MANY DISCOVERIES ARE PREDICTED. 227 



number of instances to prove it ; and the greater the law, 

 the more gradual and laborious the process. 



Far from being matters of accident, some discoveries 

 are successfully predicted ; but usually this can only occur 

 in cases where the essential conditions upon which the 

 phenomena depend are clearly imagined or known. The 

 discovery of the planet Neptune, by Le Verrier and 

 Adams, is a well-known instance of this kind. The pro- 

 bability of the existence of a disturbing body beyond 

 Uranus was imagined by M. A. Bouvard and Mr. Hussey as 

 early as the year 1834. Mr. Adams soon after set to work 

 to ascertain the nature of this disturbance ; and as early as 

 1841 he conjectured the existence of an exterior planet, 

 and decided to examine its effect. In 1844 he applied to 

 the Astronomer Royal for recorded observations, to assist 

 him; and at the end of October 1845, he had calculated 

 the elements of the supposed planet, and gave as its longi- 

 tude 3231. On June 1, 1846, a memoir of M. Le Verrier 

 was published, containing his first predictions respecting 

 the expected planet. On July 29, Professor Challis, of 

 Cambridge, began to look for the planet, and actually saw 

 it on August 4 and* 12, and reserved it, with others, for 

 examination; but did not then recognise it, in conse- 

 quence of his method of observation. On September 23, 

 M. Gralle, of Berlin, received an express application from 

 M. Le Verrier, of Paris, to try and recognise it by its 

 having (like all other planets) a visible disc, and M. Galle 

 saw and distinguished it on that very same day. On 

 September 29, Professor Challis, having seen a paper of 

 Le Verrier's, of August 31, singled out the planet by its 

 appearing to have a visible disc ; and in the same month, 

 Sir J. Herschel remarked, at a meeting of the British 

 Association, at Southampton : ' We see it ' (i.e. the planet) 

 ' as Columbus saw America from the shores of Spain. Its 



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