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CORNELL UNIVERSITY LIBRARY THIS BOOK IS ONE OF A COLLECTION MADE BY BENNO LOEWY I854-I9I9 AND BEQUEATHED TO CORNELL UNIVERSITY Ebysicaf Sc«encK^ Jbraty CHART OF SPEC 2Ki 3Ni 4 Li 5Ca 6Sr 7Ba 8T1 9 In lOStrhis MeWs 12. H 13. N 14M&S Haat-art Hth. ASTRONOMY BY J. RAMBOSSON, LAUREATE OF THE INSTITUTE OF FRANCE (ACADEMIC FRAH(;AISE AND ACAD^MIE BES SCIElTkES). TRANSLATED BY C. /fe. PITMAN, WITH NUMEROUS ILLUSTRATIONS. ilonKoit : CHATTO AND WINDUS, PICCADILLY. 1878. LONDON : BRADBURY, ACNEW, Al CO., PUINTERS, WHITEFRIARS. PKEFACE. The late Father Gratry, Member of the French Academy, once said : " It is astonishing how ignorant the great bulk of the public are in regard to astro- nomy. I have known men of education maintain that the ancient system of astronomy (which they con- sidered to be the most philosophical, and branded mc as an emijiric for not agi'eeing with them) was the true one ; that the sun revolved round the earth, not the earth round the sun. " Thus is it that this science, so simple, easy, re- gular, luminous, majestic and religious, so full, in all its details of the deepest interest, a model of all other sciences and a masterpiece of human intelligence has not only failed to become popular, but is even un- known to the great majority of those who have re- ceived a liberal education. It is true that the fault mainly lies in the defective manner of its teaching." * These remarks are, unfortunately, borne out by ex- perience, and therefore justify me in adding another * See Father Gratry's Sources, Part i., p. 136. IV PREFACE. to the list of publications, many of them very excellent ones, which have been put forth with the view of popularising this science. It, of all others, inspires the most grandiose ideas, calculated at once to elevate the mind and develop the intelligence ; it is also, as I believe, one of the branches of human knowledge, the principles of which can be the most easily compre- hended when they are expounded in clear and simple language, divested, that is to say, of technical terms. This volume has long been in preparation. So early as 1852 I pointed out in an influential news- paper the progress of science, and in the columns of La Science pour tous, of which I was editor-in-chief for several years, the successive discoveries of the astronomers were carefully registered. In 1865 I published an elementary treatise, which had an ex- ceptionally large circulation, and since then, a more complete work, which was adopted by the official committee of the Ministry of Public Instruction.* * It Avas in reference to this putlication that M. Bahinet, of tlie Institute, ■wrote me the following lines : — " My dear E.vjrr.ossox : " I have read your work on Astronomy with much interest, and have satisfied myself that the clearness of the language has not prevented it from being scientifically exact. I have in particular noticed that you have set forth the most recent advances in Astronomy, so as to hriiig them within the reach of ordinaiy intelligence, &c. "Ever yours, " PAr.is, March 8, 1866. " Babixet, of the Institute." PREFACE. V My final task is therefore facilitated, as my func- tions and inclination have led me to note each new fact as it appears, and these will be found chronicled in the following pages. I have also made a conscien- tious compendium of the most advanced and best authenticated theories, to which I have adjoined my own personal observations. My labour, too, has been materially lightened by the house, or rather dynasty, which has undertaken the publication of this work ; and I have also to thank the artists who have supplied me with the illustrations, which will be found of unusual excellence. It only remains for me to express a hope that the public to whom I address myself will find this book worthy of their notice. CONTENTS. CHAPTER I. ORIGIN AND TKOGRESS OF ASTRONOMY. Origin of Astronomy— Antediluiaan traditions — Astronomy amongst the Indians — Astronomy amongst the Chinese — Tlie Emperor Chun's Sphere — Tire Astronomers Hi and Ho put to Death for having neglected then- duty— A remarkable Antique Text — Astro- nomy amongst the Chaldseans — Abraham as an Astronomer — The Book of Job — Astronomy 2000 years before our Era — Ancient collection of Observations taken at Babylon — The Egyptians and Astrology — The circular Zodiac of Denderah — The great Pyramid of Gizeh, from an Astronomical point of view — The Ptolemies, Kings of Egypt, Patrons of Sciences and Letters — Thales, Anaxi- mander, Anaxagoras, Pythagoras, Pytheas, Aristarehus, Aristotlu, Hipparchus, and his Catalogue of 1022 Stars — Julius Caisar, Sosigenesthe Astronomer, and the learned Achorteus — Pompey and the gifted observer of silent Olympus— Ptolemy and his School — The Arabs and Almagestes — Copernicus, his System, his Appre- hensions, and his Swan-like Song before Death — Galileo's exclama- tion : "Oh! Nicholas Copernicus!" — Tycho-Brahe and his System — Uranienburg — Descartes, Philosopher and Savant — Newton and his splendid Discoveries CHAPTER n. THE SOLAR SYSTEM. The Sun— Tlie eight principal planets — The smaller planets — The satellites— formation of the solar system 33 Vlll CONTENTS. CHAPTER III. LIGHT AND THE DETAILS OF SPECTRUM ANALYSIS. PAoa Light enables us to recognise the stars — Revelations hy spectram analj'sis — Hypothesis of the emission, and that of imilulatious — Laws of light — Various measui-ements of its speed — The solar spectrum — Chemical action of light — Length of luminous waves — Analogy between sound and light — Molecular vibrations and atomic vibrations— Mode of propagating light — Refraction and reflection — Luminous interferences — How light added to light produces darkness— Triumphal eiitiy of the spectrum analysis into science — Its history — Metals revealed by analysis of the spectrum Avhich they elicit during combustion — Important chemical and spectral laws — Curious experiments in physiology — Undreamed of horizons in astronomy — Ignition in the celestial domain — Spectra of planets, moons, and stars — Substances discovered in the stars— Nature of nebula; — Movements of the stars revealed by their spectra — Discovery of the nature of comets — Matter fol- lowing in the track of the aerolites — Unity of composition extending to all the luminaries in the universe — The inhr.bited celestial spaces 40 CHAPTER IV. THE SUN. Its nature — Light and Aspect presented by its Surface — Grains of Rice, ■\Villow-leaves, Straw-motes, &c. — Pores, Faculse, and Spots in the Sun — Fonnation, nature, and motion of Spots — Rose-colomed Shadows, Red Protuberances — Cliange of Shape in the Spots— The Sun obfuscated by their enormous quantity — Rotation of the Sun — S}'nodical Revolution — Sidereal Revolution — Periodicity of the Spots — Solar Electricity and Hydrogen upon the Earth, and in the Planetaiy Regions — Solar Explosions — Constitution of the Sun — Two opposite hypotheses— Is the Sun inhabited? — Curious Anecdote — Recently acquired notions about the Sun — Tempera- ture of the Sun— Curious Calculations — Is there any probability of the Sun ever ceasing to shed light, heat, and life upon the Earth — Lucretius and our modem Astronomers — Zodiacal Light : its nature— Some parts of the Sun more brilliant than others — Atmosphere of the Sun — Various elements of which the Sun is composed — The Seasons — Dimensions and distance of the Sun — Variation of its Diameter — Its influence upon the Earth — Recapitulation . . 66 CONTENTS. IX CHAPTEE V. MERCURY. PAGE Its phases — Ti'uncation of its crescent— Prodigious height of its moun- tains — Mercury's passage across the Sun — Its volcanoes — Its distance from the Sun — Its seasons— Its density, mass, dimensions, and motions— Strange peculiarities of this planet— Is it inhabited ? — Fontenelle's opinion 113 CHAPTEE VI. VENUS. Different names of this planet — Its distance from the Sun — Its trans- latory motion — Why does it seem to vaiy in size ? — Dull and pale light occasionally emitted by its obscure part— Visible in full day- light — Curious facts : jEneas in his voyage to Italy, and General Bonaparte at the Luxemburg— Discovery of the phases of Venus — Curious anagram — Spots observed in Venus — Its gigantic moun- tains — Explanation of its phases — Its passage across the Sun's disc — Its atmosphere — "Why does it seem to remain longer to the east and west of the Sun than it takes time to revolve around it?— Means of ascertaining the Earth's distance from the Sun by the passage of Venus— Halley, Le Geutil, Chappe— Curious facts — Its rotatory motion around an axis — Its days and seasons — Description of tliis planet and its possible inhabitants . . . 118 CHAPTEE VII. THE EARTH. Its origin — Its transformations — Summary of what is known concerning the globe's crust, by Elie de Beaumont — Cooling of the globe — Temperature of the Celestial regions — Shape and dimensions of the Earth — Its chief divisions : continents and se:is — Proofs that the E;uih is almost spherical — Flattened shape at the Poles — Attraction — Its various kinds — Exact date of the establishment of the law of attraction — Scicntitic hypothesis as to this law — History of M. Bertrand's nieasurenicnt of the Earth — Various motions of the Earth — Kepler, his genius and discoveries — The seasons — Variations of day and niglit— History of the Earth's tviiuslatory motion round the Sun, by Aiago 1 23 X COXTENTS. CHAPTER VIII. THE MOOlf. Nature of the Moon — Its size — The light ■which we receive froni it ■«hen it is at its hrightcst — Heat reflected from the Moon ; history of the discovery — Shadows, spots, craters, mountains, and extinct volcanoes, observed upon the Moon's disc— Its various motions — Sidci-ual revolution — Surprising velocity of the Moon's motion — Is it possible for the Moon to fall on to the Earth— Successive applications of the principle of gravity in explanation of the solar system — Problem of the three bodies — Simple and easy experi- ments in explanation of the Moon's phases — Ashy light— Symboli- sation of the Moon ; curious passage from Sophocles . CHAPTER IX. THE ECLIPSES. Principal eclipses — Occultation — Theory of the eclipses of the Sun and the Moon — Partial, total, or central eclipse — Appulse — Luminous corona, protuberances, prominences, rose-coloured flames noticed during eclipses — The most remarkable solar eclipses — Measure of the eclipses — Immersion and emersion — History of the iiifoi-ma- tion about eclipses from what has been remarked dui'ing their occurrence — Terror which eclipses formerly inspired — Curious facts — Mcton's cycle — The golden number^Saros — Instruments for indicating past and future eclipses — The utility of eclipses in fixing doubtful dates — Historical facts — Christopher Columbus and the islanders — Pericles and his pilot — Pelopidas and an eclipse of the Sun— The soldiers of Paulus Emilius and an eclipse of the Jloon — Terror of Kicias, the Athenian general — Picmarkable passage from Plutarch 204 CHAPTER X. THE TIDES. Their nature — The first of the Greeks who incjuircd into the causes of tliis r'l'cuomeuon— Passage from Lucanus — Influence of the Moon and tlie Sun upon the waters — Theoiy of tides — M. De- launay on tlie tides — Solar and lunar tides — Obstacles to tides — Height of tides in the Moon — Flood-bar or " bore '' — M. liabinet's dcscriiitiun— Utility of tides — A charming allegory . . . 228 CONTENTS. XI CHAPTEE XI. THE PLANET MAES. Itecent observations of the plimet Mars — Its close analogies with the Earth — Its reddish aspect — Its atmosphere — Its soil — Its different names— Curious mistakes to which the distances of Mars from the Earth may give rise — The seasons in Mars — Its polos of ice ■ and snow — Its forests, seas, and islands — Dimensions, translation, rotation, and phases of Mars 238 CHAPTER XII. JUPITER, SATURN, URANUS, NEPTUNE. Jupiter — Its distance from the Sun — Its motions — Aspect of its surface — Its dimensions — Its satellites — Their eclipses and the velocity of light — Saturn — Its distance from the Earth and from the Sun— Saturn's ring— Nature of this ring— Its aspect — Its dimensions — Various hypotheses — Uranus— Its motions — Its dimensions — Its satellites — Neptune — Its distance from the Sun — Its rotatory motion round the Son- Its perturbations 246 CHAPTEE Xin. THE STARS. Fixed stars — AVandering stars — Number of stars — The stars seen through the telescope —Illusion caused by the aspect of the celes- tial vault — Distance of the stars from the Earth— Bewildering calculation — The stars nearest to the Earth — Stars of different sizes — ^The method of classifying and cjilculating them — Number of stars of each order — The Milky Way — Its nature and shape — From one sm-prise to another — The rank occupied by our Sun in the creation— Incalculable number of suns— Ideas formerly entertained concerning fixed stars — General motion of the whole solar system in space — The laws of attraction in the most remote regions of the sky — Planetary system of the stars — Double and treble stars — Splendid revelation of spectrum analysis — Elements of which the stars are composed — Types to which they appertain — Ideas concerning immensity, and the stellar bodies which it contains — Division of the stars into constellations — The constel- lations in ancient times — Historical and legendary ideas — Nortliern constellations situated above the zodiac — Constellations of the zodiac— Constellations situated below the zodiac .... 258 XU CONTENTS. CHAPTER XIV. THE COMETS. PAGE Description of a comet; its different parts — The nature of comets — The opinions of modern ami ancient astronomers — Terror which comets formerly inspired — Eecent comets — Periodic comets Changes to which comets are liahle, both in regard to their shape, motion, and course — Their volume and mass — Possibility of a comet coming into contact with the Earth — Result of the shock — Density of the various portions of a comet — The passage of the Earth through the tail of a comet — Account of the chief periodical comets — The comets of Halley, Encke, Biela or Gam- bard, Faye, Brorsen, d' Arrest, Tuttle, and Winnecke . . .239 CHAPTER XV. SHOOTING-STAKS, BOLIDES, METEOEITES, ETC. These phenomena alluded to by Homer, Ossian, Milton — Phenomena which must not be confounded with each other — Meteorites and their sub-divisions — Gaseous or pulvemlent matters reaching the ten-estrial atmosphere from the planetary regions — Showers of Sahara sand observed at a great distance from the desert — Appari- tions, motion, number, shape, composition, and weight of meteorites — History of principal meteorites — Meteorites of the rivers iEgos-Potamos and Abydos — Cybele and the Sun wor- shipped in the form of meteorites — Extraordinary bolides, as related by Plutarch — Meteorites in the Paris Exhibition of 1867 — Modern savants and the meteorites ; the pertinacity of M. Chladni — Shower of aerolites in 1803, which were observed by M. Biot — Hypotheses suggested in explanation of these phenomena— Are meteorites found in the atmosphere ? — Are they asteroids or small planets ? — The Moon, a troublesome neighbour — Analogy between meteorites and comets — Has the Biela comet been transformed iuto meteorites ? — Periodical apparitions — Eadiant points — Periodic and sporadic shooting-stars — Days and months when shooting- stars are most numerous — The influence of the precession upon their apparition — Shooting-stars and the Chinese — Meteoric currents — Shooting-stars subject to the gfeneral laws of the universe , 317 CONTENTS. XUi CHAPTEK XVI. THE DIVISION OF TIME. Viae Divuion of time ; the day, week, and month— The year ; that of the EgjTitians and of the Chaldseans— The Olympiads — The Roman year — Months added by Numa— Curious passage from Plutarch ; different lengths of the years — The Julian year— The Gregorian calendar — Reckonings of the Sun and Moon amongst the Mexicans — Russian and Greek dates — The solar cycle — The lunar cycle or golden number — Period called Saros by the ancients— The epacts — Composition of the calendar — The calendar and meridian — The absolute time and the mean time — Precession of the equinoxes — Great year, or the world's year 343 CHAPTEE XVII. ASTROLOGY. Dogmas of the astrologers— Curious facts — Natural astrology — Judicial astrology — Hippocrates — Virgil — Horace — Juvenal — Plutarch — Tacitus — Tiberius and Thrasylhis— A.strology in Mexico — Monte- zuma and the astrologers — Marsilius Ficinius — Pensa — Doctrines of the astrologers — Albert the Great — Thnrneisen— Catherine de Mcdicis —Sensible advice given by Horace , . . . .357 ASTRONOMY. CHAPTER I. ORIGIN AND PROGRESS OF ASTRONOMY. diigin of Astronomy — Antediluvian traditions — Astronomy amongst the Indians — ^Astronomy amongst the Chinese — ^The Emperor Chan's Sphere — The astronomers Ei and Bo put to Death for having neglected their duty — ^A remarkable Antique Text — ^Astronomy amongst the Choldsans — ^Abraham as an Astronomer — ^The Book of Job — ^Astronomy 2000 years before our Era — Ancient collection ,of Observations taken at Babylon — The Egyptians and Astrology — ^The circular Zodiac of Senderah — The great Pyramid of Gizeh, from kn. Astronomical point of view — The Ftolemys, Kings of Egypt, Patrons of Sciences and Letters — Thales, Anazimander, Anaxagoras, Pythagoras, Fytbeas, Aristarchus, Aristotle, Hipparchus, and his Catalogue of 1022 Stars — Julius Csesar, Sosigenes the astronomer, and the learned Achorteus — Fompey and the gifted observer of silent Olympus — Ptolemy and his School — The Arabs and Almagestes — Copernicus, his System, his Apprehensions, and his Swan- ' like Song- before Death-r-Galileo's exclamation: "Oh! Nicholas Co- pemicos I" — Tycho-Brahe and his System — Uranienburg — Descartes, Philosopher and Savant — Newton and his splendid Discoveries. I. - AsTEGNOMT, fls indicated by its Greek derivation (cum/jp, Blar, vo/to;, law), is the science of the motions of celestial liodies. The purely descriptive part is often designated by the names of Uranography and Cosmography. Its origin amongst all nations is lost in antiquity ; and this may be readily understood, for the first impulse of man must naturally be to observe the sky, the rising and setting of the stars. i ASTRONOMY. their respective and varying positions, their relation to the seasons, their diverse influences, &c. Thus Bailly retraces the origin of this science to certain antediluvian traditions, saved out of the general wreck ; and Josephus, in his Jewish Chronicles, cites, as an instance of the attraction which the phenomena of the firmament pos- sessed for the Patriarchs, the debris of a column which he states to have been still extant in his time in Syria, and upon which the descendants of Seth, third son of Adam and Eve, had engraved, several centuries before the deluge, tht leading results of their observations. " The spectacle of the firmament must," says Laplace, " have taken the attention of primitive man, especially in countries where the serenity of the air lent to the observa- tion of the stars. In the pursuit of agriculture, he must have needed to know the various seasons, and to be able to prognosticate their recurrence. He would soon have dis- covered that the rising and setting of the principal stars at the moment of their plunging within the solar rays, or of their emerging there&om, facilitated him in his purpose. Thus it is that we find the traces of astronomical observa< tions dating back to such a remote period in the history of every nation." India, the earliest civilized part of the Old World, presents a subject of study replete with interest for any one who will give it his special attention ; and though I do not pretend to possess any great knowledge of Hindoo astro- nomy, it is impossible to doubt that this people was deeply versed in this science, which was specially cultivated by the guardians of the holy places. Cassini, Bailly, and Flayfair are of opinion that the Hindoos nave transmitted toTIS'o bB OP ■ yalloUii UKen morethan 3000 B.C., and though this date ia ORIGIN AND PROGRESS OF ASTRONOMY. 3 contested by other emment^aua2iivthej_fln admit the great antiquity "of the documents in .which these observations are coD^rised._ Benthey, the most ardent adversary of Ihe Pig. 1.— Zodiac and solar systems of the Hindoos. Hindoo pretensions, himself admits that their division of the ecliptic into twenty-seven lunar stations must have been made in the year 1142 of our era ; and this division could oiily have been the result of a vast number of observations extending over an immense period. The astronomical law laid down by the Vedas for the arrangement of the calendar must necessarily date back to the fourteenth century before Christ, and Parasara, the first known Hindoo author who wrote on astronomy, probably lived about the same time. It seems astonishing that the Hindoos, well versed as B 2 k. ASTKONOMY. they were in science generally, should have been so ignorant., in matters relating to our globe. Fig. 2 represents Mount , Meroo, which, as they believed, formed the centre of the., world. It was said to be a conical-shaped mountain, and its sides to be covered with precious stones, its summit Fig, 2,— The Beven regions of the npper world, according to the Hindoo tradition being a sort of terrestrial paradise. This idea may have been inspired hy the sight of the lofty mountains which ran along the northern frontier of India ; but,' for all tliat, Mount Meroo has no existence save in the imagination of the Hindoo mythologists. They supposed it to be surrounded by seven concentric belts of habitable lands, divided from each other by seven oceans. The central belt was said to comprise India, and to be surrounded by a salt-\yater sea ; the other belts being sepai-at^d ORIGIN AND PROGRESS OP ASTRONOMY. 5 'froin-.eacli other by seas of inilk, wine, molasses,* &b, la .conti'ast to fhese errors, we find in the astronomical writings of the Hindoos the proofs of a truly wonderful amount of learning. II. To the Chinese we owe the earliest information upon astronomy which has any claim to exactitude, From time immemorial they have celebrated the epoch of the solstice, in the hope of seducing the sun, by their dances and festi- vities, to delay his departure towards the equinoxes ; and, though the first eclipses to which their annals allude are only vaguely mentioned, they prove nevertheless that in the time of the Emperor Yao, 2000 B.C., astronomy was made the subject of special study. When Yao, whose reign had been entii-ely devoted to the welfare of his subjects, died, at the age of 118, they wore mourning for him during a space of three years. " On the first day of spring (2255 B.C.) Chun was installed as heir of the emperor in the hall of his ancestors. Examining the instrument decorated with precious stones wJdch represents the stars and the movable tuhe which enables one to observe them, he regarded the seven planets* This done, he offered sacrifice to the Supreme Lord' of Heaven, and went through the usual ceremonies iii honour of the six great spirits, as well as those usually performed in respect of the mountains, the rivers, and the spirits generally." t ^■'' This instrument,- representing the stars, was a sphere of the heavens, termed Sicoun-Ki. The likeness of it is pre- served by the Chinese in several editions of the Chou-King, . • India, by MM. Dubois de Jancigny and X. Raymond, t Oliou-King, chapter 2. 6 ASTHOInOMY. and is accurately reproduced in Fig. 3. This sphere ■represents the vault of the firmament divided into sections, Fig. 3. — Tho Emperor Chun's sphere. with the earth in the centre, and the sun, the moon, the planets, and the stars in the places assigned to them in the Ptolemtean system. If this sphere is really authentic, it -proves a high degree of astronomical Imowledge for ■ so remote an epoch.* The Chinese were acquainted with the use of the diaV; they measured time by tha aid of the clepsydi'a. They * Pautliior's Uistoire do la Ohiius (Firniin-Didot). ■ ORIGIN AND PEOGEESS OF ASTRONOMY. , 7 constructed instruments for measuring the angular distances of the stars, and ascertained that the solar year, which they made begin with the winter solstice, was about six hours longer than 865 days. Their civil year was lunar, and, to bring it back to the solar year, they used the period of nineteen solar years,- which is equal to 235 lunations — a period exactly equivalent to that which Callippus, sixteen centuries later, introduced into the Greek calendar. They also divided the equator into twelve immovable signs and twenty-eight constellations, in which they carefully assigned the position of the solstices. III. Their estimates have been confirmed in the cases of thirty-one out of the thirty-six eclipses, the elements of which have been handed down to us, and which they state to have been observed between the years 776 — 480 B.C. The Chou-Iiing alludes to an eclipse of the sun which occarred in the reign of Tchoung-King, who, in connection v.ith it, put to death Hi and Ho, two functionaries who were at once astronomer's and directors of religious cere- monies. They were accused of preferring the pleasures of the table to the observance of their astronomical duties, and it is interesting to cite the Chinese text bearing upon this matter: — "At that time, Hi and Ho, abandoning themselves to vice, neglected their duties; they besotted themselves with wine; they neglected the duties of their bf&ce, and degraded themselves from their rank; they created confusion in the celestial chain, and left their func- tions unfulfilled. Upon the first day of the third moon of autumn, the Tchin (a conjunction of stars) was not in harmony with the constellation Fang (Scorpio). Tlie blinrl 8 ASTRONOMY. man sounded tlio drum (the drummer was always a blind man) ; the . magistrates and people assembled in haste, like a horse let loose. Hi and Ho were like slaves at work, they neither heard nor saw anything. Having become unable to distinguish the celestial signs and appearances> they incurred the penalty decreed by the kings our predB- cessors. The Tehingtien says: * Whoso advances -the march of time shall be put to death; whoso retards it shall also be put to death.' "*■ . . , Father Gaubil, in his " History of Chinese Astronomy," fixes the date of this eclipse as fyv back as 2155 b.o. In China an eclipse of the sun has always been looked upon as a matter of importance to the State. Upon the occasion referred to, the mandarins,' when the sun- was suddenly veiled from their eyes, were obliged. to. repair in hot haste to the palace, and to provide themselves with bows and arrows to protect the emperor, who- is looked upon as the image of the sun. The, bandmaster, who was a blind man, sounded the drum, the mandarins offered presents in honour of the spirit, whilst the emperor and court grandees put on plain apparel, and remained fasting. The nature of the laws edicted by the early kings against the calculators who made a mistake in their observations indicates the great antiquity of Chinese astronomy. It is also worthy of notice that when the Chinese astronomers were merely culpable of slight negligence, or a trifling miscalculation) they were not visited with a heavier punishment than that of fine, reprimand, or dismissal. The punishment of death was reserved for other misdeeds committed by the astro- nomer-in-chief, t , . * Pauthlsr's ITistoire do ta Ohine, pp. 68 and followiiig (Firmin-'bidot). t Father Gniibil's Bistoire de I'Astronomie Ohinoise. ORIGIN AND PROGRESS OF ASTRONOMY. IV. < The Chaldteans come next, some of theiir observations 'being said to date back to. nine centuries before Alexander the' Great. When that monarch made his victorious entry into Babylon in 331 BtC, some of the astronomers presented .him with a series.of observations extending oVe^ 1903 years, and dating back to the time of Nimrod. Calisthenes, who : accompanied the Macedonian, sent them by his orc^er to •Aristotle. The latter tells that some observations- still more ancient were lost, and he mentions a cii'cumferencie Tof the earth, the measure of which relates to the climate of .Tartary ; but he does not' say how or by whom it had been calculated, while the annals of. China explain the opera^tion •without giving the result. ' ■ Periods, which it must have taken centmies of observation io discover ahd calculate, were in use amongst the Chaldceans. They were acquainted with the ancient planets ; they pos- .'sessed a zodiac divided into twelve constellations, and a sphere which has served as a mo'del to our own ; they were 'also able to predict eclipses. Ptolemy testifies that they had, from the most remote ages, been in the habit of observing the occultation of stars by the moori, and that ttey were familiar with usage of sun-dials. They knew, too, 'the celebrated period called Saros, adopted by the Greeks, which includes aibout 18 solar years, 15 days, and 10 hours. The recurrence of this period had the effect of bringing the return of the eclipses upon the same day and in the same order as in the preceding period. It is. the cycle which Meton revealed to the Greeks, and the calculations of which the Athenians caused to be engraved in letters of gold. Bailly asserts that this period was taught to the Chaldseans by a more ancient race ; and it is remarkable that the same 10 ASTRONOMY. period was known to the Chinese, the Hindoos, and other peoples being far apart, and, to aU appearances, holding no sonununication with each other. Yet, if we are to believe the great majority of historians, the fertile plains of the Tigris and Euphrates were the cradle of astronomy. Abraham, born at Ur of the Chaldees, about 2040 B.C., is the starting-point of Jewish history ; he knew the true God, and led a holy life. The great Patriarch was alwaj'S famous in the East ; the Chaldteans, his compatriots, con- sidered him as one of their most eminent astronomers. Need I cite that magnificent passage in the Book of Job ? " Canst thou bind the sweet influences of the Pleiades, or loose the bands of Orion ? " Canst thou bring forth Mazzaroth in his season ? or canst thou guide Arctm'us with his sons ? "Knowest thou the ordinances of heaven? canst thou set the dominion thereof in the earth ? " Canst thou lift up thy voice to the clouds, that abundance of waters may cover thee ? " Canst thou send lightnings that they may go, and say unto thee, Here we are ? " Who hath put wisdom in the inward parts ? or who hath given understanding to the heart ? " * It is believed that Job inhabited Arabia, not far from the borders of Chaldeea, in the eighteenth century before our era, M. Elie de Beaumont says :— " The first astronomical observations are lost in antiquity. The Chaldseans were possessed of certam notions, by no means inaccurate, as to the stars, and the laws to which their movements are sub- jected. For the last three thousand years astronomical observations have been accumulating in enonnous pro- • Book of Job, chapter xxxviii. versos 31 — 30. ORIGIN AND PROGRESS OF ASTRONOMY. 11 portions, and becoming more precise. Tables of the results attained have been compiled, each more perfect than the preceding one, so that it is now possible to predict with certainty what position each of the stars in the solar system will occupy at a given moment. They supply the materials for calendars and almanacs, the infallibility of which has become proverbial."* The knowledge of astronomy soon spread from Chaldsea into Phoenicia and Egypt. The observations taken by the Egypiians of the motion round the sun of the planets which we call Mercury and Venus, proves how successful they were in this direction, and their priests were more par-\ ticularly renowned; but under the name of astrology, they made the movement of the stars to bear upon the/ various events of life, and so claimed to foretell the futurey Thus it was in the sanctuary for the most part that .the exact sciences were studied and perfected, an(f their results made to serve some purpose of general useful- ness. The astronomers were brought up to the priest- hood, and . the vast platforms (or flat roofs) of the temples w.ere useful as observatories. Their observations in time taught them that the rising of the same stars ceased, after the lapse of several centuries, to correspond to the same seasons, and they noticed their change of position. They divided the heavens into constellations, the names and figures of which were taken from Egyptian subjects. They constructed a zodiac which is thought to have been in existence earlier than 2500 b.c. The civil calendar was arranged, the year being composed of 365 days, which were divided into twelve months, each of thii-ty days, and " Elie de Beaumont's Hloge Histo-ique de Jean Plana, read before the Academie des Sciences, November 25tli, 1872'. ]3 ASTRONOMY. followed by five supplementary days. They also made use of the week, or period of seven days.* V. Astrology, as I have remarked, was in vogue amongst the Egyptians, as is to be proved from the zodiacs which haye been handed down to us. Fig. 4 is a miniature engraving of the circular zodiac of Denderah.t At first sight it seems to. be nothing more than a conglomeration of various figures Eurrounded with sacred letters; but on closer inspection we notice an outer cu'cle, with an inscription traced in. sacred letters, and intersected at equal distances by figui'es with the head of a woman looking upwards, or that of a hawk in. a reclining posture. These figures, their arms raised aloft, hold up a shield entu'ely covered with signs of vaiious kinds. To the l^t, a little below the centre of this shield, which repre' sents the firmament, is seen a lion, followed by a woman ani treading upon a serpent— rthis is the zodiacal sign of Leo. Behind this group is a woman holding in her hand a wheat- ear — this is Virgo. We then notice, going from the right to left, lAhra with its two scales ; Scorpio, Sagittaritis, in the shape of a winged centaur ; Capricorn, half goat half fish ; Aquarius, pouring water out of two vessels ; the Pisces, con- nected with each other by triangular lines ; next to them Aries, Taunts, and the Gemini, which represent two men wa,lking in company ; and, in succession, Cancer just above the head of the Lion. Leo is the fii'st sign i?i this system of the zodiac ; within and beyond the spiral line formed by the twelve signs, come the chief extra-zodiacal , constellations, and as the gigantic animal, moving on its liind legs near the ' Egtjpte, l)y diampollioii-Figcac, p. 95. -; + See RcdwclKS mr I'Egypie, by Latronne. ORIGIN AND PK03RESS 01" ASTRONOMY. 13 centre of the disc, has been generally admitted to represent what we know as Ursa Major, the North Pole . must be . placed in close proximity to this figure. The discovery of - the Denderah and Esnah zodiacs * led to very fierce discusr- sions, which, however, need not be referred to in these pages. , , Fig. 4. — Tlio Circular Zodiac of Denderah. The pyramids are very remarkable from an astronomical point of view ; they represent the most ancient monument of human sldll in the known world. The largest, that of Gizeh, is the most carefully constructed, being so built that each of its four corners faces exactly one of the four cardinal points. Even in the present day we should find it djfl^cult to trace so vast a meridian without deviating a. * See EedurcJtcs mr VE(iypte,\yj Latronno, p, \L 14. ASTRONOMY. single fraction. From this situation of the great pyramid has been drawn the conclusion, very important as bearing, on the physical history of the globe, that the position of the earth's axis cannot have varied to any perceptible extent for thousands of years. It may be added that the great pyramid is the only monument in the world which, from its antiquity, is capable of supplying the means for ascertaining such a fact. The appended illustration, after Champollion, represents the present appearance of this pyramid and of the Sphynx, which stands in close proximity to it. Pig. 5. — ^The great Pyramid of Gizeh and tho Sphynx The encouragement offered by the Ptolemys — more espe- cially by Ptolemy Philadelphus — to science and art, gave a great impluse to the development of intellectual knowledge. We know that the latter king attracted to his capital the most learned men in Greece, that he lodged them in his palace. ORIGIN AND PROGRESS OP ASTRONOMY. 15 arid furnished them with every facility for carrying out their scientific studies and researches. The Greeks did not cultivate astronomy until long after the Egyptians, whose pupils they were. In the year 640 B.C. Thales of Miletus .went to Egypt to study, and he afterwards founded the Ionian school, in which he taught the spherical shape of the earth, the obliquity of the ediptic, and the causes which superinduce eclipses of the sun and of the moon. After him came Anaximander and Anaxagoras, to the first of whom is credited the invention of the dial, the terrestrial globe, and geographical charts. Soon after, this school produced Pythagoras of Samos, a pupil of Thales, who made several journeys to Egypt and India. On his return home he was driven into exHe, and went to Italy, then called Greece Major, where he founded the school named after him. In addition to the subjects taught' by the Ionian school, he expounded the two distinct, motions of the earth, one upon its own axis and another round the sun. He asserted that the stars were suns, the centres of so many planetary systems. Thus we see that the fundamental principles of the astronomical system now universally accepted were in general use five hundred years before the birth of Christ. From the time of Pythagoras it was maintained that the regular movement of the celestial bodies through space produced an ineffable harmony which was termed the har- mony of the spheres. In the first place, the aspects were held to have an affinity with the intervals of the tones. Thus, the quadrate aspect, or the quadrature, is, as com- 16 ASTRONOMY. pared to the sextile aspect, or 60 degrees, as 8 is to" 2 — ^tliis ; is the affinity which forms the fifth ■(ci'>''i'>^i^) ^ music.. Kepler himself endeavoured to establish a relation between the distances of the planets from each other and the inter- vals of music ; but these comparisons are very arbitraiy and incomplete. In proportion as the theory of music has been made more perfect,- the ideas formed as to this harmony, have undergone great modifications. It was supposed that, the moon, as the lowest of the planets, that is, the nearest . to us,- answered to the not& mi, Mercuiy to fa, Venus to so J, the Sun to la, Jupiter to ut, Saturn to, re; and the orbit, of the fixed stars, as being the most distant (or highest) of. all, to mi or to the octave. The most celebrated astronomers, after Pythagoras, were • Pytheas, who taught the way to classify climates by the length of the days and nights ; Aristarchus of Samos, who fixed the apparent diameter of the sun in the year 281 b.c.,. and calculated the distance of that planet from the earth ;- and, thu'dly, Aristotle, the disciple of Plato, who set himself to ascertain, by a series of astronomical observations, the. shape and size of our planet. ~ They were followed by Hipparchus of Bithynia, who- aichieved great renown in the celebrated school of Alex- ahdi'ia 140 years b.c. This great astronomer, looking upon- pl-evious researches as unreliable, determined to go oyer the whole ground afresh, and to admit as genuine only those which should be confirmed by his own experience. He fixed to a nicety the extent of the tropical year^ discovered tlie precession of the equinoxes, and it is to him that we. owe the use of latitudes and longitudes. A new star having^ suddenly appeared, he composed a catalogue of 1022 dif-" ferent stars, which he calculated f6r the 128th year before ORIGIN AND PROGRESS OP ASTRONOMY. 17 the Christian era. This Pliny terms " an enterprise worthy of the gods, for Hipporchus thus afforded the means for ascertaining hereafter whether certain stars disappeared, and whether they underwent a change of position, size, or light; he left, in fact, the heavens as an inheritance to those who should come after him, and who possessed suffi- cient genius to turn his lahonrs to good account." There was an interval of about three centuries between Hipparchus and Ptolemy, to whom I shall presently refer. During this period there was no lack of astronomers who made discoveries of more or less value. It was at this time, indeed, that Poseidonius discovered the causes of the ebb and tide in the sea, and that the calendar underwent "the Julian reform," as it was termed, by Julius Caesar, who initiated it, entrusting the work to Sosigenes in the year 46 b.c. Eudoxus fixed the duration of the year at 365^ days, which was admitted as correct by Sosigenes, and adopted in the Julian calendar. The Julian year lasted 365, and, once in four years, 366 days, which caused a miscalculation of one day every 134 years. This mistake was set right iu the Gregorian calendar.* VII. History tells us that Julius Csesar was an ardent lover of the sciences, astronomy in particular. In his interview with the learned Achorseus, he is reported to have said : — " I came to Egypt to encounter Pompey, but your renown was not altogether foreign to my determination. In the midst of war, I have always studied the movements in the heavens, * See Chapter 16, on the IHmnmi ofTirm. 18 ASTRONOMY. the course of the stars, and the secrets of the gods. My arrangement of time is at least equal to the fasti of Eudoxns, &c." AchoriEus, in reply, alluding to the ideas then preva- lent as to the solar system, pointed out that " the stars which alone modify the volitation of the heavens, and which extend towards the pole, are supposed to possess varied influences. The sun divides the year into seasons, regu- lates the interchange of day and night, holds the stars captive by the power of its rays, and limits their waj^ard course to its centre. The moon, with its varied phases, mingles the land and the sea. Saturn influences the cold regions and the snowy zone; Mars, the winds and the thunder ; Jupiter, the air and the unchanging ether ; the productive Venus preserves the germ of universal life ; Mercury is supreme over the vast expanse so soon as it reaches the region of the sky, where the constellation of Leo is lost in that of Cancer, where Sirius pours forth his darting fires, where the changing cycle of the year is efiected in (Egoceros and Cancer, mysterious witness of the sources of the Nile." I We know that Csesar took a real interest in astronomy, and that he wrote a treatise about it. Pompey, too, was attracted by this science. Leaving Lesbos, bowed down by grief at the defeat of his army, and looking with apprehen- sion at the future, he endeavoured to find distraction firom his cares in a conversation with the pilot, and his mind became partially reassured by contemplation of the starry sky : — " He then talked with the pilot about all the stars, inquired how he ascertained the approach to land, and the means of measuring by the heavens the distance which the ship had accomplished ; what stars indicated Syria and Libya. The pilot's answer was that he and his fellows ORIGIN AND PKOGEESS OF ASTRONOMY. 10 never allowed themselves to be guided by a star slowly declining in the firmament, for they would only deceive the stulor, who preferred to trust the never-setting axis which receives light from the twofold Arctos " vm. The remainder of astronomy may be divided into five leading systems — those of Ptolemy, Copernicus, Tycho- Brahe, Descartes, and Newton. Ptolemy was a celebrated mathematician. He was bom at Pelusium, though Theodorus Meliteniotes states that he was a ThebarQ, and that he first saw the light at Ptolemais, the capital of that province. At an early age he went to Alexandria, where he founded a school, which enjoyed great celebrity about 175 b.c. En- dowed perhaps with more application than genius, he collected and arranged the works of his predecessors, espe- cially those of Hipparchus, and though he did not correct all their mistakes, he was nevertheless the greatest astro- nomer of his day, and his definite system has continued to be named after him. These his labours have been pre- served to us by the Arabs in a well-known work Called the Almagestes. His theory was, that the world is composed of two regions : the elementary and the ethereal. The first com- prised bodies which the ancients regarded as the four elements : to \^it, the earth, motionless, in the centre of the world ; water, covering a great part of the earth's surface ; air, which is above the earth ; and fire, which is above the air. The ethereal region, snrroimding the elementary region, was composed of eleven skies, which revolved around the 02 20 ASTRONOMY. earth as around a common centre. Beyond the eleven sides was the empyrean, or abode of the blest. All the celestial bodies moved around the earth, which was motionless in the centre of the world. This system lasted more than fourteen hundred years. He had an ingenious explanation of the positions and retro- gradations of the celestial bodies, for an epoch when no conception had been formed as to the immensity of the heavens and the enormous distance of the stars. IX. Copernicus was bom at Thorn, Poland, in the year 1472, and he died in 1543. It redounds to his fame that he was the son of a Polish baker, and that by the unaided force of his own genius he raised himself to the highest rank as a savant. He visited Italy in order to consult with the most famous astronomers, and, after spending some time at Borne as a teacher of mathematics, returned to Frauenburg, in his own country, where his uncle, who was a bishop, provided him with a canoury. Copernicus submitted all the then known systems of astronomy to the test of a &esh examination. He disco- vered the germs of the system which bears his name in the researches of several ancient astronomers, Philolaus more especially; but he made it really his own by the application of countless observations and calculations. Apprehensive of contradiction, he did not pubHsh his ideas till towards the close of his life, and did not, in fact, receive a copy of the book in which they were embodied until the day of his death. G. Donner, in a letter to the Duke of Prussia, says that " the honourable and worthy ORIGIN AND PROGRESS OF ASTRONOMY. 21 Dr. Nicholas Copernicus has let his work appear a few days before his departure from earth, like the death-dirge of the swan." Fontenelle deems Copernicus happy in the period of his death. " Copernicus," he writes, " was himself veiy appre- hensive as to the reception which his ideas would meet with. For a long time he was unwilling to pubHsh them, and only yielded at last to the solicitations of some persons of influence. But what did he do on the very day that he received the first copy of his book in print ? Why, he died, and so cleverly avoided the shower of contradictions which he foresaw as certain to be brought forward." * The lines in which he gives utterance to his doubts will be read with interest : — " I was long in doubt as to whether I should publish my commentaries upon the motions of celestial bodies, or whether it would not be better to imitate the example of certain followers of Pythagoras, who, instead of committing their ideas to writing, imparted them ver- bally, communicating them to the adept, and to those who felt an interest in the mysteries of philosophy, as may be gathered from the letter of Lysis to Hipparchus. This they did, not as some surmise, through an excessive spirit of jealousy, but in order that the gravest of questions, which had been deeply studied by men of undoubted capa- cities, should not be made sport of by the idlers, who have no taste for serious works which bring no gain with them ; or by men of limited intelligence, who, giving themselves up to the nominal study of science, make their way into the midst of the philosophers like drones amongst bees. The more I hesitated and resisted, the more my Mends * Fontenelle's Pluralili dci Afonda. 33 ASTRONOMY. pressed me. Nicholas Schomberg, Cardinal of Capua, a man of deep erudition, and my most intimate friend Tide- man Gysius, Bishop of Culm, as well read in the Scriptures as he was learned in the sciences, were specially urgent in their appeals. The latter put so much pressure upon me that at last I agreed to publish the work which for seven- and-twenty years I had kept to myself." Fig. 6. — Portrait of Coporniciis, engraved by J, Falck.* In Copernicus' system, the sun is motionless in the * The portraits of Copernicus and the three which follow are from the collection of M. Amhroise Krinin-Didot. ORIGIN AND PROGRESS OP ASTRONOMY. 23 centre of the universe, the earth is classed amongst the planets, the moon is one of the earth's satellites ; all the planets revolve around the sun, which is the general centre of the universe ; they traverse, at different times, orbits , oval or elliptical. ' The earth is subject to three motions, which explain the annual and daily motions of the heavens. The first, one of rotation upon its axis, is from west to east, describing the equinoctial circle in the course of the day and night. One effect of this rotation is, that the sun and the stars, though motionless, seem to rise and set each day, and to foUow a fixed inclination from east to west. The second is an annual motion of the earth round the sun, by which, in the space of 865 days 16 hours, it accomplishes its course in the ecliptic circle, but in the inverse direction to the order of the signs ; that is to say, that when itself in Capricorn, the sign of the zodiac which answers to winter, it sees the sun in the summer sign. Cancer, and is in the summer season. When it corresponds to Cancer, it sees the sun in the winter sign, Capricorn, and is in the winter season. The third is a motion of the earth upon itself, by which, while keeping its axis continually turned towards the same point of the sky, it successively exposes each part of its surface to the sun in the com'se of the year. These two latter motions, combined, are the cause of the unequal length of the days and nights, and the vicissitude of the seasons. Copernicus is compelled to place the stars at an incalcu- lable distance, because the earth traverses each year, in itSv revolution round the sun, an orbit of more tnan 699,581,708 miles; so that, at the end of six months, 24) ASTRONOMY. it must be nearly 209,858,595 miles distant from the spot where it then was. This is of no consequence, and does not prevent his system, established upon a mathe- matical basis, from being the simplest, the most' natural, and the truest in the world. Copernicus must, therefore, be looked upon as the founder of modern astronomy. His contradictors said : — " If it were true that the sun was in the centre of the planetary system, and that Mercury and Venus revolved around it, in an orbit nearer to it than that of the earth, these two planets would have phases of their own. When Venus is on this side of the sun, she would have a crescent shape, like the moon going down at night ; when she forms a right angle between the sun and us, she would be in shape like the first quarter of the moon. And yet such a thing is never seen." Copernicus' reply was that such undoubtedly was the case, as would be seen some day if instruments could be brought to a sufficient degree of perfection. And so it happened at I'lorence seventy years later. Galileo, exploring the heavens with a newly-con- structed glass, in the end of September, 1610, perceived that Venus had phases like the moon. He could not restrain the ejaculation, " Oh ! Nicholas Copernicus, could you but have lived to enjoy this recent discovery, which so fully confirms your ideas ! " X. Tycho-Brahe was born at Scania, in 1546, and "belonged to one of the noblest families in Denmark. From infancy he displayed a strong taste for astronomical observations. He travelled aU thi'ough Germany and Switzerland to visit the different observatories and learn the methods then in use, and he was entrusted with this mission by the King of ORIGIN AND PROGEESS OP ASTRONOMY. 25 Denmark, who gave him the Island of Huen to taie his observations. There he built the celebrated observatory Fig. 7.— Portrait of Tyclio-Brali^, engraved by de Gheyn. which he called Uranienburg, residing there for seventeen years. At the death of the king, his successor, Frederick, 26 ASTRONOMY. showed him less favour ; so he left Denmark, and proceeded to Bohemia, where the Emperor Joseph II. afforded him a permanent hospitality. He died at Prague in 1601. To him we owe numerous observations, the fruit of his twenty years' residence in the Island of Huen, and many of them, marvellously exact, have assisted Kepler to his discoveries. When a new star appeared in Cassiopea, in 1572, Tycho- Brahd compiled a catalogue, in which the position of more than a thousand stars was fixed with a precision most remarkable at a period antecedent to telescopic observation. Tycho-Brahe attempted to upset the system of Coper- nicus, then in great repute, and to connect it with that of Ptolemy ; wherein, of course, he failed. He maintained that the distance from the fixed stars to the sun, as laid down by Copernicus, was very improbable ; and, desirous . of upholding certain Scriptural passages which were incor- rectly said to contradict this system, he re-established the earth in its old position, placing it motionless in the centre of the world, and making the moon, the planets, and fixed stars revolve round the sun, while the latter in turn moved round the earth with all its planetary cortege. Thus he was one with Copernicus in looking upon the sim as the centre of the constellations we have named, and with Ptolemy in holding that the earth is motionless, the sun and the stars revolving around it. Li this hypothesis Venus and Mercury pass, dm-ing part of their revolution, between the sun and the earth, which , explains pretty correctly their phases, as seen thi'ough a glass, which are very like those of the moon. This system, ' though it did credit to Tycho-Brahe's ingenuity, was imiver- bally rejected. OEIGIN AND PROGRESS OP ASTRONOMY. S7 XI. I Descartes, the great French philosopher, was born at Lahaye (Touraine) in the year 1596. In early life he followed the profession of arms, serving as a volunteer under Maurice of Nassau and the Duke of Bavaria ; but he soon left the service. He then travelled through Ger- many, Holland, and Italy, and paid several visits to Paris, where he formed the acquaintance of several scientific men. After remaining for several years undecided as to the choice of a career, he resolved to give himself up to solitary study, and for this end left France for Holland, where he lived for some time in the strictest seclusion. Descartes' works earned him great renown, but they also drew down upon him many conti'adictions, and even per- secution. The Princess Elizabeth, daughter of the Palatine Elector Frederick V., delighted in his society. Mazarin gave him a pension of a thousand crowns, and Queen Christina invited him to reside at her Court. Flattered by this request, Descartes went to Stockholm, in the winter of 1649, but he died there a few months later from the severity of the climate, at the age of fifty-four. His remains were brought back to France in 1667, and entombed with great ceremony in St. Genevilve. He is looked upon as the father of modern philosophy. To him we owe the application of algebra to geometry ; he first discovered and proved the existence of the centrifugal forces, which maintain the universal equilibrium by balancing in all directions the action of gravity. His system, gene- rally known as the vortices of Descartes {tourbillons de Descaiies), is very similar to that of Copernicus. 28 ASTRONOMY. The word vortex, thus used, is intended to signify tt certain quantity of matter divided into an infinity of very small particles, which revolve aU together around one com- nu)n centre, while each one of them revolves round a centre of its own. For instance, applying this kind of motion to the stars, the vortex (tourhillon) in which we are placed is composed of the sun and the planets which revolve around it, as they do also upon themselves. Descartes Fig. 8.— Portrait of Descartes, engraved by Jonas SuyderhoeiT. admits three kinds of celestial bodies— 1st; the fixed stars, all of which are suns ; 2nd, the planets, which revolve* ORIGIN AND PROGRESS 01* ASTRONOMY. 29 round the suns ; 8rd, the moons, which revolve round the planets. I ' This system did not come scathless out of the analytic examination to which he himself subjected it, but this great thinker, by his creative researches and fruitful discoveries, gave a great impulse to human thought, and a spur as deep as it was durable to science and philosophy. XII. Newton, the most illustrious of English savants, was bom in the year 1642, at Woolstrop, Lincolnshire. He is placed in the first rank of mathematicians, natural philo- sophers, and astronomers, yet it may be said that his dis- coveries were, to a certain point, led up to by Descartes. His mother had intended to make a farmer of him, but as he showed no aptitude for this calling, she allowed him to follow his own inclination. He was sent, at the age of thirteen, to Cambridge, where Dr. Barrow was his mathe- matical tutor. He soon learnt more than his tutor knew, and made his greatest discoveries in mathematics before he was three-and-twenty ; in particular that of the binomial named after him, and of the infinitesimal calculation which he called the calculus of fluxions (differential calculus). . In 1665 he left Cambridge, in consequence of the plague, and returned to Woolstrop, where, it is said, he saw the apple fall, which led to his first ideas as to universal gravity and the system of the universe. , It seems that in 1692 his reason momentarily gave way, either as the result of a fire which destroyed several of his papers, or because he had laboured too hai'd, and after that time he did not publish any original work- of importance, 30 ASTRONOMY, contenting himself with a revision of previous publications. In 1699 the French Academy of Sciences elected him a foreign associate, and in 1703 the Eoj'al Society chose him as President — ^^vhich title he retained until his death. His Fig. 9.— Portrait of Newton, engraved by J. Smith. latter years were embittered by a dispute in which he was engaged with Leibnitz, whom he accused of plagiarism, the result of which was that, while Newton was admitted to have the priority, Leibnitz, on the other hand, proved that he had also made the self-same discovery. An English poet has termed him a man of pm*e intelli- ORIGIN AND PROGRESS OF ASTRONOMY. 31 gence, ssnt to man by the Creator to explain the works which He had created. His profound knowledge of mathematics led him to the discovery of the curve described by a body in its revolution round a' centre, to which it is attracted by a force propor- iional to the mass of the central body, and decreasing according to the laws of gravity. He thus ascertained that all the celestial bodies revolve in the four principal curves of the conic sections, viz., the planets in ellipses, the satellites in circles, the comets parabolicaUy or hyper- bolically. The summary of his system is this : Just as all weighty bodies gravitate to the earth's centre, so do the bodies which compose the universe gravitate, by the force of attraction, towards the sun, which is their common centre. But as the planets, if they were only governed by the force , of attraction, that is to saj', by the force which the sun exercised in attracting them towards itself, they would gradually be drawn into that celestial body. Newton adduced two moving powers given them by the Creator at the beginning of the world, the first of which was a centri- petal force impelling the planets towards the sun ; the second a centrifugal force, which hm-ried them away from it, the one counterbalancing the other. Thus the earth, instead of being earned far away from the sun by the centrifugal force, is maintained, by the action of the two combined, in its orbit, and compelled to describe around it an ellipsis of which it occupies one of the foci. Newton also calculated the 'motions of the satellites and the routes followed by the planets with an. accuracy confirmed by subsequent observations. The flood and ebb of the sea, the precession of the equi- 32 ASTRONOMY. noxes, the nutation of the earth's axis, the difference hetween the ti-ue and the mean time, are but effects evolved from the law of universal gravitation. In the course of this work I shall have an opportunity of developing ideas which can only be glanced at in a rapid review of the history of astronomy. CHAPTER 11. THE SOLAR SYSTEM. TI16 Sun— The eight principal planets — The smaller planets— The satellites — Formation of the solar system. The Sun, and its cortege'oi orbs, which do not emit any light of themselves, constitute what we call the solar system. It is composed, firstly, of the Sun, which, for astronomical purposes, is generally designated by the sign ©, the diameter of which is 108 times that of the Earth, and which revolves upon its own axis once in about 25 days, 10 hours ; secondly, of eight priacipal planets, and 128 smaller or telescopic planets, the orbits of which are embraced between those of Mars and Jupiter, at about co-equal distances from the Sun. The principal planets, enumerating them according to their increasing distance from the Sun, have been called : — 1st, Mercury, represented by ?, whose mean (Jistance from the sun is 35,393,000 miles, with a revolution round that luminary of 87 days, 23 hours, 15 minutes, and a dia- meter two-thirds that of the Earth. 2nd, Venus, 9, with a mean distance from the Sun of 66,130,000 mUes, a revolution of 224 days, 16 hours, 48 minutes, and a diameter nearly equal to that of th© Earth. 34 ASTEOKOMY. 3rd, the Earth, ©, with a mean distance from the Sun of 91,430,000 miles, a revolution of 365 days, 6 hours, 9 minutes, and a diameter of 7,901 miles. 4th, Mars, S, with a mean distance from the Sim of 139,812,000 mUes, a revolution of 686 days, 23 hours, 31 minutes, and a diameter half that of the Earth, 6th, Jupiter, %, with a mean distance from the Sun of 475,693,000 miles, a revolution of 4,332 days, 14 hours, 2 minutes, and a diameter eleven times that of the Earth. 6th, Satwn, Tj, with a mean distance from the Sun of 872,135,000 leagues, a revolution of 10,759 days, 5 hours, 16 minutes, and a diameter nearly ten times that of the Earth. 7th, Uranus, 9, with a mean distance from the sun of 1,752,851,000 miles, a' revolution of .30,686 .days, 17 hours, 21 minutes, and a diameter fom* times that of the Earth. 8th, Nejitwne, T, with a mean distance from the Sim of 2,746,271,000 miles, a revolution of. 60,118 days, and a diameter nearly five times that of the' Earth. The solar system is fm-ther composed of 21 planetaiy satellites : 1 for the Earth (the Moon, represented" by the figure ($) ; 4 for Jupiter, ^ for Saturn, 6 for Uranus, 2. for Neptune, and Comets the » number of which increase every day. Kepler suspected the existence of a planet between Mars and Jupiter, and Father Secchi says that he was the nrst to discover a certain regularity in the distribution of the planets. There seemed, however, to be some anomaly in the distance which separated Mars from Jupiter, and, upon this ground, he predicted that some hitherto unknown stav would eventually be discovered in the space between them : a prediction which was more than verified two centuries THE SOLAR SYSTEM. 35 afterwards,* for no less than 128 have already come tmder observation, as Avill be seen by the appended table, and fresh ones are being discovered every day. M. Le Verrier has calculated that they are not, put together, equivalent to a body one-third the size of the Earth, for he argues that if they were larger than that theii- attraction would have led to greater variations in the motions of the perihelion of Mars than have hitherto been noticed.f II. MINOE OR TELESCOPIC PLANETS JUPITEPv. ® Ceres. Pallas. @ Juno. © Vesta, (o) Astiiea, © Hejje, © Iris. @ Flaia. @ Metis. @ Hygeia. (n)- Parthenqpe. (u) "Victoria. @ Egeria. (H) Irene. @ Euuomia. @ Psyche. @ Thetis. ^ Melpomcuo, (19) Fortuna. (m) Massilia. @ Lutetia. @ Calliope. @ Thalia. (iS) Themis. Phocea. @ Proserpine. @ Euterpe. sb) BcUona. @ Amphitrite. @ Urania. ^ Euphrosyne. ^55) Pomona. @ Polyhymnia. @ Circe. @ Leucothaa. (as) .Atalanta. @ Fides. @ Leda, @ Lajtitia. . @ Harmonia. @ Daphne. @ Isis. @ Ariadno. @ Nysa. @ Eugenia. @ Hestia. @ Aglaia. @ Doris. (3) Pales. @ Virginia. BETWEEN MARS AND @ N4mausa. @ Europa. (S3) Calypso. @ Alexandra. @ Pandora. @ Melete. @ Mnemosyne. @ Concordia. @ Olympia. @ Danae. .@ Echo. @ Erato. (m) Ausonia. (^ Angelina. («S) MaximUinna. (m) Maia.j @ Asia, (ra) Leto. @ Hesperia. @ Panopca. (tT) Nioho. @ Feronia, @ Clytie. @ Galatea. @ Eurydice. • Father Socchi on T/m Sun. t Volume xxxvii. p. 793, of the Journal oi the Acadimk des Sciences, p. 33i. . d2 36 @ Freia. @ Friga. @ Diana. @ Eurymonc. @ Sappho. (S) Terpsiclioro. @ Alcmeno. @ Beatrix. @ Clio. @ lo. @ Semelo. (St) Sylvia. @ Tliisbo. @ Julia. @ Antiope. @ Egina. Q Undiua. @ Minerva. Planets 115, 118, and the remainder have not yet been named. ASTRONOMY. (S) Aurora. @ Ipliigcnia. ■(Si) Arethusa. @ Amaltliea. @ jEgle. (m) Cassandra. @ Clotlio. ® (S) lanthe. @ Sirona. @ Dike. @ Lomio. (W) Hecate. ® @ Helena. ® @ Miriam. @ © Hera. ® @ Clymeno. @ @ Artemis. ® @ Sylvia. @ @ Camilla. © (g) Hecuba. @ @ Felicitas. @ @ Lydia, @ @ Ate. III. The revolution of the Earth round the Sun, &c., &c., is now well known ; but what is not so generally known, and what creates some surprise amongst novices, is that the solar system, that is to say, the Sun and the Earth, the planets and the moons, have an incessant tendency to shift towards the constellation of Hercules. Herschel proved that the apparently inextricable irre- gularities of so many special stellar motions are mainly due to the displacement of the solar system ; and that the point in space, towards wliich we are advancing each year, is situated in the constellation of Hercules. These results are mag- nificent. The discovery of the specific movement of our system will always count as one of Herschel's greatest THE SOLAR SYSTEM. 87 achievements, notwithstanding the previous conjectures of Fontenelle, Bradley, Mayer, and others. In juxtaposition to this great discovery must he placed another which seems destined to yield still more magnificent results, and which was given to the world in 1803. I refer to the discovery that certain stars are reciprocally dependent, connecte(J with each other as the planets of our system and theii satellites are to the Sun.* Father Secchi . has set forth in such simple yet clear terms, the ideas generally accepted as correct by modern science concerning the formation of the solar system, that we cannot do better than quote him : — "Astronomers of the present day are agreed that our solar system is due to the condensation of a nebula, which formerly extended beyond the limits now occupied by the most distant planets. This nebula was originally endowed with a slow rotatory motion, which must have gradually become accelerated. According to a law of mechanics, known as the law of svperficies, every disengaged particle must move in such a way that its radius-vector will describe eqas,l superficies at equal intervals ; whence it follows that, the radius being gradually lessened by progressive contraction, the arc described dming the unity of time must have in- creased, so as to keep the superficies from undergoing variation. An augmentation of centrifugal force results from this increase of speed, and when this force comes to equal the force of gravitation, rings are formed which remain suspended around the central mass. As the speed goes on increasing, these rings are broken, and the different fragn^ents, each one in obedience to the laws of attraction, • AstronomU Pojmlaire, 38 ASTRONOMY. in their turn form new masses which are isolated from each other, and whibh become centres of action similar to the principal ceiitre. • These masses ha;ve in turn been sur- rounded by rings of a secondary order, some of which are still in existence, while others, getting dispersed, have gone to form satellites. "This theory, expounded by Kant, Herschelj and Lap- lace, has been confirmed by the ingenious experiments of M. Plateau. A mass of oil poured into a mixture of water and alcohol of the same density as itself, will be seen to assume the spherical shape which molecular attraction would tend to give. it. If it is made to turn around ita vertical diameter at an increased rate of speed, the sphere will be found first to flatten, and after a certain time to detach a ring similar to that of Saturn. As the speed is still further increased, the ring will be broken up, and form into small spheres revolving upon their own axis and around the main mass. " The matter which composed the primitive nebula must have been far more rarefied than that which we are able to obtain with the best of pneuinatic machines, and has since become enormously contracted and condensed, leaving the planets and the satellites at various distances. The Sun is the still incandescent and gaseous residue of the primitive mass. We find in the sidereal world traces of this forma- tion, in our planetary world, the rings around Saturn, in the stellar world, the annular nebulae. These masses are composed of a matter which is still gaseous, and they soem to constitute worlds in course of formation." * M. Inrichs arrives at the important conclusion that the • Father Secclii on The Sun, p. 332. Also Vestiges of Creation. THE SOLAR SYSTEM. 39 law of progressive condensation is bound up with the thii'd law of Kepler, and that the latter is itself the outcome of universal gravity acting in direct ratio with the masses and in inverse ratio with the square of the distances. Thus the law as to the formation of the . planetary system would be but a consequence of universal gravity. ■M. Delaunay, of the French Institute,, also remarks: " The great law of universal gravity, which we owe to the genius of Newton, has introduced unity into the science of astronomy, and enabled us to attach to one particular cause all the peculiarities presented by the motions of the celestial bodies."* * Rapport sur Uprogris de V Astronomic, p. 1. Fig. 10.— Urania, antique statue, now in Sweden. CHAPTER III. LIGHT AND THE DETAILS OF SPECTRUM ANALYSIS. Light enables us to recognise the stars — Revelations by spectrum analysis — Hypothesis of the emission, and that of undulations — Laws of light — Various measurements of its speed — ^The solar spectrum — Chemical action of light — Length of luminous waves — ^Analogy between sound and light — Molecular vibrations and atomic vibrations — Mode of propagating light — Eefraction and reflection — Luminous interferences — How ligh^ added to light produces darkness — ^Triumphal entry of the spectrum analysis into science — Its history — Metals revealed by analysis of the sjiectrum which they elicit dm-ing combustion — Important chemical and spectral laws — Curious experiments in physiology — Undreamed of horizons in astro- . nomy — Ignition in the celestial domain — Spectra of planets, moons, and stars — Substances discovered in the stars — Nature of nebulce — Movements of the stars revealed by their specti-a — Discovery of the natm'6 of comets — Matter following in the track of the aerolites — Unity of composition extending to all the luminaries in the universe — The inhabited celestial spaces, I. It is light which reveals to us the stars, and tlierefore it is by its aid that we must commence our study of them. Spectrum analysis has opened up a new field to astronomy, and laid bare horizons of which no one had ever dreamed. It is wonderful to reflect that a mere ray of light, that is to say, an imperceptible movement communicated to ether in boundless space, is transformed, by the aid of modern science, into a telegram sent from th^ worlds of the infinite to instruct us, not only as to the nature of the elements which compose them, but as to the motions which hm'ry them through space, and which we should be unable to ascertain with the most perfect of telescopic instruments. LIGHT. 41 These revelations, exceeding all that the wildest imagination could have conceived, are absolutely stupefying; and, as an elementary knowledge of the subject is almost a necessity even for those who do not dabble in science, I may, after giving a few notions about light in general, expose very succinctly the most important points of this procedure, which is enriching astronomy each day with new and bril- liant conquests. Few sciences give rise to so many surprises as that of light, in regard to which two very different hypotheses have been formed — that of emission, to which the name of Newton long lent great authority, and that of undulations, started by Descartes, and now very generally adopted. The hypothesis of emission supposes a luminous body to propel in different directions a material substance of extreme tenuity, so subtle as to render it impossible to ascertain its weight and solidity ; a substance which, passing through certain bodies without any loss of velocity, may yet be brought to a stop by others. When this substance comes in contact with an organ of the vision, part of it makes its way into the interior of the eye, and produces the sensation of sight. In the hypothesis of undulations, instead of supposing the transport of a material agent to great distances, it is held that the vibrations of luminous bodies are communicated to the atoms of an all-pervading ethereal fluid. These vibra- tions, propagated through this fluid, reach the organ of vision, which in turn transmits them to the optic nerve. In this hypothesis, the nature and the transmission of light would be analogous to the nature and transmission of sound, light being produced by atomic, and sound by molecular vibrations. 42 ASTRONOMY. The most recent experiments of the savants — the re- seai'ches upon interferences,' amongst others — have led to a general acceptance of this latter hypothesis. II. It is known that light declines or diminishes in force or intensity in proportion as it recedes from the point whence it emanated. This diminution is in direct ratio to the square of the distance ; for instance, if the distances are 1, 2, 3, 4, &c., the quantities of light received at distances 2, 3, 4, &c., win be 4, 9, 16 times, &c., less than at distance 1. Eoemer, the Danish astronomer, by an observation of the eclipses of Jupiter, succeeded in ascertaining the speed of light. He noticed that it always took 16 minutes and 86 seconds longer to reach the Earth when Jupiter was in conjunction with the Sun, upon the other side of the ecliptic, than when that planet was upon our side in oppo- sition to the Sun. Hence he concluded that light must take that time to traverse the whole diameter of the terrestrial orbit, that is to say, about 182,000,000 miles, and that consequently it must be 8 minutes 13 seconds in coming from the Sun. Until the experiments of M. Fizeau it was never supposed that the speed of light could be arrived at by terrestrial observations; but by an extremely simple procedure he proved that the luminous movement travelled llj miles in the -TB'rro of a second. This figure is almost the same as ♦hat arrived at in antecedent observations, but a slight want j jf clearness in the impressions obtained prevents this mea- ' surement from being so precise as those taken in the heavens. LIGHT. 43 M. Foucauk tried a new mode of measurement, in 1862, by means of a revolving mirror, and ascertained that light had a velocity of 186,250 miles a second. The procedure alluded to above gives 192,500 miles, so that there is a marked difference in the results. Many objections have been taken to this hitter method. The light had only traversed a space of 37, feet, and in this short distance it had undergone five reflections and pierced an object-glass, which, it was suggested, might have occasioned a diminution of velocity. It was urged also that no one can tell what is' the totality of phenomena which take place m a reflection, and that in fact the result "of the experiment did not or might not apply to light in an open space. Nor can one place unlimited reliance upon the extreme micrometrical divisions which it was necessary to make use of. M. Gomu made some furUier experiments for the purpose of ascertaining the exact rate at which .light travels, the results of which he has recently laid, before the French Academy of Sciences. His mode of observation is in the main similar to that adopted by M. Fizeau, except that he has perfected certain details, and amongst them may be pointed out a, means of registering by electricity the speed of the notched wheel, which it is all the more necessary to ascertain, for this wheel is used for making a direct comparison as to the velocity of light. His own point of observation was an attic in the Polytechnic School, and the other point a room in the barracks on Mont Val^rien. In this way M. Comu has obtained sufficient data concerning the velocity of light to enable liim to decide between the rate of 192,500 or 193,750 miles, as given in former calculations, and that of 44 ASTRONOMY. 186,250 miles, as established by M. Foucault in his experi- ments with the revolving mirror. To quote his own words: — "My observations concord with those of M. Foucault, though it is necessary to point out that his experiments required verification, both becr,u8e their details and the mode of procedure, not having been made public, had not been subjected to discussion, and because the method of the revolving mirror is open to grave objections, into the nature of which I cannot enter at present. M. Fizean's method is, on the other hand, free from these objections. Astronomers, again, will find in this new valuation of the velocity of light a strong confirmation as to the parallax of the Sun 8".86, which is obtained by com- bining this number with the constant of aberration. This was ascertained by M. le Yerrier, after three series of observations relative to the movement of the planets, especially of Mars and Venus. So we see that in astronomy it is impossible to overestimate the importance of deter- mining precisely the velocity of light." * III. When objects are looked at through a glass prism, not only do they appear to have been considerably displaced by the deviation of the luminous streaks which pierce the prism, but also to be covered with bands of the brightest colours. If a prism be so arranged that a luminous streak shall fall obliquely upon one of its facets, and, as it emerges, be caught upon a screen or a picture, the result wUl be an * Paper read at the Acadimie des Sciences, Feb. 10, 1873. LIGHT. . 45 oblong figure of a thousand variegated coloiurs, which has received the name of solar spectrum.* The law of reflection and refraction, and the various ways of magnifying objects by means of the lens, were known before Newton's time, but the nature of light and the origin of colours were still a mystery. Without questioning the fact of their being produced in this way, no one had sus- pected that a ray of white light was composed of a large number of single rays, each of itself capable of imparting a colour of its own. Delille expresses this idea in the lines : " Dam les mains d'un enfant, nn globe de savon D^ loDgtemps prdc^da le prisme de Newton, £t longtemps, sans monter a sa sonrce premiere, Un enfant dans ses jcnx dissequa la luniire. Kewton seid I'apeifnt, tant le progris de I'art Est le fruit de I'Stode et sonTent du liasard." There are seven shades especially noticeable in solar light as decomposed by the prism, which have on this account been termed the principal orders. Taking them in their natural order, they are : red, orange, yellow, green, blue, intKgo, and violet. In explanation of these phenomena, white light is looked upon as-composed of an infinity of rays, differing in colour, and more or less susceptible of re&action, which separate from each other as they pass through the prism. The first point noticed in solar radiation is the light which we receive, and the heat which accompanies it ; but there is also a third and very important order of phenomena, viz., chemical action. Its influences are not a thing apart, but the varying effects of one single cause, consisting simply • See same author's Hisloin du MiUores, fig. 4, p. 76. 46 ASTEONOMY. of a series of undulations which only diifer from each other in regard to their length and the rapidity with which they ai'e produced. Fig. 11. — Recomposition of liglit. The ^vaves, whose length is comprised between 768 and ,369;milli6n fractions of a milUmetre ('03987 of an inch), ,have the power of maldng our optic nerve vibrate, and thus .produce the sensation pi light. • The diversity of coloui's. de- pends only upon the length of the waves, the longest being red, and gradually toning oiF to violet. With the colours ex- tending from the green to the violet, the luminous waves also have the power of separating the molecular groups, and pro- LIGHT. 47 ducing chemical effects. These luiJiinouS waves exteftd far beyond the visible spectrum into a region whither the eye cannot penetrate, but they can be recognised by the agency of photogenical preparations. From the green to the red the waves become longer, and possess the property of agitating the molecular groups by a purely physical action, but, . in most cases, without decomposing them. These waves also extend beyond the red, and thus form a second part of the spectrum, which is invisible.* We know that soimd is produced by molecular, and light by atomic vibrations, and that the general laws relating to .'these different Idnds'of vibrations are the same; but it is -worthy of remark that the sound vibrations extend much further than those of light, since the latter, sensible to the ej'e, do not embrace more than what is known in acoustics as an octave. IV. ' Light always travels in a straight line, through a region which is perfectly and uniformly diaphanous. . But a mere difference of density is sufficient to bring about its deviation . from the straight line, and to break it up into heterogeneous quantities. As the various strata of the air all vary in density, it foUows that the light of the stars does not come to us in a straight line, and that we do not see them in the position which they actually occupy. At the horizpn there are at least 33' of refraction, or, in other words, rather more than the lai'gest diameter of the Sun or the Moon.' Thus, when the lower margins of the latter luminaries seem to * Father Secchi ou T/ui Sun, Part II, 48/ ASTRONOMY. be just upon the hprizoh, in realitythe whole of their disc is submerged beneath it (see fig. 12). The same illusions take place in our observations of the stars arid of far distant Fig. 12.— Phenomena of refraction. bodies. But as light, in its passage through the various strata of the atmosphere, does not encounter any sudden change of density, so it does not break suddenly off, as, for instance, in its passage from the air into water or a tumbler. It follows a curved line, and not a broken one. The refrac- tion of the light from the Sun and Moon gives us the benefit of their presence for a longer period, as it anticipates the Licm. 50 ASTKONOMY. hour of their rising, and delays that of their setting. It is to refraction that we owe the dawn which heralds the brightness of day, and the twilight which precedes the darkness of night. The refraction of light also flattens these luminaries, and makes their disc look oval. This is all the more pronounced the nearer they are to the horizon, and, as Biot has remarked, " when seen from eminences near the sea-shore it sometimes equals a fifth of the apparent diameter of the Sun. The Moon's disc presents the same phenomenon." Light which, having passed through a very refracting region, reaches a space where the refraction is not so greati will sometimes stop at the point which separates the two spaces, undergo a total reflexion, and travel back through the space which it had already traversed. This peculiar phenomenon takes place whenever the rays reach the surface of emersion at too great an angle of obliquity. The fact of this total reflexion explains all the varieties of magical phenomena known as mirages. In my work on the ** History of Meteors," a chapter is specially devoted to this subject, and contains many facts connected therewith which I ascertained during my travels in the New World, V. The phenomena of Inminous interferences will appear to people who are not well acqudnted with optic discoveries, perhaps even more marvellous and incredible than those of the mirage. Supposing a bright ray of solar light to come in direct encounter with a sheet of white paper, for instance, it LIGHT. 5i follows that the portion of the paper on which the ra}' strikes will be made to shimmer. But what seems incredible is, that this portion may be made quite dark without touching the paper, without arresting or diminishing the luminous ray, but, on the contrary, by increasing it. The magic mode of changing light into shade, day into night, is even more remarkable for its simplicity than for its prodigious effects, consisting, as it does, in conducting on to the paper, by a slightly different trajectory, a second , luminous ray, which of itself would have made the paper still more brilliant. One might imagine that the two rays, when they met, would have this effect ; but, on the contrary, light added to light produces darkness ! The movements of these rays neutralise each other, and the light ceases to cast any lustre. It will happen, however, that these lumi- nous rays, by reason of their direction, only partially neutralise each other, in which case there is merely a dimi- nution of the lustre. These are the curious phenomena, which, obliterating or diminishing light by the adjunction of a luminous ray, have received the name of inUrfereneet. Of the thousand rays of variegated shade and re&angibility which compose colourless (or white) light, those only are capable of neutralising each other which possess co-ordinate colours and ifeirangibility. Thus, a red ray cannot be made to obliterate a green ray. If, again, two white rays cross each other at a given point, it may happen that, in the infinite series of various coloured lights of which they are composed, the red ray, for instance, will alone disappear, and that the point of their intersection will become green — green being white nunus the red. s 2 63 ASTRONOMY. Two rays which have been changed from the -state of natural light into polarised rays following the same direc- tion, will still possess the property of interference, com- bining or neutralising each other like, and under the same conditions as, ordinary rays. Two rays which pass, without any intermediate stage, from their natural state into that of rays polarised at right angles, are definitely deprived of the property of intoi/erence. Though their course, the character, and the thickness of the spaces they traverse be modified in a thousand ways ; though they be brought back to the state of reflections com- bined of the requisite properties, or to parallel polarisations, it will be impossible to make them obliterate each other. But if two rays which were polarised in two rectangular directions, and which could not, therefore, have any influence upon each other, had in the first place undergone parallel polarisations when transformed from their natural condition, they could be made to obliterate each other merely by restoring them to the state of polarisation with which they were originally endowed. It is impossible to avoid a feeling of surprise when one - learns, for the first time, that two luminous rays are capable of destroying each other, and that darkness may result &om the conjunction of two lights ; but it is still more extraor- dinary that this property can be given and theti taken away from them, in some cases momentarily, in others altogether. The theory of interferences, looked at from this point of view, might seem rather the product of a diseased brain, to use Arago's expression, than the strict and inevitable outcome of numerous experiments which cannot be con- troverted. It was in this theory that Fresnel foimd the key to all LIGHT. 53 the beaatiful phenomena of colouration engendered foy the crystallised plates, or films, which are endowed with doable re&action, proving that they were special instances of interference. Dr. S. Young has achieved his greatest renown by his experimental and complete demonstrations con- cerning interferences, and the principles which he laid down were set forth and still further applied by Fresnel. The theory of undtUation suggested by Descartes in explanation of the phenomena of light, had been already accepted by the savants, and the most recent experiments in regard to interference confirm its perfect accuracy. Those who would reconcile the Bible with modem science will gather, from "what I have stated as to the nature of light, that Moses may have said with truth that light was created before the celestial luminaries, inasmuch as it is but a manifestation of undulations. VI. We now come to the question of the spectrum analysis. It is not only the sunlight which can be decomposed and jnade to produce a spectrum, but any other kind of light as well. It is, however, very remarkable that these lights, when decomposed, give different spectra, and by examining their character that of the bodies which produce them can also be ascertained. The study of a body by the analysis of its spectrum is termed the spectrum analysis. M. Delaunay, of the Institute, gives the following r^mmS of this discovery, so fruitful in its consequences. In order to separate, if possible, the partial images produced by the various unmixed lights, of which imcoloured (or white) 54 ASTRONOMY. light is composed, one naturally endeavours to make each of these images as narrow as possible, to prevent them from encroaching upon each other. This may be done by ad- mitting the light from mthout through an opening in the shape of a narrow chink, standing some distance back to examine it, and placing the prism in such a way that its square edges will be parallel to the length of the chink. WoUaston tried this plan in 1802, a century after Newton had discovered the solar spectrum ; and the result partially answered his expectations. Submitting certain artificial lights, such as the flame of a candle and an electric spark, he obtained results analogous to, but not identical with, that yielded by the sunlight. Yet Wollaston, in thus looking with the naked eye through the prism, obtained but a vague glimpse of the phenomenon which he was in research of, and which Fraunhofer was destined to discover in all its splendour. In 1815 that noted Munich optician, without being aware of the essay made by Wollaston thirteen years previously, endeavoured to scan through a prism the spectral outline of a ray of light admitted through a narrow chink; and to observe it more closely he made use of a glass. The spectrum then appeared to him traversed, not by four or five dark rays, as it did to Wollaston, but by a far greater number. He saw, in fact, as many as five or six hundred. He then proceeded to measure accurately the relative posi- tions of a large number of these rays, and made a drawing of the spectrum, with 354 of them in their respective places. He also ascertained, like Wollaston, that the spectra given by artificial lights are distinguished by a special disposition of these rays, and, in some cases, even by a complete absence of them. LIGHT, 55 Fraunhofer's discovery drew the attention of scientific men to the subject, and numerous experiments were imder- taken for the purpose of studying the curious phenomena which he had revealed to the world. It must be added, that a very remarkable relation has been established between the brilliant rays produced by gas in a state of combustion, and the obscure rays which this same gas creates in the spectrum of a light passing through it. These rays, brilliant in the one case, obscure in the other, occupy absolutely identical places in the spectrum. Where hydrogen gas, for instance, produces a brilliant ray when on fire, the same gas produces an obscure ray by the absorption which it exercises upon an extraneous light passing through it. TioB remarkable circumstance, known as the inversion of tiie spectrum, was first pointed out by Foucault, in 1849, and definitely established by Eirchhoff * ten years later. Such are the various phases which the question of spectrum analysis has passed through. vn. Thus we find that every substance in a state of ignition yields its special spectrum, and without seeing the body which is burning, we can, by a mere examination of its spectrum, tell to a certainty how it is composed. Gold, when incandescent, emits a spectrum different from that of silver, and that of silver differs again from other metals, &c. The main properties of some metals are so alike that it is almost impossible not to confound one with the other in the ordinary mode of investigation, and metallurgists have * Delaunay's Notice sur V Analyse Spectrale. 56 ASTRONOMY. consequently resorted to an examination of their spectra when incandescent. By means of spectral comparison and analysis they have discovered differences not to he traced in the suhstances themselves, and by this procedure have already discovered three new metals : rubidium, ccBsium, and thallium. Spectrum analysis also leads us to an important chemical discovery, which M. Dumas laid before the French Academy of Sciences in 1871. M. Lecoq de Boisbaudran had, some years ago, demonstrated the affinity between spectra of alkaline metals and those of metals coming out of an alkaline soil. He had shown that the displacement of characteristic rays was effected in accordance with the same law as the modifications in the weight of the elementary substances. Messrs. Toost and Hautefeuille, on the one hand, and M. Ditte on the other, having followed up these re- searches, ascertained that the course of the rays towards the ultra-violet hue manifests itself exactly in the same way as the increase of the atomic weights does for carbon, silica, titanium, tin, and zirconium upon the one hand, and for sulphur, selenium, and tellurium on the other. M. Dumas pointed out that this is but another proof, amongst the many which science already possessed, as to the truth of the prin- ciple upon which, in 1827, he established his classification of simple bodies into natural categories. Spectrum analysis may also be very useful in physiology and medicine. In some cases of poisoning,, the toxical element can be ascertained by burning a part of the flesh or excrement, and by decomposing through the prism the light produced by the combustion. It was in this way that M. Lany discovered thallium in the organs of animals to which that substance had been administered. LIGHT. 57 But It is in astronomy that its results have heen most remarkable. This science has made use of the spectrum analysis to extend its researches over milliards of millions of leagues, and by its means has ascertained the character and elements of the countless luminaries which pervade space. Here is an instance : a brilliant star having suddenly made its appearance (May, 1866) in the constellation of the Corona Borealis, and almost as suddenly vanished, its light was at once submitted to spectrum analysis, and it was proved that this star was one already known as emitting a very dim light, which had momentarily brightened. Its spectrum had great analogy with that of our sun, and we are told by Messrs. Huggins and Miller that "the spectrum of this star, coupled vrith the sudden outburst of its light, and its almost equally rapid diminution in intensity, make it probable that owing to some vast internal convulsion, enormous quantities of gas were emitted from it, the hydrogen in which must have taken fire by its mixture with some other element, and caused the light represented by the brilliant rays, the flames raising the solid matter in the photosphere of the star to a white heat. The hydrogen burnt out, the light would then have gradually diminished in brilliancy, and the star would resume its normal appearance." It must be borne in mind that, owing to the immense distance of the star in which this jire, (to use a word which designates the phenomenon with great accuracy) took place, the light must have taken a considerable period to reach us, and that it had been at an end ten, twenty, a hundred years or more, before we were made acquainted with it.* • Delannay's Notice sur T Analyse Speclrale. 58 ASTRONOMY. VIII. Spectnim analysis, by enabling ns to read in a ray of light the nature of the body which produces it, the elements constituting that body, and the changes that take place in it, becomes as it were a messenger from the stai's, the confidant of infinite space, the telegraph firom incalculable distances, the revealer of the closest secrets, and even a relentless denunciator. Mr. Huggins has published a work upon the spectrum analysis which contains many new and important facts, and to it we shall often have occasion to refer. The various spectra differ from each other in many important respects, but they may all be divided into three categories. 1st. The distinctive character of spectra of tiie first order consists in the continuity of those coloured bands broken by no ray, brilliant or obscure. "Whence we learn that the light in which this spectrum has its origin is emitted by an opaque body, which probably exists in a solid or liquid state. A spectrum of this order does not disclose to us the chemical nature of the incandescent body whence the light proceeds. 2nd. The spectra of the second order are formed firom coloured rays of light isolated from each other. They indicate to us that the bnlliant matter emitted by the light is in a gaseous state ; and as each element and each compo- nent body, which has become luminous without being decomposed, is distinguishable when in a gaseous state by certain rays peculiar to it, it follows that these rays are capable of revealing to us the nature of the bodies from which they are emitted. 3rd. The third order comprises the spectra of incan- LIGHT. 59 descent bodies, solid or liquid, in which the continuity of the coloured bands is broken by sombre rays. These latter are not produced by the source of light, but by vapours through which the light has passed on its way. Spectra of this kind are yielded by the light of the Sun and of the stars. The group of sombre rays produced by each vapour is identical, as regards their number and their place in the spectrum, with the group of brilliant rays of which the light is composed when the vapour has become luminous. rx. As the Moon and the planets have no light of their own, and only shine with the reflected light of the Sun, their spectrum must consequently resemble that of the Sun, modified only by the passage of the light through the atmo- spheres of the planets or by the reflexion on their surface. The spectrum of the Moon does not indicate that our satellite is surrounded by any atmosphere, nor any other distinctive feature. In that of Jupiter there is a dark band corresponding to certain atmospheric terrestrial rays, and indicating therefore the presence of vapours sinular to those in our own atmosphere. Another band denotes the pre- sence of certain gases and vapours which do not exist there. The spectrum of Saturn is somewhat faint, but it contains certain rays similar to those in the spectrum of Jupiter. M. Janssen ascertained that many of the atmospheric rays are produced by vapour of water, and it is probable that this aqueous vapour does exist in the atmospheres of Jupiter and Saturn. He goes on to say: — "During my recent mission to Italy and Greece, I took observations of several planets in regard to this point, notably from the CO ASTRONOMY. summit of Etna, where the influence of the atmosphere is ahnost nullified. These observations, and some subsequent ones, made with the most powerful instruments, indicate the presence of vapour of water in the atmospheres of Mars and Saturn, thus adding a new and important feature to the already close analogies which connect the planets of our system. So we see that all these planets form as it were one family, circulating around one focus, which distributes amongst them heat and light. Each of them has its year, its seasons, its atmosphere, and many of them are known to contain clouds within their atmosphere. " And, in addition, water, which is so important an element in the economy of every organism, is also comtaon to them. These facts give us strong ground for supposing that life is not the exclusive privilege of our little Earth, herself but one of the younger sisters in the great fanuly of planets ! " * Certain rose-tinted groups have been remarked in the spectrum of Mars, which may have some connection with the red hue distinctive of this planet. The spectrum of Tenus does not show any additional ray to denote the presence of an atmosphere. The absence of these rays may perhaps arise from the fact that the light is probably reflected, not by the surface of the planet, but by clouds some height above it. X. The stars, though much farther from us than the Moon and the planets, possessing, as they do, their own som'ce of * Memoir read to the Acadlmie des Sciences. LIGHT. 61 Ught, furnish us with more detailed information as to the nature of the elements of which they are constituted. Until the secret of spectrum analysis was discovered, we knew nothing about the stars, beyond the fact of their im- measurable distance, and their striking beauty ; now, we are in a position to learn some details of their real character. Spectrum observations tell us that the stars resemble the Sun, as to the general character of their composition. Their light, like that of the Sun, emanates from a matter raised to an intense white heat, and traverses an atmosphere of absorbing vapours. Yet, notwithstanding this structural unity, each star differs from the other, though not to an essential degree, in its chemical composition. Judging them by their spectra, they may be divided into four per- fectly distinct types, though a few of the spectra, instead of belonging to one of these four categories, seem to be intermediate between them.* With a few exceptions, the terrestrial elements which are most largely distributed amongst the stars are precisely those which are essential to life as it is on the Earth, such as hydrogen, sodium, mag- nesium, and iron. The hydrogen, sodium, and magnesium also represent the ocean, which is an essential part of a world constituted as the Earth is. Looking at the stars generally, they seem brilliant, like colourless diamonds, red, orange, or yellow tinted ; but this is not so if they are carefully watched, or observed through a glass, for then we can see next to the red or orange-^ted stars others of a blue, green, or purple colour. The spectrum analysis shows us that these diverse colours are produced by the vapours in suspense in their atmo- spheres, and we know that the composition of a stellar ♦ rather SeccW on TTie Swn, p. 800, 62 ASTRONOMY. atmosphere is in turn dependent upon the elements which constitute the star, and upon its temperature. Spectrum analysis of the variable transient stars also reveals to us the phenomena produced by the incessant changes that react upon the rays which these stars trans- mit to us. Thus it is that we have received tidings of the great perturbations taking place in the brilliant star Corona, which was only recently observed, and which has already decreased in bnlliancy. XI. During the last 150 years asti'onomers have been con- stantly revolving in their minds the veritable nature of the slightly luminous nebulosities (nebula;) which stand out from the dark surface of the firmament — conglomerations so filmy in substance as to remind one of the comets. This question has become all the more interesting now that they are held to be a part of original matter — embryo stars. The telescope has failed to enlighten us on this head, though it is true that since the object glasses have been made larger, many of these bodies have turned out to be actual stars. But at the same time other nebulosities, hitherto undiscovered, have been brought within the field of vision, to say nothing of other fantastic figures (aggrega- tions of diffused light), which it is impossible to look upon as the produce of the combined brilliancy of countless suns situated at distances more or less unfathomable. Spectrum analysis has settled this long-vexed question ; has shown us that certain nebulsa were not masses of distinct stars, but matter in a gaseous state, and has even enabled us to ascertain their elements. LIGHT. 63 ' Mr. Huggins, describing his first experiment on this head, in August, 1864, says : — " I selected one of the luminaries in the class of nebulse, which was very small, but relatively brilliant. Great was my astonishment when, on ■ looking through the small glass of the spectrum apparatus, I discovered that its spectrum had no longer the appearance of a luminous coloured band such as a star would have pro- duced, and that in place of the continuous band there was nothing to be seen but three bright rays, isolated from each other." This spectrum must, so far as it was possible to judge, have proceeded from a light emitted by matter in a gaseous state. The most brilliant of its rays was produced by a body analogous to nitrogen, or, as Mr. Huggins thinks, even more elementary than nitrogen, and which we have not as yet been able to analyse. The weakest of its rays corre- sponded with the green of hydrogen. The mean ray of the gronp was almost, but not quite, identical with the ray of barium. In addition to these brilliant rays, there was a continuous spectrum, singularly faint, proceeding from a diffused light, which seems to correspond with the centre of the nebula. This is proof that it contains a nucleus, very small, but more brilliant than the rest of its mass. Mr. Huggins has since analysed the spectra of more than sixty nebula or stellar masses, which may be divided into two groups; the first comprising the nebulee, which give a spectrum such as that just described, or, at all events, a spectrum comprising one or two of the three rays in question. Of the sixty which he examined, about a third belong to the class of gaseous bodies: the light of the remaining forty produces spectra of apparent continuity. 64 ASTRONOSiy. The harmony between the results of spectrosocpic and tele- scopic ohservations, in regard to what is common to both, is a proof of their accuracy; half of the nebulae with a continuous spectrum have been shown to be stars, and in process of time another third will probably be added to the number. But Lord Eosse failed to ascertain this definitely concemuig a single one of the gaseous nebulce. XII. Mr. Huggins was the first to apply the spectrum analysis to the study, not of matter, but of the motion of the stars. If they possess any motion of relative importance, their rays must undergo a certain amount of displacement, and by this means these motions may be estimated. In respect to some stars, Sirius in particular, his researches have established the motion of these rays beyond all doubt. He also applied this effective procedure of analysis to the observation of the comets, and arrived at the strange con- clusion that the central part has a light of its own analogous to the flame of compound carburets, whereas the nebulosity emits only a light received from the Sun. This delicate distinction, says M. Faye, is of the highest importance in studying the physical constitution of these bodies. I may add that the spectrum experiments conducted by Alexander Herschel have proved that sodiimx exists in a state of luminous vapour in the train of many aerolites. The result of the spectrum studies goes, therefore, to prove that the stars only vary from each other, and from the Sim, in special and minor ways, and that there are no important and essential differences in their constitution. M. Faye, in one of his reports to the French Academy of LIGHT. 65 Sciences, says : " Thus we see extended to all the stars of the universe that unity of composition, which distinguishes our solar world and the aerolites ; unity which is, however, compatible with many variations as singular as they are unlooked for." These same results lend a great semblance of tnith to the supposition that the stars have a function analogous to that of our Sun; and that they are, lilce it, surrounded by planets which they keep in place by force of attraction, and which they illumine and vivify by theii- light and heat. So it may well be — and eminent astronomers have given such an opinion their sanction — that these distant regions are inhabited by beings intelligent like ourselves, capable of studying the harmony of creation, and of appreciating the power of its supreme Author. Fig. 11. — Eclios (the Sun). lUiodcs Medal in tlie Britisli Musensi. CHAPTER IV. THE SUN. II.; nature— Light and Aspect presented by its Surface — Grains of Rice, Willow-leaves, Straw-motes, &c. — Pores, Faculffi, and Spots in the Sun — Formation, nature, and motion of Spots — Rose-coloured Shadows, Red Protuberances — Change of Shape in the Spots — The Sun obfuscated by their enormous quantity — Rotation of the Sun— Synodical Revolution — Sidereal Revolution — Periodicity of the Spots — Solar Electricity and Hydrogen upon the Earth, aud in the Planetary Regions — Solar Explo- sions — Constitution of the Sun — ^Two opposite hypotheses— Is the Sun inhabited I — Curious Anecdote— Recently acquired notions abont the Sun — Temperature of the Sun — Curious Calculations — Is there any probability of the Sun ever ceasing to shed light, heat, and life upon the Earth — Lucretius and our modem Astronomers — Zodiacal Light : its nature — Some parts of the Sun more briUiant than others — Atmosphere of the Sun — ^Various elements of which the Sun is composed — The Seasons — Dimensions and distance of the Sun — Variation of its Diameter — Its influence upon the Earth — Recapitulation. I. " This star among the stars," to use Arago's expression — " the light of the world," as Copernicus calls it — " the heart of the universe," in the language of Theon of Smyrna — we see shining like a luminous globe, incessantly darting in every direction its rays, which transmit vrith inconceivable rapidity both light and heat. Most of the ancient philosophers looked upon it as a burning body, which lighted the world with the emanation of its substances. In modem days there prevail two opinions, which I set forth in the chapter on Light ; one being that the Sun does, as a matter of fact, emit luminous matter emanating THE SUN. 67 from its own disc; the other, that space is filled with a rarefied and elastic substance, which is called ether. This substance, by the vibratory motions which it transmits with great rapidity, produces to the eye the phenomenon of light, much in the same way as the vibrations of the air produce to the ear that of sound. This latter theory is the one now aniversally adopted. If the Sun's surface is examined through the powerful instruments which science has at command, it will be found to present the irregular and undulating appearance of a stormy sea, covered with crevices and hillocks, and dotted over with spots of a more or less dark colour. At times may be seen, particularly close to the edge of the spots, luminous masses, known as faculce. To obtaiu a more complete knowledge of the Sun's structure, it must be examined through a powerful ocular glass, at a time when the atmosphere is perfectly still. Then it will be found that the surface is covered with a multitude of small grains, all of about the same dimensions, but varying very much in Bhape, though the majority of them are oval. A somewhat Bimilar result is arrived at by examining through the microscope milk that has been standing some time, the globules of which have lost their regularity of shape. These grains sometimes unite in small groups, when they present a more brilliant appearance, while their oval shape has led to their beiug compared to grains of rice and willow leaves. The collections of these grains near the solar spots ore somewhat different, for they seem to be longer, and to be joined to each other, perpendicularly to the edges of the penumbra, and look like straw-motes of unequal length, or the thatch on a roof seen through the telescope ; the Sun also seems to be dotted with spots, of various shapes and size, less 68 ASTRONOMY. brilliant than the rest of its disc. They are simply solutions of continuity in the solar photosphere, caused by clouds composed of metallic vapours. I'ig. 15. — AjJiicarause o£ tLo Suii'a surface as seen tliioiigli powerful glasses. II. The solar spots generally look lilce round black patches, though it often happens that they are clustered in such a way as to form an' irregular figure, with the central part, which is called the nucletis or urnbra, black ; the contour, which is mezzo-tinto, is termed the penumbra, the whitish spots being known as faculse. THE SUN.' 69 These spots were at one time supposed to be satellites revolving round the Sun, and, subsequently, to be clouds floating in its atmosphere, and even masses of scoriae floating Fig. 16, — Front view of a spot on tlie Sun. upon the sea of fii'e which constitutes its surface ; or even mountains whose beetling flanks may have produced the phenomenon of the penumbra. It is now about a centuiy since Wilson (England) proved to demonstration that the spots were due to cavities of which he had been able to ascertain the precise depth ; and he at the same time gave an exact idea as to the composition of the photosphere — that is to say, the luminous stratum which envelops the Sun. The dimensions of the spots vary veiy much, some being mere black spots known as pores, others having a surface much larger than that of the Earth, some few being four or five times larger than the surface of our globe. The 70 ASTRONOMY. spots are not equally distributed all over the disc. There are not many in the immediate vicinity of the equator, and next to none in the latitudes exceeding 35 or 40 ^ 1 ^s 1 9 Fii'. 17. — Spot observed close to the edge of the Sun. degrees, but they are much more abundant in the two symmetrical zones comprised between 10 and 80 degrees of latitude. Their number is also very variable ; sometimes there are so many of them that in a single observation one can ascertaiia the zones which usually contain them. In 1637 they were so numerous that the heat and brilliancy of the Sun were perceptibly diminished, and history records many similar obfuscations brought about by the same cause. At other times they are so rare that a whole year passes away without one of them being seen. The phenomena which they present seem at times to have only a superficial influence, but, generally speaking, it extends to the depth of the solar body, which is often agitated and heaved up over a wide expanse, amounting occasionally to a quarter of the whole disc. Thus it is possible that these spots may be the outcome of a violent agitation amongst the THE SUN. 71 matter of which the Sun is composed. The most plausible hypothesis is that attributing them to the influence of the planets (of Jupiter, Venus, and Mercury, in par- ticular), the attraction of which create regular tides on the solar globe and the great disturbances already men- tioned. III. Father Secchi, whose opinions, the result of most careful observation, are shared by many astronomers, looks upon them merely as solutions of continuity in the stratum of mists or luminous vapoiurs which form the photosphere. These clouds differ from ours in two respects, being com- posed not of vapour of water, but of the vapour of metallic substances, and, by reason of their elevated temperature, they are luminous of themselves, but are less brilliant than photosphere. So far as the external aspect goes, it is the completely identical ; the Earth covered with clouds, would appear mammiform in structure like the Sun to any one placed at some distance firom it, and the phenomenon has even been remarked from mountain summits, especially during a thunderstorm. This theory, as Father Secchi points out, explains, with- out having re^jourse to fabulous rates of speed, the rapidity with which certain changes in the shape of the spots take place. The apparent displacement of a cloud may be under- stood without supposing that the substance has traversed the same space as the contour of the cloud, for it may be accounted for by- a change of temperature, producing upon the one hand condensation, upon the other, dissolu- tion of the vapour over a considerable surface. He puts 72 ASTRONOMY. ' the question as to the nature of the spots in this way — " Are they caused by an obscure substance, rising above the luminous substance, or is it not rather the luminous matter which penetrates into an obscure region ?" He goes on to point out that all the phenomena alluded to are only to be explained by the second hypothesis : that there exists in the spots a luminous substance which pene- trates into a less brilliant region — call the clouds an obscure part if you will, but it is none the less true that the luminous part penetrates thither. The spots must contain a transparent substance, less brilliant than the photosphere, and of a gaseous character. Our atmosphere -would appear the same to a spectator looking into it from outside, say from the Moon; the clouds lighted up by the Sun would seem brilliant, while he would see black spots at the points where the air was transparent,* M. Faye, of the French Institute, has, on the other hand, propounded the hypothesis that the spots are whu-lwinds caused by the unequal speed of the successive zones of the photosphere, the angular rotation of which diminishes in speed from the equator to the poles, and that their law of motion denotes at the same time their distribution over the solar surface. In a report read to the Academie des Sciences (Dec. 80th, 1872) he says that this is naturally the case, because these spots are neither more nor less than whirl- winds engendered directly in the photosphere by the unequal speed of its parallels. In another memoir, he indicates a very curious similitude between solar and terrestrial cyclones, the laws of these two orders of pihenomena seeming almost identical. In reality. Father Secchi's theory is not incompatible • Father Secchi on Due Bun, p. 77. THE SUN. 73 with that o{ M. Faye, for the former, in reply to the argu- ments quoted above, says — " The question as to whether the spots are whirlwinds is but of secondary importance, for, even admitting them to be so, the only cause by which they could be originated would be an eruption."* IV. The spots often change in shape and vanish after having appeared for a short time, or traverse the whole visible surface of the Sun, following a line oblique to the diurnal motion and the plane of the ecliptic, and reappearing in l^#l^E.ij^l'i^'''l*V''"''7'''''l''ij!'!;lSir'f;' '"- Fig. 18. — ^Tho same spot seen at different points of the Sun. their original condition at the expiration of twelve or thirteen days. The motion of these spots has revealed to us the remark' able phenomenon of the Sun's rotation upon itself. Jordan Bruno, of Naples, author of a " Treatise upon * Father Secchi's Memoir to the Acad4mie des Sciences, March 3, 1873. 74 ASTRONOMY. the Universe," published in 1591, was the first to suspect this fact, which was definitely ascertained to be correct by Jean Fabricius, from whose memoir, published in 1611, I quote the followiug passage : — " I conceived the idea of attracting the Sun's rays through a very small aperture to a darkened chamber on to a sheet of white paper. I noticed that this spot (one which Fabricius had discovered in the Sun) had taken the shape of an elongated cloud. After an interruption of three days, caused by the bad weather, my observations showed me that the spot had made an oblique movement westward. I also noticed a smaller one close to the edge of the Sun, which in a few days reached its centre, and after that a third. The first of the three soon dis- appeared, and the others at an interval of two or three days. " I was apprehensive that they might not return, but at the end of ten days the first one reappeared in the east. It then became clear to me that these spots were accomplishing a revolution, and my opinion was confirmed by other persons to whom I pointed them out. I hesitated for some time to publish my observations, the accuracy of which seemed affected by the fact that these spots did not maintain the same distances &om one another, and that they underwent a change of shape and speed. It was, there- fore, all the more gratifying for me to remember that, as the spots are apparently on the actual body of the Sun, which is spherical and solid, they must necessarily diminish in size and slacken their speed as they reach its edges." * From a close observation of the spots it has been con- cluded that the Sun revolves upon itself in a periodof about twenty-five days, and like the Earth from east to west. Scheiner puts the synodical, that is to saj', the apparent * Fabricius, translated by Lalandc, THE SUN. 75 revolution, in which the spot seems to an observer to return to the same point upon the disc, at twenty-seven days. This gives twenty-five days and a third for the duration of the sidereal revolution — ^that is to say, the time taken by a given point of the Sim to describe a complete circle. Thus, in place of observing the rotatory motion of the solar body itself, we are compelled to study that of its atmo- sphere, being, in fact, similarly placed to an astronomer who, to ascertain the rotatory motion of the Earth, had taken up his position in the Moon, with a cloud as his point of com- parison. He would first of all have to study the atmospheric circulation and discover the laws by which it was governed — a task so difficult under such circumstances, as to be well- nigh impossible.* V. Schwabe has compiled a long series of statistics concerning the periodicity of the solar spots, having from 1826 to 1868 observed the Sun every day that the state of weather per- mitted. The result of his observations was that he recog- nised the existence of a positive periodicity, very marked maxima and minima succeeding each other at an interval of about ten years. This decennial period coincides very unexpectedly with several meteorological phenomena on the earth, — amongst others, as recent observations testify, with the variations of magnetic force and the periodicity of aurorje boreales. The periodicity of the spots indicates, as Father Secchi remarks, a periodicity in the solar activity, and the variations of this activity may well be communicated to the Earth, * Father Secchi on 2%« Sun, p. 111. 70 ASTRONOMY, either by means of heat or some other channel as yet unknown, such, for instance, as electro- dynamic induction, thus producing upon our globe meteorological or electric phenomena.* The theory expounded by M. Becquerel at the Academic des Sciences, in 1871, is in confirmation of this view. He maintains that all the causes which elicit electricity firom the Earth's surface would be insufficient to supply the enormous quantities which are diffused in the planetary regions, and even in our atmosphere. He goes on to show that the hitherto unknown origin of this electricity can be none other than the Sun. The spots upon this luminary, some of them 40,000 miles in extent, seem to be the cavities from which the hydrogen and the various substances composing the solar atmosphere are emitted. Now, hydrogen conveys with it the positive electricity which becomes diffused in the planetary regions, permeating thence to the terrestrial atmosphere, and even to the earth itself. The matter which electricity carries with it suffices for its transmission, it having been proved that electricity has the property of becoming diffused in a void space if it has any accompanying matter. The phenomena of the polar aurora produced by electric discharge also prove, in M. Becquerel's opinion, the existence of gaseous matter in space, far beyond the bounds assigned to the terrestrial atmosphere, it being certain that these aurorse are at least 125 miles distant from the earth's surface. M. Charles Sainte- Claire Deville points out that the facts adduced by M. Becqaerel in support of the celestial origin of atmospheric electricity confirm his own hj-pothesis as to the celestial origin of the variations of atmospheric tempera- * Father Secchi on TJie Sun, p. 33C. THE SUN. 77 ture, and, especially, as to the influence which the periodical apparition of cosmical suhstances in the interplanetai-y regions may have upon these phenomena. The following facts will fit in harmoniously at this point. Signer Tacchini wrote from Palermo to the Acaddmie des Sciences, that the aurora borealis of February 4th, 1872, was a phenomenon so extraordinary as to have few parallels in scientific annals, and tliat its apparition was accompanied by corresponding movements upon the surface of the Sun. The bad weather prevented Signer Tacchini from taking spectrum observations on the 8rd and 4th of February, but he noticed, on the mprning of the 5th, that the whole Fig. 19.— Motions noticed vpon the Sun's surface by Signor Tacchini, during the auiora borealis of February 4th, 1872. surface of the Sun was in an abnormal condition. The rim was covered with bright flames ; towards tlie north pole they exceeded 20 seconds, with an arc of 36' to rijght and to left, corresponding Avith a bright region of magnesium which, at the western riin, extended almost to the equator. In this portion, at 50 degrees from the pole, was seen a magnificent prominence, which rose to 2' 40", and from this point, at an arc of 40 degrees, the rim was lighted with brilliant flames, while the atmosphere was studded with small lumi- nous filaments, or bright specks, about 2 minutes in height. 78 ASTRONOMY. Signor Taccliini also sent a drawing to illustrate these phenomena (see fig. 19). M. Cheux, communicating to the Academic des Sciences the features of a white aurora horealis observed near Angers, on the 8th of August, 1872, states that the Sun had been for some time in a very effervescent state, and that on examining it with a Foucault telescope on the 9th of August, he saw about 24 spots, one of which, a deep black, was very beautiful. The drawing which accompanied his description is reproduced in fig. 22, p. 88. Senor Capello, of Lisbon, also sent some drawings of the Sun as it appeared on the 8th, 9th, 10th, and 11th of August, after the same aurora boreahs (see figs. 23-26, p. 94). Father Sanna-Solaro, in a memoir on the same subject, argued that if the Sun is taken to be the principal source of atmospheric electricity, the facts, otherwise most difficult to co-ordinate, immediately link themselves into the chain of phenomena, conveying with them, so to speak, their own explanation. Father Sanna-Solaro is, in my opinion, one of the highest authorities on meteorological subjects, and his work upon the " Causes and Laws of the Movements in the Atmosphere " may be studied with profit. VI. At the same time, there is anything but an unanimity of opinion as to whether the Sun is the main cause of the elec- tricity by which we are surrounded. M. Faye, in a very able treatise in support of his own theory, says : " We know that there is a fundamental dilference between electricity and heat or light. The greater the vacuum the more rapid is the propagation of light and heat, so much so that certain THE SUK. 79 specialists, supposing a material medium to be necessary, propounded the idea of filling the infinity of space with absolutely- imponderable ether. But electricity requires ponderable matter in order to manifest itself in the shape of currents or of simple force, attractive or repulsive. Electric experiments carried on in a place where there is an approach to a void, are very feeble in their results, and they come to a full stop in a laboratory where a complete void has been created. Thus, I repeat, the electric agencies in question must be conducted in a ponderable space. Now, we have seen that if the celestial regions are furrowed in all directions by numberless corpuscles, shooting stars, aerolites, remains of comets, and even, perhaps, by solar hydrogen, etc., these small masses of ponderable matter, accomplishing their distinct orbits round the Sun, could not possibly form a continuous mean like the air in which we set electricity into action. " I should not have thought it necessary to insist upon this idea, but that my colleague, M. Becquerel, had recently brought it into prominence by his endeavour to connect the Sun with our own atmospheric electricity. M. Becquerel admits that the solar mass is incessantly emitting hydrogen which becomes diffused in space, conveying with it its own electricity, essentially positive, and communicating it to the stars on its passage, without, however, coming in contact with their atmospheres. I do not intend to discuss these ideas, merely wishing to point out that the hydrogenous emanations of the Sun would not constitute a continuous mean capable of serving as a vehicle of communication to electric repulsions or attractions. Eepulsed from the Sun by the supposed electricity of the chromosphere, or rather, perhaps, by the repulsive force of the photosphere, these 80 ASTRONOMY. molecules would also be endowed with rotatory speed ; they would, therefore, describe hyperbolic curves convex towards the Sun, and branching towards all parts of the universe. Thus they would speed along in separate directions, gradually getting further from each other, without being capable of exercising the mutual reactions which constitute an electric mean or a gas."* M. Becquerel still maintains his theory that the Sun is the probable origin of electricity, and has answered M. Faye's arguments in a treatise laid before the Academic des Sciences, in November, 1872. VII. Before proceeding further, it will be as well to explain what is meant by the solar atmosphere, and the phenomena to which it gives rise. This atmosphere is double; the first part, which envelops the centre of the Sim, is called the photosphere. Like the region which it surroimds, it is the seat of vast chemical processes. As it radiates towards the celestial regions it loses a portion of its heat, while the gaseous bodies which it contains, becoming cool and condensing into vapour, relapse into the interior of the Sun, to return a&esh into the photosphere, and recommence the same trans- formation. Such is the explanation given of the first atmosphere by those who are of opinion that the Stm has no solid nucleus — an opinion which is generally accepted in the present day, and the reasons for and against which will be found below. The solar photosphere, according to Father Secchi.t must * M. Fayc, and the Academie des Sciences, Oct 9tb, 1872. + Father Sccchi on The Stcn^ p. 219. THE SUN. 81 contain vapours of every variety, which, owing to their extreme levity, must attain a great altitude. If a large number of bodies, looked upon by mineralogists as elementary —precious metals in particular — ^have not been discovered in the Sun, it does not follow that there are none, for it may well be that these metals, owing to the great density of their vapours, are detained in profound regions inacces- sible to spectriim analysis. The following is the nomen- clature given to substances known to exist in the Sun, arranging them in the order of their atomic weight, firom the lighter to the heavier : — hydrogen, sodium, magnesium, aluminium, silicon, potassium, calcium, chromium, manganese, iron, copper, zinc, and harium. Beyond this luminous envelope or photosphere, which forms the apparent limit of the solar disc, is an atmosphere properly so called, transparent, but endowed with a sufficient power of absorption to arrest a part of the solar rays. It is not of uniform altitude, attaining the maximum of height at the equator and near the spots, and the minimum at the poles. This atmosphere, which completely envelops the Sun, is almost entirely composed of hydrogen of a very high temperature ; it also contains a small quantity of sodium and magnesimn vapour, and even of vapour of water. In it may be noticed aggregations of rose-hued patches, analogous to the flames which may be seen round the moon's disc during the solar eclipses, and which are known as red protuberances. Hydrogen is the main element of these phenomena. 83 ASTRONOMY. VIII. Father Secchi points out tliat, by vii-tue of the law of superficies, the inner strata of the sim must have a rotatory motion more rapid than the outer ones, and that the friction has not perhaps set up a motion of identical character throughout the whole mass. The points situated at the equator must be vested with a speed greater than those nearer to the poles, as is proved by the motion of the spots. He admits, at the same time, that the exact theory of circulation in the solar mass is not yet completely solved, and that we must for the present put up with hypotheses on the subject. It also results from his observations, that the length of the solar diameter is to be decided by the amount of activity of the sun itself, and that the diameter of the disc is least where the activity is greatest. This unlooked-for conclusion concords with the general comparisons that have been made between the length of the diameters and the number of the protuberances.* The Sun is the seat of explosions which seem to be connected with the production of spots. Father Secchi has even succeeded in obtaining some definite information as to one of these phenomena, the results of which he laid before the Academie des Sciences (August 6th, 1872), accompanied by the drawing reproduced in figs. 20 and 21. This phenomenon occurred at 3.30 p.m. on the 7th of July, 1872. At 2.40 p.m. there was nothing but a small luminous jet; the internal motions of the incandescent vapours were so intense that the luminous clouds were seen to change in shape in a moment ; and at 4.15 p.m. they had ' Academic des Sciences, Sept. 9tli, 1S72. THE SUN. 83 reached an altitude ten times greaiter tliaii tlie diameter of the Earth, or, in other words, of 79,000 miles. This Kg. 20. — Solar explosions. eruption lasted two hours, and was repeated the next day at the same point in the Sun. Father Secchi adds, that at the same date an Aurora Borealis was seen at Madrid and many other places in Europe, and the phenomenon was also accompanied by violent magnetic perturbations in many places. The zodiacal light also extended over an unusually wide space, whence he concludes that these various phenomena are connected with each other, and that the great motions of the solar photosphere have their comiter- part upon the earth. An examination of figs. 20 and 21 will show that the large cumulus-shaped cloud (a), which at 3.50 p.m. was just above the jets, was formed bj the 84 ASTRONOMY. 1 entanglement and fusion of the jets themselves, and that when the mass had risen and spread itself out, the cloud Fig. 21. — Solar explosions. seemed to hreak up into filaments, curved in shape like the acanthus leaves of a Corinthian pillar (figs, b, c, d, e). At the same time, the ciu'ves of these jets are not only parabolic, but actuallj' spiral, for the volute is seen to be forming at the exti'emities of the filaments. This fact, first pointed out by Young, has been confirmed beyond the pos- sibility of a doubt during the eruption of July 13th (see fig. 21). Fig. A represents the aspect of the strange phenomenon at 8.50 p.m., on the 7th of July ; fig. b at 4.15 ; fig. c at 4.30; fig. D at 5.10; fig. p at 6.30. Fig. p represents the last traces of the eruption of the 7th, suspended in the air above some faint flames. Fig. g represents the eruption of July 18th, at 11.85 a.m. ; fig. h at 4.35 p.m.; and fig. i at 6.20 p.m. Fig. k represents a spot displaying traces of the eruption observed upon the 11th of July near the Sun's edge. IX. The question as to how the Sun is constituted was con- sidered by the most distinguished of astronomers long THE SUN. 85 before the discovery of the spectrum analysis enabled the enquirer, seated in his study, to ascertain what was taking place millions and milliards of miles away, and the result naturally was that many diverse theories were propounded. Herschel, Laplace, and several other astronomers of mark, held that the Sun consisted of an obscure body, surrounded by an atmosphere in which floated a deep stratum of clouds, and only the upper part of which was in ignition ; whence it would follow that the Sun might be inhabited. This ingenious theory takes account of the various appearances presented by the spots with which the solar body is often studded, and acquires great probability &om the polarizing experiments made by Arago. It has, however, been called in question of late years, chiefly owing to the results obtained from the spectrum analysis. If a flame containing metallic vapours is submitted to this analysis, their presence is denoted by characteristic coloured rays ; but if behind this flame is a second luminous source more intense than the first, and containing the same metallic vapours, instead of the superposed rays receiving an accretion of brilliancy, the rays of the fainter focus wiU absorb those emanating from the more ardent one, and in the place of luminous there will be obscure raj-s. And, as the solar rays emit precisely similar beams, and give what is technically termed an inverse or reversed spectrum, Kirchoff has concluded that the body of the Sun must be more incandescent than its atmosphere. But M. Petit, a former director of the Toulouse Observatory, in a memoir communicated to the Academic des Sciences, points out that this theory takes no account either of the spots, penumbrte, faculse or luculiE, or of the absence of polarization. And as the total eclipses of 86 ASTEONOMY. tlie Sun have recently revealed that the photosphere is surrounded by a second aeriform envelope, luminous like the first, though in a minor degree, he very justly says that the inverse spectrum of the sun is easily to be explained, if ■we suppose that the second atmosphere contains metallic vapours of the same nature as those in the first. He concludes, therefore, that it is not necessary to admit that the solar nucleus is in a state of fusion, and Herschel's opinion as to the possibility of the Sun being inhabited need not be called in question. He adds, " Instead of an incandescent body, which must in the nature of things become cool and die out, we might thus imagine an incessant revivifj'ing of the combustible properties, by organised beings inhabiting the surface of the solar nucleus, and maintaining the equilibrium, just as the plants and animals do in our own atmosphere." Arago says, " If I were asked the simple question — ' Is the Sun inhabited ? ' I should reply that I did not know. But if I were asked whether the Sun is habitable for beings organised like ourselves, I should have no hesitation in answering in the affirmative. The existence in the Sun of an obscure central nucleus, .enveloped in an opaque atmo- sphere, separated by a considerable space from a luminous atmosphere, is by no means inconsistent with such a supposition. Herschel believed that the Sun was inhabited. He maintained that if the solar atmosphere, in which the luminous chemical reaction takes place, is a million leagues deep, there is no reason why the brilliancy should anywhere exceed that of an ordinary Aurora Borealis. The arguments upon which he relied as proof that the solar nucleus is not necessarily very hot, notwithstanding the incandescence of the atmosphere, are neither the only nor the most powerful THE SUN. 87 ones that might be adduced. The direct observation, taken by Father Secchi, as to the lessened temperature of those points on the solar disc where the spots are noticed, is, in regard to this fact, of more importance than all the theoretical arguments put together. Dr. EUiott asserted, as early as 1787, that the sunlight was given by what he called a dense and universal Aurora Boreahs, and he held, with the ancient philosophers, that the Sun might be inhabited. When he was tried at the Old Bailey for the murder of Miss Boydell, his friends — ^Dr. Simon amongst others — declared that he was out of his mind, and cited as a clear proof of his insanity the pages in which the opinions just quoted were embodied. The ideas of a madman are nearly always adopted. This anecdote seems fitted to figure in the annals of science, and I have taken it from ' Brewster's Encyclopedia.' "* X. The astronomers of the present day are less unanimous than they were in Arago's time as to the possibility of the Sun being inhabited; butM. Vicaire, in a communication to the Academie des Sciences, endeavours to show that we must go back to the theory of Wilson, Herschel, and Arago, as to the existence, within the photosphere, of a nucleus comparatively cool and obscure. To use his own language : " The prmcipal objection that has been advanced against the hypothesis is that this nucleus, subject to the radiation of the photosphere, would lonf since have acquired the same temperature. This • Arago's Astronomic PopuUire. 88 ASTRONOMY. objection falls to the ground if the heat received by this nucleus is' employed in vaporizing the liquid of which it is formed. Moreover, this heat may and must be only a trifling fraction of what is emitted by the photosphere, absorbed as it is by the intermediate stratum, which is incessantly re-conducting it into the photosphere. As to the length of time that the nucleus has been subject to this volatilization, there is nothing to prove that it is to be measured by the total duration of the earth. I believe, on the contrary, that the sun, as at present constituted, has only shone upon this globe since the most recent geological periods.."* Many astronomers, Father Secchi and M. Faye among them, believe that the whole mass of the Sun is gaseous. Fig. 22.— Surface of the Sun on the 9th of August, 1872, at 6 a.m., as observed by M. Cheux. and that the speed of its various strata increases from the surface to the centre. The former says : " When this Sun * Acadimic des Sciences, Aug. 26th, 1872. THE SUN. 89 at the epoch of its formation had reached a volume about equivalent to that which it now possesses, its temperature would have been at least 500 million degrees, and, moreover, we know by experiments that even now its surface tem- perature amounts to several million degrees ; that of the interior is probably higher still. We must conclude from these facts that the Sun cannot be composed of a solid mass ; nor, enormous as may be the pressure existent in this mass, it cannot possibly, so to speak, be in a liquid state. "Whence we are necessarily led to the supposition that it is gaseous, notwithstanding its extreme condensation."* M. Delaunay, of the Institute, says : " The enormous temperature which the Sun must possess, renders very probable the existence in its atmosphere of the various bodies just mentioned (different metals). Upon the other hand, as the voliune of the Sun is 1,260,000 times that of the terrestrial globe, and as its mass is only 314,760 times that of the Earth, the mean density of the Sun is only a quarter that of the Earth, and consequently not much greater than that of water. Such being the case, it is difficult to believe that the Sun is a solid body enveloped in a covering of brilliant clouds, constituting what is termed the photosphere. I am inclined, rather, to agree with M. Faye, that the Sun is a gaseous mass with a very elevated temperature, which prevents the elementary substances that enter into its composition from consolidating ; while their decrease in heat superficially, brought about by the radiation into the celestial spaces beyond, would facilitate the production of combinations, which in turn, owing to the formation of solid and pulverulent precipitates, disseminated • Father Secclii on Tin Sun, p. 289. 90 ASTRONOMY. in the outer strata of the gaseous mass, would produce the brilliant light of the photosphere. These solid precipitates would, by reason of their greater density, gradually descend into the inner portion of the mass, where they would be decomposed by the high temperature and again become gaseous. Moreover, these descending currents would cause the formation of ascending currents, by means of which the matters in the inner part would be brought to the surface, so that the whole gaseous mass would in this way contribute to sustain the vast production of heat and light upon the Sun's surface. The spots, varying in number, position, shape, and size, which are generally visible in the Sun, would merely be gaps accidentally made, amidst the reful- gent clouds of the photosphere, by the currents alluded to above."* For my own part, after comparing the various solutions that have been proposed, I must pronounce for the gaseous nature of the Sun. XI. In a letter to M. Dumas, which was read at the Acaddmie des Sciences, in the early part of 1869, M. Janssen, whose researches in connexion with the spectrum analysis have obtained great notoriety, supports this view by a summary of the knowledge hitherto acquired as to the constitution of the Sun. This commimication, stated succinctly, shows that modem research, interpreted by M. Faye's theory, tends to the conclusion that the Sun is essentially a gaseous globe, with a temperature of its own so elevated that no * Dclaunay's Notice sur t Analyse SpcctrdU. THE SUN. 91 substance or body can exist there, save in a very gasiform state. But it is known that gases, even when raised to a very high temperature, are but faintly luminous when they do not contain particles of a fixed body — that is to say, of one not reduced to gas. How, then, are we to explain the brilliancy of the Sun ? In this way. The region in which the solar globe moves causes a diminution of temperature upon the surface of that luminary, suflScient to condense within it the gaseous elements and reduce them into solid dust. This dust, mixed up with the incandescent gases, gives them the effulgency and radiation which we perceive, just as carbon, Ume, and magnesium impart the luminous property to the dull flames of our own gases. Thus, by a relative decHne of temperature, the gaseous globe is surrounded by a very luminous envelope ; this is the photosphere, or visible part of the Sun — ^the Sun itself as it appears to the general public. In this photosphere are visible spots and rents which have attracted the careful attention of astronomers. These rents in the luminous envelope, the diameter of which is often double or treble that of the earth, enable us to ascertain that the central gaseous nucleus is relatively obscure ; their motions have revealed the law of the superficial rotation of the Sun — a rotation, the speed of which varies according to the latitudes, and thus have supplied us with one of the most striking proofs of the gaseous character of the Sun. It is the examination of the spots, too, that has led astronomers to admit the existence of an atmosphere around the luminous envelope. But the existence of this atmo- sphere, which has since been revealed by the phenomena of refraction noticed on the photosphere, and by the effects of absorption remarked upon the edges of the solar disc, 92 ASTRONOMY. was only guessed at, and its nature, its altitude, and its com- position were the objects of the most contradictory state- ments. As to those singular luminous appendages or pro- tuberances which have been observed during the latest total eclipses, absolutely nothing was known about them. Such was the state of things when the great eclipse of August 18th, 1868, supplied the first opportunity for applying the new method of analysis to these phenomena. Analysis of the light of these protuberances revealed first of aU their character and their gaseous composition. These large appendages are almost exclusively composed of in- candescent hydrogen. It has also been remarked that this hydrogen exists over the whole circumference of the Sun, and that the protuberances are but the more prominent parts of this hydrogenic atmosphere. When this interesting memoir, here summarized, was read, M. Leverrier remarked that the theory which consisted in treating the Sun, in regard to its luminous portion, as an incandescent globe, covered with a small gaseous atmo- sphere, to which part of the phenomena observed upon its surface are attributable, has been established beyond the possibility of doubt by the observations taken during the total eclipse of 1860. The important point ascertained during the eclipse of 1868 is as to the nature of this atmosphere; and M. Janssen, by making it possible to observe, at any period, phenomena which had before been visible only at the moment of a general eclipse, had rendered a great service to science. Upon the same occasion, M. Leverrier read a memoir firom M. Eoyet, in which it is shown that the yellow ray discovered by the spectrum analysis is visible upon the whole contour of the Sun ; whence he concludes that the incandescent gas to which it corre- THE SUN. 93 sponds is, upon the same principle as hydrogen, a constitutive element of the solar atmosphere. At the same time, ve do not at present know what this gas is, for the ray in question does not coincide with the yellow ray of sodium. XII. The question naturally arises : Why is the temperatm-e of the Sun so enormous ? It may have been caused by the very force of the gravity which conjoined the elements that formed the central point of the solar system. In the first instance, the temperature thus mechanically acquired must have been much higher than it is at present, now that the Sun is getting cooler. At the same time, the diminution of its heat, great as it may be, is almost imperceptible to us, being, as it is, so gradual, and partially compensated by the transforming of a portion of the solar mass into various chemical combinations. It may also be that certain foreign bodies, attracted into the Sun, help to maintain its inces- sant combustion. There is a great variety of opinion as to the sum of the solar temperature, and it is very asto- nishing to find that the researches of the specialists lead them to such widely different conclusions. During the last few years much has been written upon this subject, and one of the most recent treatises is that of M. Yicaire, who points out that Father Secchi estimates the temperature at 10,000,000 degrees Cent., while M. Spcerer puts it at not more than 87,000. And if to these opinions I add that of M. Pouillet, who thinks that it is not less than 1,461, or more than 1,761 degrees, my readers will see that science has not yet reached any satisfactory conclusion in regard to this matter. It is even more surprising that the most opposite results. 94 ASTKONOMY. those of Secchi and of Pouillet, have been deduced from the same phenomenon, viz., the calorific radiation of the Sun, the intensity of which they estimated by an ahnost identical process. As M. Vicaire remarks, so enormous a diiference August 8tli, 7h, 29m. p.m. August 9th, 7h. ISm. p.m. August 10th,' 8h. Bm. p.m. August 11th, 9h. 42m. p.m. Figs. 22a to 25.— Aspects of the Sun during the Aurora Borealis of August, 1872. in the results evidently cannot be due to the observations, but to the manner in which they have been interpreted ; and, after careful consideration, he arrives at the conclusion that Pouillet's evaluation is far nearer the truth than that of Father Secclii. Upon this, M. Elie de Beaumont pointed out how Sir William Thomson had shown that the Sun's THE SON. 95 temperature cannot be so very much higher than that attained in certain manufacturing processes, and adverted to his treatise upon solar heat, in which he states that the quantity emitted, according to PouiJlet, is not equal to more than a seven-thousand horse power to each square foot of its surface. Coal burnt at the rate of a pound in two seconds would produce almost the same result, and Eankine has estimated that in the locomotives coal is con- sumed at a rate not greater than a pound per square foot in from thirty to ninety seconds. This great problem as to the surface temperature of the Sun is, as M. Elie de Beaumont adds, more accessible now than it once was. This is principally due to the astrono- mical expeditions for studying, at the epoch of total eclipses, the physical constitution of the Sun, not the least important of which was that of 1858 to Fanaragua in Brazil. The result of M. Becquerel's researches in regard to the question of high temperatures, and the phenomena of irra- diation which accompany them, leads him to the conclusion that the highest temperatures which can be produced by combustion or electric agency do not exceed 2000 or 2500 degrees Cent., and that consequently the solar temperature, which is not so widely removed as might be supposed from the temperatures of these sources, would not exceed 8000 degrees. M. Fizeau thinks that if the solar radiation is, as a matter of fact, greater than the most intense sources of light which the Earth can produce, it has not, nevertheless, been found more than double or treble that of the light proceeding from them all. Thus these two sources oi light are in all points comparable, whence it is to be inferred that their respective temperatures cannot differ verj' widely. 96 ASTRONOMY. as certain estimates recently formed about the temperature of the solar surface would tend to prove. M. Fizeau's argument seems to me very conclusive. M. H. Sainte-Claire Deville says, that to speak of very elevated temperatures and their measurement is to admit that the gases are capable of dilation or compression by heat to an indefinite extent — a fact which is not proved ; or else that there is no limit to the chemical combinations, of which there is even less evidence. He also points out that to calculate the temperature of any given point of the Sun's mass is to neglect altogether the influence of the stratum — a very deep one, for all we know — of obscure solar matter which, so flEiT as we can judge, overspreads the incandescent stratum, and the radiation of which towards the Earth is also eliminated. He goes on to notice a fresh experiment which might help to settle the question. The hydrogen rays emitted by certain points of the Sun's incandescent matter, have been ascertained by astronomical observations ; Frankland and Lockyer found them present in hydrogen flame subjected to a certain pressure, and it follows that the combustible temperature of hydrogen at this same pressure can be calculated, and, as a necessary consequence, the character and pressure of the gases at those points of tho solar atmosphere where the hydrogen rays have been noticed. The result of the first experiments upon this head induce biTTi to believe that the temperature is somewhere about 2500 or 2800 degrees, which corresponds with the subse- quent experiments of Btmsen and Debray. THE SUN. 97 XIII. To complete this summary of the opinions arrived at by the most eminent astronomers, I will now add the conclu- sions come to by Father Secchi concerning solar tempera- ture, its origin, and its sustenance. 1st. The solar temperature is of several million degrees, though it is impossible to say precisely how many.* 2nd. This temperature is to all appearances the result of gravity, and must have been produced by the collapse of the matter which constituted the primitive nebula and which now composes the Sun and the planets. 3rd. At this epoch of formation the temperature must have been much higher tJian it now is : therefore the Sun is in process of cooling. 4th. Though the Sun is continually losing vast quantities of heat, the diminution of temperature is almost impercep- tible, not exceeding one degree ia four thousand years. This is due to the state of disjunction in which the matter remains under the action of the heat. 5th. Though the temperature of the Sun is not altogether invaiiable, its secular variations axe, at the same time, slighter than the frequent fluctuations which we remark without being able to iavestigate them completely. There- fore we may take for granted that our planet will continue to be habitable for a long series of ages. He proceeds to say that " though the temperature of the Sun is not altogether invariable, yet the variations are so trifling that they are only perceptible after many thousands of years. After a stiU greater lapse of time — after many millions of centuries, for instance — the Sun will become much cooler ; and a time wiU, no doubt, arrive when it will • This estimate is, as mentioned above, very much contested. H 98 ASTEONOMy. no longer possess the property of sustaining life upon the surface of the planets. It is possible that the Creator has thus ordered things from the beginning, with the purpose of repairing its activity by some extraordinary phenomenon, such, for instance, as the fall of a nebula. But these are points upon which it is unnecessary for us to dwell. Who can say whether the order which now reigns in our solar system is intended to last indefinitely ? As we know from geology, the present state of things has not always been going on, and as it has had a beginning, why should it not have an end ?" * XIV. Lucretius forestalled oiu- modem astronomers when he said : " I am aware how novel and incredible an opinion I express in predicting the future collapse of the Heavens and the Earth, and how difficult it will be for me to convince people of its truth. This is always the case when one pro- pounds a truth to which utterance has not yet been given, and which, moreover, is not susceptible to the ear or the touch — the two sole conductors of evidence into the sanctuary of the human mind. . . . You believe, perhaps, that the Earth and the Sun, the Heavens and the Sea, the Moon and the Stars are divine substances, destined to be eternal; that it is, consequently, an act of impiety, equal to that of the Giants, and meriting the severest punishment, to dare by vain arguments to shake the vault of the world, to extinguish the Sun which shines in the heavens, and to subject im- mortal beings to destruction. But all these bodies are so far from having anything in common with the divine nature, and so unworthy to be placed in the rank of Gods, that they * Fathez Secclii on Tlx Sun, p. 292. THE SUK. 99 are rather calculated to give us an idea of brute and inani- mate matter; for you must not suppose that feeling and intelligence are common to all bodies alike. " Moreover, if the Heavens and the Earth have never had an origin, if they subsist since all eternity, how comes it that there was no poet to celebrate the achievements pre- ceding the war of Thebes and the downfall of Troy ? How is it that so many heroic deeds are buried in oblivion and excluded for ever from the eternal annals of fame ? I am certain that our world is new ; it is yet in its infancy, and its origin does not d^e far back. This is why certain arts are perfected and others only invented to-day ; navigation is but just beginning to progress ; the science of harmony is a discovery of our own time ; and, lastly, that philosophy, the principles of which I expound, is but of recent date, and I am the first of my countrjTnen who has been able to discourse about it." * This train of reasoning is not very conclusive, but the quotation just given expresses in beautiful language the ideas which were current upon this topic in the days of Lucretius. To bring the subject of solar heat to a conclusion, I may add that researches about the Sun date from a very early period. Lucas Valerius remarked that its image was more brilliant at the centre than at the edges. Tliis important fact was called in question by Galileo, but it is correct, as has been proved by recent observations, those of Father Secchi amongst others. The latter also show : — 1st. That all radiations undergo a considerable absorp* tlon, -which increases froni the centre of the solar disc to the edge, where it is at its maximum. 2ud. That the equatorial regions are of a higher tempc- * Lucretius, Book iv. 100 ASTRONOMY. rature than the regions situated beyond the 80th degree of latitude, the difference being at least 1-16. 3rd. That the temperature is a trifle higher in the Northern than it is in the Southern hemispheres. 4th. That just as the spots emit less light, so also do they emit less heat than the other regions.* XV. This is the place for a few remarks concerning the zodiacal light. This light* is a phenomenon which generally accom- panies sunrise and sunset, about the period of the equinoxes, that of spring more especially ; it is seen in the form of a cone of whitish light, which is visible in the direction of the zodiac, being brightest in the regions where the sky is very limpid. I have observed, it, j;^der specially favourable conditions, upon the Atlanticv^jurin the Southern Seas. In length it sometimes seemkyrwiscj^B an arc of ninety degrees. The ancients iesigaaiMo/^uLa^nS^hy the name of trabes (rafter), The first savants^Tvro^ttempted to give it a scientific explanation seem to ht^e been J. D. Cassini and Mairan. Cassini supposed the Sun to be enveloped in a nebulous stratum, in shape like a very flattened and nearly lenticular spheroid, extending beyond the orbits of Minerva and Venus to that of the Earth. De Mairan, who had even taken detailed observations. of this phenomenon, gives a de' scription of it corresponding to that of Humboldt, and, like Cassini, he also connects it with the solar atmosphere, higher around its equator, on account of its rotation, which would account for its elongated form, visible only when the points- of observation are not plunged in this atmosphere. • Father Secchi on The Sun, p. 133. THE SUK. 101 M. Liais, in the course of his numerous sea-voyages, devoted special attention to this phenomenon, and he com- municated the result of his observations to the Academic des Sciences in 1858. " I have demonstrated," he says, "that one can only account for the zodiacal light by admitting that it is due to an imponderable substance forming around the Sun a sort of nebulosity, in which the Earth is completely lilunged. The annular aspect of this nebulosity is caused by its forming a sort of flattened eUipsoid round the Sun, that is to say, a thin stratum of matter very slightly inclined towards the terrestrial orbit, which is entirely contained in the interior of this stratum. If, therefore, we look in the direction of the flattening, or, in other words, of the ecliptic, we remark a greater thickness of matter than exists in any other direction. Consequently, we receive more light &om the side of the zodiac than we do from other quarters, so that this zone appears to us more luminous than the other parts of the sky, without being so in reality. Everybody must have remai'ked that Avhen the weather is very clear no part of the celestial vault is completely sombre. Owing to the limpidity of the air, the light from the nadir is also more pronomiced at the tropics than in the temperate regions. This comes from the solar nebula, to the glimmer of which is conjoined the slight quantity of light transmitted to us by the stars. " The zodiacal light, when a jgood view can be got of it, as in the inteiiropical zone, is the most beautiful of all phenomena. In colour it is pure white, though, as seen in Europe, certain observers have thought that they could discern a reddish tint. This latter has not, however, any real existence; for if so it would be better seen at the tropics, as colouration always becomes more marked in ASTRONOMY.. THE SUN. 103 proportion to its intensity. I believe that observers have in this instance, confounded the zodiacal light with the last red traces of twilight. At the tropics themselves, in the months of July and August for that of Capricorn, and in the months of January and February for that of Cancer, the zodiacal light is visible in the evening after sunset, perpen- dicular to the horizon. When night sets fully in, there rises in the west a white vertical column, the central axis of which equals and even exceeds in intensity the most brilliant parts of the milky way. Upon the edges of tliis column the light gradually tones off to the faint glimmer ol the heavens. It differs in this respect from the milky way, the edges of which at certain points present a striking con- trast to the surrounding sky, as in the black aperture of the Southern Cross called the coal-sack,"* Silbermann deduces from the observations which he has made that the zodiacal light has close affinity with the affluence of shooting stars and the apparition of Aurorse Boreales. In a memoir communicated to the Academie des Sciences, he says : " Whenever there is an affluence of shooting stars, there is an Aurora Borealis, either luminous or else merely cloudy, in the mean latitudes. Numerous facts make me think that such is also the case with the zodiacal light, and this recalls to my mind that the zodiacal light, like the Aurorse, concurs with sudden oscillations of the barometer, and that it is sometimes also, like the Aurorse, of a bright red colour. . . , The sudden changes of intensity, as well as the appearance of undu- latory motions, were observed by Himiboldt. A zodiacal light, extending from one edge of the horizon to the other, like that seen by Beguelin, was observed by M. Liais. * Liais, Espaoe ClUste, p. 131. 104 ASTRONOMY. Respighi, again, has recently ascertained by spectrum analysis that the zodiacal light offers the brilliant ray of nitrogen discovered by Augtrom in the Aurorse Boreales. All these facts, as well as the coincidence of the zodiacal light ■with the affluences of shooting stars and the Auroras Boreales, tend to show that this light is in reality a zodiacal Aurora, corresponding to the tide waye, and not to that of cosmical matter. It is known, too, that Laplace would not admit that the zodiacal light might be a wide extension of tlie Sun's atmosphere.* XVI. This notice would not be complete without a succinct summary of the scientific notions concerning the Sun which have now long been acquired and popularised in elementary works of education. The Sun is incessantly darting its rays from all points of its surface, and there is not an instant during which its light ceases to permeate every corner of the universe. From the close of June it undergoes a daily decrease of elevation, but the heat, nevertheless, continues to increase during the summer. And this is easy of comprehension, for we know that a body wanned by the Sun retains its heat for some time after it has ceased to be exposed to the solar rays. If a good-sized piece of metal is exposed to the Sun dm-ing a very hot summer day, it will be found to retain a certain amount of heat an hour after sunset. It therefore follows that the Earth, which is so much larger, will retain during the night, and even until the following morning, part of the heat communicated to it by the Sun on the previous * Acaddmie dcs Sciences, April 8tli, 1872. THE SUN. 105 day. The Sun adds a fresh amount to that already existing, and BO the Earth obtains an increasing balance of heat. In this way the heat goes on increasing in the bosom of the Earth, or in the air to which it communicates itself, until the nights get longer, when our globe gradually loses the heat which it had contracted during the summer. The Sun is placed in the centre of our planetary system, the Earth revolving around it in the space of about 365 days 6 hours. Until the time of Copernicus it v,as generally believed that the Earth was motionless, the Sun revolving around it ; but at that period people were ignorant as to the immense distance of the Sun from us, and of its real size (1,260,000 times larger than the Earth), so that they did not see any reason why it should not revolve around our planet. How could it be possible for a body so enormous as the Sun to travel an orbit of 500,000,000 miles in twenty-four hours ? The stars, immense globes, whose exact size we are unable to ascertain, would, to speak only of those that are least remote from us, have to travel 125,000,000 miles per second. And, lastly, how could the radiant globe of the Sun circulate around a body so small as the Earth without dragging it from its place, if it were united to it by invisible ties ? Or, if the Sun were not attached to the Earth, would it not pursue its course in space, leaving our planet hopelessly in the rear ? If two stones tied together are thrown into the air they will be seen to circulate around a point comprised in the interval between them, and which is their common centre of gravity. If one is much heavier than the other, the centre of gravity will be proportionately nearer to the former, and may even be situated within it, in which case 100 ASTRONOMY. the small one will seem to circulate by itself around the larger one, which will only be slightly displaced. Physics teach us that the centre of gravity of two bodies is to be ascertained by dividing their mutual distance in inverse ratio to their weight or volume, and by means of this calcu- lation we learn that the proportion of the Sun's mass to that of the Earth is as 854,936 is to 1. It follows, then, that the common centre of gravity of these two bodies is situated at 243 miles from the Sun's centre. The latter, therefore, does not move, the Earth revolving around it in the space of about 365 days, and turning upon its own axis every twenty-four hours. The first impression of our eyesight would of course lead us to suppose that the Sun and the other planets revolve round the Earth, and it is this illusion which led the ancient astronomers into error. The Sun's distance from the Earth is about 91,430,000 miles ; a cannon-ball travelling at the rate of 1,637J miles an hour, or 39,750 a day, would take 6 years and 110 days to reach it. The Sun's diameter is 852,584 miles, or nearly four times the distance between us and the Moon. Its dis- tance varies with the different seasons, and this is why the apparent diameter of the Sun is not always of the same dimen- sions. This remarkable phenomenon is occasioned by the translation of the Earth in an elhptic cui-ve which brings us nearer to the Sun in summer than in winter ; whence it is that the solar disc seems larger to us in the former than in the latter season. Now if we compare the Sun with other bodies which i people the immensity of space, we are taught by science that it is but an insignificant star amongst the countless legion of luminaries which shine before our eyes. This THE SUN. 107 Fig. 27. — ^Proportional size of tlio Sun as seen from tlie different planets, From Neptune. From Mercury. „ Uranus. ,, Saturn. ,, Venus. , Jnpiter. „ Hygeia. „ tlieEai-tli. ~ „ Flora. „ Mars. ] OS ASTRONOMY. subject will be treated at greater length in the chapter on the stars. XVII. Not only is the Sun the centre around which the planets describe their orbits ; it is also their centre of Hfe. Nothing can breathe or live without the beneficent influence of its rays, Lavoisier gave expression to this idea when he said, " Organism, feeling, spontaneous motion, and life, onlj' exist upon the surface of the Earth and in regions exposed to the light. One might fancy that the fable of Prometheus was the expression of a pliilosophic truth which had not escaped the notice of our forefathers. Without light, nature was lifeless, dead, and inanimate. A beneficent Being, in providing the Earth's surface with light, endowed it with organism, feeling, and thought." In my work upon the " Laws of Life,"* I dwelt at length upon the physiological influence of the agents of nature, a subject to which I can only allude casually in these pages. Speaking generally, it may be said that the life of every creature is more perfect in proportion to the amount of light which it can command, and it even seems that life is not possible without its influence, for we meet with nothing but inorganic bodies in the entrails of the Earth, or in the deep caverns to which it cannot penetrate. In them is no breath- ing or sentient thing ; at most they contain certain kinds of mosses or lichens, which form the first and most imperfect phase of vegetation, and on minuter examination it is seen that most of these plants (if indeed they are plants) only grow upon or close to rotten timber. And even upon the * Lcs Lois dc la Tic, et VArt de prolongcr scs Jours, a work crowned by tlie French Academy (Fiiniin-DiJot & Co.). THE SUN. 109 Earth's surface, if a vegetable or animal substance is de- prived of daylight, it will successively lose its colour and vigour, then stop growing and become stunted, no matter how carefully it may be niu'tured and tended. Man himself, when deprived of light, becomes pale, enervated, decrepit, and eventually loses his energy, as is unhappily too clearly proved in the case of persons who have been confined for a long period in a dungeon, of miners, ship's stokers, workmen in badly-lighted factories, and the inhabitants of cellars or narrow streets. Heat, which, it may be, is only light in another form, is not less needful for life ; it alone can develop the first germs of being. Heat begets life and life begets heat, an indissoluble bond connecting these two phenomena. It would, in fact, be difficult to say which of the two is cause and which efiect ; all we know is that wherever there is life, there also is, more or less, heat. M. Badau, in an excellent work upon the subject, saj'S that " the influence which the Sun exercises upon vegetation is greater than was formerly supposed to be the case. Not only does it supply the heat which hatches the germs de- posited in the ground ; it also fosters the respiration of the plants, and, in a certain degree, their growth. And as our alimentary and combustible substances proceed dii-ectly or by successive transformations from the vegetable kingdom, it may be said that they represent an amount of active power borrowed from the Sun in the shape of luminous vibrations, when the elements of which the plants are formed are in the act of grouping and combining together. The forces stored up by this gradual process of chemical affinities reappear, partially at least, in the mechanical efforts which the animal being is constantly making, and in the shape of ] 10 ASTEONOMY. wliich he expends a part of his o'.vn substance. They also reappear in the working of machines fed with coal. They are transformed into heat when wood is burned in a fire- place, or a nutritive substance burnt in the blood of a living thing which has the faculty of respiration, but not of motion. Thus it is that light, by making the plants to grow and flourish, prepares their nourishment for the inliabitants of the Earth, and provides them with an inexhaustible source of mechanical power."* When winter has plunged nature into apparent death, the mild temperature of spring is suflScient to reawaken its deadened forces. Beneath its gentle influences the days lengthen, the Sun's rays strike us more vertically, and as their brilliancy increases the fields become bright with flowers, and the birds gladden the woods with their song. Gradually the sun reaches its greatest elevation, and begins to decline throughout the autumn, until winter is once more upon us. The nearer we approach the poles, the nearer do we seem to the empire of death, and thei'e are regions where no plant or insect can live, and which are only inhabited by whales, bears, and other animals capable of engendering heat, and preserving a sufl&cient store of it to protect them against the rigom-s of the climate. XVIII. 1 will conclude this chapter by ab extract from Fathei* Secchi's work on " The Sun," in which he summarizes the facts hitherto ascertained concerning the great orb of day. " That igneous globe, a source of life, and cause of motion amongst the planets, was once a nebulous mass like those Eadau's Derniera Progris de la Science, p. 46. THE SUN. Ill which we now see in the depths of the sky. This mass as it grew cool gave birth to the planets and their satellites. It still preserves in its midst all the heat which must have resulted from its condensation and the collapse of its different particles, which, from the furthest limits of its domain, have, in obedience to the law of attraction, fallen towards the centre. " This enormous mass, undergoing the phases of gradual cooling through which the planets around it have passed, may one day lose the whole of its present brilliancy, but it will yet be millions upon millions of years before this takes place. "Whether something will then occur to restore its primitive powers, we cannot say, for the world's existence has had a beginning, and may, for all we know to the con- trary, have an end. " The gaseous composition of the Sun accounts for the phenomena which we notice upon its sui'face. The part which is exposed on the outside to radiation towards the regions beyond loses its gaseous constitution as it gets cool ; it remains condensed in the shape of masses, vaporous but incEindescent, in the gaseous and transparent atmosphere by which the globe is surrounded, forming a brilliant stratum which we call the photosphere. This stratum, like the interior of the solar body itself, is the seat of vast chemical processes and physical movements of a very complicated character. Causes as yet unknown, transporting considerable masses from the interior to the exterior, create immense gaps in the luminous stratum, and so give rise to the spots. The centre of these gaps, more obscure and more absorbent, cuts off from us the great majority of the luminous rays emanating from the central nucleus, composed as they are of a gaseous matter and quite isolated from each other. 113 ASTRONOMY. " Above tliis luminous stratum spreads the atmosphere, formed of transparent vapours, which attain various degrees of altitude according to their specific weight. Hydrogen, being the least dense of all these substances, floats at a great altitude, forming columns and clouds which constitute the red prominences seen about the Sun during an eclipse. Iron and calcium are the substances most abundant in the hollow of the spots and ui the rents of tlie photosphere. " The Sun's atmosphere is vast, extending to a distance equal to the foui'th of the solar radius ; it is elliptic in shape, with a greater elevation at the equator than at the poles. In the equatorial regions, in the vicinity of the spots more especially, there is a higher degree of activity than at the poles, as .is seen by the greater brilliancy and altitude of 'the atmospheric envelope itself. " The spectroscope, in revealing to us the chemical com- position of the Sun, has taught us that the substances of which it is formed are identical with those which constitute the terrestrial bodies. And j'et we are far fl'om possessing a knowledge as to the nature of all these substances." The information contained in this chapter shows the progress made in the researches as to the Sun, and the rapidity with which they have been prosecuted since the discovery of spectrum analysis. Fig, 28.— Horro (the Seasons) from a medal of the time of Commodua, CHAPTER V. MERCUKY. Its phases— Truncation of its crescent— Prodigious height of its mountains—. Mercury's passage across tlie Sun — Its volcanoes— Its distance from the Sun — Its seasons — Its density, mass, dimensions, and motions — Strange peculiarities of this planet — Is it inhahited? — Fontenelle's opinion. I. Meecuey is the smallest of the principal planets and the one nearest to the Sun. It is always so immersed in the rays of the latter that there is great difficulty in seeing it with t^ie naked eye, even at the period when it is most distant irom the Sun. Yet the Greeks, struck by the occasional intensity of its light, bestowed upon it the adjective, gKttering. Seen through the telescope, Merciuy has phases like the Moon, a fact which proves its opaqueness ; it is also because of this latter quality that it presents the shape of a black spot in its passage across the solar disc. Its crescent exhibits a horny truncation, discovered by Schroeter, which tends to show that this planet has mountains 53,000 feet high, or even more. Mercury's passages across the Sun take place but rarely, because of the inclination of the orbit, occurring at intervals of three, seven, ten years, &c., and lasting less than three hours. The luminous points noticed upoia its obscure disc lU ASTRONOMY. on these occasions have led to the supposition that it must contain volcanos in a state of activity- Fig. 29.— The phases o£ Mercuiy. The mean distance of Mercury from the Sun is 35,393,000 miles, whence it follows that the Smi's diameter, seen from Mercury, appears thrice as large as it does to us, aiid that the temperatm'e is seven times that of our torrid zone. This temperature, much greater than that of boiling water, is no doubt mitigated by an extensive atmosphere. Its seasons are very pronounced, for at the epoch of the solstices, for instance, the Sun attains an altitude, as compared to the polar horizons, not merely of 23° 27", as upon the Earth, but of 70 degrees. MERCUEY. 115 n. This planet must be of a very dense character, for if the materials of which it is composed were liable to become heated like those of the Earth, they would be melted and vitrified in a very short space of time. We know, in fact, from experiments made, that its density is one and a half times greater than the mean density of the Earth ; its mass is only a tweUth that of the Earth ; its volume sixteen, and its weight fifty-seven times less than that of our planet. It traverses, in the space of 88 days an orbit of 230,208,000 miles round the Sun, or 100,000 miles an hour. It was because of this enormous velocity that the Greeks called this planet Mercury, the messenger of the gods. It accomplishes in 24 hours 5 minutes and 28 seconds a rotatory movement around an axis 7 degrees inclined to the plane of the equator, so that there must be a great inequality in the days and seasons. In diameter it is about 2,962 miles, and at its least distant point, 47,229,000 miles {torn the Earth. It is a peculiarity of this planet that at its perigee, that is to say the point when it is nearest to the Earth, it seems smaller than at its apogee, the point when it is farthest off, and the reason of this is that when in its perigee it is not ImuinouB on the side towards us, while when at its apogee that portion of its disc lighted by the Sun faces the Earth. It is somewhat strange that Copernicus, who deduced from the motion of Mercury so powerful an argument against the Ptolomsean system, lamented upon his death-bed that, in spite of all his efforts, he had never been able to see this I 2 116 ASTRONOMY. planet. Yet its existence was known in the most remote ages, and the ancients, who did not comprehend the real system o the world, deceived by the double apparition of Mercury, sometimes after sunset, sometimes before simrise, supposed at first that there were two distinct stars, one of which they called Apollo, god of the day and of light, the other, Mercury, god of thieves. The Indians and Egyptians, who worshipped this planet, also gave it two different names. But it was ultimately remarked that only one was visible at a time, and that the apparition of the second coincided almost exactly with the disappearance of the first; so it was discovered that they were one and the same star. III. If Mercury is inhabited, it must be by people constituted very differently from ourselves. Much allowance must be made for the imaginative powers of painters and poets, and with this reserve Fontenelle's description of its supposed inhabitants is worth quoting. He says : " They are not half so far off from the Sun as we are ; it seems to them nine times as large, and floods them with a light so potent that the brightest of terrestrial days would appear but dim twilight to them, if not night itself. The heat to which they are accustomed is so great that the climate even of central Africa would freeze them through. It must be taken for granted that our iron, silver, and gold, would melt in thek world, and only appear as a liquid, like water. The dwellers in Mercury would be unable to comprehend that in another world these same liquids, wliich perhaps form their rivers, are the hardest substances with which its inhabitants are acquainted. They must be MERCUrvY. 117 SO vivacious as to be mad in our meaning of the term. I believe that they have no more memory than most negroes, that they have not the faculty of thought, that they only act by fits and starts, and that in Mercury, Bedlam is the universe." This portrait of the supposed inhabitants of Mercury is the reverse of flattering, and there is no reason why the harmony Avhich would be likely to subsist between their organism and climate should not admit of their intellectual and moral faculties being developed as perfectly as our own, if not more so. CHAPTER VI. VENUS. DifTpicnt iinmcR of Uiis planet— Its distance from the Sun— Its translator motion — Why does it seem to vary in size ? — Dull and pale light occa- sionally emitted by its obscure part— Visible in full daylight — Curious facts : jEneas in his voyage to Italy, and General Bonaparte at tlio Luxemburg — Discovery of the phases of Venus — Curious anagram — Spots observed in Venus — Its gigantic monntaios- Explanation of its phases — Its passage across the Sun's disc — Its atmosphere — "Why does it seem to remain longer to the east and west of the Sun than it takes time to revolve around it? — Means of ascertaining the Earth's distance from the Sun by the passage of Venus — Halley, Le Gentil, Chappc — Curious facts — Its rotatory motion around an axis — Its days and seasons — Description of this planet and iU possible inhabitants. I. Venus is the only planet spoken of by Homer, who de- signates it by an epithet signifying beautj'. It has also been called Juno and Isis. The identity of the brilliant stars seen, sometimes of a morning and sometimes of an evening, was not originally known, and thus the ancients called it Vesper, or the evening star, when it set some time after the Sun ; Lucifer, or the morning star, when it preceded the sunrise. Venus was called Sukra, that is to say, the brilliant, by the Indians, and everyone is aware that it is often termed the Shepherd's star. Micrometrical measurement shows that the apparent diameter of Venus is comprised between 9" "5 and 62". VENUS. 119 This enormous difference is due to the fact that it comes mthin 23,309,000 miles of our globe, and recedes to as much as 159,551,000 miles &om it. It is about 66,131,000 miles distant from the Sun, round which it accomplishes, in 224 days, 14 hours, 49 minutes^ an orbit of 482,000,000 miles, travelling, therefore, at the rate of 80,000 miles an hour or 1338 a minute. So far as we can judge, Venus has a smaller diameter, and consequently a lesser volume than the Earth, but the differ- ence is so slight that the observations from which it has been deduced may not be altogether trustworthy. The quantity of light and heat which Venus receives from the Sun is nearly double that which reaches the Earth. The obscure part of this planet is occasionally noticed to shed in the sky a dull, deadened kind of light, which some astronomers have attributed to the phosphorescence of the atmosphere or the solid part of this planet. This curious phenomenon may also be the result of a certain ash'coloured light, analogous to that of the Moon, and which would be caused by the light reflected from the Earth or Mercury to Venus. Perhaps, too, the atmosphere of the planet may be in certain cases the seat of lights analogous to those which, on the Earth, constitute Aurorse Boreales, II. Venus is sometimes so resplendent as to be visible at mid-day to the naked eye, and the uninformed masses have linked its appearance with important contemporary events, just as has been the case with comets. The ancients remarked that at night when there was no moon, the light of Venus often projected shadows, ^neas, 120 ASTRONOMY. as we are told by Varro, during his voyage from Troy to Italy saw this planet the whole time, even while the Sun was above the horizon. The same author, in one of his works that is now not extant, is reported by St. Augustine to have said that Venus had, at an epoch long before his own time, undergone a change of colour and intensity. General Bonaparte, on his way to a fete at the Luxemburg Palace, was struck by the attitude of a crowd in the Rue de Toumon, which had assembled to gaze at a star, which was visible, though it was then mid-day. This planet, which was Venus, they took to be the guiding star of the celebrated general, who had just returned from his Italian campaign. It is a singular fact that it was not until some time after its discovery that Galileo thought of observing whether or not Venus had phases, a point which he settled in the affirmative on the 10th of September, 1610. In order to follow up and verify this discovery without running the risk of having it appropriated by others, he concealed it under the following anagram : — ffax immatura. a me jam fnistra Icrjmtur. o, y. Changing the order of letters, Galileo read the line thus : — Cythia figxiras emilatur mater amorum. Father Castelli asked Galileo, in November, 1610, whether Venus and Mars did not both present phases, to which the astronomer of Florence, who at first gave an equivocal answer, replied a month later by announcing the discovery of phases in Venus. VENDS. 121 III. The dark spots noticeable in Venus extend over a large part of its diameter ; their extremities are not very sharply defined. Branchini noticed, in 1726, seven spots in the centre of Venus, which he termed seas communicating with one another by means of straits, and exhibiting eight distinct promontories. He drew illustrations of them, and named them after his patron, the King of Portugal, and the most distinguished navigators. In 1700, La Hire, observing Venus by daylight near its lower conjunction, with a mag- nifying glass of 90 degrees, noticed upon the inside of the crescent an unevenness of surface which could only be due to the presence of mountains higher than those of the moon. Schroeter, directing his attention to that paiii of the crescent nearest its horns, noticed that they were occasion- ally truncated. Upon the 28th of December, 1789, January 80th, 1790, and February 27th, 1793, he remarked near the southern horn a luminous point entirely isolated, that is to say, sepa- rated by an obscure patch from the rest of the crescent. If the planet were free from rugosities and perfectly smooth, the crescent would invariably terminate in two ex- tremities quite parallel and very pointed ; but if Venus is covered with mountains, their interception of the luminous rays proceeding from the sim will at times prevent one or even both of these horns from assuming their regular shape, and the crescent will not therefore be completely sym- metrical. 123 ASTRONOMY, As a matter of fact, Venus is not a smooth body ; it lias mountains upon its surface, and these mountains far exceed in height those of the earth. From measurements taken it seems that they are 145,200 feet high, or five times the altitude of the highest mountains iipoii the earth. Fig. 30. — The phases of Veniis. When Venus sinks of a morning Into the Sun's rays, or when, of an evening, it emerges from them, its diameter is very small, and its disc nearly roimd. This diiimeter is much larger, and the planet seems very concave, lilce the moon under similar circumstances, when it disappears of an evening, or emerges of a morning from ouS of the twihght-dawn. VENUS. 133 The concavity of its crescent faces to the east of an even- nig, and to the west of a morning. It is half full at the liy. 31.— llie patsago of Youus acio&i the Cun, 12i ASTRONOMY. periods intermediate between those mentioned, phenomena which admit of a very simple explanation, if we suppose that Venus circulates in a closed curve, with the Sun inside, that it is not luminous of itself, and that the greater part of the hght which we see there is borrowed from the Sun. As Venus is situated beyond the Sun in the same latitude, and crosses the meridian at noon, it is then said to be in upper conjunction ; the lower conjunction also occurs at noon, at the epoch when Venus and the Sun have the same latitude, the former occupying a position between the latter and the earth. Venus passes across the Sun's disc in the direction of left to right, like aldack spot with an apparent diameter of 59 seconds. Its passages are of very rare occurrence ; the first that was made the subject of observation took place in November, 1631 ; the next on June 5th, 1761, and the third on Jime 3rd, 1769. After occurring at an interval of eight years, there was a lapse of one hundred and thirteen and a half years before the next, which takes place upon the 8th of December, 1874, and wUl be followed as before, by another passage in 1882. This is the order of their periodicity, the cause of which is the inclination of Venus to the ecliptic. It is worthy of notice that its passages across the solar disc serve to ascertain the Sun's parallax, and its distance from the earth. Fig. 81 represents the passage of Venus across the Sun, observed from three different points, A, B, C. At the moment of its passage it is about two and a half times nearer to us than the Sun is. Its parallax is therefore very considerable. Let us suppose two observers, A and B, to be placed at the extremities of a terrestrial diameter, and maldng allowance for the rotatory motion of the earth, each of them will be able to measure the chord described by the VENUS. ia5 planet, either directly, or by estimating the time occupied in its passage, for the angular motion being well known, the time taken will show what space has been traversed. The length of two chords starting from a h being known, their distance a h will easily be ascertained, and, by means of two triangles with bases A fc B and A a B, it will be found that the distance of the chords is equal to five times tiie radius of the earth. Therefore the angle at which the distance a 6 is' seen from the earth is five times greater than the angle at which the terrestrial radius would be seen from the Sun, or five times the solar parallax. Thus, by taking the fifth of the distance, a b, we obtain the parallax of Venus. IV. Halley, the 'great English astronomer, was the first to indicate the passage of Venus as a means of obtaining the parallax of the Sun or its distance from the earth. Though Halley must have known that his method could not be employed in his own lifetime, he nevertheless strongly recommended it, thinking more of the service he could render humanity than of lamenting that the brevity of human life would prevent him from reaping the benefits of his discovery."* The importance of the passage of Venus across the Sun from a scientific point of view has been the cause of many perilous expeditions. As the author just quoted remarks : Imitating the heroic devotion to duty displayed by Halley, astronomers scoured the whole globe to observe the pas- sages of this planet. One of them, Le Gentil de la Galai- • Treatise on Astronomy, y Petit, ex-director of the Toulonse Observatory vol. u. p. 137. 12G ASTKONOMT. Biere, starting from India in March, 1760, and hindered hy the war then going on between the French and English, had the xiatience to await at Pondicherry for eight long years the passage of 1769, risking the loss of his post at the Paris Aeademie des Sciences, where, in default of news from him, his vacant place was filled up. Thus he risked his patrimony, and failed after all in the object of his research, for after obtaining but a cursory glimpse of the passage of 1761, from the deck of a ship, he was altogether prevented from observing that of 1769, owing to the cloudy state of the sky." The Abbe Chappe d'Auteroche, after making the journey to Siberia, in order to observe the passage of Venus in 1761, died of yellow fever in California, on the 1st of August, 1769, at the age of 41, and this because he would insist upon remaining an extra fortnight in the tainted district, in order to observe an eclipse of the moon, in addition to the passage of Venus. Many other men of science also visited the most distant parts of the continent, in order to take observations, and their labour was not unrewarded, enabling them, as it did, to determine with precision the unity of the celestial longi- tudes, and the actual distance from the earth to the Sun. The accuracy of their measurements will, beyond doubt, be confirmed on the occasion of the coming passages this year (1874), and in 1882. M. Faye, in his final discourse as President of the French Academy of Sciences, stated that the committee appointed for observing this phenomenon, though much hampered by the painful events of the last few years, had taken every measure for obtaining a successful result. VENUS. 127 V. It has been calculated that Venus has an atmosphere very similar to that of the earth as regards its extent and dif- fractive force, the estimate being based on the shadow which appears on the Sun's surface a few seconds before the dark body of Venus reaches the solar edges on the occasion of its This observation is further confirmed by the law as to the gradual variation of the light as it passes from the side which is illuminated to that which is not. During 190 days alternatively, it appeal's as a morning and evening star, and though it may seem surprising that it should appear to remain longer to the east and west of the Sun than it takes time to accomplish its period around that luminary, this difference is easy of comprehension when we remember that the earth itself revolves round the Sun, and that it follows Venus in its course, but at a lower rate of velocity. ' Dominico Cassini discovered the fact of its rotatory motion around an axis forming a sharp angle with the ecliptic, which must cause, as in Mercury, a great inequality in the days and seasons. The duration of this rotatory motion has been fixed at 23 hours, 21 minutes, 7 seconds. I will conclude this chapter with a passage from the author of Harmonies de la Nature, descriptive of this planet and its possible inhabitants : " Venus must be studded with islands, each of them con- taining mountain peaks five or six times higher than that of Teneriffe, their sides bright with verdure and flowers. " Its seas must present the most attractive spectacle. Imagine the Swiss glaciers, with their torrents, their lakes, their meadows and their pinewoods in the midst of a southern 128 ASTRONOMY. sea; add to their sides the Loire hills, crowned with vines and fimit-trees, and to their bases the tropical produce of the Moluccas and the bright-plumed birds of Java. Imagine their shores overshadowed with cocoa-trees, studded with oyster-beds, madrepores and corals growing, amidst perpetual summer, to the height of large trees in the bosom of the ocean, rising above the water at the ebbing of the tide, which lasts for 25 days, and harmonizing their scarlet and purple hues with the verdure of the palm-trees.* And imagine, finally, currents of transparent water which reflect all these beautiful spectacles, ebbing and flowing &om isle to isle with a flood of twelve days and a reflux of twelve nights, and even with all this you will have but a very faint idea of the landscape in Venus. As the Sun at the solstice rises more than 71 degrees above its equator, the pole which illuminates it must possess a temperature much milder than our spring. Though the long nights in this planet have no moons to light them. Mercury, by reason of its brilliancy and close vicinity, and the earth, by reason of its size, must be more than equal to two moons. " Its inhabitants, about the same size as ourselves, since they dwell in a planet of the same diameter, but in a more favoured celestial zone, must devote all their time to love. Some, feeding their flocks upon the hill sides, lead the life of a shepherd; others, upon the shores of their fruitful islands, join in dancing and feasting, and pass the time in sing- ing or swimming for prizes, like the inhabitants of Tahiti." It is no exaggeration to say that in this case the imagina- tion probably falls below the reality. • For an accurate description of these marvels, see Eistoire des Pierres prleUuses, p. 139. (Firmin-Didot 4 Co.) CHAPTER VII. THE EARTH. Its origin — ^Its ti'ansformations— Summary of what is known concerning the globe's crust, by EUe de Beaumont — Cooling of the globe — Temperature of the Celestial regions — Shape and dimensions of the Earth — Its chief divisions : continents and seas — Proofs that the Earth is almost spherical — Flattened shape at the Poles — Attraction — Its various kinds — Exact date of the establishment of the law of attraction — Scientific hypothesis as to this law — History of M. Bertrand's measurement of the Earth — Various motions of tlic Earth — Kepler, his genius and discoveries — The seasons — Variations of day and night — History of the Earth's translatory motion round the Sun, by Arago. I. The Earth, our common mother, to boiTow a term from the ancients, has naturally been the subject of study amongst men of science from the earliest ages. The observations of geologists have shown that our planet has only reached its present condition after imdergoing, for an incalculable period, numerous revolutions, traces of wliich are everywhere to be found. Everything tends to prove that the Earth was in the first place incandescent, and has since gradually become cooler ; the existence of an internal focus is shown by the increase of heat that takes place in the various strata of the globe in proportion to their greater depth, and this increase is about one degree centigrade for every thirty-two yards in depth. AH the luminaries in our planetary system appear to have 130 ASTRONOMY. a common origin. In conformity with these ideas, it seems i-ational to refer the chaos spoken of in the Bible to the existence of a vast nebula, which, turning upon its own axis, and very much flattened by the effect of the centri- fugal forces caused by its rotation, would, dui'ing the suc- cessive phases of its cooling, have cast off several of its strata, the accumulation of which in globules, corresponding with the separation of darkness firom light, would be the origin of the earth, the other planets, and the sateUites. This view, which is not the least inconsistent with the strictest tenets of religion, is held by many of the most distinguished astronomers and geologists, Father Secchi, director of the Eoman observatory, among them. (See pp. 37 and 38.) This theory is confirmed by all the known phenomena ; the roimded surface of the globe, the flattening at the poles, central heat, the parallelism of the indentations which, as M. Ehe de Beaumont has proved, have been formed at each cataclysm of the globe's surface, the analogy with what takes place in the heavens when stars are in process of formation, &c., &c. Thus the earth must have passed in succession from the gaseous to the liquid and solid state ; and even now every- thing tends to show that, under the relatively thin crust of 49,500 yards, which we inhabit and cultivate, the substances composing it are, if not in a liquid, at all events in a pulpy state. n. Elie de Beaumont, summing up the knowledge which we possess concerning the earth's crust, points out that if THE EARTH. 131 the terrestrial rind results from the superficial cooling of the substances in a state of fusion which originally consti- tuted the exterior envelope of the globe, the action attribu- table to the attractive forces upon those parts which are not yet cool, cannot form ground for surprise. The data gene- rally accepted as correct forbid the supposition that the crust is more than 49,500 yards thick, or in other terms, y\-^ of the terrestrial radius. Such a shell is, by comparison, thinner than that of an egg. Split in all directions, like the rocks which we see upon the surface of the globe, a vault of such sUght thickness could not hold up without some supports, and must give in such a way as to bear upon the incandescent substances beneath it. These substances are consequently exposed to great pressure, which must very much reduce the mobility of their molecules, and give them almost the properties of a solid body. The refrigerated crust becomes, so to speak, em- bodied with this incandescent substance, which, though not actually in fusion because of the pressure upon it, is at fusion temperature. Hence it results that the whole mass of the globe undergoes the action of the attractive forces as if it were a solid body. It must possess, however, a certain degree of malleability, as is denoted by the remarkable affinity which M. Alexis Perrey has shown to exist between the frequency of earthquakes and the Moon's phases. The refrigerated crust of our globe, getting gradually thicker as the cooling process continues, would eventually acquire sufficient rigidity to maintain itself without extra- neous support. The less refrigerated substances beneath it would then be released from the pressure to which they are now subjected, and an annular void might even be estab- lished between the solid crust and the substances still suffi- K 2 132 ASTRONOMY. ciently inflamed to remain liquid, close to theii- surface at least, in the absence of any pressure. But we must hope that the cooling of the globe has not yet reached this point, which would probably cause a catastrophe of unexampled magnitude. It would be due to the introduction of the sea into the empty space between the lower and still incan- descent surface of the solidified crust and the upper surface of the substances still in a state of fusion. The final phase of the relative refrigeration of the whole mass and surface of the globe, as given by Plana, will not be complete until 156 milliards of years, counting from the time when the cooling process began.* The recent re- searches of M. Poisson lead to the conclusion that all the geological phenomena which have hitherto occun-ed may be comprised in a period of a hundred million years, or even less.t m. Those parts of the mineral crust of the globe which geo- logists call sedimentary rocks were not formed all at once. Science furnishes us with the following details upon this subject. At one time the regions now situated in the centre of the continent were covered on more than one occasion with water, which deposited there thin horizontal strata of various kinds of rock. These rocks, placed one upon another, like the stones in a wall, must not be taken to be all alike, and, in fact, the diiference between them must strilie the least practised eye. The crystalline granitic rocks, upon which the sea made its first deposits, have * The printed text in the Turin Academy gives ninety-six milliards ot years, liut the correct cakulatiou is that !;iven above, t AcacUmie dcs Sciences, first half of 1S71. THE EARTH. 133 never exhibited any vestige of a living thing. These ves- tiges are only to be found in the sedimentary strata. Vegetable debris is the only thing to be met with in the oldest strata deposited by the waters, and even they belong to plants of the simplest composition, such as ferns, rushes, and lycopods. Vegetation becomes more and more composite in the upper strata, and in the most recent it may be compared to the vegetation of the present day, with, however, the signi- ficant restriction that certain vegetables which only exist in the south, such as the large palm-trees, are to be found in a fossil state at all latitudes, even in the midst of the icy regions of Siberia. In the primitive world, therefore, these hyperborean countries had a temj^erature at least as elevated as that of the parallels where the palm-trees now flourish ; Tobolsk, for instance, must have had as warm a climate as that of Alicante or Algiers in the present day. A careful examination of vegetable substances confirms this view. Thus, though shave-grass and rushes, ferns, and lycopods are to be met with in tlie present day in Europe as well as in the equinoctial regions, they never attain the same dimensions in the former as in the latter countries. To compare tlie dimensions of the same plants is equivalent to a comparison of the temperature of the regions in which they grow. But if we compare the fossil plants of our coal-producing regions with the plants which grow in the richest parts of South America, it will be seen that the former are far and away the largest. The fossil flora of France, England, Germany, and Scan^ dinavia, contain ferns fifteen yards high, the stems of which are a yard in diameter. 134 ASTBONOMY, The lycopods, which, in cold or temperate countries ai"e creeping plants, hardly rising four inches above the soil, and which even at the equator do not attain a height of more tlian three feet, grew to a height of eighty feet in the primi- tive world, even in Europe. These enormous dimensions are an additional proof of the elevated temperature which reigned there previous to the last invasion of the ocean. By studying the fossil animals we arrive at similar results. Amongst the hones contained in the soil nearest to the pre- sent surface of the globe, are the remains of hippopotami, elephants, and rhinoceros. These remains of animals indi- genous to a hot country, are to be found under all latitudes, even at Melville Isle, where the temperature now falls to fifty degrees centigrade below zero. In Siberia they are so abundant as to have been the object of a trade speculation ; and upon the cliffs bordering upon the frozen strait are to be found not merely skeletons, but whole elephants with their skin and flesh in a state of perfect preservation. Thus the polar regions have, in the course of time, undergone an enormous process of refrigeration, caused not by any change of the Sun, but by the dissipation of an original heat of their own, or with which the Earth was once impregnated. Even before the discovery of the elephants in Siberia, science had conceived the idea that the globe must have had a heat of its own, in proof of which Mairan and Buffon instanced the high temperature of certain deep mines, Giromagny amongst others. Fourier was one of the first to examine this question, and he pointed out the great influence of the temperature of the celestial regions, aiTiidst which the Earth describes its immense orbit round the Sun. Meteorologists had supposed, when they saw even at the THE EARTH. 135 equator certain mountains covered with perpetual snow, and when they observed the rapid decrease in temperature of the atmospheric strata during a balloon ascent, that the regions bej'ond the atmosphere must be enveloped in him- dreds and thousands of degrees of cold. But Fourier's minute investigations taught us that stellar radiation main- tains the regions traversed by the planets of our system at from fifty to sixty degrees centigraAebelow zero. The tempera- ture of the earth increases by a degree at every thirty or forty yards depth below the surface, according to the nature of the soil ; the temperature of the air diminishes in the same proportion at every 160 or 200 yards of altitude. IV. The shape of the earth is that of a spheroid, flattened at the poles, and bulged out at the equator, the flattening being about ^^ of the radius. The inhabitants of the Earth, who are diametrically opposite to each other in respect to the regions which they inhabit, are called antipodes, as also are the places in which they live. The point of the sky situated directly above their heads is called their zenith, and the name of nadir has been given to the opposite point. The Earth's circumference is about 24,000 miles, and the highest moimtains do not reach five miles, an altitude which, being not quite the five-thousandth part of the circumference, is very slight in comparison to the extent of the Earth, and makes no more alteration in its shape than an eminence of '03937 of an inch would upon a globe 17 feet in circumference. 136 ASTRONOMY. A few grains of sand upon a ball, or tlie unevenness upon the contoui- of an orange do not prevent those bodies from being round, and such is exactly the case with the moun- tains upon the terrestrial surface. The Earth's shape is j)recisely that which would be pre- sented by a fluid mass, endowed with a rotatory motion around a fixed axis. The air which envelops the Eailh upon every side, like the solid or liquid parts which obej- the laws of gravity, must have the same shape. In i)roportion as we recede from a body, the details be- come eflfaced, and the main features more and more appa- rent. Thus the Earth, as seen from a great distance, the Moon for instance, would present the aspect of a spherical globe, round and luminous like the Moon itself. I will now proceed to mention the chief arguments ad- vanced to prove that it is nearly spherical or round. To convince us of the fact, let it be imagined that the Earth was a plane or quite flat. In that case, as soon as the Sun appeared upon the horizon, its light would be immediately difi'used over the whole terrestrial surface alike. This, as we know, does not take place, and proves therefore that the Earth is more or less convex. A vessel sailing away from us would seem to decrease only in size if the Earth was level, but, as a matter of fact, the hull first disappears, then the sails, and, last of all, the masts ; and in coming towards us a vessel seems gi'adually to rise out of the water. This can only be accounted for by convexitj' of the Earth's surface ; and as it occurs everywhere alike, the Earth must necessarily be spherical. Magellan, the first traveller who made the voyage round the world, recognised this fact. Stai-ting from Spain west- ward, one of his vessels returned to Europe in an opposite THE EARTH. 137 direction, that is to say, as if it was coming from the East. The change in the aspect of the sky as one reccfles from Fig, 32,— The Eartb, aa sees fiom tbe Uoou 138 ASTEONOMY. the spot which formed the starting-point is a furtlier proof of the Earth's convexity. No matter in what direction we travel, fresh stars become visible ; those towards which we advance seem to rise, and those from which we recede to sink in the sky and at last become invisible beneath the horizon. The curvature of the Earth alone produces these pheno- mena. The spherically-shaped shadow which the Earth projects against the Moon when there is an eclipse of the latter, that is to say when the Earth comes between the Sun and the Moon, and intercepts the rays of the former, proves to demonstration the sphericity of the Earth, for it is only a sphere wliich, no matter how it is placed, can produce a round shadow. Fig. 33. — Phenomena produced by the sphericity of the Earlli. The flattening of the poles is also clearly proved by the attractive influence of the Earth upon the Moon. M. Delaunay, referring to this subject, says : — " As the THE EARTH. 139 Earth is a globe, slightly flattened towards the poles, and bulging out at the equator, its influence upon the Moon is not quite the same as it would be were it altogether spherical in shape. There must consequently exist in the Moon's motion some indications of this flattening of the terrestrial globe, and if it is possible by observation to determine the proportions of the effect caused by this depression of the Earth, it follows that the extent of the depression itself may be deduced therefrom. This Laplace demonstrated, and his calculation is almost identical with that which has been arrived at by various measurements of the terrestrial surface. We may even coincide with that celebrated geometer in his opinion that a study of the Moon's motion is for this purpose far preferable to geo- desical measurements, because it is the depression of the globe as a whole, and apart from any small local iiregu- larities, which is manifested in the Moon's motion ; whereas the geodesical measurements taken at the various points of the Earth's surface are moi'e or less affected by these local irregularities." * V. If the earth is globular, how comes it that houses, men, animals, and all tlie objects upon its surface keep their balance ? Why do not the waters of the seas and rivers run out of their beds ? The answer is siinple enough. Everybody must be ac- quainted with the effect of a loadstone. Place some iron- filings in close proximity to it, they would be attracted by it, and only those upon which the loadstone failed to exer- • Dclaunay's AnuKaire du Bureau dcs longiiiiclcs, 186S, p. 462. 140 ASTRONOMY. cise sufficient attractive force would fall off. The Eni'th possesses force of a similar kind, by means of which it attracts to its centre all the bodies upon its sm-face, and when one of them falls it is always tswards the Earth's centre. The frnit from its stem, the stone from the hand which held it, full to the surface of the Earth, impelled by that hidden force which has been termed attraction. This force is resident in all the bodies of nature. It exercises its influence upon the lai'gest masses as well as upon the most minute particles of matter. This it is which gives harmony to the universe, and explains the formation of bodies of all kinds. It makes itself felt throughout all matter just as if that matter had no existence, so that to discern the effect pro- duced by a spherical stratum upon a point beyond, it is necessar}' to add together the influence of all its elements, without making any distinction between those which act directly or indirectly. Attraction takes different names according to the kind ot action which it exercises. When it merely unites the different molecules which con- stitute a body, it is molecular attraction. When it is the invisible bond of union between the diverse elements which constitute our globe, or the force which precipitates to its sui-face the bodies which had been separated from it, it is gravity. And, lastly, when it presides over the preservation of the order reigning in the universe, by retention of the celestial bodies in the limits of their accustomed com-se, it takes the name of celestial gravity, and fui'nishes the prin- cipal laws of astronomy. THE EAETH. 141 VI. The motions of the celestial bodies, since the time when they were first observed, accord to demonstrate the truth of two principles discovered by Newton, which may be stated as follows : — 1st. Bodies exercise attraction in direct ratio to their mass. — For instance, a body weighing a pound, attracts lite a pound; if it weighs two,. its attractive force is doubled; if three, it is trebled, and so on. 2nd. Bodies exercise attraction in inverse ratio to the square of their distances. — The square of a number is the product of that number multiplied by itself. Thus, tlie square of 2 is 4 ; of 3, 9 ; of 4, 16, and so on. Conse- quently, at double the distance, the attractive force is four times less ; at treble the distance, nine times less, &c., &c. Let us suppose the mass of one body to be four times that of another, it will attract with four times the force, and if the two bodies are both movable, that of which the mass is four times greater than the other will only be displaced one-fourth as much. Moreover, if the distance separating the two bodies is four, five, ten times greater, they will attract sixteen, twenty-five, a hundred times less. A body which upon the earth weighed 3,600 pounds, would only weigh one pound if it was as far off as the Moon ; that is to say, it would be 3,600 times less attracted by the Earth, and might, to use Euler's expression, be held up with one finger. The fall of bodies to the ground follows the same laws. If, for instance, a stone is launched into the air, there will be a free exchange of attraction between^^it and the Earth, but as attraction is in direct ratio to the masses, the Earth, 142 ASTRONOMY. haviiig a mass infinitely larger than the stone, will not be displaced to any appreciable extent. Gravity imparts equal degiees of speed to all bodies fall- ing from the same height, whatever may be their character, shape, or volume. This is easily proved by placing within a long glass tube bodies of various kinds, such as lead, cork, paper, and feathers, and then extracting the air from it with a pneumatic machine. When the void has been created, the tube is placed in a vertical position, and tmned upside down, when the lead, cork, paper, &c., descend with the same velocity as if they were one undi- vided body. If the air is readmitted into the tube, the lighter bodies will again be distanced by the heavier sub- stances, and the differences between them will go on increas- ing until the air inside the tube has acquired the density of that outside. M. Babinet, whose recent death deprived the French Institute of a very valued member, wrote as follows con- cerning the discoveiy of the law of attraction : — " In 1666, Newton, while living in retirement in the coimtry, gave his attention for the first time to the system of the world. Several authors had already asserted that the law of attraction was in inverse ratio to the square of the dis- tance. Newton, in essaying the tmth of this law by com- paring the fall of the Moon to the fall of weighty bodies, foimd it to be false, and did not, therefore, prosecute the inquii'y any further. Four years later, he ascertained, by means of Picard's French measurement, that this important law was perfectly correct, and from that time, but not before, the law of attraction was an established fact. It is well known that when Newton received the results of Picard's measui-ements, he was so excited that he was obliged to THE EARTH. 143 ask one of his friends to comjilete the simple calculation which verifiecl this important law, which, accurately speak- ing, dates from 1670." VII. M. Emanuel Keller has furnished the Academic des Sciences with a paper upon the cause of gravity and the effects attributable to universal attraction, of which the fol- lowing interesting paragraph is an extract : — " Newton, during the last fifty years (1675—1726) of his life was always studying the cause of gravity, at one time examining its motions, at another the difference in the density of the ether, and, though he failed to assign them their precise places, he was anxious that nobody should suspect him of having ever given serious belief to the hypothesis of attraction without contact. This is evident in several of his works, notably in the second edition of the Optics, and in his letter to Bentley, wherein he says : ' It is absurd to suppose that inert nature can exercise any action save by contact; and the idea that gravity should be an innate quality, inherent, essential to bodies and permitting them to react upon each other from a distance, and with a void between them, without any intermediary for transmitting this force, seems to me so ridiculous that it is not worth while to waste time in discussing it.' " M. Lame, in his Legons sur I'Elasticite, propounds the same idea : — " The existence of the ethereal fluid is proved beyond question by the propagation of light in the planetary regions, as also by the simple yet convincing phenomena of diffi-action in the theory of undulations ; and the laws of double refrac- Ui ASTRONOMY. tion prove not less surely that ether exists in all the dia- phanous regions. Thus ponderahle nature is not alone in the imiverse ; its particles swim, so to spealc, in the midst of a fluid. If this fluid is not the only cause of all the facts that have been observed, it must at aU events modify and multiply them, and complicate their laws. Thus we cannot obtain a rational and complete explanation of the phenomena of physical nature without taking into account this agent, always and necessarily present. And there can be no doubt that through it will be discovered the veritable origin of the effects attributed to calorics, electricity, mag- netism, universal attraction, cohesion, and chemical affini- ties ; for all these mysterious and incomprehensible crea- tions are, after aU, mere co-ordinating hypotheses, useful, no doubt, in our present ignorant condition, but which will be displaced by the ultimate discoveries of true science." From these statements, carrying great authority with them, we may infer that gravity is to be explained by the intervention of ether, and it is only as to the form of this intervention that there can be any doubt. M. Keller, holds that every weighty article is subject, in the midst of the ether, like a vessel upon the water, to two orders of forces, the one circular, the other perpendicular, and that the lattet produces the motion called gravity. VIII. M. Bertrand, of the Institute, lectiu-lng upon this subject at the Sorbonne, says that the Earth has long been known to be spherical in shape, and the ancients endeavoured even to ascertain its dimensions. Aristotle estimated the circumference of our globe at 40,000 stadia THE EARTH. 145 (a stadion is 606 feet 9 inches), which was much helow the mark, just as the calculation of Archimedes was far too excessive. Louis 'XIV., in founding the Academy of Sciences, enjoined it to ascertain the true dimensions of the Earth; and Picard, by taking a direct measurement of several degrees, enabled that body to arrive at a fairly accu- rate conclusion. In none of these experiments did anything occur to raise a doubt as to the perfect sphericity of the Earth. But M. Eichet, the astronomer, on his arrival at Cayenne to take some observations, was astonished to find that the pendulum of his clock, which marked the seconds very accurately in France, did not oscillate so rapidly in Guiana, and he was obliged to shorten it a full length in order to procure a swing lasting exactly a second. Upon his return to France, the inverse phenomenon oc- curred, and he was compelled to lengthen it by just as much as he had shortened it in Guiana. As a pendulum is caused to oscillate by the force of gravity, or, in other- terms, of terrestrial attraction, it seemed as if there must be a diminution of gravity in the equatorial region. Fontenelle said that this was an exception which theory had not foreseen, but he was mistaken in this respect, as Huyghens and Newton had indicated, and even calculated the degree of gravity ia the region of the equator. We know, in fact, that when a body revolves around a centre describing a circumference, there is a development of what is called centrifugal force, which is constantly tending to make it deviate according to the tangent from the circum- ference which it is describing. The greater the extent of the circumference described, the greater is the centrifugal force. 146 ASTRONOMY. We know, too, that the Earth has a rotatory motion upon itself which takes place around an axis passing through the poles. All bodies, therefore, placed upon the surface of our globe describe each day a circumference, which is zero at the pole itself, but which increases to the equator. This rotatoiy movement engenders a centrifugal force, which ca.uses a corresponding diminution of gravity, and the decrease at the equator itself is ttto of ^^ weight of the body. Here we have one cause of the diminution in the rapidity of the pendular swing, but it only accounts for two-thirds of the effects remarked. We must therefore look for a second cause in explanation of the third effect, and this also has been indicated by Newton and Huyghens, viz., the flattening of the Earth at the poles, on account of which an object placed at the poles is nearer to the centre of the Earth, and consequently more attracted than a body placed at the equator. Newton's theory was universally accepted, and one neces- sary deduction from it was that the degrees must be longer at the poles than at the equator. But Cassini, in his measurements of the degrees from Paris to the Pyrenees, when executing a map of France, found that the degrees increased in length as he moved southward. This fact he communicated to the Academy of Sciences, which hesitated to accept it as correct, because it was opposed to Newton's theory ; but Cassini, continuing his measurements north- ward, from Paris to Dunkirk, an-ived at the same result. The conclusion, of course, was that the Earth instead of being depressed at the poles, as Newton asserted, must be, on the contrary, elongated ; and the subject created a great division of sentiment, one party advocating the accuracy of THE EARTH. 147 Cassini's calculations, the other upholding Newton. In order to settle the question, the Academy decided, in 1736, to entrust the task of measuring the degrees to two com- mittees, one of which, presided over by Clairault and Mau- pertuis, proceeded to the polar regions ; the other, with La Condamine at its head, to the inter-tropical regions. It is scarcelj- necessary to say that the arc of a meridian extending to any considerable distance cannot be measured with a chain like a plot of land, for, to say nothing of the unevenness of the ground which must be taken into account, the imperfect character of our measuring instruments would cause the grossest en-ors to be made. The mode of pro- cedure is as foUows : a base not less than five miles long is selected, choice being made of a perfectly flat surface, and this base is measured with the most perfect instruments obtainable. From each extremity of this base a common point of view is fixed upon, so that the two visual rays which reach this landmark form, with the base itself, a triangle of which one side and two angles are known. A simple sum in trigonometry will ascertain the three remaining elements of tliis triangle, that is to say, the other two sides and the third angle. The proportions of this first triangle fixed, a second is constructed in a similar way upon one side of the first ; then a third upon the second, a fourth upon the third, and so on. In this way there is formed a body of triangles, so placed as to be pierced by the meridian line, which it is sought to measure, and permitting of an exact calculation being made as to the length of this line between two given points. The committee, after selecting a base, went on to draw the triangles. In the course of geodesical operations in France, a church tower was always selected as a land- L 2 148 ASTROKOMY. mark ; but this was impossible in Lapland, and there was great difficulty in obtaining a point of view, for the countiy was covered with forests. It was found necessary to cut down trees upon the hill-tops, and construct scaffoldings to act as landmarks. The Sun, too, was veiy scoil'ching, and the mosquitoes j)roved very troublesome. Finally, how- ever, the triangles were completed, and the committee returned to their starting-point to measure the base. But in the meanwhile winter had come on, and they suffered as much from the cold as they had previously done from the heat. Still, by making the best use of the twelve minutes clear light, which was all they could count upon at this season, and assisted by the Aurora Borealis, always so frequent during the long polar nights, they were enabled to measure the base in seven days, and ascertained it to be 14,800 yards. They divided themselves into two parties to take this measurement, one party measuring from right to left, and the other from left to right, so that there might be no mistake. Their respective measurements coincided exactly, and the conclusion of their long labour was that the degree in Lapland, close to the pole, was 1012 j'ards longer than the French degree as measured by Cassini. The committee despatched to the regions of the equator, arrived at a result which coincided very accurately with the above, for they found that the degree was about a thousand yards shorter than in France. Thus, it was established that the degree increases in length from the equator to the pole.* • Ccrtnind's Claimult ct la ilc:a,.x dc U "crre. THE EAKTH. IID IX. Various procedures, yielding different results, have been adopted to calculate the mean density of the Earth, and, as a natural consequence, its weight. Calculations based upon the attraction of mountains, upon the pendulum, upon the torsion-balance, and the sub- terranean pendulum, have all been employed. M. Faye, in a communication to the Academie des Sciences (April 11th, 1873), mentions all the estimates hitherto formed, which it may be interesting to reproduce. Carlini and Plana, by experiments with the pendulum on Mont Cenis, wei'e led to put the Earth's density at 4'39 ; Maskelyne, Ilutton, and Playfair, by the deviation from the vertical on Mount Schehallion, estimated it at 4'71 ; Sir H. James, by the deviation from the vertical on .\i-thm''s seat (Edinburgh), at 5"32 ; Eeich, by Mitchell's torsion-balance, 5'44 ; Cavendish and Bailly, by the same method, at 6 '40 and 5' 66 ; Airy, by the pendulum and a mine-shaft, 400 metres deep, at 6"57. MM. Cornu and Bailie have published the results of recent experiments, whence they gather that the mean density of the Earth is represented by 5"56; and, by a careful interpretation of Bailly's observations, they re- establish a complete concordance between all the results obtained up to the present time. The Earth, being at a mean distance of 91,430,000 miles from the Sun, must traverse in one year an orbit of more than 595,850,000 miles; that is 632,000 miles a daj', or 68,000 miles an hour. Such a rate of speed, though a hundred times greater than that of a cannon ball, is only half that of Mercury in its orbit. 150 ASTRONOMY. THE EARTH. 131 Owing .to its rotation upon its axis, each point of the equator ti'avels about 24,000 miles in twenty-four hours, or 16^ miles a minute, which is about the velocity of a cannon ball. This rotation, taking place in the direction of west to east, gives rise to the apparent motion of all the celestial bodies from east to west. The Earth moves without concussion; its motion is common to both solid and liquid masses, to the air and the clouds, and that is the reason why we do not feel it. We have continually the same landscape before us ; the neigh- bourhood in which we are placed invariably retains the same situation as regards ourselves, and thus it is that we do not remark that we change place relatively to the heavens to the extent of 1,450,000 miles in the terrestrial orbit, and nearly 16^- miles a miiiute at the equator, borne along as v/e are by the Earth's motion around its axis. The Earth's motion in its orbit can only be attributed to the Sun, with which our planet is closely connected, and which exercises its powerful attraction upon it. Its prodi- gious moss, placed in the centre of our planetary system, keeps up in the bodies around it the impulsion which God gave them in the beginning, and maintains between them that admirable equilibrium without which the world could not exist. X. Keplei', the pupil of Tycho-Brah6, discovered the immu- table laws of the planetary motions. Born in 1571, at Weildiestadt (Wiirtemburg), he was one of those rare men of genius who work out the great theories only half pre- pared by the labours of earlier generations. Upon the 24th 152 ASTRONOMY. Kg. 35.— Monument erected to Kepler, at 'Weildiestadt, liis native town, THE EARTH. 153 of June, 1870, a monument in his memory was unveiled in his native town, which does not count more than two thou- sand inhabitants. Upon the house in which he was born is the following inscription : — " From this modest dwelling- place came the great Kepler, the father of untrammelled science, Avho, by the power of his genius penetrated the sublime majesty and the secrets of the Creator. This is why so humble a spot will be celebrated in the ages which are j'et to come." Frisch, who has just terminated the publication of Kepler's complete works, begun in 1854, took for the text of his discourse the words of the poet : " The spot in- habited by a great man is sacred. A century after his death, his words and his deeds still echo in the ears of pos- terity." I will quote a few sentences of his remarks " Kepler's genius was scarcely appreciated during his life- time. After the publication of the works which contained his greatest discoveries, he replied to a person who wrote to inform him of a friend's death; 'I have lost my only reader.' And he also wrote these prophetic words : ' I am quite indifi'erent as to whether my works are read or not during my lifetime. I am sure they will be in a hundred years' time.' Kepler's necessities compelled him to study astrology, which he found far more profitable than true science. In one of his letters, he says : ' Where would real Astronomy be if she had not a harum-scarum daughter, such as astrology ? The salary of the philosopher is so meagre, that the mother v.ould starve unless she had the daughter to support her ! ' " The voluminous correspondence which he has left is full of interest, for in it we see the man. His works reveal the philosopher; in his correspondence we admire the noble 154 ASTRONOMY. qualities of the father, husband, and son, and his conduct in very trying cii-cumstances. We love and esteem the man who was devoted to his mother, the modest philosopher the same with the great as he was with the lowly, who re- mained fast to liis convictions and earned respect both for his personal and scientific merits." Upon a raised pedestal of elegant shape is placed a bronze statue, about four feet in height, of the celebrated astro- nomer. He is represented in a sitting posture, holding in his left hand, Avliich rests upon a celestial globe, a parch- ment containing the dravdng of an ellipse. In the right is an open compass. The four niches of the pedestal are filled with statues two feet high, of Michel Mupsklin, the Tubingen professor wliu tauglit him mathematics, Nicholas Copernicus, Tyclio-Brahe, and Jobst Byrg, the mechanician who aided him in constructing his oi)tical and astronomical instruments. On the centre is engraved the word " KeiJler^" and upon each side are bas-reliefs representing various scenes in his life. On the front is engraved Plii/sica calestis, and beneath is a bas-relief representing Urania measuring space. Upon the right side is inscribed the word Mathc- vtatica, and underneath is Kepler, at the age of 17, entering upon his studies at Tiibingen, under Professor Moesklin. The latter is holding him by the hand and explaining to him the system of Copernicus, a plan of which is given, and a group of fellow-students is gathered around the Professor. Two other bas-reliefs represent : one, the discussion between Tycho-Cralie and Kepler as to the world's system, in the presence of Emperor lludolpli and Wallenstein, with men engaged in printing the astronomical tables called TdlmUn Ihtdolpliliuc ; while in the otlier, Kepler and Byrg, in their Prague workshop, are using their newly-completed telescope THE EARTH. 155 to observe the stars. Above these bas-reliefs are engraved the words Astronomia et Optica. XI. M. Petit, formerly director of the Toulouse Observatory, very truly remarks that, living upon the borders of two centimes, during which the cosmogonic conceptions of the human mind were very strikingly marked out, Kepler, who lighted the torch which was destined to shed such lustre upon the future, could scarcely be expected to escape the prejudices created by the d.arkness which had gone before. Endowed with an ardent imagination, possessed of an in- quiring spirit, burning with the desire to achieve fame, and originally intended for the religious profession, he was dis- tinguished as a preacher at the age of 22 ; when Mceskhn, his professor, obtained for him a post as mathematical master at Gratz, and induced him to abandon the church for astronomy. Henceforward, led to make researches into the first causes, he endeavoured to find an explanation for every fact, and this is why his first works contain many singular theories. Fortunately for him, Tycho-Brahe, who had settled in Germany after his twenty years' occupation of the Dutch Observatory (see p. 26), discovered the genius of the young astronomer by the veiy errors which he com- mitted. He procured for him the ajipointment of mathe- matician to the Emperor, and induced him to take up his residence at Prague. Thus Kepler was put in possession of the valuable materials which Tycho-Brahe had amassed, and which were of great service to him in after life. The following lines, written by Kepler himself, will give 156 ASTRONOMY. an idea of the enthusiasm by which he was animated in search of truth : " Within the last eight inonths, I have seen the first ray of light; within three, daylight ; and within the last few days, the Sun, to my great and exceeding wonder. Nothing shall re- strain me from indulging in my enthusiasm. I wish to insult mankind by the ingenuous confession that I have spoiled the Egj'ptians of their gold, in order to create a tabernacle for my God, far from the confines of Egj^pt. If you forgive me I shall be all the better pleased, but if not, I must endure your reproaches as best I can. Aleajacta est; I write my book. It will be read either by the present or by a future generation ; I don't care which. It can bide its time. Did not God remain for six thousand years in contemplation of his works ! . . . " The life of this great man was far fi'om a happy one in a material point of view ; yet he says " I would not exchange my discoveries for the duchy of Saxony." And he was, of course, quite right. Still he cannot helj) complaining of " the hard times which prevent the Treasuiy from efiecting a regular payment of his salary as mathematician to the Emperor." It was with a view of obtaining the back pay- ments of this pension that Kepler, after putting up with great privations for eleven j-ears. Went from Prague to Eatisbon in November, 1631. But broken down by suffer- ing, mental as well as physical, he was unable to resist the fatigue of the long cold journey upon horseback, and on the 13th of that month he died, far from all his friends, at the age of 60 ; and his Inst moments were further embittered by the reflection that the remembrance of his name would perhaps be of small service to the loved ones who survived him. His presentiments, alas, wore only too true ; and THE EARTH. 157 such was the fate of one who has been truly termed " the Legislator of the Stars." * Tlie concluding lines which Kepler wrote on terminating his works on Astronomy, reveal his great natural piety, while they prove how much real pleasure he derived from his studies. " Before rising from this table upon which I have conducted all my researches, I have but to raise my hands and my ej-es towards heaven, and address my humble prayer to the autlior of all light. O Thou, who by the shedding of light upon nature, dost elevate our desires to the divine hght of grace, so that we may be transported into the eternal hght of Thy glory ; I thank Thee, O Lord and Creator, for all the joy which I have felt in the contempla- tion of the work of Thy hands. In this book which contains the result of my endeavours to show man the greatness thereof, I have tried to guard against presumption, and, so far as my limited capacities permitted, to fathom the mysteries of infinity." t XII. Kepler believed, like his predecessors, that the motion of the celestial bodies must be circular and uniform, and he made several efforts to prove that the motion of the planets was the same, but, after many unsuccessful experiments, he pierced the error which had been committed by previous gene- rations, and arrived at the three important discoveries, since called the Kepler Laws, which are based upon the elliptic motion of the planets around the Sun. These laws are so precise that they enable tlie calculator to name the exact * Petit's Treatise on Astronomy, p. 245. + Kengsienbcrgs ev. Kerclied-ziq. 1S20, p. 411. 158 ASTRONOMY. date at which a planet will return to any given point of its orbit. Kepler, however, was unable to discover the forces which produced the motions he had so accuratelj' defined. He expended gi-eat labour upon this task, but it only resulted in speculations far i*emoved from the reality, and it was left for Newton to disclose the general principle of the celestial motions. Just as all weighty bodies tend to the centre of the Earth, so do the bodies which compose the solar system tend, by force of attraction, towards the sun, which is their common centre. But the planets, if they were governed only by the force of atti-action, that is to say, by the force with which the Sim attracts them towaids him, would gradually be pre- cipitated into that luminary ; and Newton found that there were two motive powers with which they had been endowed by God from the beginning. The first of these is centripetal force, which attracts or carries the planets towards the Sun, their centre ; the second is centrifugal force, which causes them to recede firom it. These two forces counterbalance each other. Thus, the Earth, instead of being transported to a great distance from the Sun by centrifugal force, or dashed against it by centripetal force, is maintained in its orbit by the combined action of the two, and made to describe around the Sun an ellipse, of which it occupies one of the foci. It is to these motions in the heavens that Lamartine's beautiful lines refer : Ces spheres, dont I'ether est le bouillonnement, Ont emprunt6 do Dieu leur premier mouvemcnt. Avcz-vous calculiS parfois, dans vos pensees, La force de ce bras qui les a balances ? Yous ramassez souvent dans la frondc ou la main La noix du rieux noyer, le cailloa du cliemiu : THE EARTH. 159 Imprimant votre effort au poignct qui Ics lance, Vous mcsnrez, enfants, la force a la distance ; L'une tonibc a vos pietls, I'aiitre vole k cent pas, Et vous (lites: " Co bras est phis fort que nion hrris." Kli liien, si par leurs jets vous comparez vos fronilis, Qu'cst-cc (lone que la main qui, lan9ant tous ces iiiomlos, Ces mcncles dent I'esprit no pent porter le poiils, Conimc le jarJinier qui senio au cliamp ses pois, I.es fait feuilrc le vide et tourner sur cux-mOuies, Par I'elau piiniitif sorti du bras suprSme, Aller ot revenir, deseeudre et romonter Pendant des temps sans fin, que lui seul sait compter, De I'espace, et du poids, et des sieclcs se joue, Et fait qu'au firmament ces mille chars sans roue Sont portes sans ornieres et tourncnt sans essicu ? Courbons-nous, mes eufants, c'est la force de Dieu. Newton did not confine his labours to the principal planets ; he calculated the motion of the satelhtes, and the routes of the comets with an accuracy confirmed by subse- quent observations. The ebb and flow of the sea, the pre- cession of the equinoxes, the nutation of the Earth's axis, &c., are all effects of attraction and centrifugal force. XIII. The Earth is nearest to the Sun about the 1st of Januaiy, and furthest away about the 1st of July. In the month of January, the Earth's distance from the Sun is 89,895,000 miles, and in the month of July, 92,965,000 miles. So that there is a difference of nearly 3,000,000 miles. It seems strange that the Earth should be further from the Sun in summer than in winter, yet it is perfectly comprehensible when it is remembered that the heat we receive from the Sun is due, not so much to its proximity as to its elevation above our horizon and the time it remains there. Above the 66th 160 ASTBONOMY, THE EABTH. 161 degree, the Sun does not set when it has entered the sign of Cancer. Below the 64th degree, it only disappears at 10.10 P.M., reappearing 50 minutes later, for, though in reality it remains 3 hours and 40 minutes helow the horizon, the reflection of its rays upon the mountain-tops, and the light shed upon the horizon hy the twilight, enable one to read and write without artificial^light. The inhabitants take advantage of this to shoot and flsh all night, while navigators are enabled to pass through the ice floes. Though the sim never sets in midsummer, its light is not so brilliant in the evening as at noon ; its bril- liancy diminishes correspondingly with its disc, and becomes mild like moonlight, so much so that one can look straight at it without being dazzled. These countries, which have nightless days, have also dayless nights. In midwinter, the only substitute for the Sun is a faint twilight, emanating from the reflection of the rays which it lets fall upon the lofty mountains and the thick mists that compose the atmosphere of the glacial zone. The nights are never so dark at the poles as in other regions, for the moon and the stars seem to possess twice as much light and scintillation, while their rays, reflected by the snow and ice with which the ground is covered, shed so bright a glow that one can see one's way, or even read with- out the aid of a candle. During the Sun's disappearance, the Moon is nearly always efliilgent in these regions, and, in addition to it, there is a continuous light in the north, the vai'ied shades and play of which are amongst the strangest phenomena of nature. The Sun, with all its varieties of light, presents us with a 162 ASTRONOMY. marvellous spectacle, and I shall never forget the splendour of the sky in the polar regions, with its vast sheets of opal, sapphire, emerald, and ruby, amidst which the Sun, after disappeai'ing beneath the horizon, seems to shed its brilliant glow long after it has ceased to be visible. But I prefer to give the reader, in place of my own impressions, an extract from M. Mai'mier's introductory speech when he was re- ceived a member of the French Academy. He says : " There is a sight in the far North which, though it recurs every year, cannot be witnessed without admiration. In summer time, as night approaches, the Sun gradually sinks towards the horizon. Darkness does not spread itself over the land, but upon the surface of the sky appears a white veil which modifies the light, and a deep silence reigns in the woods, the fields, and the waters. Nature is at rest. Then, all at once, the East becomes bright with purple, the luminous rays reappear, and motion begins again. It is the dawn of one day which follows close upon the footsteps of the other. As I recall to my mind this spectacle which I have so oiten witnessed in Sweden and Norway, it seems to me that nations, in their summer time, imdergo phases when their vital force seems numbed, when the Sim of their glory seems to be departing from them. But, yet a little while, and that immortal Sun, which no ocean can extinguish, no dai'kness obscure, will shine forth again in all its splendour." XIV. Arago, in his work on Popular Astronomy, gives an a«y count of the Earth's translation around the Sun, of which the following is an abridgment. THE EARTH. 163 Aristai'chus of Samos (280 b.c.)> insisted, as we leam fi'om Plutarch, that the Earth moved round the Sun, for which he was accused of impiety. Cleanthes of Assos (260 B.C.), is said by the same autho- rity to have been the first to explain the phenomena of the stany sky, by the hypothesis of the Earth's motion around the Sun, combined with a rotatory motion of the Earth itself upon its own axis. This explanation, Plutarch says, was so novel and so much opposed to the ideas generally received, that many philosophers were anxious to prosecute Cleanthes for impiety, as had been done with Aristarchus twenty years before. The planetary system of the ancients, as transmitted to 11$ by Ptolemy, represents the Earth as the centre around which move the seven planets, called the Moon, Mercury, Yenus, the Sun, Mars, Jupiter, and Saturn. But while they looked upon the Earth as the centre of the planetary motions, and itself stationary, the ancients saw that there was a certain distinction between the motions of the planets and the apparent motion of the Sun, but they were imable to disentangle the complicated details of the world's system. Copernicus, in the sixteenth century, endeavoured to solve the difficult problem by reverting to the ideas formerly expressed by Philolaus, the Pythagorean philosopher, who had maintained that the Ealth was a planet circulating round the Sun. In his great work, De revolutionibus, Co- pernicus began by examining whether this opinion was con- sistent with the results of observation. He found that the hypothesis of the Earth traversing an orbit placed round the Sun, gives a basis by which the relative distances of the planets from the Sun may be accurately determined, and he was enabled to construct a system which will bear u 2 164 ASTKONOMT. the severest scrutiny in all future time. In his sj'stem, the Earth circulates round the Sun, taking with it the Moon as its satellite. To Kepler belongs the credit of having established the true planetaiy system, following up the ideas of Copernicus, upon the central position of the Sun with the planets circulating around it, and of having abandoned the old hypothesis, as to circular uniform motions around an imagi- nary eccentric point void of all matter. He also dismissed as illusionary the supposed motions in an epicycle, and con- cluded that the Sun is the centre of the planetary motions which take place along the circumferences of ellipses in which it occupies one of the foci. To place this supposition beyond the reach of criticism, and to establish it as an immutable verity, he took infinite pains in the compilation of a vast mass of calculations, based principally upon Tyeho- Brah^'s observations of Mars. He succeeded in explaining aU the peculiarities of motion in that planet which had baffled the efforts of earlier astronomers. Thus he dis- covered the three great laws which bear his name, and which more recent discoveries have fully confirmed. XV. Water occupies three-fourths of the Earth's surface, dry land only a fourth. There is four times more dry land in the Northern than in the Southern hemisphere. The Eastern hemisphere contains the largest extent of land — about two-thirds of its whole mass — so that the sea predo- minates in the Western hemisphere. Internal influences have led to variations in the level of the Earth's crust, to which no doubt must be attributed the THE EARTH. 165 phenomena remarked in many places. For instance, in Sweden the soil rises about two yards every century above the sea-level; at Eavenna, the floor of the cathedral is several inches below the level of the Adriatic, though when the cathedral was built the reverse was the case ; the palace of Tiberius, at Capreje, is also below the sea-level, and at Cadiz the temple of Hercules, now submerged in the sea, is only visible at low water. The surface of the Earth is studded with rugosities, elevations, depressions, horizontal tracts inclined in different directions, &c. At first sight one is scarcely able to conceive the haimony which the con- texture of the terrestrial crust, external as well as internal, displays to an experienced observer. It might be sup- posed that if there was such a thing as hazard in nature, the word might well be applied to the mountains and water- courses, the various mines and rocks. But such is not the case, for they are governed by laws as rigorous as those which dii'ect the stars in their courses. But it is hardly twenty years since the bases of this science, as simple as it is grand, were laid down, and to the lamented M. £lie de Beaumont belongs the credit of solving the puzzle and tabulating the laws which preside over the position of mountains and the large masses on the globe's surface. Following in the footsteps of one who has been termed the father of modern geology, there is no longer any need to grope one's waj% and the details of this science may be grasped with logical accuracy by any one who will take the trouble to study his teachings.* • Sapport sur Uprogrlsde la stratigraphic, hy Elie de Beaumont. 1C6 ASTRONOMY. XVI. The embossment of the globe has a great Influence upon the climate and the nature of the soil. There are the low- lands, consisting either of plains, undidated ground, or hills and valleys ; there are the highlands, consisting of a large extent of elevated ground, to which the name plateau has been given, or vast salient masses which form the mountains. One of the most remarkable features in the high mountains are the glaciers, which are composed of masses of snow that successive frosts and thaws have transformed into ice. Their thickness varies according to their size, often exceed- ing a hundred feet, while at certain points on the Mer de Glace, at the foot of Montanvert, it reaches from 650 to 800 feet. These glaciers form an interesting subject for study, and it appears from a commimication of M. Grad to the Academy of Sciences (first half of 1871) that the masses of snow accumulated in the lofty glens of the Vosges imdergo the same transfoimation as glaciers far higher up in the Alps. These accumulations form small glaciers, which do not last very long, though it has been remarked that their stratification corresponds to several successive falls of snow, separated by intervals of milder weather. Their transformation is caused by the fusion of the snow on the surface, which, filtering into the mass, gradually changes it into ice, more or less compact at the surface of the soil. All these changes impart to the small glaciers a temporarj' propelling motion, similai' to that of the large glaciers. This motion, appreciable even in the glaciers where there is but a slight declivity, causes a transfer of the THE EARTH. 167 ooooo O CO 0(M CO rH O O » 1;* 09 If) lO "^Vi I ooooeo 3 • ,i r* "* o ofl ^ ill «> s ia< -.ji -**i ■^ «o M^Wf iH S -a) inn . <5 (o t^ ot3 o o ^ •-< i-( i-< o-i r-t WCO-* lA ^, 168 ASTRONOMY. maximum of thickness from the upper to the lower portion of the mass during the interval hetween spring and summer. Thus the Vosges glaciers present upon a small scale the same transformations as those in the Alps, except that their transformations are more rapid, because of the more elevated temperature. They are, in fact, almost at an end in the Vosges, when they are only just beginning in the upper regions of the Alps ; as for instance, on the Col Theodule, where, at an altitude of 9,900 feet, the glacial embryo, when submitted to experiment, is found not to begin its transformation imtil June. A remai'kable fact in connection with the glaciers, which must have been noticed by all tra- vellers to Chamounix during the summer, is the progressive diminution of the two principal glaciers in that valley, the Mer de Glace and the Glacier des Bossons. Persons return- ing to Chamounix after an absence of ten or fifteen years, have remarked a great change, and the observations extend- ing over a period of forty years, taken by a resident, prove that, with the exception of partial oscillations, probably due to the severity of particular winters, the same phenomenon has been going on for the whole of that period. The diminution of glaciers on the northern slope of Mont Blanc forms a striking contrast to the encroachment of the glaciers on the northern slope of Mont Eosa.* The co- existence of these two facts leads to the supposition that the oscillation of the glaciers is mainly due to local causes, engendering either a decrease or increase of temperature. Nevertheless, the contraction of the Chamounix glaciers seems to be merely an instance of the elevation of tem- perature which is said to have taken place witliin this gene- * U. Bey de Morande, aud the Acadimie des Sciences, 1869. THE EARTH. 169 ration throughout various districts of Upper Savoy. Abhe Vaullet, after forty years of thermometric obseiTations, regularly repeated twice a day, and by studying the growth of plants, arrived at this conclusion so far as the neighbour- hood of Annecy was concerned, and he attributed the change first to the clearing of forests ; secondly to the cultivation of waste lands ; thirdly, to the opening up of roads ; fourthly, to the removal of so many hedges. XVII. M. Grad, in a fuiiher comiriunication to the Academy of Sciences upon the limits of perpetual snow, makes some valuable contributions to our stock of knowledge. Bouguer states that this limit corresponds, all over the Earth, to the height at which the mean annual temperature stands at zero (centigrade). Alexander von Humboldt and Leopold Buch fix the limit at the 7nean temperature of zero (centigrade) during the stimmer; and M. Eenou affirms that in all countries the limit of perpetual snow is the altitude where, during the warmest half of the year, the mean temperature equals that at which ice melts. M. Grad insists that what little positive information we have is insufficient to permit of our establishing relation between the temperature of the air and the lowest limit of perpetual snow. The observations taken in various parts of the Alps in- cline him to think that the line of neves,* as indicated by Hugi, who was the first to study this question, is most likely to be the lowest limit of perpetual snow. * itevi is a substance half anow, half ice. 170 ASTRONOMY. Neve is snow in a grainy condition, partially trans- formed into ice, and forming upon the surface of glaciers a series of successive annual strata, the outlines of which are easy of recognition. The contour of the most recent stratum constitutes the lowest limit of perpetual snow, the precise altitude of which has only been measured at a few points, and the figures given in the works on geogi-aphy must only be taken, therefore, as approximative. In the Alps the mean height is from 3,300 to 3,630 yards in the Alpe^ Maritimes and the Alps near Cotte ; 3,080 on the northern, and 3,520 on the southern, slope of the Valais Alps ; 2,860 or 2,970 in the Glarus Alps. In the Scandinavian Alps, where the temperature is higher, and upon the western slope, which is also exposed to mild winds, the snow-limit, at 67 degrees latitude, is as littie as 1,100 yards ; and upon the eastern, which is bot^ drier and warmer, it is only 1,320 yards. Upon the southern slope of the Himalayas, comparatively warm and wet, perpetual snow has a limit of 5,745 yards ; and upoi) the eastern slope, which is both colder and drier, it is 6,180 yards. Such is also the case on many other points of the globe. The lowest limit of perpetual snow does not, a^ M. Grad points out, depend merely upon the temperature, for it varies verj' much in the same latitude, according to the amount of snow-precipitations. The highest limit ip 6,812 yards, upon the southern slope of the Kara-Koroum mountains in the interior of Asia, between 35 and 36 degrees of N. latitude. Its lowest is 6,600 yards, in the Andes near Quito, upon the equator. Upon no point of our globe does the limit of perpetual snow reach the level of the sea, not even in the regions where the climate during the waimest half of the year is below zero, as in Greenland or THE EARTH. 171 Spitzbergen. The glaciers alone descend to the level of the sea, in 43 degrees of latitude, in Patagonia, and in 60 degi'ees latitude on the western coast of America ; and this is owing to the great precipitations of snow which are caused by the moist winds.* The tropical countries containing high mountains possess a very varied type of beauty, for they may be said to enjoy all the four seasons simultaneously. Upon the summits of the mountains glitter ice and snow, while at their feet pre- vails a tropical heat, so that in the course of a quarter of an hour's walk there is a marked change of temperature. The inhabitants profit by this valuable disposition of nature, to have houses at two or three different altitudes, by which means they can enjoy perpetual spring. XVIII. Three-foui-ths of the Earth's surface are a vast sheet of salt water. The presence of the salt is accounted for by the supposition that the waters once covered the whole globe, and thus dissolved all the saline masses upon its surface. It has also been attributed to the presence of inexhaustible salt-banks in the bed of the ocean. Sea water, transparent and colourless when a small quantity is submitted to examination, is very varied in colour when looked at in a mass. At one moment its tints are azure blue, at another emerald green. It exhibits, too, all the colours which can be comprised between these two tints : dark blue, grey blue, green blue, dark green, pale green, etc. — the latter colour being especially remarkable all along the Needles. * Acadimk dea Sciences, MarcU 24tli, IS 73. 17^ ASTRONOMY. The explanation hitlierto given of the cause which gives rise to this diversity of tints has been very unsatisfactory, but it is certain that they are produced by matters of various kinds which the ocean holds in suspension. The phenomenon of the sea's phosphorescence is one of the most beautiful in nature. Wlien manifested in all its Fig. 38.— Nereus, the Sea-god. Fanofka. Blacas Mnseum, plate 20. splendour, the surface of the ocean is as magnificent as that of the starry sky. XIX. The mj'sterious depths which separate the continents of our globe are almost unexplored, and the science of marine geography is yet in its infancy. We cannot carry our inves- tigations fnr beyond the coast without meeting with difficulties as yet insuperable. Still, by means of sounding, considerable results have been achieved, and during a voyage from Eio Janeiro to the Cape, in October, 1852, the souiiding-line attained a deptli of 46,230 feet. In deep water it is difficult to reach the bottom, but the English Channel has THE EAETH. 173 been so completely sounded, tliat navigators know every inch of it. But the friction of the ^Yater, and the weight of the cord itself, make it impossible to tell exactly when the sound touches the bottom. Moreover, the cord does not descend in a straight line, being carried in different direc- tions by the influence of the under-currents. Thus it is impossible to rely very closely upon the results obtained, and it may even be wisest to discai'd altogether the result of certain soundings in the Atlantic which attained incredible depths. The system now adopted by the American Navy seems at once the simplest and the most accurate; A cannon-bdl is thrown into the sea, attached to a very thin cord. The cannon-ball sinks with a gradually, increased velocity until it reaches the bottom. The cord will continue to unroll even after the cannon-ball has reached the bottom, being borne along by the powerful currents. Still, as the speed of these currents is a known quantity, and incomparably less than that of a cannon-ball projected from a great altitude, any hydrographer is capable of distinguishing between the two periods, and so of telling when the action of the cannon- ball upon the cord ceases. This cannon-ball is so con- structed that Avhen it reaches the bed of the sea it unfastens itself from the cord, which brings up a small C3'linder con- taining substances from the bottom. In this way, specimens can be obtained from very deep parts of the ocean. Nature seemed to indicate Ireland and Newfoundland as the two starting points of the line which was to unite the continents of which they form the advanced sentries, and the study of hydrography led to the same conclusion. The bed of tlie sea sinks very rapidly on leaving the Irish coast, but it soon reaches a depth which varies very little most of 174 ASTRONOMY. the way across. This marine plain, called the tclegi-aphic plateau, is about 9,900 feet below the level of the ocean. More level and vaster than the steppes and deserts of our continents, the sound has not discovered there either sand Fig, 39,— Foraminifera, brought up from the beJ of the ocean dtu-ing the laying of the Transatlantic Cable. or Clay, and it is composed entirely, of the microscopic animals known as foraminifera. t These animalculee, which, during their ephemeral existence, cover the warm waters of the tropical seas, sink after death to the bottom, and the submarine currents carry them to these still depths, where their delicate carapaces are perpetually shielded from the tempests which convulse the surface of the waters. The bed of the sea, which, in the middle of the Atlantic, is 9,900 feet deep, gradually rises on nearing America until Newfoundland, where it forms a steep decline, as upon the Irish coast. XX. In another work, Les Mondes Scientifiques, I mentioned some curious experiments which had been made at Wharf- THE EARTH. 175 Eoad Dock, London, for the purpose of ascertaining the effects of the pressure upon a cable submerged in the Atlantic to the depth of two and a quarter miles. The experiments were made with Reed's hydraulic press, which is capable of exercising a pressure of about 10,000 lbs. to the square inch. The cable used was a piece of that which has been laid in the Gulf of Persia, with a covering of gutta-percha a centimetre (two-fifths of an inch) thick. It was subjected for an hoiu: to a pressure equal to that of a body of sea-water two and a quarter miles deep. Professor Thomson having previously tested its conductibility with a reflecting galvanometer. Some electricians expected that this enormous pressm'e (5,000 lbs. to the square inch) would force the water into the interior of the cable, and that it would consequently be deteriorated, if not destroyed. The «xperiment did not waiTant these forebodings, for it was found that the pressure had, on the contrary, improved the cable, at least so far as its conductibility was concerned. It is said that a bottle of wine, carefully corked, was plunged to a great depth in the Atlantic, and that when it was drawn up the wine had all disappeared, and its place taken by salt-water. Also, that a carefully-corked empty bottle was let down, and drawn up full of water without the cork being removed. In another experiment, six bottles of pale ale, all care- fully corked and covered with capsules, were let down ; as also a number of bottles of lemonade and ginger-beer, with wire over the corks, like champagne. In one of the empty bottles was placed a wooden cylinder, resting on the bottom and supporting the cork. All these bottles were submitted for an hour to the pressure of a column of water two and a half miles deep. 'When drawn up, the pale ale bottles were 176 ASTRONOMY. found to be unchanged, as also were the lemonade and ginger- beer bottles. The small space that had been left between tlie cork and the liquid was filled up, and tliis was all. The cork in the first empty bottle had been forced in, and it was, of course, full of water. The champagne-cork in the bottle which contained the wooden cylinder was paiiially forced in, and it came up full like the rest. These facts, due to the pressm-e of the water, are not without their interest and instruction. I will terminate this section of the chapter on " The Earth " by an account of the coral bank near Haiti, which Mr. Green, the well-known diver, contributed to the Panama Star in 1868. " The bank of coral to which I allude is forty miles long by &om ten to twenty wide, and it is one of the most beautiful spectacles which the eye of a diver ever contem- plated. The depth of the water varies firom 10 to 110 feet, and it is so clear that one can see a distance of three or four hundred feet when in the water. The bed of the ocean is, in certain places, as level as a marble floor ; at others, it is studded with columns of coral from 10 to 110 feet high, and about a foot in diameter. The summits of the highest columns support thousands of pendants, which are in turn decorated with thousands of still smaller ones. At other points, the pendants form arches upon arches, and the diver, standing upon the bed of the sea and looking through these sinuous passages, is reminded of a cathedral sub- merged beneath the ocean. Here and there the coral rises to the surface of the water, just as if the loftiest columns were the towers of majestic temples, now in ruins. There is a countless variety of shrubs in every crevice of the coral where the water has deposited any soil. Though of a very THE EARTH. 177 faint colour, owing to the small amount of light which they receive, there is every variety of shade, and all of them differ from the plants seen on dry land. One of these shrubs particularly attracted my attention. It resembled a large fan, of very varied colours and tints. The fish upon these Silver Banks are also as varied in shape and dimen- sions as the region which they move about in, from the symmetrical goby to the globular sun-fish, some being of a very dull colour, and others of a hue as changing as that of the dolphin." XXI. The temperatm'e of the sea varies, but at the bottom it is generally four degrees (centigi-ade) above zero, whatever may be the surface-temperature. Such a temperature naturally sets up numerous motions in this vast extent of water, which is incessantly tending to an equilibrium. It is possible that these motions give rise to the currents which are remarked in the ocean, and which are vast streams whose progi'ess is only arrested by bodies of water denser than themselves. It is also thought that the shape of the land and the attractive action of the Sun and the Moon contribute to produce these currents. When navigators use the thermometer, they are able to distinguish with ease the great oceanic cun'ents of tepid water which are encircled by the cold waters, and which, flowing back in the track along which they came, form a sort of inteiminable stream. In addition to the great currents there are many secondary ones, notably in the seas between the Tristan d'Acunha 178 ASTRONOMY. islands and the Cape of Good Hope. These currents are easy of recogmtion, for they make the ocean look as if it was divided into bands of different hues, and a vessel passing through them is driven more or less out of the straight course. The piincipal of these currents, the only important one in fact that has been thoroughly studied, is that which starts from the Gulf of Mexico, and which extends all along the coast of the United States to the northern region, where its waters still maintain a relatively warm temperature, and create a space of open sea amid the polar ice. Mr. James Croll has published several works upon this vast marine current, which is known as the Gulf Stream ; and he has made some interesting calculations as to the quantity of heat which its waters are capable of trans- mitting. He estimates that the total volume of the waters com- posing this stream is equivalent to a canal about fifty miles long and 1,000 feet deep, in which the water moved at a rate of four miles an hour. The mean temperature of this liquid mass, when it flows from the Gulf of Mexico into the Florida Keys, is not less than 18° 3 (centigrade). It seems certain that these waters, when they return from the north, have a mean temperature of 4° 4*=, a diminution, that is, of 13° 9*^. Thus eveiy cubic metre of water conveys from the tropics to the north 13,900 calorics, representing a djTiamic force of 6,907,000 kilogrammeters, at the rate of 425 kilogrammeters to each caloric. By the same cal- culation, it is estimated that the cmTent must embody 156,900,000,000 cubic metres of water per hour, or 3,766,000,000,000 per day. At this rate, the quantity of heat which the Gulf Sti'eam subtracts each day from the THE EARTH. 179 equatorial region amounts to 52,250,000,000,000,000 calorics, or 22,250,000,000,000,000,000 kilogrammeters. Sir John Herschel's and Pouillet's researches as to the quantity of heat transmitted directly by the Sun, show that if a portion of it was not intercepted by the atmosphere, a square yard, exposed to the Sun's rays, would receive per second a quantity of heat equivalent to 237 lbs. But while Mr. Meech has estimated the quantity of caloiic intercepted by the atmosphere at nearly ^ of the quantity emitted by the Sun, M. PouiUet puts it at ^V- Taking the former calculation, we find that the heat received per second by a square yard of ground, with the Sun at the zenith, is equivalent to a mechanical force of 96 kilogi-ammeters, 2'. If the Sun remained stationary at the zenith for twelve hours, it would amount to 4,168,000 kilogrammeters.* In Kohl's History of the Gulf Stream it is pointed out that the name of this remarkable Atlantic current dates back to 1748, when the Swede, Peter Kalm, wrote a book of travels, in which he aUuded to the debris of trees, plants, etc., which were washed from the Gulf of Mexico to the Faroe Islands and Iceland. The first navigator who turned the current to his profit was Alaminos, the pilot of a vessel convej'ing despatches from Fernand Cortes to Spain, in 1519. For the next two centuries, the American whalers were the only persons who seem to have been acquainted with it, and they, by keeping out of its course during their voyage back from Europe, reached America a fortnight quicker than the English mail-packets. Franklin, when postmaster-general, had a chart of the Gulf Stream executed by these fishermen, and sent it to the English authorities, but they do not seem • Les Mondcs aeUiUifiiues, 1870, p. 204. S3 180 ASTRONOMY. to have placed any reliance upon it. Franklin, too, was the first to take steps for ascertaining the course followed bj' the current, which he did by finding out in what parts of the ocean the water was warmer than elsewhere.* xxn. M. Grad, in a communication to the Academy of Sciences (1871), points out that if the nature of this marine current is imderstood over the first half of its course, such is not the case in the northern half. Flowing from the Gulf of Mexico into the Florida Keys, it runs parallel with the coast of the United States as far as Cape Hatteras. In this part of its course, the temperature is nowhere less than 25 degrees (centigrade), and is frequently higher, even in winter, when, upon the African coast, of the same latitude, the mean January temperature is only 12 degrees. After leaving Cape Hatteras, the Gulf Stream deviates eastward from the American coast towards Newfoundland, where, at 42 degrees western longitude from Paris, its temperature varies between 19 and 24 degrees (centigrade) from the months of January to July. Its waters then flow in a north- easterly direction, embracing the coast of Europe and the heart of the Polar Seas. But for the Gulf Stream, the climates of England and Germany would be as inhospitable as that of Labrador : the Scandinavian peninsula would be, like Greenland, a mass of ice. The northern part of Norway, where the sun is invisible for a whole month, would be so cold that mercury would freeze there, as is the case in the same parallel in Asia and America ; whereas, thanks to the * Cosmos, 1868, p. 205. THE EARTH. 181 GuK Stream, the sea at Fauholnn has a temperature of more than three degrees (centigrade) above zero. Thus, the Gulf Stream is a permanent source of heat, which Mr. James CroU estimates as equal to the quantity emitted by the Sun over a surface of five million square miles at the equator. Following the march of the temperature at the furthest ramifications of the Gulf Stream, the author gives the pro- portions of the warm cmrent to the limits of the stationary or floating ice between Greenland and the north of Europe for the last two years. The results of frequent soundings show that the Gulf Stream touches the bottom of the Atlantic Ocean between the Hebiides and the Faroe Islands, at 60 degrees latitude, with a temperatiu'e of 5° "3 at a depth of 770 fathoms. A degree fm-ther north, between the Faroe and Shetland Islands, the cun-ent is only 200 fathoms deep, and the colder water of the polar regions extend beneath it to a depth of 640 fathoms. Fmrther north still, at 60 degrees latitude, Admii-al Irmenger found that at 60 fathoms depth there was a temperature of 7° '5, as against 10 degi-ees at the sm-face. The waters of the Gulf Stream, as M. Grad proceeds to state, then divide into two arms, one flowing northward along the western coast of Spitzbergen, the other eastward along the shores of Nova Zembla. The western, or Spitz- bergen, arm reaches as fai' as 80 degrees latitude in summer, where it has a temperature of two degrees (centigTade). The limits of the Gulf Stream are more clearly defined in winter than in summer, and the motion of the floating ice ceases diu'ing the cold season, as out of a hundred avalanches encountered by vessels in the North Atlantic, ninety were met with in the months between April and August, and ten only during the remainder of the year. The tepid waters of 182 ASTRONOMY. Cfaaplan Sea Slack Sen Dlack Sea Oerraati Ocean H qq Baltic Sea Daltlc Sen »tk. Siberian Sea MMItcrrancan Sea i Baltic Sea Bnglish Channel 3. 5 "Wliltc Sea Adriatic Sea 1 3. a Mediterranean Sea German Oecan Mcdltcri-ancan Sea Blue Sea Seaorukotsk r Siberian Sea ffi Yellow Sea i- Bay of Bengal Bo. Sf Sea of Oman i Persian Gulf Lake Anil Persian Gulf > Dead Sea o Southern Ocean g Murray Murray 1 n Mediterraneaa Gulf of Guinea Atlantic Ocean Do. 1 a Do Mediterranean Sea Mississippi River GulfofMexleu 1 Atlantic Ocean Do. Do. V Pacific Ocean Mississippi Biver Gulf of California Pacific Ocean Atlantic Ocean Do. THE EARTH. 183 the Gulf Stream penetrate into the middle of the polar seas as far as 80 degrees latitude to the west of Spitzbergen, and as far as 76 degrees latitude on the western coast of Nova Zembla. In some of the extreme ramifications of the Gulf Stream there are spots where the water is colder, and as it moves slowly there is some difficulty in ascertaining the dii'ection of the currents. But, notwithstanding these diffi- culties, the fact of the Gulf Stream extending as far as Nova Zembla has been proved by the presence of timber, bamboo-canes, etc., which must have been floated thither from Brazil ; as also by the picking up of nets, and other fishing apparatus, drifted from the Loffi)den Isles, or Finmark. In a note to the Academy of Sciences, 1871, M. Marie Davy points out that the meteorology of Eiirope is regulated by the atmospheric circulation, ahd by the marine circulation which it engenders. He insists that the main regulator of the climate in France and the neighbouring countries is the aerial stream, known as the equatorial cuirent, in its first part, and the polar current in its latter part. The efforts of the meteorologist must, therefore, be concentrated upon the study of this great current : its origin, the causes which affect its quantity, as well as the direction and amplitude of its trajectory, and the laws which govern these changes, together with the signs which denote their being about to take place. He must also study the origin, nature, laws, and premonitory signs of the local phenomena which occur within the moving aerial mass, and are the causes of so much variety in the meteorology of Europe ; and, lastlj^, he must study the fluctuations of the Gulf Stream, which result from the fluctuations of the aerial cuiTent, and react in their turn upon the latter. 184 ASTRONOMY. - XXIII. Elvers generally follow the direction of mountains and flow from east to west, though there are few whose course is either northward or southward. Several of the largest rivers — such as the Nile, the Indus, the Po, etc. — overflow their banks at certain seasons of the year, ■ Nearly all the ancient poets ascribed to, these rivers the honours of divinity. Painters and poets alilce represented them as venerable old men, with long flowing beards and a crown of rushes upon theii' heads. Eeclining among the reeds, their hand rested upon a pitcher with water flowing from it, the pitcher being more or less out of the pei-pendicular, according to the rapidity of the river represented. Upon medals these divinities are placed right , or left, accordingly as the rivers which they typify flow east or west. Every river had its peculiar characteristic, generally taken from the animals indigenous to its basin, or the fishes in its water. Fig. 41, — Khea (from an Adrian medal). CHAPTER VIII. THE MOON. Natiu'o of the Moon — Its size — The light which wo receive from it when it is nt its briglitcst — Heat reflected from the Jloon ; liistory of the discovery — Shadows, spots, craters, mountains, and extinct volcanoes, observed upon the Moon's disc — Its various motions — Sidereal revolution — Sin'- prising velocity of tho Moon's motion — Is it possible for the Moon to fall on to the Earth— Successive applications of the principle of gravity in explanation of, the solar system — Problem of the three bodies — Simple and easy experiments in explanation of the Moon's i)hasos — Ashy liglit — Symbolisation of the Moon ; curious passage from Sophocles. I. • The Moon is, next to the Sun, the celestial body most calculated to arrest our notice, both in respect to its apparent size and the peculiar phenomena which we see during its course. Though it appears to us much larger than the stars, it is in reality smaller than any one of them, and its appa- rent size is caused by its relative proximity to the Earth, from which it is dista'iit only 238,793 miles. The diameter of the Moon, which does not always seem to us the same, for this luminary is at one time nearer to us than the other, is 32 minutes; that of the Earth seen from the Moon would be 1 degree 64 minutes. The diameter of the Moon is 2,153 miles ; its circumference, 6,500 ; its surface, only -'^ that of the Earth ; its volume, -^ ; its mass, -gV' Like the Earth, it is an opaque body, having no light of its own, but receiving and reflecting that of the Sun. We see it at its 186 ASTRONOMY. brightest when it is full, and its light has been calculated even then to be only -j-Jr t^i^t o^ ^^^ Sun. Volpicelli, in a communication to the Academy of Sciences (first half of 1870), gives some interesting facts relative to the heat emitted by the Moon. He says that Melloni was the first to furnish experimental proof of this phenomenon, which Virgil, Dante, Guarini, and other Latin or Eoman poets, had denied. Many philosophers, such as Ai-istotle, Thomas Aquinas, Pic de la Mirandole, and Jerome Cardan among them, asserted the contrary ; but in default of the thermometer by which the fact has since been ascertained, they were unable to prove that statement. The English philosophei", Hooke, points out how feeble is the direct calorific effect of the Moon upon the Earth. Montanori, of Modena, states that by means of an air-thermometer and a large mirror it was ascertained that the radiation of the Moon caused a rise of several degrees in the temperature. But as he does not state how tlie experiment was conducted, and as the thermometer was during his time still very im- perfect, Volpicelli discredits his assertion altogether, and believes that he is justified in saying Melloni was the first to give experimental and incontestable demonstration of the heat of the Moon's rays (March 23rd, 1846). This discovery was attended with greater difficulties than might at first sight have been expected ; for the most sensi- tive of thermometers, placed in the focus of a mirror or an eye-glass is incapable of manifesting the existence of heat in the solar radiation. The latest experiments of M. Bailie, conducted by means of an ingenious apparatus, which I have no space to describe here, corroborated as they are by the experiments of Lord Eosse, Piazzi Smyth, and Marie Davy, show that the full Moon at Paris emits during the THE MOON. 187 summer months the same quantity of heat as a black sur- face of the same size kept at 100 degrees (centigrade) and 88 yards distance. II. The Moon's surface is covered with black spots, which are visible to the naked eye, and which cause reflections of light, varying according to the position of the Moon in respect to the Sim. Seen through a telescope, they are far more numerous, extending all over its surface, and present- ing a volcanic character like the crater of Vesuvius, or the hilly ground in the department of Puy-de-dome (France). Some of these points have the aspect of lofty mountains, chiefly distinguishable by the triangular shadow which they reflect in the opposite direction to the Sun. As a general rule the mountains of the Moon seem loftier than those of the Earth. Many of them are thought by astrtinomers to be twenty-five or thirty thousand feet high, whereas the loftiest of th» American Cordilleras is only four miles (22,120 feet) above the level of the sea. At certain times one can distinguish, beyond the limit of the Moon's light, brilliant points, which seem to be detached from its disc, as if they were stars situated close to it. They are, in reality, mountains in the obscure part of its surface, but so lofty that their summits are lighted by the Sun, while their base remains in obscurity. All these asperities and unevenness of surface explain the indentations often visible upon the bright edge of the Moon. These indentations have often been mistaken for volcanos, though M. de Crety, while watching the eclipse at Aden, on the 18th of August, 1868, believed that he could 188 ASTKONOMY. distinguish, subsequent to the totality of the eclipse, three triangular prominences upon the edge of the Moon, with which they kept close order, and which seemed to be lunar volcauos in a state of activity. Judging from the descrip- tion given, these prominences must have been gaseous, or at all events formed of very divisible matter. This appari- tion is not, necessarily, an optical illusion; it has been thought to indicate the existence, upon the posterior surface of the Moon and close, to the edge,, of a chain of volcanos in a state of activity at the moment of the eclipse, and that their smoke and ashes must have been hurled beyond the edge by some unknown force, with the natm-e of which we are still unacquamted. III. Galileo first attempted to measure the height of the lunar mountains, and his example was followed by many other astronomers, so that their altitude was known even before that of many mountains of the Earth. Hevelius, in liis chart of the Moon, adopting the height assigned by Gralileo to the lunar mountains, gave to them names taken from geographers, fearing to create a feeling of jealousy if he bestowed upon them the names of rival astronomers, but since that time a different sjstem has been followed. M. Petit, of the Toulouse Observatory, in his work upon astronomy, says that, after a careful inspection of the shape of the shadows in the Moon, and that of the heights which project them, it is easily seen that most of those heights ai'e composed of a circular enclosure, the inner part of which is generally lower than the mean surface of the Moon, THE MOON. 189 Graler of Albategnius. li\%. i2 represents the full Moon as Explanation : The capital Marshes, St I. Qlacial Sea. . II. Gulf of (low. III. Half of flowers. IV. Marsh of fogs. V. Sea of rains. VI. Carpathian mounts. A'll. Ocean of tempests. Tlir. Midland Sea. IX. Sea of clouds. IX.6 Sea of vapours. X. Sea. nf darkness. Crater of Eratostlienes. seen through a powerful glass. (Gujnemcr. numbers begin at the top to the left. 'MS, Lakes, Oulfs,