432 JOURNAL OF FORESTRY 



the periods might be: For cutting the veterans, 20 years; mature, 20 

 years; young merchantable, 40 years; and immature, 40 years. The 

 question now arises: is the usual method of computing the volume of 

 the age class or group to the middle of the period siifficiently accurate? 

 Is it better than figuring the volimie to the initial year of the period 

 and adding half the growth (computed by subtracting the yield table 

 figures for the initial period year from that of the last year, or vice versa, 

 if the stand is decadent) ? Of these two methods, the writer prefers the 

 simpler and shorter "middle of the period" method to the "half the 

 growth" scheme; but for accurate computations with long periods the 

 system that follows is preferable to either. 



The new formula^ merely takes the empirical yield table yield for 

 the middle of each decade of the period and multiplies this figtire by 

 the number of decades left in the period divided by the ctimulative 

 totals of the number of decades. With a 40-year period the formula 

 would read : 



ani-\-hnz-\-cn-i-[-dni 

 fii+nz+ni+nx 



Where a, h, c, d equal the yield at the middle of each decade of the 

 period counting from the initial age of the stand as it enters the period; 

 «4, W3, M2, wi the number of decades left in the period including the 

 first decade. To illustrate this simple formula further, reference is made 

 to figure 1. 



Figure 1 



This represents graphically the age class or group which is to be cut 

 over in 40 years. It is evident that if the young merchantable group 

 (to be cut in their period of 40 years), is 80 years old, the first year of 

 the period (if the period starts January 1, and the cutting is in the 



* When the formula was first drawn up I used the yield table figure at the begin- 

 ning of each decade so as not to split decade figures, but Chapman called my atten- 

 tion to the inconsistency of not using the middle of the decade figure here, even if it 

 must be obtained by a slight additional computation. 



