14 INSTRUMENTS USED IN MENSURATION. 



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

 a smooth surface maj^ be prepared with a sharp knife. 



4. IiistriLincnfs for the Measurement of the Length of FclleJ 



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 w^ell 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). 



a. Geomelrical Height Pleasuring. 

 If a horizontal plane is drawn from the eye of the observer 

 to a tree, it will hit the same, according to the position of 

 the observer, either between the top and the foot of the tree, 

 thus dividing it into two parts, one of which is situated above 

 and the other Ijelow the horizontal plane ; or a])ove the top ; 

 or below the foot. If the observer holds a plumb-line at 

 some distance from his eye, it may be considered parallel 

 to the axis of the tree ; hence, by looking at the top and foot 

 of the tree, similar triangles are formed, which are used for 

 the determination of the height of the tree. 

 Let A B (Fig. 7) be the height of the tree. 



D B, a. ray from the eye of the observer to the top of 

 the tree. 



