^46 THE REVOLUTIOSS OF THE CRUST OF THE EARTH. 



glacial epoch should coiucide with the Miocene formation. The eccen- 

 tricity then attained its maximum ; it was 0.074664. 



The deterioration of the climatic conditions of the globe is not con- 

 stant in its persistence upon a hemisphere during the entire continuance 

 of great eccentricity ; it alternates by the combination of the eccen- 

 tricity with the precession of the equinoxes. Let us consider the influ- 

 ence of the latter. 



The precession of the equinoxes is produced by the attraction of the 

 sun upon the protuberant part of the earth, which tends to restore the 

 equator to the plane of the ecliptic, which would take place without the 

 rotation of the earth. At every j)eriod of the year except at the equi- 

 noxes, every part of the equator above and below the plane of the eclip- 

 tic tends to fall into this plane, but at the same time a counteracting 

 influence exists in the diurnal rotation ; these two forces combined pro- 

 duce a resultant which cuts the ecliptic at a point much nearer than the 

 place where the intersection would have occurred if the protuberance 

 had not existed. The consequence is a displacement of the plane of the 

 equator; the poles describe upon the starry vault a circle completed in 

 about 25,900 years. This is the period of the precession of the equi- 

 noxes, if compared with the stars, but it is much less if compared with 

 the great axis of the earth's orbit, which moves in the contrary direction. 

 It is then only 21,000 years. 



It was in the year 1248 of our era that the first day of the boreal 

 winter coincided with the passage of the earth to the perihelion.* 

 From this time until 1869 the major axis and the radius vector of the 

 spring equinox approached 10° 40' 39". Observation showed that it 

 was also from this period that the heat of the boreal hemisphere com- 

 menced gradually to diminish, while that of the austral hemisphere in- 

 creased in proportion. It is certainly not merely a chance coincidence 

 that in general the climate of Europe is deteriorating, that Greenland 

 is covered with ice, that the colony of Iceland is disappearing, that ice 

 encumbers the Spitzbergen Island, Behring's Strait, and Baffin's Bay. 

 It should also be observed, that it was about this period that the gla- 

 ciers of the Alps commenced to extend more and more, and that the 

 culture of the vine has disappeared in many localities of France. On 

 the other hand, to what must be attributed the diminution of the aus- 

 tral glacier, evident from comparison of the reports of Captain Cook 

 with those of modern travelers, if not to an amelioration of the climatic 

 condition of the austral hemisphere f Can we seriously attribute the 

 retreat of the line of permanent snow in the Cordilleras to an elevation 

 of this mountain chain"? 



All these facts must convince the mind that in fact the climate of the 

 boreal hemisphere is deteriorating, while that of the opposite hemisphere 

 is gradually improving, commencing at an epoch which coincides very 

 ''J. Adh6inar, Les E^colutions de la Mer, 2d editiou, p. 25. 



