Bailly, Jean-Sylvain

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Bailly, Jean-Sylvain

(b. Paris, France, 15 September 1736; d. Paris, 12 November 1793)


Bailly was born in the Louvre, where his father, Jacques, as keeper of the king’s paintings, had an apartment. His father also owned a house in Chaillot, a fashionable suburb of Paris. Bailly later took up residence there, where he met Madame Gaye, a widow whom he married in 1787. Little is known of his youth, but it appears that he received schooling that would prepare him to assume his father’s office, a position held by a member of the family since Bailly’s great-grandfather. He succeeded to the title in 1768. Bailly was relieved of his functions in 1783 but continued to receive the pension accorded to the position and to be known as honorary keeper. When he was about twenty, he received mathematics lessons from Montcarville of the Collège Royal. Shortly thereafter he met Nicolas de Lacaille, France’s greatest observational astronomer, and Alexis Clairaut, France’s greatest theoretical astronomer. The Further studies that he pursued with them directed him into his lifework.

Bailly undertook his first astronomical research in 1759—concerned, appropriately, with Halley’s comet. His paper on the comet’s theory, read to and published by the Acadèmie des Sciences, pointed out that one could not conclude the duration of its revolution from that of its visibility. This paper was based upon the observations of Lacaille.

In 1760 Bailly established his own observatory in the Louvre. Although not ideal, the site served him well, and he felt it important for an astronomer to base his theories on his own observations. He now began to do this with Jupiter’s satellites, reading two papers on this subject in 1762 and adding a third near the end of 1763.

By the time he read the third report, Bailly had become a member of the Acadèmie. In January 1763 Bailly was elected to the vacancy created by the death of Lacaille, probably because he had been Lacaille’s protègè and was engaged in editing an unfinished work of his. He had also shown greater promise than his competitors, Jeaurat and Messier, of significant theoretical researches.

Bailly approached the problem of inequalities in the motions of the four known satellites of Jupiter with Clairaut’s lunar theory in mind. Improvements had been made in tables of the motions since Cassini’s 1668 ephemerides, but these improvements had been made empirically. Bailly was the first to attempt to achieve better tables theoretically, by treating each satellite in turn as the third body in a three-body problem. His success was not complete—there were considerable discrepancies between his theoretical formulation of orbitary elements and their observed values—but he did demonstrate that the problem was amenable to solution by Newtonian principles.

Bailly’s memoirs sparked interest in this subject, and the Acadèmie made it the topic of its essay contest for 1766. As an Academician, Bailly could not win the prize; nevertheless, he considered himself in competition with the astronomers who entered, and therefore synthesized and extended his earlier researches to produce his major Essai Although excellent, this work was greatly overshadowed by the prize-winning entry of Lagrange. As Bailly himself later admitted, Lagrange’s use of the new method of the variation of parameters allowed him to resolve almost completely the problem of five simultaneously perturbed bodies, as opposed to his own continuing series of three-body treatments.

In 1766 there arose the possibility of Bailly’s succeeding the ailing Grandjean de Fouchy as perpetual secretary of the Acadèmie. To prepare for this position, Bailly wrote several èloges. He did not achieve his goal, however; in 1770 Fouchy named Condorcet as his assistant.

Bailly followed this setback with one of his best scientific papers, his 1771 memoir on the inequalities of the light of Jupiter’s satellites. Fouchy had earlier noted that a satellite disappeared before its total immersion and that, at its emersion, it was observed only when a small segment had already emerged form the planet’s shadow. He concluded that the size of the segment involved depended upon the amount of the particular satellite’s light. Utilizing a new observational technique, Bailly confirmed Fouchy’s theory while coupling his study of light intensities with determinations of the diameters of the satellites. The resultant paper added greatly to contemporary knowledge of Jupiter’s satellites and suggested a standard observing method to reduce instrument and observer errors. Because the fourth satellite had not been eclipsing during the period of his observations, the memoir dealt only with the first three satellites, Bailly never completed the study, for this paper was his last theoretical effort.

Having moved to his father’s house in Chaillot, Bailly turned his attention to literary pursuits. These were given direction by his scientific activity, as shown in the four-volume history of astronomy that he published between 1775 and 1782. These tomes represent his most lasting achievement and were responsible for additional honors. In 1783 he was elected to the Acadèmie Française, and two years later he was named to the Acadèmie des Inscriptions et Belles-Lettres. This made him a triple academician, the only Frenchman besides Bernard de Fontenelle to achieve this distinction.

Meanwhile, Bailly continued to work within the Acadèmie des Sciences, although not at astronomical pursuits. In 1784 he was named to the commission appointed to investigate the extravagant claims then being made for “animal magnetism” by’Mesmer and others, and he drafted the commissions’ damning report on that alleged phenomenon. It was probably for this service that, near the end of 1784, Bailly was made a supernumerary pensioner within the Acadèmie. A year later he was appointed to a commission to investigate the Hôtel Dieu, the hospital of the poor of Paris; again he prepared the findings. Three reports submitted between 1786 and 1788 deplored the miserable conditions existing at the hospital and suggested means for their correction.

It was chiefly acclaim through these reports that catapulted Bailly into public affairs at the beginning of the French Revolution; the movement culminated on 15 July 1789 in his unanimous proclamation as the first mayor of Paris. He was reelected for a second term in August 1790, but in this second year he lost popularity, particularly after the unfortunate massacre of the Champ-de-Mars. Although he retired from office in November 1791 and left Paris in July 1792, he was not forgotten. Arrested in September 1793, Bailly was soon tried, found guilty, and condemned to the guillotine. Like those of his victims, his head fell on the Champ-de-Mars.


1.Original Works. Bailly’s first paper, the “Mémoire sur la théorie de la comete de 1759,” was published in the Mémoires de mathématiques et de physique présentés a l’Académie Royale des sciences par divers savans et his dans ses assemblées, 5 (1768), 12-18. The Académie employed this journal, generally known as its collection of memoirs par savants étrangers, as an outlet for papers by scientists who, like Bailly at that time, were not members of the Académie and, thus, were “foreign” to it. This would have been the case with his first two papers “sur la théorie des satellites de Jupiter,” except that he reread them to the Académie after attaining membership; thus, these and the third were published in the regular Mémoires de l’Académie Royale des sciences for 1763. He also placed many subsequent papers there, including, most importantly, his “Mémoire sur les inégalités de la lumiére des satellites de Jupiter, sur la mesure de leur diamétres, et sur un moyen aussi simple que commode de rendre les observations comparables, en remédiant a la différence des vues et des lunettes”(1771), pp. 580-667. His Essai sur la théorie des satellites de Jupiter, on the other hand, was issued as a separate volume with privilége granted by the Académie (Paris, 1766).

Two of Bailly’s éloges, those of Corneille and Leibniz, won prizes. They were published separately by the appropriate institutions: the Académie des Sciences, BellesLetters & Arts of Rouen (Rouen, 1768) and the Académie Royald des Sciences et des Belles-Lettres of Berlin (Berlin, 1769), respectively. These were incorporated with others, including that of Lacaille, in a collection (Berlin, 1770). But all of these and some later ones may be most conveniently consulted in Vol. 1 of the two-volume Discours et mémoires (Paris, 1790).

The first of Bailly’s historical works was the Histoire de l’astronomie ancienne, depuis son origine jusqu’a l’etablissement de l’école d’Alexandre(Paris, 1775). Its argument for an antediluvean astronomy that prepared the way for the astronomers of recorded history brought him into conflict with Voltaire and resulted in the publication of his Lettres sur l’origine des sciences (Paris-London, 1777) and the similar Lettres sur l’Atlandtide de Platon (Paris -London, 1778). These were sidelights, however, and he continued to work on the larger theme. His Histoire de l’astronomie moderne appeared in two stages. Two volumes, devoted to the period up to 1730, were published in 1779, and were followed by a third that brought the story up to 1782, the year of its publication. To these he added a special Traité de l’astronomie indienne et orientale (Paris, 1787).

The Rapport des comvnissaires charges par le roi de l’examen du magnetisme animal was published separately by order of the king (paris, 1784), as were the three hospital reports (Paris, 1786, 1787, 1788). The latter were subsequently reprinted in the Histoire de l’Acadèmie Royale des sciences, which prefaces each volume of the regular Mèmoires, but in association with incorrect years, as follows:(1785), 1-110; (1786), 1-9, 13-40. As with his earlier èloges, all of these and other reports may be consulted in his Discours ei mèmoires, Vol.II.

Bailly’s own treatment of his political career is contained in the three-volume Mèmoires de Bailly, avec une notice sur la vie, des notes et des èlaircissemens historiques(Paris, 1821), which constitute Vols. VIII-X of Berville and Barrière’s Collection des mèmoires relatifs á la révolution francaise....

II.Secondary Literature. A definitive biography has not yet been written. The most important general account by a contemporary is that by Bailly’s friend Mérard de Saint-Just: Éloge historique de Jean-Sylvain Bailly au nom de la rèpublique de lettres, par une sociètè de gens de lettres; suivi de notes et de quelques pieces en prose et en vers (London, 1794). Providing more insight into his astronimical work are J.J. Lalande’s “Éloge de Bailly,” originally published in the Dècade philosophique of 30 pluviose an III. 4 (1795), 321-330, but more easily consulted in his Bibliographie astronomique... (Paris, 1803), pp. 730-736; J.B.J. Delambre’s treatment inhis Histoire de l’astronomie au dix-huitiél;me siècle (Paris, 1827), pp. 735-748; and D.F. Arago’s “Bailly, biographie lue en sèance publique de I’acadèmie des Sciences, le 26 fèvrier 1844,” in his Oeuvres complètes, II (Paris, 1854), 247-426. On Bailly’s observatory, see G. Bigourdan, Histoire de l’astronomie d’observation et des observatoires en France, part 2 (Paris, 1930), 125-132.

Ignoring works that concentrate on his political career, the best modern study is E.B. Smith, “Jean-Sylvain Bailly; Astronomer, Mystic, Revolutionary, 1736-1793,” in Transactions of the American Philosophical Society, n.s.,44 (1954), 427-538. Based entirely upon printed sources—and, in fact, providing a comprehensive checklist of Bailly’s printed works and an extensive bibliography—Smith’s work should be supplemented by an article devoted to demostrating the existence of various as yet unused manuscript sources on Bailly: R. Hahn, “Quelques nouveausx documents sur Jean-Sylvain Bailly,” in Revue d’histoire des sciences,8 (1955), 338-353.

Seymour L. Chapin

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Bailly, Jean-Sylvain

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