GO Mt)TtON OF rut SUM. 



Dr. Wilfon, of Glafgow, upon theoretical principles; and its 

 probability, from reafons of the fame nature, by M. de Lalande. 

 Probability that The rotatory motion of the fun, from which the latter con- 

 the fun has pro- eludes a difplacing of the folar centre, indicates a motion of 

 asrotatory rao- tfanfialion in fpace ; for it is not very probable that the tne- 

 ^oiif chanical impreffion which gave the former, fliould not alfooc- 



cafion the latter. This however can be admitted only as a 

 plaufiblehypothefis, until we attain a knowledge of the caufe 

 of the rotatory motion, 

 —and the vari- "^ '^'''' argument might be ftrengthened by clofely obferving 

 able itar« alfo. the flars which change their magnitudes periodically; for if 

 thefe changes arife from a rotatory motion,* a real motion in 

 fpace may be expeded to attend it ; and the multitude of thefe 

 ftars is fo great, that their concurrent teftimony is defirable. 

 Three forts of But fetting afide theoretical arguments, the Do^or notices 

 motions of ftarsi that as all parallactic motions indicate the obferver not to be 

 at reft, it may be neceiTary to explain three forts of motionSj^ 

 which will frequently be alluded to in the following difcuflion. 

 Fata'.haic, real, ijuppofe the folar fyflem to move towards a certain part of 

 and apparent. the heavens, the ftars, to an inhabitant of the earth, will ap- 

 pear to move in an oppofite direftion. Let s p (PI. II. Fig. 1.) 

 rcprefent the parallactic motion of a ftar; which, if the ftar 

 have no real motion^ will alfo belts apparent motion; but if 

 it (hould have a real motion, which in the fame lime that it 

 could have gone from s to p, would havecarried it from s to r, 

 it will be feen to move along the diagonal s a; and p a, being 

 para[l«l and equal to s r will reprefent its real motion. The 

 triangle s p a }s fuppofed to be formed in the concave of the 

 heavens by three arches of great circles, the obferver being in 

 the centre, and *p repreCents the parallaClic, pa the real, and 

 5 a the apparent motion of the flar. The Uluation and lepgth 

 of thefe arches in feconds of a degree will reprefent thedirec^ 

 tion and quantity of each motion; and calling the folar motion 

 S, the diflance of the ftar from the fun d, and the fine of the 

 ftar's diftance from the point towards which the fun is moving 



(p ; the parallactic motion will be exprelTed thqs : -^zzs p^ 



v.d 

 The largcft ftars A motion of (he fun will occafion parallaCiic motions of the 

 {hert'hlfan''s ^^''^' ^"^ vice verfa ; but to afcertain if parallaaic motions 

 motion, exift, fuch ftafs ftiould be examined as are moft vifibly affeCied 



* See Phil, Tra«f. for the year 1795, p. 68, and our Journal, 

 XI. 271. 



by 



