476 Proceedings. 
were Leverrier, of Paris, and Adams, 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, Venus, 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 ; and it 
was conjectured that these might be due to some unknown planetary mass far 
beyond the orbit of Uranus. lLeverrier 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 :—Given 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 ; 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 
