84 A TEXTBOOK OF OCEANOGRAPHY 



pure ice is, according to Bunsen, 09167; for sea-ice it varies 

 from 0903 to 0959 ; probably 092 is a fair average. 



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

 calculated from theoretical considérations. Witii an average 

 température of 5° below the freezing-point of sea-water ice 

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

 200 days 100 centimètres. If the température fall to 20° below, 

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

 200 days 201 centimètres, and after 300 days 246 centimètres 

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

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

 régions from i to i| mètres is the rule, since hère the tempéra- 

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

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

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

 rapid — from the 2nd to the 20th December 25-4 centimètres ; 

 from the 20th December to the I9th February 56"4 centimètres ; 

 to the 22nd Mardi 73 centimètres ; then to the end of May 

 72 centimètres ; after which in June it rapidly commenced 

 to melt. 



Snow is a bad conductor of beat, 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 pièces of fîeld-ice frozen or pressed 

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

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

 superfîcially 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 considérable 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 considérable danger 



