FIG. 27 



FOREST MANAGEMENT 121 



Stem measurement. The cubic contents of the stem of a 

 single felled tree are determined accurately enough for 

 ordinary purposes by multi- 

 plying the middle sectional 

 area by the length. At a 

 point taken by measure- 

 ment half-way along the 

 stem, the diameter is found 

 most conveniently by 

 callipers. The area of the 

 cross-section is then calcu- 

 lated (obtained by squaring 

 the radius and multiplying 

 the result by 3-14) and the 

 result multiplied by the 

 length of the stem. Speci- 

 ally prepared tables may be used to accelerate the operation. 



In Britain the practice is materially different, the quarter- 

 girth system being in vogue. By it the length is multiplied 

 by the square of the mean quarter-girth. Thus a tree 

 32 feet long, having an average circumference under bark 

 of 44 inches, is calculated as 



1 1 2 x = 27 cubic feet. 

 144 



This is often supposed to give the actual cubic contents, 

 whereas the result thus obtained is about one-fifth less than 

 the actual volume. To state it otherwise, timber reckoned 

 by quarter-girth measurement to have a volume of 100 cubic 

 feet contains in reality about 125 cubic feet. The difference 

 is understood to equalise itself in the price obtained, and that 

 the surplus allowed to the purchaser provides for loss and 

 waste in squaring the stem. Only the force of custom, 

 however, can defend the use of such an incorrect system. 



For scientific investigation the stem is measured as though 

 divided into short sections say, 6 feet in length. The 

 diameter is taken at the middle of each piece, thus at 3 feet ? 



