François Arago
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François Arago |
François Jean Dominique Arago (
February 26,
1786 –
October 2,
1853) was a
French mathematician,
physicist,
astronomer, and
politician.
Arago was born at
Estagel, a small village near
Perpignan, in the
département of
Pyrenees-Orientales,
France. He was the eldest of four brothers. Jean (
1788 -
1836) emigrated to
North America and became a general in the
Mexican army.
Jacques Etienne Victor (
1799 -
1855) took part in
Louis de Freycinet's exploring voyage in the
Uranie from as
1817 to
1821, and on his return to France devoted himself to his journalism and the drama. The fourth brother, Etienne Vincent de (
1802 -
1892), is said to have collaborated with
Honoré de Balzac in
The Heiress of Birague, and from
1822 to
1847 wrote a great number of light dramatic pieces, mostly in collaboration. Showing decided military tastes François Arago was sent to the municipal college of
Perpignan, where he began to study
mathematics in preparation for the entrance examination of the polytechnic school. 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 at
Toulouse, he astounded his examiner by his knowledge of
J. L. Lagrange. Towards the close of
1803 he entered the
École Polytechnique,
Paris, but apparently found the professors there incapable of imparting knowledge or maintaining discipline. The artillery service was his ambition, and in
1804, through the advice and recommendation of
Simeon Poisson, he received the appointment of secretary to the
Paris Observatory. He now became acquainted with
Pierre-Simon Laplace, and through his influence was commissioned, with
Jean Baptiste Biot, to complete the
meridianal measurements which had been begun by
J. B. J. Delambre, and interrupted since the death of P. F. A. Méchain in
1804). Arago and Biot left Paris in
1806 and began operations along the mountains of
Spain. Biot returned to Paris after they had determined the latitude of
Formentera, the southernmost point to which they were to carry the survey. Arago continued the work until
1809, his purpose being to measure a meridian
arc in order to determine the exact length of a
metre.
After Biot's departure, the political ferment caused by the entrance of the French into Spain extended to the
Balearic Islands, and the population suspected Arago's movements and his lighting of fires on the top of
Mount Galatzo as the activities of a spy for the invading army. Their reaction was such that he was obliged to give himself up for imprisonment in the fortress of
Bellver in June
1808. On
July 28 he escaped from the island in a fishing-boat, and after adventurous voyage he reached
Algiers on
August 3. From there he obtained a passage in a vessel bound for
Marseille, but on
August 16, just as the vessel was nearing Marseille, it fell into the hands of a Spanish
corsair. With the rest the crew, Arago was taken to
Roses, 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 to
Palamos. After three months' imprisonment they were released on the demand of the
dey of Algiers, and again set sail for Marseille on the
November 28, but then within sight of their port they were driven back by a northerly wind to
Bougie 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 Moslem priest, and reached it on
Christmas Day. After six months in Algiers he once again, on the
June 21,
1809, set sail for Marseille, where he had to undergo a monotonous and inhospitable quarantine in the
lazaretto, before his difficulties were over. The first letter he received, while in the lazaretto, was from
Alexander 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.
Arago had succeeded in preserving the records of his survey; and his first act on his return home was to deposit them in the
Bureau des Longitudes at
Paris. As a reward for his adventurous conduct in the cause science, he was elected a member of the
Academy of Sciences, at the remarkably early age of twenty-three, and before the close of 1809 he was chosen by the council of the polytechnic school to succeed
Gaspard Monge in the chair of
analytical geometry. At the same time he was named by the emperor one of the astronomers of the Royal Observatory, which was accordingly his residence till his death, and it was in this capacity that he delivered his remarkably successful series of popular lectures in astronomy, which were continued from
1812 to
1845.
In
1816, along with
Joseph Louis Gay-Lussac, he started the
Annales de chemie et de physique, and in
1818 or
1819 he proceeded along with Biot to execute geodetic operations on the coasts of France, England and Scotland. They measured the length of the seconds-pendulum at
Leith,
Scotland, and in the
Shetland 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 and meteorology and occasionally on civil engineering, as well as interesting memoirs of members of the Academy.
Arago's earliest physical researches were on the
pressure of
steam at different
temperatures, and the
velocity of
sound,
1818 to
1822. His
magnetic observations mostly took place from
1823 to
1826. He discovered what has been called
rotatory magnetism, and the fact that most bodies could be magnetized; these discoveries were completed and explained by
Michael Faraday. He warmly supported
Jean-Augustin Fresnel's
optical theories, helping to confirm Fresnel's wave theory of light by observing what is now known as
the spot of Arago. The two philosophers conducted together those experiments on the
polarization of
light which led to the inference that the
vibrations of the
luminiferous ether were transverse to the direction of
motion, and that polarization consisted in a resolution of rectilinear motion into components at right angles to each other. The subsequent invention of the
polariscope and discovery of
rotatory polarization are due to Arago. The general idea of the experimental determination of the velocity of light in the manner subsequently effected by
Hippolyte Fizeau and
Léon Foucault was suggested by him in
1838, but his failing eyesight prevented his arranging the details or making the experiments.
Nearly all the mathematical work of the "French school" was done before the year
1830. They are the direct successors of the French writers who flourished at the beginning of the nineteenth century, and seem to have been out of touch with the great German mathematicians of the early part of it, on whose researches much of the best work of that century is based; they are thus placed here, though their writings are in some cases of a later date than those of
Gauss,
Abel and
Jacobi.
In
1830, Arago, who always professed liberal opinions of the breme republican type, was elected a member of the chamber of deputies for the
Pyrénées-Orientales département, 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 the most creditable national enterprises, dating from this period, are due to his advocacy - such as the reward to
Louis-Jacques Daguerre for the invention of
photography, the grant for the publication of the works of Fermat and Laplace, the acquisition of the museum of Cluny, the development of railways and electric telegraphs, the improvement of the reneile. In 1830 also he 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 of
J. B. J. Fourier. Arago threw his whole soul into its service, and by his faculty of making friends he gained at once for it and for himself a world-wide reputation. As perpetual secretary it was his duty to pronounce historical éloges 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. In 1834 he again visited Scotland, to attend the meeting of the
British Association at
Edinburgh. 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 of
Louis-Philippe he left his laboratory to join the Provisional Government (February 24, 1848). He was entrusted with two important functions, that had never before been given to one person, viz. the ministry of marine and colonies (February 24, 1848 - May 11, 1848) and ministry of war (April 5, 1848 - May 11, 1848); in the former capacity he improved of 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 the abolition of negro slavery in the French colonies. On May 10, 1848, he was elected a member of the Executive Power Commission, a governing body of the French Republic. He was made President of the Executive Power Commission (May 11, 1848) and served in this capacity as provisional head of state until June 24, 1848, when collective resignation of the Commission was submitted to the National Constituent Assembly. At the beginning of May 1852, when the government of
Louis 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's fame as an experimenter and discoverer rests mainly on his contributions to magnetism and still more to optics. He showed that a magnetic needle, made to oscillate over nonruginous 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 the
Copley Medal of the
Royal 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 ("magnetism of rotation"). Arago is also fairly entitled to be regarded as having proved the long-suspected connexion between the
aurora borealis and the variations of the magnetic pa ments[?]. In optics we owe to him not only important optical discoveries of his own, but the credit of stimulating the genius of
Jean-Augustin Fresnel, with whose history, as well as with that of
Etienne-Louis Malus and of
Thomas 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 the undulatory, 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. 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. To him also due the discovery of the power of rotatory polarization exhibited by
quartz, and last of all, among his many contributions to the support of the undulatory hypothesis, comes the
experimentum crucis which he proposed to carry out for measuring directly the velocity of light in air and in water 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 by
Charles 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 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.
He remained a consistent republican to the end, and after the
coup d'état of
1852, though suffering first from
diabetes, then from
Bright's disease, complicated by dropsy, he resigned his post as astronomer rather than take the oath 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 eastern Pyrenees, but it was ineffective and he died in Paris.
Arago's works were published after his death under the direction J. A. Barral, in 17 vols., 8vo, 1854-1862; also separately his
Astronomie 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.; and
Tables analytiques et documents importants (with portrait), in 1 vol. English translations of the following portions of his works have appeared :
Treatise on Comets, by C. Gold, C.B. (London, 1833); also translated 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 d 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 by Humboldt, translated under the superintendence of Colonel Sabine ondon, 1855), and
Arago's Biographies of Scientific Men, translated by Smyth, Powell and Grant, 8vo (London, 1857).
Craters on
Mars and the
Moon, and a
ring of Neptune, are named after him.
*
Obituaries
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MNRAS 14 (1854) 102
