Internal Waves 557 



boundary conditions have to be satisfied, so that W = for z = as well 

 as for x = 0. This leads to a determinant D, which has to be a minimum 

 for the period T = (In /a) of the free oscillation of the system. 



The orographic configuration, of the Gulf of California can be compared 

 with a canal. Its depth of 1800 m remains roughly constant to' about 700 km 

 from the mouth, then it decreases quickly to zero, towards the inner end 

 (near the islands of San Estobal and San Lorenzo). For the outer part, where 

 the depth remains fairly constant, Fjelstad's theory gives a velocity of progress 

 of the wave of first order of 167 cm/sec, and for the depth of the greatest 

 vertical displacement 675 m; which is in good agreement with the observa- 

 tions of 175 cm/sec and 500-600 m depth. For that part of the Gulf of Ca- 

 lifornia where the depth decreases to zero the computation of the above- 

 mentioned determinant shows that for waves of the first order, a minimum 

 occurs for a period of 7 days; for waves of the second order for a period 

 of 14-8 days, which corresponds to the observations. It is likely that the 

 more important period of 7 days rather than the period of 14-8 days is caused 

 by the tide generating force of the lunar tide of 13 6 days, since 2x 7 = 14 

 is closer to 13 6 than the period of the standing wave of the second order. 

 Therefore the 7-day oscillation will dominate, which is in agreement with 

 the observed arrangements of the sediments in the Gulf. 



4. Causes of Internal Waves, Particularly of Those with a Tidal Character 



In order to investigate closer the causes of the formation of internal waves, 

 it has to be kept in mind that even small amounts of energy are sufficient, 

 to cause oscillations of boundary surfaces between liquids of different density, 

 or vertical displacements in a water-mass where the density increases con- 

 tinuously with depth. Even small pressure disturbances at the surface are al- 

 ready sufficient to cause large vertical displacements within a stratified medium. 

 In the case of the disturbance on the "Meteor" Anchor Station 254 we en- 

 countered (p. 527) a condition where in all probability a powerful internal 

 wave was caused by a small gust of wind, started by comparatively small 

 forces; such disturbances of the internal equilibrium of a stratified water-mass, 

 may, under favourable conditions like resonance, increase to an enormous 

 amplitude. Thus it can be expected, that in the case of disturbances of the 

 internal equilibrium in a stratified sea, the return to a new state of equilibrium 

 will be accompanied by internal waves, which are gradually dampened be- 

 cause of the frictional influences. 



Since the internal waves observed in the ocean have frequently a decidedly 

 tidal character the question arises, whether internal tide waves in the ocean 

 might be caused by the horizontal and vertical tide generating forces of the 

 sun and the moon. This would explain the tidal character right away. 

 O. Pettersson (1909, 1930, 1933, 1934, 1935) thought that especially the vert- 

 ical component of the tide generating force was the cause of the internal tide 



