and Velocity of the Solar Motion . 
227 
Considerations of the attractive Power required for a sufficient 
Velocity of the sidereal Motions. 
As attraction is a power that acts at all distances, we ought 
to begin by examining whether the motions of our stars can 
be accounted for by the mutual gravitation of neighbouring 
stars towards each other, or by a periodical binal revolution 
of them about a common centre of gravity ; or whether we 
ought not rather to have recourse to some very distant 
attractive centre. This may be decided by a calculation of 
the effects arising from the laws according to which the prin- 
ciple of attraction is known to act. For instance, let the sun 
and Sirius be two equal bodies placed in the most favourable 
situation to permit a mutual approach by attraction : that is, 
let them be without projectile motions, and removed from all 
other stars which might impede their progress towards each 
other, by opposite attractions. Then, by calculation, the space 
over which one of them would move in a year, were the 
matter of both collected in the other as an attractive centre, 
would be less than a five thousand millionth part of a second; 
supposing that motion to be seen by an eye at the distance of 
Sirius, and admitting the parallax of the whole orbit of the 
earth on this star to be one second. 
This proves evidently that the mere attraction of neigh- 
bouring stars acting upon each other cannot be the cause of 
the sidereal motions that have been observed. 
In the case of supposed periodical binal revolutions of stars 
about a common centre of gravity, where consequently pro- 
jectile motions must be admitted, the united power of the 
connected stars, provided the mass of either of them did not 
Gg 2 
