x The Hydrosphere 173 



water, one above another, the greater salinity of the warmer 

 water making its density greater than the colder but fresher 

 water above. A temperature curve of such a region 

 ("Atlantic 71 N. lat.") is shown in Fig. 33. The ends of 

 great glaciers reaching to the sea break off in the water 

 and float away in summer as icebergs ( 338). In the 

 Arctic regions the icebergs are lofty pinnacled masses, 

 often resembling cathedrals or castles several hundred feet 

 in height, with a covering of dazzlingly white snow, but 

 showing the true ice-colour of intense blue in their cracks 

 and caves. Lofty as these icebergs are, we know that as 

 ice has a density of about 0-9, only one-ninth of its volume 

 floats above water. The Antarctic icebergs are usually 

 flat-topped and table-like, but are far larger and of a deeper 

 blue colour than those of the Arctic regions. 35^" 



235. Temperature of Ocean Depths. The hot surface 

 water in the tropical zone is merely a film covering a vast 

 depth of cold water. Even although the surface is at 70 

 or 80, temperature of 40 or less is found at the depth of from 

 300 to 400 fathoms in almost all parts of the ocean. The 



FIG. 33. Curves of vertical distribution of Temperature in the ocean. Tempera- 

 ture is shown along the top ; depth down the side. The middle curve, for 

 example, shows 80 at the surface, 60 at 100 fathoms, 50 at 200 fathoms, 40 

 at 600 fathoms, and 35 at the bottom. 



fall of temperature is consequently very rapid from the sur- 

 face down to 400 fathoms in the tropics ; but much less 

 abrupt in the cooler regions to the north and south. Below 

 400 fathoms the fall of temperature to the bottom is every- 

 where very slight and gradual (Curve, "Pacific 5 N. lat.," 

 Fig. 33), and no matter how great the depth may be the 



