26 INSTRUMENTS USED IN MENSURATION. 



same position when at rest. Oscillations can be arrested by a 

 stop (see at * in figure). That point of the wheel which 

 corresponds with the horizontal line of vision is marked as 

 zero, and from this point the wheel is graduated to 60 up 

 and down. A lens is fastened alongside the eye-piece, to 

 facilitate the reading of the angle on the wheel. By directing 

 the tube to any point the angle can be easily read off on the 

 wheel, which preserves the same position while the instrument 

 is being raised or lowered. The wheel is placed in a firm 

 metal case. 



In using the instrument, any convenient position, where the 

 top and foot of the tree can be seen, is chosen, the angles to 

 the top and foot of the tree read off, and the distance from the 

 eye of the observer to the foot of the tree measured. The 

 height is then found by the formula (see Fig. 18) : 



jj- _ DA x sin (u + I) 

 cos. u 



For convenience sake a little table accompanies the instru- 

 ment, in which the heights corresponding to various distances 

 and upper plus lower angles are given. In order to reduce 

 this table as much as possible, it gives only upper angles from 

 40 to 50 in intervals of 2, and lower angles from to 25 

 in intervals of 5. This necessitates placing a staff, on which 



