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 



