with the atheistical opinions he is commonly believed to have
held. His character, notwithstanding the egotism by which it
was disfigured, had an amiable and engaging side. Young
men of science found in him an active benefactor. His relations
with these “adopted children of his thought” possessed a singular
charm of affectionate simplicity; their intellectual progress
and material interests were objects of equal solicitude to him,
and he demanded in return only diligence in the pursuit of
knowledge. Biot relates that, when he himself was beginning
his career, Laplace introduced him at the Institute for the
purpose of explaining his supposed discovery of equations of
mixed differences, and afterwards showed him, under a strict
pledge of secrecy, the papers, then yellow with age, in which he
had long before obtained the same results. This instance of
abnegation is the more worthy of record that it formed a marked
exception to Laplace’s usual course. Between him and A. M. Legendre there was a feeling of “more than coldness,” owing
to his appropriation, with scant acknowledgment, of the fruits
of the other’s labours; and Dr Thomas Young counted himself,
rightly or wrongly, amongst the number of those similarly
aggrieved by him. With Lagrange, on the other hand, he
always remained on the best of terms. Laplace left a son, Charles
Emile Pierre Joseph Laplace (1789–1874), who succeeded to his
title, and rose to the rank of general in the artillery.
It might be said that Laplace was a great mathematician by the original structure of his mind, and became a great discoverer through the sentiment which animated it. The regulated enthusiasm with which he regarded the system of nature was with him from first to last. It can be traced in his earliest essay, and it dictated the ravings of his final illness. By it his extraordinary analytical powers became strictly subordinated to physical investigations. To this lofty quality of intellect he added a rare sagacity in perceiving analogies, and in detecting the new truths that lay concealed in his formulae, and a tenacity of mental grip, by which problems, once seized, were held fast, year after year, until they yielded up their solutions. In every branch of physical astronomy, accordingly, deep traces of his work are visible. “He would have completed the science of the skies,” Baron Fourier remarked, “had the science been capable of completion.”
It may be added that he first examined the conditions of stability of the system formed by Saturn’s rings, pointed out the necessity for their rotation, and fixed for it a period (10h 33m) virtually identical with that established by the observations of Herschel; that he detected the existence in the solar system of an invariable plane such that the sum of the products of the planetary masses by the projections upon it of the areas described by their radii vectores in a given time is a maximum; and made notable advances in the theory of astronomical refraction (Méc. cél. tom. iv. p. 258), besides constructing satisfactory formulae for the barometrical determination of heights (Méc. cél. tom. iv. p. 324). His removal of the considerable discrepancy between the actual and Newtonian velocities of sound,[1] by taking into account the increase of elasticity due to the heat of compression, would alone have sufficed to illustrate a lesser name. Molecular physics also attracted his notice, and he announced in 1824 his purpose of treating the subject in a separate work. With A. Lavoisier he made an important series of experiments on specific heat (1782–1784), in the course of which the “ice calorimeter” was invented; and they contributed jointly to the Memoirs of the Academy (1781) a paper on the development of electricity by evaporation. Laplace was, moreover, the first to offer a complete analysis of capillary action based upon a definite hypothesis—that of forces “sensible only at insensible distances”; and he made strenuous but unsuccessful efforts to explain the phenomena of light on an identical principle. It was a favourite idea of his that chemical affinity and capillary attraction would eventually be included under the same law, and it was perhaps because of its recalcitrance to this cherished generalization that the undulatory theory of light was distasteful to him.
The investigation of the figure of equilibrium of a rotating fluid mass engaged the persistent attention of Laplace. His first memoir was communicated to the Academy in 1773, when he was only twenty-four, his last in 1817, when he was sixty-eight. The results of his many papers on this subject—characterized by him as “un des points les plus intéressans du système du monde”—are embodied in the Mécanique céleste, and furnish one of the most remarkable proofs of his analytical genius. C. Maclaurin, Legendre and d’Alembert had furnished partial solutions of the problem, confining their attention to the possible figures which would satisfy the conditions of equilibrium. Laplace treated the subject from the point of view of the gradual aggregation and cooling of a mass of matter, and demonstrated that the form which such a mass would ultimately assume must be an ellipsoid of revolution whose equator was determined by the primitive plane of maximum areas.
The related subject of the attraction of spheroids was also signally promoted by him. Legendre, in 1783, extended Maclaurin’s theorem concerning ellipsoids of revolution to the case of any spheroid of revolution where the attracted point, instead of being limited to the axis or equator, occupied any position in space; and Laplace, in his treatise Théorie du mouvement et de la figure elliptique des planètes (published in 1784), effected a still further generalization by proving, what had been suspected by Legendre, that the theorem was equally true for any confocal ellipsoids. Finally, in a celebrated memoir, Théorie des attractions des sphéroides et de la figure des planètes, published in 1785 among the Paris Memoirs for the year 1782, although written after the treatise of 1784, Laplace treated exhaustively the general problem of the attraction of any spheroid upon a particle situated outside or upon its surface.
These researches derive additional importance from having introduced two powerful engines of analysis for the treatment of physical problems, Laplace’s coefficients and the potential function. By his discovery that the attracting force in any direction of a mass upon a particle could be obtained by the direct process of differentiating a single function, Laplace laid the foundations of the mathematical sciences of heat, electricity and magnetism. The expressions designated by Dr Whewell, Laplace’s coefficients (see Spherical Harmonics) were definitely introduced in the memoir of 1785 on attractions above referred to. In the figure of the earth, the theory of attractions, and the sciences of electricity and magnetism this powerful calculus occupies a prominent place. C. F. Gauss in particular employed it in the calculation of the magnetic potential of the earth, and it received new light from Clerk Maxwell’s interpretation of harmonics with reference to poles on the sphere.
Laplace nowhere displayed the massiveness of his genius more conspicuously than in the theory of probabilities. The science which B. Pascal and P. de Fermat had initiated he brought very nearly to perfection; but the demonstrations are so involved, and the omissions in the chain of reasoning so frequent, that the Théorie analytique (1812) is to the best mathematicians a work requiring most arduous study. The theory of probabilities, which Laplace described as common sense expressed in mathematical language, engaged his attention from its importance in physics and astronomy; and he applied his theory, not only to the ordinary problems of chances, but also to the inquiry into the causes of phenomena, vital statistics and future events.
The device known as the method of least squares, for reducing numerous equations of condition to the number of unknown quantities to be determined, had been adopted as a practically convenient rule by Gauss and Legendre; but Laplace first treated it as a problem in probabilities, and proved by an intricate and difficult course of reasoning that it was also the most advantageous, the mean of the probabilities of error in the determination of the elements being thereby reduced to a minimum.
Laplace published in 1779 the method of generating functions, the foundation of his theory of probabilities, and the first part of his Théorie analytique is devoted to the exposition of its principles, which in their simplest form consist in treating the successive values of any function as the coefficients in the expansion of another function with reference to a different variable. The latter is therefore called the generating function of the former. A direct and an inverse calculus is thus created, the object of the former being to determine the coefficients from the generating function, of the latter to discover the generating function from the coefficients. The one is a problem of interpolation, the other a step towards the solution of an equation in finite differences. The method, however, is now obsolete owing to the more extended facilities afforded by the calculus of operations.
The first formal proof of Lagrange’s theorem for the development in a series of an implicit function was furnished by Laplace, who gave to it an extended generality. He also showed that every equation of an even degree must have at least one real quadratic factor, reduced the solution of linear differential equations to definite integrals, and furnished an elegant method by which the linear partial differential equation of the second order might be solved. He was also the first to consider the difficult problems involved in equations of mixed differences, and to prove that an equation in finite differences of the first degree and the second order might always be converted into a continued fraction.
In 1842, the works of Laplace being nearly out of print, his widow was about to sell a farm to procure funds for a new impression, when the government of Louis Philippe took the matter in hand. A grant of 40,000 francs having been obtained from the chamber, a national edition was issued in seven 4to vols., bearing the title Œuvres de Laplace (1843–1847). The Mécanique céleste with its four supplements occupies the first 5 vols., the 6th contains the Système du monde, and the 7th the Th. des probabilités, to which the more popular Essai philosophique forms an introduction. Of the four supplements added by the author (1816–1825) he tells us that the problems in the
- ↑ Annales de chimie et de physique (1816), tom. iii. p. 238.
