2lg CAUSE d% THE CHANGES OF RBtATIVE 



Obfcryations n will be remarked, when we come to compare fingM 



and inferences ^ i • i i i , ,.^ .... 



refpefting the "^calures which have been taken on diJfTerent nights, that they 



changes of re- are fomewhat more difcordant ; but I have not ventured to 

 ftrrrextr"emery ^^i^^ t^^m on that account, except in cafes where it was 

 near each other* pretty evident that fome miftake in reading off, or other ac- 

 cident to which all aftronomical obfervations are liable, was 

 to be apprehended. Nor can fuch difagreements materially 

 afFe€t the conclufions I have drawn, when it appears that the 

 deviations happen Ibmetiraes to be on one fide, and fometimes 

 on the other fide, of the true angle of pofition. For, fince 

 that angle is not a thing that will change in the courfe of a 

 few nights, the excefs of one meafure will ferve to correal the 

 defed of another; and we are not to think it extraordinary, 

 when flars are fo near together, and their motion through the 

 field of view (in confequence of the high magnifying power 

 ^e are obliged to ufe) fo quick, that we (hould now and then 

 even fall fliort of that general accuracy which may be had by 

 a careful ufe of the micrometer. 



I ftiall now enter into an examination of the caufe of the 

 change in the angle of pofition of the fmall ftar near Caftor. 



A revolving ftar, it is evident, would explain in a moll 

 fatisfadlory manner, a continual change in the angle of pofition, 

 without an alteration of the diftance. But this, being a cir- 

 cumftance of which we have no precedent, ought not to be 

 admitted without the fulieft evidence. It will therefore be 

 right to examine, whether the related phenomena cannot be 

 fatisfadtorily explained by the proper motions of the ftars, or 

 of the fun. 



Single Motions, 

 ' (a) The three bodies we have to confider, are O, a, andx; 

 and, fuppofing them to be placed as they were obferved to be 

 in the year 1779; the angle x a>?, in Fig. 1, will be 32'^ 47' 

 north preceding. We are at liberty to let the angle PaO be 

 what will beft anfwer the purpofe. Then, in order to examine 

 the various hypothefes that may be formed, according to the 

 arrangement of the principles we have given, we ftiall begin 

 with No. 1 ; and, as this admits that all phenomena may be 

 refolved by a proper motion of x, let us fuppofe thisfiar to be 

 placed any wliere far beyond a, but fo as to have been feen, in 

 the year 1779, wheie the angle of pofition, ti2^ 47' north pre- 

 ceding 



