Long Waves in Canals and Standing Waves in Closed Basins 199 



Darser Sill — Mecklenburg Bay and Kiel Bay, T = 27-70 h by inserting in 

 (VI. 85) the numerical values provided by the survey of the Baltic. However, 

 one can also compute from the observations the water-masses transported 

 back and forth across the Darser sill during one period of oscillation and, 

 with these values as the western boundary condition, determine the period 

 after Defant's method. One then obtains as the period of the uni-nodal 

 oscillation 27-6 ± 0-7 h, and for the two-nodal oscillation 17-5 ± 0-6 h. These 

 values are in good agreement with the observed free periods of the Baltic. 

 The Adriatic Sea, due to its regular shape, is particularly suited for free 

 oscillations. As a matter of fact there are indications of such seiches in the 

 tidal gauge registrations, which often attain a very large amplitude. Because 

 the range of the tide in this sea is not negligible, these variations in level 

 should be studied in the following way. The partial tides computed from 

 the observations are used to construct a tidal curve. The difference between 

 this theoretical and observed tidal curve is plotted and analysed. In this 

 manner, von Kesslitz (1910) has found for Pola and Vercelli (1941, 

 p. 32) for Zara oscillations of the following periods: 



(1) About 22 h; it occurs especially at the northern end of the Adria with 

 regular wave trains of a large amplitude (30 cm and above), particularly 

 with sirocco and bora weather. 



(2) About 11 h; it is less frequent than the former and it seems to occur 

 only with special weather conditions (bora in the northern section, stormy 

 sirocco in the southern section of the Adria) with rather irregular amplitudes. 



(3) About 8| h. 



(4) Finally 3-2|- h which appear more frequently than the two previous 

 ones, and they are more discernible in the registrations because of their 

 shorter duration. Figure 87 gives a good example of the 22 h oscillation. 



This long oscillation seems to be the uni-nodal seiche of the entire Adria 

 with a nodal line at the opening Otranto-Valona. For / = 802 km and an 



Fig. 87. Record of the tide at Pola from 22 to 24 March 1906. The dotted line is the 



theoretical tide curve. The full drawn line is the observed curve. Below; the difference 



between both curves. 3 waves with an average period of 22-2 h. 



