G92 JOURNAL OF FORESTRY 



It is evident that for a constant form factor this board foot-cubic 

 foot ratio may vary widely within a single diameter class, increasing 

 with height. The variation between two and eight log trees, as above 

 indicated, is nearly 65 per cent, or one-half as much again as the varia- 

 tion given in Terry's table resulting from a change in diameter from 

 12 to 36 inches. It should be noted, however, that Terry's trees of the 

 32-inch class only range from about 4 to 6 logs, and that the increase 

 within this narrower range is only about 1 1 per cent. This is doubtless 

 why the fallacy in his assumption did not come to light in his study. 

 Nevertheless, in any other region where the height range for each 

 diameter class is great, as is the case both on the West Coast and in the 

 Inland Empire, the danger of this seemingly innocent hypothesis is 

 apparent. 



If this ratio is not constant within each diameter class the fact that 

 the form factor for the class is constant makes the product of form 

 factor and ratio variable with height, and the method if accepted can 

 be considered only a rough approximation. This can be seen from 

 another consideration, as well. The formula developed finally takes 

 the form V = &H (where V = volume in board feet, H^ height in 

 feet, and h = "board-foot form factor," which is constant for each 

 diameter class). But this is the equation of a straight line passing 

 through the intersection of the V and H axes. If the formula is cor- 

 rect, therefore, all the diameter class curves of the graphed volume 

 table would be portions of straight lines which, if produced, would 

 radiate from the origin. I have redrawn the curves of a number of the 

 best volume tables in my possession and find no such tendency appar- 

 ent. Occasionally the straight line form of curve is approached (this 

 is always true in the case of tables prepared by the frustum-form- 

 factor method), hut these straight lines either do not radiate from a 

 common point or, if they do, that point is remote from the origin. The 

 evidence of existing tables, then, seems once more to contradict the 

 fundamental hypothesis of Terry's method. 



Of course, if the diameter class curves in any table are very short, a 

 straight line passing through the origin could be substituted for each of 

 them without introducing any very large errors, and this is exactly 

 what the method under discussion does. It appears, however, that the 

 utility of the procedure is limited to regions where tlie height range 

 for each diameter class is exceedinglv small. 



