360 



Tides and Tidal Currents in the Proximity of Land 



be possible to determine the extent of the vertical tide if we have a sufficient 

 number of tidal current measurements. The adjacent and boundary seas 

 extend mostly on to the shelf, and their depth seldom exceeds 200 m; therefore, 

 current measurements can be made without any difficulty. This enables us 

 to determine the distribution of the tide over the entire area of the adjacent 



■30 -20 -10 



Fig. 151. Disturbance of co-tidal lines by a Bay. 



sea. There exist nowadays for many adjacent seas and canals representations of 

 the currents, which give for every hour of the tide a picture of the actual 

 currents. Although these tidal current charts serve only practical purposes, 

 and their basic data cannot always be too accurate, they can, nevertheless, 

 be used in drawing conclusions as to the vertical tide within these seas. It can 

 be expected that the increasing number of current measurements and the 

 improvement of their accuracy will make such current charts a reliable aid 

 also in purely scientific questions in the near future. 



Two different methods have been used for determining the tide from tidal 

 current measurements. One of these is based on a study of the tides in the 

 North Sea by Defant (1923); it is mainly based on the equation of continuity 

 and constitutes principally a differentiation process. The other method was 

 used by Proudman and Doodson (1924, p. 185) in their study of the tides 

 of the North Sea, and it uses only the equations of motion; therefore it is 



