560 Royal Astronomical Society. 



" Simeon Denis Poisson (born June 21, 1781, died April 25, 1840) 

 was placed, by common consent, at the head of European analysts 

 on the death of Laplace. He was of humble birth, and was admitted 

 in 1793 a student of the Ecole Poly technique, then newly esta- 

 blished. It is stated, by the historian of this school, that, at the 

 age of eighteen, he submitted to his professor some ameliorations in 

 the method of demonstrating the binomial theorem ; which that 

 teacher, who was no other than Lagrange, read publicly at his next 

 lecture, and which he declared his intention of adopting in future. 



" The life of Poisson was one of quiet and uninterrupted study. 

 He never held any situation connected with politics, nor was in any 

 way, during thirty years, prevented from pursuing his one great ob- 

 ject, the application of the most abstruse and newest developments 

 of the integral calculus to problems of physics. The number of his 

 memoirs is enormous ; to which must be added his elementary trea- 

 tise on mechanics (which stands at the head of all elementary 

 writings on the application of pure analysis to the properties of 

 matter), his treatises on capillary attraction, on heat*, and on the 

 theory of probabilities. 



" It is well known that the energies of Euler, Clairaut, D'Alem- 

 bert, and the younger Bernoullis, had organised the application of 

 mathematics in a manner which made the subsequent triumphs of 

 Lagrange and Laplace seem almost beyond expectation. The power 

 of the pure mathematics seemed to flag, when Fourier first came 

 forward with his applications of definite integrals and periodic series 

 to questions of physics, which seemed to be unconquerable, and of 

 which the difficulties seemed to be altogether inexpressible by ordi- 

 nary analysis. A new school of mathematicians was rapidly formed, 

 in whose hands the mode of expression by definite integrals added 

 one more to the instances in which the happy enunciation of ques- 

 tions was all but their solution. Poisson was one of the first of this 

 school in point of time, and by far the greatest in power. Through- 

 out the major part of his writings we trace the same capability of 

 explaining the most abstruse points with fluent clearness and rigid 

 accuracy, combined with that of conquering the physical difficulties 

 of his problem by the most happy art of adaptation. 



" Many of his memoirs are on the great questions of physical astro- 

 nomy, and it is here that he shows that he was not the accident of 

 a fortunate epoch, but that he could handle the instruments of his 

 two great predecessors with skill resembling their own. Perhaps 

 his greatest achievement in this line is the extension of our know- 

 ledge respecting the stability of the solar system, as far as it may 

 be affected by perturbations of the mean orbital motions, or of the 

 axical rotations. This question does not, as many imagine, owe all 

 its interest either to the predictive power which is sought, or to the 

 grandeur of the problem considered as the path to such a power. It 

 is to be remembered that the connecting constants between the 

 oldest and most recent astronomy are the lengths of the sidereal year 

 and of the day ; and that we cannot assume to talk a common lan- 



[* See Taylor's Scientific Memoirs, vol. i. p. 122.— Ed.] 



