m] MENSURATION AND INCREMENT 13 



most useful for these purposes, and are available now for each 

 different species in each sort of locality, grown in each kind of 

 crop, age, and so on, and are based on the records of thousands 

 of measurements. In these tables, form-factors are given 

 separately for each species, with different factors corresponding 

 to differences in height. 



Where such form-factors exist, and are reliable, it is only 

 necessary to know the diameter at breast-height {4 feet 3 inches 

 exactly), and the timber-height, measured by a dendrometer, 

 thus: 

 Contents in cubic feet 



_ ^ (Diam. at B.H. in ins.)^ tt , . , 



= F-f X :rr. X 7 X height, 



144 4 



which would be equivalent to 



(Diam. in ins. at mid-timber-height)^ n- , • , , 

 — X - X height, 



or 



(Girth at mid-timber-height in ins.)*" i , . , ,.,,-, 



^^ X X timber height m feet. 



144 4.7r ^ 



It is of course only in very uniform crops grown in fully 

 stocked, close-canopied high-forest that form-factors and volume- 

 tables, which generally refer to the height of the tree, could be 

 safely applied, and even then they would only give good results 

 when applied to a large number of trees. 



However, for the present we must generally be content to do 

 without these convenient helps to investigations regarding the 

 volume of standing timber, and find some other means of 

 estimating the cubic contents of a standing crop. This is best 

 done by finding out the exact size of a sufficient number of 

 sample trees; and then to fell several trees of these sizes, and 

 cut them into small sections, and measure them carefully, timber 

 and branch-wood. 



The usual procedure is as follows. An enumeration is made of 

 the trees forming the crop, which are then totalled up for each 

 size class — generally i-inch diameter-classes, or 3-inch girth- 

 classes — and the size-classes are grouped together so as to form 

 say from three to six groups of equal range: the basal area 

 corresponding to each size is taken from tables, and entered in 



