20 
THE OCEAN WORLD. 
and that the fresh water of the rivers constantly replaces the water 
which disappears by evaporation — we have the true explanation ol 
the saltness of sea water. “ It is a very simple theory, it is true,” adds 
M. I iguier, “ but one that we have found nowhere, and the responsi- 
bility of which we therefore claim. The chloride of sodium is by no 
means the only substance dissolved in sea water. It contains, besides, 
many other mineral substances; in short, every soluble salt on the 
face of the globe, and, along with them, portions of different metals in 
infinitely small quantities.” 
I he mean temperature of the surface of the sea is nearly the same 
as the atmosphere, so long as no currents of heat or cold interpose 
their perturbing influence. In the neighbourhood of the Tropics, it ap- 
pears that the surface of the water is slightly warmer than the ambient 
air, but experiments on the temperature of the soa from the surface to 
the bottom reveal, according to our author* “some evidence which 
establishes a curious law. In very deep water a perfectly uniform 
temperature of four degrees below zero prevails, which corresponds, as 
physics have established, to the maximum density of water. Under the 
Equator this temperature exists at the depth of seven thousand feet. In 
the Polar regions, where water is colder at the surface, this temperature 
is maintained at four thousand six hundred feet. The isothermal 
lines of four degrees form a line of demarcation between the Zones, 
where the surface of the sea is colder, and those where it is warmer 
than the bed of four degrees below zero.” This is more clearly shown 
in h lg. 4, which lepiesents a section of the ocean, the curved line 
which touches two points at the surface indicating the depths where 
the temperature is constantly fixed at four degrees. 
Dr. Maury s account of this phenomenon is asserted with less confi- 
dence. I he existence of an isothermal floor of the ocean, as he calls 
it, was first suggested by the observations of Kotzebue, Admiral 
Beechey. and Sir James C. Boss. “ Its temperature, according to 
Kotzebue, is thirty-six degrees Fahr., or four degrees Cent. ; the 
depth of this bed, of invariable and uniform temperature, is twelve 
hundred fathoms at the Equator; thence it gradually rises to the 
parallel ol about fifty-six degrees north and south, when it crops out, 
and there the temperature of the sea from top to bottom is conjectured 
* “ La Terre et leg Mens,” p. 517 . Troisieme Ed. 
