174 <*• H. KNIBBS. 



Airy's two suppositions is prima facie the most probable, i.e., 



R.A. = 259-°2, D.= +32-°l for the epoch 1840. 



Reducing to 1850 the mean of Argelander's, Lundahl's, and 



O. Struve's results as L; putting Galloway's reduced result from 



southern stars as II.; and the mean of Airy's two results also 



reduced to 1850 as III., he adopted the general mean for 1850, 



as shewn hereunder, 



R.A. D. + . 



I. 259°-41' 34°33' 



II. 260-33 34-20 



III. 259-18 3205 



Mean 259-51 33-39 



or say R.A. = 259°-85, D. = + 33-°65 



Hall considered the possible case of the sun and nearer stars 

 revolving about a gigantic central body, and also of their revolving 

 about their common centre of inertia. Using the parallaxes of 

 a Centauri, and 61 Cygni, assumed as 0-"936 and 0-"422 respec- 

 tively, to determine the constants of his equations, and comparing 

 the observed with his computed motions, he concluded that the 

 centre lies towards Andromeda, instead of toward Hydra, and 

 that the motion is about a common centre of inertia rather than 

 about some gigantic mass. The place assigned for the centre was 



for 1850, 



RA=10-°4, D.= +27-°8 



A second calculation gave 



R.A. = 9°2, D.= +26-°5 



The angular velocity of the sun about this centre was 0-"06612 



per annum, 20 million years -constituting the ' Annus Magnus ' 



required to complete a revolution, whose radius was 31 million 



times the earth's mean distance from the sun. The whole gravi- 



tative mass was estimated to be 78 million times that of the sun, 



although the distribution at f of a sidereal unit apart would 



indicate only 34 million. It should be added that in his discussion, 



Hall availed himself of existing knowledge of the radial velocities 



of stars, which, taken with parallax and proper motion, permitted 



of the absolute velocities being computed. 



