COMPASS AND PLUMB-BOB METHODS 



147 



reverse again when the survey spot in the hole has been 

 reached, thus pressing them out against the borehole walls. 

 The worm wheels thus move back on the spindles and press 

 back the contact springs p interrupting the direct-current 

 supply so that only the current to the plumb g and contacts 

 j, k and I remains. The motors can be set in action at any 



I 



Idl : 



:■ g 



^4 



Sec+ion G-H 



^ Section E-F 



Section C'D 



FlG.1 



Plate X. — Kegel's apparatus. 



time by means of another current supply from outside; 

 thus the centering may be actuated at will. Instead of 

 the plumb g and its connections, a gyroscope or direction 

 indicator (magnetic needle) may be employed which will 

 maintain a definite horizontal direction by the action 

 of an electric motor or plummet, giving thus not only the 

 amount of dip but its direction also. For example, 

 the plumb g can be held to a definite orientation by a 

 gyroscope and the upper plunger rod of the plumb can 

 give a definite dip. By varying the dip and bearing of 

 this plunger the direction of the attached boring tool can 



