126 DEEP BOREHOLE SURVEYS AND PROBLEMS 



line, and the direction and extent of deviation can be 

 ascertained from the mark of the plumb on the disc u. 



The plumbing plate is scored with concentric rings 2 mm. 

 apart and has north-south and east-west lines, their inter- 

 section being directly under the plumb point of the appara- 

 tus when the latter is in the vertical position. The plumb 

 takes about 15 min. to come to rest, so that oil is often put 

 in the housing to damp the swing to about 2 min. 



The idea is sound and suited to precision work. If its 

 application shows only approximate results this is due 

 chiefly to magnetic surroundings or to its being impossible 

 to set the axis exactly on the borehole axis. 



Figure 69 shows the entire apparatus with its small 

 conical tripod legs for fitting to the non-magnetic tube 

 shown in Fig. 70. 



Marriott's Instruments. — H. F. Marriott invented his 

 well-known borehole survey apparatuses in 1904 and they 

 have successfully withstood the severe test of prolonged 

 application, particularly in the gold fields of South Africa, 

 for many years. He produced two electrical devices: (1) 

 a continuously working instrument and (2) an intermit- 

 tently working one which sufficed only for single readings 

 at individual points being surveyed. 



Marriott's continuously recording instrument for obtaining 

 the deflections of a borehole refers particularly to surveying 

 the amount of dip,^ not its direction, and is illustrated in 

 Fig. 71. Figure 71 shows the modified form of the instru- 

 ment which is essentially a method wherein a pendulum 

 varies the resistance by a rheostat. 



Here we have three plumbs F pivoted to the vertical rod 

 E by the connecting rods /' instead of one plumb, as in 

 the earlier design. The strong outside gun-metal casing 

 A of the instrument has a hollow brass hemicylinder B 

 pivoted truly axially inside it on pins b, h' . The securing 

 pivot screw h' in the base disc a' is insulated in an ebony 

 bush. So also are the discs h^ and h^ in the ends of B 

 insulated. An ebony disc c in the top of tube A carries 



1 Trans. Inst. Min. and Met., Feb. 16, 1905. 



