FLUID METHODS OF SURVEYING BOREHOLES 



99 



of to 100 deg. and a movable circular scale on its base d. 

 For a horizontal position c coincides with a vertical line 

 etched on d, which must always correspond with the 

 0-deg. point of the fixed scale. A rubber plate tightens 

 the cover of c with the aid of a pressure screw s and a lead 

 cone e. On the center plate Y is the compass / in a glass 

 housing, its needle g being arrestable by the lever h when 

 the forked end of the lever is pressed upward. This is done 

 by means of a spring j which usually holds the lever off 



Fig. 49. 



Fig. 50. 



against its pressure and is disengaged by the clock i. There 

 is a to 100-deg. graduation on the top of the compass 

 which agrees exactly, in the vertical direction, with the fixed 

 100 scale in the base plate Z. The clock hanging on upper 

 plate X is set to actuate the arrest about 15 min. after the 

 instrument has reached the spot in the hole where the read- 

 ing is being taken. Now from the position of the needle 

 and marks, as in Figs. 49 and 50, and the graduated base 

 plate we can get the deviation direction. We can obtain 

 the amount of incUnation by taking the highest and lowest 

 points on the line. If the inclination varies as in the case 

 of a hole with changing dip direction, we get the deviation 

 curves etched sometimes as in Fig. 50. 



The accuracy of the measurements may be enhanced by 

 taking a plaster cast of the cylinder and lines and magnify- 

 ing them, then measuring with a cathetometer. Without 

 the compass the apparatus is not reliable for tests of the 

 direction of deflection owing to the effects of capillarity, 

 turning of the apparatus and the thickness of the etched 

 mark. The apparatus has been tested for water-tightness 

 at depths of over 3,000 ft. and has given satisfactory 

 results. 



