THE DIRECTION OF MOTION. 237 



pretable in pursuance of the above general laws. Each por- 

 tion of such vapour-like matter must begin to move towards 

 the common centre of gravity. The tractive forces which 

 would of themselves carry it in a straight line to the centre 

 of gravity, are opposed by the resistant forces of the medium 

 through which it is drawn. The direction of movement 

 must be the resultant of these — a resultant which, in conse- 

 quence of the unsymmetrical form of the flocculus, must be 

 a curve directed, not to the centre of gravity, but towards 

 one side of it. And it may be readily shown that in an aggre- 

 gation of such flocculi, severally thus moving, there must, 

 by composition of forces, eventually result a rotation of the 

 whole nebula in one direction. 



Merely noting this hypothetical illustration for the pur- 

 pose of showing how the law applies to the case of nebular 

 evolution, supposing it to have taken place, let us pass to 

 the phenomena of the Solar System as now exhibited. Here 

 the general principles above set forth are every instant ex- 

 emplified. Each planet and satellite has a momentum which 

 would, if acting alone, carry it forward in the direction it is 

 at any instant pursuing. This momentum hence acts as a 

 resistance to motion in any other direction. Each planet 

 and satellite, however, is drawn by a force which, if unop- 

 posed, would take it in a straight line towards its primary. 

 And the resultant of these two forces is that curve which it 

 describes — a curve manifestly consequent on the unsym- 

 metrical distribution of the forces around its path. This 

 path, when more closely examined, supplies us with further 

 illustrations. For it is not an exact circle or ellipse; which 

 it would be were the tangential and centripetal forces the 

 only ones concerned. Adjacent members of the Solar Sys- 

 tem, ever varying in their relative positions, cause wdiat we 

 call perturbations; that is, slight divergences in various direc- 

 tions from that circle or ellipse which the two chief forces 

 would produce. These perturbations severally show us in 

 minor degrees, how the line of movement is the resultant 



