BIOGRAPHICAL NOTICE. xxxi 



which will explain the error of longitude will also explain the error of radius vector. If, 

 after Adams had satisfactorily explained the error of longitude, he had (with the numerical 

 values of the elements of the two planets so found) converted his formulae for pertur- 

 bation of radius vector into numbers, and if these numbers had been discordant with the 

 observed numbers of discordance of radius vector, then the theory would have been false, not 

 from any error of Adams's, but from a failure in the law of gravitation. On this question 

 therefore turned the continuance or fall of the law of gravitation. This, it appears to 

 me, has been totally overlooked in your letter. It was a question of vast importance. 



" The progress of science almost always depends on questions of this kind. Thus, 

 in Chemistry, the phlogistic theory explained the concurring facts of oxidation of metals 

 and vitiation of air, or gaseous formation in water. But did it also account for the 

 increased weight of the metal ? No. Then it was false. Laplace's notion of forces gave 

 an explanation of the course of extraordinary pencils of light. But did it or could it 

 give an explanation also of the separation of pencils and of their polarisation ? No. Then 

 it was false. 



"The theory of gravitation might have been in the same predicament with regard 

 to Uranus. Adams's answer would have made this satisfactory. . . . What could be the 

 reason of Adams's silence, I could not guess. It was so far unfortunate that it inter- 

 posed an effectual barrier to all further communication. It was clearly impossible for 

 me to write to him again." 



Looking back now upon Adams's achievement, which, as has been truly said, belongs 

 at once to the science and to the romance of astronomy, there are several points that 

 stand out as very remarkable : his extreme youth when he attacked, unaided, so difficult a 

 problem, and steadily carried it through to success; his complete faith in the Newtonian 

 law and in the results of his own mathematics; and his extreme modesty. As soon as he 

 took his degree in 1843 he devoted his whole leisure, in term time at Cambridge, and 

 in vacations in Cornwall, to the new planet's orbit, without assistance or encouragement 

 from anyone. How quietly and unassumingly he pursued his investigations is shown 

 by the fact that at the time of the finding of the planet his name was only known 

 to Airy, Challis, Herschel, Earnshaw, and a few intimate university friends of his own 

 standing. He was perfectly convinced of the reality of the planet from the first, and 

 of the approximate accuracy of the place he had assigned to it; and in the paper 

 which he placed in the hands of Challis in September, 1845, he used the words " the 

 new planet." 



Although containing no new facts it may be well to conclude the account of 

 Adams's researches on the new planet with the following extract from a letter written 

 by him at the time (November 26, 1846) to Professor James Thomson: 



" On considering the subject it appeared to me that by far the most probable 

 hypothesis that could be formed to account for these irregularities was that of the existence 

 of an exterior undiscovered planet whose action on Uranus produced the disturbances in 

 question. None of the other hypotheses that had been thrown out seemed to possess the 

 slightest claims to attention, as they were all improbable in themselves, and incapable of 

 being tested by any exact calculation. Some had even supposed that, at the great distance 

 of Uranus from the Sun, the law of attraction became different from that of the inverse 

 square of the distance, but the law of gravitation was too firmly established for this to 

 A. e 



