Long Waves in Canals and Standing Waves in Closed Basins 229 



very large, so that with a rotating earth the period of the forced waves nears 

 rapidly the period of pure inertia oscillations. 



It is probable that in narrow, oblong seas (e.g. Adria, Kattegat, etc), 

 this kind of transverse oscillations is created by atmospheric disturbances, 

 which are particularly favoured by the dimensions of the canal. In the Kat- 

 tegat there is frequently an approximately 4-hourly oscillation, and La Cour 

 (1917) and Thorade (1918, p. 234) have explained this in a similar way. 

 Following the storm tide of 15 January 1916, were oscillations of a 4-hourly 

 period with a nodal line in the north-south direction in the southern Kattegat. 

 Fig. 98 gives the variations in the level of the surface for Aarhus and Horn- 



-20 



Time. 



Fig. 98. Variations in the water level at Aarhus and Hornback during the storm flood of 



15 January 1916 (La Cour). The curves indicate the variation of the level from 1 h before 



to 1 h after each time moment. 



back ; they show clearly the behaviour of the oscillation at both sides of the 

 canal and its period. It also fits quite well the conditions of width and depth 

 existing in the southern Kattegat: 2a = 100 km and h = 20 m, which give 

 T = 3-97 h. In this particular case, the rotation of the earth will only be of 

 little importance, as T t for <p = 55° has the value of 14 7 h. As shown by 

 Jacobsen, who analysed tidal currents in this area, the M 6 tide, which has 

 a period of 4 h, is particularly pronounced. The M e tide is also quite pre- 

 dominant at Aarhus. 



For the transversal seiches and their generation the reader is referred 

 also to Servais (1957). 

 (c) Storm Surges 



The earliest methods to study the meteorological disturbances of sea 

 level are to tabulate the storm effect in terms of barometric pressure, wind 



