Prof. Cliallis's Observations relative to the New Planet. 37 



the evidence which the theory gave of the existence of the 

 planet, that its position was determined but roughly, and that 

 a search for it must necessarily be long and laborious. This 

 was the view I took, and consequently 1 had no thought of 

 commencing the search in 1845, the planet being considerably 

 past opposition at the time Mr. Adams completed his calcu- 

 lations. The succeeding interval to midsummer of 1846 

 was a period of great astronomical activity, the planet Astraea, 

 Biela's double comet, and several other comets successively 

 demanding attention. During this time I had little commu- 

 nication with Mr. Adams respecting the new planet. Atten- 

 tion was again called to the subject by the publication of M. 

 Le Verrier's first researches in the Comptes Rendus for June I, 

 1846. At a meeting of the Greenwich Board of Visitors held 

 on June 29, at which I was present, Mr. Airy announced that 

 M. Le Verrierhad obtained very nearly the same longitude of 

 the supposed planet as that given by Mr. Adams. On July 9 

 I received a letter from Mr. Airy, in which he suggested em- 

 ploying the Northumberland telescope in a systematic search 

 lor the planet, offering at the same time to send an assistant 

 from Greenwich in case I declined undertaking the observa- 

 tions. This letter was followed by another dated July 13, 

 containing suggestions respecting the mode of conducting the 

 observations, and an estimation of the amount of work they 

 might be expected to require. In my answer, dated July 18, 

 I signified the determination I had come to of undertakinc; 

 the search. Various reasons led me to this conclusion. I 

 had already, as Mr. Adams can testify, entertained the idea 

 of making these observations; the most convenient time for 

 commencing them was now approaching ; and the confirma- 

 tion of Mr. Adams's theoretical position, by the calculations 

 of M. Le Verriei", appeared to add very greatly to the proba- 

 bility of success. 1 had no answer to make to Mr. Airy's 

 offer of sending an assistant, as I understood the acceptance 

 of it to imply the relinquishing on my part of the undertaking. 

 I have now to speak of the observations. The plan of 

 operations was formed mainly on the suggestions contained 

 in Mr. Airy's note of July 13. It was recommended to sweep 

 over three times at least, a zodiacal belt 30° long and 10° 

 broad, having the theoretical place of the planet at its centre; 

 to complete one sweep before commencing the next ; and to 

 map the positions of the stars. The three sweeps, it was cal- 

 culated, would take 300 hours of observing. 'Ihis extent of 

 work, which will serve to show the idea entertained of the 

 difliculty ol' the undertaking before the planet was discovered, 

 did not appear to me greater than the case required. It will 



