equal to the area of the section measured. When the section is 

 elliptical, the mean of the longest and shortest diameters should 

 be taken | _) and the area of the section is then assumed to be 



r 

 - x 



2 

 2Vd 



Now as the real area of the ellipse is - 



* 



the mode of measurement recommended gives an excess of 

 \ (4-) , tnat " to sa y an exces s equal to the area of a circle 

 whose diameter is equal to half the difference of the two measured 

 diameters. Save in very exceptional cases, this difference is small 

 enough to be disregarded. The area of sections of irregular contour 

 can be determined from the mean of three diameters, but the 

 result thus obtained will generally be found to be somewhat too 

 high. 



Diameters are measured with a calliper resembling, in all its essen- 

 tial parts, a shoemaker's measure. The pattern of calliper invented 

 by Friedrich is one of the best. It consists of a graduated rule 

 A A, to one end of which is fixed the arm BB. CC is a moveable 

 arm capable of sliding backwards and forwards on the rule A A 

 which passes through the hole a b cd in it. To enable the arm to 

 slide freely the hole is made oblique to its inner face, but iu such 

 a> manner that as soon as it comes -in contact with the log to be 

 pmbrnoed, the arm is pushed back and rests perpendicularly on 

 the graduated rule, with which it remains in contact only along the 

 edges b and c. 



A si 



