Cir. XIII.] 



THE PRECESSION OF THE EQUINOXES. 



275 



said to hold true during' vast periods, so slow are the changes 



com 



cession, two 



movement 



usually run through while the excentricity remains essentially 

 the same. 



This is a point of great geological importance when we are 

 considering to which of three causes fluctuations in climate 



at successive epochs may have been mainly due ; namely, 

 whether to precession, or to variations in excentricity, or to 

 changes in the geography of the globe. To a certain extent 

 all these three causes must combine to produce the climate 

 of any given period. Thus, for example, at present it may 



be stated, first, that the arctic and antarctic regions 



from 



the poles to the 60th parallels of latitude, are in an ex- 



ceptional state in 



regard 



to the proportion of land to 



sea which they contain, and that there is consequently a 

 smaller proportion of land in equatorial regions where its 



Hence the existing: 



reo- 



when the condition of things was less abnormal. 



presence would augment the heat, 

 graphical conditions favour less genial climates in polar and 

 temperate latitudes than those which have usually prevailed 



Secondly, 



the present excentricity of our orbit, though moderate in 

 degree, causes the earth to receive more heat when it is 

 nearer, and less when it is farther from the sun ; and Thirdly, 

 in the northern hemisphere, in the present state of precession, 

 winter occurring in perihelion is rendered milder and shorter 

 than it would otherwise be. 



The present excentricity of the earth's orbit amounts, as 

 before stated, p. 271, to no more than a million and a half of 

 miles in opposite directions from its mean distance from the 

 sun, which is ninety-one millions. A good illustration is af- 

 forded of the slight influence on climate of this moderate ex- 

 centricity as compared to that of geographical conditions, by 

 the result of Dove's observations on the mean temperature of 

 the whole surface of the globe in perihelion as contrasted with 

 its temperature in aphelion. The difference of distance from 

 the sun at these two periods is no less than -^th of the mean 

 distance ; the planet therefore ought to be colder at the one 

 time and hotter at the other, not merely by - 3 \th of the heat 



T 2 



