10 INSTKUMENTS USED IN MENSURATION. 



on moving the 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. 



Fricdrich's Calliper. — In this instrument the section of the 

 rule has the shape of a rectangle, while the opening of the 

 movable 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 h (above). As 

 these points are liable to wear away, thus causing the arm to 

 assume a position, which is no longer at right angles to the 

 rule, Bohmerle has added a spring at h, which can be moved 

 by a screw, until the true position of the arm is established. 



h. Accuranj of Measuremenla wilh tlie CaJJiper. 

 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. 

 (3.) In the case of eccentric or elliptic trees, two diameters 



at right angles to each other must be measured and 



the mean taken. 

 (4.) The height fixed for the measurement must be strictly 



adhered to. 

 (5.) In the case of a tr&e which is divided into two or more 



limbs below the fixed height of measurement, each 



limb must be measured and recorded as a sejtarate 



tree. 

 (6.) The calliper must be placed at right angles to the axis 



of the tree, and the rule must touch the tree. 



