vii LAW AND PRINCIPLE 175 



dread in which they had been held may be said to have 

 begun with Newton's discovery. 



A notable example of the prediction of the existence 

 of a body from consideration of its gravitational influence 

 was afforded by the discovery of the planet Neptune. 

 After the planet Uranus had been discovered in 1781 

 by Sir William Herschel, astronomers calculated the 

 path in which it moved, and predicted the positions it 

 should occupy from time to time. The planet was found, 

 however, to be slightly behind or ahead of its calculated 

 position. If we know a train is moving at a certain 

 rate, we can say where it ought to be on the line at a 

 certain time, and if it is not there we assume that some- 

 thing has happened. So it was with the planet Uranus : 

 the difference between the actual and the calculated 

 positions of the planet was assumed to be due to. the 

 disturbing influence of some unknown body beyond it. 



There was every confidence that a massive globe 

 somewhere in the darkness of space was making its 

 presence manifest ; and though the problem of finding 

 the place of the disturber was very difficult, it was solved 

 by two mathematicians one an Englishman named 

 Adams and the other a Frenchman named Le Verrier. 

 In September, 1846, Sir John Herschel, son of Sir William 

 Herschel, said : " We see it [the probable new planet] as 

 Columbus saw America from the shores of Spain. Its 

 movements have been felt trembling along the far- 

 reaching line of our analysis with a certainty hardly 

 inferior to that of ocular demonstration." On the 

 twenty-third of the same month a German astronomer, 

 Dr. Galle, found the new body close to the place which 

 calculation had shown it ought to occupy. This is 

 probably the greatest triumph of mathematics applied 

 to the law of gravitation yet achieved. 



