Development of Forest Mensuration. 125 



versa! forest measure" or caliper. The improvement of 

 calipers to their modem efiBciency has been carried on 

 since 1840 by Garl and Oustwo Beyer and by many 

 others until now a self-recording caliper (by Rev£s, etc.) 

 has become a practical instrument. For measuring the 

 heights of trees, HossfeM had already a satisfactory ia- 

 strument in 1800, a very large number of improvements 

 ia 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 Ruber, using the average area multiplied by length 

 was definitely introduced in the Prussian practice in 

 1817. The names of Smdlian, 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 Ruber, 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 Rossfeld (1812) and 

 Rundeshagen and Koenig occupied themsdves with 

 daborating form factors. Smdlian (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 



