146 



UNITS OF MEASUREMENT FOR STANDING TIMBER 



while the average taper may be somewhere between these limits and the volume 

 be given incorrectly by either the upper or lower class. A tree 20 inches at the top 

 of the first log will be classed as having a taper per log of 1 inch, 2 inches or 3 inches. 

 At the top of the fourth log, the first tree will measure 17 inches, the second tree, 

 14 inches, and the third, 11 inches. The actual average top diameter may fall at 

 12 inches or at 15 inches. 



123. Substitution of Mill Factor for Log Rules in Universal Tables. In the 

 above tables, the contents of the logs are determined by the standard log rule 

 used in scaling. Dr. C. A. Schenck substituted what is termed the mill factor 

 for the log rule, thus basing the volume of the tree upon the sawed output (§ 116). 

 Assuming, as a basis, that the cubic contents of the cylinder measured at the small 

 end of the log, when multiplied by 12, gives the maximum board-foot contents 

 (§ 12), the waste for slabbing, edging and saw kerf, independent of taper, which is 

 not considered, will reduce this output to from 8 to 5 board feet per cubic foot. 

 The per cents of cubic contents of the cylinder based on small end of log, which 

 these mill factors represent are: 



An example of these mill-factor tables is given on page 147, for logs 16 feet long: 



To determine these values the volume in cubic feet of the cylinder was mul- 

 tiplied by 5, 6, 7 and 8 respectively. These tables give the cruiser the oppor- 

 tunity to substitute a fixed per cent of utilization, as indicated above, for a log 

 rule. The other three variables remain the same, namely, diameter, number of 

 logs and rate of taper per log. 



It is assumed that the mill factor can be chosen to suit the local conditions of 

 milling, the factor 8 or 66| per cent representing the use of band saws in large mills, 

 while the factor 5 approximates the conditions in small local circular-saw hard- 

 wood mills, thus making the cruiser independent of log rules. This apparent 

 advantage is nullified by two serious defects: First, the taper of the log is neglected, 

 and this frequently produces a mill factor of 10 for large logs. Second, the board- 

 foot contents is assumed to vary directly as the cubic contents, so that the tables 

 force the use of log rules based on cubic rather than sawed products and introduce 

 the errors of this method. Mill factors increase directly with the average diameter 

 of the log independent of mill practice. It is not sufficient merely to know the 

 general character of the milling, but the sizes of the timber must also be known. 

 An average mill factor based on both of these variables may be seriously in error 

 and the use of different mill factors for logs or trees of different sizes is apparently 

 necessary to secure accuracy. The use of these tables is therefore not as satis- 

 factory as their apparent simplicity seems to indicate. 



