ASTRONOMY OF THE NINETEENTH CENTURY. 



423 



an arc which, measured from the sun, would include 290 of the whole 

 circle of 360. The year 1858 was distinguished by the appearance 

 of a small comet with an intensely bright nucleus, which has a calcu- 

 lated period of about 2,000 years. It was first observed by Dr. G. B. 

 Donati at Florence, and it is known as Donati's Comet. The great 

 comet of 1 86 1 was first discovered by an amateur astronomer in New 

 South Wales on the i3th of May. It formed a splendid object in the 

 summer evening skies. Its tail at one time was more than 72 in 

 length. 



The history of science can hardly boast of a more brilliant discovery 

 than that which added a new planet to our system in 1845. 

 honour of the discovery belongs 

 equally to two men, both young 

 at the time, the one a French- 

 man, the other an Englishman. 

 This discovery was the conse- 

 quent of another, which has 

 been already mentioned, namely, 

 that of Uranus by Sir W. Herschel 

 (page 260). The motion of Ura- 

 nus was calculated by Delambre 

 in 1790, but observation soon 

 showed that the tables of the 

 planet's positions which he drew 

 up were inexact to such an extent 

 that it became necessary to re- 

 calculate them upon better data. 

 This was undertaken by a French 

 astronomer named Bouvard. He 

 based his investigations only on 

 the recent observations of Ura- 

 nus, rejecting those which were 

 recorded previous to 1781, when 

 it had been seen and registered as 

 a star. He found it, in fact, impossible to reconcile these different 

 observations with any assignable orbit. In 1821 he produced tables 

 which for a short time fairly represented the motion of the planet. 

 But Uranus soon began to depart from the course which had been 

 traced out for him ; and his irregularities increased at length so much, 

 that astronomers despaired of representing his movements by any 

 simple formula. Many hypotheses were put forward to explain the 

 discrepancies between theory and observation. One suggested that 

 the ether, or some such interstellar medium, was retarding the planet's 

 motion ; another, that possibly Uranus might be yoked to some un- 

 wieldy but hitherto unobserved satellite; another, that possibly at so 

 remote a distance from the sun the law of gravitation might be in some 



FIG. iS8. JOHN C. ADAMS. 



