QUALITY INCREMENT. 187 



If, in the course of n years, the net vahie of the unit of 

 voUime rises from q to Q, then the quahty increment is 

 — Q — q^ and the corresponding per cent, is obtained by the 

 formula — 



(J = q X l-Oj>,;' ; 

 and 



or 



logdOO +,>„) = 2 + '^^^^^'^ 



An approximately correct value for 2^q is obtained by the 

 formula — 



The quality increment may be rising, falling, or its move- 

 ments may be more or less irregular ; hence, they must be 

 ascertained in each case. 



Woods grown for firewood only show little or no quality 

 increment after middle age ; except, perhaps, in so far as the 

 percentage of stem- to branch wood increases. The latest 

 investigations seem to indicate even, that wood taken from 

 middle-aged trees has a higher heating power than wood taken 

 from older trees, although the latter may be perfectly sound. 



Matters are different in the case of timber forests; here 

 the quality increment rises, in the majority of cases, to an 

 advanced age, because : 



(1.) Trees of large dimensions are, on the whole, more 

 valuable per unit of volume than those of small dimen- 

 sions, though exceptions to this rule occur frequently. 



(2.) The percentage of timber to firewood increases, at any 

 rate up to a certain age. 



The quality increment per cent, sinks, on the whole, with 

 advancing age, though more or less irregularly; it can become 

 nil and even negative, if the timber commences to decay, while 

 the volume increment is still above zero. 



