the Planet exterior to Uranus, 529 



more distant than Uranus. Already I have made trial of two 

 different methods of observing. In one method, recommended 

 by Mr. Airy * * * I met with a difficulty which I had 

 anticipated. * * * I adopted a second method."' 



From a subsequent letter (to be cited hereafter), it appears 

 that Professor Challis had commenced the search on July 29, 

 and had actually observed the planet on August 4, 1846. 



Mr. Main's answer to the other parts of this letter, written 

 by my direction, is dated August 8. 



At Wiesbaden (which place I left on September 7), I re- 

 ceived the following letter from Professor Challis: — 



No. 19. Professor Challis to G. B. Airy. 

 [extract.] 



" Cambridge Observatory, Sept. 2, 1846. 

 "I have lost no opportunity of searching for the planet; 

 and the nights having been generally pretty good, I have 

 taken a considerable number of observations : but I get over 

 the ground very slowly, thinking it right to include all stars 

 to 10-1 1 magnitude ; and I find that to scrutinize thoroughly 

 in this way the proposed portion of the heavens will require 

 many more observations than I can take this year." 



On the same day on which Professor Challis wrote this 

 letter, Mr. Adams, who was not aware of my absence from 

 England, addressed the following very important letter to 

 Greenwich : — 



No. 20. J. C. ddams, Esq. to G. B. Airy. 



"St. John's College, Cambridge, Sept. 2, 1846. 

 " In the investigation, the results of which I communicated 

 to you last October, the mean distance of the supposed dis- 

 turbing planet is assumed to be twice that of Uranus. Some 

 assumption is necessary in the first instance, and Bode's law 

 renders it probable that the above distance is not very remote 

 from the truth : but the investigation could scarcely be con- 

 sidered satisfactory while based on anything arbitrary ; and I 

 therefore determined to repeat the calculation, making a dif- 

 ferent hypothesis as to the mean distance. The excentricity 

 also resulting from my former calculations was far too large 

 to be probable; and I found that, although the agreement 

 between theory and observation continued very satisfactory 

 down to 1840, the difference in subsequent years was be- 

 coming very sensible, and I hoped that these errors, as well 

 as the excentricity, might be diminished by taking a different 

 mean distance. Not to make too violent a change, 1 assumed 



