106 



THE SCALING OP DEFECTIVE LOGS 



from dead trees, or for surface checking. The per cent of the scale 

 thus deducted varies for every diameter of log, and for each difference 

 in the number of inches subtracted. This method of deduction should 

 not be used to offset some interior defect. By this method, a 20-inch 

 log by Scribner's Rule would give the following deductions from scale 

 in per cents. For other diameters, the per cents would differ: 



This method should usually be rejected in favor of one of the other 

 three, since it substitutes a guess for an accurate deduction. 



Use of Diagrams. The diagram method is the most accurate way 

 of computing the actual number of board feet to deduct from a log 

 for a given defect. The cross section of the defective area is blocked 

 out as a square or rectangle, and its length decided upon, whether 

 running completely through the log or only part way through. For 

 rules based on J-inch saw kerf, 20 per cent of the cross section of this 

 area must be deducted to get the net volume of 1-inch boards to be 

 deducted from the scale. 



This is expressed by formula when 

 a-& = cross sectional area in inches, 

 1 = length of defective section in feet, 

 2/ = cubic contents of the section in board feet, 

 x = volume of section, sawed into 1-in. boards, 



a-b-l 



Then 



or 



i-in. 



saw kerf. 



y- 



x = y- 



x=- 



12 



■ 202/ = 



a-b-l 

 To -- 



80y, 



In using a decimal rule, the resultant volume is rounded off to the 

 nearest 10 or "decimal value" before subtracting it from the log scale. 



