MEASUREMENT OF DISTANCE 
9 
Electricity. A little caution is necessary in handling 
the compass in order that the glass cover shall not be elec¬ 
trified by the friction of cloth or the hand, so as to attract 
the needle to its under surface. If, however, the glass does 
become electric, the trouble may be removed by breathing 
upon it, or by touching different parts of its surface with 
the moistened finger. 
Difference in Instruments. It is a well-known fact that 
different instruments do not always give the same bearing 
when read on the same marks at the same time. A differ¬ 
ence of 15' is not uncommon. 
Summary. The magnetic needle is thus seen to be sub¬ 
ject to numerous variations and irregularities, and on that 
account work with the needle compass cannot be expected 
to give the most accurate results. The instrument has 
great advantages, however, and a very large field of legiti¬ 
mate use. It gives an approximately true direction from a 
detached point. Except on open ground, it furnishes the 
quickest and cheapest means of turning an angle or pro¬ 
longing a line. Most authoritative land surveys have 
been made with the needle compass and their renewal is 
best accomplished by use of the same instrument. The 
special advantages of the compass in forest conditions and 
its most effective use therein are discussed under the head 
of Surveying Practice. 
SECTION III 
MEASUREMENT OF DISTANCE 
1. The Surveyor’s Chain 
The word ‘‘chain” in connection with land surveying is 
used to represent two things: a distance of 4 rods or 66 
feet, and an instrument for measuring distance. The 
chain in use for general land surveying is 66 feet long and 
divided into 100 links, but woodsmen working in rough 
ground find the 33 foot or half chain with 50 links much 
more convenient. 
A chain for surveying purposes should be made of steel 
wire, and its links should be brazed to prevent stretching 
