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GALILEO GALILEI (Galileo, by which he is commonly known, being his Christian name), an Italian philosopher and mathematician, born in Pisa, Feb. 15, 1564, died in Arcetri, Jan. 8, 1642. He came of a noble Florentine family, whose original name was Bonajuti, which they exchanged for that of Galilei about the middle of the 14th century. Vincenzo, the father of the philosopher, was a man of learning and the author of a number of treatises on music. He was unable to give his sons a thorough education, but Galileo acquired, amid various discouragements, a fair knowledge of the classics and the common branches of learning, and also of music, drawing, and painting. The last named art he seems to have resolved upon cultivating as a profession, but his father sent him to Pisa to study medicine, where he was matriculated at the university as a scholar in arts, Nov. 5, 1581, and became a pupil of the celebrated botanist Cæsalpinus. He still employed his leisure in his favorite branches of the fine arts, and his love of drawing led him to study geometry. After many fruitless remonstrances his father left him to the natural bent of his genius. His first discovery was about 1583, when he was led to infer the isochronism of the vibration of the pendulum by noticing the regular swinging of a lamp in the cathedral of Pisa. Though it was 50 years before the philosopher applied his discovery to clockwork, he at once perceived its importance, and caused it to be employed by physicians in counting the pulses of their patients. Some time afterward, having read the treatise of Archimedes on floating bodies, he invented a hydrostatic balance, and wrote a description of it, which introduced him to the friendship of Guido Ubaldi, the mechanist and mathematician. A paper on the centre of gravity was indirectly the means of securing for him at the age of 25 a professorship of mathematics in the university of Pisa. The salary was but 60 crowns, and he had to look for his support partly to private pupils. His sarcastic attacks upon the notions of the Aristotelians, although his arguments were fortified with careful experiments, raised up a host of enemies, whose animosity pursued him for the rest of his life. He demonstrated the error of supposing that the velocity of falling bodies is proportional to their weight, by letting fall unequal weights at the same time from the top of the leaning tower of Pisa, explaining that the trifling difference of time noticed in their respective descents was owing solely to the resistance of the air. The death of his father in 1591 imposed upon him the duty of supporting the family. Soon after this the interest of Ubaldi procured him the appointment of professor of mathematics for six years in the university of Padua. This new position, upon which he entered in September, 1592, gave him a salary of 180 florins, and enabled him to remove from a city where the hostility of the Aristotelians embittered his existence. He constructed several useful machines for the state, and composed treatises on gnomonics, astronomy, mechanics, architecture, and even fortification, which he delivered in the form of lectures. In 1597 he made a kind of thermometer in which both air and water were employed. During this period he began a friendly correspondence with Kepler, which continued until the death of the latter; and about the same time appeared a treatise on the sphere after the Ptolemaic system, which has been attributed to Galileo on rather insufficient grounds. It was published from a MS. in the library of Somaschi at Venice (Rome, 1656). Probably between the years 1593 and 1597 Galileo became a convert to the Copernican theory of the revolution of the earth about the sun; but it is impossible to fix the date of this important event in his life, for he says in a letter to Kepler (1597), that in deference to public opinion he did not declare his conviction of the truth of the new doctrines for some years after he had formed it. On the expiration of the term of his professorship the Venetian senate appointed him for six years more, and raised his salary to 320 florins. In 1604, a new star of remarkable brilliancy having appeared in the constellation Serpentarius, he attacked the popular notion that it was a meteor, and proved by the absence of parallax that it was far beyond the limits of our system. His appointment at the university was again renewed, with an addition of 200 florins to his salary. The crowds that came to hear him were now so great that he was sometimes obliged to lecture in the open air. In 1609 a report reached him at Venice that a Dutchman had constructed an instrument which had the property of making distant objects seem near. On his return to Padua the philosopher immediately applied himself to the solution of the mystery, and, after trying several combinations of lenses, succeeded in making an instrument which magnified three times. It consisted merely of a leaden organ pipe, with a plano-convex glass at one end and a plano-concave at the other. This he carried to Venice, where it at once became an object of the intensest public curiosity. He presented it to the senate, who thereupon confirmed him in his professorship for life, and raised his salary to 1,000 florins. Galileo soon constructed another telescope which magnified eight times, and at length a third which had a power of 30. The wonders of the heavens now unfolded to him, which no man had ever seen before, filled him with “incredible delight.” His earliest observations were upon the moon, whose inequalities of surface he was the first to trace. He saw myriads of stars in the milky way, counted 40 in the Pleiades, and at length, on Jan. 13, 1610, after six nights' observation, discovered the revolution of four satellites around the planet Jupiter. He did not publish this intelligence until by repeated examination, up to March 22, he had insured himself against the danger of mistake. The account of his discoveries, which he entitled Sidereus Nuncius, the “Sidereal Messenger,” was received by the astronomers of the old school with insults and incredulity. Some exclaimed against the impiety of scooping out valleys from the fair face of the moon; some attempted to explain away the satellites of Jupiter as mere appearances caused by reflected light. A professor in the university of Padua argued that as there were only seven metals, seven days in the week, and seven apertures in a man's head, so there could be but seven planets; and when forced to admit the visibility of the satellites through the telescope, he reasoned that, being invisible to the naked eye, they were useless, and consequently did not exist. Several persons claimed a prior discovery of the “Jovian planets,” and the astronomer Zach, as late as 1788, claimed for Thomas Harriot the credit of having observed them on Jan. 16, 1610, some time before Galileo's discovery was made known. According to Sir David Brewster, however, Harriot did not see them till Oct. 17. Viviani, in his life of Galileo, tells us that the telescope led him to the construction of the compound microscope, and that he presented one to the king of Poland. The grand duke of Tuscany gave Galileo 1,000 florins, and made him his philosopher and mathematician with a liberal salary and nominal duties. He now removed to Florence. To guard against future attempts to steal his laurels, he published his subsequent discoveries in enigmas, and thus in the course of the same year he announced that Saturn was “triple,” an appearance which Huygens subsequently showed was caused by that planet's rings. Galileo was the first to notice that Venus exhibits phases like those of the moon; and if not the first to descry spots on the sun's disk, he was at least the first to note their peculiarities, and to infer from them the sun's rotation. Some of these observations were made in 1611 at Rome, which he then visited for the first time, where he erected his telescope in the Quirinal garden belonging to Cardinal Bandini. He was received with the highest honors, and became a member of the famous Lincean academy. In 1612 he combated in his work on the laws of floating bodies the common opinion that the tendency of substances to sink or swim in water depends on their shape. With this period in his life the philosopher may be said to have reached the zenith of his prosperity, while at the same time the malice of his enemies began to acquire a dangerous intensity. The Copernican system, which he had long taught in public, afforded a good pretext for attacking him. The sun's revolution round the earth was thought to be a truth of Scripture. Certain Tuscan ecclesiastics began to preach against the wickedness of sending our world spinning through space, and a sarcastic Dominican hurled a sermon at Galileo from the text: Viri Galilæi, quid statis adspicientes in cœlum? “Ye men of Galilee, why stand ye looking up into heaven?” In 1613 Galileo addressed a letter to his pupil Castelli, showing that the language of the Bible should be interpreted according to popular ideas, and that the Ptolemaic system is really as much at variance with it as the Copernican. This was followed by one to Christina, grand duchess dowager of Tuscany, reiterating his views, and supporting them by quotations from the writings of the fathers. A Dominican, Lorini, laid a copy of the Castelli letter before the Roman inquisition in February, 1615, but the inquisitors refused to act in the matter, remarking that by confining himself to the system and its demonstration, and letting alone the Scriptures, Galileo would be secure from molestation. His enemies, however, continued their intrigues, and about the end of 1615 he went to Rome, either to obtain a formal sanction of his opinions, or in obedience to a summons. His case came again before the holy office in February, 1616. He was charged with teaching that the sun is the centre of the planetary system, and interpreting Scripture to suit his own theory. The qualifiers of the inquisition pronounced the obnoxious doctrines “formally heretical, because expressly contrary to Holy Scriptures.” Galileo's letters to Castelli and the grand duchess, Copernicus's work on the revolution of the heavenly bodies, and Kepler's epitome of the Copernican theory, were placed on the Index Expurgatorius, whence they were not removed until the time of Benedict XIV.; and Galileo himself was forbidden ever again to teach the motion of the earth and the stability of the sun. Thenceforward he was not permitted to express himself as though Copernicanism were, in the words of the Roman curia, “an actually grounded hypothesis.” But he was permitted and encouraged to use the hypothesis most actively as his clue to fresh scientific results, and to treat with the most ample justice the scientific arguments for and against. He was permitted to maintain that Copernicanism was scientifically likely in the highest possible degree; but he was not at liberty to teach expressly that it had received absolute and irrefragable proof. He had an audience of the pope, however, who assured him of his protection, and in 1617 he returned to Florence. Sickness prevented him from observing the three comets which appeared in 1618, but he entered warmly into discussions about them, and is supposed to have had the chief share in a lecture delivered by his friend Guiducci and printed in 1619, in which they are held to be only meteors. This discourse was attacked by the Jesuit Grassi under the pseudonyme of Lotario Sarsi, and defended by Galileo in his Saggiatore (“Assayer”), one of the most beautifully written of his works. On the accession of his friend Cardinal Barberini to the pontificate under the title of Urban VIII., he went to Rome to offer his congratulations, arriving in the spring of 1624, and receiving during the two months that he remained every mark of esteem and liberality. The pope granted him a pension of 100 crowns, and one of 60 crowns to his son. He now set about composing a work in which he might sum up all the arguments for and against his favorite theory. It was written in the form of dialogues, and accompanied by a preface in which he protested ironically against the idea that the decision of the inquisition in 1616 was rendered through ignorance or passion. He says that, on the contrary, its officers listened with attention to his statement of the scientific arguments on which his theory was based, and maintains that the grounds upon which this decision was justified were entirely religious. The book was published at Florence in 1632 under the title of “Dialogue on the two Principal Systems of the World, the Ptolemaic and Copernican.” This being regarded as a violation of the injunction, Galileo was ordered to appear in person at Rome, where he arrived in February, 1633, and took up his quarters with the Tuscan ambassador. His trial was short. The principal ground of complaint was the disobedience of the command of 1616, and the scientific reasons which Galileo again urged in support of his theory were not appreciated any better than before, but were met with religious arguments. The sentence was solemnly pronounced June 22. It set forth the offence of the accused in teaching a condemned proposition, violating his pledge, and obtaining a sanction for his book by improper means, declared him to be vehemently suspected of heresy, required him to abjure his errors and all other heresies against the Catholic church, prohibited his “Dialogue,” and condemned him to be imprisoned at the inquisition during pleasure, and to recite once a week for three years the seven penitential psalms. Galileo made his abjuration with all the formality which commonly attended such proceedings. Clad in sackcloth and kneeling, he swore upon the Gospels never again to teach the earth's motion and the sun's stability; he declared his detestation of the proscribed opinions, and promised to perform the penance laid upon him. Then rising from the ground, he is said to have exclaimed in an under tone: E pur si muove—“It does move, for all that!” After four days' confinement under the eyes of the holy office, Galileo returned to the Tuscan ambassador's, but for the rest of his life he was kept under surveillance. He passed some time in Siena, in the archbishop's palace, and in December reëntered his own house at Arcetri, near Florence, where he remained until the close of his life. The death of his favorite daughter Maria so affected his already broken health that he begged permission to visit Florence for medical assistance. It was only after four years (1638) that he obtained it, and then under severe restrictions. He seems now to have paid little attention to astronomy, but employed himself in other branches of natural philosophy. In 1638 his book of “Dialogues on Local Motion,” completed two years before, which he prized above all his other works, was printed at Amsterdam by Louis Elzevir. In 1636 also he discovered the moon's diurnal libration. In 1637 a disease which had impaired his right eye for some years attacked the left also, and in a few months he became totally blind. The severity of the inquisition was somewhat relaxed in his affliction; he was visited by eminent men of his own and foreign countries, among whom were Milton, Gassendi, and Diodati, and in the last years of his life his pupils Viviani and Torricelli formed part of his household. Almost complete deafness afterward came upon him, and at last, while preparing for a continuation of his “Dialogues on Motion,” he died of fever and palpitation of the heart.—Galileo was of middle size, well formed, with fair complexion and penetrating eyes. He was cheerful, frank, and amiable; frugal and abstemious, but fond of gay company and good wine, and profuse in his hospitality. He was unmarried, but left three natural children. His temper was quick, but placable, and his general accomplishments made him a favorite in mixed circles. His scientific writings were marked by a clear, elegant, and spirited style, which he owed to a careful study of the literature of his country. He was a great admirer of Ariosto, whose Orlando furioso, it is said, he knew by heart, and wrote severe “Considerations on Tasso” (Venice, 1793), to show that author's imitation of his favorite poet.—The following is a list of his principal works which were printed separately: Operazioni del compasso geometrico e militare (Venice, 1606); Difesa contra alle calumnie ed imposture di Balt. Capra nella considerazione astronomica sopra la nuova stella del 1604 (1607); Sidereus Nuncius (Florence, Venice, and Frankfort, 1610); Discorso intorno alle cose che stanno in sul' acqua e che in quella si muovono (Florence, 1612); Epistola ad M. Velserum de Maculis Solaribus (1612); De Maculis Solaribus et Stellis circa Jovem errantibus accuratior Disquisitio (Augsburg, 1612); Istoria e dimostrazioni intorno alle machie solari e loro accidenti (Rome, 1613); Dissertatio de Cometa Anni 1619 (Florence); Il saggiatore (Rome, 1623); Dialogo sopra i due massimi sistemi del mondo, Tolemaico e Copernicano (Florence, 1632; a Latin translation by Bernegger, entitled Systema Cosmicum, &c., Strasburg, 1635; an English version, “The Systeme of the World, in four Dialogues, Inglished from the Original Italian Copy by Thomas Salusbury,” London, 1661); Discorsi e dimostrazioni matematiche attenenti alla mecanica ed i movimenti locali (Leyden, 1638; an English translation under the title “Mathematical Discourses of Mechanics,” by Thomas Weston, London, 1730); Epistolæ tres de Conciliatione Sacæ Scripturæ cum Systemate Telluris Mobilis (printed with Gassendi's Apologia, Lyons, 1649). Collections of Galileo's works were published at Bologna by Manolessi (2 vols. 4to, 1656); Florence, by Bottari (3 vols. 4to, 1718); Padua (4 vols. 4to, 1744); Milan (13 vols. 8vo, 1808-'11). Eugenio Alberi edited a complete edition, with the life by Viviani (16 vols., Florence, 1842-'56).—For lives of Galileo see Viviani, Vita del Galilei, in the Fasti consolari dell' accademia Fiorentina; Frisi, Elogio del Galileo (Leghorn, 1775); Brenna, in Fabroni's Vitæ Italorum; Nelli, Vita e commercio letterario di Galileo Galilei (2 vols. 4to, Lausanne, 1793); Lord Brougham's “Life of Galileo” (1829); Libri, Histoire de la vie et des œuvres de Galileo Galilei (Paris, 1841); Biot, in Michaud's Biographie universelle; Drinkwater-Bethune, “Life of Galileo,” in the “Library of Useful Knowledge;” Sir David Brewster, in Lardner's “Cabinet Cyclopædia,” reprinted with lives of Tycho Brahe and Kepler under the title “Martyrs of Science” (London, 1841). Among recent biographies are those of Philarète Chasles (1862), Madden (1863), Trouessard (1856), Pauhappe (1868), and “The Private Life of Galileo” (London and Boston, 1870); also Botta's “Italian Philosophy,” in vol. ii. of Ueberweg's “History of Philosophy,” translated by George S. Morris (New York, 1874).