174 The Realm of Nature CHAP. 



bottom temperature of the open ocean remains near the 

 freezing point of fresh water. Five-sixths of the mass of the 

 ocean has a temperature under 40 F., so that taken as a 

 whole the hydrosphere is a body of cold water, its average 

 temperature being probably about 38 or 39. The prevail- 

 ing low temperature of the hydrosphere is explained mainly 

 by the great surface of water exposed in the Southern 

 Ocean to the influence of the cold Antarctic ice-continent, 

 and in less degree to the still colder winter weather within 

 the Arctic Circle. The surface drift of warm salt water 

 carried into the Southern Ocean from the north grows 

 gradually cooler and therefore denser, and sinks about 

 latitude 50 S. About this latitude also the comparatively 

 fresh and cold water drifting northward from the Antarctic 

 regions grows salter and sinks on account of the consequent 

 increase of density. The sinking water appears to be 

 drawn back by slow and massive movements to north and 

 south, thus maintaining the circulation of the ocean to its 

 greatest depths. 



236. Temperature in Enclosed Seas. Except in 

 polar regions the temperature at the bottom of the deep 

 ocean is much lower than the average winter temperature 

 of the air at sea-level ; but this is not the case for deep 

 enclosed seas. The common form of enclosed seas is 

 that of a basin, often descending to oceanic depths, but 

 barred off from the ocean by a sill. The Red Sea, for 

 example, is separated from the Indian Ocean at the Strait 

 of Bab-el-Mandeb by a sill rising to within 200 fathoms of 

 the surface, while it attains a depth of 1200 fathoms near 

 the centre, and the Indian Ocean in the Gulf of Aden is 

 still deeper. In the Red Sea the temperature at the surface 

 varies from over 85 in summer to about 70 in winter. 

 At the hottest season the rate of cooling is comparatively 

 rapid to a depth of 200 fathoms, where the temperature is 

 70, and from that level right down to the bottom the 

 temperature remains uniform all the year through. The 

 basin of the Red Sea is thus filled up to the lip with 

 uniformly warm water, whereas, as shown in Fig. 34, the 

 water of the Indian Ocean, nearer the equator, and with 



