12- and Sit-hour periocle, the difficulty has been to show that 

 those waves have their origin in the tidal forces, Defant 

 (1932) and others have sho^^rn ^hat the tidal force is not suffi- 

 cient to account for the observed amplitudes of these waves 

 directly. He suggests that these oscillations may not be due 

 to true Internal wave motion at .?.ll, but rather are the result 

 of periodic variations witn time of the horizontal tidal currents, 

 which cause periodic variations of the inclinations of an exist- 

 ing surface of discontinuity. 



Another objection to the thesis that these waves are of a 

 tidal nature has been the fact that when the velocity of propa- 

 gation of these waves was computed, it was found to be much less 

 than the velocity of the surface progressive tidal wave. There- 

 fore, it was concluded that no resonance between the tidal forces 

 and the responding medium could occur, Sl.i.ce the vertical component 

 of the tidal force is not sufficient to cause the wave directly, and 

 since no resonance could take place, it seemed to some highly unlike- 

 ly that these waves had their origin in the tidal forces. 



This problem has been attacked by Haurwitz (:95C);, who at- 

 tempted to show that resonance could occur if one considered the 

 effect of the earth's rotation. When this factor is cors'ldered, 

 one arri^^'ee. at free wave periods which are much closor in mag- 

 nitude to the semidiurnal and diurnal periods, Haurvi-z also 

 sugp;ests that^ if the procedure were altered to Include consid- 

 eration of a rotating spherical earth rather than a rotating 

 disc, much closer agreement might be obtained. Thic is still to 

 be shox'jn; the present investigators have made an attenptj but 



35 



