1902 Encyclopedia > Joseph-Louis Lagrange

Joseph-Louis Lagrange
Italian-French mathematician and astronomer
(1736-1813)




JOSEPH LOUIS LAGRANGE (1736-1813), a mathe-matician of the highest rank, was born at Turin, January 25, 1736. He was of French extraction, his great grand-father, a cavalry captain, having passed from the service of France to that of Sardinia, and settled in Turin under Emmanuel II. His father, Joseph Louis Lagrange, married Maria Theresa Gros, only daughter of a rich physician at Cambiano, and had by her eleven children, of whom only the eldest (the subject of this notice) and the youngest survived infancy. From his post as treasurer at war, as well as through his wife, he derived ample means, which he, however, lost by rash speculations, a circumstance regarded by his son as the prelude to his own good fortune; for had he been rich, he used to say, he might never have known mathematics.

The geniug of Lagrange did not at once take its true bent. His earliest tastes were literary rather than scientific, and he learned the rudiments of geometry during his first year at the college of Turin, without difficulty, but without distinction. The perusal of a tract by Halley (Phil. Trans., vol. xviii. p. 960) roused his enthusiasm for the analytical method, of which he was destined to develop the utmost capabilities. He now entered, without aid or guidance save those afforded by his own unerring tact and vivid apprehension, upon a course of study which, in two years, placed hirn on a level with the greatest of his contemporaries. At the age of nineteen he communicated to the celebrated Euler his idea of a general method of dealing with " isoperimetrical" problems, known later as the Calculus of Variations. It was eagerly welcomed by the Berlin mathematician, who had the generosity to with-hold from publication his own further researches on the subject, until his youthful correspondent should have had time to complete and opportunity to claim the invention. This prosperous opening gave the key-note to Lagrange's career. Appointed, in 1754, professor of geometry in the royal school of artillery, he formed with some of his pupils—for the most part his seniors—friendships based on community of scientific ardour. With the aid of the Marquis de Saluces and the eminent anatomist Cigna, he founded in 1758 a society which rose later to the dignity of the Turin Academy of Sciences. The first volume of its memoirs, published in the following year, contained a paper by Lagrange entitled Pecherches sur la nature et la propagation du son, in which the power of his analysis and his address in its application were equally conspicuous. Without assumption, but without hesitation, he made his first appearance in public as the critic of Newton, and the arbiter between D'Alembert and Euler. By considering only the particles of air found in a right line, he reduced the problem of the propagation of sound to the solution of the same partial differential equations that include the motions of vibrating strings, and demonstrated the insuffi-ciency of tbe methods employed by both his great contem-poraries in dealing with the latter subject. He further treated in a masterly manner of echoes and the mixture of sounds, and explained the phenomenon of grave harmonics as due to the occurrence of beats so rapid as to generate a musical note. This was followed, in the second volume of the Miscellanea Taurinensia (1762) by his "Essai d'une nouvelle méthode pour déterminer les maxima et les minima des formules intégrales indéfinies," together with the application of this important development of analysis to the solution of several dynamical problems, as well as to the demonstration of the mechanical principle of "least action." The essential point in his advance on Euler's mode of investigating curves of maximum or minimum consisted in his purely analytical conception of the subject. He not only freed it from all trammels of geometrical construction, but by the introduction of the symbol S gave it the efficacy of a new calculus. He is thus justly re-garded as the inventor of the "method of variations"— a name supplied by Eulor in 1766.

By these performances Lagrange found himself, at the age of twenty-six, on the summit of European fame. But such a height had not been reached without cost. Intense application during early youth had checked his growth, and weakened a constitution never robust. Accesses of feverish exaltation culminated, in the spring of 1761, in an attack of bilious hypochondria, which permanently lowered the tone of his nervous system, and rendered him liable, throughout his life, to recurrences of the same complaint at the same time of year. Best and exercise, however, temporarily restored his health, and he gave proof of the undiminished vigour of his powers by carrying off, in 1764, the prize offered by the Paris Academy of Sciences for the best essay on the libration of the moon. His treatise was remarkable, not only as offering a satis-factory explanation of the coincidence between the lunar periods of rotation and revolution, but as containing the first employment of his radical formula of mechanics, obtained by combining with the principle of D'Alembert that of virtual velocities. His success encouraged the Academy to propose, in 1766, as a theme for competition, the hitherto unattempted theory of the Jovian system. The prize was again awarded to Lagrange ; and he subsequently earned the same distinction with essays on the problem of three bodies in 1772, on the secular equation of the moon in 1774, and in 1778 on the theory of cometary perturba-tions.

He had in the meantime gratified a long felt desire by a visit to Paris, where he enjoyed the keen and stimulating-delight of conversing with such mathematicians as Clairaut, D'Alembert, Condorcet, and the Abbé Marie. An attack of illness frustrated his design of extending his journey to London, and he returned, though not for long, to the comparative isolation of the Piedmontese capital. The post of director of the mathematical department of the Berlin Academy (of which he had been a member since 1759) becoming vacant by the removal of Euler to St Petersburg, both he and D'Alembert united, by unpreme-ditated concert, to recommend Lagrange as his successor. Euler's eulogium was enhanced by his desire to quit Berlin, D'Alembert's by his dread of a royal command to repair thither ; and the result was that an invitation, conveying the wish of the " greatest king in Europe " to have the " greatest mathematician " at his court, was sent to Turin. On November 6, 1766, Lagrange was installed in his new position, with a salary of 6000 francs, ample leisure for scientific research, and an amount of royal favour sufficient to secure him respect without exciting envy. The national jealousy of foreigners, it is true, was at first a source of annoyance to him ; but such prejudices were gradually disarmed by the mild inoffensiveness of his demeanour, and by his strict adherence to a policy of non-intervention outside his owm immediate domain. We are told that the universal example of his colleagues, rather than any desire for female society, impelled him to matrimony ; an excess of home-sickness, however, probably directed his choice towards a lady of the Conti family (related to his own by a previous alliance), who, by his request, joined him at Berlin. The experiment was cut short by a lingering illness, during which he devoted all his time, and a con-siderable store of medical knowledge, to the care of the dying woman.





The long series of memoirs—some of them complete treatises of great moment in the history of science— communicated by Lagrange to the Berlin Academy between the years 1767 and 1787 were not the only fruits of his exile on the banks of the Spree. His Mécanique Analytique, the production in which his genius most fully and characteristically displayed itself, was due to the same period. This great work was the perfect realization of a design present to the mind of its author almost from boy-hood, and of which he had given a clear though concise sketch in his first published essay.1 Its scope may be briefly described as the reduction of the theory of mechanics to certain general formulas, from the simple development of which should be derived the equations necessary for the solution of each separate problem. From the fundamental principle of virtual velocities, which thus acquired a new significance, Lagrange deduced, with the aid of the calculus of variations, the whole system of mechanical truths, by processes so elegant, lucid, and harmonious as to constitute, in Sir William Hamilton's words, "a kind of scientific poem." This unification of method was one of matter also. By his mode of regarding a liquid as a material system characterized by the unshackled mobility of its minutest parts, the separation between the mechanics of matter in different forms of aggregation finally disappeared, and the fundamental equation of forces was for the first time extended to hydrostatics and hydrodynamics. Thus a universal science of matter and motion was derived, by an unbroken sequence of deduction, from one radical principle ; and analytical mechanics assumed the clear and complete form of logical perfection which it now wears.

A publisher having with some difficulty been found, the book appeared in Paris, under the supervision of Legendre, in 1788. But before that time Lagrange himself was on the spot. After the death of Frederick the Great, his presence was competed for by the courts of France, Spain, and Naples, and a residence in Berlin having ceased to possess any attraction for him, he removed to Paris in 1787. His reception was most flattering. Marie Antoinette warmly patronized him. He was lodged in the Louvre, received the grant of an income equal to that hitherto enjoyed by him, and, with the title of "veteran pensioner " in lieu of that of " foreign associate " (conferred in 1772), the right of voting at the deliberations of the Academy. In the midst of these distinctions, a profound melancholy seized upon him. His mathematical enthu-siasm, hitherto the happiness of his life, was for the time completely quenched, and during two years the printed volum.3 of his Mécanique, which he had seen only in manuscript, lay unopened beside him. He relieved his dejection with miscellaneous studies, especially with that of chemistry, which, in the new form given to it by Lavoisier, he found " aisée comme l'algèbre." The disastrous crisis of the Revolution roused him once more to activity and cheerfulness. Curiosity impelled him to remain and watch the progress of such a novel pheno-menon ; but curiosity was changed into dismay as the terrific character of the phenomenon unfolded itself. He now bitterly regretted his temerity in braving the danger. " Tu l'as voulu " he would repeat self-reproachfully. Even from revolutionary tribunals, however, the name of Lagrange uniformly commanded respect. His pension was continued by the National Assembly, and he was partially indemnified for the depreciation of the currency by remunerative appointments. Nominated president of the Academical commission for the reform of weights and measures, his services were retained when its " purification " by the Jacobins removed his most distinguished colleagues. He again sat on the commission of 1799 for the actual construction of the metrical system, and by his zealous advocacy of the decimal principle of subdivision largely contributed to its adoption.

The interval had, however, been marked by some of the most considerable events in the placid life of our mathema-tician. On the 31st of May 1792 he married Mademoiselle Lemonnier, daughter of the astronomer of that name, a young and beautiful girl, whose devotion ignored disparity of years, and formed the one tie with life which Lagrange found it hard to break. He had no children by either marriage, and never regretted their absence. Although specially exempted from the operation of the decree of October 1793, imposing banishment on foreign residents, he took alarm at the fate of Bailly and Lavoisier, and pre-pared to resume his former situation in Berlin. His design was frustrated by the establishment of and his official connexion with the École Normale, and then the Ecole Polytechnique. The former institution had an ephemeral existence, and his lectures there were consequently few and elementary ; but amongst the benefits derived from the foundation of the École Polytechnique one of the greatest, it has been observed, was the restoration of Lagrange to mathematics. The remembrance of his teachings was long treasured by such of his auditors—amongst whom were Delambre and Lacroix—as were capable of appreciating them. In expounding the principles of the differential calculus, he started, as it were, from the level of his pupils, and ascended with them by almost insensible gradations from elementary to abstruse conceptions. He seemed, not a professor amongst students, but a learner amongst learners ; pauses for thought alternated with luminous exposition ; invention accompanied demonstra-tion ; and thus originated his Théorie des fonctions analy-tiques (Paris, 1797). The leading idea of this remarkable work was contained in a paper published in the Berlin Memoirs for 1772. Its object was the elimination of the to some minds unsatisfactory conception of the infinite from the metaphysics of the higher mathematics, and the substitution for the differential and integral calculus of an analogous method depending wholly on the serial develop-ment of algebraical functions. By means of this " calculus of derived functions " Lagrange hoped to give to the solu-tion of all analytical problems the utmost " rigour of the demonstrations of the ancients";3 but it cannot be said that the attempt was successful. The validity of his funda-mental position was impaired by the absence of a well-constituted theory of series ; the notation employed was inconvenient, and was abandoned by its inventor in the second edition of his Mécanique ; while his scruples as to the admission into analytical investigations of the idea of limits or vanishing ratios have long since been laid aside as idle. Nowhere, however, were the keenness and clearness of his intellect more conspicuous than in this brilliant effort, which, if it failed in its immediate object, was highly effective in secondary results. His purely abstract mode of regarding functions, apart from any mechanical or geometrical considerations, led the way to a new and sharply characterized development of the higher analysis in the hands of Cauchy, Jacobi, and others.4 The Théorie des Fonctions is divided into three parts, of which the first explains the general doctrine of functions, the second deals with its application to geometry, and the third with its bearings on mechanics.

On the establishment of the Institute, Lagrange was placed at the head of the section of geometry ; he was one of the first members of the Bureau des Longitudes ; and his name appeared in 1791 on the list of foreign members of the Royal Society. On the annexation of Piedmont to France in 1796. a touching compliment was paid to him in the person of his aged father. By direction of Talleyrand, then minister for foreign affairs, the French commissary repaired in state to the old man's residence in Turin, to congratulate him on the merits of his son, whom they declared " to have done honour to mankind by his genius, and whom Piedmont was proud to have produced, and France to possess." Bonaparte, who styled him " la haute pyramide des sciences mathématiques," loaded him with personal favours and official distinctions. He became a senator, a count of the empire, a grand officer of the legion of honour, and just before his death received the grand cross of the order of reunion.

The preparation of a new edition of his Mécanique, to which he devoted himself with extraordinary zeal, exhausted his already failing powers. Frequent fainting fits gave presage of a speedy end, and on the 8th of April 1813 he had a final interview with his friends Lacépède, Monge, and Chaptal. He spoke with the utmost calm of his approaching death ; " c'est une dernière fonction," he said, "qui n'est ni pénible ni désagréable." He, however, looked forward to a future meeting, when he promised to complete the autobiographical details which weakness obliged him to interrupt. They remained untold, for he died two days later, April 10, at the age of seventy-seven, and was buried in the Pantheon, the funeral oration being pronounced by Laplace and Lacépède.





Lagrange would never allow his portrait to be painted, holding that a man's works, not his features, deserve remembrance. From a sketch, however, obtained by stealth at a meeting of the Institute, coupled with the descriptions of those who knew him, we can, in some sort, construct an image of his mild and venerable aspect. He was of the middle height, with a slight, well-proportioned figure. His head was finely formed, though not massive ; his features strongly marked, with a stamp of grave and noble beauty; eyes ashy blue, habitually cast down in meditation, but when raised, clear and penetrating ; com-plexion pale and faded. The whole physiognomy was more expressive of benignity than of strength, and his social attitude was one of deprecation rather than of self-assertion. He was timid and affable in conversation, slow to give his opinion, though frequently betraying, by his remarks even on subjects alien to his habitual studies, unexpected stores of information and depths of thought. The phrase " Je ne sais pas " became habitual with him, serving to express his sense of failure in the search for words to fit accurately with ideas always precise. Of music he used to say " Je l'aime, parce qu'elle m' isole " ; and his most abstruse reasonings were frequently pursued under its soothing influence. The sight of suffering was intolerable to him ; he abhorred controversy, tolerated—perhaps unduly—what he could not approve, and was emphatically, in his own phrase, " philosophe sans crier."

The delicacy of his health demanded precautions ex-aggerated, under the influence of nervous anxiety, into minute watchfulness. He observed a scrupulous regimen, living mainly on fruit and vegetables, and his temperance doubtless helped to keep his faculties unimpaired to the last. By self-imposed rules of study, he regulated his vast capability of work as strictly as if it had been a machine entrusted to his care. It was one of his maxims that the mind gains full command over its powers only by exercise and discipline. He had learned from Frederick the Great always to do the same things at the same hours, assigning the most difficult to the morning. Each day he set himself a task for the next, and from the first aimed at mastering certain points of his subject, with a view to inventing improvements. He always read with a pen in his hand, developing the methods of his author as he proceeded ; and his own works were so profoundly meditated that they were usually written without erasures.

Amongst the brilliant group of mathematicians whose magnani-mous rivalry contributed to accomplish the task of generalization and deduction reserved for the 18th century Lagrange occupies an eminent place. It is, however, by no means easy to distinguish and apportion the respective merits of the competitors. This is especially the case between Lagrange and Euler on the one side, and between Lagrange and Laplace on the other. The calculus of varia-tions lay undeveloped in Euler's mode of treating isoperimetrical problems. The fruitful method, again, of the variation of elements was introduced by Euler, but adopted and perfected by Lagrange, who first recognized its supreme importance to the analytical investigation of the planetary movements. Finally, of the grand series of researches by which the stability of the solar system was ascertained the glory must be almost equally divided between Lagrange and Laplace. In analytical invention, and mastery over the calculus, the Turin mathematician was admittedly unrivalled. Laplace owned that he had despaired of effecting the integration of the differential equations relative to secular inequalities until Lagrange showed him the way. On the other hand, Laplace unquestionably surpassed his rival in practical sagacity, and the intuition of physical truth. Lagrange saw in the problems of nature so many occasions for analytical triumphs ; Laplace regarded analytical triumphs as the means of solving the problems of nature. One mind seemed, as it were, the complement of the other ; and both united in honourable rivalry formed an instrument of unexampled perfection for the investigation of the celestial machinery. "What may be called Lagrange's first period of research into planetary perturbations extended from 1774 to 1784 (see ASTRONOMY, vol. ii. p. 761). The remarkable group of treatises communicated, 1781-84, to the Berlin Academy was designed, but did not prove, to be his final contribution to the theory of the planets. After an interval of twenty-four years the subject, having been reopened by Poisson in a paper read June 20, 1808, was once more attacked by Lagrange with all his pristine vigour and fertility of invention. Resuming the inquiry into the invariability of mean motions, Poisson carried the approximation, with Lagrange's formulée, as far as the squares of the disturbing forces, hitherto neglected, with the same result as to the stability of the system. He had not, however, attempted to include in his calculations the orbital variations of the disturbing bodies ; but Lagrange, by the happy artifice of transferring the origin of coordinates from the centre of the sun to the centre of gravity of the sun and planets, obtained a simplification of the formulae, by which the same analysis was rendered equally applicable to each of the planets severally. It deserves to be recorded as one of the numerous coin-cidences of discovery that Laplace, on being made acquainted by Lagrange with his new method, produced analogous expressions, to which his independent researches had led him. The final achieve-ment of Lagrange in this direction was the extension of the method of the variation of arbitrary constants, successfully used by him in the investigation of periodical as well as of secular inequalities, to any system whatever of mutually interacting bodies. "Not without astonishment," even to himself, regard being had to the great generality of the differential equations, he reached a result so wide as to include, as a particular ease, the solution of the planetary problem recently obtained by him. He proposed to apply the same principles to the calculation of the disturbances produced in the rotation of the planets by external action on their equatorial pro-tuberances, but was anticipated by Poisson, who gave formulas for the variation of the elements of rotation strictly corresponding with those found by Lagrange for the variation of the elements of revolution. The revision of the Mécanique Analytique was undertaken mainly for the purpose of embodying in it these new methods and final results, but was interrupted, when two-thirds completed, by the death of its author.

In the advancement of almost every branch of pure mathematics Lagrange took a conspicuous part. The calculus of variations is indissolubly associated with his name. In the theory of numbers he furnished solutions of many of Fermat's theorems, and added some of his own. In algebra he discovered the method of approxi-mating to the real roots of an equation by means of continued frac-tions, and imagined a general process of solving algebraical equations of every degree. The method indeed fails for equations of an order above the fourth, because it then involves the solution of an equa-tion of higher dimensions than the proposed. It possesses, however, the great and characteristic merit of generalizing the solutions of his predecessors, exhibiting them all as modifications of one prin-ciple. To Lagrange, perhaps more than to any other, the theory of differential equations is indebted for its position as a science, rather than a collection of ingenious artifices for the solution of particular problems. To the calculus of finite differences he contributed the beautiful formula of interpolation which bears his name ; although substantially the same result seemed to have been previously obtained by Euler. But it was in the application to mechanical questions of the instrument which he thus helped to form that his singular merit lay. It was his just boast to have transformed mechanics (defined by him as a "geometry of four dimensions") into a branch of analysis, and to have exhibited the so-called mechanical "principles" as simple results of the calculus. The method of "generalized coordinates," as it is now called, by which he attained this result, is the most brilliant achievement of the analytical method. Instead of following the motion of each indi-vidual part of a material system, he showed that, if we determine its configuration by a sufficient number of variables, whose number is that of the degrees of freedom to move (there being as many equations as the system has degrees of freedom), the kinetic and potential energies of the system can be expressed in terms of these, and the differential equations of motion thence deduced by simple differentiation. Besides this most important contribution to the general fabric of dynamical science, we owe to Lagrange several minor theorems of great elegance—among which may be mentioned his theorem that the kinetic energy imparted by given impulses to a material system under given constraints is a maximum. To this entire branch of knowledge, in short, he successfully imparted that character of generality and completeness towards which his labours invariably tended.

His share in the gigantic task of verifying the Newtonian theory would alone suffice to immortalize his name. His co-operation was indeed more indispensable than at first sight appears. Much as was done by him, what was done through him was still more import-ant. Some of his brilliant rival's most conspicuous discoveries were implicitly contained in his writings, and wanted but one step for completion. But that one step, from the abstract to the concrete, was precisely that which the character of Lagrange's mind indis-posed him to make. As notable instances may be mentioned Laplace's discoveries relating to the velocity of sound and the secular acceleration of the moon, both of which were led close up to by Lagrange's analytical demonstrations. In the Berlin Memoirs for 1778 and 1783 Lagrange gave the first direct and theoreti-cally perfect method of determining cometary orbits. It has not indeed proved practically available ; but his system of calcu-lating cometary perturbations by means of " mechanical quadra-tures " has formed the starting-point of all subsequent researches on the subject. His determination of maximum and minimum values for the slowdy varying planetary eccentricities was the earliest attempt to deal with the problem. Without a more accurate knowledge of the masses of the planets than was then possessed a satisfactory solution was indeed imyjossible ; but the upper limits assigned by him agreed closely with those obtained later by Leverrier. As a mathematical writer Lagrange has per-haps never been surpassed. His treatises are not only storehouses of ingenious methods, but models of symmetrical form. The clear-ness, elegance, and originality of his mode of presentation give lucidity to what is obscure, novelty to what is familiar, and sim-plicity to what is abstruse. His genius was one of generalization and abstraction ; and the aspirations of the time towards unity and perfection received, by his serene labours, an embodiment denied to them in the troubled world of polities.

Lagrange's numerous scattered memoirs have been collected and published in seven handsome 4to volumes, under the title Œuvres de Lagrange, publiées sous les soins de M. J. A. Berret, Paris, 1867-77. The first, second, and third sections of this important publication comprise respectively the papers communicated by him to the Academies of Sciences of Turin, Berlin, and Paris ; the fourth includes his miscellaneous contributions to other scientific collections, together with his additions to Euler's Algebra, and his Leçons Elémentaires at the Ecole Normale in 1795. Delambre's notice of his life, extracted from the Mèm. de Vïnstitui, 1S12, is prefixed to the first volume. Besides the separate works already named are Résolution des Équations Numériques, 1798 (2d ed. 1808, 3d ed. 1826), and Leçons sur le Calcul des Fondions, 1S05 (2d ed. 1806), designed as a commentary and supplement to the first part of the Théorie des Fonctions. The first volume of the enlarged edition of the Mécanique appeared in 1811, the second, of which the revision was completed by MM. Prony and Binet, in 1815; A third edition, in 2 vols. 4to, was issued in 1853-55, and a second of the Théorie des Fonctions in 1813. See also Virey and Potel, Précis Historique, 1813; Thomson's Annals of Philosophy, 1813-20, vols. ii. and iv. ; Suter, Geschichte der Math. Wiss.. 1873; Duhring, Kritische Gesch. der allgemeinen Principien der Mechanic, 1877 (2d éd.); Gautier, Essai Historique sur le Problème des trois Corps, 1817; Grant, History of Physical Astronomy, &c. (A. M. C.)


Footnote

4 Suter, Geschichte der math. Wiss., ii. pp. 222-23.

The above article was written by Miss Agnes M. Clerke; made astronomical observations at the Cape of Good Hope, 1888; gained the Actonian Prize for her works on astronomy, 1893; author of A Popular History of Astronomy during the Nineteenth Century and Problems in Astrophysics.




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