RHYTHM 



epochs of strongly contrasted seasons, through 

 which alternately each hemisphere passes, must 

 grow more and more different in the degrees 

 of their contrast ; and contrariwise during de- 

 crease of the eccentricity. So that in the quan- 

 tity of light and heat which any portion of the 

 earth receives from the sun, there goes on a 

 quadruple rhythm : that of day and night ; that 

 of summer and winter ; that due to the chang- 

 ing position of the axis at perihelion and aphe- 

 lion, taking 21,000 years to complete ; and that 

 involved by the variation of the orbit's eccen- 

 tricity, gone through in millions of years." 1 



The astronomic rhythms here enumerated are 

 peculiarly interesting from the fact that, owing 

 to their comparatively simple character, they 

 are susceptible of mathematical treatment, so 

 that their direct dependence on the principle 

 of the persistence of force can be quantitatively 

 demonstrated. In ascending to the order of 

 phenomena next above them in point of com- 

 plexity — the geologic phenomena occurring on 

 the earth's surface — we enter a region where 

 such quantitative proof, save of a very crude 

 sort, cannot be obtained. The great complex- 

 ity of geologic as contrasted with astronomic 

 rhythms is shown by the fact that whereas on 

 the one hand we can readily calculate the vari- 

 ations of eccentricity in the earth's orbit which 



1 First Principles, pp. 256, 257. 

 173 



