442 APPENDIX. 



4. These conditions all reversed would produce a maximum of glacia- 

 tion in the north temperate and polar regions such as evidently existed 

 at the beginning of the Post Tertiary period. 



It is probably within the power of physical astronomy to calculate the 

 epochs at which these maxima and minima have occurred. It is, how- 

 ever, a problem of considerable difficulty, involving, as it does, the rate of 

 precession of the equinoxes, the proper motion of the apsides, and the sec- 

 ular change in the obliquity of the ecliptic, none of which data are per- 

 fectly constant. According to recent determinations, the equinox com- 

 pletes a revolution in 25,868 years. The apsides move forward to meet 

 the equinox, so that perihelion has the same longitude once in 21,066 

 3 7 ears. The obliquity of the ecliptic returns to the same value in about 

 100,000 years. 



M. Adhemar has based an explanation of the occurrence of glacial pe- 

 riods upon the climatic effects of the precession of the equinoxes alone. 

 As the earth's axis is inclined to the ecliptic, the hemisphere which has 

 its winter in aphelion is not only farther from the sun than the other hem- 

 isphere during its winter, but also experiences a winter having about eight 

 days longer duration. The excess in the duration of its winter is partly 

 caused by the slower motion of the earth on that side of the equinoxes 

 which embraces the upper apsis, and partly also by the greater length of 

 the path on that side. This hemisphere is therefore subjected to an ex- 

 cess of cold. For reverse reasons, the other hemisphere enjoys more than 

 the mean warmth. 



In consequence, however, of the gyration of the axis in a period which 

 Adhe'mar takes at 21,000 years, it follows that at the end of 10,500 years 

 the hemisphere which had been turned away from the sun at aphelion be- 

 comes turned toward him. In other words, the climatic inequalities of 

 the two hemispheres become reversed. That hemisphere which for 10,500 

 years had been subjected to excessive glaciation, now enjoys excessive 

 warmth, and that which had enjoyed excessive warmth is visited by ex- 

 cessive cold. There are, therefore, two great seasons for each hemisphere 

 during the progress of the Annus Magnus, or Great Year. The summer 

 has a duration of 10,500 years, and the winter an equal duration. 



One thing farther should be remarked in connection with the accumu- 

 lation of masses of ice and snow about either pole, from whatever cause 

 the accumulation proceeds. Such an accumulation must necessarily 

 change the position of the centre of gravity of the earth-mass. That 

 centre must move toward the pole thus burdened. The fluent waters 

 upon the earth's surface, free to adjust themselves in equilibrium about 

 the centre of gravity, must change their distribution as the place of the 

 centre of gravity changes. During the glaciation of the southern hemi- 

 sphere, the waters will accumulate about the south pole ; and during the 

 glaciation of the northern hemisphere, they must accumulate about the 

 north pole. These alternating accumulations of the waters are adequate. 



