226 Dr. Herschel on the Quantity 
we have shown that the celestial bodies of the solar system 
actually have these similar projectile motions, it may be re- 
quired that we should also prove that the stars have them 
likewise ; since the appearances in Fig. 5 may otherwise be 
looked upon as merely the consequence of the assumed solar 
motion. To this I answer, that setting aside the solar motion, 
and allowing the observations of astronomers on the proper 
motion of the stars to give us the real direction and angular 
quantity of these motions, even then the same similarity will 
equally remain to be accounted for. In my examination of 
Fig. 1 and 3, it has been shown that we ought to ascribe the 
similar directions of the sidereal motions to some physical 
cause, which probably exerts its influence also on the solar 
motion ; therefore in reverting to those figures I may be said 
to appeal to the actual state of the heavens, for a proof of 
what has been advanced, with respect to the similarity of the 
directions of projectile motions. 
Having thus examined one cause of the sidereal motions, 
and shown that as far as we are acquainted with its mode of 
acting in the solar system, it is favourable to a similarity 
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 towards each other. This is an acknowledged prin- 
ciple of motion, and the laws of its exertion being perfectly 
known, we shall in this inquiry meet with no difficulty relating 
to its direction, which is always towards the attracting body. 
