Deceased Fellows : — M. Poisson. 75 



gress which he made at this celebrated school surpassed the most 

 sanguine expectations of his kind patron, M. Billy, and secured him 

 the steady friendship and support of the most distinguished of his 

 teachers. 



In the year 1800, he presented to the Institute a memoir " Sur 

 le nombre d'integrales completes dont les equations aux differences 

 finies sont susceptibles," which cleared up a very difficult and ob- 

 scure point of analysis. It was printed on the recommendation of 

 Laplace and Lagrange in the Memoires des Savaiis Etrangers, an 

 unexampled honour to be conferred on so young a man. 



Stimulated by this first success, we find him presenting a succes- 

 sion of memoirs to the Institute on the most important points of 

 analysis, and rapidly assuming the rank of one of the first geometers 

 of his age. He was successively made Repetiteur and then Professor 

 of the Polytechnic School, Professor at the College de France and 

 the Faculte des Sciences, Member of the Bureau des Longitudes, 

 and finally, in 1812, Member of the Institute. 



His celebrated memoir on the invariability of the major axes of 

 the planetary orbits, which received the emphatic approbation of 

 Laplace, and secured him throughout his life the zealous patronage 

 of that great philosopher, was presented to the Institute in the year 

 1808. Laplace had shown that the periodicity of the changes 

 of the other elements, such as the eccentricity and inclina- 

 tion, depends on the periodicity of the changes of the major 

 axis ; a condition, therefore, which constitutes the true basis 

 of the proof of the stability and permanence of the system of 

 the universe. Lagrange had considered this great problem in the 

 Berlin Memoirs for 1776, and had shown that, by neglecting certain 

 quantities which might possibly modify the result, the expression 

 for the major axis involved periodical inequalities only, and that 

 they were consequently incapable of indefinite increase or dimi- 

 nution. It was reserved to Poisson to demonstrate d 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 tiiencef'orward 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- 



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

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

 three of his noblest memoirs. Poisson, in his " M^moire sur le Mouve- 

 ment 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 fourtli ; a demonsaation of this most important proposition has 

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

 present year. [L. E. & L). Phil. Mag. vol.xvii. p. 338.] 



