112 



DEEP BOREHOLE SURVEYS AND PROBLEMS 



rent variation in resistance wires which dipped into a bath 

 of mercury, the immersion of^ the wires deciding the 

 resistance. The direction of the deviation was obtained 

 at the same time by a freed needle. Two steel cylinders 

 a and b (Fig. 61) are assembled, b inside a, with a fitting 

 closely in the borehole and with b fixed to the boring rods. 

 The lower part is filled with mercury which, when the 

 apparatus is in the perpendicular position, 

 reaches up to the lower edge e of the upper- 

 most of the two insulated strips e and /. 

 Another insulated strip is at g. The iron 

 wires h and hi are insulated down to a short 

 distance (about 1 in.) over the mercury and 

 joined together in a copper wire i which 

 goes through the rods to bank where it is 

 connected to a tangent galvanometer and 

 battery. 



If we now turn the rods slowly in a hole 

 deviating from the perpendicular, the thin 

 piece of wire k, between e and /, emerges 

 partially from the mercury. In this way 

 the resistance to the passage of current will be increased 

 and accordingly the deflection of the galvanometer will 

 be diminished. When this deflection has reached a mini- 

 mum, e and g lie in the plane of greatest inclination of the 

 borehole. 



From the difference between the maximum and minimum 

 throws of the galvanometer we may determine, once for 

 all, what resistance a given length of wire k offers and so 

 get the inclined position of the hole. A rubber boat I 

 floats on the mercury carrying a magnetic needle. This 

 float is guided by rods mm. The hard rubber ring R stand- 

 ing up on two points at about 90 deg. to the piece of wire 

 k is horizontal if e, f and g lie in the plane of greatest inclina- 

 tion of the hole. A steel wire encircles the lower side of R 

 and is intersected by e and / at opposite places and is 



^See also Redmayne, R. A. S., "Modern Practice in Mining," Vol. 1, p. 

 177, and F. Freise, "Stratameter," p. 43, 1906. 



Fig. 61. — Macfar- 

 lane's apparatus. 



