i46 HISTORY OF PHYSICAL ASTRONOMY. 



which masKs the other causes of inequality, by determining the direc- 

 tion of the motions of the perijove and node of each satellite. 



Sect. 5. Application of the Newtonian Theory to the New Planed. 



WE are now so accmstomed to consider the Newtonian theory as 

 true, that we can hardly imagine to ourselves the possibility that those 

 planets which were not discovered when the theory was founded, should 

 contradict its doctrines. We can scarcely conceive it possible that 

 Uranus or Ceres should have been found to violate Kepler's laws, or to 

 move without suffering perturbations from Jupiter and Saturn. Yet if 

 we can suppose men to have had any doubt of the exact and universal 

 truth of the doctrine of universal gravitation, at the period of these 

 discoveries, they must have scrutinized the motions of these new bodies 

 with an interest far more lively than that with which we now look for the 

 predicted return of a comet. The solid establishment of the Newtonian 

 theory is thus shown by the manner in which we take it for granted 

 not only in our reasonings, but in our feelings. But though this is so, 

 a short notice of the process by which the new planets were brought 

 within the domain of the theory may properly find a place here. 



William Herschel, a man of great energy and ingenuity, who had 

 made material improvements in reflecting telescopes, observing at Bath 

 on the 13th of March, 1781, discovered, in the constellation Gemini, a 

 star larger and less luminous than the fixed stars. On the application 

 of a more powerful telescope, it was seen magnified, and two days after- 

 wards he perceived that it had changed its place. The attention of 

 the astronomical world was directed to this new object, and the best 

 astronomers in every part of Europe employed themselves in following 

 it along the sky. 31 



The admission of an eighth planet into the long-established list, was 

 a notion so foreign to men's thoughts at that time, that other supposi- 

 tions were first tried. The orbit of the new body was at first calculated 

 as if it had been a comet running in a parabolic path. But in a few 

 days the star deviated from the course thus assigned it : and it was in 

 vain that in order to represent the observations, the perihelion distance 

 of the parabola was increased from fourteen to eighteen times the 

 earth's distance from the sun. Saron, of the Academy of Sciences of 

 Paris, is said 22 to have been the first person who perceived that the 



Voiron, Hist, Ast. p. 12. 3= ibid 



