BY YIELD TABLES. 



105 



words, it must be ascertained, to which quality class the 

 wood belongs. 



The best way of doing this is, to measure the volume of 

 a normal sample plot in the wood and compare it with the 

 volumes given in the tables for the same age and the different 

 quality classes. If it agrees with one of these volumes, the 

 two are of the same quality class, and the increment shown in 

 the table applies also to the wood in question. 



If the volume of the wood does not agi-ee with any of the 

 volumes in the tables, then that quality class is selected which 

 comes nearest to it, and the increment is ascertained in pro- 

 portion to the two volumes. Let ?'« be the present volume of 

 the wood, V„ the nearest volume given in the table ; F« + n the 

 volume given in the same table for the year a -\- n ; and 

 Va + ,1 the desired volume of the wood in the year a + n, then 

 the following equation may be assumed to hold good (see 

 Fig. 39):- 



and 

 and 



t;, : i\, = y 



Vu + a= Vu + n X ^ 



I = Increment in n years = r„ _, ,j — r„ 



V„ + n X V, 



I = v,(_-"-l 



