278 SOLAR MOTI057. 



Investigation I may be Said to ajipeal to the actual state of the heavens^ 

 motion of'tf'^ ^^^ ^ proof of what has been advanced, with respect to 

 •uu. the similarity of the directions of projectile motions. 



• Having thus examined one cause of the sidereal mo- 



X tions, and shown that as far as we are acquainted with 



its mode of acting in the solar system, it is favourable to 

 a similarly of direction ; and that moreover, if we ascribe 

 the motion of the stars to it, we have also good reason, 

 from observation, to believe it to be in favour of the same 

 similarity ; we may in the next place proceed to consider 

 the mutual gravitation of the stars toward each other. 

 This is an acknowledged principle of motion, and the 

 laws of its exertion being perfectly known, we shall in 

 this inquiry meet with no difficulty relating to its direc- 

 tion, which is always toward the attracting body. 



Considerations of the attractive Power required for a 

 sufficient Velocity of the sidereal Motions* 



As attraction is a power that acts at all distances, w© 

 ought to begin by examining whether the motions of our 

 stars can be accounted for by the mutual gravitation of 

 jieighbouring stars toward each other, or by a periodi- 

 cal binal revolution of them about a common centre of 

 gravity; or whether we ought not rather to have re- 

 course to some very distant attractive centre. This may 

 be decided by a calculation of the effects arising from the 

 laws according to which the principle 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 toward 

 each other, by opposite attractions. Then, by calcula- 

 tion, 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 mil- 

 lionth part of a second ; supposing that motion to be seen 

 by an eye at the distance of Sirius, and admitting th^ 

 parallax of the whole orbit of the earth on this star to b« 

 one second. 



This 



