CHAPTER I. 



INSTRUMENTS USED IN FOREST MENSURATION. 



Instrujjknts are required to measure the circumference or 

 diameter of logs and trees, the length of logs, the height of 

 trees and the increment. Such measurements have for their 

 ohject, either to ascertain the various dimensions, or to 

 calculate from them the volume ; in the latter case the 

 measurement of the girth or diameter is used to calculate the 

 area of the cross-cut section, on the assumption that it forms 

 a circle. This is generallj^ called the sectional or basal area. 



The instruments may be classified as follows : — 



1, Instruments for the Measurement of the Girth. 



The girth may be measured with a tape, or with a string 

 and tape. 



The tape consists of a band, about half an inch in 

 breadth, so constructed that it alters its length as little 

 as possible when moist. It is divided on one side into 

 feet, inches and, if necessary, decimals of inches ; on the 

 other side the sectional areas corresponding to the length of 

 girth are sometimes noted. It is useful to have a small 

 hook on one end, which can be pressed into the bark when 

 the girth exceeds 5 feet. Long tapes are rolled up in cases, 

 which are made of leather, wood', or metal. Of late years, 

 flexible steel tapes have come much into use. 



The advantages of the tape are, that it is easy to handle 

 and convenient to carry. Measurements with it are, how^ever, 

 subject to various sources of inaccuracy, amongst which the 

 following deserve to be mentioned : — 



(a.) The sections of most trees are not circles. 



