328 DETERMINATION AND REGULATION OF THE YIELD. 



that age ; the latter divided by the former gives the 

 mean annual increment. In order to avoid having to 

 calculate the increment year by year, it is generally 

 calculated for a number of years, which may be called 

 a'. If an abnormal wood is cut over during the a' years 

 at an age differing from the normal, and a normal 

 wood grows up in its place, the increment must be 

 calculated separately for each part of a' years. 

 {(-.) The normal and real growing stocks are calculated as 

 for the Austrian method ; the former is placed 



= — - , where / represents the normal final mean 

 'A 



increment ; the latter is obtained by multiplying the 



real final mean increment of each wood by its age. 



The difference between the real and normal growing 



stock is removed, as may be approved by the owner, 



say, in equal amounts in the course of a years. 



{(1.) The theoretical yield is then fixed by the formula — 



y _ Real Increment of a' years , G,,,a — Cr„„,,„ 



a a 



If d' is placed equal to a, that is to say, if the real 



increment is calculated for the number of years, during 



which any difference between the real and normal 



growing stock is to be removed, the above formula 



goes over into — 



{e.) The next step is to ascertain, whether the woods 

 preliminarily placed into the several periods are suf- 

 ficient to meet the yield during each period, as cal- 

 culated by the formula under id), or whether they 

 contain too much or too little volume ; in the latter 

 case, suitable shif tings must be made, which neces- 

 sitate, of course, fresh calculations of the increment 

 and real growing stock, as the final ages of some of 



