48$ Dr. Herschel's Catalogue 
observations made on double stars, whereby it will be seen, that 
many of them have actually changed their situation with regard to 
each other , in a progressive course , denoting a periodical revo- 
lution round each other ; and that the ?notion of some of them is 
direct , while that of others is retrograde. Should these observa- 
tions be found sufficiently conclusive, we may already have their 
periodical times near enough to calculate, within a certain de- 
gree of approximation, the parallax and mutual distance of the 
stars which compose these systems, by measuring their orbits, 
which subtend a visible angle. 
Before we leave the subject of binary systems, I should 
remark, that it evidently appears, that our sun does not enter 
into a combination with any other star, so as to form one of 
these systems with it. This could not take place without our 
immediately perceiving it; and, though we may have good 
reason to believe that our system is not perfectly-at rest, yet the 
causes of its proper motion are more probably to be ascribed to 
some perturbations arising from the proper motion of , neigh- 
bouring stars or systems, than to be placed to the account of a 
periodical revolution round some imaginary distant centre. 
HI. Of more complicated sidereal Systems, or treble , quadruple , 
quintuple, and multiple Stars. 
Those who have admitted our arguments for the existence of 
real double stars, will easily advance a step farther, and allow 
that three stars may be connected in one mutual system of re- 
ciprocal attraction. And, as we have from theory pointed out, 
in figures 1, 2, 3, and 4, how two stars may be maintained 
in a binary system, we shall here shew that three stars may 
