186 THE INCREMENT. 



In order to avoid the use of logarithms, several formulae 

 have been evolved, which give approximately accurate results. 

 Pressler obtained such a formula by assuming that the incre- 

 ment during the n years is laid on in annually equal quantities, 

 and by comparing the increment with the volume which is 

 present in the middle of the period of n years. He thus obtains 

 the proportion 



Capital : annual increment = 100 : p v 



and 



V-v 200 

 n * 



This formula gives p v somewhat too small ; but the difference 

 is so slight, that it can be neglected for all practical purposes. 



Example : 



Let v (in the year 70) = 3820 cubic feet ; 



V (in the year 80) = 4260 

 then 



or 



V-v 200 4260-3820 200 w 

 *- V+ v X T = 4260 + 3820 X lo = 



If any thinnings have been made during the n years, their 

 amount must be added to F, before the increment per cent. 

 is calculated. Supposing that in the above case 327 cubic feet 

 were cut between the years 60 and 70, then 



or 



4260 + 327-3820. 200 



- 



- 



