Rev. E. Hill—LEccentricity and Glacial Epochs. 17 
_ Our verdict, then, on Dr. Croll’s celebrated theory, must still be 
“Not Proven.” It would be sad indeed to send to the waste-paper 
basket those splendid tables of varying eccentricities. It would go 
to the heart of a mathematician to dismiss the question from the 
dominion of mathematics, and to abandon hope of calculating an era 
for a chronology. I cannot bring myself to this. I think that Dr. 
Croll has attacked the problem in a right manner, and that a solution 
will be found based on gain and loss of heat. J am even inclined to 
believe his opinion to be the true one, and that variations in eccen- 
tricity did bring about the Glacial Period. Extremes of heat and 
cold would tend to produce atmospheric disturbances, and these seem 
practically inimical to warmth. Extremes might also destroy vege- 
tation and so affect climates for the worse. 
There is, however, another mode of action, which, so far as I know, 
has not before been suggested. The earth loses the heat which it 
receives mainly or entirely by radiation; we have not yet considered 
the laws of this radiation. The amount increases as the temperature 
rises, decreases as it falls. If these changes were proportional, there 
would be nothing to need consideration. But they are not propor- 
tional. The amount of radiation increases faster than the temperature. 
The heat sent off in summer is more than in proper proportion to 
that sent off during winter. Suppose a uniform supply of heat. 
Then the earth would reach a steady temperature, such that there 
- would be a uniform emission of heat equal to the uniform supply. 
Now, change this uniform supply into a supply so fluctuating, that 
the total annual receipt of heat is unchanged. The temperature will 
fluctuate, and consequently the emission of heat. The excess above 
the mean of the heat emitted during the warm part of the year will 
_ exceed the deficiency below the mean of the emission during the cold 
_ part. The storage of heat during the summer will therefore fall 
short of the consumption during the winter, and the average tempe- 
rature must fall. Or, putting the action in a different aspect, as 
the supply of heat to a body is increased, the temperature increases 
in a continually diminishing ratio. Thus any increase in the supply 
of heat would not increase the temperature quite so much as an equal 
decrease in the supply would decrease the temperature. Accordingly 
fluctuations in the rate of supply, even when not affecting the 
average amount supplied, must diminish the average temperature. 
The Equator receives its heat uniformly compared to the Poles, which 
are subjected to extreme vicissitudes. Hence the Equatorial mean 
temperature ought to exceed that of the Poles, even if the total 
amount received were the same for both. And in the same way 
increase of eccentricity, aggravating the extremes of temperature, 
' must produce a diminution in the mean. What will be the extent 
of the diminution is a problem which deserves attention. So far as 
a yet been able to form an estimate, it is appreciable, but not 
arge. 
This, then, is the position in which the question at present stands. 
Increase of eccentricity acts in the direction of a Glacial period, pro- 
bably through increased reflexion of heat by snow and ice, almost 
DECADE II.—VOL. VII.—NO. I. 2 
