20 



INSTRUMENTS USED IN MENSURATION. 



and in a A C D 



hence, ^^ C = DC X tau. a : 



A C + B C = height = 11 = 1) (' (tan. a + tan. h). 

 If the horizontal h"ne of vision passes below the foot of the 

 tree, the above formula becomes — 



H = D C (tan. h - tan. a). 

 If it passes above the tree — 



H = D C {ta)i. a - tan. h). 



B 



# 



^:: 1,- J -^. 





In each of these cases the measuring of the horizontal line 

 D C can be avoided b}' placing a staff of known length along- 

 side the tree. In that case (Fig. 13) — 



and 



hence, 



M C = D C X tan. m ; K C = 1> C X tan. n 

 M X = 1 = I) C {fan. m + tan. n) ; 

 I 



DC = 



tan. ni -\- tan. n' 



