8 



4. Estimation by rc/-height, in which, besides the girth 



at breast-height, the height at which the stem tapers 

 down to half that girth is measured. 



5. Estimation with the help of the height of the tree and 



several girths, the lowest of which is measured at breast- 

 height. 



1. Ocular Estimation. 



Practised wood-cutters are able to estimate more or It ss accur-. 

 ately with the eye alone the contents of trees belonging to species 

 that they are familiar with, and growing in localities with the 

 peculiarities of which they are acquainted. It is needless to say 

 that the most experienced are liable to commit large errors, and 

 that the inexperienced should never employ this method. 



2. Estimation with the help of Mass-tables. 



The mass-tables drawn up with great labour for the forests of 

 the kingdom of Bavaria give the cubical contents of trees of known 

 height, diameter, and age-class. They comprise averages deduced 

 from the measurements of 40,000 trees. On this account, although 

 they give accurate results for a large number of trees taken to- 

 gether, they are not to be relied on for cubing trees singly, as the 

 single tree in any given case may ditfer very widely from the 

 average tree. 



3. Estimation by weans of Form- factors. 



If a = sectional area of the trunk at breast-height, h = height 

 of the tree, c = the true contents of the tree or tree-part consider- 

 ed, and C = volume of an ideal cylinder whose basal area is a and 

 height k t then we have the following formulae : 



rf 



where/ is a constant termed the form-factor, and is deduced as an 

 average from the measurement of a sufficiently large number of 

 type trees. Type trees are selected, felled and measured separately 

 for each age or si/te-class, and for each species or -group of species. 



Form-factors may be deduced, according to the requirements of 

 the case, for the stem only, or for the whole tree, or for the timber 



