Arago was the eldest of six brothers.Jean (1788–1836) emigrated to North America and became a general in the Mexican army.Jacques (1790–1854) took part inLouis de Freycinet's exploring voyage in theUranie from 1817 to 1821, and on his return to France devoted himself to his journalism and the drama.Victor (1792–1867) was a French soldier.Joseph (1796–1860) became a soldier in the Mexican army.Étienne (1802–1892), is said to have collaborated withHonoré de Balzac inThe Heiress of Birague, and from 1822 to 1847 wrote a great number of light dramatic pieces, mostly in collaboration.[4]
Showing decided military tastes, François Arago was sent to the municipal college ofPerpignan, where he began to studymathematics in preparation for the entrance examination of theÉcole Polytechnique. Within two years and a half he had mastered all the subjects prescribed for examination, and a great deal more, and, on going up for examination atToulouse, he astounded his examiner by his knowledge ofJ.-L. Lagrange's work.[4][5]
Towards the close of 1803, Arago entered theÉcole Polytechnique, Paris, but apparently found the professors there incapable of imparting knowledge or maintaining discipline. Theartillery service was his ambition, and in 1804, through the advice and recommendation ofSiméon Poisson, he received the appointment of secretary to theParis Observatory. He now became acquainted withPierre-Simon Laplace, and through his influence was commissioned, withJean-Baptiste Biot, to complete themeridian arc measurements which had been begun byJ. B. J. Delambre, and interrupted since the death ofP. F. A. Méchain in 1804 (themeridian arc of Delambre and Méchain). Arago and Biot left Paris in 1806 and began operations along the mountains of Spain. Biot returned to Paris after they haddetermined the latitude ofFormentera, the southernmost point to which they were to carry the survey.[4] Arago continued the work until 1809, his purpose being tomeasure a meridian arc in order to determine the exact length of a metre (seeParis meridian#History).
After Biot's departure, the political ferment caused by the entrance of the French into Spain extended to theBalearic Islands, and the population suspected Arago's movements and his lighting of fires on the top of Mount Galatzó (Catalan: Mola de l'Esclop) as the activities of a spy for the invading army.[5] Their reaction was such that he was obliged to give himself up for imprisonment in the fortress ofBellver in June 1808. On 28 July he escaped from the island in a fishing-boat, and after an adventurous voyage he reachedAlgiers on 3 August. From there he obtained a passage in a vessel bound forMarseille, but on 16 August, just as the vessel was nearing Marseille, it fell into the hands of a Spanishcorsair. With the rest the crew, Arago was taken toRoses, and imprisoned first in a windmill, and afterwards in a fortress, until the town fell into the hands of the French, when the prisoners were transferred toPalamos.[4][5]
After three months' imprisonment, Arago and the others were released on the demand of thedey of Algiers, and again set sail for Marseille on 28 November, but then within sight of their port they were driven back by a northerly wind toBougie on the coast of Africa. Transport to Algiers by sea from this place would have occasioned a weary delay of three months; Arago, therefore, set out over land, guided by a Muslim priest, and reached it on Christmas Day. After six months in Algiers he once again, on 21 June 1809, set sail for Marseille, where he had to undergo a monotonous and inhospitable quarantine in thelazaretto, before his difficulties were over. The first letter he received, while in the lazaretto, was fromAlexander von Humboldt; and this was the origin of a connection which, in Arago's words, "lasted over forty years without a single cloud ever having troubled it."[4]
Arago had succeeded in preserving the records of his survey; and his first act on his return home was to deposit them in theBureau des Longitudes at Paris. As a reward for his adventurous conduct in the cause of science, he was elected a member of theFrench Academy of Sciences, at the remarkably early age of twenty-three, and before the close of 1809 he was chosen by the council of theÉcole Polytechnique to succeedGaspard Monge in the chair ofanalytical geometry. At the same time he was named by the emperor one of the astronomers of theParis Observatory, which was accordingly his residence till his death. It was in this capacity that he delivered his remarkably successful series of popular lectures in astronomy, which were continued from 1812 to 1845.[4]
In 1818 or 1819 he proceeded along with Biot to executegeodetic operations on the coasts of France, England and Scotland. They measured the length of theseconds pendulum atLeith, Scotland, and in theShetland Islands, the results of the observations being published in 1821, along with those made in Spain. Arago was elected a member of the Bureau des Longitudes immediately afterwards, and contributed to each of its Annuals, for about twenty-two years, important scientific notices on astronomy andmeteorology and occasionally oncivil engineering, as well as interesting memoirs of members of the Academy.[4]
Arago's earliest physical researches were on thepressure ofsteam at different temperatures, and thevelocity of sound, 1818 to 1822. Hismagnetic observations mostly took place from 1823 to 1826. He discovered rotatory magnetism, what has been calledArago's rotations, and the fact that most bodies could be magnetized; these discoveries were completed and explained byMichael Faraday.
Charles Babbage,Account of the repetition of M. Arago's experiments on the magnetism manifested by various substances during the act of rotation, 1825
The general idea of the experimental determination of thevelocity of light in the manner subsequently effected byHippolyte Fizeau andLéon Foucault was suggested by Arago in 1838, but his failing eyesight prevented his arranging the details or making the experiments.
François Arago
Arago's fame as an experimenter and discoverer rests mainly on his contributions to magnetism in the co-discovery withLéon Foucault ofeddy currents, and still more tooptics. He showed that a magnetic needle, made to oscillate over nonferrous surfaces, such as water, glass, copper, etc., falls more rapidly in the extent of its oscillations according as it is more or less approached to the surface. This discovery, which earned him theCopley Medal of theRoyal Society in 1825, was followed by another, that a rotating plate of copper tends to communicate its motion to a magnetic needle suspended over it, which he called "magnetism of rotation"[8][9][10] but (after Faraday's explanation of 1832[11]: 283 ) is now known aseddy current. Arago is also fairly entitled to be regarded as having proved the long-suspected connexion between theaurora borealis and the variations of the magnetic elements.[4] In 1827 he was elected an associated member of the Royal Institute of the Netherlands, when that institute became theRoyal Netherlands Academy of Arts and Sciences in 1851, he became foreign member.[12] In 1828, he was elected a foreign member of theRoyal Swedish Academy of Sciences.
In optics, Arago not only made important optical discoveries on his own, but is credited with stimulating the genius ofJean-Augustin Fresnel, with whose history, as well as that ofÉtienne-Louis Malus andThomas Young, this part of his life is closely interwoven.
Shortly after the beginning of the 19th century the labours of at least three philosophers were shaping the doctrine of theundulatory, or wave, theory of light. Fresnel's arguments in favour of that theory found little favour with Laplace, Poisson and Biot, the champions of the emission theory; but they were ardently espoused by Humboldt and by Arago, who had been appointed by the Academy to report on the paper.[4] This was the foundation of an intimate friendship between Arago and Fresnel, and of a determination to carry on together further fundamental laws of the polarization of light known by their means. As a result of this work, Arago constructed a polariscope, which he used for some interesting observations on the polarization of the light of the sky. He also discovered the power of rotatory polarization exhibited byquartz.[13]
Among Arago's many contributions to the support of the undulatory hypothesis, comes theexperimentum crucis which he proposed to carry out for measuring directly the velocity of light in air and in water and glass. On the emission theory the velocity should be accelerated by an increase of density in the medium; on the wave theory, it should be retarded. In 1838 he communicated to the Academy the details of his apparatus, which utilized the relaying mirrors employed byCharles Wheatstone in 1835 for measuring the velocity of the electric discharge; but owing to the great care required in the carrying out of the project, and to the interruption to his labours caused by the revolution of 1848, it was the spring of 1850 before he was ready to put his idea to the test; and then his eyesight suddenly gave way. Before his death, however, the retardation of light in denser media was demonstrated by the experiments of H. L. Fizeau and B. L. Foucault, which, with improvements in detail, were based on the plan proposed by him.[4]
One of the 135 Arago medallions set along theParis Meridian for 9.2 km (6 mi), in memorial to Arago and his work on the meridian and his measurements of the Earth.
In 1830, Arago, who always professed liberal opinions of therepublican type, was elected a member of the chamber of deputies for thePyrénées-Orientalesdépartement, and he employed his talents of eloquence and scientific knowledge in all questions connected with public education, the rewards of inventors, and the encouragement of the mechanical and practical sciences. Many of the most creditable national enterprises, dating from this period, are due to his advocacy – such as the grant for the publication of the works ofFermat andLaplace, the acquisition of the museum of Cluny, the development of railways and electric telegraphs, and the reward toLouis Daguerre for the invention of photography which, in 1839, Arago reported to stunned listeners of a joint meeting of the academies of Arts and Sciences.
In 1830, Arago also was appointed director of the Observatory, and as a member of the chamber of deputies he was able to obtain grants of money for rebuilding it in part, and for the addition of magnificent instruments. In the same year, too, he was chosen perpetual secretary of the Academy of Sciences, the place ofJoseph Fourier. Arago threw himself into its service, and by his faculty of making friends he gained at once for it and for himself a worldwide reputation. As perpetual secretary it was his duty to pronounce historicaleulogies on deceased members; and for this duty his rapidity and facility of thought, and his happy piquancy of style, and his extensive knowledge peculiarly adapted him. He was elected a Foreign Honorary Member of theAmerican Academy of Arts and Sciences in 1832.[14]
In 1834, Arago again visitedScotland, to attend the meeting of theBritish Association atEdinburgh. From this time till 1848 he led a life of comparative quiet – although he continued to work within the Academy and the Observatory to produce a multitude of contributions to all departments of physical science – but on the fall ofLouis-Philippe he left his laboratory to join the Provisional Government (24 February 1848). He was entrusted with two important functions, that had never before been given to one person, viz. the ministry of marine and colonies (24 February 1848 – 11 May 1848) and ministry of war (5 April 1848 – 11 May 1848); in the former capacity he improved rations in the navy and abolished flogging. He also abolished political oaths of all kinds and, against an array of moneyed interests, succeeded in procuring theabolition of slavery in theFrench colonies.
On 10 May 1848, Arago was elected a member of theExecutive Power Commission, a governing body of the French Republic. He was made President of the Executive Power Commission (11 May 1848) and served in this capacity as provisional head of state until 24 June 1848, when collective resignation of the commission was submitted to the National Constituent Assembly. At the beginning of May 1852, when the government ofLouis Napoleon required an oath of allegiance from all its functionaries, Arago peremptorily refused, and sent in his resignation of his post as astronomer at the Bureau des Longitudes. This, however, the prince president declined to accept, and made "an exception in favour of a savant whose works had thrown lustre on France, and whose existence the government would regret to embitter."
Arago remained a consistent republican to the end, and after the coup d'état of 1852, though suffering first fromdiabetes, then fromBright's disease, complicated bydropsy, he resigned his post as astronomer rather than take theoath of allegiance.Napoleon III gave directions that the old man should be in no way disturbed, and should be left free to say and do what he liked. In the summer of 1853 Arago was advised by his physicians to try the effect of his native air, and he accordingly set out to the easternPyrenees, but this was ineffective and he died in Paris. His grave is at the famousPère Lachaise Cemetery in Paris. Arago was an atheist.[16]
Arago's works were published after his death under the direction J. A. Barral, in 17 vols., 8vo, 1854–1862 (F. Arago (1854),Jean-Augustin Barral (ed.),Œuvres complètes de Francois Arago (in French), Paris, Leipzig: Gide et J. Baudry,doi:10.5962/BHL.TITLE.20983,WikidataQ51430135); also separately hisAstronomie populaire, in 4 vols.;Notices biographiques, in 3 vols.;Indices scientifiques, in 5 vols.;Voyages scientifiques, in 1 vol.;Grimoires scientifiques, in 2 vols.;Mélanges, in I vol.; andTables analytiques et documents importants (with portrait), in 1 vol.
English translations of the following portions of Arago's works have appeared:
Treatise on Comets, by C. Gold, C.B. (London, 1833); also translatedW. H. Smyth and Grant (London, 1861)
Euloge of James Watt, by Muirhead (London, 1839); also translated, with notes, by Brougham
Popular Lectures on Astronomy, by Walter Kelly and Rev. L. Tomlinson (London, 1854); also translated by Dr W. H. Smyth and Prof. R. Grant, 2 vols. (London, 1855)
Arago's Autography, translated by the Rev.Baden Powell (London, 1855, 58)
Arago's Meteorological Essays, with introduction byAlexander von Humboldt, translated under the supervision of ColonelEdward Sabine (London, 1855)
^Annales de chimie et de physique (1824), vol. 27,page 363: "M. Arago communique verbalement les résultats de quelques expériences qu'il a faites sur l'influence que les métaux et beaucoup d'autres substances exercent sur l'aiguille aimantée, et qui a pour effet de diminuer rapidement l'amplitude des oscillations sans altérer sensiblement leur durée. Il promet, à ce sujet, un Mémoire détaillé." (Mr. Arago orally communicates the results of some experiments that he has conducted on the influence that metals and many other substances exert on a magnetic needle, which has the effect of rapidly reducing the amplitude of the oscillations without altering significantly their duration. He promises, on this subject, a detailed memoir.)
^"The same Arago who spent his time criticizing unfounded myths now peddled them. Arago the atheist now spoke of souls." Theresa Levitt, The shadow of enlightenment: optical and political transparency in France, 1789–1848, page 105.
Lequeux, James (2008),François Arago, un savant généreux, Paris: EDP-Sciences,ISBN978-2-86883-999-2
Walter Baily,A Mode of producing Arago's Rotation. 28 June 1879. (Philosophical magazine: a journal of theoretical, experimental and applied physics. Taylor & Francis., 1879)
The 0 meridian in Paris misused inThe Da Vinci Code is in fact an art project by the Dutch artistJan Dibbets (1941) made in 1987 as a tribute to the astronomer François Arago (1786–1853)
