1 1 4 LECTURES 



the tables soon revealed the fact that no orbit could be assigned which 

 would harmonize the observations. Bourvard himself was t'np first to 

 suggest that the motion of Uranus might be disturbed by an unknoion 

 'planet outside of it. This soon became the prevailing impression 

 among astronomers. So confident had astronomers become of its 

 existence that just two weeks before its discovery Sir John Herschel, 

 in addressing the British Association at Southampton, (September 

 10, 1846,) says, " its movements have been felt trembling along the 

 far-reaching line of our analysis with a certainty hardly inferior to 

 occular demonstration." 



Supposing such a planet to exist, the problem of determining its po- 

 sition was one of extreme difficulty, yielding only to the very highest 

 powers of mathematical analysis. It would seem that but few astrono- 

 mers in Europe had the hardihood to engage in the enterprise. The 

 illustrious Bessel had it in contemplation, but death removed him too 

 soon from the scene of his labors and left the task to other hands. 



The problem, as is well known, was oslved, separately and almost 

 simultaneously, by two youthful astronomers, Adams and Leverrier, 

 '■^ par ncbile fratrnm." Leverrier was the first to announce his result. 

 He wrote to his friend Dr. Galle, of Berlin, assigning the place of 

 the new planet. "Turn your telescope," said he, "to that point, 

 and there you will see it." The letter was received in Berlin on the 

 23d of September, 1846. On the same evening Dr. Galle turned his 

 telescope to the point designated by Leverrier, and both he and the 

 veteran Enke saw a star of the eighth magnitude in a position where 

 no star was laid down in Dr. Brenicker's chart, then recently pub- 

 lished by the Berlin Academy. It was the veritable planet which 

 had so long had a finger in disturbing the good order of Uranus. 

 The heliocentric longitude of the new planet, as fixed by Leverrier, 

 was 326° 0'. That assigned by Adams, without any knowledge of 

 Leverrier's work, was 329° 19'. The same, deduced from Dr. Galle's 

 observation, was 326° 52', being within a single degree of the place 

 assigned it by the computation of Leverrier. Never had science a 

 prouder triumph than was achieved on that memorable night. 



On examining the record of Lalande, it was found that he had 

 twice observed this planet as a star, viz : on the 8tli and 10th of May, 

 1*795. I may remark, in closing, that the first computation of the orbit 

 of Neptune was by an eminent American astronomer, the late Sears C. 

 Walker, and that Professor Pierce, of Harvard University, solved the 

 inverse problem, namely, that of deducing from the data the pertur- 

 bations which Neptune ought to produce on the planet Uranus.* 



SOLAR PARALLAX AND PLANETARY DISTANCES. 



In the two preceding lectures I have considered the figure and mag- 

 nitude of the earth and the laiv of gravitation. I shall this evening 

 call your attention to the method of determining the solar parallax 

 and planetary distances. 



One of the first wants of astronomy was, to ascertain the distances 

 of the moon, sun, and planets from the earth. It was long, however, 

 before any considerable progress was made in the solution of this 

 important problem. The first and indispensable requisite for solving 



«SmitliODkn Ecpoi-t for lS53,.page 161. 



