PERTURBATION OF URANUS. 



which, its elliptic orbit was calculated, and the disturbances pro- 

 duced upon it by the masses of Jupiter and Saturn ascertained, 

 the other planets of the system, by reason of their remoteness, 

 and the comparative minuteness of their masses, not producing 

 any sensible effects. Tables founded on these results were 

 computed, and ephemerides constructed, in which the places at 

 which the planet ought to be found from day to day for the future 

 were duly registered. 



The same kind of calculations which enabled the astronomer 

 thus to predict the future places of the planet, would, as is 

 evident, equally enable him to ascertain the places which had 

 been occupied by the planet in times past. By thus examining, 

 retrospectively, the apparent course of the planet over the firma- 

 ment, and comparing its computed places at particular epochs 

 with those of stars which had been observed, and which had 

 subsequently disappeared, it was ascertained that several of these 

 stars had in fact been Uranus itself, whose planetary character 

 had not been recognised from its appearance, owing to the imper- 

 fection of the telescopes then in use ; nor from its apparent motion, 

 owing to the observations not having been sufficiently continuous 

 and multiplied. 



In this way it was ascertained, that Uranus had been observed, 

 and its position recorded as a fixed star, six times by Flamstead : 

 viz., once in 1690, once in 1712, and four times in 1715; once 

 by Bradley in 1753, once by Mayer in 1756, and twelve times by 

 Lemonnier between 1750 and 1771. 



Now, although the observed positions of these objects, combined 

 with their subsequent disappearance, left no doubt whatever of 

 their identity with the planet, their observed places deviated 

 sensibly from the places which the planet ought to have had, 

 according to the computations founded upon its motions after 

 its discovery in 1781. If these deviations could have been 

 shown to be irregular and governed by no law, they would be 

 ascribed to errors of observation. If, on the other hand, they 

 were found to follow a regular course of increase and decrease 

 in determinate directions, they would be ascribed to the agency 

 of some undiscovered disturbing cause, whose action at the 

 epochs of the ancient observations was different from its action at 

 more recent periods. 



The ancient observations were, however, too limited in number 

 and too discontinuous to demonstrate in a satisfactory manner 

 the irregularity or the regularity of the deviation. Nevertheless, 

 the circumstance raised much doubt and misgiving in the mind 

 of Bouvard, by whom the tables of Uranus, based upon the 

 modern observations, were constructed ; and he stated that he 



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