n.ve.s.tyolCalton.aLosAnpeles L 005 331 482 9 361 Southern Branch of the University of California Los Angeles FormLl This book is DUE on the last date stamped below FEB 21 3 0 I94f College library ORION LD/URL Form L-9-15m-8,'26 QOLUBL SCIENCE FOR THE YOUNG; Oil, THE FUNDAMENTAL PRINCIPLES OF MODERN PHILOSOPHY EXPLAINED AND ILLUSTRATED IN CONVERSATIONS AND EXPERIMENTS. AND IN NARRATIVES OF TRAVEL AND ADVENTURE BY YOUNG PERSONS IN PURSUIT OF KNOWLEDGE. VOL. II.— LIGHT. I/ SCIENCE FOR THE YOUNG. LIGHT. BY JACOB ABBOTT, AUTHOR OF 'THE FRANCONIA STORIES," "MARCO PAUL SERIES," "YOUNG CHRISTIAN SERIES," "HARPER'S STORY BOOKS," "ABBOTT'S ILLUSTRATED HISTORIES," &c WITH NUMEROUS ENGRA VINGS. NEW YORK: HARPER & BROTHERS, PUBLISHERS, FRANKLIN SQUARE. NOV 19 1900 Entered according to Act of Confess, in the year 1871, by HARPER & BROTHERS, In the Office of the Librarian ot Congress, at NYasnington. Qc 3t I A/3 OBJECT OF THE WORK. THE object of this series, though it has been prepared with special reference to the young, and is written to a considerable extent in a narrative form, is not mainly to amuse the readers with the interest of incident and ad- venture, nor even to entertain them with accounts of cu- rious or wonderful phenomena, but to give to those who, though perhaps still young, have attained, in respect to their powers of observation and reflection, to a certain degree of development, some substantial and thorough instruction in respect to the fundamental principles of the sciences treated of in the several volumes. The pleas- ure, therefore, which the readers of these pages will de- rive from the perusal of them, so far as the object which the author has in view is attained, will be that of under- standing principles which will be in some respects new to them, and which it will often require careful attention on their part fully to comprehend, and of perceiving sub- sequently by means of these principles the import and significance of phenomena occurring around them which had before been mysterious or unmeaning. In the preparation of the volumes the author has been greatly indebted to the works of recent European, and especially French writers, both for the clear and succinct expositions they have given of the results of modern in- vestigations and discoveries, and also for the designs and engravings with which they have illustrated them. CONTENTS. I. RADIATION 13 II. WONDER AND MYSTERY 21 III. THE VELOCITY OF LIGHT 30 IV. THE LAW OF THE SQUARES OF THE DISTANCES 36 V. CANDLES TOO TALL , 44 VI. INTENSITY OF LIGHT. .r>4 VII. CANDLES AND LAMPS 60 VIII. THE ARGAND BURNER 71 IX. INTERMINGLING OF UNDULATIONS 75 X. REFLECTED AND TRANSMITTED LIGHT 86 XI. SPECTRES AND GHOSTS 97 XII. THE POLYTECHNIC INSTITUTION 105 XIII. VERY BRIGHT LIGHTS 11G XIV. COMBUSTION OF MAGNESIUM 124 XV. THE MAGNESIUM LAMP 131 XVI. INCANDESCENCE 130 XVII. FOLKESTONE 1 4 1 XVIII. THE CHANNEL AT NIGHT 154 XIX. THE ELECTRIC LIGHT 163 XX. THE CORRELATION OF FORCE 1 75 XXI. FRE8NEL 183 XXII. COLOR 194 XXIII. FLIPPY 205 XXIV. ILLUSIONS EXPLAINED 215 XXV. FORMATION OF IMAGES 227 XXVI. LAWS OF REFLECTION AND REFRACTION 240 XXVII. THE EYE 249 XXVIII. THE RETURN 263 XXIX. FAREWELL TO FLIPPY 274 XXX. UP THE NORTH RIVER 283 XXXI. LIGHTING BY GAS 295 XXXII. CONCLUSION SOU A 2 ILLUSTRATIONS. Lawrence and John Frontispiece. The Inverted Image 17 Illuminated Vapors 25 Illuminated Spheres 26 Velocity of Light— Astronomical Determination 31 Velocity of Light— Experimental Determination 32 Reading the Articles 41 Enlargement as the Squares of the Distances 48 Practical Results 49 Degree of Illumination 51 Comparison of Shadows 55 Law Verified 55 Comparison of Lights 56 Ritchie's Photometer 57 Ancient Lamp 65 The Police of old Times 67 The Ducks on the Lake 78 Angles of Incidence and Reflection 87 Mode of Measurement 88 Apparent Direction 91 Reflection from Water 94 The Ghost Illusion on the Stage. 102 Spectres by a Double Reflection 112 The Magic Lantern 117 Ancient Light-house 118 The Magnesium Lamp 132 Water from Fire. 137 Manufacture of Lamp-black 141 The Bat's Wing 142 Internal Supply of Air 158 Electrified Points 165 Charcoal Points— magnified 168 In a Vacuum ..; 170 xii ILLUSTRATIONS. Pag» Light-house on a Rock .^ 1 77 Magneto-electric Machine.' ; . . .". . * . k 1* Parabolic Reflector lf Flashing Light by Reflectors 186 Convergence of Rays 1* Fresnel's Idea 188 Signal Lantern 190 Effect of the Prisms 191 Pole seeming to be bent 195 Refraction of Solar Ray ! 196 Recomposition of Light 197 Newton's Disk 198 Lines of the Solar Spectrum 199 The Spectroscope 200 The Gardens of the Tuileries 208 Masts of near and distant Ships 217 Near and distant Birds 218 Reflection from the inner Surface 222 The Plane Mirror 229 Seeing through a Stone 234 Image reversed 236 Enlargement in a Concave Mirror 237 Distorted Picture reflected true 238 Diagram. Law of Reflection 243 Reflection from a Concave Surface 244 Diagram. Law of Refraction 245 Image of the Lily 251 Section of the Eye 252 Camera Obscura in a Box 254 Camera Obscura in its own Building 255 Camera Obscura in a Tent 257 Photographic Room 259 The Thaumatrope 2G1 Atmospherical Refraction 2G5 Blowing Bubbles 2G9 Newton's Bubble 271 Rough Plays 277 Flippy when he was little 279 Ancient Lake filled up 291 A Wagon-load of Gas 303 TL^ Locomotive OAQ /fl 2. CHAPTER I. RADIATION. LIGHT proceeding from a luminous object tends to radi- ate in all directions. If the luminous object is a candle, the rays can only diffuse themselves upward and on the sides, those tending downward being intercepted by the candlestick, the table, and the ground. If the candle, so shining, is supposed to be at the surface of the earth, or upon any horizontal plane, and there is nothing to intercept its rays upward or on any side, then it is plain that the space which the rays illuminate will be of the form of a hemisphere, with a radius equal to the dis- tance through the air to which the light could penetrate. The base of the hemisphere would coincide with the ground, or the horizontal plane, whatever it might be, on which the candle was placed, while the spherical surface of it would extend into the air, forming a great dome over and around the candle, like a kind of lower sky. Let us suppose that the atmosphere at the time in ques- tion is so clear that the light of such a candle would be visible for a distance of half a mile. Then the radius of the hemisphere in the atmosphere which would be illumin- ated— that is, the distance from the centre to the outer boundary of it, would be half a mile. 14 RADIATION. But, now, if we suppose that instead of a candle at the surface of the earth we have a flame, or other incandescent object, of the same size, and of precisely the same power to radiate light, in the air half a mile above the surface of the ground, then the hemisphere that was illuminated would become a sphere or globe, the diameter of which would be a mile, the distance from the centre to the cir- cumference being on every side half a mile. The light of this candle, or of the luminous object, what- ever it might be, so placed, would be barely visible to any one on the earth looking upward, for, by the supposition, half a mile is the limit of the distance to which the rays could penetrate through the atmosphere and retain suffi- cient force to produce their proper eifect on the human eye. An eye placed any where else, also, on the margin of the illuminated sphere, and directed toward the centre, would see the light. So, also, if an eye were placed any where within the outer boundary of the sphere, and were directed toward the centre of it, it would see the light, the impression being the more vivid as the eye making the ob- servation moved in from the outer boundary toward the centre. We must not suppose, however, that such an illuminated sphere as we have described would have any precise or definite boundary. Some human eyes are much more sen- sitive than others, and can see a much fainter light, or, in other words, can see a given luminous object at a much greater distance than others. The eyes of some animals, such as insects, night birds, or beasts of prey, are probably more sensitive than any human eyes. And even beyond the limit at which the light would cease to produce an ef- fect upon any organ of vision, some of its radiations may penetrate and produce other effects of which we have no cognizance. So that the magnitude of the sphere which AN ILLUMINATED SPHERE. 15 would be occupied by the radiance would be estimated very differently according to the different tests of the pres- ence of light which we might apply. Still the portion of space that the radiation would fill would in all cases be a sphere, with the luminous source itself in the centre of it, since the limits would be at an equal distance from the centre on every side, whatever might be the test by which the limits were determined. Thus every luminous point, the radiation from which is not interrupted on any side, is the centre of an illuminated sphere — illuminated in a certain sense, as will be presently explained — which sphere is larger or smaller according to the intensity of the light and the transparency of the me- dium surrounding it. In the case of a common candle, and in an ordinary condition of the atmosphere, this sphere might perhaps be a mile in diameter, supposing the limits of it to be determined by the powers of human vision. The sphere thus surrounding the luminous point is filled with light — that is, filled in a certain sense, which will also, like the sense in which it is illuminated, be presently ex- plained. A light bright enough to be seen at a distance oftfive miles would in the same manner, if its radiance were not obstructed in any direction, form the centre of an illu- minated sphere ten miles in diameter. The sense in which this sphere is illuminated or filled with light is this, namely, that if an eye is placed any where within it, and is turned toward the centre, it will see the light — that is, there is no part of it in which there would be found any space as large as the pupil of the hu- man eye, and probably not any as large as the area in- cluded by the eye of the smallest insect, that would not furnish rays enough to form an image upon the retina so as to produce vision. And here I must pause a moment to explain how it is ] Q RADIATION. that an image is formed upon the retina of the eye so as to produce vision. If you examine one of the glasses of a pair of spectacles such as are used by elderly persons, and sometimes, indeed, by persons who are still young, but not near-sighted, you will see that the glass is thicker in the middle than at the edges. Such a glass is called a convex lens. If, now, in the evening, you remove or extinguish all the lights in the room but one, and put that light at one side, or in one cor- ner, and then proceed to the opposite corner, or into the darkest part of the room, and there hold a small piece of white paper against the wall, and one of the glasses of the spectacles between it and the light at the proper distance, you will find that an image of the candle, inverted, will bo formed upon the paper or card. The image may be small, but if the experiment is carefully performed it will be beau- tifully distinct and clear. The lens collects and concen- trates the light, and forms an image of the candle upon the paper or the card, which serves as a screen to receive it. Of course this experiment can be performed on a larger scale, and in a much more satisfactory manner, with a proper lens and other convenient apparatus, as shown in the following engraving. Now in the eye there is just such a lens and just such a screen — that is, just such in respect to function. The lens is in the front part of the eye, and the screen, which is called the retina, is in the back part ; and it is by means of this image on the retina that the picture of the outward object is conveyed to the mind. Now when it is said that the whole of the illuminated sphere surrounding a source of light as described is, in a certain seme, filled with light, the meaning is that there is no part of the whole space where an opening no larger than the pupil of the human eye will not take in enough to EXPLANATION OF THE EYE. 17 THE INVESTED IMAGE. form, by their concentration upon the retina, an image of the luminous point from which they proceed, just as a lens, held in the manner I have described, will gather rays enough coming from the candle on the other side of the room to form an image of the candle on the paper screen. "Lawrence," said John, one day, as he had been reading about this in a book, " here's a nice experiment for me to try, if I only had a pair of spectacles." " Would my eye-glass answer the purpose ?" asked Law- rence. " No," replied John, " I think not. 1 suppose your eye- glass is concave, and what I require is a convex lens. Let me take it a moment, and I can soon tell." Lawrence was lying, or, rather, reclining on a sofa in the corner of the room near a window, with his head toward the window, so that the light fell fair upon the page of the book which he was reading. John was sitting at a table near. Lawrence unhooked his glass from the cord to which jg RADIATION. it was attached, and handed it to John, saying at the same time,* " There it is ; but find out whether it is convex or con- cave with your eyes, if you can, and not with your fingers." " Why not with my fingers ?" asked John. " You can feel of it if you find it necessary," said Law- rence, "but the less we touch polished glass with our hands the better. There are always particles of dust floating in the air, and these alight on our fingers and on the glass, and when we rub our fingers over the glass we rub the surface with these." "And does that do any harm?" asked John. " It depends upon what the particles of dust are com- posed of," replied Lawrence. " Some of them are minute fragments of cotton or woolen fibres worn off" from clothes. They would not do much harm. Some are minute spores of plants." " What are spores ?" asked John. " A kind of seeds," said Lawrence. " They are from such plants as form mould and mildew ; and some smaller still — so small, indeed, that the plants themselves can not be seen except with a microscope ; and you can judge how small the seeds must be. These would not do much harm any more than the woolen and cotton abrasions. But there is another kind of dust which comes from the road, and which consists of minute scales of iron, from the shoes of the horses, and the tires of the wheels, or, what is still worse, of fragments of stone from the pavements, some of which are siliceous — that is, of the nature of flint, and are exceedingly hard. When you rub these over the glass with your fingers, or with a cloth, or a piece of leather, al- though no one rubbing produces any perceptible effect, after a time the fine polish begins to be dimmed. * See Frontispiece. TAKING CAKE OF LENSES. 19 " So that, if you are going to study optics," continued Lawrence, " and are to have any nice lenses and prisms to make experiments with, I advise you to be very careful how you rub them with dusty fingers or dusty cloths." "Yes, I will," said John. " A good lens," said Lawrence, " is a -very delicate thing, and sometimes a very costly thing. It requires, in the first place, a very nice preparation and mixture of the materials out of which the glass is made, and great care in the mak- ing of it, to secure its being uniform and homogeneous throughout, so as to act upon the light in the same way in every part. Then it is a very nice operation to grind it precisely to the true form, and to polish it perfectly. So that, when you get a good lens, if you ever do get one, you can't be too careful of it." While Lawrence had been saying these things, John had been attentively examining the eye-glass, without, how- ever, touching the glass at all. " Yes," said he, " yours is a concave lens ; it is thinner in the middle than at the edges. I want one which is thicker in the middle than at the edges." "Perhaps the landlady will lend you her spectacles," said Lawrence. Lawrence and John were at this time in lodgings in Lon- don. The keeper of the lodging-house where they had taken their rooms was quite an elderly woman, and very soon after he had given Lawrence's eye-glass back to him, John heard her footsteps in his bedroom, which was a small room adjoining their sitting-room. John went in immediately, and asked her if she used spectacles. She said she did sometimes. John asked her if she was willing to lend him her spectacles a few minutes; he wished to make an experiment on light with them. " Certainly," she said. He could have them as well as 20 RADIATION. not ; but they were a poor old pair, very loose in the joints, and she was afraid they would hardly be of any use to him. John said that the looseness of the joints would not be of any consequence. So the old lady went out and pretty soon returned with the spectacles. John's plan was to go into his own room to try his ex- periment, as it would be necessary to darken the room, he said, and he did not wish to interrupt his cousin's reading. But Lawrence said he would like to see the experiment himself. So John lighted a candle and closed the shutters, following the directions given in his book. Then, placing the candle on one corner of the mantel-piece, and going to the farther corner of the room, with the spectacles and one of Lawrence's cards in his hand, he attempted to form an image on the paper in the manner we have already de- scribed. JOHN'S EXPERIMENT. 21 CHAPTER H WONDER AND MYSTERY. JOHN was quite surprised at one phenomenon which pre- sented itself to his attention in performing his experiment. While he was making his preparations, Lawrence remained on the sofa, intending, as soon as he found that John had succeeded in bringing the light to a focus and producing an image of the candle, to go and see it. But John seemed to encounter some difficulty, and presently he said that he could not manage the spectacles, for he could not keep the bow out of the way. " If I take them by one of the glasses," he said, " and hold the other glass up for the light to shine through, the upper bow falls down over it, the joints are so loose." " Never mind," replied Lawrence ; " let it fall." " Then that will make a blank bar across my picture," said John. " No," replied Lawrence ; " try it and see." So John held one of the glasses up, with the bow belong- ing on that side hanging down over it, and then, placing his card against the wall, he moved the glass to and fro, so as to find the right distance for producing a distinct image. He expected, of course, that the shadow of the bow would be seen extending from above down over the picture, if he succeeded in producing any picture. But, quite to his surprise, he soon obtained a very excel- lent image of the candle, and without any shadow, or dark bar, or any other indication of the bow at all, to disfigure it. The image was reversed, it is true — that is, it was up- side down — but it was very distinct and very beautiful. 22 WONDER AND MYSTEKT. There was the flame perfectly formed, though pointing downward, and the wick (which appeared like a slender black line in the middle of it), and the top of the candle (which was rendered bright for a little distance by the translucency of the wax at the margin), all plainly to be seen. John was very much pleased to find his experiment so successful, and he called Lawrence to come and see it. Lawrence came, and he showed John how he could vary the effect by changing the distance of the lens from the candle, though this made it necessary also to change the distance from the lens to the screen. The nearer the can- dle was to the lens on one side, the farther it was necessary to place the screen on the other, in order to bring the rays to a, focus, as it is called— that is, to make the image dis- tinct. John was, however, very much surprised to find that there was no dark line across the picture of the candle corresponding to the bar formed by the bow of the spec- tacles. Lawrence told him it would be the same with any opaque substance at the surface of the glass. He might put a patch directly upon the glass itself, and it would not show as a spot of shadow on his picture. John tried this experiment. He cut out a small round piece of paper about as large as the section of a pea, and then, wetting it to make it adhere, he put it on the glass. Notwithstanding what Lawrence had told him, he could not help expecting to see it produce a round black spot upon the image of the candle. But it did not do so. The image became somewhat less bright than before, it is true, but there was no appearance upon it of any shadow, either from the bar or from the paper patch. Lawrence explained to him how this was, and I intend to repeat the explanation in a future chapter ; but now I DIVERGENCE OP KAYS. 23 must return to the illumined sphere, which, I have said, always surrounds every luminous point, so far as there is no object intervening to intercept the rays. This sphere, as has already been said, is filled with light in the sense that in no part of it can an eye be placed where there will not be rays enough, if the eye is turned toward the luminous point, to enter the pupil and form an image of the source of the light on the retina, as John formed an image of the candle on his card by the lens. And it is in this sense only that the space within the sphere is illumined, namely, that it is completely filled with rays of light proceeding in close proximity to each other from the centre to the circumference. These rays, it is true, diminish in intensity, in some mys- terious way, as they proceed from the centre to the circum- ference ; but, in whatever way this diminution of intensity is eifected, it is not done by a separation of the rays from each other as they diverge, so as to leave some parts of the space empty. Radiation from a luminous point is, indeed, often in books represented by lines diverging from each other as tliey re- cede from the centre, and this, however closely the lines are together in the centre, gives us the idea of a necessary separation between them toward the outer portions. But we must not imagine that the diminution in the intensity of light, as the distance from the source increases, is pro- duced by any separation of the rays. Exactly how we are to picture this diminution of intensity to our minds it is difficult to say, but it is certain that it is not the result of the separation of lines of radiance from each other as they recede, leaving intervals between them dark. In describing the phenomena we use the word rays, and we represent the radiation by lines ; but we must conceive of it, so far as we can, as homogeneous throughout, and as 24 WONDER AND MYSTERY. diminishing in intensity, when it does so diminish, without the least interruption of continuity. The sphere thus is illuminated only in this sense, that an eye in any part of it, turned toward the centre, would see the light ; looking in any other direction through the sphere, it would see nothing. We may, however, conceive such a sphere to be illuminated in another sense, as follows : If, for example, the spherical space were filled with dust or smoke, or any other substance consisting of fine parti- cles, and if the light in the centre wrere increased in intens- ity just enough to make up for the loss that would be oc- casioned by the intercepting of the light by sucli particles, then the space included would be illuminated in another way. The sphere itself would then become visible, just as the sunbeams do when shining through a crevice into a dusty or smoky room, or the rays of the sun when they il- luminate the mistiness floating among the clouds at even- ing in the western sky, and which people call " drawing water," under the erroneous idea that those lines of light are streams of vapor ascending into the air. The effect is produced by the rays of light passing through the inter- stices in the clouds, and then shining upon and being re- flected by the particles of mist which they meet with on the way. It is true that the direction of the illuminated lines is generally downward, as there is usually more mist- iness in the atmosphere near the earth than above, though they are sometimes seen ascending as well as descending, as is represented in the following engraving. If now the sphere surrounding the luminous point which I have been describing were illuminated in this manner — that is, by having particles floating in the air to receive and reflect portions of the light — with a sufficiently in- creased intensity at the source to just make up for the loss, the form and the extent of it — that is, of the whole UNSEEN LIGHT. 25 ILLUMINATED VAPOE8. sphere— would be visible to the eye, appearing like a vast ball of light a mile in diameter, bright at the centre, and gradually diminishing in brightness from the centre to the outer surface, where the light, by insensible gradations, without any definite boundary, would melt into the dark- ness and disappear. It must be understood, however, that this sphere would be thus visible to us, not by means of any of the rays of light which were passing out from the centre to the cir- cumference of the sphere on their regular course, but only by means of that portion of them which was intercepted on the way and reflected to the eye by the solid particles. And this brings to our minds a principle of fundamental importance, namely, that no light produces any effect upon our vision except such as passes into the eye. It may pass before us or across our field of view in any quantity and of any intensity without being perceptible to us at all. It is only when it enters the eye, and falls upon the screen called the retina, in the back part of it, that we can have any consciousness of its presence. Thus, if such a candle as we have supposed were sur- B 26 WONDER AXD 3IYSTEKY. rounded by an atmosphere so far transparent as not to contain any substances capable of sensibly reflecting the light, the light would go on diminishing in intensity as it receded from the centre until it disappeared, and in that sense the whole sphere would be illuminated — that is, it would befitted with a radiation of light; but in looking toward it we should see nothing except the luminous point in the centre, and we should not see that unless our eyes were turned directly toward it. And now let us suppose that, instead of one luminous point or candle flame, there were two, and that they were placed at the distance of half a mile from each other; the two illuminated spheres would then interpenetrate each other, so to speak, to the extent of half their diameters. The rays from A, proceeding in the direction toward B, would encounter those of B coming toward A. The en- counter would be direct on the line joining the two central points, and in all other parts it would be indirect, and the crossing would be at various angles. LT.CMINATEB SPHEEE8. In those parts of the space common to both spheres which are equidistant from the two centres, the two ra- diances encountering each other would be equal. In those parts which were nearer to one than to the other, the light EMANATIONS OF LIGHT. 27 coming from the nearest point would be strongest. Thus the radiances issuing from the two centres would encounter each other, in the space common to both, at every possible angle and with every possible disparity of force. And yet, so far as we can discover, such rays do not in- terfere with each other in the least ; for, wherever you put your eye within the portion traversed by the light from both the centres, if you turn your eye to either, you have its image as clearly and distinctly painted on your retina as if the other did not exist ; that is to say, the radiance which comes from one of the points, though its track has been crossed all the way by the emanation from the other — both filling the space completely — is not disturbed or in- terfered with by the other in the slightest degree. This is wonderful, and it is for the purpose of furnishing a clear, simple, and precise idea of the nature of this mys- tery, as the foundation of a right understanding of what is to follow, that I have made these suppositions of candles in the air. If you have followed what I have said closely enough to have received distinctly an idea of the nature of this won- der of the non-interference, in the ordinary sense, of lumin- ous emanations moving in contrary directions, and cross- ing each other at every possible angle and on the same track, you are prepared to appreciate in some degree the amazing magnitude and extent of it. Every star in the sky is the centre of a sphere that is illuminated by its ra- diation in the manner I have explained — a sphere, too, which is so enormous in extent that its magnitude and grandeur surpass all human conception. Light is proved to move at the rate of between one and two hundred thousand miles in a second, which is sufficient to carry it round the earth in the seventh part of a second, and there are stars so remote that it would require Hun- 28 WONDER AND MYSTERY. dreds of years for their light, moving at that inconceivably great velocity, to reach the earth ! Think of the enormous magnitude of a sphere traversed by the radiance of such a star ! Now every one of the millions of stars — and if we include, as we ought to do, with those that can be seen by the naked eye, those which are brought to view by the tel escope, either as single stars or are resolved from nebulae, the number is to be reckoned at thousands of millions — is surrounded by a sphere of radiance which must extend to us, and all these spheres occupy the same space, entering into, crossing, and interpenetrating each other in every di- rection, so that, when you hold up a needle in the evening air on a clear night, there are millions upon millions of dis- tinct radiations passing through the eye of it in both direc- tions, encountering each other at every possible angle and with every conceivable disparity of force. And yet each one of these radiations maintains its way so entirely unin- terrupted and undisturbed by the rest, that you can select any one of them you choose, and, by conducting a suffi- cient portion of it from the space around, by means of the telescope, to your eye, you can there produce a picture of its source upon the retina as clear, and distinct, and as sharply defined as if its own radiance was the only one emitted, and had the entire and exclusive occupation of the field. There is no way of escaping from or diminishing the un- utterable wonderfulness of these facts. I have only called the emission a radiance — that is, something radiated — without intending to say in what it consists. It has been thought to consist of streams of infinitely minute particles of matter. It is now generally considered as an undula- tion or vibration in some extremely subtle medium dif- fused through space, to which the name luminiferous ether has been given, which phrase means, simply, the unknown SYMBOLS FOB DESIGNATING LIGHT. 29 something which transmits the light. It is supposed that one or the other of these two suppositions must be correct, because these are the only two ways in which we can con- ceive that action of any kind can be conveyed through space. But whether there may not be modes of transmis- sion for force that man, with his present mental constitu- tion, can not conceive of, is a grave question. At any rate, it is now universally agreed among scien- tific men to regard light as transmitted by a series of un- dulatory movements in an intervening medium, and all the calculations and all the language used in describing the phenomena are based at the present day on this hy- pothesis. In all the drawings, however, and other illustra- tions intended to represent the action of light to the eye, the radiation is represented by lines, which are more ap- propriate to the idea of a progressive motion of streams of particles than to that of undulations. We are obliged to use both these modes, as the best symbols of thought at our command ; but when we attempt to pass from these symbols to the realities which they are intended to repre- sent, we are lost in wondering what the actual nature of emanations can be that can thus meet, and cross, and en- counter each other in every imaginable way — in such countless numbers, within such inconceivably narrow lim- its, and at such inconceivably rapid rates of motion — and each of the millions of separate motions pursue its own way without being in the least degree deranged, disturbed, or interfered with by the rest. We ask ourselves in amaze- ment, What can the emanation be, in its intrinsic nature, that can exist under such conditions as these? What is light ? We can not tell. We can really know nothing of its essential nature. We can only study such of its modes of action and such of its effects as come within the reach of our senses and of our half-developed reasoning powers. 30 THE VELOCITY OF LIGHT. CHAPTER III. THE VELOCITY OF LIGHT. IT may seem Btrange, as, indeed, it really is, that, since light moves at such a velocity as to carry it seven times around the earth in a second, there can be any possible way by which its velocity can be measured. But many ways of doing this have been discovered or devised. Some of these methods are astronomical — that is, the velocity of light is determined by observations of certain movements and appearances among the heavenly bodies, and by computations made from them. Fully to under- stand these computations, and the astronomical principles on which they are based, requires a degree of mathematical and astronomical knowledge which few persons have time to acquire. But astronomers have given such abundant proof of the soundness and trustworthiness of their meth- ods, in the exactness — to a second — with which their pre- dictions in respect to eclipses, transits, occultations, and other celestial phenomena are always fulfilled, that when they agree in assuring us that they have determined any point connected with celestial phenomena, we have every possible reason for placing confidence in the result. A general idea, moreover, of one of the methods adopt- ed can be obtained by the aid of the following engraving. The method consists in making first an exact computation of the time when some astronomical phenomenon will actu- ally occur, and then observing the difference in the time in which it is seen to occur by an observer on the earth when the earth is on opposite sides of its orbit. The phe- ECLIPSE OF JUPITER'S SATELLITE. 31 nomenon most convenient for such purposes as this is an eclipse of some one of the satellites of Jupiter. VELOCITY OF LIGHT — ASTRONOMICAL DETERMINATION. In the engraving, e represents the satellite about enter- ing the shadow of the planet. The precise moment at which it really enters is known beforehand, and it is found by accurate observation that the apparent time of its en- tering, as seen from the earth when it is at T, in that part of its orbit which is nearest the planet, is a certain number of minutes sooner than when it is observed when the earth is at t — that is, in the part of its orbit which is most re- mote. From these data, the time required for the light to pass through the diameter of the earth's orbit is deter- mined. The conclusions deduced from astronomical observations like these have been abundantly confirmed by ingenious devices which have been contrived for measuring the ve- locity of light on the earth's surface. The engraving on the following page represents one of these methods, the principle of which, with a little attention, can be easily un- derstood, though it would require a great deal of practical experience and skill, and very delicate powers of observa- tion, to enable any person to perform the experiment suc- cessfully with it, so as to arrive at a satisfactory result. You will see by the engraving that the apparatus con- sists of two separate parts, which are connected by dotted 32 THE VELOCITY OF LIGHT. TELOCITY OF LIG11T— EXPERIMENTAL DETERMINATION. lines. In using it, the two parts are placed at the distance of several miles from each other, the separation being rep- resented by the break in the dotted lines. The left-hand part of the apparatus is simply a hollow tube, having at the right-hand end of it a lens, and at the other end a mir- ror, which are so adjusted that a beam of light entering through the lens shall be brought to a focus on the mirror, and then reflected back through the lens again on the same path by which it came in. That an incoming and an outgoing radiation can thus pass through the same tube, in contrary directions, at the same time, without in the least degree interfering with or deranging each other, is only another example of the won- derful action of this mysterious power that was described in the last chapter. The portion of the apparatus toward the right is set at the place where the observation is to be made, the other part, as has already been said, being placed at as great a distance as possible, but within view. This second part of the apparatus, like the other, consists of a tube, with a branch near one end of it, which is open toward the source APPARATUS FOE TIMING LIGHT. 38 of light. As the light enters the tube, the rays, of course, are diverging. Near the entrance they pass through a lens by which they are made parallel. A little farther on they pass through another lens, by which, from, being par- allel, they are made to converge, but before coming to a focus they strike the plate of glass, M. This glass, though not strictly a mirror — not being silvered on the back — re- flects a large portion of the rays, turning them into the main tube, but without, however, changing their convergency. On the other side of the tube is a system of clock-work moved by a weight attached to a cord that is wound round a drum seen at the end. Those who are interested in tracing out the connections of machinery will see that there are four axles in this work. The first is the axle of the drum. Near the end of this axle, toward the right, is a large toothed icheel, which carries a small wheel upon tho end of the next axle. On the left-hand end of this second axle is another large wheel, which carries a small wheel on the third axle. This same system of large wheels carrying small ones is carried through the fourth and fifth axles, and thus a very great velocity is imparted to the fifth by the descent of the weight. On the left-hand end of the fifth axle is a large wheel, which you see enters into the tube through a slit made in the side of the tube for admitting it. The margin, or cir- cumference of this wheel, is cut into alternate notches and teeth, square in form and equal to each other, and the wheel is so adjusted in respect to the tube at the point where the beam of light passing through is just coming to a focus, that each tooth, as it moves by, shall stop a beam of light, while the notch that follows shall allow it to pass. Thus, when the wheel is rapidly revolving, a succession of flashes will pass out through the tube, following each other with inconceivable rapidity. R ? 34 THE VELOCITY OF LIGHT. Let us now suppose for a moment that the weight is dis- connected from the drum, but that there is a handle at- tached to the axis of it, by means of which we can turn the wheel at any rate of velocity we choose. Let us also sup- pose that the other part of the instrument — that is, the part that we placed on a distant hill, is so far away that it would require one second for the light to pass over the adjoining country to it, enter the tube, be reflected to the end of it, and return. This would be impossible in fact, since light travels at a rate which would carry it seven times round the earth in that time. We can, however, sup- pose it for the purpose of illustrating the action of the ap- paratus. Let us suppose that we allow a flash to pass out through a notch, and that an observer with his eye at A is watching for its return. At the expiration of the second, the time required for its journey, he would see it through the glass, M, which, it will be remembered, was not silver- ed, but only polished, so that, while it reflected a portion of the light, it also allowed a portion to pass through. But if, on the other hand, the wheel were to be moved while the ray of light was gone, so that, on the return, it should find a tooth in its way to stop it, instead of an open- ing to allow it to pass through, it is plain that the «ye at A would see nothing. And, moreover, if the wheel were made to revolve regularly at a rate which should bring a notch and a tooth alternately into the path of the ray at intervals of a second, then every flash which went out through the notch would find a tooth in its way to intei-- cept it when it came in, and the eye at A would not see the light at all. If it required less than a second, as, indeed, it actually must, for the ray of light to pass to and fro, then all that would be required to stop the flashes on their return would be to make the wheel turn faster; and it is easy to see RESULT OF THE EXPERIMENT. 35 that, from the degree of speed which it would be found necessary to give to the wheel in order to bring the teeth up rapidly enough into the path to stop every flash on its return, it would be easy to determine the time required for making the journey. You will see, by a careful inspec- tion of the figure, that there are two little indexes at the ends of the fourth and fifth axles, by which the speed of the wheels in this instrument is registered, so that the com- putation can easily be made. The result of a trial made with the apparatus near Paris corresponded very nearly, in respect to the velocity of light, with those which had been obtained by the astronomical calculations. I have described this contrivance in detail, both because it is very useful to learn to understand tho nature and ac- tion of mechanism from engravings and descriptions, and also because this case is a striking instance of the ingenu- ity and skill which have been exercised by scientific men in discovering secrets of nature which we might have thought it hopeless to attempt to unfold. The idea of at- tempting to find any means of actually measuring, with- in a space of a few miles, the velocity of a motion swift enough to pass seven times round the earth in a second, would have seemed to every one, at first view, to be utter- ly chimerical. Do not forget the result, which is, that the velocity with which light moves is such as to carry it about 175,000 miles, or seven times round the earth, in a second. The rays require about eight minutes to come to us from the sun. 36 THE LAW OF THE SQUARES OF THE DISTANCES. CHAPTER IV. THE LAW OF THE SQUARES OF THE DISTANCES. I MUST admit that the title of this chapter is not, at the first view, at all an attractive one. It sounds very mathe- matical. But then there is an interest and a beauty in a mathematical principle when it is once understood, and as this one, known as the Law of the Squares of the Distances, can "be easily understood when properly explained, and as it is one of fundamental importance, not only in its appli- cation to the subject of light, but in countless other cases where we may observe its operations in the phenomena of nature, I hope that none of the more intelligent and thoughtful of the readers of this book will be alarmed at the mathematical aspect of its name. The circumstances under which John's attention was first called to it were somewhat curious. He and his cousin Lawrence had been making an excursion that day to the Tower of London, a famous old structure, which was used in former times as a fortress to defend the city from hostile vessels coming up the river. Of course, since this was its object, it was below the city at the time when it was built, but the city has now extended far below the spot on which it stands. I: is, for other reasons also, now useless for any purposes of defense, but it is still preserved, and is used as a museum of curiosities, and contains vast collections of ancient arms and armor, and of a great many other relics of old times which are very curious to see. Lawrence and John had been to visit it that day, and had stopped on their return to their lodgings to dine at a coffee-house ; for, as it was uncertain what time they would JOHN'S TOUR ABROAD. 37 return, they had concluded not to make arrangements for having dinner at home. It was eight o'clock when they arrived, and, when they went into their sitting-room, the housemaid went before them and lighted the candles which stood on the table in the middle of the room. They were two very tall candles, in two very tall candlesticks, so that the flames of the candles were about two feet above the table. John went to the sofa and sat down upon it, as if he were glad to find a place where he could rest. " I'm tired," said he, " and yet I've an hour's work to do before I go to bed." " How is that ?" asked Lawrence. " Why, I have a half hour more of study to do, and then it will take me full half an hour to write about our visit to the Tower in my journal. I must not let my journal get behindhand." In order to explain John's remark that he had half an hour more to study that evening, I must relate how it was that he came to make this voyage to Europe with his cousin. His cousin had just graduated at the scientific school, and had formed a plan to go and spend some months in Paris, in order to pursue still farther certain branches of science for which there were great facilities in that city, and also to visit and examine certain great engi- neering works which had been constructed in England and France. He was, in fact, educating himself to be an engi- neer. John, when he heard of his cousin's design, felt a strong desire to accompany him. He proposed the plan to his mother. She was at first somewhat surprised at the prop- osition, but, the more she thought of it, the more she was pleased with the idea. She said that she would speak to his father about it. 38 THE LAW OF THE SQUARES OF THE DISTANCES. When she proposed the plan to her husband the next morning at breakfast, he at first shook his head somewhat doubtfully, saying, " It Avill make a great interruption in his studies." " It will help him very much in his French, at least," said John's mother. "True," said his father; "it will help him decidedly in his French." " And it will be a great advantage to him every way," said his mother, " to see a little of the world." "And then, besides," said John, "I can go on with my studies in other things. Cousin Lawrence is an excellent teacher." "Can you study while you are traveling?" asked his father. "Yes, sir," said John, promptly. "I can have my book and study my lesson in the cars just as well as any where else. That would not prevent my looking out of the win- dow now and then." "Well," said his father, after a moment's pause, "I'll see. Talk with your cousin about it, and see what he says. Form a definite plan, and show it to me, and I will con- sider it." So John went that same day to find his cousin, and brought the question before him. His cousin seemed very much pleased with the idea of having John for his compan- ion, and said that he would draw up some kind of a plan in the form of conditions, and that then, if John agreed to them, he could offer them to his father. A few days after this, Lawrence presented a paper to John containing the conditions, asking him to examine them and see if he was willing to agree to them, or whether he would wish to have any alterations made. John examined the conditions attentively. There was THE CONTRACT WITH LAWRENCE. 39 only one alteration that he suggested, and that was, that in the article which specified that he was to study so much every day, the words Sundays excepted should be inserted. Lawrence said that was meant to be understood, but that it would be better to have it expressed. So the words were put in. Some other minor changes were also made. "And now," said Lawrence, when the last interlineation was made, " we are ready to pass it to be engrossed." " What does that mean ?" asked John. " To have a fair copy made," said Lawrence. " In legis- lative bodies, when they have made all the amendments and alterations they wish in any proposed law, they pass it to be engrossed — that is, to have a fair copy made, in a plain hand, so that it can be easily read. When this copy is made they have it read again, and, if it is all right, they pass it to be enacted" The paper expressing the proposed agreement between Lawrence and John, when engrossed, read as follows : " I propose to take John Wollaston with me to Europe, with his father's and mother's consent, on the following conditions in respect to his studies : "1. That he is to study three hours every day, subject entirely to my direction, Sundays excepted. " 2. He is never to intermit his studies on account of traveling, whether on foot, by railway, or by steamer. " 3. He is never to ask to be excused from his study on account of his not feeling well. If I think he is so unwell on any day that he ought not to study, it will be my duty to say so. "4. Time spent in reading attentively such books as I shall direct, answering questions in respect to what he has read, writing notes and abstracts of the same, and listening to additional explanations from me, is to be reckoned as 40 THE LAW OF THE SQUARES OF THE DISTANCES. study hours. Time spent in writing letters or journals, or in reading books chosen by him for his entertainment, is not to be so reckoned. " 5. The whole responsibility of keeping an account of the time, and of seeing to it that he studies three hours every day, devolves on him, and not at all on me. " 6. He may gain to the extent of one hour for any par- ticular day, if he wishes, by studying over his time on the preceding ones ; but if he falls short of his time any day, he can not make it up on succeeding ones. " 7. Inasmuch as all human plans and arrangements are subject to unforeseen and unavoidable difficulties and in- terruptions, an allowance is made of one day each fort- night for failures. If the accidental failures do not exceed this number, the engagement on his part will be under- stood to be faithfully kept. " 8. In case of failures greater in number than this, or in case of general remissness and neglect on his part in the fulfillment of his duty as herein stipulated, there is to be ho penalty whatever, except the loss of credit which he will sustain as a young man to be relied upon for honora- bly fulfilling his engagements. " LAWRENCE WOLLASTON. " Agreed to by me, "JOHN WOLLASTON." John's father, when these articles were presented to him, read them very attentively. John stood by watching him, to observe the effect. "The penalty does not seem to be very heavy," said he. " Now, father," said John, " I think it is very heavy in- deed. I would not lose my credit with my cousin Law- rence for honorably fulfilling my engagements on any ac- count whatever." STUDY HOURS. 43 Mr. Wollaston was glad to hear John say this, and, after some farther consideration and reflection, it was decided that John should go. And it was under the operation of this agreement that John had half an hour more of study to provide for before he went to bed, on the evening of his return from his visit to the Tower, as described at the com- mencement of the chapter. But I have occupied so large a portion of this chapter in explaining the nature of the agreement between Law- rence and John, and the circumstances under which it was made, that the explanation of the law of the squares of the distances must go over to the next. I shall, however, let the title stand ; we shall come to the subject in due time. 44 CANDLES TOO TALL. CHAPTER V. CANDLES TOO TALL. *' I WISH they would not have such tall candles and can- dlesticks in our rooms," said John, as he took his seat at the table. " The light is away up in the air, and I want it down here on the table, where I am going to write." So saying, John began arranging his books and papers on the table, looking up, at the same time, with an expres- sion of dissatisfaction on his countenance, toward the light. He, however, made no more complaint, but said, " I am going to do my half hour's study first, and after- ward write in my journal. I want to do the hardest first." "I advise you to write in your journal first to-night," said Lawrence ; " I have a particular reason, which I will explain to you by-and-by." John was quite inclined in all cases to follow Lawrence's advice, as he had always found his "particular reasons'1' very satisfactory. So he wrote for half an hour in his jour- nal, while Lawrence sat near, in a large arm-chair, reading the papers. As John shut up his journal and prepared to commence his half hour's study, he looked up at the tall candles again. " It would have done just as well," said he, "if these can- dles had only been half as high, and then I should have had twice as much light." '•'•Four times as much," said Lawrence. " Twice as much," said John ; " they would have been twice as near, and so would have given twice as much light." SIMILAR, IN A GEOMETRIC SENSE. 45 " Being twice as near," said Lawrence, " would make them give four times as much light. This is a case in which the law of the squares of the distances comes in. The square of two is four." Lawrence explained this principle to John as follows : "Light, as we all know, spreads itself in both directions as it recedes from the luminous point — that is, laterally, which means from side to side, and also up and down. If it spread only laterally, then the same light would, at double the distance, fall on double the space, and would consequently be weakened one half. But it spreads in the other direction also — that is, up and down ; so that at twice the distance it will spread over four times the space." " That is curious," said John. "Yes," said Lawrence, "and it is more curious still, as it is only a single case of a universal law. The two surfaces that the same portion of light from a candle would shine upon at different distances are similar, in the geometrical sense. Do you know what the wrord similar means, in a geometrical sense ?" John said he supposed it meant alike, or somewhat alike. "It means exactly alike inform? said Lawrence, " with- out any regard to size. Thus an egg and a ball are simi- lar, in common language, being both rounded, but they are not similar in the geometrical sense, because they are not exactly alike in form. A globe made to represent the earth, if it was made a perfect sphere, would, in common parlance, be similar to the earth. It would be made, in fact, expressly in resemblance of it, but it would not bb similar in a geometrical sense, for the earth is not a perfect sphere. " So with surfaces. Two kites of exactly the same size and nearly the same shape would be similar, in common 46 CANDLES TOO TALL. language, while, on the other hand, if, of these two kites of exactly the same shape, one was only a little toy an inch long, and the other were six feet long, we should not ordi- narily say that they were similar. We should say that they were very different. In a geometrical sense, however, they would be similar, while the two that differed in form, however slightly, though of the same size, would not be similar. " Now," continued Lawrence, " if we cut a square hole in a piece of paper, and let a light shine through it upon a card, or sheet of paper, for a screen, held behind it, the bright spot made on the screen will be similar to the open- ing, provided the screen is always held square to the light. The size of the bright spot would, however, be very differ- ent at different distances, and the law of increase would be as the squares of the distances— that is, at twice the dis- tance it would be four times as great; at three times the distance, nine times as great; at five times the distance, twenty-five times as great, and so on in all cases." " I mean to try it," said John. So saying, he rose from his seat, and, procuring a card, cut a small square hole in the middle of it. He then put one of the candles away in the closet, reserving the other to form the source of light. The hole which he made in the paper was about an inch square. He then put the reserved candle on the floor, and near it placed a chair. On the chair he placed a big book, on one end, in such a manner that he could slip the card between the leaves at the other end,by which ingenious contrivance the card was supported at about the height of the candle. He placed the book so that the card should be at the dis- tance of a foot from the light, and then held a sheet of white paper at the distance of another foot. He found, as Lawrence had said, that the bright spot was two inches in DIVERGING EATS. 47 dimension each way, making the spot illuminated by the light on the sheet four times as large as the hole through which the light came. "We might know that it must be so," said Lawrence, " since the rays of light proceed in straight lines, and so diverge from each other at the same rate at every distance. It follows from this that, since that portion of rays which pass through the square hole have diverged from each other one inch in passing one foot from the source — and those that pass through the hole must do exactly that — they will diverge from each other two inches in passing through two feet. This will, of course, make the bright spot twice as long, and also twice as wide, for the diver- gence is the same in both dimensions, and thus the bright spot will be four times as large as the opening through which the light passed to make it. By the same reasoning, if the distance were three feet, it would be nine times as large, for the bright spot would contain three rows of spaces as large as the opening, and there would be three spaces in each row. If the distance were five feet, the il- luminated space would be twenty-five times as large as the opening. And so in all cases. The space illuminated by any particular portion of the light from any point will be as the square of the distance ; and as the intensity of the light, supposing that none of it is lost, would be dimin- ished just in proportion to its diffusion, the intensity upon any given space will be inversely as the square of the dis- tance:'' The principle is the same, whatever is the form of the opening through which the light shines, whether square, or round, or of any irregular figure. "Whatever the shape of the opening may be, the surface that it illuminates will be of the same shape — that is, mathematically similar, and it must be enlarged, so far as it is enlarged at all, in two 48 CANDLES TOO TALL. dimensions— that is, in what the mathematicians call a do« plicate, or double ratio. ENLARGEMENT AS THE SQUARES OF THE DISTAKOE8. Thus, in the engraving, if the distance from S to M is twice as great as from S to m, M will be doubled in two dimensions, and will, consequently, be four times as great as m. John was much interested in the experiment which he had made, and still more in the general statement of the law which it illustrated. It is, indeed, very useful to know this law, as our action in certain cases will be much influ- enced by it. And few persons, unless they have had in- struction on the subject, are aware of it. It is true that every body knows that the nearer we are to the light the better we can see; but it is not every body that knows how much better — that is, every one is not aware that by diminishing the distance one half between the light and his book, he makes the brightness of it upon the page four times as great as it was before. A gentleman who had occasion to travel much in country places where he often found it difficult to obtain a good light for certain work of writing which he had to do, had a flat THE PORTABLE CANDLESTICKS. 49 tin box made of an oval form, about four inches by three, with a socket for a short candle near one end of it on the inside. He also had a paper shade, which could be fixed to the candle, to throw the light down upon his paper. By this arrangement his light was brought within six inches of his paper, and as the effect was nearly doubled by the reflection from the inner surface of the shade, which was white, his one candle flame threw nearly as strong a light upon his paper as eight candles would have done at the ordinary height of one foot. It would have given as much light as four candles without the shade, on the principle above explained of the law of the squares of the distances. The cover of the box was made of the same form with the bottom of it, with a socket in it, also near one end, By this arrangement the sockets did not interfere with each other when the cover was put on, and the gentleman, if the light from one candle, near as he brought it to the paper, was not enough, could at any time have two, the box serv- ing as one candlestick, and the cover as the other. The PRACTICAL RESULTS. c 50 CANDLES TOO TALL. two would, of course, give him as strong a light on his pa* per as sixteen candles at the ordinary height would have done. Lawrence talked with John about the law of the squares of the distances for some time, showing him that it applied to all cases of the emanation of any force from a centre, such, for example, as heat and gravitation ; and this for the simple reason that the force, or influence, whatever it might be, in receding from the centre, was expanded in two di- mensions, length and breadth, and so the surface within which any given portion of it was included was enlarged in two dimensions, which caused the surface to increase not simply as the distance, but as the square of the dis- tance. And as the intensity of the influence would be di- minished just in proportion as it was diffused over a great- er space, the intensity — that is, the force at any one point, would be inversely as the square of the distance. This principle of the very great difference in the bright- ness of the light at different distances from the source of it — a difference far greater than one, without understand- ing the principle, would suppose — is of great importance for all who have to do any work by artificial light, as, in many cases, by diminishing their distance from the light, they can gain a much greater advantage than they would at first imagine. There is another principle, also, which it is very impor- tant to understand, and that is the illumination of the pa- per, or the page, or whatever else it is that the light shines upon, depends not merely upon the distance, but also upou the angle at which the rays fall. This will be plainly seen by the engraving on the opposite page, which shows that when the same book is held oblique- ly, as it is at "the left, it receives but half as much light as when it is at right angles to the rays, as shown on the right. WASTED LIGHT. 51 Lawrence, moreover, explained to John that this same principle of the effect of an increase in two dimensions, in respect to any quantity, had a very wide application. It applied, in fact, to all similar surfaces — that is, similar in a geometrical sense. If, for instance, we have two rooms, and one is twice as long as the other, but is of exactly the same shape — that is, if it is twice as great in all its other dimensions, it will take, not twice as much, but four times as much carpet to carpet it. A person, without reflection, might have said that it would have taken twice as much ; but, with a little consideration, we see that if it had been twice as long and only just as wide, it would have required double the quantity of carpeting, but, being twice as long and twice as wide both, it will take four times as much. It makes no difference what the shape of the two sur- faces may be, provided that they are of similar shapes. A boy has a kite a foot long. He wishes to make one of the same shape two feet long. It will require four times as much paper. If he requires his new kite to be three times as long as the other, and every thing in proportion, it will require nine times as much paper. So with the covering of a ball. There will be four times as much leather in the covering of a foot-ball ten inches in diameter as there would be in one of five inches ; for the square of five is twenty-five, and the square of ten is one hundred, and one hundred is four times twenty-five. It is true that the diameter of the balls are not lines in 52 CANDLES TOO TALL. the covering, but that makes no difference. The areas of surfaces are as the squares of any corresponding lines— that is, any lines bearing the same relation to the two sur- faces compared. Lawrence explained these things to John, who listened with close attention, and asked many questions, and at length said, " Now take your pencil and write what I shall dictate to you, to be copied into your book of notes." So John took his pencil, and Lawrence dictated as fol- lows : " FUNDAMENTAL PRINCIPLE. " In all cases of force or influence of any kind radiating from a centre, and not intercepted on its way, the intensity 'at any point is inversely as the square of the distance from the centre." " And now," said John, after he had written this, " it is high time for me to begin my half hour's study." " But your half hour's study is over," said Lawrence. "Over?" said John, surprised. " I think so," said Lawrence. " Let me see." So saying, he took out his watch and said, "Yes, and ten minutes more. Listening to instructive explanations from me, you know, is to be counted for study, if you listen at- tentively and try to understand them." " Good !" said John, in a tone expressive of great exulta- tion ; " I thought I should have half an hour more of arith- metic before I could go to bed ; but now I can go to bed at once, for I am tired and sleepy." So saying, he put away his books and papers, and pre- pared to go to his room. " But, Lawrence," said he, " what was the particular JOHN'S STUDIES. 53 reason you had for wishing me to write in my journal first?" " It was because I was going to explain to you the law of the squares of the distances in relation to radiation for your study this evening, and I thought you would like your journal work done first." " Yes," said John ; " I ana very glad that you planned it so." 54 INTENSITY OF LIGHT. CHAPTER VL INTENSITY OF LIGHT. THE art of measuring the intensity of light 5s called photometry. The word comes from the Greek word pho- tos, which means of light, and the word metros, measure- ment. On the same principle, the word photometer would mean a light-measurer, just as thermometer is a heat-meas- urer, and barometer a weight-measurer, and dynamometer a strength -measurer, from Greek words meaning heat, weight, and strength. Some very curious devices have been contrived for meas- uring the comparative intensity of different lights. In some of thes-e devices the observation is made by examining the shadows cast by the two sources of light to be compared. How this is done is shown by the engraving. There is a stand with an upright rod (m) fitted to it, and beyond the rod a screen, made usually of a plate of ground glass, to receive the shadows. Any white surface would answer well enough for such a screen, but ground glass is found to possess some peculiar advantages for this purpose. The two lights to be compared are placed at a distance from the upright rod on the side opposite to the screen, so as to cast the shadows a and d upon it. In the engraving the sources of light are a lamp (L) and a candle (B). The shadows seem to be of nearly the same intensity. If, on careful examination, they are found to be as nearly as possible alike, and if the lamp, as would seein to be the case, is nearly twice as far from the screen as the candle, then it would show that the light from the lamp THE PHOTOMETER. 55 COMPARISON OF SHADOWS. would be nearly four times as great as that from the can- dle. Of course, by exactly measuring the two distances and squaring the numbers expressing them, the exact ratio would be ascertained. It would be found, in using this instrument, that if, in- stead of a lamp at A, candles of the same kind as the one at B are used, and if the distance of A from the bar m, which intercepts the light, is made double that of B, there must be four candles at A to make the shadows equal. LAW VEBIFIED. There have been various other methods devised of meas- uring the comparative intensity of light. One more I will describe. 56 INTENSITY OF LIGHT. It is represented in the engraving, where you see on the left a screen similar to the one in the last figure, being made of a plate of ground glass set in a frame. From the centre of the plate there extends forward a shade or screen of black pasteboard, which divides the ground glass plate into two parts, and confines the light coming from each of the two sources that are to be compared to its own side of the screen. In the engraving, the light shining on one side of the screen is seen to come from a jet of gas, while that on the farther side is the light of a candle. On the table, in lines extending from the glass to the lights, are scales of inches, or other equal divisions, by which the distances of the lights from the glass respectively can be at once determined. Thus one half of the glass plate is illuminated by one light, and the other by the other, and, by looking at the two parts from the outer side, a very exact comparison can be made between them. One or the other of the lights must be moved until the two illuminations are pre- TESTING ILLUMINATING GAS. 57 cisely equal, and then, by observing the distances at which the two lights are placed, and squaring the numbers repre- senting them, we get their relative intensities. Another instrument still, which helps to show how many methods have been devised to accomplish this purpose, is known as Ritchie's Photometer. BITCniE'8 PI1OTOM The engraving shows it in section. It consists of a box (a 5), with openings on the opposite sides for the admission of light from the two sources that are to be compared. In the centre, above, is a conical tube, open at the top at d. Here the eye of the observer is to be placed to compare the effects of the two lights, which shine upon two slopes of white paper, e/and e g, which come together at e. One light or the other is to be moved until the degree of il- lumination produced by them upon the paper is the same. The intensity of the radiance, then, from the two sources will be in proportion to the squares of their distances from the centre of the box. Instruments constructed on these principles, but quite complicated in their details, are fitted up in gas-works to determine the quality of the gas. In France the intensity of the light is estimated by comparing it with that fur- nished by a certain amount and quality of oil burning at a certain rate per hour, and in England the standard of C2 58 INTENSITY OF LIGHT. comparison is the light furnished by a certain kind of can- dle. The lamp or the candle is placed upon one scale of a balance, with the proper weight in the other scale. The gas-burner is placed by the side of it, and the issue of gas is so adjusted as to make the two lights equal, as shown by the photometer placed near. Both lights are then al- lowed to burn until the scale containing the lamp or can- dle rises, showing that the prescribed amount of oil or of spermaceti has been consumed. The gas is then shut off, and the metre shows how much gas has been consumed. By this means the quantity of light which the gas affords per cubic foot is easily computed. Photometers, besides being useful in determining the light-giving power of different kinds of candles and differ- ent qualities of gas, have also been employed in comparing the light coming from various other natural and artificial sources. Those who have made these observations have come to the conclusion that the light of the sun is equal to that of between five and six thousand of the standard candles, when placed at the distance of eighteen inches ; that is, that to throw a light equal to the full blaze of the sun upon a sheet of paper would require the combined power of no fewer than five to six thousand candles placed at the distance mentioned. Of course it would be practi- cally impossible to place that number of candles so that their light could be combined. The experiment only shows what number would be necessary if the combination were possible. As for the light of the moon, even when full, every one knows that it is vastly inferior to that of the sun, but few are aware how very much inferior it is. The experiments of scientific men with photometers, and the computations which they have made from their observations, vary con- siderably in their results, as was to have been expected, INTENSITY OF SUNLIGHT. 59 but the average of them makes the light of the sun about jive hundred thousand times as great as that of the full moon on the brightest nights. Of course this can only be considered as an approximate result, as it would be impos- sible, with the means yet devised, to estimate such enor- mous differences of intensity between two lights with much accuracy. 60 CANDLES AND LAMPS. CHAPTER VH. CANDLES AND LAMPS. THEKE is a very near and intimate relation between heat and light. Both come together from the sun, and both are subject, in many respects, to the same laws. In other re- spects, the modes of action which they present are striking- ly different. The prevailing opinion among scientific men at the present day is, that the phenomena of heat and of light are produced by the same agent, modified in its ac- tion in some mysterious way, the secret of which has not yet been discovered. We shall see, in another chapter, the curious relation which heat and light bear to each other, as they come to us together in the radiance of the sun. One of the most striking differences between heat and light is, that heat can be absorbed by any substance and afterward given out again slowly, but light, apparently, is not subject to this mode of action, except in a few special cases, and in these only to a very limited extent. If you put a brick or any other substance in the rays of a hot sun, or before a bright fire, it will absorb the heat, and then, if afterward you take it to a cool place and hold your hand before it, you will feel the heat which it has absorbed radiating from it and warming your hand ; but if you take it into a dark place, your eye will not detect any luminous radiance from it — that is, there will be no evidence to the senses that it absorbed light as well as heat, so as afterward to emit it. But perhaps we can not certainly infer, from the fact that our senses do not detect EMISSION OF LIGHT AND HEAT. 61 any such radiance, that there can not be any. It is con- ceivable, certainly, that the two kinds of radiance may be absorbed and afterward emitted together, and that the hand is much more sensitive to the one kind than the eye is to the other ; in other words, that the radiance, acting as heat, will produce the sensation of warmth in the nerves of feeling while its intensity is yet low, and yet will not, acting as light, produce the sensation of vision in the nerves of sight until its intensity is very high. However this may be in the case of radiance of low in- tensity, we know that, when the radiance is of high in- tensity, the heat and light bear a very intimate relation to each other, so much so that the degree of heat is expressed often by the kind and intensity of the light that is emitted. Blacksmiths and machinists say "red hot" and "white hot" to indicate different degrees of temperature, and also "cher- ry-red" and a "low red in the dark." There is a curious difference between solids and gases at high temperatures in respect to their power of emitting light. When any substance is so intensely hot that it emits bright light, we say that it is incandescent / when a gas is incandescent, it forms flame. Young persons, often, in look- ing at a flickering flame blazing up from the fire, or at that rising from a candle or a lamp, wonder what it is. Now it is simply incandescent gas — a kind of inflammable air called hydrogen gas, which, in burning — that is, in combining with the oxygen of the air — is heated to such a degree as to be- come incandescent. Burning is simply a chemical action. It is usually the combining of some combustible with the chemical sub- stance called oxygen. There are certain very curious con- siderations connected with the fact that so much heat is developed by the combination of oxygen with combustible substances, but I have not space to explain them here. All 62 CANDLES AND LAMPS. that is necessary to enable the reader to understand what I am going to say about light is, that combustion is a pro- cess that develops great heat, and that the intensity of the heat depends in a great measure on the rapidity and abun- dance of the supply of oxygen. The intensity of the light which is developed by the heat depends, in a great measure, on the substance heated con- sisting of solid particles, for, at the same temperature, the particles of a solid substance are found generally to emit a stronger light than those of a gaseous one. But then, on the other hand, certain gaseous substances emit a greater degree of heat in combustion than most solid ones. It results from this that, in order to have an intense light, one way, at least, would be to have a gaseous sub- stance to burn in order to produce the heat, and some solid particles, or solid substance, to be heated by it, to af- ford the light. This is very simple, and yet this is the philosophy of the modes generally adopted to produce artificial light and to increase the intensity of it. Take a common lamp or candle, for example, burning with a naked flame — that is, without any glass chimney. The tallow, or wax, or spermaceti, or paraifine, or oil, or kerosene, or whatever other similar combustible is used, is composed chiefly of carbon and hydrogen, and all these substances are called, accordingly, hydrocarbons. When they are burned in the wick of the candle or lamp, the hy- drogen, which is the gas, burns and produces a great heat, and the floating particles of carbon, which, though exceed- ingly minute — too minute altogether to be seen by the naked eye— are yet solid, become intensely heated, and it is they that emit the bright light. Hydrogen, burning alone, emits a very feeble light. We sometimes see, in a wood fire, faint blue flames here and COMBUSTION AND ILLUMINATION. 63 there which have very little illuminating power. These are usually flames of hydrogen, and are produced in places where, for some accidental reason, hydrogen only for a few minutes happens to issue. They would be found to be very hot if we had any way of testing their temperature, but they would afford but a very feeble light to write by if by anv means we could bring one of them to the table. The flame of an alcohol lamp is almost entirely a hydro- gen flame, and, though it is very hot, it gives very little light, on account of there being no solid particles of car- bon in it to be intensely heated by it and to emit their superior light. It is all the better, on this account, for the purposes that the alcohol lamp is used for — namely, for producing heat; for, if there were solid particles of carbon in the flame, just so far as the force of the heat should be expended in heat- ing them so as to give light, there would bo less heat for the water, or the coffee, or the blowpipe, or for any of the other heating purposes for which the flame was used. And then, besides, the floating particles of carbon in the flame, if intercepted by any substance before they are con- sumed, blacken it, or, as AVC say, smoke it. If you hold a piece of cold iron or any other such substance in the flame of a candle or lamp, it becomes smoked, as we say. The philosophy of this is, that a great many of the floating par- ticles of carbon are intercepted by the cold substance be- fore they are consumed, and so become attached to it, and blacken it. This proves that the particles of carbon are really in the flame all the time, though we do not see them, nor see any indications of their presence, except in the in- creased brightness of the flame, in consequence of their be- ing themselves heated intensely hot in it and in process of being consumed. But by holding the iron, or any cold substance, in the flame, wre at once cool all the particles 64 CANDLES AND LAMPS. that come in contact with it, and so stop their combustion, and then their true character is at once revealed. Very often a portion of the particles of carbon escape from the flame themselves without being burnt, and go up the chimney in the form of a blue smoke. The white vapors which are seen arising sometimes from a fire are vapors of water or steam, but the blue fumes are composed of parti- cles of carbon, some of which escape out of the chimney into the air, while a portion of them lodge upon the sides of it, forming soot. Some substances give out a much greater quantity of carbon in burning than others, as, for example, birch bark, pitch-pine knots, and the "light-wood," so called, of the Southern States. A great portion of this carbon is made incandescent in the flame, and gives out great light. That is the reason why those substances make such excellent torches. Of the carbon which is thus made incandescent in these flames, some is burned — that is, it finds oxygen enough to combine writh it in the flame — and so disappears as carbon, and forms another substance. But some of the particles which are made incandescent — that is, red hot — in the flame, and so help to emit light, are not burned, be- cause there is not oxygen enough for all. This portion, then, escapes into the air, where it cools and becomes black again — that is to say, each separate particle becomes black ; but generally, when it comes from a common fire, being more or less mingled with a certain portion of watery va- por, which is white, the mixture assumes a bluish hue. When, however, it is not so modified — as, for instance, sometimes when issuing from the smoke-pipe of a steamer — it shows, by its very dark bluish color, what its true character is. The cause of this escape of carbon unconsumed is that the supply of oxygen for the flame is insufficient ; for, IMPERFECT LAMPS. whenever a particle of carbon becomes red hot in the pres- ence of oxygen, it immediately combines with it, and forms another substance which is entirely invisible. There have been devised in modern times many modes of furnishing supplies of oxygen for flames in a more rapid and abundant manner, so as to prevent the escape of any unconsumed carbon, but in early times no method was known of doing this. Indeed, the ne- cessity or desirableness of doing it was not understood, for scarcely any thing was known before the middle of the last century in regard to the true nature of flame, or of the conditions on which the greater or less degree of light which could be de- rived from it depended. Accordingly, in early times lamps were used, quite artistic sometimes in exter- nal form, but very rude and imperfect in respect to the principle on which they op- erated. There was no ar- rangement to facilitate the supply of oxygen, nor to prevent the disturbing and cooling eflect of currents of I air upon the flame, so that a faint and flickering light, accompanied by a great deal ANCIENT LAMP. 66 CANDLES AND LAMPS. of unconsumed carbon in the form of smoke, was the cer- tain result. The only light for the streets of cities in Europe two or three hundred years ago was furnished by great flaming and smoking torches carried in the hand. The darkness at night, of course, afforded great facilities for the commis- sion of all kinds of crime, and robberies, murders, and as- sassinations increased to such a degree that the govern- ment of Paris at one time organized a guard of armed men to patrol the streets in search of the criminals, lighting their way, of course, by the only kind of illumination they then knew how to produce, viz., that of blazing and smoking torches, which the link-man carried before them in his hand. The true remedy for this state of things was to dispel the darkness which occasioned it by devising some way to increase the brightness of the light which could be given by a flame, and then lighting the streets by placing a fixed burner of this increased brightness at every corner. The first method of attempting to do this was by means of a reflector placed behind the flame, so as to throw all that part of the sphere of light issuing from the flame, which would naturally go back toward the wall, where it was not wanted, forward into the street. But very soon the attention of scientific men began to be turned to the question whether the intensity of the light itself could not be increased by increasing the intensity of the heat pro- duced, and then promoting the rapidity of the combustion by a more complete and rapid supply of oxygen. There would evidently be a double advantage in this, for, by fur- nishing a full supply of oxygen, all the carbon would be consumed, instead of being allowed in part to escape un- consumed as smoke, and then, moreover, the particles which were consumed would be raised to a higher intensity of heat, and so would become more highly luminous. TUB POLICE OF OLD TIMES. BLOWING THE FLAME OUT. 69 Now, in the case of an ordinary fire of wood or of coal, the way to increase the supply of oxygen is to blow it with the bellows ; that is, to send in, by means of the bellows, a rapid current of air containing the necessary oxygen. But it is a curious circumstance that, while the blowing of a solid fire makes it burn all the brighter, blowing the flame of a candle puts it out. What is the reason of this ? Fully to understand the reason, it must be observed that blowing a fire has three different effects upon it — first, to supply oxygen to it, and so make it burn faster ; secondly, to cool it ; and, thirdly, by its mechanical impulse, to blow the burning fuel away. In the case of the blacksmith's forge, only the first of these effects is produced to any con- siderable extent. The current of air supplies oxygen to increase the combustion, which greatly increases the heat. It brings coolness too, and so prevents the heat from be- coming as great as it would be if the bellows could blow hot air instead of cold ; but the influence of the greater supply of oxygen in promoting the combustion is vastly greater in increasing the heat than the cooling effect, even in the coldest winter day, is in diminishing it. And as to the third effect, the coals being solid and comparatively heavy, the current of air is not strong enough to blow them away. If, however, we imagine that the blast was so powerful as to blow the coals from the forge all over the black- smith's shop, the fire would be put out by it at once — that is, as soon as the individual coals had time to go out in their new places, scattered over the bench and floor. If the coals were very small, this would be very soon ; and if we imagine each one of them to be no larger than a par- ticle of dust, the extinguishment would be almost instan- taneous. This is precisely what happens when we blow out a can- 70 CANDLES AXD LAMPS. die. The flame is a burning or incandescent gas, with ex- tremely minute particles of solid carbon, infinitely finer than any visible dust, pervading it. When you blow it, therefore, with a strong puff of air, the whole incandescent gaseous mass is blown away, and is instantly cooled below the point of combustion ; in other words, it goes out. If there is at the time, however, a portion of the wick in- candescent, as there usually is, that, as it can not be blown away, remains burning, and the more you blow upon it the brighter it glows, until, as fast as successive portions of it become loosened and driven off, the incandescent mass is diminished; and as the coolness of the blast prevents the combustion from extending itself to portions below, the wick, as well as the flame, is soon entirely extinguished. So much for the philosophy of blowing out a candls. FUKNACE BLOWERS. 71 CHAPTER THE ABGAND BURNER. 1$ view of the facts and explanations given in the last chapter, it is easy to understand that one way, at least, of attempting to increase the light given out by any flame is to continue some mode of increasing the supply of oxygen for it without dispersing or scattering the burning materials / in other words, of " blowing" the candle or fire without blowing it out. It was a Swiss inventor named Argand who first con trived to do this, and the contrivance which he devised is called the Argand burner to this day. But, in order that you may clearly understand the prin- ciples of his invention, I must first say that there are two ways of " blowing" fires in furnaces and forges : one by driving in the current of air by the force of propulsion be- low, and the other by drawing it in, by the force of ex- haustion in the chimney above. The former is effected by means of bellows, and some- times by another contrivance called a fan-blower, by either of which a strong blast is forced into the fire at the grate. In some furnaces where a very great heat is required, the air is heated before it is driven into the furnace, so that the full effect of the additional supply of oxygen may be secured without any diminution being caused by the cool- ness of the current of air. The latter of the modes above mentioned — that is, the drawing of air in by the force of exhaustion in the chim- ney above, is effected by making the chimney very tall. 72 THE ABOARD BURNER. The air within the chimney, being heated, is light and buoy- ant, and, of course, the taller the chimney, the more buoy- ancy there is, and the greater the draft— that is, the faster the air is " drawn in," as we usually express it, though the real mode of operation is that the pressure of the atmos- phere above the fire being taken off, in part, by the buoy- ancy of the hot air in the chimney, the air is forced in to the fire by the atmospheric pressure which acts on the or- ifice below. Now Argand's plan was to furnish the increased supply of oxygen to the fire in the flame of the lamp or candle by " drawing it in" from below by means of a chimney, and he also conceived the thought of bringing in the current in the middle of the flame instead of around the outside of it. Argand, as has already been said, was a Swiss. He was of quite humble origin, but he received a scientific educa- tion, and in the earlier part of his life he was engaged very successfully in the southern part of France in connection with industrial occupations, in which his scientific knowl- edge, and especially his knowledge of chemistry, were of great service. His attention was called, while thus employed, to the subject of light, especially for use in manufacturing and other such establishments; for in those days — near the close of the last century — there was nothing in use for ar- tificial light but such naked, smoking, and flickering flames as are given out by common lamps, torches, and flambeaux. His knowledge of chemistry showed him that the reason why the flames were not bright was the scantiness of the supply of air, which could only reach the flame on the out- side. It had been discovered some time previously that an ordinary flame was hollow — being bright only on the outer surface of it — as, of course, it must be, as in the case of such a flame there is no access to the air within. THE TROUBLES OP THE INVENTOR. 73 So Argand set himself at work to contrive a way by which to admit air to the centre of the flame ; and after a great many experiments and a great deal of contrivance, he succeeded in producing a cylindrical wick which was to be inclosed between two concentric tubes, with an opening at the bottom of the inner tube for a supply of air. He also provided suitable mechanism for raising and lowering the wick5>and fitted a sheet-iron chimney over it to in- crease the draft up through the inner tube. He made his chimney of sheet-iron, because in those days they had no means of making glass chimneys that would stand so great a heat without breaking. Of course it was necessary to place the chimney so that the lower edge of it should be just above the upper edge of the flame, in or- der that the light might not be intercepted. Not long after this the glass-makers contrived to make 'glass chimneys which would stand great heat provided they were heated gradually, and then Argand's invention was complete. But the invention, great as its value has proved to be for mankind, was the source to the unhappy inventor of it of nothing but trouble and sorrow. He became involved in disputes and lawsuits with other men, especially with a Frenchman, whose name is spelled Quinquct, and is pro- nounced, as nearly as can be represented by English sym- bols, Kaingkay. Quinquet, it would seem, drew Argand's idea from him in conversation, or, at least, obtained such glimpses of it as enabled him to produce a lamp of the same character; and he harassed and thwarted Argand in alibis attempts to obtain what would correspond to a. pat- ent right to it at the present day. Argand went to En- gland, and there was more successful. His invention was adopted in that country, and was recognized as his, and the contrivance is called the Argand burner there and iu D 74 THE ARGAND BURNER. America to this day. But in France the name of Quinquet finally carried the day, and a lamp there, with a burner on this principle, is always called a Quinquet. Argand was worn out, mind and body, by his long-con- tinued disappointments and troubles, and when he was only a little past middle life he returned to the home of his childhood in Switzerland, poor, disheartened, and miserable, and died in the imbecility and wretchedness of. a prema- ture old age. And now, nearly a century since his death, they who un- derstand these facts, after they have been reading for an hour in the evening by the bright light which his simple and beautiful contrivance has given them, sometimes pay a brief tribute to hiss memory by observing for a moment in silence the brilliant and beautiful effect produced by the double current of air, intensified in its action by the draft of the chimney, and then saying to themselves, " Poor Ar- gand !" LUMINIFEROUS ETHER. 75 CHAPTER IX. INTERMINGLING OF UNDULATIONS. As has already been stated, there are, or, rather, have been, two theories in respect to the physical nature of light — one, that it consists in the emanation of streams of exceedingly minute particles, which fly through the air with inconceivable swiftness, having in some mysterious way the power of passing through glass and all transpa- rent bodies ; and the other, that it consists in a vibratory or undulatory motion in a subtle medium, which, in order to have a name for it, has been called ether. The existence of this ether is only imaginary, however, as nothing is di- rectly known in respect to it, and it is only supposed to exist, as the sole means that we can conceive of to render the transmission of luminous undulations possible. It seems, however, as has already been said, very diffi- cult to conceive of the possibility of undulations in such infinite number and variety as must be moving at every point in space, if this theory is true, meeting, and encoun- tering, and crossing each other without in the least degree interfering with or disturbing each other's motions. Still we can not say that this would be impossible. There is complete and positive proof that sound is produced by vi- brations in the air; and yet, on a calm summer morning, we can, by listening, hear a great many different sounds, all clear and distinct, and each produced by its own undu- lations, coming through the same medium with all the rest, and each without being sensibly disturbed by the others. We can hear the songs of two or three different birds, the 76 INTERMINGLING OF UNDULATIONS. talk of children at play, the whistle of a distant locomo- tive, the bark of a dog, the crowing of a cock, the chirp of a cricket, and the faint tones of the bell in the village spire, miles away. Though we can not well attend to all these sounds at once, we can hear them all, and, if we select any one to listen to specially, we can hear it distinctly and clearly, showing that the undulations which produce it come to us through the air undisturbed by the undulations of all the rest, which, however, they must necessarily trav- erse at every conceivable angle on the way. John had a curious opportunity to observe the phenom- enon of undulations crossing each other without serious interference one evening while he was with Lawrence in London. It was in St. James's Park. There are several large parks in London where people go for recreation and amusement. The nearest, and, in some respects, the most attractive of these, is St. James's Park. This park is smaller than any of the others, but it is nearer the heart of the town, and so is more accessible to large numbers of people. The queen's palace and gar- dens are near it on one side, the houses of Parliament, and Westminster Abbey, and the Horse Guards (the great head- quarters of the army) on another, and the streets all around it are lined with gay shops and elegant residences. In the park is a long and beautiful lake, crossed in the middle by a suspension bridge. There are walks along the margin of the lake, arid chairs for people who wish to sit and rest, and beds and borders of flowers, and swans, and ducks, and other kinds of swimming birds upon the water, and on pleasant summer evenings the grounds are full of ladies, and gentlemen, and children walking about and amusing themselves in various ways. One evening, about an hour before the sun went down, as Lawrence and John were walking together in one of the AN EXPERIMENT BY THE DUCKS. 77 streets in that part of the town, on their way home from Westminster Abbey, where they had been spending an hour wandering about through the aisles, and transepts, and chapels, looking at the monuments and other curious things to be seen there, Lawrence stopped, and, pointing to a side street, said, "I am going to turn off here and go into the park. There is an experiment that I am going to have performed there for you." " Who is going to perform it ?" asked John. "A couple of ducks," said Lawrence, gravely. John laughed, but he turned very readily in the direc- tion which Lawrence indicated. They soon entered the park by a ponderous iron gate, and, after walking a little way over a broad gravel walk well filled with parties of ladies and gentlemen, and boys and girls, going to and fro, and separated on each side from the shrubberies, and lawns, and beds of flowers by an open iron fence, they came to a suspension bridge lead- ing over a narrow portion of the lake. They crossed this bridge, and then, after proceeding a little farther, they found a row of chairs, which were placed by the side of the walk and facing the water. They took their seats in two of these chairs, and looked out upon the little lake. Immediately before them, across the walk, was a band of green, with large trees here and there upon it, so near, however, that their branches intermingled. Under these trees there was a view of the water, with ducks swimming here and there over the surface of it. The sheet of water was not very wide, and beyond it, the farther shore was covered with groves of trees and thickets of shrubbery. "Well," said John, as soon as they were seated and had viewed the landscape before them for a moment, "and INTERMINGLING OF UNDULATIONS. TI1E DUCKS ON THE LAKE. what is the experiment that the ducks are going to per- form?" " It is an experiment on the crossing of undulations" said Lawrence. "You see these ducks are swimming about in all directions, and each one, as he parts the water with his breast and his paddling legs, makes two lines of waves, or undulations, which diverge from each other as they re- cede behind him, or, rather, as he advances and leaves them. There is something very curious in the laws of motion that govern the formation and the spread of these lines; but I am not going to say any thing about that now, but only to have you see what the effect is when two of these lines of waves cross each other. You -would think, in such a case, that they would disturb and destroy each other, as one would suppose the undulations or vibrations of light would do. But you will see, when we get a good chance — that is, Avhen two ducks happen to come along side by side, so that the lines of waves cross each other — that there is much less interference than one would sup- FIRST AND SECOND CLASS CHAIRS. 79 pose, and that the different lines go on after the crossing much as before." Just at this time a neatly but plainly dressed woman came along the walk, having a little leathern bag hanging by her side. She advanced to Lawrence, and held out her hand for the money to pay for the chairs. " How much ?" asked Lawrence. " Two pence," said the woman — or, rather, as she pro- nounced it, "tuppence." Lawrence gave her the money, and she went away. " I thought they were only a penny apiece," said John. " That's for the common chairs," said Lawrence. " We have taken arm-chairs, and so have made ourselves first- class people. "We might as well have taken the common chairs," said John ; " they would have been just as good for us to sit here and see the ducks." "Exactly," said Lawrence; "only then we should have marked ourselves as second-class people. Every thing is managed in England on the principle of social classifica- tion. When other people don't class you, you have to class yourself. Americans almost always prefer to pay the difference, rather than make themselves second-class people. But the English don't care so much ; they are used to such distinctions." "I don't care much," said John. "The difference does not amount to much in such a case as this," said Lawrence, "but it is worth thinking of some- times, as, for instance, on a long journey. When we go to Paris, you can save a pound or two, I suppose — which would be equal to five or ten dollars — by going second class, and so have that amount to spend for apparatus or books in Paris." "Then I'll do it," said John, jumping up suddenly from 80 INTERMINGLING OF UNDULATIONS. his chair. "I'll certainly do it ; I'd as lief go second ciasa as not." "Very well," said Lawrence. "But now look at the ducks; there are two coming now directly before us in just the right position." The two ducks that Lawrence referred to were twin* ming along nearly side by side, at a short distance from the shore, and the little line of waves which passed off from the left side of one crossed that which came from the right side of the other, but the two lines seemed scarcely to interfere with each other at all. They ap- peared to go on after the crossing, each on its way, as if it had been very little disturbed by the other. This is only one among the innumerable cases occurring in nature which show the possibility of the coexistence of different vibrations among the same set of particles — that is, in the same substance — with a degree of independence of each other which, without proof from experiment, we should have thought impossible. John was very much surprised to see how little disturbed the diverging lines of waves made by the two ducks were in crossing each other. It is true that afterward, when he saw several ducks swimming this way and that, in all directions, a good deal of irregular commotion was produced on the surface of the water ; but this apparent confusion seemed to be caused quite as much by the difficulty of following with the eye, and separating by the mind, all the differ- ent lines, as by any actual interference in the undulatory actions. The case of different sounds coming through the air to the ear, which has already been referred to, is another in- stance of the coexistence of different vibrations in the same substance, each preserving unimpaired its own dis- tinctive character. So, when a bell is struck— especially HARMONICS. 81 if it is a large and heavy bell — besides the general vibra- tion which emits the principal and dominant tone, there are always a great many others which blend with the principal one, and combine with it in the effect of produc- ing the general sound. These subordinate tones are called the harmonics. It is so with a musical string or wire ; it has its harmonics as well as its principal sound, and they all come together through the air to the ear without in- terfering with each other. So, when a band of one hun- dred performers is playing, it is wonderful to think what an immensely complicated mass of vibrations, produced by so many kinds of instruments and playing so many differ- ent parts, must come to the ear through the air at the same time ; and though many of them may mingle and blend so as to produce a delightful harmony, they do not disturb or derange each other at all, for if they did so the result would be only a discordant noise. "Such cases as these," said Lawrence, "make it just pos- sible for us to conceive that the emanations of light, mov- ing constantly, as they do, with such amazing velocity, and in every direction through the starry heavens, without in- terfering with each other at all, may be propagated, as sci- entific men now suppose, by undulations or oscillations ir» a very subtle and highly elastic medium. "At any rate," he added, "this much is certain, that these emanations are propagated, in some way most mys- terious to us, in right lines, diverging in every direction from the centre ; that they expand in two dimensions as they advance, and so each portion of them occupies a space that increases as the square of the distance, and, of course, that the intensity on a surface of given magnitude diminishes as the square of the distance, or, as the mathe- maticians say, is inversely as the square of the distance ; and, finally, that these rays are wholly invisible to us, ex- D2 82 INTERMINGLING OF UNDULATIONS. cept so far as they are intercepted in their passage and re* fleeted, so as to come to us and enter our eyes, where they form an image on the retina and produce vision." When Lawrence had said this, he took out his watch, looked at it, and said, "Well, we have been here looking at the ducks, and talk- ing about the intermingling of vibrations and undulations, about twenty minutes, or twenty-five. Shall we call this time study hours or not ?" "That is just as you say," replied John. "I've learned something, at any rate, and something that I did not know before." "And you have listened attentively to it," replied Law- rence. "I think it would be fair to consider it study hours. And there is one thing more that I should like to explain, which will take about five minutes, if you are not too tired, and that will just make up the half hour." John said he was not too tired, and would like very well to make up the half hour. So Lawrence explained that, although the emanations of light, whether they were really of the nature of undulations or not, did not appear, in ordi- nary cases, to interfere with each other at all, however nu- merous and complicated their intercrossings might be, that still there was an optical phenomenon which was called in- terference, although, strictly speaking, it was not interfer- ence in the common sense, since both of the radiations in these cases produced its own full effect, though the two effects combined produced a somewhat remarkable result. Lawrence explained the principle by asking John to im- agine that two ducks were swimming over the water in such directions that the undulations should not cross each other, but should follow each other in long lines exactly parallel. "Now we may suppose," he added, "that these lines are INTERFERENCE OF LIGHT. 83 more or less near to each other. We may imagine them to coincide exactly — that is, that the crest of the wave of one shall coincide exactly with the crest of the wave of the other, and the hollow Avith the hollow — and that by this coincidence the height of the wave would be increased, and also the depth of the hollow, so that the two undulations combined should form a united one of double intensity. We can also, on the other hand, imagine that the two sets of undulations may be separated from each other by half the breadth of the tcave, so that the hollow of one should just correspond with the swell of the other, and thus that each should counteract the effect of the other, and the water consequently remain smooth." "Oh, Lawrence," said John, "it would not be possible to fit the two lines of the waves so exactly as that." " True," replied Lawrence ; " but is it impossible to con- ceive of it ?" " Xo," rejoined John, " I don't think it is impossible to conceive of it." "You are right, probably," added Lawrence, "in saying that it would be impossible to perform this experiment with any waves and by means of ducks, but it can be rep- resented perfectly by artificial waves." " Artificial waves ?" repeated John. " Yes," replied Lawrence ; " there is an article of appa- ratus by which the action and appearance of waves can be produced by means of bars of wood rising and falling, so as to illustrate the laws of their motion. You turn a crank, and a motion representing a wave runs along the machine. With this they can show very plainly what I have been explaining to you. There are two sets of mov- ing bars, or two systems, either of which alone makes a line of waves. When they are combined in a way to make the elevations and depressions of both systems correspond^ the 84 INTERMINGLING OF UNDULATIONS. waves are of double height ; but when they are combined so as to make the elevations of one correspond with the depressions of the other, then there are no waves at all. The mechanism moves, but the surface of the water, or that which represents the surface of the water, remains smooth." " That's curious," said John. "And so with light," added Lawrence. "There is a way of contriving to make the two sets of luminous undulations unite in such a manner as to produce darkness. I can not explain to you now how it is done ; only remember that in the books it is called interference. It is only interference, however, in one sense. Strictly speaking, neither interferes with or hinders the real and proper action of the other, but the two actions combined produce a remarkable result. "And now," added Lawrence, rising, "your five minutes are out, and more too, and it is time for us to go home. Only you must try to remember exactly what is meant philosophically by interference, as the word is used, in the science of light." As Lawrence and John walked over the suspension bridge on their return home, they stopped to watch the motions of the ducks and swans which were swimming about in one place near the shore. John looked attentive- ly to see whether he could detect any thing like the phe- nomenon of interference in the optical sense, but he could not. " At any rate," said he, " I should like to have one of those wave machines that you described." "You can buy one, perhaps, when you get to Paris," said Lawrence, " with the money that you save by going second class." John's father was a very wealthy man, and was perfect- EARNING PLEASURES. 85 ly willing to supply his son with all the money that he could judiciously use, and Lawrence had full authority to furnish John with whatever he thought was judicious. But they both had the good sense to know that a boy enjoys any acquisition that he may make much more, and feels a more real and substantial sense of property in it, if he has done something to earn it himself by some kind of effort, or sacrifice, or self-denial. A father who supplies his son freely and thoughtlessly with all the money that he wants acts very unphilosophically. He is as unphilosophical as he would be in thinking that it would do just as well to give his boy money to buy fish of a vendor going by, as to let him take his rod and line and go a fishing himself to catch them. 86 REFLECTED AND TRANSMITTED LIGHT. CHAPTER X. REFLECTED AND TRANSMITTED LIGHT. A GREAT many curious and beautiful illusions are pro- duced by the reflection of light. One of the most remark- able of these is the exhibition of pretended ghosts and hob- goblins at places of public entertainment. John went with Lawrence to witness one of these exhibitions at a place of instruction and amusement in London called the Polytech- nic Institution, where the whole process was explained. I shall presently give an account of this visit, but in the mean time, in order that the reader may clearly under- stand the nature of the phenomenon, it is necessary that he should pay attention for a moment to a certain mathemat- ical principle. If you throw a ball from your hand to the floor direcf.li/ downward — that is, at right angles to the floor, its tenden- cy is to rebound directly upward — that is, to come up as it went down, namely, at right angles. On the other hand, if you throw the ball somewhat for- ward, so that it shall strike the floor at some distance be- fore you, it will, in rebounding, go still farther on. In this case the ball strikes the floor at an oblique angle, and, on rebounding, or being reflected, as we might say, it rises at the same angle on the other side. It follows from this that if a boy and a girl standing at a little distance apart are playing with a ball, and the boy wishes to throw the ball so that the girl may easily catch it on the rebound, he must throw it so that it shall strike the floor as nearly as possible midway between them, so that it may have a horizontal distance to rise in equal to THE BOUNCING BALL. 87 that which was occupied in its descent ; for this it must have if it is to rise at the same angle and attain to the same height. There is the same tendency, substantially, when the ball is thrown against a perpendicular wall, though in this case the eftect is modified to a greater extent by the weight of the ball. In the case of the reflection of liglit, the effect has noth- ing to interfere with it, and the result is, that universally the angle at which the light comes to the reflecting sur- face on one side is the same as that at which it leaves it on the other, or, as it is expressed scientifically, The angle of incidence is equal to the angle of reflection. This is one of the fundamental laws of optics. The principle is made plain by the engraving, where C ANGLES OF INCIDENCE AND REFLECTION. REFLECTED AND TRANSMITTED LIGHT. represents a small hole in a shutter admitting a ray of light from the sun into a, darkened room. The light falls upon a mirror lying horizontally upon a table, making with the perpendicular, A D, the angle of incidence, C D A. The ray is reflected in the direction D B, as far to the right of the perpendicular as it came in on the left ; in other words, making the angle of reflection, A D B, equal to the angle of incidence, ADC. An instrument has been devised for showing that these angles are exactly equal, so far as mathematical principle of this kind can be shown by experiment. The next en- graving represents this instrument. It consists of a graduated circle, in the centre of which the mirror is placed, as shown at n. Attached to the graduated circle are two slender tubes, A and B, made movable upon the arc. The distance of each from the central point above can be easily determined by the grad- uation. It is found, in experimenting with this instrument, that when one of the tubes, A, is so adjusted on the arc upon one side, and the instrument is so placed upon the table that a ray of light from the sun passes through the tube to the mir- ror, and the other tube is placed at the same distance on the other side, then, and only then, will the ray, after re- flection, pass out through the other tube, B. In the same manner, when the two tubes are placed at the same distance from the upper middle point of the arc, no matter what the distance is, a person looking through one of them will see any small object — a key, for example, held at the opening of the other, showing that in all cases the angles of incidence and reflection are equal. MODE OF MEASUREMENT. REFLECTING SURFACES. 89 Almost all substances reflect a greater or less portion of the light that falls upon them. When a substance does not reflect light at all, it appears black. When the surface is very smooth — as smooth as it is when we say it is pol- ished, and is at the same time plane — then it reflects the light regularly and uniformly, and we have an image of the luminous source in it. This is because a sufficient num- ber of the rays are simply turned from their coui-se and di- rected toward the eye to form vision. They move at the same angle, in relation to each other, after they are reflect- ed as before, and so enter the eye just as they would have done had they not been reflected, but only came from a dif- ferent direction. But when, on the other hand, the surface is not polished, then the portions of the surface on which the light falls are broken and irregular; for, even though we call it smooth, it is not perfectly smooth, and the different por- tions of it reflect the light each in its own way. Thus the light comes to our eyes in a confused manner. You can get a clear idea of this by first supposing that a looking-glass is lying in the bottom of a basket, whole, so as to reflect clearly and distinctly objects seen in it, and then that afterward it is broken into pieces, and the pieces lie in confusion in the basket, all, however, right side up. The reflections would, in this last case, be confused. You would see a general light, but no distinct image. This is precisely what happens in the case of ice. When it is whole, and has a smooth and level surface, it reflects the ice like a mirror; but when it is broken up into fine pieces, it presents a general white appearance, like snow, and that is all. Snow itself consists in minute sheets and needles of ice, any one of which, by itself, reflects light like a mirror ; but it is too small to reflect any complete object, however minute, and so all the reflections coming conr 90 REFLECTED AND TRANSMITTED LIGHT. fusedly together to the eye produce the sensation of white- There is another thing very curious about reflection, which it is necessary to understand, in order that the man- ner in which the ghost illusion is produced may be fully intelligible. It is this, that the light reflected from any mirror into the eye appears to come from an object lying in the direction of the rays as they enter the eye. This must of course be so, for the vision is produced in the eye, and by the rays which enter into it, and its character is of course determined entirely by the character of the rays and the direction in which they are moving when they enter it. In other words, the impression made upon the eye is determined entirely by the condition of the rays of light after they enter the organ. It can take no cognizance of any changes these rays may have undergone on the way. This is illustrated in the engraving, where a portion of the light from the candle is reflected from the mirror and enters the eye. But you will see that it enters the eye from such a direction, and with such a degree of conver- gence— as is sho-vn by the upper arrow — as if it had come from behind the glass. Of course it produces such an im- age in the eye, and such a conception in the mind, as if it had really come from that position ; just as if the glass, instead of being a mirror, had been transparent, and the light had come from an object behind it in the position and in the light necessary to send rays directly to the eye, such as were really sent to it by the reflection. In the engraving, only that portion of the rays from the candle are represented which comes to the eye. Of course, in fact, the radiance, in such a case, emanates from the flame in every direction, and some portion of it is reflected from every part of the surface of the glass, and is dispersed to almost every part of the room. It follows from this EXPLANATION OF VISION. f •.. .1 .',« APPARENT DIRECTION. that an eye in any place to which the reflected rays come, it' turned toward the mirror, would have an image of the candle formed within it by means of a different beam, and, of course, the candle would appear to each observer in a different place behind the mirror, namely, in a place cor- responding to the course of his own particular beam. Thus, if there were twenty persons looking at the reflec- tion of the same candle in the same glass, there would be twenty images, distinct from each other in this sense, name- ly, that each would be seen reflected in a different part of the glass from the other, and would be produced by a dif- ferent beam of light. You must remember, then, that the mental perception we have of any object, formed by the image of it in the eye, depends entirely upon the manner in which the light 92 REFLECTED AND TRANSMITTED LIGHT. from it enters the eye, and not at all upon any changes in direction or in character that it may have undergone be- fore it enters. This is one of the principles on which illu- sions are created, namely, by causing the light to enter the eye in the way it would enter if it came really from such a source as would correspond with the intended illu- sion. There is one other curious thing to be observed in order to understand clearly the philosophy of the ghost illusion, and that is, that while opaque substances, such as the met- als, will only reflect light, without transmitting any — that is, will throw back what falls upon its surface, but will not allow any to pass through its substance from the other side, transparent substances, like glass and water, will re- flect apart of the light which falls upon them, and allow a part to pass through. Thus we can look out through a window into the street, and see what is there, and we can also often see the reflection of the fire, or any other bright object in the room, on the inside. When you look out through a window from a room, and, still more, when you look into a shop window from the street, it often happens that you can not -see clearly on account of the "glare." This glare is only the reflection of light from the glass on your side of it, which reflection prevents your seeing clear- ly the transmitted light, or, rather, seeing clearly the objects on the other side, which can only be seen by transmitted light. Thus, when you look into any such glass — that of a win- dow, for instance, some reflected and some transmitted light comes to your eye— that is, some light from your face, and the other objects on the same side of the glass with your- self, which is reflected, and some from the objects on the other side which is transmitted, or, in other words, which passes through : of these two it is the strongest that con- WHICH IS THE MOKE POWERFUL? 93 quers — that is, it is the light from those objects which are most strongly illuminated, on whichever side they may hap- pen to be, that makes the greatest impression on the eye. Thus the objects in the open air are usually much more brightly lighted than those in the room, and so, in looking out at a window, we see the trees, and the houses, and the people in the street, the light from which is strong, and is transmitted to us through the glass, while we do not see the objects in the room reflected in the glass, because the light from them is much feebler ; and though it is reflected just the same, notwithstanding that the transmitted light is coming through, it does not affect our vision much in presence of the brighter light that is transmitted. But if you hold a candle, a lighted match, or any other bright light before a window, you see the reflection of it at onee in the pane, even in the daytime. You can also see even a piece of white paper reflected in a window-pane, unless there is a very bright light outside. So, in the evening, when the room is lighted up, while it is nearly dark outside, the objects in the room are then the best illuminated, and the light reflected from them over- powers altogether, in the effect produced on the retina of the eye, that coming from the street, and it is much more difficult to see any thing outside on account of this reflec- tion. You have to hold your hands up on each side of your face to cut off" the light from the objects in the room, which would otherwise be reflected too strongly. This explains the reason why it is sometimes, especially in a bright day, difficult to see plainly what is in the shop- windows along the street, especially if the panes are of plate-glass. You see the objects in the street by the re- flected light in the panes, because the light from them is stronger than the light from the objects within. So, in the daytime, when the light is stronger without 94 REFLECTED AND TRANSMITTED LIGHT. than it is within the room, those in the room can see what is passing in the street very well, while those in the street can not see distinctly what is passing in the room. In the evening, however, the case is reversed. Then, the room being better lighted than the street, we have always to close the curtains, for otherwise, the light being greater now within than without, those who are within can not see easily the objects without, while those without can see very plainly what is passing within. This principle, simple as it is, operates in a thousand dif- ferent ways, and intelligent young persons, who once un- derstand it. will take pleasure in discovering examples of it which will be continually coming under their observa- tion, if they have only learned " how to observe." It is often, for example, well illustrated in the case of water. "Water is transparent, and the surface of it, when left in repose, makes plane and polishes itself, thus forming a mirror; so that we can see objects on the bottom by light which comes up through it, or objects on the banks BEFLKCT10N FRC REFLECTIONS FROM THE WATER. 95 by light which is reflected, which we see most distinctly depends upon what is most brightly illuminated. When the water is tolerably deep, so that the bottom receives little light, then the reflected light predominates in the eye, and we see, as in the engraving, the buildings, and the trees, and the grass along the shore, and even the forms of the ducks swimming on the surface, by reflected light. The light, too, is reflected in accordance with the principle already explained, namely, so as to make the angle of incidence equal to the angle of reflection. The course of only two rays is given in the engraving — one from each of two points in the eaves of the buildings, to show the principle. In truth, however, there would be an infinite number of rays from each of these points, diverging in every direction, and striking the water at every angle, all of them being reflected on the other side of the perpen- dicular to each, at the same angle they made in coming to the surface. But only those which fell upon the water at such an angle as to cause them to proceed, after reflection, to the eye of the boy, would take effect in his vision, and he would see the image of the points from which they pro- ceeded in the direction in which they came to him, as shown by the dotted prolongation of the lines beneath the water. In the same manner, from every other point in the build- ings and in the adjoining trees light would be radiated, and would fall upon the water at every angle, and those rays that fell at the right angle to be carried, after reflec- tion, to the boy, would aid in completing the picture of the landscape in his eye. Thus he would have an image of the entire scene upon his retina, the several parts of it appear- ing as far below the water as the real object was above, except so far as the effect would be modified by the higher or lower portion of his eye. 96 REFLECTED AND TRANSMITTED LIGHT. _a=^/* If the -water were perfectly smooth, then the image would be perfectly clear and distinct. But this can never be, as the surface of water is never perfectly smooth. It is ruf- fled by slight movements of the air in the calmest days. In this case, the ripples made by the swimming of the ducks affect it •- and even the swimming of fishes or of frogs, far below, is sufficient to produce some change in the surface, and to prevent absolute repose. When water is deep and the bottom is dark, we see the reflection most clearly, for then there is very little trans- mitted light coming up through to disturb it ; but when the water is shallow, and the objects on the bottom are bright and clear, then we see the reflection indistinctly or not at all, for the reflected rays are overpowered by those which are transmitted from below. These are, then, the fundamental principles on which the ghost illusion depends, namely, that any material which is at once transparent in substance, and also plane and pol- ished upon its surface, like a plate of glass, for example, will transmit and also reflect light at the same time, so that you can see objects through it and objects reflected in it together, and the comparative distinctness with which you see them depends upon the comparative intensity of the light with which they are respectively illuminated. BEGENT STREET. 97 CHAPTER XI. SPECTRES AND GHOSTS. ONE of the principal cross streets of London is Regent Street. It crosses the great thoroughfares which run lengthwise through the vast city, and which are the scenes of the principal movement to and fro, being filled in all the business hours of the day with long lines of cabs, and om- nibuses, and drays, and coaches, and carriages of all sorts. These long thoroughfares, extending for many miles, are lined with shops of every kind; but Regent Street, which crosses them in the gayest and most fashionable part of the town, is emphatically the street of shops, or stores as we call them in America ; for the goods, and wares, and ob- jects of interest and curiosity, and the novelties of dress, and articles for presents, and books, and engravings, are more numerous and splendid, and more tastefully arranged behind the great windows of plate-glass here than in any other part of London. If you wish to buy any curious or pretty thing to bring home with you as a souvenir of Lon- don, there is not a better place to look for it than Regent Street. There is one part of the street that is more splendid than the rest. At the lower end of it, where it approaches the region of the houses of Parliament, the club-houses, and the palaces of the old nobility, it makes a grand sweep in the form of a quarter of a circle. This part of the street is known, in fact, as the Quadrant ; and as the buildings on each side are uniform in architecture throughout the whole length of it, and as the shops are, if possible, more gay and E 98 SPECTKES AND GHOSTS. splendid than those in other parts of the street, the Quad- rant is quite a celebrated place for going a shopping in, es- pecially for strangers and visitors in London. The Polytechnic Institution, or, as it is more commonly called by abbreviation, the Polytechnic, is situated in Re- gent Street, though it is in the straight part of it, at a con- siderable distance from the Quadrant. John saw one day an advertisement in the papers of an entertainment that was given there every evening, in which the manner in which ghost illusions are produced on the stage was to be explained, and he felt a strong desire to go and see it. " Yes," said Lawrence, " I should like to go too ; but, so far as the showing of a ghost is concerned, I can do that here myself for you in this very room." " But you can't do it as they do it at the Polytechnic," said John. " On the same principle," said Lawrence. " I could not do it on so large a scale, and I could not make the illusion complete, because I could not conceal from you the means by which I should do it ; but I should do it in pi'ecisely the same way, so far as the optical principles are concerned. Still, I should like very much to see how they do it at the Polytechnic." " And I should like to see how you do it your way," said John. " Very well," said LaAvrence. So saying, Lawrence took from the table a card, and cut out from it a figure which formed a rude resemblance to a ghost — that is, a fantastic figure, with its arms extended, and covered with a sheet. There was a shelf outside the window of the room that Lawrence and John were in, placed there apparently to hold flower-pots, though there were at that time no flower- pots upon it. There were also upon the mantel-piece a THE CAT AND THE GHOST. 99 number of ornaments, and among the rest an image of a cat, in a sitting posture. Lawrence went to the mantel- piece and took this image from it, and then opened the window and set the image outside upon the edge of the shelf. " There," said he, " I am going to see if I can't frighten that cat with a ghost." He then lighted one of the candles by means of a match, and brought it near to the window, holding his ghost in one hand and the candle in the other. He placed John near the window, directing him to look at the cat. Then he held the figure of the ghost a little behind John, and over his head, and also held the candle in such a position that it should shine upon the figure, and yet not shine into John's eyes. The consequence was that a strong light was thrown upon the paper ghost, which made the reflection of it plainly visible near the place where the cat was seen by direct vision. In other words, the light from the image of the cat came through the glass from the outside by transmission, while that from the ghost, from the inside, on striking the glass, came back to the eye by reflection ; and thus the two images were produced side by side on the ret- ina of the eye. " Is that the way they do it ?" asked John. "Yes," said Lawrence, " that is exactly the way in prin- ciple, only they do it on a much larger scale, and they con- trive to conceal the means. Here, for instance, the ghost, being only a paper card, is lifeless, and the cat is lifeless too ; whereas the effect would be the same if they were both alive, and could move, so as to increase the appear- ance of reality." " A cat would not do for that," said John, " for cats are not afraid of ghosts." "How do you know?" asked Lawrence. 100 SPECTRES AND GHOSTS. " Because, if they were," said John, " they would not dare to go about nights as much as they do." " They could have a man, then," said Lawrence, " and he could pretend to shoot the ghost, which it would be very safe for him to do, since he would only shoot at nothing — in the air. The real person by which the appearance was produced would be on the other side of the glass, far away." " He might break the glass, then," said John. " No," replied Lawrence, " for the image is not in the glass, but far behind it — as far behind it, in fact, as the real object is before it. The image of my little ghost, for in- stance, appears, not on the surface of the window-pane, but out on the shelf, where the cat is — as far, in fact, from the glass on the outside as I hold the paper from it on the in- side." Here Lawrence moved his card backward and forward, nearer to and farther from the glass, and showed that the image seemed to advance and recede in a manner exactly corresponding to the movement of the object. Any i-eader of this book can see how this was done by cutting out such an image and holding it up near the win- dow, with a lamp or candle near, to illuminate it strongly. The effect will be greater if this is done in the evening, be- fore it is quite dark, so that there shall be no bright light shining upon the objects in the street, but only enough to make them visible. "If I wished to make a representation of the ghost at the other side of the street," continued Lawrence, " then I should have to carry back my paper ghost to the back side of the room, and make it a great deal larger. I might have a real person, with a sheet over his arms, to represent the ghost, so that he might make gestures and walk about, but the principle would be the same in that case as in this. THE ENGRAVING EXPLAINED. 103 But even then the illusion would not be complete, for the real ghost, and the light shining upon him, and also the glass of the window in which he was reflected, would all be seen. In the contrivances for producing these illusions on the stage, all these things are concealed." So saying, Lawrence took a book out of his trunk and opened to an engraving in which the arrangements usually made for producing such illusions on the stage were repre- sented and fully explained. You see here a copy of this engraving. The glass is a lai-ge piece of plate-glass, like a very large looking-glass without any silvering on the back, and it covers the whole front of the stage. In the engraving the edges of the glass are seen, but in the actual performance these edges are concealed by the curtains coming close to them on each side. Of course the glass must be extremely large. It is placed, too, in an inclined position,, so that all the light shining upon it from the side toward the audience is re- flected downward, and thus it produces no "glare." The figure which is to form the image of the ghost in the glass is in front of the stage, and is placed low enough to be entirely concealed from the view of the spectators. It is strongly illuminated by a very bright light, which is also concealed under the stage. The light, of course, radi- ates in every direcfion from the figure, and spreads over the whole surface of the glass, whence it is reflected all over the theatre ; and a certain portion of it, sufficient to form an image upon the retina, reaches every spectator, and such an image, moreover, as if the light, instead of coming up from beneath the stage, and being reflected by the glass, had really come from an equal distance behind the stage — that is, from the place where the image seems to stand, near the man who is pointing a pistol at it. This man would, however, not see any image at all near 104 SPECTRES AND GHOSTS. him, any more than a person behind a looking-glass stand- ing out upon a floor would see the images reflected in it to those before it. The figure is represented there only to show how it wrould appear to the people in front. In the same manner, the outline of the figure upon the glass itself is only imaginary, being sketched there only to show how and where the light is reflected. There would be no such image really there. The only things that would actually exist would be the figure below, the light emana- ting from it, and the images of it in the eyes of the specta- tor. In other words, what the spectators really see is the figure itself, only they see it by light coming to them in zigzag lines, or, at least, lines taking one sharp turn, as it is reflected in the glass, instead of in direct lines ; and as the short turn taken by the rays of light at the glass is al- together outside of the eye, their sense takes no cognizance of it, and the object appears to them as if it were seen di- rectly before them on the. stage. A VISIT TO THE POLYTECHNIC. 105 CHAPTER XH. THE POLYTECHNIC INSTITUTION. LAWKENCE and John set off one evening about seven o'clock to go to the Polytechnic. They went up through Regent Street, and, as they had plenty of time before them, they stopped along the way to look into tl e shop-windows, to examine and admire the curious and beautiful things that were to be seen in them. At length they arrived at their destination. The building presented somewhat the appearance of a church. They passed in through the porch in company with many other persons, and stopped on one side, as they went in, at a little office to buy their tickets. The tickets were a shilling — that is, an English shilling — each. The value of an English shilling is about a quarter of a dollar. When they were fairly within, they found that the inte- rior of the principal room was still more similar, in its ar- rangements, to a church than the exterior had been, for it was of an oblong shape, with an open floor below, and gal- leries above all around supported by pillars ; only, instead of pews, the floor below was filled with small steam-engines and large articles of apparatus ; and the floor of the gal- lery was also flat, and had a range of curiosities and little machines over the balustrade in front, and upon shelves against the wall behind, with an open space for people to walk in between. Where the pulpit usually is in a church there was a great tank, ten feet square and very deep, which was filled with water, and over it there was hanging an immense diving-bell, ready to go down. E2 106 THE POLYTECHNIC INSTITUTION. Lawrence and John walked about for some time among these things, observing and examining all that they saw, until at length they came back to what would be called in a church the singers' gallery, where there was a seat, upon which they could sit and rest themselves, and, at the same time, look down over the balustrade to the scene below. Immediately beneath them, on the lower floor, was a steam-engine, and various other machines connected with it. A little farther on was a large table with a blowpipe, and several other contrivances attached to it for glass- blowing. The glass-blower was sitting at the table at work, making a great many curious things, and a number of persons — young men, young women, and children — were standing around the table watching the operations, and buying the articles which the man made as soon as fin- ished, or selecting them from a large supply of similar ar- ticles which he had on the table before him, and which he had previously made. It was curious, John thought, to see the process of spin- ning glass silk as this man performed it. He had on his left hand a wheel, about a foot in diameter, which was mounted on a stand, and made to turn by a crank. To spin the glass thread, or silk, he held the end of a glass rod in the flame of the blowpipe before him, to keep a portion of it melted, or at least softened enough to be drawn out into a fine thread, and this thread, as fast as it was drawn, he wound round the wheel, turning the wheel all the time with his left hand. Of course Lawrence and John could not see the thread from the distance at which they were sitting, it was so fine ; but they could see the glass rod, and the glow of light at the end of it, where it touched the point of the flame of the blowpipe, and they could also see the wheel turn, which served as a reel to wind the skein upon. GLASS SPUN INTO THREADS. 107 " I should not have thought it would be possible to spin glass in that way," said John, " and certainly not to spin it so even and true. I looked at a piece of it when I was down there, and it was just as even in every part as a hair, and very fine, and yet the man does not seern to take any pains to make it so even." " It evens itself," said Lawrence. " There are one or two very curious principles involved. There is one that you will think is specially curious, if I can only explain it so that you will understand it. You see that, the hotter the glass is, the weaker it is, because it is more fluid ; while the colder it is, the stronger and stiffer it is. Now, in draw- ing out the glass, it gets cold and strong just in proportion as it gets thin, and that holds it in the thin and slender places, and prevents it getting thinner. "In other words," continued Lawrence, "just so fast as any parts of the thread get thinner than the rest, they cool at once, and become stronger, and that brings the force to act upon the parts which are thicker, and, of course, more soft, and they are drawn out until they are as thin as the rest." The same principle operates on a larger scale in pulling candy, and it is by the effect of it that it is so easy to make the candy into sticks so uniform and even. Just at this moment a bell was rung, and there was ah immediate movement among the audience toward a certain door behind the place where Lawrence and John had been sitting. They immediately rose and followed the multi- tude, supposing that the bell was a summons to hear the lecture on ghosts. There was a great crowding and jam- ming in getting through the door, each one apparently be- ing eager to obtain a good seat. Lawrence and John pressed onward with the crowd, and, when they at last entered the room, they found that 108 THE POLYTECHNIC INSTITUTION. it was a lecture-room arranged somewhat like a theatre. There was a small stage at the farther side, with a curtain drawn before it. In front of the stage was a narrow area, which was open, and a desk, or small pulpit, with a table by the side of it, for the lecturer, at one end, and something that looked like a large and tall box on wheels at the other end. Immediately back of this open area were the seats for the audience, wThich rose one above another by a rapid ascent, so that every body could see. There were various exhibitions and performances during the lecture, which continued for about half an hour. The lecturer was the celebrated Professor Pepper, who is dis- tinguished for his tact and skill in explaining and elucida- ting philosophical principles, and making every thing clear. He had an assistant with him, and the first thing to be done was to darken the room, and then throw a beam of very strong light from a kind of lantern that the assistant had upon the table across the area — or, rather, along the area from one side of the room to the other — before the specta- tors. The beam made a round and very bright spot upon the wall, but was not visible on its way through the air, or scarcely visible, because there was nothing there to in- tercept and reflect it to the eyes of the spectators. For you will recollect that, as has been explained before, light is not visible by itself as it passes through space before us, but only so far as it is intercepted by some substance and turned from its course, and so directed into the eye. There were, however, some few minute motes and parti- cles of dust floating in the air of the room over the area, which served this purpose of intercepting and reflecting the light in some slight degree, so that the path of the beam through the air was not wholly invisible. The as- sistant, however, soon brought it very clearly into view by using something which emitted a thick smoke or dust of EXHIBITION OF THE GHOSTS. 109 some kind, and which he beat in the path of the beam of light so as to make it very distinctly and beautifully visi- ble. The professor then held mirrors and lenses of various kinds in the path of the beam, so as to turn the light in va- rious directions, and change the condition of it in various ways, thus causing it sometimes to converge and come to a point, and sometimes to diverge and diffuse itself, in which case it made a very large and bright spot upon the ceiling overhead, or upon any part of the wall on which he caused it to fall. The assistant all the time continued to puff the smoke or dust into the path of light, so as to make its course distinctly visible. After some farther experiments and illustrations of this kind, the time came for the ghosts. The curtain rose and brought to view a small stage, like that of a theatre, only the front of it was closed by an immense pane of plate- glass, which must have been some ten feet square. This glass was, however, not at all noticed by the audience, for it was inclined forward at such an angle as to throw the reflections of all the light that came from the side toward the audience down toward the floor, and under the front of the stage, so that none except those who were in the secret had any idea that there was any glass there. The edges of it, at the ends, were well concealed by curtains coming up close to it. The spectators, therefore, did not see either the glass itself, or any thing reflected in it. Their reflections were all thrown downward. It is true that if there had been any thing bright down beneath the front of the stage they would have seen the reflection of it coming up to them ; bnt good care had been taken to prevent that by making it dark there. The ghost was there, or, rather, the person who was to represent the ghost, but there was no light yet shining upon him to be reflected by the glass toward the HO THE POLYTECHNIC INSTITUTION. audience, and so the audience saw nothing in the glass, but only saw through it, and, of course, only saw what was actually upon the stage before them. Things being thus arranged, all that was necessary was to allow the real person on the stage to talk and act as usual until the time came for the ghosts or hobgoblins to appear, when all at once a very bright light was thrown upon the objects representing these things under the stage, when all the spectators in the seats would see them re- flected in the glass ; and, as the images of them would appear as far behind the glass as the objects themselves were before it, they would seem to be back upon the stage, among the real actors. There is one curious difficulty, however, in the manage- ment of such an exhibition, and that is, that without some special contrivance to prevent the effect, the position of the mirror, inclined at an angle of forty-five degrees more or less, would have the effect of making the floor on which the personations of ghosts and goblins stood under the front of the stage appear in the glass in a perpendicular position — that is, up and down — so that the images in it would appear standing out upon the wall, in an impossible attitude. John had observed in the advertisement of the exhibition in the papers that the ghosts would " dance on walls and ceilings," and he had at first imagined that the being able to make them do so would be the special won- der of the performance, and would require very particular and extraordinary, and even, perhaps, quite complicated optical arrangements, instead of being, as it really is, a very difficult thing to avoid. Any one can see this for himself by means of any look- ing-glass— a small one will answer the purpose perfectly well. You place this glass on a table before you, first hold- ing it in an upright position. You place any object before EXPERIMENT WITH A MIRROR. 113 it, a small doll, for example. Now, so long as the glass and the doll are both upright, the image of the doll in the glass will appear upright, and the table, as reflected in the glass, will appear level, as it is in reality. But the moment that you begin to tip the glass forward, the reflected por- tion of the table will begin to rise up, and the reflected image of the doll will incline forward, and what at first thought seems singular, the apparent movement of what is seen in the mirror upward and forward will be twice as great as that of the mirror itself forward and downward; so that when the mirror is inclined at an angle of forty- five degrees — that is, half way down to the table — the re fleeted part of the table will be perpendicular, and the doll, instead of standing upright, will be projected forward, as if she were standing on a wall. A very good thing to try this experiment with, especial- ly when older brothers or sisters wish to show it to the younger children, is an image of a mouse, and then the mouse will seem, when reflected, as if running up or com- ing down a wall. Thus, instead of there being any difficulty in represent- ing the ghosts and goblins as appearing to be on a wall, the real difficulty is to make them appear to be on a level floor. There are various means and contrivances used to ac- complish this last purpose, one of which is to have another glass, to reflect the light a second time, and so bring the position right. Another is to have the person representing the ghost, or the figure, whatever it is, placed horizontally on the floor, and thus it will appear, when reflected, as if standing back against the wall. You can obtain a general idea how this is done by holding the looking-glass in an inclined position before you on the table, and then placing the doll on its 114 THE POLYTECHNIC INSTITUTION. back on the table, with its feet toward the glass. The doll will thus appear in an upright position in the glass. In these expei-iments which you make with the glass, a common table will be found too low, except for young children whose heads just come up above the level of the upper surface of it ; for the head of the spectator ought to be about on a level with the middle of the glass. A chair placed upon a table — a kitchen table, for example — will bring the glass, perhaps, at about the right height for young persons from twelve to fifteen years of age. In the exhibition which Lawrence and John witnessed at the Polytechnic there were several different perform- ances, in which quite a number and variety of phantasms were made to appear. One was the figure of a statue, which had the appearance of standing back against the wall of a painter's studio. Of course it was produced by some kind of statue in pasteboard, which was lying in a horizontal position beneath the front of the stage. It could be made to appear and disappear at pleasure by throwing a strong light upon it or shutting the light off. Then there were figures also — some that represented hobgoblins that ran about upon the wall. One was in the form of a monstrous fat lizard, with four paws and a long tail, that crawled about in a most mysterious manner as he appeared " reflected in the glass. Of course this animal was really a boy, with an artificial shell or coat to represent an uncouth green monster. Then there were a number of very pretty and agile lit- tle fairies in gorgeous dresses, that danced about in the most fantastic manner, so much so that it was difficult to follow them, and to tell whether they were upon the wall, in the air, or upon the floor. After the exhibition had been continued for some min- utes, and it was time to bring the lecture to a close, Pro- THE LITTLE FAIRIES. 115 fessor Pepper caused a small curtain in front of and below the stage to be lifted up, so that by looking down the spec- tators could see the objects directly that they had before seen reflected in the glass above — the pasteboard statue, the lizard, and the other similar monsters, and, what was prettier than all, the little fairies, who proved to be young and agile girls, dressed gayly as dancers. There was a very bright light shining upon them, and the girls bowed and smiled, and made salutations to the audience in a charming manner. A moment afterward the light was suddenly shut off, the fairies and hobgoblins all vanished in an instant, the curtain which had concealed them was dropped, the gas was turned on above so as to brighten up the whole room, and the performance was over. 116 VERY HEIGHT LIGHTS. CHAPTER VERY BRIGHT LIGHTS. FOR such performances as those which Lawrence and John witnessed at the Polytechnic, and also for many oth- er purposes, a very bright light is required. There are modes of producing artificial light of such intense bril- liancy that you can not look upon it directly with the naked eye for a moment. But, though we can not look upon the light itself with- out dazzling the eye, the illumination which it produces when shining upon other objects, though exceedingly bright, is very beautiful to see. Then, moreover, when objects are to be seen by reflection in a glass, there is great advantage in being able to illuminate them by a light so strong that it can not be viewed directly without dazzling the eyes. Besides this, there are a great many cases in which light is diminished by diffusion instead of by reflection, and as the diffusion weakens it, as has already been explained, in the ratio, for similar purposes, of the squares of correspond- ing lines, the light must be very bright indeed at the source, in order that it may be bright enough after diffu- sion. The engraving on the opposite page, for example, repre- sents what is called a magic lantern. It consists of a kind of lantern, with an apparatus within it capable of produc- ing an intense light, and also of concentrating this light at a point, from which it afterward diverges in such a manner as to produce an enlarged shadow, or image, of any minute LIGHT-HOUSES. 117 TUB MAQIO I.ANTEBN. object placed near the focus of the light, and throwing it upon a screen across the room, where many persons can see it together. The first purpose, however, for which the need of a very bright light was felt by mankind was to increase the range of illumination spread over the sea from the lanterns in light-houses. Light-houses on sea-coasts have been in use from very ancient times. It is true that, before the inven- tion of the mariner's compass, ships were very seldom taken intentionally far out of sight of land. But they were at any time liable to be driven off the coast by sudden storms, or to have shores that were near hidden from view by mists, or fogs, or driving rain ; and sometimes their voyage would be protracted by unfavorable winds, so that night would come on before they had entered their destined port. 118 VERY BRIGHT LIGHTS. From the effect of these and other similar causes, lights placed at certain points along frequented coasts were very eai'ly used, long before any means were known of produc- ing any light brighter than that afforded by an ordinary fire, or, rather, from such as could be produced by the most combustible natural substances that could be obtained, such as resinous wood, or porous materials saturated with pitch, or bitumen, or oil. These substances were placed sometimes in an iron receptacle called a cresset, which was raised upon the summit of a high tower, the system requir- ing, of course, the constant attendance of a guardian to watch and continually replenish the fire. The vessel containing the fire was called a cresset, from the word croisette, a little cross, the iron-work being often sur- mounted with a cross, in token of the dependence of the poor mariners, in their exposures to the terrible dangers of the sea, on the special protection of heaven. There is one thing which it is very important to observe in respect to the manner in which lights upon a sea-coast aid the mariner in finding his way over the dark waters, and that is, that the object is entirely different from that of light in other cases, as, for instance, those in the street, or in a room. These last are intended to illuminate the surrounding objects so that they can be seen. In these and in all other ordinary cases, the use of the light is not to make itself seen, but to illu- minate the objects that it shines upon so that they can ba ANCIENT LIGHT-HOUSE. OBJECT OP THE LIGHT-HOUSE. 119 seen. But the purpose of the light from a light-house is not to enable the observer to see any thing except itself, but to see itself only for the purpose of enabling him to determine where he is. It does not make visible to him the entrance into the harbor, nor show him the rocks and shoals which he is to avoid, but only to show itself, and, by so doing, to mark a point, for the purpose simply of mak- ing known to the mariner where he is. Being guided in this way only in determining his position, he must depend upon his chart, or his own knowledge of the coast lines near, for his guidance into the entrance of his haven. Thus it happens that, for a beacon on the shore of the sea, there is required, not a diffused, but a highly concen- trated light, to show itself to the mariner simply as a star beaming from the midst of surrounding darkness. All that the mariner requires of it is that it should show itself to him. He does not expect that it will reveal to him any of the surrounding objects. These he must have in his memory, or in the mental conceptions which he forms from his chart. The light is only to enable him to place himself properly among them. There is another thing that is remarkable and is very important to be understood in respect to such a light, and that is, that it is only that portion of it which shines in certain limited directions that is useful for the purpose re- quired— namely, that which goes forward over the sea — and of this only that portion which passes along at a mod- erate distance above the surface of the water. The light from any luminous point radiates naturally, as has already been explained, in every direction, so as to illuminate a complete sphere. A very large portion of this sphere is cut off, of course, by the ground. Half of it would be so cut off if the light was at the surface of the ground and the ground was level ; but, as the light is raised above the 120 VEKY BRIGHT LIGHTS. surface, an amount less than one half, though still a very large portion, is thus intercepted. Then, moreover, as the light of a light-house is not in- tended to guide travelers by land, all that would naturally shine on the landward side, if it were allowed to have its own way, would be entirely lost. In the same manner, as it is not intended to shine for the benefit of the birds in the air, all that would go upward would be lost. In a word, it is only that comparatively small portion of the sphere of radiance that extends forward over the surface of the sea, and at a small distance above it — as high as the deck of any vessel — that is of any use for the purpose designed. Now in ancient times, when these lights consisted sim- ply of blazing fires on the summit of a tower, all except this small portion was lost; but in modern times means have been found to avoid this loss by bending that portion of the rays that would naturally take a wrong direction into the right one — that is, by intercepting all or nearly all those rays which would go back over the land, or down into the ground, or up into the air, and turning them in the direction where their services are required — that is, out over the water. This is done by certain extremely inge- nious contrivances, through the effect of which the rays which issue from the source of light are collected on all sides and made to shoot forward over the sea, so that, in- stead of forming a sphere, the range of illumination takes the form of a flat wheel, or, rather, half a wheel, extending forward over the water, and lying very low. And inasmuch as we can only see any object when the rays from it enter the eye, we can only see the light from a light-house when we are placed within this range. Thus people on the land behind a light-house would not see the light of it at all, nor would birds in the air. A bird that had alighted on the mast-head of a ship coming in a dark STAGE-LIGHTS. 121 night toward the coast would see the light of the light- house like a very brilliant star in the horizon ; but if she should leave her perch and fly a few hundred feet into the air, she would lose sight of it, and she might well wonder what had become of it. The truth would be, that all the light which would naturally have come to that point would have been bent downward near to the surface of the sea, for the benefit of the mariners on the decks of their vessels, leaving the regions of the upper air in darkness, the illumination not being intended for the benefit of the birds. There is required, of course, a very bright and concen- trated light for such purposes as this, in order that the necessary amount of illumination may be brought within such a compass that the apparatus within which it is con- tained, and the lenses and reflectors required for throwing all the radiation from it out over the sea, may not be of an inconvenient or unmanageable size. A very bright light is also required for the spectral illu- sions exhibited on the stage, which have been described in a former chapter ; for, as it was there explained, it is only a part of the light that falls upon a glass plate that is re- flected from it, and, consequently, any object that is to be seen by reflection must be strongly illuminated. This is especially the case when, as has already been ex- plained, a double reflection is required to produce the de- sired effect in the best manner. You will recollect that, by one reflection only, in an inclined glass, objects that are perpendicular in reality are made to appear horizontal. To remedy this difficulty, and bring the image into a right po- sition, a second reflection is necessary. When, in order to reflect this, two plates of glass are used, as shown in the last chapter, a specially bright light is required to supply the necessary quantity for the double reflection. F 122 VERY BKIGHT LIGHTS. You must understand, however, that, as was explained in describing such spectral illusions, nothing of all these arrangements and effects is seen by the spectators in front, except the ultimate image seen in the upper glass, and ap- pearing as if it stood upon the stage. The plates of glass j the course of the rays, from the source of light in the in- strument where the man is sitting, through its zigzag course to the eyes of the spectators ; the two images, one on the lower and one on the upper glass, as well as the form and position of the glasses themselves, are all shown in the engraving of this double reflection for the purpose of showing what the actual track of the rays through the air is in such a case. But we never really see rays passing thus through the air before us. The eye takes no cognizance of any rays except those which actually enter it, and are concentrated by the lens into an image upon the -retina. Thus the spectators, in the case of an illusion like this, would be wholly unconscious of all these movements of the light, and even of the existence of the glasses, although one of them would be full before them. The light would only enter their eyes as it was reflected the last time, which would be exactly as if it came from a figure standing behind the glass upon the stage, and thus the illusion is created. In other words, the ghost seen is simply the reflection of a real figure in a mirror. In ordinary cases we know that the reflection seen in a mirror is an illusion, for the mirror is silvered on the back so as to allow no light from any thing really behind it to pass through, and thus only the objects that are reflected in it can be seen. We see the frame of it, moreover, so that we know that the mirror is there. But, in the case of these spectres, the plates of glass have no frames, and the edges of them are concealed, and we see, moreover, objects through the glass as well as those reflected in it. In ordinary cases, when we see ob EXPERIMENT AT THE WINDOW. 123 jects through a glass, we do not see those reflected in it, because the light shining on the objects beyond that are seen through the glass is usually sufficient to overpower, or nearly overpower, the reflected light ; but, by throwing a very strong light upon any object that is to be reflected, we can remedy this, and enable ourselves to see the image of the illuminated object by reflection as plainly as we do those beyond the glass directly, as can be shown in a very simple and conclusive manner by the experiment ah'eady explained of holding a piece of paper, with a lamp or can- dle shining directly upon it, near a pane of glass in the window in the daytime. The paper thus illuminated will be very distinctly seen reflected in the glass. Indeed, white paper emits usually so much light that it can ordi- narily be seen faintly reflected m the glass, if it is held near, without any artificial illumination ; but the bright- ness of the image will be greatly increased by the bright- ness of the light shining upon it. On the same principle, if you stand near a window, with your back toward it, and hold up a pane of glass, or any small piece of glass, before your eyes, you will see the ob- jects out of doors very plainly reflected in it, especially if it is a bright day. You can also see through the glass the objects that are in the room, but the objects outside will be seen too, very distinctly, and the more distinctly in pro- portion to the brightness of the light which shines upon them. Thus, when for any reason we wish to see any object dis- tinctly by reflected light in a glass which is not silvered, we require a very bright light to shine upon it, and this is con- sequently one of the purposes for which a very bright light is required. On what principle and by what methods these very bright lights are obtained will appear in the next chapters. 124 COMBUSTION OF MAGNESIUM. CHAPTER XIV. COMBUSTION OF MAGNESIUM. ONE day, just before the time for dinner, John came home from a ramble which he had been taking through the streets in London. The table was set for dinner, and Lawrence was reading a newspaper, having comfortably established himself in a large arm-chair near a window. When Lawrence heard the rap which John gave at the knocker at the door, he said, " There comes John." He knew him by his rap. It is surprising how many different modes there are in use among mankind for communicating ideas and intelli- gence as substitutes for language. One very striking in- stance is that of the boatswain's pipe, on board ships at sea, which interested John very much on his voyage across the Atlantic in the steamer. The boatswain, as perhaps the reader knows, is the officer on board a ship who has charge of the sails and rigging; and as the winds and waves are often so boisterous that no human voice could be heard at the distance at which commands often have to be given, the custom has grown up among all European nations of the boatswain's giving his directions to the men by means of a peculiar kind of whistle, called the boat- swain's pipe, which makes a very shrill and piercing sound, not loud, but so penetrating that it can be heard in the stormiest times above, or, rather, through the sound of the heaviest roaring and thundering of the winds and waves. John had been quite surprised dui'ing the voyage at two SIGNIFICANCE OF KNOCKING. 125 things in respect to the boatswain's pipe : first, at the dis- tinctness with which a sound so slender and thin could be heard amidst the wildest commotion of the elements, and also at the great variety of commands that the boatswain could give with his pipe, by means of variations and mod- ulations in the tone of it, which he made by the motion of his hand over a little hole in a part of the pipe from which the air issued. The number and variety of the orders and directions that he could give by this means constituted quite a language. In the same way, when he landed in England, John was much interested and amused in observing to how great an extent the knockers on the doors were used as a means of communicating intelligence. A single blow with a knock- er is the rap of " tradespeople," as they call them — that is, porters bringing parcels, or messengers from the butchers or grocers, or persons having any thing to sell. A double rap is reserved exclusively for the postman. When that sound is heard everybody in the house knows that the let- ters have come, and the person that attends the door must go at once, so as not to keep so important an official wait- ing. The two strokes generally come very close together — rat-tat — as quick almost as you could possibly speak those two syllables ; but, however rapidly they are given, so long as there are two, every body knows that it is the postman. Then, if it is a gentleman or lady, whether be- longing to the house or a visitor, there is quite a prolonged rapping, the strokes being usually quite rapid at first, and more deliberate and emphatic at the end — more or less so according to the rank and importance of the person knock- ing, thus, Rat-tat-tat-a-tat-tat-tat, tat! TAT! Now, in beating such a tattoo as this with the knocker of the door, no two persons, of course, do it alike, and the length and complicateness of the series of strokes admits 126 COMBUSTION OF MAGNESIUM. of so much variety in the knocks of different persons, with- out the danger of any of them being confounded with the ti-adesrnen's or postman's knock, that almost every indi- vidual comes to have his own peculiar rap, and thus the knocker has a language, as it were, of its own, notifying those who are in the house of the rank and position of the person at the door, and, in case it is any inmate of the fam- ily, making it known at once who it is. And this is how it happened that Lawrence, on hearing the knocker sounded at the door while he was reading his newspaper, said at once, " Here comes John." All this explanation, however, of the language of the boatswain's whistle at sea, and of the knocker on the doors of English houses, is a digression, and it would be some- what irrelevant in a scientific treatise on light were it not that a language belonging to this same class, and of substan- tially the same character in respect to its principles, and of, perhaps, about the same scope as to copiousness and ex- tent, has gradually grown up among the light-houses on sea-coasts, by means of which some simple but very im- portant information can be communicated to ships at sea through variations in the light, as will hereafter be more particularly explained. John very soon came into the room, and as he entered the door he held up a small object between his thumb and finger. " See," said he. Lawrence looked up. John advanced toward him, hold- ing out what had much the appearance of a watch-spring coiled up, except that the color was of a bluish-gray. "What is it?" asked Lawrence. " Magnesium," replied John. " It is a yard long when it is uncoiled. I bought it for sixpence." " That's cheap," said Lawrence. FUMES AND OTHER PRODUCTS. 127 "Yes," said John; "of course I mean an English six- pence. I saw it in a window, and I went in and bought some of it ; I am going to burn it, and make a bright light ; of course I am not going to burn it all at once, here ; I am only going to burn a small piece — half an inch long, per- haps, just for an experiment, and the rest I'm going to take to America." Lawrence approved of this arrangement, and it was agreed that they would try the experiment that evening after dinner. There was some question about the fumes which might arise, but Lawrence said he thought that there would be no fumes, as the product of the combustion of magnesium was simply magnesia, which was a harmless white powder; in other words, a finely comminuted solid. Fumes arose from combustion, he said, only when the products, or some of them, were gaseous, so that they might rise and float in the air. It is true that sometimes, when the products of the com- bustion, or the substances set free by it are solid, they are developed in the form of a powder so fine as to be borne upward by the current of hot air, so as to produce the ap- pearance of fumes, and sometimes they mingle with true fumes actually produced. This happens very strikingly in the case of the combustion of wood or coal, in which very fine particles of carbon, detached from the substance of the wood or coal, are carried up among the fumes of carbonic acid gas and the vapor of water, which are really the pro- ducts of the combustion. Now combustion, as probably the readers of this book remember, is only the rapid combination of a substance with the element called oxygen, which exists abundantly in the air, and has such an eager affinity for many other substances, especially when they are heated up to a certain 128 COMBUSTION OF MAGNESIUM. Doint, as to combine with them with great rapidity and violence. In doing this they develop or expend so much force as to produce a great quantity of light and heat, which are considered as only two of the many forms of force. To commence this process of rapid combination with oxygen, a portion of the substance must first be heat- ed to the requisite point ; but, when it is once commenced., it goes on, the heat developed by the combustion raising the successive portions to the right temperature for con- tinuing the process. This heating a portion of the com- bustible in order to commence the process is what we call kindling the fire. All this has already been explained, and must not be forgotten. Now, when substances are burned — that is, are delivered over to this eager and fierce seizure of their particles by oxygen, the compounds that are produced are called the products of combustion, and these products, of course, vary very much according to the nature of the substances com- bined. Sometimes they are gases which rise into the air. Sometimes they are powdered solids. In the case of mag- nesium, the product is the well-known white powder mag- nesia, which is, in chemical language, the oxide of magne- sium, or, as it now is sometimes proposed to call it, magne- sium oxide. John knew all this, so that when Lawrence told him there Avould be no danger from fumes in burning his mag- nesium, he was ready to assent to it at once. "But, then," said Lawrence, "there is sometimes a possi- bility that some fused portion of the substance to be burned may fall down, and do harm in that way. This happens when the heat produced melts the substance faster than there is oxygen at hand to combine with it. I do not know how this may be with magnesium, and so, in order to make our experiment perfectly safe, we will ask the landlady to ASHES. 129 lend us an old kitchen plate, and burn our magnesium over that." This was accordingly done. The landlady, when the table was cleared, brought in a plate. John broke off about an inch in length from the end of his little ribbon of magnesium, and for a handle he used a match, first breaking off the phosphoric end, and then making a little cleft with his pocket knife in the wood, by which means he formed a kind of extemporized forceps to hold the mag- nesium. When all was ready, Lawrence lighted another match and set the end of the magnesium on fire, while John held it over the plate. It kindled with some difficul- ty, as if the end of the metal required to be raised to a great heat before the process of combining with the oxy- gen of the air could begin ; but, when it was once begun, it went on with a very intense action, producing a light so vivid and dazzling that it was almost impossible to look at it. The piece of magnesium which was burnt was very short, and it was, moreover, very narrow and exceedingly thin, so that it was soon expended. John uttered some excla- mations of delight while the burning was going on, and when it went out he looked attentively at what Avas left. It was a white substance of exactly the same form with the little ribbon of magnesium, but, on touching it, it fell to an impalpable powder. " What white ashes !" said John. " No," replied Lawrence, " that is not properly ashes at all. The ashes left in burning wood are not produced by tiie combustion, but only left by it. The substances which are produced by the combustion in the case of wood go off into the air as gases; the ash is only the incombustible substance that is left behind. But the white powder in this case is formed by the combustion — that is, it is com- F2 130 COMBUSTION OF MAGNESIUM. posed of the magnesium itself, combined with the oxy- gen." " Yes," said John, " I know. It is magnesia." "It is well enough to call it the ashes in common par- lance," continued Lawrence, " on account of its resemblance to the ashes of wood or paper in its apparent origin and in its form, if we only know that it is formed, chemically, in quite a different way." To have been perfectly precise in his statement, Law- rence might have added that the ash left in the burning of wood is mostly composed of compounds of certain metals with oxygen, formed by some previous process analogous to combustion, and left in the ground, whence they were taken up by the rootlets of the plants, and built, so to speak, into the wood. But the combustion, if it really was a process of combustion, by which they were originally produced, was not the burning of the wood, but took place long before. In the combustion of the wood they were simply passed over and left, whereas, in case of magnesium, the magnesia which results is produced at the time, and by the very process of the burning. "It did not drop upon the plate after all," said John, looking at the plate, which remained perfectly clean after the experiment. "No," replied Lawrence; "I was almost sure that it would not. I wras very confident that the burning would keep well in advance of any tendency to melting ; but, in trying philosophical experiments in a parlor, it is always best to take measures for guarding against even the most improbable contingencies." BED HEAT AND WHITE HEAT. 131 CHAPTER XV. THE MAGNESIUM LAMP. THE brightness of the light produced in any sense by combustion seems to depend upon two things — first, the intensity of the heat developed by the combustion ; and, secondly, upon the presence of solid particles to be raised to what is called a white heat by this intensity. Gaseous substances, though the heat may be very great, emit usual- ly a comparatively faint light, as is observed in the case of the flame of hydrogen or of alcohol, which substances in combustion, though the heat produced is very great, give rise chiefly to incandescent gases. But in the case of mag- nesium there is not only a Very intense heat, but this heat takes effect upon the solid particles of magnesia as fast as they are produced, and causes them to emit a light of the greatest possible brilliancy. When any solid is heated in a furnace, we observe that it first begins to emit a reddish-colored light, or, as we say, it becomes red hot. When the heat is raised to a much higher degree, the light that radiates from it becomes brighter and whiter, and we say it is white hot. This would seem to be the secret of the very intense light given out by the combustion of magnesium. The com- bustion produces an extremely high degree of heat, and this takes effect on the solid particles of magnesia as fast as they are produced, raises them to the most intense in- candescence, and causes them to emit the very brilliant and dazzling radiation which we see. This white heat, moreover, is not only different in degree 132 THE MAGNESIUM LAMP. from the reel heat, but it seems to be, in some way, of a dif- ferent kind — at least it is found capable of producing dif- ferent effects, and it is in consequence of these peculiar ef- fects that the magnesium can be made very useful for cer- tain philosophical purposes. One would suppose that it would be very difficult to devise a lamp for burning a solid metal in the form of a ribbon of wire, or, indeed, in any other form, but the difficulty has been surmounted in va- rious ways. One of the modes by which this has been ac- complished is shown in the engraving. The instrument is called the magnesium lamp. The metal is used in the form of a wire, which is wound upon the wheel A, which wheel thus takes the place of the reservoir containing the oil in a common lamp. The wire is drawn off from the wheel slowly by clock-work contained in the box B; within the 'box it passes between two wheels USES OF THE MAGNESIUM LIGHT. 133 made of gutta-percha, or fitted with gutta-percha surfaces, which substance holds it with a sufficiently firm grasp to draw it forward between them as the wheels revolve. G is the key by which the clock-work is wound up when it runs down, and at T is the tongue of a little catch by means of which the clock-work may be set going or stopped at pleasure. The wire of magnesium is burned at the end C, which protrudes in front of the concave mir- ror, being pushed forward by the clock-work as fast as it burns, while the magnesia that results from the combus- tion falls down into the pan E below. F is a thumb-screw connected with rack -work, by which the mirror can be moved backward or forward as required. The whole can be taken up by the handle, which serves, when the lamp is stationary, as one of the legs. The magnesium light is used chiefly as a substitute for the light of the sun in photography, especially in cases where the light of the sun is not at command, as, for in- stance, in caverns, and mines, and other dark places. Its intensity, and certain chemical properties which result from, or, at least, accompany this intensity, fit it to answer these purposes extremely well. It has been used in this way very successfully in photo- graphing interior views of the great pyramid in Egypt, and in many other similar cases, where none but artificial light could possibly be obtained. It is also sometimes used for engineering and military purposes, such as for illuminating works of construction when it becomes necessary to carry them on at night, and also for showing the position and movements of the enemy in case of nocturnal operations in war. When, for example, the garrison of a besieged city wish to make a sortie at night, if they can send off in ad- vance, or at a little distance from them on one side, an in- tensely brilliant light, their enterprise is greatly aided, and 134 THE MAGNESIUM LAMP. that in two ways. The light, while they themselves re- main behind it in the shade, shows them the enemy and the defenses, if there are any, which they are to attack, very clearly, and at the same time dazzles the eyes of the enemy, bewilders their vision, and confuses their aim. In order fully to understand what is to follow, the reader must not lose sight of the principle on which the magne- sium light is produced — namely, by the intense avidity with which the oxygen of the air seeks to enter into re- combination with the magnesium, from which it was sep- arated by the use of great force when the metal was pre- pared, and the consequent heat, which raises the solid par- ticles of magnesia to a dazzling incandescence as fast as they are formed. Magnesium is never found in its metal- lic form in nature. It is always found already in combina- tion with oxygen, either in magnesia, which is the simple oxyde, or in some other form or combination in which it is already oxydated; and the oxygen with which it is com- bined clings to it with such tenacity that it requires a very great chemical force to separate it, so as to produce the metal in a pure and isolated state. I mean by a great chemical force a force which, though really very great, is exercised within such extremely small limits in respect to distance as to be entirely unapprecia- ble by the senses. We have an example of a force in some respects analogous to this in the freezing of water, by which the particles are forced apart only to an inconceiv- ably minute distance from each other, but yet with so much force as to lift and displace the heaviest walls if they rest upon ground that the frost can reach, or to break asunder the strongest vessels when the freezing water is confined in them ; and so, also, with the force with which the juices are drawn up in the vessels of plants and trees in the pro- cess of vegetation. This force, though inappreciable to our DIFFICULTY OF OBTAINING MAGNESIUM. 135 senses, is sufficient to move the heaviest stones, to lift and tear up pavements, and to push up and sustain the materi- als of which the branches and leaves of the tree are com- posed, hundreds of feet into the air. It is by a force somewhat analogous to these in respect to the minuteness of the limits through which it operates, and the vastness of the power which it exerts within those lim- its, that the particles of the metallic magnesium are held in combination with those of oxygen in all the substances in which it is found in a state of nature. And so firmly is it held by this force, that, though innumerable experiments were made with the substances in which it was combined, it was a very long time before the existence of the hidden metal in these substances was discovered. The discovery was at length made in 1827. Small portions were separa- ted, and the metal, as a metal, brought to view ; but it was not until quite recently that methods were devised by which any great quantities could be produced. Of course, in these attempts, the substance of the magne- sium could be brought into its metallic form only by sep- arating the oxygen from it, and this could be done only by applying a greater force to the oxygen than that by which it was united with the magnesium. This force was, as has already been said, very great. Indeed, the eagerness with which it returns to the combination, and which is the cause of the great development of heat and light, is the measure of this force. Thus the chemist, in separating the magne- sium from its oxygen in its natural combinations, forces the substances apart for the sake of witnessing the effects produced by the violence with which they come together again. The operation is very analogous to that of lifting a stone high into the air in order to observe the force of the concussion with which it strikes the ground in falling. 136 INCANDESCENCE. CHAPTER XVI. INCANDESCENCE. THERE are various methods by which an intense white light is produced by artificial means, but most of them, if not all, depend on the same principle as that already ex- plained in the case of the magnesium light — that is, in rais- ing particles of a solid substance to incandescence. The two essentials are, first, some method of producing an in- tense heat ; and, secondly, the presence of some solid sub- stance to receive the heat and to emit the light developed by it ; for light, for some mysterious reason, is emitted much more powerfully from a solid substance, however minutely subdivided, than from a gas. The same general principle, indeed, is seen to operate in the case of light derived from the lower, as well as in that from the higher temperatures produced by combustion. This is shown quite clearly in the flame of a common lamp or candle. The materials used for burning in lamps and candles, as the reader will recollect, usually belong to a class of sub- stances called hydrocarbons. They are so called because they are chiefly composed of hydrogen and carbon. Their burning is, of course, the combination of these substances with the oxygen of the surrounding atmosphere. Now hydrogen, in combining with oxygen, produces, un- der favorable circumstances, a very intense heat, and forms by the combination the vapor of water. This vapor rises from the flame and is diffused through the atmosphere. "We do not see it as it arises, but we can show it very plain- WATER FROM FIRE. 137 ly by holding a cold iron over the flame, at a little distance above it, when we shall find it will be almost immediately covered by a dew formed by the condensation of the vapor into a film of exceedingly minute drops of liquid water. And so, when you light a lamp in cold weather, the glass chimney, if put on cold, becomes for a moment bedimmed with a dew produced by the condensation of the aqueous vapor formed by the combustion of the hydrogen. As soon as the glass becomes warm the vapor is no longer condensed, though it continues to be formed as before. This phenomenon may be shown in a still more perfect manner by burning a candle for a few minutes under a cold bell glass, and observ- ing the deposition of the water on the interior of the glass, which will sometimes be so abundant as to cause drops to trickle down the ATKU FROM FIKE. This experiment of condensing water from the products of flame, which any one can easily perform, will succeed better if the iron, or other condensing substance, has some thickness, so as not to become warmed itself too soon, and so cease to condense the vapor; and if it has also a polished surface, as such a surface, by its brightness being dimmed, will show the presence of very small quantities of vapor. Sometimes children, when they are writing a letter, and are in haste for the writing to dry, hold it at a distance over the flame of a lamp, not knowing that the hydrogen, which forms a large part of the oil, produces water by its combination with oxygen in the burning, and that this water, in the form of vapor, rises directly to the place where they are holding their writing to dry ! In other words, they hold their paper in a very damp, though in a 138 INCANDESCENCE. very hot place, as is shown at once by holding there a cold chisel, or hatchet, or large carving-knife, or any other piece of polished and cold iron. The other substance contained in the hydrocarbon burned in the lamp or candle is carbon. This, so far as it is really burned — that is, so far as it finds oxygen to unite with it, forms a suffocating gas, called carbonic acid gas, which it is very injurious to breathe. This gas rises with the vapor of water into the air, and is diffused in the upper part of the room till it gets cold, when it descends and gradually escapes through open doors, or windows, or up the chim- ney. If there is no way of escape for it open, or if there are many lamps, or candles, or gas jets burning in the room, the air becomes gradually so charged with it as to be un- comfortable and unhealthy to breathe. But all the carbon does not at once find oxygen enough at hand to combine with it. A portion of it remains for a moment in the flame, where it serves the purpose of fur- nishing a supply of solid particles to emit light. They can not all burn at once, because there is not oxygen enough for all ; so that, while some are burning, and evolving great heat in so doing, the others, while waiting their turn to be supplied with oxygen, as if not willing to be idle and use- less even for a moment, employ themselves in producing and emitting light, which the heat that is supplied to them empowers them to do. At length, however, when they reach the upper and outer margin of the flame, they too obtain their supply of oxygen from the air, and, combining with it, give out more heat, and also form more carbonic acid gas, which arises with the rest into the air. Thus, on their way through the flame, after being liber- ated from their previous combination with hydrogen in the hydrocarbon, and before their turn comes to be supplied with oxygen to enable them to form a new combination, CARBOX IX THE FLAME. 139 they serve as solid particles, to emit light by their incan- descence. It is from these solid particles — individually solid, though inconceivably minute — that the chief portion of the light of such a flame comes. The combustion of the hydrogen alone, or of any other gaseous substance, though it would produce great heat, would afford very little light. For some mysterious reason, it is necessary that there should be solid particles present to transform, as it were, a portion of the heat into light, and emit it in that form. These solid particles of carbon in the flame are not di- rectly visible, but, as in the case of vapor of water, we can easily, by the use of proper means, bring them into view. If, instead of holding the cold iron in the air above the flame, we hold it, or any other solid substance, actually in the flame, the black particles are suddenly cooled by it, and deposited upon its surface as soot is upon the back of the chimney. This black substance, on account of its being produced in this way, is called lamp-black. The process Avhich thus takes place in the burning of a candle is quite a complicated, and a very curious one, and if, in watching it, our powers of vision were sufficiently acute to enable us to distinguish the several steps, we should be greatly interested in observing it. In the first place, we should distinguish in the oil, slowly coming up the wick, particles of carbon and hydrogen conjoined. We must not, however, conceive of the particles of carbon as black ; they are black when separated from their combina- tions and existing in a certain form by themselves, but they may be of any other color. Color, as will be ex- plained more fully in a future chapter, depends altogether upon the manner in which any substance absorbs or re- fiects the light, and this does not depend upon its intrinsic character at all, but apparently upon the mechanical ar- 140 INCANDESCENCE. rangement of its particles. Thus sugar, which is white in the lump, when dissolved in water and diffused through it, loses its whiteness entirely and has no color at all. The particles of carbon which, combined with the hydro- gen, form the oil, have only the color of the oil while in this combination. When they come up to the flame, the action of the heat, in some mysterious way not at all un- derstood, has the effect of developing in them a strong ten- dency to separate from each other, and to enter severally into combination with the oxygen of the air which is near. In combining with the oxygen, we should see them seize it with great avidity and violence, and the force which they thus expend we should see taking the form of heat, which would act upon the next portion of oil which came up, and produce the same effect upon the carbon and hy- drogen in that ; and thus the process would go on. The hydrogen which was thus separated from the oil, we should see, would seize upon the oxygen with the great- est avidity, and procure the largest share, or, at least, the earliest. The carbon particles would have to wait, it would seem, for their supply until the hydrogen was satisfied. The consequence of this is, as we should see, that while the hydrogen combines at once with the portion which it re- quires, thus becoming transformed into the vapor oftoater, the carbon particles, or, at least, a very large portion of them, pass up through the flame intensely heated, and, by the superior power of a solid to radiate light, become the source of nearly all the light which the flame affords. All this we should see if we had senses acute enough to perceive what really takes place in the burning of a lamp or candle. The particles of carbon which pass up thus through the flame, though while so hot they emit the yellow color of the flame, in other words, are themselves of a yellow color, MAKIXG LAMP-BLACK. 141 become intensely black if they are interrupted on the way, and suddenly cooled before they find oxygen to combine themselves with. They are, moreover, so inconceivably minute, that when assembled together they form an impal- pable powder, far softer and finer in the minuteness of the division than it would be possible to make masses of car- bon by any artificial process of pulverization. This mode, accordingly, of procuring a black powder for paint, and for painter's work, is practically employed to a great extent. The engraving shows how lamp-black is manufactured on a large scale. The fire is made in the little grate at «/ it is made of pitch, or tar, or some other hydro- carbon containing a large propor- tion of carbon. The substance is heated by a fire below it, and then is set on fire above, and is furnished with a limited supply of oxygen through small holes made for the purpose. In consequence of this lim- ited supply of oxygen, the combus- tion is imperfect, and a large por- form of a dense black smoke into the chamber b c, where it attaches itself to the walls, and also to the sides of the cone - hich the incident ray I o had been refracted, will, at the instant of its issuing from the water, o, be held back, as it were, by the superior influence over it of the greater mass of water on the side W that is near enough to act upon it at the moment of emerging, than by that on the side w, and so will be drawn down into the direction o I, which is precise- ly the same as that of the incident ray. Thus the action of the water on entering and departing rays is equal and reciprocal. This mode of stating the case is very indefinite and vague, and would be wholly unsatisfactory considered in a scientific point of view. It helps us very much, however, in fixing in our minds the general law, to consider that, in passing from a rare to a dense medium, the ray is bent to ward the side where the mass of the dense medium lies nearest to the course it is following. Stated scientifically, however, the law is, that the ray, in passing from a rare to a dense medium, is bent toward the perpendicular drawn from the point at which it enters. In passing from a dense to a rare one, it is bent/rom the per- pendicular to precisely the same extent. The diagram already referred to shows this very clearly. The dotted line pp represents the perpendicular; lo is the incident ray, entering the water at o. Instead of going on in a straight course, as represented by the dotted line o v, it is bent toward the perpendicular into the line o R. A ray transmitted in the contrary direction — that is, from R to o, instead of continuing, when it emerges from the water, GENERAL PRINCIPLE. 247 in the same direction, is bent away from the perpendicular and into the line o I. In almost all cases of light passing from one transparent medium into another of a different internal constitution, the ray is bent on this principle. The effect in most cases seems to depend upon the difference of density iu the two media, but not always. There is some mysterious quality or condition of structure, not well understood, on which the effect depends. The substance ordinarily employed for refracting light is glass, and a glass formed with curved surfaces to produce any of the various effects which may be required is called a lens. Lenses are of various kinds, according to the effect which it is desired to produce. A double convex lens, for example, is convex on both sides. Its effect upon parallel rays, according to the principle al- ready explained, namely, that the light is bent toward the side where the largest portion of the substance of the glass comes nearest to it, or, in other words, toward the perpen- dicular at the point where it enters, is to draw the rays all inward, as we see is the case with a sun-glass, which is an example of a double convex lens. On the other hand, the same principle, in the case of a double concave lens, which is the kind used for the eye-glasses of near-sighted persons, will tend to spread the rays instead of drawing them together; for while, in the convex lens, the thickness of the glass increases toward the centre, in one that is con- cave the thickness increases from the centre to the circum- ference, and the light is drawn away toward the side where the greatest thickness lies at the point at which it enters. It is easy, in the same manner, for one who has the prin- ciple of reflection in mind, as it has been explained in this chapter, to see in what way light will be reflected from a polished surface in any specified position or of any speci- 248 LAWS OF REFLECTION AND REFRACTION. fied curvature. He has only to consider in what way solid bodies, impelled against such a surface, would rebound from it. Thus a concave mirror acts to gather together parallel rays falling upon it, just as a shower of peas falling upon the inner sides of a saucer or a bowl would rebound to- ward the.centre. On the other hand, a convex mirror will reflect rays in a divergent direction, just as peas falling in a shower upon the outside of a bowl, placed bottom up- ward, would rebound away from it in every direction. THE BUTTERCUP EXPERIMENT. 249 CHAPTER XXVH. THE EYE. WHEN a child holds a buttercup, in a bright sunny day, under the chin of another child, if the light happens to come right, a slight yellow tinge appears upon the skin opposite to it. There are two explanations of this phenomenon. One is the notional, and the other the scientific one. The notional explanation, which is the one generally adopted by children, is, that the child on whom the exper- iment is performed " loves butter." The scientific explanation is, that the petals of the but- tercup having, in some mysterious way which no one pre- tends to understand, the power of separating the rays of white light which fall upon them from the sun into their component parts, and of absorbing all but the yellow rays, these yellow rays are reflected upward, and, falling upon the chin at a place somewhat sheltered from the bright light of the sun, are reflected to the eyes of the children looking on. This second reflection depends altogether upon the brightness of the light shining upon the butter- cup and the relative position of the surfaces on which the light shines, and not at all on the taste or inclination of the subject of the experiment in respect to butter. This case is a pretty fair illustration of the difference be- tween the notional and the scientifical explanations of the phenomena taking place in nature all around us at all times. The yellow rays, as we call them — though we must not L2 250 THE EYE. forget that what we mean by this term is, not that the rays are yellow in themselves, but only that they have the power of producing that sensation in us when they enter our eyes — undergo, in the case of the buttercup and the chin, two rejections : one from the petals of the flower, by which they are separated from the other rays, and the oth- er from the chin. Any color may be thus reflected a sec- ond time, and the effect is more or less distinct accord- ing as the second surface is more or less shaded from all extraneous light — that is, light coming to it from other sources. Thus almost any object held near a red curtain which the sun shines upon will look red by reflected light. In this case the red light from the curtain constitutes so large a portion of all the light which the object receives that the reflection of it becomes visible. In the same manner, all the objects in an ordinary room reflect each its own colored rays to every part of the room. These rays mingle and blend with each other in passing through the air, and if we hold up a sheet of paper as a screen, they all fall upon it together, in combination with the white light of the sun, so that the paper reflects only a mingled and general light to our eyes. But if a lens is interposed in a proper manner between the paper and any group of these objects, and all other light is excluded, then the differently colored rays from all these objects, and from the different parts of the same object, are made to converge, and are brought to a focus, each in its proper place, and a distinct image of the whole group is formed, with all the parts in their proper position, and of their proper color. How this is effected is shown very clearly in the image of the lily in the engraving. The rays from only two points (A and B) are shown, but the same effect takes place in respect to the light issuing from every other point in the flower. All these rays, in passing through the air, FORMATION OP AX IMAGE. 251 MAGK OF THE LILY. are completely intermingled, though each one, wonderful as it seems, pursues its way uninterrupted and undisturbed by the rest. A screen held at the place of the lens would receive them blended together, and would reflect their united light oi>!y as a general illumination. But the lens causes each separate pencil, coming from every different point, to converge each toward its own central line. The result is that the colors are all separated; and if the screen is held iu the proper place to receive them, and all light from other sources is excluded, a perfect image of the lily is formed, only it is inverted, since the several pencils cross each other at o in traversing the lens; those from A, for example, coming to a focus at a, and those from B at b. This experiment can be easily performed by means of any convex lens, such as a reading-glass, a sun-glass, or even one of the glasses of a pair of spectacles such as are used by elderly persons. Near-sighted glasses, being con- cave instead of convex, and so causing the rays to diverge instead of to converge, of course will not answer. The only difficulty in making this experiment perfectly successful is that of keeping all other light except that which 'comes through the. lens away from the screen, or from whatever serves as a screen, to receive the image 252 THE EYE. But if you make the experiment in the evening, and with only one lamp in the room, or, when there are more than one, if they are placed near together, and thi'ow the image on the wall, or on a sheet of paper held near the wall, an inverted picture of the flame or flames, of beautiful dis- tinctness, will be formed. Images of any other objects, as well as of bright flames, can be produced in this way, if only all extraneous light can be excluded. This is exactly what is done in the eye. The eye is simply a space inclosed, with an opening in the front part of it, where a double lens is placed to receive the light, all other light, except the rays that come through the lens, being excluded. An image, then, of any outward object toward which the eye is directed, is formed upon a peculiar membrane at the back of it called the retina, which serves as a screen. Of course the image is invert- EOTION OF TIIK EYE. THE CAMERA OBSCUEA. 253 ed. How it happens that we see things right side up when the picture that is formed in the eye by which we see them is upside down, is a mystery which greatly puz- zles the philosophers. If an exact model of the eye were made of porcelain and glass, with a little peep-hole upon one side, so that we could look in, we should see in the interior of it, on the back side, a most perfect and beautiful picture of any external scene or object toward which the opening in front might be turned. A great variety of optical instruments have been invented by man which act on the same principle as the eye. There is a lens to concentrate the rays, a screen to receive the image, and an inclosure to exclude all other light except what comes through the lens. There is also often a mirror to reflect the image, so that the screen that receives it may be placed where it may be most conven- iently viewed. There is another advantage in the use of the mirror in these cases, for, by reflection in it, the image may be thrown upon a horizontal screen, and in that case it may be looked at from the side that will bring it right side up. There are many ways by which these arrangements are inclosed for the purpose of excluding the outside light; for, in order to produce the full effect, it is necessary that all light, except what comes from the objects to be viewed, should be excluded. Indeed, these instruments all take their name from the Latin words meaning dark chamber, or, rather, chamber dark, which words are camera obscura. The following engraving shows one of the forms in which the camera obscura is often made. The rays R, which enter the tube B, are made to converge — that is, all which come from any one point in the object are made to converge, and they would fall upon the back of the box O, and form an image there, were it not that they are reflected" by the 254 THE EYE. CAMERA OBSC sloping mirror M up to a sheet of thin paper laid upon a glass plate above, where the observer can, if he pleases, make a tracing with his pencil of the picture they produce. Sometimes the inclosure to exclude light from the sides consists of a darkened room. The apparatus, however, for forming the image in such a case is substantially the same, consisting of a lens to form the image, and a mirror to pro- ject it where it is most convenient to place the screen. Sometimes the entering rays are reflected in the mirror first, and afterward passed through the lens. It is neces- sary in all these cases that the room should be darkened by means of shutters, or in some other way. The appara- tus is usually fitted into an opening made in one of the shutters, while the others are entirely closed. In order to avoid the inconvenience of darkening a large room in this way, a small building, like a summer-house, is sometimes devoted exclusively to the purpose of a camera obscura ; this is often done in large public gardens or pleasure-grounds. THE ARTIST'S TENT. 257 Sometimes a camera obscura is fitted up upon wheels, like a traveling photographic apparatus, for a show ; and sometimes in a tent, for the use of artists ; only in this case it is necessary that the tent-cloth should be perfectly opaque and dark. CAMERA OB8CCBA IN A TENT. The tent arrangement is attended with the great advan- tage that it can be removed from place to place, and can be set up in situations inaccessible to a wheeled carriage. In the engraving the tent is open in front, being drawn so in order to allow us to see the interior; and the cloth does not quite come to the ground, in order that we may see the supports. In actual use it ought to be closed en- tirely, except at the opening in the apparatus at the top to admit the light which is to form the picture. The process of photography consists simply of fixing the image produced in the camera obscura. The box used is 258 THE EYE. essentially the same with the one above described, but the paper on which the image is finally received, when all is ready and the picture is to be taken, is made sensitive by being covered with a chemical substance which is affected by the light. There is an immense number of optical instruments, greatly varied in their construction and in the purposes which they serve, which, however, all depend upon the op- eration of the simple principles of reflection and refraction which have been explained. To enter into a description, or even an enumeration of them, would be foreign to the purpose of this work, which is simply to unfold and explain the grand fundamental principles that are exemplified in the action of light, as it exhibits itself to us in the phenom- ena of nature around us. There is one principle which is, however, only in a sec- ondary sense a property of light, which I must explain be- fore closing this chapter, and that on account of the great interest which John and Flippy took in it, and in making a certain class of toys illustrative of it. The principle is called the Persistence of Vision. The phrase denotes a certain property of the retina of the eye, or of the nervous connection between the retina and the mind, or rather, perhaps, a property of light in relation to these, by which the impression made upon the mind does not instantly cease when the image is made to vanish. Thus a succession of very rapid flashes always appear to us like a continued light, as the impression left by one does not fade before another comes to renew it. A great many ingenious toys are constructed on this principle. The kind which principally struck Flippy's fan- cy— chiefly, I suppose, because he could manufacture them himself — consisted in making two different pictures on the two sides of a card, and then, by attaching strings at the THE TWIRLING CARD. 261 ends, and spinning the cards rapidly by means of the strings, the impressions of the two pictures would be com- bined in the eye, on account of the image produced by one not fading from the mind before the other came to join it. One of the pairs of pictures which the boys thus made con- sisted of a man on one side brandishing a stick, and on the other side a pig running away. Thus, when the card was twirled, you saw one picture consisting of a man driving a Pig- The boys made these pictures by cutting out the figures in black paper, and then pasting them upon the cards. The figures were not very well shaped, but Flippy said that that was no matter; they were just as funny for all that. Sometimes they drew the pictures with pen and ink, and sometimes they painted them in colors. One which they drew consisted of an empty cage on one side, and a bird, which they painted of a bright blue, on the other. When the card was twirled the bird was seen in the cage. The scientific name for this contrivance is the Thauma- trope. THE THAUMATEOPE. £62 THE EYE. They also painted spots of different colors on the oppo- site sides of the cards, in order to see what compound color would be produced in blending them by the twirling of the card. The boys spent two whole days, during which they were confined within doors by rain, in making a number of these cards of various styles and patterns. They made them to carry home with them to America. Lawrence highly approved of this amusement. He said that such cards were worthy of being regarded with special respect, in view of their being capable of fulfilling a double func- tion. They were equally adapted to interest children as amusing toys, and philosophers as articles of apparatus il- lustrating the persistence of vision. AEEIVAL AT LIVERPOOL. 263 CHAPTER XXVIII. THE EETUKN. WHEN at length the time arrived for Lawrence and John to set out on their return to America, John had learned so much about the philosophy of light, both from the books which he had read upon the subject, and from the conver- sations which he had held with Lawrence, and he had, moreover, fixed so firmly in his mind what he had learned by the notes of conversations, and the other articles which he had written in his book, that he was greatly interested in the subject. Indeed, there were some indications, once or twice, as Lawrence observed, of his beginning to feel a little vanity and self-conceit in view of the progress which he had made. After traveling slowly and by a somewhat circuitous route through France and England, Lawrence and John ar- rived at length at Liverpool, a day or two before the sail- ing of the steamer in which they had taken passage for America. The appointed day at length arrived, and they went on board with the other passengers, and the steamer set sail. It was late in the fall when this return voyage was made, and the weather for several days was stormy, and the sea rough. On this account, and also because this was the re- turn voyage, the passengers were more quiet, and kept by themselves more than on the voyage out — that is, out as the Americans call it, though the English always call the voyage to America the outward one, and that from Amer- ica to England the voyage home. The Americans are usu- ally much more quiet, and much less inclined to make ac- 264 THE KETUKX. quaintance with each other on the voyage back to Amer< ica, at the end of the tour, than when crossing the ocean on their way to Europe, at the beginning. They are tired of excitement and change, and their minds are occupied with recollections of the scenes they have visited, the pleas- ures they have enjoyed, the vexations and disappointments which they have suffered, and with thoughts of home. After a time the wind and the waves subsided, and Law- rence and John began to come up oftener to the deck. One day when they were sitting there, about noon, waiting for the officers who were engaged at their observations to "make it 12 o'clock," and for the luncheon bell, which they knew would be rung as soon as the waiters should hear eight bells strike, John took out his watch, and, finding that it was after twelve by it, he asked Lawrence why they did not strike eight bells. "It is after twelve," said he. " But you have not got our true time," said Lawrence. " Yes," replied John, "I set my watch by the ship's clock this morning." "Ah ! that \vas yesterday's time," said Lawrence. " We have run two or three hundred miles to the westward since yesterday, arid it will not be twelve o'clock here until the sun has had time to come all that distance from where we were when it was noon yesterday." John then asked some questions about the mode of mak- ing observations at sea. Lawrence said that the midday observation was for the latitude only, which they deter- mined by the altitude of the sun at noon. The altitude of the sun, when it passes the meridian, varies from day to day for the same place, and it also varies with the distance of the place from the equator ; so that by finding the alti- tude by means of the sextant, and looking in the tables, the particular latitude which gives that altitude on that REFRACTION BY THE ATMOSPHERE. 265 day is readily found. There are, however, several correc- tions to be made. Lawrence explained some of these corrections, and there was one — namely, that for refraction, as it is called — which John was much interested in, because his studies in respect to light enabled him to understand it very clearly. It seems that the rays of light from the sun, in passing from the inter-planetary space into the earth's atmosphere, are refracted, and thus bent downward more and more in passing through the increasing density of it. This effect is greatest when the sun is near the horizon, and it makes the sun appear higher than it actually is. In fact, it brings his disk into view before he has really risen. ATMOSPHERICAL KEFRACT This is made plain by the engraving, where the ray of light from the sun (S), while it is below the line of the horizon (H), is bent downward in passing through the suc- cessive portions of the atmosphere enveloping the earth, so as to come to the eye of the spectator at A as if it real- ly proceeded from a higher point, and thus brings the sun into view while it is actually below the horizon. M 266 THE KETURN. The effect is greater when the sun is low, and continual- ly diminishes with its increasing altitude; but the navi- gators' books contain a table in which they can find the proper correction to be made in every case. John was quite pleased to find that he could understand this explanation, and the drawing which Lawrence made to represent it, so easily, and said, after a moment's pause, that he thought that he had learned a good deal about light since he had been on that tour. " Yes," said Lawrence, " you have indeed. You have made an excellent beginning. But the field of knowledge widens more and more the farther we advance into it. You have learned a great many of the first principles, and these, being fundamental, are of great importance. But when you go farther, and study the construction and phi- losophy of the microscope, the telescope, the magic lan- tern, the stereoscope, and the analysis by the spectroscope of the chemical composition of incandescent substances, however remote from us, you will think that what you have yet learned is, after all, very little. And when you come to investigate the phenomena of diffraction, and in- terference, and polarization, you will almost conclude that you know now nothing at all." "What are all those things, any how ?" asked John. " What they call diffraction" said Lawrence, " is a change produced in some mysterious -way in the move- ment of rays of light when a very slender beam passes through a very narrow slit or opening, or by the side of a very narrow obstruction, so as to produce fringes of differ- ent colors. You can see these fringes sometimes, though very irregularly, when you look at a bright light with your eyes almost shut, so as to see it through your eyelashes. There are ways of producing them very regularly and beautifully on a screen by means of suitable apparatus, POLARIZATION. — INTERFERENCE. 267 but to understand clearly how they are produced requires a great deal of mathematical knowledge. It is in some way by which the pulsations or vibrations of different rays mingle or combine their actions so as to produce new and strange effects. Sometimes two rays entirely neutralize each other, so that two lights make darkness. This is what they call interference. "As to polarization" continued Lawrence, "that is more difficult still to understand. It forms quite a science by it- self, and one, too, of a highly mathematical character. Po- larized light is light which has been changed in a certain way, so that it acts differently from light in its ordinary state, and produces certain beautiful and very wonderful effects in the microscope, and reveals in a marvelous man- ner certain differences in the internal constitution of dif- ferent transparent substances which could be discovered in no other way. " What they call interference," continued Lawrence, " is, as I have already said, a kind of combination of the waves, by which the swell of one is made to correspond with the hollow of another, as it were, and so they are both extin- guished. Imagine that you throw a stone into a pond and set in motion circles of waves, and then suppose that an- other stone is thrown in so as to strike at precisely the right instant to make a second set of waves that shall ex- actly coincide with the first set. This would tend to in- crease the height of them." "Yes," said John, "I admit that." "But now," continued Lawrence, "suppose the stone were thrown in at the right instant to make the hollows of the second set coincide with the swellings of the first. The two sets of impulses would then neutralize each other — or interfere, as they call it when speaking of light, and the water would remain level." 268 THE KETUKN. " It could not be done," said John. " True," said Lawrence ; " but can it not be imagined ?" " I don't think I can hardly imagine it," said John. " Not even as an illustration ?" said Lawrence. "I don't know," said John, speaking doubtfully. " You'll have to imagine it," said Lawrence, " if you wish to get an idea of what is meant by interference in the case of light. Besides, though you say it would be impossible, perhaps, to do this with waves of water, the effect can be produced exactly by a mechanical apparatus to make arti- ficial representations of waves." " I should like to see that apparatus," said John. "At any rate," continued Lawrence, "it is found that rays of light, or luminous impulses following each other in a certain way, do extinguish each other. The experiments are very complicated and very curious, but they are thought to prove positively that light really consists of a rapid suc- cession of some kind of undulations or waves." " Do you think they do really prove that ?" asked John. " I think they prove the existence of some kind of inter- mittent action, with alternating conditions capable of in- tensifying or neutralizing each other, according as they agree or disagree ; but whether the successive impulses are of the character of vibrations or undulations in a sub- tle ether, I do not know." Lawrence was right, perhaps, in saying that he was not entirely satisfied in respect to the precise nature of this mysterious action ; but, at any rate, it seems to be proved that there is an excessively rapid intermittent force of some kind or other that is concerned in the production of light, and the length of the several pulses, and the number which are produced in a second, seem to have been quite exactly ascertained, on the principle of determining the interval in time and distance which is requisite to produce BLOWING BUBBLES. 269 the nterference. The effects of this interference are mani- fested in many very remarkable and very curious phenom.' ena. John said that he should like to see some of them. Lawrence replied that it was easy to see them, but not so easy to understand how they were produced. "They appear in various colored fringes," said Law- rence, " in almost all transparent substances which are made extremely thin — so thin that the light, in being re- flected back and forth from one surface to the other, is caused to ' interfere.' We can make a thin film of air which will show them by pressing two plates of glass to- gether which have surfaces that are not exactly parallel. We see them in very thin plates of mica, and in a thin film of oil or other such substance, floating upon water; and, [SLOWING IIUJtm.K8. 270 THE RETURN. better still, children observe and admire them in the soap bubbles which they blow. The colors come out when the bubble grows so large that the water inclosing it becomes extremely thin. "I have seen the colors in the bubbles very often," said John, " but I don't understand how they can be produced by any kind of interference of waves." " No," replied Lawrence, " I do not wonder that you do not. It requires a very profound mathematical study to understand it. Newton studied it in that way — " " What ! with a soap-bubble ?" asked John. " Yes," replied Lawrence ; " but the colors moved about so much when the bubble was floating in the open air, and the water dried from the surface so as to cause it to burst so soon, that at first he met with a good deal of difficulty. He saw that it was necessary to contrive some way to remedy these evils, so he blew his bubble in a glass globe, with very transparent sides, which served to protect it from the air, and which he previously filled with moist air in order to prevent the evaporation." He found that, when thus covered, the bubble was much more permanent than when exposed in the open air, and the colors arranged themselves in the most symmetrical and beautiful manner. " I mean to try it when I get home," said John. "I would do so, if I were you," replied Lawrence. " Only," said John, " I don't know how I can get a glass globe." " Any kind of bottle or jar would do, I suppose," said Lawrence ; " only you must have a stopper, and pass the tube that you blow your bubble with through it, so as to keep the moisture all in the jar, in order to prevent the water of the bubble from evaporating. You must also stop the opening into the pipe, for there is a certain con- THE BUBBLE PROTECTED. 271 fTOJS 8 HUBBLE. tractile force in a babble which gradually begins to drive the air out of it when you stop blowing in, if you leave the pipe open. You see this by the bubble's growing gradu- ally smaller and smaller." When Lawrence and John had arrived at about this point in their conversation, the officer in charge struck eight bells. Those who had been making observations im- mediately went below with their sextants and the lunch- eon-bell ran^. 272 THE RETURN. The voyage went on very smoothly and pleasantly aftei this, though every one seemed more than usually impa- tient to reach the land. At length, just before the time arrived for the land to come in sight, a pilot-boat appeared. The passengers were all very much interested in the coming of the pilot, for they expected that he would bring them the news which had been passing under the Atlantic from Europe to Amer- ica, on the telegraph wire, since they left Liverpool ; and as this was the year of the great French and German war, they were very anxious to learn what had happened since they left the English shores. When the pilot came, how- ever, they were much disappointed at learning that his boat left New York only the day after the steamer had left Liverpool, so that he could give the passengers only one day's later news. It was a joyful hour for all the passengers when the steamer was sailing up the harbor. Home seemed to them more attractive, after all, than any of the scenes of novelty and beauty which had enticed them abroad. The immense steamer came up very slowly and with much difficulty to the pier. There were many lines taken out in boats to the pier and fastened there, and hard pull- ing upon them by the sailors at windlasses and capstans, and much alternate stopping, and backing, and going for- ward of the engine. There were crowds of people all this time upon the pier waving hats and handkerchiefs, which salutations were responded to by the passengers on board, who crowded the promenade deck and leaned over the railings at every point where they could see. At length the bow of the steamer was brought up in an awkward position among the piles at the head of the pier, and a broad plank platform was laid across for the transfer of the baggage on shore. There were no facilities yet for FLIPPY AGAIN. 273 the passengers to land, or for any of their friends to come •,on board. A few adventurous gentlemen, however, more 'bold or more agile than the rest, were soon seen clamber- ing up over the piles and getting from them into the rig- ging, so as to come on board. Among them John's eye fell upon a boy who was stopping one of these gentlemen and asking him to give him his hand to help him across a very dangerous place. " Look ! Lawrence, look !" said John ; " there's Flippy ! I verily believe that's Flippy !" It was indeed Flippy. He had seen in the newspaper the names of Lawrence and John in the list of the passen- gers that were to cross the Atlantic in that steamer, which had been telegraphed to New York, and, being in New York at the time, had come down to welcome them to their native land. M2 274 FAREWELL TO FLIPPY. CHAPTER XXIX. FAREWELL TO FLIPPY. A FEW days after the return of Lawrence and John to New York, they went on board a North River steam-boat to go up the river on their way to their home in the coun- try. It was late in the afternoon when they went on board. On their way from the hotel to the pier, John said to Law- rence, in the carriage, " It would have been better for us to have planned to go up in the day-boat." " Why so ?" asked Lawrence. " Because then we could have seen the scenery better," said John. "That is not a settled question," replied Lawrence. " Some people think that the scenery in the evening, by starlight or moonlight, is a great deal more grand and sublime, especially in passing through the Highlands." " I don't care much about that," said John. " I like to see them stop at the landings, and watch the people going off and the others coming on in the day-time, when I can see them plainly." "Yes," rejoined Lawrence, "I should expect that you would take more interest in such scenes now than in mountains by moonlight. You have not yet attained to the romantic age." " The romantic age ?" repeated John. " Yes," said Lawrence. " I divide the period of child- hood and youth into four ages. First comes the Wonder- ON BOARD. 275 ing Age, then the Noisy Age, then the Teasing Age, and, last of all, the Romantic Age. The Romantic Age has not come for you yet." At this point in the conversation the carriage stopped. They had arrived, it seemed, at the pier. So they descend- ed from the carnage, and, after paying the fare and attend- ing to their baggage, they went on board. " Lead the way, John," said Lawrence, as they stepped from the gang-plank to the deck, " and find the place where you would like to sit. We have more than half an hour yet before the steamer will start." John replied that he would like to sit where he could see the people come on board ; and, so saying, he led the way up to the after promenade deck, and there, choosing two comfortable arm-chairs, he brought them to the side of the deck next the pier, where he could see the carriages and carts as they arrived, and the foot people, and the orange-women, and the news-boys, and witness at his ease all the exciting scenes and incidents which usually attend the sailing of a North River steamer from a New York pier. As soon as he and Lawrence were comfortably established in their seats, he asked Lawrence to go on with what he was saying about the ages of childhood and youth. So Lawrence went on to explain what he meant by the various ages that he had specified. The Wondering Age, he said, continued from infancy till the boy was seven or eight years old. Up to that time the world was all new to him, and his mind was chiefly occupied with curiosity and wonder. He went about prying into every thing. He believed every thing that he heard, so that it was very easy to make a fool of him. He liked fairy tales, and the more absurd and impossible they were, the better he was pleased with them. " Next comes the period from seven or eight to ten or 276 FAREWELL TO FLIPPY. twelve," continued Lawrence, "which I call the Noisy Age. The boy has by this time become somewhat accustomed to the strange world that he finds himself brought into, and feels more at home in it, and begins to see more clear- ly the difference between truth and falsehood in it. His powers and faculties have become enlarged and developed, his strength is increased, and he begins to like to produce sensations and effects. One of the easiest effects that he can produce is noise. He likes to hear it, and he makes a great deal of it. Indeed, the more bustle and noise there is, the better, especially if he makes it himself. So I call this the Noisy Age. In this age the boy, if left to him- self, and is strong and healthy, breaks into a room rudely where people are quietly talking, and if reproved and asked to be more quiet, he goes out sometimes slamming the door, and making more noise in going out than he did in coming in." "Yes," said John, smiling, and at the same time looking a little ashamed, " I used to do so." " In this age, too," continued Lawrence, " boys are fond of rough and noisy plays. They are always pushing each other, chasing each other, and tripping each other up, with a vast amount of shouting and hallooing by way of music- al accompaniment. "Next comes the Teasing Age," continued Lawrence. "The boy's mental faculties have now become somewhat more fully developed, and the effects that he now likes to produce are such as relate somewhat more to the minds of people than merely to their eyes and ears. He takes special pleasure in making fools of people, in getting boys or dumb animals angry with each other, and seeing them fight. If he has any sisters, he seems sometimes to take special pleasure in teasing them. That is the reason why I call it the Teasing Age." THE DIFFERENT AGES. 277 EOUGII PLAYS. " I think you ought to call it the Ugly Age," said John. " No," rejoined Lawrence, "it is not exactly from a spirit of ugliness that he does these things, but only from the pleasure of exercising his growing powers in new forms. To produce a disturbance or an excitement in a person's mind involves the exercise of a higher class of faculties than merely to make a din in their ears, and the boy likes to exercise his highest powers. The Teasing Age comes generally between ten or twelve and sixteen. After six- teen the boy generally becomes too much of a gentleman to take pleasure in troubling people in any way, especially his sisters. He then becomes ambitious of making himself agreeable instead of disagreeable." "J'ra between ten and sixteen," said John, " so I am in the Teasing Age." 278 FAREWELL TO FLIPPY. "Your case is an exception to the general rule," said Lawrence. " I think there are a great many exceptions," said John. " I think so too," replied Lawrence ; " indeed, it would not be a good specimen of a general rule if there were not a great many exceptions." " Look ! look ! Lawrence," said John, suddenly interrupt- ing and pointing toward the pier ; " there comes Flippy !" It was indeed Flippy. He was coming down the pier with a parcel in his hand. John ran to meet him. In a few minutes he returned, bringing Flippy to the place where Lawrence was sitting. Flippy placed his par- cel in Lawrence's hands, saying at the same time, " There is something for you ; but you must not open it until you get home." "Is it a present for me from you ?" asked Lawrence. "Yes," replied Flippy; "though my father gave me the money to buy it, because you were so kind to me and taught me so much while we were on the voyage. And I want to go home with you now," he continued, " to where you live." " Oh no !" rejoined Lawrence ; " it is too far ; it is more than two hundred miles from here." " No matter for that," said Flippy ; " I can write back to my father at the first place where we stop, and he will send me some money. He won't care, so long as he knows that I am witli you." " But your mother would care," replied Lawrence ; " she would be very anxious and very much worried about you." " No matter," said Flippy. " She would find out after a while that I was all right." Lawrence replied that, though his mother might find out that he was all right, as he called it, in the end, she would endure a great deal of suffering in the mean time through THE BONNE. 279 her suspense and anxiety ; and then, in order to see if he could not awaken some sentiment of gratitude in his mind toward his mother, he reminded him of his obligations to her for all the care and trouble which she had borne for him in former years, when he was a little child ; how she had attended him and watched over him in sickness, and sat by his bedside at night, and provided for all his wants. " My mother never did any of those things for me," said Flippy. " Who did them, then ?" asked Lawrence. " Bonney," replied the boy. "And who was Bonney?" asked Lawrence. "She was a girl, or perhaps a woman," said Flippy. "My mother called her the bonne, but I generally called her Bonney — generally, but not always, for sometimes when she scolded me I used to call her Bony." " Did she use to scold you ?" asked Lawrence. " Sometimes," said Flippy, " especially when she caught me sliding down the banisters." FLIPPY WHEN HE WAS LITTLE. "It seems to me it was hardly right to call her a bad 280 FAREWELL TO FLIPPY. name," said Lawrence, " because she wished to prevent you from sliding down the banisters. It was only out of re- gard for your safety that she did it. I knew a boy once who fell and broke his leg by sliding down the banisters." "I know," said Flippy; "but I could poise myself ex- actly; besides, it was not a very bad name for her, for she was really rather bony." Just at this moment the bell rang, and a steward called out, "All ashore that's going !" So Flippy rose, and, bid- ding Lawrence good-by, he and John went down the com- panion-way to the main deck, and there Flippy fell into the current of people that were pouring in a continued stream over the plank to the pier. The last thing that Flippy said was that he wished Lawrence had allowed him to go with him and John. " I might have gone just as well as not," said he, " and I could have written to my father at the first stopping-place to send me some money and a trunk full of clothes." Before John had made his way back to where Lawrence was sitting, the steam-boat had begun to move away from the pier, and very soon began to glide very swiftly past the long line of ships, and ferry-boats, and canal-boats, and sloops which lay at the wharves and filled the docks which here formed the margin of the river. " I like Flippy pretty well," said John, as soon as he had resumed his seat, " but I don't think he is very grateful to his mother." " It is partly because he does not know how much she has done and suffered for him," replied Lawrence. " There seems to be a principle of gratitude in his heart, or else he would not have thought of bringing a present to me, on account, as he says, of my having been kind to him." Here Lawrence held up the parcel which Flippy had given him, and which was still lying in his lap. THE PARCEL. 281 " What I have done for him," he added, " little as it is, he knows and appreciates, and so he is grateful for it. But his mother has perhaps not done much to win his affec- tions of late years. It is very likely that, since he was old enough to be put under the charge of a bonne, she has not had much to do with him except to watch him and check him when he is doing any thing wrong, and he has not the least idea how much she must have done and suffered for him before that time. What he wants is light. When lie grows older, and understands how much he owes his moth- er, it is very probable that he will be grateful for it all, and he may then become a great comfort to her. I am sure I hope he will." " I wonder what the present is that he has brought for you !" said John. "Let's open it now." " No," replied Lawrence ; " I was not to open it till we got home." Here John took lip the parcel and began to feel of it, in hopes of being able to ascertain in that way what it was. "I thought it was books," said he, "but it is some kind of box — a pasteboard box. I wonder what is in it ! If I were you, I would open it now and see." " I was not to open it until we got home," said Law- rence. " You did not promise him that you would not," replied John. " No," rejoined Lawrence, " I did not promise in words, but I received the package on that implied condition." " He would not care," said John. " All he wanted was that you should not open it while he was by. I don't see what possible harm it could do for you to open it now." " I do," said Lawrence. " What harm ?" asked John. " Guess," said Lawrence. 282 FAREWELL TO FLIPPY. "That it might be some delicate thing that would get injured by being opened here?" suggested John, speaking in the tone of a question. "That is a pretty good reason," said Lawrence, " but that is not what I meant." " Then I give it up," rejoined John. "It would injure my credit and character for trustwor- thiness and faithfulness with you," said Lawrence. " If you found that I would take a thing from Flippy on certain conditions, understood, and then would not observe the conditions because he was no^ there to see, you could never have full confidence in my faithfully fulfilling any condi- tions that I should make with yow." John pondered somewhat thoughtfully upon this view of the case, but he did not reply. Indeed, it was pretty evident that there was nothing that could be very well said in reply. Lawrence attached great importance to the idea of sus- taining the character of perfect trustworthiness in the esti- mation of all who knew him. He was particularly desirous that John should at all times have entire confidence in him. He knew, moreover, that the only sure way of mak- ing all who know us believe that we are thoroughly honest and true, is to be in reality thoroughly honest and true, QUESTION ABOUT A NAME. 283 CHAPTER XXX. UP THE NORTH KIVER. FOR twenty or thirty miles above New York, the North River, as it is there called, is of its ordinary width, and runs in a pretty straight course, with a range of lofty and precipitous cliffs on one side, and a series of charming land- scapes, consisting of groves, gardens, pleasure-grounds, vil- las, public institutions, and pretty little landings leading to them, on the other. For an hour after leaving the pier at New York, Lawrence and John remained at their seats upon the upper deck, in the midst of many animated groups formed of the other passengers — some talking, some read- ing, some sitting quietly in silence, but all enjoying the mild and balmy air of the evening and the beauty of the scenery. " Why do they call this river the North River in New York," asked John, " while every where else it is called the Hudson River ?" " That is certainly very singular," said Lawrence. " Even the same people," continued John, " call it the North River when they are here, and call it the Hudson River when they are in Boston." " Not always," said Lawrence. " No, not always," replied John ; " but why do they ever? What is the use of having two names for the same river at all ?" "It is very common," said Lawrence, "to have two names for the same thing, to be used indiscriminately; but this seems to be a case where the use of one word or 284 UP THE NORTH RIVER. the other depends in some degree upon the place we hap- pen to be in when we use it. That's a curious philological phenomenon." " Philological ?" repeated John. " Yes," replied Lawrence ; " philology is the science that treats of the origin and the meaning of words, and the changes they undergo in the spelling and the use of them. It is a very curious subject. You will be very much in- terested in studying it one of these days, when you get older." " Should not I be interested in it now ?" asked John. " Perhaps so," replied Lawrence. "You might try. You might begin by looking into the histories of New York and of the early settlements of this country, and see if you can find out when and why this river received its two names, and also see if you can think of any other cases where we have two different names for the same thing, according to the place we happen to be in when we are speaking of it." "Do you know of any such cases?" asked John. "I know of one," replied Lawrence. "When we are not in the cars, we commonly call the stopping-places of the trains depots, but when we are in them we call such places stations. We never ask, for example, when we are travel- ing, ' What depot is this ?' or say that we are going to stop at the next depot, but always stativn. And yet, when out of the cars, at a hotel, or in the streets of a town, people almost always say depot." " That's curious," said John ; "I wonder what the reason is!" "I think there must be some reason, or at least some ex- planation of such a usage," replied Lawrence. " It would be a good plan for you, some time when you have nothing to do, to think of it, and see if you can study it out." Lawrence did not think it at all necessary that he should SELF-CONCEIT AND VANITY. 285 try to give some kind of explanation, satisfactory or other- wise, of every remarkable appearance or phenomena which they chanced to observe, especially when the questions which arose in connection with them related to branches of knowledge which John, in the course of his education, had not yet reached. He was very willing to open before him, from time to time, glimpses of fields of investigation to the very boundaries of which he had not yet attained. There was a double advantage in this. In the first place, the bringing to his view in this way curious and interest- ing questions connected with scenes which he had not be- gun to study, and of the very nature of which he had but little idea, expanded his ideas in respect to the vast extent of the field of knowledge which he had yet to explore, and increased his interest in going forward. Then, in the sec- ond place, showing him the boundlessness of the field be- fore him tended to prevent his becoming vain and con- ceited in thinking of the acquisitions that he had already made. I say only that it tended to produce this last good re- sult, for it is almost impossible to accomplish it entirely. Boys like John, who take a great interest in learning all they can, and who, of course, make rapid progress in learn- ing, almost always, for a time, become more or less conceit- ed. It is not at all surprising that it should be so, since their appreciation of what is contained within the little field which they have already explored is necessarily so much more vivid and distinct than any conceptions which they can form of what is before them in the boundless re- gions into which they have not yet entered. About twenty or thirty miles above New York the river expands into a broad and spacious lake, called the Tappan Sea. " We are coming to the Tappan Sea," said John. " Let 286 UP THB NORTH EIVER. us go forward, so that we can look out ahead and see the vessels on the water." So they rose from their seats and walked through the long upper saloon to the forward part of the steam-boat. This saloon was richly decorated, carpeted, and furnished, and many groups of gentlemen and ladies were seated upon the sofas, and lounges, and comfortable chairs, and parties of children were playing together here and there upon the floor. Along the sides of the room were ranges of doors opening into the different staterooms. Ihe room was very long, and had a very rich and elegant appear- ance, but the whole expression of the interior was entirely different from that of the main cabins of a sea-going steam- er. There every thing is solid, massive, strong, and firmly secured ; here the style was comparatively light, airy, and graceful, and to the eyes of Lawrence and John, accus- tomed, as they were, to the shocks, and concussions, and general rough usage which the Scotia or the Cuba had had to sustain from the billows of the Atlantic, seemed exceed- ingly frail. From the forward end of this saloon Lawrence and John passed out through a door to an open part of the deck over the bows, where they had a very fine view of the grand ex- panse of water before them. "What a splendid lake!" said John; "and how many steam-boats and vessels !" "Yes," replied Lawrence; "isn't it a pity that is all going to be filled up?" "Going to be filled up !" repeated John, much surprised; " what are they going to fill it up for?" "They are not going to do it. It is the river that will do it," replied Lawrence. " The river will fill it all up, except a winding channel that it will leave through the land that it makes for its own flow. All the rest FILLING UP OF LAKES. 287 will be filled up and formed into a region of level green fields." John was much surprised at this statement, and asked how it would be done. Lawrence explained to him that the lake was a vast hollow in the land filled with water, and that the river was all the time bringing down sand, and pebbles, and sediments of various kinds from the coun- try above ; and that, though some of these materials were carried through and borne out through the lower end of the lake, and so onward into the sea, some portion must necessarily be left behind, and in process of time the whole lake must be filled. "Nonsense!" said John; "such a great lake as this could never be filled in this way. There would not be sediment enough brought down to fill it — not in a thou- sand years !" " Perhaps not," said Lawrence ; " but if the river could not fill it in a thousand years, it might in ten thousand." " No," rejoined John, " I don't believe it would fill it even in ten thousand." " Then ten million," replied Lawrence. " You can have as many years as you want. There are plenty of them coming. If there is any deposit at all left in the lake, and nothing to take it away, the lake must some time or other become filled up." The conversation on this subject was continued between Lawrence and John for some time, and in the course of it Lawrence explained somewhat at length the manner in which natural depressions in the surface of the land which occur in the course of the current of a river, or wridenings of the valley through which it flows, and which at first be- come, of course, so many reservoirs of water supplied by the river, thus forming lakes, are gradually filled up by de- posits of sand and soil, so as to form in the end broad plains 288 UP THE NORTH KIVEK. bordering the river, covei'ed with verdure and trees, and with a tortuous channel through the centre of them kept open for the passage of the water. The process is a very curious one, and has been observed, and the different steps of the progress of it in particular instances have been carefully noted and recorded by men of science. The philosophy of the operation is this : All rivers in their flow bring down with them a great deal of sediment- ary matter, which results in part from the disintegration of the rocks and mountains among which their several branches take their rise, and also from dust blown into them by the wind, and from decayed animal and vegetable substances brought into them by the rains. The heavier portions of these substances sink rapidly, and are rolled along the bottoms of the rivers in the form of pebbles and sand. Those that are not so heavy sink more slowly, and where the flow of the stream is rapid and turbulent, their complete subsidence is entirely prevented by the surging and whirl of the water; and in general, the tendency to subsidence on the part of the solid matter held in suspension is determined in a great measure by the slowness or swiftness of the current. Now, in all those places where the river is very broad and deep, the motion of the water is very slow, on account of the space through which it moves being so vast, and the quantity moving being so great, that the whole amount that has to pass through during a given time can be trans- mitted by a very slow motion. Of course, in all those places where the space is so wide and deep as to form a lake, the deposition takes place much more rapidly than in other places ; and, unless something interferes with the process, the lake, after a certain time, becomes entirely filled up. CHANNEL KEPT OPEN. 289 " Then I don't see," said John, when Lawrence had ar- rived at this point in his explanation, "how any channel is left for the passage of the water." " There is something very curious and remarkable about that," replied Lawrence. " You see that the tendency to deposit is greatest where the water is most nearly in a state of repose, and least along the line of swiftest motion. Where this line of swiftest motion would be would depend much upon the conformation of the shores, but it would in general tend to pass somewhere through the middle of the lake. Of course, as the progress of the deposition goes on nearer the shores and in all the stiller portions of the water, the space which the whole volume of the water will have for its flow will be more and more contracted, and the current along it will become swifter and swifter, and thus, as the channel becomes contracted and defined, there will be an increasing force in the flow of the water to keep it from being closed entirely. " At last," continued Lawrence, " things would come in such a case into a state of equilibrium — that is, the tenden- cy of the sediment to subside through the water by its weight, and to be borne onward by the swiftness of the current, would balance each other, and the channel of the river would then become in some measure permanent as to its size — that is, as to what is called the area of its section, only now, instead of forming a lake, it would flow mean- deringly through a level plain, over which every freshet .vould deposit a fresh layer of fertilizing soil, until it was raised far above the level of the ordinary flow of the river." Lawrence went on farther to explain that this process of filling up all the natural depressions in the land through which rivers flow, and which originally formed the beds of lakes, had been going on for thousands of years, and N 290 UP THE NORTH EIVER. that there were now found along the courses of all rivers a great many places where, according to every appearance, there had formerly been depressions which the river orig- inally filled with water, so as to form lakes and ponds, but which are now filled up nearly to the height of the highest freshets, and have become smooth and level plains, covered with grass and trees. Such grounds as these are called meadows and intervals, and sometimes river bottoms. The river flows through these fluviatile lands — that is, river- made lands, by a very devious and winding channel, which is continually changing. " Why does not it flow straight, and keep always to the same channel ?" asked John. " Ah ! that is a very important question," replied Law- rence, " though I have not time to explain the case to you now, for it is about time for the gong to sound for tea. We shall have an excellent opportunity to study the op- eration of the water in a river channel at Carlton, when we get there, for you remember the river twists and winds about there through the meadows in front of our house, and wears away the banks on one side or the other inces- santly." "Yes," replied John, "it twines about in great sweeps, and the banks in the hollow of the sweeps are caving in." " It is almost always so," rejoined Lawrence, " with the course of a river through the lands which it has made itself. There is a splendid opportunity to see this from the top of Mount Holyoke, where we look down upon a re- gion which seems once to have been a great lake, but which now consists of a plain formed of the most fertile and beautiful meadows in the world, the river flowing through them with the most extraordinary windings." The engraving gives us a glimpse of these lands, and of the windings of the river through them, as seen from near MOUNT HOLYOKE. 293 the summit of Mount Holyoke. It presents to our view a very perfect example of an ancient lake filled up, and the river flowing through the new ground in a tortuous chan- nel. " Are we going by Mount Holyoke on our way home ?" asked John. " We are going pretty near to it," said Lawrence. " Then let us stop and go up," said John ; " I like to climb mountains and see the views." " Very well," replied Lawrence. " The view from Mount Holyoke is very beautiful, and it is very instructive, too, for one who is studying these subjects. But we can see the operation of the process to better advantage at Carl- ton, for there every thing is on a smaller scale, and the changes are more perceptible. The great principles are the same in all cases, from the smallest brooks to the mightiest rivers. But why does not the gong sound ?" " I wish it would sound," replied John, " for I'm hungry for supper." " The general principle is this," resumed Lawrence, re- verting to the subject of the flow of rivers : " The true and ultimate function of brooks and rivers is to remove the mountains to the sea ! Of course they can not carry them down whole, but the frost, and the ice, and the rain disin- tegrate and wear away the rocks, and deliver the materials into the streams in such a form that the water can carry them on. The river first employs these materials in filling up all the hollows and depressions in the ground that it meets with on its way. But it does not leave any single portion of them long there, for, by twisting and winding in its course, it continually washes away and carries down the stream successive portions of the land it formed years before, and replaces what is thus removed from one side of the river by new formations, which it gradually builds up 294 UP THE NORTH KIVER. on the other side from fresh materials. We shall be able to see all this work going on, upon a comparatively small scale, when we get to Carlton." Carlton was the name which I give to the town where Lawrence and John lived. It was situated among the mountains in the interior of New England. " I mean to watch the river when I get home," said John, " and see how it works." "You can even do more than that," rejoined Lawrence ; " you can actually experiment with a stream yourself, if you take one small enough ; for the laws which govern the flow of water, and the transportation of solid matter sus- pended in it, or borne along by it, are the same, and the effects that result are analogous, whatever is the size of the stream; only in the smaller streams the changes are more rapid, and being, moreover, comprised within a nar- rower area, are more easy of observation." " Yes," said John, " there's the Beaver Brook, where I used to have my dam. I mean to go and see how it is on the Beaver Brook as soon as I get home." The conversation on this subject was here suddenly in- terrupted by the sound of the gong, on hearing which John rose at once with great alacrity, and, followed by Lawrence, went down to supper. He, however, did not forget what Lawrence had explained to him about the action of rivers in filling up such natural depressions in the land as came in their course, and forming green and fertile meadows in the places they had occupied, nor the resolution which he had made to investigate the subject by observations and experiments upon the streams in the neighborhood when he should reach home. An account of the results of these observations and experiments will be contained in the next volume of this series, which is to be entitled WATER AND LAND. AT THE SUPPEK-TABLE. 295 CHAPTER XXXI. LIGHTING BY GAS. THE sun had gone down and the twilight was far ad- vanced before the gong was sounded which summoned the passengers on board the steamer to supper, and when Law- rence and John went below they found the supper-tables lighted by a long row of candles. "Why don't they light the cabin with gas?" asked John, as soon as they were seated at the table. " Oh ! I might have known myself," he added, after a moment's reflec- tion ; " they could not bring the pipes on board." " True," replied Lawrence, " they could not bring the gas in by pipes from the mains in the city, but there are other ways in which we can conceive of gas being brought on board a steamer besides drawing it from the great city gasometers. In the form in which it exists in these gasom- eters, it is altogether too much expanded and too bulky to be conveniently transported or stored, but there are two modes of bringing it in a more compact form : first, by in- troducing it in what may be called the original packages, and, secondly, by packing it anew expressly for the pur- pose." John did not know at all what Lawrence meant by this language. He did not understand, he said, how such a sub- stance as gas could be packed at all. So Lawrence ex- plained to him what he meant. He did this in conversa- tion which was partly held at the supper-table, and partly afterward in the saloon above, when they went up after the supper was concluded. The substance of the conver- sation was this : 296 LIGHTING BY GAS. One would not suppose that such a substance as gas could be packed very easily in any way, and yet Nature has the art of stowing it in a very compact form in all that class of substances which have already been described as hydrocarbons — that is to say, in almost all natural sub- stances that are inflammable. It is packed very closely in wood, in all bituminous coal, and in all such substances as resin, pitch, wax, and tallow. "Indeed," said Lawrence, pointing to one of the tall can- dles which stood upon the table before them while they were at supper, " providing these candles is only a mode of bringing gas on board in a compact and manageable form. The paraffine of which these candles are made is a hydrocarbon — that is, it is composed chiefly of hydrogen and carbon combined with each other and packed very closely together. The heat of the burning wick liberates them and restores them to their gaseous form, and they then burn, just as the gas in the cities does from a jet; only, in the case of the candle, the gas is burned directly as fast as it is set free, and in the place where it is set free, instead" of being saved and stored in a great reservoir, and then conveyed in pipes to be burned in different places at a distance from where it is produced. In a philosophical point of view, and in all essential respects, the burning of a candle is the same as burning gas from a jet." " That's curious," said John ; " and is it the same with a lamp ?" " Precisely the same," replied Lawrence ; " only, in the case of the lamp, the material from which the gas is dis- tilled is a liquid, instead of being a solid, as it is in the case of the candle. " Thus, in point of fact," continued Lawrence, " they do burn gas in this steamer. They bring it on board packed very snugly in the paraffine of the candles. They might, NATURAL CONDENSATION. 297 even, in fact, bring it packed in coal, were it not for the in- convenience they would incur in that case in the work of unpacking it." In speaking thus of hydrogen and carbon, which are the constituents of illuminating gas, as packed in paraffine and in coal, Lawrence used language, it must be confessed, in a somewhat figurative sense ; but these materials do certain- ly exist in these substances in a very highly condensed and concentrated condition. Indeed, Nature seems to have the power of carrying into eifect this kind of packing in a most extraordinary degree. Water, for example, is composed of the two substances oxygen Snd hydrogen, both of which in their ordinary con- dition, as known to us, appear in the form of a gas. Na- ture, in combining these substances in the form of water, brings enormous volumes of them into very small compass, and retains them in that condition without any external force of compression or any means of confinement what- ever. Man can not produce this condensation by a press- ure of a hundred and fifty atmospheres. I shall presently explain what is meant by an atmosphere as a measure of pressure, though the explanation will not help the reader to form any distinct conception of what a pressure of a hundred and fifty atmospheres is, as no one can form any adequate idea of such enormous forces ex- cept those who have witnessed the production of them and observed practically some of their effects. Somewhat in the same way by which the powers of na- ture hold the naturally gaseous substances of oxygen and hydrogen in so very compact and concentrated a condition in water, do they also hold the carbureted hydrogen in the paraffine of the -candle and in coal. In the case of coal, the quantity held within a given space varies much, according to the different qualities of the coal, and to other circunv N2 298 LIGHTING BY GAS. stances ; but it is not uncommon to find a quantity of il- luminating gas sufficient to fill a room thirty feet square and ten feet high so closely compressed in the coal con- taining it, that if, while it was in that state, it could be separated from the other constituents of the coal, it would form a solid block which a man could easily lift. Thus, as Lawrence said, bringing the gas on board the vessel packed in paraffine or in coal is altogether a more convenient mode than to attempt to bring it in pure, in its natural form and of its natural bulk, as gas. In the form of paraffine it is much more expensive, in the first instance, than as one of the constituents of coal, but then it is much more easily extracted, or, perhaps, it would be Better to say, developed, from that substance than from coal ; for, in the case of paraffine, or wax, or tallow, or any other such substance, all that is necessary is to have a wick passing up through it and set on fire, and the process of melting successive portions of the substance, and converting them into an illuminating gas, goes on of itself, without any ap- paratus or machinery whatever. Whereas, on the other hand, although people might ob- tain the necessary supply of gas in coal cheaper than in any of those other forms, there would be required a com- plicated, and expensive, and bulky, and even somewhat dangerous apparatus to distill it. There would have to be a furnace to heat the coal, and tight iron 'retorts to contain it so as to prevent the gas from being burned in the fur- nace as fast as it was produced, and a reservoir to store it, and pipes to convey it to the different parts of the vessel ivhere it might be required, all of which would involve much trouble and expense. " That would not do at all," said John, when Lawrence explained these things to him. "Especially," he added, after thinking a moment, "in THE LIGHT OF LAMPS AND CANDLES. 299 the case of a steamer at sea, tossing and pitching about in a storm." Besides these objections which Lawrence pointed out, we may add that the process of preparing gas from coal, or, as Lawrence called it, the work of " unpacking it," not only involves the use of complicated machinery, but re- quires skilled workmen to manage the machinery and to conduct the process. And these men must devote, too, all their time to the work, and must be well paid, so that it is, on every account, much better to produce the gas for illumination from some of the substances that can be used in the form of candles or in lamps, though they cost more at the outset. It is only when very large quantities of gas are required, and in places, too, where there is ample room for all the machinery and appointments, that it can be profitably obtained from coal. Thus it can be manufactured advantageously on a great scale for lighting cities and towns, and even for extensive private establishments where there is plenty of space at command for the necessary works; but for single dwell- ings, or small establishments of every kind, if they are to be lighted artificially at all, the gas must be brought in packed, as Lawrence called it, in paraffine, or wax, or sper- maceti, or tallow, or oil, or kerosene, or some other similar hydrocarbon. "I never thought before," remarked John, when Law- rence had made these explanations to him, " that, when we were burning lamps or candles, we were really burning gas." "Yes," replied Lawrence; "what is actually burnt in both cases is essentially the same, only, in the case of a candle or lamp, the gas is burned as fast as it is set free, while in the case of regular gas-works it is kept from be- ing burned for a time after it is set free, and is conveyed 300 LIGHTING BY GAS. in pipes wherever the light from it is wanted. Even the flame of burning wood from a fire is the flame of gas, you recollect." " I remember you told me once," replied John, " how I might draw it off from the fire through a pipe-stem, and burn it at the end of the stem." "Yes," rejoined Lawrence; "and we might easily draw it off farther than that, if we chose, by means of an India- rubber tube. " Only, in that case," added Lawrence, " it would be bet- ter to take some other larger and stronger receptacle than the bowl of a pipe for a retort — a gun-barrel, for instance. Chemists employ gun-barrels very often for such experi- ments. An old gun-barrel which is past service for shoot- ing, such as can generally be obtained at a gunsmith's, will make a very good retort for such purposes." Lawrence went on to explain that, by taking such a gun- barrel, and, after plugging up the touch-hole, filling it half full of some hydrocarbon and connecting a long India-rub- ber tube with the outer end of it, the gas could be con- veyed away to any distance — to a stand of some kind, for example, upon a table in the middle of a room — and there burned just like gas from a pipe laid in the street. John said that he should like very much to see that done. "Very well," replied Lawrence ; " we can do it, or, rath- er, you can do it yourself under my direction, when we get home. I mean to fit up a little laboratory and workshop in Carlton, and you can then perform as many such ex- periments as you like." " I mean to make some gas, at any rate, for one thing," replied John. " Only," he added, after reflecting a mo- ment, " I should think that the end of the India-rubber tube, where it is slipped over the end of the gun-barrel, KEEPING THE IKON COOL. 301 would begin to melt or burn pretty soon. You see, if the butt end of the barrel was in the burning coals, the muzzle end would get quite hot in a very short time." " Certainly," replied Lawrence, " unless we devised some way to keep it cool. There are a great many practical dif- ficulties of this kind to be encountered in making chemical experiments, and it requires sometimes a good deal of in- genuity to contrive means to surmount them. That is one reason why making chemical experiments is so useful to a boy so soon as he- is old enough for such work. It sharp- ens his wits. " As to keeping the end of the gun-barrel cool," contin- ued Lawrence, " there is a simple mode of doing that. We have only to wrap the outer end of the barrel, where the India-rubber tube joins it, 'with a strip of cotton cloth, winding it round and round in the form of a bandage, and then keeping the cloth wet by pouring on a little water from time to time out of a pitcher." " Yes," replied John, " that would keep it cool." " Water has a wonderful power to keep any thing cool," said Lawrence, " even though it is hot water." " That is very strange," said John. " I mean," said Lawrence, " to keep any thing from get- ting very hot — red hot, or hot enough to melt or bum India-rubber, for example; for, before the iron around which the wet cloth is bound becomes hot enough for that, it will be hot enough to boil the water, and water absorbs such an enormous quantity of heat in boiling as to keep the temperature of the iron down to a comparative low point. Of course, as fast as the water in the cloth is boiled and converted into steam, you must pour on more, so as to keep the cloth all the time wet." u That would be a great deal of trouble," said John. " Yes," replied Lawrence, " and there would be a great 302 .LIGHTING BY GAS. many other troubles and inconveniences in the attempt to produce gas on a small scale for any practical purposes, but we might be willing to take the trouble once for the sake of performing the experiment." " Oh yes," replied John ; " and I mean to try it if you will help me. I mean to have a small pitcher and pour a little water on every few minutes." Before leaving this subject of the management of gas, I will add that there is an artificial mode of packing this bulky commodity after it is evolved, by compressing it, with great force, in metallic reservoirs made prodigiously strong to resist the pressure. The gas is driven into these reservoirs by means of forcing-pumps working with great power. The French have adopted this system in Paris to a considerable extent. The engraving represents a wagon loaded with gas thus compressed. The interior of the wagon is occupied by nine cylinders, which are made of copper, and are of enormous strength. There is forced into each cylinder ten or twelve times as much gas as it would naturally contain if the gas were of its ordinary density; and as the expansive pressure of the gas is in proportion to the quantity of it that is forced into a given space, the whole interior surface of each cylinder has a force pressing upon it from within outward, and so tending to burst it, often or twelve atmospheres! For you must understand that pressure in mechanics is measured by atmospheres. The pressure of the atmosphere is reckoned at fifteen pounds to the square inch. The actual pressure of the atmosphere varies from day to day in the same place according to the quantity of air that there may happen to be for the time being over the place, and in different places according to their elevation above the level of the sea ; but fifteen pounds to the square inch is taken as the standard of measurement, or, in other words, MEASUREMENT OF PRESSURE. 305 fifteen pounds to the square inch is an atmosphere of press- ure. This is a fundamental principle or fact which it is very important to remember. It comes continually into philosophical and mechanical calculations. The meaning of the principle thus stated is, that, light and rare as the atmosphere seems to us, in moving through it, it extends to so great a height above the surface of the earth, and the quantity is in the whole so great, that the weight of it is equal to fifteen pounds upon every square inch that it presses upon. That is to say, if you place a small block of wood an inch square upon a table, and a fif- teen pound weight upon the block, the additional pressure would be that of one atmosphere; and this additional press- ure would be just equal to the original pressure of the at- mosphere itself, so that, with the pressure of the weight, the whole pressure would be exactly doubled. Now this original pressure, great as it is, is not felt by us, because it acts in every direction ; just as a fish swim- ming in the water does not feel the weight of the water over him, because, the water being so perfect a fluid, the pressure resulting from the weight diffuses itself and bal- ances itself in every direction, so that the fish floats in it, as it were — in the pressure, I mean, not the water — and is not sensible of it at all ; so wre ourselves float, as it were, in the pressure of the air, which acts from above and be- low, upon every substance and upon every side of it, equal- ly, and even from the pores and interstices within it out- wardly, so that it produces, in ordinary cases, no percepti- ble effect. The amount of it, however, in every direction and from every side, is fifteen pounds to the square inch. Now this pressure is really much greater than one would at first imagine. The surface of one side of a man's hand, for example, contains not less, including the fingers, than twenty square inches; consequently, the weight of the air 306 LIGHTING BY GAS. pressing upon the hand when the man holds it out hori- zontally before him is not less than twenty times fifteen pounds — that is, three hundred pounds ! Now, if this down- ward pressure upon the upper surface were not balanced and counteracted by an equal upward pressure upon the under surface, and also from certain resisting pressures ex- erted by the fluids within the hand, no man could hold his hand out horizontally in that manner for a moment. We see what the prodigious force of this pressure is, when not counterbalanced, by the action of certain steam- engines which are worked on the principle of producing a vacuum upon each side of the piston in the cylinder alter- nately, by which means the counterbalancing pressure is taken off, and the pressure of the air on the other side is al- lowed to act without any thing to oppose it. By this means the piston is driven to and fro with prodigious force, de- veloping a power that is sufficient to work the heaviest ma- chinery, and all by the simple pressure of the atmosphere upon one side of the piston when the balancing resistance on the other side is taken away. And yet this is only the pressure of one atmosphere. It is precisely this amount of pressure which is exerted both on the inside and on the outside of a glass bottle, or any other receptable the interior of which has an open and free communication with the outside air. If, however, this free communication is closed, and a double quantity of air is forced into the receptacle through a pipe fitted to it for the purpose, then we should have the pressure of two atmospheres on the inside and only one on the outside, and there would be a surplus force of fifteen pounds upon every square inch of the in- ternal surface, tending to burst the vessel. If treble the quantity were introduced, then there would be a prepon- derance of two atmospheres — that is, of thirty pounds to GAS COMPRESSED. 307 the square inch. This amount of pressure on every square inch of a vessel of the size of a barrel, for example, would constitute an enormous bursting force— af force of forty or fifty thousand pounds ! It is on this principle, however, that the copper cylinders in the gas wagon shown in the engraving were filled, and yet so prodigiously heavy and strong were they made, that sometimes, as has already been said, ten or twelve volumes of gas were forced into them. If the number is taken as eleven, then, allowing one to balance the ordinary atmos- pheric pressure on the outside, we should have an expan- sive force of a hundred and fifty pounds to the square inch acting all the time upon the interior surfaces of all the cyl- inders. The gas, in this compact form, was conveyed about the city and delivered to the consumers. Those who chose to take their gas of this company, of course, were obliged to provide the means of receiving it in the form of a gasome- ter, or of some very strong and well-secured receptacle, for the cylinders in the wagon were altogether too massive, solid, and heavy to be removed. When the wagon arrived at the door of one of the customers, a pipe from one of the cylinders in the wagon was connected with one communi- cating with the reservoir within, and then, when the stop- cock was open, the gas from the cylinder would rush in by its own expansive force until the quantity in the two re- ceptacles was equal — that is, in case the receptacles them- selves were equal — and the pressure would be that of five atmospheres in each. Then, of course, no more would flow from that cylinder, but an additional quantity could be thrown in from a fresh cylinder where the pressure of the whole ten atmospheres was still entire. The first cylinder, moreover, which had delivered half its gas, could be made to deliver more at 308 LIGHTING BY GAS. the establishment of the next customer, whose receptacle, being empty, would be ready to take half of that which still remained. Thus, while the cylinder in the wagon would deliver five atmospheres at the first customer's, it would deliver two and a half, which would be half of the remainder, at the second, and so on. In this way, with proper management, a large portion of the load could be delivered, and the residue, which was not delivered, would not be lost, but would remain in the cylinders as so much toward the next filling. The plan, however, after all, was not found to be practi- cally successful. There were so many difficulties and in- cumbrances to interfere with the easy and convenient management of it that it never was carried into extensive operation. One good, however, results from the experi- ment : it affords an excellent illustration to aid the young student to understand the nature and the operation of pressures, and the modes of measuring them, and Law- rence made very good use of it for this purpose in hie conversation with John. THE JOYOUS LOCOMOTIVE. 309 CHAPTER XXXIL CONCLUSION. THE steamer by which Lawrence and John made their passage up the North River arrived at Albany early in the morning. From Albany they were to continue their journey by land. Their route lay to the eastward, toward New England. The scenery along the road was very pic- turesque and beautiful, and the locomotive, as if equally proud of the large company of neatly-dressed passengers under his charge, filling the long train of cars which he THE LOCOMOTIVE. 310 CONCLUSION. had to draw, and of the beauty of the country through which he had to take them, ran whistling along his way as if his heart was filled with gladness and joy, now winding around the point of a rocky hill, now running with redoubled speed down a long incline, but always bringing, at every moment, new scenes of fertility and beauty into view — smiling valleys, pretty towns, and for- est-covered hills. John was much interested, as they went on, in observing all the streams flowing through the valleys which they could overlook from the windows of the car, and he saw many examples of such streams pursuing a very meander- ing course through level meadow-lands, which had every appearance of having been formed by the filling up of an- cient lakes or ponds. The case in which this effect was manifested on the grandest scale was that of the windings of the Connecticut River at the foot of Mount Holyoke. The travelers stopped over one train expressly to obtain a good view of this valley, which object they attained by going partly up Mount Holyoke. They did not have time to go to the top. When at length they took their places in the train again to resume their journey, John amused himself with reading for a time, and then finally shut his book and said he was very tired. " I suppose you did not sleep very well last night on board the steam-boat," said Lawrence ; " and, besides, we have had a somewhat fatiguing time of it to-day." So Lawrence proposed that John should place himself in a comfortable position and see if he could not go to sleep. John said he was sure he could not go to sleep, for he was not sleepy. " You can put yourself in a comfortable position, at any rate," said Lawrence," and then I will tell you a story." LEBON. 3U John said that that was exactly what he should like. So he placed his feet upon the valise, and leaned his head upon Lawrence's shoulder, and Lawrence began : " I'll tell you the story," said he, " of the man who first discovered the mode of lighting by gas. His name was Lebon. He was a Frenchman, and an engineer by profes- sion. He was in the government employ, being engaged in superintending certain public works and manufactures. But, besides his regular business, he was greatly interested in making investigations and experiments." " That was a good thing," said John. " Yes," replied Lawrence, " if it was not carried too far. He was charged with neglecting his regular duties in or- der to gain time to make his experiments. I do not know whether the charge was just or riot, but I advise you, if you make any experiments this winter, not to let them in- terfere with your regular studies." John did not answer. The truth was, he was beginning to feel a little sleepy. " The first experiment that he made in relation to gas," continued Lawrence, " was something like our plan of dis- tilling gas in a pipe, only he used a glass bottle instead of a pipe. He observed, in watching the fire, that the flame sometimes seemed to nicker in the air at a little distance from the wood, and he conceived the idea of separating it entirely. So he filled a glass bottle with sawdust, and fitted some kind of a tube into the mouth of it, and then put the bottle into the fire among the burning coals." " But, Lawrence," said John, partially arousing himself, " the bottle would break." " Yes," said Lawrence, " if he put it in suddenly it would break, but you can heat glass very hot if you heat it very gradually, and Lebon, no doubt, took all necessary precau- tions. His experiment succeeded very well, Then he tried 312 CONCLUSION. it on a larger scale ; "but the gas, as he first formed it, had many impurities combined with it which gave it a bad smell. He had a great deal of trouble in contriving modes of freeing it from these impurities, but he succeeded toler- ably well at last. " He had, however, a great many difficulties to contend with. His salary was very small, and the condition of the government at that time in France was so unsettled, that what was due him was very slowly and irregularly paid. All his friends and acquaintances laughed at him, too, as a visionary schemer. " He, however, persevered, and at length succeeded in getting his invention so far perfected that he constructed an apparatus sufficient for lighting a house which he hired for the purpose, and then he advertised his plan and opened his house once a week or so to the public, on the payment of three francs admission. Do you remember how much three francs is, of our money ?" John did not answer. "I verily believe the boy is asleep," said Lawrence, speak- ing to himself; "so much the better. Sleep will do him more good than any story." So Lawrence did not disturb him, but let him sleep on, and John did not wake until he so nearly reached home that he did not ask for the rest of the story. I will, how- ever, add that poor Lebon did not live to see the final suc- cess of his invention. In the midst of his active efforts to induce the government to make arrangements for giving his new mode of illumination a fair trial on a proper scale, he was found one morning murdered in a public park in Paris, or, rather, in a wood which has since become a pub- lic park of great celebrity, but which was in those days des- olate and lonely, and the resort of thieves and robbers. It was supposed that, in crossing this ground on his way to ARRIVAL AT HOME. 313 his home, he was waylaid and killed by the highwaymen that infested the place at night, on account of its very dark- ness and obscurity. He lost his life thus for want of the safeguard in lone- some places which simple illumination affords — a safeguard which in those days could not be provided, but which, through his discoveries, was soon to be introduced into all the principal cities of the world. 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