CHAPTER VI 



THE CONSTRUCTION OF LOG RULES FOR BOARD-FOOT 



CONTENTS 



55. Methods Used in Constructing Log Rules for Board Feet. The 



great variation in the contents of different log rules for board feet, and 

 the variation in accuracy and consistency of these rules is due to the 

 methods used in their construction as well as to the factor of over-run 

 resulting from closer utilization. 



Four general methods have been used in constructing such rules. 

 These are: 



1. By mathematical formulae. A formula is used, which derives the 

 board-foot contents of the log directly from its diameter and length, by 

 allowing for reductions from D 2 XL for cubic volume, waste in saw 

 kerf, waste in slabs, and reduction of residual volume to board feet. If 

 the principles used in making these reductions (§ 54) are correct and the 

 amounts used are also correct, such log rules are superior to diagram 

 rules, but if errors in either principles or amounts of deduction are 

 introduced into the formula, the rule is worse than useless. 



2. By diagrams. Full-sized circles of all diameters are drawn on 

 large sheets of paper, representing the top ends of the logs. On these 

 cross sections of the log the ends or cross sections of the boards which 

 could be sawed from these logs are drawn, leaving between each board a 

 space equal to the width of the saw kerf. The area of boards in square 

 inches is then reduced to board feet by the factor -fax length in feet, for 

 logs of a standard length, and from this, for logs of all lengths. 



3. By tallying the actual sawed contents of logs at the mill for differ- 

 ent diameters and lengths. Owing to the variables introduced by the 

 thickness of lumber sawed, and by taper, this method has seldom been 

 accepted as the sole basis for a log rule, but has been extensively used 

 to check the accuracy of rules made by the preceding two methods. 



4. By conversion of the cubic contents of logs into board feet, after 

 deducting a fixed per cent of this total cubic contents for waste in saw- 

 ing and slabbing. As shown in Chapter V, all board-foot log rules 

 constructed on this basis are fundamentally wrong. 



A fifth method has been used, which is a combination of methods 

 1 and 2 or 3, namely, to alter or correct the values of an existing log rule, 

 by means of mill tallies obtained in sawing. The author of such cor- 



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