£28 Dr. Herschel on the Quantity 
greatly exceed that of the sun, would fall very short of the 
attraction required to give a sufficient velocity to their motions. 
The star Arcturus for example, which happens to move, as is 
required, in an opposite direction to the proposed solar mo- 
tion, were it connected with the sun, and had the proper 
degree of necessary projectile motion, could not describe an 
arch of 1" of its orbit, about their common centre, in less 
than 102 years ; and though the opposite motion of the sun, 
by a parallactic effect would double that quantity, it still would 
fall short of the change we observe in this star in the course 
of a single year. 
Other considerations are still more against the admission 
of such partial connections : they would intirely oppose the 
similarity of the directions of the sidereal motions that have 
been proved to exist, and which we are now endeavouring to 
explain. 
Let us then examine in what manner a distant centre of 
attraction may be the cause of the required motions. By 
admitting this centre to be at a great distance, we shall have 
its influence extended over a space that will take in a whole 
group of stars, and thus the similar directions of their motions 
will be accounted for. Their velocities also may be ascribed 
to the energy of the centre, which may be sufficiently great 
for all the purposes of the required motions. A circumstance, 
however, attends the directions of the motions to be ex- 
plained, which shows that a distant centre of attraction alone 
will not be sufficient; for these motions, as we may see in 
Fig. 3, though pretty similar in their directions, still are di- 
verging ; whereas if they were solely caused by attraction, 
they would converge towards the attracting centre, and point 
