192 Long Waves in Canals and Standing Waves in Closed Basins 



partly from the unperiodical variations of the ocean surface of the entire 

 Aegean Sea and partly from the piling up of the water by the wind. 



The basis of explanation of the entire phenomenon is the fact that there 

 can be no uniform oscillation in the area of the narrows of Khalkis. Both 

 canals leading up to Khalkis from the north and the south oscillate indi- 

 vidually, and the ensuing differences in level north and south of the narrows 

 are compensated by currents between them. It is easy to understand that 

 the great frictional influences in the shallow and narrow straits prevent free 

 oscillations, and that there can only be an aperiodical adjustment of the dif- 

 ferences in level, 

 (c) Oscillations of the "Haffe" 



Most interesting are the standing waves in "Haffe" which are mostly 

 outstretched water-masses of small depth, which are connected with the 

 open sea by one or several openings. In the Baltic the most typical examples 

 are the Frisches Haff and the Kurisches Haff, which are connected with the 

 open Baltic by the Pillauer Channel and the Memeler Channel respectively. 



In the Frisches Haff (length 90 km, mean depth less than 3 m) there are, 

 according to the gauge registrations at Piilau and other localities, seiches 

 with a duration of 8 h, and — although less often — seiches of 5 h. The 

 first is probably the oscillations of the entire, more or less enclosed water- 

 mass of the Haff with a nodal line lying approximately at the Passage Mouth, 

 whereas the latter period might be the two-nodal seiches, (See Lettau, 1932, 

 p. 229, and Moeller, 1937, p. 262). The oscillations are strongly damped 

 and show a logarithmic decrement of 0-229. 



The gauge placed in the Kurisches Haff in the channel from Memel to the 

 Baltic shows, as an average of a large number of cases, a period of oscilla- 

 tion of 9-2 h, which might correspond to that of the uni-nodal seiche, and 

 besides, a period of oscillation of 4- 12 h, which will be that of the two-nodal 

 seiche (see Lettau, 1932). Here also the damping is great, considering the 

 shallowness of the Haff (length 85 km, mean depth 40 m) (according to 

 Endros, ;. = 0-25). 



The impedance method can be used to good advantage for a theoretical 

 investigation on the period of the free oscillations of incompletely enclosed 

 lakes. A lake with one side or one end open will loose or receive periodically 

 at each oscillation a certain amount of water, and this process must in- 

 fluence the period of the free oscillations of the lake. One is inclined to assume 

 that such process generally causes an increase of the period of a lake sup- 

 posedly completely enclosed. However, Neumann (1944, p. 200) was able 

 to prove that this is not the case. The impedance theory clearly shows that 

 there is a decrease of the period, according to the position of the opening 

 with respect to the nodes and antinodes. Lettau (1932) has tried to solve 

 this problem by starting from the principle of conservation of energy. The 

 result was an increase in the period of the lake, which does not seem right. 



