130 



DEEP BOREHOLE SURVEYS AND PROBLEMS 



which it is desired to investigate. A strong current is 

 passed through the resistance frame J and allowed on for a 

 length of time sufficient to enable it to liquefy the wax 

 in the tube A. When this is considered accomplished 

 the current is cut off, whereupon the magnetic compass L 

 assumes the magnetic north and south directions. The wax 

 is then allowed to cool and resolidify, after which the instru- 

 ment is withdrawn from the hole. 



The direction of dip is obtained by noting the declina- 

 tion of the silver mirror M from the horizontal with regard 



Fig. 76. 



Fig. 77. 



Fig. 78. 



Fig. 79 



to the direction of the compass L (Fig. 75). The dip is 

 obtained in some forms of this instrument by means of a 

 gimbaled dish instead of the plate L, which like the plate 

 will become horizontal in whatever position the instrument 

 may be placed when the paraffin wax surrounding it is 

 melted by the current. A little melted paraffin is also 

 poured into the dish or pla.te so as to seal the needle when 

 it cools. This separation of the needle from the mass 

 of the paraffin prevents the needle from being disturbed 

 by the liquefying or solidifying of the paraffin. It is usual 

 to fill the space about the coil and outside the dish up to 

 the halfway mark, so fixing and warming it. Simple 

 protractors may be used for the two fundamental angular 

 measurements at the surface. Figure 80 shows plan and 

 section of a survey, by these methods, of a borehole on the 

 property of the Turf Mines, Ltd., Johannesburg. The 

 apparatus, though one of the most reliable instruments of 

 its class, is a time consumer in that there is no continuous 

 record of both amount and direction of deviation down or 

 up the borehole. The first instrument detailed registers 



