36 



ANGULAR SURVEYING 



the bearing of the line. For example, in Fig. 3, the bearing is 

 N 65 E. The north end of the needle may be recognized by 

 p the absence of the coil s. This coil is 



wound around the south half in order 

 to balance the inclination of the needle 

 in a vertical plane, called the dip of the 

 needle. 



Local Attraction. The compass 

 needle may be deflected from its natural 

 direction by the attraction of any mag- 

 netic substance near it, such as iron 

 ore, the rails of a railway, etc. This 

 disturbing influence, called local attrac- 

 tion, is very frequently met with, and 

 the surveyor should take special care 

 to avoid the errors to which it may give 

 rise. W hen the bearing determined by a backsight does not equal 

 that obtained by a foresight, with the letters N, S'and E,W inter- 

 changed , the usual cause of the difference is local attraction. To 

 determine whether the disturbing influence is at the end or the 

 beginning of the line, set the compass at an intermediate point 

 and take a sight on both points, when it will usually be found 

 that the bearing thus obtained agrees with one of the bearings 



FIG. 3 



FIG. 4 



previously found. Should this not be the case, it would tend 

 to show that local attraction exists at both the beginning and 



