During Post-Miocene Times. 15 



amounted to thirty-six days, and, on the other hand, times 

 when the difference was only forty-eight hours. 



Indirectly the temperature of the earth is greatly affected 

 by these variations in the length of the seasons ; and when 

 the winter of either hemisphere is prolonged by thirty-six 

 days the earth absorbs less heat than it does now by one- 

 fifth part (Croll, C. and T., p. 56). Whenever the orbit 

 attains to a high degree of eccentricity the climate of one 

 hemisphere becomes intensely cold, while the other grows 

 hot, moist, and winterless. The one which cools dowu is 

 the one which has wintered in aphelion ; it finds itself 

 furthest from the sun just when it most needs its warmth. 

 The other hemisphere enters on its warm season as it is 

 approaching the sun, and it is heated up excessively in 

 consequence. 



Owing to the effects of precession, each hemisphere 

 exchanges places and climates with the other about every 

 10,500 years, so that this period is the term during which 

 either hemisphere can experience extreme heat or extreme cold. 

 But the orbital eccentricity may last many times longer, 

 and while it lasts the earth's climate will be marked by 

 remarkable oscillations of temperatures between the two 

 hemispheres. 



The last epoch of high eccentricity commenced 240,000 

 years ago, and, having lasted about 160,000 years, came to 

 a close 80,000 years since (Croll, G. and T., p. 325). During 

 this epoch each hemisphere . experienced about seven 

 oscillations of climate, passing through seven glacial and 

 seven sultry periods. The winter of one hemisphere was 

 always between eighteen and twenty-six days longer than 

 its summer, while the other hemisphere had its summers 

 from eighteen to twenty-six days longer than its winters. 

 In consequence of this the temperature of the cold hemi- 

 sphere was lowered from 29.5° to 37.7° F. below the present 

 winter temperature (G. and T., p. 320). 



But the full effect of these changes was attained indirectly. 

 The long warm period in one hemisphere caused its polar 

 ice-cap to disappear after a time, and as it melted off one pole 

 it accumulated around the other. Thus the earth's centre 

 of gravity became disturbed ; it shifted towards the loaded 

 pole, and, as it moved, it drew after it the fluids of the globe — 

 the sea, the plastic nucleus, if there be one, and perhaps the 

 atmosphere. As the ocean readjusted itself to the new 

 centre, by heaping up its waters around it, the low lands of 



