148 ELEMENTARY FORESTRY. 
The Diameters of Trees and Logs are taken with a 
pair of wooden calipers of convenient size for the timber of 
the district. A limb or scale bar, graduated in inches and tenths, 
has a fixed arm standing out at right angles at one end, while 
a second arm is movable along the bar so that the trunk of a 
tree may be inclosed between them and the diameter read 
directly from the scale. The fixed arm is held in place by a 
Figure 39. Calipering a tree. 
screw so that it may be removed for packing and transportation, 
or so that a broken part may be replaced. The other arm has 
an adjustable plate which keeps it at right angles to the scale bar 
when pressed against the tree. Sometimes the circumference of 
the tree is measured with a steel tape, one side of which is grad- 
uated to give diameters of circles whose circumferences are read 
from the other side. 
The Heights of Trees are determined by means of a most 
convenient and useful little instrument, called Faustman’s mir- 
ror hypsometer. The distance of the observer from the tree is 
