16 PHYSICAL GEOGRAPHY OP THE SEA. 



The temperature of the sea apparently never sinks at any 

 depth below — 3-5° C. This is about the temperature of the 

 maximum density of sea water, which contracts steadily till just 

 above its freezing point ( — 3-67° C), when kept perfectly still. 



If we include in the tropical seas all that part of the ocean 

 where the surface temperature never falls below 68° F., and 

 where consequently living coral reefs may occur, we find that it 

 nearly equals in size the temperate and cold ocean-regions 

 added together. This distribution of the waters over the surface 

 of the globe is of the highest importance to mankind ; for the 

 immense extent of the tropical ocean, where, of course, the 

 strongest evaporation takes place, furnishes our temperate zone 

 with the necessary quantity of rain, and tends by its cooling 

 influence to diminish the otherwise unbearable heat of the 

 equatorial lands. 



The circumstance of ice being lighter than water also con- 

 tributes to the habitability of our earth. Ice is a bad con- 

 ductor of heat; consequently it shields the subjacent waters 

 from the influence of frost, and prevents its penetrating to 

 considerable depths. If ice had been heavier than water, 

 the sea-bottom, in higher latitudes, would have been covered 

 with solid crystal at the very beginning of the cold season; 

 and during "the whole length of the polar winter, the per- 

 petually consolidating surface-waters would have been con- 

 stantly precipitated, till finally the whole sea, far within the 

 present temperate zone, would have formed one solid mass of 

 ice. The sun would have been as powerless to melt this pro- 

 digious body, as it is to dissolve the glaciers of the Alps, and 

 the cold radiating from its surface would have rendered all the 

 neighbouring lands uninhabitable. 



The mixture of the water of rivers with that of the sea pre- 

 sents some hydrostatic phenomena which it is jurious enough 

 to observe. Fresh water being lighter, ought to keep at the 

 surface, while the salt water, from its weight, should form the 

 deepest strata. This, in fact, is what Mr. Stephenson observed 

 in 1818 in the harbour of Aberdeen at the mouth of the Dee, 

 and also in the Thames near London and "Woolwich. By taking 

 up water from different depths with an instrument invented for 

 the purpose, Mr. Stephenson found that at a certain distance 



