190 PRINCIPLES OF AMERICAN FORESTRY. 



The Heights of Trees are determined by means of a 

 most convenient and useful little instrument, called Faust- 

 man's mirror hypsometer. The distance of the observer 

 from the tree is measured with a steel tape, and the instru- 

 ment adjusted to that distance by the slide and vertical 

 scale. The top and bottom of the tree are then sighted 



FIG. 63. Calipering a tree. 



and the readings of the marginal scale where the plumb- 

 line crosses it added to or subtracted from each other, 

 according as the eye of the observer is above or below 

 the level of the tree. The vertical scale on the right-hand 

 side runs upward from zero to 60, and is continued on the 

 left-hand side to 110. If the distance is less than sixty, 

 the lower index mark of the slide where the plumb-line 

 is attached should be brought in a position coinciding 

 with the required reading on the right-hand scale. If 



