FOREST MANAGEMENT 



117 



and tenths. Upon the rule and at right-angles to it are two 

 arms one, placed at the end, being fixed, while the other 

 is movable. The movable arm is slid along the rule 

 until the object which it is desired to measure is clasped 

 between it and the fixed arm, when the diameter can at once 

 be read from the main bar of the instrument (Fig. 22). 



Callipers are usually from three to four feet long, with the 

 arms about half the length of the rule. They are generally 

 made of wood, though aluminium is also suitable. The con- 

 struction of Heyer's calliper is shown at Fig. 23 ; the 



FIG. 22 



FIG. 23 



a. Movable arm. b. Graduated rule. c. Metal wedge, d. Screw. 



rule b is seen to have the form of a trapezium in cross- 

 section ; the wedge c is of metal and is attached to a screw 

 d) which may be moved backward and forward to counteract 

 the influences of swelling and shrinkage due to the wetting 

 and drying of the wood. 



For measuring the diameter-increment of standing trees 

 Pressler's Increment 'Borer (Fig. 24) is used. It consists 

 essentially of three parts : (i) a hollow boring tube which is 

 slightly conical towards the point ; (2) the handle, which, 

 being hollow, accommodates the other parts of the instru- 



