THE THEORY AND PRACTICE OF WORKING PLANS 119 



yet mature. It is an unnecessarily narrow concept of sustained 

 yield; it does not even secure normality, for volume, i.e., grow- 

 ing stock, alone is no criterion of normality. 



III. The area-and-volume period method (" Kombiniertes 

 Fachwerk ") aims to combine the area " framework " and the 

 volume " framework " so that each period will contain approx- 

 imately equal areas and volumes. 



Theoretically this distribution of volumes is for the whole 

 rotation and is achieved for the I period by means of volumetric 

 surveys, for the other periods by means of yield tables. Areas 

 and volumes are then adjusted as in the area " framework " 

 and the volume " framework." The annual cut is then obtained 

 by dividing the area and the volume of the I period by the 

 number of years contained therein (usually twenty) and let- 

 ting the two factors of area and volume act as a mutual check.* 



In practice the difficulty of predicting volumes for a whole 

 rotation and of equalizing volumes and areas, led to an im- 

 portant modification whereby the volumes are calculated for 

 only the I period or, at most, the I and II periods; the areas, 

 however, dehneated, roughly, for the whole rotation so as to 

 insure a sustained yield. 



This method was founded by Heinrich Cotta in 1804. The 

 important modification of restricting the volumes to the I or 

 I and II periods dates from von Klipstein in 1823 and von 

 Grebe in 1867. 



This method possesses the combined advantages of the area 

 and the volume " framework "; it secures a greater regularity 

 of volume yield than does the former and a quicker approach 

 toward normahty than does the latter. Combined with a proper 

 distribution of the age classes and a liberal interpretation of 

 equality in the periods, the method secures good results. But 



*A number of variations have been suggested, e.g., annual cut = volume of 

 period-;- years of period (Prussian practice); annual cut = area of period -^ years 

 of period (Auhagen); annual cut = area of (I or I and II) periods-;- years and 

 reduced to volume (von Stockhausen and von Grebe). In practice both factors 

 are regarded as local conditions demand. 



