206 THE RHYTHM OF MOTION. 



sents 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 altera- 

 tions 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. Xor is this all. There is even a 

 variation of this variation. For the summers and winters of 

 the whole Earth become more or less strongly contrasted, as 

 the excentricity of its orbit increases and decreases. Hence 

 during increase of the excentricity, the epochs of moderately 

 contrasted seasons and epochs of strongly contrasted sea- 

 sons, through which alternately each hemisphere passes, 

 must grow more and more different in the degrees of their 

 contrasts; and contrariwise during decrease of the excen- 

 tricity. So that in the quantity 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 changing position of the axis 

 at perihelion and aphelion, taking 21,000 years to complete; 

 and that involved by the variation of the orbit's excen- 

 tricity, gone through in millions of years. 



§ 84. Those terrestrial processes whose dependence on 

 the solar heat is direct, of course exhibit a rhythm that cor- 

 responds to the periodically changing amount of heat which 

 each part of the Earth receives. The simplest, though the 

 least obtrusive, instance is supplied by the magnetic varia- 

 tions. In these there is a diurnal increase and decrease, an 

 annual increase and decrease, and a decennial increase and 

 decrease ; the latter answering to a period during which the 

 solar spots become alternately abundant and scarce : besides 

 which known variations there are probably others corre- 

 sponding with the astronomical cycles just described. More 



