Royal Astronomical Society, 529 



apparent, and owing to the real revolution of the solar system 

 round it. 



Applying these remarks to the case of the star 61 Cygni, and 

 assuming Bessel's value of the parallax, and the usually assumed 

 elements of the orbit of this binary system, it appears evident that 

 this system is unconnected with the solar system. It does not, how- 

 ever, appear impossible that both systems revolve round a third at an 

 immensely greater distance than that of the sun from the earth. 



The author, in conclusion, adverts to the great importance, in 

 the present advanced state of practical astronomy, of noting the po- 

 sitions of the stars having the greatest proper motions with all pos- 

 sible accuracy, and of rigorously comparing the deduced proper mo- 

 tions at equal intervals of time, for the purpose of discovering whether 

 the motions are performed in one plane, and whether they are uni- 

 form ; and also to the importance of having a catalogue of stars 

 accurately arranged in order of brilliancy by means of photometrical 

 observations, as an essentially requisite element in the determination 

 of their relative distances from the earth. 



II. Second Note on the Mass of Venus. By R. W. Rothman, 

 Esq. 



In a Note on the Masses of Mercury and Venus, read at the Meet- 

 ing of this Society on the 14th of January*, I stated that a consi- 

 deration of the motion of the perihelion of Venus had led me to con- 

 clude, that it was necessary to diminish the mass of Mercury by a 



4 

 quantity estimated approximately at — This would make the mass 



in question ■■ ■ ■. I may observe in passing, that in the notice 

 3182843 



of the meeting of the 14th of January, page 132, there is a misprint 

 in the algebraical formula for the motion of the perihelion ; but this 

 is merely a typographical error, and the calculations are correct. 

 At the same meeting there was read an extract of a letter from Pro- 

 fessor Encke to the Astronomer Royal, from which it appears that 

 Professor Encke, guided by very different considerations, has been 



led to fix the mass of Mercury in the first instance at , and 



' 3091947 



subsequently at 



4865751 



At the end of my Note I stated that the secular equations affect- 

 ing the orbit of Mercury appeared to confirm the necessity of an 

 augmentation of the mass of Venus, to which I have been led by 

 an examination of the secular motion of the node of the latter planet. 

 But, in fact, this deserves somewhat further development. 



I have calculated the secular equation of the node of Mercury 

 with the same planetary masses as those assumed in my first 

 node, excepting that of Mercury, which I have supposed equal to 



1 

 3182843' 



* See present volume, p. 398. 

 Phil. Mag. S. 3. No. 141. Suppl. Vol. 21. 2 N 



