Chap. 8] 



MAGNETIC METHOD 



333 



notebooks, and the like. The instrument case, Helmholtz-coil galvanom- 

 eters, and so on should be set up a sufficient distance from the magne- 

 tometer (see page 373). Stations should be sufficiently removed from 

 fences, well casing, pipe lines, derricks, boilers, tanks, mine shafts, bridges, 

 railroads, culverts, and the like, and from electric railways, power stations, 

 magnetic separators, and other mine plants. 



The following steps are followed in operating the instrument: (1) Re- 

 move instrument from case so that it may acquire the temperature of the 

 air. (2) Set up and level tripod. Orient tripod head with compass into 

 the magnetic prime vertical. Clamp tripod head; read and note position 

 of index. (3) Set up instrument on tripod, level with tube levels. (4) 

 Orient into magnetic prime vertical (N toward E). (5) Adjust eyepiece 

 position and mirror until scale is in focus and evenly lighted. (6) Take 

 several readings (at least three), arresting and releasing the system for 

 each. (7) Read and note time and temperature. (8) Clamp system, 

 rotate to N in W position. (9) Repeat (6) in this position. ^^ (10) Clamp 

 magnetic system. Push in safety lock. (11) Dismantle station. Do 

 not jar the instrument when placing it back in its ease. 



Table 41 is a specimen field record and office calculation. The first 

 occupies the left, the second the right side of a notebook in the correspond- 

 ing horizontal columns. Anomalies are figured in reference to a base 

 station. The forms given here may be readily modified to suit special 

 conditions. 



OFFICE RECORD 



30.0 



AnomaxiT 

 h) 



+460 



'* In rapid reconnaissance surveys and when anomalies are large, one or two 

 readings in each azimuth are sufficient. For further increase in speed, other types 

 of magnetometers not requiring a tripod (dip needle, author's Cardan-suspension 

 magnetometer) should be used if they meet the accuracy requirements. In a Schmidt 

 balance the time required for setting up, leveling, and orienting is out of proportion 

 compared to the time consumed in taking the readings. 



