43 



+ ^A), 



5 ,*/ (/ + </A) 



In the last formula, when A = 0, or something very small, 

 then 



li- 



*n ~~ */ 







y volume\ . 



The mean annual increment cf a tree ( ire / ls ^ en as - 

 sumed to be the current annual increment. This as- 

 sumption is justifiable in the case of crops that are near 

 the age of exploitability, but should never be made in the 

 case of individual trees , the mean annual increment of 

 which, provided they enjoy a fair supply of ligh- on 

 every side, goes on increasing up to a great age. 



D. Expression of the increment as a percentage. 



To render what follows more easily comprehensible by the stu- 

 dent, we will here recapitulate briefly the general principles of 

 interest. 



If r = the rate per cent, per annum, then I'D r - - 



the amount at the end. of a year. In the case of simple interest, 

 we have the rule 



j /Total interestX 1 



' \ Principal / 1UU X p 



Put in most cases it is necessary to assume compound interest for 

 which we have the following formula? : 



Amounts J = P(l-Or), or l-0r=// , orr = 



For the easy determination of V, Pressler has invented a formula 

 based on the following two assumptions': (1), that the amount at 

 the middle of the period of years is equal to | (A + P), and 



('2), that the annual increment is equal to I (A P\. He thus 



n 

 gets v 



r : ioo. :: I (A P): 4 



AP 200 

 = X 



