540 



Internal Waves 



was hardly to be expected. Consequently, the current here must be caused 

 essentially by the internal waves. 



Fjelstad published also the observations on another station, no. 3, about 

 12-3 km distant from station no. 1. The vertical displacements show quite 

 similar values for both stations. The velocity of propagation of the waves 

 on station 1 is known; consequently, the expected waves for station 3 can 

 be computed. A comparison of computed and observed waves is only possible 

 if the observations are simultaneous, which was not the case. Therefore, only 

 relative values can be compared, i.e. the variation of the amplitudes with 

 depth and the phase angles. Table 90 presents such a comparison, which 

 shows nearly complete agreement of the computed and the observed ampli- 

 tudes. The current measurements on station 3 are not as complete as would 

 be desired; moreover, the current velocities are only small. Nevertheless, 

 observations and theory agree quite well. 



The first more exhaustive analysis of periodic variations of oceanographic 

 properties on two stations fairly close to each other in the open ocean was 

 carried out by Helland-Hansen (1930). He worked up the observations 

 of the "Michael Sars" station 115 and the simultaneous observations of the 

 Scotch research vessel "Goldseeker" in the Faero-Shetland Channel. The 



Table 90. Internal waves in Herdlefjord (Norway), 

 summer 1934 

 Station 3 : 33 h observations 



ships were stationed at a distance of about 106 km from each other. Table 91 

 contains the results of the harmonical analysis for the waves of a period 

 of 12 and 24 lunar hours respectively. Essentially the observed variations 

 can be reproduced by superposing the two internal waves of tidal character. 

 The variation in the vertical displacement with depth is quite different on the 

 two stations. The phases also differ strongly so that it may be assumed that 

 the observed variations are caused by progressive waves. From the velocities 

 of the internal waves of the first to the fourth order, which were theoretically 



