CH. xxxi. LAGRANGE AND LAPLACE. 277 



there he went to the Academy of Sciences at Berlin, and 

 remained twenty years, during which time he worked 

 out most of his celebrated problems. In 1787 he settled 

 in Paris, where he died in 1813, at the age of seventy 

 seven. 



Pierre Simon Laplace was the son of a farmer, and was 

 born at Beaumont en-Auge, near Honjleur, in 1749. He, 

 too, began work very early in life, for in 1769 the famous 

 geometer D'A.embert was so struck with his talents that he 

 procured for him the chair of Mathematics in the Military 

 School of Paris, and from that time, for more than fifty years, 

 Laplace devoted himself to the pursuit of science, never 

 letting his active life as a politician interfere with his 

 scientific studies. He died in 1827. 



The work which was done both by Lagrange and Laplace 

 in astronomy was purely mathematical, and dealing as it did 

 with some of the most complicated movements of the 

 heavenly bodies, it cannot be rightly understood by any but 

 mathematicians. But some general idea may be formed of 

 the problems they solved, and we will take these in the 

 order of time, for they treated so much of the same ques- 

 tions, one taking up the subject where the other left it, that 

 it is difficult to separate their work. 



Libration of the Moon accounted for by Lagrange, 

 1764-1780. Long before the time of Lagrange it had been 

 known from observation that the moon always turns the 

 same side of her globe towards our earth as she goes round 

 it so that we never see, and never can see, more than one 

 side of her surface, so long as she has the same movement 

 as at present. In 1764 the Academic des Sciences offered 

 a prize for a complete explanation of this curious fact, and 

 Lagrange was thus led to study the question, which he 

 solved quite satisfactorily in 1780. 



