27S Mr. herschel on the proper Motion 
idea of the ftars being collected into fyftems (Phil. Tranf. vol. LI, p. 249.) 
appears to be extremely well-founded, and is every day more confirmed by obfer- 
vations : though this does not, in my opinion, take away the probability of many 
ftars being Hill as it were folitary , or, if I may life the exprefiion, interfyftcmatical. 
It occurs then naturally, that by the principle of gravitation, which is never at 
reft, and which we have no reafon to doubt extends to all poffible diftanees, one 
fyftem of ftars will aft on another as if the ftars of each fyftern were all collefted 
into the center of gravity of each refpeftive fyftem. Hence then will arife this 
evident confluence, that a fur, or fun, fuch as ours, may have a proper motion 
within its own fyftem of ftars, while at the fame time the whole Harry fyftem to 
which it belongs may have another proper motion, totally different in quantity 
pud direftion. It will require no little abftraft confideration to conceive the 
poffibility of what may be thus furmifed ; therefore an inftance or two, to eluci- 
date the matter, may not he improper. If an inhabitant of the 5th fatellite of 
Saturn fhouid have dilcovered, that his little world revolves at a great diftance 
round a planet, and to his great affonifhment fhouid alfo have found, that this 
planet again revolves round the fun; — if, farther, our hypothefis of the folar 
motion Ihould prove to be well-founded (which, in forne of the ftars, fuppoling 
them to be funs .furrounded with planets and fatellites, mull certainly be the cafe) ; 
then a third capital motion will be introduced to this inhabitant of Saturn’s fatel- 
lite; and he will experience, in a narrow compafs, what we now furmife may poflibly 
be our cafe upon a more extended fcale, by the motion of the whole fyftem of 
ftars to which our fun may belong. Another view' may, perhaps, Hill better 
throw a light upon the fubjeft. Let us admit that a very final 1 nebula may be a 
colleftion of a thoufand ftars : and if Mr. michell’s opinion of our fyftem of 
ftars, which he aiTumes to be about a thoufand (Phil. Tranf. vol. LVII. p. 255.) 
has any foundation, all thefe ftars taken together will only fubtend an angle of 
barely a minute to an eye placed 3438 times as far from the center of the fyftem 
as the two fartheft ftars in it are from each other. Now' as 1 have found fome of 
thefe nebulae that are fo fmall, that a tolerably good telefcope cannot diftinguifh 
them from a Angle ftar, whole fyftems of ftars, when prefen ted to our imagination 
under this diminutive fliape of nebula?, will eafily, I believe, be admitted among 
the number of thofe celeftial bodies that may have a proper motion. I ought to 
carry this hint a little farther, juft to fliew that it may poflibly be applied to the 
■fubjeft of refolving a number of concurrent proper motions of the fixed ftars 
into apparent ones ; and thereby, in procefs of time, to arrive at the knowledge 
of all the real complicated motions of the planet we inhabit ; of the folar fyftem 
to 
