768 



[chap. 23 



method applied in a wide field of aerodynamics should be mentioned. Likewise 

 hydraulic or electric models are occasionally used for tidal investigations, 

 especially in estuaries. 



In any case the importance of each theory should be measured by the 

 possibility of reproducing observed tides and tidal currents in oceans and seas. 

 The final problem of oceanic tides may be formulated as follows. The tides and 

 tidal currents in the actual ocean have to be computed as a whole without 

 using any tidal observations. For this computation only the well-known tidal 

 generating forces, the distributions of depth, and the shape of the coastline 

 along which the normal component of velocity is assumed zero are available. 

 The efficiency of such a theory can be proved by means of tidal observations. 

 It seems necessary for the future to solve this problem completely. To begin 

 with, the work should be concentrated on more simple problems as they arise 

 in adjacent and tributary seas, in estuaries or in special parts of the oceans, 

 taking into account all work which has been done on this subject. In any case it 

 is necessary to gather information on tides along those parts of boundaries 

 which are not coastlines ; usually there is a complete lack of observations, and 

 data must be estimated. 



3. Tidal Observation 



In numerous coastal locations, on islands in the ocean, in open seas and in 

 inlets, bights and tidal rivers all over the world, the elevation of sea-level 

 caused by tidal forces is recorded. These observations are analyzed for each 

 station, assuming that the sea-level ^ can be represented by a series of simple 

 harmonic functions with frequencies a, which are known from the tidal poten- 

 tial F [equations (1) and (2)]. The determination of harmonic constants is made 

 by national authorities. The results are sent to the International Hydrographic 



1/4 depth 



1/2 depth 



O 20 40 60 



cm/sec 



3/4 depth Bottom 



Lot. = 54" 16' N; Long. = T 46' E. 



Fig. 1. Observed tidal currents in the neighbourhood of Heligoland. The figures on the 

 arrows give hours before or after the moon's transit of Greenwich. 



