CUBIC MEASURE. 53 



CUBIC CONTENTS OF SQUARE TIMBER IN ROUND 

 LOGS. 



The most common methods of determining the cubic contents 

 of square timber that may be cut from round logs is the so-called 

 Two-thirds Rule, and the Inscribed Square Rule. 



The Two=thirds Rule. 



In the Two-thirds Rule the diameter of the log is taken at its 

 middle point, or the diameters of the two ends of the log are aver- 

 aged. The diameter of the log is reduced one- third to allow for 

 slab and the remaining two-thirds is taken as the width of the 

 square piece which may be hewed or sawed out of the log. The 

 cubic contents of the squared log are then obtained by squaring 

 this width and multiplying by the length of the log. 



This rule gives smaller results than the Inscribed Square Rule, 

 which shows the contents of a square piece that may be exactly 

 inscribed in a cylinder of the same diameter as the log. In sup- 

 port of the Two-thirds Rule it is claimed that there is a certain 

 amount of waste, due to the fact that logs are seldom perfectly 

 round and straight, and that the rule makes approximately the 

 correct allowance for such irregularities. 



The Two-thirds Rule is sometimes called the Big Sandy Cube 

 Rule. 



The Inscribed Square Rule. 



The Inscribed Square Rule gives the cubic contents of square 

 pieces which can be exactly inscribed in cylinders of different 

 sizes. The width of this square piece is usually obtained by mul- 

 tiplying the diameter of the cylinder by 17 and dividing the result 

 by 24, or by multiplying the diameter by 0.7071. This rule of 

 thumb for calculating the width of the inscribed square piece is 

 based on the fact that one side of the square inscribed in a circle 

 24 inches in diameter is 17 inches long. 



The exact mathematical rule for determining the side of a square 

 inscribed in a circle is to square the diameter, divide by 2, and 

 extract the square root. The table following was computed by 

 this method. 



Practically the same results are obtained by the Seventeen-inch 

 Rule, which is based on the fact that a 17-inch log will square 12 

 inches. According to the Seventeen-inch Rule the cubic contents 

 of a log are obtained as follows: Multiply the square of the diameter 

 of the log by its length and divide by the square of 17. 



