29 



by the formula C = c — . If the trees falling under one and the 

 same girth-gradation are of very difEerent heights, this fact 

 must be borne in mind in selecting the sample trees, or the girth- 

 gradation may be divided into sub-classes according to height and 

 each sub-class treated in the ^ay described. 



The great advantage which Draudt's method offers is, that all 

 the sample stems for the whole crop may be measured up together 

 and their contents determined, not only in one lump tignre, but also 

 according to the different classes of produce they yield, thereby 

 enabling as to estimate by meaus of a few easy multiplications the 

 contents of the entire crop. 



Uetch's Method. 



We liave seen that in Draudt's method fractions in the quotient 

 of (— ) are got rid of by taking the nearest whole number, but where 

 the quotient is much less than unity, several quotients are added 

 together, and the result worked out for a group of girth -gra- 

 dations. This procedure is obviously not quite logical, and hence 

 Urich has modified it so as to secure greater consistency. He 

 adopts the latter principle throughout, and his system is according- 

 ly always to carry over all fractions to the next class. The sample 

 trees are hence seldom required to represent a separate girth- 

 gradation, but nearly always a group composed of the whole or 

 portions of two or more such gradations. The girth of the 

 sample trees for each group is accordingly determined by the pro- 

 portion of the respective numbers of the several gradations compos- 

 ing the group. TVhen the formula C = c — is employed, it is 

 not so necessary that the sample tree should be representative of 

 the group in respect of volume as in respect of height and form- 

 factor. 



The advantages of Urich's method are the same as those of 

 Draudt's. 



In its valuation surveys the Commission for Forest Research in 

 Germany adopts five girth groups, and fells from two to three 

 sample stems for each group. This is a combination of CJrich's 

 principle with the method of girth-classes. 



