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*5 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." l 



1 Dr. Wallich : North Atlantic Sea-bed, p. 99. 



