SECT.VJII.] ASTRONOMICAL TABLES. 69 



have been computed for eleven planets, besides the moon 

 and the satellites of Jupiter. In the present state of astro- 

 nomy, the masses and elements of the orbits are pretty well 

 known, so that the tables only require to be corrected from 

 time to time as observations become more accurate. Those 

 containing the motions of Jupiter, Saturn, and Uranus, have 

 already been twice constructed within the last thirty years. 

 The tables of Jupiter and Saturn agree almost perfectly with 

 modern observation ; those of Uranus, however, are already 

 defective, probably, because the discovery of that planet in 

 1781 is too recent to admit of much precision in the deter- 

 mination of its motions, or that possibly it may be subject 

 to disturbances from some unseen planet revolving about 

 the sun beyond the present boundaries of our system. If, 

 after a lapse of years, the tables formed from a combination 

 of numerous observations should be still inadequate to re- 

 present the motions of Uranus, the discrepancies may reveal 

 the existence, nay even the mass and orbit of a body placed 

 for ever beyond the sphere of vision. 



That prediction has been fulfilled since the last edition 

 of this book was published. Not only the existence of Nep-- 

 tune, revolving at the distance of three thousand millions of 

 miles from the sun, has been discovered from his disturbing 

 action on Uranus, but his mass, the form and position of 

 his orbit in space, and his periodic time had been deter- 

 mined before the planet had been seen, and the planet itself 

 was discovered in the very point of the heavens which had 

 been assigned to it. It had been noticed for years that the 

 perturbation of Uranus had increased in an unaccountable 

 manner. After the disturbing action of all the known 

 planets had been determined, it was found that, between the 

 years 1833 and 1837, the observed and computed distance 

 of Uranus from the sun differed by 24,000 miles, which is 

 about the mean distance of the moon from the earth, while, 

 in 1841, the error in the geocentric longitude of the planet 

 amounted to 96". These discrepancies were therefore attri- 



