CH. vii] CALCULATION OF THE POSSIBILITY 57 



is made of the volume of timber which will be available during 

 the coming rotation from the standards that will be extracted. 

 No rigid prescription as to the exact volume to be removed 

 annually should issue, as there is always a good deal to be left 

 to the appreciation of the local forester, with regard both to 

 cultural and economic considerations, in dealing with the reser- 

 vation and realisation of the standards. 



50. Regulation by volume of growing stock. 



Secondly, by volume. These methods are based either on the 

 whole growing stock, or on the increment, or on both: The deter- 

 mination of the yield from the estimated volume of the whole 

 wood-capital may be effected by dividing the total estimated 

 cubic contents of the growing stock by half the number of years 

 in the rotation. We have already considered this on page 9. 

 This method may serve as a check to other methods but it is 

 not very practicable, as it involves an enumeration of the whole 

 growing stock, and is only true on the assumption that the 

 actual increment bears to the actual growing stock the same 

 relation as the normal increment bears to the normal growing 



stock. 



51. Regulation by increment. 



If the determination of the yield is based on increment, the 

 average current annual increment may be obtained for each of 

 four or five age classes, by using the borer on sample trees, or 

 by felling and measuring sample trees ; this average increment 

 per acre is, if necessary, reduced by a factor for density, unless 

 the whole area is fully stocked, and the increment thus modified 

 is multiplied by the number of acres of each size or age-class, 

 and the whole then totalled up, and divided by the number of 

 acres in the felling-series. This method too is chiefly useful as a 

 check on other methods. 



Then lastly, if the yield is to be determined by volume, based 

 on both increment and growing stock, it will be calculated by the 

 formula { . x^V-nV) 



x 



where i is the actual mean annual increment during an arbitrary 

 time x, which is chosen as a convenient period for the distribution 

 of the excess or deficiency of the actual growing stock as com- 



