270 



The Tide-generating Forces 



in Marburg a. L. The most important features can be found in Fig. 115. 

 It shows the variation of the direction and force of the tide-generating force 

 with time for the principal lunar component M 2 in Freiburg and Marburg, 

 derived by harmonic analysis. The results are similar for the other semi- 



Fig. 115. Variation in direction and intensity of the horizontal component of the tide 

 generating force for the principal lunar tide M 2 . Th: computed values for a rigid earth; 

 Mand F: observed values at Marburg a. L. and Freiberg i. S. (Tomaschek and Schaffernicht). 



diurnal and diurnal components. There is no doubt that the plumb-line 

 deviates through the action of the tidal forces. This proves the presence of 

 the horizontal components of the tidal forces. However, the ellipses are con- 

 siderably smaller than they should be theoretically, and there also appear 

 slight deviations in the phase. Mean values from several components give 

 as ratio of the amplitudes 0-6, but here a difference between semi-diurnal 

 and diurnal terms cannot be established for sure. There is also a phase shift of 

 about 1 J h, which may be somewhat smaller for diurnal components. The 

 decrease of the amplitudes can only be explained if the earth is not completely 

 rigid, but that it yields somewhat to the tide-generating forces; hence, that 

 there are tides of the solid earth. 



This result has been fully confirmed by measurements of the periodic 

 variations of gravity caused by the vertical component of the tide-generating 

 forces. 



In recent times Tomaschek and Schaffernicht were successful in construct- 

 ing an instrument which provides useful results, namely, the Bifilargravimeter. 



The discussion of the long series of measurements made in Marburg and 

 in Berchtesgaden has shown that the observed periodical variations in gravity 

 are only about two-thirds of the value computed for a rigid earth and that 

 there are also small phase displacements. Figure 116 gives for the principal 

 lunar tide M 2 the curves from observations made simultaneously at both 



