COMPASS AND PLUMB-BOB METHODS 



149 



such as the true north. The angular displacements of the 

 apparatus in the hole are found thus: When plumb h is in 

 last contact {i.e., the last touching position before disen- 

 gagement) with the sloping side of 6, we are at the limiting 

 position at which the circuit is closed and the galvanometer 



i}^ 



Fig. 



Fig. 89. 



deflects. By slowly hauling up the plumb bob we can find 

 this spot, for, after it, the contact is interrupted and the 

 needle of the galvanometer adjusts itself back to zero. 

 We thus know the length of the plumb line hanging in h 

 from /, because we know the amount hauled out to make 

 last contact. Thus it will be seen that any angular posi- 

 tions ax, 0:2 ... «„ given by the apparatus correspond to 

 certain critical lengths U, h . . . l„ of the cable g. These 



