18 HISTORY OF ASTRONOMY. 



would produce inequalities having a very short period ; 

 while the observed anomalies of Uranus are precisely 

 the reverse. Moreover, it would be necessary to assign 

 it such dimensions that it could not fail to have been 

 visible in our telescopes. Has a comet impinged upon 

 Uranus, and changed the form of its orbit? Such a 

 cause might render it impossible to represent the en- 

 tire series of observations by a single elliptic orbit; 

 but the observations before the supposed collision, would 

 all be consistent with each other, and the observations 

 after collision would also be consistent with each other. 

 Yet the observations of Uranus from 1781 to 1821, as 

 may be seen from the diagram, page 12, accord neither 

 with the earlier observations nor with the more recent 

 ones. 



There seems to remain no other probable supposition 

 than that of an undiscovered planet. But if these disturb- 

 ances are due to such a body, we can not suppose it 

 situated within the orbit of Saturn. This would disturb 

 the orbit of Saturn more than that of Uranus, while we 

 know that its influence on Saturn is inappreciable, for 

 Saturn's motion is well represented by the tables. Can 

 this body be situated between Saturn and Uranus? We 

 must then place it much nearer Uranus than Saturn, for 

 the reason already assigned, in which case its mass must 

 be supposed to be small, or it would produce too great 

 an effect upon Uranus. Under these circumstances, its 

 action would only be appreciable when in the imme- 

 diate neighborhood of Uranus, which supposition does 



