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CHAPTER IX. 



EXTERNAL FACTORS. 



§ 148. When illustrating the rhythm of motion (First 

 Principles, § 83) it was pointed out that besides the daily 

 and annual alternations in the quantities of light and heat 

 which any portion of the Earth's surface receives from the 

 Sun, there are alternations which require immensely-greater 

 periods to complete. Reference was made to the fact that 

 " 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 it 

 causes in some planets no sensible alterations of climate, in- 

 volves, in the case of the Earth, an epoch of 21,000 years 

 during which each hemisphere goes through a cycle of tem- 

 perate seasons, and seasons that are extreme in their heat 

 and cold." Further, we saw that there is a variation of this 

 variation. The slow rhythm of temperate and intemperate 

 climates, which takes 21,000 years to complete itself, under- 

 goes exaggeration and mitigation during epochs that are far 

 longer. The Earth's orbit slowly alters in form: now ap- 

 proximating to a circle, and now becoming more eccentric. 

 During the period in which the Earth's orbit has least 

 eccentricity, the temperate and intemperate climates which 

 repeat their cycle in 21,000 years, are severally less tem- 



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