THE ENGINE OF THE AIR 165 



the slow cosmic forces of the sun and moon, the earth takes 

 about 26,000 years to complete the cycle of variations of its 

 orbit, so that approximately every 13,000 years the earth will 

 pass through its perihelion, with its northern and southern 

 hemispheres alternately turned toward the sun. This is called 

 the procession of the equinox. In addition to this change, dur- 

 ing every 40,000 years the earth's inclination of axis of rota- 

 tion completes a cycle of changes in relation to the plane of 

 its orbit. The existence of winter and summer are due to this 

 inclination, hence any change will affect the climate. In addi- 

 tion to this, Huntington makes a case for variations in sun 

 spots that cause fluctuations in the radiation of solar energy 

 and hence in the convection of heat in air and sea from south 

 to north. Other scientists believe that the changes in the 

 icecap can be explained by geological phenomena like the 

 opening or closing of the isthmus at Panama. 



Von Konow makes an interesting case for this last con- 

 jecture. He writes that during the last glacial period the waters 

 of the Equatorial Current flowed through the Caribbean into 

 the Pacific, and there was no Gulf Stream as we know it today. 

 This caused a drop of mean temperature to the north and a 

 consequent extraordinary spread of the arctic icecap. This 

 additional cold in the north created greater convection, which 

 drew off to higher regions the normal heat transference from 

 the earth. It is likely that, even with the icecap extending down 

 over the continent, the part of North America free of ice may 

 have had a warmer climate, due to the fact that with the 

 Gulf Stream gone the cold Labrador Current did not come 

 down our eastern coasts. 



The final factor that helped form the icecaps is not dis- 

 puted, namely, that the rise of new mountain areas like the 

 Alps in Europe was favorable to the formation of extensive 

 glaciers. Only when the now high mountain ranges have been 

 leveled by long erosion, and glaciation has disappeared from 

 the face of the earth, will the milder winds and currents of 



