SEA-ICE . 83 



Firth of Forth as 100 : 925 ; and pure water compared with 

 sea-water taken outside the Firth as 100 : 92. The transmission 

 of waves of sound through water dépends on the non-com- 

 pressibihty of the water. Sound travels through water four 

 times as rapidly as through air. 



Electric Conductivity and Radioactivity . 

 If the electric conductivity of pure water be taken as nil, 

 then that of sea-water at 0° C. and of salinity 35 per mille will 

 be 00293 ohms. Radioactivity is not observable in sea-water. 



Sea-Ice. 



In the neighbourhood of the Pôles the température of the 

 sea is so low that large areas become frozen. This sea-ice, 

 together with river and glacier ice derived from the land, is 

 influenced by the prevailing océan currents, and is met with, 

 especially in the form of icebergs, distributed over extensive 

 areas in high latitudes in both the Northern and Southern 

 Hémisphères. Of the three kinds of ice mentioned, river-ice 

 is the least important. It is missing altogether in the Antarctic 

 régions, where there are no rivers ; in the Arctic it is formed 

 in the great rivers of the Siberian and North American plains. 



Sea-ice is frozen sea-water. Sea-water does not freeze at 

 0° C. (p. 72), but at an appreciably lower température, this 

 température depending on the salinity. Ground-ice is fre- 

 quently formed in shallow water close inshore in the Baltic 

 before the surface water is frozen over. In winter the fisher- 

 men's nets are frequently hauled in covered with ice and with 

 the fish entangled quite frozen. 



The formation of field-ice, as the flat ice formed inshore is 

 called, prevents, or at any rate hinders, the freezing of the sea 

 to any great depth — firstly because the ice is a bad conductor 

 of heat and prevents the loss of beat from the underlying water 

 by radiation, and secondly because the salts extruded from the 

 field-ice make the underlying water of higher salinity and 

 consequently lower freezing-point. The spécifie weight of 



