PLANTING AND CARE OF TREES 



23 



top of the tree is sighted along the straight edge AF which may 

 more conveniently be fitted with some simple form of sights. At 

 the point B, where the plumb line crosses the line CK the height 

 of the tree above the eye is indicated in feet. The height of the 

 eye above the ground is added to this reading if the measurement 

 is on the level, or a second sight may be made to the base of the 

 tree and the reading noted on CH added or subtracted, according 

 to whether the base of the tree is below or above the eye level. If 

 in the instrument as figured, the base line had been 100 ft. as 

 suggested, the height of the tree above the eye is given as 40 ft. 

 If, however, the base line had been 50 ft. the height is given at 20 

 ft., the reading being taken at the intersection of the thread with 

 the line running lengthwise through whatever number on MC has 

 been used to represent the base line. The two smaller diagrams 

 show the instrument in position for sighting to top and base of tree 

 respectively. In figure 13 the eye is assumed to be on the level 



/ 



," 



./ 



.y 



/ 



W 





#5ar-^' 



Fig. 13. Height Measurer in position for sighting to top of tree. 



with the base of the tree and only a single sight is necessary. In 

 figure 14, the base of tree is below eye level and the distance CD 

 must be found by a downward sight and added to the reading 

 obtained for BC. It can be readily seen from inspection of the 

 figures that the method is based upon the similarity of the triangles 

 ABC and ACD with the smaller triangles abc and acd. 



