269 



by Schroter, a quadrant by Bird, an admirable sextant by Troughton, 

 and a clock by Castens of Bremen. He possessed no transit instru- 

 ment or fixed instruments of any kind ; yet he speedily availed him- 

 self of the circumstances of his locality to determine his time with 

 great accuracy, as well as nearly every element which the peculiar 

 character of his oTDservations rendered necessary ; so fertile are the 

 resources of genius and enterprize to overcome difficulties, which 

 by ordinary men would be abandoned as altogether insuperable. 



Simeon Denis Poisson, one of the most illustrious men of science 

 that Europe has produced, was born at Pithiviers on the 21st of 

 June, 1781, of very humble parentage, and was placed, at the age 

 of fourteen, under the care of his uncle, M. L'Enfant, surgeon, at 

 Fontainebleau, with a view to the study of his profession. It was 

 at the central school of this place that he was introduced to the no- 

 tice of M. Billy, a mathematician of some eminence, who speedily 

 discovered and fostered his extraordinary capacity for mathematical 

 studies. In 1793 he was elected a pupil of the Ecole Polytechnique, 

 which was then at the summit of its reputation, counting amongst 

 its professors Laplace, Lagrange, Fourier, Monge, Prony, BerthoUet, 

 Fourcroy, Vauquelin, Guyton Morveau, and Chaptal. The pro- 

 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 Savans Etra7igers, 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- 



