BY THE MIDDLE SECTION. 



89 



By graphically representing the volume of wood at the several 

 ages, Fig. 31 is obtained, which, with the previous diagrams, 

 gives the following data : — 



ii. Determination of the Increment by the Middle Section. 



If somewhat less accurate results suffice, the volumes can be 



ascertained by multiplying the basal area in the middle by the 



height. Let T^ be the volume of the tree at the present time, 



V that n years ago, H and // the corresponding heights. Fig. 32, 



and *S' and s the corresponding basal areas at -- and -', then 

 I = V- r = S X JI - s X h. 



TT 



The height and basal area at — can easily be measured ; 



A 

 h is ascertained by cutting off a piece from the top downwards, 

 and repeating the operation, until the point has been ascer- 

 tained, where the basal area contains ii concentric rings. 



