10 



INSTRUMENTS USED IN MENSURATION. 



wedge from right to left, it releases the rule, and enables it to 



move more freely. 



To force the rule to follow the backward movement of the 



wedge, a spring is fastened at e, which pushes it from right to 



left, so that it always must be in 

 touch with the wedge. 



Friedrich's Calliper. In this 

 instrument the section of the 

 rule has the shape of a rectangle, 

 while the opening of the mov- 

 able arm is larger than the 

 section of the rule, and placed 

 slanting towards it. At the same 

 time it is so shaped, that, on 

 being pressed against the tree, 

 it assumes a position which is at 

 right angles to the rule (Fig. 1). 

 In this position the arm rests on 

 the two points, a (below) and b 

 (above). As these points are 

 liable to wear awa} r , thus causing 

 Fig. 3. Heyer's Calliper. the arm to assume a position 

 which is no longer at right 



angles to the rule, Bohmerle has added a spring at b, which 



can be moved by a screw, until the true position of the arm is 



established. 



b. Accuracy of Measurements icith the Calliper. 



To insure the greatest possible accuracy, the following 

 precautions must be taken : 



(1) Moss, creepers, etc., found on the tree must be removed 



before measurement. 



(2) In the case of an abnormal swelling or indenture, the 



measurement must be taken above or below it, or both, 

 and the average taken. 



