MEASUREMENT OF THE DIAMETER. 11 



(7.) The reading must be taken while the caUiper rests on 

 the tree, and not after it has been withdrawn. 



r. The Tree Compass. 



The shape of this instrument will be understood on reference 

 to Fig. 4. The diameter of the tree or log is taken by the 

 two points r and d, while it can be read off at // on the are/, g. 



In order to produce sufficient stiffness in the arms of the 

 compass, they have to be made of 

 metal, which makes the instrument 

 very heavy and unsuited for con- 

 tinued use. 



ih Dendromcfrrs. 



In some cases certain dendro- 

 meters are used to measure the 

 diameter of trees at some height 

 from the ground. The theory is 

 this : — 



The angle which is formed by 

 two rays running to the two sides 

 of the tree is measured, as well as 

 the distance of the eye of the 

 observer from the tree. From 

 these data the diameter is calcu- pig. 4._Tiie Tree Compass. 

 lated. Instead of the angle, the 



distance ah between the two lines of sight can be measured, 

 in which case the diameter is obtained in the following way 

 (Fig. 5 on next page) :— 



C A : C a = AB : a h, 

 and 



AB = %^Xa h. 

 C a 



If, therefore, the instrument gives a h and (' (/, and the dis- 

 tance C A has been measured, the diameter can be calculated. 

 So far, instruments of this class have not obtained a footing 



