,nebul4 ,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 alluded4 ,they are what are called nebul1 or little clouds2 and in one sense they are justly called little1 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 air4 ,the nebul are1 however1 objects of the most stupendous proportions4 ,were our earth and thousands of millions of bodies quite as big all put together1 they would not be nearly so great as one of these nebul4 ,astronomers reckon up the various nebul by thousands1 but ,i must add that most of them are apparently faint and uninteresting4 ,a nebula is sometimes liable to be mistaken for a comet4 ,the comet is1 as ,i have already explained1 at once distinguished by the fact that it is moving and changing its appearance from hour to hour1 while scores of years elapse without changes in the aspect or position of a nebula4 ,the most powerful telescopes are employed in observing these faint objects4 ,i take this opportunity of showing a picture of an instrument suitable for such observations4 ,it is the great reflector of the ,paris ,observatory 7,fig4 #hg74 ,illustration3 ,,fig4 #hf4--,the ,ring ,nebula in ,lyra1 under ,different ,telescopic ,powers4 ,there are such multitudes of nebul that ,i can only show a few of the more remarkable kinds4 ,in ,fig4 #hf will be seen pictures of a curious object in the constellation of ,lyra seen under different telescopic powers4 ,this is a gigantic ring of luminous gas4 ,to judge of the size of this ring let us suppose that a railway were laid across it1 and the train you entered at one side was not to stop until it reached the other side1 how long do you think this journey would require8 ,i recollect some time ago a picture in ,punch which showed a train about to start from ,london to ,brighton1 and the guard walking up and down announcing to the passengers the alarming fact that 8this train stops nowhere40 ,an old gentleman was seen vainly gesticulating out of the window and imploring to be let out ere the frightful journey was commenced4 ,in the nebular railway the passengers would almost require such a warning4 ,illustration3 ,,fig4 #hg4--;,a ,great ,reflecting ,telescope4 ,let the train start at a speed of a mile a minute1 you would think1 surely1 that it must soon cross the ring4 ,but the minutes pass1 an hour has elapsed2 so the distance must be sixty miles1 at all events4 ,the hours creep on into days1 the days advance into years1 and still the train goes on4 ,the years would lengthen out into centuries1 and even when the train had been rushing on for a thousand years with an unabated speed of a mile a minute1 the journey would certainly not have been completed4 ,nor do ,i venture to say what ages must elapse ere the terminus at the other side of the ring nebula would be reached4 ,a cluster of stars viewed in a small telescope will often seem like a nebula1 for the rays of the stars become blended4 ,a powerful telescope will1 however1 dispel the illusion and reveal the separate stars4 ,it was1 therefore1 thought that all the nebul might be merely clusters so exceedingly remote that our mightiest instruments failed to resolve them into stars4 ,but this is now known not to be the case4 ,many of these objects are really masses of glowing gas2 such are1 for instance1 the ring nebul1 of which ,i have just spoken1 and the form of which ,i can simulate by a pretty experiment4 ,illustration3 ,,fig4 #hh4--;,how to make the ,smoke-rings4 ,we take a large box with a round hole cut in one face1 and a canvas back at the opposite side4 ,i first fill this box with smoke1 and there are different ways of doing so4 ,burning brown paper does not answer well1 because the supply of smoke is too irregular and the paper itself is apt to blaze4 ,a little bit of phosphorus set on fire yields copious smoke1 but it would be apt to make people cough1 and1 besides1 phosphorus is a dangerous thing to handle incautiously1 and ,i do not want to suggest anything which might be productive of disaster if the experiment was repeated at home4 ,a little wisp of hay1 slightly damped and lighted1 will safely yield a sufficient supply1 and you need not have an elaborate box like this2 any kind of old packing-case1 or even a band-box with a duster stretched across its open top and a round hole cut in the bottom1 will answer capitally4 ,while ,i have been speaking1 my assistant has kindly filled this box with smoke1 and in order to have a sufficient supply1 and one which shall be as little disagreeable as possible1 he has mixed together the fumes of hydrochloric acid and ammonia from two retorts shown in ,fig4 #hh4 ,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 vitriol2 this is put into the box1 and over the floor of the box common smelling-salts is to be scattered4 ,you see there are dense volumes of white smoke escaping from every corner of the box4 ,i uncover the opening and give a push to the canvas1 and you see a beautiful ring flying across the room2 another ring and another follow4 ,if you were near enough to feel the ring1 you would experience a little puff of wind2 ,i can show this by blowing out a candle which is at the other end of the table4 ,these rings are made by the air which goes into a sort of eddy as it passes through the hole4 ,all the smoke does is to render the air visible4 ,the smoke-ring is indeed quite elastic4 ,if we send a second ring hurriedly after the first1 we can produce a collision1 and you see each of the two rings remains unbroken1 though both are quivering from the effects of the blow4 ,they are beautifully shown along the beam of the electric lamp1 or1 better still1 along a sunbeam4 ,we can make many experiments with smoke-rings4 ,here1 for instance1 ,i take an empty box1 so far as smoke is concerned1 but air-rings can be driven forth from it1 though you cannot see them1 but you can feel them even at the other side of the room1 and they will1 as you see1 blow out a candle4 ,i can also shoot invisible air-rings at a column of smoke1 and when the missile strikes the smoke it produces a little commotion and emerges on the other side1 carrying with it enough of the smoke to render itself visible1 while the solid black looking ring of air is seen in the interior4 ,still more striking is another way of producing these rings1 for ,i charge this box with ammonia1 and the rings from it you cannot see4 ,there is a column of the vapor of hydrochloric acid that also you cannot see2 but when the invisible ring enters the invisible column1 then a sudden union takes place between the vapor of the ammonia and the vapor of the hydrochloric acid2 the result is a solid white substance in extremely fine dust which renders the ring instantly visible4 ,,what ,,the ,,nebul ,,are ,,made ,,of4 ,there is a fundamental difference between the illumination of these little rings that ,i have shown you and the great rings in the heavens4 ,i had to illuminate our smoke with the help of the electric light1 for1 unless ,i had done so1 you would not have been able to see them4 ,this white substance formed by the union of ammonia and hydrochloric acid has1 of course1 no more light of its own than a piece of chalk2 it requires other light falling upon it to make it visible4 ,were the ring nebula in ,lyra composed of this material1 we could not see it4 ,the sunlight which illuminates the planets might1 of course1 light up such an object as the ring1 if it were comparatively near us2 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 star4 ,should we be able to show our smoke-rings1 for instance1 if1 instead of having the electric light1 ,i merely cut a hole in the ceiling and allowed the feeble twinkle of a star in the ,great ,bear to shine through8 ,in a similar way the sunbeams would be utterly powerless to effect any illumination of objects in these stellar distances4 ,if the sun were to be extinguished altogether1 the calamity would no doubt be a very dire one so far as we are concerned1 but the effect on the other celestial bodies 7moon and planets excepted7 would be of the slightest possible description4 ,all the stars of heaven would continue to shine as before4 ,not a point in one of the constellations would be altered1 not a variation in the brightness1 not a change in the hue of any star could be noticed4 ,the thousands of nebul and clusters would be absolutely unaltered2 in fact1 the total extinction of the sun would be hardly remarked in the newspapers published in the ,pleiades or in ,orion4 ,there might possibly be a little line somewhere in an odd corner to the effect 8,mr4 ,so-and-,so1 our well-known astronomer1 has noticed that a tiny star1 inconspicuous to the eye1 and absolutely of no importance whatever1 has now become invisible40 ,if1 therefore1 it be not the sun which lights up this nebula1 where else can be the source of its illumination8 ,there can be no other star in the neighborhood adequate to the purpose1 for1 of course1 such an object would be brilliant to us if it were large enough and bright enough to impart sufficient illumination to the nebula4 ,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 visible4 ,the actual facts are1 of course1 the other way2 the candle might be visible1 when it was impossible to discern the face which it lighted4 ,hence we learn that the ring nebula must shine by some light of its own1 and now we have to consider how it can be possible for such material to be self-luminous4 ,the light of a nebula does not seem to be like flame2 it can1 perhaps1 be better represented by the pretty electrical experiment with ,geissler's tubes4 ,these are glass vessels of various shapes1 and they are all very nearly empty1 as you will understand when ,i tell you the way in which they have been prepared4 ,a little gas was allowed into each tube1 and then almost all the gas was taken out again1 so that only a mere trace was left4 ,i pass a current of electricity through these tubes1 and now you see they are glowing with beautiful colors4 ,the different gases give out lights of different hues1 and the optician has exerted his skill so as to make the effect as beautiful as possible4 ,the electricity1 in passing through these tubes1 heats the gas which they contain1 and makes it glow2 and just as this gas can1 when heated sufficiently1 give out light1 so does the great nebula1 which is a mass of gas poised in space1 become visible in virtue of the heat which it contains4 ,we are not left quite in doubt as to the constitution of these gaseous nebul1 for we can submit their light to the prism in the way ,i explained when we were speaking of the stars4 ,distant though that ring in ,lyra may be1 it is interesting to learn that the ingredients from which it is made are not entirely different from substances we know on our earth4 ,the water in this glass1 and every drop of water1 is formed by the union of two gases1 of which one is hydrogen4 ,this is an extremely light material1 as you see by a little balloon which ascends so prettily when filled with it4 ,hydrogen also burns very readily1 though the flame is almost invisible4 ,when ,i blow a jet of oxygen through the hydrogen1 ,i produce a little flame with a very intense heat4 ,for instance1 ,i hold a steel pen in the flame1 and it glows and sputters1 and falls down in white-hot drops4 ,it is needless to say that1 as a constituent of water1 hydrogen is one of the most important elements on this earth4 ,it is1 therefore1 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 revealed4 ,,photographing ,,the ,,nebul4 ,of late years we have learned a great deal about nebul1 by the help which photography has given to us4 ,look at this group of stars which constitutes that beautiful little configuration known as the ,pleiades 7,fig4 #hi74 ,it looks like a miniature representation of the ,great ,bear2 in fact1 it would be far more appropriate to call the ,pleiades the ,little ,bear than to apply that title to another quite different constellation1 as has unfortunately been done4 ,the ,pleiades form a group containing six or seven stars visible to the ordinary eye1 though persons endowed with exceptionally good vision can usually see a few more4 ,in an opera-glass the ,pleiades becomes a beautiful spectacle1 though in a large telescope the stars appear too far apart to make a really effective cluster4 ,when ,mr4 ,roberts took a photograph of the ,pleiades he placed a highly sensitive plate in his telescope1 and on that plate the ,pleiades engraved their picture with their own light4 ,he left the plate exposed for hours1 and on developing it not only were the stars seen1 but there were also patches of faint light due to the presence of nebula4 ,it could not be said that the objects on the plate were fallacious1 for another photograph was taken1 when the same appearances were reproduced4 ,illustration3 ,,fig4 #hi4--,the ,pleiades4 ,when we look at that pretty group of stars which has attracted admiration during all time1 we are to think that some of those stars are merely the bright points in a vast nebula1 invisible to our unaided eyes or even to our mighty telescopes1 though capable of recording its trace on the photographic plate4 ,does not this give us a greatly increased notion of the extent of the universe1 when we reflect that by photography we are enabled to see much which the mightiest of telescopes had previously failed to disclose8 ,of all the nebul1 now numbering some thousands1 there is but a single one which can be seen without a telescope4 ,it is in the constellation of ,andromeda1 and on a clear dark night can just be seen with the unaided eye as a faint stain of light on the sky4 ,it has happened before now that persons noticing this nebula for the first time have thought they had discovered a comet4 ,i would like you to try and find out this object for yourselves4 ,if you look at it with an opera-glass it appears to be distinctly elongated4 ,you can see more of its structure when you view it in larger instruments1 but its nature was never made clear until some beautiful photographs were taken by ,mr4 ,roberts 7,fig4 #ij74 ,unfortunately1 the nebula in ,andromeda has not been placed in the best position for its portrait from our point of view4 ,it seems as if it were a rather flat-shaped object1 turned nearly edgewise towards us4 ,to look at the pattern on a plate1 you would naturally hold the plate so as to be able to look at it squarely4 ,the pattern would not be seen well if the plate were so tilted that its edge was turned towards you4 ,that seems to be nearly the way in which we are forced to view the nebula in ,andromeda4 ,we can trace in the photograph some divisions extending entirely round the nebula1 showing that it seems to be formed of a series of rings2 and there are some outlying portions which form part of the same system4 ,truly this is a marvellous object4 ,it is impossible for us to form any conception of the true dimensions of this gigantic nebula2 it is so far off that we have never yet been able to determine its distance4 ,indeed1 ,i may take this opportunity of remarking that no astronomer has yet succeeded in ascertaining the distance of any nebula4 ,everything1 however1 points to the conclusion that they are at least as far as the stars4 ,illustration3 ,,fig4 #ij4--,the ,great ,nebula in ,andromeda4 7,from ,mr4 ,roberts' ,photograph47 ,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 nebul4 ,these flimsy bodies are usually too ill-defined to admit of being measured with the precision and the delicacy required for the determination of distance4 ,the measurements necessary for this purpose can only be made from one star-like point to another similar point4 ,if we could choose a star in the nebula and determine its distance1 then1 of course1 we should have the distance of the nebula itself2 but the difficulty is that we have1 in general1 no means of knowing whether the star does actually lie in the object4 ,it may1 for anything we can tell1 lie billions of miles nearer to us1 or billions of miles further off1 and1 by merely happening to lie in the line of sight1 appear to glimmer in the nebula itself4 ,illustration3 ,,fig4 #ia4--,to show how ,small the ,solar ,system is compared with a ,great ,nebula4 ,if we have any assurance that the star is surrounded by a mass of this glowing vapor1 then it may be possible to measure that nebula's distance4 ,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 therein1 and is not merely seen in the same direction4 ,there are hundreds of stars visible on a good drawing or a good photograph of the famous object in ,andromeda1 and doubtless large numbers of these are merely stars which happen to lie in the same line of sight4 ,the peculiar circumstances attending the history of one star seem1 however1 to warrant us in making the assumption that it was certainly in the nebula4 ,the history of this star is a remarkable one4 ,it suddenly kindled from invisibility into brilliancy4 ,how is a change so rapid in the lustre of a star to be accounted for8 ,in a few days its brightness had undergone an extraordinary increase4 ,of course1 this does not tell us for certain that the star lay in the glowing gas2 but the most rational explanation that ,i have heard offered of this occurrence is that due1 ,i believe1 to my friend ,mr4 ,monck4 ,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 atmosphere4 ,if a dark star1 moving along with terrific speed through space1 were suddenly to plunge into a dense region of the nebula1 heat and light must be evolved in sufficient abundance to transform the star into a brilliant object4 ,if1 therefore1 we knew the distance of this star at the time it was in ,andromeda1 we should1 of course1 learn the distance of that interesting object4 ,this has been attempted1 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 notion4 ,we thus realize the enormous size of the ,great ,nebula4 ,it appears that if1 on a map of this object1 we were to lay down1 accurately to scale1 a map of the solar system1 putting the sun in the centre and all the planets around in their true proportions out to the boundary traced by ,neptune1 this area1 vast though it is1 would be a mere speck on the drawing of the object4 ,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 objects4 ,here is a sketch of a nebula 7,fig4 #ia71 and near it ,i have marked a dot which is to indicate our solar system4 ,we may feel confident that the ,great ,nebula is at the very least as mighty as this proportion would indicate4 ,,conclusion4 ,and now1 my young friends1 ,i am drawing near the close of that course of lectures which has occupied us1 ,i hope you will think not unprofitably1 for a portion of our ,christmas holidays4 ,we have spoken of the sun and of the moon1 of comets and of stars1 and ,i have frequently had occasion to allude to the relative position of our earth in the universe4 ,no doubt it is a noble globe which we inhabit1 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 space4 ,we have1 however1 been endowed with a feeling of curiosity which makes us long to know of things beyond the confines of our own earth4 ,astronomers can tell us a little1 but too often only a little4 ,they will say--,that is a star1 and ,that is a planet1 and ,that is so big1 and ,that so far2 such is the meagre style of information with which we often have to be content4 ,the astronomers who live on other worlds1 if their faculties be in any degree comparable with ours1 must be similarly ignorant with regard to this earth4 ,inhabitants of our fellow-planets can know hardly anything more than that the earth on which we dwell is a globe #hjjj miles across1 with many clouds around us4 ,some of the planets would not even pay us the compliment of recognizing our existence2 while from the other systems--the countless other systems--of space we are absolutely imperceptible and unknown4 ,out of all the millions of bodies which we can see1 you could very nearly count on your fingers those from which our earth would be visible4 ,this reflection is calculated to show us how vast must be the real extent of that universe around us4 ,here is our globe1 with its inhabitants1 with its great continents1 with its oceans1 with its empires1 its kingdoms1 with its arts1 its commerce1 its literature1 its sciences1 and yet it would seem that all these things are absolutely unknown to any inhabitants that may exist elsewhere4 ,i do not think that any reasonable person will doubt that there must be inhabitants elsewhere4 ,there are millions of globes1 many of them more splendid than ours4 ,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 life1 with all that life involves1 is manifested4 ,you will rather think that our globe is but one in the mighty fabric1 and that other globes may teem with interest just as ours does4 ,we can1 of course1 make no conjecture as to what the nature of the life may be elsewhere4 ,could a traveller visit some other globes and bring back specimens of the natural objects that he found there1 no collections that the world has ever seen could rival them in interest4 ,when ,i go into the ,british ,natural ,history ,museum and look around that marvellous collection1 it awakens in me a feeling of solemnity4 ,i see there the remains of mighty extinct animals which once roamed over this earth2 also objects which have been dredged from the bottom of the sea at a depth of some miles2 there ,i can examine crystals which have required incalculable ages for their formation2 and there ,i look at meteorites which have travelled from the heavens above down on to the earth beneath4 ,such sights1 and the reflections they awaken1 bring before us in an imposing manner the phenomena of our earth1 and the extent and interest of its past history4 ,oliver ,wendell ,holmes said that the only way to see the ,british ,museum was to take lodgings close by when you were a boy1 and to stay in the ,museum from nine to five every day until you were an old man2 then you would begin to have some notion of what this ,institution contains4 ,think what millions of ,british ,museums would be required were the universe to be adequately illustrated3 one museum for the earth1 another for ,mars1 another for ,venus--but it would be useless attempting to enumerate them6 ,most of us must be content with acquiring the merest shred of information with regard even to our own earth4 ,perhaps a schoolboy will think it fortunate that we are so ignorant with respect to the celestial bodies4 ,what an awful vista of lessons to be learned would open before his view1 if only we had a competent knowledge of the other globes which surround us in space6 ,i should like to illustrate the extent of the universe by following this reflection a little further4 ,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 know4 ,of course1 all of us learn geography and history4 ,we must know the geography of the leading countries of the globe1 and we must have some knowledge of their inhabitants and of their government1 their resources and their civilization4 ,it would seem shockingly ignorant not to know something about ,china1 or not to have some ideas on the subject of ,india or ,egypt4 ,the discovery of the ,new ,world also involves matters on which every boy and girl has to be instructed4 ,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 history1 of the creatures that inhabit them1 of their different products and climates1 then everybody would be anxious to learn those particulars2 and even when the novelty had worn off1 it would still be right for us to know something about countries perhaps more populous than ,china1 about nations more opulent than our own1 about battles mightier than ,waterloo1 about animals and plants far stranger than any we have ever dreamt of4 ,an outline of all such matters should1 of course1 be learned1 and as the amount of information would be rather extensive1 we will try to condense it as much as possible4 ,to aid us in realizing the full magnificence of that scheme in the heavens of which we form a part1 ,i shall venture to give an illustration4 ,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 us4 ,these stars being suns1 and many of them being brighter and larger than our own sun1 it is but reasonable to presume that they may be attended by planetary systems4 ,i do not say that we have any right to infer that such systems are like ours4 ,it is not improbable that many of the suns around us have a much poorer retinue than that which dignifies our sun4 ,on the other hand1 it is just as likely that many of these other suns may be the centres of systems far more brilliant and interesting1 with far greater diversity of structure1 with far more intensity and variety of life and intelligence than are found in the system of which we form a part4 ,it is only reasonable for us to suppose that1 as our earth is an average planet1 so our sun is an average star both in size and in the importance of its attendants4 ,we may take the number of stars in the sky at about one hundred millions2 and thus we see that the books which are to contain a description of the entire universe--or rather1 ,i should say1 of the entire universe that we see--must describe #ajj1jjj1jjj times as much as is contained in our single system4 ,of course1 we know next to nothing of what the books should contain2 but we can form some conjecture of the number of those books1 and this is the notion to which ,i now ask your attention4 ,so vast is the field of knowledge that has to be traversed1 that we should be obliged to compress our descriptions into the narrowest compass4 ,we begin with a description of our earth1 for nearly all the books in the libraries that exist at this moment are devoted to subjects connected with this earth4 ,they include various branches of history1 innumerable languages and literatures and religions1 everything relating to life on this globe1 to its history in past geological times1 to its geography1 to its politics1 to every variety of manufacture and agriculture1 and all the innumerable matters which concern our earth's inhabitants1 past and present4 ,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 library4 ,i can only allow to the earth one volume of about #ejj pages4 ,everything that has to be said about our earth must be packed within this compass4 ,all terrestrial languages1 histories1 and sciences that cannot be included between its covers can find no other place on our shelves4 ,i cannot spare any more room4 ,our celestial library will be big enough1 as you shall presently see4 ,i am claiming a good deal for our earth when ,i regard it as one of the most important bodies in the solar system4 ,of course it is not the biggest--very far from it2 but it seems as if the big planets and the sun were not likely to be inhabited1 so that if we allow one other volume to the rest of the solar system1 it will perhaps be sufficient1 though it must be admitted that ,venus1 of which we know next to nothing1 except that it is as large as the earth1 may also be quite as full of life and interest4 ,mars and ,mercury are also among the planets with possible inhabitants4 ,we are1 therefore1 restricting the importance of the solar system as much as possible1 perhaps even too much1 by allowing it two4 ,within those two volumes every conceivable thing about the entire solar system--sun1 planets 7great and small71 moons1 comets1 and meteors--must be included1 or else it would not be represented at all in the great celestial library4 ,we shall deal on similar principles with the other systems through space4 ,each of the #ajj1jjj1jjj stars will have two volumes allotted to it4 ,within the two volumes devoted to each star we must compress our description of the body itself and of the system which surrounds it2 the planets1 their inhabitants1 histories1 arts1 sciences1 and all other information4 ,i am not1 remember1 discussing the contents1 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 heavens4 ,let us try to form some estimate as to the kind of library that would be required to accommodate #bjj1jjj1jjj volumes4 ,i suppose a long straight hall1 so lofty that there could be fifty shelves of books on each side4 ,as you enter you look on the right hand and on the left1 and you see it packed from floor to ceiling with volumes4 ,we have arranged them according to the constellations4 ,all the shelves in one part contain the volumes relating to the worlds in the ,great ,bear1 while upon the other side may repose ranks upon ranks of volumes relating to the constellation of ,orion4 ,i shall suppose that the volumes are each about an inch and a half thick1 and as there are fifty shelves on each side1 you will easily see that for each foot of its length the hall will accommodate #hjj books4 ,we can make a little calculation as to the length of this library1 which1 as we walk down through it1 stretches out before us in a majestic corridor1 with books1 books everywhere4 ,let us continue our stroll1 and as we pass by we find the shelves on both sides packed with their thousands of volumes2 and we walk on and on1 and still see no end to the vista that ever opens before us4 ,in fact1 no building that was ever yet constructed would hold this stupendous library4 ,let the hall begin on the furthest outskirts of the west of ,london1 carry it through the heart of the city1 and away to the utmost limits of the east--not a half of the entire books could be accommodated4 ,the mighty corridor would have to be fifty miles long1 and to be packed from floor to ceiling with fifty shelves of books on each side1 if it is to contain even this very inadequate description of the contents of the visible universe4 ,imagine the solemn feelings with which we should enter such a library1 could it be created by some miracle6 ,as we took down one of the volumes1 with what mysterious awe should we open it1 and read therein of some vast world which eye had never seen6 ,there we might learn strange problems in philosophy1 astonishing developments in natural history2 with what breathless interest we should read of inhabitants of an organization utterly unknown to our merely terrestrial experience6 ,notwithstanding the vast size of the library1 the description of each globe would have to be very scanty4 ,thus1 for instance1 in the single book which referred to the earth ,i suppose a little chapter might be spared to an island called ,england1 and possibly a page or so to its capital1 ,london4 ,similarly meagre would have to be the accounts of the other bodies in the universe2 and yet1 for this most inadequate of abstracts1 a library fifty miles long1 and lined closely with fifty shelves of books on each side1 would be required6 ,methuselah lived1 we are told1 nine hundred and sixty-nine years2 but even if he had attained his thousandth birthday he would have had to read about #cjj of these books through every day of his life before he accomplished the task of learning even the merest outline about the contents of space4 ,if1 indeed1 we were to have a competent knowledge of all these other globes1 of all their countries1 their geographies1 their nations1 their climates1 their plants1 their animals1 their sciences1 languages1 arts and literatures1 it is not a volume1 or a score of volumes1 that would be required1 but thousands of books would have to be devoted to the description of each world alone1 just as thousands of volumes have been devoted to the affairs of this earth without exhausting the subjects of interest it presents4 ,hundreds of thousands of libraries1 each as large as the ,british ,museum1 would not contain all that should be written1 were we to have anything like a detailed description of the universe which we see4 ,i specially emphasize the words just written1 and ,i do so because the grandest thought of all1 and that thought with which ,i conclude1 brings before us the overwhelming extent of the unseen universe4 ,our telescopes can1 no doubt1 carry our vision to an immeasurable distance into the depths of space4 ,but there are1 doubtless1 stars beyond the reach of our mightiest telescopes4 ,there are stars so remote that they cannot record themselves on the most sensitive of photographic plates4 ,on the blackboard ,i draw a little circle with a piece of chalk4 ,i think of our earth as the centre1 and this circle will mark for us the limit to which our greatest telescopes can sound4 ,every star which we see1 or which the photographic plate sees1 lies within this circle2 but1 are there no stars outside8 ,it is true that we can never see them1 but it is impossible to believe that space is utterly void and empty where it lies beyond the view of our telescopes4 ,are we to say that inside this circle stars1 worlds1 nebul1 and clusters are crowded1 and that outside there is nothing8 ,everything teaches us that this is not so4 ,we occasionally gain accession to our power by adding perhaps an inch to the diameter of our object-glass1 or by erecting a telescope in an improved situation on a lofty mountain peak1 or by procuring a photographic plate of increased sensibility4 ,it thus happens that we are enabled to extend our vision a little further and to make this circle a little larger1 and thus to add a little more to the known inside which has been won from the unknown outside4 ,whenever this is done we invariably find that the new region thus conquered is also densely filled with stars1 with clusters1 and with nebul2 it is thus unreasonable to doubt that the rest of space also contains untold myriads of objects1 even though they may never1 by any conceivable improvement in our instruments1 be brought within the range of our observation4 ,reflect that this circle is comparatively small with respect to the space outside4 ,it occupied but a small spot on this blackboard1 the blackboard itself occupies only a small part of the end of the theatre1 while the end of the theatre is an area very small compared with that of ,london1 of ,england1 of the world1 of the solar system1 of the actual distance of the stars4 ,in a similar way the region of space which is open to our inspection is an inconceivably small portion of the entire extent of space4 ,the unknown outside is so much larger than the known inside1 it is impossible to express the proportion4 ,i write down unity in this corner and a cipher after it to make ten1 and six ciphers again to make ten millions1 and again1 six ciphers more to make ten billions2 but ,i might write six more1 ay1 ,i might cover the whole of this blackboard with ciphers1 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 see4 ,if1 therefore1 we admit the fact1 which no reasonable person can doubt1 that this outside1 this unknown1 this unreachable and1 to us1 invisible space does really contain worlds and systems as does this small portion of space in which we happen to be placed--then1 indeed1 we shall begin truly to comprehend the majesty of the universe4 ,what figures are to express the myriads of stars that should form a suitable population for a space inconceivably greater than that which contains #ajj1jjj1jjj stars8 ,but our imagination will extend still further4 ,it brings before us these myriads of unseen stars with their associated worlds1 it leads us to think that these worlds may be full to the brim with interests as great as those which exist on our world4 ,when we remember that1 for an adequate description of the worlds which we can see1 one hundred thousand libraries1 each greater than any library on earth1 would be utterly insufficient1 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 universe8 ,let us conceive that omniscience granted to us an adequate revelation of the ample glories of the heavens1 both in that universe which we do see and in that infinitely greater universe which we do not see4 ,let a full inventory be made of all those innumerable worlds1 with descriptions of their features and accounts of their inhabitants and their civilizations1 their geology and their natural history1 and all the boundless points of interest of every kind which a world in the sense in which we understand it does most naturally possess4 ,let those things be written every one1 then may we say that were this whole earth of ours covered with vast buildings1 lined from floor to ceiling with book-shelves--were every one of these shelves stored full with volumes1 yet1 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 heavens4 ,,concluding ,,chapter4 ,,how ,,to ,,name ,,the ,,stars4 ,every one who wishes to learn something about astronomy should make a determined effort to become acquainted with the principal constellations1 and to find out the names of the brighter and more interesting stars4 ,i have therefore added to ,,star-,,land this little chapter1 in which ,i have tried to make the study of the stars so simple that1 by taking advantage of a few clear nights1 there ought to be no difficulty in obtaining a knowledge of a few constellations4 ,the first step is to become familiar with the ,great ,bear1 or ,ursa ,major1 as astronomers more generally call the group4 ,we begin with this1 because after it has been once recognized1 then you will find it quite easy to make out the other constellations and stars4 ,it may save you some trouble if you can get some one to point out to you the ,great ,bear2 but even without such aid1 ,i think you will be able to make out the seven bright stars which form this remarkable group1 from the figure here given 7,fig4 #ib74 ,of course1 the position of this constellation1 as of every other in the heavens1 changes with the hour of the night1 and changes with the seasons of the year4 ,about ,april the constellation at #aa o'clock at night is high over your head4 ,in ,september at the same hour1 the ,great ,bear is low down in the north4 ,it is to be seen in the west in ,july1 and at ,christmas it lies in the east at convenient hours in the evening for observation4 ,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 sets4 ,whenever the sky is clear after nightfall1 the ,great ,bear is to be seen somewhere1 while the brightness of its component stars makes it a conspicuous object4 ,indeed1 there is only one constellation in the sky1 namely1 that of ,orion1 which is a more brilliant group than the ,great ,bear4 ,we shall tell you about ,orion presently1 but it does not suit to begin with1 because it can only be seen in winter1 and is then placed very low down in the heavens4 ,illustration3 ,,fig4 #ib4--,the ,great ,bear and the ,pole ,star4 ,your next lesson will be to utilize the ,great ,bear for the purpose of pointing out the ,pole ,star4 ,look at the two stars marked and 4 ,they are called the 8,pointers10 because if you follow the direction they indicate along the dotted line in the figure1 they will conduct your glance to the ,pole4 ,this is the most important star in the heavens to astronomers1 because it happens to mark very nearly the position of the ,pole on the sky4 ,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 night4 ,it will appear to remain in the same place in the sky1 while the other stars change their places from hour to hour4 ,it is very fortunate that we have a star like this in the northern heavens2 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 north4 ,the ,pole ,star belongs to a constellation which we call the ,little ,bear2 two other conspicuous members of this group are the two 8,guards02 you will see how they are situated from ,fig4 #hb1 p4 #cbb4 ,they lie nearly midway between the ,pole ,star and the last of the three stars which form the ,great ,bear's ,tail4 ,the same figure will also introduce us to another beautiful constellation1 namely1 ,cassiopeia4 ,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 ,bear4 ,illustration3 ,,fig4 #ic4--,the ,great ,square of ,pegasus4 ,cassiopeia is also one of the constellations that never set to ,british observers2 but now we have to speak of groups which do set1 and which1 therefore1 can only be observed when the proper season comes round4 ,the first of these is 8the ,great ,square of ,pegasus02 you cannot see this group conveniently in the spring or summer1 but during the autumn and winter it is well placed after nightfall4 ,there are four conspicuous stars forming the corners of the square1 and then three others marked 1 1 and 7,fig4 #ic71 which form a sort of handle to the square4 ,in fact1 if you once recognize this group1 you will perhaps see in it a resemblance to a great saucepan with a somewhat bent handle1 and then you will be acquainted with a large tract of ,star-land near the ,square of ,pegasus4 ,from the figure you will see that a line imagined to be drawn from the ,pole ,star over the end of ,cassiopeia1 and then produced as far again1 will just lead to the ,great ,square4 ,i have also marked on this figure two objects that are of great telescopic interest2 one of them is the ,nebula in ,andromeda1 of which we had an account in the last lecture4 ,you see it lies halfway between the corner of the square and the group of ,cassiopeia4 ,another interesting object is the star marked ,andromed4 ,the telescope shows it to consist of a pair of stars1 the colors of which are beautifully contrasted4 ,at the end of this handle to the ,great ,square of ,pegasus is the star 1 in the constellation of ,perseus4 ,it lies between two other stars and 4 ,we refer to ,fig4 #hd1 in which these stars are shown4 ,we there employed the figure to indicate the position of ,algol1 the remarkable variable star4 ,your map will also point out some other important stellar features4 ,if we curve round the three marked 1 1 and of ,perseus1 the eye is conducted to ,capella1 a gem of the first magnitude in the constellation of ,auriga4 ,close to ,capella is a long triangle1 the corners of which are the 8,hdi10 the three kids--which ,capella is supposed to nurture4 ,if we carry a curve through 1 1 1 of ,perseus1 and now bend it in the opposite way1 the eye is led through and in the same constellation1 and then on to the ,pleiades1 of which we have already spoken4 ,illustration3 ,,fig4 #id4--,orion and ,sirius4 ,perseus lies in one of the richest parts of the heavens4 ,the ,milky ,way stretches across the group1 and the sky is strewn with stars beyond number4 ,even an opera-glass directed to this teeming constellation cannot fail to afford the observer a delightful glimpse of celestial scenery4 ,we may1 however1 specially remind the beginner that the objects on this map are not always to be seen1 and as an illustration of the way in which the situation and the visibility of the constellations are affected by the time of year1 ,i shall take the case of the ,pleiades and follow them through a season4 ,let us suppose that we make a search for this group at #aa ,p4,m4 every night4 ,on the #ast of ,january1 the ,pleiades will be found high up in the sky in the southwest4 ,on the #ast of ,march1 they will be setting in the west at the same hour4 ,on the #ast of ,may1 the ,pleiades are not visible1 neither are they on the #ast of ,july4 ,on the #ast of ,september1 they will be seen low down in the east4 ,on the #ast of ,november1 they will be high in the heavens in the southeast4 ,on the ensuing #ast of ,january1 the ,pleiades will be found back in the same place which they occupied on the same date in the preceding year1 and so on throughout the cycle4 ,of course1 you will not suppose that their changes are due to actual motions in the group of stars themselves4 ,they are merely apparent1 and are to be explained by the motion of the earth round its axis1 and the revolution around the sun4 ,next we are to become acquainted with the glory of our winter skies1 the constellation of ,orion1 ,fig4 #id4 ,i dare say many of my readers are already familiar with the well-known twin stars which form the belt of ,orion1 but if not1 they will be able to recognize it by the help of the groups already learned4 ,imagine a line drawn from the ,pole ,star through ,capella1 and then produced as much further again1 and we shall be conducted into the precincts of ,orion4 ,this group lies on the equator1 and1 consequently1 it is equally familiar to southern astronomers and to those of the north4 ,it can be best seen by those who observe it from or near the equator4 ,the brightest star in ,orion is known either as ,orionis or as ,betelgeuze1 by which name it is represented in the figure4 ,this star is of the first magnitude1 and so is ,rigel on the opposite side of the belt4 ,the three stars of the belt and the two others1 and 1 at which they point above and below1 are of second magnitude4 ,the owner of a telescope finds especial attractions in this constellation4 ,notably among the subjects which will interest him is the ,great ,nebula1 the position of which is indicated in our figure4 ,under the middle of the belt are a few stars1 around which is a hazy light that is perceptible with the smallest telescopic aid4 ,viewed by instruments of adequate proportions1 these have developed into a marvellous nebula of glowing gas1 attaining to dimensions so vast that no one has yet ever attempted to estimate them4 ,the vicinity of ,orion is also enriched with some of the most interesting stellar objects4 ,follow the line of the belt upwards to the right1 and your eye is conducted to a ruddy first magnitude star named ,aldebaran1 in the constellation of the ,bull4 ,this is a pleasing object1 which the beginner will sometimes be apt to confuse with the planet ,mars1 to which1 under certain circumstances1 it certainly bears a resemblance4 ,another very pleasing little group1 known as the ,hyades1 will be found near ,aldebaran4 ,if the line of the belt of ,orion be carried down to the left1 it will be found to point to ,sirius1 or the ,dog ,star4 ,you will find it an interesting occupation to make for yourself maps of small parts of the heavens4 ,first copy out the chief stars in their proper places from the star atlas1 and then fill in the smaller stars with your own observations4 ,try first on some limited region of the heavens2 take the figure of ,cassiopeia1 for instance1 or the ,square of ,pegasus1 and see if you can produce a fair representation of those groups by marking in the stars that your instrument will show you2 or take the ,pleiades and make a tracing of the principal stars of the group from the sketch that we have given 7,fig4 #hi71 then take an opera-glass and fill in as carefully as you can all that it will show4 ,i can assure you that you will find a little definite work of this kind full of interest and instruction4 ,but ,i hope you will desire to advance further in the study of the heavens4 ,it is to be remembered that with even the most moderate instruments there is much to be done4 ,many comets have been detected1 and many planets have been discovered1 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 over4 ,it remains for me to add a few words which will help you in finding the planets4 ,it is1 of course1 impossible to represent such objects as ,jupiter1 ,saturn1 ,venus1 ,mars1 and ,mercury on maps of the heavens1 because these bodies are constantly moving about1 and if their places were right to-day they would be wrong to-morrow4 ,the almanac will be necessary for you here4 ,you must find out by its help what planets are visible and in what part of the sky they are placed4 ,then you will have to compare your maps with the heavens1 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 planet4 ,although these objects are so star-like to the unaided eye1 yet the resemblance is at once dispelled when we use a telescope4 ,the star is only a bright point of light and white1 the planet shows a visible shape4 ,this is1 at least1 the case with the five planets ,i have named2 for there are others1 such as ,uranus and ,neptune1 which are too far to be much more than star-like points in ordinary telescopes4 ,the minor planets would not interest you4 ,i hope that the reading of ,,star-,,land will1 at all events1 induce you to make a beginning of the study of the heavens1 if you have not already done so4 ,if you have the advantage of a telescope1 so much the better2 but1 if this is impossible1 make the best use of your own eyes4 ,do not put it off or wait till you get some one to teach you4 ,if it be clear this very night1 go out and find the ,great ,bear and the ,pole ,star1 and as many of the other constellations as you can1 and at once commence your career as an astronomer4 ,,table ,,of ,,useful ,,astronomical ,,facts4 ,the sun's mean distance from the earth is #ib1gjj1jjj miles2 his diameter is #hfe1jjj miles1 and he rotates in a period between #be and #bf days4 ,the moon's mean distance from the earth is #bch1jjj miles2 the diameter of the moon is #bafj miles1 and the time of revolution round the earth is #bg4cbb days4 ,,the ,,planets4 ---------------------------------------- -------------------------- ,mean ,distance ,periodic ,time ,diameter from the ,sun of ,revolution of ,planet ,axial ,rotation4 in ,millions in ,days4 in ,miles4 of ,miles4 ---------------------------------------- -------------------------- ,hrs4 ,mins4 ,secs4 ,mercury #ce4i #hg4ifi #b1iib ,uncertain4 ,venus #fg4j #bbd4gj #g1ffj ,uncertain4 ,earth #ib4g #cfe4bf #g1iah #bc #ef #d4ji ,mars #ada #fhf4ih #d1bjj #bd #cg #bb4g ,jupiter #dhb #d1ccb4f #he1jjj #i #ee -- ,saturn #hhd #aj1gei #ga1jjj #aj #ad #bc4h ,uranus #a1ghj #cj1fhg #ca1gjj ,unknown4 ,neptune #b1ghj #fj1abg #cd1ejj ,unknown4 ---------------------------------------- -------------------------- ,,the ,,satellites ,,of ,,mars4 ---------------------------------------- --------- ,name4 ,mean ,distance from ,periodic ,time4 ,centre of ,mars4 ---------------------------------------- --------- ,hrs4 ,mins4 ,secs4 ,phobos #e1hjj miles4 #g #ci #ad ,deimos #ad1ejj 0 #cj #ag #ed ---------------------------------------- --------- ,,the ,,satellites ,,of ,,jupiter4 ---------------------------------------- --------------- ,name4 ,mean ,distance from ,periodic ,time4 ,centre of ,jupiter4 ---------------------------------------- --------------- ,days ,hrs4 ,mins4 ,secs4 ,i #bfb1jjj miles4 #a #ah #bg #cd ,,ii #dag1jjj 0 #c #ac #ac #db ,,iii #ffd1jjj 0 #g #c #db #cc ,,iv #a1agj1jjj 0 #af #af #cb #aa ,v #aab1djj 0 -- #aa #eg 787 ---------------------------------------- ---------------- ,,the ,,satellites ,,of ,,saturn4 ---------------------------------------- --------------- ,name4 ,mean ,distance from ,periodic ,time4 ,centre of ,saturn4 ---------------------------------------- --------------- ,days ,hrs4 ,mins4 ,secs4 ,mimas #aah1jjj miles4 #j #bb #cg #bg4i ,enceladus #aeb1jjj 0 #a #h #ec #f4g ,tethys #ahh1jjj 0 #a #ba #ah #be4g ,dione #bda1jjj 0 #b #ag #da #h4i ,rhea #ccg1jjj 0 #d #ab #be #aj4h ,titan #gha1jjj 0 #ae #bb #da #be4b ,hyperion #idf1jjj 0 #ba #g #g #dj4h ,iapetus #b1bhj1jjj 0 #gi #g #ed #dj4d ---------------------------------------- --------------- ,a ninth satellite was discovered in ,august1 #ahih1 by ,prof4 ,w4 ,h4 ,pickering1 but its mean distance and periodic time have not yet been determined with precision4 ,,the ,,satellites ,,of ,,uranus4 ---------------------------------------- ----- ,name4 ,mean ,distance from ,periodic ,time4 ,centre of ,uranus4 ,days4 ---------------------------------------- ----- ,ariel #aai1jjj miles4 #b4ebjchc ,umbriel #aff1jjj 0 #d4addaba ,titania #bgb1jjj 0 #h4gjehig ,oberon #cfc1jjj 0 #ac4dfcbfi ---------------------------------------- ----- ,,the ,,satellite ,,of ,,neptune4 ---------------------------------------- ----- ,name4 ,mean ,distance from ,periodic ,time4 ,centre of ,neptune4 ,days4 ---------------------------------------- ----- ,anonymous #bbj1jjj miles4 #e4hgfij ---------------------------------------- ----- ,,index4 ,a4 ,active ,volcanoes1 ,number of1 #aab4 ,adams1 of ,cambridge1 and ,leverrier1 of ,paris1 #bdi4 ,address of ,mr4 ,john ,smith1 #ccj4 ,africa would be better known if on ,moon1 #aje4 ,air as ,blanket to keep ,earth ,warm1 #h4 ,air not ,transparent1 #abe4 0 ,pump1 #ahc4 0 ,resistance of1 #ahb4 ,alcor1 #cdc4 ,aldebaran1 #chi4 ,algol1 #cdb1 #che4 ,alpha ,centauri1 #cai2 ,railway to1 #cch4 ,ancient ,theory to account for ,rising and ,setting of ,sun1 #dh4 ,andromeda1 #che4 ,andromeda1 ,nebula in1 photographed by ,mr4 ,roberts1 #cfd4 ,andromedes1 #caj4 ,annual ,motion of ,earth round ,sun1 #ef4 ,annular ,eclipses1 #hh4 ,apparent ,smallness of ,distant ,objects1 #be4 ,appearance of the ,sun1 #ce4 ,appearance of the ,sun during a ,total ,eclipse1 #dj4 ,arctic ,sun1 #ff4 ,area of ,moon's ,surface1 #hb4 ,ariel1 #bdc4 0 ,distance and ,period of1 #cib4 ,arthur's ,seat1 ,volcano1 #aad4 ,asaph ,hall1 ,professor1 #aie4 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,park1 #aae4 ,yerkes ,observatory1 ,chicago1 ,telescope at1 #ih4 ,transcriber's ,notes ,the ,table of ,contents does not mention the 8,table of ,useful ,astronomical ,facts01 which begins on page #cia1 immediately preceding the ,index4 ,punctuation1 hyphenation1 and spelling were made consistent when a predominant preference was found in the original book2 otherwise they were not changed4 ,simple typographical errors were corrected2 unbalanced quotation marks were remedied when the change was obvious1 and otherwise left unbalanced4 ,illustrations in this e,book have been positioned between paragraphs and outside quotations4 ,in versions of this e,book that support hyperlinks1 the page references in the ,list of ,illustrations lead to the corresponding illustrations4 ,the index was not checked for proper alphabetization or correct page references4 ,page #bjh3 ,the reference to page #abd should be to page #abh4