8 FIRST PRINCIPLES [CH. 



normal under one rotation and one method of treatment, but 

 it would at once become abnormal if either of these conditions 

 was changed, and the forest required to be re-constituted or 

 organised on a different pattern. Normal volume means the 

 totaL cubic contents of the whole growing stock of a normal 

 forest, which results from its being formed of a normal (that is, 

 complete and regular, with an equal area of each) succession of 

 age-classes, and from its having a normal increment, that is, a 

 maximum possible annual rate of production. It is of course 

 not necessary that the different age-classes should be arranged 

 contiguously in regular succession of age on the ground, nor is 

 it even necessary that each age-class should be contained on a 

 single area all of one holding, but the normal state may, and 

 always ought to, exist, even in an irregular forest, where trees 

 of all ages are mixed up anyhow, and growing one above another 

 all over the whole area, although in this case no separate 

 age-classes are visible. Unless this normal series of age-classes 

 exists although invisible the full equal yield cannot be realised 

 every year for ever. 



8. Relation between wood-capital and increment. 



To make this clear, let us take an example. Suppose that we 

 are working a forest on a twenty year rotation, and for that 

 purpose have divided the ground up into twenty equal areas. 



We can represent our growing stock diagrammatically as in the 

 figure on page 9. The horizontal co-ordinate represents the area 

 divided into 20 equal parts, and the vertical co-ordinate re- 

 presents the volume of timber produced by the growth of the 

 forest year by year. We have then 20 crops of equal area forming 

 a regular succession of ages. The first area, on the left-hand side, 

 is i year old, the second 2 years old, the third 3 years old, and 

 so on up to the twentieth area which is 20 years old. The volume 

 of the growing stock or wood-capital is represented by the area 

 of the triangle ABC, and the yield, which is equal to the annual 

 increment over the whole area, is represented by the rectangle 

 AEFB, which is formed of 20 years' accumulated growth on 

 one-twentieth of the area, and is equal to the sum of the annual 

 increments (the portions shaded along the diagonal A C) for one 



