$232] Hallefs Comet: D'Alembert 295 



perihelion (the point of its orbit nearest the sun, i> in fig. 80) 

 about April i3th of the following year, though owing to 

 various defects in his calculation there might be an error of 

 a month either way. The comet was anxiously watched for 

 by the astronomical world, and was actually discovered by 

 an amateur, George Palitzsch (1723-1788) of Saxony, on 

 Christmas Day, 1758; it passed its perihelion just a month 

 and a day before the time assigned by Clairaut. 



Halley's brilliant conjecture was thus justified ; a new 

 member was added to the solar system, and hopes were 

 raised to be afterwards amply fulfilled that in other 

 cases also the motions of comets might be reduced to 

 rule, and calculated according to the same principles as 

 those of less erratic bodies. The superstitions attached 

 to comets were- of course at the same time still further 

 shaken. 



Clairaut appears to have had great personal charm and 

 to have been a conspicuous figure in Paris society. Un- 

 fortunately his strength was not equal to the combined 

 claims of social and scientific labours, and he died in 1765 

 at an age when much might still have been hoped from his 

 extraordinary abilities.* 



232. Jean-le-Rond D'Alembert was found in 1717 as an 

 infant on the steps of the church of St. Jean-le-Rond in 

 Paris, but was afterwards recognised, and to some extent 

 provided for, by his father, though his home was with his 

 foster-parents. After receiving a fair school education, 

 he studied law and medicine, but then turned his attention 

 to mathematics. He first attracted notice in mathematical 

 circles by a paper written in 1738, and was admitted to 

 the Academy of Sciences two years afterwards. His earliest 

 important work was the Traite de Dynamique (1743), whic'i 

 contained, among other contributions to the subject, the 

 first statement of a dynamical principle which bears his 

 name, and which, though in one sense only a corollary 

 from Newton's Third Law of Motion, has proved to be of 

 immense service in nearly all general dynamical problems, 



* Longevity has been a remarkable characteristic of the great 

 mathematical astronomers : Newton died in his 85th year ; Euler, 

 Lagrange, and Laplace lived to be more than 75, and D'Alembert 

 was almost 66 at his death. 



