HEYER'S METHOD. 329 



(d) The theoretical yield is then fixed by the formula 

 y_Real Increment of a' yea,YS. rea iG 



If a 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 expression 

 goes over into : 



Y real ~^~ real -^ a norm. 



a 



(e) The next step is to ascertain whether the woods pre- 

 liminarily placed into the several periods are sufficient 

 to meet the yield during each period, as calculated by 

 the formula under (d), or whether they contain too 

 much or too little volume ; in the latter case, suitable 

 shiftings must be made, which necessitate, of course, 

 fresh calculations of the increment and real growing- 

 stock, as the final ages of some of the woods are thereby 

 altered. This process is continued until the require- 

 ments of the method are realized, that is to say, until 

 each period contains the same area, and at the same 

 time the volume necessary to meet the yield as cal- 

 culated under (d). As already indicated, the forester 

 must in this respect be satisfied with approximate 

 results. 



(/) The regulation of the yield is restricted to final returns. 

 The intermediate returns are estimated only for the 

 first period, or part of it, by means of yield tables, or 

 past experience, and added to the final yield. 



3. Merits of the Method. 



The method is one of great precision. On the other hand, 

 it is very complicated, and it calculates the increment, and 

 normal and real growing stock incorrectly, as in the case of 



