AGE OF WOODS. 77 



All these formulfe are somewhat trouhlesome. 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 — 



^ ^ a , 4- a, + aa + . . • . . . (5.) 

 n 



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

 as the case may be. 



Finally, the age of the arithmetical mean sample tree can 

 be taken as the mean age of the wood. 



Example : — 



Years. 



(2) A -^X^QX50+3X 150X60 + 2X100X70+1X50X80 



2 X 80+3 X 1 50+2 X 100+1 X 50 



= 61-6 



/ox f _ 2 X 50 + 3 X 60 + 2 X 70 + 1 X 80 ^^.^ 



^'^^'-- • 2 + 3 + 2 + 1 7 '='^' 



.^s ^ _ 1,500X50+1,600X60+800X70+300X80 _ rn.o 

 1,500 + 1,600 + 800 + 300 '^ 



(5) A= . . . 50 + 60+70 + 80 ^ ^. 



