Long Waves in Canals and Standing Waxes in Closed Basins 221 



17 1 1920 hr 18 1 1920 •» W-BBO 



12 14 16 18 20 22 2 4 6 8 10 12 14 16 18 20 22 

 20 



16 



12 



8 



4 







gSiSftffi 



Fig. 94. Seiches in Lake Vattern (Sweden). Records at Bastedalen: above: caused by 

 wind, below: caused by a sudden variation in pressure. 



They were able to determine, on the strength of accurate meteorological 

 registrations, that in this instance it was essentially the points 1 , 2 and 3 which 

 were responsible for the seiche. 

 (b) Action of Wind and Atmospheric Pressure on a Lake Surface 



Any force acting on the water surface of a lake or a bay will cause the 

 water surface to deviate from its normal position of equilibrium. As long 

 as this force lasts, the water surface will remain in this new position. However, 

 when this force subsides or ceases completely, the water surface tends to re- 

 sume its former state of equilibrium. The frictional forces being small, this 

 does not happen aperiodically, but in oscillations whose period corresponds 

 to the period of free oscillation of the system. A periodic disturbing force 

 provokes forced oscillations with a period corresponding to the period of 

 the disturbing force. The amplitude will increase if the period of the force 

 approximates the period of one of the natural modes of free oscillation. But 

 these forced oscillations will not last long in a lake which is exactly tuned 

 to certain periods. The forced progressive waves developed in the lake are 

 rapidly dampened by reflection on the shores, and those which correspond 

 to the dimensions of the lake will be least affected. 



It is rather simple to examine theoretically and thoroughly the action of 

 changes in atmospheric pressure and wind on the oscillatory processes of 

 water surface in more or less closed basins. One can compute accurately the 

 oscillations already developed if, besides the dimensions of the oscillating 

 mass, we have the atmospheric pressure and wind as functions of locality 



