57 



The increment for the n years will hence be 



T D = (P, + F a + )-(*, + * 2 + ). 



Lastly, if it is required to ascertain the probable increment 

 for the next n years, we must determine the probabU 

 volumes F/, F a ',...of the several ste'm-ctasses at the end 

 of n years by the same formula of Pressler's 

 r/ 



1 

 The increment for the years will be 



I B ' = (v\ + i\ )- ( !\ + F 2 )= r.-r. 



The rate per cent, of increase for the same period will be 



, r. + r. 220. 

 P ' ' r\ v t "V 



From all the preceding expressions it will be obvious that th e 

 increment per cent, of the whole crop can be equal to 

 the arithmetical mean of the percentages of increase of 



the sample stems only when F t V^ = F 3 ; and 



this mean will be approximately correct if the cumber of 

 sample stems is very large and they are mostly taken from 

 the dominant class. 



II. "WlTH THB AID OF PERCENTAGES THAT HAVE BEEN PRE- 

 VIOUSLY OBTAINED FROM ACTUAL INVESTIGATIONS IN SIMILAR 



CROPS. In this case three facts have to be borne in mind, 

 viz., (\) that the percentage of increase falls as the age of 

 the crop increases, and can be assumed as constant only for 

 short periods; (2) that the percentage falls niore rapidly, 

 the more quickly the individual trees increase iu diumete 1 * 

 and volume (crops of poor upward growth yield a higher 

 percentage than vigorous crops of equal age) ; and (3) 

 that the lighter crop has a larger percentage than the 

 denser one. 



This method is employed in open crops and coppice standards in 

 neither of which cases are tables of yield directly applicable. 



III. WITH THE AID OP TABLES OF YIELD. When such tablet art 



used, the locality, density and age of the crop in question 

 and of the crops from which the tables have been 

 calculated must correspond very closely. 



IV. BY ASSUMING THE INCREMENT BOUGHT TO BE TUB MIAN 



ANNUAL INCREMENT AT THE PRESENT AGB OP TH1 CROP. 



This method is applicable to exploitable crops. 



