THE ENGINE OF THE AIR 163 



of Cro-Magnon man. In geological time this icecap is a mod- 

 em phenomenon. The history of modern man begins with 

 the recession of the cap about 15,000 years ago. How modern 

 this is from a geological point of view is best stated by measur- 

 ing the entire period of the earth's existence from the Archean 

 or earliest geological times as one year. This would give a value 

 of only a little over a minute to the existence of historic time, 

 or the last 8,000 years; hence we can feel very much up-to-the- 

 minute in starting the history of modern weather conditions 

 over the Ocean River with the recession of the most recent 

 icecap, which extended through the Great Lakes to Boston 

 and covered England just south of London and extended 

 thence through the Baltic. 



Today we live in a glacial climate — as defined by the pres- 

 ence of icecaps at the poles. But this kind of climate is not 

 normal if we consider the calendar of geological time: glacial 

 climates have existed less than one percent of that total time. 

 Our planet has experienced these periodic times of ice because 

 the greater part of it is covered with water which in the polar 

 regions never varies far from freezing temperature. A relatively 

 slight variation in the uniform temperature of the ocean due 

 to the tipping of the earth's axis, a warmer current from the 

 tropics due to increased solar radiation, or sudden geological 

 changes in the earth's crust, could make the difference be- 

 tween a shrinking or expanding icecap and thus affect the 

 climate of the whole world. 



R. J. Russell, of the Soil Conservation Service, explains the 

 fluctuation of an icecap — according to Brooks — as follows: 

 ''As long as winter temperatures remain above 28 degrees F., 

 the approximate freezing point of ocean water, the polar seas 

 remain open. At slightly lower temperatures, the ice frozen 

 during the winter melts again in the following summer, and 

 the seas remain effectively open. But if the winter temperature 

 falls about 5 degrees F. lower than the freezing point of ocean 

 water, an icecap will form. Its growth will be slow at first . . . 



