THE PLANET NEPTUNE. 25 



to deduce the form of the orbit from the small arc 

 described by the planet since its discovery, but he found 

 himself unable to obtain a reliable value for the eccen- 

 tricity. It became now a question of the greatest in- 

 terest to determine with precision the elliptic elements 

 of the planet. This, however, was an object of unusual 

 difficulty, on account of the slow motion of the planet, 

 its heliocentric motion amounting to only about two 

 degrees in a year. The result derived from observations 

 embraced within a short interval of time must therefore 

 be received with great distrust, since a slight error in 

 the measurement of a minute portion of the orbit leads 

 to a much larger error in the computed length of the 

 remainder of the path. To furnish the orbit with much 

 precision, we must have observations extending over a 

 long series of years. Under these circumstances, it be- 

 came a question of the highest interest, whether this 

 body may not have been observed by astronomers of 

 former years, and mistaken for a fixed star. If we could 

 obtain one good observation, made some time in the 

 last century, it would enable us at once to determine the 

 orbit with nearly the same precision as that of Jupiter 

 itself. It will then be presumed that astronomers have 

 not neglected to explore the records of the past, to dis- 

 cover if possible some chance observation of the new 

 planet. 



Mr. Hind, of London, adopting the predicted elements 

 of Le Terrier, examined Lalande's and other observations 

 for this purpose, and satisfied himself that the new planet 



