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 that 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 1 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 vised. 
* See page 21. 
