METHOD OP SEVERAL HEIGHT CLASSES. 65 



These data show that form factors for Scotch pine obtained 

 from measurements in Germany are well applicable to woods 

 in England. 



B. The Height is not a Function of the Diameter. 



If it is found that in the case of equal diameters the heights 

 differ considerably, then height classes must be formed in 

 addition to diameter classes. 



In some cases it happens that the different height classes 

 are separated according to area, for instance where a marked 

 change in the quality of the locality occurs,. due to change in the 

 soil or subsoil, aspect, etc. In such cases the wood is divided 

 into as many parts as different height classes appear, and each 

 is treated as a separate wood. 



If the several height classes are mixed over the whole area, 

 a case which is comparatively rare, as in irregular selection 

 forests, then the diameter and height must be measured in 

 each case. Where only two height classes are adopted, the 

 height may be estimated while the diameter is measured, and 

 the tree placed in the one or the other height class. Each of 

 the latter is then considered as a separate wood, and its 

 volume ascertained according to one of the methods described 

 under A. 



Cases where more than two height classes, in addition to 

 diameter classes, are called for, are very rare. Generally, the 

 distinction of height classes is a matter of considerable diffi- 

 culty, unless the heights are measured. They are only neces- 

 sary where a very high degree of accuracy is aimed at. 



The following example will show the manner of booking in 

 the case of two height classes : 



VOL. III. 



