PHYSICAL AND LITERARY. 199 



kind is the motion lately difcovered by 

 Mr llradley, which, on this account, lay 

 fo long concealed from ^{Ironqmers- Of 

 the lame kind is the motion pt the fixed 

 ilars, that ought to arife from the paral- 

 lax of the earth's orbit. And of this 

 kind I fufpecl the variation of the obli- 

 quity of the ecliptic to be. 



To proceed therefore to what I take to 

 be the caufe of this motion, Sir Ifaac 

 Jslewton has found, that the fan and 

 planets gravitate all tovsrards each other 

 mutually ; that it is neither the fun, (acr 

 cording to the Copernican fyitem), nor 

 jhe earth, (according to Ptolooiyj, that 

 is the centre of the fyllem. or fixed point ; 

 but the centre of gravity of the whole fy« 

 ftem. That the lun therefore moves a- 

 bout this centre ; and that, when Jupiter 

 and Saturn, the two biggeft planets, are 

 in the fame right line, on the fame fide 

 of the fun, the centre of the fun will be 

 almoft a diameter of the lun, diftant from 

 this fixed point. Hence, though we fup- 

 pofe the earth to move always in the 

 fame plane, the fun will appear to have 



different 



