32 MEASUREMENT OF FELLED TREES. 



taken, the error generally does not exceed 2 per cent, of the 

 true value. 



In Britain and in India, the sectional area in the middle is 

 calculated hy the method of the quarter girth, that is to say, 

 by the formula — 



,S = (|)^ = -0625 X[f. 



In comparing this with the real sectional area = "0796 X^r, 

 it is found, that the quarter girth method gives only 78i per 

 cent, of the true volume, omitting 21^ per cent. The method is 

 based upon the assumption, that this amount represents the 

 waste incurred in squaring the timber. Quantities calculated 

 by the exact method can be converted into the quantities cor- 

 responding to the other, by deducting 21|^ per cent, of the 

 volume. If the bark has been omitted in the measurements 

 in both cases, no further correction is required. In measure- 

 ments according to the quarter girth, it is usual to deduct for 

 the bark one inch out of every t\Yelve of the quarter girth, 

 before the latter is squared. Hence the deduction on account 

 of bark comes to 12 X 12 — 11 X 11 = 144 — 121 = 23 square 

 inches out of every 144. In other words, a reduction of 

 16 per cent, is made on account of bark. This may be too 

 much in some cases and too little in others. It is a much 

 better plan, in the case of felled logs, to take off a ring of 

 bark, so as to measure the girth over the wood only. 



Generally speaking, it may be said, that the volume calcu- 

 lated according to the quarter girth method is equal to three- 

 fourths of the volume in the round. 



2. Volume of Branch and Root Wood. 



In some cases, pieces of branch and root wood are of a 

 sufficiently regular shape to measure and calculate their 

 volume in the manner given above. As a general rule, how- 

 ever, such wood requires a different treatment* Its volume is 

 ascertained by shaping it according to custom and stacking 



