LE TERRIER AND ADAMS 7 PLANET. 



the disturbances of the motion of Uranus, so far as these were 

 ascertained by observation, would be produced without sensible 

 difference, either by the actual planet which has been discovered, 

 or by either of the planets assigned by MM. Le Terrier and 

 Adams, or even by an indefinite number of others which might 

 be assigned, either within the path of Neptune, or between it and 

 that of Adams' planet, or, in fine, beyond the latter within certain 

 assignable limits. 



15. That the planets assigned by MM. Le Terrier and Adams 

 are not identical with the planet to the discovery of which their 

 researches have conducted practical observers is, therefore, true ; 

 but it is also true that, if they or either of them had been iden- 

 tical with it, such excessive amount of agreement would have 

 been purely accidental, and not at all the result of the sagacity 

 of the mathematician. All that human sagacity could do with 

 the data presented by observation was done. Among an indefinite 

 number of possible planets capable of producing the disturbing 

 action, two were assigned, both of which were, for all the purposes 

 of the inquiry, so nearly coincident with the real planet as inevit- 

 ably and immediately to lead to its discovery. 



16. It might appear from considering merely the enormous dis- 

 tance of this planet from the earth, that the problem to ascertain 

 the rate of its motion, and the time it takes to make a complete 

 revolution round the sun, would be attended with great diffi- 

 culty; nothing, on the contrary, can be more easy or simple. 

 By observing the place of the planet with precision on any given 

 night, say, for example, on the 1st January, 1853, and again 

 on the 1st January, 1854, it will be found to have moved through 

 2-187 ; we infer, therefore, that this is the rate of its annual 

 motion; and this inference would be verified by repeating the 

 same observation on the 1st of January, 1855, and, in a word, on 

 the same night in each succeeding year. 



Having thus ascertained that Neptune moves round the sun at 

 the rate of 2-187 per annum, the question is : how long he will 

 take to make a complete revolution that is 360 round the sun ; 

 and this, it is clear, is a question in the simple Rule of Three, 

 that, can be solved by any school-boy, and is thus stated 



2-187 : 360 :: 1 year : 36 = 164-6 years. 



Thus it appears that this planet will make a complete revolution 

 round the sun in about 164 years and 7 months, and although 

 not more than a few years have elapsed since the date of its dis- 

 covery, we are just as certain, that it will complete its revolution 

 in that interval as our posterity will be, who will witness the 

 completion of its period, in the year of our Lord 2011. 

 184 



