166 



DEEP BOREHOLE SURVEYS AND PROBLEMS 



2. It is heavy and cumbersome and thus not suited for 

 great depths. 



3. It is not easy to manufacture and in some cases, i.e., 

 big deviations, will be difficult to manipulate. 



4. It uses up more time than a lighter and simpler 

 device. 



5. It has too many movable parts. 



The Denis-Foraky Teleclinograph. — This is a pendulum 

 apparatus and one of the best known of the modern 

 precision devices employed in freezing shaft boreholes.^ 

 It is remarkably accurate, being in many cases somewhat 

 of the order 1 in 3,000. ^ The principle is best understood 

 as follows: 



Imagine a cylindrical tube (Fig. 102) of length AO with 

 a system of rigidly orientated coordinates XF on one end 

 when in situ in the hole. Knowing the 

 coordinates of o' and the projection of A 

 on the coordinate plane, we also get the 

 position of the axis zz' of the tube which 

 on a centered plumb is the hole axis also. 

 Then by making a series of 10-m. interval 

 observations we can get the borehole trace 

 in 10-m. stretches projected on the horizon- 

 tal plane. The freely oscillating pendulum 

 A will, if given an initial impulse, describe 

 a surface the trace of which on plane XY 

 will be an ellipse with center o', which is the 

 vertical projection of A. More correctly, 

 but differing not sufficiently to affect the results with such 

 small angles involved, it is the sphere to which the above 

 plane is a tangent upon which the trace is generated. On 

 the sphere parallels are traced to the axes XX' and YY' at 

 a distance k and actually occupied by the conducting bars 

 (reglets) on which the pendulum point contacts every time 

 it crosses one, closing a circuit with a registering apparatus. 



^ See a full description in Prospectus of Foraky, Societe anonyme d'entre- 

 prise de forage et de foncage, Brussels. 



2 Adam, D., Colliery Engineering, p. 414, Nov. 24, 1924. 



Fig. 102. 



