46 THE MEASUREMENT OF LOGS— BOARD-FOOT CONTENTS 



45. Middle Diameter as a Basis for Board-foot Contents. In some 

 regions no attempt is made to divide long logs in scaling. While short 

 logs are scaled at the end, logs over a given length are measured once at 

 the middle and the scale applied to the entire log. In cypress this 

 measurement is sometimes taken at a point distant from the small end 

 by one-third of the total length. This practice of substituting middle 

 for end diameters on long logs and scaling the log as one long cylinder 

 whose diameter is thus obtained assumes that the loss in sawing the 

 smaller top section will be offset by gain from taper in the butt portion. 

 The total scale by this method exceeds that obtained by scaling the log 

 as the sum of separate cylinders. 



In theory this measurement of logs for board-foot contents at the middle diameter 

 should possess the same advantage over measurement at the small end as for cubic 

 contents. But for the former purpose, the factor of waste exercises a definite influ- 

 ence on the method of scaling adopted, where for cubic contents it does not. 



With very close utilization of short lengths, it may be assumed that the sawed 

 output of two logs of the same middle diameter, one of which tapers rapidly, the 

 other gradually, would be nearly equal, since what is lost at the small end of the 

 rapidly tapering log would be saved at the larger end. That this is approximately 

 true is the premise on which Tiemann based his board-foot log rule (§ 63) on middle 

 diameter. 



If, on the other hand, the minimum length of board corresponds with the ordinary 

 length of log sawed, the log with rapid taper loses a far greater percent than that 

 with small taper, and two logs whose diameters at their small end are the same 

 would give equal sawed contents regardless of differences in taper. Since the latter 

 condition held when the log rules in common use were invented, this fact, and not 

 the difficulty of scaling logs at the middle point, explains the general adoption of 

 the custom of basing the contents upon the diameter at the small end. 



46. Definition and Basis of Over-run. The purpose of all log rules 

 is to furnish a standard of measurement for logs, fair alike to buyer and 

 seller. For board-foot log rules this is best accomplished when the 

 rule measures accurately the amount of lumber that may be sawed from 

 straight, sound logs. It was the intention and the claim that each of the 

 fifty or more log rules extant should perform this service under the con- 

 ditions for which it was made; j^et in spite of this fact, the contents of 

 sound logs of the same dimensions, as measured by different rules, may 

 diiTer more than 100 per cent. 



While some rules based on incorrect premises never were accurate, most of the 

 rules as checked by actual mill tests were probably satisfactory when first employed. 

 But these rules were not changed to keep pace with the closer utilization brought 

 about by the improvements in machinery, methods and markets. Although obso- 

 lete as a mea.sure of actual product, they have been retained through custom. It is 

 difficult to supplant or alter a commonly accepted standard of measure, even if 

 grossly inconsistent and inaccurate. 



Antiquated log rules thus cease to perform the true function for which they 



