SOURCKS 01- HEAT IN TUB SEA 40 



SOURCES OF HEAT ix 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 temperature of the earth increases 

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

 of temperature in the bottom layers of oceanic water as a result 

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

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

 use were not sufficiently accurate. More recently indications 

 of a rise of temperature towards the bottom have been observed, 

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

 earth would both tend to increase the temperature of the bottom 

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

 due to each cause separately. 



Certainly the increase in pressure causes the bottom-water 

 temperatures 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 temperature 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 temperature 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 

 metres (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 temperature of the 

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



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