8 INSTRUMENTS USED IN MENSURATION. 



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. 



Where large numbers of trees are to be measured, it is 

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



Vertical Section of Movable 

 Arm. 



Fig. 1. Friedrich's Calliper. 



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

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

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

 from J to 1J inches are recorded as having a diameter of 

 1 inch, those from 1J to 2J inches as 2 inches, and so on. 

 A good calliper must fulfil the following conditions : 



(1) It must be sufficiently light so as not to fatigue the 



labourer, and yet sufficiently strong to resist the wear 

 and tear which it is likely to be subjected to. 



(2) The two arms must be at right angles to the rule, or at 



least parallel to each other, when pressed on to the tree. 



