62 DEEP BOREHOLE SURVEYS AND PROBLEMS 



the divisions is 1 hr. The glass-covered clock Ues together 

 with the compass in the frame G (Fig. 3). Four projec- 

 tions m are provided in the housing so that the mutual 

 positions of the clock and compass are maintained by fitting 

 into corresponding niches in the frame. In order that the 

 frame itself be immovable with the gun-metal box it has 

 four studs n fitting in recesses in K.^ 



For setting up the apparatus the angle between the niche 

 in the end of the chisel and the north end of the needle 

 must be known. Koebrich himself gave the direction 

 of the chisel the 12 o'clock line of the compass and let 

 the niche of the chisel direct itself with respect to the north 

 end of the needle. (The angle thus indicated is deg). 

 In order to avoid errors in this indication the inventor 

 has replaced all screw joints with conical joints. In action 

 the procedure is as follows: Adjust the arresting device to 

 act about 15 min. after all the rods have been let down 

 in the hole. Then assemble the apparatus as in Fig. 3 

 and suspend it in the hole until about three-quarters 

 of the stroke off the floor of the hole. Now a powerful 

 blow is struck with the chisel, producing a mark (Fig. 6) 

 on the rock which will show the nick mark of the chisel 

 in suitable rock. After the blow the chisel remains until 

 the needle is set and is then pulled out and the indicated 

 time read off the compass. In the usual way of core boring 

 a core is made, lifted out and orientated. The free-playing 

 compass is held so that the angle between the north end 

 of the needle and the nick projection marked on the core 

 has the same value as when the apparatus was assembled, 

 and according to Koebrich deg. The compass is turned 

 so that the needle reads the previously indicated hour, 

 thus giving the position of the core as it had formerly been 

 in the hole. The dip and strike are now easily obtained. 



Koebrich's apparatus was a big step forward over preced- 

 ing methods both in respect to protection from internal 

 injury by boring operations and in respect to the trust- 



1 Freise, F., Die Entwicklung der Stratameter, Osterr. Z. Berg-Hiittenw., 

 No. 42, p. 546, 1906. 



