QUALITY INCREMENT. 193 



An approximately correct value for p q is obtained by the 

 formula : 



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

 ments may be more or less irregular ; hence it is impossible 

 to indicate these movements in a mathematical form. 



Woods grown for firewood only show little or no quality 

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

 per-centage of stem- to branch wood increases. The latest 

 investigations seem even to indicate that wood taken from 

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

 from older trees although they 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 

 dimensions. 



(2) The per centage 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 quantity increment is still above nil. 



Example. A Scotch pine wood 60 years old contains : 



Timber = 3,300 cubic feet, worth 4d. per cubic foot. 

 Firewood = 760 Id. 



Hence, mean quality : 



3300x4 + 760x1 

 " -406CT -= 3 ' 44 P* nce - 



The same wood in the year 70 has : 



Timber = 3,820 cubic feet, worth 5d. a cubic foot. 

 Firewood = 710 Id. 



VOL. III. O 



