76 SEA FISHERIES 



point of the ocean thus assumes the greatest impor- 

 tance in this respect, for it indicates the progress of 

 the currents and explains the rising of waters to the 

 surface. This is not the vertical movement from 

 bottom to surface of a deep sheet of still water, but a 

 sort of lowering, by evaporation, of the surface of the 

 sea itself, which is continually compensated by a general 

 rising of the subjacent waters. 



The variations of the symbol nS 4 9 are not the sole 

 causes of oceanic currents. They are in evidence 

 everywhere, but they do not constitute a unique mode 

 of movement. Other causes are the rotation of the 

 earth, the tides, and the winds. It has been calculated 

 that a mass of water 1,350 fathoms in depth would 

 be animated by a movement from east to west below the 

 equator and from west to east in higher latitudes. The 

 oscillation of the liquid mass, provoked twice daily by 

 the attraction of the moon and the sun, has the effect of 

 producing a double node of undulation separated by two 

 depressions. From this, especially in the shallower seas 

 in the neighbourhood of continents, results a regular 

 progressive movement of the waters striving to pass, 

 in order to re-establish the level, from the crest to the 

 hollow, from the hollow to the crest. 1 Finally, the winds 



1 As a matter of fact such a tendency would obviously be a purely 

 static tendency, opposed to and in equilibrium with the force of 

 gravity. That is, the water tends to flow from the crest to the level 

 but does not do so, or there would be no tide. The actual course o 

 tides in shallow waters is easily understood. The waters become 

 deeper, or in other words the crest of the wave rolls over them, and 

 more water is required that the wave may rise and pass on ; this 

 extra water is pulled over the surrounding level, to flow back when 

 the tidal pull has passed. The pull of the moon causes the tidal 

 stream to flow ; the pull of the earth, or the weight of the water, 

 causes it to ebb. [TRANS.] 



