COMPASS AND PLUMB-BOB METHODS 



129 



tube A, while in the lower end is screwed a similar plug 

 C washered at E. The wiring connection is similar in 

 construction to that previously described above. These 

 wires d, d' are carried in two opposite longitudinal grooves 

 a^, a^ removing them from the circumference of the tube A 

 from which they are insulated (Fig. 75). One wire a^ 

 is connected with the terminal G and the other, a^, with the 



Fig. 74. 



terminal H. These terminals G and H are connected 

 to the resistance coil J, and projecting well up through 

 this coil is a needle K affixed rigidly to the base plug C. 

 When assembling the parts, K is screwed in through the base 

 by means of the nut k' beneath. Balanced on the point of 

 K (Figs. 78, 79) is a magnetic compass L, attached to a 

 conical base I. K balances this hollow base inside, and 

 horizontally over the needle L lies the mirror M. 



When making a survey, the instrument is lowered into 

 the hole by means of the cable hawser to the point at 



