Ecclesiastical Attempt to enforce the Geocentric Doctrine that the Earth is the Centre of the Universe, and the most important Body in it.
The Heliocentric Doctrine that the Sun is the Centre of the Solar System, and the Earth a small Planet, comes gradually into Prominence.
Struggle between the Ecclesiastical and Astronomical Parties.—Activity of the Inquisition.—Burning of Bruno.—Imprisonment of Galileo.
Invention of the Telescope.—Complete Overthrow of the Ecclesiastical Idea.—Rise of Physical Astronomy.—Newton.—Rapid and resistless Development of all Branches of Natural Philosophy.
Final Establishment of the Doctrine that the Universe is under the Dominion of mathematical, and, therefore, necessary Laws.
Progress of Man from the Anthropocentric Ideas to the Discovery of his true Position and Insignificance in the Universe.
An astronomical problem. [252] The Age of Reason in Europe was ushered in by an astronomical controversy.
Is the earth the greatest and most noble body in the universe, around which, as an immovable centre, the sun, and the various planets, and stars revolve, ministering by their light and other qualities to the wants and pleasures of man, or is it an insignificant orb—a mere point—submissively revolving, among a crowd of compeers and superiors, around a central sun? The former of these views was authoritatively asserted by the Church; the latter, timidly suggested by a few thoughtful and religious men at first, in the end gathered strength, and carried the day.
Its important consequences. [253] Behind this physical question—a mere scientific problem—lay something of the utmost importance—the position of man in the universe. The conflict broke out upon an ostensible issue, but every one saw what was the real point in the dispute.
Treatment of the Age of Reason.In the history of the Age of Reason in Europe, which is to fill the remaining pages of this book, I am constrained to commence with this astronomical controversy, and have therefore been led by that circumstance to complete the survey of the entire period from the same, that is, the scientific point of view. Many different modes of treating it spontaneously present themselves; but so vast are the subjects to be brought under consideration, so numerous their connexions, and so limited the space at my disposal, that I must give the preference to one which, with sufficient copiousness, offers also precision. Whoever will examine the progress of European intellectual advancement thus far manifested will find that it has concerned itself with three great questions: 1. The ascertainment of the position of the earth in the universe; 2. The history of the earth in time; 3. The position of man among living beings. Under this last is ranged all that he has done in scientific discovery, and all those inventions which are the characteristics of the present industrial age.
What am I? Where am I? we may imagine to have been the first exclamations of the first man awakening to conscious existence. Here, in our Age of Reason, we have been dealing with the same thoughts. They are the same which, as we have seen, occupied Greek intellectual life.
Roman astronomical ideas.When Halley's comet appeared in 1456, it was described by those who saw it as an object of "unheard-of magnitude;" its tail, which shook down "diseases, pestilence, and war" upon earth, reached over a third part of the heavens. It was considered as connected with the progress of Mohammed II., who had just then taken Constantinople. It struck terror into all people. From his seat, invisible to it, in Italy, the sovereign pontiff, Calixtus III., issued his ecclesiastical fulminations; but the comet in the heavens, like the sultan [254] on the earth, pursued its course undeterred. In vain were all the bells in Europe ordered to be rung to scare it away; in vain was it anathematized; in vain were prayers put up in all directions to stop it. True to its time, it punctually returns from the abysses of space, uninfluenced by anything save agencies of a material kind. A signal lesson for the meditations of every religious man.
More correct ideas among some of the clergy.Among the clergy there were, however, some who had more correct cosmic ideas than those of Calixtus. A century before Copernicus, Cardinal de Cusa had partially adopted the heliocentric theory, as taught in the old times by Philolaus, Pythagoras, and Archimedes. He ascribed to the earth a globular form, rotation on its axis, and a movement in space; he believed that it moves round the sun, and both together round the pole of the universe.
The geocentric and heliocentric theories.By geocentric theory is meant that doctrine which asserts the earth to be the immovable centre of the universe; by heliocentric theory that which demonstrates the sun to be the centre of our planetary system, implying, as a necessary influence, that the earth is a very small and subordinate body revolving round the sun.
The geocentric doctrine adopted by the Church.I have already, in sufficient detail, described how the Roman Church had been constrained by her position to uphold the geocentric doctrine. She had come to regard it as absolutely essential to her system, the intellectual basis of which she held would be sapped if this doctrine should be undermined. Hence it was that such an alarm was shown at the assertion of the globular form of the earth, and hence the surpassing importance of the successful voyage of Magellan's ship. That indisputable demonstration of the globular figure was ever a solid support to the scientific party in the portentous approaching conflict.
Preparations for the heliocentric doctrine.Preparations had been silently making for a scientific revolution in various directions. The five memoirs of Cardinal Alliacus "On the Concordance of Astronomy with Theology," show the turn that thought was taking. His "Imago Mundi" was published in 1460, and is said to have [255] been a favourite work with Columbus. In the very Cathedral of Florence, Toscanelli had constructed his celebrated gnomon, 1468, a sun-ray, auspicious omen! being admitted through a plate of brass in the lantern of the cupola. John Muller, better known as Regiomontanus, had published an abridgment of Ptolemy's "Almagest," 1520. Euclid had been printed with diagrams on copper as long before as 1482, and again in Venice twenty-three years subsequently. The Optics of Vitello had been published 1533. Fernel, physician to Henry II. of France, had even ventured so far, supported by Magellan's voyage, as to measure, 1527, the size of the earth, his method being to observe the height of the pole at Paris, then to proceed northward until its elevation was increased exactly one degree, and to ascertain the distance between the stations by the number of revolutions of his carriage wheel. He concluded that it is 24,480 Italian miles round the globe. The last attempt of the kind had been that of the Khalif Almaimon seven hundred years previously on the shore of the Red Sea, and with nearly the same result. The mathematical sciences were undergoing rapid advancement. Rhæticus had published his trigonometrical tables; Cardan, Tartaglia, Scipio Ferreo, and Stefel were greatly improving algebra.
Copernicus, the works of.The first formal assertion of the heliocentric theory was made in a timid manner, strikingly illustrative of the expected opposition. It was by Copernicus, a Prussian, speaking of the revolutions of the heavenly bodies; the year was about 1536. In his preface, addressed to Pope Paul III., whether written by himself, or, as some have affirmed, for him by Andreas Osiander, he complains of the imperfections of the existing system, states that he has sought among ancient writers for a better way, and so had learned the heliocentric doctrine. "Then I too began to meditate on the motion of the earth, and, though it appeared an absurd opinion, yet since I knew that in previous times others had been allowed the privilege of feigning what circles they chose in order to explain the phenomena, I conceived that I might take the liberty of trying whether, on the supposition of the earth's motion, it was possible to find [256] better explanations than the ancient ones of the revolutions of the celestial orbs."
"Having, then, assumed the motions of the earth, which are hereafter explained, by laborious and long observation I at length found that, if the motions of the other planets be compared with the revolution of the earth, not only their phenomena follow from the suppositions, but also that the several orbs and the whole system are so connected in order and magnitude that no one point can be transposed without disturbing the rest, and introducing confusion into the whole universe."
Introduction of his system.The apologetic air with which he thus introduces his doctrine is again remarked in his statement that he had kept his book for thirty-six years, and only now published it at the entreaty of Cardinal Schomberg. The cardinal had begged of him a manuscript copy. "Though I know that the thoughts of a philosopher do not depend on the judgment of the many, his study being to seek out truth in all things as far as is permitted by God to human reason, yet, when I considered how absurd my doctrine would appear, I long hesitated whether I should publish my book, or whether it were not better to follow the example of the Pythagoreans and others, who delivered their doctrine only by tradition and to friends." He fears being accused of heresy. He concludes: "If there be vain babblers who, knowing nothing of mathematics, yet assume the right of judging on account of some place of Scripture perversely wrested to their purpose, and who blame and attack my undertaking, I heed them not, and look upon their judgments as rash and contemptible."
Copernicus clearly recognized not only the relative position of the earth, but also her relative magnitude. He says the magnitude of the world is so great that the distance of the earth from the sun has no apparent magnitude when compared with the sphere of the fixed stars.
Early correction of the Copernican theory.To the earth Copernicus attributed a triple motion—a daily rotation on her axis, an annual motion round the sun, a motion of declination of the axis. The latter seemed to be necessary to account for the constant direction of the pole; but as this was soon found to be a misconception, [257] the theory was relieved of it. With this correction, the doctrine of Copernicus presents a clear and great advance, though in the state in which he offered it he was obliged to retain the mechanism of epicycles and eccentrics, because he considered the planetary motions to be circular. It was the notion that, since the circle is the most simple of all geometrical forms, it must therefore be the most natural, which led to this imperfection. His work was published in 1543. He died a few days after he had seen a copy.
Against the opposition it had to encounter, the heliocentric theory made its way slowly at first. Among those who did adopt it were some whose connexion served rather to retard its progress, because of the ultraism of their views, or the doubtfulness of their social position. Giordano Bruno of Nola. Such was Bruno, who contributed largely to its introduction into England, and who was the author of a work on the Plurality of Worlds, and of the conception that every star is a sun, having opaque planets revolving round it—a conception to which the Copernican system suggestively leads. Bruno was born seven years after the death of Copernicus. He became a Dominican, but, like so many other thoughtful men of the times, was led into heresy on the doctrine of transubstantiation. Not concealing his opinions, he was persecuted, fled, and led a vagabond life in foreign countries, testifying that wherever he went he found scepticism under the polish of hypocrisy, and that he fought not against the belief of men, but against their pretended belief. He teaches the heliocentric theory, For teaching the rotation of the earth he had to flee to Switzerland, and thence to England, where, at Oxford, he gave lectures on cosmology. Driven from England, France, and Germany in succession, he ventured in his extremity to return to Italy, and was arrested in Venice, where he was kept in prison in the Piombi for six years without books, or paper, or friends. Meantime the Inquisition demanded him as having written heretical works. He was therefore surrendered to Rome, and, after a farther imprisonment of two years, tried, excommunicated, and delivered over to the secular authorities, to be punished "as mercifully as possible, and without the shedding of [258] his blood," the abominable formula for burning a man alive. He had collected all the observations that had been made respecting the new star in Cassiopeia, 1572; he had taught that space is infinite, and that it is filled with self-luminous and opaque worlds, many of them inhabited—this being his capital offence. He believed that the world is animated by an intelligent soul, the cause of forms but not of matter; that it lives in all things, even such as seem not to live; that every thing is ready to become organized; that matter is the mother of forms and then their grave; that matter and the soul of the world together constitute God. His ideas were therefore pantheistic, "Est Deus in nobis." In his "Cena de le Cenere" he insists that the Scripture was not intended to teach science, but morals only. The severity with which he was treated was provoked by his asseverations that he was struggling with an orthodoxy that had neither morality nor belief. This was the aim of his work entitled "The triumphant Beast." and is burnt alive as a heretic. He was burnt at Rome, February 16, 1600. With both a present and prophetic truth, he nobly responded, when the atrocious sentence was passed upon him, "Perhaps it is with greater fear that ye pass this sentence upon me than I receive it." His tormentors jocosely observed, as the flames shut him out forever from view, that he had gone to the imaginary worlds he had so wickedly feigned.
This vigorous but spasmodic determination of the Church to defend herself was not without effect. It enabled her to hold fast the timid, the time-servers, the superficial. Lord Bacon. Rejects the Copernican doctrine. Among such may be mentioned Lord Bacon, who never received the Copernican system. With the audacity of ignorance, he presumed to criticize what he did not understand, and, with a superb conceit, disparaged the great Copernicus. He says, "In the system of Copernicus there are many and grave difficulties; for the threefold motion with which he encumbers the earth is a serious inconvenience, and the separation of the sun from the planets, with which he has so many affections in common, is likewise a harsh step; and the introduction of so many immovable bodies in nature, as when he makes the sun and stars [259] immovable, the bodies which are peculiarly lucid and radiant, and his making the moon adhere to the earth in a sort of epicycle, and some other things which he assumes, are proceedings which mark a man who thinks nothing of introducing fictions of any kind into nature, provided his calculations turn out well." The more closely we examine the writings of Lord Bacon, the more unworthy does he seem to have been of the great reputation which has been awarded to him. The popular delusion to which he owes so much originated at a time when the history of science was unknown. They who first brought him into notice knew nothing of the old school of Alexandria. This boasted founder of a new philosophy could not comprehend, and would not accept, the greatest of all scientific doctrines when it was plainly set before his eyes.
It has been represented that the invention of the true method of physical science was an amusement of Bacon's hours of relaxation from the more laborious studies of law and duties of a court. His chief admirers have been persons of a literary turn, who have an idea that scientific discoveries are accomplished by a mechanico-mental operation. The practical uselessness of his philosophy. Bacon never produced any great practical result himself, no great physicist has ever made any use of his method. He has had the same to do with the development of modern science that the inventor of the orrery has had to do with the discovery of the mechanism of the world. Of all the important physical discoveries, there is not one which shows that its author made it by the Baconian instrument. Newton never seems to have been aware that he was under any obligation to Bacon. Archimedes, and the Alexandrians, and the Arabians, and Leonardo da Vinci did very well before he was born; the discovery of America by Columbus and the circumnavigation by Magellan can hardly be attributed to him, yet they were the consequences of a truly philosophical reasoning. But the investigation of nature is an affair of genius, not of rules. No man can invent an organon for writing tragedies and Epic poems. Bacon's system is, in it own terms, an idol of the theatre. It would scarcely guide a man to a solution of the riddle of Ælia Lælia Crispis, or to that of the charade of Sir Hilary.
His scientific errors. [260] Few scientific pretenders have made more mistakes than Lord Bacon. He rejected the Copernican system, and spoke insolently of its great author; he undertook to criticise adversely Gilbert's treatise "De Magnete;" he was occupied in the condemnation of any investigation of final causes, while Harvey was deducing the circulation of the blood from Aquapendente's discovery of the valves in the veins; he was doubtful whether instruments were of any advantage, while Galileo was investigating the heavens with the telescope. Ignorant himself of every branch of mathematics, he presumed that they were useless in science, but a few years before Newton achieved by their aid his immortal discoveries. It is time that the sacred name of philosophy should be severed from its long connexion with that of one who was a pretender in science, a time-serving politician, an insidious lawyer, a corrupt judge, a treacherous friend, a bad man.
Adoption of the Copernican doctrine.But others were not so obtuse as Bacon. Gilbert, one of the best of the early English experimentalists, an excellent writer on magnetism, adopted the views of Copernicus. Milton, in "Paradise Lost," set forth in language such as he only could use the objections to the Ptolemaic, and the probabilities of the Copernican system. Some of the more liberal ecclesiastics gave their adhesion. Bishop Wilkins not only presented it in a very popular way, but also made some sensible suggestions explanatory of the supposed contradictions of the new theory to the Holy Scriptures. It was, however, among geometricians, as Napier, Briggs, Horrox, that it met with its best support. On the continent the doctrine was daily making converts, and nightly gathering strength from the accordance of the tables of the motions of the heavenly bodies calculated upon its principles with actual observation.
Invention of the telescope.It is by no means uninteresting to notice the different classes of men among whom this great theory was steadily winning its way. Experimental philosophers, Republican poets, Episcopal clergymen, Scotch lords, West of England schoolmasters, Italian physicists, Polish pedants, painstaking Germans, each from his own special point of view, was gradually receiving the light, and doubtless, [261] from such varied influence, the doctrine would have vindicated its supremacy at last, though it might have taken a long time. On a sudden, however, there occurred a fortunate event, which led forthwith to that result by a new train of evidence, bringing the matter, under the most brilliant circumstances, clearly to the apprehension of every one. This great and fortunate event was the invention of the telescope.
Galileo constructs one.It is needless to enter on any examination of the authorship of this invention. It is enough for our purpose to know that Lippershey, a Dutchman, had made one toward the close of 1608, and that Galileo, hearing of the circumstance, but without knowing the particulars of the construction, in April or May of the following year invented a form of it for himself. Not content with admiring how close and large it made terrestrial objects, he employed it for examining the heavens. Telescopic astronomical discoveries. On turning it to the moon, he found that she has mountains casting shadows, and valleys like those of the earth. The discovery of innumerable fixed stars—not fewer than forty were counted by him in the well-known group of the Pleiades—up to that time unseen by man, was felt at once to offer an insuperable argument against the opinion that these bodies were created only to illuminate the night; indeed, it may be said that this was a death-blow to the time-honoured doctrine of the human destiny of the universe. Already Galileo began to encounter vulgar indignation, which accused him loudly of impiety. On January 7th, 1610, he discovered three of Jupiter's satellites, and a few days later the fourth. To these he gave the designation of the Medicean stars, and in his "Sidereal Messenger" published an account of the facts he had thus far observed. As it was perceived at once that this planet offered a miniature representation of the ideas of Copernicus respecting the solar system, this discovery was received by the astronomical party with the liveliest pleasure, by the ecclesiastical with the most bitter opposition, some declaring that it was a mere optical deception, some a purposed fraud, some that it was sheer blasphemy, and some, fairly carrying out to its consequences the absurd philosophy of the day, asserted that, since [262] the pretended satellites were invisible to the naked eye, they must be useless, and, being useless, they could not exist. Continuing his observations, Galileo found that Saturn differs in an extraordinary manner from other planets; but the telescope he used not being sufficient to demonstrate the ring, he fell into the mistake that the body of the planet is triple. This was soon followed by the discovery of the phases of Venus, which indisputably established for her a motion round the sun, and actually converted what had hitherto, on all hands, been regarded as one of the weightiest objections against the Copernican theory, into a most solid support. "If the doctrine of Copernicus be true, the planet Venus ought to show phases like the moon, which is not the case;" so said the objectors. Copernicus himself saw the difficulty, and tried to remove it by suggesting that the planet might be transparent. The telescope of Galileo for ever settled the question by showing that the expected phases do actually exist.
Commencing opposition to Galileo.In the garden of Cardinal Bandini at Rome, A.D. 1611, Galileo publicly exhibited the spots upon the sun. He had observed them the preceding year. Goaded on by the opposition his astronomical discoveries were bringing upon him, he addressed a letter in 1613 to the Abbe Castelli, for the purpose of showing that the Scriptures were not intended as a scientific authority. This was repeating Bruno's offence. Hereupon the Dominicans, taking alarm, commenced to attack him from their pulpits. It shows how reluctantly, and with what misgivings the higher ecclesiastics entered upon the quarrel, that Maraffi, the general of the Dominicans, apologized to Galileo for what had taken place. The astronomer now published another letter reiterating his former opinions, asserting that the Scriptures were only intended for our salvation, and otherwise defending himself, and recalling the fact that Copernicus had dedicated his book to Pope Paul III.
He is summoned to Rome. Through the suggestion of the Dominicans, Galileo was now summoned to Rome to account for his conduct and opinions before the Inquisition. He was accused of having taught that the earth moves; that the sun is stationary; and of having attempted [263] to reconcile these doctrines with the Scriptures. The sentence was that he must renounce these heretical opinions, and pledge himself that he would neither publish nor defend them for the future. Is condemned by the Inquisition, In the event of his refusal he was to be imprisoned. With the fate of Bruno in his recollection, he assented to the required recantation, and gave the promise demanded. The Inquisition then proceeded to deal with the Copernican system, condemning it as heretical; the letters of Galileo, which had given rise to the trouble, were prohibited; also Kepler's epitome of the Copernican theory, and also the work of Copernicus. which condemns the Copernican system. In their decree prohibiting this work "De Revolutionibus," the Congregation of the Index, March 5, 1616, denounced the new system of the universe as "that false Pythagorean doctrine utterly contrary to the Holy Scriptures."
Again it appears how reluctant the Roman authorities were to interfere, and how they were impelled rather by the necessity of their position than by their personal belief in the course they had been obliged to take. The personal sentiments of the Popes. After all that had passed, the Pope, Paul V., admitted Galileo to an audience, at which he professed to him personally the kindest sentiments, and assured him of safety. When Urban VIII. succeeded to the pontifical chair, Galileo received the distinction of not less than six audiences; the Pope conferred on him several presents, and added the promise of a pension for his son. In a letter to the Duke of Florence his Holiness used the most liberal language, stated how dear to him Galileo was, that he had very lovingly embraced him, and requested the duke to show him every favour.
Galileo publishes "The System of the World."Whether it was that, under these auspicious circumstances, Galileo believed he could with impunity break through the engagement he had made, or whether an instinctive hatred of that intellectual despotism and hypocrisy which was weighing upon Europe became irrepressible in his breast, in 1632 he ventured on the publication of his work, entitled "The System of the World," its object being to establish the truth of the Copernican doctrine. It is [264] composed in the dialogue form, three speakers being introduced, two of them true philosophers, the third an objector. Whatever may have been the personal opinion of the Pope, there can be no doubt that his duty rendered it necessary for him to act. Galileo was therefore again summoned before the Inquisition, the Tuscan ambassador expostulating against the inhumanity of thus dealing with an old man in ill health. But no such considerations were listened to, and Galileo was compelled to appear at Rome, February, 1633, and surrender himself to the Holy Office. The Pope's nephew did all in his power to meet the necessity of the Church and yet to spare the dignity of science. He paid every attention to the personal comfort of the accused. When the time came for Galileo to be put into solitary confinement, he endeavoured to render the imprisonment as light as possible; but, finding it to prey upon the spirits of the aged philosopher, he, on his own responsibility, liberated him, permitting him to reside in the house of the Tuscan ambassador. Is again condemned by the Inquisition. The trial being completed, Galileo was directed to appear, on June 22nd, to hear his sentence. Clothed in the penitential garment, he received judgment. His heretical offences were specified, the pledges he had violated recited; he was declared to have brought upon himself strong suspicions of heresy, and to be liable to the penalties thereof; but from these he might be absolved if, with a sincere heart, he would abjure and curse his heresies. However, that his offences might not altogether go unpunished, and that he might be a warning to others, he was condemned to imprisonment during the pleasure of the Inquisition, his dialogues were prohibited by public edict, and for three years he was directed to recite, once a week, the seven penitential psalms.
His degradation and punishment.In his garment of disgrace the aged philosopher was now made to fall upon his knees before the assembled cardinals, and, with his hand on the Gospels, to make the required abjuration of the heliocentric doctrine, and to give the pledges demanded. He was then committed to the prison of the Inquisition; the persons who had been concerned in the printing of his book were punished; and the sentence and abjuration [265] were formally promulgated, and ordered to be publicly read in the universities. In Florence, the adherents of Galileo were ordered to attend in the Church of Santa Croce to witness his disgrace. After a short imprisonment in the jail of the Inquisition, he was ordered to Arcetri, and confined in his own house. Here severe misfortunes awaited him; his favourite daughter died; he fell into a state of melancholy; an application that he might go to Florence for the sake of medical advice was refused. It became evident that there was an intention to treat him with inexorable severity. After five years of confinement, permission was reluctantly accorded to him to remove to Florence for his health; but still he was forbidden to leave his house, or receive his friends, or even to attend mass during Passion Week without a special order. The Grand-duke tried to abate this excessive severity, directing his ambassador at the court of Rome to plead the venerable age and ill health of the immortal convict, and that it was desirable to permit him to communicate certain scientific discoveries he had made to some other person, such as Father Castelli. Not even that was accorded unless the interview took place in the presence of an official of the Inquisition. Soon after Galileo was remanded to Arcetri. He spent the weary hours in composing his work on Local Motion, his friends causing it to be surreptitiously published in Holland. The calamities of his old age. His infirmities and misfortunes now increased. In 1637 he became totally blind. In a letter he plaintively says, referring to this calamity, "So it pleases God, it shall therefore please me also." The exquisite refinement of ecclesiastical vengeance pursued him remorselessly, and now gave him permission to see his friends when sight was no longer possible. It was at this period that an illustrious stranger, the author of "Paradise Lost," visited him. Shortly after he became totally deaf; but to the last he occupied himself with investigations respecting the force of percussion. His death; is refused burial. He died, January, 1642, in the seventy-eighth year of his age, the prisoner of the Inquisition. True to its instincts, that infernal institution followed him beyond the grave, disputing his right to make a will, and denying him burial in [266] consecrated ground. The pope also prohibited his friends from raising to him a monument in the church of Santa Croce, in Florence. It was reserved for the nineteenth century to erect a suitable memorial in his honour.
Steady advance of the Copernican system.The result of the discoveries of Copernicus and Galileo was thus to bring the earth to her real position of subordination and to give sublimer views of the universe. Mœstlin expresses correctly the state of the case when he says, "What is the earth and the ambient air with respect to the immensity of space? It is a point, a punctule, or something, if there be any thing, less." It had been brought down to the condition of one of the members of a family—the solar system. And since it could be no longer regarded as holding all other bodies in submissive attendance upon it, dominating over their movements, there was reason to suppose that it would be found to maintain interconnexions with them in the attitude of an equal or subordinate; in other words, that general relations would be discovered expressive of the manner in which all the planetary members of the solar system sustain their movements round the sun.
Kepler, his mode of inquiry.Among those whose minds were thoroughly occupied with this idea, Kepler stands pre-eminently conspicuous. It is not at all surprising, considering the tone of thought of those times, that he regarded his subject with a certain mysticism. They who condemn his manner of thus viewing things do not duly appreciate the mental condition of the generation in which he lived. Whatever may be said on that point, no one can deny him a marvellous patience, and almost superhuman painstaking disposition. Guess after guess, hypothesis after hypothesis, he submitted to computations of infinite labour, and doubtless he speaks the melancholy truth when he says, "I considered and reflected till I was almost mad." Yet, in the midst of repeated disappointment, he held, with a truly philosophical determination, firmly to the belief that there must be some physical interconnexion among the parts of the solar system, and that it would certainly be displayed by the discovery of laws presiding over the distances, times, and velocities [267] of the planets. In these speculations he was immersed before the publications of Galileo. In his "Mysterium Cosmographicum" he says, "In the year 1595 I was brooding with the whole energy of my mind on the subject of the Copernican system."
Discovery of Kepler's laws.In 1609 he published his work entitled "On the Motion of Mars." This was the result of an attempt, upon which he had been engaged since the beginning of the century, to reconcile the motions of that planet to the hypothesis of eccentrics and epicycles. It ended in the abandonment of that hypothesis, and in the discovery of the two great laws now known as the first and second laws of Kepler. They are respectively that the orbits of the planets are elliptical, and that the areas described by a line drawn from the planet to the sun are proportional to the times.
In 1617 he was again rewarded by the discovery which passes under the designation of Kepler's third law: it expresses the relation of the mean distances of the planets from the sun with the times of their revolutions—"the squares of the periodic times are in the same proportion as the cubes of the distances." In his "Epitome of the Copernican Astronomy," published 1622, he showed that this law likewise holds good for the satellites of Jupiter as regards their primary.
His remonstrance with the Church.Humboldt, referring to the movement of Jupiter's satellites, remarks: "It was this which led Kepler, in his 'Harmonices Mundi,' to state, with the firm confidence and security of a German spirit of philosophical independence, to those whose opinions bore sway beyond the Alps, 'Eighty years have elapsed during which the doctrines of Copernicus regarding the movement of the earth and the immobility of the sun have been promulgated without hindrance, because it was deemed allowable to dispute concerning natural things and to elucidate the works of God, and, now that new testimony is discovered in proof of the truth of those doctrines—testimony which was not known to the spiritual judges, ye would prohibit the promulgation of the true system of the structure of the universe.'"
Rectification of the Copernican theory.Thus we see that the heliocentric theory, as proposed [268] by Copernicus, was undergoing rectification. The circular movements admitted into it, and which had burdened it with infinite perplexity, though they had hitherto been recommended by an illusive simplicity, were demonstrated to be incorrect. They were replaced by the real ones, the elliptical. Kepler, as was his custom, ingenuously related his trials and disappointments. Alluding on one occasion to this, he says: "My first error was that the path of a planet is a perfect circle—an opinion which was a more mischievous thief of my time, in proportion as it was supported by the authority of all philosophers, and apparently agreeable to metaphysics."
The philosophical import of these laws.The philosophical significance of Kepler's discoveries was not recognized by the ecclesiastical party at first. It is chiefly this, that they constitute a most important step to the establishment of the doctrine of the government of the world by law. But it was impossible to receive these laws without seeking for their cause. The result to which that search eventually conducted not only explained their origin, but also showed that, as laws, they must, in the necessity of nature, exist. It may be truly said that the mathematical exposition of their origin constitutes the most splendid monument of the intellectual power of man.
Necessity for mechanical science.Before the heliocentric theory could be developed and made to furnish a clear exposition of the solar system, which is obviously the first step to just views of the universe, it was necessary that the science of mechanics should be greatly improved—indeed, it might be said, created; for during those dreary ages following the establishment of Byzantine power, nothing had been done toward the acquisition of correct views either in statics or dynamics. It was impossible that Europe, in her lower states of life, could produce men capable of commencing where Archimedes had left off. She had to wait for the approach of her Age of Reason for that.
Leonardo da Vinci.The man of capacity at last came. Leonardo da Vinci was born A.D. 1452. The historian Hallam, enumerating some of his works, observes, "His knowledge was almost preternatural." Many of his [269] writings still remain unpublished. Long before Bacon, he laid down the maxim that experience and observation must be the foundation of all reasoning in science; that experiment is the only interpreter of nature, and is essential to the ascertainment of laws. Unlike Bacon, who was ignorant of mathematics, and even disparaged them, he points out their supreme advantage. Seven years after the voyage of Columbus, this great man—great at once as an artist, mathematician, and engineer—gave a clear exposition of the theory of forces obliquely applied on a lever; a few years later he was well acquainted with the earth's annual motion. He knew the laws of friction, subsequently demonstrated by Amontons, and the principle of virtual velocities; he described the camera obscura before Baptista Porta, understood aerial perspective, the nature of coloured shadows, the use of the iris, and the effects of the duration of visible impressions on the eye. He wrote well on fortification, anticipated Castelli on hydraulics, occupied himself with the fall of bodies on the hypothesis of the earth's rotation, treated of the times of descent along inclined planes and circular arcs, and of the nature of machines. He considered, with singular clearness, respiration and combustion, and foreshadowed one of the great hypotheses of geology, the elevation of continents.
Stevinus continues the movement in Natural Philosophy.This was the commencement of the movement in Natural Philosophy; it was followed up by the publication of a work on the principles of equilibrium by Stevinus, 1586. In this the author established the fundamental property of the inclined plane, and solved, in a general manner, the cases of forces acting obliquely. Six years later Galileo's treatise on mechanics appeared, a fitting commencement of that career which, even had it not been adorned with such brilliant astronomical discoveries, would alone have conferred the most illustrious distinction upon him.
Discovery of the laws of motion.The dynamical branch of Mechanics is that which is under most obligation to Galileo. To him is due the establishment of the three laws of motion. They are to the following effect, as given by Newton:
(1.) Every body perseveres in its state of rest or [270] of uniform motion in a right line unless it is compelled to change that state by forces impressed thereon.
(2.) The alteration of motion is ever proportional to the motive force impressed, and is made in the direction of the right line in which that force is impressed.
(3.) To every action there is always opposed an equal reaction, or the mutual actions of two bodies upon each other are always equal, and directed to contrary parts.
Up to this time it was the general idea that motion can only be maintained by a perpetual application, impression, or expenditure of force. Galileo himself for many years entertained that error, but in 1638 he plainly states in his "Dialogues on Mechanics" the true law of the uniformity and perpetuity of motion. Such a view necessarily implies a correct and clear appreciation of the nature of resistances. No experimental motion that man can establish is unrestrained. But a perception of the uniformity and perpetuity of motion lies at the very basis of physical astronomy. With difficulty the true idea was attained. The same may be said as respects rectilinear direction, for many supposed that uniform motion can only take place in a circle.
Establishment of the first law of motion,The establishment of the first law of motion was essential to the discovery of the laws of falling bodies, in which the descent is made under the influence of a continually acting force, the velocity increasing in consequence thereof. Galileo saw clearly that, whether a body is moving slowly or swiftly, it will be equally affected by gravity. This principle was with difficulty admitted by some, who were disposed to believe that a swiftly moving body would not be as much affected by a constant force like gravity as one the motion of which is slower. With difficulty, also, was the old Aristotelian error eradicated that a heavy body falls more swiftly than a light one.
and of the second,The second law of motion was also established and illustrated by Galileo. In his "Dialogues" he shows that a body projected horizontally must have, from what has been said, a uniform horizontal motion, but that it will also have compounded therewith an accelerated motion downward. Here again we perceive it is[271] necessary to retain a steady conception of this intermingling of forces without deterioration, and, though it may seem simple enough to us, there were some eminent men of those times who did not receive it as true. The special case offered by Galileo is theoretically connected with the paths of military projectiles, though in practice, since they move in a resisting medium, the air, their path is essentially different from the parabola. Curvilinear motions, which necessarily arise from the constant action of a central force, making a body depart from the rectilinear path it must otherwise take, are chiefly of interest, as we shall presently find, in the movements of the celestial bodies.
and of the third.A thorough exposition of the third law of motion was left by Galileo to his successors, who had directed their attention especially to the determination of the laws of impact. Indeed, the whole subject was illustrated and the truth of the three laws verified in many different cases by an examination of the phenomena of freely falling bodies, pendulums, projectiles, and the like. Among those who occupied themselves with such labours may be mentioned Torricelli, Castelli, Viviani, Borelli, Gassendi. Through the investigations of these, and other Italian, French, and English natural philosophers, the principles of Mechanics were solidly established, and a necessary preparation made for their application in astronomy. By this time every one had become ready to admit that the motion of the planetary bodies would find an explanation on these principles.
Application of Mechanics to the celestial motions.The steps thus far taken for an explanation of the movements of the planets in curvilinear paths therefore consisted in the removal of the old misconception that for a body to continue its motion forward in a straight line a continued application of force is necessary, the first law of motion disposing of that error. In the next place, it was necessary that clear and distinct ideas should be held of the combination or composition of forces, each continuing to exercise its influence without deterioration or diminution by the other. The time had now come for it to be shown that the perpetual movement of the planets is a consequence of the first law [272] of motion; their elliptic paths, such as had been determined by Kepler, a consequence of the second. Several persons almost simultaneously had been brought nearly to this conclusion without being able to solve the problem completely. Thus Borelli, A.D. 1666, in treating of the motions of Jupiter's satellites, distinctly shows how a circular motion may arise under the influence of a central force; he even uses the illustration so frequently introduced of a stone whirled round in a sling. In the same year a paper was presented to the Royal Society by Mr. Hooke, "explicating the inflection of a direct motion into a circular by a supervening attractive principle." Huygens also, in his "Horologium Oscillatorium," had published some theorems on circular motions, but no one as yet had been able to show how elliptical orbits could, upon these principles, be accounted for, though very many had become satisfied that the solution of this problem would before long be given.
Newton; publication of the "Principia."In April, 1686, the "Principia" of Newton was presented to the Royal Society. This immortal work not only laid the foundation of Physical Astronomy, it also carried the structure thereof very far toward its completion. It unfolded the mechanical theory of universal gravitation upon the principle that all bodies tend to approach each other with forces directly as their masses, and inversely as the squares of their distances.
Propounds the theory of universal gravitation.To the force producing this tendency of bodies to approach each other the designation of attraction of gravitation, or gravity, is given. All heavy bodies fall to the earth in such a way that the direction of their movement is toward its centre. Newton proved that this is the direction in which they must necessarily move under the influence of an attraction of every one of the particles of which the earth is composed, the attraction of a sphere taking effect as if all its particles were concentrated in its centre.
Preparation for Newton.Galileo had already examined the manner in which gravity acts upon bodies as an accelerating force, and had determined the connexion between the spaces of descent and the times. He illustrated such facts experimentally by the use of inclined planes, by the aid [273] of which the velocity may be conveniently diminished without otherwise changing the nature of the result. He had also demonstrated that the earth's attraction acts equally on all bodies. This he proved by inclosing various substances in hollow spheres, and showing that, when they were suspended by strings of equal length and made to vibrate, the time of oscillation was the same for all. On the invention of the air-pump, a more popular demonstration of the same fact was given by the experiment proving that a gold coin and a feather fall equally swiftly in an exhausted receiver. Galileo had also proved, by experiments on the leaning tower of Pisa, that the velocity of falling bodies is independent of their weight. It was for these experiments that he was expelled from that city.
Extension of attraction or gravity.Up to the time of Newton there were only very vague ideas that the earth's attraction extended to any considerable distance. Newton was led to his discovery by reflecting that at all altitudes accessible to man, gravity appears to be undiminished, and that, therefore, it may possibly extend as far as the moon, and actually be the force which deflects her from a rectilinear path, and makes her revolve in an orbit round the earth. Admitting the truth of the law of the inverse squares, it is easy to compute whether the moon falls from the tangent she would describe if the earth ceased to act upon her by a quantity proportional to that observed in the case of bodies falling near the surface. In the first calculations made by Newton, he found that the moon is deflected from the tangent thirteen feet every minute; but, if the hypothesis of gravitation were true, her deflection should be fifteen feet. It is no trifling evidence of the scrupulous science of this great philosopher that hereupon he put aside the subject for several years, without, however, abandoning it. At length, in 1682, learning the result of the measures of a degree which Picard had executed in France, and which affected the estimate of the magnitude of the earth he had used, and therefore the distance of the moon, he repeated the calculations with these improved data. It is related that "he went home, took out his old papers, and resumed his calculations. As they drew to a close, he became so much agitated that he was obliged [274] to desire a friend to finish them." The expected coincidence was verified. And thus it appeared that the moon is retained in her orbit and made to revolve round the earth by the force of terrestrial gravity.
The cause of Kepler's laws.These calculations were founded upon the hypothesis that the moon moves in a circular orbit with a uniform velocity. But in the "Principia" it was demonstrated that when a body moves under the influence of an attractive force, varying as the inverse square of the distances, it must describe a conic section, with a focus at the centre of force, and under the circumstances designated by Kepler's laws. Newton, therefore, did far more than furnish the expected solution of the problem of elliptical motion, and it was now apparent that the existence of those laws might have been foreseen, since they arise in the very necessities of the case.
Resistless spread of the heliocentric theory.This point gained, it is obvious that the evidence was becoming unquestionable, that as the moon is made to revolve round the earth through the influence of an attractive force exercised by the earth, so likewise each of the planets is compelled to move in an elliptical orbit round the sun by his attractive force. The heliocentric theory, at this stage, was presenting physical evidence of its truth. It was also becoming plain that the force we call gravitation must be imputed to the sun, and to all the planetary bodies as well as to the earth. Accordingly, this was what Newton asserted in respect to all material substance.
Perturbations accounted for.But it is a necessary consequence of this theory that many apparent irregularities and perturbations of the bodies of the solar system must take place by reason of the attraction of each upon all the others. If there were but one planet revolving round the sun, its orbit might be a mathematically perfect ellipse; but the moment a second is introduced, perturbation takes place in a variable manner as the bodies change their positions or distances. An excessive complication must therefore be the consequence when the number of bodies is great. Indeed, so insurmountable would these difficulties be, that the mathematical solution of the general problem of the solar system would be hopeless were it not for the fact that [275] the planetary bodies are at very great distances from one another, and their masses, compared with the mass of the sun, very small.
Results of the theory of gravitation.Taking the theory of gravitation in its universal acceptation, Newton, in a manner that looks as if he were divinely inspired, succeeded in demonstrating the chief inequalities of the moon and planetary bodies; in determining the figure of the earth—that it is not a perfect sphere, but an oblate spheroid; in explaining the precession of the equinoxes and the tides of the ocean. To such perfection have succeeding mathematicians brought his theory, that the most complicated movements and irregularities of the solar system have been satisfactorily accounted for and reduced to computation. Trusting to these principles, not only has it been found possible, knowing the mass of a given planet, to determine the perturbations it may produce in adjacent ones, but even the inverse problem has been successfully attacked, and from the perturbations the place and mass of a hitherto unknown planet determined. It was thus that, from the deviations of Uranus from his theoretical place, the necessary existence of an exterior disturbing planet was foreseen, and our times have witnessed the intellectual triumph of mathematicians directing where the telescope should point in order to find a new planet. The discovery of Neptune was thus accomplished.
It adds to our admiration of the wonderful intellectual powers of Newton to know that the mathematical instrument he used was the ancient geometry. Not until subsequently was the analytical method resorted to and cultivated. This method possesses the inappreciable advantage of relieving us from the mental strain which would otherwise oppress us. It has been truly said that the symbols think for us. The "Principia;" its incomparable merit. Mr. Whewell observes: "No one for sixty years after the publication of the 'Principia,' and, with Newton's methods, no one up to the present day, has added any thing of value to his deductions. We know that he calculated all the principal lunar inequalities; in many of the cases he has given us his processes, in others only his results. But who has presented in his beautiful geometry or [276] deduced from his simple principles any of the inequalities which he left untouched? The ponderous instrument of synthesis, so effective in his hands, has never since been grasped by any one who could use it for such purposes; and we gaze at it with admiring curiosity, as on some gigantic implement of war which stands idle among the memorials of ancient days, and makes us wonder what manner of man he was who could wield as a weapon what we can hardly lift as a burden."
Philosophical import of Newton's discoveries.Such was the physical meaning of Newton's discoveries; their philosophical meaning was of even greater importance. The paramount truth was resistlessly coming into prominence—that the government of the solar system is under necessity, and that it is mathematically impossible for the laws presiding over it to be other than they are.
Thus it appears that the law of gravitation holds good throughout our solar system. But the heliocentric theory, in its most general acceptation, considers every fixed star as being, like the sun, a planetary centre. Unity of idea in the construction of the universe. Hence, before it can be asserted that the theory of gravitation is truly universal, it must be shown that it holds good in the case of all other such systems. The evidence offered in proof of this is altogether based upon the observations of the two Herschels on the motions of the double stars. Among the stars there are some in such close proximity to each other that Sir W. Herschel was led to suppose it would be possible, from observations upon them, to ascertain the stellar parallax. While engaged in these inquiries, which occupied him for many years, he discovered that many of these stars are not merely optically in proximity, as being accidentally in the same line of view, but are actually connected physically, revolving round each other in regular orbits. The motion of these double suns is, however, in many instances so slow as to require many years for a satisfactory determination. Gravitation of double stars. Sir J. Herschel therefore continued the observations of his father, and with other mathematicians, investigated the characteristics of these motions. The first instance in which the true elliptic elements of the orbit of a binary star were [277] determined was given by M. Savary in the case of chi Ursæ Majoris, indicating an elliptic orbit of 58 ¼ years. But the period of others, since determined, is very much longer; thus, in sigma Coronæ, it is, according to Mr. Hind, more than 736 years. From the fact that the orbits in which these stars move round each other are elliptical, it necessarily follows that the law of gravitation, according to the inverse square, holds good in them. Considering the prodigious distances of these bodies, and the departure, as regards structure of the systems to which they belong, from the conditions obtaining in our unisolar system, we may perhaps assert the prevalence of the law of gravitation throughout the universe.
Coloured light of double stars.If, in association with these double suns—sometimes, indeed, they are triple, and occasionally, as in the case of epsilon Lyræ, quadruple—there are opaque planetary globes, such solar systems differ from ours not only in having several suns instead of a single one, but, since the light emitted is often of different tints, one star shining with a crimson and another with a blue light, the colours not always complementary to one another, a wonderful variety of phenomena must be the result, especially in their organic creations; for organic forms, both vegetable and animal, primarily depend on the relations of coloured light. How varied the effects where there are double, triple, or even quadruple sunrises, and sunsets, and noons; and the hours marked off by red, or purple, or blue tints.
Grandeur of Newton's discoveries.It is impossible to look back on the history of the theory of gravitation without sentiments of admiration and, indeed, of pride. How felicitous has been the manner in which have been explained the inequalities of a satellite like the moon under the disturbing influence of the sun; the correspondence between the calculated and observed quantities of these inequalities; the extension of the doctrine to satellites of other planets, as those of Jupiter; the determination of the earth's figure; the causes of the tides; the different force of gravity in different latitudes, and a multitude of other phenomena. The theory asserted for itself that authority which belongs to intrinsic truth. It [278] enabled mathematicians to point out facts not yet observed, and to foretell future events.
And yet how hard it is for truth to force its way when bigotry resists. In 1771, the University of Salamanca, being urged to teach physical science, refused, and this was its answer; "Newton teaches nothing that would make a good logician or metaphysician; and Gassendi and Descartes do not agree so well with revealed truth as Aristotle does."
The earth in time.Among the interesting results of Newton's theory may be mentioned its application to secular inequalities, such as the acceleration of the moon's mean motion, that satellite moving somewhat quicker now than she did ages ago. Laplace detected the cause of this phenomenon in the influence of the sun upon the moon, combined with the secular variation of the eccentricity of the earth's orbit. Moreover, he showed that this secular inequality of the motion of the moon is periodical, that it requires millions of years to re-establish itself, and that, after an almost inconceivable time, the acceleration becomes a retardation. In like manner, the same mathematician explained the observed acceleration in the mean motion of Jupiter, and retardation of that of Saturn, as arising from the mutual attraction of the two planets, and showed that this secular inequality has a period of 929 ½ years. With such slow movements may be mentioned the diminution of the obliquity of the ecliptic, which has been proceeding for ages, but which will reach a limit and then commence to increase. These secular motions ought not to be without interest to those who suffer themselves to adopt the patristic chronology of the world, who suppose that the earth is only six thousand years old, and that it will come to an end in about one thousand years more. They must accept, along with that preposterous delusion, its necessary consequences, that the universe has been so badly constructed, and is such a rickety machine, that it can not hold together long enough for some of its wheels to begin to revolve. Astronomy offers us many illustrations of the scale upon which the world is constructed as to time, as well as that upon which it is constructed as to space.
Dominion of law in the universe.From what has been said, the conclusion forces [279] itself upon us that the general laws obtaining as respects the earth, hold good likewise for all other parts of the universe; a conclusion sustained not only by the mechanism of such motions as we have been considering, but also by all evidence of a physical kind accessible to us. The circumstances under which our sun emits light and heat, and thereby vivifies his attendant planets, are indisputably the same as those obtaining in the case of every fixed star, each of which is a self-luminous sun. There is thus an aspect of homogeneousness in the structure of all systems in the universe, which, though some have spoken of it as if it were the indication of a uniformity of plan, and therefore the evidence of a primordial idea, is rather to be looked upon as the proof of unchangeable and resistless law.
Ruin of anthropocentric ideas.What, therefore, now becomes of the doctrine authoritatively put forth, and made to hold its sway for so many centuries, that the earth is not only the central-body of the universe, but in reality, the most noble body in it; that the sun and other stars are mere ministers or attendants for human use? In the place of these utterly erroneous and unworthy views, far different conceptions must be substituted. Man, when he looks upon the countless multitude of stars—when he reflects that all he sees is only a little portion of those which exist, yet that each is a light and life-giving sun to multitudes of opaque, and therefore, invisible worlds—when he considers the enormous size of these various bodies and their immeasurable distance from one another, may form an estimate of the scale on which the world is constructed, and learn therefrom his own unspeakable insignificance.
Aids for measurements in the universe.In one beat of a pendulum a ray of light would pass eight times round the circumference of the earth. Thus we may take the sunbeam as a carpenter does his measuring-rule; it serves as a gauge in our measurements of the universe. A sunbeam would require more than three years to reach us from alpha Centauri; nine and a quarter years from 61 Cygni; from alpha Lyræ twelve years. These are stars whose parallax has been determined, and which are therefore nearest to us.
Clusters of stars. [280] Of suns visible to the naked eye there are about 8000, but the telescope can discern in the Milky Way more than eighteen millions, the number visible increasing as more powerful instruments are used. Our cluster of stars is a disc divided into two branches at about one-third of its length. In the midst of innumerable compeers and superiors, the sun is not far from the place of bifurcation, and at about the middle of the thickness. Outside the plane of the Milky Way the appearance would be like a ring, and, still farther off, a nebulous disc.
Distribution of matter and force in space.From the contemplation of isolated suns and congregated clusters we are led to the stupendous problem of the distribution of matter and force in space, and to the interpretation of those apparent phantoms of self-luminous vapour, circular and elliptic discs, spiral wreaths, rings and fans, whose edges fade doubtfully away, twins and triplets of phosphorescent haze connected together by threads of light and grotesque forms of indescribable complexity. Perhaps in some of these gleaming apparitions we see the genesis, in some the melting away of universes. There is nothing motionless in the sky. In every direction vast transformations are occurring, yet all things proclaim the eternity of matter and the undiminished perpetuity of force.
Limit of the theory of gravitation.The theory of gravitation, as delivered by Newton, thus leads us to a knowledge of the mathematical construction of the solar system, and inferentially likewise to that of other systems; but it leaves without explanation a large number of singular facts. It explains the existing conditions of equilibrium of the heavenly bodies, but it tells us nothing of their genesis; or, at the best, in that particular it falls back on the simple fiat of God.
Phenomena of the solar system.The facts here referred to conduct us, however, to another and far higher point of view. Some of them, as enumerated by Laplace, are the following:—1. All the planets and their satellites move in ellipses of such small eccentricity that they are nearly circles; 2. The movements of the planets are in the same direction and nearly in the same plane; 3. The movements of the satellites are in the same direction as those of [281] the planets; 4. The movements of rotation of these various bodies and of the sun are in the same direction as their orbitual motions, and in planes little different.
The nebular hypothesis.The nebular hypothesis requires us to admit that all the ponderable material now constituting the various bodies of the solar system once extended in a rarefied or nebulous and rotating condition, beyond the confines of the most distant planet. That postulate granted; the structure and present condition of the system may be mathematically deduced.
For, as the vast rotating spheroid lost its heat by radiation, it contracted, and its velocity of rotation was necessarily increased; and thus were left behind from its equatorial zone, by reason of the centrifugal force, rotating rings, the same result occurring periodically again and again. These rings must lie all in one plane. They might break, collapsing into one rotating spheroid, a planet; or into many, asteroids; or maintain the ring-like form. From the larger of these secondary rotating spheroids other rings might be thrown off, as from the parent mass; these, in their turn breaking and becoming spheroids, constitute satellites, whose movements correspond to those of their primaries.
We might, indeed, advance a step farther, and show how, by the radiation of heat from a motionless nebula, a movement of rotation in a determinate direction could be engendered, and that upon these principles, the existence of a nebulous matter admitted, and the present laws and forces of nature regarded as having been unchanged, the manner of origin of the solar system might be deduced, and all those singular facts previously alluded to explained; and not only so, but there is spontaneously suggested the cause of many minor peculiarities not yet mentioned.
Facts accounted for by it.For it follows from the nebular hypothesis that the large planets should rotate rapidly, and the small ones more slowly; that the outer planets and satellites should be larger than the inner ones. Of the satellites of Saturn, the largest is the outermost; of those of Jupiter, the largest is the outermost save one. Of the planets themselves, Jupiter is the largest, and outermost save three. These cannot be coincidences, [282] but must be due to law. The number of satellites of each planet, with the doubtful exception of Venus, might be foreseen, the presence of satellites and their number being determined by the centrifugal force of their primary. The hypothesis also points out the time of revolution of the planets in their orbits, and of the satellites in theirs; it furnishes a reason for the genesis and existence of Saturn's rings, which are indeed its remaining witnesses—their position and movements answering to its requirements. It accounts for the physical state of the sun, and also for the physical state of the earth and moon as indicated by their geology. It is also not without furnishing reasons for the existence of comets as integrant members of our system; for their singular physical state; for the eccentric, almost parabolic orbits of so many of them; for the fact that there are as many of them with a retrograde as with a direct motion; for their more frequent occurrence about the axis of the solar system than in its plane; and for their general antithetical relations to planets.
Whether nebulæ actually exist.If these and very many other apparently disconnected facts follow as the mechanical necessities of the admission of a gravitating nebula—a very simple postulate—it becomes important to ascertain whether, by actual observation, the existence of such material forms may be demonstrated in any part of the universe. It was the actual telescopic observation of such objects that led Herschel to the nebular hypothesis. He concluded that there are two distinct kinds of nebulæ, one consisting of clusters of stars so remote that they could not be discerned individually, but that these may be discerned by sufficient telescopic power; the other being of a hazy nature, and incapable of resolution. Nebulæ do not occur at random in the heavens: the regions poorest in stars are richest in them; they are few in the plane of our sidereal system, but numerous about its poles, in that respect answering to the occurrence of comets in the solar system. The resolution of many of these hazy patches of light into stars by no means disproves the truly nebulous condition of many others.
Fortunately, however, other means than telescopic observation for the settlement of this question are [283] available. In 1846, it was discovered by the author of this book that the spectrum of an ignited solid is continuous, that is, has neither dark nor bright fixed lines. Fraunhofer had previously made known that the spectrum of ignited gases is discontinuous. Here, then, is the means of determining whether the light emitted by a given nebula comes from an incandescent gas, or from a congeries of ignited solids, stars, or suns. If its spectrum be discontinuous, it is a true nebula or gas; if continuous, a congeries of stars.
In 1864, Mr. Huggins made this examination in the case of a nebula in the constellation Draco. It proved to be gaseous.
Subsequent observations have shown that of sixty nebulæ examined, nineteen give discontinuous or gaseous spectra; the remainder continuous ones.
It may, therefore, be admitted that physical evidence has at length been obtained, demonstrating the existence of vast masses of matter in a gaseous condition, and at a temperature of incandescence. The hypothesis of Laplace has thus a firm basis.
Opposition to the nebular hypothesis.Notwithstanding the great authority of the astronomers who introduced it, the nebular hypothesis has encountered much adverse criticism; not so much, however, from its obvious scientific defects, such as its inability to deal with the cases of Uranus and Neptune, as from moral and extraneous considerations. There is a line in Aristophanes which points out precisely the difficulty: