16 PHYSICAL GEOGRAPHY OF 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 surious 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 
