60 CALCULATION OF THE POSSIBILITY [CH. 



are relatively proportional to the three divisions of the triangle, 

 and which therefore bear to one another the proportion of five to 

 three, and three to one. 



If now it is found that the proportion of five to three does not 

 exist between the oldest and the middle-aged thirds of the crop, 

 the difference is subtracted from the third in excess and added 

 to the third showing deficiency, so as to obtain the correct 

 proportion. When this adjustment has been made, the modified 

 total volume of the oldest third, plus the addition of a modest 

 increment for half the time, is divided by the number of years 

 in one third of the rotation, and this gives the annual yield for 

 that time. If timber extracted in thinnings or improvement 

 fellings carried out in the middle-aged parts of the forest is of 

 a size to bring it to be included in the possibility as prescribed, 

 provision should be made for this in the calculation of the yield 

 by adding to the total volume of timber in the oldest third of 

 the crop a suitable proportion of the estimated future increment 

 of the middle-aged third during the same period. In France the 

 allowance for future increment is sometimes neglected, so as to 

 keep well within the possibility, and to have a reserve always 

 in hand wherewith to meet unforseen contingencies. This cal- 

 culation of the annual yield is revised every ten years. 



54. Calculation of yield under Selection method. 



Under the Selection method we have already the following 

 three limitations; firstly, area as determined by the size of the 

 felling-series and the number of years in the felling-cycle 

 adopted; secondly, a girth limit, which is the size adopted as 

 the size of exploitability, on attaining which size each tree is 

 fit for felling; and thirdly, cultural rules, which, although they 

 are of paramount importance, do not directly affect the present 

 calculation. Now it is evident that the first two limitations 

 already define exactly the annual yield, so that it is unnecessary 

 in addition to prescribe a fixed number of trees. It is, however, 

 usually convenient to know what the average annual out-turn 

 is going to be, but this number of trees cannot logically be en- 

 forced by a rigid prescription, though it may be prescribed 



