168 A MANUAL FOR NORTHERN WOODSMEN 
was known that single trees depart widely from the 
type, it was assumed that for trees having the same di¬ 
ameter and height an average volume could be ascer¬ 
tained which would hold approximately throughout the 
distribution of the species. Proceeding on this assump¬ 
tion, tables were worked out for the different tree species 
and these when applied in actual business proved close to 
the fact and vastly improved the work of timber valuation 
in Germany a hundred years ago. 
European measurements of logs and standing timber do 
not recognize anything corresponding to the board foot, 
but everything is reckoned in solid contents. The same 
rule holds in the scientific study of tree form in all coun¬ 
tries where it has been pursued, the unit in the United 
States being the cubic foot. For all such studies, too, the 
total height of the tree as a well-defined factor capable 
of ready measurement has usually been employed rather 
than any size limit set part way up, and a diameter breast 
high, or 4| feet above the ground, has been settled upon 
as the basis of all diameter comparisons. The area of a 
cross-section of a tree at this point is called the basal area, 
and the same term is applied to a number of trees or to a 
stand of timber. In the study of tree form, the term form 
factor has proved to be a useful one. The form factor of a 
tree is the percentage which the volume of any tree (usu¬ 
ally reckoned in cubic feet, outside the bark) makes,of 
the volume of a cylinder having the same height and the 
tree’s breast diameter. Illustration: A tree 15 inches in 
breast diameter and 75 feet high may, after caliper meas¬ 
urement every 4 feet along it, prove to have 38.6 cubic feet 
in it. A cylinder of these dimensions contains 92 cubic 
feet. The form factor, therefore, is .42. 
For many years past the study of tree form has been 
ardently pursued, and many interesting facts and laws 
have been ascertained. In large measure these results 
have been brought to bear on the actual business of Euro¬ 
pean countries where timber is grown as a crop under 
uniform conditions. In this country, where the forests 
are natural and as a rule irregular, it will be many years 
before the same can be true. The following, however, 
