COMMENT ON "A FORMULA METHOD OF ESTIMATING 



TIMBER" 



Bv Donald Bruce 

 Associate Professor of Forestry, University of California 



Professor Terry in his article in the April issue of the Journal 

 proposes a new method of preparing volume tables and of computing 

 estimates which, if it could be proven sound, would result in a very 

 material saving of time and effort in these two labor-consuming opera- 

 tions. As he requests criticism and suggestion, the following comments 

 are oft'ered : 



A fundamental assumption of his work is "that for a given species 

 in a given region . . . the ratio between the cubic and board foot 

 contents (considering only the merchantable stem) of the trees of each 

 diameter class will be practically constant." No evidence is given in 

 support other than the argument that this hypothesis "seems to be 

 reasonable." In the opinion of the writer, analysis will not confirm 

 this statement. 



Let us take a diameter class at random, say 32-inch, and see what the 

 ratio actually would be for trees within it which are of the same form 

 but of varying heights. The simplest form to compute is that of a tree 

 of absolutely regular taper to a fixed top, say 8 inch — that is, the frus- 

 tum of the cone. The merchantable form factor of such a tree will be 



volume of frustum of cone 

 volume of cylinder 



— (32^ -fS^'-f 32X8) height 



— X 32' X height 



.44. 



This value is constant for the class, since it is independent of height. 

 But if such trees are scaled as 16-foot logs, the following results: 



Height in 16-foot logs. 



Scale of tree in 16-foot 

 logs, Scribner Decimal C. 



Volume in cubic feet of 

 corresponding cylinder . . 



Ratio, scale to cubic vol- 

 ume 



1220 

 446.8 

 2.73 



1530 

 536.2 



2.85 



691 



