214 Nature's .fllMractee. 



the glacial period is the change in the distri- 

 bution of land and water, which is supposed to 

 affect the distribution of heat over the earth's 

 surface. There is much in this theory that 

 commends itself as plausible. Another theory 

 supposes that the land in northern Europe and 

 America was elevated to a higher level at that 

 time than it is now. Others attribute it to 

 variation of temperature in space and of the 

 amount of heat radiated by the sun. The final 

 theory for accounting for the ice age is at- 

 tributed to what is termed the precession of 

 the equinoxes. In short, the precession of the 

 equinoxes means that the division between 

 summer and winter is changing gradually, so 

 that during a period of 10,500 years the sum- 

 mers are growing longer in the northern hemi- 

 sphere and the winters shorter. We are now 

 in the period of long summers, but in another 

 10,000 years we shall be in the period of short 

 summers and long winters. This difference of 

 time between the winters and the summers is 

 supposed to be sufficient to change the thermal 

 conditions sufficiently to produce an ice age. 



It is true that the conditions now are very 

 evenly balanced, so much so that in Switzer- 

 land the glaciers will increase for some years 

 together, when the conditions will change, 

 causing them to gradually recede. Several of 

 the theories that have been advanced present 

 evidences that are entitled to careful consider- 



