S4. Prof. Challis's Observations relative to the New Planet. 



senate, I considered it to be my duty at once to comply with 

 this request. A new body of the solar system has been dis- 

 covered, by means depending on the furthest advances hitherto 

 made in theoretical and practical astronomy, and confirming 

 in a most remarkable manner the theory of universal gravita- 

 tion. It is therefore on every account desirable that the 

 members of the senate should be made fully accjuainted with 

 the part which has been taken by the Cambridge Observatory 

 relatively to this important extension of astronomical science. 

 The observations 1 shall have to speak of, and the reasons 

 for undertaking them, are so closely connected with theore- 

 tical calculations performed by a member of this university 

 to account for anomalies in the motion of the planet Uranus, 

 that the history of the former necessarily involves that of the 

 latter. I hope that for this reason, and because of the pecu- 

 liar nature of the circumstances, I may be allowed to make a 

 communication less formal and restricted in its character than 

 a mere report of observatory proceedings. 



The tables with which the observations of the planet Uranus 

 have been uniformly compared were published by A. Bouvard 

 in 1821. They are founded on a continued series of obser- 

 vations extending ironi 1781, the year of its discovery, to 

 1821. Previous to 1781, it had been accidentally observed 

 seventeen times as a fixed star, the earliest observation of this 

 kind being one by Flamsteed in 1690. Bouvard met with a 

 difficulty in forming his tables. On an attempt to found them 

 upon the ancient as well as the modern observations, it ap- 

 peared that the theoretical did not agree with the observed 

 course of the planet. He thought this might be attributed to 

 the imperfection of the ancient observations, and consequently 

 rejected all previous to 1781 in the formation of the tables 

 finally published. These tables represent well enough the 

 observations in the forty years from 1781 to 1821 ; but very 

 soon after the latter year new errors began to show them- 

 selves, which have gone on increasing to the present time. It 

 was now evident that the ancient observations had been re- 

 jected on insufficient grounds, and that from some unknown 

 cause the theorj' was in fault. Were the tables calculated 

 inaccurately? The difference between observation and theory 

 (amounting in 1841 to 9G" of geocentric longitude) was too 

 great, and Bouvard's calculations were made with too much 

 care to allow of this ex|)lanation. The elfiect of small terms 

 neglected in the calculation of the perturbations caused by 

 Jupiter and Saturn, could not be supposed to bear any con- 

 siderable proportion to the observed amount of error. This 

 state of the theory suggested to several astronomers the idea 



