476 Proceedings. 



were Leverrier, of Paris, and A.dams, of Cambridge. The planet Uranus had 

 been discovered by the elder Herschel in 1781. During the ensuing half 

 century its orbit had been observed and calculated and recalculated over and 

 over again. Its theoretical orbit is of course an ellipse, but its actual and 

 observed orbit differs from its theoretical orbit, that is, its orbit as it ought 

 to be is found to be disturbed or perturbed. These perturbations, as they are 

 called, were accurately observed and recorded not only by Herschel himself 

 but by hosts of astronomers in'all parts of Europe. Now all except a fraction 

 of these perturbations were capable of being accounted for and laid down with 

 precision, as caused by the attraction of Saturn and Jupiter. The combined 

 influence of the smaller planets — Mars, the Earth, Yenus, and Mercury — was 

 so small as to be left out of account ; for, besides their immense distance from 

 Uranus, the size of the Earth as compared with that of Jupiter is about that 

 of a pea to a moderate-sized orange. But it was found that after giving due 

 effect to the attraction of Jupiter and Saturn — which could be exactly 

 estimated — there remained certain perturbations still unaccounted for j and it 

 was conjectured that these might be due to some unknown planetary mass far 

 beyond the orbit of Uranus. Leverrier and Adams, unknown to each other, 

 imposed each upon himself the gigantic labour of determining the place of this 

 unknown planetary mass, by inference from the known and accurately recorded 

 perturbations. The converse process was familiar to mathematical astronomers, 

 that is : — Griven the mass and density of a planet, and its distance from the 

 affected body, the perturbation could be found. But no one, I believe, before 

 their time had had the courage to grapple with the problem : — Given the 

 perturbation of the affected planet, what is the place of the unknown 

 disturbing body ? However, after labour which is almost incomprehensible to 

 persons not in some degree familiar with such calculations, Adams and 

 Leverrier both came to nearly the same conclusion at the same time, the 

 difference between the two results being very trifling. 



In September, 1846, Leverrier wrote to Dr. Galle, of Berlin, announcing 

 the result at which he had arrived, giving him the heliocentric longitude of 

 the supposed planet for the 23rd September, and requesting him to look for 

 the disturbing body in or near the place pointed out. On the 23rd September 

 Dr. Galle, assisted by M. Encke, discovered what then appeared to be a star 

 of the eighth magnitude very near the place indicated j but either from its 

 distance or from the insufficient power of the instruments it did not exhibit 

 a defined disc, so as to enable the observers at once to determine its planetary 

 character. There was, however, no star in that place in the most recent 

 catalogue. Star or planet was a question which could not be solved at once, 

 and Galle had to wait until the following night with what patience he could. 

 Then, indeed, the newly discovered body had moved in its orbit, and its true 



