OF THE LATE PROFESSOR ROBISON, 5lt 



these oppositions in the case of this planet, there were yet only 

 four which had been actually observed. Dr Herschell had, 

 however, discovered the planet soon after the opposition of 

 1781 was passed, and though of course that opposition was 

 not seen, yet from the observations that were made so soon 

 after, Professor Robison thought he could deduce the time with 

 sufficient accuracy. The opposition of the winter 1786 he 

 observed himself; for though there was, unfortunately, no ob- 

 servatory at Edinburgh, he endeavoured to supply that defect 

 on the present occasion by a very simple apparatus, viz. a tele- 

 scope on an equatorial stand, which served to compare the right 

 ascension and declination of the planet with those of some 

 known stars which it happened to be near. His general solu- 

 tion of the problem is very deserving of praise ; and though the 

 method pursued is in its principle the same with all those which 

 ever since the time of Kepler have been employed for find- 

 ing the elements of a planetary orbit, it appears here in a very 

 simple form, the construction being wholly geometrical, and 

 easily understood. The elements, as he found them, are not 

 very different from those that have since been determined 

 from more numerous and more accurate observations* 



When Dr Herschel first made known this most distant of 

 the planets, many astronomers believed that they had disco- 

 vered the source of those disturbances in our system, which 

 had not yet been explained. Professor Robison was of this 

 number ; for he tells us, in the beginning of his paper, that 

 he had long thought that the irregularities in the motion 

 of Jupiter and Saturn, which had not been explained by the 

 mutual gravitation of the known planets, were to be accounted 

 for by the action of planets of considerable magnitude, beyond 

 the orbit of Saturn. Subsequent inquiry, however, has not ve- 



Vol.VILP.II. 3U rifled 



