AGE OF WOODS. 77 



n x + n 2 + n 3 + . . . 



All these formulae are somewhat troublesome. Formulae 

 (1) and (2) demand a knowledge of the volume, and (2) besides 

 of the areas occupied by each age class. Formula (3) necessi- 

 tates also a knowledge of the areas, while for formula (4) the 

 numbers of trees in each age class must be ascertained. In 

 practice the mean age is frequently taken as equal to the average 

 age of the sample trees, or of the age classes, according to the 

 formula : 



where n represents the number of sample trees or age classes. 



Finally, the age of the arithmetical mean sample tree can be 

 taken as the mean age of the wood. 



Example : 



Let 



E\C\ fWi - O QPYPQ />? 



i t)\J //t/i *-i clCl t/o /c-i 



% = 70 m ;} = 2 HS= 800 



^ = 80 Wl4 = 1 fj TO4= 300 



Mean age according to formula : 



, 4000 + 9000 + 7000 + 4000 _ 24000 = 63-2 



4000 9000 , 7000 4000 380 

 "50" 60 " 70 " 80 



A _2 x 80 x 50 + 3 x 150 x 60 + 2 x 100x70 + 1x50x80 = 61 . 6 

 2x80 + 3x150 + 2x100+1x50 



_ 2 x 50 + 3 x 60 + 2 x 70 + 1 x 80 = 62>5 



2 + 3 + 2 + 1 



/ 4 \ J - 150 x 50 + 160 x 60 + 80 x 70 + 300 x 80 = 5g . g 

 1500 + 1600 + 800 + 300 



50 + 60 + 70 + 80 = 65 



\) " A 



