Royal Astronomical Society. 



69 



M. Lamont does not consider his results as very certain, since I. 

 and II. gave somewhat discordant results ; but he is of opinion that 

 Bouvard's mass is certainly too large. 



The determination of the mass of Uranus has become of great and 

 pressing interest, since the theoretical researches of MM. Le Verrier 

 and Adams into the apparently irregular motion of the planet, have 

 led to the memorable discovery of Neptune. 



If we suppose the two planets to be equal, it is clear that the effect 

 of Uranus to perturb the motion of Neptune will be more sensible 

 than the reciprocal effect of Neptune upon Uranus, on account of 

 the greater distance of Neptune from the sun. The theory of Nep- 

 tune, therefore, must continue imperfect until a correct value of the 

 mass of Uranus can be assigned. With this object in view, M. Otto 

 Struve has undertaken a series of micrometrical measurements of the 

 two principal satellites, which he hopes to complete about the be- 

 ginning of February 1848. But though the planet is now 5° or 6° 

 north of the equator, and therefore far more favourably placed than 

 when Sir J. Herschel pursued his investigations, still the observa- 

 tions are very difficult, requiring steady images and a perfectly trans- 

 parent sky. In this unfavourable autumn only fourteen nights have 

 hitherto been sufficiently fine. Generally speaking, the distances of 

 II. have been measured as satisfactorily as the angles of position, 

 and only twice or thrice has the object been too faint to be well 

 observed. The reduction is delayed until the series is completed. 



When M. Struve had become familiarly acquainted with these two 

 satellites, he turned his attention to their supplementary companions. 

 After being twice deceived in measuring a small star instead of a 

 satellite, he saw on October 8 an object, of which he made the fol- 

 lowing measures in position : — 



Oct. 8 



fa 



11 



16 Pulkowa M. T. 



18 



35 



36 



42 



Position =17«7 

 186-5 

 178-8 

 182-3 

 176-7 



An estimate of comparative distance made the distance = 



14"-2. 



These observations do not, indeed, prove the object to have been a 

 satellite ; but the difference between the first and last angles of po- 

 sition, if it had been a star, would, from the motion alone of Uranus, 

 have been 9°. The following observations leave no doubt that a 

 satellite was observed : — 



Pulkowa M. T. Position. 



No. obs. Pulkowa M. T. Distance. No. obs. 



