en. ii.] RHYTHM. 303 



is the revolution of the nodes — a slow change in the position 

 of the orbit-plane, which after completing 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 extreme 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 com- 

 plex oscillations. And further we have variations in the 

 directions of the planetary axes — that known as nutation, 

 and that larger gyration which, in the case of the earth, 

 causes the precession of the equinoxes. 



" These rhythms, already more or less compound, are 

 compounded with each other. Such an instance as the secular 

 acceleration and retardation of the moon, consequent on the 

 varying eccentricity of the earth's orbit, is one of the 

 simplest. Another, having more important consequences, 

 results from the changing direction of the axes of rotation in 

 planets whose orbits are decidedly eccentric. Every planet, 

 during a certain long period, presents more of its northern 

 than of its southern hemisphere to the sun at the time of its 

 nearest approach to him ; and then again, during a like 

 period, presents more of its southern hemisphere than of its 

 northern — a recurring coincidence which, though causing in 

 some planets no sensible alterations of climate, involves in 

 the case of the earth an epoch of 21,000 years, during which 

 each hemisphere goes through a cycle of temperate seasons, 

 and seasons that are extreme in their heat and cold. Nor is 

 this all. There is even a variation of this variation. Tor 

 the summers and winters of the whole earth become more or 

 less strongly contrasted, as the eccentricity of its orbit 

 increases and decreases. Hence during increase of the 

 eccentricity, the epochs of moderately contrasted seasons 



