AUXILIARY REGISTRATIONS IN BOREHOLE SURVEYS 45 



Errors of measurement arise from the following sources. 



a. Incorrect Adjustment of the Plumb in the Hole. — This 

 arises often in unlined boreholes which frequently prevent 

 the plumb fitting the hole like a piston. This mostly arises 

 in chisel-bored holes which tend to ovality in cross section. 

 Ten millimeters inexactitude renders the method unadopta- 

 ble. The application of spring-centering mechanism to 

 remedy this is not to be recommended. 



b. Sag of the Rope. — This occurs with long ropes holding 

 small weights and it renders false readings of m. These 

 errors increase, with the slope of the hole and its depth, 

 according to the catenary law. The rope should be very 

 light compared with the weight; or it may be ridded by 

 centrally fixing the plumb at the measuring place and 

 tensioning the rope.^ 



c. Incorrect Readings. — Inexactitude in reading m in- 

 creases as m diminishes, greatly influencing the coordinates. 

 Repeated micrometer readings should be made and the 

 mean taken. 



1 See Wache's device, German Patent No. 3859, or Gliickavf, No. 46, 1904. 



