8 A MANUAL FOR NORTHERN WOODSMEN 



the north end of the needle begins to swing to the east, 

 reaching its maximum position between eight and ten 

 o'clock in the forenoon. It then swings west to a maximum 

 westerly position reached from one to two o'clock p. M. 

 Then it swings slowly east again to a mean position reached 

 between six and eight p. M., at which point it remains 

 practically steady during the night. 



The effect of this variation is such that if a surveyor 

 starts a line in the morning and runs one course all day, he 

 runs, not a straight line, but a long curve. This variation, 

 however, like the slight variation thut occurs during the 

 course of the year, is in woods work commonly disregarded. 



Irregular Changes. The needle is subject occasionally 

 to sudden and irregular changes in direction. They some- 

 times occur during thunder storms, and at other times are 

 attributed to so-called magnetic storms, related perhaps 

 to the aurora borealis. Trouble from this source is not 

 often experienced by the surveyor, but it is a matter which 

 needs to be understood and watched for. 



Local Attractions. All users of the compass are on 

 guard against the disturbance caused by iron in its vicinity, 

 in the form, for instance, of chains, axes, and steel rails. 

 In addition, there are in most countries regions of greater 

 or less extent where the needle is subject to irregularities. 

 These are due to iron ore or other magnetic material located 

 in the vicinity, or to unknown causes. 



A local disturbance is indicated when the compass does 

 not read the same on the two ends of a line, and in compass 

 running error from this source is guarded against by keep- 

 ing careful watch of the backsight. Local disturbances 

 vary much in intensity. When very strong, they are readily 

 detected, and if confined in area present little difficulty to 

 the surveyor, who will clear out his line across them with 

 especial care, and either picket ! through or set the compass 

 by backsight. Slight disturbances are harder to detect. 

 If the area of disturbance is large, particularly if the ground 

 is broken, the compass cannot be depended on to carry a 

 line through with accuracy, and a transit or solar instru- 

 ment must be used. 



1 See page 21. 



