ARCTIC SEA ICE 99 



in their transpositions and alterations, bring about the breaking 

 up of the protruding edges of ice. As a consequence of this the 

 mechanical destruction and partial disappearance of the ice proceeds, 

 and, therefore, the number and size of areas of open water increase. 

 This in turn affords more liberty for the ice to move, helping pressure 

 ridges and pressure areas of larger extent to reform and thus increas- 

 ing the strength of the Arctic Pack to the point where its annual cycle 

 of life may begin anew. 



As to the composition of the ice of the Arctic Pack the measure- 

 ments and survey by Makarov in 1899, referred to above, gave 

 the following results. Late in June (June 19, in latitude 79° 10' N. 

 and longitude 9° 5' E.) the Arctic Pack was composed of the following 

 ice: 



Ice of an average thickness of 2 meters . . . 70% of the total area surveyed 

 Ice of an average thickness of 1.3 meters . . 25% of the total area surveyed 

 Open water and leads 5% of the total area surveyed 



THE PACK ICE 



In the pack ice only under conditions of calm water and low tem- 

 perature of air does the ice finally get so strong that it solidly cements 

 together the separate pieces into more extensive and stable areas. 

 Up to May the ice increases in thickness, until it attains about 2 

 meters on the average. These areas, however, in their turn undergo 

 breaking up and heaping up throughout the winter and spring, until 

 the breaking up of the sea in summer, i. e. until the breaking up of 

 the fast-ice, whereupon the winter pack ice receives more liberty 

 of motion and consequently is subjected to more frequent and strong 

 shocks and pressure, a circumstance which, together with the process 

 of melting, breaks it into smaller constituents, which partly are 

 destroyed and disappear entirely, forming the larger areas of open 

 water, and partly are left till the time of formation of the ice, supply- 

 ing the next cycle with pieces of many-years-old ice for insertion into 

 all the three classes of ice that make up the cover of the Arctic Sea. 



THE FAST-ICE 



The annual life cycle of the fast-ice (Fig. 8) embraces its formation, 

 development, existence, and disappearance as a separate class of the 

 ice cover of the Arctic Sea, which, being destroyed in summer, partly 

 disappears but, once broken up and detached from the shore, passes 

 over into the pack ice and, as a component of the latter, partly dis- 

 appears, partly passes over into the many-years-old forms. 



The factors favorable for the freezing of sea water and the forma- 

 tion of new ice in general are (besides the physico-chemical properties 

 of sea water) the following physico-geographical conditions: lowering 



