204 BRITISH FISHERIES 



fisheries scientific undertaking, are of such interest 

 that I may profitably treat them in some detail. 1 



The characters of sea-water which are made use 

 of by the hydrographer in tracing the movements 

 of large bodies of water are : 



1 . The temperature at the surface and at a number 

 of intermediate depths down to the bottom. The 

 temperature is always changing. Generally it is 

 warmer at the surface than at the bottom. In all 

 deep oceans the temperature of the water at the 

 bottom is very little above freezing-point, and it 

 undergoes hardly any variations. 



2. The density. Sea-water is heavier than fresh 

 water, because of the salt and other substances 

 dissolved in it. The number representing the 

 density expresses how much heavier is a certain 

 bulk of sea- water (say a gallon), as it exists in the 

 sea, than an equal bulk of pure water at the 

 temperature of 4 C. 



3. The salinity that is, the salt-contents. The 

 number representing the salinity expresses how 

 many grammes of solid matter are dissolved in 

 1000 grammes of sea-water. The salinity varies 

 with the source of the water. In North European 

 seas (excepting the Skagerack, the Cattegat, and 

 the Baltic, where the water may at times be very 

 fresh) it may be as low as 30 and as high as 35*5. 



1 See Dickson in Geographical Journal for March 1896; and an 

 excellent review by Cunningham in Journal Mar. BioL Association^ 

 vol. iv. p. 233, 1896. 



