14 INSTRUMENTS USED IN MENSURATION. 



the column of wood and the inner wall of the borer, with its 

 toothed side towards the former, and firmly pressed in. This 

 prevents the cylinder from turning round inside the borer 

 during the following operation. The borer is now screwed 

 backward one or two turns, whereby the cylinder of wood is 

 severed at its base from the tree. The borer is now screwed 

 further in, which causes the severed cylinder of wood to 

 be pushed back, until it can easily be withdrawn and placed 

 into the cradle. In this way a column of wood is obtained of 

 about *2 inches diameter and from 2 to 6 inches long accord- 

 ing to the length of the borer. The breadth of the con- 

 centric rings is then measured. If the rings are not distinct, 

 a smooth surface may be prepared with a sharp knife. 



4. Instruments for the Measurement of the Length of Felled 



Trees and Logs. 



The length of felled trees and logs is measured with the 

 tape or measuring staff. The former has already been described. 

 The staff varies in length up to about 15 feet ; it should be 

 made of hard, straight-grained, well-seasoned wood, and well 

 varnished to protect it against moisture. The ends may 

 usefully be capped with metal plates. 



5. Instruments for the Measurement of the Height of Standing 



Trees. 



The instruments which have been designed for measuring 

 the height of standing trees are very numerous, but they are 

 all based upon one of two principles : either they determine 

 the height by means of similar triangles (geometrical height 

 measuring), or they serve to measure the angles of elevation 

 and depression (trigonometrical height measuring). 



