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 inclination, 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-
