COMPASS AND PLUMB-BOB METHODS 123 



deflections and thus produced a series of curves. The com- 

 pass was then stationed at the points C and D in the mine 

 and the deflections noted. The points CD were marked 

 on the plan and a tracing of the magnetic curves set over 

 each point as shown. The observed deflection at C was 

 3 deg., and at B, Q}4 deg. The bore E was found where 

 the curves corresponding to these deflections crossed one 

 another, being about 8 ft. from the expected position. 



Although this was a relatively short borehole it well 

 illustrates the truth that nearly all boreholes deflect in 

 greater or less degree. One has but to lay out a few 

 hundred feet of securely jointed drill rods on an uneven 

 ground surface to note how easily this virtual wire will 

 sag and bend to accommodate itself to the local contour. 



The Otto-Gothan Apparatus.— This is essentially an 

 improvement or addition to Gothan's original stratameter 

 described in Chap. IV (Plate IV) and the same lettering 

 and terms apply in Fig. 68 as there. The addition con- 

 sists of a bolt q, which, during the running of the clockwork 

 i, is pressed by the end of rod p as a result of the action of 

 spring n which actuates the time indicator r. The bolt q 

 is pressed against the action of spring c so that the bolt is 

 always in contact with another fixed point opposite thereto. 

 On these two points a ring t is supported, forming the upper 

 end of a plumb line which swings above the disc u of wax 

 or any other soft material. The rod p is provided at its 

 lower end with a groove or slot z, which, on the arresting 

 of the clockwork, is raised by the spring n until the bolt q 

 can pass through the slot z, so that the latter is pressed 

 out of engagement with the ring t by means of a spring c, 

 the plumb line being thereby allowed to fall a short dis- 

 tance and to make a mark on the soft material u. 



The magnetic needle I is provided with small pinlike 

 projections, which, when the shaft is raised by the turning 

 of the cam s, become inserted in a bar or pad m cut from or 

 covered with cork, paper, leather or other suitable material, 

 and the needle is thus held securely in position until the 

 apparatus is brought, for inspection, to the top of the boring 



