CHAP. VII.] DEEP-SEA TEMPERATURES. 303 
Were it not for the operation of the law on which 
the latter phenomenon depends, the entire ocean 
would long since have become solidified, and both 
sea and land rendered unfit for the habitation of 
living organisms. Unlike other bodies which ex- 
pand and become lighter with every rise in tempera- 
ture, water attains its maximum density, not under 
the lowest degree of cold, but at 39°5 Fahrenheit ; 
and consequently so soon as the superficial layer 
of sea is cooled down to this degree, it descends, 
and allows a fresh portion to ascend and be in 
turn cooled. This process is continued until the 
whole upper stratum is reduced in temperature to 
39°°5, when, instead of contracting further, it begins 
to expand and get lighter than the water beneath, 
floats on it, becomes further cooled down, and at 
28°65 is. converted into ice...,. = Thus. under the 
operation of an apparently exceptional law, the 
equilibrium of the oceanic circulation is maintained ; 
for whilst at the equator the mean temperature of 
the surface layer of water, which is 82°, gradually 
decreases, until at a depth of 1,200 fathoms it be- 
comes stationary at 39°5, and retains that tempera- 
ture to the bottom, within the Polar regions and 
extending to lat. 56° 25’ in either hemisphere, the 
temperature increases from the surface downwards 
to the isothermal line, beyond which it remains 
uniform as in the former case. Hence in lat. 56° 25’ 
the temperature is uniform the whole way from the 
surface to the bottom ; and as has been found by 
observation about lat. 70°, the isothermal line occurs 
at 750 fathoms below the surface.’’’ 
1 Dr. Wallich: North Atlantic Sea-bed, p. 99. 
