ESTIMATE BY MEANS OF FOmi FACTORS. 



87 



Form factors for l)ranch wood, fagots, or root wood only are, 

 as a rule, not used ; their volume is ascertained by utilising 

 the results of actual fellings and determining their proportion 

 to the volume of timber. 



As it would be highly inconvenient to measure the diameter, 

 or girth, of the tree close to the surface of the ground, where 

 it is usually cut, it has been agreed to take the measurement 

 at a convenient height. 

 According as to whether 

 that point is fixed or vari- 

 able, the following kinds 

 of form factors may be 

 distinguished : — 



(1.) Absolute Form Fac- 

 tors. — The diameter (or 

 girth) is measured at any 

 convenient height above 

 the ground, and the form 

 factor refers only to the 

 part a of the tree above 

 that point (Fig. 24), while 

 the volume of the piece b 

 below it is ascertained by 

 separate measurement and 

 added to the rest. This 

 is evidently troublesome 

 and takes extra time. 



(2.) True or Normal Form Factors. — The diameter (or girth) 

 is measured at a constant proportion of the height of the tree, 

 say, i\)th, 2^oth, etc. (Figs. 24 and 25). In this case the height 

 of the ideal cylinder is equal to the height of the tree. Such 

 form factors, it was believed, would have the advantage, that 

 all trees of the same shape would have the same form factor, 

 since they have been measured at a height, which bears in all 

 cases the same proportion to the total height. There are, 

 however, various drawbacks to the employment of these form 



Fitr. '25. 



