164 FOKESTRY WORK 



If it is not desirable to fell the sample poles, the average 

 cubic contents may be computed by means of form factors. 

 On the Continent, tables of form factors have been com- 

 piled, and this method is mostly used in calculating con- 

 tents of trees, but unfortunately very little data is avail- 

 able in this country upon which to compile such tables. 

 A form factor is a number representing the proportion 

 ■which the contents of a tree bear to a true cylinder of the 

 same height and girth or basal area at breast height. 



The Continental tables are all worked on basal area, 

 and no figures are available for J girth. 



To find the form factor for a tree (| girth measurement), 

 the total length of the tree is measured, and also the 

 J girth at breast height. For example we will take a tree 

 50 feet high by 8| inches I girth. The length of timber 

 is then taken, say 40 feet to where it is 3 inches in dia- 

 meter; the I girth at midway, 20 feet, is 6| inches. 



A cylinder 50 feet long by 8| inches would contain 

 25 cubic feet. The actual contents of the tree are 11 

 cubic feet 8 inches. 



25 feet = 300 inches; 11 feet 8 inches = 140 inches. 



Therefore, the relation of the tree to a true cyhnder 

 erected on the same base is expressed by the fraction ^^, 

 which, being reduced to decimal figures = 



300)1400(-466 

 1200 



2000 or say -47, the form factor, 

 1800 



2000 



