president's address. 23 



small limits. The construction or improvement of these theories 

 from the comparison of the observed and tabular places obtained 

 over a long series of years, is the highest class of modern 

 astronomical research, and it is only undertaken by mathema- 

 ticians specially conversant with gravitational astronomy. 



Perhaps I could not illustrate more clearly the perfect 

 reasoning employed in some of these difficult problems of 

 mathematical astronomy, than by referring to that great triumph 

 of human intellect which culminated in the discovery of the 

 planet Neptune, the most distant known member of the solar 

 system. The problem was indeed a difficult one to solve. For 

 if we wish to determine in what way two known planets of given 

 distance, mass, and other ascertained elements will affect each 

 other, the most skilful mathematicians sometimes fail in explain- 

 ing certain marked peculiarities in their movements, although 

 they are necessary consequences of the relations already known 

 to exist between the two bodies. How much more difficult then 

 it must be to infer from the observed irregularities in the motion 

 of one planet, the distance, mass, and position of another planet 

 hitherto unknow^n. This was, however, the problem that two 

 mathematicians independently attempted to solve. 



Since the publication in 1821 of M. Bouvard's tables of 

 Uranus, the apparently great irregularities in the motion of this 

 planet caused considerable interest, and various explanations 

 were suggested to account for this irregular motion. About the 

 year 1843 it occurred to Mr. J. Couch Adams, a name honoured 

 by all Cornishmen, who had just taken the highest mathematical 

 honours at Cambridge, and shortly afterwards to M. Le Verrier, 

 of Paris, that by taking these apparent deviations from the 

 planet's true motion as a basis of calculation, they might be 

 able, on the assumption that the irregularities were produced by 

 perturbations caused by the attraction of an exterior planet, to 

 point out, by an inverse process of calculation, the exact position 

 in the heavens where such an unknown attracting body would 

 probably be found. Each of the two astronomers was fully 

 convinced in his own mind of the reality of the problem, a belief 

 afterwards confirmed by the discovery of the suspected planet 

 very near the identical places in the heavens indicated by them. 



