HYDROGRAPHY 159 



sea from a river. A submarine is less buoyant, when passing 

 from the sea at a density of 1-026 into fresh water, by 26 tons 

 in a thousand, and vice versa. So that when a submarine 

 of 1,000 tons leaves a river for the sea, she must take in an 

 extra 26 tons of ballast to keep her down, and when she 

 returns she must get rid of 26 tons, or she will sink deeper in 

 the fresh water. 



It has been pointed out by Buchanan that coastal waters 

 are areas of minimum density, while the areas of maximum 

 surface density are in the centres of the five tropical oceans, 

 North and South Atlantic, North and South Pacific, and 

 Indian Ocean. From these areas the denser surface water 

 flows outwards in all directions. 



. Where layers of water of different densities and tempera- 

 tures lie one upon another, the " discontinuity " line is often 

 the boundary between two very different assemblages of 

 organisms. It may also be the layer along which submarine 

 waves are formed, as has been shown by Dr. Otto Pettersson 

 on the west coast of Sweden, where these submarine waves 

 of inflowing Salter " Bank " water from the North Sea, 

 underneath the surface fresher coastal water, bring shoals of 

 herrings to constitute the important winter fisheries of the 

 Skagerak. 



Pressure. 



We exist at the sea-level under the pressure of one atmo- 

 sphere, which amounts to nearly 15 lb. on the square 

 inch. At any depth in the sea there is the added weight of 

 the water above, so the pressure increases greatly with the 

 depth. A cubic foot of sea- water weighs 64 lb., and the 

 pressure increases by one additional atmosphere for each 

 10 metres (or 33 feet) in depth — at 1,000 metres the pressure 

 is that of 100 atmospheres. A diver at a depth of 30 fathoms 

 sustains a pressure of 80 lb. per square inch, and at the 

 greatest depths of over six miles the pressure is about 

 Q^ tons on every square inch. 



