84 A TEXTBOOK OF OCEANOGRAPHY 



pure ice is, according- to Bunsen, 0-9167; for sea-ice it varies 

 from 0-903 to o - 959 ; probably 0*92 is a fair average. 



The rate of growth of sea-ice in thickness has been 

 calculated from theoretical considerations. With an average 

 temperature of 5 below the freezing-point of sea-water ice 

 forms at the following rate : for 100 days 71 centimetres; for 

 200 days 100 centimetres. If the temperature fall to 20 below, 

 then the ice thickness is after 100 days 142 centimetres, after 

 200 days 201 centimetres, and after 300 days 246 centimetres 

 thick. Ice-shoals of one winter's growth are rarely more than 

 2 metres thick, and scarcely ever exceed 3 metres. In Antarctic 

 regions from i to ij metres is the rule, since here the tempera- 

 tures are not so low and the salinity is higher than in the 

 Arctic. Accurate observations on the rate of growth of sea-ice 

 have been made by Drygalski. At first the growth is very 

 rapid from the 2nd to the 2oth December 25-4 centimetres; 

 from the 2Oth December to the i9th February 56*4 centimetres ; 

 to the 22nd March 73 centimetres; then to the end of May 

 72 centimetres ; after which in June it rapidly commenced 

 to melt. 



Snow is a bad conductor of heat, its coefficient being about 

 one-tenth that of ice. So there are strong agencies preventing 

 the freezing of the sea below a small depth. 



The field-ice, which is usually formed near the shore, soon 

 becomes disturbed by wind and sea, and it becomes uneven. 

 Floe-ice consists of several pieces of field-ice frozen or pressed 

 together. Pack-ice is formed from broken-up floes which have 

 to a certain extent closed together again. This pack-ice is 

 superficially very uneven, as it is driven together and heaped 

 up by winter storms. When there are leads or lanes of water 

 forming more or less navigable channels the pack-ice is said 

 to be open; when it is not possible to navigate the pack, it is 

 said to be close. This pack-ice, which is in constant movement, 

 exercises considerable pressure on ships embedded in it in 

 fact, unless ships are specially built to withstand this pressure, 

 as was Nansen's ship the Fram, there is considerable danger 



