220 Long Waves in Canals and Standing Waves in Closed Basins 



apparently is of secondary importance. Chrystal (1908, p. 464) arrived 

 at the same conclusion in his thorough investigations of the seiches of Loch 

 Earn. The following disturbing forces can generally be regarded as the causes 

 of oscillatory motions in lakes and bays: 



(1) The sudden return to its equilibrium of a surface previously disturbed 

 by the passage of an atmospheric disturbance over a section of the lake. 



(2) The sudden oscillation back to a state of equilibrium of a watermass 

 previously piled up by wind. 



(3) Sudden or rapid receding of an accumulation of water produced by 

 an extremely rapid influx across a section of the lake (violent rainfalls). 



(4) Shocks of rain drops falling on the water surface. 



(5) Sudden changes in air pressure, as expressed in microbarograms 

 (registrations of variographs). 



(6) Shock pressures of wind gusts on the lake surface. 



(7) Subsiding of the electrical attraction on the surface by thunder clouds. 

 One should always remember that, in general, these disturbing forces 



have little intensity. On the other hand, it should also be borne in mind that, 

 as shown by Emden (1905), small quantities of energy are capable of 

 developing seiches. Chrystal emphasizes that for Loch Earn which, as the 

 Lake of Geneva, must be counted among the deep lakes, the disturbing 

 forces 1, 5 and 6 cause seiches. He also corroborates Russel's opinion already 

 expressed in 1890, that slight pressure disturbances across the lake surface 

 develop standing waves, especially when there is a certain synchronism with 

 the natural modes of oscillations of the lake. For the influence of seismic 

 waves, see Oddone (1910, p. 115). Bergsten (1926) has been able to prove 

 that the seiches in the Lake Vattern are attributable mainly to piling up effect 

 by wind. This contrast to the theory of Forel and Chrystal is explained by 

 the fact that the Lake Vattern with its large water surface is a shallow lake, 

 and the effect of piling up by wind is, therefore, much more apparent than 

 in deep lakes, where the current in deeper layers can compensate more rapidly 

 this effect. Figure 94 (above) illustrates the start of a seiche by the influence of the 

 wind. On 17 January 1920, at 2 p.m., there blew a south-westerly wind of 

 a force of 8 Beaufort, which at 9 p.m. turned to W 9. The limnogram shows 

 how with the change of the wind at 9 p.m. the water level receded rapidly 

 and the potential energy of the wind drift caused a uninodal seiche of 5-6 cm. 

 In many cases it can be proved that sudden changes in pressure can also cause 

 seiches. Figure 94 (below) illustrates the case on 31 May 1925 in Bastedalen. 

 The microbarographic disturbance at about 10 a.m. shows approximately 

 the same period as the uni-nodal seiche of the lake and thus contributes to 

 the further development of the oscillation already present. 



The action of the separate forces causing seiches was investigated by Okada, 

 Fujiwhara and Maeda (1913, p. 210) in a special case, in which a thunder- 

 storm on Lake Biwa in Central Japan caused a seiche of 17 cm amplitude. 



