Laplace, PIERRE SIMON, MARQUIS DE, the greatest mathematician and theoretical astronomer since Sir Isaac Newton, born 28th March 1749, was the son of a poor farmer at Beaumont near Trouville, in Normandy. He studied at Caen, through the assistance of some charitable neighbours, and, after teaching mathematics at a military school in his native town, went to Paris and attracted the notice of D'Alembert by a paper on dynamics. When appointed professor in the Royal Military School he soon acquired a reputation by his mastery of the whole range of mathematical science and its application to certain difficulties in practical astronomy—solving a problem which both Euler and Lagrange had grappled with in vain. Chosen an associate of the Academy of Sciences in 1773 and member in 1785, he meanwhile, by his powerful grasp of the analytic method of dealing with gravitating masses, established the great generalisation that our planetary system is stable—that what had been termed irregularities were not disturbing the general equilibrium, but, on the contrary, necessary to it. This complete solution of the 'mechanical problem of the solar system,' as he termed it, has bestowed upon astronomy the 'Three Laws of Laplace.' Here, as well as in his great treatise to be presently mentioned, the special service of Laplace was that he set forth comprehensively in one homogeneous work the leading results which had severally been attained by Newton, Halley, Clairaut, and Euler, at the same time proving their harmony and interdependence. The singular insight of Laplace as an astronomer was apparent in his explanation of the 'secular inequalities' shown by ancient and modern observations in the motions of the planets Jupiter and Saturn. He was the first to construct a complete theory of the satellites of Jupiter, and his investigation of the tidal theory has been characterised by Airy as 'one of the most splendid works' in the history of mathematics.
The successive governments of France agreed in honouring Laplace. He helped to establish the Polytechnic and Normal Schools in Paris, became one of the first members of the Bureau des Longitudes, and soon after was appointed president. After the 18th Brumaire Bonaparte made Laplace Minister of the Interior, though only to supersede him in six weeks' time. In 1799 Laplace entered the senate, where he made a report on the necessity of returning from the Revolution calendar to the Gregorian; in 1803 he was appointed chancellor of the senate. He was created count under the empire, and in 1815 a peer, in 1817 a marquis, by Louis XVIII. His opponents attributed the latter honour to his having voted for the deposition of Napoleon in 1814, accusing him of servility, which was also alleged in 1827 when he became an 'ultra-royalist.' Elected to the Academy in 1816, he was next year appointed president. In his memoir on the 'attraction of spheroids' are first set forth the two celebrated means of applying analysis to physical problems—Laplace's coefficients and the potential function—which are requisite in the theory of attractions and in the more abstruse parts of electrical science.
Besides many original treatises on the application of mathematical methods to lunar and planetary problems, molecular physics, electricity, and magnetism—mostly memoirs to the French academies—Laplace published the four following books. The Mécanique Céleste, with supplements (5 vols. Paris, 1799–1825), stands alone amongst works on mathe- matical astronomy as a systematic demonstration of the highest results in natural philosophy. The Exposition du Système du Monde (1796; 6th ed. 1824) was written for non-mathematicians, and has been admired for the excellent style as well as for its clear and concise statement of all the leading astronomical facts and theories. In a note at the end of the later editions occurs the famous Nebular Hypothesis (see NEBULÆ), which many have deemed to be of not less importance than many of the results obtained by great mathematic effort. As early as 1784 Laplace issued his Théorie du Mouvement et de la Figure des Planètes, and in 1812-14-20 his Théorie analytique des Probabilités. The last remains a classical work to algebraists, though extremely difficult, the theory being applied not only to ordinary chances and averages, but to causes of phenomena and vital statistics.
Laplace was gifted with great power of memory and keen scientific sagacity, as well as with singular skill in interpreting nature by means of the higher mathematics. He showed some personal vanity, but was of an amiable disposition, frequently assisting young men of promising parts. His constant good health was partly attributable to his abstemiousness. Laplace died at Paris, 5th March 1827. In 1878 the Academy undertook a 13-vol. edition of his Œuvres complètes.