INSTRUMENTS USED IN MENSURATION. 



at some distance from the observer, and moved backward and 

 forward until the top of the tree is seen along the upper edge a, 

 and the foot along the lower edge b ; then the point is marked 



5 - 



2O_; 

 25 



Fig. 16. Christen's Hypsometer. 



on the instrument, where a ray from the eye to the top of the 

 staff hits the instrument at c. In this way similar triangles 

 are formed, in which the following equation holds good : 



or 



A B 



If now a 

 values for A 



= 12 inches, and Z = 4 3 r ards, and successive 

 = height of tree, are introduced, corresponding 



