MEASUREMENT OF THE DIAMETER. 7 



of inaccuracy, amongst which the following deserve to be 

 mentioned : 



(a) The sections of most trees are not circles. 

 (6) Owing to the presence of a rough bark, the measured 

 girth is too large. 



(c) Irregularities in the tree are difficult to avoid. 



(d) The tape is frequently not applied at right angles to the 



axis of the tree. 



In order to avoid some of the disadvantages of tape measure- 

 ments, a thin string is sometimes used, which is then held 

 parallel to a graduated tape or rule. In this way more 

 accurate results may be obtained, but the procedure takes 

 more time, and is therefore not employed where large numbers 

 of trees have to be measured. 



2. Instruments for the Measurement of the Diameter. 



The diameter of sections of trees is measured with an 

 ordinary rule or a tape ; in all other cases the calliper is used, 

 or sometimes the tree compasSr 



a. The Calliper or Diameter Gauge. 



It consists of a graduated rule and two arms. Of the latter, 

 one is fixed at one end at right angles to the rule, so that its 

 inner plane lies in the starting point of the graduated scale ; 

 the other arm moves along the rule, parallel to the fixed 

 arm. 



In using the calliper, the tree is brought between the two 

 arms until it touches the rule, then the fixed arm is pressed 

 against the tree on one side and the movable arm shifted until 

 it touches the tree on the other side. The diameter can then 

 be read off on the rule (see Fig. 1 on next page). 



The length of the rule and of the arms depends on the size 

 of the trees to be measured ; each arm should be at least half 

 the length of the rule. Callipers exceeding 4 feet in length are 



