FURTHER COMMENT ON "A FORMULA METHOD OF 

 ESTIMATING TIMBER." 



By E. I. Terry. 



I appreciate the criticism of my formula for estimating timber made 

 by Professor Bruce in the October number of the Journai<. My study 

 was based entirely upon the western yellow pine of Colorado and T 

 anticipated some criticism from the Pacific Coast along the lines which 

 Bruce has made. I accept without question the validity of his objec- 

 tions but believe that the defects which he points out and which be- 

 come apparent when applied to very large timber may be corrected 

 without changing the fundamental principle of the formula. 



In the first of his two specific criticisms, Bruce takes exception to 

 my statement that for a given species in a given region the ratio be- 

 tween the cubic and board-foot contents of the merchantable stem 

 of each diameter class will be practically constant. He shows by an 

 example that the ratio increases with increasing length of the merchant- 

 able stem, and that for large trees having a wide range in merchant- 

 able height this increase may be considerable. According to my con- 

 ception of the ratio between board and cubic feet, I would compare 

 the scale of a tree of given size, not, as Bruce does, with the cubic 

 volume of the corresponding cylinder, but with the actual cubic contents 

 of the same merchantable stem. In Bruce's example the ratio of board 

 feet to the cubic foot for a 33-inch five-log tree is 2.73, which appears 

 to me to be extremely low. For the thirty 32-inch yellow pines upon 

 which my computations for that diameter class were based — ranging 

 from 4 to 6 logs, with an average height of 4.5 logs — I found the ratio 

 to be 6.7 board feet to the cubic foot (Table 2, page 418, Journal of 

 Forestry, for April, 1919). I do not, however, deny that for a given 

 diameter the ratio of board to cubic feet will vary with increasing 

 height, but for the Rocky Mountain pine I found the variation to be so 

 slight that it could be neglected without appreciable error. 



In his second criticism, Bruce points out that the equation V = 6 H 

 is the equation of a straight line when b, the board-foot form factor, is 

 constant for each diameter class. It is in all probability true that for a 

 given diameter the increase in volume with increase in merchantable 



160 



