the Planet exterior to Uranus. 515 



since the receipt of your letter have looked more carefully to 

 it. It is a puzzling subject, but I give it as my opinion, with- 

 out hesitation, that it is not yet in such a state as to give the 

 smallest hope of making out the nature of any external action 

 on the planet. Flamsteed's observations I reject (for the pre- 

 sent) without ceremony ; but the two observations by Bradley 

 and Mayer cannot be rejected. Thus the state of things is 

 this, — the mean motion and other elements derived from the 

 observations between 1781 and 1825 give considerable errors 

 in 1750, and give nearly the same errors in 1834, 'when the 

 planet is at nearly the same part of its orbit. If the mean 

 motion had been determined by 1750 and 18 34, this would 

 have indicated nothing; but the fact is, that the mean 

 motions were determined (as I have said) independently. 

 This does not look like irregular perturbation. The obser- 

 vations would be well reconciled if we could from theory 

 bring in two terms, one a small error in Bouvard's excen- 

 tricity and perihelion, the other a term depending on twice 

 the longitude. The former, of course, we could do; of the 

 latter there are two, viz. a term in the equation of the centre, 

 and a term in the perturbations by Saturn. The first I 

 have verified completely (formula and numbers); the second 

 I have verified generally, but not completely: I shall, when 

 I have an opportunity, look at it thoroughly. So much for 

 my doubts as to the certainty of any extraneous action. But 

 if it were certain that there were any extraneous action, 1 

 doubt much the possibility of determining the place of a planet 

 which produced it. I am sure it could not be done till the 

 nature of the irregularity was well determined from several 

 successive revolutions." 



It will readily be understood that I do not quote this letter 

 as a testimony to my own sagacity ; but I think it deserving 

 of production, as showing the struggle which was made twelve 

 years ago to explain the motions of Uranus, and the difficulty 

 which seemed to envelope the subject. With regard to my 

 last sentence, I think it likely that the same difficulty would 

 still have been felt, if the theorists who entered seriously upon 

 the explanation of the perturbations had not trusted more 

 confidently to Bode's law of distances than I did myself. 



In the year 1836, having quitted the Observatory of Cam- 

 bridge, I completed the reduction of the planetary observa- 

 tions made there during the years 1833, 1834, 1835, in such 

 a form as to exhibit the heliocentric errors of the tabular 

 places of Uranus, together with theeffect of errors of the tabular 

 radius vector. The memoir containing these reductions was 



