34 DEEP BOREHOLE SURVEYS AND PROBLEMS 



being at g. Two parallel resistance conductors a^ and a^ 

 and a steel needle c pass through the fiber lid I. The 

 needle connects the mercury to earth by way of the trun- 

 nion n, the case e and the borehole lining. By dipping 

 into the mercury the conductors are connected in parallel. 

 Any change of inclination alters the length of conductors 

 immersed, and thus the relation between the resistance of 

 the conductors is a direct function of the tilt. This rela- 

 tion is determined by means of a Wheatstone bridge which 

 will be detailed later when discussing Professor Brigg's 

 "clinophone" for vertical boreholes. The most disagree- 

 able feature of the apparatus is the employment of mercury, 

 which is an unsatisfactory medium to employ in mining 

 owing to its so easily becoming dirtied and thence unreliable. 



THERMAL SURVEYS 



These are usually resorted to in cases where we need 



1. The geothermal gradient of the strata of a given area. 



2. To investigate the frost columns in a freezing shaft. 



3. To employ geophysical data in oil zones, etc. 



4. Purely scientific researches. 



They are purely thermometer surveys undertaken with 

 some special form of maximum or minimum thermometer 

 using various fluids and systems of calibration. Numerous 

 devices^ have been invented to meet these needs, and in 

 all cases it is necessary for the apparatus to remain in the 

 hole some hours in order to acquire the temperature of its 

 surroundings. 



a. Measuring Decrease of Temperature. — The Mom- 

 mertz apparatus (Fig. 14) is one of the best known low- 

 temperature contrivances used in borehole temperature 

 surveys, i.e., in freezing shafts. A sheet-iron flask a 

 contains a liquor which can withstand great cold, and this 

 vessel is closed by means of a wooden plug. It hangs inside 

 another flask c and between them is an insulating space 

 on the vacuum-flask principle of exhausted air. The outer 



1 See the final chapter of Ambronn and Cobb's "Elements of Geophysics " 

 McGraw-Hill Book Company, Inc., New York. 



