PENDULUM METHODS 155 



lamps i for illuminating the glass plate e, the index i and the 

 plumb line g. There is also here a camera A; and a rolling 

 film n driven by clockwork I and electrically controlled 

 by the cable fine m. The frame d and glass plate e and 

 the plumb line g can be placed at an angle in the tube a 

 by means of the spring o on rod p and spring q bearing on 

 plate c. 



The staple r is arranged to carry a lowering rope. If the 

 apparatus is suspended by rod p the frame and springs will 

 occupy the positions shown bold in Fig. 93, the springs 

 being compressed and the rod / being parallel to the walls. 

 If, however, the apparatus is suspended from the staple r 

 the springs are released to the dotted position of Fig. 93, 

 forcing the frame to the inclined position. 



To make a reading the appliance is suspended by rod p 

 with two external points on its casing in the meridian. 

 Then plumb fine g takes the position of the dip, and so the 

 position of the dip of the borehole orifice is found. This 

 position is photographed from below. Suspend the appli- 

 ance on the staple r without turning and bring the plumb 

 line spindle / into the inclined position. The plumb line 

 g now assumes a position which is determined by the dip 

 of the borehole and the inclined position of 

 the axis /in accordance with the parallelo- 

 gram of displacements. The film n is 

 advanced by electrically releasing the 

 clockwork I ; the lamps j are again switched 

 in and the new position of the plumb line 

 recorded photographically. By compar- 

 ing the two readings a diagram of the type 

 shown in Fig. 94 is obtained, from which the deviation 

 is found. The extent of the dip is calculated from the 

 amount of deflection of the plumb line from the center of 

 the scale on e and i and from the length of the plumb line 

 itself. The distance of the bottom of the plumb line is 

 read on a special scale on i. 



The objections to the apparatus are as follows: 



1. Double suspension is liable to introduce turning errors. 



