44 VOLUME OF WHOLE WOODS. 



trees which show, at any rate approximately, the same basal 

 area, height, and form factor, so that they can be thrown together 

 and dealt with in an uniform manner ; in other words all trees 

 of a wood which show the same base, height, and form factor, 

 are joined into one class ; the volume of one tree (or of a 

 few trees) is ascertained, and the volume of the whole class 

 obtained by multiplying the former by the number of trees 

 in the class. If every class is dealt with in the same way, the 

 volume of the whole wood is obtained by adding together the 

 volumes of the several classes. 



So far, however, little or no advantage is gained, because it 

 would be necessary to ascertain the base, height, and form 

 factor of each tree in order to put it into its proper class, and 

 when this has once been done, the volume of each tree may 

 just as well be calculated separately. Moreover, in crowded 

 woods the height is not always easy to measure, and the form 

 factor could only be estimated, unless it is taken from a table. 

 Only the basal area is easily ascertainable by measuring either 

 the diameter or the girth. 



Here, experience had to be called in, which fortunately 

 showed that in regularly-grown crowded woods, the height 

 and form factor are approximately functions of the diameter 

 of the tree; in other words, trees of the same diameter 

 have approximately the same height and form factor. At any 

 rate this is found to hold good to a sufficient extent, so as to 

 justify a classification according to diameter classes only. 



In open woods, however, the height and form factor vary 

 within much wider limits, so that, besides diameter classes, at 

 any rate also height classes must be formed. Hence, the two 

 cases must be dealt with separately. 



A. The Height is a Function of the Diameter. 

 1. Description of the General Method. 



a. Formation of Diameter Classes. 



The number of classes depends on the difference between 

 the largest and smallest trees of a wood, and the desired degree 



