Miscellaneous Subsurface Methods 



559 



degree angle-unit instrument is shown in figure 280. Drift is read by 

 counting the rings from the center and interpolating when necessary. The 

 direction bearing is obtained by prolonging a line from the center dot 

 through the intersection of the plumb-bob cross hairs to the edge of the 

 compass card. 



The compass cards of all single-shot instruments used in California 



Figure 280. Single-shot disc, direction N. 45° E., drift 6° 00'. 



are corrected for local magnetic declination. Instruments rented in other 

 parts of the United States have compasses which indicate magnetic bear- 

 ings. The true bearings are obtained from the latter readings by apply- 

 ing a correction for local magnetic declination. 



Angle units are interchangeable in maximum drift values of 10, 18, 

 and 90 degrees. The first two units are constructed as described above. 

 The 90-degree unit, a disc from which is shown in figure 281, is a much 

 more intricate mechanism completely immersed in compass fluid (fig. 



