Tides and Tidal Currents in the Proximity of Land 



359 



matical developments are not simple, for which reason we will only discuss 

 here the most important results. Figure 149 illustrates the case of a wave 

 with a straight wave crest progressing from right to left, which is confronted 

 with a small island, whose dimensions are small compared to the wave length. 

 The current ellipses rotating to the right are indicated on the right side of 

 the picture by the two semi-axes of the ellipse. We see that the tide wave 

 travels slower on both sides around the island than farther outside (in the 

 manner of Fig. 148), whereby the ranges on the left side are increased, those 

 on the right side reduced. This can be so strong that a curved nodal line is 

 formed, within which the tide proceeds inversely. 



Fig. 150. Disturbance of co-tidal lines by an island elliptically shaped (Proudman). 



Figure 150 shows the co-tidal lines in the immediate vicinity of an elliptic 

 island, whereas Fig. 151 indicates how the co-tidal lines are changed when 

 the tide wave passes from right to left in front of the opening of a bay. This 

 disturbance corresponds exactly to the conditions in Fig. 148, where the 

 co-tidal lines diverge within the bay and converge at the corners. Proudman 

 has also dealt with the case of a straight coast where another sea is connected 

 through an opening. In the undisturbed state there is an antinode at a straight 

 coast. According to whether a ^ /, the passing of the tide wave through this 

 opening is different. 



7. Computation of the Tide from Tidal Current Measurements 



The equations of motion and of continuity form a link between the tidal 

 currents and the vertical displacement caused by the tide. Therefore, it must 



