98 DETERMINATION OF THE INCREMENT. 



years old, the same volume as the present 20 years old wood 

 has ; again, the 60 years old wood the same volume, when it 

 was 40 3'ears old, as the present 40 years old wood, etc. Or, 

 to put the matter differently, the 20 years old wood should 

 have, when it becomes 40 years old, the same volume as the 

 40 years old wood has now, etc. In addition, the progress of 

 the increment should be steady throughout. 



Although it is difficult to select localities on these lines, 

 which are of exactly the same quality, or woods which will 

 develop in the same manner, there can be no doubt, that 

 ultimately satisfactory yield tables can be obtained only by 

 observing and periodically measuring suitable woods for a 

 series of years. Hence, the method is actually followed. For 

 each quality class and age gradation several sample plots are 

 selected, and these are periodically measured and the mean 

 taken. In this way, yield tables will ultimately be obtained. 

 It is necessary to take several plots for each quality and age 

 gradation, so as to obtain average results, and because one or 

 other may become unfit for the purpose in consequence of 

 unforeseen events. 



c. 3Ieasuremcni of a large number of Woods of different Ages once, 

 so that Yield Tables are obtained immediately. 



Until yield tables, prepared as indicated under h, become 

 available, others for immediate use are required. These are 

 obtained by measuring fully stocked sample plots in a sufficient 

 number of woods, representing all ages with moderate intervals. 

 Out of the data thus obtained, steady curves and tables are 

 prepared. A separate set of woods is required for each quality 

 class, and the great difficulty consists in selecting for each set 

 localities of the same quality. For this purpose, various 

 methods have been suggested. Most of these start from 

 an indicating wood, while one, specially elaborated by Baur,* 

 starts from a different principle ; the latter will be dealt with 

 in detail further on, as it is the most practical method. 



* Late Professor of Forestry at the University of Munich. 



