11 



Without being guilty of any important error, we may assume 

 that in one and the same crop trees of like girth and height 

 do not differ greatly either as respects cubical contents or form- 

 factor. Hence, we may divide the component trees of a crop into 

 classes based on equality of diameter and height combined, and 

 find the contents of each class by selecting trees fairly representa- 

 tive of that class {sample or ti/pe trees) and measuring these sepa- 

 rately. The average contents of the sample trees, multiplied by 

 the number of trees composing the class, will give very approxi- 

 mately the true contents of the whole class. 



If all the component trees of a crop were of like height, dia- 

 meter, and form, the measurement of a single sample stem would 

 suflaee. In reality, however, the trees of a crop are of unequal 

 development, and must be divided into classes comprising indivi* 

 duals of equal diameter and height. Nevertheless, it may some- 

 times be possible in a more or less irregular crop to find a tree 

 such that its contents are equal to the mean contents of all the 

 trees comprising the crop. Such a tree we may term an average 

 tree, and the method of measurement in which nverage trees are 

 employed may accordingly be termed Valuation by average tree*. 

 Suppose c = the contents of the average tree, C, = the contents of 

 the whole crop, and « = the total number of trees, then — 



C 

 C=~ 



It is, however, rare to find a single tree such that its contents are 

 equal to the mean contents of all the trees of the crop; while, on 

 the other hand, the establishment of as many classes as there are 

 different girths and heights present in the forest would involve 

 .enormous expenditure of time and labour. Hence the adoption of 

 a middle course is to be recommended. Firstly, more compre- 

 hf'usive girtb-and-height classes should be established, each class 

 comprisino' trees not precisely of one and the same dimensions, 

 but of different heights and girths varying between a maximum 

 and a minimum that are sufficiently close together to ensure the 

 necessary degree of accuracy ; and, secondly, these girth-and-height 

 classes being established, the average tree for each class should be 

 obtained by calculation. For convenience' sake we may designate 

 this method of measuring crops. Valuation ly means of girlh-aad- 

 keight gradations. 



