8 INSTRUMENTS USED TN MENSURATION. 



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). 



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



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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 rarely used. The rule is divided into units, which depend 

 on the desired degree of accuracy. Ordinarily they will be 

 inches or two inches ; in some cases half inches, and for very 

 accurate measurements decimals of inches. 



T\'here large numbers of trees are to be measured, it is 

 desirable to round off the limits of each unit ; for instance, if 

 the rule is divided into intervals of inches, the first division 

 line is placed at ^ inch from zero, the second at 1^, the third 

 at '2h, and so on (Fig. 2). In this way all trees measuring from 



