SOURCES OF HEAT IN THE SEA 49 



Sources of Heat in the Sea. 

 • There are two possible sources of heat for oceanic water — 



1. The inner heat of the earth. 



2. The sun's rays. 



According to Geikie, the température of the earth increases 

 one degree Fahrenheit for every 64 feet of descent. An increase 

 of température in the bottom layers of oceanic water as a resuit 

 of the internai heat of the earth has long been suspected. In 

 1840 Aimé looked for it, but at that time the thermometers in 

 use were not sufficiently accurate. More recently indications 

 of a rise of température towards the bottom hâve been observed, 

 but since the increase in pressure and the internai heat of the 

 earth would both tend to increase the température of the bottom 

 layers of water, it is impossible to say at présent how much is 

 due to each cause separately. 



Certainly the increase in pressure causes the bottom-water 

 températures to be higher than they otherwise would be. The 

 inner heat of the earth passes into the lower layers of water by 

 convection and probably increases the température there to a 

 small amount, an amount which has not yet been accurately 

 ascertained. 



The second source of heat — the sun's rays — is by far the 

 more important. 



Occasionally the surface layers may be heated by contact 

 with warm air, but as a rule the température of the sea is 

 higher than that of the air above it. 



The sun's rays penetrate the water, the dark heat rays are 

 absorbed near the surface, while the light rays, which also 

 contain some heat, penetrate to a depth of several hundred 

 mètres (p. 76) before disappearing altogether. 



The sun's rays are strongest in the tropics, diminishing to 

 the north and south. 



There is a small daily variation in the température of the 

 surface layer of water in the océan. The average variation is 



4 



