MEASUREMENT OF TIMBER. 63 



pendicularly into the ground, and screwing on the 

 board, he next measures the distance from the 

 base of the tree ; he then returns to the instru- 

 ment, and being cautious not to move the stock 

 out of the perpendicular, he looks through the pin- 

 hole in the brass upright D, and turns the board 

 round, elevating it in front, until the pinhole, 

 the wire at E, and the top of the tree are in line. 

 He now glances at the figure on the side, which 

 corresponds with his distance from the tree, and 

 looking along its line to the point at which the 

 free swinging cord intersects it, reads off the 

 height on the perpendicular line intersecting this 

 point. 



The board is once more placed horizontally, and 

 the operator, glancing along it to the trunk of the 

 tree, gets his assistant to mark the spot, the 

 distance of which from the ground is added to 

 the figures previously read off, which gives the 

 height. 



In the following diagram, if the distance from 

 the tree be 20 feet, the height given is 9 feet ; 

 if 30 feet, 13 feet; if 50 feet, 21 feet; if 90 feet, 

 39 feet; to which measurements have to be added 

 the height of the operator's eye above the base of 

 the tree, as before explained. 



If the instrument were elevated until the line 

 fell as in the dotted line C, then at 20 feet dis- 



