The Tide-generating Forces 



269 



instruments are able to determine such extraordinarily small variations and 

 that first a very advanced measuring technic should be developed, in order 

 to obtain perfect results. 



Horizontal pendulums are used for measuring small periodical disturb- 

 ances of the plumb-line. Such a pendulum consists in principle of a rod mov- 

 able in horizontal direction and which can turn around an almost perpendicu- 

 lar axis (see Fig. 1 14). If this axis were exactly perpendicular, such a pendulum 



Fig. 114. Basic idea of Zollner horizontal pendulum. 



could be swung by any lateral force, however small, but it would soon be 

 immobilized in this position after the force ceases to act (indifferent equi- 

 librium). However, if the axis A A of the pendulum is slightly inclined against 

 the vertical line VV (by a small angle /), small lateral forces still can move 

 the pendulum out of its position of rest; however, when the force ceases, 

 it returns automatically to the initial position of rest by a component of the 

 force of gravity. The amount of the deviation from the position of rest is 

 an indication as to the intensity of the force. Bifilar suspension of the rod 

 and photographic registration strongly reduce frictional influences. As only 

 forces transverse to the plane of the pendulum cause the latter to rotate, two 

 horizontal pendulums placed perpendicularly to each other are needed for 

 measuring the west-east and the north-south component of the tide-generating 

 force. Great precautions are to be taken in these measurements, because the 

 apparatus, of course, reacts upon any inclinations of the base supporting it. 

 Such inclinations can also, by all kinds of external circumstances (e.g. tem- 

 perature variations of the upper layers of earth, etc.), easily take such propor- 

 tions as to cause deflections which are greater than those of the tidal forces. 

 For a review of measurements on gravity, see Schweydar (1921). A long 

 series of measurements was made by Schweydar (1921) in a mine gallery 

 (depth 189 m) in Freiburg (Saxony), and by Schaffernicht (1937, p. 349); 

 (see also Tomaschek and Schaffernicht, 1932, p. 787) in a 25 m deep cave 



