236 THE DIRECTION OF MOTION. 



along certain lines, determines their continuance along 

 those lines. 



It further follows from the conditions, that the direction 

 of movement can rarely if ever be perfectly straight. For 

 matter in motion to pursue continuously the exact line in 

 which it sets out, the forces of attraction and repulsion must 

 be symmetrically disposed around its path ; and the chances 

 against this are infinitely great. The impossibility of mak- 

 ing an absolutely true edge to a bar of metal — the fact 

 that all which can be done by the best mechanical appli- 

 ances, is to reduce the irregularities of such an edge to 

 amounts that cannot be perceived without magnifiers — suffi- 

 ciently exemplifies how, in consequence of the unsymmetri- 

 cal distribution of forces around the line of movement, the 

 movement is rendered more or less indirect. It 



may be well to add that in proportion as the forces at work 

 are numerous and varied, the curve a moving body describes 

 is necessarily complex: witness the contrast between the 

 flight of an arrow and the gyrations of a stick tossed about 

 by breakers. 



As a step towards unification of knowledge we have now 

 to trace these general laws throughout the various orders of 

 changes which the Cosmos exhibits. We have to note how 

 every motion takes place along the line of greatest traction, 

 of least resistance, or of their resultant ; how the setting up 

 of motion along a certain line, becomes a cause of its con- 

 tinuance along that line ; how, nevertheless, change of rela- 

 tions to external forces, always renders this line indirect ; and 

 how the degree of its indirectness increases with every addi- 

 tion to the number of influences at work. 



§ 76. If we assume the first stage in nebular condensa- 

 tion to be the precipitation into flocculi of denser matter pre- 

 viously diffused through a rarer medium, (a supposition both 

 physically justified, and in harmony with certain astronomi- 

 cal observations,) we shall find that nebular motion is inter- 



