9 



only, or for the branches, or for the roots, or for all and each 

 severally. 



In the formulae above we have supposed that the girth and 

 sectional measurements have been taken at the height of a man's 

 chest above the ground, assumed, for the sake' of uniformity, to be 

 4 feet 3 inches. But it is obvious that any other conventional 

 height would serve the purpose, although it is usual and most 

 convenient to employ the one we have adopted. We need refer to 

 only one other convention which is sometimes used. The girth 

 may be measured at a constant fraction (say, for instance, one- 

 twentieth) of the height of the tree, in which case the form-factors 

 obtained are termed normal. Normal form-factors yield perfectly 

 correct results, but they are not practical owing to the difficulty 

 and trouble of measuring at such various heights, many of which 

 cannot be conveniently reached. 



Form-factors are said to be absolute when the base of the ideal 

 cylinder is assumed to be in the same plane as the girth which 

 is measured. In this case the contents of the portion of the stem 

 below the plane must be calculated separately. 



Like mass-tables, form-factors give closer results for an entire 

 forest than for individual trees. 



'The preparation of a complete set of form-factors requires great 

 care and experience, as their correctness depends entirely on the 

 selection of the type trees, whose dimensions serve as the basis of 

 all the calculations. In some cases the trees of a crop have been 

 classified into various classes according to their height and shape, 

 and a separate form-factor calculated for each class. The most 

 recent investigations prove that form-factors vary chiefly with the 

 height of the trees. 



4. Estimation by r\c\\i-height. 



By the term n'cM-height we mean that height at which the 

 stem of the tree measured has a diameter equal to half the diameter 

 at some point near the ground. If h r = the m-M-height h g =the 

 height at which the diameter near the ground is measured, and 

 a the sectional area of the stem at this height, then, according 

 to Pressler, the contents of the stem 



as ij ah r + ah g . 



This formula is based on the fact that the first term represents 



