78 



Four cafes of 

 •rbits. 



Tn thefe cafes 

 die center of 

 gravity is confi 

 derabiy remote 

 from the larger 

 ftar. 



ON THE CONSTRUCTION OF THE HEAVENS. 



mutual gravitation towards each other. This fhould be called 

 a real double ftar j and any two ftars that are thus mutually 

 connected, form the binary fidereal fyftem which we are now 

 to confider. 



It is ea fy to prove, from the doctrine of gravitation, that two 

 ftars may be fo connected together as to perform circles, or 

 fimilar ellipfes, round their common centre of gravity. In this 

 cafe, they will always move in directions oppofite and parallel 

 to each other ; and their fyftem, if not deftroyed by fome fo- 

 reign caufe, will remain permanent. 



Figure 1 (Plate V.) represents two equal ftars a and /;, 

 moving in one common circular orbit round the centre o, but 

 in the oppofite directions of at and b t. In Fig. 2. we have a 

 fimilar connection of the two ftars ab ; but, as they are of dif- 

 ferent magnitudes, or contain unequal quantities of matter, 

 they will move in circular orbits of different dimenfions round 

 their common center of gravity o. Fig. 3. reprefents equal, 

 and Fig. 4. unequal ftars, moving in fimilar elliptical orbits 

 round a common centre ; and, in all thefe cafes, the directions 

 of the tangents tt t in the places a b, where the ftars are, wiU 

 be oppofite and parallel, as will be more fully explained here- 

 after. 



Thefe four orbits, fimpleas they are, open an extenfive field 

 for reflection, and, I may add, for calculation. They fhew, even 

 before we come to more complicated combinations, where the 

 fame will be confirmed, that there is an efTential difference 

 between the construction of folar and fidereal fyftems. In 

 each folar fyftem, we have a very ponderous attractive centre, 

 by which all the planets, fatellites, and comets are governed, 

 and kept in their orbits. Sidereal fyftems take a greater fcope : 

 the ftars of which they are compofed move round an empty- 

 centre, to which they are nevertheless as firmly bound as the 

 planets to their mafly one. It is however not neceflary here 

 to enlarge on diftinctions which will hereafter be ftrongly fup- 

 ported by fact.;, when clufters of ftars come to be considered. 

 I (hall only add, that in the fubordinate bodies of the folar 

 fyftem itfelf, we have already inftances, in miniature, as it may- 

 be called, of the principle whereby the laws of attraction are 

 applicable to the folution of the moft complicated phenomena 

 of the heavens, by means of revolutions round empty centres. 

 For, although both the earth and its moon are retained in their 

 4 orbits 



