CHAPTER VII 

 PENDULUM METHODS 



Introductory Note. — The physical features of the pendu- 

 lum which are essentially those of the plummet have been 

 among the great attractions of physicists for the last 300 

 years. The outstanding features marking the discoveries 

 of Newton, Foucault and Kater are all incorporated in 

 modern borehole survey instruments of this class. 



Our reason for distinguishing this suite of apparatuses 

 from the compass and plumb bob section is that generally 

 the plumb bob is used as a dropping pricker, a plunger 

 pricker, a balanced vertical bar, or in some other way not 

 fully utilizing its oscillatory properties. This is not a rigid 

 statement, since many compass devices also apply the 

 swinging bob. 



The pendulum proper is being understood when we con- 

 sider the elliptic or circular paths of a hanging bob or rigid- 

 limbed pendulum. It has the outstanding advantage of 

 independence of the magnetic north or the constitution of 

 its surroundings, working and obeying its astronomical 

 north-seeking faculty as well in magnetically disturbed 

 regions as without them. Its possibilities are evidenced by 

 the success of submarine and aerial navigation, since 

 gyrocompass action is an adaptation of the pendulum 

 principle. 



Koemer's Apparatus for Measuring Deviation. — This 

 device, which is essentially a spring pendulum apparatus, 

 was invented in 1906 by G. Koerner, an engineer of Nord- 

 hausen, Prussia, the suspension of the plumb line or pendu- 

 lum being altered by mechanical means and the oblique 

 positions of the same recorded photographically. 



In Fig. 93 the tube a is kept to the hole center by the 

 feeler spring wheels 6 pressing on the sides of the borehole. 



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