The Arago family were involved in politics, having leftist republican views, and also had military connections. At first François Arago showed an interest in a military career and it was with this in mind that he was educated at the Municipal College of Perpignan where he came to love mathematics. In 1803 he was examined at Toulouse for admission to the École Polytechnique in Paris. His examiner was Louis Monge, the brother of Gaspard Monge who was at the time director of the École Polytechnique, and Arago was placed as one of the top five students in the ranked entrance list. He entered the École Polytechnique in 1803 and took up lodgings in a Paris apartment owned by a friend of his father. This friend introduced Arago to Poisson who was five years older than Arago and had been appointed at an assistant professor at the École Polytechnique in the year before Arago entered the school. Arago and Poisson became friends, not really behaving as student and teacher. Poisson would frequently visit Arago in his apartment in the evenings and the two would discuss politics and mathematics. Although, as we have said, the two were not like student and teacher, nevertheless Poisson did influence his younger friend very considerably. Despite being taught by some of the leading mathematicians in the world, Arago did not find their abilities to teach come anywhere close to their abilities for research. In 1805 Poisson was able to offer Arago a task of far more importance than one would have expected a young student to be asked to undertake. He asked Arago to help in measuring the meridian.
Delambre and Méchain measured the meridian from Dunkerque (Dunkirk) to Barcelona between 1792 and 1798. Using their data the length of the metre was set but Méchain had remained keen to obtain more data. In September 1801 the Bureau des longitudes requested that an expedition be sent to extend the meridian measurements south of Barcelona. Méchain was keen to lead this new expedition and in April of 1803 he left for Spain. He triangulated down the Catalan coast in June, and by January of 1804 he had crossed to Ibiza and Mallorca (Majorca). However he died in September of 1804 with the task incomplete. Laplace asked Poisson to find someone who would continue the work, and Poisson proposed his young friend Arago. The challenge was a daunting one, and Arago took some time to make a decision. However he finally decided that he would accept the prestigious offer, was nominated formally on 22 February 1805 and then moved into the Paris Observatory which would become his headquarters. Biot was assigned to become the second member of the team and the pair spent about eighteen months learning the necessary techniques. In fact they were asked to carry out a second scientific task at the same time for they were asked to use a pendulum to measure the force of gravity at the various locations on their travels so that the data might be used for a more accurate estimate of the exact shape of the Earth.
On 3 September 1806 Arago and Biot set out for Spain. They continued the task which Méchain had been undertaking on his final expedition and by 1808 they were on Mallorca, an important point which allowed the Paris meridian to be continued south of Barcelona. They had been operating in Spain at an extremely difficult time, given that they were French. Napoleon had turned his attention towards Spain and Portugal in 1807 and marched his armies through Spain to Portugal in October 1807. They conquered Portugal and occupied parts of Spain. In May 1808 Napoleon declared his brother Joseph Bonaparte as Spanish ruler and the War of Independence began. Biot and Arago must have looked extremely suspicious; two Frenchmen with sophisticated measuring instruments working on Spanish territory. Biot fled back to France but Arago remained on Mallorca, disguised as a Spaniard, trying to complete his measurements which he had recorded in a logbook. However lighting of fires on the top of Mount Galatzo was pretty suspicious so he was arrested as a spy and put in prison.
Arago managed to persuade the commander of the prison that he was a scientist, not a spy, and the commander agreed to give Arago a chance to escape. He did so on 29 July 1808 and, still carrying his precious logbook, managed to find a fishing boat heading for Algiers, which he boarded. Reaching Algiers on 3 August he went to the French consul who supplied him with a forged Austrian passport and by 16 August he was on a boat heading to Marseille. This might have been a remarkable adventure had it ended at that point, but more drama was to come. The boat on which Arago was sailing was captured while on its way to France by a Spanish warship and he was back in captivity again. Arago was held in a Spanish prison in Roses but after only a short spell the Spanish decided to send their prisoners to Palamos since the French armies were advancing through Spain. However Arago was lucky and, having been recognised by the authorities, was released an put on another boat for Marseille on 28 November.
It was not to be, however, for again Arago failed to reach his homeland. A storm blew the boat back to Bougie on the north African coast where he was captured by Moslems. After further adventures during which he persuaded his captors that he wished to convert to Islam to obtain favourable treatment, he was allowed to return to Algiers which he did overland, arriving there on 25 December. A new local leader in Algiers was opposed to the French and Arago found himself in prison waiting to be shipped off to a penal colony. However the French consul again came to his rescue and, on 21 June 1809, Arago was put, for the third time, on a ship bound for Marseille. This time he reached his destination without mishap and on 2 July 1809 he was standing on French soil.
Arriving back in Paris with his logbook containing the measurements he was treated as a hero. He became an assistant professor at the École Polytechnique and a member of the Academy of Sciences. He also worked at the Paris Observatory for the rest of his career. One experiment which he carried out at the Observatory in 1810 concerned light. He reasoned that since light was refracted by a prism through an angle depending on the ratio of the velocity of light outside the prism to that inside it, then he should be able to detect differences in the velocity of light depending on the speed of the Earth towards and away from the source.
Of course we now know, from Einstein, that this reasoning is incorrect. Arago knew roughly what the velocities should be, so he knew that the effect he was looking for was well within what he should be able to detect. In fact detecting the differences in velocities due to the rotation of the Earth should have been possible. Arago made observations of stars on 19 and 27 March 1810. He made observations when the Earth was rotating towards the sources and also when it was rotating away from the same sources. To his surprise he did not detect any differences in the velocity of light. He observed the same sources on 8 October when the Earth when the difference in velocity should have been twice the Earth's orbital velocity. Again he was puzzled that the velocity of light seemed unaffected by the velocity of the observer towards or away from the source. He presented his results to the Academy of Sciences on 10 December 1810. How did Arago explain the puzzling results? He suggested that light particles travel at a variety of different velocities but the human eye of only sensitive to light travelling at one particular velocity. This would explain why the velocity appeared independent of the velocity of the observer.
Arago married Lucie Carrier-Bescombes in 1811; one of their sons, Alfred Arago (1816-92), became an Inspector of Fine Arts, another son Emmanuel Arago served with his father on the National Assembly for many years. In 1813 Arago began to give popular lectures on astronomy. He continued to give these lectures every year until 1845. He became Director of the Paris Observatory. Arago was also active in other ways. He served for many years as secretary of the Academy of Sciences and in this capacity he inaugurated the Proceedings of the Academy of Sciences in 1835.
He was active politically for the republican cause and filled several political roles in government in addition to his scientific duties. His political career began in 1830 when he was elected as a deputy from the Pyrénées-Orientales region. Later he served as a deputy from Paris. In the provisional government which took power after the 1848 Revolution he was elected on 23 April and he served as minister of war and the navy. He introduced many reforms while holding this government position including the abolition of slavery throughout France and its many possessions. This government established universal manhood suffrage in France, a cause championed by Arago. He was elected to the Executive Power Commission on 10 May 1848, being named as president of the Commission since he was elected with the largest number of votes. We should also note that he used his political positions to advance science, such as obtaining money to fund the publication of the works of Fermat and Laplace, supporting the development of railways and of the telegraph. He was also able to get funding for the latest astronomical instruments for the Observatory and able to gain financial support for the Academy of Sciences.
Arago made early discoveries on the corpuscular theory of light in 1811. Working with Fresnel he discovered that two beams of light polarised in perpendicular directions do not interfere, leading to the transverse theory of light waves. His theory of light predicted that the velocity of light should decrease as it passes into a denser medium. In 1838 he described a test to compare the velocity of light in air and in water or glass. However difficulties with the experiment meant that Arago was not in a position to try his experiment until 1850. By this time however his sight had become poor, so it was left to others to carry out a refined version of the experiment. Successful results were obtained by Fizeau and Léon Foucault before Arago died. Bertrand, in his obituary of Arago, wrote:-
Arago smiled at the beautiful experiment [of Fizeau and Foucault] which, with its well deserved praise, brought back pleasant memories of his own glory days when he beat Laplace, Poisson, and Biot, to gain his place in the Academy of Sciences.In 1820 the Danish physicist H C Orsted produced experimental results on electricity and magnetism. Arago carried out further experiments of this type and demonstrated several effects which led Faraday later to explain them as induction. It appears that Arago was the first person to construct an electromagnet.
Working with Biot, Arago made measurements of arc length on the Earth which led to the standardisation of the metric system of lengths. We described above the adventures he had in taking readings in the south, but later, in 1821, the two extended their results to the north making measurements of the force of gravity using a pendulum in Scotland at Leith, near Edinburgh, and in the Shetland Islands.
He suggested that his student Le Verrier investigate irregularities in Uranus's orbit and, after Neptune was discovered, participated in the argument regarding naming the planet and disputed with Adams regarding priority. Although many have criticised Arago for his part in this dispute, particularly for his efforts to get the new planet named Le Verrier, one has to feel that he did the right thing in championing his own student who had made the discovery following his advice.
Other contributions made by Arago include work on the polarization of light, investigations of the solar corona and chromosphere, and measurements of the diameters of the planets. He also worked on the velocity of sound.
On 2 December 1851 there was a coup d'état in France with Louis-Napoleon Bonaparte assuming absolute power and dissolving the National Assembly. Exactly one year later he became Emperor taking the title Napoleon III. Leaders in various field were required to swear allegiance and Arago held two such positions, as Director of the Observatory and as secretary of the Academy of Sciences. Arago, a staunch republican, refused to swear allegiance. He remained at the Paris Observatory, which Napoleon II had renamed the Imperial Observatory. Arago requested that the Academy of Sciences replace him as Secretary, not because of the problems with Napoleon III, but rather because he felt that his health was failing, in particular his eyesight was by now very weak, and he could not properly carry out his duties. He was suffering from a build-up of fluid between his tissues, diabetes, and inflammation of the kidneys.
On 22 August 1853 Arago attended the Academy of Sciences for the final time then, despite his health problems, he set out on the difficult journey to his family in the Roussillon region. After giving them advice on a wide range of matters he returned to Paris where he died shortly after. He was buried in the Père Lachaise cemetery in Paris.
Arago was honoured with the award of the Royal Society Copley Medal in 1825 and the Rumford Medal in 1850. Craters on both the Moon and Mars have been named after him, as has a ring of Neptune. The Academy of Sciences inaugurated the Arago Medal in 1893.
Article by: J J O'Connor and E F Robertson