310 MAN AND THE GLACIAL PERIOD. 



longer summers, when it is so much farther away from 

 the sun. Thus' the theory rests at last upon the question 

 what would become of the heat reaching the earth in 

 these differing conditions. It is plausibly urged by Mr. 

 Croll that when a hemisphere of the earth is passing 

 through a period of long winters the radiation of heat will 

 be so excessive that the temperature would fall much 

 below what it would during the shorter winters; and 

 so ice and snow would accumulate far beyond the usual 

 amount. It is also supposed that the effect of the sum- 

 mer's sun in melting the ice during the short summer 

 would be diminished through natural increase of the 

 amount of foggy and cloudy weather. 



Adhemar's theory is supposed by Sir Robert Ball, 

 Royal Astronomer of Ireland, to be considerably re-enforced 

 by a discovery which he has made concerning the dis- 

 tribution of heat upon the earth during the seasons cul- 

 minating in the summer and winter solstices. Croll had 

 assumed, on the authority of Herschel, that a hemisphere 

 of the earth during the longer winter in aphelion would 

 receive the same actual amount of heat which would fall 

 upon it during the shorter summer in perihelion ; whereas, 

 according to Dr. Ball's discoverv, " of the total amount of 

 heat received from the sun on a hemisphere of the earth 

 in the course of a year, sixty-three per cent is received dur- 

 ing the summer and thirty-seven per cent during the win- 

 ter."* When, therefore, the summers occur in perihelion 

 the heat is more intense than Croll had supposed, and, 

 at the same time, the winters occurring in aphelion 

 are more deficient in heat than he had assumed. This 

 discovery of Dr. Ball will not, however, materially affect 

 the discussion of Croll's theory upon its inherent mer- 

 its, since it is simply an intensification of the causes 

 invoked by him. We will therefore let it stand or fall 



* Cause of an Ice Age, p. 90. 



