448 



General Theory of Ocean Currents in a Homogeneous Sea 



cases there was no doubt of the presence of pure inertia movements. The best example 

 is that contained in the measurements of 17-24 August. The recordings were made 

 between Gotland and the mainland (57-8° N. 17-8° E., depth 100 m) at a depth of 

 14 m over a period of 162 h. The structure of the sea showed a well-developed density 

 transition layer (thermocline) at 25 m and an almost homogeneous top layer. The 

 variations in direction and strength of the current can be given in the form of a pro- 

 gressive vector diagram which shows the track of a single small water element. 

 On the current directed towards the NNW there is superimposed an oscillation 

 rotating to the right, at first increasing and then decreasing (Fig. 195). The changes 



Fig. 195. Inertia oscillations in the Baltic in hodograph representation (according to 



Gustafson and Kullenberg). 



with time in the two velocity components are shown in Fig. 196. This diagram is 

 particularly reminiscent of the theoretically derived oscillation due to a suddenly 

 starting wind or to a suddenly imposed pressure gradient (Figs. 192, 193). If the first 

 waves of the excitation period are omitted the period of the oscillations is 14-0 h as 

 compared with 14-14 h for the inertia oscillation. The phase difference is almost 

 exactly a quarter of a period and the amplitudes are very nearly equal. 



The meteorological observations taken at the same time do not permit any deduc- 

 tions about the origin of this inertia wave. The question concerning the horizontal 



