42 THE MEASUREMENT OF LOGS— BOARD-FOOT CONTENTS 



It is practically impossible to secure closer utilization than 70 per 

 cent of actual total cubic contents of logs in the form of sawed inch 

 lumber exclusive of the utilization of slabs, edgings and sawdust when 

 circular saws whose kerf is j inch or more are used. By using band 

 saws which cut a |-inch kerf and by producing a large per cent of timbers 

 and boards thicker than 1 inch, thus reducing the waste from saw kerf, 

 the utilization may rise as high as 80 per cent for the larger logs. 



42. Errors in Use of Cubic Rules for Board Feet. By comparing the 

 per cent of possible utilization in Table III (§ 41) with the per cents 

 given for cubic log rules in Table II (§ 38) the character and relative 

 accuracy of these log rules can be judged. For the Blodgett Rule, with 

 a ratio of 115 units to 1000 board feet measured at middle diameter, 

 the ratio or per cent scaled is 51.9 for all classes and sizes of logs. By 

 comparison with Tiemann's Rule this rule is shown to be correct for 

 logs between 9 and 10 inches in diameter, but over-scales smaller logs, 

 and under-scales larger logs. The original Blodgett ratio of 100 : 1000 

 gives a per cent of 59.7. This is correct for 16-inch logs, too high for 

 all logs of smaller diameter and too low for larger logs. 



AMien the point of measurement is shifted to the small end of log, the 

 diameter measurement is correspondingly reduced. ^Vhen the scale of 

 board-foot contents thus determined is compared with this smaller 

 cylinder, the per cent of utilization can be expressed for such log rules 

 and applies uniformly to logs of all sizes, but only to the small cylinder 

 thus measured (§ 81). 



A comparison of the Blodgett Rule applied at the small end of log, 

 with the Tiemann rule applied at the middle of log, is shown below. The 

 per cents will apply to logs of all lengths whose total taper is but 2 inches. 



TABLE IV 

 CoMPAHisox OF Blodgett and Tiemann Log Rules for Certain Logs 



Cubic rules, as a class, when converted to read in terms of board feet, 

 thus tend to over-scale small logs and under-scale large logs, whether 



