RHYTHxM 



according to the structural complication of the 

 bodies in question. Let us exhibit a few in- 

 stances of this rhythmical action, before we pass 

 to the stupendous consequences of the theorem 

 which I have been endeavouring to elucidate. 

 Some of the chief instances to be gathered from 

 astronomic phenomena have been so admira- 

 bly presented by Mr. Spencer, that I cannot do 

 better than to quote in full his concise state- 

 ment. 



Along with the planetary revolutions which 

 furnish the illustration with which I began this 

 chapter, " the solar system presents us with va- 

 rious rhythms of a less manifest and more com- 

 plex kind. In each planet and satellite there is 

 the revolution of the nodes — a slow change in 

 the position of the orbit-plane, which after com- 

 pleting itself commences afresh. There is the 

 gradual alteration in the length of the axis 

 major of the orbit, and also of its eccentricity, 

 both of which are rhythmical alike in the sense 

 that they alternate between maxima and minima, 

 and in the sense that the progress from one ex- 

 treme to the other is not uniform, but is made 

 with fluctuating velocity. Then, too, there is 

 the revolution of the line of apsides, which in 

 course of time moves round the heavens — not 

 regularly, but through complex oscillations. 

 And further we have variations in the directions 

 of the planetary axes — that known as nutation, 

 171 



