14 MENSURATION AND INCREMENT [CH. 



a column against that class, so that by multiplying this basal 

 area by the number of trees in each size-class, and adding them 

 together, the aggregate basal area of each group is found. 

 Dividing this total by the number of trees in the group, the basal 

 area of the average tree of the group is found, and its corre- 

 sponding girth or diameter is taken from the tables. This gives 

 us the exact size of the sample trees which we now have to find 

 in the crop, and one or two sample trees as nearly as possible 

 of exactly this size are selected for each group, and felled and 

 cut up and carefully measured. This gives the cubic contents in 

 timber and in small wood of the average tree for each group. 

 This volume, multiplied by the proportion borne by the basal 

 area of the whole group to the basal area of the sample trees 

 felled and measured, gives the volume for each group in solid 

 cubic feet. These have only to be totalled to get the total volume 

 for the whole area enumerated. Columns can, if desired, be 

 added for height, age, and form-factor. 



This procedure would not be practicable in a very mixed and 

 utterly irregular crop, as it assumes a certain degree of uni- 

 formity. If the crop were quite irregular in all respects, no 

 accurate estimate could be arrived at except by estimating the 

 volume of each tree separately. 



If, on the other hand, the crop were practically even-aged and 

 quite uniform, it would not be necessary to make groups, but 

 one average tree for the whole crop could be taken. It will be 

 noticed in the method described that no account is taken of 

 height. If, however, the crop contains distinct height-classes, 

 these must be dealt with separately in the same way. 



Lastly, it is to be noted that, apart from the question of girth 

 or diameter, care must be taken in selecting the sample tree, as 

 any tree of this size would not do. It must be a tree representing 

 in shape and development all the conditions of growth existing 

 in the crop, especially with respect to the result of the density 

 of the crop and the crowding together of the stems in their 

 growth. The average density of the crop is of course a matter 

 of first importance as determining the size, shape, and develop- 

 ment, of the representative sample tree. 



If the woodland area dealt with is of small extent, a complete 



