
_ Ser. II] CLIMATE IN ITS GEOLOGICAL RELATIONS. 23 
geologists were satisfied that no important change of climate could 
be attributed to change of eccentricity. 
It is to the luminous memoirs of Dr. James Croll that geology 
is indebted for the first fruitful suggestion in this matter, and for 
the subsequent elaborate development of the whole subject of the 
physical causes on which climate depends. His researches will be 
found in detail in his volume, Clamate and Time, 1875. He has been 
good enough, however, to draw up the following abstract of them for | 
the present work. 
“ Assuming the mean distance of the sun to be 92,400,000 miles, 
then when the eccentricity is at its superior limit, ‘07775, the 
distance of the sun from the earth, when the latter is in the aphelion 
of its orbit,is no less than 99,584,100 miles, and when in the 
perihelion it is only 85,215,900 miles. The earth is, therefore, 
14,368,200 miles farther from the sun in the former than in the 
latter position. The direct heat of the sun being inversely as the 


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Perihe 

N. WINTER SOLSTICE IN APHELION. N. WINTER SOLSTICE IN PERIHELION. 
Fic. 1.—EccentrIcITy oF THE EARTH’S ORBIT IN RELATION TO CLIMATE. 
square of the distance, it follows that the amount of heat received by 
the earth in these two positions will be as 19 to 26. The present 
eccentricity being -0168, the earth’s distance during our northern 
winter is 90,847,680 miles. Suppose now that, from the precession 
of the equinoxes, winter in our northern hemisphere should happen 
when the earth is in the aphelion of its orbit, at the time that the 
orbit is at its greatest eccentricity; the earth would then be 
8,736,420 miles farther from the sun in winter than it is at present. 
The direct heat of the sun would therefore, during winter, be one- 
fifth less and during summer one-fifth greater than now. This 
enormous difference would necessarily affect the climate to a very 
great extent. Were the winters under these circumstances to occur 
