CHAP. II., 2.] 



PHYSICAL ASTKONOMY LAPLACE. 



23 



Trait'e 



eanique 

 CHleste. 



ir less 

 original 

 than the 

 Principia . 



yet gives 

 him justly 

 a high re- 

 putation. 



dynamics and optics ; and we shall find very few 

 important branches of general physics in which he 

 has not left some permanent record of his interest, 

 in the course of his career of fifty-five years of 

 anxious devotion to science. 



His largest and most systematic work, the Traiti 

 de Mecanique Celeste, in five quarto volumes, was 

 not only most ably executed, but exceedingly well 

 timed. The applications of analysis to physical astro- 

 nomy had been accumulating for nearly a century. 

 Hundreds of memoirs relating to them, dispersed 

 through many volumes in different languages, written 

 with varying ability, in various stages of scientific pro- 

 gress, and with differing notations, presented a mass 

 of reading almost beyond the reach of the most reso- 

 lute student. Laplace undertook to digest the whole 

 into one body of doctrine, composed throughout on 

 a uniform plan, with the best mathematical aids which 

 were known at the commencement of this century. 

 And though improvements and discoveries have been 

 since made, the methods and most of the results of 

 the Mecanique Celeste remain, with little variation, 

 the preferable ones of our own time. As a work of 

 labour, it may compete with the Principia of New- 

 ton ; as an original work, it is of course immeasur- 

 ably inferior. Its principles are, in fact, the same 

 with those laid down in that immortal code, and its 

 deductions are collected (as we have said) from the 

 writings of Clairaut, D' Alembert, Euler,and Lagrange, 

 as well as from the previous memoirs of the author 

 himself. Laplace has been too sparing of his cita- 

 tions and acknowledgments, and a consequence of 

 this literary avarice has been that he is sometimes 

 considered as more of a compiler and less of a dis- 

 coverer than is justly his due. For however ill he 

 could have dispensed with the skilful preparations 

 of his illustrious rivals and contemporaries, his pre- 

 eminent sagacity furnished on several occasions the 

 key-stone of the arch which imparted at once strength 

 and completeness to the fabric. We have seen in 

 the last section that though the credit of the theo- 

 rems respecting the stability of the solar system is very 

 generally attributed to Lagrange, who, indeed, prin- 

 cipally furnished the methods, and gave great gene- 

 rality to the results, yet the capital discovery of the 

 invariability of the major axes of the planetary orbits 

 is due to Laplace. It was he, again, who removed 

 from the theory of gravity the two greatest and 

 most impracticable difficulties with which it had 

 ever been assailed the anomaly of the lunar ac- 

 celerations, and the great inequalities of Jupiter and 

 Saturn, and by so doing rendered it almost infinitely 

 improbable that any future discrepancy should more 

 than temporarily embarrass a theory which had tri- 



umphed in succession over such formidable causes of 

 doubt. True that Lagrange, in his memoir of 1783, 

 had come within a single step of the first of these 

 discoveries, and, by a process of exclusion, had almost 

 forced attention in the right direction respecting the 

 latter ; still Laplace seized the prize in both cases, 

 after a fair, prolonged, and arduous struggle. Now 

 these three discoveries were the greatest in physical 

 astronomy between those of D' Alembert and Clairaut 

 on the precession of the equinoxes, the motion of the 

 lunar apse, and the periodicity of comets, and that 

 of Leverrier and Adams on the perturbations of 

 Uranus about a century later. 



The universal testimony of mathematicians is to 

 the effect that Lagrange was unrivalled as a 

 analyst, in his power of generalization, and in the with La- 

 inherent elegance of his methods ; that Laplace, S ran g e - 

 with nearly equal power in using the calculus, had 

 more sagacity in its mechanical and astronomical 

 applications, or rather, perhaps, we should say, in 

 directing it to the discrimination of causes, and the 

 revelation of consequences. 



In other respects he differed far more widely from (91.) 

 his illustrious compeer. He rather courted popu- His P ul >Kc 

 larity, and was pleased at being considered worthy 

 of political distinction. For a short time he was one 

 of Napoleon's ministers ; but the Emperor, it is said, 

 was more satisfied with his mathematical than with 

 his diplomatic talents. He had none of the shyness of 

 Lagrange, nor his repugnance to general society. He 

 received with affability and kindness those who were 

 introduced to him, and his attentions were after- 

 wards recollected with gratitude by rising men of 

 science abroad. He had a villa at Arceuil, adjoining 

 that of Berthollet, and was one of the original mem- 

 bers of the " Societe d' Arceuil," to whose memoirs 

 he contributed. He exercised a powerful influence and as a 

 in the Academy of Sciences, of which for. a time he ^ ember of 



, ,,tne Aco- 



acted as dictator, and he was not very tolerant ot demy of 

 views in science opposed to his own. The undula- Sciences, 

 tory theory of light he always opposed, and was 

 mainly determined in doing so by the facility with 

 which the attraction of luminous corpuscles could be 

 subjected to calculation. 



The weak point of his scientific character was one so (92.) 

 natural, and perhaps so inseparable from his prevail ^^y^^ 

 ing studies, that it is not fair to criticise it too severely, display. 

 This was a love of analytical display in treating ques- 

 tions which it rather embarrassed than illustrated ; 

 and generally, a disposition to overrate the sphere of 

 mathematical discovery. This he had in common 

 with Euler, to whom he was very superior in physical 

 attainments and sagacity. His language, and that of 

 his eulogists, 1 often amounts to the assumption that 



1 For instance Arago says (speaking of the invariability of the major axes) : " Enfin par la toute-puissance d'une formule 

 mathematique, le monde materiel se trouva raffermi sur ses fondements" (Annuaire, 1844, p. 304). This is indeed the idolatry 

 of mathematics. Many examples may be found in Laplace's writings on Probability ; which occasioned Mr Ellis to say of him, that 

 " to Laplace all the lessons of History were merely confirmations of the ' resultats de calcul.' " To the same effect was the mot 

 of Napoleon, " that Laplace carried into the art of government the principles of the infinitesimal calculus.'' 



