THE CAUSE OF THE GLACIAL PERIOD. ibT 



from September 22d to March 20th is only one hundred and 

 seventy-nine days, or seven days less. So that, while the 

 eartb is farther away from the sun during the summer, and 

 receiving daily less heat, the additional seven days occupied 

 by that part of the journey makes ample compensation, and 

 the absolute amount of heat received during the longer time 

 exactly equals that received by the earth during the shorter 

 half of its journey. It will be observed, also, that when the 

 summer in the northern hemisphere occurs at aphelion, the 

 summer in the southern hemisphere occurs in perihelion, thus 

 exactly reversing the conditions. 



Now it is claimed by Mr. Croll and others that when, in 

 either hemisphere, the winter is short and occurs in perihelion, 

 the climate of that hemisphere will be less favorable to the 

 production of glaciers than in the hemisphere where the 

 winter is long and in aphelion. Consequently, according to 

 their theory, the situation of the earth is now favorable to the 

 production of glaciers in the southern hemisphere, and un- 

 favorable to their production in the northern. This theory- 

 is not based, however, on the idea that the hemisphere whose 

 winter is in aphelion receives less heat from the sun than the 

 other hemisphere during that season, but upon the supposition 

 that the greater period occupied by the sun in passing through 

 aphelion when the winter nights are long, gives more oppor- 

 tunity for the loss of heat during winter in that hemisphere 

 by radiation. 



Xow, if it be correct that a winter in aphelion is f avoidable 

 to glaciation, and a winter in perihelion unfavorable, then, 

 from the astronomical changes which transfer this condition 

 periodically from one hemisphere to another, we can reason 

 that the northern and southern hemispheres are alternately 

 subjected to conditions favorable to glaciation. For, through 

 what is called the " precession of the equinoxes,'' the periods 

 of aphelion and perihelion are exactly reversed in their rela- 

 tions to the two hemispheres every 10,500 years. The points 

 at which the equinoxes occur are slowly slipping around, 

 making a revolution once in about 21,000 years; so that. 



