12 INSTRUMENTS USED IN MENSURATION. 



of the observer from the tree. From these data the diameter 

 is calculated. Instead of the angle, the distance a b between 

 the two lines of sight can be measured, in which case the 

 diameter is obtained in the following way (Fig. 5) : 



and 



A B = G - -4 x a 6. 

 Oa 



If, therefore, the instrument gives a b and Ca, and the dis 

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



Fig. 5. 



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

 in practice, because those at present available do not work with 

 sufficient accuracy and C A is difficult to ascertain. 



3. Instruments for the Measurement of the Diameter Increment. 



The diameter increment of prepared sections is measured 

 with an ordinary rule, or with a pair of compasses and a rule. 

 Such rules are made of metal or wood, and are sufficiently 

 sub-divided. 



