268 



CHAPTEE XXIL 



VICISSITUDES m CLIMATE HOW FAR INFLUENCED BY 



ASTRONOMICAL CHANGES. 



THE PRECESSION OF THE EQUINOXES, AND VARIATIONS IN THE EXCENTRICITY 

 OF THE EARTH'S ORBIT CONSIDERED AS AFFECTING CLIMATE. — UNDER WHAT 



CONDITIONS EXTREME EXCENTRICITY MAY EXAGGERATE COLD. MEASUREMENT 



OF HEAT.— TEMPERATURE OF SPACE. CLIMATES OF SUCCESSIVE PHASES OF 



PRECESSION. — VARIATION IN THE OBLIQUITY OF THE ECLIPTIC. RADIATION OF 



HEAT IMPEDED BY A COVERING OF SNOW. — QUANTITY OF POLAR ICE AND ITS 



INFLUENCE IN ALTERING THE LEVEL OF THE OCEAN. MIGRATIONS OF THE 



GREENLAND WHALE. —LIQUEFACTION AND EVAPORATION OF SNOW. HOW FAR 



THE DATES OF FORMER GLACIAL PERIODS MAY BE FIXED BY COMPUTING THE 

 ERAS OF MAXIMUM EXCENTRICITY. DATES OF THE NEOLITHIC AND PALEO- 

 LITHIC ERAS. OF THE INTENSITY OF GLACIAL COLD. DURATION OF THE 



GLACIAL PERIOD AS COMPARED TO SUCCESSIVE TERTIARY, SECONDARY, AND 

 PRIMARY EPOCHS. — SUPPOSED VARIATIONS IN THE TEMPERATURE OF SPACE. — 

 SOLAR MAGNETIC PERIODS AND VARIABLE SPLENDOUR OF THE STARS.— SUPPOSED 

 GRADUAL DIMINUTION OF THE EARTH'S PRIMITIVE HEAT. — SUPPOSED CHANGE 

 IN THE POSITION OF THE AXIS OF THE EARTH* S CRUST. 



the excen- 

 mate. — In 



The precession of the equinoxes and variat 

 tricity of the earth's orbit considered as aff< 

 the last chapter we were chiefly occupied in considering 

 how far changes in physical geography or in the position of 

 land and sea may account for those variations of climate to 

 which geology bears testimony. I endeavoured to show that 

 this class of causes must always have exerted a dominant 

 influence ; and we may now consider how far those variations 

 in the relative position of our planet to the other heavenly 

 bodies which astronomy reveals to us, 

 climate. In other words, to what extent may the precession 

 of the equinoxes, the revolution of the apsides, and the 

 excentricity of the earth's orbit, have co-operated with geo- 

 graphical conditions in bringing about fluctuations of tempe- 

 rature in the habitable parts of the globe in former ages. 



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