532 JOURNAL OF FORESTRY 



cipally in the top diameters and is therefore of little or no importance 

 for practical purposes. For form class 0.70 the two series are : 



Spruce 100 95.2 89.9 84.0 77.5 70.0 61.4 51.3 38.3 22.7 



Pine 100 95.3 90.0 84.1 77.5 70.0 61. 1 50.6 37.2 19.4 



Difference in per cent o.i o.i o.i 0.3 0.7 1.6 3.5 



Table IV gives a comparison between the measured and mathemat- 

 ically computed diameters and the absolute form factors. 



Tables of Taper 



Similar as for spruce, tables of taper have been constructed for pine 

 inside bark. 



The difference in the values given in the two tables is very small, 

 however, and in the book of tables adopted for use by the Swedish 

 Forest Service the table based on the formula for the stem form of 

 Scotch pine is included under the following title : "Taper table for 

 Scotch pine and spruce inside bark. The table is also applicable to 

 spruce and other thin-barked species (not Scotch pine) outside bark." 



Since measurements on standing trees are taken outside bark, it was 

 necessary to study the thickness of bark of Scotch pine in different 

 portions of the stem, and especially at breast height. Jonson examined 

 some 4,000 trees in regard to this. 



Other very important developments are Jonson's method of deter- 

 mining the form class of standing trees and the construction and use 

 of his taper and volume tables. Jonson's method of determining form 

 class (the table referred to in this description is the volume table pub- 

 lished by Professor Jonson) is as follows: 



The position and form of the crown is a very good indirect measure 

 of the stem form of a tree. According to Metzger, the pressure of the 

 wind on the tree crown constitutes a force which compels the tree to 

 construct its stem in such a manner that the same relative resistance to 

 fracture sets in in all parts, the smallest possible amount of material 

 being used. As the concentrated force of the wind strikes a point situ- 

 ated lower or higher on the tree, we get larger or smaller taper, which 

 means poor or good form class. 



With the help of the equation for the stem curve and recognized 

 mechanical laws governing stresses, it has been possible to calculate the 

 normal relation between crown and stem form. As the location of the 

 point of attack of the bending force is determinative of form, this point 

 is called the "form point" and its height is expressed in percentage of 

 the total height of the trees. 



When deciding on form class, the point — "the form point" where the 



