80 THE CONSTRUCTION OF LOG RULES 



expressed in two forms, one for application to diameter at small end, inside bark, 

 the other to diameter outside bark at middle of log. The latter form would apply 

 only to species with bark of similar average thickness to the second-growth white 

 pine on which the latter is based. The utility of this rule as a standard is inter- 

 fered with by the fact that a certain per cent, not stated, of IJ-inch and 2i-inch 

 lumber was included with 1-inch boards in its construction. The results are there- 

 fore somewhat too high for 1-inch lumber. 



This log rule indicates that the contents of logs measuring from 4 to 10 inches 

 in diameter at small end are from 20 to 50 per cent greater when scaled by this rule 

 than by the International |-inch rule. Above 12 inches, the excess is not over 

 10 per cent. Since these boards are measured at their average face, taper is fully 

 utilized, while waste from slabs and edging is reduced to a minimum. The result- 

 ant per cent of utilization is very consistent for logs of all sizes; hence it shows a 

 marked gain in the small sizes over the per cents utilized in square-edged boards as 

 shown in Table III. 



The importance of a log rule of this character in scaling the boardrfoot contents 

 of second-growth timber in regions utilizing round-edged boards is obvious. Rules 

 of this character are nearly as satisfactory as the cubic foot in measuring small timber. 

 For complete accuracy in applying this rule to other species, the average taper 

 must be known, or the average thickness of bark. Similar local log rules have 

 been made for loblolly or old field pine in the Atlantic Coast States. 



76. Conversion of Values of a Standard Rule to Apply to Different 

 Widths of Saw Kerf and Thickness of Lumber. Where over-run or 

 under-run is caused by a difference in the width of saw kerf used, or in 

 the thickness of lumber sawed, from the standards used in the log rule, 

 the per cent of this difference between scaled and sawed contents due to 

 these factors may be easily determined, and applied, if desired, to the 

 scale; or it may be incorporated in a new set of values or local log rule 

 similar to those made from mill tallies. 



For saws of different widths. 



Let iv = width of saw 'kerf in standard rule; 



K' = width of saw kerf used in sawing. 

 Then 



— =per cent of lumber, minus saw kerf by standard rule; 



1-l-A. 



— =per cent of lumber using different saw kerf. 



1+A 



The correction to apply to the standard rule in terms of per cent is: 



1 



Per cent correction = 100 X , 



T+A' 

 e.g., the International rule, j-inch kerf plus iV-inch shrinkage = i^-inch = .3125, . 



1 



100 X =76. 3 per cent. 



1.3125 ^ 



