134 ELEMENTARY FORESTRY. 
mately the same dimensions and grown under the same condi- 
tions. The form factor is expressed as a decimal, and is the ratio 
of the mean volume of the sample trees to the volume of a cylin- 
der with the same diameter as the diameter of the mean sample 
tree at breast height, and whose length is equal to the height of 
the tree. For example: A tamarack measures 6.9 inches in diam- 
eter, breast high, and the height of the tree is fifty-one feet. Its 
volume by accurate measurement of the felled tree is 7.21 cubic 
feet, and the volume of a cylinder with a diameter of 6.9 inches 
and a length of fifty-one feet is 13.24 cubic feet. The form 
factor, or factor of shape, is therefore 7.21+13.24=.54, and if 
this tamarack represents the mean of a large number of trees 
of approximately the same dimensions, the factor may be applied 
to all of them, or to all trees of the same size and grown under 
the same conditions. In the same way factors are determined 
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Tigure 37. Determining the volume of a felled tree 
for all sizes, and tabulated for future use. In application the 
volume of a tree 6.9 inches in diameter, breast high, and fifty- 
one feet high would be found thus: Volume of cylinder & form 
factor equals volume of tree, or 13.24X.54=7.21. This method 
gives a much closer approximation than could be obtained by 
using a geometric figure supposed to represent the shape of the 
tree. . 
The Volume of a Felled Tree may be determined more 
accurately. It is considered in sections, or log lengths, and the 
volume of each section is found by multiplying the middle cross- 
sectional area by the length. The degree of accuracy of this 
method depends on the length of the sections; the shorter they 
are the more accurate the result. The last section at the top, 
when small, may be treated as a cone whose volume is equal to 
the basal areca times one-third its length; or when large and 
tapering off suddenly it may be considered as a paraboloid whose 
volume is equal to the basal area times one-half its length. The 
