CHAPTER IX. 



EXTERNAL FACTORS. 



§ 148. When illustrating the rhythm of motion {First 

 PHiidples, § 94) 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 co-incidence which, though 

 causing 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, it; was pointed out that there is a varia- 

 tion of this variation. The slow rhythm of temperate and in- 

 temperate climates, which takes 21,000 years to complete, 

 itself undergoes exaggeration and mitigation, during epochs 

 that are far longer. The Earth's orbit slowly alters in 

 form : now approximating to a circle ; and now becoming 

 more eccentric. During the period at which the Earth's 

 orbit has least eccentricity, the temperate and intemperate 

 climates which repeat their cycle in 21.000 years, are 



