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-The Project Gutenberg EBook of Star-land, by Robert Stawell Ball
-
-This eBook is for the use of anyone anywhere in the United States and most
-other parts of the world at no cost and with almost no restrictions
-whatsoever.  You may copy it, give it away or re-use it under the terms of
-the Project Gutenberg License included with this eBook or online at
-www.gutenberg.org.  If you are not located in the United States, you'll have
-to check the laws of the country where you are located before using this ebook.
-
-Title: Star-land
-
-Author: Robert Stawell Ball
-
-Release Date: September 18, 2019 [EBook #60318]
-
-Language: English
-
-Character set encoding: UTF-8
-
-*** START OF THIS PROJECT GUTENBERG EBOOK STAR-LAND ***
-
-
-
-
-Produced by deaurider, Charlie Howard, and the Online
-Distributed Proofreading Team at http://www.pgdp.net (This
-file was produced from images generously made available
-by The Internet Archive)
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-[Illustration]
-
-
-
-
-  STAR-LAND
-
-  _BEING TALKS WITH YOUNG PEOPLE ABOUT THE
-  WONDERS OF THE HEAVENS_
-
-
-  BY
-
-  SIR ROBERT STAWELL BALL, F.R.S.
-
-  LOWNDEAN PROFESSOR OF ASTRONOMY IN
-  THE UNIVERSITY OF CAMBRIDGE
-  AUTHOR OF “THE STORY OF THE HEAVENS,” ETC.
-
-
-  Illustrated
-
-
-  NEW AND REVISED EDITION
-
-
-  BOSTON, U.S.A., AND LONDON
-  GINN & COMPANY, PUBLISHERS
-  The Athenæum Press
-  1899
-
-
-
-
-  ENTERED AT STATIONERS’ HALL
-
-  COPYRIGHT, 1899, BY
-  CASSELL & COMPANY, LIMITED
-
-  ALL RIGHTS RESERVED
-
-
-
-
-  To
-  THOSE YOUNG FRIENDS
-  WHO HAVE ATTENDED MY CHRISTMAS LECTURES
-  THIS LITTLE BOOK
-  IS DEDICATED
-
-
-
-
-PREFACE TO FIRST EDITION.
-
-
-It has long been the custom at the Royal Institution of Great Britain
-to provide each Christmastide a course of Lectures specially addressed
-to a juvenile audience.
-
-On two occasions, namely, in 1881 and in 1887, the Managers entrusted
-this honorable duty to me. The second course was in the main a
-repetition of the first; and on my notes and recollections of both the
-present little volume has been founded.
-
-I am indebted to my friends Rev. MAXWELL CLOSE, Mr. ARTHUR RAMBAUT, and
-Dr. JOHN TODHUNTER for their kindness in reading the proofs.
-
-                                        ROBERT S. BALL.
-
-  OBSERVATORY,
-      CO. DUBLIN,
-        _Oct. 22, 1889_.
-
-
-
-
-CONTENTS.
-
-
-  LECTURE I.
-
-  THE SUN.
-                                                                    PAGE
-
-  The Heat and Brightness of the Sun--Further Benefits that we
-  receive from the Sun--The Distance of the Sun--How Astronomers
-  measure the Distances of the Heavenly Bodies--The Apparent
-  Smallness of Distant Objects--The Shape and Size of the Sun--
-  The Spots on the Sun--Appearances seen during a Total Eclipse
-  of the Sun--Night and Day--The Daily Rotation of the Earth--
-  The Annual Motion of the Earth round the Sun--The Changes of
-  the Seasons--Sunshine at the North Pole                              1
-
-
-  LECTURE II.
-
-  THE MOON.
-
-  The Phases of our Attendant the Moon--The Size of the Moon--How
-  Eclipses are produced--Effect of the Moon’s Distance on its
-  Appearance--A Talk about Telescopes--How the Telescope aids us
-  in Viewing the Moon--Telescopic Views of the Lunar Scenery--On
-  the Origin of the Lunar Craters--The Movements of the Moon--On
-  the Possibility of Life in the Moon                                 74
-
-
-  LECTURE III.
-
-  THE INNER PLANETS.
-
-  Mercury, Venus, and Mars--How to make a Drawing of our System--
-  The Planet Mercury--The Planet Venus--The Transit of Venus--
-  Venus as a World--The Planet Mars and his Movements--The
-  Ellipse--The Discoveries made by Tycho and Kepler--The
-  Discoveries made by Newton--The Geography of Mars--The
-  Satellites of Mars--How the Telescope aids in Viewing Faint
-  Objects--The Asteroids, or Small Planets                           134
-
-
-  LECTURE IV.
-
-  THE GIANT PLANETS.
-
-  Jupiter, Saturn, Uranus, Neptune--Jupiter--The Satellites of
-  Jupiter--Saturn--The Nature of the Rings--William Herschel--
-  The Discovery of Uranus--The Satellites of Uranus--The
-  Discovery of Neptune                                               212
-
-
-  LECTURE V.
-
-  COMETS AND SHOOTING STARS.
-
-  The Movements of a Comet--Encke’s Comet--The Great Comet of
-  Halley--How the Telegraph is used for Comets--The Parabola--
-  The Materials of a Comet--Meteors--What becomes of the Shooting
-  Stars--Grand Meteors--The Great November Showers--Other Great
-  Showers--Meteorites                                                255
-
-
-  LECTURE VI.
-
-  STARS.
-
-  We try to make a Map--The Stars are Suns--The Numbers of the
-  Stars--The Clusters of Stars--The Rank of the Earth as a Globe
-  in Space--The Distances of the Stars--The Brightness and Color
-  of Stars--Double Stars--How we find what the Stars are made
-  of--The Nebulæ--What the Nebulæ are made of--Photographing the
-  Nebulæ--Conclusion                                                 318
-
-
-  CONCLUDING CHAPTER.
-
-  HOW TO NAME THE STARS.                                             381
-
-
-
-
-STAR-LAND.
-
-
-
-
-LECTURE I.
-
-THE SUN.
-
-  The Heat and Brightness of the Sun--Further Benefits that we receive
-      from the Sun--The Distance of the Sun--How Astronomers measure
-      the Distances of the Heavenly Bodies--The Apparent Smallness of
-      Distant Objects--The Shape and Size of the Sun--The Spots on the
-      Sun--Appearances seen during a Total Eclipse of the Sun--Night
-      and Day--The Daily Rotation of the Earth--The Annual Motion of
-      the Earth round the Sun--The Changes of the Seasons--Sunshine at
-      the North Pole.
-
-
-THE HEAT AND BRIGHTNESS OF THE SUN.
-
-We can all feel that the sun is very hot, and we know that it is very
-big and a long way off. Let us first talk about the heat from the sun.
-On a cold day it is pleasant to go into a room with a good fire, and
-everybody knows that the nearer we go to the fire, the more strongly
-we feel the heat. The boy who is at the far end of the room may be
-shivering with cold, while those close to the fire are as hot as they
-find to be pleasant. If we could draw much nearer to the sun than we
-actually are, we should find the heat greatly increased. Indeed, if we
-went close enough, the temperature would rise so much that we could
-not endure it; we should be roasted. On the other hand, we should
-certainly be frozen to death if we were transported much further away
-from the sun than we are now. We are able to live comfortably, because
-our bodies are just arranged to suit the warmth which the sun sends to
-that distance from it at which the earth is actually placed.
-
-Suppose you were able to endure any degree of heat, and that you had
-some way of setting out on a voyage to the sun. Take with you a wax
-candle, a leaden bullet, a penny, a poker, and a flint. Soon after
-you have started you find the warmth from the sun increasing, and the
-candle begins to get soft and melt away. Still, on you go, and you
-notice that the leaden bullet gets hotter and hotter, until it becomes
-too hot to touch, until at last the lead has melted, as the wax had
-previously done. However, you are still a very long way from the sun,
-and you have the penny, the poker, and the flint remaining. As you
-approach closer to the luminary the heat is ever increasing, and at
-last you notice that the penny is beginning to get red-hot; go still
-nearer, and it melts away, and follows the example of the bullet and
-the candle. If you still press onwards, you find that the iron poker,
-which was red-hot when the penny melted, begins to get brighter and
-brighter, till at last it is brilliantly white, and becomes so dazzling
-that you can hardly bear to look at it; then melting commences, and the
-poker is changed into liquid like the penny, the lead, and the wax.
-Yet a little nearer you may carry the flint, which is now glowing with
-the same fervor which fused the poker, but even the flint itself will
-have to yield at last and become, not merely a liquid like water, but
-a vapor like steam.
-
-You will ask, how do we learn all this? As nobody could ever make such
-a journey, how can we feel certain that the sun is so excessively
-hot? I know that what I say is true for various reasons, but I will
-only mention one, which is derived from an experiment with the
-burning-glass, that most boys have often tried.
-
-[Illustration: FIG. 1.--How to use the Burning-glass.]
-
-[Illustration: FIG. 2.--The Noonday Gun.]
-
-We may use one of those large lenses that are intended for magnifying
-photographs. But almost any kind of lens will do, except it be too
-flat, as those in spectacles generally are. On a fine sunny day in
-summer, you turn the burning-glass to the sun, and by holding a
-piece of paper at the proper distance a bright spot will be obtained
-(Fig. 1). At that spot there is intense heat, by which a match can be
-lighted, gunpowder exploded, or the paper itself kindled. The broad
-lens collects together the rays from the sun that fall upon it, and
-concentrates them in one spot, which consequently becomes hot and
-bright. If we merely used a flat piece of glass the sunbeams would go
-straight through; they would not be gathered together, and they would
-not be strong enough to burn the paper. The lens, you see, is not
-flat; its faces are curved, and they thus acquire the power of bending
-in rays of light or heat, so as to unite their effect on that one
-point which we call the _focus_. When a great number of rays are thus
-collected on the same spot, each of them contributes a little warmth.
-
-[Illustration: FIG. 3.--A Tell-tale for the Sun.]
-
-Some ingenious person has turned this principle to an odd use, by
-arranging a burning-glass over a cannon in such a way that just when
-noon arrived the spot of light should reach the touch-hole of the
-cannon and fire it off. Thus the sun itself is made to announce the
-middle of the day (Fig. 2).
-
-Another application of the burning-glass is to obtain a record of the
-number of hours of sunshine in each day. You will understand the
-apparatus from Fig. 3; the lens is here replaced by a glass globe,
-which acts as a burning-glass. As the sun moves over the sky the bright
-spot of light also moves, and therefore burns its track on a sheet of
-paper marked with lines corresponding to the hours. When the sun is
-hidden by clouds the burning ceases, so by preserving each day the
-piece of paper, we have an unerring tell-tale, which shows us during
-what hours the sun was shining brightly, and the hours during which he
-was hidden. You see, the burning-glass is not merely a toy, it can be
-made useful in helping us to learn something about the weather.
-
-Another experiment with the burning-glass will also teach us something.
-Take a candle, and from its flame you can get a bright point at the
-focus. It may fall upon your hand, but you can hardly feel it, and
-you will readily believe that the focus is not nearly so hot as the
-candle. Even when a burning-glass is held in front of a bright fire
-there is comparatively little heat in the focus. By using a lens to
-condense the beams from an electric lamp, Professor Tyndall has shown
-how to light a piece of paper, and to produce many other effects.
-But, nevertheless, the focus is not nearly so hot as the arc between
-the two glowing carbons. You might move your finger through the focus
-without much inconvenience, but I would not recommend you to trust your
-finger between the poles of the electric light itself. The temperature
-obtained at the focus of a burning-glass seems thus to be always less
-than that prevailing at the source of heat itself. This principle
-will be equally true when we turn a burning-glass to the sun, and
-hence we know that the sun must be hotter than any heat which can be
-obtained by the biggest burning-glass on the brightest of summer days.
-But burning-glasses a yard wide have been made, and astonishing heat
-effects have been produced. Steel has thus been melted by the sunbeams,
-and so have other substances which even our greatest furnaces cannot
-fuse. Therefore the sun must have a higher temperature than that of
-molten steel; higher, indeed, than any temperature we can produce on
-the earth.
-
-I have tried to prove to you that the sun is very hot; but it would be
-well to see what arguments might be used on the other side. Indeed, it
-is by considering objections that we often learn. So I shall tell you
-of a difficulty that was once raised when I was endeavoring to explain
-the heat of the sun to an intelligent man. “I am sure,” said my friend,
-“that you must be quite wrong. You said that the nearer you got to
-the sun the hotter it would be; but I know this to be a mistake. When
-tourists go to Switzerland, they sometimes climb very high mountains.
-But the top of a mountain, of course, is nearer the sun than below; and
-so, if the sun were really hot, the climber should have found it much
-warmer on the top of the mountain than at its base. But every one knows
-that there is abundant ice and snow on lofty Alpine summits, while down
-below in the valleys there may be at the same time excessively warm
-weather. Does it not therefore seem that the nearer we go to the sun
-the colder it is, and the further we are from the sun the warmer it
-is?”
-
-But my friend was quite wrong in his argument. The coldness of the
-mountain tops depends upon something which he had not taken into
-account. There is something else besides the sun which helps to make
-us so warm and comfortable. This other essential thing is more or less
-deficient at great heights. You know that we live by breathing air, and
-we find air wherever we go, over land and sea, all round the earth.
-Those who ascend in balloons are borne upwards by the air, and thus we
-can show that air extends for miles and miles over our heads, though it
-becomes lighter and thinner the loftier the elevation.
-
-We not only utilize the air for breathing, but it is also of
-indispensable service to us in another way. It acts as a blanket to
-keep the earth warm; indeed, we ought rather to describe the air as
-a pile of blankets one over the other. These air blankets enable the
-earth to preserve the heat received from the sunbeams by preventing it
-from escaping back again into space. Thus warmth is maintained, and
-our globe is rendered habitable. You see then, that for our comfort
-we require not only the sun to give us the heat, but also the set of
-blankets to keep it when we have got it. If we threw off the blankets
-we should be uncomfortable, though the sun were as bright as before.
-A man who goes to the top of a mountain at mid-day does approach the
-sun to some extent, and, so far as this goes, he ought no doubt to
-feel warmer, but the gain is far too small to be thought of. Even at
-the top of Mont Blanc the increase in heat due to the approach to the
-sun would be only one ten-millionth part of the whole. This would be
-utterly inappreciable; even a thermometer would not be delicate enough
-to show it. On the other hand, by ascending to the top of the mountain,
-the climber has got above the lower regions of the air; he has not, it
-is true, reached even halfway to the upper surface--that is still very
-far over his head--but the higher layers of the atmosphere are so very
-thin that they form most indifferent blankets. The Alpine climber on
-the top of the mountain has thus thrown off the best portion of his
-blankets, and receives a chill; while the gain of heat arising from his
-closer approach to the sun is imperceptible. Perhaps you will now be
-able to understand why eternal snow rests on the summits of the great
-mountains. They are chilled because they have not so many air blankets
-as the snug valleys beneath.
-
-The brightness of the sun is among the most wonderful things in nature,
-and there are three points that I ask you to remember, and then indeed
-you will agree with Milton, that the sun is “with surpassing glory
-crowned.” First think of the beauty and brilliancy of a lovely day in
-June. Then remember that all this flood of light comes from a single
-lamp at a most tremendous distance; and thirdly, recollect that the
-sun is not like a bull’s-eye lantern, concentrating all his light
-_specially_ for our benefit, but that he diffuses it equally around,
-and that we do not get on this earth the two-thousand-millionth part
-of what he gives out so plenteously! When we think of the brightness
-of day, of the distance from which the light has come, though Nature
-has not adjusted any vast lenses to direct the light specially in our
-direction, we begin to comprehend the sun’s true magnificence.
-
-
-FURTHER BENEFITS THAT WE RECEIVE FROM THE SUN.
-
-I want to show you how great should be the extent of our gratitude to
-the sun. Of course, on a bright summer’s day, when we are revelling in
-the genial warmth and enjoying the gladness of sunshine, it needs no
-words to convince us of the utility and of the beneficence of sunbeams.
-So we will not take midsummer. Let us take midwinter. Take this very
-Christmas season when the days are short and cheerless, the nights
-are long and dark and cold. We might be tempted to think that the sun
-had well-nigh forgotten us. It is true he only seems to pay us very
-occasional visits, and between fogs and clouds we in England see but
-little of him; but, visible or invisible, the sun incessantly tends
-us, and provides for our welfare in ways that perhaps we do not always
-remember.
-
-Let me give an illustration of what I mean. You will go back this dull
-and cold afternoon to the happy home where your Christmas holidays are
-being enjoyed. It will be quite dark ere you get there, for the sun in
-these wintry days sets so very early. You will gather around a cheerful
-fire. The curtains will be drawn, the lamps will be lighted, and the
-disagreeable weather outside will be forgotten in the pleasant warmth
-and light within. Five o’clock has arrived, the pretty wicker table has
-been placed near mamma’s chair; on it are the cups and saucers and the
-fancy teapot. Under the table is a little shelf, with some tempting
-cakes and a tender muffin. Two or three welcome friends have joined the
-little group, and a delightful half-hour is sure to follow.
-
-But you may say, “What have tea and muffins, lamps and fireplaces to do
-with the sun? Are they not all mere artificial devices, as far removed
-as possible from the sunbeams or the natural beauties which sunbeams
-create?” Well, not so far, perhaps, as you may think. Let us see.
-
-Poke up the fire, and while it is throwing forth that delicious warmth,
-and charming but flickering light, we will try to discover where that
-light and heat have come from. No doubt they have come from the coal,
-but then, whence came the coal? It came from the mine, where brave
-colliers hewed it out deep under the ground, and then it was hoisted
-to the surface by steam engines. Our inquiry must not stop here, for
-another question immediately arises, as to how this wonderful fuel
-came into the earth? When we examine coal carefully, by using the
-microscope to see its structure, we find that it is not like a stone;
-it is composed of trees and other plants, the leaves and stems of which
-can be sometimes recognized. Indeed, the fossil trunks and roots of
-the great trees are occasionally conspicuous in the coal-pit. It is
-quite plain that these are only the remains of a vegetation which was
-formerly growing and flourishing, and on further inquiry we learn that
-coal must have been produced in the following manner:--
-
-Once upon a time a great forest flourished. The sun shone down on this
-forest, and it was watered by genial showers, while insects and other
-creatures sported in its shades. It is true that the trees and plants
-were not like those we now see about us. They were more like ferns and
-mare’s-tails and gigantic club-mosses. In the fulness of time they
-died, and fell, and decayed, and others sprang up to meet the like end.
-Thus it happened that, in course of ages, the remains of leaves, and
-fruits, and trunks accumulated over the soil. The forest was situated
-near the seashore, and then a remarkable change took place--the land
-began slowly to sink. You need not think that this is impossible. Land
-has often been known to change its level gradually. In fact, a sinking
-process is slowly going on now in many places on the earth, while
-the land is rising in other localities. As the forest gradually sank
-lower and lower, the sea-water began to inundate it, and all the trees
-perished until, at last, deep water submerged the surface which had
-once been covered by a fine forest. At the bottom of this sea lay the
-decaying vegetation.
-
-That which was the destruction of the growing forest, proved to be the
-means of preserving its remains, for, then as now, the rivers flowed
-into the sea, and the waters of the rivers, especially in times of
-flood, carried down with them clay or mud, held in suspension. Upon
-the floor of the ocean this material was slowly deposited; and thus
-a coating of mud overlay the remains of the forest. In the course of
-ages, these layers grew thick and heavy, and hardened into a great flat
-rock, while the trunks and leaves underneath were squeezed together by
-the weight, and packed into a solid mass which became black, and in
-the course of time was transformed into coal.
-
-After ages and ages had passed by, the bed of the sea ceased to sink,
-and began slowly to rise. The water over the newly made layers of stone
-became shallower, and at last the floor was raised until it emerged
-from the sea. But, of course, it would not be the original ground
-which formed the surface of the newly uncovered land. The sheets of
-consolidated clay lay on the top; over the fresh surface life gradually
-spread, until man himself came to dwell there, while far beneath his
-feet the remains of the ancient vegetation were buried.
-
-When we now dig down through the rocks we come upon the portions of
-trees and other plants which the lapse of time, and the influence of
-pressure, have turned from leaves and wood into our familiar coal.
-
-That ancient forest grew because sunbeams abounded in those early
-times, and nourished a luxuriant vegetation. The heat and the light
-then expended so liberally by the sun were seized by the leaves of
-flourishing plants, and were stored away in their stems and foliage.
-Thus it is that the ancient sunbeams have been preserved in our
-coal-beds for uncounted thousands of years. When we put a lump of coal
-on our fire this evening, and when it sends forth a grateful warmth and
-cheerful light, it but reproduces for our benefit some of that store of
-preserved sunbeams of which our earth holds so large a treasure. Thus,
-the sun has contributed very materially to our comfort, for it has
-provided the fire to keep us warm.
-
-The orb of day has, however, ministered further to our tea party,
-for has it not produced the tea itself? The tea grew a long way off,
-most likely in China, where the plant was matured by the warmth of the
-sunbeams. From China the tea-chests were brought by a sailing vessel to
-London; the ship performed this long voyage by the use of sails, blown
-by what we call wind, which is merely the passage of great volumes of
-air as they hurry from one part of the earth to another.
-
-We may ask what makes the air move, for it will not rush about in
-this way unless there be considerable force to drive it. Here again
-we perceive the influence of the sun. Tracts of land are warmed by
-the genial sunbeams. The air receives the heat from the land, and the
-warm air is buoyant and ascends, while cooler air continually flows in
-to supply its place. To do this it has, of course, to rush across the
-country, and thus wind is caused. All the air currents on our earth are
-consequently due to the sun. You see, therefore, how greatly we are
-indebted to our brilliant luminary for the enjoyment of our tea-table.
-Not only has the sun given us the coal and the tea, but it has actually
-provided the means by which the tea was carried all the way from China
-to our own shores.
-
-We can also trace the connection between the hot water and the sun. Of
-course, the water has come immediately from the kettle, and that has
-been taken from the fire, and the fire was produced by sunbeams. Thus
-we learn that it is the warmth of the sun that has made the water boil.
-If you visit the water-works you will see great reservoirs. In some
-cases they have been filled by a river, sometimes the water is pumped
-from a deep well in the ground, sometimes it is the surface-water
-caught on a mountain side. Whatever be the immediate source of our
-water supply, the real origin is to be sought, not in the earth
-beneath, but in the heavens above. All the water we use day by day has
-come from the clouds. It is the clouds which sent down the rain, or
-sometimes the snow, or the hail, and it is this water from the clouds
-which fills our rivers. It is this water also which sinks deep into the
-earth and supplies our wells, so that from whatever apparent source the
-water seems to have come, it is indeed the clouds which have been the
-real benefactors. The water in your teacup to-night was, a little while
-ago, in a cloud, floating far overhead in the sky.
-
-We may look a little further and find whence the clouds have come. It
-is certain that clouds are merely a form of steam or vapor of water,
-and as they are so continually sending down rain on the earth, there
-must be some means by which their supply will be replenished. Here
-again our excellent friend the sun is to be found ever helping us
-secretly, if not helping us openly. He pours down his rich and warm
-beams on the great oceans, and the heat turns some of the water into
-vapor, which, being lighter than the air, ascends upwards for miles.
-There the vapor often passes into the form of clouds, and the winds
-waft these clouds to refresh the thirsty lands of the earth. Thus, you
-see, it is the sun which procures for us water from the great oceans
-which cover so much of our globe, and sends it on by the winds to
-supply our water-works, and fill our teapots. Notice another little
-kindliness of our great benefactor. The water of the oceans is quite
-salt. But we could not make tea with salt water, so the sun, when
-lifting the vapor from the sea, most thoughtfully leaves all the salt
-behind, and thus provides us with the purest of sweet water.
-
-That nice muffin was baked by the sun, toasted by the sun, and made
-from wheat grown by the sun. If the wheat was ground in a wind-mill,
-then the sun raised the wind which turned the mill. Perhaps the
-flour-mill was driven by steam, in which case the sun, long ago,
-provided the coal for the boiler. The miller might have lived on a
-river and used a water-mill, but if he did, then here again the sun
-actually did the work. The sun raised the water to the clouds, and
-after it had fallen in rain, and was on its way back to the sea, its
-descent was utilized to turn the water-wheel. The water derives its
-power to turn the mill from the fact that it is running downhill,
-but it could not run down unless it had first been raised up; and
-thus it is indeed the sun which drives the water-wheel. Nor can the
-baker dispense with the sun’s aid even if he rejected wind-mills, or
-steam-mills, or water-mills, and determined to grind the corn himself
-with a pestle and mortar. Here, at least, it might be thought that it
-is a man’s sinews and muscles that are doing the work, and so no doubt
-they are. But you are mistaken if you think the sun has not rendered
-indispensable aid. The sun has just as surely provided the power
-which moves the baker’s arms as it has raised the wind which turned
-the wind-mill. The force exerted in grinding with the pestle has been
-derived from the food that the man has eaten; that food was grown by
-the sun, and the man received from the food the energy it had derived
-from the sun’s heat. So that, look at it any way you please, even for
-the grinding of the wheat to make the muffin for your tea party, you
-are wholly indebted to the sun.
-
-It is the sun which has bleached the tablecloth to that snowy
-whiteness. The sun has given those bright colors which look so pretty
-in the girls’ dresses. With how much significance can we say and feel
-that light is pleasant to the eye, and what prettier name than Little
-Sunbeam can we have for the darling child who makes our home so bright?
-
-
-THE DISTANCE OF THE SUN.
-
-The sun is a very long way off. It is not easy for you to imagine a
-distance so great, but if you want to learn astronomy you must make
-the attempt. This is the first measurement that we shall have to make
-on our way to that far-off country called Star-Land; but long as we
-shall find it to be, we shall afterwards have to consider distances
-very much longer. When you are out in the street, or taking a walk in
-the country, you can see at once that this man is near, or that house
-is far, or that mountain is many miles away. This is because you have
-other objects between to help you to judge of the distances of these
-different objects. You will see, for example, that there are many
-houses or farmyards, and you will notice hedges dividing different
-fields between you and the mountain. You also see that there are
-woods and parks, and perhaps stretches of moorland extending up the
-slopes. You have an impression that the farmyards and fields are of
-considerable size, and that the woods or moors are wide and extensive;
-and putting these things together, you realize that the mountain must
-be miles away.
-
-But when we look at the sun we have no aids conveniently placed to
-help us in judging his distance. There are no intervening objects, and
-merely gazing at the sun helps us but little in obtaining any accurate
-knowledge. We must go to the astronomer and ask him to tell us how far
-he has found the sun to be, and then we must also beg from him some
-explanation of the method he has used in making his measurements.
-
-It has been found that the sun is, on the average, about ninety-three
-millions of miles from the earth; but sometimes it is a little further
-and sometimes it is a little nearer. Let us first try to count
-93,000,000. The easiest way will be to get the clock to do this for
-us; and here is a sum that I would suggest for you to work out. How
-long will the clock have to tick before it has made as many ticks as
-there are miles between the earth and the sun? Every minute the clock,
-of course, makes 60 ticks, and in 24 hours the total number will reach
-86,400. By dividing this into 93,000,000 you will find that more than
-1076 days, or nearly three years, will be required for the clock to
-perform the task.
-
-We may consider the subject in another way, and find how long an
-express train would take to go all the way from the earth to the sun.
-We shall suppose the speed of the train to be 40 miles an hour; and
-if the train ran for a whole day and a whole night without stopping,
-it would then accomplish 960 miles. In a year the distance travelled
-would reach 350,400 miles, and by dividing this into 93,000,000 we
-arrive at the conclusion that a train would have to travel at a pace
-of 40 miles an hour, not alone for days and for weeks and for years,
-but even for centuries. Indeed, not until 265 years had elapsed would
-the mighty journey have been ended. Even though King Charles I. had
-been present when the train began to move, the destination would not
-yet have been reached. No one who started in the train could expect to
-reach the end of the trip. That would not occur till the time of his
-great-great-grandchildren.
-
-
-HOW ASTRONOMERS MEASURE THE DISTANCES OF THE HEAVENLY BODIES.
-
-I shall so often have to speak of the distances of the celestial bodies
-that I may once for all explain how it is that we have been able to
-discover what these distances are. This would be a very puzzling matter
-if we were to try and describe it fully, but the principle of the
-method is not at all difficult. Do you know why you have been provided
-with two eyes? It is undoubted that one of the reasons is to aid you in
-estimating distances. You see this boy (Fig. 4) judges of the distance
-of his finger by the inclination of his two eyes when directed at it.
-In a similar way we judge of the distance of a heavenly body by making
-observations on it from two different stations.
-
-[Illustration: FIG. 4.--Two Eyes are better than One.]
-
-[Illustration: FIG. 5.--How we measured the Height of the Ball.]
-
-I shall illustrate our method of measuring the actual distance of a
-body in the heavens by showing you how we can find the height of that
-large india-rubber ball which is hanging from the ceiling. Of course,
-I do not intend to have a measuring tape from the ball itself, because
-I want to solve the problem on the same principle as that by which we
-measure the distance of the sun or of any other celestial body which we
-cannot reach. I will ask the aid of a boy and a girl, who will please
-stand one at each end of the lecture table. The apparatus we shall want
-is very simple; it consists of two cards and a pair of scissors. The
-boy will kindly shape his card to such an angle that when he holds it
-to his eye one side of the angle shall point straight at the little
-girl, and the other side shall point straight at the ball, just as you
-see in the picture (Fig. 5). The girl will also please do the same
-with her card, so that along one side she just sees the little boy’s
-face, while the other side points up to the ball. It will be necessary
-to cut these angles properly. If the angle be too big, then when one
-side points to the boy’s face, the other will be directed above the
-ball. If the angle on the card be too small, then one side will be
-directed below the ball, while the other is pointed to the boy. The
-whole accuracy of our little observations depends upon cutting the
-card angles properly. When they have been truly shaped it will be easy
-to find the distance of the ball. We first take a foot rule and measure
-the length of our table from one of our young friends to the other.
-That length is twelve feet, and to discover the distance of the ball
-we must make a drawing. We get a sheet of paper, and first rule a line
-twelve inches long. That will represent the length of the table, it
-being understood that each inch of the drawing is to correspond to a
-foot of the actual table. Let the end where the girl stood be marked
-B, and that of the boy, A, and now bring the cards and place them on
-the line just as shown in the figure. The card the girl has shaped is
-to be put so that the corner of it lies at B, and one edge along B A.
-Then the boy’s card is to be so put that its corner is at A and one
-edge along A B. Next with a pencil we rule lines on the other edges of
-the cards, taking care that they are kept all the time in their proper
-positions. These two lines carried on will meet at C; and this must be
-the position of the ball on the scale of our little sketch. It only now
-remains to take the foot rule and measure on the drawing the length
-from A to C. I find it to be twenty inches, and I have so arranged it
-that the distance from B to C is the same.
-
-[Illustration: FIG. 6.--This is what we wanted the Cards for.]
-
-I do not intend to trouble you much with Euclid in these lectures, but
-as many of my young friends have learned the sixth book, I will just
-refer to the well-known proposition, which tells us that the lengths
-of the corresponding sides of two similar triangles are proportional.
-We have here two similar triangles. There is the big one with the boy
-at one corner, the girl at the other, and the ball overhead. Here
-is the small triangle which we have just drawn. These triangles are
-similar because they have got the same angles, and it was to insure
-that they should have the same angles that we were so careful in
-shaping the cards. As these two triangles are similar, their sides must
-be proportional. We have agreed that the line A B, which is twelve
-inches long, is to represent the length of the table between the little
-boy and girl. Hence the distance, A C, must, on the same scale, be the
-interval between the ball and the boy at the end. This is twenty inches
-on the drawing, and therefore the actual distance from the end of the
-table to the ball is twenty feet.
-
-Hence you see that without going up to the ball or having a string from
-it, or in any other way making direct communication with it, we have
-been able to ascertain how far up in the air the ball is actually hung.
-This simple illustration explains the principle of the method by which
-astronomers are able to learn the distances of the different celestial
-bodies from the earth. You must think of the sun, the moon, and the
-stars as globes supported in some manner over our heads, and we seek to
-discover their distances from measurements of angles made at the ends
-of a base-line.
-
-[Illustration: FIG. 7.--This would be our Base-line when finding the
-Sun’s Distance.]
-
-Of course, astronomers must choose two stations which are far more
-widely separated than are those in our little experiment. In fact, the
-greater the interval between the two stations, the better. Astronomers
-require a much longer distance than from one side of this room to
-the other, or from one side of London to the other side. If it were
-merely a balloon at which we were looking, then, when one observer
-at one side of London and another at the opposite side shaped their
-cards carefully, we should be able to tell the height of the balloon
-very easily. But as the sun is so much further off than any balloon
-could ever be, we must separate the observers much more widely.
-Even the breadth of England would not be enough, so we have to make
-them separate more and more until they are as widely divided as it
-is possible for any two people on this earth to be. One astronomer
-takes up his position at A (Fig. 7), and the other at the opposite
-side at B, so that they can both see the sun. They are obliged to
-use a much more accurate way of measuring the angles than by cutting
-out cards with pairs of scissors; and as the astronomer at A is not
-able to see his friend at B, it becomes no easy matter to measure the
-angles accurately. However, we shall not now trouble ourselves about
-such difficulties. It may suffice for the present to know that the
-angles are measured by delicate and very accurate instruments used
-by astronomers. They will not, indeed, make a little sketch such as
-sufficed for our purpose. They make a calculation which is a much more
-accurate way of effecting true measurement. The astronomers know the
-size of the earth, and thus they know how many thousands of miles lie
-between the two stations where the observations are made. This distance
-means in their calculation just what the length of the table did in our
-sketch. From each end of the line they set off an angle just as we did,
-and the astronomer must use the principle of similar triangles which he
-finds in Euclid, just we had to do. At last, when they have calculated
-the sides of their triangle, they obtain the distance of the sun.
-
-
-THE APPARENT SMALLNESS OF DISTANT OBJECTS.
-
-[Illustration: FIG. 8.--The nearer you are, the bigger the Globe looks.]
-
-[Illustration: FIG. 9.--The Globe is so far off that it lies beyond the
-Picture. The dotted lines show how small it seems.]
-
-I ought here to explain a principle which those who are learning
-about the stars must always bear in mind. The principle asserts that
-the further a body is, the smaller it looks. Perhaps this will be
-understood from the adjoining little sketch (Fig. 8). It represents a
-great globe, on which oceans and continents are shown, and you see
-a little boy and a little girl are looking at the globe. The girl
-stands quite close to it, and I have drawn two dotted lines from her
-eye, one to the top of the globe, and the other to the under surface.
-If she wants to examine the entire side of the globe which is visible
-to her, she must first look along the upper dotted line, and then she
-must turn her glance downwards until she comes to the lower line, and
-having to turn her eyes thus up and down she will think the globe is
-very big, and she will be quite right. The boy is, as you see, on the
-other side of the globe, but I have put him much further off than the
-girl. I have also drawn two dotted lines from his eye to the globe, and
-it is plain that he will not have to turn his head much up and down to
-see the whole globe. He can take it all in at a glance, and to him,
-therefore, the globe will appear to be comparatively small, because he
-is sufficiently far from it. The more distant he is, the smaller it
-will appear. You can easily imagine that, if the globe were far enough,
-the two lines that would include the whole would be like those shown
-(Fig. 9), in which the globe is so distant that it cannot be seen in
-the picture. The apparent size of the globe, which is really measured
-by the angle between these two lines, would always be smaller and
-smaller according as the distance was greater. Now you can understand
-why an object seems smaller the further away it is; indeed, when
-sufficiently far, the object ceases to be visible at all.
-
-I could give many illustrations of the diminution of size by distance,
-and so, doubtless, could you. Every boy knows that his kite looks
-smaller and smaller the greater the length of string that he lets
-out. I have seen in the West of Ireland a bird that seemed like a
-little speck high up near the clouds, but from its flight and other
-circumstances I knew that the speck was not a little bird. It was,
-indeed, a great eagle, which was dwarfed by the elevation to which it
-had soared.
-
-It is in astronomy that we have the best illustrations of this
-principle. Enormous objects seem to be small because they are so very
-far off. You must therefore always remember that although an object may
-appear to be small, this appearance may be only a delusion. It may be
-that the object is very big, but very distant. In astronomy, this is
-almost always the case, there is so much room above us, around us, on
-all sides in space. Look up at the ceiling. It certainly does not bound
-space, for there is another side to it; and then there is the roof of
-the house. But the roof is not a boundary, for, of course, there is the
-air above it, and then, higher up still, there are the clouds, and so
-we can carry our imagination on and on through and beyond the air up
-to where the stars are, and still on and on. And as there is unlimited
-room, the celestial bodies take advantage of it, and are, generally
-speaking, at distances so gigantic that, no matter how small they may
-appear, their smallness is merely deceptive.
-
-Let us try to illustrate in another way the exceeding remoteness of the
-sun. So please imagine that you were on the sun, and that you took a
-view of our earth from that distance. To find out what we must expect
-to see, let us think of a balloon voyage. If you were to go up in a
-balloon, you would at first see only the houses, or objects immediately
-about you, but as you rose the view would become wider and wider. You
-would see that London was surrounded by the country, and then, as you
-still soared up and up, the sea would become visible, and you would
-be able to trace out the coasts, east and west and south. If, in some
-way, you could soar higher than any balloon could carry you, the whole
-of the British Islands would presently lie spread like a map beneath.
-Still on and on, and then the continent of Europe would be gradually
-opened out, until the great oceans, and even other continents, would
-at last be caught sight of, and then you would perceive that our whole
-earth was indeed a globe. The higher you went, the less distinctly
-would you be able to see the details on the surface. At last the
-outlines of the continents and oceans would fade, and you would begin
-to lose any perception of the shape of the earth itself. Long ere you
-had reached the distance of the sun, the earth would look merely as the
-planet Venus now does to us. It is instructive to consider how small
-our earth would seem if it were possible to view it from the sun. Think
-of that very familiar little globe, a lawn-tennis ball, which is two
-and three-quarter inches in diameter. But suppose a tennis ball were
-at the opposite side of the street, or still further away; suppose,
-for example, that it were half a mile away, what could you expect to
-see of it? And yet the earth, as seen from the sun, would appear to be
-no larger than a tennis ball would look when viewed from a distance of
-half a mile.
-
-
-THE SHAPE AND SIZE OF THE SUN.
-
-We have spoken of the heat of the sun, how hot he is; of the distance
-of the sun, how far he is; and now we must say a little about the size
-of the sun; and also about his shape. It is plain that the sun is
-round, that it has the shape of a ball. We are sure of this because,
-though a plate is circular, yet, if it were placed so that we only
-saw it edgeways from a distance, it would not appear to be round. The
-sun is always rotating, and as it always seems to be a circle, we are
-therefore certain that the true shape of the sun must be globular, and
-not merely circular like a flat plate.
-
-In the middle of the day, when the sun is high in the heavens, it is
-impossible for us to form a notion of the size of the sun. People
-will form very different estimates as to his apparent bigness. Some
-will say he looks as large as a dinner plate, but such statements are
-meaningless, unless we say where the plate is to be held. If it be near
-the eye, of course the plate may hide the sun, and, for that matter,
-everything else also. If the plate were about a hundred feet away, then
-it would often hide the sun. If the plate were more than a hundred feet
-distant, then it could not hide the sun entirely, and the further the
-plate, the smaller it would seem.
-
-No means of estimating the sun’s size are available when his orb stands
-high in the heavens. But when he is rising or setting, we see that he
-passes behind trees or mountains, so that there are intervening objects
-with which we can compare him; then we have actual proof that the sun
-must be a very large body indeed.
-
-[Illustration: FIG. 10.--A Sunset viewed from Marseilles (_Marcus
-Codde_).]
-
-I give here a picture, by Marcus Codde, taken from a French journal,
-_l’Astronomie_, which gives a charming illustration of a sunset at
-Marseilles (Fig. 10). If you wish to see that the sun is bigger than
-a mountain, you may go to the top of Notre Dame de la Garde, but you
-must choose either the 10th of February or the 31st of October for your
-visit, because it is only on the evenings of those days that the sun
-sets in the right position.
-
-On both these evenings the sun sinks directly behind Mount Carigou in
-the Pyrenees; this mountain is a long way from Marseilles--no less,
-indeed, than one hundred and fifty-eight miles. But the mountain is so
-lofty, that when the sky is clear, the summit can be distinctly seen
-upon the sun as a background, in the way shown in the picture. This
-must be a very pretty sight, and it teaches us an important lesson.
-The sun is further away than the mountain, and yet you see the sun on
-both sides of the mountain, and above it. Here, then, we learn without
-any calculations, that the sun must be bigger than the upper part of a
-great mountain in the Pyrenees.
-
-When we calculate the size of the sun from the measurements made by
-astronomers, we discover that it is much bigger than Mount Carigou; we
-see that even the entire range of the Pyrenees, the whole of Europe,
-and even our whole globe, are insignificant by comparison.
-
-[Illustration: FIG. 11.--How we compare the Earth and the Sun.]
-
-There is a football on the table, shown in Fig. 11. We shall suppose it
-to represent the sun; we shall now choose something else to represent
-the earth. We must, however, exhibit the proportions accurately. A
-tennis ball will not do; it is far too large. The fact is, the width
-of the earth is less than the one-hundredth part of the width of the
-sun. The tennis ball is, however, only a quarter the width of the
-football, so we must choose something a good deal smaller. I try with
-a marble, even with the smallest marble I can find, but when I measure
-it, I find that one hundred such marbles, placed side by side, would
-be far longer than the width of the football; I must therefore look
-for something still smaller. A grain of small-sized shot will give
-the right size for the model of our earth. About one hundred of these
-grains placed side by side will extend to a length equal to the width
-of the football. Now you will be able to form some conception of how
-enormous the sun really is. Think of this earth, how big we find it
-when we begin to travel. What a tremendous voyage we have to take to
-get to New Zealand, and even then we have only got halfway round the
-globe. Then think that the sun is in the same proportion bigger than
-the earth as that football is bigger than that grain of shot. If a
-million of such grains of shot were melted and cast into one globe,
-it would not be so large as that football. If a million globes, as
-large as our earth, could be united together, no doubt a vast globe
-would be produced, but it would not be so large as the sun. Think of a
-single house, with three or four people living in it, and then think of
-this mighty London, with its millions of inhabitants. The house will
-represent our earth, while great London represents the sun!
-
-
-THE SPOTS ON THE SUN.
-
-I have shown you that the sun is intensely hot, and a very long way
-off, and enormously big. And now we have to describe the appearance of
-the surface of the sun when we examine it closely.
-
-[Illustration: FIG. 12.--Looking at the Sun.]
-
-If you get a piece of very dark glass, or if you smoke a piece of glass
-over a candle, then you can look directly at the sun with comfort. A
-nicer plan is to prick a pinhole in a card, through which you can look
-at the sun without any inconvenience. Generally speaking, a view of
-the sun in this way will show you only a uniformly bright surface. To
-study the face of our great luminary carefully, you must use the aid
-which the telescope gives to the astronomer. A very good way of doing
-this is shown in Fig. 12. A small telescope, fixed on a stand, is
-pointed to the sun, and, the eyepiece being drawn out somewhat further
-than when direct observations are being made, the sun draws its own
-picture on a screen. This may be examined without any inconvenience, or
-without the necessity for any protection to the eye, and a number of
-young astronomers can all view the sun at the same moment. On such a
-picture you will generally see the brilliant surface marked with dark
-spots, which are sometimes as numerous as in the case represented in
-Fig. 13. These spots present very different appearances according to
-circumstances. One such spot when seen with a very powerful telescope
-showed the wonderful structure which is represented in Fig. 14.
-
-[Illustration: FIG. 13.--This is what the Sun sometimes looks like.]
-
-The visible surface of the sun is entirely formed of intensely heated
-vapors. We might almost say that the spots are holes, by which we can
-look through the brilliant surface to the interior and darker parts.
-Sometimes the spots close up, and fresh ones will open elsewhere. Now
-and then the whole surface is mottled over in a remarkable way. I give
-here a picture which was taken from Mr. Nasmyth’s beautiful drawing,
-in which he shows how the sun sometimes assumes the appearance which
-has been likened to willow leaves (Fig. 15). This appearance was very
-noticeable in the great spot of September, 1898.
-
-[Illustration: FIG. 14.--A Sun-spot (_after Janssen_).]
-
-[Illustration: FIG. 15.--Nasmyth’s Drawing of the Willow-leaved
-Structure of the Sun.]
-
-[Illustration: FIG. 16.--Spot nearing the Sun’s Edge.]
-
-The spots often last long enough to demonstrate a remarkable fact. We
-must remember that the sun is a great globe, and that it is poised
-freely in space. There is nothing to hold it up, and there is nothing
-to prevent it from turning round. That it does turn round, we can prove
-by careful observation of the spots. I can best illustrate what I want
-by Fig. 17, which shows six imaginary pictures. The first represents
-the sun on the 1st day of the month; the next shows it five days later,
-on the 6th; another view is five days later still, on the 11th; and
-so on until the last picture, which corresponds to the 26th. You see,
-on the first day there is a spot near the left edge; by the 6th,
-this spot is near the middle; by the 11th, it is near the right edge;
-then you do not see it at all on the 16th, or on the 21st; but on the
-26th it is back in the same place from which it started. We find
-other spots to have a similar history. They appear to move across the
-face, and then to return in a little less than four weeks to the same
-place where they were originally noticed. These appearances can be
-illustrated very simply by cutting a small hole through the rind of an
-orange down to the white interior skin, which may be darkened with ink.
-Put a knitting needle through the axis of the orange, and then turn it
-slowly round. The spot will be found to go through the changes that we
-have seen. We start with the spot near the left, it moves across the
-face, and then passes to invisibility by moving behind the globe until
-it reappears again, after having moved round the back. As the same may
-be observed with every spot which lasts long enough, we learn that the
-changes in the places must be produced by the turning round of the sun.
-Here you see is the way in which an astronomical discovery is made. We
-first observe the fact that the spots do always appear to move. Then
-we try to account for this, and we find a very simple explanation, by
-supposing that the whole sun, spots and all, turns steadily round and
-round. It can also be proved in a very conclusive manner that no other
-explanation is possible. This rotation of the sun is always going on
-uniformly, and some curious consequences follow from it. The view of
-the sun which is turned towards us to-day is quite different from that
-which was towards us a fortnight ago, or from that which we shall see
-in a fortnight hence. There is no actual or visible axis about which
-the sun rotates. In this the sun is like the earth and other celestial
-bodies.
-
-[Illustration: FIG. 17.--How the Sun turns round.]
-
-
-APPEARANCES SEEN DURING A TOTAL ECLIPSE OF THE SUN.
-
-For a great deal of our knowledge about the sun we are indebted to
-the moon. It will sometimes happen that the moon comes in between
-us and the sun, and produces an eclipse. At first you might think
-that an eclipse would only have the effect of preventing us from
-seeing anything of the sun, but it really reveals most beautiful
-and interesting objects, of whose existence we should otherwise be
-ignorant. The great luminary has curious appendages which are quite
-hidden under ordinary circumstances. In the full glare of day the
-dazzling splendor of the sun obliterates and renders invisible these
-appendages, which only shine with comparatively feeble light. It
-fortunately happens that the moon is just large enough to intercept
-the whole of the direct light from the sun, or rather, I should
-say, from the central parts of the sun. Surrounding that central
-and more familiar part from which the brilliancy is chiefly derived
-is a remarkable fringe of delicate and beautiful objects which are
-self-luminous no doubt, but with a light so feeble that when presented
-to us amid the full blaze of sunlight they are invisible. When,
-however, the moon so kindly stops all the stronger beams, then these
-faint objects spring into visibility, and we have the exquisite
-spectacle of a total eclipse. The objects that I desire to mention
-particularly are the corona and the prominences.
-
-[Illustration: FIG. 18.--Total Eclipse of the Sun, May 6, 1883 (_drawn
-by Trouvelot_).]
-
-A pretty picture of the total eclipse of the sun which occurred on May
-6, 1883, is here shown (Fig. 18). It is taken from a drawing made by
-M. Trouvelot, who was sent out with a French observing party. They
-went a very long way to see an eclipse, but what they saw recompensed
-them for all their trouble. The track along which the phenomenon could
-be best seen lay in the Pacific Ocean, and a place had to be selected
-which was so situated that the sun should be high in the heavens at
-the important moment, and also that the duration while the total
-eclipse lasted should be as long as possible. They accordingly went to
-Caroline Island, and all this journey to the other side of the earth
-was taken to witness a phenomenon that only lasted five minutes and
-twenty-three seconds. Short though these precious minutes were, they
-were long enough to enable good work to be done. Careful preparations
-had been made so that not a moment should be thrown away. Each member
-of the party had his special duty allotted to him, and this had been
-rehearsed so carefully beforehand that when the long-expected moment
-of “totality” arrived there was neither haste nor confusion; every one
-carefully went through his part of the programme. M. Trouvelot, for
-instance, occupied himself for two minutes and a few seconds in making
-the sketch that we now show. No doubt an accomplished astronomical
-artist like M. Trouvelot would gladly have taken longer time for his
-sketch of so unique a sight, but brevity was imperative. He had already
-had experience of similar eclipses, so that he was prepared at once
-to note what ought to be noted, and the picture we have shown is the
-result. This was completed within less than half of the duration of
-totality, and the artist had still three minutes left to devote to
-another and quite different part of the work, which does not concern us
-at present.
-
-[Illustration: FIG. 19.--The Corona of the Sun, 1882 (_by Schuster_).]
-
-I want you particularly to look at these long branches or projections
-which we see surrounding the sun when totally eclipsed. They shine
-with a pearly light, and, in fact, it is stated that even during the
-gloomiest portion of the time there was still as much illumination as
-on a bright moonlight night. All that light came from this glorious
-halo round the sun which astronomers call the “corona.” We do not
-under ordinary circumstances obtain even the slightest glimpse of this
-object. Even during a partial eclipse of the sun it is not visible, but
-directly the moon quite covers the sun, so as to cut off all the direct
-light, then the corona springs into visibility. It is always there, no
-doubt, though we cannot see it.
-
-One of the most interesting photographs of the eclipsed sun which has
-ever been taken was that by Professor Schuster in 1882 (Fig. 19). The
-corona is well shown, and also a comet.
-
-The other appendages to the sun which can be seen during an eclipse are
-the objects which we call “prominences.” They are of a ruddy color, and
-seem to be great flames, which leap upwards from the glowing surface
-of the sun below. Though the existence of the prominences was first
-discovered by their presence during eclipses, it fortunately happens
-that we are no longer wholly dependent on eclipses for the purpose
-of making our observations of these remarkable objects. It is true
-that we may look at the sun with even the biggest and most powerful
-telescope in the world, and still not be able to perceive anything of
-the prominences. We require the aid of a special appliance called the
-spectroscope to render them visible. But I am not now going to describe
-this ingenious contrivance. I am only going to speak of the results
-which have been obtained by its means. We shall here again avail
-ourselves of the experience of M. Trouvelot for a picture of two of
-these wonderful appendages.
-
-[Illustration: FIG. 20.--Solar Prominences (_drawn by Trouvelot_).]
-
-The view (Fig. 20) shows the ordinary aspect of the sun diversified
-with groups of dark spots. The fringe around the margin of the globe
-is of some ruddy material, forming the base of the flames which rise
-from the glowing surface. No doubt these flames are also often present
-on the face of the sun, but we cannot see them against the brilliant
-background. They are only perceptible when shown against the sky
-behind. At two points of this ruddy fringe, which happen curiously
-enough to be nearly opposite to each other, two colossal flames have
-burst forth. They extend to a vast distance, which is quite one-third
-of the width of the sun. The vigor of these outbreaks may be estimated
-by the remarkable changes which are incessantly going on. These great
-flames may indeed be said to flicker; only, considering their size, we
-must allow them a little more time than is demanded for the movements
-of flames of ordinary dimensions. The great flame on the left was
-obviously declining in brilliancy when first seen. In a quarter of an
-hour it had broken up into fragments, some of which were still to be
-seen floating in the sun’s atmosphere. In ten minutes more the light of
-this flame had almost entirely vanished. Surely these are changes of
-extraordinary rapidity when we remember the size of this prominence. It
-was nearly 300,000 miles in height--that is to say, about thirty-seven
-times the width of our earth.
-
-Great as are these prominences, others have been recorded which are
-even larger. One of them has been seen to rush up with a speed of
-200,000 miles an hour--that is, with more than two hundred times the
-pace of the swiftest of rifle-bullets.
-
-
-NIGHT AND DAY.
-
-The sun is bright, and the earth is dark. The sun gives light and heat,
-and the earth receives light and heat. We should be in utter darkness
-were it not for the sun; at least, all the light we should have, beyond
-our trivial artificial light, would come from the feeble twinkle of
-the stars. The moon would be no use, for the brightness of the moon is
-merely the reflection of the sunbeams. Were the sun’s light completely
-extinguished we could never again see the moon, and we should also miss
-from the sky a few other bodies, which we call planets, such as Jupiter
-and Venus, Mars and Saturn. But the stars would be the same as before,
-for they do not depend upon the sun for their light. We shall, indeed,
-afterwards see that each star is itself a sun.
-
-Picture to yourself the earth as receiving a stream of sunbeams. These
-beams fall on one half of our globe, and give to it the brilliance
-of day. The other half of the earth of course receives no sunlight.
-It is in the shadow, and consequently the darkness of night there
-prevails. The boundary between light and darkness is not quite sharply
-defined, for the pleasant twilight softens it a little, so that we
-pass gradually from day to night. Looking at the progress of the sun
-in the course of the day, we see that he rises far away in the east,
-then he gradually moves across the heavens past the south, and in the
-evening declines to the west, sets, and disappears. All through the
-night the sun is gradually moving round the opposite side of the earth,
-illuminating New Zealand and Japan and other remote countries, and then
-gradually working round to the east, where he starts afresh to give us
-a new day here.
-
-Our ancestors many ages ago did not know that the earth was round. They
-thought it was a great flat plain, and that it extended endlessly in
-every direction. They were, however, much puzzled about the sun. They
-could see from the coasts of France and Spain or Britain that the sun
-gradually disappeared in the ocean; they thought that it actually took
-a plunge into the sea. This would certainly quench the glowing sun;
-and some of the ancients used to think they heard the dreadful hissing
-noise when the great red-hot body dropped into the Atlantic. But there
-was here a difficulty. If the sun were to be chilled down every evening
-by dropping into the water hundreds of miles away to the west, how did
-it happen that early the next morning he came up as fresh and as hot as
-ever, hundreds of miles away to the east? For this, indeed, it seemed
-hard to account. Some said that we had an entirely new sun every day.
-The gods started the sun far off in the east, and after having run
-its course it perished in the west. All the night the gods were busy
-preparing a new sun to be used on the succeeding day. But this was
-thought to be such a waste of good suns that a more economical theory
-was afterwards proposed. The ancients believed that the continents of
-the earth, so far as they knew them, were surrounded by a limitless
-ocean. At the north, there were high mountains and ice and snow, which
-they thought prevented access to this ocean from civilized regions.
-Vulcan was the presiding deity who navigated those wastes of waters,
-and to him was intrusted the responsible duty of saving the sun from
-extinction. He had a great boat ready, so that when the sun was just
-dropping into the ocean at sunset he caught it, and during all the
-night he paddled with his glorious cargo round by the north. The glow
-of the sun during the voyage could even be sometimes traced in summer
-over the great highlands to the north. This, at all events, was their
-way of accounting for the long midsummer twilight. After a tedious
-night’s voyage Vulcan got round to the east in good time for sunrise.
-Then he shot the sun up with such terrific force that it would go
-across the whole sky, and then the industrious deity paddled back with
-all his might by the way he had come, so as to be ready to catch the
-sun in the evening and thus repeat his never-ending task.
-
-
-THE DAILY ROTATION OF THE EARTH.
-
-Vulcan and his boat seemed a pretty way of accounting for the sun’s
-apparent motion. The chief drawback was that it was all work and
-no play for poor Vulcan. There were also a few other difficulties.
-Captains of ships told us that they had sailed out on the great sea,
-and that so far from finding that the ocean extended on and on in one
-flat plain forever, the water seemed to bend round, so that, in fact,
-after sailing far enough in the same direction, they found that they
-would be brought back again to the place from which they started. They
-also knew a little about the north. They told us that there could be no
-such ocean as that which Vulcan in this fable was supposed to navigate.
-It also appeared that ships had been voyaging all over the globe
-night and day in every direction, and that no captain had ever seen
-the sun coming down to the sea, and still less had he ever met with
-Vulcan in the course of his incessant voyages. Thus it was discovered
-that the earth could not be a never-ending flat, but that it must be
-a globe, poised freely in space without any attachment to hold it up.
-It was thought that the change from day to night might be accounted
-for by supposing that the sun actually went round the earth through
-the space underneath our feet. This is, indeed, what it seems to do.
-But there was a great difficulty about this explanation, which began
-to be perceived when the size and distance of the sun were considered.
-It required the sun to possess an alarming activity. He would actually
-have to rush round a circle one hundred and eighty million miles in
-diameter and complete this astonishing voyage once every day.
-
-[Illustration: FIG. 21.--How we illustrate the Changes between Day and
-Night.]
-
-A little reflection will show that a very much simpler explanation was
-available. It was shown that the sun need not revolve round the earth
-once every day, but that everything would be explained if the earth
-itself turned round in such a way as to produce the changes from day
-to night. We may illustrate the case by this figure (Fig. 21). The
-small globe is the earth, which I can turn by the handle. The lamp will
-represent the sun, and, as at present shown, the side of the earth,
-on which England lies, is towards the lamp and in full day. On the
-opposite side of the globe are other countries such as New Zealand, and
-there it is dark. You see that by simply turning the handle I can move
-England gradually round so that it passes into the dark side, and then
-night falls over the country. At the same time New Zealand is turned
-round to enjoy the smiles of day. This is a very simple method of
-accounting for the succession of day and night, and it is also the true
-method. We have already seen that the sun turns round, and now we find
-that the earth also turns, but the little body, the earth, goes much
-the faster, for it makes twenty-five turns while the sun goes round
-once.
-
-Our earth is at this moment spinning round at a speed so great that
-London moves many hundreds of miles every hour. A town near the
-equator would gallop round at a pace of more than a thousand miles an
-hour--quicker, in fact, than a rifle-bullet. Don’t you think that we
-ought to perceive that we are being whirled about in this terrific
-fashion? We know that when we are flying along in a railway train, we
-feel the jolting and we hear the noise, and we feel the blast of air
-if we put our heads out of window, and we see the trees as they appear
-to rush past. All these things tell us that we are in rapid motion.
-But suppose these sensations were absent. Imagine a line so perfectly
-laid that no jolts are perceptible, and that no racket is heard; draw
-down the blinds so that nothing can be seen, how then are we to know
-that we are moving? Indeed, your grandfathers used to be able to enjoy
-such a tranquil locomotion. I remember seeing in my childhood the
-fly-boats, as they were called, on the Royal Canal, wherein passengers
-were conveyed from Dublin to the West of Ireland, before the railway
-was made. The fly-boat was a sort of Noah’s ark in appearance, drawn
-by a horse cantering along the towing-path. In the cabin of such
-a vessel, where there was not the slightest motion of rolling or
-pitching--nothing but noiseless gliding along the canal--no one would
-be conscious of motion, so long as he did not look through the cabin
-windows. No one was ever seasick in a fly-boat; it was the perfection
-of travelling for those who loved ease and quiet.
-
-The motion of the earth round its axis is, so far, like that of the
-fly-boat. It is so absolutely smooth that we do not feel anything, and
-we only become conscious of it by looking at outside objects. These
-are the sun, or the moon, or the stars. We see these bodies apparently
-going through their unvarying rising and setting, just as, in looking
-out from the fly-boat, the passengers in that quaint old conveyance
-could see the houses and trees as they passed.
-
-Seeing is believing; and I should like here, in this very theatre, to
-show you that we are actually turning round; and this I am enabled to
-do by the kindness of my distinguished friend, Professor Dewar.
-
-[Illustration: FIG. 22.--A Pendulum.]
-
-I am tempted to wish that I had Aladdin’s lamp for the moment, for I
-would rub it, and when the great genie appeared, I would bid him take
-the Royal Institution, and all of us here, to a place which everybody
-has heard of, and nobody has seen--I mean the North Pole. It would be
-so easy to describe the experiment I am about to show you, there. It is
-not so easy here. But it will be sufficiently accurate for our purpose
-to suppose that we actually have made the voyage, and that this is the
-Pole at the centre of the lecture-table. The direction of the axis
-round which the earth is turning is a line pointing up straight to the
-ceiling. This lecture-table and all the rest of the theatre is going
-round. In about six hours it will have moved a quarter of the way, and
-in twenty-four hours it will have gone completely round. That is, at
-least, what would happen if we were actually at the Pole. As we are
-not there, for the Pole is many miles away from the Royal Institution,
-I must slightly modify this statement, and say that the table here
-takes more than twenty-four hours to go round. And now I want some
-way of proving that such is actually the case. There is no use in our
-merely looking at it, because we ourselves, and this whole building,
-and the whole of London, are all turning together. What we want is
-something which does not partake of the motion. Here is a heavy leaden
-ball (Fig. 22). It is fastened to the roof by a fine steel wire, and
-you see it swings to and fro with a deliberate and graceful motion. I
-want it to oscillate very steadily, so I draw it to one side and tie
-it by a piece of thread to a support, and then I burn the thread, and
-the great ball begins to swing to and fro. It would continue to do so
-for an hour, or indeed for several hours, and it is a peculiarity of
-this motion that the vibration always remains in the same direction
-in space. Even the rotation of the earth will not affect the plane of
-this great pendulum, so far at least as our experiment is concerned.
-Here, then, we have a method of testing my assertion about the turning
-round of this theatre. I mark a line on the table, directly underneath
-the motion of the ball to and fro. If we could wait for an hour or so,
-we should see that the motion of the ball seemed to have altered to
-a direction inclined to its original position, but it is really the
-table that has moved, for the direction of the motion of the ball is
-unaltered. We cannot, however, wait so long, therefore I show you the
-ingenious method which Professor Dewar has devised. By a beam from
-the electric light, he has succeeded in so magnifying the effect that
-even in a single minute it is quite obvious that the whole of this
-room is distinctly turning round, with respect to the oscillations of
-the pendulum. This celebrated experiment proves by actual inspection
-that the earth must be rotating. By measuring the motion we might even
-calculate the length of the day, though I do not say it would be an
-accurate method of doing so.
-
-The proper way of finding how long the earth takes to turn round is
-by observing the stars. Fix on any star you please, and note it in a
-certain position to-night; if you then observe the moment when the star
-is in the same place to-morrow, the interval of time that has elapsed
-is the true duration of one complete rotation. When accurately measured
-its length is found to be 23 hours 56 minutes 4 seconds, or about four
-minutes shorter than the ordinary day, measured from one noon to the
-next.
-
-
-ANNUAL MOTION OF THE EARTH ROUND THE SUN.
-
-[Illustration: FIG. 23.--The Changes of the Sun with the Seasons.]
-
-I have as yet only been speaking of the _daily_ movements by which the
-sun appears to go across the heavens between morning and evening. We
-next consider the annual movements which give rise to the changes of
-the seasons. It is now Christmastide, when the days are short and dark,
-while six months ago the days were long and glorious in the warmth
-and brightness of summer. A similar recurrence of the seasons takes
-place every year, and thus we learn that some great changes alter the
-relation between the earth and the sun year after year. We must try and
-explain this. Why is it that we enjoy warmth at one season, and suffer
-from frost and snow at another?
-
-Note first a great difference between the sun in summer and the sun in
-winter. I will ask you to look out at noon any day when the clouds are
-absent, and you will then find the sun at the highest point it reaches
-during the day. All the morning the sun has been gradually climbing
-from the east; all the afternoon it will be gradually sinking down to
-the west. Let us make the same observation at different parts of the
-year. Suppose we take the shortest day in December. You will look out
-about twelve o’clock from some situation which affords a view towards
-the south, and there, as shown in the adjoining sketch (Fig. 23), is
-the midwinter sun.
-
-But now the spring approaches, and the days begin to lengthen. If you
-watch the sun you will see it pass higher and higher every noon until
-Midsummer Day is reached, and then the sun at noon is found quite high
-up in the sky. As autumn draws near, the sun at noon creeps downwards
-again until, when the next shortest day has come round, we find that
-it passes just where it did at the previous midwinter. With unceasing
-regularity year after year the sun goes through these changes. When he
-is high at noon we have days both long and warm; when he is low at noon
-we have days both short and cold.
-
-Vulcan with his golden boat was naturally expected to give an
-explanation of this. As the summer drew on, each day Vulcan shot out
-the sun with a stronger impulse, so that it should ascend higher and
-higher. His greatest effort was made on Midsummer Day, when, after
-rowing but a little way round from the north towards the east, he drove
-off the sun with a terrific effort. The sun soared aloft to the utmost
-height it could reach, and in the meantime Vulcan returned to the west
-to be ready to catch the sun as it descended. On the other hand, in
-midwinter, he came round much further through the east to the south,
-and then shot up the sun with his feeblest effort, and had to paddle as
-hard as ever he could so as to complete his long return voyage during
-the brief day.
-
-[Illustration: FIG. 24.--How the Stars are to be seen in broad
-Daylight.]
-
-It is evident that there are two quite distinct kinds of motion of the
-sun. There is first the daily rising and setting, for which we have
-accounted by showing that it is merely an appearance produced by the
-fact that the earth is turning round. But now we have been considering
-quite a different motion by which the sun seems to creep up and down in
-the heavens, and this takes a whole year to go through its changes.
-
-There is still another point which we must consider before we can
-understand all these puzzling movements of the sun. We shall ask the
-stars to help us by their familiar constellations. You know, perhaps,
-the Great Bear, or the Plough as it is often called, and Orion. There
-are also Aries the Ram, Taurus the Bull, and other fancifully named
-systems. These constellations have been known for countless ages, and
-for our present purposes we may think of them as permanent groups
-in the heavens, which do not alter either their own shapes or their
-positions relatively to each other. These groups of stars extend all
-around the sky. They are not only over our heads and on all sides down
-to the horizon, but if we could dig a deep hole through the earth,
-coming out somewhere near New Zealand, and if we then looked through,
-we should see that there was another vault of stars beneath us. We
-stand on our comparatively little earth in what seems the centre of
-this great universe of stars all around. It is true we do not often
-see the stars in broad daylight, but they are there nevertheless. The
-blaze of sunlight makes them invisible. A good telescope will always
-show the stars, and even without a telescope they can sometimes be
-seen in daylight in rather an odd way. If you can obtain a glimpse of
-the blue sky on a fine day from the bottom of a coal pit, stars are
-often visible. The top of the shaft is, however, generally obstructed
-by the machinery for hoisting up the coal, but the stars may be seen
-occasionally through the tall chimney attached to a manufactory when an
-opportune disuse of the chimney permits of the observation being made
-(Fig. 24). The fact is that the long tube has the effect of completely
-screening from the eye the direct light of the sun. The eye thus
-becomes more sensitive, and the feeble light from the stars can make
-its impression, and produce vision. From all these various lines of
-reasoning we see that there can be no doubt of the continuous presence
-of stars above and around us, and below us, on every side, and at all
-times.
-
-[Illustration: FIG. 25.--The Sun seems to revolve around the Earth.]
-
-If you look out at Christmas time, towards the south, you will see the
-Belt of Orion and the Dog Star in a splendid portion of the heavens.
-These stars you will see every winter in the same place. But you may
-look in vain for them in summer. No doubt you can see stars in the
-summer evenings, but they will be totally different from those that
-adorned the skies in winter. Each season has its own constellations.
-This simple fact was known to the ancients, and we shall find
-its explanation full of meaning. Let us select four well-known
-constellations which will best answer our purpose. They lie in a circle
-round the heavens. They are Orion, Virgo, Scorpio, and Pisces. I am
-supposing that you are looking out at midnight towards the south. In
-December you will see Orion; in March, Virgo; in June, Scorpio; and in
-September, Pisces; and then next December you will be looking at Orion
-again. See what this proves. At midnight, of course, the sun is at the
-other side of the earth, so that if I am looking at Orion in midwinter
-the sun must be behind my back. Look at our little picture (Fig. 25).
-The earth is in the middle, and the sun must be on the opposite side
-to Orion. That is, the sun must be somewhere about the position I have
-marked at A. In March we see Virgo in the south at midnight, when, of
-course, the sun is at the other side of the earth; so that the sun must
-be somewhere at B. In June Scorpio is seen, so that the sun must be at
-the other side, at C. That is to say, in midsummer the sun is in that
-part of the sky where Orion is situated. If, therefore, on a bright
-June day we could see the stars, we should find Orion in the south.
-But, of course, the light of the sun makes Orion invisible. We can,
-however, see the stars by our telescopes, and on rare occasions an
-eclipse of the sun will occur, by which he is temporarily extinguished,
-and then we can see the stars without the help of a telescope, even
-though it is daytime.
-
-[Illustration: FIG. 26.--The Earth, however, really revolves around the
-Sun.]
-
-Thus it would seem as if the sun were first at A and then at B, C, and
-D, and then began to go round again. I say it would _seem_ as if the
-sun had these movements, and the ancients thought there was no doubt
-about the matter. Even after it was plain that the earth turned round
-on its axis so as to give the changes of day and night, it was still
-thought necessary to suppose that the sun went round the earth once in
-the year, in order to explain how the changes in the stars during the
-different seasons were produced.
-
-Here is another case in which we must be careful to distinguish between
-what appears to be true and what is actually the case. Everything that
-we undoubtedly see would be just as well explained by supposing that
-the sun remained at rest, and that the earth revolved around it, as
-in Fig. 26. If, for instance, the earth were at A in midwinter, then
-the sun is on the opposite side to Orion, and of course at midnight we
-shall be able to see Orion. So in spring the earth is at B, and we see
-Virgo, and similarly in summer we have Scorpio, and in autumn Pisces.
-Thus all that is actually visible could be fully accounted for by
-regarding the sun as fixed in the centre, and the earth as travelling
-round it from A to B, to C and to D respectively, and completing the
-journey in a twelvemonth. Which idea are we to adopt? Shall we say that
-the earth goes round the sun, or the sun goes round the earth?
-
-I remember an old college story, which I cannot help giving you at this
-place. It may serve to lighten what I fear you must otherwise have
-thought rather a tedious part of our subject. There were three students
-brought up for examination in astronomy, and they showed a lamentable
-ignorance of the subject, but the examiner, being a kind-hearted man,
-wished, if possible, to pass them; and so he proposed to the three
-youths the very simplest question that he could think of. Accordingly,
-addressing the first student, he said: “Now tell me, does the earth go
-round the sun, or the sun go round the earth?” “It is--the earth--goes
-round the sun.” “What do you say?” he inquired, turning rather suddenly
-on the next, who gasped out: “Oh, sir--of course--it is the sun goes
-round the earth.” “What do you say?” he shouted at the third unhappy
-victim. “Oh, sir, it is--sometimes one way, sir, and sometimes the
-other!”
-
-But which is it? Well, we must remember that the earth is comparatively
-a very little body and the sun a very big one, so it is not at all
-surprising to learn that the earth goes round the sun, which remains,
-practically speaking, at rest in the centre. Thus our great earth and
-all it contains are continually bound in what is very nearly a circular
-course round the great luminary. You will find it instructive to work
-out this little sum. How fast is the earth moving, or how far do we go
-in a second? We are about 93,000,000 miles from the sun, and the great
-circle that we go round has a diameter twice as great as this--that is,
-about 186,000,000 miles. The circumference of a circle is nearly three
-and one-seventh times its diameter, and accordingly the whole length
-of the voyage in the year is about 585,000,000 miles. This has to be
-accomplished in 365 days, so that the daily run must be about 1,600,000
-miles. We divide this by 24, to find the distance journeyed each hour,
-which we find to be about 67,000 miles; and we must divide this again
-by 60 to find the length covered in a minute, and by 60 again for the
-progress made each second. It is truly startling to find that, night
-and day, this great earth has to travel more than eighteen miles every
-second in order to get round its mighty path in the allotted time.
-
-I began this lecture about forty minutes ago, and I think from what I
-have said you will be able to calculate a result that will, I dare say,
-astonish you. In these forty minutes we have moved about 45,000 miles.
-No doubt my lecture commenced in this hall, and in your presence; but
-can I truly say I began it _here_? Well, no; I began it not here, but
-at a place 45,000 miles away; but we have all been travelling together,
-and the journey has been so very smooth and free from all jolts, that
-we never thought anything about the motion.
-
-I am sure many of those to whom I am now speaking have read accounts
-of voyages in the Arctic regions. You have been told of the sufferings
-of the crews during the long winters, amid the ice and snow; and you
-have heard how, during that dismal period, there is total darkness,
-for the sun never rises for weeks and months together. On the other
-hand, these northern regions often present a more cheerful picture.
-During midsummer, the long darkness of winter is atoned for by
-perpetual sunshine. At midnight there is still the full brilliance
-of day, and the sun, though low, no doubt, has not passed below the
-horizon. Even in the northerly parts of Europe we can see the midnight
-sun. Lord Dufferin, in his delightful narrative of a cruise, entitled
-“Letters from High Latitudes,” gives an interesting illustration
-of the perplexities arising from endless daylight. It appears that
-everything went on happily until the fatal moment when the yacht
-crossed the Arctic circle. Then it was that dire tribulation arose
-among the poultry. A fine cock was the cause of the trouble. Knowing
-his duty, he always liked to be particular about performing the
-important task of crowing at sunrise. This he could do regularly, so
-long as the yacht remained in reasonable latitudes, where the sun
-behaved properly. But when they crossed the Arctic circle, the cock
-was confronted with a wholly new experience. The sun never set in the
-evening, and consequently never had to rise in the morning. What was
-the distracted bird to do? He did everything. He burst into occasional
-fits of terrific crowing at all sorts of hours, then he gave up crowing
-altogether, but finding that did not mend matters, he took to crowing
-incessantly. Exhaustion was succeeded by delirium, and rather than
-live any longer in a universe where the sun was capable of pranks
-so heartless, the indignant fowl flung himself from the vessel and
-perished in the Arctic Ocean.
-
-
-THE CHANGES OF THE SEASONS.
-
-[Illustration: FIG. 27.--The Changes of the Seasons.]
-
-In the adjoining figure, I show a little sketch (Fig. 27), by which I
-try to explain the changes of the seasons. It exhibits four positions
-of the earth, one on each side of the sun. The left. A, represents the
-earth when summer gladdens the northern hemisphere; while the right,
-C, shows winter in the same region. You will see the two central lines
-which represent the axis about which the earth rotates. Of course,
-the earth has no visible axis. The line which runs through the globe
-from the North to the South Pole is imaginary. It remains fixed in the
-earth, for we can prove in our observatories that the Pole does not
-shift its position to any considerable extent in the earth itself.
-In fact, if we could reach the North Pole and drive a peg into the
-ground year after year to mark the exact spot, we should find that
-the position of the Pole was sensibly the same. Does it not seem
-strange that we should be able to know so much about the Pole, though
-we have never been able to get there; have never, in fact, been able
-to get within less than 400 miles of it? I think you will be able to
-understand the point quite easily. The latitude of a place, as you know
-from your geography, is the number of degrees, and parts of a degree,
-between that place and the equator. In our observatories, we can
-determine this so accurately that the difference between the latitude
-of one side of a room and of the other side of the same room is quite
-perceptible. As we find that the latitudes of our observatories remain
-sensibly unchanged from year to year, we are certain that the Pole
-must remain in the same place. Indeed, if the Pole were to alter its
-position by the distance of a stone’s throw, the careful watchers in
-many observatories would speedily detect the occurrence.
-
-And now I must direct your attention to something apparently quite
-different. When the battle of Waterloo was fought, the great victory
-was won with the aid of the old-fashioned musket, a smooth-bore gun
-which was loaded at the muzzle with a good charge of powder, and then a
-round bullet was rammed down. “Brown Bess,” as the musket was called,
-was a most efficient weapon at close quarters, and indeed at any
-distance _when the bullet hit_; but there was the difficulty. The round
-bullets, rushing up the tube and out into the air in a somewhat vague
-manner, had a habit of roaming about, which was quite incompatible with
-the accurate shooting of our modern rifles.
-
-One great improvement in small arms consisted in giving to the bullet a
-rapid rotation about an axis which is in the line of fire. This is what
-the _rifle_ accomplishes. The grooves in the barrel of the rifle twist
-round, and though they only give half a complete turn in the length of
-the barrel, yet the speed of the bullet is so great that when it flies
-off it is actually spinning with the tremendous velocity of about one
-hundred and fifty revolutions a second. Even with the old-fashioned
-round bullet, the rifling of the barrel effected great improvement in
-the accuracy of the shooting. The introduction of the elongated bullets
-was another great improvement, while the adaptation of breech-loading
-enabled a bullet to be used rather larger than that which could have
-been forced down the barrel, and thus it was insured that the grooves
-should bite into the bullet as it hurries past and impart the necessary
-spin.
-
-A body rapidly rotating about an axis has a tendency to preserve the
-direction of that axis, and powerfully resists any attempt to change
-it. Our earth is spinning in this fashion. It is true that the rotation
-is, in one sense, a slow one, for it requires almost an entire day for
-each rotation. But when we remember the dimensions of our earth, we
-shall modify this notion. We have already stated that any place on the
-equator has to travel more than one thousand miles each hour in order
-to accomplish the journey within the required time. So far, therefore,
-the earth moves like a rifle-bullet, and the direction of its axis
-remains constant.
-
-In the course of the great voyage between summer and winter, the earth
-travels from one side of the sun to the opposite side, and in doing
-so it still continues to spin about an axis parallel to the original
-direction. See the consequences which follow. The sun illuminates half
-the earth, and in the left position in Fig. 27, representing summer,
-the North Pole is turned over towards the sun, and lies in the bright
-half of the earth. There is continual day at the North Pole, and night
-is unknown there at this time of year, because the turning of the earth
-about its axis will not bring the Pole nor the regions near the Pole
-into the dark hemisphere. Thus it is that the Arctic regions enjoy
-perpetual day at this season. Look now at the position of England when
-the northern hemisphere is tilted towards the sun, and is consequently
-enjoying the full splendor of midsummer. As the earth turns round,
-England will gradually cross the boundary between light and shade, and
-will enter upon the darkened hemisphere. Then there will be night in
-England, but you will see from the figure that the day is much longer
-than the night, and hence it is that we enjoy the fine long days in
-summer.
-
-We next look at a different scene six months later. The earth has
-reached the other side of the sun, but the axis has remained parallel
-to itself, consequently the North Pole is now inclined entirely away
-from the sun. The earth continues to turn round as before, but its
-movements do not bring the North Pole or the surrounding Arctic regions
-out of the dark hemisphere, and consequently the night must be unbroken
-in these dismal circumstances. The long continuous day which forms the
-Polar midsummer is dearly purchased by the gloom and cold of a winter
-in which there is no sun for many weeks in succession. Observe also the
-changed circumstances of England. In the course of each twenty-four
-hours it lies much longer in the dark half of the earth than in the
-bright, and consequently there is only a short day succeeded by a long
-night.
-
-
-SUNSHINE AT THE NORTH POLE.
-
-It is a privilege of astronomers to be able to predict events that will
-happen in thousands of years to come, and to describe things accurately
-though they never saw them, and though nobody else has ever seen them
-either. No one has ever yet got to the North Pole, but whenever they
-do, we are able to tell them much of what they will see there. We may
-leave it to Jules Verne to describe how the journey is to be made, and
-how the party are to be kept alive at the North Pole. I shall give a
-picture of the changes of the seasons, and of the appearance in the
-stars, as seen from thence.
-
-We shall, therefore, prepare to make observations from that very
-particular spot on this earth--the North Pole. I suppose that eternal
-ice and snow abide there. I don’t think it would be a pleasant
-residence. However, we shall arrange to arrive on Midsummer Day,
-prepared to make a year’s sojourn. The first question to be settled is
-the erection of the hut. In a cold country it is important to give the
-right aspect, and we are in the habit of saying that a southerly aspect
-is the best and warmest, while the north and the east are suggestive
-only of chills and discomfort. But what is a southerly aspect at the
-North Pole, or, rather, what is not a southerly aspect? Whatever way
-we look from the North Pole we are facing due south. There is no such
-thing as east or west; every way is the southward way. This is truly
-an odd part of the earth. The only other locality at all resembling it
-would be the South Pole, from which all directions would be north.
-
-The sun would be moving all through the day in a fashion utterly unlike
-its behavior in our latitudes. There would, of course, be no such thing
-as rising and setting. The sun would, indeed, at first seem neither
-to go any nearer to the horizon nor to rise any higher above it, but
-would simply go round and round the sky. Then it would gradually get
-lower and lower, moving round day after day in a sort of spiral, until
-at last it would get down so low that it would just graze the horizon,
-right round which it would circulate till half the sun was below, and
-then until the whole disk had disappeared. Even though the sun had now
-vanished, a twilight glow would for some time be continuous. It would
-seem to come from a source moving round and round below the horizon,
-then gradually the light would become fainter and fainter until at last
-the winter of utter and continuous blackness had set in. The first
-indications of the return of spring would be detected by a feeble
-glow near the horizon, which would seem to move round and round day
-after day. Then this glow would pass into a continuous dawn, gradually
-increasing until the sun’s edge crept into visibility, and the great
-globe would at last begin to climb the heavens by its continual spiral
-until midsummer was reached, when the change would go on again as
-before.
-
-Our first excursion to the country of Star-land has now been taken, and
-we have naturally commenced by studying that sun to which we owe so
-much. But we shall have to learn that though our sun is of such vital
-importance to us, yet, in magnificence and size, he has many rivals
-among the host of stars.
-
-
-
-
-LECTURE II.
-
-THE MOON.
-
-  The Phases of our Attendant, the Moon--The Size of the Moon--How
-      Eclipses are produced--Effect of the Moon’s Distance on its
-      Appearance--A Talk about Telescopes--How the Telescope aids us in
-      Viewing the Moon--Telescopic Views of the Lunar Scenery--On the
-      Origin of the Lunar Craters--The Movements of the Moon--On the
-      Possibility of Life in the Moon.
-
-
-THE PHASES OF OUR ATTENDANT, THE MOON.
-
-The first day of the week is related to the greatest body in the
-heavens--the sun--and accordingly we call that day Sun-day. The second
-day of the week is similarly called after the next most important
-celestial body--the moon--and though we do not actually say Moon-day,
-we do say Monday, which is very nearly the same. In French, too, we
-have _lune_ for moon, and _Lundi_ signifies our Monday. The other days
-of the week also have names derived from the heavens, but of these we
-shall speak hereafter. We are now going to talk about the moon.
-
-We can divide the objects in this room into two classes. There are the
-bright faces in front of me, and there are the bright electric lights
-above. The electric lights give light, and the faces receive it. I can
-see both lights and faces; but I see the electric lamps by the light
-which they themselves give. I see the faces by the illumination which
-they have received from the electric lights. This is a very simple
-distinction, but it is a very important one in Star-land. Among all
-these bodies which glitter in the heavens there are some which shine
-by their own light, like the lamps. There are others only brilliant by
-reflected light, like the faces. It seems impossible for us to confuse
-the brightness of a pleasant face with the beam from a pretty lamp, but
-it is often not very easy to distinguish in the heavens between a body
-which shines by its own light and a body which merely shines by some
-other light reflected from it. I think many people would make great
-mistakes if asked to point out which objects on the sky were really
-self-luminous and which objects were merely lighted up by other bodies.
-Astronomers themselves have been sometimes deceived in this way.
-
-The easiest example we can give of bodies so contrasted is found in the
-case of the sun and the moon. Of course, as we have already seen, the
-sun is the splendid source of light which it scatters all around. Some
-of that light falls on our earth to give us the glories of the day;
-some of the sunbeams fall on the moon, and though the moon has itself
-no more light than earth or stones, yet when exposed to a torrent of
-sunbeams, she enjoys a day as we do. One side of her is brilliantly
-lighted; and this it is which renders our satellite visible.
-
-Hence we explain the marked contrast between the sun and the moon. The
-whole of the sun is always bright; while half of the moon is always in
-darkness. When the bright side of the moon is turned directly towards
-us, then, no doubt, we see a complete circle, and we say the moon
-is full. On other occasions a portion only of the bright surface is
-directed to us, and thus are produced the beautiful crescents and
-semicircles and other phases of the moon.
-
-[Illustration: FIG. 28.--To show that the Moon is lighted by Sunbeams.]
-
-A simple apparatus (illustrated in Fig. 28) will explain their various
-appearances. The large india-rubber ball there shown represents the
-moon, which I shall illuminate by a beam from the electric light. The
-side of the ball turned towards the light is glowing brilliantly, and
-from the right side of the room you see nearly the whole of the bright
-side. To you the moon is nearly full. From the centre of the room you
-see the moon like a semicircle, and from the left it appears a thin
-crescent of light. I alter the position of the ball with respect to the
-lamp, and now you see the phases are quite changed. To those on my left
-our mimic moon is now full; to those on my right the moon is almost
-new, or is visible with only a slender crescent. From the centre of the
-room the quarter is visible as before. We can also show the same series
-of changes by a little contrivance of Figs. 29 and 30.
-
-Thus every phase of the moon (Fig. 31), from the thinnest beautiful
-crescent of light that you can just see low in the west after sunset
-up to the splendor of the full moon, can be completely accounted for
-by the different aspects of a globe, of which one-half is brilliantly
-illuminated.
-
-[Illustration:
-
-  FIG. 29.      FIG. 30.
-
-The Phases of the Moon.]
-
-We can now explain a beautiful phenomenon that you will see when the
-moon is still quite young. We fancifully describe the old moon as lying
-in the new moon’s arms when we observe the faintly illuminated portion
-of the rest of that circle, of which a part is the brilliant crescent.
-This can only be explained by showing how some light has fallen on the
-shadowed side; for nothing which is not itself a source of light can
-ever become visible unless illuminated by light from some other body.
-
-[Illustration: FIG. 31.--The Changes in the Moon.]
-
-Let us suppose that there is a man on the moon who is looking at the
-earth. To him the earth will appear in the same way as the moon appears
-to us, only very much larger. At the time of new moon the bright side
-of the earth will be turned directly towards him, so that the man in
-the moon will see an earth nearly full, and consequently pouring
-forth a large flood of light. Think of the brightest of all the bright
-moonlight nights you have ever seen on earth, and then think of a light
-which would be produced if you had thirteen moons, all as big and as
-bright as our full moon, shining together. How splendid the night
-would then be! You would be able to read a book quite easily! Well,
-that is the sort of illumination which the lunar man will enjoy under
-these circumstances; all the features of his country will be brightly
-lighted up by the full earth. Of course, this earth-lighted side of
-the moon cannot be compared in brilliancy with the sun-lighted side,
-but the brightness will still be perceptible, so that when from the
-earth we look at the moon, we see this glow distributed all over the
-dark portion; that is, we observe the feebly lighted globe clasped in
-the brilliant arms of the crescent. At a later phase the dark part of
-the moon entirely ceases to be visible, and this for a double reason:
-firstly, the bright side of the earth is then not so fully turned to
-the moon, and therefore the illumination it receives from earth-shine
-is not so great; and, secondly, the increasing size of the sun-lighted
-part of the moon has such an augmented glow that the fainter light is
-overpowered by contrast. You must remember that more light does not
-always increase the number of things that can be seen. It has sometimes
-the opposite effect. Have we not already mentioned how the brightness
-of day makes the stars invisible? The moon herself, seen in full
-daylight, seems no brighter than a small particle of white cloud.
-
-
-THE SIZE OF THE MOON.
-
-It is not easy to answer the question which I am sometimes asked, “Is
-the moon very big?” I would meet that question by another, “Is a cat a
-big animal?” The fact is, there is no such thing as absolute bigness or
-smallness. The cat is no doubt a small animal when compared with the
-tiger, but I think a mouse would probably tell you that the cat was
-quite a big animal--rather too big, indeed, in the mouse’s opinion. And
-the tiger himself is small compared with an elephant, while the mouse
-is large as compared with a fly.
-
-When we talk of the bigness or the smallness of a body, we must always
-consider what we are going to compare it with. It is natural in
-speaking of the moon to compare it with our own globe, and then we can
-say that the moon is a small body.
-
-The relative sizes of the earth and the moon may be illustrated by
-objects of very much smaller dimensions. Both a tennis ball and a
-football are no doubt familiar objects to everybody. If the earth be
-represented by the football, then the moon would be about as large as
-the lawn-tennis ball. But this proportion is not quite accurate, so
-I will suggest to you an instructive way of making a better pair of
-models of the earth and the moon. In fact, experiments somewhat similar
-to those I describe have been actually going on in every kitchen in the
-land during this festive season. For have not globes and balls of all
-sorts and sizes been made of plum-pudding, and it will only require a
-little care on the part of the cook to make a pair of luscious spheres
-that shall fairly set forth the sizes of the earth and the moon. There
-is first to be a nice little round plum-pudding, three inches in
-diameter. It is just a little bigger than a cricket ball. It should,
-however, only make its appearance at a bachelor’s table. Were it set
-down before a hearty circle on Christmas Day, dire disappointment would
-result. One boy of sound constitution could eat it all. Perhaps it
-would weigh about three-quarters of a pound. This little globe is to
-represent the moon.
-
-[Illustration: FIG. 32.--Relative Sizes of the Earth and Moon.]
-
-Another plum-pudding is to be constructed which shall represent
-the earth (Fig. 32). We must, however, beg the cook to observe the
-proportions. The width of the earth, or the diameter, to use the proper
-word, is about four times the diameter of the moon. Hence, as the
-small plum-pudding was three inches across, the large one must have
-a diameter of twelve inches. This will be a family pudding of truly
-satisfactory dimensions; perhaps the cook will be a little surprised
-to find the alarming quantity of materials that will be required to
-complete a sphere of plum-pudding a foot in diameter.
-
-These models having been duly made, and boiled, and placed on the
-table, we are now to propose the following problem:--
-
-“If one schoolboy could eat the small plum-pudding, how many boys would
-be required to dispose of the large one?”
-
-The hasty person, who does not reflect, will at once dash out the
-answer, “Four!” He will say, “It is quite plain that, since one of
-the puddings has four times the diameter of the other, it must be
-four times as big; and therefore, as one boy is able to eat the small
-pudding, four boys will be adequate for the large one.” But the hasty
-person will, as usual, be quite wrong. His argument would be sound
-if it were merely two pieces of sugar-stick that he was comparing;
-no doubt there is only four times as much material in a piece twelve
-inches long as there is in a piece three inches long. But the
-plum-puddings have breadth and depth, which are in the same proportions
-as the length, and the consequence is that the large plum-pudding is
-far more than four times as big as the small one. No four boys, however
-admirable their capacities, would be equal to the task of consuming it.
-Nor even if four more boys were called in to help would the dish be
-cleared. Twenty boys, forty boys, fifty boys would not be enough. It
-would take sixty-four boys to demolish the magnificent plum-pudding one
-foot in diameter.
-
-If the cook will try the experiment, she will find that by taking the
-materials sufficient for sixty-four small plum-puddings all of the
-same size, and mixing them together, she will, no doubt, make a large
-plum-pudding, but its diameter will only be four times that of the
-small puddings.
-
-As a matter of fact, the moon is 2160 miles in diameter, and the earth
-is 7918 miles. These numbers are so nearly 2000 and 8000 respectively,
-that for simplicity I have spoken of the earth as having a diameter
-four times as great as the moon. If we want to be very accurate, we
-ought to determine the ratio of the two quantities from the figures
-just given. Our illustration of the plum-puddings must, therefore, be
-a little modified. The earth is not quite so much as sixty-four times
-as big as the moon; but this figure is sufficiently accurate for our
-present purpose.
-
-Another interesting question may be proposed, namely: How much land
-is there on the moon? We might state the answer in acres or in square
-miles; but it will, perhaps, be more instructive to make a comparison
-between the moon and the earth.
-
-Here also I shall use an illustration; and we shall again consider
-two globes which are respectively three inches and twelve inches in
-diameter. The globes I use this time are hollow balls of india-rubber.
-These will represent the earth and the moon with sufficient accuracy,
-and the relative surfaces of these two globes is what I want to find.
-There are different ways in which the comparison might be made. I
-might, for instance, paint the two globes and see the quantity of
-paint that each requires. If I did this, I should find that the great
-globe took just sixteen times as much paint as the small one. We can
-adopt a simpler plan. The india-rubber in one of these balls has the
-same thickness as in the other, as they are each hollow, so that the
-quantity which is required for each ball may be taken to represent its
-surface. By simply weighing the two balls, I perceive that the large
-one is sixteen times as heavy as the small one. You notice here the
-difference between the comparative weights of two hollow balls and
-two solid ones of the same material. Had these globes been of solid
-india-rubber, the large one would have weighed sixty-four times as much
-as the small one, just as in the case of the plum-puddings; but being
-hollow, the ratio of their weights is only the square of the ratio of
-their diameters--that is to say, four times four, or sixteen.
-
-We are thus taught that if the moon were exactly one-fourth of the
-diameter of the earth, its surface would be one-sixteenth part of
-that of the earth. It would, no doubt, have made our subject a little
-easier and simpler if the moon had been created somewhat smaller than
-it is. As, however, the universe has not been solely constructed for
-the purpose of these talks about Star-land, we must take things as we
-find them. This proportion is not four; it is more nearly 3⅔, and the
-relative surfaces of the two bodies is the square of 11/3, or about
-13½. In other words, the entire extent of the surface of our globe is
-about thirteen and a half times that of the moon.
-
-The face of the full moon, being half the entire extent of the
-surface, is, therefore, about one-twenty-seventh part of the earth’s
-surface--continents, oceans, seas, and islands all taken together. The
-British Empire and the Russian Empire are each of them as large as the
-face of the full moon.
-
-
-HOW ECLIPSES ARE PRODUCED.
-
-The moon is the attendant, or the satellite of the earth, ministering
-to the wants of the earth by mitigating the darkness of our nights. The
-earth goes around the sun in its annual journey of 365 days. The moon
-revolves around the earth once every twenty-seven days. The motion of
-the moon is thus a very complicated one, for it is, in fact, moving
-round a body which is itself in constant motion (Fig. 33).
-
-You will see by your almanacs every year that certain eclipses are to
-take place; and after what we have said about the sun and the moon, it
-will be easy to understand how eclipses arise. There are two different
-kinds. You will sometimes see an eclipse of the moon, and sometimes
-those eclipses of the sun of which we have spoken in the last Lecture.
-You may be surprised to find with what accuracy the eclipses can be
-predicted. We can tell not only those that will occur this year and
-next year, but we could also foretell the eclipses that will appear
-in a hundred or a thousand years to come; or we can, with equal ease,
-calculate backwards, so as to find the circumstances of eclipses that
-happened thousands of years ago. This shows how well we have learned
-the way the moon moves.
-
-[Illustration: FIG. 33.--To show how the Earth goes round the Sun and
-the Moon round the Earth.]
-
-[Illustration: FIG. 34.--A Total Eclipse to the Girl and a Partial
-Eclipse to the Boy.]
-
-[Illustration:
-
-  Partial.       Annular.
-
-FIG. 35.--Different Kinds of Solar Eclipse.]
-
-An eclipse of the sun is the simpler occurrence, so we shall describe
-it first. It happens when the moon comes between the earth and the sun.
-Look at our little astronomers shown in Fig. 34. A boy and a girl are
-both gazing at the sun, when the moon comes between. To the boy the
-moon appears to take a great bite out of the sun, so that it looks like
-the left-hand picture in Fig. 35. (I have drawn a line from the end
-of the telescope in Fig. 34, which shows how much of the sun is cut
-off.) This would be called a partial eclipse of the sun. The almanac
-will sometimes describe the eclipses as visible in London, or visible
-at Greenwich; but that need not be taken so literally as was supposed
-by a Kensington gentleman, who, on noticing that the almanac said an
-eclipse was to be visible in London, called a cab and drove into the
-city to look for it. His almanac had not mentioned that it would be
-visible from his own house. You may usually take for granted that when
-an eclipse is said to be visible from London or Greenwich, it will be
-more or less visible all over England. Most of these eclipses are only
-partial, and though they are interesting to watch they do not teach us
-much. By far the most wonderful kind of eclipse is that in which the
-whole of the bright part of the sun is blotted out. Then, indeed, we do
-see wonders. But such eclipses are rare, and even when they do occur
-they only last a very few minutes. The sights that are displayed are so
-interesting that astronomers often travel thousands of miles to reach
-a suitable locality for making observations.
-
-The girl in Fig. 34 is placed in the best possible position for seeing
-the eclipse. There you find her right in the line of the sun and moon;
-and I think you will agree that she cannot see any part of the sun, for
-the moon is altogether in the way. I have drawn two dotted lines, one
-at each side. All that she can see beyond the moon must lie outside
-these dotted lines, and she will be in the dark as long as the moon
-stays in the way. When the eclipse is complete, comparative darkness
-steals over the land. The birds are deceived, and fly home to the trees
-to roost. The owls and the bats, thinking their time has arrived,
-venture forth on their nocturnal business. Even flowers close their
-petals, only to open a few minutes later when the sun again bursts
-forth. Other flowers that give forth their fragrance at night are also
-sweetly perceptible so long as the sun remains obscured. An unruly
-cow, accustomed to break into a meadow at night, was found there after
-an eclipse was over; while I learn from the same authority that a man
-rushed over in great excitement to see what his chickens were doing,
-but came back much disappointed on finding them pecking away as if
-nothing had happened.
-
-It will sometimes happen that the moon is so placed that the edge of
-the sun can be seen all round it. A case of the kind is shown in the
-right-hand picture of Fig. 35. It is called an annular, or ring-shaped
-eclipse.
-
-The eclipses of which we have been speaking are, of course, only to be
-seen during the day when the sun must be up. The lunar eclipses, which
-are visible at night, are due to the interposition of the earth between
-the sun and the moon. The sun is at night-time under our feet at the
-other side of the earth, and the earth throws a long shadow upwards.
-If the moon enter into this shadow, it is plain that the sunlight is
-partly or wholly cut off, and since the moon shines by no light of her
-own, but only by light borrowed from the sun, it follows that when
-she is buried in the shadow all the direct light is intercepted, and
-she must lose her brilliancy. Thus we obtain what is called a lunar
-eclipse. It is total if the moon be entirely in the shadow. The eclipse
-is partial if the moon be only partly in the shadow. The lunar eclipse
-is visible to everybody on the dark hemisphere of the earth if the
-clouds will keep out of the way, so that usually a great many more
-people can see a lunar eclipse than a solar eclipse, which is only
-visible from a limited part of the earth. It thus happens that the
-lunar eclipse is the more familiar spectacle of the two.
-
-When the moon is entirely in the shadow, one might naturally think
-that it would become totally invisible. This is not always the case.
-It is a curious fact that in the depth of a total eclipse the moon is
-often still visible, for she glows with a copper-colored light, which
-is bright enough to render some of the chief marks on her surface
-distinctly discernible.
-
-
-EFFECT OF THE MOON’S DISTANCE ON ITS APPEARANCE.
-
-We are now about to take a good look at the moon and examine the
-different objects which are marked upon it. There is a peculiar
-interest attached to this particular orb, because it is much the
-nearest of all the heavenly bodies to our globe, and therefore the one
-that we can see the best. Every other object--sun, star, or planet--is
-hundreds, or perhaps thousands, of times as far off as the moon. It is
-right that we should desire to learn all we can about the bodies in
-space. We know that the earth is a great ball, and we see that there
-are many other such bodies. Some of them are much larger, and some of
-them are smaller than the globe on which we dwell; some of them are
-dark bodies like the earth, and among them the moon is one. Is it not
-reasonable that we should make special efforts to find out all we can
-about this interesting neighbor?
-
-Though the moon is so close to us relatively to other objects in space,
-yet when we express its distance in the ordinary methods of measurement
-it is a very long way off--about 240,000 miles--a length nearly as
-great as that of all the railways in the world put together. An express
-train which runs forty miles an hour would travel 240 miles in six
-hours, and the whole distance to the moon would be accomplished in
-6000 hours, so that travelling by night and day incessantly you would
-accomplish the journey in 250 days. To take another illustration, if
-you wrapped a thread ten times round the equator of the earth, it would
-be long enough to stretch from the earth to the moon. Or suppose a
-cannon could be made sufficiently strong to be fired with a report loud
-enough to be audible 240,000 miles away. The sound would only be heard
-at that distance a fortnight after the discharge had taken place.
-
-The moon is too far for us to examine the particular features on its
-surface by the unaided eye. Suppose that there was a mighty city
-like London on the moon, with great buildings and teeming millions
-of people, and you went out on a fine night to take a look at our
-neighbor. What do you think you would be able to see of the great lunar
-metropolis? Would you be able to see its streets full of omnibuses, or
-even its great buildings? Would you see St. Paul’s and Westminster--the
-great parks and the river? Of all these things your unaided eye would
-show you almost nothing. I can give you a little illustration. Suppose
-that you made a tiny model of London; imagine this little structure
-all complete, so that the streets, the buildings, the bridges,
-the railways, the parks, and the Thames were placed in their true
-proportions; suppose that the miniature city was so small that it could
-stand on a penny postage stamp, surely everything would look very
-insignificant, even if you had the model in your hand and looked at
-it with the aid of a magnifying glass. But suppose it were put on the
-other side of the table or on the other side of the room, or the other
-side of the street. Even St. Paul’s Cathedral itself would have ceased
-to be distinguishable; but yet the distance is not nearly great enough.
-You would have to put the little model a quarter of a mile away before
-it would be in the right position to illustrate the appearance of a
-lunar London to the unaided eye.
-
-
-A TALK ABOUT TELESCOPES.
-
-The astronomer will not be contented with a mere naked-eye inspection
-of a world so interesting as the moon. He will get a telescope to help
-his vision. The word “telescope” means a contrivance for looking at
-objects which are a long way off. We have explained that the further
-an object is, the smaller it appears to be. The telescope enables us
-to largely overcome this inconvenience. It has the effect of making a
-distant object look larger.
-
-There are great differences in the forms of telescopes; and some
-instruments are large and some small, according to the purposes for
-which they are required. Perhaps the most useful practical application
-of the telescope is by the officer on duty on board a ship. He is
-generally provided with a pair of these instruments bound together to
-form the “binocular.”
-
-You are all acquainted with this useful contrivance, or at all
-events with the opera-glass, that is used for purposes with which
-landsmen are more familiar. The ship’s telescope, or the binocular,
-or the opera-glass, is feeble in power when compared with the great
-instruments of the Observatory. The officer on the ship will generally
-be satisfied with a telescope which shall show the objects with which
-he is concerned at about one-third of their actual distance. Thus,
-suppose his attention is directed to a great steamer three miles
-away, he wishes to see her more clearly, and accordingly he takes a
-view through his binocular. Immediately the vessel is so transformed
-that it seems to be only one mile away. The apparent dimensions of
-the object are increased threefold. The hull is three times as long,
-the masts and the funnel are three times as high, the sailors are
-three times as tall; various objects on the ship too small to be seen
-at three miles would be visible from one mile, and to that apparent
-distance the ship has now been brought.
-
-If the sailor desires to reduce the apparent distance of objects, how
-much more keenly does the astronomer feel the same want? At best, the
-sailor only has to scan a range of a few miles with his glass, but what
-are a few miles to the astronomer? It is true that he can count the
-distance of the moon by thousands of miles, a good many thousands, no
-doubt, but for all other objects he must use millions, while for most
-bodies in space, millions of millions of miles are the figures we are
-constrained to employ. Need it be said that the astronomer must resort
-to every device he can to make the body appear closer. He does not
-despise the modest binocular. It is often a useful instrument in the
-Observatory. It gives most beautiful pictures of the celestial scenery,
-and you would be amazed to find how many thousands of stars you can
-see with its help which your unaided eye would not show you at all.
-The binocular will also greatly improve the appearance of the moon,
-but still its powers fall far short of what we require for the study
-of lunar landscapes. Even though we can reduce the moon’s apparent
-distance to one-third its actual amount, yet still that third is a
-very considerable distance. One-third of 240,000 is 80,000, so that we
-can see the moon no better with a binocular than we should see it were
-it 80,000 miles away, and were we viewing it with the unaided eye.
-
-[Illustration: FIG. 36.--The Dome at Dunsink Observatory.]
-
-[Illustration: FIG. 37.--The Equatorial at Dunsink Observatory.]
-
-I am not going to enter here upon any detailed account of the
-telescope, because I shall say a little more on the subject in a later
-lecture; at present I only describe that form of instrument which is
-most convenient for studying the moon. I take as an illustration the
-South Equatorial at Dunsink Observatory, which belongs to Trinity
-College, Dublin.
-
-This telescope has a building to itself, which stands on the lawn in
-front of the house. The site is open and elevated, so as to command an
-extensive prospect of the heavens. You will see in Fig. 36 a picture
-of the structure. It is circular in form and is entered by the little
-porch. The most peculiar feature of an edifice intended to contain
-this kind of telescope is its roof, or _Dome_, as we call it. It is of
-a hemispherical shape with a projecting rim at the bottom. But no one
-would go to the trouble and expense of making a round dome like that
-over the Observatory if it were not necessary for a particular purpose.
-The dome is very unlike ordinary roofs, not only in appearance, but
-also because it can turn round. In the next figure you will see a
-section through the building, and the wheels are exposed by which the
-dome is carried. These wheels run easily on rails, so that when the
-attendant pulls the rope which you see in his hands, he turns round a
-large pulley, and that operates a little cogwheel which works into a
-rack, and thus makes the dome revolve. The roof is built of timber,
-covered with copper; it weighs more than six tons, but the machinery
-is so nicely adjusted, that a child four years old can easily set
-the whole in motion. The object of all this machinery is seen when
-we learn that there is only one opening in the dome. It is covered
-by the shutter shown over the doorway in Fig. 36. When opened to the
-top, it gives a long and wide aperture, through which the astronomer
-can look out at the heavens. Of course the dome has to be turned until
-the opening has been brought to face the required aspect. The big
-telescope can thus be directed to any object above the horizon. You
-see a gentleman using the telescope (Fig. 37), and this shows that
-the great instrument is nearly three times as long as the astronomer
-himself! No doubt the telescope seems to be composed of a good many
-different parts, but the essential portions of the instrument are
-comparatively few and simple. At the upper end is the object glass,
-which consists of two lenses, one of flint glass and the other of crown
-glass. Both of these must be of exceptional purity, and the shape to
-be given to the lenses is a matter of the utmost importance. It is in
-the making of this pair of glasses that the skill of the optician has
-to be specially put forth. So valuable indeed is an object glass which
-fulfils all the requirements, that it is by far the most costly part
-of the instrument. There are no glasses in the interior of the tube
-until you come to the end where the observer is looking in. This is
-closed by an eyepiece consisting of a lens, or a pair of lenses. There
-are usually many different eyepieces for a telescope, and they contain
-lenses of varied powers, to be used according to the state of the
-atmosphere, or to the particular kinds of observation in progress.
-
-If you point a big telescope to the sky, and see therein the sun or
-the moon or any of the stars, you will speedily find that the objects
-pass away out of view. Remember our earth is constantly turning
-round, and bears, of course, the Observatory with it, so that though
-the telescope be rightly pointed to the heavens at one moment, by
-the next it will have been turned aside. To you who are using the
-telescope, the appearance produced is as if the heavenly bodies were
-themselves moving. We can counteract this inconvenience. The telescope
-is supported on a pedestal, which is built on masonry, that goes down
-through the floor to its foundation on the solid rock beneath. In the
-iron casing at the top of the pedestal you will see a little window,
-and inside is clockwork driven by a heavy weight. This clockwork turns
-the whole telescope round in the opposite direction to that in which
-the earth is moving. The consequence is that the telescope remains
-constantly pointed to the same part of the heavens.
-
-[Illustration: FIG. 38.--The Yerkes Telescope, University of Chicago.]
-
-This instrument is no doubt a large one, but of late years many much
-greater have been built. The most powerful telescope that has ever been
-erected is the great Yerkes instrument belonging to the University of
-Chicago, of which a picture is shown in Fig. 38. The object glass is 40
-inches across.
-
-
-HOW THE TELESCOPE AIDS US IN VIEWING THE MOON.
-
-Those who are in charge of an observatory are often visited by persons
-who, coming to see the wonders of the heavens, and finding instruments
-of such great proportions, not unnaturally expect the views they are to
-obtain of the celestial bodies shall be of corresponding magnificence.
-So they are, no doubt, but then it frequently happens that the pictures
-which even the greatest telescope can display will fall far short of
-the ideal pictures which the visitors have conjured up in their own
-imaginations, so that they are often sadly disappointed. Especially is
-this true with regard to the moon. I have seen people who, when they
-had a view of the moon through a great telescope, were surprised not
-to find vast ranges of mountains which looked to them as big as the
-Alps, or mighty deserts, over which the eye could roam for thousands of
-miles. They have sometimes expected to behold stupendous volcanoes that
-not only were, but that looked to be as big as Vesuvius. Others seem
-to have thought they ought to see the moon with such clearness that
-the fields were to be quite visible, and some would not have been much
-astonished if they had observed houses and farmyards, and, perhaps,
-even cocks and hens.
-
-There are different ways of estimating the apparent dimensions of
-an object, but the size the moon appears to me to have in a great
-telescope may be illustrated by taking an orange in your hand and
-looking at the innumerable little marks and spots on its surface. The
-amount of detail that the eye will show on the orange is about equal
-to the amount of detail that a good telescope will show on the moon. A
-desert on the moon, which really is a hundred miles across, will then
-correspond to a mark about an eighth of an inch in diameter on the
-orange. Some of you may ask what is gained by the use of a telescope,
-for the moon looks to us as large as a plate with the unaided eye, and
-now we hear it only looks as big as an orange in the telescope. But
-where is the plate with which you compare your moon supposed to be
-held? It is surely not in your hand. It is imagined to be up in the
-sky, a very long way off. Though an orange is much smaller than a
-plate, yet you will be able to see many more details in the orange by
-taking it in your hand than you could see on a plate which was at the
-other side of the street.
-
-[Illustration: FIG. 39.--The Advantage of using a Telescope.]
-
-I sometimes find that people will not believe how much the telescope
-that they are using is magnifying the moon until they use both eyes
-together, of which one is applied to the telescope, while the other
-is directed to the moon. It will then be seen, even with a very small
-instrument, that the telescopic moon is as big as the larger of the two
-crescents in the adjoining figure (Fig. 39), while the naked-eye moon
-is like the smaller.
-
-The greatest telescopes are capable of reducing the apparent distance
-of an object to about one-thousandth part of its actual amount. If,
-therefore, a body were a thousand miles away, it would, when viewed by
-one of these mighty instruments, be seen as large as our unaided vision
-would show it, were the body only a single mile distant. No doubt this
-is a large accession to our power, but it often falls far short of what
-the astronomer would desire. The distances of the stars are all so
-great that even when divided by one thousand, they are still enormous.
-If you have a number expressed by 100,000,000,000,000, then dividing it
-by a thousand merely means taking off three of the ciphers, and there
-is still a large number left. We are, however, at present concerned
-with the moon, and, as its distance is about 240,000 miles, the effect
-of the best telescope is to reduce this distance apparently to 240
-miles. Here, then, we find a limit to what the best of all telescopes
-can do. It can never show us the moon better than, hardly indeed so
-well as, we could see it with our unaided eye were it only 240 miles
-over our heads. We cannot expect the most powerful instruments to
-reveal any object on the moon unless that object were big enough to be
-seen by the unaided eye when 240 miles away. What could we expect to
-see at a distance of 240 miles?
-
-Here is a little experiment which I made to study this point. I marked
-a round black dot on a sheet of white paper. The dot was a quarter of
-an inch in diameter, and then I fastened this on a door in the garden,
-and walked backwards until the dot ceased to be visible. I found this
-distance to be about thirty-six yards. I tried a little boy of eight
-years old, and it appeared that the dot became invisible to him about
-the same time as it did to me. “What has this to do with the moon?” you
-will say. Well, we shall soon see. In thirty-six yards there are 5184
-quarters of an inch, and as it is unnecessary to be very particular
-about the figures, we may say, in round numbers, that the distance when
-we ceased to be able to distinguish the dot was about five thousand
-times as great as the width of the dot itself. You need not, therefore,
-expect to see anything on the moon or on anything else which is not at
-least as wide as the five-thousandth part of the distance from which
-we are viewing it. The great telescope practically places the moon at
-a distance of 240 miles, and the five-thousandth part of that is about
-eighty yards; consequently a round object on the moon about eighty
-yards in diameter would be just glimpsed as the merest dot in the most
-powerful telescope. To attract attention, a lunar object should be
-much larger than this. If St. Paul’s Cathedral stood on a lunar plain,
-it would be visible in our great telescopes. It is true that we could
-not see any details. We should not be able to distinguish between a
-Cathedral and a Town-hall. There would just be something visible, so
-that the artist who was making a sketch of that part would put down a
-mark with his pencil to show that something was there. This will show
-us that we need not expect to see objects on the moon, even with the
-mightiest of telescopes, unless they are of great size.
-
-
-TELESCOPIC VIEWS OF LUNAR SCENERY.
-
-I have already warned you not to expect too much, even with the biggest
-of telescopes; and just as a caution, I may, perhaps, tell you a story
-I once heard of an astronomer who had a great telescope. It was a very
-famous instrument, and people often came to the Observatory at night
-to enjoy a look at the heavens. Sometimes these visitors were grave
-philosophers, but frequently they were not very accomplished men of
-science. One evening such a visitor came to the Observatory, and sent
-in his name and an introduction to the astronomer, with a request
-that he might enter the temple of mystery. The astronomer courteously
-welcomed the stranger, and asked him what he specially desired to see.
-
-“Oh!” said the visitor, “I have specially come to see the moon--that is
-the object I am particularly interested about.”
-
-“But,” said the astronomer, “my dear sir, I would show you the moon
-with pleasure, if you were here at the proper time; but what brings you
-here now? Look up; the evening is fine. There are the stars shining
-brightly, but where is the moon? You see it is not up at present. In
-fact, it won’t rise till about half-past two to-morrow morning, and it
-is only nine o’clock now. Come back again in five or six hours, and you
-shall observe the moon with the great telescope.”
-
-But the visitor evidently thought the astronomer was merely trying to
-get rid of him by a pretext. And he was equal to the occasion--he was
-not going to be put off in that way.
-
-“Of course, the moon is not up,” he replied; “any one can see that, and
-that is the reason why I have come, for _if the moon had been up, I
-could have seen it without your telescope at all_!”
-
-Although no explorer can ever reach our satellite, yet it is hardly
-an exaggeration to say that in some respects we know the geography of
-the moon a good deal better than we know the geography of the earth.
-Think of the continent of Africa. In that great country there are
-mighty tracts, there are vast lakes and ranges of mountains, of which
-we know but little. We could make a better map of Africa, so far at
-least as its broad outlines are concerned, if it were fastened up on
-our side of the moon than we actually possess at this moment. There
-is no spot on the nearer side of the moon as large as an ordinary
-parish in this country which has not been surveyed. There are maps
-and charts of the moon showing every part of it, which is as big as a
-good-sized field. Indeed, as there are no lunar clouds, the features
-of its surface are never obscured whenever our own atmosphere will
-permit us to make our observation. Artists have frequently sketched
-the lunar features, and there is plenty of material for them to work
-on. We have also had photographs taken of the moon, but there is a
-special difficulty to be encountered in taking photographs of celestial
-bodies which photographers of familiar objects on this earth do not
-experience. For a photograph to be successful, everybody knows that
-the first requisite is for the sitter to stay quiet while the plate
-is being exposed. This is, unhappily, just what the moon cannot do. We
-endeavor to obviate the difficulty by moving the telescope round so as
-to follow the moon in its progress. This can be done with considerable
-accuracy, but, unfortunately, there is another difficulty which lies
-entirely beyond our control. As the rays of light from the moon perform
-their journey through hundreds of miles of unsteady air, the rays are
-bent hither and thither, so that the picture is more affected by the
-atmosphere than in the case of a photographer’s portrait taken in the
-studio. If we are merely _viewing_ the moon through the telescope, the
-quivering effect on the rays of this long atmospheric voyage, though
-rather inconvenient, does not prevent us from seeing the object, and we
-can readily detect the true shape of each feature in spite of incessant
-fluctuations. When, however, these rays fall not on the eye, but on
-the photographic plate, they produce by their motion a picture which
-cannot be much magnified without becoming very confused and wanting in
-sharpness. Nevertheless, for the general outlines of our satellite’s
-appearance and for the portraiture of its splendid features we have
-derived the greatest assistance from photography.
-
-[Illustration: FIG. 40.--The Full Moon.]
-
-The adjoining picture (Fig. 40) gives a fair idea of what the full moon
-looks like when viewed through a small telescope. I do not, however,
-say that the lunar objects can then be observed under favorable
-conditions; for when the moon is full is the very worst time for making
-observations of our satellite. In fact, at this phase you can hardly
-see anything except slight differences between the colors of different
-parts. The best time for observing the moon is at the first quarter;
-but even then you can only observe satisfactorily those objects which
-happen to lie along the border between light and shade. To study the
-moon properly you must, therefore, watch it during several different
-phases, from the time when it presents a thin and delicate crescent
-(just after new moon) until it has again waned to a thin and delicate
-crescent (just before the next new moon). We want the relief given by
-shadows to bring out the full beauty of lunar scenery.
-
-On the map you will first notice the large dark-colored patches which
-are so conspicuous on the moon’s face. They are, apparently, the empty
-basins which great seas once filled. But if water was ever there it has
-at all events now quite disappeared. These dark parts are, no doubt, a
-good deal smoother than the rest of the surface; but we can see many
-little irregularities which tell us that we are not looking at oceans.
-The chief features I want you to observe are the curious rings which
-you see in the figure; there is a very well-marked one a little below
-the centre, and in the upper part many rings--large and small--are
-crowded together. We call them lunar craters. You will see what they
-are like from the model, of which a picture is shown in Fig. 42. But to
-realize from this picture the proper scale of the object, you should
-imagine it to be some miles in width. The cliffs which rise all round
-to form the wall, as well as the mountain which adorns the centre, are
-quite as high as any of the mountains in Great Britain.
-
-[Illustration: FIG. 41.--View on the Moon.
-
-(_By Lœwy and Puiseux, Paris Observatory._)
-
-The large central crater is Hipparchus and above it is Albategnius.]
-
-[Illustration: FIG. 42.--Our Model of a Lunar Crater.]
-
-You may desire to know how we are able to measure the heights of
-mountains on the moon. That is what I am now going to show you; and for
-this purpose we shall look at our imitation lunar crater. Here is the
-great ring, or circular enclosure, surrounded by cliffs, and here is a
-sharp mountain peak rising in the centre. I shall ask to have the beam
-from the electric lamp turned on our model. You see how prettily it
-is lighted up. I have placed the lamp so that the beams are sloping;
-and I have done this with the express object of making the shadows
-long. In fact, as we look at a lunar crater, which lies on the border
-between light and shade, the sun illuminates the object under the same
-conditions as those shown in the figure. I dare say you have often
-noticed what long shadows are cast at sunset. Similar shadows are made
-to teach the astronomer the altitudes of the lunar mountains; for he
-measures the length of the shadow, and then by a little calculation he
-can find the height of the object by which that shadow has been cast.
-I shall suppose that we want to measure the height of a flagstaff
-(Fig. 43). It is quite possible to do this by merely measuring the
-length of the shadow which that flagstaff casts at noon. It would not
-be correct to say that the height of the flagstaff is the length of
-its shadow. This will, indeed, be the case if you are fortunate enough
-to make your measurement at or near London on either the 6th of
-April or the 5th of September. On all other days in the year a little
-calculation must be made, which I need not now mention, but which the
-astronomer, with the aid of his Nautical Almanac, can do in a very few
-minutes. In a similar manner, by measuring the lengths of the shadows
-on the moon, and by finding the number of miles in the shadow, we are
-able to calculate the altitudes of the lunar mountains and of the
-ranges of cliffs by which the walled plains are surrounded.
-
-[Illustration: FIG. 43.--How we found the Height of the Flagstaff.]
-
-
-ON THE ORIGIN OF THE LUNAR CRATERS.
-
-We have now to offer an explanation of the curious rings which are
-the most characteristic features on the moon. To account for them we
-must look for a moment at some objects on the earth. You have all
-heard of volcanoes or burning mountains, such as Vesuvius or Etna,
-which occasionally break out into violent eruptions, and send forth
-great showers of ashes and torrents of molten lava. In the Sandwich
-Islands there is a celebrated volcano called Kilauea. It is like a
-vast lake of lava, so hot that it is actually molten, and glows with
-heat like red-hot iron. The adventurous tourist who visits this crater
-can climb to the brink of a lofty range of cliffs which surround it,
-and gaze down upon the fervid sea beneath. Suppose that by some great
-change the internal heat which keeps this mighty basin glowing were
-to decline and go out, the sea of lava would cease to be liquid, and
-would ultimately grow hard and cold, and we should then have an immense
-flat plain, surrounded by a range of cliffs. Elsewhere in the Sandwich
-Islands examples of extinct craters may be found at the present day.
-Those who have studied these interesting localities point out how such
-terrestrial craters explain the ringed plains in the moon. It seems
-certain that in ancient days great volcanoes abounded on our satellite,
-and the rings were often much larger than those on the Sandwich
-Islands, some of them being one hundred miles or more in diameter.
-The volcanoes must long ago have been raging on the moon with a fury
-altogether unknown in any active volcanoes which this earth can now
-show. We can also surmise how the lofty mountain peak, which so often
-rises in the centre of a lunar ring, has been upheaved. When the fires
-had almost subsided, and the floor had grown nearly cold, one last
-and expiring effort is made by which the congealing surface is burst
-through at the centre, and materials are shot forth which remain as the
-central mountain to the present day.
-
-I must, however, impress upon you that even our greatest telescopes
-never exhibit to us any volcanic eruptions at present going on in the
-moon; in fact, it is most doubtful if any change has been noticed in
-the features on its surface since the date of the invention of the
-telescope. The volcanoes sculptured the crust of the moon into the form
-in which we see it, and that form our satellite has preserved for ages,
-of which we cannot estimate the duration. All the craters and all the
-volcanoes in the moon can only be described as extinct.
-
-It would be interesting for us to compare the present condition of
-the volcanoes in the earth with that of the ringed craters in the
-moon. The noisy volcanoes on our globe are those most talked about; we
-often hear of Vesuvius being in eruption, and in August, 1883, there
-was a terrific eruption at Krakatoa, during which a large quantity
-of dust was shot up into the air, to such a height that it was borne
-right round the earth, and produced beautiful sunsets and unwonted sky
-hues in almost every country in the world. The explosion at Krakatoa
-made the loudest noise that history has recorded. Fortunately such
-convulsions of the earth do not often happen, for, on that occasion,
-the sea rushed in on the land, and thousands of lives were lost. There
-are said to be one hundred and fifty volcanoes on different parts of
-the earth, which are more or less active, but there are many others in
-which the fire has gone out, and which seem to be just as cold and
-just as extinct as any volcanoes in the moon. Even in our own islands
-there are abundant remains of ancient volcanoes. Masses of lava are
-found in many places where now there is no trace of an active volcano.
-Perhaps there is no more remarkable sight in the British Isles than
-that lofty rock which is crowned by Edinburgh Castle; it is the remnant
-of a former volcano, while Arthur’s Seat, close by, is another. In
-the centre of France is the beautiful district of Auvergne, in which
-ancient volcanoes abound; and the lava streams can be traced for miles
-across the country. These volcanoes have been extinct for thousands of
-years, during which time the lava has become largely covered with soil
-and vegetation, and in some places vineyards are cultivated upon it.
-
-We are now able to contrast the earth with the moon, in so far as
-volcanoes are concerned. On the earth we have some that are active,
-and a much greater number that are extinct. On the moon we find no
-active volcanoes, for there all are extinct. I can explain how this
-difference has arisen, but first let me show you a simple experiment.
-My assistant will kindly bring to me from that furnace two iron balls,
-which we placed there before the commencement of this lecture; there
-they are, you see, both glowing with a bright red heat, for at present
-they are equally hot. We will place them on these stands, and allow
-them to grow cold. One of these balls is a small cannon-ball, four
-inches in diameter, while the other is only one inch. They are in the
-same proportion as the earth is to the moon; but look, even while I am
-speaking the balls have ceased to preserve the same temperature, for
-the little one has become almost black from loss of its heat, while the
-large one still looks nearly as red as it did at the beginning; this
-simple experiment will illustrate the principle that two heated bodies
-will cool at very different rates, if their sizes be different, while
-the other conditions are the same. The small body will always cool
-faster than the large one. They need not be globes for this experiment;
-if you put a poker and a knitting needle into the fire, and leave both
-there until they are red-hot, and then put them out into the fender,
-you will speedily find that though they were at the same temperature
-when drawn from the fire, they do not long remain so; indeed, the
-knitting needle has become cold enough to handle before the poker has
-ceased to glow. Our experiments have been made with, no doubt, small
-objects only, but the law about which they inform us will remain true,
-even for the greatest objects.
-
-Our earth at the present day shows many indications of being much
-hotter within than it is on the surface. The volcanoes themselves are
-mere outbreaks of incandescent material from inside. Then there are
-hot springs of water at Bath, which gush out from the earth. There
-are geysers of hot water in Iceland and in the Yellowstone Park in
-America, and in other places. And there are other indications also,
-with which every miner is familiar. Wherever a deep pit is sunk into
-the earth, the rocks below are always found to be warmer than those
-on the surface, and the deeper the pit the greater is the heat that
-is encountered. Thus, from all over the world we obtain proofs of
-the present existence of internal heat. Great as is the earth, we
-must still apply the simple common-sense principles that we use in
-our everyday life here. Let me give an illustration. Suppose that a
-servant came into the room and placed a jug of water on the table, and
-that an hour afterwards you went to the jug of water and found it to
-be cold, you would not from that fact alone be able to infer anything
-with certainty, as to whether the water had been warm or cold when it
-was brought in. It might have been perfectly cold, as it is at present,
-though on the other hand the water might have been warm at first, and
-have since cooled down to the temperature of the room during the hour.
-
-Suppose, however, that when you went to the jug of water, which had
-stood on the table for an hour, you found it tepid, no matter how
-slightly its temperature might be above that of the room, do you not
-see the inference you would be able to draw? You would argue in this
-way: that water has still some heat; it must, of course, be gradually
-cooling, and therefore it was hotter a minute ago than it is now; it
-was hotter still two minutes ago, or ten minutes; and must have been
-very hot and perhaps boiling when it was brought in an hour ago.
-
-I want you to apply exactly the same reasoning to our earth. It is,
-as I have shown you, still hot and warm inside. Of course, that heat
-is gradually becoming lost; so that the earth will from year to year
-gradually cool down, though at an extremely slow rate. But we must
-look back into what has happened during past ages. Just as we inferred
-that the jug must have contained very hot water an hour before from
-the mere fact that the water was still warm, so we are entitled to
-infer, from the fact that the earth still retains some heat, that it
-must in ages gone by have been exceedingly hot. In fact, the further
-we look back, the hotter do we see the earth growing, until at last we
-are constrained to think of a period, in the excessively remote past,
-long ere life began to dawn on this earth, when even the surface of the
-earth was hot. Back further still we see the earth no longer covered
-with the hard, the dark, and the cold surface we now find; we are to
-think of it in these primitive times as a huge glowing mass, in which
-all the substances that now form the rocks were then incandescent, and
-even molten material.
-
-There is good reason for knowing that in those early times the moon
-also was molten with heat; and thus our reasoning has led us to think
-of a period when there were two great red-hot globes--one of which had
-about four times the diameter of the other--starting on their career
-of gradually cooling down. Recall our little experiment with the two
-cooling globes of iron; imagine these globes to preserve their relative
-proportions, but that one of them was 8000 miles and the other 2000
-miles across. Ages will, no doubt, elapse ere they part with their
-heat sufficiently to allow the surfaces to cool and to consolidate. We
-may, however, be sure that the small globe will cool the faster, that
-its outside will become hard sooner than will the surface of the large
-one, and long after the small globe has become cold to the centre,
-the large one may continue to retain some of its primeval heat. We
-can thus readily understand why all the volcanoes on the moon have
-ceased--their day is over. It is over because the moon, being so small,
-has grown so cold that it no longer sustains the internal fires which
-are necessary for volcanic outbreaks. Our earth, in consequence of its
-much greater size, has grown cold more slowly. It has no doubt lost the
-high temperature on the exterior, and its volcanic energy has probably
-abated from what it once was. But there is still sufficient power in
-the subterranean fires to awaken us occasionally by a Krakatoa, or
-to supply Vesuvius with sufficient materials and vigor for its more
-frequent outbursts. The argument shows us that the time will at last
-come when this earth shall have parted with so large a proportion of
-its heat that it will be no longer able to provide volcanic phenomena,
-and then we shall pass into the exhausted stage which the moon attained
-ages ago.
-
-
-THE MOVEMENTS OF THE MOON.
-
-Though the moon is going round and round the earth incessantly, yet it
-always manages to avoid affording us a view of what is on the other
-side. Our satellite always directs the same face towards the earth, and
-we may reasonably conjecture that the other side is covered, like the
-side we know, with rings and other traces of former volcanoes. In this
-respect the moon is quite a peculiar object. The other great celestial
-bodies, such as the sun or Jupiter, turn round on their axes, and show
-us now one side and then the other, with complete impartiality. The way
-in which the moon revolves may be illustrated by taking your watch
-and chain, and as you hold the chain at the centre making the watch
-revolve in a circular path. At every point of its path the ring of the
-watch is, of course, pointed to the centre where the chain is held. If
-you imagine your eye placed at the centre, to represent the earth, the
-movements of the watch would exemplify the way the moon turns round it.
-
-One more point I must explain about the moon before we close this
-lecture. There is nothing more familiar than the fact that a heavy body
-will fall to the ground. Indeed, it hardly matters what the material
-of the body may be, for you see I have a small iron ball in one hand
-and I hold a cork in the other. I drop them at the same moment, and
-they reach the ground together. Perhaps you would have expected that
-the cork would have lagged behind the iron. I try the experiment
-again and again, and you can see no difference in the times of their
-falling, though I do not say this would be true if they were dropped
-from the top of the Monument. In general we may say that bodies let
-drop will fall sixteen feet in the first second. Even a bit of paper
-and a penny piece will fall through the same height in the same time
-if you can get over the difficulty of the resistance of the air. This
-is easily managed. Cut a small piece of tissue paper which will lie
-flat on the top of the penny, and hold the penny horizontal with the
-paper uppermost. Though there is nothing to fasten the paper to the
-penny, you will find that they fall together. If we could conduct the
-experiment of dropping the penny and the bit of paper in a vacuum,
-then, whether the paper was laid on the penny or placed in any other
-way, the two objects would reach the table at the same moment if
-released at the same moment at equal heights.
-
-Wherever we go we find that bodies will always tend to fall in towards
-the centre of the earth; thus in New Zealand, at the opposite side of
-our globe from where we are now standing, bodies will fall up towards
-us, and this law of falling is obeyed at the top of a mountain as it is
-down here. No matter how high may be the ascent made in a balloon, a
-body released will fall towards the earth’s centre. Of course, we can
-only ascend some five or six miles high, even in the most buoyant of
-balloons; but we know that the attraction by which bodies are pulled
-downwards towards the earth extends far beyond this limit. If we could
-go ten, twenty, or fifty miles up, we should still find that the earth
-tried to pull us down. Nor, even if you could imagine an ascent made to
-the height of 1000 miles, would gravitation have ceased. A cork or an
-iron ball, or any other object dropped from the height of 1000 miles,
-would assuredly tumble down on the ground below.
-
-Suppose that by some device we were able to soar aloft to a height
-of 4000 miles. I name that elevation because we should then be as
-high above the earth as the centre of the earth is below our feet. We
-should have doubled our distance from the centre of the earth, and the
-intensity of the gravitation would have decreased to one-quarter of
-what it is at the surface. A body which at the earth’s surface falls
-sixteen feet in a second would there fall only four feet in a second,
-and the apparent weight of any body would be so much reduced that it
-would seem to weigh only a quarter of what it weighs down here. Thus,
-the higher and higher we go, the less and less does gravity become; but
-it does not cease, even at a distance of millions of miles. Therefore
-you might say that as gravity tries to pull everything down, wherever
-it may be, why does it not pull down the moon? This is a difficulty
-which we must carefully consider. Supposing that the earth and the moon
-were simply held apart, both being at rest, and that then the moon
-were to be let go, it would no doubt drop down directly on the earth.
-The movement of the moon would, however, be very different if, instead
-of being merely let fall, it was thrown sideways. The effect of the
-earth’s pull upon the moon would then be shown in keeping the moon
-revolving around us instead of allowing it to fly away altogether, as
-it would have done had the earth not been there to attract it.
-
-[Illustration: FIG. 44.--An Illustration to explain the Movement of the
-Moon]
-
-We can explain this by an illustration. On the top of a mountain I have
-placed a big cannon (Fig. 44). We fire off the cannon, and the bullet
-flies away in a curved path, with a gradual descent until it falls to
-the ground. I have made the mountain look hundreds of times larger than
-any mountain could possibly be; and now I want you to imagine a cannon
-far stronger and gunpowder more potent than any powder or cannon that
-has ever yet been manufactured. Fire off a bullet with a still greater
-charge than the last time, and now the path is a much longer one, but
-still the bullet curves down so as ultimately to fall on the earth. But
-make now one final shot with a charge sufficiently powerful, and away
-flies the bullet, following this time the curvature of the earth, for
-the earth’s attraction has the effect of bending the path of the bullet
-from a straight line into this circular form. By the time the bullet
-has travelled a quarter of the way round, it is no nearer to the earth
-than it was at first, nor has it parted with any of its original speed.
-Thus, notwithstanding its long journey, the bullet has practically just
-as much energy as when it first left the muzzle of the cannon. Away it
-will fly round another quarter of the earth, and still in the same
-condition it will accomplish the third and the fourth quarters, thus
-returning to the point from which it started. If we have cleared the
-cannon out of the way, the bullet will fly again over the mountain top
-without having lost any of its speed by its voyage round the earth.
-Therefore it will be in a condition to start again, and thus to revolve
-around the earth permanently. If, then, from the top of a mountain
-240,000 miles high a great bullet 2000 miles in diameter had once been
-projected with the proper velocity, that bullet would continue forever
-to circle round and round the earth, and even though the mountain and
-the cannon disappeared, the motion would be preserved indefinitely.
-This illustration will, at all events, show how a continuous revolution
-of the moon round the earth can exist, notwithstanding that the earth
-is constantly pulling our satellite down towards its surface.
-
-
-ON THE POSSIBILITY OF LIFE IN THE MOON.
-
-Astronomers are often asked whether any animals can be living on the
-moon. No observations we can make with the telescope can answer that
-question directly. There are great plains to be seen on the moon, of
-course, but even if there were elephants tramping over those plains,
-our telescopes could not show them. Nor will our instruments pronounce
-at once whether plants or trees flourish on the moon. The mammoth trees
-of California are so big that a tunnel has been cut through the trunk
-of one large enough to give passage for a carriage and pair. Even if
-there were trees as big as this on the moon, they would not be visible
-from the most famous observatories.
-
-Let us think what we should ourselves experience if we could in some
-marvellous manner be transferred from the earth to its satellite, and
-tried to explore that new and wonderful country. Alas, we should find
-it utterly impossible to live there for an hour, or even for a minute!
-Troops of difficulties would immediately beset us. The very first would
-be the want of air. Ponder for a moment on the invariable presence
-of air around our own globe. Even if you climb to the top of a high
-mountain, or if you take a lofty voyage in a balloon, you are all the
-time bathed in air. It is air which supports the balloon, just as a
-cork is buoyed up by water. In all circumstances, we must have air to
-breathe. In that air is oxygen gas, and we must have oxygen incessantly
-supplied to our lungs to reinvigorate our blood. We require, too, that
-this oxygen shall be diluted with a much larger amount of nitrogen
-gas, for our lungs and system of circulation are adapted for abode in
-that particular mixture of gases which we find here. The atmosphere
-becomes more and more rarefied the higher we ascend, and apparently
-terminates altogether some two or three hundred miles over our heads.
-Beyond the limits of the atmosphere it seems as if empty space would be
-met with all the way from the earth to the moon. We could not procure
-a single breath of air, and life would be, of course, impossible. Even
-at a height of three or four miles, respiration becomes difficult,
-and doubtless life could not possibly be sustained at a height of ten
-miles.
-
-It is therefore plain that for a voyage to the moon we should require
-an ample supply of air, or, at least, of life-giving oxygen, which in
-some way or other was to be inhaled during the progress of the journey.
-When at length 240,000 miles had been traversed, and we were about
-to land on the moon, we would first of all ascertain whether it was
-surrounded with a coating of air. Most of the globes through space
-are, so far as we can learn, covered and warmed with an enveloping
-atmosphere of some kind; but, unhappily, the poor moon has been left
-entirely, or almost entirely, without any such clothing. She is quite
-bare of atmosphere at all comparable in density or in volume to that
-which surrounds us, though possibly we do now and then perceive some
-traces of air, or of some kind of gas, in small quantities in the lunar
-valleys.
-
-I am sure each intelligent boy or girl will want to know how we are
-able to tell all this. We have never been at the moon, and how then can
-we say that it is nearly destitute of air? Nor can our telescope answer
-this question immediately, for you could hardly expect to see air,
-even if it were there. How then are we able to make such assertions?
-There are many different ways in which we have learned the absence of
-air from the moon. I will tell you one of the easiest and the most
-certain of these methods. First let me say that air is not perfectly
-transparent. No doubt I can see you, and you can see me, though a good
-many feet of air may lie between us; but when we deal with distances
-much greater, there is a very simple way in which we can show that air
-is not quite transparent. In the evening, when the sun is setting and
-the sky is clear, you can look at him without discomfort; but in the
-middle of the day you know that it is impossible to look at the sun
-without shading your eyes with smoked glass or protecting them by some
-similar contrivance. The reason is, that when the sun is either setting
-or rising we look at it through an immense thickness of air, which not
-being perfectly transparent stops some of the light. Thus it is that
-the sun in these circumstances loses its dazzling brilliancy, and we
-can view it without discomfort.
-
-At the seaside you can notice the same effect in a different manner. Go
-out on a fine and clear night, when the stars in their thousands are
-glittering overhead, and then look down gradually towards the horizon,
-and you will find the stars becoming fainter and fainter. Indeed, even
-the brightest star cannot be seen when it is at the horizon, because an
-immense thickness of the atmosphere is not transparent.
-
-We can now state the argument by which we may prove that there is
-little or no air on our satellite. The moon will frequently pass
-between the earth and a star, and when the star is a really bright one
-the observations that can be made are of great interest. Let me first
-describe what we actually see. The star is shining brightly until the
-moment when the moon eclipses it. Generally speaking, its disappearance
-is instantaneous. But this would not be the case if the moon were
-encircled with an atmosphere. If the moon were coated with air, the
-light from the star would not be extinguished _instantly_; it would
-gradually decline, according as it had to pass through more and more
-of the moon’s atmosphere. Thus you would find that the star dwindled
-down in brightness before the solid body of the moon had advanced far
-enough to shut it out. The sudden extinction of the stars demonstrates
-the airless state of our satellite.
-
-There would be another insuperable difficulty in adopting the moon
-as a residence, even supposing that you could get there. Water is
-absent from its surface. We have examined every part of it, and we
-find no evidences of seas or of oceans, of lakes or rivers; we never
-see anything like clouds or mists, which are, of course, only water in
-the vaporous form. We are, therefore, assured that, so far as water is
-concerned, the moon is an absolute desert. This is, perhaps, the most
-striking contrast between the aspect of the earth and the aspect of the
-moon. Were an astronomer on the moon to look at our earth he would find
-most of its surface concealed beneath clouds, and through the openings
-in these clouds he would see that by far the greater part of this globe
-was covered by the expanse of ocean; in fact, when the lunar astronomer
-had realized the prevalence of water upon this earth, either in the
-form of ocean or cloud, I feel sure he would come to the conclusion
-that nothing could live here except seals or other amphibious animals.
-
-Owing to the absence of air and water, the moon would be totally
-disqualified for the support of life of those types in which we know
-it. For air and water are necessary to every animal, from the humblest
-animalcule up to whales or elephants. Air and water are necessary
-for every form of vegetable life, from the lichen which grows on a
-stone up to the noble old oak of the forest. But even supposing that
-we could land on the moon, bearing with us an ample supply of oxygen
-to breathe, and of water to drink, we should find ourselves perplexed
-and embarrassed, to say the very least of it, by an extraordinary
-difference that would immediately attract our notice. That familiar
-experience of gravity, or the weights of things, which we have
-acquired in our residence on a great globe like the earth, would seem
-ludicrously altered when we began to walk about on a little globe
-like the moon. We should be astonished at the transformation by which
-the weight of everything was much lessened; when you pulled out your
-watch you would hardly feel it at the end of the chain; it would seem
-like a mere shell; but yet the watch is all right, it is going as well
-as ever. Nothing has altered about it except its weight. A big stone
-attracts your notice, and, to your amazement, you find that it does
-not weigh so much as a piece of wood of the same size would weigh down
-here. A stone that you could hardly stir on the earth, you can carry
-about on the moon. Nor is this to be explained by any peculiarity in
-the constitution of the lunar stone. Most probably it will be not very
-dissimilar to some of the rocks on the earth. The relative lightness
-of a lunar stone is not due to its being formed of some very special
-material; we must seek for some other explanation. Every object on
-the moon would be found only one-sixth as heavy as the same object on
-the earth. A sturdy laborer at one of the docks can carry one sack
-of corn on his back here, and he finds that this load is as much as
-is convenient. He would, however, discover, were he placed on the
-moon, that his load had suddenly become lightened to one-sixth part
-(Fig. 45). The laborer would find that he could carry six sacks of
-corn on the moon without making a greater effort than the support of
-a single sack on the earth cost him. To explain how such a change as
-this has occurred, look at these two pictures: one shows the laborer
-on a small body like the moon, the other shows him on a great globe
-like the earth. What the laborer actually does feel is not quite so
-simple a thing as he imagines. He imagines that it is the weight of the
-corn, and the corn alone, which produces that pressure on his shoulders
-which he knows so well. But that is not exactly the manner in which
-the philosopher will look at the same question. What the laborer does
-actually feel is the attraction between the earth beneath his feet and
-the corn on his back. It is this force which produces the pressure on
-his shoulders. Its magnitude no doubt depends upon the quantity of corn
-in the sack, but it also depends on the quantity of matter on the earth
-beneath his feet. In fact, the force between two attracting bodies
-depends upon the masses of both the attracting bodies. When the laborer
-is transferred to the moon, of which the mass is so much less than that
-of the earth, the attraction is less there than it is here, even though
-the corn is the same in the two cases.
-
-[Illustration: FIG. 45.--The Lessened Gravitation on the Moon.]
-
-Many odd instances could be given of the extraordinary consequences
-of life on a world where all weights are reduced to a sixth part. One
-occurred to me the other day when I saw a postman going his rounds with
-an amazing load of Christmas presents and parcels. I thought, how much
-happier must be the lot of a postman on the moon, if such functionaries
-are wanted there! All the presents of toys or more substantial
-donations might be the same as before, the only alteration would be
-that they would not feel nearly so heavy. A box which contains a pound
-of chocolate bonbons might still contain exactly the same quantity of
-sweetmeat on the moon, but the exertion of carrying it would be reduced
-to one-sixth. It would only weigh as much as two or three ounces do
-on the earth. Our streets provide another admirable illustration of
-the drawbacks of our life here as compared with the facilities offered
-by life on the moon. I feel quite confident that no perambulators can
-be necessary there. I cannot indeed say that there are babies to be
-found on the moon, but of this I am certain, that even if the lunar
-babies were as plump and as sturdy as ours, they must still only weigh
-about a sixth as much as ours do. A lunar nurse would scorn to use a
-perambulator, even for a pair of twins; she might take them both out
-on her arm for an airing, and even then only bear one-third of the
-load that her terrestrial sister must sustain if she is carrying but a
-single child.
-
-The lightness of bodies in the moon would entirely transform many of
-our most familiar games. In cricket, for instance, I don’t think the
-bowling would be so much affected, but the hits on the moon would
-be truly terrific. I believe an exceptionally good throw of the
-cricket-ball here is about a hundred yards, but the same man, using
-the same ball and applying the same force to it, would send the ball
-six hundred yards on the moon. So, too, every hit would in the lunar
-game carry the ball to six times the distance it does here. Football
-would show a striking development in lunar play; a good kick would not
-only send the ball over the cross-bar, but it would go soaring over the
-houses, and perhaps drop in the next parish.
-
-Our own bodies would, of course, participate in the general buoyancy,
-so that, while muscular power remained unabated, we should be almost
-able to run and jump as if we had on the famous seven-league boots. I
-have seen an athlete in a circus jump over ten horses placed side by
-side. The same athlete, making the same effort, would jump over sixty
-horses on the moon.
-
-A run with a pack of lunar foxhounds would indeed be a marvellous
-spectacle. There need be no looking round by timid horsemen to find
-open roads or easy gaps. The five-barred gate itself would be utterly
-despised by a huntsman who could easily clear a hay-rick. It would
-hardly be worth taking a serious jump to clear a canal unless there was
-a road and a railway or so, which could be disposed of at the same time.
-
-To illustrate this subject of gravitation in another way, suppose that
-we were to be transferred from this earth to some globe much greater
-than the earth--to a globe, for instance, as large and massive as
-the sun. We can then show that the weight of every object would be
-increased. Indeed, everything would weigh about twenty-seven times as
-much as we find it does here. To pull out your watch would be to hoist
-a weight of about five or six pounds out of your pocket. Indeed, I do
-not see how you could draw out your watch, for even to raise your arm
-would be impossible; it would feel heavier by far than if it were made
-of solid lead. It is, perhaps, conceivable that you might stand upright
-for a moment, particularly if you had a wall to lean up against; but of
-this I feel certain, that if you once got down on the ground, it would
-be utterly out of your power to rise again.
-
-These illustrations will at least answer one purpose: they will show
-how difficult it is for us to form any opinion as to the presence or
-the absence of life on the other globes in space. We are just adapted
-in every way for a residence on this particular earth of a particular
-size and climate, and with atmosphere of a particular composition.
-Within certain slender limits our vital powers can become accommodated
-to change, but the conditions of other worlds seem to be so utterly
-different from those we find here, that it would probably be quite
-impossible for beings constituted as we are to remain alive for five
-minutes on any other globe in space.
-
-It is, however, quite another question as to whether there may not
-be inhabitants of some kind on many of the other splendid globes. We
-have through the wide extent of space inconceivable myriads of worlds,
-presenting, no doubt, every variety of size and climate, of atmosphere
-and soil. It seems quite preposterous to imagine that among all these
-globes ours alone should be the abode of life. The most reasonable
-conclusion for us to come to is that these bodies may be endowed with
-life of types which are just as appropriate to the physical conditions
-around them as is the life, both animal and vegetable, on this globe to
-the special circumstances in which it is placed.
-
-
-
-
-LECTURE III.
-
-THE INNER PLANETS.
-
-  Mercury, Venus, and Mars--How to make a Drawing of our System--The
-      Planet Mercury--The Planet Venus--The Transit of Venus--Venus
-      as a World--The Planet Mars and his Movements--The Ellipse--The
-      Discoveries made by Tycho and Kepler--The Discoveries made by
-      Newton--The Geography of Mars--The Satellites of Mars--How the
-      Telescope aids in Viewing Faint Objects--The Asteroids, or Small
-      Planets.
-
-
-MERCURY, VENUS, AND MARS.
-
-We can hardly think of either the sun or the moon as a world in the
-sense in which our earth is a world, but there are some bodies called
-planets which seem more like worlds, and it is about them that we
-are now going to talk. Besides our Earth there are seven planets of
-considerable size, and a whole host of insignificant little ones. These
-planets are like ours in a good many respects. One of them, Venus, is
-about the same size as this earth; but the two others, Mercury and
-Mars, are very much smaller. There are also some planets very much
-larger than any of these, namely, Jupiter, Saturn, Uranus, and Neptune.
-We shall in this lecture chiefly discuss three bodies, namely, Mercury,
-Venus, and Mars, which, with the earth, form the group of “inner”
-planets.
-
-The planets are all members of the great family dependent on the sun.
-Venus and the earth may be considered the pair of twins, alike in size
-and weight. Mercury and Mars are the babies of the system. The big
-brothers are Jupiter and Saturn. All the planets revolve round the
-sun, and derive their light and their heat from his beams. We should
-like to get a little closer to some of our fellow-planets and learn
-their actual geography. Unfortunately, even under the most favorable
-circumstances, they are a very long way off. They are many millions of
-miles distant, and are always at least a hundred times as far as the
-moon. But far as the planets may be, astronomers have been familiar
-with their existence for ages past. I can give you a curious proof of
-this. You remember how we said the first and the second days of the
-week were called after the sun and the moon, Sun-day and Moon-day, or
-Monday, respectively. Let us see about the other days. Tuesday is not
-quite so obvious, but translate it into French and we have at once
-_Mardi_; this word means nothing but Mars’ day, and our Tuesday means
-exactly the same. Wednesday is also readily interpreted by the French
-word _Mercredi_, or Mercury’s day, while Venus corresponds to Friday.
-Jupiter’s day is Thursday, while Saturn’s day is naturally Saturday.
-The familiar names of the days of the week are thus associated with the
-seven moving celestial bodies which have been known for uncounted ages.
-
-
-HOW TO MAKE A DRAWING OF OUR SYSTEM.
-
-I want every one who reads this book to make a little drawing of the
-sun and the planets. The apparatus that you will need is a pair of
-compasses; any sort of compasses that will carry a bit of pencil will
-do. You must also get a little scale that has inches and parts of
-inches divided upon it; any carpenter’s rule will answer. The drawing
-is intended to give a notion of the true sizes and positions of the
-fine family of which the earth is one member. The figure I have given
-(Fig. 46) is not on so large a scale as that which I ask you to use,
-and which I shall here mention. Try and do the work neatly, and then
-pin up your little drawings where you will be able to see them every
-day until you are quite familiar with the notion of what we mean by our
-solar system.
-
-[Illustration: FIG. 46.--The Orbits of the Four Inner Planets.]
-
-First open the compasses one inch, and then describe a circle, and mark
-a dot on this as “MERCURY,” in neat letters, and also write on the
-circle “88 days.” At the centre you are to show the “SUN.” This circle
-gives the track followed by Mercury in its journey round the sun in the
-period of 88 days. Next open your compasses to 1¾ in., which you must
-do accurately by the scale. The circle drawn with this radius shows the
-relative size of the path of Venus, and to indicate the periodic time,
-you should mark it, “225 days.” The next circle you have to draw is a
-very interesting one. The compass is to be opened 2½ in. this time,
-and the path that it makes is to be marked “365 days.” This shows the
-high road along which we ourselves journey every year, along which we
-are, indeed, journeying at this moment. If you wanted to obtain from
-your figure any notions of the true dimensions of the system, the path
-of the earth will be the most convenient means of doing so. The earth
-is 93,000,000 miles from the sun, and our drawing shows its orbit as a
-circle of 2½ in. radius. It follows that each inch on our little scale
-will correspond to about 37,000,000 miles. As, therefore, the radius of
-the orbit of Mercury has been taken to be one inch, it follows that the
-distance of Mercury from the sun is about 37,000,000 miles.
-
-We have, however, still one more circle to draw before we complete
-this little sketch. The compass must now open to four inches, and a
-circle which represents the orbit of Mars is then to be drawn. We mark
-on this “687 days,” and the inner part of the solar system is then
-fully represented. You see, this diagram shows how our earth is in
-every sense a planet. It happens that one of the four planets revolves
-outside the earth’s path, while there are two inside. By marking the
-days on the circles which show the periods of the planets, you perceive
-that the further a planet is from the sun, the longer is the time that
-it takes to go round. Perhaps you will not be surprised at this, for
-the length of the journey is, of course, greater in the greater orbits;
-but this consideration will not entirely explain the augmentation of
-the time of revolution. The further a planet is from the sun, the more
-slowly does it actually move, and therefore, for a double reason, the
-larger orbit will take a longer time. From London to Brighton is a
-much longer journey than from London to Greenwich, and, therefore, the
-journey by rail to Brighton will, of course, be a longer one than by
-rail to Greenwich. But suppose that you compared the railway journey
-to Greenwich with the journey, not by rail, but by coach, to Brighton,
-here the comparative slowness of the coach would form another reason
-besides the greater length of the journey for making the Brighton trip
-a much more tedious one than that to Greenwich. Mars may be likened to
-the coach which has to go all the way to Brighton, while Mercury may be
-likened to the train which flies along over the very short journey to
-Greenwich.
-
-[Illustration: FIG. 47.--Comparative Sizes of the Planets.]
-
-We can easily show from our little sketch that Mercury must be
-moving more quickly than Mars, for the radii of the two circles are
-respectively one inch and four inches, and therefore the path of Mars
-must be four times as long as the orbit of Mercury. If Mars moved as
-fast as Mercury, he would, of course, require only four times as many
-days to complete his large path as Mercury takes for his small path;
-but four times 88 is 352, and, consequently, Mars ought to get round
-in 352 days if he moved as fast as Mercury does. As a matter of fact,
-Mars requires nearly twice that number of days; indeed, no less than
-687, and hence we infer that the average speed of Mars cannot be much
-more than half that of Mercury.
-
-[Illustration: FIG. 48.--Phases of an Inferior Planet.]
-
-To appreciate duly the position of the earth with regard to its
-brothers and sisters in the sun’s family it will be necessary to use
-your compasses in drawing another little sketch, by which the sizes of
-the four bodies themselves shall be fairly represented. Remember that
-the last drawing showed nothing whatever about the sizes of the bodies;
-it merely exhibited the dimensions of the paths in which they moved. As
-Mercury is the smallest globe of the four, we shall open the compasses
-half an inch and describe a circle to represent it. The earth and Venus
-are so nearly the same size (though the earth is a trifle the larger)
-that it is not necessary to attempt to exhibit the difference between
-them, so we shall represent both bodies by circles, each 1¼ inches
-in radius. Mars, like Mercury, is one of the globes smaller than the
-earth, and the circle that represents it will have a radius of ¾ of an
-inch. You should draw these figures neatly, and by a little shading
-make them look like globes. It would be better still if you were to
-make actual models, taking care, of course, to give each of them the
-exact size. A comparative view of the principal planets is shown in
-Fig. 47.
-
-
-THE PLANET MERCURY.
-
-Quicksilver is a bright and pretty metal, and, unlike every other
-metal, it is a liquid under ordinary circumstances. If you spill
-quicksilver, it is a difficult task to gather the liquid up again. It
-breaks into little drops, and you cannot easily lift them with your
-fingers; they slip away and escape your grasp. Quicksilver will run
-easily through a hole so small that water would hardly pass, and it
-is so heavy that an iron nail or a bunch of keys will float upon it.
-Now, this heavy, bright, nimble metal is known by another name besides
-quicksilver; a chemist would call it mercury, and the astronomers use
-exactly the same word to denote a pretty, bright, nimble, and heavy
-planet which seems to try to elude our vision. Though Mercury is so
-hard to see, yet it was discovered so long ago, that all record is lost
-of who the discoverer was.
-
-You must take special pains if you want to see the planet Mercury, for
-during the greater part of the year it is not to be seen at all. Every
-now and then a glimpse is to be had, but you must be on the alert to
-look out just after sunset, or you must be up very early in the morning
-so as to see it just before sunrise. Mercury is always found to be in
-attendance on the sun, so that you must search for him near the sun;
-that is, low down in the west in the evenings, or low down in the east
-in the mornings. To ascertain the proper time of the year at which to
-look for him you must refer to the almanac.
-
-We have seen how Mercury revolves in a path inside that of Venus, and
-it is therefore nearer to the sun. Indeed, Mercury is so close to the
-sun that it is generally overpowered by his brilliance and cannot
-be seen at all. Like every other planet, Mercury is lighted by the
-sun’s rays, and shows phases in the telescope just as the moon does
-(Fig. 48). In this figure the different apparent sizes of the planet at
-different parts of its path are shown. Of course the nearer Mercury is
-to the earth the larger does it seem.
-
-If we can only see Mercury so rarely, and if even then it is a
-very long way off, does it not seem strange that we can tell how
-heavy it is? Even if we had a pair of scales big enough to hold a
-planet, what, it may be asked, would be the use of the scales when
-the body to be weighed was about a hundred millions of miles away?
-Of course the weighing of a planet must be conducted in some manner
-totally different from the kind of weighing that we ordinarily use.
-Astronomers have, however, various methods for weighing these big
-globes, even though they can never touch them. We do not, of course,
-want to know how many pounds, or how many millions of tons they
-contain; there is but little use in trying to express the weight in
-that way. It gives no conception of a planet’s true importance. One
-world must be compared with another world, and we therefore estimate
-the weights of the other worlds by comparing them with that of our own.
-We accordingly have to consider Mercury placed beside the earth, and
-to see which of the two bodies is the bigger and the heavier, or what
-is the proportion between them. It so happens that Mercury, viewed as
-a world, is a very small body. It is a good deal less in size than our
-earth, and it is not nearly so massive. To show you how we found out
-the mass of Mercury I shall venture on a little story. It will explain
-one of the strange devices that astronomers have to use when they want
-to weigh a distant body in space.
-
-There was once, and there is still, a little comet which flits about
-the sky; we shall call it after the name of its discoverer, Encke.
-There are sometimes splendid comets which everybody can see--we will
-talk about these afterwards--but Encke is not such a one. It is very
-faint and delicate, but astronomers are interested in it, and they
-always look out for it with their telescopes; indeed, they could not
-see the poor little thing without them. Encke goes for long journeys
-through space--so far that it becomes quite invisible, and remains
-out of sight for two or three years. All this time it is tearing along
-at a tremendous speed. If you were to take a ride on the comet, it
-would whirl you along far more swiftly than if you were sitting on a
-cannon-ball. When the comet has reached the end of its journey, then
-it turns round and returns by a different road, until at last it comes
-near enough to show itself. Astronomers give it all the welcome they
-can, but it won’t remain; sometimes it will hardly stay long enough for
-us to observe that it has come at all, and sometimes it is so thin and
-worn after all its wanderings that we are hardly able to see it. The
-comet never takes any rest; even during its brief visit to us it is
-scampering along all the time, and then again it darts off, gradually
-to sink into the depths of space, whither even our best telescopes
-cannot follow it. No more is there to be seen of Encke for another
-three years, when again it will come back for a while. Encke is like
-the cuckoo, which only comes for a brief visit every spring, and even
-then is often not heard by many who dearly love his welcome note;
-but Encke is a greater stranger than the cuckoo, for the comet never
-repeats his visit of a few weeks more than once in three years; and he
-is then so shy that usually very few catch a glimpse of him.
-
-An astronomer and a mathematician were great friends, and they used to
-help each other in their work. The astronomer watched Encke’s comet,
-noted exactly where it was, on each night it was visible, and then told
-the mathematician all he had seen. Provided with this information the
-mathematician sharpens his pencil, sits down at his desk, and begins
-to work long columns of figures, until at length he discovers how to
-make a time table which shall set forth the wanderings of Encke. He is
-able to verify the accuracy of his table in a very unmistakable way by
-venturing upon prophecies. The mathematician predicts to the astronomer
-the very day and the very hour at which the comet will reappear. He
-even indicates the very part of the heavens to which the telescope
-must be directed, in order to greet the wanderer on his return. When
-the time comes the astronomer finds that his friend has been a true
-prophet; there is the comet on the expected day, and in the expected
-constellation.
-
-This happens again and again, so that the mathematician, with his
-pencil and his figures, marks stage by stage the progress of Encke
-through the years of his invisible voyage. At each moment he knows
-where the comet is situated, though utterly unable to see it.
-
-The joint labors of the two friends having thus discovered law and
-order in the movements of the comet, you may judge of their dismay
-when on one occasion Encke disappointed them. He appeared, it is
-true, but then he was a little late, and he was also not in the spot
-where he was expected. There was nearly being a serious difference
-between the two friends. The astronomer accused the mathematician of
-having made mistakes in his figures, the mathematician retorted that
-the astronomer must have made some blunder in his observations. A
-quarrel was imminent, when finally it was suggested to interrogate
-Encke himself, and see whether he could offer any explanation. The
-mathematician employed peculiar methods that I could not explain, so I
-shall transform his processes into a dialogue between himself and the
-offending comet.
-
-“You are late,” said he to the comet. “You have not turned up at the
-time I expected you, nor are you exactly in the right place; nor,
-indeed, for that matter, are you now moving exactly as you ought to do.
-In fact, you are entirely out of order, and what explanation have you
-to give of this irregularity?”
-
-You see the questioner felt quite confident that there must have been
-some cause at work that he did not know of. Mathematicians have one
-great privilege; they are the only people in the world who never make
-any mistakes. If they knew accurately all the various influences that
-were at work on the comet, they could, by working out the figures, have
-found exactly where the comet would be placed. If the comet was not
-there, it is inevitable that there must have been something or other
-acting upon the comet, of which the mathematician was in ignorance.
-
-The comet, like every other transgressor, immediately began to make
-excuses, and to shuffle off the blame on somebody else. “I was,” said
-Encke, “going quietly on my rounds as usual. I was following out stage
-by stage the track that you know so well, and I would certainly have
-completed my journey and have arrived here in good time and in the
-spot where you expected me had I been let alone, but unfortunately I
-was not let alone. In the course of my long travels--but at a time
-when you could not have seen me--I had the misfortune to come very
-close to a planet, of which I dare say you have heard--it is called
-Mercury. I did not want to interfere with Mercury; I was only anxious
-to hurry past and keep on my journey, but he was meddlesome, and began
-to pull me about, and I had a great deal of trouble to get free from
-him, but at last I did shake him off. I kept my pace as well as I could
-afterwards, but I could not make up the lost time, and consequently I
-am here a little late. I know I am not just where I ought to be, nor am
-I now moving quite as you expect me to do; the fact is, I have not yet
-quite recovered from the bad treatment I have experienced.”
-
-The astronomer and the mathematician proceeded to test this story. They
-found out what Mercury was doing; they knew where he was at the time,
-and they ascertained that what the comet had said was true, and that
-it had come very close indeed to the planet. The astronomer was quite
-satisfied, and was proposing to turn to some other matter, when the
-mathematician said:--
-
-“Tarry a moment, my friend. It is the part of a wise man to extract
-special benefit from mishaps and disasters. Let us see whether the
-tribulations of poor Encke cannot be made to afford some very valuable
-information. We expected to find Encke here. Well, he is not here--he
-is there, a little way off. Let us measure the distance between the
-place where Encke is, and the place where he ought to have been.”
-
-This the astronomer did. “Well,” he said, “what will this tell you? It
-merely expresses the amount of delinquency on the part of Encke.”
-
-“No doubt,” said the mathematician, “that is so; but we must remember
-that the delinquency, as you call it, was caused by Mercury. The bigger
-and the heavier Mercury was, the greater would be his power of doing
-mischief, the more would he have troubled poor Encke, and the larger
-would be the derangement of the comet in consequence of the unfortunate
-incident. We have measured how much Encke has actually been led astray.
-Had Mercury been heavier than he is, that distance would have been
-larger; and if Mercury had been lighter than he is, you would not, of
-course, have found so large an error in the comet.”
-
-We may illustrate what is meant in this way. A steamer sails from
-Liverpool to New York, and in favorable circumstances the voyage across
-the Atlantic should be accomplished within a week. But supposing that
-in the middle of the ocean a storm is encountered, by which the ship is
-driven from her course. She will, of course, be delayed, and her voyage
-will be lengthened. A trifling storm, perhaps, she will not mind, but a
-heavy storm might delay her six hours; a still greater storm might keep
-her back half a day; while cases are not infrequent in which the delay
-has amounted to one day, or two days, or even more.
-
-The delay which the ship has experienced may be taken as a measure of
-the vehemence of the storm. I am not supposing that her machinery has
-broken down; of course, that sometimes happens at sea, as do calamities
-of a far more tragic nature. I am merely supposing the ship to be
-exposed to very heavy weather, from which she emerges just as sound as
-she was when the storm began. In such cases as this we may reasonably
-measure the intensity of the storm by the number of hours’ delay to
-which the passengers were subjected. “The weather we had was much worse
-than the weather you had,” one traveller may say to another. “Our ship
-was two days late, while you escaped with a loss of one day.”
-
-When the comet at last returned to the earth after a cruise of three
-years through space, the number of hours by which it was late expressed
-the vehemence of the storm it experienced. The only storm that the
-comet would have met with, at least in so far as our present object
-is concerned, was the trouble that it had with Mercury. The mass of
-Mercury was, therefore, involved in the delay of the comet. In fact,
-the delay was a measure of the mass of the planet. I do not attempt to
-describe to you all the long work through which the mathematician had
-to plod before he could ascertain the mass of Mercury. It was a very
-tedious and a very hard sum, but at last his calculations arrived at
-the answer, and showed that Mercury must be a light globe compared to
-the earth. In fact, it would take twenty-five globes, each equal to
-Mercury, to weigh as much as the earth.
-
-I dare say you will think that this was a very long and roundabout way
-of weighing. Supposing, however, we had to weigh a mountain, or rather
-a body which was bigger than fifty thousand mountains, and which was
-also many millions of miles away, all sorts of expedients would have
-to be resorted to. I have told you one of them. If you feel any doubts
-as to the accuracy with which such weighings can be made, then I must
-tell you that there are many other methods, and that these all agree in
-giving concordant results.
-
-[Illustration: FIG. 49.--Relative Weights of Mercury and the Earth.]
-
-We hardly know anything as to what the globe of Mercury may be like.
-We can see little or nothing of the nature of its surface. We only
-perceive the planet to be a ball, brightly lighted by the sun, and we
-cannot satisfactorily discern permanent features thereon, as we are
-able to do on some of the other planets.
-
-
-THE PLANET VENUS.
-
-You will have no difficulty in recognizing Venus, but you must choose
-the right time to look out for her. In the first place, you need never
-expect to see Venus very late at night. You should look for the planet
-in the evening, as soon as it is dark, towards the west, or in the
-morning, a little before sunrise, towards the east. I do not, however,
-say that you can always see Venus, either before sunrise or after
-sunset. In fact, for a large part of the year, this planet is not to be
-seen at all. You should therefore consult the almanac, and unless you
-find that Venus is stated to be an evening star or a morning star, you
-need not trouble to search for it. I may, however, tell you that Venus
-can never be an evening star and a morning star at the same time. If
-you can see it this evening after sundown, there is no use in getting
-up early in the morning to look out for it again. The planet will
-remain for several weeks a splendid object after sunset, and then will
-gradually disappear from the west, and in a couple of months later will
-be the morning star in the east. Venus requires a year and seven months
-to run through her changes, so that if you find her a bright evening
-star to-night, you may feel sure that she was a bright evening star a
-year and seven months ago, and that she will be a bright evening star
-in a year and seven months to come. Nor must you ever expect to see her
-right overhead; she is always to the west or to the east.
-
-The splendor of Venus, when at her best, will prevent you at such times
-from mistaking this planet for an ordinary star. She is then more than
-twenty times as bright as any star in the heavens. The most conclusive
-proof of the unrivalled brightness of Venus is found in the fact that
-she can be recognized in broad daylight without a telescope. Even on
-the brightest June afternoons the lovely planet is sometimes to be
-discerned like a morsel of white cloud on the perfect azure of the sky.
-
-Venus is so brilliant that perhaps you will hardly credit me when I
-tell you that she has no more light of her own than has a stone or a
-handful of earth, or a button. Is it possible that this is the case,
-you will say, for as we see the planet so exquisitely beautiful, how
-can she be merely a huge stone high up in the heavens? The fact is that
-Venus shines by light not her own, but by light which falls upon her
-from the sun. She is lighted up just as the moon, or just as our own
-earth is lighted. Her radiance merely arises from the sunbeams which
-fall upon her. It seems at first surprising that mere sunbeams on the
-planet can give her the brilliancy that is sometimes so attractive.
-Let me show you an illustration which will, I trust, convince you that
-sunbeams will be adequate even for the glory of Venus.
-
-Here is a button. I hang it by a piece of fine thread, and when I
-dip it into the beam from the electric lamp, look at the brilliancy
-with which the mimic planet glitters. You cannot see the shape of the
-button; it is too small for that; you merely see it as a brilliant gem,
-radiating light all around. Therefore, we need not be surprised to
-learn that the brilliancy of the evening star is borrowed from the sun,
-and that if, while we are looking at the planet in the evening, the sun
-were to be suddenly extinguished, the planet would also vanish from
-view, though the stars would shine as before.
-
-Thus we explain the appearance of Venus. The evening star is a
-beautiful, luminous point, but it has no shape which can be discerned
-with the unaided eye. When, however, the telescope is turned towards
-Venus we have the delightful spectacle of a tiny moon, which goes
-through its phases just as does our own satellite. When first seen as
-an evening star Venus will often be like the moon at the quarter, and
-then it will pass to the crescent shape. Then the crescent becomes
-gradually thinner, and next will follow a brief period of invisibility
-before the appearance of Venus as the morning star. It seems at first
-a little strange that Venus when brightest should not be full like the
-moon, which in similar circumstances is, of course, a complete circle
-of light. The planet, however, has a very marked crescent-shaped form
-in these circumstances. But at this time the planet is so near us
-that the gain of brilliancy from the diminution of distance more than
-compensates for the small part of the illuminated side which is turned
-towards us.
-
-You ought all to try to get some one to show you Venus through a
-telescope. A very large instrument is not necessary, and I feel sure
-you will be delighted to see the beautiful moon-shaped planet. You will
-then have no difficulty in understanding how the brightness of the
-planet has come from the sun. The changes in the crescent merely depend
-upon the proportion of the illuminated side which is turned towards us.
-Were Venus itself a sunlike body we should, of course, see no crescent,
-but only a bright circle of light.
-
-In Fig. 50 you will notice an imaginary picture of a young astronomer
-surveying Venus with a telescope. I have not, as is obvious, attempted
-to show the different objects in their proper proportions. The sun is
-supposed to have set, so that his beams do not reach the astronomer.
-Night has begun at his observatory; but the sunbeams fall on Venus,
-and light her up on that side turned towards the sun. A part of this
-lighted side is, of course, seen by the telescope which the astronomer
-is using, and thus the planet seems to him like a crescent of light.
-
-[Illustration: FIG. 50.--To show that Venus shines by Sunlight.]
-
-
-THE TRANSIT OF VENUS.
-
-We might naturally think from Fig. 46 that Venus must pass at every
-revolution directly between the earth and the sun; and therefore it
-might appear that what is called the transit of Venus across the sun
-ought to occur every time between the appearance of the planet as the
-evening star and the next following appearance as the morning star.
-No doubt on each of these occasions Venus seems to approach the sun
-closely; but the orbits of Venus and the Earth do not lie quite in the
-same plane, and hence the planet usually passes just over or just under
-the sun, so that it is a very rare event indeed for her to come right
-in front of the sun. But this does sometimes happen. It happened, for
-instance, in the year 1874, and again in the year 1882; but, alas!
-I cannot hold out to you the prospect of ever seeing another such
-spectacle. There will be no further occurrence of the transit of Venus
-until the year 2004, though there will be another eight years later, in
-2012.
-
-It seems rather odd that one transit of Venus should be followed by
-another after an interval of eight years, and that then a period of
-much more than a century should have to elapse before there will be
-a repetition of a similar pair. This is in consequence of a curious
-relation between the motion of Venus and the motion of the Earth, which
-I must endeavor to explain with the help of a little illustration.
-
-Let us suppose a clock with ordinary numbers round the dial, but so
-arranged that the slowly moving short hand requires 365.26 days to
-complete one revolution round the dial, while the more rapidly moving
-long hand revolves in 224.70 days. The short hand will then go round
-once in a year, and the long hand once during the revolution of Venus.
-Let us suppose that both hands start together from XII, then in 224.70
-days the long hand is round to XII again, but the short hand will have
-only advanced to about VII, and by the time it reaches XII the long
-hand will have completed a large part of a second circuit. It happens
-that the two numbers 224.70 and 365.26 are very nearly in the ratio
-of 8 to 13. In fact, if the numbers had only been 224.8 and 365.3
-respectively, they would be exactly in the proportion of 8 to 13. It,
-therefore, follows that eight revolutions of the short hand must occupy
-very nearly the same time as thirteen revolutions of the long hand.
-After eight years the short hand will of course be found again at XII;
-and at the same moment the long hand will also be back at XII, after
-completing thirteen revolutions.
-
-We can now understand why the transits, when they do occur, generally
-arrive in pairs at an interval of eight years. Suppose that at a
-certain time Venus happens to interpose itself directly between the
-earth and the sun, then, when eight years have elapsed, the earth is,
-of course, restored for the eighth time since the first transit to the
-same place, and Venus has returned to almost the same spot for the
-thirteenth time. The two bodies are practically in the same condition
-as they were at first, and, therefore, Venus again intervenes, and the
-planet is beheld as a black spot on the sun’s surface. We must not
-push this argument too far; the relation between the two periods of
-revolution, though nearly, is not exactly 8 to 13. The consequence is
-that when another eight years have elapsed, the planet passes a little
-above the sun or a little below the sun, and thus a third occurrence of
-the transit is avoided for more than a century. The next transit will
-take place at the opposite side of the path.
-
-We were fortunate enough to be able to see the transit of Venus in 1882
-from Great Britain. Perhaps I should say a part of the transit, for the
-sun had set long before the planet had finished its journey across the
-disk. Venus looked like a small round black spot, stealing in on the
-bright surface of the sun and gradually advancing along the short chord
-that formed its track.
-
-[Illustration: FIG. 51.--Venus in Transit across the Sun.]
-
-An immense deal of trouble was taken in 1882, as well as in 1874, to
-observe this rare occurrence. Expeditions were sent to various places
-over the earth where the circumstances were favorable. Indeed, I do not
-suppose that there was ever any other celestial event about which so
-much interest was created. The reason why the event attracted so much
-attention was not solely on account of its beauty or its singularity;
-it was because the transit of Venus affords us a valuable means of
-learning the distance of the sun. When observations of the transit of
-Venus made at opposite sides of the earth are brought together, we are
-enabled to calculate from them the distance of Venus, and knowing that,
-we can find the distance of the sun and the distances and the sizes of
-the planets. This is very valuable information; but you would have to
-read some rather hard books on astronomy if you wanted to understand
-clearly how it is that the transit of Venus tells us all these
-wonderful things. I may, however, say that the principle of the method
-is really the same as that mentioned on pp. 19–25. When you remember
-that not we ourselves, nor our children, and hardly our grandchildren,
-will ever be able to see another transit of Venus, you will, perhaps,
-not be surprised that we tried to make the most of such transits as
-have occurred in our time.
-
-
-VENUS AS A WORLD.
-
-Though Venus exhibits such pretty crescents in the telescope, yet
-I must say that in other respects a view of the planet is rather
-disappointing. Venus is adorned by such a very bright dress of sunbeams
-that we can see but little more than those sunbeams, and we can hardly
-make out anything of the actual nature of the planet itself. We can
-sometimes discern faint marks upon the globe, but it is impossible
-even to make a conjecture of what the Venus country is like. This is
-greatly to be regretted, for Venus approaches comparatively close
-to the earth, and is a world so like our own in size and other
-circumstances that we feel a legitimate curiosity to learn something
-more about her.
-
-But the marks on the planet, though very faint, are still sufficiently
-definite to have enabled some sharp-sighted astronomers to answer a
-question of much interest. They have made it plain that in one most
-important respect Venus is very unlike our Earth. Our globe, of course,
-rotates on its axis once each day, but Venus requires no less than 225
-days to complete each rotation. In fact, this planet rotates in such a
-fashion that she always keeps the same face to the sun. The inhabitants
-of Venus will therefore find that it is perennial day on one side of
-this globe and everlasting night on the other.
-
-Venus is one of the few globes which might conceivably be the abode
-of beings not very widely different from ourselves. In one condition
-especially--namely, that of weight--she resembles the earth so closely
-that those bodies which we actually possess would probably be adapted,
-so far as strength is concerned, for a residence on the sister planet.
-Our present muscles would not be unnecessarily strong, as they would be
-on the moon, nor should we find them too weak, as they would certainly
-prove to be were we placed on one of the very heavy bodies of our
-system. Nor need the temperature of Venus be regarded as presenting any
-insuperable difficulties. She is, of course, nearer to the sun than
-we are, but then climate depends on other conditions besides nearness
-to the sun, so that the question as to whether Venus would be too hot
-for our abode could not be readily decided. The composition of the
-atmosphere surrounding the planet would be the most material point in
-deciding whether terrestrial beings could live there. I think it to
-be in the highest degree unlikely that the atmosphere of Venus should
-chance to suit us in the requisite particulars, and therefore I think
-there is not much likelihood that Venus is inhabited by any men, women,
-or children resembling those on this earth.
-
-
-THE PLANET MARS AND HIS MOVEMENTS.
-
-The path of the earth lies between the orbits of the planets Venus and
-Mars. It is natural for us to endeavor to learn what we can about our
-neighbors. We ought to know something, at all events, as to the people
-who live next door to us on each side. I have, however, already said
-that we cannot observe very much upon Venus. The case is very different
-with respect to Mars. He is a planet which we are fortunately enabled
-to study minutely, and he is full of interest when we examine him
-through a good telescope.
-
-The right season for observing Mars must, of course, be awaited, as he
-is not always visible. Such seasons recur about every two years, and
-then for months together Mars will be a brilliant object in the skies
-every night. Nor has Mars necessarily to be sought in the early morn or
-immediately after sunset, in the manner we have already described for
-Venus and Mercury. At the time Mars is at his best he comes into the
-highest position at midnight, and he can generally be seen for hours
-before, and be followed for hours subsequently. You may, however, find
-some difficulty in recognizing him. You probably would not at first be
-able to distinguish Mars from a fixed star. No doubt this planet is a
-ruddy object, but some stars are also ruddy, and this is at the best a
-very insecure characteristic for identification. I cannot give you any
-more general directions, except that you should get your papa to point
-out Mars to you the next time it is visible. It is just conceivable
-that papa himself might not know how to find Mars. If so, the sooner he
-gets a set of star maps and begins to teach himself and to teach you,
-the better it will be for you both.
-
-Mars, though apparently so like a star, differs in some essential
-points from any star in the sky. The stars proper are all fixed in the
-constellations, and they never change their relative positions. The
-groups which form the Great Bear or the Belt of Orion do not alter,
-they are just the same now as they were centuries ago. But the case is
-very different with a planet such as Mars. The very word planet means a
-_wanderer_, and it is justly applied, because Mars, instead of staying
-permanently in any one constellation, goes constantly roaming from one
-group to the other. He is a very restless body; sometimes he pays his
-respects to the heavenly Twins, and is found near Castor and Pollux in
-Gemini, then he goes off and has a brief sojourn with the Bull, but it
-looks as if that fierce animal got tired of his company and hunted
-him off to the Lion. His quarters then become still more critical.
-Sometimes it looks as if he desired to seek for peace beneath the
-waters, and so he visits Aquarius, while at other times he is found in
-dangerous proximity to the claws of the Crab.
-
-Mars cannot even make up his mind to run steadily round the heavens in
-one direction; sometimes he will bolt off rapidly, then pause for a
-while, and turn back again; then the original impulse will return, and
-he will resume his journey in the direction he at first intended. It
-is no wonder that I am not able to give you very explicit directions
-as to how you may secure a sight of a truant whose wanderings are
-apparently so uncertain. Yet there is a definite order underlying all
-his movements. Astronomers, who make it their business to study the
-movements of Mars, can follow him on his way; they know exactly where
-he is now, and where he will be every night for years and years to
-come. The people who make the almanacs come to the astronomers and get
-hints from them as to what Mars intends to do, so that the almanacs
-announce the positions in which the planet will be found with as much
-regularity as if he was in the habit of behaving with the orderly
-propriety of the sun or the moon.
-
-We must not lay all the blame on Mars for the eccentricities of his
-movements. Our earth is to a very large extent responsible. What we
-think to be Mars’ vagaries are often to be explained by the fact that
-we ourselves on the earth are rapidly shifting about and altering our
-point of view.
-
-[Illustration: FIG. 52.--How the Tree seems to move about.]
-
-I was driving down a pretty country road with a little girl three years
-old beside me, when I was addressed with the little remark, “Look at
-the tree going about in the field.” Now, you or I, with our longer
-experience of the world around us, know that it is not the custom of
-trees to take themselves up and walk about the fields. But this was
-what this little girl saw, or rather what she thought she saw; and very
-often what we do see is something very different from what we think we
-see. We think we see Mars performing all these extraordinary movements,
-as the little girl thought she saw the tree moving about. But just as
-that little girl, when she grew to be a big girl, found that what she
-thought was a tree walking across the field must really have some quite
-different explanation, so we, too, find that what Mars seems to do is
-one thing, and what Mars actually does is quite another thing.
-
-Let us see what the little girl noticed. She was looking at the tree,
-and first she saw it on one side of the house, and then she saw it on
-the opposite side (Fig. 52). If it had been a cow instead of a tree, of
-course the natural supposition would have been that the cow had walked.
-Our little friend may, perhaps, have thought it unusual for a tree to
-walk, but still she saw the undoubted fact that the tree had shifted to
-the other side of the house, and therefore, perhaps, remembering what
-the cow could do, she said the tree had moved.
-
-[Illustration: FIG. 53.--A Specimen of the Track of Mars.]
-
-The little girl did not stop to reflect that she herself had entirely
-changed her position, and hence arose the surprising phenomenon of a
-tree that could move about. You will understand this, at once, from the
-two positions of the car here shown. In the first position, as the girl
-looks at the tree, the dotted line shows the direction of her glance,
-and the other dotted line shows how the apparent places of the tree and
-the house have altered. It is her change of place that has accomplished
-the transformation. Observe also that the tree appeared to her to move
-in the direction opposite to that in which she is going.
-
-Mars generally appears to move round among the stars from west to east.
-In fact, if we were viewing him from the sun he would always seem to
-move in this manner. But at certain seasons our earth is moving very
-fast past Mars, and this will make him appear to move in the opposite
-direction. This apparent motion is sometimes so much in excess of his
-real motion, that it may give us an entirely incorrect idea of what the
-planet is actually doing.
-
-Thus, notwithstanding that Mars is moving one way, he may appear to us
-who dwell on the earth to be going in the opposite way. This illusion
-only happens for a short time, just when we are passing Mars, as we
-do every two years. The effect on the planet is to make the path he
-pursues at this time something like that shown in Fig. 53. The planet
-is nearest to us at the time he is moving in this loop. He is then
-to be seen at his best in the telescope, so that it is especially
-interesting to watch Mars through this critical part of his career.
-
-I want to show you how to make a little calculation which will explain
-the law by which the seasons when we can see Mars best will follow
-each other. The period he requires for a voyage round the sun is not
-quite two years, for that would be 730 days, and Mars only takes 687
-days for his journey. It is, however, true that 1-15/17 years is very
-nearly the period of Mars. Hence, every 32 years Mars will complete 17
-rounds. From this we shall be able to see how long it will take after
-the earth once passes Mars before they pass again. I shall suppose
-there is a circular course, around which two boys start together to
-run a race. One of these boys is such a good runner that he will get
-quite round in 17 minutes; but the other boy can hardly run more than
-half as quickly, for he will require 32 minutes to complete one circle.
-Here then is the question. Suppose the two boys to start together: how
-long will it be before the faster runner gains one complete circuit on
-the other? By the time the good runner (A) has completed one circuit,
-the bad runner (B) has only got a little more than halfway. When A
-has completed his second circuit, he has, of course, run for twice
-17 minutes--that is, for 34 minutes. This is two minutes longer than
-the time B requires to get round once; therefore B is only ahead by
-a distance which A could cover in about one minute; but B will have
-advanced during this minute a distance for which A will require another
-half-minute, during which B covers a distance for which A will need a
-further quarter, and so on. But all these intervals--one minute, half
-a minute, a quarter of a minute, one-eighth, one-sixteenth, and so
-on--added together amount to two minutes, and hence it follows that B
-will not be overtaken until about two minutes after A has completed his
-second round--that is, in 36 minutes altogether.
-
-We can pass from this illustration to the case of the planet Mars
-and the earth. The orbit of the earth is traversed in a year, and
-therefore, after the earth has once passed Mars, which is then, as
-astronomers would say, in _opposition_, about two years and the eighth
-of a year--that is, two years and six or seven weeks--will elapse
-before Mars is again favorably placed. You will thus see that we need
-not expect to observe Mars under the best conditions every year.
-Besides, the distance of the planet from the earth at opposition
-varies so greatly that some oppositions are more favorable than others.
-
-The time has come when I must tell you something about the shapes
-of the paths in which the earth and the other planets perform their
-great journeys round the sun. Perhaps you will think that I am going
-to contradict some of the things that I have told you before. I have
-often represented the orbits of the planets as circles, and now I am
-going to tell you that this is not correct. The fact is that the paths
-are nearly circles; but, still, there is some departure from the exact
-circular shape. Mars, in particular, moves in a path which is more
-different from a circle than the path of the earth, and consequently it
-is appropriate to introduce this subject when we are engaged about Mars.
-
-[Illustration: FIG. 54.--How to draw an Ellipse.]
-
-We must first take another lesson in drawing, and the appliances I
-want you to use for the purpose are very simple. You must have a smooth
-board and some tacks or drawing-pins, besides paper, pencil, and twine.
-
-[Illustration: FIG. 55.--Specimens of Ellipses.]
-
-We first lay a sheet of paper on the board, and then put in two tacks
-through the paper and into the board. It does not much matter where
-we put them in. Next we take a piece of twine and tie the two ends
-together so as to form a loop, which we pass round the two tacks
-(Fig. 54). In the loop I place the pencil, and then you see I move it
-round, taking care to keep the twine stretched. Thus I produce a pretty
-curve, which we call the ellipse. I must ask all of you to practise
-this experiment. Try with different lengths of string, and try using
-different distances between the tacks. Here are some sketches of two
-shapes of ellipse and a parabola (Fig. 55). Elliptic curves can be made
-almost circles by putting the two tacks close together, or they can be
-made very long in comparison with their width. They are all pretty and
-graceful figures, and are often useful for ornamental work. The ellipse
-is a pretty shape for beds of flowers in a grass-plot.
-
-The importance of the ellipse to astronomers is greater than that of
-any other geometrical figure. In fact, all the planets, as they perform
-their long and unceasing journeys round the sun, move in ellipses; and
-though it is true that these ellipses are very nearly circles, yet the
-difference is quite appreciable.
-
-It is also important to observe that the sun is not in the centre
-of the ellipse which the planet describes. The sun is nearer to one
-end than to the other. And the actual position of the sun must be
-particularly noted. Suppose that some mighty giant were preparing to
-draw an exact path for the earth, or for Mars, of course he would
-want to have millions of miles of string for producing a big enough
-curve, and one of the nails that he used would have to be driven right
-into the sun. The following is the astronomer’s more accurate method
-of stating the facts. He calls each of the points represented by the
-tacks around which the string is looped a _focus_ of the ellipse; the
-two points together are said to be the _foci_; and as the planet is
-describing its orbit, the position of the sun will lie exactly at one
-of the foci.
-
-The ellipse is a curve that nature is very fond of reproducing. From
-an electric light, a brilliant beam will diverge. If you hold a globe
-in the beam, and let the shadow fall on a sheet of paper, it forms an
-ellipse. If you hold the sheet squarely, the shadow is a circle; but as
-you incline it, you obtain a beautiful oval, and by gradually altering
-the position, you can get a greatly elongated curve. Indeed, you can
-thus produce an ellipse of almost any form. The electric light is not
-indispensable for this purpose; any ordinary bright lamp with a small
-flame will answer, and by taking different sized balls and putting them
-in various positions, you can make many ellipses, great and small.
-
-
-THE DISCOVERIES MADE BY TYCHO AND KEPLER.
-
-It was by the observations of a celebrated old astronomer, named Tycho
-Brahe, that the true shape of a planet’s path came to be afterwards
-determined. Tycho lived in days before telescopes were invented. He had
-few of the excellent contrivances for measuring which we have in our
-observatories. We shall take a look at this fine old astronomer, as he
-sits amid his curious astronomical machines.
-
-[Illustration: FIG. 56.--Tycho Brahe in his Observatory.]
-
-He lived on an island near Copenhagen, and he has given us a picture
-of himself (Fig. 56), as he is seated with his quaint apparatus, and
-his assistants around him, busily engaged in observing the heavens.
-You see the walls of his observatory are decorated with pictures; and
-one of the great Danish hounds which the King of Denmark had presented
-to him lies asleep at his feet. I do not think we should now encourage
-big dogs in the observatory at night. Nor do modern astronomers put on
-their velvet robes of state, as Tycho was said to have done when he
-entered into the presence of the stars, as, by so doing, he showed his
-respect for the heavens. Astronomers, nowadays, rather prefer to wear
-some comfortable coat which shall keep out the cold, no matter what may
-be its appearance from the picturesque point of view. In this wonderful
-contrivance, you see Tycho Brahe did not use any actual telescope. He
-observed through a small opening in the wall, and lest there should be
-any mistake as to what is going on, you see he is pointing towards it,
-and giving his three assistants their instructions. The most important
-work is being done by the man on the right. He is engaged in making the
-actual observation. But he has no aid from magnifying lenses. All he
-can do is to slide a pointer up or down till it is just in line with
-the planet or star as he sees it through the hole opposite.
-
-On the circle a number of marks have been engraved, and there are
-numbers placed opposite to the marks; it is by these that the position
-of the object is to be ascertained. If the object is high, then the
-pointer will be low; and if the object is low, then the pointer will
-be high. The observer calls out the position when he has found it, and
-there, you see, is a man ready with writing materials to take down
-the observation. Notice also the other astronomer who is looking at
-the clock. He gives the time, which must also be recorded accurately.
-In fact, the entire process of finding the place of a heavenly body
-consists in two observations--one from the circle and the other from
-the clock; so that though Tycho had no telescope to aid his vision, yet
-the principle on which his work was done was the same as that which we
-use in our observatories at this moment.
-
-You may think that such a concern would hardly be capable of producing
-much reliable work. However, Tycho compensated in a great degree for
-the imperfection of his instrument by the skill with which he used it.
-He had a noble determination to do his very best. Perseverance will
-accomplish wonders even with very imperfect means. A great astronomer
-has said that a skilful observer ought to be able to make valuable
-measurements with a common cart-wheel!
-
-It was with instruments on the principle of that which I have here
-shown that Tycho made his celebrated observations of Mars. Week
-after week, month after month, year after year, did the patient old
-astronomer track the planet through his capricious wanderings.
-
-Before we try to explain anything, it is of course necessary to
-ascertain, with all available accuracy, what the thing actually is.
-Therefore, when we seek to explain the irregular movements of a planet,
-the first thing to be done is to make a careful examination of the
-nature of those irregularities. And this was what Tycho strove to do
-with the best means at his disposal.
-
-The full benefit of Tycho’s work was realized by Kepler when he
-commenced to search out the kind of figure in which Mars was moving.
-First he tried various circles, and then he sought, by placing the
-centre in different positions, to see whether it would not be possible
-to account thus for the irregularities of the wayward planet. It would
-not do; the movement was not circular. This was thought very strange
-in those days, for the circle was regarded as the only perfect curve,
-and it was considered quite impossible for a planet to have any motion
-except it were the most perfect. There was, however, no help for
-it; so Kepler sagaciously tried the ellipse, which he considered to
-be the most perfect curve next to the circle. He continued his long
-calculations, until at last he succeeded in finding one particular
-ellipse, placed in one particular position, which would just explain
-the strange wanderings of our erratic neighbor. It was not alone
-that the motion of the planet traced out an ellipse; it was further
-discovered that the sun lies at one of the foci of the curve. If the
-sun were anywhere else, the motion of the planet would have been
-different from that which Tycho had found it to be.
-
-You must know that this discovery is one of the very greatest that
-have ever been made in the whole extent of human knowledge. After it
-had been proved that the orbit of Mars was elliptic, it became plain
-that the same path must be traced by every planet. There are very big
-planets, and there are small ones; there are planets which move in
-very large orbits, and there are planets whose paths are comparatively
-small. In all cases the high road which the planet follows is
-invariably an ellipse, and the sun is invariably to be found situated
-at the focus. It is surely interesting to find that these beautiful
-ellipses which we can draw so simply with a piece of twine and a pencil
-should be also the very same figures which our great earth and all the
-other bodies which revolve around the sun are ever compelled to follow.
-
-Kepler also made another great discovery in connection with the same
-subject. If the planet moved in a circle with the sun in the centre,
-then there would be very good reason to expect that it would always
-move at the same speed, for there would be no reason why it should go
-faster at one place than at another. In fact, the planet would then be
-revolving always at the same distance from the sun, and every part of
-its path would be exactly like every other part. But when we consider
-that the motion is performed in an ellipse, so that the planet is
-curving round more rapidly at the extremities of its path than in the
-other parts where the curvature is less perceptible, we have no reason
-to expect that the speed shall remain the same all round.
-
-We know that the engine-driver of a railway train always has to slacken
-speed when he is going round a sharp curve. If he did not do so, his
-train would be very likely to run off the line, and a dreadful accident
-would follow. The engine-driver is well aware that the conditions of
-pace are dependent on the curvature of his line. The planet finds that
-it, too, must pay attention to the curves; but the extraordinary
-point is that the planet acts exactly in the opposite way to the
-engine-driver. The planet puts on its highest pace at one of the most
-critical curves in the whole journey. There are two specially sharp
-curves in the planet’s path. These are, of course, the two extremities
-of the ellipse which it follows. The cautious engine-driver would, of
-course, creep round these with equal care, and no doubt the planet
-goes slowly enough about that end of the ellipse which is farthest
-from the sun. There its pace is slower than anywhere else; but from
-that moment onwards the planet steadily applies itself to getting
-up more and more speed. As it traverses the comparatively straight
-portion of the celestial road, the pace is ever accelerating until the
-sharp curve near the sun is being approached; then the velocity gets
-more and more alarming, until at last, in utter defiance of all rules
-of engine-driving, the planet rushes round one of the worst parts of
-the orbit at the highest possible speed. And yet no accident happens,
-though the planet has no nicely laid lines to keep it on the track.
-
-If lines are necessary to save a railway train from destruction, how
-can we possibly escape when we have no similar assistance to keep us
-from flying away from the sun and off into infinite space? Kepler
-has taught us to measure the changes in the speed of the body with
-precision. He has shown that the planet must, at every point of its
-long journey, possess exactly the right speed; otherwise everything
-would go wrong. I dare say you have seen, at different points along
-a line of railway, boards put up here and there, with notices like,
-“Ten miles an hour.” These words are, of course, an intimation to the
-engine-driver that he is not to vary from the speed thus stated. Kepler
-has given us a law which is equivalent to a large number of caution
-boards, fixed all round the planet’s path, indicating the safe speed
-for the journey at every stage. It is fortunate for us that the planet
-is careful to observe these regulations. If the earth were to leave
-her track, the consequences would be far worse than those of the most
-frightful railway accident that ever happened. Whichever side we took
-would be almost equally disastrous. If we went inwards we should plunge
-into the sun, and if we went outwards we should be frozen by cold.
-
-We owe our safety to the care with which the speed of the earth is
-prescribed. When near the sun, the earth is pulled inwards with
-exceptionally strong attraction. We are often told that when a strong
-temptation seizes us, the wisest thing that we can do is to run away
-as hard as possible. This is just what the laws of dynamics cause the
-earth to do at this critical time. She puts on her very best pace, and
-only slackens when she has got well away from the danger.
-
-The peril that we are exposed to when the earth is at the other end
-of the orbit is of an opposite character. We are then a long way
-from the sun, and the pull which it can exercise upon the earth is
-correspondingly lessened. Care is then required lest we should escape
-altogether from the sun’s warmth and his guidance. We must therefore
-give time to the sun to exercise his power, so as to enable the earth
-to be recalled; accordingly we move as slowly as possible until the
-sun conquers the earth’s disposition to fly off, and we begin to return.
-
-You may remember that when we were speaking about the moon, I showed
-you how a body might revolve around the earth in a circle under the
-influence of an attraction towards the earth’s centre. So long as the
-path is really a circle, then the power with which the earth is drawing
-the body remains the same. In a precisely similar way, a body could
-revolve around the sun in a circle, in which case also the attraction
-of the sun will remain the same all round. But now we have a very much
-more difficult case to consider. If the body does not always remain at
-the same distance, the power of the sun will not be the same at the
-different places. Whenever the object is near the sun, the attraction
-will be greater than when it is farther off. For example, when the
-distance between the two bodies is doubled, then the pull is reduced to
-the fourth part of what it was before.
-
-
-THE DISCOVERIES MADE BY NEWTON.
-
-I have now some great discoveries to talk to you about, which were made
-by Sir Isaac Newton. He was not an astronomer who looked much through
-a telescope, though he made many remarkable experiments. He used to
-sit in his study and think, and then he used to draw figures with his
-pencil, and make long calculations. At last he was able to give answers
-to the questions: What is the reason why the planet moves in an
-ellipse? Why should it move in this curve rather than in any other? Why
-should this ellipse be so placed that the sun lies at one of the foci?
-
-If the planet had run uniformly round its course, Newton would have
-found his task an impossible one. But I have already explained that
-the motion is not uniform. I described how the planet hurried along
-with extra speed at certain parts of its path; how it lingered at
-other parts; how, in fact, it never preserved the same rate for even a
-single minute during the whole journey. Kepler had shown how to make a
-time-table for the whole journey. In fact, just as a captain on a long
-voyage keeps a record of each day’s run, and shows how to-day he makes
-170 miles, and to-morrow perhaps 200, and the next day 210, while the
-day after he may fall back to 120, so Kepler gave rules by which the
-log of a planet in its voyage round the sun might be so faithfully kept
-that every day’s run would be accurately recorded.
-
-When Newton commenced his work, one of the first questions he had to
-consider was the following: Suppose that a great globe like a planet,
-or a small globe like a marble, or an irregular body like an ordinary
-stone, were to be thrown into space, and were then to be left to follow
-its course without any force whatever acting upon it, where would it go
-to?
-
-You may say, at once, that a body under such circumstances will
-presently fall down to the ground; and so, of course, it will, if it be
-near the earth. I am not, however, talking of anything near the earth;
-I want you to imagine a body far off in the depths of space, among the
-stars. Such a body need not necessarily fall down here, for you see the
-moon does not fall, and the sun does not.
-
-If you were at a great distance from our globe and from all other large
-globes--so far, indeed, that their attractions were imperceptible--you
-could try the experiment that I wish now to describe. Throw a stone
-as hard as ever you can, and what will happen? Of course, when you do
-it down here, it moves in a pretty curve through the air, and tumbles
-to the ground; but away in open space, what will the stone do? There
-will be no such motion as up or down, as we ordinarily understand it;
-for though the earth, no doubt, will lie in one particular direction
-at a great distance, yet there will be other bodies just as large in
-other directions; and there is no reason why the stone should move
-towards one of these rather than to another; in fact, if they are all
-far enough, as the stars are from us, their attractions will be quite
-inappreciable. There is, therefore, not the slightest reason why the
-stone should swerve to one side more than to another. There is no more
-reason why it should turn to the right than why it should turn to the
-left. Nor could you throw the stone so as to make it follow a curved
-path. You can, of course, make it describe a curve while it remains in
-your hand, but the moment the stone has left your hand, it proceeds on
-its journey by a law over which you have no control. As the direction
-cannot be changed towards one side more than towards the other, the
-stone must simply follow a straight line from the very moment when it
-is released from your hand.
-
-The speed with which the stone is started will also not change. You
-might at first think that it would gradually abate, and ultimately
-cease. No doubt a stone thrown along the road will behave in this way,
-but that is because the stone rubs against the ground. If you throw
-a stone across a sheet of ice, then it will run a very long distance
-before it stops, and all the time it will be moving in a straight line.
-In this case there is but little loss by rubbing against the ice,
-because it is so smooth. Thus we see that if the path be exceedingly
-smooth, the body will run a long way before it stops. Think of the
-distance a railway train will run if, while travelling at full speed
-along a level line, the steam is turned off.
-
-These illustrations all show that if you let a body alone, after having
-once started it, and do not try to pull it this way or that way, and
-do not make it rub against things, that body will move on continually
-in a straight line, and will keep up a uniform speed. We can apply
-this reasoning to a stone out in space. It would certainly move in a
-straight line, and would go on and on forever, without losing any of
-its pace.
-
-I need hardly tell you that no one has ever been able to try this
-experiment. In the first place, we reside upon the surface of the
-earth, and we have no means of ascending into those elevated regions
-where the stone is supposed to be projected. There is also another
-difficulty which we cannot entirely avoid, and that arises from the
-resistance of the air. All movements down here are impeded because
-the body has to force its way through the air; and in doing so it
-invariably loses some of its speed. Out in open space there is, of
-course, no air, and no loss of speed can therefore arise from this
-cause.
-
-[Illustration: FIG. 57.--The Humming-top.]
-
-There are, however, several actual experiments by which we can assure
-ourselves of the general truth. Set a humming-top spinning (Fig. 57);
-it gradually comes to rest, partly because of the rubbing of its point
-on the table, and partly because it has to force its way through the
-air. In fact, the hum of the top that you hear is only produced at the
-expense of its motion. Supposing I use a much heavier top; if I set it
-spinning it will keep up for many minutes, because its weight gives it
-a better store of power wherewith to overcome the resistance of the
-air. I remember hearing a story about Professor Clerk-Maxwell. He had,
-when at Cambridge, invented one of these large and heavy tops, which
-would spin for a long time. One evening the top was left spinning on a
-plate in his room when his friends took their departure, and no doubt
-it came to rest in due time. Early the next morning, Professor Maxwell,
-hearing the same friends coming up to his rooms again, jumped out of
-bed, set the top spinning, and then got back to bed, and pretended to
-be asleep. He thus astounded his friends, who, of course, imagined that
-the top must have been spinning all the night long!
-
-If we spin a top under the receiver of an air pump (Fig. 58), it will
-keep up its motion for a very much longer time after the air has been
-exhausted than it would in ordinary circumstances. Such experiments
-prove that the motion of a body will not of itself naturally die out,
-and that if we could only keep away the interfering forces altogether,
-the motion would continue indefinitely with unabated speed. What I have
-been endeavoring to illustrate is called the first law of motion. It is
-written thus:--
-
-“_Every body continues in its state of rest or of uniform motion in a
-straight line, except in so far as it may be compelled by impressed
-forces to change that state._”
-
-[Illustration: FIG. 58.--To illustrate the First Law of Motion.]
-
-I would recommend you to learn this by heart. I can assure you it is
-quite as well worth knowing as those rules in the Latin Grammar with
-which many of you, I have no doubt, are acquainted. The best proof
-of the first law of motion is derived, not from any experiments, but
-from astronomy. We make many calculations about the movements of the
-sun, the moon, the stars, and then we venture on predictions, and
-we find those predictions verified. Thus we had a transit of Venus
-across the sun in 1882, and every astronomer knew that this was going
-to occur, and many went to the ends of the earth so that they might
-see it favorably. Their anticipations were realized; they always are.
-Astronomers make no mistakes in these matters. They know that there
-will be another transit of Venus in the year 2004, but not sooner. The
-calculations by which these accurate prophecies are made involve this
-first law of motion; and as we find that such prophecies are always
-fulfilled, we know that the first law of motion must be true also.
-
-Newton knew that if a planet were merely left alone in space, it would
-continue to move on forever in a straight line. But Kepler had shown
-that the planet did not move in a straight line, but that it described
-an ellipse. One conclusion was obvious. There must be some force acting
-upon the planet which pulls it away from the straight line it would
-otherwise pursue. We may, for the sake of illustration, imagine this
-force to be applied by a rope attached to the planet so that at every
-moment it is dragged by some unseen hand. To find the direction this
-rope must have, we take the law of Kepler, which explains the rules
-according to which the planet varies its speed. I cannot enter into
-the question fully, as it would be too difficult for us to discuss
-now. I should have to talk a great deal more about mathematics than
-would be convenient just at present; but I think you can all understand
-the result to which Newton was led. He showed that the rope must always
-be directed towards the sun. In other words, suppose that there was
-no sun, but that in the place which it occupied there was a strong
-enough giant constantly pulling away at the planet, then we should find
-that the speed of the planet would alter just in the way it actually
-does. Thus we learn that some force must reside in the sun by which
-the planet is drawn, and this force is exerted, although there is no
-visible bond between the sun and the planet.
-
-There is another fact to be learned about the sun’s attraction, and
-this time we obtain it by knowing the shape of the curve followed by
-the planet. The laws by which the planet’s speed is regulated prove
-that the force emanates from the sun. We shall now learn much more when
-we take into account that the path of the planet is an ellipse, of
-which the sun lies at the focus. Nothing has been said as yet regarding
-the magnitude of the pull which is being exerted by the sun. Is that
-pull to be always the same, or is it to be greater at some times than
-at other times? Newton showed that no ellipse other than a circle
-could be described, if the pull from the sun were always the same. Its
-magnitude must be continually changed, and the nearer the planet lies
-to the sun, the more vehement is the pull it receives. Newton laid down
-the exact law by which the force on the planet at any one place in
-its path could be compared with the force at any other position. Let
-us suppose that the planet is in a certain position, and that it then
-passes into a second position, which is twice as far from the sun. The
-pull upon the planet at the shorter distance is not only greater than
-the pull at the longer distance, but it is actually four times as much.
-Stating this result a little more generally, we assert, in the language
-of astronomers, that the _attraction varies inversely as the square of
-the distance_. If this law were departed from, then I do not say that
-it would be impossible for the planet to revolve around the sun in some
-fashion, but the motion would not be performed in an ellipse described
-around the sun in the focus.
-
-You see how very instructive are the laws which Kepler discovered.
-From the first of them we were able to infer that the sun attracts the
-planets; from the second, we have learned how the magnitude of the
-attracting force varies.
-
-The true importance of these great discoveries will be manifest when
-we compare them with what we have already learned with regard to the
-movements of the moon. As the moon revolves around the earth it is held
-by the earth’s attraction, and the moon follows a path which, though
-nearly a circle, is really an ellipse. This orbit is described around
-the earth just as the earth describes its path around the sun. That
-law by which a stone falls to the ground in consequence of the earth’s
-attraction is merely an illustration of a great general principle.
-Every body in the whole universe attracts every other body.
-
-Think of two weights lying on the table. They no doubt attract each
-other, but the force is an extremely small one--so small, indeed, that
-you could not measure it by any ordinary appliance. One or both of the
-attracting masses must be enormously big if their mutual gravitation is
-to be readily appreciable. The attraction of the earth on a stone is
-a considerable force, because the earth is so large, even though the
-stone may be small. Imagine a pair of colossal solid iron cannon-balls,
-each 53 yards in diameter, and weighing about 417,000 tons. Suppose
-these two globes were placed a mile apart, the pull of one of them on
-the other by gravitation would be just a pound weight. Notwithstanding
-the size of these masses, the hand of a child could prevent any motion
-of one ball by the attraction of the other. If, however, they were
-quite free to move, and there was absolutely no friction, the balls
-would begin to draw together; at first they would creep so slowly that
-the motion would hardly be noticed. The pace would no doubt continue to
-improve slowly, but still not less than three or four days must elapse
-before they will have come together.
-
-By the kindness of Professor Dewar, I am enabled to exhibit a
-contrivance with which we can illustrate the motion of a planet around
-the sun. Here is a long wire suspended from the roof of this theatre,
-and attached to its lower end is an iron ball, made hollow for the sake
-of lightness. When I draw the ball aside, it swings to and fro with
-the regularity of a great pendulum. But when I place a powerful magnet
-in its neighborhood (Fig. 59), you see that as soon as the ball gets
-near the magnet it is violently drawn to one side, and follows a curved
-path. This magnet may be taken to represent the sun, while the ball is
-like our earth, or any other planet, which would move in a straight
-line were it not for the attraction of the sun which draws the body
-aside.
-
-[Illustration: FIG. 59.--The Effect of Attraction.]
-
-
-THE GEOGRAPHY OF MARS.
-
-We will now say something with respect to the geography of our
-fellow-planet, a subject which seems all the more interesting because
-Mars is so like the earth in many respects. We require a fairly
-good telescope for the purpose of seeing him well, but when such an
-instrument is directed to the planet, a beautiful picture of another
-world is unfolded (Fig. 60). There are many things visible on his
-surface, but we must always remember that even with our most powerful
-telescopes the planet still appears a long way off.
-
-[Illustration: FIG. 60.--Views of Mars.]
-
-[Illustration: FIG. 61.--Mars.
-
-(_By Douglass, Lowell Observatory._)]
-
-In the most favorable circumstances, Mars is at least one hundred times
-as far from us as the moon. But we know that an object on the moon must
-be as large as St. Paul’s Cathedral if it is to be visible in our
-telescopes. An object on Mars must be, therefore, at least one hundred
-times as broad and one hundred times as long as St. Paul’s Cathedral if
-it is to be discernible by astronomers on our earth. We can, therefore,
-only expect to see the general features of our fellow-planet. Were we
-looking at our earth from a similar distance, and with equally good
-telescopes, the continents and oceans, and the larger seas and islands,
-would all be large enough to be conspicuous. It is, however, doubtful
-whether they could ever be properly revealed through the serious
-impediment to vision which our atmosphere would offer.
-
-It fortunately happens that the surface of Mars is only obscured
-by clouds to a very trifling extent, and we are thus able to see a
-panorama of our neighboring globe laid before us. Mars is not nearly
-so large as our earth, the diameters of the two bodies being nearly
-as two to one. It follows that the number of acres on the planet is
-only a quarter of the number of acres on the earth. Careful telescopic
-scrutiny shows that the chief features which we see on Mars are of a
-permanent character. In this respect Mars is much more like the moon
-than the sun. The latter presents to us merely glowing vapors, with
-hardly more permanence than is possessed by the clouds in our own sky.
-On the other hand, the entire absence of clouds from the moon enables
-us to see the permanent features on its surface. Most of the visible
-features on Mars are also invariable; though occasionally it would seem
-that the climate produces some changes in its appearance.
-
-[Illustration: FIG. 62.--The South Pole of Mars, September, 1877
-(_Green_).]
-
-We first notice that there are differently colored parts on Mars. The
-darkish or bluish regions are usually spoken of as seas or oceans;
-though we should be going beyond our strict knowledge were we to assert
-that water is actually found there. Look at the horn-shaped object in
-the centre of the lower picture in Fig. 60. We call it the Kaiser Sea,
-and it is so strongly marked that even in a small telescope it can be
-often seen. You must not, however, always expect to notice this feature
-when you look at the planet through a telescope, for it turns round
-and round. We can make a globe representing Mars. On this are to be
-depicted this great sea and the other characteristic objects. But as
-we turn the globe around, the opposite side of the planet is brought
-into view, and other features are revealed like those represented in
-the upper figure. Mars requires 24 hours 37 minutes 22.7 seconds to
-complete a single rotation. It is somewhat remarkable that this only
-differs from the earth’s period of rotation by a little more than half
-an hour.
-
-Mars contains what we call continents as well as oceans, and we also
-find there lakes and seas and straits. These objects are indicated in
-the drawings that are here represented. But the most striking features
-which the planet displays are the marvellous white regions, which are
-seen both at its North Pole and at its South Pole (Fig. 62). If we
-were able to soar aloft above our earth and take a bird’s-eye view of
-our own polar regions, we should see a white cap at the middle of the
-arctic circle. This appearance would be produced by the eternal ice and
-snow. It would increase during the long, dark winter, and be somewhat
-reduced by melting during the continuously bright summer. Though we
-cannot thus see our earth, yet we can sometimes observe one Pole of
-Mars and sometimes the other, and we find each of these Poles crowned
-with a dense white cap, which increases during the severity of its
-winter, and which declines again with the warmth of the ensuing summer.
-
-Sketches of Mars have been made by many astronomers; among them we may
-mention Mr. Green, who made a beautiful series of pictures at Madeira
-in 1877. These may be supplemented by the drawings of Mr. Knobel in
-1884, when the opposite Pole of the planet was turned to view. The
-drawings show the polar snows, and there seem to be some elevated
-districts in his arctic regions which retain a little patch of snow
-after the main body of the ice cap has shrunk within its summer limits.
-An interesting case of this kind is shown in Fig. 62, which has been
-copied from one of Mr. Green’s drawings.
-
-It has lately been surmised that the continents on Mars are
-occasionally inundated by floods of water. There are also indications
-of clouds hanging over the Martian lands, but the inhabitants of
-that planet, in this respect, escape much better than we do. A
-certain amount of atmosphere always surrounds Mars, though it is much
-less copious than that we have here. As to the composition of this
-atmosphere we know nothing. For anything we can tell, it might be a
-gas so poisonous that a single inspiration would be fatal to us; or if
-it contained oxygen in much larger proportion than our air does, it
-might be fatal from the mere excitement to our circulation which an
-over-supply of stimulant would produce. I do not think it the least
-likely that our existence could be supported on Mars, even if we could
-get there. We also require certain conditions of climate, which would
-probably be totally different from those we should find on Mars.
-
-Many remarkable observations of Mars have been lately made by Mr.
-Percival Lowell. It seems very doubtful how far our former division
-of continents and oceans on Mars can be maintained. Mr. Lowell has
-paid special attention to a wonderful system of lines on the planet’s
-surface to which the name of “canals” has been given, which often show
-such a degree of regularity as would almost suggest the idea that they
-had been laid down by intelligent guidance.
-
-
-THE SATELLITES OF MARS.
-
-When Mars appeared in his full splendor in 1877, he was for the first
-time honored with the notice of instruments capable of doing him
-justice. I do not, however, mean that in former apparitions he was
-not also carefully observed, but a great improvement had recently
-taken place in telescopes, and it was thus under specially favorable
-auspices that his return was welcomed in 1877. This year will be always
-celebrated in astronomical history for a beautiful discovery made by
-Professor Asaph Hall, the illustrious astronomer at Washington.
-
-Before I can explain what this discovery was, I must have a little talk
-about moons, or satellites as they are often called. You know that
-we have one moon, which is constantly revolving round the earth, and
-accompanies the earth in its long voyage round the sun. But the earth
-is only a planet, and there are many other planets which are worlds
-like ours. It is natural to compare these worlds, and as we have one
-moon, why should not the other planets also have moons? If there are
-children in one house in a square, why should there not be children
-in the other houses? We find that some of the other planets have
-satellites, but they do not seem to be distributed very regularly. In
-fact, they are almost as capriciously allotted as the children would be
-in eight houses that you might take at random.
-
-[Illustration: FIG. 63.--Mars and his Two Satellites.]
-
-In Number One there lives an old bachelor, and in Number Two a single
-lady. These are Mercury and Venus, and of course there are no children
-in either of these houses. Number Three is inhabited by old mother
-Earth, and she has got a fine big son, called the Moon. Number Four is
-a nice little house inhabited by Mars. There are to be found a pair
-of little twins, and nimble creatures they are too. Number Five is a
-great mansion. A very big man lives here, called Jupiter, with four
-robust sons and daughters that everybody knows. I fancy they must go
-to many dancing parties, for every night they may be seen whirling
-round and round. For three hundred years these four moons have been
-known to astronomers, but in 1892 there was an addition to the family
-in the shape of a tiny moon which had never been seen up to that time.
-Number Six is also a fine big house, though not quite so big as Number
-Five, but larger than any of the others. It is inhabited by Saturn,
-and contains the biggest family of all. Up till the other day eight
-sons and daughters were known to live here, but they are not nearly so
-sturdy as Jupiter’s children; in fact, the young Saturns do not make
-much display, and some of them are so delicate that they are hardly
-ever seen. In this household, too, a new member has recently appeared.
-For fifty years the family was known to consist of these eight sons
-and daughters, but in August, 1898, when they were being photographed
-in a group, it was discovered that a ninth moon had been added. Number
-Seven is also a fine large house; but Uranus, who lives there, is such
-a recluse that unless you carefully keep your eye on his house, you
-will hardly ever catch a glimpse of him. There are four children in
-that house, I believe, but we hardly know them. They move in circles
-of their own, and apparently have seen a good deal of trouble. Only
-one more house is to be mentioned, and that is Number Eight, inhabited
-by Neptune. It contains one child, but we are hardly on visiting terms
-with this household, and we know next to nothing about it.
-
-Before 1877, Mars appeared to be in the same condition as Venus or
-Mercury--that is, devoid of the dignity of attendants. There was,
-however, good reason for thinking that there might be some satellites
-to Mars, only that we had not seen them. You see that, as Number Three
-had one child, and Numbers Five, Six, Seven, and Eight had each one, or
-more than one, it seemed hard that poor Number Four should have none
-at all. It was, however, certain that if there were any satellites to
-Mars, they must be comparatively small things; for if Mars had even
-one considerable moon, it must have been discovered long ago.
-
-On the memorable occasion in 1877, Professor Hall discovered that the
-ruddy planet Mars was attended, not alone by one moon, but by two.
-Their behavior was most extraordinary. It appeared to him at first
-almost as if one of these little moons was playing at hide-and-seek.
-Sometimes it would peep out at one side of the planet, and sometimes
-at the other side. I have here a picture (Fig. 63) which shows how
-these moons of Mars revolve. That is the globe of the planet himself
-in the middle, and he is turning round steadily in a period which is
-nearly the same as our day. But the remarkable point is that the inner
-of the moons of Mars runs round the planet in 7 hours 39 minutes. It
-would seem very strange in our sky if we had a little moon which rose
-in the west instead of in the east, and which galloped right across
-the heavens three times every day--and this is what Mars has. The
-outer moon takes a more leisurely journey, for he requires 30 hours 18
-minutes to complete a circuit. If for no other reason than to see these
-wonderful moons, it would be very interesting to visit Mars.
-
-The satellites of this planet are in contrast to our moon. In the
-first place, our moon takes 27 days to go round the earth, and is
-comparatively a long way off. The moons of Mars are much nearer to
-their planet, and they go round much more quickly. There is also
-another difference. The moons of Mars are much smaller bodies than
-our moon. If we represent Mars by a good-sized football, his moons,
-on the same scale, would be hardly so big as the smallest-sized
-grains of shot. Does it not speak well for the power of telescopes
-in these modern days that objects so small as the satellites of Mars
-should be seen at all? You remember, of course, that neither Mars
-himself nor his moons have any light of their own. They shine solely
-in consequence of the sunlight which falls upon them. They are merely
-lighted like the earth itself, or like the moon. The difficulty about
-observing the satellites is all the greater because they are seen in
-the telescope close to such a brilliant body as Mars. The glare from
-the bright planet is such that when we want to see faint objects like
-the satellites we have to hide Mars, so as to get a comparatively dark
-space in which to search.
-
-Now that they know exactly what to look for, a good many astronomers
-have observed the satellites of Mars. A superb telescope is
-nevertheless required. And, in fact, you could not find a better
-test for the excellence of an instrument than to try if it will show
-these delicate objects. But do not imagine that merely having a
-good telescope and a clear sky is all that is requisite for making
-astronomical discoveries. You might just as well say that by putting
-a first-rate cricket-bat in any man’s hands will ensure his making a
-grand score. Every boy knows that the bat does not make the cricketer,
-and I can assure him that neither will the telescope make the
-astronomer. In both cases, no doubt, there is some element of luck. But
-of this you may be certain: that as it is the man that makes the score,
-and not the bat, so it is the astronomer that makes the discovery, and
-not his telescope.
-
-Deimos and Phobos were the names of the two personages, according to
-Homer, whose duty it was to attend on the god Mars, and to yoke his
-steeds. A conclave of classical scholars and astronomers appropriately
-decided that Deimos and Phobos must be the names of the two satellites
-to the planet which bears the name of Mars.
-
-
-HOW THE TELESCOPE AIDS IN VIEWING FAINT OBJECTS.
-
-We have been hitherto talking about large planets, which, if not as big
-as our earth, are at least as big as our moon. But now we have to say a
-few words about a number of little planets, many of them being so very
-small that a million rolled together would not form a globe so big as
-this earth. These little objects you cannot see with your unaided eye,
-and even with a telescope they only look like very small stars.
-
-I have often been asked why it is that a telescope enables us to
-see objects, both faint and small, which our unaided eyes fail to
-show. Perhaps this will be a good opportunity to say a few words on
-the subject. I think we can explain the utility of the telescope by
-examining our own eyes. The eye undergoes a remarkable transformation
-when its owner passes from darkness into a brilliantly lighted room
-(Fig. 64). Here you see two views of an eye, and you notice the great
-difference between them. They are not intended to be the eyes of two
-different people, or the two eyes of the same person; they are merely
-two conditions of the same eye. They are intended to illustrate two
-different states of the eye of a collier. The right shows his eye when
-he is above ground in bright daylight; the left is his eye when he has
-gone down the coal-pit to his useful work in the dark regions below. I
-remember when I went down a coal-pit I was lowered down a long shaft,
-and when the bottom was reached a safety lamp was handed to me. The
-gloom was such, that at first I found some little difficulty in guiding
-my steps, but the capable guide beside me said in an encouraging voice,
-“You will be all right, sir, in a few moments, for you will get your
-_pit-eyes_.” I did get my “pit-eyes,” as he promised, and was able to
-see my way along sufficiently to enjoy the wonderful sights that are
-met with in the depths below.
-
-[Illustration: This shows the Eye in the Dark.
-
-This shows the Eye in the Daylight.
-
-FIG. 64.]
-
-The change that came over my eyes is that which these two pictures
-illustrate: the black, round spot in the centre is an opening covered
-with a transparent window, by which light enters the eye; the black
-spot is called the pupil, and nature has provided a beautiful
-contrivance by which the pupil can get larger or smaller, so as to make
-vision agreeable. When there is a great deal of light we limit the
-amount that enters by contracting the pupil so as to make the opening
-smaller. Thus the picture with the small pupil represents the state of
-the collier’s eye when he is above ground in bright sunlight. When he
-descends into the pit, where the light is very scanty, then he wants to
-grasp as much of it as ever he can, and consequently his pupil enlarges
-so as to make a wider opening, and this is what he calls getting his
-“pit-eyes.”
-
-But you need not go down a coal-mine to see the use of the iris--for so
-that pretty membrane is called which surrounds the pupil. Every time
-you pass from light into darkness the same thing can be perceived.
-When we turn down the lights in a room, so that we are in comparative
-darkness, our pupils gradually expand. As soon as the lights are
-turned up again, then our pupils begin to contract. Other animals have
-the same contrivance in their eyes. You may notice in the Zoölogical
-Gardens how quickly the pupil of the lion contracts when he raises his
-eyes to the light. The power of rapidly changing the pupil might be
-of service to a beast of prey. Imagine him crouching in a dense shade
-to wait for his dinner; then of course the pupil will be large from
-deficiency of light; but when he springs out suddenly on his victim, in
-bright light, it would surely be of advantage to him to be able at once
-to see clearly. Accordingly his pupil adjusts itself to the altered
-conditions with a rapidity that might not be necessary for creatures of
-less predaceous habits.
-
-These changes of the pupil explain how the telescope aids our eyes
-when we want to discern any faint objects, like the little planets.
-Such bodies are not visible to the unaided eyes, because our pupils
-are not large enough to grasp sufficient light for the purpose. Even
-when they are opened to the utmost, we want something that shall enable
-them to open wider still. We must therefore borrow assistance from
-some device which shall have an effect equivalent to an enlargement
-of the pupil beyond the limits that nature has actually assigned to
-it. What we want is something like a funnel which shall transform a
-large beam of rays into a small one. I may explain what I mean by the
-following illustration: Suppose that it is raining heavily, and that
-you want to fill a bucket with water. If you merely put the bucket out
-in the middle of a field, it will never be filled; but bring it to
-where the rain-shoot from a house-top is running down, and then your
-bucket will be running over in a few moments. The reason, of course,
-is that the broad top of the house has caught a vast number of drops
-and brought them together in the narrow shoot, and so the bucket is
-filled. In the same way the telescope gathers the rays of light that
-fall on the object glass, and condenses them into a small beam which
-can enter the eye. We thus have what is nearly equivalent to an eye
-with a pupil as big as the object glass. Thus the effect of a grand
-telescope amounts to a practical increase of the pupil from the size of
-a threepenny-piece up to that of a dinner-plate, or even much larger
-still.
-
-
-THE ASTEROIDS OR SMALL PLANETS.
-
-An asteroid is like a tiny star, and in fact the two bodies are very
-often mistaken. If we could get close to the objects, we should see a
-wide difference between them. We should find the asteroid to be a dark
-planet like our earth, lighted only by the rays from the sun. The star,
-small and faint though it may seem, is itself a bright sun, at such a
-vast distance that it is only visible as a small point. The star is
-millions of times as far from us as the planet, and utterly different
-in every respect.
-
-It is a curious fact that the planets should happen to resemble the
-stars so closely. We can find an analogous fact in quite another part
-of nature. In visiting a good entomological collection, you will be
-shown some of the wonderful leaf-like insects. These creatures have
-wings, exactly formed to imitate leaves of trees, with the stalks and
-veins completely represented. When one of these insects lies at rest,
-with its wings folded, among a number of leaves, it would be almost
-impossible to penetrate the disguise. This mimicry is no doubt an
-ingenious artifice to deceive the birds or other enemies that want to
-eat the insect. There is, however, one test which the cunning bird
-could apply: the leaves do not move about of their own accord, but the
-leaf-insects do. If therefore the bird will only have the patience
-to wait, he will see a pair of the seeming leaves move, and then the
-deception will be to him a deception no longer, and he will gobble up
-the poor insect.
-
-In our attempts to discover the planets we experience just the same
-difficulties as the insect-eating bird. Wide as is the true difference
-between a planet and a star, there is yet such a seeming resemblance
-between them that we are often puzzled to know which is which. The
-planets imitate the stars so successfully, that when one of them
-is presented to us among myriads of stars it is impossible for us
-to detect the planet by its appearance. But we can be cunning--we
-can steadily watch, and the moment we find one of these star-like
-points beginning to creep about we can pounce upon it. We know by its
-movements that it is only disguised as a star, but that it is really
-one of the planets.
-
-It is not always easy to discover the asteroids even by this principle,
-for unfortunately these bodies move very slowly. If you have a planet
-in the field of view, it will creep along so gradually that an hour or
-more must have elapsed before it has shifted its position with respect
-to the neighboring stars to any appreciable extent. The search for such
-little planets is therefore a tedious one, but there are two methods of
-conducting it: the new one, which has only recently come into use, and
-the old one. I shall speak of the old one first.
-
-Although the body’s motion is so slow, yet when sufficient time is
-allowed, the planet will not only move away from the stars close by,
-but will even journey round the entire heavens. The surest way of
-making the discovery is to study a small part of the heavens now and
-to examine the same locality again months or years afterwards. Memory
-will not suffice for this purpose. No one could recollect all the stars
-he saw with sufficient distinctness to be confident that the field of
-the telescope on the second occasion contained either more or fewer
-stars than it did on the first. The only way of doing this work is to
-draw a map of the stars very carefully. This is a tedious business,
-for the stars are so numerous that even in a small part of the heavens
-there will be many thousands of stars visible in the telescope. All of
-these will have to be entered faithfully in their true places on the
-map. When this has been done the map must be laid aside for a season,
-and then it is brought out again and compared with the sky. No doubt
-the great majority of the objects will be found just as they were
-before. These are the stars, the distant suns, and our concern is not
-at present with them. Sometimes it will happen that an object marked
-on the first map has left a vacant place on the second. This, however,
-does not help us much, for, whatever the object was, it has vanished
-into obscurity, and a new planet could hardly be discovered in this
-way. But sometimes it will happen that there is a small point of light
-seen in the second map which has no corresponding point in the first.
-Then, indeed, the expectation of the astronomer is aroused; he may be
-on the brink of a discovery. Of course he watches accurately the little
-stranger. It might be some star that had been accidentally overlooked
-when forming the map, or it might possibly be a star that has become
-bright in the interval. But here is a ready test: is the body moving?
-He looks at it very carefully, and notes its position with respect to
-the adjacent stars. In an hour or two his suspicions may be confirmed;
-if the object be in motion, then it is really a planet. A few further
-observations, made on subsequent days, will show the path of the body.
-And the astronomer has only to assure himself that the object is not
-one of the planets that have been already found before he announces his
-discovery.
-
-The new method of searching for small planets, which has only come into
-use in recent years, is a very beautiful one, and renders the process
-of making such discoveries much more easy than the older method which I
-have just described.
-
-We can take photographs of the heavenly bodies by adjusting a sensitive
-plate in the telescope so that the images of the objects we desire to
-see shall fall upon it. The method will apply to very small stars, if
-by excellent clockwork and careful guiding we can keep the telescope
-constantly pointing to the same spot until the stars have had time to
-imprint their little images. Thus we obtain a map of the heavens, made
-in a thoroughly accurate manner. Indeed, the delicacy of photography
-for this purpose is so great, that the plates show many stars which
-cannot be seen with even the greatest of telescopes. Suppose that
-a little planet happened to lie among the stars which are being
-photographed. All the time that the plate is being exposed the wanderer
-is, of course, creeping along, and after an hour (exposures even longer
-are often used), it may have moved through a distance sufficient to
-ensure its detection. The plate will, therefore, show the stars as
-points, but the planet will betray its presence by producing a streak.
-
-The asteroids now known number between 400 and 500. Out of this host
-a few afford some information to the astronomer, but the majority of
-them are objects possessing individually only the slightest interest.
-No small planet is worth looking at as a telescopic picture. We should
-consider that asteroid to be a large one which possessed a surface
-altogether as great as England or France. Many of these planets have a
-superficial extent not so large as some of our great counties. A globe
-which was just big enough to be covered by Yorkshire--if you could
-imagine that large county neatly folded round it--would make a very
-respectable minor planet.
-
-We know hardly anything of the nature of these small worlds, but it is
-certain that any living beings they could support must have a totally
-different nature from the creatures that we know on this earth. We can
-easily prove this by making a calculation. I shall suppose a small
-planet one hundred miles wide, its diameter being, therefore, the
-one-eightieth part of the diameter of the earth. If we were landed
-on such a globe, we should be far more puzzled by the extraordinary
-lightness of everything than we should be in the similar case of the
-moon to which I referred (p. 124). If we suppose the planet to be
-constructed of materials which had the same density as those of which
-the earth is made, then every weight would be reduced to the eightieth
-part of what it is here.
-
-There would be one curious consequence of residence on such a globe.
-We have heard of attempts to make flying machines, or to provide a
-man with wings by which he shall soar aloft like the birds. All such
-contrivances have hitherto failed. It may be possible to make a pair of
-wings by which a man can fly down, but it is quite another matter when
-he tries to fly up again. Suppose, however, we were living on a small
-planet, it would be perfectly easy to fly, for as our bodies would only
-seem to weigh a couple of pounds, we ought to be able to flap a pair of
-wings strong enough to overcome so trivial a force. I should, however,
-add that this is on the supposition that the atmosphere has the same
-density as our own.
-
-Life on these small planets would indeed be extraordinary. Let us take,
-for example, Flora, and see how a game of lawn tennis on that body
-would be managed. The very slightest blow of the racket would drive the
-ball a prodigious distance before it could touch the ground; indeed,
-unless the courts were about half a mile long, it would be impossible
-to serve any ball that was not a fault. Nor is there any great exertion
-necessary for playing lawn tennis on Flora, even though the courts are
-several hundred acres in extent. As a young lady ran to meet the ball
-and return it, each of her steps might cover a hundred yards or so
-without extra effort; and should she have the misfortune to get a fall,
-her descent to the ground would be as gentle as if she was seeking
-repose on a bed of the softest swan’s-down.
-
-These little planets cluster together in a certain part of our
-system. Inside are the four inner planets, of which we have already
-spoken; outside are the four outer planets, of which we have soon to
-speak. Between these two groups there was a vacant space. It seemed
-unreasonable that where there was room for planets, planets should
-not be found. Accordingly the search was made, and these objects
-were discovered. Even at the present day, more and more are being
-constantly added to the list.
-
-Up till quite recently all the small planets which had been discovered
-confined themselves to the space lying between the paths of the
-major planets Mars and Jupiter. This invariable rule was, however,
-departed from in the case of one of these bodies which was discovered
-in August, 1898. This little body, which was known for some time by
-the provisional appellation of D Q, and which has now been definitely
-christened Eros, is an exception to this rule. It travels at an average
-distance from the sun actually less than that of Mars, and at the
-nearest point can come within 15,000,000 miles of the earth.
-
-We occasionally get information from these little bodies; for in their
-revolutions through the solar system, they sometimes pick up scraps of
-useful knowledge, which we can elicit from them by careful examination.
-For example, one of the most important problems in the whole of
-astronomy is to determine the sun’s distance. I have already mentioned
-one of the ways of doing this, which is given by the transit of Venus.
-Astronomers never like to rely on a single method; we are therefore
-glad to discover any other means of solving the same problem. This
-it is which the little planets will sometimes do for us. Juno on one
-occasion approached very close to the earth, and astronomers in various
-parts of the globe observed her at the same time. When they compared
-their observations they measured the sun’s distance. But I am not going
-to trouble you now with a matter so difficult. Suffice it to say,
-that for this, as for all similar investigations, the observers were
-constrained to use the very same principle as that which we illustrated
-in Fig. 5.
-
-Let me rather close this lecture with the remark that we have here been
-considering only the lesser members of the great family which circulate
-round the sun, and that we shall speak in our next lecture of the giant
-members of our system.
-
-
-
-
-LECTURE IV.
-
-JUPITER, SATURN, URANUS, NEPTUNE.
-
-  Jupiter, Saturn, Uranus, Neptune--Jupiter--The Satellites of
-      Jupiter--Saturn--The Nature of the Rings--William Herschel--The
-      Discovery of Uranus--The Satellites of Uranus--The Discovery of
-      Neptune.
-
-
-Our lecture to-day ought to make us take a very humble view of the size
-of our earth. Mercury, Venus, and Mars may be regarded as the earth’s
-peers, though we are slightly larger than Venus, and a good deal larger
-than Mercury or Mars; but all these four globes are insignificant in
-comparison with the gigantic planets which lie in the outer parts of
-our system. These great bodies do not enjoy the benefits of the sun to
-the same extent that we are permitted to do; they are so far off that
-the sun’s rays become greatly enfeebled before they can traverse the
-distance; but the gloom of their situation seems to matter but little,
-for it is highly improbable that any of these bodies could be inhabited.
-
-A view of parts of the paths of these four great planets is shown
-in Fig. 65. The innermost is Jupiter, which completes a circuit in
-about twelve years; then comes Saturn, revolving in an orbit so great
-that twenty-nine years and a half are required before the complete
-journey is finished. Still further outside is Uranus, which has a
-longer journey than Saturn, and moves so much more slowly that a man
-would have to live to the ripe old age of eighty-four if a complete
-revolution of Uranus was to be accomplished during his lifetime. At the
-boundary of our system revolves the planet Neptune, and though it is a
-mighty globe, yet we cannot see it without a telescope. It is invisible
-to the naked eye for two reasons: first of all, because it is so far
-from the sun that the light which illuminates it is excessively feeble;
-and, secondly, because it is so far from us that whatever brilliancy it
-has is largely reduced.
-
-[Illustration: FIG. 65.--The Orbits of the Four Giant Planets.]
-
-
-JUPITER.
-
-Of all these bodies Jupiter is by far the greatest; he is, indeed,
-greater than all the other planets rolled into one. The relative
-insignificance of the earth when compared with Jupiter is well
-illustrated by the fact that if we took 1200 globes each as big as our
-earth, and made them into a single globe, it would only be as large as
-the greatest of the planets. A view of the comparative sizes of the
-earth and Jupiter is shown in Fig. 66.
-
-[Illustration: FIG. 66.--Jupiter and the Earth compared.]
-
-Fig. 67 shows a picture of Jupiter as seen through the telescope.
-First, you will notice that the outline of the planet’s shape is not
-circular, for it is plain that the vertical diameter in this picture
-is shorter than the horizontal one; in fact, Jupiter is flattened at
-the Poles and bulges out at the equator, so that a section through the
-Poles is an ellipse. Jupiter is turning round rapidly on his axis, and
-this will account for the protuberance. We find that the planet has
-assumed almost the same form as if it were actually a liquid. This
-we can illustrate by a globe of oil which is poised in a mixture of
-spirits of wine and water so carefully adjusted that the oil has no
-tendency to rise or fall. As we make the globe of oil rotate, which we
-can easily do by passing a spindle through it, we see that it bulges
-out in the form that Jupiter as well as other planets have taken.
-
-[Illustration: FIG. 67.--The Clouds of Jupiter.]
-
-On the picture of the planets you will see shaded bands. These are
-constantly changing their aspect, and for a double reason. In the first
-place, they change because Jupiter is rotating so quickly that in
-five hours the whole side of the planet which is towards us has been
-carried out of sight. In another five hours the original side of the
-globe will be back again, for the entire rotation occupies about 10
-hours, or, more precisely, 9 hours 55 minutes 21 seconds.
-
-But these bands are themselves not permanent objects. They have no more
-permanence than the clouds over our own sky. Sometimes Jupiter’s clouds
-are more strongly marked than on other occasions. Sometimes, indeed,
-they are hardly to be seen at all. It is from this we learn that those
-markings which we see when we look at the great planet are merely the
-masses of cloud which surround and obscure whatever may constitute his
-interior.
-
-There is a circumstance which demonstrates that Jupiter must be an
-object exceedingly different from the earth, though both bodies agree
-in so far as having clouds are concerned. What would you think when I
-tell you that we were able to weigh Jupiter by the aid of his little
-moons, of which I shall afterwards speak? These little bodies inform us
-that Jupiter is about 300 times as heavy as our earth, and we have no
-doubt about this, for it has been confirmed in other ways. But we have
-found by actual measurement that Jupiter is 1200 times as big as the
-earth, and therefore, if he were constituted like the earth, he ought
-to be 1200 times as heavy. This is, I think, quite plain; for if two
-cakes were made of the same material, and one contained twice the bulk
-of the other, then it would certainly be twice as heavy. If there be
-two balls of iron, one twice the bulk of the other, then, of course,
-one has twice the weight of the other. But if a ball of lead have
-twice the bulk of a ball of iron, then the leaden ball would be more
-than twice as heavy as the iron, because lead is the heavier material.
-In the same way, the weights of the earth and Jupiter are not what we
-might expect from their relative sizes. If the two bodies were made
-of the same materials and in the same state, then Jupiter would be
-certainly four times as heavy as we find him to be. We are, therefore,
-led to the belief that Jupiter is not a solid body, at least in its
-outer portions. The masses of cloud which surround the planet seem
-to be immensely thick, and as clouds are, of course, light bodies in
-comparison with their bulk, they have the effect of largely increasing
-the apparent size of Jupiter, while adding very little to his weight.
-There is thus a great deal of mere inflation about this planet, by
-which he looks much bigger than his actual materials would warrant if
-he were constituted like the earth.
-
-These facts suggest an interesting question. Why has Jupiter such an
-immense atmosphere, if we may so call it? The clouds we are so familiar
-with down here on the earth are produced by the heat of the sun, which
-beats down upon the wide surface of the ocean, evaporates the water,
-and raises the vapor up to where it forms the clouds. Heat, therefore,
-is necessary for the formation of cloud; and with clouds so dense and
-so massive as those on Jupiter, more heat would apparently be necessary
-than is required for the moderate clouds on this earth. Whence is
-Jupiter to get this heat? Have we not seen that the great planet is
-far more distant from the sun than we are? In fact, the intensity of
-the sun’s heat on Jupiter is not more than the twenty-fifth part of
-what we derive from the same source. We can hardly believe that the
-sun supplied the heat to make those big clouds on the great planet; so
-we must cast about for an additional source, which can only be inside
-the planet itself. So far as his internal heat is concerned, Jupiter
-seems to be in much the same condition now as our earth was once, ages
-ago, before its surface had cooled down to the present temperature.
-As Jupiter is so much larger than the earth, he has been slower in
-parting with his heat. The planet seems not yet to have had time to
-cool sufficiently to enable water to remain on his surface. Thus the
-internal heat of the planet supplies an explanation of his clouds. We
-may also remark that as the present condition of Jupiter illustrates
-the early condition of our earth, so the present condition of the earth
-foreshadows the future reserved for Jupiter when he shall have had time
-to cool down, and when the waters that now exist in the form of vapor
-shall be condensed into oceans on his surface.
-
-
-THE SATELLITES OF JUPITER.
-
-Every owner of a telescope delights to turn it on the planet Jupiter,
-both for the spectacle the globe itself affords him, and for a view of
-the wonderful system of moons by which the giant planet is attended.
-Fortunately the four satellites of Jupiter lie within reach of even
-the most modest telescope, and their incessant changes relative to
-Jupiter and each other give them a never-ending interest for the
-astronomer. Compared with the torpid performance of our moon, which
-requires a month to complete a circuit around the earth, Jupiter’s
-moons are wonderfully brisk and lively. Nor are they small bodies like
-the satellites of Mars, for the second of Jupiter’s satellites is quite
-as big as our moon, and the other three are very much larger. It is,
-however, true that his satellites appear insignificant when compared
-with Jupiter’s own enormous bulk.
-
-The innermost of these little bodies flies right round in a period of
-one day and eighteen or nineteen hours, while the outermost of them
-takes a little more than a fortnight--that is, rather more than half
-the time that our moon demands for a complete revolution. Jupiter’s
-satellites are too far off for us to see much with respect to their
-structure or appearance even with mighty telescopes. It is, of course,
-their great distance from us that makes them look insignificant. They
-would, however, be bright enough to be seen like small stars were
-it not that, being so close to Jupiter, his overpowering brightness
-renders such faint objects in his vicinity invisible.
-
-It was by means of the satellites of Jupiter that one of the most
-beautiful scientific discoveries was made. As a satellite revolves
-round the giant planet it often happens that the little body enters
-into the shadow of the great planet. No sunlight will then fall upon
-the satellite, and as it has no light of its own, it disappears from
-sight until it has passed through the shadow and again receives
-sunlight on the other side. We can watch these eclipses with our
-telescopes, and there can be no more interesting employment for
-a small telescope. The movements of these bodies are now known so
-thoroughly that the occurrence of the eclipses can be predicted. The
-almanacs will tell when the satellite is calculated to disappear, and
-when it ought again to return to visibility. When astronomers first
-began to make these computations a couple of hundred years ago, the
-little satellites gave a great deal of trouble. They would not keep
-their time. Sometimes they were a quarter of an hour too soon, and
-sometimes a quarter of an hour too late. At last, however, the reason
-for these irregularities was discovered, and a wonderful reason it was.
-
-Suppose there were a number of cannons all over Hyde Park, and that
-these cannons were fired at the same moment by electricity. Though the
-sounds would all be produced simultaneously, yet, no matter where you
-stood, you would not hear them altogether; the noise from the cannons
-close at hand would reach your ears first, and the more distant reports
-would come in subsequently. You can calculate the distance of a flash
-of lightning if you allow a mile for every five seconds that elapse
-between the time you saw the flash and the time you heard the peal
-of thunder which followed it. The light and the noise were produced
-simultaneously, but the sound takes five seconds to pass over every
-mile, while the light, in comparison to sound, may be said to move
-instantaneously. That sound travelled with a limited velocity was
-always obvious, but never until the discrepancies arose about Jupiter’s
-satellites was it learned that light also takes time to travel. It is
-true that light travels much more quickly than sound--indeed, about
-a million times as fast. Light goes so quickly, that it would rush
-more than seven times around the earth in a single second. So far as
-terrestrial distances are concerned, the velocity of light is such
-that the time required for a journey is inappreciable. The distances,
-however, between one celestial body and another are so enormous, that
-even a ray of light, moving as quickly as it alone can move, will
-occupy a measurable time on the way. Our moon is comparatively so near
-us, that light takes little more than a second to cover that short
-distance. Eight minutes are, however, required for light to travel from
-the sun to the earth; in fact, the sunbeams that now come into our
-eyes left the sun eight minutes ago. If the sun were to be suddenly
-extinguished, it would still seem to shine as brightly as ever in the
-eyes of the inhabitants of this earth for eight minutes longer. As
-Jupiter is five times as far from the sun as we are, it follows that
-the light from the sun to Jupiter will spend forty minutes on the
-journey, and the light from Jupiter to the earth will take a somewhat
-similar time. When we look at Jupiter and his moons, we do not see him
-as he is now, we see him as he was more than half an hour ago, but
-the interval will vary somewhat according to our different distances
-from the planet. Sometimes the light from Jupiter will reach us in as
-little as thirty-two minutes, while sometimes it will take as much as
-forty-eight--that is, the light sometimes requires for its journey a
-quarter of an hour more than is sufficient at other times.
-
-We can therefore understand that irregularity of Jupiter’s satellites
-which puzzled the early astronomers. An eclipse sometimes appeared a
-quarter of an hour before it was expected; because the earth was then
-as near as it could be to Jupiter, while the calculations had been
-made from observations when Jupiter was at his greatest distance.
-It was these eclipses of the satellites which first suggested the
-possibility that light must have a measurable speed. When this was
-taken into account, then the occasional delay of the eclipses was found
-to be satisfactorily explained. Confirmation flowed in from other
-sources, and thus the discovery of the velocity of light was completely
-established.
-
-Professor Barnard, when studying Jupiter in 1892 with the splendid
-refractor at the Lick Observatory, saw a very small point of light
-nearer to the planet than the nearest of the four satellites already
-known. Further examination showed that this little object was indeed
-another satellite. Thus Jupiter has a fifth moon in addition to the
-four which have been known so long. This little body is so small and
-faint that it can only be discerned under the most favorable conditions
-by the most powerful telescopes.
-
-
-SATURN.
-
-Next outside Jupiter, on the confines of the ancient planetary system,
-revolves another grand planet, called Saturn. His distance from us is
-sometimes nearly a thousand millions of miles, and he requires more
-than a quarter of a century for the completion of each revolution.
-Sometimes people do not pronounce the names of the planets quite
-correctly. I have heard of a gardener who has a taste for astronomy,
-and sometimes begins to talk about the planets Juniper and Citron.
-Probably you will know what he meant to say. The ancients had
-discovered Saturn to be a planet, for though he looked like a star, yet
-his movement through the constellations could not escape their notice
-when attention was paid to the heavens.
-
-[Illustration: FIG. 68.--Saturn and the Earth compared.]
-
-In the matter of size Saturn is only surpassed by Jupiter among the
-planets. He is about 600 times as large as the earth; the small object,
-E, shown in Fig. 68, represents our earth in its true comparative size
-to the ringed planet; but Saturn is so far off, that even at his best
-he is never so bright as Venus, or Mars, or Jupiter become when they
-are favorably situated. On the globe of Saturn we can sometimes see a
-few bands, but they are faint compared with those on Jupiter. There
-is, however, no doubt that what we see upon Saturn is a dense mass of
-cloud. Indeed, he can have comparatively little solid matter inside,
-for this planet does not weigh so much as a ball of water the same size
-would do. Saturn, like Jupiter, must be highly heated in his interior.
-
-The ring, or rather series of rings, by which the planet is surrounded
-are also shown in Fig. 68: these appendages are not fastened to the
-globe of Saturn by any material bonds; they are poised in space,
-without any support, while the globe or planet proper is placed
-symmetrically in the interior.
-
-I have made a model which shows Saturn with his rings, but it is
-necessary for me to fasten the rings by little pieces of wire to the
-globe, for there is no mechanical means by which the rings of the model
-could be poised without support, as they are around the planet. If
-we throw the beam of the electric lamp on the little planet, we see
-the shadow which the planet casts on its ring. Similar shadows can be
-observed in the actual Saturn of the sky, and this is a proof that
-the planet does not shine by its own light, but by the light of the
-sun which falls upon it. Here again we illustrate the wide difference
-between a planet and a star, for were our sun to be put out, Saturn
-and all the other planets in the sky would vanish from sight, while
-the stars would, of course, twinkle on as before. There are three
-rings round Saturn; they all lie in the same plane, and they are so
-thin, that when turned edgewise towards us the whole system almost
-disappears, except in very powerful telescopes. The outer and the
-inner bright rings are divided by a dark line, which can be traced
-entirely round. At the inner edge of the inner ring begins that strange
-structure called the _crape_ ring, which extends halfway towards the
-globe of the planet. The most remarkable point about the crape ring is
-its semi-transparency, for we can sometimes see the globe of the planet
-through this strange curtain. The crape ring can only be observed with
-a powerful telescope. The other two rings are within the power of very
-moderate instruments.
-
-
-THE NATURE OF THE RINGS.
-
-For the explanation of the nature of Saturn’s rings we are indebted
-to the calculations of mathematicians. You might have thought,
-perhaps, that nothing would be simpler than to suppose the rings
-were stiff plates made from solid material. But the question cannot
-be thus settled. We know that the ring could not bear the strain
-of the planet’s attraction upon it if it were a solid body. I may
-illustrate the argument by familiar facts about bridges. Where the
-span is but a small one, as, for instance, when a road has to cross
-a railway, a canal, or a river, the arch is, of course, the proper
-kind of structure. There is, for example, a specially beautiful arch
-over the river Dee at Chester. But if the bridge be longer than this,
-masonry arches are not suitable. Where a considerable span has to be
-crossed, as at the Menai Straits, or a gigantic one, as at the Firth
-of Forth, then arches have to be abandoned, and iron bridges of a
-totally different construction have to be employed. Arches cannot be
-used beyond a limited span, because the strain upon the materials
-becomes too great for their powers of resistance to withstand. Each of
-the stones in an arch is squeezed by intense pressure, and there is a
-limit beyond which even the stoutest stones cannot be relied upon. As
-soon, therefore, as the span of the arch is so great that the stones
-it contains are squeezed as far as is compatible with safety, then the
-limit of size for that form of arch has been reached.
-
-Suppose that you stood on Saturn at his equator, and looked up at the
-mighty ring which would stretch edgewise across your sky. It would rise
-up from the horizon on one side, and, passing over your head, would
-slope down to the horizon on the other. You would, in fact, be under an
-arch of which the span was about 100,000 miles. Owing to the attraction
-of Saturn, every part of that structure would be pulled forcibly
-towards his surface, and thus the materials of the arch, if it were a
-solid body, would be compressed with terrific force.
-
-It does not really signify that the arch I am now speaking of is half
-of a ring the other half of which is below the globe of the planet.
-That is only a difference with respect to the support of two ends of
-the arch, and does not affect the question as to the pressure upon
-its materials; nor does the fact that the ring is revolving remove
-the difficulty, though it undoubtedly lessens it. We know no solid
-substance which could endure the pressure. Even the toughest steel
-that ever was made would bend up like dough under such conditions. We
-cannot, therefore, account for Saturn’s ring by supposing it to be a
-solid, for no solid would be strong enough.
-
-Do you not remember the old fable of the oak tree and the pliant
-reed--how when the storm was about to arise the oak laughed at the
-poor reed, and said it would never be able to withstand the blasts?
-But matters did not so turn out. The mighty oak, which would not
-yield to the storm, was blown down, while the slender reed bent to
-the wind and suffered no injury. This gives us a hint as to the true
-constitution of Saturn’s ring; it is not a solid body, trying to resist
-by mere strength; it is rather to be explained as an excessively pliant
-structure. Indeed, I ought not to call it a structure at all; it is
-rather a multitude of small bodies not in the least attached together.
-I do not know what the size of these bodies may be. For anything we can
-tell, they may be no larger than the pebbles you find on a gravel walk.
-
-Let us see how we could encircle our earth with rings like those which
-surround Saturn. I shall ask you to be provided with a sufficiently
-large number of pebbles, and you must also imagine that I have the
-means of ascending high up into space, halfway from here to the moon.
-Suppose I went up there and simply dropped the pebble, of course, it
-would tumble straight down to the earth again. If, however, I threw it
-out with proper speed and in the proper direction, I could start it
-off like a little moon, and it would go on round and round our earth
-in a circle. I mention a pebble, but really it is little matter what
-the size of the object may be--it may be as small as a grain of shot
-or as big as a cannon-ball. Now take another pebble. Cast it also in a
-somewhat similar path, taking care, however, that the planes of the two
-orbits shall be the same. Each of these little bodies shall pursue its
-journey without interference from the other. Then proceed in the same
-way with a third, a fourth, with thousands and millions and billions of
-pebbles, until at last the small bodies will become so numerous that
-they almost fill a large part of the plane with a continuous shoal.
-Each little object, guided entirely by the earth’s attraction, will
-pursue its path with undeviating regularity. Its neighbors will not
-interfere with it, nor will it interfere with them. Let us circumscribe
-the limits of our flat shoal of moonlets. We first take away all those
-that lie outside a certain large circle; then we shall clear away
-sufficient to make a vacant space between the outer ring and the inner
-ring, and thus the two conspicuous rings have been made; at the inside
-of the inner ring we shall take out numbers of pebbles here and there,
-so as to make this part much less dense than the outer portions, and
-thus produce a semi-transparent crape ring; then we shall clear away
-those that come too close to the planet, and form a neat inner boundary.
-
-Could we then view our handiwork from the standpoint of another planet,
-what appearance would our earth present? The several pebbles, though
-individually so small, would yet, by their countless numbers, reflect
-the sun’s light so as to produce the appearance of a continuous sheet.
-Thus we should find a large bright outer ring surrounding the earth,
-separated by a dark interval from the inner ring, and at the margin of
-the inner ring the pebbles would be so much more sparsely distributed
-that we should be able to see through them to some extent. That
-beautiful system of rings which Saturn displays is undoubtedly of a
-similar character to the hypothetical system which I have endeavored to
-describe. No other explanation will account for the facts, especially
-for the semi-transparency of the crape ring. The separate bodies from
-which Saturn’s rings are constituted seem, however, so small that we
-are not able to see them individually. There are some other fine lines
-running round the rings beside the great division, and these can also
-be explained by the theory I have stated.
-
-Saturn has other claims on our attention besides those of its rings.
-It has an elaborate retinue of satellites--no fewer, indeed, than
-nine; but some of them are very faint objects, and not by any means
-so interesting as the system by which Jupiter is attended. The ninth
-of these was discovered quite recently by Professor W. H. Pickering,
-of Harvard College Observatory. This little moon, for which the name
-“Phoebe” has been suggested, is further from the planet than any of the
-others. It is a minute object shining as a star of the 15th or 16th
-magnitude, and moves around the planet in a period of about sixteen
-months.
-
-Saturn was the last and outermost of the planets with which the
-ancients were acquainted. Its path lay on the frontiers of the then
-known solar system, and the magnificence of the planet itself, with
-its attendant luminaries and its marvellous rings, rendered it worthy
-indeed of a position so dignified. These five planets--namely, Mercury,
-Venus, Mars, Jupiter, and Saturn--made up with the sun and the moon
-the seven “planets” of the ancients. They were supposed to complete
-the solar system, and, furthermore, the existence of other members was
-thought to be impossible. In modern times it has been discovered that
-there are yet two more planets. I do not now refer to those little
-bodies which run about in scores between Mars and Jupiter. I mean two
-grand first-class planets, far bigger than our earth. One of them is
-Uranus, which revolves far outside Saturn, and the other is Neptune,
-which is much further still, and whose mighty orbit includes the whole
-planetary system in its circuit. To complete its journey round the sun
-not less than 165 years is required.
-
-
-WILLIAM HERSCHEL.
-
-I have to begin the account of this discovery by telling you a little
-story. In the middle of the last century there lived at Hanover a
-teacher of music whose name was Isaac Herschel. He had a family of ten
-children, and he did the best for them that his scanty means would
-permit. Of his children William was the fourth, and he inherited his
-father’s talents for music, as did most of his brothers and sisters.
-He was a bright, clever boy at school, and he made such good progress
-in his music that by the time he was fourteen years old he was able
-to play in the military band of the Hanoverian Guards. War broke out
-between France and England, and as Hanover was then under the English
-crown, the French invaded it, and a battle was fought in which the poor
-Hanoverian Guards suffered very terribly. Young Herschel spent the
-night after the battle in a ditch, and he came to the conclusion that
-he did not like fighting, though he was only a member of the band, and
-he resolved to change his profession. That was not so easy to do just
-then, for even a bandsman cannot leave the service in war time at his
-own free will. William Herschel, however, showed all through his life
-that he was not the man to be baffled by difficulties. I do not know
-whether he asked for leave, but at all events he took it. He deserted,
-in fact, and his friends succeeded in sending him away to England.
-
-He was nineteen years old when he commenced to look for a career over
-here, and certainly he found his prospects in the musical profession
-very discouraging. Herschel was, however, very industrious; and at last
-he succeeded in getting appointed as organist of the Octagon Chapel at
-Bath. He gradually became famous for his musical skill, and had numbers
-of pupils. He used also to conduct concerts and oratorios, and was
-well known in this way over the West of England. Busy as Herschel was
-with his profession, he still retained his love of reading and study.
-Every moment he could spare from his duties he devoted to his books.
-It was natural that a musician should specially desire to study the
-theory of music, and to understand it properly you should know Euclid
-and algebra, and, indeed, higher branches of mathematics as well.
-Herschel did not know these things at first; he had not the means of
-learning them when he was a boy, so he worked very hard after he became
-a man. And he studied with such success that he made fair progress in
-mathematics, and then it appeared to him that it would be interesting
-to learn something about astronomy. After he had begun to read about
-the stars, he thought he would like to see them, and so he borrowed a
-telescope. It was only a little instrument, but it delighted him so
-much that he said he must have one for himself. So he wrote to London
-to make inquiries.
-
-Telescopes were much dearer in those days than they are now, and
-Herschel could not give the price that the opticians demanded. Here
-again his invincible determination came to his aid. What was there to
-prevent him from making a telescope? he asked himself; and forthwith
-he began the attempt. You will think it strange, perhaps, that a
-music-teacher who had no special training as a mechanic should at once
-commence so delicate and difficult a task; but it is not really so hard
-to make a telescope as might be imagined. The amateur cannot make such
-a pretty-looking instrument as he is able to buy at the shops--the
-tubes will not be so beautifully polished and the finish will be such
-as a trained workman would be ashamed of--but the essential part of a
-telescope is comparatively easy to make; at least, I should say of a
-reflecting telescope, which is the kind Herschel attempted to make,
-and succeeded in making. You must know that there are two kinds of
-telescopes. The commoner one with which you are more familiar is called
-the refracting telescope, and it has glass lenses. It was an instrument
-constructed on this principle that we spoke of in a former lecture
-(p. 97). The reflecting telescope depends for its power upon a bright
-mirror at the lower end, and when using this instrument you look at
-the reflection of the stars in the mirror. It was a reflector like
-this that Herschel began to construct, and he engaged in the task with
-enthusiasm. His sister Caroline had come to live with him, and she used
-to help him at his work. So much in earnest was he that he used to rush
-into his workshop directly he came home from a concert, and without
-taking off his best clothes he would plunge into the grinding and
-polishing of his mirrors. His sister tried to keep the house as tidy as
-possible, but Herschel put up a carpenter’s shop in the drawing-room,
-and turning-lathes in the best bedroom. At last he succeeded. He made
-a mirror of the right shape, and found that it exhibited the stars
-properly. It was not a looking-glass in the ordinary sense, with glass
-on one side and quicksilver on the other. The mirror that Herschel
-constructed was entirely of metal. It consisted of a mixture of two
-parts of copper with one of tin.
-
-[Illustration: FIG. 69.--The Mirror.]
-
-The copper has first to be melted in a furnace, for the metal must be
-above a red heat before it will begin to run. Then the tin has to be
-carefully added, and the casting of the mirror is effected by pouring
-the molten metal into a flat mould. Thus the rough mirror is obtained,
-which in Herschel’s earlier telescopes seems to have been about six
-or seven inches in diameter, and nearly an inch thick. Though copper
-is such a tough substance, and though tin is also tough, yet when
-melted together to make speculum metal, as this alloy is called, they
-produce an exceedingly hard and brittle material. When we remember
-that we could never break a copper penny piece by throwing it down on
-the flags, it may seem strange that the “speculum metal” should be so
-exceedingly brittle. A piece the size of a penny would be more brittle
-than a bit of glass of the same dimensions, and when the speculum is
-cast, unless it is cooled very carefully, it will fly into pieces.
-Herein lay one of the difficulties that Herschel encountered. Speculum
-metal must be put into an oven as soon as the casting has become solid,
-and then the heat is gradually allowed to abate. When the speculum has
-been at last obtained, next follows the labor of giving it the true
-figure and polish. It is not only more fragile than glass, but it is
-also quite as hard, and therefore the grinding is a tedious operation.
-First the surface has to be ground with coarse sand, and then with
-emery, which is gradually made finer and finer until the true figure
-has been given (Fig. 69). The mirror is then somewhat basin-shaped,
-but the depression is very slight. For example, in a mirror six inches
-across the depression at the centre would perhaps be not more than the
-twentieth of an inch. Small though this depression is, yet it has to be
-made with exactness. In fact, if it were wrong at any point by so much
-as the tenth of the thickness of this sheet of paper, the telescope
-would not perform accurately. The tool that is used in grinding is
-made of cast iron, and has been turned in a lathe to the right shape.
-It is divided into squares in the manner shown in Fig. 70. After the
-grinding comes the polishing, and this is effected with a tool like the
-grinder in shape. This has to be covered over with little squares of
-pitch, so that when warmed and put down on the mirror it is soft enough
-to receive the right shape. Some rouge and water is spread over the
-mirror, and the polisher is worked backward and forward with the hand
-until a brilliant surface is obtained.
-
-[Illustration: FIG. 70.--The Grinding Tool.]
-
-When the amateur astronomer has completed this part of the task, all
-the great difficulties about his telescope are conquered. The tube may
-be made of wood, and, indeed, a square tube will do just as well as a
-round one. He must also provide for the top of the tube a small mirror,
-which has to be perfectly flat. The preparation of this requires much
-care, because it is not so easy as one might suppose to obtain an
-accurately flat surface. One way of doing this is to get three pieces,
-and grind each two of them together until every pair will touch all
-over; then they will certainly all be flat. One more part you want,
-and that is an eyepiece. This presents no difficulty. A single glass
-lens can be made to answer and your telescope is complete.
-
-
-THE DISCOVERY OF URANUS.
-
-It was in the year 1774 that Herschel first had a view of the heavens
-through the telescope he had himself constructed. During the early part
-of his career he does not seem to have made any important discoveries.
-He was gradually preparing himself for the great achievement by which
-his name became famous.
-
-It was on the 13th of March, 1781, that the organist of the Octagon
-Chapel at Bath turned his telescope on the constellation of the Twins,
-and began to look at one star after another. You must know that a star
-merely looks like a little point in a telescope; even the greatest
-instrument will only make a star look brighter, and will never show
-it with a perceptible disk. In looking over the stars that night,
-Herschel’s attention was arrested by one object that did look larger
-when magnified, and therefore was not a star. The only other objects
-which would behave in this way were the planets, or possibly a comet.
-Indeed, at first Herschel imagined that what he saw must be a comet. It
-could hardly have occurred to him that he was to have such good fortune
-as to discover a new planet. The five great planets had been known from
-all antiquity. Was it reasonable to suppose that there could be yet
-another that had never been perceived? Fortunately, there was a test
-available. A star remains in the same place from night to night and
-from year to year; while a planet, as we have already had occasion to
-mention, is a body which is wandering about. The movements of a planet
-are, however, not at all like those of a comet. To decide on the nature
-of Herschel’s newly discovered body, it was sufficient to observe the
-character of its motion. A few nights sufficed to do this. The position
-of the body was carefully marked relatively to the neighboring stars,
-and it was soon shown that it was a planet.
-
-Here, then, a great discovery was made. A new planet, now called
-Uranus, was added to our system. It would be nothing to discover a
-new star. You might as well talk of discovering a new grain of sand
-on the seashore. The stars are in untold myriads. They are so far off
-that they have no relation whatever to our system, which is presided
-over by the sun. But by the detection of a new planet, revolving far
-outside Saturn, Herschel showed that a new and most interesting member
-had to be added to the five old planets which have been known from the
-earliest records of history.
-
-It may well be imagined that a discovery so startling as this excited
-astonishment throughout the scientific world. “Who is this Bath
-organist?” everybody asked. Accounts of him and his discoveries
-appeared in the papers. His fellow-citizens were not so familiar with
-the name as we are, happily, now; and the spelling of the unusual name
-showed many varieties. When George III. heard of Herschel’s great
-achievement, he directed the astronomer to be summoned to Windsor,
-that the King might receive an account of the wonderful discovery from
-the lips of the discoverer himself. Herschel of course obeyed, and he
-brought with him his famous telescope, and also a map of the whole
-solar system, to show to the King. No doubt he thought that his Majesty
-had probably not paid much attention to astronomy. Herschel was,
-therefore, prepared to explain to the King what it would be necessary
-for him to know before he could fully appreciate the magnitude of the
-discovery.
-
-You will remember that Herschel while still a boy had deserted from the
-army, many years previously. It appears that the King had learned this
-fact in some way, so that when Herschel was ushered into his presence
-his Majesty said that before the great astronomer could discuss science
-there was a little matter of business that must be disposed of. The
-King accordingly handed Herschel a paper, in which he was, I dare say,
-greatly surprised to find a pardon to the deserter written out by the
-King himself.
-
-Then Herschel unfolded his wonderful discovery, which the King
-thoroughly appreciated, and in the evening the telescope was set up in
-the gardens, and the glories of the heavens were displayed. Herschel
-made a most favorable impression on his Majesty, and when the King
-told the ladies of the Castle next day of all that Herschel had shown
-him, their astronomical ardor was also aroused, and they asked to see
-through the marvellous tube. Of course Herschel was ready to comply,
-and the telescope was accordingly carried to the windows of the Queen’s
-apartments at Windsor, which would have commanded a fine view if the
-clouds had not been in the way, which they unfortunately were. Even
-for royalty the clouds would not disperse, so what was to be done?
-Herschel was equal to the occasion. He specially wanted to exhibit
-Saturn, for it is one of the most beautiful objects in the sky, and
-will fascinate any intelligent beholder. No astronomers would have been
-able to see Saturn through the clouds, but Herschel did not disappoint
-his visitors; he directed the instrument, not to the sky (nothing was
-there to be seen); he turned it towards a distant garden wall. Now what
-would you expect to see by looking through a telescope at a garden
-wall--bricks, perhaps, or ivy? What these ladies saw was a beautiful
-image of Saturn, his globe in the centre and his rings all complete,
-forming so true a resemblance to the planet that even an experienced
-astronomer might have been deceived. In the afternoon Herschel had seen
-that the clouds were thick, and that there would be little probability
-of using the telescope properly. Accordingly he cut out a little
-image of Saturn, illuminated it by lamps, and set it up at a suitable
-distance on a garden wall.
-
-Herschel’s visit to Windsor was productive of important consequences.
-The King said it was a pity that so great an astronomer should devote
-himself to music, and that it would be far better for him to give up
-that profession and come and live at Windsor. His Majesty promised that
-he would pay him a salary, and he also undertook to provide the cost of
-erecting great telescopes. His faithful sister Caroline came with him
-as his assistant, and also received some bounty from the King. From
-that moment Herschel renounced all his musical business, and devoted
-himself to his great life-task of observing the heavens.
-
-He built telescopes of proportions far exceeding those that had ever
-been then thought of. He used to stand at night in the open air from
-dusk to dawn gazing down the tube of his mighty reflector, watching the
-stars and other objects in the heavens as they moved past. He would
-dictate what he saw to Caroline, who sat near him. It was her business
-to write down his notes and to record the position of the objects which
-he was describing. Sometimes, she tells us, the cold was so great that
-the ink used to freeze in her pen when she was at this work. Until he
-became a very old man, Herschel devoted himself to his astronomical
-labors. His discoveries are to be counted by thousands, though not one
-of them was so striking or so important as the detection of the new
-planet which first brought him fame.
-
-The question of a name for the addition to the sun’s family had, of
-course, to be settled. Herschel had surely a right to be heard at the
-christening, and as a compliment to his Majesty he named the stranger
-the Georgium Sidus. So, indeed, for a brief while, the planet was
-actually styled. The Continental astronomers, however, would not accept
-this designation; all the other planets were named after ancient
-divinities, and it was thought that the King of England would seem
-oddly associated with Jupiter and Saturn; perhaps also they considered
-that the British dominions, on which the sun never sets, were already
-quite large enough, without further extension to the celestial
-regions. Accordingly a consultation was held, the result of which was
-that George III. was deprived of his planetary honors, and the body was
-given the name of Uranus, which, by universal consent, it now bears.
-
-The planet Uranus lies just on the verge of visibility with the
-unaided eye. It can sometimes be glimpsed like a faint star, and, of
-course, with a telescope it is readily perceived. Many generations of
-astronomers before Herschel’s time had been observing the heavens,
-making maps of the stars, and compiling great catalogues in which the
-places of the stars were accurately put down. It often happened that
-Uranus came under their notice, but it never occurred to them that
-what seemed so like a star was really a planet. I have, no doubt, said
-that Uranus looked unlike a star when Herschel examined it; but then
-that was because Herschel was a particularly skilful astronomer. To
-an observer of a more ordinary type Uranus would not present any very
-remarkable appearance, and would be passed over merely as a small star.
-In fact, the planet was thus observed not once or twice, but no fewer
-than seventeen times, before the acute eye of Herschel perceived its
-true character. On many previous occasions the planet had been noted
-as a star by astronomers who are in every way entitled to our respect.
-It required a Herschel, determined to see everything in the very best
-manner, to grasp the discovery which eluded so many others.
-
-When Uranus was observed on these former occasions and mistaken for
-a star, its place had been carefully put down. These records are at
-present of the utmost use, because they show the past history of the
-planet; and they appear all the more valuable when we remember that
-Uranus requires no less than eighty-four years to accomplish a single
-revolution around the sun. Thus, since the planet was discovered in
-1781, it had completed one revolution by 1865, and is now (1899) about
-one-third of the way around another. The earlier observations extend
-backwards almost 200 years, so that altogether we have more or less
-information about the movements of the planet during the completion of
-two circuits and a half.
-
-Uranus is a great deal bigger than the earth, as you will see in the
-view of the comparative sizes of the planets (Fig. 47). It appears to
-be of a bluish hue, but we cannot tell whether it turns round on its
-axis, or rather, I should say, we are not able to _see_ whether it
-turns round on its axis; for we can hardly doubt that it does so.
-
-Notwithstanding that Uranus is at so great a distance from the earth,
-we have been able to put this planet, no less than the nearer ones,
-in the weighing scales, and we assert with confidence that Uranus is
-fifteen times as heavy as our earth. We are indebted to the satellites
-for this information.
-
-
-THE SATELLITES OF URANUS.
-
-You must use a very good telescope to see the satellites of Uranus.
-They are four in number, bearing the names of Ariel, Umbriel, Titania,
-and Oberon. The innermost of these, Ariel, completes a journey round
-the planet in two days and a half; Oberon, the most distant, requires
-thirteen days and a half. A planet is always tending to pull its
-satellite down, and the satellite is kept from falling by the speed
-with which it revolves. The heavier the planet, the faster must its
-satellites go round. Thus, to take an illustration from our own moon,
-we know that, if the earth were to be made four times heavier than it
-is, the moon would have to spin round twice as fast as it does, in
-order to remain in the same orbit. The speed with which the satellites
-of Uranus revolve accordingly affords a measure of the mass of the
-planet. Were Uranus heavier than he is, his satellites would revolve
-more quickly than they do; were he lighter, the satellites would take a
-longer period to go round.
-
-Uranus also seems to be greatly swollen by clouds, in the same manner
-as are both Jupiter and Saturn; in fact, if our earth was as big as
-Uranus, it would weigh four or five times as much as Uranus does. Hence
-we are certain that Uranus must consist of materials less dense on the
-whole than are those of which our earth is made.
-
-There is another singular circumstance connected with the moons of
-Uranus. I have told you how every body revolving round another by
-gravitation will describe an ellipse; but, of course, there are many
-different kinds of this curve, and some may be nearly circles. There
-is nothing whatever to prevent a satellite from revolving around its
-primary in an exact circle if it be started properly; that is, in the
-right direction and with the right speed. All the four satellites of
-this planet seem to revolve in circles so perfect that we can make an
-accurate picture of this system with a pair of compasses. It is further
-to be noticed that the four circles seem to lie exactly in the same
-plane. The orbits of the other great planets and of their satellites
-lie in planes inclined at angles of less than 35° to the ecliptic, the
-plane in which the earth moves. Here again the satellites of Uranus are
-exceptional. The plane in which they are contained stands up almost
-squarely to the plane in which the motion of the planet is performed.
-The moons of Uranus seem to have got a twist, from some accidental
-circumstance for which we are not able to account.
-
-
-THE DISCOVERY OF NEPTUNE.
-
-The boundaries of the solar system had been much extended by the
-discovery of Uranus, but they were destined to receive still further
-enlargement by the detection of another vast planet, revolving far
-outside Uranus, the orbit of which forms, according to our present
-knowledge, the outline of the planetary system.
-
-I have here to describe one of the greatest discoveries that have ever
-been made. It is not the magnificence of the outermost planet itself
-that I refer to, though, indeed, it is bigger than Uranus. I am rather
-thinking of the _way_ in which the discovery was made. I do not mean
-any disrespect to Herschel when I say that the discovery of Uranus was
-chiefly a stroke of good fortune; but I may be permitted to describe
-it in this manner by way of emphasizing as strongly as I can how
-utterly different was the train of ideas which led to the discovery of
-Neptune. Herschel merely looked at one star after another till suddenly
-he dropped on the planet, having beforehand not the slightest notion
-that any such planet was likely to exist. But Neptune was shown to
-exist before it was ever seen, and, in fact, the man that first saw
-the planet, and knew it to be a planet, was not the discoverer. This
-is rather a difficult subject; and it would take you years of hard
-study to be able to understand the train of reasoning by which Neptune
-was found. I shall, however, make an attempt to explain this matter
-sufficiently to give at least some idea of the kind of problem that had
-to be solved.
-
-You will remember that law of Kepler which tells us that every planet
-moves round the sun in an ellipse. If the planet be uninterfered
-with in any way and guided only by the attraction of the sun, it
-will forever continue to describe precisely the same ellipse without
-the slightest alteration. It was ascertained that the path which
-Uranus followed was not always regular. The early observations of the
-planet, when it was mistaken for a star, have here been of the utmost
-service. They have indicated the ellipse which Uranus described the
-last time it went round, and our modern observations have taught us
-the path which the planet is at present describing. These two ellipses
-are slightly different, and the consequence is that, supposing we
-take the observations of Uranus made 100 years ago, and calculate
-from them where Uranus ought to be now, we find that the planet is
-a little astray. Astronomers are not accustomed to be wrong in such
-calculations, and when discrepancies arise, the first thing to be
-done is to see what has caused them. It is certain that the position
-in which Uranus is found this very night, for example, is not what it
-would have been had the sun alone been guiding the planet. Perhaps you
-will think that it is impossible for reliable computations to be made
-about such matters; but I assure you they can, and the very fact that
-the motion of Uranus appeared to be irregular made it interesting to
-try and find out the cause of the disturbance.
-
-I have already explained, when speaking about Mars (p. 187), that
-there is an attraction between every two bodies, but in the group of
-planets to which the earth belongs the sun’s attraction is so much
-stronger than any other force that all the movements are guided mainly
-by it. Nevertheless it is true that not only does the sun pull our
-earth and all the other planets as well, but all the planets, including
-the earth, are pulling one another. In fact, there is an incessant
-struggle going on in the family party. Fortunately the sun is so much
-more powerful than any other member, that he keeps them all pretty
-well in order; and unless you look very carefully you will not see the
-effects of the little struggles that are going on between every pair
-of the system. Our earth itself is pulled and swayed to and fro by the
-actions of its brothers and sisters. It is dragged perhaps a thousand
-or two thousand miles this way by Jupiter, or it gets a good tug in the
-other direction by Venus. Mars and Saturn also do their little best
-to force the earth away from its strict path. However, our earth does
-not suffer much from these irregularities. It pursues its route fairly
-enough, just as a coach from London to Brighton will get safely to its
-destination notwithstanding the fact that it has to swerve a little
-from its path whenever it meets other vehicles on the way, or when the
-coachman wishes to avoid a piece of the road on which stones have been
-freshly laid down.
-
-The track followed by Uranus was found to be somewhat irregular,
-like that of every other planet. Jupiter gave it a pull, and so did
-Saturn, and at first it was thought that the irregularities which were
-perceived could be explained by the action of these planets, so big and
-so well known. Here is a question for calculation; it involves a very
-long and a very hard piece of work, but it is possible to estimate how
-far each of the other planets is capable of dragging Uranus from its
-path. Is it not remarkable that by working out long calculations we
-should be able to find what one planet hundreds of millions of miles
-away was able to do to another planet still further off, and not only
-for to-day or yesterday, but for past time extending over more than a
-century? If, however, you will listen to me a little longer, I think I
-shall give you a proof that these sums could be worked out correctly.
-
-When the calculations had been made which showed how much the known
-planets could disturb Uranus, it was found that there were still some
-deviations of the planet that remained unexplained. They were not
-large; they only amounted to showing that the body was just a little
-astray from the spot where the calculations indicated it should be. The
-rest of astronomy was so perfect, and the law of attraction prevailed
-so universally, that it was thought the law of attraction must provide
-some way of explaining the behavior of Uranus. He could not have left
-his track of his own accord; therefore there must be some agency at
-work upon him of which we did not know. What could this unknown source
-of disturbance be? Every such trouble had hitherto been found to be a
-consequence of the attraction of gravitation; therefore there must be
-some unknown body pulling at Uranus which no one had ever seen. Where
-could it be? How was it to be discovered? Such were the questions that
-were asked, and they were answered in a most satisfactory manner.
-
-First of all, what sort of body could it be that was pulling Uranus?
-It is obvious that none of the stars would be competent to produce
-so great an effect; they are all so far off that they have nothing
-whatever to say to any of the domestic matters in our little solar
-system, which is simply a group by itself. It would be more reasonable
-to suppose that there must be yet another planet which nobody had ever
-recognized, but which affected Uranus so as to account for his truant
-behavior. To begin to search for this planet with telescopes without
-some guidance would be futile; in fact, astronomers had been scanning
-the heavens for planets for nearly fifty years, and though several
-had been discovered, they all belonged to the zone of little planets,
-and none of them were big enough to pull Uranus about appreciably. Of
-course, if all the stars could be blotted out of the sky, so that
-nothing but planets were left, then, by sweeping the telescope over
-the heavens, every planet that exists might be speedily found. The
-difficulty is that the planets, which are either small or very distant,
-look so like the stars that it is impossible to recognize them among
-the millions of glittering points in the sky. It was, however, hoped
-that the unknown planet would be large enough to be visible in the
-telescope, if only we knew exactly where to point it.
-
-Two illustrious astronomers, Adams of Cambridge, and Leverrier of
-Paris, both separately undertook an astonishing piece of calculation.
-They tried to find out the position of the unknown planet from the
-mere fact that it deranged Uranus in a particular way. I dare say many
-of those who are reading this book have learned simple equations in
-algebra, and they have worked such questions as to find the length
-of a pole, half of which is in mud, a quarter in water, and ten
-feet above the water. Those who know this much can perhaps realize
-the problem that had to be solved in trying to discover the unknown
-planet. So difficult a question as this had to be solved in a way that
-your masters would hardly allow you to use when working your sums in
-algebra. I do not think they would let you make a series of guesses.
-Let us try 20 feet, for instance, as the length of the pole; that will
-make 10 feet in the mud, 5 feet in the water, and 5 feet outside. This
-will not do; it is not enough; we must try again; and after another
-guess or two, we see that a pole 40 feet long will exactly answer. We
-do not use this method of guessing in algebra, because solving the
-simple equation is a much better method. Adams and Leverrier found
-that to discover the unknown planet was a question so very difficult,
-that they were obliged to use a sort of guessing, but very intelligent
-guessing, I need hardly assure you. They proceeded in this way
-(Fig. 71). They would draw a circle outside the path of Uranus, and
-then suppose that a planet was revolving in that circle. Its effect
-upon Uranus would then be calculated, and it would be found whether
-the observed irregularities could be in this manner accounted for. The
-first planet they tried was not the right one; then they began again
-with another, until at last, after many trials and much very hard work,
-they saw that there might be a planet in a particular path far outside
-Uranus, such that if this planet were of the right weight and moving
-with the right speed, then it would pull Uranus exactly in the way
-that astronomers had observed it to be pulled. They found at last that
-there could be little doubt about the matter; for this unknown body
-would account for all the facts. Then, indeed, they had solved their
-equation; they had found the unknown.
-
-[Illustration: FIG. 71.--Orbits of Uranus and Neptune.]
-
-The two great astronomers had thus discovered a planet, but as yet it
-was only a planet on paper. Those who could judge of the subject had
-no doubt that the planet was really in the sky; but just as you like
-to prove that you have found the correct answer to your sum, so people
-were naturally anxious to prove the truth of this wonderful sum that
-Adams and Leverrier had worked out. This was to be done by actually
-seeing the planet of which the astronomers had asserted the existence.
-Leverrier calculated that the new planet in a certain night would be in
-a particular position on the sky. Accordingly he wrote to Dr. Galle, of
-the observatory at Berlin, requesting him on the evening in question
-to point his telescope to the very spot indicated, and there he would
-see a planet which human eyes had never before beheld. Of course, Dr.
-Galle was only too delighted to undertake so marvellous a commission.
-The evening was fine; the telescope was opened; it was directed towards
-the heavens; and there, in the very spot which the calculations
-of Leverrier had indicated, shone the beautiful little planet. At
-Cambridge arrangements had also been made to search for the new member
-of the solar system, in accordance with Professor Adams’ calculations.
-There also the planet that had given all this trouble to Uranus was
-brought to light. At first it looked like a star, as all such planets
-do; but that it was not a star was speedily proved, by the two tests
-which are sure indications of a planet. First the body was so moving
-that its position with respect to the adjacent stars was constantly
-changing. Then, when a strong magnifying power was placed on the
-telescope, the little object was seen, not to be a mere starlike point,
-but to expand into the little disk which shows us we are not looking at
-a distant sun, but at a world like our own.
-
-Was not this truly a great discovery? Have we not shown you how
-entitled the calculations of astronomers are to our respect, when we
-find that they actually discovered the existence of a majestic planet
-before the telescope had revealed it? See also the greatly increased
-interest that belongs to Herschel’s discovery of Uranus. We can hardly
-imagine anything that would have given more gratification to this old
-astronomer than to think that his Uranus should have given rise to a
-discovery even more splendid than his own. He died, however, more than
-twenty years before this achievement.
-
-The authorities who decide on such matters christened the new planet
-Neptune; and this body wanders round on the outskirts of our solar
-system, requiring for each journey a period of no less than 165 years.
-The circle thus described has a radius thirty times as great as that of
-the earth’s track.
-
-Neptune is altogether invisible to the unaided eye, but it is
-sufficiently bright to have been occasionally recorded as a star.
-Indeed, nearly fifty years before it was actually discovered to be a
-planet it had been included by the astronomer Lalande in a list of
-stars he was observing. A curious circumstance was afterwards brought
-to light. When reference was made to the books in which Lalande’s
-observations were written, it was found that he had observed this
-object twice, namely, on May 8 and May 10, 1785. Of course, if the
-object had indeed been a star its position on the two days would have
-been the same, but being a planet it had moved. When Lalande, on
-looking over his papers, saw that the places of this supposed star
-were different on the two nights, he concluded that he must have made
-a mistake on the first night, and accordingly treated the object as
-if the place on the 10th was the right one. Just think how narrowly
-Lalande missed making a discovery! Unhappily for his renown, he took it
-for granted that one or both of his observations were erroneous, and so
-they must have been if the object had been a star. But they were both
-right; it was the planet which had moved in the interval.
-
-As Neptune is half as far again from the earth as Uranus, we can hardly
-expect to learn much about the actual nature of the planet. We do know
-that it has four times the diameter of the earth, so that it exceeds
-the earth in the same proportion that the earth is larger than the moon.
-
-Like the other great planets, Neptune is also enveloped with copious
-clouds; in fact, it only weighs one-fifth part as much as it would do
-if it were made of materials as substantial as are those of the earth.
-Like our earth, Neptune is attended by one moon, which revolves round
-the planet in a little more than six days.
-
-The orbit of this great planet marks the boundary of our known system
-of planets. We have seen how the five great planets of antiquity have
-been increased in these modern days by the addition of two more, Uranus
-and Neptune, while the discovery of a multitude of small planets has
-given a further increase to the number of the sun’s family. We have
-still some other objects in our solar system to describe; some of them
-are excessively big; these are the comets. Some of them are exceedingly
-small; they are the shooting stars. We shall talk about comets and
-shooting stars in our next lecture.
-
-
-
-
-LECTURE V.
-
-COMETS AND SHOOTING STARS.
-
-  The Movements of a Comet--Encke’s Comet--The Great Comet of
-      Halley--How the Telegraph is used for Comets--The Parabola--The
-      Materials of a Comet--Meteors--What becomes of the Shooting
-      Stars--Grand Meteors--The Great November Showers--Other Great
-      Showers--Meteorites.
-
-
-THE MOVEMENTS OF A COMET.
-
-The planets are all massive globes, more or less flattened at the
-Poles; but now we have to talk about a multitude of objects of the
-most irregular shapes, and of the most flimsy description. We call
-them _comets_, and they exist in such numbers that an old astronomer
-has said “there were more comets in the sky than fishes in the sea,”
-though I think we cannot quite believe him. There is also another
-wide difference between planets and comets: planets move round in
-nearly circular ellipses, and not only do we know where a planet is
-to-night, but we know where it was a month ago, or a hundred years
-ago, or where it will be in a hundred years or a thousand years to
-come. All such movements are conducted with conspicuous regularity
-and order; but now we are to speak of bodies which generally come in
-upon us in the most uncertain and irregular fashion. They visit us we
-hardly know whence, except that it is from outer space, and they are
-adorned in a glittering raiment, almost spiritual in its texture.
-They are always changing their appearance in a baffling, but still
-very fascinating manner. If an artist tries to draw a comet, he will
-have hardly finished his picture of it in one charming robe before he
-finds it arrayed in another. The astronomer has also his complaints to
-make against the comets. I have told you how thoroughly we can rely on
-the movements of the planets, but comets often play sad pranks with
-our calculations. They sometimes take the astronomers by surprise, and
-blaze out with their long tails just when we do not expect them. Then
-by way of compensation they frequently disappoint us by not appearing
-when they have been most anxiously looked for.
-
-After a voyage through space the comet at length begins to draw in
-towards the central parts of our system, and as it approaches the sun,
-its pace becomes gradually greater and greater; in fact, as the body
-sweeps round the sun the speed is sometimes 20,000 times faster than
-that of an express train. It is sometimes more than 1000 times as fast
-as the swiftest of rifle bullets, occasionally attaining the rate of
-200 miles a second. The closer the comet goes to the sun, the faster
-it moves; and a case has been known in which a comet, after coming in
-for an incalculable duration of time towards the sun, has acquired a
-speed so tremendous, that in two hours it has whirled round the sun and
-has commenced to return to the depths of outer space. This terrific
-outburst of speed does not last long. A pace which near the sun is
-20,000 times that of our express trains diminishes to 10,000 times, to
-fifty times, to ten times that pace; while in the outermost part of
-its path the comet seems to creep along so slowly that we might think
-it had been fatigued by its previous exertions.
-
-[Illustration: FIG. 72.--How the Comet’s Tail is disposed.]
-
-We have so often seen a stream of sparks stretching out along the track
-of a sky-rocket, that we might naturally suppose the tail of a comet
-streamed out along its path in a somewhat similar manner. This would
-be quite wrong. You see from Fig. 72 that the tail does not lie along
-the comet’s path, but is always directed outwards from the sun. If you
-will draw a line from the sun to the head of the comet and follow the
-direction of the line, it shows the way in which the tail is arranged.
-You will also notice how the tail of the comet seems to grow in length
-as it approaches the sun. When the comet is first seen, the tail is
-often a very insignificant affair, but it shoots out with enormous
-rapidity until it becomes many millions of miles long by the time the
-comet is whirling round the sun. Those glories soon begin to wane as
-the comet flies outward; the tail gradually vanishes, and the wanderer
-retreats again to the depths of space in the same undecorated condition
-as that in which it first approached.
-
-When a comet appears, it is always a matter of interest to see whether
-it is an entirely new object, or whether it may not be only another
-return of a comet which has paid us one or more previous visits. The
-question then arises as to how they are to be identified. Here we see a
-wide contrast between unsubstantial bodies like comets and the weighty
-and stately planets. Sketches of the various planets or of the face of
-the sun, though they might show slight differences from time to time,
-are still always sufficiently characteristic, just as a photographic
-portrait will identify the individual, even though the lapse of years
-will bring some changes in his appearance. But the drawing of a comet
-is almost useless for identification. You might as well try to identify
-a cloud or a puff of smoke by making a picture of it. Make a drawing of
-a comet at one appearance, and sketch particularly the ample tail with
-which it is provided. The next time the comet comes round it may very
-possibly have two tails, or possibly no tail at all. We are therefore
-unable to place any reliance on the comet’s personal appearance in our
-efforts to identify it. The highway which it follows through the sky
-affords the only means of recognition; for the comet, if undisturbed
-by other objects, will never change its actual orbit. But even this
-method of identification often fails, for it not unfrequently happens
-that during its erratic movements the comet gets into fearful trouble
-with other heavenly bodies. In such cases the poor comet is sometimes
-driven so completely out of its road that it has to make for itself
-an entirely new path, and our efforts to identify it are plunged in
-confusion. It has happened that a second comet or even a third will be
-found in nearly the same track, but whether these are wholly different,
-or whether they are merely parts of the same original object, it is
-often impossible to determine.
-
-The great majority of comets are only to be seen with a telescope, and
-hardly a year passes without the detection of at least a few of these
-faint objects. The number of really brilliant comets that can be seen
-in a lifetime could, however, be counted on the fingers.
-
-
-ENCKE’S COMET.
-
-We have already alluded to a little body called Encke’s comet, which
-was discovered by an astronomer at Marseilles. It was in the year 1818
-that he was scanning the heavens with a small telescope, when an object
-attracted his attention. It was not one of those grand long-tailed
-comets which every one notices; this body was so faint that it merely
-appeared as a very small cloud of light, and was recognized as a comet
-by the fact that it was moving. It happens that there are other bodies
-in the sky very like comets; we call them nebulæ, and we shall have
-something to say about them afterwards. But it is remarkable that
-just as a planet is liable to be mistaken for a star, so a comet is
-liable to be mistaken for a nebula. However, in each case the fact of
-its movement is the test by which the planet or the comet is at once
-detected. A nebula stays always in the same spot, like a star, while a
-comet is incessantly moving. In fact, with a telescope you can actually
-watch a comet stealing past the stars that lie near it. You know that
-an object a very long way off may appear to move slowly, though in
-reality it is moving very rapidly. Look at a steamer near the horizon
-at sea. In the course of a minute or two it will not appear to have
-shifted its position to any appreciable extent, but that is because
-it is far off. If you were near the ship, you would see that it was
-dashing along at the rate of perhaps fifteen or twenty miles an hour.
-In a similar manner the comet seems to move slowly, because it is at
-such a great distance. As a matter of fact it is moving faster at the
-time we see it than any steamer, faster than any express train, faster
-than any cannon-ball. There were special reasons why the movements
-of Encke’s comet should be watched with peculiar care, and the track
-which it pursued be ascertained. If you can observe a comet three
-times and measure its position in the sky, the movement of that comet
-is completely determined. Perhaps I should say would be determined if
-the comet were let alone, which, unfortunately, is not often the case.
-Indeed, you may remember how I told you some of the misadventures of
-this very comet when we were speaking about the planet Mercury. Encke’s
-comet comes round in a period of a little more than three years, and it
-gives us some curious information that has been ascertained during its
-journeys. One of the facts we have thus learned is so important that we
-cannot omit to notice it (Fig. 73).
-
-[Illustration: FIG. 73.--The Orbit of Encke’s Comet.]
-
-At increasing heights above the earth’s surface there is gradually
-less and less air; until at last, at about 200 or 300 miles above the
-surface on which we dwell, there would be none. You might as well try
-to quench your thirst by drinking out of an empty cup as attempt to
-breathe in the open space which begins a few hundred miles aloft.
-In open space motion could take place quite freely. Down here the
-resistance of the air is a great impediment to movement, especially
-when very rapid. A heavy cannon-ball is checked and robbed of its pace
-by having to plough its way through our dense atmosphere. The motion
-is arrested in the same way, though not of course to the same degree,
-as if the cannon-ball had been fired into water. Unsubstantial objects
-are, of course, impeded by the air to a far greater extent than such
-heavy bodies as cannon-balls. You know that you cannot throw a handful
-of feathers across the road in the same way that you could throw a
-handful of gravel. The light feathers cannot force their way through
-the air so well as the pebbles. A body so flimsy as a comet would
-never be able to push its way through an atmosphere like ours; but out
-in empty space the comet meets with no resistance during the greater
-part of its path. Accordingly, though it has little more substance
-than a will-o’-the-wisp, the comet pursues its journey with as much
-resolute dignity as if it were made of cast iron. If in any part of
-its track the body should have to pierce its way through any material
-like even the thinnest possible air, then the unsubstantial nature of
-the cometary materials would be at once shown. The motion would be
-impeded, and the body’s path would be changed. In this way a comet may
-be made very instructive, for it will show whether space is really
-so empty as we sometimes suppose it to be. During the greater part
-of its course the flimsy little Encke tears along with such ease and
-speed that there seems to be nothing to impede it, and thus we learn
-that space is generally empty. However, when the comet begins to wheel
-around the sun, the freedom of its movements seems to receive a check.
-The unsubstantial object has to force its way with a difficulty that
-it did not experience so long as it was moving round the greater part
-of its orbit. We thus learn that there is a thin diffused atmosphere
-surrounding the sun. We cannot, indeed, say that it is like our air.
-Its composition is quite different, and almost the only way we know
-of its existence is by the evidence which this comet affords. In a
-former lecture I showed how Encke’s comet told us the mass of the
-planet Mercury. Now we see how the travels of the same body give us
-information about the sun himself. I ought, however, to add that some
-more recent observations seem not to have confirmed the belief that
-there is the resistance of the kind we have just been considering.
-
-
-THE GREAT COMET OF HALLEY.
-
-I dare say you would think it more interesting to talk about some big
-and bright comets rather than about objects so faint as that of Encke.
-It unfortunately happens that most of the fine comets pay our system
-only a single visit. There is only one of the really splendid objects
-of this kind that comes back to us with anything like regularity.
-
-It was last seen in the year 1835, and I am glad to tell you that it
-is coming again; it is expected about the year 1910. You may ask, How
-can we feel sure that such a prediction as I have mentioned will turn
-out correctly? The fact is that this comet has been watched for a great
-many centuries. We find ancient records, some of them nearly 2000 years
-old, of the appearance of grand comets, and several of these are found
-to fit in with the supposition that there is a body which accomplishes
-its journey in a period of about seventy-five or seventy-six years. Of
-course there are thousands of other comets recorded in these old books
-as well; but what I mean is that among the records many are found which
-clearly indicate some successive returns of this particular body.
-
-I will explain how the movements of this comet were discovered. There
-was a great astronomer called Halley, who lived two hundred years
-ago, and in the year 1682 he, like every one else, was looking with
-admiration at a splendid comet with a magnificent tail which adorned
-the sky in that year. At the observatories, of course, they diligently
-set down the positions of the comet, which they ascertained by
-carefully measuring it with telescopes. Halley first calculated the
-highway which this comet followed through the heavens, and then he
-looked at the list of old comets that had been seen before. He thus
-found that in 1607--that was, seventy-five years earlier--a great
-comet had also appeared, the path of which seemed much the same as
-that which he found for the body that he had himself observed. This
-was a remarkable fact, and it became still more significant when he
-discovered that seventy-six years earlier--namely, in 1531--another
-great comet had been recorded, which moved in a path also agreeing
-with those of 1607 and 1682. It then occurred to Halley that possibly
-these were not three different objects, but only different exhibitions
-of one and the same, which moved round in the period of seventy-five or
-seventy-six years.
-
-There is a test which an astronomer can often apply in the proof
-of his theory, and it is a very severe test. He will not only show
-himself to be wrong if it fails, but he will also make himself somewhat
-ridiculous. Halley ventured to submit his reputation to this ordeal. He
-prophesied that the comet would appear again in another seventy-five
-or seventy-six years. He knew that he would, of course, be dead long
-before 1758 should arrive; but when he ventured to make the prediction,
-he said that he hoped posterity would not refuse to admit that this
-discovery had been made by an Englishman.
-
-You can easily imagine that as 1758 drew near, great interest was
-excited among astronomers to see if the prediction of Halley would be
-fulfilled. We are accustomed in these days to find many astronomical
-events foretold with the same sort of punctuality as we expect in
-railway time-tables. The Nautical Almanac is full of such prophecies,
-and we find them universally fulfilled. Even now, however, we are not
-able to set forth our time-tables for comets with the same confidence
-that we show when issuing them for the sun, the moon, or the stars.
-How astonishing, then, must Halley’s prediction have seemed! Here was
-a vast comet which had to make a voyage through space to the extent of
-many hundreds of millions of miles. For three-quarters of a century
-it would be utterly invisible in the greatest telescopes, and the only
-way in which it could be perceived was by figures and calculations
-which enabled the mind’s eye to follow the hidden body all around its
-mysterious track. For fifty, or sixty, or seventy years nothing had
-been seen of the comet, nor, indeed, was anything expected to be seen
-of it; but as seventy-one, and seventy-two, and seventy-three years
-had passed, it was felt that the wanderer, though still unseen, must
-be rapidly drawing near. The problem was made more difficult for those
-skilful mathematicians who essayed to calculate it by the fact that the
-comet approached the thoroughfares where the planets circulate; and, of
-course, the flimsy object would be pulled hither and thither out of its
-path by the attractions of the weighty bodies. It was computed that the
-influence of Saturn alone was sufficient to delay the comet for more
-than three months, while it appeared that the attraction of Jupiter was
-potent enough to retard the expected event for a year and a half more.
-Was it not wonderful that mathematicians should be able to find out all
-these facts from merely knowing the track which the comet was expected
-to follow? Clairaut, who devoted himself to this problem, suggested
-that there might also be some disturbances from other causes of which
-he did not know, and that consequently the expected return of the comet
-might be a month wrong either way. Great indeed was the admiration in
-astronomical circles when, true to prediction, the comet blazed upon
-the world within the limits of time Clairaut had specified.
-
-The remarkable fulfilment of this prophecy entitles us to speak with
-confidence about the past performances of this comet. Among all the
-apparitions of Halley’s comet for the last two thousand years, perhaps
-the most remarkable is that which took place in the year 1066. I am
-sure you will all remember this date in your English history; it
-was the year of the Conquest. In those days they did not understand
-astronomy as we understand it now; they used to think of a comet as
-a fearful portent of evil, sent to threaten some frightful calamity;
-such as a pestilence, a war, a famine, or something else equally
-disagreeable. Hence in the year of the Conquest the appearance of so
-terrific an object in the sky was a very significant omen. Attention
-was concentrated upon the spectacle, and a picture of Halley’s comet
-as it appeared to the somewhat terrified imaginations of the people of
-those days has been preserved. There is a celebrated tapestry at Bayeux
-on which historical incidents are represented by beautifully worked
-pictures. On this fabric we have a view of Halley’s comet in a quaint
-and rather ludicrous aspect. You will read of this comet also in the
-early pages of Tennyson’s “Harold.”
-
-
-HOW THE TELEGRAPH IS USED FOR COMETS.
-
-In these days the study of comets is prosecuted with energy. Over the
-world observatories are situated, and whenever a comet is discovered,
-tidings of the event are diffused among those likely to be interested.
-Suppose that one is discovered in the southern hemisphere, the
-astronomers then write to warn the northern observatories of the
-event. But comets often move faster than her Majesty’s mails, so that
-the telegraph has to be put into requisition. The kind of message is
-one which shall show the position and the movements of the body. It
-necessarily involves a good many figures and words, and consequently
-it is desirable to abbreviate as much as possible for the sake of
-economy. There is a further difficulty in using the telegraph, because
-the messages are not of an intelligible description to those not
-specially versed in astronomy. Skilful as the telegraph clerks are,
-they can hardly be expected to be familiar with the technicalities of
-astronomers. The clerk at the receiving end is handed a message which
-he does not understand very clearly. The clerk at the other end does
-not understand the message which is delivered to him, and between them
-it has happened that they have transformed the message into something
-which not only they do not understand, but which, unfortunately, nobody
-else can understand either. These difficulties have been surmounted by
-an agreement between astronomers, which is so simple and interesting
-that I must mention it.
-
-The kind of message that expresses the place of a comet will consist of
-sentences something of this kind: “One hundred and twenty-three degrees
-and forty-five minutes.” Surely it would be an advantage to be able to
-replace all these words by a single word, particularly if by doing so
-the risk of error would be diminished. This is what the astronomers’
-telegraphic arrangement enables them to accomplish. There is a certain
-excellent Dictionary known as Worcester’s. I am sure when Mr. Worcester
-arranged this work, he had not the slightest anticipation of an odd
-use to which it would occasionally be put. Every astronomer who is
-co-operating in the comet scheme must have a copy of the book. To send
-the message I have just referred to, he has to take up his Dictionary
-and look out page 123. Then he will count down the column until he
-comes to the forty-fifth word on that page, which he finds to be
-“constituent,” and according to this plan the message, or at least this
-part of it, is merely that one word, “constituent.” The astronomer who
-receives this message and wishes to interpret it takes up his copy of
-Worcester’s Dictionary and looks out for “constituent.” He sees that it
-is on page 123, and that it is the forty-fifth word down on that page;
-and therefore he knows that the interpretation of the message is to be
-one hundred and twenty-three degrees and forty-five minutes.
-
-
-THE PARABOLA.
-
-Generally speaking, great comets come to us once and are then never
-seen again. Such bodies do not move in closed ovals or ellipses, they
-follow another kind of curve, like that represented in Fig. 74. It
-is one that every boy ought to know. In fact, in one of his earliest
-accomplishments he learned how to make a parabola. It is true he did
-not call it by any name so fine as this, but every time a ball is
-thrown into the air it describes a part of the beautiful curve which
-geometers know by this word (Fig. 74). In fact, you could not throw
-a ball so that it should describe any other curve except a parabola.
-No boy could throw a stone in a truly horizontal line. It will always
-curve down a little, will always, in fact, be a portion of a parabola.
-
-[Illustration: FIG. 74.--The Path of a Projectile is a Parabola.]
-
-There are big parabolas and there are small ones. One of the shells
-which are thrown into a town when bombarded from a distance describes,
-as it rises and then slopes down again, part of a mighty parabola. So
-does a tennis ball thrown by the hand or struck by the racket; though
-here, indeed, I admit that a spin may be given to the ball which will
-somewhat detract from the simplicity of its movement. In playing
-baseball, a large part of the skill of the pitcher consists in throwing
-the ball in such a way that it shall not move in a parabola, but in
-some twisting curve by which he hopes to baffle his adversary. Setting
-aside these exceptions, and such another as the case of a body tossed
-straight up or dropped straight down, we may assert that the path of a
-projectile is a parabola.
-
-[Illustration: FIG. 75.--The Lighthouse Reflector.]
-
-There are some remarkable applications of the same curve for practical
-purposes. From our lighthouses we want to send beams off to sea, so
-as to guide ships into port. If we merely employed a lamp without
-concentrating its rays, we should have a very imperfect lighthouse,
-for the lamp scatters light about in all directions. Much of it goes
-straight up into the air, much of it would be directed inland; in fact,
-there is only an extremely small part of the entire number of rays
-that will naturally take the useful direction. We therefore require
-something round the lamp which shall catch the truant rays that are
-running away to idleness and loss, and shall concentrate them into the
-direction in which they will be useful to the mariner. An effective
-way of doing this is to furnish the lamp with a reflector. On its
-bright surface (Fig. 75) all the rays fall which would otherwise
-have gone astray, and each of them is properly redirected, where the
-sailors can see it. It is essential that the mirror shall do this work
-accurately, and this it will only do when it has been truly shaped so
-as to be a parabola.
-
-You will remember, also, how I described to you the reflector which
-Herschel made for his great telescope. The shape of the mirror must
-be most accurately worked, and it, too, must have a parabola for its
-section; so that you see this curve is one of importance in a variety
-of ways.
-
-But the grandest of all parabolas are those which the comets pursue.
-Unlike the ellipse, the parabola is an open curve; it has two branches
-stretching away and away forever, and always getting further apart. Of
-course, in the examples of this curve that I have given it is only a
-small part of the figure that is concerned. When you throw a stone it
-only describes that part of the parabola that lies between your hand
-and the spot where the stone hits the ground. It is just a part of the
-curve in the same way that a crescent may be a bit of a circle. It is
-to comets that we must look for the most complete illustration of the
-ample extent of a parabola.
-
-The shape of this grand curve will explain why so many comets only
-appear to us once. It is quite clear that if you begin to run round a
-closed racecourse, you may, if you continue your career long enough,
-pass and repass the starting-post thousands of times. Thus, comets
-which move in ellipses, and are consequently tracing closed curves,
-will pass the earth times without number. For this reason we may see
-them over and over again, as we do Encke’s comet or Halley’s comet. But
-suppose you were travelling along a road which, no matter how it may
-turn, never leads again into itself, then it is quite plain that, even
-if you were to continue your journey forever, you can never twice pass
-the same house on the roadside. That is exactly the condition in which
-most of the comets are moving. Their orbits are parabolas which bend
-round the sun; and, generally speaking, the sun is very close to the
-turning-point. The earth is also, comparatively speaking, close to the
-sun; so that while the comet is in that neighborhood we can sometimes
-see it. We do not see the comet for a long time before it approaches
-the sun, or for a long time after it has passed the sun. All we know,
-therefore, of its journey is that the two ends of the parabola stretch
-on and on forever into space. The comet is first perceived coming in
-along one of these branches to whirl round the sun; and after doing so,
-it retreats along the other branch, and gradually sinks into the depths
-of space.
-
-Why one of these mysterious wanderers should approach in such a hurry,
-and then why it should fly back again, can be partially explained
-without the aid of mathematics.
-
-Let us suppose that, at a distance of thousands of millions of miles,
-there floated a mass of flimsy material resembling that from which
-comets are made. Notwithstanding its vast distance from the sun, the
-attraction of that great body will extend thither. It is true the
-pull of the sun on the comet will be of the feeblest and slightest
-description, on account of the enormously great distance. Still, the
-comet will respond in some degree, and will commence gradually to move
-in the direction in which the sun invites it. Perhaps centuries, or
-perhaps thousands, or even tens of thousands, of years will elapse
-before the object has gained the solar system. By that time its speed
-will be augmented to such a degree, that after a terrific whirl around
-the sun, it will at once fly off again along the other branch of the
-parabola. Perhaps you will wonder why it does not tumble straight
-into the sun. It would do so, no doubt, if it started at first from a
-position of rest; generally, however, the comet has a motion to begin
-with which would not be directed exactly to the luminary. This it is
-which causes the comet to miss actually hitting the sun.
-
-It may also be difficult to understand why the sun does not keep the
-comet when at last it has arrived. Why should the wandering body be in
-such a hurry to recede? Surely it might be expected that the attraction
-of the sun ought to hold it. If something were to check the pace of
-the comet in its terrific dash round the sun, then, no doubt, the
-object would simply tumble down into the sun and be lost. The sun has,
-however, not time to pull in the comet when it comes up with a speed
-20,000 times that of an express train. But the sun does succeed in
-altering the _direction_ of the motion of the comet, and the attraction
-has shown itself in that way.
-
-I can illustrate what happens in this manner. Here is a heavy weight
-suspended from the ceiling by a wire; it hangs straight down, of
-course, and there it is kept by the pull of the earth. Supposing I draw
-the weight aside and allow it to swing to and fro, then the motion
-continues like the beat of a pendulum. The weight is always pulled
-down as near to the earth as possible, but when it gets to the lowest
-point, it does not stay there, it goes through that point, and rises
-up at the other side. The reason is that the weight has acquired speed
-by the time it reaches the lowest point; and that, in virtue of its
-speed, it passes through the position in which it would naturally rest,
-and actually ascends the other side in opposition to the earth’s pull,
-which is dragging it back all the time. This will illustrate how the
-comet can pass by and even recede from the body which is continually
-attracting it.
-
-Just a few words of caution must be added. Suppose you had an ellipse
-so long that the comet would take thousands and thousands of years to
-complete a circuit, then the part of the ellipse in which the comet
-moves during the time when we can see it is so like a parabola, that
-we might possibly be mistaken in the matter. In fact, a geometer will
-tell us that if one end of an ellipse was to go further and further
-away, the end that stayed with us would gradually become more and more
-like this curve. Therefore, some of those comets which seem to move in
-parabolas may really be moving in extremely elongated ellipses, and
-thus, after excessively long periods of time, may come back to revisit
-us.
-
-
-THE MATERIALS OF A COMET.
-
-A comet is made of very unsubstantial material. This we can show in a
-very interesting manner, when we see it moving over the sky between
-the earth and the stars. Sometimes a comet will pass over a cluster of
-very small stars, so faint that the very lightest cloud that is ever
-in the sky would be quite sufficient to hide them. Yet the stars are
-distinctly visible right through the comet, notwithstanding that it may
-be hundreds of thousands of miles thick. This shows us how excessively
-flimsy is the substance of a comet, for while a few feet of haze or
-mist suffice to extinguish the brightest of stars, this immense curtain
-of comet stuff, whatever it may be made of, is practically transparent.
-
-I have often told you that we are able to weigh the heavenly bodies,
-but a comet gives us a great deal of trouble. You see that the weighing
-machine must be of a very delicate kind if you are going to weigh a
-very light object. Take, for example, a little lock of golden hair,
-which no doubt has generally a value quite independent of the number of
-grains that it contains. Suppose, however, that we are so curious as to
-desire to know its weight, then one of those beautiful balances in our
-laboratories will tell us. In fact, if you snipped a little fragment
-from a single hair, the balance would be sensitive enough to weigh it.
-If, however, you were only provided with a common pair of scales like
-those which are suited for the parcel post, then you could never weigh
-anything so light as a lock of hair. You have not small enough weights
-to begin with, and even if you had they would be of no use, for the
-scale is too coarse to estimate such a trifle. This is precisely the
-sort of difficulty we experience when we try to weigh a comet. The
-body, though so big, is very light, and our scales are so cumbersome
-that we are in a position of one who would try to weigh a lock of hair
-with a parcel-post balance. We cannot always find suitable scales for
-weighing celestial bodies. We have to use for the purpose whatever
-methods of discovering the weights happen to be available. So far, the
-methods I have mentioned are of the rudest description; they serve well
-enough for weighing heavy masses like planets, but they will not do for
-such unsubstantial bodies as comets.
-
-But, though we fail in this endeavor, _i.e._ to weigh comets, yet
-skilful astronomers have succeeded in something which at first you
-might think to be almost impossible. They have actually been able to
-discover some of the ingredients of which a comet is made. This is so
-important a subject that I must explain it fully.
-
-The most instructive comet which we have seen in modern days is that
-which appeared in the year 1882. It was an object so great that its
-tail alone was double as long as from the earth to the sun. It was
-discovered at the observatories in the southern hemisphere early in
-September of that year. A little later it was observed in the northern
-hemisphere in extraordinary circumstances. It must be remembered that
-a comet is generally a faint object, and that even those comets which
-are large enough and bright enough to form glorious spectacles in the
-sky at night are usually invisible during the brightness of day. For a
-comet to be seen in daylight was indeed an unusual occurrence; but on
-the forenoon of Sunday, September 17, Mr. Common at Ealing saw a great
-comet close to the sun. Unfortunately clouds intervened, and he was
-prevented from observing the critical occurrence just approaching. An
-astronomer at the Cape of Good Hope--Mr. Finlay--who had also been one
-of the earliest discoverers of the comet, was watching the body on the
-same day. He followed it as it advanced close up to the sun; bright
-indeed must that comet have been which permitted such a wonderful
-observation. At the sun’s edge the comet disappeared instantly; in
-fact, the observers thought that it must have gone behind the sun. They
-could not otherwise account for the suddenness with which it vanished.
-This was not what really happened. It was afterwards ascertained that
-the comet had not passed behind the sun; it had, indeed, come between
-us and our luminary. In its further progress this body showed in a
-striking degree the incoherent nature of the materials of which a comet
-is composed. It seemed to throw off portions of its mass along its
-track, each of which continued an independent journey. Even the central
-part in the head of the comet--the nucleus, as it is called--showed
-itself to be of a widely different nature from a substantial planetary
-body. The nucleus divided into two, three, four, or even five distinct
-parts, which seemed, in the words of one observer, to be connected
-together like pearls on a string.
-
-The comet of 1882 was also very instructive with regard to the actual
-materials from which such bodies are made. Astronomers have a beautiful
-method by which they find out the substances present in a heavenly
-body, even though they never can get a specimen of the body into their
-hands. We know at least three materials which were present in this
-comet. The first of them is an ingredient which is very commonly found
-in comets--a chemist calls it carbon. It is an extremely familiar
-material on the earth; for instance, coal is chiefly composed of
-carbon. Charcoal when burned leaves only a few ashes. All the substance
-that has vanished during combustion is carbon; in fact, it is not too
-much to say that carbon is found abundantly not only in wood, but
-in almost every form of vegetable matter. The food we eat contains
-abundant carbon, and it is an important constituent in the building up
-of our own bodies. Generally speaking, carbon is not found in a pure
-state--it is associated with other substances. Soot and lampblack are
-largely composed of it; but the purest form of this element carbon that
-we know is the diamond.
-
-It is interesting to note that carbon is certainly found as a frequent
-constituent of comets. The great comet of 1882 undoubtedly contained
-it, as well as certain other substances. Of these we know two: the
-first is the element sodium, an extremely abundant material on earth,
-inasmuch as the salt in the sea is mainly composed of it. It was also
-discovered that the same great comet contained another substance very
-common here and extremely useful to mankind. Dr. Copeland and Dr. Lohse
-at Dunecht showed that iron was present in this body which had come in
-to visit us from the depths of space.
-
-These discoveries are especially interesting because they seem to show
-the uniformity of material composing our system. We already knew that
-sodium and iron abounded in the sun, and now we have learned that
-these bodies and carbon as well are present in the comets. In the
-next chapter we shall learn that the very same materials--sodium and
-iron--are met with in bodies far more remote from us than any bodies of
-our own system.
-
-Comets have such a capricious habit of dashing into the solar system
-at any time and from any direction, that it is worth while asking
-whether a comet might not sometimes happen to come into collision with
-the earth. There is nothing impossible in such an occurrence. There
-is, however, no reason to apprehend that any disastrous consequences
-would ensue to the earth. Man has lived on this globe for many, many
-thousands of years, and the rocks are full of the remains of fossil
-animals which have flourished during past ages; indeed, we cannot
-possibly estimate the number of millions of years that have elapsed
-since living things first crawled about this globe. There has never
-been any complete break in the succession of life, consequently during
-all those millions of years we are certain that no such dire calamity
-has happened to the earth as a frightful collision would have produced,
-and we need not apprehend any such catastrophe in the future.
-
-I do not mean, however, that harmless collisions with comets may not
-have occasionally happened; in fact, there is good reason for knowing
-that they have actually taken place. In the year 1861 a fine comet
-appeared; and it is not so well remembered as its merits deserve,
-because it happened, unfortunately for its own renown, to appear
-just three years after the comet of 1858, which was one of the most
-gorgeous objects of this kind in modern times. But in 1861 we had a
-novel experience. On a Sunday evening in midsummer of that year, we
-dashed into the comet, or it dashed into us. We were not, it is true,
-in collision with its densest part; it was rather the end of the tail
-which we encountered. There were, fortunately, no very serious results.
-Indeed, most of us never knew that anything had happened at all, and
-the rest only learned of the accident long after it was all over.
-For a couple of hours that night it would seem that we were actually
-in the tail of the comet, but so far as I know no one was injured or
-experienced any alarming inconvenience. Indeed, I have only heard of
-one calamity arising from the collision. A clergyman tells us that at
-midsummer he was always able in ordinary years to read his sermon at
-evening service without artificial light. On this particular occasion,
-however, the sky was overcast with a peculiar glow, while the ordinary
-light was so much interfered with that the sexton had to provide a pair
-of candles to enable him to get through the sermon. The expense of
-those candles was, I believe, the only loss to the earth in consequence
-of its collision with the comet of 1861.
-
-[Illustration: FIG. 76.--How the Tail of a Comet arises.]
-
-The tail of a comet appears to develop under the influence of the sun.
-As the wandering body approaches the source of central heat it grows
-warm, and as it gets closer and closer to the sun, the fervor becomes
-greater and greater, until sometimes the comet experiences a heat more
-violent than any we could produce in our furnaces. The most infusible
-substances, such as stones or earth, would be heated white-hot and
-melted, and even driven off into vapor, under the intense heat to which
-a comet is sometimes exposed. Comets, indeed, have been known to sweep
-round the sun so closely as to pass within a seventh part of its radius
-from the surface. It seems that certain materials present in the comet,
-when heated to this extraordinary temperature, are driven away from
-the head, and thus form the tail (Fig. 76). Hence we see that the tail
-consists of a stream of vaporous particles, dashing away from the sun
-as if the heat which had called them into being was a torment from
-which they were endeavoring to escape.
-
-The tail of a comet is, therefore, not a permanent part of the body.
-It is more like the smoke from a great chimney. The smoke is being
-incessantly renewed at one end as the column gets dispersed into the
-air at the other. As the comet retreats, the sun’s heat loses its
-power. In the chills of space there is, therefore, no tail-making
-in progress, while the small mass of the comet renders it unable to
-gather back again by its attraction the materials which have been
-expelled. Should it happen that the comet moves in an elliptic orbit,
-and thus comes back time after time to be invigorated by a good
-roasting from the sun, it will, of course, endeavor to manufacture a
-tail each time that it approaches the source of heat. The quantity
-of material available for the formation of tails is limited to the
-amount with which the comet originally started; no fresh supply can be
-added. If, therefore, the comet expends a portion of this every time
-it comes round, an inevitable consequence seems to follow. Suppose a
-boy receives a sovereign when he goes back to school, and that every
-time he passes the pastry-cook’s shop some of his money disappears in
-a manner that I dare say you can conjecture, I need not tell you that
-before long the sovereign will have totally vanished. In a similar way
-comets cannot escape the natural consequences of their extravagance;
-their store of tail-making substance must, therefore, gradually
-diminish. At each successive return the tails produced must generally
-decline in size and magnificence, until at last the necessary
-materials have been all squandered, and we have the pitiful spectacle
-of a comet without any tail at all.
-
-The gigantic size of comets must excite our astonishment. A pebble
-tossed into a river would not be more completely engulfed than is our
-whole earth when it enters the tail of one of these bodies. But we now
-pass by a sudden transition to speak of the very smallest bodies, of
-little objects so minute that you could carry them in your waistcoat
-pocket. You will perhaps be surprised that such things can play an
-important part in our system and have a momentous connection with
-mighty comets.
-
-
-METEORS.
-
-If you look out from your window at the midnight sky, or take a walk
-on a fine clear night, you will occasionally see a streak of light
-dash over the heavens, thus forming what is called a falling, or a
-shooting, star (Fig. 77). It is not really one of the regular stars
-that has darted from its place. The objects we are now talking of are
-quite different from stars proper. To begin with, the shooting stars
-are comparatively close to us when we see them, and they are very
-small, whereas the stars themselves are enormous globes, far bigger
-than our earth, or often even bigger than the sun. Sometimes a great
-shooting star is seen which makes a tremendous blaze of light as bright
-as the moon, or even brighter still. These objects we call meteors,
-and you will be very fortunate if you can ever see a really fine one.
-Astronomers cannot predict these things as they predict the appearance
-of the planets. Bright meteors consequently appear quite unexpectedly,
-and it is a matter of chance as to who shall enjoy the privilege of
-beholding them. But it is not about the great meteors that we are now
-going to speak particularly; they are often not so interesting as the
-small ones.
-
-[Illustration: FIG. 77.--A Brilliant Meteor.]
-
-These little meteoroids, as we shall call them, have a curious history.
-They become visible to us only at the very last moment of their
-existence--in fact, the streak of light which forms a shooting star is
-merely the destruction of a meteoroid. You must always remember that
-we are here living at the bottom of a great ocean of air, and above
-the air extends the empty space. Air is a great impediment to motion;
-a large part of the power of a locomotive engine has to be expended
-solely in pushing the air out of the way so as to allow the train to
-get through. The faster the speed, the greater is the tax which the
-air imposes on the moving body. A cannon-ball, for instance, loses
-an immensity of its speed, and consequently of its power, by having
-to bore its way through the air. In outer space beyond the limits of
-this atmosphere, a freedom of movement can be enjoyed of which we
-know nothing down here. I spoke of this when discussing the movements
-of Encke’s comet. Even this very unsubstantial body could dash along
-without appreciable resistance until it traversed the atmosphere
-surrounding the sun. But now we have to speak of the motion of a little
-object both small and dense, resembling perhaps a pebble or a fragment
-of iron, or some substance of that description. It is a little body
-such as this which produces a shooting star.
-
-For ages and ages the meteoroid has been moving freely through space.
-The speed with which it dashes along greatly exceeds that of any of the
-motions with which we are familiar. It is about 100 times as swift as
-the pace of a rifle-bullet. About twenty miles would be covered in a
-second. You can hardly imagine what that speed is capable of. Suppose
-that you put one of these flying meteoroids beside an express train to
-race from London to Edinburgh, the meteoroid would have won the race
-before the train had got out of the station. Or suppose that a shooting
-star determined to make the circuit of the earth, it might, so far as
-pace is concerned, go entirely around the globe and back to the point
-from which it started in a little more than twenty minutes. But the
-fact is, you could not make any object down here move as fast as a
-shooting star. No gunpowder that could be made would be strong enough,
-in the first place, and even if the body could once receive the speed,
-it would never be able to force its way through the air uninjured.
-
-So long as a little shooting star is tearing away through open space
-we are not able to see it. The largest telescope in the world would
-not reveal a glimpse of anything so small. The meteoroid has no light
-of its own, and it is not big enough to exhibit the light reflected
-from the sun in the same manner as a little planet would do. It is
-only at the moment when it begins to be destroyed that its visibility
-commences. If the little object can succeed in dashing past our earth
-without becoming entangled in the atmosphere, then it will pursue its
-track with perhaps only a slight alteration in its path, due to the
-pull exercised by the earth. The air which surrounds our globe may be
-likened to a vast net, in which if any little meteor becomes caught
-its career is over. For when the little body, after rejoicing in the
-freedom of open space, dashes into air, immediately it experiences a
-terrific resistance; it has to force the particles of air out of the
-way so as to make room for itself, and in doing so it rubs against them
-with such vehemence that heat is produced.
-
-I am sure every boy knows that if he rubs a button upon a board it
-becomes very hot, in consequence of the friction. There are many other
-ways in which we can illustrate the production of heat in the same
-manner. One is a contrivance by which we spin round rapidly a piece of
-stick pressed against a board. Quantities of heat are thus produced
-by the friction, and volumes of smoke rise up. We have read how some
-savages are able to produce fire by means of friction in a somewhat
-similar manner, but to do so requires a rare amount of skill and
-patience. There is another illustration by which to show how heat can
-be produced by friction. A brass tube full of water is so arranged that
-it can be turned around very rapidly by the whirling table. We apply
-pressure to the tube, and after a minute or two the water begins to get
-hot, and then at last to boil, until ultimately the cork is driven out
-and a diminutive and, fortunately, harmless explosion of the friction
-boiler takes place. Engineers are aware how frequently heat is produced
-by friction, when it is very inconvenient or dangerous. Indeed, unless
-the wheels of railway carriages are kept well greased, the rubbing
-of the axle may generate so much heat that conflagrations in the
-carriage will ensue. Nature, in the little shooting star, gives us a
-striking illustration of the same fact. Perhaps you may be surprised to
-hear that the whole brilliancy of the shooting star is simply due to
-friction. As the little body dashes through the air it becomes first
-red-hot, then white-hot, until at last it is melted and turned into
-vapor. Thus is formed that glowing streak which we, standing very many
-miles below, see as a shooting star.
-
-A bullet when fired from a rifle will fly into pieces after it has
-struck against the target, and if you quickly pick up one of these
-pieces you will generally find it quite hot. Whence comes this heat?
-The bullet, of course, was cold before the rifleman pulled the
-trigger. No doubt there was a considerable amount of heat developed
-by the burning of the gunpowder, but the bullet was so short a time in
-contact with the wad, through which so little heat would pass, that we
-must look to some other source for the warmth that has been acquired.
-Friction against the barrel as the bullet passed to the mouth must have
-warmed the missile a good deal, and when rubbing against the air the
-same influence must have added still further to its temperature, while
-the blow against the target would finally warm it yet more.
-
-In comparing the shooting star with the rifle-bullet we must remember
-that the celestial object is travelling with a pace 100 times as swift
-as the utmost velocity that the rifle can produce, and the heat which
-is generated by friction is increased in still greater proportion.
-If we double the speed, we shall increase the quantity of heat by
-friction fourfold; if we increase the speed three times, then friction
-will be capable of producing nine times as much heat. In fact, we must
-multiply the number expressing the relative speed by itself--that is,
-we must form its square--if we want to find an accurate measure for
-the quantity of heat which friction is able to produce when a rapidly
-moving body is being stopped by its aid. The shooting star may have
-a pace 100 times that of the rifle-bullet, and if we multiply 100 by
-100 we get 10,000; consequently we see that the heat produced by the
-shooting star before its motion was arrested in dashing through the air
-would be 10,000 times that gained by the rifle-bullet in its flight.
-If the temperature of the rifle-bullet only rose a single degree by
-friction, it would thus be possible for the shooting star to gain
-10,000 degrees, and this would be enough to melt and boil away any
-object which ever existed. Thus we need not be surprised that friction
-through the air, and friction alone, has proved an adequate cause
-for the production of all the heat necessary to account for the most
-brilliant of meteors.
-
-It is rather fortunate for us that the meteors do dash in with this
-frightful speed; had the little bodies only moved as quickly as a
-rifle-bullet, or even only four or five times as fast, they would have
-pelted down on the earth in solid form. Indeed, on rare occasions it
-does happen that bodies from the heavens strike down on the ground.
-The great majority of those that fall on the ground, however, become
-entirely transformed into harmless vapor. The earth would, indeed,
-be almost uninhabitable from this cause alone were it not for the
-protection that the air affords us. All day and all night innumerable
-missiles would be whizzing about us, and though many of them are
-undoubtedly very small, yet as their speed is 100 times that of a
-rifle-bullet, the fusillade would be very unpleasant. It is, indeed,
-the intense hurry of these celestial bullets to get at us which is the
-very source of our safety. It dissipates the meteors into streaks of
-harmless vapor.
-
-
-WHAT BECOMES OF THE SHOOTING STARS.
-
-When we throw a lump of coal on the fire, all that is soon left is a
-little pinch of ashes, and the rest of the coal has vanished. You might
-think it had been altogether annihilated, but that is not nature’s
-way. Nothing is ever completely destroyed; it is merely transformed or
-changed into something else. The greater part of the coal has united
-with the oxygen which it has obtained from the air, and has formed a
-new gas, which has ascended the chimney. Every particle that was in the
-coal can be thus accounted for, and in the act of transformation heat
-is given out.
-
-A meteor also becomes transformed, but the substance it contains is not
-lost, though it is changed into glowing vapors. It is known that with
-heat enough any substance can be turned into vapor, just as water can
-be boiled into steam. Look at an electric light flashing between two
-pieces of carbon. Though carbon is one of the most difficult substances
-to melt, yet such is the intense heat generated by the electric
-current that the carbon is not only melted, but is actually turned
-into a vapor, and it is this vapor glowing with heat that gives us the
-brilliant light. In a similar manner iron can be rendered red-hot,
-white-hot, and then boiled and transformed into an iron vapor, if we
-may so call it. There is, indeed, plenty of such iron vapor in the
-universe. Quantities of it surround the sun and some of the stars.
-
-When ordinary steam is chilled it condenses into little drops of water.
-So, too, if iron be heated until it is transformed into vapor, and if
-that vapor be allowed to condense, it will ultimately form a dust,
-consisting of bits of iron so small that you would require a microscope
-to examine them. There is iron present in the small shooting stars.
-Other substances are also contained therein, and all these materials,
-after being boiled by the intense heat, are transformed into vapor.
-When the heat subsides, the vapor condenses again into fine dust,
-so that the ultimate effect of the atmosphere on a shooting star is
-to grind the little object into excessively fine powder, which is
-scattered along the track which the object has pursued. Sometimes this
-powder will continue to glow for minutes after the meteor has vanished,
-and in the case of some great meteors this stream of luminous dust in
-the air forms a very striking spectacle. A great meteor, or fire-ball
-as it is often called, appeared on the 6th of November, 1869. It flew
-over Devonshire and Cornwall, and left a track fifty miles long and
-four miles wide. The dust remained visible all along the great highway
-for nearly an hour; it formed a glowing cloud hanging in the sky, and
-though originally nearly straight, it became bent and twisted by the
-winds before it finally disappeared from view.
-
-We have now to see what becomes of this meteoric dust which is being
-incessantly poured into the air from external space. None of it ever
-gets away again; for whenever an unfortunate meteor just touches the
-air it is inevitably captured and pulverized. That dust subsides
-slowly, but we do not find it easy to distinguish the particles which
-have come from the shooting stars, because there is so much floating
-dust which has come from other sources.
-
-A sunbeam is the prettiest way of revealing the existence of the motes
-with which the air is charged. The sunbeam renders these motes visible
-exactly in the same way as planets become visible when sunbeams fall
-on them in far-distant space. But if we have not the sunbeams here,
-we can throw across the room a beam of electric light, and it is seen
-glowing all along its track, simply because the air of the room, like
-air everywhere, is charged with myriads of small floating particles.
-If you hold the flame of a spirit-lamp beneath this beam, you will see
-what seems like columns of black smoke ascending through it. But these
-columns are not smoke, they are pure air, or rather air in which the
-solid particles have been transformed into vapor by the heat from the
-spirit-flame.
-
-The motes abound everywhere in the air. We take thousands of them
-into our lungs every time we breathe. They are on the whole gradually
-sinking and subsiding downwards, but they yield to every slightest
-current, so that when looking at a sunbeam you will find them moving in
-all directions. It is sometimes hard to believe that the little objects
-are tending downwards, but if you look on the top of a book that has
-lain for a time on a book-shelf, you find there a quantity of dust,
-produced by the motes which have gradually subsided where they found a
-quiet spot and were allowed sufficient time to do so.
-
-The great majority of these particles consist, no doubt, of fragments
-of terrestrial objects. The dust from the roads, the smoke from the
-factories, and numerous other sources, are incessantly adding their
-objectionable particles to the air. There can be no doubt that the
-shooting stars also contribute their mites to the dust with which the
-atmosphere is ever charged. The motes in the murky air of our towns
-have no doubt chiefly originated from sources on this earth. Many
-of these sources it would be impossible to regard as of a romantic
-description. We may, however, feel confident that among those teeming
-myriads of small floating objects are many little particles which,
-having had their origin from shooting stars, are now gradually sinking
-to the earth.
-
-This is not a mere surmise, for dust has been collected from lofty
-Alpine snows, from the depths of the sea, and from other localities far
-removed from the haunts of men. From such collections, tiny particles
-of iron have been obtained, which have evidently been once in a molten
-condition. There is no conceivable explanation for the origin of iron
-fragments in such situations, except that they have been dropped from
-shooting stars.
-
-I am sure you have often helped in the making of a gigantic snowball.
-You begin with a small quantity of snow that can be worked with your
-hands. Then you have rolled it along the ground until it has become
-so big and so heavy that you must get a few playmates to help you,
-until at last it has grown so unwieldy that you can move it no longer,
-and then you apply your artistic powers to carving out a statue. The
-snowball has grown by the addition of material to it from without,
-and as it became heavier and heavier, it lapped up more and more of
-the snow as it rolled along; so that with each increase of size, its
-capacity for becoming still larger has also increased. I want to liken
-our earth to a snowball which goes rolling on through space, and
-every day, every hour, every minute, is gathering up and taking into
-it the little shooting stars that it meets with on its way. No doubt
-the annual accumulation is a very small quantity when compared with the
-whole size of the earth; but the earth is always drawing in, and now,
-at all events, never giving back again; so that when this process is
-carried on long enough, astonishing results may be obtained.
-
-You have all heard many maxims on this subject--how every little saving
-will at length reach a respectable or a gigantic total. Nature abounds
-with illustrations of the principle. All the water that thunders over
-Niagara is merely a sufficient number of little drops of rain collected
-together. Our earth has been gradually hoarding up, during countless
-ages, all the meteor dust that has rained upon it; and the larger the
-earth grows, the bigger is the net which it spreads, and the greater is
-the power it has to capture the wandering bodies. Thus, our earth, ages
-and ages ago, may have been considerably smaller than it is at present;
-in fact, a large proportion of this globe on which we dwell may have
-been derived from the little shooting stars which incessantly rain in
-upon its surface.
-
-
-GRAND METEORS.
-
-I dare say that many of those present will, in the course of their
-lives, have opportunities of seeing some of the great meteors, or
-fire-balls, which are occasionally displayed. Generally speaking, about
-one hundred or so of these splendid objects are recorded every year.
-We are never apprised that they are coming; they take us unawares, and
-therefore we have no opportunity to make proper arrangements for seeing
-them. There is only the chance that such persons as have been fortunate
-enough to see them will have noted the circumstances with sufficient
-accuracy to enable us to make use of their observations.
-
-[Illustration: FIG. 78.--How to find the Height of a Meteor.]
-
-The chief point to determine is the height of the meteor above the
-earth. For this we must have two observations at least, made in places
-as far asunder as possible. Suppose an observer at London and an
-observer at York were both witnesses of a splendid meteor; if they
-find, on subsequent comparison, that their observations were made at
-the same moment, there is no reasonable doubt that it was the same
-object they both saw. The observer at York describes the meteor as
-lying to the south, halfway down from the point directly over his head
-towards the horizon. The London observer speaks of the meteor as being
-to the north; and to him also it appeared that the object was halfway
-down towards his horizon from the point directly over his head. If you
-know a little Euclid, you can easily show from these facts that the
-height of the meteor must have been half the distance between London
-and York, that is, 85 miles (Fig. 78).
-
-I do not mean to say that the mode of discovering the meteor’s height
-will be always quite such a simple process as it has been in the
-case of the London and York observations. The principle is, however,
-the same--that whenever from two sufficiently distant positions the
-direction of the meteor has been observed, its path is known--just as
-on p. 21 we showed how the height of the suspended ball was obtained
-from observations at each end of the table. Generally speaking, bright
-meteors begin at an elevation of between fifty and one hundred miles,
-and they become extinguished before they are within twenty miles of the
-ground.
-
-Sometimes a tremendous explosion will take place during the passage of
-a meteor through the air. There was a celebrated instance in America
-on the 21st of December, 1876, which will give an idea of one of these
-objects possessing exceptional magnificence. It began in Kansas about
-seventy-five miles high, and thence it flew for a thousand miles at a
-speed of ten or fifteen miles a second, until it disappeared somewhere
-near Lake Ontario. Over a certain region between Chicago and St.
-Louis, the great ball of fire burst into a number of pieces, and
-formed a cluster of glowing stars that seemed to chase each other over
-the sky. This cluster must have been about forty miles long and five
-miles wide, and when the explosion occurred a most terrific noise was
-produced, so loud that many thought it was an earthquake. A remarkable
-circumstance illustrates the tremendous height at which this explosion
-occurred. The meteor had burst into pieces, the display was all over,
-and was beginning to be forgotten, and yet nothing had been _heard_. It
-was not until a quarter of an hour after the explosion had been _seen_
-that a fearful crash was heard at Bloomington. The explosion actually
-occurred 180 miles from the spot, and as sound takes five seconds to
-travel a mile, you can easily calculate that the noise required a
-quarter of an hour for its journey. What a tremendous noise it must
-have been!
-
-Shooting stars are of every grade of brightness. Beginning with the
-more gorgeous objects which have been compared with the moon or
-even with the sun himself, we descend to others as bright as Venus
-or as Jupiter; others are as bright as stars of various degrees of
-brilliancy. Fainter shooting stars are much more numerous than the
-conspicuous ones; in fact, there are multitudes of these objects so
-extremely feeble that the unaided eye would not show them. They only
-become perceptible in a telescope. It is not uncommon while watching
-the heavens at night to notice a faint streak of light dashing across
-the field of the instrument. This is a shooting star which is invisible
-except through the telescope.
-
-
-THE GREAT NOVEMBER SHOWERS.
-
-[Illustration: FIG. 79.--A Great Shower of Shooting Stars.]
-
-Occasionally we have the superb spectacle of a shower of shooting
-stars. None of you, my young friends, can as yet have had the good
-fortune to witness one of the specially grand displays, but you may
-live in hope; there are still showers to come. Astronomers have
-ventured on the prophecy that in or about the year 1899 you will have
-the opportunity of seeing a magnificent exhibition of this kind. There
-is only one ground for anxiety, and that is as to whether the clouds
-will keep out of the way for the occasion. I think I cannot explain
-my subject better than by taking you into my confidence and showing
-you the reasons on which we base this prediction. The last great
-shooting-star shower took place in the year 1866, or, perhaps, I should
-rather say that this was the last display from the same shooting-star
-system as that about which we are now going to speak. On the night of
-the 13th of November, 1866, astronomers were everywhere delighted by
-a superb spectacle. Enjoyment of the wondrous sight was not only for
-astronomers. Every one who loves to see the great sights of nature
-will have good reason for remembering that night. I certainly shall
-never forget it. It was about ten o’clock when a brilliant meteor or
-two first flashed across the sky, then presently they came in twos and
-threes, and later on in dozens, in scores, in hundreds. These meteors
-were brilliant objects, any one of which would have extorted admiration
-on an ordinary night. What, then, was the splendor of the display when
-they came on in multitudes? For two or three hours the great shower
-lasted, and then gradually subsided.
-
-We were not taken unawares on this occasion, for the shower was
-expected, and had been, in fact, awaited with eager anticipation. It
-should first be noticed that each year some shooting stars may always
-be looked for on or about the 13th of November. Every thirty-three
-years, or thereabouts, the ordinary spectacle breaks out into a
-magnificent display. It has also been found that for nearly 1000 years
-there have been occasional grand showers of meteors at the time of year
-mentioned, and all these incidents agree with the supposition that
-they are merely repetitions of the regular thirty-three-year shower.
-The first was in the year A.D. 902, which an old chronicle speaks
-of as the “year of the stars,” from the extraordinary display which
-then took place. I do not think the good people 1000 years ago fully
-appreciated the astronomical interest of such spectacles; in fact,
-they were often frightened out of their wits, and thought the end of
-the world had come. Doubtless many ancient showers have taken place
-of which we have no record whatever. In more modern days we have had
-somewhat fuller information; for example, on the night between the 12th
-and 13th of November, 1833, a shower was magnificently seen in America.
-Mr. Kirkwood tells us that a gentleman of South Carolina described the
-effect on the negroes of his plantation as follows: “I was suddenly
-awakened by the most distressing cries that ever fell on my ears.
-Shrieks of horror and cries for mercy I could hear from most of the
-negroes of the three plantations, amounting in all to about 600 or 800.
-While earnestly listening for the cause, I heard a faint voice near the
-door calling my name. I arose, and taking my sword, stood at the door.
-At this moment I heard the same voice still beseeching me to arise,
-and crying out that the world was on fire. I then opened the door, and
-it is difficult to say which excited me the most--the awfulness of the
-scene or the distressed cries of the negroes. Upwards of a hundred lay
-prostrate on the ground, some speechless, and some with the bitterest
-cries, but with their hands raised praying for mercy. The scene was
-truly awful, for never did rain fall much thicker than the meteors fell
-towards the earth.”
-
-By the study of many records of great showers it was learned that the
-interval at which these grand displays succeeded one another was about
-thirty-three years; and when it was remembered that the last great
-shower was in 1833 it was confidently expected that another similar
-display would take place in 1866. This was fully confirmed. Yet another
-thirty-three years brings us to 1899, when we have good reason for
-looking forward to a grand shower of these bodies. It may be expected
-to occur on November 14 or November 15. It may, however, possibly be
-that a shower will occur on the same days of the succeeding year.
-
-[Illustration: FIG. 80.--The Earth crossing the Track of Meteors.]
-
-We know a good deal now as regards the movements of these little
-objects. I want you to think of a vast swarm, something like a flock
-of birds, which I dare say you have often seen flying high in the air;
-the difference, however, is that the flock of meteors is enormously
-greater than any flock of birds ever was; and the meteors, too, are
-scattered so widely apart, that each one may be miles away from its
-next neighbors. Usually the meteoric shoal is many millions of miles
-long, and perhaps a hundred thousand miles in width. The great flock
-of meteors travels through space in a certain definite track. We have
-learned how the sun guides a planet, and forces the planet to move
-around him in an ellipse. But our sun will also condescend to guide
-an object no bigger than a shooting star. A bullet, a pea, or even
-a grain of sand will be held to an elliptic course around the sun as
-carefully as the great Jupiter himself. The entire shoal of meteors may
-therefore pursue their common journey around the sun as if inspired by
-a common purpose, each individual member of the host being, however,
-guided by the sun, and performing its path in real independence of its
-neighbors. The orbit followed by this shoal of meteors is enormously
-large and wide. Here is a sketch of the path (Fig. 80), and I have
-laid down the position of the orbit of the earth, but not on the same
-scale. The ellipse is elongated, so that while the shoal approaches
-comparatively close to the sun at one end of its journey, at the other
-end it goes out to an enormous distance, far beyond the orbit of the
-earth--beyond, indeed, the orbit of Jupiter or Saturn; in fact, it
-reaches to the path of Uranus. To accomplish so vast a journey as this
-thirty-three years and a quarter are required, and now you will be
-easily able to see why we get periodical visits from the shoal.
-
-It is, however, a mere piece of good fortune that we ever encounter the
-November meteors. Probably there are numerous other shoals of meteors
-quite as important which we never see, just in the same way as there
-are many shoals of fish in the sea that never come into our net. The
-earth moves round the sun in a path which is very nearly a circle, and
-the shoal moves round in this long oval. We cannot easily represent
-the true state of things by mere diagrams which show all these objects
-on the same plane. This does not give an accurate representation of
-the orbits. I think you will better understand what I mean by means of
-some wire rings. Make a round one to represent the path of the earth,
-and a long oval one to represent the path of the meteors. There is to
-be a small opening in the circular ring so that we can slip one of the
-orbits inside the other. If we are to see the meteors, it is of course
-necessary that they should strike the earth’s atmosphere, for they are
-not visible to us when they lie at a distance like the moon or like the
-planets. It is necessary that there be a collision between the earth
-and the shoal of meteors. But there never could be a collision between
-two trains unless the lines on which these trains run meet each other;
-therefore, it is necessary that this long ellipse shall actually cross
-the earth’s track; it will not do to have it pass inside like the two
-links of a chain; our earth would then miss the meteors altogether,
-and we should never see them. There are very likely many of such shoals
-of meteors revolving in this way, and thus escaping our notice entirely.
-
-You will also understand why there is no use in looking for these
-showers except on a particular day of November. On that day, and on
-that day alone, the earth appears at that particular point of its
-route where the latter crosses the track of the shoal. On the 1st of
-November, for instance, the earth has not yet reached the point where
-it could meet with these bodies. By the end of November it has passed
-too far. But even supposing that the earth is crossing the track of
-the meteors on the 13th of November, it is still possible that only
-a few, or none at all, shall be seen. The shoal may not happen to be
-at that spot at the right time. For a display of meteors to occur, it
-is therefore necessary that the shoal shall happen to be passing this
-particular stage of the journey on the 13th of November. In 1866 the
-earth dipped through the shoal and caught a great many of these meteors
-in its net. For a few hours the earth was engaged in the capture, until
-it emerged on the other side of the shoal, and the display was at an
-end.
-
-Sometimes it happens that in two years following each other, grand
-showers of meteors are seen. The reason of this is that the shoal is
-very long and thin, and consequently if the earth passes through the
-beginning of the shoal one year, it may have returned to the same point
-next year before the whole length of the shoal has completely passed.
-In this case we shall have two great showers in consecutive years.
-Thus a very fine display was seen in America on the proper day in 1867,
-while many stragglers were also observed during the three subsequent
-recurrences of the same date.
-
-Whenever the 13th of November comes round we generally meet with at
-least five shooting stars belonging to this same system, and we must
-explain how this occurs. Suppose there is a small racecourse so that
-the competitors will have to run a great many times round before the
-race is over. Let there be a very large number of entries, and let
-the majority of the athletes be fairly good runners, while a few are
-exceptionally good with varying degrees of excellence, and a few are
-very bad, some being worse than others. The whole group starts together
-in a cluster at the signal, and perhaps for the first round or two they
-may keep tolerably well together. It will be noticed the cluster begins
-to elongate as one circuit after another is made; the better runners
-draw out to the front, and the slower runners lag further and further
-behind; at last it may happen that those at the head will have gained
-a whole round on those at the tail, while the other runners of varying
-degrees of speed will be scattered all round the course. The majority
-of the runners, if of nearly equal speed, may continue in a pretty
-dense group.
-
-Precisely similar has been the great celestial race which these meteors
-are running. They started on their grand career centuries ago, and ever
-since then they have been flying round and round their mighty course.
-The greater proportion of the meteors still stay close together, and
-their pace is nearly uniform. The exceptionally smart ones have shot
-ahead, the exceptionally slow ones have lagged behind, and thus it
-happens that, after fifty or more revolutions have been completed,
-the shape of the original swarm has become considerably modified.
-Its length has been drawn out, while the stragglers and the fastest
-runners have been scattered all around the path. Across this course
-our earth carries us every November; there we usually encounter some
-of the members of this swarm which have strayed from the great host;
-they flash into the air, and thus it is that some of these bodies are
-generally seen every November.
-
-[Illustration: FIG. 81.--The Radiant.]
-
-During a shooting-star shower it is interesting to notice that all the
-meteors seem to diverge from a single point. In the adjoining figure
-(Fig. 81), which shows the directions of a number of meteors’ tracks,
-you will notice that every one seems to radiate from a certain point of
-the sky. In the case of the shower of the 13th–15th of November this
-point lies in the constellation Leo. I must refer you to the Appendix
-for a description of the way to find Leo or the Lion. The radiant
-point, as we term it, of this system of meteors is there situated. It
-is true that the meteors themselves do not generally seem to come all
-the way from this place. It is the direction of their luminous trails
-produced backward that carries the eye to the radiant (Fig. 81). If a
-meteor were actually seen there, it would be certainly coming straight
-towards us; it would not then appear as a streak of light at all; it
-would merely seem like a star which suddenly blazed into splendor and
-then again sank down into invisibility. Every meteor which appeared
-near this point would be directed very nearly at the observer, and its
-path would therefore seem very much foreshortened. I can illustrate
-this with a long straight rod. If I point it directly at you, you can
-only see the end. If I point it nearly at you, it will seem very much
-shortened. During the great shower in 1866 many of the meteors could be
-observed so close to the radiant in Leo that they seemed merely like
-very short marks in the sky; some of them, indeed, seemed to be merely
-starlike points swelling up into brilliance and then going out. Hence
-it is that we call this system of shooting stars the “Leonids.” They
-bear this name because their radiant lies in the constellation Leo,
-and unless the direction of a shooting star emanates from this point it
-does not belong to the Leonids. Even if it did so, the meteor would not
-be a Leonid unless the date was right, namely, on the 13th of November,
-or within a day thereof. We thus have two characteristics which belong
-to a system of shooting stars; there is the date on which they occur
-and the point from which they radiate.
-
-
-OTHER GREAT SHOWERS.
-
-To illustrate what I have said, we will speak about another system of
-shooting stars; they are due every August, from the 9th to the 11th,
-and their directions diverge from a point in the constellation of
-Perseus. I may remind you of the dates of the recurrence of this shower
-as well as of the November meteors of which we have just spoken, by
-quoting the following production:--
-
-    If you November’s stars would see,
-    About the fourteenth watching be.
-    In August, too, stars shine through heaven,
-    On nights between nine and eleven.
-
-It may be worth your while to remember these lines, and always to keep
-a look-out on the days named. The August meteors, the Perseids we
-often call them, do not make gorgeous displays, in particular years,
-with the regularity of the Leonids. There have been, no doubt, some
-exceptionally grand showers between the 9th and the 11th of August, but
-we cannot predict when the next splendid one is due. There are vast
-numbers of stragglers all round the track of the Perseids. In fact,
-it would seem as if the great race had gone on for such a long period
-that the cluster had to a great extent broken up, and that a large
-proportion of the meteors were now scattered the whole way around the
-course with tolerable uniformity. This being so, it follows that every
-time we cross the track we are nearly certain to fall in with a few of
-the stragglers, though we may never enjoy the tremendous spectacle of a
-plunge through a dense host of meteoroids.
-
-There are many other showers besides the two I have mentioned. Some
-shooting stars are to be seen almost every fine night, and those
-astronomers who pay particular attention to this subject are able
-to make out scores of small showers which might not interest you.
-Each of these is fully defined by the night of the year on which it
-occurs and the position of the point in the heavens from which the
-meteors radiate. Of these I must mention one. It is not usually very
-attractive, but it has a particular interest, as I shall now explain.
-
-On the 27th of November, 1872, a beautiful meteoric shower took place.
-You will notice that though the month is the same, the day is entirely
-different from that on which the Leonids appear. This shower of the
-27th is called the Andromedes, because the lines of direction of the
-shooting stars of which it is composed seem to diverge from a point in
-the constellation Andromeda. Ordinarily speaking, there is no special
-display of meteors connected with the annual return of this day; but in
-1872 astronomers were astonished by an exhibition of shooting stars
-belonging to this system. They were not at all bright when compared
-with the Leonid meteors. They were, however, sufficiently numerous to
-arrest the attention of very many, even among those who do not usually
-pay much attention to the heavens.
-
-The chief interest of the shower of Andromedes centers in a remarkable
-discovery connecting meteors and comets. There is a comet which was
-discovered by the astronomer Biela. It is a small object, requiring a
-telescope to show it. This comet completes each revolution in a period
-of about seven years; or rather, I should say, that was the time which
-the comet used to spend on its journey, for a life of trouble and
-disaster seems of late to have nearly extinguished the unfortunate
-object. In 1872 the comet was due in our neighborhood, and on the
-night of the 27th of November, in the same year, the earth crossed the
-track, and, in doing so, the shower of shooting stars was seen. This
-was a remarkable coincidence. We crossed the path of the comet at the
-time when we knew the comet ought to be there; and though we did not
-then see the comet, we saw a shower of shooting stars, and a wonderful
-shower too. A circumstance so peculiar suggested at once that the comet
-and the shooting stars must in some way or other be connected together.
-This is a suggestion we can test in another manner. We know the history
-of the comet, and we are aware that at the very time of the shower,
-the comet was approaching from the direction of the constellation of
-Andromeda. It was coming, in fact, from the very quarter whence the
-shooting stars have themselves travelled. Taking all these things
-together, it seems impossible to doubt that the shoal of shooting stars
-was, if not actually the comet itself, something closely connected with
-that famous body.
-
-
-METEORITES.
-
-Some years ago, a farmer living near Rowton, in Shropshire, noticed
-on a path in a field a hole which had been suddenly made by some
-mysterious and unknown agent. The laborers who were near told him they
-had just heard a remarkable noise; and when the farmer put his hand
-down into the hole, he felt something hot at the bottom of it. He took
-a spade and dug up the strange body, and found it to be a piece of
-iron, weighing about seven pounds. He was naturally amazed at such an
-occurrence, and brought the body home with him.
-
-Where did that piece of iron come from? It is plain that it could not
-have been always in the ground. The noise and the recently made hole
-showed that was not the case, and that is confirmed by the fact that
-the iron was hot. A piece of iron within a few feet of the earth’s
-surface cannot have remained warm for any length of time. It is
-therefore clear that the iron must have tumbled from the sky. This is
-a marvellous notion; in fact, it seems so incredible that at first
-people refused to believe that such things as stones or solid lumps of
-iron could have fallen from the heavens to the earth. But they had to
-believe it; the evidence was too conclusive. Fortunately, however,
-the occurrence is a comparatively rare one; indeed, our life on this
-globe would have an intolerable anxiety added to it if showers of iron
-hailstones like that at Rowton were at all of frequent occurrence. We
-should want umbrellas of a more substantial description than those
-which suffice for the rains we actually experience. There are, indeed,
-instances on record of persons having been killed by the fearful blows
-given by these bodies in falling.
-
-The Rowton siderite is a comparatively small one; pieces weighing
-hundredweights, and even tons, have been collected together in our
-museums. I would recommend you to pay a visit to that interesting room
-in our great British Museum in which these meteorites are exhibited.
-There we see actual specimens of celestial bodies which we can feel
-or weigh, and which our chemists can analyze. It may be noticed that
-they only contain substances that we already know on this earth. This
-celestial iron has often been made use of in primitive times before
-man understood how to smelt iron from its ore and how to transform it
-from cast iron to wrought iron. Nature seems to have taken heed of
-their wants, and occasionally to have thrown down a lump or two for the
-benefit of those who were so fortunate as to secure them.
-
-That these stones or irons drop from the sky is absolutely certain,
-but when we try to find out their earlier history we become involved
-in not a few difficulties. Nobody really knows the true history of
-these objects, but the view of their origin which seems to me to
-possess fewer difficulties than any other view is that which we may
-call the Columbiad Theory. I use this expression because every boy or
-girl listening to me ought to have read Jules Verne’s wonderful book,
-“From the Earth to the Moon,” and if any of you have not read it, the
-sooner you do so the better. It is there narrated how the gun club of
-Baltimore designed a magnificent cannon which was sunk deep into the
-ground, and then received a terrific charge of guncotton, on which a
-great hollow projectile was carefully lowered, containing inside the
-three adventurous explorers who desired to visit the moon. Calculations
-were produced with a view of showing that by firing on a particular
-day the explosion would drive the projectile up to the moon. There
-was, however, the necessary condition that the speed of projection
-should be great enough. The gun club were accurate in saying that if
-the cannon were able to discharge the projectile with a speed twenty
-or thirty times as great as that which had ever been obtained with
-any other cannon, then the missile would ascend up and up forever if
-no further influence were exerted on it. No doubt we have to overlook
-the resistance on the air and a few other little difficulties, but to
-this extent, at all events, the gun club were right: that a velocity of
-about six or seven miles a second would suffice to carry a body away
-from the gravitation of the earth.
-
-No one supposes that there were ever Columbiad cannons on our globe by
-which projectiles were shot up into space; but it seems possible that
-there may have been in very ancient days volcanoes on the earth with a
-shooting power as great as that which President Barbicane designed for
-the big cannon.
-
-Even now we have some active volcanoes of great energy on our earth,
-and we know that in former days the volcanoes must have been still more
-powerful; that, in fact, the Vesuvius of the present must be merely
-a popgun in comparison with volcanoes which have shaken the earth in
-those primitive days when it had just cooled down from its original
-fiery condition. Some of these early volcanoes, in the throes of their
-mighty eruptions, appear to have shot forth pieces of iron and volcanic
-substances with a violence great enough to carry them off into space.
-
-Suppose that a missile were projected upwards, it would ascend higher
-and higher, and gravity would, of course, tend to drag it back again
-down to earth. It can be shown that with an initial speed of six or
-seven miles a second the missiles would never return to the earth if
-only influenced by its attraction. The subsequent history of such a
-projectile would be guided by the laws according to which a planet
-moves. The body is understood to escape the destination which was aimed
-at by the Columbiad. I mean, of course, that it is not supposed to hit
-the moon. Of course, this might conceivably happen; but most of the
-projectiles would go quite wide of the mark, and would travel off into
-space.
-
-Though the earth would be unable to recall the projectile, the
-attraction of the sun would still guide it, whether it was as big as
-a paving-stone or ever so much larger or smaller. The body would be
-constrained to follow a path like a little planet around the sun. This
-track it would steadily pursue for ages. The wanderer would, however,
-cross the earth’s track once during each of its revolutions at the
-point from which it was projected. Of course, it will generally happen
-that the earth will not be there at the time the meteorite is crossing,
-and the meteorite will not be there at the time the earth is crossing.
-Nothing will therefore happen, and the object goes again on its long
-rounds. But sometimes it must occur that a meteor does not get past
-the junction without coming so close to the earth that it plunges into
-the air, often producing a noise and generating a streak of light like
-a shooting star. Then it tumbles down, and is restored to that earth
-whence it originally came.
-
-If this be the true view--and I think there are less weighty objections
-to it than to any other I know of--then the history of the piece
-of iron that was found in Shropshire would be somewhat as follows.
-Many millions of years ago, when the fires of our earth were much
-more vigorous than they are in these dull times, a terrific volcanic
-outbreak took place, and vast quantities of material were shot into
-space, of which this is one of the fragments. During all the ages that
-have since elapsed this piece of iron has followed its lonely track. In
-a thousandth part of the time rust and decay would have destroyed it
-had it lain on the earth, but in the solitudes of space there was found
-no air or damp to produce corrosion. At last, after the completion of
-its long travels, it again crashed down on the earth.
-
-We have now briefly surveyed the extent of the solar system. We began
-with the sun, which presides over all, and then we discussed the
-various planets with their satellites, next we considered the eccentric
-comets, and finally the minute bodies which, as shooting stars or
-meteorites, must be regarded as forming part of the Sun’s system. In
-our closing lecture we shall have to deal with objects of a far more
-magnificent character.
-
-
-
-
-LECTURE VI.
-
-STARS.
-
-  We try to make a Map--The Stars are Suns--The Numbers of the
-      Stars--The Clusters of Stars--The Rank of the Earth as a Globe
-      in Space--The Distances of the Stars--The Brightness and Color
-      of Stars--Double Stars--How we find what the Stars are made
-      of--The Nebulæ--What the Nebulæ are made of--Photographing the
-      Nebulæ--Conclusion.
-
-
-WE TRY TO MAKE A MAP.
-
-The group of bodies which cluster around our sun forms a little island,
-so to speak, in the extent of infinite space. We may illustrate this
-by a map in which we shall endeavor to show the stars placed at their
-proper relative distances. We first open the compasses one inch, and
-thus draw a little circle to represent the path of the earth. We are
-not going to put in all the planets. We take Neptune, the outermost, at
-once. To draw its path I open the compasses to thirty inches, and draw
-a circle with that radius. That will do for our solar system, though
-the comets no doubt will roam beyond these limits. To complete our map
-we ought of course to put in some stars. There are a hundred million to
-choose from, and we shall begin with the brightest. It is often called
-the Dog Star, but astronomers know it better as Sirius. Let us see
-where it is to be placed on our map. Sirius is beyond Neptune, so it
-must be outside somewhere. Indeed, it is a good deal further off than
-Neptune; so I try at the edge of the drawing-board; I have got a method
-of making a little calculation that I do not intend to trouble you
-with, but I can assure you that the results it leads me to are quite
-correct; they show me that this board is not big enough. But could a
-board which was big enough fit into this lecture theatre? Here, again,
-I make my little calculations, and I find that there would not be room
-for a board sufficiently great; in fact, if I put the sun here at one
-end, with its planets around it, Sirius would be too near on the same
-scale if it were at the further corner. The board would have to go out
-through the wall of the theatre, out through London. Indeed, big as
-London is, it would not be large enough to contain the drawing-board
-that I should require. It would have to stretch about twenty miles from
-where we are now assembled. We may therefore dismiss any hope of making
-a practical map of our system on this scale if Sirius is to have its
-proper place. Let us, then, take some other star. We shall naturally
-try with the nearest of all. It is one that we do not know in this
-part of the world, but those that live in the southern hemisphere are
-well acquainted with it. The name of this star is Alpha Centauri. Even
-for this star, we should require a drawing three or four miles long
-if the distance from the earth to the sun is to be taken as one inch.
-You see what an isolated position our sun and his planets occupy. The
-members of the family are all close together, and the nearest neighbors
-are situated at enormous distances. There is a good reason for this
-separation. The stars are very pretty and perfectly harmless to us
-where they are at present situated. They might be very troublesome
-neighbors if they were very much closer to our system. It is therefore
-well they are so far off; they would be constantly making disturbance
-in the sun’s family if they were near at hand. Sometimes they would be
-dragging us into unpleasantly great heat by bringing us too close to
-the sun, or producing a coolness by pulling us away from the sun, which
-would be quite as disagreeable.
-
-
-THE STARS ARE SUNS.
-
-We are about to discuss one of the grandest truths in the whole
-of nature. We have had occasion to see that this sun of ours is a
-magnificent globe immensely larger than the greatest of his planets,
-while the greatest of these planets is immensely larger than this
-earth; but now we are to learn that our sun is, indeed, only a star
-not nearly so bright as many of those which shine over our heads every
-night. We are comparatively close to the sun, so that we are able
-to enjoy his beautiful light and cheering heat. Each of those other
-myriads of stars is a sun, and the splendor of those distant suns is
-often far greater than that of our own. We are, however, so enormously
-far from them that they appear dwindled down to insignificance. To
-judge impartially between our sun or star and such a sun or star as
-Sirius we should stand halfway between the two; it is impossible to
-make a fair estimate when we find ourselves situated close to one star
-and a million times as far from the other. After allowance is made for
-the imperfections of our point of view, we are enabled to realize the
-majestic truth that the sun is no more than a star, and that the other
-stars are no less than suns. This gives us an imposing idea of the
-extent and the magnificence of the universe in which we are situated.
-Look up at the sky at night--you will see a host of stars; try to think
-that every one of them is itself a sun. It may probably be that those
-suns have planets circulating round them, but it is hopeless for us to
-expect to see such planets. Were you standing on one of those stars
-and looking towards our system, you would not perceive the sun to be
-the brilliant and gorgeous object that we knew so well. If you could
-see him at all, he would merely seem like a star, not nearly so bright
-as many of those you can see at night. Even if you had the biggest
-of telescopes to aid your vision, you could never discern from one
-of these bodies the planets which surround the sun. No astronomer in
-the stars could see Jupiter even if his sight were a thousand times
-as good or his telescopes a thousand times as powerful as any sight
-or telescope that we know. So minute an object as our earth would, of
-course, be still more hopelessly beyond the possibility of vision.
-
-
-THE NUMBERS OF THE STARS.
-
-To count the stars involves a task which lies beyond the power of man
-to accomplish. Even without the aid of any telescope, we can see a
-great multitude of stars from this part of the world. There are also
-many constellations in the southern hemisphere which never appear
-above our horizon. If, however, we were to go to the equator, then,
-by waiting there for a twelve-month, all the stars in the heavens
-would have been successively exposed to view. An astronomer, Houzeau,
-with the patience to count them, enumerated about 6000. This is the
-naked-eye estimate of the star-population of the heavens; but if,
-instead of relying on unaided vision, you get the assistance of a
-little telescope, you will be astounded at the enormous multitude of
-stars which are disclosed.
-
-[Illustration: FIG. 82.--The Great Bear and the Pole.]
-
-An ordinary opera-glass or binocular is a very useful instrument for
-looking at the stars in the heavens. If you employ an instrument of
-this sort, you will be amazed to find that the heavens teem with
-additional hosts of stars that your unaided vision would never have
-given you knowledge of. Any part of the sky may be observed; but, just
-to give an illustration, I shall take one special region, namely, that
-of the Great Bear (Fig. 82). The seven well-known stars are here shown,
-four of which form a sort of oblong, while the other three represent
-the tail. I would like you to make this little experiment. On a fine
-clear night, count how many stars there are within this oblong; they
-are all very faint, but you will be able to see a few, and, with good
-sight, and on a clear night, you may see perhaps ten. Next take your
-opera-glass and sweep it over the same region; if you will carefully
-count the stars it shows, you will find fully 200; so that the
-opera-glass has, in this part of the sky, revealed nearly twenty times
-as many stars as could be seen without its aid. As 6000 stars can be
-seen by the eye all over the heavens, we may fairly expect that twenty
-times that number--that is to say, 120,000 stars--could be shown by the
-opera-glass over the entire sky. Let us go a step further, and employ
-a telescope, the object-glass of which is three inches across. This is
-a useful telescope to have, and, if a good one, will show multitudes
-of pleasing objects, though an astronomer would not consider it very
-powerful. An instrument like this, small enough to be carried in the
-hand, has been applied to the task of enumerating the stars in the
-northern half of the sky, and 320,000 stars were counted. Indeed,
-the actual number that might have been seen with it is considerably
-greater, for when the astronomer Argelander made this memorable
-investigation he was unable to reckon many of the stars in localities
-where they lay very close together. This grand count only extended to
-half the sky, and, assuming that the other half is as richly inlaid
-with stars, we see that a little telescope like that we have supposed
-will, when swept over the heavens, reveal a number of stars which
-exceeds that of the population of any city in England except London. It
-exhibits more than one hundred times as many stars as our eyes could
-possibly reveal. Still, we are only at the beginning of the count; the
-very great telescopes add largely to the number. There are multitudes
-of stars which in small instruments we cannot see, but which are
-distinctly visible from our great observatories. That telescope would
-be still but a comparatively small one which would show as many stars
-in the sky as there are people living in this mighty city of London;
-and with the greatest instruments, the tale of stars has risen to a
-number far greater than that of the entire population of Great Britain.
-
-In addition to those stars which the largest telescopes show us, there
-are myriads which make their presence evident in a wholly different
-way. It is only in quite recent times that an attempt has been made to
-develop fully the powers of photography in representing the celestial
-objects. On a photographic plate which has been exposed to the sky in
-a great telescope the stars are recorded by thousands. Many of these
-may, of course, be observed with a good telescope, but there are not a
-few others which no one ever saw in a telescope, which apparently no
-one ever could see, though the photograph is able to show them. We do
-not, however, employ a camera like that which the photographer uses who
-is going to take your portrait. The astronomer’s plate is put into his
-telescope, and then the telescope is turned towards the sky. On that
-plate the stars produce their images, each by its own light. Some of
-these images are excessively faint, but we give a very long exposure
-of an hour or two hours; sometimes as much as four hours’ exposure
-is given to a plate so sensitive that a mere fraction of a second
-would sufficiently expose it during the ordinary practice of taking a
-photograph in daylight. We thus afford sufficient time to enable the
-fainter objects to indicate their presence upon the sensitive film.
-Even with an exposure of a single hour a picture exhibiting 16,000
-stars has been taken by Mr. Isaac Roberts, of Liverpool. Yet the
-portion of the sky which it represents is only one ten-thousandth part
-of the entire heavens. It should be added that the region which Mr.
-Roberts has photographed is furnished with stars in rather exceptional
-profusion.
-
-Here, at last, we have obtained some conception of the sublime scale on
-which the stellar universe is constructed. Yet even these plates cannot
-represent all the stars that the heavens contain. We have every reason
-for knowing that with larger telescopes, with more sensitive plates,
-with more prolonged exposures, ever fresh myriads of stars will be
-brought within our view.
-
-You must remember that every one of these stars is truly a sun, a
-lamp, as it were, which doubtless gives light to other objects in its
-neighborhood as our sun sheds light upon this earth and the other
-planets. In fact, to realize the glories of the heavens you should try
-to think that the brilliant points you see are merely the luminous
-points of the otherwise invisible universe.
-
-Standing one fine night on the deck of a Cunarder we passed in open
-ocean another great Atlantic steamer. The vessel was near enough for
-us to see not only the light from the mast-head but also the little
-beams from the several cabin ports; and we could see nothing of the
-ship herself. Her very existence was only known to us by the twinkle
-of these lights. Doubtless her passengers could see, and did see, the
-similar lights from our own vessel, and they probably drew the correct
-inference that these lights indicated a great ship.
-
-Consider the multiplicity of beings and objects in a ship: the captain
-and the crew, the passengers, the cabins, the engines, the boats,
-the rigging, and the stores. Think of all the varied interests there
-collected and then reflect that out on the ocean, at night, the sole
-indication of the existence of this elaborate structure was given by
-the few beams of light that happened to radiate from it. Now raise your
-eyes to the stars; there are the twinkling lights. We cannot see what
-those lights illuminate, we can only conjecture what untold wealth of
-non-luminous bodies may also lie in their vicinity; we may, however,
-feel certain that just as the few gleaming lights from a ship are
-utterly inadequate to give a notion of the nature and the contents of
-an Atlantic steamer, so are the twinkling stars utterly inadequate to
-give even the faintest conception of the extent and the interest of
-the universe. We merely see self-luminous bodies, but of the multitudes
-of objects and the elaborate systems of which these bodies are only
-the conspicuous points we see nothing and we know very little. We are,
-however, entitled to infer from an examination of our own star--the
-sun--and of the beautiful system by which it is surrounded, that these
-other suns may be also splendidly attended. This is quite as reasonable
-a supposition as that a set of lights seen at night on the Atlantic
-Ocean indicates the existence of a fine ship.
-
-
-THE CLUSTERS OF STARS.
-
-On a clear night you can often see, stretching across the sky, a track
-of faint light, which is known to astronomers as the “Milky Way.” It
-extends below the horizon and then round the earth to form a girdle
-about the heavens. When we examine the Milky Way with a telescope we
-find, to our amazement, that it consists of myriads of stars, so small
-and so faint that we are not able to distinguish them individually; we
-merely see the glow produced from their collective rays. Remembering
-that our sun is a star, and that the Milky Way surrounds us, it would
-almost seem as if our sun were but one of the host of stars which form
-this cluster.
-
-[Illustration: FIG. 83.--Globular Cluster in the Centaur.]
-
-There are also other clusters of stars, some of which are exquisitely
-beautiful telescopic spectacles. I may mention a celebrated pair of
-these objects which lies in the constellation of Perseus. The sight of
-them in a great telescope is so imposing that no one who is fit to
-look through a telescope could resist a shout of wonder and admiration
-when first they burst on his view. But there are other clusters. Here
-is a picture of one which is known as the “Globular Cluster in the
-Centaur” (Fig. 83). It consists of a ball of stars, so far off that,
-however large these several suns may actually be, they have dwindled
-down to extremely small points of light. A homely illustration may
-serve to show the appearance which a globular cluster presents in a
-good telescope. I take a pepper-castor and on a sheet of white paper
-I begin to shake out the pepper until there is a little heap at the
-centre and other grains are scattered loosely about. Imagine that
-every one of those grains of pepper was to be transformed into a tiny
-electric light, and then you have some idea of what a cluster of stars
-would look like when viewed through a telescope of sufficient power.
-There are multitudes of such groups scattered through the depths of
-space. They require our biggest telescopes to show them adequately.
-We have seen that our sun is a star, being only one of a magnificent
-cluster that form the Milky Way. We have also seen that there are other
-groups scattered through the length and depth of space. It is thus we
-obtain a notion of the rank which our earth holds in the scheme of
-things celestial.
-
-
-THE RANK OF THE EARTH AS A GLOBE IN SPACE.
-
-Let me give an illustration with the view of explaining more fully
-the nature of the relation which the earth bears to the other globes
-which abound through space, and you must allow me to draw a little
-upon my imagination. I shall suppose that Her Majesty’s mails extend
-not only over this globe, but that they also communicate with other
-worlds; that postal arrangements exist between Mars and the earth,
-between the sun and Orion--in fact, everywhere throughout the whole
-extent of the universe. We shall consider how our letters are to be
-addressed. Let us take the case of Mr. John Smith, merchant, who lives
-at 1001, Piccadilly; and let us suppose that Mr. John Smith’s business
-transactions are of such an extensive nature that they reach not only
-all over this globe, but away throughout space. I shall suppose that
-the firm has a correspondent residing--let us say in the constellation
-of the Great Bear; and when this man of business wants to write to Mr.
-Smith from these remote regions, what address must he put upon the
-letter, so that the Postmaster-General of the universe shall make no
-mistake about its delivery? He will write as follows:--
-
-  MR. JOHN SMITH,
-      1001 Piccadilly,
-          London,
-              England,
-                  Europe,
-                      Earth,
-                          Near the Sun,
-                              Milky Way,
-                                  The Universe.
-
-Let us now see what the several lines of this address mean. Of course
-we put down the name of Mr. John Smith in the first line, and then
-we will add “1001 Piccadilly” for the second; but as the people in
-the Great Bear are not likely to know where Piccadilly is, we shall
-add “London” underneath. As even London itself cannot be well known
-everywhere, it is better to write “England.” This would surely find
-Mr. John Smith from any post-office on this globe. From other globes,
-however, the supreme importance of England may not be so immediately
-recognized, and therefore it is as well to add another line, “Europe.”
-This ought to be sufficient, I think, for any post-office in the solar
-system. Europe is big enough to be visible from Mars or Venus, and
-should be known to the post-office people there, just as we know and
-have names for the continents on Mars. But further away there might
-be a little difficulty; from Uranus and Neptune the different regions
-on our earth can never have been distinguished, and therefore we must
-add another line to indicate the particular globe of the solar system
-which contains Europe. Mark Twain tells us that there was always
-one thing in astronomy which specially puzzled him, and that was to
-know how we found out the names of the stars. We are, of course, in
-hopeless ignorance of the name by which this earth is called among
-other intelligent beings elsewhere who can see it. I can only adopt
-the title of “Earth,” and therefore I add this line. Now our address
-is so complete that from anywhere in the solar system--from Mercury,
-from Jupiter, or Neptune--there ought to be no mistake about the letter
-finding its way to Mr. John Smith. But from his correspondent in the
-Great Bear this address would be still incomplete; they cannot see our
-earth from there, and even the sun himself only looks like a small
-star--like one, in fact, of thousands of stars elsewhere. However, each
-star can be distinguished, and our sun may, for instance, be recognized
-from the Great Bear by some designation. We shall add the line “Near
-the Sun,” and then I think that from this constellation, or from any
-of the other stars around us, the address of Mr. John Smith may be
-regarded as complete. But Mr. Smith’s correspondence may be still
-wider. He may have an agent living in the cluster of Perseus or on some
-other objects still fainter and more distant; then “Near the Sun” is
-utterly inadequate as a concluding line to the address, for the sun,
-if it can be seen at all from thence, will be only of the significance
-of an excessively minute star, no more to be designated by a special
-name than are each of the several leaves on the trees of a forest. What
-this distant correspondent will be acquainted with is not the earth or
-the sun, but only the cluster of stars among which the sun is but a
-unit. Again we use our own name to denote the cluster, and we call it
-the “Milky Way.” When we add this line, we have made the address of Mr.
-John Smith as complete as circumstances will permit. I think a letter
-posted to him anywhere ought to reach its destination. To perfect it,
-however, we will finish up with one line more--“_The Universe_.”
-
-
-THE DISTANCES OF THE STARS.
-
-I must now tell you something about the distances of the stars. I
-shall not make the attempt to explain fully how astronomers make such
-measurements, but I will give you some notion of how it is done. You
-may remember I showed you how we found the distance of a globe that
-was hung from the ceiling. The principle of the method for finding the
-distance of a star is somewhat similar, except that we make the two
-observations not from the two ends of a table, not even from opposite
-sides of the earth, but from two opposite points on the earth’s orbit,
-which are therefore at a distance of 186,000,000 miles. Imagine that on
-Midsummer Day, when standing on the earth here, I measure with a piece
-of card the angle between the star and the sun. Six months later, on
-Midwinter Day, when the earth is at the opposite point of its orbit,
-I again measure the angle between the same star and the sun, and we
-can now determine the star’s distance by making a triangle. I draw a
-line a foot long, and we will take this foot to represent 186,000,000
-miles, the distance between the two stations; then placing the cards
-at the corners, I rule the two sides and complete the triangle, and the
-star must be at the remaining corner; then I measure the sides of the
-triangle, and find how many feet they contain, and recollecting that
-each foot corresponds to 186,000,000 miles, we discover the distance of
-the star. If the stars were comparatively near us, the process would be
-a very simple one; but, unfortunately, the stars are so extremely far
-off that this triangle, even with a base of only one foot, must have
-its sides many miles long. Indeed, astronomers will tell you that there
-is no more delicate or troublesome work in the whole of their science
-than that of discovering the distance of a star.
-
-In all such measurements we take the distance from the earth to the
-sun as a conveniently long measuring-rod, whereby to express the
-results. The nearest stars are still hundreds of thousands of times
-as far off as the sun. Let us ponder for a little on the vastness of
-these distances. We shall first express them in miles. Taking the sun’s
-distance to be 93,000,000 miles, then the distance of the nearest fixed
-star is about twenty millions of millions of miles--that is to say,
-we express this by putting down a 2 first, and then writing thirteen
-ciphers after it. It is, no doubt, easy to speak of such figures, but
-it is a very different matter when we endeavor to imagine the awful
-magnitude which such a number indicates. I must try to give some
-illustrations which will enable you to form a notion of it. At first I
-was going to ask you to try and count this number, but when I found it
-would require at least 300,000 years, counting day and night without
-stopping, before the task was over, it became necessary to adopt some
-other method.
-
-When on a visit in Lancashire I was once kindly permitted to visit a
-cotton mill, and I learned that the cotton yarn there produced in a
-single day would be long enough to wind round this earth twenty-seven
-times at the equator. It appears that the total production of cotton
-yarn each day in all the mills together would be on the average about
-155,000,000 miles. In fact, if they would only spin about one-fifth
-more, we could assert that Great Britain produced enough cotton yarn
-every day to stretch from the earth to the sun and back again! It is
-not hard to find from these figures how long it would take for all the
-mills in Lancashire to produce a piece of yarn long enough to reach
-from our earth to the nearest of the stars. If the spinners worked
-as hard as ever they could for a year, and if all the pieces were
-then tied together, they would extend to only a small fraction of the
-distance; nor if they worked for ten years, or for twenty years, would
-the task be fully accomplished. Indeed, upwards of 400 years would be
-necessary before enough cotton could be grown in America and spun in
-this country to stretch over a distance so enormous. All the spinning
-that has ever yet been done in the world has not formed a long enough
-thread!
-
-There is another way in which we can form some notion of the immensity
-of these sidereal distances. You will recollect that, when we were
-speaking of Jupiter’s moons (p. 219), I told you of the beautiful
-discovery which their eclipses enabled astronomers to make. It was
-thus found that light travels at the enormous speed of about 185,000
-miles per second. It moves so quickly that within a single second a ray
-would flash two hundred times from London to Edinburgh and back again.
-
-We said that a meteor travels one hundred times as swiftly as a
-rifle-bullet; but even this great speed seems almost nothing when
-compared with the speed of light, which is 10,000 times as great.
-Suppose some brilliant outbreak of light were to take place in a
-distant star--an outbreak which would be of such intensity that the
-flash from it would extend far and wide throughout the universe.
-The light would start forth on its voyage with terrific speed. Any
-neighboring star which was at a distance of less than 185,000 miles
-would, of course, see the flash within a second after it had been
-produced. More distant bodies would receive the intimation after
-intervals of time proportional to their distances. Thus, if a body
-were 1,000,000 miles away the light would reach it in from five to six
-seconds, while over a distance as great as that which separates the
-earth from the sun the news would be carried in about eight minutes. We
-can calculate how long a time must elapse ere the light shall travel
-over a distance so great as that between the star and our earth. You
-will find that from the nearest of the stars the time required for the
-journey will be over three years. Ponder on all that this involves.
-That outbreak in the star might be great enough to be visible here,
-but we could never become aware of it till three years after it had
-happened. When we are looking at such a star to-night we do not see it
-as it is at present, for the light that is at this moment entering
-our eyes has travelled so far that it has been three years on the way.
-Therefore, when we look at the star now we see it as it was three years
-previously. In fact, if the star were to go out altogether, we might
-still continue to see it twinkling for a period of three years longer,
-because a certain amount of light was on its way to us at the moment
-of extinction, and so long as that light keeps arriving here, so long
-shall we see the star showing as brightly as ever. When, therefore, you
-look at the thousands of stars in the sky to-night, there is not one
-that you see as it is now, but as it was years ago.
-
-I have been speaking of the stars that are nearest to us, but there
-are others much farther off. It is true we cannot find the distance
-of these more remote objects with any degree of accuracy, but we
-can convince ourselves how great that distance is by the following
-reasoning. Look at one of the brightest stars. Try to conceive that the
-object was carried away further into the depths of space, until it was
-ten times as far from us as it is at present, it would still remain
-bright enough to be recognized in quite a small telescope; even if it
-were taken to one hundred times its original distance it would not have
-withdrawn from the view of a good telescope; while if it retreated
-one thousand times as far as it was at first it would still be a
-recognizable point in our mightiest instruments. Among the stars which
-we can see with our telescopes, we feel confident there must be many
-from which the light has expended hundreds of years, or even thousands
-of years, on the journey. When, therefore, we look at such objects, we
-see them, not as they are now, but as they were ages ago; in fact, a
-star might have ceased to exist for thousands of years, and still be
-seen by us every night as a twinkling point in our great telescopes.
-
-Remembering these facts, you will, I think, look at the heavens with a
-new interest. There is a bright star, Vega or Alpha Lyræ, a beautiful
-gem, so far off that the light from it which now reaches our eyes
-started before many of my audience were born. Suppose that there are
-astronomers residing on worlds amid the stars, and that they have
-sufficiently powerful telescopes to view this globe, what do you think
-they would observe? They will not see our earth as it is at present,
-they will see it as it was years, and sometimes many years, ago. There
-are stars from which, if England could now be seen, the whole of the
-country would be observed at this present moment to be in a great
-state of excitement at a very auspicious event. Distant astronomers
-might notice a great procession in London, and they could watch the
-coronation of a youthful queen amid the enthusiasm of a nation. There
-are other stars still further, from which, if the inhabitants had good
-enough telescopes, they would now see a mighty battle in progress not
-far from Brussels. One splendid army could be beheld hurling itself
-time after time against the immovable ranks of the other. They would
-not, indeed, be able to hear the ever-memorable, “Up, Guards, and at
-them!” but there can be no doubt that there are stars so far away that
-the rays of light which started from the earth on the day of the
-battle of Waterloo are only just arriving there. Further off still,
-there are stars from which a bird’s-eye view could be taken at this
-very moment of the signing of Magna Charta. There are even stars from
-which England, if it could be seen at all, would now appear, not as the
-great England we know, but as a country covered by dense forests, and
-inhabited by painted savages, who waged incessant war with wild beasts
-that roamed through the island. The geological problems that now puzzle
-us would be quickly solved could we only go far enough into space and
-had we only powerful enough telescopes. We should then be able to view
-our earth through the successive epochs of past geological time; we
-should be actually able to see those great animals whose fossil remains
-are treasured in our museums tramping about over the earth’s surface,
-splashing across its swamps, or swimming with broad flippers through
-its oceans. Indeed, if we could view our own earth reflected from
-mirrors in the stars, we might still see Moses crossing the Red Sea, or
-Adam and Eve being expelled from Eden.
-
-So important is the subject of star distance that I am tempted to give
-one more illustration in order to bring before you some conception of
-how vast such distances are. I shall take, as before, the nearest of
-the stars so far as known to us, and I hope to be forgiven for taking
-an illustration of a practical and a commercial kind instead of one
-more purely scientific. I shall suppose that a railway is about to
-be made from London to Alpha Centauri. The length of that railway,
-of course, we have already stated: it is twenty billions of miles.
-So I am now going to ask your attention to the simple question as to
-the fare which it would be reasonable to charge for the journey. We
-shall choose a very cheap scale on which to compute the price of a
-ticket. The parliamentary rate here is, I believe, a penny for every
-mile. We will make our interstellar railway fares much less even than
-this; we shall arrange to travel at the rate of one hundred miles for
-every penny. That, surely, is moderate enough. If the charges were so
-low that the journey from London to Edinburgh only cost fourpence,
-then even the most unreasonable passenger would be surely contented.
-On these terms how much do you think the fare from London to this
-star ought to be? I know of one way in which to make our answer
-intelligible. There is a National Debt with which your fathers are,
-unhappily, only too well acquainted; you will know quite enough about
-it yourselves in those days when you have to pay income tax. This debt
-is so vast that the interest upon it is about sixty thousand pounds
-a day, the whole amount of the National Debt being six hundred and
-thirty-eight millions of pounds (April, 1898).
-
-If you went to the booking office with the whole of this mighty sum
-in your pocket--but stop a moment; could you carry it in your pocket?
-Certainly not, if it were in sovereigns. You would find that after you
-had as many sovereigns as you could conveniently carry there would
-still be some left--so many, indeed, that it would be necessary to get
-a cart to help you on with the rest. When the cart had as great a load
-of sovereigns as the horse could draw there would be still some more,
-and you would have to get another cart; but ten carts, twenty carts,
-fifty carts, would not be enough. You would want five thousand of these
-before you would be able to move off towards the station with your
-money. When you did get there and asked for a ticket at the rate of one
-hundred miles for a penny, do you think you would get any change? No
-doubt some little time would be required to count the money, but when
-it was counted the clerk would tell you that there was not enough, that
-he must have nearly two hundred millions of pounds more.
-
-That will give some notion of the distance of the nearest star, and we
-may multiply it by ten, by one hundred, and even by one thousand, and
-still not attain to the distance of some of the more remote stars that
-the telescope shows us.
-
-On account of the immense distances of the stars we can only perceive
-them to be mere points of light. We can never see a star to be a globe
-with marks on it like the moon, or like one of the planets--in fact,
-the better the telescope the smaller does the star seem, though,
-of course, its brightness is increased with every addition to the
-light-grasping power of the instrument.
-
-
-THE BRIGHTNESS AND COLOR OF STARS.
-
-Another point to be noticed is the arrangement of stars in classes,
-according to their lustre. The brightest stars, of which there are
-about twenty, are said to be of the first magnitude. Those just
-inferior to the first magnitude are ranked as the second; and those
-just lower than the second are estimated as the third; and so on. The
-smallest points that your unaided eyes will show you are of about the
-sixth magnitude. Then the telescope will reveal stars still fainter and
-fainter, down to what we term the seventeenth or eighteenth magnitudes,
-or even lower still. The number of stars of each magnitude increases
-very much in the classes of small ones.
-
-Most of the stars are white, but many are of a somewhat ruddy hue.
-There are a few telescopic points which are intensely red, some exhibit
-beautiful golden tints, while others are blue or green.
-
-There are some curious stars which regularly change their brilliancy.
-Let me try to illustrate the nature of these variables. Suppose that
-you were looking at a street gas-lamp from a very long distance, so
-that it seemed a little twinkling light; and suppose that some one was
-preparing to turn the gas-cock up and down. Or, better still, imagine
-a little machine which would act regularly so as to keep the light
-first of all at its full brightness for two days and a half, and then
-gradually turn it down until in three or four hours it declines to a
-feeble glimmer. In this low state the light remains for twenty minutes;
-then during three or four hours the gas is to be slowly turned on again
-until it is full. In this condition the light will remain for two days
-and a half, and then the same series of changes is to recommence. This
-would be a very odd form of gas-lamp. There would be periods of two
-days and a half during which it would remain at its full; these would
-be separated by intervals of about seven hours, when the gradual
-turning down and turning up again would be in progress.
-
-[Illustration: FIG. 84.--Perseus and its Neighboring Stars, including
-Algol.]
-
-The imaginary gas-lamp is exactly paralleled by a star Algol, in the
-constellation of Perseus (Fig. 84), which goes through the series of
-changes I have indicated. Ordinarily speaking, it is a bright star of
-the second magnitude, and, whatever be the cause, the star performs
-its variations with marvellous uniformity. In fact, Algol has always
-arrested the attention of those who observed the heavens, and in early
-times was looked on as the eye of a Demon. There are many other stars
-which also change their brilliancy. Most of them require much longer
-periods than Algol, and sometimes a new star which nobody has ever
-seen before will suddenly kindle into brilliancy. It is now known that
-the bright star Algol is attended by a dark companion. This dark star
-sometimes comes between Algol and the observer and cuts off the light.
-Thus it is that the diminution of brightness is produced.
-
-
-DOUBLE STARS.
-
-Whenever you have a chance of looking at the heavens through a
-telescope, you should ask to be shown what is called _a double star_.
-There are many stars in the heavens which present no remarkable
-appearance to the unaided eye, but which a good telescope at once
-shows to be of quite a complex nature. These are what we call double
-stars, in which two quite distinct stars are placed so close together
-that the unaided eye is unable to separate them. Under the magnifying
-power of the telescope, however, they are seen to be distinct. In
-order to give some notion of what these objects are like, I shall
-briefly describe three of them. The first lies in that best known of
-constellations, the Great Bear. If you look at his tail, which consists
-of three stars, you will see that near the middle one of the three a
-small star is situated; we call this little star Alcor, but it is the
-brighter one near Alcor to which I specially call your attention. The
-sharpest eye would never suspect that it was composed of two stars
-placed close together. Even a small telescope will, however, show this
-to be the case, and this is the easiest and the first observation that
-a young astronomer should make when beginning to turn a telescope to
-the heavens. Of course, you will not imagine that I mean Alcor to be
-the second component of the double star; it is the bright star near
-Alcor which is the double. Here are two marbles, and these marbles are
-fastened an inch apart. You can see them, of course, to be separate;
-but if the pair were moved further and further away, then you would
-soon not be able to distinguish between them, though the actual
-distance between the marbles had not altered. Look at these two wax
-tapers which are now lighted; the little flames are an inch apart. You
-would have to view them from a station a third of a mile away if the
-distance between the two flames were to appear the same as that between
-the two components of this double star. Your eye would never be able
-to discriminate between two lights only an inch apart at so great a
-distance; a telescope would, however, enable you to do so, and this is
-the reason why we have to use telescopes to show us double stars.
-
-You might look at that double star year after year throughout the
-course of a long life without finding any appreciable change in the
-relative positions of its components. But we know that there is no
-such thing as rest in the universe; even if you could balance a body
-so as to leave it for a moment at rest, it would not stay there, for
-the simple reason that all the bodies round it in every direction are
-pulling at it, and it is certain that the pull in one direction will
-preponderate, so that move it must. Especially is this true in the case
-of two suns like those forming a double star. Placed comparatively near
-each other they could not remain permanently in that position; they
-must gradually draw together and come into collision with an awful
-crash. There is only one way by which such a disaster could be averted.
-That is by making one of these stars revolve around the other just as
-the earth revolves around the sun, or the moon revolves around the
-earth. Some motion must, therefore, be going on in every genuine double
-star, whether we have been able to see that motion or not.
-
-Let us now look at another double star of a different kind. This time
-it is in the constellation of Gemini. The heavenly twins are called
-Castor and Pollux. Of these, Castor is a very beautiful double star,
-consisting of two bright points, a great deal closer together than were
-those in the Great Bear; consequently a better telescope is required
-for the purpose of showing them separately. Castor has been watched
-for many years, and it can be seen that one of these stars is slowly
-revolving around the other; but it takes a very long time, amounting
-to hundreds of years, for a complete circuit to be accomplished. This
-seems very astonishing, but when you remember how exceedingly far
-Castor is, you will perceive that that pair of stars which appear so
-close together that it requires a telescope to show them apart must
-indeed be separated by hundreds of millions of miles. Let us try to
-conceive our own system transformed into a double star. If we took
-our outermost planet--Neptune--and enlarged him a good deal, and then
-heated him sufficiently to make him glow like a sun, he would still
-continue to revolve round our sun at the same distance, and thus a
-double star would be produced. An inhabitant of Castor who turned his
-telescope towards us would be able to see the sun as a star. He would
-not, of course, be able to see the earth, but he might see Neptune like
-another small star close to the sun. If generations of astronomers in
-Castor continued their observations of our system, they would find a
-binary star, of which one component took a century and a half to go
-round the other. Need we then be surprised that when we look at Castor
-we observe movements that seem very slow?
-
-There is often so much diffused light about the bright stars seen in
-a telescope, and so much twinkling in some states of the atmosphere,
-that stars appear to dance about in rather a puzzling fashion,
-especially to one who is not accustomed to astronomical observations.
-I remember hearing how a gentleman once came to visit an observatory.
-The astronomer showed him Castor through a powerful telescope as a fine
-specimen of a double star, and then, by way of improving his little
-lesson, the astronomer mentioned that one of these stars was revolving
-around the other. “Oh, yes,” said the visitor, “I saw them going round
-and round in the telescope.” He would, however, have had to wait for a
-few centuries with his eye to the instrument before he would have been
-entitled to make this assertion.
-
-Double stars also frequently delight us by giving beautifully
-contrasted colors. I dare say you have often noticed the red and the
-green lights that are used on railways in the signal lamps. Imagine one
-of those red and one of those green lights away far up in the sky and
-placed close together, then you would have some idea of the appearance
-that a colored double star presents, though, perhaps, I should add that
-the hues in the heavenly bodies are not so vividly different as are
-those which our railway people find necessary. There is a particularly
-beautiful double star of this kind in the constellation of the Swan.
-You could make an imitation of it by boring two holes, with a red-hot
-needle, in a piece of card, and then covering one of these holes with a
-small bit of the topaz-colored gelatine with which Christmas crackers
-are made. The other star is to be similarly colored with blue gelatine.
-A slide made on this principle placed in the lantern gives a very good
-representation of these two stars on the screen. There are many other
-colored doubles besides this one; and, indeed, it is noteworthy that
-we hardly ever find a blue or a green star by itself in the sky; it is
-always as a member of one of these pairs.
-
-
-HOW WE FIND WHAT THE STARS ARE MADE OF.
-
-Here is a piece of stone. If I wanted to know what it was composed
-of, I should ask a chemist to tell me. He would take it into his
-laboratory, and first crush it into powder, and then, with his test
-tubes, and with the liquids which his bottles contain, and his
-weighing scales, and other apparatus, he will tell all about it;
-there is so much of this, and so much of that, and plenty of this, and
-none at all of that. But now, suppose you ask this chemist to tell
-you what the sun is made of, or one of the stars. Of course, you have
-not a sample of it to give him; how, then, can he possibly find out
-anything about it? Well, he can tell you something, and this is the
-wonderful discovery that I want to explain to you. We now put down the
-gas, and I kindle a brilliant red light. Perhaps some of those whom
-I see before me have occasionally ventured on the somewhat dangerous
-practice of making fireworks. If there is any boy here who has ever
-constructed sky-rockets, and put the little balls into the top which
-are to burn with such vivid colors when the explosion takes place, he
-will know that the substance which tinged that red fire must have been
-_strontium_. He will recognize it by the color; because _strontium_
-gives a red light which nothing else will give. Here are some of these
-lightning papers, as they are called; they are very pretty and very
-harmless; and these, too, give brilliant red flashes as I throw them.
-The red tint has, no doubt, been produced by _strontium_ also. You
-see we recognized the substance simply by the color of the light it
-produced when burning.
-
-Perhaps some of you have tried to make a ghost at Christmas by dressing
-up in a sheet, and bearing in your hand a ladle blazing with a mixture
-of common salt and spirits of wine, the effect produced being a most
-ghastly one. Some mammas will hardly thank me for this suggestion,
-unless I add that the ghost must walk about cautiously, for otherwise
-the blazing spirit would be very apt to produce conflagrations of a
-kind more extensive than those intended. However, by the kindness of
-Professor Dewar, I am enabled to show the phenomenon on a splendid
-scale, and also free from all danger. I kindle a vivid flame of an
-intensely yellow color, which I think the ladies will unanimously
-agree is not at all becoming to their complexions, while the pretty
-dresses have lost their variety of colors. Here is a nice bouquet,
-and yet you can hardly distinguish the green of the leaves from the
-brilliant colors of the flowers, except by trifling differences of
-shade. Expose to this light a number of pieces of variously colored
-ribbon, pink and red and green and blue, and their beauty is gone; and
-yet we are told that this yellow is a perfectly pure color; in fact,
-the purest color that can be produced. I think we have to be thankful
-that the light which our good sun sends us does not possess purity
-of that description. There is one substance which will produce that
-yellow light; it is a curious metal called sodium--a metal so soft
-that you can cut it with a knife, and so light that it will float on
-water; while, still more strange, it actually takes fire the moment it
-is dropped on the water. It is only in a chemical laboratory that you
-will be likely to meet with the actual metallic sodium, yet in other
-forms the substance is one of the most abundant in nature. Indeed,
-common salt is nothing but sodium closely united with a most poisonous
-gas, a few respirations of which would kill you. But this strange metal
-and this noxious gas, when united, become simply the salt for our eggs
-at breakfast. This pure yellow light, wherever it is seen, either in
-the flame of spirits of wine mixed with salt or in that great blaze at
-which we have been looking, is characteristic of sodium. Wherever you
-see that particular kind of light, you know that sodium must have been
-present in the body from which it came.
-
-We have accordingly learned to recognize two substances, namely,
-_strontium_ and _sodium_, by the different lights which they give out
-when burning. To these two metals we may add a third. Here is a strip
-of white metallic ribbon. It is called magnesium. It seems like a bit
-of tin at the first glance, but indeed it is a very different substance
-from tin; for, look, when I hold it in the spirit-lamp, the strip of
-metal immediately takes fire, and burns with a white light so dazzling
-that it pales the gas-flames to insignificance. There is no other
-substance which will, when kindled, give that particular kind of light
-which we see from magnesium. I can recommend this little experiment
-as quite suitable for trying at home; you can buy a bit of magnesium
-ribbon for a trifle at the optician’s; it cannot explode or do any
-harm, nor will you get into any trouble with the authorities provided
-you hold it when burning over a tray or a newspaper, so as to prevent
-the white ashes from falling on the carpet.
-
-There are, in nature, a number of simple bodies called elements. Every
-one of these, when ignited under suitable conditions, emits a light
-which belongs to it alone, and by which it can be distinguished from
-every other substance. I do not say that we can try the experiments in
-the simple way I have here indicated. Many of the materials will yield
-light which will require to be studied by much more elaborate artifices
-than those which have sufficed for us. But you see that the method
-affords a means of finding out the actual substances present in the
-sun or in the stars. There is a practical difficulty in the fact that
-each of the heavenly bodies contains a number of different elements; so
-that in the light it sends us the hues arising from distinct substances
-are blended into one beam. The first thing to be done is to get some
-way of splitting up a beam of light, so as to discover the components
-of which it is made. You might have a skein of silks of different hues
-tangled together, and this would be like the sunbeam as we receive it
-in its unsorted condition. How shall we untangle the light from the
-sun or a star? I will show you by a simple experiment. Here is a beam
-from the electric light; beautifully white and bright, is it not? It
-looks so pure and simple, but yet that beam is composed of all sorts of
-colors mingled together, in such proportions as to form white light. I
-take a wedge-shaped piece of glass called a prism, and when I introduce
-it into the course of the beam, you see the transformation that has
-taken place (Fig. 85). Instead of the white light you have now all
-the colors of the rainbow--red, orange, yellow, green, blue, indigo,
-violet, marked by their initial letters in the figure. These colors are
-very beautiful, but they are transient, for the moment we take away
-the prism they all unite again to form white light. You see what the
-prism has done; it has bent all the light in passing through it; but it
-is more effective in bending the blue than the red, and consequently
-the blue is carried away much further than the red. Such is the way in
-which we study the composition of a heavenly body. We take a beam of
-its light, we pass it through a prism, and immediately it is separated
-into its components; then we compare what we find with the lights
-given by the different elements, and thus we are enabled to discover
-the substances which exist in the distant object whose light we have
-examined. I do not mean to say that the method is a simple one; all
-I am endeavoring to show is a general outline of the way in which we
-have discovered the materials present in the stars. The instrument that
-is employed for this purpose is called the spectroscope. And perhaps
-you may remember that name by these lines, which I have heard from an
-astronomical friend:--
-
-[Illustration: FIG. 85.--How to split up a Ray of Light.]
-
-   “Twinkle, twinkle, little star,
-    Now we find out what you are,
-    When unto the midnight sky,
-    We the spectroscope apply.”
-
-I am sure it will interest everybody to know that the elements which
-the stars contain are not altogether different from those of which the
-earth is made. It is true there may be substances in the stars of which
-we know nothing here; but it is certain that many of the most common
-elements on the earth are present in the most distant bodies. I shall
-only mention one, the metal iron. That useful substance has been found
-in some of the stars which lie at almost incalculable distances from
-the earth.
-
-
-THE NEBULÆ.
-
-In drawing towards the close of these lectures I must say a few words
-about some dim and mysterious objects to which we have not yet alluded.
-They are what are called nebulæ, or little clouds; and in one sense
-they are justly called little, for each of them occupies but a very
-small spot in the sky as compared with that which would be filled by
-an ordinary cloud in our air. The nebulæ are, however, objects of the
-most stupendous proportions. Were our earth and thousands of millions
-of bodies quite as big all put together, they would not be nearly so
-great as one of these nebulæ. Astronomers reckon up the various nebulæ
-by thousands, but I must add that most of them are apparently faint and
-uninteresting. A nebula is sometimes liable to be mistaken for a comet.
-The comet is, as I have already explained, at once distinguished by the
-fact that it is moving and changing its appearance from hour to hour,
-while scores of years elapse without changes in the aspect or position
-of a nebula. The most powerful telescopes are employed in observing
-these faint objects. I take this opportunity of showing a picture of an
-instrument suitable for such observations. It is the great reflector of
-the Paris Observatory (Fig. 87).
-
-[Illustration: FIG. 86.--The Ring Nebula in Lyra, under Different
-Telescopic Powers.]
-
-There are such multitudes of nebulæ that I can only show a few of the
-more remarkable kinds. In Fig. 86 will be seen pictures of a curious
-object in the constellation of Lyra seen under different telescopic
-powers. This is a gigantic ring of luminous gas. To judge of the size
-of this ring let us suppose that a railway were laid across it, and
-the train you entered at one side was not to stop until it reached
-the other side, how long do you think this journey would require?
-I recollect some time ago a picture in _Punch_ which showed a train
-about to start from London to Brighton, and the guard walking up and
-down announcing to the passengers the alarming fact that “this train
-stops nowhere.” An old gentleman was seen vainly gesticulating out of
-the window and imploring to be let out ere the frightful journey was
-commenced. In the nebular railway the passengers would almost require
-such a warning.
-
-[Illustration: FIG. 87.--A Great Reflecting Telescope.]
-
-Let the train start at a speed of a mile a minute, you would think,
-surely, that it must soon cross the ring. But the minutes pass, an
-hour has elapsed; so the distance must be sixty miles, at all events.
-The hours creep on into days, the days advance into years, and still
-the train goes on. The years would lengthen out into centuries, and
-even when the train had been rushing on for a thousand years with an
-unabated speed of a mile a minute, the journey would certainly not have
-been completed. Nor do I venture to say what ages must elapse ere the
-terminus at the other side of the ring nebula would be reached.
-
-A cluster of stars viewed in a small telescope will often seem like a
-nebula, for the rays of the stars become blended. A powerful telescope
-will, however, dispel the illusion and reveal the separate stars. It
-was, therefore, thought that all the nebulæ might be merely clusters
-so exceedingly remote that our mightiest instruments failed to resolve
-them into stars. But this is now known not to be the case. Many of
-these objects are really masses of glowing gas; such are, for instance,
-the ring nebulæ, of which I have just spoken, and the form of which I
-can simulate by a pretty experiment.
-
-[Illustration: FIG. 88.--How to make the Smoke-rings.]
-
-We take a large box with a round hole cut in one face, and a canvas
-back at the opposite side. I first fill this box with smoke, and there
-are different ways of doing so. Burning brown paper does not answer
-well, because the supply of smoke is too irregular and the paper
-itself is apt to blaze. A little bit of phosphorus set on fire yields
-copious smoke, but it would be apt to make people cough, and, besides,
-phosphorus is a dangerous thing to handle incautiously, and I do not
-want to suggest anything which might be productive of disaster if the
-experiment was repeated at home. A little wisp of hay, slightly damped
-and lighted, will safely yield a sufficient supply, and you need not
-have an elaborate box like this; any kind of old packing-case, or even
-a band-box with a duster stretched across its open top and a round hole
-cut in the bottom, will answer capitally. While I have been speaking,
-my assistant has kindly filled this box with smoke, and in order to
-have a sufficient supply, and one which shall be as little disagreeable
-as possible, he has mixed together the fumes of hydrochloric acid and
-ammonia from two retorts shown in Fig. 88. A still simpler way of doing
-the same thing is to put a little common salt in a saucer and pour
-over it a little oil of vitriol; this is put into the box, and over
-the floor of the box common smelling-salts is to be scattered. You see
-there are dense volumes of white smoke escaping from every corner of
-the box. I uncover the opening and give a push to the canvas, and you
-see a beautiful ring flying across the room; another ring and another
-follow. If you were near enough to feel the ring, you would experience
-a little puff of wind; I can show this by blowing out a candle which
-is at the other end of the table. These rings are made by the air
-which goes into a sort of eddy as it passes through the hole. All the
-smoke does is to render the air visible. The smoke-ring is indeed
-quite elastic. If we send a second ring hurriedly after the first, we
-can produce a collision, and you see each of the two rings remains
-unbroken, though both are quivering from the effects of the blow. They
-are beautifully shown along the beam of the electric lamp, or, better
-still, along a sunbeam.
-
-We can make many experiments with smoke-rings. Here, for instance, I
-take an empty box, so far as smoke is concerned, but air-rings can be
-driven forth from it, though you cannot see them, but you can feel them
-even at the other side of the room, and they will, as you see, blow out
-a candle. I can also shoot invisible air-rings at a column of smoke,
-and when the missile strikes the smoke it produces a little commotion
-and emerges on the other side, carrying with it enough of the smoke to
-render itself visible, while the solid black looking ring of air is
-seen in the interior. Still more striking is another way of producing
-these rings, for I charge this box with ammonia, and the rings from
-it you cannot see. There is a column of the vapor of hydrochloric
-acid that also you cannot see; but when the invisible ring enters the
-invisible column, then a sudden union takes place between the vapor
-of the ammonia and the vapor of the hydrochloric acid; the result is
-a solid white substance in extremely fine dust which renders the ring
-instantly visible.
-
-
-WHAT THE NEBULÆ ARE MADE OF.
-
-There is a fundamental difference between the illumination of these
-little rings that I have shown you and the great rings in the heavens.
-I had to illuminate our smoke with the help of the electric light,
-for, unless I had done so, you would not have been able to see them.
-This white substance formed by the union of ammonia and hydrochloric
-acid has, of course, no more light of its own than a piece of chalk;
-it requires other light falling upon it to make it visible. Were the
-ring nebula in Lyra composed of this material, we could not see it.
-The sunlight which illuminates the planets might, of course, light up
-such an object as the ring, if it were comparatively near us; but Lyra
-is at such a stupendous distance that any light which the sun could
-send out there would be just as feeble as the light we receive from a
-fixed star. Should we be able to show our smoke-rings, for instance,
-if, instead of having the electric light, I merely cut a hole in the
-ceiling and allowed the feeble twinkle of a star in the Great Bear to
-shine through? In a similar way the sunbeams would be utterly powerless
-to effect any illumination of objects in these stellar distances. If
-the sun were to be extinguished altogether, the calamity would no doubt
-be a very dire one so far as we are concerned, but the effect on the
-other celestial bodies (moon and planets excepted) would be of the
-slightest possible description. All the stars of heaven would continue
-to shine as before. Not a point in one of the constellations would be
-altered, not a variation in the brightness, not a change in the hue of
-any star could be noticed. The thousands of nebulæ and clusters would
-be absolutely unaltered; in fact, the total extinction of the sun would
-be hardly remarked in the newspapers published in the Pleiades or in
-Orion. There might possibly be a little line somewhere in an odd corner
-to the effect “Mr. So-and-So, our well-known astronomer, has noticed
-that a tiny star, inconspicuous to the eye, and absolutely of no
-importance whatever, has now become invisible.”
-
-If, therefore, it be not the sun which lights up this nebula, where
-else can be the source of its illumination? There can be no other star
-in the neighborhood adequate to the purpose, for, of course, such an
-object would be brilliant to us if it were large enough and bright
-enough to impart sufficient illumination to the nebula. It would be
-absurd to say that you could see a man’s face by the light of a candle
-while the candle itself was too faint or too distant to be visible.
-The actual facts are, of course, the other way; the candle might be
-visible, when it was impossible to discern the face which it lighted.
-
-Hence we learn that the ring nebula must shine by some light of its
-own, and now we have to consider how it can be possible for such
-material to be self-luminous. The light of a nebula does not seem to
-be like flame; it can, perhaps, be better represented by the pretty
-electrical experiment with Geissler’s tubes. These are glass vessels
-of various shapes, and they are all very nearly empty, as you will
-understand when I tell you the way in which they have been prepared. A
-little gas was allowed into each tube, and then almost all the gas was
-taken out again, so that only a mere trace was left. I pass a current
-of electricity through these tubes, and now you see they are glowing
-with beautiful colors. The different gases give out lights of different
-hues, and the optician has exerted his skill so as to make the effect
-as beautiful as possible. The electricity, in passing through these
-tubes, heats the gas which they contain, and makes it glow; and just
-as this gas can, when heated sufficiently, give out light, so does the
-great nebula, which is a mass of gas poised in space, become visible in
-virtue of the heat which it contains.
-
-We are not left quite in doubt as to the constitution of these gaseous
-nebulæ, for we can submit their light to the prism in the way I
-explained when we were speaking of the stars. Distant though that ring
-in Lyra may be, it is interesting to learn that the ingredients from
-which it is made are not entirely different from substances we know
-on our earth. The water in this glass, and every drop of water, is
-formed by the union of two gases, of which one is hydrogen. This is an
-extremely light material, as you see by a little balloon which ascends
-so prettily when filled with it. Hydrogen also burns very readily,
-though the flame is almost invisible. When I blow a jet of oxygen
-through the hydrogen, I produce a little flame with a very intense
-heat. For instance, I hold a steel pen in the flame, and it glows and
-sputters, and falls down in white-hot drops. It is needless to say
-that, as a constituent of water, hydrogen is one of the most important
-elements on this earth. It is, therefore, of interest to learn that
-hydrogen in some form or other is a constituent of the most distant
-objects in space that the telescope has revealed.
-
-
-PHOTOGRAPHING THE NEBULÆ.
-
-Of late years we have learned a great deal about nebulæ, by the help
-which photography has given to us. Look at this group of stars which
-constitutes that beautiful little configuration known as the Pleiades
-(Fig. 89). It looks like a miniature representation of the Great
-Bear; in fact, it would be far more appropriate to call the Pleiades
-the Little Bear than to apply that title to another quite different
-constellation, as has unfortunately been done. The Pleiades form a
-group containing six or seven stars visible to the ordinary eye, though
-persons endowed with exceptionally good vision can usually see a few
-more. In an opera-glass the Pleiades becomes a beautiful spectacle,
-though in a large telescope the stars appear too far apart to make a
-really effective cluster. When Mr. Roberts took a photograph of the
-Pleiades he placed a highly sensitive plate in his telescope, and on
-that plate the Pleiades engraved their picture with their own light.
-He left the plate exposed for hours, and on developing it not only
-were the stars seen, but there were also patches of faint light due to
-the presence of nebula. It could not be said that the objects on the
-plate were fallacious, for another photograph was taken, when the same
-appearances were reproduced.
-
-[Illustration: FIG. 89.--The Pleiades.]
-
-When we look at that pretty group of stars which has attracted
-admiration during all time, we are to think that some of those stars
-are merely the bright points in a vast nebula, invisible to our unaided
-eyes or even to our mighty telescopes, though capable of recording
-its trace on the photographic plate. Does not this give us a greatly
-increased notion of the extent of the universe, when we reflect that
-by photography we are enabled to see much which the mightiest of
-telescopes had previously failed to disclose?
-
-Of all the nebulæ, now numbering some thousands, there is but a single
-one which can be seen without a telescope. It is in the constellation
-of Andromeda, and on a clear dark night can just be seen with the
-unaided eye as a faint stain of light on the sky. It has happened
-before now that persons noticing this nebula for the first time have
-thought they had discovered a comet. I would like you to try and find
-out this object for yourselves.
-
-If you look at it with an opera-glass it appears to be distinctly
-elongated. You can see more of its structure when you view it in larger
-instruments, but its nature was never made clear until some beautiful
-photographs were taken by Mr. Roberts (Fig. 90). Unfortunately, the
-nebula in Andromeda has not been placed in the best position for its
-portrait from our point of view. It seems as if it were a rather
-flat-shaped object, turned nearly edgewise towards us. To look at the
-pattern on a plate, you would naturally hold the plate so as to be
-able to look at it squarely. The pattern would not be seen well if
-the plate were so tilted that its edge was turned towards you. That
-seems to be nearly the way in which we are forced to view the nebula
-in Andromeda. We can trace in the photograph some divisions extending
-entirely round the nebula, showing that it seems to be formed of a
-series of rings; and there are some outlying portions which form part
-of the same system. Truly this is a marvellous object. It is impossible
-for us to form any conception of the true dimensions of this gigantic
-nebula; it is so far off that we have never yet been able to determine
-its distance. Indeed, I may take this opportunity of remarking that
-no astronomer has yet succeeded in ascertaining the distance of any
-nebula. Everything, however, points to the conclusion that they are at
-least as far as the stars.
-
-[Illustration: FIG. 90.--The Great Nebula in Andromeda.
-
-(_From Mr. Roberts’ Photograph._)]
-
-It is almost impossible to apply the methods which we use in finding
-the distance of a star to the discovery of the distance of the
-nebulæ. These flimsy bodies are usually too ill-defined to admit of
-being measured with the precision and the delicacy required for the
-determination of distance. The measurements necessary for this purpose
-can only be made from one star-like point to another similar point. If
-we could choose a star in the nebula and determine its distance, then,
-of course, we should have the distance of the nebula itself; but the
-difficulty is that we have, in general, no means of knowing whether the
-star does actually lie in the object. It may, for anything we can tell,
-lie billions of miles nearer to us, or billions of miles further off,
-and, by merely happening to lie in the line of sight, appear to glimmer
-in the nebula itself.
-
-[Illustration: FIG. 91.--To show how Small the Solar System is compared
-with a Great Nebula.]
-
-If we have any assurance that the star is surrounded by a mass of
-this glowing vapor, then it may be possible to measure that nebula’s
-distance. It will occasionally happen that grounds can be found for
-believing that a star which appears to be in the glowing gas does
-veritably lie therein, and is not merely seen in the same direction.
-There are hundreds of stars visible on a good drawing or a good
-photograph of the famous object in Andromeda, and doubtless large
-numbers of these are merely stars which happen to lie in the same line
-of sight. The peculiar circumstances attending the history of one star
-seem, however, to warrant us in making the assumption that it was
-certainly in the nebula. The history of this star is a remarkable one.
-It suddenly kindled from invisibility into brilliancy. How is a change
-so rapid in the lustre of a star to be accounted for? In a few days
-its brightness had undergone an extraordinary increase. Of course,
-this does not tell us for certain that the star lay in the glowing
-gas; but the most rational explanation that I have heard offered of
-this occurrence is that due, I believe, to my friend Mr. Monck. He has
-suggested that the sudden outbreak in brilliancy might be accounted for
-on the same principles as those by which we explain the ignition of
-meteors in our atmosphere. If a dark star, moving along with terrific
-speed through space, were suddenly to plunge into a dense region of
-the nebula, heat and light must be evolved in sufficient abundance to
-transform the star into a brilliant object. If, therefore, we knew the
-distance of this star at the time it was in Andromeda, we should, of
-course, learn the distance of that interesting object. This has been
-attempted, and it has thus been proved that the Great Nebula must
-be very much further from us than is that star of whose distance I
-attempted some time ago to give you a notion.
-
-We thus realize the enormous size of the Great Nebula. It appears that
-if, on a map of this object, we were to lay down, accurately to scale,
-a map of the solar system, putting the sun in the centre and all the
-planets around in their true proportions out to the boundary traced by
-Neptune, this area, vast though it is, would be a mere speck on the
-drawing of the object. Our system would have to be enormously bigger
-before it sufficed to cover anything like the area of the sky included
-in one of these great objects. Here is a sketch of a nebula (Fig. 91),
-and near it I have marked a dot which is to indicate our solar system.
-We may feel confident that the Great Nebula is at the very least as
-mighty as this proportion would indicate.
-
-
-CONCLUSION.
-
-And now, my young friends, I am drawing near the close of that
-course of lectures which has occupied us, I hope you will think
-not unprofitably, for a portion of our Christmas holidays. We have
-spoken of the sun and of the moon, of comets and of stars, and I have
-frequently had occasion to allude to the relative position of our earth
-in the universe. No doubt it is a noble globe which we inhabit, but I
-have failed in my purpose if I have not shown you how insignificant
-is this earth when compared with the vast extent of some of the other
-bodies that abound in space. We have, however, been endowed with a
-feeling of curiosity which makes us long to know of things beyond
-the confines of our own earth. Astronomers can tell us a little, but
-too often only a little. They will say--That is a star, and That is
-a planet, and That is so big, and That so far; such is the meagre
-style of information with which we often have to be content. The
-astronomers who live on other worlds, if their faculties be in any
-degree comparable with ours, must be similarly ignorant with regard to
-this earth. Inhabitants of our fellow-planets can know hardly anything
-more than that the earth on which we dwell is a globe 8000 miles
-across, with many clouds around us. Some of the planets would not even
-pay us the compliment of recognizing our existence; while from the
-other systems--the countless other systems--of space we are absolutely
-imperceptible and unknown.
-
-Out of all the millions of bodies which we can see, you could very
-nearly count on your fingers those from which our earth would be
-visible. This reflection is calculated to show us how vast must be
-the real extent of that universe around us. Here is our globe, with
-its inhabitants, with its great continents, with its oceans, with its
-empires, its kingdoms, with its arts, its commerce, its literature, its
-sciences, and yet it would seem that all these things are absolutely
-unknown to any inhabitants that may exist elsewhere. I do not think
-that any reasonable person will doubt that there must be inhabitants
-elsewhere. There are millions of globes, many of them more splendid
-than ours. Surely it would be presumptuous to say that this is the
-only one of all the bodies in the universe on the surface of which
-life, with all that life involves, is manifested. You will rather think
-that our globe is but one in the mighty fabric, and that other globes
-may teem with interest just as ours does. We can, of course, make no
-conjecture as to what the nature of the life may be elsewhere. Could
-a traveller visit some other globes and bring back specimens of the
-natural objects that he found there, no collections that the world has
-ever seen could rival them in interest. When I go into the British
-Natural History Museum and look around that marvellous collection,
-it awakens in me a feeling of solemnity. I see there the remains of
-mighty extinct animals which once roamed over this earth; also objects
-which have been dredged from the bottom of the sea at a depth of some
-miles; there I can examine crystals which have required incalculable
-ages for their formation; and there I look at meteorites which have
-travelled from the heavens above down on to the earth beneath. Such
-sights, and the reflections they awaken, bring before us in an imposing
-manner the phenomena of our earth, and the extent and interest of its
-past history. Oliver Wendell Holmes said that the only way to see the
-British Museum was to take lodgings close by when you were a boy, and
-to stay in the Museum from nine to five every day until you were an old
-man; then you would begin to have some notion of what this Institution
-contains. Think what millions of British Museums would be required were
-the universe to be adequately illustrated: one museum for the earth,
-another for Mars, another for Venus--but it would be useless attempting
-to enumerate them!
-
-Most of us must be content with acquiring the merest shred of
-information with regard even to our own earth. Perhaps a schoolboy
-will think it fortunate that we are so ignorant with respect to the
-celestial bodies. What an awful vista of lessons to be learned would
-open before his view, if only we had a competent knowledge of the other
-globes which surround us in space! I should like to illustrate the
-extent of the universe by following this reflection a little further. I
-shall just ask you to join with me in making a little calculation as to
-the extent of the lessons you would have to learn if astronomers should
-succeed in discovering some of the things they want to know.
-
-Of course, all of us learn geography and history. We must know the
-geography of the leading countries of the globe, and we must have some
-knowledge of their inhabitants and of their government, their resources
-and their civilization. It would seem shockingly ignorant not to know
-something about China, or not to have some ideas on the subject of
-India or Egypt. The discovery of the New World also involves matters
-on which every boy and girl has to be instructed. Supposing we were
-so far acquainted with the other globes scattered through space that
-we were able to gain some adequate knowledge of their geography and
-natural history, of the creatures that inhabit them, of their different
-products and climates, then everybody would be anxious to learn those
-particulars; and even when the novelty had worn off, it would still be
-right for us to know something about countries perhaps more populous
-than China, about nations more opulent than our own, about battles
-mightier than Waterloo, about animals and plants far stranger than
-any we have ever dreamt of. An outline of all such matters should, of
-course, be learned, and as the amount of information would be rather
-extensive, we will try to condense it as much as possible.
-
-To aid us in realizing the full magnificence of that scheme in
-the heavens of which we form a part, I shall venture to give an
-illustration. Let us attempt to form some slight conception of the
-number and of the bulk of the books which would be necessary for
-conveying an adequate description of that marvellous universe of stars
-which surround us. These stars being suns, and many of them being
-brighter and larger than our own sun, it is but reasonable to presume
-that they may be attended by planetary systems. I do not say that we
-have any right to infer that such systems are like ours. It is not
-improbable that many of the suns around us have a much poorer retinue
-than that which dignifies our sun. On the other hand, it is just as
-likely that many of these other suns may be the centres of systems
-far more brilliant and interesting, with far greater diversity of
-structure, with far more intensity and variety of life and intelligence
-than are found in the system of which we form a part. It is only
-reasonable for us to suppose that, as our earth is an average planet,
-so our sun is an average star both in size and in the importance of its
-attendants. We may take the number of stars in the sky at about one
-hundred millions; and thus we see that the books which are to contain
-a description of the entire universe--or rather, I should say, of the
-entire universe that we see--must describe 100,000,000 times as much as
-is contained in our single system. Of course, we know next to nothing
-of what the books should contain; but we can form some conjecture of
-the number of those books, and this is the notion to which I now ask
-your attention.
-
-So vast is the field of knowledge that has to be traversed, that we
-should be obliged to compress our descriptions into the narrowest
-compass. We begin with a description of our earth, for nearly all
-the books in the libraries that exist at this moment are devoted to
-subjects connected with this earth. They include various branches
-of history, innumerable languages and literatures and religions,
-everything relating to life on this globe, to its history in past
-geological times, to its geography, to its politics, to every variety
-of manufacture and agriculture, and all the innumerable matters which
-concern our earth’s inhabitants, past and present. But this tremendous
-body of knowledge must be much condensed before it would be small
-enough to retire to its just position in the great celestial library. I
-can only allow to the earth one volume of about 500 pages. Everything
-that has to be said about our earth must be packed within this compass.
-All terrestrial languages, histories, and sciences that cannot be
-included between its covers can find no other place on our shelves. I
-cannot spare any more room. Our celestial library will be big enough,
-as you shall presently see. I am claiming a good deal for our earth
-when I regard it as one of the most important bodies in the solar
-system. Of course it is not the biggest--very far from it; but it seems
-as if the big planets and the sun were not likely to be inhabited, so
-that if we allow one other volume to the rest of the solar system, it
-will perhaps be sufficient, though it must be admitted that Venus, of
-which we know next to nothing, except that it is as large as the earth,
-may also be quite as full of life and interest. Mars and Mercury are
-also among the planets with possible inhabitants. We are, therefore,
-restricting the importance of the solar system as much as possible,
-perhaps even too much, by allowing it two. Within those two volumes
-every conceivable thing about the entire solar system--sun, planets
-(great and small), moons, comets, and meteors--must be included, or
-else it would not be represented at all in the great celestial library.
-
-We shall deal on similar principles with the other systems through
-space. Each of the 100,000,000 stars will have two volumes allotted
-to it. Within the two volumes devoted to each star we must compress
-our description of the body itself and of the system which surrounds
-it; the planets, their inhabitants, histories, arts, sciences, and all
-other information. I am not, remember, discussing the contents, but
-only the number of books we should have to read ere we could obtain
-even the merest outline of the true magnificence of the heavens. Let
-us try to form some estimate as to the kind of library that would be
-required to accommodate 200,000,000 volumes. I suppose a long straight
-hall, so lofty that there could be fifty shelves of books on each side.
-As you enter you look on the right hand and on the left, and you see
-it packed from floor to ceiling with volumes. We have arranged them
-according to the constellations. All the shelves in one part contain
-the volumes relating to the worlds in the Great Bear, while upon the
-other side may repose ranks upon ranks of volumes relating to the
-constellation of Orion.
-
-I shall suppose that the volumes are each about an inch and a half
-thick, and as there are fifty shelves on each side, you will easily
-see that for each foot of its length the hall will accommodate 800
-books. We can make a little calculation as to the length of this
-library, which, as we walk down through it, stretches out before us
-in a majestic corridor, with books, books everywhere. Let us continue
-our stroll, and as we pass by we find the shelves on both sides packed
-with their thousands of volumes; and we walk on and on, and still see
-no end to the vista that ever opens before us. In fact, no building
-that was ever yet constructed would hold this stupendous library. Let
-the hall begin on the furthest outskirts of the west of London, carry
-it through the heart of the city, and away to the utmost limits of
-the east--not a half of the entire books could be accommodated. The
-mighty corridor would have to be fifty miles long, and to be packed
-from floor to ceiling with fifty shelves of books on each side, if it
-is to contain even this very inadequate description of the contents
-of the visible universe. Imagine the solemn feelings with which we
-should enter such a library, could it be created by some miracle! As
-we took down one of the volumes, with what mysterious awe should we
-open it, and read therein of some vast world which eye had never
-seen! There we might learn strange problems in philosophy, astonishing
-developments in natural history; with what breathless interest we
-should read of inhabitants of an organization utterly unknown to our
-merely terrestrial experience! Notwithstanding the vast size of the
-library, the description of each globe would have to be very scanty.
-Thus, for instance, in the single book which referred to the earth I
-suppose a little chapter might be spared to an island called England,
-and possibly a page or so to its capital, London. Similarly meagre
-would have to be the accounts of the other bodies in the universe; and
-yet, for this most inadequate of abstracts, a library fifty miles long,
-and lined closely with fifty shelves of books on each side, would be
-required!
-
-Methuselah lived, we are told, nine hundred and sixty-nine years; but
-even if he had attained his thousandth birthday he would have had to
-read about 300 of these books through every day of his life before he
-accomplished the task of learning even the merest outline about the
-contents of space.
-
-If, indeed, we were to have a competent knowledge of all these other
-globes, of all their countries, their geographies, their nations, their
-climates, their plants, their animals, their sciences, languages, arts
-and literatures, it is not a volume, or a score of volumes, that would
-be required, but thousands of books would have to be devoted to the
-description of each world alone, just as thousands of volumes have been
-devoted to the affairs of this earth without exhausting the subjects
-of interest it presents. Hundreds of thousands of libraries, each
-as large as the British Museum, would not contain all that should be
-written, were we to have anything like a detailed description of the
-universe _which we see_. I specially emphasize the words just written,
-and I do so because the grandest thought of all, and that thought
-with which I conclude, brings before us the overwhelming extent of
-the unseen universe. Our telescopes can, no doubt, carry our vision
-to an immeasurable distance into the depths of space. But there are,
-doubtless, stars beyond the reach of our mightiest telescopes. There
-are stars so remote that they cannot record themselves on the most
-sensitive of photographic plates.
-
-On the blackboard I draw a little circle with a piece of chalk. I think
-of our earth as the centre, and this circle will mark for us the limit
-to which our greatest telescopes can sound. Every star which we see,
-or which the photographic plate sees, lies within this circle; but,
-are there no stars outside? It is true that we can never see them,
-but it is impossible to believe that space is utterly void and empty
-where it lies beyond the view of our telescopes. Are we to say that
-inside this circle stars, worlds, nebulæ, and clusters are crowded, and
-that outside there is nothing? Everything teaches us that this is not
-so. We occasionally gain accession to our power by adding perhaps an
-inch to the diameter of our object-glass, or by erecting a telescope
-in an improved situation on a lofty mountain peak, or by procuring a
-photographic plate of increased sensibility. It thus happens that we
-are enabled to extend our vision a little further and to make this
-circle a little larger, and thus to add a little more to the known
-inside which has been won from the unknown outside. Whenever this is
-done we invariably find that the new region thus conquered is also
-densely filled with stars, with clusters, and with nebulæ; it is thus
-unreasonable to doubt that the rest of space also contains untold
-myriads of objects, even though they may never, by any conceivable
-improvement in our instruments, be brought within the range of our
-observation. Reflect that this circle is comparatively small with
-respect to the space outside. It occupied but a small spot on this
-blackboard, the blackboard itself occupies only a small part of the
-end of the theatre, while the end of the theatre is an area very small
-compared with that of London, of England, of the world, of the solar
-system, of the actual distance of the stars. In a similar way the
-region of space which is open to our inspection is an inconceivably
-small portion of the entire extent of space. The unknown outside is
-so much larger than the known inside, it is impossible to express the
-proportion. I write down unity in this corner and a cipher after it
-to make ten, and six ciphers again to make ten millions, and again,
-six ciphers more to make ten billions; but I might write six more,
-ay, I might cover the whole of this blackboard with ciphers, and even
-then I should not have got a number big enough to express how greatly
-the extent of the space we cannot see exceeds that of the space we
-see. If, therefore, we admit the fact, which no reasonable person can
-doubt, that this outside, this unknown, this unreachable and, to us,
-invisible space does really contain worlds and systems as does this
-small portion of space in which we happen to be placed--then, indeed,
-we shall begin truly to comprehend the majesty of the universe. What
-figures are to express the myriads of stars that should form a suitable
-population for a space inconceivably greater than that which contains
-100,000,000 stars? But our imagination will extend still further. It
-brings before us these myriads of unseen stars with their associated
-worlds, it leads us to think that these worlds may be full to the brim
-with interests as great as those which exist on our world. When we
-remember that, for an adequate description of the worlds which we can
-see, one hundred thousand libraries, each greater than any library on
-earth, would be utterly insufficient, what conception are we to form
-when we now learn that even this would only amount to a description of
-an inconceivably small fragment of the entire universe?
-
-Let us conceive that omniscience granted to us an adequate revelation
-of the ample glories of the heavens, both in that universe which we
-do see and in that infinitely greater universe which we do not see.
-Let a full inventory be made of all those innumerable worlds, with
-descriptions of their features and accounts of their inhabitants and
-their civilizations, their geology and their natural history, and all
-the boundless points of interest of every kind which a world in the
-sense in which we understand it does most naturally possess. Let those
-things be written every one, then may we say that were this whole earth
-of ours covered with vast buildings, lined from floor to ceiling with
-book-shelves--were every one of these shelves stored full with volumes,
-yet, even then this library would be inadequate to receive the books
-that would be necessary to contain a description of the glories of the
-sidereal heavens.
-
-
-
-
-CONCLUDING CHAPTER.
-
-HOW TO NAME THE STARS.
-
-
-Every one who wishes to learn something about astronomy should
-make a determined effort to become acquainted with the principal
-constellations, and to find out the names of the brighter and more
-interesting stars. I have therefore added to STAR-LAND this little
-chapter, in which I have tried to make the study of the stars so simple
-that, by taking advantage of a few clear nights, there ought to be no
-difficulty in obtaining a knowledge of a few constellations.
-
-The first step is to become familiar with the Great Bear, or Ursa
-Major, as astronomers more generally call the group. We begin with
-this, because after it has been once recognized, then you will find
-it quite easy to make out the other constellations and stars. It may
-save you some trouble if you can get some one to point out to you
-the Great Bear; but even without such aid, I think you will be able
-to make out the seven bright stars which form this remarkable group,
-from the figure here given (Fig. 92). Of course, the position of this
-constellation, as of every other in the heavens, changes with the hour
-of the night, and changes with the seasons of the year. About April
-the constellation at 11 o’clock at night is high over your head. In
-September at the same hour, the Great Bear is low down in the north.
-It is to be seen in the west in July, and at Christmas it lies in
-the east at convenient hours in the evening for observation. One of
-the advantages of using the Great Bear as the foundation of our study
-of the stars arises from the fact that to an observer in the British
-Islands or in similar latitudes this group never sets. Whenever the sky
-is clear after nightfall, the Great Bear is to be seen somewhere, while
-the brightness of its component stars makes it a conspicuous object.
-Indeed, there is only one constellation in the sky, namely, that of
-Orion, which is a more brilliant group than the Great Bear. We shall
-tell you about Orion presently, but it does not suit to begin with,
-because it can only be seen in winter, and is then placed very low down
-in the heavens.
-
-[Illustration: FIG. 92.--The Great Bear and the Pole Star.]
-
-Your next lesson will be to utilize the Great Bear for the purpose of
-pointing out the Pole Star. Look at the two stars marked α and β. They
-are called the “Pointers,” because if you follow the direction they
-indicate along the dotted line in the figure, they will conduct your
-glance to the Pole. This is the most important star in the heavens to
-astronomers, because it happens to mark very nearly the position of
-the Pole on the sky. You will easily note the peculiarity of the Pole
-Star if you will look at it two or three times in the course of the
-night. It will appear to remain in the same place in the sky, while the
-other stars change their places from hour to hour. It is very fortunate
-that we have a star like this in the northern heavens; the astronomers
-in Australia or New Zealand can see no bright star lying near the
-Southern Pole which will answer the purposes that the Pole Star does so
-conveniently for us in the north.
-
-The Pole Star belongs to a constellation which we call the Little Bear;
-two other conspicuous members of this group are the two “Guards”; you
-will see how they are situated from Fig. 82, p. 322. They lie nearly
-midway between the Pole Star and the last of the three stars which
-form the Great Bear’s Tail. The same figure will also introduce us to
-another beautiful constellation, namely, Cassiopeia. You will never
-find any difficulty in identifying the figure that marks this group if
-you will notice that the Pole lies midway between it and the Great Bear.
-
-[Illustration: FIG. 93.--The Great Square of Pegasus.]
-
-Cassiopeia is also one of the constellations that never set to British
-observers; but now we have to speak of groups which do set, and which,
-therefore, can only be observed when the proper season comes round. The
-first of these is “the Great Square of Pegasus”; you cannot see this
-group conveniently in the spring or summer, but during the autumn and
-winter it is well placed after nightfall. There are four conspicuous
-stars forming the corners of the square, and then three others marked
-α, γ, and β (Fig. 93), which form a sort of handle to the square. In
-fact, if you once recognize this group, you will perhaps see in it a
-resemblance to a great saucepan with a somewhat bent handle, and then
-you will be acquainted with a large tract of Star-land near the Square
-of Pegasus. From the figure you will see that a line imagined to be
-drawn from the Pole Star over the end of Cassiopeia, and then produced
-as far again, will just lead to the Great Square. I have also marked on
-this figure two objects that are of great telescopic interest; one of
-them is the Nebula in Andromeda, of which we had an account in the last
-lecture. You see it lies halfway between the corner α of the square and
-the group of Cassiopeia. Another interesting object is the star marked
-γ Andromedæ. The telescope shows it to consist of a pair of stars, the
-colors of which are beautifully contrasted.
-
-At the end of this handle to the Great Square of Pegasus is the star
-α, in the constellation of Perseus. It lies between two other stars γ
-and δ. We refer to Fig. 84, in which these stars are shown. We there
-employed the figure to indicate the position of Algol, the remarkable
-variable star. Your map will also point out some other important
-stellar features. If we curve round the three marked γ, α, and δ of
-Perseus, the eye is conducted to Capella, a gem of the first magnitude
-in the constellation of Auriga. Close to Capella is a long triangle,
-the corners of which are the “Hœdi,” the three kids--which Capella is
-supposed to nurture.
-
-If we carry a curve through γ, α, δ, of Perseus, and now bend it in the
-opposite way, the eye is led through ε and ζ in the same constellation,
-and then on to the Pleiades, of which we have already spoken.
-
-[Illustration: FIG. 94.--Orion and Sirius.]
-
-Perseus lies in one of the richest parts of the heavens. The Milky
-Way stretches across the group, and the sky is strewn with stars beyond
-number. Even an opera-glass directed to this teeming constellation
-cannot fail to afford the observer a delightful glimpse of celestial
-scenery.
-
-We may, however, specially remind the beginner that the objects on
-this map are not always to be seen, and as an illustration of the way
-in which the situation and the visibility of the constellations are
-affected by the time of year, I shall take the case of the Pleiades and
-follow them through a season. Let us suppose that we make a search for
-this group at 11 P.M. every night. On the 1st of January, the Pleiades
-will be found high up in the sky in the southwest. On the 1st of March,
-they will be setting in the west at the same hour. On the 1st of May,
-the Pleiades are not visible, neither are they on the 1st of July. On
-the 1st of September, they will be seen low down in the east. On the
-1st of November, they will be high in the heavens in the southeast. On
-the ensuing 1st of January, the Pleiades will be found back in the same
-place which they occupied on the same date in the preceding year, and
-so on throughout the cycle. Of course, you will not suppose that their
-changes are due to actual motions in the group of stars themselves.
-They are merely apparent, and are to be explained by the motion of the
-earth round its axis, and the revolution around the sun.
-
-Next we are to become acquainted with the glory of our winter skies,
-the constellation of Orion, Fig. 94. I dare say many of my readers are
-already familiar with the well-known twin stars which form the belt of
-Orion, but if not, they will be able to recognize it by the help of the
-groups already learned. Imagine a line drawn from the Pole Star through
-Capella, and then produced as much further again, and we shall be
-conducted into the precincts of Orion. This group lies on the equator,
-and, consequently, it is equally familiar to southern astronomers and
-to those of the north. It can be best seen by those who observe it from
-or near the equator.
-
-The brightest star in Orion is known either as α Orionis or as
-Betelgeuze, by which name it is represented in the figure. This star
-is of the first magnitude, and so is Rigel on the opposite side of the
-belt. The three stars of the belt and the two others, γ and κ, at which
-they point above and below, are of second magnitude.
-
-The owner of a telescope finds especial attractions in this
-constellation. Notably among the subjects which will interest him is
-the Great Nebula, the position of which is indicated in our figure.
-Under the middle of the belt are a few stars, around which is a hazy
-light that is perceptible with the smallest telescopic aid. Viewed
-by instruments of adequate proportions, these have developed into a
-marvellous nebula of glowing gas, attaining to dimensions so vast that
-no one has yet ever attempted to estimate them.
-
-The vicinity of Orion is also enriched with some of the most
-interesting stellar objects. Follow the line of the belt upwards to
-the right, and your eye is conducted to a ruddy first magnitude star
-named Aldebaran, in the constellation of the Bull. This is a pleasing
-object, which the beginner will sometimes be apt to confuse with the
-planet Mars, to which, under certain circumstances, it certainly bears
-a resemblance. Another very pleasing little group, known as the Hyades,
-will be found near Aldebaran. If the line of the belt of Orion be
-carried down to the left, it will be found to point to Sirius, or the
-Dog Star.
-
-You will find it an interesting occupation to make for yourself maps
-of small parts of the heavens. First copy out the chief stars in their
-proper places from the star atlas, and then fill in the smaller stars
-with your own observations. Try first on some limited region of the
-heavens; take the figure of Cassiopeia, for instance, or the Square
-of Pegasus, and see if you can produce a fair representation of those
-groups by marking in the stars that your instrument will show you;
-or take the Pleiades and make a tracing of the principal stars of
-the group from the sketch that we have given (Fig. 89), then take an
-opera-glass and fill in as carefully as you can all that it will show.
-I can assure you that you will find a little definite work of this kind
-full of interest and instruction.
-
-But I hope you will desire to advance further in the study of the
-heavens. It is to be remembered that with even the most moderate
-instruments there is much to be done. Many comets have been detected,
-and many planets have been discovered, by the use of telescopes so
-small that they could be easily carried out from the house for the
-evening’s work and brought back again after the observations were over.
-
-It remains for me to add a few words which will help you in finding
-the planets. It is, of course, impossible to represent such objects
-as Jupiter, Saturn, Venus, Mars, and Mercury on maps of the heavens,
-because these bodies are constantly moving about, and if their places
-were right to-day they would be wrong to-morrow. The almanac will be
-necessary for you here. You must find out by its help what planets are
-visible and in what part of the sky they are placed. Then you will
-have to compare your maps with the heavens, and when you find a bright
-star-like body that is not shown on your maps you may conclude at once
-that it is the planet. Although these objects are so star-like to the
-unaided eye, yet the resemblance is at once dispelled when we use a
-telescope. The star is only a bright point of light and white, the
-planet shows a visible shape. This is, at least, the case with the five
-planets I have named; for there are others, such as Uranus and Neptune,
-which are too far to be much more than star-like points in ordinary
-telescopes. The minor planets would not interest you.
-
-I hope that the reading of STAR-LAND will, at all events, induce you to
-make a beginning of the study of the heavens, if you have not already
-done so. If you have the advantage of a telescope, so much the better;
-but, if this is impossible, make the best use of your own eyes. Do not
-put it off or wait till you get some one to teach you. If it be clear
-this very night, go out and find the Great Bear and the Pole Star, and
-as many of the other constellations as you can, and at once commence
-your career as an astronomer.
-
-
-
-
-TABLE OF USEFUL ASTRONOMICAL FACTS.
-
-
-The sun’s mean distance from the earth is 92,700,000 miles; his
-diameter is 865,000 miles, and he rotates in a period between 25 and 26
-days.
-
-The moon’s mean distance from the earth is 238,000 miles; the diameter
-of the moon is 2160 miles, and the time of revolution round the earth
-is 27.322 days.
-
-
-THE PLANETS.
-
-  --------+---------------+---------------+-----------+-----------------
-          | Mean Distance | Periodic Time | Diameter  |
-          | from the Sun  | of Revolution | of Planet | Axial Rotation.
-          | in Millions   |   in Days.    | in Miles. |
-          |   of Miles.   |               |           |
-  --------+---------------+---------------+-----------+-----------------
-          |               |               |           | Hrs. Mins. Secs.
-  Mercury |        35.9   |     87.969    |   2,992   |    Uncertain.
-  Venus   |        67.0   |    224.70     |   7,660   |    Uncertain.
-  Earth   |        92.7   |    365.26     |   7,918   |  23   56    4.09
-  Mars    |       141     |    686.98     |   4,200   |  24   37   22.7
-  Jupiter |       482     |  4,332.6      |  85,000   |   9   55    --
-  Saturn  |       884     | 10,759        |  71,000   |  10   14   23.8
-  Uranus  |     1,780     | 30,687        |  31,700   |     Unknown.
-  Neptune |     2,780     | 60,127        |  34,500   |     Unknown.
-  --------+---------------+---------------+-----------+-----------------
-
-
-THE SATELLITES OF MARS.
-
-  ----------+--------------------+-------------------
-    Name.   | Mean Distance from |  Periodic Time.
-            |  Centre of Mars.   |
-  ----------+--------------------+-------------------
-            |                    | Hrs. Mins. Secs.
-  Phobos    |   5,800 miles.     |  7    39    14
-  Deimos    |  14,500   ”        | 30    17    54
-  ----------+--------------------+-------------------
-
-
-THE SATELLITES OF JUPITER.
-
-  ----------+--------------------+-------------------------
-    Name.   | Mean Distance from |     Periodic Time.
-            | Centre of Jupiter. |
-  ----------+--------------------+-------------------------
-            |                    | Days  Hrs.  Mins.  Secs.
-  I         |    262,000 miles.  |   1    18    27     34
-  II        |    417,000   ”     |   3    13    13     42
-  III       |    664,000   ”     |   7     3    42     33
-  IV        |  1,170,000   ”     |  16    16    32     11
-  V         |    112,400   ”     |  --    11    57    (?)
-  ----------+--------------------+--------------------------
-
-
-THE SATELLITES OF SATURN.
-
-  ----------+--------------------+-------------------------
-    Name.   | Mean Distance from |     Periodic Time.
-            | Centre of Saturn.  |
-  ----------+--------------------+-------------------------
-            |                    | Days  Hrs.  Mins.  Secs.
-  Mimas     |    118,000 miles.  |   0    22    37     27.9
-  Enceladus |    152,000   ”     |   1     8    53      6.7
-  Tethys    |    188,000   ”     |   1    21    18     25.7
-  Dione     |    241,000   ”     |   2    17    41      8.9
-  Rhea      |    337,000   ”     |   4    12    25     10.8
-  Titan     |    781,000   ”     |  15    22    41     25.2
-  Hyperion  |    946,000   ”     |  21     7     7     40.8
-  Iapetus   |  2,280,000   ”     |  79     7    54     40.4
-  ----------+--------------------+-------------------------
-
-A ninth satellite was discovered in August, 1898, by Prof. W. H.
-Pickering, but its mean distance and periodic time have not yet been
-determined with precision.
-
-
-THE SATELLITES OF URANUS.
-
-  ----------+--------------------+---------------
-    Name.   | Mean Distance from | Periodic Time.
-            | Centre of Uranus.  |     Days.
-  ----------+--------------------+---------------
-  Ariel     |   119,000 miles.   |    2.520383
-  Umbriel   |   166,000   ”      |    4.144121
-  Titania   |   272,000   ”      |    8.705897
-  Oberon    |   363,000   ”      |   13.463269
-  ----------+--------------------+---------------
-
-
-THE SATELLITE OF NEPTUNE.
-
-  ----------+--------------------+---------------
-    Name.   | Mean Distance from | Periodic Time.
-            | Centre of Neptune. |     Days.
-  ----------+--------------------+---------------
-  Anonymous |   220,000 miles.   |    5.87690
-  ----------+--------------------+---------------
-
-
-
-
-INDEX.
-
-
-  A.
-
-  Active Volcanoes, Number of, 112.
-
-  Adams, of Cambridge, and Leverrier, of Paris, 249.
-
-  Address of Mr. John Smith, 330.
-
-  Africa would be better known if on Moon, 105.
-
-  Air as Blanket to keep Earth Warm, 8.
-
-  Air not Transparent, 125.
-
-   ”  Pump, 183.
-
-   ”  Resistance of, 182.
-
-  Alcor, 343.
-
-  Aldebaran, 389.
-
-  Algol, 342, 385.
-
-  Alpha Centauri, 319;
-    Railway to, 338.
-
-  Ancient Theory to account for Rising and Setting of Sun, 48.
-
-  Andromeda, 385.
-
-  Andromeda, Nebula in, photographed by Mr. Roberts, 364.
-
-  Andromedes, 310.
-
-  Annual Motion of Earth round Sun, 56.
-
-  Annular Eclipses, 88.
-
-  Apparent Smallness of Distant Objects, 25.
-
-  Appearance of the Sun, 35.
-
-  Appearance of the Sun during a Total Eclipse, 40.
-
-  Arctic Sun, 66.
-
-  Area of Moon’s Surface, 82.
-
-  Ariel, 243.
-
-    ”    Distance and Period of, 392.
-
-  Arthur’s Seat, Volcano, 114.
-
-  Asaph Hall, Professor, 195.
-
-  Asteroids or Small Planets, 203.
-
-  Astronomer and Mathematician, 144.
-
-  Astronomers, How they measure the Distances of the Heavenly Bodies, 19.
-
-  Astronomical Facts, Table of, 391.
-
-  _Astronomie, l’_, A French Journal, 30.
-
-  Athlete on Moon, 131.
-
-  Atlantic, Sun dropped into, 48.
-
-  Atmosphere of Moon, 125.
-
-  Attraction of Gravitation, 119, 186.
-
-  August Meteors, 309.
-
-  Auriga, 385.
-
-  Auvergne, Ancient Volcanoes in, 114.
-
-  Awful Vista of Lessons, 371.
-
-  Axis of Earth Constant in Direction, 69.
-
-
-  B.
-
-  Babies on the Moon, 130.
-
-  Balloon, How supported, 124.
-
-  Bands on Saturn, 224.
-
-  Bear, Great, 381;
-    Little, 383.
-
-  Belts on Jupiter, 215.
-
-  Benefits that we receive from the Sun, 10.
-
-  Betelgeuze, 388.
-
-  Biela’s Comet, 311.
-
-  Blanket to keep Earth Warm, 8.
-
-  Books, Number of, Necessary to describe Universe, 372.
-
-  Brahe, Tycho, 170.
-
-  Brightness of Saturn, 223.
-
-      ”      ”  the Stars, 340.
-
-      ”      ”   ”  Sun, 1.
-
-  Brighton Coach, 138.
-
-  British Museum Collection of Meteorites, 313.
-
-  British Natural History Museum, 370.
-
-  “Brown Bess,” 68.
-
-  Brussels, 328.
-
-  Bull, Constellation of the, 388.
-
-  Burning-glass, Experiment with, 3.
-
-  Button illuminated, 152.
-
-
-  C.
-
-  Cambridge Observations of Neptune, 251.
-
-  Capella, 385.
-
-  Carigou, Mount, in Pyrenees, 31.
-
-  Caroline Island, 42.
-
-  Cart-wheel, Measurements with, 173.
-
-  Cassiopeia, 384.
-
-  Castor and Pollux, 345.
-
-  Celestial Library, 372.
-
-      ”        ”     Size of, 373.
-
-  Changes of the Seasons, 66.
-
-  Changes of the Sun with the Seasons, 57.
-
-  Charles I., 19.
-
-  Christmas Time, What the Sun does for us at, 10.
-
-  Clerk-Maxwell’s Top, 182.
-
-  Clock to count Sun’s Distance, 18.
-
-  Clouds a Form of Steam, 15.
-
-    ”    fill our Rivers, etc., 15.
-
-    ”    on Jupiter, 216.
-
-    ”    ”  Mars, 191.
-
-    ”    ”  Neptune, 253.
-
-    ”    ”  Saturn, 224.
-
-    ”    ”  Uranus, 243.
-
-  Cluster in the Centaur, 328.
-
-  Clusters of Stars, 327.
-
-  Coal, Mode of Production of, 11.
-
-   ”    Whence came it?, 11.
-
-  Coal-pit, 201.
-
-  Cock and the Sun, 66.
-
-  Codde, Marcus, Picture of Sunset, 30.
-
-  Coldness of Mountain Tops, 8.
-
-  Collier, Eye of a, 201.
-
-  Color of Stars, 340.
-
-  Columbiad Theory of Meteorites, 314.
-
-  Comet attracted by Sun, 274.
-
-    ”   colliding with Earth, 281.
-
-    ”   Encke’s, 143, 259.
-
-  Comet, Great, of September, 1882, 277.
-
-  Comet, Halley’s, 263.
-
-    ”    Identification of a, 258.
-
-    ”    Materials of a, 276.
-
-    ”    Movements of a, 255.
-
-    ”    of Biela, 311.
-
-    ”    of 1861, 281.
-
-    ”    seen at Sun’s Edge, 278.
-
-    ”    Speed of a, 256.
-
-    ”    Weighing Scales for a, 276.
-
-  Comets and Shooting Stars, 255.
-
-    ”    Disposition of Tails of, 257.
-
-    ”    Extravagance of, 283.
-
-    ”    Tails of, 281.
-
-  Common, Mr., 278.
-
-  Comparative Sizes of Planets, 139.
-
-  Comparison of Solar System and Nebula, 367.
-
-  Cooling of Earth and Moon, Illustration of, 116.
-
-  Copenhagen, Residence of Tycho, 170.
-
-  Corona of the Sun, 44.
-
-  Cotton Yarn, 334.
-
-  Crape Ring of Saturn, 225.
-
-  Craters on the Moon, 108.
-
-  Craters, Terrestrial and Lunar, compared, 113.
-
-  Cricket on the Moon, 131.
-
-  Cunarder’s Lights at Sea, 326.
-
-
-  D.
-
-  Danish Hounds of Tycho Brahe, 172.
-
-  Day and Night, 46, 51.
-
-  Daylight, Stars seen in, 59.
-
-  Deimos and Phobos, Satellites of Mars, 200.
-
-  Deimos, Distance and Period of, 391.
-
-  Desertion, Herschel’s, 231.
-
-  Dewar, Professor, 53, 55, 187.
-
-  Dictionary, Worcester’s, Use of, 269.
-
-  Dione, Distance and Period of, 392.
-
-  Direction of Earth’s Axis Constant, 69.
-
-  Discoveries of Kepler, 174.
-
-       ”      ”  Newton, 178.
-
-  Disguised Stars, 205.
-
-  Disposition of Comets’ Tails, 257.
-
-  Distance of the Sun, 17, 210, 391.
-
-  Distances of Heavenly Bodies, How measured, 19.
-
-  Distances of Nebulæ, 365.
-
-      ”     ”  the Stars, 332.
-
-  Distant Objects, Apparent Smallness of, 25.
-
-  Double Stars, 342;
-    Motion of, 344.
-
-  D. Q., 210.
-
-  Drawing of the Solar System, 135.
-
-  Dufferin, Lord, 65.
-
-  Dunsink Observatory, Telescope of, 95.
-
-  Dust from Meteors, 292.
-
-
-  E.
-
-  Eagle in West of Ireland, 27.
-
-  Earth, Annual Motion of, 56.
-
-    ”    colliding with Comet, 281.
-
-    ”    Diameter of, 391.
-
-    ”    Distance of, from Sun, 391.
-
-  Earth, History of, as seen from Stars, 337.
-
-  Earth, Internal Heat of, 115.
-
-    ”    Moon-view of, 78.
-
-  Earth, Period of, 391.
-
-    ”    Rank of, in Space, 329.
-
-    ”    Rotation of, 49, 391.
-
-    ”    viewed from Sun, 28.
-
-    ”    Visibility of, 369.
-
-  Eclipse, Total, Appearances seen during, 40.
-
-  Eclipses, How produced, 84.
-
-     ”      Annular, 88.
-
-     ”      of Jupiter’s Satellites, 219.
-
-     ”      ”  Moon, 89.
-
-  Edinburgh Castle, 114.
-
-  Effect of Moon’s Distance on its Appearance, 89.
-
-  Electric Lamp, Heating Effect of, 6.
-
-  Ellipse, 167;
-    Importance of, 169.
-
-     ”     and Parabola, 169, 275.
-
-  Elliptic Paths of Planets, 169.
-
-  Enceladus, Distance and Period of, 392.
-
-  Encke’s Comet, 143, 259;
-    Periodicity of, 260.
-
-  Eros, 210.
-
-  Eternal Snow on Mountain Summits, 9.
-
-  Euclid, 22.
-
-  Evening Star, 151.
-
-  Examining Moon, Quarter the Best Time for, 107.
-
-  Experiment with Burning-glass, 3.
-
-  Explosion of Krakatoa, 113.
-
-  Express Train to the Sun, 19.
-
-  Extinct Craters on the Earth, 114;
-    on the Moon, 114.
-
-  Eye of a Collier, 201.
-
-  Eyes, Use of Two, 19.
-
-
-  F.
-
-  Facts, Table of Useful Astronomical, 391.
-
-  Five o’Clock Tea, Sun’s Share in, 10.
-
-  Finlay, Mr., 278.
-
-  Flagstaff, Height of, 110.
-
-  Flora, Lawn-tennis on, 209.
-
-  Fly-boats on Royal Canal, Smooth Motion of, 52.
-
-  Flying Machines, 208.
-
-  Focus, 5;
-    of Ellipse, 170.
-
-  Football on Moon, 131.
-
-  Fossil Trees, 11.
-
-  France, Extinct Craters in, 114.
-
-  Friday, 135.
-
-
-  G.
-
-  Galle of Berlin finds Neptune, 251.
-
-  Geissler’s Tubes, 361.
-
-  Geography of Mars, 188.
-
-      ”     ”  the Moon, 105.
-
-  George III. and Herschel, 238.
-
-  Georgium Sidus, 240.
-
-  Giant Planets, 212;
-    Orbits of, 213.
-
-  Globe, Shadow of, 170.
-
-  Grand Meteors, 295.
-
-  Gravitation, 120, 186.
-
-       ”       on Moon, 128.
-
-       ”       ”  Small Planets, 209.
-
-       ”       ”  the Sun, 132.
-
-  Great Bear, 161;
-    Number of Stars in, 323;
-    How to find, 381.
-
-  Green, Mr., 193.
-
-  Greenwich, 138.
-
-  Grinding Specula, 235.
-
-  “Guards,” 383.
-
-
-  H.
-
-  Habitability of Moon, 123.
-
-       ”       ”  Other Worlds, 133.
-
-  Hall, Professor Asaph, 195.
-
-  Halley’s Comet, 263;
-    Reappearance of, 266.
-
-  Heat, Internal, of Earth, 115.
-
-   ”    of the Sun, 1.
-
-  Heating Effect of Electric Lamp, 6.
-
-  Height of Flagstaff, 110.
-
-    ”    ”  India-rubber Ball, 19.
-
-    ”    ”  Meteor, 296.
-
-    ”    ”  the Sun, 57.
-
-  Herschel, Caroline, 233, 239.
-
-     ”      William, 230.
-
-     ”         ”     at Windsor, 238.
-
-  Herschel’s Saturn, 239.
-
-  “Hœdi,” or Kids, 385.
-
-  Holmes, Oliver Wendell, 370.
-
-  Hot Water and the Sun, 14.
-
-  Houzeau, 322.
-
-  How Astronomers measure the Distances of the Heavenly Bodies, 19.
-
-  How Planets are weighed, 142.
-
-   ”  to find the Planets, 390.
-
-   ”  ”  name the Stars, 381.
-
-   ”  ”  split up a Ray of Light, 351.
-
-  Humming-top, 182.
-
-  Hundreds of Thousands of Libraries required, 376.
-
-  Hyperion, Distance and Period of, 392.
-
-
-  I.
-
-  Iapetus, Distance and Period of, 392.
-
-  Identification of Comets, 258.
-
-  India-rubber Ball, Height of, 19.
-
-  Inner Planets, The, 134.
-
-  Insects, Leaf-like, 204.
-
-  Institution, Royal, 53.
-
-  Internal Heat of Earth, 115.
-
-  Inventory of Worlds, 379.
-
-  Iris of the Eye, 202.
-
-  Iron Vapor, 291.
-
-  Island, Caroline, 42.
-
-
-  J.
-
-  Janssen’s Picture of Sun-spot, 36.
-
-  Jupiter, 214.
-
-     ”     compared with Earth, 214.
-
-  Jupiter’s Belts, 215.
-
-      ”     Clouds, 216.
-
-      ”     Diameter, 391.
-
-      ”     Distance, 391.
-
-      ”     Internal Heat, 218.
-
-  Jupiter’s Period of Revolution, 212, 391.
-
-  Jupiter’s Rotation, 215, 391.
-
-     ”      Satellites, 218.
-
-     ”          ”       Eclipses of, 220.
-
-  Jupiter’s Satellites, Distances and Periods of, 392.
-
-  Jupiter’s Weight, 214.
-
-
-  K.
-
-  Kaiser Sea, 192.
-
-  Kepler, 174;
-    His Laws, 177, 186.
-
-  Kilauea, Volcano, 112.
-
-  Kirkwood’s Description of Meteor Shower, 1833, 301.
-
-  Krakatoa Eruption, 113.
-
-
-  L.
-
-  Lalande’s Observations of Neptune, 253.
-
-  Latitude defined, 68.
-
-  Lawn-tennis on Flora, 209.
-
-  Law of Motion, First, 183.
-
-  Laws of Kepler, 177.
-
-  Leaf-like Insects, 204.
-
-  Leonids, 308.
-
-     ”     Orbit of, 302.
-
-  “Letters from High Latitudes,” 65.
-
-  Leverrier, of Paris, and Adams, of Cambridge, 249.
-
-  Life on the Moon, 123.
-
-   ”   ”  Other Worlds, 369.
-
-   ”   ”  Small Planets, 209.
-
-  Light, Velocity of, 219.
-
-  Limit of Visibility, 102.
-
-  Lion, Eyes of, 202.
-
-  Little Bear, 383.
-
-  Little Sunbeam, 17.
-
-  London, Model of, 91.
-
-  Lowell, Mr., 194.
-
-  Lunar Athlete, 131.
-
-    ”   Babies, 130.
-
-    ”   Craters, 108.
-
-    ”      ”     Origin of, 111.
-
-    ”   Cricket and Football, 131.
-
-    ”   Eclipses, 89.
-
-    ”   Foxhounds, 131.
-
-    ”   Geography, 105.
-
-    ”   Postman, 130.
-
-    ”   Seas, 108.
-
-
-  M.
-
-  Magnesium, 350.
-
-  Magnet attracting Ball, 187.
-
-  Man on the Moon, 77.
-
-  Maps of the Stars, 206, 318;
-    How to make, 389.
-
-  Mars, 134, 160.
-
-   ”    and his Movements, 160.
-
-   ”    Atmosphere of, 194.
-
-   ”    Color of, 161.
-
-  Mars, Diameter and Distance of, 391.
-
-  Mars, General Direction of Motion of, 165.
-
-  Mars, Geography of, 188.
-
-   ”    Period of, 391.
-
-   ”    Polar Snows on, 193.
-
-   ”    Retrograde Motion of, 162.
-
-   ”    Rotation of, 192, 391.
-
-   ”    Satellites of, 194.
-
-   ”    Seas on, 193.
-
-   ”    Views of, 189, 190.
-
-   ”    When to observe, 160.
-
-  Materials of a Comet, 276.
-
-  Mathematician and Astronomer, 144.
-
-  Measurements with a Cart-wheel, 173.
-
-  Measuring-rod used by Astronomers, 333.
-
-  Mercury, 134, 141.
-
-  Mercury, Diameter, Distance, Period, and Rotation of, 391.
-
-  Mercury, Transit of, 142.
-
-     ”     Weight of, 142.
-
-     ”     Where to find, 142.
-
-  Meteor, Height of a, 296.
-
-    ”     of Dec. 21, 1876, 297.
-
-    ”     ”  Nov. 6, 1869, 292.
-
-  Meteoric Dust, 292.
-
-  Meteorites, 312.
-
-  Meteorites, Columbiad Theory of, 314.
-
-  Meteorites in British Museum, 313.
-
-  Meteoroids, 285.
-
-  Meteoroids heated by Friction of Air, 288.
-
-  Meteoroids, Velocity of, 286.
-
-  Meteors, 284.
-
-     ”     August, 309.
-
-  Methuselah, 376.
-
-  Milky Way, 327.
-
-  Mimas, Distance and Period of, 392.
-
-  Minor Planets, Size and Number of, 207.
-
-  Mirror for Reflecting Telescope, 234.
-
-  Mode of Production of Coal, 11.
-
-  Model of Lunar Crater, 110.
-
-  Monday, Why so called, 74, 135.
-
-  Mont Blanc, 8.
-
-  Moon always shows Same Face, 118.
-
-  Moon, Imaginary Voyage to, 124.
-
-   ”    Life on, 123.
-
-   ”    rising in West, 198.
-
-   ”    Size of, 79, 101.
-
-  Moon’s Appearance, Effect of Distance on, 89.
-
-  Moon’s Area, 82.
-
-    ”    Atmosphere, 125.
-
-    ”    Diameter, 391.
-
-    ”    Distance, 391.
-
-    ”    Movements, 84, 118.
-
-    ”    Phases, 74.
-
-    ”    Time of Revolution, 391.
-
-  Moon-view of Earth, 78.
-
-  Morning Star, 151.
-
-  Motes in Sunbeam, 292.
-
-  Motion, Annual, of Earth, 56.
-
-  Motion of Planet round Sun illustrated, 188.
-
-  Mount Carigou in the Pyrenees, 31.
-
-  Mountains on the Moon, How measured, 108.
-
-  Mountain Tops, Coldness of, 8.
-
-
-  N.
-
-  Naming the Stars, 381.
-
-  Nasmyth, 35.
-
-  National Debt, 339.
-
-  Nature of Saturn’s Rings, 225.
-
-  Nebula in Orion, 388.
-
-    ”    Ring, in Lyra, 354.
-
-  Nebulæ, 353.
-
-  Nebulæ and Solar System compared, 367.
-
-  Nebulæ, Distances of, 366.
-
-    ”     Photographs of, 362.
-
-  Nebulæ, Stars in, 366.
-
-    ”     What made of, 359.
-
-  Neptune, Discovery of, 244.
-
-  Neptune, Former Observations of, 252.
-
-  Neptune’s Brightness, 252.
-
-      ”     Clouds, 253.
-
-  Neptune’s Diameter, Distance, Period, and Rotation, 391.
-
-  Neptune’s Size, 253.
-
-      ”     Satellite, 253.
-
-  Neptune’s Satellite, Distance and Period of, 393.
-
-  Neptune’s Time of Revolution, 213.
-
-  Newton’s Discoveries, 178.
-
-  Night and Day, 46, 51.
-
-  Noonday Gun, 5.
-
-  North Pole, 53;
-    Continual Day at, 70;
-    Sunshine at, 71.
-
-  November Showers of Meteors, 299.
-
-  Number of Books Necessary to describe Universe, 372.
-
-  Number of Minor Planets, 207.
-
-    ”    ”  Stars, 321.
-
-
-  O.
-
-  Oberon, 243;
-    Distance and Period of, 392.
-
-  Objects, Distant, Apparent Smallness of, 25.
-
-  Observing Robes, 172.
-
-  Occultation of Star by Moon, 126.
-
-  Octagon Chapel, Bath, Organist of, 231.
-
-  Old Moon in New Moon’s Arms, 77.
-
-  Orbit of Leonids, 302.
-
-  Orbits of Giant Planets, 213.
-
-    ”    ”  Uranus and Neptune, 250.
-
-  Orion, 387.
-
-  Oxygen Necessary to Life, 124.
-
-
-  P.
-
-  Pacific Ocean, Track of Eclipse across, 42.
-
-  Parabola, 169, 269.
-
-     ”      and Ellipse, 169, 275.
-
-  Parabolic Reflectors, 272.
-
-  Pegasus, Square of, 384.
-
-  Pendulum, 54.
-
-  Periodicity of Comets, 263.
-
-  Perseids, 309.
-
-  Perseus, 327, 385.
-
-  Perturbation of Encke’s Comet, 146.
-
-  Phases of Mercury, 142.
-
-    ”    ”  the Moon, 74.
-
-  Phases of Venus, 152.
-
-  Phobos and Deimos, Satellites of Mars, 200.
-
-  Phobos, Distance and Period of, 391.
-
-  Phœbe, 229.
-
-  Photographic Search for Planets, 207.
-
-  Photographing the Nebulæ, 362.
-
-  Photographs of the Heavenly Bodies, 207.
-
-  Photographs of the Moon, 107, 109.
-
-  Pit-eyes, 201.
-
-  Planetary Time Table, 179.
-
-  Planets, How to find the, 390.
-
-     ”     Small, or Asteroids, 203.
-
-     ”     Small, Search for, 205.
-
-  Planets, Small, Size and Number of, 207.
-
-  Pleiades, 363, 385;
-    Apparent Change in Position of, 387.
-
-  “Pointers,” 383.
-
-  Polar Snows on Mars, 193.
-
-  Pole, North, 53;
-    Continual Day at, 70;
-    Sunshine at, 71.
-
-  Pole Star, 383.
-
-  Postman on the Moon, 130.
-
-  Prediction of Halley, 264.
-
-  Preserved Sunbeams, 13.
-
-  Prism, Refraction of Light through, 351.
-
-  Prominences on Sun, 44.
-
-  Proportion of Sunlight received by Earth, 9.
-
-  Pupil of the Eye, 201.
-
-  Pupil of the Eye as Large as a Dinner Plate, 203.
-
-
-  Q.
-
-  Quarter the Best Time for examining the Moon, 107.
-
-  Quicksilver, 141.
-
-
-  R.
-
-  Radiant Point of Meteor Shower, 308.
-
-  Railway to Alpha Centauri, 338.
-
-  Rank of Earth in Space, 329.
-
-  Reappearance of Halley’s Comet, 267.
-
-  Recorder, Sunshine, 5.
-
-  Reflectors for Lighthouses, 271.
-
-  Refraction of Light through Prism, 351.
-
-  Relative Sizes of Earth and Sun, 31.
-
-  Requisites for Astronomical Discoveries, 199.
-
-  Resistance of Air, 182.
-
-  Retrograde Motion of Mars, 162.
-
-  Rhea, Distance and Period of, 392.
-
-  Rifle, 68.
-
-  Rigel, 388.
-
-  Ring Nebula in Lyra, 354.
-
-  Rings of Saturn, 224.
-
-  Rings of Smoke, 357.
-
-  Rising and Setting of Sun, Ancient Theories of, 48.
-
-  Roberts, Mr. Isaac, 325;
-    His Photograph of Great Andromeda Nebula 364.
-
-  Rotating Globe of Oil, 215.
-
-  Rotation of Earth, 49.
-
-  Rotation of Earth, Illustration of, 53.
-
-  Rotation of Jupiter, 215.
-
-     ”     ”  Mars, 192.
-
-     ”     ”  Sun, 39.
-
-  Rowton Meteorite, 313.
-
-  Royal Canal Fly-boats, 52.
-
-  Royal Institution, 53.
-
-
-  S.
-
-  St. Paul’s Cathedral on the Moon, 103.
-
-  Sandwich Isles, Crater in the, 112.
-
-  Satellites, 195.
-
-      ”       of Jupiter, 218.
-
-      ”       ”  Mars, 194.
-
-      ”       ”  Saturn, 229.
-
-  Satellites of Uranus, Revolution of, 243.
-
-  Saturday, 135.
-
-  Saturn, 222.
-
-    ”     and Earth, 223.
-
-  Saturn’s Bands and Clouds, 224.
-
-     ”     Brightness, 223.
-
-  Saturn’s Diameter and Distance, 391.
-
-  Saturn’s Internal Heat, 224.
-
-     ”     Period, 391.
-
-     ”     Rings, 224.
-
-     ”     Rotation, 391.
-
-     ”     Satellites, 229.
-
-  Saturn’s Satellites, Distances and Periods of, 392.
-
-  Saturn’s Time of Revolution, 212.
-
-  Scale of Universe, 319.
-
-  Search for Small Planets, 205.
-
-  Seas on Mars, 193.
-
-   ”   ”  the Moon, 108.
-
-  Seasons, Changes of the, 66.
-
-  Seasons, Changes of the Sun with the, 57.
-
-  Seen and Unseen Universe, 377.
-
-  Shadow of Globe, 170.
-
-  Shape and Size of the Sun, 29.
-
-  Shooting Stars and Comets, 255.
-
-  Shooting Stars, What becomes of them, 290.
-
-  Sidus, Georgium, 240.
-
-  Sirius, 318.
-
-    ”     How to find, 389.
-
-  Sirius, Position of, on Map of Universe, 319.
-
-  Size of Celestial Library, 373.
-
-     ”    Minor Planets, 207.
-
-     ”    Moon, 79, 101.
-
-     ”    the Sun, 29.
-
-  Sizes, Comparative, of Planets, 139.
-
-  Sizes, Comparative, of Earth and Sun, 31.
-
-  Smith, Mr. John, his Address, 330.
-
-  Smoke Rings, 357.
-
-  Snow on Mountain Tops, 9.
-
-  Snowball, 294.
-
-  Sodium, 350.
-
-  Solar Gravitation, 132.
-
-    ”   Prominences, 44.
-
-  Solar System and Nebula compared, 367.
-
-  Solar System, Drawing of the, 135.
-
-  Spectroscope, 352.
-
-  Speculum Grinding, 235.
-
-     ”     Metal, 235.
-
-  Speed of Planets, 138.
-
-  Splendor of Venus, 151.
-
-  Spots on the Sun, 33.
-
-  Square of Pegasus, 384.
-
-  Star eclipsed or occulted by Moon, 126.
-
-  Star Maps, 206, 318;
-    How to make, 389.
-
-  Stars, 318;
-    Number of, 321.
-
-    ”    are Suns, 320.
-
-    ”    Clusters of, 327.
-
-    ”    Disguised, 205.
-
-    ”    Distances of, 332.
-
-    ”    Double, 342;
-             Motion of, 344.
-
-    ”    How to name the, 381.
-
-    ”    seen in Daylight, 59.
-
-    ”    Variable, 341.
-
-    ”    What made of, 347.
-
-  Steam, Clouds a Form of, 15.
-
-  Steel melted by Sunbeams, 7.
-
-  Strontium, 348.
-
-  Sun and Sirius compared in Brightness, 320.
-
-  Sun, Benefits that we receive from the, 10.
-
-  Sun, Express Train to, 18.
-
-  Sun must be Hotter than Molten Steel, 7.
-
-  Sun’s Appearance, 35.
-
-    ”   Attraction on a Comet, 274.
-
-    ”   Corona, 44.
-
-    ”   Diameter, 391.
-
-    ”   Distance, 17, 158, 210, 391.
-
-    ”   Heat and Brightness, 1.
-
-    ”   Rotation, 39, 391.
-
-    ”   Shape and Size, 29.
-
-    ”   Share in Five o’Clock Tea, 10.
-
-    ”   Spots, 33.
-
-    ”   Sunbeams, Preserved, 13.
-
-  Sunday, 74, 135.
-
-  Sunlight, Proportion of, received by Earth, 9.
-
-  Sunset at Marseilles, 30.
-
-  Sunshine at North Pole, 71.
-
-     ”     Recorder, 5.
-
-
-  T.
-
-  Tails of Comets, 281.
-
-  Telegraph used for Comets, 267.
-
-  Telescope of Dunsink Observatory, 96.
-
-  Telescope of Yerkes Observatory, 98.
-
-  Telescopes, 92;
-    Reflecting, 232;
-    Herschel’s, 233.
-
-  Telescopic Aid, 200.
-
-      ”      View of Moon, 101, 104.
-
-  Tethys, Distance and Period of, 392.
-
-  Theory to account for the Sun’s Rising and Setting, 48.
-
-  Thursday, 135.
-
-  Ticket to Alpha Centauri, 339.
-
-  Time occupied by Light and Stars, 335.
-
-  Titan, Distance and Period of, 392.
-
-  Titania, 243.
-
-  Titania, Distance and Period of, 392.
-
-  Top-spinning under Air Pump, 183.
-
-  Total Eclipse of the Sun, Appearances during, 40.
-
-  Toy, Burning-glass not merely a, 6.
-
-  Train to the Sun, 18.
-
-  Transit of Venus, 154.
-
-  Trouvelot’s Drawing of Eclipse, 41.
-
-  Trouvelot’s Drawing of Solar Prominences, 45.
-
-  Tuesday, 135.
-
-  Twilight, 47.
-
-  Two Eyes Better than One, 19.
-
-  Tycho Brahe, 170.
-
-  Tycho’s Method of Observing, 172.
-
-  Tyndall’s, Professor, Heating Effect of Electric Lamp, 6.
-
-
-  U.
-
-  Umbriel, 243.
-
-  Umbriel, Distance and Period of, 392.
-
-  Universe, Seen and Unseen, 377.
-
-  “Up, Guards, and at them,” 337.
-
-  Uranus and Earth compared, 243.
-
-  Uranus, Clouds on, 243.
-
-    ”     Color of, 242.
-
-    ”     Diameter of, 391.
-
-    ”     Discovery of, 237.
-
-    ”     Distance of, 391.
-
-  Uranus, Former Observations of, 241.
-
-  Uranus, Period of, 391.
-
-    ”     Rotation of, 391.
-
-    ”     Satellites of, 242.
-
-  Uranus, Satellites of, Distances and Periods of, 392.
-
-  Uranus, Time of Revolution, 213.
-
-    ”     Weight of, 243.
-
-  Ursa Major, or Great Bear, 381.
-
-  Use of Two Eyes, 19.
-
-  Utility of Telescope, 200.
-
-
-  V.
-
-  Vapor of Iron, 291.
-
-  Variable Stars, 341.
-
-  Vega, 337.
-
-  Velocity of a Planet, 138.
-
-     ”     ”  Light, 219.
-
-  Venus, 134, 150.
-
-  Venus, Appearance of, during Transit, 157.
-
-  Venus as a World, 158.
-
-    ”   Composition of Air on, 160.
-
-  Venus, Diameter and Distance of, 391.
-
-  Venus, Gravitation on, 159.
-
-    ”    Greatest Brilliance of, 152.
-
-  Venus, in Telescope, 151.
-
-    ”    Period and Rotation of, 391.
-
-  Venus, Sun’s Distance deduced from Transit of, 158.
-
-  Venus, Temperature of, 159.
-
-    ”    Transit of, 154.
-
-  Venus, When and where to seek, 150.
-
-  View of Earth from the Sun, 28.
-
-  Views of Lunar Scenery, 104, 109.
-
-    ”   ”  Mars, 189, 190.
-
-  Visibility, Limit of, 102.
-
-      ”       of Earth, 369.
-
-  Volcanoes, Number of, 112.
-
-  Voyage to the Moon, 124.
-
-    ”    ”  Sun, 2.
-
-  Vulcan, 48, 49, 57.
-
-
-  W.
-
-  Waste of the Tail-making Material of Comets, 283.
-
-  Water, Hot, and the Sun, 14.
-
-  Water-mill turned by Sun, 16.
-
-  Water on Moon, 127.
-
-  Waterloo, 68, 337.
-
-  Wednesday, 135.
-
-  Weighed, How Planets are, 142.
-
-  Weighing Scales for Comets, 276.
-
-  Weight of Jupiter, 214.
-
-    ”    ”  Mercury, 149.
-
-    ”    ”  Uranus, 243.
-
-  Weights on the Moon, 127.
-
-     ”    ”   ”  Sun, 132.
-
-  What Nebulæ are made of, 359.
-
-   ”   Stars are made of, 347.
-
-  Wind, Cause of, 14.
-
-  Wind-mill turned by Sun, 15.
-
-  Worcester’s Dictionary, Use of, 269.
-
-
-  Y.
-
-  Yellowstone Park, 115.
-
-  Yerkes Observatory, Chicago, Telescope at, 98.
-
-
-
-
-Transcriber’s Notes
-
-
-The Table of Contents does not mention the “Table of Useful
-Astronomical Facts”, which begins on page 391, immediately preceding
-the Index.
-
-Punctuation, hyphenation, and spelling were made consistent when a
-predominant preference was found in the original book; otherwise they
-were not changed.
-
-Simple typographical errors were corrected; unbalanced quotation
-marks were remedied when the change was obvious, and otherwise left
-unbalanced.
-
-Illustrations in this eBook have been positioned between paragraphs
-and outside quotations. In versions of this eBook that support
-hyperlinks, the page references in the List of Illustrations lead to
-the corresponding illustrations.
-
-The index was not checked for proper alphabetization or correct page
-references.
-
-Page 208: The reference to page 124 should be to page 128.
-
-
-
-
-
-End of the Project Gutenberg EBook of Star-land, by Robert Stawell Ball
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