FREEZING OF SEA-WATER. 173 



and below, to be only 2°«2 C. (35°'96 F.) The 

 water of the ocean does not possess the property, 

 which fresh water has, of attaining its greatest 

 density at a certain temperature ; it becomes denser 

 and denser the more it is cooled, till at last at a 

 temperature of- 3° or-4° (26°'6 or 24°*8 P.) it 

 solidifies. At the moment of freezing the greatest 

 portion of its dissolved salt separates, so that the 

 ice formed from sea- water possesses almost perfectly 

 the properties of, and is quite as light as, ice ob- 

 tained from pure water. Thus sea-water, so 'long 

 as it remains liquid, while cooling goes on, increases 

 in weight, and sinks, leaving its place to the warmer 

 liquid that flows in to supply it ; and so the cooling 

 of its surface must reach gradually to very great 

 depths, and therefore can go on but very slowly at the 

 surface itself. This then explains how it is that we 

 find open water, at some distance from the coast, 

 even within the Arctic Circle in the middle of 

 winter. In the Polar Sea, too, at several points, 

 the temperature has been examined even to the 

 depth of 6000 feet. It seems that it cannot any- 

 where sink much below— 2° C. (28°*4 F.) The 

 bottom of the sea has nowhere been found frozen. 



The following table shows the most probable 

 mean temperatures at the surface of the Atlantic 

 Ocean in different latitudes : — 



