Long Waves in Canals and Standing Waves in Closed Basins 219 



oscillation thus assumes the nature of a rotatory wave; however, the devia- 

 tions from a simple standing wave are small. 



The increase of the period caused by the earth's rotation will be unim- 

 portant. An evaluation can be made from the relation (VI. 129). We simplify 

 the Baltic as a rectangular basin of the length 2/ = 1500 km, a width, 



10cm 



25 26 27 28 29 30 XI 37 





 lOcm 



10 



121-34 



Fig. 93. Uni-nodal oscillation of the system Baltic — Gulf of Finland at Helsinki and Tallinn 



(Neumann). 



B = 1 50 km and a depth h =11 m. The period of the uni-nodal longitudinal 

 oscillation is then 29-7 against 27-5 h of the real Baltic. As /= l-24xl0 -4 

 and c = (ji/B)y(gh) and in first approximation a =f/c = 0-2 and / = 5n 

 the relation (VI. 129) gives al/c = 1 -56 so that a/c = 00993 and r/(rat rest) = 

 1 007. 



The increase of the period caused by the earth's rotation, therefore, is 

 only about 07%, which is very little. Nevertheless, this influence makes 

 itself felt very definitely in the time of high water on both sides of the Baltic. 



6. Generation and Growth of the Oscillations of the Sea and Lake Surface 



(a) Causes of the Seiches 



Forel showed that sudden changes in the atmospheric pressure cause 

 frequently seiches of great amplitude, whereas the influence of the wind 



