492 LEVEBRIER AND HIS WORK. 



l3ut the record of imdcrtakings carried to successful endings, and together 

 aggregating the noblest astronomical achievement ever accomplished. 



Urbain Jean Joseph Leverrier was born at St. Lo, in the old Department 

 of JSTorrnand}', France, on March 11, 1811. He was a chosen student and 

 obtained honors in the Polytechnic School, which entitled him to a choice 

 of employment in any of the select branches of the public service he might 

 ■desire. Choosing the position of engineer attached to the administration of 

 'the government tobacco monopoly, in order that he might possess the nec- 

 essary facilities for the continuation of his studies, his attention was first 

 directed to chemical experimentation, .and in 1837 he published his first 

 original investigations, announcing a new combination of phosphorus and 

 oxygen. His jireference, however, was for mathematics, and in 1839 he 

 liegan the colossal astronomical task, the termination of which he himself 

 announced to the French Academy of Sciences on December 21, 1874. 



In order to reach a just estimate of this vast work, it is necessary to recall 

 the fact that in the solar system the mass of the sun is so great that that 

 luminary is capable of swaying the motion of all the planets without being 

 himself disturbed. Although the planets exert an attractive power on the 

 sun, still if their joint attraction were exercised upon him in a straight line, 

 he would not be disturbed by a space equal to his own radius. So vast then 

 is the controlling power of the sun that even the greatest disturbance in the 

 •entire sj'stem (that resulting from the mutual attraction of Jupiter and Sat- 

 urn) is inconsiderably small in comparison. But the fact still remains that 

 the planets do disturb each other in varj^ing degrees, and the more massive 

 the planet the greater its influence upon its neighbors. Consequently and 

 conversely, if we know how much one planet disturbs another, we have a 

 means of determining the mass of the influencing bod3^ 



This determination was the object of Leverrier's inquiry, and he set to 

 work to examine into the motions of the seven planets known at the period 

 when his labors began. It is scarcely possible for any one, not conversant 

 with the delicate and intricate toil of the astronomer, to appreciate the 

 multitudinous perturbing causes which in such an investigation it becomes 

 necessary to take into account. Some idea may, however, be gained from 

 the fact that in determining the earth's motion around the sun — but one 

 part of his subject — Leverrier reviewed and discussed nine thousand distinct 

 observations. "Our conclusion is," he says, referring to these, "that the 

 observations of the sun leave much to be desired, on account of sj'stematic 

 -errors aftecting them ; and there is no discordance between theory and ob- 

 servation which cannot be attributed to errors in observing." 



Still, from these imperfect data, he estimated the sun's apparent monthly 

 displacements and deduced therefrom an estimate of the distance of the sun, 

 showing that the generall}" accepted flgures were too large by between three 

 and four millions of miles. 



Meanwhile, by a most careful analj'sis of all available observations of 

 Uranus, Leverrier had satisfied himself that that planet was imdergoing 



