LA METHODE DU CONTBOLE. 105 



since this increment represents the results of both the spontaneous 

 and solicited effort on the forest. It is an extraordinary delicate 

 indication of the growth and vigour of trees and as such must 

 form the subject of continuous study on the part of the silvicul- 

 turist. Hence enumeration, methodical and repeated, is the 

 fundamental operation of the method of control. The modifications 

 to which the growing stock of a forest is subjected are of two 

 classes increases and decreases. Increases are caused by the 

 increment and by the entry of new individuals, decreases are due 

 to removal either accidental or intentional. It is therefore neces- 

 sary to make inventories of the material retained (entries) and of 

 material removed (exits). It goes without saying of course that 

 these two operations, the basis of the whole method, must be made- 

 in exactly the same manner, and with the same units if their results 

 are to be of any value." 



The calculation of the Increment. This consists in comparing 

 two successive periodic enumerations, taking account of trees 

 exploited. 



Let m be the volume at the beginning of one period. 

 M the next 



E of the material removed during the period. 



,, A be the increment. 



Then A=M + E m. 



This gives the increment of the division as a whole, but it is 

 also carried out for classes and species. 



The method has not so far been used in Indian working plans.' 

 It is complicated by a calculation^ the growing stock of the old method* 

 and middle sized groups, based on the theory that these should 

 contain volumes in the proportion of 15:' 3 and that their sum 

 should equal the total volume which would exist if half of the 

 entire area were covered with trees of just exploitable size. If 

 this is so, then 



v + (ix 



Yield = - -f- 



Where v= volume of old group 



