102 H& MANAGEMENT OF WOODLANDS. 



profitable to fell a mature crop now or n years hence, one can 

 easily find this out by means of a simple formula giving p the 

 percentage of increment obtaining throughout any given period 

 of n years and expressed in terms of the capital (in timber or in 

 money) at the middle of such period : 



future yield - present yield 200 

 p - . - x - 

 future yield + present yield n years 



This is simply an expansion of the formula for the percentage 

 of increment now accruing annually on the present cubic con- 

 tents, from the simple proportion that the present cubic contents 

 are to the present annual increment as 100 to the percentage of 



annual increment 



increment (p) ; therefore p =. - - x 100 for any 



cubic contents 



one year; but if a period of 5 or 10 years or more is being 

 dealt with, then the average for such period is 



mean annual increment 



= - -- x 100. Thus, if C be 



cubic contents at middle of period 



the present capital in wood (or in its equivalent, money or net 

 income) and Ca+6 what it will have grown to n years hence, 

 then (Co + Ca+b) + 2 will be the mean capital or cubic contents 

 aver n years, and (Ca+b- C a ) + n the mean annual increment; 



Ca + Ca+b Ca+b Ca 



therefore - - : 100 :: -- : p; and;?, the indicating 

 2 n 



percentage of growth or increment over n years, 



_Ca+b-Ca 200 

 ~ Ca+b + Ca n 



For example, suppose a timber-crop is worth <100 per acre 

 to-day, and is likely to be worth 140 ten years hence, what is 



40 200 

 its mean percentage 6f increment 1 Here p = x - = 



If, therefore, the owner thinks timber-growing an unprofitable 

 business unless he can get a return of 3 per cent on the 



