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SU BAILLY

La vita di Bailly potrebbe essere descritto in termini di duplice attrazione di scetticismo e credenza. Questi furono i due poli tra i quali fluttuava il suo pensiero. Qualche volta, come nell' Éloge de Leibnitz, egli era attratto dallo scetticismo. Altre volte invece, come nell' Histoire de l'astronomie ancienne e nelle Letters a Voltaire, sotto l'influenza di Court de Gébelin, egli lo respinse. A Voltaire scrisse: «Il dubbio deve avere limiti; non tutte le verità possono essere provate come verità matematiche».^[1] Bailly sapeva comunque certamente dubitare ed essere scettico quando la ragione lo richiedeva. La partecipazione all'indagine ufficiale sul mesmerismo ne fu una prova, e gli permise anche di dissipare l'illusione che fosse un frère illuminé.

In prima istanza, si sarebbe potuto immaginare che Bailly si potesse disporre a favore del magnetismo animale, affascinato dal concetto, presente nelle teorie di Mesmer, di armonia universale, dalla soluzione semplice di fenomeni complessi, dalla riconciliazione della Terra e le forze celesti, che egli andava cercando. Infatti il suo mentore, Court de Gébelin, aveva abbracciato il magnetismo animale come una nuova "fede" che aveva l'obiettivo di «ristabilire l'armonia primitiva che regnava tra l'uomo e l'universo».^[2] Il fatto che un membro dell'altra commissione, Jussieu, pubblicò un parere dissenziente a quello degli altro, era comunque la prova giusto che ad essi fu permesso di operare in completata libertà.

La Fête de la Fédération

Quando iniziò a farsi largo l'idea di organizzare una festa a Parigi, per convocare una federazione di delegati politici e della Garde nationale da tutta la nazione, la Comune di Parigi adottò questi progetto. Si nominò una delegazione, guidata da Bailly, per presentare il progetto all'Assemblea nazionale costituente la quale l'approvò il 5 giugno.^[3]

La Fête de la Fédération si tenne il 14 luglio 1790, ad un anno esatto dalla Presa della Bastiglia, e vi parteciparono i rappresentanti di tutte le province della Francia per assistere al solenne giuramento di fedeltà che sarebbe stato pronunciato dal generale La Fayette, da Luigi XVI e da Talleyrand, vescovo di Autun. La cerimonia si svolse al Campo di Marte, dove per l'occasione fu costruito un grande anfiteatro in grado di ospitare 400'000 persone.^[4]

Sulle disuguaglianze

Perhaps it was the momentum of work already in hand; perhaps it was a desperate bid for favor; perhaps it was a determination to rise above the political bicker- ing that led Bailly to produce one of his best scientific papers, the "Mémoire sur les inégalités de la lumière des satellites de Jupiter, sur la mesure de leur diamètre, et sur un moyen aussi simple que commode de rendre les observations comparables, en remédiant à la différence des vues et des lunettes."18

Lalande, Laplace, Delambre, Arago-all the astronomers who have evaluated Bailly's work-are in accord on the excellence of this mnemoire. Lalande wrote of it:

Ce travail, plein de sagacité, ne pouvait être fait que par un de nos plus grands astronomes; et je lui disais, dans le temps de sa gloire, que j'aimerais mieux l'avoir fait que d'avoir été le premier sur la liste des présidents des Etats généraux et des maires de Paris, quoique son mérite l'y efit place.19

The time of apparent eclipse of a satellite precedes the time of real eclipse, because the observer sees only the illuminated segment of the satellite. The apparent size of this segment varies with the brilliance of the satellite, the intensity of light on Jupiter, the distance of the satellite at Jupiter's limb, the height of the eclipse above the earth's horizon, the power of the telescope used and the observer's personal equation. Similarly the apparent end of the eclipse follows the real emergence. Grandjean de Fouchy had long before tackled the problem and offered a partial solution in these terms:

Si cette partie visible était toujours de même grandeur, elle ne troublerait en rien le calcul, puisque ce ne serait qu'une quantité constante à ajouter au temps de l'emersion, et a soustraire au temps de l'immersion; mais cette moindre partie visible doit varier suivant l'intensité de la lumière des satellites... Cette intensité doit varier 1° en raison inverse des carrés de la distance de Jupiter au soleil, 2° en raison inverse des carrés de la distance de Jupiter à la terre.20

In order to construct tables of error for the eclipses of the satellites, Fouchy devised an ingenious system for determining for a given position the time by which the real eclipse lagged behind the apparent eclipse. Using two telescopes of equal power, he applied to the objective of one a diaphragm of such dimensions that the two apertures were in the same ratio as the greatest and shortest distances of Jupiter from the earth; the time lapse between the apparent eclipses observed with these two telescopes, he felt, should give the quantity of the equation for the invisible segment of the satellite. For forty years no further experiments were made, because, occupied with the secretariat of the Academy, Fouchy had neither the time nor the inclination to con- tinue them. And his discovery was not put to use, because although he had indicated a method for establishing an equation, he had not determined the quantity. Bailly says that he began to work on Fouchy's ideas as early as 1765.21 Unlike Fouchy, he used a single telescope for his observations. By means of a dia- phragm applied to the objective of the instrument, he diminished the aperture in the same ratio as Jupiter's greatest distance from the earth to its actual distance. When there was to be an eclipse of a satellite, he observed the moment of contact through the diminished aperture, then removed the diaphragm and timed the interval to the second or "true" contact. Such observations, conducted from 1768 on, enabled Bailly to confirm Fouchy's theory of the intensity of light, but showed no correlation between it and the equation of error for the eclipses.

Comme toutes ces formules supposent que l'on connaisse le diamètre des satellites et la grandeur du segment éclairé, qui devient insensible, il s'agissait de chercher les moyens de déterminer ces deux inconnues. J'ai pensé qu'on pouvait imiter, dans tous les moments, ce qui arrive dans les éclipses où la lumière diminue par degrés, et qu'en diminuant de même l'ouverture de la lunette, on parviendrait peut-être à faire disparaître le satellite.22

This eclipse-at-will was produced by a series of diaphragms of graduated size removed in rapid succession from the objective of the telescope. Bailly's first discovery as a result of this procedure was that the point of disappearance of the third satellite was at 1/64 its maximum intensity; for the other three, at 1/16; however he called the first satellite the largest and accounted for its lesser brilliance by its proximity to Jupiter.23

The measurement of the diameters of the satellites was in terms of their appearance from the center of Jupiter, and was determined by the time each takes to enter completely into Jupiter's shadow:

Ayant trouvé par l'observation le diaphragme qui fait disparaître le satellite, je connais le rapport du segment invisible au disque entier, au moment où le satellite disparaîtra; je couvre ensuite l'objectif de ma lunette d'un diaphragme un peu plus grand, qui me laisse apercevoir le satellite, mais faible et très petit, de manière que ce satellite cesse d'être visible dès que sa lumière sera tant soit peu diminuée. Je suis ainsi averti du moment où il commence à toucher l'ombre et l'intervalle de temps écoulé entre cet instant et celui de la véritable immersion me donne la mesure d'une grande partie du diamètre, d'où il est aisé de conclure le diamètre entier.24

Bailly supposed the area of the invisible portion of the satellite to be in inverse ratio to the square of the aperture, and he prepared a set of tables25 for computing the true diameter from the observed diameter. A by-product of this research was the discovery that the equation of error varied in conformity with Bouguer's tables of refraction,26 and Bailly computed his tables at two-degree intervals from the horizon to the zenith. It followed from Bailly's formula for the invisible portion of the satellite that, if the invisible segment had a fixed relation with the light-gathering power of a telescope, the relative errors of different telescopes could be precisely determined. With this idea in view, Bailly and Messier 27 together conducted a series of experiments with both refracting and reflecting telescopes. They further compared the results of their observations to determine the personal factor affecting their timing. Bailly concludes his mémoire with a number of suggestions for standard observing practice, designed to reduce errors of the instrument and of the observer. Although much of Bailly's work has been superseded and forgotten, there can be no doubt that it was extremely useful in its time. Bailly had not been able to make observations of the fourth satellite while working on this paper, and Lalande asked his permission to carry on his work in that field. Delambre and Maskelyne 28 continued the same line of investigation for a while, until it became apparent that basing the formula on the aperture of the diaphragm was not a sound procedure.29 We may note in passing that the summary of Bailly's long and painstaking paper which appeared in the history of the Academy for 1771 is unusually terse. The only word of praise is for Bailly's "recherches également ingénieuses et fines". The summary was written by Condorcet, who had officially become Secretaire Per- petuel in February 1773, when Fouchy went into re- tirement.30 The growing awareness of hostility in the Academy is apparent in Bailly's writings of this period. For one thing, he turned to a wider audience and, in due course, to a broader field of interest. Furthermore, he displays a new attitude of independence and self-justification bordering, in one case, on bitterness. Sometime during 1772, Bailly wrote a detailed letter to the Royal Society, outlining his methods for the study of the light of Jupiter's satellites. This letter was read before the Royal Society February 18 and 25, 1773, and published in the Philosophical Transactions for that year together with "Notes on the foregoing paper" by the Reverend Samuel Horsley,32 who, expressing certain reservations on matters of detail, nevertheless voices the highest opinion of Bailly's work.

• 18 MEM AC SCI 1771: 580-667.
• 19 Eloge, 323.
• 20 MEM AC SCI 1732: 42
• 21 MEM AC SCI 1771: 581.
• 22 Ibid., 588.
• 23 This is in accord with Galileo's findings, but contrary to modern knowledge, which makes the third and fourth satellites of approximately equal size and larger than the first and second.
• 24 MEM AC SCI 1771: 615.
• 25 Ibid., 612-613. 26 Pierre Bouguer (1698-1758), Traite de la gradation de la lumiere, Paris, 1760.
• 27 Charles Messier (1730-1817), astronomer and fellow-academician.
• 28 Nevil Maskelyne (1732-1811), F. R. S., Astronomer Royal.
• 29 Delambre, Histoire de I'astronomie au 186 sie'cle, 745 ff.
• 30 The volume for 1771 was published in 1774.

Elogi

Gli elogi testimoniano lo sviluppo del pensiero di Bailly. Tratto tipico della sua epoca, Bailly è meno interessato alla forma estetica che nello scopo didattico dell'arte. Convinto della superiorità della sua epoca illuminata, Bailly è anche affascinato dal concetto di "uomo naturale", con il quale si intende non l'uomo primitivo, ma il denominatore comune degli uomini in tutte le società in tutti i periodi. Storia, diritto, arte, e scienza sono tutti visti come espressioni del progresso fisico e morale dell'uomo.

La composizione dell′Éloge de Leibnitz in particolare avrebbe avuto un effetto profondo e duraturo sul pensiero di Bailly. Anche se dà voce al credo del philosophe, dell'uomo universale, è stato il système ad averlo attratto, la «verosimigianza sostituita alla verità inaccessibile». Al periodo di questi éloges si potrebbe datare anche la germinazione di numerose idee di Bailly che trovarono la luce nelle opere successive, come l′Histoire de l'astronomie ancienne, le Lettres sur l'Atlantide de Platon e l′Essai sur les fables et sur leur histoire.

Bailly godette di un moderato successo con i suoi éloges. Dei quattro presentati nelle varie competizioni, uno solo vinse il prix d'eloquence, mentre altri due hanno ricevuto una menzione d'onore. Anche se quelli di Carlo V, Molière e Lacaille non sembra fossero andati immediatamente in stampa, gli altri due, quello su Corneille e quello su Leibniz, furono invece pubblicati ognuno per due edizioni. Nel 1770 fu inoltre pubblicata a Berlino e a Parigi da Delalain un'ulteriore edizione che raccoglieva tutti e cinque gli éloges. Eppure, se Bailly era alla ricerca di fama e fortuna con questo tipo di scrittura, si deve comunque ammettere che non colse nel segno. Se il suo obiettivo era quello di ottenere la segreteria dell'Accademia francese delle scienze, come gli era stato promesso da D'Alembert che poi però lo aveva tradito preferendogli Condorcet, non ci riuscì. Alcuni aneddoti, riportati dal biografo di Bailly, Michel de Cubières, sembrano infatti riflettere anche una certa disillusione da parte dell'insigne astronomo sia nei confronti degli éloges sia verso il mondo accademico più in generale.^[5] A quanto pare, infatti, Bailly arrivò a dire che «i premi accademici non provano nulla» e che la maggior parte degli éloges non erano altro che «folies de jeuness» ovvero "follie di gioventù".^[5]

Bailly philosophe

da pag 43

About the time of Bailly's retirement to Chaillot, we can detect a change in his thinking which was, for a time, to widen the gap between him and the philo- sophes. Bailly seems always to have needed a guide and mentor-first Lacaille, then Clairaut, d'Alembert, and Buffon. The first two directed his efforts in the fields of astronomy and mathematics; d'Alembert and Buffon enocuraged him in the literary field. When he turned to the popularization of astronomy, he came under the influence of Court de Gebelin 51 whose nine volume Monde primitif was just appearing. This work was an attempt to reduce the complexities of civilization, its customs, traditions, speech, etc. to a universal theme. Court de Gebelin felt that the key to the mysteries of nature was to be found in the history of antiquity and that history, properly understood, might lead humanity to a new golden age. His vision of le grand ordre was utopian, and much of his history is inaccurate; but in many ways Court de Gebelin was a precursor of modern thinkers. His theory of fables as allegorical documents foreshadows the work of modern folklorists. Similarly his search for the primitive language which was the source of all languages anticipated the linguistic research of the nineteenth century. If Bailly had acquired from his astronomical research a conviction that truth was basically simple, he had also learned from Leibnitz to substitute "vraisemblance" for "la verite inaccessible." The influence of Court de Ge'belin was to encourage this speculative bent and cause Bailly to be branded "frere illumine" by the more ardent philosophes. Yet the illuminism of Bailly's work, if it can be called that, is not so far removed from the rationalism of his detractors, but is rather symptomatic of the deterioration of systematic doubt which was common towards the end of the century.

Note

1. ^ Jean Sylvain Bailly, Lettres sur l'Atlantide de Platon.
2. ^ Lettre de l'auteur du Monde primitif a messieurs ses souscripteurs sur le magnétisme animal, 42, Parigi, Valleyre, 1784.
3. ^ Fête de la Fédération su cosmovisions.com
4. ^ 14 luglio 1790: la Festa della Federazione di Giovanni Ligasacchi
5. Michel de Cubières, Recueil des pièces intéressantes sur les arts, les sciences et la littérature, ouvrage posthume de Sylvain Bailly, précédé de la vie littéraire et politique de cet homme illustre, 1810, XX-XXII.