210 THE NORMAL AGE CLASSES. 



Another waj^ is to fix the number of age classes : in that 

 case n is variable, but this procedure is not to be recommended, 

 as it is likely to lead to confusion. 



It is usual to take for » a round number, say 10, 20, or 

 even 30; in coppice woods, n is usually taken as = 5. The age 

 classes are numbered. It is best to call the youngest I., the 

 next youngest 11. , and so on ; for instance, if n = 20 — 



First age class I., contains all \¥Oods up to 20 years old. 

 Second ,, II., ,, „ from 21 to 40 years old. 



Third „ III., „ „ „ 41 to 60 



And so on. 



In this way the number of the age class indicates directly 

 the limit of ages of the woods contained in it. The reverse 

 method, of calling the oldest age class I., the next oldest II., 

 etc., is less desirable, but unfortunately it has been largely 

 adopted. 



The number of years included in an age class is called a 

 " period," and the area dealt with in the course of a period is 

 called a "periodic coupe" (French: "Affectation"). 



The area of the age classes under the several methods of 

 treatment will be as follows : — 



a. Char ChiUinij w Hifih Forest. 

 The area of each age class, C, in a normal state, is — 



C = n X c = )i X ~, or C = n x 



r + s 



according as to whether each clearing is at once re-stocked, or 



allowed to lie fallow for .s years. 



Example : — 



Let area A =1,000 acres. 



Rotation r = 100 years. 



s = „ 



11 = 20 „ 

 Then, 



Annual age gradation = ^ = -'— - =10 acres; 



