UNIVERSITY OF CALIFORNIA AT LOS ANGELES THE LIBRARY OF THE UNIVERSITY OF CALIFORNIA LOS ANGELES S266 1 1 Marie pinx' HALO THE ATMOSPHERE. TRANSLATED FROM THE FRENCH CAMILLE FLAMMARION, EDITED BY JAMES GLAISHER, F.R.S., SUPERINTENDENT OF THE MAGNETICAL AND METEOROLOGICAL DEPARTMENT OF THE ROYAL OBSERVATORY AT GREENWICH. WITH TEN CHROMO-LITHOGRAPHS AND EIGHTY - SIX WOODCUTS NEW YORK: HARPER & BROTHERS, PUBLISHERS, FRANKLIN SQUARE. 1874. PREFACE BY THE EDITOR. THE following work is translated and abridged from M. Flammarion's LJ 'Atmosphere, Paris, 1872. That some curtailment of the text of the original work was requisite will be apparent when it is stated that the French Edition contains 824 large pages of closely printed matter, and is of more than twice the extent of the present volume. Not only was some compression necessary in order to bring the work within a rea- sonable compass, but, independently of this, one or two chapters, such as that on the Eespiration and Alimentation of Plants, appeared to have so remote a connection with the subject of the work the Atmosphere that their omission would in any case have been desirable. Every one who has any acquaintance with French popular works on Science is aware that very many exhibit a tendency to imaginative, or, to express my meaning colloquially, "fine" writing, which ill accords with the precision and accuracy that ought to be a characteristic of scientific information, even when expressed in language free from tech- nicalities. There is a good deal of this exalted kind of composition in M. Flammarion's book, which even in the French not very agreeable to an English reader becomes, when translated, intolerable. I have, therefore, omitted these rhapsodies very freely, though traces enough of them will be found here and there to betray the French origin of the work. I may add that the task of editing has not been a light one ; besides the necessity for compression and the consequent selection of the mat- ter to be included, I have been obliged to exercise some sort of censor- ship over the facts contained in the work. It is impossible for any one man to have a complete knowledge of so great a variety of subjects as are treated of by M. Flammarion, and the compiler of such a book must include many things taken from others, of the accuracy of which he is not fully competent to judge. In cases where a statement contained in the original work appeared to me clearly erroneous, I have corrected it, 445073 4 PREFACE BY THE EDITOR. appended a note, or omitted it altogether ; and in cases where I have been doubtful of the accuracy of a passage, or have differed in opinion from the author, I have not considered myself justified in making an alteration, so long as there was no strong primd facie presumption that the original was incorrect. In spite of obvious blemishes, inseparable from a translation, and a certain want of continuity in a few places, which is due to the omission of portions of the book as originally written, I believe the volume will be found to be readable, popular, and accurate, and it covers ground not occupied by any one work in our language. The work treats on the form, dimensions, and movements of the earth, and of the influence exerted on meteorology by the physical conformation of our globe; of the figure, height, color, weight, and chemical components of the atmosphere; of the meteorological phe- nomena induced by the action of light, and the optical appearances which objects present as seen through different atmospheric strata ; of the phenomena connected with heat, wind, clouds, rain, and electricity, including the subjects of the laws of climate: the contents are there- fore of deep importance to all classes of persons, especially to the ob- server of nature, the agriculturist, and the navigator. The whole is explained in a very popular manner, and as free as pos- sible from all technicalities ; the object having been to produce a work giving a broad outline of the causes which give rise to facts of every- day occurrence in the atmosphere, in such a form that any reader who wished to obtain a general view of such phenomena and their origin would be readily enabled to do so. The great number of subjects treated of will thus, to the majority of readers, who merely desire an insight into the general principles that produce phenomena which every one has seen or heard of, be found to be rather an advantage, as the whole range of atmospheric action is thus displayed in the same volume in moderate compass, without so much detail being anywhere given as to make the book other than interesting to even the most casual reader. The translation was made by Mr. C. B. PITMAN. January, 1873. PREFACE. In esi vivimus, movemur et sumus. OF all the various subjects which invite a studious examination, it is impossible to select one possessing a more direct, a more permanent, or a more real interest than that which forms the subject of this work. The Atmosphere gives life to earth, ocean, lakes, rivers, streams, forests, plants, animals, and men ; in and by the Atmosphere every thing has its being. It is an ethereal sea reaching over the whole world; its waves wash the mountains and the valleys, and we live beneath it and are penetrated by it. It is the Atmosphere which makes its way as a life-giving fluid into our lungs, which gives an impulse to the frail ex- istence of the new-born babe, and receives the last gasp of the dying man upon his bed of pain. It is the Atmosphere which imparts verd- ure to the fertile fields, nourishing at once the tiny flower and the mighty tree ; which stores up the solar rays in order to give us the benefit of them in the future. It is the Atmosphere which adorns with an azure vault the planet in which we move, and makes us an abode in the midst of which we act as if we were the sole tenants of the infinite the masters of the universe. It is the Atmosphere which illuminates this vault with the soft glitter of twilight, with the waving splendors of the aurora borealis, with the quivering of the lightning and the multi- form phenomena of the heavens. At one moment it inundates us with light and warmth, at another it causes the rain to pour down in torrents upon the thirsty land. It is the channel by which the sweet perfumes descend from the hills, and the vehicle of the sound which permits human beings to communicate with each other, of the song of the birds, of the sighing of the wind among the trees, of the moaning of the waves. Without it, our planet would be inert and arid, silent and lifeless. By it the globe is peopled with inhabitants of every kind. Its indestructi- ble atoms incorporate themselves in the various living organisms; the Q PREFACE. particle which escapes with our breath takes refuge in a plant, and, after a long journey, returns to other human bodies ; that which we breathe, eat, and drink has already been inhaled, eaten, and drunk mill- ions of times : dead and living, we are all formed of the same sub- stances. .... What study can possess a vaster or more direct interest than that of the vital fluid to which we owe the manner of our being and the maintenance qf our life? The study of the Atmosphere, of its physical condition, of its move- ments, of its functions, and of the laws which regulate its phenomena, forms a special branch of human research. This science, which since the days of Aristotle has been designated Meteorology, belongs in part to Astronomy, which shows the movements of our planet around the sun movements to which we owe day and night, season, climates, solar action, or, in a word, the basis of the subject. On the other hand, it appertains to Natural Philosophy and Mechanics, which explain and measure the forces brought into play. As it exists in the present day, Meteorology is a new science, of recent establishment, scarcely as yet fixed in its elementary principles. We are assisting at its elaboration, at its struggling into life. The present generation has seen the establishment of meteorological societies throughout the different nations of Europe, and of sp.ecial observatories for the exclusive study of the problems relating to the Atmosphere. The analysis of climates, seasons, currents, and periodical phenomena is scarcely terminated. The examination of atmospheric disturbances, of tempestuous movements, and of storms, has been made, so to speak, before our own eyes. The science of the Atmosphere is the question of the day. We are just now, in regard to this study, in an analogous situation to that of modern Astronomy in the days of Kepler. Astron- omy was founded in the seventeenth century. Meteorology will be the work of the nineteenth. I have endeavored to collect in this work all that is at present posi- tively known about this important subject, to represent as completely as possible the actual state of our knowledge about the Atmosphere and its work that is, about the air, the seasons, the climates, the winds, the clouds, the rain, the hurricanes, the storms, the lightning, the me- teorsin a word, the phenomena of time, and above all, the general upholding of terrestrial life. It is, in fact, a synthesis of the research effected during the last half century (especially during the latter portion of it) as to the great phenomena of terrestrial nature, and the forces PREFACE. 7 .which produce them. The great majority of us, inhabitants of the earth, no matter to what nation we belong, pass our lives without at- tempting to form an idea of our actual position, without asking ourselves what is the force which prepares for us our daily bread, ripens for us the grapes that give the wine, presides over the change in the seasons, and alternates the exhilarating blue sky with the rains and cold of in- hospitable winter. Yet, why should we live in such a state of igno- rance? I venture to hope that after perusing this work there will be no difficulty in understanding the life and movements of the globe. Every thing which takes place around us is interesting when, instead of remaining as one born blind, a man has learned to appreciate external things and to keep himself in intelligent communication with Nature. I could have wished to keep this work, destined as it is for the gen- eral public, free from scientific terms and figures which constitute its basis. I have done so as far as possible, but without in any point sacri- ficing accuracy and precision in respect to observed facts. It seems to. me, too, that what is termed the public (that is, every one) has become somewhat scientific itself, since so many excellent works have popular- ized ideas previously reserved for a small circle of the elect. CAMILLE FLAMMARION. PARIS : November, 1871. CONTENTS. BOOK FIRST. OUR PLANET AND ITS VITAL FLUID. CHAP. PAGE I. THE TERRESTRIAL GLOBE 17 II. THE ATMOSPHERIC ENVELOPE 23 III. THE HEIGHT OF THE ATMOSPHERE 28 IY. WEIGHT OF THE TERRESTRIAL ATMOSPHERE THE BAROMETER AND ATMOSPHERIC PRESSURE ...... 38 V. CHEMICAL COMPONENTS OF THE AIR 57 VI. SOUND AND THE VOICE 75 VII. AERONAUTICAL ASCENTS . . 85 BOOK SECOND. LIGHT AND THE OPTICAL PHENOMENA OF THE AIR. I. THE DAY 103 II. EVENING 113 III. THE RAINBOW 121 IV. ANTHELIA : SPECTRE-SHADOWS UPON MOUNTAINS THE ULLOA CIRCLE CIRCLE SEEN FROM A BALLOON 127 V. HALOS : PARHELIA PARASELENES CIRCLES SURROUNDING AND TRAVERSING THE SuN CORONAS COLUMNS VARI- OUS PHENOMENA 137 VI. THE MIRAGE.. ... H9 IQ CONTENTS. CHAP. PAGE VII SHOOTING-STARS BOLIDES AEROLITES STONES FALLING FROM THE SKY 163 VIIL THE ZODIACAL LIGHT 174 BOOK THIRD. TEMPERATURE. I. HEAT: THE THERMOMETER QUANTITY OF HEAT RECEIVED TEMPERATURE OF THE SUN TEMPERATURE OF SPACE 181 II. HEAT IN THE ATMOSPHERE 190 III. THE TEMPERATURE OF THE AIR : ITS MEAN CONDITION DAILY AND MONTHLY VARIATIONS OF THE TEMPERATURE TEM- PERATURE OF EACH SUMMER, WINTER, AND YEAR AT PARIS AND AT GREENWICH SINCE THE LAST CENTURY DAILY AND MONTHLY VARIATIONS OF THE BAROMETER 202 IV. REMARKABLE SUMMERS THE HIGHEST KNOWN TEMPERATURES 218 V. AUTUMN WINTER : WINTER LANDSCAPES COLD SNOW ICE HOAR-FROST, RIME, ETC. REMARKABLE WINTERS THE LOWEST KNOWN TEMPERATURES 229 VI. CLIMATE : DISTRIBUTION OF TEMPERATURE OVER THE GLOBE ISOTHERMAL LINES THE EQUATOR THE TROPICS THE TEMPERATE REGIONS THE POLES THE CLIMATE OF FRANCE . 245 BOOK FOURTH. THE WIND. I. THE WIND AND ITS CAUSES: GENERAL CIRCULATION OF THE ATMOSPHERE THE REGULAR AND PERIODICAL WINDS TRADE-WINDS THE MONSOON BREEZES 269 II. THE SEA CURRENTS : METEOROLOGY OF THE OCEAN MARITIME ROUTES THE GULF STREAM 284 CONTENTS. 11 CHAP. PAGE III. THE VARIABLE WINDS THE WIND IN OUR CLIMATES MEAN DIRECTIONS IN EUROPE AND IN FRANCE RELATIVE FRE- QUENCY OF DIFFERENT WlNDS RlSE OF THE WlNDS ACCORD- ING TO THE TlMES AND PLACES MONTHLY AND DlURNAL VA- RIATION IN INTENSITY . . 297 IV. RESPECTING CERTAIN SPECIAL WINDS : THE BISE THE BORA THE GALLEGO THE MISTRAL THE HARMATTAN THE SI- MOOM THE KHAMSEEN THE SIROCCO THE SOLANO 318 V. THE POWER OF THE AIR: THE HURRICANE THE CYCLONE THE TEMPEST 327 VI. TROMBES, WHIRLWINDS, OR WATER-SPOUTS 337 BOOK FIFTH. WATER CLO UDSRAIN. I. THE WATER UPON THE SURFACE OF THE EARTH AND IN THE AT- MOSPHERE : THE EARTH VOLUME AND WEIGHT OF THE WA- TER THROUGHOUT THE GLOBE PERPETUAL CIRCULATION VAPOR OF WATER IN THE ATMOSPHERE ITS VARIATIONS AC- CORDING TO THE HEIGHT, THE LOCALITY, AND THE WEATHER THE HYGROMETER DEW WHITE FROST 355 II. THE CLOUDS : WHAT A CLOUD is THE MANNER OF ITS FORMA- TION MIST OBSERVATIONS TAKEN FROM A BALLOON AND FROM MOUNTAINS DIFFERENT KINDS OF CLOUDS THEIR SHAPES THEIR HEIGHTS 363 III. RAIN: GENERAL CONDITIONS OF THE FORMATION OF RAIN ITS DISTRIBUTION OVER THE GLOBE RAIN IN EUROPE 381 IV. HAIL: PRODUCTION OF HAIL COURSE OF HAILSTORMS VARY- ING DISTRIBUTION OF HAILSTORMS IN DIFFERENT PARTS OF THE COUNTRY HEAVIEST HAILSTORMS KNOWN NATURE, SIZE, AND SHAPE OF HAILSTONES PERIODS OF THEIR OCCUR- RENCE 390 V. PRODIGIES: SHOWERS OF BLOOD OF EARTH OF SULPHUR OF PLANTS OF FROGS OF FISH OF VARIOUS KINDS OF AN- IMALS.. . 401 12 CONTENTS. BOOK SIXTH. ELECTRICITY, THUNDER-STORMS, AND LIGHTNING. CHAP. PAGE I. ELECTRICITY UPON THE EAKTH AND IN THE ATMOSPHERE : ELEC- TRIC CONDITION OF THE TERRESTRIAL GLOBE DISCOVERY OF ATMOSPHERIC ELECTRICITY EXPERIMENTS OF OTTO DE GUE- RICKE, WALL, NOLLET, FRANKLIN, ROMAS, RICHMANN, SAUS- SURE, ETC. ELECTRICITY OF THE SOIL, OF THE CLOUDS, OF THE AIR FORMATION OF THUNDER-STORMS. , 423 IL LIGHTNING AND THUNDER 431 III. THE SAINT ELMO FIRES AND THE JACK-O'-LANTERNS 441 IV. AURORA BOREALES .445 ILLUSTRATIONS. CHR OMO-LITHO GRAPHS. FIG. PAGE 1. Halo Frontispiece. 2. Sunset at Sea Toface 119 3. The Rainbow " 121 4. Lunar Rainbow seen at Compiegne " 126 5. Sunrise from the Righi " 127 149 218 6. African Mirage 7. Summer Landscape 8. Winter Landscape 9. The Storm 10. Aurora Borealis seen at Paris, May 13, 1869. 423 445 WOOD-CUTS. 1. Mathematical Limit of the Shape of the Atmosphere 29 2. Measure of the Height of the Atmosphere, according to the Length of Twilight 32 3. Thickness of the Earth's Crust, of our Atmosphere, and of a higher Atmosphere 34 4. Suction-Pump 39 5. Suction and Forcing Pump 40 6. Torricelli inventing the Barometer 41 7. Barometer Tube full of Quicksilver 43 8. The Tube in the Basin 43 9. Otto de Guericke's Experiment 45 10. The Magdeburg Hemispheres .' .' 46 11. Atmospheric Pressure. Rupture of Equilibrium 47 12. Atmospheric Pressure under an inverted Glass 47 13. Diagram showing the Decrease of atmospheric Pressure, according to Height 51 14. Variation in the atmospheric Pressure at the Level of the Sea 52 15. Lavoisier analyzing atmospheric Air 56 16. Matrass or Glass Vessel 58 17. The Apparatus for Analysis of Air 58 18. Mercury-Eudiometer, for analyzing Air 59 19. Apparatus for analyzing Air by the Method of Weight 60 20. Apparatus for obtaining the Proportion of carbonic Acid in Air 61 21. Apparatus for separating the Oxygen from the Nitrogen 62 22. Vibrations of a Blade 75 23. Vibration of a Cord 76 24. Illustration of Hawksbee's Experiment 78 25. Baroscope 86 26. Soap-bubbles inflated with Hydrogen 88 27. Distribution of Kinds of Birds according to Height of Flight 97 28. Lunar Day Ill 29. Atmospheric Refraction 114 30. Simple Reflection of Rays in a Drop of Rain 121 14 LIST OF ILLUSTRATIONS. PAGE FIO- -, 9 q 31. Formation of the Rainbow 32. Double Keflection of Rays in a Drop of Rain 1^ 33. Theory of the two Arches of a Rainbow 12 * 34. Triple Rainbow 12 35. The Spectre of the Brocken 12 36. The Ulloa Circle 132 37. Theory of the Halo u 38. Halo seen in Norway ^ 39. Corona formed around the Moon by Diffraction 147 40. Explanation of the ordinary Mirage 152 41. Mirage seen at Paris in 1869 158 42. Lateral Mirage seen on the Lake of Geneva 160 43. La Fata Morgana I 62 44. Shooting-stars 165 45. Fall of a Bolide in the Daytime 170 46. The Caille Aerolite, weighing 12^ cwt 172 47. The Pyrheliometer 183 48. Relative Intensity of the calorific, luminous, and chemical Rays of the Sun 192 49. Inequality of the Thickness of Air traversed by the Sun 196 50. Regular Diurnal Oscillation of the Barometer 213 51. Regular Monthly Oscillation of the Barometer 216 52. Snow Crystals 231 53. Winter. The Seine full of floating Ice 235 54. Comparative Temperatures of Rome, London, Paris, Vienna, St. Petersburg 251 55. The last human Dwelling-places. Esquimaux of the Polar Regions 262 56. Ice at the Pole 264 57. Section of the Atmosphere, showing its general Circulation 272 58. Average annual Prevalence of different Winds at London 305 59. Average annual Prevalence of the different Winds at Brussels 305 60. Monthly Intensity of the Winds 307 61. Diurnal Intensity of the Winds 307 62. The Simoom 323 63. Whirlwind 346 64. Sand Whirlwind 348 65. Water-spout at Sea 350 66. Intense Fog in one of the Islands of the Antipodes 368 67. Intense Fog in the Spitzbergen Mountains 369 68. Formation of a Thunder-cloud 377 69. Above and below the Rain-cloud 380 70. Diminution in the Rain-fall from the Tropics to the Poles 383 71. Increase of Rain, according to the Undulations of the Soil 384 72. Comparative Depths of Rain-fall 385 73. Section of Hailstones, showing their ordinary interior Structure 398 74. Section of a Hailstone, enlarged 399 75. Different Forms of Hail 400 76. Rain of Blood in Provence, July, 1608 404 77. Shower of Locusts 417 78. Shower of Cock-chafers 418 79. Experiments of Franklin and Romas 424 80. Richmann, of St. Petersburg, struck by Lightning during an electrical Experiment. . 426 81. Harvesters killed by Lightning 438 82. Curious Freak of Lightning * 440 83. Saint Elmo Fire over the Spire of Notre-Dame, Paris 442 84. An Aurora Borealis over the Polar Sea 447 85. Aurora Borealis observed at Bossekop (Spitzbergen), January 6, 1839 449 86. Aurora Borealis observed at Bossekop (Spitzbergen), January 21, 1839 451 BOOK FIRST. OUR PLANET AND ITS VITAL FLUID. THE ATMOSPHERE. CHAPTER I. THE TERRESTRIAL GLOBE. BORNE forward in space, in obedience to the mysterious laws of uni- versal gravity, our globe travels therein with a rapidity that our closest study can scarcely conceive. Let us imagine a sphere absolutely free, isolated on all sides, without any prop or stay, placed in the midst of space. If this sphere were alone in the immensity, it would remain thus suspended, motionless, without power to incline to this side or to that. Eternally fixed, it would constitute in itself the whole of crea- tion ; astronomy and physics, mechanics and biology, would all be in- cluded in its conception. But the earth is not the only world existing in space. Millions of celestial bodies have been formed, like itself, in the infinite heavens, and their co-existence establishes between them relations inherent in the very constitution of matter. The earth, in particular, belongs to a system of planets analogous to itself, having the same origin and the same destiny, situated at various distances around the same centre, and governed by the same motive power. Our planet- ary system is composed essentially of eight worlds, made to revolve in successive orbits, the exterior one of which is seven thousand million leagues in extent. The sun, a colossal star nearly a million and a half times larger than the earth, and 350,000 times as heavy, occupies the centre of these orbits ; or, to speak more accurately, a focus of one of the nearly circular ellipses which they describe. It is around this gi- gantic star that take place the revolutions of the planets, which are per- formed with an indescribable speed on account of the length of the cir- cumference to be traversed. Far from being motionless, as it appears to us, the globe which we inhabit revolves at an average distance of ninety-one and a half millions of miles from the sun, and over an orbit THE ATMOSPHERE. which does not measure less than 587 millions of miles. These are traversed in 365 days and six hours-that is to say, that we move through space with a speed of more than one and a half million o miles per day, or more than 66,000 miles an hour. The most rapid of express trains can scarcely accomplish more than twenty-five leagues an hour. Upon the invisible roads of the heavens the earth moves with a speed eleven hundred times greater. The difference is so enor- mous, that it is impossible to express it in this work by a geometrical figure. If the distance traversed in an hour by a locomotive was rep- resented by one tenth of an inch, it would be necessary to trace a line more than nine feet long to indicate the comparative advance made by our planet during the same space of time. I will add, as a point of comparison, that the movement of the tortoise is about eleven hundred times less rapid than that of an express train. Consequently, were an express train to be sent in pursuit of the earth, it would be as a tortoise in pursuit of an express train. Situated as we are about the globe, infinitely small mollusks, made to adhere to its surface by its oentral attraction, and carried away with it, we are unable to appreciate this movement or form a direct idea con- cerning it. It is only by the observation of the corresponding change of position in the celestial perspectives, and by calculations based thereon, that we have been able and this only during the last few centuries to acquire a knowledge of its nature, its form, and its importance. From the deck of a ship, from a railway-carriage, or the car of a balloon, we are alike unable to form an idea of the movement that is transferring us from one place -to another, because we participate in it ; and with- out some object of comparison not partaking of the motion, it is impos- sible for us to appreciate it. To form an idea of the rapidity of the earth's motion, we must imagine ourselves placed not upon the earth's surface but outside it, in space itself, not far from the course along which it hurries so impetuously. Then we should see far in the distance to our left, I will suppose a little star shining amidst the rest in the gloom of space. Then this little star would seem to grow larger, and to draw nearer to us. Soon there would be perceptible a disk like that of the moon, upon which we should also recognize spots formed by the op- tical difference between continents and seas, by the polar snows and the cloudy bands of the tropics. We should endeavor to distinguish upon this gradually swelling globe the principal geographical shapes visi- ble athwart the vapors and clouds of the atmosphere, when suddenly, THE TERRESTRIAL GLOBE. 19 standing out against the sky and covering the immensity of its dome, the globe would meet our affrighted gaze, as if it were a giant emer- ging from the abysses of space. Then, rapidly, without giving us time to recognize it, the colossus would rush away to our right, quickly diminishing in size, and silently burying itself in the dark depths be- yond. So moves the globe we inhabit, and we are borne along by it like so many grains of. dust adhering to the whirling surface of a can- non-ball projected into space. How great a difference there is between this truth and the ancient fallacy which represented the earth as the support of the firmament! During the reign of illusion so old, and yet so difficult to dispel, even in our epoch, from certain minds the earth was believed to form in it- self alone the living universe, and to represent the whole of nature. It was the centre and objective of all creation, while the rest of space was but a vast and silent solitude. There was a higher region in the uni verse viz., the heavens, the empyreum ; a lower region viz., the earth, hell. Mysticism had created the world for terrestrial humanity alone, as being the centre of Divine Will. In the present day we know that the heavens are but boundless space, and that the earth is in the heav- ens just as the other stars ; we contemplate in the firmament worlds sim- ilar to our own, and the starry night addresses itself to our minds with a new eloquence. The terrestrial globe, with its humanity, is no longer more than an atom cast into the infinite one of the countless fly-wheels which, in tens of thousands, constitute the mysterious mechanism of the physical world. Our planetary system, despite its vastness, compared to the microscopical volume of this earth, is, sun and all, eclipsed in the presence of the extent and number of the stars, which are solar centres of systems distinct from ours. The astonished gaze encounters distant suns whose light takes hundreds and thousands of years to reach us, not- withstanding its wondrous speed of 186,000 miles a second ; farther still the eye may contemplate pale masses of stars which, seen nearer, would resemble our Milky Way, and would be found to be composed of mill- ions of suns and systems ; beyond these, again, the eye and the mind still seek to discover more distant creations, but the sweep of our fa- tigued conceptions soon falls to a lower level, worn out and lost by this interminable flight into the regions of infinity. An invisible star, lost in the myriads of stars, the earth is borne along in the heavens by various movements far more numerous and peculiar than most people would be inclined to suppose. The most important THE ATMOSPHERE. is that of revolution, which we have noticed above, and by virtue * the earth moves round the sun at the rate of one and a half miUion of miles a day. A second movement, that of. te*m, causes it " u n round its own axis in the course of every four-and-twenty hours t may be at once seen, in examining this movement of the globe that the different points of the terrestrial surface have a difierent speed, ac- cording to their distance from the axis of rotation. At the equator where the speed is greatest, the terrestrial surface has to traverse 2o,000 miles in twenty-four hours; that is, more than 1040 miles an hour, or about seventeen a minute. In the latitude of London, where the circle is perceptibly smaller, the speed is eleven miles a minute. At Bekia- witz one of the towns almost in the heart of the polar region, the speed is seven and a half miles a minute; and finally, at the poles themselves, it is nil. A third movement, that which constitutes the precession of the equinoxes, causes the terrestrial axis to accomplish a slow rotation, which occupies not less than 25,868 years, and in virtue of which all the stars of heaven annually seem to change their position, to return to the same point only at the close of this great secular cycle. A fourth movement gradually makes a change in the position of the perihelion, which makes the circuit of the orbit in 20,984 years, so that in this other cycle the seasons successively take the place the one of the other. A fifth move- ment causes the plane of the earth's orbit, which it describes around the sun, to oscillate, and diminishes the obliquity of the ecliptic at present, to increase it in the future. A sixth movement, due to the action of the moon, and called nutation, causes the pole of the equator to describe upon the celestial sphere a small ellipse in eighteen years and eight months. A seventh movement, caused by the attraction of the planets, and principally by the gigantic world of Jupiter and our neighbor Ve- nus, occasions perturbations, calculable beforehand, in the curve de- scribed by our planet around the sun, swelling or flattening it, according to the variations of distance. An eighth movement, more considerable and less exactly measured than the preceding ones, though its existence is incontestable, is the transport of the whole planetary system in space. The sun is thus not motionless, but traverses an immense orbital line, the direction of which is at present toward the constellation of Her- cules. The speed of this general movement is estimated at 487,000 miles a day. The laws of motion would incline one to believe that the sun gravitates around a centre as yet unknown to us. If so, how vast must be the extent of the circumference of the ellipse which it describes, THE TERRESTRIAL GLOBE. 21 since for the last century it has followed, as far as we can judge, a per- fectly straight line ! These different movements, which cause the earth to travel in space, are ascertained with certainty, thanks to the vast number of the ob- servations of the stars made for more than 4000 years, and to the defi- nite nature of the modern principles of celestial mechanics. The knowl- edge of these constitutes the essential basis of the highest and most substantial of sciences. The earth is henceforth inscribed in the ranks of the stars, in spite of the evidence of the senses, in spite of secular illusions and errors, and, above all, in spite of human conceit, which had for a long time complacently formed a creation for man alone. Drawn here and there by these diverse movements some of which, such as that of the perturbations, are extremely complicated the terres- trial globe travels onward, whirling along, balancing itself under the in- fluence of varied forces, rushing with an incomprehensible rapidity to- ward an unknown goal. Since the beginning of the world, the earth has not twice passed the same spot, and the place which we occupy at this very moment is rapidly sinking behind into our track never to re- turn. The very terrestrial surface, too, undergoes changes every centu- ry, every year, every day, and the conditions of life change throughout eternity as throughout space. After having thus examined the move- ment of the earth in space, we must join to it, in order to complete its astronomical aspect, the motion of the moon round the earth in twenty- nine days and a half. The moon is only -$ of the size, and -g^- of the weight of the earth. Its action upon the ocean and the atmosphere is, nevertheless, comparable with that of the sun, and is even more impor- tant as regards the production of tides : it is as useful to know its move- ment about us as to know that of our planet about its primary. The revolution of the moon around the earth takes place really in twenty- seven days and eight hours, but during these twenty-seven days the earth has not been motionless, but, on the contrary, has advanced a cer- tain distance. The moon employs about two days more to complete its revolution and to return to the same point in relation to the sun, which gives twenty-nine days and thirteen hours for the lunation or the cycle of phases. The revolution in twenty-seven days is called the sidereal revolution, because in that time the moon returns upon the celestial sphere to the same position in relation to the stars. We see that to re- turn to the same position in relation to the sun, and to accomplish its synodical revolution, our satellite must make more than a circle upon 22 THE ATMOSPHERE. the celestial sphere, and pass over in addition the distance which the earth has traveled during that time. If we suppose the earth motion- less, the movement of the moon round it may be nearly represented by a circle. In reality, it is a sinuous line, resulting from the combination of the two movements. Three stars thus command our attention in the general history of na- turethe sun, the earth, and the moon. They are held up, isolated, in space in a manner dependent on tfceir respective weights. The sun weighs two quadrillions of tons (two followed by twenty-four zeros). The sun is 355,000 times heavier than the earth,'the latter eighty times more so than the moon. The sun holds the earth at arms-length, so to speak, ninety-one and a half millions of miles distant ; the earth holds the moon also by the influence of its mass at a distance of 237,000 miles. In gravitating around our luminary, the earth, constantly immersed in its rays, brings the different portions of its surface successively into its fertilizing emanations. Morning succeeds evening, and spring au- tumn. Night, like winter, is but the transition from one light to anoth- er. The solar heat keeps in continual work the mighty factory of the terrestrial atmosphere, forming the currents, the winds, the tempests, and the breezes ; preserving the water liquid and the air gaseous, rais- ing water from the inexhaustible wells of the ocean, producing the mists, the clouds, the rains, and the storms ; organizing, in a word, the permanent system of the vital circulation of the globe. It is this system of circulation, with the varied phenomena of the atmospheric world, which we are about to study in this work. The subject is vast and grand, for upon it depends all terrestrial life. In studying it we learn, therefore, the very organism of existence upon the planet we inhabit. THE ATMOSPHERIC ENVELOPE. 23 CHAPTER II. THE ATMOSPHERIC ENVELOPE. OUR globe, the motions of which we have been explaining, is encir- cled by a gaseous film which adheres to its entire spherical surface. This layer of fluid extends with uniform thickness all round the globe, covering it on every side. We have already compared the earth in the midst of space to a cannon-ball launched into the air ; by imagining this cannon-ball surrounded by a thin ring of smoke not more than jfa of an inch thick, we may form some idea of the position of the atmos- phere around the terrestrial globe. It is, indeed, from this position that the atmosphere derives its name ('ATJUOC, vapor ; and > S^aT/oa, sphere), being, as it were, a second sphere of vapor concentric with the solid sphere of the globe itself. As a rule, sufficient importance is not at- tached to the functions of this atmospheric envelope. It is from it that we draw our beipg. Plants, animals, and men imbibe therefrom the first elements of their existence. The earth's organization is so ordered that the atmosphere is sovereign of all things, and that 'the savant can say of it as the theologian said of God: "In it we live and move, and have our being." The air is the first bond of society. Were the atmosphere to vanish into space, an eternal silence would be the lot of the terrestrial surface. We may not think of the fact with our forgetfulness of nature, but none the less the air is the great medium of sound, the liquid channel in which our words travel, the vehicle of language, of ideas, and of social communication. It is also the first element of our bodily tissues. Breathing affords three-quarters of our nourishment; the other quarter we obtain in the aliment, solid and fluid, in which oxygen, hydrogen, nitrogen, and car- bonic acid are the chief component parts. Further, the particles which are at the present moment incorporated in our organism will make their escape either in perspiration or in the process of breathing; and, after having sojourned for a certain time in the atmosphere, will be re- incorporated in some other organism, either of plant, animal, or man. With the unceasing metamorphoses in beings and in things, there is 24 THE ATMOSPHERE. at the same time going on a continuous exchange between the products of nature and the moving flood of the atmosphere, by virtue of which the gases of the air take up their abode in the animal, the plant, or the stone, while the primitive elements, momentarily incorporated in an organism, or in the terrestrial strata, effect their release and help to re- compose the aerial fluid. Each atom of air, therefore, passes from life to life, as it escapes from death after death ; being in turn wind, flood, earth, animal, or flower, it is successively employed in the composition of a thousand different beings. The inexhaustible source whence ev- ery thing that lives draws breath, the air is, besides, an immense reser- voir into which every thing that dies pours its last breath ; under its action, vegetables and animals and various organisms are brought into existence, and then perish. Life and death are alike in the air which we breathe, and perpetually succeed the one to the other by the ex- change of gaseous particles ; thus the atom of oxygen which escapes from the ancient oak may make its way into the lungs of the infant in the cradle, and the last sigh of the dying man may go to nourish the brilliant petal of a flower. The breeze which caresses the blades of grass goes on its way until it becomes a tempest that uproots the forest- trees and strews the shore with shipwrecks; and so, by an infinite con- centration of partial death, the atmosphere provides an unfailing sup- ply of aliment for the universal life spread over the surface of the earth. It is this unceasing activity of the aerial envelope of gas which forms, nourishes, and sustains the vegetable carpet that extends over the sur- face of the dry land. From the meanest blade of grass to the colossal Baobab, this rich and diversified covering draws all its sustenance from the air. And while it keeps up the vital circulation of the earth by incessant exchanges of which it is the vehicle, the atmosphere is also the aerial laboratory of that splendid world of colors which brightens the surface of our planet. It is owing to the reflection of the blue rays that the sky and the distant heights near the horizon assume their lovely azure tint, which varies according to the altitude of the spot and the abun- dance of the exhalations ; and to it also we owe the contrast of the clouds. It is in consequence of the refraction of the luminous rays, as they pass obliquely across the aerial strata, that the sun announces its approach every morning by the soft and pure melody of the glowing dawn, and makes its appearance before the astronomical hour at which THE ATMOSPHERIC ENVELOPE. 25 it should rise ; it is owing to a similar phenomenon that, toward even- ing, it apparently slackens the speed of its descent beneath the horizon, and, when it has disappeared, leaves floating upon the western heights the fantastic fragments of its blazoned bed. Without the gaseous en- velope of our planet, we should never have that varied play of light, those changing harmonies of color, those gradual transformations of delicate shades which lighten up the world, from the gleaming bright- ness of the summer sun down to the shadows which cover, as with a veil, the forest depths. The study of the atmosphere embraces also the general conditions of terrestrial existence. The notion of life is so bound up in all our con- ceptions with that of the forces which we see ever at work in nature, that the myths of the early inhabitants of the world always attributed to these forces the generation of plants and animals, and imagined the epoch anterior to life as that of primitive chaos and struggle of the elements. " If we do not consider," says Humboldt, " the study of phys- ical phenomena so much as bearing on our material wants as in their general influence upon the intellectual progress of humanity, it will be found that the highest and most important result of our investigation will be the knowledge of the intercommunication of the forces of na- ture, and the certainty of their mutual dependence upon each other. It is the perception of these relations which enlarges the views and en- nobles our enjoyment of them. This enlargement of the view is the result of observation, of meditation, and of the spirit of the age in which all the directions of thought concentrate themselves. History teaches him who can travel back through the strata of preceding centuries to the farthest roots of knowledge how, for thousands of years, the human race has labored to grasp, through ever-recurring changes, the fixity of the laws of nature, and to gradually conquer a large portion of the physical world by the force of intelligence." The most important result of a rational examination of nature is, that it leads one to comprehend unity and harmony in this immense assem- bly of things and forces, to embrace with equal ardor what is due to the discoveries of past ages and to those of our own time, and to analyze the details of phenomena without succumbing beneath their weight. It is thus that it has been given to man to show himself worthy of his high destiny, by penetrating into the meaning of nature, unveiling its secrets, and mastering by thought the materials collected by obser- vation. 2g . THE ATMOSPHERE. We may now contemplate our planet traveling in space, and keeping about it the aerial envelope which adheres to its surface. Our imagina- tion can easily comprehend the general shape of this gaseous sphere which encircles the solid globe, and which is comparatively thin and of slight bulk. The exterior surface of the atmosphere is therefore curved like that of the sea, for, like water, the external layer of air tends to a level, all points of which are at equal distances from the centre. To the eyes of novices, it seems difficult to reconcile the idea of the spherical surface of the ocean with what is commonly termed a level; the idea that the air has a horizontal level like water, and that, like an aerial ocean, this level is always tending to an equilibrium, seems at first sight somewhat obscure. Nevertheless, not only does the air possess to an unlimited degree all the properties of elasticity and mobility of a fluid seeking equilibrium, but, different in this respect from water and other liquids, it is extremely capable of compression and, consequently, susceptible of extreme expansion. These are facts which must always be kept in mind, for they will assist in the understanding of a great number of at- mospheric conditions explained in future chapters of this work. What, then, is the thickness of this gaseous stratum which envelops our globe? This is the point which we shall examine in the next chapter. To ascertain the height to which the atmosphere extends, it would be necessary to calculate the density of the air at different elevations in the average state, leaving out of consideration accidental disturbances. This can be done when we know the temperature of the air, its pressure, and the tension of the vapor of water which it contains. It would, further, be necessary, in order to obtain an exact determination, to take account, first, of the gradual diminution in weight as the distance from the cen- tre of the earth is increased ; secondly, of the variation in the centrifu- gal force according to the latitude. These variations are, however, slight, and scarcely affect the calculation, in consequence of the coat of air being of such insignificant thickness as compared to the radius of the globe. The height of the atmosphere has its limits, which, as we shall see, are somewhat confined. If the air had no elasticity, its limit would be at a distance where the centrifugal force was in equilibrium with the weight; but as this condition does not exist, its elasticity must necessarily be coun- terbalanced by a force of some kind, and this force is the weight of the strata of air which are above the particular one we are considering. But THE ATMOSPHERIC ENVELOPE. 27 the higher we ascend the more rarefied does the air become, and when the last strata are reached there is nothing to keep them down. Nev- ertheless, the atmosphere being limited, as we shall presently see, these strata can not be lost in space ; and it is probable that in consequence of their rarefaction and the great decline in their temperature, their phys- ical condition is so modified that the elastic force becomes nil. Laplace has pointed out this indispensable condition ; Poisson has specified it, by showing that the equilibrium would still be possible with a very consid- erable limiting density, provided that the fluid was not capable of ex- pansion ; and Biot, who has summed up these conditions, clearly indi- cates the state of these external inexpansible strata in his remark that they must be like " a liquid which does not evaporate." We will now examine the mechanical and physical conditions of this aerial envelope, estimate its exterior shape, and measure its height. 23 THE ATMOSPHERE. CHAPTER III. THE HEIGHT OF THE ATMOSPHERE. As the earth travels in space with enormous swiftness, carrying along with it, adhering to its surface, the gaseous body that encircles it, it naturally follows that this latter does not extend indefinitely into space, but ceases to exist at a certain distance from the surface. How far can it extend ? Carried along by the rotation of the globe in its daily movement, we may conclude that at a certain -height above the ground the movement of the atmosphere is so rapid that the centrifugal force which it acquires would hurl into space the outside particles of air, which would then cease to adhere to the surface and, for the same rea- son, to form part of the atmosphere. Certain inventors of methods of aerial navigation have vaguely im- agined that the atmosphere does not entirely turn round with the earth, so that, by rising to a certain height, we could see the globe moving around beneath our feet, and should only have to wait until the me- ridian, where we wished to alight, passed under the balloon, to find ourselves transported there by the rotation of the globe. Such an idea is, of course, absurd, as the atmosphere and all that it contains partake equally with the earth in the rotation of the latter. The centrifugal force increases as the square of the velocity, and at the equator its amount is -^ part of that of gravity, so that a body at the equator weighs less than the same body at either of the poles by -j-g-g- of its weight. If, therefore,- the earth rotated on its axis seventeen times as fast as it does, since seventeen times seventeen is equal to two hundred and eighty-nine, a body at the equator would not have any weight. A stone, for instance, detached from the ground by the action of the hand, would not fall down again ; we should become so feath- er-like, that, in dancing upon the surface, we should resemble aerial nymphs displaced by the wind. As the circumferences of circles vary as their radii, at seventeen times the distance from the surface to the centre of the earth that is to say, at a height of about sixteen times the radius of the earth, or about 63,000 miles if the other quantities involved remained unchanged, the atmosphere would cease to rotate THE HEIGHT OF THE ATMOSPHERE. 29 with the earth; but, in point of fact, the weight does not remain un- changed, but diminishes as the distance from the centre of attraction is increased. By combining this diminution with the increase of centrifugal force, we find that at a distance of about 6"61 times the radius of the earth from its centre, which corresponds to a height above its surface of about 21,000 miles, the centrifugal force is equal to the weight, and conse- quently the aerial particles which might happen to be in these regions must of necessity escape. This is the distance at which a satellite would gravitate in exactly twenty-three hours fifty-six minutes, the time occupied by our planet in its rotation. It is, theoretically, the max- imum limit of the atmosphere, which, however, as a matter of fact, is far from extending to so great a height, as we shall see; but, mathe- matically, it might do so, and it is only at this enormous distance that the centrifugal force would be sufficiently great to prevent the atmos- phere from existing as such. Such is the extreme and maximum limit of the atmosphere; but it is at a far lower elevation that the air we breathe really ceases. Thus, at the height of 10,000 feet the height of Mount ^Etna there is beneath the mountaineer nearly a third of the aerial mass ; at 18,000 feet, which is less than that of the peaks of many mountains, the column of air which presses upon the soil has already lost half its weight, and conse- , quently at this point the whole gaseous mass, which reaches far up into the sky, does not weigh more than the strata which are compressed into the region below. In consequence of the forces that act upon it, the shape of the atmos- phere is not absolutely spherical, but swollen out at the equator, where it is much higher than at the poles. The maximum limit of this figure, in the case where the flattening is greatest, has been E given by Laplace. The diameter of the atmosphere at the equator is a third greater than at the poles.* It is the mathematical limit, beyond which the terrestrial Fig. 1 Mathematical limit of the shape of the at- mosphere. * [This is inaccurate. Laplace proved that the ratio of the least (the polar) diameter to the greatest (the equatorial) diameter could not be less than (not f, as in the text). Fig. 1 is THE ATMOSPHERE. 30 atmosphere can not pass. But it has not this exaggerated shape, though in reality it is perceptibly denser at the equator than at the poles. It may be remarked that it is probable that a detached train of the lighter gases remains constantly in the rear of the globe during its rapid- revolution around the sun. It need scarcely be added that the shape of the atmosphere undergoes further change, owing to the atmospheric tides, which are due to the varying attraction of the sun and the moon. The decreasing weight of the atmospheric strata affords us the first means of calculating a minimum limit of the height of the atmosphere. Mechanics have given us the maximum limit, and it is in this instance to physics that we shall have recourse. Consider a vertical column of air, then the pressure at any point must be equal to the weight of air above ; or, in other words, any por- tion of the column measured from the ground supports all the rest of the column above; the lower strata of the atmosphere are, therefore, more pressed down (and consequently denser), because they have a greater weight resting on them. The barometer, which measures this pressure of the air, is higher at the foot than at the summit of a mount- ain ; and the relation which exists between the pressure and the height is so close, that the difference in level between two points may be de- -duced from the difference in the heights of the barometrical columns simultaneously placed at these two stations. The smaller the pressure the more dilated is the air ; so that, at first sight, it would seem as if the atmosphere must extend to an immense distance. A celebrated natural philosopher, Mariotte, first determined the law of the compression of gases ; and the result of his researches shows that the quantity of air contained in the same volume or, in other words, the density of the air is proportionate to the pressure to which it is subjected. Until within the last few years this law was considered en- tirely accurate; but recently it has appeared most difficult to conceive why the terrestrial atmosphere does not extend very far into space ; while other considerations indicate that it is necessarily limited, and therefore incorrectly drawn, as the protuberance should be considerably greater. It may be mentioned that one consequence deduced by Laplace from his result is, that the Zodiacal light can not be produced by reflection on the atmosphere of the sun, as the former always appears in the form of a thin lens, the ratio of the polar to the equatorial diameter being'much less than . Laplace's investigation is given in vol. ii., pp. 194-197 of the Mecanigue Celeste (National Edition). ED.] THE HEIGHT OF THE ATMOSPHERE. 31 ceases at a short distance above the ground. This apparent contra- diction was the result of a too extensive generalization of Mariotte's law, which is simply relative instead of rigorously definite; and Eeg- nault has studied the differences which exist between the theoretical law and the facts of the case. Subsequently to these investigations, M. Liais has ascertained, by in- troducing very small portions of air into a large barometrical instru- ment made for the purpose, that the differences between the results of observation and the theory usually adopted are still greater. By di- minishing sufficiently the quantity of air, it has been possible to find a limit at which the particles, far from separating from each other, as would happen were the gases capable of indefinite dilatation, seem, on the contrary, to have a mutual tendency to adhesion similar to that of the molecules in a viscous liquid. The elasticity of the air, producing expansion, ceases, therefore, at a certain degree of dilatation, from which point this gas assumes the character of a liquid, but a liquid out of all comparison lighter than those with which we are acquainted. By means of this decrease in the density of the air in proportion to its height, Biot has, by an examination of the physical conditions of equilibrium and a complete discussion of the observations obtained at different degrees of altitude by Gay-Lussac, Humboldt, and Boussin- gault, demonstrated that the minimum height of the atmosphere is 160,000 feet, or about thirty miles. At that height the air must be as rarefied as beneath the exhausted receiver of an air-pump; that is to say, as rarefied as the air in the nearest approach to a vacuum that we can make. Thus the minimum height of the atmosphere is thirty miles, and the maximum 21,000. Hence we have two defined limits, but with a great distance between them. There are, however, other methods by which we can get nearer to the truth. Efforts have been made to measure the height of the atmosphere optically, by studying the length of the twi- light and the length of time during which the solar rays continue to reach the aerial regions when the luminary himself has sunk below the horizon. If the atmosphere were unlimited, the phenomenon of night would be entirely unknown to us; the light of the sun, reaching the strata of air which are sufficiently distant from the earth, would be continuously sent on to us by reflection from these strata. On the other hand, the absence of any aerial envelope would cause the night to begin exactly 32 THE ATMOSPHERE. at sunset and the light of day to burst upon us immediately the sun rose As it is, every one knows that the twilight of evening and the morning dawn prolong the time during which we enjoy the solar light. It will be readily imagined that the observation of these phenomena at once suggested the idea of seeking to resolve, by their agency, the height to which the atmosphere extended. Suppose the earth to be represented by the circle, radius o A, and that its atmosphere is limited by the circumference F G H I c. It is evident that, when the sun has sunk beneath the horizon F A c B of the place A, it will only give light to a portion of the atmosphere. Thus, when the sun arrives at j, if we imagine a tangent cone to the earth, hav- ing the sun for its sum- mit, all those parts of the atmosphere situated below J G will be de- prived of light, and the part c I H G will alone Fi" 2 -Measure of the height of the atmosphere, according to be illuminated. Later on, when the sun reaches j', the portion bounded by c I H will alone be subject to its light; later still, only from c to I ; and finally, when the sun gets to j'", upon the tangent line from c, the intersection of the plane of the horizon FACE and the limiting sphere of the atmosphere, the twilight ceases. From the moment, therefore, that the sun sets, we ought to see a sort of arc appear on the opposite side of the horizon, rising gradually until it reaches the zenith, and then slowly descend until it finally disap- pears. Such is the theory that the earliest astronomers conceived as to the phenomenon of twilight. In the optics of Alhasen (in the tenth century) we find that the angle of the sun's declivity for the close of the twilight or the break of dawn was taken as 18, and this estimate is still adopted by modern astronomers as the average amount. In our climate it is difficult to distinguish with accuracy the limit of separation between that part of the atmosphere which is lighted by the sun and that which does not receive its rays directly. But Lacaille, in his voyage to the Cape of Good Hope, recognized all the phases which THE HEIGHT OF THE ATMOSPHERE. 33 have been enumerated theoretically. He says: "Upon the 16th and 17th of April, 1751, while at sea and in calm weather, the sky being ex- tremely clear and serene, at the point where I could distinguish Venus at the horizon as a star of the second magnitude, I saw the twilight ter- minated in the arc of a circle as regularly as possible. Having regula- ted my watch by the exact hour, according to sunset, I saw this arc lost in the horizon, and I calculated, by the hour at which I made this ob- servation, that the sun had descended below the horizon, on the 16th of April, 16 38', and on the 17th, 17 13V Other observations have since been made, as we shall see further on. It is easy to understand that, once having ascertained the apparent daily circle described by the sun upon a certain date, and the position of the observer upon the earth, we can calculate, by the time that has elapsed between the hour of sunset and the moment of the crepuscular arc's disappearance, the angle traversed by the sun below the hori- zon. It will also be understood that, according to the time and place, there will be found a difference both in regard to twilight and dawn, since the variations in the relative position of the sun and the state of the air must necessarily influence the direction and quantity of the light which, after countless reflections and refractions, reaches the ob- server. We will study, in the second book, the optical effects of twilight; at present we are only concerned with -the relation existing between its duration and the height of the atmosphere. Now, the time during which the sun, after sinking below the horizon of a particular spot, continues to give light directly to part of the at- mosphere visible from this place, depends upon the thickness of the aerial strata which envelop the earth. Let us suppose, for instance, that we pass a plane (Fig. 2) through the place A, the centre, o, of the earth and the centre of the sun ; this plane will cut the earth in the cir- cle o A. Let F A B be the intersection of the horizon of the spot A with this same plane ; from c draw the tangent C D to the earth ; all that part of the atmosphere visible at A will cease to be illuminated by the sun when, in its apparent diurnal movement, it has sunk below CD j'". Now we have seen that, from the duration of twilight, it was concluded that it came to an end when the angle BC j'" of descent below the ho- rizon was 18. As the angle o A c is a right angle, and as o A is the radius of the earth, we know one side and the angles of the triangle o AC, and consequently are enabled to calculate the other parts, oc 3 34 THE ATMOSPHERE. may therefore be regarded as known, and thence it results that we have the height, E c, of the atmosphere, for E c = o c o E. Such is the method devised by Kepler for deducing the height of the atmosphere from the phenomena of twilight. The results which it has furnished agree with the preceding, and give our atmosphere a height of from thirty to thirty-seven miles.* The average radius of the earth being 3908 miles, it will be seen that this height is but a little more than the 130th part of this radius; that is to say, that if the earth were represented by a sphere about twenty-two feet in diameter, the atmosphere would be like a coat of vapor adhering to the surface, with a thickness of about one inch. Figure 3 represents exactly this relation. It shows firstly, the incandescent interior of the globe, which is a; secondly, the solid crust, b, on which we live (it is but twelve leagues, or thirty miles, thick, as, in consequence of the increased tem- perature of one degree (Fahrenheit) for fifty feet, minerals fuse at this depth) ;f thirdly, the thickness of the aerial layer which we breathe, and which is represented by c; and, fourthly, the probable height of a very light atmosphere, d, over and above ours, of which we are about to treat. It may be further mentioned, in reference to the ig. S Section showing J the relative thickness of measurement of the height of the atmosphere by the earth's crust, of our . . , atmosphere, and of a the duration of twilight, that certain observers have higher atmosphere. obtained, as the result of similar researches, an ele- vation much greater than that given above, affording a clear proof that ihe twelve leagues actually represents the minimum only. M. Liais has made a direct calculation of this height by observing the duration of * [It is to be noted that different methods give different heights for the atmosphere, but ihere is no discrepancy, as different things are meant. Thus, if experiments on twilight give forty miles as the height, this implies that the air above this elevation reflects no appreciable ;:mount of light ; while, if we define the height to be to the point where the friction will not et light to a meteor, we have about seventy miles; but, of course, there is no reason why there : hould not be some air at much greater heights. ED.] t [This is the observed rate of decrease at the surface of the earth, but it is not true that :he thickness of the crust must be as stated in the text. It follows, from several considera- lions of other kinds, that the thickness of the crust is in all probability not less than 600 miles. ED.] THE HEIGHT OF THE ATMOSPHERE. 85 twilight and of the crepuscular curve, which colors the sky with that lovely rose tint which is so remarkable, especially in southern countries. These observations have been made both on the Atlantic, during a voyage from France to Eio Janeiro, and in the bay upon the shores of which the last-named city stands. They give, as a minimum, 180 miles, and, as a probable height, 204 miles. By observing, from the summit of the Faulhorn, the course of the crepuscular arcs, Bravais obtained a height of seventy-one and a half miles. The height, however, varies according to the temperature of the seasons, and remains always greatest at the equator. Another method, different from the preceding, consists in measuring the thickness of the penumbra which surrounds the earth's shadow on the moon during lu- nar eclipses, as well as the phenomena of refraction produced. This measurement gives from fifty to sixty miles as the thickness of the terrestrial atmosphere, the influence of which is felt under this special aspect. The observations which accord the atmosphere a height far greater than the theoretical thirty-eight miles have been for many years the object of special discussion. Quetelet, director of the Brussels Observa- tory, has, after much research on this head, arrived at the conclusion that it does indeed extend much higher than had been supposed, but that the upper strata are not quite of the same nature as those nearer the earth. This addition is supposed to be due to an ethereal atmosphere, very rarefied and differing from the lower atmosphere in which we live. It is the region where are mostly seen the shooting stars, which afterward disappear when they reach the terrestrial atmosphere. The upper atmosphere* is still, the lower in continual motion. The special movements caused by the action of the winds and tempests are limited in their height by the effect of the seasons. Thus, as regards our climate, the agitated portion, in the vicinity of the earth, would not be more than from seven to ten miles high during the winter, while its height must be almost double in summer. All that part of the atmos- phere which is above the latter would only experience a very slight and scarcely sensible movement, arising from the movable basis upon which it reposes. The continual disturbances going on in the lower regions cause the air in the inferior atmosphere to be very much alike in its chemical * [The existence of such an atmosphere seems to me very uncertain. ED.] gg THE ATMOSPHERE. components. No difference has been discovered at the various eleva- tions which it is possible to attain for the purpose of collecting air and submitting it to analysis. In the upper atmosphere the phenomena, of which we are scarcely able to form an idea by judging them from the surface of our globe, take place. There, also, appear the shooting stars; descending from a still greater height, the aurora borealis, and those mighty luminous phenomena which we often witness without having the power to sub- mit them directly to the test of experiment. All these facts do not es- cape us altogether, especially as regards the aurora borealis and the magnetic phenomena. If we can not determine the cause, we can at least feel the effect with sufficient force to be in a position to appre- ciate them. Sir John Herschel, De la Eive, and Hansteen seem to share upon this point the opinion of Quetelet. We can quite admit that, above our at- mosphere ot oxygen, nitrogen, and vapor of water, there exists an at- mosphere excessively light, which may extend two hundred miles in height, and which is naturally composed of the very lightest gases. The terrestrial globe being about 8000 miles in diameter, this total thickness represents the fortieth of the globe's diameter. The simulta- neous existence of these two atmospheres is, therefore, the general con- clusion at which we will, momentarily at least, stop. As to the basis of the atmosphere, we may now inquire if it ceases at the surface of the ground, and does not descend into the interior of the globe itself. Pressing upon all bodies upon the surface of the earth, it tends to penetrate in all directions between the molecules of liquids as into the interstices of the rocks. It is to be found in water as in all vegetables and all organic structures ; the earth and the porous stones are impreg- nated with it, and that in proportion to the force with which it presses. It will be seen, therefore, that the air is not limited to the part which is, so to speak, a gaseous envelope, and that a sensible fraction of its constituent elements penetrates the waters of the ocean and the inter- stices of the ground. Certain savants have imagined that the air of which the atmosphere is composed is but the continuation of an inte- rior atmosphere ; but the rise in the temperature, due to the central heat, would prevent the condensation of gases, and must limit the pres- ence of air in the under strata. A rough estimate of the quantity of air which is thus introduced into THE HEIGHT OF THE ATMOSPHERE. 37 the waters of the ocean may be formed by measuring the absorption of gases by various liquids. Under ordinary pressure, sea-water absorbs from two to three per cent, of its volume, only the proportion of oxygen is much greater than in the ordinary air. The result of the calculation is, that the quantity of air absorbed by the ocean is not above a three- hundredth part of the atmosphere. We thus have a tolerably complete determination both as to the height and shape of this terrestrial atmosphere. OQ THE ATMOSPHERE. oo CHAPTER IV. WEIGHT OF THfc TERRESTRIAL ATMOSPHERE-THE BAROMETER AND ATMOSPHERIC PRESSURE. WHILE treating of the height of the atmosphere, we have already seen that the air is denser in the lower regions of the aerial ocean-that is to say, near the surface of the earth than in the higher regions. The air, light and unsubstantial as it may appear to us to be, has consequently a positive weight. Each square foot of the earth's surface sustains a con- siderable pressure, the amount of which we shall presently attempt to estimate, corresponding to the height and density of the column of air above it. Our ancestors were not able to measure the atmospheric pressure; but we must not conclude from this that they were ignorant of the effects which it exercised, especially when the wind was violent. Yet this force, which every one felt without being able to measure, was not ren- dered determinate until the middle of the seventeenth century. In 1640, the Grand Duke of Tuscany having ordered the construc- tion of fountains upon the terrace of the palace, if was found impossible to make the water rise more than thirty-two feet. The duke wrote to Galileo in reference to this strange refusal of the water to obey the pumps. Torricelli, the pupil and friend of Galileo, gave the true ex- planation of the fact, and proved, as we shall see, that this column of water of thirty-two feet was in equilibrium with the weight of the at- mosphere. The celebrated invention of Torricelli has sometimes been erroneous- ly attributed to Pascal. The French philosopher himself alludes to the mistake, and shows how much of the merit is due to him in the follow- ing terms : " The report of my experiments having been spread abroad in Paris, they have been confounded with those made in Italy ; and, thanks to this misunderstanding, some, according me an honor to which I can lay no claim, attributed the Italian experiment to me, while oth- ers unjustly deprived me of the credit of those to which I was really en- titled. To give to others and to myself the justice due to us, I published, in 1647, the experiments which I had made the year before in Norman- WEIGHT OF THE ATMOSPHERE. 39 dy ; and that they might not be confounded with one made in Italy, 1 gave the latter separately and in italics, whereas mine were printed in Eoman letters. Not content with giving it these distinctive marks, I have stated in so many words that I am not the inventor of the barom- eter ; that it was made in Italy four years previously, and was the cause of my making similar experiments." It was, then, the refusal of the water to rise more than thirty-two feet, in obedience to the pumps, which revealed to Torricelli the fact that the atmosphere had weight, and that its whole weight was balanced by a column of water thirty-two feet in height. Let us then examine for a moment the mechanism and action of the pump. Every one knows that these simple and old-fashioned contrivances serve to raise water either by suction or pressure, or by both combined. Hence their classification as suction-pumps, forcing-pumps, and suction and forcing pumps. Before Galileo's day, the ascension of water in the suction-pump was ascribed to the fact of nature abhorring a vacuum ; but it is, in reality, merely an effect of atmospheric pressure. Take a tube, at the lower extremity of which is a piston, and place this lower end in water. If the piston is drawn up, a vacuum is created below, and the atmospheric pressure, acting upon the surface of the liquid external to the pump, makes it rise in the tube and follow the movement of the piston. Herein lies the principle of the suction- pump, which is essentially composed of the body of the pump, in which a piston moves, communicating by a tube with a reservoir of water (see Fig. 4). At the point where the body of the pump and the suction-tube join is placed a valve, opening upward, and in the body of the piston there is an opening formed by a similar valve. For water to reach the body of the pump, the suction-valve must be less than thirty-two or thirty-three feet above the level of the wa- ter in the well, otherwise the water would cease to rise at a certain point in the tube, Fig. 4.-suction-pnmp. and the motion of the piston would be unable to raise it any farther, 40 TUB ATMOSPHERE. In addition, to insure raising at each ascent of the piston a volume of water equal to the volume of the body of the pump, the spout must be placed at a less height than thirty-two feet above the reservoir. Thus the suction-pump will not raise water to a height of more than thirty-two feet ; but the water having once passed above the pis- ton, the height to which it can then be raised depends solely upon the force which drives the piston. The suction and force pump (see Fig. 5) raises water both by suction and pressure. At the base of the body of the pump, over the orifice of the suction-pipe, is, as before, a valve opening upward. Another valve, also open- ing upward, closes the aperture of the bent tube, which runs into a receptacle called the air-vessel.* Then from this reservoir there starts a pipe which serves to raise the water !. to the required height. Finally, the farce- pump only acts mechanically, and does not atmospheric pressura ft differs on] y from the other in that it has no suction-pipe, its body going right into the water which is to be drawn up. In reference to this elevation of the water only to a certain height, Torricelli, throwing aside, like his master, all idea of a hidden cause, showed- that the pressure of the air compels the water to mount up into the pipe from which the air is withdrawn, until the weight of water raised into the pipe is equivalent to that of the ^ir which presses upon an equal section of the reservoir from which the water is being raised. By the aid of this principle he was led to invent the barometer. To exer- cise equal pressures, the liquid columns must be of heights inversely proportional to their density. Thus, a liquid twice as heavy as water would, with a column of sixteen feet, be in equilibrium with the atmos- phere; and quicksilver, which is nearly thirteen and a half times as heavy as water, would be in equilibrium if the height of the column were diminished in this proportion that is, to about twenty-nine inches. * [The air-vessel is not essential to the principle of the pnmp ; if it were not used the supply of water would be intermittent, as in the common suction-pump, but the effect of the elasticity of the air in the air-vessel is to render the stream of water continuous.* ED.] Fig. ^.-Suction and forcing pump. Pig. 6. Torricelli inventing the Barometer. WEIGHT OF THE ATMOSPHERE. 43 This conclusion is easily verified. Take a glass tube, three feet in length, and open only at one end ; fill it with quicksilver, and then, placing the finger on the open end (see Fig. 7), put the lower portion of the tube into a basin filled with the same liquid, with the end closed by the fin- rig. T. The tube full of quicksilver. Fig. 8. The tube in the basin. ger downward. Immediately the finger is removed, the quicksilver inside will descend several inches and then stop (see Fig. 8 ). The equilibrium is established, and the liquid column which remains sus- pended in the pipe is a true balance, for the weight of the column of mercury is exactly in equilibrium with the atmospheric pressure. Torricelli gave to this tube of quicksilver, thus placed vertically in a basin of quicksilver, the name of Barometer; that is to say, a contriv- ance to indicate the weight of the air, from the Greek flapog, weight, and fjLirpov, measure. Its invention by Torricelli dates from 1643. Three years later, Pascal repeated the experiment in France with a wa- ter-barometer, and even a wine-barometer. This was at Rouen. His tube was forty-nine feet long, and to avoid the difficulty, insurmounta- 44 THE ATMOSPHERE. ble in that day, of exhausting the air in it directly, he had it sealed at one end, filled it with wine, and closed the other end with a cork. Then, by means of cords and pulleys, the tube was placed upright and the lower end put into a vessel full of water. As soon as the cork that kept it closed was removed, the whole liquid column in the tube fell, until its surface was about thirty-three feet above the level of the water in the vessel. The remaining sixteen feet above were destitute of air. Consequently, the liquid column itself formed an equilibrium to the at- mospheric pressure, and from this he drew the conclusion that a column of water (or of wine of the same density) thirty-two feet high weighs as much as a column of air on the same base. The surface of the earth is pressed upon as if it was covered with a body of water thirty-two or thirty-three feet deep, and we who live upon the bed of this ocean of air undergo the same pressure. If it is the pressure of the air which causes the elevation of the quick- silver or the water, as we ascend into the atmosphere, the weight of the column of quicksilver raised, and consequently the height of this col- umn, must gradually diminish in a manner dependent on the strata of air left beneath it. The experiment was made on the Puy-de-D6me, ac- cording to the instructions of Pascal, by his brother-in-law, Florin Pe- rier, upon the 19th of September, 1648, and repeated by Pascal himself on the Tour St. Jacques at Paris. . The results were decisive, and the barometer became an easy and accurate means of measuring the total weight of the atmosphere, and the variations in the pressure which it exerts at different times and places upon the surface of the globe. We thus see that it was between 1643 and 1648 that the atmospheric press- ure was demonstrated by the construction of the barometer and the ex- periments which its discoverers at once entered upon. By a coincidence not at all unusual in the history of science, while the indications of the barometer were being studied in Italy and France, experiments were being made in Holland to ascertain the pre- cise weight of the air, but by quite a different process. In 1650, Otto de Guericke, burgomaster of Magdeburg, invented the air-pump, by which the air may be exhausted from any receptacle and a nearly absolute vacuum created. The ingenious inventor conceived in the same year the idea of weigh- ing a globe of glass, first leaving in it the air which it contained, and then weighing it again when the air had been removed by the air- pump. The globe, when emptied of air, was found to be less heavy WEIGHT OF THE ATMOSPHERE. 45 by about one-third of a grain for every cubic inch of the globe's ca- pacity. Aristotle had long before suspected that air had weight, and to make sure of the fact, he weighed a leather bottle, first empty and afterward when inflated with air; for, he remarked, if the air has weight, the leather bottle will be heavier when weighed the second time than it was the first time. The experiment not confirming his supposition, he concluded that the air had no weight. Nevertheless, several of the ancient philosophers admitted the material nature of air as a fact. Thus the Epicureans compared the effects of the wind with those of water in motion, and considered the elements of the air as invisible bodies. During the reign of the peripatetic philosophy, however, it was assumed that air was without weight, and there were but few philosophers who did not share this erroneous opinion. We have seen that, by repeating judiciously the experiment of Aris- totle, Otto de Guericke demonstrated the real weight of air. If Aris- totle's experiment led to a contrary result, it must be attributed to the change in the volume of the leather bottle during his two trials, for every body, when weighed in a fluid, loses in weight a quantity equal to the weight of the fluid displaced. The leather bottle made use of by Aris- totle would have shown an increase of weight if weighed in a vacuum. Let us suppose that about 1835 cubic inch- es, of air were introduced into it by in- spiration; its weight would have in- creased by about 550 grains, but at the same time the bottle would become in- flated, and its volume, being increased by 1835 cubic inches, would have dis- placed a volume of air of equal weight, so that its loss in weight would be also 550 grains, and the weight of the air and bottle together would consequent- ly remain the same as before. But in the experiment of Otto de Guericke the globe was always of the same size, Whether empty Or full of air, and its Fig. 9._Otto de Guericke' 8 experiment loss in weight through the displacement of the air being in each case ^g THE ATMOSPHERE. the same, there was, of course, a difference, which proved that air had weight. Otto de Guericke, at the same time, conceived the idea of the Magdeburg Hemispheres, so called from the town in which he invented them, and which consist of two hollow hemispheres of copper, with a diameter of from four to five inches. The hemispheres fit each other hermetically. One of them has attached to it a cock that screws on to the plate of an air-pump, and the other a ring which acts as a handle to move it backward or forward. As long as the two hem- ispheres, when in contact, contain air within them they can easily be separated, for there is equilibrium between the expansive force of the interior air and the outside pressure of the atmosphere, but when once a vacuum is formed by the exhaustion of the air, it requires a considerable effort to draw them apart. In one of these experiments, the learned burgomaster had each hemisphere pulled by four strong horses without succeeding in sep- arating them. The diameter was more than two feet, which gives a total of more than three and a quarter tons as the atmospheric pressure brought to bear in the way of resistance. The pressure of the atmosphere on a square inch is equivalent to the weight of a column of quicksilver with a volume of 29'92 cubic inches, viz., about fifteen pounds. It is easy and interesting to draw from this the conclusion that, as the superficies of an average human body Fig. 10.-The Magdeburg Hemispheres. ig sixteen Q ^^ Q ^ W(J may each of us be said to be subject to a pressure of about fifteen tons. That we are not crushed by this enormous pressure, is because it does not all press vertically down on us. As the air surrounds us on all sides, its pressure is transmitted over our body in all directions, and, in consequence, becomes neutralized. Air penetrates readily and with full pressure into the profoundest cavities of our organism ; hence we have the same pressure inside and outside, and thus these weights be- come exactly balanced. This is easily proved by the experiment of bursting a bladder under the receiver of an air-pump. Take a cylin- drical glass vessel, hermetically closed at the upper end by a piece of gold-beater's skin, with the other end placed (see Fig. 11) on the plate of an air-pump ; as soon as the air begins to be exhausted from the si, the gold-beater's skin becomes depressed under the influence of WEIGHT OF THE ATMOSPHERE. 47 the atmospheric pressure upon it from above, and soon bursts. The opposite result occurs if the pressure from out- side is lessened. If a bird is placed in the vac- uum of an air-pump, its body will be seen to swell, its blood to spurt out with violence, and in a short time it perishes, a victim to a kind of explosion the inverse of that just described. This fact is confirmed, as we shall see farther on, by the ascents that have been made to -great elevations. Upon reaching the regions where the air is much rarefied, the limbs swell, and the blood has a tendency to force its way through the skin, in consequence of the want of equilibrium Fig. ii.-Atmospheric press- between its own tension and that of the external ure; rupture of equilibrium. ^ Any one can show the effect of atmospheric pressure by a very simple experiment. This consists in filling a glass quite with water and laying over the top a sheet of paper. It can then be turned over without spilling any of the liquid, a fact which must be attributed ^ to the pressure which the atmosphere exercises upon the sheet of paper. It was stated above that, where a vacuum is created, the atmospheric pressure is about fifteen pounds to the square inch. It is this pressure which causes the limpet to adhere to the rock, when this mollusk has by contraction created a s lass - vacuum under its shell. The fly, excluding the air from between its feet and the ceiling, is enabled, apparently, to violate the laws of grav- ity. Cupping-glasses, when applied to the body, act on this same principle, and we can not take a step without observing some fact which is founded on the effects of atmospheric pressure. Such are the general facts and experiments which demonstrated that the air had weight, and gave birth to the instrument wherewith this weight was to be determined, viz., the barometer. It now remains to apply these ideas to the whole atmosphere, the extent of which we endeav- ored to explain in the preceding chapter. * [I have neither experienced any of these symptoms myself, nor have I observed them in others. ED.] ^g THE ATMOSPHERE. At the level of the sea the pressure, upon the average, sustains the barometrical column at a height of about 29'92 inches. Experiments frequently repeated by physical philosophers and the accuracy of which has been verified have proved that the weight of the air at 32 (Fahr.) of temperature, and under a pressure of 29'92 inches of mercury, is to the weight of an equal volume of quicksilver in the proportion of unity to 10,509 that is to say, that 10,509 cubic inches of air have the same weight as one cubic inch of mercury. If the density of the strata of air were everywhere the same, it would be easy to deduce from the above result not only the height of a given spot by the aid of the barometer reading there, but also the total height of the atmosphere. It is, indeed, evident that if a fall of an inch in the height of the barometer corresponded to a change of height of 10,509 inches, a fall of 29-92 inches, which is the total height of the barome- ter, would correspond to 29'92 times 10,509 inches that is, about five miles. Such would be the height of the atmosphere if its density re- mained the same from top to bottom, but we have seen that its lower strata are denser than the higher. It follows, therefore, that, to pro- cure a fall of an inch in the mercury of the barometer, it is necessary to traverse a greater distance above the level of the ground or the sea. Halley was the first to deduce a formula by which heights might be obtained by means of the barometer. We have seen in the previous chapters that, since the experiments of Mariotte, it has been recognized that air becomes compressed in pro- portion to the weight above, or to the pressure exerted upon it. Thence it is inferred that, in rising vertically in the atmosphere to suc- cessive elevations, increasing in arithmetical progression, the density of the corresponding strata of air would diminish in geometrical pro- gression. This would be accurate if the temperature were everywhere the same, and the difference in height would scarcely be any more com- plicated than if the density were constant. But the temperature of the air diminishes with increased height, so that the variation in density is not so simple, as the upper strata are more condensed by their lower temperatures than those below. The relation between temperature and height is rather complicated, as we shall see farther on ; and this, of course, renders more difficult the process of measuring heights by the barometer. At the same time, the atmospheric strata always contain a certain quantity of aqueous vapor, the weight of which must be added to that of the air. WEIGHT OF THE ATMOSPHERE. 49 Furthermore, the weight of any body, and consequently that of a stratum of air, is proportionately less as the body in question is farther removed from the centre of the earth. And as the weight of bodies varies also according to the latitude on account of centrifugal force, it becomes evident that, for a single formula to be in general use for ob- servations made at different points of the globe, it is indispensable that it should include the latitude of the place of observation. Laplace has given, in the "M6canique Celeste," the corrections ren- dered necessary by these different causes in measuring height, and has deduced from theory alone a formula the accuracy of which has been confirmed by numerous experiments. To determine the height of a mountain it is necessary that two per- sons take simultaneous observations of the readings of the barometer, one at its foot, the other at its summit. They must be careful, at the same time, to read the thermometers attached to the barometers, as well as others to determine the temperature of the surrounding air. Two observations will be sufficient ; but it is better to have several. A single observer can also ascertain the difference in level between two stations, not very distant the one from the other, with very fair ac- curacy, if he takes care to observe the thermometer and barometer at the lower stations, both when he leaves it and returns to it, and infers, from the difference, the reading at the lower when taking that at the higher station. When, by a long series of observations, the average readings of the barometer and thermometer at a given place have been determined, they may be employed to calculate the absolute elevation of the place above the level of the sea by taking corresponding observations at the level of the ocean. Sufficient barometrical observations have al- ready been made at various elevations for us to be in a position to rep- resent this decrease of atmospheric pressure, with increase of elevation, no longer theoretically, but from direct observation. From a series of observations, made at very different elevations, the table on the following page has been formed. This satisfactory series of barometrical observations, which we are able to establish by means of numerous ascents, either in the balloon or up the mountain path, and by researches of several observers in inhabited regions far above the level of the sea, enables us also to en- deavor to represent, by a curve and a tint, this rapid decrease in the weight of the atmosphere. In Fig. 13, the horizontal line which forms 4 THE ATMOSPHERE. . . Height above the Sea. Mean Reading. Feet. 159 213 472 650 804 1,339 2,067 3,937 4,856 6,837 8,130 8,773 9,541 10,893 13,124 15,748 20,014 22,113 22,966 22,966 26,247 29,000 37,000 Inches. 29-92 29-74 29-68 29-57 29-37 29-21 28-58 27-91 25-98 25-24 23-62 22-17 21-85 21-02 20-08 18-70 16-69 14-17 13-39 12-79 12-60 10-79 9-75 7-00 MeTn btrometrilTreading at Greenwich Observatory Dijon (Perrey) Geneva Observatory (Plantamour) TJndpz (Blondeau} Summit of Vesuvius (Palmieri) Guatemala (R. P. Canudas) Guanaxuato (Humboldt) The Monastery of the Great St. Bernard The Summit of the Faulhorn (Bi avals; Town of Quito (Fouque') In several aeronautical ascents (llammaiion) Summit of Mont Blanc (Ch. Mai tins) \""\( The summit of Ibi-Gamin (the highest mountain that has been In the highest ascent (Glaisher) the base represents the mean state of the barometer at the level of the sea (29-92 inches). Each other horizontal line indicates the reading of the barometer 'corresponding to the elevation which is shown by the vertical line. In this way, or by the aid of the tinted portion, it will be noticed that at 8200 feet the pressure is diminished by one-quarter, at 18,000 feet by one-half, and at 31,168 feet by three-quarters. The reading of the barometer diminishes, therefore, rapidly as we rise above the level of the sea. But even there it is not the same all over the globe's surface. It is lower at the equator than between the tropics ; at the equator it is about 29'84 inches ; it then increases up to the 33d degree of latitude, where it is 30'16 inches ; then decreases un- til the 43d degree (30'00 inches), toward which point it becomes sta- tionary, and so remains up to the forty-eighth degree. Thence it con- tinues to decrease so far as sixty-four degrees, where it stands at 29'65 inches. Lastly, it again increases from that point as far as the remotest latitudes at Spitzbergen (seventy-fifth degree), where the height of the barometer is 29-84 inches. Between the pressures at the thirty-third degree and the sixty-fourth degree of latitude, tliere is, therefore, a dif- ference of half an inch. I have laid down these results on a diagram, and traced the following curve (see Fig. 14, p. 52): These variations in the atmospheric pressure are probably caused by WEIGHT OF THE ATMOSPHERE. 51 the trade-winds and upper currents of air, which slightly raise the whole mass of the atmosphere. It is easy to conceive that the latitude may exercise some influence upon the pressure of the air, inasmuch as the conditions of temperature, pressure, and rotary movement vary with it. It is less, easy to explain part of the weight of the earth. If all this mass of air were agglomerated into a single ball, it would weigh as much as a ball of copper with a diameter of sixty-two miles. Thus the weight of the air is far from being insignificant. Fig. 15. Lavoisier analyzing Atmospheric Air. CHEMICAL COMPONENTS OF THE AI2t. 57 CHAPTER Y. CHEMICAL COMPONENTS OF THE AIR. IT is to the great French chemist Lavoisier that science owes the dis- covery of the chemical components of the air.- Let us go back to the researches of this laborious observer, and hear from his own lips the recapitulation of his interesting studies. Our atmosphere, he remarks, must be made up of all the substances capable of remaining in an aeriform state at the ordinary degree of tem- perature and atmospheric pressure which we experience. These fluids form a mass, almost homogeneous,* from the surface of the earth to the highest elevation which man has ever reached, and the density of which decreases with elevation. But it is possible that above our atmosphere there are several strata of very different fluids. What is the number, and what is the nature, of the elastic fluids which compose this lower stratum that we inhabit? After having established the fact that chemistry offers two methods essential for the study of bodies that is to say, analysis and synthesis Lavoisier describes as follows the celebrated experiment of the first analysis of air : " Taking a vessel, or long-necked tube, with a bell or globe at its ex- tremity, containing about thirty-six cubic inches (see Fig. 16, p. 58), I bent it (see Fig. 17, p. 58) so as to place it in the furnace while the extreme end of the neck was under a glass cover, which was placed in a basin of mer- cury. Into this vessel I poured four ounces of very pure mercury ; and then, by means of a siphon, I raised tne mercury to about three-quarters the height of the glass cover, and marked the level by gumming on a strip of paper. I then lighted the fire in the furnace, and kept it up in- cessantly for twelve days, the mercury being just sufficiently heated to boil. At the expiration of the second day, small red particles formed upon the surface of the mercury, and increased in size and number for the next four or five days, when they became stationary. At the end * [Homogeneous must be understood to mean that the components of the atmosphere are found mixed in the same proportion at all heights. Its usual meaning is, of course, "of uni- form density." ED.] THE ATMOSPHERE. of the twelve days, seeing that the calcination of the mercury made no further progress, I let out the fire and set the vessels to cool. The vol- ume of air contained in the body and neck of the vessel before the op- eration was fifty cubic inches; and this was reduced by evaporation to forty-two or forty-three. On the other hand, I found, upon carefully collecting the red particles out of the melted mercury, that their weight was about forty-five grains. The air which remained after this opera- tion, and which had lost a sixth of its volume by the calcination of the mercury, was no longer fit for respiration or combustion, as animals placed in it died at once, and a candle was extinguished as if it had been plunged in water. Taking the forty-five grains of red particles, and placing them in a small glass vessel, to which was adapted an apparatus for receiving the liquids and aeriform bodies which might become sepa- Pig. 16. The glass vessel. Fig. 17. The apparatus. rated, and having lighted the fire in the furnace, I observed that the more the red matter became heated, the deeper became its color. When the vessel approached incandescence, the red matter commenced to be- come smaller, and in a few minutes had quite disappeared ; and at the same time forty-one and a half grains of mercury became condensed in the small receiver, and from seven to eight cubic inches of an elastic fluid, better adapted than the air of the atmosphere to supply the respi- ration of animals and combustion, passed under the glass cover. From the consideration of this experiment, we see that the mercury, while it is being calcined, absorbs the only portion of the air fit for respiration, or, to speak more correctly, the base of this portion ; and the rest of the air which remains is unable to support combustion or undergo respira- tion. Atmospheric air is, therefore, composed of two elastic fluids of different, and even opposite, natures." CHEMICAL COMPONENTS OF THE AIR. 59 The nature of air was thus clearly established by these experiments, which were made in 1777. Its real components were not, however, completely ascertained until the present century. The first exact analy- sis of air is scarcely fifty years old, and is due to Gay-Lussac and Hum- boldt, who analyzed it by the use of the eudiometer. Fig. 18. Mercury-Eudiometer, for analyzing air. When an equal mixture of air and pure hydrogen are set fire to in the eudiometer, all the oxygen disappears in the shape of water, which becomes condensed into dew, the volume of which is insensible, and there remains a mixture formed of nitrogen and the excess of hydro- gen employed. Now the hydrogen causes a volume of oxygen equal to half itself to disappear as water; whence it follows that the volume of oxygen contained in the measured air is equal to one-third of the volume that has disappeared. If the measures of the air, the hydro- gen, and the gases after explosion, are made at the same pressure and the same temperature, and if, in addition, the gases were saturated with humidity before explosion, the determination would require no correction. Such is the principle of the method. Gay-Lussac and Humboldt found that there was twenty-one per cent, of oxygen, and seventy-nine per cent, of nitrogen, in the air. This analysis has since been confirmed by nearly all chemists. There is another method by means of which the relative quantities of oxygen and nitrogen con- tained in the air of the atmosphere can be weighed a process which gives results far more accurate than the measuring of the volumes (al- 60 THE ATMOSPHERE. ways very small) of the gases employed in the other processes. The apparatus used is composed-first, of a tube which brings m the air from outside of the room where the operation is proceeding; secondly, of a set of Liebig balls, L, containing a concentrated solution of caustic potash; thirdly, of a tube,/ in the shape of the letter U several times repeated, and filled with fragments of caustic potash; fourthly, of a second set of balls, o, containing concentrated sulphuric acid; fifthly, of a second tube, Z, of the same shape as the one above mentioned, filled with pumice-stone steeped in concentrated sulphuric acid; sixth- ly, of a straight tube, T, of hard glass. This tube is filled with copper filings, and laid upon a long iron furnace, so that it can be heated Fig. 19. Apparatus for analyzing air by the method of weight throughout its whole length, and is moreover furnished at its extremi- ties with two taps, r and r', which admit of its being emptied ; seventh- ly, of a glass globe, B, holding from two to three gallons, and the neck of which is fitted with a tap, R. To perform the experiment, as complete a vacuum as possible is made in the tube T ; the two taps are closed tight, and the tube, thus emptied of air, is weighed. The glass ball B, having been emptied of air, is also weighed. The various portions are then put together in the order described, and the tube T is made red-hot. Then the taps r r' of the tube T, and the tap R of the glass ball, are successively opened. The air, entering by the suction-tube to the right, traverses first of all the balls L and the tube /, where it parts with its carbonic acid ; then it passes into the second set of balls, o, and into the tube Z, CHEMICAL COMPONENTS OF THE AIR. 61 where the sulphuric acid removes all the vapor of water it contains. Separated from these, the air makes its way into the tube T, containing the red-hot copper, which retains the oxygen, and then passes into the empty glass ball in a state of pure nitrogen. The increase of weight in the tube clearly gives the weight of the oxygen which has been de- posited in the operation. The difference between the weight of the globe when empty and when full of nitrogen as clearly represents the weight of this gas. By means of this analysis, made with every con- ceivable precaution, MM. Dumas and Boussingault ascertained that one hundred parts of air contain Oxygen, 23 in weight; 20 '8 in volume. Nitrogen, 77 " 79 '2 " The difference between the proportion of weight and that of volume is due to the fact that oxygen is rather heavier than nitrogen. These, therefore, are the two fundamental elements of the chemical constitution of air. But there exist other elements in far smaller quantities; such, for instance, as carbonic acid and aqueous vapor. Their quan- tity is determined by the ap- paratus described for finding the weight of the oxygen and nitrogen in the air. (See Fig. 20.) An iron vessel is filled with water, and emptied by \ uSI^t-- "k- means of a tap inserted in the lower part The water which runs out is gradually replaced J r Fig 20 Apparatus for obtaining the proportion of car- by external air, which has to bonic acid in air. pass through the six curved tubes before it reaches the reservoir. The first two of these are filled with pumice-stone steeped in sulphuric acid, and the air, on its way through them, leaves behind the water which was mixed with it. The two middle tubes are filled with a concentrated solution of potash, which absorbs the carbonic acid. Of the last two tubes, containing pumice-stone steeped in sulphuric acid, the first is intended to extract the humidity which the potash has im- parted to the air, and the other to prevent the humidity from making g2 THE ATMOSPHERE. its way back from the sucker into the tubes. By weighing, before and after the experiment, the series of analyzing tubes, we obtain the weight of the water and the weight of the carbonic acid contained in a volume of air equal to that of the reservoir. The atmosphere contains about -^^ of its volume of carbonic acid. There is also a very simple process by which the oxygen and the nitrogen can be separated. Into a graduated tube, containing a certain volume of air, with its open end placed in a vessel containing water or mercury, is inserted a long stick of phosphorus. (Fig. 21.) At the expiration of six or seven hours, as a rule, the oxygen is absorbed, and the stick of phosphorus may be withdrawn, and the gas which re- mains that is to say, the nitrogen measured. The absorption is con- sidered to be complete (the appara- Fig. 21._Apparatn 8 for separating the oxygen tUS being placed ill the dark) when there ceases to be any glimmer upon the surface "of the phosphorus. The rapid absorption of the oxygen by the phosphorus may be shown by .heating the gas in a bell-glass into which a fragment of phosphorus has been introduced ; the phos- phorus is heated by an alcohol-lamp, and a portion of it volatilized ; and when the flame has reached all the space occupied by the gas, the experiment is complete. Time is left for it to get cool ; .the volume of nitrogen is transferred into a graduated tube and measured, the dif- ference from the original weight giving the quantity of. oxygen. Oxygen and nitrogen are two permanent Bases'that is-'to say, it has been found impossible hitherto, either by compression or cold, to de- stroy their gaseous form. The first, oxygen, is the ordinary agent of combustion, whether of the kind which takes place in our fire-places or in our organisms. The second, nitrogen, exercises a moderating influence over the first Carbonic acid, which exists in quantities varying according to time and place, but always very small in amount, has been liquefied under a strong pressure conjoined to intense cold ; it has even been solidified. In that state it has the appearance of light and very compressible snow, CHEMICAL COMPONENTS OF THE AIR. 63 the contact of which with the skin produces a burning sensation, this excessive cold acting upon the epidermis in the same way as great heat.* In the small quantities in which it is found, carbonic acid pro- duces no ill effects ; in larger quantities it is hurtful to the breathing, and finally produces asphyxia. Emanations from the earth, the abundant sources of carbonic acid, are often met with in volcanic districts. When M. Boussingault ex- plored the craters at the equator, he was shown a locality where no animals could remain ; this was at Tunguravilla, not far from the vol- cano of Tunguragua. He thus describes his visit of 1851 : " Our horses soon gave us indications that we were approaching it ; they refused to obey the spur, and threw up their heads in a most disagreeable fashion. The ground was strewn with dead birds, among which was a magnifi- cent black-cock, that our guides at once picked up. Among the vic- tims were also several reptiles and a multitude of butterflies. The sport was good, and the game did not seem too high. An old In- dian, Quichua, who accompanied us, declared that, to procure a good sleep, there was nothing like making one's bed upon the Tungura- villa." This deleterious emanation made itself manifest by the sterility of the ground for a circle of some hundred yards ; it was especially great at a point where there were many large trees lying dried up and half buried in the vegetable earth, which implies that these trees had flour- ished upon the spot where they have been lying since the eruption of the carbonic acid. This gas, like that which is also met with in similar circumstances in various regions of the globe, is carbonic acid more or less mixed with air, according to its distance above the soil. Carbonic acid exercises a directly deleterious effect upon the nerves and brain. Hence the anaesthetic effects which it may produce, and which all visitors to.Pouzzoles, near Naples, may have seen at a grotto which has become famous from this cause. The keeper has a dog whose legs he ties together, to prevent his runhing away ; he then places him in the middle of the grotto. The animal displays evident fear, struggles to escape, and soon appears to be dying. His master then takes him out into the open air, where he gradually recovers himself. One'of these dogs has been used for this purpose more than three years. It is all but proved now that the con- * [The snow-like flakes can be handled with impunity ; it is only when forcibly pressed against the skin that a blister is produced. ED.] 64 THE ATMOSPHERE. vulsions of the pythonesses charged with expounding the decrees of the gods were produced by the priests with carbonic gas. This grotto is situated upon the slope of a very fertile hill, opposite, and not far from, Lake Agnano. The entrance is closed by a gate of which the keeper retains the key. It has the appearance and shape of a small cell the walls and vault of which have been rudely cut in the rock. It is about one yard wide, three deep, and one and a half high, and it is difficult to judge from its aspect whether it is the work of man or of nature. The ground in this cavern is very earthy, damp, black, and at times heated. It is, as it were, steeped in a whitish mist, in which can be distinguished small bubbles. This mist is composed of carbonic acid gas, which is colored by a small quantity of aqueous vapor. The stratum of gas is from ten to twenty-five inches high. It represents, therefore, an inclined plane the highest part of which cor- responds to the deepest portion of the grotto, and this is a physical consequence of the formation of the ground. The grotto being about on the same level as the opening leading into it, the gas finds its way out at the door, and flows like a rivulet along the hill-path. The stream may be traced for a long distance, and a candle dipped into it at a distance of more than six or seven feet from the grotto is extin- guished at once. A dog dies in the grotto in three minutes, a cat in four, a rabbit in seventy-five seconds. A man could not live more than ten minutes if he were to lie down upon this fatal ground. It is said that the Emperor Tiberius had two slaves chained up there, and that they perished at once ; and that Peter of Toledo, Viceroy of Na- ples, shut up in the grotto two men condemned to death, whose end was as rapid. Two analyses of the air in this grotto, which had been collected at different times (see Ch. Ste. 01. Devi lie and F. Le Blanc), gave in vol- ume Carbonic acid 67'1 73'6 Oxygen 6'5 5'3 Nitrogen 26'4 21-1 lOO'O 100-0 It is not necessary to travel so far for this predominance of carbonic acid. At Montrouge, near Paris, and in the neighborhood, there are large quarries, and even cellars, which are filled from time to time with this mephitic gas. Upon the borders of Lake Laacher, near the Rhine, and at Aigue- CHEMICAL COMPONENTS OF THE AIR. 65 perse, in Auvergne, there are two sources of carbonic acid so abundant that they give rise to accidents in the open country. The gas rises out of small hollows in the ground, where the vegetation is very rich ; the insects and small animals, attracted by the richness of the verdure, seek shelter there, and are at once asphyxiated. Their bodies attract the birds, which also perish. In former times the accidents caused by this gas in caves, mines, and even in wells, gave rise to the most extravagant stories. Such locali- ties were said to be haunted by demons, gnomes, or genii, the guardians of subterranean treasures, whose glance alone caused death, as no trace of lesion or bruise was to be found on the unfortunate persons so sud- denly struck down. In addition to the oxygen, nitrogen, and carbonic acid, the air con- tains a certain number of other substances, in smaller and very varying quantities. The most important is aqueous vapor, of which I have spoken above in describing the method of analysis for determining its presence. The air always contains a certain proportion of aqueous vapor in a state of solution, and invisible. When this water passes into the state termed vesicular, it constitutes clouds or mists. The quantity of aqueous vapor varies with the seasons, the temperature, the altitude, the geographical position, etc. At the same temperature and under the. same pressure the maximum quantity capable of being mixed with the air is invaria- ble. The hygrometrical state of the air, for a given temperature, is but the relation between the quantity of moisture really existing in the air and the quantity which would exist if the air were saturated at the same temperature. The millions of cubic feet of vapor of water which, mix- ing with the air, form the clouds and the rain constitute the most im- portant element of the atmosphere in respect to the circulation of life. Therefore water will be in a subsequent chapter the object of special study. The quantity of heat necessary for the evaporation of the water from the earth's surface has been ascertained. The volume annually evaporated may be represented by the volume of water which falls from the atmosphere in that space of time ; and, in comparing the results of observations taken at different latitudes and in both hemispheres, we are led to estimate this volume as corresponding to a depth of fifty-four and a quarter inches over the whole earth. The amount of heat neces- sary to evaporate such a volume of water would suffice, according to Daubree, to liquefy a thickness of ice of nearly thirty -three feet in depth 5 QQ THE ATMOSPHERE. enveloping the whole globe. From the calculations of Dalton, the at- mosphere contains about the 0'0142th part of its weight in water: the upper strata are nearly free from water. What other .substances are there to be found in the atmosphere? It unquestionably contains small quantities of ammonia, partially in a state of carbonate of ammonia; perhaps, too, partially in a state of ni- trate, or even nitrite, of ammonia. The origin of this substance must evidently be attributed principally to the decomposition of vegetable and animal matter ; and its presence in the air is of peculiar importance in regard to the phenomena of vegetation and the chemical statics of plants. Several chemists have attempted to determine its exact propor- tion, which does not seem to exceed a few millionths of the volume of the air. The quantity of ammonia found in different waters is (in weight) : Rain-water O'OOOOOOS Fresh-water 0'0000002 Spring-water O'OOOOOOl From one to two grains of ammonia per cubic foot have been found in sea- water. This is, no doubt, a very trifling quantity ; but when we reflect that the ocean covers more than three-quarters of the globe, and when we consider also its enormous mass, it may be fairly looked upon as a vast reservoir of ammoniacal salts, whence the atmosphere can make good the losses which it is continually undergoing. The streams, too, carry to the sea prodigious quantities of ammoniacal matter. I will give one instance. According to M. Desfontaines, the engineer, the Rhine at Lauterburg has, on the average, a flow of 39,000 cubic feet of water a second; and from a careful examination, of the amount of ammonia contained in the water, it results that the Rhine, in its passage by Lauterburg. carries down with it every twenty -four hours at least 22,500 Ibs. of ammonia that is, 13,000,000 Ibs. a year. The atmosphere, incessantly undergoing change (although its constitu- tion remains unaltered) by the immense labor of human beings who, like so many chemical pairs of bellows, are in continual motion on the bed of the aerial ocean, is the theatre of accidental chemical modifica- tions which play their part in the general organization. We see rising from the ground aqueous vapor, effluvia of carbonic acid gas, nearly always unmixed with nitrogen, sulphureted hydrogen gas, sulphurous vapors ; less frequently we notice vapors of sulphuric or hydrochloric acid; and, lastly, carbureted hydrogen gas, which has for thousands of CHEMICAL COMPONENTS OF THE AIR. 67 years been in use among different nations for the purposes of producing warmth and light. Of all these gaseous emanations the most numerous and abundant are those of carbonic acid. In former ages, the greater heat of the globe and the large number of crevices that the igneous rocks had not yet covered contributed considerably to these emissions. Large quantities of hot vapor and of this gas became mixed with the aerial fluid, and produced that exuberant vegetation of pit-coal and lignites which is nearly an inexhaustible source of physical strength for a nation. The enormous quantity of carbonic acid the combination of which with lime has produced the chalky rocks then rose out of the bosom of the earth under the predominant influence of volcanic forces. What the alkaline soils could not absorb spread itself into the air, whence the vegetable matter of the Old World drew continuous sustenance. Then, too, abun- dant emissions of sulphuric acid in vapor have led to the destruction of mollusks and fish, and to the formation of beds of gypsum. Humboldt adds, that the introduction of carbonate of ammonia into the air is prob- ably anterior to the appearance of organic life upon the globe's surface. Besides the ammoniacal vapors, the atmosphere also contains many traces of nitrogen, and even nitric acid. Several observers have also demonstrated, especially in large towns, the presence of a small quanti- ty of hydrogen in some form, probably carbureted. M. Boussingault was the first to prove, by precise experiments, the presence of a hydrogenous gas or vapor equal, at the most, to a 10 5 00 part of the air in volume. Analysis has also brought to light a certain quantity of iodine. The entire, or nearly entire, absence of iodine in the air or water of certain mountainous countries has, according to M. Chatin, a close connection with the existence of goitre among the inhabitants of these countries. His conclusions have been received, as a rule, with incredulity by chemists. Yet, when we consider that rain-water collected in a plu- viometer contains various kinds of salts, which arise from the washing of the dust suspended in the atmosphere, and that chemists have often found evidence of the presence of iodine in rain-water, there can be no difficulty in admitting that the presence in the air of iodine, free or in combination, may be, if not a normal, at least an occasional occurrence. We now arrive at the last element ascertained by special investigations to be existent in the atmosphere, viz., ozone. Van Marum, about the year 1780, by means of powerful electric machines, excited a large number of sparks in a tube full of oxygen, gg THE ATMOSPHERE. about six or seven inches long. After passing about five hundred sparks into the tube, he found that the gas had acquired a very strong smell, which, to use his own words, " seemed clearly the smell of elec- tric matter." Every one, indeed, is aware that if lightning strikes any object it leaves behind it what is commonly called a sulphurous smell. Van Marum also found that the gas acquired, after the experiment, the property of oxidizing mercury without heat. Nearly sixty years later, in 1839, M. Schoenbein, professor at Basle, informed the Academy of Sciences at Munich that, having decomposed some water, he had been struck by the smell of gas emitted. After a few researches he drew the conclusion that a new body was brought to light by his experi- ment, which he called ozone, from o%w (to emit an odor). A large number of contributions have been subsequently made to the subject by various savants. Ozone is interesting in a chemical point of view, both in its nature and its energetic affinities, for it oxidizes directly both silver and mer- cury, at least when these metals are moist. It also liberates iodine from potassic iodide, and forms, with the metal, an oxide which, doubt- less, contains far more oxygen than the potash. The hydracids impart to it their hydrogen. The salts of magnesium become decomposed by its contact with the formation of peroxide. Chlorine, bromine, and iodine, pass, when moist, under the influence of ozone, into chloric, bromic, and iodic acid. This agent has an exciting effect upon the lungs, provokes coughing and suffocation, and presents all the characteristics of a poisonous sub- stance. Notwithstanding all the researches that have been made in reference to ozone, the knowledge of it is, from a physical and chemical point of view, very imperfect ; a fact easy to understand when I state that it is impossible, even with the most perfect methods, to transform more than T-^ of a mass of oxygen into pure ozone. This maximum reached, action ceases. How can it be easy to study a body which is spread over at least 1800 times its own volume of another gas?* It has occurred to several experimentalists, such as Schoenbein, Berigny, Pouriau, Bceckel, Houzeau, and Scoutetten, to join to the ordinary meteorological observations ozonometrical observations also. * [By a continuous electrical discharge, maintained for many hours, Andrews and Tait were enabled to transform into ozone one-twelfth of the volume of oxygen operated on Phil. Trans., I860. ED.] CHEMICAL COMPONENTS OF THE AIR. 69 M. Schoenbein, in his experiments, boiled 'one part of potassic iodide, ten parts of starch, and two hundred of water, a preparation of "Jo- seph's paper " being afterward steeped in it. The latter is dried in a close room, and then cut up into small strips. This paper becomes blue by contact with the ozone, for the iodine is set at liberty and re- acts upon the starch. The deepness of the tint, however, depends upon the quantity of oxygen which has been turned into ozone. A small strip is exposed each day for twelve hours, sheltered both from the sun's rays and the rain, and its tint is then compared with a scale of ten colors, varying from white to indigo. In l^ol, MM. Marignac and De la Rive undertook several experi- mental researches as to ozone ; and their conclusion was, that this sub- stance must be simply oxygen in a particular condition of chemical ac- tivity, determined by electricity. Berzelius and Faraday gave their ad- hesion to this opinion of the Geneva savants; and MM. Fremy and Bec- querel demonstrated, by fresh experiments in 1852, its legitimacy. The works of Thomas Andrews, published in 1855, leave no doubt upon this head. Ozone, no matter from what source it is derived, is a unique and separate body, with identical properties and the same constitution ; it is not a composite body, but an allotropic condition of oxygen. This allotropic condition is due to the action of electricity upon the oxygen. This opinion, based upon the best experiments, has now been univer- sally accepted, and this constitution of ozone appears incontestable. Let us further add to all these divers substances the presence of oxy- genated water, as indicated by M. Struve, director of the Pulkowa Ob- servatory. While engaged in a chemical analysis of the water in the River Kusa, M. Struve was struck with the presence of a certain quan- tity of nitrite of ammonia, which was only to be found after a fall of snow or of rain. Soon after the downfall had ceased, all trace of this substance had again disappeared; M. Struve therefore supposed that the nitrite of ammonia existed in the air, and that it had been brought away by the snow or the rain. He entered upon researches on the sub- ject, and in the course of them made the interesting discovery of the presence of oxygenated water in the atmosphere. From these research- es may be drawn the following conclusions : 1st. Oxygenated water is formed in the atmosphere like ozone and nitrite of ammonia, and be- comes separated from the air through the atmospheric deposits. 2d. Ozone, oxygenated water, and nitrite of ammonia, are always intimately connected. 3d. The alterations which the atmospheric air brings about -Q THE ATMOSPHERE, in the starch-iodine papers are caused by the ozone and oxygenated Wa One word more. In absorbing into our lungs the quantity of air due to us we often unwittingly inhale whole hosts of microscopical animals which are in suspension in the atmospheric fluid, and even portions of antediluvian animals, mummies, and skeletons of past ages! Paris is nearly entirely built with chalky microscopical skeletons and tortoise-shells. The shells of the /oramm/era, for instance, m a fossil state by themselves form entire chains of lofty hills and immense beds of building-stone. The rough chalk in the neighborhood of Paris is in some places so full of these remains that a cubic inch in the Gentilly quarries contains at least 100,000 of them. When we pass close by a house that is being pulled down, or one in course of construction, and find ourselves enveloped in a cloud of dust that penetrates down our throats, we often, beyond a doubt, inhale hundreds of these tiny atoms. Each day and each hour we inhale and take into our chest legions of animal and vegetable life. There are the living microzoa, several spe- cies of which are the fish of our 'blood ; there are the vibriones, which attach themselves to our teeth like oyster-banks to rocks. Then, again, there is the dust of microscopical . animalcules, so small that it takes 75,000,000 to make a grain ; and, besides these, there are the grains of pollen which, germinating in our lungs, further the spread of parasite life, which is out of all comparison more developed than the normal life visible to our eyes. The winds and storms, by their violent agitation of the atmosphere ; the ascending currents due to the inequalities of temperature ; the vol- canoes, by their incessant emission of gas, vapors, and ashes, so finely divided that they often fall at a prodigious distance, carry up and main- tain in the higher regions corpuscles drawn away from the surface of the ground, or forced out of the internal and, perhaps, still incandescent portion of the globe. In the phenomena connected with the organism of plants and animals, these substances, so slight and of such different origins, the vehicle of communication for which is the air, very proba- bly exercise a far more pronounced action than is generally believed. Their permanence is, too, placed beyond doubt by the mere evidence of the senses, when a ray of sun penetrates a darkened room. As M. Boussingault remarks, "The imagination may conceive very readily, though not without a certain disgust, what is contained in these morsels of dust which we are incessantly inhaling, and which have been aptly CHEMICAL COMPONENTS OF THE AIR. 71 denominated the refuse of the atmosphere. They establish, in a certain sense, a contact between individuals far removed from each other ; and though their proportion, their nature, and, consequently, their effects, are so varied, it is not too much to attribute to them a part of the insa- lubrity which generally manifests itself in all great agglomerations of human beings." Eain carries away these morsels of dust, while it dissolves their solu- ble matter, among which are found ammoniacal salts, as they also dis- solve the vapor of carbonate of ammonia and the carbonic acid gas diffused in the air. There must, therefore, exist in a fall of rain, at its commencement, more soluble substances than at its close ; and if the rain continues uninterruptedly in calm weather, after a certain interval there can only be very insignificant indications of the existence of the substances. Miasmas, the propagators of epidemics, are superinduced by the aerial currents ; the cholera, the small-pox, the yellow fever, and the diseases which periodically attack a district, seem to have their princi- pal source of propagation in the atmosphere the factory of death as it is of life. The rate of mortality, which was so heavy in Paris during the early part of 1870, in consequence of small-pox, pleurisy, and in- flammation of the lungs, was especially severe in the northern districts of the city, over which the southerly wind spread the miasmas of the whole town, and where there was scarcely any ozone. A knowledge of the conditions of public health will be furnished in part by a study of the relations of meteorology to the variations in the rate of mortal- ity, which is as continually oscillating under the slight breath of the wind as under the trifling alterations in barometrical pressure. The air which Gay-Lussac brought down with him from his aero- nautical voyage, and which was collected at a height of 23,000 feet, had the same composition as that which floats upon the earth's sur- face. The experiments of M. Boussingault in America, and those of M. Brunner in the Alps, lead to the same conclusions. This similarity in results arises from the fact that currents of air and continual varia- tions in density are unceasingly mixing up together the atmospheric strata. Is it the same at a greater height ? It is scarcely probable, for the nitrogen and oxygen being in a state of mixture, and not chemically combined, the gases must be ranged according to their density, allow- ing, of course, for the law of expansion ; that is to say, there are, as it 72 THE ATMOSPHERE. were two distinct atmospheres, the least dense of which does not ex- tend 'so far as the other, so that the proportion of nitrogen, the density of which is 0-972, that of the air being 1, must increase the higher one rises in the atmosphere; while the oxygen, the density of which is 1-057 (and which is the denser of the two), must be in a greater pro- portion near the surface. According to this hypothesis, the latter gas, at 23,000 feet, would constitute only Vw of the volume of air ; but at present experiment has failed to note so great a difference, because this calculation supposes the air to be in a state of tranquillity, whereas at these heights it is, as a matter of fact, in a continuous state of agi- tation. The composition of the air varies very little : when it rains, the con- densed water dissolves more oxygen than nitrogen ; in frost, the water leaves these two gases alone ; the water which evaporates returns then to the atmosphere. We may now ask ourselves, in terminating this study of the chem- ical composition of the air, if this constitution is variable over the ter- restrial globe. By virtue of one of the great natural harmonies which unite the animal and the vegetable kingdoms, while the animals act as combustion-machines, taking the oxygen from the air and throwing it back into the atmosphere in the state of carbonic acid, the vegetables play the reverse part, acting as reducing-machines. Under the influ- ence of the solar rays, the green portions of the plants react upon the carbonic acid, decompose it, concentrate the carbon, and restore the oxygen to the air. The atmosphere, vitiated by the animals, is puri- fied by the action of the vegetables. The chemical equilibrium of the air's components has thus a tendency to self-preservation by virtue of this inverse action brought to bear upon its constituent elements. Certain phenomena due to the decomposition of rocks through oxida- tion seemed, at first sight, calculated to modify in the long run the com- position of the air ; but a series of inverse actions of reduction tends to restore, in the shape of carbonic acid, the oxygen that has disappeared. As Ebelmen has pointed out,' in his memoir upon changes in rocks, the process of reactions in the mineral matter upon the globe's surface seems also calculated to establish a compensation which maintains the chem- ical composition of the atmosphere. The question is whether this compensation is complete. Supposing it does not take place as, indeed, is possible does the quantity of oxy- gen diminish? As Thenard has remarked, "This is a very important CHEMICAL COMPONENTS OF THE AIR. 73 question, the solution of which can only be arrived at in the course of several centuries, because of the enormous volume of air by which our planet is surrounded." In their remarkable memoir upon the true constitution of the atmos- pheric air, MM. Dumas and Boussingault thus expressed themselves in 1841: "Some calculations, which, though not of absolute precision, never- theless are based upon sufficiently certain grounds, tend to prove how far an analysis should extend to reach the limit at which the varia- tions 'in oxygen would be sensibly manifest. The atmosphere is un- ceasingly agitated; the currents, stirred up by heat, by winds, by elec- tric phenomena, are continually being mixed up and confusing to- gether the various strata. The whole mass would, therefore, have to be changed in order to admit of an analysis indicating the difference between one epoch and another. But this mass is' enormous. If we could place the whole atmosphere into a balloon, and suspend it in one side of a pair of scales, it would be necessary to put on the other side 138,000 cubes of copper (each a mile in length, breadth, and thickness) to balance it. Let us now suppose that each man consumes a little more than two pounds of oxygen a day, that there are a thousand millions of men upon the earth, and that, through the respiration of animals and the putrefaction of organic matter, this consumption attributed to man be quadrupled. Let us further suppose that the oxygen disengaged from plants is only the compensating agent of the causes of absorp- tion omitted in our calculation, which would assuredly be putting the chances of alteration of the air in the strongest light. Well, even 'on this overdrawn hypothesis, at the end of a century the whole human race, and three times its equivalent, would only have absorbed a quan- tity of oxygen equal to fourteen or fifteen of the cubic miles of copper. "Thus, to assert that, with their utmost efforts, the animals which people the face of the earth could in a century render the air they breathe impure, to the extent of depriving it of the -^^ part of the oxygen that nature has placed there, is to make a supposition far be- yond the reality." In habitations badly ventilated, the effects of the breathing of men or animals, and the phenomena of the combustion of coal or of com- bustible matters, may cause a sensible alteration in the state of the air. Thus, in barracks, hospital rooms, theatres, wells, mines, etc., chemical analysis, when it is accurate enough, indicates a different composition 74 THE ATMOSPHERE. from that of the open air. Furthermore, in habitations even out of the influence of the presence of sick persons, the animal emanations which escape with the aqueous vapor in respiration and perspiration may ex- ercise an incontestable physiological influence, often more injurious than that caused by the production of carbonic acid or the disappear- ance of the oxygen in small quantities. It is especially when the air arrives at a state of saturation from the causes cited above that there is reason to consider it deleterious. There is an unanimity of opinion in the present day that, to avoid a disas- trous influence upon the organic economy, dwelling-houses, and espe- cially hospitals, should be so constructed as to give more than 20,000 cubic feet of air per day to each individual. SOUND AND THE VOICE. 75 CHAPTER VI. SOUND AND THE VOICE. AMONG the works of the atmosphere in terrestrial life, one of the most important is unquestionably that of serving as a vehicle for hu- man thought, and enveloping the world in a sphere of harmony and activity which could not exist without it. What is sound? It is a movement produced in the air, and transmitted therein by successive undulations. To be perceived by the ear, this vibratory movement must be neither too slow nor too rapid. When the air, agitated by sound, vibrates at the rate of sixty undulations a second, it emits the dullest sound which can reach the ear. When the vibrations are 40,000 per second, they convey the sharpest sound which the auditory nerves can perceive. To appreciate the nature of the sonorous movement, let us suppose that between the chaps in a vise, A (see Fig. 22), is fixed one of the extremities, c, of an elastic blade, c D ; that the upper end, D, is pulled back to D', and then let go. By virtue of its elasticity, the blade will return to its primi- tive position; but in consequence of the speed it has acquired, it will pass it and go on to D", executing on both sides of c D a series of oscillations the amplitude of which will gradually decrease, and in a more or Fig. 22,-Vibrations of a blade. ^^ ^^ space Qf ^ altogether cease< The longer the elastic blade is, the slower will be the vibrations; while, in proportion as the blade is shortened, the vibratory movement will become more rapid, and at a certain point will be imperceptible to the eye. But when the organ of vision ceases to play a part, so to speak, that of the organ of hearing begins, and the ear can distinctly ,-Q THE ATMOSPHERE. catch a sound, the nature of which depends upon the physical condi- tions of the vibrating body. Another instance of the production of sound is furnished by the vibration of a piece of cord fas- tened at its extremities, A B, and pulled in the middle.* Its vibration is rendered perceptible by the fact of the cord presenting the shape of a bobbin. By reason of the persistent impression upon the retina, and the speed of the vibratory movement, the eye sees the cord in all its positions together, as it were, the time of a vibration being less than that of a luminous impression, which is the tenth of a second. Sound, therefore, is but an im- pression upon the organ of hearing, caused by the vibra- ting movement of a given body. But the existence of a vibratory body on the one hand, and of an ear on the other, is not enough to cause an impression: a relation must be established between that body and the organ of hearing, and this is effected by a ponderable medium, liquid or gaseous, constituted of more or less elastic matter. If we im- agine a body vibrating in a complete vacuum, or in the centre of a space entirely devoid of elasticity, the ear, at a certain distance off, would catch no sound. Sound, in the proper sense of the word, does not exist in such a case. We may in fact form, from what is mentioned above, the following- definition of sound: Sound is an impression produced by the vibrations of a body transmitted to the organ of hearing by the intervention of a ponderabk and elastic me- dium. At what rate is sound propagated ? The first exact measurements were made in 1738 by a commission of the Academy of Sciences, of which Lacaille and Cassini de Thury were members. Several pieces of ordnance were placed upon the heights of Montmartre (then outside the walls of Paris) and at Mon- tlhery (an elevated position in the department of the Seine-et-Oise, dis- tant about 16 miles from Paris), and it was arranged that from a given hour a gun should be fired at equal stated intervals. The persons en- gaged in the experiment counted the .time that elapsed between the flash and the arrival of the report; and this was. found to be, on an * [This is, of course, the principle of all stringed instruments the harp, violin, etc. It is difficult to hold the cord sufficiently tight by the hand to produce a note. ED.] SOUND AND THE VOICE. 77 average, 1 minute 24 seconds for a distance of about 95,000 feet, which is at the rate of 1037 feet per second. These experiments were repeated in 1822 by the Bureau des Longi- tudes a section of the Academy of Sciences the persons taking part in them being Arago, Gay-Lussac, Humboldt, Prony, Bouvard, and Mathieu. Villejuif and Montlhery, distant from each other 61,000 feet, were the places selected ; and it was found that at a temperature of 61 the velocity of transmission was 1047 feet a second. A great number of similar experiments have been made in different countries. Yery recently, M. Regnault investigated this subject, em- ploying all the resources of modern physics, and especially telegraphic signals, for registering instantaneously the discharge and the arrival of the sound. The velocity of sound varies with the density and the elasticity of the air, and therefore with its temperature. According to the most accurate measurements, the following table may be given in reference thereto : Temperature (Fahr.) Velocity per Second. Temperature (Fahr.) Velocity per Second. 5 14 23 . 32 41 50 68 1056 feet. 1070 1079 1089 1096 1102 1112 68 77 80 95 104 113 122 1122 feet. 1132 1142 1152 1161 1171 1181 Sound is propagated in the air by successive undulations, which may be roughly compared to the circular waves which are produced on the surface of water around a point disturbed by the fall of a stone. But they are, in reality, very different phenomena. In the liquid waves, the molecules are alternately raised and lowered in regard to the gen- eral level, but undergo no change of density ;"" while this change is, on the contrary, a characteristic of the waves of sound. There is, however, one circumstance common to both these phenomena which is worth pointing out and that is, that the wave causes no real progressive movement. Thus, when waves of water follow each other, if we notice any small floating object, it is seen to alternately rise and fall, but it re- mains in the same place upon the surface of the water. Similarly, in the waves of sound, the molecules of the air execute oscillatory move- ments in regard to the propagation of sound, but the centre of these movements remains unchanged. irg THE ATMOSPHERE. Scientific education should teach us to behold in nature the invisible as well as the visible to depict to the eyes of the intellect what escapes the eyes of the body. We may, with a little application, form a true idea of a sound-wave ; we may mentally see the molecules of air first pressed the one against the other; then, immediately after, this con- densation brought away again by an opposite effect of dilatation or rarefaction. We thus learn that a wave of sound is composed of two parts : in one the air is condensed ; while in the other, on the contrary, it is rarefied. A condensation and a dilatation are then the essential constituents of a sound-wave. But, if the air is necessary to the propa- gation of sound, what happens when a sounding body, such as the bell of a clock, is placed in a space destitute of air ? The result is that no sound proceeds from the empty space ; the hammer strikes the bell, but silently. Hawksbee demonstrated this fact in a memorable experi- ment in 1705, before the Eoyal Society of London. He placed a clock under the receiver of an air-pump, in such a way that the striking of the clapper would continue after the air had been exhausted. While the receiver was full of air, the sound was quite audible; but it was no longer so (or at least in a very slight degree) when a vacuum had been created. The appended illustration is that of a contrivance which en- ables us to repeat Hawksbee's experiment in an improved manner. Under the re- ceiver B, placed firmly on the plate of an air-pump, will be seen the works of a strik- ing clock, A. The hammer is kept back by a spring and ratchet, c. As much as possi- ble of the air is exhausted ; then, by means of a stem, ^, which passes out through the top of the receiver, without letting in the exterior air, the trigger d, which holds back the hammer b, is pulled. The bell, a, vi- brates silently. But if we let the air into the recipient, we at once hear a sound, very feeble at first, but growing louder as the air becomes denser. At great heights in the at- Pig ^ mosphere, the intensity of sound is notably less. According to the calculations of Saus- sure, the detonation of a pistol upon the summit of Mont Blanc is about equal in intensity to that of a common cracker at the level of the sea SOUND AND THE VOICE. 79 Since it is proved that there is no sound in a vacuum, fearful catas- trophes might take place in the planetary regions without the slightest audible notice of them reaching the surface of the earth. The vibratory movement of the air has been represented as being a circular wave, which spreads out in all directions with equal velocity, and diminishes in intensity as it advances. Where does it cease ? where is it extinguished ? We must regard this as taking place at the point in space where it is no longer sensible to the most delicate ear ; and we all know how much this limit varies with the organization and habits of different individuals. At the same time, there can be no doubt that the aerial wave continues to spread out after the most prac- ticed ear has ceased to be sensible of it. In the places where there is a numerous population, the incessant noise kept up in the air by so many thousands of people creates a characteristic difference between day and night ; the noises become confounded together, and are propa- gated in a confused mass. During the night there is nothing to lessen the intensity of sound, and the ear perceives in all their force the howl- ing of the tempest, the blast of the winds, the roaring of the waves, the shrill cry of the bird of prey or the wild beast ; and it is then that pusillanimous fears and superstitious terror take possession of the timid. Traveling in a balloon over the plains of Charente, the stream of a river seemed to make as much noise as that of a great cascade, and the croaking of the frogs were audible at the height of 3000 feet. Above two miles all noise ceases. I never encountered a silence more com- plete and solemn than in the heights of the atmosphere in those chill- ing solitudes to which no terrestrial sound reaches. "Two conditions determine essentially," says Tyndall, "the velocity of the sound-wave, viz., the elasticity and density of the medium which it passes through." The elasticity of the air is measured by the press- ure which it supports, and to which it forms an equilibrium. We have seen that, at the level of the sea, this pressure is equal to that of a col- umn of quicksilver 29'92 inches high. Upon the summit of Mont Blanc the barometrical column scarcely exceeds half this height, and, therefore, at the highest point of this mountain, the elasticity of the air is only half what it is upon the sea-coast. If we could increase the elasticity of the air without at the same time augmenting its density, we should increase the velocity of sound. We should also effect that object if we could diminish the density without making any change in the elasticity. The air heated in a closed vessel, gO THE ATMOSPHERE. in which it can not become dilated, has its elasticity increased by the warmth, while its density remains the same. Sound will, therefore, be propagated more rapidly through air thus heated than through the air at its normal temperature. In like manner, air which is free to dilate has its density diminished by heat,* while its elasticity remains the same, and consequently it will propagate sound more rapidly than cold air this, indeed, takes place when our atmosphere is heated by the sun ; the air becomes dilated and much lighter, volume for volume, while its pressure, or, in other words, its elasticity, remains the same. This is the explanation of the statement that the velocity of sound in air is 1090 feet a second at the temperature of melting ice. At a lower temperature the velocity is less, and at higher temperatures greater, with an average difference of about one foot for one degree (Fahr.). Under the same pressure that is to say, with the same elasticity the density of hydrogen is much less than that of the air, and, in conse- quence, the velocity of sound through hydrogen gas considerably ex- ceeds its velocity through air. The reverse is the case with carbonic gas, which is denser than air ; for under the same pressure sound trav- els less rapidly through this gas than through air. The fact that air, even when very rarefied, can transmit intense sounds, is proved by the explosion of meteors at a great height above the earth, though it is true that, for this to be the case, the initial cause of the atmospheric disturbance must be very violent. The movement of sound, like all others, is less in amount when it communicates from a light body to one more dense. The action of hydrogen on the voice is a phenomenon of this kind. The voice is formed by the injection of air from the lungs into the larynx ; in its passage through this organ the air is set vibrating by the vocal chords, which thus give rise to sound ; and if one wishes to speak when the lungs are full of hydrogen, the vocal chords still impress their move- ment on the hydrogen, which transmits it to the air outside. But this transmission of a light gas to one much denser causes a considerable diminution in the intensity of the sound. The effect of this is very re- markable. Tyndall demonstrated it to the Royal Institution in Lon- don. Having, by a great effort of inhalation, filled his lungs with hydrogen, he began to speak, and his voice, generally powerful, was hoarse and hollow ; there was no ring in it ; it seemed to issue from the depths of the grave. * [The air must be contained in a vessel so constructed that the elasticity (pressure) is kept the same (for instance, in a cylinder in which fits a piston of constant weight). ED.] SOUND AND THE VOICE. gl The intensity of sound mainly depends upon the density of the air from which it proceeds, not on that of the air in which it is heard. The wave of sound, propagated in all directions from the point where the sound has been produced, diffuses itself in the mass of air in which the motion takes place, and consequently lessens the amount of move- ment at any point. Let us imagine around the centre of disturbance a spherical layer of air, with a radius of a yard ; another layer of the same thickness, with a radius of two yards, contains four times as much air; one with a radius of three yards contains nine times as much; one with a radius of four yards, sixteen times as much ; and so on. The quantity of matter set in motion increases, therefore, as the square of the distance from the centre of disturbance ; the intensity of the sound diminishes in the same degree. This law is expressed by the statement that the intensity of sound varies inversely as the square of the dis- tance from the point of initial disturbance. The decrease in the sound in inverse ratio to the square of the distance would not occur if the- sound-wave spread in such a way as to prevent its being diffused later- ally. By producing a sound in a tube the interior surface of which is perfectly smooth these conditions may be realized, and the wave thus confined reaches a great distance, with but a slight loss of intensity. In this way Biot, noting the transmission of sound through the conduit pipes that supply Paris with water, found that he could carry on a con- versation in a low tone at a distance of 3300 feet ; the faintest murmur of the voice was heard at this distance, and the firing of a pistol at one end of the pipes extinguished a candle placed at the other end. Echoes depend, in a great measure, upon the compressibility and elasticity of the air. The sound-wave, as has been stated, spreads in- definitely, and is finally lost in space; but if it encounters a body capa- ble of opposing it, it undergoes a reflection like that of light when it falls upon a smooth surface. For an echo to be distinctly produced, there must be a distance of fifty -five feet at least the tenth of a sec- ond in time between the person speaking and the reflecting surface. When the former is nearer, the echo is replaced by a confused reso- nance, which, in some buildings, renders it impossible for a speaker to make himself heard. Whether acute or grave, sounds have the same velocity* that of * [This is proved by the fact that, if a band of music be heard at a distance, the sounds are not confused, the distinctness of the tune being unaffected by the distance, though the loudness is of course diminished. ED.] G go THE ATMOSPHERE. 1115 feet a second in air of 61 (Fahr.). At half this distance the echo aives back four syllables rapidly pronounced; at a greater distance it will distinctly reflect a larger number of syllables and whole phrases. The echo in Woodstock Park repeats seventeen syllables in the day- time and sixty at night. Pliny tells us that a portico was built at Olympia which repeated sounds twenty times. The echo at the Cha- teau de Simonetti was said to repeat the same word forty times. The theory is the same for the multiplied echoes; they result from the re- flecting surfaces against which the aerial wave is thrown back several times from the one to the other, like a ray of light between two parallel glass plates. Perceptible sounds are included between the limits of 60,000 and 40,000 simple vibrations a second, except in the case of ears which are exceptionally sharp. The undulations of the ether, which produce light, are far more rapid.* Visible colors are the result of vibrations so rapid that between 400 and 800 billions take place in a second. Of perceptible sounds, the extreme limits of the human voice are the lowest, /a, of 87, and the highest, ut, of 4200 vibrations. Sound has four fundamental properties duration, height, intensity, and timbre or quality. The first three are defined by the words used to express them. As to the timbre, it is the resonance peculiar to each instrument and to each voice which enables us to clearly distinguish the sounds of a violin from those of a clarionet or a flute, and to recog- nize a person by hearing him speak or sing.f The timbre of sounds has long been an insoluble enigma to natural philosophers and physiologists. It is only within the last few years that the excellent experiments of Helmholtz have proved that it de- pends upon the number of harmonic sounds which are produced simul- taneously with the fundamental tone, and upon their relative intensity. The intensity of sounds generated upon the surface of the earth spreads upward far more readily than in any other direction, and is transmitted to great heights in the atmosphere. Citing some few in- stances from my aeronautical travels, I will, in the first place, mention * [It must be borne in mind that there is only a general analogy between light and sound. In the latter, the vibrations consist of condensations and rarefactions in the air (or other gas). which are longitudinal i. e., take place in the direction in which the sound is proceeding: while, in light, the vibrations are transversal (i. e., perpendicular to the direction of the ray), and take place in an ether which is supposed to pervade all space. ED.] t It is, of course, more difficult to recognize a person by his song than by his speech. SOUND AND THE VOICE. 83, that a noise, immense, colossal, and indescribable, is ever to be heard at 1000 to 1500 feet above Paris. In rising from a relatively quiet garden as from the Observatory we are astonished to hear a chaos of sound and a thousand various noises. The following details will, however, illustrate more strikingly this ascent of sound : The whistle of a steam-engine may be heard at 10,000 feet ; the noise of a train at 8200 ;* the barking of a dog at 6000 ; the report of a gun attains the same height ; the shouts of people sometimes are audible at 5000 feet, as also the crowing of a cock or the tolling of a bell. At 4500 feet the beating of a drum and the sound of a band are audible ; at 3900 feet the rumble of vehicles upon the pavement ; and at 3300 feet the shout of a single individual. At this last height, during the silence of the night, the current of a stream at all rapid produces the same effect as the rush of a cascade ; and at 2950 feet the croaking of frogs is plaintively distinct. At 2620 feet the slight noises made by the cricket are heard very plainly. This does not hold good of sound when descending. While we hear distinctly the voice of a person speaking from 1600 feet underneath us, it is impossible to catch what is said at a height of more than 300 feet above us. The occasion upon which I was most struck by this astonishing transmission of sounds vertically upward was in an ascent that took place on June 23, 1867. Having been in the midst of the clouds for several minutes, we were surrounded by a white and opaque veil that concealed both the sky and the earth, when I noticed with surprise a singular increase of light taking place around us, and all at once the sounds of a band reached our ears. We could follow the piece of music as distinctly as if the band had been in the clouds, a few yards distant from us. We were then just above Antony, a village near Paris. Hav- ing mentioned the fact in a newspaper, I was glad to receive, a few days afterward, a letter from the President of the Philharmonic Society in that place, informing me that his society had seen the balloon above them, and had purposely played a very soft piece, in the hope that they might be of service to us in our researches. In this case the balloon was about 2950 feet above the place. At 3280, 3940, and even 4590 feet, the parts were still distinctly audible. Far from being an obstacle to the transmission of sound, the clouds in- creased its intensity, and made the band seem close to us. * [On June 26, 1863, 1 heard a rail-way train when at the height of 22,000 feet. ED.] 34 THE ATMOSPHERE, When sound has ceased, there still continues in the air a movement which may cause to vibrate membranes placed to receive and to inter- pret these impressions. M. Eegnault has measured these silent waves ; he has determined the distance traversed both by the sonorous wave and the silent wave which continues after the former has ceased. In a gas-pipe, twelve inches in diameter, a pistol, with a charge of fifteen grains of gunpowder, was heard at the other extremity, 6250 feet off; and when the pipe was closed with an iron plate, the echo of the report was perceptible to any one listening attentively. The limit of the so- norous wave was therefore, in this instance, 12,600 feet; that of siknt waves is much greater. Air, the vehicle of sound, is at the same time the vehicle of smells and of all the emanations that are exhaled from the terrestrial surface. But smells are due not only to the vibratory movement, like sound and light. Fourcroy was the first to establish the fact that they are in part caused by the volatilization of vegetables or other matter ; that smells are caused by actual molecules suspended in the air material particles, very slender and volatilized in the atmosphere. But the matter seems to become almost intangible. Nothing can give a more faithful idea of the divisibility of matter than the diffusion of smells. Three-quarters of a grain of musk placed in a room develop a very strong smell in it for a considerable time, without the musk perceptibly losing weight, and the box containing the musk will retain the perfume almost indefinitely. Haller states that papers perfumed with a grain of ambergris were quite odoriferous at the expiration of forty years. I remember purchasing upon the quay in Paris, some twelve years ago, a pamphlet which had a pro- nounced odor of musk about it. It had, no doubt, been there many months, exposed to the sun, the wind, and the rain. Since that time it has remained upon a library shelf, where the air has full access to it, and having just opened its pages, I find it as fully scented as ever. Smells are transported by the air to great distances. A dog can rec- ognize his master's approach from a distance ; and it is asserted that at twenty-five miles from the coast of Ceylon the delicious perfume of its balmy forests is still borne upon the wind. These sweet perfumes, like the harmony and the activity of the terrestrial surface, we owe to the atmosphere. AERONAUTICAL ASCENTS. 35 CHAPTER VII. AERONAUTICAL ASCENTS. THE air being a fluid possessing weight, analogous to water in regard to the principles of pressure,* but, as we have seen, very much lighter, an instant's reflection will suffice to show that, if a body lighter than air be placed in the atmosphere, it will rise just as a body lighter than water such as wood or cork will, if placed at the bottom, at once ascend to the surface, because of its less specific gravity. If the atmosphere formed a homogeneous ocean above the surface of the globe, equally dense throughout, arid terminated, like the sea, by a defined surface, every body the density of which was less than the density of this aerial ocean would rise, when left to itself, by the as- censional force of a pressure dependent on the difference of densities, and would remain floating upon the upper surface of this atmosphere. This was the notion of several of the predecessors of Montgolfier ; among others, of the worthy Father Galien, in his fantastic scheme for aerial navigation, published in 1755. His famous ship was to contain " fifty-four times as much weight as Noah's ark," its dimensions were to be equal to those of the town of Avignon ; for the hypothesis of this excellent ecclesiastic was that this vast iron vessel would float in the atmosphere in virtue of the same principle as that by which a ship floats upon the ocean. But as the density of the atmospheric strata diminishes with elevation, all objects lighter than the lower strata mount merely to the region the density of which is such that the weight of the body is equal to the weight of the volume of fluid dis- placed. Archimedes established for liquids a principle which we can apply with precision to the atmospheric fluid, enunciating it as follows: All bodies situated in the atmosphere lose a portion of their absolute weight, equal to the weight of the air which they displace. This actual loss of weight in the air is proved by means of a pair of scales specially constructed for the purpose, as the name indicates, * [It must be borne in mind that water is very slightly compressible indeed ; while air is an elastic fluid, capable of almost indefinite compression or expansion. ED.] 86 THE ATMOSPHERE. Pig. 25. The Baroscope. of seeing the weight-the baroscope. One extremity of the beam has attached to it a hollow copper sphere; the other end carries a small piece of lead, balancing in the air the copper sphere. If this apparatus is placed under the glass-receiver of an air-pump, as soon as a vacuum has been created the balance inclines to the side of the sphere, showing that in reality it weighs more than the mass of lead which was in equilibrium with it when in the air ; or, in other words, that it has lost in the air a portion of its weight, because its volume was larger than that of the piece of lead. To verify, by means of the same apparatus, that this loss is just equal to the weight of the air displaced, the volume of the sphere must be measured, and if it holds say about a pint, or 34 -6 cubic inches, the weight of this volume of air being 11-3 grains, the corresponding weight must be attached to the piece of lead, and the equilibrium will be re-established in the vacuum, but will be destroyed upon the re- introduction of air. Let us note, en passant, in reference to this subject, that when any object is weighed in scales it is never its exact weight which is ob- tained, but its apparent weight. To get at the actual weight, the ob- ject must be weighed in vacuum. This is a source of continual error which is rarely taken into consideration. But, on the other hand, it may be asked, what is the real weight of any particular body ? and the reply must be, there is no such thing. It is a purely relative matter, resulting from the volume and density of the planet which we inhabit. A pound weight does not constitute an absolute quantity, notwith- standing appearances to the contrary. The proof of this is, that if a pound weight were transported to the surface of the sun it would weigh nearly twenty-eight pounds ;* whereas it would weigh two pounds and three quarters, nearly, upon the surface of Jupiter, and only one-sixth of a pound at the moon ! And even without going so far as this, if we imagine our atmosphere gradually becoming denser and denser, we * [The weighing must, of course, be made by means of a spring-balance, or other balance of the same kind. If a certain object balances a pound weight on the earth in a pair of scales, it would do so also anywhere else on the sun, moon, etc. ED.] AERONAUTICAL ASCENTS. 87 should, in that case, become lighter; or, again, if the earth revolved seventeen times faster than it does, the inhabitants of tropical countries would have no weight at all, and only weigh a few grains in the lati- tude of London or Paris. This may serve to confirm the doctrine of those English philosophers who, with Berkeley at their head, argued that the only real fact is, that there is nothing real in the world. But let us return to the weight of the air. A balloon is, in fact, merely a body lighter than the weight of the air which it displaces, and which consequently rises in search of its equilibrium into higher re- gions of less density, where it will only displace a volume of air equal to its own weight. It is clear that, far from being in opposition to the laws of gravity, the ascent of balloons is, on the contrary, a special con- firmation of them. Whatever may be the substance which is used for filling a globe of silk or other material, if the whole apparatus the gas which fills the envelope, the car, the net to which it is attached, the aeronauts, etc. weighs less than the air which it displaces, it constitutes by that very fact an aerostatical machine, and rises in the atmosphere. When Montgolfier launched, for the first time, a balloon into the air, his balloon was simply inflated with hot air. The density of air heated up to 122 (Fahr.) is 0-84, that of air at 32 being represented by 1. The density at 212, the temperature of boiling-water, is 072, giving scarcely a difference of one-third for the ascensional force. The den- sity of pure hydrogen is only O07 ; that is, one-fourteenth of that of air. The density of carbureted hydrogen is about 0'55; that is, about one- half the density of air. The latter of these two gases is generally used for filling balloons. By a happy coincidence not rare in the history of science, hydrogen gas was discovered almost simultaneously with the invention of bal- loons. In 1782, Cavallo exhibited before audiences, at his London lectures, soap-bubbles formed of hydrogen, which rose by their less specific gravity up to the ceiling of the hall. In the following year (June 5, 1783) Montgolfier launched the first aerostat. With a little study and energy, Cavallo might have deprived the Annonay manu- facturer of the immortality of his invention. A balloon inflated with hot air is still often called a Montgolfier bal- loon, after its inventor. A balloon inflated with gas is denominated a gas-balloon, and often, popularly, an air-balloon. Gas has been adopted almost exclusively since its first trial, which was made at THE ATMOSPHERE. Fig. 20. Soap-babbles Inflated with hydrogen. Paris, on the 27th of August, 1783, by M. Charles, Member of the Academy of Sciences, and the Brothers Robert. The first time that a car was suspended to a balloon was on the 19th of September, 1783, in presence of Louis XVI. and Marie Antoinette, at Versailles ; and the earliest passengers were a sheep, a cock, and a duck. The first real aerial voyage was accomplished on the 21st of October following, by Pilatre des Hosiers and the Marquis d'Arlandes, who rose, by means of a fire-balloon, from the Chateau de la Muette (the Bois de Boulogne), and made their descent at Montrouge (on the south side of Paris), after having crossed the capital. To say that one/eefe one's self being carried up by a balloon perhaps scarcely gives a correct idea of the situation. It is better to say, sees one's self carried up, for the voyager feels no kind of movement, and the earth seems to him to be descending. As personal impressions are unquestionably those the recital of which comes nearest to the reality, I will take the liberty of citing some. My first ascent took place on Ascension-Day (May 25) in 1867. Eugene Godard, the aeronaut, having verified the perfect equilibrium of the balloon, orders the four assistants to let slip through their hands, without losing hold of them, the ropes which secured the car, and thus we find ourselves a few yards above the ground. The sky is clear, the AERONAUTICAL ASCENTS. 89 wind light, and the balloon, filled with hydrogen gas, becomes impa- tient and endeavors to rise. Then, taking a sack of ballast in his hand, Godard gives the word to ." let go," throwing over a few pounds of sand, and the aerostat rises with majestic ease. The balloon rises in an oblique curve, caused by two component forces its ascensional power on the one hand, and the velocity of the wind on the other. If, as is proper from all points of view, we take care to let the balloon have only a slight ascensional force, the most magnificent of panoramas is slowly developed before the charmed gaze. If we wish only to ascend to a height of 3000 to 4000 feet, the balloon is allowed to move horizontally as soon as i-t reaches an atmospheric stratum of this elevation, whose density -is then equal to that of the balloon. For higher ascents, the balloon is lightened by throwing out ballast. The aeronaut, the meteorologist, or the astronomer who thus hovers in the air, is in a most enviable position for studying the atmosphere. Penetrating into the very midst of the clouds, traversing them to de- termine the light and heat which influence them, following the storm in its mysterious formation, studying the production of rain, snow, and the hail, transporting himself, in fact, into the very regions where these phenomena are occurring, it is there alone that the observer is really master of the globe. The savant may in vain spend years by his fire- side in forming hypotheses by the aid of books and apparatus ; but in this, as in most other things, the surest method of ascertaining what is going on, is " to go and see for one's self," as the old proverb has it. And, assuredly, no attempt can yield more fruitful results. I do not intend to revert to a subject which was largely and com- pletely dealt with in 1870 in a work specially devoted thereto. The purpose of this chapter is not to record my travels in the air; the scien- tific results flowing from them will be found embodied in the various explanations which compose the present book. It was merely necessa- ry to lay down the general theory of the ascent of a balloon in its rela- tions to the study of the atmosphere, and to give some idea of the effects of the higher regions. If aerial travels may be profitably applied to the study of the forces at work in the atmosphere, and of the laws which preside over its mul- tiform movements, they are also a special subject of interest for the ob- server, and open for him an exclusive vista of vast and useful contem- plation. Borne into the fields of the sky by the invisible breath of the go THE ATMOSPHERE. winds, the solitary balloon rises above the earth, and the traveler views its surface as a map stretched out on a boundless plain seen with all the characteristics of its local topography. Capitals situated on the banks of rivers, the central cities of provinces, innumerable villages dissemi- nated over the country, and succeeding each other in hundreds like the little chateaux one used to see dotted down in old-fashioned maps, hill- sides brown with the vine, furrows golden with grain, verdant mead- ows, cragged mountains whose tops are covered with sombre forests, sparkling streams and sinuous rivers running to the distant ocean all the charms, soft or stern, of landscape and perspective are slowly re- vealed to the delighted gaze of the aeronaut who, without feeling the slightest movement, hovers as in a dream until he again sets foot upon the earth that he has been contemplating from on high. A less power- ful impression, but of a similar kind, is derived from a mountain ascent. The purity of the upper air, and the variation in atmospheric press- ure, are physical elements which must be taken into account in order to explain the benefit of a sojourn at a moderate altitude. The peculiar action which may be exercised upon impressionable organizations by the contemplation of mountains, where nature has bestowed so liberally that mixture of the gracious and the terrible which tends to make up the picturesque, is undeniable. J. J. Rousseau says : " Every one must feel, though he may not observe it, that in the purer and more subtle air of the mountains he has a greater facility of breathing, more nimble- ness in the body, more serenity of mind; the pleasures are less ardent there, as the passions are more subdued. Meditation assumes a certain tranquil voluptuousness, which is not in the least sensuous or bitter. It seems that, as we rise above the abode of man, we leave all terrestrial and base sentiments behind, and as we approach the ethereal regions, the soul gains something of their inalterable purity. We become grave without being melancholy, placid without indolence, content to live and to think. I doubt whether any violent agitation, any hysterical affec- tion, could hold out against a lengthened sojourn there ; and I am as- tonished that a bath of the healthy mountain air is not one of the great- est medical remedies." It is, however, proper to state that, beyond moderate altitudes, the human organism is susceptible of a deleterious influence, owing to the change in atmospheric pressure, the dryness of the air, and the cold. The physiological uneasiness and disturbances which are felt at great heights have long been ascertained facts. As early as the fifteenth cen- AERONAUTICAL ASCENTS. 91 tury they were observed and described by Da Costa, under the name of mal de montagne. Later, all mountain explorers in the Alps, the Andes, and the Himalayas, as well as aeronauts, have noted these singular per- turbations of organism, and have published theories more or less plau- sible in explanation of them. The principal cause assigned since De Saussure has been merely the rarefaction of the air ; but by what series of actions and reactions does this rarefaction affect the human body ? That was the point which needed elucidation. In 1804, Gay-Lussac and Biot rose as high as 13,000 feet in a balloon. Gay-Lussac's pulse went up from 62 to 80 a minute ; that of Biot from 79 to 111. In the memorable ascent of July 17, 1862, Messrs. Glaisher and Coxwell attained the enormous elevation of 37,000 feet. Previous to the start, Glaisher's pulse stood at 76 beats a minute, Mr. Coxwell's at 74. At 17,000 feet the pulse of the former was at 84 of the latter, at 100; at 19,000 feet Glaisher's hands and lips were quite blue, but not his face ; at 21,000 feet he heard his heart beating, and his breath- ing was becoming oppressed ; at 29,000 feet he became senseless, and only returned to himself when the balloon had come down again to the same level ; at 37,000 feet the aeronaut could no longer use his hands, and was obliged to pull the string of the valve with his teeth. A few minutes later he would have swooned away, and probably lost his life; The temperature of the air was at this time 12 below zero. In aero- stats, however, the explorer remains motionless, expending little or none of his strength, and he can therefore reach a greater elevation before feeling the disturbance which brings to a halt at a far lower level the traveler who ascends by the sole strength of his muscles the steep sides of a mountain. De Saussure, in his ascent of Mont Blanc on the 2d of August, 1787, has given an account of the uneasiness which his companions and him- self began to experience when a long distance from the summit. Thus, at 13,000 feet, upon the Petit-Plateau, where he passed the night, the hardy guides who accompanied him, to whom the few hours' previous marching was absolutely child's play, had only removed five or six spadefuls of snow in order to pitch the tent, when they were obliged to give in and take a rest, while several felt so indisposed that they were compelled to lie upon the snow to prevent themselves from faint- ing. "The next day," De Saussure tells us, "in mounting the last ridge which leads to the summit, I was obliged to halt for breath at every fifteen or sixteen paces, generally remaining upright and leaning 92 THE ATMOSPHERE. on my stock; but on more than one occasion I had to lie down, as I felt an absolute need of repose. If I attempted to surmount the feel- in o 1 , my legs refused to perform their functions; I had an initiatory feeling of faintness, and was dazzled in a way quite independent of the action of the light, for the double crape over my face entirely sheltered the eyes. As I saw with regret the time which I had intended for ex- periments upon the summit slipping away, I made several attempts to shorten these intervals of rest. I tried, for instance, a momentary stoppage every four or five paces, instead of going to the limit of my strength, but to no purpose, as at the end of the fifteen or sixteen paces I was obliged to rest again for as long a time as if Lhad done them at a stretch ; indeed, the uneasy feeling was strongest about eight or ten seconds after a stoppage. The only thing which refreshed me and augmented my strength was the fresh wind from the north. When, in mounting, I had this in my face, and could swallow it down in gulps, I could take twenty-five or twenty-six paces without stopping." Bravais, Martins, and Le Pileur, in their celebrated expedition to Mont Blanc in 1844, experienced and investigated the same phenome- na upon the Grand Plateau. In clearing the tent, which was half filled with snow, the guides had continually to stop for breath. An internal uneasiness, according to Martins, made itself apparent in many different ways. The appetite was gone. The strongest, biggest, and most har- dy of the guides fell upon the snow, and was nearly in a fit when the doctor, Le Pileur, felt his pulse. On nearing the summit, Bravais was anxious to see how far he could go without a rest ; at the thirty-second step he was obliged to stop short. All the indispositions felt by the savans of whom we have been speaking, and by many other travelers, at great elevations, have been classed in the following list: Breathing. The breathing is accelerated, impeded, laborious; and there is a feeling of extreme dyspnoea at the least movement. Circulation. The great majority of travelers have noticed palpita- tions, quickening of the pulse, beating of the carotids, a sensation of plenitude in the vessels, and sometimes the imminent approach of suf- focation and various kinds of hemorrhage. Innervation.'VQTy painful headache, a sometimes irresistible desire to sleep, dullness of the senses, loss of memory, and moral prostration. Digestion. Thirst, strong desire for cooling drinks, dry ness of the tongue, distaste for solid food, nausea, and eructations. AERONAUTICAL ASCENTS. 93 Functions of Locomotion. Pains more or less severe in the knees and legs; walking causing great fatigue and exhausting all strength. These disturbances are not regular, they do not all come on at once, and evidently depend a good deal upon the strength, the age, the hab- its, and the previous actions of the individual. They seem to have a greater effect upon Alpine climbers than in other mountainous regions. Thus, at the Great St. Bernard, the monastery of which has an altitude of only 8117 feet, most of the monks become asthmatic. They are compelled to descend frequently into the valley of the Khone to regain their health, and at the end of ten or twelve years' service to quit the monastery for good, under penalty of becoming quite infirm ; and yet, in the Andes and Thibet, there are whole cities where people can en- joy as good health as anywhere else. Boussingault says, that " when one has seen the activity which goes on in towns like Bogota, Micui- pampa, Potosi, etc., which have a height of from 8500 feet to 13,000 feet ; has witnessed the strength and agility of the toreadors in a bull- fight at Quito (which is 9541 feet); when one has seen young and deli- cate women dance for the whole night long in localities almost as lofty as Mont Blanc, where De Saussure had scarcely the strength to read his instruments, and where the vigorous mountaineers fainted; when one remembers that a celebrated combat, that of Pichincha, took place at a height as great as that of Monte Eosa (15,000 feet), it will be admit- ted that man can become habituated to the rarefied air of the highest mountains." The same writer is also of opinion that in the vast fields of snow, the discomfort is increased by an emission of vitiated air under the action of the solar rays, and he bases this impression upon an experi- ment of De Saussure, who found the air near the surface of snow to contain less oxygen than that of the surrounding atmosphere. In cer- tain hollows and inclosed valleys of the higher part of Mont Blanc in the Corridor, for instance people generally feel so unwell in trav- ersing it, that the guides long thought that this part of the mountain was impregnated with some mephitic exhalation. Thus, even now, whenever the weather permits, people ascend by the Bosses ridge, where a purer air prevents the physiological disturbances from being so intense. Notwithstanding that one may become gradually accustomed to the attenuated air of high elevations, certain animals can not live there. Thus cats, taken up to the altitude of 13,000 feet, invariably succumb, g^ THE ATMOSPHERE. after having been subject to singular attacks of tetanus, of gradually increasing intensity; and, after making tremendous leaps, succumb from fatigue, and die in convulsions. We will conclude these remarks by mentioning that the highest in- habited spot in the world is the Buddhist cloister of Hanle (Thibet), where twenty priests live at the enormous height of 16,500 feet. There are other cloisters built at a nearly equal height in the province of Guari Khorsum, upon the banks of the lakes Monsaraour and Ba- kous, and they are inhabited all the year round. In these equatorial regions one can live very easily for ten or twelve days at an altitude of 18,000 feet, but not for a longer time. The Brothers Schlagintweit, when they explored the glaciers of the Ibi-Gamin in Thibet, encamped and passed the night, with eight men of their expedition, from the 13th to the 23d of August, 1855, at these exceptional elevations, which are rarely visited by a human being. For ten days their encampments varied from 18,000 to 21,000 feet ; that is to say, the greatest altitude at which a European ever passed the night These three brothers succeeded, on August 19, 1856, in mounting to an elevation of 24,339 f ee t_farther than man has ever yet reached. At first they suffered a good deal when they got to 17,000 feet ; but, after a few days, they felt nothing but a passing uneasiness even at 19,000 feet. It is, however, probable that a prolonged stay at this altitude would have produced ill effects. Three or four years ago, Professor Tyndall, in order to take scientific observations, passed the whole night upon the summit of Moni Blanc, sheltered only by a small tent. The guides who accompanied him were so unwell that the next morning they were obliged to make their way downward as quickly as possible. A year or two ago, M. Lortel, who had several times ascended to 14,000 feet upon Mont Blanc without discomfort, and who doubted whether another 1600 feet could superinduce the symptoms asserted, went to the summit to judge for himself. He writes: "I am now con- vinced, and am compelled to admit, de visu and rather at my expense, that there really do exist causes of disturbance at this height which af- fect a person who ascends so far, especially if he is in motion, in this rarefied air. This is also the result of my personal observations ; and I have satisfied myself that it is much less hurtful to the organic func- tions to rise to great heights when sitting still in a car than by climbing over the snows." AERONAUTICAL ASCENTS. 95 To complete our atmospheric panorama, it is interesting to see what are the highest points of the mountainous peaks upon which man is liv- ing, and what are the highest points of the mountain chains which raise into the rarefied atmosphere their silent and icy peaks. The highest spots of the earth which are inhabited are : The Buddhist cloister of Hanle (Thibet) 16,532 feet. Cloisters on the sides of the Himalaya 14,764 to 16,404 " The post-house of Apo (Peru) 14,377 " The post-house of Ancomarca (do.) 14,206 " The village of Tacora (do.) 13,691 " The town of Calamarca (Bolivia) 13,651 " The vineyard of Antisana (Republic of Ecuador) 13,455 " The town of Potosi (Bolivia), ancient pop. : 100,000 13,323 " The town of Puno (Peru) 12,871 " The town of Oruro (Bolivia) 12,455 " The town of La Paz (do.) 12,225 " Quito, capital of the Ecuador Eepublic, is situated at an altitude of 9541 feet ; La Plata, capital of Bolivia, at 9331 feet ; Santa Fe de Bo- gota, at 8730 feet. The highest inhabited spot in Europe is the Monas- tery of Mount St. Bernard, which is 8117 feet high. The highest passes of the Alps are the pass of Mount Cervin, 11,188 feet ; the Great St. Bernard, 8110 feet ; the Col de Seigne, 8074 feet ; and the Furka, 8002 feet. The highest passes in the Pyrenees are the Port d'Oo, 9843 feet ; the Port Viel d'Estaube, 8402 feet ; and the Port de Pinede, 8202 feet. The highest mountains in the world are : Asia : The Gaurisankar, or Mount Everest (Himalaya) 29,003 feet. The Kanchinjinga (Sikkim, Himalaya) 28,156 " The Dhaulagiri (Nepaul, do. ) 26,825 " The Juwahir (Kemaon, do. ) 25,670 " Choomalari (Thibet, do. ) 23,945 " America: The Aconcagua (Chili) 22,422 " The Sahama (Peru) 22,349 " The Chimborazo (Republic of Ecuador) 21,424 " The Sorota (Bolivia) 21,283 " Africa: The Kilimanjaro 20,001 " Mount Woso (Ethiopia) 16,601 " Oceania: The Mownna-Roa, volcano (Sandwich Isles) 15,874 " Europe: Mont Blanc 15,797 " Monte Rosa 15,211 " The birds, of course, represent the population of the very highest altitudes. In the Andes the condor, in the Alps the eagle and the vult- Q* THE ATMOSPHERE. t/O ure, hover above the topmost peaks. Fitted for the longest journeys, they are the greatest sailors in the atmospheric ocean, just as the petrels and the gigantic sea-swallows are the great sailors over the Atlantic. The choucas (a kind of jackdaw), with its intensely black plumage and yellow beak and red legs, does not rise so high into the atmosphere, but it is especially the bird of the highest peaks, of the region of snows and barren cones. It is met with at the summit of Monte Eosa and at the Col du Geant, at over 11,500 feet. There are also birds more graceful in form which live in the region of hoar-frost, and lend a little animation to those bleak and unchanging landscapes. The snow-chaffinch has so great a preference for this cold region that he rarely descends to the zone of the woods. The accenteur of the Alps also follows him to great elevations, preferring the stony and barren region which separates the zone of vegetation from that of perpetual snow, and both of these birds sometimes soar as high as 11,000 to 15,000 feet in "pursuit of insects. The engraving (see Fig. 27) represents the principal kinds of birds according to the maximum height to which they fly. The earth has its birds, like the air. Certain kinds never use their wings but for a few moments, when it is impossible for them to move along the ground; for instance, all the gallinaceous kinds. The region of snow has its own kind, just as it has its characteristic sparrows. The ptarmigan, or snow-hen, is met with in Iceland as in Switzerland. It soars far above the everlasting hoar-frosts, and is so fond of the snow that at the ap- proach of summer it mounts farther in search of it, plunging into it with evident delight. A few lichens, grains brought up there by the air, suffice for its food. It looks for insects, with which it nourishes its young. The insects are, indeed, the only animals which are abundant in these bleak regions a fresh analogy with the polar countries. It is also the class of coleoptera which predominate in the higher Alpine regions. They attain to 9800 feet on the southern slope, and to 7900 feet on the opposite side. Their wings are so short that they scarcely seem to have any ; one would imagine that nature had intended to protect them from the strong currents of air which would undoubtedly carry them away if their wings had not been, so to speak, reefed. One does every now and then encounter other insects, neuroptera and butterflies, which the winds have taken up to these heights, and which are afterward lost amidst the snows. The seas of ice are covered with victims that have Fig. 27. Distribution of kinds of Birds according to height of flight. Condor (has been seen as high as 9000 metres, or 29,500 feet) ; 2. Griffon ; 3. Vulture ; 4. Sarcoromphns ; 5. Eagle ; 6. Urndn ; 7. Kite ; 8. Falcon , 9. Sparrow-hawk ; 10. Fly-bird ; 11. Pigeon ; 12. Buzzard ; 13. Swallow ; 14. Heron ; 15. Crane ; 16. Duck an.l Swan (found in lakes at an altitude of 1800 metres, or 5900 feet) ; 17. Crow; 18. Lark; 19. Quail; 20. Parrot; 21. Partridges and Pheasants ; 22. Penguin. 7 AERONAUTICAL ASCENTS. 99 perished in this way. Nevertheless, there are certain kinds which ap- pear to travel freely as high as 13,000 or 16,800 feet. In my aerial voyages, I have met with butterflies at heights to which the birds of our latitudes do not ascend, and at more than 9800 feet above the ground. Dr. J. D. Hooker noticed some at Mount Momay, at an alti- tude of more than 17,700 feet. Such is the scale of animal life in these Alpine zones, where the fauna gradually becomes scarcer, finally giving way to solitude and desolation. Beyond the last stage of vegetation, beyond the extreme region attained by the insect and mammifers, all becomes silent and uninhabited ; yet the air is still full of microscopic animalcules, which the wind raises up like dust, and which are dissemi- nated to an unknown height. BOOK SECOND, LIGHT AND THE OPTICAL" PHENOMENA OF THE AIR. THE DAY. 103 CHAPTER I THE DAY. As the atmosphere is the organizer of life; as all beings, animal and vegetable, are so constituted as to be able to breathe in its midst and construct, by means of its fluid molecules, the solid tissue of their or- ganisms, we must now turn our attention with admiration to the atmos- phere, as being still further the ornament of nature, and we shall see that we owe to it not only the picture, but also the frame. Whether the sky be clear or cloudy, it always seems to us to have the shape of an elliptic arch ; far from having the form of a circular arch, it always seems flattened and depressed above our heads, and gradually to become farther removed toward the horizon. Our ances- tors imagined that this blue vault was really what the eye would lead them to believe it to be ; but, as Voltaire remarks, this is about as rea- sonable as if a silk- worm took his web for the limits of the universe. The Greek astronomers represented it as formed of a solid crystal sub- stance ; and so recently as Copernicus, a large number of astronomers thought it was as solid as plate-glass. The Latin poets placed the di- vinities of Olympus and the stately mythological court upon this vault, above the planets and the fixed stars. Previous to the knowledge that the earth was moving in space, and that space is everywhere, theologi- ans had installed the Trinity in the empyrean, the angelic hierarchy, the saints, and all the heavenly host. ... A missionary of the Middle Ages even tells us that, in one of his voyages in search of the terres- trial paradise, he reached the horizon where the earth and the heavens met, and that he discovered a certain point where they were not joined together, and where, by stooping, he passed under the roof of the heav- ens. . . . And yet this vault has, in fact, no real existence! I have myself risen higher in a balloon than the Greek Olympus was sup- posed to be situated, without being able to reach this limit, which, of course, recedes in proportion as one travels in pursuit of it like the apples of Tantalus. What, then, is this blue, which certainly does exist, and which veils from us the stars during the day ? -^04 THE ATMOSPHERE. The vault which we behold is formed by the atmospheric strata which, in reflecting the light that emanates from the sun, interpose be- tween space and ourselves a sort of fluid veil, which varies in intensity and height with the density of the aerial zones. The illusion referred to above took a long time to dispel, and it was also a work of time to make it known that the shape and dimensions of the celestial vault change with the constitution of the atmosphere, with its state of trans- parency and its degree of illumination. One part of the celestial rays sent from the sun to our planet is absorbed by the air, the other part is reflected ; the air, nevertheless, does not act equally on all the col- ored rays of which white light is composed, but acts like a glass, al- lowing the rays toward the red end of the solar spectrum to pass more readily than those in the neighborhood of the blue end. This differ- ence is only perceptible when the light passes through a great thick- ness of air. De Saussure pointed out that the blue color of the sky was due to the reflection of light, and not to a hue peculiar to aerial par- ticles. "If the air were blue," he says, "the distant mountains, which are covered with snow, would appear blue also, which is not the case." An experiment made by Hassenfratz also proves that the blue ray is more reflected; in fact, the thicker the atmospheric stratum is which a ray traverses, the more do the blue rays disappear to give place to the red ; and as, when the sun is near to the horizon, the ray has to traverse a greater thickness of air, the sun therefore appears red, purple, or yellow. The blue rays are also frequently absent in rainbows which make their appearance just before sunset. We shall see further on that it is the vapor of water accumulated in the air which plays the principal part in this reflection of the light, to which we owe the azure of the sky and the brightness of day. Very recently, Professor Tyndall reproduced the blue of the sky and the tint of the clouds in an experiment at the Royal Institution. Va- por of different substances, of nitrite of butylene, of benzene, and of carbonic sulphide, is introduced into a glass tube ; a succession of elec- tric sparks is then passed through it, and the condensation and rarefac- tion of the vapor augmented ad libitum. As soon as the vapors em- ployed, no matter what their nature is, are sufficiently attenuated, the reflection of the light first manifests itself by the formation of a blue like that of the sky. There is, I will suppose, in the tube a half atmos- phere of air mixed with vapor, and another half atmosphere of air that has passed through hydrochloric acid. The proportion and density of the gas can, of course, be varied. THE DAY. 105 The vaporish cloud, after having first assumed the blue tint, becomes more condensed and white, and as it thickens, becomes exactly like real cloud, presenting, as regards polarization, the same variation of phe- nomena. The atmospheric air is one of the most transparent bodies known. When it is not charged with mist or obscured by other bodies, we can see objects at an immense distance, and mountains only disappear from our view when they are below the horizon ; but, in spite of its slight power of absorption, the air is not completely transparent ; its mole- cules absorb a portion of the light which they receive, permit the pas- sage of another part, and reflect the third; and hence it is that they give rise to what appears a vault, that they light up terrestrial objects which the sun does not reach directly, and effect an imperceptible tran- sition between day and night. It is easy to convince one's self of the decrease in the intensity of the solar light during its passage through the atmosphere by daily ob- servations. If an object situated near the horizon is watched for sev- eral days together, it will be seen that it is more visible at one time than at another. The distance at which its details fade out of sight is at one moment less than at another, as may be proved by direct meas- urement; the transparency of the air can be even expressed numeric- ally, as has been done by De Saussure through the instrumentality of the diaphanometer. The distance at which objects disappear does not depend upon the angle of vision alone, but also upon the manner of their illumination, and the contrast which their color offers to surround- ing objects. This explains why the stars, despite their small diameter, are so visible in the vault of heaven. It is the same with some terres- trial objects. It is difficult to distinguish a man, as he stands out in the fields, as against dark surfaces ; but he is very easily seen if he is placed upon an elevation so as to stand out against the clear sky. Hence the optical illusions so common in mountainous countries. While the chain of the Alps, seen from the plain at a great distance, is visible in its minute details, the spectator standing upon one of its peaks can distinguish hardly any thing in the plain. From the Faul- horn, for instance, it is easy to make out very distinctly the chain of the high Alps ; but every thing in the valley below is dim and con- fused. The summits of the Pilate, the Black Forest, and the Yosges are clearly defined at a great distance, whereas nothing can be distin- guished in the plain between the Alps and the Jura. Any one who JQ6 THE ATMOSPHERE. has passed a few months amidst the lakes and mountains of Switzer- land must have noticed the same variations in the visibility of objects. To measure the intensity of the blue color, De Saussure invented the cyanometer, which is composed simply of a strip of paper divided into thirty rectangles, the first of which is of the deepest cobalt blue, while the last is nearly white, the intermediate colors offering every conceiv- able shade between dark blue and white. If it be found that the blue of one of these rectangles is identical with that of the sky, this identi- ty is then represented by a number corresponding to one of the rect- angles, and all that remains to be done is to arrange the scale of the instrument. Humboldt perfected this apparatus, and rendered it capable of giving very precise measurements of the blue tint. The mere contemplation of the heavens tells us that their color is not the same at every altitude, being generally deeper at the zenith, and gradually becoming lighter toward the horizon, where it is often nearly white. The contrast is rendered the more striking by the use of the cyanometer. Thus it will be found that sometimes the color corre- sponds to the number twenty-three in the neighborhood of the zenith, and to the number four near the horizon. But the color of the same part of the sky also changes pretty regularly during the day, as it be- comes darker from morning until noon, and lighter again from noon until evening. In our climates the deepest blue is when, after several days of rain, the wind drives away the clouds. The color of the sky is modified by the combination of three tints the blue, which is reflected by the aerial particles; the black of infinite space; and the white of the vesicles of mists and snow-flakes which float at the high elevations. If we rise sufficiently high in the atmos- phere, we leave a part of the vesicles of vapor below us. Thus the white rays reach the eye in a lesser proportion, and, the sky being cov- ered with fewer particles which reflect the light, its color becomes of a deeper blue. The nature of the ground also plays an important part in these effects of reflection and atmospheric transparency. In the regions where there are vast surfaces devoid of vegetation, as in a great part of Africa, the air is very dry, and loses part of its trans- parency, especially in consequence of the dust borne by the winds and the absence of heavy rain to cleanse the air. In the other parts of the intertropical zone, upon the Atlantic, on the American continent, in the THE DAT. 107 South Sea Islands, and in certain regions of India, aqueous vapor, in a state of transparent gas, is abundantly mixed with the air ; and in place of the grayish hue which it possesses in our climates and in sandy des- erts, the sky presents a strongly -marked tint of azure blue, which spe- cially characterizes the regions about the zenith, and sometimes even the sky near the horizon. The limiting surface of the atmosphere being parallel to that of the earth, and the visible portion being that only which is above the plane of the horizon, it is clear that rays of light reaching the eye in different directions have traversed different thicknesses of air. If the sun were at the zenith, its rays would pass through the thinnest stratum of air ; the nearer the sun approaches the horizon, the thicker becomes the mass of air which its rays have to pierce, and consequently the weaker its rays become. The light of the sun at its meridian passage is dazzling, whereas we can look at it with the naked eye when near the horizon ; and for the same reason the regions situated near the horizon seem always to be without stars. The color of the sky is thus explained by the reflection of light upon the molecules of the vapor of water which invisibly pervades the air. How are we now to explain the very perceptible shape of an ellip- tical vault which the sky presents, whether cloudy or entirely clear? This may be explained as a simple effect of perspective. I will suppose we have before us an avenue of poplars, all of the same height. Every one knows that this height will apparently de- crease with distance, and that the top of the trees at the extreme end of the avenue will appear to be at the height of our eyes. The trees' roots are upon a horizontal surface, because we ourselves are upon the ground. It is by the top line that the inclination toward the ground operates. If we were in the upper branches of the nearest tree, then it would be from below that the perspective inclination would operate. The same train of reasoning may be applied to the clouds. Starting from those which are vertically above our heads, they succes- sively decline in height according to their distances above the horizon. When we are above the clouds in a balloon, they no longer seem to sink toward the earth like a vault, but to extend like the plane surface of an immense ocean of snow. When but a few miles above them, they describe a curve in the contrary direction.* * [Having been led theoretically to expect such a phenomenon, I always, when some miles above the clouds, attentively looked for its appearance, and invariably without success. It 10g THE ATMOSPHERE. With a clear sky, the surface of the earth, seen from a great height, is hollow underneath the car of a balloon, and gradually rises around up to the circular horizon. Far from appearing convex, as might be ex- pected if one imagined that at a great height in the atmosphere the spherical shape of the globe would be recognized, the surface of the ground is hollowed out underneath us, rising till it reaches the horizon, which seems always to be on a level with the eye. This aspect of the earth, hollowed out like a basin, surprised me very much the first time I saw it from a balloon, for at the height which I had attained I had expected to see it convex. Thus the sinking of the apparent vault of the sky above our heads is due to an effect of perspective, as we can not estimate vertical heights in the same way as horizontal lengths. A tree forty-five feet high seems much longer on the ground than when standing. A tower three hundred feet high would appear far more if laid along the ground than when vertical. Being in the habit of walking along the ground, and not of soaring into the air, we appreciate lengths at their true estimate, whereas heights are beyond our powers of direct judgment. It results from the apparent shape of the celestial vault that the con- stellations seem to us much larger toward the horizon than at the ze- nith (as, for instance, the Great Bear when it skirts the horizon, and Orion when he rises), and that the sun and the moon appear to have larger disks at their rising and setting than at their culminating points. It further results that we are constantly in error in estimating the height of stars above the horizon. A star which is at 45 of altitude that is, just half-way between the horizon and the zenith seems to us much higher ; and when we point out a star as being at 45, it may happen that it is only at 30.* Modern treatises on physics and meteorology have not gone into this is true that, the dip of the horizon being very small, objects on the horizon practically ap- pear to be on the same level as the eye, while the ground underneath of course seems far be- low, so that, in this sense, the appearance of the earth is cup-shaped. But, in point of fact, if the day be clear, the distance of the horizon is so much greater than is that of the ground below, that the effect is no more noticeable than it is from the top of a hill. If the air be not clear, all traces of the appearance are of course absent. ED.] * [Most people imagine they are looking at the zenith when they are looking at a point 10 or 20 below it, and on this account their estimates of heights are too great. As regards the shape of the celestial sphere, it may be remarked that the distance to the horizon would appear greater than to the zenith, if it were only because of the intervening objects which occur in the former case; while, looking upward, there is nothing to aid the eye in its estimation. ED.] THE DAT. 109 curious question of the aspect of the sky. I find it discussed in certain works of the seventeenth and eighteenth centuries, but rather from a philosophical point of view than in its purely geometrical aspect. Af- ter a long dispute between Mallebranche and Eegis upon this point, Eobert Smith examined it in his "Optics" (1728), and concluded that the horizontal diameter of the celestial vault must seem to us six times as long as the vertical diameter. He is of opinion that this is due to the fact that " our view does not extend distinctly to the point at which the objects form an angle of the 8000th part of an inch in our eye, so that all objects seem to us to sink under the horizon at a distance of 25,000 yards." The mathematician Euler, in his "Letters to a German Princess" (1762), devotes several chapters to an explanation of it, which may be stated in a few words. First, the light of the stars which are near the horizon is much weakened, because their rays have a greater distance to travel through our lower atmosphere than those which are at a great- er height; secondly, being less luminous, we deem them to be farther off, because we always take the objects which are most clear to be near- est to us (for instance, a conflagration at night seems much closer to us than it really is) ; thirdly, this apparent distance of the celestial objects which are near the horizon gives rise to the imaginary elliptic vault of the heavens. The logical arrangement of these last two points seems the inverse of the theory explained above, yet it may be seen that these two facts do not follow the one from the other, but are simultaneous in our ob- servation. Perspective is due to the distance and to the diminution in brightness, and it gives a clear explanation of the apparent shape pre- sented by the atmospheric strata, and the variation in size according to the elevation above the horizon. There is, so to speak, a double effect of geometrical and luminous perspective. We do not appreciate the beauty or the practical importance of the diffusion of light by the air, because it is always present to us. A so- journ of a few hours in our neighbor the moon would suffice to show us the enormous difference there is between an atmospheric day and one without air. As Biot remarked, in a very correct simile, the air is around the earth a sort of brilliant veil, which multiplies and disperses the sunlight by an infinity of repercussions. It is to it that we owe the light which we enjoy when the sun is below the horizon. After the latter has risen HO THE ATMOSPHERE. there is no spot so secluded, provided the air has access to it, which does not receive some light, although the sun's rajs may not reach it directly. If the atmosphere did not exist, each point of the terrestrial surface would only receive the light reaching it directly from the sun. The strange effect of the absence of the atmosphere would be far more complete and striking if we had the power of transporting our- selves into our satellite. Let us compare the cheerful spectacle that the earth presents, partly covered with its humid and wavy mantle, and decked with flowers, to the aspect of the moon, with its stony or metallic surface, abounding with crevasses and vast mountainous des- erts, with its extinct volcanoes and peaks that seem like gigantic tombs, with its sky invariably black and shapeless, in which reign, day and night, stars without scintillation, the sun and the earth. There day- time is, so to speak, nothing but night lighted up by a rayless sun. No dawn in the morning, no twilight in the evening. The nights are pitch-dark. Those parts of the lunar hemisphere which are toward us are lighted by an earth-light, the first quarter of which coincides with sunset, the full earth with midnight, and the new earth with sunrise.* In day-time the solar rays are lost against the jagged ridges, the sharp points of the rocks, or the steep sides of their abysses, designing here and there grotesque shapes against the angular contours, and only strik- ing the surfaces exposed to their action to become at once reflected and lose themselves in space fantastic shadows standing out in the midst of a sepulchral world. Fig. 28 represents a landscape taken in the moon, in the centre of the mountainous region of Aristarchus. There is nothing but white and black. The rocks reflect passively the light of the sun ; the craters remain partially wrapped in shade ; fantastic steeples seem to stand out like phantoms in this glacial cemetery ; the absence of the atmosphere leaves the black space of the starry heaven perpetually hanging over this dismal region, to which, fortunately, the earth can offer no sort of analogy. * [The moon always turns the same face to the earth ; so that there is one-half of the moon's surface that has never been seen from the earth. The words one-half must not be taken quite literally, as, owing to a slight oscillatory motion of the moon, called libration, we sometimes see a little more round the comer, as it were, than at other times. Speaking generally, therefore, an inhabitant of the moon, if he saw the earth at all (f. e., was on the hemisphere turned toward us), would always see it in the same position in the sky (and in size about four times as large as the moon appears to us). The statement in the text is only true for a spectator placed at the middle point of the visible hemisphere of the moon ; the lunar day is of course about four weeks. ED.] Fig. 23. Lunar Day. EVENING. CHAPTER II. * EVENING. LIGHT, that imponderable agent which enables us to see objects, and which by its qualities illuminates the magnificent atmospheric world in which we live, gives rise to an ever-changing series of effects. The at- mosphere not only bathes the landscape with light by reflection, but also decomposes it by refraction, and gives additional variety to the beauties of the earth and sky. When a ray of light passes from one transparent medium to another, it undergoes a deviation caused by the difference of density of the two media.* In passing from air to water the ray is bent toward the ver- tical, because water is denser than air. It is the same with a ray which passes from a higher to a lower stratum of air, for, as we have seen, the lower strata are denser than those above. If a ray of common light be admitted through a small hole in a dark- ened room, and, after passing through a glass prism, be received on a screen, it will be seen that the ray of white light has been decomposed by refraction through the prism into seven colors violet, indigo, brown, green, yellow, orange, red which occupy different positions, in the above order, on the screen. The red rays, being the least bent from the direction of the original ray, are said to be least refrangible, and the violet rays, which form the other end of the spectrum, are said to be most refrangible. In refracting light the air produces two distinct effects. On the one hand, it causes a ray of light which has its origin beyond the earth's at- * [M. Flammarion here adds the sentence, "A stick plunged into water appears bent at the surface of the liquid, and the immersed portion appears more nearly vertical." As this illus- tration of the effect of refraction is given in many popular works, I think it worth while to point .out its inaccuracy. A ray of light entering a denser fluid (the surface of which is hori- zontal) is bent nearer to the vertical ; but a stick is not a ray of light, and in no way resembles one. The immersed portion of the stick is seen by rays that have been refracted at the surface of the water ; and it easily follows, from the principles of optics, that the part under water ap- pears bent from (not toward) the vertical. This any one can verify for himself experimentally. The sentence quoted above is therefore not only erroneous in theory, but also incorrect in fact. The apparent bending of the stick is only indirectly due to refraction. ED.] H4. THE ATMOSPHERE. mosphere to become bent as it approaches the earth, so that we see the sun, moon, planets, comets, and the stars, as if they were higher in the heavens than they really are. On the other hand, it causes a more or less considerable separation between the various rays that constitute white light, according to its state of transparency and density. The first effect mainly produces twilight ; the second gives that soft, undulating beauty which is seen in the serenity of the evening. Eefraction is greater or less, in proportion as the luminous ray trav- erses the atmosphere in a direction more or less inclined to the ver- tical, being greatest for horizontal and vanishing for vertical rays. As- tronomical observations would all be false with regard to the positions of objects if they were not corrected for the effect of refraction. Thus, Fig. 29. Atmospheric refraction. for instance, the star A is seen at A'; the star B at B'; at the zenith alone stars are where they appear to be, there being no alteration in the direction of the ray of light due to refraction. To make these neces- sary corrections, tables have been constructed giving refractions, based upon the hypothesis of a uniform disposition of the different strata of air lying one above the other. The refracting power of the air is determined on the hypothesis that it contains only oxygen and nitro- gen ; but we have seen that it further contains from four to six parts in 10,000 of carbonic acid, and an ever-varying quantity of the vapor of water. The refracting power of the vapor of water differs so little from that of air properly so called, that the correction depending on it need not, as a rule, be taken into the calculation. To calculate the amount of correction to be applied to any observa- EVENING. 115 tion, it is only necessary to note at the time the temperature of the air and the pressure of the atmosphere at the place of observation. To illustrate the effect of refraction, I have selected from a table of refractions a few numbers, at different zenith distances. They show to what extent objects are apparently raised by its influence : TABLE OF EEFRACTIONS. Distances from the Zeuith. Refractions. Distances from the Zenith. Refractions. 90 deg. 33 min. 47 sec. 74 deg. 3 min. 20 sec. 89 24 22 72 2 57 88 18 23 70 2 38 87 14 28 65 2 4 86 11 48 60 1 40 85 9 54 1 55 1 23 84 8 30 50 1 9 83 7 25 45 58 82 6 34 40 * 48 81 5 53 30 33 80 5 20 20 21 78 4 28 10 10 76 3 50 From this table we see that an object situated just upon the horizon is raised by more than 33', or about -j-^j- of the distance from the hori- zon to the zenith. Neither the sun nor the moon is 33' in diameter. When, therefore, they appear to have just risen, they are still entirely below the horizon. In the same way, the sun does not appear to begin to set until after sunset has actually taken place. It follows from the.se considerations that the sun may be seen in the west and the moon in the east at the time of full moon, and even an eclipse of the moon may be visible while the sun is still above the hori- zon, although the earth is then exactly between the two luminaries, and the latter are both, astronomically speaking, below the horizon. This is due to refraction. This curious circumstance was noted during eclipses of the moon on June 16, 1666, and May 26, 1668. Owing to the same cause, the sun and the moon seem to be flattened both at their rising and setting, the rays proceeding from the lower edge of the luminary being more refracted than those proceeding from the upper, so that the apparent vertical diameter is diminished, while the horizontal diameter remains, of course, unaltered. The length of the day is thus increased, and that of the night decreased. It is for this rea- son that at Paris the longest day of the year is sixteen hours seven min- utes, and the shortest eight hours eleven minutes, instead of being fif- -Qg THE ATMOSPHERE. teen hours fifty-eight minutes, and eight hours two minutes. We see that the length of the day at Paris at the time of the solstices is thus prolonged by nine minutes, and by seven minutes at the equinoxes. At the North Pole the sun seems to be in the horizon, not when it ar- rives at the spring equinox, nor when its angular distance from the North Pole is 90, but when it is 90 33' ; it then remains visible until, having passed to the autumnal equinox, its polar distance has again be- come equal to 90 33'. Care must always be taken to keep account of refraction in calculating the hours of sunrise and sunset. Twilight is that light which remains after the sun has set or which is seen before sunrise. The duration of twilight is, in many respects, a useful element to be acquainted with. It depends chiefly upon the an- gle to which the sun has descended below the horizon ; but it is modi- fied by several circumstances, the chief of which is the degree of clear- ness of the atmosphere. The direct light of the sun at the time of sun- set reaches to the west ; as the sun sinks, its boundary-line rises, and some little time afterward crosses the zenith, when civil twilight ends; the planets and large stars then become visible to the naked eye. The eastern half of the sky being thus first deprived of direct solar light, night begins there. Afterward, the boundary -line (the crepuscular curve) itself disappears in the west; then the astronomical twilight ceases and night has fully set in. Twilight begins or ends when the sun is at a certain distance below the horizon ; this distance is variable, depending upon the state of the atmosphere. It may be taken that civil twilight ends when the sun is about 8 below the horizon, and that as- tronomical twilight ends when the sun is about 18 below the horizon. The phenomena of twilight are hardly known in tropical climates ; as soon as the sun has descended below the horizon, darkness sets in sud- denly. This was remarked by Bruce at Senegal, where, however, the air is so transparent that Yenus may sometimes be distinguished at mid- day, and in the interior of Africa night succeeds day almost immedi- ately after sunset. At Cumana, Humboldt tells us, twilight lasts but a very few minutes, although the atmosphere is higher under the tropics than in other regions. The following tables give the length of the civil and astronomical twilight in France for the various seasons and for the fifteenth day of each month. By adding the duration of twilight to the hour of sunset, the time at which each of the twilights terminates is readily obtained, and subtracting it from the hour of sunrise, the times of their com- EVENING'. 117 mencement are found. France, from the Pyrenees to Dunkirk, is with- in the 41st and 42d degrees of latitude. It will be seen that, even within these trifling limits, there is a perceptible difference. The short- est civil twilights take place on the 29th of September and the 15th of March, the longest on the 21st of June ; the shortest astronomical twi- lights fall upon the 7th of October and the 6th of March, the longest on the 21st of June. North of 50 latitude, the astronomical twilight con- tinues all night for some time both before and after the summer solstice. TABLE OF THE LENGTHS OF THE LONGEST AND SHORTEST DAYS. Latitude. Length of the Day. The longest : June 21. The shortest : . December 21. 42 degrees. 44 15 hrs. 13 min. 15 " 28 " 9 hrs. 8 " 47 min. 46 15 " 44 " 8 " 30 " 48 " 16 " 2 " 8 " 14 " 50 " 16 " 24 " 7 " 55 " TABLE OF THE DURATION OF CIVIL TWILIGHT. Month. Latitude. 42 deg. 44 deg. 46 d< 36 m 34 33 34 38 41 39 36 33 33 35 37 g. 48 deg. 60 drg. 34m 32 31 32 35 37 36 33 31 31 33 34 n. 35 min. 33 " 32 " 33 " 36 ' 39 ' 38 ' 34 ' 32 ' 32 " 34 " 36 " in. < ! 38m 35 34 36 40 44 42 37 34 35 37 39 n. 40m 37 35 36 42 46 44 39 36 36 39 41 n. February March April May June Julv August October November TABLE OF THE DURATION OF ASTRONOMICAL TWILIGHT. Latitude. 42 deg. 44 deg. 46 cleg. 48 deg. 50 deg. January 31 1 33 1 36 1 40 1 45 February March 24 24 1 26 1 26 1 29 1 29 1 32 1 33 1 36 1 37 April . . 33 1 35 1 39 1 44 1 50 May 46 1 52 2 1 2 11 2 26 June July 56 48 2 5 1 54 2 19 2 4 2 36 2 14 3 13 2 31 qt> 1 37 42 47 54 September .... 24 1 26 30 34 38 October 23 1 25 29 33 36 November 30 1 32 39 43 December 34 1 36 40 45 50 -Qg THE ATMOSPHERE. In warm countries, the presence of humidity in the air not only gives to the sky its dark azure tint, but also has the effect of modifying the vital power of the solar rays. At the equator it adds to the thousand other wonders of nature an incomparably beautiful display of light both at sunrise and sunset The sunset, in particular, affords a spectacle indescribably magnificent a superiority over sunrise attributable to the presence of moisture in the air. This is more abundant in the evening, after the heat of the da} 7 , than in the morning, when it is par- tially condensed into dew by the effect of the cooler temperature of night. It is not in our climate that the finest sunsets are seen. The celestial blue of distant mountains, the rose or violet tints which in turn tinge the nearer hills, and the warm tones of the soil, harmonize in a mar- velous manner, when the sun disappears below the horizon, with the gleaming gold of the west, the red or roseate tints that crown it in the sky, the dark azure of the zenith, and the more sombre and often, in contrast to the others, greenish hue which prevails in the east. In the equinoctial regions, these soft and delicate tints, joined to the varied as- pect of the earth's configuration and the richness of vegetation, produce more striking effects than with us. Sometimes light and roseate clouds, fringed with a coppery red, produce peculiar effects, similar to certain sunsets in our regions ; but whenever the sky is clear the shades differ entirely from those of the temperate zone, and present a special charac- ter. Sometimes, too, the indentations of mountains situated below the horizon, or invisible clouds intercepting a part of the solar rays which, after sunset, still reach the elevated regions of the atmosphere, give rise to the curious phenomenon of crepuscular rays. Then may be seen, starting from the point where the sun has disappeared, a series of rays, or rather of diverging "glories," which sometimes extend as far as 90, and even in some instances are prolonged as far as the point opposite to the sun. "Upon the ocean," as M. Liais remarks, "when the sky, near the equator, is free from cloud in the visible part, and when the diverging rays mingle with the crepuscular arcs, the play of light as- sumes a form and brilliance which defy all description or pictorial illus- tration. How, indeed, is it possible to depict completely the rosy tints of the arc fringed by the crepuscular rays that border the segment which is still strongly lighted up from the west, the segment itself being tinged with a bright gold hue? How, above all, is it possible to de- scribe the tint of an inimitable blue, different from that of noonday, and EVENING. H9 occupying that portion of the sky which is included between the ordi- nary azure and the crepuscular arc? " To all this splendor of the western sky must be added the descrip- tion of its fires as reflected upon the surface of the waters agitated by the trade-wind, the dark blue color of the sea to the east, the white foam of the wave, which sharply defines upon this gloomy background the pale roseate arc of the eastern sky, and the sombre and greenish seg- ment of the horizon." What spectacle can be more sublime than a sunset at sea? We have attempted in the illustration to recall this beautiful spectacle. The col- ored clouds which float in this western sky are cirro-cumuli, which will be described in the chapter upon the Clouds. The setting sun is nearly always accompanied by these cirro-cumuli clouds, which serve to display those aspects of the sky which are of so remarkable a beauty in the west. In consequence of the curvature of the earth, sea-clouds which are sometimes seen from Paris are more than two miles above the ocean, and are formed of ice and snow, even in the month of July. These are nearly the highest clouds, and pro- duce the varied forms of mountains, fishes, animals, and other fantastic shapes, which one may discern of an evening upon a bright and rich ground of every tint that light can give. To the preceding remarks may be added one of a more general and curious nature, in reference to the influence of the evening light in the construction of cities. Towns grow in a westward direction. Paris, the cradle of which was the lie de la Cite, has, in its successive aggran- dizements, constantly extended toward the west. Two thousand years ago, Paris was situated on the north-east slope of Mount St. Genevi&ve, where the arenas have recently been discovered. Under the Merovin- gians it commenced its descent toward the west, and has unceasingly progressed in that direction ever since. The wealthy classes have a pronounced tendency to emigrate westward, leaving the eastern districts for the laboring populations. This remark applies not only to Paris, but to most great cities London, Vienna, Berlin, St. Petersburg, Turin, Liege, Toulouse, Montpellier, Caen, and even Pompeii. Whence arises this tendency ? A fact so universal can not be due to accident. Is it the stream of the Seine which has taken Paris westward in its wake? Not so, for the Thames flows in a contrary direction, while London has none the less extended to the west like Paris. Twelve years ago, Doctor Junod (Comptes-Rendus of the Academy of Sciences 12Q THE ATMOSPHERE. in 1858) offered, as an explanation of this fact, the statement that the east wind is that which raises in the greatest degree the barometrical column, while the west wind lowers it the most, and therefore inundates the eastern part of a town with deleterious gases, so that the latter has to put up not only with its own smoke and miasmas, but also with those coming from the western portion. It may, in fact, be admitted that people prefer going where fresh air is to be found, and in the direction from which the wind blows most frequently. But the wind is not the same in all countries. For my own part, I am more inclined to see in this fact an evidence of the attraction of light. And the suggestion is an extremely simple one. It may be re- marked that people, as a rule, take their promenade of an evening, and not of a morning, and always, or nearly always, in the direction of sun- set. This disposition has led to the formation of gardens, country houses, and places of public resort, and, little by little, the wealthy pop- ulation of a large city extends in this direction- 1'- X .-; THE RAINBOW. CHAPTER III. THE RAINBOW. THE general action of light in nature is always evident to our eyes ; its effects in the atmosphere are of very different kinds, and produce a thousand optical phenomena, always curious, often fantastic, but all capable of explanation in these days by physical laws. We shall de- vote the following chapters to the examination of the phenomena that are due to this agent, at once so powerful and so delicate. The most common of these phenomena is the rainbow, and the ex- planation of it will aid us in understanding the others. There are few persons who have not remarked in water falling from a fountain or cascade the production of a miniature rainbow analogous to the majestic arch which crosses the sky. Whenever these small rainbows are seen, three circumstances will be observed in connection with them: first, that drops of water must be present; secondly, that the sun must be shining ; and, thirdly, that the observer must be be- tween the sun and the water. These three conditions in regard to the production of the rainbow will explain the phenomenon in which the Jewish religion saw the presence of Jehovah, and the Greek mythology the auspicious influence of the goddess Iris. In order to see a rainbow as a result of the action of light, whether on artificial rain or on the drops of rain falling from the clouds in the atmosphere, the spectator's back must be to the sun. In this position, the solar rays which shine upon the drops of water are reflected and refracted as follows : Let us suppose a drop of water, A 1 1', in the atmosphere. A solar ray reaches this drop at I, and pass- es into it, being deflected from a Straight line by refraction, Fig. 30.-Simple reflection of rays in a drop of rain. 122 THE ATMOSPHERE. inasmuch as the ray passes into a medium of different density. Arriv- in at A on the surface of the small sphere of liquid which constitutes the drop, it is reflected and returns in the direction of A i', being refract- ed on emergence into the direction i' M. This ray, so decomposed by refraction, presents all the colors ar- ranged in regular order, as each color possesses a different degree of re- frangibility. The inclination increases from red to violet; that is to say, that if the red ray from a particular drop reaches the eye, the other ravs proceeding from the same drop can not reach it too; but a drop at a less elevation in the air can send a violet ray which will be visible at the same time. Thus, the observer sees, in the direction of these drops, a red hue above and a violet hue below. The intermediate drops simi- larly emit rays which, when seen by the eye, are of the colors included between red and violet, forming a solar spectrum, the colors of which, starting from the lowest arc, are v iolet, indigo, blue, green, yellow, orange, red. Let us now imagine a conical surface passing through the drop, and having for axis the straight line drawn from the eye of the observer to the sun. Every drop of water which is upon this surface of the cone produces the same effect, so that there is a mass of spectra forming a circular band, in which the simple colors succeed each other in the or- der indicated, the violet, a (see Fig. 33, p. 124), being inside, and the red, b, outside. The phenomenon continues as long as the drops of water go on fall- ing in the same region of space, the luminous appearance being inces- santly renewed by the falling of the drops, so that the arch appears per- manent while the rain lasts. Calculation has shown that the angle of the cone of the red rays is 42 20', and that of the violet rays 40 30'. This is, therefore, the dis- tance from the arc to the centre or the point of the sky on which the shadow of the head of the spectator, p (see Fig. 33), would be cast. The diameter, H H' (see Fig. 33), of the whole arc subtends an angle of about 84, the width of the arc being 2, or nearly four times the apparent diameter of the sun. The rainbow, therefore, demonstrates the existence of small spheres of liquid water, falling as rain in the midst of the atmosphere. The arch is more brilliant as their size increases. They must be much larger than those which form the clouds for the eye to be able to distin- guish the colors, and that is the reason why mists and clouds do not THE RAINBOW. 123 produce any rainbow. Knowing that the rainbow is caused by the re- fraction of the sun's rays through drops of rain as they fall, we may de- duce therefrom not only the size of this arch, but also the conditions without which it could not exist. If the sun were on the horizon, the shadow of the spectator's head would be cast there also, and as the axis of the cone would be horizontal, it follows that we should see a semi- circle of an apparent radius of 41. ' As the sun rises, the axis of the cone is inclined, and the arch becomes smaller; and finally, when the sun reaches a height of 41, the axis of the cone forms the same angle with the plane of the horizon, and the top of the arch just touches this Fig. 31. Formation of the rainbow. latter plane. If the sun were still higher, the arch would be projected upon the ground. The phenomenon is rarely visible under this last con- dition. The secondary rainbow, of which I am about to speak, disap- pears when the sun reaches an altitude of 52, for which reason a rain- bow can not be seen at noon in summer. The observer standing upon the earth can, therefore, never see more than half a circumference (viz., when the sun is on the horizon) ; and, as a rule, the arch is only 100 to 150 in length. When the earth does not stand in the way of the production of the lower part, more than a semi-circumference, and even a whole circumference, may be seen. This occurred to me once in a 124: THE ATMOSPHERE. balloon ; and by a curious coincidence (the upper part being concealed), I saw a rainbow upside down, in which the violet color was inside. A second arch, in which the colors appear in an inverse order to those in the rainbow described above, is frequently remarked. This second arch is explained by a double reflection, s I A B i' M (see Fig. 32) and sVo, s'b'o (see Fig. Fig. 32.-Donble reflection of rays in a drop of rain. g3) j n ^ ^^ the deyiations of the rays after they emerge from the liquid sphere are 51 for the red rays, and 54 for the violet rays. This secondary arch is always paler than the first The zone comprised between the principal and the secondary arch is generally darker than the rest of the sky, and appears to me, after nu- merous observations, to be a region of absorption for the luminous rays. It is ascertained that a larger number of reflections may be produced, and that other arches, more and more pale in hue, may exist. But the diffused light prevents them from being seen. How- ever, a third has been seen, at 40 from the sun. By causing the solar rays to fall upon a jet of water in a dark place, as many as seventeen rainbows have been counted. It may happen that the sun is reflected toward a cloud by the surface of a piece of still water; and then this reflection will also give rise to a rainbow. It has been found that in this case the rain- bow must cut the arch formed by the direct rays at a height dependent upon that of the sun. If the two phenomena produce a secondary arch, the four curves intertwined form a very beautiful spectacle. A case in Fig. 33 Theory of the two arches of a rainbow. THE RAINBOW. 125 which they were quite complete and perfectly distinct is cited by Monge. Halley observed three arches, one of which was formed by the rays reflected upon a river. This arch first intersected the exterior arch so as to divide it into three equal parts. When the sun sunk to- ward the horizon, the points of meeting were drawn close together. There soon was seen but one single arch, and as the colors were in in- verse order, pure white was formed by the superposition of the two se- ries. The sun, too, may produce, after being reflected upon a piece of water, a complete circle, the upper part of which being sometimes invis- Pig. 34. Triple rainbow. ible, gives rise to the singular phenomenon of a rainbow upside down. The Academicians dispatched to the polar regions to measure an arc of the meridian, observed upon the Ketima Mountain, on July 17, 1736, a triple rainbow analogous to that of which Halley speaks. In the lower bow the violet was underneath, the red outside as usual : this was the principal arch. The second, which was parallel to it, was the secondary arch. In this the red was underneath and the violet at the top. The third arch, starting from the extremities of the first, crossed the second, -^26 THE ATMOSPHERE. and had, like the principal one, the violet inside and the red outside. This is the phenomenon drawn in Fig. 34. Seeing, then, that the rainbow is due to the refraction and reflection of the solar rays upon little drops of water falling in the air, it is easy to conceive that moonlight may cause an analogous appearance, though less intense ; and this indeed is the case, though a lunar rainbow is not very common. The illustration represents a lunar rainbow which I had an opportunity of remarking one spring evening at Compiegne. Many observers have remarked and described this nocturnal rain- bow. I gather from the writings of Americ Vespuce (1501) that he had several times observed "the iris at night." He considers that the red of the arch is due to fire, the green to the earth, the white to the air, and the blue to the water; and, he adds, "this sign will cease to ap- pear when the elements are used up, forty years before the end of the world." I notice in an ancient treatise on meteorology (that of P. Cotte) that, in addition to the ordinary rainbow, the secondary rainbow, the reflect- ed arches, and the lunar rainbow, there has been mentioned yet another optical effect, called the " marine rainbow," formed upon the surface of the sea, and composed of a large number of zones. It sometimes ap- pears upon wet meadows lying opposite to the sun. This fifth aspect is a kind of anthelion, which I will allude to in the next chapter. The name of "white rainbow" has also been given to the anthelical circle, which will also be considered in the same chapter. Lastly, there are sometimes seen colored bands below the violet of the ordinary rainbow, which appear to belong to an arch lying over the first. This arch then takes the name of supernumerary arch, and is due to very complex effects of interference of light, explainable on the un- dulatory theory. The first person who attempted to explain the phenomenon of the rainbow by the reflection of light upon the.interior of the drops of rain was a German monk of the name of Theodoric ; the second an arch- bishop, A. De Dominis (1611). But the true theory was first given by Descartes, with the exception of the separation of colors, which was only determined by the discovery of Newton as to the unequal refrangibility of the rays of the solar spectrum. A Mori* piavr' chromolitk' LUNAR RAINBOW SEEN' AT COMPiEGNE ANTHELIA. 127 CHAPTER IV. ANTHELIA: SPECTRE-SHADOWS UPON MOUNTAINS THE ULLOA CIRCLE CIRCLE SEEN FROM A BALLOON. TREATISES on meteorology have not, up to the present day, classified with sufficient regularity the diverse optical phenomena of the air. Some of these phenomena have, however, been seen but rarely, and have not been sufficiently studied to admit of their classification. We have examined the common phenomenon of the rainbow, and we have seen that it is due to the refraction and reflection of light on drops of water, and that it is seen upon the opposite side of the sky to the sun in day-time or the moon at night. We are now about to consider an order of phenomena which are of rarer occurrence, but which have this prop- erty in common with the rainbow, viz., that they take place also upon the side of the sky opposite to the sun. These different optical effects are classed together under the name of anihelia (from avOt, opposite to, and rjAioc, the sun). The optical phenomena which occur on the same side as, or around the sun, such as halos, parhelia, etc., will form the subject of the next chapter. Before coming to the anthelia, properly so called, or to the colored rings which appear around a shadow, it is as well first to note the effects produced on the clouds and mists that are facing the sun when it rises or sets. Upon high mountains, the shadow of the mountain is often seen thrown either upon the surface of the lower mists or upon the neigh- boring mountains, and projected opposite to the sun almost horizon- tally. I once saw the shadow of the Righi very distinctly traced upon Mount Pilate, which is situated to the west of the Righi, on the other side of the Lake of Lucerne. This phenomenon occurs a few minutes after sunrise, and the triangular form of Righi is delineated in a shape very easy to recognize. The shadow of Mont Blanc is discerned more easily at sunset. MM. Bravais and Martins, in one of their scientific ascents, noticed it under specially favorable circumstances, the shadow being thrown upon the snow-covered mountains, and gradually rising in the atmosphere until it J28 THE ATMOSPHERE. reached a height of 1, still remaining quite visible. The air above the cone of the shadow was tinted with that rosy purple which is seen, in a fine sunset, coloring the lofty peaks. "Imagine," says Bravais, "the other mountains also projecting, at the same moment, their shadows into the atmosphere, the lower parts dark and slightly greenish, and above each of these shadows the rosy purple surface, with the deeper rose of the belt which separates it from them ; add to this the regular contour of the cones of the shadow, principally at their upper edge, and lastly, the laws of perspective causing all these lines to converge the one to the other toward the very summit of the shadow of Mont Blanc ; that is to say, to the point of the sky where the shadows of our own selves were ; and even then one will have but a faint idea of the richness of the me- teorological phenomenon displayed before our eyes for a few instants. It seemed as though an invisible being was seated upon a throne sur- rounded by fire, and that angels with glittering wings were kneeling before him in adoration." Among the natural phenomena which now attract our attention, but fail to excite our surprise, there are some which possess the characteris- tics of a supernatural intervention. The names which they have re- ceived still bear witness to the terror which they once inspired ; and even to-day, when science has stripped them of their marvelous origin, and explained the causes of their production, these phenomena have re- tained a part of their primitive importance, and are welcomed by the savant with as much interest as when they were attributed to divine agency. Out of a large and very diverse number, I will first select the Spectre of the Brocken. The Brocken is the highest mountain in the picturesque Hartz chain, running through Hanover, being 3300 feet above the level of the sea. One of the best descriptions of this phenomenon is given by the trav- eller Hane, who witnessed it on the 23d of May, 1797. After having ascended no less than thirty times to the summit, he had the good for- tune at last to contemplate the object of his curiosity. The sun rose at about four o'clock, the weather being fine, and the wind driving off to the west the transparent vapors which had not yet had time to be con- densed into clouds. About a quarter past, four, Hane saw in this di- rection a human figure of enormous dimensions. A gust of wind near- ly blowing off his hat at that moment, he raised his hand to secure it, and the colossal figure imitated his action. Hane, noticing this, at once made a stooping movement, and this was also reproduced by the Fig. 35. The Spectre of the Brocken. ANTHELIA. 131 spectre. He then called another person to him, and placing themselves in the very spot where the apparition was first seen, the pair kept their eyes fixed on the Achtermanrishohe, but saw nothing. After a short interval, however, two colossal figures appeared, which repeated the gestures made by them, and then disappeared. Some few years ago, in the summer of 1862, a French artist, M. Stroobant, witnessed and carefully sketched this phenomenon, which is drawn in Fig. 35. He had slept at the inn of the Brocken, and rising at two in the morning, he repaired to the plateau upon the summit in the company of a guide. They reached the highest point just as the first glimmer of the rising sun enabled them to distinguish clearly ob- jects at a great distance. To use M. Stroobant's own words, "My guide, who had for some time appeared to be walking in search of some- thing, suddenly led me to an elevation whence I had the singular priv- ilege of contemplating for a few instants the magnificent effect of mi- rage, which is termed the Spectre of the Brocken. The appearance is most striking. A thick mist, which seemed to emerge from the clouds like an immense curtain, suddenly rose to the west of the mountain, a rainbow was formed, then certain indistinct shapes were delineated. First, the large tower of the inn was reproduced upon a gigantic scale ; after that we saw our two selves in a more vague and less exact shape, and these shadows were in each instance surrounded by the colors of the rainbow, which served as a frame to this fairy picture. Some tour- ists who were staying at the inn had seen the sun rise from their win- dows, but no one had witnessed the magnificent spectacle which- had taken place on the other side of the mountain." Sometimes these spectres are surrounded by colored concentric arcs. Since the beginning of the present century, treatises on meteorology designate, under the name of the Ulloa circle, the pale external arch which surrounds the phenomenon, and this same circle has sometimes been called the " white rainbow." But it is not formed at the same an- gular distance as the rainbow, and, although pale, it often envelops a series of interior colored arcs. Ulloa, being in company with six fellow - travelers upon the Pam- bamarca at day-break one morning, observed that the summit of the mountain was entirely covered with thick clouds, and that the sun, when it rose, dissipated them, leaving only in their stead light vapors, which it was almost impossible to distinguish. Suddenly, in the opposite direc- tion to where the sun was rising, "each of the travelers beheld, at about 132 THE ATMOSPHERE. seventy feet from where he was standing, his own image reflected in the air as in a mirror. The image was in the centre of three rainbows of different colors, and surrounded at a certain distance by a fourth bow with only one color. The inside color of each bow was carnation or red, the next shade was violet, the third yellow, the fourth straw color, the last green. All these bows were perpendicular to the horizon; they moved in the direction of, and followed, the image of the person whom they enveloped as with a glory." The most remarkable point was that, although the seven spectators were standing in a group, each Fig. 36. The Ulloa circle. person only saw the phenomenon in regard to his own person, and was disposed to disbelieve that it was repeated in respect to his companions. The extent of the bows increased continually and in proportion to the height of the sun ; at the same time their colors faded away, the spec- tres became paler and more indistinct, and finally the phenomenon dis- appeared altogether. At the first appearance the shape of the bows was oval, but toward the end they became quite circular. The same appa- rition was observed in the polar regions by Scoresby, and described by him. He states that the phenomenon appears whenever there is mist and at the same time shining sun. In the polar seas, whenever a rather ANTHELIA. ^33 thick mist rises over the ocean, an observer, placed on the mast, sees one or several circles upon the mist. These circles are concentric, and their common centre is in the straight line joining the eye of the observer to the sun, and extended from the sun toward the mist. The number of circles varies from one to five; they are particularly numerous and well colored when the sun is very brilliant and the mist thick and low. On July 23, 1821, Scoresby saw four concentric circles around his head. The colors of the first and of the second were very well defined ; those of the third, only visible at intervals, were very faint, and the fourth only showed a slight greenish tint The meteorologist Kaemtz has often observed the same fact in the Alps. Whenever his shadow was projected upon a cloud, his head ap- peared surrounded by a luminous aureola. To what action of light is this phenomenon due? Bouguer is of opinion that it must be attributed to the passage of light through icy particles. Such, also, is the opinion of De Saussure, Scoresby, and oth- er meteorologists. In regard to the mountains, as we can not assure- ourselves directly of the fact by entering into the clouds, we are reduced to conjecture. The aerostat traversing the clouds completely, and passing by the very point where the apparition is seen, affords one an opportunity of ascertaining the state of the cloud. This observation I have been able to make, and so to offer an explanation of the phenomenon.* As the balloon sails on, borne forward by the wind, its shadow trav- els either on the ground or on the clouds. This shadow is, as a rule, black, like all others; but it frequently happens that it appears alone on the surface of the ground, and thus appears luminous. Examining this shadow by the aid of a telescope, I have noticed that it is often com- posed of a dark nucleus and a penumbra of the shape of an aureola. This aureola, frequently very large in proportion to the diameter of the central nucleus, eclipses it to the naked eye, so that the whole shadow appears like a nebulous circle projected in yellow upon the green ground of the woods and meadows. I have noticed, too, that this lu- minous shadow is generally all the more strongly marked in proportion to the greater humidity of the surface of the ground. Seen upon the clouds, this shadow sometimes presents a curious as- * [The explanation of the phenomenon offered by M. Flammarion (viz., that it is due to dif- fraction) was generally recognized long previous to M. Flammarion's ascents. ED.] Ig4: THE ATMOSPHERE. peek I have often, when the balloon emerged from the clouds into the clear sky, suddenly perceived, at twenty or thirty yards' distance, a sec- ond balloon distinctly delineated, and apparently of a grayish color, against the white ground of the clouds. This phenomenon manifests itself at the moment when the sun re-appears. The smallest details of the car can be made out clearly, and our gestures are strikingly repro- duced by the shadow. On April 15, 1868, at about half- past three in the afternoon, we emerged from a stratum of clouds, when the shadow of the balloon was seen by us, surrounded by colored concentric circles, of which the car formed the centre. It was very plainly visible upon a yellowish white ground. A first circle of pale blue encompassed this ground and the car in a kind of ring. Around this ring was a second of a deeper yel- low, then a grayish red zone, and lastly, as the exterior circumference, a fourth circle, violet in hue, and imperceptibly toning down into the gray tint of the clouds. The slightest details were clearly discernible net, ropes, and instruments. Every one of our gestures was instantane- ously reproduced by the aerial spectres. The anthelion remained upon the clouds sufficiently distinct, and for a sufficiently long time, to permit of my taking a sketch in my journal and studying the physical condi- tion of the clouds upon which it was produced.* I was able to deter- mine directly the circumstances of its production. Indeed, as this brill- iant phenomenon occurred in the midst of the very clouds which I was traversing, it was easy for me to ascertain that these clouds were not formed of frozen particles. The thermometer marked 2 above zero. The hygrometer marked a maximum of humidity experienced, namely, 77 at 3770 feet, and the balloon was then at 4600 feet, where the hu- midity was only 73. It is therefore certain that this is a phenomenon of the diffraction of light simply produced by the vesicles of the mist. The name of diffraction is given to all the modifications which the lu- minous rays undergo when they come in contact with the surface of bodies. Light, under these circumstances, is subject to a sort of devia- tion, at the same time becoming decomposed, whence result those curi- ous appearances in the shadows of objects which were observed for the first time by Grimaldi and Newton. The most interesting phenomena of diffraction are those presented by * A colored illustration of this remarkable phenomenon is given in the Voyages Aeriens, which was published by MM. Glaisher, De Fonvielle, and G. Tissandier, in conjunction with myself, part 2, p. 292. ANTHELIA. 135 gratings, as. are technically denominated the systems of linear and very narrow openings situated parallel to one another and at very small in- tervals. A system of this kind may be realized by tracing with a dia- mond, for instance, on a pane of glass equidistant lines very close to- gether. As the light would be able to pass in the interstices between the strokes, whereas it would be stopped in the points corresponding to those where the glass was not smooth, there is, in reality, an effect pro- duced as if there were a series of openings very near to each other. A hundred strokes, about -^ of an inch in length, may thus be drawn without difficulty. The light is then decomposed in spectra, each over- lapping the other. It is a phenomenon of this kind which is seen when we look into the light with the eye half closed ; the eyelashes, in this case, acting as a net-work or grating. These net-works may also be produced by reflection, and it is to this circumstance that are due the brilliant colors observed when a pencil of luminous rays is reflected on a metallic surface regularly striated. To the phenomena of gratings must be attributed, too, the colors, oft- en so brilliant, to be seen in mother-of-pearl. This substance is of a laminated structure ; so much so, that in carving it the different folds are often cut in such a way as to form a regular net- work upon the sur- face. It is, again, to a phenomenon of this sort that are due the rain- bow hues seen in the feathers of certain birds, and sometimes in spiders' webs. The latter, although very fine, are not simple, for they are com- posed of a large number of pieces joined together by a viscous sub- stance, and thus constitute a kind of net-work. If the sun is near the horizon, and the shadow of the observer falls upon the grass, upon a field of corn, or other surface covered with dew, there is visible an aureola, the light of which is especially bright about the head, but which diminishes from below the middle of the body. This light is due to the reflection of light by the moist stubble .and the drops of due. It is brighter about the head, because the blades that are near where the shadow of the head falls expose to it all that part of them which is lighted up, whereas those farther off expose not only the part which is lighted up, but other parts which are not, and this dimin- ishes the brightness in proportion as their distance from the head in- creases. The phenomenon is seen whenever there is simultaneously mist and sun. This fact is easily verified upon a mountain. As soon as the shadow of the mountaineer is projected upon a mist, his head gives rise to a shadow surrounded by a luminous aureola. 136 THE ATMOSPHERE. The Illustrated London News of July 8, 1871, illustrates one of these apparitions, " The Fog Bow, seen from the Matterhorn," observed by E. Whymper in this celebrated region of the Alps. The observation was taken just after the catastrophe of July 14, 1865 ; and by a curious co- incidence, two immense white aerial crosses projected into the interior of the external arc. These two crosses were no doubt formed by the intersection of circles, the remaining parts of which were invisible. The apparition was of a grand and solemn character, further increased by the silence of the fathomless abyss into which the four ill-fated tourists had just been precipitated. Other optical appearances of an analogous kind are manifested under different conditions. Thus, for instance, if any one, turning his back to the sun, looks into water, he will perceive the shadow of his head, but always very much deformed. At the same time he will see starting from this shadow what seem to be luminous -bodies, which dart their rays in all directions with inconceivable rapidity, and to a great dis- tance. These luminous appearances these aureola rays have, in ad- dition to the darting movement, a rapid rotatory movement around the head. SALOS. 137 CHAPTER Y. HALOS : PARHELIA PARASELENES CIRCLES SURROUNDING AND TRAV- ERSING THE SUN CORONAS COLUMNS VARIOUS PHENOMENA. THE description of optical phenomena now brings us to one of the most singular and complicated effects of the reflection of light in the at- mosphere. Under the name of halo (aXwg, area) is designated a brill- iant circle which, under certain atmospheric conditions, surrounds the sun at a distance of 22 or 46 ; while, under the name of parhelia, or mock suns (jrapa, near, and rjXtoc, sun), are designated luminous circu- lar spaces, generally of a red, yellow, or greenish color, which appear both to the right and to the left of the sun, at the same distance (viz., about 22), bearing a sort of rough resemblance to the sun itself. The same appearances may be seen about the moon ; and it is, indeed, easier to observe them, as the diminished brilliancy of the moon's light ren- ders an examination of the area around it less difficult. These lumi- nous spaces are called paraselenes (irapa, near, and