ARAGO, FRANCOIS JEAN DOMINIQUE. 



ARAQO, FRANCOIS JEAN DOMINIQUE. 



proprietor, who was a mathematician, he familiarised himself with the 

 writings of Legendre, Lacroix and Gamier. His real master, to quote 

 a passage from his autobiography, " was a cover of Garnier's ' Treatise 

 of Algebra.' This cover consisted of a printed sheet, on the outside 

 of which blue paper was patted. The reading of the uncovered side 

 inspired me with a desire to know what the blue paper concealed. 

 I damped it, and removing it with care, read underneath this advice 

 given by D'Alembert to a young man who was telling him of the diffi- 

 culties he met with in his studies : ' Keep on, sir, keep on, and con- 

 viction will come to you ; ' which was for me a ray of light. Instead 

 of trying obstinately to comprehend at first sight the propositions 

 that came before me, I kept on, and was astonished the next day at 

 understanding perfectly that which, the evening before, had appeared 

 to me wrapped in thick clouds." 



In eighteen months Arago was ready for his examination, but the 

 examiner having been detained by illness, a delay occurred, during 

 which his friends sought to divert him from the pursuit he had 

 chosen. He kept on, however, and studied the works of Euler and 

 Laplace, and took lessons in fencing and dancing, having heard that 

 these accomplishments were essential to an officer. In the summer of 

 1803 he was examined by Monge at the university of Toulouse, and 

 passed with high commendations first of his class. He repaired forth- 

 with to Paris, and entered the Polytechnic School, where, after a 

 few months, he came off as triumphantly from an examination by 

 Legendre as from that at Toulouse. In either case, his readiness and 

 familiarity with the subjects required, overcame the prejudices of the 

 examiners. 



Hi vas studying for the artillery branch of the service when, in 

 1804, the post of Secretary to the Observatory at Paris, then under 

 Bouvard's direction, having fallen vacant, he was persuaded, but with 

 great reluctance on his part, to undertake the duties. The temporary 

 appointment, as he thought it, effected an entire change in his pur- 

 suits, for he remained attached to the Observatory for the rest of his 

 life. At the instance of Laplace he worked with Biot, who was assistant- 

 observer, at experimental researches for determining the refractive 

 power of different gases an inquiry commenced by Borda the results 

 of which formed the subject of a paper presented to the Academy of 

 Sciences, and printed in their ' Memoirs ' for 1806. In the same year 

 the two young men were appointed by the government to extend and 

 complete the measurement of the arc of the meridian, whith, carried 

 from Dunkirk to Barcelona by Delambre and Mdcbain, had been inter- 

 rupted by the death of the latter. It was now to be extended from 

 Barcelona to the Balearic Isles, and from thence to Formentera, by an 

 immense triangle, the measurement of which had been thought impos- 

 sible. The fatigues of this survey in a wild mountain region, exposed 

 to heat, cold, and storm, were excessive. For six months Arago was 

 stationed on an elevated peak in the Desierto de las Patinas, watching 

 for the light set up on Ivica, which, owing to a defect in fixing the 

 mirror, was seldom visible. A space of about seventy-five square yards 

 was all the ground he had for exercise ; and two Carthusian monks, 

 who, forgetting their vow of silence, used to ascend the mountain to 

 converse with him in the evenings, were his only society. The work 

 involved frequent journeys, in which, apart from the fierce heats, 

 much risk was incurred owing to the hostile feeling between France 

 and Spain, and from parties of brigands. On two occasions a noto- 

 rious robber-chief intruded himself as a nightly guest on the zealous 

 surveyor. 



The geodesical union from the mainland to Ivica, and thence to 

 Formentera an arc of parallel of one degree and a half in one 

 triangle was successfully accomplished. Biot had returned to Paris 

 when, in the summer of 180S, the fire-signals on Mount Galazo in 

 Majorca were suspected to be advices to the French army then invading 

 the Peninsula, and Arago was denounced as a spy. To escape the 

 threatened violence, he obtained permission from the governor to 

 imprison himself in the citadel of Belver. Having a safety-pass from 

 the English Admiralty, he escaped in a half-decked boat to Algiers in 

 July. In August he sailed for France in an Algerine frigate, ajid was 

 in sight of the coast of Provence when the vessel was captured by a 

 Spanish privateer, and carried into Rosas. Here he was again exposed 

 to great danger : the authorities, bitterly suspicious, subjected him to 

 repeated examination!), and consigned him to the hulks at Palamos, 

 where hU sufferings from want of food were, as he tells us, aggravated 

 by the sight of the Pyrenees, and the thought that his mother might 

 then be looking up at their peaks, anxious for her son. 



Being liberated on demand of the dey, he sailed once more for 

 France on September 28, and was off the port of Marseille when the 

 ship, caught by the mistral, was drifted all across the Mediterra- 

 nean to the coast of Africa. Arago landed at Bougie, and having 

 travelled to Algiers, found a new dey in power, who would have sent 

 him to the galleys but for consular interference. Here he lingered, 

 waiting for an opportunity to return home, until June, 1809, when he 

 again sailed, and though chased by an English cruiser, landed at Mar- 

 seille on the 2nd of July, with his manuscripts and instruments. For 

 eleven months had he been tossed about amid hardships and priva- 

 tions, of all of which he has left a narrative, interesting as a romance, 

 in hi* ' L'Hi.itoire do ma Jeuueaso." 



While yet in the lazaretto, he received a letter from Humboltlt the 

 commencement of a lasting friendship with the illustrious Prussian. 



BIOQ. DIV. VOt. J. 



Tenderly attached to his mother, his first visit was to her at Perpignan. 

 She had mourned him as dead. 



Arago hastened to Paris to communicate his observations to the 

 Academy and the Bureau des Longitudes. Though but twenty-three 

 years of age, he had already gained a reputation by his labours and 

 misfortunes; and the death of Lalande having left a vacancy in the 

 Academy, he was elected a member by 47 out of 50 votes on the 17th 

 of September, and had the honour of the usual presentation to the 

 emperor. Thereafter Arago's influence was felt in the learned body ; 

 and his opposition to unworthy candidates brought him at times into 

 collision with some of the most eminent of his colleagues. Before the 

 close of 1809 he was appointed assistant astronomer to the Observatory, 

 and to succeed Monge in the chair of analytical mathematics at the 

 Polytechnic School. 



In 1811, taking up the researches of Malus, he read a paper to the 

 Academy in which knowledge of the laws of light was greatly extended, 

 and the changes described that take place in polarised rays on passing 

 through different kinds of crystalline plates. The phenomena of 

 colour, of intensity, of rotation, and of reflection were examined, and 

 in a way that laid the foundation of that branch of physical optics 

 known as ' chromatic polarisation ; ' and the interesting fact was first 

 announced, that " while the light from a clouded sky undergoes no 

 modification, that reflected from the atmosphere when the sky is 

 unclouded is polarised, the intensity of the polarisation varying with 

 the hour of the day and the position of the point with respect to the 

 sun." 



In 1812, authorised by the Bureau des Longitudes, Arago com- 

 menced that course of lectures on astronomy and kindred subjects 

 which he continued up to 1845 with the most brilliant success. The 

 high and the low thronged to hear him ; the learned to catch his ani- 

 mated manner and lucid style the many to be charmed. As the 

 present emperor said, when a captive at Ham, Arago " possessed in a 

 high degree those two faculties so difficult to meet with in the same 

 man that of being the grand-priest of science, and of being able to 

 initiate the vulgar into its mysteries." The effect was heightened by 

 the tall commanding form of the lecturer, his full sonorous voice, his 

 striking features, and dark piercing eyes, shaded by thick bushy brows. 



Conjointly with Gay-Lussac, Arago established the ' Annales de 

 Chimie et de Physique" in 1816 a serial still published, and much 

 valued by scientific men. In the same year he announced what has 

 been received as a crucial experiment, demonstrating the truth of the 

 undulatory theory of light over the rival theory of emission. Young 

 had shown in his experiments that the interposition of an opaque 

 screen in the path of a ray under certain circumstances, prevents the 

 formation of fringes. Arago found that the ray was only retarded, 

 and that by a modification of the apparatus the fringes were still 

 discernible. 



In 1816, also, Arago for the first time visited England, whera he 

 made the acquaintance of Young and other eminent men of science. 

 With a Frenchman's feeling, he had a painful dislike to hear any allu- 

 sion to the battle of Waterloo ; and while in London he positively 

 refused an invitation to see Waterloo Bridge. His entertainers adopted 

 the stratagem of proposing an excursion on the Thames, which, being 

 accepted, the party descended the river admiring the prospect, and 

 presently coming to the imposing structure of granite then stretching 

 fresh and new from side to side, Arago was asked for his opinion of it. 

 He perceived the trick, and replied " Your bridge has at least an arch 

 too many ; and that one, to be in its place, should be transported to 

 Berlin." 



Another task commenced by Arago in 1818, again in conjunction 

 with Biot, was the connection of the French arc with the English arc 

 by a system of signals and measurements from one side of the channel 

 to the other. The results, together with those of the Spanish triangu- 

 lation, were published by order of the Bureau des Longitudes, in a 

 volume entitled ' Recueil d'Observatious Ge"odesiques, Aetronomiques, 

 et Phys'ques." In 1319, jointly with Fresuel, Arago published a series 

 of experiments on the action exercised by polarised rays on each other, 

 singularly remarkable for the ingenuity of the methods employed. 

 Space fails here to give the details ; but it was by means of these experi- 

 ments that Fresnel was enabled to give a complete explanation of the 

 production of colours in crystalline plates, which had been referred by 

 Young to the interference of transmitted rays. The co-operation of 

 the two savants produced happy results ; for Arago, though rich in 

 inventive faculty, lacked the perseverance which works a thought out 

 to its ultimate consequences. " We complete one another," he used 

 to say ; " I know how to point out the difficulty, and Fresnel how to 

 conquer it." 



In 1820 Arago took up a new line of inquiry. Having witnessed a 

 demonstration of Oersted's discovery at Geneva, he repeated it before 

 the Academy, and with further results. The Danish philosopher had 

 shown that a voltaic current passing along a wire would deflect a 

 magnetic needle : "Arago found that non-magnetic substances were 

 equally affected ; that bars of iron and steel became temporary mag- 

 nets, and lost their magnetism with the cessation of the current. He 

 proved moreover the best magnet to be a steel bar inclosed by a helix 

 of copper-wire, to which we owe the discovery of the electro-magnet, 

 and all that has since been accomplished thereby. Four years later 

 other facts were published. Arago showed that metals not magnetic 



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