158 



DEEP BOREHOLE SURVEYS AND PROBLEMS 



f which actuates two drums on which a thin wire rope h 

 to the head of the lower tube is wound. For closing the 

 lenses up again spiral springs on the drums coil up the wire 

 automatically. On the top end of each tube is a headpiece 



Side View Section C-D 



C — 



Section E'F 

 Fig. 98. — Centering device. 



Section A-B 

 -Erlinghagen's electromagnef. 



X in which the measuring apparatus (Fig. 97) is guided by 

 the thin ropes h exactly on the center line of the tubes or 

 lenses. The lower spiral spring i and the levers k serve to 

 hold the lowest lens of the telescope exactly in the middle 

 of the borehole when in the extended condition. It will 

 be seen that in small diameter boreholes the brake loosening 

 device and telescope lenses would be inadvisable owing to 



