392 ASTROMETRY 



the existing tables of motion were replaced by more accurate ones, 

 as, for example, Hansen's and Olufson's tables of the sun, etc., and 

 Hansen began a new theory of the motions of Venus and Saturn, 

 which he published in his Berlin prize memoirs. Nothing, however, 

 forming a congruous whole was at this time accomplished. The 

 uncertainty of the astronomical constants, the inconsistency between 

 the different determinations of them, the need of a more accurate 

 knowledge of the masses of the large planets for the investigation of 

 the motions of the small planets and comets, the especially unsatis- 

 factory theory of the planet Uranus, all these urged investigators 

 to a thorough revision of the theory of the large planets. At this 

 time, at the opportune moment, appeared the astronomical giant, 

 Leverrier. He was already known to the astronomical world by his 

 work on the secular variations of the orbits of the inner planets, 

 published in the year 1839, when by his wonderful investigations 

 on the motion of the planet Uranus he not only established the exist- 

 ence of an outer planet, but also gave its position so accurately that 

 it was only necessary to direct the telescope to this point of the 

 heavens in order to find it. In connection with the discovery of the 

 planet Neptune, which furnishes one of the most brilliant chapters 

 of the century in celestial mechanics, justice demands that we also 

 mention with equal praise the name of Adams. 



Shortly after the discovery of Neptune, Leverrier began the colos- 

 sal work of the revision of the planetary system, which he was enabled 

 to bring to a conclusion. Leverrier planned his work clearly and 

 systematically, and clearly and systematically carried it out. La- 

 grange's method of the variation of constants proved its power in 

 the most splendid manner. Mathematician and astronomer, Lever- 

 rier gave in a peculiarly harmonious combination only the necessary 

 formulae and these in the simplest manner, and so arranged the astro- 

 nomical material as most completely to suit his problem. I am con- 

 vinced that, no matter how many new revisions may be necessary, 

 Leverrier's Annales de I'Observatoire de Paris will never be forgotten. 



The theory of the inner planets was completed at the end of the 

 sixties, whence new and much-needed values of the masses of the 

 planets Mercury and Venus, as well as of the solar parallax, were 

 obtained. As a result of this investigation it was discovered that the 

 motion of the line of apsides of the planet Mercury was not represented 

 satisfactorily by the theory, whence Leverrier assumed the exist- 

 ence of an unknown intra-Mercurial planet as the cause of the per- 

 turbations. 



What disappointment Leverrier met at that time is well known, but 

 it is also well known that nothing could turn him from his devotion 

 to the great problem, nothing could bend the force of his great genius. 

 Even after his flight from Paris a lead pencil and a copy of logarithm 



