xviii BIOGRAPHICAL NOTICE. 



errors of the mean longitude of Uranus, after taking account of the disturbing effect 

 of the new planet, the errors being small except in the case of Flamsteed's observation 



of 1690 \ 



On November 10, 1845, Le Verrier presented to the French Academy an elaborate 

 investigation of the perturbations of Uranus produced by Jupiter and Saturn, in which 

 he pointed out several small inequalities which had previously been neglected. After 

 taking these into account he still found that the theory was quite incapable of 

 explaining the observed irregularities of the motion of Uranus. 



On June 1, 1846, Le Verrier presented to the French Academy his second memoir 

 on the theory of Uranus. After reducing afresh nearly all the existing observations, 

 he came to the conclusion that there was no other possible explanation of the 

 discordances except that of a disturbing planet exterior to Uranus. He investigated 

 the elements of the orbit of such a planet, and assuming its mean distance to be 

 double that of Uranus, and its orbit to be in the plane of the ecliptic, he gave as 

 the most probable result that the value of the true longitude of the disturbing body 

 for January 1, 1847 was about 325, and that it was not likely that this place was in 

 error by so much as 10. Neither the elements of the orbit nor the mass of the 

 planet were given. 



The position thus assigned by Le Verrier to the disturbing planet differed by only 

 1 from that given by Adams in the paper which he had left at the Royal Observatory 

 more than seven months before. As will be mentioned subsequently, Le Verrier's third 

 memoir, containing the elements of the orbit, was communicated to the French Academy 

 on August 31, 1846. 



On July 9, 1846 the Astronomer Royal, who was then staying with Dean Peacock 

 at Ely, wrote a letter to Challis suggesting that search should be made for the new 

 planet with the Northumberland Equatorial at Cambridge, and offering to supply him 

 with an assistant if he were unable himself to make the examination ; and on July 13 

 he transmitted to Challis a paper of suggestions with respect to the proposed sweep for 

 the planet, which was to extend over a part of the heavens 30 long in the direction of 

 the ecliptic, and 10 broad, having the theoretical place of the planet as its centre. On 

 July 18, Challis, who had been absent from Cambridge, replied to these communications, 

 stating that he had determined to sweep for the hypothetical planet himself, and that 

 he should therefore not require the services of an assistant. The actual search for the 

 planet was commenced by Challis with the Northumberland telescope on July 29, 1846, 

 three weeks before the planet was in opposition, and the observations were continued 

 steadily until September 29. The plan adopted was to make three sweeps over the 

 whole zone, completing one sweep before commencing the next, and mapping the positions 

 of the stars. When the observations were completed, a planet could be at once detected 

 by its motion in the interval. For the first few nights the telescope was directed to the 

 part of the zone in the immediate neighbourhood of the place indicated for the planet 

 by theory. 



On September 2, in a letter to the Astronomer Royal, Challis said that he had lost 

 no opportunity of searching for the planet, and that the nights being pretty good he had 



1 A facsimile of this paper is given after p. liv. 



