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nution. It was reserved to Poisson to demonstrate a priori that the 

 non-periodic terms of the order which he considered would mu- 

 tually destroy each other ; a most important conclusion, which re- 

 moved the principal objection that existed to the validity of the 

 demonstration of Lagrange*. 



This brilliant success of Poisson in one of the most difficult pro- 

 blems of physical astronomy, would appear to have influenced him in 

 devoting himself thenceforward almost exclusively to the application 

 of mathematics to physical science ; and the vast number of memoirs 

 and works (amounting to more than 300 in number) which he pub- 

 lished during the last thirty years of his life, made this department 

 of mathematical science, and more particularly whatever related to 

 the action of molecular forces, pre-eminently his own. They compre- 

 hend the theory of waves and of the vibrations of elastic substances, 

 the laws of the distribution of electricity and magnetism, the pro- 

 pagation of heat, the theory of capillary attraction, the attraction 

 of spheroids, the local magnetic attraction of ships, important pro- 

 blems on chances, and a multitude of other subjects, which the 

 time allowed for this notice will not permit me to mention. His 

 well-known treatise on Mechanics is incomparably superior to every 

 similar publication in the clear and decided exposition of principles 

 and methods, and in the happy and luminous combination of the 

 most general theories with their particular and most instructive ap- 

 plications. 



Poisson was not a philosopher who courted the credit of propound- 

 ing original views which did not arise naturally out of the immediate 

 subjects of his researches ; and he was more disposed to extend and 

 perifect the application of known methods of analysis to important 

 physical problems, than to indulge in speculations on the inven- 

 tion or transformation of formulae, which, however new and elegant, 

 appeared to give him no obvious increase of mathematical power in 

 the prosecution of his inquiries. His delight was to grapple with 

 difficulties which had embarrassed the greatest of his predecessors, 

 and to bring to bear upon them those vast resources of analysis, and 

 those clear views of mechanical and physical principles in their most 

 refined and difficult applications, which have secured him the most 

 brilliant triumphs in nearly every department of physical science. 



The confidence which he was accustomed to feel in the results of 

 his analysis — the natural result of his own clear perception of the 

 necessary dependence of the several steps by which they were de- 

 duced — led him sometimes to accept conclusions of a somewhat 



* The publication of this memoir recalled the attention of this illustrious 

 mathematician to a subject which he had long neglected, and gave rise to 

 three of his noblest memoirs. Poisson, in his " Memoire sur le Mouve- 

 Dient de la Lune autour de la Terre," has not satisfactorily shown that 

 the major axis of the moon's orbit contains no argument of long period 

 amongst the terms which involve lower powers of a certain quantity m, 

 which denotes the ratio of the sun's mean motion to that of the moon, 

 than the fourth ; a demonstration of this most important proposition has 

 been given by Sir John Lubbock in the Philosophical Magazine for the 

 present year. 



