154 



DEEP BOREHOLE SURVEYS AND PROBLEMS 



The central plate c holds a frame d carrying a graduated 

 glass plate e. A rotatable spindle / in the center of these 



& 



Fig. 93. 



plates c and e carries a plumb line g on an arm h, and also 

 a graduated index i slotted to take the plumb line. In 

 the bottom of the tube are four electric incandescent 



