THE ICE AGE. 



595 



The gradual decrease of temperature upon the earth, which was the 

 cause of the Glacial period extending over the north of Europe, has been 

 attributed to the eccentricity of the earth's orbit ; and here astronomy steps 

 in to our assistance. We have read in the chapters on Astronomy, how the 

 movement of the earth, like a top near the end of its " spin," causes the 

 " precession of the equinoxes," and in connection with this phenomenon the 

 earth's orbit becomes more and more circular at certain periods of thousands 

 of years, and goes away from the sun. We therefore receive the light and 

 heat at a greater angle. Consequently, less heat is received, and ice is 

 formed, as at the North and South Poles at present. 



Fig. 685. Drift Ice. 



Doctor Croll has pointed out that the great eccentricity of the earth's 

 orbit existed about 210,000 years ago, when there was a difference between 

 the nearest and farthest position of the earth and the sun of 12,000,000 of 

 miles at least.* This is a very considerable distance even in the enormous 

 spaces which intervene between us and the other planets of the solar system, 

 and about that time the Glacial period arrived. Perhaps we may make this 

 clearer by going back to the precession of the equinoxes. 



The earth moves in an orbit called an ellipse, and the sun is not in the 

 centre of this nearly circular path. We can now understand that the earth 

 comes nearer to the sun sometimes and recedes at others. These points of 



* It might reach 14,000,000 of miles at a maximum. 



