Development of Forest Mensuration. 125 
versal forest measure” or caliper. The improvement of 
calipers to their modern efficiency has been carried on 
since 1840 by Carl and Gustav Heyer and by many 
others until now a self-recording caliper (by Reuss, etc.) 
has become a practical instrument. For measuring the 
heights of trees, Hossfeld had already a satisfactory in- 
strument in 1800, a very large number of improvements 
in great variety having followed, with Faustmann’s mir- 
ror hypsometer probably in the lead. As a special de- 
velopment for measuring diameters at varying heights 
Pressler’s instrument should be mentioned, and a very 
complicated but extremely accurate one constructed by 
Breymann. 
Various formulas for the computation of the con- 
tents of felled trees had already been developed by Oet- 
telt and others in the eighteenth century and a formula 
by Huber, using the average area multiplied by length 
was definitely introduced in the Prussian practice in 
1817. The names of Smalian, Hossfeld, Pressler and 
others are connected with improvements in these direc- 
tions. 
The idea of form factors and their use was first devel- 
oped by Huber, who made three tree classes, according 
to the length of crowns, measured the diameters six feet 
above ground, and used reduction factors of .75, .66, 50 
for the three classes. But the first formula for determin- 
ing form factors is credited to Hossfeld (1812) and 
Hundeshagen and Koenig occupied themselves with 
elaborating form factors. Smalian (1837) introduced 
the conception of the normal or true form factor relating 
it to the area at one-twentieth of the height. An en- 
tirely new idea has lately been introduced by Schiffel 
