184 SPARKS FROM A GEOLOGIST'S HAMMER. 



The glaciating action would alternate annually, and only 

 as the seasons alternate.* 



As to the precession of the equinoxes, this results in 

 a change in the attitude of the earth's axis when in the 

 apsides. At present, when the earth is in perihelion, the 

 north pole is turned away from the sun, and the north- 

 ern hemisphere has winter. Suppose that in the course 

 of ages the north pole should become turned toward the 

 sun at time of perihelion: then, the obliquity remaining 

 the same, the force of the sun's rays would be increased 

 in the polar regions during the winter by all the amount 

 of the difference in the sun's summer and winter dis- 

 tances from the earth. If, for instance, the sun is now 

 three millions of miles nearer the earth in winter than 

 in summer, in the case supposed the sun would be three 

 millions of miles nearer the earth in summer than in 

 winter. That is, it would be six million miles nearer the 

 earth than at present in summer, and the same amount 

 remoter in winter. But it is in summer that the sun's 

 effects are produced on polar glaciation. The result would 

 therefore be to diminish northern glaciation. During 

 winter, as the sun is permanently below the horizon, or 

 near the horizon, it is comparatively immaterial whether 

 three millions of miles more remote or not. Half a cycle 

 in the precession of the equinoxes would therefore di- 

 minish northern glaciation and correspondingly increase 

 southern glaciation. The complete cycle of precession is 

 about 21,000 years ; hence from this cause we should 

 have in the northern hemisphere a secular winter every 

 21,000 years, followed, after 10,500 years, by a secular 

 summer. The southern hemisphere would have secular 

 * See a paper by James Croll in Geological Magazine, London, Sept. 1878. 



